Reduced graphite oxide/SnO₂/Au hybrid nanomaterials for NO₂ sensing performance at relatively low operating temperature

Abstract

The reduced graphene oxide/SnO₂/Au (rGO/SnO₂/Au) hybrid nanomaterials have been prepared by a one-step hydrothermal process with a property of gas sensing at a relatively low operating temperature. The structure and morphology characteristics of the resultant product were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The sizes of the Au nanoparticles of rGO/SnO₂/Au-1 to rGO/SnO₂/Au-4 range from about 12 to 90 nm, respectively. To demonstrate the usage of such hybrid nanomaterials, chemical gas sensors have been fabricated and investigated for NO₂ detection. It is found that the rGO/SnO₂/Au sensor exhibits much better response/recovery time which is 19/20 s at the optimal operating temperature of 50 °C to 5 ppm NO₂, compared with the pure rGO (798/8319 s) and rGO/SnO₂ (427/908 s). The enhanced sensing features can be attributed to the heterojunctions with the highly conductive graphene, SnO₂ thin film and Au nanoparticles.

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1. Introduction

NO₂ as a typical air pollutant mainly comes from the exhaust gases of combustion processes, which severely affects the respiratory system of human beings and animals.^1,2 Large amounts of NO₂ have been released to the environment by combustion system and vehicle every year. Therefore, the detection of NO₂ is imperatively necessary for industrial and domestic appliances. Numerous techniques have been developed to detect trace levels of NO₂, such as chemiluminescence,³ ion chromatography,⁴ spectrophotometry,⁵ etc. However, with the growing awareness about environmental pollution and occupational safety issues over the recent decades, how to make high performance sensors for NO₂ has become an important subject.

Graphene, known as “the thinnest material in our universe” with only one-atom thickness,⁶ has attracted huge attention for its high electron mobility since discovery.^7–9 Owing to the 2D structure, graphene can adsorb highly sensitive molecules which treat every carbon atom as a surface atom. Because of its unique features of high surface area, light weight, high electron mobility and mechanical strength, graphene can make a highly promising range of applications such as supercapacitors, nanoelectronics, sensors, hydrogen storage and so forth.^6–13 Graphene oxide (GO) can form well-dispersed aqueous colloids without stabilizers,^14–16 during to electrostatic repulsion of the versatile oxygen-containing groups (OCGs). Also it is of particular interest that the aqueous colloidal dispersion of GO can be used as the starting support material for fabricating advanced graphene-based nanomaterials.¹⁷ Recently, the synthesis of hybrid nanomaterials based on graphene has flashed enormous research interest.^10,17–20 Compared with the graphene, reduced graphene oxide is expected to be a promising sensing materials due to its semiconductor properties.^21–25 To promote the sensitivity and particularly the selectivity of the rGO based sensors, rGO decorated with noble metals and metal oxides have been proposed.²⁶ Chen et al. recently reported a room temperature NO₂ gas sensor using a Co₃O₄ doped-graphene film.²⁷

In this paper, we have fabricated rGO/SnO₂ nanocomposites decorated with Au nanoparticles in high concentration of a GO precursor which was achieved by introducing a certain amount of HAuCl₄ and SnCl₂ into the reaction system. The structural and morphology characteristics have been characterized. The resulting hybrid nanomaterials have been used as gas sensors, which exhibit good performance to NO₂ at operating temperature (50 °C). The effect of the ratio of Au on the sensitivity is investigated. The influences of different operating temperature and gas concentration on the sensitive characteristics have also been investigated. In addition, the sensing mechanism for the detection of NO₂ is also discussed.

2. Experimental

2.1. Chemicals

Graphite powder, H₂O₂ (30%), NaNO₃, H₂SO₄ (98%), SnCl₂, KMnO₄ and HAuCl₄ were purchased from Beijing Chemical Corp. All chemicals were used as received without further purification.

