Recent advances in the fabrication and structure-specific applications of graphene-based inorganic hybrid membranes.

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Recent advances in the fabrication and structure-specific applications of graphene-based inorganic hybrid membranes Xinne Zhao,‡a Panpan Zhang,‡a Yuting Chen,a Zhiqiang Su*a and Gang Wei*b

Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/

There are increasing interests for the preparation and applications of graphene (G)-based materials due to their unique electronic, optical, magnetic, thermal and mechanical properties. Compared to G-based hybrid and composite materials, G-based inorganic hybrid membrane (GIHM) shows enormous advantages ascribing to its simplicity of synthesis, planar two-dimensional multilayer, high specific surface area, mechanical stability, as well as unique optical and mechanical properties. In this review, we demonstrated the recent advances in the technical fabrication and structure-specific applications of GIHMs with desirable thickness and compositions. In addition, the advantages and disadvantages of these utilized methods for creating GIHMs were discussed in detail. Finally, the potential applications and key challenges of GIHMs for future technical applications were mentioned.

1. Introduction Graphene (G), as the basic unit of graphite-like materials, has the uniform two-dimensional (2D) honeycomb lattice structure with rigorously sp2 hybridization.1-4 Inspired by the unique properties of G, such as large surface area, superior mechanical strength, extraordinary electronic and catalytic properties, as well as excellent thermal and chemical stabilities, many efforts have been performed to explore the potential academic and technological applications of G-based materials since its discovery in 2004.5-7 When different types of nanostructures like metallic nanoparticles (NPs), nanowires (NWs), nanotubes (NTs), polymer NPs, quantum dots, and non-metallic NPs integrate with G, the created G-based hybrid materials can present fascinating applications in optoelectronic devices, electrochemical biosensors, reinforced functional materials, drug carries, lithium-ion batteries, supercapacitors, and energy storage.8-12 With the advantages of low price, high quality, and simple synthesis process, G-based materials hold greater potentials than other carbon-based materials.13 In addition, it is now easy to chemically synthesize G and graphene oxide (GO) according to recent technical progress.14-16 GO can not only reserve some properties of G but also extend the physical and chemical modifications for fabricating some novel functional materials. The decoration of G with oxygenic functional groups endows GO a lot of functions in three reasons: (i) GO can be dispersed uniformly in most of solvents; (ii) The oxygenic functional

This journal is © The Royal Society of Chemistry 2013

Xinne Zhao is currently a master degree candidate under the supervision of Prof. Su at Beijing University of Chemical Technology, China. Her research interests are focused on functional graphene-based hybrid nanomaterials, organic-inorganic nanofibrous materials by electrospinning, and biosensors.

Panpan Zhang received his bachelor degree in Polymer Materials and Engineering from Beijing University of Chemical Technology, China (2012). Currently, he is pursuing a master degree in Materials Science and Engineering under the supervision of Prof. Zhiqiang Su. His research interests focus on novel carbon and metal nanomaterials for the applications in electrochemical biosensors, energy storage, and conversion.

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Published on 12 February 2015. Downloaded by Kingston University on 12/02/2015 18:05:34.

Review

Nanoscale

ARTICLE DOI: 10.1039/C5NR00084J Zhiqiang Su is a Professor at Beijing University of Chemical Technology, China. His research is focused on nanohybrids, biomedical materials, polymer materials, biosensors, and bioelectronics. So far, he has published more than 60 papers in the international peer reviewed journals. The papers have been cited for more than 450 times with an h-index of 15.

Gang Wei is currently a senior researcher and group leader in Hybrid Materials Interfaces Group at University of Bremen, Germany. His research interests include carbon-based nanomaterials, nanofibrous biomaterials, biosensors, and single molecule force spectroscopy. He has published more than 60 papers in the international peer reviewed journals and the published papers have been cited more than 850 times with an h-index of 19. groups are beneficial to some chemical reactions; (iii) The surface groups of GO can mediate the combination between GO with polymers, ceramics, and metal matrices.17 As highly oxidized nanosheet of sp2-hybridized carbon, GO can be transformed into reduced graphene oxide (RGO) by chemical and thermal reduction.18 Compared with GO, RGO can partly recover and acquire the outstanding conductivity, which is important for the fabrication of functional nanomaterials and nanodevices.19-22 The hybridization of G (G and RGO) with inorganic nanostructures can be described as three typical ways based on the reaction procedures: (i) GO was reduced to RGO firstly, and then the inorganic nanostructures were prepared and attached onto RGO;23-26 (ii) The inorganic nanostructures grew up on GO, and then GO/nanostructure hybrids were reduced to RGO/nanostructure simultaneously;27-31 (iii) One-step synthesis and reduction of GO and inorganic ions precursors were carried out in one system.32-35 Since 2006, Ruoff’s group first reported a G–polystyrene nanocomposite,8 varieties of G-based nanocomposites have been widely synthesized and applied. For example, polymeric membranes containing G-based materials, having several applications in energy and environment related areas (fuel cells, sensors, water purification).36-40 What’s more，the introduction of inorganic nanostructures onto the surface of G can not only reserve the remarkable performances of G, but also bring some


new properties into the created nanohybrids. One of the main functions of G is to provide a support for the binding of inorganic nanostructures. According to the components of inorganic nanostructures, the fabricated G-based inorganic nanohybrids can be classified to three main types by hybridizing G with metallic nanostructures, metallic compound nanostructures, and non-metallic nanostructures. Recently, there are increasing interests on the fabrication and applications of G-based inorganic hybrid membranes (GIHMs). First of all, the created GIHMs preserve all the advantages from individual components, and the hybridization of G with other nanoscale building blocks can bring additional and surprising properties. The continuous and conductive GIHM makes it possible for photovoltaic applications and provides a large surface area for attaching with other materials.41 At the same time, the controllable thickness of GIHM makes it suitable for fabricating functional materials with flexible plasticity and unique optical transparent properties. Furthermore, the hybrid structure of GIHM makes it with high efficiency in electrical conductivity. Finally, the created GIHM is very light, which can be used to replace other materials in portable devices. A survey of the open publications related to the keywords “graphene, hybrid, membrane” and “graphene, hybrid, film” in the past 7 years with SciFinder Scholar was performed and the result is shown in Fig. 1. It is clear that the fabrication and application of 2- and 3-D GIHMs exhibit increasing interests in recent years.

