International Journal of Pure and Applied Sciences and Technology

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1 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), pp International Journal of Pure and Applied Sciences and Technology ISSN Available online at Research Paper Temperature Sensor based on Nano-Sized Zinc Oxide Synthesized via Drop wise Method Richa Srivastava Nanomaterials and Sensors Research Laboratory, Department of Physics, University of Lucknow, Lucknow , U.P., India Corresponding author, (Received: ; Accepted: ) Abstract: Nano-sized ZnO was synthesized through the hydroxide precipitation using drop wise method. The variations in resistance of sensing pellet at different temperatures were recorded. The relative resistance was decreased linearly with increasing temperatures over the range, 150 C-260 C. The activation energy of ZnO calculated from Arrhenius plot was found 0.007eV. Temperature response in terms of the relative variation, R, of sensor resistance to a given temperature was measured. Sensing element has maximum average sensitivity 0.33%/ C to temperature. SEM and XRD of the sensing material were studied. Scanning electron micrographs show that synthesized zinc oxide is in the form of nanosheets. Minimum average thickness of ZnO nanosheets is found to be 35nm. Optical characterization of the sensing material was carried out by UV-visible spectrophotometer. By UV-Vis spectra, the estimated value of band gap of ZnO was found 4.59 ev. Keywords: Resistance, surface morphology, Temperature sensor, Activation energy, nanostructure. 1. Introduction: We have tried to emphasize the synergistic role of synthetic techniques in the preparation of excellent temperature sensor. Synthesis Methods play very important role to control the size and surface area of materials [1]. Present paper reported temperature sensing performances of zinc oxide synthesized through hydroxide route. Nano sized zinc oxide due to the large band gap 3.37 ev and high exciton binding energy of 60 mev shows various useful properties and gives large and diverse range of growth of different type of morphologies such as nanosheets, nanocombs, nanobelts, nanowires and nanorings, which may be used in various applications [2-4]. It is one of the promising materials among metal oxides for use in humidity sensors [5-10] and gas sensors [11-18]. As the temperature is an important parameter for measuring the different properties of materials. Because of various useful properties of ZnO, it behaves as good temperature sensing applications [19-20].

2 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), Experimental 2.1 Synthesis of Material ZnO is prepared by conventional precipitation method using sodium hydroxide. For the preparation of zinc hydroxide, sodium hydroxide solution of 1M was added drop wise to zinc sulphate of 0.5M with the molecular ratio 1:2.2 under vigorous stirring. After completion of reaction it was kept for 24 h and then it was filtered and washed with deionized water to remove sodium ions. Subsequent calcinations gave zinc oxide in powder form. Chemical reaction taking place is given below: 2NaOH+ ZnSO 4 Na 2 SO 4 + Zn(OH) 2 Heat treatment ZnO + H 2 O Precipitate This powder was mixed well with 10% glass powder. Addition of glass powder as a permanent binder during the process played a major role in getting mechanical strength of pellet. The pellet having thickness 4 mm and diameter 10 mm was prepared under pressure 616 MPa at room temperature. 2.2 Characterizations of n-type ZnO Scanning Electron Micrographs The morphology of sensing material in the form of pellet was also investigated. Fig. 1 shows scanning electron micrographs of ZnO in the form of pellet. SEM studies show that the material has networks of nanosheet. At room temperature, the thickness of ZnO nanosheets in the form of pellet was found to be laying in the range nm. Figure1 Scanning electron micrographs of ZnO in the form of pellet (a) At microscale (b) At nanoscale Fig. 1 Scanning electron micrograph of ZnO in the form of pellet X-Ray Diffraction Patterns X-ray diffraction shows the presence of ZnO phase. Fig. 2 shows the XRD pattern of sample prepared at room temperature. The synthesized material is pure and less crystalline as shown in Fig. 2. The crystallite size of ZnO is found to be 35 nm corresponding to the peak having the values of d

3 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), spacing and FWHM of zinc oxide are Å and respectively at the plane (1 0 0) for 2θ=31.7. Fig. 2 X-Ray Diffraction of synthesized ZnO powder prepared at room temperature UV-Visible Spectroscopy Optical characterization of the sensing element was done by using UV-visible spectrophotometer (Varian, Carry-50Bio). The band gaps estimated from the UV-Vis spectra as shown in Fig. 3 of ZnO was 4.59 ev. The increase in band gap due to quantum confinement effect in nanoparticle. Absorbance (a.u.) W a v e le n g th (n m ) 2.3 Experimental Procedure Fig. 3 UV Spectra of ZnO. Each circular pellet having diameter 10 mm and thickness 4 mm was made by using hydraulic pressing machine (M.B. Instruments, Delhi) under the pressure of 616 MPa at room temperature. Further the pellet was put within Ag-Pellet-Ag electrode configuration as shown in the Fig. 4 and this configuration was put inside the electrical furnace for temperature sensing and variation of resistance with different temperatures 150, 160, 180, 200, 220, 240, 260 C were recorded. The used heating rate was 2 C/minute.

