Structure design, controllable synthesis, and application of metal-semiconductor heterostructure nanoparticles

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Structure design, controllable synthesis, and application of metalsemiconductor heterostructure nanoparticles Yuepeng Lv1, Sibin Duan1, Rongming Wang∗ Beijing Advanced Innovation Center for Materials Genome Engineering, Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, 100083, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-semiconductor Heterostructure Structure design Synthesis method Application

Metal-semiconductor heterostructure nanoparticles (NPs) have attracted much interest and extensively been investigated due to their promising potentials in the fields of catalysis, energy conversion and storage. Due to the synergistic effect and coupling effect between metal and semiconductor, designing and controlling the interface structure of metal-semiconductor NPs and understanding the structure-property relationship have great significance for optimizing their physicochemical performances. Metal NPs coupled with semiconductors forming several kinds of interface structures, including core-shell, yolk-shell, Janus and oligomer-like structures are summarized in this review. The controllable synthesis methods to obtain metal-semiconductor heterostructure NPs with fantastic properties are presented in detail. Moreover, the structure-property relationship and their applications in the fields of catalysis, energy conversion and storage are also exhibited. A brief outlook is given on the challenges and possible solutions in future development of metal-semiconductor NPs.

1. Introduction

metal-semiconductor NPs at atomic scale is also a problem to solve. Semiconductor nanomaterials are widely used in many fields. Usually, high-performance materials can be obtained by controlling the morphology, component and atomic structure of semiconductors [9]. Designing heterostructure of metal and semiconductor NPs through suitable process, and utilizing the synergistic effect of the two parts are also practical methods to improve the physicochemical performances of semiconductor [4–8,10–12]. Metal-semiconductor heterostructures are widely used in many fields, especially in the fields of optics-related, electrocatalysis and energy storage, such as solar cell [13,14], photocatalysis [15–23], hydrogen evolution reaction (HER) [4,24,25], oxygen evolution reaction (OER) [7,26], oxygen reduction reaction (ORR) [27–29], supercapacitors [5,8,30,31], ions batteries [32–39] and so on. In this review, we will provide an overview of metal-semiconductor heterostructure NPs. The structure features of NPs with core-shell, yolkshell, Janus and oligomer-like nanostructures are summarized in Section 2. In Section 3, the controlled synthesis methods are presented, including physical methods and chemical methods. And in Section 4, we will discuss the unique physicochemical properties of metal-semiconductor heterostructure NPs, especially in the catalytic and energy related fields. In the last section, the scientific problems in the field is analyzed and a brief outlook based on the previous summaries is given

Nano-heterostructures refer to the composite structures formed by the combination two or more components of nanomaterials through specific interfacial contacting. Metal-semiconductor heterostructure is a composite structure system formed by metal and semiconductor components through specific interface, which is one of the most effective ways to construct nanomaterials with specific physicochemical properties [1–3]. Due to the coupling and synergistic effects resulting from heterogeneous interfaces, the system not only possesses the corresponding physicochemical properties of its individual component, but also exhibits superior or completely new characteristics. By controlling the components and the interface structure of heterostructure NPs, the properties of metal-semiconductor materials can be effectively optimized. The metal-semiconductor NPs can be designed by adjusting the surface structure, interface structure, crystal structure and electronic structure [4,5]. Many interface interactions, such as charge transfer, interface strain, and exciton-plasmon interaction, have been comprehensively studied in metal-semiconductor nanostructures [6–8]. Moreover, due to the differences of crystal structure, physical and chemical characters between metal and semiconductor, the design and synthesis of metal-semiconductor NPs is of great challenge for researchers, especially designing the interface structure and electronic structure of ∗

Corresponding author. E-mail address: [email protected] (R. Wang). 1 These authors contributed equally to this paper. https://doi.org/10.1016/j.pnsc.2019.12.005 Received 29 December 2019; Received in revised form 31 December 2019; Accepted 31 December 2019 1002-0071/ © 2020 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Yuepeng Lv, Sibin Duan and Rongming Wang, Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.12.005

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surface and inhibit the growth of particles on specific crystal facts [55,56].