Fig. 1 Comparison of the annual number of scientific publications with the keywords of “graphene, hybrid, membrane” and “graphene, hybrid, film”, as searched by SciFinder Scholar. There are already about 417 related publications in 2014 (until 16 November).

In this review, we focus on the recent advances in the technical fabrication and applications of GIHMs with desirable thickness and compositions. We would like to demonstrate the fabrication of GIHMs by hybridizing G with three different types of nanomaterials, including metallic, metallic compound, and non-metallic nanostructures. The advantages and disadvantages of these utilized methods for the synthesis of GIHMs were introduced and discussed in detail. In addition, the potential applications and key challenges of GIHMs for future technical applications were mentioned.

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2. GIHMs fabricated with G and metallic nanostructures The metallic nanostructures can be classified as single- and multi-metallic systems (NPs, NWs, and NTs). With nanoscale size, metallic nanostructures have unique optical, magnetic, electrical and chemical properties.42 Previously, many methods have been utilized to fabricate GIHMs by hybridizing G with single-metallic23-25,27,28,32-34,43-63 and multi-metallic35,64,65 nanostructures. 2.1 Chemical vapor deposition (CVD) CVD has made a great contribution to prepare the G-based membranes. Hydrocarbon gases through CVD consolidating on metal surfaces are going to form uniform, large size and single-layered G membranes, which can be transferred onto arbitrary substrates that decorated with metallic NPs.43 The sheet resistance of G getting through CVD is much lower than that of G synthesized with chemical method.44 These characteristics may have huge potentials for the applications of flexible and large-scale functional devices. In order to fabricate GIHMs with CVD method, the created G membrane needs to be transferred onto a flexible substrate. Previously, several transfer techniques like roll-to-roll transfer, polymer-assisted transfer, thermal release tape (TRT), and hot-pressing remove have been utilized to combine with CVD for fabrication of GIHMs.45,46 The TRT-supported G membrane can be easily transferred onto the glass substrate coated with Ag nanostructure.45 For example, Du and co-workers fabricated a GIHM based on G and AuNPs by CVD-TRT method, which has potential application as an active substrate for surface-enhanced Raman scattering (SERS) detection of analytes.47 Furthermore, Liu and co-workers fabricated a G/AgNW hybrid membrane with a two-step method.48 They created a G membrane via CVD, which was further coated with polymethyl methacrylate (PMMA) monolayer. After that, the PMMA-coated G membrane was adhered onto the AgNW film to form G/AgNW hybrid membrane by dissolving PMMA in acetone. With the same strategy, Xu et al. obtained the PMMA-coated G membrane and transferred it onto the AgNWs, which were synthesized through a solvothermal method.49 The obtained GIHM can be used as electrodes for flexible and transparent loudspeakers. In another case, Mulpur and co-workers fabricated G membrane with the help of CVD method and the Ag thin film was deposited onto Pyrex microscope slides by physical vapor deposition system. Finally, the GIHM was obtained by transferring G membrane onto Ag film substrate and further applied for amplifying surface plasmon coupled emission.50 At present, CVD method is still the most common technique for fabricating GIHMs with large-scale production, but it is pretty complex since the membrane transfer process is required. In addition, the removal of precursors costs a lot of time and the


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ARTICLE DOI: 10.1039/C5NR00084J experimental procedure is not environment-friendly. Moreover, the reaction condition is slightly rigorous. 2.2 Substrate-assisted dry- and wet-transfer Dry- and wet-transfer techniques have been used to improve the transfer of large-scale G membrane. The dry-transfer method allows G membrane to transfer onto shallow depressions while the wet-transfer one enables G membrane to transfer onto perforated and flat substrates.66 The typical processes are shown in Fig. 2. For the dry-transfer process, a thick PMMA is coated on a G/Cu foil firstly (Fig. 2a). After that, a polydimethylsiloxane (PDMS) frame is used as a flexible support for G/Cu/PMMA membrane. With this support, G membrane will not be destroyed or wrinkled when removing from the etchant. After etching the Cu, the G/PMMA/PDMS membrane is dried and placed onto the target substrate. At the end, the wavy and rough membrane is heated to make itself strong enough when peeling off the PDMS.

Fig. 2 Schematic illustrations of (a) dry- and (b and c) wet-transfer processes for fabricating GIHMs.

For wet-transfer (Fig. 2b and c), G membrane is obtained in water, and a needle is utilized for positioning the membrane on a substrate (Fig. 2c). In this strategy, the assisted substrate can be regarded as a physical support for carrying G, and this substrate should be removed after the final combination of G with metallic NPs. Some typical substrates like flexible glass, normal rigid glass, and polymer membrane can be used as the assisted substrates. For example, Kholmanov et al. used the dry-transfer method for the fabrication of GIHM with RGO and CuNWs for the first time (Fig. 3).23 The thin film of CuNWs and RGO membrane were first fabricated by spray-coating and spin-coating, respectively. Then PMMA/RGO membrane was obtained by spin-coating PMMA onto RGO membrane. After a series of treating processes, PMMA/RGO membrane was transferred onto the surface of CuNW film by dry-transfer method. PMMA layer was removed with acetone, which eventually resulted in the formation of RGO/CuNW hybrid membrane. The corresponding electrical and optical tests

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ARTICLE DOI: 10.1039/C5NR00084J Furthermore, the great potential application as electrochemical biosensor was demonstrated.32 Compared with previous methods, our strategy for producing GIHM has several advantages such as environmental friendliness, cost-effectiveness, high efficiency, and time-saving.

Fig. 3 Schematic presentation for the fabrication of RGO/CuNW hybrid membrane.

