- Email: [email protected]
Fabrication and gas sensing properties of Au-loaded SnO2 composite nanoparticles for highly sensitive hydrogen detection
Sensors and Actuators B 240 (2017) 664–673
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication and gas sensing properties of Au-loaded SnO2 composite nanoparticles for highly sensitive hydrogen detection Ying Wang a , Zhenting Zhao a , Yongjiao Sun a , Pengwei Li a , Jianlong Ji a , Yong Chen b,c , Wendong Zhang a,∗ , Jie Hu a,∗ a Micro and Nano System Research Center, Key Lab. of Advanced Transducers and Intelligent Control System (Ministry of Education) & College of Information Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, Hubei, China c Ecole Normale Supérieure, CNRS-ENS-UPMC UMR 8640, Paris 75005, France
a r t i c l e
i n f o
Article history: Received 14 April 2016 Received in revised form 5 September 2016 Accepted 6 September 2016 Available online 8 September 2016 Keywords: Hydrothermal method Au-loaded SnO2 Gas sensing performance Hydrogen sensor
a b s t r a c t Pristine tin dioxide (SnO2 ) and Au-loaded SnO2 composite nanoparticles were synthesized by a simple hydrothermal method. The phase structure, composition, and morphology of synthesized Au-loaded SnO2 composite nanoparticles were comprehensively investigated. Furthermore, the gas sensing performance of the as-prepared pristine and Au-loaded SnO2 gas sensors toward low concentration of hydrogen (H2 ) were systematically evaluated. The results indicated that compared to the pristine SnO2 gas sensor, the Au-loaded SnO2 composite nanoparticles could not only significantly improve the gas sensing response, but also decrease the optimum working temperature. Moreover, the experimental results showed that the 4.0 at.% Au-loaded SnO2 gas sensor exhibited the highest response (25) to 100 ppm H2 at 250 ◦ C, which was about five times higher than that of the pristine one. In addition, it also provided a rapid response/recovery time (1 s/3 s) and a low detection limit (1 ppb). Therefore, Au-loaded SnO2 composite nanoparticles are more suited for hydrogen detection compared to pristine SnO2 gas sensor. © 2016 Published by Elsevier B.V.
1. Introduction Hydrogen has attracted significant attention in recent years as one of the cleanest, superiorly efficient, abundant, and renewable energy sources [1–4]. Hydrogen is used extensively in scientific research and industry as the fuel for the internal combustion engines, rocket propellant, glass and steel manufacturing, shielding gas in atomic hydrogen welding, rotor coolant in electrical generators, and refining of petroleum products [5]. However, Hydrogen is odorless, colorless, and tasteless gas, which is extremely explosive in a wide range of concentration (4–75%) [6,7]. Further, dangers associated with hydrogen include high permeability through many materials and flammability. Therefore, development of rapid, accurate, and highly sensitive hydrogen sensors to detect a leakage for safe storage, delivery, and usage of hydrogen is highly desirable in order to achieve safe and efficient processing of hydrogen on massive scale. Recently, different hydrogen sensors based on thermal conductivity, catalytic combustion, thermoelectric effects, and
∗ Corresponding authors. E-mail addresses: [email protected] (W. Zhang), [email protected], [email protected] (J. Hu). http://dx.doi.org/10.1016/j.snb.2016.09.024 0925-4005/© 2016 Published by Elsevier B.V.
surface plasmon resonance have been reported [8–12]. Besides, hydrogen sensors based on metal oxide semiconductor nanomaterials such as zinc oxide (ZnO), tin dioxide (SnO2 ), indium(III) oxide (In2 O3 ), titania (TiO2 ), and tungsten oxide (WO2 ) have also attracted significant interest because of the possible advantages such as high sensitivity, long-term stability, and low fabrication cost [4,13–17]. Among different metal oxide semiconductor materials, SnO2 is particular interesting due to its high electron mobility, low cost and good chemical properties [18]. However, pristine SnO2 sensors exhibit poor selectivity, long response/recovery time, and high working temperature, which might limit its applications [19–21]. Accordingly, various methods, including surface functionalization, heterostructures fabrication, and metal doping, have been used to enhance the gas-sensing properties of SnO2 [22–27]. Among them, metal doping or loading is one of the most effective approaches to enhance the gas-sensing performances. For example, Xu et al. [22] studied aluminum (Al) doping in SnO2 nanofibers with several dopants concentrations, and showed that 1 at.% Al-doped SnO2 exhibited the highest response value (7.7) toward 100 ppm H2 at 340 ◦ C. Xiao et al. [15] reported 1.0 mol% palladium (Pd)-doped SnO2 hollow microcubes, and the measured results exhibited the highest response value of 90 along with a short response and recov-
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
ery time (3 s, 22 s) toward 200 ppm ethanol at 300 ◦ C. Katoch et al. [27] investigated gold (Au)-doped SnO2 nanofibers, and the gas response could reach 84 to 5 ppm carbon monoxide (CO) under the optimized temperature (300 ◦ C). Although much recent research efforts have been devoted to the investigation of the influence of noble metal elements to the sensing performances, few works put forward the optimization of Au concentration in SnO2 , which might be important to improve the gas sensing properties for hydrogen detection. In this study, pristine SnO2 and Au-loaded SnO2 composite nanoparticles were synthesized by hydrothermal method. The gas sensing properties of the sensors toward hydrogen were comprehensively studied and the effects of Au loading on SnO2 -based hydrogen detection response were analyzed, which showed a significant dependence of sensing performance on Au concentration. Moreover, the measured results show that the 4.0 at.% Au-loaded SnO2 gas sensor exhibit high response, low detection limit, fast rapid response/recovery, good selectivity and stability for the detection H2 gas.
2. Experimental 2.1. Chemicals Hydrogen tetrachloroaurate (III) hydrate (HAuCl4 ·4H2 O, 99.9%) and potassium stannatetrihydrate (K2 SnO3 ·3H2 O, 99.5%) and were purchased from Aladdin Ltd. (Shanghai, China). Glucose monohydrate (C6 H12 O6 ·H2 O) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals were reagent grade and were used without further purification
2.2. Synthesis of the sensing materials In a typical process, 4.87 mmol of K2 SnO3 ·3H2 O and 22 mmol of glucose monohydrate were dissolved into 30 ml of deionized water by ultrasonic dispersion. Then, definite amount (0, 0.5 at.%, 1.0 at.%, 2.0 at.%, 4.0 at.%, and 7.0 at.%) of HAuCl4 were added into the above mentioned solution. After stirring vigorously for 10 min, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave and heated at 180 ◦ C for 6 h. After cooling to room temperature, the brown-gray products were centrifuged and washed with deionized water and ethanol for several times and dried in an oven at 60 ◦ C for 6 h. Finally, the obtained products were calcined at 550 ◦ C for 5 h in air. For convenience, pristine SnO2 and different contents of Au-loaded SnO2 samples were labeled as SNAx (x = 0, 0.5, 1, 2, 4, and 7), corresponding to [Au/Sn] ratios of 0, 0.5, 1.0, 2.0, 4.0, and 7.0 at.%.
2.3. Characterization of the sensing materials The phase of the obtained products were investigated by Xray diffractometry (XRD, DRIGC-Y 2000A) with Cu-K␣1 radiation ( = 1.5406 Å), and the scanning speed was 0.1◦ s−1 for 2 in the range of 20◦ –80◦ . The energy dispersive spectroscopy (EDS, Bruker) was introduced to identify the chemical composition with a 10 kV accelerating voltage. The morphologies and structural characterization of the samples were characterized by scanning electron microscopy (SEM, JEM-7100F) and transmission electron microscope (TEM, JEM-2100F). Moreover, X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.) with monochromatic Al Ka (1486.6 eV) irradiation.
665
2.4. Sensor fabrication and measurements The fabrication process steps of pristine and Au-loaded SnO2 gas sensors can be found as follows: the as-calcined products were mixed with terpineol and ethyl cellulose (in a weight ratio of 2:7:1), and ground with deionized water in an agate mortar to form a paste. The paste was then coated on a ceramic tube with a pair of Au electrodes and Pt wires to form a sensing film (Fig. 1(a)). After that, the ceramic tube coated with sensing film was subsequently dried and annealed at 600 ◦ C for 2 h in order to strengthen the bonding between the sensing film and tube. Then, a Ni-Cr heating wire (diameter = 0.5 mm, resistance = 42 ) was inserted into the ceramic tube to heat the gas sensor. In order to improve the longterm stability, the gas sensors were aged at 300 ◦ C for 7 days before the gas sensing measurement. Fig. 1(b) shows the photograph of the integrated gas sensor. The gas sensing properties of the sensor was investigated by a commercial WS-30A static gas sensing measurement system (Weisheng Electronics Co., Ltd., Henan, China) at a fixed relative humidity (RH) of 30% ± 5%, and the detailed measurement information was described in Fig. S1 (Supporting information S1). The sensor gas response (S) is defined as S = Ra /Rg , where Ra and Rg are the resistance in air and in the testing gas, respectively. The response/recovery time is defined as the time taken for the sensor output to reach 90% of the total resistance change in the case of adsorption and desorption, respectively.
3. Results and discussion 3.1. Structural and morphological characterization The structures and crystalline phases of the synthesized composite nanoparticles were analyzed by XRD, as shown in Fig. 2(a). The detected diffraction peaks for SNA0 , SNA0.5 , and SNA1 samples can be indexed to tetragonal SnO2 (JCPDS card no.41-1445, space group: P4/mnm, a = 4.738 Å, and c = 3.187 Å), and no obvious characteristic peaks of Au can be observed for as-prepared samples, which is probably due to relatively low concentrations of Au in samples [13,28]. However, for SNA2 , SNA4 , and SNA7 samples, the XRD pattern not only includes all the peaks of SnO2 , but also other peaks located at 44.39◦ , 64.58◦ , and 77.57◦ , which are well consistent with (200), (220), and (311) lattice planes of Au (JCPDS card no. 65-2870), respectively. For the further investigation of the composite nanoparticles, the EDS measurements were performed on the SNA4 sample. From the measured results (Fig. 2(b)), the 4.0 at.% Au-loaded SnO2 composite nanoparticles are only composed of Sn, O, and Au elements, while the Si element was from the substrate. The morphologies of the as-prepared pristine and Au-loaded SnO2 composite nanoparticles were observed by SEM. Fig. 3(a) displays the pristine SnO2 sample, exhibiting a rough surface with uniform nanoparticle size of about 70 nm diameter. Fig. 3(b) exhibits the SNA0.5 nanoparticle, which presents the similar morphology as the pristine SnO2 with the particle diameters of 35–50 nm. However, with the increasing Au concentration, the observed morphologies of the SNAx (x = 1, 2, 4, and 7) samples exhibit smaller particle size, as shown in Fig. 3(c–f). Meanwhile, the Au-loaded SnO2 sample was further confirmed by the TEM (Supporting information S2). Fig. 4(a and b) shows the typical TEM images of 4.0 at.% Au-loaded SnO2 sample, and it can be found that Au nanoparticles are randomly distributed in SnO2 sample. From the HRTEM image of the selected areas, the measured interplanar spacings are about 0.33 nm and 0.27 nm (Fig. 4c), which is assigned to the (110) and (101) planes of SnO2 , respectively. Crystalline ordering is also observed with the lattice spacing of 0.23 nm, which corresponds to the (111) plane of Au (Fig. 4(d)).
