Article pubs.acs.org/acssensors
Detection of Methanol with Fast Response by Monodispersed Indium Tungsten Oxide Ellipsoidal Nanospheres Chong Wang,†,‡ Xueying Kou,†,‡ Ning Xie,†,‡ Lanlan Guo,†,‡ Yanfeng Sun,*,†,‡ Xiaohong Chuai,†,‡ Jian Ma,†,‡ Peng Sun,†,‡ Yue Wang,§ and Geyu Lu*,†,‡ †
State Key Laboratory of Automotive Simulation and Control, Jilin University, 5988 Renmin Avenue, Changchun 130012, China College of Electronic Science and Engineering and §State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China
‡
S Supporting Information *
ABSTRACT: Indium tungsten oxide ellipsoidal nanospheres were prepared with different In/W ratios by using a simple hydrothermal method without any surfactant for the first time. Sensors based on different In/W ratios samples were fabricated, and one of the samples exhibited better response to methanol compared with others. High content of defective oxygen (Ov) and proper output proportion of In to W might be the main reasons for the better gas sensing properties. The length of the nanosphere was about 150−200 nm, and the width was about 100 nm. Various techniques were applied to investigate the nanospheres. Sensing characteristics toward methanol were investigated. Significantly, the sensor exhibited ultrafast response to methanol. The response time to 400 ppm methanol was no more than 2 s and the recovery time was 9 s at 312 °C. Most importantly, the humidity almost had no effect on the response of the sensor fabricated here, which is hard to achieve in gas-sensing applications. KEYWORDS: indium tungsten oxide, ellipsoidal nanospheres, methanol sensor, fast response, humidity independent
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and In2O3.35 Disadvantages still exist if we want to put these sensors into practical application eventually. Especially, humidity is one of the major obstacles that significantly affects the sensing performance including the response and the response/recovery time. Due to the ubiquity of water gas and the interaction between water and the sensing materials, it is pernicious to detect targeting gas.36 One solution for these issues is to fabricate new type of sensing materials which can overcome these problems without further doing catalytic additives through using facile synthesis process. Complex metal oxides possessed much more advantages such as thermal stability and crystalline form by contrast with simple metal oxides.37 A growing number of different synthesis methods was applied for the preparation of gas sensing material.32 Hydrothermal method has always been popular with high praise because the products synthesized by such method have pure composition and good dispersion.38−40 The advantages about hydrothermal method have been reported in detail in the literatures.38−40 The morphology, the aggregation levels, and the purity of the sensing body affect the sensing abilities of the material.38 Hydrothermal method affords excellent control of these aspects to enhance the sensing abilities of the material
n this information age, people are becoming more and more dependent on acquiring information with fast speed. Gas sensors, as a device to transport information about the concentration and species of the detecting gas, have been widely used in our daily life.1−8 Owing to their wide range of applications, it seems to be such a meaningful task to develop sensors with high performance in the field of scientific research, and many researchers have devoted themselves to design highly sensitive gas sensors with fast response.9−12 Methanol is toxic to human beings. Excessive inhalation of methanol could cause headache, vertigo, and nausea.13 Methanol is also harmful to eyesight; even 20 mL can blind someone.14 Thus, a highly sensitive and stable methanol gas sensor has always been the pursuit in many aspects.15−19 The time weighted average (TWA) levels for methanol is 200 ppm, which is announced by the Occupational Safety and Health Administration (OSHA). The explosion limit for methanol is 6.7−36% (V/V). Long exposure to 1200−2300 ppm methanol will harm the visual system and skin. Up to now, many kinds of materials have been applied to develop gas sensors such as semiconducting metal oxides,20−22 silicon,23−25 polyaniline,26−28 and fiber optic chemical materials29−31 People have paid much attention on semiconducting oxides not only because of the low price, but also the promising properties for developing stable and sensitive gas sensors in the future.32 As reported before, some metal oxide semiconductor nanostructures have been used to fabricate methanol gas sensors, for example, SnO2,33 ZnO,34 © 2017 American Chemical Society
Received: January 15, 2017 Accepted: April 13, 2017 Published: April 13, 2017 648
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compared with other methods. All these merits make the hydrothermal method a preferred choice to synthesize sensing material for gas sensors. In this report, a simple hydrothermal method was applied to synthesize indium tungsten oxide ellipsoidal nanospheres without any surfactant. The sensors based on the ellipsoidal nanospheres exhibit excellent sensing performance to methanol. Moreover, it is worth noting that the humidity in environment almost had no effect on the sensing parameters of the sensor including response and response/recovery time.
