25th anniversary article: hybrid nanostructures based on two-dimensional nanomaterials.

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25th Anniversary Article: Hybrid Nanostructures Based on Two-Dimensional Nanomaterials Xiao Huang, Chaoliang Tan, Zongyou Yin, and Hua Zhang* and chemical vapor deposition (CVD).[55,56] In addition to those 2D materials derived Two-dimensional (2D) nanomaterials, such as graphene and transition metal from layered crystals, many nanosheets/ dichalcogenides (TMDs), receive a lot of attention, because of their intriguing nanoplates can be prepared directly from properties and wide applications in catalysis, energy-storage devices, elecwet-chemical synthesis, such as Au and tronics, optoelectronics, and so on. To further enhance the performance of Pd nanosheets,[9,29] CuS nanosheets,[57] their application, these 2D nanomaterials are hybridized with other functional and TiO2 nanoplates.[20,58] The ability to nanostructures. In this review, the latest studies of 2D nanomaterial-based prepare a variety of 2D nanomaterials has enabled the rapid development in hybrid nanostructures are discussed, focusing on their preparation methods, their fundamental research and practical properties, and applications. applications.[20,28,40,42–51,57,58] Inspired by the successful preparation of graphene-based hybrid materials,[13,59–63] other 2D nanomaterials have also been considered 1. Introduction promising building blocks for construction of various complex nanostructures for realizing the improved or even novel propHybrid nanostructures are a class of materials that are comerties. Generally, there are three types of hybrid nanostrucposed of two or several components with at least one of them tures based on 2D nanomaterials: i) controlled growth of other owning a dimension in the nanoscale regime. A hybrid nanofunctional materials on 2D nanomaterials as synthetic temstructure possesses the advantages of its individual compoplates,[9–12,64] ii) growth and/or assembly of 2D nanomaterials nents, and at the same time may exhibit new properties and functions for practical applications.[1–8] The components of on 1D and 3D templating structures to form porous hierarchical architectures,[18–24,30,31] and iii) formation of heterostruca hybrid structure can be chosen from a wide range of mate[ 9–11 ] [ 12,13 ] rials, including metals, tures by stacking different kinds of 2D materials to artificially metal oxides, metal chalcogecreate van der Waals layered crystals.[65–77] In this review, we nides,[14,15] polymers,[16,17] carbonaceous materials,[18–21] etc. Importantly, their morphologies are versatile, such as zero aim to describe various kinds of hybrid nanostructures based dimensional (0D) nanoparticles, 1D nanowires/rods/belts,[22–24] on 2D nanomaterials, particularly focusing on their preparation methods and applications in catalysis, energy storage, and elec2D nanosheets/plates[9,25–29] and 3D porous frameworks/ tronic/optoelectronic devices. networks.[30–39] Over the last decade, graphene, the thinnest and most representative 2D material, has aroused tremendous research interest because of its exceptional electronic, optical and 2. Preparation of Hybrid Nanostructures Based on mechanical properties.[40,41] Other kinds of 2D materials, 2D Nanomaterials such as hexagonal BN (h-BN) and transition metal dichalco[ 25–28 ] genides (TMDs) derived from their layered bulk crystals, 2.1. Graphene-Based Hybrid Nanostructures have also been intensively investigated in recent years due to their promising properties and a broad range of applications Featuring the excellent electronic properties, large specific such as electronics, optoelectronics, catalysis, energy storage surface area and good thermal and mechanical stability, gradevices and so on.[28,40,42–51] A number of methods have therephene and its derivatives are ideal templating materials to facilitate the nucleation and growth of inorganic nanocrystals fore been developed to prepare these 2D nanomaterials, such for producing functional hybrid nanostructures via diversified as mechanical exfoliation,[26,44,48,49,52,53] electrochemical Li-intermethods.[59,78–80] In this section, we will use some typical examcalation and exfoliation,[28,43] direct sonication in solvents[42,54] ples to discuss the preparation methods for various graphenebased hybrid materials. The more detailed description can be Dr. X. Huang,[+] C. L. Tan,[+] Dr. Z. Y. Yin,[+] Prof. H. Zhang School of Materials Science and Engineering found in our previous review article.[59] Nanyang Technological University 50 Nanyang Avenue Singapore 639798, Singapore E-mail: [email protected]; [email protected]

[+]These

authors contributed equally to this work

DOI: 10.1002/adma.201304964

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2.1.1. Graphene–metal Hybrids The controlled synthesis of metal nanostructures has received much interest since their physical and chemical properties are

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dependent on their size, shape, composition and structure.[81,82] To date, various metallic nanostructures like Au,[9,83–93] Ag,[88,94–96] Pt,[87,97–101] Pd,[102–105] Fe,[106] Ge,[107] Sn,[108] Si,[109,110] Ni,[111] Ru[112] and bi-/tri-metallic ones such as FePt,[113] PtPd,[114] PdAg[115] and Pt/CuPd[10] have been successfully prepared on graphene-based nanosheets, aiming to achieve enhanced performances in applications like catalysis, electronics and sensors. The in situ chemical reduction method is the most popular strategy used for synthesis of graphene–metal hybrid nanostructures. The direct reduction of metal precursors (e.g., HAuCl4, AgNO3, K2PtCl4, H2PdCl6 or CuCl2) in the presence of graphene oxide (GO) or reduced graphene oxide (rGO) sheets by using reducing agent like amines, NaBH4, and ascorbic acid in solution has been applied for synthesis of various nanostructures on graphene. For instance, Au nanoparticles (NPs) were synthesized on rGO by reduction of AuCl4− with NaBH4 in an rGO/octadecylamine (ODA) solution.[93] Other techniques such as electroless deposition[94] and electrochemical deposition[100] have also been applied to grow metal nanocrystals on graphene. As one of the early demonstrations of electroless deposition, our group showed that by simply heating GO or rGO sheets immobilized on a 3-aminopropyltriethoxysilane (APTES)-modified-Si/SiO2 substrate in AgNO3 solution, Ag NP-decorated GO or rGO could be obtained without the addition of any reduction agents (Figure 1a).[94] Electrochemical deposition is another useful method for the reduction of metal precursors on graphene surface. For example, H2PtCl6 can be electrochemically reduced to prepare nanoflowers on rGO which also served as the conductive electrode for the deposition.[100] Alternatively, many other approaches such as photochemical reduction,[85] microwave assisted synthesis,[116] laser irradiation[117] and thermal evaporation[118] have also been developed to prepare graphene–metal hybrid nanostructures. Besides spherical metal NPs, graphene-supported growth of anisotropic metal nanostructures such as nanorods,[119,120] nanowires,[89] nanosheets[9] and nanorings[115] has also been successfully achieved. Our group has demonstrated that by using GO or rGO as dispersible templates, novel anisotropic Au nanostructures (nanowires[89] and nanosheets[9]) can be synthesized. For example, for the first time, hexagonal closepacked (hcp) square-like Au nanosheets with an edge length of 200–500 nm and a thickness of ca. 2.4 nm were prepared by heating the mixture of HAuCl4 and oleylamine in the presence of GO sheets (Figure 1b).[9] As for bimetallic nanostructures, recently, Pd-Ag nanorings were synthesized on graphene nanosheets by Liu et al. via the galvanic displacement reaction between the pre-synthesized Ag NPs and Pd ions.[115] In addition, Hu et al. prepared the ternary Pt/PdCu hollow nanoboxes on three-dimensional (3D) graphene framework (3DGF) (Figure 1c),[10] by first forming binary PdCu nanocubes on 3DGF and then subsequently depositing Pt on the cubic PdCu via a successive solvothermal process. The ex situ hybridization, as an alternative method to the in situ growth, allows for the pre-selection of nanostructures with desirable properties or functions. Generally, the pre-synthesized or commercial available metal nanostructures were mixed with graphene or its derivatives, prior to which the surface of metal nanocrytals and/or graphene sheets is functionalized to ensure good bonding between them via covalent or non-covalent