2.2. Preparation of GO

GO was prepared from natural graphite powder through a modified Hummers’ method.²⁸ In a typical synthesis, 1 g of graphite was added into 23 mL of H₂SO₄, 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. After that, 46 mL of water was added into the 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 H₂O₂ 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⁻¹ with the aid of ultrasound for further use.

2.3. Preparation of rGO/SnO₂/Au nanocomposites

The rGO/SnO₂/Au nanocomposites were prepared according to previous report.²⁹ In a typical synthesis, 0.5 ml of GO (1 mg mL⁻¹) were added into 20 ml of water, followed by stirring for 10 min to get a homogeneous yellow-brown colloidal. Then 0.024 g of SnCl₂ and HCl (38%) was introduced into the GO solution, which was sonicated for 40 min, followed by addition of HAuCl₄ (0.01 M). After 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 several hours. Four different rGO/SnO₂/rGO samples with the Au ratio of 0.5%, 1%, 2% and 5% by weight were obtained, designated as rGO/SnO₂/Au-1, rGO/SnO₂/Au-2, rGO/SnO₂/Au-3 and rGO/SnO₂/Au-4, respectively. Similarly, the rGO and rGO/SnO₂ were also prepared.

2.4. Characterizations

The composition and phase of the products were characterized by X-ray powder diffraction (XRD, Rigaku D/max-2500) with graphite monochromatized and Cu Kα λ = 0.15418 nm and X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy, respectively. The structure and morphology of the samples were obtained by transmission electron microscopy (TEM, JEOL JEM-3010).

2.5. Fabrication and measurement of gas sensors

The prepared rGO/SnO₂/Au nanocomposites were mixed with deionized water to obtain a solution in a mortar. Fig. 1a and b shows a schematic image and photographs of this sensor substrate. The droplet has covered a ceramic plate (1 mm × 1.5 mm), which was previously covered with gold electrodes and ruthenium oxides as heater on front and back sides by screen printing technique, followed by drying at room temperature. Fig. 1c shows the schematic diagram of the test circuit. The operating temperature of the rGO/SnO₂/Au sensors was adjusted by varying the heating voltage (V_H). By monitoring V_S, the resistance of the sensor in air or a test gas can be measured.³⁰ The response of the sensor was defined as the ratio of the sensor resistance in dry air (R_a) to that in target gas (R_g), which is measured between 30 and 60 °C. The time taken by the sensor to achieve 90% of the total resistance change was defined as the response time in the case of adsorption or the recovery time in the case of desorption.³¹ The gas-sensing properties of the sensors were measured using a CGS-8 gas-sensing characterization system.

Fig. 1 (a) A schematic illustration of the sensor. (b) Photographs of the sensor substrate. (c) Theoretic diagram of the test circuit. The gas-sensing properties of the samples were measured using a RQ-2 gas-sensing characterization system. The sensitivity of the sensor was defined as the ratio (S = R_a/R_g for the reduce gas or S = R_g/R_a for the oxide gas) of the sensor resistance in dry air (R_a) to that in the testing gas (R_g). The time taken by the sensor to achieve 90% of the total resistance change was defined as the response time in the case of adsorption and the recovery time in the case of desorption.

3. Results and discussions

3.1. Structural and morphological characteristics

The phase composition of the hybrid structures is analyzed by XRD. Fig. 2a shows the XRD patterns of GO. It is seen that a strong peak of 2θ at 11.26° corresponding to the (002) interlayer d spacing of 7.85 Å was observed, indicating the successful preparation of GO by oxidation of graphite.³² However, no diffraction peaks can be ascribed to graphene in Fig. 2a, probably due to self-reassembling of exfoliated graphite oxide or the reduction of GO by hydrothermal treatment.³³ Fig. 2b–f demonstrate the XRD patterns of rGO/SnO₂ and rGO/SnO₂/Au and show four highly broadened peaks, 26.56, 33.82, 51.50 and 65.42°, which are attributed to the (100), (101), (211) and (112) planes of tetragonal rutile SnO₂ (JCPDS Card no. 41-1049),³⁴ indicating the formation of SnO₂ crystals. Fig. 2g shows the EDX pattern of the rGO/SnO₂/Au-2 sample. EDX analysis shows the rGO/SnO₂/Au-2 sample was composed of C, Sn, O and Au element. Weak peaks for Au could be seen in Fig. 2g. This may be due to the low content of Au nanoparticles in the composite materials.

