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Sensitive Room-Temperature H2S Gas Sensors Employing SnO2 Quantum Wire/Reduced Graphene Oxide Nanocomposites Zhilong Song, Zeru Wei, Baocun Wang, Zhen Luo, Songman Xu, Wenkai Zhang, Haoxiong Yu, Min Li, Zhao Huang, Jianfeng Zang, Fei Yi, and Huan Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04850 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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Chemistry of Materials

Sensitive Room-Temperature H2S Gas Sensors Employing SnO2 Quantum Wire/Reduced Graphene Oxide Nanocomposites Zhilong Song, Zeru Wei, Baocun Wang, Zhen Luo, Songman Xu, Wenkai Zhang, Haoxiong Yu, Min Li, Zhao Huang, Jianfeng Zang, Fei Yi, Huan Liu* School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei, 430074, P. R. China ABSTRACT: Metal oxide/ graphene nanocomposites are emerging as one of the promising candidate materials for developing high performance gas sensors. Here, we demonstrated sensitive room-temperature H2S gas sensors based on SnO2 quantum wires that were anchored on reduced graphene oxide (rGO) nanosheets. Using a one-step colloidal synthesis strategy, the morphology-related quantum confinement of SnO2 was well controlled by tuning the reaction time due to the steric hindrance effect of rGO. The as-synthesized SnO2 quantum wires/rGO nanocomposites were spin-coated onto ceramics substrates without further sintering to construct chemiresistive gas sensors. o The optimal sensor response toward 50 ppm of H2S was 33 in 2 s, and it was fully reversible upon H2S release at 22 C. In addition to the excellent gas adsorption of ultrathin SnO2 quantum wires, the superior sensing performance of SnO2 quantum wire/rGO nanocomposites was attributed to the enhanced electron transport resulting from the favorable charge transfer of SnO2/rGO interfaces and the superb transport capability of rGO. The easy fabrication and room-temperature operation make our sensors highly attractive for ultrasensitive H2S gas detection with less power consumption.

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1. Introduction Because of the low cost, easy production, compact size and simple measuring electronics, metal oxide semiconductor-based gas sensors are widely used to detect varieties of inflammable and toxic 1-4 gases. As a typical n-type wide bandgap semiconductor, tin oxide (SnO2, Eg= 3.6 eV at 300 K) has been immensely investi5-7 gated for the detection of H2S, a highly toxic gas usually pro8 duced in sewage plants, coal mines, and oil industries. However, o their high operating temperature (usually above 100 C) sets a huge limitation for achieving wide applications due to the power consumption issue, combined with the difficulty in integration and miniaturization. To develop sensitive room-temperature gas detection, two9 dimensional (2-D) nanostructures including graphene-based and 10-12 graphene-like layered nanomaterials have been explored as novel building blocks for gas sensors because of their high specific surface area and fast electron transport kinetics. However, their sensitivity was insufficient and the recoverability was poor at room temperature, possibly due to their high conductivity and strong gas adsorption. Instead, the newly emerged oxide/graphene nanocomposites exhibited great potentials in room-temperature gas sensors. For 13 example, the reduced graphene oxide (rGO)-sensitized In2O3 and 15 14 16 Cu2O nanowires , graphene-SnO2 , and graphene-ZnO were sensitive toward low concentration of NO2, liquefied petroleum gas and formaldehyde at room temperature, respectively. In the 17 case of H2S, however, SnO2 nanofibers/rGO and SnO2 nanoparti-

