Research

, Volume: 19( 6)

• View PDF
• Download PDF

Structure Analysis of Zinc Oxide Nanorods Synthesized by Cost Effective Precipitation Method Applicable for UV Protection

*Correspondence:

AP Sunitha, AP Sunitha, E-mail: sunithaganesh@gvc.ac.in

Abstract

Zinc Oxide (ZnO) has very attractive properties among nanomaterials with wide verity of applications. In this article simple and cost effective synthesis and detailed structure analysis of nanorods of ZnO prepared by chemical precipitation method at room temperature is reported. Powder of as synthesized ZnO nanorods are characterized by XRD, absorption spectra, dynamic light scattering (DLS), TEM and SAED. The XRD pattern shows the wurtzite crystal structure with average crystalline size as 29 nm by Debye-Scherrer formula and 24.5 nm by Williamson-Hall (W. H) plot. W. H plot also give a strain of -0.0004. From XRD, dislocation energies, lattice constants, d space are also tabulated. Broad peak in absorption spectra in ultra violet range indicate that the nanorods are applicable in textiles, sun creams and UV protection windows, industries as an ultraviolet absorber. Ultra violet radiation is very hazardous to human body. So, materials, especially nanomaterials with large surface to volume ratio and ultra violet absorption property have got a lot of acceptance. TEM characterizations also prove the nanorod structure with diameter of the order of 50 nm and length greater than 200 nm. DLS experiment estimated the size of synthesized structure. Selected area electron diffraction (SAED) pattern verifies the polycrystalline nature of ZnO nanorods and got an interplanar distance of 0.26 nm.

Abstract

Zinc Oxide (ZnO) has very attractive properties among nanomaterials with wide verity of applications. In this article simple and cost effective synthesis and detailed structure analysis of nanorods of ZnO prepared by chemical precipitation method at room temperature is reported. Powder of as synthesized ZnO nanorods are characterized by XRD, absorption spectra, dynamic light scattering (DLS), TEM and SAED. The XRD pattern shows the wurtzite crystal structure with average crystalline size as 29 nm by Debye-Scherrer formula and 24.5 nm by Williamson-Hall (W. H) plot. W. H plot also give a strain of -0.0004. From XRD, dislocation energies, lattice constants, d space are also tabulated. Broad peak in absorption spectra in ultra violet range indicate that the nanorods are applicable in textiles, sun creams and UV protection windows, industries as an ultraviolet absorber. Ultra violet radiation is very hazardous to human body. So, materials, especially nanomaterials with large surface to volume ratio and ultra violet absorption property have got a lot of acceptance. TEM characterizations also prove the nanorod structure with diameter of the order of 50 nm and length greater than 200 nm. DLS experiment estimated the size of synthesized structure. Selected area electron diffraction (SAED) pattern verifies the polycrystalline nature of ZnO nanorods and got an interplanar distance of 0.26 nm.

Keywords

Zinc oxide nanorods; Precipitation; Dynamic Light Scattering (DLS)

Introduction

Among II-VI group compounds, zinc oxide (ZnO) is very popular with large number of applications [1-3]. In bulk form, it possesses a wide direct bandgap of 3.3 eV [4-6]. In nanoform, the bandgap can be varied by varying the size of nanoparticle. The bandgap engineering property of nanostructures makes them possible to use in verity of applications [2]. Properties of nanostructures of ZnO can be tuned by changing its morphology and size [7-8]. Most common structure of ZnO is hexagonal wurtzite with lattice constants a and b equal to 3.2495 Å and c equals 5.2062 Å [9-10]. The ratio of c/a is 1.6. Wurtzite structure has no center of symmetry [11]. Hexagonal close packed zinc and oxygen lattices intertwined to form wurtzite structure [2]. Both zinc and oxygen ion has a coordination number of 4. Sp3 hybridized crystals generally show n-type conductivity [2,12].

ZnO has been recognized as pigment in paintings from earlier period itself. It is also applied as vulcanization accelerator in rubber industries, as chemical in ointments and in skin protectors [2,9] etc. Recently, properties like high electron mobility, stability and large specific surface area of ZnO are used in gas sensing applications [13-17]. Organic pollutants can be removed from water by making use of ZnO as photocatalyst for photodegradation [18-19]. Because of high electron mobility of ZnO, it can be used in excitonic solar cells like hybrid [1] and dye sensitized solar cells [20] as charge transport material. Low refractive index of 2.05, and high exciton binding energy of 60 meV attracts application of ZnO to Optical devices [2]. Since the exciton binding energy is inversely proportional to the square of Bohr exciton radius, large exciton binding energy indicates the tight binding among excitons. So thermal degradation of exciton will not occur at room temperature and this will help to design UV laser and detectors at room temperature.

