An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite

Open AccessArticle An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite by Cheng ZhangCheng Zhang SciProfiles Scilit Preprints.org Google Scholar 1,2, Ze

ZhangZe Zhang SciProfiles Scilit Preprints.org Google Scholar 1,2, Yao TianYao Tian SciProfiles Scilit Preprints.org Google Scholar 1,2, Lingmin YuLingmin Yu SciProfiles Scilit Preprints.org

Google Scholar 3 and Hairong WangHairong Wang SciProfiles Scilit Preprints.org Google Scholar 1,2,* 1 State Key Laboratory for Manufacturing Systems Engineering, Xi’an 710049, China 2

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China 3 School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China * Author

to whom correspondence should be addressed. Sensors 2024, 24(23), 7823; https://doi.org/10.3390/s24237823 Submission received: 11 November 2024 / Revised: 29 November 2024 / Accepted: 5

December 2024 / Published: 7 December 2024 (This article belongs to the Section Chemical Sensors) Download keyboard_arrow_down Download PDF Download PDF with Cover Download XML Download Epub

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ethanol, an ultrasensitive ethanol sensor was developed based on a dual-nanoparticle In2O3/SnO2 composite that was prepared by hydrothermal synthesis, and its suspension was dipped on a flat

electrode to form a gas sensor. The nanocomposite was characterized by an SEM (scanning electron microscope), XRD (X-ray diffraction), and a TEM (transmission electron microscope), and the

nanoparticle structure was observed. The experimental results showed that gas sensors based on the In2O3/SnO2 nanocomposite had higher responses compared to sensors based on pure In2O3.

Among the nanocomposites, the one with a In2O3-to-SnO2 mol ratio of 1:8 was used in the sensor with the highest response of 1.41 to 100 ppb ethanol at 150 °C, which also exhibited good

repeatability. The ultrasensitive response to ethanol can be attributed to the faster electron migration rate and the increase in oxygen-absorbing sites caused by the n-n heterojunction in

the nanocomposite. Due to its low detection limit, good repeatability, and relatively high responses in high humidity, this sensor has a potential application in exhaled breath detection.

Keywords: nanocomposite; n-n heterojunction; gas sensors; ethanol; low detection limit 1. IntroductionEthanol is a VOC (volatile organic compound), and it is widely used in many fields such

as chemical industries, pharmacology, motor vehicle fuel, food industries, liquors, beverages, and so on [1]. Ethanol gas is also present in breath exhaled from the human body [2]. Like many

other kinds of VOCs in human exhaled gases [3,4,5,6], the concentration of ethanol may correspond to the health of the human body [7,8,9,10]. For example, ethanol gas has been used as a

biomarker of liver disease [11]. To realize disease screening by exhaled breath analysis, one precondition is to detect the concentration of ethanol gas fast and accurately. Turner C et al.

used SIFT-MS (selected-ion flow-tube mass spectrometry) to monitor the volatile compounds in the exhaled breath of 30 volunteers (19 males and 11 females) over a 6-month period and found

that the mean ethanol level for all samples was 196 ppb (parts per billion) with a standard deviation of 244 ppb, and the ethanol concentrations of all breath samples ranged from 0 to 1663

ppb [12]. To detect the trace-level ethanol that exists in complex exhaled gases, a sensor must have a very low detection limit, high selectivity, and good anti-humidity ability. So far, it

is still quite difficult to find a gas sensor with all these merits.Many studies have shown that MOS (metal oxide semiconductor) sensors are effective for ethanol gas detection [13]. In

1962, Seiyama et al. developed the first metal oxide-based gas sensor [14]. Nowadays, metal oxide sensors have been thoroughly studied and widely applied [15,16,17,18,19,20,21].

