异质同质相结高效光催化的研究进展以相变构建为例

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Chinese Journal of Catalysis 40 (2019) 796–818

催化学报 2019年第40卷第6期 | www.cjcatal.org

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Review (Special Column on the 11th National Conference on Environmental Catalysis and Eco-Materials)

Review on heterophase/homophase junctions for efficient photocatalysis: The case of phase transition construction

Kai Yang a,c, Xiaoxiao Li a, Debin Zeng a, Fanyun Chen a, Changlin Yu a,b,*, Kailian Zhang a, Weiya Huang a

School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China

b Key Laboratory of Petrochemical Pollution Process and Control, Faculty of Environmental Science and Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, Guangdong, China

c Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, Fujian, China

ARTICLE INFO

ABSTRACT

Article history:

Received 9 Novermber 2018 Accepted 26 December 2018 Published 5 June 2019 Keywords:

Phase transformation Phase junction Photocatalysis

Efficient charge transfer

Semiconductor photocatalysts are extensively applied in environmental treatment and energy con-version. However, one of their major disadvantages is their relatively low photocatalytic perfor-mance owing to the recombination of generated electron-hole pairs. The presence of the phasejunction is an effective way to promote the photocatalytic activity by increasing the separation effi-ciency of the electron-hole pairs. Accordingly, extensive research has been conducted on the design of phase junctions of photocatalysts to improve their charge transfer properties and efficiencies. Therefore, for the design of an appropriate phase junction and the understanding of the mechanism of electron-hole separation, the development of the photocatalytic phase junction, including the preparation methods of the heterogeneous materials, is tremendously important and helpful. Here-in, the commonly used, externally induced phase transformation fabrication techniques and the primary components of the semiconductors are reviewed. Future directions will still focus on the design and optimization of the phase junction of photocatalytic materials according to the phase transition with higher efficiencies for broadband responses and solar energy utilization. Addition-ally, the most popular phase transformation fabrication techniques of phase junctions are briefly reviewed from the application viewpoint.

1. Introduction

The efficient utilization of solar energy is a frontier research field in the 21st century with intense competition in various

countries. Accordingly, photocatalysis is a very attractive approach used for the utilization of solar energy [1–4]. Given that photocatalytic materials are usually faced with various problems, including narrow spectral responses, low-quantum

* Corresponding author. Tel/Fax: +86-797-8312334; E-mail: yuchanglinjx@163.com

This work was supported by the National Natural Science Foundation of China (21707055, 21567008, 216070), Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology, Program of 5511 Talents in Scientific and Technological Innovation of Jiangxi Province (20165BCB18014), Academic and Technical Leaders of the Main Disciplines in Jiangxi Province (20172BCB22018), Jiangxi Province Natural Science Foundation (20161BAB203090, 20181BAB213010, 20181BAB203018), Young Science Foundation of Jiangxi Province Education Office (GJJ160671), and Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201712) in Fuzhou University. DOI: S1872-2067(19)63290-0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 6, June 2019

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efficiency, poor stability, and recycling difficulties [2,5–7], considerable strides ought to be expended to achieve the desired environmental treatment and the production of clean and sustainable energy. The latter could be achieved by converting solar energy efficiently on a large scale and at a low cost utilizing multiple methodologies, including photocatalytic pollutant degradation [8–12], photocatalytic water splitting [13–18], and generation of valuable chemical fuels from photocatalytic CO2 reduction [19–21]. Therefore, the development of photocatalytic materials with wide spectral responses (corresponding solar spectrum), increased activity, and increased light stability plays key roles in improving the utilization efficiency of solar energy, and in the expansion of the use of photocatalysis.

At present, semiconductors that are used as photocatalysts mainly include metal oxides (TiO2 [22] and ZnO [23]), noble metal compounds (Ag3PO4 [24] and AgCO3 [25]), sulfides (CdS [26] and ZnS [27]), nonmetallic elementary substances (elemental phosphorus [28,29] and S8 [30]), and polymers (g-C3N4 [31]). This particular area has become a hot-spot research area in recent years. Among these types of semiconductors, metal oxides generally have a wide bandgap and can only use the sun’s ultraviolet light radiation, thus limiting their efficiency. Noble metal compounds are not suitable for large-scale applications because of their high cost. Sulfides or nonmetallic elements are prone to photo-corrosion and have poor stability. Additionally, the photo-generated electron and hole recombination rates of the polymers are high. The maximum energy conversion from solar energy is in the range of 1%–5% and does not meet the commercial requirement (≥10%) [32]. Development of highly active, stable, and inexpensive photocatalytic materials is a core task associated with the improvement of solar energy efficiency. Visible light absorption can be improved by doping or by surface modifications, but the photo-induced electron and hole recombination is still a serious issue in a single semiconductor.

In addition, a single semiconductor cannot have a narrower bandgap, a negative conduction band, and a more positive valence band at the same time. Correspondingly, the heterophase/homophase junction is the most commonly used way to improve the photocatalytic performance in semiconductors. It improves the photocatalytic activity in two ways: one is to widen the photoresponse range of the semiconductor materials, and the other is to improve the efficiency of the separated photogenerated electrons from holes.

2. Advantages and challenges of

heterophase/homophase junction in photocatalysts The construction of the phase junction is one of the methods used to effectively improve the photocatalytic activity and ameliorate the aforementioned problems associated with these materials. These materials can be classified (a) into three types (I, II, and III) [33] according to the relative position of the two semiconductor bands, and (b) into the p-n type and Z scheme [34–36] according to the different electron transfer pathways, as shown in Fig. 1. When different semiconductors, such as the p-type (holes being the majority charge carriers) or n-type (electrons being the majority charge carriers), come into close contact, a junction will be formed, and a potential difference will be established on both sides of the junction owing to the differences in their respective energy bands and other properties. The existence of the space potential difference can make the photo-generating carrier inject charged particles from one semiconductor energy level to another semiconductor energy level, thus facilitating the separation of electrons and holes, and improving the efficiency of photocatalysis [4,37]. Considering the p-n junction as an example, when the two types of carriers coexist in the two regions, a thin p-n junction is formed at their boundary layer. Owing to the high concentration of holes in the p-type region

Fig. 1. Schematic representation of five types of heterojunction systems.

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and electrons in the n-type region, diffusion drives the motion of electrons and holes toward the junction. A spatially charged region is formed near the boundary of the p-n junction. The p-type region has a negative charge, while the n-type region has a positive charge, thereby forming a strong local electric field across the junction. Subject to the local electric field of the junction, charge accumulates at both ends of the junction to form a potential difference, which can then be used as a driving force to effectively separate the photo-induced charges [38–40].

In most cases, the construction of the phase junction requires special preparation technology and a good lattice mismatch [41–45]. Additionally, the nanosized semiconductor requirements for lattice matching are not stringent, and the nano-heterojunction interface, in conjunction with more reliable material selections and preparation methods, is extensively used. Nano-heterojunction materials have the advantage of the small sizes and particle confined effects combined with the characteristics of fast moving carriers, which can lead to unique properties that single-component nanomaterials or body phase heterojunctions do not have [46–48]. Nanocrystalline heterostructures are generally designed and used. For example, semiconductor nanocarbon and nanometal heterostructures can facilitate the rapid transfer of photogenerated electrons [49]. Therefore, considerable efforts have been expended in recent years on the design and fabrication of nanophase junctions for improving the photocatalytic activity. Although the heterojunction has an excellent performance, it is not composed of a single element, and its construction is based on a step-by-step reaction process. First, it is necessary to synthesize the two components of the heterojunction separately. The heterojunction is formed subsequently by using a second binding process. The reaction is complex, time consuming, and not environmentally friendly. Compared to the heterojunction, the homogenous junction is constructed with the same material, and based on the established phase transition, it can also achieve a highly efficient separation of photogenerated carriers. Homogenization does not require the introduction of other elements and has, thus, attracted considerable attention [50–52]. However, the formation of the homojunction is very difficult. Most of the homojunction structures are formed by semiconductors with different crystal phases in the phase transition process, such as anatase/rutile TiO2, α/β phase Ga2O3, or hexagonal/cubic CdS. Because of the similar chemical compositions, the band structures of these semiconductor materials cannot be easily modified. Therefore, only a few studies exist in the literature on the homojunction of homocrystalline materials.

Despite the existence of several review papers on phase junctions, such as semiconductor/semiconductor junctions, semiconductor/metal junctions, semiconductor/nonmetal junctions, and surface heterojunctions [4,14–15,47,53–59], an extensive review of the rational designs of phase junctions using fabrication methods, based on externally induced phase transitions, is still necessary to provide readers with an understanding of the current developments in this dynamic

research field. In this study, the current research status is discussed in relation to the progress of low-cost applications with the efficient use of constructed nanophase junctions in photocatalysis.

