A Review of Direct Z‐Scheme Photocatalysts

REVIEW

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A Review of Direct Z-Scheme Photocatalysts

Jingxiang Low, Chuanjia Jiang, Bei Cheng, Swelm Wageh, Ahmed A. Al-Ghamdi, and Jiaguo Yu*

DOI: 10.1002/smtd.201700080

emerged as one of the most promising photoconversion technologies because of its ability to directly utilize solar energy for producing solar fuels such as hydrogen (H2), methane (CH4), methanol (CH3OH), formic acid (HCOOH), and formaldehyde (CH2O), and alleviating environmental pollution.[19–23] Despite the significant progress in photocatalysis that has been achieved in recent years, the photoconversion efficiency of photocatalytic reactions is still low, and they are far from practical application, because of the rapid electron–hole recombination and poor light utilization of semiconductors.[24–27] Therefore, it is of importance to search for advanced photocatalytic systems with efficient light utilization and electron–hole separation.

In the past several decades, diverse strategies have been studied to enhance the photoconversion efficiency of photo

catalysts, including doping, metal loading, heterojunction construction, etc. Among these proposed strategies, the combination of two semiconductors to form a typeII heterojunction photocatalyst is one of the most facile ways for enhancing photocatalytic performance, due to its effectiveness for spatially separating the photogenerated electron–hole pairs through the band alignment between two semiconductors.[25,28] For a typical typeII heterojunction, both the conduction band (CB) and valence band (VB) of semiconductor A are higher than those of semiconductor B (Figure 1a).[29–31] Therefore, the photogenerated electrons in the CB of semiconductor A will migrate to the CB of semiconductor B under light irradiation due to band alignment. Meanwhile, the photogenerated holes in the VB of semiconductor B will transfer to the VB of semiconductor A. Since photogenerated electrons and holes respectively accumulate on semiconductor B and semiconductor A, spatial separation of the electrons and holes can be achieved with a typeII heterojunction photocatalyst for enhancing the photocatalytic activity.[32–34] However, there are several obvious problems that restrict the wide application of typeII heterojunction photocatalysts. In detail, the reduction and oxidation reactions of typeII heterojunction photocatalysts, respectively, occur for semiconductor B with a lower reduction potential and semiconductor A with a lower oxidation potential.[35,36] Therefore, the redox ability of typeII heterojunction photocatalysts will be greatly reduced.[37] In addition, it is difficult for electrons in semiconductor A and holes in semiconductor B to respectively migrate to the electronrich CB of semiconductor B and the holerich

Recently, great attention has been paid to fabricating direct Z-scheme photo-catalysts for solar-energy conversion due to their effectiveness for spatially separating photogenerated electron–hole pairs and optimizing the reduc-tion and oxidation ability of the photocatalytic system. Here, the historical development of the Z-scheme photocatalytic system is summarized, from its first generation (liquid-phase Z-scheme photocatalytic system) to its current third generation (direct Z-scheme photocatalyst). The advantages of direct Z-scheme photocatalysts are also discussed against their predecessors, including conventional heterojunction, liquid-phase Z-scheme, and all-solid-state (ASS) Z-scheme photocatalytic systems. Furthermore, characteriza-tion methods and applications of direct Z-scheme photocatalysts are also summarized. Finally, conclusions and perspectives on the challenges of this emerging research direction are presented. Insights and up-to-date informa-tion are provided to give the scientific community the ability to fully explore the potential of direct Z-scheme photocatalysts in renewable energy produc-tion and environmental remediation.

Photocatalysts

J. X. Low, Dr. C. J. Jiang, Prof. B. Cheng, Prof. J. G. YuState Key Laboratory of Advanced Technology for Materials Synthesis and ProcessingWuhan University of Technology122 Luoshi Road, Wuhan 430070, P. R. ChinaE-mail: [email protected], [email protected]. S. Wageh, Prof. A. A. Al-Ghamdi, Prof. J. G. YuDepartment of PhysicsFaculty of ScienceKing Abdulaziz UniversityJeddah 21589, Saudi Arabia

The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/smtd.201700080.

1. Introduction

Due to rapid industrialization and population growth, the global energy crisis and environmental pollution have become two of the greatest challenges of human society in the 21st century.[1–5] The utilization of solar energy, which is a powerful, affordable, and renewable energy source, for energy production and pollutant elimination, is regarded as the best solution to these critical problems.[6–9] Therefore, much effort has been devoted toward converting solar energy into an applicable energy medium through various technologies, such as photocatalysis,[10,11] solar cells,[12,13] photoelectrochemical cells,[14–16] and so on.[17,18] Among them, photocatalysis has

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VB of semiconductor A, due to electrostatic repulsion between electron–electron or hole–hole. Thus, it is highly desired to prepare new heterostructured photocatalytic systems to overcome these problems.

Biomimetic artificial photosynthesis through building of the direct Zscheme photocatalyst represents a viable strategy for enhancing the photocatalytic performance.[38,39] In 2013, the concept of a direct Zscheme photocatalyst was proposed by Yu et al. for explaining the high photocatalytic formaldehyde (HCHO) degradation performance of the TiO2/gC3N4 composite. Specifically, the structure of a direct Zscheme photocatalyst is similar to that of a typeII heterojunction photocatalyst (Figure 1a,b), but its chargecarrier migration mechanism is different. Specifically, a typical direct Zscheme system has a chargecarrier migration pathway that resembles the letter “Z” (Figure 1b).[40] During the photocatalytic reaction, the photogenerated electrons in semiconductor B, with lower reduction ability, recombine with the photogenerated holes in semiconductor A with a lower oxidation ability (Figure 1b).[41] Therefore, the photogenerated electrons in semiconductor A with high reduction ability and photogenerated holes in semiconductor B with a high oxidation ability can be maintained. As a result, the redox ability of the direct Zscheme photocatalyst can be optimized. In addition, it should be noted that chargecarrier migration for the direct Zscheme photocatalyst is physically more feasible than that for typeII heterojunction photocatalysts, since the migration of photogenerated electrons from the CB of semiconductor B to the photogenerated holerich VB of semiconductor A is favorable due to the electrostatic attraction between the electron and hole.

Since 2013, huge accomplishments related to direct Zscheme photocatalysts for photocatalytic applications have been achieved by numerous research groups. Therefore, it is of significance to summarize the recent discoveries and achievements in the field of direct Zscheme photocatalysts. Here, the development and basic principle of direct Zscheme photocatalysts are discussed. Then, the characterization methods for direct Zscheme photocatalysts are summarized. The recent advances and trends in the study of direct Zscheme photocatalysts for various photocatalytic applications are also presented. Finally, the conclusions and perspectives for future research on the direct Zscheme photocatalyst are provided.

2. Historical Development of Direct Z-Scheme Photocatalysts

In order to have a comprehensive understanding of direct Zscheme photocatalysts, it is of significance to discuss development of the Zscheme photocatalytic system from the 1st generation to the current 3rd generation (Figure 2). The concept of the Zscheme photocatalytic system was originally proposed by Bard in 1979.[42] As shown in Figure 3, the liquidphase Zscheme photocatalytic system is built by combining two different semiconductors with a shuttle redox mediator (viz. an electron acceptor/donor (A/D) pair). Under light irradiation, both semiconductor A and semiconductor B are photoexcited, thereby generating electrons and holes respectively in their CB and VB. Then, the photogenerated electrons from

semiconductor B will transfer to the VB of semiconductor A via a shuttle redox mediator according to Reaction (1) and (2), leaving photogenerated holes in the VB of semiconductor B.

Jingxiang Low obtained his B.Eng. (Hons) from Multimedia University, Malaysia in 2011 and his M.S. in materials science from Wuhan University of Technology. He is currently a Ph.D. candidate under the supervision of Prof. Jiaguo Yu at the State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology. His current research includes photocatalytic H2 production and CO2 reduction. See more details at: http://www.researcherid.com/rid/N-2381-2014.

