A Mini Review on Bismuth-Based Z-Scheme Photocatalysts

Recently, the bismuth-based (Bi-based) Z-scheme photocatalysts have been paid great attention due to their good solar energy utilization capacity, the high separation rate of their photogenerated hole-electron pairs, and strong redox ability. They are considerably more promising materials than single semiconductors for alleviating the energy crisis and environmental deterioration by efficiently utilizing sunlight to motivate various photocatalytic reactions for energy production and pollutant removal. In this review, the traits and recent research progress of Bi-based semiconductors and recent achievements in the synthesis methods of Bi-based direct Z-scheme heterojunction photocatalysts are explored. The recent photocatalytic applications development of Bi-based Z-scheme heterojunction photocatalysts in environmental pollutants removal and detection, water splitting, CO2 reduction, and air (NOx) purification are also described concisely. The challenges and future perspective in the studies of Bi-based Z-scheme heterojunction photocatalysts are discussed and summarized in the conclusion of this mini review.


Introduction
The rapid growth of industry and the population has resulted in the over-consumption, unreasonable exploitation and utilization of fossil fuel resources. The global energy crisis and environmental deterioration have become two primary challenges of the 21st century in human society [1,2]. Therefore, it is necessary to develop renewable energy sources that can replace fossil fuels. As a powerful and inexhaustible renewable energy source, solar energy is considered as one of the best methods to mitigate these problems. It can be exploited and utilized for energy production (for example H 2 [3][4][5][6], hydrocarbon fuel [7,8] and electric energy [9,10] production) and removal of pollutants (such as CO 2 [11][12][13][14][15], organic contaminants in water [16][17][18][19][20][21] or air [22][23][24][25][26], emerging micropollutants [27,28]) by using photocatalytic, photovoltaic and other light-conversion technologies [29,30]. As one of the most promising light conversion technologies, photocatalytic technology only needs the appropriate In recent years, tremendous attempts have been made to improve the visible-light absorption ability, light-harvesting efficiency, and photogenerated carriers' separation of Bi-based semiconductors. Rongan He, Jiaguo Yu, and Wingkei Ho et al. have reviewed the strategies for enhancing the performance of bulk Bi-based semiconductors including component adjustment, morphology control, heterojunction construction, and surface modification [57]. Although the component adjustment, morphology control, and surface modification can improve the photocatalytic performance of bulk Bi-based semiconductors with a single component to some extent, the first inherent limitation is still not overcome. However, heterojunction construction, especially Zscheme heterojunction construction, is considered a promising strategy to conquer the conflict between good solar energy utilization ability and excellent redox capacity for single-component Bibased semiconductor. From the development history of the Z-scheme heterojunction, it can be divided into three generations: liquid-phase Z-scheme, all-solid-state Z-scheme and direct Z-scheme photocatalyst [58]. Among them, the third generation Z-scheme heterojunction, direct Z-scheme photocatalyst, possesses the largest application range, the fastest charge-carrier migration rate, and the lowest fabrication cost because, unlike the first two generations with the help of electron mediator to transfer charge carriers, its charge-carrier migration driving force is the internal electric field which is formed due to the charge redistribution upon the contact of semiconductor components. The comprehensive description of the basic principle of the Z-scheme heterojunction can be found in some published review articles [59,60].
The present mini review focuses on recent achievements in a number of synthesis methods of Bi-based direct Z-scheme heterojunction photocatalysts and different photocatalytic applications of Bi-based Z-scheme heterojunction photocatalysts, such as photocatalytic degradation and photoelectrocatalytic detection of environmental pollutants, water splitting, CO2 reduction, and air (NOx) purification. Finally, the challenges, prospects, and future directions for Bi-based Z-scheme heterojunction photocatalysts are presented.

Synthesis Methods of Bi-Based Direct Z-Scheme Photocatalysts
Different synthesis methods can obtain Bi-based direct Z-scheme photocatalysts with different interfacial properties, geometrical configurations (see Figure 2), morphology, and crystallinity and so The present mini review focuses on recent achievements in a number of synthesis methods of Bi-based direct Z-scheme heterojunction photocatalysts and different photocatalytic applications of Bi-based Z-scheme heterojunction photocatalysts, such as photocatalytic degradation and photoelectrocatalytic detection of environmental pollutants, water splitting, CO 2 reduction, and air (NO x ) purification. Finally, the challenges, prospects, and future directions for Bi-based Z-scheme heterojunction photocatalysts are presented.

