This work presents a gas-phase approach for the synthesis of Cu 2 O/TiO 2 powder-based photocatalysts using atomic layer deposition (ALD). The process is carried out in a fluidized bed reactor working at atmospheric pressure using (trimethylvinylsilyl)-hexafluoroacetulacetonate copper(I) as the Cu-precursor and H 2 O vapor as the oxidizer. The saturating regime of the chemical reactions and the linear growth of ALD are achieved. In combination with the unsaturated regime, the ALD approach enables the deposition of ultrasmall Cu 2 O clusters with average diameters in the range of 1.3-2.0 nm, narrow particle size distributions and tunable Cu 2 O loadings on P25 TiO 2 nanoparticles. The photocatalytic performance of Cu 2 O/TiO 2 photocatalysts is investigated by the degradation of organic dyes, including Rhodamine B (RhB), methyl orange, and methylene blue; the results demonstrate that the surface modification of TiO 2 nanoparticles by Cu 2 O nanoclusters significantly enhances the photocatalytic activity of TiO 2 . This is attributed to the efficient charge transfer between Cu 2 O and TiO 2 that reduces the charge recombination. The photocatalytic reaction mechanism is further investigated for the degradation of RhB, revealing the dominating role of holes, which contribute to both direct hole oxidation and indirect oxidation (i.e. via the formation of hydroxyl radicals). Our approach provides a fast, scalable and efficient process to deposit ultrasmall Cu 2 O clusters in a controllable fashion for surface engineering and modification.