mixed sno2 tio2 included with carbon nanotubes for gas-sensing application

Mixed SnO 2 /TiO 2 included with carbon nanotubes for gas-sensing application Nguyen Van Duy a , Nguyen Van Hieu b,c, Ã , Pham Thanh Huy b,c , Nguyen Duc Chien c,d , M. Thamilselvan a , Junsin Yi a a School of Information and Communication Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon 440746, South Korea b International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Vietnam c Hanoi Advanced School of Science and Technology (HAST), Hanoi University of Technology (HUT), Vietnam d Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Vietnam article info Article history: Received 21 April 2008 Received in revised form 9 July 2008 Accepted 9 July 2008 Available online 25 September 2008 PACS: 61.48.De 07.07.df 81.07.De Keywords: Mixed SnO 2 –TiO 2 Carbon nanotubes Gas sensor abstract TiO 2 and SnO 2 are the well-known sensing m aterials with a good thermal stability of the former and a high sensitivity of the latter. Carbon nanotubes (CNTs) have also gas sensing ability at room temperature. CNTs-included SnO 2 /TiO 2 material was a new exploration to combine the advantages of three kinds of materials for gas-sensing property. In this work, a uniform SnO 2 /TiO 2 solution was prepared by the sol–gel process with the ratio 3:7 in mole. The CNTs with contents in the range of 0.001–0.5 wt% were dispersed in a mixed SnO 2 /TiO 2 matrix by using an immersion-probe ultrasonic. The SnO 2 –TiO 2 and the CNTs-included SnO 2 –TiO 2 thin films were fabricated by the sol–gel spin-coating method over Pt-interdigitated electrode for gas-sensor device fabrication and they were heat treated at 500 1C for 30 min. FE-SEM and XRD characterizations indicated that the inclusion of CNTs did not affect the particle size as well as the morphology of the thin film. The sensing properties of all as-fabricated sensors were investigated with different ethanol concentrations and operating temperatures. An interesting sensing characteristic of mixed SnO 2 /TiO 2 sensors was that there was a two-peak shape in the sensitivity versus operating temperature curve. At the region of low operating temperature (below 2801C), the hybrid sensors show improvement of sensing property. This result gives a prospect of the stable gas sensors with working temperatures below 250 1C. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor metal oxide gas sensors have been investigated extensively since the past decades owing to their advantages of high sensitivity to pollutant gases, fast response and recovery, low cost, easy implementation, and small size [1,2]. Gas sensors based on SnO 2 materials have been commercially available [3,4]. Thin- film gas sensors have improved the gas-sensing properties from bulk or thick- film ones. They not only give a high sensitivity but also have very fast response and recovery times. However, there still exist great disadvantages of SnO 2 and TiO 2 materials. SnO 2 is thermally unstable and its electrical properties can be degener- ated upon prolonged thermal treatment in reducing the gas atmosphere [1]. On the other hand, in spite of the high thermal and chemical stability, the gas sensors based on TiO 2 materials require high operating temperatures (normally up to 400 1C). This would result in high power consumption and difficulty of packaging. Mixed oxide has been studied to combine the advantages of the sensing property of each oxide component [4–6]. The formation of mixed oxide is classified into three types as follows: (1) Chemical compound. (2) Solid solution. (3) Mix of (1) and (2) types. SnO 2 –TiO 2 falls into the second category. The use of mixed oxides in gas detection has been tried successfully in some systems such as SnO 2 –WO 3 [5], TiO 2 –WO 3 [5–7], TiO 2 –SnO 2 [5,8]. Among these mixed oxides, the SnO 2 –TiO 2 system has been investigated more extensively for gas-sensing applications [5,8–12]. Carbon nanotubes (CNTs) have been the most actively studied material in recent years due to their unique electrical, mechanical and chemical properties, and much attention has been paid to their application in various