3.2.4. Atomic Layer Deposition

The atomic layer deposition (ALD) method is a recently developed and facile strategy for the element doping of semiconductors. Actually, ALD is a gas-phase deposition process based on alternate surface reactions of the substrates, and the ALD method possesses several advantages, such as good reproducibility, considerable conformality, and excellent uniformity [53]. Consequently, the ALD method is considered as a promising strategy for the preparation of doped and composite photocatalysts [54]. For example, Pore et al. [55] prepared a series of N-TiO2 films via employing the ALD processes. In this study, TiCl4 was used as the titanium precursor and there were two ALD cycles during the fabrication process: (i) a thin layer of TiN was grown from the TiCl4 and NH3; and (ii) TiO2 was deposited on the surface of TiN layer from TiCl4 and H2O, meanwhile the as-prepared TiN layer was part-oxidized to TiO2, thus resulting in the TiO2−xNx. Moreover, the nitrogen concentration of the obtained TiO2−xNx could be well controlled via changing the ratio of TiN and TiO2 deposition cycles. Similarly, Lee et al. [56] reported a facile and effective vapor-phase synthesis strategy to prepare a conformal N-TiO2 thin film based on the ALD process. As shown in Figure 7, the fabrication process of the corresponding N-TiO2 film involved four main steps: (i) pulse the TiCl4 vapor on the surfaces of a substrate to produce a monolayer of chemisorbed TiClx species; (ii) remove the remaining unreacted TiCl4 and corresponding HCl byproducts using nitrogen gas; (iii) NH4OH as the nitrogen source was subsequently pulsed to generate a mixture of gaseous H2O and NH3, which react with the as-prepared TiClx species to obtain the N-TiO2; and (iv) remove the unreacted precursors and HCl byproducts again. This cycle could be repeated to achieve the N-TiO2 film with the desired thickness. The as-prepared N-TiO2 exhibited significantly enhanced photocatalytic degradation performance for organic pollutants solely driven by the solar irradiation.

**Figure 7.** Schematic illustration demonstrating the synthesis process of the N-doped TiO2 conformal film via the ALD method. Adapted with permission from Reference [56]. Copyright (2017) Wiley.
