Au Nanocomposite for

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Jul 22, 2016 - developed to reduce the operating temperature and improve the sensitivity and stability of gas sensors [11–18]. Graphene, known as “the ...
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Reduced Graphene Oxide/Au Nanocomposite for NO2 Sensing at Low Operating Temperature Hao Zhang 1,2 , Qun Li 3 , Jinyu Huang 3 , Yu Du 3, * and Shuang Chen Ruan 1, * 1 2 3

*

Shenzhen Key Laboratory of Laser Engineering, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China Shenzhen Key Laboratory of Sensor Technology, College of Physics Science and Technology, Shenzhen University, Shenzhen 518060, China; [email protected] (Q.L.); [email protected] (J.H.) Correspondence: [email protected] (Y.D.); [email protected] (S.C.R.); Tel.: +86-755-2653-8886 (S.C.R.)

Academic Editors: Eduard Llobet and Stella Vallejos Received: 30 June 2016; Accepted: 21 July 2016; Published: 22 July 2016

Abstract: A reduced grapheme oxide (rGO)/Au hybrid nanocomposite has been synthesized by hydrothermal treatment using graphite and HAuCl4 as the precursors. Characterization, including X-ray diffraction (XRD), Raman spectra, X-ray photoelecton spectroscopy (XPS) and transmission electron microscopy (TEM), indicates the formation of rGO/Au. A gas sensor fabricated with rGO/Au nanocomposite was applied for NO2 detection at 50 ˝ C. Compared with pure rGO, rGO/Au nanocomposite exhibits higher sensitivity, a more rapid response–recovery process and excellent reproducibility. Keywords: graphene; nanocomposite; NO2 sensing; low operating temperature

1. Introduction Since air pollution has become an urgent global problem with the development of industry and technology, detecting gases, especially toxic gases, as the basis for controlling air pollution, has become increasingly significant. NO2 is a toxic compound produced by combustion in power plants and combustion engines. This gas is harmful to the environment and is a major cause of acid rain, photochemical smog and pollution haze. Up to now, metal oxides (MOS) semiconductor sensors have been widely used in NO2 sensing. In most cases, the operating temperature of MOS sensors is over 200 ˝ C, which creates obstacles for fabricating integrated circuits and increases the energy consumption. Recently, a range of techniques such as surface functionalization with novel metals including Pd, Pt and Au or doping with novel metals [1–7], MEMS fabrication [8], nano-sensing materials [9], application of electrostatic fields [10], and ultraviolet (UV) irradiation have been developed to reduce the operating temperature and improve the sensitivity and stability of gas sensors [11–18]. Graphene, known as “the thinnest material in our universe” with only one-atom thickness, has attracted huge attention for its high electron mobility since its discovery. Because of its unique features of high surface area, light weight, high electron mobility and mechanical strength, graphene represents a very promising platform to load metal or semiconductor nanoparticles, organic and biological molecules for photocatalytic, optoelectronic, cellular imaging, and biosensor applications [19–22]. Due to its 2D structure, graphene can adsorb highly sensitive molecules which treat every carbon atom as a surface atom. Among the different methods to prepare graphene, chemical and thermally reduced graphene oxide (rGO) derived from the graphene oxide (GO) process based on Hummers’ method is mostly used [23]. Due to electrostatic repulsion of the versatile oxygen-containing groups (OCGs), Sensors 2016, 16, 1152; doi:10.3390/s16071152

