Electrochemical processes for nanomaterials and their properties

Resume : Anodization of Ti in aqueous and organic electrolytes is a powerful approach to generating high aspect ratio TiO2 nanotubes with high degree of orientation. These materials have generated a great deal of interest in areas such as photocatalysis, photoelectrochemical generation of fuels and dye-sensitized solar cells. In this context, the performance of mesoporous TiO2 materials is strongly determined by the mobility of charge carriers, particularly electrons. Carrier transport in these complex architectures is determined by deep trap states arising from local crystal disorder and grain boundaries. In this contribution, we shall provide a detailed analysis of deep trap states in TiO2 nanotubes as a function of the anodization conditions employing dynamic photoelectrochemical measurements. TiO2 nanotubes were obtained by anodization of Ti in solutions of NH4F in ethylene glycol, followed by annealing in air at various temperatures. We correlate structural information obtained by high resolution electron microscopy with dynamic photoelectrochemical measurements as a function of the anodization condition. Analysis of photocurrent transients in the presence of hole scavengers (SO32-) as a function of the applied potential allow generating quantitative diagrams of deep trap state density. We examine correlations between the density of deep trap states and parameters such as average nanotube length and other parameters controlled by the anodization conditions.

Resume : Pt is an expensive rare metal although it is known to be one of the best catalysts for various electrochemical reactions. Therefore, reduction of the consumption amount of Pt without loosing the catalytic activities is of great interest. For this purpose, we have studied about an electrodeposited mesoporous Pt film which possess not only a large surface area but also extremely high catalytic activity for methanol oxidation(1). Recently, we focus our attention to spherical configuration to make most of the mesoporous Pt effectively (2). In this presentation, we will report spherical mesoporou Pt structures on various substrates, and their better catalytic activities for methanol oxidation, iodine reduction, and so on than that in film configuration. (1) H. Wang et al., “Synthesis of Mesoporous Pt Films with Tunable Pore Sizes from Aqueous Surfactant Solutions,” Chem. Mater., 2012, 24, 1591. (2) M. Endo, H. Aoyama, M. Matsunaga, ”Catalytic Activity of Spherical Meso-Porous Pt films-deposited Flon-doped Tin Oxide Substrate Covered by Multi-Walled Carbon Nanotube for Iodine Reduction,” ECS 226 meeting, Cancun, Mexico, 7 Oct., 2014

Resume : Au-Ag alloy is a very suitable starting material to synthesize highly porous Au structures, allowing wet-chemical dealloying and exhibiting complete solubility over the whole composition range. Nanoporous Au is characterized by an extremely high surface-to-volume ratio and good electrical conductivity, and exhibits unique properties for applications e.g. in optics, catalysis, sensorics, and energy-harvesting. The porosity of the material depends on the initial Au:Ag concentration and distribution of the atoms. At the nano-scale, various processes such as oxidation, phase segregation and surface diffusion should influence the final Au:Ag composition and distribution in the porous nanostructures. Furthermore, the initial size and geometry of the object also affect the final morphology. Here, we report the investigation of the dealloying process of Au1-xAgx alloy nanowires in an acidic solution as a function of initial wire composition and dimension. We synthesize cylindrical Au-Ag nanowires with controlled dimensions (diameter: 25, 45, and 80 nm, length ~ 30 µm) and different Au:Ag ratios (60:40 and 40:60 at. %) by electrodeposition from a single-bath electrolyte in the cylindrical pores of etched ion-track membranes. After deposition, the polymer membrane is dissolved and the nanowires are transferred onto transmission electron microscopy (TEM) grids. The selective dissolution of silver in nitric acid leads to rough and porous nanowires depending on the initial Au:Ag composition. To gain a detailed insight into the dealloying process we investigate surface morphology, size, elemental distribution and crystallographic properties of the nanowires before and after Ag dissolution.

Resume : Chalcogenides, such as bismuth telluride (Bi2Te3) or bismuth telluride selenide (Bi2Te3-xSex), are among the most used thermoelectric materials for devices for room temperature applications. These applications include waste heat recovery, peltier devices, among others. Films of these materials can be fabricated by electrochemical deposition, which provides an inexpensive and easily scalable technique already used in industry. But the thermoelectric efficiency of those films is around 25% of what can be achieved by other more expensive techniques. Therefore, any improvement in the performance of these electrochemical deposited films would be welcome for applications.The enhancement of the thermoelectric properties of Bi2Te3 and Bi2Te3-xSex can be done through improving the morphology and stoichiometry of the electrodeposited films. This has been already achieved within our group with the addition of different elements (surfactants, complexing agents) These studies lead to an improvement of the Seebeck coefficients of the films, but the values of bulk Bi2Te3 and Bi2Te3-xSex were not reached. Therefore, we propose here a thorough study of how other factors present in the electrochemical process can lead to a further improvement of the Seebeck coefficient of the electrodeposited films. The characterization of the films has been done through SEM, EDS, TXRF,XRD and their thermoelectric performance has been studied measuring their Seebeck coefficient and electrical conductivity.