fields of nanotechnology [13,14]. Moreover, they have nanoscale size and large surface area that can ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.07.007 Ã Corresponding author at: International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT), No. 1 Dai Co Viet Road, Hanoi, Vietnam. Tel.: +84 4 8680787; fax: +84 4 8692963. E-mail addresses: hieu@itims.edu.vn, hieunv-itims@mail.hut.edu.vn (N. Van Hieu). Physica E 41 (2008) 258–263 provide excellent gas absorption properties. These extreme absorption properties make CNTs advantageous for use in many areas of applications. For example, the gas absorption of CNTs at room temperature will change its electric properties with fast response time, which can enable CNTs to be a good candidate of gas-sensing applications [15–17]. For the gas-sensing materials, there are various approaches using CNTs as the solution such as CNTs for dispersion, CNTs for composite, CNTs for filling, CNTs for coating, etc [18–26]. It has been recently reported in the literature that single-wall CNTs (SWCNTs) doping on SnO 2 can significantly improve SnO 2 gas-sensor performance, and especially the sensor can function at room temperature with sufficient sensitivity [27]. Some other endeavors on including CNTs into tungsten tri-oxide (WO 3 ) [25], polymethylmethacrylate [28], polypyrrole [29], etc. have been published. In our previous work, we have demonstrated the improvement of performance of the TiO 2 -based sensor by including CNT s [30] and high performance of the room-temperature NH 3 gas sensor b y u sing SnO 2 /CNT s composites [31].Inthis work, w e e xplor e possibilities to impr ov e the performance and t o reduce the operating temperature of the SnO 2 –TiO 2 ethanol sensors by adding CNTs. 2. Experimental 2.1. Materials synthesis and characterizations SnO 2 –TiO 2 sol was prepared by the sol–gel method. The precursors used to fabricate the solutions were tetra propylortho titanate Ti(OC 3 H 7 ) 4 (99%), tin ethylhexanoate Sn(OOCC 7 H 15 ) 2 and isopropanol C 3 H 7 OH (99.5%). To synthesize the hybrid SWCNT/ SnO 2 –TiO 2 material, the SWCNTs with a diameter lower than 2 nm and multi-wall CNTs (MWCNTs) with diameters ranging from 20 to 40 nm purchased from Shenzhen Nanotech Port Ltd. Co. (Shenzhen China) were introduced in the SnO 2 –TiO 2 sol solution by an ultrasonic shaker at a power of 100 W for 10 min. The CNTs content was varied in the range of 0.001–0.5 wt%. The film was deposited by spin coating on silica substrate at 4000 rpm for 20 s and a film thickness of around 320 nm was obtained. The sensors realized with different SWCNTs contents were signed as S0–S7. Meanwhile, the sensors with various MWCNTs contents were signed as M0–M7. As-deposited films were dried for 30min at 60 1C and then they were annealed at 500 1C for 30 min. The morphology and the crystalline phase of the films were char- acterized using a field emission scanning electron microscope (FE- SEM; 4800 Hitachi, Japan). The microstructure of the sintered film was characterized by X-ray diffraction (XRD), using a Bruker-AXS D5005. 2.2. Gas sensor fabrication and testing The fabrication of the gas sensor was carried out in the following manner: (i) the inter-digitated electrode was fabricated using a conventional photolithographic method with a finger width of 100 m m and a gap size of 70 m m. The fingers of the inter- digitated electrode were made by sputtering 10nm Ti and 200 nm Pt on a layer of silicon dioxide (SiO 2 ) with a thickness of about 100 nm thermally grown on top of a silicon wafer; (ii) the sensing layers were deposited on top of the electrode with subsequent heat treatment at 500 1C for 30 min. The sensor under test was placed on top of a hot plate and held by two tungsten needles. Then they were loaded in a glass chamber with a volume of 4 L as shown in Fig. 1. More details about the measurement set-up can be found elsewhere [31]. The desired ethanol concentrations were obtained by mixing ethanol gas with air using a mass flow control system with computer control (AALBORG model GFC17S-VALD2-A0200) and subse- quently injected into the chamber. The chamber was purged with air and the experiment was repeated. The electrical resistance response during testing was monitored by a precision semicon- ductor parameter analyzer HP4156A, which can be used to detect a very low electrical current (around 10 À12 A). This allows us to measure the high resistance of the mixed oxide films. The resistance responses of the sensor in air ambient and upon exposure of ethanol pulses were monitored. The sensor response (S) was defined as the ratio of the sensor resistance in air (R a ) and in ethanol gas (R g ). 