Frequently used metal oxide materials include SnO2 [22,23], In2O3 [24], ZnO [25], TiO2, and so on [26,27,28,29,30,31,32]. In2O3 is a typical n-type material with a wide band gap and low

resistivity. P. Van Tong et al. synthesized porous In2O3 nanorods and developed a gas sensor based on this nanomaterial, which has excellent CO-sensing performance [33]. Furthermore, In2O3

is also used to detect gas at ultralow concentrations. A gas sensor based on the hierarchical branch-like In2O3 nanomaterial has obvious responses to ppb-level ozone [34]. Different

microstructures can also be beneficial for gas responses. A larger specific surface area can adsorb more gas molecules, resulting in higher response values and lower detection limits. W. Zhu

et al. found that In2O3 nanospheres have high responses, long-term stability, a short response time for ethanol, and good selectivity compared to other materials [35]. In order to obtain

better response characteristics for ethanol, researchers turned to heterojunctions by combining In2O3 with other metal oxides; e.g., O. K. Zhang et al. prepared hierarchical ZnO-In2O3

heterojunctions, and gas sensors based on these heterojunctions had high sensitivity and selectivity to ethanol [36]. The work functions of the two metal oxide materials are different, and

the energy band bends at contact. Electron mobility increases, so more oxygen vacancies are generated when adsorbing air, more free electrons are released when contacting the target gas, and

better response characteristics are obtained. Due to its wide band gap and stable chemical properties, SnO2 is widely used in VOC gas sensors [37]. The responses of gas sensors based on

In2O3-SnO2 nanocomposites are higher than those of pure SnO2 for detecting formaldehyde, and their improved sensing properties were actually attributed to the n-n heterojunction and the

synergistic interactions between In2O3 and SnO2 [38]. D. An et al. prepared a series of materials. In2O3-SnO2 nanocomposites were synthesized by the hydrothermal method. A sensor based on an

In2O3–SnO2 composite exhibited high responses to n-butanol gas. [39]. Y. Liu et al. prepared In2O3-SnO2 by the hydrothermal method, and the test results showed that a sensor made of this

material had a good response performance for ethanol gas [40]. Illuminated by these findings, in this study, we developed an ultrasensitive ethanol sensor based on a nanocomposite formed by

the nanoparticle metal oxides In2O3 and SnO2, which were prepared via hydrothermal synthesis, and attempted to detect ethanol at the hundred ppb level in a humid environment like exhaled

breath. 2. Experimental Section 2.1. Nanocomposite PreparationWe prepared the dual-nanoparticle In2O3/SnO2 composite by the hydrothermal method as follows [41]. First, 100 mL of distilled

water was placed in a beaker. Then, solutions of 10 mmol NaCO3 and 5 mmol SnCl4·5H2O were put into the beaker. It was stirred in a magnetic mixer for 2 h. After that, the mixed solutio

n in the vessel was poured into a PTFE (polytetrafluoroethylene)-lined reactor. Then, the reactor was placed in a drying oven and heated at a constant temperature of 160 °C for 12 h [39].

After naturally cooling to room temperature, a suspension was obtained, and it was processed by washing in anhydrous ethanol and deionized water several times. The resultant was

ultrasonically cleaned for 10 min and was then centrifuged at 4000× g rpm (revolutions per minute) for 30 min. The above cleaning process was repeated a couple of times. The remaining

suspension was placed in a drying oven to thermostatically dry at 80 °C for 12 h until the suspension completely became a white powder. Finally, to calcinate the precipitate, the heating

rate was set as 2 °C min−1. The temperature was increased to 400 °C and maintained for 2 h. The result was a pure SnO2 nanostructure, which was marked as 1#.Then, solutions of 10 mmol NaCO3

and 5 mmol In(NO3)3 were put into a vessel with 100 mL of distilled water, and they were stirred vigorously in a magnetic mixer for 2 h. The mixed solution was put into the PTFE-lined

reactor, and the reactor was heated at a constant temperature of 160 °C for 12 h in a drying oven [42]. Using the same process as before, after naturally cooling to room temperature, a

suspension was obtained, and it was processed by washing with anhydrous ethanol and deionized water several times. The resultant was cleaned via centrifugation for 10 min. The above cleaning

step was repeated a couple of times. The remaining suspension was placed in a drying oven to thermostatically dry at 80 °C for 12 h until the suspension completely became a white powder.