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an aqueous colloidal dispersion of GO can be used as the starting support material for fabricating advanced graphene-based nanomaterials [24]. Reduced graphene oxide is expected to be a promising sensing material due to its semiconductor properties. To date, although rGO has a relatively weak response compared to SnS2 with similar 2D structure [25], it could detect gases at low operating temperatures, even room temperature. In this work, a rGO/Au nanocomposite was synthesized through a facile one-step hydrothermal method. Sensors based on the rGO/Au and rGO were also fabricated, and their NO2 sensing performance was investigated. The rGO/Au exhibited p-type semiconductor behavior in the gas sensing process. Investigations on the gas sensors showed that sensors based on rGO/Au composites exhibited shorter response and recovery times compared with those of pure rGO at low operating temperature. Furthermore, a possible sensing mechanism for the detection of NO2 is also discussed. 2. Materials and Methods 2.1. Chemicals All chemicals were of analytical grade and were used as received without further purification. Graphite and HAuCl4 , were supplied by Beijing Chemical Corp, Ltd. (Beijing, China). The water used throughout all experiments was purified through a Millipore system (Millipore, Bedford, MA, USA). 2.2. Preparation of GO GO was prepared from natural graphite powder through a modified Hummers’ method [26]. In a typical synthesis, 1 g of graphite was added into 23 mL of H2 SO4 , followed by stirring at room temperature for 24 h. After that, 100 mg of NaNO3 was introduced into the mixture and stirred for 30 min. Subsequently, the mixture was kept below 5 ˝ C by ice bath, and 3 g of KMnO4 was slowly added into the mixture. After being heated to 35–40 ˝ C, the mixture was stirred for another 30 min. 46 mL of water was then added into above mixture during a period of 25 min and the mixture was heated to 95 ˝ C under stirring for 15 min. Finally, 140 mL of water and 10 mL of H2 O2 were added into the mixture to stop the reaction. After the unexploited graphite in the resulting mixture was removed by centrifugation, as-synthesized GO was dispersed into individual sheets in distilled water at a concentration of 1 mg/mL with the aid of ultrasound for further use. 2.3. Preparation of rGO/Au Nanocomposite rGO/Au composite was prepared by in situ production of Au nanoparticles on the surface of GO. In a typical synthesis, 0.5 mL of GO (1 mg/mL) were added into 20 mL of deionized water, followed by stirring for 10 min to get homogeneous yellow-brown colloidal. Then HAuCl4 (0.01 M) and sodium citrate was introduced into the GO solution, which was sonicated for 40 min. After that, further sonicating for 30 min, the aqueous dispersion was transferred into a 40 mL Teflon-lined, stainless-steel autoclave and heated at 180 ˝ C for 12 h. The black product was harvested by centrifugation and washed with water and ethanol several times, and dried at 60˝ C for 12 h. For comparison, the rGO was prepared by the similar method without addition of HAuCl4 . 2.4. Characterizations Powder X-ray diffraction (XRD) data were recorded on a D/Max-2550 diffractometer (Rigaku, Tokyo, Japan) with Cu-Kα radiation (λ = 0.15418 nm). The transmission electron microscopic (TEM) images were performed on a JEM-3010 TEM microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of colloidal solution on carbon-coated copper grid and dried at room temperature. X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALABMK IIX-ray photoelectron spectrometer using Mg as the mexciting source. Raman spectra were obtained on a J-YT64000 Raman spectrometer with 514.5 nm wavelength incident laser light.

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2.5.Fabrication Fabricationand andGas GasSensing SensingMeasurements Measurements 2.5. Fabrication and Gas Sensing Measurements 2.5. The prepared prepared rGO/Au rGO/Au nanocomposite nanocomposite was mixed mixedin inaa mortar mortar with with deionized deionizedwater water to to obtain obtain The prepared rGO/Au nanocompositewas mortar with deionized water to obtain The solution. Then Then the the droplet droplet has has placed placed on on aaa ceramic ceramic plate plate (1 (1 mm mmˆ 1.5 mm), mm), which which was was previously previously solution. Then the droplet has placed on ceramic 1.5 mm), which was previously solution. ×× 1.5 covered with gold electrodes and ruthenium oxides as heater on frontal and back sides by screen screen covered with with gold gold electrodes electrodes and and ruthenium ruthenium oxides oxides as as heater heater on on frontal frontal and and back back sides sides by by screen covered printing technique, followed by dryness at room temperature. Figure 1 shows a schematic illustration room temperature. temperature. Figure Figure 11 shows shows aa schematic schematic illustration illustration printing technique, followed by dryness at room ofthe thesensor sensorcoated coatedwith withthe thesensing sensingmaterial. material.The Theresponse responseof ofthe thegas gassensor sensoris isdefined definedas asthe theratio ratio of the sensor coated with the sensing material. The response of the gas sensor is defined as the ratio of of the resistance of the sensor in air (R a ) to that in the tested gases (R g ). For oxidizing tested gases, the of the the resistance resistanceof ofthe thesensor sensorininair air(R(R ) to that tested gases oxidizing tested gases, of a) ato that in in thethe tested gases (Rg(R ). For oxidizing tested gases, the g ). For expression Response a/R forthe thefor reducing testedgases, gases, Response /Ra.a.The time the expression is Response =gg,R,while /Rg , for while the reducing testedititgases, it is Response =The Rgtime /R expression isisResponse ==RRa/R reducing tested isisResponse ==RRgg/R awhile a. taken by the sensor to achieve 90% of the total resistance change was defined as the response time in The time taken by the sensor to achieve 90% of the total resistance change was defined as the response taken by the sensor to achieve 90% of the total resistance change was defined as the response time in thecase case ofcase adsorption orthe theor recovery timein inthe the case ofdesorption. desorption. TheThe gas-sensing properties of time in the of adsorption the recovery time incase the case of desorption. gas-sensing properties the of adsorption or recovery time of The gas-sensing properties of sensors were measured using CGS-8 gas-sensing characterization system. of sensors were measured using a CGS-8 gas-sensing characterization system. sensors were measured using aaCGS-8 gas-sensing characterization system.

