Mechanochemically designed bismuth-based halide perovskites for efficient photocatalytic oxidation of vanillyl alcohol

Halide perovskite materials (HPMs) have been recently employed as photocatalysts in H2 generation, CO2 reduction and organic synthesis. However, the high toxicity of lead is directing research towards Pb-free halide perovskites with bismuth as the main candidate to replace Pb. This contribution discloses the synthesis of two bismuth-based halide perovskites with chemical compositions Cs2AgBiBr6 and Cs3Bi2Br9via a solvent-free mechanochemical process in a ball mill. The obtained perovskite powders were characterized via X-ray diffraction, scanning electron microscopy (SEM) and absorption and photoluminescence (PL) steady-state and time-resolved spectroscopy. Cs2AgBiBr6 was able to absorb more in the visible region (Eg = 2.12 eV) as compared to Cs3Bi2Br9 (Eg = 2.53 eV). Additionally, PL time decays were considerably longer for Cs2AgBiBr6 (τav = 740 ns) with respect to Cs3Bi2Br9 (τav = 0.3 ns). Both photo-systems were employed in the oxidation of vanillyl alcohol to vanillin, an aldehyde derivative, under UV or visible illumination. Moderate values of photocatalytic conversion (15-30%) were observed except for Cs2AgBiBr6 under visible light irradiation, where 95% conversion could be obtained after only 80 minutes of exposition. PL measurements with the fluorescent probe hydroethidine and electron spin resonance (ESR) demonstrated the formation of superoxide radical species (˙O2−) after photoexcitation, with a larger concentration observed for Cs2AgBiBr6 under visible light due to higher absorption and longer lifetime of the photogenerated charge carriers. Time-resolved PL measurements of both catalysts mixed with vanillyl alcohol powder shed light on the oxidation step upon irradiation taking place due to a hole transfer process from the valence band of the catalysts. © 2022 The Royal Society of Chemistry