Structural and optical properties of ZnO nanorods by electrochemical growth using multi-walled carbon nanotube-composed seed layers
© Ko et al; licensee Springer. 2012
Received: 2 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
We reported the enhancement of the structural and optical properties of electrochemically synthesized zinc oxide [ZnO] nanorod arrays [NRAs] using the multi-walled carbon nanotube [MWCNT]-composed seed layers, which were formed by spin-coating the aqueous seed solution containing MWCNTs on the indium tin oxide-coated glass substrate. The MWCNT-composed seed layer served as the efficient nucleation surface as well as the film with better electrical conductivity, thus leading to a more uniform high-density ZnO NRAs with an improved crystal quality during the electrochemical deposition process. For ZnO NRAs grown on the seed layer containing MWCNTs (2 wt.%), the photoluminescence peak intensity of the near-band-edge emission at a wavelength of approximately 375 nm was enhanced by 2.8 times compared with that of the ZnO nanorods grown without the seed layer due to the high crystallinity of ZnO NRAs and the surface plasmon-meditated emission enhancement by MWCNTs. The effect of the MWCNT-composed seed layer on the surface wettability was also investigated.
PACS: 81.07.-b; 81.16.-c; 81.07.Pr; 61.48.De.
In the past decade, various ZnO nanostructures including nanowires, nanorods, nanosheets, nanoflowers, and nanotubes have received intensive attention because of their excellent physical properties for a wide range of practical device applications such as ultraviolet photodetectors, field-effect transistors, light-emitting diodes, and biological and chemical sensors [1–3]. Among many fabrication approaches, a hydrothermal or electrochemical deposition method has been considered as an efficient way to grow ZnO nanostructures since it is a simple, low-temperature, large-scale, and cost-effective process [4, 5]. In order to grow high-quality ZnO nanostructures in such chemical synthesis methods, the seed layer is very important because the orientation and crystallinity of ZnO nanostructures depend on the conditions of the underlying seed layer . For this reason, the radio frequency [rf] sputtering or atomic layer deposition and subsequent thermal annealing treatment have been employed to form a good seed layer with excellent step coverage, thickness controllability, and reproducibility. However, it requires a somewhat complicated procedure and high-vacuum environment.
Meanwhile, carbon nanotubes [CNTs] have been one of the most advanced functional materials because of their superior electronic property, good thermal/chemical stability, high mechanical strength, and large surface area [7–9]. Recently, the ZnO/CNT composites and hybrid nanostructures have been considered as a promising candidate for improving the device efficiency in the electronic and optoelectronic devices because these structures can provide the enhanced electrical and optical properties by the cooperative physical interaction between ZnO nanostructures and CNTs . However, achieving good control over the size and morphology of the ZnO/CNT hybrid structures is still difficult. Thus, the investigation of the seed layer containing CNTs for growing the ZnO nanostructures is very interesting. In this work, the electrochemically synthesized ZnO nanorod arrays [NRAs] after spin-coating an aqueous seed solution containing MWCNTs, which can be expected to be a facile and efficient process for the fabrication of high-quality ZnO NRAs, were studied. Their structural and optical properties were also evaluated.
All chemicals were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA) and Kojundo Chemical Laboratory Co., Ltd. (Saitama, Japan), which were of analytical grade and used without further purification. The MWCNTs and indium tin oxide [ITO]-coated soda-lime glasses were also purchased from Hanwha-Nanotech (Incheon, South Korea) and Samsung Corning (Seoul, South Korea), respectively. The ITO coated on the soda-lime glass (i.e., ITO/glass) substrate was fabricated by rf magnetron sputtering. The samples were cleaned by acetone, methanol, and deionized [DI] water under sonication. To prepare an aqueous seed solution, the 0.1 M zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was dissolved in DI water. A sonication process was performed while slowly adding the MWCNT paste which was grown by a thermal chemical vapor deposition method. Then, the ITO/glass substrate was spin-coated with this seed solution at 3,000 rpm for 90 s and dried at a hot plate of 80°C for 10 min. After the spin-coating-and-drying procedure was repeated successively five times for a uniform seed layer coating, the sample was heated at 200°C for 1 h to increase the adhesion between the seed layer containing MWCNTs and the substrate. In order to electrochemically grow the ZnO NRAs, the seed layer-coated ITO/glass and platinum [Pt] electrode were immersed into the electrolyte solution containing 2 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 2 mM hexamethylenetetramine (C6H12N4), and DI water. During the electrochemical growth, the temperature of the electrolyte solution and the applied cathodic voltage were kept at 80°C and -2 V, respectively. After the synthesis of ZnO NRAs, the sample was rinsed with flowing DI water and dried by flowing nitrogen gas.
The morphology and structural properties of the fabricated samples were analyzed using a field-emission scanning electron microscope [FE-SEM] (LEO SUPRA 55, Carl Zeiss, Oberkochen, Baden-Württemberg, Germany) with an operating voltage of 15 kV. To prevent or reduce the electric charge accumulation, the samples were coated by Pt sputtering. The orientation and crystallinity of the samples were characterized using an X-ray diffractometer (M18XHF-SRA, Mac Science, Yokohama, Japan) with a monochromated Cu Kα line source (λ = 0.154178 nm). The photoluminescence [PL] measurements were performed using a PL mapping system (RPM 2000, Accent Optics, Denver, CO, USA) with a laser source emitting at the wavelength of 266 nm at room temperature. The macroscopic surface property on wettability was characterized from the measurement of the contact angles with the water droplet on the surface of the samples using a contact angle measurement system (Phoenix-300, SEO Co., Ltd., Gyeonggi-do, South Korea) with a measurement accuracy of ± 0.1°.
Results and discussion
The structural and optical properties of electrochemically synthesized ZnO NRAs on the ITO/glass substrate using MWCNT-composed seed layers formed by a simple method were investigated. It was found that the morphology of ZnO NRAs strongly depends on the kinds of seed layers. The optimized MWCNT-composed seed layer resulted in the high-density, well-aligned ZnO NRAs with a high crystallinity. The PL peak intensity of the NBE UV emission in ZnO/MWCNT hybrid nanostructures was significantly increased due to the surface plasmon-meditated emission enhancement by MWCNTs as well as the improved crystallization property. Also, the surface macroscopic property on the wettability could be modified with more hydrophilic characteristics. This simple electrochemical fabrication method using the seed layer containing CNTs is very useful to grow high-quality ZnO NRAs on an ITO/glass substrate for various optoelectronic device applications.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2010-0016930 and no. 2010-0025071).
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