Synthesis Methods of Bi-Based Direct Z-Scheme Photocatalysts
Different synthesis methods can obtain Bi-based direct Z-scheme photocatalysts with different interfacial properties, geometrical configurations (see Figure 2), morphology, and crystallinity and so on, which eventually affect the photocatalytic performance of the prepared Bi-based direct Z-scheme photocatalysts. It is widely known that both morphology (size, shape and dimensionality) and crystallinity have a substantial influence on the properties of semiconductor oxides and further on the photocatalytic performance of the direct Z-scheme photocatalysts that they make up. In order to optimize the performance of semiconductor components, a variety of synthesis methods have been developed to control the morphology and crystallinity of semiconductor components by adjusting the parameters of synthesis methods. For example, Yongfa Zhu et al. [61] prepared a series of Bi 2 MoO 6 with different morphologies (from 2D nanosheet to 1D microrod structures) by adjusting the pH value of the reactant through a hydrothermal method. Similarly, uniform BiOCl hierarchical microspheres assembled by nanosheets with tunable thickness were synthesized via a simple solvothermal route [62]. Bi 2 O 3 with different hierarchitectures were reported to be controllably synthesized by modulating the experimental conditions of the template-free method, such as VO 3 − concentration, the reaction temperature, and the pH values [63]. with different morphologies (from 2D nanosheet to 1D microrod structures) by adjusting the pH value of the reactant through a hydrothermal method. Similarly, uniform BiOCl hierarchical microspheres assembled by nanosheets with tunable thickness were synthesized via a simple solvothermal route [62]. Bi2O3 with different hierarchitectures were reported to be controllably synthesized by modulating the experimental conditions of the template-free method, such as VO3 − concentration, the reaction temperature, and the pH values [63]. However, for a direct Z-scheme photocatalyst, just optimizing the properties of its semiconductor components is not enough, the interfacial condition among the semiconductor components is crucial, considering the charge-carrier migration driving force of direct Z-scheme photocatalyst is related to the contact of semiconductor components. It is noteworthy that the stronger the interaction and intimate interface among the components of the Z-scheme heterojunction, the higher the efficiency of charge carrier separation and transfer [64]. Moreover, the geometrical configuration of direct Z-scheme photocatalyst is also important because it affects the contact mode of semiconductor components in Bi-based direct Z-scheme photocatalysts. In the surface decorated structure (Figure 2a), the components of the Bi-based Z-scheme heterojunction are all exposed to the reaction environment and can be excited by light and participate the redox reactions. However, excess amount of decorated component will inhibit the light absorption of other components, which can be improved by Janus structure (Figure 2b). In a core-shell structure ( Figure  2c), the core component can be protected from light corrosion or undesired dissolution. Nevertheless, because of the protection, the core component cannot be excited by light and participate the redox reactions, which is not of benefit to the charge consumption and further charge carrier transfer between the core and shell component. Table 1 lists the advantages and shortcomings of extensively used synthesis methods of Bi-based direct Z-scheme photocatalysts including hydrothermal and However, for a direct Z-scheme photocatalyst, just optimizing the properties of its semiconductor components is not enough, the interfacial condition among the semiconductor components is crucial, considering the charge-carrier migration driving force of direct Z-scheme photocatalyst is related to the contact of semiconductor components. It is noteworthy that the stronger the interaction and intimate interface among the components of the Z-scheme heterojunction, the higher the efficiency of charge carrier separation and transfer [64]. Moreover, the geometrical configuration of direct Z-scheme photocatalyst is also important because it affects the contact mode of semiconductor components in Bi-based direct Z-scheme photocatalysts. In the surface decorated structure (Figure 2a), the components of the Bi-based Z-scheme heterojunction are all exposed to the reaction environment and can be excited by light and participate the redox reactions. However, excess amount of decorated component will inhibit the light absorption of other components, which can be improved by Janus structure (Figure 2b). In a core-shell structure (Figure 2c), the core component can be protected from light corrosion or undesired dissolution. Nevertheless, because of the protection, the core component cannot be excited by light and participate the redox reactions, which is not of benefit to the charge consumption and further charge carrier transfer between the core and shell component. Table 1 lists the advantages and shortcomings of extensively used synthesis methods of Bi-based direct Z-scheme photocatalysts including hydrothermal and solvothermal method, solid-state synthesis, deposition-precipitation method, cation exchange method, electrospinning method, self-assembly method, mechanical agitation method, and ultrasonic chemical method. The interfacial properties and geometrical configurations of the prepared Z-scheme heterojunction by these synthesis methods are also compared in Table 1 [65][66][67][68][69]. In the tables of Section 3, we summarize the representative examples of the aforementioned synthesis methods of Bi-based direct Z-scheme photocatalysts from recent related works.

Applications of Bi-Based Z-Scheme Photocatalysts
Due to the excellent redox capacity, good solar energy utilization ability, and efficient hole-electron separation ability, superior photocatalytic performance, Bi-based Z-scheme heterojunctions have been achieved in a wide range of applications, such as degradation of pollutants [70], water splitting [68], CO 2 reduction [71], detection of environmental pollutants [72], removal of NO x [73], etc. In this section, various photocatalytic applications of Bi-based Z-scheme photocatalysts are summarized and briefly presented.

Degradation of Pollutants in Water
With the rapid development of the economy, large-scale industrialization and urbanization make environmental pollution a very serious problem, which not only hinders the sustainable development of society, but also threatens the life and safety of human beings [74]. Owing to the unique structure of a Bi-based Z-scheme photocatalyst, the redox potential of the heterojunction can be maximized, the solar energy can be almost utilized completely, and the photogenerated electrons and holes can be separated efficiently. The Bi-based Z-scheme photocatalyst is reported to be a promising photocatalyst for photocatalytic degradation of various environmental pollutants. Table 2 lists some of the latest and most representative research on degradation of pollutants by Bi-based Z-scheme photocatalyst. As shown in Table 2, the photocatalytic efficiency usually was expressed by the degradation rate of pollutants (%) or the kinetic constant k (min −1 ). It has been shown that compared with corresponding single Bi-based photocatalyst, the Z-scheme heterojunction greatly improves the photocatalytic degradation efficiency of pollutants. For example, Liu et al. [75] prepared Z-scheme Bi 3 O 4 Cl/CdS by the simple hydrothermal method. Briefly, CdS nanospheres and Bi 3 O 4 Cl nanosheets were