ZnO nanocrystalline powder synthesized by ultrasonic mist-chemical vapour deposition

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Optical Materials 30 (2008) 1316–1322 www.elsevier.com/locate/optmat

ZnO nanocrystalline powder synthesized by ultrasonic mist-chemical vapour deposition Preetam Singh, Ashvani Kumar, Deepak, Davinder Kaur * Center of Nanotechnology, Department of Physics, Indian Institute of Technology Roorkee, Roorkee, India Received 8 March 2007; received in revised form 22 May 2007; accepted 19 June 2007 Available online 13 August 2007

Abstract In this paper, we report on the synthesis and characterization of ZnO nanocrystalline powder grown by ultrasonic mist-chemical vapour deposition (UM-CVD) which is a promising method for large-area deposition at low temperatures taking into account of its simplicity, inexpensiveness and safety. The morphology and crystallite size of the ZnO nanopowder characterized by FESEM and TEM revealed that the powder consisted of the mixture of nanoparticles with particle size of 50–100 nm. The XRD results indicated that the synthesized ZnO powder had the pure wurtzite structure with lattice parameters a and c of 3.244 and 5.297 nm, and c/a ratio of 1.6, respectively. High temperature XRD studies of ZnO nanopowder showed that the crystallite size increased with increasing temperature with a systematic shift in peak positions towards lower 2h values due to change in lattice parameters. Temperature dependence of the lattice constants shows linear increase in their values. Diffraction patterns of ZnO nanopowder obtained from TEM were also in agreement with the XRD results. The synthesized powder exhibited the estimated direct band gap (Eg) of 3.43 eV. The optical band gap calculated from Tauc’s relation and the band gap calculated from the particle size inferred from XRD were in agreement with each other. Ó 2007 Elsevier B.V. All rights reserved. Keywords: ZnO nanopowder; Ultrasonic mist-chemical vapour deposition; High temperature XRD

1. Introduction ZnO has attracted much attention from last few decades due to its wide variety of applications [1]. Combine with the high conductivity that can be achieved by doping, this leads to applications in surface acoustic wave devices and transparent conducting electrodes. It is a strong piezoelectric [2], in which the piezoelectric properties can change the characteristics of potential energy barriers at interfaces. The resulting piezoresistance is exploited in metal oxide varistors, which can dissipate large amounts of power in short response times and are commonly found as electrical surge protectors [3]. Semiconductor nanostructures are promising candidates for future electronic and photonic devices [4,5]. ZnO powders have been applied for varistors and

*

Corresponding author. Tel.: +91 1332 285407; fax: +91 1332 273560. E-mail address: [email protected] (D. Kaur).

0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.06.012

other functional devices, and also can be used as enforcement phase, wear resistant phase and anti-sliding phase in composites in consequence of its high elastic modulus and strength [6]. As n type semiconducting material ZnO powders can absorb infrared electromagnetic wave in the range of 2.45–18 GHz. Due to the various attractive properties and potential applications of ZnO, there have been much attention paid on the fabrication of ZnO films and nanopowder in recent years. A variety of techniques have been used to fabricate ZnO nanostructures like dc magnetron sputtering [7], sol–gel [8], spray pyrolysis [9] metal organic chemical vapour deposition [10] . Because nanostructure ZnO has many applications in industrial area so the preparation of ZnO powder is an important issue. Different method for the preparation of ZnO powder have been investigated before [11–13]. Among them are mechanochemical processing, sonochemical processing, microemulsion, precipitation route, etc. However, these methods are not economically

