Cu-Deficiency in the p-Type Semiconductor Cu_5–xTa₁₁O₃₀: Impact on Its Crystalline Structure, Surfaces, and Photoelectrochemical Properties

Abstract

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(5−2x) 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 visiblelight 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 3d 10 -orbitals as well as O 2p 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.

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode

“…Powder X-ray diffraction (PXRD) data, see the Supporting Information, show that these corresponded to growing amounts of surface oxidation of Cu(I) to Cu(II) cations and a subsequent phase segregation at the surfaces. These results are consistent with a similar surface-mediated oxidation pathway as found previously for CuNb 3 O 8 , Cu 3 VO 4 , Cu 5 Ta 11 O 30 , and Cu 2 WO 4 [4,6,16,17]; i.e., heat treatments in air of these Cu(I)-containing semiconductors result in their surface oxidation, with the formation of CuO nano-islands. The preferential growth of the nano-islands over the edges arises because of the facile diffusion path of copper cations within the layers that are aligned with the ab-plane of the structure, as investigated by electron microscopy and described previously for the related Cu 5 Ta 11 O 30 , which contains structurally similar layers of pentagonal bipyramids [17].…”

“…For example, when Cu 2 Ta 4 O 11 is heated in a vacuum, the following decomposition occurs: Cu 2 Ta 4 O 11 → Cu 2 O + 2 Ta 2 O 5 . This reaction, and other alternative decomposition pathways, have been observed in all Cu(I)-containing niobates, tantalates, and vanadates [4][5][6][15][16][17][18]. For example, Cu 2 Ta 4 O 11 is calculated to be metastable by ~0.04 eV atom −1 , and thus its decomposition reaction is thermodynamically favorable.…”

“…The latter is enhanced by mild oxidation of their surfaces in air. For example, recent studies on several Cu-containing semiconductors, including CuNb 3 O 8 [16,20], Cu 3 VO 4 [6], Cu 5 Ta 11 O 30 [17], and CuBi 2 O 4 [21] have consistently shown that their decomposition and/or oxidation in air leads to the formation of CuO nano-islands on their surfaces. This has been found, in each case, to result in higher photocurrents, because of the increased charge separation efficiency arising from the type-II band alignments between the underlying semiconductor and the CuO islands at their surfaces.…”

Flux-mediated syntheses, structural characterization and low-temperature polymorphism of the p-type semiconductor Cu2Ta4O11

“…The second order Jahn-Teller distortions have typically been reported and observed for octahedral d 0 cations with high valency, with the magnitude of the distortion increasing in the following order: Figure 4B. Furthermore, the preferential directions of the [24].…”

“…In order to target the discovery of new p-type metal-oxide semiconductors, the Maggard research group has investigated new high-purity syntheses within several Cu(I)-M(V) (M = V, Nb, Ta) oxide systems using both solid state and molten-salt flux methods, such as for Cu 3 VO 4 [19], CuNb 3 O 8 [20], Cu 2 Nb 8 O 21 [17], Cu 3 Ta 7 O 19 [21], and β-Cu 2 Ta 4 O 11 [22,23]. The use of a CuCl molten salt (melting point ~426 °C) within these systems significantly shortens their reaction times down to ~15-30 min, with nanoparticle reactants rapidly growing to micron-sized single-crystal particles, e.g., as reported for Cu 2 [19,20,24]. Flux synthesis of these phases as highly-faceted single-crystal particles has been critical for elucidating the role of the surface CuO nanoparticles.…”