synthesized firstly by the hydrothermal method. Then, with the assist of polyvinyl pyrrolidone (PVP), Z-scheme Bi 3 O 4 Cl/CdS was synthesized by a facile surfactant-free hydrothermal treatment. The results showed that the CdS nanospheres are successfully and uniformly loaded on the surface of Bi 3 O 4 Cl nanosheets forming a surface-decorated heterostructure and an efficiently intimate heterojunction interface (see Figure 3a). The heterojunction presents an obviously enhanced absorption in the visible region compared to pure Bi 3 O 4 Cl. The significant improvement of charge transfer and separation of the composite was proved by photocurrent (see Figure 3b) and electrochemical impedance spectra (EIS) measurements. Under visible light illumination, Bi 3 O 4 Cl/CdS composite displays higher photocatalytic activity towards the ciprofloxacin (CIP) and tetracycline (TC) degradation than pure Bi 3 O 4 Cl, which is ascribed to the direct Z-scheme mechanism (See Figure 3c). The direct Z-scheme mechanism was proved by active species trapping experiments and electron spin resonance (ESR) technology. As shown in Figure 3c, the redox potential of the Z-scheme Bi 3 O 4 Cl/CdS heterojunction can be maximized sufficiently to produce OH (2.40 V vs. NHE (normal hydrogen electrode)) and O 2 − (−0.33 V vs. NHE).   Zeng et al. [83] fabricated ternary Z-scheme heterojunction (meso-tetra (4-carboxyphenyl) porphyrin (TCPP)/reduced graphene oxide (rGO)/Bi2WO6 (BWO)) via an ultrasonic chemical Zeng et al. [83] fabricated ternary Z-scheme heterojunction (meso-tetra (4-carboxyphenyl) porphyrin (TCPP)/reduced graphene oxide (rGO)/Bi 2 WO 6 (BWO)) via an ultrasonic chemical method. Firstly, rGO/BWO was prepared by the hydrothermal method. Then, rGO/BWO was added to absolute ethanol and ultrasonicated for dispersal. TCPP was added to the above suspension and kept in ultra-sonication then stirred until the solvent was completely volatilized. Finally, the product was dried to obtain the TCPP/rGO/BWO. During the synthetic process, the carboxylic groups of TCPP make it combine with the BWO tightly. The introduction of rGO further improves the photocatalytic performance of the composite because of its π-π structure for efficient contaminants adsorption, great photo-response property for expanding visible-light response range, and high electron mobility for promoting charge transfer and separation. The structure of the prepared TCPP/rGO/BWO is a surface-decorated heterostructure, which was proved by scanning electron microscopy (SEM) results. The TCPP0.25/rGO/BWO has the best photocatalytic performance for the degradation of tetracycline NHE). Therefore, a Z-scheme mechanism is proposed to elucidate the charge transfer process in the TCPP0.25/rGO/BWO system to ensure the efficient charge separation and sufficient redox potential.

Water Splitting
Solar-water splitting can convert solar energy into clean, carbon-neutral and storable chemical energy (hydrogen fuel) without using fossil fuels and causing carbon emissions. Therefore, solar-water splitting has attracted much attention [95].
The photocatalytic (PC) and photoelectrocatalytic (PEC) methods are two simple, efficient, low-cost and environmentally benign means for achieving solar-water splitting. As shown in Figure 4a, during the photocatalytic process, the photocatalysts that are highly dispersed in solution can be excited by solar light and produce electron-hole pairs which further participate in the redox reactions to split water to hydrogen (H 2 ) and oxygen (O 2 ). Unlike in the PC system where photocatalysts are dispersed in the solution, in the PEC system the photocatalysts should be attached on the working electrode to construct the PEC system and in order to apply an external bias. Although the external bias is beneficial to promote the charge separation and reaction kinetics and attached photocatalysts are easy to reuse, the specific surface area and the photocatalytic active sites of attached photocatalysts on the electrode is far less than dispersed photocatalysts in the solution. In the PEC water splitting system, at least one photoelectrode should be required for utilizing solar energy. Depending on the type of photocatalyst on the photoelectrode, PEC water splitting system has different hydrogen production principles. As displayed in Figure 4b, when the photoelectrode consisting of n-type semiconductor photocatalyst as photoanode is excited by solar energy, the photogenerated holes will oxidise water molecule to hydrogen ions and O 2 , see Equation (1). The photogenerated electrons will transfer to the counter electrode (cathode) via an external circuit to reduce the hydrogen ions to hydrogen, see Equation (2) [96]. By contrast, when the photocatalyst is p-type semiconductor and as photocathode, the H 2 and O 2 will evolve at the surface of photocathode (see Equation (2)) and anode (see Equation (1)), respectively (see Figure 4c). The two basic semi-reactions and their overall reaction (Equation (3)) are as follows: anode : Overall : Therefore, a Z-scheme mechanism is proposed to elucidate the charge transfer process in the TCPP0.25/rGO/BWO system to ensure the efficient charge separation and sufficient redox potential.

Water Splitting
Solar-water splitting can convert solar energy into clean, carbon-neutral and storable chemical energy (hydrogen fuel) without using fossil fuels and causing carbon emissions. Therefore, solarwater splitting has attracted much attention [95].
The photocatalytic (PC) and photoelectrocatalytic (PEC) methods are two simple, efficient, lowcost and environmentally benign means for achieving solar-water splitting. As shown in Figure 4a, during the photocatalytic process, the photocatalysts that are highly dispersed in solution can be excited by solar light and produce electron-hole pairs which further participate in the redox reactions to split water to hydrogen (H2) and oxygen (O2). Unlike in the PC system where photocatalysts are dispersed in the solution, in the PEC system the photocatalysts should be attached on the working electrode to construct the PEC system and in order to apply an external bias. Although the external bias is beneficial to promote the charge separation and reaction kinetics and attached photocatalysts are easy to reuse, the specific surface area and the photocatalytic active sites of attached photocatalysts on the electrode is far less than dispersed photocatalysts in the solution. In the PEC water splitting system, at least one photoelectrode should be required for utilizing solar energy. Depending on the type of photocatalyst on the photoelectrode, PEC water splitting system has different hydrogen production principles. As displayed in Figure 4b, when the photoelectrode consisting of n-type semiconductor photocatalyst as photoanode is excited by solar energy, the photogenerated holes will oxidise water molecule to hydrogen ions and O2, see Equation (1). The photogenerated electrons will transfer to the counter electrode (cathode) via an external circuit to reduce the hydrogen ions to hydrogen, see Equation (2) [96]. By contrast, when the photocatalyst is p-type semiconductor and as photocathode, the H2 and O2 will evolve at the surface of photocathode (see Equation (2)) and anode (see Equation (1)), respectively (see Figure 4c). The two basic semi-reactions and their overall reaction (Equation (3)) are as follows: . Schematic diagram of photocatalytic hydrogen production principle of photocatalyst dispersed in electrolyte (a), schematic diagram of hydrogen production principle in a photoelectrocatalytic system composed of a n-type semiconductor as photoanode (b), and a p-type semiconductor as photocathode (c), respectively.
