Synthesis and highly enhanced acetylene sensing properties of Au nanoparticle-decorated hexagonal ZnO nanorings

Abstract

Hexagonal ZnO nanorings were synthesized using a one-step hydrothermal method and Au nanoparticles were decorated on the surface of the ZnO nanorings through a facile deposition process. The as-prepared ZnO nanorings showed a well-defined hexagonal shape with a width of 0.75–1.4 μm, a thickness of 0.17–0.33 μm and a hollow size of 0.2–1 μm. For the Au nanoparticle-decorated hexagonal ZnO nanorings (Au–ZnO nanorings), Au nanoparticles with a size of 3–10 nm were distributed discretely on the surface of the ZnO nanorings. The acetylene sensing performance was tested for the ZnO nanorings and Au–ZnO nanorings. The results indicated that the Au–ZnO nanorings showed a higher response (28 to 100 ppm acetylene), lower operating temperature (255 °C), faster response/recovery speed (less than 9 s and 5 s, respectively), and lower minimum detectable acetylene concentration (about 1 ppm). In addition, the mechanism for the enhanced acetylene-sensing performance of the Au–ZnO nanorings was discussed.

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Introduction

Zinc oxide (ZnO), an n-type metal oxide semiconductor with a wide direct band gap (3.37 eV), high exciton binding energy (60 meV) and piezoelectricity at room temperature,^1–3 has attracted great interest for catalysis,^4,5 photodetection,^6,7 solar cells,^8,9 pressure sensors^10,11 and lithium ion batteries^12,13 etc. In the field of gas-sensing, ZnO has been recognized as an excellent gas-sensing material for the detection of combustible and toxic gases, and volatile organic compounds including hydrogen,¹⁴ carbon monoxide,¹⁵ nitric oxide,¹⁶ ethanol,¹⁷ acetone,^18,19 and formaldehyde²⁰ etc. due to its biocompatibility, non-toxicity, stability, low-cost, ease of large scale fabrication and superior sensing properties.

In recent years, nano- and micro-scale ZnO with various morphologies such as nanoparticles,¹⁷ nanorods,¹⁵ nanowires,²¹ nanofibers,²² nanoplates²³ and nanosheets,²⁴ etc. have been synthesized. Compared with these morphologies, nanorings shows a specific two-dimensional (2D) structure that can be regarded as a one-dimensional (1D) nanostructure bending into a 2D nanostructure in a plane. Such a structure gives nanorings the advantages of having 1D, 2D and hollow nanostructures including slight agglomeration, high specific surface area and excellent porosity etc., which are beneficial for accelerating gas diffusion and improving the gas-sensing performance for gas-sensing materials.

For many gas sensors, the response is determined by the efficiency of catalytic reactions with the detected gas molecules taking place at the surface of the gas-sensing material.²⁵ However, in practice, the widely used gas-sensing metal oxides exhibit poor catalytic activity and sensitivity to the detected gas. In recent years, it has been proved that metallic catalysts such as Ag,²⁶ Au,^27–29 Pd,³⁰ and Pt³¹ nanoparticles loaded onto the surfaces of metal oxide supports serve as sensitizers or promoters, dramatically improving the sensitivity, response and recovery time, and operating temperature. Moreover, the catalytic activity of noble metals is sharply enhanced with a decrease in their size and noble metal nanoparticles with a small size and discrete distribution have great advantages in the field of gas-sensing.³²

At present, many gas sensors with good properties have been obtained for detecting ethanol, acetone, nitric oxide, xylene and carbon monoxide etc. However, there is insufficient research on acetylene (C₂H₂). Acetylene is the most effective and versatile fuel gas, enabling its application in welding, cutting, straightening and other localized heating processes. Moreover, acetylene is a colourless and odorless gas which is not easy to detect in air and is highly combustible and explosive. Therefore, great effort is required to fabricate practical acetylene sensors.

In this work, ZnO nanorings with a well-defined hexagonal shape were synthesized using a one-step hydrothermal method. Then Au nanoparticles with a size of 3–10 nm were discretely decorated on the surface of the ZnO nanorings through a facile deposition process. The acetylene sensing performance was tested for the ZnO nanorings and Au–ZnO nanorings and the results indicated that the Au–ZnO nanorings showed a great improvement including having the higher response, lower operating temperature, faster response/recovery speed and lower minimum detectable acetylene concentration. In addition, the mechanism for the enhanced acetylene-sensing performance of the Au–ZnO nanorings was discussed.

