3D Architectured Graphene/Metal Oxide Hybrids for ...

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sensors Review

3D Architectured Graphene/Metal Oxide Hybrids for Gas Sensors: A Review Yi Xia 1,2,3, *, Ran Li 1,4 , Ruosong Chen 3 , Jing Wang 3,5, * and Lan Xiang 3, * 1 2 3 4 5

*

Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China The Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming 650093, China Department of Chemical Engineering, Tsinghua University, Beijing 100084, China; [email protected] Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China Correspondence: [email protected] (Y.X.); [email protected] (J.W.); [email protected] (L.X.); Tel.: +86-10-6278-8984 (L.X.)

Received: 27 March 2018; Accepted: 3 May 2018; Published: 7 May 2018

 

Abstract: Graphene/metal oxide-based materials have been demonstrated as promising candidates for gas sensing applications due to the enhanced sensing performance and synergetic effects of the two components. Plenty of metal oxides such as SnO2 , ZnO, WO3 , etc. have been hybridized with graphene to improve the gas sensing properties. However, graphene/metal oxide nanohybridbased gas sensors still have several limitations in practical application such as the insufficient sensitivity and response rate, and long recovery time in some cases. To achieve higher sensing performances of graphene/metal oxides nanocomposites, many recent efforts have been devoted to the controllable synthesis of 3D graphene/metal oxides architectures owing to their large surface area and well-organized structure for the enhanced gas adsorption/diffusion on sensing films. This review summarizes recent advances in the synthesis, assembly, and applications of 3D architectured graphene/metal oxide hybrids for gas sensing. Keywords: 3D architectured hybrids; graphene; metal oxide; gas sensor

1. Introduction With the rapid development of modern industry, the detection of hazardous gases has become an important issue for human health and environmental protection. A variety of materials such as carbon-based materials, noble metals, metal oxides or sulfides and organic semiconductors have been explored to fabricate gas sensors [1–15]. Among them, graphene, including reduced graphene oxide (rGO)/metal oxide-based hybrid-structures, has been proved as a potential sensing material for gas sensors due to its low temperature sensitivity and fast carrier transportation properties [16–25]. In the past decades, graphene has received more and more attention for application in gas sensors because of its high electrical conductivity, surface area (2630 m2 /g), and charge carrier mobility (15,000 cm2 ·V−1 ·s−1 ) at room temperature [16,19,26–28]. The superiority of graphene for gas sensing relies on two basic factors associated with its 2D dimensions, i.e., the ultrahigh surface area per atom and high electron transport along the graphene base-plane. However, further efforts are required to solve some issues, e.g., the insufficient sensitivities, long dynamic responses, poor repeatability and selectivity of rGO-based gas sensors [10,25,27–29]. On the other hand, graphene exhibits excellent anchoring ability as a substrate for chemical functionalities or nanomaterials and, thus, fabrication Sensors 2018, 18, 1456; doi:10.3390/s18051456

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of novel graphene-based nanohybrids has been an effective approach for improving the gas sensing properties [30]. Metal oxide (MOx) semiconductors represent promising materials for gas sensing because of their high sensitivity, selectivity to gas molecules, good stability, low cost and various controllable nanostructures [31–40]. Hence, graphene/MOx-based nanohybrids have been identified as promising candidates for gas sensing application due to their enhanced sensing performance and synergetic effects between the two components. Numerous MOx such as SnO2 , ZnO, WO3 , etc. were utilized to form hybrids with graphene for improving the gas sensing properties [41–44]. Nevertheless, as sensing materials graphene/metal oxide nanocomposites still have several limitations. For example, the sensitivity and response rates are still insufficient; illumination or thermal treatment is required for recovery in some sensors [41–46] so the construction of highly sensitive and rapid-response graphene/MOx nanohybrid-based gas sensors still remains a challenge. 3D hierarchical structures have recently attracted much attention for the synthesis of gas sensors owing to their large surface area and well-organized porous structure which improves the gas adsorption/diffusion on sensing films [47–50]. Thus, researchers have been shifting their interest to the construction of well-formed 3D graphene or reduced graphene oxide (rGO)/MOx hybrids for highly sensitive, selective and cost-effective gas sensors. Up to now, several reviews have been published on the design of rGO/MOx-based nanostructures for gas sensors with various morphologies [25,51–55]. However, only a small part of these studies were aimed at the construction of 3D graphene/MOx nanohybrids for gas sensing applications. Herein, we present a review on 3D architectured graphene/metal oxide hybrids for gas sensors. First, various combination strategies for preparing different 3D rGO/MOx nanostructures are reviewed, including composites combining 2D or 3D graphene with dimensionally different metal oxides such as nanorods, nanosheets, and hierarchical structures, etc. Then, gas sensing applications of 3D architectured graphene/MOx hybrids, especially the development of gas detection at room temperature, are discussed. 2. Construction of 3D Graphene/MOx Nanostructures 2.1. 2D Graphene/1D MOx Based Architectures One dimensional metal oxides show many advantages in gas sensing applications because of their high surface-to-volume ratio, abundant surface states and potential to assemble hierarchical structures [9]. Nanostructures such as nanotubes, nanowires, nanorods, and nanofibers have been widely utilized for the construction of 3D hybrid sensing materials with graphene under template-assisted or multistep sequential growth synthesis conditions. 2.1.1. Template-Assisted Synthesis Template-assisted methods have been widely used in fabricating 3D architectures, due to the advantages of the diverse morphology of the available templates and large-scale synthesis [56]. Choi et al. reported 3D WO3 hemitubes functionalized by graphene with high surface area made using a nonwoven polymeric fiber composed of polyvinylpyrollidone (PVP)/poly(methyl methacrylate) (PMMA) composite as template under O2 plasma treatment conditions [57]. A schematic illustration of the graphene/WO3 3D structure formation process is shown in Figure 1a–f. Firstly, WO3 hemitube structures with wrinkled, bumpy surface topology were achieved by RF-sputtering WO3 films onto the O2 treated PVP/PMMA composite nanofiber templates, followed by high temperature calcination to remove the polymeric template; Finally, graphene was homogenously mixed with WO3 hemitubes to form 3D nanocomposites owing to the heterojunction between WO3 hemitubes and graphene induced by the charge transportation [57]. The morphology of 3D graphene/WO3 hemitubes is showed in Figure 1g–i.

