Hydrothermally Grown ZnO Micro/Nanotube Arrays and Their Properties
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We reported the optical and wettability properties of aligned zinc oxide micro/nanotube arrays, which were synthesized on zinc foil via a simple hydrothermal method. As-synthesized ZnO micro/nanotubes have uniform growth directions along the  orientations with diameters in the range of 100–700 nm. These micro/nanotubes showed a strong emission peak at 387 nm and two weak emission peaks at 422 and 485 nm, respectively, and have the hydrophobic properties with a contact angle of 121°. Single ZnO micro/nanotube-based field-effect transistor was also fabricated, which shows typical n-type semiconducting behavior.
KeywordsZnO Nanotubes Arrays
One-dimensional (1-D) nanostructures become the focus of current research because of the unique properties related with their special structures, such as high surface-to-volume ratios, special compositions, etc. [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. They play important roles in fabricating nanoscale functional electronic, optoelectronic, electrochemical, and mechanical devices. Among the numerous 1-D nanostructures, 1-D metal oxide nanostructures have been widely investigated and now have been widely used in many areas, such as catalysts, sensors, ceramics, transparent oxide conductive films (TOC) and electronic devices [16, 17, 18, 19].
With a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, zinc oxide (ZnO)  has been attracting attention in both fundamental research and practical applications and has been considered as a promising material for gas sensors, varistors, and optical devices [5, 12]. Many kinds of 1-D ZnO nanostructures have been synthesized till now, such as nanowires, nanotubes, nanobelts, nanorings, nanonails and so on, by many groups including ours [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40]. It was found that the properties of 1-D ZnO nanostructures are affected by many factors, such as morphologies, compositions, and alignments [30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40].
In this paper, we present a simple hydrothermal method for the synthesis of aligned ZnO micro/nanotube arrays on zinc foil. These micro/nanotubes have uniform growth directions along the  orientations with diameters in the range of 100–700 nm. Room-temperature photoluminescence (PL) properties of these micro/nanotubes were investigated and they showed a strong emission peak at 387 nm and 2 weak emission peaks at 422 and 485 nm, respectively. As-synthesized ZnO micro/nanotube arrays also show hydrophobic properties with a contact angle of 121°. Finally, single micro/nanotube-based field-effect transistor was also fabricated to investigate their electronic transport properties.
All chemical reagents used in the experiment are of analytical grade and used without further purification. In a typical procedure, 5 mmol zinc acetate and equal amount of hexamethylenetetramine (HMT) were dissolved in 30 ml deionized water under stirring. And then 10 mL ammonia was put into the above mixed solution. Keeping stirring for 15 min, the transparent solution was transferred into a PTFE-lined autoclave with volume of 50 ml. A zinc foil after ultrasonic treatment was then put into the autoclave. After sealed, the autoclave was put in an oven and heated at 130°C for 7 h. After reaction, the foil was taken out of the autoclave and washed with water for several times and then dried in air.
X-ray diffraction (XRD) pattern of the products was carried out on a D/max-rB of Kaisha X-ray diffractometer with 2θ in the range of 30°–80°. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were taken on a Hitachi S-4800 field-emission scanning electron microscope equipped with an energy-dispersion X-ray detector. The microstructures of the product were investigated using a high-resolution transmission electron microscope (HRTEM, JEM-3000F). Room-temperature photoluminescence (PL) was curried out with a SPEX FL-2T2 fluorophotometer with an excited wavelength of 325 nm. Water contact angle (CA) of the ZnO micro/nanotube arrays was measured by using a Dataphysics OCA20 contact angle system at ambient temperature. All the electrical measurements were carried out using a semiconductor parameter analyzer (Agilent 4156B apparatus).
Results and Discussion
In summary, ZnO micro/nanotube arrays have been successfully synthesized via a simple hydrothermal method on a zinc foil substrate. As-synthesized micro/nanotubes are single crystals with growth directions along the  orientations. These micro/nanotubes show strong near-band-edge emission at 387 nm and weak defects-related emissions at 422 and 485 nm, respectively. Contact angle result indicates they have good hydrophobic properties. Field-effect transistors were fabricated based on the ZnO micro/nanotubes, which show n-type semiconducting characteristics. Our results show that the hydrothermally synthesized ZnO micro/nanotubes may be used as self-cleaning photocatalysts as well as building blocks for nanoscale electronic and optoelectronic devices.
This work was supported by the High-level Talent Recruitment Foundation of Huazhong University of Science and Technology, the Basic Scientific Research Funds for Central Colleges (Q2009043), the Natural Science Foundation of Hubei Province (2009CDB326), the Doctor Start-up Fund of Harbin Normal University (KGB200802), the National Natural Science Foundation of China (20871037), the Natural Science Foundation of Heilongjiang Province (B2007-2) and the Science Technology and Research Project of Education Bureau, Heilongjiang Province (11531229, 12531236).
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