cles/rGO gas sensors still have to be operated at temperatures o high as 200 and 100 C, respectively. While Cu2O nanocrystals grown on rGO were sensitive to 5 ppb of H2S at room tempera19 ture, the response time was long (~2 min). For the gas sensors employing oxide/graphene nanocomposites, highly sensitive and rapid response for room-temperature H2S detection still remain challenging. In this work, we take the view that the benefits of oxide/rGO nanocomposites for gas sensors may be enhanced through a controllable material synthesis. We employed the one-step colloidal synthesis protocol in which the morphology of SnO2 was easily controlled by tuning the reaction time. The as-synthesized SnO2/rGO nanocomposites, owing to their excellent solution processability, were directly fabricated into sensing layers on sub20-24 strates without further sintering . In this way, the materials properties of SnO2/rGO nanocomposites were conserved from solution to film. Therefore, the benefits of SnO2/rGO nanocomposites in real gas sensors could be optimized through the controllable materials synthesis. We conducted a systematical comparison study and demonstrated ultrasensitive H2S room-temperature gas sensors; the mechanism underlying their enhanced roomtemperature H2S-sensing performance was also proposed. 2. Experimental 2.1. Sensor fabrication and test The whole procedure for the material synthesis, sensor fabrication and gas-sensing test was shown in Scheme 1. For the synthesis of rGO, the graphene oxide (GO) synthesized from worm-like exfo-

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liated graphite powder following the modified Hummers meth26 o od were thermally reduced at 300 C for 30 min in an Ar atmosphere, resulting in the rGO nanosheets. In the one-step colloidal . synthesis, typically, 0.6 g SnCl4 5H2O was dissolved into 20 mL oleic acid (OA) and 2.5 mL oleylamine to form a transparent solution. Then, 1.0 mg rGO (in 1.0 mL H2O) and 10 mL ethanol were added with mild stirring. The mixture was transferred into a 50 o mL Teflon-lined stainless steel autoclave to react at 180 C for 3-8 h and then transferred to a cold water bath for cooling down to room temperature. The product was rinsed with toluene and final-1 ly dispersed in ethanol at a concentration of 20 mg mL , in which the nominal ratio of rGO to SnO2 was 3.88 wt%.

backscattering geometry using the 514.5 nm line of Ar -laser as an excitation source. The shape and size of the SnO2/rGO nanocomposites were investigated by high resolution transmission electron microscope (HR-TEM) on a JEOL-2100 using an accelerating voltage of 200 kV. SEM images were obtained by a FEI Sirion 200 scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed with a VG Multilab 2000 system with Al source; all of the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. Work functions were measured by a KP 020 K probe (KP 127 Technology, U.K.).

Materials for the device fabrication included the SnO2/rGO -1 nanocomposites, Cu(NO3)2 diluted in methanol (0.05 mmol mL ), and anhydrous methanol. The layer-by-layer spin-coating deposition technique was carried out on a spin coater inside a fume hood at room temperature according to the following steps: 1) Three drops of SnO2/rGO solution were dropped onto the Al2O3 substrates pre-patterned with interdigital Ag electrodes and then spun -1 at 1250 rpm for 20 s, it was repeated twice; 2) 10 mg mL Cu(NO3)2 in methanol was added dropwise to the substrates, and spun at 1250 rpm for 20 s after a 45 s wait, the Cu(NO3)2 treatment was repeated twice; 3) The film was washed by methanol flush and then spun dry for three times.

3.1. Morphology and structure structure of the SnO2/rGO nanocomposites

3. Results and discussion We varied the reaction time of the one-step colloidal synthesis to investigate the morphology and structure evolution of the SnO2/rGO nanocomposites. We also prepared pristine SnO2 samples for comparison in order to identify the role of rGO in the one-step synthesis. Since the rGO was dispersed in de-ionized water, equal amount of de-ionized water was added for the synthesis of pristine SnO2 so as to exclude the effect of water in our comparison.

The gas sensors were tested by a commercial computerconnected Keithley 2450 source meter (Keithley Instrument, USA) system under static conditions with the relative humidity being o 56~60% at 22 C. The sensor response was defined as the ratio of Ra to Rg, where Ra is the baseline resistance in presence of clean air and Rg the resistance of the sensor device in presence of target gas. The response time is defined as the time required to reach 90% of the final response upon target gas exposure, and the recovery time as the time interval over which the sensor response drops to 10% of the stabilized response in the target gas when placed in clean air.

