Reduced Graphene

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Confined formation of ultrathin ZnO nanorods/reduced graphene oxide mesoporous nanocomposites for high-performance room-temperature NO2 sensors Yi Xia, Jing Wang, Jianlong Xu, Xian Li, Dan Xie, Lan Xiang, and Sridhar Komarneni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12501 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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ACS Applied Materials & Interfaces

Confined formation of ultrathin ZnO nanorods/reduced graphene oxide mesoporous nanocomposites for high-performance room-temperature NO2 sensors Yi Xiaa, b, #, Jing Wanga, c, #, *, Jian-Long Xud, Xian Lie, Dan Xiee, Lan Xianga, *, Sridhar Komarnenic, * a

Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China

b

Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming

650093, China c

Department of Ecosystem Science and Management and Materials Research Institute, Materials Research

Laboratory, The Pennsylvania State University, University Park, PA 16802, USA d

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based

Functional Materials and Devices, Soochow University, Suzhou, Jiangsu 215123, China e

Institute of Microelectronics, Tsinghua University, Beijing, 100084, China

Keywords: ZnO, reduced graphene oxide, spatial confinement growth, room-temperature sensor, NO2 Abstract Here we demonstrate high-performance room-temperature NO2 sensors based on ultrathin ZnO nanorods/reduced graphene oxide (rGO) mesoporous nanocomposites. Ultrathin ZnO nanorods were loaded on rGO nanosheets by a facile two-step additive-free solution synthesis involving anchored seeding followed by oriented growth. The ZnO nanorod diameters were simply controlled by the seed diameters associated with the spatial confinement effects of graphene oxide (GO) nanosheets. Compared to the solely ZnO nanorods and rGO based sensors, the optimal sensor based on ultrathin ZnO nanorods/rGO nanocomposites exhibited higher sensitivity and quicker p-type response to ppm level of NO2 at room temperature, and the sensitivity to 1 ppm

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of NO2 was 119% with the response and recovery time being 75 and 132 s. Moreover, the sensor exhibited full reversibility, excellent selectivity and a low detection-limit (50 ppb) to NO2 at room temperature. In addition to the high transport capability of rGO as well as excellent NO2 adsorption ability derived from ultrathin ZnO nanorods and mesoporous structures, the superior sensing performance of the nanocomposites was attributed to the synergetic effect of ZnO and rGO, which was realized by the electron transfer across the ZnO-rGO interfaces through band energy alignment. 1. Introduction Nitrogen dioxide (NO2) is one of the most toxic gaseous pollutants released from combustion or automotive emissions, which directly affects human health at very low concentrations of ppm level.1,2 The detection of NO2 has been an important environmental issue and attracted increasing attention because this pollutant causes acid rain. NO2 sensors based on metal oxides such as SnO2, WO3, ZnO, etc. have achieved great success owing to their excellent performances such as high sensitivity and selectivity.3-7 Unfortunately, their high operating temperatures (typically 200-600 °C) set a huge limitation for achieving wider applications, because of energy consumption issue, the risk of gas explosion as well as the difficulty in integration. These limitations have recently motivated researchers, who are engaged in realization of high-performance room-temperature NO2 sensors.8-11 Graphene has been demonstrated as a promising candidate for NO2 sensors owing to its room-temperature sensitivity and fast carrier transport kinetics.12-16 However, their sensing performances were usually limited at room temperature. Many graphene-based NO2 sensors exhibited insufficient sensitivities and long response/recovery time of up to tens of minutes.12-14 In addition, the problem of poor reversibility and selectivity also restricted their practical applications.14-16 Ideally a room-temperature NO2 sensor should be highly sensitive


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and selective, fast-responsive and fully reversible. Therefore, the newly emerged oxide/graphene nanocomposites may have great potential for designing room-temperature NO2 sensors owing to the combined advantages and synergetic effects of the two materials. Various metal oxides such as SnO2, WO3, ZnO, In2O3, etc. were hybridized with graphene and they showed enhanced NO2 sensing performances.17-26 Nevertheless, oxide/graphene nanocomposites as sensing materials still have several limitations. For example, the sensitivity and response rate were still insufficient;17,25,26 illumination or thermal treatment was required for recovery in some sensors.24,27,28 Till now, the fabrication of highly sensitive and rapid-response sensors based on oxide/graphene nanocomposites for room-temperature NO2 detection still remains a challenge. To achieve higher sensing performances of oxide/graphene nanocomposites, many recent efforts have been devoted to the controllable synthesis of metal oxides with confined sizes and desired morphologies in the nanocomposites. For example, Mao et al. reported the enhanced sensitivity and selectivity of rGO-based NO2 sensor by decoration of ultrafine (3-6 nm) SnO2 nanocrystals.19 Wang et al. systematically optimized the NO2 sensing performance of ln2O3/rGO nanocomposites by tuning the morphologies and distributions of ln2O3 nanocubes embedded in the rGO networks.21 3D nanoflowers consisting of 5-9 nm CuxO nanoparticles were homogeneously grown on multilayer graphene for sensitive and fast NOx detection.22 On the other hand, the construction of porous structures was also demonstrated to be effective so as to enhance the benefits of oxide/graphene nanocomposites for NO2 sensors. For example, rGO conjugated Cu2O nanowire mesocrystals were hydrothermally fabricated and these nanocomposites exhibited high sensitivity toward NO2 at room temperature owing to the unique nanowire-assembled mesoporous structures.23 Three-dimensional mesoporous rGO aerogels embedded with SnO2 or ZnO were also employed for room-temperature NO2 detection.24-26 Inspired by the above studies, herein, we propose a novel high-performance room-temperature NO2 sensor


