Preparation of zinc oxide nanoparticle–reduced graphene oxide–gold nanoparticle hybrids for detection of NO₂

Abstract

In this paper, in pursuit of developing high-performance graphene-based gas sensors, a novel NO₂ gas sensor has been successfully fabricated using ZnO nanoparticles and Au nanoparticles modified reduced graphene oxide (ZnO–rGO–Au) ternary hybrids as sensing materials. The ZnO–rGO–Au hybrids were prepared by deposition of Au nanoparticles on the surface of ZnO–rGO hybrids by the wet chemical method. The combined characterizations of X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive spectroscopy, as well as N₂ sorption isotherm indicate the successful preparation of ZnO–rGO–Au hybrids. Most importantly, ZnO–rGO–Au hybrids exhibit good sensing performances for detection of NO₂ at relatively low operating temperature (80 °C), such as high response, fast response and recovery rate, and good selectivity. Furthermore, the excellent sensing performances of sensor based on ZnO–rGO–Au hybrids also render it suitable for development of high performance gas sensors.

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Introduction

During the past few years, considerable interest has been focused on the development of carbon-based materials with novel structures, such as graphene, carbon nanotubes (CNTs) and fullerene.^1–3 Among them, graphene-based materials have attracted increasing attention since the first discovered in 2004 due to their outstanding properties of high surface area, high carrier mobility and other excellent chemical and physical properties, compared to CNTs and fullerene.^4,5 Up to now, graphene-based materials have been widely examined as functional materials for various applications in the fields of electronics,⁶ energy storage and conversion,⁷ gas sensing,⁸ photonics,⁹ electrocatalysis,¹⁰ photocatalysis,¹¹ electrochemical sensing,¹² and so on.

In particular, graphene-based materials are also promising materials for fabrication of high-performance gas sensors, especially for detection of gases at low operating temperature, even at room temperature. Two excellent properties ensure graphene-based materials as good candidates for fabrication of high-performance gas sensors operating at low operating temperature.^13–15 (1) High carrier mobility of graphene at room temperature ensures the observation of resistance change during the detection process. (2) Detectable change in their resistance after adsorption or desorption of guest gases leading to observing the resistance change with increasing or decreasing the concentration of target gases. Although the pure graphene-based materials (including graphene prepared by the chemical vapor deposition, and reduced graphene oxide obtained by reduction of graphene oxide) exhibit obvious response for targets at room temperature, the poor sensing performances should be further improved, such as weak sensitivity, slow response and recovery rate.

Recent researches have shown that the incorporation of graphene with metal oxides could improve the sensing performances of graphene-based gas sensors. For example, Wang and co-authors prepared of In₂O₃–rGO nanocomposites by the hydrothermal method for detection of NO₂ at room temperature.¹⁶ Wang et al. prepared flower-like Ni(OH)₃ modified rGO for detection of NO₂ at room temperature.¹⁷ Zhang et al. prepared NiO–rGO nanocomposite for room temperature detection of NO₂.¹⁸ Our group also reported the preparation of SnO₂ nanoparticles–rGO nanocomposites, ZnO nanoparticles–rGO hybrids for detection of NO₂ at low temperature as well as room temperature.^19,20 Unfortunately, sensing performances (sensitivity, response time and recovery time) of these sensors still need further be enhanced to satisfy the criterion for practical applications.

Recently, Pinna and co-workers have developed gas sensors using Pt nanoparticles, SnO₂ nanoparticles and rGO ternary nanostructures as sensing materials for detection of H₂ at room temperature,²¹ paving the pathway for preparation of ternary graphene-based materials for gas sensing. Although graphene-based ternary hybrids have been widely prepared and used as novel functional materials for various fields, to the best of our knowledge, few works have been reported for development of rGO-based ternary hybrids for gas sensing operating at low temperature.

In this work, a novel rGO-based gas sensor has been fabricated using ZnO nanoparticles–rGO modified with Au nanoparticles (ZnO–rGO–Au) ternary hybrids as sensing materials. ZnO–rGO–Au hybrids were prepared by deposition of Au nanoparticles on the surface of ZnO–rGO hybrids by the wet chemical method. It is also found that the ZnO–rGO–Au hybrids exhibit good sensing performance for detection of NO₂ operating at relatively low operating temperature (80 °C), opening the door for development of rGO-based ternary hybrids for fabrication of high-performance gas sensors.

