Fabrication and characterization of a low power consumption ethanol gas sensor based on a suspended micro-hotplate

Abstract

A low power consumption ethanol gas sensor based on a suspended micro-hotplate was fabricated using the droplet guiding deposition technique. FESEM and XRD characteristics show that the host material, SnO₂, maintained the rutile structure and a particle size of ∼40 nm. HRSEM and TEM images illustrate the porous and permeable morphologies of SnO₂/TiO₂ and SnO₂/CNT. The optimal sintering and operating temperatures for detecting ethanol are 450 °C and 300 °C respectively. The response was defined as the ratio of sensor resistance measured in air and ethanol (R_a/R_g), and it was improved by adding 2 wt% TiO₂ and 1 wt% CNT. The responses of SnO₂/TiO₂, SnO₂/CNT, and pure SnO₂ to the lowest concentration of 1 ppm are about 5.2, 4.8 and 2.2 respectively. With an increase in the concentration to 500 ppm ethanol, the responses increase rapidly and reach about 21.5, 19.4 and 15.2. In the low range from 1 ppm to 50 ppm, the variation curves of response are basically linear. The dynamic responses indicate fast response/recovery speed, relatively steady baseline and good repeatability. The experimental results show that the MHP-based sensors made of SnO₂/TiO₂ or SnO₂/CNT have good sensitivity and selectivity for detecting ethanol in a wide range from 1 ppm to 500 ppm while consuming only 12 mW of power.

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1. Introduction

With the rapid development of the auto industry, the number of accidents caused by drunk driving has increased dramatically, and ethanol sensors based on semiconductors have drawn more and more attention around the world. Nevertheless, most sensors are fabricated based on the traditional technique of coating the sensing material onto the surface of a ceramic tube with electrodes and a heater.^1–3 Due to small dimensions, low power consumption and possibility of being integrated with other devices, gas sensors based on the micro-hotplate (MHP) have attracted the attention of more and more scholars since 1990s.^4–6 Yaowu Mo et al. deposited SnO₂ with the help of an electron beam evaporation device to detect ethanol.⁶ Wöllenstein et al. designed gas sensor arrays by means of reactive magnetron sputtering for detecting H₂, CO, NO₂ and NH₃.⁷ Wang et al. utilized electrospinning technology to deposit sensing film onto electrodes for relative humidity detection.⁸ The above mentioned methods involve either expensive equipment or high voltage, which limits their popularization. In this study, we concentrated on the study of an easy and low-cost coating technique, in which only a PicoTipTM emitter with 8 μm inner diameter was employed.

As an n-type wide-band gap (E_g = 3.6 eV) semiconductor, SnO₂ is intensively used in many applications such as gas sensors, solar cells and photovoltaics.^9–13 Pristine SnO₂ has poor selectivity and requires high working temperature; therefore, many studies have been have been conducted to improve the sensing characteristics of SnO₂ sensors by doping with precious metals such as Pt,¹⁴ Pd,¹⁵ Au,¹⁶ as well as transition metal oxides such as copper oxide,¹⁷ lanthanum oxide,¹⁸ and silica.¹⁹ However, there are not many reports for SnO₂ thick film sensors processed by the powder mixing of SnO₂ and TiO₂, or SnO₂ and CNT.²⁰

In this study, we are presenting a coating technique called the droplet guiding deposition (DDG) to fabricate a MHP-based gas sensor for monitoring ethanol vapour. To improve the sensitivity and selectivity, TiO₂ and CNT, respectively, are selected as the additives to the host material of SnO₂. For elevating the response to ethanol gas, the optimal sintering temperature, operating temperature, as well as the optimal mixing ratios of TiO₂ and CNT, have been determined to be 450 °C, 300 °C, 2 wt% and 1 wt%, respectively. The underlying mechanism of ethanol gas adsorption–desorption is also discussed.

2. Experimental

2.1 Synthesis and characterization

The synthesis of SnO₂ nanoparticles was carried out by a sol–gel method. 3.5 g of SnCl₄·5H₂O (GR grade) and 0.4 g citric acid (GR grade) were completely dissolved in deionized water (100 mL). Aqueous ammonia (0.3 M) was added as a precipitator under strong stirring until the pH of the solution reached 1.5–2. The pasty precipitate was washed using deionized water several times to eliminate the chloride ions. When being heated to 80 °C, the pH of the product was adjusted to 1.5–2 by adding saturated oxalic acid solution, and the completely transparent Sn(OH)₄ aqueous colloids were prepared. The Sn(OH)₄ aqueous colloids were dried at 80 °C, then were ground in an agate mortar and finally calcined at 500 °C in a muffle furnace. Commercial TiO₂ (99%) and CNT (95%) were ground with pure SnO₂ in an agate mortar for 2 hours to get a good mixture. The structure of SnO₂ was characterized by an X-ray diffraction instrument (XRD: D/Max 2400, Rigaku, Japan) in the 2θ region of 20–80° with Cu Kα1 radiation. The morphologies of the sensing materials were obtained using field emission scanning electron microscopy (FE-SEM: NOVA NanoSEM 450, FEI USA) and transmission electron microscopy (TEM: HT7700, HITACHI Japan).