Experimental section

Materials

All of the starting reagents were used without purification. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (C₆H₁₂N₄, HMTA) were purchased from the Xilong Chemical Co., Ltd. Polyvinyl alcohol 1788 (PVA-1788) and polyvinylpyrrolidone (PVP, K 88-96) were purchased from Aladdin chemistry Co., Ltd.

Preparation of hexagonal ZnO nanorings

The typical preparation of hexagonal ZnO nanorings is described as follows: 0.297 g of Zn(NO₃)₂·6H₂O, 0.28 g of HMTA and 0.4 g of PVP were successively dissolved in 20 mL of deionized water under stirring at room temperature. When the PVP had completely dissolved, a certain amount of PVA-1788 was added into the above solution under stirring. After the PVA-1788 had dissolved completely, the resulting solution was gradually heated to 90 °C with a water bath and maintained for 10 min under stirring. Then the resulting white suspension was transferred into a 30 mL Teflon-lined stainless steel autoclave for hydrothermal treatment at 90 °C for 6 h. After cooling down to room temperature, the products were rinsed with hot deionized water several times followed by drying in air at 60 °C overnight. The formation mechanism of the ZnO nanorings is presented in Fig. S1.†

Preparation of Au nanoparticle-decorated ZnO nanorings

Au nanoparticles were decorated on the surface of the ZnO nanorings according to the procedure in the literature.³³ In a typical synthesis, 0.0163 g (0.20 mmol) of as-prepared ZnO nanorings were dispersed in 11.50 mL deionized water under ultrasonic treatment, then 7.50 mL of HAuCl₄ (0.02 M) aqueous solution and 1.00 mL of NH₃·H₂O were added in order to the suspension. After stirring for about 3 h, the precipitate was collected by centrifugation and washed with deionized water several times. Finally, the Au nanoparticle-decorated ZnO nanorings (denoted as Au–ZnO nanorings) were obtained through annealing the precursor at 350 °C for 1 h in a muffle furnace. The mechanism of the Au decoration is presented in Fig. S1.†

Characterization

The samples were characterized using powder X-ray diffraction (XRD, Shimadzu XRD-6000 with Cu Kα radiation, λ = 0.154178 nm), scanning electron microscopy (SEM, JSM6700F), energy-dispersive X-ray diffraction (EDX), selected area electron diffraction (SAED) and transmission electron microscopy (TEM, FEI Tecnai F20).

Gas sensor fabrication and testing

The fabrication process of sensors in brief: the as-prepared ZnO was mixed with deionized water in a weight ratio of 100 : 25 to form a paste. The paste was coated onto a ceramic tube on which a pair of gold electrodes was previously printed, and then a Ni–Cr heating wire was inserted into the tube to form a side-heated gas sensor. The fabricated sensor is shown in Fig. 1.

Fig. 1 Sketch of the gas sensor.

Gas sensing properties were measured using a CGS-8 (Chemical gas sensor-8) intelligent gas sensing analysis system (Beijing Elite Tech Co. Ltd., China). The gas response was defined as S = R_a/R_g, where R_g and R_a are the resistance values of the sensors in the presence and absence of the target gas, respectively. 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. The minimum detectable is defined as the concentration which makes the response of the sensor reach 3.

Results and discussion

The crystal structures of the ZnO nanorings and Au–ZnO nanorings were examined using XRD, as shown in Fig. 2. It can be seen that the peaks in the diffraction pattern of the ZnO nanorings, at 2θ = 31.8°, 34.4°, 36.8°, 47.5°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 71.6° and 77.0° correspond to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) characteristic peaks of wurtzite ZnO (JCPDS: 36-1451), respectively. For the Au–ZnO nanorings, all of the diffraction peaks belong to ZnO and no diffraction peaks corresponding to Au can be found which may be attributed to the discrete distribution and small size of the Au nanoparticles.²⁶

Fig. 2 XRD patterns of the ZnO and Au–ZnO nanorings.

