Synthesis, Structural, Optical and Photocatalytic Studies of Nanostructured

Mechanics, Materials Science & Engineering, May 2017

ISSN 2412-5954

Synthesis, Structural, Optical and Photocatalytic Studies of Nanostructured Cadmium Doped ZnO Nanorods by Hydrothermal Method 12 P. Logamani1, G. Poongodi2, R. Rajeswari3,a 1

Department of Chemistry, Bharathiyar University, Coimbatore, India

2

Department of Chemistry, Quaid-e-Millath Govt. College for Women, Chennai, India

3

Department of Physics, Quaid-e-Millath Govt. College for Women, Chennai, India

a

rajikanna99@yahoo.co.in DOI 10.2412/mmse.37.62.535 provided by Seo4U.link

Keywords: cadmium doped ZnO, nanorods, hydrothermal method, FESEM.

ABSTRACT. Nowadays, considerable attention has been paid to the eradication of hazardous substances in the environment especially in the wastewater. The photocatalytic reaction is used to mineralize the hazardous recalcitrant pollutants in to simple and harmless compounds and has been enhanced by the application of nanoparticles. Zinc oxide (ZnO) is a nontoxic wide band gap semiconductor photocatalyst, having unique properties such as high mobility, excellent chemical and thermal stability, high transparency and biocompatibility. To enhance its photocatalytic activity in the visible region ZnO can be doped with metals and non-metals. In the present work, pure and cadmium doped ZnO nanorods were prepared by hydrothermal method and characterized by X-ray diffraction, field-emission scanning electron microscopy with EDAX and UV Vis spectroscopy. The XRD results showed that the grown nanorods were well crystalline with hexagonal wurtzite structure. FESEM images confirm the nanorod structure. UV-Vis transmission spectra show that the substitution of Cd in ZnO leads to band gap reduction. The Cd doped ZnO nanorods were found to exhibit improved photocatalytic activity for the degradation of methylene blue dye under visible light in comparison with the undoped ZnO.

Introduction. Semiconductor photocatalysts have been extensively studied to remove harmful organic pollutant as well as energy production, since the photocatalytic splitting of water on TiO2 electrode has been reported by Fujishima et al (1972) [1]. The semiconductors as photocatalysts have shown excellent utility in the complete mineralization of various environmental pollutants such as dyes, detergents and volatile organic compounds [2]. In recent years, ZnO has been studied as photocatalyst, for the destruction of wide range of organic pollutants [3]. One of the main limitations of ZnO is the photo-instability in aqueous solution, when it is exposed to UV irradiation, the photocatalytic activity of ZnO gets decreased [4]. Several efforts have been made to reduce the instability of ZnO, among which transition metal doping is the simple and efficient technique to reduce photo instability. Furthermore, the optical absorption and photocatalytic performance of ZnO can be improved by transition metal dopants [5, 6]. In particular, cadmium is considered as a potential material for its abundant electron states and large solubility into ZnO matrix [7]. During the photocatalytic reaction, cadmium in ZnO matrix acts as an electron sink to enhance the separation of photo-excited electrons from holes, which favors the photocatalytic activity. In this study, the nanostructured pure and Cd doped ZnO nanorod samples were prepared by hydrothermal method.

The influence of cadmium doping on the structural, optical and photocatalytic properties of ZnO has been studied.

Experimental detail 12

icle under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, May 2017

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Nanostructured Cd doped ZnO powder was prepared by hydrothermal method. All the reagents were of analytical grade purity. All the reagents were of analytical grade purity. Aqueous solutions of 0.1 M zinc chloride and 0.01 M cadmium chloride were mixed under continuous stirring for 45 min at room temperature. After stirring, aqueous solution of 0.1 M hexamethylenetetramine (HMTA) was added in the previous solution and the resultant solution was again stirred for 30 min. The pH of the solution was maintained to 8.0 by adding few drops of ammonium hydroxide. The final solution was again vigorously stirred for 30 min and consequently transferred to Teflon lined autoclave which was then sealed and heated up to150 for 5 h. and the same procedure was repeated without cadmium chloride for pure ZnO. After terminating the reaction, the autoclave was allowed to cool at roomtemperature and the obtained products were washed several times with deionised water and ethanol.

Finally the prepared products were dried at 60 .

Characterization. The nanocrystalline structure of Cd doped ZnO powder was investigated byXRD. The surface morphology and elemental confirmation of nanorods were studied using FEI Quanta FEG 200 model FESEM operated at 30 kV equipped with an energy-dispersive X-ray (EDAX) detector.