Xiao Huang received her bachelor degree at School of Materials Science and Engineering of Nanyang Technological University in Singapore in 2006 and completed her Ph.D. at the same school under supervision of Professors Hua Zhang and Freddy Boey in 2011. She is currently working as a postdoctoral fellow in Prof. Hua Zhang's group. Her research interests include the synthesis and applications of hybrid nanostructures based on two-dimensional nanomaterials. Chaoliang Tan received his B.E. degree from Hunan University of Science and Technology in 2009. After he got his M.S. degree from South China Normal University, he moved in 2012 to the School of Materials Science and Engineering of Nanyang Technological University in Singapore as a Ph.D. student under the supervision of Professor Hua Zhang. His research interests focus on the synthesis, assembly, and applications of two-dimensional nanosheet-based composite materials. Hua Zhang studied at Nanjing University (B.S., M.S.), and completed his Ph.D. at Peking University (with Zhongfan Liu) in 1998. After postdoctoral work at Katholieke Universiteit Leuven (with Frans De Schryver) and Northwestern University (with Chad Mirkin), and working at NanoInk Inc. and the Institute of Bioengineering and Nanotechnology (Singapore), he joined Nanyang Technological University (2006) where currently he is the full Professor. His current research interests focus on synthesis of two-dimensional nanomaterials (graphene and transition metal dichalcogenides) and their composites, and their applications in nano- and bio-sensors, clean energy, water remediation, etc. Zongyou Yin studied at Jilin University in China for his B.E. and M.S., and completed his Ph.D. at Nanyang Technological University in Singapore (2008). After his PhD, he worked as a Research fellow at Prof. Hua Zhang’s group, and currently he is working as Scientist II in Institute of Materials Research and Engineering in Singapore. His research interests include the low-dimensional nanomaterials and their applications.


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REVIEW Figure 1. a) SEM image of Ag NPs deposited on GO. Reproduced with permission.[94] Copyright 2009, American Chemical Society. b) TEM image of Au square sheets synthesized on GO. Reproduced with permission.[9] Copyright 2011, Nature Publishing Group. c) TEM image of Pt/PdCu nanocubes on graphene sheets. Reproduced with permission.[10] Copyright 2012, John Wiley & Sons, Inc. d) TEM image of Co3O4/graphene hybrid material. Inset: the corresponding SAED pattern. Reproduced with permission.[12] Copyright 2011, Nature Publishing Group. e) TEM image of TiO2/MoS2/graphene hybrid. Reproduced with permission.[15] Copyright 2012, American Chemical Society. f) TEM image of TiN/N-doped graphene hybrid. Reproduced with permission.[197] Copyright 2011, John Wiley & Sons, Inc.

interactions. For example, several kinds of NPs including Au, Ag, Pt and Pd can be easily loaded onto bovine serum albumin (BSA) protein-functionalized rGO surface via the π–π interaction.[121] In addition, Nafion-coated rGO sheets were used to prepare rGO/TiO2 composites.[122] Alternatively, by modification of pre-synthesized NPs, 2-mercaptopyridine-modified Au NPs[123] or benzyl mercaptan-capped CdS NPs[124] have been successfully attached to GO or rGO surfaces via π−π stacking.

2.1.2. Graphene–metal Oxide Hybrids Metal oxides have been studied intensively for several decades due to their broad applications in electronics, photocatalysis, electrocatalysis, energy storage and conversion devices etc.[125,126] To further enhance their properties and performances, up till now, a number of metal oxides, such as TiO2,[127–131] SiO2,[132–135] ZnO,[13,136–144] SnO2,[145–151] MnO2,[152–156] Co3O4,[157–159] Fe3O4,[160–165] NiO,[166] Cu2O,[167,168] EuO[169] and RuO2,[170] have been grown on graphene-based nanosheets via different synthetic approaches. Wet chemical oxidation method, hydro/solvothermal method and sol-gel method are the most popular approaches used for synthesis of metal oxide nanostructures on graphene surface. Recently, Dai and co-workers prepared different kinds of metal oxides on GO or rGO via a two-step solution-phase reaction.[12,171] For example, to prepare Co3O4 NPs, Co(OAc)2 was firstly oxidized on GO sheets by hydrolysis and oxidation at a relative low temperature (e.g., 80 °C),[12] followed by a hydrothermal process for the crystallization of Co3O4 NPs and reduction of GO (Figure 1d).[12] By using this similar approach, other graphene–metal oxide or hydroxide hybrid

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materials such as Mn3O4-rGO[171] and Ni(OH)2-rGO[172] have also been successfully prepared by the same group. The sol-gel method, on the other hand, is usually used for preparation of TiO2[127,130,173–176] and SiO2[177] nanostructures through a series of hydrolysis and polycondensation reactions by using metal alkoxides, chlorides or fluorides as precursors (e.g., titanium isopropoxide,[127] titanium butoxide,[176] and tetraethylorthosilicate (TEOS)).[177] The OH groups on GO or rGO surface is advantageous for the sol-gel method because they can act as the nucleation sites for the hydrolysis, facilitating the strong bonding between the metal oxides and graphene. Different from the solution-phase synthesis on dispersible graphene templates, the electrochemical deposition,[13] molecular beam epitaxy (MBE)[169] and metal–organic vapor-phase epitaxy (MOVPE)[144] are mainly used for deposition of metal oxides on graphene thin films. As a typical example, our group reported that the ZnO nanorod-decorated rGO thin film on quartz can be fabricated via electrochemical deposition in the oxygen-saturated aqueous solution of ZnCl2 and KCl.[13] Importantly, the epitaxial growth of metal oxides on graphene was demonstrated by Yi and coworkers, where ZnO nanostructures (e.g., nanowires, nanowalls, and nanotubes) were grown on pristine graphene nanosheets through the metal–organic vaporphase epitaxy.[144,178] The resulting ZnO/graphene hybrid film was further used as a template to direct the growth of epitaxial GaAs thin film,[178] or GaN/InxGa1–xN/GaN[144] quantum nanostructures for light emitting diodes (LEDs). Very recently, we demonstrated that by using metal organic frameworks (MOFs) as precursors, porous metal oxide nanostructures can be prepared on 3D graphene networks (3DGNs), which are produced by CVD growth of graphene on porous 3D Ni foam.[136] For example, ZIF-8 or MIL-88-Fe crystals were



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first synthesized on 3DGN, and then annealed in air to give ZnO/3DGN or Fe2O3/3DGN hybrids, which were applied in photocatalysis or LIBs, respectively.[136]

2.1.3. Graphene–metal Chalcogenide Hybrids Semiconducting metal chalcogenide nanostructures are particularly promising in applications like bio-imaging,[179] transistors,[180] solar cells,[181] and light-emitting diodes.[182] To date, metal chalcogenide nanocrystals that have been synthesized on GO or rGO nanosheets include CdS,[183–185] CdSe,[186–190] CdTe,[191] MoS2,[14,15] PbS,[192,193] ZnS[194,195] and ZnSe,[196] Note that hybrid materials based on 2D TMD nanosheets will be described in details in Section 2.2. Hydro/solvothermal method is most commonly used to synthesize metal chalcogenide nanocrystals. For example, Dai and co-workers reported that MoS2 NPs can be easily deposited on rGO surface by a one-pot solvothermal reaction between (NH4)2MoS4 and hydrazine in a N,N-dimethylformamide (DMF) solution of GO.[14] The similar strategy was used to prepare MoS2/rGO composite which was subsequently used for further deposition of TiO2 NPs to prepare the TiO2/MoS2/graphene trihybrid material (Figure 1e).[15] Recently, Chen et al. reported the in situ growth of ZnSe NPs on nitrogen-doped graphene sheets by a one-pot hydrothermal process, where [ZnSe](diethylenetriamine)0.5 nanobelts acted as not only the precursor for preparation of ZnSe NPs but also the nitrogen source for doping GO.[196] 2.1.4. Other Graphene-Based Hybrids Besides metals, metal oxides and chalcogenides, the preparation of other kinds of inorganic nanocrystals on graphene has also been realized recently.[197–199] For example, TiN NP-modified nitrogen-doped graphene hybrid was synthesized by Wen et al. (Figure 1f),[197] by using the C3N4 polymer as the nitrogen source for both the formation of TiN NPs and doping of graphene. In another interesting work, Luo et al. prepared rGOsupported Sn-core/C-sheath nanocables by a two-step process, in which SnO2 NPs were first deposited on rGO via hydrolysis, and then the as-prepared rGO-SnO2 hybrid was further heated under a gas mixture of C2H2 and Ar at 600 °C. During the heating, the droplets of molten Sn catalyzed the growth of 1D carbon nanotubes (CNTs), and driven by the capillary force, the Sn droplets were further drawn into the tubular CNT shells to eventually lead to the rGO-supported Sn-core/C-sheath nanocables (i.e., rGO-Sn@C).[199] In addition, the formation of aerogels/hydrogels with high porosity and good mechanical properties is an attractive method to the construction of graphene-based 3D porous hybrids, such as graphene/polypyrrole,[200] GO/epoxy,[201] GO/DNA,[202] and graphene/CNT hybrids.[203] For example, Shi and coworkers recently prepared GO/DNA hydrogel by heating a solution containing GO sheets and double-strand (ds) DNAs. During their experiement, the dsDNAs unwound to the single-strand (ss) DNAs and bridged the adjacent GO sheets via strong noncovalent interactions.[202] The resulting GO/DNA hydrogel can be used in various biological and environmental applications, such as tissue engineering and removal of organic pollutant.

