Synthesis and Characterization of ZnO Nanorods by the Hydroxide ...

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 37:417–423, 2007 Copyright # 2007 Taylor & Francis Group, LLC ISSN: 1553-3174 print/1553-3182 online DOI: 10.1080/15533170701465937

Synthesis and Characterization of ZnO Nanorods by the Hydroxide Route and Their Application as Humidity Sensors B. C. Yadav, Richa Srivatava, and C. D. Dwivedi Materials and Sensors Research Laboratory, Department of Physics, University of Lucknow, Lucknow, India

ZnO nanorods have been synthesized through the hydroxide route, and to that, 10% glass powder was mixed. Pellets of powder were made and exposed to humidity. It is observed that as Relative Humidity (RH%) inside the chamber increases from 5% to 95%, resistance of pellets decreases successively. Further pellets were annealed for 3 hours at temperatures of 15088 C, 30088 C, 45088 C and 55088 C in an electric furnace successively. After each and every time of annealing, pellets were exposed to humidity and variations in resistance with humidity have been observed. SEM, XRD and percentage weight loss of the sample have been studied. Average diameter of nanorods is found approximately 50 nm and length around 250– 350 nm. Keywords

humidity sensor, pellets, annealing, resistance

INTRODUCTION Recently, one-dimensional nanoscale materials have received considerable attention due to their remarkable properties as applied in optoelectronic and electronic nanodevices.[1 – 5] There has been an explosion of fundamental and applied research into the synthesis of materials of different morphologies with nanometer dimensions, due to their importance in mesoscopic physics and device applications. The n-type semi conducting materials such as stannous oxide, zinc oxide and titanium dioxide are promising materials[6 – 18] for gas and humidity sensors. Of all the materials investigated, ZnO is a material that has fascinated researchers with a wide variety of morphologies and range of promising device applications.[10 – 18] It is a wide direct band gap (3.37 eV) semiconductor with a high chemical stability and a relatively high excitation binding energy (60 meV).

Received 31 January 2007; accepted 03 April 2007. The authors are highly grateful to Vice-Chancellor Prof. R. P. Singh and Head, Department of Physics, Prof. G. P. Gupta for constant encouragement and support. Address correspondence to B. C. Yadav, Materials and Sensors Research Laboratory, Department of Physics, University of Lucknow, Lucknow 226007, U.P. India. E-mail: balchandra_yadav@ rediffmail.com

ZnO is used in many applications such as surface acoustic wave (SAW) devices, laser devices, solar cells, humidity sensors and gas sensor devices and MEMS. Wires, tubes springs, needles, belts, cables, tetra pods and flowers[2,12 – 18] all have been synthesized in nanosize morphologies and shown to be potential candidates in optoelectronic applications. Another special morphology of ZnO is the ZnO micro-cage. In any material, hollow structures are very different from their solid counterparts. Until now, hollow and solid ZnO spheres have been synthesized by various techniques,[14] and also there are different methods for synthesizing ZnO nanorods, such as hydrothermal synthesis, salvothermal synthesis, the micro-emulsion hydrothermal process, chemical vapor deposition (CVD) and a catalyst-free CVD method. Control and measurement of water vapors and gases in various industries e.g., manufacturing of products such as textiles, leathers, food processing paper industry and storage of cereals, semiconductors and petrochemicals are very important. Humidity is an essential constituent of our environment. It plays a very important role in human life. Since water present in all living organisms, from the simplest to the most complex, such as human beings drastically influences their lives and working efficiencies. The measurement of humidity gives us an estimate of the amount of water vapours present in air.[19 – 21] There are several methods to measure relative humidity.[22 – 24] A wide range of humidity sensors have been developed in recent years in response to these needs. In this paper we have studied the synthesis and characterization of ZnO nanomaterials and their applications as humidity sensors. SEM and XRD studies of samples have been done. Annealing effects on morphology at temperatures 1508C, 3008C, 4508C, and 5508C of materials have also been studied. Average diameter of nanorods is found approximately 50 nm and length around 250 –350 nm. RESULTS AND DISCUSSION Variations in resistance with the variation in relative humidity for the sensing element of zinc oxide prepared by hydroxide route have been plotted in Figure 1. The plots for sensor annealed at different temperatures show similar