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Figure1.1.(a–e) (a–e) Schematic illustration of formation ofgraphene-WO 3D graphene-WO architectures; 3 hemitube Figure Schematic illustration of formation of 3D 3 hemitube architectures; (g–i) (g–i) SEM and TEM image with SAED pattern in the inset of graphene-WO hemitube composite. SEM and TEM image with SAED pattern in the inset of graphene-WO3 3hemitube composite. Reproducedwith withpermission permission from from [57], [57], © © 2012 2012 American American Chemical Chemical Society Society Reproduced

2.1.2.Multistep MultistepSequential SequentialGrowth Growth 2.1.2. Althoughtemplate-based template-basedmethods methodshave havebeen beenone oneof ofthe themost mostpromising promisingroutes routesto to fabricate fabricate3D 3D Although hybrids, some some problems problems such such as as tedious tedious experimental experimental procedures, procedures, the the high high cost cost of of templates, templates, and and hybrids, residual impurities its its development in application [58]. Therefore, convenient and efficient residual impuritieshave havelimited limited development in application [58]. Therefore, convenient and multistep approaches have been developed to produce desired hybrid3D structures [59]. efficient multistep approaches have been developed to many produce many3D desired hybrid structures [59]. Deng et al. developed 3D rGO-conjugated Cu2 O-nanowire mesoporous hybrids in the presence of graphene (GO) and in a one-pot hydrothermal treatmenthybrids [60]. The mesocrystals Deng et oxide al. developed 3Do-anisidine rGO-conjugated Cu2O-nanowire mesoporous in the presence consisted ofoxide highly anisotropic nanowires building blocks andtreatment possessed[60]. a distinct octahedral of graphene (GO) and o-anisidine in a as one-pot hydrothermal The mesocrystals morphology with eight {111} equivalent crystal faces [60]. consisted of highly anisotropic nanowires as building blocks and possessed a distinct octahedral The multistep sequential growth mechanism of the mesoporous hybrids is as follows (Figure 2a–f): morphology with eight {111} equivalent crystal faces [60]. Firstly, agglomeration of amorphous Cu2 O nanohybrids particlesisatasthe primary stage TheGO-induced multistep sequential growth mechanismspherical of the mesoporous follows (Figure resulting in the transitionagglomeration of a growth mechanism fromspherical conventional growth particle 2a–f): Firstly, GO-induced of amorphous Cu2O ion-by-ion nano particles at thetoprimary mediated crystallization; then, the developed into hierarchical stage resulting in the transition of aformed growthamorphous mechanismmicrospheres from conventional ion-by-ion growth to mesoporous nanowire assemblies through mesoscale transformation Ostwald ripening; finally,into the particle mediated crystallization; then, the formed amorphous by microspheres developed porous 3D framework structures interspersed among 2D rGOmesoscale sheets leading to the self-organization of hierarchical mesoporous nanowire assemblies through transformation by Ostwald large-scale 3D mesoporous architecture where the GO was reduced simultaneously [60]. to ripening; finally, the poroushybrid 3D framework structures interspersed among 2D rGO sheets leading the self-organization of large-scale 3D mesoporous hybrid architecture where the GO was reduced simultaneously [60].

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Figure 2. (a–f) Schematic illustration of the Cu2O crystallization process assisted by o-anisidine and Figure 2. (a–f) Schematic illustration of the Cu2 O crystallization process assisted by o-anisidine and GO. GO. Reproduced permission [60], © 2012 American Chemical Society. Reproduced with with permission fromfrom [60], © 2012 American Chemical Society.

