Phase Pure Silver Pyrovanadate (Ag4V2O7) Thin Films: Synthesis via a Hybrid Cathodic/Anodic Approach and Characterization

Abstract

Combining two or more binary compounds to generate a multinary compound is a strategy for enhancing the photoelectrochemical (PEC) performance and properties of the parent binary compounds. In this vein, multinary compound semiconductors have recently come under the spotlight in a variety of device applications. In particular, the low energy band gap of the Ag-V-O ternary family (Ag3VO4, Ag4V2O7, AgVO3, Ag2V4O11, and AgV7O18) has recently sparked interest in using these visible light active semiconductor for photovoltaic solar cells, solar water splitting, CO2 photoreduction, and environmental remediation applications. In particular, Ag4V2O7, a visible light active semiconductor with a low band gap (2.49 eV) has been synthesized in powder form using solid state, hydrothermal, and precipitation methods.1,2 However, for device applications, deposition techniques that facilitate direct synthesis of semiconductor in film form, are attractive.In this vein, electrochemical deposition is a fast, low temperature, and scalable method to prepare large active area films of a given semiconductor. Therefore, this paper presents a hybrid cathodic and anodic route for the electrodeposition of Ag4V2O7 thin films and its subsequent physical and optoelectronic characterizations. Ag4V2O7 thin film on fluorine-doped tin oxide (FTO) substrate was synthesized using a hybrid cathodic and anodic route previously developed in our laboratory.3 In the cathodic step, a metallic silver thin film was electrosynthesized from silver ions in a nonaqueous medium on fluorine-doped tin oxide (FTO) support. In the anodic step, the generated silver ion (Ag+) from anodic stripping of the silver thin film in a pyrovanadate ion (V2O7 4-)containing-solution resulted in an in situ precipitation reaction with V2O7 4- to form Ag4V2O7 thin film on FTO. Electrochemical quartz crystal nanogravimetry (EQCN) confirmed one and four electron stoichiometry for the cathodic and anodic steps respectively.Energy-dispersive X-ray analysis (EDX) of as-prepared sample gave the Ag/V ratio: 1.95 ± 0.02, close to the expected stoichiometric Ag/V ratio for Ag4V2O7. X-ray diffraction analyses of the as-synthesized sample significantly confirmed that room temperature electrodeposited sample was crystalline and phase-pure. To measure the direct and indirect energy band gaps, Tauc plots were constructed from diffuse reflectance spectroscopy (DRS) data. The estimated direct and indirect optical transition energies of Ag4V2O7 thin films were 2.58 ± 0.04 and 2.52 ± 0.02 eV respectively. A valence band energy position as -4.88 ± 0.03 eV (on the vacuum energy scale) was estimated for Ag4V2O7 using atmospheric photoemission spectroscopy (APS) results. By combining the DRS and APS results, the calculated conduction band edge position of Ag4V2O7 was determined to be -2.36 eV on the vacuum energy scale. Kelvin-probe measurement located the Fermi level of silver pyrovanadate thin films at -4.82 ± 0.02 eV on the vacuum energy scale. The corresponding EF for Ag4V2O7 was only 0.06 eV above the valence band position consistent with a moderately-doped p-type semiconductor. These new results will be discussed in the framework of the application of silver pyrovanadate for PEC reduction processes such as hydrogen evolution or CO2 reduction.