Applied Surface Science 256 (2009) 298304

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Applied Surface Science

Synthesis of ZnO2 nanoparticles by laser ablation in liquid and their annealing transformation into ZnO nanoparticles
M.A. Gondal a,b,*, Q.A. Drmosh a,b, Z.H. Yamani a,b, T.A. Saleh b,c
a

Laser Research Laboratory, Physics Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia c Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 31 July 2009 Received in revised form 4 August 2009 Accepted 4 August 2009 Available online 11 August 2009 Keywords: Nanotechnology Nanoparticles Pulsed laser applications Zinc oxide Zinc peroxide Optical properties of ZnO

A pulsed laser emitting UV radiations generated by the third harmonic of Nd:YAG was applied for the synthesis of nano-structured ZnO2 and ZnO. For the synthesis of nanoparticles of ZnO2, a high-purity metallic plate of Zn target was xed at the bottom of a glass cell, in the presence of deionized water mixed with oxidizing agent H2O2, under repeated laser irradiation. The optical properties, size and the morphology of the synthesized ZnO2 and ZnO by laser ablation was inuenced strongly by postannealing conditions which is not previously reported. By annealing ZnO2 at 200 8C for 8 h, the product (ZnO2) synthesized primarily was converted completely to ZnO. By variation of the annealing temperatures from 200 to 600 8C, the grain size of ZnO changes from 5 to 19 nm with a change in lattice parameters, the band gap and some other optical properties of nano-ZnO. 2009 Elsevier B.V. All rights reserved.

1. Introduction Material properties are size-dependent, and changing the particle size of any material can actually alter properties which were formally thought to be constant for a given material. In semiconductors, for example, band gaps have been found to be particle size-dependent. Studies on particle size variation reveal that the reactivity of nanoparticles decreases as their size increases [1]. Nanocrystalline materials have gained importance in recent years, as they represent a class of material with new exciting properties and wide technological applications such as photocatalysis, chemical remediation, photo-initiazation of polymerization reactions, quantum dot devices, and solar energy conversion. In addition, the semiconductor nanocrystals are attracting much attention because of their size-dependent electrical and optical properties [2,3]. Zinc oxide (ZnO) is a unique material that exhibits dual semiconducting and piezoelectric properties. Compared with other semiconductor materials, ZnO has higher exciton binding

* Corresponding author at: Laser Research Laboratory, Physics Department, King Fahd University of Petroleum & Minerals, Mail Box 372, Eastern Province, Dhahran 31261, Saudi Arabia. Tel.: +966 38602351; fax: +966 38602293. E-mail address: magondal@kfupm.edu.sa (M.A. Gondal). 0169-4332/$ see front matter 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.019

energy (60 meV) and has been studied as an optoelectronic, transparent conducting, and piezoelectric material. In the past few years, numerous studies have been conducted on both production as well as electronic and optoelectronic applications of nano-structured ZnO [46]. Different chemical methods have been reported for the synthesis of nano-structured ZnO, like the solgel method precipitation in alcoholic medium, polyol synthesis, etc. In addition, other methods have been applied for the synthesis of ZnO nanostructures such as sputtering, molecular beam epitaxy, hydrothermal and electro-deposition [713]. However, most of these techniques are costly and complex in nature, specically to control the particle size and the size uniformity. Yet, laser ablation is one of the emerging techniques for the synthesis of nano-structured ZnO. Recently pulsed laser ablation (PLA) technique in liquid media has been applied for the controlled fabrication of nanomaterials via rapid reactive quenching of ablated species at the interface between the plasma and liquid [1418]. The important features of PLA technique are that one can prepare well crystallized nanoparticles (NP) which are pure without by-products. Moreover, one can add surfactants to liquids to control the size and the aggregation state of NP by changing the surface charge of the nuclei [19]. Recently the synthesis of nano-ZnO has been reported by Zeng et al. [2022] where they applied sodium dodecyle sulfate