Chuanjia Jiang received his B.S. and M.S. in environ-mental engineering from Tsinghua University, Beijing in 2009 and 2011, respectively, and his Ph.D. in civil and environmental engineering in 2016 from Duke University. In 2016, he joined Wuhan University of Technology as an Assistant Professor. His current research inter-

ests include materials for air and water pollution con-trol. See more details at: http://www.researcherid.com/rid/C-9398-2014.

Jiaguo Yu received his B.S. and M.S. in chemistry from Central China Normal University and Xi’an Jiaotong University, respectively, and his Ph.D. in materials sci-ence in 2000 from Wuhan University of Technology. In 2000, he became a Professor at Wuhan University of Technology. He was a post-doctoral fellow at the Chinese

University of Hong Kong from 2001 to 2004, a visiting sci-entist from 2005 to 2006 at the University of Bristol, and a visiting scholar from 2007 to 2008 at the University of Texas at Austin. His current research interests include semicon-ductor photocatalysis, photocatalytic hydrogen production, CO2 reduction to hydrocarbon fuels, and so on. See more details at: http://www.researcherid.com/rid/G-4317-2010.


http://www.researcherid.com/rid/N-2381-2014

http://www.researcherid.com/rid/N-2381-2014

http://www.researcherid.com/rid/C-9398-2014

http://www.researcherid.com/rid/C-9398-2014

http://www.researcherid.com/rid/G-4317-2010

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( )+ →−A e @CB of semiconductor B D (1)

( )+ →+D h @VB of semiconductor A A (2)

Therefore, the photogenerated electrons remain on semiconductor A with the higher reduction potential, while the photogenerated holes remain on semiconductor B with the higher oxidation potential, thereby achieving the optimization of the redox potential of the photocatalytic system.

Although the 1st generation Zscheme photocatalytic system is efficient for photocatalytic reactions, it suffers from several obvious problems. First, backward reactions exist due to the use of reversible redox mediators such as I−/IO3− and Fe2+/Fe3+. For example, the acceptor, such as Fe3+, and the donor, such as Fe2+, will compete with the reactants for reduction and oxidation reaction, respectively during the photocatalytic reaction. Therefore, the photoconversion efficiency of the Zscheme photocatalytic system will be greatly reduced. Second, the 1st generation Zscheme photocatalytic system can only be applied in the liquid phase. Therefore, its practical application in the

gas and solid phases is greatly limited. In order to solve these problems, Tada et al. proposed the concept of the 2nd generation Zscheme photocatalytic system, namely the allsolidstate (ASS) Zscheme photocatalytic system, in 2006 (Figure 4).[43] An ASS Zscheme photocatalytic system is composed of two different semiconductors and a noblemetal nanoparticle (NP) as the electron mediator. Since the noblemetal NP is used as an electron mediator in the ASS Zscheme photocatalytic system, the backward reaction of the firstgeneration Zscheme photocatalytic system can be inhibited. However, the use of noble metals, which are rare and expensive, greatly limits the wide application of the ASS Zscheme photocatalytic system. Moreover, noblemetal NPs are normally strong light absorbers.[44,45] Therefore, the lightabsorption ability of the photocatalyst will be also greatly reduced by construction of the ASS Zscheme photocatalyst.

Thereafter, Wang et al. prepared a mediatorfree ASS Zscheme photocatalytic system in 2009.[46] It was found that the Zscheme electron–hole transfer mechanism can be applied to ZnO and CdS when they are in intimate contact. In 2013, our group proposed the concept of the thirdgeneration Zscheme photocatalytic system, namely the direct Zscheme photocatalyst (Figure 1b).[47] The advantages of the previous two generations of Zscheme photocatalytic systems are fully inherited by the direct Zscheme photocatalyst, including improved electron–hole separation efficiency and optimized redox potential. In detail, a direct Zscheme photocatalyst consists of only two semiconductors that have direct contact at their interface. In comparison with the previous two generations, electron or hole mediators are not required in the direct Zscheme photocatalyst. Therefore, the construction cost of the Zscheme photocatalytic system can be greatly reduced. Moreover, the lightshielding effect caused by the loading of the metalbased mediator can also be overcome by building the direct Zscheme photocatalyst. Owing to these advantages, the direct Zscheme photocatalyst has been widely studied for various photocatalytic applications (see Table 1).[40,48,49]

3. Characterization Methods for Direct Z-Scheme Photocatalysts

As discussed in Section 1, the structure of a direct Zscheme photocatalyst is similar to that of a typeII heterojunction photocatalyst. Therefore, it is imperative to investigate the chargecarrier migration mechanism for the direct Zscheme photocatalyst through various characterization methods, in order to differentiate it from typeII heterojunction photocatalysts. To date, various characterization methods have been proposed for this purpose, including photocatalyticreduction testing, radical species trapping, metal loading, Xray photoelectron spectroscopy (XPS), effective mass calculation, and internal electricfield simulation. The following subsections elucidate these characterization methods. It should be kept


Figure 1. Comparison of the charge-carrier separation mechanism on the type-II heterojunction (a) and the direct Z-scheme (b) photocatalyst built on two different semiconductors (SCs).

Figure 2. The roadmap of the evolution of Z-scheme photocatalytic system from the 1st gen-eration to the 3rd generation.



in mind that utilizing only a single characterization method cannot give precise information on the chargecarrier migration mechanism for the direct Zscheme photocatalyst. Thus, a comprehensive investigation is always required to confirm the formation of direct Zscheme photocatalysts through a combination of different characterization methods.

3.1. Photocatalytic-Reduction Testing

It is well known that not all the photogenerated electrons reaching the surface of a photocatalyst can be utilized for the photocatalyticreduction reaction. Specifically, only the photogenerated electrons in a semiconductor with sufficient reduction potential can be applied for specific photocatalyticreduction reactions. The standard redox potentials of various photocatalytic reduction reaction are shown in Reaction (3) to (8):

+ →= − =

+ −2H 2e H ,0.41V vs NHE at pH 7

2

0E

(3)

+ + →= − =

+ −CO 2H 2e HCOOH,0.61V vs NHE at pH 7

2

0E

(4)

+ + → += − =

+ −CO 2H 2e CO H O,0.53V vs NHE at pH 7

2 2

0E

(5)

+ + → += − =

+ −CO 4H 4e HCHO H O,0.48V vs NHE at pH 7

2 2

0E

(6)

+ + → += − =

+ −CO 6H 6e CH OH H O,0.38V vs NHE at pH 7

2 3 2

0E

(7)

+ + → += − =

+ −CO 8H 8e CH 2H O,0.24V vs NHE at pH 7

2 4 2

0E

(8)

Therefore, it is feasible to determine the accumulation of photogenerated electrons in specific semiconductors of a direct Zscheme photocatalyst through an investigation of the final product of the photocatalyticreduction reaction. For example, our group confirmed the formation of the CdS/WO3 direct Zscheme photocatalyst by determining the products of photocatalytic CO2 reduction (Figure 5a,b).[61] It was found that the WO3 shows no photocatalytic CO2 activity for CH4 production due to its low reduction potential (0.5 V vs normal hydrogen electrode (NHE)) (Figure 5c). Meanwhile, the prepared CdS exhibited good photocatalytic CO2reduction activity for CH4 production because of its sufficient reduction potential (−0.6 V vs NHE) (Figure 5c). Moreover, all the prepared CdS/WO3 composites exhibited better CH4 production efficiency. This result is because of the formation of the direct Zscheme heterojunction between CdS and WO3, which can enhance the electron–hole separation efficiency and optimize the reduction ability of the CdS/WO3. In detail, according to the direct Zscheme mechanism, the photogenerated electrons will accumulate on the CdS, which has sufficient reduction potential for reduction of CO2 into CH4. If the photogenerated electrons migrate according to the conventional typeII heterojunction, no CH4 can be produced by using CdS/WO3. This is because, according to the conventional heterojunction mechanism, the photogenerated electrons will accumulate on the WO3, which does not have sufficient reduction potential. This result confirms the formation of a direct Zscheme photocatalytic system instead of a typeII heterojunction between the CdS and the WO3 (Figure 5c). Moreover, it should be noted that CH4, which require more electrons (8 electrons) and less reduction potential (−0.24 V vs NHE) is the main reaction product during the photocatalytic CO2reduction test using the CdS/WO3 composite (Figure 5a). This is due to the accumulation of photogenerated electrons on a specific semiconductor, which is beneficial for the multielectron reaction of CH4 production. This study demonstrates that the photocatalytic reduction test can be a viable method to investigate the chargecarrier migration mechanism for the direct Zscheme photocatalyst.

