Journal of Science & Technology 99 (2014) 079-083
Gas Hybrid Materials Made of TiOz Nanowires and Carbon Nanotubes for Sensing Ethanol at Low Temperature
Nguyen Van Duy, Dang Thi Thanh Le, Bui Thi Thanh Binh, Do Due Dai, Nguyen Due Hoa*, Nguyen Van Hieu*
International Training Institute for Materials Science (ITIMS), Hanoi University ofScience and Technology No. I. Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam
Abstract
Semiconductor metal oxide/carbon nanotube hybrids generally have superior properiies to those of their individual constituents. In this work, high-quality Ti02 nanowires (NWs) and their hybrid with multiwalted carbon nanotubes (MWCNTs) were fabricated for use as effective low-temperature ethanol-gas sensors TiOi NWs with high quality were prepared by the hydrothenval method using Ti02 powders as a precursor, and hybnd materials were obtained by mixing TiOi NWs with MWCNT (2 - 50 wt.% in content) in an absolute isopropanol solution using an ultrasonic probe. The hybrid matenals were then deposited onto Pt- interdigitated electrodes for gas-sensing characterizations. The as-fabricated sensors were heat treated at 500 "C for 30 min to enhance the adhesion of materials onto the substrate and stabilize devices. Scanning electron microscopy and transmission electron microscopy were used to morphologically characterize the materials. The sensing properiies of the sensors were investigated with different ethanol concentrations at different working temperatures. Results revealed that the combination of Ti02 NWs and MWCNTs as a hybrid material was effective for low-temperature ethanol-gas sensors.
Keywords: Ti02 nanowire; Carbon nanotube; Hybrid Materials; Gas sensor
1. Introduction
The effective monitoring of toxic and flammable gases such as CO, NH3, NO2, C2H5OH, CH4, C3H8, and H2 is critical because they contribute to global warming, climate change, and damage to human health [ I j . Many studies have focused on early gas leakage alarming and reducing envu-onmental pollution, leading to the development of advanced and cost-effective sensors with fast response and recovery times, low power consumption, and high sensitivity and stability [2,3].
Resistive gas sensors based on different forms of nanostructured metal oxides have been investigated for monitoring au- quality [3,4]
Ti02 is a less common material than Sn02 for gas sensor applications but is more thennodynamically stable and expected to possess higher stability as a sensing device. Ti02 is also believed to exhibit a higher sensitivity for reducing gases because its dielectric constant (e = 100) is higher than that of Sn02 {£ ~ 24) [5,6]. Ti02 nanowires (NWs) and their composites have been tested for detecting various types of gases, and they are found to easily sense formaldehyde, NHj, and H2.
' Correspondmg author: Tel: (+844) 3868.0787 Email: [email protected]; [email protected]
However, the high working temperature of above 200 "C and low conductivity of TiO; materials limit their application in resistive-type gas sensors because they provide a low signal and consume more power, causing difficulties in designing the read-out system [7].
In another strategy, carbon nanotubes (CNTs) having excellent properties of high electrical conductivity, good mechanical strength, and high specific area are believed to provide a large number of adsorption sites for gas reactions, high sensitivity, and effective stability [8]. CNT-based gas sensors reportedly work well at room temperamre, which enables safe detection of flammable gases and reduced power consumption of devices [9], A number of reports have been made on CNT-based gas sensors for NH3, NOi, ethanol, and H2 gas detection [10-12].
However, the drawbacks of CNT-based gas sensors are their long response and recovery time ascribed to the stiong interaction between gaseous molecules and the surface of CNTs, Recently, the use of CNTs and metal oxide nanocomposites or hybrids as sensing materials for gas sensors have been reported to exhibit enhanced sensitivity [13,14]. Comprehensive reviews have shown that the combination of CNTs and various types of metal oxides reduce the working temperatures of gas sensors [15].