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(SDS) as a surfactant to control the particle size and studied the optical properties of nano-Zn/ZnO composit synthesized using laser ablation. In addition, the entire product could be collected in solution, and the obtained colloid solution is very easy to handle. Another advantage of this technique is that one does not require costly chambers and high vacuum pumps [23]. Most of the work on ZnO post-annealing and other effects have been carried out on thin lms of ZnO. Little work has been reported on bulk nanostructured ZnO (powders) especially using laser ablation process. Previous studies presented controversial results annealing ZnO directly in oxygen environments. Some reports claim that oxygen is depleted from ZnO; other claim oxygen content is enhanced [24]. The main purpose of the present research is to study the post-annealing temperature effect on nanoparticle size and energy band gap of ZnO synthesized indirectly from ZnO2 prepared by laser ablation method in presence of an oxidizing agent (H2O2). In this work, we applied 355 nm wavelength laser for generation of ZnO2 nanoparticles in a mixture of deionized water and 3% of H2O2, by focusing the laser beam on a pure zinc plate. By post-annealing of the ZnO2 particles at different temperatures, ZnO nanoparticles were prepared. The effect of the post-annealing temperature on the nano-structure, optical properties and energy band gap of ZnO nanoparticles synthesized from ZnO2 were investigated. 2. Materials and methods For the synthesis of nanostructure ZnO particles, a laser-based setup was designed and constructed [25]. For ablation of Zn particles, a Q-switched Nd-YAG laser (Spectra physics Model GCR 100) operating at 355 nm wavelength using third harmonic generator was employed. This laser can deliver maximum pulse energy of 300 mJ with a pulse width of 8 ns and operates at a 10 Hz pulse repetition rate. The collimated beam is tightly focused on the target (Zn) sample using a convex lens to create a spark or breakdown in the sample. The laser energy was measured with a calibrated energy meter (Ophir model 300) for the study of dependence of nanoparticle yield on incident laser energy. In order to get a uniform laser beam shape, a 2 mm diameter aperture was placed inside the path of the laser beam. For the synthesis of nanoparticles, a high-purity metallic zinc foil, 1 mm thick, and purity 99.99% (Sigma Aldrich Company) was xed on a magnetic holder at the bottom of a glass cell as the target, and was rotated to avoid deep ablation crusts. Typical laser pulse energy for PLA process was between 40 and 130 mJ per pulse. The laser beam was focused by a lens with a focal length of 250 mm in order to get sufcient laser intensity for ablation. The typical diameter of the laser spot on a bulk target was 0.08 mm and the typical liquid volume was 10 ml. The addition of an oxidizing agent like H2O2 helps in synthesis and conversion of ZnO to ZnO2. After 40 min laser irradiation time, a milky colloidal solution of peroxide-based nanomaterials was obtained. The colloidal suspension was separated from water after laser irradiation using centrifuge. In order to synthesize the nanoparticles of ZnO2, H2O2 was added in 3% concentration to the distilled water and the laser energy was kept at 100 mJ and the laser irradiation time was 40 min. For synthesis of zinc oxide, the synthesized ZnO2 powder was heated at different temperatures. It is worth mentioning that ZnO (Fig. 1a) could be produced directly by irradiation of zinc foil under pure deionized water environment without any addition of H2O2. However, this is not the objective of this work, because we like to be able to modify the parameters of ZnO nanoparticles without transforming them into Zn upon annealing. The post-annealing temperature effect on the morphology, band gap and other optical properties of the ZnO2 were carefully

Fig. 1. X-ray diffraction of (a) ZnO prepared by pulsed laser ablation in deionized water and (b) ZnO2 prepared in 3% H2O2 water solution.