According to the aforementioned semi-reactions, in order to realize overall water splitting, the selected ideal photocatalyst should satisfy the requirements that the CB edge potential of the photoelectrode semiconductor should be above the energy level of the H + /H2 (0 V vs. NHE at pH = 0) . Schematic diagram of photocatalytic hydrogen production principle of photocatalyst dispersed in electrolyte (a), schematic diagram of hydrogen production principle in a photoelectrocatalytic system composed of a n-type semiconductor as photoanode (b), and a p-type semiconductor as photocathode (c), respectively.
According to the aforementioned semi-reactions, in order to realize overall water splitting, the selected ideal photocatalyst should satisfy the requirements that the CB edge potential of the photoelectrode semiconductor should be above the energy level of the H + /H 2 (0 V vs. NHE at pH = 0) and the VB edge potential should be below the energy level of the O 2 /H 2 O (1.23 V vs. NHE at pH = 0) [97]. The Z-scheme heterojunction provides a promising way to overcome the thermodynamic energy barrier for solar-water splitting by combining a narrow bandgap semiconductor which ensures the maximized utilization solar energy and one or more paired semiconductors with appropriate energy band structure to broaden the redox range of the photocatalyst composite. Bi-based semiconductors are promising candidates for a Z-scheme heterojunction attributed to their unique energy band structure and other attractive advantages. Table 3 summarizes the latest development of Bi-based Z-scheme photocatalysts applied for solar-water splitting. The Bi-based Z-scheme photocatalyst and its synthesis method, the conditions of the solar-water splitting process, the utilization of co-catalyst, the products and yields, and apparent quantum yield (AQY) of all the examples are presented in Table 3. From the data of Table 3, it is seen that Bi-based Z-scheme photocatalysts exhibit promising performance for solar-water splitting. Most research using Bi-based Z-scheme photocatalysts can obtain good H 2 evolution yield. Chou et al. [98] made SnS 2 self-growth on the BiPO 4 nanosheets to form three dimensions (3D) flower heterogeneous composite by a multi-step solvothermal method. According to the radical-trapping experiment results and band structure analysis, a Z-scheme heterojunction was formed between SnS 2 and BiPO 4 . Under visible light, the formed Z-scheme heterojunction showed the highest H 2 evolution rate of 303 µmol h −1 g −1 , which is about 1.43 and 2.01 times higher than that of pure SnS 2 and pure BiPO 4 , respectively. Due to the high energy barrier of its four-electron transfer process, the O 2 -evolution half reaction in overall solar-water splitting is much more challenging than the H 2 -evolution half reaction, which dramatically suppresses the efficiency of the overall solar-water splitting. Usually, most semiconductor photocatalysts have low activity for O 2 evolution [109]. Besides the overall solar-water splitting, H 2 -evolution half reaction using a sacrificial agent is another efficient way to harvest and convert solar energy to H 2 . The sacrificial agent plays a significant role in the H 2 -evolution half reaction. Suitable sacrificial reagents can improve the H 2 -evolution efficiency remarkably by scavenging the photogenerated holes to reduce the charge carrier recombination significantly. Zhu et al. investigated the effect of the different organic compounds on the hydrogen production rate of ZnIn 2 S 4 /RGO/BiVO 4 [106]. As shown in Figure 5a, in the presence of organic compounds such as formaldehyde, methanol, formic acid, acetaldehyde and ethanol and so on, the hydrogen production rate has been promoted to a different extent. Organic compounds including alcohols (for example methanol [99]), aldehydes (for example formaldehyde [106]), organic acids (for example lactic acid [103]) have been extensively used as efficient hole scavengers for H 2 production in Bi-based Z-scheme photocatalysts systems. Due to the high energy barrier of its four-electron transfer process, the O2-evolution half reaction in overall solar-water splitting is much more challenging than the H2-evolution half reaction, which dramatically suppresses the efficiency of the overall solar-water splitting. Usually, most semiconductor photocatalysts have low activity for O2 evolution [109]. Besides the overall solar-water splitting, H2-evolution half reaction using a sacrificial agent is another efficient way to harvest and convert solar energy to H2. The sacrificial agent plays a significant role in the H2-evolution half reaction. Suitable sacrificial reagents can improve the H2-evolution efficiency remarkably by scavenging the photogenerated holes to reduce the charge carrier recombination significantly. Zhu et al. investigated the effect of the different organic compounds on the hydrogen production rate of ZnIn2S4/RGO/BiVO4 [106]. As shown in Figure 5a, in the presence of organic compounds such as formaldehyde, methanol, formic acid, acetaldehyde and ethanol and so on, the hydrogen production rate has been promoted to a different extent. Organic compounds including alcohols (for example methanol [99]), aldehydes (for example formaldehyde [106]), organic acids (for example lactic acid [103]) have been extensively used as efficient hole scavengers for H2 production in Bi-based Z-scheme photocatalysts systems.  Furthermore, owing to the sufficient redox ability of the Bi-based Z-scheme photocatalyst, even refractory organic pollutants can also be successfully used as hole scavengers to achieve simultaneous H 2 generation and pollutant degradation. Very recently, Liu et al. constructed a Cu 2 O/BiVO 4 Z-scheme heterojunction by using reduced graphene oxide (rGO) as an adhesive via a two-step solvothermal method. This Bi-based Z-scheme photocatalyst exhibits excellent photocatalytic performance on simultaneous tetracycline (TC) degradation and H 2 production under visible light irradiation [108].

