Electrosynthesis of Crystalline and Phase Pure Silver Orthovanadate (Ag3VO4)-a Visible Light-Active P-Type Semiconductor
Authors: Vali Abbas , Rajeshwar Krishnan 
Published: December 2020

Silver-based ternary oxides are promising candidates for use as a visible light active semiconductor since their band gaps are lowered to the visible wavelength range due to the mixing of Ag 4d and O 2p orbitals in the valence band. In particular, Ag3VO4 with the highest ratio of Ag2O: V2O5 stoichiometry (3:1) among Ag-V-O ternary compounds generated from the two binary oxides (Ag2O and V2O5), is a visible light active semiconductor with a low band gap (2.18 eV).1

This paper describes the electrodeposition of Ag3VO4 film and its subsequent characterization. Electrodeposition is a fast and low temperature method which can be deployed to prepare thin films of a given semiconductor. In addition, 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.
A two-step electrodeposition strategy previously developed in this laboratory, for ternary oxides2 was applied in this study, to prepare Ag3VO4 thin films on fluorine doped tin oxide (FTO) substrate. In the first step, a silver thin film was deposited cathodically from silver nitrate in a non-aqueous medium (to preclude silver oxide formation). In the second step, the silver thin film was stripped anodically in sodium orthovanadate (Na3VO4) solution to generate Ag+ and subsequent in situ precipitation with VO4
3- to yield Ag3VO4 thin film on the substrate. Electrochemical crystal microgravimetry confirmed one and three electron stoichiometry for the first and second steps respectively.
Rather surprisingly, X-ray diffraction analyses of the as-prepared film showed that it was crystalline
even without thermal anneal in phase-pure α-Ag3VO4 form
. Compositional assays using energy-dispersive X-ray analysis (EDX) averaged from different spots of the as-prepared sample were consistent with the Ag/V ratio: 2.92 ± 0.02. The direct and indirect energy band gaps of α-Ag3VO4
thin films were measured by diffuse reflectance spectroscopy (DRS) and found to be 2.58 ± 0.04 and 2.37 ± 0.02 eV respectively. The surface band gap of as-prepared α-Ag3VO4 was measured by surface photovoltage spectroscopy (SPS) and found to be 2.16 ± 0.03 eV. The positive polarity of the photovoltage at excitation energies higher than the valence band threshold was diagnostic of p-type semiconductor behavior for α-Ag3VO4. Atmospheric photoemission spectroscopy (APS) showed the valence band position of α-Ag3VO4
to be -4.82 ± 0.03 eV on the vacuum energy scale. By combining the SPS and APS results, the calculated conduction band edge position of α-Ag3VO4 was determined to be -2.66 eV on the vacuum energy scale. These results will be discussed in the context of the application of this semiconductor for photoelectrochemical reduction processes such as hydrogen evolution or CO2 reduction.

References

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