3.2. Radical Species Trapping Test

It is well established that the hydroxyl (·OH) radical can be generated on semiconductors that can produce photogenerated


Figure 3. Schematic illustration of the 1st generation liquid-phase Z-scheme photocatalytic system, where A and D respectively represent the electron acceptor and donor.

Figure 4. Schematic illustration of the 2nd generation Z-scheme photo-catalytic system, all-solid-state (ASS) Z-scheme photocatalytic system.

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Table 1. Studies of direct Z-scheme heterojunction for various photocatalytic applications.

Photocatalyst Light source Application Performance Ref.

Ag2CrO4–graphene oxide 300 W xenon arc lamp

(400 nm cutoff filter)

Methylene blue degradation k = 2.8 × 10−1 min−1 [50]

g-C3N4/TiO2 3 W 365 nm UV lamp Brilliant red X3B degradation k = 5.1 × 10−2 min−1 [51]

CaIn2S4/TiO2 200 W mercury vapor lamp Metronidazole degradation k = 1.91 × 10−2 min−1 [52]

Bi2O3/g-C3N4 500 W xenon arc lamp (400 nm cutoff filter) Rhodamine B degradation k = 1.01 × 10−2 min−1 [53]

g-C3N4–TiO2 15 W 365 nm UV lamp Formaldehyde degradation k = 7.36 × 10−2 min−1 [47]

ZnIn2S4/Bi2WO6 500 W tungsten–halogen lamp Metronidazole degradation k = 1.66 × 10−2 min−1 [54]

Bi2O3/NaNbO3 375 W mercury lamp Rhodamine B degradation k = 5.86 × 10−2 min−1 [55]

g-C3N4–Ag3PO4 300 W xenon arc lamp C2H4 degradation k = 1.05 h−1 [56]

NaNbO3/WO3 375 W mercury lamp Rhodamine B and Methylene

blue degradationk = 3.7 × 10−2 min−1 and

9.6 × 10−2 min−1

[57]

g-C3N4/Ag2CO3 300 W xenon arc lamp (400 nm cutoff filter) Rhodamine B degradation k = 1.36 × 10−1 min−1 [58]

ZnO/CdS 400 W xenon arc lamp H2 production 1007 mmol h−1g−1 [59]

Anatase/rutile TiO2 350 W xenon arc lamp H2 production 324 μmol h−1g−1 [60]

C,N-TiO2/g-C3N4 300 W xenon arc lamp (400 nm cutoff filter) H2 production 39.18 mmol h−1g−1 [27]

CdS–WO3 300 W xenon arc lamp (420 nm cutoff filter) CO2 reduction 1.02 μmol h−1g−1 CH4 [61]

g-C3N4/ZnO 300 W xenon arc lamp CO2 reduction 0.6 μmol h−1g−1 CH3OH [62]

Cu2O/Fe2O3 300 W xenon arc lamp (400 nm cutoff filter) CO2 reduction 1.67 μmol h−1gcat−1 CO [63]

Figure 5. a) Gas-chromatography detection of the photocatalytic CO2 reduction products over 5 mol% CdS-loaded WO3 (C5). b) Performance of photo-catalytic CO2 reduction to CH4 over WO3 (C0), pure CdS (C100), and CdS-loaded WO3 samples with different CdS loadings: 1 mol% (C1), 2 mol% (C2), C5, 10 mol% (C10), 20 mol% (C20). c) Schematic illustration of the charge-carrier migration mechanism on the CdS/WO3 type-II heterojunction and direct Z-scheme photocatalyst. Reproduced with permission.[61] Copyright 2015, Wiley-VCH.



holes with sufficient oxidation potential (2.4 V vs NHE), while the superoxide (·O2−) radical can be produced from photogenerated electrons in a semiconductor with sufficient reduction potential (−0.33 V vs NHE) under light irradiation.[64–66] Since direct Zscheme photocatalysts and typeII heterojunction photo catalysts can respectively maximize and minimize the redox potential of the photocatalyst, it is feasible to differentiate these two photocatalytic systems by investigating the production efficiency of the radical species on the photocatalyst.

3.2.1. ·OH-Radical Trapping Test

The hydroxylradical trapping test is one of the most commonly applied methods to confirm the formation of a direct Zscheme photocatalyst. Specifically, the ·OH radical can be produced by the reaction of OH−/H2O with the photogenerated hole on the semiconductor with an oxidation potential greater than 2.4 V vs NHE. Therefore, the ·OH radical trapping test is a simple yet effective method to confirm the accumulation of photogenerated holes on either semiconductor with a high oxidation potential or a low oxidation potential. Typically, the production of ·OH radicals can be investigated by photoluminescence (PL) spectroscopy, using terephthalic acid (TA) as a probe molecule. The nonfluorescent TA molecules can react with an ·OH radical to produce highly fluorescent 2hydroxyterephthalic acid (HTA), which is detectable by the PL spectrometer. The PL peak intensity of the HTA is in proportion to the number of produced ·OH radicals. For example, the PL spectral changes of the gC3N4/TiO2 direct Zscheme photo catalyst with different illumination times are shown in Figure 6a.[47] Obviously, the PL intensity of the HTA in the sample increases with increasing illumination time. This result indicates that the photogenerated holes on the gC3N4/TiO2 composite has sufficient oxidation potential for generating ·OH radicals, which can react with TA to produce HTA. Since the photogenerated holes on gC3N4 do not have sufficient oxidation potential to react with OH−/H2O for generating ·OH radicals, it can be concluded that the photogenerated holes accumulate on the TiO2 (Figure 6b). The accumulation of photogenerated holes on TiO2 is in accordance with the chargecarrier migration mechanism of the direct Zscheme

photocatalyst. Therefore, the formation of the direct Zscheme system instead of the typeII heterojunction between TiO2 and gC3N4 can be confirmed.

3.2.2. ·O2− Radical Trapping Test

Similarly, the electrongeneration efficiency of the photocatalyst can be examined by investigating the generation of ·O2− radical ions, which are normally determined by electron spin resonance (ESR) spectroscopy. It should be noted that the ·O2− radical ions are not directly detectable by the ESR spectrometer. Therefore, 5,5dimethylpyrroline Noxide (DMPO) is used to react with ·O2− to form DMPO·O2−, which has obvious ESR characteristic peaks. For instance, the ESR signals of DMPO·O2− produced by gC3N4, Ag3PO4/gC3N4, and Ag3PO4 under 30 s light irradiation in methanol are shown in Figure 7a.[56] Basically, six ESR characteristic peaks of DMPO·O2− can be found for pure gC3N4 and the Ag3PO4/gC3N4 composite. Meanwhile, no peak can be observed in the ESR spectra of Ag3PO4 and the blank test, indicating that no ·O2− is produced for these samples. The results indicate that the photogenerated electrons mainly accumulate in the gC3N4 of the Ag3PO4/gC3N4 composite which has sufficiently high reduction potential to produce ·O2−. Therefore, the migration of photogenerated electrons in the Ag3PO4/gC3N4 follows the direct Zscheme instead of the typeII heterojunction transfer mechanism (Figure 7b).

