- Email: [email protected]
Synthesis, characterization, electrical and sensing properties of ZnO nanoparticles
Advanced Powder Technology 21 (2010) 609–613
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis, characterization, electrical and sensing properties of ZnO nanoparticles AK Singh * Defence Institute of Advanced Technology, Girinagar, Pune 411025, India
a r t i c l e
i n f o
Article history: Received 11 December 2009 Accepted 2 February 2010
Keywords: ZnO Nanofluids Photoluminescence UV–VIS Impedance spectroscopy Thermal conductivity
a b s t r a c t The present study investigates the electrical and sensing properties of mechanically compacted pellets of nanosized zinc oxide powders synthesized by chemical method at room temperature in alcohol base using Triethanolamine (TEA) as capping agent. Synthesized ZnO particles has been characterized for its optical, structural, morphological properties using UV–VIS spectrophotometer, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The ZnO particles have hexagonal wurtzite structure and the particles are of 20–30 nm in size. The electrical properties of the prepared material have been investigated with Impedance Spectroscopy at different temperatures and frequencies and other laboratory setup. Resistivity, I–V curves, AC impedance of ZnO nanoparticles pellets with temperature was investigated and response was compared with commercial ZnO. Piezoelectric and oxygen sensing property of ZnO were also examined. Dynamic hysteresis of sintered ZnO pellet using axis ACCT TF analyzer 2000HS did not show polarization retention by sample. Oxygen sensing of ZnO pellet has been investigated for different concentrations of oxygen for the temperature range of 200–350 °C. The decrease of the current flow through the ZnO pellet with increasing oxygen concentration indicates the application of ZnO in oxygen sensing. The prepared ZnO particles were also used for preparing nanofluids of different concentrations and were characterized by measuring thermal conductivity using hot wire method which shows sigmoidal behavior over a temperature range of 10–50 °C. Ó 2010 The Society of Powder Technology Japan.. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction In the past few years, intense research activities have been devoted to the synthesis, structural characterization and investigation of physical and other properties of nanostructured materials. These nano materials has also been used in preparing Nanofluids (colloidal suspensions of nanoparticles) for efficient heat transfer applications. Zinc oxide NPs exhibits remarkable physical properties that make metal oxide nanoparticles strong candidates in nanofluids and other applications. ZnO, a key technological material, has received wide attention due to its specific chemical, electrical, surface and microstructural properties. ZnO being direct band gap semiconductor has high exciton binding energy (60 meV) would allow excitonic transitions at room temperature, meaning high radiative recombination efficiency for spontaneous emission as well as lower threshold voltage for emission. ZnO is considered as a substitute to GaN and ITO, as transparent conducting oxide. It has high electrochemical stability and control over resistance (103 to 105 O). The microstructural and physical properties of ZnO can be modified by introducing changes into the pro-
* Tel.: +91 20 24304173; fax: +91 20 24389411. E-mail addresses: [email protected], [email protected]
cedure of its chemical synthesis. The studies for nanosized ZnO powders have been conducted due to their size-dependent electronic and optical properties, which offer possibilities for microelectronic devices [1,2]. As one of the major materials for solid state gas sensor, bulk and thin films of ZnO have been proposed for its sensing applications. ZnO sensor elements have been fabricated in various forms including single crystal [3,4] sinter pellet [5–7] thin film [8,9] and thick film [10]. The preparation methods of nanosized metal oxide particles have been extensively studied for the precise control of the morphology at the nanometric scale. In this study, ZnO nanoparticles are synthesized by wet chemical method, under ambient atmosphere at room temperature which is a promising option for large scale production. Also, properties of ZnO can be tailored other than the advantage of range of precursors, temperature, time, pH. etc. To restrict the growth of the particles, TEA has been used as capping agent. Synthesized ZnO powder has been used to prepare a pellet. Mechanically compacted pellet has been studied for electrical, piezoelectric and oxygen gas sensing properties. Effect of annealing in air atmosphere was also studied on the electrical properties of ZnO nanomaterial. Prepared ZnO response has been studied for three different concentrations (100 ml, 150 ml and 200 ml) of oxygen in the temperature range of 200–350 °C. Synthesized ZnO has also been used for preparing nanofluids and have been characterized