(See Fig. 1). 2. Structure feature

3. Controllable synthesis Metal-semiconductor heterostructure NPs can be classified into core-shell, yolk-shell, Janus and oligomer-like structures according to their geometrical configurations. For the metal-semiconductor core-shell structure, the core is coated and completely contacted with the shell, suggesting that this structure has one kind of surface composed of semiconductor component and a fully contacted metal-semiconductor interface. Forming core-shell structure can inhibit the aggregation of high surface energy metal NPs and protect them from environmental corrosion. Due to the complete contact between the metal and the semiconductor parts, the core-shell structure has the largest interface area, which maximize the electronic transfer along the interface and reduce the charge transfer resistance of the material [8]. The preparation method of core-shell structure of metal-semiconductor NPs, especially those with single crystal shell, is very complex. The commonly used method to construct metal-semiconductor core-shell nanostructure is seeded growth method. Because of the need of small lattice mismatch between the core and shell and the harsh reaction conditions, many preparation methods have been proposed, such as in-situ conversion method [4,7,8], ion exchange [10,11,40,41], sol-gel method [42]. The metal-semiconductor NPs with yolk-shell structure have similar structure with the core-shell NPs, where the metal core NPs are located at the inner part of the semiconductor shells. The difference is that the core and the shell are not completely contacted, leaving a hollow space between the core and shell. Metal-semiconductor yolk-shell structure has one kind of surface composed of semiconductor component and an incomplete contacted metal-semiconductor interface. This structure can not only inhibit the aggregation of metal particles with high surface energy and protect metal NPs from environmental corrosion, but also can be applied in many fields due to its unique hollow structure. For the photocatalytic reaction, due to the incomplete contact between the core and the shell, the recombination of photo generated electron-hole pairs is inhibited, the lifetime of photo-generated electron-hole pairs can be prolonged, and the photocatalytic activities of the material are improved [15]. When the electromagnetic wave enters the yolk-shell structure, the hollow structure can increase the transmission distance of the electromagnetic wave and increase the absorption of it [16]. For some yolk-shell structures, there are holes on the shell, through which other materials can be transferred into the hollow space between metal core and semiconductor shell. Yolk-shell structure metal-semiconductor NPs can be prepared by template method [15,16,43]. Synthesis approaches, i.e., in-situ conversion method, ion exchange method, etc. based on Kirkendall effect, have been proven to be effective to obtain yolk-shell structure [4,44]. Janus and oligomer-like nanostructures have similar structures, both of which are the growth of metal NPs on the surface of semiconductors or the growth of semiconductor NPs on the surface of metals. Therefore, it can reduce the contact with environment and improve the stability of the material performance. In the Janus and oligomer-like nanostructures both the metal and semiconductor parts contact with the external environment, while the relative contact area is smaller than that of core-shell structure and yolk-shell structure. Janus structure and oligomer-like structure metal-semiconductor NPs are usually prepared by chemical deposition method [27,45–48], seeded growth methods [49,50], chemical reduction method [51,52], sol-gel method [53,54]. The growth location of secondary component on the surface of as-prepared component, such as the top of nanorods, the top or the center of tetrahedron is a key factor to control their physicochemical performances. In order to control the growth locations, it is necessary to control the free energies of the material surface and interface. The surfactants are widely used to control the surface energy and interface energy, change the wettability of materials, cover specific planes

There are several ways to control the structure of metal-semiconductor NPs, such as choosing the suitable materials, which can effectively reduce the lattice mismatch between metal and semiconductor, thus the interface energy controlling and changing, special synthesis methods and reaction mechanisms also can be used to design the structure of metal semiconductor materials. For the controllable synthesis of metal-semiconductor heterostructures, several kinds of methods have been developed, including physical methods and chemical methods. Physical synthesis methods are usually simple, mainly physical deposition methods, which are beneficial to obtaining metal-semiconductor heterostructures with clean surface. Chemical synthesis methods are widely used, which are relatively complex and numerous compared with physical methods, including chemical deposition method, chemical reduction method, solgel method, template method, hydrothermal/solvothermal method, etc. 3.1. Physical synthesis methods Physical deposition methods have been developed many years to uniformly grow metal NPs on the surface of semiconductors [57–63]. It can be divided into Physical Vapor Deposition (PVD), Pulsed Laser Deposition (PLD), Molecular Beam Epitaxy (MBE) and so on. Physical deposition is one of the main methods for the preparation of highperformance metal-semiconductor films and devices. The advantages of these methods are that no other impurities will be introduced, the basic form of the semiconductor does not change, and the size of the metal NPs can be easily controlled. Hou et al. prepared TiO2 nanotube array by an anodizing process. The Au NPs were coated on the TiO2 nanotube array by the PVD method [57]. The size of the Au NPs was controlled by controlling the current and deposition time. Ma et al. prepared Bi@ Bi2O3 by PLD method. Bi NPs were prepared by PLD firstly, and then the surfaces of Bi particles were oxidized in ambient air to produce Bi2O3 layers. The thicknesses of the Bi2O3 layers were controlled by the temperature of the substrate [62]. Sasak et al. prepared Sn-doped nGa2O3 simple metal-semiconductor field-effect transistor by MBE method. Sn-doped n-Ga2O3 MESFET layers were formed on Mg-doped b-Ga2O3 substrates by MBE, and then Pt/Ti/Au were deposited to form Schottky gates [63]. 3.2. Chemical synthesis methods Although the physical method could be used to prepare metalsemiconductor NPs, it is difficult to control the structure of metalsemiconductor NPs. The preparation of metal-semiconductor NPs by chemical method is more complex, and more parameters could be used to control the nanostructure, component and shape of metal-semiconductor heterostructure NPs. 3.2.1. Chemical deposition method Chemical deposition method is a common and simple method to prepare metal-semiconductor heterostructures, which includes Chemical Vapor Deposition (CVD), Liquid-Phase Deposition (LPD), etc. Photochemical deposition is a chemical deposition method without complex reaction conditions, and can be used in both gas and liquid phases [27,45–48,55]. Photo-generated electrons generated from semiconductor can act as a reducing agent under illumination of light with a suitable wavelength. The advantage of this method is that it does not require an additional reducing agent or stabilizer, so that a pure composite can be obtained and a clean interface can be obtained. The size of the metal NPs can be controlled by different irradiation times, irradiation energy, and precursor concentration. Generally, the shape 2

