Probing the surface states in nano ZnO powder synthesized by sonication method: Photo and thermo-luminescence studies

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Probing the surface states in nano ZnO powder synthesized by sonication method: Photo and thermo-luminescence studies Dojalisa Sahu a, B.S. Acharya a,n, B.P. Bag a, Th. Basanta Singh b, R.K. Gartia c a

Institute of Minerals & Materials Technology, Bhubaneswar Luminescence Dating Laboratory, Manipur University, Imphal c Department of Physics, Manipur University, Imphal b

a r t i c l e in fo

abstract

Article history: Received 30 September 2009 Received in revised form 19 February 2010 Accepted 22 February 2010 Available online 15 March 2010

Zinc oxide, a transparent conducting oxide, has been synthesized in a novel route by application of continuous and pulsed mode ultrasonication. The powders prepared in this method are found to be nano particles of 24 and 20 nm respectively. The behaviour of two powders is found to be different when X-ray diffraction, photoluminescence, and Fourier transform infrared spectra were recorded. The thermo luminescence behaviour was also found to be different. It has been possible to incorporate H ion into the system by sonication process. Surface states created by sonication process are found to influence the photo and thermo luminescence of the system. & 2010 Elsevier B.V. All rights reserved.

Keywords: Sonication Surface states Photoluminescence Thermo luminescence ZnO

1. Introduction Oxygen vacancies in metal oxide have been investigated extensively to link its role in electronic optical and electrical properties of such systems. In nano metal oxide such as zinc oxide, the role of oxygen vacancies has been debated [1]. As the native oxide has been found in wurtzite form zinc blende structure formation cannot be ruled out. The nature of intrinsic defects in ZnO still remains elusive as the interstitial Zn atom has been argued to be responsible for its n-type properties [2,3,18,19]. Simultaneously the oxygen vacancies have been found to be responsible for green luminescence in this oxide. As one moves from micro to nano system, crystalline to amorphous phase, the surface area increases and plays an important role in deciding the nano and bulk properties. Intrinsic defects or interstitials zinc at the surface has been found to be responsible for magnetic phase [4,5]. In oxide crystal of MgO, color centers like F and F + , which are formed due to oxygen vacancies, have been observed with one or two electrons [6], respectively. Nano structures of ZnO have been investigated by several workers using photo and cathode luminescence [7–9]. However, thermo luminescence studies on ZnO nano powders are scarce [10,11]. The present study intends to reveal the role of surface defects in transparent conducting oxide (TCO) material, such as ZnO. Photo and thermo-

n

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0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.02.049

luminescence studies have been carried out on ZnO powders prepared by sonication method. Recently, role of surface in ZnO nano rods has been reported by Zhao et al. [12] using time resolved photo-luminescence. Studies on nano powders synthesized by sonication process have been reported by few investigators [21–23]. Thus, it becomes important to study the effect of surface states in the photo and thermo-luminescence of nano ZnO powders synthesized by sonication method.

2. Experimental The synthesis of nano ZnO powders was performed using a sonicator bath operated under continuous and pulsed mode. In this method, 0.2 M zinc nitrate (AR grade) was dissolved in 10 mL distilled water. 2 mL of 25% NH3 (GR grade) was added drop by drop till mild precipitation occured and the 10 drops of NH3 were added to make the solution clear. This solution was subjected to continuous and pulsed sonication in a sonicator model VC-375 (Sonic & Material Inc.) operating at 112.5 W. The frequency of the sonicator was maintained at 20 kHz750 Hz. In continuous method (CS) sonication was carried out for 1 h whereas in the pulsed mode (PS) it was 2.5 h. After 15 min of sonication, the clear solution became pale white and gradually a solid suspension was formed as the sonication continued. During sonication, the temperature in the sonication bath was maintained around 70 1C. The heat generated during this process was responsible

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for release of ammonia from the solution resulting in the dissociation of ZnðNH3 Þ24 þ complex. The reaction can be enumerated as follows:

reading is always kept constant at about 10 min. The heating rates used for the TL measurement are 5 1C s  1. All data presented are after subtraction of the background emission.