3D core-shell rGO/MOx structures can also be achieved through a multistep strategy. For 3D core-shell rGO/MOx structures can also be achieved through a multistep strategy. For example, example, Abideen et al. fabricated rGO nanosheet-loaded ZnO core-shell nanofibers using a simple Abideen et al. fabricated rGO nanosheet-loaded ZnO core-shell nanofibers using a simple electrospinning method [61]. The two-step formation process is shown in Figure 3a. Firstly, the electrospinning method [61]. The two-step formation process is shown in Figure 3a. Firstly, the precursor solution was obtained by mixing a Zn2+-containing solution with rGO for a certain time, precursor solution was obtained by mixing a Zn2+ -containing solution with rGO for a certain time, leading to the formation of the ZnO/rGO precursor. Then, the 3D rGO/ZnO core-shell nanofibers leading to the formation of the ZnO/rGO precursor. Then, the 3D rGO/ZnO core-shell nanofibers were formed under electrospinning treatment. The morphology of as-prepared samples is shown in were formed under electrospinning treatment. The to morphology of as-prepared shown in Figure 3b–d. A similar strategy has also been used conjugate rGO with SnO2,samples In2O3, Feis2O 3 and so on [62–64].

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Figure 3b–d. A similar strategy has also been used to conjugate rGO with SnO2 , In2 O3 , Fe2 O3 and so Sensors 2018, 18, x FOR PEER REVIEW on [62–64].

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Figure Figure3. 3.(a) (a)Schematic Schematicillustration illustrationof offormation formationof ofrGO rGOnanosheet-loaded nanosheet-loadedZnO ZnOcore-shell core-shellnanofibers; nanofibers; (b–d) (b–d) SEM SEM and and TEM TEM pictures pictures of of samples. samples. Reproduced Reproduced with with permission permission from from [61], [61], © 2015 Elsevier.

We We recently recently successfully successfully developed developed aa facile facile and and efficient efficient two-step two-step solution solution method methodto tosynthesize synthesize 3D mesoporous rGO/ultrathin ZnO nanorods nanocomposites (rGO/UT-ZNR) in 10 min ◦ C[65]. 3D mesoporous rGO/ultrathin ZnO nanorods nanocomposites (rGO/UT-ZNR) in 10 minat at80 80°C [65]. rGO/UT-ZNR were obtained via the in-situ growth of ZnO nanoseeds on GO nanosheets followed rGO/UT-ZNR were obtained via the in-situ growth of ZnO nanoseeds on GO nanosheets followed by oriented growth of the nanoseeds into ZnO nanorods in a Zn(OH)2/NaOH mixed suspension (Figure 4a). We firstly prepared the Zn(Ac)2 methanol solution added with GO under continuous stirring to achieve adsorption equilibrium. Zinc ions were firmly absorbed on the surface of GO nanosheets owing to the strong metal ion anchoring ability of the functional groups from GO (step 1). After that, ZnO nanoseeds layer was formed on the GO nanosheets in NaOH methanol solution

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by oriented growth of the nanoseeds into ZnO nanorods in a Zn(OH)2 /NaOH mixed suspension (Figure 4a). We firstly prepared the Zn(Ac)2 methanol solution added with GO under continuous stirring to 18, achieve adsorption GO Sensors 2018, x FOR PEER REVIEW equilibrium. Zinc ions were firmly absorbed on the surface of 6 of 20 nanosheets owing to the strong metal ion anchoring ability of the functional groups from GO (step 1). (step 2). Then, as-prepared nanoseeds was added to an aqueous suspension After that, ZnOthe nanoseeds layerGO/ZnO was formed on themixture GO nanosheets in NaOH methanol solution containing NaOH/Zn(OH) 2 precursor fornanoseeds the further evolution of thetoZnO nanoparticles into (step 2). Then, the as-prepared GO/ZnO mixture was added an aqueous suspension ultrathin ZnO nanorods, while the in-situ reduction of GO to rGO occurred simultaneously (step 3) containing NaOH/Zn(OH) precursor for the further evolution of the ZnO nanoparticles into ultrathin 2 [65]. ZnO nanorods, while the in-situ reduction of GO to rGO occurred simultaneously (step 3) [65]. In our case, GO nanosheets nanosheets not not only onlyevolved evolvedinto intorGO rGOininthe thenanohybrids, nanohybrids,but butalso alsoprovided provideda aconfined confinedspace spacefor forion ionadsorption, adsorption,anchored anchorednucleation nucleationand andsubsequent subsequentgrowth growth of of ZnO ZnO nanorods, morphology of resulting in the formation formation of of 3D 3D rGO/UT-ZNR rGO/UT-ZNRmesoporous mesoporousnanohybrids nanohybrids[65]. [65].The The morphology rGO/UT-ZNR is is shown 3D rGO/UT-ZNR rGO/UT-ZNR of rGO/UT-ZNR shownininFigure Figure4d. 4d. Furthermore, Furthermore, gram-scale gram-scale (ca. (ca. 1.2 1.2 g) 3D successfully produced after only only 10 10 min min of of reaction. reaction. Such efficient large-scale large-scale nanohybrids can be successfully of 3D 3D rGO/MOx rGO/MOx architectures may allow the opportunity for commercial application as production of materials. sensing materials.

