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Structural and Moisture Sensing Properties of WO3-ZnO Nanocomposites Synthesized by a Soft Chemical Route
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 9082–9088
www.materialstoday.com/proceedings
NCNN 2017
Structural and Moisture Sensing Properties of WO3-ZnO Nanocomposites Synthesized by a Soft Chemical Route Vandna Shakya* and Narendra Kumar Pandey Sensors and Materials Research Laboratory, Department of Physics, University of Lucknow, U.P., Pin-226007, India
Abstract WO3 ceramic oxide which is sensitive to atmospheric moisture levels have been synthesized and tested. WO3-ZnO nanocomposites have been prepared in the form of pellets by WO3 obtained from synthesized route method mixed with 2 weight % of ZnO powder. These pellets have been annealed at temperatures 300 °C to 600 °C. When samples have been exposed to humidity, the change in the resistance of pellet depends on the amount of adsorbed water vapour. It is observed that as relative humidity (% RH) increases, there is a decrease in the resistance for the entire range of humidity from 15 % to 95 %. For the sensing element of ZnO doped WO3, the repeatability over different cyclic operations is within ±1.82% and ±8.64% of the measured values of sensitivity after four and six months respectively. WO3-ZnO composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moisture sensing properties of the composites were investigated, including sensitivity, humidity hysteresis, repeatability and response properties. The composite with 2 weight% of ZnO shows the maximum sensitivity 16.42 M Ω/% RH at 600°C annealing temperature, narrow hysteresis (1.09%) and highest repeatability of the response. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 ).
Keywords: Humidity Sensing, WO3, ZnO, Hysteresis, Sensitivity, Annealing Temperature
*Email address: [email protected]
2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 )
Vandna Shakya etal/ Materials Today: Proceedings 5 (2018) 9082–9088
9083
1. Introduction Research has been going on to find suitable materials that show good sensitivity over a wide range of humidity, low hysteresis, long-term stability, low power consumption and properties which are stable to thermal cycling and exposure to the various chemicals likely to be present in the environment. Ceramic such as WO3, ZnO, SnO2, Al2O3, TiO2 etc are most investigated sensing materials for relative humidity sensors. Ceramic materials have been attracting attention for their intrinsic characteristics; they are suitable for the reproducibility of the electrical properties, mechanical strength, chemical and physical stability. These materials possess a unique structure consisting of grains, grain boundary surfaces, and pores, which make them suitable for adsorption of water molecules because of the high surface exposure [1]. Tungsten trioxide (WO3) is a n-type semiconductor metal oxide having wide band gap. Tungsten oxide shows optical, electrical, structural, morphological properties, chemical and physical stability. All these properties of WO3 can be controlled by the method of preparation adopted and make easier the use of tungsten trioxide in wide range applications [2]. Doped WO3 with other oxides exhibit change their properties, morphologies and have many applications. ZnO is one the most extensively studied for sensing purpose because its remarkable optical and electrical properties. Suman Pokhrel et al. reported the electrical and humidity sensing properties of Cr2O3-WO3 composites. Cr2O3-WO3 composites were made by mixing Cr2O3 and WO3 in different molar ratios and pressing at 100 M Pa to form cylindrical pellets and sintered at temperature range 373-673 K and then their humidity sensing properties studied [3]. M.V.N. Ambika Prasad et al. presented humidity sensors based on polyaniline/WO3 composites by in situ' polymerization. Composite with 50 weight % of WO3 film on SiNPA through electron-beam evaporation process and the humidity sensing was studied [4]. Dewyani Patil et al. prepared humidity sensitive poly (2, 5-dimethoxyaniline)/WO3 (PDMA/WO3) composite with different weight percents of WO3 by a simple mechanical mixing method. The PDMA/WO3 composites show better sensing properties than pure PDMA, such as higher sensitivity, quicker response and small hysteresis [5]. Noubeil Guermat et al. report the study of humidity-sensitive and electrical properties of plasma polymerization of hexamethyldisiloxane (pp- HMDSO) thin film-based sensors. The humidity sensitive film was deposited by glow discharge at low frequency power (19 kHz) in a capacitive-coupled parallel plate plasma reactor. The deposited film sensor exhibited a small hysteresis (2% RH) and fast response (8 and 34 s for adsorption and desorption between 35 and 95% RH, respectively) [6]. Md Sin et al. have reported about the electrical characteristics of aluminium (Al)doped zinc oxide (ZnO) thin film for high sensitivity humidity sensors. Parameter 0.6 at% aluminium doped ZnO showed high sensitivity of 2.32 MΩ/%RH and suitable for humidity sensor [7]. Richa Srivastava et al. have reported comparative performance of n-type ZnO, ZnO-TiO2 and ZnO-Nb2O5 nanomaterials as humidity sensors. Average sensitivity achieved 8 MΩ/%RH for the n type ZnO annealed at 550°C. After chemical mixing of TiO2, the sensitivity increased to 18 MΩ/%RH and after Nb2O5, it was found to be 19 MΩ/%RH [8]. We report here morphological and moisture sensing studies of WO3-ZnO nanocomposites prepared by synthesized route. 2.