3.3. Metal Loading

The photodeposition of metal NPs is widely applied to directly confirm the reduction site of photocatalytic system, thus providing information on the migration pathway of charge carriers. In detail, metal precursors such as HAuCl4, H2PtCl6, and AgNO3 are firstly dissolved in water to create a metalion solution. Under light irradiation, these metal ions will be reduced into metal NPs and in situ deposited at the electronrich reduction site of the photocatalytic system by reacting with the photogenerated electrons according to Reaction (9):

+ →+ − 0M ne Mn

(9)


Figure 6. a) Photoluminescence (PL) spectra of the TiO2/g-C3N4 under different light irradiation periods in the NaOH solution (2 × 10−3 m) in the presence of terephthalic acid (5 × 10−4 m), and b) comparison of the redox potential of TiO2 and g-C3N4 for producing ·OH radicals and ·O2− radicals. Reproduced with permission.[47] Copyright 2013, The Royal Society of Chemistry.

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Therefore, the accumulation of photogenerated electrons on a specific semiconductor of the direct Zscheme photocatalyst can be confirmed. For example, Xu et al. applied the metal photodeposition method to confirm the electronmigration pathway for the rutile/anatase TiO2 direct Zscheme photocatalyst.[60] As shown in the transmission electron microscopy (TEM) and highresolution TEM images (Figure 8), the Pt NPs are selectively loaded on rutile TiO2, indicating that the photogenerated electrons transfer to rutile TiO2 instead of anatase TiO2, which is consistent with the electronmigration mechanism of the direct Zscheme photocatalyst.

3.4. XPS Characterization

XPS characterization has been extensively used to determine the surface chemistry of materials. Recently, the application of the XPS characterization has been extended to investigate the changes in the electronic density on the different surfaces of a photocatalyst through investigating the shift in the binding energies. In detail, the introduction of a foreign material on a semiconductor can cause a shift in the binding energy of a specific element of the semiconductor if the electron migration occurs on its surface. Specifically, a positive shift in the binding energy indicates a decrease of the electron density, whereas a negative shift indicates an increase in the electron density. Therefore, the shift of binding energy in the XPS spectra can be utilized for determining the electronmigration pathway for a direct Zscheme photocatalyst. For instance, Peng and coworkers confirmed the formation of a direct Zscheme heterojunction between gC3N4 and ZnO through XPS characterization.[62] The highresolution XPS spectra of N 1s of gC3N4 and the ZnO/gC3N4 composite are shown in Figure 9a, and those of O 1s of ZnO and the ZnO/gC3N4 composite are shown in Figure 9b. Compared with pure gC3N4, the N 1s states of the ZnO/gC3N4 composite were found to shift toward the lowerenergy region (Figure 9a). Meanwhile, the O 1s states of the ZnO/gC3N4 composite shifted toward the higherenergy region in comparison with that of pure ZnO (Figure 9b). This result indicates the migration of electrons from ZnO to gC3N4 on the ZnO/gC3N4 composite. This migration mechanism of electrons is attributed to the formation of a weak internal electric field between the ZnO and the gC3N4. Specifically, the Fermi level of gC3N4 is higher than that of ZnO (Figure 9c). When ZnO is coupled with gC3N4, electrons on the gC3N4 will migrate to ZnO to obtain a Fermi level equilibrium, leaving a positive charge on the gC3N4 (Figure 9d). Thus, the interface between the ZnO and the gC3N4 is charged, thereby creating a weak internal electric field at the interface. During XPS characterization, both the ZnO and the gC3N4 are photoexcited, and the photogenerated electrons will migrate from the ZnO to the gC3N4 under the influence of the internal electric field (Figure 9e). This migration mechanism of the photogenerated electrons is in accordance with the direct Zscheme mechanism. Therefore, the formation of the direct Zscheme instead of a typeII heterojunction between the ZnO and the gC3N4 can be confirmed.


Figure 7. a) Electron spin resonance (ESR) spectra for DMPO-·O2− signal of different samples under 30 s light illumination, and b) schematic illus-tration for the charge-carrier transfer mechanism for a Ag3PO4/g-C3N4 type-II heterojunction and a direct Z-scheme photocatalyst. Reproduced with permission.[56] Copyright 2015, The Royal Society of Chemistry.

Figure 8. a,b) TEM (a) and high-resolution TEM (b) images of the rutile/anatase TiO2 direct Z-scheme photocatalyst with the loading of Pt NPs. Reproduced with permission.[60] Copyright 2014, Elsevier B.V.



3.5. Effective-Mass Calculation

Supplementary to the above experimental methods, theoretical simulation is also a powerful method to evaluate the chargecarriermigration mechanism in a photocatalytic system. Generally, firstprinciples simulation based on a density functional theory (DFT) calculation is carried out to analyze the chargecarrier generation and migration for a specific photocatalyst by analyzing the effective masses of its charge carriers. For example, Yu et al. demonstrated that theoretical material simulation is an effective method for investigating the formation of the direct Zscheme system for a ZnO/gC3N4 composite.[62] In detail, the electronicband structure of ZnO and gC3N4 were firstly attained using firstprinciples simulation based on DFT calculation (Figure 10). Then, the effective masses of the charge carriers of the ZnO and the gC3N4 can be respectively calculated via the parabolic fitting of their CB minimum and VB maximum according to Equation (10) and (11):

= −(d /d )* 2 2 2 1m E k (10)

= / *v k m (11)

where ħ, d2E/dk2, k, m*, and v respectively represent the reduced Planck constant, the coefficient of the secondorder term in a quadratic fit of E(k) curves for the band edge, the wave vector, the effective mass of a charge carrier, and the transfer rate of a charge carrier. Generally, the smaller the charge carrier’s effective mass, the faster the chargecarrier transfer. In order to give a clearer image of the chargecarrier separation efficiency for the ZnO/gC3N4 nanocomposite, the separation rate of the photogenerated holes to the electrons can be also calculated based on Equation (12):

= /* *D m mh e (12)

The calculated effective masses of electrons and holes and their separation rate in both the GZ and the GF directions in the reciprocal space are given in Table 2. It is obvious that the effective mass of the electron in the GZ direction of ZnO (0.034) is much less than that of gC3N4 (3.9), indicating that the photogenerated electrons of the ZnO in the GZ direction show higher tendency to transfer than those of the gC3N4 at the ZnO/gC3N4 interface. These simulation results indicate that chargecarrier migration for ZnO and gC3N4 is intrinsically feasible and follows the Zscheme mechanism instead of the typeII heterojunction mechanism.

3.6. Internal Electric-Field Simulation

Other than the effective masses of the photogenerated charge carriers, the surface energy and the interface formation energy, etc. are also found to have a great impact on the photocatalytic performance of a direct Zscheme photocatalyst. For instance, Liu et al. found that the formation of an internal electric field between the gC3N4 and the TiO2 is beneficial for facilitating the direct Zscheme chargecarrier transfer efficiency.[67] In their study, the TiO2 (100) surface was chosen for constructing a TiO2/gC3N4 heterostructure due to its high reactivity. The 4 × 2 unit of monolayer gC3N4 and 4 × 1 unit of the TiO2 (100) surface were created for simulating the structure of the gC3N4/TiO2 heterostructure (Figure 11a–c). The interface formation energy of the TiO2/gC3N4 was calculated to be −1.16 eV. The negative value of the interface formation energy reveals that the TiO2/gC3N4 has a stable interface, indicating that


Figure 9. a,b) High-resolution XPS spectra of the N 1s for g-C3N4 and ZnO/g-C3N4 (a) and of O 1s for ZnO and ZnO/g-C3N4 (b). c–e) Schematic illustration of the Fermi level of ZnO and g-C3N4 (c), formation of the weak internal electric field on ZnO/g-C3N4 (d), and charge-carrier separation on the ZnO/g-C3N4 (e). Reproduced with permission.[62] Copyright 2015, The Royal Society of Chemistry.