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan.. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.02.002
610
AK Singh / Advanced Powder Technology 21 (2010) 609–613
for thermal behavior under low concentrations (loading) of 0.075, 0.25 and 0.5%. 2. Synthesis methodology Zincacetatedihydrate, Zn(Ac)22H2O dissolved in Dimethylsulphoxide (DMSO) and Potassiumhydroxide (KOH) dissolved in Ethanol is used to synthesize ZnO and then Triethanolamine (TEA) is added as capping agent. 0.2 M Zincacetatedihydrate, Zn(Ac)22H2O is prepared in 20 ml Dimethylsulpoxide (DMSO) and stirred till it is completely dissolved and forms a clear solution. 1.2 M solution of Potassiumhydroxide in 10 ml Ethanol is added to the solution of Zincacetate drop wise till the solution becomes milky white under slow stirring condition until it is uniformly white. 0.12 ml TEA is added and stirring is continued for proper mixing of the capping agent in the milky white solution. The precipitate is separated by centrifugation and then washed at least three times and then naturally dried to get fine white powder. The dried synthesized ZnO nanoparticles are made into pellet using hydraulic press under 7500 psi. The pellet was silver painted using silver conducting paint. The dimensions of the pellet are 0.62 mm thickness and 13.11 mm diameter. All the chemicals used were purchased from the leading suppliers without further purification. Synthesized ZnO has also been used for preparing stable nanofluids in DI water of different concentrations. Acetylacetone has been used for stabilization of ZnO nanoparticles in DI water.
Fig. 1. UV–Vis absorption spectra of ZnO nanoparticles.
3. Measurements and analysis UV–VIS absorption study has been carried out using Nanodrop 1000 spectrophotometer by dispersing nanoparticles in methanol. Photoluminescence (PL) measurements have been done at room temperature using Perkin–Elmer (LS-55) Luminescence Spectrophotometer. Analysis of crystal structure, crystal size and morphology has been carried out by XRD and SEM. Stoichiometric analysis has been done using EDAX. From Scherrer’s formula [11] using FWHM of XRD patterns, size of particles is calculated
0:9k D¼ b cos h
ð1Þ
where k is wave length of X-ray source, b is full width at half maximum in radians, h is Bragg’s diffraction angle. Impedance spectroscopy measurements were performed using Broadband Dielectric Spectrometer of Novocontrol Model (Alpha ATB). Other electrical and gas sensing properties has been measured by using laboratory setup having provision for four probe resistance measurement, controlled heating of pellet and flow of gas. The gas adsorbs on the ZnO sensing layer and causes a change in resistance depending on the gas concentration. Piezoelectric properties of the sample has been analyzed using axis ACCT TF analyzer 2000HS by dynamic hysteresis of sintered ZnO pellet after polling. Hotwire method [12] has been used to measure thermal conductivity of nanofluid.
Fig. 2. Room temperature PL of ZnO nanoparticles.
oxygen and zinc interstials), a weak peak at 519 nm (green emission attributed to singly ionized oxygen vacancies) and low intensity peaks at 412 nm (attributed to zinc vacancies) and 393 nm (UV emission attributed to free exciton recombination) [13]. 4.2. Particle size and structural properties Fig. 3 shows SEM images of ZnO capped with 0.12 ml of TEA. Observation of figure shows that ZnO particles are spherical in nat-
4. Results and discussion 4.1. Optical properties Fig. 1 shows the UV absorption spectra of ZnO capped with TEA having absorption peak at 360 nm which is blue shifted to, from 375 nm absorption wavelength (expected for bulk ZnO having direct band gap of 3.3 eV) and showing sharp excitonic peaks at 360 nm. Fig. 2 shows the room temperature photoluminescence spectra of ZnO excited at 322 nm for ZnO nanoparticles after annealing for 03 h at 673 K. A strong peak has been observed at 471.5 nm (blue emission attributed to intrinsic defects such as
Fig. 3. SEM of ZnO nanoparticles.