3. Results and discussion 3.1. Microstructure characterizations The formation of the SnO 2 –TiO 2 solid solution can be seen by an XRD pattern in Fig. 2. With the mole ratio of SnO 2 :TiO 2 at 3:7, it shows that the diffraction peaks of the oxide solution follow Vegard’s law. A similar result has been seen for SnO 2 –TiO 2 deposited by sol–gel [11] and sputtering methods [5]. The solution is formed by mixing SnO 2 and TiO 2 lattices in the rutile phase in which both the materials are in the tetragonal structure. From XRD peaks, we get the inter-planar spacing values of SnO 2 –TiO 2 - mixed oxide as shown in Table 1. The peak shift is explained by the substitution of Sn 4+ for Ti 4+ in the TiO 2 crystal structure. Because of the larger radii of Sn 4+ , the lattice spacing increases when the substitution occurs. In the sol–gel process, the chemical reaction controlled at low speed gives the possibility of a homogenous mixed solution. Sn–O and Ti–O bonding disperse uniformly during stirring and hydrolysis reaction. From the peak broadening, the crystallite size estimated by the Scherrer equation was found to be about 5.5 nm. XRD was carried out with the highest SWCNTs content of 0.5 wt% (sample S7); it is understandable that the SWCNTs peaks were not detected in the XRD pattern. FE-SEM images show the surface morphology of the thin films after heat treatment. They exhibit that the particle size is around 10 nm. These results may be caused by the impeding of the polycrystalline aggregate process of each other SnO 2 and TiO 2 . This grain size is approximately two times the Debye length for the depletion layer on the surface. It implies that the surface- sensing mechanism is more effective in these films. Another result is that all the films’ surfaces are highly porous and uniform in granular shape. The high porosity of the thin film makes it more easy to adsorb and desorb gas molecules. All these characteristics promise good gas sensing properties of the material. The FE-SEM ARTICLE IN PRESS H P 4 1 5 6 A Delta Electronic ES30-5 Power Supply Exhaust Target Gas Rotary Pump MFC Mass Flow Controller Fig. 1. Apparatus for gas-sensor testing. N. Van Duy et al. / Physica E 41 (2008) 258–263 259 images of S0 and S4 samples as shown in Fig. 3a and b indicate that the film morphology was not clearly different between the undoped and the SWCNTs-doped samples. CNTs trace cannot be seen in the FE-SEM image of 0.1wt% SWCNTs/SnO 2 –TiO 2 (S6) after annealing at 500 1C for 30 min. We suggest that at low content of CNTs, they are embedded in the oxide matrix. In addition, SWCNTs–TiO 2 and SWCNTs–SnO 2 bondings can be formed naturally through some physicochemical interactions such as Van der Waals force, H bonding and other bondings. The interaction between –OH groups in the course of the hydrolysis reaction of Sn(OC 7 H 15 ) 2 , Ti(OC 3 H 7 ) 4 and –COOH, –OH groups on SWCNTs formed by the purification process can be a case for explanation. This indicated that the crystallites would grow up and enclose SWCNTs during the heat treatment. Therefore, it is very difficult to find CNTs on the film surface. In general, the trace of CNTs on the film surface could be seen in the composite material in which the CNTs’ content would normally be higher than 5 wt%. 