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based on oxide/graphene nanocomposites with characteristics of size-confined oxide and mesoporous structure simultaneously. In contrast to many reported controllable syntheses employing surfactants,22,29 templates30,31 or high-temperature hydro-/solvo-thermal processes,23-26 in this work, a low-temperature additive-free solution approach to oxide/graphene nanocomposites was developed. We tried to take dual advantages of the GO nanosheets, which not only converted to rGO in the nanocomposites, but also served as substrates for ion adsorption, anchored nucleation and subsequent spatially confined growth of ZnO nanorods, leading to the formation of ultrathin ZnO nanorods/rGO mesoporous nanocomposites. High-performance room-temperature NO2 sensors based on the as-fabricated nanocomposites were demonstrated, and the mechanism underlying the enhanced NO2 sensing properties was also proposed. 2. Experimental 2.1 Synthesis Chemicals. Commercial chemicals of analytical grade and deionized water were used in all the experiments. Graphene oxide (GO) was synthesized from natural graphite powder (>99.8 %, Alfa Aesar) by a modified Hummers’ method.32 The obtained GO nanosheets were re-dispersed in deionized water with a concentration of 5 mg·mL-1 for further experiments. Fabrication of GO-supported ZnO nanoseeds. ZnO nanoseeds were synthesized by a hydrolysis method in the presence and absence of GO. 5 mL of GO suspension was added into 100 mL zinc acetate dihydrate solution in methanol (0.02 mol∙L-1) and stirred at room temperature for 1 h to achieve adsorption equilibrium. After that, 100 mL of NaOH solution in methanol (0.0002-0.002 mol∙L-1) was added and the mixed solution was then maintained at room temperature for 2 h to allow the formation of ZnO nanoseeds on the GO nanosheets. The obtained GO-supported ZnO nanoseeds were centrifuged, washed and re-dispersed in 10 ml of deionized


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water for further experiments. For ZnO nanoseeds fabrication, the same procedure was employed without the addition of GO. Fabrication of ZnO nanorods/rGO nanocomposites. ε-Zn(OH)2 was used in the current work as a precursor for growth of ZnO nanorods on the seeded GO. ε-Zn(OH)2 was prepared by drop-wise addition of 50 mL of 2.0 mol∙L-1 ZnSO4 into 50 mL of 4.0 mol L−1 NaOH at 25 °C under stirring, then the precipitate was filtrated, washed and dried at 25 °C for 24.0 h. 6.4 g NaOH, 3.5 g ε-Zn(OH)2 and the above obtained 10 ml suspension of ZnO nanoseeds or GO-supported ZnO nanoseeds were added separately into 30 ml of deionized water and stirred for 30 min. Then the obtained suspension was aged at 80 °C for 30 min for oriented growth of ZnO nanorods. Finally, the obtained products were centrifuged, washed and dried at 25 °C overnight. 2.2 Characterization The morphology and microstructures of the samples were examined with a field emission scanning electron microscope (FESEM, JSM 7401F, JEOL, Japan) and a high-resolution transmission electron microscope (HRTEM, JEM-2010, JEOL, Japan), respectively. Powder X-ray diffraction (XRD) was used for phase identification using an X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα (λ = 0.154178 nm) radiation. Raman spectra were recorded using He-Ne laser excitation at 532 nm with a Horiba Jobin Yvon LabRAM HR800 Raman spectrometer. Surface composition of the samples was characterized by X-ray photoelectron spectrometer (XPS, PHI-5300, PHI, USA). The Brunauer-Emmett-Teller (BET) surface areas of the products were determined using a nitrogen adsorption analyzer (Quadrasorb-S1, Quantachrome, USA) and the pore-size distribution was estimated by the Barrett-Joyner-Halenda method. Photoluminescence (PL) spectra of the samples were measured at room temperature on a FLS920 fluorescence spectrophotometer (Edinburgh Instruments, UK) under the excitation of 325 nm.


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3.3 Gas sensor fabrication and tests N-type (100) oriented silicon wafers were used as the sensor substrates with a 300 nm thick silicon dioxide (SiO2) dielectric-layer that thermally grown on the surfaces. The interdigital electrodes with a finger width of 10 μm and gap width of 10 μm were patterned and formed by standard photolithography, RF sputtering of titanium/gold (Ti/Au, 20 nm/100 nm) electrodes and the following lift-off process. The sensing materials of ZnO and ZnO/rGO nanocomposites were dispersed in the mixture of deionized water and ethanol (1:1) to achieve a concentration of 3.0 g·L–1 (the concentration of rGO suspension was 0.5 g·L–1). For preparing the sensing film, the suspensions were spin-coated onto the interdigital electrodes at 1000 rpm for 6 s and 3000 rpm for 30 s, respectively, then dried on a hot-plate at 90 oC for 30 min. The fabricated sensors were placed in a homemade chamber with gas inlet and outlet. Dry air was used as carrier gas. The input concentration of NO2 was 1.0-10 ppm and the flow rate of the mixed gas was 200 mL·min–1. The resistance of the films was measured and recorded by a Keithley 2700 multimeter/data acquisition system. All electrical tests were performed at 25 °C. The sensitivity (S) of the sensor is defined as

𝑅𝑎 −𝑅𝑔

𝑅𝑎 −𝑅𝑔

𝑅𝑔

𝑅𝑎

× 100 (%) for p-type response while

× 100 (%) for n-

type response, where Ra is the initial resistance in dry air, and Rg is the resistance of the sensitive film after exposing to NO2 atmosphere. The response time is defined as the time required until 90% of the whole response is reached while the recovery time denotes the time needed until 90% of the signal is recovered.

