*4.2. New Generation of Heterojunctions*

Although the conventional type-II heterojunctions are capable of spatially separating the photo-generated electron–hole pairs, there remain several critical limitations, such as the relatively weak redox ability due to the lower reduction and oxidation potentials, and the suppressed migration of electrons and holes due to the electrostatic repulsion [59]. Recently, in order to overcome the abovementioned limitations, a new generation of heterojunctions have been developed, including the p-n heterojunctions, the surface heterojunctions, the Z-scheme heterojunctions, and the semiconductor/carbon heterojunctions. Here we will give a brief introduction of each kind of these newly developed heterojunctions.

#### 4.2.1. p–n Heterojunctions

The p-n heterojunctions could be obtained by incorporating a p-type semiconductor with an n-type semiconductor, and it has been proved that the formation of p-n heterojunctions are effective for improving the photocatalytic performance of composite catalysts [65,66]. In general, before the irradiation of light, there is an internal electric field in the region closed to the p-n interface due to the electron–hole diffusion tendency of the composite semiconductors system with unequal Fermi levels [59,67]. Alternatively, when the composite semiconductors are irradiated by a light, and the energy state of the photon is beyond the band gaps of both p-type and n-type semiconductors, electron–hole pairs will be generated in the corresponding semiconductors. However, due to the presence of an internal electric field, the photo-generated electrons and holes will transfer to the CB of the n-type semiconductor and the VB of p-type semiconductor, respectively. Furthermore, it has been proved that this spatial separation of the photo-generated electron–hole pairs is much more efficient compared with that of conventional type-II heterojunction because of the synergy of the internal electric field and band alignment [59,68]. As a result, a variety of composite semiconductors with the p-n heterojunctions have been created for the application of photocatalysis. For example, Wen et al. [69] reported the fabrication of a BiOI/CeO2 p-n junction using a facile in situ chemical bath method. The result demonstrated that the BiOI/CeO2 composite with a mole ratio of 1:1 exhibited a superior photocatalytic performance for the decomposition of bisphenol A (BPA) and methylene orange under visible light irradiation. Most recently, as shown in Figure 11, our group reported a facile method for the preparation of SnS2/MoO3 hollow nanotubes based on the hydrothermal method [70]. The obtained SnS2/MoO3 hollow nanotubes exhibit a typical p-n heterojunction structure, and a synergistic effect between MoO3 and SnS2 was proven to yield an optimal hydrogen peroxide production performance.

**Figure 11.** Schematic illustration of the SnS2/MoO3 hollow nanotubes and its photocatalysis mechanism with a two-channel pathway. Adapted with permission from Reference [70]. Copyright (2018) Royal Society of Chemistry.

#### 4.2.2. Surface Heterojunctions

As reported before, a surface heterojunction can be created between two crystal facets of a single semiconductor [59,71]. For example, Yu et al. [72] proved that the formation of a heterojunction between the (001) and (101) facets in TiO2 contribute significantly toward the enhancement of photocatalytic activity. This method enables the construction of a heterojunction on the surface of a single semiconductor, which is less costly because only one semiconductor is used. They also demonstrate that there is an optimal ratio for the (001) and (101) facets in the anatase TiO2 for the improvement of its photocatalysis performance. Subsequently, Gao et al. [73] found that the surface heterojunction of TiO2 could be self-adjusted, and its photocatalytic activity could be significantly improved via combining a proper surface heterojunction with the Schottky junction. Apart from the TiO2, Bi-based semiconductors could also be employed for the design of photocatalysts with surface heterojunctions. Most recently, as shown in Figure 12, Lu et al. [74] synthesized a tetragonal BiOI photocatalyst by regulating the amount of water in the hydrolysis process at room temperature. The as-prepared photocatalyst possessed a typical surface heterojunction structure between (001) facets and (110) facets, and exhibited a promoted photocatalytic performance for the degradation of organic contaminants in water under visible light.

**Figure 12.** (**a**,**b**) Schematic illustrating the growth of TiO2 nanosheets at different conditions. (**c**) UV-vis images of the related samples. (**d**) Photocatalytic degradation efficiency of different catalysts for methyl orange. (**e**) Schematic demonstrating the migration of electrons and holes in the surface heterojunction. Adapted with permission from Reference [74]. Copyright (2018) Elsevier.

#### 4.2.3. Z-Scheme Heterojunctions

Z-scheme heterojunctions were constructed to overcome the limitation of the lower redox potential of the heterojunction systems. [59,75] In general, the Z-scheme heterojunction is composed of two different semiconductors and an electron acceptor/donor pair. During the photocatalysis process, the photo-generated electrons/holes will transfer from the matrix semiconductor to the coupled semiconductor through the electron acceptor/donor pair or an electron mediator. As a result, the electrons/holes will accumulate on different semiconductors with higher redox potentiasl, and an effective spatial separation of electron–hole pairs is also realized. Up to now, the Z-scheme heterojunctions have been well developed, and various photocatalysts with well-designed Z-scheme heterojunctions have been invented for the wastewater treatment. [75] For example, Wu et al. [76] reported the fabrication of the Ag2CO3/Ag/AgNCO composite photocatalyst via a simple in situ ion exchange method. The obtained composite photocatalyst possessed the Z-scheme heterojunction and exhibited a highly efficient degradation ratio of rhodamine B and the reduction of Cr (VI) under the driving of visible light. They proved that the significantly enhanced photocatalytic activity could be attributed to the low resistance for the interfacial charge transfer and desirable absorption capability. Recently, considering the relative high cost of the common used electron mediators (e.g., Pt, Ag, and Au), a new generation of Z-heterojunctions without the electron mediators have been invented for

wastewater treatment, which is named as the direct Z-scheme system [59]. For example, Lu et al. [77] synthesized a CuInS2/Bi2WO6 composite catalyst with a direct Z-scheme heterojunction via the in situ hydrothermal growth of Bi2WO6 on the surface of CuInS2 networks. The obtained composite photocatalysts with an optimal Z-scheme exhibited a superior visible light degradation performance of the tetracycline hydrochloride in water than that of the pristine CuInS2 and Bi2WO6. The improved photocatalytic activity was attributed to the formed intimate interface contact, which ensured a good interfacial charge transfer ability (Figure 13).

**Figure 13.** Schematic illustrating the interfacial electron transfer process and possible photocatalytic mechanism of CuInS2/Bi2WO6 with the Z-scheme heterojunction. Adapted with permission from Reference [77]. Copyright (2019) Elsevier.