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chargecarrier migration can be easily achieved between TiO2 and gC3N4. Finally, the threedimensional chargedensity difference was calculated to give a clear picture of the chargecarrier migration for the TiO2/gC3N4 (Figure 11d). It is obvious that the interface between the gC3N4 and the TiO2 is the center of the chargecarrier redistribution. Meanwhile, a negligible change of the charge density can be observed in the interior of the TiO2 due to the weak interface formation energy between the gC3N4 and the TiO2. The planaraveraged chargedensity difference along the Z direction is also given (Figure 11e). The electrons on the gC3N4 tend to migrate to TiO2 via the TiO2/gC3N4 interface, leaving holes on the gC3N4. The chargecarrier diffusion between the TiO2 and the gC3N4 will continue until the equilibrium state of the system is obtained. As a result, the net chargecarrier accumulation causes the formation of an internal electric field at the

TiO2/gC3N4 interface (Figure 11e). Upon light irradiation, the photogenerated electrons will migrate to the gC3N4 under the influence of this internal electric field (see Figure 11f). The formation of the internal electric field is beneficial for accelerating the chargecarrier separation across the TiO2/gC3N4 interface according to the direct Zscheme mechanism.

4. Application of Direct Z-Scheme Photocatalysts

4.1. Photocatalytic Pollutant Degradation

Thousands of different organic pollutants enter the air, soil, and water everyday as a result of daily human activities.[68–70]


Figure 10. a,b) The crystal structures of ZnO (a) and g-C3N4 (b), where the red, light gray, blue, and dark-gray balls represent oxygen, zinc, nitrogen, and carbon atoms, respectively. c,d) Electronic band structures of ZnO (c) and g-C3N4 (d). Reproduced with permission.[62] Copyright 2015, The Royal Society of Chemistry.

Table 2. The effective masses of the photogenerated electrons and holes of ZnO and g-C3N4 calculated through parabolic fitting of the CB minimum and VB maximum along a specific direction in the reciprocal space (see Figure 10c,d); mh

*, me*, and D represent the effective mass of

holes and electrons, and mh*/me

*, respectively.

Species Effective mass G-Z direction G-F direction

ZnO mh* 0.72 10.3

me* 0.034 0.9

D 20.9 11.5

g-C3N4 mh* 29 0.64

me* 3.9 0.41

D 7.4 1.6

Figure 11. a–c) Schematic illustration for the 4 × 2 unit of monolayer g-C3N4 (a), 4 × 1 unit of the TiO2 (100) surface (b), and structure of g-C3N4/TiO2 composite (c). d,e) The simulated three-dimensional elec-tron-density difference (d) and planar-averaged electron density differ-ence (e) for g-C3N4/TiO2, where the yellow and cyan areas respectively indicate electron accumulation and depletion. f) Schematic illustration of the charge-carrier migration mechanism on g-C3N4/TiO2 under the influence of the internal electric field. Reproduced with permission.[67] Copyright 2016, The Royal Society of Chemistry.



These organic pollutants can greatly harm the environment and cause negative effects on human health. In order to eliminate these pollutants from the environment, various technologies have been proposed, including photocatalytic degradation,[71,72] biological degradation,[73] physical adsorption,[6,74,75] and filtration.[76,77] Among them, semiconductorbased photocatalysis has captured substantial attention because of its ability to utilize sustainable solar energy for degradation of organic pollutants without causing any side effects to the environment. Although various semiconductors have been studied for photocatalytic degradation of pollutants, their photocatalytic performance remains unsatisfactory.[35,78,79] Therefore, researchers have made enormous efforts toward developing novel photocatalytic systems with high photocatalytic activities. Particularly, the development of direct Zscheme photocatalysts has shown potential for use in enhancing the efficiency of photocatalytic pollutant degradation through the promotion of the separation of photogenerated electron–hole pairs and maximizing the redox potential of the photocatalytic system.

For example, our group reported a TiO2/gC3N4 direct Zscheme photocatalyst for photocatalytic HCHO decomposition.[47] The TiO2/gC3N4 direct Zscheme photocatalyst was prepared by a simple calcination route using P25 and urea as the TiO2 and gC3N4 sources, respectively. Upon calcination, urea was polycondensed into gC3N4 and uniformly deposited onto the surface of the TiO2 (Figure 12a,b). The thermogravimetric analysis was carried out to confirm the actual loading content of gC3N4 in the TiO2/gC3N4 composite. As shown in Figure 12c, the gC3N4 can be fully decomposed at ca. 600 °C in

air. Meanwhile, the TiO2 exhibited negligible weight loss under similar conditions. Therefore, the actual gC3N4 loading content can be simply estimated by calculating the weight differences of the samples before and after calcination at ca. 600 °C in air. According to this proposed method, the actual loading contents of gC3N4 on U20, U100, U200, and U500 were respectively estimated to be 3, 12, 18, and 26 wt%. Then, the photocatalytic performance of the samples was investigated by the photocatalytic decomposition of HCHO in air. It was found that the photocatalytic HCHO decomposition efficiency of the TiO2 increased as the content of gC3N4 increased from 0 to 12 wt% (Figure 12d). According to the ·OH radical trapping test, this is attributed to the formation of a direct Zscheme heterojunction between the gC3N4 and the TiO2 (Figure 12e). However, further increase of the loading content of gC3N4 caused a decrease in the photocatalytic performance of the samples. This is because overloading of the gC3N4 caused a shielding effect on the TiO2 and inhibited the contact between the TiO2 and the reactant (Figure 12f). This work suggested that careful tuning of the content of an individual semiconductor in a direct Zscheme photocatalyst is crucial to optimize its photocatalytic performance.

Furthermore, Zhu et al. reported the gC3N4/Ag2WO4 direct Zscheme photocatalyst for photocatalytic methyl orange degradation.[80] The gC3N4/Ag2WO4 was prepared through the in situ precipitation method (Figure 13a). Specifically, gC3N4 was firstly dispersed into the water to form negatively charged gC3N4. Then, AgNO3 was added into the gC3N4 suspension, and the dissolved Ag+ ions could be easily anchored on


Figure 12. a,b) Low-resolution (a) and high-resolution (b) TEM images of optimized g-C3N4/TiO2 (U100), in which the g-C3N4 loading content on the g-C3N4/TiO2 was simply tuned by changing the urea/P25 weight ratio from 0 (U0), 20 (U20), 100 (U100), and 200 (U200), to 500 (U500). c) Ther-mogravimetric analysis (TGA) spectra of the U0, U100 and g-C3N4. d) Comparison of photocatalytic formaldehyde degradation performance of U0, U20, U100, U200, U500, and g-C3N4. e,f) Schematic illustration for charge-carrier separation on the U100 (e) and U500 (f) samples. Reproduced with permission.[47] Copyright 2013, The Royal Society of Chemistry.

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gC3N4 due to the electrostatic attraction between the Ag+ ions and the negatively charged gC3N4. When Na2WO4 was introduced into the suspension, the Ag+ ions can react in situ with the WO4

2− ions to form Ag2WO4. Therefore, the Ag2WO4 NPs

can be uniformly anchored on the surface of the gC3N4 (Figure 13b,c), providing a greater number of surface active sites for the photocatalytic reaction. After loading of Ag2WO4, the PL fluorescence intensity of the gC3N4 was greatly reduced (Figure 13d), indicating the high electron–hole separation rate for the gC3N4/Ag2WO4. This result is attributed to the formation of a direct Zscheme heterojunction between the gC3N4 and the Ag2WO4, which can spatially separate the photogenerated electrons and holes. As a result, the photocatalytic performance of the gC3N4 for degradation of methyl orange was greatly enhanced by the loading of Ag2WO4 (Figure 13e).

Thereafter, Jo and Natarajan systematically investigated the effect of the TiO2 morphology on the photocatalytic performance of the direct Zscheme gC3N4/TiO2 for the degradation of isoniazid.[81] The gC3N4 was coupled with TiO2 NPs and TiO2 NTs via the wetness impregnation method to respectively form TiO2 NPs/gC3N4 and TiO2 NTs/gC3N4 composites (Figure 14). The photocatalytic performance of the TiO2 NPs/gC3N4 and the TiO2 NTs/gC3N4 is better than that of pure gC3N4. According to the radical trapping test, this is due to the formation of a direct Zscheme heterojunction between the gC3N4 and the TiO2, which can reduce the electron–hole recombination rate and maximize the redox potential of the photocatalyst. Moreover, the photocatalytic performance of the TiO2 NTs/gC3N4 was higher than that of the TiO2 NPs/gC3N4 because the TiO2 NTs/gC3N4 has a larger specific surface area than TiO2 NPs/gC3N4 to provide a greater

number of surface active sites for photocatalytic reaction. This work demonstrated that the rational tuning of the morphology of individual semiconductor in a direct Zscheme photocatalyst is important for enhancing its photocatalytic performance.


