Extended Data Fig. 10: Cocatalyst deposition and photocatalytic performance.

a–c, SEM (a), AFM (b), and KPFM (c) images for EH-Cu₂O particles with photodeposition of Au (EH-Cu₂O/Au). Scale bars, 2 μm. d, CPD distribution along the line in c. The data show the local increase in the surface potential at Au sites, demonstrating an enhancement of the built-in electric field. e, f, SEM images for E-Cu₂O (e) and H-Cu₂O (f) particles photodeposited with Au. They are denoted as E-Cu₂O/Au and H-Cu₂O/Au, respectively. g, SPVM image of an H-Cu₂O/Au particle. Inset, the corresponding AFM image. h, SPV distributions across the {111} and {001} facets obtained before and after the Au deposition on H-Cu₂O particles. i, Determination of the driving force of the anisotropic charge transfer in Cu₂O photocatalytic particles by calculating the differences of SPV signals between different facets. j, Time course of photocatalytic H₂ evolution for different Cu₂O photocatalyst particles. The lines represent linear fits for determining the rates of H₂ generation. The association between anisotropic SPV signals and photocatalytic activities can be understood as follows. A photocatalytic process requires photogenerated electrons and holes at surface to drive photooxidation and photoreduction reactions simultaneously. Therefore, effective charge separation refers to creating photogenerated electrons and holes that are localized on the spatially separated surface of the photocatalyst. For cubic Cu₂O, the SPV vectors of different facets are cancelled out due to symmetry considerations and the SPV difference equals to 0, which means no driving force for effective charge separation. In this case, photogenerated electrons are distributed at surface whereas holes are confined in the bulk by the symmetric surface built-in electric field, rendering the photocatalytic reaction inactive. Facet engineering yields an inter-facet built-in electric field for effective electron–hole separation between different facets, resulting in the observed anisotropic SPV signals and a detectable photocatalytic reaction rate (E-Cu₂O). However, the SPV vectors of different facets are partially offset, only resulting in a small driving force. Nevertheless, the conjoint facet engineering and defect engineering enable effective accumulations of electrons and holes at different facets via a synergistic effect of inter-facet built-in electric field and anisotropic trapping. Consequently, the SPV vectors are aligned, leading to significant enhancements of anisotropic SPV signals and photocatalytic activity for EH–Cu₂O. A further selective cocatalyst assembly enhances both the positive SPV signals of {111} facets and negative SPV signals of {001} facets by ~50%, and therefore facilitates both photogenerated electrons and holes accumulations at surfaces, further improving the photocatalytic activity by 50% for EH-Cu₂O/Au.