Synthesis and Characterization of ZnO Nanorods and Nanodisks from Zinc Chloride Aqueous Solution

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.


Introduction
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) [15], chemical vapor deposition [16], spray pyrolysis [17,18], thermal evaporation [19], 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.

Experimental
ZnCl 2 , hexamethylenetetramine (HMT, C 6 H 12 N 4 ), 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 ZnCl 2 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 ZnCl 2 -HMT mixture and the solution was kept at 95°C for 12 h in a sealed silicateglass 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,SHI-MADZU). 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 NO x analyzer (Yanaco, ECL-88A) [25]. Figure 1 shows the morphologies of the samples prepared in aqueous solutions of equimolar of ZnCl 2 -HMT mixture with and without 50 vol.% EG additive. The initial concentrations of Zn 2? were 0.01 M, 0.05 M, and 0.1 M, respectively. The morphology of the product changed greatly depending on EG additive and concentration of Zn 2? . In the absence of EG, at such low initial ZnCl 2 -HMT concentration of 0.01 M, the product exhibited uniform spindle-like structure with the length of more than 20 lm (Fig. 1a). When ZnCl 2 -HMT concentration increased to 0.05 M, hexagonal plate-like morphology with diameter of about 50 lm was formed (Fig. 1b, c).

Results and Discussion
In contrast, with 50 vol.% EG additive in the 0.01 M ZnCl 2 -HMT mixed aqueous solution, instead of spindlelike structure, ellipse-like head rod structure with smaller size (half length to that of spindle-like structure (Fig. 1a) was formed (Fig. 1d). When the initial concentration of ZnCl 2 -HMT reached 0.05 M, the morphology changed to a rod-like hexagonal structure (Fig. 1e). When ZnCl 2 -HMT concentration was 0.1 M, as shown in Fig. 1f, the product exhibited similar hexagonal plate-like morphology to that in the absence of EG, whereas the plate-like particle size decreased to about half to that prepared at the same zinc concentration in the absence of EG (Fig. 1c). These results indicated that zinc ion concentration and EG additive acted very important roles in the morphological control of particles during the solution synthesis process. The existence of EG made the crystals to grow homogeneously due to its good dispersibility and glutinosity. It was also found that in the absence of HMT in 0.1 M ZnCl 2 solution, only small amount of products with nonhomogeneous morphologies consisted of spherical and rod-like structure could be obtained, although the image was not shown here.  2 and Zn(CH 3 COO) 2 as Zn 2? source. The platelike structure strongly related to the formation of simonkolleite possessing layered structure [26]. It might be inferred that the high concentration of zinc ion and chloride ion preferred to form simonkolleite structure. Since the formations of ZnO and Zn 5 (OH) 8 The samples with various morphologies prepared in aqueous solution and 50% EG aqueous solution were characterized. Figure 3 shows the FT-IR spectra of the prepared rod-like ZnO and plate-like Zn 5 (OH) 8 Cl 2 Á H 2 O samples. It is accepted that zincite (ZnO) only has the bands in the 350-600 cm -1 region corresponds to the zincoxygen bond [27]. It was obvious that Fig. 3a , c showed the similar spectra with that of ZnO crystal. On the other hand, the samples b and d in Fig. 3 showed quite different spectra to those of Fig. 3a, c. A strong band due to deformation vibration of H 2 O molecules at 1,630 cm -1 , an absorption band centered at 3,434 cm -1 represented the characteristic of an O-H stretching vibration, and intensive bands at 898 and 720 cm -1 due to stretching vibration modes of chloride ion [28][29][30] were observed. These results indicated that simonkolleite (Zn 5 (OH) 8 Cl 2 Á H 2 O) preferred to be formed at increased chloride ion concentration. These FT-IR data agreed with the XRD patterns shown in Fig. 2. Figure 4 showed the TG-DTA curves of the hexagonal plate-like simonkolleite samples. The samples prepared in different zinc ion concentration showed similar TG-DTA curves. The weight loss until 100-120°C was related to the adsorbed water molecules on the surface of the samples. Other two obvious weight loss accompanying with endothermic peak could be observed around 200 and 380°C, respectively. These weight loss might be ascribed to the dehydration from OHgroups and release of hydrogen chloride from the simonkolleite Zn 5 (OH) 8 Fig. 4 were 26-28 wt% which agreed well to the calculated value.
To confirm the morphological change and phase transformation behavior of the hexagonal plate-like simonkolleite, the sample was heat treated in air at 300, 600, and 1200°C, and their XRD patterns and SEM photographs are shown in Figs. 5 and 6, respectively. As shown in Fig. 5  might be observed that the (002) peak of the sample prepared by calcination showed higher intensity than those of (100) and (101) peaks, indicating the preferred orientation of ZnO particles, remember usually the ZnO possessed comparatively weak (002) peak intensity (ICSD No. 89-1397). Figure 6 indicated that the morphology also changed during the heat treatment process. After heat treatment at 300°C, some weak trace of porous structure on the surface of the samples could be observed. With the increment of treatment temperature, the porosity on the sample surface increased. This behavior related to the decomposition of simonkolleite structure, i.e., the increment of porosity might be caused by the release of water and hydrogen chloride from Zn 5 (OH) 8 Cl 2 Á H 2 O at high temperature. Figure 7 shows the TEM photographs of the plate-like Zn 5 (OH) 8 Cl 2 Á H 2 O and those after thermal treatment at different temperatures. It is obvious that plate-like Zn 5 (OH) 8 Cl 2 Á H 2 O possessed smooth surface and those after thermal treatment possessed porous structure, and the porosity increased with treatment temperature.
The SSA and deNO x photocatalytic activities of the asprepared samples with different morphologies were characterized. For comparison, a standard titania photocatalyst aeroxide Ò P25 was also characterized and the results are summarized in Table 1. The aeroxide Ò \P25 titania powder is usually used as a reference sample in photocatalytic   research, because of its high photocatalytic activity. As shown in the Table 1, the Zn 5 (OH) 8 Cl 2 Á H 2 O with hexagonal plate-like structure prepared in both water or 50vol.% EG aqueous solution showed such high BET specific surface areas as 19.4 m 2 /g and 19.7 m 2 /g, respectively, while those of the as-prepared rod-like ZnO showed low values of 3-4 m 2 /g, however, ZnO samples showed higher deNO x activity than those of Zn 5 (OH) 8 Cl 2 Á H 2 O. The high SSA of the simonkolleite might be related to its very thin plate-like structure. Although the prepared samples showed lower photocatalytic activity compared with commercial titania powders, it was notable that about 18% of 1 ppm NO was continuously removed under UV light irradiation. Figure 8 shows the DRS spectra of the samples prepared at 95°C in ZnCl 2 -HMT mixed aqueous solution and 50 vol.% EG aqueous solution together with that of commercial ZnO. The spindle-like ZnO showed similar DRS