SYNTHESIS AND CHARACTERIZATION OF Al DOPED ZnO NANOPARTICLES

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1 International Conference on Ceramics, Bikaner, India International Journal of Modern Physics: Conference Series Vol. 22 (2013) World Scientific Publishing Company DOI: /S SYNTHESIS AND CHARACTERIZATION OF Al DOPED ZnO NANOPARTICLES WASI KHAN*, Z. A. KHAN, A. A. SAAD, S. SHERVANI, A. SALEEM, A. H. NAQVI Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Z.H. College of Engg. & Technology, Aligarh Muslim University, Aligarh , India * Undoped and Al-doped ZnO (AZO) nanoparticles (NPs) have been successfully synthesized by the simple sol-gel method. The NPs have been characterized by a number of techniques as x-ray diffraction (XRD), UV-visible spectroscopy and scanning electron microscopy (SEM) at room temperature for 0%, 0.5%, 1% and 2% of Al concentration. The structural characteristics were examined using XRD and SEM with EDS. XRD analysis reveals that all samples crystallizes in polycrystalline nature with wurtzite lattice and exhibit no other impurity phase. The average crystallite size decreases with increase in Al concentration. The absorption spectra indicate increase in optical energy gap (E g ) with increase in Al ion doped into the ZnO lattice site. Keywords: ZnO Nanoparticles; Sol-Gel; XRD; UV-Vis, SEM. 1. Introduction ZnO is an n-type semiconductor with wide direct bandgap energy (3.37 ev) and a larger binding energy (60 mev). Due to its unique characteristics like low cost, non toxicity, abundance in nature, suitability to doping, this material has got wide applications in electronic and optoelectronic devices such as ultraviolet light-emitters, piezoelectric transducers and solar cells [1-4]. ZnO nano powders doped with different ions such as Al, In, Ga, etc. have improved electrical, optical, and catalytic properties [5-7]. Among these, Al-doped ZnO (AZO) nanopowders are both conductive and transparent in the visible region and thus can be utilized in transparent conductive pastes [6, 9]. Nowadays various methods have been presented for preparing AZO nanopowders such as sol gel [5], spray pyrolysis [6], precipitation [10], and hydrothermal processes [8]. Among these methods, the sol gel process had attracted considerable attention because of its simplicity, acceptable costs and the crystalline quality of the ZnO prepared by the sol gel process. For understanding the effect of concentration sols with metal dopant on the structural characteristics and physical properties of ZnO nanoparticles, this study doped Al (0, 0.5, 1and 2 at%) into ZnO nanoparticles by the sol gel process to investigate the * Corresponding author. Tel./Fax.:

2 Synthesis and Characterization of Al Doped ZnO Nanoparticles 631 microstructural variations and optical property were systematically characterized of AZO nanoparticles. 2. Experimental Procedure To prepared the un-doped zinc oxide (ZnO) nanoparticles by adding 1M zinc acetate dehydrate (Zn(CH 3 COO) 2 2H 2 O) in to 50 ml de-ionized water, using Sol-Gel method synthesized it. For the AZO nanoparticles, aluminum nitrate nanohydrate (Al(NO 3 ) 3 9H 2 O) was added into the solution to serve as the aluminum source. Three Al atomic contents of 0.5 at%, 1 at% and 2 at% were chosen, stirred in clear solution at 80 C till gel formation. Meanwhile, citric acid (C 6 H 8 O 7 ) was dropped into these solutions slowly until its ph reaches to 1.5 and after this NH 3 (10 ml) also added. After that, the solutions were placed in an oven at 100 C until the solvent evaporated. Finally, it putted into an oxidizing furnace to perform heat treatment at 600 C and after this thoroughly grinded it. Final product formed nanoparticles of AZO. The flow chart of the samples preparation is shown in Fig. 1. Zn(CH 3 COO) 2 2H 2 O (1 M) 10 ml NH 3 for gel formation Mixed with 50 ml distilled water Add Citric acid (C 6 H 8 O 7 ) to adjust ph ~1.5 Centrifuge o C Grinding Al(NO 3 ) 3 9H 2 O (0, 0.5, 1& 2%) Stirring for o C Calcination at C for 4 hrs AZO nanoparticles Fig. 1. Flow chart of ZnO and AZO nanoparticles synthesis. The structural characteristics were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and the optical properties were measured by UV-Vis. absorption spectrometer at room temperature.

3 632 W. Khan et al. 3. Results and Discussion Fig. 2 shows the XRD patterns of undoped and Al doped ZnO nanocrystalline powders for different Al concentrations in the 2θ range degree. The patterns were indexed using PowderX software and all peaks were well matched with hexagonal structure of ZnO using the standard data (JCPDS ). Here, no other impurity peaks were observed in the XRD patterns which show the purity of the nanoparticles formation. This result suggested that the nano powders have a polycrystalline hexagonal wurtzite structure with well developed crystallinity. The average crystallite sizes (D) was calculated using well known Scherrer s equation D = 0.9λ / (β cos θ) (1) where λ, β and θ are the x-ray wavelength (0.154 nm), full width at half maxima (FWHM) of the diffraction peak and the Bragg s diffraction angle, respectively. Crystallite size of each samples was calculated using most intense peak (101) of ZnO and AZO nanoparticles and tabulate in table 1. No considerable changes in the lattice parameters are found for different Al doping concentrations. Fig. 2. XRD spectra of undoped and Al doped ZnO nanoparticles.