Experimental section

Materials

Zn(OAc)₂·2H₂O, methanol, KMnO₄, H₂O₂ (30 wt%), NaNO₃, HAuCl₄·4H₂O, NaBH₄ and H₂SO₄ (98%) were purchased from Beijing Chemical Corp (Beijing, China). Hydrazine hydrate and KOH were purchased from Shanghai Chemical Corp. (Shanghai, China). Graphite powder was purchased from Aladdin Ltd. (Shanghai, China). All chemicals were used without any further purification. The water used throughout all experiments was purified through a Millipore system.

Preparation of ZnO–rGO–Au hybrids

GO was prepared from natural graphite powder through a modified Hummers' method.²² ZnO–rGO–Au hybrids were prepared by deposition of Au nanoparticles on the surface of ZnO–rGO hybrids. ZnO–rGO hybrids were prepared by the two-step wet chemical method according to the previous report.²⁰ In typical synthesis of ZnO–rGO–Au hybrids, 2 mL of ZnO–rGO dispersion was added into 20 mL of H₂O, followed by addition of 20 μL of HAuCl₄ solution (24.3 mM). After sonication for 5 min, 5 mL of NaBH₄ solution (9.193 mg NaBH₄ dissolved in 5 mL of H₂O) was added into the mixture, followed by further sonication for 5 min. The ZnO–rGO–Au hybrids were collected by centrifugation at 10 000 rpm for 10 min and washed with water for twice. The ZnO–rGO–Au hybrids were dispersed into H₂O for further use and characterization.

Characterizations

Powder X-ray diffraction (XRD) datum was recorded on a RigakuD/MAX 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray photoelectron spectrometer using Mg as the exciting source. Transmission electron microscopy (TEM) measurement was made on a HITACHI H-8100 electron microscopy (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. Nitrogen sorption isotherm was obtained at −196 °C with a JW-BK 132F analyzer. Samples were prepared for measurement by treating at 150 °C under nitrogen atmosphere for 12 h. Pore size distribution was calculated using Barrett–Joyner–Halenda (BJH) method. The chemical composition mapping through energy-dispersive spectroscopy (EDS) of the sample was examined by field emission scanning electron microscopy (JSM-6700F).

Fabrication of gas sensors

The sensors were obtained by dropping ZnO–rGO–Au dispersion onto the ceramic substrate. Before dropping the sensing materials, two pairs of Au electrodes were printed on one side of ceramic substrate as signal electrode and another side of ceramic substrate as heating electrode. For dropping sensing materials onto electrode, 0.3 μL of ZnO–rGO–Au aqueous dispersion was dropped on the surface of signal electrode, followed by dryness at room temperature. The detailed structure of the sensor was showed in our previous publications.^19,20

Gas sensing properties were examined using a static test system by CGS-8 intelligent test meter (Beijing Elite Tech. Co., Ltd, China). Target gas with various concentrations was prepared by injection of various volume of NO₂ gas with fixed concentration into a test chamber (about 1 L in volume). After fully mixed with air (relative humidity was about 25%), the sensor was put into the test chamber. When the resistance of sensor reached a constant value, the sensor was taken out to recover in air. The response of the sensor was defined as S = (R_a − R_g)/R_g × 100, where R_a is the resistance of the sensor in air and R_g is the resistance of the sensor in target gas. 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.

Results and discussion

Due to the different formation mechanism of ZnO nanoparticles and Au nanoparticles, two-step wet chemical method was developed for preparation of ZnO–rGO–Au hybrids. Scheme 1 shows the scheme to illustrate the preparation of ZnO–rGO–Au hybrids. Firstly, ZnO–rGO hybrids was prepared by deposition of ZnO nanoparticles on the surface of GO by heating Zn(OAc)₂ in methanol in the presence of alkaline, followed by reduction of GO into rGO using N₂H₄ as reducing agent. Then, Au nanoparticles were deposited on the surface of ZnO–rGO hybrids by sonication-assisted reduction of HAuCl₄ using NaBH₄ as reducing agent.

Scheme 1 A scheme to illustrate the preparation of ZnO–rGO–Au hybrids.

