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 °C in order to investigate the effects of its reported film preparation conditions as a p-type photoelectrode. At 650 °C and higher temperatures, Cu5Ta11O30 is found to decompose into CuTa2O6 and Ta2O5. At lower temperatures of 250 to 550 °C, nanosized CuIIO surface islands and a Cu-deficient Cu5–xTa11O30 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 Cu5Ta11O30 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 TaO7 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 CuI(5–2x)CuIIxTa11O30. Polycrystalline films of Cu5–xTa11O30 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 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/cm2 (radiant power density of ∼500 mW/cm2). 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 3d10-orbitals as well as O 2p orbitals. Thus, these findings help shed new light into the role of Cu-deficiency and CuIIO surface islands on the p-type photoelectrode films for solar energy conversion systems.

本研究以p型半导体Cu₅Ta₁₁O₃₀为对象，探究了铜脱除行为对其晶体结构、表面化学性质及光电化学性能的影响。本研究采用氯化铜（CuCl）介导的熔剂合成法制备了高纯度Cu₅Ta₁₁O₃₀物相，随后将产物置于空气中于250~750 ℃范围内进行焙烧，以探究其作为p型光电极的已报道薄膜制备条件的影响效应。当温度达到650 ℃及以上时，Cu₅Ta₁₁O₃₀会分解为CuTa₂O₆与Ta₂O₅。在250~550 ℃的低温区间内，通过电子显微镜表征与里德维尔（Rietveld）结构精修结果分别观测到了纳米级氧化铜(II)（CuO）表面岛状结构，以及贫铜型Cu₅₋ₓTa₁₁O₃₀晶体结构（例如，在空气中于450 ℃焙烧3 h后，x≈1.8(1)）。随着反应温度升高与反应时间延长，由于晶格内Cu(I)离子的不断脱除，该晶体的晶胞体积逐渐减小。Cu₅Ta₁₁O₃₀的原始晶体结构在铜空位浓度达到约50%时仍得以保留，但此时会出现一个显著缩短的Cu-O键长（缩短约0.26 Å），同时五角双锥型TaO₇层内的Ta-O键长也会发生相应变化，变化幅度约为0.29 Å~0.36 Å。通过X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）可观测到其表面Cu(I)被脱除并氧化为Cu(II)阳离子的过程；而磁化率数据则与晶格内Cu(I)的氧化过程相吻合，该过程可表示为Cuᴵ(5−2ₓ)CuᴵᴵₓTa₁₁O₃₀。本研究通过烧结法制备了Cu₅₋ₓTa₁₁O₃₀多晶薄膜，并将其分别置于空气中于350 ℃、450 ℃、550 ℃三个温度下焙烧，每个温度下分别保温15 min、30 min与60 min。研究发现，随着Cu₅₋ₓTa₁₁O₃₀晶格内贫铜程度与CuO表面岛状结构占比的提升，其p型可见光光电流显著增强，最高可达−2.5 mA/cm²（辐射功率密度约为500 mW/cm²）。当在Cu₅Ta₁₁O₃₀薄膜表面沉积含量逐渐增加的CuO纳米颗粒时，同样可观测到较高的p型光电流，这表明光电流增强主要源于CuO纳米颗粒的存在：其可提供适宜的能带偏移，从而驱动表面的电子-空穴分离过程。与之相反，不含CuO纳米颗粒的贫铜型Cu₅₋ₓTa₁₁O₃₀样品几乎未观测到光电流。电子结构计算结果表明，铜空位浓度的升高会使费米能级向低能方向偏移，进而导致主要的Cu 3d¹⁰轨道与O 2p轨道发生电子占据数减少的现象。综上，本研究的发现为明晰贫铜结构与CuO表面岛状结构在用于太阳能转换系统的p型光电极薄膜中的作用机制提供了新的视角。