Figure 1 HRTEM images and SAED patterns of the pristine SnO2 (a, o c, e) and SnO2/rGO nanocomposites (b, d, f) synthesized at 180 C for 3, 6 and 8 h, respectively.

Scheme 1 The oneone-step synthesis of SnO2 quantum wires/rGO gas-nanocomposites, followed by their sensor fabrication and gas sensing test. 2.2. Microstructure characteri characterization haracterization X-ray diffraction (XRD) measurements were performed using a diffractometer (MAXima_X XRD-7000, Shimadzu, Japan) with o Cu Kα radiation in the 2θ range of 20~80 . UV-vis absorption spectra were measured using a PerkinElmer Lambda 950 UVvis/NIR spectrophotometer. The Raman spectroscopy was performed by Lab RAM HR 800 Microlaser Raman system in

As shown in Figure 1, ultrathin nanowires were observed for the pristine SnO2 and the SnO2/rGO nanocomposites synthesized o at 180 C for 3, 6 and 8 h, respectively. The formation of SnO2 nanowires was probably due to the presence of water in the precursor which was suspected to coordinate to the SnO2 nanoseed surface and thus would take part in the oriented attachment of separate nanoseeds, finally forming nanowires under the annealing 27 effect in the synthesis reaction. The well-dispersed SnO2 nanowires, both in the pristine and nanocomposite samples, exhibited a high degree crystallinity; the lattice fringes with interplanar spacing of 0.334 nm and 0.267 nm corresponded to the (110) and


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(101) planes of rutile SnO2, respectively. The four clear diffraction rings of the selected area electron diffraction (SAED) pattern shown in each inset corresponded to the (110), (101), (211) and (112) planes, respectively, confirming the tetragonal rutile structure of the SnO2 nanowires. As the reaction time increased, the diffraction rings corresponding to the (200) and (002) planes of SnO2 became more clear, suggestive of increasing crystallinity of SnO2. For the SnO2/rGO nanocomposites, the (002) diffraction ring of rGO was not observed. The HRTEM images of SnO2/rGO nanocomposites indicated that the SnO2 nanowires were well spread out on the rGO nanosheets. The stability of the SnO2/rGO nanocomposites against sonication demonstrated the strong binding between the SnO2 nanowires and rGO nanosheets. The presence of rGO in the SnO2/rGO nanocomposites was further confirmed by Raman spectra (Figure 2a). Both the pristine rGO and the SnO2/rGO nanocomposites exhibited two major peaks ascribed to the D band at 1 1 1345 cm− and the G band at 1598 cm− , corresponding to the defect formation and the first-order scattering of the E2g mode of 2 28 sp domains in the rGO nanosheets, respectively. For the pristine rGO, the intensity ratio of the D and G band (ID/IG) was 1.249, suggestive of a partial reduction of graphene oxide in preparation of rGO nanosheets. Meanwhile, after the one-step colloidal synthesis of SnO2/rGO nanocomposites, the ID/IG ratio decreased to 0.976, suggestive of a further reduction of the rGO in the one-step colloidal synthesis of SnO2/rGO nanocomposites. The second1 order D band (2D) of the pristine rGO at 2671 cm− was not observed on the SnO2/rGO nanocomposites, possibly due to the small amount of rGO in the nanocomposites. The good distribution and adhesion of SnO2 nanowires on rGO nanosheets may be attributed to the hydrophobic basal plane and 29, hydrophilic edges of rGO nanosheets which probably served as a 4+ surface ligand binding to the Sn and thereby confined the nucleation of SnO2 nanoseed and the growth of SnO2 nanowires on its surface. Interestingly, the average diameter of the SnO2 nanowires estimated from the HRTEM images was 3.5, 3.8 and 4.2 nm for o the pristine sample synthesized at 180 C for 3, 6 and 8 h, but for the SnO2/rGO nanocomposites it was 3.0, 3.6 and 4.0 nm, respectively (Figure S1 in Supporting Information). The results indicated that the diameter of SnO2 nanowires was narrowed due to the presence of rGO, suggestive of the steric hindrance effect of rGO nanosheets that helped the formation of ultrathin SnO2 nanowires in the one-step synthesis of SnO2/rGO nanocomposites. We conducted time-resolved XRD analysis and UV-vis absorption characterization to seek deep insights into the effect of rGO on the morphology evolution of SnO2 nanowires in the one-step synthesis. The XRD patterns (Figure 2b) of the pristine SnO2 and SnO2/rGO nanocomposites had same peak positions at Bragg angles (2θ) of 26.7°, 33.6°, 51.8°, and 64.8° regardless of the reaction time, corresponding to the (110), (101), (211) and (112) planes of the tetragonal rutile SnO2 structure (JCPDS 41-1445), respectively. Consistent with the SAED results, the XRD peaks at 38.0° and 57.8° corresponding to the (200) and (002) planes of SnO2 became more clear as the reaction time increased; for the SnO2/rGO nanocomposites, the (002) Bragg peak of rGO was not observed, most likely due to its low content in the XRD sample beyond detection