2.2 MHP-based sensor fabrication by means of DDG

As shown in Fig. 1(a), an array of two MHPs (each of size 250 μm × 250 μm) was designed using microelectronic fabrication technology.²¹ Each MHP, composed of dielectric film, was supported by four bridge-shaped arms, to be suspended above the silicon substrate (the height of the airspace below was 0.5 μm). Fig. 1(b) shows a magnified image of a single MHP. At the central operating area (with the size of 100 μm × 100 μm), a snake-shaped platinum electrode for heating was fabricated and interdigital electrodes were made upon the heater, separated by an insulation layer. Both the width of the snake-shaped heater and the space occupied by the interdigital electrodes were 10 μm. Fig. 1(c) shows the cross-section of a MHP-based sensor. The relation curve between operating temperature and power consumption is shown in Fig. 1(d). It can be seen that only 12 mW of power consumption could bring about an operating temperature as high as 300 °C.

Fig. 1 (a) An array of two MHPs, (b) magnified image of single MHP, (c) cross-section of MHP-based sensor, (d) curve of operating temperature and power consumption.

Because of the small scale, it was rather hard to implant the sensing material into such a tiny area as 100 μm × 100 μm. A DDG micro-dropping technique was developed to fulfill this task. First, a PicoTipTM emitter with 8 μm inner diameter, as shown in Fig. 2(a), was linked to an injector through a bidirectional peek pipeline, as shown as Fig. 2(b). The injector full of deionized water was driven slowly by a pump and the water dripped out from the needle tip. With the help of a microscope, the droplet could be located on the particular area of the MHP accurately. Second, a certain quantity of sensing powder was served in a medicine spoon and then scattered evenly onto the water droplet to form a sticky paste. After the drying process, the water evaporated and the sensor was placed upside down to get rid of the redundant powder. Lastly, the sensor was exposed to temperatures as high as hundreds of degrees centigrade in a muffle furnace and the bond mechanism was activated, which would finally lead to good adhesion between the sensing membrane and the substrate of MHP. Fig. 3(a) and (b) show the 3D photograph (by means of optical microscopy) of the sensing membranes sintered at 300 °C and 450 °C and the insets show the top view of the sensing membranes. It can be seen that the thicknesses of the sensing membranes are measured to be 188.9 μm and 223.8 μm, and the bottom outlines present an approximate circle with a diameter of more than 100 μm, which roughly agrees with the size of the MHP's operating area. The thicknesses of the sensing membranes sintered at 300 °C, 450 °C and 600 °C are summarized in Table 1. It can be seen that the thicknesses of the sensing membranes sintered at different temperatures are roughly the same and the relative discrepancies might come from the inconsistency of the planting powder process.

Fig. 2 (a) Image of a PicoTipTM emitter with 8 μm inner diameter, (b) illustrated diagram of the emitter in combination with the injector.

Fig. 3 3D image of the sensing film under an optical microscope (a) sintered at 300 °C, (b) sintered at 450 °C.

Table 1 Thickness of membrane vs. sintering temperature

Thickness of membrane	Sintering temperature
188.9 μm	300 °C
223.8 μm	450 °C
218.3 μm	600 °C

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2.3 Sensor measurements

The gas sensing properties of the MHP-based sensors were measured using a static liquid–gas distribution method, wherein a given amount of target gas was injected into a test box by a syringe. Sensors were exposed to air by removing the rubber plug after the measurement. As shown in Fig. 4, the voltage on the MHP-based sensor was measured using a voltage division circuit and each sensor (R) was connected in series with an external resistor (R_O), where U_H was the heating voltage of MHP, U_T was the testing voltage and U_O was the export voltage. The relation between U_T and U_O is given by U_O = (R_O × U_T)/(R_O + R). The gas response was defined as S = R_a/R_g, where R_a and R_g were sensor resistances measured in air and in ethanol atmosphere.

Fig. 4 Measurement principle of the sensor array based on MHP.