Finally, to calcinate the precipitate, the heating rate was set as 2 °C min−1. The temperature was increased to 400 °C and maintained for 2 h. The result was a pure In2O3 nanostructure,

which was marked as 6#.To prepare the In2O3/SnO2 nanocomposites, four groups of solutions, which were 1 mmol SnCl4·5H2O and 4 mmol In(NO3)3; 2 mmol SnCl4·5H2O and 3 mmol In(NO3)3; 3 mmol

SnCl4·5H2O and 2 mmol In(NO3)3; and 4 mmol SnCl4·5H2O, 1 mmol In(NO3)3, and 10 mmol NaCO3, were selected and poured into vessels with 100 mL of distilled water. With the same processes used

for samples 1# and 6#, four kinds of In2O3/SnO2 nanocomposites with different mol ratios were prepared and marked as 2#, 3#, 4#, and 5#, as shown in Table 1. The nanocomposite preparation

process is shown schematically in Figure 1. 2.2. Preparation of the Gas Sensors and Test MethodsThe powders of the above sensitive nanomaterials were put into test tubes, and a moderate

amount of deionized water was added to prepare suspensions. A planar ceramic sheet measuring 3 × 3 × 0.25 mm had two sensitive electrodes on one side and heating electrodes on another side.

A suspension of sensitive material was manually coated on the sensitive electrodes by drop coating. The sensitive-material coating was heated in an oven at 500 °C for 2 h. Finally, the four

pins of

the ceramic sheet were welded to the sensor TO packaging with soldering tin. The preparation process is shown in Figure 2.The sensors were tested using a gas distribution system, as shown in

Figure 3. Two mass flow meters (Sevenstar CS200A) controlled by a computer provided different flow rates for ethanol and dry air. The mixed gas flowed into the cylindrical gas chamber in

which the gas sensor was installed at 200 sccm (standard cubic centimeters per minute). A DC power supply was used to apply a voltage to the two heating electrodes, providing working

temperatures for the gas sensor. The resistance between the two sensitive electrodes of the gas sensor was monitored by a digital multimeter (Agilent 34970 A). For the gas sensor, the

resistance between the two sensitive electrodes in pure air was Ra, the resistance in an atmosphere with the target gas was Rg, and the response of the sensor based on an n-type metal oxide

was defined using Equation (1). R e s p o n s e = R a / R g (1) In the test, 2 ppm ethanol in dry air was marked as gas 1 and dry air was marked as gas 2. The ethanol concentration was

changed by controlling the flow rate ratio of the two mass flow meters; e.g., an ethanol concentration of 100 ppb could be achieved with 5% gas 1 (2 ppm ethanol) and 95% gas 2 (dry air). 3.

Results and Discussion 3.1. CharacterizationThe prepared materials were then characterized and analyzed in different ways. FESEM (field emission scanning electron microscopy) images were

obtained on a Quanta 250FEG microscope with an operating voltage of 20 kV. As shown in Figure 4, the In2O3, SnO2, and In2O3/SnO2 composites were irregular nanoparticles with diameters of

about 10 nm. The same 500 nm scale bar is used for all images. Some of the particles were clustered together, most of them showed bumpy surfaces, and the sizes of several of the more obvious

nanoparticles were measured (about ten nm to tens of nm).The XRD (X-ray diffraction) patterns of the In2O3, SnO2, and In2O3/SnO2 composites are shown in Figure 5. They were obtained using a

Bruker D8 ADVANC instrument (Cu Kα, 2.2 kW, λ = 1.54056 Å) at 2θ from 20° to 70°. The angles of the main characteristic peaks were observed at 2θ values of 30.63°, 35.54°, and 51.04°, which

corresponded to the (222), (400), and (440) planes, respectively, when compared with an In2O3 standard card (JCPDS71-2195). The material belonged to a cubic crystal-type iron manganese ore

type with an average grain size of 29 nm. The SnO2 pattern was compared with the standard card JCPDS72-1147, and the angles of the main characteristic peaks were observed at 2θ values of