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2.2. Hybrid Nanostructures Based on Other 2D Nanomaterials 2.2.1. 2D Nanomaterial Templated Growth Similar to graphene, other 2D nanomaterials can also be used as templates to assist the growth of functional nanostructures, such as metals[11,64,204,205] and semiconductors.[206] Despite the great efforts made in graphene-based synthesis, the epitaxial growth of inorganic nanostructures was only realized on mechanical exfoliated graphene[144,178] or CVD-graphene.[55,207] The dispersible rGO nanosheets, on the other hand, lose the long range order of the graphitic lattice due to the pre-treatment with strong oxidizing agents, and thus could not induce solutionphase epitaxial growth of nanocrystals. Importantly, we recently demonstrated that single-layer MoS2 nanosheets can direct the growth of a series of noble metal nanostructures, such as Ag, Au, Pt and Pd. Particularly, Pd NPs with sizes of ca. 5nm and Pt NPs with sizes of 1–3 nm were epitaxially grown on the MoS2 nanosheets (Figure 2a–d). Besides the spherical NPs, anisotropic structures such as Ag nanoplates were also grown and epitaxially aligned on the MoS2 surface (Figure 2e–h).[11] Dependent on the kind of metals, different synthetic methods and conditions were applied. For example, Pd NPs were synthesized in situ by reduction of K2PdCl4 with ascorbic acid in the presence of poly(vinylpyrrolidinone) (PVP) and MoS2, while Pt NPs were obtained by photochemical reduction of K2PtCl4 in the presence of sodium citrate. It is noteworthy that, compared to the conventional solid-state epitaxial growth of metals on bulk substrates, the wet-chemical epitaxial synthesis enables large-scale production at a relatively low cost, which is essential for many practical applications, such as fuel cells and photocatalysis. Very recently, Jin and coworkers demonstrated the epitaxial growth of vertically aligned ZnO nanowires (NWs) on CuGaO2 (CGO) nanoplates,[206] by using three different methods in the controlled manner. The first method is called the “suspended” synthesis, where CuGaO2 nanoplates immobilized on a Au-coated Si/SiO2 substrate were immersed facing down in a growth solution for ZnO (Figure 3a,b). In the second approach, similar substrates as in the first method were mounted with facing down in a continuous flow reactor (CFR, Figure 3c). This solid-phase reaction promotes the screw-dislocation driven growth of high aspect ratio ZnO NWs. The third method is called the “floating” synthesis. As-prepared CuGaO2 nanoplates dispersed in ethanol were drop-casted and floating on top of the ZnO precursor solution, which was then heated to 90 °C to realize the downward growth of ZnO NWs into the solution (Figure 3d–g). The produced heterostructures can be scooped by a conducting substrate which is in contact with the ZnO nanowires (Figure 3k). This unique geometry of CuGaO2 nanoplate/ZnO NWs/Au is advantageous for directly studying electrical properties of the nanoplate-nanowire junction.[206]

2.2.2. Hierarchical Architectures The large specific surface area is one of the most appealing features of 2D nanomaterials. However, when the thin films made of these 2D nanomaterials used as hybrid electrodes and applied in catalysis, batteries and supercapacitors, restacking


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REVIEW Figure 2. a) TEM image of Pd NPs synthesized on an MoS2 nanosheet. b) SAED pattern of a Pd-MoS2 hybrid nanosheet with the electron beam perpendicular to the basal plane of the MoS2 nanosheet. c) TEM image of Pt NPs synthesized on an MoS2 nanosheet. d) SAED pattern of a Pt-MoS2 hybrid nanosheet with the electron beam perpendicular to the basal plane of the MoS2 nanosheet. e) TEM image of Ag nanoplates synthesized on MoS2 nanosheet. f) TEM image of a typical Ag nanoplate on MoS2 nanosheet. g) FFT-generated SAED pattern of (f). h) Filtered HRTEM image of the Ag nanoplate in (f). Inset in (e): Photograph of the Ag-MoS2 solution. Reproduced with permission.[11] Copyright 2013, Nature Publishing Group.

of the nanosheets is inevitable, causing a decrease in their effective surface area. Assembly of 2D nanomaterials to the free-standing 3D hierarchical architectures in the presence of a template structure can greatly preserve the individual sheet structure with largely exposed surface area. To date, various 2D nanomaterials have been grown and assembled on 1D nanostructures, such as carbon nanotubes (CNTs),[18,19] carbon fibers,[20,21] and semiconducting nanowires[22,23] or nanobelts.[24] For example, Shi et al. prepared 2D MoSx (2 < x < 3) nanosheet-coated CNTs via a solvothermal method at 200 °C with (NH4)2MoS4 as the source reactant and N,N-dimethylformamide (DMF) as solvent in the presence of CNTs.[18] It is worth mentioning that the synthetic method adopted in this work is critical to produce MoSx nanosheets with edges extending away from the CNT surface to ensure a large surface area and plenty active edge sites, rather than forming concentric MoS2 nanotubes.[208,209] Recently, our group successfully coated few-layer MoS2 nanosheets onto TiO2 nanobelts to form TiO2@MoS2 heterostructures via a hydrothermal reaction involving Na2MoO4·2H2O and C2H5NS (Figure 4a–c).[24] Unlike CNTs, which can induce the growth of layered nanostructures,[209,210] un-treated TiO2 nanobelts with smooth surfaces could not provide high density of nucleation sites, and therefore, acid treatment of the TiO2 nanobelts was carried out to roughen their exposed surfaces and create abundent nucleation sites for MoS2 nanosheets.[24] Besides MoS2 nanosheets, other 2D nanomaterials have also been synthesized and assembled on 1D templating structures to form hierarchical architectures, such as CNT@Ni3S2,[19] carbon nanofiber@SnO2,[21] and carbon nanofiber@TiO2.[20] In an interesting work reported by Lou and coworkers,[21] SnO2 nanoplates were firstly deposited on carbon nanofibers via a simple hydrothermal reaction, and then transformed to SnO2 hierarchical tubular (HT) structure by annealing in air, during which the sacrificial carbon nanofiber templates were removed by combustion. After that, a thin layer of amorphous carbon was coated onto the SnO2

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nanoplates by hydrothermal carbonization of glucose to result in the final SnO2@C-HTs (Figure 4d–f).[21] The hierarchical architectures based on 2D nanosheets are not limited to two-component systems. Our group recently developed a full solution-processable strategy to fabricate wellaligned, tree-like nanostructure consisting of triple components, including 1D ZnO nanowires (NWs) as the tree trunk, 2D TiO2 nanosheets (NSs) as the tree bark and 1D CuO nanoneedles (NNs) as the tree branches on an FTO substrate.[22] Such tree like structure can be used as photoanode in photoelectrochemical cell (PEC) for water splitting, which will be discussed in Section 3.3. Another useful templating material that is able to assist the growth and assembly of 2D nanomaterials is the 3D graphene network. Onto the 3D graphene surface, 2D nanomaterials such as MoS2 can be deposited via CVD growth (Figure 5a,b), and the resulting architecture is applicable as hybrid electrode for lithium-ion batteries[30] or electrocatalytic hydrogen evolution.[32] Recently, we also showed that Ni(OH)2 nanosheets coated on Ni3S2 nanorods were grown on the surface of a 3D graphene network, referred to as Ni3S2@Ni(OH)2/3DGN (Figure 5c,d), by immersing the graphene/Ni foam in C2H5NS solution heated in an autoclave at 180 °C for 12 h.[31] Note that the resulting structure composition is strongly related to the reaction time (Figure 5c), that is, 6 h of reaction resulted in only Ni3S2 nanorods whereas 16 h led to only Ni(OH)2 nanosheets on the 3D graphene.