limit of the diffractometer. As the reaction time increased, we observed gradual narrowing of the SnO2 peaks both for the pristine and nanocomposite samples, suggestive of the increased diameter and crystallinity in the continuing growth of SnO2 nanowires within the period of reaction time we investigated. The observations from XRD patterns suggested that the addition of rGO did not change the lattice structure, consistent with the HRTEM and SEAD results.

a)

b)

Figure 2 a) Raman spectra of pristine rGO and the pristine SnO2, o SnO2/rGO nanocomposites synthesized at 180 C for 8 h, b) XRD patterns of pristine rGO and the pristine SnO2, SnO2/rGO nanocompoo sites synthesized at 180 C for 3, 6 and 8 h, respectively.

The role of rGO in narrowing the size of SnO2 nanowires was further demonstrated through the quantum confinement investigation. Figure 3 showed the time-resolved UV-vis absorption spectra of pristine SnO2 and SnO2/rGO nanocomposites; their bandgaps were calculated according to the absorption spectra and summarized in Table 1. Because the diameters of SnO2 nanowires were 24, 30 well below twice of the exciton Bohr radius of SnO2 (2.7 nm), we had reasons to expect quantum confinement of SnO2 nanowires, as evidenced by the observation that the bandgaps of all the as-synthesized materials were broadened compared to the 3.6 eV for bulk SnO2. Therefore, the ultrathin SnO2 nanowires we synthesized in this study could be termed as quantum wires. While the energy bandgap of the SnO2 quantum wires, both in the pristine and nanocomposite samples, decreased as the reaction time increased due to their continuing growth, the SnO2 quantum wire/rGO nanocomposites exhibited slightly lager bandgap compared to the pristine SnO2. The results unambiguously confirmed the role of rGO in limiting the radial growth of SnO2 quantum wires in the one-step synthesis. Specifically, for the thinnest quantum wires yielded by shortest reaction time (3 h), the SnO2 quantum wires/rGO nanocomposites exhibited the first exciton absorption peak (260 nm) which was not observed in the pristine counterpart, suggestive of their strongest quantum confinement effect


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among all the samples. While the exact formation mechanism of SnO2 quantum wires still needed further investigation, their morphology-related quantum confinement was clearly observed and could be easily tuned by the reaction time as well as the addition of rGO. They are supposed to be ideal models for realizing the controllable synthesis of various quantum materials of metal oxides. Overall, the one-step colloidal synthesis provided a facile way to the stable nanocomposites incorporating ultrathin SnO2 quantum wires well distributed on rGO nanosheets, which may guarantee excellent gas-sensing properties for their excellent gas adsorption and electron transport.