3.2. Ethanol sensing properties We have measured the responses of all sensors to ethanol at different concentrations ranging from 125 to 1000 ppm and at operating temperature in a range from 210 to 40 0 1C to investigate the gas-sensing properties. The sensor responses at various operating temperatures are shown in Fig. 4. It was found that the response and recovery times of the sensors are less than 10 s. We have observed that the metal oxide thin-film sensor show a relatively low response-recovery time, and the hybrid CNTs/metal oxide thin-film sensor show even lower values. This observation was also previously reported [20,24,26,32]. ARTICLE IN PRESS Table 1 The interplanar spacing values of SnO 2 –TiO 2 mixed oxide calculated by Vegard’s law are close to the measured values d(110) (A ˚ ) d(101) (A ˚ ) d(2 0 0) (A ˚ ) d(211) (A ˚ ) SnO 2 3.35 2.64 2.37 1.76 TiO 2 3.24 2.48 2.30 1.68 SnO 2 –TiO 2 3.28 2.51 2.32 1.70 Calculation with Vegard law 3.27 2.53 2.32 1.71 Fig. 3. FESEM images depict the uniform and highly porous surface of blank (a) and hybrid (b) 0.1% SWCNTs/SnO 2 –TiO 2 samples. 1M 0 50 100 150 200 250 300 350 10M 100M 1G Air Air 500 ppm 1000 ppm 375 ppm 250 ppm 125 ppm 305 ° C 335 ° C 365 ° C 400 ° C R ( Ω ) t (s) Fig. 4. Ethanol response characteristics of sensor S4 at different temperatures show fast response and recovery times less than 10s. SnO 2 TiO 2 2-Theta - Scale Intensity (Counts) 20 25 30 35 40 45 50 55 60 Fig. 2. X-ray diffraction pattern of SnO 2 –TiO 2 (at ratio 3:7 in mole) shows the diffraction peaks of solid solution following Vegard’s law. Dot lines indicate SnO 2 rutile peaks and dash lines indicate TiO 2 rutile peaks. N. Van Duy et al. / Physica E 41 (2008) 258–263260 The stepwise decrease in electrical resistance obtained with increasing ethanol concentration from air to 1000 ppm ethanol gas in air, and after several cycles of the gas injection, the resistance turns back to the original value when the sensor is exposed to air. These characteristics indicated that the hybrid sensor has relatively stable response. However, the high resistance of around 10 9 O at an operating temperature below 300 1Cisa drawback of the hybrid material. Working temperature is one of the most important parameters for gas sensors. The conventional gas sensors based on SnO 2 and TiO 2 materials operate at the temperature region from 300 to 400 1C. The response versus operating temperature (S–T) curves of our sensors at 100 0 ppm ethanol depict the two-peak shape characteristic. The first maximum in response appears at an operating temperature of around 260 1C and the second peak is around 380 1C. This can be seen clearly with S1 and S4 sensors based on the SWCNTs/SnO 2 –TiO 2 material, as shown in Fig. 5. For the SWCNTs content of 0.001 wt% (S1) and 0.025 wt% (S4), we get the response to 1000 ppm ethanol of 11.1 and 9.6 at operating temperature of 2601C, 32 and 41 at an operating temperature of 380 1C, respectively. Meanwhile, at higher content of SWCNTs, there is a strong degradation in the response. This observation cannot be clearly explained yet. A plausible explanation for the observed effect is that the addition of SWCNTs considerably increases the surface adsorption area of the mixed oxide and added more p/n junction of SWCNTs/SnO 2 –TiO 2 as discussed below. However, when the CNT content is sufficiently high, the SWCNTs begin to connect together and results in a shorter resistance path that shunts the gas-sensing current of the mixed oxide layer. Thus, the gas sensitivity is reduced for a very high SWCNT content. The dependence of the response on ethanol concentration at operating temperatures of 260 and 380 1C is given in Fig. 6. It can be seen that all the sensors present more or less linear characteristic in the investigated range from 125 to 1000 ppm ethanol, which makes their use more convenient. Once again, S1 and S4 dedicate the best in slope than the others. The slope values of fit lines are given in Table 2. We have also surveyed the ARTICLE IN PRESS 200 0 5 10 15 20 25 30 35 40 45 S (R air /R ethanol ) S0 S1 S3 S4 S7 0 5 10 15 20 25 30 35 T ( o C) S (R kk /R Ethanol ) M0 M2 M3 M4 M7 420 400 380360 T (°C) 340320 300 280260240 220 200 420 400 380360340320 300 280260240 220 Fig. 5. The dependence of response on operating temperature depicts the two maximum characteristics on both SWCNTs/SnO 2 –TiO 2 (a) and MWCNTs/SnO 2 –TiO 2 (b) systems. The first peak is around 260 1C and the second one is around 380 1C. 5 10 15 20 25 