Time-resolved microwave conductivity (TRMC) analysis has been utilized to investigate the photogenerated charge carrier dynamics and predict the power conversion efficiency of organic photovoltaics. This technique allows facile evaluation of the product of the quantum yield (φ) and the sum of charge carrier mobilities Σμ (= μ₊ + μ_–) without the need for fabricating the actual device, and is therefore applicable to photocatalysts in powder form. Several groups have reported the TRMC evaluation of oxide semiconductors, mainly focusing on TiO₂ nanoparticles. However, the relationship between the TRMC photoconductivity (φΣμ) and the photocatalytic performance has not yet been fully understood. In this study, we explore the relationship between the photocatalytic activity of an oxyhalide Bi₄TaO₈Cl with the kinetic parameters derived from the analysis of TRMC transients. This photocatalyst was synthesized via a solid-state reaction (SSR) at different calcination temperatures (700–900 °C) to comprehensively examine the crystallinity and stoichiometry (Cl/Bi ratio). Figure 1a shows the φΣμ transients of Bi₄TaO₈Cl powders measured by TRMC analysis. The φΣμ maxima (φΣμ_max) and half lifetimes (τ_1/2) of the TRMC transients increase and decrease, respectively, with the increasing calcination temperature (Figure 1b). The increased φΣμ_max is reasonably attributed to the improved crystallinity, while the decreased τ_1/2 is presumably ascribed to increased charge carrier trapping and/or charge recombination due to the increased halogen defects (Figure 1c). Finally, we calculated the product of φΣμ_max and τ_1/2 (Figure 1d). A good correlation between φΣμ_max × τ_1/2 and the photocatalytic activity of Bi₄TaO₈Cl was found for the first time. Our work opens up the application of TRMC analysis to enable not only the speedy optimization of photocatalysts but also the efficient screening of other potential semiconducting photocatalysts.

Figure 1 (a) TRMC transients of Bi<sub>4</sub>TaO<sub>8</sub>Cl samples calcined at different temperatures. (b) TRMC transient maxima (<i>φ</i>Σ<i>μ</i><sub>max</sub>) and half-lifetimes (<i>τ</i><sub>1/2</sub>). (c) Crystallite size and Cl/Bi atomic ratio of the samples. The sizes were estimated using the Scherrer equation from the XRD patterns. The Cl/Bi atomic ratios were determined from XPS analysis. (d) <i>φ</i>Σ<i>μ</i><sub>max</sub> × <i>τ</i><sub>1/2</sub> (solid bars) and the O<sub>2</sub> evolution rate over the samples in aqueous AgNO<sub>3</sub> solution (striped bars).

Figure 1 (a) TRMC transients of Bi₄TaO₈Cl samples calcined at different temperatures. (b) TRMC transient maxima (φΣμ_max) and half-lifetimes (τ_1/2). (c) Crystallite size and Cl/Bi atomic ratio of the samples. The sizes were estimated using the Scherrer equation from the XRD patterns. The Cl/Bi atomic ratios were determined from XPS analysis. (d) φΣμ_max × τ_1/2 (solid bars) and the O₂ evolution rate over the samples in aqueous AgNO₃ solution (striped bars).

Ryu Abe

Graduate School of Engineering, Kyoto University

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