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Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances Huanli Wang,a Lisha Zhang,*a Zhigang Chen,b Junqing Hu,b Shijie Li,a Zhaohui Wang,a Jianshe Liu*a and Xinchen Wang*c Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor–semiconductor heterojunction, the semiconductor–metal heterojunction, the semiconductor–carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems

Received 8th April 2014

are also discussed in relation to the environmental and energy applications, such as degradation of

DOI: 10.1039/c4cs00126e

pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a

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summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research.

Key learning points (1) (2) (3) (4) (5)

The The The The The

fundamental principles of semiconductor photocatalysis design and construction of semiconductor–semiconductor heterojunctions design and construction of semiconductor–metal heterojunctions design and construction of semiconductor–carbon heterojunctions design and construction of multicomponent heterojunctions

1. Introduction In recent years, energy shortages and environmental pollution have become the focus of world attention. As one of the most promising solutions for these problems, semiconductor photocatalysis has attracted much attention, since it is a ‘‘green’’ technology for decomposing water into hydrogen and oxygen, inactivating viruses and/or completely eliminating all kinds a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China. E-mail: [email protected], [email protected]; Fax: +86 21 67792522; Tel: +86 21 67792523 b College of Materials Science and Engineering, Donghua University, Shanghai 201620, China c State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, China. E-mail: [email protected]

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of contaminants, under the illumination of sunlight under ambient conditions.1 To date, the TiO2 semiconductor has undoubtedly proven to be one of the excellent photocatalysts for water splitting and the oxidative decomposition of many organic compounds.2 However, due to its wide band-gap of 3.2 eV, TiO2 can only be excited by ultraviolet or near-ultraviolet radiation, which occupies only 4% of the incoming solar light spectrum on the earth.3 To efficiently utilize the visible region (l 4 400 nm) which covers the largest proportion of the solar spectrum, the development of visible-light-driven (VLD) photocatalysts is the current trend. Many groups have reported some novel VLD photocatalysts, such as simple oxides (Bi2O34 and WO35), sulfides (CdS6), complex oxides (Bi2WO67 and Zn:In(OH)ySz8) and nitrides (C3N49). Up to date, although much efforts have been devoted to preparing different VLD photocatalysts, there are still some drawbacks hindering their practical application, such as short photogenerated electron–hole pair lifetimes and the limited visible-light absorption.

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It is still necessary to design novel VLD photocatalyst systems to improve photocatalytic efficiencies for the requirements of future environmental and energy technologies. During the past decade, a variety of strategies have been employed to improve the photocatalytic efficiencies of photocatalysts, for example, via suitable textural design, doping, and forming a semiconductor heterojunction by combining them with metal and/or other semiconductors. Among these, the construction of a semiconductor heterojunction has attracted a lot of attention due to its perfect effectiveness in improving the photocatalytic activity. Many important findings have been reported on the semiconductor heterojunction photocatalysts (SHPs) during the past few years. We have also developed nanoCu2O–carbon nanotube composites,10 a C@TiO2 catalyst,11 an AgBr–Ag–Bi2WO6 three-component nanojunction,12,13 Bi2WO6 superstructure decorated with Bi2O3 nanoparticles14 and metal–C3N4 materials,15 for the degradation of organic pollutants. Furthermore, a C3N4/sulfur-mediated C3N4 (CN/CNS) isotype heterojunction has also been developed for the degradation of organic pollutants or photocatalytic hydrogen evolution.16 We believe that a comprehensive review on this subject is timely to promote further developments in this exciting and still emerging area of research. This review focuses on the recent progress in the rational design, fabrication and applications of semiconductor heterojunction photocatalysts, while providing some stimulating perspectives on the future developments.

2. Fundamental principles of semiconductor photocatalysis 2.1 Fundamental principles of semiconductor heterojunction photocatalysts From the point of view of semiconductor photochemistry, the role of photocatalysis is to initiate or accelerate specific reduction and oxidation (redox) reactions in the presence of irradiated semiconductors. When the semiconductor catalyst is illuminated with photons whose energy is equal to or greater than their band-gap energy (Eg), firstly, an electron (ecb ) is promoted from the valence band (VB) into the conduction band (CB), leaving a hole (hvb+) behind (see Fig. 1I).1 Secondly, the excited electrons and holes migrate to the surface. Thirdly, the electrons (ecb ) in the CB should have a chemical potential of +0.5 to 1.5 V versus the normal hydrogen electrode (NHE) and exhibit a strong reduction capacity; while holes (hvb+) in the VB should have a chemical potential of +1.0 to +3.5 V versus the NHE and exhibit a strong oxidative potential. Electrons (ecb ) and holes (hvb+) can act as the reductant (Fig. 1III) and oxidant (Fig. 1IV) to react with electron donors (D) and electron acceptors (A) adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles, respectively. Besides, they can get trapped in metastable surface states (Fig. 1VI and VII). It should be noted that, in the second step, the excited state conduction-band electrons and valenceband holes can recombine and dissipate the input energy in the

