STRUCTURAL, COMPOSITIONAL,
MORPHOLOGICAL AND OPTICAL PROPERTIES OF ZnO NANORODS
R. B. Kale
1, Shih-Yuan Lu
21Department of Physics, Institute of Science, Mumbai, India (M. S.)
2Department of Chemical Engineering, National Tsing-Hua University, Hsinchu, Taiwan 30013, Republic of China
ABSTRACT
A simple, low cost, green hydrothermal route has been developed to synthesize ultra-long ZnO nanorods at relatively low temperature. Zinc sulfate was used as the zinc ion source and sodium hydroxide solution was used as a source of hydroxide ions as well as to increase the pH of the resultant precursor. The as-synthesized ZnO product was characterized with several analytical instruments. X-ray diffraction study reveals well crystallized hexagonal crystal structure of ZnO product without post-heat treatment. Scanning electron observations showed the uniformity in the ultra-long ZnO nanorods of high aspect ratios with a hundred percent morphological yield. Energy dispersive X-ray analyses confirmed that the product was slightly deficient in zinc species. Ultraviolet-visible absorption study showed the excitonic peak near the bulk band gap value.
Photoluminescence spectroscopic study revealed that the light emission was due to transition of electrons from conduction band to valance band and the emission due to the defect states originated during the hydrothermal growth of nanorods.
Keywords: Semiconductors; Hydrothermal synthesis; Structural characterization; Optical properties
1. INTRODUCTION
Presently and in the forthcoming days nanomaterials are key issues of research because of the efforts that are being continued to fabricate commercially useful nanodevices, using these nanomaterials. Since nanomaterials possess special size and shape dependent optoelectronic properties. These materials have exhibited distinct properties that differ from those of their bulk counterparts, due to their small sizes and large surface to volume ratios. Particularly, one dimensional (1D) products such as nanorods, nanowires, nanotubes, nanobelts have received considerable research attention owing to their unique applications in mesoscopic physics and fabrication of nanodevices [1].
Zinc oxide, with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, is an important semiconducting and piezoelectric material. It is a smart material that has versatile applications in the fields of optoelectronics, piezoelectric sensors, transducers, and resonators. The wide band
gap and large exciton binding energy make ZnO suitable for short wavelength optoelectronic applications and ensure an efficient excitonic emission at room temperature. ZnO is transparent to visible light and can be made highly conductive by doping. It is also cheap, easily grown, biosafe and biocompatible. It can be directly used in biomedical applications without any coating [2-4]. Apart from this, 1D ZnO nanostructures proved to be valuable candidates for field emission devices [5]. Many researchers worked on the synthesis of 1D ZnO nanomaterials using different methods and the results have been reported. They used different methods such as conventional sputter deposition technique [6], chemical vapor deposition (CVD) [7], thermal evaporation [8], pulsed laser deposition [9], molecular beam epitaxy [10] and physical vapor deposition [11]. However, these methods require special equipments, complex process to control gas flow with high temperatures and are expensive that is unfavorable for an industrialized process. Recently, a lot of reports were available on the preparation of 1D ZnO nanostructures through hydrothermal routes with or without structure directing agents [12-18]. It is worth to mention that the products synthesized using hydrothermal methods often originate with flowerlike micron-sized 3D structure and thus face the problem of morphological non-uniformity. A simple process to synthesize ZnO nanorods with high aspect ratios by a one-step, hydrothermal method, without templates or substrates, has been scarcely reported.
In this research, a simple, inexpensive hydrothermal route was developed to grow ZnO nanorods of fascinating high aspect ratios without using surfactants, catalysts, toxic solvents, etc.
II. EXPERIMENTAL
The procedure to prepare high quality ZnO nanorods is as follows. Initially, an appropriate amount of ZnSO4 (0.05 M, 60 ml) and NaOH (1 M, 60 ml) aqueous solutions was separately prepared in de-ionized water. Then NaOH solution is dripped into ZnSO4 solution under constant stirring. Then the resultant solution is transferred to Teflon-lined stainless steel autoclave Then the autoclave was sealed tightly and maintained at 170 OC for 24 h, and then cooled to room temperature naturally. The white precipitate was collected from the bottom of the container. The precipitate was washed with ethanol followed by water repeatedly and dried in air at 80 OC for 12 h.
The crystal structure of the product was analyzed using X-ray diffractometer (Phillips PW-1710) with CuKα (λ
= 1.5406 Å) radiation. The microstructure of the ZnO product was studied using a scanning electron microscope (SEM) (JEOL, JSM-5600) equipped with an energy dispersive X-ray analyzer (EDAX). UV-Vis-NIR spectrophotometer (Hitachi Model-330, Japan) was used to measure the optical absorbance. Photoluminescence (PL) study was carried out with fluorescence spectrometer (Hitachi F-4500) equipped with a xenon lamp (150 W).
III. RESULTS AND DISCUSSION
3.1 Structural Study
The X-ray diffraction (XRD) pattern and relevant information are shown in Fig. 1. The well resolved sharp XRD peak reveals the well-crystallized nature of the as-synthesized ZnO product. The comparison of observed and standard (JCPD data file no. 36-1451) 2θ values of the peaks, confirms the well known hexagonal crystal structure with the lattice constant a = 3.229 Å and c = 5.177 Å. The values of the lattice constants were slightly less than the standard bulk values (a = 3.2498 Å and c = 5.2066 Å) and the peaks were slightly shifted to higher angles (inset of Fig. 1). It may be due to strains originated from nanosized ZnO products during the hydrothermal growth.