The wet-transfer method has also been used to produce GIHM based on G with AgNWs. For example, Choi et al. transferred CVD-grown G membrane to PMMA with the wet-transfer method.52 AgNWs were deposited onto the G membrane by a simple spin-coating. As a matter of fact, substrate-assisted dry- or wet-transfer method is very effective procedures to fabricate functional GIHMs by combining other membrane-preparing techniques like CVD, spin-coating, and spray-coating. 2.3 Self-assembly Self-assembly is a very simple and time saving method to prepare large-scale GIHMs by controlling the experimental parameters. For example, Zhou et al. reported an electrostatic self-assembly method for the fabrication of RGO/AgNP hybrid membrane.28 They firstly synthesized polycation-modified AgNPs and conjugated the prepared NPs onto GO sheet. The reduction of GO and absorption of AgNPs occurred simultaneously, resulting in the formation of RGO/AgNP hybrid membrane. Their results showed that AgNPs with high density and uniform size distribution dispersed very well on RGO sheets. The fabricated GIHM with higher transmittance and lower sheet resistance holds great potential for solar cells as metallic electrode and anti-reflection coating. Recently, we have developed a more convenient and environmentally friendly strategy to synthesize self-assembled RGO/AuNP hybrid membrane, which only needs one step to achieve the preparation and reduction simutaneously.32,33 Fig. 4 shows the schematic model for the synthesis of RGO/AuNP hybrid membrane with self-assembly technique.32 GO and HAuCl4 were mixed together in aqueous solution and reduced into RGO and AuNPs with glucose under heat treatment. Continuous evaporation makes the created RGO interact with AuNPs, and then self-assemble into a multilayer structure. Finally, a PET film was used to harvest the formed RGO/AuNP hybrid membrane. The fabricated GIHM with this method is stable, flexible, semi-transparent, and size-adjustable.


Fig. 4 Schematic presentation for one-pot green synthesis of self-assembled RGO/AuNP hybrid membrane.

In other group, Bramhaiah et al. fabricated GIHM with RGO and multi-metallic (Au, Ag, and Pt) NPs at an aqueous-organic interface via self-assembly.64 Organic phase contained toluene solution of Ag(PPh3)4NO3 or Au(PPh3)Cl or Pd(PPh3)2Cl2 (PPh3 is triphenylphosphine), and aqueous phase contained GO with tetrakis hydroxymethyl phosphonium chloride. The ultra-thin films were formed at the interface of two phases followed by in-situ chemical reduction with sodium borohydride (NaBH4), leading to the fabrication of GIHM with RGO and metallic NPs. With the simple interfacial self-assembly method, Hoseini et al. also fabricated a novel RGO/Pt3Sn hybrid membrane,35 which means reducing the organometallic precursors and GO with NaBH4 at toluene-water interface. This route for fabricating GIHM is easy and inexpensive, which has great potential for the construction of functional membrane-based devices. Though the self-assembly method for the fabrication of GIHMs is simple and time-saving, it still has some disadvantages. The GIHMs fabricated via self-assembly are crisp, weak, and low-efficient. In addition, self-assembly usually needs surfactants to promote the combination of G with NPs. 2.4 Filtration method This method can obtain GIHMs without polymeric or surfactant stabilizers and the fabrication process can be simple, low-cost, and convenient. In addition, the thickness and constitution of the fabricated GIHMs by this method are controllable. Huang et al.55 and Chen et al.56 both obtained G membrane by filtration method. The same point of their studies is that they used hydrazine to reduce GO dispersion. The RGO membrane was obtained via filtration and followed by decorated with

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indicated that the electrical conductivity, oxidation resistance, substrate adhesion, and stability of the fabricated GIHM in harsh environment have been improved greatly. This interesting work expands the application of GIHMs for working as transparent electrodes in Prussian blue-based electrochromic devices.


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Journal Name AuNPs or NiNPs for fabrication of GIHMs, which can be respectively used in bioelectrochemistry, biosensor or electrocatalyst. In addition, Xu et al.57 and Tien et al.58 fabricated GIHMs with the help of vacuum filtration of G/AgNP dispersion. The G/AgNPs dispersion was firstly synthesized by adding silver nitrate aqueous solution into GO dispersion, and then reduced by hydrazine and NaBH4-ethylene glycol, respectively. The GIHM they obtained can be separately used in SERS or used as transparent and conductive films. Previously, Kong et al. used this method to produce layer-by-layer (LBL) GIHMs, in which the G and AuNP layers are alternating arrangement.25 The fabrication includes a typical two-step process. Firstly, a thin G membrane on a quartz substrate was produced by using the vacuum filtration of a RGO solution, and then AuNPs were synthesized by spontaneous reducing of Au ions on the created G membranes. In their study, the vacuum filtration method for LBL-assembly of AuNP-decorated G thin membranes does not require any agents, such as linkers or reducing agents, for the reduction of Au ions when compared to self-assembly method. In this technique, the RGO thin membrane shows more electrically conductive than the GO sheet, which is applicable for sensors and electronic devices. This strategy is simple and direct, and the fabricated GIHMs have potentials in application such as AuNP-oligonucleotide complexes for intracellular gene regulation and probes for DNA microarrays. One of the clear disadvantages of this method is that the size and shape of the fabricated GIHMs are limited by the vacuum filtration system. 2.5 Electrochemical deposition Electrochemical deposition is a novel route for synthesizing G/metallic NP hybrid membrane, which is also a simple, efficient, low-cost, and environmentally friendly route. It has enormous potential to be applied in environmental monitoring, food safety, disease detection, and so on. For instance, Zhao et al. used this method to produce a RGO/AgNP membrane.34 They first mixed and sonicated GO aqueous solution and AgNO3 solution together, and then placed the mixed solution overnight. The mixed solution with Nafion was dropped onto the surface of glassy carbon electrode (GCE), which was polished in advance. After the solution was completely dry, the GCE was used for cyclic voltammetry (CV) scanning in a phosphate buffer saline solution. Their results indicated that the GIHM created on GCE is 3D LBL structure. The CV and I-T tests confirm that the fabricated electrochemical sensor was fast, stable, and reliable, which makes it possible to be applied for chemical or biological determinations. In another work, Hu et al. also reported an electrochemical process for fabricating size-controlled GIHM with AuNPs on G surface.24 The GO membranes were fabricated first, and Au ions were obtained through anode oxidation of bulk Au in HCl aqueous solution, which growing into AuNPs on the RGO/PET membrane by electrochemical reduction. The unique of this method is that the requirement of Au ion precursor can be omitted and the hybrid membranes can be directly used as a uniform and highly active


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ARTICLE DOI: 10.1039/C5NR00084J SERS substrate without any surface modification and further treatment. However, the electrochemical method also has its limitations for the large-scale fabrication of hybrid membranes. 2.6 One-step pulsed power wire (PWE) evaporation PWE evaporation is a simple and economic method for synthesizing G/metallic NP hybrid membranes in an aqueous solution. Myekhlai et al. for the first time utilized this method to create a flexible transparent conductive GIHM and further used it as a novel optoelectronics material.60 The experimental process in schematic diagram is described in Fig. 5a.

Fig. 5 (a) Schematic experimental process for preparing G/AgNP hybrid membrane. (b) SEM image of G/AgNP hybrid membrane.

