Photocatalytic materials for energy and environment

Resume : Exploration of low-cost and earth-abundant photocatalysts for highly efficient solar photocatalytic water splitting is of great importance. Although transition metal dichalcogenides (TMDs) showed outstanding performance as cocatalysts for hydrogen evolution reaction (HER), designing TMD-hybridized photocatalysts with abundant active sites for HER still remains challenge. Here, a facile one-pot wet chemical method is developed to prepare MS2?CdS (M = W or Mo) nanohybrids. Surprisedly, in the obtained nanohybrids, single-layer MS2 nanosheets with lateral size of 4?10 nm selectively grow on the Cd-rich (0001) surface of wurtzite CdS nanocrystals. These MS2?CdS nanohybrids possess large number of edge sites in the MS2 layers, which are active sites for HER. The photocatalytic performances of WS2?CdS and MoS2?CdS nanohybrids towards HER under visible light irradiation (>420 nm) are about 16 and 12 times of that of pure CdS, respectively. Importantly, the MS2?CdS nanohybrids showed enhanced stability after long-time test (16 h), and 70% of catalytic activity still remained.

Resume : TiO2- based photocatalysis was applied successfully to several processes, including hydrogen production via water splitting and decomposition of organic pollutants in air or water. Enhanced photocatalytic activity can be obtained by decreasing the photocatalyst band-gap by joining TiO2 to noble metal nanoparticles or other oxides. Another approach to increase the catalytic performance of TiO2 is the introduction of defects in the crystalline structure of titania. In this work different titania colloids were subjected to a laser irradiation process by using nanosecond pulsed laser beams. As revealed by the characterization techniques (SEM, Raman, XPS), the TiO2 modified samples contain a higher amount of oxygen vacancies and Ti3+ defects. The catalytic activity in the water splitting reaction pointed out an increase of hydrogen production for laser modified titania with respect to untreated samples mainly under UV irradiation. To enhance the hydrogen evolution under visible light irradiation the parameters of the laser irradiation process, such as laser fluence, wavelength and solvent, were modified. In addition, gold and silver nanoparticles were added on laser modified titania samples, to exploit also the surface Plasmon resonance effect of these doping agents. The as-developed strategies may open up a new avenue for designing TiO2 materials in order to enhance the visible light absorption and the photocatalytic performance.

Resume : The semiconducor hematite (alpha-Fe2O3) as non-toxic, earth abundant and low cost material represents a promising photoanode material for solar water splitting. However, its applicability is so far diminished by an overall low charge carrier mobility as well as high intrinsic recombination rates. Assemblies of single-crystalline hematite nanowires might here excel coping with the short charge carrier lifetime, exhibiting a superior surface to volume ratio and supporting efficient light trapping. Here, we present a straightforward thermal vapour-solid synthesis of single-crystalline hematite nanowires using just plain low-carbon steel sheets (200 µm), and iron foils of various thickness (25 to 250 µm). After acetone and HCl cleaning, the substrates were placed in a quartz tube furnace and oxidized in ambient air or H2O vapour at temperatures of ranging from 300 to 800°C at atmospheric pressure. Single-crystalline hematite nanowires evolved during the oxidation procedure in high density and reproducibility with diameters of 10 to 200 nm and nanowire length of 1 to 12 µm. In dependence on the synthesis parameters and the substrate material also other morphologies can be triggered challenging the existing growth model relying solely on compressive stress driven diffusion. The morphological studies are furthermore accompanied by photo-electrochemical characterizations using NaOH as electrolyte and by ambient-photoelectron-spectroscopy to determine the surface Fermi level position.

Resume : The design of highly efficient electrodes for photo-electrochemical water splitting is of both fundamental and practical importance [1,2]. We show in this presentation that the water splitting activity on hematite can be strongly enhanced by modification of the structure and the local chemistry. We have calculated the oxygen evolution reaction (OER) overpotentials with Density Functional Theory. We will discuss the effects of surface orientation, active surface sites, presence of surface steps, lateral interaction, and oxygen vacancies on the OER activity [3,4]. Particularly, the presence of oxygen vacancies is found to be most efficient to control the OER activity. An overpotential of as low as 0.47 V was obtained for an optimal oxygen vacancy concentration of 1.26/nm2. The obtained results are also used to calculate the rate constants of the OER steps which will serve as input for state-space modeling [5] to simulate experimental, electrochemical impedance data. We have chosen hematite as a model system. However, the approach allows for identifying reaction rate limiting parameters and optimizing OER materials in general. (1) van de Krol, Grätzel, Photoelectrochemical Hydrogen Production, Springer, 2012. (2) Zhang, X.; Bieberle-Hütter, A., ChemSusChem 9 (2016) 1223. (3) Zhang, X. et al., J. Phys. Chem. C 120 (2016) 18201. (4) Zhang, X.; Cao, C.; Bieberle-Hütter, A., J. Phys. Chem. C 120 (2016) 28649. (5) Bieberle, A.; Gauckler, L. J. Solid State Ionics 146 (2002) 23.

Resume : We investigated the effect of hematite reduction and doping on water splitting properties using the unique possibilities of synchrotron-based characterization techniques. Model photoanodes based on hematite (?-Fe2O3) thin films were elaborated by atomic oxygen molecular beam epitaxy. Two different hematite reduction pathways are tested: in situ thermal reduction by annealing in vacuum or chemical reduction by surface etching. Our results take advantage of the capabilities offered by the following techniques for the study of such systems: X-ray Absorption Spectroscopy (XAS), Extended X-ray Absorption Fine Structure (EXAFS), Resonant PhotoEmission Spectroscopy (RPES) and X-ray PhotoEmitted Electron Microscopy (X-PEEM). On the one hand, the interplay between stoichiometry and crystallographic structure was studied by EXAFS for both undoped and Ti-doped hematite films [1,2]. On the other hand, spectro-microscopy based on absorption and resonant photoemission in XPEEM revealed the origin of photocurrent improvement for Ti-doped hematite layers which underwent partial chemical etching [3]. For each system studied, photocurrent measurements allowed to correlate the information provided by the synchrotron-based techniques with the water splitting performances [1-4]. [1] M. Rioult et al., J. Phys. Chem. C 120, 7482 (2016). [2] H. Magnan et al., Appl. Phys. Lett. 101, 133908 (2012). [3] M. Rioult et al., Surf. Sci. 641, 310 (2015). [4] M. Rioult et al., J. Phys. Chem. C 118, 3007 (2014).

Resume : Given large number of applications of tungsten trioxide, WO3, thin films including, among others, optical devices, flat panel displays, different types of microsensors, continuous efforts are directed towards improvement of their optoelectronic properties. The use of WO3, thin films in photoelectrochemical devices, e.g., as photoanodes for water splitting requires combination of large absorbance of light within the semiconductor band-gap energies with efficient photogeneration and separation of charge carriers that implies efficient kinetics of oxygen evolution reaction. These requirements are fulfilled for the mesoporous WO3 films synthesized by a sol-gel method from the precursors containing structure directing agents that favor formation of pores during annealing. Typical films with thicknesses of the order of 1 micron or less, formed on fluorine doped tin oxide (FTO) conductive glass, are deposited either by doctor-blade technique or by spraying and a subsequent annealing in oxygen at a range of temperatures above 500°C. These preparation conditions ensure structural ordering into highly crystalline monoclinic form of WO3. Due also to a large porosity of the films that allows a good permeation of the electrolyte, excellent water oxidation photocurrents reaching 4 mA/cm2at 1.2 V vs. RHE are regularly attained under standard AM 1.5G solar light (100 mW/cm2). Acknowledgment This research was supported by the National Science Centre grant, MAESTRO UMO-2013/10/A/ST5/00245.

Resume : The release of CO2¬ from the combustion of fossil fuels is the primary cause of global warming. To mitigate the potentially catastrophic effects of climate change an immediate and extensive reduction in CO2¬ emission must occur. Learning from nature, artificial photosynthetic methods are being explored as a route to producing renewable fuels, such as the photolysis of water to produce H2 fuel. Many metal oxide semiconductors are capable of splitting water. They are often durable, possess low toxicity and can be grown by low cost and upscalable methodologies such as chemical vapour deposition (CVD). Recent research has shown that a very promising method, perhaps vital for producing the most efficient devices, is to stack layers of different semiconducting materials and form a heterojunction. In this talk we explore the WO3/ BiVO4 heterojunction system grown using CVD for solar water splitting applications, with a focus on how nanostructural control and layer thickness affects efficiency. The charge transfer behaviour and reaction kinetics of metal oxide heterojunctions have rarely been studied. Transient absorption spectroscopy – a laser flash spectroscopy that can monitor the generation, recombination, trapping, charge transfer etc of photo-generated charges – was used to study the behaviour of photo-generated charge in this system. This allowed us to determine how this heterojunction system functions in improving the photocatalytic efficiency compared with its individual components, and helped us form design strategies to further improve this system.

Resume : During the past two decades, our group has been focusing on the development of molecular catalysts for water oxidation and hydrogen generation inspired by the active sites of respective OEC in Photosystem II and [FeFe]-Hydrogenases. These molecular catalysts are based on Mn, Ru, Fe, Cu, Ni and Co, with some water oxidation catalysts showing record high turnover frequency (TOF) of >1000 s-1 in pH 1 solutions with chemical driven. With the deep understandings of O-O bond and H-H bond formation mechanisms from those molecular catalysts, we have recently developed a series of nanostructured first row transition metal oxides/hydroxides as electrocatalysts for water oxidation and hydrogen generation with low overpotentials and high current densities. Light driven total water splitting in photoelectrochemical (PEC) cells have been demonstrated in our group and will be presented during the symposium. References [1] L. Duan, L. Wang, F. Li, F. Li, L. Sun, Acc. Chem. Res. 2015, 48, 2084. [2] F. Li, K. Fan, B. Xu, E. Gabrielsson, Q. Daniel, L. Li, L. Sun, J. Am. Chem. Soc. 2015, 137, 9153. [4] L. Sun, Science 2015, 348, 635. [5] K. Fan, H. Chen, Y. Ji, H. Huang, P. M. Classon, Q. Daniel, B. Philippe, H. Rensmo, F. Li, Y. Luo, L. Sun, Nature Commun. 2016, 7, 11981. [6] P. Zhang, M. Wang, H. Chen, Y. Yang, Y. Liang, L. Sun, Adv. Energy Mater. 2016, 6, 1502319. [7] Q. Daniel, R. B. Ambre, B. Zhang, B. Philippe, H. Chen, F. Li, K. Fan, S. Ahmadi, H. Rensmo, L. Sun, ACS Catal. 2017, 7, 1143.

Resume : Hydrogen is a promising potential clean source of energy which can be produced using abundant and renewable resources (e.g. water and sunlight). Water reduction catalysts are usually based on non-precious metals among which cobalt has been the most studied because of its abundance and low price. However the development of highly active and stable catalysts that can operate in purely aqueous solutions still remains a great challenge. The most active molecular cobalt catalysts reported in literature present a common architecture: a tetra- or pentapyridyl ligand inducing a distorted octahedral geometry, with the remaining coordination sites occupied by labile ligands (e.g. water). To further investigate the effect of coordination geometry on the catalytic activity, we designed a new ligand with six coordination sites that lead to the formation of an unusual heptacoordinate Co(II) complex. Under visible light irradiation in water, this complex can efficiently catalyze the production of H2 with a turnover number (TON) > 16000 mol H2 (mol cat)-1 and nearly 90% of H2 evolved within the first hour of irradiation, i.e. a turnover frequency (TOF) > 4000 mol H2 (mol cat)-1 h-1. These results suggest that heptacoordinate cobalt complexes, never used so far in the field of light-driven hydrogen evolution, represent a promising alternative platform for the development of highly active and stable photocatalysts.

Resume : Solar fuel generation is an actively researched subject, as it may facilitate a renewable energy-based economy. Several semiconductors have been proposed as light absorbers for this purpose; the best known one, TiO2, is very active, stable and inexpensive, but only converts UV light. Some sulphides like In2S3 and SnS2, with bandgaps of respectively 2.0 and 2.2 eV, have shown better photo-activity with visible light (using efficiently photons in their whole absorption range) and stability against corrosion (in oxidative abatement of organics) than e.g. the toxic compound CdS. Here we will describe our recent results on the photocatalytic production of H2 in aqueous suspension using a combination of a Ni-Fe hydrogenase enzyme (no precious or heavy metal involved) and In2S3, and will report new results on the photon-assisted (at lower potentials than in the dark) electrochemical generation of O2 using an electrode containing In2S3 as light absorber and a Cu-based laccase enzyme anchored to it which acts as molecular catalyst for O2 evolution from water with good faradaic efficiency.

Resume : The dwindling of fossil fuels combined with environmental factors caused by their combustion has led to an increase in the research into solar fuels, allowing the storage of the most abundant renewable energy resources. A viable solution to produce a fuel that is both clean and environmental friendly is solar water splitting for generating hydrogen (H2). We aim at using silicon semiconductor for light harvesting and to decorate them with molecular catalysts that will generate H2. Besides natural abundance, a key advantage of using silicon as a photoelectrode material relates to its low band gap of 1.12 eV along with high light absorption properties. Literature studies based on light-driven water splitting using molecular catalysts on silicon and related inorganic semi-conductors are limited [1], mainly because these materials get oxidized when immersed in an electrolyte solution. In order to avoid such a process, we developed metal oxide coatings to protect and passivate the semiconductor surface before molecular catalysts are immobilized onto these photocathodes. The attachment of molecular catalysts such as cobaloxime [2] and/or cobalt diimine-dioxime catalysts [3,4] proceeded through carboxylic acid or phosphonate functional groups which strongly bind to these metal oxide coatings. Such systems proved efficient for H2 production under irradiation. 1. Queyriaux, N., Kaeffer, N., Morozan, A., Chavarot-Kerlidou, M. and Artero, V., 2015. J. Photochem. Photobiol. C., Photochemistry Reviews, 25, pp.90-105. 2. Gu, J., Yan, Y., Young, J.L., Steirer, K.X., Neale, N.R. and Turner, J.A., 2016. Nat. Mater., 15(4), pp.456-460. 3. Kaeffer, N., Chavarot-Kerlidou, M. and Artero, V., 2015. Acc. Chem. Res., 48(5), pp.1286-1295 4. Coutard, N., Kaeffer, N. and Artero, V., 2016. Chem. Commun., 52(95), pp.13728-13748.

Resume : Photocatalytic water splitting remains an important option for the harnessing of solar energy for renewable fuel production. Whilst the best studied photocatalysts are based on inorganic semiconductors, there is growing interest in utilising organic photocatalysts for light-driven water splitting. Some of the most promising materials are linear conjugated polymers that demonstrate stable hydrogen production in water in the presence of a hole scavenger. Moreover, strong variations in hydrogen evolution rate (HER) are achieved through variations in chemical structure of the polymer, suggesting improved designs could be available through better understanding of the structure-property relationship and the mechanisms. We use fs-ms transient optical spectroscopy, molecular dynamics and quantum chemical calculations to investigate the impact of chemical structure on excited state dynamics and molecular organisation in the solvent mixture. We find a strong correlation of long-lived charged species on the polymer to the HER. From the simulations, we find interactions between the polymer and the solvents are apparently important to achieve efficient photoinduced charge transfer. It appears that the best performing materials contain polar dibenzosulfone groups that structure the solvent mixture to make both water and hole scavenger accessible to the photoexcited polymer. Possible mechanisms for the photocatalytic activity and potential improved polymer photocatalysts are discussed.

Resume : We report on a device for bias-free conversion of CO2 to syngas (H2 + CO), which by design, is integrated, scalable to large areas, and compatible with state-of-the-art photovoltaics and electrocatalysts. Within this contribution mainly three aspects will be addressed: adaption and integration of silicon HIT solar cells as photoanodes, cathode material development, and reactor assembly. Using the silicon HIT technology, which has recently acquired an industry leading level, we show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm2 can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we investigate different non-precious metallic (e.g. Ni, Cr, or Ti) protection layers and explore the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles (Mo, Fe) as OER catalyst. We demonstrate an OER overpotential below 500 mV along with a high photoanode operation stability (>100 hours). For improving the cathode performance, the deposition of single and compound nanosized catalysts, such as Ag, Zn, and Cu2O onto Ni-and Cu-foams by means of thermal- and electro-deposition is evaluated. Low overpotentials and stable H2:CO ratios between 1 and 3 with high Faradaic efficiencies are exhibited. Finally, we analyze whether two-compartments (anolyte, catholyte) or one-electrolyte solution will be beneficial in terms of device performance, considering the adequate membrane for each case.