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acts as surfactant, Cu2O NPs are grown on the apexes. When hexadecyl trimethyl ammonium Bromide (CTAB)/sodium dodecyl sulfate (SDS) is used as surfactant, Cu2O covers the hexahedral Au NP without apexes. If the surfactant is SDS, Cu2O will cover the whole surface of Au NPs. Different adsorption of surfactants on the site of high curvature has a great effect on the deposition of Cu2O [64–66]. 3.2.2. Chemical reduction method Generally, chemical reduction means that the as-prepared metal or semiconductor NPs are immersed in solution containing the precursors of another component. The precursors are adsorbed on the surface of the NPs, and then reduced by appropriate chemical reductant [28,51,52]. Commonly used reductants include sodium borohydride, sodium citrate and ascorbic acid. The reaction conditions of chemical reduction are mild and the method is simple. Janus and oligomer-like heterostructure metal-semiconductor can be prepared with this method. Camargo et al. prepared TiO2spheres decorated with Au in water using HAuCl4 as Au precursor, ascorbic acid as the reductant and PVP as the stabilizer [51]. With this method, the size and amount of attached Au NPs can be controlled by the concentration and volume of the reductant. It has also been applied to synthesis Pd, Pt and Ag decorated TiO2 NPs. Pandikumar et al. used chemical reduction method to reduce HAuCl•H2O by using NaBH4 to obtain Au–TiO2nanocomposite [52]. Fig. 1. Schematic representation of the structures and the applications of metalsemiconductor heterostructure NPs summarized in this review.

3.2.3. Sol-gel method Sol-gel method is based on ligand or surfactant molecules [42,53,54,56,67]. In a typical route, metal or semiconductor NPs are added to the precursor solution containing ligands or surfactants to form colloids by seeded growth or redox reaction, and then metalsemiconductor heterostructure NPs are obtained by hydrolysis and condensation of the intermediates. The morphology and structure of the obtained metal-semiconductor mainly depend on the choice and usage of the precursors and surfactants. Sonawane et al. prepared Au–TiO2 heterostructure by heating the prepared Au–TiO2 dry gel at different temperature [53]. Han et al. prepared Au@TiO2-graphene core-shell structure by the hydrolysis of the intermediates formed by TiCl3 solution [42] (Fig. 3a). Han et al. prepared three kinds of Aunanorod-TiO2 heterostructure NPs by the hydrolysis of precursor [56] (Fig. 3b ~ f). The morphology of products is controlled by changing the volume of titanium di-isopropoxide bis(acetylacetone) (TDAA) solution. Because of the hydrophobicity of TDAA, Au nanorods do not have good wettability with the hydrophilic surface, which can influence the interfacial energy of Au–TiO2. When larger volume TDAA solution is added, the concentric geometry turned to the most stable configuration. At this time, the total energy is the lowest. When the volume of TDAA solution

and structure of heterostructures will not be destroyed during photochemical deposition. Wang et al. prepared Pt–TiO2/C by photochemical deposition method [27]. The solution was irradiated by mercury lamp to deposit Pt NPs on TiO2/CNPs. The morphology of the material does not change after the reaction. Zeng et al. prepared Au/ZnO nano-pyramids by photochemical deposition method [48]. The Au NPs were deposited on the vertexes of the ZnO nano-pyramids by UV irradiation. For the ZnO pyramid, the highest occupied state is the O-p state at the bottom of the pyramid, and the lowest unoccupied state is the Zn-sp at the apex of the pyramid. The excited electrons are transferred from the (001)-O surface to the (001)-Zn surface under UV irradiation. Au ions adsorbed on the (001)-Zn surface were easily reduced by bringing the excited electrons to their empty state. The filled state of Au is lower than the Fermi level, and the empty state is located in the bulk band gap of ZnO. This causes the reduced Au NP to replace the capped (001)-Zn surface. Au NP Therefore, Au NPs grow at the vertexes of ZnO nanopyramids. Han et al. prepared hexahedral Au–Cu2O heterostructures by chemical deposition method [55] (Fig. 2). By selecting the type of surfactant, Cu2O NPs were grown in a particular location. When PVP

Fig. 2. (a) Schematic illustration of the formation of Au–Cu2O hetero-nanocrystals with different growth modes of Cu2O. SEM images (b–d), TEM images (e–g), and HAADF-STEM and EDS elemental mapping images (h–j) of Auvertex-Cu2O (b, e, and h), Auvertex-exp-Cu2O (c, f, and i), and AuHOH@Cu2O (d, g, and j) heteronanocrystals. Reproduced with permission from Ref. [55]. Copyright: 2016 American Chemical Society. 3

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Fig. 3. (a) Schematic illustration of the synthesis process of G-AuONC@TiO2 CSNs. (b–d) TEM images of Au nanorod-TiO2 Janus, eccentric, and concentric geometries. (e) Schematic representations of the Au nanorod-TiO2 with various geometries. (f) Schematic representation of the model of a Au nanorod-TiO2 used for theoretical energy calculations. (g) Plots of normalized total energy (sum of interfacial and elastic energies) of the system versus L1/L2 for various values of V/V0 = (L12+L22)/ 2 L2, which is related to the volume of TDAA precursor solution added. (g) Schematics illustrating the measures for targeting the typical problems in the synthesis of core-shell nanostructures. Reproduced with permission from Refs. [42,56,67]. Copyright: 2011 The Royal Society of Chemistry, 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2013 American Chemical Society.

