Β-Cu2V2O7 Thin Films By a Hybrid Electrochemical/Thermal Route: Preparation and Characterization
Authors: Vali Abbas , Rajeshwar Krishnan 
Published: December 2020

Solar water splitting offers an attractive solution for renewable energy storage. Ternary copper vanadates have drawn attention for use in photoelectrochemical (PEC) water oxidation due to their low band gap and photostability.1 In particular, Cu2V2O7 with a ratio of CuO: V2O5 stoichiometry (2:1) is a visible light active n-type semiconductor with a low band gap (see below), and has seen use in PEC devices because of its high stability for PEC water oxidation.2

This paper describes the development of a hybrid electrochemical/thermal route for the preparation of Cu2V2O7 thin films. Electrodeposition is a facile, low temperature method which can be deployed to prepare thin films of a target semiconductor. Also, the optoelectronic parameters (e.g., photocurrent, flat band potential) of the thin film semiconductors prepared by electrodeposition can be measured easily without any more processing steps. In addition, the thickness of a given thin film semiconductor, a crucial factor for PEC reactions, can be controlled easily in an electrodeposition process.

A two-step electrodeposition strategy for ternary oxides3 was deployed in this study, to prepare Cu2V2O7 thin films on fluorine doped tin oxide (FTO) substrate. In the first step, a copper thin film was deposited cathodically from copper(II) nitrate in a non-aqueous medium to preclude copper oxide formation. In the second step, the copper thin film was stripped anodically in ammonium metavanadate (NH4VO3) aqueous solution to generate Cu+ and subsequent in situ precipitation with VO3
- species to yield a green CuVO3 film on the substrate. Electrochemical quartz crystal microgravimetry confirmed two- and one-electron stoichiometry for the first and second steps respectively.

Compositional assays by X-ray energy-dispersive analyses (EDX), averaged from different spots of as-prepared sample, were consistent with a Cu/V ratio: 0.98 ± 0.02. X-ray diffraction pattern for the as-prepared film showed that it was amorphous. Then, the green amorphous CuVO3 thin film was oxidized to a dark reddish crystalline phase-pure β-Cu2V2O7 product by thermal anneal at 450 oC for 30 min in the presence of oxygen. The direct and indirect energy band gaps of β-Cu2V2O7 were measured by diffuse reflectance spectroscopy (DRS) and Tauc analyses yielded values of 2.26 ± 0.03 and 2.08 ± 0.02 eV respectively. The PEC activity of β-Cu2V2O7
was measured at different thicknesses to find the optimum photoelectrode film thickness for water oxidation; this was found to be ~250 nm. These new results will be discussed in the context of the prospects for this semiconductor candidate for PEC water oxidation.

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