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.

This work presents a novel chemical vapor deposition (CVD) approach that enables the deposition of ultrathin and conformal SiO₂ layers on TiO₂ anatase nanoparticles at room temperature using SiCl₄ and air containing water without the use of a catalyst. The morphology of the CVD-grown SiO₂ layers was found to be strongly dependent on the initial surface states of the TiO₂ nanopowders, which could be altered by applying a simple heat pretreatment. The deposition on untreated TiO₂ resulted in granular films, whereas on preheated TiO₂ highly uniform and conformal SiO₂ layers were obtained. By varying the SiCl₄ precursor dosing time and the number of CVD cycles, the thickness of the SiO₂ could be controlled at the nanometer level, which allowed us to investigate the influence of film thickness on the photocatalytic suppression ability. We found that a conformal SiO₂ layer with a thickness of 3 nm could sufficiently suppress the photocatalytic activity of anatase TiO₂ nanoparticles, which was demonstrated by the photodegradation of Rhodamine B. Our approach offers a simple, fast, feasible and low-temperature deposition method which can be directly applied to SiO₂ coating on nanoparticles in pigments and other fields, particularly heat-sensitive materials, and further developed for large-scale production.