666
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Fig 1. (a) Schematic diagram of SnO2 gas sensing element (b) and photograph of an integrated gas sensor.
Fig. 2. (a) XRD patterns of pristine and Au-loaded SnO2 samples, (b) EDS spectrum of SNA4 nanostructures.
Fig. 3. SEM images of pristine and Au-loaded SnO2 samples: (a) SNA0 , (b) SNA0.5 , (c) SNA1 , (d) SNA2 , (e) SNA4 and (f) SNA7 .
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
667
Fig. 4. TEM images of 4.0 at.% Au-loaded SnO2 composite nanoparticles; (a-b) low resolution images, (c) high resolution image SnO2 , and (d) Au.
To further analyze the compositions and chemical states of Auloaded SnO2 composite nanoparticles, XPS measurements were also conducted on the SNA7 sample after being annealed at 550 ◦ C for 5 h. The binding energies of the peaks were calibrated using the C1 s peak of the carbon contamination at 284.6 eV as a reference. Fig. 5(a) shows the low-resolution full range XPS spectrum of the sample. As expected, all the measured peaks demonstrate the presence of Sn, O, and Au elements, which is in good agreement with the results of XRD. Fig. 5(b) reveals the high resolution XPS spectra of Sn 3d energy state, revealing two symmetric peaks corresponding to Sn 3d5/2 and Sn 3d3/2 located at 485.90 and 494.30 eV,
respectively, which can be attributed to the binding energy of Sn–O bond. Fig. 5(c) presents two distinct peaks for O 1s state centered at 529.9 eV and 530.70 eV. The lower binding energy peak centered at 529.90 eV corresponds to the bulk lattice oxygen, and the higher energy peak positioned at 530.7 eV is associated with the chemisorbed oxygen ions such as O− and O2 − states in SnO2 . Fig. 5(d) displays the Au 4f XPS spectrum, which can be deconvoluted into two major peaks with binding energies at 82.65 eV and 86.15 eV, corresponding to Au 4f7/2 and Au 4f5/2 , respectively. The measured results indicated the existence of Au element in the sample, which was consistent with the results of XRD and EDS.
668
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Fig. 5. XPS spectra of the as-synthesized SNA7 sample: (a) full survey scan spectrum, (b) Sn3d, (c) O1s and (d) Au 4f.
Fig. 6. Responses of the as-prepared gas sensors to 100 ppm H2 at different working temperatures.
3.2. Gas sensing properties To determine the optimum working temperature for gas detection, gas sensors based on pristine and Au-loaded SnO2 composite nanoparticles were fabricated and their gas sensing properties were investigated by a WS-30A gas sensing measurement system. Fig. 6 shows the responses of all the sensors to 100 ppm H2 under different working temperatures in the range 125 to 350 ◦ C. Obviously, the gas responses of all the sensors increase at the beginning and then decrease with further increase in the working temperature. Moreover, all the Au-loaded SnO2 gas sensors exhibit the maximum response at the working temperature of 250 ◦ C, which is higher than that of pristine sensor. Consequently, an optimum working temperature of 250 ◦ C was chosen for the remainder of the experiments. Notably, the SNA4 sensor exhibits a maximum response of about 25, which is almost five times higher than that of the SNA0 (about 4.5 at 275 ◦ C). Thus, these results indicate that the loading of Au on
SnO2 gas sensors not only significantly contributed to enhance the response, but also considerably decreased the optimum working temperature. Meanwhile, to further analysis the response transients of sensor under actual operating temperatures, the behaviors of the change in sensor resistance level were also investigated on all the as-prepared gas sensors in air and hydrogen, as shown in Fig. S3 (Supporting information S3). The dynamic response/recovery transient experiments were conducted on pristine and Au-loaded SnO2 gas sensors. Fig. 7(a) shows the response/recovery curves of as-prepared sensors toward H2 with different concentrations varying from 5 ppm to 1000 ppm under their optimum working temperatures, respectively. Clearly, all the as-fabricated sensors exhibit a rapid response with the increasing concentration of H2 gas, and they can recover to their initial values after many cycles between exposure to H2 and air. The inset displays the magnified response of gas sensors at low concentrations of H2 (5–20 ppm). At the same time, the dynamic resistivity transients of pristine and Au-loaded SnO2 were also determined to H2 in the concentration range of 5–1000 ppm, and the resistance change upon Au loading concentration were shown in Fig. S4 (Supporting information S4). Simultaneously, to further investigate the gas sensing performances, the responses of all the as-prepared sensors were plotted as a function of the gas concentration, as shown in Fig. 7(b). The measured results revealed that the response of Au-loaded SnO2 gas sensors increased more sharply compared to the pristine SnO2 sensor, which displayed the superiority of these sensors over pristine SnO2 sensor. In particular, the SNA4 sensor exhibits the highest responses to different concentrations of H2 , and the response is about 6 times higher than that of SNA0 sensor to 1000 ppm H2 at 250 ◦ C. Moreover, the response of all the as-fabricated sensors also exhibits good linear relationships under different concentrations of H2 , as shown in Fig. S5 (Supporting information S5). In order to further evaluate the gas sensing properties of the as-fabricated gas sensors, the gas sensing experiments were performed at low concentrations (1 ppb–1 ppm) of H2 under
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
669
Fig. 7. (a) Dynamic response-recovery curves of the as-fabricated sensors to H2 in a range of 5–1000 ppm under their optimum working temperatures. (b) Dependence of the gas sensors response under different concentrations of H2 .
Fig. 8. Real-time response of the as-fabricated gas sensors to different low concentrations (1 ppb, 10 ppb, 100 ppb and 1 ppm) of hydrogen at their optimum working temperatures; (a) SNA0 , (b) SNA0.5 , (c) SNA1 , (d) SNA2 , (e) SNA4 , (f) SNA7 .
their optimum working temperatures (Fig. 8). Clearly, no obvious response changes are observed for SNA0 and SNA0.5 sensors, when the gas sensors are exposed to low concentrations H2 atmosphere (Fig. 8(a and b)). However, with the increasing concentration of Au, the as-fabricated gas sensors exhibit improvement gas sensing properties (Fig. 8(c–f)). Especially, the SNA4 sensor exhibits the highest gas sensing performance, and the measured response can reach to 2.1 for 1 ppm H2 . Moreover, the detection limit can also be down to 1 ppb, which suggests that the SNA4 gas sensor can be used as a promising candidate for low concentration of H2 detection. The response and recovery characteristics are critical parameters for evaluation the gas sensing performances of gas sensors. Fig. 9 presents the typical response-recovery curves of all the as-prepared sensors to 100 ppm H2 at their optimum working temperature. The response and recovery time of SNA4 gas sensor was about 1 and 3 s, respectively, which is shorter than that of pristine SnO2 sensor (2 s and 4 s). Moreover, the detailed corresponding response and recovery time for all the as-fabricated sensors is listed in Table S2 (Supporting information Table S2), and the measured results indicate that all the as-fabricated SnO2 gas sensors exhibit rapid response-recovery characteristics to H2 detection.
Fig. 9. Response–recovery characteristics of the as-prepared gas sensors to 100 ppm hydrogen under optimum working temperatures.
Selectivity is also significantly important in gas sensing properties for practical applications. Therefore, the gas sensing selectivity of pristine and Au-loaded SnO2 gas sensors was further tested
670
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
3.3. Gas sensing mechanism
Fig. 10. Selectivity of the as-prepared gas sensors to different kinds of gases at 100 ppm under their optimum working temperatures, respectively.
Fig. 11. The long-term stability of the as-prepared gas sensors to 100 ppm H2 under their optimum temperatures.
at their optimum working temperatures by exposing the sensors to 100 ppm potential interfering gases including nitrogen dioxide (NO2 ), carbon monoxide (CO), ammonia (NH3 ), and methane (CH4 ). Fig. 10 shows the histogram of the gas responses of as-fabricated sensors, thus demonstrating that the SNA4 gas sensor shows the highest response toward H2 . Moreover, it has a low response to the other interfering gases at the same temperature, which indicates that the sensor exhibits high selectivity toward H2 than other gases. Furthermore, the long-term stability of the sensors was also determined. Fig. 11 depicts the responses of all the fabricated sensors to 100 ppm H2 at their optimum working temperatures every 10 days. The measured results demonstrate that the maximal deviations of the response is less than 10% over 40 days, which confirms the as-prepared sensors have good long-term stability for H2 detection. In order to demonstrate the gas sensing performances of the fabricated SNA4 gas sensor, we summarized the gas sensing performances of the previously reported hydrogen gas sensors as well as the as-fabricated SNA4 in Table 1. The comparative results indicate that SNA4 gas sensor exhibits obvious advantages, such as the high response, rapid response/recovery times, and ultra-low limit of detection. Although, the optimal working temperature of the SNA4 is higher than that of ZnO–SnO2 composite and SnO2 /carbon [32,33]. The obtained results indicate that the SNA4 gas sensor is extremely promising for the detection of H2 .