AK Singh / Advanced Powder Technology 21 (2010) 609–613
Fig. 4. X-ray diffraction pattern of ZnO nanoparticles.
ure and size of the particles is in the range of 40–50 nm. Fig. 4 shows the XRD pattern of TEA capped ZnO nanoparticle. The XRD pattern shows broadening of the peaks indicating ultra fine nature of the crystallites. The peaks assigned to diffractions from various planes correspond to hexagonal structure of ZnO. The crystallite size estimated for the same sample from Scherrer’s formula using FWHM [11] from XRD patterns is of the order of 12–20 nm from different peaks, giving average size of 15.4 nm from all the peaks. The crystallite sizes obtained indirectly by XRD and directly by electron microscopy may not always exactly match. Whereas the electron microscopy can be used to determine almost any crystallite size, X-ray peak broadening methods give the most correct results for crystallite sizes in the range of 10–100 nm. 4.3. Electrical properties Fig. 5 shows the resistivity of ZnO pellet with temperature in the range of 85–335 °C when biased with 20 V. Figure shows decrease in resistivity with temperature which is a characteristic of the semiconductor. There are two distinct zones of conduction i.e. electronic conduction and ionic conduction. Electronic conduction is referred to as n- or p-hopping-type depending on whether the principal charge carriers are electrons or holes, respectively. This is present at low temperatures and the second conduction region which is prevalent at high temperatures is the ionic conduction. Ionic conduction takes place when ions can hop from site to
Fig. 5. Variation of resistivity of pellet with temperature prepared using ZnO nanoparticles.
611
Fig. 6. I–V characteristics of nano ZnO pellet before and after annealing at room temperature.
site within a crystal lattice as a result of thermal activation. Observation of Fig. 6 shows that at lower temperatures the resistivity of the annealed (400 °C for 3 h) ZnO is much lower than that of the un-annealed ZnO. This is because the un-annealed ZnO is much more porous than annealed ZnO which results higher resistivity. In the annealed ZnO the boundaries of the grain get diffused thereby resulting in reduction in the size of the pores. At higher temperatures the resistivity of the annealed ZnO becomes stable. This is because of lack of vacant oxygen ions in annealed ZnO, which have been removed in the annealing process. Fig. 6 shows the I–V characteristics of un-annealed and annealed sample at constant temperature (27 °C). It is observed that the breakdown voltage of annealed ZnO is much lower than that of un-annealed ZnO. It is because of the lower porosity of the annealed sample which results in the better conductivity. Fig. 7 shows the variation of impedance with temperature (30– 300 °C) for different frequencies (1–18 MHz). It has been found that the impedance at lower frequency is higher than that at the higher frequency with sharp decrease in impedance at 275 °C this is due to sharp increase in capacitance at 275 °C (variation of capacitance with temperature not shown here). The impedance of the commercial ZnO (particle size of 2–5 lm) is higher than that of ZnO nanoparticles as shown in Fig. 8 for 100 kHz frequency. The peak for ZnO nano pellet is sharp and well formed.
Fig. 7. Variation of impedance of nano ZnO pellet with temperature and at different frequencies.
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AK Singh / Advanced Powder Technology 21 (2010) 609–613
The permittivity of a material describes an AC signal transmission speed and dielectric material capacitance. The measurement are taken for both nano ZnO and Commercial ZnO and compared at 1 kHz frequency as shown in Fig. 9. It is found that the permittivity of the nano size ZnO is much higher than that of commercial ZnO, because of small particle size i.e. higher porosity compared to commercial ZnO. Therefore the nano ZnO is much better dielectric material than commercially available ZnO. 4.4. Piezoelectric properties
Fig. 8. Variation of impedance of commercial ZnO nanoparticle ZnO with temperature at different frequencies.
Piezoelectric behavior of ZnO has been analyzed after sintering the sample at 800 °C. Poling was done at 90 °C, 500 V for 20 min, then 10 min at 1 KV beyond which current conduction occured and poling could not be carried out. Fig. 10 shows the polarization voltage hysteresis curve of sintered ZnO pallet upto voltage application of 1100 V indicating the absence of polarization retention by sample. 4.5. Gas sensing properties Fig. 11 shows variation of current with temperature for different concentration of gas i.e. 0.2, 0.4 and 0.8 volume percentage and is well below the explosion limit. Here a constant voltage of 20 V is applied to the pellet and the current is monitored. The gas in addition to the surface adsorbed oxygen, deplete the electrons from the conduction bands of ZnO and thus the currents reducing with the increase in the concentration of the oxygen. The decrease of the current flow through the ZnO pallet with increasing oxygen concentration indicates the application of ZnO in oxygen sensing. 4.6. Thermal conductivity of ZnO nanofluids
Fig. 9. Variation of permittivity of commercial ZnO and nanoparticle ZnO.
