Materials development for solar fuel production and energy conversion

Resume : For energy conversion the oxygen reduction and oxygen evolution reactions are of extreme interest. For both reactions a huge over potential is needed to obtain a reasonable current, and in both cases this is mainly due to sluggish catalysis. I study the reasons for the over potential on the basis of density functional simulations and determine activity descriptors, which are easy to calculate and therefore suited for computational screening for more active catalyst materials. Universal trends in the activity for the oxygen reactions on metals and oxides are identified. I show examples on design of new catalysts which were suggested by the simulations and later in rotating disk experiments showed an increased activity. Furthermore, some directions to improve eletrocatalysis are suggested.

Resume : Metal (oxy)nitride photocatalysts have attracted much attention in terms of solar hydrogen production because of their band structures suitable for water splitting under visible light irradiation. (Ga1-xZnx)(N1-xOx) is capable of overall water splitting when being modified with hydrogen evolution cocatalysts. Ta3N5 modified with proper cocatalysts is applicable as an oxygen evolution photocatalyst in Z-scheme water splitting. Modification of (oxy)nitride photocatalysts with oxygen evolution cocatalysts is important for the water splitting reaction because they are thermodynamically less stable than oxides. The reactivity of photoexcited holes must be controlled kinetically so that water oxidation is facilitated while self-oxidation can be suppressed. Recently, it was found that coloading (Ga1-xZnx)(N1-xOx) with oxygen evolution cocatalysts such as Mn3O4, RuO2, and IrO2 not only improved the photocatalytic activity for overall water splitting but also the durability of the oxynitride. Similar effect was also observed for the TaON photocatalyst. Additionally, CoOx deposited via a heat treatment under an NH3 flow was found to work as an efficient oxygen evolution cocatalyst for various (oxy)nitrides such as LaTiO2N, Ta3N5, and BaNbO2N. It was revealed that the lifetime of photoexcited carriers could be extended significantly by loading oxygen evolution cocatalysts. In this talk, recent advancement in oxygen evolution cocatalysts and (oxy)nitride photocatalysts will be presented.

Resume : Solar hydrogen generation is considered as key challenge for renewable fuel production and reliable energy storage. We estimate the band-alignment of III-V ternary compounds relative to the redox potential of water from available literature data and suggest a photochemical diode based on dilute nitride GaPN grown lattice-matched on Si(100)?a tandem configuration with bandgaps predicted to be close to theoretical optimum for light-induced water splitting [1]. Recently, we showed that the atomic order at GaP(100) surfaces has a significant impact on the interface formation with water[2]. The influence of N on the GaPN surface preparation during metalorganic vapor phase epitaxy (MOVPE) therefore is extremely important. We benchmark the in situ reflection anisotropy (RA) spectra of two different GaPN/Si(100) surfaces to photoelectron spectroscopy (PES) and low-energy electron diffraction (accessible after MOVPE-to-UHV transfer) which enables in situ control during surface preparation in VPE ambient. The Ga-rich GaPN/Si(100) surface may be prepared analogously to GaP(100) without significant N depletion in both bulk and surface layers (evidenced by PES and x-ray diffraction), while excess N impedes group-V-rich surface preparation. We attribute a feature in the RA spectra close to the E1 transition of GaP to N incorporation which allows to study GaPN growth in situ during MOVPE. [1] S. Hu et al., Energy Environ. Sci. 6, 2984 (2013) [2] MM. May et al., New J. Phys. 15, 103003 (2013)

Resume : Tantalum nitride Ta-Nx ultra-thin films present a large variety of nanostructure depending of the nitrogen content and the deposition technique. This impacts directly their optical gap from zero for metallic Ta and TaN to 2.2eV for the semi conductive Ta3N5. That latter nanostructure is of special interest for solar production of hydrogen by water splitting or as a compound for photovoltaic multilayer system. We will show that Ta3N5 thin films may be elaborated using High Power Pulsed Magnetron Sputtering HIPPIMS deposition while it is not possible by conventional magnetron sputtering. HIPPIMS is of special interest for that materials production because it allows a larger incorporation of nitrogen inside the films and generates specific nanostructures. Therefore, the study is based on plasma diagnostic, nuclear analysis for concentration and density, grazing incidence X-ray scattering GIWAXS for the nanostructure, high resolved electron scanning microscopy SEM-Feg for the morphology and XPS for the local bonding. We will present our first results concerning the optical properties followed by Ellipsometry and reflexion-tranmission measurements.