3. Results and discussion

3.1 Materials characterization

Fig. 5(a)–(c) give the SEM images of SnO₂ powder derived from the as-deposited sensing layers after being sintered at 300 °C, 450 °C and 600 °C, respectively. As shown here, the particle size is estimated to be approximately 40 nm. The samples sintered at 300 °C and 450 °C have even size distribution and good dispersion, which imply a large porous surface structure. However, besides the slight enlargement of particle size, the agglomeration of the sample sintered at 600 °C gets worse. It is likely that the surface free energy of the crystal boundary could lead to migration and merging among particles. The higher the sintering temperature, the stronger the surface free energy, and the worse the migration and merging, which cause size enlargement and agglomeration of particles.

Fig. 5 FESEM images of SnO₂ sintered (a) at 300 °C, (b) at 450 °C, (c) at 600 °C.

The typical X-ray diffraction (XRD) patterns of SnO₂ are shown in Fig. 6, in which all the diffraction peaks are similar to those of bulk SnO₂. We can see that the XRD spectra of SnO₂ sintered at different temperatures coincide with the standard spectrum. The characteristic peak of the rutile structure is shown. The mean crystallite sizes of the SnO₂ nanoparticles, as estimated from the width at the half-maximum of the main diffraction peaks according to the Scherrer equation, (D = 0.89λ/β cos θ, where D is the crystallite size, λ the wavelength of X-ray, β the line broadening at full width half-maximum, and θ the Bragg diffraction angle of the peak), are about 40 nm, 43 nm, 44 nm, respectively, which are in good agreement with the particle sizes as estimated from FEM analysis.

Fig. 6 XRD patterns of SnO₂ sintered at 300 °C, 450 °C and 600 °C.

The morphology and microstructure of SnO₂/TiO₂ and SnO₂/CNT sintered at 450 °C were illustrated by FESEM and TEM observations. The micrographs in Fig. 7(a) and (b) show the presence of SnO₂ (small particles) and TiO₂ (big particles), which provide evidence of the large porous surface structure. In most cases, many SnO₂ particles surround some TiO₂ particles and come into good contact with each other, which facilitates the possible formation of a heterojunction. The case of SnO₂/CNT shown in Fig. 7(c) and (d) is similar and the only difference is that CNT takes on the morphology of porous fibre, which could enhance the porosity and permeability of this composite.

Fig. 7 (a and b) FESEM image (inset shows high magnification FESEM of selected area) and TEM image of SnO₂/TiO₂ sintered at 450 °C; (c and d) FESEM image (inset shows high magnification FESEM of selected area) and TEM image of SnO₂/CNT sintered at 450 °C.

3.2 Ethanol sensing properties

The sintering temperature exercises great influence on the sensitivity of the semiconductor gas sensor. The MHP-based sensor was sintered at 200 °C, 300 °C, 450 °C and 600 °C, and the gas sensing experiments in response to 500 ppm ethanol were performed. The experimental data are shown in Table 2, where R_O and R_a are a good impedance match. Fig. 8 shows the relation curve between the response to 500 ppm ethanol and sintering temperature. It can be seen that the response continues to increase with the increase in sintering temperature and reaches a peak value of 28.33 at 450 °C, then decreases slightly with a further increase in temperature. This is probably because the sintering process could increase the quantity of absorbed oxygen on the surface of the sensing film, and the maximum number is observed at 450 °C.

Table 2 Experimental data at different sintering temperatures

Sintering temperature	UO (V) (air)	UO (V) (ethanol)	RO (MΩ)	Ra (MΩ)	Rg (MΩ)
200 °C	1.63	2.20	35	36.88	17.92
300 °C	1.62	2.68	42	43.55	9.71
450 °C	1.53	3.17	21	24.36	0.86
600 °C	1.72	3.19	35	31.82	1.19

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Fig. 8 Curve showing the relationship between the response to 500 ppm ethanol and sintering temperature.

Operating temperature is another important factor, which has a significant impact on the response of gas sensors.^22,23 At a too low temperature, the reaction between the target gas molecules and sensing material is too weak to be detected. However, at a too high temperature, the desorption of gas molecules on the surface of the material is very strong for diffusion through the sensing material. Fig. 9 shows the changing curve of response with operating temperature. It is obvious that the response increases rapidly and reaches its maximum value of 17.33 at 300 °C, and then decreases dramatically with a further increase in operating temperature. According to the calibration curve for operating temperature and power consumption, only 12 mW of power is consumed to get a temperature of 300 °C, which is broadly in line with the data reported in literature.⁴

Fig. 9 Curve between the response and operating temperature towards 500 ppm ethanol.