2.2.3. Hybridization of Organic Materials and 2D Inorganic Nanomaterials 2D inorganic nanomaterials have also been hybridized with organic materials, examples including MoS2/polyaniline (PANi) nanowires,[17] MoS2/poly(vinylpyrrolidinone) (PVP) nanosheets,[16] and Co9S8-oleylamine nanoplates.[211] Xie and coworkers demonstrated a unique in-plane co-assembly



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Figure 3. a–g) Schemes illustrating the three different synthetic methods to grow ZnO NW arrays on CGO nanoplates: a,b) “Suspended” synthesis; c) CFR method; d–g) “floating” synthesis. e,f) CGO nanoplates float on top of the ZnO growth solution as shown in the digital photograph (e) and bright-field optical micrograph (f). h–k) SEM images of CuGaO2 nanoplate/ZnO NWs heterostructures prepared by the “suspended” method (h,i), CFR method (j) and the floating method (k). Reproduced with permission.[206] Copyright 2013, American Chemical Society.

strategy for hybridization of organic materials and 2D inorganic nanosheets consisting of small 2D TMD nanoplates (e.g., Co9S8) and alkylamines. This process takes advantage of the alkylamine (CnH2n−xNH2 with n ≥ 12) that can selectively adsorb on the corner sites and side surface sites of the nanoplates, and then connect with each other in-plane to give the co-assembled inorganic-organic hybrid nanosheets (Figure 6). It has to be noted that, to realize the in-plane assembly, the amine molecules should be long enough (i.e., CnH2n−xNH2 with n ≥ 12). Otherwise, the 2D nanoplates could not be sufficiently stabilized, resulting in the aggregation of 2D nanoplates. 2.2.4. Stacking of Different 2D Nanomaterials Recently, by stacking of different 2D nanomaterials, the obtained heterostructures, with unusual properties and new

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phenomena to be explored, have drawn much attention.[65–67] The large-scale preparation of stacked 2D nanomaterials can be realized via solution-phase hybridization. For example, Wang et al. mixed GO solution with (NH4)2MoS4 and hexadecyltrimethylammonium bromide (CTAB) to form MoS42−-CTA−-GO complex, which was then reduced by hydrazine hydrate refluxed at 95 °C for 8 h. The resulting product was then heat treated at 800 °C for 2 h in nitrogen to form the hybrid of single-layer MoS2 and graphene nanosheets.[212] The resulting layered structure showed an interlayer spacing of ca. 1.1 nm confirmed by both XRD analysis and HRTEM measurement, suggesting a possible MoS2-carbon-MoS2 sandwiched structure.[212,213] Alternatively, MoS2 nanosheets exfoliated after the lithium intercalation were directly mixed with GO sheets followed by reduction to prepare the MoS2-rGO hybrid material.[214] Recently, Gao et al. physically exfoliated bulk h-BN and graphite powders


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REVIEW Figure 4. a,b) SEM (a) and TEM (b) images of TiO2@MoS2 heterostructures (50 wt% of MoS2). c) HRTEM image of few-layer MoS2 nanosheets synthesized on TiO2. Reproduced with permission.[24] Copyright 2012, John Wiley & Sons, Inc. d,e) SEM (d) and TEM (e) images showing SnO2@C hierarchical tubular structures. f) HRTEM image indicating that the amorphous carbon overlayer is ca. 2 nm in thickness. Reproduced with permission.[21] Copyright 2013, John Wiley & Sons, Inc.

in common solvents, such as isopropyl alcohol (IPA), to yield single, double and few-layered h-BN or graphene nanosheets, which were then mixed in solution to prepare hybrid h-BN/graphene layered solids.[215] In order to fabricate heterosturctures for electronic and optoelectronic devices, 2D nanosheets obtained from CVD process or mechanical exfoliation are more attractive compared to those prepared from solution-phase synthesis or chemicalintercalation and exfoliation due to the better structural and electrical properties. Up till now, several types of solid-phase vertically stacked 2D nanomaterials have been reported such as graphene/h-BN,[68–71] graphene/h-BN/graphene,[73] graphene/ WS2/graphene,[74,75] and MoS2/graphene.[76,77] A commonly used transfer method to prepare these heterostructures is called the “dry” transfer (Figure 7).[67,68,74] For example, to make a graphene/h-BN heterostructure, a graphene layer is first exfoliated on a Si/SiO2 substrate coated with a polymer stack consisting of a water-soluble layer (Mitsubishi Rayon aquaSAVE) and a poly(methyl methacrylate) (PMMA) layer. This substrate coated with double polymer layers and then graphene is then placed and floated on the surface of a water bath (Figure 7a(i)). After the water-soluble polymer is dissolved, the Si/SiO2 substrate sinks to the bottom of the water bath while the hydrophobic PMMA/graphene remains floating on the water surface (Figure 7a(ii)). The PMMA/graphene layer is then adhered to a

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glass transfer slide (Figure 7 a(iii)), which is clamped onto the arm of a micromanipulator mounted on an optical microscope. Finally, the graphene sheet on PMMA is optically located, and brought into contact with a target h-BN film pre-exfoliated on another Si/SiO2 substrate (Figure 7a(iv)).[68] To make multilayer stacks, after each transfer, the polymer should be dissolved and the heterostructure annealed in a mixture of H2/Ar at 250 °C before the subsequent transfer of another layer.[74] So far, the heterostructure that consists of the largest number of 2D sheets is the superlattice made from six alternating bilayers of graphene and h-BN (Figure 7b).[70] In fact, graphene-based heterostructures can be directly obtained by vapor phase epitaxial growth.[55,216,217] For example, Shi et al. showed that MoS2 can be deposited on a CVD-graphene film via the van der Waals epitaxial growth, through the adsorption and decomposition of (NH4)2MoS4 vapor on graphene.[55] The direct formation of the graphene/MoS2 hybrid film might have some applications based on the highly conductive and transparent graphene electrode coupled with MoS2 as catalyst or active redox centers. 2.2.5. In-Plane Heterostructures Apart from the aforementioned heterostructures, in-plane h-BN/graphene heterostructures were recently reported by



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Figure 5. a) SEM image of MoS2/3D graphene network composite. The arrow indicates the microsized MoS2 flakes. b) TEM image of small MoS2 particles on the surface of graphene. Inset in (b) is an HRTEM image of an MoS2 particle. Reproduced with permission.[30] Copyright 2013, John Wiley & Sons, Inc. c) Proposed mechanism for the growth of the Ni3S2@Ni(OH)2/3DGN structure. d) SEM image of Ni(OH)2 nanosheets grown on Ni3S2 nanorods. Inset: TEM image of a Ni3S2 nanorod with surface deposited Ni(OH)2 nanosheets. Reproduced with permission.[31] Copyright 2013, The Royal Society of Chemistry.

Liu et al.[218] After the h-BN films were first deposited using the CVD method on copper/nickel foils, they were selectively etched with exposed regions defined by laser-cut masks. Fewlayer graphene was then deposited on the etched regions at 1000 °C using CH4 as the carbon source and Ar/H2 as carrier gas. Various graphene/h-BN heterostructures with maskdefined patterns can thus be generated by this method, such as comb, bars and rings (Figure 7c–e).