RGO-
Moreover, the introduction of a suitable co-catalyst can further improve the solar-water splitting efficiency of a Bi-based Z-scheme photocatalyst. The reason for this improvement may be attributed to the fact that co-catalysts can efficiently collect photogenerated carriers and catalyze H 2 or O 2 evolution as well as improve the stability of photocatalysts by suppressing photocorrosion [110]. Usually, noble metals and especially Pt have been extensively used as co-catalyst for solar-water splitting [111]. For example, Zhou et al. [103] prepared hierarchical CdS/BiVO 4 hybrid composed of CdS nanoparticles decorated on BiVO 4 nanowires (NWs) by the solvothermal method. The strongly chemical interaction between CdS and BiVO 4 was confirmed by XPS (X-ray photoelectron spectroscopy) results. As illustrated in Figure 5b, under visible light irradiation and in a lactic acid electrolyte, no hydrogen generation was observed on pure BiVO 4 even loaded with Pt as co-catalyst, attributed to the CB edge potential of pure BiVO 4 being lower than the energy level of H + /H 2 . The bare CdS showed a weak hydrogen production capacity. After forming a CdS/BiVO 4 Z-scheme heterojunction, the hydrogen production capacity was enhanced, whereas the improvement of photocatalytic H 2 activity was by orders of magnitude after loading Pt. At an optimized condition, CdS (50 wt.%) /BiVO 4 NWs with loading 2 wt.% Pt exhibited the fastest photocatalytic H 2 generation rate which is 9.30 times of that of CdS (50 wt.%)/BiVO 4 NWs without loading Pt co-catalyst. Therefore, due to their low overpotential for H 2 evolution and excellent electron-accepting capacity, using noble metal as a co-catalyst is an effective method to improve the photocatalytic hydrogen evolution, which is conducive to the electron generated by the photocatalyst to transfer to the noble metal to catalyze H 2 evolution.
However, the high price and scarcity of noble metals limit their large-scale practical application in photocatalytic H 2 generation as co-catalysts. In recent years, many efforts were devoted to develop co-catalysts composed of abundant and inexpensive elements to assist Bi-based Z-scheme photocatalyst for solar-water splitting. Recently, Xu et al. [99] prepared Bi/Bi 5 O 7 I/Sn 3 O 4 by hydrothermal method (See Figure 6a). The band structure of Bi 5 O 7 I matches well with that of Sn 3 O 4 to constitute a direct Z-scheme heterojunction. Metallic Bi which is evenly covered on the surface of Z-scheme Bi 5 O 7 I/Sn 3 O 4 photocatalyst and comes from the reduction reaction between Sn 3 O 4 and Bi 5 O 7 I during the synthesis process was demonstrated to be a good substitute for noble metals as co-catalyst to further improve the H 2 generation and extend the light absorption range. The X-ray diffraction (XRD) results (see Figure 6b) confirmed the presence of Bi metal. As shown in Figure 6b, as increasing the amount of Bi 5 O 7 I, the characteristic peak of Bi gradually appears and increases. The photocatalytic hydrogen production experiments under visible light irradiation demonstrated that the highest hydrogen evolution of this Z-scheme catalyst reached 325.9 µmol h −1 ·g −1 without any noble metal co-catalyst and exceeded the rate on pure Sn 3 O 4 by 5 times (See Figure 6c).
Great processes have been made to construct double Z-scheme heterojunction structures to make full use of solar energy to apply in water splitting. Drmosh et al. [104] prepared Bi 2 S 3 /MoS 2 /TiO 2 (MBT) by a facile microwave-assisted hydrothermal method. The double Z-scheme heterojunction structure was constructed due to the matched band structure among Bi 2 S 3 nanorods, MoS 2 nanometer sheets and TiO 2 nanotubes. As exhibited in Figure 6d Reproduced with permission from [104]. Copyright Elsevier, 2020.
Great processes have been made to construct double Z-scheme heterojunction structures to make full use of solar energy to apply in water splitting. Drmosh et al. [104] prepared Bi2S3/MoS2/TiO2 (MBT) by a facile microwave-assisted hydrothermal method. The double Z-scheme heterojunction structure was constructed due to the matched band structure among Bi2S3 nanorods, MoS2 nanometer sheets and TiO2 nanotubes. As exhibited in Figure 6d, photogenerated electrons in the CB of TiO2 can recombine with photogenerated holes in the VB of MoS2, and photogenerated electrons in the CB of MoS2 can recombine with photogenerated holes in the VB of Bi2S3, preserving the photogenerated electrons and holes with the strongest reduction and oxidization power. The constructed direct double Z-scheme heterojunction extends the light-harvesting capability, couples the respective  [104]. Copyright Elsevier, 2020.

CO 2 Reduction
Nowadays, excessive carbon dioxide (CO 2 ) emission is a key reason for global warming. Proposing environmentally friendly and efficient strategies to dramatically reduce atmospheric CO 2 is essential and urgent. Photocatalysis seems a propitious and appealing strategy because it can directly make use of inexhaustible solar energy to convert CO 2 in the atmosphere into hydrocarbons such as CH 4 , HCOOH, CO, CH 2 O and CH 3 O (Equations (4)-(9)) [33,112], alleviating the greenhouse effect and energy crisis simultaneously. The product species of photocatalytic CO 2 reduction depend on the relationship between the CB edge potential of the photocatalyst and the reduction potentials of the desired CO 2 reduced product. Since CO 2 is well known as one of the most thermodynamically stable chemical species [113,114], to satisfy thermodynamic requirements, the more negative the CB edge potential of the photocatalyst, the higher the possibility to drive the CO 2 reduction process. On the other hand, the photocatalytic CO 2 reduction system ultimately needs the use of water as the electron source (Equation (10)); a semiconductor photocatalyst has to satisfy band edge potentials that straddle both the water oxidation and CO 2 reduction potentials.