The structure of the hybrids was first characterized by the XRD technique. Fig. 1 shows the XRD pattern of the hybrids powder. It is seen that seven diffraction peaks at 2θ of 31.72, 34.40, 36.08, 47.36, 56.37, 62.70 and 67.85° are observed for the final hybrids, which are indexed as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) diffractions of hexagonal phase with Wurtzite crystal structure of ZnO (space group: C_6v (P6₃mc), JCPDS file no. 0-3-0888).²³ Furthermore, four obvious diffraction peaks centered at 2θ of 38.16, 44.59, 65.17 and 77.44° are observed, which are attributed to (111), (200), (220) and (311) planes of face-centered cubic (FCC) structure of Au nanoparticles (JCPDS no. 65-2870).²⁴ It is also found that a broad diffraction peak centered at 2θ of 22.51° is also observed, which is associated with the diffraction peak of rGO. All these observations indicate the successful preparation of ZnO–rGO–Au hybrids.

Fig. 1 XRD pattern of ZnO–rGO–Au hybrids.

It is well known that XPS measurement has been proven as an effective technique to examine elemental composition and chemical status for functional materials, especially for rGO-based materials. Thus, the ZnO–rGO–Au hybrids were characterized by XPS technique. Fig. 2a shows XPS profile of C1s for ZnO–rGO–Au hybrids. It is seen that C1s spectrum could be deconvoluted into three peaks at 284.6 eV, 286.6 eV, and 288.9 eV, associated with C–C, C–O, and C=O bands, respectively.²⁵ It is noted that the intensities of C–O, and C=O bands are much lower than that of C–C band, compared to GO used as precursor for preparation of ZnO–rGO–Au hybrids. Furthermore, the ratio of C–O and C=O in the carbon-based bands is also much lower than that of rGO obtained by the chemical reduction of GO.²⁵Fig. 2b shows the O1s XPS spectrum of ZnO–rGO–Au hybrids. It is deduced that two kinds of oxygen-containing bands are existed in the hybrids: the O-containing bands in rGO and ZnO crystal. It has been reported that there are C–O and C=O bands in rGO,²⁶ and O ion in ZnO lattice (O_L), O ions in oxygen-deficient lattice in ZnO (O_V) and chemisorbed oxygen species (O_C).²⁷ In the present work, a peak at 533.42 eV is observed for ZnO–rGO–Au hybrids, which is attributed to the C–O band in rGO. However, no peak attributed to C=O band is observed, which may be attributed to the low content of C=O in rGO. Furthermore, two peaks at 532.51 eV and 531.25 eV are observed, which are associated with O_C and O_L, respectively, further confirming the formation of ZnO in hybrids.

Fig. 2 (a) C1s XPS spectrum, (b) O1s XPS spectrum, (c) Zn2p XPS spectrum, and (d) Au4f XPS spectrum of ZnO–rGO–Au hybrids.

To examine the structure of Zn element in the hybrids, the Zn2p XPS spectrum is also investigated, as shown in Fig. 2c. It is found that two peaks at 1022.41 eV and 1045.59 eV are observed, which are associated with Zn2p_3/2 and Zn2p_1/2, respectively, further indicating the preparation of ZnO. Additionally, Au4f XPS spectrum of the hybrids is also examined, as shown in Fig. 2d. Two separated peaks at 84.11 eV and 87.87 eV attributed to Au4f_7/2, and Au4f_5/2 are observed, suggesting the formation of metal Au in the hybrids. Due to the closely peaks compared to Au4f peaks, two peaks at 89.40 eV, and 92.14 eV associated with Zn3p_3/2 and Zn3p_1/2 are also observed. All these observations indicate the successful preparation of ZnO–rGO–Au hybrids by the two-step wet chemical method. Based on the results of XPS spectra, the composition of ZnO–rGO–Au hybrids is C 37.80%, O 7.52%, Zn 3.95%, and Au 0.74%.

The composition of ZnO–rGO–Au hybrids was further characterized by the EDS mapping to probe the elements' location. As shown in Fig. 3a–e, ZnO–rGO–Au hybrids are composited by C, O, Zn, and Au elements distributing the whole samples. Furthermore, the corresponding EDS spectrum (Fig. 3f) also indicates that the samples are comprised of C, O, Zn, and Au elements. The peak attributed to Si element is also observed, which is associated with the substrate used for EDS characterization.

Fig. 3 The EDS mapping for (a) all, (b) C element, (c) O element, (d) Zn element, (e) Au element, and (f) corresponding EDS spectrum.