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Article
RESULTS AND DISCUSSION
Structural and Morphological Characteristics. The XRD patterns of five samples are given in Figure 2a. It was found that with, increasing the input amount of In(NO3)3· 4.5H2O, peaks at about 29° and 50° began to appear. But the diffraction peaks could not be indexed to any existing substance. After the treatment of HCl, the weights of S1, S2, S3, S4 and S5 were reduced and all the diffraction peaks could be attributed to orthorhombic WO3 by XRD characterization (JPCDS card no. 89-4477), shown in Figure 2b. Revealing all the samples transformed into WO3. As the weight of WO3 could be known by weighing, we could calculate the weight of W. The weight loss during the treatment of HCl is considered as the weight of In. Thus, we could calculate the output molar proportion. The relationship between the input proportion of In to W and the output proportion of In to W is given in Figure 2c. The input proportion of In to W trends to decrease with increasing the output proportion of In to W, then increases with further increasing the output proportion of In to W. Sample S3 shows the smallest output proportion of In to W. The morphologies and microstructures of samples S1−S5 are shown in Figure 3. They were observed by FESEM with different magnifications. From the panorama image, a number of ellipsoidal nanosphere with a length of 150−200 nm and width of 100 nm could be clearly observed. These abundant nanospheres indicated a high yield of the ellipsoidal nanostructures. The high-magnification FESEM image of a single ellipsoidal nanosphere is shown in Figure 3a3−e3, which exhibit detailed morphological information on the nanosphere. For all the samples, the surface of the nanosphere is rough. This rough surface is composed of numerous nanoparticles with even smaller size. In addition, sample S1 has the biggest length−diameter ratio. And with increasing the amount of In, the length−diameter ratio trends to decrease. These experimental results also indicate that even though the input amount of In was different, the morphology of as-prepared samples are similar to each other except the minor changes of length− diameter ratio. Sample S1 was then investigated TEM. Figure 4a is the low-magnification TEM image, from which abundant ellipsoidal nanospheres can be found obviously. SAED pattern is shown in Figure 4b, confirming these nanospheres are polycrystalline. Corresponding HRTEM image is given in Figure 4c, revealing the presence of highly crystalline nanospheres with random orientation and mesopores, as marked by the red circle. Figure 4d−g shows the EDX elemental scanning images, and different colors represent
EXPERIMENTAL SECTION
Preparation of Indium Tungsten Oxide Ellipsoidal Nanosphere. An amount of 0.166 g of Na2WO4·2H2O was dissolved in 25 mL of deionized water. Then, different amounts of In(NO3)3·4.5H2O (0.025, 0.25, 0.5, 2, 4 mmol) was dropped into the solution, respectively. After stirring for 5 min, the above solutions were transformed into Teflon-lined stainless steel autoclaves. Then the Teflon-lined stainless autoclaves were put into an oven and the temperature of the oven was set as 200 °C for 20 h. After 20 h, the autoclaves began to cool down until the temperature was as low as room temperature. The precipitates were collected. After being washed with deionized water and ethanol, the precipitates were dried in air. After that, the dried precipitates were annealed at 500 °C for 2 h in air atmosphere with a heating rate of 2 °C/min and labeled as S1, S2, S3, S4, and S5. Then, they were sealed in HCl, respectively. After washing with deionized water several times, yellow precipitates were collected. Characterization. The X-ray diffraction (XRD) patterns were recorded, and field emission scanning electron microscopy (FESEM) images and transmission electron microscopy (TEM) and highresolution TEM images were obtained to study the structure of the samples. X-ray photoelectron spectroscopy (XPS) measurements and Brunauer−Emmett−Teller (BET) measurements were also performed Fabrication and Measurement of Gas Sensor. The obtained powders were employed to fabricate gas sensor devices. The fabrication process and definition of response/reponse/recovery time were introduced in our previous report.41 A schematic structure of the gas sensor is shown in Figure 1.
Figure 1. Schematic structure of the gas sensor.
Figure 2. (a) X-ray diffraction patterns of the five samples. (b) X-ray diffraction patterns of the samples after immersing in HCl. (c) Relationship between input proportion of In to W and output proportion of In to W. 649
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Figure 3. SEM image of (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5 with different magnification.