with water to obtain Ag@AgCl cubic cage NPs. For the case of using ethylene glycol reduction method, Ag NPs were directly deposited on AgCl cages, and NaCl templates were removed at the same time. Using the template method, the shape of metal-semiconductor heterostructures can be well controlled by choosing suitable template [69,71]. However, if the etch process is incomplete, impurities are introduced into the sample, and excessive etching may damage the structure.

is less, the dipolymer structure NPs are obtained. TiO2 NPs nucleate and grow on one side of the gold nanorods, which reduces the Au–TiO2 interface area. Therefore, bending the nanorods is required to minimize the interfacial energy. Chen et al. prepared Au@ZnO core-shell structure by the hydrolysis of precursor. PVP was used to tune the interfacial energy, stabilize NPs to prevent aggregation, interfere with ZnO crystal formation [67] (Fig. 3g). The advantages of sol-gel method are that the interface energy between metal and semiconductor NPs could be effectively controlled by the introduction of ligands or surfactants, thus the thickness of the shell and size of metal NPs would be well regulated. However, the necessary calcination process could make the morphology and structure of metal-semiconductor NPs changed at high temperature.

3.2.5. Hydrothermal/solvothermal method 3.2.5.1. Seeded growth. Seeded growth method is a commonly used method for preparing metal-semiconductor heterostructures [4,13,17,49,50,72]. Epitaxial growth is a typical seeded growth strategy. Shi et al. have prepared Au–Cu3InSnSe5 and Pt–Cu3InSnSe5 core-shell structures using seeded growth method [13]. Au NPs and Pt NPs are firstly prepared as the seeds by the reduction of H2PtCl6•6H2O with Oleylamine. The Cu3InSnSe5 shell then grow on the surface of Au or Pt NPs to form core-shell NPs. In order to obtain high-quality single crystal heterostructure, the epitaxial growth method is commonly used. For epitaxial growth, the lattice mismatch between the metal and semiconductor component is a key parameter to determine the interface structure and the ultimate morphology of metal-semiconductor heterostructure NPs. If the mismatch is relatively large, it is difficult to obtain the core-shell or yolk-shell NPs especially the NPs with single crystal shells. The surfactants can be used to alter the wettability of the NPs surface and reduce the surface energy, thus promote the epitaxial growth mechanism to form core-shell or yolk shell NPs. Zeng et al. prepared four kinds of Au–Fe3O4 nanocrystals with different morphologies [49] (Fig. 5). Due to the small lattice mismatch between 2d111 (Au) and d111 (Fe3O4), Fe3O4 can epitaxially grow on Au NPs to obtain spherical, cubic, Janus and dumbbell-shaped Au–Fe3O4 heterostructures.

3.2.4. Template method Templated methods, including hard template method and soft template method have been developed for the nanomaterial synthesis during the last few years [68–71]. Typically, the materials that are easily etched, such as carbon, polystyrene, and SiO2 spheres, are used as templates. Choosing the appropriate template, the shell grows on the surface of the template. Then the template is etched off, leaving the core-shell or yolk-shell NPs. Zaera et al. synthesized Au/TiO2 yolk-shell nanostructures by template method [68]. Au@SiO2 NPs were firstly obtained by coating SiO2 on Au NPs, then TiO2grew on the as-prepared Au@SiO2 NPs. Finally, after SiO2 templates were etched off by NaOH solution, the Au@TiO2 yolk-shell NPs were left. Sum et al. also prepared Ag@AgCl cubic cage NPs by template method [70] (Fig. 4). During the synthesis, NaCl single crystals were used as the temples. AgCl grew on NaCl template to form NaCl@AgCl cubic cages, after which Ag NPs were deposited on the as-prepared NaCl@AgCl cubic cages by photoreduction method. Then NaCl was removed by washing the product 4

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Fig. 4. Schematic illustration of the water-soluble sacrificial salt-crystal-template route for the formation of Ag@AgCl cubic cages. Two methods have been selected to generate Ag NPs: photoreduction and ethylene glycol-assisted reduction. Reproduced with permission from Ref. [70]. Copyright: 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

catalysis [44]. After annealing, NiAu-based materials are transformed into NiAu@Au@NiO core-shell structure. Using the prepared NiAu@Au@NiO NPs to catalyze NaBH4 hydrogen production and under the driving of the catalytic reaction, the Ni atoms moved to the outside of the NPs and the O atoms moved to the inside of the NPs, finally forming the Au@NiO yolk-shell structure.