Resume : In this paper, a novel overlapping junction of ZnO Nanowires (ZnO NWS-OLJ) was successfully synthesized on FTO glass (1.5×3.0 cm2, 10Ω/cm) via a facile and cheap solution chemical strategy. ZnO nanowires overlap and touch each other during their growth, and form ZnO NWS-OLJ. Meanwhile, we also successfully prepared ZnO NWS-OLJ / Au heterostructures and ZnO Nanorod (ZnO NRD) array on FTO glass for compare. Photocurrent density–Time (J-T) measurements for the as-fabricated ZnO NWS-OLJ and ZnO NRD array clearly showing that ZnO NWS-OLJ photoanode yields the higher photocurrent density than the ZnO NRD array. The average photocurrent density of the ZnO NWS-OLJ electrode reaches 40 uA•cm-2, which is 2 times higher than that (~20 uA•cm-2) of ZnO NRD array. Photocurrent density of ZnO NWS-OLJ / Au heterostructures is similar to that of pure ZnO NWS-OLJ. However, ZnO NWS-OLJ / Au heterostructures shows the better stability than ZnO NWS-OLJ and ZnO NRD array during Photocurrent density–Voltage (J-V) measurements. The enhanced photoelectrochemical (PEC) efficiency of ZnO NWS-OLJ is ascribed to their novel structure. The ZnO NWS-OLJ has many nanowire/nanowire junctions. These junctions could increase electrical conducting paths, and improve the separation efficiency of photoinduced charges. Furthermore, the surface plasmon resonance (SPR) effect of Au NPs can effectively increase the light absorption and enhance the PEC performance in ZnO NWS-OLJ / Au heterostructures. This study not only provides a high efficiency and stability PEC materials, but also provides a new idea to build novel structure in designing photoactive materials.

Resume : In the search of novel photocatalytic materials, the integrated control of nanoscale morphology, structure and composition is particularly useful to attain properties suitable for improving the system performances. Here we show an approach based on the pulsed laser deposition (PLD) of TiO2 films with hierarchically organized nano- and mesostructures combined with hydrogenation/reduction during the synthesis step and/or after a post-annealing treatment. Tuning of TiO2 hierarchical morphology was achieved by control of deposition pressure during laser ablation in an oxygen-poor atmosphere, which was followed by thermal treatments in a Ar/H2 mixture or in vacuum. Morphology, structure and optical absorption are investigated by SEM, Raman spectroscopy, XRD and UV-vis-NIR spectroscopy, whereas photocurrent measurements under solar simulator illumination with a three-electrode cell are employed to assess the material photoresponse. Notably, appropriate oxygen-poor deposition conditions were found to be beneficial for the material photocatalytic activity (i.e. combination between Ti or TiO2 target material and optimized O2 partial pressure in a Ar/O2 mixture as background gas); moreover, a thermal treatment in Ar/H2 or in vacuum leads to the appearance of an optical absorption tail towards the visible region and to a significant enhancement of the material quantum efficiency, if preceded by thermal sintering in air.

Resume : One promising approach to produce hydrogen and other fuels is the use of photo-electro-catalytic (PEC) cells for the direct conversion of water and/or CO2 using solar energy. A PEC cell consists of a photovoltaic cell (PV) that is directly combined with an electrolysis cell (EC), where the reaction takes place. In the case of multi-junction Si based photoelectrodes [1] used in the present work, annual variations in the solar spectral quality alter the portions of photogenerated charge carriers in each sub cell. This defines annual energy conversion, and thus the photoelectrode and PEC device performance under varied spectral conditions, which differs from obtained under AM1.5G spectrum. Our model evaluates annual performance of the PEC device based on the spectral changes in illumination for a range of average photon energies of the spectrum over a year time. This allows to study the electrical output of the photoelectrode, which is then combined with the current voltage behavior of the electrolysis, enabling the annual performance of the entire PEC device to be evaluated. We report on the annual evaluation of the hydrogen production by an integrated PEC device employing various types [1] of multi-junction silicon thin film solar cells in respect of the influencing parameters. The annual output of PEC devices based on these multi-junction cells is compared and discussed for different catalyst systems with either earth-abundant or precious (noble) metal catalysts. [1] F. Urbain, V. Smirnov, J.P. Becker et al, Energy Environ. Sci. 2016, 9, 145–154

Resume : Photoelectrochemical water splitting driven by solar light offers promises as a clean and effective method to convert solar energy into chemical energy. According to theoretical calculations, the oxynitride semiconductor LaTaON2 promises good performance as a photoanode for solar light driven photoelectrochemical water splitting. Conventional electrocatalysts for oxygen evolution reaction such as IrOx,1, 2 CoOx,2, 3 Co−Pi (inorganic cobalt phosphate)4 and NiOx5, 6 are often also used as cocatalysts for photoanodes to promote the kinetics of water oxidation and/or suppress the recombination of photo−generated electrons and holes by passivating the surface recombination centers.7 By loading CoOx or IrOx cocatalysts on TaON, Ta3N5, BaTaO2N, SrNbO2N and LaTiO2N, 50−400% higher photocurrents and more negative onset potentials for water splitting were usually observed.1, 2, 8-10 However, a previous research on LaTaON2 photoanode showed that no obvious improvement in photocurrent was obtained after loading cocatalysts Co3O4, Co(OH)2 or Co−Pi.11 Here we would like to investigate whether and how proper cocatalysts can be found to further improve the PEC performance of LaTaON2. A comparative study was carried out by loading NiOx, Ni0.7Fe0.3Ox, CoOx and IrOx as cocatalysts on LaTaON2 photoanode. The highest improvement in the photoelectrochemical performance for solar water splitting was achieved by using Ni−based oxides as cocatalysts, while CoOx and IrOx also enhance the performance (though to a lower extent), but more pseudocapacitive current was introduced thus resulting in an inefficient utilization of the photo−generated holes. By repetitive voltage cycling between 1.0 VRHE and 1.6 VRHE, NiOx and Ni0.7Fe0.3Ox are transformed into oxyhydroxides, which are known to possess higher catalytic activities. However, compared to the as−loaded oxides, the oxyhydroxides lead to lower photoelectrochemical performance, which is ascribed to the decay of the passivation centers at the photoelectrode−cocatalyst interface. It is concluded that without sufficient passivation of surface recombination states, high catalytic activities cannot be achieved. We suggest the use of a thin layer of TiO2 or Al2O3 deposited by atomic layer deposition in-between photoelectrode and cocatalysts to ensure a stable passivation. Despite the photoelectrochemical performance of LaTaON2 can be improved by cocatalysts, the maximum achievable photocurrent density is still not comparable to that reported for other oxynitride compounds. Poor electronic conductivity or severe bulk recombination of the photo−generated electron−hole pairs are the main limiting factors for the photon−to−current conversion efficiency in LaTaON2 photoanodes. 1. M. Higashi, K. Domen and R. Abe, Energy Environ. Sci., 2011, 4, 4138-4147. 2. M. Higashi, K. Domen and R. Abe, J. Am. Chem. Soc., 2013, 135, 10238-10241. 3. M. Higashi, K. Domen and R. Abe, J. Am. Chem. Soc., 2012, 134, 6968-6971. 4. C. Wang, T. Hisatomi, T. Minegishi, Q. Wang, M. Zhong, M. Katayama, J. Kubota and K. Domen, J. Phys. Chem. C., 2016, 120, 15758-15764. 5. C. G. Morales-Guio, M. T. Mayer, A. Yella, S. D. Tilley, M. Grätzel and X. Hu, J. Am. Chem. Soc., 2015, 137, 9927-9936. 6. Y. Liang and J. Messinger, Phys. Chem. Chem. Phys., 2014, 16, 12014-12020. 7. J. Yang, D. Wang, H. Han and C. Li, Acc. Chem. Res., 2012, 46, 1900-1909. 8. R. Abe, M. Higashi and K. Domen, J. Am. Chem. Soc., 2010, 132, 11828-11829. 9. K. Maeda, M. Higashi, B. Siritanaratkul, R. Abe and K. Domen, J. Am. Chem. Soc., 2011, 133, 12334-12337. 10. S. Landsmann, A. E. Maegli, M. Trottmann, C. Battaglia, A. Weidenkaff and S. Pokrant, ChemSusChem, 2015, 8, 3451-3458. 11. L. Zhang, Y. Song, J. Feng, T. Fang, Y. Zhong, Z. Li and Z. Zou, Int. J. Hydrogen. Energ, 2014, 39, 7697-7704.

Resume : Nowadays, the major challenge is to find new environmentally friendly ways to produce energy that may cover the global consumption. The direct conversion of solar energy though an energy carrier (fuel), storable and usable upon request, appears as an interesting alternative. Photocatalysis is an innovative and promising way to produce hydrogen from renewable energy sources. Indeed, the water dissociation (water-splitting) highlighted by Fujishima and Honda in a photoelectrocatalytic cell opened a promising way to produce hydrogen from light energy. In our study, we will focus on a gC3N4-TiO2 based system decorated with Au nanoparticles. With a band gap of 2.7 eV, g-C3N4 allows the valorization of an important part of the visible light spectra.The innovative aspect is to optimize every components in order to take advantages of each component and then to intimately associate them in hierarchical structure for obtaining competitive rates of hydrogen production at room temperature under solar and visible illumination. These optimized Au/gC3N4/TiO2 nanocomposites allowed to reach high quantum yield compared to the reference Au/TiO2 and Au/gC3N4 photocatalysts. Finally, photocatalytic activities of these composites will be correlated with their physico-chemical properties.

Resume : Since the discovery of water splitting on TiO2 under UV light by Fujishima in 1972, paramount research has been carried out on modification of TiO2 in order to exploit its photocatalytic ability under visible light. Among them sensitization of a large bandgap semiconductor with a small band gap semiconductor is a promising technique due to the efficient exciton seperation and absorption of visible light over a wide range. The present study explores a cost effective noble metal free system capable of promoting water splitting under visible light irradiation. A novel, single step, energy savvy solution combustion synthesis has been employed to synthesize CdS/TiO2/MS (M=Ni, Co, Cu) ternary hybrid materials. As the single step synthesis involves the simultaneous nucleation of CdS, TiO2 and metal sulfide it leads to the proper connectivity between the constituent materials. The as synthesized catalysts were characterized by using several techniques in order to understand the structural, optical and morphological characteristics of the catalysts. Photocatalytic H2 evolution from water splitting reveals the superior performance of these catalysts than CdS/TiO2/Pt composite. Among the three ternary hybrids CdS/TiO2/Cu2S shows the best H2 evolution from water splitting. The present synthetic strategy which is devoid of molecular linkers at interface is more advantageous for solar applications, where rate of electron transfer at the hetero junctions plays a major role.

Resume : Tantalum oxide (Ta2O5) is one of the most active photocatalsts for water splitting under UV irradiation because of its unique physical and chemical properties. However, the solar light-harvesting ability of Ta2O5 is inefficient owing to its wide band gap (> 3.8 eV). Thus, we have developed the hybrid materials of Ta2O5 nanotubes attached with CuO (with a band gap of 1.73 eV) nanoparticles to enhance the efficiency of H2 production. The Ta2O5 nanotubes were first grown by anodization. Then the CuO nanoparticles were grown on Ta2O5 nanotubes by chemical vapor deposition (CVD) to form heterostructure of CuO nanoparticles/Ta2O5 nanotubes. With the heterostructure of CuO NPs/Ta2O5 NTs, the evolution rate of H2 is significantly increased. The as-prepared CuO NPs/Ta2O5 NTs show an improved light absorption and electron-hole separation due to their reduced band gap and well-grown interface. Consequently, the photocatalytic activity for hydrogen production of the heterostructure of CuO NPs/Ta2O5 NTs was enhanced 70% more than as-grown Ta2O5 nanotubes without any assistance of co-catalysts. The results indicate that the as-obtained CuO NPs/Ta2O5 NTs are promising photocatalytic materials for hydrogen production.

Resume : Nowadays, the research to find materials for energy conversion has raised a huge interest in the scientific community[1]. Particularly, a high number of investigations are focused to find suitable compounds for photoassisted generation of hydrogen at the interface semiconductor/aqueous electrolyte[2] being that the most interesting and cleanest alternative to produce this energy carrier. In particular, the transition metal trisulfides (TMT) offer the possibility to find suitable materials to dissociate H2O molecule in a photoelectrochemical cell (PEC)[3,4]. In this work, we have synthesized ternary transition metal trisulfides (Ti1-xNbxS3, 0< x< 1) that belongs to a large class of low-dimensional compounds (MX3, M=Ti, Zr, Hf, Nb, Ta, W and X= S, Se, Te). These ternary compounds exhibit properties that fit the requirements to be used in energy conversion through photoelectrochemical processes in PECs, i.e. a layered morphology that provides a huge specific area, an adequate band gap, optimal transport properties, etc. In this work, the ability of these ternary materials to photogenerate H2 is compared with that of binary trisulfides (TiS3 and NbS3). To this aim, we have determined their flatband potentials, and we have measured the open circuit photovoltages and the photogenerated hydrogen flows (quantified by QMS) of the different samples under 200mW/cm2 visible light illumination in a PEC, at different bias potentials. [1] A. Castellanos-Gomez, Nat. Photonics 2016, 10, 202. [2] A. Fujishima, K. Honda, Nature 1972, 238, 37. [3] M. Barawi, E. Flores, I. J. Ferrer, J. R. Ares, C. Sánchez, J Mater Chem A 2015, 3, 7959. [4] E. Flores, J. R. Ares, I. J. Ferrer, C. Sánchez, Phys. Status Solidi RRL - Rapid Res. Lett. 2016, 10, 802.

Resume : Thin conformal films of ternary oxide materials can fill a multitude of roles in devices for renewable energy harvesting and conversion. This presentation will give examples of our work applying mixed oxide films prepared by atomic layer deposition (ALD) to the problem of electrocatalysis for a non-fossil energy economy: We have used ALD to prepare the n-type semiconducting spinel ZnFe2O4 as a photoabsorber for light-driven water oxidation. Compared to hematite (Fe2O3), a well-studied model system, the ternary oxide benefits from a lower rate of surface electron-hole recombination, resulting in a higher hole transfer efficiency. Additionally, using a macroporous Sb:SnO2 substrate, the light harvesting could be considerably improved, resulting in a 5-fold enhancement in photocurrent. The ternary oxides include numerous transparent conductors (TCOs). By combining ALD with templating methods, large-surface-area porous TCO structures are accessible, which can be used as scaffolds and current collectors for environmental electro- and photoelectrocatalysis.

Resume : The development and deployment of renewable energies that use hydrogen as a fuel or CO2 as a feedstock will enable a sustainable approach to generating, storing and using energy for the benefit of society as a whole. However, there is still much work to be done, in particular producing hydrogen from water and activating CO2. We are undertaking modelling studies using density functional theory to develop new metal oxide composites based on modifying TiO2 surfaces with metal oxide nanoclusters. We are exploring the potential for these composites to activate water using visible light photoexcitation or solar thermochemical processes to produce hydrogen and to activate CO2 to produce methanol or methane. We will present our DFT results on the fundamental properties of these novel catalysts, focusing on modification of rutile and anatase TiO2 with CeO2, SnO, Bi2O3 and alkaline earth metal oxides. From a photocatalysis perspective, this includes any band gap modification that red shifts light absorption into the visible region and the location of photoinduced electrons and holes. From the photothermal perspective we use the reducibility, as measured by the cost to form oxygen vacancies, as a descriptor of the redox properties of these materials to be activated. We then present our results on two pathways for hydrogen production, namely water oxidation and water splitting by filling oxygen vacancies. Finally, we discuss composites that can activate CO2 which will allow production of useful chemicals. The results from this DFT study allow us to assess the ability of these composite systems to activate water and CO2, thus allowing the prediction of composite materials that will drive hydrogen production and CO2 transformation.