The SEM images of the ZnO nanorings and Au–ZnO nanorings are shown in Fig. 3(a) and (b). From Fig. 3(a), it can be seen that the ZnO nanorings show a hexagonal ring-like morphology with a width of 0.75–1.4 μm, a thickness of 0.17–0.33 μm and a hollow size of 0.2–1 μm. After the Au nanoparticle-decoration process, the Au–ZnO nanorings retained the original hexagonal ZnO nanoring morphology. From the high magnification SEM images of the Au–ZnO nanorings in insets (i) and (ii), it can be seen that Au nanoparticles with a size range from 3 nm to 10 nm were discretely distributed onto the surface of the ZnO nanorings. Such a small size can result in the Au nanoparticles showing an excellent catalytic activity and thus enhancing the sensing properties of the ZnO nanorings. In addition, the films composed of ZnO and Au–ZnO nanorings are both porous which facilitates the diffusion and distribution of the surrounding gas phase to the surface of the internal films, which will increase the number of reaction sites and contribute greatly to improving the gas sensing properties.

Fig. 3 Low- and high-magnification SEM images of (a) the ZnO nanorings and (b) the Au–ZnO nanorings.

In order to obtain more details, transmission electron microscopy TEM, selected area electron diffraction (SAED) and energy-dispersive X-ray diffraction (EDX) were employed to analyze the Au–ZnO nanorings. Fig. 4(a) shows a low-magnification TEM image of a single Au–ZnO nanoring and a clear hexagonal ring-like outline can be observed. In addition, Au nanoparticles attached to the surface of the ZnO nanoring can be seen just next to the outline. The SAED pattern shown in Fig. 4(b) can be indexed to the [0001] zone axis of a single-crystal hexagonal ZnO nanoring, and this implies that the as-prepared ZnO nanorings are single-crystalline and the single Au–ZnO nanoring shown in Fig. 4(a) is viewed along the [0001] direction. In order to observe the microstructure of the Au–ZnO nanorings, a high-magnification TEM (HRTEM) image is shown in Fig. 4(c). In the HRTEM image, the interplanar distance of 0.281 nm is close to the d spacings of the (101̄0) planes of the hexagonal structured ZnO nanoring. In addition, as shown in Fig. 4(c), the interplanar distance of 0.238 nm is close to the d spacings of the (111) planes of Au, and the size of the Au nanoparticle is about 10 nm which is in accordance with the results of SEM. The corresponding EDX spectrum of the Au–ZnO nanorings is shown in Fig. 4(d). As can be seen, the EDX peaks around 1.03 keV, 8.64 keV and 9.53 keV can be indexed to Zn, and the EDX peaks around 0.52 keV can be indexed to O. Moreover, the obvious EDX peaks of Au are found at around 2.10 keV, 9.71 keV and 11.58 keV, certifying the presence of Au nanoparticles and the Au to Zn atomic ratio is about 6.6 : 100.

Fig. 4 (a) The low-magnification TEM image of a single Au–ZnO nanoring; (b) the corresponding SAED pattern; (c) the high-magnification TEM image; (d) the EDX spectrum of the Au–ZnO nanorings.

In order to find the optimum operating temperature of the sensors based on the hexagonal ZnO and Au–ZnO nanorings, their responses to 100 ppm acetylene at different operating temperatures (from 205 to 350 °C) were collected. As shown in Fig. 5, the response of the sensors to acetylene increases with the increase of the operating temperature and attains the maximum values at a certain temperature, followed by a decrease. It can be seen that the Au–ZnO nanorings obtain the maximum response of 28 at 255 °C which is much higher than a response of 12 at 302 °C for the ZnO nanorings, which indicates that the Au nanoparticles contribute to the decrease in the operating temperature and the increase in the response to acetylene. So, 255 °C and 302 °C were found to be the optimal operating temperatures of the Au–ZnO and ZnO nanorings respectively and the following acetylene-sensing performances were tested at their optimal operating temperatures.

Fig. 5 Response of the ZnO and Au–ZnO nanorings to 100 ppm acetylene as a function of the operating temperature.

The responses of the ZnO and Au–ZnO nanorings to different concentrations of acetylene are shown in Fig. 6. It can be easily seen that the responses increased rapidly with increasing acetylene concentration, then gradually slowed down nearing saturation when the concentration reached above 1000 ppm. Moreover, the Au–ZnO nanorings had a higher response than the ZnO nanorings at each acetylene concentration.

