Methods for the photoreduction of molecules are provided. The methods use diamond having a negative electron affinity as a photocatalyst, taking advantage of its ability to act as a solid-state electron emitter that is capable of inducing reductions without the need for reactants to adsorb onto its surface. The methods comprise illuminating a fluid sample comprising the molecules to be reduced and hydrogen surface-terminated diamond having a negative electron affinity with light comprising a wavelength that induces the emission of electrons from the diamond directly into the fluid sample. The emitted electrons induce the reduction of the molecules to form a reduction product.