It is well known that the gas sensing mechanism of SnO2 gas sensor belongs to the surface-controlled type, and the gas sensing performance is primarily determined by the species and the amount of chemisorbed oxygen density on the surface area of SnO2 . When pristine SnO2 gas sensor was exposed to ambient air (Fig. 12(a)), oxygen molecules are adsorbed on active sites of SnO2 nanoparticles to generate chemisorbed oxygen species (O2 − , O− , and O2− ) by trapping electrons from the SnO2 conduction band to generate the potential barrier, which results in the increase of the resistance. When SnO2 nanoparticles are exposed to a reducing gas, such as H2 (Fig. 12(b)), the chemisorbed oxygen anions at the surface of SnO2 react with reducing gas (H2 ). The gas removes chemisorbed oxygen anions and is oxidized. As a result, these free electrons trapped by chemisorbed oxygen species are released and return to conduction band of SnO2 , which leads to a decrease in the resistance. Compared to the pristine SnO2 gas sensor, the Au-loaded SnO2 composite nanoparticles exhibit significantly enhanced gas sensing properties. The improvement of gas sensing performance can be attributed to the following aspects. Firstly, the gas sensing mechanism could be explained by the catalytic activity of Au nanoparticles, which have been reported in previous studies [27,34]. The Au nanoparticles show high availability for the catalytic activation of the dissociation of molecular oxygen, and the activated oxygen species are then spilled onto the surface of SnO2 , which interacted with the absorbed oxygen, which result in an increase in width of the depleted layer. Secondly, the work function of SnO2 (4.5 eV) is lower than that of Au nanoparticles (5.1 eV), and the Schottky junctions would form between Au and SnO2 , which causes the electrons transfer from SnO2 to Au nanoparticles [18,35]. Both the factors led to much thicker electron depletion regions. Therefore, the resistance of the Au-loaded SnO2 gas sensors became significantly larger than that of pristine SnO2 sensor in ambient air (Fig. 12(c)). When the sensors were transferred to reductive H2 atmosphere, the thickness of the magnified depletion layer decreased sharply due to the reaction with adsorbed oxygen species, which resulted in further increase in the conductance and enhancement in the gas sensing response compared to pristine SnO2 (Fig. 12(d)). Thirdly, the smaller size of Au-loaded SnO2 composite nanoparticles are advantageous because they will provide the larger surface area for the reaction with target gas and the longer two-phase boundaries between Au and SnO2 , which cause the enhancement of gas sensing performance. In addition, the measured results also show that Au-loaded SnO2 composite nanoparticles also exhibit high selectivity toward H2 gas, which can be attributed to the H2 and O2 spillover reactions on Au surfaces. Because Au nanoparticles is a good catalyst, it can serve as specific absorption site to dissociate molecular oxygen (O2 ) as well as dissociation and ionization of H2 molecules into H atoms due to spillover effect [36–43]. Therefore, more oxygen and H2 molecules get faster adsorbed and dissociated, and the dissociated H atoms react with the oxygen ionic species (O− ), then release free electron (e− ) to conduction band. Accordingly, the resistance of the Au-loaded SnO2 composite nanoparticles sensor decreases, and response increases. Moreover, the measured results also demonstrate that the appropriate loading of Au nanoparticles could significantly improve the gas sensing properties, which was proved in Fig. 6. However, the excess loading of Au nanoparticles will results in less capability of conduction electrons at the surface to capture adsorbed oxygen, since there were not enough exposed surfaces of SnO2 to receive dissociated oxygen adsorbs [44]. And less exposed surfaces of SnO2 also affected narrow depletion layer width of the gas sensors, which will cause the deterioration of gas sensing performance.
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
671
Table 1 Comparison of the sensing performance of various gas sensors based on SnO2 nanostructures toward hydrogen. Materials
S (C)
tres
trec
T (◦ C)
LOD
Ref.
SnO2 nanorod SnO2 nanowire NiO/SnO2 nanofibers Al-doped SnO2 Pd-SnO2 composite nanofiber Co-doped SnO2 ZnO-SnO2 composite SnO2 /carbon nanotubes SNA4
1.4 (500 ppm) 4.25 (1000 ppm) 19 (500 ppm) 7.7 (100 ppm) 8.2 (100 ppm) 24 (100 ppm) 10 (10000 ppm) 1.3 (100 ppm) 25 (100 ppm)
– – – 3s 9s 2s 60 s – 1s
– – – 2s 9s 3s 75 s – 3s
300 300 320 340 280 330 150 100 250
– 10 ppm 5 ppm 10 ppm 4.5 ppm – – 100 ppm 1 ppb
[29] [29] [30] [22] [20] [31] [32] [33] This work
tres/rec : response/recovery time; S: response; C: gas concentration; T: temperature; LOD: limit of detection.
Fig. 12. Schematic band diagrams of SNA0 sensor exposed to (a) air, (b) hydrogen gas ambient, and Au-loaded SnO2 gas sensor to (c) air and (d) H2 gas ambient.
4. Conclusions In summary, pristine and Au-loaded SnO2 composite nanoparticles were synthesized by a facile hydrothermal process. The structure, composition, and morphology of synthesized Au-loaded SnO2 were characterized by means of XRD, SEM, TEM and XPS. The gas sensors based on pristine and Au-loaded SnO2 composite nanoparticles were fabricated and tested towards H2 gas. The as-fabricated Au-loaded SnO2 gas sensors present enhanced gas sensing performance. Especially, the 4.0 at.% Au-loaded SnO2 gas sensor showed the high response, low detection limit, fast rapid response/recovery time and good selectivity for H2 detection. The good performance can be attributed to the improved catalytic activation and formation of Schottky junctions at the Au and SnO2 interfaces. The present results imply the potentialities of 4.0 at.% Au-loaded SnO2 gas sensor for detecting of H2 gas.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 51205274 and 51205276), the National Natural Science of Shanxi Province (2016011039), the Talent project of Shanxi Province (201605D211020), Technology
Foundation for Selected Overseas Shanxi Scholar ([2014] 95), Science and Technology Major Project of the Shanxi Science and Technology Department (Grant no. 20121101004) and Key Disciplines Construction in Colleges and Universities of Shanxi (Grant no. [2012] 45).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.09.024.
References [1] S. Dhall, N. Jaggi, R. Nathawat, Functionalized multiwalled carbon nanotubes based hydrogen gas sensor, Sensor Actuators A: Phys. 201 (10) (2013) 321–327. [2] J.J. Hassan, M. Mahdi, A. Chin, C.W. Chin, H. Abu-Hassan, Z. Hassan, A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays, Sens. Actuators B: Chem. 176 (6) (2012) 360–367. [3] R.A. Rani, A.S. Zoolfakar, J.Z. Ou, M.R. Field, M. Austin, K. Kalantar-zadeh, Nanoporous Nb2 O5 hydrogen gas sensor, Sens. Actuators B: Chem. 176 (1) (2013) 149–156. [4] A. Esfandiar, A. Irajizad, O. Akhavan, S. Ghasemi, M.R. Gholami, Pd–WO3 /reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors, Int. J. Hydrogen Energy 39 (15) (2014) 8169–8179.
672
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
[5] M. Ramanathan, G. Skudlarek, H. Wang, S.B. Darling, Crossover behavior in the hydrogen sensing mechanism for palladium ultrathin films, Nanotechnology 21 (12) (2010) 125501–125506 (6). [6] J. Wang, B. Singh, J.H. Park, S. Rathi, I. Lee, S. Maeng, H. Joh, C. Lee, G. Kim, Dielectrophoresis of graphene oxide nanostructures for hydrogen gas sensor at room temperature, Sens. Actuators B: Chem. 194 (4) (2014) 296–302. [7] Z. Wang, Z. Li, T. Jiang, X. Xu, C. Wang, Ultrasensitive hydrogen sensor based on Pd0 -loaded SnO2 electrospun nanofibers at room temperature, Appl. Mater. Interfaces 5 (6) (2013) 2013–2021. [8] R. Tabassum, B.D. Gupta, Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite, Appl. Opt. 54 (5) (2015) 1032–1040. [9] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors–a review, Sens. Actuators B: Chem. 157 (2) (2011) 329–352. [10] A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, K. Watanabe, A surface plasmon resonance hydrogen sensor using Au/Ta2 O5 /Pd multi-layers on hetero-core optical fiber structures, Sens. Actuators B: Chem. 185 (2013) 53–58. [11] E. Brauns, E. Morsbach, S. Kunz, M. Bäumer, W. Lang, A fast and sensitive catalytic gas sensors for hydrogen detection based on stabilized nanoparticles as catalytic layer, Sens. Actuators B: Chem. 193 (3) (2014) 895–903. [12] C.H. Han, D.W. Hong, S.D. Han, J. Gwak, K.C. Singh, Catalytic combustion type hydrogen gas sensor using TiO2 and UV-LED, Sens. Actuators B: Chem. 125 (1) (2007) 224–228. [13] J. Hu, F. Gao, S. Sang, P. Li, X. Deng, We Zhang, Y. Chen, K. Lian, Optimization of Pd content in ZnO