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in relative humidity (RH%), i.e., DR MV=RH% DRH% This sensor shows highest sensitivity over the entire range of RH and applicable for commercial production. The sensitivity of sensor at different annealing temperatures is shown in the table: S¼

Annealing temperature Average sensitivity (M V/RH%)

FIG. 1. Variations of resistance in MV against RH% for sensing element annealed at temperatures 1508C, 3008C, 4508C, and 5508C.

nature. Curve for annealing temperature u ¼ 1508C shows that as RH increases, resistance decreases slightly up to 80% RH and shows lesser sensitivity in this range, then it decreases rapidly up to 95%RH. Curve for annealing temperature u ¼ 3008C and u ¼ 4508C shows similar character with slightly enhanced sensitivity. Curve for annealing temperature u ¼ 5508C shows that as RH increases, resistance decreases sharply up to 50%RH and shows highest sensitivity (12.74 MV/RH%) in this range, then it decreases less rapidly up to 95%RH as relative humidity increases. It is observed that as annealing temperature increases, resistance of the material decreases.[4] It is shown in Figure 2. We found that with increase in temperature, pore diameters inside the pellet have been increased, which results enhancement in sensitivity. Sensitivity of humidity sensor has been defined as the change in resistance (DR) of sensing element per unit change

FIG. 2. Variations of resistance in MV against annealing temperatures at 5%RH.

1508C 3008C 4508C 5508C 4.6 4.7 5.49 7.95

It has also been shown in Figure 3. We observe that as annealing temperature increases sensitivity of sensor increases. The results obtained are found to be reproducible and show low hysterisis. It is shown in Figure 4 for sensing element annealed at u ¼ 5508C. Experiments have been repeated after 2 months with the same sensing element but no aging effect was observed. Ceramic humidity sensors show chemical resistance.[22 – 24] Conductivity of such sensing materials varies with amount of water adsorbed by it. This principle is employed for the

FIG. 3. Variations of sensitivity of sensor with annealing temperatures.

FIG. 4. Variations of resistance in MV against RH% for sensing element showing.

ZnO NANOROD SYNTHESIS AND CHARACTERIZATION

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FIG. 5. Scanning electron micrograph of ZnO pellet at (a) micro-scale, (b) micro-scale in high magnification, and (c) nanoscale.

measurement of humidity in resistive-type humidity sensors. This type of sensor can be subdivided into ionic type and electronic-type humidity sensor depending upon their conduction mechanism. These sensors mostly work at low temperatures. The change in impedance of porous ceramics at different environmental humidity values is related to the water adsorption mechanism on the oxide surface.[24] At low humidities, conduction is due to proton hopping between hydroxyl ion on the first layer of chemisorbed water, while at higher humidities, protons hop between physisorbed molecules with a Grotthuss chain reaction mechanism.[21] The morphology of the sensing element influences water vapor adsorption and desorption. The condensation of water vapors occurs as a result of capillary action. The behavior of this condensation is a function of ceramic pore size and its distribution. Therefore, in this case as humidity inside the chamber increases, adsorption of water vapor increases, consequently, resistance of sensor decreases.

FIG. 6. Scanning electron micrograph of ZnO film using sol-gel technique.

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FIG. 7. Scanning electron micrograph of pellet (a) pellet at 1508C, (b) pellet annealed at 3008C, (c) pellet annealed at 4508C, (d) pellet annealed at 5508C in microscale, and (e) pellet annealed at 5508C.


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FIG. 8. XRD of ZnO powder (a) without annealed, (b) annealed at 1508C, and (C) annealed at 5508C.