Fig. 2 The XRD patterns of the products: (a) GO, (b) rGO/SnO₂, (c) rGO/SnO₂/Au-1, (d) rGO/SnO₂/Au-2, (e) rGO/SnO₂/Au-3 and, (f) rGO/SnO₂/Au-4. (g) EDX pattern of the rGO/SnO₂/Au-2 sample.

Fig. 3 shows the Raman spectroscopy spectra confirming the reduction of GO. The two strong peaks of the D-band (∼1351 cm⁻¹) and G-band (∼1595 cm⁻¹) were observed, which correspond to the diamondoid and graphitic graphene structures, respectively. It is well known that the intensity ratio of the D and G band (I_D/I_G) is strongly related to the quantity of functional groups of rGO, and compared with the I_D/I_G value of GO (1.088), the increased value of rGO (1.453) suggests restoration of C=C bonds after hydrothermal reduction.³⁵

Fig. 3 Raman spectroscopy spectra of the GO (black line) and rGO (red line) samples.

Fig. 4 displays the typical TEM images of the rGO/SnO₂/Au hybrid nanocomposite. The nanosheets have a size of several micrometers, with some surface wrinkles in some places. Compared with Fig. 4a, it can be observed in Fig. 4b that a large amount of SnO₂ nanoparticle dots are attached on the surfaces of the nanosheets, with a size of about several nanometers, which is also reflected by the wide diffraction peaks of the XRD pattern. By close observation of Fig. 4c–f, some black dots with a darker contrast against the background can be observed, which are Au nanoparticles. It should be noted that Au nanoparticles can be distinguished from the nanoparticle dots. Interestingly, as the ratio of HAuCl₄ increases, the size of Au nanoparticles also becomes larger and the sizes of the Au nanoparticles of rGO/SnO₂/Au-1 to rGO/SnO₂/Au-4 are about 12, 20, 42 and 90 nm, respectively.

Fig. 4 TEM images of the samples: (a) rGO, (b) rGO/SnO₂, (c) rGO/SnO₂/Au-1, (d) rGO/SnO₂/Au-2, (e) rGO/SnO₂/Au-3, (f) rGO/SnO₂/Au-4.

Fig. 5 reveals the typical TEM and the HRTEM images of the rGO/SnO₂/Au-2 samples. Again the Au nanoparticles were clearly discovered to be located on the surface of the rGO/SnO₂/Au-2 samples (Fig. 5a and b). Fig. 5c shows that interplanar distances of 0.23 nm and 0.333 nm correspond to the (111) planes of Au and (110) planes of SnO₂.

Fig. 5 (a) TEM images of the rGO/SnO₂/Au-2 samples and HRTEM images of (b) Au nanoparticles and (c) SnO₂ nanoparticles.

To further investigate the characteristics of the products, XPS technique was used to analyze the chemical state of GO and rGO/SnO₂/Au nanocomposites. Fig. 6a and Fig. 6b reveal the C1s spectra of GO and rGO/SnO₂/Au samples, exhibiting three peaks at 284.6, 286.6 and 288.4 eV, attributed to the C–C, C–O and C=O bands in graphene-based materials.³⁶ Note that the peak intensity of C–O and C=O is strong in GO. Compared with that, after hydrothermal treatment, the peak intensity of C–O and C=O decreases remarkably, suggesting most oxygen-containing functional groups are successfully removed after hydrothermal treatment and Sn(IV). Fig. 6c shows the Sn 3d spectrum of rGO/SnO₂/Au, showing two strong bands at 486.9 and 495.3 eV, which indicates the formation of SnO₂.³⁷ Fig. 6d confirms the presence of Au in the hybrids, with significant signals at 83.3 and 86.9 eV corresponding to metallic Au. A minor peak at 85.3 eV owing to Au(I) is also detected. The intensity of the Au 4f signal is higher than that of Au(I), indicating that most of Au is of zero valence and exits in the metallic form. All these observations indicate the successful formation of Au and SnO₂ and reduction of GO by hydrothermal treatment of GO, SnCl₂ and HAuCl₄.