microstructures for high-performance gas detection, J. Mater. Sci. 50 (4) (2015) 1935–1942. [14] P. Rai, Y.S. Kim, H.M. Song, M.K. Song, Y.T. Yu, The role of gold catalyst on the sensing behavior of ZnO nanorods for CO and NO2 gases, Sens. Actuators B: Chem. 165 (1) (2012) 133–142. [15] L. Xiao, S. Shu, S. Liu, A facile synthesis of Pd-doped SnO2 hollow microcubes with enhanced sensing performance, Sens. Actuators B: Chem. 221 (31) (2015) 120–126. [16] T. Hyodo, S. Furuno, E. Fujii, K. Matsuoa, S. Motokuchoa, K. Kojioa, Y. Shimizua, Porous In2 O3 powders prepared by ultrasonic-spray pyrolysis as a NO2 -sensing material: utilization of polymethyl methacrylate microspheres synthesized by ultrasonic-assisted emulsion polymerization as a template, Sens. Actuators B: Chem. 187 (10) (2013) 495–502. [17] Y. Kwon, H. Kim, S. Lee, I. Chin, T. Seong, W.I. Lee, C. Lee, Enhanced ethanol sensing properties of TiO2 nanotube sensors, Sens. Actuators B: Chem. 173 (12) (2012) 441–446. [18] X. Wang, S. Qiu, C. He, G. Lu, W. Liu, J. Liu, Synthesis of Au decorated SnO2 mesoporous spheres with enhanced gas sensing performance, Advances 3 (41) (2013) 19002–19008. [19] N. Ma, K. Suematsu, M. Yuasa, K. Shimanoe, Pd size effect on the gas sensing properties of Pd-loaded SnO2 in humid atmosphere, Appl. Mater. Interfaces 7 (28) (2015) 15618–15625. [20] H. Zhang, Z. Li, L. Liu, X. Xua, Z. Wang, W. Wang, W. Zheng, B. Dong, C. Wang, Enhancement of hydrogen monitoring properties based on Pd-SnO2 composite nanofibers, Sens. Actuators B: Chem. 147 (1) (2010) 111–115. [21] F. Gao, G. Qin, Y. Li, Q. Jiang, L. Luo, K. Zhao, Y. Liuc, H. Zhao, One-pot synthesis of La-doped SnO2 layered nanoarrays with enhanced gas-sensing performance toward acetone, Advances 6 (13) (2016) 10298–10310. [22] X. Xu, J. Sun, H. Zhang, Z. Wang, B. Dong, T. Jiang, W. Wang, Z. Li, C. Wang, Effects of Al doping on SnO2 nanofibers in hydrogen sensor, Sens. Actuators B: Chem. 160 (1) (2011) 858–863. [23] D. Shaposhnik, R. Pavelko, E. Llobet, F. Gispert-Guirado, X. Vilanova, Hydrogen sensors on the basis of SnO2 -TiO2 systems, Sens. Actuators B: Chem. 174 (1) (2012) 527–534. [24] L. Xu, R. Xing, J. Song, W. Xu, H. Song, Nanotubes surface engineered by Ag nanoparticles: synthesis, characterization, and highly enhanced HCHO gas sensing properties, J. Mater. Chem. C 1 (11) (2013) 2174–2182. [25] L. Trakhtenberg, G. Gerasimov, V. Gromov, T.V. Belysheva, O.J. Ilegbusi, Effect of composition on sensing properties of SnO2 + In2 O3 mixed nanostructured films, Sens. Actuators B: Chem. 169 (4) (2012) 32–38. [26] S.W. Choi, A. Katoch, G.J. Sun, S.S. Kim, Synthesis and gas sensing performance of ZnO–SnO2 nanofiber-nanowire stem-branch heterostructure, Sens. Actuators B: Chem. 181 (2013) 787–794. [27] A. Katoch, J.H. Byun, S.W. Choi, S.S. Kim, One-pot synthesis of Au-loaded SnO2 nanofibers and their gas sensing properities, Sens. Actuators B: Chem. 202 (1) (2014) 38–45. [28] G. Qin, F. Gao, Q. Jiang, Y. Li, Y. Liu, L. Luo, K. Zhao, H. Zhao, Well-aligned Nd-doped SnO2 nanorods layered array: preparation, characterization and enhanced alcohol-gas sensing performance, Phys. Chem. Chem. Phys. 18 (7) (2016) 5537–5549. [29] B. Wang, L.F. Zhu, Y.H. Yang, N.S. Xu, G.W. Yang, Fabrication of a SnO2 nanowire gas sensor and sensor performance for hydrogen, J. Phys. Chem. C 112 (17) (2008) 6643–6647. [30] Z. Wang, Z. Li, J. Sun, H. Zhang, W. Wang, W. Zheng, C. Wang, Improved hydrogen monitoring properties based on p-NiO/n-SnO2 heterojunction composite nanofibers, J. Phys. Chem. C 114 (13) (2010) 6100–6105. [31] L. Liu, C. Guo, S. Li, L. Wang, Q. Dong, W. Li, Improved H2 sensing properties of Co-doped SnO2 nanofibers, Sens. Actuators B: Chem. 150 (2) (2010) 806–810. [32] B. Mondal, B. Basumatari, J. Das, C. Roychaudhury, H. Saha, N. Mukherjee, ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing, Sens. Actuators B: Chem. 194 (194) (2014) 389–396.
[33] M. Yang, D.H. Kim, W.S. Kim, T.J. Kang, B.Y. Lee, S. Hong, Y.H. Kim, S.H. Hong, H2 sensing characteristics of SnO2 coated single wall carbon nanotube network sensors, Nanotechnology 21 (21) (2010) 215501. [34] X. Li, X. Zhou, H. Guo, C. Wang, J. Liu, P. Sun, F. Liu, G. Lu, Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties, Appl. Mater. Interfaces 6 (21) (2014) 18661–18667. [35] L. Wang, H. Dou, Z. Lou, T. Zhang, Encapsuled nanoreactors (Au@SnO2 ): a new sensing material for chemical sensors, Nanoscale 5 (7) (2013) 2686–2691. [36] S.M. Majhi, P. Rai, Y.T. Yu, Facile approach to synthesize Au@ZnO core-shell nanoparticles and their application for highly sensitive and selective gas sensors, Appl. Mater. Interfaces 7 (18) (2015) 9462–9468. [37] R.K. Chava, S.Y. Oh, Y.T. Yu, Enhanced H2 gas sensing properties of Au@In2 O3 core–shell hybrid metal–semiconductor heteronanostructures, CrysEngComm 18 (20) (2016) 3655–3666. [38] X. Wang, W. Wang, Y. Liu, Enhanced acetone sensing performance of Au nanoparticles functionalized flower-like ZnO, Sens. Actuators B: Chem. 168 (8) (2012) 39–45. [39] Y.S. Shim, L. Zhang, D.H. Kim, Y.H. Kim, R.C. You, S.H. Nahm, C.Y. Kang, W. Lee, H.O. Jang, Highly sensitive and selective H2 , and NO2 , gas sensors based on surface-decorated WO3 , nanoigloos, Sens. Actuators B: Chem. 198 (4) (2014) 294–301. [40] X. Li, J. Liu, H. Guo, X. Zhou, C. Wang, P. Sun, X. Hu, G. Lu, Au@In2 O3 core–shell composites: a metal–semiconductor heterostructure for gas sensing applications, Advances 5 (1) (2014) 545–551. [41] S. Vallejos, P. Umek, T. Stoycheva, F. Annanouch, E. Llobet, X. Correig, P.D. Marco, C. Bittencourt, C. Blankman, Single-step deposition of Au- and Pt-nanoparticle-functionalized tungsten oxide nanoneedles synthesized via aerosol-assisted CVD, and used for fabrication of selective gas microsensor arrays, Adv. Funct. Mater. 23 (10) (2013) 1313–1322. [42] Q. Xiang, G.F. Meng, H.B. Zhao, Y. Zhang, H. Li, W.J. Ma, J.Q. Xu, Au nanoparticle modified WO3 nanorods with their enhanced properties for photocatalysis and gas sensing, J. Phys. Chem. C 114 (5) (2010) 2049–2055. [43] J. Guo, J. Zhang, M. Zhu, D. Ju, H. Xu, B. Cao, High-performance gas sensor based on ZnO nanowires functionalized by Au nanoparticles, Sens. Actuators B: Chem. 199 (4) (2014) 339–345. [44] E. Wongrat, N. Hongsith, D. Wongratanaphisan, A. Gardchareon, S. Choopun, Control of depletion layer width via amount of AuNPs for sensor response enhancement in ZnO nanostructure sensor, Sens. Actuators B: Chem. 171 (8) (2012) 230–237.
Biographies
Ying Wang received her B.S. degree from the Hohai University in 2010. She is majoying in Micro and Nano System Research Center under a PhD candidate at Taiyuan University of Technology. Her main research fields are semiconductor metal oxide gas sensor and synthesis of micro/nano functional materials.
Zhenting Zhao received his M.S. degree from the Taiyuan University of Technology in 2012. He is currently working toward the Ph.D. degree at the Micro and Nano System Research Center of Taiyuan University of Technology, China. His main researches focus on electroanalytical chemistry, electrochemical sensors and synthesis of material.
Yongjiao Sun received her B.S. degree from the Taiyuan University of Technology in 2012. She is currently working toward the Ph.D. degree at the Micro and Nano System Research Center, Taiyuan University of Technology, China. She main researches focus on semiconductor metal oxide gas sensor and fabrication of micro/nano functional materials.
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Pengwei Li received his Ph.D. degree from Beihang University China, in 2010. He is now an associate professor in the Micro and Nano System Research Center of Taiyuan University of Technology, China. His research interests focused on various functional nanomaterials and their related micro-nano electronic devices.
Jianlong Ji received his Ph.D. in Taiyuan University of technology and joined the Micro-nano system research center in 2014. He has united research experience in Tsinghua University, current research focus mainly on micro/nano-electromechanical systems (MEMS/NEMS) and microfluidic devices.
Yong Chen received his Ph.D. degree from University of Montpellier in 1986. He joins the Centre National de Recherche Scientifique (CNRS) in 1990 and he is now research director (1st class) of the CNRS and Group Leader of Pôle Microfluidique at the Ecole Normale Supérieure (ENS) of Paris. He is also Guest Professor of the Institute for Integrated Cell-Material Science (iCeMS) at Kyoto University and Changjian Visiting Scholar of Peking University (PKU). He has expertise in condensed matter physics, nanotechnology and nanobiotechnology, microfluidic and biomedical devices, stem cells and cell-material interaction.