Figure 4. (a) Schematic illustration of fabrication of rGO/UT-ZNR; (b–d) TEM images of GO, GO/ZnO Figure 4. (a) Schematic illustration of fabrication of rGO/UT-ZNR; (b–d) TEM images of GO, nanoseeds and rGO/UT-ZNR. Reproduced with permission from [65], © 2016 American Chemical GO/ZnO nanoseeds and rGO/UT-ZNR. Reproduced with permission from [65], © 2016 American Society. Chemical Society.

2.2. 2D Graphene/2D MOx Based Architectures 2.2. 2D Graphene/2D MOx Based Architectures MOx nanosheets are other potential units for the formation of 3D rGO/ZnO hybrids for gas MOx nanosheets are other potential units for the formation of 3D rGO/ZnO hybrids for gas sensors owing to their large surface area [66–70]. However, 2D MOx nanostructures are difficult to sensors owing to their large surface area [66–70]. However, 2D MOx nanostructures are difficult to hybridize with graphene due to the weaker affinity between them [53,54]. Up to now, just a few hybridize with graphene due to the weaker affinity between them [53,54]. Up to now, just a few studies studies were reported in this area [71–73]. were reported in this area [71–73]. For instance, Hoa et al. developed novel 3D porous composites consisting of 2D graphene and For instance, Hoa et al. developed novel 3D porous composites consisting of 2D graphene and 2D NiO nanosheets (NSs) using a low-cost and large area scalable solution-based process at low 2D NiO nanosheets (NSs) using a low-cost and large area scalable solution-based process at low temperature [71]. The 3D hybrid architectures were obtained in two steps. First, GO films were spray temperature [71]. The 3D hybrid architectures were obtained in two steps. First, GO films were spray coated on the electrodes and reduced to rGO via a heating treatment. Then Ni seeds were coated and coated on the electrodes and reduced to rGO via a heating treatment. Then Ni seeds were coated and annealed on the rGO films, followed by the formation of NiO nanosheets after the reaction in precursor solution (Figure 5a). The morphology of 3D porous rGO/NiO hybrids is shown in Figure 5b.

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annealed on the rGO films, followed by the formation of NiO nanosheets after the reaction in precursor solution (Figure 5a).PEER TheREVIEW morphology of 3D porous rGO/NiO hybrids is shown in Figure 5b. 7 of 20 Sensors 2018, 18, x FOR

Figure 5. 5. (a) (a) Schematic Schematic illustration illustration of of formation formationof of3D 3Dporous porousrGO/NiO rGO/NiO nanosheets nanosheets hybrids; hybrids; (b) (b) SEM SEM Figure pictures of samples. Reproduced with permission from [71], © 2013 Elsevier. pictures of samples. Reproduced with permission from [71], © 2013 Elsevier.