Zn(NO3)2 + 2NH4OH +H2O-Zn(OH)2 +2NH4NO3 + H2O

2.1. Theoretical techniques used for analysis

Zn(OH)2 +2NH4NO3 +H2O+ 2NH4OH-[Zn(NH3)4](NO3)2 +H2O

The theoretical technique used for the analysis of the glow curves has been given in detail in the recent paper [16]. The equation governing the TL process for general order kinetics (1obr2) following Pagonis et al. [17] can be written as   E IðTÞ ¼ n0 s00 exp  kT 2 3=ð b Þ b1   ZT s00 ðb1Þ E 6 07 ð3Þ exp  0 dT 5  41 þ b kT

Pptclear solution at pH410 only Clear solution-subjected to sonication [Zn(NH3)4](NO3)2 + H2O-Zn(OH)2 + 2NH4NO3 + xNH3 -(2 x) NH4OH +H2O Zn(OH)2-ZnO+ H2O x will depend upon time of sonication and resulting temperature of the bath. Since the temperature of the bath was maintained at 70 1C no ammonia will be present. The pH of the solution was measured at the beginning and at the end of the experiment. In both the cases the starting point pH was 10.4, but the pH comes down to 9.6 at the end of continuous mode and 8.4 for pulsed mode. The powder obtained from sonication method was washed 2–3 times with distilled water and alcohol by centrifugation. The precipitate was collected in a petri dish and dried in an oven at 130 1C overnight. The powder was characterized by XRD technique for structure verification. For this purpose,XRD patterns were recorded in 20–801 2y with a step size of 0.05 02y. The diffraction lines so obtained were matched with the standard diffraction pattern (PDF Card no. 00-005-0664). The crystallite sizes of these samples were obtained by using the Scherer equation: D ¼ kl=b cos y

ð1Þ

where D is crystallite size, l is the radiation wavelength ˚ b is the peak half-width, y is the diffracting angle (1.5406 A), and k¼0.94 for spherical shape particle. Strain was calculated using the following formula:

e ¼ b=4tan y

ð2Þ

where e is the strain. Size and strain values obtained from the above formulas were compared with those obtained from the Williamson–Hall plot. The optical absorption spectra of the powder sample were taken in the wavelength range 320–800 nm by dispersing the sample in acetone. The spectra were recorded in Shimadzu UV–vis 1700 (Pharmaspec) spectrophotometer. The absorbance spectra of different concentrations were recorded for ZnO powder synthesized by continuous and pulsed modes. Photoluminescence spectra of the powder sample were recorded at room temperature ( E26 1C) with the help of a Fluoromax-4 (Perkin Elmer) spectroflurometer. The powder sample was excited at 364 nm and the emission spectra were recorded. The excitation spectra were recorded by keeping the monochromator at 560 nm. Both the emission band width and excitation band width were 5.0 nm. The Thermoluminescence (TL) measurement was performed using the commercial TL/OSL reader (model no. Risø TL/OSL reader TL-DA-15) [14,15]. The samples were irradiated at room temperature with an inbuilt beta irradiation (90Sr) source at a dose rate of 0.084 Gy s  1. The irradiated samples were read out in flowing nitrogen atmosphere. Standard clean glass filters (combination of Schott UG – 11 and BG – 39) are always installed in the reader between the sample and the photomultiplier tube (EMI 9635). These filters allow the light wavelength to range from 300 to 400 nm. This eliminates the unwanted radiations emitted from the heater. The duration between irradiation and TL

T0

where E is the activation energy or trap depth (eV); k ¼the Boltzmann’s constant (eV K  1); T ¼the absolute temperature (K), T¼T0 + bt where b ¼dT/dt, heating rate; t the time (s); T0 the temperature at time t ¼0 (K); n0 the number of trapped electrons at time t ¼0 (m  3); b the kinetic order, a parameter with values typically between 1 and 2; s0 the so-called effective preexponential factor for general order kinetics (m3(b  1)s  1);  1) , an empirical parameter acting as an ‘‘effective s00 ¼s0 n(b 0 frequency factor’’ for generalorder kinetics (in s  1).