3 Results and discussion 3.1 Confined formation of ultrathin ZnO nanorods/rGO mesoporous nanocomposites


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Scheme 1. Schematic illustration of the fabrication of ultrathin ZnO nanorods/rGO nanocomposites. Ultrathin ZnO nanorods/rGO (UT-ZNR/rGO) nanocomposites were synthesized via deposition of ZnO nanoseeds on GO nanosheets followed by oriented growth of the nanoseeds into ZnO nanorods in a Zn(OH)2/NaOH mixed suspension, as illustrated in Scheme 1. First, GO dispersion was added to the Zn(Ac)2 methanol solution and stirred continuously to achieve adsorption equilibrium. Due to a strong coordination interaction between the metal ions and oxygen groups (e.g. -COOH) from GO,33,34 zinc ions were favorably adsorbed onto both sides of the GO nanosheets (Scheme 1, step 1). Second, a certain amount of NaOH methanol solution was added to initiate the formation of ZnO nanoparticles, resulting in ZnO nanoseed layers supported on the GO nanosheets (Scheme 1, step 2). Then, the obtained ZnO nanoseeds/GO gel was centrifuged, isolated from the mother solution, and added into an aqueous suspension containing NaOH and Zn(OH)2 precursor for oriented growth of the nanoseeds into ultrathin ZnO nanorods, while the in-situ reduction of GO to rGO occurred simultaneously (Scheme 1, step 3). Generally, graphene oxide layers were easily crinkled and folded regularly to form a honeycomb-like framework, hence the two-dimensional surfaces of GO would be spatially limited. In the present case, the oxygen-groups of graphene oxide may contribute to the adsorption and anchoring of metal cations, and then the crinkled sites or in-plane pores served as nanoreactors for spatially confined nucleation and growth of ZnO nanorods, thereby resulting in a strong affinity and uniform dispersion of the as-grown ultrathin ZnO nanorods within the mesoporous graphene framework, leading to the formation of ZnO/rGO nanocomposites with well-defined interfaces. The spatial confinement effects of GO nanosheets helped the


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formation of ultrasmall nanoseeds and their subsequent development into ultrathin ZnO nanorods. In addition, the randomly oriented ZnO nanorods, which were distributed on both sides of rGO acted as a spacer and protective layer to prevent the severe agglomeration of rGO during the drying process, thus contributing to the generation of abundant mesopores within the nanorod-networks. As a result, a new kind of UT-ZNR/rGO nanocomposite with mesoporous structure was obtained.

Figure 1. (a-d) TEM images of the ZnO nanoseeds with varying diameters supported on GO nanosheets; (e, f) TEM image of the nanoseeds formed under the same condition as that in Figure 1c and d but without GO. Zn2+/OH- ratio: (a) 100; (b) 40; (c-f) 10. The GO-supported ultrasmall ZnO nanoseeds played the key roles in the formation of UT-ZNR/rGO nanocomposites. We varied the average size of ZnO nanoseeds supported on GO nanosheets to investigate the


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effect of seed size on the formation of ZnO nanorods. By simply adjusting the Zn2+/OH- ratio as 100, 40, 10 in the sol-gel process, ZnO nanoseeds with average diameters of 45, 22 and 6 nm were deposited onto GO nanosheets, respectively, as shown in Figure 1a-d (denoted as seed-1/GO, seed-2/GO and seed-3/GO, respectively). The coating of ZnO nanoseeds on GO nanosheets was found to form a homogenous single seed layer in all cases. Neither overlapped nanoparticles or isolated nanoparticles outside of the GO nanosheets were observed, which indicated that the nucleation and growth of ZnO nanoseeds only occurred on the GO surfaces from the adsorbed zinc ions, suggesting the strong binding and anchoring ability of GO. TEM images (Figures 1a and b) reveal that ZnO nanoseeds with larger diameters aggregated in a side-by-side manner nearly fully covering the GO surface in the cases of Zn2+/OH-=100 and 40, while ultrasmall ZnO nanoseeds (Figures 1c and d) with uniform diameters of 4-8 nm were dispersedly decorated on the GO surface at Zn2+/OH-=10. For comparison, we also prepared ZnO nanoseeds in the absence of GO in order to identify the roles of GO. In sharp contrast, under the same conditions to that of Figures 1c and d but without the help of GO, inhomogeneous growth of ZnO nanoseeds and their aggregation into micro-sized agglomerates were observed (Figure 1e). Enlarged TEM image (Figure 1f) reveals that the agglomerates in Figure 1e were assembled by nanoparticles with nonuniform diameters ranging from 5-15 nm, obviously larger than that of the nanoparticles anchored on GO nanosheets. Correspondingly, the variation of nanoseed sizes could also be detected by XRD analyses (Figure S1a). The nanoseeds formed on GO nanosheets showed broader diffraction peaks than those of nanoseeds in solution, implying the smaller crystallite size. The calculated (100) crystallite sizes agreed well with the TEM observed particle sizes of different nanoseeds and the results were summarized in Figure S1b. Interestingly, some ultrasmall ZnO nanoseeds were located on the edge of GO and showed some extent of aggregation (Figure 1c), which implied the weaker effects of the GO edges, suggesting that the two-dimensional


