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Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2
Accepted Manuscript Title: Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2 Author: Zahra Karami Horastani S. Masoud Sayedi M. Hossein Sheikhi PII: DOI: Reference:
S0925-4005(14)00639-X http://dx.doi.org/doi:10.1016/j.snb.2014.05.100 SNB 16974
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-3-2014 22-5-2014 23-5-2014
Please cite this article as: Z.K. Horastani, S.M. Sayedi, M.H. Sheikhi, Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2 , Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.05.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2 Zahra Karami Horastania , S. Masoud Sayedia, M. Hossein Sheikhib* Department of Elec. & Comp. Eng., Isfahan University of Technology, 8415683111 Isfahan, Iran
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School of Elec. & Comp. Eng., Shiraz University, 7194684471 Shiraz, Iran Shiraz Nanotechnology Research Institute, Shiraz University, 7194684471 Shiraz, Iran
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Highlights
The effect of SWCNT adding in electrical conductivity of SnO2 is studied.
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The effect of SWCNT adding in methane sensitivity of SnO2 is investigated.
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An electrical circuit model based on obtained results is proposed.
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The role of SWCNT in sensitivity enhancement of SnO2 gas sensor is studied.
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Abstract
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The effect of single wall carbon nanotube (SWCNT) additive on electrical conductivity and methane sensitivity of SnO2 gas sensor has been investigated. Sensors were prepared through powder pressing procedure and the morphology and microstructure of the materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The electrical resistivity and methane sensitivity of the samples were examined in the temperature interval of 100 to 450°C. Results show that in the air ambient the electrical resistivity of the samples increases with the increase of SWCNT concentration. Also, comparative results show that by increasing of SWCNT concentration up to 1.2%wt, the methane sensitivity of the samples increases, and after that by further increase of SWNT concentration the sensitivity decreases. Based on the SEM images of the sensors, and also the measurement results, three types of resistance, related to metal oxide, SWCNT bundle, and SWCNT/ SnO2 grain junction, were realized in the sensors and based on that, a simple *
Corresponding author. Tel.: +98-711-7255024. Fax: +98-711-6474280.
E-mail address: [email protected] (M. H. Sheikhi). 1
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electrical circuit model is proposed. Results show that SWCNT/ SnO2 grain junction resistance is the main reason for the higher electrical resistivity of the samples with higher SWCNT concentrations.
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Key words: SnO2, Single wall carbon nanotube, Electrical conductivity, Gas sensor, Electrical model.
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1. Introduction Metal oxide semiconductors have been used widely in the gas sensing applications. Among them, tin oxide (SnO2) with low cost and good sensitivity is one of the most interesting ones. Tin oxide as a wide band gap (3.6 eV) n-type semiconductor constitutes a major part of different forms from ceramic conductive to nano structure sensors.
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market and publication [1, 2]. It is sensitive to many different gases, and fabricated in many
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One traditional way to improve sensitivity, selectivity, response time and operating temperature of metal oxide gas sensors is the use of additives [3-9]. Recently, much attention
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has been focused on the carbon nanotubes (CNTs) as the additive. Collins et al. demonstrated that CNTs exhibit very good adsorption property due to their high specific surface areas and
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nano scale structures, and that they can respond to both reducing and oxidizing gases through a charge transferring reaction with the gas molecules that changes their conductivity [10]. Previous works have demonstrated that the hybrid of CNT and metal oxides can be used for
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detection of various gases such as NO2, liquid petroleum gas (LPG), i-butane, methanol and ethanol. It was reported that the hybrid material has a better performance compared to pure
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metal oxide resistive sensor. Wie et. al [11] fabricated hybrid single wall carbon nano tube (SWCNT)/SnO2 gas sensors with 0.003 and 0.03%wt. of SWCNT to detect NO2 at room
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temperature, and compared the results with pure SnO2. They found that the hybrid sensors exhibit higher sensitivity compared to pure SnO2, and that the sensor with higher SWCNT
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concentration has better performance. Hieu et al. [12] investigated the effect of multi wall carbon nanotube (MWCNT) on the ethanol and LPG sensing property of SnO2. They found that the responses are improved compared to pure SnO2 sensor, and that the MWCNT adding act differently for ethanol and LPG. Aroutiounian et al. [13] developed a gas sensor based on SnO2/MWCNT (0.1%wt MWCNT) composite for which electrical conductance was changed when exposed to i-butane. They couldn’t detect i-butane with pure SnO2 or pure CNT. Wongchoosuk et al. [14] investigated the effect of MWCNT concentration (0.5, 1%wt) on the methanol and ethanol sensing property of SnO2. They found that adding MWCNT to SnO2 matrix improves gas sensitivity of SnO2, and that the quantity of MWCNT changes the selectivity of the sensor. More information related to above studies is presented in table 1. Methane (CH4) gas can be found in environment, industrial and domestic areas. It is a colorless, inflammable, nontoxic gas with a sweet, oil type odor. It is the simplest alkene, and the main component of natural gas. Due to its inflammable nature, on one hand, and its wide 3
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applications (in chemical factories, mines, automobile, household etc.) in other hand, development of a reliable methane sensor is essential for the safe use of it. As noted before CNT adding to SnO2 can improve the working characteristic of the SnO2based gas sensors in detecting NO2, ethanol, methanol, LPG and i- butane gases. The focus of
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the present work is to investigate how the SWCNT adding influences the methane sensitivity and electrical conductivity of SnO2. The hybrid SnO2-CNT sensors with different SWCNT
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concentrations were prepared by powder pressing method which is an inexpensive and reproducible way in the gas sensor fabrication process.
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In the following and in section 2 a brief description of SnO2 and SWCNT as the basic components of the investigated sensors is presented. In section 3 experimental procedures
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including sensor fabrication and measurement setup are explained. Experimental results, a model that justifies sensor’s resistivity vs. temperature curve in the air ambient, and a discussion on sensing mechanism are presented in sections 4 to 6. And finally, conclusion is
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provided in section 7.
2. A brief discussion on SnO2 and SWCNT as two gas sensing components
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a) SnO2. It is well known that the electrical conductivity of a polycrystalline metal oxide such
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as SnO2 is mainly determined by its grain boundary conductance rather than its grain conductance. Therefore, its conductivity can be controlled by some surface reactions like
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surface molecule adsorption. At temperatures between 100 to 500°C, oxygen molecules are adsorbed on metal oxide surface through the following reactions [15]: O2(gas)↔O2(ads) ↔O-2(ads) ↔O-(ads) ↔O2-(ads) ↔O2-(lattice)
Below 150°C the adsorption is mainly in the form of O2-, and above 150°C, O- and O2- are mostly involved. These elements increase the barriers between the grains, and as a result, change the conductivity (Fig. 1). When the temperature of a polycrystalline gas sensor increases above room temperature, due to the increase of the carriers with sufficient energy to overcome the schottky barriers in the grain boundaries, and also due to the semiconducting behavior of SnO2, its conductivity increases. However, with more increase of temperature, the adsorption of oxygen on the grain surface increases, causing the enhance of the barrier height between the grains. So there is a competition between the increase of carrier concentration, that leads to a higher conductivity, and the enhancement of barrier height that
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leads to a lower conductivity. The conductance of polycrystalline SnO2, can be expressed as: [15] Eq. 1
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In which
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Eq. 2
where Ф is the barrier height between the grains, T is the temperature in Kelvin, k is the
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Boltzmann constant, q is the elementary charge, nb is the bulk electron concentration, and m* is the effective electron mass. As Eq. 1 shows, the conductance depends on the barrier height exponentially, and variation in this parameter, compared to carrier concentration, is dominant
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one. Thus, with increase of temperature, and increase of barrier height, eventually the conductivity decreases. However, with further increase of temperature, the condition changes
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and desorption of the previously adsorbed oxygens happens. This causes decrease in barrier height, and as a result, increase in the conductivity.
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Schottky barrier height between the grains can be estimated by plotting the logarithm of conductance as a reverse function of temperature (Arrhenius plot). This estimation is true if
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following three conditions are fulfilled within the temperature range: [16]
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a) No gas adsorption/desorption takes place during heating and cooling process, b) Only thermionic conduction is relevant, c) No oxygen diffusion into or out of the grain occurs during heating or cooling process.
It should be noticed that above conditions are only fulfilled in a small temperature range above room temperature, thus, barrier height calculation can be estimated only in this temperature range.
For the gas sensing mechanism, two general steps can be considered. The first is barrier formation in air environment due to oxygen adsorption as discussed above, and the second is reaction of gas molecules with adsorbed oxygen. In the case of reducing gas such as CH4, this reaction causes reinjection of electrons to the grain: CH4+4O-→CO2+2H2O+4e-
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This decreases barrier height, and as a result, decreases resistance of the sensor compared to that of in the air environment [6, 15]. b) SWCNT. SWCNT, which is a graphene sheet rolled into a cylindrical shape, can be used as gas sensing component. This component can be divided into two groups of metal and
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semiconductor, depending on its diameter and chirality (n, m). If n-m is multiple of three, the SWCNT is metallic, otherwise, it is a semiconducting material [17]. The band gap of
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semiconducting SWCNT can be expressed as Eg=0.7/D, where D is the diameter length [18].