3. Fabrication strategies of heterophase/homophase junction photocatalysts

As mentioned above, the creation of heterophase/homophase structures on the photocatalysts plays important roles in their photocatalytic activities [60,61]. From the viewpoint of the structural origin, the heterojunctions/homojunctions can be categorized into the composite phase (consisted of two or three different phases), the in-situ growth composite phase, and the phase-, facet- and van der Waals type junctions of a single substance. The junctions of a single substance can be constructed from different crystalline phases of the same chemical composition, like anatase/rutile TiO2 [62], α/β-Bi2O3 [63], and the α/γ-Bi2O3 homojunction [], whereby different crystal facets of the single same semiconductor [65,66] are stacked on a layer-by-layer basis on two-dimensional materials [67,68]. 3.1. Heterophase junctions of two or three different phases Based on the recombination processes in different semi-conductors that aim to the formation of heterophase junction structures, the optical absorption threshold can be effectively expanded, and the separation efficiency of the photogenerated carriers can be improved. Semiconductor recombination can transfer photogenerated electrons from the conduction band with a higher conductance to the band with a lower conduct-ance so that the electrons and holes cannot be recombined, thus improving the photocatalytic activity of the photocatalyst. Numerous techniques have been developed to establish nanostructured heterophase junctions as well as the heteroge-neous photoelectrodes using facile, low-cost, and ener-gy-efficient methods, such as hydrothermal/solvothermal methods, electrochemical deposition, chemical bath deposition, sol-gel processes, and chemical precipitation [69–101]. Previ-ously, we reviewed some typical heterophase junctions [9], such as TiO2-based, Ag-based [100], ZnO-based, and met-al-sulfide-based heterophase junctions. Additionally, many heterojunction semiconductors have been studied by combin-ing two semiconductor types, as shown in Table 1. For example, in the CdS-TiO2 composite photocatalyst [103,104], the compo-site CdS is excited by visible light to produce photogenerated electrons and holes. Because the lower energy level of the con-duction band of CdS is higher than that of TiO2, the photogen-erated electrons will be transmitted to the conduction band of TiO2 through the two-phase interface, and the photogenerated holes will be left on the CdS, thus realizing the spatial separa-tion of the positions of the photogenerated charges. Two of the most important issues in the system are lattice matching and contact tightness. Chen et al. [104] found that the CdS-TiO2 obtained using a single-step method can result in a better in-terfacial contact performance than the two-step method. Thus,

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Table 1

Typical nano-heterophase junctions of photocatalysts. Nano-hetero-

Production of H2 from water splitting

phase junctions p-n type Cu2S/CdS [69], MoS2/CdS [70], CuO/ZnO [71]Z scheme Type I Type II WO3/g-C3N4 [72], ZnO/Cd/CdS [73], g-C3N4/Au/CdS [74], CdS/SiC [75] TiO2/CuO [76], CdSe/CdS [77], MoS2/g-C3N4

[78], CdS/ZnS [79] g-C3N4/g-C3N4 [80], g-C3N4/polypyrrole [81],

Generation of CxHyOz from CO2 reduction CuO/TiO2–xN [84], CuInS2/TiO2 [85]

Photocatalytic degradation

of pollutions

Ag2O and NaTaO3 [92], CuInSe2/ZnO

[93], BiOI/ZnO [94]

Ag3PO4/Ag/g-C3N4 [86], Fe2V4O13/RGO/CdS [87], Ag2CrO4/GO [95], C3N4/TiO2 [96]

TaON/Ag/Ru BLRu′ [88]

Bi2S3/CdS []

ZnO/g-C3N4 [90], Cu2O/TiO2 [91]

Sb2S3/TiO2 [97], TiO2/CuO [98] Bi2WO6/WO3 [99], BiOI/SnS2 [100],

g-C3N4/SrTiO3 [82], CdS/g-C3N4 [83]

higher photocatalytic activity was obtained. Inserting narrow bandgap semiconductors between layers of inorganic layered materials, such as CdS into H4Nb6O17, H2Ti4O9, KTiNbO5, or mi-croporous/mesoporous materials, can also realize the close contact of the catalyst interface and the directional transfer of charge, thus improving the effect of the photocatalytic charge separation.

3.2. In situ growth of phase junction

The properties of the phase junction interface determine the resistance of electron transfer. The ohmic contact can change the path of the electron transfer by optimizing the preparation or treatment conditions. Compared to this particular phase junction, the conductor transfer electron method is more pow-erful and can form a more reliable ohmic contact. In situ growth of phase junctions of semiconductor hetero-phase/homophase structures can be controlled by the micro-structural or surface morphologies, as shown in Fig. 2. The mo-tion patterns of the electrons and holes generated by photoex-citation are dependent on the crystallization and surface state of the materials [105–108]. For a single semiconductor, the microstructure or surface morphologies of the crystal can be controlled effectively, and the photoelectron and hole pairs can be separated along the internal or surface directions, thereby allowing the quantification of the photocatalytic activity.

Fig. 2. Nanoscale heterogeneity. (a) Mixed type, (b) Laminated type, (c)Core-shell type, (d) Coaxial type, (e) Surface dispersion type. Reprinted with permission from Ref. [110]. Copyright 2009 China National Knowledge Internet.

BiOI/ZnTiO3 [101]

In fact, for a semiconductor material, there are usually dif-ferent crystal forms, and the transition requires time if the re-action needs to be controlled at the time at which the phase transition occurs. A semiconductor material with a mixed phase can be obtained. It is worth noting that the band-edge positions of different crystal phases of uniform materials usu-ally change slightly because of the changes of the crystal lattice parameters. Therefore, \"in situ junctions\" can be produced by controlling the formation of different crystalline phases in the same substance. The most successful example of this theory is the formation of a surface heterogeneous junction between TiO2 anatase and rutile [57], which is further demonstrated by the α/β Ga2O3 surface junction [109].

The formation of \"in situ junctions\" can be achieved by con-trolling the surface morphology of the semiconductor. The abil-ity of the surface to regulate the electrons and holes will change owing to the arrangement of atoms, the state of the surface bonding, and the coordination condition. In other words, the density of electron states on different surfaces is not the same. Therefore, the generated electric potentials on adjacent crystal planes (surfaces) that drive the electrons and holes in the di-rection of the junction also vary. Accordingly, this can also achieve an effective spatial charge separation.

The methods described above mainly aim at the enhance-ment of the ability of photocharged migration on the surface of the catalyst. The actual acceleration point of the catalytic reac-tion should be the acceleration of the surface chemical reaction. Therefore, almost all of the above \"in situ junctions\" can be formed by combining homogenous and other (different) phas-es, or different homogenous surfaces. To improve the catalytic efficiency, it is necessary to accelerate the rate of the interfacial chemical reaction. As mentioned earlier, the reaction rate has increased to a new value by the introduction of sacrificial agents in the solution (loading Pt nanoparticles by the addition of chloroplatinic acid for hydrogen production). Correspond-ingly, it is necessary to introduce the co-catalyst by construct-ing the \"in situ Schottky junction\" [111,112]. Precious metal nanoparticles have a lower superpotential. Accordingly, the overpotential required for hydrogen production can be re-duced. Owing to the existence of the Schottky barrier, a single electron is transferred from the semiconductor to the metal nanoparticle, but cannot be recombined with photogenerated holes. The metal nanoparticles can act as the capture center of

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the electron, and the photogenerated charge is thus spatially separated [112].

3.3. In situ phase transformation (close interface)

The regulation and control of the crystal type includes two aspects: one of them is the achievement of the mixed crystal for the enhanced charge separation, and the other is the identification of the temperature-dependence law or the externally induced synthesis of the phase transition. Numerous facile, low-cost, and energy efficient techniques of phase transformations, such as ion doping [113,114], oxide regulations [115,116], wet chemical or solution-phase methods [117–120], pressure-induced phase transitions [121,122], calcination-induced phase transitions [123–125], self-induced self-surface phase junctions [126,127], pulse laser irradiation or UV/ozone-assistants [128,129], mechanical alloying [130], molten salt auxiliary [131], and electrochemistry methods [132,133], have been extensively developed to fabricate heterojunctions/homojunctions. A large number of experimental studies have defined some of these laws that possess increased significance for the rational design of the crystal structure, the guidance of the synthesis, and the optimization of the catalytic performance of the catalyst [134]. 3.3.1. Anionic and cationic regulations

Introducing a small number of anions into the semiconduc-tor photocatalyst will influence the phase transition of undoped semiconductors. For example, the introduction of small, low-valence anions (such as Cl–, F–) can promote the formation of the rutile phase in TiO2. However, the introduction of high-valence anions (such as PO43– and SO42–) with large radii is not conducive to the transformation of anatase to rutile [135,136]. The percentage of anatase in the surface region can increase from 2% to 75% with the increase of the content of SO42– in TiO2 from 0 to 3 wt%. In other words, smaller anionic valency makes the formation of rutile titanium dioxide more favorable. This also explains why the sulfuric acid process can-not easily generate rutile products, while the chlorination pro-cess can be easily executed in an industrial setting. In effect, when the doped ion radius is greater than or less than the Ti4+ radius, the doped ion enters the TiO2 matrix and generates lat-tice defects that improve the lattice energy. This energy should be released before the phase transition, which makes the ana-tase TiO2 more stable. Accordingly, this leads to the increase of the anatase-rutile phase transition temperature. However, it is worth noting that different phase structures of TiO2–SO42− samples can be obtained by this method at the same increased temperature (Fig. 3), thus resulting in activities that are six times higher than those of P25 in the photocatalytic H2 produc-tion of the reformation of methanol. In the chart on the right part of Fig. 3, small anatase particles were well dispersed on the large rutile particles to construct an intimate anatase-rutile phase junction, which is conducive to the efficient charge transfer.