Figure1. 1.A Aschematic schematicillustration illustrationof ofthe thesensor sensorcoated coatedwith withthe thesensing sensingmaterial. material. Figure Figure 1. A schematic illustration of the sensor coated with the sensing material.

3. Results 3. 3. Results Results 3.1.Structural Structuraland andMorphological MorphologicalCharacteristics Characteristics 3.1. 3.1. Structural and Morphological Characteristics Thephase phasecomposition composition ofof the hybrid structures analyzed byXRD. XRD. strong peakat at2θ 2θof of 11.26° The the hybrid structures isisanalyzed by AAstrong peak The phase compositionof the hybrid structures is analyzed by XRD. A strong peak at11.26° 2θ of ˝ corresponding to the (002) interlayer d spacing of 7.85 Å is observed in Figure 2, indicating the corresponding to the (002) d spacing of 7.85ofÅ7.85 is observed in Figure 2, indicating the 11.26 corresponding to theinterlayer (002) interlayer d spacing Å is observed in Figure 2, indicating successful preparation of GO by oxidation of graphite [27]. However, no diffraction peaks can be successful preparation of GO by by oxidation of of graphite [27]. the successful preparation of GO oxidation graphite [27].However, However,no nodiffraction diffractionpeaks peakscan can be be ascribedto tographene, graphene,probably probablydue dueto toself-reassembling self-reassemblingof ofexfoliated exfoliatedgraphite graphiteoxide oxideor orthe thereduction reduction ascribed ascribed to graphene, probably due to self-reassembling of exfoliated graphite oxide or the reduction of GO by hydrothermal treatment [28,29]. Furthermore, several peaks are observed at 43.1°, 44°,˝64.2°, of [28,29]. Furthermore, several peaks are observed at 43.1°, 44°, of GO GOby byhydrothermal hydrothermaltreatment treatment [28,29]. Furthermore, several peaks are observed at 43.1 ,64.2°, 44˝ , ˝ ˝ ˝ 70.1°, 77.2° which are attributed to Au nanoparticles (JCPDS card No. 01-1172) indicating the 70.1°, 77.2° , which are attributed to Au nanoparticles (JCPDS card 64.2 , 70.1 77.2 which are attributed to Au nanoparticles (JCPDS cardNo. No.01-1172) 01-1172)indicating indicating the the formationof ofAu Aucrystals crystalsin inthe thecomposite compositematerial. material. formation formation of Au

Figure2. TheXRD XRDpatterns patternsof ofGO GO(black (blackline) line)and andrGO/Au rGO/Au(red (redline). line). Figure rGO/Au Figure 2.2.The The XRD patterns of GO (black line) and (red line).