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Fig. 1 Schematic illustration of the principle of semiconductor photocatalysis: (I) the formation of charge carriers by a photon; (II) the charge carrier recombination to liberate heat; (III) the initiation of a reductive pathway by a conduction-band electron; (IV) the initiation of an oxidative pathway by a valence-band hole; (V) the further thermal (e.g., hydrolysis or reaction with active oxygen species) and photocatalytic reactions to yield mineralization products; (VI) the trapping of a conduction band electron in a dangling surficial bond; (VII) the trapping of a valence-band hole at the surface of the semiconductor.

form of heat or emitted light (Fig. 1II). The recombination is often facilitated by a scavenger or crystalline defects which can trap the electron or the hole. Therefore, a better crystallinity with few defects can usually minimize the trapping states and recombination sites, resulting in an increased efficiency in the usage of the photogenerated carriers for desired photoreactions. Based on the fundamental principles of semiconductor photocatalysis, the recombination between the electron and the hole is detrimental to the efficiency of a semiconductor photocatalyst. For higher photocatalytic efficiency, the electron–hole pairs should be efficiently separated, and charges should be rapidly transferred across the surface/interface to restrain the recombination. To improve the photocatalytic performance, the approach that has generally been applied is to form a semiconductor heterojunction by coupling with a secondary substance (noble metal, other semiconductors and so on), which will be discussed in detail in the following section. 2.2 The categories of semiconductor heterojunction photocatalysts In recent years, considerable efforts have been placed on the design and fabrication of heterojunctions for improving the photocatalytic activity.17 Overall, there are four typical categories of heterojunction photocatalysts, including: (1) the semiconductor– semiconductor (abbreviated as S–S) heterojunction; (2) the semiconductor–metal (abbreviated as S–M) heterojunction; (3) the semiconductor–carbon group (abbreviated as S–C) heterojunction (carbon group: activated carbon, carbon nanotubes (CNTs) and graphene); (4) the multicomponent heterojunction. The design principle of the above-mentioned four categories of SHPs will be discussed in detail in the following passage.

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3. The design and construction of semiconductor–semiconductor (S–S) heterojunctions

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3.1

The design principle of S–S heterojunctions

In general, the S–S heterojunction systems can be divided into two different types: p–n semiconductor heterojunction (Fig. 2) and non-p–n heterojunction systems (Fig. 3). The semiconductor p–n junction is an effective architecture for the highly efficient charge collection and separation. In general, when the p- and n-type semiconductors are in contact, they form a p–n junction with a space-charge region at the interfaces due to the diffusion of electrons and holes, and thus create a built-in electrical potential that can direct the electrons and holes to travel in the opposite direction (Fig. 2). When the p–n heterojunction is irradiated by photons with energy higher or equal to the bandgaps of the photocatalysts, the photogenerated electron–hole pairs can be quickly separated by the built-in electric field within the space charge region. Driven by the electric field, the electrons are transferred to the CB of the n-type semiconductors and the holes to the VB of the p-type semiconductors.18 In this p–n type heterostructure, several advantages can be obtained: (1) a more effective charge separation; (2) a rapid charge transfer to the catalyst; (3) a longer lifetime of the charge carriers; and (4) a separation of locally

Fig. 2 Schematic diagram showing the energy band structure and electron– hole pair separation in the p–n heterojunction.

Fig. 3 Schematic diagram showing the energy band structure and electron– hole pair separation in the non-p–n heterojunction.

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incompatible reduction and oxidation reactions in nanospace. All of these features endow the p–n type heterostructures with an enhanced photocatalytic performance. In addition to the p–n type heterostructure, there are also other non-p–n type heterojunction systems, where the most suitable for photocatalytic applications is the staggered bandgap type (Fig. 3). In this type, the semiconductors A and B with matching band potentials are tightly bonded to construct the efficient heterostructure. When the CB level of semiconductor-B is lower than that of semiconductor-A, electrons in the CB of semiconductor-A can be transferred to that of semiconductor-B under visible light irradiation. If the VB level of semiconductor-B is lower than that of semiconductor-A, holes in the VB of semiconductor-B can be transferred to that of semiconductor-A. As a result, the separation and migration of photogenerated carriers can be promoted by the internal field, so less of a barrier exists. Therefore, the probability of electron–hole recombination can be reduced. A larger number of electrons on the semiconductor-B surface and holes on the semiconductor-A surface can, respectively, participate in photoredox reactions to directly or indirectly degrade organic pollution, and thus the photocatalytic reaction can be enhanced greatly. 3.2