The anions NO3−, HCO3−, PO43−, SiO32−, and MoO42− can be utilized to regulate the anatase-rutile phase of TiO2 [137]. Larger anions are more favorable to the formation of ana-tase-type titanium dioxide. Experiments by Devi et al. [138] confirmed that only anatase exists at 700 °C for TiO2 doped Mo6+, and the rutile phase appears at 700 °C for doped Mn2+. This may be owing to the fact that the doping of high-valence

Fig. 3. (a) Overall photocatalytic activity of H2 production and (b) surface-specific photocatalytic activity of H2 production of Pt/P25, Pt/P25-H2O-700, and Pt/P25-x%SO42–-700. The solid line in (b) indicates the surface area of the samples. (c) CO selectivities of Pt/P25, Pt/P25-H2O-700, and Pt/P25-x%SO42–-700 catalysts. (d) TEM image of typical agglomerates of nanoparticles, (e) fast Fourier transform of the HRTEM image, (f) HRTEM image and colorization of anatase (A, blue) and rutile (R, red) by reverse FFT based on the respective anatase and rutile spots observed in (e) (for interpretation of the references to different colors in this figure, the reader is referred to the web version of this article). Reprinted with permission from Refs. [136] and [119]. Copyright 2012 Royal Society of Chemistry, Copyright 2014 Elsevier.

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ions (Mo6+) makes the Fermi energy level drift upward, raises the surface barrier, narrows the charge region, and ensures an effective separation of the photogenerated electrons and holes in the presence of a strong electric field, thus enhancing the photodegradation effect. Liu et al. [119] found that N3– doping can promote the formation of the rutile phase to form hetero-junctions, reduce the bandgap of the TiO2 semiconductor, and expand the irradiation light from ultraviolet to visible light. Tian et al. [139] has studied the effect of S2– doping on the ratio of anatase to rutile. The results show that the higher the S2- doping is, the higher the ratio of anatase to rutile is, which is more favorable to the photocatalytic activity. Pichavant et al. [140] obtained the interfaced anatase-rutile photocatalysts by thermohydrolysis using very low levels (less than 0.5%) of Sn(IV) that acted as a rutile phase promoter, as shown in Fig. 3. This process is much more direct than the coprecipitation of the amorphous product and is followed by the hydrothermal treatment process.

The Sn(IV) content can accurately control the anatase-rutile ratio and the nanoparticles obtained (diameters of 10 nm) ex-hibit epitaxial relationships between the anatase (200) and rutile (210) planes, thus building porous submicronic compo-site agglomerates with increased specific surface area (200 m2/g). Su et al. [141] modified the Bi atoms on the BiVO4 sur-face by ZnO coating and etching treatments. Based on this coating process, the Zn doping on the surface lowered the Fer-mi level of BiVO4 and a BiVO4/Zn:BiVO4 homojunction with a type II band alignment was formed. The construction of the BiVO4 homojunction improved the charge transfer process and avoided the surface electron trapping, thereby exhibiting an enhanced PEC performance. Therefore, anionic and cationic doping can effectively regulate the phase transition and im-prove the photocatalytic activity of semiconductor photocata-lysts.

3.3.2. Oxide regulations

Introducing oxide in the photocatalyst can also regulate its phase transition and influence the photocatalytic activity. Jung et al. [142] showed that the addition of Al2O3 into TiO2 for the thorough transformation of anatase-rutile types has an obvious inhibitory effect, the calcining temperature is as high as 800 °C,

and anatase TiO2 is still the main crystal structure with a spe-cific spatial extent regarding the rutile phase. Li et al. [143] found that the addition of Nd2O3 has an effect on the surface phase of TiO2 and on the photocatalytic activity studied by UV Raman spectroscopy. They showed that the amount of Nd2O3 that was well dispersed onto the surface of Nd-TiO2 particles had a higher surface area and a wider optical response, and could tune the crystalline phase and composition of Nd-TiO2 calcined by high temperatures, inhibit the crystal size agglom-eration, and transform the anatase phase to the rutile phase. Additionally, Nd-TiO2 nanoparticles are more reactive in the photocatalytic degradation of RhB. Nonmetal oxide, such as SiO2, can also regulate and effectively control the crystal type and the grain growth of TiO2 and improve the catalytic perfor-mance. SiO2 composite particles can effectively improve the lifetime of TiO2 catalysts, and the TiO2–SiO2 composite has a larger specific surface area and is more acidic than TiO2. The superior photocatalytic performance may be attributed to the bridging oxygen structures of Ti–O–Si from TiO2 and SiO2. This led to increased specific surface area and surface defects, which is advantageous to the photogenerated electron-hole separa-tion, and promoted the photocatalytic reaction [144,145]. Fur-thermore, Ti–O–Si effectively inhibited the thorough phase transition of anatase to rutile, increased the stability of anatase, and prevented the accumulation of the TiO2 grain growth [146]. Periyat et al. [115] studied different silicon contents on the effect of the stability of the TiO2 anatase type. When the mass fraction of SiO2 was 15%, rutile appeared and was maintained up to a calcined temperature of 1000 °C.

3.3.3. Soft-chemical-route-induced phase junctions

Soft chemical route uses the circuitous steps to prepare the nanomaterials based on a slow reaction process in the presence of mild reaction conditions. Common soft chemical methods include the sol-gel, hydrothermal, microemulsion, low-temperature solution phase, vapor deposition, precursor, microwave-assisted synthesis methods, and others. Sun et al. [58] successfully prepared α-/γ-Bi2O3 homojunctions via a hy-drothermal method that exhibited a synergetic effect on the photocatalytic activity. The diffuse reflectance and impedance spectra determined the homojunction between the α-Bi2O3 and

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γ-Bi2O3. Based on these, they pointed out the mechanism of the charge transfer and the separation on the interface of α-/γ-Bi2O3. Bera et al. [147] and Shi et al. [59] also successfully synthesized the α/β-Bi2O3 homojunction photocatalyst with the use of novel solvothermal-calcination or one-pot hydrothermal methods. Correspondingly, the material exhibited exceptional and synergistic photocatalytic activity and a fairly stable per-formance for the removal of 17α-ethinylestradiol and rhoda-mine B from aqueous solutions. Fig. 4 shows a possible photo-catalytic mechanism and a clear phase junction between α-Bi2O3 and β-Bi2O3. It suggests that the approach is practicable toward the design and construction of heterogeneous photo-catalysts. Because of the different electronic structures of the layered 2H/1T-MoS2 with the semiconducting 2H (trigonal prismatic D3h) phase and the 1T (octahedral Oh) metallic phase, Wu et al. [148] reported earth-abundant 1T-MoS2 and graphitic C3N4 heterojunctions formed by a facile hydrothermal method in thioacetamide and molybdenum pentachloride. These allowed faster photogeneration of electrons for a drasti-cally improved photocatalytic hydrogen evolution activity compared to that of the 2H counterpart (Fig. 5). Furthermore, mixed-phase TiO2 powders with controllable anatase/rutile ratios could be easily controlled by the hydrothermal or sol-vothermal method, such as the pH [149,150], solvent type [151], and precursor [152]. For instance, Li et al. [149] reported anatase and rutile mixed-phase TiO2 photocatalysts by the tar-taric acid as a phase regulator in the hydrothermal environ-ment and found that the heterojunction catalyst with an ana-

tase to rutile content ratio of 77% to 23% exhibited the highest photocatalytic activity. Li et al. [152] also prepared under con-trolled conditions a pure phase and mixtures of four TiO2 pol-ymorphs (anatase, rutile, brookite, and TiO2(B)) based on hy-drothermal reactions in an acidic potassium titanate solution as the precursor. In Fig. 6, the acid concentrations of the HCl, HNO3, and H2SO4 solutions can change the compositions of the mixed-phase anatase/TiO2(B), rutile/brookite, and TiO2(B)/rutile/rookite hierarchical nanostructures. Accord-ingly, the samples with 59% anatase were obtained and led to the highest photocatalytic activity, which was 1.43 times that of P25. Similarly, Sun et al. [153] fabricated the heterostructured Bi2O2CO3/Bi2O4 photocatalysts by a facile one-pot hydrother-mal method, where Bi2O4 nanorods and particles were depos-ited on the surfaces of the Bi2O2CO3 plates, and the material exhibited remarkably enhanced activity compared to Bi2O2CO3 and Bi2O4.