673
Wendong Zhang received the Ph.D. degree from the Beijing Institute of Technology, Beijing, China, in 1995. He was with École Normale Supérieure, Paris, France, and the Massachusetts Institute of Technology, Cambridge, MA, USA, as a Senior Visiting Scholar, in 2006 and 2009, respectively. He served as the Headmaster of the North University of China, Taiyuan, China, and the Taiyuan University of Technology, Taiyuan, in 2004 and 2010, respectively. He is the Technical Chief of the National Security Key Basic Research Project. He was a recipient of the State Technological Invention Award three times and the State Technological Invention Award once. His current research interests include MEMS/NEMS and optical gyroscope. Jie Hu received his Ph.D. degree from Université Pierre et Marie Curie, France in 2011. After this, he worked as a short-term researcher at Institute for Integrated CellMaterial Sciences, Kyoto University, Japan. He is currently an associate professor in the Micro and Nano System Research Center of Taiyuan University of Technology, China. His research interests focuses on the synthesis of metal oxides and functional nanomaterials for their applications in gas sensor and electrochemical sensor.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication and gas sensing properties of Au-loaded SnO2 composite nanoparticles for highly sensitive hydrogen detection Ying Wang a , Zhenting Zhao a , Yongjiao Sun a , Pengwei Li a , Jianlong Ji a , Yong Chen b,c , Wendong Zhang a,∗ , Jie Hu a,∗ a Micro and Nano System Research Center, Key Lab. of Advanced Transducers and Intelligent Control System (Ministry of Education) & College of Information Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b Institute for Interdisciplinary Research, Jianghan University, Wuhan 430056, Hubei, China c Ecole Normale Supérieure, CNRS-ENS-UPMC UMR 8640, Paris 75005, France
a r t i c l e
i n f o
Article history: Received 14 April 2016 Received in revised form 5 September 2016 Accepted 6 September 2016 Available online 8 September 2016 Keywords: Hydrothermal method Au-loaded SnO2 Gas sensing performance Hydrogen sensor
a b s t r a c t Pristine tin dioxide (SnO2 ) and Au-loaded SnO2 composite nanoparticles were synthesized by a simple hydrothermal method. The phase structure, composition, and morphology of synthesized Au-loaded SnO2 composite nanoparticles were comprehensively investigated. Furthermore, the gas sensing performance of the as-prepared pristine and Au-loaded SnO2 gas sensors toward low concentration of hydrogen (H2 ) were systematically evaluated. The results indicated that compared to the pristine SnO2 gas sensor, the Au-loaded SnO2 composite nanoparticles could not only significantly improve the gas sensing response, but also decrease the optimum working temperature. Moreover, the experimental results showed that the 4.0 at.% Au-loaded SnO2 gas sensor exhibited the highest response (25) to 100 ppm H2 at 250 ◦ C, which was about five times higher than that of the pristine one. In addition, it also provided a rapid response/recovery time (1 s/3 s) and a low detection limit (1 ppb). Therefore, Au-loaded SnO2 composite nanoparticles are more suited for hydrogen detection compared to pristine SnO2 gas sensor. © 2016 Published by Elsevier B.V.
1. Introduction Hydrogen has attracted significant attention in recent years as one of the cleanest, superiorly efficient, abundant, and renewable energy sources [1–4]. Hydrogen is used extensively in scientific research and industry as the fuel for the internal combustion engines, rocket propellant, glass and steel manufacturing, shielding gas in atomic hydrogen welding, rotor coolant in electrical generators, and refining of petroleum products [5]. However, Hydrogen is odorless, colorless, and tasteless gas, which is extremely explosive in a wide range of concentration (4–75%) [6,7]. Further, dangers associated with hydrogen include high permeability through many materials and flammability. Therefore, development of rapid, accurate, and highly sensitive hydrogen sensors to detect a leakage for safe storage, delivery, and usage of hydrogen is highly desirable in order to achieve safe and efficient processing of hydrogen on massive scale. Recently, different hydrogen sensors based on thermal conductivity, catalytic combustion, thermoelectric effects, and
∗ Corresponding authors. E-mail addresses: [email protected] (W. Zhang), [email protected], [email protected] (J. Hu). http://dx.doi.org/10.1016/j.snb.2016.09.024 0925-4005/© 2016 Published by Elsevier B.V.
surface plasmon resonance have been reported [8–12]. Besides, hydrogen sensors based on metal oxide semiconductor nanomaterials such as zinc oxide (ZnO), tin dioxide (SnO2 ), indium(III) oxide (In2 O3 ), titania (TiO2 ), and tungsten oxide (WO2 ) have also attracted significant interest because of the possible advantages such as high sensitivity, long-term stability, and low fabrication cost [4,13–17]. Among different metal oxide semiconductor materials, SnO2 is particular interesting due to its high electron mobility, low cost and good chemical properties [18]. However, pristine SnO2 sensors exhibit poor selectivity, long response/recovery time, and high working temperature, which might limit its applications [19–21]. Accordingly, various methods, including surface functionalization, heterostructures fabrication, and metal doping, have been used to enhance the gas-sensing properties of SnO2 [22–27]. Among them, metal doping or loading is one of the most effective approaches to enhance the gas-sensing performances. For example, Xu et al. [22] studied aluminum (Al) doping in SnO2 nanofibers with several dopants concentrations, and showed that 1 at.% Al-doped SnO2 exhibited the highest response value (7.7) toward 100 ppm H2 at 340 ◦ C. Xiao et al. [15] reported 1.0 mol% palladium (Pd)-doped SnO2 hollow microcubes, and the measured results exhibited the highest response value of 90 along with a short response and recov-
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
ery time (3 s, 22 s) toward 200 ppm ethanol at 300 ◦ C. Katoch et al. [27] investigated gold (Au)-doped SnO2 nanofibers, and the gas response could reach 84 to 5 ppm carbon monoxide (CO) under the optimized temperature (300 ◦ C). Although much recent research efforts have been devoted to the investigation of the influence of noble metal elements to the sensing performances, few works put forward the optimization of Au concentration in SnO2 , which might be important to improve the gas sensing properties for hydrogen detection. In this study, pristine SnO2 and Au-loaded SnO2 composite nanoparticles were synthesized by hydrothermal method. The gas sensing properties of the sensors toward hydrogen were comprehensively studied and the effects of Au loading on SnO2 -based hydrogen detection response were analyzed, which showed a significant dependence of sensing performance on Au concentration. Moreover, the measured results show that the 4.0 at.% Au-loaded SnO2 gas sensor exhibit high response, low detection limit, fast rapid response/recovery, good selectivity and stability for the detection H2 gas.
2. Experimental 2.1. Chemicals Hydrogen tetrachloroaurate (III) hydrate (HAuCl4 ·4H2 O, 99.9%) and potassium stannatetrihydrate (K2 SnO3 ·3H2 O, 99.5%) and were purchased from Aladdin Ltd. (Shanghai, China). Glucose monohydrate (C6 H12 O6 ·H2 O) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals were reagent grade and were used without further purification
2.2. Synthesis of the sensing materials In a typical process, 4.87 mmol of K2 SnO3 ·3H2 O and 22 mmol of glucose monohydrate were dissolved into 30 ml of deionized water by ultrasonic dispersion. Then, definite amount (0, 0.5 at.%, 1.0 at.%, 2.0 at.%, 4.0 at.%, and 7.0 at.%) of HAuCl4 were added into the above mentioned solution. After stirring vigorously for 10 min, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave and heated at 180 ◦ C for 6 h. After cooling to room temperature, the brown-gray products were centrifuged and washed with deionized water and ethanol for several times and dried in an oven at 60 ◦ C for 6 h. Finally, the obtained products were calcined at 550 ◦ C for 5 h in air. For convenience, pristine SnO2 and different contents of Au-loaded SnO2 samples were labeled as SNAx (x = 0, 0.5, 1, 2, 4, and 7), corresponding to [Au/Sn] ratios of 0, 0.5, 1.0, 2.0, 4.0, and 7.0 at.%.
2.3. Characterization of the sensing materials The phase of the obtained products were investigated by Xray diffractometry (XRD, DRIGC-Y 2000A) with Cu-K␣1 radiation ( = 1.5406 Å), and the scanning speed was 0.1◦ s−1 for 2 in the range of 20◦ –80◦ . The energy dispersive spectroscopy (EDS, Bruker) was introduced to identify the chemical composition with a 10 kV accelerating voltage. The morphologies and structural characterization of the samples were characterized by scanning electron microscopy (SEM, JEM-7100F) and transmission electron microscope (TEM, JEM-2100F). Moreover, X-ray photoelectron spectroscopy (XPS) measurement was performed on a VG ESCALAB 250 spectrometer (Thermo Electron, U.K.) with monochromatic Al Ka (1486.6 eV) irradiation.
665
2.4. Sensor fabrication and measurements The fabrication process steps of pristine and Au-loaded SnO2 gas sensors can be found as follows: the as-calcined products were mixed with terpineol and ethyl cellulose (in a weight ratio of 2:7:1), and ground with deionized water in an agate mortar to form a paste. The paste was then coated on a ceramic tube with a pair of Au electrodes and Pt wires to form a sensing film (Fig. 1(a)). After that, the ceramic tube coated with sensing film was subsequently dried and annealed at 600 ◦ C for 2 h in order to strengthen the bonding between the sensing film and tube. Then, a Ni-Cr heating wire (diameter = 0.5 mm, resistance = 42 ) was inserted into the ceramic tube to heat the gas sensor. In order to improve the longterm stability, the gas sensors were aged at 300 ◦ C for 7 days before the gas sensing measurement. Fig. 1(b) shows the photograph of the integrated gas sensor. The gas sensing properties of the sensor was investigated by a commercial WS-30A static gas sensing measurement system (Weisheng Electronics Co., Ltd., Henan, China) at a fixed relative humidity (RH) of 30% ± 5%, and the detailed measurement information was described in Fig. S1 (Supporting information S1). The sensor gas response (S) is defined as S = Ra /Rg , where Ra and Rg are the resistance in air and in the testing gas, respectively. The response/recovery time is defined as the time taken for the sensor output to reach 90% of the total resistance change in the case of adsorption and desorption, respectively.
3. Results and discussion 3.1. Structural and morphological characterization The structures and crystalline phases of the synthesized composite nanoparticles were analyzed by XRD, as shown in Fig. 2(a). The detected diffraction peaks for SNA0 , SNA0.5 , and SNA1 samples can be indexed to tetragonal SnO2 (JCPDS card no.41-1445, space group: P4/mnm, a = 4.738 Å, and c = 3.187 Å), and no obvious characteristic peaks of Au can be observed for as-prepared samples, which is probably due to relatively low concentrations of Au in samples [13,28]. However, for SNA2 , SNA4 , and SNA7 samples, the XRD pattern not only includes all the peaks of SnO2 , but also other peaks located at 44.39◦ , 64.58◦ , and 77.57◦ , which are well consistent with (200), (220), and (311) lattice planes of Au (JCPDS card no. 65-2870), respectively. For the further investigation of the composite nanoparticles, the EDS measurements were performed on the SNA4 sample. From the measured results (Fig. 2(b)), the 4.0 at.% Au-loaded SnO2 composite nanoparticles are only composed of Sn, O, and Au elements, while the Si element was from the substrate. The morphologies of the as-prepared pristine and Au-loaded SnO2 composite nanoparticles were observed by SEM. Fig. 3(a) displays the pristine SnO2 sample, exhibiting a rough surface with uniform nanoparticle size of about 70 nm diameter. Fig. 3(b) exhibits the SNA0.5 nanoparticle, which presents the similar morphology as the pristine SnO2 with the particle diameters of 35–50 nm. However, with the increasing Au concentration, the observed morphologies of the SNAx (x = 1, 2, 4, and 7) samples exhibit smaller particle size, as shown in Fig. 3(c–f). Meanwhile, the Au-loaded SnO2 sample was further confirmed by the TEM (Supporting information S2). Fig. 4(a and b) shows the typical TEM images of 4.0 at.% Au-loaded SnO2 sample, and it can be found that Au nanoparticles are randomly distributed in SnO2 sample. From the HRTEM image of the selected areas, the measured interplanar spacings are about 0.33 nm and 0.27 nm (Fig. 4c), which is assigned to the (110) and (101) planes of SnO2 , respectively. Crystalline ordering is also observed with the lattice spacing of 0.23 nm, which corresponds to the (111) plane of Au (Fig. 4(d)).