The photocatalytic CO 2 reduction reaction usually includes four main steps as follows: (1) CO 2 adsorption process on the active sites of photocatalyst; (2) absorption of sufficient incident photon energy by the photocatalyst to generate electron-hole pairs; (3) charge separation and migration to the surface of the photocatalyst; (4) surface reactions for CO 2 reduction and products desorption. In order to efficiently photocatalytically reduce CO 2 , the photocatalyst should possess all the features to successfully achieve each above step: including strong CO 2 selective adsorption capacity, broad light response range, high charge separation efficiency, and sufficiently strong redox activity. Due to the aforementioned last three virtues, Z-scheme photocatalytic systems have been extensively investigated and applied for CO 2 reduction in recent years. Based on the prominent photocatalytic activity nature of bismuth semiconductor, the representative progresses of Bi-based Z-scheme photocatalytic systems for the application of CO 2 reduction are summarized in Table 4. The CO 2 adsorption process can be improved by using the photocatalyst with large surface area. Modulating the morphology is an efficient way to obtain the photocatalyst with large surface area. Jung's group reported Z-scheme BiVO 4 /carbon-coated Cu 2 O (BVO/C/Cu 2 O) nanowire arrays (NWAs) with a three-dimensional (3D) structure for efficient photoconversion of CO 2 to CO and CH 4 [124]. The highest CO formation rate on this BVO/C/Cu 2 O NWAs reached 3.01 µmol h −1 ·g −1 , which is about 9.4 and 4.7 times on Cu 2 O mesh and Cu 2 O NWAs, respectively. The dramatically enhanced photocatalytic activity was ascribed to the construction of a Z-scheme on a 3D NWAs structure. Enlarged surface area and enhanced charge-carrier transfer of 3D NWAs structure was evidenced by the electrochemical surface area method and photocurrent experiment results, respectively. Owing to the large surface area, enhanced charge-transport property, and light scattering or reflecting effect of the 3D NWAs structure, the combination of unique 3D morphology with a Z-scheme charge flow is not only beneficial for the efficient charge separation and transfer [125], but also favorable for facilitating the light absorption and CO 2 adsorption by providing ample active sites [126,127]. Moreover, the thermodynamic feasibility of the photocatalytic reduction of CO 2 and water oxidation on this BVO/C/Cu 2 O NWAs was verified by the band edge configuration via ultraviolet photoelectron spectroscopy. The Z-scheme charge-transfer mechanism was confirmed by investigating the energy level of the photoinduced hole via a photoluminescence (PL) experiment using coumarin as a probe molecule in water.
Apart from morphology modulation of the photocatalyst, introducing a co-catalyst in the Bi-based Z-scheme system to improve the photocatalytic CO 2 reduction process is another efficient way. Recently, Jo and coworkers [122] constructed a Z-scheme Bi 2 WO 6 /rGO/g-C 3 N 4 (BWO/rGO/CN) for photocatalytic CO 2 reduction. Therein, rGO with excellent conductivity and large specific area was used as a co-catalyst for the Z-scheme photocatalyst to not only facilitate charge-carrier migration in Z-scheme mode, but also benefit the CO 2 adsorption and electron capture by establishing the unique π-π conjugation interaction and providing abundant active sites and further promoting the CO 2 photoreduction. In addition, attributed to forming large intimate interfaces, the 2D/2D/2D configuration of BWO/rGO/CN possesses strong light absorption in the visible region and strong electron shuttling at the interfaces hindering the direct recombination of charge carriers. As a consequence, the optimized Z-scheme BWO/rGO/CN displayed a remarkable photocatalytic performance for not only CO 2 reduction but also water splitting. The AQY of 0.75% at 400 nm was higher than the other state-of-the art CO 2 photoreduction catalyst system. Moreover, integrating other techniques with photocatalytic CO 2 reduction over a direct Z-scheme system will obtain unexpected results. The recently reported photothermal synergic enhancement of photocatalytic CO 2 reduction performance of a direct Z-scheme Bi 4 TaO 8 Cl/W 18 O 49 (BiW) system is an interesting example of the integration of the photocatalytic process with external heating [120]. The direct Z-scheme heterojunction was successfully fabricated by growing W 18 O 49 nanostructures on the surface of a Bi 4 TaO 8 Cl nanosheet (See Figure 7a). The well matched energy band of these two semiconductors makes the constructed Z-scheme heterojunction a promising photocatalyst for CO 2 reduction and H 2 O oxidation under visible light. Under photothermal conditions, the CO yield of the optimal BiW was increased surprisingly by 87 times over photocatalytic conditions (see Figure 7b). Interestingly, after light irradiation, long-lasting catalytic reduction of CO 2 in the dark was observed. The possible reasons for these experimental results are that external heating enhances the Z-scheme behavior of the BiW heterostructure by helping electrons at electron traps detrap to the surface of photocatalyst to increase the efficiency of electron utilization and promote the CO 2 reduction reaction, at the same time, decreasing the activation energy of lattice oxygen to promote oxidation reactions at the other reaction sites. The photoexcited electrons stored at the oxygen vacancy defects of W 18 O 49 can be released and excited to the conduction band of Bi 4 TaO 8 Cl by heating to reduce CO 2 and produce considerable CO in the dark after light irradiation (See Figure 7c). The design of this photothermal catalyst provides a novel and