P. Singh et al. / Optical Materials 30 (2008) 1316–1322

feasible to be applied in a nanopowder mass production process. For this purpose spray CVD [14] is a simple and inexpensive deposition method for the deposition of nanopowder. It is a combination of spray pyrolysis and CVD, allowing the deposition to occur from the vapour phase using a spraying solution. The precursor droplets were introduced from bottom upwards by spraying through a well-defined temperature profile resulting in the formation of a nanocrystalline powder. The quality of powder has been identified to be better than that deposited by spray pyrolysis, yet the system is inexpensive and simple. Earlier we have reported the successful fabrication of superconducting Josephson junctions devices [15] and HTSC coated conductors [16] prepared via ultrasonic spray pyrolysis technique. In the present study, we have developed modified, much flexible ultrasonic mist-chemical vapour deposition system (UM-CVD) based on similar spray-CVD dynamics [34] to grow ZnO nanocrystalline powder with particle size in the range 50–100 nm. In situ high temperature XRD studies were performed in the temperature range (room temperature–1000 °C) to study the effect of temperature on phase, crystallite size and lattice constants of the ZnO nanocrystalline powder. 2. Experimental To synthesize ZnO nanopowder we have developed cost effective ultrasonic mist-chemical vapour deposition method. The set up was composed of three zones, i.e. ultrasonic spray zone, evaporation zone and deposition zone. Each zone is important for controlling deposition. The first was the ultrasonic spray zone which consists of the mist generating system of liquid source with ultrasonic atomizer and misted droplet carrying system with air carrier gas. The second was the heating zone in which the misted droplet was pyrolyzed in a preheated reactor kept at 400 °C. The last and the third zone was for the trapping of produced powder. The whole system was enclosed in an exhaust hood. To prepare ZnO nanopowder zinc nitrate (Zn(NO3)2  6H2O) (purity > 99% , Sigma–Aldrich, USA) was used as precursor. The solution was prepared from dissolution of metal nitrate hydrate in pure water to a concentration of 0.1 M. The aqueous zinc nitrate solution was delivered into the reactor by liquid atomization. The atomized droplets containing the precursor were passed through a reactor and collected on a special geometry. The chemical solution was atomized into the stream of the fine droplets via ultrasonic nebulizer operated to an atomizing frequency of 1.7 MHz. Nitrates precursor solution was poured in to the vessel from inlet side. The aerosol was generated from the vibration of the transducer. The average diameter of the misted droplet can be approximately calculated from an expression given by Lang [17] 

8pc Dd ¼ 0:34 qf 2

13

1317

where Dd is the droplet diameter, c is the solution surface tension (72.9  103 N/m), q is the solution density (1030.3 kg/m3), and f is the applied ultrasonic frequency (1.7 MHz). The diameter of the misted droplets was calculated using the above parameters and was found to be 2.8 lm. The produced droplets were then carried into the chamber by air carrier gas with flow rate of 1 dm3/min and pyrolyzed at a predetermined temperature of 400 °C. As a result, zinc oxide powder was formed. The orientation and crystallinity of the powder was studied using Bruker D-8 advanced diffractrometer with high temperature attachment in h–2h geometry. The X-ray source used for ˚ ) with XRD measurements was Cu Ka radiation (1.54 A the scanning speed 2h = 1°/min in the angle range between 10° and 60°. To obtain a profile fitting with good signal, a polycrystalline Si powder was used for instrumental correction. The high temperature stage allows samples to be measured at tightly controlled temperatures from room temperature to 1600 °C in open air, under vacuum, or in a purge gas. The microstructure was studied using field emission scanning electron microscope (FESEM). The micrographs and diffraction pattern of nanopowder was studied using transmission electron microscope (Philips EM400). Perkin–Elmer Lambda 25 UV–Visible spectrometer was used to study the optical properties of nanopowder. 3. Results and discussion For high temperature XRD studies, ZnO nanopowder mixed with zapon lacquer was applied on a platinum strip, which was used as a sample holder cum heater for high temperature XRD. The powder was heated in vacuum in the temperature range varying from room temperature to 1000 °C at the rate of 10 °C/min. Fig. 1a shows the XRD pattern taken at room temperature and Fig. 1b shows the results of in situ high temperature X-ray diffraction for ZnO nanopowder at increasing temperature from 100 °C to 1000 °C. It has been observed that the XRD peak broadening decreases with increase of temperature. The observed reflections at room temperature (Fig. 1a) were (1 0 0), (0 0 2) and (1 0 1) reflection which were similar to the observed reflections in ZnO bulk.The intensity of these reflections increases with rise in temperature as shown in Fig. 1b. Mondelaers et al. [18] have observed these reflections in case of their HTXRD pattern of ZnO nanopowder prepared via aqueous carboxylate gelate route only at temperature above 400 °C as the precursor was amorphous below 400 °C. This confirms that growth kinetics of an oxide product is precursor dependent [19]. In our case precursor was zinc nitrate which get decomposed very easily in reactor itself at temperature below 200 °C and we were able to observe ZnO reflections in HTXRD spectra even at room temperature. No phase change was observed in XRD pattern even after in situ heating of the powder to 1000 °C (Fig. 1b). At temperature above 700 °C, the reflection due to Zn2PtO4 phase was detected. It is well known that the lattice parameters are temperature dependent, i.e. an increase in temperature leads to

