The promotional effects on photocatalytic hydrogen production of Cu_xO clusters deposited using atomic layer deposition (ALD) on P25 TiO₂ are presented. The structural and surface chemistry study of Cu_xO/TiO₂ samples, along with first principles density functional theory simulations, reveal the strong interaction of ALD deposited Cu_xO with TiO₂, leading to the stabilization of Cu_xO clusters on the surface; it also demonstrated substantial reduction of Ti⁴⁺ to Ti³⁺ on the surface of Cu_xO/TiO₂ samples after Cu_xO ALD. The Cu_xO/TiO₂ photocatalysts showed remarkable improvement in hydrogen productivity, with 11 times greater hydrogen production for the optimum sample compared to unmodified P25. With the combination of the hydrogen production data and characterization of Cu_xO/TiO₂ photocatalysts, we inferred that ALD deposited Cu_xO clusters have a dual promotional effect: increased charge carrier separation and improved light absorption, consistent with known copper promoted TiO₂ photocatalysts and generation of a substantial amount of surface Ti³⁺ which results in self-doping of TiO₂ and improves its photo-activity for hydrogen production. The obtained data were also employed to modify the previously proposed expanding photocatalytic area and overlap model to describe the effect of cocatalyst size and weight loading on photocatalyst activity. Comparing the trend of surface Ti³⁺ content increase and the photocatalytically promoted area, calculated with our model, suggests that the depletion zone formed around the heterojunction of Cu_xO-TiO₂ is the main active area for hydrogen production, and the hydrogen productivity of the photocatalyst depends on the surface coverage by this active area. However, the overlap of these areas suppresses the activity of the photocatalyst.

The photocatalytic mechanism of TiO₂ (P25) nanoparticles coated with SiO₂ (SiO₂:TiO₂) by atomic layer deposition was investigated. The deposition of SiO₂ on TiO₂ not only gives a photocatalytic improvement for the degradation of both Rhodamine B (3.6–fold) and Acid Blue 9 (3–fold). SiO₂ deposition also changes the mechanism from direct oxidation of the pollutant at the surface of TiO₂ to a predominantly OH^⁎ radical based degradation of the pollutants originating from SiO₂:TiO₂. Low SiO₂ loadings on TiO₂, where the coating is incomplete, improve the OH^⁎ radicals generation due to the higher number of acidic Si–OH groups combined with the facilitated charge separation at the TiO₂–SiO₂ interface. As a consequence of incomplete coverage, the TiO₂ surface remains accessible, which allows both the oxidation and reduction reactions at the SiO₂:TiO₂ surface. On the other hand, high loading of SiO₂ (>3 wt.% Si) results in photocatalytic suppression due to the coverage of TiO₂ surface by SiO₂. The degradation of differently charged dyes on the SiO₂:TiO₂ surface demonstrates the independence of the adsorption properties on the photocatalytic improvement. Simultaneous degradation of two dyes demonstrated the advantage of SiO₂:TiO₂ being less selective and, therefore, better suited for general water purification.

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.

Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO₂:TiO₂ (P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO₂:TiO₂, and Pt:TiO₂ ) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO₂ layers on TiO₂ nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si–OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO₂ (P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO₂:TiO₂ (P25) nanopowder exceeded the dye degradation performance of both the individual SiO₂:TiO₂ (P25) (1.5 fold) and Pt:TiO₂ (P25) (4-fold) catalysts by 6-fold as compared to TiO₂ (P25). This approach thus shows that by understanding the individual materials’ behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.

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.