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.