Sol–gel synthesized zinc oxide nanorods and their structural and optical investigation for optoelectronic application
© Foo et al.; licensee Springer. 2014
Received: 29 January 2014
Accepted: 14 August 2014
Published: 25 August 2014
Nanostructured zinc oxide (ZnO) nanorods (NRs) with hexagonal wurtzite structures were synthesized using an easy and low-cost bottom-up hydrothermal growth technique. ZnO thin films were prepared with the use of four different solvents, namely, methanol, ethanol, isopropanol, and 2-methoxyethanol, and then used as seed layer templates for the subsequent growth of the ZnO NRs. The influences of the different solvents on the structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, and photoluminescence. The obtained X-ray diffraction patterns showed that the synthesized ZnO NRs were single crystals and exhibited a preferred orientation along the (002) plane. In addition, the calculated results from the specific models of the refractive index are consistent with the experimental data. The ZnO NRs that grew from the 2-methoxyethanol seeded layer exhibited the smallest grain size (39.18 nm), largest diffracted intensities on the (002) plane, and highest bandgap (3.21 eV).
Top-down and bottom-up methods are two types of approaches used in nanotechnology and nanofabrication . The bottom-up approach is more advantageous than the top-down approach because the former has a better chance of producing nanostructures with less defects, more homogenous chemical composition, and better short- and long-range ordering . Semiconductor nanorods (NRs) and nanowires possess convenient and useful physical, electrical, and optoelectronic properties, and thus, they are highly suitable for diverse applications [3, 4].
ZnO, one of the II-VI semiconductor materials, has attracted considerable interest because of its wide bandgap (approximately 3.37 eV), high exciton binding energy (approximately 60 meV), and long-term stability [5, 6]. ZnO has been applied in various applications, such as in light-emitting diode , gas and chemical sensors [8–10], ultraviolet (UV) detector [11, 12], solar cell [13, 14], and biomolecular sensors [15, 16]. To create high-quality ZnO NRs, various techniques have been proposed, such as the aqueous hydrothermal growth , metal-organic chemical vapor deposition , vapor phase epitaxy , vapor phase transport , and vapor–liquid-solid method .
Among these methods, the aqueous hydrothermal technique is an easy and convenient method for the cultivation of ZnO NRs. In addition, this technique had some promising advantages, like its capability for large-scale production at low temperature and the production of epitaxial, anisotropic ZnO NRs [21, 22]. By using this method and varying the chemical use, reaction temperature, molarity, and pH of the solution, a variety of ZnO nanostructures can be formed, such as nanowires (NWs) [16, 23], nanoflakes , nanorods , nanobelts , and nanotubes .
In this study, we demonstrated a low-cost hydrothermal growth method to synthesize ZnO NRs on a Si substrate, with the use of different types of solvents. Moreover, the effects of the solvents on the structural and optical properties were investigated. Studying the solvents is important because this factor remarkably affects the structural and optical properties of the ZnO NRs. To the best of our knowledge, no published literature is available that analyzed the effects of different seeded layers on the structural and optical properties of ZnO NRs. Moreover, a comparison of such NRs with the specific models of the refractive index has not been published.
ZnO seed solution preparation
Homogenous and uniform ZnO nanoparticles were deposited using the sol–gel spin coating method . Before seed layer deposition, the ZnO solution was prepared using zinc acetate dihydrate [Zn (CH3COO)2 · 2H2O] as a precursor and monoethanolamine (MEA) as a stabilizer. In this study, methanol (MeOH), ethanol (EtOH), isopropanol (IPA), and 2-methoxyethanol (2-ME) were used as solvents. All of the chemicals were used without further purification. ZnO sol (0.2 M) was obtained by mixing 4.4 g of zinc acetate dihydrate with 100 ml of solvent. To ensure that the zinc powder was completely dissolved in the solvent, the mixed solution was stirred on a hot plate at 60°C for 20 min. Then, 1.2216 g of MEA was gradually added to the ZnO solution, while stirring constantly at 60°C for 2 h. The milky solution was then changed into a homogenous and transparent ZnO solution. The solution was stored for 24 h to age at room temperature (RT) before deposition.