The structure of the ZnO–rGO–Au hybrids was further examined by the TEM images. Fig. 4a shows the low magnification TEM image of ZnO–rGO–Au hybrids. A wrinkled substrate is observed, indicating the formation of rGO–based materials. Several nanoparticles are also observed on the surface of rGO. A higher magnification TEM image (Fig. 4b) further reveals the formation of nanoparticles on rGO and there are two kinds of nanoparticles on rGO according to the different contrast by electron beam irradiation. According to the results mentioned above, these two different nanoparticles are attributed to ZnO nanoparticles and Au nanoparticles. The Au nanoparticles exhibit deeper color than that of ZnO nanoparticles, which is could be attributed to the good conductivity of Au nanoparticles. The particle size of small Au nanoparticles is about 10–20 nm, while the large one for ZnO nanoparticle has the particle size about 30–40 nm. Fig. 4c shows the high resolution TEM image of one nanoparticle on rGO, revealing obvious lattice plane of Au nanoparticle, further indicating the formation of Au nanoparticles in hybrids. The TEM image of edge of the Au nanoparticle is shown in Fig. 4d.

Fig. 4 (a and b) Low and (c and d) high magnification TEM images of ZnO–rGO–Au hybrids.

Furthermore, the SEM images of rGO and ZnO–rGO have been examined as shown in Fig. S1.† It is seen that rGO exhibits plate-like morphology formed by the aggregation of rGO due to the strong π–π interaction between rGO sheets. Furthermore, it is seen that the ZnO–rGO hybrids are consisting of nanoparticles and rGO sheets, which is attributed to the assembly of ZnO and rGO.

It is deduced that modification of rGO with ZnO nanoparticles and Au nanoparticles could prevent agglomerate of rGO affecting by the π–π interaction between them, leading to the formation of porous structure. Thus, the porous structure of ZnO–rGO–Au hybrids was characterized by the N₂ sorption isotherm. Fig. 5 shows N₂ adsorption/desorption isotherm of ZnO–rGO–Au hybrids. It is seen that the N₂ amount adsorbed by hybrids increases with increasing the relative pressure, indicating the formation of porous structure. The inset of Fig. 5 shows the corresponding pore size distribution curve of ZnO–rGO–Au hybrids. It is seen that BET surface area, pore size, and pore volume of ZnO–rGO–Au hybrids are 84.1 m² g⁻¹, 2.6 nm, and 0.17 cm³ g⁻¹, respectively. All these observations suggest that modification of rGO with ZnO nanoparticles and Au nanoparticles results in formation of porous structure.

Fig. 5 (a) N₂ sorption isotherms and (b) pore size distribution curve of ZnO–rGO–Au hybrids.

It is well known that rGO-based materials are promising materials for fabrication of NO₂ gas sensors. Thus, the sensing application of ZnO–rGO–Au hybrids was investigated for detection of NO₂. Notably, previous reports have demonstrated that rGO-based materials can be used for fabrication of gas sensors for detection of NO₂ at room temperature. However, the relatively slow response and recovery rate still prevents their wide applications. To overcome this problem, increase of operating temperature has been adopted for enhancing the sensing performances of rGO-based gas sensor in this work.

The effect of the operating temperature on the sensing performances of ZnO–rGO–Au hybrids-based sensor was examined. Fig. 6 shows the response and recovery curves of the sensors based on ZnO–rGO–Au hybrids to 100 ppm NO₂ operating at various temperatures. It is seen that response, response time and recovery time of the sensor based on ZnO–rGO–Au hybrids to 100 ppm operating at 60 °C are 49.08%, 49 s, and 471 s, respectively. By increasing the operating temperature to 70 °C, the response of sensor to 100 ppm NO₂ decreases down to 42.79%; while the response time and recovery time decrease to 33 s and 212 s, respectively. A further increase of operating temperature to 80 °C, the response decreases down to 32.55%, and the response time and recovery time are also decreased to 27 s, and 86 s. When further increasing the operating temperature to 90 °C, a fast recovery time of 26 s is obtained. However, the sensor exhibits a relative low response (17.05%). Based on the above observations, it is concluded that the responses to NO₂ decrease with increasing the operating temperature along with decreasing the response time and recovery time. Thus, in this wok, the operating temperature is adopted at 80 °C and the following sensing performances are obtained at 80 °C. Notably, the operating temperature in this work is much lower than the NO₂ sensors based on metal oxides. Although the operating temperature is slightly higher than the rGO-based NO₂ sensors operated at room temperature, the response time and recovery time are much shorter than that of the previously reported rGO-based NO₂ sensors operating at room temperature.

