The contact of two semiconductors with different band structures forms the interface region, which is called the heterojunction. Since Fermi energy levels representing carrier concentrations vary for different semiconductors, carriers will transfer between two semiconductors when they contact each other. According to the transfer characteristics of photogenerated electrons and holes at the heterojunction interface, the researchers divide heterojunction into such types: type-I, type-II, p-n and Z-scheme. Although carrier transfer characteristics vary, the photocatalytic degradation of pollutants via semiconductors involves at least five steps, as shown in
Figure 4: (1) light absorption of semiconductors; (2) generation of the photogenerated carriers; (3) migration and recombination of photogenerated carriers; (4) adsorption of pollutants and desorption of products and (5) redox reaction on the semiconductor surface. In this section, we summarize the research progress of these four different types of heterojunctions.
3.1. Straddling Gap Heterojunctions
Type-I and type-II both belong to the traditional heterojunctions. Due to the different relative positions of the two semiconductor bands, the photogenerated carrier transmission routes are different, so the characteristics of the redox reaction are different [
26]. Generally, the heterojunction formed by the conduction band energy level of semiconductor II (SC II) is lower than that of semiconductor I (SC I) and the valence band is higher than that of SC I and is called a straddling gap (type-I) heterostructured photocatalyst. After light absorption, the photogenerated carriers will accumulate on SC II, and the corresponding photocatalytic redox reaction will also take place on SC II with lower redox potential. The transfer routes of conventional heterojunctions are shown in
Figure 5.
Chen et al. [
28] prepared a chromium-modified Bi
4Ti
3O
12 photocatalyst for pollutant via sol–gel hydrothermal technique. Bismuth titanate (Bi
4Ti
3O
12) is often used for photocatalytic degradation of organic pollutants owing to its excellent photocatalytic activity and stability. However, the high rate of recombination between photogenerated carriers has a negative effect on its photocatalytic activity, so it is necessary to enhance its absorption of light and ensure the separation of photogenerated carriers by doping chromium without changing the crystal structure. Chen applied a novel low-temperature sol–gel hydrothermal technique to reduce the reaction temperature to 160 °C, ensuring crystal perfection while improving the absorbance of Bi
4Ti
3O
12 in the visible region. The degradation of methyl orange (MO) in the aqueous solution by photocatalyst varies with the increase in Cr content. The addition of Cr can effectively improve the degradation ability of the photocatalyst to MO, but meanwhile leads to the accumulation of particles and reduces the surface activity of the photocatalyst. Moreover, the degradation of MO by the Cr-modified Bi
4Ti
3O
12 photocatalyst is only three times above that without Cr-modified Bi
4Ti
3O
12, so it is still challenging to satisfy the requirements of commercial application.
In 2017, Li et al. [
29] prepared the LaFe
1-xMn
xO
3/Attapulgite (ATP) nanocomposite photocatalyst, combining the sol–gel method and co-deposition method. It is applied to the photocatalytic NO reduction at low temperatures with specific selectivity. While regulating the amount of Mn deposition to form heterojunctions, they find that the photocatalyst has the highest activity when x = 0.6, and the conversion of NO is up to 85%. As shown in
Figure 6a, the NO conversion rate with different Mn-deposition is tested. It is found that both the photocatalytic activity and stability are improved with the increase in Mn-deposition. However, the NO conversion sharply decreases when x > 0.6, which indicates that excessive LaMnO
3 deposition may hinder the light absorption or heterojunction formation, thus reducing the catalytic efficiency. By calculating the bandgap of LaMnO
3 and predicting the bandgap structure of LaFe
1−xMn
xO
3, the team proposed a photosensitive catalytic mechanism for NO, as shown in
Figure 6b. The LaMnO
3/ATP nanocomposite photocatalyst exhibits remarkable room temperature catalytic performance, which broadens its application prospect. Huang et al. [
30] successfully prepared different heterojunctions in Bi
xO
yI
z/g-C
3N
4. The results show that the type-II heterojunction has higher catalytic activity and more types of pollutants for degradation. It indicates that the low separation efficiency of photogenerated hole-electron pairs will limit the development of type-I heterojunctions.
Due to the defects of the type-I heterojunctions, there is little research on type-I heterostructured photocatalysts, which are listed in
Table 2.