Resume : Although Cu2O is a commonly used narrow band gap semiconductor to fabricate visible response photocatalyst, up to date there is only a few reports on Ag co-catalysed TiO2-Cu2O nanocomposites despite the promising synergetic effect of coupling and plasmonic co-catalysis. Herein we report a simple wet chemical synthesis method coupled with a UV treatment step for TiO2-Ag-Cu2O ternary hybrid nanomaterials. The effect of Ag content and the synthesis sequence of Ag deposition on the photocatalytic performance was investigated. The size distribution, crystal phase, optical and dark adsorption properties of the nanostructures were characterized by SEM, XRD and diffuse reflectance, respectively. Due to the mixed indirect and direct nature of the nanocomposites, the band gap estimation was performed by using both Tauc plot and differential reflectance model. The apparent visible activities followed pseudo-zero order kinetics where TiO2-Ag(3%)-Cu2O catalyst exhibited the highest rate constant, which was ca. two times as high as that of the binary TiO2-Cu2O catalyst. The synthesis sequence of the Ag deposition step significantly altered the material properties resulting in different dark adsorption and apparent visible activities. The improved performance of TiO2-Ag-Cu2O ternary hybrid materials could be related to the level of dark adsorption and could be attributed to the sophisticated charge separation framework between TiO2 and Cu2O and between Cu2O and Ag.

Resume : Titanium dioxide is a widely studied material with applications in fields such as photocatalysis and energy storage. Recent experimental and computational work has suggested that fluorination of TiO2 bulk nanoparticles can lead to a disordered structure, which in turn reduces the material’s bandgap and can stabilize the otherwise reactive {001} surface [1,2]. This is a promising approach to tune these materials’ properties, the disorder and varied chemical composition introduced at interfaces. These modifications to the surface in turn influence the structure to the electric double layer, further complicating a complete understanding of the surface chemistry. Together, these molecular details may lead to unexpected chemical reactivity and dynamics at the interface and remain to be thoroughly studied. We present here the results of density functional theory based molecular dynamics simulations along with free energy perturbation methods used to determine the intrinsic surface acidity (pKa) and redox potentials at the aqueous interface. We compare of the clean, aqueous anatase TiO2 interface to that of an aqueous, disordered fluorinated anatase. This first-principles approach, which takes into account the electronic polarization explicitly, allows for the hybridization of localized electronic states of the fluorinated surface and the water molecules at the surface, and thus provides insight into the relationship between the structural disorder present in the fluorinated surface and its surface chemistry. [1] Corradini, D.; Dambournet, D.; Salanne, M. Sci. Rep. 2015, 5, 11553. [2] Li, W.; Corradini, D.; Body, M.; Legein, C.; Salanne, M.; Ma, J.; Chapman, K. W.; Chupas, P. J.; Rollet, A. L.; Julien, C.; Zhagib, K.; Duttine, M.; Demourgues, A.; Groult, H.; Dambournet, D. Chem. Mater. 2015, 27 (14), 5014.

Resume : Amorphous molybdenum sulphides are an emerging class of inorganic polymers which are predominantly utilized for their superior catalytic properties in the hydrogen evolution reaction. In this work we investigate interactions between water vapours and polymeric MoS3⅔. We report that MoS3⅔ is a highly hygroscopic semiconductor which can reversibly bind up to 7% of its weight as surface water. The reversibly bound moisture is determined to predominantly interact through van der Waals forces and hydrogen bonds with the inorganic matrix. The presence of surface water was found to have profound influence on the semiconductors properties, modulating the materials photoluminescence by over one order of magnitude and its conductivity in excess of 3 orders of magnitude. In-depth analysis of the moisture adsorption process implicate shared and bridging disulphide ligands as the binding site. We utilized these newly discovered properties to design low energy dehumidifiers, highly sensitive conductometric moisture sensors and an ink based electrolyte less water splitting photocatalyst that relies entirely on the hygroscopic nature of MoS3⅔ as the water source. While the efficiency of the hydrogen evolution process is lower than that observed when catalysis is conducted in liquid suspension, it is surprising that a process entirely reliant on the hygroscopic properties of MoS3⅔ as a water source can lead to sustained hydrogen production. Overall the findings are relevant to the field of photocatalytic hydrogen production, while also hinting at additional future applications of amorphous molybdenum sulphides.

Resume : Electronic properties of BiVO4 and WO3 are essential for photocatalytic water splitting. Modulated and continuous wave (cw) surface photovoltage (SPV) spectroscopy allow for the characterization of electronic states in low and wide band gap semiconductor layers, single crystals and powders. Numerous powders of BiVO4, WO3 and H2WO4 have been compared with each other. In-phase and phase-shifted by 90° modulated SPV signals were correlated with (quasi) cw SPV measurements. Strong differences in the onset energy, being related to the band gap, and in SPV signals below and above the band gap were observed and discussed on the basis of trap limited transport. Modulated and (quasi) cw signals were analyzed with a model based on random walks of charge carriers through electronic states with distributions in space and energy. Furthermore, the SPV signals were also correlated with photocurrent measurements in electrolytes.

Resume : Hematite has been intensively studied in the field of photocatalysis and photoelectrolysis of water over the course of the last decades due to its almost ideal optical properties, e.g. band gap. However, limitations in transport properties could not be overcome completely, limiting the performance considerably, yet. One key material characteristic governing the charge transport characteristic is the electronic density of states (DOS) in the valence band. Photoelectron Spectroscopy (PES) is an ideal method to access the DOS of a material. Decades of research with this method on Hematite have shown the valence band DOS depend on the oxygen environment of iron. The influence of the crystallographic structure and orientation on the DOS is therefore very high. There are, however, huge differences in the spectra of Hematite in literature with a lack of structural clearance. Therefore, this work uses a combination of structural and spectroscopic characterization methods (e.g. XRD, Raman,UV-Vis Optical Transmission, XPS) to reveal how the sample crystallographic properties define the electronic structure. It can be shown that a crystalline Hematite thin film shows a much different electronic structure than an amorphous one. These differences are attributed to the bonding environment of iron and will be discussed with regard towards the use of Hematite as photoanode.

Resume : High-efficiency direct solar-to-fuel energy conversion can be achieved using direct semiconductor liquid junctions. However, the development of robust and inexpensive materials that also operate at high efficiency are needed to make this approach economically viable. In this presentation our laboratory?s progress in the development new materials will be discussed along with the application toward overall solar water splitting. Specifically, this talk will focus on the application of solvent-exfoliated 2D semiconducting transition metal dichalcogenides (TMDs). TMDs like MoS2 and WSe2 generally have intriguing electronic properties making them promising candidates for high-efficiency solar energy conversion. While semiconducting TMDs can be solution processing into dispersed 2D nanoflakes, the high concentration of edge sites act as recombination sites for photogenerated carriers. The challenges with charge transport, separation, recombination and water redox catalysis in these systems will be discussed herein with respect to the 2D flake size and defect passivation treatment. Overall it is shown that TMDs can achieve internal quantum efficiency for photon harvesting similar to bulk single crystal samples.

Resume : The electronic band structure of TiO2 or SnO2 is of fundamental implication for conversion of solar energy to electricity and fuels. The position of conduction band (CB) edge controls the potential of dye-sensitized solar cell, recombination blocking in perovskite solar cells and H2O or CO2 conversion to solar fuels. Electrochemical tools for monitoring the CB edge are the flatband potential (measured by impedance spectroscopy), onset of water oxidation or dark H+ reduction, cyclic voltammetric mapping of DOS including the electron trap states and spectroelectrochemical determination of CB electrons. Photoelectron spectroscopy (PES) and most DFT simulations support the higher CB position of rutile against that of anatase, but electrochemical studies indicate just the opposite. The controversy can be explained by adsorption of OH– and H+ ions on the titania surface. Cyclic voltammetry, electrochemical impedance spectroscopy, Hall measurements and PES allow analysing the band energetics in nm-thick TiO2/SnO2 layers grown by ALD. The low-temperature ALD-SnO2 layers are amorphous and perfectly pinhole-free for thicknesses down to 2 nm. The compact nature and blocking function of thin SnO2 films is not perturbed by annealing at 450 oC, which is a significant benefit compared to other amorphous ALD oxides. Amorphous and crystalline ALD SnO2 films substantially differ in their CB positions – a finding to be taken into account when considering band alignment engineering in various solar devices. Acknowledgement: This work was supported and by the Czech National Science Foundation (contract No. 13-07724S).

Resume : To enable high-efficiency semiconductor photoabsorbers for water splitting, protective and conductive coatings are needed stable from acidic to basic electrolytes while transparent to light and with proper band alignment. One of the best candidates is Titanium Dioxide, as it is known to be stable in wide range of pH and has shown effective protection both for HER and OER applications. We analyze the influence of ALD growth temperature on the charge transfer across protective TiO2 coatings for silicon photocathodes and photoanodes, and we use conductivity AFM to determine the conduction path across these TiO2 layers. As temperature is increased, TiO2 phase transition from amorphous to Anatase has a significant impact on the layer behaviour, affecting charge transfer and stability, main parameters for optimal device fabrication. Au/TiO2/Ti/Si structures have been prepared and studied for determining the I(V) curves, showing a hysteresis behavior correlated with oxygen vacancies movement. In cathodic applications, conduction across the protective layer can be increased by using higher deposition temperatures with more stable TiO2 phases and reducing defects and charge traps, obtaining higher fill factors up to 0.73 and 9 % half-cell Solar-to-Hydrogen conversion efficiencies. Also, crystallized TiO2 is demonstrated to be mandatory for long term stability, and over 300h continuous operation is proven. NiOOH catalysts help improving photoanodes efficiency in combination with TiO2.

Resume : The photocatalytic reduction of carbon dioxide (CO2) into solar fuels has attracted a lot of attention due to solving the global climate change and energy crisis. Various inorganic semiconductor composites have been reported such as heterojunction of semiconductors, semiconductor-metal and semiconductor-conductive carbon. Among them, a graphene-based photocatalyst is a promising composite due to its outstanding electrical properties, large surface area, high chemical stability and good template for nanomaterials. Although graphene offers the opportunity to efficient light harvesting, its efficiency for CO2 reduction remains a big challenge. Here we demonstrate that chemical functionalization of a graphene based photocatalyst could enhance the photoreduction of CO2. The amine-functionalized graphene/CdS composite (AG-CdS) was obtained by ethylenediamine coupling reaction with the carboxylic group on the surface of a graphene/CdS composite (G-CdS). The amine functionalized surface shows enhanced CO2 capture capacity, and consequently, could activate the CO2, which is the rate determining steps. In addition, the amine-functionalized graphene retains unique light harvesting property of graphene-based composite. Furthermore, the methane formation rate for AG-CdS under visible light is four times higher compared to G-CdS and it showed high stability during the photoconversion reaction.

Resume : The influence of the synthesis method on pure and Cu-modified titania for H2-generation via photocatalytic ethanol reforming is firstly analyzed. Concerning to the pure TiO2 based samples, enhanced photocatalytic behavior is obtained through the combination of sol-gel (SG) and hydrothermal (HT) methods. More precisely, the H2 productivity responds to a sequence: TiO2-P25 < TiO2-SG < TiO2-SG/HT. Also for the Cu-modified samples, the HT method leads to a better performance compared to the wet-impregnation method. This might be explained by the favored Cu insertion into the lattice with the corresponding structural modification e.g., crystallite size decrease, besides the beneficial effects associated to the enhanced charge transfer mechanism (formation of metal/TiO2 and/or p/n CuOx/TiO2 heterojunctions). Lastly, an interesting comparison between Ag and Pt as noble metals (NM) over Cu-modified TiO2-SG/HT is carried out. Although both metals entail significant catalytic improvements, the Pt supremacy for H2 evolution is clearly stablished. Summarizing, the obtained outcomes point out the importance of the synthesis method for controlling the electronic and structural properties as well as the key role of the noble metal and metal/support interface for achieving enhanced photocatalysts for ethanol reforming.

Resume : Abstract In this work, a noble-metal-free homogeneous system was constructed in one step with Keggin-type polyoxometalate derivates (POM) (??K7?xNaxPW11O39?14H2O (B-1) and ??A?(nBu4N)3[PW9O34(tBuSiOH)3] (B-3)) as a catalyst, , triethanolamine (TEOA) as a sacrificial electron donor and Eosin Y as a photosensitizer for water splitting to produce hydrogen under visible-light irradiation. POMs is produced by photosensitization under visible-light irradiation. The effect of various component concentrations and POMs with different addenda atoms on hydrogen production was discussed. POM based this system made of earth-abundant elements is expected to contribute toward the development of functional and efficient artificial photocatalytic system [1-2]. [1] D.Li, Y. Guo, C. Hu?, L. Mao, E. Wang, Photocatalytic degradation of aqueous formic acid over the silica composite films based on lacunary Keggin-type polyoxometalates, Applied Catalysis A: General 235 (2002) 11?20 [2] H.Yang, T. Liu, M. Cao, H. Li, S.Gao, R. Cao, A water-insoluble and visible light induced polyoxometalate-based Photocatalyst, Chem. Commun., 2010, 46, 2429?2431.

Resume : Tin disulfide (SnS2) with favored band alignment for the enhancement of visible-light absorption and catalytic activities makes it an ideal catalyst for photochemical CO2 reduction compared with many other known catalysts. Crystal orientation plays a critical role on catalytic properties since current density and overpotential are affected by different active sites on different crystal planes exposed. Therefore, in this study, we would like to investigate the effects of crystal orientations of SnS2 thin films to catalytic performance of CO2 reduction reaction. SnS2 thin films with different preferred orientations are prepared by metal-organic chemical vapor deposition (MOCVD) technique using [(CH3)2N]4Sn and H2S as precursors in order to study on their catalytic properties. MOCVD allows us to control the growth of SnS2 thin films precisely in kinematical approach as well as depositing SnS2 at low temperature. In addition, SnS2 textural thin films are deposited on various substrates such as Si, sapphire, and quartz for the sake of tuning different preferred-orientated thin films. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses are used to decide the texture of SnS2 thin films. Overpotential and current density of each films are analyzed by linear sweep voltammetry (LSV). LSV plot can then be converted into Tafel plot. The intrinsic activity of each textured film and the reaction pathway of CO2 reductions can therefore be detected. Catalytic properties of SnS2 thin films with different orientations can be explained by the reaction pathway. Through understanding the effect of different orientations of SnS2 and its catalytic properties, we are able to design and improve the catalytic performance of our materials.

Resume : Flower like metal oxides (such as Tungsten oxide, Nickel Oxide and Zinc oxide) were synthesized by acidic or basic precipitation at mild temperature. The structure and morphology of Flowerlike Metal Oxides have characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. We also study the photocatalytic performance for the photodegradation of 4- Nitrophenol.

Resume : Photocatalysis is a promising approach to improve both air and health quality. Materials with novel photocatalytic properties under indoor light irradiation leads to smart coatings, which are benchmark materials suitable for their indoor applications. In this contribution we focus on modified TiO2 as catalyst in heterogeneous photo-catalytic processes and address the efficiency of TiO2 -based building and construction materials on the removal of environmental pollutants indoors and outdoors. We also present data on the presence of eventually formed, toxicologically relevant and harmful by-products as the result of the photo-induced degradation of pollutants in an effort for better evaluation of induced risks for human health from the application of TiO2 modified materials. Finally, we present recent results on the disinfection performance of these material and the inactivation of severe pathogens contained in indoor air environments.