Figure33shows showsthe theRaman Ramanspectra spectraconfirming confirmingthe thereduction reductionof ofGO. GO.Two Twostrong strongpeaks peaksof ofthe theDD Figure −1 −1 band(~1351 (~1351cm cm−1))and andGGband band(~1595 (~1595cm cm−1))were wereobserved, observed,which whichcorrespond correspondto tothe thediamondoid diamondoidand and band graphitic graphene structures, respectively. It is well known that the intensity ratio of the D and graphitic graphene structures, respectively. It is well known that the intensity ratio of the D and GG

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Figure 3 shows the Raman spectra confirming the reduction of GO. Two strong peaks of the D ´1 ´1 band (~1351 Sensors 2016, 16, cm 1152 ) and G band (~1595 cm ) were observed, which correspond to the diamondoid 4 of 9 Sensors 2016, 16, 1152 of 9 and graphitic graphene structures, respectively. It is well known that the intensity ratio of the D 4and

band (ID/IG) is strongly related to to the G band (ID/IG) is strongly related thequantity quantityofoffunctional functionalgroups groupsofofrGO, rGO,and and compared compared with band (ID/IG) strongly to the value quantity of functional groupsrestoration of rGO, and compared with ID/IG value (0.796), the (1.131) of C=C bonds ID/IG valueof ofisGO GO (0.796),related theincreased increased value of of rGO rGO (1.131) suggests suggests restoration C=C bonds after after ID/IG value ofreduction GO (0.796), the increased value of rGO (1.131) suggests restoration of C=C bonds after hydrothermal [30]. hydrothermal reduction [30].

Figure 3. Raman spectroscopy of the GO (black line) and rGO/Au (red line) samples. Figure 3. Raman spectroscopy of the GO (black line) and rGO/Au (red line) samples. Figure 3. Raman spectroscopy of the GO (black line) and rGO/Au (red line) samples.

The typical TEM image of rGO/Au is shown in Figure 4. It is clearly seen that the rGO has been The TEM image is shown shown in in Figure Figure 4. It It is is clearly clearly seen seen that that the the rGO rGO has The typical typical image of ofofrGO/Au rGO/Au is has been been decorated with aTEM few amount Au nanoparticles in Figure4.4a. Almost all the Au nanoparticles are decorated with a few amount of Au nanoparticles in Figure 4a. Almost all the Au nanoparticles are decorated with a few amount of Au nanoparticles in Figure 4a. Almost all the Au nanoparticles are distributed uniformly on the rGO surface and have a size of about 10 nm (Figure 4b). The presence distributed uniformly on on the the rGO surface and have aa size of about 10 nm (Figure 4b). The presence distributed uniformly rGO surface and have size of about 10 nm (Figure 4b). The presence of isolated Au nanoparticles reveals that hydrothermal treatment of GO and HAuCl4 solution is an of Au reveals that hydrothermal treatment of isolated isolated Au nanoparticles nanoparticles reveals of that hydrothermal treatment of of GO GO and and HAuCl HAuCl44 solution solution is is an an effective method for the preparation rGO/Au nanocomposite. effective nanocomposite. effective method method for for the the preparation preparation of of rGO/Au rGO/Au nanocomposite.

Figure 4. (a) A TEM images of rGO/Au; (b) An enlarged image of selected area. Figure 4. (a) A TEM images of rGO/Au; (b) An enlarged image of selected area. Figure 4. (a) A TEM images of rGO/Au; (b) An enlarged image of selected area.