3. Results From the X-ray diffraction data (Fig. 1 and Table 1a and 1b) it can be seen that all the reflecting planes of ZnO with wurtzite structure are present in both the powders synthesized through continuous and pulsed mode sonication. However, the relative intensities of different reflections differ considerably. Although (1 0 1) direction intensity is highest for both the samples, the relative intensities along (1 1 0), (1 0 3), (1 1 2) and (2 0 1) increase considerably for pulsed sonicated sample compared to the relative intensities of these above reflections in continuous mode sonication sample. It is interesting to note that relative intensity along (0 0 2) and (1 0 0) planes decreases in the pulsed sonicated sample. Annealing both the powders at 400 1C for 2 h makes the relative intensities comparable for continuous and

Fig. 1. XRD patterns of ZnO powder: (a) NIST standard ZnO sample, (b) ZnO powder obtained through continuous sonication, (c) ZnO powder obtained through pulsed sonication.

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Table 1a Lattice parameter, crystallite size and strain of ZnO powder synthesized by sonication method. Parameters

Continuous sonication

Pulsed sonication

A(A1) C(A1) U A/C Crystallite size from XRD(nm) 7 1 nm Crystallite size from Williamson–Hull plot(nm) Crystallite size from TEM (nm) Strain(%) from XRD

3.239 5.205 0.379 1.606 24 23

3.257 5.217 0.379 1.602 20 20

24 0.0027

20 0.034

Table 1b Relative intensity of different reflections for powder synthesized by sonication method. Reflection plane 100 002 101 102 110 103 200 112 201 004 202 104

Continuous sonication

Pulsed sonication

64 65 100 23 34 29 5 22 14 2 3 2

57 42 100 24 42 34 6 32 16 2 5 2

pulsed sample. The average crystallite size calculated from XRD data for continuous and pulsed mode samples was found to be 2471 and 20 71 nm, respectively. Assuming the shape of the particle to be spherical the crystallite size and strain have been calculated and it is observed that the strain value is higher in pulsed mode sample compared to continuous sample. Like relative intensities, the strain value becomes same after annealing the sample at 400 1C for 2 h. It is well-known that in sonication process mechanical shock waves directly exert pressure on the surface of the nano particles. Cavitations are normally accepted as one mechanism, where nano and micro size bubble or cavities are generated at several hundreds or thousands of atmosphere that create local heating with temperatures as high as 4000–5000 K. Such local heating causes the particles to collapse and aggregate with a smooth surface. During such localized high temperature and pressure it is expected that Zn blende structure may be formed along with wurtzite structure. This leads to increase in relative intensities in pulsed sample. From the strain value one can see that the structure is getting more stressed, which results in more strain in the sample. Simultaneously, the crystallite size is less and the relative intensities increase along different directions, which hint on the formation of Zn blende structure in the pulsed mode sample. The behaviour of two powders when dissolved in acetone is quite interesting. This has been depicted in Fig. 2. It is observed that dispersing the same amount of ZnO powder in the acetone, the optical absorption spectra are quite different for two samples. For continuous mode sample the absorbance is quite high compared to pulsed sample. To see the effect of concentration on the optical absorbance of ZnO powders, different amounts of ZnO powder dispersed in acetone for continuous and pulse mode were measured. This has been shown in Fig. 2A and B. For powder synthesized by pulsed mode, the optical absorbance increases linearly in the concentration range studied in this investigation,