In this study, different mass ratios of TiO₂ and CNT, such as 1 wt%, 2 wt% and 5 wt%, were mixed with SnO₂. Fig. 10(a) shows the response curve with different quantities of TiO₂ at 300 °C. It is obvious that the response reaches a peak of 21.5 when the addition ratio of TiO₂ is 2 wt%. Fig. 10(b) shows the response curve with different quantities of CNT and the response reaches a peak of 19.4 with 1 wt% CNT added at 300 °C.

Fig. 10 Curve showing the relationship between the response to 500 ppm ethanol and addition ratios of (a) TiO₂, (b) CNT at 300 °C.

Fig. 11(a) shows the relation curves of the response of the MHP-based sensor at 450 °C sintering and 300 °C operating temperature vs. ethanol concentration from 1 ppm to 500 ppm. Enlarged and linear fitted curves from 1 ppm to 50 ppm are shown in Fig. 11(b). It can be seen that the response curves are basically linear with the ethanol concentration in the range of 1–50 ppm. The responses of SnO₂/TiO₂, SnO₂/CNT and pure SnO₂ to the lowest concentration of 1 ppm are about 5.2, 4.8 and 2.2, respectively. With the increase in the ethanol concentration, the responses to 500 ppm ethanol increase rapidly and reach about 21.5, 19.4 and 15.2. The responses of SnO₂/TiO₂, SnO₂/CNT are higher than that of SnO₂.

Fig. 11 Response of SnO₂/TiO₂, SnO₂/CNT and SnO₂ gas sensors to ethanol concentration: (a) 1–500 ppm, (b) 1–50 ppm.

According to international conventions, the ethanol concentration between 20 mg/100 mL and 80 mg/100 mL in a driver's blood is judged as drink driving, while the concentration of 80 mg/100 mL and above is judged as drunk driving. The ratio between the ethanol concentration in the blood of a drunk driver and that in the breath exhaled by him is 2100 : 1 physiologically. The converted ethanol concentration between 46 ppm and 185 ppm in the breath is the judging range to screen drink driving, and the lower limit value to judge drunk driving is about 185 ppm. Based on our experimental data, the MHP-based sensor has good sensitivity for detecting ethanol in a wide range from 1 ppm to 500 ppm, which consumes only 12 mW of power. Along with the convenience of being integrated with other devices, it is reasonable to believe that the MHP-based sensor has great potential in portable application for judging a drunk driver.

The response and recovery characteristics were further investigated with the sensors being exposed to different concentrations at 300 °C, as shown as Fig. 12. It can be seen that the characteristics of response and recovery were almost reproducible with a nearly square response shape observed, which indicated that the sensor responded rapidly to ethanol gas and quickly achieved a near steady state. The output voltage then changed slowly because the variation in sensor resistance slowed down when the ethanol gas diffused through the sensitive material and occupied the remaining surface reaction sites. When the sensor was exposed to air, the output voltage returned to near baseline level. It is obvious that the responses of SnO₂/TiO₂ and SnO₂/CNT to different concentrations of ethanol are superior to that of SnO₂.

Fig. 12 Typical dynamic response curves of SnO₂/TiO₂, SnO₂/CNT and SnO₂ toward ethanol with increasing concentrations at 300 °C.

Response repeatability and baseline drift are also important features for the practical applications of gas sensors. After exposure to the target gas, a gradual reduction of the response could be noted. Moreover, baseline drift is a common problem with most metal oxide sensors such as copper oxide.^24,25 During repeated pulses, the short-term stability in the response and baseline were investigated and the results are shown in Fig. 13(a) and (b). It is indicated that SnO₂/TiO₂ and SnO₂/CNT based MHP sensors have good repeatability and reliability.

Fig. 13 Short-term evaluation of the sensor response and baseline stability: (a) SnO₂/TiO₂, (b) SnO₂/CNT.

The response of SnO₂/TiO₂, SnO₂/CNT and SnO₂ sensors to ethanol, methanol, acetone, formaldehyde, ammonia and toluene with a concentration of 250 ppm at 300 °C are given in Fig. 14. The results show that the sensors were less sensitive to methanol, acetone, formaldehyde and almost insensitive to ammonia, and toluene. SnO₂ can accept a lone pair of electron, and organic vapours such as acetone and ethanol always include carbonyl or OH groups that have lone-paired electrons to donate, which may be responsible for the strong adsorption of these vapours. In that sense, the sensors based on SnO₂/TiO₂ and SnO₂/CNT showed an obvious advantage in the selective detection of ethanol.

