Charge separation manipulation in lead-free Cs3Sb2Br9/WO3 heterojunction for efficient green-light-driven oxidation of benzyl alcohol and mechanistic insights

Abstract

Herein, we report an effective strategy to suppress charge recombination in the Cs₃Sb₂Br₉ (CSB) photocatalyst by integrating it with an electron-storing semiconductor, WO₃. The resulting Cs₃Sb₂Br₉/WO₃ heterojunction (CSBW) is investigated for its charge carrier dynamics and photocatalytic activity via the selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). The CSBW heterojunction exhibits nearly three-fold enhancement in the BA conversion rate (7995 µmol g_cat⁻¹ h⁻¹) compared to the pristine CSB (2892 µmol g_cat⁻¹ h⁻¹) with high selectivity to BAD (≥95%), while WO₃ shows negligible activity. The apparent quantum yield over the CSBW photocatalyst is approximately 2.7%, compared to 0.96% for pristine CSB under the conditions studied. By selectively using green light to mainly activate the CSB component, we clearly isolated the electron-accepting ability of WO₃, uncovering the role of WO₃ in the CSBW heterojunction. Both experimental and theoretical studies reveal that electrons, transferred from the CSB component, are trapped at the reduced W⁵⁺ and W⁴⁺ sites within the WO₃, thereby verifying an electron sink function of WO₃ which effectively suppresses undesirable charge carrier recombination in the CSBW photocatalyst. Through this electron transfer and trapping mechanism presumably via Sb–O–W and Cs–O–W bonds, the photoinduced holes remaining on the CSB are efficiently isolated and participate in the selective oxidation, promoting the overall photocatalytic activity of the CSBW heterostructure. Mechanistic studies employing Hammett plot analysis, radical trapping and scavenger tests, and DFT calculations indicate that BA oxidation proceeds predominantly via a carbon-centered radical intermediate (PhCH˙OH), where photoinduced holes serve as the key active species. This work demonstrates an alternative approach to efficiently suppress the rapid electron–hole recombination which is a critical bottleneck in low-dimensional, lead-free halide perovskites, thereby advancing their utilization in visible-light-driven photocatalysis and related applications.