The planetary ball milling method was used to modify the surface of pristine G to form ground G. The surface of pristine G was modified by planetary ball milling method to form ground G, after that the ground G is highly dispersed in water under ultrasonication. AgNPs were prepared previously by PWE method67 and synthesized in the prepared G aqueous solution, and then the mixture was stirred by a magnetic stirrer to prevent the agglomeration. After that, the nanofluid composite containing of G/AgNP was evaporated under mild temperature and enhanced by thermal treatment. Furthermore, the G/AgNP nanohybrids were decorated on flexible PET film from its aqueous solution by spin-coating. The morphological and structural characteristics of the fabricated GIHM were investigated by scanning electron microscopy (SEM), and corresponding results confirmed that AgNPs were successfully decorated on the surface of G (Fig. 5b). It should be noted that the fabricated GIHM was harvested by a PET film and it was hard to separate PET from GIHM, which will limit the applications of GIHMs fabricated by this method. 2.7 Comparisons of fabrication techniques According to the introductions above, it was found that there are many strategies can be utilized to fabricate GIHMs based on

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ARTICLE DOI: 10.1039/C5NR00084J

G or RGO and inorganic single-metallic nanostructures. The process for preparing GIHMs can be simply classified into two types. The first one is preparing G or RGO membrane first, and then adding the as-synthesized inorganic particles onto G or RGO membranes to fabricate GIHMs.23-25,45,48-50,52,55,56 The other one is preparing the membrane by combining GO or RGO and inorganic NPs at the same time.28,32-35,57,58,60,64 For the first type, G and RGO membranes can usually be produced through CVD, Langmuir-Blodgett (L-B), vacuum filtration, spin or spray-coating before decorating inorganic particles.23,25,68 The GO surface structure can be accurately controlled in molecular level by L-B method, and it is the only technique to get GO sheets in LBL deposition manner. Through sequential LBL deposition of close-packed GO monolayers, multilayer GO membranes were obtained for producing uniform coverage and transparent conductors with low optical scattering loss after being transferred onto a substrate.69 For instance, Shi et al. got hybrid membranes by producing GO membranes through L-B technique, which were then decorated with AgNWs for further chemical reduction with hydrazine hydrate.27 In addition, another novel way for fabricating GIHM with metallic nanostructure is constantly being discovered. For example, using biocompatible ingredients, such as chitosan, peptide nanofiber and protein like fibrinogen and bovine serum albumin to form a stable dispersion and accelerate fabrication of large area membranes.61-63,70 The advantages and disadvantages of each method are presented in Table 1 clearly. Table 1 Comparisons of fabrication techniques for GIHMs with G and metallic nanostructures.

Advantages

Disadvantages

CVD43,44,46-50

common, large-scale production

time-consuming, high cost, rigorous reaction condition, thickness uncontrollable

Substrate-assisted dry- or wet-transfer23,52,66

large-scale production, effective procedures

complex process, time-consuming

Self-assembly28,32,33 ,35,64

simple, time saving, large-scale production

needs surfactants, crisp, weak and waste of materials

Filtration25,55-58

simple, low-cost, and convenient, controllable thickness and constitution

size and shape are limited by used instrument

Electrochemical deposition24,34

simple, efficient, low-cost, and eco-friendly

difficult for the large-scale production

PWE evaporation60,67

simple and eco-friendly

hard to separate from substrate


3. GIHMs fabricated with G and metallic-compound nanostructures The metallic compounds to hybridize with G (G and RGO) include oxides,26,29,30,71-81 hydroxides,82,83 mineral salts,84,85 and so on. Here, we would like to introduce how to add the metallic compound nanostructures onto G to fabricate GIHMs. 3.1 Filtration method This method is simple and efficient, which is most widely used for fabricating GIHMs based on G and metallic compounds since filtration can assist and direct the assembly of G and metallic compounds. Zhou et al.71 prepared a G/MnO2 nanosphere hybrid membrane by means of a typical filtration method, which can also be used for fabrication of RGO and other metal oxide hybrid membrane electrodes. For example, with the help of filtration method, Shao et al.72 obtained flexible, free-standing, and 3D porous GIHM based on G and MnO2 nanorod as the positive electrodes, and Kim et al.84 fabricated a RGO/titanate nanosheets hybrid membrane. Thu et al.73 synthesised superparamagnetic Fe3O4 nanocrystals anchored on GO sheets by a co-precipitation reaction and obtained hybrid membrane via vacuum filtration. In the study of Li et al., Ni(OH)2 nanoplates were firstly intercalated into G sheets, and with help of an extended filtration-assisted method, the excellent flexible GIHM was fabricated and used as positive electrode for supercapacitor.83 In addition, Zhang et al. demonstrated the vacuum filtration method to fabricate G/montmorillonite hybrid membranes with excellent properties of flexibility, electrical conductivity, and fire resistance.85

Fig. 6 Schematic fabrication of (a) G membrane and (b) MnO2NT thin membranes to LBL structure by filtration method. (c) SEM image of thin G/MnO2NT hybrid membrane.

Yu et al.74 produced a novel GIHM of G and MnO2NTs by filtration free-standing technique and further utilized the created