1318

P. Singh et al. / Optical Materials 30 (2008) 1316–1322

1 (100)

d 21 0 1

RT

(102)

400

(110)

600

d¼

200

30

40

50

60

Pt (111)

(101)

4

*

1000°C

Intensity (a.u.)

*

2

(002)

* Zn PtO

(100)

2θ (degree)

b

900°C 800°C 700°C 600°C 500°C 400 °C 300° C 200° C

100° C

26

28

30

¼

  4 1 1 þ 2 3 a2 c

where d is the interplaner distance, aand c are q the ffiffi lattice parameters (being hexagonal structure ac ¼ 83Þ. The XRD spectra were used to calculate the size of the ZnO nanoparticles with increase in temperature using Scherrer’s formula [22]:

(002)

800

Intensity (a.u.)

(101)

a

32

34

36

38

40

42

2θ (degree) Fig. 1. (a) Room temperature XRD pattern of ZnO nanopowder and (b) high temperature X-ray diffraction (HT-XRD) pattern of the ZnO nanopowder at various temperatures.

expansion of the lattice [20,21]. In case of ZnO nanopowder, it was observed that the particle size and lattice parameters increased with increasing temperature as mentioned in Table 1. The increase of the lattice parameter of ZnO nanopowder with the increase in temperature was calculated using the equation,

Table 1 Measured properties of the ZnO nanopowder

0:9k B cos hB

˚ ), where k, hB and B are the X-ray wavelength (1.54056 A Bragg diffraction angle and line width at half maximum, respectively. We have also incorporated the instrumental broadening of 0.1° for the size calculations. The XRD results indicated that the synthesized ZnO powders had the pure wurtzite structure with lattice parameters a and c of 3.244 and 5.297 nm, respectively. The lattice parameters (‘a’ and ‘c’) and particle size as the function of temperature are shown in Fig. 2 and it was observed that there is a continuous increase in the lattice parameter and particle size with temperature as shown in Table 1. It was found that the size of ZnO nanoparticles was around 14.9 nm at 27 °C which increased to 35.7 nm when the sample was heated to 1000 °C. Carnes and Klabunde [33] have also observed the similar trend of increase in crystallite size from 3 nm to 17 nm on heating their ZnO nanocrystalline powder synthesized via sol–gel process, from room temperature to 1000 °C, respectively. According to Ostwald ripening, the increase in the particle size is due to the merging of the smaller particles into larger ones [23] and is a result of potential energy difference between small and large particles and can occur through solid state diffusion. To examine this let us consider a particle, P of diameter R0. The particle P can be changed into n smaller identical unit cells of edges, a and c without changing its volume,  3 4 R0 p ¼ nV where V 3 2 pffiffiffi 3 3 2 a c ðfor hexagonal structureÞ ¼ 2 i.e.