CHARACTERIZATION The morphology of sensing material was investigated with a scanning electron microscope (SEM, LEO-0430, Cambridge). Figures 5a, 5b, and 5c show the morphology of sensing element in the form of pellet. Figures 5a and 5b are on micro scale while Figure 5c is on nano scale. The product consists of nanorods with a length of around 250– 350 nm and an average diameter between 40 – 50 nm. Blade-like nanoflakes are discernible on the rough surface of such nanorods under higher magnification (Figure 6). Heat treatment of the sensing element at different temperatures gives different morphologies of materials. SEM micrographs are as shown in Figures 7a, 7b, 7c, 7d, and 7e. We found that with increase in temperature, pore diameters have been increased. Figures 8a, 8b, and 8c show the XRD

Patterns. These patterns show crystallinity of the sensing material in the form of powder at different temperatures. Their crystallinity decreases as annealing temperature increases and molecules are combining each other form clusters leaving more space for pores, which is why samples annealed at higher temperatures i.e., 5508C become more sensitive. Figure 9 shows variations in percentage weight of the sample with different temperatures. We observed that percentage weight loss is the function of temperature,[12] and it is maximum (18%) in the range between 1508C –3008C.

Device Assembly Device assembly is shown in Figure 10. A controlled humidity chamber has been designed. Potassium hydroxide is

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powder has been prepared using hydraulic pressing machine at normal pressure (3MPascal) and at room temperature 308C. This pellet has been subjected for annealing at different temperatures. CONCLUSION The sensing element annealed at temperature u ¼ 5508C is found to have better sensitivity than other temperatures. It has high sensitivity (12.74 MV/RH%) in the lower humidity range i.e., up to 50% RH. Its average sensitivity is 7.95 MV /RH% over the entire range of R.H. i.e., from 5% to 95%. Thus the humidity sensor reported here based on electrical resistance is robust, cost-effective and user friendly and can be used for a wide range of relative humidity. FIG. 9. Percentage weights of the sample with different temperatures.

REFERENCES

FIG. 10. Device assembly

used as dehumidifier and potassium sulphate is used as humidifier. Variations in resistance have been noted by using a digital multimeter (VC9808, India). Relative humidity is measured using standard hygrometer associated with thermometer (Huger, Germany). The humidifier/dehumidifier is kept in a dish over a stand. In the process the temperature of the chamber remains same throughout the experiment. The chamber is then dehumidified first up to 10% RH by using the dehumidifier potassium hydroxide and then up to 5% RH by carrying out the heat-cleaning cycle of the sensing element. The lowest count hygrometer used here is 1% RH and that of the thermometer 18C. EXPERIMENTAL ZnO is prepared by a conventional precipitation route. In this method sodium hydroxide solution is suddenly mixed with zinc sulphate solution at room temperature in the molar ratio 1:2.2. It yields precipitate of zinc hydroxide. Precipitate is filtered out and washed with deionized water until the SO2 4 ions are not completely removed and its subsequent calcinations give zinc oxide in powder form. This powder is mixed well with 10% glass powder. Addition of glass powder as a permanent binder during the process plays a major role in getting adhesion of the material for pellet formation. The pellet of this