3. Applications of Hybrid Nanostructures Based on 2D Nanomaterials 3.1. Lithium-Ion Batteries Lithium-ion batteries (LIBs) are currently one of the most promising rechargeable battery systems, particularly important for powering electronic devices used in our daily life.[219] Electrodes consisting of nanostructures have shown high redox activity, large electrode/electrolyte contact area and short paths

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for charge transport.[220,221] However, the lithium-insertion reaction during the charge/discharge process often leads to the aggregation and structural deterioration of nanostructured electrodes. To overcome these problems, graphene and its derivatives, with the excellent conductivity, high specific surface area and flexibility have been employed as supporting materials for preparation of graphene-based hybrid electrodes for LIBs. Examples of metal oxides or metals that have been synthesized on or physically mixed with graphene sheets are SnO2,[145,149–151] Co3O4,[157,158] Mn3O4,[171] MoO2,[222] TiO2,[130,223] Fe3O4,[165] Fe2O3,[224–226] V2O5,[227] CuO,[228,229] Cu2O,[230] CoO,[231] NiO,[166] Sn,[108,232,233] and Ge.[107] Generally, the advantages of graphenebased hybrid electrodes can be summarized as follows. First, the conductivity of inorganic nanostructures can be significantly enhanced after hybridization with graphene.[171] Second, the graphene supporting layer can effectively protect the nanostructures from aggregation during the charge/discharge process.[234] Third, the volume expansion of the nanocrystals can be accommodated as a result of the excellent flexibility of graphene and its derivatives.[165] For example, Mn3O4 is a


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REVIEW Figure 6. a) Schematic illustration of the in-plane coassembly route to the inorganic–organic hybrid nanosheets with atomic thickness in the presence of long-chain alkylamine. b,c) HRTEM images of the Co9S8–OA nanosheet with different magnification. Inset of (c) is the HRTEM image of a typical 2D Co9S8 nanoplate. Reproduced with permission.[211] Copyright 2013, American Chemical Society.

promising anode material with a high theoretical capacity of ca. 936 mA h g−1. However, its practical capacity only reaches about 400 mA h g−1 because of its poor conductivity.[171] Dai and co-workers reported that after deposition of Mn3O4 NPs on graphene, the resultant hybrid anode presented a much higher specific capacity of ca. 900 mA h g−1 compared to the pure Mn3O4 electrode does.[171] It is suggested that the significantly

enhanced performance could be mostly attributed to the excellent conductivity of graphene, facilitating efficient conduction of charge carries. Recently, graphene-encapsulated structures have also been successfully used for LIBs which exhibited much improved performances, such as graphene encapsulated Co3O4 NPs,[234] Fe3O4 nanospheres,[165] Si nanowires,[110] and LiFePO4 particles.[235] For example, Müllen and coworkers

Figure 7. a) Schematic illustration of the transfer process used to fabricate graphene/h-BN heterostructure. Reproduced with permission.[68] Copyright 2010, Nature Publishing Group. b) Bright-field cross sectional STEM image of a stack of graphene and h-BN bilayers with the layer sequence schematically shown to the left. Reproduced with permission.[70] Copyright 2012, Nature Publishing Group. c) Optical image of the as-grown graphene/h-BN patterned layers (shaped as combs, bars and rings) on a copper foil. Light areas are h-BN and dark areas are graphene. d) Optical image of a graphene/h-BN film separated from copper, on water, after coating with PMMA and etching the copper foil. e) SEM image showing an h-BN ring surrounded by graphene. Reproduced with permission.[218] Copyright 2013, Nature Publishing Group.

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prepared Fe3O4 nanospheres wrapped by graphene nanosheets (GS) and further embedded in a 3D graphene foam (GF) to form a 3D hybrid electrode (Fe3O4@GS/GF).[165] The GS/GF hybrid matrix served as a double protection against the volume changes of Fe3O4 spheres during charge/discharge processes. In addition, the 3D porous graphene framework improved the conductivity and greatly increased the specific surface area of the electrode. The resultant Fe3O4@GS/GF was capable to deliver a high reversible capacity of 1059 mA h g−1 at 93 mA g−1 over 150 cycles (Figure 8). The very early demonstration of lithium intercalation into layered inorganic materials (e.g., TiS2) dates back to the 1970s, marking the birth of the insertion type of LIBs.[236,237] Unfortunately, this type of batteries usually suffer from low conductivity and poor cyclic stability. Recently, the application of layered metal dichacolgenides, especially MoS2, in LIBs has gained renewed attention, and the improved energy storage performance has been realized by using hybrid nanostructured electrodes. Some examples of MoS2-based hybrid electrodes for LIBs are listed in Table 1. Like many other inorganic battery materials, MoS2 nanosheets have been combined with graphene nanosheets for the improved conductivity and stability. Such hybrid materials can be obtained by directly mixing pre-exfoliated MoS2 nanosheets and GO,[214] or in situ growth of MoS2 nanosheets on GO which was then subsequently reduced to rGO.[212,238] For example, Chang and Chen prepared few-layer MoS2/rGO hybrids via the L-Cysteine-assisted hydrothermal process. The MoS2/rGO-based anode with an Mo:C molar ratio of 1:2 exhibited a high specific capacity of ca. 1100 mA h g−1 at a current density of 100 mA g−1 for 100 cycles (Figure 9).[238] It was suggested that the graphene nanosheets can provide conductive paths for the hybrid electrode,[238] and simultaneously serve as a cushion to accommodate volume changes of the active 2D nanomaterials.[212] Recently, our group also showed that 3D graphene network can support the CVD-growth of MoS2 flakes for LIBs, providing good electrical contact between the MoS2 flakes and Ni foam, (i.e., the current collector).[30] In the aforementioned MoS2/graphene hybrid materials, a key problem is that most of the MoS2 nanosheets consisting of a few to multiple layers were in-plane grown or assembled on graphene, leading to the much reduced surface area for lithium storage. In addition, the conductivity of TMDs is low especially in the direction normal to their basal planes.[17] Therefore, construction of 3D hierarchical porous and loose electrodes has been explored, such as MoS2/CNT hybrid,[18] MoS2 spheres in carbon matrix[239] and MoS2/PANi nanowires.[17] First, in these structures, nanosheets/nanoplates with largely exposed surface area possess short distance for the diffusion of Li+ ions and large electrode-electrolyte contact area for the flux of Li+ ions across the interface, leading to the enhanced rate capability.[17,21,239] Second, the conductive matrix such as amorphous carbon[239] and conductive polymer[17] can provide improved conductivity and prevent nanosheets from aggregation. In addition, the voids between the nanosheets and matrix materials in the 3D hierarchical structure can accommodate the volume change and protect the active material from pulverization during discharge/ charge process.[17,21,239]