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but for powder synthesized by continuous mode it increases sharply and finally decreases to initial value. It was also observed that even after dispersing the powder in acetone for sometime (i.e. 10 min), the precipitates settle down when absorption was measured. With the increase in time of sonication for both the sample, clear solutions with white precipitate were observed. The optical absorption spectra of such solution were taken and have been given in Fig. 2B. It can be seen that the optical absorption band around 300 nm grows with the increase in time of sonication. However, when the powders of ZnO were precipitated, filtered, dried and then their absorption spectra in acetone medium were taken, very interesting features appeared. ZnO produced through continuous process behaved differently when compared with the ZnO powder obtained by pulsed mode sonication. In pulsed mode powder a broad band around 360 nm grows with the increase in concentration of powder in acetone solution, but in powder prepared by continuous mode the absorbance was found to be more compared to pulsed mode powder (at a particular concentration) and the broad band shifts towards higher wavelength side. Maximum absorbance was observed for 0.0045 g of ZnO dispersed in 10 ml acetone whereas in pulsed mode sample this is observed for 0.0095 g of ZnO dispersed in 10 ml of acetone. The infrared spectra of the powders were recorded with the help of a Perkins Elmer FTIR spectrometer (Model-Spectrum GX) in the range between 400 and 4000 cm  1 in diffuse reflectance mode. This is shown in Fig. 3. Several bands have been observed for both the samples. The FTIR spectra of nano ZnO powder show interesting features. Besides the various OH mode and metal– oxygen bands, bands at 1505 and at 1769 cm  1 were observed for pulsed powder. In continuous mode powder bands at 1505 and 1753 cm  1 were observed with low intensity. Further, a triplet around 2103 cm  1 was observed in pulsed mode sample whereas this band was missing in continuous mode sample. The photoluminescence spectra of nano ZnO powders synthesized by sonication method are shown in Fig. 4. The spectra were recorded by exciting at 364 nm band light. The spectrum was found to consist of several bands with a very broad band in the region 500–700 nm. Thus, deconvolution of the spectra was carried out assuming the emission peaks to be Gaussian. The deconvoluted spectrum is shown in Fig. 5 and is given in a tabular form in Table 2. Although both the spectra consist of six emission bands 2.28 eV band is conspicuously absent in pulsed mode sample and a new band at 2.41 eV appears in the spectrum. Thermo-luminescence spectra of two powders were taken with the help of TL/OSL-DA-15 reader of TL/OSL using internal Sr Ug-90 b source from room temperature to 50 1C. The powders were irradiated with various doses of g range from Sr g-90 bsource and were recorded with a heating rate 5 1C/s. The recorded spectra are shown in Figs. 6 and 7. The glow curves show three peaks prominently when irradiation dose is increased. The peak positions were found to be different for continuous and pulsed mode powders. A computerized glow curve method adopted by Gartia et al. [16] was followed to find out the peak position and their activation energies (Fig. 8). This is given in Table 3. It can be seen that the low temperature glow peak grows linearly with dose for continuous mode sample whereas this saturates with dose for powder prepared by pulsed mode. The intensity of the glow peak for pulsed mode sample is 10 times higher than that of continuous mode sample.

4. Discussions Normally, zinc oxide crystallizes in two structures, i.e. wurtzite and zinc blende, at room temperature [1]. However, other high

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Fig. 2. A: Optical absorption spectra of ZnO synthesized by (a) continuous and (b) pulsed sonication in acetone medium. Curves 1,2,3,4,5, and 6 represent for 0.002, 0.0035, 0.0045, 0.0065, 0.0075 and 0.0095 g in 10 ml acetone, respectively. B: Growth of UV absorbance spectra of the solution immediately after sonication: (a) continuous mode and (b) pulsed mode.

Fig. 3. FTIR spectra of ZnO powder synthesized by (a) continuous and (b) pulsed mode sonication.

Fig. 4. Photoluminescence spectra of ZnO powder prepared by sonication method: (a) continuous mode and (b) pulsed mode.

pressure form (rock salt structure) is not stable at room temperature. Under pressure ZnO undergoes first order phase transition to form rock salt structure with atoms having 4-fold coordination. Under extreme high pressure, CsCl structure results and possesses six-fold co-ordinations. Recently Catlow et al. [7] have proposed several polymorphs of ZnO from theoretical calculations. But it is a well-established fact that wurtzite or zincite structure is the most stable structure at RT and the next stable structure is zinc blende, which can be synthesized under stringent conditions [20,24,26]. Irrespective of their structure, oxygen vacancies, interstitial and other intrinsic defects have been found to play important roles in determining its optical, electrical and catalytic properties. The luminescence in ZnO is still debatable and considerable efforts have been made to understand the origin of emission bands in this system. The method of sample preparation plays an important role in the observed surface structure. In this present investigation, the samples have been prepared by ultrasonic application. In the method of sonication,

extra force comes from mechanical shock waves directly exerted on the protrusions of the nano structure. Cavitations are normally used to explain the creation of high pressure and local heating temperature produced during sonication process [13,21–23,25]. In this process local heating and high pressure may cause the formation of zinc blend structure in ZnO powder. If such a formation is possible it will be reflected in the X-ray diffraction pattern of ZnO powder prepared by the sonication method. A comparison of XRD reflection given in Table 1a and 1b reflects this information. The relative intensities of 110, 112 and 201 reflection have been found to be more in pulsed sonication method compared to those of ZnO obtained from continuous sonication method. The intensities of 100 and 002 planes decrease in the pulsed sonicated sample. A cursory look into Table 1a and 1b indicates dilation of the lattice along a and c axes, although the c/a ratio remains constant. This dilation can occur through pulsed sonication, which stabilizes the formation of basal planes in their lowest energy configuration. Both the samples show wurtzite