To further investigate the characteristics of the products, XPS technique was used to analyze the To further the characteristics of the5aproducts, XPS technique used to analyze the chemical state ofinvestigate GO and rGO/Au samples. Figure shows the XPS spectrumwas of rGO-Au composite. To further investigate the characteristics of the products, XPS technique was used to analyze the chemical state of GO and rGO/Au samples. Figure 5a shows the XPS spectrum of rGO-Au composite. Two peaks at about 284.6 eV and 532.0 eV are observed, which are attributed to C1s and O1s bands, chemical state of GO and rGO/Au samples. Figure 5a shows the XPS spectrum of rGO-Au composite. Two peaks atNote aboutthat 284.6 eVsignificant and 532.0 signals eV are observed, which are attributed to C1s O1s eV bands, respectively. two at 83.3 and 86.9 eV, and a miner peakand at 85.3 are Two peaks at about 284.6 eV and 532.0 eV are observed, which are attributed to C1s and O1s bands, respectively. Note that two significant signals at 83.3 and 86.9 eV, and a miner peak at 85.3 eV are exhibited corresponding to metallic Au and Au(I), respectively (Figure 5b), which further confirms respectively. Note that two significant signals at 83.3 and 86.9 eV, and a miner peak at 85.3 eV are exhibited corresponding to metallic Au and Au(I), respectively (Figure 5b), which further confirms the presence of Au element in the final samples. The intensity of the Au4f signal is higher than that exhibited corresponding to metallic Au and Au(I), respectively (Figure 5b), which further confirms the theAu(I), presence of Au that element samples. Theand intensity themetallic Au4f signal higher than that of indicating mostin ofthe Aufinal is of zero valence exits inofthe form.isFigure 5c,d reveal presence of Au element in the final samples. The intensity of the Au4f signal is higher than that of Au(I), of Au(I), indicating that most of Au is of zero valence and exits in the metallic form. Figure 5c,d reveal the C1s spectra of GO and rGO/Au samples, exhibiting three peaks at 284.6, 286.6 and 288.4 eV, indicating that most of Au is of zero valence and exits in the metallic form. Figure 5c,d reveal the C1s the C1s spectra GOC–O andand rGO/Au samples, exhibiting threematerials peaks at[31]. 284.6, 286.6 and 288.4that eV, attributed to the of C–C, C=O bands in graphene-based It is worth noting spectra of GO and rGO/Au samples, exhibiting three peaks at 284.6, 286.6 and 288.4 eV, attributed to attributed to to the and C=O bands graphene-based [31]. It is worth noting that compared the C–C, peak C–O intensity of C–O andinC=O in GO, those materials of rGO–Au decrease tremendously, the C–C, C–O and C=O bands in graphene-based materials [31]. It is worth noting that compared to the compared to the peak intensity of C–O and C=O in GO, those of rGO–Au decrease tremendously, which suggests that most oxygen-containing functional groups were successfully removed after which suggeststreatment. that mostAll oxygen-containing functional groups were successfully removed hydrothermal these observations indicate the successful formation of Au after and hydrothermal treatment. All these observations indicate the successful formation of Au and reduction of GO by hydrothermal reaction of GO and HAuCl4. reduction of GO by hydrothermal reaction of GO and HAuCl4.

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peak intensity of C–O and C=O in GO, those of rGO–Au decrease tremendously, which suggests that most oxygen-containing functional groups were successfully removed after hydrothermal treatment. All these observations indicate the successful formation of Au and reduction of GO by hydrothermal Sensors 2016, 2016, 16, 1152 of 99 reaction of16, GO and HAuCl4 . Sensors 1152 55 of

Figure 5. 5. (a) (a) XPS XPS spectras spectras of of rGO/Au; rGO/Au; (b) (b) Au4f spectrum of of rGO/Au; rGO/Au; (c) (c) C1s C1s spectrum spectrum of (d) C1s C1s Figure (a) XPS spectras Au4f spectrum spectrum of rGO/Au; of GO; GO; (d) (d) C1s Figure 5. of rGO/Au; spectrum of rGO/Au. spectrum of rGO/Au. spectrum of rGO/Au.