Experimental Methods
WO3 nanocrystalline powders were obtained by a soft chemistry route based tungstic acid (H2WO4). H2WO4 (10g) was mixed with 100ml of an aqueous solution of H2O2 (30% PH = 3.9) then PTA formed with PH= 2 (stirred continuously for 4h). The solution was heated to dryness to obtain a precursor powder, which was annealed at 600°C in air for 3h to obtain nanocrystalline pure WO3 powder. To make the samples of WO3-ZnO nanocomposites, 2 weight % (sample WZ-2) of ZnO powder (Loba Chemie, 99.9%) has been added in synthesized nanocrystalline WO3 powder.10% weight of polyvinyl alcohol was added as binder to increase the strength of the sample. Mixed powder was grinded to uniformly for three hours. The resultant powder was pressed into pellet shape by uniaxially applying pressure of 267 M Pa in a hydraulic press machine (M.B. Instruments, Delhi, India) at room temperature. After sintering, each sensing sample was placed in a same humidity chamber and its sensing outputs were obtained under different humidity levels. Inside the humidity chamber, a thermometer (±1°C) and standard hygrometer (Huger, Germany, ±1% RH) are placed for the purpose of calibration. Variation in resistance has been recorded with change in relative humidity. Relative humidity has been measured using the standard hygrometer. Variation in resistance of the pellets has been recorded using a resistance meter (Sino meter, ±1 MΩ, model: VC-9808). Copper electrode has been used to measure the resistance of the pellets. The resistance of the pellets has been measured normal to the cross-section of the pellets. To see the effect of aging, the sensing properties of these elements have been examined again in the humidity control chamber after four and six months. The response and recovery time for the sensing elements are also observed.
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To study grain micro-structure of sensing sample ZnO doped WO3 annealed at 600°C was carried out using SEM (LEO-430, Cambridge, England). The micrographs were taken for the 35.00 Kx magnifications. SEM micrographs shows that the sensing sample manifest porous structure having granulation and tendency to agglomerate. SEM micrograph for the sensing sample WZ annealed at 600°C is shown in Figure 1. It can be observed that inter granular pores are linked through the large pores. The pore structures should be regarded as inter connected voids that form a kind of capillary tubes. Sensing sample annealed at 600°C has the maximum void concentration and porosity. The average grain size calculated from SEM micrograph for this sensing element is 230 nm. In this process, Due to the presence of larger grains, more of the surface areas of the sensing elements are exposed leading to more adsorption of water molecules. This increases sensitivity of the sensing elements. It also helps in reducing response and recovery times because it enhances diffusion rate of water into or out-off the porous structure.
Fig. 1. SEM micrograph of sensing sample of ZnO doped WO3 for annealing temperature 600 °C.