3.2.5.2. In-situ conversion. For the seeded growth method, the closely matched lattice parameters of core and shell components are crucial to obtain metal-semiconductor NPs with single crystal shells. In most cases, the crystalline defects induce weakened material properties and degraded device performances. In order to solve the problem, a new preparation method, in-situ conversion, has been developed recently [4,7,8,44,73]. Wang et al. developed this method to synthesize Au/ Ni12P5 core-shell NPs [8] (Fig. 6). Since the lattice mismatch between Au and Ni12P5 is large, it is difficult to prepare core-shell structure by seeded growth. To obtain Au@Ni12P5 core-shell NPs with single crystal shells, their corresponding Au–Ni bimetallic NPs with dumbbell heterostructure. Due to the strong capping effect of TPP, it acts as a capping agent on the surface of the Au atom, which is very important for the formation of the Au core. As the temperature increases, the recrystallization process is accompanied by the migration of Ni atoms. triphenyl phosphine (TPP) is simultaneously used as a phosphorus source to convert Ni into a single crystal Ni12P5 shell to form an Au/ Ni12P5 core-shell structure. It is a new method for preparing metalsemiconductor NPs, which solves the problem of large lattice mismatch between metal and semiconductor and preparing core-shell structures. Qiao et al. transformed NiAu-based material structure into Au@NiO yolk-shell structure using in situ conversion method by annealing and

3.2.5.3. Ion exchange. For several metal-semiconductor heterostructures, it is quite difficult to be prepared directly, while the preparation method of metal-semiconductor NPs with similar composition with the target product is mature. Then the target product can be obtained indirectly by changing the anion or cation of the semiconductor material using the ion exchange method [10,11,15,16,40,41,43]. Yu et al. prepared Au@CdS core-shell nanorods by cation-exchange [11]. Au@Ag2S nanorods were prepared by the sulfuration of Au@Ag nanorods. Subsequently, Cd2+ was used to exchange with Ag+ to obtain Au@CdS nanorods. Zhang et al. used soft acid-base coordination reactions to prepare core-shell structured metalsemiconductor NPs with large lattice mismatch. The amorphous layer was formed outside the metal core, and then the single crystal shell was obtained by ion exchange [41]. Zhang et al. prepared Au@MS (M = Cu, Cd, and Sn) yolk-shell structure heterostructures using ion exchange

Fig. 5. (a, b) HRTEM images and electron diffraction patterns from spherical Au@Fe3O4 core-shell NPs, (c, d) cubic Au@Fe3O4 core-shell NPs, (e)Janus structure Au–Fe3O4 NPs, (f) dumbbell-like Au–Fe3O4 NPs. The scale bars are 4 nm. Reproduced with permission from Ref. [49]. Copyright: 2006 American Chemical Society. 5

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Fig. 6. (a) Schematic illustration of the formation of the Au/Ni12P5 core-shell NPs. (b–d) TEM images of the product at different times after adding TPP, indicating that the final core-shell Au/Ni12P5 NPs are converted from heterodimer Au–Ni NPs. Reproduced with permission from Ref. [8]. Copyright: 2014 Nature Publishing Group.

Fig. 7. (a) Schematic of the preparation of the Au@MS (M = Cu2-x, Cd, and Sn) yolk-shell Nanocrystals via sulfuration and versatile phosphine-initialized cation exchange; (b) TEM image of the as-prepared Au@Cu2O core-shell Nanocrystals. (c ~ e) TEM image of the Au@Cu2xS yolk-shell Nanocrystals, the Au@CdS yolk-shell Nanocrystals and the Au@SnS yolk-shell Nanocrystals. Reproduced with permission from Ref. [43]. Copyright: 2017 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

sensitized solar cells (DSSC), the preparation of high-performance solar cells has attracted much attention from researchers. The counter electrode is an important part of dye-sensitized solar cells. The preparation of high-performance counter electrode is of great significance to improve the performance of solar cells [13,14]. Metal-semiconductor NPs possess better performance than pure semiconductors because of the synergistic effect and Mott Schottky effect, which can reduce the internal resistance of materials and improve the conductivity of materials [14]. Therefore, metal-semiconductor heterostructure NPs have been designed and widely used as the counter electrodes of solar cells. Li et al. prepared Pt–Cu3InSnSe5 and Au–Cu3InSnSe5 core-shell structure by solvothermal method [13] (Fig. 8). Compared with Cu3InSnSe5 NPs, DSSC device tests exhibited that power conversion efficiencies of Pt/ Cu3InSnSe5 and Au/Cu3InSnSe5 core-shell NPs increased from 5.8% to 7.6% and 6.5%, respectively. The enhanced performances were resulted from the reduced internal resistance and improved conductivity with the beneficial of metal-semiconductor interface. Chen et al. prepared Ni3S4–Pt2Fe1 nanorods as the counter electrodes of solar cells by solvothermal method [14]. The power conversion efficiency of Ni3S4–Pt2Fe1 is 8.79%, better than that of Pt2Fe1 (7.91%), Pt (7.83%) and Ni3S4 (7.31%). As the Mott–Schottky effect can generate enhanced internal electric field at the interface between metal and semiconductor. The metal-semiconductor interface also promotes the

method [43] (Fig. 7). Firstly, the Au@CuO core-shell structure was synthesized. Then, using Na2S solution as an ion exchanger, i.e., S2− substituted O2− in the solution. The Au@CuS yolk-shell structure was obtained with the help with the Kirkendall effect. If Cu2+was replaced with Cd2+, Sn2+, and Zn2+, the target product Au@MS yolk-shell structures were also successfully synthesized. 4. Applications The metal-semiconductor heterostructure NPs are unique system containing specific metal-semiconductor interfaces, which determine the physicochemical performance of the NPs to some extent. To investigate the structure-performance relationship is beneficial not only to deeply understanding the intrinsic mechanism, but also to guide the design of interface structure for enhanced capabilities in many fields of applications. In this review, we focus on the application of metalsemiconductor composite structures in the field of optics-related catalysis [13–23], electrocatalysis [4,7,24–29] and energy-storage related fields [5,8,30–39]. 4.1. Solar cell Due to the low cost and being environment friendly of dye6