The optical transmission spectra were recorded using LABINDIA T90+ UV-Vis spectrophotometer in the wavelength range 300-800 nm. The photocatalytic activity of pure and Cd doped ZnO was carried out by degradation of methylene blue (MB) (1 10-5 M) in aqueous solution. The absorption spectra of MB at different irradiating intervals using pure and Cd doped ZnO samples as photocatalyst were recorded by using UV-Vis spectrophotometer. The photocatalytic degradation was evaluated by measuring the absorbance of MB solution at 665 nm. The degradation efficiency of MB was calculated using the relation [8], Degradation (%) = (C0

Ct)/C0 x 100 = (A0

At)/A0 x 100

where C0 is the initial concentration; Ct is the concentration after t min; A0 is the initial absorbance; At is the absorbance after t min. reaction of MB solution at the characteristic absorption wavelength of 665 nm.

Result and discussion Structural studies. The crystal structures of pure Cd doped ZnO samples were confirmed by XRD analysis.

Fig. 1. XRD patterns of pure and Cd doped ZnO. MMSE Journal. Open Access www.mmse.xyz

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Fig. 1 shows the powder XRD patterns of Cd doped ZnO samples. All the diffraction peaks of grown samples were indexed and found to be Wurtzite hexagonal structure (JCPDS No.36-1451). It is evident from the XRD data that no extra peaks related to cadmium metal, other oxides phase were detected, which illustrates that the obtained are of single phase. But a small lower angle shift was obseved for Cd doped ZnO sample when compared to pure sample which may be caused by the substitution of Cd in ZnO (inset of Fig.1). This slight variation indicates that the incorporation of cadmium ion into the ZnO lattice. Fig. 2 shows the FESEM image of the prepared Cd doped ZnO sample. The morphology of Cd doped ZnO sample was one dimensional rod structure. The typical length of the nanorods is in the range of 500 550nm, while the diameters are in the range of 50-80 nm.

Fig. 2. FESEM image of Cd doped ZnO. The EDAX analysis was performed to confirm the presence of Cd in ZnO sample. The EDAX spectra of pure and Cd doped ZnO samples are shown in Fig.s 3. The results showed that the samples consist of Zn, Cd and O, which confirms the substitution of cadmium in ZnO.

Fig. 3. EDAX Spectra of pure and Cd doped ZnO. Optical studies The optical UV-Vis transmittance spectra of pure and Cd doped ZnO samples were recorded in the wavelength range 300 800 nm are shown in Fig. 4. The optical band gap energy values (Eg) were 2

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observed that the band gap values of pure and Cd doped ZnO samples are 3.18eV and 3.14eV. The reduction in the band gap is likely to originate from the sp-d exchange interaction between the localized d-electrons of Cd+ ions and band electrons of ZnO as well as the lattice expansion [9]. This band gap narrowing is expected to find important application in photocatalysis, since more solar energy might be effectively used for photocatalytic reaction.

Fig. 4. Optical transmission spectra of pure and Cd doped ZnO (Inset: Tauc plot between Eg and 2 ).

Photocatalytic activity.

The photocatalytic activity of pure and Cd doped ZnO samples were investigated using MB degradation under visible light irradiation. The result reveals that the Cd doped ZnO sample exhibit higher photocatalytic activity than pure ZnO (Fig.5). The presence of Cd in ZnO is attributed to improve the absorption in visible range and the large content of oxygen vacancies or defects act as trapping centers for the photogenerated electrons. This can lead to a reduction in the recombination of photo-generated electron pairs [10], there by promoting an interfacial charge transfer and hence, the rate of degradation of MB was significantly increased.

Fig. 5. Photocatalytic activity and degradation efficiency of MB with pure and Cd doped ZnO. Summary. Nanocrystalline pure and Cd doped ZnO nano powder samples were prepared by hydrothermal method. The XRD studies revealed that the prepared samples exhibit hexagonal wurtzite structure. FESEM image showed that the Cd doped ZnO sample consist of nanorod structure.

EDAX spectra confirmed the presence of Cd in ZnO. The optical studies revealed that the band gap MMSE Journal. Open Access www.mmse.xyz

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energy was reduced, indicating that Cd doping influences the energy band structure of ZnO. Cadmium doping in ZnO enhances the photocatalytic activity of ZnO by inhibiting electron hole recombination.

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