XRD of the samples were studied using XPERT PRO-Analytical XRD system (Netherlands). The wavelength of the source CuKα used is 1.54060 Å. The average crystalline size of the samples has been calculated using Scherrer’s formula D = Kλ/B cos θ
(1)
Here, D is the crystallite size, K a fixed number of 0·9, λ the X-ray wavelength, θ the Bragg angle and B the full width at half maximum of the peak. Figure 2 shows the XRD pattern of sensing sample ZnO doped WO3 for the annealing temperature 600 ̊C. High intensity, large number of peaks and sharpness of peaks show high crystallinity of the sensing sample WZ annealed at 600◦C [9,10]. XRD shows some peaks are found broad and the broadness of the peak indicates that crystallites are of small size or nano-size. From Scherer’s formula we can see that broader the diffraction peak (i.e., greater the full-width at half-maximum (FWHM) value) smaller the crystal size. The crystallite size for sensing sample ZnO doped WO3 annealed at 600°C calculated from Scherrer's formula is in the 12-121 nm range.
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Fig. 2. XRD pattern for sensing sample of ZnO doped WO3 for annealing temperature 600 °C.
3.
Results and Discussion
Variation in resistance with the change in % RH for sensing sample ZnO doped WO3 is shown in Figure 3 for the humidification process. Figure 3 shows large decrease in the value of the resistance for initial values of the relative humidity range from 15% ̶ 40% RH while in 40% –95% RH range, the fall in resistance is slow observed for all the sensing elements. As semiconducting dry oxide of WO3 nanocomposite is brought in contact with humid air, water molecules chemisorb on the available sites of the oxide surface. The adsorption of water molecules on the surface takes place via a dissociative chemisorptions process. Hence the electrons are accumulated at the WO3 surface and consequently, the resistance of the sensing element decreases with increase in relative humidity. Hysteresis effect is an important factor in metal oxides and in binary systems of metal oxides. The phenomenon of hysteresis may be attributed to the initial chemisorptions on the surface of the sensing elements. This chemisorbed layer, once formed is not further affected by exposure to or removal of humidity, it can be thermally desorbed only. For the sensing sample ZnO doped WO3 the hysteresis values are 3.01%, 1.81%, 1.77% and 1.09% for the annealing temperature 300°C, 400 °C, 500°C and 600°C, respectively. Thus, there is a definite decrease in the hysteresis when temperature increases from 300°C to 600°C. The reason behind that, external energy required for the removal of chemisorbed layer in comparison to the energy required for the removal of physisorbed layer. With the increase in the annealing temperature, the porosity increases surface-to volume ratio of the material. Due to which more of the surface areas of the sensing elements are exposed leading to more adsorption of water molecules i.e physisorption process increases as compared to the chemisorption process. Thus the hysteresis decreases with the increased annealing temperature. It has been observed that all sensing elements show low hysteresis value within acceptable range. Fig. 4 shows the hysteresis graph of the sensing sample ZnO doped WO3 for the annealing temperature 600°C. This figure shows the graphs both for the increasing and decreasing cycles of relative humidity.
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Fig. 3. Humidification graphs for sensing sample ZnO doped WO3 for different annealing temperatures.
Fig. 4. Hysteresis graph for sensing sample ZnO doped WO3 for annealing temperature 600 °C; a: an increasing cycle of % RH; b: decreasing cycle of % RH.
To check the effect of ageing and reproducibility, the sensing elements were examined again in the humidity chamber after 6 months and the variation of resistance with %RH recorded. The results in terms of resistance vs. %RH have been found to be generally reproducible over different cycles of operation. Fig. 5 shows the repeatability graph for the sensing sample WZ annealed at 600°C. In this figure, a represents an initial increasing cycle and b represent an increasing cycle after six months. The results generally show that the annealing temperature increases from 300 ̊C- 600 ̊C the sensitivity of the sensing sample ZnO doped WO3 increases. The sample WZ shows results with maximum sensitivity of 16.42MΩ/%RH when annealed at 600°C. Figure 6 shows the sensitivity vs. temperature graphs for all sensing sample WZ when annealed at different temperatures from 300°C to 600°C.
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Fig. 5. Ageing graph for sensing sample of ZnO doped WO3 for annealing temperature 600 °C; a: initial humidification cycle of % RH; a-4: humidification cycle of % RH after four months; a-6: humidification cycle of % RH after six months.
Fig. 6. Average sensitivity versus annealing temperature for the humidity range of 15%–95% RH: for sensing sample of ZnO doped WO3.
