enhanced photocatalytic activity of rare earth metal (nd ...scanning electron microscopy (fe-sem)...

Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

MMSE Journal. Open Access www.mmse.xyz

Enhanced Photocatalytic Activity of Rare Earth Metal (Nd and Gd) doped ZnO Nanostructures23

P. Logamani1, R. Rajeswari2, G. Poongodi3, 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 – [email protected]

DOI 10.2412/mmse.89.80.76 provided by Seo4U.link

Keywords: ZnO, rare earth dopants, photocatalytic activity, FESEM.

ABSTRACT. Presence of harmful organic pollutants in wastewater effluents causes serious environmental problems and therefore purification of this contaminated water by a cost effective treatment method is one of the most important issue which is in urgent need of scientific research. One such promising treatment technique uses semiconductor photocatalyst for the reduction of recalcitrant pollutants in water. In the present work, rare earth metals (Nd and Gd) doped ZnO nanostructured photocatalyst have been synthesized by wet chemical method. The prepared samples were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDS). The XRD results showed that the prepared samples were well crystalline with hexagonal Wurtzite structure. The results of EDS revealed that rare earth elements were doped into ZnO structure. The effect of rare earth dopant on morphology and photocatalytic degradation properties of the prepared samples were studied and discussed. The results revealed that the rare earth metal doped ZnO samples showed enhanced visible light photocatalytic activity for the degradation of methylene blue dye than pure nano ZnO photocatalyst.

Introduction. The photocatalytic degradation of organic pollutants such as dyes or pesticides from water using semiconductor materials has recently attracted a lot of attention. Among photocatalysts, Zinc oxide (ZnO) have received much attention owing to its stable structure, wide direct bandgap, nontoxicity, high photocatalytic activity, mild reaction conditions and reasonable cost[1]. However, in practical applications, the photocatalytic activity of ZnO is greatly limited by its wide band-gap (3.37 eV) which makes it poor response towards visible light and rapid recombination rate of photogenerated electron-hole pairs which inhibits its photocatalytic reaction. Hence, various strategies have been adopted to enhance the photocatalytic activity of ZnO. It is well known that doping is an effective method to improve the photocatalytic activities. Recent studies revels that the rare earth (RE) ions doping on ZnO can introduce impurity energy levels in band gap and expanded its visible light response [2-6]. Furthermore RE ions doping can produce traps for photogenerated charge carriers and decreased the electron–hole pairs recombination rate.

Several methods have been adopted to synthesize pure and RE doped ZnO structures, including microwave heating process, hydrothermal method, chemical co precipitation method, chemical vapour synthesis and sol-gel method. In this paper a template free precipitation method was used to prepare RE doped ZnO nanopowders. This method is simple, inexpensive and high yield providing room temperature synthesis of pure and doped ZnO nano powder.

Experimental Details. Pure and RE (Nd and Gd) doped ZnO nano powder were synthesized using wet chemical precipitation method. Initially, 0.25M of ZnCl2 and 2mol% of each doping rare earth

23© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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element precursor [((CH3CO2)3 Nd•3H2O) and (Gd (NO3)3•6H2O)] were also dissolved in distilled water. Each of the as obtained solution was dropped into 50ml of 0.1M NaOH solution under magnetic stirring. The precipitated materials were centrifuged and washed with distilled water many times to remove the unwanted ions. The wet samples were dried at 800 for 24h. The dried powders were then calcined at 4000 for 2h.

The crystalline structure of the pure and RE doped ZnO samples were characterised byXRD. The surface morphology and elemental confirmation of the samples were studied using field emission scanning electron microscopy (FE-SEM) equipped with an energy dispersive X-ray (EDS) detector. Optical transmission spectra were taken using LABINDIA T90+ UV-Vis spectrophotometer in the wavelength range of 300-800 nm. Photocatalytic activity of the pure and RE doped ZnO photocatalysts was studied by degrading an aqueous solution of methylene blue (MB) ( (1×10-5 M) dye under visible light. Prior to the light irradiation, 50 ml of MB solution with 0.1 g of photocatalysts was continuously stirred using a magnetic stirrer and kept in the dark for 30 minutes in order to reach adsorption equilibrium. 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 [7],

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 and At is the absorbance after ‘t’ min. reaction of MB solution at the characteristic absorption wavelength of 665 nm.