Construction and performance of S–S heterojunctions

During the past few years, many groups have reported different types of S–S heterojunction photocatalysts. These research studies greatly improve the efficiencies of the photocatalysts and promote their applications in the energy production and environmental protection. The following section will discuss the highlighted findings in this subject. 3.2.1 TiO2 based S–S heterojunctions. In recent years, tremendous efforts have been made in surface modification of TiO2 nanomaterials with other semiconductors. Table 1 summarizes and compares the typical TiO2-based heterojunction systems. Most of these systems possess high dye adsorption capacity, an extended light absorption range, enhanced charge separation, promoted mass-transfer and thus improved photocatalytic efficiency. For example, Wang’s group successfully synthesized Bi2WO6–TiO2 hierarchical heterostructure through a simple and practical electrospinning-assisted route (Fig. 4A and B).19 As shown in Fig. 4A, Bi2WO6 nanoplates grew aslant on the primary TiO2 nanofibers. These three dimensional (3D) Bi2WO6–TiO2 hierarchical heterostructures exhibited enhanced VLD photocatalytic activity for the decomposition of CH3CHO, which was almost 8 times higher than that of the Bi2WO6 sample, and the decomposition rate by the bare TiO2 could be neglected under visible light irradiation. This high photocatalytic activity was ascribed to the reduced probability of electron–hole recombination and the promoted migration of photogenerated carriers. Similarly, Wang et al.20 fabricated SnO2–TiO2 heterostructured photocatalysts based on TiO2 nanofibers by combining the electrospinning technique with the hydrothermal method (Fig. 4C and D). This SnO2–TiO2 composite possessed a high photocatalytic activity for the degradation of Rhodamine B (RhB) dye under UV light irradiation, which was almost 2.5 times higher than that of the bare TiO2.

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Comparison of the typical TiO2-based heterojunction systems

Types

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Preparation

Morphology

The improved performances

TiO2 (anatase)– Coating method TiO2 (rutile)

Films

4.5 times than the sum of TiO2 (anatase) The increase in charge-separation 21 and (rutile) for the decomposition of efficiency resulting from interfacial CH3CHO in 3 hours electron transfer from TiO2 (anatase) to TiO2 (rutile)

Bi2WO6–TiO2

Electrospinningassisted route

Bi2WO6 nanoplates 8 times than that of Bi2WO6 sample grew aslant on TiO2 for the decomposition of CH3CHO in substrates 120 min under visible light, and the decomposition rate by the bare TiO2 could be neglected under visible light irradiation

The reduced probability of electron– 19 hole recombination

SnO2–TiO2

Electrospinning technique and hydrothermal method

SnO2 nanostructures About 2.5 times than that of TiO2 for grown on TiO2 the degradation of RhB in 60 min nanofibers

The efficient charge separation

CdS–TiO2

MicroemulsionNanocrystals mediated solvothermal method

Almost 8 times than CdS for the decom- The efficient charge separation by position of methylene blue (MB) in minimizing the electron–hole 60 min under UV-vis light irradiation recombination

Fig. 4 SEM (A) and HRTEM (B) images of Bi2WO6/TiO2 nanofiber from ref. 19; SEM (C) and HRTEM (D) images of SnO2/TiO2 heterostructures from ref. 20. Reprinted with permission. Copyright 2009, American Chemical Society.

The enhanced photocatalytic efficiency was attributed to the improvement of the separation of photogenerated electrons and holes as discussed in Section 3.1. 3.2.2 Other S–S heterojunctions. Apart from TiO2 based S–S heterojunctions, other S–S heterojunctions have also been reported, such as Bi2O3–Bi2WO6,14 WO3–BiVO423 and C3N4based heterojunctions.16 These S–S heterojunctions exhibited excellent photocatalytic performances. For example, we have reported C3N4-sulfur-modified-C3N416 and C3N4–MoS224 for hydrogen-evolution. By coating trithiocyanuric acid (TCA) on the surface of C3N4, and then subjecting them to a heat treatment at 600 1C, an isotype C3N4-sulfur-modified-C3N4 heterojunction (termed the CN–CNS heterojunction) was fabricated.16 These CN–CNS heterojunctions exhibited an enhanced H2 evolution activity over their corresponding host substrates. The CN–CNS (the coating

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The proposed reasons

Ref.

20

22

amount of TCA was 20 mg) showed the highest performance, where 11 times higher performance than that of C3N4 was obtained. Furthermore, the H2 produced amount increased steadily with irradiation time, without a noticeable deactivation by four consecutive cycle tests. The enhanced performance was attributed to the promoted charge separation which arose from the band offsets. Besides the S–S SHPs mentioned above, novel S–S SHPs with tuned structure have also been proposed by taking into account of both surface junctions and the texture engineering of the photocatalysts,25–31 including multi-layer-film SHPs,23 and SHPs with spatially separated co-catalysts31 (Table 2). For example, Hong et al. fabricated WO3–BiVO4 heterojunction film electrodes by the layer-by-layer deposition of WO3 and BiVO4 on a conducting glass.23 The electrode with the optimal composition (four layers of WO3 covered by a single layer of BiVO4) exhibited 1.74 and 7.3 times increase of photocurrent relative to bare WO3 and bare BiVO4, respectively, under simulated solar light. Furthermore, by taking BiVO4 as a model semiconductor, Prof. Li’s group confirmed the existence of oxidation and reduction facets,29 and they rationally designed and prepared a series of BiVO4-based SHPs (M/MnOx/BiVO4 and M/Co3O4/BiVO4, M stands for noble metals) by selective deposition of reduction and oxidation cocatalysts on the {010} and {110} facets of BiVO4.30 These SHPs exhibited remarkably enhanced photocatalytic activities for photocatalytic water oxidation and photocatalytic degradation of methyl orange and RhB, which were not only due to the intrinsic nature of charge separation between the {010} and {110} facets of BiVO4, but also the synergistic effect of dual-cocatalysts deposited on different facets of BiVO4. Recently, Prof. Choi’s group reported BiVO4-based SHPs by coupling two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, for solar water splitting.26 The introduction of OEC can reduce the interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction, resulting in the improvement of the propensity for surface-reaching holes to instigate water-splitting chemistry. The resulting BiVO4/FeOOH/NiOOH