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Fig. 8. (a) J−V characteristics of DSSCs devices with counter electrodes made of conventional Pt, neat Mo-coated glass, and Mo-coated glass with Pt/Cu3InSnSe5, Au/Cu3InSnSe5, and Cu3InSnSe5 NP films, respectively. (b) Plots of PCE of DSSCs devices with counter electrodes made of Mo-coated glass with Pt–Cu3InSnSe5, Au–Cu3InSnSe5, and Cu3InSnSe5 NP films as a function of time, respectively. Reproduced with permission from Ref. [13]. Copyright: 2017 American Chemical Society.

4.3. HER

electron transfer, thus improves the power conversion efficiency of materials.

HER is a half reaction of water splitting reaction, which can be used to prepare clean energy-hydrogen. The metal-semiconductor heterostructure is designed to realize the charge transfer between metal and semiconductor, optimize the electronic structure of materials, and regulate the adsorption desorption of ions and groups, so as to improve the catalytic performance of materials. Wang et al. prepared Au-NiSx core-shell, yolk-shell and oligomer-like structure NPs by seeded growth method and in-situ conversion method [4] (Fig. 10). The HER activity of Au@NiSx core-shell NPs is the highest among the three kinds of heterostructures. The HER catalysts results indicate that the catalytic activities of the Au-NiSx NPs are predominately dependent on their interface structures. The Au@NiSx core-shell NPs outperform the other two metal-semiconductor resulting from the synergistic effect and coupling effect between Au cores and NiSx shells, which is confirmed by the X-ray photoelectron spectroscopy. Wang et al. prepared NiSe2/Ni ultrafine NPs by high temperature treatment method [25]. The NiSe2/ Ni-NSC has lower overpotential, Tafel slope and impedance toward HER than other materials such as Ni NC. The strong interaction is due to the coupling of metallic Ni and NiSe2. The unique core-shell structure features endow NiSe2/Ni-NSC with more accessible active sites and a more convenient change transfer approach. Ying et al. prepared Co/ TiO2 Schottky catalyst by solvothermal method [24]. The Co/TiO2 has an overpotential of 229 mV, which is lower than that of Co + TiO2, pure TiO2 and pure Co. The interface Schottky junction can effectively adjust the electronic properties of Co/TiO2 and improve the local electronic density of CO surface. A weaker and optimized free energy of hydrogen adsorption is obtained, thus its HER activity is improved.

4.2. Photocatalysis Photocatalysis is a technology that uses sunlight to degrade organic pollutants or decompose water to produce hydrogen. Because it is characteristics of environment-friendly and low energy consumption, the preparation of high-performance photocatalytic materials has become a hot spot in material science research field. The special interface structure and synergistic effect are used to control the photocatalytic properties of the materials. Zhang et al. synthesized Au@ZnS–AgAuS yolk-shell NPs by ion exchange method [15] (Fig. 9). The preparation of these yolk-shell structures has achieved the optimization of plasmaexciton coupling and plasma enhanced electron-hole separation, which has a high photocatalytic activity under visible light irradiation. Au nanorod-CdS yolk-shell NPs and Au nanorod@CdS core-shell NPs were prepared by Han et al. using ion exchange method [16]. Compared with the core-shell structure, yolk-shell structure has higher photocatalytic activity, and its catalytic capacity was about 27 times of the core-shell structure. The studies have demonstrated that the higher photocatalytic activity of yolk-shell nanostructure is due to the synergistic effect between the radiative relaxation of the plasmon energy of the Au cores and the multiple reflections of the incident light in the hollow provided by yolk-shell structure. This promotes the light absorption of CdS shells, which drives photocatalysis.

Fig. 9. (a) UV–vis extinction spectra of Au NCs and as-prepared Au@ZnS–AgAuS Yolk-shell NPs NCs. (b) Photodegradation curve of methyl blue (MB). (c) Visible light photocatalytic H2-production of Au@ZnS–AgAuS (140 °C 4 h) yolk-shell NPs, Au@ZnS core-shell NPs. Reproduced with permission from Ref. [15]. Copyright: 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 7

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Fig. 10. (a~c) TEM images of Au@NiSx core-shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like NPs. Electrochemical performances of the HER catalysts: (d) polarization curves for the Au@NiSx core-shell, Au-NiSx yolk-shell, Au-NiSx oligomer-like and NiSx NPs on GC electrodes. (e) Tafel curves of the four sample, and (f) Nyquist plots of the four sample and a simulated circuit diagram. Reproduced with permission from Ref. [4]. Copyright: 2019 Elsevier.

