Structural and photoelectrochemical properties in the thin film system Cu-Fe-V-O and its ternary subsystems Fe-V-O and Cu-V-O

Thin-film material libraries in the ternary and quaternary metal oxide systems Fe-V-O, Cu-V-O, and Cu-Fe-V-O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu-Fe-V-O, the compositions (Fe₆₁V₃₉)O_x and (Cu₅₂V₄₈)O_x with promising photoactivity were identified from their ternary subsystems Fe-V-O and Cu-V-O, respectively. Two Cu-Fe-V-O material libraries with (Cu_29-72Fe_4-27V_22-57)O_x and (Cu_11-55Fe_27-73V_12-34)O_x composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu₃FeV₆O₂₆/FeVO₄), II (Cu₅V₂O₁₀/FeVO₄/α-Cu₃Fe₄V₆O₂₆), III (Cu₅V₂O₁₀), IV (Cu₅V₂O₁₀/FeVO₄, V (FeVO₄/γ-Cu₂V₂O₇/α-Cu₃Fe₄V₆O₂₆), VI (β-Cu₂V₂O₇/α-Cu₃Fe₄V₆O₂₆/FeVO₄), and VII (β-Cu₃Fe₄V₆O₂₆/FeVO₄). In the investigated composition range, two photoactive regions, (Cu₅₃Fe₇V₄₀)O_x and (Cu₄₅Fe₂₁V₃₄)O_x, were identified, exhibiting 103 μA/cm² and 108 μA/cm² photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu₄₅Fe₂₁V₃₄)O_x comprises the dominant α-Cu₃Fe₄V₆O₂₄ phase and minor FeVO₄ phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm.