Zn-doped and La/Zn co-doped TiO2 nanoparticles were prepared by sol-gel method and utilized as the photocatalysts for the isomerization of norbornadiene to quadricyclane that has significant potential for solar energy storage and high-energy fuel synthesis. For Zn-doped samples, Zn ions do not enter the TiO2 lattice, but distribute on the particle surface in the form of ZnO crystallites. These crystallites inhibit the agglomeration, growth and anatase-to-rutile phase transformation of TiO2. The prepared particles contain considerable amount of surface-bound OHs, especially for 1% Zn/TiO2. A red shift in the optical absorption is observed due to the electron transfer between TiO2 and ZnO. In the photocatalytic isomerization reaction, Zn-doped TiO2 exhibits higher activity than homogenous sensitizer like Ethyl Michler's Keton, and 1%Zn/TiO2 produces the highest yield of quadricyclane. To further enhance the activity, 1%Zn/TiO2 was co-doped with La. La2O3 crystallites also distribute on the surface of TiO2, similar to the case of ZnO. The particle size is reduced to <7 nm but the surface-bound OHs are decreased to some degree. There is a significant blue shift in the optical absorption with a sharply increased absorbance in the UV region due to the quantum-size effect. 5%La-1%Zn/TiO2 and 3%La-1%Zn/TiO2 exhibit higher activity compared with 1%Zn/TiO2, but higher or lower content of La is detrimental to the reaction. It is concluded that doping Zn can significantly increase the surface-bound OHs, whereas doping La reduces the particle to quantum-size at the expense of surface-bound OHs. A good compromise between the two factors eventually provides a high activity. The isomerization reaction over semiconductors is proposed to proceed through an exciplex (charge-transfer) intermediate. (C) 2008 Elsevier B.V. All rights reserved.