3.2. Electrocatalysis The key to advancing renewable-energy technologies is the development of electrocatalysts with high catalytic activity, low cost and excellent stability. Noble metals, particularly Pt and Pd, are well known electrocatalysts that are commercially available. To minimize the use of Pt or Pd and thus reduce cost, they have been hybridized with other materials. Meanwhile, non-precious metals or metal-free catalysts with high catalytic activity are also under intensive exploration. The excellent conductivity and large specific surface area of graphene make it an attractive matrix for synthesis of hybrid materials for electrocatalytic reactions. Besides, graphene derivatives also possess good electrocatalytic activities for various redox reactions.[240] To date, metals such as Pt,[99,100,241–243] Pd,[104,244] Au,[87,245] FePt[113] and Pt/CuPd,[10] metal oxides such as Co3O4,[12] Fe3O4,[164] MnCo2O4[246] and Co/CoO,[247] and other inorganic materials such as MoS2[14] and TiN[197] have been coupled with graphene for preparation of novel electrocatalysts. For example, the Pt-graphene hybrid catalyst has exhibited higher catalytic activity and better stability compared to the commercial Pt-C in catalytic reactions like methanol oxidation[248] and the oxygen reduction reaction (ORR).[99] Additionally, Chen et al. reported that Pd NP-graphene hybrid showed superior performance toward both formic acid and ethanol oxidation compared to the Pd-C catalyst.[249] In their work, it is worth mentioning that the Pd NPs were synthesized by directly mixing K2PdCl4 and GO sheets via the electroless deposition. Therefore, the obtained Pd NP-graphene hybrid was free from surface ligands, leading to the much enhanced catalytic activity. Although the relatively better performance has been achieved by using graphene as the catalyst support, the high cost arising from the use of noble metals (e.g., Pt and Pd) restricts their wide application. Therefore, much effort has been devoted to the development of non-precious metal or metal oxide catalysts for electrocatalytic reactions. For example, Parvez et al. found that the nitrogen-doped graphene (NG) showed a stable methanol crossover effect, high current density (6.67 mA cm−2) and good durability (ca. 87% after 10 000 cycles) when it catalyzed ORR in alkaline solution.[106] Moreover, a further enhanced methanol crossover effect and higher current density (8.20 mA cm−2) can be achieved by loading 5 wt% Fe NPs on NG, and the resultant hybrid showed faster electron transfer and higher stability in both alkaline and acidic solutions than that of Pt or NG alone.[106] In addition, graphene–metal oxide hybrids are also emerging and promising electrocatalysts. As a representative example, Dai and co-workers reported that Co3O4 NPs hybridized with N-doped-rGO could be used as a bi-functional catalyst for ORR and OER in alkaline solutions, and the ORR catalytic activity is comparable with the commercial 20 wt% Pt on Vulcan XC-72.[12] The high catalytic activity of this composite might be attributed to the synergistic coupling between Co3O4 NPs and graphene. Later on, the same group demonstrated that the activity of the hybrid material for ORR could be further enhanced by partial substitution of Co with Mn to synthesize MnCo2O4/N-doped graphene hybrid.[246] Moreover, Fe3O4 NPs loaded in a 3D N-doped graphene aegogel were demonstrated by Müllen and coworkers for ORR, which exhibited a better durability than does the commercial Pt-C catalyst.[164]


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REVIEW Figure 8. a,b) SEM (a) and TEM (b) images of Fe3O4 nanospheres embedded in graphene nanosheets. c) Cycling performance of Fe3O4@GS/GF, Fe3O4 @GS, and Fe3O4 nanospheres at a current density of 93 mA g−1. d) Cycling performance of Fe3O4 @GS/GF and Fe3O4@GS at various current densities. Reproduced with permission.[165] Copyright 2013, John Wiley & Sons, Inc.

Some layered TMDs like MoS2 and WS2 are well known electrocatalysts and have drawn much attention recently due to their low cost, high chemical stability, and good catalytic properties toward hydrogen evolution reaction (HER).[14,250–252] Their catalytic performance can be further improved by the structural and morphology control, as well as the formation of composites. Recently, MoS2 NPs were synthesized in situ on graphene surface by Dai and coworkers.[14] Superior catalytic activity was obtained from this MoS2/rGO hybrid for HER, exhibiting a Tafel slope of only ca. 41 mV decade−1. The excellent performance has been attributed to the abundant catalytic edge sites on the MoS2 NPs and the good electrical coupling between the NPs and rGO sheets. To further increase the loading of catalysts on graphene, Li and co-workers recently grew MoSx (x ≥ 2)

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sheets on 3D graphene coated Ni foam.[32] It was found that the loading weight of the MoSx catalyst on 3D graphene/Ni foam is larger than that on other carbon-based electrodes such as carbon paper, carbon cloth and graphite mats. Apart from the increased catalyst loading that led to the larger catalytic current, the much reduced resistance of the electrode consisting of Ni and CVD-graphene also contributed to the superior performance of the hybrid electrode.[32] Similar to graphene, TMD nanosheets have been used to support the growth of noble metals for the enhanced hydrogen production. Some examples include the composites of Pt/ MoS2, Pt/TiS2, Pt/TaS2, Au/MoS2 and Au/WS2.[11,64,205] Our recent work indicated that single-layer MoS2 nanosheets can template the epitaxial growth of well dispersed 1–3 nm Pt NPs



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www.advmat.de Table 1. List of MoS2-based hybrid structures for LIBs and their performance. Materials MoS2/3D graphene MoS2/graphene nanosheets

Preparation method

Capacity and cyclic performance

Ref.

CVD growth

877 mA h g−1 at 100 mA g−1 after 50 cycles

[30]

in situ solution growth of MoS2 on GO followed by heat treatment

808 mA h g−1 at 100 mA g−1 after 100 cycles

MoS2/graphene nanosheets

Hydrothermal synthesis of MoS2 in the presence of GO, followed by annealing

MoS2/graphene nanosheets

Mixing chemically exfoliated MoS2 with GO followed by reduction

MoS2 spheres in carbon matrix

Hydrothermal synthesis of MoS2 in the presence of PVP

MoSx nanosheets coated on CNTs

Hydrothermal synthesis of MoSx in the presence of CNTs

MoS2/PANI nanowires

Hydrothermal reaction involving MoOx/PANi nanowires and thiourea

with exposed highly active edge facets. The resulting Pt-MoS2 hybrid catalyst showed a Tafel slope of ca. 40 mV decade−1, and much enhanced catalytic activity compared to the commercial Pt-C catalyst on the basis of equal Pt loading.[11] In the case of Au NP-decorated MoS2 or WS2 nanosheets,[205] although Au NPs do not provide high catalytic activity for hydrogen evolution, their decoration on MoS2 or WS2 greatly improved the charge transport between neighboring nanosheets, as indicated by a reduced charge-transport impedance, leading to the much enhanced catalytic performance.[205]

3.3. Photocatalysis Due to their low cost, chemical inertness and photostability, inorganic nanocrystals, especially metal oxide and chalcogenide semiconductors, have been considered the most suitable photocatalysts for widespread environmental applications.[125,126] Hybrid nanostructures are particularly important in photocatalysis, because a single component catalyst might not be able to simultaneously meet the several requirements for high-performance photocatalysis (i.e., broad visible light adsorption, large specific surface area, effective electron-hole pair generation and minimum charge carrier recombination). As a result, graphene and its derivatives have been incorporated with different semiconductor nancrystals such as TiO2,[15,127,131,173,253] ZnSe,[196] CdS,[254] MoS2[15] and Ag/AgX (X = Cl, Br)[255] to prepare advanced photocatalysts. Generally, these hybrid photocatalyts have acquired enhanced photocatalytic activities as a result of graphene's outstanding electronaccepting and transporting property. For example, graphenesupported TiO2 NPs as a visible light photocatalyst exhibited a higher catalytic activity than that of TiO2-carbon nanotube hybrid toward selective oxidation of alcohols.[173] The beneficial effect from graphene as an electron collector is also demonstrated by Li et al. by using CdS cluster-modified graphene for visible-light-driven photocatalytic H2 production, where a high H2-production rate (1.12 mmol h−1) that is about 4.87 times than that of pure CdS NPs has been achieved.[254] In another example, Yu and co-workers reported that ZnSe NP-decorated N-doped graphene composite showed superior activity for the decomposition of methyl orange (MO) under visible-light irradiation.[196] Other types of 2D nanosheets, on the other hand, have been mostly grown and assembled on 1D nanostructures

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−1

1100 mA h g at 100 mA

g−1

after 100 cycles

915 mA h g−1 at 500 mA g−1 after 700 cycles 813.5 mA h

g−1

−1

at 1000 mA g after 100 cycles

1000 mA h g−1 at 50 mA g−1 after 45 cycles 959.6 mA h

g−1

at 100 mA

g−1

after 50 cycles

[212] [238]