Fig. 12 shows the variation of thermal conductivity(K) of ZnO nanofluids for different concentrations in the temperature range of 10–40 °C. Thermal conductivity exhibits slow increase for 0.075% and 0.25% loading nanofluids while 0.5% nanofluid shows relatively fast increase in K for the given temperature range. Beyond 30 °C temperature, K is found to increase abruptly for nano-
Fig. 10. Polarization–voltage hysteresis curve for nano ZnO pellet after sintering.
AK Singh / Advanced Powder Technology 21 (2010) 609–613
Fig. 11. Variation of current with temperature for different concentrations of oxygen gas.
613
served from the measurements of resistivity with temperature. The semi conductivity in ZnO must be due to large oxygen deficiency in it. Then behavior of the resistivity with change in temperature was studied and compared with the annealed (400 °C) ZnO and found that the at lower temperature the resistivity of annealed ZnO is lower than the un-annealed one because porosity is less in the latter which results in the better conductivity. At higher temperatures the resistivity of annealed one is higher because of lack of singly ionized oxygen vacancies. Using Impedance Spectroscopy (IS) the variation of impedance, permittivity and loss tangent is measured with change in temperature and frequencies; and then compared with the commercial ZnO which is in micro sizes. We found that impedance of commercial ZnO is far higher than that nano ZnO. Permittivity of nano ZnO is higher than commercial one therefore latter can act as good dielectric. The loss tangent of the nano ZnO is less than that of commercial ZnO, hence can be used as transmitter material. There is substantial increase in thermal conductivity which makes nanofluids attractive as cooling fluids. Acknowledgements
0.68
0.075% ZnO 0.250% ZnO 0.500% ZnO
0.66
Authors are thankful to Vice-Chancellor, DIAT, Pune, for granting permission to publish this work. Authors would like to thank Prof. SK Kulkarni, Department of Physics, University of Pune, for providing Photoluminescence facility and technical discussions; Director, DMSRDE, Kanpur, for SEM and EDAX of samples, and Dr HH Kumar, ARDE, Pune for piezoelectric characterization of samples.
K [W/mK]
0.64 0.62 0.60
References
0.58 0.56 0.54 0.52 10
20
30
40
50
0
Temperature [ C ] Fig. 12. Variation of thermal conductivity with temperature for different ZnO loading.
fluid of 0.5% loading, which can be attributed to agglomeration of ZnO nanoparticles and Brownian motion of nanoparticles as the temperature increases. At higher temperature Acetylacetone in the solution is not able to hold the ZnO nanoparticles in dispersed condition and results in agglomeration. Also, with increase in temperature Brownion motion increases causing convection which in turn increases the effective thermal conductivity of nanofluid. 5. Conclusion ZnO nanoparticles have been successfully synthesized using zinc acetate, DMSO and KOH in Ethanol at room temperature using TEA as capping agent. The semiconducting nature of ZnO is ob-
[1] S. Monticone, R. Tufeu, A.V. Kanaev, Complex nature of the UV and visible fluorescence of colloidal ZnO nanoparticles, J. Phys. Chem. B 102 (1990) 2854– 2862. [2] E.A. Meulenkamp, Synthesis and growth of ZnO nanoparticles, J. Phys. Chem. B 102 (1998) 5566–5572. [3] Y. Liu, J. Dong, P.J. Hesketh, M. Liu, Synthesis and gas sensing properties of ZnO single crystal flakes, J. Mater. Chem. 15 (2005) 2316–2320. [4] J. Xu, Y. Chen, Y. Li, J. Shen, Gas sensing properties of ZnO nanorods prepared by hydrothermal method, J. Mater. Sci. 40 (2005) 2919–2921. [5] C. Liewhiran, S. Phanichphant, Improvement of flame-made ZnO nanoparticulate thick film morphology for ethanol Sensing, Sensors 7 (2007) 650–675. [6] A. Glot, E.Di. Bartolomeo, E. Traversa, Dependence of ZnO/ZnO contact impedance on frequency and air humidity, Phys. Chem. Solid State 3 (3) (2002) 423–428. [7] B. Baruwati, D.K. Kumar, S.V. Manorama, Hydrothermal synthesis of highly crystalline ZnO nanoparticles: a competitive sensor for LPG and EtOH, Sens. Actuators, B 119 (2006) 676–682. [8] H. Xu, X. Liu, Cui D. Li, M. Jiang, A novel method for improving the performance of ZnO gas sensors, Sens. Actuators, B 114 (2006) 301–307. [9] B.B. Rao, Zinc oxide ceramic semi-conductor gas sensor for ethanol vapour, Mater. Chem. Phys. 64 (2000) 62–65. [10] C. Xiangfeng, J. Dongli, A.B. Djurišic, Y.H. Leung, Gas-sensing properties of thick film based on ZnO nano-tetrapods, Chem. Phys. Lett. 401 (2005) 426–429. [11] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, M.A, 2005. [12] A.K. Singh, V.S. Raykar, Microwave synthesis of silver Nanofluids with polyvinylpyrrolidone (PVP) and their transport properties, Colloid Polym. Sci. 286 (2008) 1667–1673. [13] A.K. Singh, V. Viswanath, V.C. Janu, Synthesis, effect of capping agents, structural, optical and photoluminescence properties of ZnO nanoparticles, J. Lumin. 129 (2009) 874–878.