3.2. NO NO222 Sensing Sensing Properties Sensing Properties Properties 3.2. Figure 666 shows shows the response and recovery times ofof sensors based onon thethe rGO andand rGO/Au to 55 shows the theresponse responseand andrecovery recoverytimes times sensors based rGO rGO/Au Figure of sensors based on the rGO and rGO/Au to ppm NO2NO 2,, respectively. respectively. TheThe response time, value (recovery time, value) of rGO/Au sensors sensors to 5 ppm response time, Tr1 value(recovery (recoverytime, time,TTTr2r2r2value) value) of of rGO/Au rGO/Au ppm NO The response time, TTr1r1 value 2 , respectively. ˝ C (Figure upon exposure exposure to to 55 ppm ppm NO NO222 gas gas is 132 (386 s) at low operating temperature of 50 °C 6b). upon gas is is 132 132 sss (386 (386 s) s) at at aaa low low operating operating temperature temperature of of 50 50 °C (Figure 6b). (Figure 6b). Compared with that, the pristine rGO sensor exhibits a longer T r1 (T r2 ) of 798 s (7312 s) in Figure 6a. (Tr2r2))of of798 798ss(7312 (7312s) s) in in Figure Figure 6a. 6a. Compared with that, the pristine rGO sensor exhibits a longer Tr1 r1 (T It is is concluded concluded that that loading loading Au Au nanoparticles nanoparticles is is an an effective effective method method for for shortening shorteningresponse/recovery response/recovery It shortening time (T (Tr1 r1/T /T graphene-based gas sensors at low operating temperature. Additionally, It is is found found /Tr2r2r2 )for forgraphene-based graphene-basedgas gassensors sensorsat ataaalow lowoperating operatingtemperature. temperature.Additionally, Additionally, It time r1 )) for that rGO/Au sensor exhibits remarkably enhanced response of 1.33 to 10 ppm NO 2 , which has been sensor exhibits exhibits remarkably remarkably enhanced enhanced response response of 1.33 to 10 ppm NO22, which has been that rGO/Au rGO/Au sensor improved up up from from 1.13 1.13 from from pure puregraphene graphenesensor. sensor. improved pure graphene sensor.

Figure 6. 6. The The response response curve curve to to 5 ppm ppm NO NO2 of of the the sensors sensors based based on on (a) (a) rGO; rGO; (b) (b) rGO/Au rGO/Au at at 50 °C. °C. Figure Figure 6. The response curve to 55 ppm NO22 of the sensors based on (a) rGO; (b) rGO/Au at 50 50 ˝ C.

Figure 7a 7a shows shows the the response response of of the the rGO/Au rGO/Au based based sensor sensor being being orderly orderly exposed exposed to to 0.5–5 0.5–5 ppm ppm Figure NO 2 gases at a low operating temperature of 50 °C. It is shown that the response and recovery NO2 gases at a low operating temperature of 50 °C. It is shown that the response and recovery characteristics are are in in agreement agreement with with the the above above analysis analysis of of Figure Figure 6b. 6b. Moreover, Moreover, itit is is also also found found that that characteristics the response response gradually gradually rises rises as as the the concentration concentration of of NO NO22 increases, increases, showing showing aa range range of of linear linear response response the (Figure 7b). The response values of the sensor to 0.5, 1, 2 and 5 ppm NO 2 gas are about 1.05, 1.12, 1.19 (Figure 7b). The response values of the sensor to 0.5, 1, 2 and 5 ppm NO2 gas are about 1.05, 1.12, 1.19 and 1.33, respectively. and 1.33, respectively.

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Figure 7a shows the response of the rGO/Au based sensor being orderly exposed to 0.5–5 ppm NO2 gases at a low operating temperature of 50 ˝ C. It is shown that the response and recovery characteristics are in agreement with the above analysis of Figure 6b. Moreover, it is also found that the response gradually rises as the concentration of NO2 increases, showing a range of linear response (Figure 7b). The response values of the sensor to 0.5, 1, 2 and 5 ppm NO2 gas are about 1.05, 1.12, 1.19 and 1.33, respectively. Sensors 2016, 16, 1152 6 of 9 Sensors 2016, 16, 1152 6 of 9

Figure 7. 7. (a) (a) Dynamic Dynamic NO NO22 sensing sensing transients curve of the rGO/Au-based sensor toto 0.5–5 ppm NO 2 at Figure NO transientscurve curveof ofthe therGO/Au-based rGO/Au-based sensor 0.5–5 ppm NO sensor to 0.5–5 ppm NO 2 at 2 sensingtransients 2 ˝ 2 at 50 °C.˝ 50 °C; (b) The responses of the rGO/Au based sensor to 0.5–5 ppm NO at The responses rGO/Au based sensor to 0.5–5 ppm at °C. 50 C. 2 at250 50 50 °C; C; (b)(b) The responses of of thethe rGO/Au based sensor to 0.5–5 ppm NONO