Resume : In the last decades, thermoelectric materials have produced great interest due to the necessity of fabricating high performance devices for energy harvesting. The principal candi-date to operate around room temperature is bismuth telluride. Bi2Te3 is a semiconductor with a small band gap of 0.15 eV. Depending on its stoichiometry these material can be n-type or p-type. Bi2Te3 nanowires with different diameters and high aspect ratio have been grown by pulsed electrodeposition according to a previously work [1]. These nanowires were grown in-to anodic aluminum oxide membranes. These AAO templates were made by a two-step ano-dization process in different electrolytes, as described previously [2, 3], in order to obtain dif-ferent membranes with different pore diameters. Also, anodic aluminum oxide commercial membranes were used. The influence of pulse time in the growth rate and applied potential in the crystallo-graphic orientation was studied. The results showed bismuth telluride nanowires with high as-pect ratio and different diameters. These nanowires are oriented all in the same direction, that means along [1 1 0] direction. The compositions, the growth rate and diameter of the nan-owires were then analyzed. References: 1. Manzano, C.V., et al., Thermoelectric properties of Bi2Te3 films by constant and pulsed electrodeposition. Journal of Solid State Electrochemistry, 2013. 17(7): p. 2071-2078. 2. Martín, J., et al., High-Aspect-Ratio and Highly Ordered 15-nm Porous Alumina Templates. ACS Applied Materials & Interfaces, 2013. 5(1): p. 72-79. 3. Manzano, C.V., J. Martín, and M.S. Martín-González, Ultra-narrow 12 nm pore diameter self-ordered anodic alumina templates. Microporous and Mesoporous Materials, 2014. 184(0): p. 177-183.

Resume : Non-aqueous electrodeposition from relatively low dielectric constant solvents is an established field however like all deposition methodologies it has its limitations. One fundamental physical constraint on deposition temperature is the critical point above which the fluid becomes supercritical, the dielectric constant drops and high pressure apparatus is required. Whilst electrodeposition from supercritical fluids [SCFs] is significantly harder than from non-supercritical fluids the properties of SCFs bring a range of important advantages. One key advantage is the ability to penetrate extremely small nanopores even if the surface energies are not favourable. This advantage combined with the large electrochemical window possible with solvents such as supercritical HFCs, the good mass transport properties of SCFs mean that supercritical fluid electrodeposition [SCFED] is particularly suited to deposition into nanopores, e.g. anodic alumina ( down to 10nm) and mesoporous silica (10-1nm). We will present the basics of how to perform SCFED and a range of our recent results on the deposition of metals, e.g Cu, p-block elements, e.g. Se and Sn, and other functional nanomaterials. We will demonstrate the ability to deposit reactive materials via Ge and how to improve the crystallinity of as grown layers. We will present SCFED Te nanowires with oriented growth not along the standard [001] direction and transistors produced from them. J, Ke et al. PCCP14 1517 (2012)

Resume : The direct conversion of CO2 into useful fuels by electroreduction at ambient conditions usually requires gas diffusion electrodes (GDE) to avoid mass transport limitations due to the low solubility of CO2 in water. Mostly, GDE are composed of electrocatalyst particles, binder and a conductive additive mixture, i.e. Nafion® or Vulcan Carbon, which can increase its fabrication cost. In this work, we report on the immobilization of Sn catalyst by electrodeposition over carbon paper to be employed as GDE on a continuous electrochemical flow cell, designed to adjust different experimental parameters. After a series of cell potentials from -1 to -3 V, maximum efficiency obtained at -1.7 V vs. Ag/AgCl for 93% CO2 conversion (21 % CO, 72 % HCOOH) during 2 hours continuous operation. Tests on flow rate ratio revealed that higher gas flows contribute to low current region owing its Tafel slope supporting an approach to 1 e- mechanism via CO2● – radical formation as the limiting step, while higher liquid flow rates enhance the medium current densities by means of final faradaic efficiency. Long term electrochemical tests also showed excellent stability on Sn-GDE catalyst without any significant catalyst dissolution at a time period of 6 hours at different cell currents from -1 to -10 mA.cm-2. Such a specific cell design and ease of catalyst deposition method could be useful for different chemistries on the search for cheaper and additive free catalysts for high value fuel synthesis.