666
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Fig 1. (a) Schematic diagram of SnO2 gas sensing element (b) and photograph of an integrated gas sensor.
Fig. 2. (a) XRD patterns of pristine and Au-loaded SnO2 samples, (b) EDS spectrum of SNA4 nanostructures.
Fig. 3. SEM images of pristine and Au-loaded SnO2 samples: (a) SNA0 , (b) SNA0.5 , (c) SNA1 , (d) SNA2 , (e) SNA4 and (f) SNA7 .
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
667
Fig. 4. TEM images of 4.0 at.% Au-loaded SnO2 composite nanoparticles; (a-b) low resolution images, (c) high resolution image SnO2 , and (d) Au.
To further analyze the compositions and chemical states of Auloaded SnO2 composite nanoparticles, XPS measurements were also conducted on the SNA7 sample after being annealed at 550 ◦ C for 5 h. The binding energies of the peaks were calibrated using the C1 s peak of the carbon contamination at 284.6 eV as a reference. Fig. 5(a) shows the low-resolution full range XPS spectrum of the sample. As expected, all the measured peaks demonstrate the presence of Sn, O, and Au elements, which is in good agreement with the results of XRD. Fig. 5(b) reveals the high resolution XPS spectra of Sn 3d energy state, revealing two symmetric peaks corresponding to Sn 3d5/2 and Sn 3d3/2 located at 485.90 and 494.30 eV,
respectively, which can be attributed to the binding energy of Sn–O bond. Fig. 5(c) presents two distinct peaks for O 1s state centered at 529.9 eV and 530.70 eV. The lower binding energy peak centered at 529.90 eV corresponds to the bulk lattice oxygen, and the higher energy peak positioned at 530.7 eV is associated with the chemisorbed oxygen ions such as O− and O2 − states in SnO2 . Fig. 5(d) displays the Au 4f XPS spectrum, which can be deconvoluted into two major peaks with binding energies at 82.65 eV and 86.15 eV, corresponding to Au 4f7/2 and Au 4f5/2 , respectively. The measured results indicated the existence of Au element in the sample, which was consistent with the results of XRD and EDS.
668
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Fig. 5. XPS spectra of the as-synthesized SNA7 sample: (a) full survey scan spectrum, (b) Sn3d, (c) O1s and (d) Au 4f.
Fig. 6. Responses of the as-prepared gas sensors to 100 ppm H2 at different working temperatures.
3.2. Gas sensing properties To determine the optimum working temperature for gas detection, gas sensors based on pristine and Au-loaded SnO2 composite nanoparticles were fabricated and their gas sensing properties were investigated by a WS-30A gas sensing measurement system. Fig. 6 shows the responses of all the sensors to 100 ppm H2 under different working temperatures in the range 125 to 350 ◦ C. Obviously, the gas responses of all the sensors increase at the beginning and then decrease with further increase in the working temperature. Moreover, all the Au-loaded SnO2 gas sensors exhibit the maximum response at the working temperature of 250 ◦ C, which is higher than that of pristine sensor. Consequently, an optimum working temperature of 250 ◦ C was chosen for the remainder of the experiments. Notably, the SNA4 sensor exhibits a maximum response of about 25, which is almost five times higher than that of the SNA0 (about 4.5 at 275 ◦ C). Thus, these results indicate that the loading of Au on
SnO2 gas sensors not only significantly contributed to enhance the response, but also considerably decreased the optimum working temperature. Meanwhile, to further analysis the response transients of sensor under actual operating temperatures, the behaviors of the change in sensor resistance level were also investigated on all the as-prepared gas sensors in air and hydrogen, as shown in Fig. S3 (Supporting information S3). The dynamic response/recovery transient experiments were conducted on pristine and Au-loaded SnO2 gas sensors. Fig. 7(a) shows the response/recovery curves of as-prepared sensors toward H2 with different concentrations varying from 5 ppm to 1000 ppm under their optimum working temperatures, respectively. Clearly, all the as-fabricated sensors exhibit a rapid response with the increasing concentration of H2 gas, and they can recover to their initial values after many cycles between exposure to H2 and air. The inset displays the magnified response of gas sensors at low concentrations of H2 (5–20 ppm). At the same time, the dynamic resistivity transients of pristine and Au-loaded SnO2 were also determined to H2 in the concentration range of 5–1000 ppm, and the resistance change upon Au loading concentration were shown in Fig. S4 (Supporting information S4). Simultaneously, to further investigate the gas sensing performances, the responses of all the as-prepared sensors were plotted as a function of the gas concentration, as shown in Fig. 7(b). The measured results revealed that the response of Au-loaded SnO2 gas sensors increased more sharply compared to the pristine SnO2 sensor, which displayed the superiority of these sensors over pristine SnO2 sensor. In particular, the SNA4 sensor exhibits the highest responses to different concentrations of H2 , and the response is about 6 times higher than that of SNA0 sensor to 1000 ppm H2 at 250 ◦ C. Moreover, the response of all the as-fabricated sensors also exhibits good linear relationships under different concentrations of H2 , as shown in Fig. S5 (Supporting information S5). In order to further evaluate the gas sensing properties of the as-fabricated gas sensors, the gas sensing experiments were performed at low concentrations (1 ppb–1 ppm) of H2 under
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
669
Fig. 7. (a) Dynamic response-recovery curves of the as-fabricated sensors to H2 in a range of 5–1000 ppm under their optimum working temperatures. (b) Dependence of the gas sensors response under different concentrations of H2 .
Fig. 8. Real-time response of the as-fabricated gas sensors to different low concentrations (1 ppb, 10 ppb, 100 ppb and 1 ppm) of hydrogen at their optimum working temperatures; (a) SNA0 , (b) SNA0.5 , (c) SNA1 , (d) SNA2 , (e) SNA4 , (f) SNA7 .
their optimum working temperatures (Fig. 8). Clearly, no obvious response changes are observed for SNA0 and SNA0.5 sensors, when the gas sensors are exposed to low concentrations H2 atmosphere (Fig. 8(a and b)). However, with the increasing concentration of Au, the as-fabricated gas sensors exhibit improvement gas sensing properties (Fig. 8(c–f)). Especially, the SNA4 sensor exhibits the highest gas sensing performance, and the measured response can reach to 2.1 for 1 ppm H2 . Moreover, the detection limit can also be down to 1 ppb, which suggests that the SNA4 gas sensor can be used as a promising candidate for low concentration of H2 detection. The response and recovery characteristics are critical parameters for evaluation the gas sensing performances of gas sensors. Fig. 9 presents the typical response-recovery curves of all the as-prepared sensors to 100 ppm H2 at their optimum working temperature. The response and recovery time of SNA4 gas sensor was about 1 and 3 s, respectively, which is shorter than that of pristine SnO2 sensor (2 s and 4 s). Moreover, the detailed corresponding response and recovery time for all the as-fabricated sensors is listed in Table S2 (Supporting information Table S2), and the measured results indicate that all the as-fabricated SnO2 gas sensors exhibit rapid response-recovery characteristics to H2 detection.
Fig. 9. Response–recovery characteristics of the as-prepared gas sensors to 100 ppm hydrogen under optimum working temperatures.
Selectivity is also significantly important in gas sensing properties for practical applications. Therefore, the gas sensing selectivity of pristine and Au-loaded SnO2 gas sensors was further tested
670
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
3.3. Gas sensing mechanism
Fig. 10. Selectivity of the as-prepared gas sensors to different kinds of gases at 100 ppm under their optimum working temperatures, respectively.
Fig. 11. The long-term stability of the as-prepared gas sensors to 100 ppm H2 under their optimum temperatures.
at their optimum working temperatures by exposing the sensors to 100 ppm potential interfering gases including nitrogen dioxide (NO2 ), carbon monoxide (CO), ammonia (NH3 ), and methane (CH4 ). Fig. 10 shows the histogram of the gas responses of as-fabricated sensors, thus demonstrating that the SNA4 gas sensor shows the highest response toward H2 . Moreover, it has a low response to the other interfering gases at the same temperature, which indicates that the sensor exhibits high selectivity toward H2 than other gases. Furthermore, the long-term stability of the sensors was also determined. Fig. 11 depicts the responses of all the fabricated sensors to 100 ppm H2 at their optimum working temperatures every 10 days. The measured results demonstrate that the maximal deviations of the response is less than 10% over 40 days, which confirms the as-prepared sensors have good long-term stability for H2 detection. In order to demonstrate the gas sensing performances of the fabricated SNA4 gas sensor, we summarized the gas sensing performances of the previously reported hydrogen gas sensors as well as the as-fabricated SNA4 in Table 1. The comparative results indicate that SNA4 gas sensor exhibits obvious advantages, such as the high response, rapid response/recovery times, and ultra-low limit of detection. Although, the optimal working temperature of the SNA4 is higher than that of ZnO–SnO2 composite and SnO2 /carbon [32,33]. The obtained results indicate that the SNA4 gas sensor is extremely promising for the detection of H2 .
