N-doped graphene derived from biomass as a visible-light photocatalyst for hydrogen generation from water/methanol mixtures.

There is much current interest in developing graphene-based materials as photocatalysts, particularly in the field of solar fuels and the photocatalytic generation of hydrogen. Graphene is a versatile material allowing different modification strategies to improve its activity. Thus, in the present manuscript we report that, in contrast to the lack of photocatalytic activity of undoped graphene, nitrogen doping introduces UV- and visible-light activity for hydrogen evolution; the efficiency of the material depends on the preparation conditions. The N-doped graphene is obtained by pyrolysis under an inert atmosphere of natural chitosan, which is considered a biomass waste, followed by ultrasound exfoliation, without the need of oxidation and reconstitution. The main parameter controlling the residual amount of nitrogen and the resulting photocatalytic activity is the pyrolysis temperature that produces an optimal material when the thermal treatment is carried out at 900 °C. Due to the fact that, in contrast to graphene oxide, N-doped graphene exhibits an almost "neutral" absorption spectrum, the material exhibits photocatalytic activity upon UV- (355 nm) and visible-light (532 nm) irradiation, and is able to generate hydrogen upon simulated sunlight illumination.