Fermi-level pinning and internal electric field engineering in S-scheme titanium dioxide/bismuth molybdate for fast mineralization of gaseous formaldehyde

Abstract

In an effort to overcome the intrinsic electronic drawbacks of TiO2 photocatalysts (e.g., limited spectral response and high electron-hole recombination rates), n-n type S-scheme TiO2/Bi2MoO6 (coded TB-x, x = 1–10 mol%) are synthesized for photocatalytic degradation (PCD) of formaldehyde (FA). The optimized TB-2 catalyst achieves 100% FA (5 ppm) degradation within 7.5 min, demonstrating a high clean air delivery rate (14.1 L min−1) and an apparent quantum yield (0.25%). This translates to a superior 1.6- to 8.8-fold enhancement over pristine counterparts. The prominent activity stems from a precisely engineered interface, where the difference in Fermi levels (–0.11 V vs. NHE for Bi2MoO6 and +0.80 V vs. NHE for TiO2) induces electron transfer upon contact, creating significant band bending and a powerful internal electric field (IEF) directed from Bi2MoO6 toward TiO2. Under light, this IEF drives the S-scheme recombination of low-energy electrons and holes while preserving the high-energy charges. Both the S-scheme charge transfer mechanism and the complete FA mineralization pathway (via dioxymethylene and formate intermediates to CO2) are unequivocally validated through an integrated approach of operando DRIFTS/KPFM and DFT simulations. This work provides a fundamental blueprint for designing high-performance photocatalytic systems, delivering the first atomic-level mechanistic validation for a Bi2MoO6/TiO2 S-scheme heterojunction.