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

The p -type semiconductor Cu 5 Ta 11 O 30 has been investigated for the effect of Cu extrusion on its crystalline structure, surface chemistry, and photoelectrochemical properties. The Cu 5 Ta 11 O 30 phase was prepared in high purity using a CuCl-mediated flux synthesis route, followed by heating the products in air from 250 to 750 °C in order to investigate the effects of its reported film preparation conditions as a p -type photoelectrode. At 650 °C and higher temperatures, Cu 5 Ta 11 O 30 is found to decompose into CuTa 2 O 6 and Ta 2 O 5 . At lower temperatures of 250 to 550 °C, nanosized Cu II O surface islands and a Cu-deficient Cu 5– x Ta 11 O 30 crystalline structure (i.e., x ∼ 1.8(1) after 450 °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 Cu 5 Ta 11 O 30 is conserved up to ∼50% Cu vacancies but with one notably shorter Cu–O distance (by ∼0.26 Å) and concomitant changes in the Ta–O distances within the pentagonal bipyramidal TaO 7 layers (by ∼0.29 Å to ∼0.36 Å). 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 I (5–2 x ) Cu II x Ta 11 O 30 . Polycrystalline films of Cu 5– x Ta 11 O 30 were prepared under similar conditions by sintering, followed by heating in air at temperatures of 350 °C, 450 °C, and 550 °C, each for 15, 30, and 60 min. An increasing amount of copper deficiency in the Cu 5– x Ta 11 O 30 structure and Cu II O 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 ∼500 mW/cm 2 ). Similarly high p -type photocurrents are also observed for Cu 5 Ta 11 O 30 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 Cu 5– x Ta 11 O 30 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 3 d 10 -orbitals as well as O 2 p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and Cu II O surface islands on the p -type photoelectrode films for solar energy conversion systems.