3.3.4. Controlling calcination-induced phase junctions

When the semiconductors were heated, the atoms or ions in the lattices vibrated violently and broke away from the original lattice to form new nuclei. They gradually grew by accepting diffused material flow. The main factors that affected semicon-ductor phase transitions were the crystal structure, internal defects, morphology (particle size, porosity, and surface condi-tion), energy state of components, as well as the heating tem-perature, time, and atmosphere. According to this phenome-non, a theoretical model of semiconductor phase transition is

Fig. 5. (a) Photocatalytic hydrogen production rates for g-C3N4 and 0.20 wt% 2H-MoS2@g-C3N4 nanosheets and 0.20 wt% 1T-MoS2@ g-C3N4nanosheets; (b) Stability test outcomes of 0.20 wt% 1T-MoS2@g-C3N4 nanosheets over a period of 12 h in 4 h cycles; (c) Schematics showing the syn-thetic process of the 1T-MoS2@g-C3N4 nanosheets. Reprinted with permission from Ref. [148]. Copyright 2017 Royal Society of Chemistry.

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Fig. 6. (a) Phase diagram of TiO2 samples prepared in HCl, HNO3, and H2SO4 solutions based on hydrothermal reactions; (b) Photocatalytic activities of the samples with different anatase contents and P25 for MB decomposition. Reprinted with permission from Ref. [152]. Copyright 2012 Royal Society of Chemistry.

Fig. 7. Theoretical model of semiconductor phase transition explainingthe construction of heterojunctions.

proposed to construct the heterojunction, as shown in Fig. 7 [123]. Herein, owing to the control of thermodynamics and phase transition kinetics, the phase transition of semiconduc-tors usually occurs in a step-by-step manner. When the tem-perature of the system rises to Temp. 1, the first phase transi-tion takes place in the semiconductor. By controlling the phase transition rate, a two-phase coexistence system can be ob-tained. Because the phase transition occurs within itself, phases 1 and 2 can generate close contact heterostructures, and the heterostructures of the crystal phase 2/crystal phase 3 or crystal phase 3/crystal phase 4 may also be formed. Once the

appropriate amount of heterogeneous phase structure is suc-cessfully constructed, a potential difference may exist between the two types of semiconductor crystal phases with energy level matching, which will establish an internal electric field, accelerate the separation of photogenerated electrons and holes, and improve the activity and stability of the photocata-lytic reaction.

For instance, our group constructed binary [123,124] and ternary [154,155] semiconductor phase formation systems with heterogeneous phase junctions based on thermodynamics and phase transformation kinetics, and found the regulation of the crystal phase interface on photoelectrons (e–) and holes (h+). Thereinto, the phase transition law of Ag2CO3 was found, and the mechanism of phase transition was established to con-struct Ag2CO3@Ag2O. Thus, the silver-based photocatalyst was obtained with high-stability and high-photocatalytic efficiency, which established Ag2CO3 as a hotspot photocatalyst in recent years. Thermogravimetric analyses in Fig. 8a show that the weightlessness in the temperature range of 180 to 250 °C cor-responds to the phase transition of Ag2CO3 → Ag2O. Meanwhile,

Fig. 8. (a, b) TG curves of Ag2CO3 in air atmosphere; (c) Schematic of the Ag2CO3@Ag2O heterostructure according to the Ag2CO3 phase transition; (d) Photograph showing the color changes of different samples; (e) Energy-level structure of Ag2CO3@Ag2O; (f) Electron and hole transfer model diagram showing the surface plasmon resonance effect. Reprinted with permission from Refs. [123,124]. Copyright 2014 Wiley and 2016 Elsevier.

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the Ag2O phase was stable in the temperature range of 250 to 300 °C, and the Ag2O → Ag phase transition corresponded to the temperature range of 300 to 425 °C. Fig. 8b shows that the production of the Ag2CO3 phase transition was completely de-pendent on Ag2O at 210 °C and occurred over a period of 22 min. The Ag2CO3 surface began to change into Ag2O when it was heated, and the new phase of Ag2O was wrapped on the Ag2CO3 surface, which prevented the escape of CO2 and the Ag2O crys-tal growth and reduced the phase transition rate. The existence of the Ag2CO3@Ag2O heterocrystalline interface was confirmed by HRTEM. Therefore, the core-shell structure of Ag2CO3@Ag2O at different phase contents could be prepared by accurately controlling the heating parameters of Ag2CO3. The calculation of electronegativity proved that the constructed Ag2CO3@Ag2O had a matching energy level structure (Fig. 8e) that can gener-ate an internal electric field. With the use of visible light irradi-ation, the photogenerated holes of Ag2CO3 could easily migrate to the Ag2O valence band, while the electron of Ag2O could move to the conduction band of Ag2CO3, thereby accelerating the separation of h+ and e–. Based on the surface plasmon res-onance effect of precious metals (Fig. 8e), Au, Ag, Au, and Pt nanoparticles were deposited on the heterogeneous boundary plane and the activity of photocatalytic reaction was further improved [154].

The Ag2CO3@Ag2O heterojunction interface has increased the photocatalytic degradation efficiency for organic pollutants, such as dyes, phenol, bisphenol A, and others. In the presence of visible light irradiation, the photocatalytic activity of the highly concentrated methyl orange dye can be completely de-graded with mineralization. The efficiency of Ag2CO3@Ag2O is 67 times and 31 times higher than those of Ag2CO3 and Ag2O, respectively, and is much higher than that of the novel Ag3PO4 photocatalyst [24]. A silver-based photocatalyst is used as a noble metal photocatalyst and has a high cost in engineering applications. Additionally, the silver ion of the binary hetero-junction photocatalyst may be reduced to a simple mass by the photogenerated electrons produced by itself during the reac-tion process. Accordingly, the active component may be lost. Meanwhile, a Zn3(VO4)2/Zn2V2O7/ZnO ternary and cheap zinc vanadate photocatalyst system has been developed [155] whose photocatalytic activity was much higher than that of the Zn3(OH)2V2O7·2H2O precursor synthesized with the use of the microwave hydrothermal synthesis and the Zn2V2O7 monoclin-ic system calcined at 800 °C. The thermodynamic calculation shows that Zn3(OH)2V2O7·2H2O is a step-by-step phase transi-tion material (Zn3(OH)2V2O7·2H2O → Zn3(VO4)2/ Zn2V2O7/ZnO → Zn2V2O7), as shown in Fig. 9a. The double Z-scheme mecha-nism in Fig. 9b shows the nontraditional transport of pho-to-induced h+ and e− for the efficient removal of the pollutants. Based on this principle, more types of bismuth-based het-erogeneous phase junction catalysts were developed with in-creased photocatalytic performance, such as BiOI/Bi5O7I, Bi5O7I/Bi2O3, BiOBr/Bi24O31Br, and Bi24O31Br/α-Bi2O3 [156–158]. Other thermodynamically unstable substances, such as other series of heterogeneous photocatalysts of sil-ver-based semiconductors, bismuth oxyhalides, bismuth oxide, and bismuth carbonate, were systematically investigated by other groups [116,159–162]. For example, Huang et al. [1] prepared three-dimensional hierarchical bismuth oxyiodides with an in situ phase transformation process based on a con-trolled calcination strategy. Accordingly, BiOI microspheres were used as a self-sacrificed template for the construction of the phase junctions, while the formation diagram of the hier-archical Bi4O5I2-Bi5O7I hetero architecture was obtained as shown in Fig. 10. The authors confirmed that the activity order was Bi4O5I2-Bi5O7I > Bi4O5I2 > Bi5O7I > BiOI, which ascribed to the efficient charge separation and transfer that originated from band alignment of the as-formed phase junction. Hou et al. [165] prepared an α-β phase Bi2O3 nanowire heterojunction by a facile hydrothermal that was responsive to visible light, and the following heat treatment process and the reaction temper-ature could readily tailor the ratio of the monoclinic α-Bi2O3 and the tetragonal β-Bi2O3, and adjust the morphology from two-dimensional β-Bi2O3 sheets to the α-/β-Bi2O3 nanowire junction. The α-/β-Bi2O3 heterojunction had an exceptional photocatalytic performance compared to that of the β-Bi2O3 sheets in the degradation of rhodamine B and methyl orange in the presence of visible light irradiation that was attributed to

Fig. 9. (a) Construction of the Zn3(VO4)2/Zn2V2O7/ZnO ternary heterojunction according to the thermal analysis of Zn3(OH)2V2O7·2H2O; (b) Possible photocatalytic process of organic pollutants over the Zn3(VO4)2/Zn2V2O7/ZnO system. Reprinted with permission from Ref. [155]. Copyright 2018 Elsevier.

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Fig. 10. Photodegradation efficiency of (a) RhB and (b) BPA over calcinated BiOI at different temperatures; (c) Formation diagram of the hierarchical architectures; (d) Bi4O5I2-Bi5O7I phase junction. Reprinted with permission from Ref. [1]. Copyright 2017 Elsevier.

the efficient charge separation and transfer. Bian et al. [126] fabricated β-Bi2O3/Bi2O2.33@Bi2O2CO3 ternary composites based on the self-integration of the slow, thermal decomposi-tion of Bi2O2CO3. Equivalently, Bi2O2CO3 → β-Bi2O3/Bi2O2.33@Bi2O2CO3 (300 °C) → β-Bi2O3/Bi2O2.33(320 °C) → β-Bi2O3 (350 °C). It was found that β-Bi2O3/Bi2O2.33@Bi2O2CO3 had a well-matched bandgap and displayed superior photocatalytic performance compared to Bi2O2CO3.