Figure 13. a) Schematic illustration for the growth mechanism of the g-C3N4/Ag2WO4. b,c) TEM images of the g-C3N4/Ag2WO4. d) PL spectra of the g-C3N4 and g-C3N4/Ag2WO4. e) Comparison of the photocatalytic performance of the g-C3N4, Ag2WO4 and g-C3N4/Ag2WO4. Reproduced with permission.[80] Copyright 2017, Elsevier B.V.

Figure 14. a,b) TEM images of TiO2 NPs/g-C3N4 (a) and TiO2 nanotubes (TiO2 NTs)/g-C3N4 (b) direct Z-scheme photocatalysts. Reproduced with permission.[81] Copyright 2015, Elsevier B.V.



Additionally, exposed facets on the semiconductor can also have great influence on the photocatalytic pollutant degradation performance of the direct Zscheme photocatalyst. For instance, Huang et al. systematically investigated the effect of exposed {001} and {101} facets of TiO2 on the photocatalytic performance of the TiO2/gC3N4 direct Zscheme photocatalyst.[51] They prepared TiO2 nanosheets (NSs) and TiO2 nanoboxes (NBs), which were coupled with gC3N4 to respectively form TiO2 NS/gC3N4 and TiO2 NB/gC3N4 (Figure 15a,b). The TiO2 NS contacted with the gC3N4 mainly through the {001} facets (Figure 15a,c), whereas the special structure of the TiO2 NBs caused TiO2 to mainly contact with gC3N4 through the {101} facets (Figure 15b,d). Under light irradiation, photogenerated electrons and holes on TiO2 accumulate at the {101} and {001} facets, respectively, due to the presence of the surface heterojunction between the {101} and {001} facets of the TiO2.[82]

Then, the photogenerated electrons that are accumulated at the {101} facets of the TiO2 NBs can easily migrate to the gC3N4 for a reduction reaction according to the direct Zscheme migration mechanism. Meanwhile, the photo generated holes remain on the {001} facets of the TiO2 NBs for an oxidation reaction. As a result, the electron–hole separation across the TiO2 NB/gC3N4 is much faster than that of the TiO2 NS/gC3N4, and TiO2 NB/gC3N4 showed the highest photocatalytic performance among all the prepared samples for degradation of Brilliant Red X3B (Figure 15e). This work showed that appropriate interface engineering of the exposed facet of an individual semiconductor of a direct Zscheme photocatalyst can greatly improve the electron–hole separation efficiency, thereby enhancing the photocatalytic performance.

Other than TiO2 and gC3N4, other semiconductorbased direct Zscheme photocatalysts have also been studied for photo catalytic pollutant degradation. For instance, Jo et al. reported a ZnIn2S4 marigold flower/Bi2WO6 direct Zscheme photocatalyst for photocatalytic degradation of metronidazole (MTZ).[54] The ZnIn2S4 marigold flower was prepared by a simple hydrothermal method. During the hydrothermal reaction, ZnIn2S4 NPs were firstly formed (Figure 16a), which further grew into a nanopetal structure. Thereafter, the ZnIn2S4


Figure 15. a,b) TEM images of TiO2 nanosheets (TiO2 NS)/g-C3N4 (a) and TiO2 hollow nanoboxes (TiO2 NB)/g-C3N4 (b). c,d) Schematic illustration of the contact interface between TiO2 and g-C3N4 on TiO2 NS/g-C3N4 (c) and TiO2 NB/g-C3N4 (d). e) Comparison of the photo-catalytic performance of the prepared samples for degradation of Brilliant Red X3B. Reproduced with permission.[51] Copyright 2015, Elsevier B.V.

Figure 16. a) Schematic illustration for the growth mechanism of ZnIn2S4 marigold flowers. b,c) Field-emission scanning electron microscopy (FESEM) images of the ZnIn2S4 marigold flower (b) and ZnIn2S4 marigold flower/Bi2WO6 direct Z-scheme photocatalyst (c). d) Light-absorption spectra of the prepared samples. Reproduced with permission.[54] Copy-right 2016, The Royal Society of Chemistry.

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nanopetals curled up under high temperature and pressure. Finally, these curled ZnIn2S4 nanopetals selfassembled and formed a hierarchical flowerlike structure (Figure 16b). After obtaining the ZnIn2S4 marigold flowers, the ZnIn2S4 marigold flower/Bi2WO6 direct Zscheme photocatalyst was obtained by wetimpregnation of the ZnIn2S4 marigold flower with a precursor of Bi2WO6 (Figure 16c). After coupling with ZnIn2S4, it was found that the lightabsorption spectrum of the Bi2WO6 was redshifted, indicating the intimate interfacial interaction between the Bi2WO6 and the ZnIn2S4 (Figure 16d). The photocatalytic MTZ degradation performance of the optimized ZnIn2S4 marigold flower/Bi2WO6 direct Zscheme photocatalyst was higher than that of pure ZnIn2S4 and Bi2WO6. This enhanced photo

catalytic activity is due to the enhanced chargecarrier separation efficiency and optimized redox potential of the direct Zscheme ZnIn2S4 marigold flower/Bi2WO6 photocatalyst.

Recently, graphenebased materials have attracted wide attention from the scientific community due to their ultralarge specific surface area and high chemical and physical stability.[83,84] Therefore, graphenebased materials such as graphene oxide (GO) have been employed, coupled with other semiconductors, forming direct Zscheme photocatalysts. For example, our group prepared Ag2CrO4/GO direct Zscheme photocatalysts through a simple selfassembly precipitation method for degradation of methylene blue (Figure 17a).[50] In detail, negatively charged GO was dispersed into a AgNO3


Figure 17. a) Schematic illustration for the formation mechanism of the Ag2CrO4/graphene oxide (GO) direct Z-scheme photocatalyst. b,c) SEM (b) and TEM images of the optimized Ag2CrO4/GO direct Z-scheme photocatalysts (G1), in which Ag2CrO4 loaded with different GO contents: 0 (G0), 0.5 (G0.5), 0.75 (G0.75), 1 (G1), 2 (G2) and 3 wt% (G3) was prepared. d) Electrochemical impedance spectra of the prepared samples. e) Comparison of the photocatalytic performance of the prepared samples and nitrogen-doped TiO2 (N-TiO2) for degradation of methylene blue. Reproduced with permission.[50] Copyright 2015, Elsevier B.V.



solution, and the Ag+ ions were adsorbed on the negatively charged GO due to electrostatic attraction. Then, a K2CrO4 solution was added into the above suspension, and K2CrO4 in situ reacted with Ag+ ions on the negatively charged GO to create the Ag2CrO4/GO direct Zscheme photocatalysts. As shown in the scanning electron microscopy (SEM) and TEM images (Figure 17b,c), Ag2CrO4 particles are successfully loaded on the GO surface, indicating the strong interaction between the Ag2CrO4 and the GO. Moreover, after coupling with the GO, the electron–hole separation efficiency of the Ag2CrO4 is greatly enhanced due to the formation of a direct Zscheme heterojunction between the Ag2CrO4 and the GO (Figure 17d). As a result, the photocatalytic performance of the optimized Ag2CrO4/GO was 3.5 times higher than that of pure Ag2CrO4 for degradation of methylene blue due to the improved electron–hole separation efficiency and the optimized redox potential of the Ag2CrO4/GO composite (Figure 17e). This work demonstrated that graphenebased material can be a potential candidate for constructing advanced direct Zscheme photocatalysts.