The selectivity selectivity of the the gas sensor sensor is also important important for practical practical application. application. Figure Figure 88 shows shows the the The The selectivity of of thegas gas sensorisisalso also importantforfor practical application. Figure 8 shows responses of of rGO/Au rGO/Au based based sensor sensor to to potential potential interference interference gases gases (Cl (Cl22,, H H22,, NO, NO, CH CH4, CO). Take Take 5 ppm responses the responses of rGO/Au based sensor to potential interference gases (Cl2 , H42, ,CO). NO, CH45, ppm CO). target gases for example, the sensor only gives a relatively lower response to interference gases, target gases for example, the sensor only gives a relatively lower response to interference gases, Take 5 ppm target gases for example, the sensor only gives a relatively lower response to interference indicating the the sensor sensor has has aa good good selectivity. selectivity. indicating gases, indicating the sensor has a good selectivity.

Figure 8. 8. The The responses responses of of rGO/Au rGO/Au based sensor sensor to 55 ppm ppm of different different gases at at 50 50 ˝°C. °C. Figure Figure 8. The responses of rGO/Au based based sensor to to 5 ppm of of different gases gases at 50 C.

The further further test test for for repeatability repeatability of of the the sensor sensor based based on on rGO/Au rGO/Au is is illustrated illustrated in in Figure Figure 9. 9. It It is is The The further test for repeatability of the sensor based on rGO/Au is illustrated in Figure 9. It is revealed that that the the sensor sensor maintains maintains its its initial initial response response amplitude amplitude without without aa clear clear decrease decrease upon upon three three revealed revealed that the sensor maintains itsNO initial response amplitude without a clear decrease upon three successive sensing tests to 5 ppm of 2, albeit the swift response and recovery process, indicating successive sensing tests to 5 ppm of NO2, albeit the swift response and recovery process, indicating successive sensing tests to 5 ppm ofrepeatability NO2 , albeit throughout the swift response and recovery process, indicating that the the sensor sensor has an an outstanding outstanding the cycle cycle test. that has repeatability throughout the test. that the sensor has an outstanding repeatability throughout the cycle test.

Figure 8. The responses of rGO/Au based sensor to 5 ppm of different gases at 50 °C.

The further test for repeatability of the sensor based on rGO/Au is illustrated in Figure 9. It is revealed that the sensor maintains its initial response amplitude without a clear decrease upon three successive sensing tests to 5 ppm of NO2, albeit the swift response and recovery process, indicating Sensors 2016, 16, 1152 7 of 9 that the sensor has an outstanding repeatability throughout the cycle test.

Figure 9. The reproducibility of the rGO/Au sensor on successive exposure (3 cycles) to 5 ppm NO2 Figure The reproducibility of the rGO/Au sensor on successive exposure (3 cycles) to 5 ppm NO2 at 509.°C. Sensors 7 of 9 ˝ C.16, 1152 at 502016,