2.3. 2D 2D Graphene/3D Graphene/3D MOx MOx Architectured Architectured Hybrids Hybrids 2.3. 3D hierarchical hierarchical structures structures have have been been recently recently attracted attracted much much attention attention for for the the fabrication fabrication of of gas gas 3D sensors because because of of their their larger larger surface surface area area and and well-constructed well-constructed 3D 3D structures structures that that improve improve gas gas sensors adsorption/diffusion on sensing films [47–50]. Various 3D MOx nanostructures have been adsorption/diffusion on sensing films [47–50]. Various 3D MOx nanostructures have been successfully successfully usedwith to combine with rGOdue nanosheets due to the of the easily-controlled used to combine rGO nanosheets to the advantage of advantage the easily-controlled morphology morphology of MOx. of MOx. 2.3.1. Graphene/Regular Graphene/Regular 3D 2.3.1. 3D MOx MOx Nanostructures Nanostructures Many studies onon thethe combination of 3D such as spheres, cubes Many studies have havebeen beenfocused focused combination of nanostructures 3D nanostructures such as spheres, and rGO nanosheets because nanostructures with regular shape are more easily covered and cubes and rGO nanosheets because nanostructures with regular shape are more easily covered and connected with with the the easily easily crinkled crinkled and and folded folded rGO rGO nanosheets nanosheets [74–82]. [74–82]. For connected For example, example, Zhang Zhang et et al. al. demonstrated rGO/α-Fe 2 O 3 composites with 3D nanostructures using a low-cost and demonstrated rGO/α-Fe2 O3 composites with 3D nanostructures using a low-cost and environmentally environmentally friendly hydrothermal method Uniform α-Fe2O3 cubes adhered uniformly on friendly hydrothermal method [74]. Uniform α-Fe[74]. 2 O3 cubes adhered uniformly on both sides of the both sidesand of the crumpled and rippled rGO [74]. sheets [74]. A novel 3D nitrogen-doped crumpled rippled rGO sheets (Figure 6a,b) A (Figure novel 3D6a,b) nitrogen-doped reduced template and reduced template and surfactant free graphene oxide (N-rGO)/NiO cube (hc-NiO) composite was surfactant free graphene oxide (N-rGO)/NiO cube (hc-NiO) composite was obtained through a facile obtained through a facile method and[75]. a post-calcination treatment in-situ hydrothermal method and ahydrothermal post-calcination treatment The in-situ growth of NiO [75]. cubesThe organized growth of NiO cubes organized by many nanoparticles on the surface of N-rGO layers can by many nanoparticles on the surface of N-rGO layers can be observed in Figure 6e [75]. rGO/In2 Obe 3 observed in Figure 6e [75]. 2O3 cube nanocomposites also one-step successfully synthesized by a cube nanocomposites wererGO/In also successfully synthesized bywere a facile microwave-assisted facile one-stepmethod microwave-assisted hydrothermal method (Figure 6f) [76]. hydrothermal (Figure 6f) [76]. MOx with spherical structures can be be also also introduced introduced to to form form 3D 3D hybrids hybrids with MOx with spherical structures can with rGO rGO nanosheets. nanosheets. For example, rGO decorated TiO 2 microspheres are obtained under a hydrothermal method [78]. rGO rGO For example, rGO decorated TiO2 microspheres are obtained under a hydrothermal method [78]. nanosheetsplay playa dual a dual role, in which they notcover onlysome cover some TiO2 balls to form a partially nanosheets role, in which they not only TiO 2 balls to form a partially “wrapping” “wrapping” microstructure (Figure 6d), but also act as a “bridge” between two neighboring oxide microstructure (Figure 6d), but also act as a “bridge” between two neighboring oxide particles particles (Figure 6c), leading to the formation of novel 3D rGO-MOx nanostructures. In the other (Figure 6c), leading to the formation of novel 3D rGO-MOx nanostructures. In the other hand, a facile hand, amethod facile sol-gel method was employed3D tographene-wrapped synthesize 3D graphene-wrapped WOcomposite 3 nanospheres sol-gel was employed to synthesize WO3 nanospheres [79]. composite [79]. Different from the partially wrapped TiO 2 spheres, the WO 3 nanospheres were Different from the partially wrapped TiO2 spheres, the WO3 nanospheres were distinctly enwrapped distinctly enwrapped gauze-like nanosheets (Figure 6g,h). Besides, Fe2O3 with gauze-like graphene with nanosheets (Figuregraphene 6g,h). Besides, Fe2 O3 nanospheres, SnO2 hollow particles nanospheres, SnO 2 hollow particles and SnO 2 discoid crystal can also be modified by rGO for novel and SnO2 discoid crystal can also be modified by rGO for novel 3D nano-hybrids [80–82]. 3D nano-hybrids [80–82].

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Figure rGO/α-Fe 2O3 cubic nanostructure [74]; (c–d) rGO decorated TiO2 Figure 6. 6. Morphology Morphologyofof(a–b) (a–b) rGO/α-Fe 2 O3 cubic nanostructure [74]; (c–d) rGO decorated microspheres [78]; (e) N-rGO/NiO cube [75]; rGO/In 3 cube nanocomposites [76]; (g–h) grapheneTiO2 microspheres [78]; (e) N-rGO/NiO (f) cube [75];2O(f) rGO/In2 O3 cube nanocomposites [76]; 3 nanospheres [79]. (a–b), (c–d) and (g–h) were reproduced with permission from wrapped WO (g–h) graphene-wrapped WO3 nanospheres [79]. (a–b), (c–d) and (g–h) were reproduced with [74,78,79], ©from Elsevier. (e) and© (f)Elsevier. were reproduced with permission fromwith [75,76] © American Chemical permission [74,78,79], (e) and (f) were reproduced permission from [75,76] Society. © American Chemical Society.