Temperature (°C)

Crystallite size (nm)

Lattice parameter, ˚) a (A

Lattice parameter, ˚) c (A

No. of unit cells, n

Band gap (XRD) eV

 3 4 R0 n¼ p 3 2V

27 100 200 300 400 500 600 700 800 900 1000

14.9 15.2 16.3 17.8 19.6 19.7 20.2 21.8 24.5 30.2 35.7

3.243 3.244 3.245 3.247 3.249 3.250 3.252 3.253 3.256 3.259 3.261

5.296 5.297 5.299 5.302 5.305 5.307 5.310 5.312 5.317 5.321 5.325

11,961 12,688 15,631 20,319 27,079 27,468 29,560 37,120 52,543 98,156 161,823

3.2278 3.2267 3.2232 3.2195 3.2160 3.2159 3.2151 3.2130 3.2102 3.2067 3.2048

We have observed an increase in the number of unit cells in the particle with an increase in temperature even after incorporating the increase in lattice parameter and is shown in Table 1. This size increment could possibly due to the merging of the smaller particles into larger particles as per Ostwald ripening. Nanocrystalline materials can be described as two component systems made up of discontinuous crystallites and continuous matrix of grain boundaries containing disordered material [24]. The density of grain boundaries is less

P. Singh et al. / Optical Materials 30 (2008) 1316–1322

a

3.264

36

3.260

Particle size (nm)

3.256 28 3.252 24 3.248 20 3.244 16

Particle size Lattice parameter

12 0

150

300

450

600

750

Lattice parameter 'a' (A°)

32

3.240

3.236 1050

900

Temperature ° C

b 36

5.330 5.325

Particle size (nm)

5.320 28

5.315 5.310

24

5.305 20 5.300 16

Particle size Lattice parameter

12 0

150

300

450

600

750

900

Lattice parameter 'c' (A°)

32

5.295

5.290 1050

Temperature °C Fig. 2. (a) Variation of particle size and lattice parameter ‘a’ and (b) variation of particle size and lattice parameter ‘c’ with temperature.

than that of perfect crystallite due to excess free volume of grain boundaries. The excess free volume associated with the grain boundaries can be calculated using the following expression [25]:

mately 50–100 nm. Fig. 5 shows the atomic force microscope (AFM) image of ZnO nanopowder. It revealed the spherical grains with average size of approximately 60 nm. The morphology and structure of powder was further investigated by TEM. Bright field TEM images and the corresponding diffraction pattern for ZnO nanopowder were shown in Fig. 6. The sample was scanned in all zones before the picture was taken. The micrographs revealed that the particles were nearly spherical in shape. The diffraction pattern shows spotty ring pattern without any additional diffraction spots and rings of secondary phases revealing their highly crystalline ZnO wurtzite structure. Three fringe patterns were observed with plane distance ˚ in the electron diffraction pattern of 2.79, 2.58 and 2.44 A which corresponds to 1 0 0, 0 0 2 and 1 0 1 planes of pure wurtzite hexagonal structure of ZnO. The grain growth kinetics of ZnO nanopowder was studied using the following phenomenological kinetic grain growth equation [32]: S n  S n0 ¼ k 0 t expðEa =RT Þ where S is an average grain size at the time t, S0 is the grain size at time t = 0, n is the kinetic grain growth exponent, Ea is the apparent activation energy for grain growth process, k0 is the pre-exponential constant, R is the gas constant, and T is the absolute temperature. Under isothermal conditions, this relation can be written as S n  S n0 ¼ kt Using this relation, the value of kinetic grain growth exponent n can be determined from the slope of log(S) versus log(t) plot. Fig. 7 shows the relation of logarithm of mean particle size as a function of logarithm of time for our ZnO powder samples, which were heated at the rate of 10 °C/ min up to 700 °C and than were kept isothermal for time varying from 1 to 120 min. A linear trend has been observed in Fig. 7, as in accordance with the above mentioned 6