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Fig. 5. A: Curve fitting of photoluminescence (emission) spectra of ZnO powder for (a) continuous and (b) pulsed mode sonication (Gaussian shape). B: Curve fitting of photoluminescence (excitation) spectra of ZnO powder for (a) continuous and (b) pulsed mode sonication (Gaussian shape)

Table 2 Spectral band positions obtained by deconvolution of emission and excitation spectra, assuming the peaks to be Gaussian. Excitation wavelength¼ 364 nm, emission wavelength ¼560 nm. Sample

Crystallite size in nm

Emission spectra in eV (nm)

Continuous mode Pulsed mode

24 20

3.00 (412) 2.99 (414)

Sample

Crystallite size in nm

Excitation spectra in eV (nm)

Continuous mode Pulsed mode

24 20

2.671 (464) 2.792 (443)

2.83 (437) 2.81 (441)

3.390 (365) 3.029 (409)

structure but the intensities of the above reflections increase, thereby indicating the formation of zinc blende structure in this process. The crystallite sizes obtained for CS and PS mode samples are 24 and 20 nm, respectively. The values obtained by the Williamson and Hall Method and transmission electron microscopy agree well with this data. The strain value obtained in PS sample is higher than CS mode sample. This indicates that pulsed mode sonication creates more strain in powders of ZnO by way of mechanical shock waves. The UV–vis spectra recorded for samples dispersed thoroughly in acetone medium shows the growth of 360 nm band to be linear (in PS mode sample) whereas in continuous mode sample it increases non-linearly and then drops to initial value. It was observed that even after 15 min of dispersion of the powder in acetone, the powders settle down when dispersal by sonication stops. This leads to quenching of absorbance in CS sample as the fine particles coagulate and settle down. In a bid to analyze the role of surface of the ZnO particles, FTIR spectra of both the powders were recorded in inert gas atmosphere. It has been reported that two surfaces, i.e. polar and nonpolar, behave differently in ZnO system. Since the powders were synthesized in water medium with NH3 as reactive reagent, it will be worthwhile to discuss the role of hydroxyl species on the chemical and optical properties of ZnO surfaces, where very little information is available. Although assignment of hydroxyl and metal–oxygen bands has been made by several workers [2,27,28,29] we have observed a band at 1505 cm  1 and this band becomes more prominent in pulsed sonicated sample. In a recent investigation by Catlow et al a band at 1507 cm  1 has been assigned to Zn–H–Zn asymmetric stretching vibration. Further,

2.65 (468) 2.66 (466)

2.28 (544) 2.41 (514)

3.291 (397) 3.179 (389)

2.17 (570) 2.19 (566)

2.00 (618) 2.02 (615)

3.383 (366)

additional band at 1777 cm  1 in both the powders can be assigned to dissociative adsorption of H2 bridging Zn–H–Zn and O–H–O species. This is in accordance with the recent result of Catlow, who has proposed two types of adsorption of hydrogen in ZnO system. Ghiotti et al. [29] have observed a band at 1475 cm  1, which evolves with time. In our study the band at 1505 cm  1 is more prominent in pulsed mode, where the time of synthesis is more (2.5 h) compared to continuous mode synthesis (1 h). The weak signal for CS mode sample may be attributed to high symmetry of hydrogen in trigonal configuration. In pulsed mode the (Zn–H–Zn) hydride, the vacancy, moves from trigonal to bridging configuration of lower symmetry resulting strong signal at 1505 cm  1. The change in configuration introduces substantial dipole moment, which results in Zn–H–Zn asymmetric stretching vibration. The band at 1777 cm  1 can be attributed to molecularly adsorbed H2O on ZnO. The band at 437, 552, 580 cm  1 can be attributed to E2 high and B1 high mode as observed in Raman spectra of ZnO [2,18,19,31]. The roles of vacancies in oxide semiconductors have been widely investigated but their role in the formation of color centers such as F-centre has not been investigated in detail. This F-centre has been found to be present in bulk ZnO material. In nano powders of ZnO, the surface plays an important role in catalytic adsorption phenomenon. Formation of oxygen vacancies in the presence of H2 can be written as ZnO(s)+ H2(g)-Vo +H2O(g) The energy required for this reaction to proceed is 0.27 eV as shown by Catlow et al [7]. So it is natural to expect oxygen