A regression analysis of the data was carried out for the humidification graph of sensing sample ZnO doped WO3. A polynomial of degree 3 fitted to the curve of relative humidity versus resistance graph of the sensing sample WZ for the annealing temperature 600 ̊C. Y= -0.010x3+2.119x2-132.0x+2628 Here, x = % Relative Humidity and y = Resistance Corresponding polynomials of degree 3 fitted to the curve of sensitivity versus temperature graph of the sensing sample WZ for different annealing temperature. Y= 6E-07x3-0.001x2+0.493x-69.0 Here, x = temperature and y = sensitivity in MΩ/% RH. The other important parameters for sensor devices are response and recovery time. To investigate these properties, the humidity in the chamber has been switched over from 15% RH (low humidity) to 95% RH (high humidity) and for calculation of recovery time the humidity in the chamber has been brought down from 95% RH (high humidity) to 15% RH (low humidity). For sensing sample ZnO doped WO3 at the annealing temperature 600 °C, the response
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time was 65 s when RH was changed from 15 to 95%, while the recovery time was 360 s when RH was changed from 95 to 15%. Adsorption and desorption of the water molecules take place at different energy levels. Adsorption is an exothermic process, where as desorption needs external energy for water molecules to depart from the metal oxide surface. As desorption is an endothermic process, it takes a longer time to desorb the water vapour; therefore, the recovery time is always greater than the response time [11]. 4.
Conclusions
Sensitivity of sensing sample ZnO doped WO3 increases with increases annealing temperature. Sensing element annealed at 600°C proves to be the best sensing element from other samples with sensitivity of 16.42 MΩ/%RH in the 15%–95% RH range. For this sensing element, the lower hysteresis is 1.09% for the annealing temperature 600°C in comparison to other lower annealing temperature. Reproducibility over cyclic operations were within ±1.82% and ±8.64% of the measured values after four and six months. XRD pattern shows formation of monoclinic tungstite, sanmartinite ZnWO4, zincite and orthorhombic tungsten oxide (W3O8). As calculated from Scherer’s formula the minimum crystallite size for this sensing element is 12nm and according to SEM micrograph grain size is 230 nm. Acknowledgements Authors would like to thank the Geological Survey of India, Lucknow for providing XRD facility and Birbal Sahni Institute of Paleobotany, Lucknow for providing SEM facility. References [1] Mario Pelino and Carlo Cantalini, Active and Passive Elec. Comp., 1994, Vol.16, pp. 69-87. [2] M. Akiyama, J. Tamaki, N. Miura, N. Yamazou, Chem. Lett. 9 (1991) 1611–1614. [3] Suman Pokhrel, K.S. Nagaraja, Sensors and Actuators B 92 (2003) 144-150. [4] Yong Fen Dong, Long Yu Li, Wei Fen Jiang, Hai Yan Wang, Xin Jian Li, Physica E 41 (2009) 711-7. [5] Dewyani Patil, You-Kyong Seo, Yong Kyu Hwang, Jong-San Chang, Pradip Patil, Sensors and Actuators B 132 (2008) 116-124. [6] Guermat Noubeil, Bellel Azzedine, Sahli Salah and Raynaud Patrice 2014 J. Chem. Sci. Technol. 3 13 [7] Md Sin N D, Fuad Kamel M, Alip Rosalena Irma, Mohamad Zulfakri and Rusop M. 2011 Adv. Mater. Sci. Eng. 2011, 5 [8] Srivastava Richa and Yadav B C 2012 Adv. Mat. Lett. 3 197. [9] Rout C S, Hegde M and Rao C N R 2008 Sens. Actuators B 128 488. [10] Na Dong-myong, Satyanarayana L., Choi Gwang-Pyo, Shin Yong-Jin and Park Jin Seong, 2005, Sensors 5 419. [11] Sundaram R, Raj E S and Nagaraja K S 2004 Sens. Actuators B 99 350.