Among wide bandgap semiconductors with similar structure and properties, ZnO with high exciton binding energy has an advantage of room temperature excitonic transitions [7, 10, 21-24]. In piezoelectric devices, the large value of piezoelectric coupling coefficient of ZnO is very useful [25-26]. The absorption capacity in UV range and non-toxic behavior [11,27] lead to wide acceptance in cosmetics. Due to the electrochemical qualities, ZnO and its nanocomposites can also be used as electrode in supercapacitors [28-29]. Studies also show that, ZnO has antibacterial properties to resist gram positive and negative bacteria, Bacillus subtilis, e coli etc. [27-30]. In textile industry, ZnO nanoparticles act as UV light absorber [31].

ZnO nanorods synthesis can be done by various methods. Y. Tak et al prepared ZnO nanorods by thermal evaporation and solution method on silicon substrate [32]. L. Wang et al. [17], and O. Akhavan [19] synthesized nanorods by hydrothermal treatment. W.I Park et al synthesized nanorods of ZnO by electron beam evaporation method [33]. In this work, ZnO nanorods are synthesized by economically feasible and simple [7] chemical precipitation method at room temperature. Because of no need of templates or catalysts, chemical precipitation method is good among large scale production methods [6]. Zinc acetate dihydrate and sodium hydroxide are used as precursors. Polyvinyl pyrrolidine (PVP) is used as capping agent. The reaction is thermodynamically favourable with releasing of energy at the time of precipitation of ZnO [34]. A. M. Pourrahimi et al. proved that nanostructure ZnO created from Zinc acetate is more stable than zinc chloride and zinc nitrate and zinc sulphate [35]. In this article, cost effective preparation of ZnO nanorods by chemical precipitation method and structure analysis of the synthesized nanostructure by various methods like XRD, W-H plot, TEM, SAED etc is reported.

Methods

0.2 molar of zinc acetate dihydrate (ZnC4H6O4).2H2O and 0.2 molar of sodium hydroxide (NaOH) were used as precursors. Poly vinyl pyrrolidine (PVP) was used as capping agent. All the chemicals were of analytical grade. To get 0.2 molar of zinc acetate solution, 2.2 g of it was dissolved in 50 ml deionised water. At the time of stirring in 1200 rpm, 1 g of PVP dissolved in 50 ml water was poured drop by drop to the solution. After getting a uniform solution, 0.2 molar sodium hydroxide solution was added drop by drop to the solution and it continually stirred for 3 hours at 1200 rpm to get precipitated. The whole process was carried out at room temperature. The supernatant precipitate containing nano size zinc oxide is separated and washed with water and iso propyl alcohol. The final product was dried in an air oven at 100°C for one hour.
The chemical reaction of synthesis is,

Crystal structure of as synthesised ZnO was examined by Xray diffractometer (PANalytical X’Pert PRO) with Cu-Kα radiation. Wave length of radiation is 0.15406 nm and scan range was 10-90 degree, glancing angle scan. UV-Vis-NIR spectrometer (Jasco-V-570, UV/VIS) with detector resolution of 0.1 nm in UV-Visible region and 0.5 nm in NIR region was used to plot absorption spectra. Dynamic light scattering characteristics was done by (DLS, SZ-100, Horiba Scientific) for 40 ns, 80 ns and 120 ns gate delay times. Transmission electron microscopic (TEM) images were taken to analyse the nanostructure formation. SAED was taken to confirm the crystalline structure and to find the d spacing.

Results and Discussions

The XRD pattern of synthesised ZnO is shown in FIG. 1a. The distinct peaks at angles (2Ѳ) 32.0499, 34.7190, 36.5377, 47.8596, 56.8586, 63.1269, 68.1995, 69.3278 indicate the structure of ZnO as wurtzite hexagonal structure. Corresponding crystallographic planes are (100), (002), (101), (102), (110), (103), (112), (203). It is verified using JCPDS data (card number: 36-1451) and listed in TABLE 1. Interplanar distance (d-spacing) is calculated using Bragg’s diffraction equation (1). Average crystalline size (D) is determined using the Debye-Scherrer equation (2) [10,36-40]. Where β is the full width at half maximum (FWHM) of various XRD peaks. Crystalline sizes estimated are listed in TABLE 1. Crystalline size may different from particle size. Crystalline size is the size of crystal with coherent diffraction [41].