In situ Observation of Nanoparticle Photocharging: Gold Nanorods as Photochemical Capacitors

Light can induce charge on plasmonic nanoparticles. This idea was used in the last years to explain experi- mental results on chemical reactions on gold and silver nanoparticles. Here, we give spectroscopic evidence for this photocharging effect and present a method to examine the charge transfer from a hole scavenger to gold nanorods in situ. In contrast to the plasmon resonance in gold nanospheres, the longitudinal resonance of gold nanorods is very sensitive to the free electron density in the metal. We show here how the illumination with light blue-shifts the resonance visualizing the transfer of electrons from ethanol in the solution to the nanoparticles and model the intensity-dependence in a capacitor model. The peak can also be shifted back again after illumination when the particles are left in dark or exposed to molecules with a low reduction potential. We discuss how particle size, oxygen in the solution or the absence of ethanol influence the particle charging. With this work, we open the door to further detailed studies that optimize the exploitation of the often neglected effect of photocharging.