synergistic effect. Wei et al. prepared Au–Co3O4 yolk-shell heterostructure by chemical reduction method [28]. The Au–Co3O4 displayed excellent catalytic performance for ORR, with the onset potential of 0.99 V, half-wave potential of 0.83 V, and limit current density of 5.3 mA·cm−2, which is close to the performance of commercial 20 wt % Pt/C. The excellent electron transfer property of Au NPs and the electronic coupling effect between Au and Co3O4are attributed to the good catalytic performances. Huang et al. prepared Pd@NiO core-shell NPs by solvothermal method [29] (Fig. 12). The Pd@NiO core-shell NPs showed better ORR catalysis activity than commercial Pt/C. The effective core-shell interface tuned Pd core, enhanced the adsorption of O2, weakened the chemical adsorption of CH3OH, and promoted the evolution of O-species. Highly efficient electron transfer was reached by d-d transfer of interfacial oxidation states. Interface engineering leads to an internal d-band-offset, which can effectively reduce the electrocatalytic barrier with extra-high current density at lower potential.

4.4. OER OER is another half reaction of water splitting reaction, which generates oxygen. Lower OER overpotential can reduce the potential of water splitting, thus saving energy. The same as the design of HER catalyst, the preparation of metal-semiconductor heterostructure can reduce the charge transfer resistance and improve the conductivity so as to improve the OER catalytic performance of the material. Pan et al. prepared M/MOx (M = Co, Ni) metal-semiconductor heterostructure NPs with a smaller overpotential of 270 mV than IrO2 [26]. The atomic arrangement and electronic structure were adjusted through the grain boundary, disordered region and vacancy defects to obtain novel properties of materials. The as-prepared M/MOx metal-semiconductor heterostructure exhibited rapid electron transfer, and the deformation of the interface electronic structure produced abundant active sites. Xu et al. prepared Au@Ni12P5 core-shell by solvothermal method [7] (Fig. 11). The overpotential of Au@Ni12P5 is 340 mV, lower than Au–Ni12P5 oligomer-like structure and pure Ni12P5. The enhanced OER activates is reasoned from the weak hydride acceptance on the surface and the strong interface coupling between the Au core and the Ni12P5 shell.

4.6. Supercapacitor Metal-semiconductor heterostructure also can be used in energy storage field because of the synergistic effect and electronic coupling effect. The interaction between metal and semiconductor at the interface can enhance the conductivity of the heterostructure, reduce its internal resistance, and make the electron transfer rapidly. Supercapacitors, known as electrochemical capacitors, store energy by electrolyte polarizing. A supercapacitor with good performance needs to have a large specific capacitance and good stability. Metalsemiconductor NPs have been reported to exhibit better supercapacitor performances than pure semiconductors because the metal-semiconductor interface reduces the electrical resistance between the current collector and active material interfaces, improves the conductivity

4.5. ORR Oxygen reduction reaction plays a key role in fuel cell and zinc-air cell. The design of metal-semiconductor interface structure controls the electronic structure of materials and improve their ORR catalytic activity. Wang et al. prepared Pt/TiO2–C heterostructure by photochemical deposition method [27]. Compared with 20 wt % Pt/C, the half-wave potential of Pt/TiO2–C showed a positive shift of 49 mV, which promotes the electron transfer in the catalytic reaction due to the 8

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Fig. 11. (a) Polarization curves, (b) Tafel plots and (d) Nyquist plots of reported Au/Ni12P5 coreshell NPs, Au–Ni12P5 oligomer-like NPs and pure Ni12P5 NPs. (c) Chronoamperometric response for Au/Ni12P5 core-shell NPs. Reproduced with permission from Ref. [6]. Copyright: 2017 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Au–Ni12P5 NPs. Therefore, by forming an Au/Ni12P5 core-shell structure, there is a synergistic effect between the Au core and the Ni12P5 shell, which enhances the performance of the Au/Ni12P5 supercapacitor. Electrochemical impedance spectrums demonstrate that the core-shell Au/Ni12P5 NPs possess the lowest charge-transfer resistance of the three samples by 2–3 orders of magnitude, which can be attributed to the enhanced electric conductivity and/or short charge diffusion lengths due to the synergistic effect between metallic cores and semiconductor shells, or the strong interfacial electronic coupling and/ or the enhanced spin-orbit coupling in metals and semiconductors at

of the semiconductor, and enhances the interface electron coupling and spin coupling. Wang et al. prepared Au/Ni12P5 core-shell NPs with single crystal shell [8] (Fig. 13). They compared the specific capacitance of Au/Ni12P5 core-shell structure with the Au/Ni12P5 oligomerlike structure and pure Ni12P5 NPs. Among the three different nanostructures, the Au/Ni12P5 core-shell NPs process the highest specific capacitance, while the Au/Ni12P5 oligomer-like structure is the worst. Compared to pure Ni12P5, the specific capacitance of the Au/Ni12P5 core-shell NP is increased by 94.8%. However, Au NP does not enhance the supercapacitor properties of nickel phosphide in the oligomer-like

Fig. 12. ORR performances of Pd@NiOx/C. (a) CV curves in N2-saturated 1 M KOH, (b) polarization curves in O2-saturated 0.1 M KOH, (c) the electron transfer number from the tests performed on RRDE and (d) the comparison of half-wave potential and mass activity of Pd@ Ni/C, [email protected]/C, Pd@ NiO-0.3/C, [email protected]/C, the commercial Pt/C and Pd/C. Reproduced with permission from Ref. [29]. Copyright: 2019 Elsevier.