studied using XRD, photoluminescence, UVvis absorption spectrophotometer and FTIR. The structure and grain size were characterized by using X-ray diffraction (Shimadzu XRD Model 6000). Typical sizes of ZnO nanoparticles of 519 nm were achieved with this method using different annealing temperatures. These UVvis optical absorbance spectra of the ZnO nanoparticles were recorded at room temperature using JASCO UV/VIS spectrophotometer using a JASCO V-570 spectrophotometer. Photoluminescence spectra were studied using a specrourometer (Shimadzu RF-5301 PC) equipped with 150-W Xe lamp as the excitation source. The sample nano-powders were also characterized by infrared spectroscopy (FTIR-100 Spectrometer using KBr pellets). 3. Results and discussions 3.1. XRD studies of ZnO2 and ZnO Fig. 1 depicts the XRD spectra of zinc oxide (ZnO) and zinc peroxide (ZnO2) nanoparticle synthesized in powder form using pulsed laser ablation. The XRD spectra clearly show the crystalline structure of the nanoparticles and various peaks of zinc oxide (ZnO) and zinc peroxide (ZnO2). The main dominant peaks of ZnO were identied at 2( = 31.38, 34.88, 36.48, 48.28, 57.08, 63.28 and 68.38 while the main dominant peaks for ZnO2 were identied at 2( = 328, 37.18, 53.48 and 63.48 respectively, as taken from the standard literature. In order to investigate the effect of post-annealing effect on the transformation of ZnO2 into ZnO, the ZnO2 powder prepared by addition of the 3% H2O2 in the ablation process was annealed at various annealing temperatures from 200 to 600 8C under 1 atmospheric pressure for 8 h. It has been reported that the formation of ZnO2 results from the photochemical reaction of H2O2 in high concentration (30%) and Zn(CH3COO2)2, and the corresponding process is considered (Eq. (1)) as following [26]: ZnCH3 COO2 2 H2 O2 -------ZnO 2HCH3 COO2 (1)

While in our case we do not need any precursor like Zn (CH3COO2)2 and the formation of ZnO2 by laser ablation of pure Zn in presence of relatively low concentration H2O2 (3%) has been achieved. In our case, the possible reactions for formation of ZnO (Eq. (2)) and ZnO2 (Eq. (3)) are followings: H2 O hnlaser at 355 nm ! H OH OH hnlaser at 355 nm ! O H Zn O ! ZnO

(2)

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Fig. 2. XRD patterns of ZnO prepared by annealing temperature of ZnO2 at (a) 200 8C (b) 300 8C, (c) 400 8C, (d) 500 8C and (e) 600 8C.

Fig. 3. Variation of grain size and energy band gap versus the annealing temperature for ZnO produced from ZnO2 prepared by pulsed laser ablation in 3% H2O2 water solution.

By addition of H2O2 Zn H2 O2 ! ZnOH2 ZnOH2 ! ZnO H2 O ZnO O ! ZnO2 (3)

Fig. 2 shows the XRD pattern of the ZnO at different annealing temperatures. As clear from the gure, there is improvement in the crystallinity of ZnO with the increase in the annealing temperature from 200 to 600 8C. This is improvement is due to the recrystallization of ZnO particles by the supply of sufcient thermal energy. 3.2. Effect of annealing temperature on grain size

usually happens at higher temperatures and some other thermal effects such as Ostwald ripening [29] where the formation of larger particles are more energetically favored than smaller particles. This stems from the fact that molecules on the surface of a particle are energetically less stable than the ones already well ordered and packed in the interior. Large particles, with their lower surface to volume ratio, result in a lower energy state (and have a lower surface energy). As the system tries to lower its overall energy, molecules on the surface of a small (energetically unfavorable) particle will tend to diffuse and add to the surface of larger particle. Therefore, the numbers of smaller particles continue to shrink while larger particles continue to grow in size [30,31]. 3.3. Effect of annealing temperature on lattice parameters

ZnO starts to emerge by annealing ZnO2 around 130 8C. ZnO2 is converted completely into ZnO (Eq. (4)) around 200 8C by following reaction [2022,27]: ZnO2 ! ZnO 1 O2 2 (4)