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Synthesis, characterization, electrical and sensing properties of ZnO nanoparticles AK Singh * Defence Institute of Advanced Technology, Girinagar, Pune 411025, India
a r t i c l e
i n f o
Article history: Received 11 December 2009 Accepted 2 February 2010
Keywords: ZnO Nanofluids Photoluminescence UV–VIS Impedance spectroscopy Thermal conductivity
a b s t r a c t The present study investigates the electrical and sensing properties of mechanically compacted pellets of nanosized zinc oxide powders synthesized by chemical method at room temperature in alcohol base using Triethanolamine (TEA) as capping agent. Synthesized ZnO particles has been characterized for its optical, structural, morphological properties using UV–VIS spectrophotometer, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The ZnO particles have hexagonal wurtzite structure and the particles are of 20–30 nm in size. The electrical properties of the prepared material have been investigated with Impedance Spectroscopy at different temperatures and frequencies and other laboratory setup. Resistivity, I–V curves, AC impedance of ZnO nanoparticles pellets with temperature was investigated and response was compared with commercial ZnO. Piezoelectric and oxygen sensing property of ZnO were also examined. Dynamic hysteresis of sintered ZnO pellet using axis ACCT TF analyzer 2000HS did not show polarization retention by sample. Oxygen sensing of ZnO pellet has been investigated for different concentrations of oxygen for the temperature range of 200–350 °C. The decrease of the current flow through the ZnO pellet with increasing oxygen concentration indicates the application of ZnO in oxygen sensing. The prepared ZnO particles were also used for preparing nanofluids of different concentrations and were characterized by measuring thermal conductivity using hot wire method which shows sigmoidal behavior over a temperature range of 10–50 °C. Ó 2010 The Society of Powder Technology Japan.. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction In the past few years, intense research activities have been devoted to the synthesis, structural characterization and investigation of physical and other properties of nanostructured materials. These nano materials has also been used in preparing Nanofluids (colloidal suspensions of nanoparticles) for efficient heat transfer applications. Zinc oxide NPs exhibits remarkable physical properties that make metal oxide nanoparticles strong candidates in nanofluids and other applications. ZnO, a key technological material, has received wide attention due to its specific chemical, electrical, surface and microstructural properties. ZnO being direct band gap semiconductor has high exciton binding energy (60 meV) would allow excitonic transitions at room temperature, meaning high radiative recombination efficiency for spontaneous emission as well as lower threshold voltage for emission. ZnO is considered as a substitute to GaN and ITO, as transparent conducting oxide. It has high electrochemical stability and control over resistance (103 to 105 O). The microstructural and physical properties of ZnO can be modified by introducing changes into the pro-
* Tel.: +91 20 24304173; fax: +91 20 24389411. E-mail addresses: [email protected], [email protected]
cedure of its chemical synthesis. The studies for nanosized ZnO powders have been conducted due to their size-dependent electronic and optical properties, which offer possibilities for microelectronic devices [1,2]. As one of the major materials for solid state gas sensor, bulk and thin films of ZnO have been proposed for its sensing applications. ZnO sensor elements have been fabricated in various forms including single crystal [3,4] sinter pellet [5–7] thin film [8,9] and thick film [10]. The preparation methods of nanosized metal oxide particles have been extensively studied for the