2

ðL þ d=2Þ  L2 L2

where L is the crystallite size and d is the mean width of the grain boundaries. In most of the prior calculations of the excess free volume, the width of the grain boundary has been assumed to be constant (d = 1 nm), independent of the grain size [26]. Fig. 3 shows the variation of free volume calculated in our case of ZnO nanopowder with crystallite size. It was observed that the grain boundaries free volume decreases with increase in crystallite size which is in confirmation with other reported results [27]. As with decrease in crystallite size the number of grain boundaries increases and hence there is a increase in grain boundary free volume. The morphology of ZnO nanopowder as revealed by FESEM (Fig. 4) showed nanoparticles of size approxi-

5

4

ΔVF %

DV F ¼

1319

3

2

1 16

20

24

28

32

36

Crystallite size (nm) Fig. 3. Variation of excess free volume of grain boundaries with crystallite size.

1320

P. Singh et al. / Optical Materials 30 (2008) 1316–1322

Fig. 4. FESEM image of ZnO nanopowder.

Fig. 6. TEM image and corresponding diffraction pattern of ZnO nanopowder.

temperature range 700–1000 °C and other two straight lines lie between 300 °C and 600 °C and below 300 °C, respectively. This indicates that different kind of growth process occur in the investigated temperature range. The Fig. 5. AFM image of ZnO nanopowder. 1.65

log S ¼ ð1=nÞ½logðk 0 Þ  0:434Ea =RT  The value of apparent activation energy can be obtained from the slope of Arrhenius plot of log S versus 1/T curve. Fig. 8 shows the plot of log(S) versus (1/T) for our ZnO powder. A linear correlation has not been observed in the complete temperature range 100–1000 °C. Three linear regimes can be empirically distinguished over the whole temperature range. The highest slopes are obtained for the

1.60 1.55 1.50

log S

empirical grain growth equation. The value of n is found to be 7.4 which is in agreement with other reported results [18,19]. For a constant value of n, the grain growth equation can be expressed as

1.45 1.40 1.35 1.30 1.25 0.0

0.5

1.0

1.5

2.0

2.5

log t Fig. 7. Plot of log S versus log t in the isothermal particle growth at 700 °C.

1.6

a 80

1.5

Transmittance %

P. Singh et al. / Optical Materials 30 (2008) 1316–1322

log S

1.4

1.3

1321

60

40

20

1.2

0 300

1.1 0.5

1.0

1.5

2.0

2.5

3.0

400

500

-3

600

700

800

900

Wavelength (nm)

1/T X 10 (1/K) Fig. 8. Plot of log S versus reciprocal temperature (1/T).

b

ahm ¼ Aðhm  Eg Þn where a is the absorption coefficient, A is a constant, h is Planck’s constant, m is the photon frequency Eg is the optical band gap and n is 1/2 for direct semiconductor. An extrapolation of the linear region of a plot of (ahm)2 on the y-axis versus photon energy (hm) on the x-axis gives the value of the optical band gap Eg. Since Eg = hm when (ahm)2 = 0. The calculated band gap was found to be near 3.43 eV of the ZnO nanopowder, which is greater than the reported

Eg= 3.43 eV 3 (αhν)2

pecularity has also been confirmed from the variation of crystallite size of ZnO powder with temperature as shown in Fig. 2. The value of apparent activation energy as obtained from the highest slope of the plot was found to be 54 kJ/mol in the temperature range 700–1000 °C. There is a large discrepancy in the reported value of the activation energy of ZnO in the literature. Audebrand et al. [19] have reported the value of activation energy varying from 13 to 167 kJ/mol for their different ZnO powder samples prepared from different precursors. Similarly Milosevic et al. [34] have reported the value of Ea varying from 70.2 to 570.4 kJ/mol for ZnO powder heated under different heating rate in the range varying from 5 to 20 °C/min. The optical transmission spectra of the ZnO nanopowder was recorded as a function of wavelength in the wavelength range 300–900 nm as shown in Fig. 9. Transmittance spectra revealed that an average value of transmittance was about 55% in the visible range of electromagnetic radiation. The transmittance is expected to depend on several factors, such as oxygen deficiency, surface roughness, and impurity centers. The lower value of transmittance in case of ZnO nanopowder could be due to light scattering at the rough surface. Transmittance spectra also presented a sharp ultraviolet cutoff at approximately 310 nm which reflects that in case of ZnO powder the absorption edge of transmittance shifts to longer wavelength region and there is increase in the optical absorption in the UV region. In order to calculate the direct band gap of ZnO powder, we have used the Tauc’s relationship [28] as follows:

4

2

1

2.6

2.8

3.0

3.2

3.4

3.6

hν (eV) Fig. 9. (a) Optical transmittance spectra and (b) energy band gap of ZnO nanopowder.

band gap value of ZnO bulk. i.e. Eg = 3.37 eV [29]. The difference in the band gap values ranging from 3.1 to 3.3 eV is one of the curious features of the literature on ZnO. Srikant and Clarke [30] investigated the optical band gap of ZnO single crystals at room temperature using a variety of optical techniques (i.e. conventional reflection and transmission absorption measurement, spectroscopic ellipsometry, Fourier transform infrared spectroscopy and photoluminescence) and reported three different Eg values of 3.1, 3.2 and 3.3 eV. They concluded that the room temperature band gap of ZnO was 3.3 eV, whereas the reports of an apparent band gap at 3.1 and 3.2 eV were due to the existence of a valence band-donor transition at 3.15 eV which can dominate the absorption spectrum when the bulk, as distinct from the surface, of a single crystal is probed. For the sake of comparison the band gap of the nanopowder was also calculated from the following relation [31]: Eg ¼ E0g þ

h2 8lR2

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P. Singh et al. / Optical Materials 30 (2008) 1316–1322 3.230

References

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[1] M. Singhal, Mater. Res. Bull. 32 (2) (1997) 236; S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293; A.R. Raju, C.N.R. Rao, Sensor. Actuat. B 3 (1991) 305. [2] V.E. Wood, A.E. Austin, Magnetoelectric Interaction Phenomena in Crystals, Gordon and Breach, London, 1975. [3] A. Ahmin, J. Am. Ceram. Soc. 72 (1989) 369. [4] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H. Lee, Appl. Phys. Lett. 81 (2002) 3648; X. Zhao, S.C. Zhang, C. Li, B. Zheng, H. Gu, J. Mater. Synth. Proc. 5 (1997) 227. [5] C.H. Chia, T. Makino, K. Tamura, Y. Segawa, M. Kawasaki, A. Ohtomo, H. Koinuma, Appl. Phys. Lett. 82 (2003) 1848; S. Singh, N. Rama, M.S. Ramachandra Raoa, Appl. Phys. Lett. 88 (2006) 222111. [6] C.M. Lieber, United States Patent 5897945, Application No. 606892, April 27, 1999. [7] A.K. Chawla, Davinder Kaur, Ramesh Chandra, Opt. Mater. 29 (2007) 995. [8] Y.M. Kim, M. Yoon, I.W. Park, Y.J. Park, J.H. Lyou, Solid State Commun. 129 (2004) 175. [9] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [10] Y. Ohya, T. Niwa, T. Ban, Y. Takahashi, Jpn. J. Appl. Phys. 40 (2001) 297. [11] J. Ding, T. Tsuzuki, P.G. McCormick, R. Street, J. Phys. D Appl. Phys. 29 (1996) 2365. [12] P.G. McCormick, J. Ding, H. Yang, Mater. Res. 1 (1996) 85. [13] M.R. Vaezi, S.K. Sadrnezhaad, Mater. Des. 28 (2007) 515. [14] J.D. Harris, K.K. Banger, D.A. Scheiman, M.A. Smith, M.H.C. Jin, A.F. Hepp, Mater. Sci. Eng. B 98 (2003) 150. [15] Davinder Kaur, A.K. Gupta, J. Phys. D: Appl. Phys. 35 (2002) 729. [16] Ashvani Kumar, Preetam Singh, Davinder Kaur, Cryogenics 46 (2006) 749. [17] R.J. Lang, J. Acoust. Soc. Am. 43 (1962) 6. [18] D. Mondelaers, G. Vonhoyland, H. Van Den Rul, L.C. Van Poucke, Mater. Res. Bull. 37 (2002) 901. [19] N. Audebrand, J.P. Auffredic, D. Louer, Chem. Mater. 10 (1998) 2450. [20] R. Lamber, S. Wetjen, N.I. Jaeger, Phys. Rev. B 51 (1995) 10968. [21] R. Banerjee, E.A. Sperling, G.B. Thompson, H.L. Fraser, S. Bose, P. Ayyub, Appl. Phys. Lett. 82 (2003) 4250. [22] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, 1970. [23] K.K. Nanda, F.E. Kruis, H. Fissan, Phys. Rev. Lett. 89 (2002) 256103. [24] S.R. Phillpot, D. Wolf, H. Gleiter, J. Appl. Phys. 78 (1995) 847. [25] R. Banerjee, E.A. Sperling, G.B. Thompson, H.L. Fraser, S. Bose, P. Ayyub, Appl. Phys. Lett. 82 (2003) 24. [26] R.W. Siegel, in: R.W. Cahn, P. Hassen, E.J. Krammer (Eds.), Processing of Metals and Alloys, VCH, Weinheim, Germany, 1991, p. 583. [27] P.P. Chattopadhyay, P.M.G. Nambissan, S.K. Pabi, I. Manna, Phys. Rev. B 63 (2001) 054107. [28] J. Tauc (Ed.), Amorphous and Liquid Semiconductor, Plenium Press, New York, 1974, p. 159. [29] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [30] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [31] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [32] R.H. Doremus, Rates of Phase Transformations, Harcourt, Brace Jovanovich, Orland, 1985. [33] C.L. Carnes, K.J. Klabunde, Langmuir 16 (2000) 3764. [34] Z.V. Marinkovic, L. Mancic, O. Milosevic, J. Euro. Ceram. Soc. 24 (2004) 1929; L. Mancic, Z. Marinkovic, O. Milosevic, J. Mining Metall. B 38 (2002) 179.