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feature of GO was critical for its anchoring ability. It was also demonstrated that GO could play similar roles in the formation of larger nanoseeds. Nonuniformly aggregated nanoparticles with diameters of 40-100 and 20-50 nm were obtained in the absence of GO (Figure S2), much larger than those shown in Figures 1a and b, respectively. The above results suggested that GO nanosheets not only captured the zinc ions for the heterogeneous nucleation and growth of ZnO nanoseeds, but also served as substrates anchoring the nanoseeds to inhibit their further growth and aggregation by spatial confinement effect. In a GO-Zn(Ac)2 methanol solution, due to the low zinc concentration and sufficient oxygen groups on GO surfaces, all the zinc ions were captured by the GO nanosheets by the strong coordination interaction. After adding NaOH, the formation of ZnO nucleus started to progress, via continuous aggregation of Zn2+ and OHions to form clusters associated with proton transfer reactions.35 As zinc ions were pre-attached onto the GO nanosheets, these reactions preferably proceeded on the GO surfaces. As a result, the adsorbed clusters gradually turned to a single layer of nanoparticles with a crystalline wurtzite structure, anchoring on the GO nanosheets. Moreover, the deprotonation of Zn2+-OH- cluster is an important step during the nucleation of ZnO,35 which could be hindered by GO due to its acidic groups (i.e. -COOH).36 Therefore, the presence of GO may inevitably affect the heterogeneous nucleation and growth rate of ZnO nanoseeds on its surfaces. In summary, owing to the combined anchoring and hindering effects induced by the rich oxygen groups on GO, as well as the limited space of the two-dimensional surfaces, the nucleation and growth of ZnO nanoseeds on GO was inhibited, resulting in their uniform and well dispersed coating onto the GO nanosheets. However, the clusters in the bulk solution grew faster than those on GO, and the initially formed nanoseeds tended to grow larger and aggregated quickly due to their high surface energy. Consequently, micro-sized ZnO nanoseed agglomerates were obtained in the absence of GO.


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The above obtained three kinds of GO supported ZnO nanoseeds were then added into aqueous solutions containing 4 molL-1 NaOH and 3.5 g Zn(OH)2 and aged at 80 oC for 30 min for oriented growth of the nanorods. The reduction of GO to rGO occurred simultaneously in this step, as revealed by the Raman spectra (Figure S3). The intensity ratio of the D and G bands (ID/IG) was 1.08 and 1.20 for ZnO nanoseeds/GO and ZnO nanorods/rGO, respectively, suggesting the removal of the oxygen upon partial reduction of GO during the nanorod growth.37 The reduction of GO to rGO was also confirmed by XPS analysis (Figure S4), which suggested a decrease of C-O and –COOH species after the growth step. Three kinds of ZnO nanorods/rGO nanocomposites formed with the addition of seed-1/GO, seed-2/GO and seed-3/GO (denoted as ZNR-1/rGO, ZNR-2/rGO and ZNR-3/rGO, respectively) are displayed in Figure 2a-c (enlarged Figure 2c and its inset were also provided as Figure S5). The SEM images reveal that randomly oriented nanorods were densely distributed on the rGO surfaces, forming hedgehog-like ZnO nanorods/rGO heteroassemblies in all cases. The 2D-feature of rGO could be still identified after growing the nanorods, while very few isolated nanorods could be observed out of the heteroassemblies, suggesting the successful oriented growth of 1D ZnO nanorods on the 2D rGO platforms. In contrast, under the same synthesis conditions of ZNR-3/rGO but without the help of GO, radially-aligned ZnO nanorod-assemblies were obtained (Figure 2d, denoted as ZNR-3). In addition, due to the severe aggregation of the nanoseeds in the absence of GO, the nanorods were formed on the surfaces of the nanoseed agglomerates, resulting in intersected urchin-like microstructures. We also found that the pre-coating of ZnO nanoseeds onto GO was critical for the formation of ZnO/rGO heteroassemblies. If the aggregated nanoseeds and GO were added separately in the growth step, mixed product containing urchin-like ZnO nanorod-assemblies and rGO nanosheets was obtained instead of heteroassemblies. These results suggested that GO served as a 2D platform to anchor ZnO nanoseeds for subsequent oriented growth of ZnO nanorods.


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Figure 2. SEM images of ZNR-1/rGO (a), ZNR-2/rGO (b), ZNR-3/rGO (c) and ZNR-3 (d); TEM and HRTEM images of ZNR-3/rGO (e) and ZNR-3 (f). HRTEM image (inset in Figure 2e) reveals the c-axis oriented growth of the ultrathin ZnO nanorods. The diameters of the ZnO nanorods on the seeded GO nanosheets were controlled by the size of the preformed ZnO nanoseeds, and the presence of GO also contributed to the further narrowing of the nanorods. The diameter distributions of the four samples in Figure 2a-d are presented in Figure S6. The average diameters of ZNR-1/rGO, ZNR-2/rGO and ZNR-3/rGO were ca. 120, 55, 12 nm, respectively, suggesting that the decrease of nanoseed diameter led to the decrease of the nanorod diameter. Moreover, the obtained ZNR-3 in the absence of GO showed an obviously larger average diameter of 20 nm compared to that of ZNR-3/rGO. The narrowed diameters of the nanorods on rGO was further confirmed by TEM observations. Low-magnitude TEM images (Figure S7) reveal that ZNR-3/rGO was composed of dense ultrathin nanorods assembled on the rGO nanosheets,