Resume : Nanotwinned Cu (nt-Cu) exhibits outstanding strength and high ductility, which are key to modern electronics applications. Besides the preferable mechanical properties, nt-Cu also demonstrates high thermal stability and electrical resistivity as low as that of bulk copper. Recently, the mechanical behaviour of highly (111)-oriented nt-Cu films has been intensively investigated in order to understand its deformation mechanism. These studies have shown that dense nanotwins, with spacing of 100 nm or less, improve strength and ductility. In this study, we report the synthesis of nt-Cu films by pulse plating. By using appropriate pulse parameters, highly textured Cu films with horizontally- or vertically-aligned nanotwins were prepared. The oriented nt-Cu films consist of coarse columnar grains in which nanotwins are uniformly formed along or perpendicularly to the growing surface, depending on the deposition potential. An increase in the off-time results in formation of more uniform and denser nanotwins, attributed to a stress-relaxation mechanism. Micropillars compression tests were conducted to assess the mechanical response of nt-Cu deposits as a function of the twin orientation and density. In this presentation, we will discuss the effects of the orientation of nt-Cu deposits on their deformation mechanism based on the most recent results of our micro mechanical tests for the (111) and (11-2)-textured nt-Cu deposits.

Resume : Carbon nanotubes decorated with palladium nanoparticles and assembled in a resistive microsensor were designed for measurements of hydrogen isotopes (Deuterium and Protium) in a heavy water detritiation facility. The aim is to find inexpensive and reliable solution for a rapid determination of the isotopes, concentrations and safety limits. Palladium nanoparticles are anchored on the carbon nanotubes surface by precipitation from chloride solutions in high ultrasonic density field. By dropcasting and dielectrophoretic alignment, Pd-CNTs are quasi-aligned between the interdigitized gold electrodes of a SiO2 substrate. The microsensor was conditioned, for thermomechanical stability, in inert atmosphere by cycling up to 4000C until the electrical resistance at RT remains constant (+/- 1%). The calibration was performed with a series of isotope concentrations diluted in inert gas. The electrical resistance: measured by 4-point method via data acquisition system. Both hydrogen isotopes have a different behavior related to the adsorption on CNT respective Pd – nanoparticle well resolved in the resistance response. Difference in sensitivity and the resistance values is higher than 10% and dependent of Pd- nanoparticle dimensions. In this respect the resistive sensor based on Pd-CNTs could be an alternative to be integrated in systems for leakage detection.

Resume : Electrochromism is a reversible change in a material’s optical properties (transmittance, absorbance and reflectance) under an applied voltage. Tungsten trioxide (WO3) is a material with remarkable electrochromic properties, suitable for application such as solar permeability control in buildings (i.e. smart windows), variable reflectance mirrors and light shutters. WO3 coatings were deposited on fluorine-doped tin oxide (FTO) coated glass substrates, using low pressure chemical vapor deposition. The structure, morphology and the subsequent electrochromic properties were found to be strongly depended on the deposition period. The samples were characterized using x-ray diffraction, Raman spectroscopy and scanning electron microscopy, while, their time response from bleached to colored state, the charge involved in the electrochromic process were determined by cyclic voltammetry. Finally, the electrochemical impedance spectroscopy was used to analyze the electronic conductivity and mobility of Li+ ions injected into the host matrixes. Acknowledgements: This project is implemented through the Operational Program "Education and Lifelong Learning" action Archimedes III and is co-financed by the European Union (European Social Fund) and Greek national funds (National Strategic Reference Framework 2007 - 2013).

Resume : We describe a novel application for highly sensitive nano-biosensor based on DNA-templated gold nanowires (AuNWs) as the conducting channel to detect biotin-streptavidin binding. The DNA-templated AuNWs were manipulated by the positively charged gold nanoparticles (AuNPs) attached along λ-DNA molecules which were precisely positioned and uniformly separated on surface of large scale substrate by surface-patterning technique and connected between the source and drain electrodes with a gap of 50 nm. The positively charged AuNWs were employed to avoid nonspecific binding of streptavidin, with attachment of biotin to the layer for specific molecular recognition. We used atomic force microscopy (AFM) to observe the configuration of nano-biosensor based on the AuNWs and HP4145 semiconductor parameter analyzer to detect the change of its electrical conductivity by specific biotin-streptavidin binding. Specific biotin-streptavidin binding was detected by the change of current in the sensor characteristic. We confirmed that the nanodevice can be operated the highly sensitive nano-biosensor extending below the picomolar concentration regime after binding with streptavidin. This approach opens up for large scale fabrication of highly sensitive biomolecule sensor chips for potential use in medicine and biotechnology.