2.3.2. Graphene/3D 2.3.2. Graphene/3DMOx MOxHierarchical HierarchicalAssemblies Assemblies To improve the gas adsorption/desorption, assemblies organized via To improve the gas adsorption/desorption, 3D 3D MOx MOx hierarchical hierarchical assemblies organized via building blocks as nanoparticles, nanoparticles, nanorods, widely developed developed by building blocks such such as nanorods, and and nanosheets nanosheets have have been been widely by combining rGO owing to their high surface area and porous structures [83–92]. combining rGO owing to their high surface area and porous structures [83–92]. Li et al. reported the preparation of novel 3D hierarchical porous ZnO nanoflowers modified Li et al. reported the preparation of novel 3D hierarchical porous ZnO nanoflowers modified with with rGO under hydrothermal reaction conditions [83]. The uniform 3D flower-like structures are rGO under hydrothermal reaction conditions [83]. The uniform 3D flower-like structures are assembled assembled by nanosheets with porous structures (Figure 7a, b). By utilizing a facile one-step by nanosheets with porous structures (Figure 7a, b). By utilizing a facile one-step hydrothermal method, hydrothermal method, Liu et al. fabricated 3D sensing materials composed of hierarchical flower-like Liu et al. fabricated 3D sensing materials composed of hierarchical flower-like In2 O3 and rGO [84]. In2O3 and rGO [84]. In the 3D rGO-In2O3 composite, as shown in Figure 7c, d, flexible and transparent In the 3D rGO-In2 O3 composite, as shown in Figure 7c, d, flexible and transparent rGO sheets were rGO sheets were placed among flower-like hierarchical In2O3 organized by nanosheets [84]. Ngo et placed among flower-like hierarchical In2 O3 organized by nanosheets [84]. Ngo et al. also developed al. also developed 3D hybrids in which NiO nanoflowers were uniformly grown on the surface of 3D hybrids in which NiO nanoflowers were uniformly grown on the surface of rGO by a facile rGO by a facile hydrothermal method followed by annealing under flowing nitrogen (Figure 7e) [85]. hydrothermal method followed by annealing under flowing nitrogen (Figure 7e) [85]. Besides these nanosheet-based 3D nanohybrids, rGO/nanorod-assemblybased 3D Besides these nanosheet-based 3D nanohybrids, rGO/nanorod-assemblybased 3D nanostructures nanostructures have been also developed [86–88]. For example, urchin-like CuO 3D structures have been also developed [86–88]. For example, urchin-like CuO 3D structures modified by rGO were modified by rGO were fabricated by a one-pot microwave-assisted hydrothermal method [86]. The fabricated by a one-pot microwave-assisted hydrothermal method [86]. The connection between CuO connection between CuO and rGO can be observed and rGO shows a crumpled layered structure and rGO can be observed and rGO shows a crumpled layered structure distributed randomly in the distributed randomly in the composites with some stacking layers (Figure 7f) [86]. composites with some stacking layers (Figure 7f) [86]. On the other hand, template-induced porous MOx structures can also be coupled with rGO to On the other hand, template-induced porous MOx structures can also be coupled with rGO to form form 3D composites [89,90]. Xue successfully produced 3D ordered mesoporous In2O3-rGO 3D composites [89,90]. Xue successfully produced 3D ordered mesoporous In2 O3 -rGO nanocomposites nanocomposites using mesoporous silica as a hard template through ultrasonic mixing [90]. Zhu et using mesoporous silica as a hard template through ultrasonic mixing [90]. Zhu et al. employed a facile al. employed a facile method to obtain rGO/SnO2 3D microporous nanocomposites by a simple method to obtain rGO/SnO2 3D microporous nanocomposites by a simple blending and deposited blending and deposited onto different microporous substrates [91]. onto different microporous substrates [91].

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Figure 7. porous ZnO nanoflowers modified with rGOrGO [83];[83]; (c– Figure 7. Morphology Morphologyofof(a–b) (a–b)3D 3Dhierarchical hierarchical porous ZnO nanoflowers modified with 2 O 3 composite [84]; (e) rGO/NiO nanoflowers [85]; (f) urchinlike CuO 3D structures d) 3D rGO-In (c–d) 3D rGO-In2 O3 composite [84]; (e) rGO/NiO nanoflowers [85]; (f) urchinlike CuO 3D structures modified by by rGO rGO [86]. [86]. (a–e) reproduced with with permission permission from from [83–85], [83–85], © © Elsevier. Elsevier. (f) were modified (a–e) were were reproduced (f) were reproduced with with permission permission from from [86] [86] © © 2014 2014 American American Chemical Chemical Society. Society. reproduced

2.3.3. Graphene-MOx Based Ternary Ternary 3D 3D Hybrids Hybrids 2.3.3. Graphene-MOx Based Single elements to to decorate thethe as-prepared rGO-MOx 3D Single elements such such as asnoble noblemetals metalscan canbebeused used decorate as-prepared rGO-MOx architectures to form a ternary composite. Uddin et al. synthesized an Ag-loaded 3D ZnO 3D architectures to form a ternary composite. Uddin et al. synthesized an Ag-loaded 3D ZnO nanostructure-rGO (Ag/ZnO (Ag/ZnO Hrc-RGO) by an an nanostructure-rGO Hrc-RGO) hybrid hybrid using using aa facile facile hydrothermal hydrothermal method method followed followed by efficient photochemical photochemical route route for for the the Ag Ag deposition deposition [93]. [93]. The The pH pH level level adjusted adjusted by by the the capping capping agent agent efficient molecules (NH 4OH) and the anisotropic growth of ZnO play very important roles in the formation molecules (NH4 OH) and the anisotropic growth of ZnO play very important roles in the formation of hierarchical 8a,8a, b show thatthat small-sized Ag of hierarchical ZnO ZnO microsphere-like microsphere-likenanosheet nanosheetassemblies. assemblies.Figure Figure b show small-sized nanoparticles with an average particle size of 40 nm are attached onto the ZnO nanosheets, closely Ag nanoparticles with an average particle size of 40 nm are attached onto the ZnO nanosheets, affixed affixed onto the 3 the to 5-layer thickthick RGO sheets [93]. Using a acontrolled closely onto 3 to 5-layer RGO sheets [93]. Using controlledhydrothermal hydrothermal process, process, Esfandiar et al. prepared 3D rGO-WO 3-Pd ternary composites in which Pd/WO3 nanostructures were Esfandiar et al. prepared 3D rGO-WO3 -Pd ternary composites in which Pd/WO3 nanostructures were incorporated on partially reduced graphene oxide (PRGO) sheets [94]. The nanostructure growth of WO3 on the graphene sheet could be improved by the addition of the PRGO during the hydrothermal

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incorporated partially reduced Sensors 2018, 18, xon FOR PEER REVIEW

graphene oxide (PRGO) sheets [94]. The nanostructure growth of 10 of 20 WO3 on the graphene sheet could be improved by the addition of the PRGO during the hydrothermal process processand andthe thefinal finalternary ternaryhybrids hybridsexhibited exhibitedaahierarchical hierarchicalnanostructure nanostructurewith withaahigh highsurface surfacearea area (Figure (Figure8c, 8c, d) d) [94]. [94].