4.2. Photocatalytic Hydrogen Production

Recently, hydrogen, a clean and highheatvalue energy carrier, has received great attention due to depletion of fossilfuel energy sources and their adverse effects on the environment.[85–87] Photocatalytic water splitting for H2 production is an ideal strategy for the generation of H2 because of its simplicity and clean reaction,[88–90] in which only the photocatalyst, sunlight, and water are required to produce H2 at room temperature.[29,91] Therefore, many semiconductorbased photocatalysts have been studied and applied in photocatalytic H2 production.[92–95] Although this solarenergy conversion strategy exhibits many advantages, H2 production through photocatalytic water splitting is still far from practical application. This is mainly due to the high electron–hole recombination rate on the photocatalysts.[96–99] Building a direct Zscheme photocatalyst can effectively suppress the electron–hole recombination and isolate the reduction and oxidation sites of the photocatalytic reaction. Therefore, various direct Zscheme photocatalysts have been studied for enhancing the efficiency of the photocatalytic production of H2.

For example, Mukhopadhyay et al. reported hierarchical ZnO/CdS nanocomposites with enhanced photocatalytic watersplitting performance.[59] ZnO nanorods and mesoporous ZnO nanoflowers were firstly prepared through sequential sonochemical and hydrothermal methods. Then, the prepared ZnO nanorods and mesoporous ZnO nanoflowers were coupled with CdS NPs to form ZC1 and ZC2 direct Zscheme photocatalysts, respectively (Figure 18a–d). It was found that the lightabsorption range of both ZnO nanorods and mesoporous ZnO nanoflowers was extended into the visiblelight range by the loading of CdS due to the relatively small bandgap value of CdS (Figure 18e). The extended lightabsorption range of the samples is beneficial for harvesting both UV and visible wavelengths of the incident solar radiation. The PL intensities of the ZC1 and ZC2 were lower than that of pure ZnO,

indicating that the electron–hole separation efficiencies of ZC1 and ZC2 are higher than that of pure ZnO. This is because the electron–hole separation efficiency of ZnO can be greatly enhanced by the CdS NPs through the fluorescence resonance energy transfer (FRET) mechanism (Figure 18f,g). In detail, the photogenerated electrons in the CB of the ZnO can transfer to the VB of the CdS, thereby reducing the fluorescence of the ZnO. More interestingly, the electron–hole recombination rate of ZC1 is lower than that of ZC2 because the absorption–emission interaction between the ZnO and the CdS of ZC1 is higher than that of ZC2 (Figure 18h). This is mainly attributed to the large contact interface between the ZnO nanorods and CdS NPs, which can greatly accelerate the migration of the charge carriers across the interface between the ZnO and the CdS. As a result, the prepared ZC1 exhibited the highest photo catalytic H2 production performance among all the prepared samples.

Recently, Chen et al. demonstrated that the combination of doping and the construction of the direct Zscheme heterojunction is a feasible way to enhance the photocatalytic performance of TiO2.[27] Specifically, a visiblelight responsive TiO2/gC3N4 direct Zscheme photocatalyst was prepared through a simple onepot solvothermal route with the assistance of concentrated nitric acid, for photocatalytic H2 production. Upon C and N doping, the C and N codopedTiO2/gC3N4 (C–N TiO2/gC3N4) exhibited a redshifted lightabsorption band edge in comparison with the pure TiO2 NPs (Figure 19a), indicating the incorporation of C and N atoms into TiO2. The extended lightabsorption range of the samples is beneficial for enhancing their visiblelight reactivity. Meanwhile, the formation of the direct Zscheme heterojunction between C–N TiO2 and gC3N4 markedly improved their charge transfer and separation efficiency for photocatalytic reaction (Figure 19b,c). As a result, the photocatalytic H2 production performance of the optimized C–N TiO2/ gC3N4 (3 wt% C–N TiO2/gC3N4) composite was 10.9 and 21.3 times higher than those of CNTiO2 and pure gC3N4, respectively (Figure 19d). Doping is deemed to be a simple method for improving the light absorption of the individual semiconductors in a direct Zscheme photocatalyst to achieve its visiblelight activity.

Thereafter, our group found that direct Zscheme photocatalysts can also be constructed from a single semiconductor with the same chemical composition but different phases.[60] In detail, anatase/rutile biphase nanofibers with different rutile contents were prepared by slow and rapid cooling of calcined electrospun TiO2 nanofibers (Figure 20a–d). Both anatase and rutilephase TiO2 can be observed in the TEM images of the AR28 and AR45 samples (Figure 20b,d), indicating that the mixedphase TiO2 nanofiber was successfully prepared with large a specific surface area (Table 3). Then, Xray diffraction (XRD) characterization also further confirmed the presence of anatase and rutile phases in AR28 and AR45 (Figure 20e). The photocatalytic H2 production test was then performed to investigate the photocatalytic performance of the prepared samples. It was found that the photocatalytic H2 production activity of the AR28 and AR45 was higher than that of the AR0 and AR100 (see Figure 20f). According to the Pt NPs photodeposition test, this is due to the formation of a direct Zscheme heterojunction between the





Figure 18. a,b) TEM images of ZnO nanorod/CdS NP (ZC-1) (a) and ZnO nanoflower/CdS NPs composites (ZC-2) (b). c,d) High-resolution TEM images of ZC-1 (c) and ZC-2 (d). e) Light-absorption spectra of ZnO rods, ZnO NPs, ZC-1 and ZC-2. f,g) PL emission spectra of ZC-1 (f) and ZC-2 (g) excited at 345 nm, in comparison with the emission spectra of ZnO and CdS. h) Emission spectra of ZC-1 and ZC-2 excited at 390 nm in comparison with the emission spectra of the CdS NPs. Reproduced with permission.[59] Copyright 2015, The Royal Society of Chemistry.




anatase and rutilephase TiO2 of the AR28 and AR45 samples (Figure 20g). Specifically, the Pt NPs were selectively deposited on the rutilephase TiO2 during the photoreduction process (see Figure 8), suggesting the accumulation of photogenerated electrons on the rutilephase TiO2. This result confirms that the chargecarrier migration mechanism on the anatase/rutile biphase nanofibers obeys the Zscheme mechanism instead of the typeIIheterojunction migration mechanism. Moreover, it was found that the photocatalytic performance of the AR45 was higher than that of the AR28. This is because the balance between the rutile and anatase phases is beneficial for the spatial separation of photogenerated electron and hole pairs.

4.3. Photocatalytic CO2 Reduction

The everrising demand for fossil fuels and a collateral increase in the concentration of atmospheric CO2 have urged the development of carbonmanagement technologies.[100–103] To this end, much research has been devoted to searching for new technologies for the reduction of CO2, for example biological conversion,[104] electrocatalysis,[105] photocatalysis, and photothermal catalysis.[106] Among them, photocatalytic CO2 reduction into valuable chemical fuels such as CH4, CH3OH, HCOOH, and CH2O has received great attention because it can alleviate the current dependence on fossil fuels of human society, as well as reducing the CO2 concentration in the

atmosphere.[107–110] Notably, hydrocarbon fuels such as CH4 or CH3OH have a higher energy density than H2, making them easier for energy storage and more suitable for practical applications. Nevertheless, compared with other technologies, photo catalytic CO2 reduction for solar fuel production is still an enormous challenge because of its low solarenergyconversion efficiency, which is primarily attributed to the rapid electron–hole recombination rate on the semiconductor.[111–113] Thus, it is necessary to develop advanced photocatalysts with good electron–hole separation efficiency.[114–116] Recent studies have indicated that building direct Zscheme photocatalysts is one of the best strategies to prepare highly efficient photocatalysts for CO2 reduction.