ScienceDirect Materials Today: Proceedings 5 (2018) 9082–9088
www.materialstoday.com/proceedings
NCNN 2017
Structural and Moisture Sensing Properties of WO3-ZnO Nanocomposites Synthesized by a Soft Chemical Route Vandna Shakya* and Narendra Kumar Pandey Sensors and Materials Research Laboratory, Department of Physics, University of Lucknow, U.P., Pin-226007, India
Abstract WO3 ceramic oxide which is sensitive to atmospheric moisture levels have been synthesized and tested. WO3-ZnO nanocomposites have been prepared in the form of pellets by WO3 obtained from synthesized route method mixed with 2 weight % of ZnO powder. These pellets have been annealed at temperatures 300 °C to 600 °C. When samples have been exposed to humidity, the change in the resistance of pellet depends on the amount of adsorbed water vapour. It is observed that as relative humidity (% RH) increases, there is a decrease in the resistance for the entire range of humidity from 15 % to 95 %. For the sensing element of ZnO doped WO3, the repeatability over different cyclic operations is within ±1.82% and ±8.64% of the measured values of sensitivity after four and six months respectively. WO3-ZnO composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moisture sensing properties of the composites were investigated, including sensitivity, humidity hysteresis, repeatability and response properties. The composite with 2 weight% of ZnO shows the maximum sensitivity 16.42 M Ω/% RH at 600°C annealing temperature, narrow hysteresis (1.09%) and highest repeatability of the response. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 ).
Keywords: Humidity Sensing, WO3, ZnO, Hysteresis, Sensitivity, Annealing Temperature
*Email address: [email protected]
2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th NATIONAL CONFERENCE ON NANOMATERIALS AND NANOTECHNOLOGY (NCNN VI - 2017 )
Vandna Shakya etal/ Materials Today: Proceedings 5 (2018) 9082–9088
9083
1. Introduction Research has been going on to find suitable materials that show good sensitivity over a wide range of humidity, low hysteresis, long-term stability, low power consumption and properties which are stable to thermal cycling and exposure to the various chemicals likely to be present in the environment. Ceramic such as WO3, ZnO, SnO2, Al2O3, TiO2 etc are most investigated sensing materials for relative humidity sensors. Ceramic materials have been attracting attention for their intrinsic characteristics; they are suitable for the reproducibility of the electrical properties, mechanical strength, chemical and physical stability. These materials possess a unique structure consisting of grains, grain boundary surfaces, and pores, which make them suitable for adsorption of water molecules because of the high surface exposure [1]. Tungsten trioxide (WO3) is a n-type semiconductor metal oxide having wide band gap. Tungsten oxide shows optical, electrical, structural, morphological properties, chemical and physical stability. All these properties of WO3 can be controlled by the method of preparation adopted and make easier the use of tungsten trioxide in wide range applications [2]. Doped WO3 with other oxides exhibit change their properties, morphologies and have many applications. ZnO is one the most extensively studied for sensing purpose because its remarkable optical and electrical properties. Suman Pokhrel et al. reported the electrical and humidity sensing properties of Cr2O3-WO3 composites. Cr2O3-WO3 composites were made by mixing Cr2O3 and WO3 in different molar ratios and pressing at 100 M Pa to form cylindrical pellets and sintered at temperature range 373-673 K and then their humidity sensing properties studied [3]. M.V.N. Ambika Prasad et al. presented humidity sensors based on polyaniline/WO3 composites by in situ' polymerization. Composite with 50 weight % of WO3 film on SiNPA through electron-beam evaporation process and the humidity sensing was studied [4]. Dewyani Patil et al. prepared humidity sensitive poly (2, 5-dimethoxyaniline)/WO3 (PDMA/WO3) composite with different weight percents of WO3 by a simple mechanical mixing method. The PDMA/WO3 composites show better sensing properties than pure PDMA, such as higher sensitivity, quicker response and small hysteresis [5]. Noubeil Guermat et al. report the study of humidity-sensitive and electrical properties of plasma polymerization of hexamethyldisiloxane (pp- HMDSO) thin film-based sensors. The humidity sensitive film was deposited by glow discharge at low frequency power (19 kHz) in a capacitive-coupled parallel plate plasma reactor. The deposited film sensor exhibited a small hysteresis (2% RH) and fast response (8 and 34 s for adsorption and desorption between 35 and 95% RH, respectively) [6]. Md Sin et al. have reported about the electrical characteristics of aluminium (Al)doped zinc oxide (ZnO) thin film for high sensitivity humidity sensors. Parameter 0.6 at% aluminium doped ZnO showed high sensitivity of 2.32 MΩ/%RH and suitable for humidity sensor [7]. Richa Srivastava et al. have reported comparative performance of n-type ZnO, ZnO-TiO2 and ZnO-Nb2O5 nanomaterials as humidity sensors. Average sensitivity achieved 8 MΩ/%RH for the n type ZnO annealed at 550°C. After chemical mixing of TiO2, the sensitivity increased to 18 MΩ/%RH and after Nb2O5, it was found to be 19 MΩ/%RH [8]. We report here morphological and moisture sensing studies of WO3-ZnO nanocomposites prepared by synthesized route. 2.
