Facet Engineering of CdS/Bi2S3 Heterojunction Photocatalysts for High-Rate, Ultraselective CH4 Production from Acetic Acid

Abstract

Photocatalytic upcycling of organic molecules to methane is challenging due to the requisite multi-electron transfers and competing formation of CO and H2. Here, we demonstrate the tunable production of CO or CH4 by the photocatalytic reforming of acetic acid over Z-scheme Bi2S3/CdS semiconductors. Electron transport and the adsorption of reactively-formed *CO over (101) facets of the CdS component favours a mixture of CH4 (1741 µmol·gcat−1·h−1) and CO (1659 µmol·gcat−1·h−1), whereas the (100) facet promotes 99% selectivity to CH4 (3024 µmol·gcat−1·h−1) outperforming state-of-the-art photocatalysts for CO2 reduction. In situ spectroscopy and quantum chemical calculations reveal electron delocalisation across (101) Cd-sites weakens *CO adsorption, while a decrease in the energy of the d-band centre and charge localisation at (100) Cd-sites strengthens *CO adsorption and lowers the energy barrier to its hydrogenation. Photoexcited holes at Bi-sites in Bi2S3 promote C-C cleavage of acetic acid to *CH3 and *CO2− intermediates, with the latter undergoing reduction to *CO over CdS. Shallow trap states in (100) facets promote migration of photoexcited electrons to surface intermediates with concomitant proton-coupled electron transfer exclusively forming CH4. Deep trap states in (101) facets favour *CO desorption. Facet engineering of Z-scheme heterojunction photocatalysts offers facile control of product selectivity.