1. Shukla, S. K.; Parashar, G. K.; Misra, P.; Yadav, B. C.; Shukla, R. K.; Bali, L. M.; Dubey, G. C. Nanolike magnisium oxide films and their significance in optical fiber humidity sensor. Sens. Actuators B. 2004, 98 (1), 5 – 11. 2. Zheng, W. P.; Shannon, M. M.; Sheng, D.; Douglas, H. L. Nanowire array gratings with ZnO combs. Nano Lett. 2005, 5 (4), 723–727. 3. Jiaqiang, X.; Yuping, C.; Daoyong, C.; Jianian, S. Hydrothermal synthesis and gas sensing characters of ZnO nano rods. Sens. Actuators B. 2006, 113, 526–531. 4. Sun, Z.; Liu, L.; Zhang, L.; Jia, D. Rapid synthesis of ZnO nanorods by one-step, room-temperature, solid-state reaction and their gas-sensing properties. Nanotechnology 2006, 17, 2266– 2270. 5. Roy, S.; Basu, S. Improved zinc oxide film for gas sensor applications. Bull. Mater. Sci. 2002, 25, 513– 515. 6. Yadav, B. C.; Pandey, N. K.; Srivastava, A. K.; Sharma, P. Study of optical humidity sensor based on titania films fabricated by solgel and thermal evaporation methods. Meas. Sci. Tech. 2007, 18, 1– 5. 7. Yadav, B. C.; Srivastava, A.; Shukla, R. K.; Dubey, G. C. Improved version of optical humidity sensor with incident light in the form of a conical beam. Ind. J. Pure Appl. Phy. 2006, 44, 694–699. 8. Yadav, B. C.; Shukla R. K.; Bali, L. M. Sol-Gel Processed TiO2 films on U-shaped glass- rods as optical humidity sensor. Ind. J. Pure Appl. Phy. 2005, 43, 51 – 55. 9. Yadav, B. C.; Shukla, R. K. Titania films deposited by thermal evaporation as humidity sensor. Ind. J. Pure Appl. Phys. 2003, 32, 13 –15. 10. Ravindran, S.; Andavan, G. T. S.; Ozkan, C. Selective and controlled self-assembly of zinc oxide hollow spheres on bundles of single-walled carbon nanotube templates. Nanotechnology. 2006, 17, 723– 727. 11. Shukla, S. K.; Parashar, G. K.; Misra, P.; Yadav, B. C.; Shukla, R. K.; Srivastava, A.; Deva, F.; Dubey, G. C. On exploring sol-gel deposited ZnO thin film as humidity sensor: an optical fiber approach. Chem. Sensors Jap., Suppl. B 2004, 20, 546– 547. 12. Daun, J.; Huang, X.; Wang, E.; Ai, H. Synthesis of hollow ZnO microspheres by an integrated autoclave and pyrolysis process. Nanotechnology 2006, 17, 1786– 1790.


13. Yongjun, H. Preparation and modification of ZnO microspheres using a Pickering emulsion as template. Mater. Lett. 2005, 59, 114 –117. 14. Pearton, S. J.; Norton, D. P.; Ip, K.; Heo, Y.; Steiner, W. Recent progress in processing and properties of ZnO. Prog. Mat. Sci. 2005, 50, 293– 340. 15. Lori, E. G.; Matt, L.; Dawud, H. T.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. General route to vertical ZnO nanowire arrays using textured ZnO seeds. Nano Lett. 2005, 5 – 7, 1231 –1236. 16. Wang, F. Z.; Ye, Z. Z.; Ma, D. W.; Zhu, L. P.; Zhuge, F. Novel morphologies of ZnO nanotetrapods. Mater. Lett. 2005, 59, 560 –563. 17. Jujia, W. P. H.; Zhengrong, R. T.; Neil, C. S.; Carolyn, M. M. Directed spatial organization of zinc oxide nanorods. Nano Lett. 2005, 5, 1, 83 – 86. 18. Lee, G. H.; Shin B. C.; Kim, I. S.; Lee, W. J.; Kyun, M. O.; Jeon, H. D. Formation of size control of ZnO nanowires on

19.

20.

21. 22. 23. 24.

423

Al-Zn-Si-Fe alloy by directed melt oxidation process. J. Electroceramo 2004, 13, 201–207. Fripiat, J. J.; Jelli, A.; Poncelet, G.; Andre, J. Thermodynamic properties of adsorbed water molecules and electrical conduction in montmorillonites and silicas. J. Phys. Chem. 1965, 69, 2185– 2197. Anderson, J. H.; Parks, G. A. The electrical conductivity of silica gel in the presence of adsorbed water. J. Phys. Chem. 1968, 72, 3362– 3368. Yadav, B. C.; Pandey, N. K.; Srivastava, A. K. Water: an unique matter. Lko. J. Sci. 2005, 2, 15 – 23. Kulwicki, B. M. Humidity sensors. J. Am. Ceram. Soc. 1991, 74, 697. Zhi, C.; Chi, L. Humidity sensors: A review of materials and mechanism. Sensors Lett. 2005, 3, 274– 295. Traversa, E. Ceramic sensors for humidity detection: the state of the art and future developments. Sens. Actuators B 1995, 23, 135– 156.