During fabrication process, there is no precise control on the chirality parameter of the
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nanotube, but statistically it can be assumed 1/3 of nanotube is metallic and the rest is semiconducting. In an SWCNT bundle (network) a mixture of metallic and semiconducting
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SWCNTs exists. However, previous studies have shown a metallic behavior for this mixture [18]. Work function of SWCNT has been measured 5.05 eV by using photo electron emission
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technique [19]. 3. Experiments 3.1. Fabrication of gas sensor
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To investigate the effect of SWCNT additive on the properties of SnO2 sensor two types of
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sensor – the pure one, and the hybrid one with different SWCNT concentrations, were fabricated. SWCNT-SnO2 mixed powder with 0.3, 0.6, 1.2, 2.4 and 3.8%wt SWCNT
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concentrations were prepared by mixing of 1.2 gr SnO2 and 0.0036, 0.0072, 0.0145 , 0.029 and .045 gr of SWCNT powder (diameter: 1-2 nm, length: 1-2 μm, purity >99%) in a deionized water. The solution was sonicated to obtain a well- mixed suspension. The suspension stirred by hot magnet stirring to make water evaporated and to obtain mixed powder. The mixed powder was thoroughly milled by grinding in mortar to have uniform distribution of SWCNT in SnO2 powder. 0.6 gr of powder was compressed into a 12-mmdiameter cylindrical pellet in a hard steel mold by a mechanical compressor which was controlled by computer and was set to pressure of 10 KN. In order to obtain mechanical strength and thermal stability, the pellets were sintered at 500˚C for 2h in a programmable furnace. The furnace temperature was gradually increased to 500˚C in 2h, stayed at 500˚C for 2h, and then decreased to 30˚C in 10h. After sintering process the electrodes of the sensor were connected by using silver paste, and then the sensor was fixed on a micro heater. The temperature of the micro heater was adjustable, up to 450˚C. The sensor was then placed in
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an oven for 20 minutes at 130˚C to anneal. A schematic of the fabricated sensor is shown in Fig. 2. 3.2. Measurement setup Fig. 3 illustrates the employed laboratory gas detection apparatus. It consists of a 2.5 litter
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chamber, two wire connections for voltage measurements connected to a digital multimeter, two wire connections for connecting the micro heater to a DC voltage supply, and a
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thermocouple for controlling the working temperature. Methane was used as the detecting gas and air as the reference gas. Concentration of the methane was controlled by the injection of a
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pre-calculated amount of saturated methane gas. To measure electrical resistance of the sensor a simple voltage divider, as shown in Fig.3, was used. In the figure Vin is the input signal with 0.5 V amplitude and 1 kHz frequency, Rs is the resistance of the sensor, Rr is the
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reference resistor and Vm is the measured voltage. Rs is calculated by using Vm which is measured by multimeter and registered by computer every 0.5 second. The sensor response
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(S) of the sensor was calculated by using following equation: S%=100×[(Rair-Rgas)/Rair]
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4. Results and discussion
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in which Rair is resistance of the sensor in the air ambient and Rgas is resistance of the sensor when methane gas presents in the ambient.
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4.1. Phase and morphology investigation 4.1.1. Material characterization
Fig. 4 shows XRD diffraction patterns of pure SWCNT, pure SnO2 and a mix of SnO2 and 3.8%wt of SWCNT. XRD result of Pure SWCNT shows a peak intensity at 26.5°, which is not clearly apparent or distinguishable in XRD of SnO2/SWCNT. This can be related to either low concentration of SWCNT or overlapping of the SWCNT peak and the first (110) peak of SnO2.To ensure that SWCNT was not burned during fabrication process, the same heat treatments that used to fabricate SnO2/SWCNT sensor, applied to a SWCNT powder. It was observed that SWCNT survived after 2 hours of heating at 500°C. 4.1.2. Surface morphology and microstructure The morphology of the pure and SWCNT doped SnO2 samples after thermal treatment at 500ºC was investigated by scanning electron microscopy (SEM). The SEM micrographs of
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the samples are shown in Fig.5. The figure shows the presence of both metal oxide grain and CNT bundle. CNTs are present in the samples after all stages of heat treatments.
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4.2. Effect of SWCNT concentration on electrical resistivity and gas sensitivity of SnO2 4.2.1. Electrical resistivity in air ambient
Fig. 6 shows the variations of electrical resistance in air ambient as a function of temperature
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for the pure SnO2 and for the SnO2/SWCNT hybrids with different SWCNT concentrations.
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The resistance of pure SnO2 decreases with the increase of temperature at low temperatures. As mentioned before electrical conductivity in polycrystalline SnO2 is mainly controlled by schottky barriers between the grains, not the resistance of the bulk. By increasing of the
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temperature the resistance decreases due to the increase of the carriers with sufficient energy to overcome the schottky barriers in the grain boundaries; and also, due to the semiconducting behavior of SnO2, i.e., increasing of carrier concentration by increase of
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temperature. By further increase in temperature, as the figure shows, the resistance starts increasing up to a maximum value. This is due to heighten schottky barriers at grain
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boundary surfaces caused by oxygen adsorption. At temperatures above 150°C oxygen in the form of O- or O2- is adsorbed chemically on the surface of SnO2 grains, traps their electrons
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[15]. This causes an upward band bending and increase of schottky barriers. With more
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increase of temperature, the resistance decreases due to desorption of previously adsorbed oxygen molecules, and also because of presence of some carriers with a kinetic energy higher than barrier height. The SnO2/SWCNT samples showed similar behavior but with higher values of resistance compared to pure SnO2. As the figure shows with increasing of SWCNT concentration, electrical resistance of the samples increases. Another result is the shift of corresponding temperature of maximum resistance from 400°C for pure SnO2 sample to 325350°C interval for SWCNT/SnO2 samples. In order to estimate the schottky barrier height between the grains, as shown in Fig.7, the logarithm of the conductance of the samples as an inverse function of temperature was plotted. The calculated barrier heights for pure sample, and 0.3, 0.6, 1.2, 2.4 and 3.8%wt SWCNT impure samples are estimated 0.14, 0.79, 1.22, 1.78, 1.86 and 1.59 eV respectively. These values show that the barrier height grows with increasing of SWCNT concentration (except the sample contain 3.8%wt SWCNT, this may be related to non-uniform distribution
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of SWCNT because of high SWCNT concentration in this sample). This confirms the increase of sensor resistance with increasing of SWCNT concentration. 4.2.2. Sensor response in the presence of methane gas Sensor response of all the prepared samples to methane gas was examined. Fig. 8 shows the
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responses for the pure SnO2 sample and the hybrid sample with 1.2%wt SWCNT. Before each measurement, sensor was exposed to air for 10 minutes to reach to a stable condition,
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and then 2000 ppm concentration of methane gas was added by injecting appropriate amount of methane with micro syringe. As shown in Fig.8, after gas injection the resistance changes
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fast to an almost stable value. In order to recover the sensor, the glass chamber door is opened.
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Response of all the samples for 2000 ppm methane gas, as a function of temperature, was measured. Maximum temperature was 450ºC and minimum temperature which was about 275˚C was determined by minimum measurable sensor response. For all the samples sensor
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response increases by increasing the temperature to an optimum value and then it decreases. Also response of the samples increases by increasing of up to 1.2%wt SWCNT concentration,
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and after that sensor response decreases. For the sample with 1.2%wt SWCNT, which has the best response, maximum response (67%) occurs at 375ºC. Fig. 9 shows sensor response
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versus SWCNT concentration in three different temperatures. Fig. 9 shows that the higher curves are related to higher working temperatures. For any selected value of sensitivity for
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pure sample, corresponding point is located at 0%wt on one of the curves in the figure. An equivalent sensitivity for a mixed sample happens at lower temperatures. This can be deduced by drawing a horizontal line in the figure representing points with sensitivity equal to the selected one. The crossing of this line and lower curves happens for mixed samples meaning that for the same sensitivity the working temperature of mixed sensors is lower than that of pure sensor. For the pure sample and the sample with 1.2%wt SWCNT, in their optimum temperatures (400 and 375ºC respectively), the sensor response as a function of methane concentration is presented in Fig. 10. As the figure shows sensor response of both sensors first increases with increasing of gas concentration and then it is saturated at some points. The saturation point for the SWCNT sensor happens at higher gas concentration. Table 2 summarizes the current results and some previously reported results on methane sensor. Comparing the sensitivity and working temperature of different sensors, shows that for the same methane concentration two cases of Ref. 5 and Ref. 21 have better performances 9
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compared to present work. Though, considering the low cost of fabrication process employed in this work and also the possibility of fabricating thin structures to improve the performance, the proposed SnO2/SWCNT sensor has potential to be an important candidate in developing methane gas sensors.
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5. Modeling of sensor’s resistivity vs. temperature behavior in the air ambient
The SEM images of the hybrid sensors show that, in terms of electrical conductivity, sensor
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structure can be divided into two different paths, the hybrid SWCNT path, and the pure SnO2 path (Fig. 11). Accordingly, conductivity of each hybrid sample can be determined by the
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conductivities of these two parallel paths. Fig.6. shows that for all the samples, the hybrid SWCNT samples and the pure SnO2 samples, at temperature interval between 150 to 250ºC, a very similar low value resistance exists. The reason is, in a hybrid SWCNT structure, with the
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presence of a pure SnO2 path parallel to hybrid path, the equivalent resistance of the sample is determined by the low value resistance of this path, regardless of the value of its parallel
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hybrid path.
For each hybrid path three types of resistances can be assumed: metal oxide resistance,
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SWCNT bundle resistance,
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SWCNT/ SnO2 grain junction resistance.
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Based on above discussion, a simple electrical circuit model as shown in Fig.12 is proposed. In the figure, RSWCNT is the resistance of SWCNT bundle, RSnO2 is the resistance of pure SnO2, RSWCNT/SnO2 is the resistance of SWCNT bundle - SnO2 junction, x is a parameter determined by SWCNT concentration, and k and k΄ are two coefficients related to x. x and k’ are equal to zero for pure sample, and they increase by increasing SWCNT concentration. Coefficient k also increases by increasing SWCNT concentration, but it is in the range of (1, ∞). The parallel of two SnO2 related resistances (i.e., k RSnO2 and k RSnO2 / k-1 in Fig.12) equals RSnO2 for all values of k. In other words, since the total amount of SnO2 material, regardless of the amount of added SWCNT, is fixed, by adding different amount of impurity only the ratio of the two SnO2 related resistances will be changed, and the equivalent parallel of the two resistances will not be changed. Accordingly, for x=0, when two x and k’ related resistances (i.e., x RSWCNT and k' RSWCNT/SnO2 in Fig.12) are zero, the total resistance of the circuit is equal to the pure sample resistance, RSnO2.