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Comparison of the typical S–S heterojunction systems

Types

Preparation

Bi2O3–Bi2WO6 Dip-coatinganneal method

Morphology

The improved performances

The proposed reasons

Flower-like

2.7 times than that of Bi2WO6 for the degradation of RhB in 60 min under visible light irradiation

The great broadening of photo14 absorption range and the existence of the p–n junction

Sheets covered C3N4-sulfurmediated C3N4 with paper-fold thin layers

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WO3–BiVO4

Layer-by-layer deposition

Pt/MnOx/BiVO4 Facet-selective photo-deposition

Ref.

11 times higher hydrogen evolution rate The promoted charge separation which 16 compared with C3N4 under visible light arose from the band offsets irradiation Films

1.74 and 7.3 times increase photocurrent The synergy of excellent charge transfer 23 compared with bare WO3 and bare BiVO4 characteristics of WO3 and good light respectively under UV light irradiation absorption capability of BiVO4

Nanoparticles deposited on crystals

Efficient charge separation More than 65 and 30 times higher O2 evolution rate compared with bare BiVO4 and MnOx/BiVO4 under visible light irradiation respectively

30

BiVO4/FeOOH/ Electrode position FeOOH and NiOOH Exhibited markedly better performance NiOOH combined with layers deposited on for water oxidation than those of BiVO4/ photodeposition BiVO4 nanoporous FeOOH and BiVO4/NiOOH network

The reduced interface recombination at 26 the BiVO4/OEC junction and a more favorable Helmholtz layer potential drop created at the OEC/electrolyte junction

Ta3N5/Pt/IrO2

The core/shell structure and the effective 31 separation and collection of the electrons and holes at the respective cocatalysts

Stober method combined with chemical vapor deposition

Core/shell

More than 6 times higher H2 evolution rate than that of bulk Ta3N5

photoanode achieved a photocurrent density of 2.73 milliamps per square centimeter at a potential as low as 0.6 V versus the reversible hydrogen electrode (RHE). Apart from promoting charge separation, coupling semiconductors with different gaps can also enhance the light absorption, promote the surface reaction kinetics, and thus improve the photocatalytic efficiency. For example, Prof. Ye’s group prepared AgX/Ag3PO4 (X = Cl, Br, I) core–shell heterostructures with an unusual rhombic dodecahedral morphology by a facile and general method.32 The pure Ag3PO4 rhombic dodecahedrons absorb solar energy with a wavelength shorter than 530 nm. After the growth of a thin AgCl and AgBr layer on their surface, the absorption edges of these heterocrystals were drastically extended to around 550 nm and 560 nm respectively. Moreover, their photocatalytic performance studies indicate that these core–shell heterostructures exhibited much higher photocatalytic activities, structural stabilities and photoelectric conversion performances than the pure Ag3PO4 catalyst.

4. The design and construction of semiconductor–metal (S–M) heterojunctions 4.1

The design principle of S–M heterojunctions

Another effective method to create a space-charge separation region (called the Schottky barrier) is to form a S–M junction. At the interface of the two materials, electrons flow from one material to the other (from the higher to the lower Fermi level) to align the Fermi energy levels. The common case is the heterojunction based on the n-type semiconductor and metal, where the ideal case is that the work function of the metal is higher than

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Fig. 5

Schematic of the Schottky barrier.

that of the n-type semiconductor (such as TiO2), and electrons will flow from the semiconductor into the metal to adjust the Fermi energy levels (Fig. 5). The formation of the Schottky barrier results in the fact that the metal has excess negative charges and the semiconductor has excess positive charges. In addition, the Schottky barrier can serve as an efficient electron trap preventing electron–hole recombination in photocatalysis, which often results in an enhanced photocatalytic performance. 4.2

Construction and performance of S–M heterojunctions

In recent years, many S–M heterojunction photocatalysts have been successfully prepared by different groups. Recently, Prof. Antonietti’s group presented the latest progress in the synthesis and catalytic performances of S–M heterojunctions, including N-doped carbon–metal and C3N4–metal heterojunctions.33 In the present review, we focus on the advances in carbon-free S–M heterojunctions, as demonstrated in Table 3. Obviously, after the deposition of noble metal, the photocatalytic efficiencies of these