[214] [239] [18] [17]

to prepare hybrid photocatalysts, such as MoS2 nanosheets/ TiO2 nanobelt,[24] TiO2 nanosheets/carbon fiber,[20] and Ni3S2 nanosheets/CNT.[19] For example, our group recently prepared few-layer MoS2 nanosheet-coated TiO2 nanobelt hybrid, referred to as TiO2@MoS2, used for both H2 evolution and degradation of dye molecules,[24] which showed the enhanced catalytic activity compared to the pure TiO2 belts. The MoS2 nanosheets showed better visible-light absorption than TiO2, and the formation of the TiO2-MoS2 heterojunction reduced the recombination of photo-generated charge carriers. It is worth noting that the highest photocatalytic activity of TiO2@MoS2 hybrid heterostructures was obtained when the loading of MoS2 was 50 wt%, resulting in a hydrogen production rate of as high as 1.6 mmol h−1 g−1. Too low MoS2 loading causes the poor visible light absorption, while excess MoS2 coating blocks the light from reaching the TiO2 nanobelts, thus suppressing the generation of photogenerated electrons/holes in TiO2. In another report, Wang and coworkers prepared micrometer- and nanometer-sized anatase TiO2 sheets, mainly enclosed by the most active (001) facets on carbon fibers (Figure 10a–c).[20] In comparison with TiO2 NSs grown on a planar substrate, the TiO2 NSs on carbon fibers exhibited a 3.38-fold improvement in photocatalytic degradation of methanol orange (MO) under UV light irradiation. Moreover, the synergistic effect of TiO2 and carbon can greatly hinder the recombination of photoinduced electrons and holes, and thus significantly enhance their photocatalytic performance. Besides the hybrid structures with two components which were developed, ternary architectures have also been prepared for photocatalysis. Our group recently developed a ZnO/TiO2/ CuO hybrid nanostructure as the photoanode in photoelectrochemical cell (PEC) for water splitting (Figure 10d–f),[22] where the 1D ZnO nanowires and CuO nanoneedles both benefit the efficient charge transport, and the 2D TiO2 nanosheets provide high photocatalytic activity due to their exposed (001) facets. In addition, the p-n junction formed between TiO2 and CuO can efficiently separate photogenerated electron-hole pairs. As a result, the PEC based on this hybrid nanostructure exhibited the excellent photocatalytic activity for H2 evolution in water splitting even in the neutral electrolyte, under the low potential of 0.3 V with Ag/AgCl as reference electrode. In another example, Hou et al. demonstrated a 2D porous g-C3N4 nanosheets/nitrogen-doped graphene/layered MoS2 (CNNS/ NRGO/MoS2) ternary nanojunction.[256] The CNNS with a large surface area can absorb visible light, together with the layered


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MoS2 to enhance light absorption and generate more photoelectrons. The charge separation and transfer were improved at the CNNS/MoS2 interface (sheet to sheet), where the NRGO worked as the electron mediator for shuttling electrons–holes between the CNNS and MoS2 sheets. Consequently, the hybrid material exhibited a high photocurrent density and photocatalytic activity for simultaneous oxidation of MB and reduction of Cr(VI) compared to other reference electrodes under simulated sunlight irradiation.

3.4. Electronic and Optoelectronic Devices The formation of artificial layered crystals from stacks of different 2D nanomaterials with dissimilar electronic properties opens a new avenue in material science, and brings unlimited

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Figure 9. a) TEM image of MoS2/rGO hybrid material with the Mo:C molar ratio of 1:2. b) Rate capability of MoS2/rGO samples at different current density: (1) MoS2/rGO (1:1); (2) MoS2/rGO (1:2); (3) MoS2/ rGO (1:4). Reproduced with permission.[238] Copyright 2011, American Chemical Society.

possibilities in creating complex structures with unusual properties and function.[65–67] Most heterostructures that have been demonstrated so far were explored to overcome the limitation of graphene devices. The first type of such structures is graphene coupled with h-BN.[68–71] It has been found that graphene devices on SiO2 substrates show inferior performance compared to suspended graphene devices, due to problems such as carrier scattering from charged surface states and impurities,[257] roughness of substrate surface [258] and optical phonons of SiO2 surface.[259] The h-BN, an insulating isomorph of graphite, is an ideal substrate for graphene devices, as it has an atomically smooth surface and expected to be free of dangling bonds and charge traps. In addition, it also has a large bandgap of 5.97 eV[260] and large phonon modes with energies two times larger than the similar modes in SiO2.[68] Recently, Hone and coworkers fabricated high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using the “dry” transfer method. It was shown that the graphene devices on h-BN substrates have mobilities and carrier inhomogeneities almost an order of magnitude higher than those on SiO2.[68] Following this work, Mayorov et al. encapsulated graphene in between two h-BN sheets, where micrometer-scale ballistic transport in the graphene at room temperature was demonstrated for a wide range of carrier concentrations.[261] In this sandwiched structure, the bottom h-BN served as the dielectric substrate whereas the top h-BN served as both a top-gate dielectric and a protective cover for the graphene from deterioration under ambient conditions. Another type of graphene-based devices built up by stacked 2D crystals is the tunneling transistor first demonstrated by Britnell et al.[73] In their work, two monolayers of graphene used as electrodes were separated by a layer of MoS2 or h-BN.[73] By applying a gate voltage between one of the graphene electrodes and a gate electrode (i.e., an isolated silicon substrate), carrier concentrations in both graphene layers were changed. The application of a bias voltage between the two graphene electrodes induced a tunneling current across the MoS2 or h-BN layer, leading to the room temperature On/Off ratio of up to ca. 10 000 and 50, respectively. Their work demonstrated that the limitation of graphene with zero bandgap can be overcome by combination of it with other 2D materials with proper structural design, even without bandgap engineering of the graphene. Later on, the MoS2 or h-BN tunneling layer was replaced by a few-layer WS2 planar crystal, which is able to switch between tunneling and thermionic transport regimes, resulting in much higher On/Off ratios and larger current in the On state.[75] As shown in Figure 11, a negative gate voltage shifts the Fermi level of both graphene layers downwards, increasing the tunneling barrier height to give an “Off” state, whereas a positive gate voltage reduces the barrier height and simultaneously induces an over-barrier thermionic current to set the transistor to “On”. The gate induced exponential change in the barrier height, together with the switching from tunneling current to therminonic current, led to the extremely large On/Off ratios up to 1 × 106 at room temperature (Figure 11e,f). Unfortunately, by adopting the tunneling configuration, the large On/Off ratio is usually achieved with sacrifice of current density. Recently, Duan and coworkers proposed a vertical

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Figure 10. a) SEM image of uniform and dense array of TiO2 NSs grown on a carbon fiber. b) Comparison of the photocatalytic degradation rates of MO for TiO2 NSs on carbon fibers (triangles), TiO2 NSs on a FTO substrate (circles), and pure carbon fiber (squares). c) Schematic representation of the same amount of TiO2 NSs grown on FTO-glass and carbon nanofiber substrates with same surface area. Reproduced with permission.[20] Copyright 2012, John Wiley & Sons, Inc. d,e) Schematic illustration and the corresponding SEM images of the tree-like ZnO/TiO2/CuO heterostructures on FTO. f) Linear sweep voltammograms for P25, TiO2, ZnO/TiO2 and ZnO/TiO2/CuO films on FTO. Reproduced with permission.[22] Copyright 2011, John Wiley & Sons, Inc.

field-effect transistor (VFET) that can simultaneously deliver a large On/Off ratio and large current density.[76] In a typical n-type VFET, the MoS2 layer, sandwiched between a singlelayer graphene and a metal thin film, serves as the semiconducting channel, instead of the tunneling barrier as in the case of a tunneling transistor. The VFET exhibited both a large room temperature On/Off ratio of more than 103 and a high current density of up to 5000 A cm−2, which is about 2–5 orders of magnitude larger than do the vertical tunneling transistors or barristers.[73,262] Importantly, while the current density in the On state showed a weak dependence on MoS2 thickness, the On/Off ratio was strongly dependent on the MoS2 thickness. In order to achieve a high On/Off ratio, a relatively thick MoS2 layer (e.g., 30–40 nm) should be used. In contrast, due to the short-channel effect, when the MoS2 thickness was reduced to 9 nm, for example, an On/Off ratio of only 3 was obtained. So far, phototransistors based on the pristine graphene have shown the low photoresponsivity (typically < 10 mA W−1) due to the poor electron-hole separation efficiency of graphene resulting from its ballistic charge transfer property.[263–265] Strategies such as bandgap engineering[266] and use of surface plasmons[267] or quantum dots[268] have been applied to enhance the responsivity of graphene-based devices. In contrast, photodetectors based on MoS2 thin layers have shown higher photoresponsivity ranging from 7.5 mA W−1 to 880 A W−1.[44,269–271] Recently, Roy et al. fabricated the graphene-on-MoS2 binary heterostructures, which showed highly sensitive photodetection and gate-tunable persistent photoconductivity.[272] In a typical device, a single-layer graphene was overlayed on a multilayer MoS2 flake which was placed on a Si/SiO2 substrate. The formation of the trap-free heterojunction between the graphene and MoS2 layer allowed for the large carrier mobility in the graphene and effective transfer of photogenerated electrons from the MoS2 to graphene under 2198


the appropriate gate electric field. The photoresponsivity of the device reached up to 1 × 1010 A W−1 at 130 K and 5 × 108 A W−1 at room temperature.[272] In another report, Britnell et al. demonstrated that a graphene/WS2/graphene sandwiched structure with appropriately positioned Fermi levels combined with doping of the two graphene layers differently has led to enhanced photon absorption and electron-hole creation.[74] The extrinsic quantum efficiency (EQE) of the device was calculated to reach up to 30%. The device performance can be further enhanced by optimizing light absorption in the active layer through the incorporation of plasmonic nanostructures. As an example, Au NPs were deposited on top of one of the hBN/ graphene/MoS2/graphene heterostructures, leading to a 10-fold increase in the photocurrent.[74]