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Journal Name GIHM for lithium-ion battery with enhanced performances. Fig. 6 shows the schematic presentation for constructing G (Fig. 6a) and MnO2 LBL thin membranes (Fig. 6b). The cross-sectional image of the LBL G/MnO2NT hybrid membrane (Fig. 6c) shows that MnO2NTs were interspersed between the layers of G. The continuity of G layer was destroyed by the long MnO2NTs, which made it difficult to be distinguished. The G layers were separated and the electrical conductivity of the thin membrane was improved due to the addition of MnO2. The possible reason is that the layered structure including MnO2NTs can reduce the charge transfer resistance between G layers and accelerate the ion movement. By vacuum filtration and thermal reduction processes, Wang et al. created a 3D conductive network of flexible free-standing hollow G/Fe3O4 hybrid membranes,29 which can be used as materials for lithium-ion batteries. Due to their complementary working potential windows, transition metal oxides and carbon materials are generally served as negative and positive electrodes in asymmetric supercapacitors. Gao and co-workers synthesized RGO by a hydrothermal synthesis, and then they mixed the created RGO with Mn(CH3COO)2·4H2O and polyethylene glycol (PEG) in NaOH for the final fabrication of RGO/Mn3O4NP hybrids.26 By filtrating the mixtures of RGO and Mn3O4NPs with cellulose acetate membrane filters, the RGO/Mn3O4NP hybrid membrane was prepared after washing, air-drying, and peeling off from the filters. Nevertheless, the size and shape of the fabricated GIHMs are limited by the filtration equipment. 3.2 Self-assembly Self-assembly technique is a simple method, by which large-scale but still uniform nanostructured membranes can be obtained without any specialized equipment.76,77 In the study of Zhu and co-workers, a permanent magnet was utilized to help collect the fabricated GIHMs for nanodevices or catalytic applications.78 Electrostatic interaction is the mainly driven force for the assembly and formation of GIHMs. The 2D RGO/Fe3O4NP membranes were obtained by the following steps. They first prepared G with natural graphite by oxidation-exfoliation-reduction route. In order to improve the solubility of RGO in water, a surfactant sodium dodecylbenzene sulfonate (SDS) was used. After that, they prepared Fe3O4NPs by reducing Fe(acac)3 in triethylene glycol. The as-prepared SDS-modified RGO is positive-charged and the synthesized Fe3O4NPs are negative-charged. Finally, they obtained the RGO/Fe3O4NP hybrid membranes by a permanent magnet. The obtained GIHMs have excellent flexibility and mechanical properties. Due to the floating feature of the charged RGO, Fe3O4NPs were assembled onto both sides of RGO sheet, which provides the same properties at both sides of the fabricated GIHMs in solution. This general method can be easily extended to fabricate other functional GIHMs by using different Co3O4, NiO, and TiO2 NPs. The disadvantage of this method is that the previous preparation for embellishing RGO and metallic compounds nanostructure is complex.



3.3 CVD CVD was mentioned for preparing G-based metal hybrid membranes in Part 2.1. This technique can also be utilized to fabricate GIHMs based on G and metal oxide nanostructures. For example, Devadoss et al. obtained G/WO3 membrane with the help of CVD.79 In their study, G monolayer was firstly synthesized on a Cu foil with the typical CVD process. Then WO3NPs were deposited onto the G membrane in an ultra-high vacuum chamber by radio frequency sputtering with the atmosphere of Ar and O2. In another work, Kim et al. reported a successful strategy to fabricate large-scale GIHM of G with CVD and VO2 with sputtering method followed by a heat-treatment, which has potential to be applied in optoelectronic devices.80 The optical response property was also investigated by changing the number of G layers and temperature. 3.4 Electrophoretic deposition (EPD) Recently, the EPD method has been widely used to fabricate GO- and G-based films and membranes, which have potential applications for supercapacitors, field emission devices, solar cells, transparent conductors, and so on.86 EPD is an economic technique for the surface modification of metal oxides and the deposition of nanostructured membranes.30 As a way to fabricate GIHM, EPD contains several excellent advantages, such as high deposition rate and production, controllable thickness, and free of adhesive agents when compared to other processing methods.81

Fig. 7 Schematic representation for the preparation of RGO membrane with EPD. (a) Solution of RGO. (b) EPD to prepare pure RGO membranes. (c) Dry RGO membrane is collected on SS. (d) RGO/SS plate floating on the surface of FeCl3 solution. (e) Separation of the RGO membrane from SS. (f) RGO membrane floating on the surface of solution. (g) Getting the RGO membrane. (h) RGO membrane is transferred to substrate; (i) Dry RGO free-standing membrane.

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Wang et al. developed a new EPD strategy to prepare pure RGO membranes.86 RGO colloidal particles were suspended in a liquid medium and migrated under an electric field, which promotes the deposition of RGO onto the stainless steel (SS) electrode to form a membrane (Fig. 7a and b). RGO/SS plate was dried and then placed in FeCl3 solution for separation of the RGO membrane from SS (Fig. 7c, d and e). Substrate was used to collect the RGO membrane, which was floating on the surface of FeCl3 solution (Fig. 7f, g, h and i). The size and quality of the prepared RGO membrane by EPD method was controllable, but the agglomeration of RGO was uncontrollable. In Fig. 7, we can clearly catch sight of the procedure of fabricating RGO membrane by EPD. EPD was also utilized to obtain GIHMs. In another work, Wang and co-workers produced G/CoFe2O4NP hybrid membranes by EPD method, which only required one moderate step and post-thermal annealing.30 The composition of electrolyte is a stable GO suspension with Fe(NO3)3·9H2O and Co(NO3)2·6H2O, which was deposited onto the Cu foil to form G/CoFe2O4NP hybrid film by EPD. In this EPD process, the working electrode was Cu foil, and the counter electrode was Pt foil. After the EPD fabrication, the obtained G/CoFe2O4NP hybrid membrane was washed, dried, and thermally annealed, which can be further used for lithium storage. EPD is an original method with the advantages of fabricating hybrid membranes as lithium-ion batteries electrode without harsh chemicals, binders, and additives.

MoO2, Co3O4, NiO, CuO, Fe2O3 or Fe3O4) hybrid membranes have broad applications in lithium-ion batteries.74 The fabrication methods have some similarities with the methods for getting GIHM with G and metallic nanostructures, such as the CVD, filtration method, and self-assembly. Nevertheless, a new technique, EDP, was mentioned in this part of review, which can be used to fabricate both pure RGO film and GIHMs. What calls for special attention is that chemical bath deposition method, which is cheap and only requires simple devices. This deposition technique was also utilized to fabricate G/Ni(OH)2NP thin membrane previously.82 Table 2 shows the comparison of different methods for the fabrication of GIHMs with G and metallic compound nanostructures.

4. GIHMs fabricated with G and inorganic non-metallic nanostructures The non-metallic nanostructures contain C (e.g. CNTs, fullerenes, carbon black (CB), carbon fiber, and carbon balls),31,87-102 Si,103-105 SiO2,106-108 Si3N4,109 and other non– metallic materials. 4.1 Original bubble deposition method (BDM) BDM is an easy and flexible technique to deposit GO membranes precisely on discretionary substrates, which allows controlling the thickness of GO membranes.

3.5 Comparisons of fabrication techniques Table 2 Comparisons of fabrication techniques for GIHMs with G and metallic compound nanostructures.