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Fig. 6. Glow curves of ZnO nano particles irradiated with various doses of b-rays (Heating rate¼ 5 1C s  1): (a) material prepared in pulsed mode and (b) material prepared in continuous mode.

Fig. 7. . Dose response curve of the main peak (  120 1C). (a) full circle – pulsed mode and (b) Hollow circle – continuous mode.

vacancies in this system. The samples irradiated by g-rays show five and four peaks, for continuous and pulsed mode powder, respectively, when curve fitted by the computerized glow curve fitting method. In both the powders, the kinetics was found to be more than unity indicating more than one process operating during thermo-luminescence phenomena. The intensity of these peaks is found to be ten times higher in pulsed mode compared to continuous mode powders. This indicates that more number of F centers (Vo – surfaces oxygen) are formed in the pulsed sonication process. The calculated energy for formation of defects in ZnO has been given by Catlow. It can be seen from his calculation that formation of oxygen and Zn interstitials requires minimum formation energy, i.e. 1.652 and 2.183 eV, respectively. Thus

during the sonication process it is likely that these two types defects will be formed more. g- Irradiation creates F and F + type centers and F + type centers are formed when an electron is trapped at vacant oxygen interstitial surface sites. Further vacant interstitial surface sites can also trap holes during irradiation. Since the mobility of holes at RT is quite high, these centers will decay very fast. Thus, when the U irradiated powder further excited with blue light such as 470 nm band and the optically stimulated luminescence decay was recorded at RT, the decay was found to follow a kinetic of intermediate order. This hints about more than one process of radiative transitions taking place in this system. This is expected as more than two types of centers were found to be formed by g-irradiation. The activation energies frequency factors and order of kinetics calculated from these TL glow curves are given in Table 3. Further, the growth of 26 1C peak was found and shows saturation tendency for pulsed mode (20 nm) and continuous mode powder (24 nm). This indicates the saturation of oxygen vacancies in pulsed mode powder and continuous mode powder by b ray irradiation. The photoluminescence in ZnO thin film has been investigated by several workers. The deep band emission (DBE), that is the green emission, has been studied extensively but no consensus has emerged about its origin [2]. Recently Mac Borseth et al. 1 , [30] have attributed the band at 2.53, 2.35 and 2.17eV to Vzn Vo- and Li related defects respectively. In an earlier investigation we had observed six emission bands in nano ZnO doped with Al impurities and a tentative model had been proposed. In the present investigation, the exciton band at 390 nm is found to be missing but bands around 412 and 414 nm have been observed for continuous and pulsed mode powder, respectively. Further, the band gap for these powders has been found to be 3.05 and 3.15 eV, respectively, from optical absorption measurement

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Fig. 8. CGCD of the glow curves of Fig. 1 irradiated at 200 Gy of b-rays (heating rate ¼5 1C s  1): (a) material prepared in pulsed mode and (b )material prepared in continuous mode K K K Experiment, — numerical best-fit.