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surface treatment to remove most oleic acid and oleylamine 20,24 through the ligand exchange. Varieties of inorganic salts (typi-1 cally 10 mg mL in absolute methanol) were used, among which the Cu(NO3)2-treatment yielded sensitive, fast and recoverable sensor response toward H2S gas at room temperature; the response of the as-deposited SnO2/rGO nanocomposite films was very weak as expected (Figure S2a in Supporting Information). XPS characterization (Figure S2b) confirmed the presence of Cu (1.49 at%) in the final sensor devices, which may be helpful considering that Cu can act as a catalyst in the SnO2-based H2S gas sensors.

a)

a)

b)

b)

c) Figure 3 UV-Vis spectra of a) pristine SnO2 quantum wires and b) o SnO2 quantum wire/rGO nanocomposites synthesized at 180 C for 3, 6 and 8 h, respectively.

Table 1 Energy bandgap (eV) of pristine SnO2 quantum wires and o SnO2 quantum wire/rGO nanocomposites synthesized at 180 C for 3, 6 and 8 h, respectively. Reaction time (h)

3

6

8

SnO2

4.50

4.05

3.86

SnO2/rGO

4.77

4.13

3.88

d)

3.2. GasGas-sensing properties of SnO2 quantum wire/rGO nanocomnanocomposites Owing to the solution processability of the SnO2 quantum wires/rGO nanocomposites, their sensor fabrication was conducted via the spin-coating method at room temperature, which was expected to bring about the benefits of nanostructure material in real sensors devices by overcoming the agglomeration issue encountered in high-temperature fabrication processing. It should be noted that the as-synthesized SnO2/rGO nanocomposites were 24 capped with abundant oleic acid and oleylamine, the long carbon chains of which might hinder both the gas adsorption and carrier transport in the sensor device. We therefore applied a film-level

Figure 4 a) Response curves of gas sensors based on SnO2/rGO o nanocomposites synthesized at 180 C for different reaction time, b) Response curves toward different concentrations of H2S, c) dependence of the response upon gas concentration, and d) selectivity of the optimal gas sensor employing SnO2/rGO nanocomposites (8 h).


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Table 2 RoomRoom-temperature H2S sensing performance of different chemiresistive gas sensors.

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Materials

Method

Substrate

Concentration

Sensor

Response/Recovery

(ppm)

response

time (s)

Reference

In2O3

Carbothermal method

Quartz tube

10

1.4

~60/~7200

31

Cu/SWCNTs

Spin coating

PET

20

1.3

10/20

32

PPy/WO3

UV-polymerization

Al2O3

1

5.3

260/12600

33

Co phthalocyanine-Au

Thermally evaporated

PET

10

5.2

540/960

34

deposition SnO2 multi-tube arrays

-

-

5

1.5

14/30

35

SnO2-CNT

Hot filament chemical

Cu/CuO

50

4

~60/~60

36

Si/SiO2

50

1.8

180/500

37

vapor deposition Quasi-2D CuO/SnO2

Electrochemical deposi-

Cu2O -FGS

Syringe dispensing

Si/SiO2

0.1

1.4

-/-

19

SnO2 nanowire/rGO

Spin coating

Al2O3

50

33

2/292

This work

tion

We first preliminarily studied the room-temperature H2Ssensing properties of the nanocomposites synthesized under different reaction time (3, 6 and 8 h); the response curves were shown in Figure 4a. It was observed that all the SnO2/rGO sensors o were sensitive to 50 ppm of H2S gas at 22 C; the sensor resistances decreased upon H2S exposure and they were recoverable upon H2S release. On the other hand, the SnO2/rGO nanocomposites (8 h) that incorporated thickest SnO2 quantum nanowires were found to be most sensitive with a response of 33. Therefore, smaller diameter of SnO2 quantum wires did not necessarily guarantee superior sensitive response of SnO2 quantum wire/rGO nanocomposites. The results suggested that the length of SnO2 quantum wires and their distribution on the rGO nanosheets, although hard to be differentiated from the HRTEM images, might also play important role in determining the sensing properties and needed further investigation. We summarized the performance of the optimal gas sensors based on the SnO2 quantum wire/rGO nanocomposites (8 h) and compared it with other room-temperature H2S gas sensors reported 19, 31-37 (Table 2). Our sensors were particularly in recent literatures attractive for their fast and sensitive response toward H2S gas at room temperature. Their continuous response curve upon H2S exposure/release cycles of different concentrations (10, 20, 40, 50, 60, 80, and 100 ppm) shown in Figure 4b indicated the excellent dynamic response and recovery properties at room temperature; the response time was 2~13 s depending on the H2S gas concentration (Figure S3 in Supporting Information). The dependence of the sensor response on the H2S gas concentration in the range of 20 to 100 ppm was approximately linear (Figure 4c), the slope and 2 -1 correlation coefficient (R ) of the linear fit was 0.69 ppm and 0.959. The limit of detection (LOD) was estimated to be 43 ppb at o 21, 24 22 C according to the equation LOD (ppm) = 3×RMSnoise/slope , where RMSnoise was the standard deviation of noise and calculated to be 0.01 based on 237 data points in the baseline of the response curve. The response curve measured at LOD indicated a response