promising method for using solar energy to catalyze the reduction of CO 2 to fuels. From Table 4, the main product of CO2 photoreduction in most Bi-based Z-scheme systems are CO and CH4. By comparing all the reaction equations of products from CO2 reduction (Equations (4)-(9)), the redox potential E 0 of CO2/CH4 (−0.24 V vs. NHE) is the least negative value, which means the reaction of producing CH4 is one in which a reaction occurs most easily from a thermodynamic viewpoint. The CO formation through simple two-electron reduction process of CO2 is easier to achieve than the other products' generation through a multiple-electron reduction process of CO2. In order to photoreduce CO2 to obtain more carbonaceous products via more difficult multi-electron-transfer pathways, further exploration is still needed such as with the help of a noble metal cocatalyst. Meanwhile, water usually serves as an electron donor during CO2 photoreduction process. In ideal conditions, H2O should be oxidized into protons and O2 (Equation (10)). However, the generated protons actually can further react with photoexcited electrons to produce H2 (Equation (11)). At the same time, O2 also can react with photoexcited electrons (Equation (12)). These reactions form competitive relationship with CO2 photoreduction (Equations (4)-(9)), which makes the electron-transfer process of CO2 photoreduction more complicate [128,129]. Therefore, to understand in depth and illuminate the actual and complicated mechanism of photocatalytic CO2 reduction is also crucial. From Table 4, the main product of CO 2 photoreduction in most Bi-based Z-scheme systems are CO and CH 4 . By comparing all the reaction equations of products from CO 2 reduction (Equations (4)-(9)), the redox potential E 0 of CO 2 /CH 4 (−0.24 V vs. NHE) is the least negative value, which means the reaction of producing CH 4 is one in which a reaction occurs most easily from a thermodynamic viewpoint. The CO formation through simple two-electron reduction process of CO 2 is easier to achieve than the other products' generation through a multiple-electron reduction process of CO 2 . In order to photoreduce CO 2 to obtain more carbonaceous products via more difficult multi-electron-transfer pathways, further exploration is still needed such as with the help of a noble metal cocatalyst. Meanwhile, water usually serves as an electron donor during CO 2 photoreduction process. In ideal conditions, H 2 O should be oxidized into protons and O 2 (Equation (10)). However, the generated protons actually can further react with photoexcited electrons to produce H 2 (Equation (11)). At the same time, O 2 also can react with photoexcited electrons (Equation (12)). These reactions form competitive relationship with CO 2 photoreduction (Equations (4)-(9)), which makes the electron-transfer process of CO 2 photoreduction more complicate [128,129]. Therefore, to understand in depth and illuminate the actual and complicated mechanism of photocatalytic CO 2 reduction is also crucial.

Removal of Gas Phase Pollutants and Other Applications
In recent years, the emission of nitrogen oxides (NO x ) from human activities into the atmosphere has become one of the major environmental problems because the great harm of NO x to human health, climate, and agriculture [130]. In addition, NO x mainly composed of NO and NO 2 can produce secondary aerosols and cause more serious environmental problems, such as acid rain, haze, photochemical smog, PM 2.5 (fine particulate matter with less than 2.5 µm diameter), and ozone accumulation, etc. [131,132]. Among various methods for NO x removal, such as physical adsorption [133], biofiltration [134], and thermal catalytic reduction [135], photocatalysis, as a green chemical method, is considered to be a promising strategy due to its high efficiency and low cost. It has been reported that Bi-based Z-scheme heterojunction systems exhibit high photocatalytic activity for pollutant removal including NO x removal because of the abundant active radicals with strong redox ability in these systems. Zhu [73]. The optimal Bi 2 MoO 6 /ZnIn 2 S 4 exhibited superior photocatalytic activity for NO removal, and the removal efficiency reached 84.94% in 80 min, attributed to the low rate of recombination of photogenerated charge carriers in the direct Z-scheme charge transfer mode. Volatile organic compounds (VOCs), such as alcohols, aldehydes, ketones, alkenes, and aromatic compounds, are known to cause enormous harm to human health [137]. However, highly effective VOC elimination still remains a challenge. Photocatalytic degradation is found to be an attractive and promising technique for the abatement of VOC because of its mild operation conditions (room temperature and atmospheric pressure), highly effective and thorough degradation, and good solar energy utilization capacity. Photocatalyst is one of the keys of this technique. Z-scheme photocatalysts with high specific surface area have been accepted as rational photocatalysts for photocatalytic degradation of VOCs. For example, it was shown that direct Z-scheme BiVO 4 /g-C 3 N 4 with coral-like structure achieved efficient mineralization of toluene under visible light illumination [138]. Brunauer-Emmett-Teller (BET) measurement provided evidence for the high specific surface area of the coral-like structure which provides more active sites for the photocatalytic oxidization toluene. According to the results of the ESR and terephthalic acid photoluminescence (TA-PL), the enhanced photocatalytic performance of BiVO 4 /g-C 3 N 4 was attributed to a direct Z-scheme migration. The degradation rate constant of the optimal BiVO 4 /g-C 3 N 4 for toluene degradation reached 0.138 h −1 .
