Atomic layer deposition of SnO2 and TiO2 on electrodeposited BiOI thin films for efficient light-driven peroxymonosulfate activation

Abstract

Light-driven peroxymonosulfate (PMS) activation is gaining traction as a green advanced oxidation strategy for degrading recalcitrant water pollutants; however, catalyst instability and sluggish charge separation still hinder its practical application. Here, we report for the first time the fabrication of ALD-engineered BiOI thin-film heterojunctions, coated with nanometric SnO2 or TiO2 layers (∼5 nm) and decorated with Pd nanoparticles (∼2 nm), which simultaneously enhance catalytic activity and stability. The BiOI/SnO2 and BiOI/TiO2 systems exhibit well-defined type-II band alignments, facilitating efficient interfacial charge transfer, while Pd nanoparticles form Schottky junctions that extract photogenerated electrons and mitigate BiOI photocorrosion. Using 20 ppm tetracycline (TC) at pH 7 as a model contaminant, TiO2-BiOI achieved 92.7 % TC removal and 84.8 % total organic carbon (TOC) mineralization within 90 min under UV-A light (365 nm) with 2.5 mM PMS. In contrast, SnO2-BiOI showed superior performance under simulated sunlight (λ > 400 nm), attaining 80.8 % degradation and 76.5 % mineralization. Radical scavenging assays revealed a threefold increase in sulfate and hydroxyl radical production compared to pristine BiOI. Pd modification reduced Bi and I leaching by more than 80 % after 360 min of continuous irradiation and preserved over 95 % of the photocatalytic activity across ten successive reuse cycles. This work establishes a modular ALD-based strategy to design stable semiconductor/oxide/metal nanointerfaces for wavelength-tunable PMS activation. The resulting thin-film catalysts, fabricated on FTO substrates with sub-nanometer precision, offer a scalable platform for solar-driven water purification and expand the material design space for sulfate-radical-based advanced oxidation processes.