SYNTHESIS OF ZINC OXIDE NANOPARTICLES BY PRECIPITATION METHOD AND SIMPLE HEATING METHOD

SYNTHESIS OF ZINC OXIDE NANOPARTICLES BY PRECIPITATION METHOD AND SIMPLE HEATING METHOD Phyu Phyu Win*, Myint Sanda Win Nanotechnology Research Department, Metallurgical Research and Development Centre, Ministry of Science and Technology, Myanmar *[email protected] ABSTACT The spherical zinc oxide nanoparticles were successfully synthesized by alkaline precipitation technique from zinc acetate dihydrate and sodium hydroxide using triethanolamine (TEA) as surface active agent in order to obtain the particles more spherical. The spherical ZnO nanoparticles with the range from 150 nm to 300 nm were observed from this method. For comparison, a simple heating of zinc acetate dehydrate precursor by adding various contents of poly ethylene glycol (PEG) was carried out to prepare the uniform, hexagonal shape and dispersed zinc oxide nanoparticles with narrow size distribution. The morphologies of prepared zinc oxide nanoparticles were determined by scanning electron microscope (SEM). The elemental composition of zinc oxide nanoparticles was carried out by SEM attached Energy Dispersive X-ray Diffractometer (EDX) and the crystallization phase structures were studied by X-ray diffraction (XRD). Keywords: Zinc oxide nanoparticles, zinc acetate, spherical, precipitation, polyethylene glycol pyrolysis, template-free method, and other routes. Nanomaterials that are made hydrothermally are generally high-quality, single crystals with a diversity of shapes and sizes. In this technology, the most significant point to be explored is a rapid and simple method for producing nanometer size particles that emit high luminescence intensity (Hsieh, 2007; Aneesh, 2007).

1. INTRODUCTION Recently, the unique properties of nanomaterials have motivated the researchers to develop the simpler and inexpensive techniques to produce nanostructures of technologically important materials (Shah, 2009). Nanostructure zinc oxide, an n-type II-VI compound semiconductor with a wide band gap of 3.3 eV, has attracted considerable more and more attention as a promising candidate for its wide range applications such as varistors, photocatalysis, gas sensors, solar cells, pigment, transducers, etc.. High crystallinity of zinc oxide nanoparticles is one of the most important factors in achieving a high UV emission efficiency (Nyffenegger, 1998; Satoshi, 2004; Husnu, 2008; Yan, 2008; Hai, 2004).

This study was aimed at the alkaline precipitation synthesis of ZnO nanoparticles from zinc acetate dihydrate and sodium hydroxide in the ethanol rich phase using triethanolamine (TEA) as surface acting agent. We are developing a method for a simple and rapid production of ZnO nanoparticles with narrow size distribution by heating the precursor zinc acetate with various amounts of polyethylene glycol polymer.

Monodispersed ZnO particles with defined morphological characteristics, such as spherical, ellipsoidal, needle, and rod-like shapes have been obtained. ZnO nanoparticles can be prepared on a large scale at low cost by simple solution methods, such as alkali precipitation, thermal decomposition, hydrothermal synthesis, sol-gel methods, microemulsions, spray

2. EXPERIMENTAL 2.1 Hydrothermal Synthesis The chemical reagents used in this work were Zn (CH3COO)2.2H2O, NaOH and TEA of 1

analytical grade purity. Zinc acetate solution was prepared by dissolving 0.001 mole of Zn(CH3COO)2.2H2O in 50 ml of water. The alkaline solution was prepared by dissolving NaOH (0.002 mole) and TEA (0.003 mole) in 950 ml of absolute ethanol under stirring until the homogeneous solution. The zinc salt solution was added into the alkaline solution under magnetic stirring at 60-70oC and this solution was continuously heated for different reaction times: 1h and 2h. It was then allowed to cool naturally at room temperature. After the reaction was complete, the resulting white product was washed with ethanol to remove the organic agent TEA and hydroxide ions. The product was filtered and dried in oven at 60oC for 3 h.