Techniques

Advantages

Disadvantages

Filtration method26,29,71-

simple and convenient, controlled thickness and constitution

size and shape are limited

facile and robust

complex preparation

CVD79,80

most common, large-scale production

time-consuming, high cost, rigorous reaction condition, thickness uncontrollable

EPD 30,81,86

economic, high deposition rate, controllable thickness, free of adhesive agents

size and quality are limited by electrode, uncontrollable RGO agglomeration

74,83-85

Self-assembly 78

In this section, we focus on the techniques for obtaining GIHMs based on G and metallic compound nanostructures. It is easy to find that transition-metal oxides are most common since the created GIHMs based on G and transition-metal oxides (e.g.


Fig. 8 Schematic representation of the BDM process: (a) Bubble formatting followed by the water drains (blue arrows). (b)The substrate moves down to make the bubble membrane as a cylinder (right) and adheres with the substrate. The membrane thickness continues to decrease (yellow arrow) and finally bursts. AFM height images of (c) deposited SWCNT monolayer (black), (d) GO sheets (yellow) deposited on the SWCNTs layer to fill the pores, and (e) last SWCNT deposition.

For instance, Azevedo et al. took advantage of this method to get hybrid membranes based on GO sheets and CNT for applications in electronics and sensors.87 Fig. 8 shows the schematic representation for the fabrication of GO/CNT hybrid membrane with the typical BDM technique. Firstly, with only GO solution, bubble was made on a closed chamber (Fig. 8a).

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Journal Name Secondly, water drainage was started and the membrane was attached onto a substrate (Fig. 8b). The residual water was evaporated at a very fast speed after bursting of the bubble. Interfacing single-walled carbon nanotubes (SWCNT) and GO layers, a sandwich structure was formed (Fig. 8c-e). The prepared hybrid structure composed of a monolayer SWCNTs and a dense GO monolayer. The pores between SWCNTs were filled by GO sheets. With the deposition of SWCNTs membrane, a sandwich structure was achieved. This novel “low-cost” BDM overcomes the disadvantages of traditional G deposition methods and has been proved to be an efficient method to assemble different nanostructures, such as SWCNTs, AuNPs, and metal oxide NWs into monolayer membranes. 4.2

Surface precipitation

Nguyen et al. presented a new available strategy for rapid fabrication of G and G/CNT hybrid membranes by low vacuum annealing of cellulose acetate on Ni surface.88 Their method of fabrication and hybridization brings a lot of advantages, such as time and energy saving, property tuneable, and toxic chemicals free. The precipitation of G on Ni surfaces was achieved with a solid carbon precursor (cellulose acetate) and it was joined with a host component CNT network, which was coated on Ni to produce G/CNT hybrid membrane. Fig. 9 shows the process to fabricate G and G/CNT hybrid membranes.

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ARTICLE DOI: 10.1039/C5NR00084J GIHMs with G and non-metallic nanostructures. The composition and structure of the obtained membranes through this method is controllable with nanoscale precision. Huang et al. prepared GO/CNT hybrid membranes by simply casting of aqueous dispersion on a Ti substrate through self-assembly as electrodes for supercapacitors.89 Firstly, the multi-walled carbon nanotubes (MWCNTs) with a diameter and length ranging between 10-20 nm and 10-50 µm were synthesized by a CVD method. To fabricate GO/CNT membrane, the synthesized CNTs were handled by ultrasonicator in distilled water and then the as-prepared GO aqueous dispersion was added. In order to obtain a highly stable GO/CNT hybrid aqueous dispersion, the mixture was put into ultrasonicator and casted on Ti sheet and dried in an oven. Finally, the dried hybrid membranes were annealed for further application as binder-free electrodes of high performance supercapacitors. By adding CNTs, the electrodes exhibit remarkably high specific capacitances. Electrostatic LBL self-assembly is a versatile fabrication technique for producing GIHMs. Yu et al. fabricated large-scale GIHM by successive self-assembling functionalized G nanosheets and MWCNTs for supercapacitors.90 The created GIHM based on G and MWCNTs is suitable for electrochemical measurements after assembling onto different substrate. This possible reason is that the hybrid membrane with interconnected network carbon structures has well-defined nanopores, which are benefited to ion diffusion. In addition, Shao et al.92 and Lv et al.94 took advantage of LBL self-assembly process to fabricate GIHM at a liquid-air interface, which have potential to be applied in electrochemical sensor.

Fig. 9 Schematic fabrication of G and G/CNT hybrid membranes.

Firstly, the contaminants resided on Ni surfaces were removed by ultrasonic in an acetone bath. Then, a roller was used to press the cellulose acetate membrane on a Ni surface by mechanical force. After that, they placed the sample in a quartz container and put them together into a lamp annealing system. The sample was heated at a vacuum atmosphere. The various annealing periods and temperatures lead to the formation of different graphitic qualities in GIHMs. As expected, the fabricated GIHM show excellent optoelectrical and field electron emission properties. Surface precipitation method is a potential route to hybridize G with various nanomaterials. 4.3 LBL self-assembly LBL self-assembly is a controllable and reproducible method of adsorbing counter charged species in an aqueous environment,106 and this method can also be used to produce


Fig. 10 (a, b) Schematic representation of RGO/MWCNT hybrid LBL membrane; (c) High-resolution SEM image of (RGO/MWCNT)5 hybrid LBL membrane without further treatment, (d) treating with thermal reduction, and (e) chemical treatment; (f) Image of (RGO/MWCNT)25 hybrid LBL membrane on a PET substrate after mild thermal treatment.

Diverse nanostructures, such as CNTs and NPs can be consolidated in a single platform by LBL self-assembly. For example, CNTs modified with carboxylic acid, amine, and polyions have been recently combined with G by LBL self-assembly technique to fabricate ultrathin electroactive multilayer GIHMs. Hong et al. obtained a GIHM based on RGO and MWCNTs with this technique.97 The schematic representation for the fabrication of RGO/MWCNT LBL membrane is shown in Fig. 10a and b. They synthesized

