After Fujishima and Honda reported their results on the photo-electrolysis of water [1], three materials have received special attention for the fabrication of photo-anodes: (i) titanium dioxide (TiO2), (ii) hematite (α-Fe2O3) and (iii) tungsten trioxide (WO3). These materials present high efficiencies for solar energy capture and conversion to chemical energy, stability under water splitting operating conditions and/or low cost and earth abundance [2]. However, when operating under illumination in the visible range (ca. 45% of the solar spectrum), these semiconductors need of an external potential (bias) to promote the oxygen evolution reaction on its surface, due to the position of their conduction and valence bands [3]. In this context, it has been observed that molybdenum trioxide (MoO3) films have good electrochromic and photochromic properties, while molybdenum dioxide (MoO2) has an unusual metal-like electrical conductivity [4,5]. Therefore, these compounds emerge as an interesting alternative for the fabrication of photo-anodes for water splitting. Photo-anodes are usually fabricated by coating or electrodepositing films of the photo-active material on a substrate (e.g. fluorine-doped tin dioxide(FTO)-coated glass) that acts as current collector and support. The technique chosen has to allow a good control of the structural parameters (particle size, crystallinity and thickness) of the photo-active films produced, since they will affect the photo-electrochemical behavior of the cell [6]. Furthermore, if the photo-anodes are to be produced on a large scale, the production rate and the reproducibility that the fabrication technique offers is crucial. Electrochemical coating processes have been applied successfully in industry for decades. Therefore, this research project aims to produce photo-anodes for water splitting by electrodeposition of MoO2/MoO3 thin films on FTO-coated glass. Fabrication of MoO2/MoO3-based photo-anodes was carried out by applying a potential of -1.377 V vs Ag/AgCl (3 M KCl) during 3 hours on an FTO-coated glass working electrode immersed in a solution containing 0.075 M molybdenum trioxide (MoO3), 0.075 M nickel sulfate (NiSO4·6H2O), 0.5 M tri-sodium citrate (Na3C6H5O7·2H2O) and 0.7 M ammonium hydroxide (NH4OH). The MoO2/MoO3 films produced are ca. 2.3 μm thick, present an irregular microstructure (determined using scanning electron microscopy, SEM) and show a dark brown pigmentation, typical of molybdenum oxides. X-ray photoelectron spectroscopy (XPS) analysis of the samples determined that these films contain molybdenum in at least two different oxidation states (IV and VI), possibly forming MoO2 and MoO3. These results are in good agreement with the energy dispersive X-ray spectroscopy (EDX) analysis of the samples, which showed that the films produced contain approximately 33.7% in weight of molybdenum. Finally, a Tauc plot (Figure 1) was generated using the UV-visible absorption spectra of the samples, which determined that the photo-anodes fabricated present an allowed direct band gap of 2.6 eV. The anodes photo-activity for oxygen evolution was studied by cyclic voltammetry using an electrochemical cell with a 0.1 M Na2SO4 solution as electrolyte. The results obtained (Figure 2) show that in the dark the oxygen evolution reaction on the surface of the MoO2/MoO3-based photo-anodes is triggered at ca. 1.2 V vs Ag/AgCl (3 M KCl), while under UV illumination (wavelength = 364 nm) this phenomenon occurs at ca. 0.6 V vs Ag/AgCl (3M KCl). From the results exposed before, it is clear that the photo-anodes produced present interesting semiconducting and catalytic properties for their application in an electrochemical cell for water splitting. Consequently, future work will be centered in the design and fabrication of a photo-electrochemical cell using these electrodes and improvements will be introduced by heat treatment or doping of the electrodeposited MoO2/MoO3 films. ACKNOWLEDGMENTS The authors thank the Chilean Government for a CONICYT studentship for MGG. REFERENCES Fujishima, A. and Honda, K. (1972). Electrochemical photolysis of water at a semiconductor electrode. Nature, 238, 37-38. Ong, CK. (2013). Design and performance of photo-electrochemical reactors with Fe2O3 photo-anodes for water splitting. PhD thesis, Imperial College London. Xu, Y. and Schoonen, M. (2000). The absolute energy positions of conduction and valence bands of selected semiconducting minerals. American Mineralogist, 85, 543-556. Al-Kandari, H., Mohamed, AM., Al-Kharafi, F., Zaki, MI. and Katrib, A. (2012). Modification of the catalytic properties of MoO2−x(OH)y dispersed on TiO2 by Pt and Cs additives. Applied Catalysis A: General, 417, 298-305. Dukstiene, N., Sinkeviciute, D. and Guobiene, A. (2012). Morphological, structural and optical properties of MoO2 films electrodeposited on SnO2∣glass plate. Central European Journal of Chemistry, 10, 1106-1118. Krol, R. and Grätzel, M. (2012). Photoelectrochemical Hydrogen Production. Springer, Nueva York. Figure 1
A speciation model is proposed for the determination of the concentration of different species formed in an aqueous solution containing Mo(VI), Ni(II), citrate, ammonia and sulfate at 298 K, 10 5 Pa and variable pH and Mo(VI)/Ni(II) activity ratios. This solution is to be used as electrolyte in the electrodeposition process of thin films of Mo and Ni oxides, which appear to be a promising material for the fabrication of photo-anodes for water splitting in photo-electrochemical cells. The speciation model comprises 53 species and their stability constants, which are related through molar and charge balances to estimate the composition of the solution given a pH value and formal concentrations. As a result, predominance and distribution diagrams are produced, based on which electrolyte conditions (pH and central species concentrations) are recommended to maximize the availability of desired species for the electrodeposition process. Eh-pH diagrams for molybdenum and nickel species at the recommended activities (10 −2 for Mo and Ni species, and 10 −1 for citrate, ammonia and sulfate species) are also produced, to determine the adequate potential range to be applied for the electrodeposition of Mo and Ni oxides films.
Photo-electrochemical cells are a promising technology for the environmentally-benign production of hydrogen using solar energy. In this kind of cells, a photo-active material is coated or deposited on a substrate (e.g. fluorine-doped tin dioxide(FTO)-coated glass) which acts as current collector and support. In this research project, a molybdenum oxide layer is electrochemically deposited on the surface of a FTO-coated glass working electrode immersed in a solution containing 0.075 M molybdate (MoO4 2-), 0.075 M nickel sulfate (NiSO4•6H2O), 0.5 M tri-sodium citrate (Na3C6H5O7•2H2O) and 0.7 M ammonium hydroxide (NH4OH), by applying a potential of -1.377 V vs Ag/AgCl (3 M KCl) during 3 hours. The characterization of the photo-anodes produced suggests that they present semiconducting and catalytic properties which make them attractive for their application in a photo-electrochemical cell for water splitting.
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