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

1. Fujishima, A. and Honda, K. (1972). Electrochemical photolysis of water at a semiconductor electrode. Nature, 238, 37-38.
2. Ong, CK. (2013). Design and performance of photo-electrochemical reactors with Fe₂O₃ photo-anodes for water splitting. PhD thesis, Imperial College London.
3. Xu, Y. and Schoonen, M. (2000). The absolute energy positions of conduction and valence bands of selected semiconducting minerals. American Mineralogist, 85, 543-556.
4. Al-Kandari, H., Mohamed, AM., Al-Kharafi, F., Zaki, MI. and Katrib, A. (2012). Modification of the catalytic properties of MoO_2−x(OH)_y dispersed on TiO₂ by Pt and Cs additives. Applied Catalysis A: General, 417, 298-305.
5. 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.
6. Krol, R. and Grätzel, M. (2012). Photoelectrochemical Hydrogen Production. Springer, Nueva York.