4 Synthesis and Characterization of Al Doped ZnO Nanoparticles 633 Table 1. Variation in crystallite size and energy band gap with Al concentration. Al concentration Crystallite size (nm) (From Scherrer formula) 0% % % % Band gap (ev) From table observed that the crystallite size in the undoped ZnO is 32 nm which increases to 27.2 nm in the 0.5 at% Al-doped nanoparticles. Increase in crystallite size on Al doping arises due to reduction in the densities of nucleation centres and thus to grow resulting in large crystallites [11]. However, on further doping, the crystallite size begins to decrease and becomes 14.9 nm for 2 at% Al-doped nanoparticles. This was due to the decreased sintering rate that occurred during the sol gel process [12]. Fig. 3. SEM images of ZnO and 2% Al doped ZnO nanoparticles. The morphological and structural studies were investigated using scanning electron microscopy and displayed in Fig. 3 for undoped and 2% Al doped ZnO. These micrographs exhibited the formation of nanoparticles of ZnO and AZO. It was noted that the agglomeration increases with increasing Al concentration. To check the chemical composition of the material, an energy dispersive X-ray (EDS) spectroscopy analysis was performed. Fig. 4 shows the EDS spectra of pure and 2% Al doped ZnO sample, which confirms the synthesis of undoped ZnO and Al doped ZnO nanoparticles. The surface of the ZnO and AZO nanoparticles also exhibited elements of C, O, Zn and Al. These results are consistent with the XRD data.

5 634 W. Khan et al. Fig. 4. EDS spectra of ZnO and 2% Al doped ZnO nanoparticles. Fig. 5 shows the optical absorption spectra of ZnO and AZO in the wavelength range 250 to 800 nm. It is observed that the absorbance increases with Al doping concentration. The optical band gap of the nanopowders was determined by applying the Tauc relation as given below. αhν= B(hν E g ) n (2) where α is the absorption coefficient (α = 2.303A/t, here A is the absorbance and t is the thickness of the cuvett), B is a constant, h is Planck s constant, ν is the photon frequency, and E g is the optical band gap. The value of n = 1/2, 3/2, 2 or 3 depending on the nature of the electronic transition responsible for absorption and n=1/2 for direct band gap semiconductor. An extrapolation of the linear region of a plot of (αhυ) 2 on the Y- axis versus photon energy (hυ) on the X-axis gives the value of the optical band gap (E g ) as shown in Fig. 6 and tabulated in table 1 for all samples. Fig. 5. Absorption as a function of wavelength of undoped and Al doped ZnO nanoparticles.

6 Synthesis and Characterization of Al Doped ZnO Nanoparticles 635 Fig. 6. Plots of (αhυ) 2 vs photon energy (hυ) for undoped and Al doped ZnO nanoparticles. From the table it was observed that the value of E g in the undoped ZnO is ~3.28 ev which is lower than its bulk value of 3.37 ev. On doping E g increases and achieving a maximum value of 3.44 ev at 2.0 at%. According to quantum confinement theory, the band gap of a semiconductor depends on the crystal size, and its value will increase as the crystal size decreases [13,14]. 4. Conclusion Undoped and Al doped ZnO NPs were synthesized by sol-gel method. The microstructural and optical properties of the NPs have been investigated. The variation of Al content did not significantly influence the phase structure or the crystalline quality. SEM images of the samples show the formation of agglomerated NPs. The optical absorption increases with increases in the Al doping, leads to increase in optical band gap. The feasibility of incorporation of Al ion in ZnO NPs implies the ability to control the energy band gap of the material. This aspect may have a number of implications.

7 636 W. Khan et al. Acknowledgments Authors are greatly thankful to the Council of Science & Technology (CST), Govt. of UP for financial support in form of Centre of Excellence in Materials Science (Nanomaterials) and DST, Govt. of India for M. Tech. Programme under Nanomission. References 1. J. Bao, M. A. Zimmler, F. Capasso, X. Wang and Z. F. Ren, Nano Lett. 6, 1719 (2006). 2. H. C. Cheng, C. F. Chen and C. Y. Tsay, Appl. Phys. Lett. 90, (2007). 3. P. P. Sahay and R. K. Nath, Sens. Actuators B 133, 222 (2008). 4. J. Xu, C. Yuping, L. Yadong and S. Jianian, J. Mater. Sci. 40, 2919 (2005). 5. H. Serier, M. Gaudon and M. Menetrier, Solid State Sci. 11, 1192 (2009). 6. A. Verma, F. Khan, D. Kar, B. C. Chakravarty, S. N. Singh and M. Husain, Thin Solid Films 518, 2649 (2010). 7. T. Ogi, D. Hidayat, F. Iskandar, A. Purwanto and K. Okuyama, Adv Powder Technol. 20, 203 (2009). 8. W. H. Zhang, W. D. Zhang and J. F. Zhou, J. Mater. Sci. 45, 209 (2010). 9. E. Hamarberg, A. P. Schwab and C. Feldmann, J. Colloid. Interface Sci. 334, 29 (2009). 10. S. F. Du, Y. J. Tian, H. D. Liu, J. Liu and Y. F. Chen, J. Am. Ceram. Soc. 89, 2440 (2006). 11. A. S. Riad, S. A. Mahmoud and A. A. Ibrahim, Physica B 296, 319 (2001). 12. M. Mazaheri, A. M. Zahedi and S. K. Sadrnezhaad, J. Am. Ceram. Soc. 91, 56 (2008). 13. Y. G. Wang, S. P. Lau, H. W. Lee, S. F. Yu, B. K. Tay, X. H. Zhang and H. H. Hng, J. Appl. Phys. 94, 354 (2004). 14. G. Srinivasan, R. T. Rajendra Kumar and J. Kumar, Opt. Mater. 30, 314 (2007).