precise control of the morphology at the nanometric scale. In this study, ZnO nanoparticles are synthesized by wet chemical method, under ambient atmosphere at room temperature which is a promising option for large scale production. Also, properties of ZnO can be tailored other than the advantage of range of precursors, temperature, time, pH. etc. To restrict the growth of the particles, TEA has been used as capping agent. Synthesized ZnO powder has been used to prepare a pellet. Mechanically compacted pellet has been studied for electrical, piezoelectric and oxygen gas sensing properties. Effect of annealing in air atmosphere was also studied on the electrical properties of ZnO nanomaterial. Prepared ZnO response has been studied for three different concentrations (100 ml, 150 ml and 200 ml) of oxygen in the temperature range of 200–350 °C. Synthesized ZnO has also been used for preparing nanofluids and have been characterized
0921-8831/$ - see front matter Ó 2010 The Society of Powder Technology Japan.. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2010.02.002
610
AK Singh / Advanced Powder Technology 21 (2010) 609–613
for thermal behavior under low concentrations (loading) of 0.075, 0.25 and 0.5%. 2. Synthesis methodology Zincacetatedihydrate, Zn(Ac)22H2O dissolved in Dimethylsulphoxide (DMSO) and Potassiumhydroxide (KOH) dissolved in Ethanol is used to synthesize ZnO and then Triethanolamine (TEA) is added as capping agent. 0.2 M Zincacetatedihydrate, Zn(Ac)22H2O is prepared in 20 ml Dimethylsulpoxide (DMSO) and stirred till it is completely dissolved and forms a clear solution. 1.2 M solution of Potassiumhydroxide in 10 ml Ethanol is added to the solution of Zincacetate drop wise till the solution becomes milky white under slow stirring condition until it is uniformly white. 0.12 ml TEA is added and stirring is continued for proper mixing of the capping agent in the milky white solution. The precipitate is separated by centrifugation and then washed at least three times and then naturally dried to get fine white powder. The dried synthesized ZnO nanoparticles are made into pellet using hydraulic press under 7500 psi. The pellet was silver painted using silver conducting paint. The dimensions of the pellet are 0.62 mm thickness and 13.11 mm diameter. All the chemicals used were purchased from the leading suppliers without further purification. Synthesized ZnO has also been used for preparing stable nanofluids in DI water of different concentrations. Acetylacetone has been used for stabilization of ZnO nanoparticles in DI water.
Fig. 1. UV–Vis absorption spectra of ZnO nanoparticles.
3. Measurements and analysis UV–VIS absorption study has been carried out using Nanodrop 1000 spectrophotometer by dispersing nanoparticles in methanol. Photoluminescence (PL) measurements have been done at room temperature using Perkin–Elmer (LS-55) Luminescence Spectrophotometer. Analysis of crystal structure, crystal size and morphology has been carried out by XRD and SEM. Stoichiometric analysis has been done using EDAX. From Scherrer’s formula [11] using FWHM of XRD patterns, size of particles is calculated
0:9k D¼ b cos h
ð1Þ
where k is wave length of X-ray source, b is full width at half maximum in radians, h is Bragg’s diffraction angle. Impedance spectroscopy measurements were performed using Broadband Dielectric Spectrometer of Novocontrol Model (Alpha ATB). Other electrical and gas sensing properties has been measured by using laboratory setup having provision for four probe resistance measurement, controlled heating of pellet and flow of gas. The gas adsorbs on the ZnO sensing layer and causes a change in resistance depending on the gas concentration. Piezoelectric properties of the sample has been analyzed using axis ACCT TF analyzer 2000HS by dynamic hysteresis of sintered ZnO pellet after polling. Hotwire method [12] has been used to measure thermal conductivity of nanofluid.