Journal ofScience & Technology 99 (2014) 079-083 In this study, T1O2 N W and multiwalled
carbon nanotube (MWCNT) hybrid materials were synthesized and used as sensmg layers for an ethanol- gas sensor. The hybrid matenals of Ti02 NWs and MWCNTs were prepared by mixing high-quality Ti02 NWs obtained by the hydrothermal method with different contents of commercial MWCNTs. The electrical ethanol-sensing properties of the hybrid materials were systematically investigated and reported.
2. Experimental
2.1 Hydrothermal synthesis ofTiOi NWs T1O2 NWs were synthesized by a hydrothermal method using commercial Ti02 powders (Merck, 98%) as precursors. In a typical synthesis, Ti02 powders (1 g) and 20 ml of concentrated alkaline solution (10 M) were stirred at room temperature for 24 h. The mixture was poured mto a Teflon-lined stainless autoclave, heated at 190 "C for 24 h, and natiirally cooled down to room temperature. The obtained mixture was filtered and washed with an acid solution (HCl) and deionized water to eliminate the alkaline ions. The obtained matenals were dried in ah at 45 °C The morphology of the materials was characterized by field-emission scanning election microscopy (FE-SEM) and transmission election microscopy (TEM).
2.2 Fabrication of sensors based on hybrid TiOi NWs and MWCNTs
Hybrid materials were obtained by mixing the prepared TiOi NWs with commercial MWCNTs (Shenzhen Nanotech Port Co., Ltd.) with a diameter of 20 nm and length of 2 pm. The mixture ofthe two components was dispersed m absolute isopropanol with an ultrasonic probe for about 30 min. The obtained solution was deposited onto thermally oxidized silicon substiate-supported Pt electrodes and then heat treated at 500 °C for 30 min in air to
enhance the adhesion between materials and substiate. The MWCNT contents were 2 wt.%, 9 wt.%, 16.67 wt.%, 28.57 wt.%, and 50 wt.% of T1O2 NWs. This heat treatment also bumed out the amorphous carbon contaminated in the MWCNTs.
The ethanol-sensing properties ofthe hybrid materials were observed fi-om tiie resistance change under an atmosphere of ethanol vapor at ethanol concentrations of 150, 300, 600, 900, and 1200 ppm, and at various operatmg temperatiu^s ranging fi'om room temperature to 400 °C. The response was defined as a ratio o{R^s/Rmt, where R3,, and Sgas are the resistance of the sensor in the presence of dry air and ethanol gas, respectively [16].
3. Results and disscution
3.1 Crystal structure and morphology of the synthesized materials
The morphology o f t h e synthesized T1O2 NWs was investigated by FE-SEM and TEM images, and the results are shown in Fig. 1. The synthesized TiOz NWs had a very smooth surface with uniform diameters along the wire axis. The average diameter of Ti02 NWs was about 20 nm, with a length reaching a few micrometers (Fig. 1(A)). Upon TEM sampling, the TiOi N W s were broken into shorter NWs (Fig. 1(B)). Investigation of Ti02 NWs obtained by the hydrothermal method confirmed the crystallinity ofthe material.
The crystal structure of the synthesized Ti02 NWs investigated by the X-ray diffi-action (XRD) is shown m Fig. 2. The XRD pattem exhibited main diffraction peaks at 26 - 25.28°, 37.8°, 48.05°, 53.89°, 55.69°, and 62.69° belonging to the reflections of the (101), (004), (200), (105), (211), and (204) planes of Ti02 anatase (JCPDS, No. 2 1 - 1272). This result was consistent with recent reports on Ti02 N W s fabricated by the alkaline hydrothermal method [17].
Fig. 1. (A) FE-SEM and (B) TEM images ofthe prepared Ti02 NWs.
Journal ofScience & Technology 99 (2014) 079-083
Fig. 2. XRD pattem ofthe prepared Ti02 NWs.
An un-indexed diffraction peak was observed in the XRD pattem at 26 =27.39°, indicating the existence of a foreign phase. However, this peak was weak compared with the other peaks of Ti02 NWs, indicating a low content of foreign phase..
