Highly sensitive and humidity-independent ethanol sensors based on In₂O₃ nanoflower/SnO₂ nanoparticle composites

Abstract

As an ethanol sensing material, the composites of In₂O₃–SnO₂ were composed of In₂O₃ microflowers and SnO₂ nanoparticles. Both In₂O₃ microflowers and SnO₂ nanoparticles were synthesized by hydrothermal method and then mixed in an ultrasonic environment. The morphology and phase composition of the as-synthesized samples were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The results on gas sensing properties showed that when the mass ratio of In₂O₃ and SnO₂ was 2 : 1, the sensor based on the as-prepared In₂O₃–SnO₂ composite exhibited high response and good selectivity to ethanol at 250 °C. The response to 100 ppm ethanol gas was 53.2. UV illumination stabilized the responses of the sensors while the relative humidity increased. The gas sensing mechanism proposed was that the addition of SnO₂ to In₂O₃ enhanced the catalytic activity for the ethanol reaction, which changed the electrical resistance of the materials. Besides, the morphology was helpful to the gas reaction on the surface of the sensing materials.

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1. Introduction

Ethanol detection of breath is an effective way to aid police in apprehending drink driving offenders.¹ In addition, as a kind of volatile organic compound (VOC), ethanol detection can potentially be utilized as a breath marker for specific diseases.² So far, several types of ethanol sensors have been reported for breath analysis.^3–5 In recent years, the semiconductor gas sensors have attracted the widespread attention of scientific researchers because of their high sensitivity, low cost and simplicity.^6–8 It is well known that the performances of sensors are significantly influenced by the components, crystallite size and morphology of the sensing materials.^9–11

Generally, a smaller crystal size owns a larger surface area, which provides more active sites for the target gas reacting with the sensing materials. However, agglomeration of fine particles greatly reduces the valid surface area. Fortunately, hierarchical nanostructure oxides have emerged due to their high active surface area and loose microstructure favourable for gas adsorption and rapid gas diffusion, and have dominated the research in enhancing the sensitivity of the metal oxide gas sensors. Another important sensing parameter is selectivity that can be improved through the use of composite metal oxides.^12–14 The physical interface between two dissimilar materials is often referred to as a heterojunction. In Akbar et al.¹⁵ review, the authors detailed the dominant electronic and chemical mechanisms that influence the performance of the metal oxide heterojunction as resistive-type gas sensors.

Up to date, many composites such as In₂O₃–CeO₂, ZnO–SnO₂, TiO₂–SnO₂, SnO₂–Fe₂O₃^16–19 have been reported to be promising sensing materials to obtain the sensitive and selective gas sensors. As two important kinds of fundamental materials, In₂O₃ and SnO₂ have been widely studied due to their responses towards different gases.^20–23 In₂O₃–SnO₂ composites^24,25 have also been proved to improve the properties of sensors. However, to the best of our knowledge, there are few reports on the morphology of In₂O₃–SnO₂ composites and no reports have shown that In₂O₃–SnO₂ composites have an excellent response and selectivity for ethanol.

Commonly, the water vapour strongly interacts with the oxide semiconductor surface, which leads to a significant deterioration of the sensor performance. There is high humidity in breath, so it is very important to reduce the influence of humidity on the performance of the sensors.

In this work, In₂O₃ microflowers and SnO₂ nanoparticles were both synthesized by hydrothermal method. Then the two kinds of materials were mixed in the ultrasound environment. The testing on gas sensing properties showed that while the mass ratio of In₂O₃ and SnO₂ was 2 : 1, the sensors based on In₂O₃–SnO₂ composite oxides showed the highest response to ethanol, the response to 100 ppm ethanol gas was 53.2 at 250 °C. UV illumination was adopted to reduce the influence of relative humidity on the performances of the sensors.

2. Experimental

2.1 Synthesis and characterization of the samples

In₂O₃ was synthesized according to the literatures.²⁶ In a typical synthesis process, 0.381 g of In(NO₃)₃·4.5H₂O and 0.15 g urea were dissolved in 36 mL deionized water with stirring until the solution was clear. Then, the mixture solution was transferred into a Teflon-lined stainless steel autoclave, heated at 160 °C for 4 h. After the autoclave was cooled to room temperature naturally, the result product was washed with deionized water and ethanol for six times alternately by centrifuge, and then dried at 80 °C for 3 h. The products were sintered to 500 °C at a rate of 2 °C min⁻¹ and then kept at 500 °C for 2 h. Then In₂O₃ microflowers were synthesized.

