This work presents a gas-phase approach for the synthesis of Cu₂O/TiO₂ 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 H₂O 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 Cu₂O clusters with average diameters in the range of 1.3-2.0 nm, narrow particle size distributions and tunable Cu₂O loadings on P25 TiO₂ nanoparticles. The photocatalytic performance of Cu₂O/TiO₂ 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 TiO₂ nanoparticles by Cu₂O nanoclusters significantly enhances the photocatalytic activity of TiO₂. This is attributed to the efficient charge transfer between Cu₂O and TiO₂ 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 Cu₂O clusters in a controllable fashion for surface engineering and modification.

We employed atomic layer deposition (ALD) to deposit ultrathin SiO₂ layers on P25 TiO₂ nanoparticles to fabricate TiO₂/SiO₂ core/shell nanostructures. The ALD process was carried out in a fluidized bed reactor working at atmospheric pressure using SiCl₄ and H₂O as precursors, enabling the deposition of SiO₂ at 100 °C with the ability to control the thickness at the sub-nanometer level. By controlling the thickness of the SiO₂ in a very narrow range, i.e., below 2 nm, the photocatalytic activity of TiO₂ can be tuned. In particular, an enhancement was obtained for the SiO₂ layers with a thickness below 1.4 nm, in which the layer with a thickness of about 0.7 nm exhibited the highest photocatalytic activity; for SiO₂ layers thicker than 1.4 nm, the photocatalytic activity was strongly suppressed. The photocatalytic activity enhancement and the degradation mechanism of RhB by the TiO₂/SiO₂ photocatalysts were investigated by combining X-ray photoelectron spectroscopy, UV–Vis absorption spectroscopy, photoluminescence spectroscopy and the aid of charge carrier and radical scavengers. Our findings have revealed an improvement of photogenerated charge separation due to the SiO₂ coating and the dominating role of hydroxyl radicals in the degradation of RhB.