Resume : Recently g-C3N4 has attracted great interest due to its low cost and visible-light activity. However, the fast electron-hole recombination and low activity for H2 evolution have restricted the use of g-C3N4 alone for solar fuels production. Herein, we present our recent work on development of decorated g-C3N4 with noble-metal-free cocatalysts, cobaloxime and NiS2.[1,2] Both co-catalysts can allow for effective receiving of excited electrons from g-C3N4 for H2 generation through efficient redox cycles. Further, we also demonstrated that the coupling of g-C3N4 coupling with other semiconductor structures, such as red phosphor, CdS, and In2O3, allows for creating heterojunctions to promote charge separation for efficient photocatalytic H2 generation and CO2 reduction.[3-5] Our studies demonstrate economic solar-to-fuels conversion platforms based on metal-free g-C3N4 photocatalysts and noble-metal-free coupling compounds and co-catalysts. 1. S. W. Cao, X. Liu, Y. Yuan, Z. Zhang, J. Fang, S. C. J. Loo,* J. Barber, T. C. Sum, C. Xue,* Phys. Chem. Chem. Phys. 2013, 15, 18363. 2. L. S. Yin, Y. Yuan, S. Cao, Z. Zhang, C. Xue*, RSC Adv. 2014, 4, 6127. 3. Y. P. Yuan, S. Cao, Y. Liao, L. Yin, C. Xue*, Appl. Catal. B: Environ. 2013, 140, 164. 4. S. W. Cao, Y. Yuan, J. Fang, F. Boey, J. Barber, S. C. J. Loo*, C. Xue*, Int. J. Hydrogen Energy 2013, 38, 1258. 5. S. W. Cao, X. Liu, Y. Yuan, Z. Zhang, J. Fang, S. C. J. Loo,* T. C. Sum, C. Xue*, Appl. Catal. B: Environ. 2014, 147, 940.

Resume : The electronic structure of the cobalt oxide based catalysts for OER is analyzed using synchrotron radiation photoelectron spectroscopy. The catalyst films are prepared by electrochemical deposition. X-ray photoelectron spectroscopy and resonant photoelectron spectroscopy is used to analyze the Co2p and O1s core levels, absorption edges and valence bands. Here we find a difference in the Co oxidation state as a function of film thickness (deposited charge). We discuss our resonant data in terms of the partial density of states of the valence and conduction band. For the individual Co3d states, we determine their configuration, their spin, and their energy level relative to the Fermi energy. At resonant excitation we find the Co2p partial DOS to exhibit sharp features next to the VBM for increased cobalt oxidation state instead for a broad emission at around 6eV below EFermi for a low oxidation state. We attribute such sharp features to the low spin (LS) configuration of Co3+. In contrast, our data prove the Co2+ ground state for thin pristine cobalt oxide films and demonstrate that it is exclusively in the Co3d7 high spin state. In addition, both cobalt oxide configurations have characteristic oxygen to Co charge transfer states in the band gap. We attribute the trivalent charge transfer state to be the active state for the oxygen evolution reaction. [1] M. Richter, M., D. Schmeißer, Applied Physics Letters 102, 253904 (2013)

Resume : Recent years have shown a renewed interest in the use of functional materials for conversion of solar to chemical energy by photocatalysis. Although these materials are, mostly, semiconductor oxides, it has been demonstrated that metal nanoparticles (NPs) improve their conversion efficiency due to their surface plasmon resonance properties. Au and Ag are of particular interest since exhibit resonant behavior in the visible range, allowing the harvesting of a large amount of the solar flux. Among semiconductor photocatalysts for water splitting, TiO2 has been investigated due to its abundance, stability, non-toxicity and high photocatalytic activity. So, composite materials fabricated by TiO2 thin films in connection with Au or Ag NPs gain attention as candidate for efficient photocatalysts devices. In this work, NPs are obtained after thermal dewetting of sputtered Au or Ag films on TiO2. We correlate the photocatalytic activity of the NPs-TiO2 composite film under UV and visible illumination with structural characteristics: crystal phase (anatase or rutile), density, size and plasmonic response of the Au and Ag NPs. Different designs are investigated: NPs simply deposited on the TiO2 surface or NPs either embedded in the TiO2 film. Structural and morphological analysis performed by XRD, SEM, AFM and RBS will be presented, so as plasmonic response evaluated by UV-visible spectroscopy. Finally, activity performance of the composite materials will be presented and discussed.

