Synthesis and Characterization of ZnO Nanorods and Nanodisks from Zinc Chloride Aqueous Solution
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- Long, T., Yin, S., Takabatake, K. et al. Nanoscale Res Lett (2009) 4: 247. doi:10.1007/s11671-008-9233-2
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ZnO nanorods and nanodisks were synthesized by solution process using zinc chloride as starting material. The morphology of ZnO crystal changed greatly depending on the concentrations of Zn2+ion and ethylene glycohol (EG) additive in the solution. The effect of thermal treatment on the morphology was investigated. Photocatalytic activities of plate-like Zn5(OH)8Cl2 · H2O and rod-like ZnO were characterized. About 18% of 1 ppm NO could be continuously removed by ZnO particles under UV light irradiation.
KeywordsZnO nanorod ZnO nanodisk Photocatalytic activity Zinc chloride
Zinc oxide with a hexagonal wurtzite crystal structure possesses unique optical and electronic properties, and wide applications on piezoelectric devices, transistors, photodiodes, photocatalysis [1, 2, 3, 4], etc. In recent years, much attention has been paid to nanostructure ZnO materials, and various morphologies of ZnO such as nanowire, nanorod, nanotube, nanobelt, nanoring, nanoneedles, and hollow structures, etc. have been developed [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Many methods have been employed for the morphological control of ZnO crystal, such as pulsed laser deposition(PLD) , chemical vapor deposition , spray pyrolysis [17, 18], thermal evaporation , wet-chemical route [20, 21], etc., in which the wet chemical route has been becoming a charming method due to the mild reaction condition and simplicity of the synthesis process. It is important to prepare well-crystallized and orientated ZnO nanoparticles. In most solution processes for the synthesis of ZnO nanoparticles, zinc acetate, and zinc nitrate are used as starting materials [21, 22, 23, 24], but using zinc chloride as a starting material was seldom reported. In the present study, ZnO with rod-like and plate-like structure were successfully synthesized from zinc chloride aqueous solution, and their photocatalytic properties were characterized.
ZnCl2, hexamethylenetetramine (HMT, C6H12N4), ethylene glycol (EG), commercial ZnO powder, butyl acetate, ethyl acetate, and nitrocellulose were used as starting materials. All these chemicals were used as delivered without further purification. Firstly, the cleaned borosilicate glass substrate was coated with thin film of ZnO nanoparticles by a spin-coater (Mikasa 1H-D7). The coating liquid was prepared by uniformly mixing 1 g commercial ZnO nano particles (Sumitomo Osaka Cement ZnO-350) with 2 g of industrial grade nitrocellulose, 5 g of ethyl acetate and 5 g of butyl acetate together with 50 g zirconia balls of 2.7 mm diameter with ball milling using a plastic bottle for 40 h. Then, the prepared substrate was calcined at 400 °C for 1 h. For the second step, the equimolar of ZnCl2 and HMT were dissolved in water or 50 vol.% EG aqueous solution. The ZnO nanoparticles coated glass substrates obtained in the first step were dipped into 50 mL of as-prepared solution containing a desired concentration of ZnCl2–HMT mixture and the solution was kept at 95 °C for 12 h in a sealed silicate-glass bottle. Finally, the glass substrate was taken out and washed with distilled water and acetone, then vacuum dried at 80 °C for 1 h. The morphology of the crystals was observed by SEM (Hitachi S-4800) and TEM (JOEL JEM-2000EX). The crystalline phase of the products was determined by X-ray diffraction analysis (XD-01,SHIMADZU). The specific surface area (SSA) was evaluated by nitrogen adsorption–desorption isothermal measurement at 77 K (NOVA-4200e). FT–IR measurements were conducted using the FTS7000 series (DIGILIB). Thermal gravimetry and differential thermal analysis (TG–DTA) curves were traced on a Rigaku Thermoflex (TG8101D) at a heating rate of 10 °C/min in air. The diffuse reflectance spectra of the samples were measured using an UV–vis spectrophotometer (Shimadzu UV-2450). The photocatalytic activity was evaluated by the oxidative destruction of nitrogen monoxide under irradiation of high pressure mercury arc of various light wavelengths using a flow type reactor with a NOx analyzer (Yanaco, ECL-88A) .
Results and Discussion
SSA and deNOxability of the samples prepared under different conditions, together with those of P25 titania
DeNOxphotocatalytic activity (%)
P25 Titania AEROXIDE®
0.01 M ZnCl2–HMT mixed aqueous solution
Zn5(OH)8Cl2 · H2O
0.05 M ZnCl2–HMT mixed aqueous solution
0.05 M ZnCl2–HMT mixed 50% EG solution
Zn5(OH)8Cl2 · H2O
0.1 M ZnCl2–HMT mixed 50% EG solution
Based on above results, the following conclusions might be drawn: The morphology and crystalline phase of the product by the heat treatment of ZnCl2–HMT aqueous solution with and without EG changed greatly depending on the concentrations of Zn2+ion and EG additive in the solution. Layered hexagonal plate-like Zn5(OH)8Cl2 · H2O were formed in 0.05 M and 0.1 M ZnCl2–HMT mixed aqueous solution and in 0.1 M ZnCl2–HMT mixed 50 vol.% EG aqueous solution. The existence of EG in the solution promote the homogeneous crystal growth, and also delay the formation of hexagonal plate-like structure. Hexagonal plate-like Zn5(OH)8Cl2 · H2O have comparatively higher SSA than that of rod-like ZnO crystal fabricated by the same method. Although the prepared ZnO samples showed lower photocatalytic activity compared with commercial titania powders, about 18% of 1 ppm NO was continuously removed.
This research was carried out as one of the projects under the Special Education and Research Expenses on “Post-Silicon Materials and Devices Research Alliance” and the JSPS Asian Core Program “Interdisciplinary Science of Nanomaterials”, JSPS Core University Program (CUP), supported by Nippon Sheet Glass Foundation for Materials Science and Engineering, Research for Promoting Technological Seeds, JST, and a Grant-in-Aid for Science Research (No.20360293).