DOI: 10.1002/chem.201102799 Kinetic and Mechanistic Investigations of the Light Induced Formation of Gold Nanoparticles on the Surface of TiO 2 Hanan H. Mohamed, [a, b] Ralf Dillert, [b] and Detlef W. Bahnemann* [b] Introduction The conversion of solar energy into environmentally friend- ly fuels most notably involving the production of molecular hydrogen from water has been achieved utilizing semicon- ductor photocatalysis. [1–6] One of the promising strategy to attain this is the adequate deposition of transition-metal clusters serving as electron-transfer catalysts on the surface of semiconductor nanoparticles. This was found to be bene- ficial for maximizing the efficiency of the photocatalytic re- actions. [7–16] Upon contact, a Schottky barrier can be formed between the semiconductor and the metal particles, leading to a rectified charge carrier transfer similar to that in cou- pled semiconductors. [17, 18] The charge equilibration between the semiconductor and the metal nanostructures drives the Fermi level close to the conduction band edge of the semi- conductor. [19] Studying the kinetics and mechanisms involved in the deposition of metal nanoclusters on nanometer-scale semi- conductors is of great importance both fundamentally and practically. The key to important catalytic properties includ- ing selectivity, [20] activity, [21] lifetime, and stability [21] depend on the catalyst size, [22] surface composition, [23] and struc- ture, [24] which in turn require greater control over the photo- catalyst synthesis. Furthermore, the deposition of metal nanoparticles improves both charge separation as well as in- terfacial charge-transfer kinetics; however, the mechanism responsible for such an improvement is yet to be fully un- derstood on the nanoscale level. One of the main reasons for this paucity of mechanistic information is the lack of ap- propriate experimental methods being able to follow these processes in real time. [25] Friedmann et al. [26] have studied the primary processes during the UV-A irradiation of the system consisting of TiO 2 nanoparticles and silver ions in the presence of poly- vinyl alcohol (PVA) using a nanosecond XeF laser as light source. They investigated the photodeposition of metallic silver clusters with more than 12 silver atoms on TiO 2 by de- tecting their absorption spectra with a maximum at 380 nm. Sahyun and Serpone [27] examined the photodeposition of Abstract: The kinetics of the formation of gold nanoparticles on the surface of pre-illuminated TiO 2 have been investi- gated using stopped-flow technique and steady state UV/Vis spectroscopy. Excess electrons were loaded on the employed nanosized titanium dioxide particles by UV-A photolysis in the presence of methanol serving as hole scavenger, stored on them in the ab- sence of oxygen and subsequently used for the reduction of Au III ions. The for- mation of gold nanoparticles with an average diameter of 5 nm was con- firmed after mixing of the TiO 2 nano- particles loaded with electrons with aqueous solution of tetrachloroaureate (HAuCl 4 ) by their surface plasmon ab- sorbance band at 530 nm, as well as by XRD and HRTEM measurements. The rate of formation of the gold nanopar- ticles was found to be a function of the concentration of the gold ions and the concentration of the stored electrons, respectively. The effect of PVA as a stabilizer of the gold nanoclusters was also studied. The observed kinetic be- havior suggests that the formation of the gold nanoparticles on the TiO 2 sur- face is an autocatalytic process com- prising of two main steps: 1) Reduction of the gold ions by the stored electrons on TiO 2 forming gold atoms that turn into gold nuclei. 2) Growth of the metal nuclei on the surface of TiO 2 forming the gold particles. Interesting- ly, at higher TiO 2 electron loading the excess electrons are subsequently trans- ferred to the deposited gold metal par- ticles resulting in “bleaching” of their surface plasmon band. This bleaching in the surface plasmon band is ex- plained by the Fermi level equilibra- tion of the Au/TiO 2 nanocomposites. Finally, the reduction of water resulting in the evolution of molecular hydrogen initiated by the excess electrons that have been transferred to the previously formed gold particles has also been ob- served. The mechanism of the underly- ing multistep electron-transfer process has been discussed in detail. Keywords: excess electrons · gold · hydrogen production · nanoparti- cles · titanium dioxide [a] Dr. H. H. Mohamed Chemistry Department, Faculty of Science Helwan University, Helwan, Cairo (Egypt) [b] Dr. H. H. Mohamed, Dr. R. Dillert, Prof. Dr. D. W. Bahnemann Institut für Technische Chemie, Leibniz Universität Hannover Callinstrasse 3, 30167 Hannover (Germany) Fax: (49) 511-762-2774 E-mail : bahnemann@iftc.uni-hannover.de  2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 4314 – 4321 4314