3.2 Compositional Study
The typical EDAX pattern of the ZnO product and its analytical information is depicted in Fig. 2. The pattern shows strong peaks of Zn and O elements without any other impurity peaks. The average atomic percentages of Zn and O species are 45.84: 54.16; slight rich in oxygen may be due to atmospheric dissolution of H2O. The peaks of C and Au are centered at 0.27 and 2.15 KeV, respectively, that are due to the carbon coated grid and gold sputtering over the sample.
Fig. 1: The XRD pattern of ZnO nanorods
Fig. 2: The EDAX pattern of ZnO nanorods
Fig. 3: SEM images of ZnO nanorods with different magnifications (a) 1.4 kX (b) 11 kX (c) 15 kX
3.3 Morphological Study
Figs. 3(a-c) shows the SEM images of ZnO synthesized using the hydrothermal method. SEM images reveal that the product was morphologically uniform and consisted of densely packed nanorods. The diameter and length of the nanorods were in the range of 100-200 nm and 4-6 μm, respectively. Hence the aspect ratio of the nanorods was in the order of 40. This type of high aspect ratio nanorods is intriguing candidates for light emitting devices. Also the nanorods showed a sharp tip at one end that corresponds to the (001) facet of the hexagonal ZnO product. A number of reports [19, 20] are available on the oriented attachment growth of ZnO nanorods grown under hydrothermal conditions. In the present study, the rods are well separated from each other and no sign of any oriented attachment growth. It may be due to the larger volume capacity of the autoclave that exerts a lower pressure during the hydrothermal growth of the ZnO nanorods. The reaction mechanism for the growth of ZnO nanorods can be expressed as:
Zn2+ + 2OH- → Zn(OH)2 (1) Zn(OH)2 + 2OH- → Zn(OH)42- (2) Zn(OH)42- → ZnO + H2O + 2OH- (3)
At the early stage of the reaction, ZnO nucleates spontaneously from the solution of Zn(OH)42- . Then, the initially formed ZnO acts as seed nuclei for the growth of 1D ZnO nanostructure along the <001> direction to form nanorods.
3.4 Optical Study
The ultra-long ZnO nanorods obtained in the present hydrothermal condition may possess interesting optical properties. To study these properties, UV-visible absorptions and photoluminescence (PL) measurements were carried out. Fig. 4 shows the UV-visible spectrum of the ZnO nanorods measured at room temperature and exhibits typical absorption characteristics of ZnO. The curve shows typical excitonic characteristics and an excitonic peak maxima centered at 372 nm in good agreement with earlier reports as well as bulk ZnO [21, 22]
and no quantum confinement effects have been observed. This confirms the well crystallized structure and high optical quality of the as-prepared ZnO nanorods.
Fig. 5 illustrates the PL spectrum of the ZnO nanorods at room temperature under photon excitation of 325 nm.
Two strong emission bands including a strong near band edge (NBE) ultraviolet emission band centered at ~ 397nm and a defect related strong blue emission band at 462 nm. It is worth to note that, as compared with previously reported results for ZnO nanorods and nano-wires [23-25], the UV emission of the present ZnO product exhibits a red-shift. Numerous reports [12, 22, 26-29] are available on the red-shift in the NBE emission peak of ZnO products with different morphologies. Previous researcher reported that the PL spectrum of ZnO is sensitive to the particle shape, size, temperature, preparation method [23, 27-29], etc. Therefore, this particular UV emission behavior may be related to the ultra-long ZnO nanorods having nanometer-sized diameters and micron-sized lengths. The UV emission should be attributed to the radiative recombination of excitons [12, 26].
The blue emission peak centered at 463 nm may correspond to the deep level or trap-state emission that is originated due to structural defects. In previous works, the blue band emissions from ZnO were also reported.
Nevertheless the exact origins have not been assigned so far [12, 14, 26, 30, 31]. Fig. 6 shows SEM image of ZnO synthesized using Zn[CH3COOH] (0.05 M, 60 ml), instead of ZnSO4 (0.05 M, 60 ml) without disturbing other experimental conditions. SEM image clearly shows noticeable changes in morphology of synthesized ZnO nanorods. It reveals not only increase in diameter and decrease in length with prismatic hexagonal facet but also spectacular flowerlike morphology. It is worth to mention that the Zinc precursor plays an important role during the growth of ZnO nanoparticles. The previous results [32] demonstrated that high aspect ratio and aggregate flower-like ZnO nanorods can be used as the gas sensing material for fabricating high performance gas sensors.
Fig. 4: UV-Visible absorption spectrum of ZnO nanorods Fig. 5: PL spectrum of ZnO nanorods
IV. CONCLUSION
The Zno nanorods of high aspect ratio with excellent crystal quality have been successfully synthesized by a simple hydrothermal, environmentally benign route. XRD study revealed well crystallized wurtzite structure of the ZnO nanorods. The UV-visible study confirms the bulk band gap value of the ZnO nanorods without any quantum confinement effects. The PL shows the strong NBE emission peak along with an emission that corresponds to defects that are originated during the hydrothermal growth of the ZnO nanorods. Zinc precursor also plays a vital role in the reaction and growth mechanism of ZnO and dramatically changes the morphology of the synthesized ZnO material.
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