Fig. 6 Curves of sensor response versus acetylene concentration at their optimum operating temperature.

As for gas sensing applications, rapid response and recovery is of great importance for real-time monitoring. To investigate the response–recovery behaviors of the hexagonal ZnO and Au–ZnO nanorings, the sensors were sequentially exposed to 0.5, 1, 3, 5, 10, 20, 40, 60, 80, 100 and 150 ppm acetylene. As shown in Fig. 7, when the sensors were exposed to acetylene the response increases rapidly and when subjected to air the sensor recovery to the initial state is also rapid. Compared with ZnO nanorings, the Au–ZnO nanorings show the higher response, faster response/recovery speed and lower minimum detectable for acetylene. The Au–ZnO nanorings have a shorter response time of 3–11 s compared to 6–15 s and a recovery time of 2–5 s compared to 5–9 s. The Au–ZnO nanorings also have a lower minimum detectable of below 1 ppm (between 0.5 ppm and 1 ppm, and the corresponding responses are 2.5 and 3.3, respectively) compared to below 10 ppm (between 5 ppm and 10 ppm, and the corresponding responses are 2.8 and 3.6, respectively). To certify the statistical significance, the sensor performance was measured 5 times in 0.5 ppm and 1 ppm ambient acetylene, respectively, as shown in Fig. S2.† In addition, the repeatability of the response of the ZnO and Au–ZnO nanorings to 100 ppm acetylene is shown in Fig. S3.† These results indicate that the Au–ZnO nanorings are more advantageous than the ZnO nanorings for acetylene detection.

Fig. 7 Transient response of the ZnO and Au–ZnO nanorings.

Comparisons between the ZnO nanorings, Au–ZnO nanorings and other reported acetylene-sensing materials are displayed in Table 1. As can be seen, the Au–ZnO nanorings have an advantage in detecting acetylene due to the high response, low detectable limit, fast response/recovery speed as well as low operating temperature.

Table 1 Comparison of acetylene sensors within this work and with some reported in the literature

Material		Operating temperature	Response	Response/recovery time	Detectable limit
a The short line denotes that the corresponding data is not spelled out in the literature.
Sm2O3-doped SnO2 (ref. 34)		180 °C	63.8	12 s/30 s	>10 ppm
Cd2Sb2O6.8 (ref. 35)		360 °C	About 9 (100 ppm)	—^a	—
2 at.% Pt/ZnO^36		300 °C	165 (500 ppm)	11 s/—	50 ppm
Ag-loaded hierarchical ZnO nanostructure-reduced graphene oxide^37		200 °C	13 (100 ppm)	About 45 s/115 s (100 ppm)	<10 ppm
ZnO/reduced graphene oxide^38		250 °C	18.2 (100 ppm)	31 s/9 s (100 ppm)	10 ppm
This work	ZnO nanorings	302 °C	12 (100 ppm)	8 s/5 s (100 ppm)	<10 ppm
	Au–ZnO nanorings	255 °C	28 (100 ppm)	4 s/3 s (100 ppm)	<1 ppm

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The gas sensing selectivity is another important parameter for evaluation of the sensing ability of oxide semiconductor materials. Fig. 8 shows the cross-sensitivities of the hexagonal ZnO nanorings and Au–ZnO nanorings to 100 ppm of various gases including acetylene (C₂H₂), toluene (C₇H₈), benzene (C₆H₆), formaldehyde (HCHO), ammonia (NH₃), carbon monoxide (CO) and methane (CH₄). It is clear that both the hexagonal ZnO nanorings and Au–ZnO nanorings exhibit the largest responses towards acetylene among the tested gases and show good selectivity to acetylene.

Fig. 8 Selectivity of the hexagonal ZnO nanorings and Au–ZnO nanorings toward acetylene (C₂H₂), toluene (C₇H₈), benzene (C₆H₆), formaldehyde (HCHO), ammonia (NH₃), carbon monoxide (CO) and methane (CH₄).