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The resistance of metal oxide, and its dependency on temperature are well-known. Also metallic behavior of SWCNT bundles is known. Based on this, addition of SWCNT to metal oxide should enhance the conductivity of the samples. However, the results show that the resistance increases by adding SWCNT. Considering above three mentioned types of
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resistance, the SWCNT/ SnO2 grain junction resistance can explain this phenomenon. The work functions of SWCNT and SnO2 are about 5.05 eV and 4.18 eV, respectively [19, 5]. In the case of semiconducting SWCNT/ SnO2 grain junction, there is a heterojunction between
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wide band gap n-type SnO2 grain (3.7 eV) and narrow band gap p- type SWCNT (0.7/D ≈ 0.35 to 0.7 eV). And in the case of metallic SWCNT there is a schottky junction. Thus,
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addition of SWCNT to the SnO2, forms some depletion regions. These regions impose an extra resistance to the structure (RSWCNT/ SnO2 in Fig.12) that increases the resistance of the
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sample. The barrier height calculated in section 4.2.1 confirms this model. Another effect of SWCNT adding to SnO2, as mentioned before, is the shift of the corresponding temperature of maximum resistance to the lower temperatures. The maximum resistance related to the
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maximum oxygen adsorption on the SnO2 grains, therefore, it can be suggested that SWCNT adding, similar to Pd and Ag adding, improves oxygen adsorption by spillover mechanism
6. Sensing Mechanism
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[4], and this increases the resistivity of the samples.
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The results of present work show that adding appropriate amount of SWCNT to SnO2 matrix enhances methane sensitivity of SnO2. Various explanations for gas sensing enhancement have been proposed in the literature: • Creation of junction between n-SnO2 and SWCNT [11]. • Oriented growth of SnO2 along SWCNT during heat treatment and enhancement of local electric field which is favorable for gas sensing reactions [26].
• Local electric field at CNTs – SnO2 junctions that enhances gas sensing reactions [14]. • Formation of nano channel in the SnO2 sample that increases gas diffusion into SnO2 [14]. • Physisorption of methane molecules on CNTs [27, 28]. 11
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• Chemisorbed oxygen molecules on the CNTs that act as reactive gas for oxidation of methane. Also, consumption of the adsorbed oxygen that can change electrical properties of CNTs and affect the sensitivity [10, 29]. Some studies examined the possibility of using carbon materials as a methane decomposition
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catalyst [30]. Catalytic properties of thermally distorted SWCNTs for methane dissociation were reported too [31].
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To interpret the promotional role of additives in metal oxide host, two different chemical and electrical mechanisms were proposed.
In the chemical mechanism, additives assist to
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dissociate molecules that causes atoms “spillover” on to the surface of metal oxides. In this mechanism the effective factor is the surface of metal oxide not the metal oxide/ additive
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interface. This means a high dispersion of additives is necessary. In the electrical mechanism, however, the interface between metal oxide and additive is the effective factor. Additives interact with metal oxide as electron acceptor or donor, depending on their work function. In
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the presence of gas, changes in the work function of the additive will change the Schottky barrier between the additive and the metal oxide. This changes sensor conductivity. This mechanism only enhances the sensitivity of the sensor if the changes of the barriers induced
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the pure metal oxide grains [4, 6].
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through interaction with surrounding gas is higher than the changes of the barriers between
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In the case of SWCNT additive the chemical mechanism is more effective due to following reasons: •
The shift of corresponding temperature of maximum resistivity from 400 to 325350˚C by increasing the SWCNT concentration (Fig. 6), may be related to spillover of oxygen molecules by SWCNT at lower temperatures.
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As mentioned before, previous research showed that SWCNT can act as a catalyst in the methane decomposition process.
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As shown in Fig. 10 for the SWCNT/SnO2 sample, saturation of the sensor response happens at a higher methane concentration. It can be due to the increase of previously adsorbed oxygen molecules that increases adsorption sites for methane radicals; or due to catalytic activity of SWCNT that increases methane radicals.
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Previous studies by Heiu et. al [12] confirm above suggestion. Their results indicated that the response of pure SnO2 sensor to 250 ppm ethanol gas is higher than that of to 2500 ppm LPG. While in the case of CNTs/SnO2, sensor response to 2500 ppm LPG is much higher than that of 250 ppm ethanol. This shows that CNT additive acts as a catalyst, and reacts differently to
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LPG compared to ethanol. Also Wongchoosuk et al. [14] reported that CNT concentration can be used in tuning of sensitivity and selectivity of SnO2 sensor for a target gas, which is
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another evidence for the catalytic activity of CNTs.
Present results show that by adding more than 1.2 %wt SWCNT the sensor response of the
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sensors decreases. This particular behavior of increase and then decrease of sensor response by increasing the additives was reported before for some other additives (Pd, Sb2O3, Co …) [32-33]. In a reducing gas ambient, resistance of the sensor is determined by the
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concentration of the carriers that released by surface reaction between oxygen adsorbents and reducing gas molecules. With increasing the SWCNT concentration, the air resistance of the sensor increases highly due to the increase of barrier height, while the increase of carrier
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concentration in gas ambient can’t follow up equivalently to decrease sensor resistivity. Therefore, sensor response decreases by adding more than an specific value (here 1.2%wt) of
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SWCNT concentration. Also, as mentioned before, in the chemical mechanism, optimum
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condition happens when high dispersion of additives occurs, which is related to an optimum
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value of SWCNT concentration.
7. Conclusion
The effect of SWCNT concentration on electrical resistivity and methane sensitivity of SnO2 gas sensor was investigated. Results showed that by adding SWCNT to SnO2 structure its electrical resistivity in air ambient increases. It was suggested that the SWCNT/ SnO2 grain junction resistance is the main reason for this increase, and based on that, a simple electrical circuit model was proposed. Regarding methane sensing behavior of the samples, comparative results revealed that the sensor response increases by increasing of up to 1.2%wt SWCNT concentration, and after that by further increase of SWNT concentration the sensor response decreases. Two possible electrical and chemical mechanisms that could interpret the sensitivity behavior of the samples were discussed. Chemical mechanism was concluded as the main reason for the methane sensitivity enhancement of the sensor.
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Acknowledgements
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The authors would like to thank Dr. Sayyed Javad Hashemifar for his valuable discussion and comments.
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[5] D. Haridas, V. Gupta, Enhanced response characteristics of SnO2 thin film based sensors loaded with Pd clusters for methane detection, Sens Actuators B Chem, 166(2012) 156-64. [6] S. Basu, P. Basu, Nanocrystalline metal oxides for methane sensors: role of noble metals, Journal of Sensors, 2009(2009). [7] J.-H. Sung, Y.-S. Lee, J.-W. Lim, Y.-H. Hong, D.-D. Lee, Sensing characteristics of tin dioxide/gold sensor prepared by coprecipitation method, Sens Actuators B Chem: Chemical, 66(2000) 149-52. [8] S. Bose, S. Chakraborty, B. Ghosh, D. Das, A. Sen, H.S. Maiti, Methane sensitivity of Fedoped SnO2 thick films, Sens Actuators B Chem, 105(2005) 346-50. [9] M. Rumyantseva, V. Kovalenko, A. Gaskov, E. Makshina, V. Yuschenko, I. Ivanova, et al., Nanocomposites SnO2-Fe2O3: Sensor and catalytic properties, Sens Actuators B Chem, 118(2006) 208-14. [10] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science, 287(2000) 1801-4. [11] B.-Y. Wei, M.-C. Hsu, P.-G. Su, H.-M. Lin, R.-J. Wu, H.-J. Lai, A novel SnO2 gas sensor doped with carbon nanotubes operating at room temperature, Sens Actuators B Chem, 101(2004) 81-9. [12] N. Van Hieu, N.A.P. Duc, T. Trung, M.A. Tuan, N.D. Chien, Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes: A competitive sensor for ethanol and liquid petroleum gas, Sens Actuators B Chem, 144(2010) 450-6. [13] V. Aroutiounian, V. Arakelyan, E. Khachaturyan, G. Shahnazaryan, M. Aleksanyan, L. Forro, et al., Manufacturing and investigations of i-butane sensor made of SnO2 multiwallcarbon-nanotube nanocomposite, Sens Actuators B Chem, 173(2012) 890-6. [14] C. Wongchoosuk, A. Wisitsoraat, A. Tuantranont, T. Kerdcharoen, Portable electronic nose based on carbon nanotube-SnO2 gas sensors and its application for detection of methanol contamination in whiskeys, Sens Actuators B Chem, 147(2010) 392-9. [15] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram, 7(2001) 143-67.
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[16] F. Schipani, C. Aldao, M. Ponce, Schottky barriers measurements through Arrhenius plots in gas sensors based on semiconductor films, AIP Adv, 2(2012) 032138--6. [17] R. Saito, G. Dresslhaus, M. S. Dresselhaus, Physical properties of carbon nanotubes, fourth ed. Imperial College Press, London, 1998. [18] S. Dehghani, M.K. Moravvej-Farshi, M.H. Sheikhi, Temperature Dependence of Electrical Resistance of Individual Carbon Nanotubes and Carbon Nanotubes Network, Mod Phys Lett B, 26(2012). [19] M. Shiraishi, M. Ata, Work function of carbon nanotubes, Carbon, 39(2001) 1913-7.