Fig. 2. Room temperature PL of ZnO nanoparticles.
oxygen and zinc interstials), a weak peak at 519 nm (green emission attributed to singly ionized oxygen vacancies) and low intensity peaks at 412 nm (attributed to zinc vacancies) and 393 nm (UV emission attributed to free exciton recombination) [13]. 4.2. Particle size and structural properties Fig. 3 shows SEM images of ZnO capped with 0.12 ml of TEA. Observation of figure shows that ZnO particles are spherical in nat-
4. Results and discussion 4.1. Optical properties Fig. 1 shows the UV absorption spectra of ZnO capped with TEA having absorption peak at 360 nm which is blue shifted to, from 375 nm absorption wavelength (expected for bulk ZnO having direct band gap of 3.3 eV) and showing sharp excitonic peaks at 360 nm. Fig. 2 shows the room temperature photoluminescence spectra of ZnO excited at 322 nm for ZnO nanoparticles after annealing for 03 h at 673 K. A strong peak has been observed at 471.5 nm (blue emission attributed to intrinsic defects such as
Fig. 3. SEM of ZnO nanoparticles.
AK Singh / Advanced Powder Technology 21 (2010) 609–613
Fig. 4. X-ray diffraction pattern of ZnO nanoparticles.
ure and size of the particles is in the range of 40–50 nm. Fig. 4 shows the XRD pattern of TEA capped ZnO nanoparticle. The XRD pattern shows broadening of the peaks indicating ultra fine nature of the crystallites. The peaks assigned to diffractions from various planes correspond to hexagonal structure of ZnO. The crystallite size estimated for the same sample from Scherrer’s formula using FWHM [11] from XRD patterns is of the order of 12–20 nm from different peaks, giving average size of 15.4 nm from all the peaks. The crystallite sizes obtained indirectly by XRD and directly by electron microscopy may not always exactly match. Whereas the electron microscopy can be used to determine almost any crystallite size, X-ray peak broadening methods give the most correct results for crystallite sizes in the range of 10–100 nm. 4.3. Electrical properties Fig. 5 shows the resistivity of ZnO pellet with temperature in the range of 85–335 °C when biased with 20 V. Figure shows decrease in resistivity with temperature which is a characteristic of the semiconductor. There are two distinct zones of conduction i.e. electronic conduction and ionic conduction. Electronic conduction is referred to as n- or p-hopping-type depending on whether the principal charge carriers are electrons or holes, respectively. This is present at low temperatures and the second conduction region which is prevalent at high temperatures is the ionic conduction. Ionic conduction takes place when ions can hop from site to
Fig. 5. Variation of resistivity of pellet with temperature prepared using ZnO nanoparticles.
611
Fig. 6. I–V characteristics of nano ZnO pellet before and after annealing at room temperature.
site within a crystal lattice as a result of thermal activation. Observation of Fig. 6 shows that at lower temperatures the resistivity of the annealed (400 °C for 3 h) ZnO is much lower than that of the un-annealed ZnO. This is because the un-annealed ZnO is much more porous than annealed ZnO which results higher resistivity. In the annealed ZnO the boundaries of the grain get diffused thereby resulting in reduction in the size of the pores. At higher temperatures the resistivity of the annealed ZnO becomes stable. This is because of lack of vacant oxygen ions in annealed ZnO, which have been removed in the annealing process. Fig. 6 shows the I–V characteristics of un-annealed and annealed sample at constant temperature (27 °C). It is observed that the breakdown voltage of annealed ZnO is much lower than that of un-annealed ZnO. It is because of the lower porosity of the annealed sample which results in the better conductivity. Fig. 7 shows the variation of impedance with temperature (30– 300 °C) for different frequencies (1–18 MHz). It has been found that the impedance at lower frequency is higher than that at the higher frequency with sharp decrease in impedance at 275 °C this is due to sharp increase in capacitance at 275 °C (variation of capacitance with temperature not shown here). The impedance of the commercial ZnO (particle size of 2–5 lm) is higher than that of ZnO nanoparticles as shown in Fig. 8 for 100 kHz frequency. The peak for ZnO nano pellet is sharp and well formed.
Fig. 7. Variation of impedance of nano ZnO pellet with temperature and at different frequencies.
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AK Singh / Advanced Powder Technology 21 (2010) 609–613
The permittivity of a material describes an AC signal transmission speed and dielectric material capacitance. The measurement are taken for both nano ZnO and Commercial ZnO and compared at 1 kHz frequency as shown in Fig. 9. It is found that the permittivity of the nano size ZnO is much higher than that of commercial ZnO, because of small particle size i.e. higher porosity compared to commercial ZnO. Therefore the nano ZnO is much better dielectric material than commercially available ZnO. 4.4. Piezoelectric properties
Fig. 8. Variation of impedance of commercial ZnO nanoparticle ZnO with temperature at different frequencies.