Then, responses trend to decrease if the temperature is higher than 312 °C. When the operating temperature is too low, there is not enough energy to make the gas molecules and surface absorbed oxygen species reactive effectively. But if the operating temperature is too high, rate of adsorption is lower than desorption, reducing the efficient use of the sensing material.45 It is to say, the optimum balance established at a certain temperature. Thus, 312 °C was regarded as the optimum operating temperature in the following investigation. In addition, among the sensors, sensor S1 shows the maximum response and S3 shows the minimum response. Response of sensors S1−S5 toward different methanol concentration ranging from 20 to 1000 ppm is shown in Figure 5b. And S1 still shows the highest response among the sensors. Figure 5c displays the dynamic resistance curve to methanol with concentrations of 20, 50, 100, 200, 400, and 1000 ppm methanol. And the responses are 1.95, 2.87, 5.38, 7.58, 12, and 18.1, respectively. Obviously, the resistance decreased when the sensor contacted with the reducing gas, this kind of behavior belongs to n-type oxide semiconductor.32 Figure 6 presents an individual resistance transient of sensor S1 to methanol with a concentration of 400 ppm. The response time was only 1 s and the recovery time was 9 s, demonstrating the relative rapid response to methanol, which is vital in the perspective of practical application. We attribute the short response and recovery time to the loose structure of the ellipsoidal
Figure 4. (a) TEM image of S1. (b) SAED pattern of an individual ellipsoidal nanoparticle. (c) HRTEM image of an individual ellipsoidal nanoparticle. (d−g) EDX elemental maps of O, In, and W elements, respectively.
different elements. These images reveal the ellipsoidal nanospheres consist of W, In and O element. Gas Sensing Properties. Gas sensing characteristics of samples S1−S5 were investigated. As reported before in literature, the response of the oxide semiconductor gas sensor is strongly dependent on the operating temperature because of its affection on the surface state of oxide semiconductor and reactions on the surface of the material.42−44 The responses to 400 ppm methanol of S1−S5 were tested at different operating temperatures in order to ascertain the optimum temperature, and the corresponding results are given in Figure 5a. Obviously, for all the sensors, the response increases with raising operating temperature in the initial stage. Sensors exhibited the highest response to methanol until the temperature rises to 312 °C. 650
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Figure 5. Response of sensors based on S1−S5 (a) to 400 ppm methanol as a function of the operating temperature and (b) toward methanol with different concentrations. (c) Dynamic resistance of S1 to different concentrations of methanol.
sensor were carried out (Figure 7d). It can be seen that the response was almost constant even after 160 cyclic tests, which indicated the sensor had excellent repeatability. Excellent stability is the pursuit for developing gas sensors. Sensing performance was tested as the extension of the aging time, and the results are given in Figure 7e. During this period, the operating temperature was set as 312 °C all the time for subsequent sensing stability tests. Then, the response to 400 ppm methanol was carried out during 45 days. Only small variations exist for the responses during these days, implying that the sensor displayed promising property candidate with excellent stability. Selectivity is also an important parameter for the gas sensor in the real application.46 Therefore, response to 100 ppm acetone, ethanol, toluene and formaldehyde were examined respectively, as shown in Figure 7(f). Thus, the ellipsoidal nanosphere has a rather good selectivity to methanol. Mechanism. To figure out what is the main factor that contributed to the response enhancement of sensor S1, nitrogen adsorption and desorption measurements were achieved to investigate the samples. Figure S1 is the typical adsorption−desorption graphs of samples S1−S5. The solid spheres represent the adsorption curve and the hollow spheres represent the desorption curve. The specific surface area calculated by the BET method was 10.13, 9.29, 12.88, 14.21, and 13.09 m2/g for the S1, S2, S3, S4, and S5 samples, respectively. Even though the S1 sensor exhibits the highest response among these sensors, the specific surface area of S1 is only medium. In addition, the differences of the specific surface areas are small, and there is no direct relationship between the input amount of In salt and the value of the specific surface area. We speculate the specific surface area is not the main reason for the high response in this work. In air ambient, oxygen molecules directly absorb on the surface of the sensing material and exist in the state of O2−(ads), O−(ads), and O2−(ads).47 On exposure to methanol, gas molecules reactive with the oxygen ions on the surface of the sensing material (eq 1).
Figure 6. (a) Individual resistancecurveto 400 ppm of methanol. (b) Period of the resistance curve to 100, 200, and 400 ppm methanol.