3.2. Staggered Gap Heterojunctions
For type-I heterojunctions with the straddling gap, the photogenerated hole–electron pairs cannot be effectively separated, which significantly limits the redox ability. The structure of type-II is different from the type-I heterojunction, and both the conduction and valance band of SC I are higher than those of SC II, as shown in
Figure 7. Therefore, under the excitation of light, the photogenerated holes migrate to the vacancy in the valence band of SC I, and the photogenerated electrons migrate to the conduction band of SC II, facilitating the spatial separation of photogenerated carriers [
15,
27]. Because of the photogenerated carriers’ transfer characteristics, the type-II heterostructured photocatalyst exhibits a more comprehensive light absorption range, a quicker transfer rate of mass and stronger photocatalytic activity.
Liu et al. [
35] prepared a heterostructured photocatalyst via a simple sol–gel method for visible light degradation of formaldehyde. They added waste zeolite to assist the adsorption of formaldehyde gas on the basis of previous studies. Sols were prepared by dispersing photocatalysts in HCl solution. Then, they combined the above sol and waste zeolite via the sol–gel method to achieve photocatalysts. As shown in the TEM images (
Figure 8a), the samples exhibit a polycrystalline structure in the selected region, suggesting that the heterostructure of the sample has an intense quantum confinement effect. The effect may be beneficial for the improvement of the photocatalytic activity. According to the simulation experiment, the efficiency of the degrading formaldehyde is related to the proportion of i10, up to 90%. Cyclic tests of formaldehyde photodegradation further investigate the durability of 90% I-WZ coatings. As shown in
Figure 8b–d, the photocatalytic activity retains over 95% after four cycles. The performance of the newly prepared g-C
3N
4-TiO
2/spent zeolite coating is significantly improved, and its reaction rate is 2.3 times that of commercial TiO
2 (P25) in formaldehyde degradation applications. This study also demonstrates the possibility of photocatalyst coatings for formaldehyde with interior lighting.
Wang et al. [
36] studied ternary heterostructured photocatalysts and prepared a ZnIn
2S
4@SiO
2@TiO
2 photocatalyst in a combination with sol–gel and solvothermal methods. The ternary heterojunction is formed by uniformly dispersing SiO
2@TiO
2 nanoparticles on a prefabricated 2D layered flower-like ZnIn
2S
4, as shown in
Figure 9a. According to the transient photocurrent response and the variation in EIS of the heterojunctions in
Figure 9b,c, it can be determined that the electron transfer rate at the semiconductor interface is enhanced, indicating that the heterojunction improves the utilization of photogenerated carriers. The photocatalytic experiments also confirm this speculation. According to the theoretical model established in the study (
Figure 9d), the heterojunction formed between SiO
2@TiO
2 and ZnIn
2S
4 is characterized by a staggered gap, which effectively decreases the recombination probability of photocatalytic carriers and enhances the degradation rate of the MB dyestuff up to 99.7%.
More research on type-II heterojunctions is briefly shown in
Table 3. Although type-II heterojunctions overcome the disadvantage of low hole–electron pair separation rate, its further application is still challenged. The redox reactions occur on the semiconductor with low reduction potential and low oxidation potential, respectively, which greatly limit the redox ability of the type-II heterostructured photocatalyst. In addition, the migration of hole to the hole-rich VB of the semiconductor and electron to the electron-rich conduction band (CB) of the semiconductor is difficult because of the electrostatic repulsion. Therefore, it is still necessary to develop a more efficient heterojunction to overcome these shortcomings, to be further applied in pollutant degradation.
3.3. p-n Heterojunctions
The emergence of p-n heterojunctions solves the above problems. The p-n heterojunction can enhance the photocatalyst efficiency by accelerating the transfer of the photogenerated carriers, which is the influence of the built-in electric field [
49]. The p-n heterojunction is composed of two independent semiconductors, namely, the p-type semiconductor with more holes and the n-type semiconductor with more electrons. When p and n-type semiconductors are in contact with each other, due to the concentration difference between electrons and holes, electrons diffuse to the p-type, meanwhile holes diffuse to the n-type until the Fermi energy level of the structure is balanced. The immovable charged particles of the two semiconductors form a region of space charge at the heterojunction, resulting in a built-in electric field owing to the interaction between positive and negative charges (from the positively charged n region to the negatively charged p region). As shown in
Figure 10, when the bandgap of the built-in electric field is smaller than the incident light energy, the semiconductor is excited to produce electrons and holes. With the built-in electric field, the n-type semiconductor attracts electrons, and the p-type semiconductor attracts holes, realizing the separation of photocatalyst carriers. Moreover, the CB and VB of the n-type semiconductor are lower than those of the p-type semiconductor, which is beneficial to the separation of photocarriers [
4,
15]. In conclusion, under the synergistic effect of the built-in electric field and band arrangement, the separation of carriers is realized to the greatest extent without decreasing the redox capacity, which integrates the advantages of the first two types [
50].
