Fe(III)-doped TiO 2 nanoparticles have been prepared in three different ways using inorganic or organic precursors, respectively. Transmission electron microscopic (TEM) structure characterization of the obtained particles reveals that TiO 2 particles with and without the Fe(III) dopant possess the crystal structure of anatase. Cryo-TEM of vitrified samples confirms earlier assumptions that TiO 2 nanoparticles tend to form threedimensional networks in solution. A novel energy transfer mechanism is suggested employing these threedimensional networks as antenna systems which lead to improved photocatalytic activities of the colloidal preparations. The performance of the nanostructured TiO 2 with and without Fe-dopant for the photocatalytic oxidation of methanol producing HCHO is strongly dependent on the preparations and the Fe-content. Fe(III)doped TiO 2 nanoparticles prepared from organic precursors by the novel method (¡2.5 atom% Fe) exhibit a greatly enhanced photocatalytic activity (quantum yield of HCHO up to ca. 15%).

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Colloidal TiO2 (2.4 nm average particle diameter) has been prepared and modified by photodeposition of Pt (PtTi-S1) and by mixing with Pt nanoparticles (PtTi-S2). Transmission electron microscopy reveals particle aggregation in colloidal TiO2 and large networks of agglomerated particles in the platinized samples. The photocatalytic activity of the samples (0.1 g L-1) has been investigated by measuring the quantum yield, φ HCHO, of HCHO formed from aqueous methanol at pH 3.5 under different conditions. In CW photolysis of the oxygenated suspensions (300−400 nm UV light, 8 × 10-7 einstein L-1 s-1 photon absorption rate) the platinized photocatalysts (1 wt % Pt) enhance φ HCHO by a factor of 1.5−1.7 with respect to neat colloidal TiO2 where φ HCHO is 0.02. In addition to the action of Pt as an electron sink, the strong promotion of photocatalytic methanol oxidation by PtTi-S2 at the small Pt/TiO2 particle ratio of ca. 1:1060 is proposed to arise from transfer of excitation energy or of photogenerated charge carriers through the particle network. Repetitive laser-pulse illumination of the oxygenated samples (351 nm, 0.5 Hz repetition frequency) increases φ HCHO by ca. 50% in comparison with CW illumination at the same average photon absorption rate of ca. 8 × 10-7 einstein L-1 s-1. As a tentative explanation an increase of the photocatalyst surface by laser-pulse stimulated deaggregation of colloidal TiO2 and fragmentation of the networks in the platinized samples is suggested. HCHO is also produced in the deoxygenated suspensions under repetitive laser-pulse illumination. PtTi-S1 and PtTi-S2 increase φ HCHO by factors of 1.8 and 1.2, respectively, in comparison with neat colloidal TiO2 where φ HCHO is 0.027. Possible mechanisms are discussed. The higher activity of PtTi-S1 is attributed to stronger electrocatalysis of H2 formation by highly dispersed Pt in PtTi-S1 as compared with the Pt particles in PtTi-S2.