Resume : Ultra-thin two-dimensional (2D) materials have triggered tremendous interest for their exceptional properties and potential applications. In this work, cadmium sulphide (CdS) nanospheres are successfully decorated with 2D organic graphitic carbon nitride (g-C3N4) and inorganic bismuth oxychloride (BiOCl) semiconductor nanosheets as visible light driven (VLD) photocatalysts by solvothermal cum co-precipitation approach. The morphologies, optical and electronic properties of all the composite samples have been studied by spectroscopic and microscopic methods. Addition of BiOCl nanopetals into CdS/g-C3N4 heterostructure showed enhanced VLD photocatalytic activity for the degradation of rhodamine B (RhB), and the photocatalytic degradation constant (PCD) of BiOCl/CdS/g-C3N4 (0.09 min-1) was found to be 90 times that of g-C3N4 (0.001 min-1) , 1.8 times of CdS/g-C3N4 (0.05 min-1) and 2.25 times of BiOCl/g-C3N4 (0.04 min-1). The photocatalytic mechanisms are analyzed by active species trapping experiments, and hydroxyl radicals seem to be more crucial than superoxide radicals and holes for enhanced photocatalytic activity. It is established that morphology and synergic interaction of BiOCl and g-C3N4 nanosheets with CdS nanoparticles play a key role in separation of photogenerated electron-hole pairs and enhanced photocatalytic activity. This study highlights the potential applications of 2D semiconductor-based photocatalysts, and provides new insights for the fabrication of photocalysts with efficient visible light activity.

Resume : In this experiment, CaO particle react with a flowing mixture of TiCl4 and Ar gas at specific temperatures (600 - 770 ?C) to synthesize with two different morphology TiO2 materials. One was composed of rutile TiO2 nanowire (NWs) with length 8-12 ?m and diameter 80-120 nm. The other one was anatase phase of TiO2 NCs with length 400-800 nm and thickness 800-850 nm. And the two different crystal phase of TiO2 which are rutile and anatase in the middle and the end of boat. They were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and photo-luminescence spectroscopy (PL). The reactions are proposed to be via a vapor-solid reaction growth (VSRG) pathway. Finally, two different phases of TiO2 were applied in photocatalytic and field emission applications.

Resume : In this work we prepared MoS2 nanoflakes modified TiO2 nanoparticles (TiO2-MoS2 nanocomposite ) with varying concentration of MoS2 nanoflakes by using two-step hydrothermal synthesis method, involving preparation of TiO2 nanoparticles and MoS2 nanoflakes. The prepared samples were characterized by using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of pristine TiO2 nanoparticles and TiO2-MoS2 nanocomposite samples was evaluated by photocatalytic degradation of Rhodamine B (RhB) whereas photo-electro-chemical activity of these samples was measured by solar water splitting experiments in terms of current-voltage characteristics under dark and visible light irradiation. The presence of photocatalytic activity was also confirmed by the measurement of photocurrent. It can be found out that TiO2-MoS2 nanocomposite with 5 % MoS2 nanoflakes in TiO2 nanoparticles exhibits highest photocatalytic activity since it has the optimum amount of MoS2 which prohibits the recombination of photogenerated electrons and holes. In addition, highly apparent photocatalytic reaction rate constant is observed for MoS2 nanoflakes modified TiO2 nanoparticles with 5% MoS2 nanoflakes which is 2.27 times than that of pristine TiO2 nanoparticles.

Resume : Water pollution due to indiscriminate disposal of industrial dyes poses serious environmental hazards nationally and internationally. Some of the dyes are potentially carcinogenic and may induce mutagenicity and genotoxicity. Recently, graphene based nanocomposite have attracted considerable interest for photocatalysis based waste water treatment. Here, we report the synthesis of Graphene Oxide based hydrophobic PAN/GO nanofibers using electrospinning technique for photocatalytic degradation of Rhodamine 6G dye under natural sunlight illumination.The synthesized nanofibers were characterized using X- Ray diffraction, EDX, Field Emission Scanning Electron Microscopy and FTIR spectroscopy. Dye removal efficiency was investigated by monitoring UV-vis absorption intensity over time. Structural change in dye was studied using FTIR analysis. Kinetics of dye degradation reaction was monitored through pseudo 1st and pseudo 2nd order kinetics model. Effects of nanofiber weight and initial dye concentration on the degradation efficiency were investigated in details. Reusability and Stability of these synthesized nanofibers in dye solution have been studied using Scanning Electron Microscopy and FTIR spectroscopy. A comparative study for dye degradation was also performed using TiO2 coated nanofibers under Visible light and UV light illumination. These large area reusable graphene nanofibers provide a scalable and novel route for photocatalytic degradation of carcinogenic dyes from industrial water.

Resume : In response to challenges of energy crisis, global warming and climate change, electrochemical carbon dioxide reduction to produce chemicals or low-carbon fuels can served as mean for carbon neutral and therefore has attracted much attention recently. The notorious greenhouse gas carbon dioxide can then become feedstock to synthesize numerous low-carbon fuels such as formate/formic acid, methanol, ethanol and others to provide renewable energy storage with high energy density forms. However, most of electrochemical reduction processes utilize precious metals (platinum or palladium) as electrode, or apply harsh temperature/pressure conditions which may limit the development of electrochemical reduction of carbon dioxide toward industrial applications. In this work, we use copper nanoparticle decorated reduced graphene oxide (rGO), which is derived from one of the most abundant elements (i.e. carbon), as the electrochemical reduction electrode. The process can achieve carbon dioxide reduction with lower working potential. Electrode materials of platinum, palladium, rGO and copper decorated rGO as working potential were compared. The influences of reduction potential, electrode material and electrolyte acidity on production yield were discussed and characterized by scanning electron microscopy and gas chromatography. In addition, this process was conducted under ambient temperature/pressure without harsh condition or complicated instruments. The rGO/Cu electrode can achieve a cost reduction of 95% in average compared to precious metal electrodes (Pt and Pd) and still have almost the same yield (at -0.2 V (vs Ag/AgCl), pH4.8). The work shall pave a new path toward developing carbon dioxide reduction electrodes and further promote development of electrochemical reduction process toward carbon dioxide derived low-carbon fuels.

Resume : This article reveals the merits of GaON/ZnO hybrid nanorods arrays (NRAs) grown over FTO substrate, as a conceivable substitute for efficient photoelectrochemical (PEC) water splitting for the first time to the best of our knowledge. We effectively fabricated an efficient nitrogen rich photoactive GaON/ZnO NRAs over FTO electrode by single step guileless solvothermal reaction at fairly lower temperature. The optimized reaction conditions were determined to be 80 oC and 6 hrs. The morphological study is provided in SEM images, which clearly indicated the ZnO nanotubes bulging out from the nanoflowers sheets of GaON. The even intermixing of ZnO and GaON is observed in all SEM micrographs. The existence of nitrogen in the hybrid product is confirmed by elemental analysis through EDX, elemental mapping and XPS techniques. The XRD and FTIR analysis give information about structural properties. The UV/Vis-DRS spectroscopy results give indication about the optical properties and suggested the decrease in the bandgap of GaON/ZnO NRAs from 3.26 eV to 2.58 eV as compared to pure ZnO. The observed photocurrent densities for ZnO and GaON/ZnO NRAs are 0.4 and 10 mA/cm2 respectively, which revealed significant 2.5 folds increase for the later with better stability. The electron impedance spectroscopy (EIS) through Nyquist plot also indicated the significant decrease in the impedance, which improved the interfacial charge transfer in the case of GaON/ZnO NRAs. This research work connotes the prominence of nitridation on the overall photochemical properties of GaON/ZnO NRAs.

Resume : For most photocatalytic reactions, charge transfer predominantly takes place at certain particular reaction sites, most commonly at surface defects. The understanding of the nature of photo-active sites is of major importance for all metal nanoparticle-semiconductor hybrid materials. Gallium nitride is one of the most stable semiconductors even in photo(electro)chemistry, when decorated with metal nanoparticles as co-catalyst. Furthermore, the band gap of 3.4 eV can be tuned by alloying and the material can be grown either p- or n-type doped with various dopant concentrations. In this work we demonstrate the elucidation of reaction pathways on clean extended GaN(0001) surfaces in the UHV with a chemical probe molecule. For this purpose CO is the molecule of choice, as it exhibits photon-stimulated desorption (PSD) upon UV irradiation on the bare semiconductor surface. Further, it is shown that the CO adsorption properties are dependent on semiconductor doping, while the PSD kinetics remain the same. By an oxidation of the nitride surface, the photo-reactivity is drastically limited, indicating that its origin is closely associated with the surface properties of the clean nitride surface.

Resume : Cupric oxide is earth-abundant compound which has been a hot topic of investigations for many years. Nanosized CuO (nano-CuO) has superior physical and chemical properties that differ from those of its micro or bulk counterparts. Nano-CuO finds multiple applications in various fields, including as photocatalyst for dye degradation and water splitting [1]. In this study the local atomic structure and dynamics of nano-CuO have been probed by the Cu K-edge x-ray absorption spectroscopy in the temperature range from 3 K to 300 K in comparison with the bulk oxide. The analysis of the extended x-ray absorption fine structure (EXAFS) indicates that the first-order Jahn-Teller (JT) effect, which is responsible for the strong distortion of CuO6 octahedra [2], dominates the size effect in nano-CuO. The presence of the JT effect makes challenging theoretical simulations of atomic structure of CuO nanoparticles. We will address this problem using molecular dynamics simulations based on the ReaxFF variable charge force field model [3] and demonstrate the use of the EXAFS data for the force-field model validation [4]. The results of the complementary EXAFS analysis by the reverse Monte Carlo simulations will be also presented. [1] Q. Zhang et al., Prog. Mater. Sci. 60 (2014) 208. [2] S. Åsbrink, L.-J. Norrby, Acta Cryst. B 26 (1970) 8. [3] A. Ahmed, P. Elvati, A. Violi, RSC Adv. 5 (2015) 35033. [4] A. Kuzmin et al., Z. Phys. Chem. 230 (2016) 537.

Resume : Barium titanate is known as a perovskite ferroelectric material with a high dielectric constant, which finds its applications mostly as a multilayer ceramic capacitor in microelectronics. Its ferroelectric properties as well as its band gap of 3.2 eV makes it also interesting for solar water splitting. The ability to create spontaneous polarization has a significant influence on surface photochemistry. Enhanced charge-carrier separation, which in turn inhibits the recombination of holes and electrons, might result in good photocatalytic activity. Spray-flame synthesis (SFS) is a gas-phase based combustion synthesis method which allows continuous production of crystalline particles with well-defined morphology and chemical composition in the nanometer range. SFS of barium titanate by is a challenging task due to large quantities of CO2 in the reactor off gas, which can result in the formation of thermally stable BaCO3 as a byproduct. It will be shown that a careful choice of solvents, precursors, and process conditions enables the formation of phase-pure crystalline powders with an average particle size of 10 nm. As the surface of the pristine particles contains adsorbed moieties of combustion products, a subsequent heat treatment at 250°C led to a surface cleaning. We will present and compare the photocatalytic activities of pristine as well as heat-treated barium titanate nanoparticles of different phase compositions and discuss strategies to improve their performance.

Resume : Solar water splitting is a promising way to sustainably produce hydrogen, a clean and storable fuel. We recently reported the application of thin-film silicon multijunction photocathodes with a solar-to-hydrogen (STH) efficiency of 9.5%.[1] The photoelectrodes were optimized on a laboratory scale device area ?1 cm². However, practical applications critically rely on approaches that are scalable to large areas. Here, we introduce two approaches for the design of suitable metal grid front contacts to address the challenges related to the current collection in photoelectrodes of areas beyond typical laboratory scale experiments. We demonstrate the successful upscale of our multijunction solar cells to an active area of 64 cm². Furthermore, we advertise a modular device setup which utilizes sheet metal electrodes to protect the photoelectrode from corrosion. Finally, we present a stand-alone integrated solar water-splitting module with an active area of 64 cm² and a long-term stable STH efficiency of approx. 5%. The modular setup of our device provides a versatile toolbox for the evaluation of upscaling concepts for photoelectrodes and catalysts. 1. F. Urbain, V. Smirnov, J.-P. Becker et al., Energy Environ. Sci., 2016, 9, 145?154. 2. J.-P. Becker, B. Turan, V. Smirnov et al., J. Mater. Chem. A, 2017, DOI: 10.1039/C6TA10688A

Resume : Metal perovskite oxynitrides – AB(ON)3 – are promising materials that may combine the advantages of oxides and nitrides. The introduction of nitrogen atoms into the anionic network by substitution makes it possible to tune the properties of these materials, leading to small band gaps able to absorb visible light, while still retaining the photocatalytic ability common to ultraviolet (UV) absorbing oxides. The importance of this is that UV light makes up only 4% of the solar spectrum, compared to 40% for visible light radiation. These visible light photocatalysts can generate reactive oxygen species, such as the superoxide radical anion and hydroxyl radicals, both powerful oxidants. This means that they can be used indoors, under ordinary illumination conditions for the degradation of organic pollutants and biological contaminants. This has applications in clinical environments, improving food manufacturing or as self-cleaning home products. Passive perovskite oxynitride visible light photocatalysts could also be incorporated into the built environment to aid air purification. In this context our work has been focussed on the formation of thin films of these perovskite metal oxynitrides with A = Ba, Ca, La, Sr and B = Nb, Ta, making use of sol-gel dip coating methods, using alumina substrates as a proxy for domestic kitchen and bathroom tiles. Here we summarise the synthesis and characterisation of these films using X-ray diffraction, and organic pollutant photocatalytic degradation.

Resume : Solar energy is one of the primary alternatives for solving the issues induced by the consumption of fossil fuels. Photocatalytic water splitting is among the major efforts to generate fuel gas like hydrogen to alleviate pollution. In this work, we developed the heterostructure of HgS nanoparticles on ZnO nanowires for water splitting. ZnO has been widely used in water splitting because of its favorable optical and electrical properties with relatively low recombination loss and high electron mobility. In addition, ZnO nanowire grown in furnace has great crystallinity resulting in decreasing charge carrier loss at the grain boundaries. On the other hand, ZnO can only be utilized in ultraviolet light region, which contributes less than 5% of the total energy of the solar spectrum, due to their wide band gap (3.3 eV). To facilitate the reaction driven by visible light, we incorporated HgS nanoparticles which has great absorption in infrared light region but weak photoluminescence because of the surface defect trapping excitons. As a result, electrons excited by sunlight would migrate from HgS nanoparticle to ZnO nanowire, then to the surface to reduce hydrogen ions. In the present work, both HgS nanoparticles and the ZnO nanowires were grown by chemical vapor deposition (CVD) methods. XRD, SEM and TEM were utilized to characterize the samples. The hydrogen production was measured with a gas chromatograph. The experimental results indeed showed the hydrogen generation rate of present structure increases significantly compared to pure ZnO nanowire irradiated by sunlight.

Resume : The electronic processes taking place in Semiconductor Quantum Dots (QDs) functionalised with transition metal complexes (dyads) are object of a growing interest because of their possible applications to water splitting. In fact, QDs have recently proved to be among the best sensitizers for light-driven hydrogen evolution in the presence of transition metal complexes. However, most of the systems reported so far are based on a multicomponent strategy, which does not enable to tune the electron transfer kinetics and limits the efficiency of the process. QD-metal complex dyads usually show superior catalytic efficiencies with respect to multicomponent systems. However, the role played by the connecting unit in the modulation of electron transfer dynamics is basically unexplored. We report the photo and electrochemical study of a family of dyads based on CdSe/ZnS core/shell QDs functionalised with several Ir(III) derivatives which differ only in the connecting unit. The most common connecting units (i.e. amine, dithiocarbamate, carboxylic and phosphonic acids) used for QDs functionalization have been investigated. The results of this study show that the chemical nature of the connecting unit induces major changes in the energy/electron transfer efficiency and suggest that the most common connecting units used for the functionalization of QDs (namely amine and dithiocarbamate) are probably not suitable for an efficient electron transfer in dyads.

Resume : Cuprous oxide (Cu2O) have many advantages including direct bandgap corresponding to visible light, high absorption coefficient, low cost, low toxicity and ease of fabrication. Nanostructured cuprous oxide possesses tunable bandgap and reduced electron-hole pair recombination rate during transmission, which is suitable for water splitting and catalytic applications. In this study, Cu/Cu2O nanowire array was fabricated by electroplating Cu nanowires into nano-porous templates, followed by appropriated oxidation treatment. The electroplated Cu nanowires possess very dense nanoscale twin defects that can modify the surface structure and change the oxidation kinetics of Cu nanowires accordingly. By controlling oxidation ambient and temperature, we can grow a uniform cuprous oxide layer on the nanotwinned Cu nanowires. The Cu/Cu2O core-shell structure demonstrates high chemical stability. The Cu/Cu2O interfacial structure was examined by high-resolution TEM. The oxidation mechanism and photoelectrochemical properties of copper/cuprous oxide nanowire arrays will be investigated.