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Table 3

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Comparison of the typical S–M heterojunction systems

Types

Preparation

Morphology

The improved performances

Au/TiO2

Multicomponent assembly approach

Mesoporous titania photocatalyst by embedding gold NPs

Almost 3 times higher than that of TiO2 Enhanced light absorption and for phenol decomposition under UV-vis improved QE irradiation

Ag–AgCl

Ion-exchange reaction NPs combined with irradiating under visible light

8 times than N-doped TiO2 for the decomposition of MO dye in solution

The plasmon resonance and electron 35 conductivity of Ag

Ag–AgBr

A one-step hydrothermal process

Colloids

13 times and 1.5 times than that of N-TiO2 and AgBr respectively for the degradation of RhB under visible light

The enhanced separation and transportation of photogenerated charge

Necklace-like

The rapid electron export through Ag 37 It can completely degrade RhB dye in only 2 min, while the pure Ag3PO4 cubes nanowires as well as highly efficient need about 8 min charge separation at the contact interfaces

Ag–Ag3PO4 A modified polyol process combined with a hetero-growth process

semiconductors have been significantly enhanced. For example, Li et al.34 synthesized highly active mesoporous Au–TiO2 nanocomposites by utilizing a multicomponent assembly approach, where surfactant, TiO2, and gold building clusters were cooperatively assembled in a one-step process (Fig. 6A and B). These unique nanocomposites show significantly improved photocatalytic

Fig. 6 (A and B) Representative TEM images of (A) Au/TiO2, and (B) highresolution image of Au/TiO2 from ref. 34; (C and D) typical SEM images of Ag–AgBr from ref. 36; inset: a HR-TEM image of one nanoparticle on AgBr; (E) SEM images of Ag@AgCl from ref. 38 (inset: a surface SEM image of the product); (F) SEM images of Ag/Ag3PO4 heterostructures from ref. 37. Reprinted with permission. Copyright 2007, 2012 and 2012, American Chemical Society, Wiley-VCH and Royal Society of Chemistry.

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The proposed reasons

Ref. 34

36

activities for the phenol oxidation and chromium reduction, which are almost 3 times higher than that of TiO2 when 0.5% of Au was doped. They believed that the enhanced light absorption and improved quantum efficiency (QE) are the main factors leading to the improved photocatalytic activity. Plasmonic photocatalysts which were developed very recently have attracted great attention as promising candidates for the development of highly efficient and stable photocatalysts.35,36,38 Ag–AgX and Ag–Ag3PO4 are the most studied and widely used plasmonic photocatalysts. For example, Wang and his coworkers have prepared Ag–AgCl35 by depositing silver NPs directly onto the surface of AgCl. These plasmonic photocatalysts have a strong adsorption in the visible region. In addition, they also exhibited far enhanced photo-oxidation capability for the decomposition of methyl orange (MO) dye in solution compared with N-doped TiO2, under visible light irradiation. However, the synthesis procedures mentioned above for the Ag–AgX (Cl, Br) plasmonic photocatalyst are complex or time-consuming. To solve these problems, Lu et al.36 reported an effective strategy for one-step preparation of Ag–AgBr by hydrothermal treatment of AgNO3 and NaBr in the presence of poly[(2-ethyldimethylammonioethylethacrylate ethyl sulfate)-co-(1-vinylpyrrolidone)] (PQ11) (Fig. 6C and D). These Ag–AgBr exhibited high efficiency for the degradation of RhB under visible light illumination, which was about 13 times and 1.5 times higher than that of N-TiO2 and AgBr respectively. Similarly, Han et al.38 have also developed a simple hydrothermalphotochemical process for the preparation of a cube-like Ag@AgCl plasmonic photocatalyst (Fig. 6E). This Ag@AgCl photocatalyst exhibited high photocatalytic activity for the photodegradation of MO dye under visible light irradiation compared to N-doped TiO2, which should be ascribed to the localized surface plasmon resonance effect from the photogenerated Ag NPs, the well defined interface, and the complexation between Ag+ ions and the nitrogen atoms in the MO dye. In addition, the photocatalytic activity of the photocatalyst has no obvious loss even after four cycles of the photodegradation of MO dye solution. In addition to the AgX (Cl, Br), another semiconductor coupled with Ag was the Ag3PO4 semiconductor. Ye’s group coupled Ag3PO4 submicro-cubes and highly conductive Ag nanowires into novel

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necklace-like coaxial heterostructures37 (Fig. 6F). With this novel necklace-like Ag nanowire-Ag3PO4 cube heterostructure as a photocatalyst, RhB dye can be completely degraded in only 2 min under visible light irradiation, while the pure Ag3PO4 cubes need about 8 min. This enhancement was attributed to the rapid electron export through Ag nanowires as well as highly efficient charge separation at the contact interfaces which was caused by the Fermi level equilibration. S–M SHPs have also been applied in carbon dioxide (CO2) photofixation.39,40 For example, Hinogami et al. prepared metalparticle-coated p-Si electrodes for photoelectrochemical reduction of CO2.40 This p-Si electrode modified with small metal (Cu, Ag, or Au) particles generated a high photovoltage of ca. 0.5 V and works as an effective electrode for the photoelectrochemical reduction of CO2.

