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Phase Transitions: A Multinational Journal Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gpht20
Dip coated TiO2 nanostructured thin film: synthesis and application a
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Manoj Vanaraja , Karthika Muthukrishnan , Shanmugam b
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Boomadevi , Rakesh Kumar Karn , Vijay Singh , Pramod K. Singh & Krishnamoorthy Pandiyan
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School of Electrical & Electronics Engineering, SASTRA University, Thanjavur, India b
Department of Physics, National Institute of Technology, Tiruchirappalli, India
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Department of Chemical Engineering, Konkuk University, Seoul, Republic of Korea d
IMST, Vestfold University College, Tonsberg, Norway
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Centre for Nonlinear Science and Engineering, SASTRA University, Thanjavur, India Published online: 20 Jul 2015.
To cite this article: Manoj Vanaraja, Karthika Muthukrishnan, Shanmugam Boomadevi, Rakesh Kumar Karn, Vijay Singh, Pramod K. Singh & Krishnamoorthy Pandiyan (2015): Dip coated TiO2 nanostructured thin film: synthesis and application, Phase Transitions: A Multinational Journal, DOI: 10.1080/01411594.2015.1053885 To link to this article: http://dx.doi.org/10.1080/01411594.2015.1053885
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Phase Transitions, 2015 http://dx.doi.org/10.1080/01411594.2015.1053885
Dip coated TiO2 nanostructured thin film: synthesis and application Manoj Vanarajaa, Karthika Muthukrishnana, Shanmugam Boomadevib, Rakesh Kumar Karna, Vijay Singhc, Pramod K. Singhd and Krishnamoorthy Pandiyana,e* a School of Electrical & Electronics Engineering, SASTRA University, Thanjavur, India; Department of Physics, National Institute of Technology, Tiruchirappalli, India; cDepartment of Chemical Engineering, Konkuk University, Seoul, Republic of Korea; dIMST, Vestfold University College, Tonsberg, Norway; eCentre for Nonlinear Science and Engineering, SASTRA University, Thanjavur, India b
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(Received 1 April 2015; accepted 18 May 2015) TiO2 thin film was fabricated by dip coating method using titanium IV chloride as precursor and sodium carboxymethyl cellulose as thickening as well as capping agent. Structural and morphological features of TiO2 thin film were characterized by X-ray diffractometer and field emission scanning electron microscope, respectively. Crystallinity of the film was confirmed with high-intensity peak at (101) plane, and its average crystallite size was found to be 28 nm. The ethanol-sensing properties of TiO2 thin film was studied by the chemiresistive method. Furthermore, various gases were tested in order to verify the selectivity of the sensor. Among the several gases, the fabricated TiO2 sensor showed very high selectivity towards ethanol at room temperature. Keywords: solgel preparation; TiO2; Na-CMC; sensors
1. Introduction Environmental pollution due to industrialization and urbanization has made it imperative to develop highly selective and sensitive metal oxide semiconductor (MOS) sensors for the detection of various harmful and combustible gases in the atmosphere. The inspiring advantages provided by the semiconducting metal oxides are its their cost effectiveness, portability, quick response to toxic gases and simple working principle.[13] Moreover, nano-structured MOSs show better sensing response, which arises due to its their small size, high density of surface sites, increased surface-to-volume ratio and catalytic effects.[4,5] Of the diverse metal oxides, TiO2 is an n-type polymorphic semiconductor which has a wide range of applications in industries as well as in day-to-day life,[6] in which anatase titania has gained interest as one of the suitable materials for gas sensors.[7,8] Among the volatile organic compounds, ethanol is a reducing gas. When it exceeds the limit (1000 ppm) mentioned by the Occupational Safety and Health Administration, it can lead to severe health problems, such as headache, eye irritation, respiratory issues, etc.[9] Some of the sources of ethanol include perfumes, explosives, ripened fruits, fuel industries and it is also found in the breath of an alcoholised person.[10,11] Hence, ethanol detectors have been used in various fields, including food quality monitoring, breath analyzers and pollution control.[12]
*Corresponding author. Email:
[email protected] Ó 2015 Taylor & Francis
