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CO₂ photoreduction is the light-driven conversion of carbon dioxide into carbon monoxide, formate, methanol, methane or longer hydrocarbons, using a semiconductor photocatalyst that absorbs photons and delivers electrons and holes to surface sites ^[1]. The reaction competes with hydrogen evolution when water is the electron donor, and the product distribution is set by the binding energies of the adsorbed intermediates, so selectivity varies strongly between catalysts and between reaction media ^[2]. Most studies employ aqueous suspensions of TiO₂, other metal oxides, metal sulfides, graphitic carbon nitride or metal-organic frameworks, often with a metal or oxide cocatalyst that separates charge and provides the active site; rates for a laboratory photocatalyst are typically on the order of micromoles of product per gram per hour, with apparent quantum yields usually below a few percent ^[3]. Because a single photon supplies about 1.8-3.2 eV while the overall reduction of CO₂ to methane requires eight electrons and eight protons, at least the corresponding number of photons is needed per molecule, which caps the attainable solar-to-fuel efficiency ^[2]. Reaction conditions also matter: the CO₂ concentration, the amount of water vapour, the pH and the light intensity all shift both the rate and the product ratio ^[4]. Stability, and the difficulty of comparing results obtained under different illumination and reactor geometries, remain open problems, and standardised reporting of rates, quantum yields and control experiments is still an active issue ^[5].