SnO₂ was synthesized according to the literatures.²⁷ In a typical synthesis process,0.526 g of SnCl₄·5H₂O, 0.6 g of cetyltrimethyl ammonium bromide (CTAB) and 0.2 g of hexamethylenetetramine (HMT) were added to 40 mL mixed solvent of ethanol and water (1 : 1, v/v) with stirring until the solution was clear. Then, the mixture solution was transferred into a Teflon-lined stainless steel autoclave, heated at 200 °C for 4 h. The processes after this were same with the process used to synthesize In₂O₃.

Then In₂O₃ microflowers and SnO₂ nanoparticles were mixed physically in certain mass ratio under ultrasound environment. Four In₂O₃–SnO₂ composites samples with mass concentration of 0% SnO₂, 33.3% SnO₂, 70% SnO₂ and 100% SnO₂ were obtained, represented by S0, S1, S2 and S3, respectively. Until this step, In₂O₃–SnO₂ composites were obtained.

2.2 Fabrication and measurement of sensors

Gas sensors were fabricated as follows: the prepared materials (S0, S1, S2 and S3) were mixed with deionized water to form something like slurry, and then spread on an alumina tube (4 mm in length, 1.2 mm in external diameter, and 0.8 mm in internal diameter, attached with a pair of gold electrodes, each electrode was connected with two Pt wire) by a small brush until forming a film thick enough to cover the gold electrode entirely. A Ni–Cr heating wire went through the tube to support heat for the sensors. The resistances of the sensors were measured by multimeter (fluke 8846A) under constant temperature and humidity. The measurement was processed by a static test system: R_a and R_g were the resistances of the sensors in air and tested gas, respectively. A certain amount of the tested gas was injected into a closed chamber, and the sensor was put into the chamber for the measurement of the sensitive performance. When the response reached a constant value, the sensor was removed to air to recover. The response of the sensor was defined as S = R_a/R_g for reducing gases or R_g/R_a for oxidizing gases. The response and recovery times are defined as the times taken by the resistance change of sensor achieving 90% of the total in the case of adsorption and desorption, respectively.

UV illumination was processed according our previous works.^28,29 UV-LED (peak wavelength = 380 nm; operating voltage = 3 V) was chosen as the light source, and the distance between the UV-LED and gas sensors was 2 mm. The measured power of the UV-LED at this distance was 0.7 Cd/m² (PR650, California, USA). Relative humidity (%RH) was obtained by using Humidity generator (Shanghai ESPEC environmental equipment corp. SETH-Z-022L).

In order to investigate the repeatability of the sensors. The fabrication and measurement of sensors were repeated 5 times, the results showed the sensors had similar properties.

3. Results and discussion

3.1 Characterization of the products

The crystal phase of In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites (S1) were investigated by XRD with a Rigaku D/max-2500 diffractometer using Cu-Kα1 radiation. From the Fig. 1, we can see that In₂O₃–SnO₂ composites has all peaks of In₂O₃ and SnO₂. They were in good agreement with the JCPDS file of In₂O₃ (JCPDS no. 71-2195) and SnO₂ (JCPDS no. 77-451).It proved that In₂O₃ microflowers and SnO₂ nanoparticles were mixed entirely by physical route and they had no chemical reacts. No diffraction peaks from any other impurities were observed, indicating the high purity of the products.

Fig. 1 XRD of In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites.

3.2 Structural and morphological characteristics

The morphologies and structures of In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites (S1) were identified by SEM using a JEOL JSM-7500F microscope with an accelerating voltage of 15 kV as shown in Fig. 2. Fig. 2(a) presents the SEM image of SnO₂ particles, it can be seen that SnO₂ particles are composed of relatively uniform SnO₂ nanoparticles with a length of 20–30 nm. It can be seen that the surfaces of nanoparticles are coarse. As is shown in Fig. 2(b), the In₂O₃ microflowers have a good dispersion and uniform size of 1–1.5 μm, it can be seen from the image that the In₂O₃ microflowers are composed of aggregated nanosheets. Fig. 2(c) shows In₂O₃–SnO₂ composites, SnO₂ nanoparticles seems to be surrounded by In₂O₃ microflowers, SnO₂ and In₂O₃ was marked with boxes. Composition of In₂O₃–SnO₂ composites (S1) had been characterized using EDS as shown in Fig. 2(d). The EDS spectrum showed that components of the materials are In, Sn and O. The Si was attributed to the substrate used in the SEM measurement.