Piezoelectric behavior of ZnO has been analyzed after sintering the sample at 800 °C. Poling was done at 90 °C, 500 V for 20 min, then 10 min at 1 KV beyond which current conduction occured and poling could not be carried out. Fig. 10 shows the polarization voltage hysteresis curve of sintered ZnO pallet upto voltage application of 1100 V indicating the absence of polarization retention by sample. 4.5. Gas sensing properties Fig. 11 shows variation of current with temperature for different concentration of gas i.e. 0.2, 0.4 and 0.8 volume percentage and is well below the explosion limit. Here a constant voltage of 20 V is applied to the pellet and the current is monitored. The gas in addition to the surface adsorbed oxygen, deplete the electrons from the conduction bands of ZnO and thus the currents reducing with the increase in the concentration of the oxygen. The decrease of the current flow through the ZnO pallet with increasing oxygen concentration indicates the application of ZnO in oxygen sensing. 4.6. Thermal conductivity of ZnO nanofluids
Fig. 9. Variation of permittivity of commercial ZnO and nanoparticle ZnO.
Fig. 12 shows the variation of thermal conductivity(K) of ZnO nanofluids for different concentrations in the temperature range of 10–40 °C. Thermal conductivity exhibits slow increase for 0.075% and 0.25% loading nanofluids while 0.5% nanofluid shows relatively fast increase in K for the given temperature range. Beyond 30 °C temperature, K is found to increase abruptly for nano-
Fig. 10. Polarization–voltage hysteresis curve for nano ZnO pellet after sintering.
AK Singh / Advanced Powder Technology 21 (2010) 609–613
Fig. 11. Variation of current with temperature for different concentrations of oxygen gas.
613
served from the measurements of resistivity with temperature. The semi conductivity in ZnO must be due to large oxygen deficiency in it. Then behavior of the resistivity with change in temperature was studied and compared with the annealed (400 °C) ZnO and found that the at lower temperature the resistivity of annealed ZnO is lower than the un-annealed one because porosity is less in the latter which results in the better conductivity. At higher temperatures the resistivity of annealed one is higher because of lack of singly ionized oxygen vacancies. Using Impedance Spectroscopy (IS) the variation of impedance, permittivity and loss tangent is measured with change in temperature and frequencies; and then compared with the commercial ZnO which is in micro sizes. We found that impedance of commercial ZnO is far higher than that nano ZnO. Permittivity of nano ZnO is higher than commercial one therefore latter can act as good dielectric. The loss tangent of the nano ZnO is less than that of commercial ZnO, hence can be used as transmitter material. There is substantial increase in thermal conductivity which makes nanofluids attractive as cooling fluids. Acknowledgements
0.68
0.075% ZnO 0.250% ZnO 0.500% ZnO
0.66
Authors are thankful to Vice-Chancellor, DIAT, Pune, for granting permission to publish this work. Authors would like to thank Prof. SK Kulkarni, Department of Physics, University of Pune, for providing Photoluminescence facility and technical discussions; Director, DMSRDE, Kanpur, for SEM and EDAX of samples, and Dr HH Kumar, ARDE, Pune for piezoelectric characterization of samples.
K [W/mK]
0.64 0.62 0.60
References
0.58 0.56 0.54 0.52 10
20
30
40
50
0
Temperature [ C ] Fig. 12. Variation of thermal conductivity with temperature for different ZnO loading.
fluid of 0.5% loading, which can be attributed to agglomeration of ZnO nanoparticles and Brownian motion of nanoparticles as the temperature increases. At higher temperature Acetylacetone in the solution is not able to hold the ZnO nanoparticles in dispersed condition and results in agglomeration. Also, with increase in temperature Brownion motion increases causing convection which in turn increases the effective thermal conductivity of nanofluid. 5. Conclusion ZnO nanoparticles have been successfully synthesized using zinc acetate, DMSO and KOH in Ethanol at room temperature using TEA as capping agent. The semiconducting nature of ZnO is ob-
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