Fig. 3. FE-SEM images of the hybrid Ti02 N W - MWCNT with different amounts of MWCNTs: (A) 0 wt.%, (B) 2 wt %, (C) 9 wt. %, (D) 16.67 wt. %, (E) 20.57 wt. %, and (F) 50 wt.%.
SEM images of the hybrid materials based sensors with different amounts of MWCNTs are shown in Fig. 3. The morphology of the pure Ti02 NW-based sensor did not change after deposition onto the substrate, indicating the stability of matenals (Fig. 3(A)). However, the morphology of the hybrid sensors slightly changed with variations in MWCNT content. As shown by the FE-SEM images, the materials were porous and had a large surface area, and the materials seemed to be agglomerated together
(Fig. 3). The average length of NWs observed in the hybrid sensors was shorter than that of pure TiO^
NWs. This phenomenon resulted from the dispersion process in which the high-power ultrasonic vibration broke the long T1O2 NWs into shorter NWs.
MWCNTs with an average diameter (20 nm) approximately equal to that of Ti02 NWs were used;
thus, differentiating them in the FE-SEM images was difficult. Ln addition, given that the weight density of MWCNTs (1.34 g/cm^) was much lower than that of TiOi NWs (4 g/cm^), the observations in the FE-SEM images were mainly of MWCNTs; they had a curved stmcture unlike the straight ones of Ti02 NWs.
3.2 Electrical and gas-sensing properties of hybrid TiOi NWs and MWCNTs
The content of MWCNTs influences the conductivity and thus the resistance of hybrid Ti02 N W - M W C N T sensors. Fig. 4 shows the relationship between the operating temperature and electrical resistance ofthe hybrid sensors. Despite the electrical resistances ofthe sensors were measured at different temperatures but they exhibited a trend of increase with decrease of MWCNT content. For instance, at room temperature, the resistance of MWCNTs (50 wt,%) sensor was about 70 £2, but this value increased to 37 k t l with a decrease of MWCNT content to 2 wt.%. The MWCNTs had a much higher conductivity than that of TiOi; therefore, in hybrid form, the MWCNT content dominated the conductivity of the sensors, as a result, the resistance of the hybrid sensors significantly decreased with increased MWCNT content. The sensor resistance of the hybrid sensors also decreased with increased temperature, suggesting the semiconducting characteristics ofthe materials.
The ethanol-gas sensing properties of the hybrid TiOi NWs-MWCNTs were compared with those of pure TiOi NWs. Fig. 5(A) shows the dynamic responses of the hybrid gas sensor (50 wt.%
MWCNTs) to ethanol vapour at room temperature.
lOOki
K Z W L % MWCNTs 1-9 wL% MWCNTi 1-16.67% MWCNTi
>-26.57% MWCNTa H 50% IMWCNTt
50 100 ISO 300 250 300 : Temperature ( C) Fig. 4. Resistance of the hybrid T1O2 N W - M W C N T sensors at different temperatures.
Journal ofScience & Technology 99 (2014) 079-083 The Fig. shows that the electtical resistance of
the sensor abmptly increased in the presence of ethanol and retumed to the original value upon exposure to air. The response curve showed that the resistance of the sensor varied over time with various cyclic tests for different ethanol concentrations (150, 300, 600, 900, and 1200 ppm). The hybrid materials exhibited a p-type semiconducting charactenstic of MWCNTs, consistent witfi otiier reports [10], The results demonstrated that the hybrid sensors with high concenti-ations of MWCNTs (50 wt.% and 28.57 vrt.%) only operated at room temperature.
