3.2.5. Sol-Gel and Combustion Method

Compared with the abovementioned synthesis approaches, the sol-gel and combustion method is a facile and low-cost strategy for the preparation of various semiconductors and the corresponding hybrid semiconductors. With the merits of simplicity and the possibility of controlling the synthesis conditions, the sol-gel methods have been well developed and several extended sol-gel techniques have been invented to fabricate new types of semiconductor photocatalysts. For example, Albrbar et al. [57] reported the synthesis of a series of mesoporous anatase TiO2 powders doped by N, and S, as well as the N,S co-doped anatase TiO2 powder using a non-hydrolytic sol-gel process. During the gel synthesis process, titaniumtetrachloride and titaniumisopropoxide were used as the precursor of Ti, dimethylsulfoxide (DMSO) was used as the precursor of S, and NH3 was used as the precursor of N. For the preparation of S-doped TiO2, the obtained gel derived from the solvent of DMSO was calcined in air, while N and S co-doped TiO2 was obtained when the gel was annealed in the atmosphere of NH3. In addition, the pristine TiO2 and corresponding N-doped TiO2 was further obtained via calcining the gel derived from the solvent of cyclohexane in air and NH3, respectively. In their studies, the photocatalytic activities of the samples were evaluated via the degradation of dye C.I. Reactive Orange16 in water under the irradiation of visible light. The obtained results showed that the N-doped TiO2 exhibited better visible-light photocatalytic activity compared with the pristine TiO2 and S-doped TiO2. Similarly, the sol-gel method is also versatile enough to be combined with other materials synthesis techniques. Most recently, Rajoriya et al. [58] successfully fabricated a samarium (Sm) and nitrogen (N) co-doped TiO2 photocatalyst through an ultrasound-assisted sol-gel process (Figure 8), where they found that after doping TiO2 with Sm and N, the photocatalytic degradation performance of the TiO2 for 4-acetamidophenol was greatly improved owing to the significantly improved separation efficiency of the photo-generated electron–hole pair.

**Figure 8.** Schematic illustrating the ultrasound assisted sol-gel synthesis process of the Sm/N doped TiO2. Adapted with permission from Reference [58]. Copyright (2019) Elsevier.

#### **4. Heterojunctions Construction**

Besides the abovementioned heteroatoms doping strategy, constructing heterojunctions in photocatalysts is also considered as one of the most promising approaches for improving the photocatalysis performance of semiconductors due to its feasibility and effectiveness for the spatial separation of electron–hole pairs. More specifically, the heterojunction is defined as the formed interface between two semiconductors with the unequal band structure, which can form band alignments [59,60]. In fact, there have been several types of heterojunction structures, which could be considered as the conventional heterojunction structures, and the new generation of heterojunction structures.

#### *4.1. Conventional Heterojunctions*

In general, the conventional heterojunctions can be classified as three types depending on the different band gaps of the composite semiconductors, which are type I with a straddling gap, type II with a staggered gap, and type III with a broken gap (Figure 9) [59]. As for the type I heterojunction, the VB and CB of semiconductor A are lower and higher than the corresponding VB and CB of semiconductor B, respectively. As a result, the photo-generated electrons and holes transfer to the CB and VB of semiconductor B, which is negative for the separation of electron–hole pairs. Moreover, the redox reaction of the composite semiconductors with a type I heterojunction will conduct on the surface of semiconductor B with a lower redox potential, therefore the redox ability of the whole photocatalyst may be suppressed. Meanwhile, in the composite semiconductor system with type II heterojunctions, the VB and CB of semiconductor A are higher than that of semiconductor B, thus the photo-generated electrons will migrate from the CB of semiconductor A to that of semiconductor B with a lower reduction potential, and the corresponding holes in the VB of semiconductor B will migrate to semiconductor A with a lower oxidation potential, thus a spatial separation of electron–hole pairs will be completed. However, the band gap of the two semiconductors will not overlap in the type III heterojunctions, and as a result, there is no transmission or separation of electrons and holes between semiconductor A and semiconductor B. Consequently, the type II heterojunction is the most effective structure for improving the photocatalysis performance of semiconductors, and has received a great deal of research attention.

**Figure 9.** Schematic illustrating the photocatalysis mechanism of the three different types of heterojunction photocatalysts: (**a**) type-I, (**b**) type-II, and (**c**) type-III. Adapted with permission from Reference [59]. Copyright (2017) Wiley.

Up to now, several type-II heterojunction photocatalysts have been developed by creating two different phases in the same semiconductor, or directly compositing different semiconductors together [60,61]. For example, Yu et al. [62] once created the anatase-brookite dual-phase in a TiO2 photocatalyst to form a type-II heterojunction via hydrolyzing the titanium tetraisopropoxide in water and an ethanol-H2O mixture solution. They found that the co-presence of brookite and anatase phases in the TiO2 significantly enhanced the photocatalysis performance. After that, Uddin et al. [63] successfully fabricated the mesoporous SnO2-ZnO heterojunction photocatalysts using a two-step synthesis strategy. Furthermore, they had carefully examined the band alignment, the results showed that the obtained SnO2-ZnO heterojunction photocatalyst possessed a type-II band alignment and exhibited higher photocatalytic activity for the degradation of methyl blue in water than that of the individual SnO2 and ZnO nanocatalysts (Figure 10). Apart from the inorganic semiconductors, organic semiconductors could also be incorporated with the semiconductors to form the type-II heterojunction. For example, Shirmardi et al. [64] used polyaniline (PANI) as the organic semiconductor combined with ZnSe nanoparticles via a simple and cost-effective co-precipitation method in the ambient conditions. The obtained ZnSe/PANI nanocomposites exhibited obvious enhancement in the photocatalytic performance compared to that of the pristine ZnSe nanoparticles.

**Figure 10.** (**a**) Nanostructures of SnO2−ZnO composite photocatalysts. (**b**) The corresponding photocatalytic performances of SnO2–ZnO (red line with square dots), SnO2 (green line with triangle dots), and ZnO (blue line with circle dots). Adapted with permission from Reference [63]. Copyright (2012) American Chemical Society.