Figure rGO-In22O Figure 8. 8. Morphology Morphologyofof(a–b) (a–b) Ag-loaded Ag-loaded 3D 3D ZnO ZnO nanostructure-rGO nanostructure-rGO [93]; [93]; (c–d) (c–d) 3D 3D rGO-In O33 composite composite[94]. [94].Reproduced Reproducedwith withpermission permissionfrom from[93,94], [93,94],© ©2015 2015and and2014 2014Elsevier. Elsevier.

2.4. 2.4.Graphene-MOx Graphene-MOxHybrids HybridsAssembled Assembledwithin within3D 3DGraphene-Multilayer Graphene-MultilayerNetwork Network Construction Constructionof ofmultilayer multilayernetworks networksfor forGO GOcan canserve serveas asaahierarchical hierarchicalspace spacefor forthe thegrowth growthof of MOx to form 3D architectured rGO/MOx hybrids [43,44,95–100]. Typically, a 3D rGO aerogel/ZnO MOx to form 3D architectured rGO/MOx hybrids [43,44,95–100]. Typically, a 3D rGO aerogel/ZnO spheres spheres composite composite isis produced produced via via aa facile facile solvothermal solvothermal method method [43]. [43]. The The formation formation process process isis described as follows: firstly, ZnCl 2 was dispersed into multilayer GO solution for ion anchoring. described as follows: firstly, ZnCl2 was dispersed into multilayer GO solution for ion anchoring. Second, Second,NaNO NaNO33and andNaAc NaAcwere wereadded addedtotothe themixed mixedsolution solutionresulting resultingin inaaprecursor precursorsolution. solution.Then, Then, the obtained mixture was transferred to a Teflon-lined stainless-steel autoclave for the in-situ growth the obtained mixture was transferred to a Teflon-lined stainless-steel autoclave for the in-situ growth of ofthe the ZnO ZnO spheres spheres under under solvothermal solvothermaltreatment treatmentconditions, conditions,while whilethe thein-situ in-situreduction reductionof ofGO GOto to rGO occurred simultaneously. Finally, a black integrated 3D graphene aerogel–ZnO was obtained rGO occurred simultaneously. Finally, a black integrated 3D graphene aerogel–ZnO was obtained via via aa freeze-drying freeze-drying process process to to maintain maintain the the 3D 3D monolithic monolithic architecture architecture [43]. [43]. The The3D 3D rGO rGO exhibits exhibits interconnected macroporous structures and the ZnO spheres featured a size of 0.5–1 μm are interconnected macroporous structures and the ZnO spheres featured a size of 0.5–1 µm areanchored anchored homogeneously homogeneouslyon onthe thesurface surfaceof ofrGO rGOlayers layers (Figure (Figure 9a–d). 9a–d).

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Figure 9. Morphology Morphologyof of3D 3DrGO rGOaerogel/ZnO aerogel/ZnO spheres composite. A photograph of composite. Figure 9. spheres composite. (a) (a) A photograph of composite. (b– (b–d) SEM images with different magnifications of rGO aerogel/ZnO spheres composite. Reproduced d) SEM images with different magnifications of rGO aerogel/ZnO spheres composite. Reproduced with with permission permission from from [43], [43], © © 2015 2015 Elsevier. Elsevier.