Fig. 14 Responses of SnO₂/TiO₂, SnO₂/CNT and SnO₂ sensors to different gases at a concentration of 250 ppm at 300 °C.

3.3 Gas sensing mechanism

SnO₂ is an n-type semiconductor whose sensing mechanism belongs to the surface-controlled type, and like all metal oxides, it absorbs oxygen from the surrounding atmosphere.²⁶ These oxygen molecules could be converted to ions by capturing a single or double electron from the conduction band of SnO₂ (converting the oxygen molecules to a single or double ion). After being sintered at a rather high temperature with the quantity of oxygen molecules chemisorbed on the sensor's surface increasing significantly, a space charge depletion layer comes into being and results in a high resistance. In the presence of ethanol, it reacts with oxygen molecules and depletes a large number of oxygen vacancies. The reaction includes two stages as follows:

First stage: dehydrogenation reaction of ethanol to acetaldehyde

C₂H₅OH(ads) + 2O²⁻(ads) → 2C₂H₄O⁻(ads) + O₂(g) + 2H₂O(g) + 2e (1)

In this stage, the C₂H₄O⁻ (ads) is unstable and easily affected by the heat:

C₂H₄O⁻(ads) → CH₃CHO(ads) + e (2)

2CH₃CHO(ads) + 5O₂²⁻(ads) → 4CO₂ + 4H₂O + 10e (3)

Second stage: dehydrogenation reaction of ethylene

C₂H₅OH(ads) → C₂H₄(g) + H₂O(g) (4)

C₂H₄(g) + 3O₂²⁻(ads) → 2CO₂ + 2H₂O + 6e (5)

The electrons generated in the abovementioned reactions are released back to the conduction band and lead to a rise in electron concentration, which eventually increases the conductivity of SnO₂. It is particularly worth mentioning that blindly increasing the sintering or operating temperature might not improve the response of the gas sensor dramatically because of the increasing crystallite dimension and decreasing specific surface area. Therefore, appropriate sintering and operating temperatures are of great importance to gas sensors for good sensitivity.

The experiments show that SnO₂/TiO₂, SnO₂/CNT and pure SnO₂ are all sensitive to ethanol. The response values of SnO₂/TiO₂ and SnO₂/CNT are higher than that of pure SnO₂, which means the adsorption capability of SnO₂/TiO₂ and SnO₂/CNT composites were greatly improved. The reason maybe as follows: TiO₂ has the same rutile type crystal structure and similar interatomic distance to SnO₂. When TiO₂ is added to SnO₂, it is easy to form a type of hetero-junction, which would greatly reduce the height of the grain boundary electron barrier and make more electrons transfer from ethanol gas to the interior of the material. Thus, the conductivity of this composite is highly enhanced and the sensitivity is improved.²⁷ The case of CNT is similar; moreover, the hollow structure and large specific surface area are helpful to enhance the porosity and permeability of the composite, as well as the better catalytic activity can promote the exchange of electrons to a certain extent.²⁸ The sensing mechanism of hetero-junction sensing materials needs further investigation.

4. Conclusions

In this study, a MHP-based gas sensor was fabricated by means of the DDG technique, which was proved to be simple, inexpensive and effective. The structure and morphology of the host material, SnO₂, were characterized by XRD and FESEM, and the results implied the rutile structure and small particle size of ∼40 nm. The morphologies of SnO₂/TiO₂ and SnO₂/CNT were characterized by HRSEM and TEM, which illustrated the porosity and permeability. The optimal sintering temperature and operating temperature of the MHP-based sensor for detecting ethanol is 450 °C and 300 °C, respectively. With 2 wt% TiO₂ and 1 wt% CNT added, the response of MHP-based sensors was relatively improved. The response of SnO₂/TiO₂, SnO₂/CNT and SnO₂ to the lowest concentration of 1 ppm is about 5.2, 4.8 and 2.2, respectively. With an increase in the ethanol concentration to 500 ppm, the responses increase rapidly and reach about 21.5, 19.4 and 15.2. In the low range of 1 ppm to 50 ppm, the variation curves of response are basically linear. The dynamic responses indicate fast response/recovery speed, relatively steady baseline and good repeatability. The experimental results show that MHP-based sensors made of SnO₂/TiO₂ or SnO₂/CNT have good sensitivity and selectivity for detecting ethanol in a wide range from 1 ppm to 500 ppm, while consuming only 12 mW of power, and therefore have great potential in portable ethanol sensing applications.

Acknowledgements

The authors thank the National Natural Science Foundation of China (61376115, 61074166, 61131004) for financial support.

Notes and references

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