30 35 40 45 S0 S1 S3 S4 S7 C ethanol (ppm) S (R air /R ethanol ) 0 2 4 6 8 10 C Ethanol (ppm) (R Air /R Ethanol ) S0 S1 S3 S4 S7 1000800600400200 0 1000800600400200 Fig. 6. Response versus on ethanol concentration characteristics in the range from 125 to 1000 ppm at operating temperatures of 240 (a) and 380 1C (b). Table 2 Fitting slope of S–C curves at operating temperatures of 240 and 380 1C S0 S1 S2 S3 S4 S5 S6 S7 At 240 1C (/100 ppm) 0.28 0.69 0.34 0.20 0.84 0.38 0.27 0.24 At 380 1C (/100 ppm) 1.91 3.78 2.12 2.78 2.74 2.07 2.07 1.09 N. Van Duy et al. / Physica E 41 (2008) 258–263 261 influence MWCNTs inclusion on the sensing properties of the mixed oxide material. The sensing properties of this hybrid material are quite similar to that of SWCNTs/SnO 2 –TiO 2 (see Fig. 5b). From the response values at operating temperatures of 240 and 380 1C given in Table 3, we can see that the best improvement in ethanol sensing is obtained for 0.01 and 0.025 wt% MWCNTs. However, the effect of MWCNTs on the ethanol sensing property of mixed oxide is not as high as SWCNTs. To summarize all the results, we plotted the maximum sensitivities versus the CNTs doping content, as seen in Fig. 7.It is easy to see how better when the CNTs-doped mixed SnO 2 –TiO 2 sensors are working at the low temperature. The best improve- ment for operating temperatures of 380 and 260 1C is achieved at SWCNTs contents of 0.001% and 0.025%, respectively. These observations are the same as in the case of MWCNTs inclusion. These results of the sensing properties at a working temperature below 250 1C even give ethanol detectability that is 20–25 times smaller compared to the CNTs/SnO 2 composite sensor prepared by electron beam evaporation [26]. 3.3. Gas sensing mechanism At first, one needs to discuss the two-peak shape of response- operating temperature curves. For the sensors based on tin oxide and titanium oxide, such results have never been seen before. We assumed that the presence of both SnO 2 and TiO 2 makes the mixed oxide material with combined properties. At the operating temperature below 500 1C, the surface sensing mechanism plays a dominant role. Ethanol vapor adsorbs on the surface grain boundaries and reacts with the adsorbed oxygen ions on the surface. It should be noted that the adsorbed oxygen ions trap electrons, inducing a surface depletion layer between the grains. This means the surface density of the negatively charged oxygen decreases by the ethanol vapor absorption, so the barrier height in the grain boundary is reduced. The reduced barrier height decreases sensor resistance. We propose that these processes take place more easily for SnO 2 than for TiO 2 due to the lower working temperature of SnO 2 [9]. The presence of both SnO 2 and TiO 2 has two effective working temperature regions. At an operating temperature of around 250 1C, the sensing properties of the mixed oxide are due to SnO 2 , while TiO 2 is more sensitive at a temperature around 380 1C. As described in the previous section, the CNTs inclusion has caused no obvious differences in surface morphology as well as particle size. Consequently, the porosity and particle size cannot result in a remarkable improvement of the hybrid CNTs/SnO 2 –- TiO 2 gas-sensor performance. The improvement of the SnO 2 –TiO 2 gas-sensor performance by including SWCNTs has not been well understood so far and not much work has been published on the subject. The model proposed by Wei et al. [27] seems to be reasonable for the explanation. This model was applied for SWCNTs-doped SnO 2 and somehow we can apply for our case. The model has been hypothesized that CNTs-doped SnO 2 –TiO 2 materials can build up p/n hetero junctions, which was formed by (n-oxide)/(p-CNT)/(n-oxide). Fig. 8 schematically depicts the changes in the electronic energy bands for two depletion layers, one is on the surface of mixed-oxide particles and the other is at the interface between CNT and mixed oxide. When the mixed oxide is exposed to ethanol gas, the gas molecules will react with oxygen ions previously adsorbed on the surface of mixed oxide. This can simply be described as [33] 2C 2 H 5 OH þ O À 