It is well known that the gas sensing mechanism of SnO2 gas sensor belongs to the surface-controlled type, and the gas sensing performance is primarily determined by the species and the amount of chemisorbed oxygen density on the surface area of SnO2 . When pristine SnO2 gas sensor was exposed to ambient air (Fig. 12(a)), oxygen molecules are adsorbed on active sites of SnO2 nanoparticles to generate chemisorbed oxygen species (O2 − , O− , and O2− ) by trapping electrons from the SnO2 conduction band to generate the potential barrier, which results in the increase of the resistance. When SnO2 nanoparticles are exposed to a reducing gas, such as H2 (Fig. 12(b)), the chemisorbed oxygen anions at the surface of SnO2 react with reducing gas (H2 ). The gas removes chemisorbed oxygen anions and is oxidized. As a result, these free electrons trapped by chemisorbed oxygen species are released and return to conduction band of SnO2 , which leads to a decrease in the resistance. Compared to the pristine SnO2 gas sensor, the Au-loaded SnO2 composite nanoparticles exhibit significantly enhanced gas sensing properties. The improvement of gas sensing performance can be attributed to the following aspects. Firstly, the gas sensing mechanism could be explained by the catalytic activity of Au nanoparticles, which have been reported in previous studies [27,34]. The Au nanoparticles show high availability for the catalytic activation of the dissociation of molecular oxygen, and the activated oxygen species are then spilled onto the surface of SnO2 , which interacted with the absorbed oxygen, which result in an increase in width of the depleted layer. Secondly, the work function of SnO2 (4.5 eV) is lower than that of Au nanoparticles (5.1 eV), and the Schottky junctions would form between Au and SnO2 , which causes the electrons transfer from SnO2 to Au nanoparticles [18,35]. Both the factors led to much thicker electron depletion regions. Therefore, the resistance of the Au-loaded SnO2 gas sensors became significantly larger than that of pristine SnO2 sensor in ambient air (Fig. 12(c)). When the sensors were transferred to reductive H2 atmosphere, the thickness of the magnified depletion layer decreased sharply due to the reaction with adsorbed oxygen species, which resulted in further increase in the conductance and enhancement in the gas sensing response compared to pristine SnO2 (Fig. 12(d)). Thirdly, the smaller size of Au-loaded SnO2 composite nanoparticles are advantageous because they will provide the larger surface area for the reaction with target gas and the longer two-phase boundaries between Au and SnO2 , which cause the enhancement of gas sensing performance. In addition, the measured results also show that Au-loaded SnO2 composite nanoparticles also exhibit high selectivity toward H2 gas, which can be attributed to the H2 and O2 spillover reactions on Au surfaces. Because Au nanoparticles is a good catalyst, it can serve as specific absorption site to dissociate molecular oxygen (O2 ) as well as dissociation and ionization of H2 molecules into H atoms due to spillover effect [36–43]. Therefore, more oxygen and H2 molecules get faster adsorbed and dissociated, and the dissociated H atoms react with the oxygen ionic species (O− ), then release free electron (e− ) to conduction band. Accordingly, the resistance of the Au-loaded SnO2 composite nanoparticles sensor decreases, and response increases. Moreover, the measured results also demonstrate that the appropriate loading of Au nanoparticles could significantly improve the gas sensing properties, which was proved in Fig. 6. However, the excess loading of Au nanoparticles will results in less capability of conduction electrons at the surface to capture adsorbed oxygen, since there were not enough exposed surfaces of SnO2 to receive dissociated oxygen adsorbs [44]. And less exposed surfaces of SnO2 also affected narrow depletion layer width of the gas sensors, which will cause the deterioration of gas sensing performance.
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
671
Table 1 Comparison of the sensing performance of various gas sensors based on SnO2 nanostructures toward hydrogen. Materials
S (C)
tres
trec
T (◦ C)
LOD
Ref.
SnO2 nanorod SnO2 nanowire NiO/SnO2 nanofibers Al-doped SnO2 Pd-SnO2 composite nanofiber Co-doped SnO2 ZnO-SnO2 composite SnO2 /carbon nanotubes SNA4
1.4 (500 ppm) 4.25 (1000 ppm) 19 (500 ppm) 7.7 (100 ppm) 8.2 (100 ppm) 24 (100 ppm) 10 (10000 ppm) 1.3 (100 ppm) 25 (100 ppm)
– – – 3s 9s 2s 60 s – 1s
– – – 2s 9s 3s 75 s – 3s
300 300 320 340 280 330 150 100 250
– 10 ppm 5 ppm 10 ppm 4.5 ppm – – 100 ppm 1 ppb
[29] [29] [30] [22] [20] [31] [32] [33] This work
tres/rec : response/recovery time; S: response; C: gas concentration; T: temperature; LOD: limit of detection.
Fig. 12. Schematic band diagrams of SNA0 sensor exposed to (a) air, (b) hydrogen gas ambient, and Au-loaded SnO2 gas sensor to (c) air and (d) H2 gas ambient.
4. Conclusions In summary, pristine and Au-loaded SnO2 composite nanoparticles were synthesized by a facile hydrothermal process. The structure, composition, and morphology of synthesized Au-loaded SnO2 were characterized by means of XRD, SEM, TEM and XPS. The gas sensors based on pristine and Au-loaded SnO2 composite nanoparticles were fabricated and tested towards H2 gas. The as-fabricated Au-loaded SnO2 gas sensors present enhanced gas sensing performance. Especially, the 4.0 at.% Au-loaded SnO2 gas sensor showed the high response, low detection limit, fast rapid response/recovery time and good selectivity for H2 detection. The good performance can be attributed to the improved catalytic activation and formation of Schottky junctions at the Au and SnO2 interfaces. The present results imply the potentialities of 4.0 at.% Au-loaded SnO2 gas sensor for detecting of H2 gas.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant nos. 51205274 and 51205276), the National Natural Science of Shanxi Province (2016011039), the Talent project of Shanxi Province (201605D211020), Technology
Foundation for Selected Overseas Shanxi Scholar ([2014] 95), Science and Technology Major Project of the Shanxi Science and Technology Department (Grant no. 20121101004) and Key Disciplines Construction in Colleges and Universities of Shanxi (Grant no. [2012] 45).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.09.024.
References [1] S. Dhall, N. Jaggi, R. Nathawat, Functionalized multiwalled carbon nanotubes based hydrogen gas sensor, Sensor Actuators A: Phys. 201 (10) (2013) 321–327. [2] J.J. Hassan, M. Mahdi, A. Chin, C.W. Chin, H. Abu-Hassan, Z. Hassan, A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays, Sens. Actuators B: Chem. 176 (6) (2012) 360–367. [3] R.A. Rani, A.S. Zoolfakar, J.Z. Ou, M.R. Field, M. Austin, K. Kalantar-zadeh, Nanoporous Nb2 O5 hydrogen gas sensor, Sens. Actuators B: Chem. 176 (1) (2013) 149–156. [4] A. Esfandiar, A. Irajizad, O. Akhavan, S. Ghasemi, M.R. Gholami, Pd–WO3 /reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors, Int. J. Hydrogen Energy 39 (15) (2014) 8169–8179.
672
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
[5] M. Ramanathan, G. Skudlarek, H. Wang, S.B. Darling, Crossover behavior in the hydrogen sensing mechanism for palladium ultrathin films, Nanotechnology 21 (12) (2010) 125501–125506 (6). [6] J. Wang, B. Singh, J.H. Park, S. Rathi, I. Lee, S. Maeng, H. Joh, C. Lee, G. Kim, Dielectrophoresis of graphene oxide nanostructures for hydrogen gas sensor at room temperature, Sens. Actuators B: Chem. 194 (4) (2014) 296–302. [7] Z. Wang, Z. Li, T. Jiang, X. Xu, C. Wang, Ultrasensitive hydrogen sensor based on Pd0 -loaded SnO2 electrospun nanofibers at room temperature, Appl. Mater. Interfaces 5 (6) (2013) 2013–2021. [8] R. Tabassum, B.D. Gupta, Surface plasmon resonance-based fiber-optic hydrogen gas sensor utilizing palladium supported zinc oxide multilayers and their nanocomposite, Appl. Opt. 54 (5) (2015) 1032–1040. [9] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors–a review, Sens. Actuators B: Chem. 157 (2) (2011) 329–352. [10] A. Hosoki, M. Nishiyama, H. Igawa, A. Seki, Y. Choi, K. Watanabe, A surface plasmon resonance hydrogen sensor using Au/Ta2 O5 /Pd multi-layers on hetero-core optical fiber structures, Sens. Actuators B: Chem. 185 (2013) 53–58. [11] E. Brauns, E. Morsbach, S. Kunz, M. Bäumer, W. Lang, A fast and sensitive catalytic gas sensors for hydrogen detection based on stabilized nanoparticles as catalytic layer, Sens. Actuators B: Chem. 193 (3) (2014) 895–903. [12] C.H. Han, D.W. Hong, S.D. Han, J. Gwak, K.C. Singh, Catalytic combustion type hydrogen gas sensor using TiO2 and UV-LED, Sens. Actuators B: Chem. 125 (1) (2007) 224–228. [13] J. Hu, F. Gao, S. Sang, P. Li, X. Deng, We Zhang, Y. Chen, K. Lian, Optimization of Pd content in ZnO microstructures for high-performance gas detection, J. Mater. Sci. 50 (4) (2015) 1935–1942. [14] P. Rai, Y.S. Kim, H.M. Song, M.K. Song, Y.T. Yu, The role of gold catalyst on the sensing behavior of ZnO nanorods for CO and NO2 gases, Sens. Actuators B: Chem. 165 (1) (2012) 133–142. [15] L. Xiao, S. Shu, S. Liu, A facile synthesis of Pd-doped SnO2 hollow microcubes with enhanced sensing performance, Sens. Actuators B: Chem. 221 (31) (2015) 120–126. [16] T. Hyodo, S. Furuno, E. Fujii, K. Matsuoa, S. Motokuchoa, K. Kojioa, Y. Shimizua, Porous In2 O3 powders prepared by ultrasonic-spray