Fig. 6 (a) C1s spectrum of GO (b) C1s spectrum of rGO/SnO₂/Au-2 samples, (c) Sn3d spectrum of rGO/SnO₂/Au-2, (d) Au4f spectrum of rGO/SnO₂/Au-2.

3.2. NO₂ sensing properties

Sensors were fabricated by dropping the rGO/SnO₂/Au dispersion onto the electrode to demonstrate NO₂ sensing application. The effect of the amounts of Au nanoparticles on the sensing performance toward NO₂ is investigated. Fig. 7 demonstrates the response/recovery curves of all rGO/SnO₂/Au-based sensors to 5 ppm NO₂ at 50 °C. It can be seen that the four sensors show relatively fast response and recovery speed. Notably, with the proportion of Au ranging from 0.5% to 5%. The maximum response with the value of 1.6 was obtained on rGO/SnO₂/Au-2, So, the rGO/SnO₂/Au-2 was chosen for the remaining test.

Fig. 7 The response curves to 5 ppm NO₂ of the sensors based on rGO/SnO₂/Au-1, rGO/SnO₂/Au-2, rGO/SnO₂/Au-3, and rGO/SnO₂/Au-4.

Moreover, the response/recovery speed is dramatically increased for the Au nanoparticles functionalized sensor. Fig. 8 shows the response and recovery times of the rGO, rGO/SnO₂ and rGO/SnO₂/Au-2-based sensors to 5 ppm NO₂, respectively. The T_r1 values of rGO/SnO₂/Au-2 sensors upon (T_r2 values during recovery after) exposure to 5 ppm NO₂ gases are 19 s (20 s) at a low operating temperature of 50 °C, which are the shortest values in the literature (Fig. 8a). The T_r1 (T_r2) values of the rGO/SnO₂ and pristine rGO sensors are relatively short 427 s (908 s) and 798 s (8319 s), (Fig. 8b and c). All these observations indicate that loading Au nanoparticles is an effective method for enhancing the response and response/recovery time (T_r1/T_r2) for graphene-based gas sensors at a low operating temperature.

Fig. 8 The response curve to 5 ppm NO₂ of the sensors at 50 °C based on (a) rGO, (b) rGO/SnO₂ and (c) rGO/SnO₂/Au-2.

Fig. 9 shows the response values of the rGO/SnO₂/Au-2-based sensor being orderly exposed to 5–50 ppm NO₂ gases at a low operating temperature of 50 °C. It is clear that the response and recovery characteristics were almost reproducible as well as the quick response and recovery times, which is in good agreement with the above analysis of Fig. 8c. Moreover, it is also found that the response gradually raises as the concentration of NO₂ increases. The response values of the sensor to 5, 10, 20 and 50 ppm NO₂ gas were about 1.67, 1.91, 2.37, and 2.68, respectively.

Fig. 9 Dynamic NO₂ sensing transients curve of the rGO/SnO₂/Au-2-based sensor to 5–50 ppm NO₂ gases at 50 °C.

We then investigated the response of three sensors at different NO₂ concentrations (Fig. 10). The responses of Au-loaded sensors were linear from 5 to 30 ppm, but increased more slowly from 30 to 100 ppm as the sensor began to saturate. Significantly, it should be noted that the responses to NO₂ were enhanced 5.85 times by employing rGO/SnO₂/Au-2, in contrast with rGO/SnO₂ and rGO. None-the-less, the sensor showed a broad range of linear response, this in turn, directly verifies the beneficial effect of Au nanoparticles.