~1.02 toward 43 ppb of H2S gas at room temperature, which was recoverable upon gas release (Figure S4 in Supporting Information). We further evaluated the selectivity of the optimal sensors toward H2S (Figure 4d). At room temperature, the sensor response toward 50 ppm of NH3, SO2, NO2 and ethanol vapor was 1.27, 1.47, 0.65 and 0.94, suggestive of excellent H2S-sensing selectivity against NH3, SO2, NO2 and ethanol vapor at room temperature. The presence of Cu which was introduced in the sensor devices via the ligand exchange may contribute to the H2S selectivity because of its well-known specificity toward H2S gas by acting as a 32,37 catalyst or forming heterojunctions. The exact mechanism of the selective H2S sensing still needed further study. In sum, sensitive and selective room-temperature H2S gas sensors were achieved through the controllable synthesis of colloidal SnO2 quantum wire/reduced graphene oxide nanocomposites. Their lower detection of limit, reduced power consumption, as well as the ease of fabrication offered potentials for practical use necessitated by the advent of internet of things. 3.3. H2S- sensing mechanism Owing to the room-temperature sensor fabrication, we were able to correlate the sensor performance with the materials properties when discussing the sensing mechanism. We examined the H2Ssensing properties of pristine rGO and SnO2 quantum wires (8 h) (Figure 5a); the response curve of the SnO2/rGO (8 h) device sample was replotted here for a clear comparison. It was observed o that at 22 C, the pristine rGO had no response toward 50 ppm of H2S; the pristine SnO2 quantum wires had a response of 20.5, which was lower compared to the SnO2/rGO nanocomposites. According to the comparison study on the concentration of rGO in the nanocomposites (Figure S5 in Supporting Information), the initial resistance of the SnO2/rGO device decreased as the amount of rGO increased, but the maximum response was observed at a moderate amount of rGO. The results suggested that for the gas sensors employing SnO2 quantum wire/rGO nanocomposites, the


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SnO2 quantum wires behaved as the key sensing materials; their ultrathin one-dimensional microstructure was helpful for increasing surface area and favoring gas adsorption. Meanwhile, the SnO2 quantum wires in the SnO2/rGO nanocomposites (8 h) actually had similar structure and morphology as those of the pristine SnO2 quantum wires according to the experimental results from HRTEM, XRD and UV-vis absorption characterizations. Therefore, we proposed that there was a synergetic effect of the SnO2/rGO nanocomposites that accounts for the performance enhancement of SnO2 quantum wire/rGO nanocomposites over the pristine SnO2 or rGO counterparts.

a)

b)