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Gas-sensing characteristics of the MOS sensor mainly depend on the crystallinity, particle size and morphology of the film.[13] These factors can be engineered in the solgel method by varying the reaction parameters and precursor concentration.[14] Hence, we have utilized the solgel dip coating method to fabricate a titania thin film sensor. In addition, an organic polymer, sodium carboxymethyl cellulose (Na-CMC), [15] was used during the sol preparation to get nano particles with a diameter below 50 nm and to improve the adhesiveness of titania nano particles on the glass substrate, as it can form a hydrogen bond due to the availability of abundant OH group in its structure. Furthermore, it can be used as a capping agent to avoid agglomeration.[16] Good sensing response, fast response time, short recovery time, selectivity and room temperature operation are some of the key parameters required for an effective sensor. Though many works related to an ethanol sensor based on TiO2 were found in the literature, they lack selectivity and their operating temperature is high.[1719] In general, selectivity is defined as the ability of a sensor to discriminate between gases in a mixture. Hence, in our work titania thin film with good selectivity towards ethanol at room temperature is reported. 2. Experimental details Initially, 2 mL of titanium tetrachloride (TiCl4) was dissolved in 30 mL of propanol to avoid an immediate reaction with the atmosphere (because TiCl4 readily reacts with atmosphere and evolves cloudy fumes), which gives a yellow solution. Later, 20 mL of deionized water was added, which resulted in color change from yellow to transparent solution. The solution was kept under constant stirring for 30 min to get a homogeneous solution. To get a viscous solution, in a separate beaker 0.37 g of Na-CMC was added to 50 mL of deionized water, which was heated at 70 C. Upon adding homogeneous solution to a beaker containing Na-CMC mixture, a white color viscous sol was obtained. The solgel obtained was washed twice with deionized water and stirred for 48 h, which was used as a source for dip coating. Dip coating was done on ultrasonically cleaned glass substrate using an automatic dip coating unit (HOLMARC, HO-TH-01). During each cycle, the substrate is dipped into the sol for 2 min and dried for 5 min at 75 C. This process was repeated for 10 cycles and at the end, the film was dried at room temperature for 1 day. Then the film was annealed at 450 C in a muffle furnace for 3 h to improve the crystallinity. The structural characteristics of TiO2 nano particles were studied by the X-ray diffraction pattern taken from the PANalytical X-ray diffractometer equipped with Cu Ka radiation as a source. The diffraction pattern was recorded for the angle 20 70 in step scan mode with the scanning rate of 12.70 sec. The surface morphology of nano particles were analyzed by FE-SEM characterization (6701F, JEOL, Japan). The image was taken in secondary electron imaging mode by applying a voltage of 3.0 KV to the sample. To avoid the discrepancy of charge accumulation, the sample was coated with a thin layer of gold prior to characterization. The gas-sensing behavior of the thin film was studied at room temperature by the chemiresistive method in the presence of gases, such as ethanol, propanol, acetone, benzene and toluene. 3. Results and discussion The XRD pattern of TiO2 thin film representing the anatase phase as shown in Figure 1. High intense plane (101) confirms the crystallinity and the existence of the tetragonal structure. The average crystallite size of the titania nano particle calculated by Scherrer’s
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Figure 1. XRD pattern of TiO2 thin film. Kλ formulae,[20] D D bcosu was found to be 28 nm. The presence of porosity and spherical morphology was confirmed with the result of the FE-SEM image shown in Figure 2(a) and 2(b). Gas-sensing behavior was studied by the chemiresistive method using the experimental setup reported in [16] at room temperature. The basic sensing phenomena involve adsorption and desorption of oxygen on the metal oxide surface.[21] This suggested that the sensing of nanostructured metal oxides is greatly influenced by its surface sites.[2] Initially, the thin film was exposed to 100 ppm of test gases: toluene, benzene, propanol, acetone and ethanol. Since these test gases were reducing gases, the titania thin film sensor responds by decreasing its resistance from the base resistance. The response of the film towards various test gases was estimated using the following equation:[22]
SD
Ra ; Rg
(1)
where Ra and Rg are the resistance of the film before and after the injection of test gas, respectively. The response of the film towards 100 ppm of test gas at room temperature is shown in Figure 3(a) with the inset representing its corresponding transient resistance change. Figure 3(a) implies that the maximum resistance change was observed for ethanol, which indicates that ethanol molecules interact more rapidly with adsorbed oxygen species on the titania surface than the other molecules. The possible chemical reaction for ethanol at room temperature is as follows:[3,23] O2 C e ¡ $ O2 ¡ ðadsÞ
(2)
C2 H5 OH C 3O2 ¡ ðadsÞ $ 2CO2 C 3H2 O C 3e ¡ :
(3)
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Figure 2. (a) and (b) FE-SEM of titania thin film at two different magnifications.