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forming a porous network structure, while ZNR-3 sample showed urchin-like morphology, consistent with the SEM observations. Enlarged TEM images (Figure 2e and f) clearly demonstrated the smaller diameters of the nanorods on rGO. This could be connected with the smaller nanoseeds formed on GO nanosheets (avg. 6 nm) than those formed bulk solution (avg. 9 nm). On the other hand, the confinement effects of GO in the oriented growth step was also believed to be important for the formation of ultrathin nanorods. We performed a timedependent TEM observation of the products during the growth of ZNR-3/rGO. With increasing reaction time, the average length and diameter of ZnO nanorods by growth on rGO gradually increased in the initial 10 min but no obvious change was observed after 10 min (Figure S8a-c). The representative HRTEM image in Figure S8d demonstrated the interplanar spacings of 0.26 nm, which was quite similar to that of (001) planes of ZnO, indicating the oriented growth of the ZnO nanorods on rGO along the c-axis. The diameter variations during the growth of ZNR-3/rGO and ZNR-3 were shown in Figure S8, the diameters of nanorods increased in the initial stage and kept almost unchanged after 10 min of reaction. The average diameter of the nanorods in ZNR-3/rGO increased slower than that in ZNR-3, suggesting that the growth of ZnO nanorods on rGO was spatially hindered by the limited space on 2D surfaces of GO. These results suggested that the spatial confinement of GO affected both the anchored seeding and oriented growth step, leading to the formation of UT-ZNR/rGO nanocomposites. The results in Figure S9 also implied the very rapid growth of ultrathin ZnO nanorods. This was quite different from some previously reported synthesis of ZnO nanorods/rGO nanocomposites, in which dilute solutions containing soluble zinc in the range of 0.01-0.1 mol∙L-1 were used as precursors, resulting in long reaction times (1-6 h) and low yields of the products.32,38,39 In the current work, we employed Zn(OH)2 as a solid-state zinc source, while NaOH solution served as solvent to dissolve Zn(OH)2 and also provided an alkaline growth condition for ZnO. The growth process could be described by the dissolution-precipitation mechanism.40


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Compared to traditional chemical bath deposition of ZnO using soluble zinc salts, the use of Zn(OH)2 could also prevent the unexpected homogeneous nucleation and growth of ZnO in bulk solution,41 ensuring the anchored growth of ZnO on rGO surfaces. In addition, gram-scale (ca. 1.2 g) UT-ZNR/rGO nanocomposites were obtained after only 10 min of reaction. Such rapid large-scale synthesis of ZnO/rGO nanocomposites may also be promising for other applications such as photocatalysts and solar cells.39,42

Figure 3. N2 adsorption–desorption isotherms (a) (inset: BET surface areas) and pore size distributions (b) of the as-prepared samples. The dense ZnO nanorods distributed on both sides of rGO may act as a spacer and protective layer to prevent the severe agglomeration of rGO during the drying process, leading to the formation of nanocomposites with high surface areas and rich pore structures. The BET surface areas of the obtained samples were measured using nitrogen sorption technique. The BET surface areas of ZNR-1/rGO, ZNR-2/rGO and ZNR-3/rGO were 27.3, 43.6 and 59.9 m2/g, respectively. By comparison, the ZNR-3 sample showed a much lower surface area of 31.2


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m2/g than that of ZNR-3/rGO. Moreover, the ZnO/rGO nanocomposite samples showed increased mesopore volumes compared to ZNR-3 samples, especially in the range of 15-50 nm, and ZNR-3/rGO sample showed the largest mesopore volume. The high surface area and rich mesoporous structure of ZNR-3/rGO could be attributed to the confined growth of the nanorods in addition to their network structure on rGO nanosheets without agglomeration. 3.2 Gas-sensing properties of ZnO nanorods/rGO nanocomposites We first preliminarily studied the room-temperature NO2 sensing properties of the as-prepared ZnO/rGO nanocomposites of three kinds; their response curves to 1.0-10.0 ppm of NO2 are shown in Figure 4a. As the gas response at room temperature is a comparatively slow process and hard to get saturated in order to make the sensing data more reliable, the NO2 feeding time of each dose was fixed at 2 min, while the intervals were fixed at 4 min to allow a full recovery.11 All the ZnO/rGO sensors were sensitive to 1.0-10.0 ppm of NO2 at 25 °C, and they showed excellent reversible response and recovery properties upon continuous NO2 exposure/release cycles. The ZNR-3/rGO nanocomposite that incorporated the thinnest ZnO nanorods were found to be most sensitive, suggesting that ultrathin ZnO nanorods necessarily guaranteed the superior sensitivities of the nanocomposites. The sensitivities vs. NO2 concentrations and their fitting curves of the three sensors are summarized and shown in Figure 4b. The dependence of the sensor response on the NO2 gas concentration in the range of 1.0−10 ppm was approximately linear, which implied the reliability of the sensors. The slope of the linear fit was 31.7% ppm1

and the correlation coefficient was 0.978. The limit of detection (LOD) was estimated to be 47 ppb according

to the following equation:43,44 LOD (ppm) = 3×

𝑆𝐷 𝑠𝑙𝑜𝑝𝑒

where SD is the standard deviation of noise in air and was calculated to be equal to 0.5% based on 200 data points


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in the baseline of the response curve. The response curve measured near LOD indicated a clear response of ca. 7% to 50 ppb of NO2 at room temperature, which was also recoverable and reproducible upon NO2 exposure/release cycles (Figure S10). The presently achieved LOD was found to be lower than those of many reported oxide/graphene nanocomposites.19,24,45-47 The high sensitivity and low detection limit are beneficial for the application of ZNR-3/rGO in practical NO2 sensors.