For example, Yu et al. prepared the ZnO/gC3N4 direct Zscheme photocatalyst for CO2 reduction. The efficiency of the photocatalytic CO2 reduction of the prepared ZnO/gC3N4 was higher than that of commercial ZnO, labsynthesized ZnO, and gC3N4 for CH3OH production (Figure 21a).[62] According to the ·OH radical trapping test, this improved photocatalytic performance of the ZnO/gC3N4 is due to the formation of a direct Zscheme heterojunction between the gC3N4 and the ZnO, which can promote electron–hole separation and optimize the redox potential of the photocatalytic system. Moreover, theoretical simulations were also carried out to further confirm the formation of the direct Zscheme heterojunction between the gC3N4 and the ZnO. Specifically, the effective masses of the photogenerated electrons and holes of the gC3N4 and the ZnO

Figure 19. a) Light-absorption spectra of g-C3N4, 1 wt% C and N co-doped-TiO2/g-C3N4 (1 C–N TiO2/g-C3N4), 2 wt% C and N co-doped-TiO2/g-C3N4 (2 C–N TiO2/g-C3N4), 3 wt% C and N co-doped-TiO2/g-C3N4 (3 C–N TiO2/g-C3N4), 5 wt% C and N co-doped-TiO2/g-C3N4 (5 C–N TiO2/g-C3N4) and TiO2 NPs. b,c) Photocurrent response (b) and electrochemical impedance (c) spectra of the prepared samples. d) Comparison of the photocatalytic H2 production performance of the prepared samples. Reproduced with permission.[27] Copyright 2015, The Royal Society of Chemistry.




were estimated through the parabolic fitting to the CB minimum and VB maximum of the simulated band structure of the gC3N4 and the ZnO. It was found that the calculated effective

masses of the photogenerated electrons in the GZ direction of the ZnO (0.034) are much lower than those of the gC3N4 (3.9), indicating that the photogenerated electrons in the GZ direction of the ZnO have a higher tendency to transfer than those of the gC3N4. Therefore, the migration of electrons from the ZnO to the gC3N4 is more favorable than that from the gC3N4 to the ZnO. Both an ·OH radical trapping test and theoretical simulation results indicated that the enhanced photocatalytic performance of the ZnO/gC3N4 is due to the formation of a direct Zscheme heterojunction (Figure 21b,c).

Then, our group reported welldispersed CdS NPs grown on hierarchical WO3 hollow spheres for photocatalytic CO2 reduction.[61] It was found that the CdS NPs were successfully deposited on WO3 nanosheets (Figure 22a,b). The close contact between the CdS and the WO3 can accelerate the

Figure 20. a–d) SEM and TEM images of AR28 (a,b) and AR45 (c,d). e) XRD patterns of AR0 (I), AR28 (II), AR45 (III), and AR100 (IV). f) Comparison of the photocatalytic H2 production performance of the prepared samples. g) Schematic illustration of the charge-carrier migration for an anatase/rutile direct Z-scheme photocatalyst. Reproduced with permission.[60] Copyright 2014, Elsevier B.V.

Table 3. Physical properties of anatase/rutile bi-phase nanofibers with 0% (AR0), 28% (AR28), 45% (AR45), and 100% rutile (AR100) content.

Sample SBET [m2 g−1]

PVa) [m3 g−1]

APSa) [nm]

AR0 31 0.02 3.1

AR28 69 0.13 7.2

AR45 57 0.11 7.6

AR100 5 0.01 5.6

a)PV and APS respectively denote pore volume and average pore size.




migration of charge carriers between the CdS and the WO3 during the photo catalytic reaction (Figure 22b). Moreover, the CO2adsorption ability of the CdS/WO3 was higher than that of WO3 because the introduction of the CdS NPs can increase the specific surface area of the WO3. The improved CO2adsorption ability of the sample is beneficial for enhancing the CO2 concentration on its surface, thereby improving the CO2conversion efficiency (Figure 5b). Furthermore, the successful formation of the direct Zscheme photocatalyst was evidenced from the ·OH radical trapping test using a terephthalic acid probe molecule. In detail, the PL signals of the HTA of the WO3 and CdS/WO3 gradually increase with irradiation time (see Figure 22c), indicating that WO3 and CdS/WO3 have sufficient oxidation potential to produce ·OH radicals (Figure 22d). Meanwhile, no PL signal can be observed for the CdS, suggesting that CdS does not have sufficient oxidation potential to produce ·OH radicals. These results demonstrate that the photogenerated holes accumulate on the WO3 instead of the CdS, indicating the formation of a direct Zscheme heterojunction (Figure 5c). Moreover, it should be noted that PL signal of the CdS/WO3 is higher than that of WO3, indicating that a higher concentration of ·OH radicals was produced by the CdS/WO3. This is due to the faster electron–hole separation for the CdS/WO3. As a result, the photocatalytic performance of the CdS/WO3 was higher than that of pure CdS and WO3 for CO2 production (Figure 5b). This work demonstrates that the coupling of two different semiconductors can not only form a direct Zscheme

photocatalyst for enhancing their electron–hole separation efficiency but also improves their physicochemical properties for different functionalities.

5. Conclusion and Future Perspectives

Here, the historical development of the direct Zscheme photo catalyst from its 1st generation to the current 3rd generation has been outlined. Then, the recent advances of the direct Zscheme photocatalyst including characterization methods and applications have been discussed. It is conspicuous that the construction of direct Zscheme photocatalysts shows great potential in various photocatalytic applications ranging from environmental remediation to solar fuel production. However, the development of direct Zscheme photocatalysts is still at an early stage. Enormous challenges exist for fully exploring the potential of direct Zscheme photocatalysts. Therefore, future development in this field is urgently needed and should be focused in following directions.

Firstly, the physical and chemical properties of the individual semiconductor in a direct Zscheme photocatalyst should be optimized. Much work is needed to optimize the physicochemical properties of each semi

conductor in a direct Zscheme photocatalyst to enhance the photoconversion efficiency. These research efforts may include the preparation of advanced semiconductors with high visiblelight utilization for high solarenergyconversion efficiency, low chargecarriertransfer resistance for fast chargecarrier migration and good physical and chemical stability for longterm application.

Secondly, the contact interface between two semiconductors in the direct Zscheme photocatalyst must be rationally controlled. The chargecarrier separation across the interface between two semiconductors is crucial for optimizing the photo catalytic performance of a direct Zscheme photocatalyst. Therefore, maximizing and optimizing the contact interface between two semiconductors in a direct Zscheme photocatalyst can be an effective method to optimize their photocatalytic performance.

Thirdly, the photocatalytic performance and selectivity of a direct Zscheme photocatalyst can be further improved by building ternary or multicomponent photocatalytic systems. One viable way is to deposit suitable oxidation and reduction cocatalysts respectively on oxidation and reduction sites of the direct Zscheme photocatalyst in order to further enhance its electron–hole separation efficiency and photocatalytic performance.

Finally, the mechanisms of direct Zscheme photocatalysts remain largely unclear and must be extensively studied. Theoretical material simulation based on firstprinciples

Figure 21. a) Comparison of the photocatalytic CO2 reduction performance of commercial ZnO, ZnO, ZnO/g-C3N3, and g-C3N4 for CH3OH production. b,c) Schematic illustrations of the electron–hole pair separation mechanism on the type-II heterojunction (b) and the direct Z-scheme photocatalyst (c). Reproduced with permission.[62] Copyright 2015, The Royal Society of Chemistry.




DFT combined with various characterization methods such as photo catalytic testing, radicalspecies trapping tests, XPS characterization, etc. can be an effective strategy for providing a full image of the photocatalytic reaction for a direct Zscheme photo catalyst. The indepth understanding of the working mechanism of direct Zscheme photocatalysts can provide researchers with various opportunities for optimizing the chargecarrier separation efficiency and tuning the selectivity of the photocatalytic reaction toward different products.

We believe that the further expansion of direct Zscheme photocatalysts from these directions will greatly stimulate the application of photocatalysis in energy conversion and environmental remediation, thereby ensuring the sustainable development of human society.

AcknowledgementsThis review was partially supported by the 973 program (2013CB632402), NSFC (21433007, 51320105001, 21573170 and 51372190), the Fundamental Research Funds for the Central Universities (2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1 and 2016-KF-17), and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001).

KeywordsCO2 reduction, direct Z-scheme, hydrogen production, photocatalysts, pollutant degradation

Received: January 16, 2017Revised: February 15, 2017

Published online: April 18, 2017

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