  Graphene has recently been shown to be permeable to thermal protons, the
nuclei of hydrogen atoms, which sparked interest in its use as a
proton-conducting membrane in relevant technologies. However, the influence of
light on proton permeation remains unknown. Here we report that proton
transport through Pt-nanoparticle-decorated graphene can be enhanced strongly
by illuminating it with visible light. Using electrical measurements and mass
spectrometry, we find a photoresponsivity of 10^4 A W-1, which translates into
a gain of 10^4 protons per photon with response times in the microsecond range.
These characteristics are competitive with those of state-of-the-art
photodetectors that are based on electron transport using silicon and novel
two-dimensional materials. The photo-proton effect can be important for
graphene's envisaged use in fuel cells and hydrogen isotope separation. Our
observations can also be of interest for other applications such as
light-induced water splitting, photocatalysis and novel photodetectors.