Experimental Methods
WO3 nanocrystalline powders were obtained by a soft chemistry route based tungstic acid (H2WO4). H2WO4 (10g) was mixed with 100ml of an aqueous solution of H2O2 (30% PH = 3.9) then PTA formed with PH= 2 (stirred continuously for 4h). The solution was heated to dryness to obtain a precursor powder, which was annealed at 600°C in air for 3h to obtain nanocrystalline pure WO3 powder. To make the samples of WO3-ZnO nanocomposites, 2 weight % (sample WZ-2) of ZnO powder (Loba Chemie, 99.9%) has been added in synthesized nanocrystalline WO3 powder.10% weight of polyvinyl alcohol was added as binder to increase the strength of the sample. Mixed powder was grinded to uniformly for three hours. The resultant powder was pressed into pellet shape by uniaxially applying pressure of 267 M Pa in a hydraulic press machine (M.B. Instruments, Delhi, India) at room temperature. After sintering, each sensing sample was placed in a same humidity chamber and its sensing outputs were obtained under different humidity levels. Inside the humidity chamber, a thermometer (±1°C) and standard hygrometer (Huger, Germany, ±1% RH) are placed for the purpose of calibration. Variation in resistance has been recorded with change in relative humidity. Relative humidity has been measured using the standard hygrometer. Variation in resistance of the pellets has been recorded using a resistance meter (Sino meter, ±1 MΩ, model: VC-9808). Copper electrode has been used to measure the resistance of the pellets. The resistance of the pellets has been measured normal to the cross-section of the pellets. To see the effect of aging, the sensing properties of these elements have been examined again in the humidity control chamber after four and six months. The response and recovery time for the sensing elements are also observed.
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Vandna Shakya et al/ Materials Today: Proceedings 5 (2018) 9082–9088
To study grain micro-structure of sensing sample ZnO doped WO3 annealed at 600°C was carried out using SEM (LEO-430, Cambridge, England). The micrographs were taken for the 35.00 Kx magnifications. SEM micrographs shows that the sensing sample manifest porous structure having granulation and tendency to agglomerate. SEM micrograph for the sensing sample WZ annealed at 600°C is shown in Figure 1. It can be observed that inter granular pores are linked through the large pores. The pore structures should be regarded as inter connected voids that form a kind of capillary tubes. Sensing sample annealed at 600°C has the maximum void concentration and porosity. The average grain size calculated from SEM micrograph for this sensing element is 230 nm. In this process, Due to the presence of larger grains, more of the surface areas of the sensing elements are exposed leading to more adsorption of water molecules. This increases sensitivity of the sensing elements. It also helps in reducing response and recovery times because it enhances diffusion rate of water into or out-off the porous structure.
Fig. 1. SEM micrograph of sensing sample of ZnO doped WO3 for annealing temperature 600 °C.