Resume : Photo-electrochemical solar fuel conversion is a seminal method to convert solar energy into a storable fuel and contributes to finding sustainable energy solutions. However, the overall efficiency of photo-electrochemical solar fuel conversion is still low and the degradation of the electrodes is high. Increasing the surface area of the electrodes is a known and effective method to increase the catalytic active area and by this to increase the performance. Usually, nanostructures are fabricated from the bottom up, i.e. by means of growing nanotubes or nanowires. In this study, we use He plasmas with ion fluxes between 10^20 m-2 s-1 and 10^23 m-2 s-1 to nanostructure surfaces from the top down. The method was already proven on bulk materials where feature sizes below 100 nm were obtained. We have now transferred the technique to thin films deposited on typical solar fuel conversion substrates, i.e. F: SnO2 (FTO) on glass. The high processing temperature and delamination of the thin films from the substrate are major concerns when thin films are used compared to bulk material. We will show results of structural and microscopical characterization of successfully plasma nanostructured metal thin films of Fe, W, Ti, Mo on FTO-glass as well of the resulting oxides after post-annealing. Photo-electrochemical characterization results prove the feasibility of using these nanostructures to significantly improve the performance of current solar to fuel conversion materials.

Resume : The transformation and storage of solar energy is a major challenge in the framework of renewable energy sources. The conversion into chemical energy stored in the form of hydrogen, through direct photoelectrochemical water splitting is a promising method. Hematite (α-Fe2O3) is a potential key photoanode material due to its optimal band gap, excellent chemical stability, abundance, non-toxicity and low cost. However, for practical applications hematite performances must be improved with respect to the absorption coefficient and holes diffusion length. Moreover, the roles of the long range crystalline order, quality and orientation remain largely unknown yet. In order to tackle these open questions we have studied the growth, the crystalline and electronic structures and the photoelectrochemical properties of epitaxial doped and undoped Fe2O3 films grown by oxygen plasma assisted molecular beam epitaxy on different substrates. This method allows obtaining perfectly single crystalline films with well-defined doping levels, defects and crystallographic structures well adapted for photoelectrochemical measurements [1]. We show that both crystalline structure (hematite, maghemite) and crystallographic orientation ((111), (001), polycrystalline)) strongly influence in a non-trivial way the photoelectrochemical properties. These results are discussed in relation with conductivity anisotropy and surface kinetic properties. [1] H. Magnan et al., Appl. Phys. Lett. 101, 133908 (2012)

Resume : Copper chalcopyrite semiconductors include a wide range of compounds that are of interest for photoelectrochemical water splitting. In the present study, we investigated the chalcopyrite, especially CuInS2 (CIS) fabricated by electrodeposition and spray pyrolysis methods as photochatodes for water splitting. Thin film of CISed was formed by stack electrodeposition of copper and indium followed by sulfurization under H2S flow. CISed loaded with Pt (Pt-CISed) worked as photocathodes for H2 generation from an aqueous solution containing 0.1 M Na2SO4 (pH = 9). Introduction of an n-type CdS layer on the CuInS2 before the Pt loading (Pt-CdS/CISed) resulted appreciable improvements of H2 liberation efficiency and a higher photocurrent onset potential. Moreover, CISed film modified with In(OHx,Sy) (Pt- In(OHx,Sy)/CISed) also worked as an efficient photocathode with photocurrent as high as 14 mA cm-2 at 0 V (vs. RHE) and maximum applied bias photon-to-current efficiency of over 1.5% at 0.25 V (vs. RHE). In addition, thin film of CISsp and its Ga alloyed film (Ga:CISsp) were synthesized by spray pyrolysis modified by n-type CdS layer and loading of Pt. Both Pt-CdS/CISsp and Pt-CdS/Ga:CISsp worked as photocathodes with photocurrent as high as ca. 6 mA cm-2 at 0 V (vs. RHE) from an aqueous solution containing 0.1 M Na2SO4 (pH = 9). The Pt-CdS/Ga:CISsp photocathode with Ga/In ratio of ca. 0.25 had a ca. 0.1 V higher photocurrent onset potential than that of Pt-CdS/CISsp.

Resume : Generation of hydrogen using solar power with the help of semiconductor photoelectrodes is an efficient and environmental friendly way to produce energy. Among the various semiconducting materials, Silicon could be one of the best choices for this application. However, certain problems like surface oxidation, charge transfer from Si surface to electrolyte, reflectance of the absorber are main issues for Si photocathodes. Surface oxidation and charge transfer can be optimized by passivation layers and by the use of catalyst on Si surface. Instead, the problem of surface reflectance can be avoided by structuring the silicon photoabsorber. Structuring of Si in the top down method is generally obtained by etching the substrate with metal catalyst on it, but this process needs several steps of sample preparation. The structuring can be also done using the electrochemical technique, without having any metal catalyst on the Si surface. In this work, we used the latter method and prepared microstructured Si photocathodes. These showed a better photo current as compared to bare Si in low pH electrolytes and an the onset potential shifted by 300mV positive in comparison to planar Si photocathodes.