In addition, other heterogeneous phase junction catalysts were evaluated. For example, Zeng et al. [166] constructed a Sr2Ta2O7−xNx/SrTaO2N heterostructured photocatalyst in situ using hydrothermally prepared Sr2Ta2O7 nanosheets in the atmosphere of NH3, thereby achieving a well-matched band structure. In the photocatalytic hydrogen evolution, the Sr2Ta2O7−xNx/SrTaO2N heterostructured photocatalyst yielded the highest rate with Sr2Ta2O7−xNx and pure SrTaO2N using similar reaction conditions. Wang et al. [109] first investigated the α-β Ga2O3 phase junction by phase transformation at ele-vated temperatures from 673 to 1073 K. Furthermore, they demonstrated that in photocatalytic water splitting, the Ga2O3 phase junction calcined at a temperature range of 863–3 K had a much higher activity than the pure α phase (673 to 773 K) or the β phase (973 to 1073 K) of Ga2O3.

On the basis of phase transitions, the mechanism of phase transition and the co-doping of elements used to improve pho-tocatalytic performance were further explored. For example, Ti2CN was used for the first time as a raw material, and C-N self-doping was realized at the same time by the precise control of the O2 atmosphere, heating time, and temperature, in the reactions relevant to the formation of the crystal phase of ana-tase and rutile TiO2 [167]. The photoelectron spectra indicated that C and N were doped in the process of the phase transition. It was further proved that C-N/TiO2 could be prepared with different C and N contents based on the accurate control of the heating temperature, time, and atmosphere of Ti2CN. C-N codoping can form impurity energy levels, reduce the gap of TiO2, accelerate the separation of h+ and e–, and improve the absorption ability. Additionally, the preparation of P and WO3 co-doped TiO2 can significantly inhibit the photogenerated electron and hole recombination [168]. Zhu et al. [169] inves-tigated the effects of heat treatment on the anatase-rutile phase transformation and the photocatalytic activity of Sn-doped TiO2 nanomaterials. It is found that at 350 °C, 1% Sn doping led to the highest degradation rate, while at 500 °C and 650 °C, 5% Sn doping led to the best photocatalytic activity. The relationship

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between the regulation of heterogeneous phase junctions and the physical properties of the compounds, such as the crystal structure, morphology, ionic radius, and decomposition kinet-ics, were also investigated. Therefore, the highly efficient het-erojunction photocatalyst was synthesized by the phase trans-formation kinetics method of heating and self-transformation. This constitutes the technological innovation of the preparation process of the heterojunction photocatalyst, whereby the qual-ity of the photocatalyst has been improved.

3.3.5. Self-surface phase junctions of crystal plane

In a photocatalytic reaction on the surface of a semiconduc-tor, it is essential to achieve an effective transfer and spatial separation of the photoinduced charges on the surface of the photocatalyst [170]. Thereinto, the surface phase junctions and the aligned built-in electric fields can drive the charge separa-tion and transfer [171–173]. The outcome of this process is the formation of two spatially separated surface locations that boost the targeted chemical reaction. Therefore, the self-surface phase junction of a single semiconductor under-lines the role of the built-in electric field in conducting the semiconductor carriers to the surface redox reaction sites. Using effective detection technology, Li et al. [57] developed ultraviolet (UV) Raman spectroscopy in the study of the surface properties of the TiO2 catalyst, which has recently proved to be a powerful tool in determining the variation of the surface phase structure. Because UV Raman spectroscopy can be more sensitive to the surface phase, it can overcome the interference of fluorescence and strengthen the Raman signals at short wavelengths. Li’s group [143] also investigated the influence of Nd2O3 on the surface phase of TiO2 using UV Raman spectros-copy and showed that the coexistence of anatase and rutile in the surface region of Nd-TiO2 determined the optimal activity. UV Raman spectra certainly justified the fact that the surface ion SO42– could restrain the surface phase transformation at elevated temperatures [145].

Based on the hydrothermal treatment strategy in the HNO3 solution using titanate nanotube as a precursor, Huang et al. [174] synthesized the highly active TiO2 photocatalysts of the anatase-brookite and anatase-rutile surface phase junction. The surface heterophase junctions of TiO2 could suppress the re-combination of photoinduced charge carriers. Additionally, TiO2 consisted of 72.9 wt% anatase, 24.6 wt% brookite, and 2.5 wt% rutile, and exhibited a higher photocatalytical activity owing to the H2 yield than that of P25 using the same reaction conditions (179 μmol h–1 g–1 vs. 45.3 μmol h–1 g–1).

According to the density functional theory (DFT), Yu et al. [172] calculated the density of states on different crystal planes of anatase TiO2. It was found that the (001) and the (101) planes can form heterojunctions of crystal planes and promote the separation and transfer of photogenerated electrons and holes (Fig. 11(a)). The results of the photocatalytic reduction of CH4 by CO2 show that when the ratio of the (101) and (001) surfaces exposed to TiO2 is close to 1:1 (optimum ratio), the photogenerated electrons gathered on the (101) plane could participate in the reduction reaction, while holes gathered on the (001) plane could participate in the oxidation reduction.

Fig. 11. (a) Spatial separation of redox sites of the exposed (101) and (001) facets on TiO2; (b) Charge separation on the (001) facet of Cu2O parallel to the light irradiation direction; SEM images of (c) Au-deposited (Cu2O/Au) and (d) dual co-catalyst deposited (Cu2O/Au/MnOx) Cu2O crystals prepared via photodeposition with the use of asymmetric illumination; SPVM images of (e) Au-deposited and (fdual co-catalyst-deposited Cu2O crystals obtained with the use of asymmetric illumination. Reprinted with permission from Refs. [60,61]. Copyright 2014 American Chemical Society and 2018 Nature.

The formation rate of CH4 reached 1.35 μmol h–1 g–1 and in-creased 21 times.

In recent years, Li et al. [175] also indicated that the pho-to-induced surface potential changes (SPV) could be imaged directly to assess the localized spatial charge separation be-havior across the spatially varying charge regions at the sur-faces of the photocatalysts. The same authors were able to identify complex charge separation and transfer processes that have been revealed in many other photocatalysts [176,177]. For instance, single cubic Cu2O crystals were prepared to reveal the influence of the intrinsic asymmetric built-in electric field on charge separation by localized SPV measurements [61]. The (001) facet was exposed to the light direction (Fig. 11(b)), and the charge separation was driven by the built-in electric field with a SPV of 20 mV. The quantitative results showed that the photogenerated charge separation was highly dependent on the length of Cu2O (i.e., the particle size), whereby holes moved toward the illuminated facet, and electrons moved toward the shadow facet. It can be deduced that the heterojunction of the crystal plane of cubic Cu2O particles under asymmetric illumi-nation was important in the photocatalyst system. The spatially separated electrons and holes were available for surface redox reactions of trivalent Au3+ reduction and Mn2+ oxidation, as shown by Fig. 11(c) and (d), whereby the shadow facets in-

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Fig. 12. SEM images of BiVO4 with and without the deposition of singlemetal/oxide. (a) BiVO4, (b) Au/BiVO4, (c) Pt/BiVO4, (d) Ag/BiVO4, (e) MnOx/BiVO4, (f) PbO2/BiVO4. The contents of the deposited met-als/metal oxides are all 5 wt%. Scale bar, 500 nm. Reprinted with per-mission from Ref. [176]. Copyright 2013 Nature.

cluded the deposition of Au particles and the illuminated facets contained the MnOx agglomerates. The charge separation be-havior was also further confirmed by SPV, as shown in Fig. 11(e) and (f).

Similarly, in Fig. 12, also shown are the spatial separations of the photogenerated electrons and holes on the (010) and (110) crystal facets of BiVO4 [60,168,177]. The reduction reac-tion of the photogenerated electrons and the oxidation reaction of the photogenerated holes can occur on the (010) and (110) facets of BiVO4 and correspond to the selective deposition of the Pt and MnOx cocatalysts. These results demonstrate that a material with appropriate self-phase-junction of crystal planes is vitally important in the photocatalytic system. This also pro-vides guidance in designing different, high-energy crystal planes for efficient charge separation and redox reactions in energy and in environmental photocatalysis.