Resume : TiO2 in its rutile form has gained considerable attention for its photocatalytic properties [1]. Due to a pronounced absorption in the visible and infrared wavelength regime as well as its n-type conductivity, reduced TiO2 is of special interest for a wide range of applications [2]. For example, Schottky diodes consisting of reduced TiO2 and a suitable metal contact can be used as building block for a photochemical cell to split water [3] or for solar light harvesting [2]. Here, we report on the fabrication of Pd/TiO2 and Pt/TiO2 Schottky diodes using rutile single crystals with different surface orientations. Prior to deposition of Pd or Pt the TiO2 crystals were reduced in an atmosphere containing H2 leading to n-type conductivity and a pronounced light absorption in the visible/infrared spectral range. The Schottky diodes exhibit a rectification of up to 5 orders of magnitude between forward and reverse bias voltage (± 4 V). However, temperature-dependent Current-Voltage measurements reveal the presence of a highly-compensated interfacial layer between the metal contact and TiO2. Impedance spectroscopy is used to investigate the influence of the highly-compensated interfacial layer on the electrical performance of the Schottky diodes. Moreover, the long-term-stability of the diodes will be discussed. [1] A. Fujishimam et al., Nature 213 (1972). [2] A. Naldoni et al., Journal of the American Chemical Society 134 (2012). [3] A. Wolcott et al., Small 5 (2009)

Resume : In the framework of research on clean, secure and efficient energy, hydrogen production by solar water splitting is a promising method. During this process, electron ? hole pairs are generated in insolated semiconductors, which will then participate to the water oxido-reduction reactions[1]. The Aqueous Chemical Growth (ACG)[2] method is an affordable and versatile growth process with low capital investment and it provides high-quality oxide films to be used as photoanodes. Hematite (?-Fe2O3) photoanodes are obtained by air-annealing (500°C for 1h) the iron oxyhydroxide (FeOOH) deposited by ACG on FTO (F doped SnO2) substrates. For this purpose, FTO substrates were immersed in low pH aqueous FeCl3 solutions and maintained at a temperature of 95°C for several hours. We obtain continuous films of perpendicularly grown and perfectly defined iron oxide nano-rods. A comparative study is reported by varying the pH and the deposition time. All samples were characterized before and after the annealing using XPS (X-ray Photoemission Spectroscopy), XRD (X-Ray Diffraction) SEM (Scanning Electron Microscopy) and AFM (Atomic Force Microscopy). Morphology, structure and chemical changes are presented, discussed and related to the photo-activity of the samples measured using a dedicated setup[3]. [1] A. Fujishima et al., Nature 238, 37 (1972) [2] L. Vayssieres, IJNT, 1, (2004); L. Vayssieres et al., Chem. Mater. 13, (2001) 233 [3] M. Rioult et al., J. Phys. Chem. C 118 (6), 3007?3014 (2014)

Resume : Recently, the reduction of atmospheric carbon dioxide (CO2) to chemically valuable fuels has attracted considerable attention. Several groups of semiconductors have also been studied for their ability act as photocathodes in photochemical or photoelectrochemical setups. Semiconductors are divided into two groups, n-type semiconductors and p-type semiconductors depending on whether the majority carriers are electrons or holes. To use a photocathode for electrochemical reduction of CO2 a p-type semiconductor is an essential material. A p-type semiconductor can act as efficient photocathode due to electron levels in the conduction band. These levels can be occupied by electrons created by irradiation of incident light. Energy of electrons is higher than the band gap energy. To date many p-type semiconductors were studied in role of photocathodes. Metal phosphides, especially gallium phosphide (GaP) and indium phosphide (InP) belong to this group of semiconductors. Both have been studied for their ability to reduce CO2 in photoelectric and photoelectrochemical setups. GaP with p doping is more suitable for application for CO2 splitting due to its higher band gap energy (2.28 eV) in comparison to InP (1.32 eV). Properties of GaP as photocathode were studied by more laboratories. Planar bulk GaP material was used for these studies. In adition, no aditional co-catalyst was used in these experiments. In 1979, Inoue et al. [1] first reported the photocatalytic reduction of CO2 in aqueous solution to produce formaldehyde (HCHO), formic acid (HCOOH), methanol (CH3OH), using various semiconductors. Here the GaP and silicon carbide (SiC) were used as photocathodes. GaP exhibited the highest selectivity for production of methanol but at exceptionally high overpotentials. Later, a pyridinium ion was found to be an efficient catalyst for the reduction of CO2 to methanol in conjuction with hydrogenated Pd electrode. The faradaic yields up to 30% have been observed [2]. Combination of GaP photocathode and pyridinium co-catalyst have been studied by Barton et al. [3]. They employed p-type of GaP wafer as photocathode in a photoelectrochemical setup, in which a photonic external quantum efficiency of 2.6% at illumination by external light source with ?=465 nm was obtained. The GaP photocathode was held at potential E=-0.5 V against the saturated calomel electrode (SCE). Pyridine was added to the water based electrolyte to act as co-catalyst. High efficiency obtained at this setup can be explained by mechanistic approach based on the idea that CO2 reacts directly at the photocathode surface via pyridinium-mediated process or pure chemical approach that pyridine is responsible for the selectivity of the process towards methanol through intermediate carbamate species. Later Keith et al. [4] presented their theoretical study of pyridinium role in photoelectrocatalytic CO2 reduction process. They applied first-principles density functional theory and continuum solvation models to calculate basic pyridinium parameters in this complicated chemical process. Their results showed that electrode surface plays a critical role in the pyridinium reduction. This is a substantial alternative to the mechanistic view on the pyridinium catalyzed photoelectrochemical CO2 reduction. We studied influence of the nanostructured surface on the GaP photocathode properties. In our paper we will present results of photoelectrochemical measurements obtained at using of GaP photocathodes with planar and by nanowires covered surface. We prepared two types of GaP based photocathodes. Basic type consists of planar GaP wafer p-type with hole Hall concentration 1-3x1018 cm-3. The second type of photocathode is p-type GaP also (the concentration of holes was 0.9?1.5x1018 cm_3) but its surface is covered by nanowires (NWs). GaP nanowires were grown on GaP substrate by MOVPE using VLS mode. It is a NW growth mechanism from vapour phase during which an alloy phase catalyzes and rapidly adsorbs a source material vapour to a supersaturation level which subsequently initiates crystal nucleation at the liquid?solid interface. GaP nucleated into the NWs at Au seed particles, which coalesced from a sub-nanometre thick Au layer deposited on the substrate by evaporation. Upon the deposition of the Au layer, the substrate was heated under a gradually increasing phosphine (PH3) flow in an AIX 200 low-pressure MOVPE reactor. Once temperature Ta = 650 °C was reached, it was annealed during 10 min under a PH3 flow. This led to the formation of Au seeds with 20-40 nm in diameter and a density of 400 seeds per 1 square micrometre. GaP NW growth was subsequently performed from (PH3) and trimethylgallium (TMGa) at a temperature of 500 °C and pressure of 100 mbar. The NWs were doped with zinc from diethylzinc (DEZn) to be p-type. The aqua based electrolytes were composed from basically 10mM pyridine supported by 0.5M KCl [3]. Concentration of pyridine was systematically changed with aim to plot its correlation with the nanostructured GaP surface. Electrochemical experiments were performed using a DY2116B potentiostat/galvanostat with a three electrode system, in which a GaP electrode, Pt wire electrode (or polished graphite electrode eventually) and Ag/AgCl electrode were used as a working electrode, counter electrode and reference electrode, respectively. We used three various light sources: solar simulator Oriel, 100 W Hg-Xe arc lamp and 50W LED white light. Acknowledgements: This research was supported by the Science and Technology Assistance Agency under grant APVV-14-0297 and by VEGA project No. 2/0104/17 and resulted from implementing a Centre of Excellence for New Technologies in Electrical Engineering project, ITMS code 26240120011, and supported by the Research Development Operational Programme funded by the ERDF. References [1] T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature 277, (1979) 637 - 638 [2] G. Seshadri, Ch. Lin, J. Electroanalytical Chemie, 372 (1994) 145-150 [3] E. E. Barton, D. M. Rampulla and A. B. Bocarsly, Journal of the American Chemical Society 130, (2008) 6342-6344. [4] J.A. Keith et al. Journal of the American Chemical Society 134, (2012) 7580-7585.

Resume : The purpose of this paper focuses on a new sol?gel synthesis of doped titania xerogels which fulfills the criteria of cost and which can be carried out without the use of organic solvents and high-temperature treatment. The preparation method was based on a sol?gel technique applied to titanium tetrabutoxide as a precursor. By controlling the hydrolysis of the precursor and by also monitoring the rate of the drying process, transparent TiO2 xerogel sheets, showing nanometric thickness, were obtained at room temperature. The nano-sheets prepared were transparent to wavelengths between 380 and 700 nm; they showed a nanoporous texture, a specific surface area of ? 240 m2 g?1 and, by X-ray diffraction, they showed a mixed anatase/amorphous structure. The X-ray diffraction spectra indicated that the anatase crystals had a size in the range of the nm. UV reflectance spectra indicated at room temperature the same characteristics of the analogous nanoparticles. Moreover NMR characterization of transparent layered structure was performed in order to better understand the role of the dopant in stabilizing the structure itself.

Resume : The search for green efficient energy sources is becoming a mater of increasing urgency. In this field, water-splitting technology, that is photoelectrochemical conversion of water into O2 and high energetic H2, shows a great interest, as it would permit a direct conversion from solar energy to chemical energy. This technology would offer a way to a sustainable production of H2. H2 is considered as a promising clean fuel candidate to replace hydrocarbons fuels in the future. The photosynthesis of this molecule from water is of major attraction to researchers. Since the discovery of the ability of TiO2 to dissociate water into O2 and H2 upon irradiation, an impressive amount of work has been devoted to the development of photocatalyst semiconductors. Although very active, TiO2?s major drawback is that it is active only with the UV part of the solar spectrum, meaning that the visible part, representing 45% of the solar spectrum is not used. To overcome this problem, the scientific community is developing new semiconductors having smaller band gaps than TiO2. In parallel of the experimental design of new semiconductors, the community of theoretical chemists brought its contribution to this work. Density functional theory (DFT) is a tool complementary to experiment to characterize new semiconductors. High accuracy of hybrid DFT to compute semiconductors properties has been assessed in several published works, paving the way to a theoretical design of semiconductors for specific applications including photocatalytic water splitting. The difference of semiconductor potential of photogenerated holes and electrons must have at least 1.23 eV. But because of the need of excess energy for charge transfer toward the electrocatalyst and kinetic consideration of the redox reaction on top of the electrocatalyst surface, the optimum bandgap for the semiconductor is considered between 1.8 eV and 2.2 eV. The properties of layered oxychalcogenides MCuOS (M = Bi, Y, La , Lu ) for water splitting is investigated. They have been studied for various applications, mostly consisting in transparent p-type semiconductors[1], and thermoelectric materials. BiCuOS compound is very promising candidate for waters splitting applications ,but the major photocatalytic drawback of BiCuOS comes from the indirect nature of its band gap and the small band gap of 1.2 eV. RECuOS (RE=La, Gd, Lu, Y) compounds have bandgaps around 3 eV. These materials have respectively too low and too high bandgaps for water splitting application. By accurate modelling and calculations we successfully design in silico solid solutions having the formula Bi1-xRExCuOS (RE=rare earth element) satisfying all the requirements for water splitting including a bandgap between 1.8 eV and 2.2 eV with high transport properties. The effect of spin-orbit coupling on the most stable configuration?s electronic and optical properties is took into consideration. Full analysis including thermodynamic stability of the solid solution, bandgaps, density of states, effective masses, dielectric constants and exciton binding energies of these materials as photocatalysis semiconductors is provided.

Resume : Rutile &anatase nanoparticles with different ratio dependent of arc discharge parameters were synthesized using electrodes carbon-titan. XRD-EDS and Raman spectroscopy show a complex structure Rutile&anatase partial doped with carbon. Band gap is dependent of synthesis conditions in submerged water ranging from 2.5-2.9 eV. Nanopowder was deposited on the Cu-substrate and sintered at 600 Celsius degrees. In a photoelectrochemical cell the water splitting yield reached up to 7%

Resume : Nowadays, the quest for photocatalytic nanomaterials working within the visible range of the solar spectrum is at its peak. The recovery of the nanomaterials after the photodegradation processes from the environment is, however, problematic. In this work we propose the enhancement of photocatalytic properties of TiO2 nanoparticles by decorating them with noble metals (Pt, Pd, Au, Ag) combined with biocompatible features for eliminating the need of recovering them. The new architectures based on TiO2 nanoparticles and decorated with noble metals have been obtained by a rapid, one-step procedure ? laser pyrolysis, as compared with chemical methods. Optical, morphological and structural characterizations of the nanocomposites have been performed, as well as their photodegradation efficiency of different organic pollutant (methyl orange, 4-clorophenol and other dyeing agents). The evaluation of the biocompatible properties of the TiO2- based nanoparticles have been done on cell culture growing of dermal fibroblasts (CCD-1070Sk) and kidney cells (HEK-293) exposed to different concentrations of NP for different periods of time.

Resume : During last years, the interest on synthesis of low dimensional compounds to be used on energy conversion field has been drastically growing. The sulfide family is especially attractive due to their abundancy, cheapness and variety of morphologies. Particularly, it becomes very interesting in the photoelectrochemical field because of its wide range of values of transport and optical properties. Vanadium tetrasulfide (VS4) is n one-dimensional compound (chain-like crystal structure) with promising electrochemical properties [1]. However, there is scarce information about its properties [2] due to its extremely slow formation kinetics and unknown growth or synthesis mechanism [2-3]. Here, we report a detailed investigation about the formation mechanism of VS4 as well as its structural, transport and photoelectrochemical properties. To this aim, VS4 films were grown by a solid-gas reaction process between sulfur and vanadium films during different reaction times. Films characterization reveals the formation of VS4 micro-rods without intermediate phases in short times. A tentative formation mechanism will be proposed. Moreover, VS4 films show an adequate band-gap to be used on photoassisted water splitting. The evolved hydrogen was quantified and results will be compared to other sulfides used to generate hydrogen. [1] C. S. Rout et al. J.Am.Chem. Soc. 2013, 135, 8720-8725 [2] M.N. Kozlova et al. Chem Eur. J. 2015, 21, 1-8 [3] G. Lui et al. Ind. Eng. Chem. Res. 2015, 54, 2682-2689

Resume : In recent years, photoelectrochemical (PEC) splitting of water with an aim to produce hydrogen (H2) as a chemical fuel has been a subject of intense research [1]. Recently, ferroelectric semiconductors are reported to exhibit enhanced PEC activity. Ferroelectric materials can be polarized with the application of an electric field resulting in a built-in potential which helps in separating out the photogenerated charge carriers. In addition to this, by changing the polarization direction, the energy band alignment at the electrode/electrolyte interface can also be modulated in a way that it can help in the easy transfer of charge carriers from electrode to electrolyte[2-4]. In this paper, we investigated the photoelectrochemical properties of ferroelectric PVDF/Cu/PVDF-NaNbO3 PEC cell and demonstrated that PEC properties can be tuned with ferroelectric polarization and piezophototronic effect. Photocurrent density has been tuned from ~0.71 mA/cm2 to 1.97 mA/cm2 by changing the polarization direction. Furthermore, due to flexibility and piezoelectric properties of PVDF/Cu/PVDF-NaNbO3 PEC cell, ~26% enhancement in the photocurrent is obtained using the piezophototronic effect. A model depicting the modulation of band alignment between PVDF and NaNbO3 with electric field is proposed to explain the observed tuning of the PEC properties. Electrochemical Impedance spectroscopy measurements supported the validity of the proposed model.