26.57°, 33.92°, and 51.74°, which corresponded to the (110), (101), and (211) planes. The crystal type was a tetragonal rutile type, and the average grain size was 6 nm. Figure 5 shows that

the peak was obvious during the detection of pure In2O3. When SnO2 was present, the corresponding peak was covered, probably because of the small content of In2O3. Finally, the c

urve of the composite material only had a relatively obvious peak of SnO2. It is also possible that the In2O3 in the composite material existed in the interior of the SnO2, making the curve

of the SnO2 more obvious. 3.2. Gas-Sensing CharacteristicsNext, a series of gas-sensitive performance tests were performed on the sensors. Figure 6 describes the relations between the

responses and the operating temperatures of the In2O3/SnO2 sensors for 1 ppm ethanol at operating temperatures ranging from 100 °C to 250 °C. The resistances of all sensors changed at

different operating temperature values, and the reason for the resistance changes induced by the changes in the sensor materials at the same operating temperatures was the n-n

heterojunction. The responses of the sensors reached their maximum values at their optimal operating temperatures. The optimal operating temperature of In2O3/SnO2 sensor sample 2# was 150

°C, as shown in Figure 6.The responses of the sensors to ethanol from 100 ppb to 2000 ppb were tested at the optimal operating temperature, as shown in Figure 7. The responses of all sensors

increased with increasing concentrations, and sample 2# had the highest response of all sensors.The previous tests showed that sample 2# performed best. As shown in Figure 8, sensor 2# had

an obvious response to a low concentration of ethanol and had good repeatability. The response of the In2O3/SnO2 sensor to 100 ppb ethanol was 1.41, while the response of the pure In2O3

sensor to 100 ppb ethanol was 1.21. The baseline resistance and the resistances of In2O3/SnO2 sensor 2# to 200 ppb ethanol at different relative humidities at 150 °C are shown in Figure 9.

The sensors can also work in a humid environment. 3.3. DiscussionFirst of all, we analyzed the process and yield of the hydrothermal preparation. About 0.64 g of composite material can be

prepared each time using a 100 mL reactor, and the total preparation time is about 48 h. If a larger reactor were used, a higher yield could be obtained per preparation. In addition, there

are several times during the preparation process that require natural cooling. It can be assumed that using a heat dissipation device would shorten the preparation time.Next, we will present

a detailed discussion and analysis based on the gas test results. The ethanol gas sensor based on the In2O3/SnO2 nanocomposite prepared by hydrothermal synthesis demonstrated high

responses, a low detection limit, and a low operating temperature.As shown in Table 2, the ethanol sensor based on In2O3/SnO2 in this work has several advantages. In2O3/SnO2 has a lower

working temperature, consumes less power, and can detect ethanol at a lower concentration of 100 ppb. Compared to pure In2O3, In2O3/SnO2 has a higher response to 2 ppm ethanol. This

In2O3/SnO2-based sensor has good repeatability and can operate in humid environments.According to the detection concentration and the signal-to-noise ratio measured at 100 ppb, the

theoretical detection lim

it was calculated. The LOD (limit of detection) of the sensor is approximately 1.5 ppb.Figure 10 shows the distribution of the O, Sn, and In elements in the sample 2# In2O3/SnO2

nanocomposite based on EDS (energy-dispersive spectroscopy). Figure 11 shows the sample 2# In2O3/SnO2 nanocomposite as characterized by a JEOL JEM-2100 TEM (transmission electron

microscope). As shown in Figure 11, at a higher magnification, the nanocomposite can be clearly seen as a particle with a diameter of about 10 nm. The two metal oxides are evenly distributed

in the picture. As shown in Figure 11c, a d-spacing of 0.236 nm was attributed to the SnO2 (200) plane and 0.413 nm was assigned to the (211) plane of In2O3. Figure 11d shows the SAED

(selected-area electron diffraction) patterns of In2O3/SnO2.These results agree well with the structural results obtained from the XRD patterns. Next, we discuss the mechanism of the

sensitive material.When the nanomaterial is in air, the surface of the material adsorbs oxygen in the air, forming oxygen vacancies, which leads to an increase in the material resistance.