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positive-charged MWCNTs (MWCNT-NH3+) and prepared negative-charged RGO (RGO-COOH) by chemical reducing GO with hydrazine in the presence of ammonia. Due to the electrostatic interactions between MWCNT-NH3+ and RGO-COO- , a multilayer GIHM based on RGO and MWCNTs was created by repeatedly spin-coating on a planar silicon wafer or quartz slide. The typical SEM images of the prepared 5-layer GIHM are shown in Fig. 10c-e. Fig. 10f shows the optical image of the created (RGO/MWCNT)25 hybrid thin membrane on a PET substrate after mild thermal treatment. The hybrid multilayer of (RGO/CNT)n membrane fabricated via LBL assembly offers an excellent control over the optical and electrical properties. Due to the highly tunable and versatile properties, the thin membranes fabricated via LBL assembly reveal a unique potential for integrating active carbon nanomaterials in the applications of advanced energy, sensors, and electronic devices. The disadvantage is that the functionalization of inorganic non-metallic nanostructures and self-assembly process cost a lot of time, which limits their practical applications. 4.4 Filtration method Filtration has been found to be a useful method to assemble Gand GO-based hybrid membranes since its relatively facile procedure. This technique can also be utilized to fabricate GIHMs with G and non-metallic nanostructures. For example, Khan et al. prepared mixed dispersions to obtain G/CNT hybrid membrane by vacuum filtration method for application as conductive coatings.98 They first dispersed SWCNT powder in N-methyl pyrrolidone (NMP) solvent by high energy ultrasonication, and then they dispersed multilayer G in NMP by low energy ultrasonication. After that, they mixed the two parts of dispersions in a round bottom flask, and then sonicated the mixture in low power sonic bath. The G/CNT hybrid membrane can be produced by filtrating it through a microporous poly(vinylidenefluoride) (PVDF) membrane. After being carefully peeled off, the free-standing membrane was dried under vacuum to remove the residual solution. The created GIHM has better mechanical property and conductivity than the membranes created with only CNTs or G.

Fig. 11 Schematic illustration of the fabrication of RGO/CB hybrid membrane.


ARTICLE DOI: 10.1039/C5NR00084J In another case, Wang and co-workers prepared a novel GIHM based on RGO and CB with the vacuum filtration method and the fabricated GIHM has potential application in flexible and high rate performance supercapacitor.31 Fig. 11 shows the schematic for the preparation of RGO/CB hybrid membrane. The GO prepared by modified Hummers' method was mixed with water at room temperature under ultrasonication. CB was dispersed in dimethylformamide (DMF) under ultrasonication. The GO dispersion and CB dispersion in DMF were mixed and hydrazine was added into the mixture with violent stirring. Finally, the RGO/CB hybrid membrane was prepared by vacuum filtration using the cellulose esters filter membrane. A simple, in-situ, and one-step filtration method was also carried out to fabricate the free-standing G/SiNP hybrid membrane for using as lithium-ion batteries, in which the SiNPs are highly encapsulated in a G nanosheet matrix.103 The G/SiNP composite suspension was filtrated with positive pressure by passing through the wetted PVDF filter which final obtained a free-standing mat of the G/SiNP composite. The resultant composite mat was washed and dried overnight to peel off from the PVDF filter. Finally, the as-prepared sample was heat-treated to remove -H and -OH groups. 4.5 Comparisons of fabrication techniques There is a general tendency that carbon material, which acts as inorganic non-metallic nanostructures, combines with G to fabricate GIHMs. The composites, particularly G and CNT, have become a new material with great potential due to their large specific surface area, high electrical conductivity and unique mechanical properties. Carbon-based materials have shown advantageous flexibility and potential for producing flexible and bendable free-standing electrodes.103 The general CVD method is also used to fabricate GIHM with carbon materials. In some studies, G membrane was first obtained via CVD for further treatment and combination with carbon nanomaterials,100,108,109 which are similar to the process for fabricating GIHMs with metal and metallic compound nanostructures. However, some new method was used in fabrication of GIHMs with carbon materials, for example, the hybrid G/amorphous carbon membrane was acquired immediately through CVD.101,102 Apart from the LBL self-assembly, CVD and filtration introduced in Part 2 and 3, new fabrication technique like BDM and surface precipitation were introduced in this part for the first time. In a recent study, GIHMs based on different types of inorganic nanostructure have been reported,79,110,111 which extends the applications of GIHMs. The following table (Table 3) shows the advantages and disadvantages of different fabrication techniques for the fabrication of GIHMs with G and non-metallic nanostructures. Table 3 Comparisons of fabrication techniques for GIHMs with G and inorganic non-metallic nanostructures.

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Techniques

Advantages

Disadvantages

BDM87

easy and flexible, discretionary substrates, controlled thickness

low production efficiency

Surface precipitation88

time and energy saving, property tunable, toxic chemicals free

circumscribed application

versatile fabrication technique

lack of practical application

simple and facile

the size and shape are limited

Self-assembly89,90,92 ,94,95 and LBL assembly97,106 Filtration31,98,99,103,10 4

5. Summary of the potential applications of GIHMs

Table 4 Summary of the fabrication strategies and corresponding applications of GIHMs.

Nanostructures for GIHMs

Fabrication strategies

Applications

AgNPs28, 50, 57,58

self-assembly, CVD, physical vapor deposition, vacuum filtration

metallic electrode for solar cells, SERS57, transparent and conductive films58, surface plasmon coupled emission

AgNWs43,49

CVD, solvothermal method

antibacterial material, electrochromic devices, electrodes for flexible and transparent loudspeakers

AuNPs25,32,47,55

filtration, self-assembly, CVD-TRT

sensors, electronic devices, biosensors, SERS

Metallic NPs34,60

electrochemical deposition, PWE evaporation

sensors and optoelectronic devices

CuNWs23

dry-transfer method

transparent electrodes for electrochromic devices

Pt3Sn35

self-assembly

membrane-based devices

NiNPs56

filtration

electrocatalyst

SiNPs

filtration

lithium-ion batteries

MnO2 nanorod72

filtration

positive electrodes

MnO2NTs74

filtration

lithium-ion batteries

Ni(OH)2 nanoplates83

filtration-assisted method

positive electrode for supercapacitor

Montmorillonit e85

vacuum filtration

fire resistance

Fe3O4NPs29,78

vacuum filtration, self-assembly

lithium-ion batteries, nanodevices, catalysis

VO2NPs80

CVD and sputtering

optoelectronic devices

CoFe2O4NPs30

EPD

lithium storage

CNT87,89,90,92,97, 98

BDM, self-assembly, filtration

sensors, supercapacitors, energy materials, electro-devices

CB31

vacuum filtration

supercapacitor

103,104

Fig. 12 Potential applications of GIHMs.