pyrolysis as a NO2 -sensing material: utilization of polymethyl methacrylate microspheres synthesized by ultrasonic-assisted emulsion polymerization as a template, Sens. Actuators B: Chem. 187 (10) (2013) 495–502. [17] Y. Kwon, H. Kim, S. Lee, I. Chin, T. Seong, W.I. Lee, C. Lee, Enhanced ethanol sensing properties of TiO2 nanotube sensors, Sens. Actuators B: Chem. 173 (12) (2012) 441–446. [18] X. Wang, S. Qiu, C. He, G. Lu, W. Liu, J. Liu, Synthesis of Au decorated SnO2 mesoporous spheres with enhanced gas sensing performance, Advances 3 (41) (2013) 19002–19008. [19] N. Ma, K. Suematsu, M. Yuasa, K. Shimanoe, Pd size effect on the gas sensing properties of Pd-loaded SnO2 in humid atmosphere, Appl. Mater. Interfaces 7 (28) (2015) 15618–15625. [20] H. Zhang, Z. Li, L. Liu, X. Xua, Z. Wang, W. Wang, W. Zheng, B. Dong, C. Wang, Enhancement of hydrogen monitoring properties based on Pd-SnO2 composite nanofibers, Sens. Actuators B: Chem. 147 (1) (2010) 111–115. [21] F. Gao, G. Qin, Y. Li, Q. Jiang, L. Luo, K. Zhao, Y. Liuc, H. Zhao, One-pot synthesis of La-doped SnO2 layered nanoarrays with enhanced gas-sensing performance toward acetone, Advances 6 (13) (2016) 10298–10310. [22] X. Xu, J. Sun, H. Zhang, Z. Wang, B. Dong, T. Jiang, W. Wang, Z. Li, C. Wang, Effects of Al doping on SnO2 nanofibers in hydrogen sensor, Sens. Actuators B: Chem. 160 (1) (2011) 858–863. [23] D. Shaposhnik, R. Pavelko, E. Llobet, F. Gispert-Guirado, X. Vilanova, Hydrogen sensors on the basis of SnO2 -TiO2 systems, Sens. Actuators B: Chem. 174 (1) (2012) 527–534. [24] L. Xu, R. Xing, J. Song, W. Xu, H. Song, Nanotubes surface engineered by Ag nanoparticles: synthesis, characterization, and highly enhanced HCHO gas sensing properties, J. Mater. Chem. C 1 (11) (2013) 2174–2182. [25] L. Trakhtenberg, G. Gerasimov, V. Gromov, T.V. Belysheva, O.J. Ilegbusi, Effect of composition on sensing properties of SnO2 + In2 O3 mixed nanostructured films, Sens. Actuators B: Chem. 169 (4) (2012) 32–38. [26] S.W. Choi, A. Katoch, G.J. Sun, S.S. Kim, Synthesis and gas sensing performance of ZnO–SnO2 nanofiber-nanowire stem-branch heterostructure, Sens. Actuators B: Chem. 181 (2013) 787–794. [27] A. Katoch, J.H. Byun, S.W. Choi, S.S. Kim, One-pot synthesis of Au-loaded SnO2 nanofibers and their gas sensing properities, Sens. Actuators B: Chem. 202 (1) (2014) 38–45. [28] G. Qin, F. Gao, Q. Jiang, Y. Li, Y. Liu, L. Luo, K. Zhao, H. Zhao, Well-aligned Nd-doped SnO2 nanorods layered array: preparation, characterization and enhanced alcohol-gas sensing performance, Phys. Chem. Chem. Phys. 18 (7) (2016) 5537–5549. [29] B. Wang, L.F. Zhu, Y.H. Yang, N.S. Xu, G.W. Yang, Fabrication of a SnO2 nanowire gas sensor and sensor performance for hydrogen, J. Phys. Chem. C 112 (17) (2008) 6643–6647. [30] Z. Wang, Z. Li, J. Sun, H. Zhang, W. Wang, W. Zheng, C. Wang, Improved hydrogen monitoring properties based on p-NiO/n-SnO2 heterojunction composite nanofibers, J. Phys. Chem. C 114 (13) (2010) 6100–6105. [31] L. Liu, C. Guo, S. Li, L. Wang, Q. Dong, W. Li, Improved H2 sensing properties of Co-doped SnO2 nanofibers, Sens. Actuators B: Chem. 150 (2) (2010) 806–810. [32] B. Mondal, B. Basumatari, J. Das, C. Roychaudhury, H. Saha, N. Mukherjee, ZnO–SnO2 based composite type gas sensor for selective hydrogen sensing, Sens. Actuators B: Chem. 194 (194) (2014) 389–396.
[33] M. Yang, D.H. Kim, W.S. Kim, T.J. Kang, B.Y. Lee, S. Hong, Y.H. Kim, S.H. Hong, H2 sensing characteristics of SnO2 coated single wall carbon nanotube network sensors, Nanotechnology 21 (21) (2010) 215501. [34] X. Li, X. Zhou, H. Guo, C. Wang, J. Liu, P. Sun, F. Liu, G. Lu, Design of Au@ZnO yolk-shell nanospheres with enhanced gas sensing properties, Appl. Mater. Interfaces 6 (21) (2014) 18661–18667. [35] L. Wang, H. Dou, Z. Lou, T. Zhang, Encapsuled nanoreactors (Au@SnO2 ): a new sensing material for chemical sensors, Nanoscale 5 (7) (2013) 2686–2691. [36] S.M. Majhi, P. Rai, Y.T. Yu, Facile approach to synthesize Au@ZnO core-shell nanoparticles and their application for highly sensitive and selective gas sensors, Appl. Mater. Interfaces 7 (18) (2015) 9462–9468. [37] R.K. Chava, S.Y. Oh, Y.T. Yu, Enhanced H2 gas sensing properties of Au@In2 O3 core–shell hybrid metal–semiconductor heteronanostructures, CrysEngComm 18 (20) (2016) 3655–3666. [38] X. Wang, W. Wang, Y. Liu, Enhanced acetone sensing performance of Au nanoparticles functionalized flower-like ZnO, Sens. Actuators B: Chem. 168 (8) (2012) 39–45. [39] Y.S. Shim, L. Zhang, D.H. Kim, Y.H. Kim, R.C. You, S.H. Nahm, C.Y. Kang, W. Lee, H.O. Jang, Highly sensitive and selective H2 , and NO2 , gas sensors based on surface-decorated WO3 , nanoigloos, Sens. Actuators B: Chem. 198 (4) (2014) 294–301. [40] X. Li, J. Liu, H. Guo, X. Zhou, C. Wang, P. Sun, X. Hu, G. Lu, Au@In2 O3 core–shell composites: a metal–semiconductor heterostructure for gas sensing applications, Advances 5 (1) (2014) 545–551. [41] S. Vallejos, P. Umek, T. Stoycheva, F. Annanouch, E. Llobet, X. Correig, P.D. Marco, C. Bittencourt, C. Blankman, Single-step deposition of Au- and Pt-nanoparticle-functionalized tungsten oxide nanoneedles synthesized via aerosol-assisted CVD, and used for fabrication of selective gas microsensor arrays, Adv. Funct. Mater. 23 (10) (2013) 1313–1322. [42] Q. Xiang, G.F. Meng, H.B. Zhao, Y. Zhang, H. Li, W.J. Ma, J.Q. Xu, Au nanoparticle modified WO3 nanorods with their enhanced properties for photocatalysis and gas sensing, J. Phys. Chem. C 114 (5) (2010) 2049–2055. [43] J. Guo, J. Zhang, M. Zhu, D. Ju, H. Xu, B. Cao, High-performance gas sensor based on ZnO nanowires functionalized by Au nanoparticles, Sens. Actuators B: Chem. 199 (4) (2014) 339–345. [44] E. Wongrat, N. Hongsith, D. Wongratanaphisan, A. Gardchareon, S. Choopun, Control of depletion layer width via amount of AuNPs for sensor response enhancement in ZnO nanostructure sensor, Sens. Actuators B: Chem. 171 (8) (2012) 230–237.
Biographies
Ying Wang received her B.S. degree from the Hohai University in 2010. She is majoying in Micro and Nano System Research Center under a PhD candidate at Taiyuan University of Technology. Her main research fields are semiconductor metal oxide gas sensor and synthesis of micro/nano functional materials.
Zhenting Zhao received his M.S. degree from the Taiyuan University of Technology in 2012. He is currently working toward the Ph.D. degree at the Micro and Nano System Research Center of Taiyuan University of Technology, China. His main researches focus on electroanalytical chemistry, electrochemical sensors and synthesis of material.
Yongjiao Sun received her B.S. degree from the Taiyuan University of Technology in 2012. She is currently working toward the Ph.D. degree at the Micro and Nano System Research Center, Taiyuan University of Technology, China. She main researches focus on semiconductor metal oxide gas sensor and fabrication of micro/nano functional materials.
Y. Wang et al. / Sensors and Actuators B 240 (2017) 664–673
Pengwei Li received his Ph.D. degree from Beihang University China, in 2010. He is now an associate professor in the Micro and Nano System Research Center of Taiyuan University of Technology, China. His research interests focused on various functional nanomaterials and their related micro-nano electronic devices.
Jianlong Ji received his Ph.D. in Taiyuan University of technology and joined the Micro-nano system research center in 2014. He has united research experience in Tsinghua University, current research focus mainly on micro/nano-electromechanical systems (MEMS/NEMS) and microfluidic devices.
Yong Chen received his Ph.D. degree from University of Montpellier in 1986. He joins the Centre National de Recherche Scientifique (CNRS) in 1990 and he is now research director (1st class) of the CNRS and Group Leader of Pôle Microfluidique at the Ecole Normale Supérieure (ENS) of Paris. He is also Guest Professor of the Institute for Integrated Cell-Material Science (iCeMS) at Kyoto University and Changjian Visiting Scholar of Peking University (PKU). He has expertise in condensed matter physics, nanotechnology and nanobiotechnology, microfluidic and biomedical devices, stem cells and cell-material interaction.
673
Wendong Zhang received the Ph.D. degree from the Beijing Institute of Technology, Beijing, China, in 1995. He was with École Normale Supérieure, Paris, France, and the Massachusetts Institute of Technology, Cambridge, MA, USA, as a Senior Visiting Scholar, in 2006 and 2009, respectively. He served as the Headmaster of the North University of China, Taiyuan, China, and the Taiyuan University of Technology, Taiyuan, in 2004 and 2010, respectively. He is the Technical Chief of the National Security Key Basic Research Project. He was a recipient of the State Technological Invention Award three times and the State Technological Invention Award once. His current research interests include MEMS/NEMS and optical gyroscope. Jie Hu received his Ph.D. degree from Université Pierre et Marie Curie, France in 2011. After this, he worked as a short-term researcher at Institute for Integrated CellMaterial Sciences, Kyoto University, Japan. He is currently an associate professor in the Micro and Nano System Research Center of Taiyuan University of Technology, China. His research interests focuses on the synthesis of metal oxides and functional nanomaterials for their applications in gas sensor and electrochemical sensor.