Facet Engineering of CdS/Bi ₂ S ₃ Heterojunction Photocatalysts for High‐Rate, Ultraselective CH ₄ Production from Acetic Acid

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·g_cat ^-1·h^-1) and CO (1659 µmol·g_cat ^-1·h^-1), whereas the (100) facet promotes 99% selectivity to CH₄ (3024 µmol·g_cat ^-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.