The average grain size d of the nanoparticle of ZnO generated by annealing the ZnO2 was estimated by using the standard Eq. (5) known as Scherrer formula [28]: d kl b cos u (5)

During this study, the effect of annealing temperature on the crystalline structure was also investigated and for this purpose the lattice parameters were estimated at different temperatures. It is well known that ZnO has hexagonal unit cell with two lattice parameters a and c that can be calculated from the XRD spectrum by using the following equations [32]: ! 2 2 2 2 1 4 h k l l 2 (6) 2 2 3 a c d

where: k = constant (0.89 < k < 1), l = wavelength of the X-ray, b = FWHM (Full Width at Half Maximum) width of the diffraction peak, u = diffraction angle. The mean grain size of (0 0 2) oriented ZnO calculated using the above mentioned Eq. (5) at angle 36.48 from Fig. 2ae are 5, 6, 9, 15 and 19 nm respectively and are listed in Table 1. The trend in increase in the grain size due to rise in annealing temperature is presented in Fig. 3. The increase in grain size with annealing temperature could be attributed to agglomeration process which
Table 1 Lattice parameters and grain size at different annealing temperatures. Temperature (8C) 200 300 400 500 600

l
sin u 2 0 0

and

a p 3 sin u1 0 0

(7)

where: (1 0 0 and (2 0 0 are the angles of the peaks 1 0 0 and 2 0 0 respectively. By applying Eqs. (6) and (7), the lattice parameters were calculated from the XRD spectra (Fig. 2) recorded at different annealing temperature for ZnO. The estimated parameters at different temperatures are plotted in Fig. 4 and listed in Table 1. It is clear from Fig. 4 that lattice parameters increase with the rise in annealing temperature. This increase of lattice parameter has been reported for different crystalline materials elsewhere [33,34]. 3.4. Photoluminescence studies A study of the photo-luminous (PL) property of any material is interesting because it can provide valuable information on the quality and purity of the material. The semiconductor ZnO nanoparticles, with sizes comparable to or below their exciton Bohr radius, have distinctive electronic and optical behaviors due

u1 0 0
31.74 31.66 31.57 31.52 31.44

) a (A 3.253 3.261 3.269 3.274 3.282

u0 0 2
34.24 34.22 34.21 34.19 34.07

) C (A 5.233 5.236 5.238 5.241 5.258

Grain size (nm) 5 6 9 15 19

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Fig. 4. Plot showing trend of lattice parameters at different temperatures.

to exciton quantum connement phenomena. For such reasons, quantum dots suitably describes these semiconductor nanoparticles which absorb light at specic wavelength and emit it at longer ones. Therefore, recording of PL spectrum is of paramount importance for estimating the size of nanoparticles and their characteristics for various applications. The photoluminescence (PL) spectra of ZnO were recorded in the UV (360400 nm) region and is depicted in Fig. 5. Clearly, the emission peak around 374 shifts to longer wavelength (red shifting) as the annealing temperature increases from 200 to 600 8C. We believe this is a result of defects related to shallow binding excitons formed during high temperature annealing. Our result is contrary to the one reported earlier [35] that increasing the annealing for 5 h at 500 8C shows increase in the intensity of UV peak and decrease in green emission, compared to the as-prepared samples. In our case the intensity of longer wavelength peak increases which indicates the improvement in the crystalline nature of the our sample. It is worth mentioning that violet photoluminenescence band at 425 nm (2.92 eV) has been also observed by Zeng et al. [22] from Zn/ZnO core shell nanoparticles and these bands were sensitive to the shell thickness and annealing conditions. Based on the electron paramagnetic resonance measurements, the violet emission was