There are numerous advantages to fabricate G-based nanocomposites into GIHMs for wide range of applications. The thickness of GIHM confer upon it flexible plasticity to contact with different surfaces. Fig. 12 summaries the potential applications of GIHMs in different fields. The unique properties of optical transparent, excellent electrical conductivity, and mechanical properties bring chance for using GIHM in optoelectronics material (organic light emitting diodes, touch panels), transparent electrodes in Prussian blue-based electrochromic devices, positive electrodes, and solar cells as metallic electrode and anti-reflection coating. The structure of the GIHM endows it with high efficiency in electrical conductivity, which can be applied in energy storage (lithium-ion batteries, supercapacitors), electrocatalyst, and electrochemical biosensor. The other excellent properties also bring huge potential for GIHMs to be used in SERS, amplifying


surface plasmon coupled emission, construction of functional membrane-based devices, fire resistance, conductive coatings, and so on. What’s more, the membrane is so extremely light that it can replace other materials in making portable devices.

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6. Conclusions and outlooks In summary, we introduced and discussed on the various techniques to fabricate GIHMs in this review. The structure-specific applications of the fabricated GIHMs were presented. In addition, the advantages and disadvantages of all the methods for creating GIHMs with G and metallic, metallic compound, and non-metallic nanostructures were commented. From this review, we can get the ideas: i) It is simple and convenient to obtain the GIHM through vacuum filtration, while the size and shape of membrane is limited by used instrument. ii) CVD can achieve large-scale production, while it can hardly obtain thin membranes. Moreover, it need post-processing, which is the same with substrate-assisted wet- or dry-transfer technique. iii) LBL self-assembly is a versatile fabrication technique, while usually accompanies with disadvantage of using surfactants and wasting materials. At the same time, the fabricated GIHMs via self-assembly are weak and crisp. iv) Spin-coating or spray-coating method is rapid, but the structure of GIHMs is difficult to control, which will influence the final property of opto-electronic. v) BDM and EPD are both neoteric techniques for fabrication of GIHM, but they still have their boundedness. The properties of GIHMs are not only dependent on the composition but also largely influenced by the way they are prepared. In order to fabricate high-quality GIHMs, several key points should be taken into consideration in advance. As one of the most important parts of GIHM, G with the structure form of individual nanosheets can most effectively enhance its function when compared with other structure forms of G. When it comes to inorganic nanostructures, a uniformly distribution on G surface is required. Furthermore, the agglomerate of NPs is also a challenge for experimenters to overcome. The key point is how to make the inorganic NPs distribute uniformly in GIHMs. Moreover, certain methods need bonders such as bifunctional linker and agglomerant to adhere NPs onto the G surface. The choice of linkers directly influences the success or failure for fabrication of GIHMs. The last but not the least, the size, thickness and component of the fabricated GIHMs should be accurately controlled for further applications. For the development of GIHMs in the future, firstly it is an increasing tendency to fabricate GIHMs in eco-friendly way, such as electrophoretic deposition, electrochemical method, and self-assembly. Secondly, the component of GIHMs will be no longer restricted to only one part. The multicomponent GIHM with different types of NPs is researched at present. Among the multicomponent hybrid membranes, the organic-inorganic hybrid membranes, which can be used as conducting material in


fuel cell, are the focus of concern. Thirdly, it will be more interesting to fabricate functional membranes or films with graphene-like materials112 (MoS2, BN, WS2, MoTe2, and so on) and the nanotructures introduced our work. Finally, the most important part is to combine theoretical research with practical application. In order to achieve the industrialization of preparing hybrid membranes, some special equipment, such as extruder and large-scale electro-deposition equipment can be widely utilized. In addition, making the productive process with the specialties of environmental protection, low energy consumption, and high efficiency is the developing trend in the future.

Main abbreviations GIHM NPs NWs NTs GO RGO CVD TRT SERS PET PMMA PDMS LBL GCE CV PWE L-B EPD CB BDM NMP PVDF

graphene-based inorganic hybrid membrane nanoparticles nanowires nanotubes graphene oxide reduced graphene oxide chemical vapor deposition thermal release tape surface-enhanced Raman scattering polyethylene terephthalate polymethyl methacrylate polydimethylsiloxane layer-by-layer glassy carbon electrode cyclic voltammetry pulsed power wire Langmuir-Blodgett Electrophoretic deposition carbon black bubble deposition method N-methyl pyrrolidone poly(vinylidenefluoride)

Acknowledgments The authors gratefully acknowledge the financial supports from the Fundamental Research Funds for the Central Universities (project no. ZZ1307). We also would like to thank the financial support from the Deutsche Akademische Austausch Dienst (DAAD) master's short-term scholarship for academic exchange in the Leibniz Institute for Solid State and Materials Research Dresden.

Notes and references a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, China. E-mail: [email protected]

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To make it more clear, the detailed applications of GIHMs fabricated with G and different nanostructures (metal, metal compound, and non-metal) were presented in Table 4.


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b Hybrid Materials Interface Group, Faculty of Production Engineering, University of Bremen, D-28359 Bremen, Germany. Fax: (+) 49 421 218 64599; E-mail: [email protected] ‡ These authors contributed to the work equally.

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Journal Name


J. Name., 2012, 00, 1-3 | 14

Recent advances in hybrid molecular imaging systems.

Recent advances in solution-processed inorganic nanofilm photodetectors.

Gold Nanoparticles: Recent Advances in the Biomedical Applications.

Recent advances in anisotropic magnetic colloids: realization, assembly and applications.

Recent advances in glycerol polymers: chemistry and biomedical applications.

Recent advances in bioprinting and applications for biosensing.

Recent advances in biocatalyst discovery, development and applications.

MicroRNAS in endometrial cancer: recent advances and potential clinical applications.

Recent advances in dialkyl carbonates synthesis and applications.

Recent Advances in Brain Signal Analysis: Methods and Applications.

Recent advances in bioprinting techniques: approaches, applications and future prospects.

Recent advances in carbon nanodots: synthesis, properties and biomedical applications.

Inorganic Hybrid Nanofibers.

Recent advances and versatility of MAGE towards industrial applications.

Catalytic applications of layered double hydroxides: recent advances and perspectives.

Recent advances in hybrid Cu2O-based heterogeneous nanostructures.

Stimuli-responsive Pickering emulsions: recent advances and potential applications.

Organic-inorganic hybrid materials: nanoparticle containing organogels with myriad applications.

Recent advances in lab-on-a-chip for biosensing applications.

Recent Advances in Biosensor Technology for Potential Applications - An Overview.

Recent advances in terahertz technology for biomedical applications.

Recent advances in M13 bacteriophage-based optical sensing applications.

Recent advances in complex networks theories with applications.

Experimental microdosimetry: history, applications and recent technical advances.