Photocatalytic CO2 reduction is the conversion of carbon dioxide into fuels or chemical feedstocks at the surface of an illuminated semiconductor, where absorbed photons generate the electron-hole pairs that drive the reduction and the accompanying oxidation. The reaction is difficult because the first electron transfer to a linear, thermodynamically stable molecule is uphill and competes with hydrogen evolution, so the product distribution is set by the surface chemistry as much as by the band positions, and the mechanistic picture, including which adsorbed intermediate forms first, is still discussed ^[1]. Copper-containing semiconductors and copper-loaded photocatalysts have been examined most often, because copper binds the reduction intermediates in a way that favours products beyond carbon monoxide ^[2]. Noble metals collect electrons efficiently but are expensive, and non-noble substitutes are being developed to replace them ^[3]. Coupling a semiconductor to a conductive carbon phase improves charge separation and has steered the reaction selectively toward methane under visible light ^[4]. Integrating the photocatalytic step with a second conversion stage, so that the chemical output becomes electrical power in one device, is an alternative direction ^[5]. Reported rates and selectivities vary widely between laboratories, partly because of the difficulty of excluding carbon contamination.