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Fig. 13. (a) Specific capacitances derived from the discharge curves. (b) Nyquist plots of the electrodes. The inset shows the equivalent circuit diagram. Reproduced with permission from Ref. [8]. Copyright: 2014 Nature Publishing Group.

Fig. 14. (a) CV and (b) charging/discharging profiles of Ag–TiO2/V2O5 hybrid architectures, (c) cycle behaviors of Ag/V2O5, TiO2/V2O5 and Ag–TiO2/V2O5 hybrid architectures as well as neat V2O5 nanosheets, and (d) rate capability of Ag–TiO2/V2O5 hybrid architectures [33]. Copyright: 2016 The Royal Society of Chemistry.

electrical conductivity of composites and inhibit irreversible capacity loss [32]. Zhu et al. prepared Ag NPs and TiO2 nanorod modified V2O5 nanosheets [33] (Fig. 14). Due to the poor electronic conductivity of V2O5, the battery performance of the sample was improved by introducing Ag NPs of good conductivity. Ag NPs can effectively conduct electrons from the current collector to the electrode. Zhang et al. prepared hollow Sn/ZnS@C metal-semiconductor NPs by ion-exchange and reduction method [39]. The hollow Sn/ZnS@C NPs are applied into sodium ion batteries. Due to its unique stable structure and synergistic effect between Sn and ZnS, the high reversible capacity, good rate performance and good cycle stability were achieved during the optimized voltage range. The synergistic effect between metal-semiconductor-carbon shell can effectively improve the charge transfer dynamics, and the presence of carbon shell can prevent the electrode from being crushed in the sodium sodiation/desodiation process, thus

the nanoscale. Zhang et al. prepared Au-embedded ZnO/NiO hybrid to improve the supercapacitor performance of ZnO/NiO hybrid [5]. During the charge process, due to the localized Schottky barrier of the Au/NiO interface, the electrons are temporarily trapped and accumulated at the Fermi level until the gap between ZnO and NiO is filled. The embedding of Au NPs can enhance the ability to release more electrons of materials and enhance its conductivity, thereby improving the capacitance performance of Au-embedded ZnO/NiO heterostructure. 4.7. Ion battery Ion battery is a secondary battery that works with lithium or other ion movement between the anode and the cathode. Metal-semiconductor heterostructure NPs demonstrate better battery performance than pure semiconductors NPs because metal NPs can enhance the 10

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obtaining excellent performances of sodium ion battery.

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5. Summary and outlook In this review, the structure design, controllable synthesis, and application of metal-semiconductor heterostructure NPs in the fields of optics-related catalysis, electrocatalysis and energy storage are summarized. Based on the interfacial structure, metal-semiconductor heterostructures can be divided into: core-shell, yolk-shell, Janus and oligomer-like nanostructures. The lattice mismatch between metal and semiconductor, the interface energy, special synthesis methods and reaction mechanisms are useful important factors to design metalsemiconductor heterostructures. The synthesis methods of metal-semiconductor heterostructure NPs are mainly chemical methods, including chemical deposition method, chemical reduction method, sol-gel method, template method, hydrothermal/solvothermal method, and so on. Metal-semiconductor heterostructure NPs with different structures show great potential in the fields of energy, catalysis, etc. through the synergistic effect between metal NPs and semiconductor NPs. Despite the numerous achievements, there are still many problems to be solved in the structure design, controllable synthesis, and application of metal-semiconductor NPs. (1) The first one is that new methods for controllable synthetic of metal-semiconductor need to be developed. In the case of large lattice mismatch, the preparation of core-shell and yolk-shell with high-quality single-crystal shell metalsemiconductor NPs is still difficult in some cases. It can be achieved by controlling the energy of the system and developing special synthesis methods combined with simulation predicting method. (2) The second one is that the growth mechanism investigation is usually conducted by time-dependent experiments. However, it results in a lack of understanding of the changes in morphology and structure during the NPs growth. The same problem also exists in the structure-property relationship study. Atomic changes in material morphology and structure during application also need to be understood. Therefore, in-situ characterization tools are necessary. Recently, environmental TEM (ETEM) and in-situ TEM technologies have been developed rapidly and proved to be effective methods to characterize the material evolution process and structure-property relationship under the simulated environment. Combined with aberration-corrected technique and advanced in situ spectroscopy tools, it is possible to dynamically understand the growth mechanism and structure-property relationship at atomic scale, which will instruct the synthesis and promote the applications of metalsemiconductor NPs in future works. Declaration of competing interest No conflict of interest. Acknowledgement This research was funded by the National Key Research and Development Program of China (No. 2018YFA0703702), the National Natural Science Foundation of China (Nos. 11674023, 51971025 and 51901012), the Fundamental Research Funds for the Central Universities (FRF-TP-19-022A2) and 111 Project (No. B170003). References [1] [2] [3] [4] [5]

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