3. Applications of of 3D 3D Graphene/MOx Graphene/MOx Hybrids Hybrids for 3. Applications for Room Room Temperature Temperature Gas Gas Sensing Sensing 3D nanocomposites have have been been widely widely used used for for the the fabrication 3D architectured architectured graphene/MOx graphene/MOx nanocomposites fabrication of of gas sensors to detect various hazardous gases such as NO , NH , HCHO, H S and so on [101–105]. [101–105]. gas sensors to detect various hazardous gases such as NO22, NH33, HCHO, H22S and so on However, gas sensors sensors require require high high operating operating temperatures temperatures (typically (typically However, many many 3D 3D graphene/MOx-based graphene/MOx-based gas ◦ C) to achieve high sensitivity and fast response. The high temperature operation brings issues >100 >100 °C) to achieve high sensitivity and fast response. The high temperature operation brings issues such such as as high high energy energy consumption consumption and and the the risk risk of of gas gas explosions. explosions. These These limitations, limitations, therefore, therefore, have have recently motivated the development of high-performance room-temperature gas sensors. NO seems 2 recently motivated the development of high-performance room-temperature gas sensors. NO2 seems to room temperature sensing target, target, not not only only because because its its toxicity, toxicity, but to be be the the most most investigated investigated room temperature sensing but also also its that improve improve the the gas reaction on on the the surface its electrophilic electrophilic characteristics characteristics that gas chemical chemical adsorption adsorption reaction surface of of sensing materials. Many efforts have been dedicated to control the structures of 3D rGO/MOx sensing sensing materials. Many efforts have been dedicated to control the structures of 3D rGO/MOx sensing materials sensing properties properties at at room room temperature materials for for enhanced enhanced NO NO2 sensing temperature [65,76,79,80,84,90,92,106–108]. [65,76,79,80,84,90,92,106–108]. The graphene and and MOx MOx are are the The synergetic synergetic effects effects between between graphene the key key factors factors for for improving improving the the room room temperature NO sensing performance. In graphene/MOx composites, the MOx nanostructures act as temperature NO22 sensing performance. In graphene/MOx composites, the MOx nanostructures act as key on the the other other hand, hand, the the highly highly conductive conductive key sensitive sensitive materials materials for for the the chemical chemical adsorption adsorption of of NO NO22;; on graphene can not only ensure the current flow across electrodes for fast response, but also graphene can not only ensure the current flow across electrodes for fast response, but also conjugate conjugate with Thus, many many efforts efforts have with MOx MOx to to form form Schottky-junctions Schottky-junctions enhancing enhancing the the electron electron capture. capture. Thus, have been been dedicated hybrids to to achieve dedicated to to optimizing optimizing the the components’ components’ structures structures in in 3D 3D rGO/MOx rGO/MOx hybrids achieve enhanced enhanced sensing sensing performance. performance. Controllable preparation of of MOx MOx with with confined confined sizes sizes in in the could be be one Controllable preparation the nanohybrids nanohybrids could one efficient efficient way to contribute contributetotoenhancing enhancingsensing sensing properties. Yang al. developed a 3D nanoflower-like way to properties. Yang et al.etdeveloped a 3D nanoflower-like CuxO

consisting of 5–9 nm ultrafine nanoparticles/multilayer graphene (CuMGC) composites as a room

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CuxO consisting of 5–9 nm ultrafine nanoparticles/multilayer graphene (CuMGC) composites as a room temperature NO sensor The 3D nanoflower-like was located in-situ on the temperature NO2 gas sensor The[92]. 3D nanoflower-like CuxO wasCuxO located in-situ on the multilayer 2 gas [92]. multilayer rGOsteps via three steps (Figure 10a). The 3D hybridsa showed a high sensitivity and fast rGO via three (Figure 10a). The 3D hybrids showed high sensitivity (95.1) and(95.1) fast response response time (9.6ppm s) toof97NO ppm of NO210b). (Figure The enhanced sensitivity is attributed to the small time (9.6 s) to 97 2 (Figure The10b). enhanced sensitivity is attributed to the small size 3D size 3D CuxO flowers with high surface area which could provide more active sites for the surface CuxO flowers with high surface area which could provide more active sites for the surface adsorption adsorption reaction of NO , while the fast is Schottky due to thecontact Schottky contactrGO between rGO size and reaction of NO 2, while the 2fast response is response due to the between and small small size CuxO leading to formation of many more donors to capture and migrate electrons from CuxO leading to formation of many more donors to capture and migrate electrons from the the conduction band. Using a similar route, Mao prepareda a3D 3D rGO-based rGO-based NO NO22 sensor sensor with conduction band. Using a similar route, Mao et etal.al.prepared sensitivity (287% (287% to 100 ppm of NO22)) and and selectivity selectivity by by decoration decoration of of ultrafine ultrafine (3–6 (3–6 nm) nm) enhanced sensitivity SnO22 nanocrystals nanocrystals [108]. [108].

Figure 10. (a) Schematic illustration of formation of 3D nanoflower-like CuxO/multilayer graphene; Figure 10. (a) Schematic illustration of formation of 3D nanoflower-like CuxO/multilayer graphene; (b) Dynamic sensing property of CuMGC at room temperature. Reproduced with permission from [92] (b) Dynamic sensing property of CuMGC at room temperature. Reproduced with permission from © 2014 Royal Society of Chemistry. [92] © 2014 Royal Society of Chemistry.

The design of porous structures was also proved to be feasible route to enhance the performance of 3D rGO/MOx nanohybrids for room temperature NO2 sensing. For instance, a gas sensor based on 3D mesoporous rGO aerogels embedded with SnO2 or ZnO showed an enhanced response rate (190

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The design of porous structures was also proved to be feasible route to enhance the performance of 3D rGO/MOx nanohybrids for room temperature NO2 sensing. For instance, a gas sensor based on Sensors 2018, 18, x FOR PEER REVIEW 13 of 20 3D mesoporous rGO aerogels embedded with SnO2 or ZnO showed an enhanced response rate (190 s and 200200 s tos 10 100 ppm of NOof [43,44]. However, the low sensitivity (

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