Figure 4. Dynamic response curves (a) and sensitivities (b) of the sensors based on the three kinds of ZnO nanorods/rGO nanocomposites to 1.0-10.0 ppm of NO2 at room temperature. We also examined the sensing properties of rGO and ZNR-3 sample to 1 ppm of NO2 at room temperature (Figure 5a); the response curve of the ZNR-3/rGO sensor was replotted here for a clear comparison. The ZNR3 sensor exhibited a n-type response (i.e., increased film resistance in NO2) due to the n-type nature of ZnO. Though the sensitivity was rather high (~78%), the sensor was dynamically slow with response and recover


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times of ca. 8 and 13 min, respectively. The rGO under ambient conditions exhibits p-type behavior due to the electron withdrawing property of residual oxygen groups which induces hole-like carriers transport behaviors.48 Thus, the exposure of rGO sensor to NO2 led to an increase of the hole carriers and a decrease of the film resistance, i.e., a p-type response. The sensing rate of rGO sensor was higher than ZnO with response and recovery time of ca. 5 and 7 min, however, the sensitivity was comparatively low (~19%). The ZNR-3/rGO sensor displayed a p-type response to NO2 with a high sensitivity of up to 119%. In contrast with some reported oxide/graphene nanocomposites exhibiting n-type response to NO2,20,31 the presently obtained p-type responses implied a different sensing mechanism, which will be discussed in detail in the later part. The ZNR-3/rGO sensor also exhibited fast sensing dynamics, with response/recovery time of 75 and 132 s, respectively. A comparison of NO2 sensing performance at room temperature between the present ZNR-3/rGO sensor and reported oxide/graphene nanocomposites-based sensors in recent literatures is summarized in Table 1. It can be noticed from this summary that though good or even very high sensitivities could be achieved by some sensors, they showed rather slow sensing dynamics or even no fully recoverable;18-21,23,24 some of the sensors exhibited fast response/recovery, however, their sensitivities were insufficient.17,22,25,26 Comparatively speaking, our sensors were highly sensitive and particularly attractive for their fast response/recovery, full reversibility as well as low LOD toward NO2 at room temperature.


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Figure 5. (a) Dynamic response curves of the sensors based on rGO, ZNR-3, and ZNR-3/rGO to 1 ppm of NO2; (b) sensitivities of the sensors based on rGO, ZNR-3, and ZNR-3/rGO to 1 ppm of NO2 and other gases; (c) reproducibility of the ZNR-3/rGO sensor exposed to 1 ppm of NO2 (for successive 6 cycles); (d) stability of the ZNR-3/rGO sensor in air; inset: response curve of the ZNR-3/rGO sensor to 1 ppm of NO2 after 30 days. Table 1. Comparison of NO2 sensing performance at room temperature between the present ZNR-3/rGO sensor and reported oxide/graphene nanocomposites-based sensors in recent literatures. Response/recovery Sensitive material

NO2 conc. (ppm)

S(%)*

Ref. time*

ZnO nanoparticles/rGO

5

25.6

165 s/499 s

17

ln2O3 nanocrystals/rGO

10

~280

4 min/24 min

18

~300s/not SnO2 nanocrystals/rGO

100

287

fully 19

recovered Flame-made SnO2 5

17100

ca. 15 min/45 min

20

nanoparticle/exfoliated graphene


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3 In2O3 nanocube/rGO

5

min/

not

fully

60.8

21 recovered

3D nanoflower-like CuxO/multilayer 97

95.1

9.6 s/ca. 200 s

22

2

67.8

ca. 5 min/8 min

23

graphene Cu2O nanowire mesocrystals/rGO

5 SnO2/rGO mesoporous aerogel

100

min/not

105

fully 24

recovered SnO2/rGO mesoporous aerogel

10

>4

190 s/224 s

25

ZnO /rGO mesoporous aerogel

100

8.9

ca. 200 s/400 s

26

1/10

119/403

75 s/132s (1 ppm)

Ultrathin ZnO nanorods/rGO

This

mesoporous nanocomposite

work

* Some values were read from the figures since the authors did not report them and may be not very accurate. Figure 5b shows the sensitivities of the three kinds of sensors to 1 ppm of NO2 and other gases including HCHO, CH4, H2S, H2 and CO. Obviously, the ZNR-3/rGO sensor exhibited very high selectivity to NO2 and the selectivity was significantly enhanced compared to ZNR-3 and rGO sensors. Figure 5c shows the response curve of the ZNR-3/rGO sensor to 1 ppm of NO2 for successive 6 cycles of gas in and off. It reveals that the sensor showed rather stable sensitivities of 115-120 % in 6 cycles, demonstrating excellent reversibility, little appreciable baseline drift and good reproducibility. In addition, neither illumination nor thermal treatment was required to recover the sensor to baseline. However, for a rGO sensor, an obvious baseline drift was observed in 5 cycles (Figure S11a). The slower sensing dynamics and the baseline drift of rGO sensor may be connected with the stacking of rGO nanosheets into nonporous films on the electrodes during the drying process (Figure