Guo et al. [
51] synthesized rGO/SnO
2 p-n heterojunction aerogels via the sol–gel method for phenol detection. The heterojunction aerogel formed by p-type graphene and n-type tin dioxide nanoparticles improves the low conductivity of metal oxide semiconductors and the low specific surface area of graphene composites, effectively improving the sensitivity of gas sensors. Meanwhile, the aerogel possesses excellent high-temperature stability. According to the detection results of phenol gas by rGO/SnO
2 heterojunction aerogels, it is found that the optimal mass ratio of SnO
2 and rGO is 7.5:1. With the above mass ratio, the sensitivity of the gas sensor hardly changes after repeated testing, showing good repeatability and stability. The gas selectivity of rGO/SnO
2 heterojunction aerogel is also tested, and the gas sensor shows better baseline stability, proving the relative selectivity to phenol.
Wei et al. [
52] developed a core-shell TiO
2@LaFeO
3 (TLFO) heterojunction nanosphere photocatalyst via the carbon-sphere-templated and sol–gel method and employed peroxomonosulfate (PMS) to initiate the reaction for atrazine removal. Compared with the LFO heterojunction synthesized by predecessors, the core-shell structure improves mass transfer and charge separation efficiency by expanding the surface area and improving the light efficiency at the same mass. The synthesis process of the TLFO hollow core-shell structure is shown in
Figure 11a. The TLFO nanospheres are obtained with the deposition of LFO. TEM images of TLFO verify the formation of the structure and the stability of the hollow structure after repeated cycling. By comparing the photocatalytic degradation performance of 2-chloro-4-methylamino-6-isopropylamine-s-triazine (ATZ) in different systems and under different conditions, as shown in
Figure 11b,c, ATZ can be completely removed by the reaction initiated by adding PMS, which confirms the synergistic effect of the Vis/PMS/TLFO system. In order to further explore the optimal degradation effect of the system on ATZ, the team changed the photocatalyst dose, PMS concentration, initial pH value, coexisting ions and other parameters to observe the change in degradation efficiency. They propose a hypothesis of a photocatalytic mechanism and verify it by EPR spectroscopy. The incident light causes the system to produce a variety of free radicals, which play a crucial role in ATZ photodegradation as ROS scavengers. Combined with the Mott-Schottky measurement, the band structure diagram of the TLFO and ROS generation process in Vis/TLFO/PMS is given in
Figure 11d.
In order to accelerate the separation of BiOBr carriers, Qu et al. [
53] prepared a polyimide aerogel/BiOBr p-n type heterojunction via the water bath method. The polyimide aerogel not only retains the advantages of polyimide’s adjustable band gap and high stability but also possesses stronger adsorption capacity due to its larger specific surface area and abundant pore structure, which can better capture pollutants. The p-n heterojunction formed between polyimide and BiOBr can effectively inhibit the recombination of photogenerated carriers and significantly improve the degradation efficiency of organic contaminants. To further improve the interfacial properties of p-n heterojunctions, a colloidal quantum dot treated with a low-temperature solution is developed [
54]. Moreover, a conjugated polyelectrolyte polymer film is added between PbS colloidal quantum dots (CQD) and ZnO layers to enhance the built-in electric field and charge selectivity of the heterojunction, which has the potential to improve the photocatalyst performance. In recent years, increasing research has been conducted on p-n heterostructured photocatalysts in degrading pollutants, and some of them are showed in
Table 4.
3.4. Direct Z-Scheme Heterojunctions
Although the above three types of heterojunctions effectively inhibit the photocatalyst carrying recombination, the redox ability is limited due to the low reduction and oxidation potential. Bard et al. [
63] proposed the idea of Z-scheme heterojunctions by simulating natural photosynthesis in 1979. They found that it had overwhelming advantages, including the maximization of system redox capacity, the widening of visible light absorption and the photogenerated electron–hole separation. According to electronic media, Z-scheme photocatalysts can be divided into three types: conventional Z-scheme heterojunctions, all solid Z-scheme heterojunctions and direct Z-scheme heterojunctions [
64,
65,
66]. Reversible redox ion pairs are the common medium of the conventional Z-scheme, and the ion pairs tend to exist in the form of the liquid phase, so this scheme is also called the liquid phase Z-scheme. Tada et al. firstly implemented the all-solid Z-scheme using solid materials such as precious metals or carbon-based materials as the charge transfer media. Although it possesses the advantages of light absorption range, strong redox potential, and separation of reduction and oxidation active sites, some problems exist: (1) the existence of the medium increases the charge transfer route and manufacturing cost of the system; (2) the conventional Z-scheme is limited to solution and is sensitive to pH and (3) the all solid Z-scheme means that the system exist in both liquid and gas phases, so the particle growth is difficult to control [
67]. Therefore, the direct Z-scheme without charge transfer media has attracted more and more attention. In this part, the direct Z scheme is mainly discussed.