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[20] C. Papadopoulos, D. Vlachos, J. Avaritsiotis, Comparative study of various metaloxide-based gas-sensor architectures, Sens Actuators B Chem, 32(1996) 61-9. [21] B.-K. Min, S.-D. Choi, Undoped and 0.1 wt.% Ca-doped Pt-catalyzed SnO2sensors for CH4 detection, Sens and Actuators B: Chem, 108(2005) 119-24.
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[22] F. Quaranta, R. Rella, P. Siciliano, S. Capone, M. Epifani, L. Vasanelli, et al., A novel gas sensor based on SnO2/Os thin film for the detection of methane at low temperature, Sens Actuators B Chem, 58(1999) 350-5. [23] D. Abbaszadeha, R. Ghasempoura, F. Rahimi, Iraji zad, A.," Effective factors on methane sensing of tin-oxide activated by palladium in sol–gel process", Sens Transducers J, 73(2006) 819-25. [24] S. Chakraborty, I. Mandal, I. Ray, S. Majumdar, A. Sen, H. Maiti, Improvement of recovery time of nanostructured tin dioxide-based thick film gas sensors through surface modification, Sens Actuators B Chem, 127(2007) 554-8. [25] T. Wagner, M. Bauer, T. Sauerwald, C.-D. Kohl, M. Tiemann, X-ray absorption nearedge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection, Thin Solid Films, 520(2011) 909-12. [26] J. Liu, Z. Guo, F. Meng, Y. Jia, J. Liu, A novel antimony-carbon nanotube-tin oxide thin film: carbon nanotubes as growth guider and energy buffer. Application for indoor air pollutants gas sensor, J Phys Chem C, 112(2008) 6119-25. [27] M.D. Ganji, A. Mirnejad, A. Najafi, Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes, Sci Tech Adv Mat, 11(2010) 045001. [28] M. Ganji, M. Asghary, A. Najafi, Interaction of methane with single-walled carbon nanotubes: role of defects, curvature and nanotubes type, Commun Theor Phys, 53(2010) 987. [29] L. Valentini, I. Armentano, L. Lozzi, S. Santucci, J. Kenny, Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption, Mat Sci Eng C, 24(2004) 527-33. [30] N. Muradov, Catalysis of methane decomposition over elemental carbon, catal commun, 2(2001) 89-94. [31] L. Bagolini, F. Gala, G. Zollo, Methane cracking on single-wall carbon nanotubes studied by semi-empirical tight binding simulations, Carbon, 50(2012) 411-20. [32] K. Chatterjee, S. Chatterjee, A. Banerjee, M. Raut, N. Pal, A. Sen, et al., The effect of palladium incorporation on methane sensitivity of antimony doped tin dioxide, Mat Chem Phys, 81(2003) 33-8. [33] G. Korotcenkov, I. Boris, V. Brinzari, S. Han, B. Cho, The role of doping effect on the response of SnO2-based thin film gas sensors: Analysis based on the results obtained for Codoped SnO2films deposited by spray pyrolysis, Sens Actuators B Chem, 182(2013) 112-24.
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Biographies
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Zahra Karami Horestani was born in Isfahan, Iran. She received her B.S. and M.S. degrees in Electrical and Electronics Engineering from Shiraz University of Technology and Khajeh Nasir Toosi University of Technology Iran, in 2006 and 2008, respectively. Currently, she is working towards her PhD in Engineering at Isfahan University of Technology, in the area of gas sensor design and fabrication.
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Sayed Masoud Sayedi was born in 1960. He received the B.S. and M.S. degrees in electrical engineering from Isfahan University of Technology, Isfahan, Iran in 1986 and 1988 respectively and the Ph.D. degree in electrical engineering from Concordia University, Montreal, Canada in 1997. He is currently an associate professor in the faculty of Electrical & Computer Engineering at Isfahan University of Technology. His research interest includes the study of VLSI fabrication processes and analysis and design of microelectronic circuits.
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Mohammad Hossein Sheikhi was born in Shiraz-Iran in 1972. He received The B.Sc., M.Sc., and Ph.D., all in Electrical Engineering, from Shiraz University, Sharif University of Technology, and Tarbiat Modarres University in 1994, 1996, and 2000, respectively. He has been with Tohoku University in Japan from 2000 to 2001, as postdoctoral research fellow. Since 2001, he has been a faculty member at Shiraz University, where currently he is an associate professor. His research interest include nanoelectronic devices, Nanosensors, and MEMS. He has been head of nanotechnology research institute at Shiraz University.
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Figure Captions:
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Figure 1: Schematic of charge carrier concentration in metal oxide grains. Negatively charged chemisorbed oxygen species cause an upward band bending and consequently a depletion layer in the near-surface region. This causes a Schottky-like barrier across grain boundaries [2].
Figure 3: Schematic of the measurement system.
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Figure 2: Schematic of the fabricated sensor.
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Figure 4: X-ray diffraction pattern of (a) SWCNT, (b) SnO2, and (c) hybrid SWCNT/SnO2. Figure 5: SEM micrographs of (a): pure SnO2 sample (b): hybrid sample containing 1.2%wt SWCNT.
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Figure 6: Variation of electrical resistance in air ambient as a function of temperature for pure SnO2 and SnO2/SWCNT with different SWCNT concentrations. Figure 7: Arrhenius plot for pure SnO2 and SnO2/SWCNT with different SWCNT
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concentrations. Schottky barriers were calculated for the section specified in the box. Figure 8: Responses of pure SnO2 sample and the hybrid sample with 1.2%wt SWCNT to 2000 ppm methane gas.
Figure 9: Sensor response versus SWCNT concentration in three different temperatures. Figure 10: Sensor response versus CH4 concentration for pure SnO2 sample and sample with 1.2%wt SWCNT.
Figure 11: Schematic of sensor microstructure showing two types of resistance path. Figure 12: Proposed electrical circuit model for the sensor.
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Biographies
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Zahra Karami Horestani was born in Isfahan, Iran. She received her B.S. and M.S. degrees in Electrical and Electronics Engineering from Shiraz University of Technology and Khajeh Nasir Toosi University of Technology Iran, in 2006 and 2008, respectively. Currently, she is working towards her PhD in Engineering at Isfahan University of Technology, in the area of gas sensor design and fabrication.
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Sayed Masoud Sayedi was born in 1960. He received the B.S. and M.S. degrees in electrical engineering from Isfahan University of Technology, Isfahan, Iran in 1986 and 1988 respectively and the Ph.D. degree in electrical engineering from Concordia University, Montreal, Canada in 1997. He is currently an associate professor in the faculty of Electrical & Computer Engineering at Isfahan University of Technology. His research interest includes the study of VLSI fabrication processes and analysis and design of microelectronic circuits.
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Mohammad Hossein Sheikhi was born in Shiraz-Iran in 1972. He received The B.Sc., M.Sc., and Ph.D., all in Electrical Engineering, from Shiraz University, Sharif University of Technology, and Tarbiat Modarres University in 1994, 1996, and 2000, respectively. He has been with Tohoku University in Japan from 2000 to 2001, as postdoctoral research fellow. Since 2001, he has been a faculty member at Shiraz University, where currently he is an associate professor. His research interest include nanoelectronic devices, Nanosensors, and MEMS. He has been head of nanotechnology research institute at Shiraz University.
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Table 1: A brief summary of reported SnO2/CNT gas sensor Technique
CNT Concentration(%wt)
Gas
Measured Parameter
SnO2/SWCNT Thin Film
Chemical Method
0.003, 0.03
NO2
Sensor Resistance
SnO2/MWCNT Thin Film
Sol-gel Method
0.1
LPG Ethanol
SnO2/MWCNT
Sol-gel Method
0.1
E- beam Evaporation
0.5, 1
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[12]
Sensor Resistance
[13]
Sensor Resistance
[14]
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Ethanol/ Methanol
[11]
Sensor Resistance
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SnO2/MWCNT Thin Film
i-butane
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Thick Film/Ceramic
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Sensing Material
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Table 2: A brief summary of results reported on SnO2 based methane sensor.
SnOx and InOx thin film/Al2O3+Pd
Technique
Gas Concentration
rf sputtering
10000 ppm
Sol- gel
1000 ppm
Co precipitation method
3000 ppm
Response
2.55 (Ra/Rg)
Temp. (°C)
Ref.
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1996
Sensing Material
60.7% ( (Ra-Rg)/Ra)
420
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Year
[20]
620 (Ra/Rg) )
2005
Fe doped SnO2 thick films
Pt/SnO2/thin film
Precipitation method Sputtering
1000 ppm
5000 ppm
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Pd loaded SnO2 thin film
Sol-gel
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2006
82% ( (Ra-Rg)/Ra)
400
[7]
350
[8]
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250
65% ( (Ra-Rg)/Ra)
2.3 (Ra/Rg) )
400
[22]
56.5% ( (Ra-Rg)/Ra)
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2005
SnO2/Au thick film
[21]
99.8% ( (Ra-Rg)/Ra)
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2000
SnO2/Os thin film
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1999
1000 ppm
4% ( (Ra-Rg)/Ra)
350
[23]
2006
Iron doped tin oxide
Precipitation method
1000 ppm
66% ( (Ra-Rg)/Ra)
350
[24]
2011
Pd / SnO2
Chemical method
6500 ppm
95% ( (Ra-Rg)/Ra)
600
[25]
2012
Pd / SnO2
rf sputtering
200 ppm
99 % ( (Ra-Rg)/Ra)
250
[5]
2014
SWCNT/ SnO2
Powder pressing
2000 ppm
67 % ( (Ra-Rg)/Ra)
350375
Present Work
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S0925-4005(14)00639-X http://dx.doi.org/doi:10.1016/j.snb.2014.05.100 SNB 16974
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
17-3-2014 22-5-2014 23-5-2014
Please cite this article as: Z.K. Horastani, S.M. Sayedi, M.H. Sheikhi, Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2 , Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.05.100 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of single wall carbon nanotube additive on electrical conductivity and methane sensitivity of SnO2 Zahra Karami Horastania , S. Masoud Sayedia, M. Hossein Sheikhib* Department of Elec. & Comp. Eng., Isfahan University of Technology, 8415683111 Isfahan, Iran
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School of Elec. & Comp. Eng., Shiraz University, 7194684471 Shiraz, Iran Shiraz Nanotechnology Research Institute, Shiraz University, 7194684471 Shiraz, Iran
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Highlights
The effect of SWCNT adding in electrical conductivity of SnO2 is studied.