Resume : The CO2 emissions from the combustion of fossil fuels are the main contributor factor of global warming. Therefore the conversion of CO2 to fuels using renewable energy is of high importance. Furthermore, the clean production of fuels would provide security of energy supply. One approach to the conversion of CO2 to fuels is ?artificial photosynthesis? utilising semiconductor photocatalysts which can harvest solar energy and reduce CO2 to fuels e.g. CO, CH4, CH3OH. There are many research groups investigating the photocatalytic reduction of CO2. However, it is difficult to compare laboratory results since different types of reactors and catalysts are used and no standard conditions are applied. In this work a fixed bed reactor was employed, operating in batch mode and titanium dioxide was utilized as a reference photocatalyst. It has been shown that modification of the parent titanium dioxide with metals increases the yields for CO2 reduction products, mainly CO and CH4. Several controls were performed for CO2 photoreduction, with TiO2 (P25) and Cu-TiO2 tested as photocatalysts. Cu-TiO2 was found to be slightly more efficient for the production of CH4 than P25. We acknowledge support from the US-Ireland R&D Partnership Program, NSF (CBET-1438721), SFI (SFI 14/US/E2915)and DfE (USI065). Keywords: CO2 photoreduction, titanium dioxide, methane, carbon monoxide

Resume : In this research we prepared various ZnOrod/MeO core-shell structures by pneumatic spray pyrolysis method. Shell layers represent ZnO thin films with various morphologies, MoO3, In2O3, and TiO2 thin films. ZnO shells have two different morphologies; a dense network of well-ordered platelets covering the side facets of the rod-like and needles-shaped ZnO crystals. These were obtained by spraying Zn(Ac)2 solution over the surface of pre-grown ZnO rods at deposition temperature of 330 °C within 5 min. All core-shell structures have been characterised by scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), UV-VIS, surface wettability measurements and XPS. Photocatalytic activity of the layers was evaluated by means of photocatalytic oxidation activity (PCO) of a sulphonamide antibiotic, sulphametisole (SMZ). The PCO results on core-shell structures are compared to the performance of bare ZnO rods. Core shell structures showed increased photocatalityc activity: for example, ZnOrod/MoO3 removed 68 % of SMZ (10 mg L-1) within 6 h, whereas plain ZnO rods could remove only 32 %. The explanation of the differences in ZnO objects photoactivity will be proposed.

Resume : Plasmonic metal/semiconductor heteronanostructures have received considerable attention for their broad applications in photocatalysis, due to the improved light-harvesting ability and efficient charge separation. Among these combinations, the integration of gold nanorods (AuNRs) with TiO2 is highly desirable for efficient hydrogen production, mainly because AuNRs possess widely tunable surface plasmon resonance (SPR) band from visible to near-infrared (NIR) range with a large cross section. The fabrication of AuNRs and TiO2 nanostructures aiming at utilizing UV-VIS-NIR light, however, is still challenging. As calcination at high temperatures (above 450 oC), which is usually required to obtain a UV-active TiO2 in the anatase phase, is detrimental for anisotropic AuNRs. In this work, we report the facile synthesis of AuNRs@crystalized TiO2 core-shell nanostructures by uniformly coating TiO2 on AuNRs through elaborate-controlled hydrolysis of titania precursor and subsequent hydrothermal treatment (~85 oC). The crystalized TiO2 shell as an effective physical barrier together with relative low temperature treatment are important in preserving the intactness of the gold core. Besides, this method allows for control of the shape, size of the gold core as well as the thickness of the TiO2 shell. The as-synthesized AuNRs@TiO2 core-shell nanoparticles show significantly enhanced H2 generation ability under UV-Vis-NIR light irradiation. The stability and reusability of AuNRs@TiO2 core-shell nanoparticles are also demonstrated. It is hoped that our work could provide a new idea on the design of anisotropic metal/ semiconductor heteronanostructures towards efficient solar energy conversion.

Resume : Hydrogen production from steam or autothermal alcohol reforming has been widely studied, but these methods require high temperatures and emit CO2. Here, we present a new strategy for the simultaneous room-temperature production of hydrogen and other chemicals without the emission of CO2, via the photoelectrochemical reforming of biomass-derived alcohols. The measured hydrogen quantum efficiencies reach around 80% across the entire visible solar spectrum from 450 to 850 nm, achieving an ultrahigh hydrogen production rate of 7.91 ?mol/(min·cm2) under AM 1.5G illumination.

Resume : Solar energy has the potential to meet the increasing global energy demands. Therefore solar energy conversion and storage, via water splitting has been attracting substantial interest over the last ten years, which can provide renewable H2 fuel with a strong potential to replace fossil fuel. This zero carbon process can not only provide a renewable fuel but also dramatically reduce CO2 emission. The key in the technology is an efficient photocatalyst. The current low efficiency in water splitting to H2 fuel process is contributed to both fast charge recombination and large bandgap of an inorganic semiconductor.1 Stimulated by our recent research outcomes on the charge dynamics in inorganic semiconductor photocatalysts,1 we developed novel materials strategies for solar driven hydrogen synthesis. One is to mitigate the charge recombination by improving the degree of polymerization of a polymer e.g. C3N4. With respect to it, one successful example of pure water splitting in a suspensions solution under visible light has been demonstrated for the first time (Figure 1).2,3 The other strategy is to narrow the bandgap of C3N4 (g-CN) by bandgap engineering. The material prepared via an oxygen rich organic precursor has a brown color, indicating a longer range absorption in the visible region, resulting into a quantum yield (QY) of 10% at 420 nm.4 References 1. J. Tang, J. R. Durrant and D. R Klug, J. Am. Chem. Soc., 2008, 130(42) 13885-13891. 2. D.J. Martin, P.J.T. Reardon, S.J.A Moniz, J. Tang. J. Am. Chem. Soc., 2014, 136, 12568-12571. 3. D.J. Martin, K. Qiu, S.A. Shevlin, A.D. Handoko, X. Chen, Z. Guo, and J. Tang. Angewandte Chemie International Edition 2014, 53, 9240-9245. 4. Y. Wang, M.K. Bayazit, S.J Moniz, Q. Ruan, C. Lau, N. Martsinovich, J. Tang, 2017, submitted.

Resume : Metal cluster-semiconductor systems such as Au/TiO2 and Rh/GaN are promising new hybrid materials for combined light harvesting and conversion of photon energy into chemical energy. The most explored model reaction is photochemical water splitting, but H2-production is still limited so far. Another renewable feedstock of interest is ethanol, because alcohols are the main product from biofermentation and are as such ideal precursors for biomass in general. In order to tailor metal cluster-semiconductor hybrid systems to a maximized hydrogen production, an understanding of the oxidation reaction on n-type semiconductor’s surface is of paramount importance. To ensure very well defined conditions, rutile TiO2(110) and GaN(0001) are studied under UHV conditions. Exemplified on titania, we demonstrate how the mechanisms of photochemical reactions can be elucidated via the judicious choice of different experiments. It is shown, that ethanol is photooxidized to acetaldehyde. Although formally two hydrogen atoms remain from the stoichiometric reaction equation, no molecular hydrogen production is observed. The hydrogen forms water on the surface which poisons the photocatalyst by site-blocking on a defect-rich surface. This mechanism enables even the interpretation of ambient photocatalytic systems. Further it is shown, that the nitride chemistry differs significantly from the oxides, since there is evidence for thermal H2-production instead of surface hydroxylation.

Resume : We fabricated 3D CNT-GR-TiO2 nanocomposite materials consisting of 0D TiO2 nanoparticles, 1D CNTs and 2D graphene nanosheets, and measured their photocatalytic activity for H2 production from water without the assistance of any noble metal co-catalyst. The nanocomposite material exhibits the highest photocatalytic activity with a maximum H2 production rate of 31 mmol h?1 g?1 under the wide spectrum of solar light irradiation. The rate of H2 production is 4.5-fold higher than commercial TiO2 and the estimated solar energy conversion efficiency is 14.6%. Here graphene acts as an electron sink and transporter, and the enhanced photocatalytic activity is due to interfacial charge transfer through Ti-C bond, which is confirmed by XPS, TEM, PL and EIS analyses. It is found that CNTs in this nanohybrid structure not only prevent the restacking of graphene nanosheets but also provide additional electron transport channels thereby suppressing the recombination rate of electron-hole pairs on the surface of GR-TiO2 composite. Furthermore, such a significant enhancement can also be attributed to multiple factors which include synergistic interaction between graphene and TiO2, presence of oxygen vacancies and Ti3 states, improved surface area, suppressed recombination rate of charge carriers, and accelerated charge transfer from TiO2 to graphene sheets through CNTs. We anticipate that the work presented here may stimulate new insights for the utilization of low-cost CNT-GR-TiO2 3D hybrid nanostructures for various important practical applications.

Resume : Hydrogen is a suitable energy carrier that can be directly produced using solar energy by means of photocatalytic water splitting. TiO2 is the most investigated photocatalyst for this purpose, but it has several disadvantages. To enhance the photocatalytic performance of TiO2, studies have been focused on changing the structure and the chemical composition of TiO2. This work reports the influence of different oxides (BiVO4, CeO2 and CuO) on the chemico-physical properties of TiO2, prepared in order to obtain a highly porous material with an Inverse Opal structure, and analyze the performance of these systems in the photocatalytic water splitting under both UV and Visible light irradiation. The characterization results pointed out that the TiO2 obtained after the synthesis exhibits an Inverse Opal porous structure with interconnected macropores. Due to the peculiar macro-mesoporosity, allowing a more efficient light absorption process, the Inverse Opal TiO2 exhibited a higher activity in the H2 production compared to the commercial TiO2 both under UV and Visible light irradiation. A further positive effect in terms of increased photoactivity was verified by addition of BiVO4 and, to a less extent, of CeO2. The combination of the benefits of the highly porous structure of Inverse Opal TiO2 and the presence of a light sensitizer as BiVO4 represents a promising strategy to synthesize efficient photocatalysts for the water splitting reaction.

Resume : Photocatalytic water splitting is the most cost-effective technology to harvest energy from the sun without any carbon footprint.[1] Theoretical simulations were performed to study the intrinsic limitations of photocatalysts for water splitting.[2] The objective is to understand why photocatalysts exhibit low quantum efficiencies. The semiconductor device equations were numerically solved to test the effects of the semiconductor properties (obtained via ab initio DFT calculations) on the quantum efficiency of the photocatalytic system. Using rigid boundary conditions and assumptions, we calculated for the first-time photocatalytic quantum efficiency trends. The results show that semiconductor-liquid junctions inhibit majority charge injection into the electrocatalysts. Charge separation may be limited to the surface; nonetheless, charge generation occurs in the bulk due to the large penetration depth of sunlight. Hence, inefficient charge separation and large recombination rates may limit the efficiency. It was not possible to obtain QE > 70% (even for ideal cases). A typical defective n-type semiconductor resulted in QE < 12%. Future particulate photocatalysts must create effective pathways for transport of electrons and holes into their respective sites via interface engineering. Simply, the photocatalytic device design must be changed. [1] K. Takanabe, Top. Curr. Chem. (2016) 371, 73. [2] A. T. Garcia-Esparza, K. Takanabe, J. Mater. Chem. A (2016) 4, 2894.

Resume : Solar water splitting, or the so-called artificial photosynthesis, is one of the most promising approaches to meet the increasing energy demand through converting solar energy into chemical energy in the form of hydrogen as a clean fuel. Due to their optical and photocatalytic properties, oxynitride materials emerged as potential photocatalysts for solar water splitting. According to theoretical calculations1, CaNbO2N is expected to be a promising photocatalyst for solar water splitting. In this work, we present for the first time the photoelectrochemical behavior of CaNbO2N photoanodes towards solar water splitting. CaNbO2N photocatalyst is prepared by thermal Ammonolysis of the oxide precursor, as confirmed by the X-ray diffraction analysis and absorbance measurements. The photoanodes are then fabricated by electrophoretic deposition of CaNbO2N photocatalyst on fluorine-doped tin oxide (FTO) followed by a post-necking treatment with TaCl5. The photoactivity of the CaNbO2N photoanodes towards solar water splitting is investigated by measuring their photoelectrochemical performance. Then, the performance of the photoanodes is enhanced by photo-assisted deposition of CoOx cocatalyst and through the atomic layer deposition of a thin passivation layer of Al2O3.

Resume : Solar fuels have been considered as one of the most promising technological concepts [1] due to their high potential use and environmental suitability, owing to the greater convenience of sunlight storage into chemicals compared to its conversion into electricity. In this context, development of devices able to photocatalytic split water into O2 and H2 is currently of primary interest. However, this is a challenging problem because the water oxidation (WO) reaction is thermodynamically uphill. Several WO photocatalysts have been developed over the last few decades: TiO2 nanostructures2 are among the most studied materials under UV irradiation and BiVO4 is one of the most promising visible-light active materials for this application.[3] In our group, the WO photocatalytic activity of TiO2 and BiVO4 has been investigated in different systems and device configurations.[4-6] The role of their chemical-physical properties (i.e. morphology, crystals orientation, doping, etc)[7] on their performance was investigated by experimental and modelling tools. Moreover, in framework of the ARTIPHYCTION project (FCH JU contract nr. 303435), in 2015, we coordinated the building of the first large-scale (area of 1.6 m2) demonstrator of photoelectrochemical water splitting: the ARTIPHYCTION prototype.[3] In such effort, the main parameters influencing and limiting the final efficiency of a BiVO4-based water-splitting device were elucidated and will be herein discussed. References: 1) J Environ Chem Eng 2016, 4, 3934-3953; 2) PCCP 2015, 17, 7775-7786; 3) Catalysts 2017, 7, 13; 4) Chem Eng J 2014, 238, 17-26; 5) Electrochim Acta 2014, 131, 184-194; 6) J Phys Chem C 2015, 119, 9916-9925; 7) Appl Catal B: Environm 2016, 180, 630-636.

Resume : In this contribution we report on the fabrication of ultra-thin (< 10 nm) films of TiO2 and Ti/TiO2 on p-Si photocathodes for water splitting, where they fulfil the double role of (a) protective coatings and (b) platforms for functionalization with co-catalysts. The films are synthesized either by radio-frequency magnetron sputtering (RF-MS) for TiO2 or thermal evaporation for Ti/TiO2, in both cases simple physical vapor deposition techniques already applied at industrial level. Structural characterization reveals compact, conformal layers which are effective in protecting both flat and low-reflective textured p-Si photocathodes from photocorrosion in a broad pH range. Coated electrodes also exhibit improved photocurrents, due to electron injection into the TiO2 conduction band favoring charge separation over recombination. Additionally, for Ti/TiO2 films, which consist mostly of native oxide, the residual metallic Ti ensures optimal electric contact with the substrate. Moreover, the coatings can be functionalized with Pt nanoparticles with good adhesion, further enhancing cathodic performance. The best case, low-reflective p-Si coated with Ti/TiO2/Pt NPs, shows an improvement in photoresponse of up to 9 times. This strategy relies on common fabrication methods and can be easily extended to other photoelectrode materials prone to photocorrosion and normally difficult to functionalize, thus introducing an extra degree of flexibility in the choice of materials for photoelectrochemical cells.

Resume : Photo-electrochemical cells that are being used for solar powered hydrogen production attract major academic interest due to their challenging combination of tandem photovoltaic and catalytic semiconductor layers. As, for industrial reasons, the semiconductor layer must be cost effective, studies focus on hematite (α– F2O3) or simply rust electrodes. However, despite the vast amount of investigations the efficiency is not yet reached technological feasibility nor the mechanism of water splitting is understood. By modifying recently published mechanism [Zandi et al, Nature Chemistry 8, 778, (2016)] for hematite electrodes and by experimentally controlling hydrogen peroxide (H2O2) in the electrolyte, we present new findings related to the governing reactions of the hole scavenger and the competition between these reactions and the photo-anodic water splitting reactions. Specifically, we cover two main features that are empirically observed: (i) the non-monotonic increase of the current density with the increase of voltage, and (ii) the abrupt effect of hydrogen peroxide addition (even at small concentrations) to the electrolyte. We believe that the proposed pathways provide a basic step toward improving the efficiency of the photo-anode.