attributed to the electronic transition from defect level, corresponding to high concentration zinc interstitials to the valence band. However Wang et al. observed violet PL at 402 nm from ZnO lms deposited by RF magnetron sputtering and attributed it to the electronic transition from conduction band tail states to valence band tail states [36]. It has been reported [3742] that stoichiometric pure ZnO thin lms usually show strong UV luminescence and no visible spectrum. If there is any visible luminescence in the spectrum, it is due to defects related to deep level emissions, such as Zn interstitials or oxygen vacancies. We did not observe any visible luminescence in our case which also indicates that ZnO prepared with our method is more pure and stoichiometrically perfect. The dependence of the emission wavelength on the particle size is evident when the particle size gets to nanolevel. Mie scattering theory, which predominantly occurs when the particle size becomes much smaller than the interacting light wavelength, could explain this size-dependent electronic transition. In general, the electric dipole term is responsible for the electronic transition and dominates in the limit that r/(!0, where r is the particle size. In such situation, crystallites behave like a molecule as far as electromagnetic radiation is concerned. In the quantum regime, the interaction Hamiltonian (Hint) between the radiation and the electrons in the ZnO nanoparticles can explain the sizedependent phenomena. Hint is a function of electric eld vector potential and the momentum vector of the electrons. When this Hint becomes small (due to the particle size), it will be small enough to be treated with familiar perturbation theory. The electric dipole transitions are strong as they appear in the rst order [43]. 3.5. UV absorption spectrum of ZnO Fig. 6 depicts comparison of the UVvis absorption spectra of as prepared ZnO (without oxidizing agent) and annealed zinc oxide at 600 8C. It is clear that the exciton absorption peak of the fresh sample (348 nm) is shifted to 373 nm with the increase in annealing temperature. In addition to the shifts of the absorption edges, there are substantial tails on the longer wavelength side of the absorbance spectra as depicted in Fig. 6. The tail may be caused by the scattering of a range of particle sizes, and/or some type of Urbach tail effect due to inter-grain depletion regions [4143].

Fig. 5. Photoluminescence emission spectra for ZnO prepared by post-annealing of ZnO2 at 200 8C, 300 8C, 400 8C, 500 8C and 600 8C. A red wavelength shift is indicated by vertical broken line.

Fig. 6. Absorption spectra of ZnO nanoparticles prepared by (a) laser ablation of zinc in deionized water and (b) post-annealing of ZnO2 (600 8C).

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M.A. Gondal et al. / Applied Surface Science 256 (2009) 298304 Table 2 Absorption bands of annealed nanoparticles measured with FTIR spectroscopy. Annealing temperature (8C) 0 200 600 Wavenumber of yZnO (cm1) 479.3 464.0 457.7

one can calculate the band gap of nanostructure materials at different sizes. Experimentally, the direct band gap of semiconductors can be estimated from the relation (9) between the absorption coefcient and UV photon energy which is given by:

a E AE Eg 1=2

(9)

Fig. 7. Variation of energy band gap versus the grain size of ZnO produced from ZnO2 prepared by pulsed laser ablation in 3% H2O2 water solution.

The band gap of the nanocrystals can be calculated using the effective mass model as reported by Brus [41]: E Ebulck g   2  p2 1 h 1 1:8 e2 2 me mh 4pee0 r 2r   2 h p2  1 1 1 2 h 4pee0 r 2 me mh 

(8)

where E is the photon energy, Eg is the direct band gap of the semiconductor, A is a constant. Therefore, a plot of (aE)2 versus photon energy E should yield a straight line that cuts the photon energy axis at the band gap. Table 1 shows experimental band gap dependence on annealing temperature. Fig. 7 depicts the dependence of the band gap (Eg) on the grain sizes. The band gap decreases with increasing temperature and with increasing grain size. This change in band gap can be understood due to electronic structure dependence on size of nanocrystals especially in ultra-ne size such as quantum dots. From the experimental data, one can see that the Eg change is more prominent when the grain sizes are less than 10 nm and this could be due to quantum effects [4446]. 3.6. FTIR absorption spectroscopy of ZnO Fourier transform infrared (FTIR) spectra were measured at room temperature with an FTIR spectrometer using the KBr pellet technique [47]. FTIR measurements are essential to conrm the formation of crystalline ZnO nanocrystals and to identify any adsorbed species onto the surface of nanoparticles. Hence, FTIR spectra of the nanoparticles products without and with annealing treatment were performed for a better comprehension of the structure and