3.3.6. Other assistant-driven phase junctions

To-this-date, other empirical and experimental methods have also been reported for the construction of phase junctions, such as electrochemistry, pulse laser irradiation or UV/ozone-driving, mechanical alloying, and pressure-induced

phase transitions. Using electrochemical methods, Liang et al. [179] prepared a mixture of anatase and rutile phases with TiO2 nanotubes. These nanotubes were synthesized with the anodic oxidation method and exhibited high-photocatalytic activity for the degradation of 2,3-dichlorophenol in an aque-ous solution. Sreekantan et al. [132] prepared anatase-rutile TiO2 nanotubes using the titanium foil anodization method in an electrochemical bath of 1 mol/L glycerol with 0.5 wt% NH4F at a pH of 6. Furthermore, it was shown that the anodization voltage could influence the morphology of the anodized and well-aligned growing titanium with pit-like oxide. Ren et al. [133] reported a one-step electrochemical oxidation method which only required a few minutes to prepare mixed-phase TiO2 porous films with oriented rutile. In the synthetic process, the additive amount of HCl or HF in the electrolyte could regu-late the orientation as well as the rutile nanocrystallites with a large specific surface area. The obtained tailor-oriented TiO2 porous films could produce an enlarged photocurrent based on the treatment of HF which was used as an additive compared to the photocurrent from randomly oriented TiO2 films. Addition-ally, the synergistic effects of orientation led to a film arrange-ment with increased exposure of the rutile (101) facets, while the F impurity of the surface showed improved photocatalytic and photoelectrochemical performances. Lv et al. [180] fabri-cated an α-β phase junction in Bi4V2O11 nanofibers based on the electrospinning retardation effect (electrospinning with sub-sequent calcination), which could transform the α-Bi4V2O11 in β-Bi4V2O11 and construct an α-β Bi4V2O11 phase junction with a well-defined type-II band alignment, as shown in Fig. 13. The α-β phase junction on the Bi4V2O11 nanofibers achieved an out-standing photocatalytic activity used in the removal of Cr(VI) and MB with the concurrent use of nitrogen fixation.

Several other studies focused on the phases that are natu-rally induced by extra pressure, induced phase transitions, wire explosion processes, or UV or pulse laser irradiations, in mate-rials such as Nb-doped TiO2 [140] and RbPbCl3 [141]. Lu et al. [140] reported enhanced electron transport in Nb-doped TiO2 nanoparticles based on pressure-induced phase transitions. The electron transport in semiconductors was related to the packing factor of the pressure-induced conductivity evolution. Huang et al. [141] also studied the generation of new pres-

Fig. 13. Formation process and fast separation and transfer of photogenerated carriers on the α-β phase junction structure of the Bi4V2O11 nanofibers. Reprinted with permission from Ref. [180]. Copyright 2018 ACS.

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sure-induced phase transitions and their photocatalytic prop-erties in RbPbCl3 using calculations based on fundamental principles. Accordingly, they predicted four new phases, in-cluding P4/mmm, C2mm, I4mm, and Cm, for RbPbCl3, whereby the electronic structural properties could be modulated by pressure at specific phases. Accordingly, the same authors showed that the C2mm, I4mm, and Cm phases were promising candidates in photocatalytic water splitting.

Based on the wire explosion process, Ranjan et al. [181] synthesized mixed phases for TiO2 nanoparticles of anatase and rutile in an oxygen atmosphere at a specific pressure. The phase and morphology of TiO2 nanoparticles were governed by the ratio of the stored quantity in the capacitor and the subli-mation quantity of the wire. Additionally, the increase of the ratio and the decrease of the oxygen pressure could promote the enhancement of the rutile content. At the two extreme con-centration conditions of 300 and 400 mg/L, it exhibited an ex-cellent performance in the photocatalytic removal of methylene blue.

In addition, Zhang et al. [128] reported a UV/ozone-assisted method to prepare mesoporous TiO2 with controlled phase composition at temperatures as low as 100 °C. The exposure time can potentially adjust and control the mixture of the crys-talline mesoporous anatase and rutile phases. Choi et al. [129] introduced a facile crystalline phase control method of electro-spinning and pulse laser irradiation in liquid to prepare the mixed anatase and rutile phases of TiO2 nanofibers to achieve enhanced photochemical responses and photocatalytic activi-ties in the decomposition of methylene blue. Similarly, Russo et al. [182] studied the phase transformation of TiO2 nanoparticles governed by the femtosecond laser ablation of P25 in aqueous solutions and the deposition on FTO conductive substrates. It was then found that the prolongation of the ablation time of the TiO2 dispersion could widen the bandgap. In other words, it is highly desirable to develop a new, low-cost and effective prep-aration method for the advancement of phase junctions. 4. Applications of heterophase/homophase junction photocatalysts

4.1. Photocatalytic H2 evolution

Photocatalytic splitting of water is used to produce hydro-gen and is considered as one of the most challenging reactions

for the preparation of fuel by solar energy. As the understand-ing of the mechanisms of operation of semiconductor photo-catalysts and co-catalysts continues to improve, research pro-gress will continue to be accomplished in the field of photoca-talysis. Li et al. [183] prepared a composite Sr2TiO4/SrTiO3(La,Cr) heterojunction photocatalyst using a simple, in situ, polymerized complex method. Well defined het-erojunctions were formed by matching the lattice fringes of SrTiO3(La,Cr) and Sr2TiO4(La,Cr) based on microscopic obser-vations. The absorption spectroscopy and Mott-Schottky plots revealed the band structure of the composite Sr2TiO4/SrTiO3(La,Cr), and the facts that the photogenerated electrons migrated from SrTiO3(La,Cr) to Sr2TiO4(La,Cr) and holes tended to move from Sr2TiO4(La,Cr) to SrTiO3(La,Cr) driven by a minor potential difference. These findings indicated the superior photocatalytic activity of photocatalytic H2 pro-duction (Fig. 14). A time-resolved FT-IR spectroscopic study confirmed the long-lived electrons and the facilitated charge transfer and separation. Li et al. [184] studied the integration of the nanorod photocatalyst with the hexagonal@cubic CdS core@ shell for highly active production of H2 with unprece-dented stability by taking into consideration advantages of core-shell structures in reference to the surface passivation. This shortened the electron diffusion length, increased light absorption, and enhanced the tunneling of charge carriers. The entire process was based on a highly effective, low-cost strate-gy of the one-pot hydrothermal method, i.e., the direct treat-ment of cadmium nitrate and thiourea precursor solution at a specific temperature with a variable Cd/S precursor molar ratio and reaction time (Fig. 15).

Liu et al. [185] also prepared CuOx/TiO2 photocatalysts which employed TiO2 with different phase structures, and CuOx/P25 with the largest population of Cu2O-anatase TiO2 heterojunction. These compounds exhibited the highest photo-catalytic production of H2. Similarly, Devaraji et al. [186] also synthesized two-dimensional mixed phase leaf-Ti1-xCuxO2 sheets. The synthesis was based on a natural leaf template, and the incorporation of Cu ions (1 wt%) into the leaf–TiO2 lattice using a deposition-precipitation method, and was followed by a calcination process. Two-dimensional mixed-phase

Fig. 14. Schematic band structure and photocatalytic H2 production activities of La and Cr codoped Sr2TiO4/SrTiO3 and its mechanism for H2 produc-tion in the presence of visible light irradiation. Reprinted with permission from Ref. [183]. Copyright 2013 RSC.

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Fig. 15. Formation process and the photocatalytic H2 production activity at different conditions pertaining to the nanorod photocatalyst with thehexagonal@cubic CdS core@ shell. Reprinted with permission from Ref. [184]. Copyright 2016 Wiley.

leaf–Ti1-xCuxO2 sheet calcination at 750 °C led to the highest hydrogen yield. This was mainly attributed to surface-phase junctions, disordering mesoporosity, 2D sheet morphology, and fast charge carriers. Xu et al. [187] constructed a bi-phasic (an-atase/rutile) nanocomposite TiO2 nanofiber photocatalyst with a direct Z scheme which attained an enhanced photocatalytic H2 production activity. A novel anatase/rutile TiO2 photoelec-trode with hydrogenated heterophase interface structures was also found, and was synthesized by the hydrothermal synthe-sis-hydrogenation-branching growth method [188]. The supe-rior photoelectrochemical (PEC) performance was attributed to the matching energy levels between anatase-branches and the hydrogenated rutile-nanorod, and to the new energy levels of oxygen vacancy and Ti−OH. Qiu et al. [1] constructed TiO2 hierarchical architecture assembled by nanowires with ana-tase/TiO2(B) phase junctions using a hydrothermal process which was followed by calcination. The optimum calcination treatment at 450 °C yielded the best photocatalytic hydrogen production activity. Wang et al. [80] constructed the isotype C3N4/sulfur-mediated C3N4 heterojunctions of polymeric car-

bon nitride between two different crystal phases of a single substance (known as crystal-phase heterojunctions) by com-bining bulk C3N4 and the molecularly engineered heptazine tectons in a sulfur medium (sulfur-mediated C3N4 can adjust its packing structure and electronic band structure). As shown in Fig. 16, such polymeric isotype heterojunctions show signifi-cant enhancements in the activities and durabilities of the H2 evolution reactions. This was attributed to the promoted exci-ton dissociation and charge separation from the band offsets. 4.2. Photocatalytic CO2 reduction

The photocatalytic reduction of CO2 is the conversion of the C=O double bonds from linear CO2 molecules to C–O single bonds through renewable solar energy, and the use of new catalysts, thereby producing methane, methanol, or olefin, and other products. This can create new chemical supplies to re-place the current dependence on oil, coal, and natural gas, and provide an economical and environmentally friendly CO2 utili-zation process. For example, small metal particle-coated p-Si

Fig. 16. Organic heterojunction and stability test of the evolution of H2 between C3N4/sulfur-mediated C3N4 photocatalysts in the presence of visi-ble-light irradiation. Reprinted with permission from Ref. [80]. Copyright 2012 Wiley.