Resume : Converting and storing solar energy has been regarded as a promising approach to address the current global reliance on fossil fuels. The generation of hydrogen from water using sunlight can form potentially the basis of a clean and renewable source of energy. Solar water splitting system requires stable light-absorbing electrodes for both the oxidative and reductive half-reactions. However, the kinetically slow oxygen evolution reaction (OER) is a bottle-neck process for producing sustainable solar-driven fuels using water. Silicon has a suitable band gap (1.12 eV) to absorb the wide range of the solar spectrum and has high carrier mobility. For this reason, silicon is an attractive candidate among the various materials such as transition metal oxides and semiconductors. Since the silicon photoanode is unstable at electrolyte and suffers from fast photocorrosion due to the position of thermodynamic redox potentials, the solar to hydrogen conversion efficiency of bare silicon photoanode is largely suppressed. To solve those problems of silicon, recent studies on silicon photoanode has focused on designing a surface passivation layer that protects against chemical and photo-induced corrosion without degrading the inherent photoactive ability of silicon and promotes the chemical adsorption of water molecules to lower the overpotential at the solid/liquid interface. Surface coating of metal oxides (e.g., MnOx, CoOx, Fe2O3) has been proved to be moderate for physical barrier between Si electrodes and electrolytes, but also act as oxygen evolution catalysts to enhance the kinetics of electron transfer at Si-electrolyte interfaces. In particular, NiOx is one of the most promising material because it is chemically stable at high pH electrolyte, optically and transparent in the visible region. Since NiO is intrinsically p-type, it provides junction photovoltage to further reduce overpotential. Herein, we report the synthesis and photoelectrochemical properties of multifunctional NiO on a n-type silicon photoanode synthesized by using a facile pulsed-electrodepostion, in which the porosity of nanostructures are tailored. The solution-processed n-Si photoanode showed higher catalytic behavior, lowering the onset, leading to highly efficient Si-based photoanodes.

Resume : Titanium dioxide (TiO2) is well-known material for photocatalytic water treatment. Due to its bandgap energy of ca. 3,0 eV, it can be excited only by light with wavelengths shorter than 400 nm, which results in limited absorption of solar light. One of the methods used to improve the optoelectronic properties of TiO2 is incorporation of plasmonic metal nanoparticles. We decorated rutile and anatase TiO2 electrodes with silver and gold particles obtained by photodeposition, electrodeposition or sputtering. We confirmed the presence of particles and examined their size and shape by scanning electron microscopy (SEM) and UV-Vis spectroscopy. The performance of modified and unmodified electrodes was compared by photoelectrochemical measurements. We have shown that both gold- and silver-modified electrodes exhibited substantial photocurrents under solar light irradiation, e.g. for the decomposition of acetic acid in water. In fact, increased incident photon to current efficiencies (IPCEs) under electrodes irradiation with light of wavelengths matching the absorption spectra of silver and gold nanoparticles were consistently observed. The latter effect was greater with the gold nanoparticles on the surface than with the silver ones and for electrodes consisting of the rutile rather than anatase. Obtained results demonstrated that repeated measurements strongly affected activity of the electrodes. The authors acknowledge funding from the MAESTRO Grant UMO-2013/10/A/ST5/00245.

Resume : Photocatalysis is widely identified as a clean technology for degrading organic pollutants including various pesticides and industrial effluents. 1-5 It can also be employed for the degradation of the bacteria present on various surfaces. Anatase phase titanium dioxide (bandgap of 3.2 eV) is capable of bacterial disinfection under UV irradiation. The efficiency of a photocatalyst mainly depends on the ability of the semiconductor to yield long-lasting electrons and holes, which result in the formation of various free radical species. As part of a programme to develop highly efficient photocatalytic materials, investigations were aimed at preparing new photo-catalytically activate antimicrobial materials for disinfection applications. High thermal stability of the anatase phase is one of the requirements for making coatings on ceramic surfaces. For example, anatase phase which is stable up to the sintering temperature of the substrate (e.g., bathroom tile) is highly beneficial for anti-bacterial building materials. The preparation of a range of novel photocatalytic materials by engineering the band-gap using various dopants such as N, S, C and F will be discussed. A range of photocatalysts with thermal stability and visible light activity will also be discussed. References 1. V. Etacheri, C. Di Valentin, J. Schneider, D. Bahnemann, and S. C. Pillai , Journal of Photochemistry and Photobiology, 25 (2015) 1?29 2. S. Banerjee, S.C. Pillai, P. Falaras, K. E O'Shea, J. A Byrne, D. D Dionysiou, J. Phys. Chem. Lett. 5 (2014) 2543?2554. 3. S. Banerjee, D. D. Dionysiou, and S. C. Pillai, Applied Catalysis B: Environmental 176 (2015) 396?428 4. R. Fagan, D. E. McCormack, D. D. Dionysiou, and S.C. Pillai Materials for semiconductor processing 42 (2016) 2?14 5. J. Podporska-Carroll, E. Panaitescu, B. Quilty, L. Wang, L. Menon, S. C. Pillai, Applied Catalysis B: Environmental 176, (2015) 70-75.

Resume : UV driven titanium dioxide photocatalysis has been shown to be effective for the disinfection of water contaminated with pathogenic microorganisms. Two approaches to improving the efficiency of photocatalytic disinfection are to modify/dope titanium dioxide or to use electrochemically assisted photocatalysis. In this work we prepared titania-graphene composites by the photocatalytic reduction of graphene oxide. The composites were tested for the inactivation of E.coli under UV-Vis and visible only irradiation. The mechanism of disinfection was investigated using probes and scavengers. The composites showed enhanced activity for disinfection under UV and visible irradiation, as compared to titanium dioxide alone. Also, aligned titania nanotubes were grown by anodic oxidation of Ti foil and doped with nitrogen by annealing in ammonia. The nanotube coated foils were used as anodes in an electrochemical cell for the photoelectrolytic inactivation of E.coli. The N doped nanotubes showed no significant visible only activity, however, there was a significant improvement for the UV-vis photoelectrolytic inactivation of E.coli. The reasons for this improvement in UV-vis activity will be discussed.

Resume : During the last few years, mesoporous single crystals have been discussed intensely in the context of solar hydrogen generation, since it is expected that this material class can contribute significantly to the improved design of highly efficient solar cell devices. Especially for photocatalytic or photoelectrochemical applications high surface area and good charge-transport properties are key features to enhanced device performance. Charge recombination is minimized in defect free structures such as single crystals, where the surface area is small. High surface area, however, is obtained best by agglomerations of nanoparticles, where conductivity is low and charge recombination high because of multiple grain boundaries. One possibility to achieve performance improvement consists in the fabrication of large single crystals (up to several µm) with a mesoporous structure fabricated often via a complex template based approach. However other reactions are thinkable, during which porous structures in photocatalytically active single crystals are formed spontaneously without the need for templates. One example is the solid state – gas phase reaction carried out for the synthesis of oxynitrides, i.e. thermal ammonolysis. However, the control of pore size and density and crystal quality requires a detailed understanding of the reaction mechanisms during synthesis. We have investigated the mechanism of pore formation during thermal ammonolysis by microscopic (mainly transmission electron microscopy) and bulk characterization techniques and demonstrate how this knowledge enabled the design of materials with enhanced photocatalytic and photoelectrochemical activity.

Resume : Direct coupling of separation and photocatalysis using membranes based on titanium dioxide is an interesting approach usually applied in water treatment devices, and recently considered for other applications, such as hydrogen production by water splitting using solar energy, which is precisely the application aimed in this study. Photocatalysis is defined as the acceleration of a chemical process by a photocatalyst, that is to say a catalyst that is capable of absorbing a wavelength equivalent to its band gap. Semiconductors such as SiO2, TiO2, WO3, SnO2, etc. are generally considered as the best photocatalysts. Among them, titanium dioxide (TiO2) is particularly interesting due to its excellent photocatalytic activity and kinetic stability at low temperature. The advantage of its immobilization on a support rather than its usage in suspensions is the facility of the integration process into multi-layered systems. In this work, TiO2 films obtained by Low Frequency Plasma Enhanced Chemical Vapor Deposition (LF-PECVD) under the conditions established by Zhou et al. [1] were optimized in terms of photocatalytic activity. In fact, the PECVD process is very competitive because it allows the synthesis of materials of high stability, integrity and compactness. Such a TiO2 Low Frequency Plasma deposition has never been mentioned in the literature yet. Generally, this semiconductor has been deposited by Radiofrequency PECVD or Microwave Chemical Vapor Deposition (MOCVD). The precursor used in this study was the Titanium Tetra-isopropoxide (TTIP), easily vaporizable when heated and giving non-toxic by-products while reacting. The optimization step was about nitrogen doping of TiO2 (N-TiO2 films) by addition of ammonia in the deposition chamber, with the aim of shifting the materials band gap from the UV region to the visible region. The thin films were characterized in terms of their structural and functional properties (XRD, SEM, XPS, FTIR, AFM, Raman, Pilkington test). The XRD patterns and the Raman spectra showed peaks shifting indicating the lattice composition modification consecutive to ammonia introduction in the deposition chamber. Further studies done by depth profiling XPS also proved nitrogen incorporation into the TiO2 lattice. The comparison between the UV/Vis spectra of the non-doped and doped TiO2 layers showed a wavelength absorption shifting towards nearly 600 nm. Lastly, N-doped TiO2 films have been proved as efficient for the photocatalytic degradation of stearic acid under visible light. As prospects, the designed N-doped TiO2 films will constitute the anode compartment in a multi-layered all-plasma assembly composed of a polymer electrolyte membrane [2] sandwiched between the TiO2 anode and a platinum cathode. This assembly will be envisaged as a photo-electrochemical cell [3] for hydrogen production / separation by solar energy irradiation. References [1] M. Zhou, S. Roualdès, J. Zhao, V. Autès, A. Ayral., Thin Solid Films, 2015, 589, 770-777. [2] J. Bassil, S. Roualdès, V. Flaud, J. Durand, Journal of Membrane Science, 2014, 461, 1-9. [3] V. Autès, Développement de Matériaux Multicouches PECVD pour la production d’hydrogène à partir de lumière solaire, Master (M2) training report obtained at University of Montpellier, 2014.

Resume : Designing textiles enabling VOC degradation and bacteria killing is a promising strategy aiming at the air quality control for comfort and health of the population. In this frame, functionalizing textiles with multi-material titania-based photocatalytic thin films is promising. Interest in Layer-by-Layer (LbL) assembly thin films has been growing for the past decade for providing properties to surfaces, being an easy building process of versatile multi-material films with tunable properties at nanoscale precision. This technique consists in alternating deposition of oppositely charged species on charged surface. Here we report on the building via LbL assembly of multi-material nanoscale films containing positively charged TiO2 NPs, Ag NPs and PSS polyanion, and exhibiting high photocatalytic activity toward VOC and bacteria. Construction of films was first monitored on model surfaces by ellipsometry, Quartz Crystal Microbalance, UV-Vis spectroscopy and microscopy, before being transferred onto textiles. UVA light photocatalytic activity of films was evaluated in terms of formic acid oxidation in gas phase and antibacterial activity. The activity towards chemicals and bacteria was strongly impacted by the film properties. The influence of the number of layers as well as the Ag NPs synthesis was studied. We showed that a catalyst–saving nanoscale film with a single layer of TiO2 (5µg/cm²) showed very high photocatalytic activity, further strongly enhanced (x3) via addition of Ag.

Resume : Who can identify a powder in an unlabeled bottle/package found in a laboratory? How can evaluate the fluctuation in quality of a sample prepared in a routine procedure? In the field of organic synthesis, it is accepted that showing reasonable fit of elemental composition and NMR data with theoretical value and authentic one, respectively, can be identification of a product. Such a difference in identification for molecules and particles must be due to the existence of surface on particles, and therefore, composition and bulk structure of particles cannot be enough to identify. We propose energy-resolved distribution of electron traps (ERDT), which may be located predominantly on the surface, measured for titania and some metal oxide powders and propose the distribution can be a finger print of powder for identification. Here, newly developed reversed double-beam photoacoustic spectroscopy (RDB-PAS) is reported as a powerful tool for ERDT measurement. A brief explanation of the principle of RDB-PAS is (i) photoacoustic detection of accumulation of electrons filling ETs from deeper level to shallower level, using scanning continuous monochromatic light (second beam) to excite valence-band electrons directly to ETs, (ii) differentiating the resultant spectra and conversion to absolute density of ETs, (iii) plotting ERDT as a function of energy difference from the top of VB and (iv) comparison with the energy of CB bottom, in reference to VB top. As a result, both ERDT and CB bottom position are plotted as a function of energy difference from the VB top of each sample. Identification of titania and the other metal oxide powders by ERDT and correlation of ERDT with photocatalytic performance are discussed.

Resume : Anthropogenic CO2 and concurrent global warming awareness have led to initiatives to reduce atmospheric CO2 emission through various green technologies. The photocatalytic conversion of CO2 to fuels, or other useful chemical products, represents an opportunity to capture and utilise CO2. New photocatalyst materials are developed within the research community with aims to increase efficiency, control product distribution or shift activity into the visible region to increase the attractiveness of photocatalytic CO2 reduction compared to other CO2 treatment technologies. In this study, we discuss changes to the distribution and yield of gaseous products CO, CH4 etc over time for TiO2 particles modified with Cu and Pd and the impact of a change in photocatalyst morphology to nanotubes. The reactor conditions entailed an immobilised catalyst plate illuminated by filtered xenon source in a water saturated inert atmosphere 20:80 CO2:Ar. Limited CO2 reduction was observed with the P25 standard catalyst, the yield and ratio of methane was observed to increase upon modification, under UV irradiation. None of the modifications investigated produced a shift in activity into the visible region of the solar spectrum. These results indicate copper and palladium cluster modification was not successful in band gap narrowing but can be used to improve UV activity in TiO2 materials. We acknowledge support from the US-Ireland R&D Partnership Program, NSF (CBET-1438721), SFI (SFI 14/US/E2915) and DfE (USI065).

Resume : Binary TiO2/SiO2 oxides were synthesized by flame spray pyrolysis as supports (and co-catalysts) for catalytic Pt deposits. As the mole ratio of titanium isopropoxide:hexamethyldisiloxane that was fed to the flame increased the following was observed: (i) the specific surface area gradually decreased from ~323 to ~58 m2/g; (ii) the crystal structure changed from a predominance of amorphous silica to a predominance of crystalline anatase; (iii) the band gap energy of Pt/TiO2-SiO2 and (iv) the occurrence of Si-O-Ti bonds were both affected. The catalytic oxidation of formic acid over Pt-TiO2/SiO2 was found to be strongly dependent on the TiO2/SiO2 ratio and was further enhanced by UV light pre-illumination. Interestingly, the Pt/SiO2 catalyst showed the highest activity under dark catalytic conditions (i.e. no UV illumination) while the Pt-TiO2/SiO2 with a TiO2:SiO2 ratio of 1:3 exhibited the highest catalytic rate under UV light pre-illumination and photocatalytic conditions. The presence of surface active oxygen (PtOads), superoxide ions and oxygen vacancies associated with Ti3+ are proposed to be beneficial for formic acid oxidation under dark and UV light pre-illumination conditions while the hydroxyl radicals act as additional oxidants under photocatalytic conditions. In addition, the size and cuboctahedral shape of the Pt deposits on the SiO2 surface for the Pt/SiO2 catalyst is believed to be a key contributor to its observed superior catalytic rates under dark condition.