When the material comes into contact with ethanol gas, ethanol reacts with oxygen to produce gains and losses of electrons, forming a large number of free electrons, which causes the

resistance of the material to decrease, and the concentration of ethanol can be obtained by testing the resistance change of the material. When the temperature is above 150 °C, oxygen mostly

exists as O − . The specific reaction process is depicted in Equations (2)–(6). O 2 g a s → O 2 a d s o r b e d (2) O 2 ( a d s o r b e d ) + e − → O 2 ( a d s o r b e d ) − (3) O 2 ( a d s

o r b e d ) − + e − → 2 O ( a d s o r b e d ) − (4) O ( a d s o r b e d ) − + e − → O ( a d s o r b e d ) 2 − (5) C 2 H 6 O ( g a s ) + 6 O − → 3 H 2 O ( g a s ) + 2 C O 2 ( g a s ) + 6 e −

(6) The composite material of In2O3/SnO2 has a larger specific surface area, which enables the sensor to absorb more gas molecules when contacting ethanol, acquiring electrons more quickly

and reducing the resistance. In addition, the work function and band gap of SnO2 and In2O3 are W1 =4.7 eV and Eg1 = 3.5 eV, and W2 = 4.3 eV and Eg2 = 3.6 eV, respectively [46].As shown in

Figure 12, the band gaps of the two metal oxides are different, leading to band bending at the junction of the two materials. As a result, the resistance of the material increases, and the

test results confirmed this. Compared with pure In2O3, the two materials In2O3 and SnO2 can form an n-n heterojunction through hydrothermal synthesis, so when exposed to ethanol gas, the

sensor can form an electron depletion zone, releasing more free electrons, thereby improving the gas-sensitive performance of the material as a whole. 4. ConclusionsAn ethanol sensor based

on In2O3/SnO2 was prepared by hydrothermal synthesis. Then, characterization of the material and tests of the gas-sensitive properties of the sensor were carried out. The experimental

results showed t

hat the sensor has a low detection limit of 100 ppb and a higher response to ethanol compared to pure In2O3. The In2O3/SnO2-based sensor also has good repeatability and can work in a humid

environment.Due to the advantages above, this ethanol sensor has a potential application in exhaled breath analysis. In the future, we will continue to investigate the responses of this

nanocomposite-based sensor and will work to improve the accuracy of detecting trace ethanol in the complex VOCs in exhaled breath. Author ContributionsData curation, C.Z.; Methodology, Z.Z.;

Software, Y.T.; Validation, L.Y.; Formal analysis, H.W. All authors have read and agreed to the published version of the manuscript.FundingThis work was supported by the National Key R&D

Program of China (Grant No. 2022YFB3206800). We also appreciated support from the International Joint Laboratory for Micro/Nano Manufacturing and Measurement Technologies.Institutional

Review Board StatementNot applicable.Informed Consent StatementNot applicable.Data Availability StatementData will be made available on request.Conflicts of InterestThe authors declare that

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based chemical gas sensors. Sens. Actuators B-Chem. 2021, 329, 129275. [Google Scholar] [CrossRef] Figure 1. A schematic of the nanocomposite preparation process. Figure 1. A schematic of

the nanocomposite preparation process. Figure 2. The process of preparing the gas sensors on the flat ceramic sheet. Figure 2. The process of preparing the gas sensors on the flat ceramic

sheet. Figure 3. The real-time dynamic valve system. Figure 3. The real-time dynamic valve system. Figure 4. SEM images of (a) pure SnO2; (b–e) In2O3/SnO2 nanocomposites with ratios of 1:8,

