Small Au nanoparticles (NPs) with mean diameter of 4.1 nm were highly deposited on TiO2 films via a simple electrostatic self-assembly method. The physically separated Au NPs, with a high surface density of 6.3 x 10(11) NPs/cm(2), were mainly distributed on the top layer of porous TiO2 films. The deposition of Au NPs induced a negative shift (similar to 100 mV) of the apparent flat band potential of Au-TiO2 electrodes. The charge separation efficiency of the TiO2 electrode increased from 72.1% to 88.5% by dispersing Au NPs. Whatever redox species were present in the electrolyte, the Au-TiO2 electrode had higher photovoltage than the TiO2 electrode. The photovoltage was very sensitive to added redox species such as O-2, O-3, and methanol, and the effect of adsorbed redox species on electron accumulation was discussed. The electrochemical impedance spectroscopic measurements revealed that the charge transfer resistance (R-ct) of Au-TiO2 films was reduced to 16% of bare TiO2 electrode, and the decreased R-ct corresponded to the increased photocatalytic activity of Au-TiO2 films. The beneficial role of uniformly dispersed small Au NPs on the charge separation was discussed. By modifying TiO2 films with small Au NPs, the photocatalytic activity of TiO2 films for formaldehyde degradation increased about 2.5 times. (C) 2011 Elsevier B.V. All rights reserved.