ZnO seed layer preparation
ZnO NRs formation
The surface morphology of the ZnO NRs was analyzed using scanning electron microscopy (SEM, Hitachi SU-70, Hitachi, Ltd, Minato-ku, Japan). X-ray diffraction (XRD, Bruker D8, Bruker AXS, Inc., Madison, WI, USA) with a Cu Kα radiation (λ = 1.54 Ǻ) was used to study the crystallization and structural properties of the NRs. The absorbed chemical compounds that exited on the surface of the ZnO NRs and SiO2/Si substrate were identified using the Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 400 spectrometer, PerkinElmer, Waltham, MA, USA). A UV-visible-near-infrared spectrophotometer from PerkinElmer was used to study the optical properties of the ZnO NRs at RT. In addition, the optical and luminescence properties of the ZnO NRs were studied through photoluminescence (PL, Horiba Fluorolog-3 for PL spectroscopy, HORIBA Jobin Yvon Inc., USA).
Results and discussion
XRD parameters of ZnO NRs
Measured structural properties of ZnO NRs using XRD for different solvents
XRD (100) peak position
XRD (002) peak position
Grain size (nm)
where λ is the X-ray wavelength of the incident Cu Kα radiation (0.154056 nm). For the bulk ZnO from the JCPDS data with card number 36–1451, the pure lattice constants a and c are 3.2498 and 5.2066 Å, respectively. Based on the results shown in Table 2, all of the ZnO NRs had lower lattice constant values compared with the bulk ZnO. The ZnO NRs prepared with MeOH (a = 3.23877 Ǻ and c = 5.20987 Ǻ) were closest to the bulk ZnO. This phenomenon can be attributed to the high-temperature annealing condition. Similar results were observed by Lupan et al. , in which the increase in temperature decreases the lattice constant of ZnO.
Direct bandgap, calculated refractive indices of ZnO NRs corresponding to optical dielectric constant
Refractive index ( n)
Optical constant (Ɛ ∞ )
where A = 25 Eg + 212, B = 0.21 Eg +4.25, and (Eg + B) refer to an appropriate average Eg of the material. The calculated refractive indices of the end-point compounds and Eg are listed in Table 3. In addition, the relation Ɛ ∞ = n2 was used to calculate the optical dielectric constant Ɛ ∞ . Our calculated refractive index values are consistent with the experimental values [23, 57–63], as shown in Table 3. Therefore, Herve and Vandamme model is an appropriate model for solar cell applications.
UV luminescence can be used to evaluate the crystal quality of a material, whereas visible luminescence can be used to determine structural defects . A study by Abdulgafour . indicates that a higher ratio of UV/visible is an indicative index of a better crystal quality. In the current study, the UV/visible ratios for the ZnO NRs prepared with the use of IPA, MeOH, 2-ME, and EtOH were 13.34, 12.15, 8.32, and 5.14, respectively. Therefore, the UV/visible ratio trend confirms the improvements in crystal quality of the ZnO NRs that were prepared using different solvents.
In this study, ZnO NRs with a highly crystalline structure were synthesized via a low-cost and convenient hydrothermal technique. The SEM images of the samples demonstrated that the diameters of the hydrothermally synthesized ZnO NRs range from 20 to 50 nm. The XRD patterns exhibited that all of the ZnO NRs had remarkably excellent crystal qualities and high c-axis orientations. The calculated bandgap values of the synthesized ZnO NRs were lower than that of the bulk ZnO. The crystal qualities, grain size, diameter, and optical bandgap of the ZnO NRs were affected by the type of solvent used in the ZnO seed layer preparation. The ZnO NRs that were synthesized with the use of 2-ME, a solvent, exhibited the most improved results, in terms of structural and optical properties; these ZnO NRs showed the smallest grain size, smallest crystallite size, and highest bandgap values. The method developed in this study provides a simple and low-cost approach to fabricate ZnO NRs with the desired properties.
The authors wish to acknowledge the financial support of the Malaysian Ministry of Higher Education (MOHE) through the FRGS grant no. 9003–00276 to Prof. Dr. Uda Hashim. The author would also like to thank the technical staff of the Institute of Nano Electronic Engineering and School of Bioprocess Engineering, University Malaysia Perlis for their kind support to smoothly perform the research.
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