5. The design and construction of semiconductor–carbon (S–C) heterojunctions 5.1

The design principle of S–C heterojunctions

For the construction of a S–C heterojunction, different types of carbons have been used, including activated carbon, CNTs and graphene. The design principles of S–C heterojunctions have slight differences, which will be discussed, respectively, in the following section. 5.1.1 Semiconductor-activated carbon heterojunctions. Activated carbon was initially used as a support for TiO2 in photodegradation studies,41 due to its very large specific surface area that is typically more than one order of magnitude larger than that of P25. Usually, the increase of the surface area leads to the improvement of the photocatalytic activity which can be concluded from the Langmuir–Hinshelwood mechanism. Therefore, it can be expected that combining semiconductors with the activated carbon yields an increase in adsorbed amounts of pollutants and thus enhances their photocatalytic activity. 5.1.2 Semiconductor–CNT heterojunctions. CNTs also provide a larger specific surface area and thus the coupling of semiconductors on it can enhance the photocatalytic degradation efficiency, as explained above. Furthermore, similar to the metals above, CNTs also exhibit metallic conductivity as one of the possible electronic structures and just like the semiconductor– metal heterojunction system. Semiconductor–CNTs can form a Schottky barrier junction which is an effective method of increasing recombination time. Furthermore, CNTs have a large electron-storage capacity and therefore can accept photon-excited electrons in mixtures or nanocomposites with semiconductors, thus retarding or hindering the recombination.1 Among the semiconductor–CNT systems, the TiO2–CNT composite system represents a good example illustrating the enhancement of the photocatalytic properties of semiconductor–CNT heterojunctions. Hoffmann et al.1 proposed a mechanism to explain the enhancement of the photocatalytic properties of TiO2–CNT composites, as shown in Fig. 7. A high-energy photon excites an electron from the VB to the CB of anatase TiO2. Photogenerated electrons formed in

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Fig. 7 The proposed mechanisms for the TiO2–CNT heterojunction.

the space-charge regions are transferred into the CNTs until the Fermi levels of the TiO2 and that of the CNTs are on the same level, and holes remain on the TiO2 to take part in redox reactions. 5.1.3 Semiconductor–graphene heterojunctions. Graphene is a single layer of graphite which possesses a unique two-dimensional structure, high conductivity, superior electron mobility and extremely high specific surface area. It can be produced on a large scale at low cost, nowadays. Thus, it has been regarded as an important guest/host component for making various functional materials. In particular, numerous attempts have been made to combine graphene with photocatalysts to enhance their photocatalytic performance.17,42 The graphene in the composites can promote the charge separation, restrain the hole–electron recombination as well as provide a large surface/interface for heterogeneous reactions at the interface, which results in an enhanced photocatalytic activity. As shown in Fig. 8, electron–hole pairs are generated within the semiconductor upon light excitation, these photogenerated electrons tend to transfer to graphene sheets, and then scavenged by dissolved oxygen, facilitating the hole–electron separation. Meanwhile, the holes leaving from the VB of the semiconductor can either react with adsorbed water (or surface hydroxyl) to form hydroxyl radicals or directly oxidize various organic compounds.

Fig. 8 The schematic structure of semiconductor–graphene heterojunctions.

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It is worth mentioning that in some cases, the electronic interactions and charge equilibration between graphene and the semiconductor can lead to a shift in the Fermi level and decrease the CB potential of the semiconductor (Fig. 8). Thus, the negative shift in the Fermi level of semiconductor–graphene and the high migration efficiency of photoinduced electrons can suppress the charge recombination effectively, resulting in the enhanced photocatalytic activity.

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5.2

Construction and performance of S–C heterojunctions

S–C heterojunction systems are currently being considered for many applications including their potential use to address environmental problems. In particular, semiconductor–CNT heterojunction systems have attracted much attention in photocatalytic applications. Recently, heterojunction systems of CNTs and TiO2 have been reviewed.43 All of these heterojunction systems exhibited improved photocatalytic efficiency compared with bare TiO2. The enhancement was due to the slowed recombination of photogenerated electron–hole pairs, the extended excitation wavelength, as well as the increased surface-adsorbed amount of reactant species.44 In addition, plenty of semiconductor–graphene heterojunctions have also been developed, and most of the graphene-based nanocomposites have been reviewed very recently.17,42 They have given a detailed description of the design, fabrication, modification and applications of these graphene-based semiconductor photocatalysts. Herein, several typical semiconductor–graphene heterojunctions have been listed in Table 4. These heterojunctions have exhibited a significant enhancement of photocatalytic efficiency compared with the bare semiconductors. For example, Li et al.46 synthesized graphene nanosheets decorated with CdS clusters through a solvothermal method in which graphene oxide (GO) served as the support while cadmium acetate (Cd(Ac)2) as the CdS precursor (Fig. 9A and B). These nanosized composites exhibited a high H2-production rate of 1.12 mmol h 1 (which was about 4.87 times higher than that of pure CdS NPs) at graphene content of 1.0 wt% and Pt 0.5 wt% under visible-light irradiation. This high photocatalytic H2-production activity was mainly attributed to the presence of graphene, which serves as an

