Figure shows high contrast for specific sites that is likely to be in agreement with the presence of Cu as atomic sites randomly distributed over the C3N4 support. It is however to be noted that distinguishing single-atomic Cu sites from few-atomic Cu sites is difficult due to the difference in the contrast generated from the support (varying thickness). We have carefully chosen only thin edges of the supported catalyst where the contrast of Cu sites could be distinguished well from that of the supports. A similar analysis was repeated for the photocatalyst after the reaction. To our delight, the HAADF-STEM image (Figure ) again revealed a similar contrast, indicating no differences for Cu on the support. The corresponding EDX elemental maps (Figure ) furthermore confirmed the presence of Cu on the support. The EDX spectra used for quantification are shown in Supplementary Figure .