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on both the inner and outer surfaces of the In2O3 microtubes) where the optimized ZnIn2S4-In2O3 exhibited an outstanding performance for CO2 reduction (3075 μmol h–1 g–1) and in-creased stability.

4.3. Photocatalytic degradation

Hou et al. [193] constructed bismuth oxychloride micro-flower/nanosheet homojunctions via a solvent-free route for the first time with the controlled reaction temperature owing to some microflowers merging into large nanosheets at T > 120 °C (Fig. 19). The increased photoactivity of the homojunction sample was attributed to the considerable differences in sizes and the exposed facets between the microflowers and the nanosheets. This allowed the determination of the increased specific surface area and the differences in their energy band levels. Furthermore, Hao et al. [194] fabricated an n–n type bismuth oxychloride (BiOCl/Bi12O17Cl2) homogeneous phase junction by facilely manipulating the basicity in a one-pot hy-drothermal process. The BiOCl/Bi12O17Cl2 phase junctions showed a much higher photocatalytic activity than the single BiOCl and Bi12O17Cl2 toward the degradation of industrial con-taminants in the presence of simulated solar light that was at-tributed to the efficient separation and transfer of pho-to-induced carriers because of the junction interface between BiOCl and Bi12O17Cl2.

Kondamareddy et al. [195] synthesized TiO2-anatase/rutile heterojunctions by incorporating tungsten (W6+) ions into the lattice of pure anatase TiO2 nanoparticles with ultra-trace con-centrations (ppm) using a simple one-step hydrothermal method. The enhancement of the photocatalytic activity under visible light irradiation for the degradation of rhodamine B was assigned to the impurity energy levels of the W6+ ions, oxygen vacancies, and the anatase-rutile heterojunction. Gao et al. [196] also investigated the effects of the C and Y doping and annealing temperatures on the structural and optical proper-ties and photocatalytic activity of mixed phase TiO2. It was found that C and Y doping can broaden the absorption spec-trum of TiO2, increase the specific surface area, and enhance the photocatalytic activity. Makal et al. [197] studied (a) TiO2 nanowires with different phases using the hydrothermal tech-nique, e.g., TiO2-B, anatase, and rutile, and (b) the subsequent thermal annealing process in vacuum at different tempera-

electrodes (Cu, Ag, or Au) were prepared for the photoelectro-chemical reduction of CO2 [190]. These particles had high pho-tovoltages of ca. 0.5 V and were used as effective electrodes for the photoelectrochemical reduction of CO2. He et al. [86] pre-pared Ag3PO4/Ag/g-C3N4 in situ using g-C3N4, AgNO3, and Na2HPO4 as raw materials. The photogenerated electrons of Ag3PO4 and photogenerated holes of g-C3N4 could transfer to Ag for the recombination (Fig. 17). In this way, the photogener-ated electrons could catalyze the reduction of CO2 at the high position of the g-C3N4 conduction band, and result in higher activities. The rate of preparation of CxHyOz was as high as 57.5 μmol h–1 g–1, and was 6.1 and 10.4 times higher than g-C3N4 and P25, respectively.

Wang et al. [191] designed hierarchical In2S3-CdIn2S4 heter-ostructured nanotubes based on a novel self-template strategy, including sequential anion- and cation-exchange reactions (Fig. 18a). These nanotubes can be efficient and stable photocata-lysts for visible light and CO2 reduction owing to the separation and migration of the photoinduced charge carriers, the adsorp-tion and concentration of CO2 molecules, and the abundant number of active sites for surface redox reactions in the hier-archical heterostructured nanotubes. Similarly, the same au-thors [192] also constructed the sandwich-like ZnIn2S4-In2O3 hierarchical tubular heterostructures based on (a) the thermal annealing treatment of the in-MIL-68 prism precursor, and (b) the subsequent hydrothermal reaction on the surfaces of the as-derived In2O3 tubes (Fig. 18b) (growing ZnIn2S4 nanosheets

Fig. 18. Schematic illustration of the synthetic process of hierarchical (a) In2S3-CdIn2S4 and (b) ZnIn2S4-In2O3 heterostructures. Reprinted with per-mission from Refs. [191,192]. Copyright 2017, 2018 ACS.

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Fig. 19. Formation and growth process, energy band levels, and photogenerated charge carrier transfer pathways of BiOCl microflower/nanosheet homojunctions. Reprinted with permission from Ref. [193]. Copyright 2018 Wiley.

tures, and investigated their photocatalytic performance. The TiO2-B/anatase heterostructured nanowires by partial phase transformation at 700 °C showed enhanced photocatalytic ca-pacity because of their broad optical band gaps.

Jo et al. [198] fabricated a unique Janus Ag2O/Ag2CO3 het-erojunction photocatalyst with improved photocatalytic prop-erties using in situ phase transformation, and Zhao et al. [199] reported Ag2O/Ag2CO3 3D hollow flower-like hierarchical mi-crospheres, which exhibited improved photocatalytic perfor-mance and stability compared to those of bare Ag2CO3. Wang et al. [200] fabricated TiO2(B)/anatase heterophase junctions in nanowires via a surface-preferred phase transformation pro-cess, i.e., a three-step synthesis method, including hydrother-mal treatment, H+ exchange, and annealing. The optimized structure with 24% TiO2(B) in the bulk and 100% anatase on the surface led to an enhanced photocatalytic activity (Fig. 20). Samsudin et al. [201] fabricated bluish-gray hydrogenated TiO2 by hydrogenating it at constant temperature and pressure with controlled hydrogenation duration. They demonstrated that the photocatalytic activity was attributed to the increased oxy-gen vacancies and Ti3+ for solar light absorption, while the ex-tended hydrogenation duration caused decreases in the activity owing to the excessive catalyst growth and loss in the total surface area. Li et al. [202] studied the charge transfer at an anatase/rutile TiO2 phase junction using time-resolved photo-luminescence spectroscopy, and found that the charge separa-

Fig. 20. Formation and growth process of the phase transformation (a) on the surface, (b) in the bulk, and (c) at the surface-preferred phase junction at 600 °C. Reprinted with permission from Ref. [200]. Copyright 2018 Elsevier.

812 Kai Yang et al. / Chinese Journal of Catalysis 40 (2019) 796–818

tion at the anatase/rutile phase junction reduced the micro-second time scale photoluminescence decay rate for the charge carriers because of the slower electron-hole recombination (Fig. 21). Moro et al. [203] studied the mesoporous and nano-crystalline titanium dioxide by a possible synergism between TiO2 and carbon nanotubes under hydrothermal conditions. Based on this study, they showed that the addition of a small amount of MWCNTs further increased the photoactivity of the nanocrystalline titania for the degradation of 4-nitrophenol. Because of the different band structures of g-C3N4 prepared separately from urea and thiourea using the same thermal con-ditions, Dong et al. [204] prepared a novel layered g-C3N4/g-C3N4 metal-free isotype heterojunction in situ with molecular composite precursors, which exhibited significantly enhanced photocatalytic activity for visible light for the remov-al of NO from air. This was directly ascribed to an efficient charge separation and transfer across the heterojunction in-terface and to the prolonged lifetime of charge carriers, as in-dicated in Fig. 22. Meanwhile, Liu et al. [205] studied an amor-phous/crystalline g-C3N4 homojunction with a matched energy level structure via a two-step polymerization of melemon at the surface of crystalline g-C3N4 (as seeds). They also optimized the ratio of crystalline g-C3N4 in the homojunction that resulted in a superior photocatalytic performance for the degradation of

Fig. 21. PL decays of TiO2 samples with different phase compositions at the excitation source of 340 nm, and charge transfer and recombination atanatase/rutile phase junction. Reprinted with permission from Ref. [202]. Copyright 2016 Elsevier.

Fig. 22. Charge transfer, visible-light photocatalytic performance, and in situ formation process of the CN-T/CN-U isotype heterojunction. Reprinted with permission from Ref. [204]. Copyright 2013 ACS.

Kai Yang et al. / Chinese Journal of Catalysis 40 (2019) 796–818 813

organic pollutants under visible light irradiation. The synthetic process and photocatalytic degradation of RhB of composites are shown in Fig. 23.

5. Conclusions and perspectives

Highly efficient photocatalysts are required to maximize the solar spectrum usage that ultimately leads to an effective sepa-ration of photo-generated electron-hole pairs. The separated electron-hole pairs can be transferred to the surface of the cat-alysts and react with substrates. Nanocrystalline heterojunc-tion photocatalytic materials, including semiconduc-tor/semiconductor, semiconductor/metal, semiconduc-tor/nonmetal, and surface heterojunctions, have successfully achieved the effective separation and long-term stability of carriers.