Unlike the hybrid sensors, the initial resistance of the pure Ti02 NW sensor was very high (-100 M£2), and no significant response to ethanol was observed (data not shown). However, the pure Ti02 NW sensor significantly responded to ethanol at a high temperature, as shown in Fig. 5(B), The pure Ti02 NW sensor showed efficient response/recovery characteristics to ethanol at 320 °C. The pure TiO:
NW sensor showed the natural sensing behaviors of an n-type semiconductor whose resistance decreases upon exposure to a reducmg gas [16]. The gas-
sensing characteristics of pure TiO; NWs can be understood by tiie surface depletion region mechanism [16]. At low temperatures, piu-e TiOj NWs had few active chemisorbed oxygen ions on the surface, and reactions between chemisorbed oxygen ions and C2H5OH molecules were difficult to mitiate;
thus, no significant change m sensor resistance was observed. At a high temperature (320 °C), the chemisorbed oxygen ions m the forms of O" and 0^"
on the surface of Ti02 N W s easily interacted with C2HiOH molecules and resulted in decreased sensor resistance. Compared with pure TiOi NWs, the hybrid sensors had a much lower electrical resistance and a capability to work at room temperature. These sensors were promising for low-power-consumption devices.
The response of the hybrid Ti02 NW- MWCNT sensor to different ethanol concentrations was also investigated at different working temperatures. Fig. 6(A) presents the dynamic response of the hybrid sensor (16.67 wt.%
MWCNTs) to ethanol at operating temperatures of 150, 200, and 250 °C, respectively. The sensor
64.a
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SOwLSMWCNTserc:
600 ppm 300 ppm 64.0' 160 ppm
1000 1S0D 20D0 2500 Time (sec)
100M-
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u
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b
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Fig. 5. Response to etiianol of (A) hybrid Ti02 N W s - M W C N T s at room temperature, and (B) pure Ti02 NWs at 320 °C.
(A) (41 (51
r ! ' ... (51 —n—«n*r 150'C
ZOO'C -a-iSO'C (1)160 ppm (Z) 300 ppm
100D 2000 3000 4000 5000 6000 7000 Time (sec)
a
J
.H,OH S 150 ppm I - 2 Wt.% MWCNTs I - 9 wL% MWCNTs -16,67% MWCNTs
"-26,67% MWCNTs K SO wt% MWCNTs
100 150 200 250 300 360 400460 Temperature (°C)
Fig. 6. (A) Response to ethanol of the hybrid Ti02 N W s - M W C N T s (16.67 wt.% MWCNTs) at different temperatures. (B) Response to ethanol of different sensors at different temperatures.
Journal ofScience & Technology 99 (2014) 079-083 worked at all measured temperatures and showed
reversible response and recovery characteristics. The response/recovery time significantly decreased with increased working temperature. The values were about 240/550 s, 100/145 s, and 26/78 s at 150, 200, and 250 °C, respectively Such characteristics can be explained by the acceleration of thermal energy for adsorption and desorption processes. The response of hybrid sensors (with variations m MWCNT content) as a fiinction of working temperature is shown in Fig.
6(B). The hybrid sensors worked at room temperature for samples having high amounts of MWCNTs (50 wt.% and 28.57 vrt.%), but the sensor response was very low. Decreasing the content of MWCNTs (2 wt,%) increased the sensor response but also required a higher working temperature. For a moderate working temperature ranging from 150 °C to 250 °C, the hybrid sensors with 9 wt,% and 16.67 wt,% content of MWCNTs were more suitable for low-power-consumption devices because they had a lower electncal resistance and a reasonable response,
4. Conclusion
In conclusion, we mtroduced the use of Ti02 NW-MWCNT hybrid materials as the sensing layers for ethanol-gas sensors. The hybrid matenals of T1O2 NWs and MWCNTs were prepared by a simple and scalable method. The hybrid matenal enabled the fabrication of gas sensors that solved the problems faced by conventional pure Ti02 or pure MWCNT sensors. This hybrid material also made it a potential candidate for gas-sensmg applications at low temperatures with low power consumption.
Acknowledgments
This work was financially supported by Vietaam's National Foundation for Science and Technology Development (Nafosted, Code' 103.02- 2011.46).
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