Resume : The conversion of solar light into electrons and holes which are used to split water into hydrogen and oxygen is one of the possible ways to address the world's pressing energy supply and storage problem. A material to be used as light harvester in a photoelectrochemical cell requires to be (i) chemical/structural stable under irradiation, and to have (ii) a band gap in the visible range with band edges well positioned with respect to the redox levels of water. In previous studies [1], we performed a computational screening for 20000 perovskites with focus on one-photon water splitting finding 20 new promising materials. Now, we extend the screening to a more general collection of materials already known to exist in nature (as described in the Materials Project database [2]). The descriptors for the screening are (i) the heat of formation, evaluated with respect to solid and dissolved phases using Pourbaix diagrams [3], (ii) the bandgap, calculated using the GLLB-SC functional [4], and (iii) the band edge positions, calculated using an empirical formula based on the electronegativities of the constituent atoms. Based on the screening, we suggest a handful of materials for further experimental investigation. [1] I.E. Castelli et al., Energy Environ. Sci., 5, 5814 (2012) and Energy Environ. Sci., 5, 9034 (2012). [2] https://www.materialsproject.org/. [3] I.E. Castelli et al., Topics in Catalysis (2013). [4] M. Kuisma et al., Phys. Rev. B 82, 115106 (2010).

Resume : Electrolysis applications like water splitting rely on efficient electro-catalysts. One state-of-the-art catalyst for the anodic side of PEM electrolysis cells are electrode coatings containing oxides of iridium. In order to reduce the content of noble metal a cheap “diluent” such as titanium can be used. Introducing defined porosity into catalytic coatings can improve the performance of IrO2 in Oxygen Evolution Reaction (OER). However, structure - performance relationships for IrO2 and Ir/TiOx catalysts in the OER remained so far largely unexplored. Control over pore size and pore connectivity of oxide coatings can be achieved when micelles of amphiphilic copolymers are employed as pore templates. We therefore synthesized templated mesoporous catalytic coatings via the Evaporation Induced Self-Assembling (EISA) approach. Dipcoated films were calcined under air flow at different temperatures between 350 and 625 °C. The material properties were analyzed by SEM, TEM, SAXS, XRD, BET and resistivity measurements. The OER activity was tested under acidic condition using a rotating disk setup. SEM images of mesoporous IrO2 catalysts calcined at 400 and 475 °C could be obtained. The catalyst films differed in pore morphology, crystallite sizes, electrical conductivity, electrochemically active surface area and OER performance. The effect of catalyst composition, crystallinity and the type of pore system on OER performance will be discussed in detail.

Resume : Recent advances in dye-sensitized photoelectrochemical synthesis cells (DSPECs) is pushing research toward new molecular materials and inorganic integration strategies that can produce efficient, stable and robust device designs for water-splitting and CO2 reduction. Photoanodes include a photoactive dye molecule and catalyst bound to a nanostructured metal oxide (e.g. mesoporous TiO2) that accepts and transport charge. Maintaining the integrity of the dye/oxide binding unit is especially challenging for water oxidation DSPECs where irradiation leads to dye desorption. Atomic layer deposition (ALD) is a sequential self-limiting reaction scheme that can precisely deposit inorganic and/or organic thin films that are highly conformal and uniform on 3D structures. Recently, our team discovered that when mesoporous TiO2 with surface-bound dye molecules is coated with a thin layer of metal oxide by ALD, the rate of dye desorption significantly decreases, even under highly oxidative conditions, with a relatively small impact on photoelectron generation. For example, by applying three ALD cycles of AlMe3-H2O (∼3 Å of Al2O3) to [Ru(bpy)2(4,4′-(PO3H2)-bpy)]2+ (RuP) dye on TiO2, the rate of dye desorption decreased significantly compared to similar electrodes without the ALD layer. Further ALD enhances stability but also increases emission and decreases injection yields from the metal-toligand charge-transfer (MLCT) excited state of RuP. We also translated this stabilization strategy to dye sensitized photovoltaic cells, and to DSPECs operating under highly oxidative pH conditions. We further find that ALD can produce highly conductive core/shell mesoporous oxides that decrease charge recombination and accelerate photoelectron transport through the device. We will describe the outstanding issues in ALD-integrated dye-sensitized devices and discuss in-situ analysis of the ALD reaction steps that promote dye stabilization.