nanospheres morphology. As marked in Figure 4a, the overlapping part of ellipsoidal nanosphere shows obviously darker color than the unsheathed part. This loose structure makes fast diffusion of the targeting gas and induced fast reaction speed. The inset is reversible response/recovery dynamic curves of the sensor to 100, 200, and 400 ppm methanol, revealing good reproducibility of the gas sensor material. Table S1 in the Supporting Information shows sensing performance of different methanol gas sensors reported in recent years, sensor we prepared here. Compared with the literatures before (Table S1), our sensor showed high response and fairly fast response and recovery time. The sensing performances of sensor S1 under different RH air (25%, 50%, 75%, and 95%) were tested. The result is shown in Figure 7a. Error bars represent the variability (standard error) obtained in three measurements. Clearly, the response was not significantly affected by humidity (the response is 15 under 25% RH and 16 under 95% RH). It is worth noting that the variations of resistance and response/recovery time with humidity are very small (Figure 7b and c). Demonstrating the gas-sensing characteristics of the material is independent of the relative humidity. To get the stability of sensor S1, the continuous cyclic tests of the responses toward 400 ppm methanol for the
CH3OH + 3O−(ads) → CO2 + 2H 2O + e−
(1)
As a result, the resistance of the sensor decreased. We reach a conclusion that abundant oxygen ions lead to sufficient reaction. The XPS spectrum of O 1s is exhibited in Figure 8b−f to verify the status of oxygen species, in which the curves could be decomposed into three fitted peaks. These three peaks can be attributed to the lattice oxygen species (OL), oxygen ions in oxygen deficient regions (OV), and chemisorbed oxygen species (OC) from lower binding energy to higher binding 651
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Figure 7. (a−c) Influence of RH on the response to 400 ppm methanol, the resistance in the air. and response/recovery time. (d) 160 continuous cylic tests to 400 ppm of methanol. (e) Response to 400 ppm methanol during 45 days. (f) Response to 100 ppm of various gases.
Figure 8. XPS spectra of O 1s of (a) S1, (b) S2, (c) S3, (d) S4, and (e) S5.
energy in turn. The relative percentages of OL, OV, and OC components for S1−S5 are listed in Table 1. The content of OV component is 39.2%, 33.77%, 31.78%, 32.41%, and 33.75%. The relationship between the gas response and the content of OV is given in Figure 9a. As reported before, high levels of OV makes the sample show high ability for adsorbing the ionized oxygen species and exhibit enhanced gas sensing performance.48 So, it is reasonable that S1 sensor exhibits the highest response among these sensors due to the highest Ov component. As shown in Figure 9b, the output proportion of
Table 1. Fitting Results of O 1s XPS Spectra of the Five Samples ① ② ③ ④ ⑤
652
OL
OV
OC
21.4 38.04 38.04 22.85 23.25
39.2 33.77 31.78 32.41 33.75
39.4 28.18 19.73 44.74 43
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.S.). *E-mail:
[email protected] (G.L.). ORCID
Chong Wang: 0000-0002-9031-9140 Geyu Lu: 0000-0002-7428-2456 Notes
The authors declare no competing financial interest.
Figure 9. (a) Relationship between gas response andcontent of OV. (b) Relationship between content of OV and output proportion of In to W.
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ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (Nos. 61573164, 61520106003, 61374218, 61134010, and 61327804), the Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT13018), the National High-Tech Research and Development Program of China (863 Program, No 2014AA06A505), and Project 2015094 supported by the Graduate Innovation Fund of Jilin University.
In/W is S3 < S2 < S1 < S4 < S5. Obviously, for all the sensors, the content of OV component increases with a raise of output proportion of In/W and reaches the maximum value as shown in S1 sample, then decreases with further increase output proportion of In/W. We come to a conclusion that output proportion of In/W have a direct relationship with the content of OV component, and proper output proportion of In/W leads the highest response. As mentioned, the response and response/recovery time were not significantly affected by the humidity, demonstrating gas-sensing characteristics of the material is independent of the relative humidity. Indium oxide is one kind of humidity sensing material because of its high sensitivity to humidity.49,50 We speculate In plays a similar role in this research. The hydroxyl groups are considered to form on the surface of In, as a result, the gas-sensing characteristics is independent of humidity. The similar results were reported before.3651,52 Lee et al. did excellent work in which NiO was regarded as an important role because of the adsorption of hydroxyl groups in the humidity atmosphere, making the water shows no effect on the sensing process.36 In our work, In is considered as the key reason to keep the stable gas-sensing characteristics even in the humidity atmosphere. This property suggests that our sensors might be beneficial to fabricate stable methanol gas sensor in the future application.
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CONCLUSION In this paper, indium tungsten oxide ellipsoidal nanospheres were successfully prepared with different In/W ratios by a simple hydrothermal method without any surfactant for the first time. One of them showed the best gas sensing properties to methanol compared with others. We attributed it to the high content of OV and proper output proportion of In to W. The length of the nanosphere was about 150−200 nm, and the width was about 100 nm. The sensor based on the indium tungsten oxide ellipsoidal nanospheres exhibited ultrafast response to methanol. Moreover, the response of the sensor was not significantly affected by humidity. And it showed excellent repeatability and stability.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.7b00028. Typical N2 adsorption−desorption isotherms of samples S1−S5; comparison of the sensing performances of various metal oxide nanostructure-based sensors toward methanol (PDF) 653
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DOI: 10.1021/acssensors.7b00028 ACS Sens. 2017, 2, 648−654
