Preparation and Photoelectrochemical Properties of p-type Cu5Ta11O30 and Cu3Ta7O19 Semiconducting Polycrystalline Films

New p-type polycrystalline films of semiconducting Cu 5 Ta 11 O 30 and Cu 3 Ta 7 O 19 were prepared on fluorine-doped tin oxide (FTO) glass starting from their CuCl-flux synthesis as highly faceted micrometer-sized particles. The particles were annealed on FTO at 400–500 °C, followed by a mild oxidation in air at between 250 and 550 °C. In an aqueous 0.5 M Na 2 SO 4 electrolyte solution (pH = 6.3), the films exhibit strong cathodic photocurrents under irradiation by visible and/or ultraviolet light, which increased with higher annealing and oxidation temperatures owing to increased p-type carrier concentration and better electrical contact between particles. Thermogravimetric analyses show that the oxidation treatments result in an oxygen uptake at concentrations of ∼3 × 10 20 cm –3 at 250 °C, to ∼4 × 10 21 cm –3 at 550 °C, with the higher temperatures leading to the decomposition of the film. The Cu 5 Ta 11 O 30 and Cu 3 Ta 7 O 19 bulk powders exhibit band-gap sizes of ∼2.59 and ∼2.47 eV, respectively, and show an onset of their cathodic photocurrents at wavelengths of ∼500–550 nm. Mott–Schottky measurements of their flat-band potentials have been used to determine the valence band positions at approximately +1.06 and +1.19 V versus RHE (pH = 6.3), and thus conduction band positions of about −1.53 and −1.28 V for Cu 5 Ta 11 O 30 and Cu 3 Ta 7 O 19, respectively. The band positions are thus suitably located for the photon-driven reduction and oxidation of water. The highest observed incident photon-to-current efficiencies (IPCE %) for hydrogen production were ∼5% at 350 nm and ∼1–2% at 500–600 nm. Electronic structure calculations based on density functional theory methods show that the conduction band states are delocalized within layers of TaO 7 pentagonal bipyramids, whereas the valence band states originate within layers of linearly coordinated Cu(I) cations. The lowest-energy band-gap transitions involve a metal-to-metal charge transfer between Cu(I) and Ta(V) cations in these two types of layers. Compared to other Cu(I) oxides, these structures possess sufficiently disperse bands for high carrier mobility within these layers, and thus the strong cathodic photocurrents of the films.