Gold Cluster Coverage Effect on H2 Production over Rutile TiO2(110)

Unlike catalytic reactions thermally driven by metal nanoparticles, reaction rates in photocatalysis do not scale with either the density of nanoparticles or their size. Because of the complexity of multicomponent photo catalysts in powder form, this lack of correlation, routinely observed for decades, is still not understood. In order to explore this phenomenon, H-2 production from ethanol over Au clusters with different coverages deposited on single-crystal rutile TiO2(110) were studied by scanning tunneling microscopy and online mass spectrometry. There is a nonlinear increase of the H-2 production with increasing gold coverage. The key determining factor appears to be the Au intercluster distance. Increasing this distance resulted in an increase in the normalized production. These results are explained in terms of competition between clusters for excited electrons to reduce H+ (of surface OH groups) to H-2. It was possible to determine the proportionality factor between the hydrogen production and the number of absorbed photons. A slope close to 1 is found, which is in line with the current doubling effect in electrocatalysis. Moreover, pump probe transient absorption spectroscopy measurements were conducted. The results show that excited electrons transfer from the conduction band of TiO2 to Au particles within the first picoseconds after UV excitation. The fact that Au metal intercluster distances directly affect the reaction rate indicates that there is an optimum arrangement between the metal and the semiconductor that could potentially be achieved by nanostructuring.