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 H. Here, we demonstrate the tunable production of CO or CH by the photocatalytic reforming of acetic acid over Z-scheme BiS/CdS semiconductors. Electron transport and the adsorption of reactively-formed CO over (101) facets of the CdS component favours a mixture of CH (1741 µmol·g ·h) and CO (1659 µmol·g ·h), whereas the (100) facet promotes 99% selectivity to CH (3024 µmol·g ·h) outperforming state-of-the-art photocatalysts for CO 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 BiS promote C-C cleavage of acetic acid to CH and CO 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 CH. Deep trap states in (101) facets favour CO desorption. Facet engineering of Z-scheme heterojunction photocatalysts offers facile control of product selectivity.