4 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), Fig. 4 Sample holder (Ag-pellet-Ag electrode configuration). Fig. 5 Variations in resistance with temperatures of ZnO sensing element. The resistance of the sensing material decreases over the entire range of temperature as shown in Fig Temperature Sensitivity The response of the sensor samples to increasing temperature has been investigated. Temperature is in an important variable to document when measuring sensitivity. The relative resistance of the samples decreased linearly with increasing temperatures over range. The temperature response can be given in terms of relative variation, R, of the sensor resistance to a given temperature, R = [R 0 - R t / R 0 ] / 100% Where R 0 is the initial resistance of the sensor and R t is the resistance at different temperature. The calibration curves for the sample were obtained by plotting R against the temperatures. Fig. 6 shows the changes in R against temperature. The sensitivity of sample is determined from the slope of R.

5 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), Activation Energy Fig. 6 Relative resistive changes in % against temperature. Electrical resistance behavior in air against the temperature of the sensing material prepared was calculated. Activation energy ( E) measures the thermal or other form of energy required to raise electrons from the donor levels to the conduction band or to accept electrons by the acceptor levels E a from the valence band respectively for n- and p- type materials. The temperature resistance plot in the form of ln ρ and (1/T), known as Arrhenius plot, has a slope of ( E/2K) according to equation ln ρ = ln ρ + Ε 2KT [21] 0 By measuring the slope of Arrhenius plot of a linear zone, we have calculated the activation energy of nanostructured ZnO. Fig. 7 Arrhenius Plot for the ZnO sensing element

6 Int. J. Pure Appl. Sci. Technol., 14(1) (2013), We observed that resistance of ZnO changes greatly with temperature. This reveals semiconducting nature of the material. Fig. 7 shows the ln R vs. 1000/T (Arrhenius plot) for ZnO. This plot shows a linear decrease of the ln R with increase in the temperature. The estimated value of activation energy was found ~ ev. Such a minimum value of activation energy is quite significant for sensors operating at room temperature. A large variation in resistance indicates the greater sensitivity of the sensing element. The resistance variation of the ZnO can be ascribed to typical band conduction. It can be noted that a change in temperature will alter the resistance because both the charge of the surface species (O 2, O 2 -, O - or O 2- ) as well as their coverage can be altered in this process. Since the conduction process in metal oxide semiconducting materials depend heavily on grain boundaries therefore large and small particle sizes of materials are responsible for deviation from straight line behavior. In the overall conduction process a contribution arising from the participation of ZnO lower average particle size and another with higher average particle size i.e., the distribution of particle size dominates in thermally activated conduction process. 3. Results and Discussion The variations in temperature response of sensing element made of ZnO to a given temperatures have been shown in Fig. 6. The results have shown that sensing element has the highest average sensitivity to temperature, 0.33%/ C. The energy transition in an investigated temperature interval ( ºC), which may be an electron excitation from valence band to conduction band. This transition controls the R-T characteristics. The activation energy determined from the slope of resistance data was found ev. Fig. 5 shows the semiconducting nature of these sensing materials. In this figures the decrease in resistance with the temperature must mainly regarded as due to the thermally activated mobility of the carriers rather than to a thermally activated generation of these. 4. Conclusion Nanosheet of zinc oxide having thickness 35nm which had good temperature sensing properties. Maximum average sensitivity 0.33%/ C was achieved over the entire range of temperature. The estimated value of activation energy for electrical conduction of charge carriers was found to be 0.007eV. Thus, we can say that zinc oxide synthesized via hydroxide route is promising materials for temperature sensing. Results are found to be reproducible and no ageing effects have been observed. Thus temperature sensor made of ZnO is cost effective, easy to fabricate and user friendly and can be used for both indoor and outdoor application 5. Acknowledgement Dr. Richa Srivastava is highly grateful to University Grants commission, Delhi for Post Doctoral Fellowship (No. F.15-79/11 (SA-II). References [1] P. Knauth and J. Schoonman, Nanostructured materials: Selected synthesis methods, properties and applications, Technology & Industrial Arts, [2] X. Feng, Y. Ke, L. Guodong, L. Qiong and Z. Ziqiang, Synthesis and field emission of four kinds of ZnO nano structure: Nanosleeve-fishes, radial nanowire arrays, nanocombs and nanoflowers, Nanotech, 17(2006), [3] Q. Wei, G. Meng, X. An, Y. Hao and L. Zang, Temperature controlled growth of ZnO nanostructure: Branched nanobelts and wide nanosheets, Nanotech, 16(2005), [4] C. Xu, M. Kim, J. Chun and D.E. Kim, The selectively manipulated growth of crystalline ZnO nanostructures, Nanotech, 16(2005),

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