Fig. 2 SEM images of (a) SnO₂ nanoparticles, (b) In₂O₃ microflowers and (c) In₂O₃–SnO₂ composites (d) EDS spectra of In₂O₃–SnO₂ composites.

Fig. 3 shows nitrogen adsorption–desorption isotherm and pore size distribution (inset of Fig. 3) of the In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites (S1). Pore size distribution curves were calculated from the desorption branch of a nitrogen isotherm by the BJH method using the Halsey equation. The BET surface area of the In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites were calculated to be 19.5 m² g⁻¹, 41.0 m² g⁻¹ and 34.4 m² g⁻¹ respectively by the Brunauer–Emmett–Teller (BET) method with a Micromeritics Gemini VII apparatus (Surface Area and Porosity System).

Fig. 3 Typical N₂ adsorption–desorption isotherms and pore size distribution of In₂O₃ microflowers, SnO₂ nanoparticles and In₂O₃–SnO₂ composites.

3.3 Gas-sensing properties

As we know, the mass ratio of In₂O₃ microflowers and SnO₂ nanoparticles and the working temperature both influenced greatly gas responses in this research, so the gas sensing properties of S0 to S3 to 100 ppm ethanol under different temperatures were investigated, the results were shown in Fig. 4.The VOCs were got from liquid evaporation, concentration calculation method is according to ideal gas equation of state (PV = nRT). The sensors based the In₂O₃–SnO₂ composites (S1) showed the best response while the mass ratio of In₂O₃ and SnO₂ was 2 : 1. The best response to 100 ppm ethanol was 53.2 at 250 °C. A comparison between the sensing performances of the sensor and literature reports^{26,27,30–32} were summarized in Table 1.

Fig. 4 Responses of sensors based on S0–S3 to 100 ppm ethanol.

Table 1 Comparison between the sensing performances of the sensor (S1) and literature reports

Material	Fabrication approach	Ethanol concentration (ppm)	Temperature (°C)	Sensor response	References
In2O3 microflowers	Hydrothermal route	100 ppm	RT	∼1.5	26
SnO2 nanostructures	Hydrothermal route	100 ppm	275 °C	∼9.6	27
SnO2–In2O3	Sol–gel and electrospinning	100 ppm	RT	∼3.0	30
In2O3 nanospheres	Hydrothermal route	100 ppm	275 °C	∼21.0	31
In2O3–SnO2 nanocomposites	Mechanochemical reaction	100 ppm	RT	∼18.7	32
In2O3–SnO2 composites	Hydrothermal route	100 ppm	250 °C	53.2	This work

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Fig. 5 shows the responses of the sensors based on S1 to different concentrations ethanol at 250 °C. Before 500 ppm, the responses increase with the ethanol concentration like a line, when the ethanol concentration is more than 500 ppm, the response is nearly saturated. Dynamic response towards 100–1000 ppm ethanol at 250 °C was shown in the inset of Fig. 5.

Fig. 5 Relationship between responses of sensors based on S1 and ethanol concentration at 250 °C, the inset displaying dynamic response.

The response and recovery characteristics were investigated which were shown in the Fig. 6(a).The results indicate that the In₂O₃–SnO₂ composites sensor based on S1 has fast response–recovery kinetics. The response and recovery times of the In₂O₃–SnO₂ composites sensors are about 15 s and 60 s, respectively. Five reversible cycles of the response curve indicate a stable and repeatable characteristic, as shown in Fig. 6(b).

Fig. 6 (a) Dynamic response resistance of the sensor (S1) and (b) five periods of response curve of the sensor to 100 ppm ethanol at 250 °C (20% RH).

Selectivity is also an important performance of the sensor based on the as-prepared In₂O₃–SnO₂ composites to various gases, such as NO, H₂S, NO₂, C₆H₅CH₃, C₃H₆O, CH₃OH, HCHO and CH₃CH₂OH.All of the gases were tested at an operating temperature of 250 °C as shown in Fig. 7. The response of the sensors based on In₂O₃–SnO₂ composites (S1) to toluene, acetone and methanol is much lower than ethanol, and the response to other gases or vapours such as NO, H₂S, NO₂ and formaldehyde is negligible. The results indicate that the as-prepared In₂O₃–SnO₂ composites display superior selectivity to ethanol against the other interference gases and was very suitable for sensing ethanol at 250 °C.

Fig. 7 Comparison of responses of In₂O₃–SnO₂ composites (S1) sensors to various gases at 250 °C.