Resume : Mesocrystals are a new class of hierarchically nanostructured solid materials and are mesoscale assemblies of mutually oriented nanoparticles. As a result of this special architecture mesocrystals display emerging properties that are different from those of the usual nanoparticulate materials or single crystalline materials, such as high porosity and surface area, unusual mechanical stability and enhanced electric and electrochemical performance. Due to these emerged properties and immense interest for their application in vast areas of nanotechnology, including gas sensing, energy storage, and photocatalysis have been whiteness in recent years. In this contribution synthesis and the formation mechanism of multicomponent (MC-) NbSb-TiO2 mesocrystals and their application in photocatalysis will be presented. MC-mesocrystals were successfully synthesized via close-control of the non-aqueous sol-gel synthesis. The structure and composition, unraveled by in-depth microscopic (HRTEM, HAADF-STEM/EDXS) analysis revealed the formation mesocrystals with mutually oriented TiO2 nanounits interspaced by layers of SbNb. Compared to the single-crystalline counterparts, (SbNb)-TiO2 MC-mesocrystals display superior photocatalytical (PC-) properties when tested for the decomposition of the organic pollutants. The enhanced PC- activity is attributed to their visible light activity as well as mesocrystals architecture, displaying high surface area and exposed surface facets.

Resume : The use of photo-electrochemical (PEC) reactions to produce clean H2 from water appears as an interesting alternative for the production of fuel directly from solar irradiation. Titanium dioxide is the most studied materials for solar to chemical energy conversion applications owing to its high (photo-)chemical stability and also its large natural abundance, ease of synthesis at low cost. Nevertheless, due to its large band gap (> 3.1 eV), only the UV part of the solar spectrum is absorbed and efficiently used. Using the properties of nanomaterials developed since a decade and the progress in solid state chemistry for engineering energetic band structure of semi-conductors, the conversion efficiency of such systems can be significantly improved. We are developing photo-anodes constituted of aligned (M,N) co-alloyed TiO2 nanotubes obtained by an anodization method and integrated in PEC cell. Due to the favourable photo-generated charge transfer, the unidimentional TiO2-NTs structures exhibit a higher photocurrent than the one measured for spherical nanoparticles (TiO2 P25). In order to extend the photo-electrochemical conversion in the visible part of the solar spectra the co-alloying approach improves the solubility of dopant in the semiconductor and the stability of the structure by favouring its charge neutrality. All these advantages result in an improved PEC conversion efficiency for (M,N) co-alloyed structure in the visible light domain when compared to undoped or N-doped TiO2-NTs. Additionally to material characterization methods, the different aspect of charge transfer and recombination are investigated by advanced electrochemical methods. We are specifically developing (photo-)electrochemical impedance spectroscopy methods such as IMPS, PEIS, IMVS and transient photocurrent measurement to study these transport phenomena in?situ during the PEC reaction. One aspect we are particularly studying is the influence of the irradiation wavelength on the transport phenomena. This approach can be of great interest to study transport phenomena in complex systems like co-alloyed TiO2 materials, but also for tandem system and hetero-junction. Indeed by choosing appropriate wavelength, it should be possible to activate separately the different components of the photo-electrode.

Resume : Monoclinic bismuth vanadate (m-BiVO4) is a promising photoelectrode for photoelectrochemical (PEC) water splitting due to its higher activity for oxygen evolution reaction (OER). However, due to its poor charge transport by alone, performance of PEC cells with a BiVO4 photoanode is reported significantly less than those expected theoretically. To challenge this issue, a metal oxide buffer layer such as WO3 and SnO2 has been inserted between BiVO4 and Fluorine-doped Tin Oxide (FTO) which is a widely-used transparent and conductive substrate in BiVO4–based PEC cells, and this approach has led to enhanced PEC performance compared to the BiVO4 alone. However, little attention has been paid to tailoring properties of the buffer layer. In this work, we investigated the effects of varying thickness of a SnO2 buffer layer in SnO2/BiVO4 photoanodes. The SnO2 buffer layers are fabricated on FTO substrates by a simple solution based method, and the SnO2 layer thickness are varied by the precursor solution concentration. Remarkably, the simple change of the SnO2 thickness led to alteration of material properties of the subsequently grown BiVO4 layer including crystalline phases, crystal orientation, grain size, film roughness, and even band gap. With the optimization of SnO2 layer thickness, PEC performances of SnO2/BiVO4 photoanodes exhibited a much enhanced photocurrent density of 0.95 and 3.76 mA cm-2 (measured at 1.23 V vs. reversible hydrogen electrode under 1 sun illumination) for water and sulfite oxidation reactions, respectively. We discover that material properties of BiVO4 are strongly influenced by the SnO2 layer emphasizing the importance of the optimization of the buffer layer. Details of material characterizations as well as PEC measurements of SnO2/BiVO4 photoanodes will be presented.

Resume : The ability to split water using sunlight is one of the most promising ways of harvesting and storing energy from the sun. Amongst other devices, photoelectrochemical cells have generated great interest due to their direct solar-to-fuel conversion but efficiencies are still quite low. The BiVO4/WO3 heterojunction has drawn much attention as a photoanode material for these systems. The coupling of these two materials is known to improve charge separation and increase the stability of BiVO4 but little is known about the microstructure of the interface that promotes this charge separation. A DFT modelling of the heterojunction paired with transient absorption spectroscopy experiments will relate the heterojunction microstructure with the physical properties of this novel material.

Resume : Despite its enormous potential, the overall conversion efficiency of the light energy into chemical one is still limited. The process requires photo-anodes based on materials having good light harvesting properties supported on conductive sheets. The photocatalysts used should be active under visible light, should have a poor photodegradability, and the photogenerated charges should have an enough long lifetime, ie the recombinations of charges should be limited. Several semiconductive nanomaterials have been already designed to address these issues, and original photo-electrochemical cells leading water oxidation at the surface of photo-anode and injecting electrons in the external circuit until a platinum cathode for the proton reduction have been proposed. Photo-anodes consist generally of a dense array of nanostructures of TiO2 supported on a transparent and conductive sheet. Nevertheless, such TiO2 photo-electrodes are only useful in the UV domain. While several visible dyes have been already used to photosensitize TiO2, we propose in this work the employment of carbon dots (C-dots). C-dots correspond to a new class of carbon nanocrystals which possess the same fascinating photophysical properties than quantum dots, but in contrast to them, they do not contain heavy metals, making them environmental friendly. They exhibit also a high chemical stability. They can be easily synthesized by thermal decomposition of glucides, urea, or organic solvents like polyols. In several cases, they strongly absorb visible light and are able to transfer their photogenerated electron to another material when they are judiciously coupled to it. TiO2-C-dots photo-anodes have been elaborated, fully characterized, and photocurrent measurements have been performed. Incorporation of polyoxometalates (POMs) on the photo-anodes will be also investigated. The role of POMs is to decrease recombination processes of photogenerated electrons-holes. Such hybrid materials are a good omen for water splitting applications. As a consequence, these materials appear as a sustainable approach for the general issue of the development of a fuel economy based on hydrogen synthesized from solar energy.

Resume : Plasmonic Ag/a-Fe2O3/TiO2 heterostructure photoanodessuccessfully engineered by In situ electrochemical anodization technique assisted by pulse sonication. The formation of heterostructure investigated through various characterization techniques to determine their structural, morphological and optical properties. Morphological features of the Ag deposited a-Fe2O3/TiO2 studied via FESEM, showed the layer distribution of a-Fe2O3 nanoparticles (NPs) over TiO2 nanotubes arrays (NTAs). The presence of silver (Ag) in Ag/a-Fe2O3/TiO2 is confirmed by EDX spectrum. XRD spectrum elucidated the formation of a-Fe2O3 and Rutile TiO2 crystalline phases. XPS analysis further established the formation of the heterostructure with expected elemental ratios and oxidation states. The DRS UV-Vis spectroscopy results of the pure TiO2 NTAs, a-Fe2O3/TiO2 and Ag deposited a-Fe2O3/TiO2 showed a gradual decrease in the band gap with enhanced absorption towards visible-region after heterostructure formation in the order Ag/a-Fe2O3/TiO2 > a-Fe2O3/TiO2 > TiO2. The Photoluminescence (PL) spectrum showed significant quenching with the assimilation of Ag in a-Fe2O3/TiO2 NTAs, which unswervingly influence the charge separation, recombination and plasmonic properties of photogenerated excitons. Photoelectrochemical (PEC) properties of Ag deposited a-Fe2O3/TiO2 heterostructure was also measured to be high for Ag deposited a-Fe2O3/TiO2 as compared to pristine TiO2 NTAs and a-Fe2O3/TiO2 due to the plasmonic nature of Ag. This study signifies the facile synthetic strategy and importance of plasmonic behavior of Ag on the overall photoelectrochemical water oxidation of Ag/a-Fe2O3/TiO2 photoanodes. The Ag/a-Fe2O3/TiO2 nanotube-arrays photoanodes has more panorama to absorb light and electrons that are supposed to be produced, via the electron transfer from Ag nanocubes to a-Fe2O3/TiO2 heterostructure, leading to ameliorated water oxidation. This result gives us a new perception of assembling heterostructure for proficient solar- driven water splitting.

Resume : Hydrogen, as a renewable and storable fuel, can be produced through light driven water splitting using the output voltage of an illuminated solar cell. Thermodynamically, a voltage of 1.23 V is needed for the water splitting reaction, but due to overpotential losses, the actual voltage required is approx. 1.6 V. Such high voltages can be achieved using multi-junction solar cells. We reported a solar-to-hydrogen (STH) efficiency of 9.5% for a photovoltaic-biased electrochemical water splitting device (PV-EC) employing a thin film silicon multi-junction solar cell.[1] The results were achieved under standard test conditions (STC: temperature 25°C, irradiance 1000 W/cm² with an AM1.5G spectrum, incident angle 0°). However, if such devices were to be used outdoors, a wide range of different illumination conditions have to be considered. Here, we report on the dependence of the STH efficiency on the illumination conditions, in particular the incident illumination angle and intensity. To predict the STH performance, we implemented the measured current-voltage curves in an electrical serial circuit model[2] and extracted the current density at the operation point of the PV-EC device (V=0). For example, the initial STH efficiency under 0° is divided in half for an illumination angle of 60°. We discuss the physical effects that lead to the observed behavior. 1. F. Urbain et al., Energy Environ. Sci., 2016, 9, 145?154. 2. J.-P. Becker et al., Phys. status solidi a, 2016, 213, 1738?1746.

Resume : Developing of new, highly efficient, photocatalytic materials depends to large extent on better understanding of the physical and chemical phenomena occurring right after photon absorption. Over the years two major techniques have been utilized using pulsed lasers beams for excitation: Time Resolved Microwave Conductivity (TRMC) and UV spectroscopy. While these two complementary methods provide important information on the life times of free carriers they are almost silent with respect to the chemical species involved in the process and with respect to specific loci at which the post-excitation processes occur. Here we present a third method for studying the excitation of photoactive and, in particular, photocatalytic materials. The method is based on measuring time resolved (5*10-9 sec in resolution) IR spectroscopy of the photocatalytic materials upon excitation with the third harmonic (355 nm) of a Nd:YAG pulsed laser. The time resolved FTIR spectra is obtained by using a “step-scan” configuration, i.e. by recording a temporal signal at a specific location of the moving mirror in the Michelson interferometer, following by altering the position of the mirror and re-measuring the temporal signal upon re-excitation. At the end of the process, an array of data points in the time-distance is obtained, facilitating to perform Fourier Transformation of all data points gathered at a specific time. In the presentation, the fundamental pros and cons of the method will be described. Results for several photocatalysts will be presented (among which BiVO4 and TiO2). A correlation between faceting effects, photocatalytic properties and ultrafast measurements will be discussed and compared with conclusions made by utilizing other techniques.

Resume : Despite recent interest into examining novel photocatalyts, TiO2 still remains to be the most researched photocatalyts. This is due to its non-toxicity, long term stability, high oxidising ability and its cost efficiency. Of the three main TiO2 phase (anatase, brookite and rutile), anatase is widely regarded as the most photocatalytic active phase. There are, however, two major drawbacks to using TiO2 as a photocatalyts. Anatase is metastable and begins to transform into rutile at temperatures above 600ºC, this is an issue when used for building materials which are normally made at much higher temperatures. TiO2 can only utilise UV light during photocatalysis due to its large band gap (3.2 eV), this is problematic when using solar light as only 4% of this is UV light. One solution for both issues is the use of metal dopants onto TiO2. From examining XRD results, it can be concluded that copper only has a small impact on the transition temperature. At 600°C all doped samples (2%, 4% and 8% Cu) were 100% anatase, while the control (0% Cu) only contained 34.3% anatase. The control (0% Cu) and 2%Cu were 100% rutile at 650⁰C, while 4% and 8%Cu still has anatase present at this temperature (27.3% and 74.3% anatase respectively). Raman spectroscopy was used to confirmed these results. XPS confirmes the presence of Cu2+. This paper will discuss the theoretical and experimental results of copper doped TiO2 and how this affected the photocatalytic activity.

Resume : Photocatalytic applications of solar energy have been presented in several publications (e.g. [1]). Frequently, heterogeneous photocatalysis is conducted in flow reactors like the prototype introduced in a previous study [2]. It was shown, how the performance of this reactor depends on photocatalyst concentration, its state of aggregation and incident UV irradiance. However, a comprehensive reactor model could not be established due to lacking knowledge of the radiation field within the reactor. This work investigates the penetration of UV light in photocatalytic flow reactors and its impact on the overall photocatalytic performance. Based on experimental data a new reactor model is proposed, which computes the influence of photocatalyst concentration and reactor thickness. The according experiments refer to the photodegradation of methylene blue under UVA in the presence of pyrogenic titanium dioxide (P25). Additional measurements dealt with the propagation of UV light in suspensions of the photocatalyst. Our experimental and theoretical findings hint on optimum values for the photocatalyst concentration and the reactor thickness. [1] S. Malato et al., Catal. Today, 122:137-149, 2007. [2] H.N. Le et al., Beilstein J. Nanotechnol., 6:2423-2430, 2015.

Resume : This work introduces a new reactor concept for the photocatalytic treatment of waste water: the fixed-bed photocatalytic membrane reactor (FPMR). The core part of the FPMR is a structurally controlled catalyst layer, which is formed by dead-end filtration on a membrane. The waste water is forced to flow through the photocatalytic layer, while it is illuminated with UV light. The FPMR is expected to overcome existing limitations of conventional reactors, such as low photocatalytic activity, low mass transfer, need for an extra separation step, inflexibility or low energy efficiency. The reactor performance was investigated for the photodegradation of organic compounds, i. a. methylene blue and oxalic acid. The photocatalytic layer was composed of pyrogenic titania. Its structural properties were controlled via titania dispersion, flocculation or by adding a second particle phase. Our experiments addressed the influence of UV irradiance, superficial velocity and structural layer properties. Besides, a mathematical model was established, which accounts for mass transfer, UV intensity and propagation of UV light through the photocatalytic layer, and which relates the intrinsic reaction rate constant with the overall rate of photodegradation. All experimental data support the validity of the model. More importantly, the overall reaction rate constant of the FPMR is around 10 [1/s], which is significantly larger than found for conventional photocatalytic reactors.

Resume : For improving the photocatalytic activity of titanium dioxide (TiO2), the use of appropriate conjugated polymers such as poly(hexyl thiophene) (P3HT) to form composites with TiO2 can extend the absorption for the light harvesting. Such photocatalytic materials have also a high charge separation ability, which increases the photocatalytic reaction. Besides, to reduce the electron-hole recombination rates, which impact strongly the photocatalytic performance, graphene by its high electrical conductivity, can be incorporated to the P3HT/TiO2 composites in order to improve the charge transport in the materials. However, the effects of the graphene size on the photocatalytic behavior of composites has not been studied so far. In this work, we have investigated TiO2/P3HT/graphene composites and we examined the effect of the graphene size on the photocatalytic performance of the materials. Optical characterizations using steady and time resolved photoluminescence, Raman, Infrared and absorption have been performed on composites synthesized with different graphene sizes (expressed by graphene weight per surface unit). It has been shown that their properties depend on the graphene concentration and also on its size. Photocatalytic activities of the composite films were examined through the degradation processes of a rhodamine solution under visible light irradiation in the presence of the composites. Highest performance was provided by composite samples containing graphene of medium size (150 m2/g). The improvement of photocatalytic activity of these composites is related to an increase in contact surface between graphene and polymer resulting in a better charge transfer between materials, enhancing thus their performance.