Eg(eV)

3.220 3.215 3.210 3.205 0

200

400

600

800

1000

Temperature (°C) Fig. 10. Variation of band gap with temperature.

where E0g was the energy band gap for bulk material, R was the radius of the particles calculated from XRD data and 1/l = 1/me + 1/mh (me and mh being the electron and hole effective masses, respectively). Here the reduced mass of the exciton was 0.242m0, where m0 is the electron mass. Thus we obtain Eg ¼ E0g þ 1:545R2 ðnmÞ. The values calculated from the above given formula were found to vary from 3.226 eV to 3.204 eV with increase in temperature from 100 °C to 1000 °C and are shown in Fig. 10. This increase in the value of band gap was attributed to the increase in the particle size with temperature. It was believed that this size increment is due to merging of small grains in to bigger ones as per Ostwald ripening. 4. Conclusion In summary, we have developed ultrasonic mist-chemical vapour deposition method to prepare ZnO nanocrystalline powder. The technique is simple and inexpensive and is useful for industrial applications of ZnO nanopowder. Further we have studied the structural properties of ZnO nanopowder in situ as a function of temperature using high temperature XRD and TEM. From the XRD and TEM analysis, it was confirmed that the resultant particles were of ZnO with wurtzite structure. High temperature XRD also confirmed that the ZnO nanoparticles are stable not only at room temperature but also at high temperature of 1000 °C. We have also studied the variation in lattice parameters with temperature and have observed a linear increase in lattice parameters with temperature along with increase in the size of grains. The size increment of the particles with temperature was explained in terms of Ostwald ripening. The optical band gap of the ZnO nanopowder calculated from Tauc relation was in agreement with the band gap calculated from XRD. Acknowledgement This work was supported by DST, India under the scheme Nanoscience and Technology Initiatives (NSTI) with reference no. DST 238.