In order to solve the problem that traditional wastewater plants could not effectively degrade ibuprofen, Ashutosh K et al. [
68] developed a type of direct Z-scheme heterojunction that can be directly recycled, which solves the inherent problems of type-II heterojunctions and made up for the recycling and complex synthesis process of g-C
3N
4/TiO
2 developed by predecessors. The team introduces Fe
3O
4@SiO
2 nanoparticles with magnetic properties and synthesizes g-C
3N
4/TiO
2/FeO@SiO
2 (gCTFS) heterojunctions via a simple sol–gel method with only a few ultra-thin g-C
3N
4 nanosheets. To remove the impurities in the synthesis process and achieve better photocatalytic efficiency, the team researched the IBU removal at different calcination temperatures, as shown in
Figure 12a. It was found that gCTFS calcined at 500 °C has the best degradation effect of IBU. Through the XRD diffraction diagram shown in
Figure 12b, it was found that gCTFS-500 has the highest crystallinity and the strongest intensity of its plane diffraction peak. This also indicates that the variation trend of photocatalytic removal efficiency is consistent with the crystallinity of the sample. High-resolution transmission electron microscopy (HRTEM) is applied to measure the gCTFS-500, as shown in
Figure 11c, which further verifies the high crystallinity of the gCTFS heterojunction. It is confirmed that the charge transfer mechanism is Z-scheme rather than type-II by •OH radical trapping tests. The gCTFS heterojunction has the advantages of high photocatalytic performance, recyclability and good stability. However, the disadvantage of low light absorption rate still exists, which may affect its practical application in environmental remediation.
Li et al. [
69] prepared a Z-scheme heterostructure of nitrogen-doped carbon quantum dots (N-CQDs)-modified PrFeO
3/palygorskite (Pal) for photocatalytic reduction in NO
X. As shown in
Figure 13a–c, increasing the doping amount of N-CQDs significantly improves the NO
X conversion rate. However, excessive N-CQDs also reduce the NO
X loading rate, and the optimal doping amount is 5 wt.%. The reason may be that excessive N-CQDs deposition on the surface of the heterojunction affects the wider absorption of the heterojunction, thus reducing the photocatalytic activity. The conversion rate of 5 wt.% N-CQDs/PrFeO
3/Pal to NO
X can still reach 89% after six cycles of testing, indicating the good stability of the catalyst. Li et al. [
70] applied a freeze-drying technology to prepare photocatalysts and synthesized a porous In
2O
3/In
2S
3 heterostructure with a three-dimensional structure, achieving the efficient photocatalytic degradation of RhB. According to the comparative analysis of the degradation process, the high photocatalytic ability of the photocatalyst is mainly attributed to: (1) the formation of heterojunctions inhibiting the electron–hole recombination; (2) the layered porous structure increasing the specific surface area and reaction sites and (3) the three-dimensional structure promoting the mass transfer. The team also tests the repeatability and degradability of the 3DHPS In
2O
3/In
2S
3 heterojunction. As shown in
Figure 13d–f, the In
2O
3/In
2S
3 heterojunction exhibits good recycling performance, making it possible to achieve practical applications.
In order to improve pollutant adsorption, Zhang et al. [
65] proposed a novel direct Z-scheme by combining 3D g-C
3N
4-ZnO with graphene aerogel. The photocatalyst is prepared by the hydrothermal method, self-assembly method and cold drying method. It realizes the efficient degradation of RhB, methyl orange and other organic dyes. In recent years, researchers have no longer been satisfied with the development of a single Z-scheme heterojunction, and new double Z-scheme photocatalytic systems are gradually being designed and prepared. Yao et al. [
71] paid more attention to the design of a novel double-Z-scheme and prepared the black phosphorus quantum dots (BPQDs)/g-C
3N
4/BiFeO
3 photocatalyst by a sol–gel method, which significantly improves its degradation ability and stability, demonstrating a new direction to the design of Z-scheme heterojunctions. In
Table 5, we have summarized recent research on Z-scheme heterojunctions.