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The effect of SWCNT adding in methane sensitivity of SnO2 is investigated.
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An electrical circuit model based on obtained results is proposed.
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The role of SWCNT in sensitivity enhancement of SnO2 gas sensor is studied.
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Abstract
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The effect of single wall carbon nanotube (SWCNT) additive on electrical conductivity and methane sensitivity of SnO2 gas sensor has been investigated. Sensors were prepared through powder pressing procedure and the morphology and microstructure of the materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The electrical resistivity and methane sensitivity of the samples were examined in the temperature interval of 100 to 450°C. Results show that in the air ambient the electrical resistivity of the samples increases with the increase of SWCNT concentration. Also, comparative results show that by increasing of SWCNT concentration up to 1.2%wt, the methane sensitivity of the samples increases, and after that by further increase of SWNT concentration the sensitivity decreases. Based on the SEM images of the sensors, and also the measurement results, three types of resistance, related to metal oxide, SWCNT bundle, and SWCNT/ SnO2 grain junction, were realized in the sensors and based on that, a simple *
Corresponding author. Tel.: +98-711-7255024. Fax: +98-711-6474280.
E-mail address: [email protected] (M. H. Sheikhi). 1
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electrical circuit model is proposed. Results show that SWCNT/ SnO2 grain junction resistance is the main reason for the higher electrical resistivity of the samples with higher SWCNT concentrations.
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Key words: SnO2, Single wall carbon nanotube, Electrical conductivity, Gas sensor, Electrical model.
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1. Introduction Metal oxide semiconductors have been used widely in the gas sensing applications. Among them, tin oxide (SnO2) with low cost and good sensitivity is one of the most interesting ones. Tin oxide as a wide band gap (3.6 eV) n-type semiconductor constitutes a major part of different forms from ceramic conductive to nano structure sensors.
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market and publication [1, 2]. It is sensitive to many different gases, and fabricated in many
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One traditional way to improve sensitivity, selectivity, response time and operating temperature of metal oxide gas sensors is the use of additives [3-9]. Recently, much attention
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has been focused on the carbon nanotubes (CNTs) as the additive. Collins et al. demonstrated that CNTs exhibit very good adsorption property due to their high specific surface areas and
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nano scale structures, and that they can respond to both reducing and oxidizing gases through a charge transferring reaction with the gas molecules that changes their conductivity [10]. Previous works have demonstrated that the hybrid of CNT and metal oxides can be used for
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detection of various gases such as NO2, liquid petroleum gas (LPG), i-butane, methanol and ethanol. It was reported that the hybrid material has a better performance compared to pure
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metal oxide resistive sensor. Wie et. al [11] fabricated hybrid single wall carbon nano tube (SWCNT)/SnO2 gas sensors with 0.003 and 0.03%wt. of SWCNT to detect NO2 at room
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temperature, and compared the results with pure SnO2. They found that the hybrid sensors exhibit higher sensitivity compared to pure SnO2, and that the sensor with higher SWCNT
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concentration has better performance. Hieu et al. [12] investigated the effect of multi wall carbon nanotube (MWCNT) on the ethanol and LPG sensing property of SnO2. They found that the responses are improved compared to pure SnO2 sensor, and that the MWCNT adding act differently for ethanol and LPG. Aroutiounian et al. [13] developed a gas sensor based on SnO2/MWCNT (0.1%wt MWCNT) composite for which electrical conductance was changed when exposed to i-butane. They couldn’t detect i-butane with pure SnO2 or pure CNT. Wongchoosuk et al. [14] investigated the effect of MWCNT concentration (0.5, 1%wt) on the methanol and ethanol sensing property of SnO2. They found that adding MWCNT to SnO2 matrix improves gas sensitivity of SnO2, and that the quantity of MWCNT changes the selectivity of the sensor. More information related to above studies is presented in table 1. Methane (CH4) gas can be found in environment, industrial and domestic areas. It is a colorless, inflammable, nontoxic gas with a sweet, oil type odor. It is the simplest alkene, and the main component of natural gas. Due to its inflammable nature, on one hand, and its wide 3
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applications (in chemical factories, mines, automobile, household etc.) in other hand, development of a reliable methane sensor is essential for the safe use of it. As noted before CNT adding to SnO2 can improve the working characteristic of the SnO2based gas sensors in detecting NO2, ethanol, methanol, LPG and i- butane gases. The focus of
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the present work is to investigate how the SWCNT adding influences the methane sensitivity and electrical conductivity of SnO2. The hybrid SnO2-CNT sensors with different SWCNT
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concentrations were prepared by powder pressing method which is an inexpensive and reproducible way in the gas sensor fabrication process.
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In the following and in section 2 a brief description of SnO2 and SWCNT as the basic components of the investigated sensors is presented. In section 3 experimental procedures
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including sensor fabrication and measurement setup are explained. Experimental results, a model that justifies sensor’s resistivity vs. temperature curve in the air ambient, and a discussion on sensing mechanism are presented in sections 4 to 6. And finally, conclusion is
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provided in section 7.
2. A brief discussion on SnO2 and SWCNT as two gas sensing components
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a) SnO2. It is well known that the electrical conductivity of a polycrystalline metal oxide such
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as SnO2 is mainly determined by its grain boundary conductance rather than its grain conductance. Therefore, its conductivity can be controlled by some surface reactions like
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surface molecule adsorption. At temperatures between 100 to 500°C, oxygen molecules are adsorbed on metal oxide surface through the following reactions [15]: O2(gas)↔O2(ads) ↔O-2(ads) ↔O-(ads) ↔O2-(ads) ↔O2-(lattice)
Below 150°C the adsorption is mainly in the form of O2-, and above 150°C, O- and O2- are mostly involved. These elements increase the barriers between the grains, and as a result, change the conductivity (Fig. 1). When the temperature of a polycrystalline gas sensor increases above room temperature, due to the increase of the carriers with sufficient energy to overcome the schottky barriers in the grain boundaries, and also due to the semiconducting behavior of SnO2, its conductivity increases. However, with more increase of temperature, the adsorption of oxygen on the grain surface increases, causing the enhance of the barrier height between the grains. So there is a competition between the increase of carrier concentration, that leads to a higher conductivity, and the enhancement of barrier height that
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leads to a lower conductivity. The conductance of polycrystalline SnO2, can be expressed as: [15] Eq. 1
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In which
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Eq. 2
where Ф is the barrier height between the grains, T is the temperature in Kelvin, k is the
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Boltzmann constant, q is the elementary charge, nb is the bulk electron concentration, and m* is the effective electron mass. As Eq. 1 shows, the conductance depends on the barrier height exponentially, and variation in this parameter, compared to carrier concentration, is dominant
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one. Thus, with increase of temperature, and increase of barrier height, eventually the conductivity decreases. However, with further increase of temperature, the condition changes
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and desorption of the previously adsorbed oxygens happens. This causes decrease in barrier height, and as a result, increase in the conductivity.
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Schottky barrier height between the grains can be estimated by plotting the logarithm of conductance as a reverse function of temperature (Arrhenius plot). This estimation is true if
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following three conditions are fulfilled within the temperature range: [16]
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a) No gas adsorption/desorption takes place during heating and cooling process, b) Only thermionic conduction is relevant, c) No oxygen diffusion into or out of the grain occurs during heating or cooling process.
It should be noticed that above conditions are only fulfilled in a small temperature range above room temperature, thus, barrier height calculation can be estimated only in this temperature range.
For the gas sensing mechanism, two general steps can be considered. The first is barrier formation in air environment due to oxygen adsorption as discussed above, and the second is reaction of gas molecules with adsorbed oxygen. In the case of reducing gas such as CH4, this reaction causes reinjection of electrons to the grain: CH4+4O-→CO2+2H2O+4e-
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This decreases barrier height, and as a result, decreases resistance of the sensor compared to that of in the air environment [6, 15]. b) SWCNT. SWCNT, which is a graphene sheet rolled into a cylindrical shape, can be used as gas sensing component. This component can be divided into two groups of metal and
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semiconductor, depending on its diameter and chirality (n, m). If n-m is multiple of three, the SWCNT is metallic, otherwise, it is a semiconducting material [17]. The band gap of
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semiconducting SWCNT can be expressed as Eg=0.7/D, where D is the diameter length [18].