Table 4

Types

Fig. 9 (A and B) TEM images and SAED pattern of graphene nanosheets decorated with CdS clusters from ref. 46; (C and D) TEM images of graphene decorated with Bi2WO6 composite from ref. 47. Reprinted with permission. Copyright (2011), American Chemical Society and PCCP Owner Societies.

electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS NPs. Bi2WO6 has also been coupled with graphene to enhance its photocatalytic efficiency. For example, Wang’s group47 prepared a graphene– Bi2WO6 composite via an in situ hydrothermal reaction (Fig. 9C and D). This graphene–Bi2WO6 photocatalyst showed significantly enhanced photocatalytic activity for the degradation of RhB under visible light (l 4 420 nm), which was 3 times greater than that of the pure Bi2WO6. The enhanced photocatalytic activity could be attributed to the negative shift in the Fermi level of graphene– Bi2WO6 and the high migration efficiency of photoinduced electrons; these electrons may not only be effectively involved in the oxygen reduction reaction but also suppress the charge recombination, as revealed in Section 5.1.3. Besides CNTs and graphene, other type carbons can also be used to construct S–C heterojunctions. For example, carbon@TiO2 dyade

Comparison of the typical graphene–S heterojunction systems

Preparation

Morphology

The improved performances

The proposed reasons

Ref.

Self-assembly Macro-mesoporous solid TiO2– graphene method films

11 times higher MB degradation The improved mass transport, the reduced the rate than that of pure mesoporous length of the mesopore channel, the increased titania films accessible surface area of the thin film and the suppressed charge recombination

P25– A one-step P25 NPs deposited on graphene hydrothermal graphene nanosheet reaction

6 times than that of P25 for the degradation of MB under visible light

Great adsorptivity of dyes, extended photorespond- 45 ing range, and enhanced charge separation and transportation properties

CdS– A solvothergraphene mal method

4.87 times higher H2-production rate than that of pure CdS NPs under visible-light irradiation

The enhanced separation of photogenerated electron and hole pairs

Graphene nanosheets decorated with CdS clusters

44

46

Bi2WO6– In situ hydro- Bi2WO6 NPs distributed on 3 times than that of Bi2WO6 for the The negative shift in the Fermi level of graphene– 47 graphene thermal the surface and edges of degradation of RhB under visible Bi2WO6 and the high migration efficiency of photoinduced electrons reaction the graphene sheets light

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structure has been prepared by a solvothermal method.11 C@TiO2 has a high surface area and anatase structure and is able to absorb a high amount of photoenergy in the visible region, driving effectively photochemical degradation reactions. The origin of photocatalytic activity under visible light is due to a direct optical charge transfer transition involving both the TiO2 and carbon phase, keeping the high reactivity of the photogenerated electron and hole.

6. The design and construction of multicomponent heterojunctions Up to date, although a variety of approaches have been developed to prepare many kinds of VLD SHPs, there are still some drawbacks, such as the limited region of visible-light photo-response. To solve these problems, multicomponent heterojunction systems have been developed,48,49 in which two or more visible-light active components and an electron-transfer system are spatially integrated. The typical schematic structure of multicomponent heterojunction systems is shown in Fig. 10. First of all, since both semiconductor A (S-A) and semiconductor B (S-B) can be excited by UV/visible light and have different photoabsorption ranges, the conjunction of their photoabsorption can broaden the range of UV/visible-light photoresponse. Secondly, the photocatalytic reaction is initiated by the absorption of UV/visible-light photons with energy equal or higher than the band-gap in both S-A and S-B, which results in the creation of photogenerated holes in their VB and electrons in their CB. On the one hand, the electrons in the CB of S-A easily flow into metal (electron transfer I: S-A - metal) through the Schottky barrier because the CB (or the Fermi level) of S-A is higher than that of the loaded metal, which is consistent with the previous study on electron transfer from the semiconductor (such as TiO2) to metal (such as Ag, Au).48,50 This process of electron transfer I is faster than the electron–hole recombination between the VB and the CB of S-A. Thus, plenty of electrons in the CB of S-A can be stored in the metal component. As a result, more holes with a strong oxidation power in

Fig. 10 Schematic structure of multicomponent heterojunction systems.