Fig. 10 The responses of the rGO, rGO/SnO₂, and rGO/SnO₂/Au-2-based sensor to different concentrations of NO₂ at 50 °C.

The selectivity of three sensors was tested by exposing the sensors to potential interference gases (NO₂, NO, Cl₂, and H₂), as shown in Fig. 11. Take 5 ppm target gases for example, the rGO/SnO₂/Au-2-based sensor only gives a negligible response to interference gases, whereas the response of the rGO/SnO₂/Au-2 sensor is significantly enhanced to NO₂ gas compared with the rGO and rGO/SnO₂ ones. The above results conclude that the rGO/SnO₂/Au-2 has high selectivity for NO₂ sensing.

Fig. 11 The responses of the rGO, rGO/SnO₂ and rGO/SnO₂/Au-2 based sensors to 5 ppm of different gases at 50 °C.

Fig. 12 illustrates the reproducibility of the rGO/SnO₂/Au-2 sensor, revealing 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 that the sensor has a good stability throughout the cycle test.

Fig. 12 The reproducibility of the rGO/SnO₂/Au-2 sensor on successive exposure (3 cycles) to 5 ppm NO₂ at 50 °C.

3.3. Gas sensing mechanism of rGO/SnO₂/Au

The possible sensing mechanism for the superior gas sensing performance is proposed as follows: firstly, the principle of gas sensing for the resistance-type sensors is based on the conductance variations of the sensing element, thus the introduction of Au and SnO₂ to rGO significantly contributes to improving the conductivity, leading to a better sensing behavior. Secondly, benefiting from the existence of SnO₂, the increasing number of dots facilitates the gas adsorption and diffusion on the active surface. In open air, the formation of oxygen adsorbates (O²⁻) on the surface of SnO₂ and rGO results in an electron-depleted surface layer owing to electron transfer from the SnO₂–rGOs to oxygen in a similar fashion as for the reported literature.³⁸ Upon exposure to an oxidizing gas (NO₂), gas molecules can easily adsorb on their active sites in the form of NO₂ (Fig. 13). The Schottky barrier around the interface of rGO and SnO₂ nanorods results in the specific capture and migration of electrons from SnO₂ nanorods to graphene. As shown in Fig. 13, the role of rGO as an electron mediator further facilitates the electron transfer from SnO₂ nanorods to NO₂ molecules. Thirdly, the enhancement of NO₂ gas sensing properties of rGO/SnO₂ nanosheets by Au-functionalization can be explained based on the model proposed for the metal catalyst-enhanced gas sensing of nanomaterials.³⁹ In the case of rGO/SnO₂/Au nanorods, Au nanoparticles over the surface spill the NO₂ gas over the rGO/SnO₂ nanorods surface and enhance the chemisorption and dissociation of NO₂ gas owing to the high catalytic or conductive nature of Au. Consequently, the number of electrons attracted to the gas increases.

Fig. 13 The scheme of the proposed gas sensing mechanism: the adsorption behaviour of NO₂ molecules on the rGO/SnO₂/Au-2 nanocomposite.

4. Conclusions

rGO/SnO₂/Au nanocomposites have been prepared by one-step hydrothermal synthesis method. The gas sensing results indicate that the introduction of the Au nanoparticles into the rGO/SnO₂ composite materials significantly enhanced the NO₂-sensing performance at a low operating temperature of 50 °C. The results showed that the rGO/SnO₂/Au-2 sensors could detect NO₂ gases as low as 1 ppm. The response and recovery times are 19 and 20 s to 5 ppm NO₂. All results indicate that Au nanoparticles loading can significantly enhance the NO₂ sensing properties of graphene-based sensing materials at a low operating temperature, which has excellent potential applications as gas sensors.

Acknowledgements

This research work was financially supported by the National Natural Science Foundation of China (Grant no. 51202085) and Program for Chang Jiang Scholars and Innovative Research Team in University (no. IRT1017).

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