4. Gas Sensing Mechanism of rGO/Au 4. Gas Sensing Mechanism of rGO/Au It is well shown that the pure rGO exhibits a relatively weak response and long response and It is well shown that the pure rGO exhibits a relatively weak response and long response and recovery times for detection of NO2 at 50 ˝°C. The unsatisfactory sensitivity of the rGO sensor results recovery for detection NO2There at 50 are C. two The types unsatisfactory of the rGO results from itstimes constituent carbon of atoms. of carbon sensitivity atoms in graphene, sp2 sensor hybridized 2 hybridized from its constituent carbon atoms. There are two types of carbon atoms in graphene, sp carbon atoms constituting the graphite structure, and sp3 hybridized carbon atoms constituting carbon atoms constituting the graphite structure, andoxygen-containing sp3 hybridized carbon constituting structural defects and forming a chemical bond with groups.atoms The adsorption structural defects and forming a chemical bond with oxygen-containing groups. The adsorption energy of the latter is larger (5.7 kcal/mol), resulting in slower adsorption and desorption [32,33]. energyThe of the latter is larger (5.7 kcal/mol), resulting in slower adsorption and desorption [32,33]. improvement of NO2 gas sensing properties of rGO nanosheets by Au-functionalization can improvement of NO sensing properties of rGOfor nanosheets by Au-functionalization be The explained as follows: firstly, the principle of gas sensing the resistance-type sensors is basedcan 2 gas beon explained as follows: firstly,ofthe of gas sensing the resistance-type sensors is based the conductance variations theprinciple sensing element, thus thefor introduction of Au to rGO contributes to improving theof conductivity, tothus a better behavior. Secondly, on onsignificantly the conductance variations the sensing leading element, the sensing introduction of Au to rGO based contributes the model to proposed for the catalyst-enhanced sensing of nanomaterials [34], thebased NO2 on gasthe significantly improving the metal conductivity, leading to agas better sensing behavior. Secondly, was proposed spilt overfor thetherGO nanosheet surface bygas Ausensing nanoparticles, and the [34], chemisorption and model metal catalyst-enhanced of nanomaterials the NO2 gas was dissociation of NO 2 gas was enhanced on the Au nanoparticle surface due to the high catalytic or spilt over the rGO nanosheet surface by Au nanoparticles, and the chemisorption and dissociation of conductive nature of Au. Consequently, the number of electrons attracted to the gas increases. NO 2 gas was enhanced on the Au nanoparticle surface due to the high catalytic or conductive nature of Thirdly, the electron transfer of from the defect statesto tothe thegas Auincreases. nanoparticles not only results in an Au. Consequently, the number electrons attracted Thirdly, the electron transfer increase in resonant density, but also creates energetic electrons in high energy state [35,36]. from the defect stateselectron to the Au nanoparticles not only results in an increase in resonant electron These resonant electrons are so active that they can escape from the surface of Au nanoparticles to density, but also creates energetic electrons in high energy state [35,36]. These resonant electrons are so the NO2. As shown in Figure 10, the role of Au as an electron mediator further facilitates the electron active that they can escape from the surface of Au nanoparticles to the NO2 . As shown in Figure 10, transfer from rGO to NO2 molecules. Therefore, the electron density rGO decreased significantly by the role of Au as an electron mediator further facilitates the electron transfer from rGO to NO2 Au-functionalization. molecules. Therefore, the electron density rGO decreased significantly by Au-functionalization.

Figure 10. The scheme of the proposed gas sensing mechanism: the adsorption behavior of NO2 Figure 10. The scheme of the proposed gas sensing mechanism: the adsorption behavior of NO2 molecules ononthe molecules therGO/Au rGO/Au nanocomposite. nanocomposite.

5. Conclusions The synthesis of well controlled rGO/Au composites by hydrothermal treatment is demonstrated. The gas sensing results showed that the rGO/Au sensors could detect NO2 gas at levels as low as 0.5 ppm. The sensitivity of rGO/Au to 5 ppm NO2 (1.33) is higher than that of rGO (1.13).

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5. Conclusions The synthesis of well controlled rGO/Au composites by hydrothermal treatment is demonstrated. The gas sensing results showed that the rGO/Au sensors could detect NO2 gas at levels as low as 0.5 ppm. The sensitivity of rGO/Au to 5 ppm NO2 (1.33) is higher than that of rGO (1.13). Moreover, the response (recovery) time towards 5 ppm NO2 improved from 798 s (7312 s) to 132 s (386 s) via the introduction of the Au nanoparticles. All results indicate that Au nanoparticle loading can significantly enhance the NO2 sensing properties of graphene-based sensing materials at a low operating temperature, which indicates excellent potential applications as gas sensors. Acknowledgments: This research work was financially supported by National Natural Science Foundation of China (Grant No. 61575129, 61275144), Guangdong Natural Science Foundation (2016A030313059), State Key Laboratory of Inorganic Synthesis and Preparative Chemistry Open Project (2015-10), and Natural Science Foundation of SZU (82700002601). Author Contributions: Hao Zhang proposed the idea, synthesized the materials, processed data and wrote the paper. Qun, Li and Jinyu Huang are Responsible for test data.Yu Du and Shuangchen Ruan directed the research as the principal investigator (PI) of the project. Conflicts of Interest: The authors declare no conflict of interest.

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