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

Investigations into new p-type metal oxides with small bandgap sizes, i.e., E-g similar to 0.9 eV to similar 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 I (4) over bar 2m (no. 121), Z = 2, a = 4.581(4) angstrom, c = 8.998(2) angstrom), was synthesized in high purity using solid-state methods and investigated for its small optical bandgap size (E-g similar to 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 degrees C in air, a Cu-deficient composition is formed according to: Cu3VO4(s) + (x/2) O-2(g) -> Cu3-xVO4(s) + x CuO(s). At 350 degrees 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., similar to 15-25 nm in width by similar to 1-5 mu m in length) from the particles' surfaces. Polycrystalline films of p-type Cu3VO4 were prepared under these conditions (i.e., heated in air at 300 degrees C or 350 degrees C) and found to yield significant cathodic photocurrents under solar-simulated, visible-light irradiation (lambda > 420 nm; AM 1.5 G filter, irradiant power density of similar to 100 mW cm(-2)). Their photocurrents increased with the heating temperature of the film and with the applied bias, e.g., similar to 0.1 mA cm(-2) at zero applied bias to similar 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 similar to 0.2 mA cm(-2) are similarly found. Mott-Schottky measurements determined the energetic potential of the Cu3VO4 conduction band to be at similar to-0.63 V versus RHE at pH = 5.8, with an acceptor concentration of similar to 1.29 x 10(17) 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 similar to 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.