Cu-Deficiency in the p-Type Semiconductor Cu5-xTa11O30: Impact on Its Crystalline Structure, Surfaces, and Photoelectrochemical Properties

The p-type semiconductor Cu5Ta11O30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu5Ta11O30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 degrees C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 degrees C and higher temperatures, Cu5Ta11O30 is found to decompose into CuTa2O6 and Ta2O5. At lower temperatures of 250 to 550 degrees C, nanosized (CuO)-O-II surface islands and a Cu-deficient Cu5-xTa11O30 crystalline structure (i.e., x similar to 1.8(1) after 450 degrees C for 3 h in air) is found by electron microscopy and Rietveld structural refinement results, respectively. Its crystalline structure exhibits a decrease in the unit cell volume with increasing reaction temperature and time, owing to the increasing removal of Cu(I) ions from its structure. The parent structure of Cu5Ta11O30 is conserved up to similar to 50% Cu vacancies but with one notably shorter Cu-O distance (by similar to 0.26 angstrom) and concomitant changes in the Ta-O distances within the pentagonal bipyramidal TaO7 layers (by ?0.29 angstrom to ?0.36 angstrom). The extrusion and oxidation of Cu(I) to Cu(II) cations at its surfaces is found by X-ray photoelectron spectroscopy, while magnetic susceptibility data are consistent with the oxidation of Cu(I) within its structure, as given by (Cu(5-2x)CuxTa11O30)-Cu-I-Ta-II. Polycrystalline films of Cu5-xTa11O30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 degrees C, 450 degrees C, and 550 degrees C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu5-xTa11O30 structure and CuIIO surface islands are found to result in significant increases in its p-type visible-light photocurrent at up to -2.5 mA/cm(2) (radiant power density of similar to 500 mW/cm(2)). Similarly high p-type photocurrents are also observed for Cu5Ta11O30 films with an increasing amount of CuO nanoparticles deposited onto their surfaces, showing that the enhancement primarily arises from the presence of the CuO nanoparticles which provide a favorable band-energy offset to drive electron-hole separation at the surfaces. By contrast, negligible photocurrents are observed for Cu-deficient Cu5-xTa11O30 without the CuO nanoparticles. Electronic structure calculations show that an increase in Cu vacancies shifts the Fermi level to lower energies, resulting in the depopulation of primarily Cu 3d(10)-orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and (CuO)-O-II surface islands on the p-type photoelectrode films for solar energy conversion systems.