During fabrication process, there is no precise control on the chirality parameter of the
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nanotube, but statistically it can be assumed 1/3 of nanotube is metallic and the rest is semiconducting. In an SWCNT bundle (network) a mixture of metallic and semiconducting
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SWCNTs exists. However, previous studies have shown a metallic behavior for this mixture [18]. Work function of SWCNT has been measured 5.05 eV by using photo electron emission
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technique [19]. 3. Experiments 3.1. Fabrication of gas sensor
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To investigate the effect of SWCNT additive on the properties of SnO2 sensor two types of
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sensor – the pure one, and the hybrid one with different SWCNT concentrations, were fabricated. SWCNT-SnO2 mixed powder with 0.3, 0.6, 1.2, 2.4 and 3.8%wt SWCNT
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concentrations were prepared by mixing of 1.2 gr SnO2 and 0.0036, 0.0072, 0.0145 , 0.029 and .045 gr of SWCNT powder (diameter: 1-2 nm, length: 1-2 μm, purity >99%) in a deionized water. The solution was sonicated to obtain a well- mixed suspension. The suspension stirred by hot magnet stirring to make water evaporated and to obtain mixed powder. The mixed powder was thoroughly milled by grinding in mortar to have uniform distribution of SWCNT in SnO2 powder. 0.6 gr of powder was compressed into a 12-mmdiameter cylindrical pellet in a hard steel mold by a mechanical compressor which was controlled by computer and was set to pressure of 10 KN. In order to obtain mechanical strength and thermal stability, the pellets were sintered at 500˚C for 2h in a programmable furnace. The furnace temperature was gradually increased to 500˚C in 2h, stayed at 500˚C for 2h, and then decreased to 30˚C in 10h. After sintering process the electrodes of the sensor were connected by using silver paste, and then the sensor was fixed on a micro heater. The temperature of the micro heater was adjustable, up to 450˚C. The sensor was then placed in
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an oven for 20 minutes at 130˚C to anneal. A schematic of the fabricated sensor is shown in Fig. 2. 3.2. Measurement setup Fig. 3 illustrates the employed laboratory gas detection apparatus. It consists of a 2.5 litter
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chamber, two wire connections for voltage measurements connected to a digital multimeter, two wire connections for connecting the micro heater to a DC voltage supply, and a
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thermocouple for controlling the working temperature. Methane was used as the detecting gas and air as the reference gas. Concentration of the methane was controlled by the injection of a
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pre-calculated amount of saturated methane gas. To measure electrical resistance of the sensor a simple voltage divider, as shown in Fig.3, was used. In the figure Vin is the input signal with 0.5 V amplitude and 1 kHz frequency, Rs is the resistance of the sensor, Rr is the
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reference resistor and Vm is the measured voltage. Rs is calculated by using Vm which is measured by multimeter and registered by computer every 0.5 second. The sensor response
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(S) of the sensor was calculated by using following equation: S%=100×[(Rair-Rgas)/Rair]
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4. Results and discussion
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4.1. Phase and morphology investigation 4.1.1. Material characterization
Fig. 4 shows XRD diffraction patterns of pure SWCNT, pure SnO2 and a mix of SnO2 and 3.8%wt of SWCNT. XRD result of Pure SWCNT shows a peak intensity at 26.5°, which is not clearly apparent or distinguishable in XRD of SnO2/SWCNT. This can be related to either low concentration of SWCNT or overlapping of the SWCNT peak and the first (110) peak of SnO2.To ensure that SWCNT was not burned during fabrication process, the same heat treatments that used to fabricate SnO2/SWCNT sensor, applied to a SWCNT powder. It was observed that SWCNT survived after 2 hours of heating at 500°C. 4.1.2. Surface morphology and microstructure The morphology of the pure and SWCNT doped SnO2 samples after thermal treatment at 500ºC was investigated by scanning electron microscopy (SEM). The SEM micrographs of
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the samples are shown in Fig.5. The figure shows the presence of both metal oxide grain and CNT bundle. CNTs are present in the samples after all stages of heat treatments.
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4.2. Effect of SWCNT concentration on electrical resistivity and gas sensitivity of SnO2 4.2.1. Electrical resistivity in air ambient
Fig. 6 shows the variations of electrical resistance in air ambient as a function of temperature
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The resistance of pure SnO2 decreases with the increase of temperature at low temperatures. As mentioned before electrical conductivity in polycrystalline SnO2 is mainly controlled by schottky barriers between the grains, not the resistance of the bulk. By increasing of the
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temperature the resistance decreases due to the increase of the carriers with sufficient energy to overcome the schottky barriers in the grain boundaries; and also, due to the semiconducting behavior of SnO2, i.e., increasing of carrier concentration by increase of
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temperature. By further increase in temperature, as the figure shows, the resistance starts increasing up to a maximum value. This is due to heighten schottky barriers at grain
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boundary surfaces caused by oxygen adsorption. At temperatures above 150°C oxygen in the form of O- or O2- is adsorbed chemically on the surface of SnO2 grains, traps their electrons
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[15]. This causes an upward band bending and increase of schottky barriers. With more
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increase of temperature, the resistance decreases due to desorption of previously adsorbed oxygen molecules, and also because of presence of some carriers with a kinetic energy higher than barrier height. The SnO2/SWCNT samples showed similar behavior but with higher values of resistance compared to pure SnO2. As the figure shows with increasing of SWCNT concentration, electrical resistance of the samples increases. Another result is the shift of corresponding temperature of maximum resistance from 400°C for pure SnO2 sample to 325350°C interval for SWCNT/SnO2 samples. In order to estimate the schottky barrier height between the grains, as shown in Fig.7, the logarithm of the conductance of the samples as an inverse function of temperature was plotted. The calculated barrier heights for pure sample, and 0.3, 0.6, 1.2, 2.4 and 3.8%wt SWCNT impure samples are estimated 0.14, 0.79, 1.22, 1.78, 1.86 and 1.59 eV respectively. These values show that the barrier height grows with increasing of SWCNT concentration (except the sample contain 3.8%wt SWCNT, this may be related to non-uniform distribution
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of SWCNT because of high SWCNT concentration in this sample). This confirms the increase of sensor resistance with increasing of SWCNT concentration. 4.2.2. Sensor response in the presence of methane gas Sensor response of all the prepared samples to methane gas was examined. Fig. 8 shows the
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responses for the pure SnO2 sample and the hybrid sample with 1.2%wt SWCNT. Before each measurement, sensor was exposed to air for 10 minutes to reach to a stable condition,
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and then 2000 ppm concentration of methane gas was added by injecting appropriate amount of methane with micro syringe. As shown in Fig.8, after gas injection the resistance changes
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fast to an almost stable value. In order to recover the sensor, the glass chamber door is opened.
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Response of all the samples for 2000 ppm methane gas, as a function of temperature, was measured. Maximum temperature was 450ºC and minimum temperature which was about 275˚C was determined by minimum measurable sensor response. For all the samples sensor
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response increases by increasing the temperature to an optimum value and then it decreases. Also response of the samples increases by increasing of up to 1.2%wt SWCNT concentration,
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and after that sensor response decreases. For the sample with 1.2%wt SWCNT, which has the best response, maximum response (67%) occurs at 375ºC. Fig. 9 shows sensor response
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versus SWCNT concentration in three different temperatures. Fig. 9 shows that the higher curves are related to higher working temperatures. For any selected value of sensitivity for
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pure sample, corresponding point is located at 0%wt on one of the curves in the figure. An equivalent sensitivity for a mixed sample happens at lower temperatures. This can be deduced by drawing a horizontal line in the figure representing points with sensitivity equal to the selected one. The crossing of this line and lower curves happens for mixed samples meaning that for the same sensitivity the working temperature of mixed sensors is lower than that of pure sensor. For the pure sample and the sample with 1.2%wt SWCNT, in their optimum temperatures (400 and 375ºC respectively), the sensor response as a function of methane concentration is presented in Fig. 10. As the figure shows sensor response of both sensors first increases with increasing of gas concentration and then it is saturated at some points. The saturation point for the SWCNT sensor happens at higher gas concentration. Table 2 summarizes the current results and some previously reported results on methane sensor. Comparing the sensitivity and working temperature of different sensors, shows that for the same methane concentration two cases of Ref. 5 and Ref. 21 have better performances 9
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compared to present work. Though, considering the low cost of fabrication process employed in this work and also the possibility of fabricating thin structures to improve the performance, the proposed SnO2/SWCNT sensor has potential to be an important candidate in developing methane gas sensors.
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5. Modeling of sensor’s resistivity vs. temperature behavior in the air ambient
The SEM images of the hybrid sensors show that, in terms of electrical conductivity, sensor
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structure can be divided into two different paths, the hybrid SWCNT path, and the pure SnO2 path (Fig. 11). Accordingly, conductivity of each hybrid sample can be determined by the
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conductivities of these two parallel paths. Fig.6. shows that for all the samples, the hybrid SWCNT samples and the pure SnO2 samples, at temperature interval between 150 to 250ºC, a very similar low value resistance exists. The reason is, in a hybrid SWCNT structure, with the
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presence of a pure SnO2 path parallel to hybrid path, the equivalent resistance of the sample is determined by the low value resistance of this path, regardless of the value of its parallel
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hybrid path.
For each hybrid path three types of resistances can be assumed: metal oxide resistance,
•
SWCNT bundle resistance,
•
SWCNT/ SnO2 grain junction resistance.
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•
Based on above discussion, a simple electrical circuit model as shown in Fig.12 is proposed. In the figure, RSWCNT is the resistance of SWCNT bundle, RSnO2 is the resistance of pure SnO2, RSWCNT/SnO2 is the resistance of SWCNT bundle - SnO2 junction, x is a parameter determined by SWCNT concentration, and k and k΄ are two coefficients related to x. x and k’ are equal to zero for pure sample, and they increase by increasing SWCNT concentration. Coefficient k also increases by increasing SWCNT concentration, but it is in the range of (1, ∞). The parallel of two SnO2 related resistances (i.e., k RSnO2 and k RSnO2 / k-1 in Fig.12) equals RSnO2 for all values of k. In other words, since the total amount of SnO2 material, regardless of the amount of added SWCNT, is fixed, by adding different amount of impurity only the ratio of the two SnO2 related resistances will be changed, and the equivalent parallel of the two resistances will not be changed. Accordingly, for x=0, when two x and k’ related resistances (i.e., x RSWCNT and k' RSWCNT/SnO2 in Fig.12) are zero, the total resistance of the circuit is equal to the pure sample resistance, RSnO2.