hydroxide using TEA as surface active agent, the ethanol rich phase is the best dispersing agent and the best solvent. The triethanolamine dissolves in the ethanol. After the ZnO particles have surface active agent TEA to adsorb in the particles surface, a diffuse layer of ethanol- rich liquid around the particles is created. The zinc cation is soluble in water and growth is controlled by the diffusion of Zn2+ ions in the ethanol-rich layer. The TEA tends to make the particles more spherical. Without the organic additives or without using alcohol as dispersion medium, rods rather than spheres formed (Hsieh, 2007). The X-ray diffraction patterns for the ZnO nanoparticles prepared at different reaction times are shown in Figure 1.

Intensity (a. u.)

(202)

(201)

(112)

(103)

(102)

(c)

(110)

Alternatively, the nanosized ZnO particles were prepared by a simple heating of the precursor Zn (CH3COO)2.2H2O with poly- ethylene glycol (Molecular weight = 2 x 104). Zinc acetate was dissolved in distilled water to prepare 10 ml of zinc salt solution (0.6 M). Various amounts of PEG: 0 g, 8 g and 12 g were added to the zinc cation solution followed by heating under stirring at 70oC for 20 min to obtain homogeneous viscous materials. The materials were heated in a furnace at 500oC and 800oC for 1 h.

(100) (002)

(101)

2.2 Simple Heating Method

(b)

(a)

2.3 Characterization 20

The crystal structure of the synthesized particles was determined by X-ray diffraction (XRD, D8 Advance Bruker Co.,) with Cu Kα radiation λ= 1.5406 Ǻ. The particle size and morphology of the products were investigated by scanning electron microscope (EVO®60 SEM) and the elemental composition was obtained from energy dispersive X-ray diffraction attached to SEM (EDX INCA Penta FETx3 OXFORD Instruments).

30

40 50 2θ (deg)

60

70

80

Figure 1. XRD patterns of ZnO nanoparticles synthesized by alkaline precipitation at 60-70oC (a) as prepared for 1 h reaction time, (b) as prepared for 2 h reaction time and (c) calcined at 200oC for 2 h. The sharp diffraction peaks of the synthesized products indicate their good crystallinity. Figure 1-a shows the XRD pattern of the as-prepared ZnO nanoparticles of 1 h reaction time and Figure 1-b shows that of 2 h reaction time. Figure 1-c shows the XRD pattern of the ZnO nanoparticles calcined at 200oC in air for 2 h

3. RESULTS AND DISCUSSION In the alkaline precipitation of ZnO nanoparticles from zinc acetate and sodium 2

and is observed the diffraction peaks with the highest intensity. All diffraction peaks in the XRD patterns of as-prepared ZnO as well as calcined ZnO can be assigned to the hexagonal structure (JCPDS 36-1451). No characteristic peaks were observed other than ZnO. It indicates that the high purity of the ZnO nanoparticles is obtained. It can be seen that the intensity of the diffraction peaks increases and becomes shaper with increasing in temperature, which implies that the crystalline structure tends to have more integrity (Aneesh, 2007).

where λ is the wavelength of the incident X-ray beam; θ, the Bragg’s diffraction angle; and β is the full width at the half-maximum (FWHM) of the ZnO (101) line in radians. As the reaction temperature increases, FWHM decreases. Thus the size of ZnO nanoparticles increases as the temperature increases. This is due to the change of growth rate between the different crystallographic planes (Aneesh, 2007). The values of β were 0.012 and 0.0104 radians. By using this equation, the average sizes of the ZnO nanoparticles synthesized were found to be in the range of 14 – 20 nm depending on the crystal growth condition.

