Interfacial electron transfer at TiO2 nanostructured electrodes modified with capped gold nanoparticles: The photoelectrochemistry of water oxidation

We have studied the photoelectrochemical behavior of nanostructured anatase electrodes modified with gold nanoparticles stabilized either with citrate (Aucit) or with tetraoctylammonium bromide (AuTOAB). An increase both in photocurrent and in photopotential was observed in 0.05 M NaOH upon modification with AuTOAB. This behavior is attributed to an efficient hole consumption at the gold nanoparticles, which implies an effective hole transport to them from the TiO2 nanostructure, probably facilitated by the capping agent. Once in the Au particles, the holes would promote an oxidation process (O2 generation), which requires that the Au nanoparticles attain a high enough local electrode potential. This is possible as long as the AuTOAB nanoparticles may behave as nanocapacitors with a very low capacity (ca. 1 aF) and thus the accumulation of a few holes may induce important changes in their potential. In acidic medium or in the presence of 0.1 M CH3OH no enhancement was detected. In the case of TiO2 modified with Aucit, no increase of the photoelectrochemical response was observed in any case. It is suggested that a proper design of the metal/semiconductor nanojunctions may lead to an enhanced charge separation and to the eventual development of photoinduced metal electrocatalysis.