2 ¼ 2CH 3 CHO þ þ 2H 2 O þ e The electrons released from the surface reaction transfer back into the conductance bands, which increase the conductivity of ARTICLE IN PRESS 1E-3 0 5 10 15 20 25 30 35 40 45 50 CNTs content (%) SWCNTs, T=240-260°C SWCNTs, T=360-880°C MWCNTs, T=240-260°C MWCNTs, T=360-880°C S (R air /R ethanol ) 0.50.050.01 Fig. 7. Maximum response of two sensor systems at low and high operating temperature regions, ethanol concentration of 1000 ppm. CNT CNT n d 1 d 2 d 3 d 4 n E e E f E v p Depletion layer Distance Potential In reducing gas In air Grain boundary TiO 2 /SnO 2 CH 3 /CHO CH 3 /CHO CH 3 /CHO CH 3 /CHO TiO 2 /SnO 2 TiO 2 /SnO 2 O 2 O 2 O 2 O O O Fig. 8. Schematic of potential barriers to electronic conduction at grain boundaries and at p–n heterojunctions for CNTs/mixed oxide; d 1 and d 3 are depletion layer widths when exposed to ethanol; d 2 and d 4 are depletion layer widths in air. Table 3 Two maximum values in response of MWCNTs/SnO 2 –TiO 2 to 1000 ppm ethanol: S m1 , S m2 T m1 (1C) S m1 T m1 (1C) S m2 M0 240 5.3 380 23.5 M2 240 5.6 365 13.8 M3 240 9.7 365 28 M4 240 9.3 365 31.4 M6 240 9.7 365 18 M7 240 6.9 380 21.9 N. Van Duy et al. / Physica E 41 (2008) 258–263262 the sensing material. It is noted that the adsorption of the ethanol gas may change the two depletions layers as described above. Before the ethanol gas is adsorbed, the widths of the depletion layers at the interface between mixed oxide grains and mixed oxide/CNT are given as d 2 and d 4 , respectively. After the adsorption, the widths of these depletion layers are d 1 and d 3 , respectively. The change in both the depletion layers at the oxide grain boundaries and the n/p junction contributed to the improved sensitivity of the sensing materials. In other words, n-type mixed oxide and p-type CNT form a heterostructure. Like the working principle of an n–p–n amplifier, the CNT works as a base, blocked electrons transfer from n (emitter) to n (collector), and thus lowering the barrier a little bit allows a large amount of electrons to pass from the emitter to the collector [24]. This amplification effect may explain the fact that the hybrid materials (SnO 2 /SWCNTs) can detect NO 2 at room temperature [27]. So the improvement of the gas sensor performance and the shift of operation temperature toward the lower temperature region in our work can be attributed to the amplification effect of the p–n junctions in addition to the effect of the grain boundaries. Meanwhile, the fact that the contribution of MWCNTs (20odo40 nm) is not as much as SWCNTs (do2 nm) can be explained based on the quantum effect as follows. The space charge layer thickness (Debye length) is around 3 nm for the metal oxides (for example SnO 2 ). So the largest distance between adjacent boundaries accessing gas molecules should be less than 6nm [34]. However, mixed oxide (SnO 2 /TiO 2 ) grains are much larger than 6 nm so that not all metal oxides can participate in the reaction when gas absorbs on it. Therefore, the mixed-oxide/ SWCNT material structure formed by inclusion of the SWCNTs with diameter lower than 2 nm will produce quantum effects between SWCNTs and mixed oxide nanoparticles. The SWCNTs with a diameter of o2 nm reduce the distance between two adjacent gas-assessing and reaction surface to be less than the space charge layer thickness. 4. Conclusion SnO 2 –TiO 2 mixed oxide has been studied at the ratio 3:7 in mole for ethanol-sensing properties. At appropriate annealing conditions, it has shown the formation of the solid solution from two components by the XRD pattern. All the film surfaces were uniform and highly porous. In addition, the grain size around 10 nm gave a high specific surface. The new explorer in the two- peak shape of the response versus operating temperature characteristics has proved the combined behavior of the mixed- oxide material. SnO 2 and TiO 2 are complementary to each other for gas-sensing properties. The inclusion of CNTs at specific contents into the mixed oxide system improved the response of the sensor in the low operating temperature region. 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