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The resistance of metal oxide, and its dependency on temperature are well-known. Also metallic behavior of SWCNT bundles is known. Based on this, addition of SWCNT to metal oxide should enhance the conductivity of the samples. However, the results show that the resistance increases by adding SWCNT. Considering above three mentioned types of
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resistance, the SWCNT/ SnO2 grain junction resistance can explain this phenomenon. The work functions of SWCNT and SnO2 are about 5.05 eV and 4.18 eV, respectively [19, 5]. In the case of semiconducting SWCNT/ SnO2 grain junction, there is a heterojunction between
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wide band gap n-type SnO2 grain (3.7 eV) and narrow band gap p- type SWCNT (0.7/D ≈ 0.35 to 0.7 eV). And in the case of metallic SWCNT there is a schottky junction. Thus,
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addition of SWCNT to the SnO2, forms some depletion regions. These regions impose an extra resistance to the structure (RSWCNT/ SnO2 in Fig.12) that increases the resistance of the
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sample. The barrier height calculated in section 4.2.1 confirms this model. Another effect of SWCNT adding to SnO2, as mentioned before, is the shift of the corresponding temperature of maximum resistance to the lower temperatures. The maximum resistance related to the
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maximum oxygen adsorption on the SnO2 grains, therefore, it can be suggested that SWCNT adding, similar to Pd and Ag adding, improves oxygen adsorption by spillover mechanism
6. Sensing Mechanism
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[4], and this increases the resistivity of the samples.
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The results of present work show that adding appropriate amount of SWCNT to SnO2 matrix enhances methane sensitivity of SnO2. Various explanations for gas sensing enhancement have been proposed in the literature: • Creation of junction between n-SnO2 and SWCNT [11]. • Oriented growth of SnO2 along SWCNT during heat treatment and enhancement of local electric field which is favorable for gas sensing reactions [26].
• Local electric field at CNTs – SnO2 junctions that enhances gas sensing reactions [14]. • Formation of nano channel in the SnO2 sample that increases gas diffusion into SnO2 [14]. • Physisorption of methane molecules on CNTs [27, 28]. 11
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• Chemisorbed oxygen molecules on the CNTs that act as reactive gas for oxidation of methane. Also, consumption of the adsorbed oxygen that can change electrical properties of CNTs and affect the sensitivity [10, 29]. Some studies examined the possibility of using carbon materials as a methane decomposition
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catalyst [30]. Catalytic properties of thermally distorted SWCNTs for methane dissociation were reported too [31].
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To interpret the promotional role of additives in metal oxide host, two different chemical and electrical mechanisms were proposed.
In the chemical mechanism, additives assist to
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dissociate molecules that causes atoms “spillover” on to the surface of metal oxides. In this mechanism the effective factor is the surface of metal oxide not the metal oxide/ additive
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interface. This means a high dispersion of additives is necessary. In the electrical mechanism, however, the interface between metal oxide and additive is the effective factor. Additives interact with metal oxide as electron acceptor or donor, depending on their work function. In
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the presence of gas, changes in the work function of the additive will change the Schottky barrier between the additive and the metal oxide. This changes sensor conductivity. This mechanism only enhances the sensitivity of the sensor if the changes of the barriers induced
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the pure metal oxide grains [4, 6].
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through interaction with surrounding gas is higher than the changes of the barriers between
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In the case of SWCNT additive the chemical mechanism is more effective due to following reasons: •
The shift of corresponding temperature of maximum resistivity from 400 to 325350˚C by increasing the SWCNT concentration (Fig. 6), may be related to spillover of oxygen molecules by SWCNT at lower temperatures.
•
As mentioned before, previous research showed that SWCNT can act as a catalyst in the methane decomposition process.
•
As shown in Fig. 10 for the SWCNT/SnO2 sample, saturation of the sensor response happens at a higher methane concentration. It can be due to the increase of previously adsorbed oxygen molecules that increases adsorption sites for methane radicals; or due to catalytic activity of SWCNT that increases methane radicals.
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Previous studies by Heiu et. al [12] confirm above suggestion. Their results indicated that the response of pure SnO2 sensor to 250 ppm ethanol gas is higher than that of to 2500 ppm LPG. While in the case of CNTs/SnO2, sensor response to 2500 ppm LPG is much higher than that of 250 ppm ethanol. This shows that CNT additive acts as a catalyst, and reacts differently to
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LPG compared to ethanol. Also Wongchoosuk et al. [14] reported that CNT concentration can be used in tuning of sensitivity and selectivity of SnO2 sensor for a target gas, which is
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another evidence for the catalytic activity of CNTs.
Present results show that by adding more than 1.2 %wt SWCNT the sensor response of the
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sensors decreases. This particular behavior of increase and then decrease of sensor response by increasing the additives was reported before for some other additives (Pd, Sb2O3, Co …) [32-33]. In a reducing gas ambient, resistance of the sensor is determined by the
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concentration of the carriers that released by surface reaction between oxygen adsorbents and reducing gas molecules. With increasing the SWCNT concentration, the air resistance of the sensor increases highly due to the increase of barrier height, while the increase of carrier
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concentration in gas ambient can’t follow up equivalently to decrease sensor resistivity. Therefore, sensor response decreases by adding more than an specific value (here 1.2%wt) of
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SWCNT concentration. Also, as mentioned before, in the chemical mechanism, optimum
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condition happens when high dispersion of additives occurs, which is related to an optimum
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value of SWCNT concentration.
7. Conclusion
The effect of SWCNT concentration on electrical resistivity and methane sensitivity of SnO2 gas sensor was investigated. Results showed that by adding SWCNT to SnO2 structure its electrical resistivity in air ambient increases. It was suggested that the SWCNT/ SnO2 grain junction resistance is the main reason for this increase, and based on that, a simple electrical circuit model was proposed. Regarding methane sensing behavior of the samples, comparative results revealed that the sensor response increases by increasing of up to 1.2%wt SWCNT concentration, and after that by further increase of SWNT concentration the sensor response decreases. Two possible electrical and chemical mechanisms that could interpret the sensitivity behavior of the samples were discussed. Chemical mechanism was concluded as the main reason for the methane sensitivity enhancement of the sensor.
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Acknowledgements
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The authors would like to thank Dr. Sayyed Javad Hashemifar for his valuable discussion and comments.
References
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[1] K.J. Choi, H.W. Jang, One-dimensional oxide nanostructures as gas-sensing materials: review and issues, Sensors, 10(2010) 4083-99. [2] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Progress in surface science, 79(2005) 47-154. [3]A. Sharma, M. Tomar, V. Gupta, Enhanced response characteristics of SnO2 thin film based NO2 gas sensor integrated with nanoscaled metal oxide clusters, Sens and Actuators B: Chem, 181(2013) 735-42. [4] N. Yamazoe, Y. Kurokawa, T. Seiyama, Effects of additives on semiconductor gas sensors, Sens and Actuators, 4(1983) 283-9.
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[5] D. Haridas, V. Gupta, Enhanced response characteristics of SnO2 thin film based sensors loaded with Pd clusters for methane detection, Sens Actuators B Chem, 166(2012) 156-64. [6] S. Basu, P. Basu, Nanocrystalline metal oxides for methane sensors: role of noble metals, Journal of Sensors, 2009(2009). [7] J.-H. Sung, Y.-S. Lee, J.-W. Lim, Y.-H. Hong, D.-D. Lee, Sensing characteristics of tin dioxide/gold sensor prepared by coprecipitation method, Sens Actuators B Chem: Chemical, 66(2000) 149-52. [8] S. Bose, S. Chakraborty, B. Ghosh, D. Das, A. Sen, H.S. Maiti, Methane sensitivity of Fedoped SnO2 thick films, Sens Actuators B Chem, 105(2005) 346-50. [9] M. Rumyantseva, V. Kovalenko, A. Gaskov, E. Makshina, V. Yuschenko, I. Ivanova, et al., Nanocomposites SnO2-Fe2O3: Sensor and catalytic properties, Sens Actuators B Chem, 118(2006) 208-14. [10] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science, 287(2000) 1801-4. [11] B.-Y. Wei, M.-C. Hsu, P.-G. Su, H.-M. Lin, R.-J. Wu, H.-J. Lai, A novel SnO2 gas sensor doped with carbon nanotubes operating at room temperature, Sens Actuators B Chem, 101(2004) 81-9. [12] N. Van Hieu, N.A.P. Duc, T. Trung, M.A. Tuan, N.D. Chien, Gas-sensing properties of tin oxide doped with metal oxides and carbon nanotubes: A competitive sensor for ethanol and liquid petroleum gas, Sens Actuators B Chem, 144(2010) 450-6. [13] V. Aroutiounian, V. Arakelyan, E. Khachaturyan, G. Shahnazaryan, M. Aleksanyan, L. Forro, et al., Manufacturing and investigations of i-butane sensor made of SnO2 multiwallcarbon-nanotube nanocomposite, Sens Actuators B Chem, 173(2012) 890-6. [14] C. Wongchoosuk, A. Wisitsoraat, A. Tuantranont, T. Kerdcharoen, Portable electronic nose based on carbon nanotube-SnO2 gas sensors and its application for detection of methanol contamination in whiskeys, Sens Actuators B Chem, 147(2010) 392-9. [15] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram, 7(2001) 143-67.
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[16] F. Schipani, C. Aldao, M. Ponce, Schottky barriers measurements through Arrhenius plots in gas sensors based on semiconductor films, AIP Adv, 2(2012) 032138--6. [17] R. Saito, G. Dresslhaus, M. S. Dresselhaus, Physical properties of carbon nanotubes, fourth ed. Imperial College Press, London, 1998. [18] S. Dehghani, M.K. Moravvej-Farshi, M.H. Sheikhi, Temperature Dependence of Electrical Resistance of Individual Carbon Nanotubes and Carbon Nanotubes Network, Mod Phys Lett B, 26(2012). [19] M. Shiraishi, M. Ata, Work function of carbon nanotubes, Carbon, 39(2001) 1913-7.