where Ebulck is the bulk energy gap = 3.37 eV [42], r is the particle g radius, me is the effective mass of the electrons = 0.24m0, mh is the effective mass of the holes = 0.45m0, e is the relative permittivity = 3.7, e0 is the permittivity of free space, h is Plancks constant, and e is the charge of the electron. The second term in Eq. (8) represents the kinetic-energy term containing the effective masses of the electron and the hole. The third and fourth terms represent the Coulomb attraction between the electrons and the spatial correlation between the electron and the hole respectively. The last two terms are usually negligible compared with the other two terms. By using the above formula,

Fig. 8. Typical FTIR spectra of ZnO: (a) bulk, (b) nanoparticles product, (c) nanoparticles product annealed at 200 8C and (d) nanoparticles product annealed at 600 8C.

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Fig. 9. Enlarged FTIR spectra, range of 430-495 cm1, stretching vibrations of ZnO (yZnO): (a) bulk, (b) nanoparticles product, (c) nanoparticles product annealed at 200 8C and (d) nanoparticles product annealed at 600 8C.

composition of these materials. The spectrum for bulk ZnO is also included for comparison purposes. Fig. 8 depicts a typical FTIR spectrum of the ZnO in various forms. Fig. 8a is the FTIR spectrum of bulk ZnO, Fig. 8b is of nanoZnO produced directly without annealing, Fig. 8c and d is of annealed ZnO generated from indirect method at 200 and 600 8C. An absorption band revealing the vibrational properties of ZnO nanocrystals is observed for each sample in the range of 430 495 cm1. This band is mainly assigned to the stretching vibrations of ZnO. Commercial bulk ZnO powders show absorption of this band at about 442.6 cm1 while in our case it is at 479.3 cm1 for nanoparticle ZnO produced by direct method. When the nano-ZnO produced through indirect method was thermally treated (annealing treatment), a shift of the IR absorption peak toward a low wavenumber (red shift) was observed. This band (479.3 cm1) is shifted to 464.0 and 457.7 cm1 after annealing at 200 and 600 8C, respectively (Table 2). Fig. 9 is an enlargement of the FTIR spectra in the 430495 cm1 region. Considering XRD measurements which show the ZnO as a major product, the more symmetrical peak at about 457.7 cm1 (annealing at 600 8C) indicates more uniformity of the ordered oxide structure. Such attractive uniformity, along with the high adsorption properties, offer great promise for using these nanoparticles for designing new sensitive sensors for different gases to be applied for different industrial and environmental applications. The broad absorption peaks in the range of 34103465 cm1 correspond to OH group, and indicates the existence of water absorbed on the surface of nanocrystals. The presence of this band can be clearly attributed to the adsorption of some atmospheric water during FTIR measurements. Those at 15001650 and at about 2370 cm1 are the C=O stretching mode arising from the absorption of atmospheric CO2 on the surface of the nanoparticles.

4. Conclusions In summary, we indirectly synthesized nano-structured ZnO by annealing ZnO2 synthesized by laser ablation method, and annealing at different temperatures. Different techniques (XRD, UV spectro-photometery PL, and FTIR) were applied for the characterization of synthesized nano-ZnO. The optical properties, grain size, lattice parameters and band gap and IR absorption band of ZnO vary with the annealing temperature. The synthesized material using this indirect method could be applicable in developing materials for sensors for various applications in industry, medicine and environment. Acknowledgments The support by the Physics Departments, Center for NanoTechnology (CENT) and King Fahd University of Petroleum and Minerals through project SABIC-090023 and KACST-28-40 and is gratefully acknowledged. One of the authors (Q. Drmosh) is thankful to Government of Yemen for nacial support for his master work. He is also thankful to KFUPM for its hospitality and permission to work at its research facilties. References
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