The average crystalline size of the products was estimated from XRD peaks by using the following Scherrer equation (Cullity, 1978):

(a)

(d)

(b)

(e)

(c)

(f)

Figure 2. SEM images of zinc oxide nanoparticles prepared (a) by alkaline precipitation for reaction time 1 h (b) for reaction time 2h, (c) after calcination at 200oC for 2h, and prepared by a simple heating method at 800oC for 1 h (d) without PEG, (e) with 8 g of PEG and (e) with 12 of PEG 3

The SEM images of the ZnO nanoparticles prepared by precipitation technique were shown in Figure 2-a to Figure 2-c. It is found that the morphology of the ZnO nanoparticles prepared is uniform and spherical shape. The sizes of the as-prepared particles by reaction times 1h are investigated less than 200 nm and the longer reaction time gave the larger particle sizes in the range from 150 nm to 250 nm. After calcinations at 200oC for 2h, the average particle size was found to be increased in the range 150 nm - 300 nm in diameter.

OXFORD Instruments) as shown in Figure 3 indicating the high purity of ZnO nanoparticles.

In this work, the hexagonal shape ZnO nanoparticles were prepared from the same precursor (zinc acetate dehydrate) by simple heating method with polyethylene glycol (PEG). The effect of polymer on particle properties such as shape and size were investigated by using different amounts of PEG: 0 g, 8 g and 12 g. All the precursors were heated at 800oC in air for 1h. Heating at temperatures of less than 500oC gave a large amount of carbon residue and the sample colour was black. Since the organic material must be removed to obtain pure oxide, precursor must be heated at high temperatures to decompose organic material so that low temperature processing is not allowed in this process. When using PEG, the heating temperature must be higher than 500oC (Abdullah, 2004).

Element

Weight%

Zn O Si Cu Al Totals

79.35 19.89 0.24 0.41 0.10 100.00

Figure 3. EDX spectrum and elemental analysis of ZnO nanoparticles prepared by alkaline precipitation

4. CONCLUSIONS

As shown in the SEM image of Figure 2-d, the larger irregular shape particles of several nanometer size were obtained without PEG. Coagulation process intensively occurred during the heating process since the absence of separating medium between particles to permit the as produced particles coalescence easily. Adding polymer in the precursor significantly decreased in particle size as shown in Figure 2e and Figure 2-f. It is clear that the uniform hexagonal ZnO particles with 200-300 nm in size were prepared by using higher amount of PEG (12 g).

ZnO nanoparticles were synthesized using alkaline precipitation method and by simple heating method using PEG polymer. In alkaline process, the effect of the reaction time on the structure was investigated. The XRD patterns and SEM images demonstrates that the spherical ZnO nanoparticles have the hexagonal structure and the particle size increases with the growth temperature. The particle sizes of the asprepared ZnO crystal are less than 200nm. By heating of zinc salt precursor with PEG is a simple and potential route for production of ZnO nanoparticles with 200-300 nm in size. This method may be rapid and economically efficient for large production.

The elemental compositions of the ZnO nanoparticles prepared by precipitation were determined by energy dispersive X-ray diffraction (EDX INCA Penta FETx3 4

fine zinc oxide nanoparticles under supercritical conditions”, 14th International Conference on the Properties of water and steam in Kyoto, 280-283 (2004).

AKNOWLEDGEMENTS This research was support by the Ministry of Science and Technology for the nanotechnology project. The authors are grateful to Metallurgical Research and Development Centre (MRDC) for supporting facilities. Mr. Khin Maung Win, XRD lab in MRDC, Dr. Aye Aye Toe, Mr. Myo Min Tun and Mr. Kyaw Naing Oo for taking keen interest in measuring of SEM/EDX and XRD in Analytical Lab, Pyin Oo Lwin.

Shah M.A. and Al-Shahry, 2009. Zinc oxide nanoparticles prepared by the reaction of zinc metal with ethanol. JKAU: Sci., 21, pp. 61-67. Yan Z., and Yingxue Z., 2008. Preparation of pure ZnO nanoparticles by a simple solid –state rection method, Appl Phys. A: Material Science & Processing, 92, pp 275-275.

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