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[20] C. Papadopoulos, D. Vlachos, J. Avaritsiotis, Comparative study of various metaloxide-based gas-sensor architectures, Sens Actuators B Chem, 32(1996) 61-9. [21] B.-K. Min, S.-D. Choi, Undoped and 0.1 wt.% Ca-doped Pt-catalyzed SnO2sensors for CH4 detection, Sens and Actuators B: Chem, 108(2005) 119-24.
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[22] F. Quaranta, R. Rella, P. Siciliano, S. Capone, M. Epifani, L. Vasanelli, et al., A novel gas sensor based on SnO2/Os thin film for the detection of methane at low temperature, Sens Actuators B Chem, 58(1999) 350-5. [23] D. Abbaszadeha, R. Ghasempoura, F. Rahimi, Iraji zad, A.," Effective factors on methane sensing of tin-oxide activated by palladium in sol–gel process", Sens Transducers J, 73(2006) 819-25. [24] S. Chakraborty, I. Mandal, I. Ray, S. Majumdar, A. Sen, H. Maiti, Improvement of recovery time of nanostructured tin dioxide-based thick film gas sensors through surface modification, Sens Actuators B Chem, 127(2007) 554-8. [25] T. Wagner, M. Bauer, T. Sauerwald, C.-D. Kohl, M. Tiemann, X-ray absorption nearedge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection, Thin Solid Films, 520(2011) 909-12. [26] J. Liu, Z. Guo, F. Meng, Y. Jia, J. Liu, A novel antimony-carbon nanotube-tin oxide thin film: carbon nanotubes as growth guider and energy buffer. Application for indoor air pollutants gas sensor, J Phys Chem C, 112(2008) 6119-25. [27] M.D. Ganji, A. Mirnejad, A. Najafi, Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes, Sci Tech Adv Mat, 11(2010) 045001. [28] M. Ganji, M. Asghary, A. Najafi, Interaction of methane with single-walled carbon nanotubes: role of defects, curvature and nanotubes type, Commun Theor Phys, 53(2010) 987. [29] L. Valentini, I. Armentano, L. Lozzi, S. Santucci, J. Kenny, Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption, Mat Sci Eng C, 24(2004) 527-33. [30] N. Muradov, Catalysis of methane decomposition over elemental carbon, catal commun, 2(2001) 89-94. [31] L. Bagolini, F. Gala, G. Zollo, Methane cracking on single-wall carbon nanotubes studied by semi-empirical tight binding simulations, Carbon, 50(2012) 411-20. [32] K. Chatterjee, S. Chatterjee, A. Banerjee, M. Raut, N. Pal, A. Sen, et al., The effect of palladium incorporation on methane sensitivity of antimony doped tin dioxide, Mat Chem Phys, 81(2003) 33-8. [33] G. Korotcenkov, I. Boris, V. Brinzari, S. Han, B. Cho, The role of doping effect on the response of SnO2-based thin film gas sensors: Analysis based on the results obtained for Codoped SnO2films deposited by spray pyrolysis, Sens Actuators B Chem, 182(2013) 112-24.
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Biographies
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Zahra Karami Horestani was born in Isfahan, Iran. She received her B.S. and M.S. degrees in Electrical and Electronics Engineering from Shiraz University of Technology and Khajeh Nasir Toosi University of Technology Iran, in 2006 and 2008, respectively. Currently, she is working towards her PhD in Engineering at Isfahan University of Technology, in the area of gas sensor design and fabrication.
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Sayed Masoud Sayedi was born in 1960. He received the B.S. and M.S. degrees in electrical engineering from Isfahan University of Technology, Isfahan, Iran in 1986 and 1988 respectively and the Ph.D. degree in electrical engineering from Concordia University, Montreal, Canada in 1997. He is currently an associate professor in the faculty of Electrical & Computer Engineering at Isfahan University of Technology. His research interest includes the study of VLSI fabrication processes and analysis and design of microelectronic circuits.
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Mohammad Hossein Sheikhi was born in Shiraz-Iran in 1972. He received The B.Sc., M.Sc., and Ph.D., all in Electrical Engineering, from Shiraz University, Sharif University of Technology, and Tarbiat Modarres University in 1994, 1996, and 2000, respectively. He has been with Tohoku University in Japan from 2000 to 2001, as postdoctoral research fellow. Since 2001, he has been a faculty member at Shiraz University, where currently he is an associate professor. His research interest include nanoelectronic devices, Nanosensors, and MEMS. He has been head of nanotechnology research institute at Shiraz University.
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Figure Captions:
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Figure 1: Schematic of charge carrier concentration in metal oxide grains. Negatively charged chemisorbed oxygen species cause an upward band bending and consequently a depletion layer in the near-surface region. This causes a Schottky-like barrier across grain boundaries [2].
Figure 3: Schematic of the measurement system.
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Figure 2: Schematic of the fabricated sensor.
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Figure 4: X-ray diffraction pattern of (a) SWCNT, (b) SnO2, and (c) hybrid SWCNT/SnO2. Figure 5: SEM micrographs of (a): pure SnO2 sample (b): hybrid sample containing 1.2%wt SWCNT.
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Figure 6: Variation of electrical resistance in air ambient as a function of temperature for pure SnO2 and SnO2/SWCNT with different SWCNT concentrations. Figure 7: Arrhenius plot for pure SnO2 and SnO2/SWCNT with different SWCNT
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concentrations. Schottky barriers were calculated for the section specified in the box. Figure 8: Responses of pure SnO2 sample and the hybrid sample with 1.2%wt SWCNT to 2000 ppm methane gas.
Figure 9: Sensor response versus SWCNT concentration in three different temperatures. Figure 10: Sensor response versus CH4 concentration for pure SnO2 sample and sample with 1.2%wt SWCNT.
Figure 11: Schematic of sensor microstructure showing two types of resistance path. Figure 12: Proposed electrical circuit model for the sensor.
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Biographies
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Zahra Karami Horestani was born in Isfahan, Iran. She received her B.S. and M.S. degrees in Electrical and Electronics Engineering from Shiraz University of Technology and Khajeh Nasir Toosi University of Technology Iran, in 2006 and 2008, respectively. Currently, she is working towards her PhD in Engineering at Isfahan University of Technology, in the area of gas sensor design and fabrication.
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Sayed Masoud Sayedi was born in 1960. He received the B.S. and M.S. degrees in electrical engineering from Isfahan University of Technology, Isfahan, Iran in 1986 and 1988 respectively and the Ph.D. degree in electrical engineering from Concordia University, Montreal, Canada in 1997. He is currently an associate professor in the faculty of Electrical & Computer Engineering at Isfahan University of Technology. His research interest includes the study of VLSI fabrication processes and analysis and design of microelectronic circuits.
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Mohammad Hossein Sheikhi was born in Shiraz-Iran in 1972. He received The B.Sc., M.Sc., and Ph.D., all in Electrical Engineering, from Shiraz University, Sharif University of Technology, and Tarbiat Modarres University in 1994, 1996, and 2000, respectively. He has been with Tohoku University in Japan from 2000 to 2001, as postdoctoral research fellow. Since 2001, he has been a faculty member at Shiraz University, where currently he is an associate professor. His research interest include nanoelectronic devices, Nanosensors, and MEMS. He has been head of nanotechnology research institute at Shiraz University.
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Table 1: A brief summary of reported SnO2/CNT gas sensor Technique
CNT Concentration(%wt)
Gas
Measured Parameter
SnO2/SWCNT Thin Film
Chemical Method
0.003, 0.03
NO2
Sensor Resistance
SnO2/MWCNT Thin Film
Sol-gel Method
0.1
LPG Ethanol
SnO2/MWCNT
Sol-gel Method
0.1
E- beam Evaporation
0.5, 1
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[12]
Sensor Resistance
[13]
Sensor Resistance
[14]
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Ethanol/ Methanol
[11]
Sensor Resistance
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SnO2/MWCNT Thin Film
i-butane
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Thick Film/Ceramic
Ref
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Sensing Material
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Table 2: A brief summary of results reported on SnO2 based methane sensor.
SnOx and InOx thin film/Al2O3+Pd
Technique
Gas Concentration
rf sputtering
10000 ppm
Sol- gel
1000 ppm
Co precipitation method
3000 ppm
Response
2.55 (Ra/Rg)
Temp. (°C)
Ref.
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1996
Sensing Material
60.7% ( (Ra-Rg)/Ra)
420
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Year
[20]
620 (Ra/Rg) )
2005
Fe doped SnO2 thick films
Pt/SnO2/thin film
Precipitation method Sputtering
1000 ppm
5000 ppm
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Pd loaded SnO2 thin film
Sol-gel
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2006
82% ( (Ra-Rg)/Ra)
400
[7]
350
[8]
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250
65% ( (Ra-Rg)/Ra)
2.3 (Ra/Rg) )
400
[22]
56.5% ( (Ra-Rg)/Ra)
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2005
SnO2/Au thick film
[21]
99.8% ( (Ra-Rg)/Ra)
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2000
SnO2/Os thin film
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1999
1000 ppm
4% ( (Ra-Rg)/Ra)
350
[23]
2006
Iron doped tin oxide
Precipitation method
1000 ppm
66% ( (Ra-Rg)/Ra)
350
[24]
2011
Pd / SnO2
Chemical method
6500 ppm
95% ( (Ra-Rg)/Ra)
600
[25]
2012
Pd / SnO2
rf sputtering
200 ppm
99 % ( (Ra-Rg)/Ra)
250
[5]
2014
SWCNT/ SnO2
Powder pressing
2000 ppm
67 % ( (Ra-Rg)/Ra)
350375
Present Work
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