The acetylene sensing mechanisms of the ZnO nanorings and Au nanoparticle-decorated ZnO nanorings can be explained as follows and are illustrated in Fig. 9(a)–(d). Fig. 9(a) illustrates the electronic transmission path within and between ZnO nanoring(s). Fig. 9(b) shows the vertical section of an edge of a ZnO nanoring. The response of semiconducting metal oxides is based on the reactions between the target gas molecule and the oxygen species on the surface of the oxides.^39,40 ZnO is an n-type semiconductor and its resistance is mainly determined by the conduction band electrons. As shown in Fig. 9(b), when the hexagonal ZnO nanorings are surrounded by air, oxygen molecules can be adsorbed on their surface to generate chemisorbed oxygen species by capturing electrons from the conduction band of ZnO. As a result, a wide electron depletion layer is generated next to the surface of the ZnO nanorings and it narrows the electronic conducting channel, which can lead to an increase in the ZnO nanoring resistance. When the sensor is exposed to acetylene, acetylene molecules can react with the chemisorbed oxygen species (O⁻ is believed to be dominant at the sensor operating temperature^41,42) and release the trapped electron back to the conduction band, which will diminish the depletion layer, widen the electronic conducting channel and result in the decrease of the ZnO nanoring resistance. When exposed to air again, chemisorbed oxygen species will increase and the ZnO nanoring resistance will recover to a high value. In other words, ZnO nanorings will respond to changes in ambience through the decrease (from air to ambient acetylene) or increase (from acetylene to ambient air) in resistance. The reaction between chemisorbed oxygen species and acetylene can be simply described as:

C₂H₂ + 5O⁻ = H₂O + 2CO₂ + 5e⁻ (1)

Fig. 9 (a) and (c) the electronic transmission path within and between the ZnO and Au–ZnO nanoring(s), respectively; (b) and (d) the acetylene-sensing mechanism of the ZnO and Au–ZnO nanorings, respectively.

The highly enhanced acetylene-sensing properties of the Au–ZnO nanorings can be mainly attributed to catalytic oxidation of acetylene induced by the Au nanoparticles. It has been reported that Au nanoparticles supported on metal oxide semiconductors show catalytic ability for acetylene oxidation. For the case of the Au–ZnO nanorings, the reaction between acetylene and O⁻ spices is promoted, and as a result, a higher response, faster response/recovery speed and lower operating temperature are obtained. Fig. 9(c) illustrates the electronic transmission path within and between the Au–ZnO nanoring(s) and Fig. 9(d) shows the vertical section of an edge of a Au–ZnO nanoring. In addition, Au nanoparticles can effectively accelerate the transformation of O₂ to O⁻ spices which will deepen the electron depletion layer (as shown in Fig. 9(d)) and cause the Au–ZnO nanorings to show a higher atmospheric resistance (as shown in Fig. S4, ESI†).^43,44 Therefore, when the Au–ZnO nanorings are exposed to acetylene, a larger resistance change i.e. a higher response is generated.

Moreover, the porous structure of the acetylene-sensing film composed of Au–ZnO nanorings is beneficial for gas diffusion and provides more active sites for oxygen adsorption and the reaction between adsorbed oxygen species and acetylene, which also contributes to the high response and rapid response/recovery rate.

Conclusions

In conclusion, hexagonal ZnO nanorings were synthesized using a one-step hydrothermal method and Au nanoparticles were decorated on the surface of ZnO nanorings through a facile deposition process. The as-prepared ZnO nanorings showed a well-defined hexagonal shape with a width of 0.75–1.4 μm, a thickness of 0.17–0.33 μm and a hollow size of 0.2–1 μm. For the Au nanoparticle-decorated ZnO nanorings (Au–ZnO nanorings), Au nanoparticles with a size of 3–10 nm were distributed discretely on the surface of the ZnO nanorings. The acetylene sensing performance was tested for the ZnO nanorings and Au–ZnO nanorings. The results indicated that the Au–ZnO nanorings showed a higher response (28 to 100 ppm acetylene), lower operating temperature (255 °C), faster response/recovery speed (less than 9 s and 5 s, respectively), and lower minimum detectable acetylene concentration (about 1 ppm).

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 61274068 and 61404058), the Project of Science and Technology Plan of Changchun City (Grant No. 14KG020) and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2013KF10).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16552k

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