32 Journal of Applied and Industrial Sciences, April, 2013, 1 (1): 32-35
Synthesis of ZnO Nanostructures and their Photocatalytic Activity Nazar Elamin1*, Ammar Elsanousi1,2 1
College of Applied and Industrial Sciences, University of Bahri, Khartoum, Sudan, Tel: 00249-123656557, E-mail:
[email protected]
2
School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, P. R. China, Tel: 0086-18222552523, E-mail:
[email protected] (Received: January 10, 2013; Accepted: February 27, 2013)
Abstract- Zinc Oxide (ZnO) nanostructured materials (nanotubes and nanosheets) have been synthesized via a simple hydrothermal method. Keeping the hydrothermal temperature fixed (at 90oC), it was found that the hydrothermal treatment duration has a strong effect on the morphological structure of the resulting product, where it was observed that nanosheets are formed at shorter treatment durations (10h) and by increasing the treatment duration (to about 14h) they transform into nanotubes, probably by the rolling mechanism. The photocatalytic activity of both nanostructures has been examined by the degradation of Methyl Orange using UV light (λ =253.7 nm), and results showed that ZnO nanosheets are more effective on the degradation of the MO than the nantubes, due to their high surface area. These results indicate that ZnO nanosheets can be a good choice for the treatment of organic waste-water in future. Keywords: Methyl Orange, Nanotubes, Nanosheets, Photocatalytic activity, Zinc Oxide.
1. INTRODUCTION In a photocatalytic system, photo-induced molecular transformation or reaction takes place at the surface of the catalyst. A basic mechanism of photocatalytic reaction on the generation of electron–hole pair and its destination is as follows: when a photocatalyst is illuminated by the light stronger than its band gap energy, electron migrates from valance band (VB) to conduction band (CB) and holes are formed in valance band; these holes can generate hydroxyl radicals which are highly oxidizing in nature. Probably hole can react with dye molecule and abstract electron from dye molecule and process of degradation start [1-5]. The most effective functional materials for photocatalytic applications are nanosized semiconductor oxides since it has a great potential to contribute to such environmental problems. Until now, many kinds of semiconductors have been studied as photocatalyst including TiO2, ZnO, CdS, WO3, etc. Most of these semiconductor photocatalysts have band gap in the ultraviolet (UV) region, i.e., equivalent to or larger than 3.2eV (λ = 387 nm). Therefore, they promote photocatalysis upon
illumination with UV radiation [6, 7]. Surface area and surface defects play an important role in the photocatalytic activities of metal oxide nanostructures; one-dimensional nanostructures like nanowires/nanorods are ideal candidates for application to photocatalysis since they offer a larger surface-to-volume ratio than nanoparticles. Recently it has been demonstrated that semi-conducting materials mediated photocatalytic oxidation of organic compounds is a successful, convention alternative to conventional methods for the removal of organic pollutants from water [8]. The uses of ZnO as a photocatalytic degradation material for environmental pollutants has also been extensively studied, because of its nontoxic nature, low cost, and high photochemical reactivity. However, for higher photocatalytic efficiency and many practical applications, it is desirable that ZnO photocatalyst should absorb not only UV but also visible light. In order to absorb visible light, band gap of ZnO has to be narrowed or split into several subgaps, which can be achieved by implanting transition metal ions or by doping nitrogen [10, 11]. ZnO probably has the most abundant forms of any known material. The properties of ZnO are strongly dependent on its structure, including the morphology, aspect ratio, size, orientation, and density of crystal [12, 13]. ZnO has emerged to be more efficient catalyst as far as water detoxification is concerned because it generates H2O2 more efficiently, it has high reaction and mineralization rates, and also it has more numbers of active sites with high surface reactivity [14-16]. 2. Experimental 2.1. Preparation of ZnO nanostructures ZnO nanosheets and nanotubes were synthesized by hydrothermal method as follows; 6 mL of ammonia (25%wt) was added slowly to 80mL of Zinc nitrate solution (Zn (NO3)2.6H2O) under continuous stirring. After 8 h, a white solution was obtained. The mixture was then transferred into
33 Journal of Applied and Industrial Sciences, April, 2013, 1 (1): 32-35 two different autoclaves (20ml). The first autoclave was heated at 90oC for 10h (ZnO nanosheets) and the other autoclave was heated at 90oC for 14h (ZnO nanotubes). The obtained precipitates were washed several times with distilled water and dried in air at 70 oC. 2.2. Characterization The resulting products were analyzed by X-ray diffraction (XRD) using Cu k radiation ( = 1.5417 Å). The overall morphology of the obtained ZnO powders was observed by Scanning Electron Microscope (SEM). The photocatalytic activity of the two samples was characterized by measuring the degradation ratio of Methyl Orange (MO). 0.04g of the powder sample was ultrasonically dispersed in 200ml of MO solution with a concentration of 20mg/L. The mixture was stirred for 30min to keep the suspension homogenous and then was placed under UV lamp (30 W, UV-C, λmax =253.7 nm). About 5ml of the solution was taken after different time intervals (0, 20, 50, 80, 110 and 150min). Each taken sample was then centrifuged at 500 rpm for 10 min to remove any ZnO precipitates and their absorption spectra were recorded using a UV-vis spectrophotometer (UV MINII 1240 Sed-Spec-48, Shimdzu).