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tween SnO2 surface and rGO was further indicated by the XPS observation (Figure S7 in Supporting Information) that the binding energies (BEs) of Sn3d5/2 and O1s in the SnO2/rGO nanocomposites were lower compared to the pristine SnO2. In the presence of H2S gas, the H2S molecules adsorbed on the SnO2 quantum wires may form donor-like surface states due to the chemisorption, thereby inducing electron transfer from H2S to the n-type SnO2 and leading to the resistance decrease, shown as the sensor response. For the SnO2 quantum wire/rGO nanocomposites, the electron flow across the H2S/SnO2 interfaces to the electrodes for collection was enhanced by the favorable electron transfer from SnO2 quantum wires to rGO nanosheets (Figure 5c); the excellent transport capability of rGO contributed to the enhanced electron transfer as well. In addition, the network of SnO2 quantum wires in the nanocomposites incorporating rGO nanosheets was probably looser compared to the pristine SnO2 (Figure S8 in Supporting Information), which thereby guaranteed excellent adsorption of H2S molecules. Overall, both the chemical and electronic interactions between SnO2 quantum wires and H2S molecules were enhanced due to the presence of rGO. Therefore, the one-step colloidal synthesis opens a facile pathway to the construction of high-performance gas sensors employing metal oxide/graphene nanocomposites. 4. Conclusions

c)

Figure 5 a) Response curves of gas sensors based on pristine rGO, SnO2 quantum wires (8 h) and SnO2/rGO nanocomposites (8 h), b) PL spectra of pristine SnO2 quantum wires (8 h) and SnO2 quantum wire/rGO nanocomposites (8 h), c) Schematic illustration of H2Ssensing mechanism of gas sensors employing SnO2 quantum wire/rGO nanocomposites.

Based on PL spectra of the SnO2 quantum wire/rGO nanocomposites and the pristine SnO2 (Figure 5b), we proposed the key mechanism underlying the synergetic effect of the SnO2 quantum wire/rGO nanocomposites to be the favorable electron transfer across SnO2/rGO interfaces. Compared to the pristine SnO2, the SnO2/rGO nanocomposites exhibited a slight blue shift of emission peak with much lower intensity. This observation suggested that the rGO served as electron acceptors for the SnO2 quantum wires distributed on them, thereby resulting in the PL quenching of SnO2 quantum wires by reducing the radiative recombination. The result was consistent with the fact that the work function of rGO nanosheets was larger than that of SnO2 quantum wires (Figure S6 in Supporting Information). The electronic interaction be-

We demonstrated the sensitive and selective H2S gas sensors employing the SnO2 quantum wire/reduced graphene oxide nanocomposites. Both the sensor fabrication and their operating were conducted at room temperature. The good distribution and adhesion of SnO2 quantum wires on rGO nanosheets were achieved through the one-step synthesis in a simple solution system. We observed morphology-related quantum confinement evolution of crystallized SnO2, which could be well controlled by tuning the reaction time with the aid of the steric hindrance of rGO in the one-step synthesis. Our systematical comparison study suggested that the excellent gas adsorption of SnO2 quantum wires and enhanced electron transport in the whole sensor, realized by the favorable electron transfer across SnO2/rGO interfaces and the superb transport capability of rGO were the key parameters that accounts for the superb room-temperature H2S sensing performance.

ASSOCIATED CONTENT SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: ××××××××. SnO2 diameter distributions (Figure S1); additional experimental data about the Cu(NO3)2 treatment (Figure S2); dependence of response time on the H2S gas concentration (Figure S3); response curve at LOD (Figure S4); additional experimental data about the concentration of rGO (Figure S5); work functions measured by a Kelvin Probe (Figure S6); binding energy analysis from XPS measurement (Figure S7), and SEM images of the gas sensors (Figure S8) (PDF)

AUTHOR INFORMATION CORRESPONDING AUTHOR


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Chemistry of Materials enhanced NO2 sensing performance at room temperature. ACS Appl.

*Huan Liu. Email: [email protected]..

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Mater. Interfaces 2014, 2014 6, 21093-21100.

Author Contributions The paper was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Fundings Research described in this paper was supported by the National Natural Science Foundation of China (61571206 and 51572096). H. L. acknowledges the Program for New Century Excellent Talents in University (NCET-12-0216).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Analytical and Testing Center of HUST and the Center for Nanoscale Characterization & Devices of WNLO for the characterization support.

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SnO2 quantum wires


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Sensitive Room-Temperature H2S Gas Sensors ... - ACS Publications

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