XRD of the samples were studied using XPERT PRO-Analytical XRD system (Netherlands). The wavelength of the source CuKα used is 1.54060 Å. The average crystalline size of the samples has been calculated using Scherrer’s formula D = Kλ/B cos θ
(1)
Here, D is the crystallite size, K a fixed number of 0·9, λ the X-ray wavelength, θ the Bragg angle and B the full width at half maximum of the peak. Figure 2 shows the XRD pattern of sensing sample ZnO doped WO3 for the annealing temperature 600 ̊C. High intensity, large number of peaks and sharpness of peaks show high crystallinity of the sensing sample WZ annealed at 600◦C [9,10]. XRD shows some peaks are found broad and the broadness of the peak indicates that crystallites are of small size or nano-size. From Scherer’s formula we can see that broader the diffraction peak (i.e., greater the full-width at half-maximum (FWHM) value) smaller the crystal size. The crystallite size for sensing sample ZnO doped WO3 annealed at 600°C calculated from Scherrer's formula is in the 12-121 nm range.
Vandna Shakya etal/ Materials Today: Proceedings 5 (2018) 9082–9088
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Fig. 2. XRD pattern for sensing sample of ZnO doped WO3 for annealing temperature 600 °C.
3.
Results and Discussion
Variation in resistance with the change in % RH for sensing sample ZnO doped WO3 is shown in Figure 3 for the humidification process. Figure 3 shows large decrease in the value of the resistance for initial values of the relative humidity range from 15% ̶ 40% RH while in 40% –95% RH range, the fall in resistance is slow observed for all the sensing elements. As semiconducting dry oxide of WO3 nanocomposite is brought in contact with humid air, water molecules chemisorb on the available sites of the oxide surface. The adsorption of water molecules on the surface takes place via a dissociative chemisorptions process. Hence the electrons are accumulated at the WO3 surface and consequently, the resistance of the sensing element decreases with increase in relative humidity. Hysteresis effect is an important factor in metal oxides and in binary systems of metal oxides. The phenomenon of hysteresis may be attributed to the initial chemisorptions on the surface of the sensing elements. This chemisorbed layer, once formed is not further affected by exposure to or removal of humidity, it can be thermally desorbed only. For the sensing sample ZnO doped WO3 the hysteresis values are 3.01%, 1.81%, 1.77% and 1.09% for the annealing temperature 300°C, 400 °C, 500°C and 600°C, respectively. Thus, there is a definite decrease in the hysteresis when temperature increases from 300°C to 600°C. The reason behind that, external energy required for the removal of chemisorbed layer in comparison to the energy required for the removal of physisorbed layer. With the increase in the annealing temperature, the porosity increases surface-to volume ratio of the material. Due to which more of the surface areas of the sensing elements are exposed leading to more adsorption of water molecules i.e physisorption process increases as compared to the chemisorption process. Thus the hysteresis decreases with the increased annealing temperature. It has been observed that all sensing elements show low hysteresis value within acceptable range. Fig. 4 shows the hysteresis graph of the sensing sample ZnO doped WO3 for the annealing temperature 600°C. This figure shows the graphs both for the increasing and decreasing cycles of relative humidity.
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Fig. 3. Humidification graphs for sensing sample ZnO doped WO3 for different annealing temperatures.
Fig. 4. Hysteresis graph for sensing sample ZnO doped WO3 for annealing temperature 600 °C; a: an increasing cycle of % RH; b: decreasing cycle of % RH.
To check the effect of ageing and reproducibility, the sensing elements were examined again in the humidity chamber after 6 months and the variation of resistance with %RH recorded. The results in terms of resistance vs. %RH have been found to be generally reproducible over different cycles of operation. Fig. 5 shows the repeatability graph for the sensing sample WZ annealed at 600°C. In this figure, a represents an initial increasing cycle and b represent an increasing cycle after six months. The results generally show that the annealing temperature increases from 300 ̊C- 600 ̊C the sensitivity of the sensing sample ZnO doped WO3 increases. The sample WZ shows results with maximum sensitivity of 16.42MΩ/%RH when annealed at 600°C. Figure 6 shows the sensitivity vs. temperature graphs for all sensing sample WZ when annealed at different temperatures from 300°C to 600°C.
Vandna Shakya etal/ Materials Today: Proceedings 5 (2018) 9082–9088
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Fig. 5. Ageing graph for sensing sample of ZnO doped WO3 for annealing temperature 600 °C; a: initial humidification cycle of % RH; a-4: humidification cycle of % RH after four months; a-6: humidification cycle of % RH after six months.