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S10b), which inhibited the gas access and diffusion within the film. Therefore, the highly improved sensing dynamics and excellent reversibility of the ZNR-3/rGO sensor may be connected with the rich mesoporous structure within the nanocomposites, which facilitated the gas transport and diffusion processes. Furthermore, we checked the stability of the ZNR-3/rGO sensor in air to find more practical application capability. The sensitivities of the sensor exposed in air with different period of time were measured and presented in Figure 5d. The sensitivity only decreased slightly with time. After 30 days in air, a sensitivity of 105% to 1 ppm of NO2 was still achieved, indicating ca. 89% of the initial sensitivity could be retained. In addition, the sensor could still accomplish effective detection with fast response and almost full recovery (inset in Figure 5d) after 30 days. The above results suggest that the ZNR-3/rGO sensor can selectively and steadily detect NO2 of ppm level at room temperature. 3.3 NO2 sensing mechanism of ZnO nanorods/rGO nanocomposites The data in Figure 5a reveal the much lower sensitivity but quicker response rate of the rGO sensor than those of the ZNR-3 sensor. Moreover, a comparison study on the concentration of rGO in the nanocomposites (Figure S12) revealed that the maximum sensitivity was achieved at a comparatively low amount of rGO (~2%). These results implied that the ultrathin ZnO nanorods contributed more than rGO to the high sensitivity of the nanocomposites. However, the nanocomposites exhibited p-type responses similar to that of rGO. Based on these facts, we proposed the synergetic roles of ZnO and rGO in the nanocomposites: the ZnO nanorods behaved as the key sensing materials which were involved in the adsorption and interaction with NO2, while rGO nanosheets with excellent transport capability served as highly conductive channels within the nanocomposite film to direct the current flow across the electrodes for response, which may overcome the transport barriers in solely ZnO film with low mobility, contributing to the improved sensing dynamics.


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The synergetic effects of ZnO and rGO derived from their strong electronic interaction. The roomtemperature operation of the sensors enabled us to correlate their optical/electronic properties with the sensing mechanism of the nanocomposites. The photoluminescence (PL) spectra of the ZNR-3 sample and ZNR-3/rGO nanocomposite (Figure 6a) revealed similar broad-band green-yellow emissions centered at ca. 550 nm, which were attributed to originate from oxygen vacancies in ZnO.49,50 However, the ZNR-3/rGO nanocomposite exhibited a much lower PL intensity compared to that of ZNR-3. The PL quenching of ZnO nanorods implied the electron transfer from ZnO to rGO which resulted in the reduced radiative recombination. The electron transfer from ZnO surface to rGO was further indicated by the XPS analysis (Figure S13) because the Zn 2p and O 1s peaks in the ZNR-3/rGO nanocomposites shifted to lower binding energies compared to that in ZNR-3. The electron transfer from n-type ZnO to p-type rGO would result in the decreased carrier concentration in both ZnO and rGO, leading to the increased resistance of the nanocomposite film in air, which could be evidenced by the I-V characteristics of three samples in Figure 6b. The ZNR-3/rGO nanocomposite film exhibited a similar IV characteristics to rGO film, however, a much lower current compared to those of both rGO and ZNR-3. These results indicated that rGO served as conductive channels and the electron donating from ZnO to rGO would lead to the increased resistance of the nanocomposite film in air.


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Figure 6. (a) Room temperature PL spectra of ZNR-3 and ZNR-3/rGO; (b) I-V curves of the rGO, ZNR-3 and ZNR-3/rGO films measured in the dark; inset: illustrating the I-V measurements; (c) schematic illustration of the energy band structures of ZnO-rGO junction and electron transfer in the nanocomposite: (1) Before contact; (2) nanocomposite in air and (3) nanocomposite exposure to NO2; Ec, Ev, and Ef are conduction band, valance band and Fermi energy, respectively. Such electron transfer derived from the fact that the work function of rGO was larger than that of ZnO, as shown in Figure 6c (1) (before contact). The work function of n-type ZnO was reported to be 4.2-4.3 eV,51,52 closely to its conduction band at 4.1 eV, while the work function of chemically reduced GO were determined ranging from 4.4 to 5.0 eV.53-55 Such energy band structure has some similarity to the case of semiconductor/metal heterojunction where the metal work function is less than that of the semiconductor, since the zero bandgap rGO could play the similar role as metals.20,56,57 When ZnO and rGO were brought into contact, it was expected that electrons would pass from ZnO to rGO until the equilibrium of Fermi level was achieved,


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as shown in Figure 6c (2). As a result, a Schottky-type junction were formed across the ZnO/rGO interface, which was characterized with a upward band bending and a depletion region in ZnO near the surface. An internal electrical field was formed in the depletion region across the ZnO-rGO interfaces. When the nanocomposite was exposed to electrophilic NO2, the electron transfer from ZnO to adsorbed NO2 would led to a decrease of the ZnO carrier concentration. Less numbers of carriers in ZnO caused an increase of Ec/EF difference in the ZnO region and thus smaller Fermi level difference between ZnO and rGO, leading to a reduction of the electric field of the junction.58 As a result, electrons would be gave back from rGO to ZnO, leading to the increased hole concentration in rGO and thus increased conductivity of the nanocomposite film, as shown in Figure 6c (3). By considering the definition of the sensitivity (Ra/Rg-1) for p-type response, the nanocomposite film with a significantly larger resistance in air as well as enhanced interaction with NO2 was expected to achieve a much higher sensitivity compare to the rGO film. The above discussion suggests that the interaction of ZnO nanorods with NO2 played the key role in determine the sensitivity of the nanocomposite. By considering this, the increase of sensitivity of the ZnO nanorods/rGO nanocomposites with the decrease of nanorod diameter could be explained by the enhanced interaction between NO2 and the nanocomposites. Moreover, the ZNR-3/rGO nanocomposite with large accessible surface area and mesopore volume facilitated the diffusion and adsorption of NO2 within the nanocomposite film, which resulted in the large reduction of the film resistance upon NO2 exposure, leading to the high sensitivity and fast sensing dynamics at room temperature. On the other hand, the remarkable selectivity to NO2 of the UT-ZNR/rGO nanocomposite could also be connected with the selective and high NO2 adsorption ability of ZnO, which was also observed previously in ZnO-based NO2 sensors.3,5 At room temperature, the adsorbed oxygen on the ZnO surface exists in the form of O2-, reducing gases such as CO and H2 could not