As a newly developed sensing technique, photoelectrochemical (PEC) sensors have attracted tremendous attention. Such PEC sensors combine the advantages of optical detection and electrochemical detection and improve their inherent defects at the same time. Owing to a detached excitation system and detection system, the PEC has high sensitivity, rapid measurement speed, and a low background signal. Additionally, in comparison to optical detection methods with costly optical imaging system and complicated image analysis software, the cost of PEC with a simple electrochemical detection system is much lower. From the sensing mechanism of PEC, the semiconductor as photoelectrode which is excited by light to generate hole-electron pairs and further produce photocurrent signal with the separation and migration of charge carriers is the core component of PEC. However, fast recombination of photogenerated electrons and holes as one of the inherent limitations of semiconductors is a big challenge for PEC sensors. The Z-scheme charge-carrier migration mode is beneficial to the charge separation and migration as well as improving the photoelectric conversion and signal generation efficiency of PEC. For instance, a Z-scheme iodine doped BiOCl/nitrogen-doped graphene quantum dots (I-BiOCl/N-GQDs) heterojunction was prepared by a one-pot precipitation method at room temperature [139]. Such a Z-scheme I-BiOCl/N-GQDs heterojunction was used as a photoelectrode to construct a "signal-off" cathodic PEC sensor for the selective detection of chlorpyrifos. The optimal I-BiOCl/N-GQDs composite exhibited the highest photocurrent signal (See Figure 8a) indicating excellent spatial separation efficiency of charge carriers which was evidenced by PL and EIS results. The Z-scheme charge carrier transfer pathway and the enhanced light harvesting can be used to explain the improved PEC performance. Figure 8b displays the PEC detection mechanism of chlorpyrifos. As shown in Figure 8b, in the presence of chlorpyrifos in the solution, the S and N atoms of chlorpyrifos will bind with the Bi(III) to form the bismuth-chlorpyrifos complex on the surface of I-BiOCl/N-GQDs, which will decrease the photocurrent signal because of the steric hindrance effect to achieve detection of chlorpyrifos. As shown in Figure 8c,d, this PEC sensor presented a wide linear detection range (0.3-80 ng·mL −1 ), considerably low detection limit (0.01 ng·mL −1 ), and good selectivity toward chlorpyrifos. Nevertheless, there is a lack of evidence for verifying the Z-scheme charge carrier transfer pathways in this work. semiconductor as photoelectrode which is excited by light to generate hole-electron pairs and further produce photocurrent signal with the separation and migration of charge carriers is the core component of PEC. However, fast recombination of photogenerated electrons and holes as one of the inherent limitations of semiconductors is a big challenge for PEC sensors. The Z-scheme chargecarrier migration mode is beneficial to the charge separation and migration as well as improving the photoelectric conversion and signal generation efficiency of PEC. For instance, a Z-scheme iodine doped BiOCl/nitrogen-doped graphene quantum dots (I-BiOCl/N-GQDs) heterojunction was prepared by a one-pot precipitation method at room temperature [139]. Such a Z-scheme I-BiOCl/N-GQDs heterojunction was used as a photoelectrode to construct a "signal-off" cathodic PEC sensor for the selective detection of chlorpyrifos. The optimal I-BiOCl/N-GQDs composite exhibited the highest photocurrent signal (See Figure 8a) indicating excellent spatial separation efficiency of charge carriers which was evidenced by PL and EIS results. The Z-scheme charge carrier transfer pathway and the enhanced light harvesting can be used to explain the improved PEC performance. Figure 8b displays the PEC detection mechanism of chlorpyrifos. As shown in Figure 8b, in the presence of chlorpyrifos in the solution, the S and N atoms of chlorpyrifos will bind with the Bi(III) to form the bismuthchlorpyrifos complex on the surface of I-BiOCl/N-GQDs, which will decrease the photocurrent signal because of the steric hindrance effect to achieve detection of chlorpyrifos. As shown in Figure  8c,d, this PEC sensor presented a wide linear detection range (0.3-80 ng·mL −1 ), considerably low detection limit (0.01 ng·mL −1 ), and good selectivity toward chlorpyrifos. Nevertheless, there is a lack of evidence for verifying the Z-scheme charge carrier transfer pathways in this work.

Conclusions and Perspectives
As one of the most promising light-conversion technologies, photocatalysis can ease the problem of energy and environmental pollution in the future by using appropriate semiconductor as photocatalysts and solar energy as energy input to drive photocatalytic reactions. Photocatalysts are a crucial factor which determine the performance of this technique. Bi-based semiconductors with advantages of non-toxicity, low cost and good thermal stability and as a new and important visible light-activated semiconductor have attracted great attention and become a research hotspot. Recently, great progress has been made in improving the visible-light absorption ability, light-harvesting efficiency, and photogenerated carriers' separation of Bi-based semiconductors. However, for a single-component Bi-based semiconductor, it is hard to simultaneously satisfy strong redox ability for a specific photocatalytic reaction and the good solar energy utilization capacity. Z-scheme heterojunction construction is considered a promising strategy to conquer this conflict via combining a narrow bandgap semiconductor and one or more paired semiconductor with appropriate energy band structure to broad the light response range and enhance the redox ability of photocatalyst composite. Bi-based Z-scheme photocatalysts exhibit good performance in various photocatalytic applications involving energy production, for example, water splitting, environmental remediation such as CO 2 reduction, NO x removal, pollutants degradation, as well as pollutant detection, for example, construction of PEC sensors.
However, the study of the Bi-based Z-scheme heterojunction is still in its infancy and has many challenges and problems: (1) selecting two or more semiconductors with good matched band structure is the prerequisite for the successful preparation of direct Z-scheme photocatalysts. Moreover, the selected semiconductor components should satisfy the requirement that one of two contacted semiconductors should have higher CB, VB position and E f than the other, whereas whether the constructed photocatalyst follows the direct Z-scheme mechanism or not can be validated by the experimental methods performing expensive instruments. Therefore, the combination theoretical calculation and simulation with experimental characterizations may be a cost-effective strategy to comprehensively understanding the mechanism of Z-scheme and successfully construct direct Z-sheme photocatalysts. (2) Optimizing and controlling the contact interface between the selected semiconductor components of a direct Z-scheme photocatalyst is necessary for high efficiency of charge carrier separation and transfer which is beneficial to enhance the photocatalytic performance of prepared direct Z-scheme photocatalysts. (3) For specific photocatalytic applications, introducing suitable, abundant, and inexpensive co-catalysts into the Bi-based Z-scheme semiconductor will further improve the efficiency of photocatalytic applications. In addition, modulating the morphology of the Bi-based Z-scheme semiconductor to obtain high specific surface area is another way to enhance the efficiency of photocatalytic applications owing to this providing more active sites for the reactive matters. (4) In practical applications, it is also necessary to consider the recovery of Bi-based Z-scheme photocatalysts. In most applications, the photocatalyst is dispersed in the solution in the form of powders, which makes recovery work after application very complicated. Thus, constructing Bi-based Z-scheme photocatalysts with magnetic or self-floating properties is a new direction.
Owing to the potential of the Bi-based Z-scheme photocatalyst, it would be promising to realize its industrialization after resolving these challenges.