Fig. 6. Average sensitivity versus annealing temperature for the humidity range of 15%–95% RH: for sensing sample of ZnO doped WO3.
A regression analysis of the data was carried out for the humidification graph of sensing sample ZnO doped WO3. A polynomial of degree 3 fitted to the curve of relative humidity versus resistance graph of the sensing sample WZ for the annealing temperature 600 ̊C. Y= -0.010x3+2.119x2-132.0x+2628 Here, x = % Relative Humidity and y = Resistance Corresponding polynomials of degree 3 fitted to the curve of sensitivity versus temperature graph of the sensing sample WZ for different annealing temperature. Y= 6E-07x3-0.001x2+0.493x-69.0 Here, x = temperature and y = sensitivity in MΩ/% RH. The other important parameters for sensor devices are response and recovery time. To investigate these properties, the humidity in the chamber has been switched over from 15% RH (low humidity) to 95% RH (high humidity) and for calculation of recovery time the humidity in the chamber has been brought down from 95% RH (high humidity) to 15% RH (low humidity). For sensing sample ZnO doped WO3 at the annealing temperature 600 °C, the response
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time was 65 s when RH was changed from 15 to 95%, while the recovery time was 360 s when RH was changed from 95 to 15%. Adsorption and desorption of the water molecules take place at different energy levels. Adsorption is an exothermic process, where as desorption needs external energy for water molecules to depart from the metal oxide surface. As desorption is an endothermic process, it takes a longer time to desorb the water vapour; therefore, the recovery time is always greater than the response time [11]. 4.
Conclusions
Sensitivity of sensing sample ZnO doped WO3 increases with increases annealing temperature. Sensing element annealed at 600°C proves to be the best sensing element from other samples with sensitivity of 16.42 MΩ/%RH in the 15%–95% RH range. For this sensing element, the lower hysteresis is 1.09% for the annealing temperature 600°C in comparison to other lower annealing temperature. Reproducibility over cyclic operations were within ±1.82% and ±8.64% of the measured values after four and six months. XRD pattern shows formation of monoclinic tungstite, sanmartinite ZnWO4, zincite and orthorhombic tungsten oxide (W3O8). As calculated from Scherer’s formula the minimum crystallite size for this sensing element is 12nm and according to SEM micrograph grain size is 230 nm. Acknowledgements Authors would like to thank the Geological Survey of India, Lucknow for providing XRD facility and Birbal Sahni Institute of Paleobotany, Lucknow for providing SEM facility. References [1] Mario Pelino and Carlo Cantalini, Active and Passive Elec. Comp., 1994, Vol.16, pp. 69-87. [2] M. Akiyama, J. Tamaki, N. Miura, N. Yamazou, Chem. Lett. 9 (1991) 1611–1614. [3] Suman Pokhrel, K.S. Nagaraja, Sensors and Actuators B 92 (2003) 144-150. [4] Yong Fen Dong, Long Yu Li, Wei Fen Jiang, Hai Yan Wang, Xin Jian Li, Physica E 41 (2009) 711-7. [5] Dewyani Patil, You-Kyong Seo, Yong Kyu Hwang, Jong-San Chang, Pradip Patil, Sensors and Actuators B 132 (2008) 116-124. [6] Guermat Noubeil, Bellel Azzedine, Sahli Salah and Raynaud Patrice 2014 J. Chem. Sci. Technol. 3 13 [7] Md Sin N D, Fuad Kamel M, Alip Rosalena Irma, Mohamad Zulfakri and Rusop M. 2011 Adv. Mater. Sci. Eng. 2011, 5 [8] Srivastava Richa and Yadav B C 2012 Adv. Mat. Lett. 3 197. [9] Rout C S, Hegde M and Rao C N R 2008 Sens. Actuators B 128 488. [10] Na Dong-myong, Satyanarayana L., Choi Gwang-Pyo, Shin Yong-Jin and Park Jin Seong, 2005, Sensors 5 419. [11] Sundaram R, Raj E S and Nagaraja K S 2004 Sens. Actuators B 99 350.