The resistance change attributed due to the interaction of ethanol with adsorbed oxygen occurs as per the following principle. When the titania film was exposed to the atmosphere, the chemisorbed oxygen molecules create a potential barrier near the surface of MOS. As this process traps electrons from the MOS surface, there occurs an increase in resistance. The point at which the resistance becomes constant in the atmosphere is noted as base resistance. After injecting the ethanol, it interacts with adsorbed oxygen and
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Figure 3. (a) Response and selectivity of titania thin film towards test gases. Inset: its transient response. (b) Transient response of titania for various concentrations of ethanol from 2 to 100 ppm. (c) Histogram representing response and recovery time of sensor towards various concentration of ethanol at room temperature.
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liberates the trapped electrons back to the MOS surface. Hence, the height of the potential barrier decreases, which is followed by the decrease in resistance.[1] The transient resistance of the titania film towards various concentrations of ethanol ranging from 2 to 100 ppm is shown in Figure 3(b). It implies that, with the increase in concentration, the resistance of the film varies greatly from its base resistance. It was due to the catalytic effect of titania nano particles, which enhance the interaction between the oxygen species and the ethanol by increasing the concentration of the adsorbed oxygen at room temperature.[6] Furthermore, it shows that the response of the film increase from 1.66 to 467 with the increase in the concentration of ethanol from 2 to 100 ppm. Apart from good response, fast response time and shorter recovery time towards test gas is one of the major requirements for a sensor. The time taken by the sensor to attain 90% of its maximum and original resistance is considered as response under recovery time.[24] The histogram representing the response and recovery time for various concentrations of ethanol at room temperature is shown in Figure 3(c). The minimum response and recovery time is found to be 21 sec (5 ppm) and 25 sec (10 ppm). From the calculated response value shown in Figure 3(a), selectivity towards ethanol was estimated using the relation, [16]
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Selethanol ð%Þ D
Sethanol ; Stoluene C Sbenzene C Spropanol C Sacetone C Sethanol
(4)
where Stoluene ; Sbenzene ; Spropanol ; Sacetone and Sethanol are the responses exhibited by the thin film sensor towards corresponding gases at room temperature. The calculated magnitude of the selectivity shown in Figure 3(a) clearly indicated that the film was highly selective to ethanol. Also, it showed comparatively very poor selectivity to all the test gases when compared with ethanol. Thus, the obtained nano spheres with porosity could make the film selective to sense ethanol in an atmosphere containing other gases, such as acetone, propanol, benzene and toluene. 4. Conclusion Titania thin film showing selective response to ethanol at room temperature was fabricated by the dip coating method. Sol for dip coating was prepared using TiCl4 and NaCMC as a starting material. The XRD and FE-SEM characterization revealed that the annealed crystalline titania nano particles were in the anatase phase with spherical morphology and porosity. The gas-sensing studies done by the chemiresistive method indicated that the nano spherical titania thin film was highly sensitive to ethanol with the selectivity of 90%. Owing to its excellent selective sensing ability at room temperature, it can be used as a good candidate for an ethanol detector. Acknowledgements The authors wish to express their sincere thanks to Dr John Bosco Balaguru, SASTRA University and the Department of Science & Technology, New Delhi, India.
Disclosure statement No potential conflict of interest was reported by the authors.
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Funding This work is financially supported by the Department of Science & Technology, New Delhi, India [ID:INT/SWD/VINN/P-04/2011 and SR/FST/ETI-284/2011(C)].
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