Table 3 Trapping parameters as obtained from computerized glow curve decomposition (CGCD) method of Fig. 6. Fig. 6(a) Tm (1C) (glow peak temp) 118 164 210 266 326

Fig. 6(b) E (eV) (activation energy)

s (s  1) (frequency)

b (order of kinetics)

Tm (glow peak temp)(1C)

E (eV) (activation energy)

s (s  1) (frequency)

b (order of kinetics)

0.99 1.00 1.00 1.00 1.60

2.10E+ 12 1.00E +11 6.53E+ 09 4.;45E +08 7.47E+ 12

1.48 1.50 1.50 1.08 1.07

126 166 230 288 –

1.09 1.00 1.01 1.09 –

2.27E + 13 8.63E + 10 2.77E + 09 1.17E + 09

1.50 1.80 1.00 1.79

(figure not shown). Thus, the band gap has been found to be compressed and accordingly the 412 nm band can be attributed to exciton only. The 437 nm or 441 nm bands can be attributed to transition between exciton level and interstitial oxygen (O2). The excitation spectra for both the samples were recorded by placing the monochromator at one of the intense emission bands (560 nm) and the common excitation band found for both the powders is 365 nm although there are other excitation bands. Thus, all the PL spectra have been recorded with 364 nm ( E3.38 eV) excitation band light. When excited with 3.38 eV light, the standard ZnO band gap energy as reported [1,2] at room temperature, the electrons are raised to conduction band and then recombine with different defect centers to give different emission bands. Thus, the 566 nm (2.19 eV) band can be attributed to transition between Vo Zni and valence band or between exciton level and anti-site oxygen (OZn). The emission band at 441 nm can be assigned to transition between exciton level and oxygen interstitial. Recent paper of Catlow also assigns the band around 2.80 eV to neutral oxygen interstitial in a split interstitial peroxy configuration. The new band appearing at 466 nm (2.66 eV) can be due to interstitial hydrogen present between two Zn dimmer vacancies. It has been argued that formation energy of two layers clustered vacancies in ZnO is 1.45 eV per dimmer. In fact, we have observed Zn–H–Zn band (1510 cm  1) in the FTIR spectra of both continuous and pulsed ZnO samples. Further, the thermo luminescence glow curves show low temperature glow peaks with activation energy  1.00 eV. This indicates the formation of

dimmer vacancies layer close to the surface and since the TL glow curves were recorded with blue filter, the centers responsible for blue light can give thermo luminescence only. The order of kinetics calculated from these glow curves hints about the complexity of the process and one can say that more than one defect centers are responsible for this band emission. In a ZnO:Li system the band around 466 nm has been assigned to Li at interstitial position [31]. We had also reported a similar band in ZnO:Li thin film system [32]. The emission band observed in the red region may be assigned to H at oxygen site. Similar result has been obtained by Dingle [33] in ZnO single crystal system. Catlow et al. while exploring the sites of oxygen adsorption on ZnO, have come up with the idea of two types of adsorption sites for hydrogen in ZnO. Type-I adsorption is reversible dissociative type and type-II is irreversible type. In type-I, adsorption of H2 on Zn–O dimmers takes place and gives rise to Zn–H or O–H species and in type-II bridging of two species Zn–H–Zn and O–H–O takes place. But both these species are in different chemical environments of the surface atoms arising from the difference in coordination and proximity to defect sites [7]. It has also been demonstrated that the peak at 1745 cm  1 (theoretically calculated) results from vacancy with a type-II hydride (AZn) and type-I (Zn–H–Zn) produces a peak at 1507 cm  1. Our experimental observation of FTIR peak at 1510 cm  1 agrees well with this value for type-I hydrogen adsorption site. The small bands in the region of 1753 cm  1 in both continuous and pulsed mode sample indicate the presence of type-II hydrogen also. Thus it becomes obvious that during the

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process of sonication in aqueous medium insertion of hydrogen in ZnO system takes place and this hydrogen occupies two positions, which contribute to the luminescence band in the red region.

5. Conclusions From the above results and discussions we are led to the following conclusions (a) Sonication is an effective way of inserting hydrogen into ZnO system. (b) Once inserted, the hydrogen atom in oxygen position takes part in the process of emission in the red region. (c) Pulsed sonication reduces the crystallite size and introduces more defects, which become active in giving thermo luminescence. (d) Surface states created by pulsed/continuous sonication take part in the thermo luminescence process, thereby giving rise to low temperature glow peaks. (e) These surface states take part in photoluminescence also. (f) The band gap compression can be explained in the light of the Burstein–Moss shift. (g) The photoluminescence emission bands can be ascribed to defects due to oxygen, in vacancies and interstials, etc. and the hydrogen replacing the oxygen sites.

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