Atomic layer deposition of ZnO on NH2-MIL-125(Ti) for enhanced selective CO2 to CH4 conversion via S-scheme heterojunction engineering

NH 2 -MIL-125(Ti) (NM) is a prototypical metal–organic framework (MOF) photocatalyst with visible-light responsiveness. However, its practical application in CO 2 reduction is significantly limited by fast charge carrier recombination and poor product selectivity. Constructing heterojunctions with appropriately matched semiconductors offers an effective strategy to overcome these limitations, where coupling material selection and precise interfacial engineering are key. ZnO, a stable n-type wide-bandgap semiconductor, not only possesses excellent electron mobility but also exhibits suitable conduction band alignment with NM, enabling favorable interfacial charge transfer. Moreover, ZnO can form an interfacial Ti–O–Zn bonding bridge with NM, facilitating directional electron transport and suppressing charge recombination. In this work, we employed atomic layer deposition (ALD) to construct uniform ZnO shells on NM with sub-nanometer precision, thereby finely tuning the ZnO loading and interface structure. A series of ZnO@NH 2 -MIL-125 (ZnO@NM) composites are obtained by controlling the ZnO loading, with the optimal sample showing the highest activity, yielding 13.30 μmol h −1  g −1 of CO and 5.47 μmol h −1  g −1 of CH 4 . Notably, CH 4 selectivity is significantly improved from 16 % to 29 %, and further increases to 31 % under gas-solid conditions. In situ Fourier Transform Infrared (FTIR) spectroscopy and Density Functional Theory (DFT) calculations reveal that ZnO not only promotes directional interfacial electron transfer but also facilitates the hydrogenation of *CO to *CHO, thus enhancing CH 4 selectivity. This study demonstrates an ALD-enabled interface engineering strategy to construct MOF-semiconductor heterojunctions, offering mechanistic insights and design principles for advancing efficient and selective photocatalytic systems for carbon dioxide reduction.