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readily react with O2- due to its low oxidation ability, thus the sensors exhibited low sensitivities to these gases. However, NO2 is a highly electrophilic gas and can directly capture the free electrons from the ZnO nanorods by a surface reaction with O2- (2NO2 + O2- + 2e- → 2NO3-). Therefore, the selective adsorption and electron transfer of NO2 on ZnO surfaces contributed to the high selectivity of the nanocomposites. Moreover, the ZnO nanorods obtained in the current work may be particularly preferable for NO2 adsorption, as the PL results in Figure 6a suggested the existence of rich oxygen-vacancies on the ultrathin ZnO nanorods. It was previously reported that the adsorption energy of NO2 on the oxygen-vacancy site of ZnO surface (−0.98 eV) is lower than that on the perfect site (−0.30 eV).59 Therefore, the oxygen-vacancies may also enhance the selective adsorption and electron transfer of NO2 on the ZnO nanorod surfaces and thus improving the sensitivity and selectivity. To sum up, the superior sensing performance of the nanocomposites was achieved by incorporating the benefits of highly sensitive ZnO and highly conductive rGO, as well as their interfacial electron transfer through band energy alignment. The presently developed simple solution synthesis provides an alternative strategy for the design and construction of novel oxide/graphene nanocomposites, which opens a facile pathway to the fabrication of high-performance room-temperature NO2 sensors. 4 Conclusion We have demonstrated high-performance room-temperature NO2 sensors based on the ultrathin ZnO nanorods/rGO mesoporous nanocomposites designed and synthesized here. The formation of ultrathin ZnO nanorods on rGO nanosheets was achieved by anchored seeding followed by oriented growth, with the aid of the spatial confinement effects of the GO/rGO nanosheets. A systematical comparison study suggested that the ZnO/rGO nanocomposites that incorporated the thinnest ZnO nanorods exhibited the highest sensitivity to ppm level NO2 at room temperature. The nanocomposite sensors were particularly attractive for their low detection


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limit, rapid response/recovery, full reversibility as well as excellent selectivity. The high transport capability of rGO as well as the excellent NO2 adsorption ability derived from ultrathin ZnO nanorods and mesoporous structures showed synergetic effect on the superior sensing performance of the nanocomposites, which was realized by the interfacial electron transfer through band energy alignment. ASSOCIATED CONTENT Supporting Information XRD patterns, calculated crystallite sizes and the TEM observed average diameters of different nanoseeds, TEM images of the seeds formed in the absence of GO, Raman spectra and C 1s XPS spectra of ZnO nanoseeds/GO and ZnO nanorods/rGO, enlarged Figure 2c and its inset, TEM and HRTEM images of ZNR-3/rGO at different reaction time, diameter distributions of the nanorods in different samples, low-magnitude TEM images of ZNR3/rGO and ZNR-3, variation of nanorod diameters during the growth of ZNR-3/rGO and ZNR-3, dynamic response curve of the ZNR-3/rGO sensor to 50 ppb NO2, reproducibility of the rGO sensor to 1 ppm of NO2 and SEM image of the rGO sensor, effect of rGO ratio on the sensitivity of ZNR-3/rGO nanocomposites, Zn 2p and O 1s XPS spectra of ZNR-3 and ZNR-3/rGO samples. AUTHOR INFORMATION Corresponding Author * Email: [email protected] (J. Wang); * Email: [email protected] (L. Xiang); * Email: [email protected] (S. Komarneni). Author Contributions # Y. X. and J. W. contributed equally to this work.


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Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the National Science Foundation of China (Nos. 51234003 and 51374138), National Science Foundation of Jiangsu Province (BK20160328) and National Key Technology Research and Development Program of China (2013BAC14B02). References (1) Das, A.; Dost, R.; Richardson, T.; Grell, M.; Morrison, J. J.; Turner, M. L. A Nitrogen Dioxide Sensor Based on An Organic Transistor Constructed from Amorphous Semiconducting Polymers. Adv. Mater. 2007, 19, 40184023. (2) Bernstein, J.; Alexis, N.; Barnes, C.; Bernstein, I.; Nel, A.; Peden, D. Health Effects of Air Pollution. J. Allergy Clin. Immunol. 2004, 114, 1116–1123. (3) Rai, P.; Kwak, W. K.; Yu, Y. T. Solvothermal Synthesis of ZnO Nanostructures and Their MorphologyDependent Gas-Sensing Properties. ACS Appl. Mater. Interfaces 2013, 5, 3026-3032. (4) Maeng, S.; Kim, S. W.; Lee, D. H.; Moon, S. E.; Kim, K. C.; Maiti, A. SnO2 Nanoslab as NO2 Sensor: Identification of the NO2 Sensing Mechanism on A SnO2 Surface. ACS Appl. Mater. Interfaces 2013, 6, 357-363. (5) Chen, M.; Wang, Z.; Han, D.; Gu, F.; Guo, G. Porous ZnO Polygonal Nanoflakes: Synthesis, Use in HighSensitivity NO2 Gas Sensor, and Proposed Mechanism of Gas Sensing. J. Phys. Chem. C 2011, 115, 1276312773. (6) Rossinyol, E.; Prim, A.; Pellicer, E.; Arbiol, J.; Hernández-Ramírez, F.; Peiro, F.; Cornet, A.; Morante, J. R.; Solovyov, L. A.; Tian, B. Synthesis and Characterization of Chromium-Doped Mesoporous Tungsten Oxide


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Reduced Graphene

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