particles were converted to nanosheets after heat treatment for10h. The growth mechanism of the nanosheets is suggested to be due to the nanoparticles aggregation and assembly in one-dimensional order [17]. However, increasing the hydrothermal duration to 14h leads to the formation of nanotubes as shown in figure 3, it was also observed that some nanotubes show a multilayered structure. It should be noted that nanosheets are rolling to form nanotubes according to the rolling mechanism proposed by Kuang, et al. [18].
3. Results and Discussion 3.1. Characterization results Figure 1 shows the XRD patterns of the as-obtained nanostructures. All the diffraction peaks can be well indexed to the hexagonal phase ZnO reported in the JCPDS (card No. 36-1451), with lattice constants of a=3.249Å, c=5.206Å. The results indicate that the samples consist of pure phase and no characteristic peaks were observed for other impurities. The narrow width of the peaks confirms that both samples are of high purity and good crystallinity.
Fig 2. SEM image of ZnO nanosheets prepared by heat treatment at 90oC for10h. It is clear from these results that the hydrothermal treatment duration has a strong effect on the morphological features of the resulting products, where it was found that the morphology of the product changes from nanosheets to nanotubes by increasing the hydrothermal duration.
tubes
sheets
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Fig 1. XRD Spectra of ZnO nanotubes and nanosheets
Figure 2 shows the SEM image of the obtained ZnO nanosheets. From the figure, we can see that all the ZnO
Fig 3. SEM image of ZnO nanotubes prepared by heat treatment at 90oC for14h.
34 Journal of Applied and Industrial Sciences, April, 2013, 1 (1): 32-35 3.2 Comparison of Photocatalytic Activity of ZnO Nanosheets and Nanotubes. 1.2
nanotubes nanosheets
1.0
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A good catalyst should be stable under operation conditions. Therefore, the chemical stability of the ZnO photocatalyst was assessed. We investigated the photocatalytic activity of ZnO nanosheets and nantubes after prolonged exposure to UV light in aqueous solution. Figure 4 shows the optical absorption spectra of the methyl orange solution at different time intervals of the photodegradation reactions over ZnO nanosheets and ZnO nantubes. For both samples, the degradation behaviors were found to be similar but with different time responses.
0.6
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Fig 5. Comparison for the photocatalytic activities of the ZnO nanosheets and nanorods. (C0 and C are the equilibrium concentrations of MO before and after UV-irradiation, respectively).
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Fig 4. UV–vis absorption spectra of MO on ZnO nanosheets sample at different reaction time
As shown in Figure 5, it was found that the ZnO nanosheets were more effective on the degradation of the MO than the nantubes. Higher photocatalytic activity of the ZnO nanosheets is considered to be due to the higher surface area than that of the ZnO nanotubes, where at higher surface areas, larger contact area between photocatalyst and target material can be obtained. It also means that higher degree of UV light absorption could occur at the smaller size in the test solution. Conclusion
A graph was drawn between time Vs C/C0 (Figure 5) to shows the photodegradation results of ZnO nanosheets and Nanotubes. The results of the Figure gave us an impression about the effect of the morphology on the degradation efficiency for the two different morphologies of ZnO catalysts. The effect of morphology on the photodegradation efficiency can be ascribed to the following reason; when the size of ZnO crystals decreases, the amount of the dispersion particles per volume in the solution will increase, resulting in the enhancement of the photon absorbance.
ZnO nanosheets and nanotubes have been hydrothermally synthesized by fixing the hydrothermal temperature of 90oC and varying the treatment duration. Characterization results showed that the nanosheets are formed at treatment duration of 10h, while the nanotubes were found to form at treatment duration of 14h, indicating that the hydrothermal treatment duration plays a crucial role on the morphological structure of the resulting product. The photocatalytic activity results showed that the nanosheets are more effective on the degradation of MO, which was attributed to their high surface area compared to nanotubes, making them promising candidates for the treatment of organic waste-water.
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