With respect to Table 2, we discussed the general strategies and recent progress with the use of efficient techniques for the construction of phase junctions for the development of highly efficient and stable photocatalysts, including: (1) the addition of a suitable amount of anions or positive ions (such as Cl–, F–, PO43–, SO42–, and Mo6+) to regulate the phase transition on un-doped semiconductor photocatalysts, (2) the implementation of oxide regulation in the phase transition process of the pho-

Table 2 Comparison of the typical heterojunctions based on an externally driven phase transformation. Formation technique Anionic and cationic regulations Oxide regulations Soft chemical route

Types Preparation Improved performance Proposed reasons Effective separation of the Mo6+, Mn2+, Cl–, F–, NO3−, HCO3−, Regulation of the composi-Higher activities than that of

3−2−2− industrial P25 in the photogenerated electron-hole PO4, SiO3, and MoO4 tion of anatase-rutile phase

photocatalytic H2 production pair of TiO2

Al2O3, Nd2O3, SiO2 Anatase-rutile type Promotion of the degradation Prevention of the accumulation

of RhB of TiO2 grain growth Enhanced separation of α-/γ-Bi2O3 homojunction, A one-pot hydrothermal Exceptional and synergistic

photocatalytic activity photogenerated electron and α/β-Bi2O3 homojunction, method or a novel

hole pairs solvothermal 2H/1T-MoS2/C3N4, mixed-phase

calcination method TiO2, TiO2(B)/rutile/rookite,

Bi2O2CO3/Bi2O4

Higher photocatalytic activity Energy level matching and Ag2CO3@Ag2O, BiOI/Bi5O7I, Control of heating

an internal electric field temperature, time, and than single primitive sample Bi5O7I/Bi2O3, BiOBr/Bi24O31Br,

accelerate the separation of atmosphere Bi24O31Br/α-Bi2O3,

photogenerated carriers β-Bi2O3/Bi2O2.33@Bi2O2CO3,

Zn3(VO4)2/Zn2V2O7/ZnO, Sr2Ta2O7−xNx/SrTaO2N, α-β Ga2O3

(101) and (001) surfaces of TiO2, Design of a single crystal Improved CO2 reduction and Spatial separation of

semiconductor (001) facet of cubic Cu2O, (010) H2 production rate photogenerated electrons and

and (110) crystal facets of BiVO4 holes on different crystal facets

Outstanding photocatalytic Well-established type-II band α-β Bi4V2O11, mixed-phase TiO2 Anodic oxidation or

alignment one-step electrochemical activity in the removal of

Cr(VI) and MB, and nitrogen oxidation

fixation

Nb-doped TiO2 Extra pressure Promotion of photocatalytic Enhanced electron transport

water splitting

Control of calcination

Self-surface phase junctions of crystal plane

Electro-chemistry

Pressure induction

814 Kai Yang et al. / Chinese Journal of Catalysis 40 (2019) 796–818

tocatalysts to improve the photocatalytic activity of the phase junction, (3) the induction of phase junctions with the soft chemical route to achieve exceptional and synergistic photo-catalytic activity, (4) the control of calcination-induced phase junctions based on the control of the phase transition rate and time, (5) the creation of self-surface phase junctions of the crystal plane to improve the separation and transportation of the electron-hole pairs, (6) the use of other assistant-driven phase junctions, such as pulse laser irradiation, UV/ozone, and electrochemistry methods, for the enhancement of the utiliza-tion of sunlight and the improvement of the separa-tion/transportation of the carriers. The efficient techniques in this field associated with phase transitions provided a promis-ing route to the preparation of the phase junctions and to the enhancement of the photocatalytic efficiencies.

The factors that affect the performance of photocatalysts are very complex. Accordingly, the photocatalytic activity was not only influenced by its phase structure, but also by its surface chemical properties. These factors introduced many difficulties in the interpretation of nanocrystalline heterostructures with the use of the traditional solid band theory. Therefore, the re-search of phase junction photocatalytic materials at the na-nometer scale may be focused on the following aspects: (1) development of simple, cheap, large-scale, and pollution-free preparation technologies of phase transitions, to regulate the energy band structure of the nanoscale heterojunction and to improve the matching degree of the energy band; (2) design of a suitable preparation method to construct a novel and efficient heterogeneous structure type according to the properties and the final structure of the related semiconductors; (3) fabrica-tion of nano-heterojunction photocatalytic materials in devices

and application to the actual photocatalysis control process. It is necessary to use the effective in situ and transient technolo-gies to study the actual charge transfer of nanocrystalline het-erostructures for the establishment of a suitable theory for nanocrystalline heterostructures. Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (21707055, 21567008, 216070), Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology, Program of 5511 Talents in Scientific and Technological Innovation of Jiangxi Province (20165BCB18014), Academic and Technical Leaders of the Main Disciplines in Jiangxi Province (20172BCB22018), Jiangxi Province Natural Science Foundation (20161BAB203090, 20181BAB213010, 20181BAB203018), Young Science Foundation of Jiangxi Province Education Office (GJJ160671), Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201712) in Fuzhou University and Doctoral Fund of Jiangxi University of Science and Technology. References

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Graphical Abstract

Chin. J. Catal., 2019, 40: 796–818 doi: S1872-2067(19)63290-0

Review on heterophase/homophase junctions for efficient photocatalysis: The case of phase transition construction Kai Yang, Xiaoxiao Li, Debin Zeng, Fanyun Chen, Changlin Yu *, Kailian Zhang, Weiya Huang

Jiangxi University of Science and Technology; Guangdong University of Petrochemical Technology; Fuzhou University

Applications of the low-cost and efficient phase junction pathway for the construction of nanophase junctions in photocatalysis.

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异质/同质相结高效光催化的研究进展: 以相变构建为例

杨凯a,c, 李笑笑a, 曾德彬a, 陈范云a, 余长林a,b,*, 张开莲a, 黄微雅a

江西理工大学冶金与化学工程学院, 江西赣州341000

广东石油化工学院环境科学工程学院, 广东省石油化工污染过程与控制重点实验室，广东茂名525000

福州大学能源与环境光催化国家重点实验室, 光催化研究所, 福建福州350002

摘要: 半导体光催化剂在环境处理和能量转换方面有着巨大的应用潜力, 但由于电子-空穴对的复合作用, 半导体光催化剂的光催化性能较低. 相结的存在是提高电子-空穴分离效率及光催化活性的有效途径, 对相结设计的深入研究是提高电荷转移性能和效率的有效手段. 因此, 相结光催化技术的发展, 对于设计一个良好的相结和了解电子-空穴分离机理具有重要的意义.

通常, 相结的构建需要特殊的制备技术以及良好的晶格匹配. 纳米异质结材料结合快速转移载流子的特点, 具有小尺寸效应和颗粒限域效应的优点, 且具有单组分纳米材料或体相异质结不具有的独特特性. 纳米晶异质结可以促进光生电子的快速转移, 根据两种半导体带的相对位置, 异质结可分为I型、II型和III型, 根据不同的电子转移途径可分为p-n型和Z-型. 当p型半导体(空穴为多数电荷载流子)或n型半导体(电子为多数电荷载流子)密切接触时, 由于能带和其它性质的差异, 会形成结, 并在结的两侧形成空间电位差. 空间电位差的存在可以使产生光生载流子从一个半导体能级注入到另一个半导体能级, 从而促进电子和空穴的分离, 提高光催化效率. 以p-n结为例, 当它们在这两个区域共存时, 它们的边界层形成一个薄的p-n结. 由于p型区空穴浓度高, n型区电子浓度高, 结处存在电子和空穴的扩散现象. 在p-n结边界附近形成空间电荷区, 从而在结内形成强的局域电场. 在结的局部电场作用下, 电荷在结两端累积形成电位差, 后者作为驱动力可以有效地分离光生电荷.

近年来, 人们在纳米相结的设计和制备上做了大量工作以提高光催化剂活性. 虽然异质结具有优良的性能, 但异质结的成分和元素并不是单一的, 它的形成也不是一步反应. 首先, 需要分别合成异质结的两个成分, 反应复杂, 耗时, 不环保. 与异质结相比, 同一材料通过相变构建的结也能实现光生载流子的高效分离. 同质化不需要引入其它要素, 因此引起了大量关注. 在相变过程中, 大多数均由不同晶相的半导体形成, 如锐钛矿型/金红石型TiO2, α-β相Ga2O3或六方/立方CdS. 由于化学成分相同, 半导体材料的能带结构不易改变. 因此, 对同晶材料的同质结研究较少. 虽然已有几篇关于异质相结的综述论文, 但通过对外部诱导相变法制备相结的回顾, 仍可为读者提供有关该领域研究进展的新的认识. 本文对低成本、高效率的相变思路在光催化领域中的应用进行了简要的总结, 并对其在光催化领域中的应用前景进行了展望. 关键词: 相变; 相结; 光催化; 有效电子转移

收稿日期: 2018-11-09. 接受日期: 2018-12-26. 出版日期: 2019-06-05.

*通讯联系人. 电话/传真: (0797)8312334; 电子信箱: yuchanglinjx@163.com

基金来源: 国家自然科学基金(21707055, 21567008, 216070); 江西省5511科技创新人才(20165BCB18014); 江西省主要学术与学科带头人(20172BCB22018); 江西省自然科学基金(20161BAB203090, 20181BAB213010, 20181BAB203018); 江西省教育厅青年基金(GJJ160671); 福州大学国家重点实验室开放基金(SKLPEE-KF201712); 江西理工大学清江优秀人才项目; 江西理工大学博士启动基金.

本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).

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异质同质相结高效光催化的研究进展以相变构建为例