Resume : This paper investigates the electrochemical response of a screen printed electrode (SPE) sensor modified with polyaniline-GOx nanocomposite for determination of a biogenic amine (e.g. histamine). The sensors were prepared by electrochemical deposition and plasma polymerization technique The polyaniline-GOx nanocomposite sensitive layers were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman & FT-IR spectroscopy and electrochemical means (cyclic voltammetry) in order to assess their synergic properties. The deposition technique has a great influence on the analyte detection; plasma polymerization technique showed some advantages as a thinner layer of sensitive material and a higher current density for the analyte. The modified SPEs have better analytical performance for detection of histamine (lower detection limit down to 0.5 ppm), fast response time and good stability. Keywords: screen-printed electrodes, sensor, histamine, cyclic voltammetry, plasma polymerization.

Resume : Although electrodeposition from aqueous media has been widely used to obtain metallic deposits, there are cases where the application of non-aqueous solutions offers advantages over the traditional baths or it even represents the only way to electrodeposit some metals. A study of the electrodeposition of Ni from various alcoholic solutions was performed. Besides methanol and ethanol as solvents, Ni electrodeposition from ethylene glycol, glycerol, 1,2 propanediol and 1,3 propanediol was also the aim of our research. A detailed cyclic voltammetry study of these solutions has been carried out by establishing the polarization characteristics of Ni deposition and dissolution in each bath. Then the surface morphology, crystal structure and texture as well magnetic properties of the deposits have also been investigated. The best results were obtained with methanol as solvent and the deposit quality was investigated for various deposition potentials. Compact Ni deposits with metallic appearance and nanocrystalline grain size were obtained in the deposition potential range of ‒1.10 V to ‒1.40 V vs. SCE. A weak (111) texture of the deposits was observed by XRD and the lattice constant corresponded well to pure face-centred cubic (fcc) Ni. From the other solvents investigated, the Ni deposits were of lower quality in most cases. The Ni content in the deposits was in some cases fairly low and the characteristic fcc-Ni lines could also not always be observed.

Resume : We present preparative methods towards a three-dimensional magnetic data storage medium based on arrays of parallel, cylindrical metallic nanowires structured along their length. The preparation utilizes porous anodic alumina as an inert template that defines the geometry (cylinder length, diameter and pitch), combined with galvanic deposition of the functional materials inside the pores. We demonstrate that the composition of nickel-cobalt alloys can be tuned systematically at one constant deposition potential, whereby the magnetocrystalline anisotropy can be minimized. The wires then display maximal shape anisotropy. The definition of bits along the 'vertical' direction of each wire is performed either by the introduction of non-magnetic segments or by modulation of the pore diameter. The former strategy is made possible by pulsed electrodeposition from a ternary electrolyte. The latter structure is achieved by combining several anodization steps. We demonstrate the presence of magnetic domain boundaries at such designed pinning points using bulk magnetometry and single-wire imaging techniques. Finally, we introduce ultrathin oxide layers as future read/write elements by atomic layer deposition.

Resume : Nanoporous materials are of great interest for various technological applications including SERS,[1] catalysis,[2] and biotechnology.[3] Currently, enormous efforts are dedicated to the development of novel type of porous materials such as porous nanowires. The main drawback of the synthesis approaches reported so far is located mainly in the short length of the porous nanowires which cannot reach the macroscopic scale. In this contribution we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with length up to several millimeters. This two-step approach consists in the growth of gold/copper alloy nanowires by magnetron co-sputtering on nanograted silicon substrate, serving as a physical template, followed by a selective electrochemical dealloying process in diluted sulfuric acid. We demonstrate that the final porosity of the nanowires can be tailored by tuning the leaching voltage and time as well as the initial gold content and diameter of the gold/copper alloy nanowires. [1] Qian et al. Appl. Phys. Lett. 2007, 90, 153120. [2] Fujita et al. Nat. Mater. 2012, 11, 775?780. [3] Shulga et al. Chem. Mater. 2007, 19, 3902?3911.