Optical, electronic, and photoelectrochemical properties of the p-type Cu3−xVO4 semiconductor

Abstract

Investigations into new p-type metal oxides with small bandgap sizes, i.e., E_g ∼0.9 eV to ∼1.5 eV, are currently needed to enable the preparation of tandem cells with high solar-to-hydrogen conversion efficiencies. The p-type Cu(I)-vanadate, Cu₃VO₄ (space group I2m (no. 121), Z = 2, a = 4.581(4) Å, c = 8.998(2) Å), was synthesized in high purity using solid-state methods and investigated for its small optical bandgap size (E_g ∼ 1.2 eV) and photoelectrochemical properties in the form of polycrystalline films. Powder X-ray diffraction and electron microscopy data show that, beginning at 300 °C in air, a Cu-deficient composition is formed according to: Cu₃VO₄(s) + (x/2) O₂(g) → Cu_3−xVO₄(s) + x CuO(s). At 350 °C the compound decomposes at the surfaces into the Cu(II)-containing oxides CuO and Cu₃V₂O₈ (3 : 1 molar ratio), the latter protruding as rods (i.e., ∼15–25 nm in width by ∼1–5 μm in length) from the particles' surfaces. Polycrystalline films of p-type Cu₃VO₄ were prepared under these conditions (i.e., heated in air at 300 °C or 350 °C) and found to yield significant cathodic photocurrents under solar-simulated, visible-light irradiation (λ > 420 nm; AM 1.5 G filter, irradiant power density of ∼100 mW cm⁻²). Their photocurrents increased with the heating temperature of the film and with the applied bias, e.g., ∼0.1 mA cm⁻² at zero applied bias to ∼0.25 mA cm⁻² at −0.2 applied bias (pH = 5.8). The photocurrent of a non-heated film was negligible compared to the films heated in air and exhibited a larger dark current. Further, when CuO nanoparticles were formed directly onto the Cu₃VO₄ films from aqueous Cu(NO₃)₂ solutions (i.e., 0.1 M and 0.25 M), cathodic photocurrents of ∼0.2 mA cm⁻² are similarly found. Mott–Schottky measurements determined the energetic potential of the Cu₃VO₄ conduction band to be at ∼−0.63 V versus RHE at pH = 5.8, with an acceptor concentration of ∼1.29 × 10¹⁷ cm⁻³. Thus, a type-II band offset is predicted to occur between the p-type Cu₃VO₄ film and the p-type CuO surface nanoparticles, and elucidating the critical role of the CuO surface nanoparticles in forming a charge rectification barrier and enhancing the charge separation at the surfaces. Electronic structure calculations show that the conduction band states of Cu₃VO₄ are delocalized within the ab-plane of the structure and exhibit an ∼2 eV band dispersion centered around the gamma k-point. The bandgap size, conduction band dispersion, and band energies of Cu₃VO₄ are thus found to be promising for further investigations into tandem n-/p-type photoelectrochemical cells for solar energy conversion applications.