4. Conclusions With the ability to prepare various nanosized materials of all dimensionalities (i.e., 0D, 1D, 2D, and porous 3D), combined with the advanced nanofabrication techniques, the current research in preparation of hybrid nanostructures lies on not only the choice of functional components, but also the spatial organization/assembly and geometric properties of the complex nanostructures. By using 2D nanomaterials with unique properties, the planar hybrids, porous hierarchical architectures and vertically stacked heterostructures have been prepared which have showed much impressing properties, enhanced functions and improved performance. Specifically, for applications in catalysis, energy storage and solar harvesting, an advanced hybrid nanostructure should generally meet the requirements of large specific surface area for reaction, ion exchange or light adsorption, a conductive network for charge transport, and an interface/heterojunction


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REVIEW Figure 11. a) Schematic of vertical architecture of transistor. b) Band diagram corresponding to no Vg and applied Vb. c) Negative Vg shifts the Fermi level of the two graphene layers down from the neutrality point, increasing the potential barrier and switching the transistor OFF. d) Applying positive Vg results in an increased current between GrB and GrT due to both thermionic (red arrow) and tunneling (blue arrow) contributions. e) I–V plots for different Vg (semi-logarithmic scale). f) Red circles: zero-bias conductivity as a function of gate voltage (measured as the slope of the I–Vb curve at zero Vb). Blue circles: conductivity measured at Vb = 0.02 V as a function of gate voltage. Inset: I–V at different Vg (linear scale). T = 300 K. Device active area: 0.25 mm2. Reproduced with permission.[75] Copyright 2013, Nature Publishing Group.

formed by two components for the effective channeling or separation of charge carriers. Among these requirements, much effort has been devoted to increasing the effective surface area of a hybrid, for example, by constructing 3D porous hierarchical architectures. However, the specific surface area could never go beyond the theoretical value of a monolayer nanosheet. One possible strategy is to create high-density nanosized holes in the nanosheet via etch-based methods. As such, the resulting nanoporous sheet structure (or nanomesh) can have a surface area much larger than the theoretical value of a seamless 2D material,[273] and simultaneously provide plenty of active edge sites for various reactions.[251] For electronic and optoelectronic devices, the verticallystacked heterostructures hold great promises. Although the investigation only started in 2010, the significant breakthrough has already been made. So far, graphene, h-BN, MoS2 and WS2 have been used for preparation of several kinds of heterostructures. More choices of 2D building blocks and their combinations are anticipated in the near future. Importantly, these artificial 3D crystals made of functional 2D nanomaterials can be further hybridized with desirable molecules, ions, metals, etc. via means like vapor-phase intercalation for further modulation of device properties and improvement of device performance.

Acknowledgements This article is part of a series celebrating the 25th anniversary of Advanced Materials. This work was supported by MOE under AcRF Tier 2 (ARC

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26/13, No. MOE2013-T2–1–034), AcRF Tier 1 (RG 61/12, RGT18/13 and RG5/13), and Start-Up Grant (M4080865.070.706022) in Singapore. This research is also funded by the Singapore National Research Foundation and the publication is supported under the Campus for Research Excellence And Technological Enterprise (CREATE) programme (Nanomaterials for Energy and Water Management). Received: October 6, 2013 Revised: December 20, 2013 Published online: March 10, 2014 [1] R. Costi, A. E. Saunders, U. Banin, Angew. Chem. Int. Ed. 2010, 49, 4878. [2] P. D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 2006, 35, 1195. [3] S. Mann, Nat. Mater. 2009, 8, 781. [4] Z. Spitalsky, D. Tasis, K. Papagelis, C. Galiotis, Prog. Polym. Sci. 2010, 35, 357. [5] S. Lee, C. Fan, T. Wu, S. L. Anderson, J. Am. Chem. Soc. 2004, 126, 5682. [6] Y. Yin, A. P. Alivisatos, Nature 2005, 437, 664. [7] K. M. K. Yu, C. M. Y. Yeung, S. C. Tsang, J. Am. Chem. Soc. 2007, 129, 6360. [8] Y. Liu, X. Dong, P. Chen, Chem. Soc. Rev. 2012, 41, 2283. [9] X. Huang, S. Li, Y. Huang, S. Wu, X. Zhou, S. Li, C. L. Gan, F. Boey, C. A. Mirkin, H. Zhang, Nat. Commun. 2011, 2, 292. [10] C. Hu, H. Cheng, Y. Zhao, Y. Hu, Y. Liu, L. Dai, L. Qu, Adv. Mater. 2012, 24, 5493. [11] X. Huang, Z. Zeng, S. Bao, M. Wang, X. Qi, Z. Fan, H. Zhang, Nat. Commun. 2013, 4, 1444. [12] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat. Mater. 2011, 10, 780.



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Simultaneous Microfabrication and Tuning of the Permselective Properties in Microporous Polymers Using X-ray Lithography S. H. Han, P. Falcaro, and co-workers

INTERNATIONAL JOURNAL OF APPLIED GLASS SCIENCE

Impact Factor: 7.823 With an Impact Factor of 7.823, Small continues to be the premier Applied journal for research at the nanoandGlass SCIENCE mircoscale.

www.small-journal.com

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Impact Factor: 0.857 Particle, a member of the Advanced journals family, focuses on all aspects of particle research. It is one of the top 10 journals in Characterization & Testing by Impact Factor and by total citations, too. ijag_4_1_oc.indd 1

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First Impact Factor: 1.548 The journal received its first Impact Factor of 1.548 and has established itself as an indispensable source of knowledge on the application of glass science and engineering across the entire materials spectrum. Published on behalf of The American Ceramic Society.

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NEW journalS Advanced Healthcare Materials

Vol. 2 • No. 6 • June • 2013

www.advhealthmat.de

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First Immediacy Index: 0.712 Launched in 2012, Advanced Healthcare Materials received its first Immediacy Index of 0.712. This inaugural value establishes Advanced Healthcare Materials as a premier journal for publishing biomedical materials research. www.advhealthmat.com Get complimentary online access in 2013: wileyonlinelibrary.com/newjournals-optin

Vol. 1 • No. 6 • June • 2013

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www.advopticalmat.de

Advanced Optical Materials First Immediacy Index will be announced in 2014. This new journal was founded in 2013 as a member of the Advanced journals family. It is covering all aspects of light-matter interactions, including topics like plasmonics, metamaterials, photonics and more. www.advopticalmat.com

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Get complimentary online access in 2013&2014: wileyonlinelibrary.com/newjournals-optin

wileyonlinelibrary.com/subject/materials *2013 Release of Journal Citation Reports® Source: Thomson Reuters 2012 Citation Data

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Materials Science

Top Journals and their 2012 Impact Factors*

25th anniversary article: label-free electrical biodetection using carbon nanostructures.

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[On the 25th anniversary of the PAPPS ].

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Electrochemical sensors and biosensors based on nanomaterials and nanostructures.

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25th anniversary article: progress in chemistry and applications of functional indigos for organic electronics.

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Editorial. The 25th Anniversary of the BMS.

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The CCN family of proteins: a 25th anniversary picture.