Combinatorial Synthesis and High-Throughput Characterization of Fe–V–O Thin-Film Materials Libraries for Solar Water Splitting

Abstract

The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe₁₀V₉₀O _x to Fe₇₉V₂₁O _x) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe₂V₄O₁₃ phase from ∼11 to 42 at. % Fe (toward low-Fe region) and the FeVO₄ phase from ∼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe₂O₃ is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO₄ phase show enhanced photocurrent densities (∼160 to 190 μA/cm²) throughout the thickness gradients whereas compositions with the Fe₂V₄O₁₃ phase show comparatively low photocurrent densities (∼28 μA/cm²). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of ∼190 μA/cm² was obtained for films with ∼54 to 66 at. % Fe for the FeVO₄ phase with ∼2.04 eV for the indirect and ∼2.80 eV for the direct band gap energies.