2:6, 3:4, and 2:1; and (f) pure In2O3. Figure 4. SEM images of (a) pure SnO2; (b–e) In2O3/SnO2 nanocomposites with ratios of 1:8, 2:6, 3:4, and 2:1; and (f) pure In2O3. Figure 5. XRD

patterns of In2O3, SnO2, and nanocomposites of In2O3/SnO2 with different ratios. Figure 5. XRD patterns of In2O3, SnO2, and nanocomposites of In2O3/SnO2 with different ratios. Figure 6. The

optimal operating temperatures of different sensors for 1 ppm ethanol. Figure 6. The optimal operating temperatures of different sensors for 1 ppm ethanol. Figure 7. The responses to

different concentrations of ethanol. Figure 7. The responses to different concentrations of ethanol. Figure 8. (a,b) The responses of sample 2 (1:8 In2O3:SnO2) to ethanol and (c,d) the

repeatability of this sample’s response to 200 ppb ethanol. Figure 8. (a,b) The responses of sample 2 (1:8 In2O3:SnO2) to ethanol and (c,d) the repeatability of this sample’s response to 200

ppb ethanol. Figure 9. Baseline resistance and balancing resistance to 200 ppb ethanol of In2O3/SnO2 sensor 2# under different relative humidities. Figure 9. Baseline resistance and

balancing resistance to 200 ppb ethanol of In2O3/SnO2 sensor 2# under different relative humidities. Figure 10. EDS of sample 2# In2O3/SnO2 nanocomposite. Figure 10. EDS of sample 2#

In2O3/SnO2 nanocomposite. Figure 11. (a,b) HRTEM images of In2O3/SnO2 (sample 2#); (c) d-spacing; and (d) SAED patterns. Figure 11. (a,b) HRTEM images of In2O3/SnO2 (sample 2#); (c)

d-spacing; and (d) SAED patterns. Figure 12. Schematic diagrams and energy band structures of the In2O3/SnO2 nanocomposite: (a) before contact; (b) in air; and (c) in ethanol. Figure 12.

Schematic diagrams and energy band structures of the In2O3/SnO2 nanocomposite: (a) before contact; (b) in air; and (c) in ethanol. Table 1. Mark numbers of different materials. Table 1. Mark

numbers of different materials. Elements’ Mol RatiosSn100%80%60%40%20%0In020%40%60%80%100%Sample numbers1#2#3#4#5#6# Table 2. Comparison of gas-sensing characteristics of ethanol sensors.

Table 2. Comparison of gas-sensing characteristics of ethanol sensors. MaterialsTarget GasConcentrationResponseTemperatureRef.In2O3Ethanol2 ppm3.4300 °C[35]ZnO-In2O3Ethanol50 ppm170240

°C[36]SnO2Ethanol50 ppm16200 °C[38]CuOEthanol100 ppm1.76250 °C[43]Mo-SnO2Ethanol100 ppm46.8220 °C[44]SnO2-CuOEthanol100 ppm8320 °C[45]In2O3/SnO2Ethanol100 ppb2 ppm1.45150 °CThis work

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Zhang, C.; Zhang, Z.; Tian, Y.; Yu, L.; Wang, H. An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite. Sensors 2024, 24, 7823.

https://doi.org/10.3390/s24237823

Zhang C, Zhang Z, Tian Y, Yu L, Wang H. An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite. Sensors. 2024; 24(23):7823. https://doi.org/10.3390/s24237823

Zhang, Cheng, Ze Zhang, Yao Tian, Lingmin Yu, and Hairong Wang. 2024. "An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite" Sensors 24, no. 23: 7823.

https://doi.org/10.3390/s24237823

Zhang, C., Zhang, Z., Tian, Y., Yu, L., & Wang, H. (2024). An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite. Sensors, 24(23), 7823.

https://doi.org/10.3390/s24237823

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