UV illumination was used to enhance the performance of the sensors (S1). Although the sensitivity and the response and recovery times were almost not affected by UV illumination, UV illumination greatly reduced the influence of humidity on the sensitivity of the sensors, the results were shown in Fig. 8. It can be seen that the sensitivity is almost stable below 80% RH under UV illumination. When the relative humidity is even up to 95% RH, the sensitivity still maintains 75%, which is higher than that without UV illumination. UV illumination can decompose the water vapour on the surface according to the mechanism of photochemical water splitting (water can be decomposed into hydrogen and oxygen under UV illumination³³).

Fig. 8 Responses of the sensors (S1) in different relative humidity, the inset is the resistances in air vs. relative humidity.

3.4 Gas mechanism

The formation of metal oxide composites is known to increase the response and selectivity to gases. For this work, both In₂O₃ and SnO₂ increase the response and selectivity to gases. For this work, both In₂O₃ and SnO₂ are n-type semiconductor oxides. The surface of the sensor material in air is covered with chemisorbed oxygen ions such as O⁻, O²⁻ and O₂²⁻, the electrons on chemisorbed oxygen ions come from the material, which decrease the electron density in n-type material and in turn increase the resistance of the oxide. If reducing gas is introduced, it reacts with the surface adsorbed oxygen, the electron will be donated back into the semiconductor that causes a decrease in the resistance. Adsorbed ethanol undergoes two possible mechanisms of dehydrogenation to an aldehyde and dehydration to an alkene, shown in reactions (1) and (2).³⁴

C₂H₅OH (g) →CH₃CHO (g) + H₂ (g) (1)

C₂H₅OH (g) → C₂H₄ (g) + H₂O (g) (2)

Following this process consecutive reactions happen which consume ionic oxygen species and release electrons, so the resistance of the oxide is reduced. The electrons only removed to a certain depth from the surface known as the depletion region. The depletion region width may change as the test gas or oxygen is adsorbed on the surface, which in turn caused a measurable change in the resistance. This process was shown in Fig. 9(a).

Fig. 9 (a) Schematic diagram of the proposed mechanism of In₂O₃–SnO₂ composites and (b) Energy band structure of In₂O₃, SnO₂ and In₂O₃–SnO₂ composites.

The enhancement in gas-sensing property on In₂O₃ and SnO₂ composites may be mainly ascribed to heterostructure.^35,36 The SnO₂ has a lower Fermi level than that of In₂O₃, In₂O₃ would receive electrons from SnO₂, leading to the formation of an accumulation layer at the In₂O₃–SnO₂ interface, as shown in Fig. 9(b). The increase of electrons on the surface of In₂O₃ is in favour of adsorption of O₂ and decrease of resistance.

Besides, In₂O₃ and SnO₂ were both active site, but each promoted different breakdown profiles of the ethanol vapours being sensed. Because of their complementary catalytic activity (catalytic activity reaches an optimum level when the mass ratio of In₂O₃ and SnO₂ is 2 : 1), the mixed oxides permitted a more complete breakdown of the ethanol vapours, leading to the observed enhancement in sensitivity. In addition, the porous structure is helpful for gas diffusion and its reaction on the surface of the sensing materials.

4. Conclusions

In summary, In₂O₃–SnO₂ composites had been successfully obtained through a mixing of In₂O₃ microflowers and SnO₂ nanoparticles. The nanostructure is in favour of permeation of the tested gas. The In₂O₃–SnO₂ sensors exhibit high response and good selectivity to ethanol at 250 °C. It can detect 100 ppm ethanol with a response value of about 53.2 and with little interference from other gases. The enhancement in gas-sensing property on In₂O₃ and SnO₂ composites may be ascribed to heterostructure. Because of the depletion layer manipulation and complementary catalytic activity, the composite oxides permitted a more complete breakdown of the ethanol vapours, leading to the observed enhancement in sensitivity and selectivity. The sensitivity is stable while the relative humidity increasing under UV illumination due to photochemical water splitting.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (nos 61074172, 61134010, and 61327804) and Program for Chang Jiang Scholars and Innovative Research Team in University (no. IRT1017). National High-Tech Research and Development Program of China (863 Program, no. 2013AA030902).

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Footnote

† Electronic supplementary information (ESI) available: Details of preparations and characterizations; the SEM images using a JEOL JSM-7500F microscope with an accelerating voltage of 15 kV; the XRD measurements with a Rigaku D/max-2500 diffractometer using Cu-Kα1 radiation; the BET measurements with a Micromeritics Gemini VII apparatus (surface area and porosity system). See DOI: 10.1039/c5ra07213a

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