Facet-dependent active sites of a single Cu₂O particle photocatalyst for CO₂ reduction to methanol

Atomic-level understanding of the active sites and transformation mechanisms under realistic working conditions is a prerequisite for rational design of high-performance photocatalysts. Here, by using correlated scanning fluorescence X-ray microscopy and environmental transmission electron microscopy at atmospheric pressure, in operando, we directly observe that the (110) facet of a single Cu₂O photocatalyst particle is photocatalytically active for CO₂ reduction to methanol while the (100) facet is inert. The oxidation state of the active sites changes from Cu(i) towards Cu(ii) due to CO₂ and H₂O co-adsorption and changes back to Cu(i) after CO₂ conversion under visible light illumination. The Cu₂O photocatalyst oxidizes water as it reduces CO₂. Concomitantly, the crystal lattice expands due to CO₂ adsorption then reverts after CO₂ conversion. The internal quantum yield for unassisted wireless photocatalytic reduction of CO₂ to methanol using Cu₂O crystals is ~72%.