Fig. 6 Response and recovery curves of the sensor based on ZnO–rGO–Au hybrids to 100 ppm NO₂ operating at 60 °C, 70 °C, 80 °C and 90 °C.

The responses of the sensors toward various concentrations of NO₂ are also examined. Fig. 7a shows the response and recovery curve of the sensor based on ZnO–rGO–Au hybrids toward various concentrations of NO₂. It is seen that the resistance of the sensor decreases after introduction of various concentration NO₂, while the resistance recovers back to the initial value after returning the sensor into air, indicating the good response and recovery properties of the sensors. Fig. 7b shows the relationship curve between the responses toward NO₂ and corresponding concentrations from 100 ppm to 20 ppm, revealing that the response of sensor increases with increasing the concentrations of NO₂.

Fig. 7 (a) Response and recovery curve of the sensor based on ZnO–rGO–Au hybrids to various NO₂ concentrations of 100 ppm, 80 ppm, 60 ppm, 40 ppm and 20 ppm at 80 °C and (b) the corresponding linear relationship between the response and concentrations of NO₂.

Furthermore, the selectivity of the sensor based on ZnO–rGO–Au hybrids toward NO₂ sensing is also examined. As shown in Fig. 8, response of the sensor toward 100 ppm NO₂ is 32.55%, which is much higher than that of Cl₂, ammonia, NO and ethanol, indicating good selectivity of the sensor based on ZnO–rGO–Au hybrids. All these observations indicate that ZnO–rGO–Au hybrids are good candidate for development of high performance NO₂ sensor with fast response and recovery rate. Fig. S2† shows the reproducibility of temporal response of ZnO–rGO–Au hybrids exposed to 100 ppm NO₂. It is seen that the sensors maintain its initial response amplitude without a clear decrease upon three successive sensing tests to 100 ppm NO₂, indicating that the ZnO–rGO–Au hybrids possesses good repeatability.

Fig. 8 The selectivity of the sensors based on ZnO–rGO–Au hybrids toward 100 ppm gases, including NO₂, Cl₂, NH₃, NO and ethanol at room temperature.

Although the NO₂ sensors based on ZnO–rGO hybrids have been reported previously,^20,28 the novelty of the present work is introduction of Au nanoparticles into sensing materials. To evaluate the advantage of the NO₂ sensor fabricated in the present work, a table has been provided as shown in Table 1. Although the previously reported NO₂ sensors based on ZnO–rGO show obvious response to NO₂, the response and recovery rate is too slow. By introduction of Au nanoparticles into NO₂ sensor, a fast response and recovery rate is obtained, which could be attributed to the excellent properties of Au nanoparticles for gas sensing. Compared with the previously reported NO₂ sensor based on ZnO–rGO hybrids, the sensor thus fabricated exhibits the fastest response and recovery rate, which paves a new way for preparation of high-performance rGO-based NO₂ sensor.

Table 1 The comparison of the prepared NO₂ sensor with the previously reported NO₂ sensor based on ZnO–rGO hybrids

Materials	Operating temperature (°C)	Concentration (ppm)	Response (%)	Response time/recovery time (s s^−1)	Ref.
a ZnO–rGO hybrids were prepared by deposition of ZnO nanoparticles on rGO in methanol solution assisted with alkaline. b ZnO–rGO hybrids were prepared by formation of rGO–Zn(OH)2 hybrid followed by heat treatment at 500 °C.
ZnO–rGO^a	RT	5	25.6	165/499	20
ZnO–rGO^b	RT	50	14.3	96/1552	28
3D graphene aerogel–ZnO	RT	50	8	132/164	29
ZnO–rGO–Au	80	100	32.55	27/86	This work

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Conclusions

In this work, ZnO–rGO–Au hybrids have been successfully prepared by the wet-chemical method, and such hybrids were further used as sensing materials for NO₂ sensing at relatively low operating temperature (80 °C). The sensing results indicate that the sensors based on ZnO–rGO–Au hybrids exhibits fast response and recovery rate, as well as good selectivity for NO₂ sensing. Our present work is important because it provides an effective sensing material for high-performance gas sensing with fast response and recovery rate.

Acknowledgements

This research work was financially supported by the National Natural Science Foundation of China (Grant No. 51202085), Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT3018) and the Open Project from State Key Laboratory of Transducer Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The SEM images of rGO and ZnO–rGO; the reproducibility of temporal response of ZnO–rGO–Au hybrids. See DOI: 10.1039/c5ra18680c

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