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the VB of S-A escape from the pair recombination and are available to oxidize the pollutants or OH . On the other hand, since the energy level of metal is above the VB of S-B, holes in the VB of S-B also easily flow into metal (electron transfer II: metal - S-B, see Fig. 10), which is faster than the electron–hole recombination between the VB and CB of S-B. More electrons with a strong reduction power in the CB of S-B can escape from the pair recombination and are available to reduce some absorbed compounds (such as O2, H+, etc.). Therefore, simultaneous electron transfer I and II (that is, vectorial electron transfer of S-A - metal - S-B in Fig. 10) can occur as a result of UV/visible-light excitation of both S-A and S-B. In these vectorial electron-transfer processes, metal in multicomponent heterojunction systems acts as a storage and/or a recombination center for electrons in the CB of S-A and holes in the VB of S-B, and contributes to enhancing interfacial charge transfer and realizing the complete separation of holes in the VB of S-A and electrons in the CB of S-B. So the multicomponent heterojunction systems can simultaneously and efficiently generate holes with a strong oxidation power in the VB of S-A and electrons with a strong reduction power in the CB of S-B, resulting in greatly enhanced photocatalytic activity, compared with the single semiconductor or semiconductor heterojunctions mentioned above. In 2006, Tada et al.48 developed a CdS–Au–TiO2 threecomponent nanojunction system using a simple photochemical technique (Fig. 11A and B). This CdS–Au–TiO2 nanojunction exhibited excellent photocatalytic activity, which was far higher than that of either the single component or two components systems. For this photocatalytic CdS–Au–TiO2 nanojunction system, 52.2% of methylviologen (MV2+) have been reduced in 100 min, which are 1.6, 1.8 and 2.3 times than that of Au/TiO2, CdS/TiO2 and TiO2 (calculated from the figure in the original publication48).

Fig. 11 TEM (A) and HRTEM (B) images of Au@CdS-TiO2 from ref. 48; SEM (C) and HRTEM (D) images of the AgBr–Ag–Bi2WO6 nanojunction system from ref. 12. Reprinted with permission. Copyright 2006 and 2009, Nature Publishing Group and Elsevier.

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Subsequently, an AgBr–Ag–Bi2WO6 nanojunction system was developed by a facile deposition–precipitation method (Fig. 11C and D).12 This AgBr–Ag–Bi2WO6 nanojunction system shows much higher VLD photocatalytic activity than a photocatalyst with single visible-light response components, such as Bi2WO6 nanostructures, Ag–Bi2WO6 and AgBr–Ag–TiO2. For example, with the AgBr–Ag–Bi2WO6 nanojunction system as the photocatalyst, the MX-5B could be photocatalytically degraded 42.8 mg L 1 within 60 min under visible-light irradiation, which is higher than that by Bi2WO6 nanostructures (2.0 mg L 1), Ag–Bi2WO6 (2.9 mg L 1) and AgBr–Ag–TiO2 (34.1 mg L 1). Furthermore, 65% of pentachlorophenol could be mineralized within 4 h by AgBr–Ag–Bi2WO6, which is much higher than that (34.5%) of the AgBr–Ag–TiO2 composite. This excellent VLD photocatalytic performance was mainly attributed to the vectorial interparticle electron transfer driven by the two-step excitation of both VLD components (AgBr and Bi2WO6).

7. Conclusions and outlook In summary, we have discussed the general strategies and recent progress in SHPs for developing highly efficient and stable photocatalysts, including: (1) the coupling of semiconductors with other semiconductors to satisfy high absorption of solar energy and to create sufficient built-in potential for redox reactions; (2) the formation of heterostructured junctions with carbon material to effectively drive the separation and transportation of the electron– hole pairs; (3) the deposition of metal to enhance the utilization of sunlight or improve the separation and transportation of the electron–hole pairs; (4) the forming of multicomponent heterojunctions for enhancing the utilization of sunlight and improving the separation/transportation of the electron–hole pairs. The achieved progress in this field indicates that forming heterojunctions affords a promising route to enhance the photocatalytic efficiencies of photocatalytic semiconductors. To date, many kinds of semiconductor heterojunctions have been reported to improve photocatalytic efficiencies by enhancing the utilization of sunlight or improving the separation/ transportation of the electron–hole pairs, and some examples are highlighted in this review. However, the studies in this area are currently unsystematic, and the heterojunction systems with high efficiency and stability are needed to be further developed. Meanwhile, the heterogeneous photocatalysts have been and will continue to be applied in solar water splitting and environmental remediation, while the photocatalytic conversion of carbon dioxide to fuels and other useful chemicals is expected to be a dominant area.

Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB632405), the National Natural Science Foundation of China (Grant No. 21033003, 21007009, 21107013, 21377023, 41273108, and 51272299), High-Tech Research and Development Program of China (Grant No. 2012AA030309),

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Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1221), Specialized Research Fund for the Doctoral Program of Higher Education (CUSF-DHD-2013054), Innovation Program of Shanghai Municipal Education Commission (Grant No. 13ZZ053), project of the Shanghai Committee of Science and Technology (13JC1400300), the Shanghai Leading Academic Discipline Project (B604), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program.

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Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances.

Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental is...
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