Result and discussion Structural and morphological studies. Fig. 1 shows X-ray diffraction patterns of RE-doped ZnO samples. All the diffraction peaks 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 RE related compounds or precipitates were detected, which illustrates that the RE atoms were incorporated into the ZnO lattice.

Fig. 1. Powder XRD patterns of pure and RE doped ZnO.

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Figs. 2 (a–c) shows the FESEM image of synthesized Pure and RE-doped ZnO samples. It can be seen that the synthesized products were rod shaped and grown in large quantity. The nanorods were 30–80nm diameter and 300–400 nm long. FESEM images reveals that the nanorods are grown in very high density, uniform size and distributed randomly. The incorporation of rare earth ion Gd and Nd in ZnO lattice slightly changes the aspect ratio of nanorods. The EDAX analysis was performed to confirm the presence of RE ions (Gd and Nd) in ZnO thin films. The results reveal that the samples consist of Zn, Gd, Nd and O which confirms the substitution of RE in ZnO.

Fig. 2. (a-c) FE-SEM images of Pure and RE doped ZnO samples with corresponding EDS.

3.2 Optical studies The optical UV –Vis transmittance spectra of pure and RE doped ZnO samples were recorded in the wavelength range 300 – 800 nm are shown in Fig. 3. The optical band gap energy values (Eg) were calculated by extrapolation of the linear part of (αhν)2 versus hν plot as shown in inset of Fig. 3. It is observed that the band gap values of pure, Gd and Nd doped ZnO samples are 3.25eV, 3.16eV and 3.06eV respectively. The reduction in band gap originated from the charge transfer between the ZnO valence band and the RE ion 4f level [8].

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Fig. 3. Optical transmittance spectra of pure and RE doped ZnO samples (Inset: Tauc plot between Eg and (αhν)2).

3.3 Photocatalytic activity The photocatalytic activity of pure and RE doped ZnO samples were investigated using MB degradation under visible light irradiation. It was found that RE doped ZnO samples exhibit enhanced photocatalytic activity than pure ZnO (Fig.4 (a - c)).

Fig. 4. Photocatalytic (a) degradation (b) degradation efficiency and (c) degradation kinetics of MB dye for pure and RE doped ZnO samples

Fig. 4 (c) shows the first order reaction kinetic model for photocatalytic degradation of MB dye. The apparent first order reaction rate constant (k), half-life value (t1/2) and linear coefficient (R2) calculated from the kinetic plot for MB dye is given in Table 1. From the table, it was observed that Nd doped ZnO sample showed higher photocatalytic activity than Gd doped ZnO and pure ZnO.

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Table 1 Kinetic parameters of pure and RE doped ZnO samples for MB dye

Sample Rate constant k (min-1) Half Life Value t1/2

(min) Linear coefficient R2

Pure ZnO 0.0099 69.94 0.960

Gd ZnO 0.0133 52.03 0.979

Nd ZnO 0.0189 36.59 0.966

The defect generated by RE doping in ZnO lattices could became the centres to capture photoinduced electrons, so that the recombination of photoinduced electrons and holes could be effectively inhibited. The trapped electron or hole will be migrated to the catalyst surfaces where it will participate in a redox reaction with the dye molecules, thereby reducing the electron and hole recombination and hence increases the photodegradation efficiency [9-11]. Moreover the reduction in the band gap energy of RE doped ZnO may also attributed to the enhanced photocatalytic activity. Summary. Pure and RE (Gd and Nd) doped ZnO nanorod samples were synthesised by wet chemical precipitation method. The XRD studies revealed that all the prepared samples exhibit hexagonal wurtzite structure. FE-SEM images revealed that the samples consist of nanorod structure. The optical studies showed reduction in the band gap. The RE doping in ZnO act as electron trapping centres and inhibit electron hole recombination, which leads to the generation of ROS and enhances the photocatalytic activity of ZnO.

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Cite the paper

P. Logamani, R. Rajeswari, G. Poongodi (2017). Enhanced Photocatalytic Activity of Rare Earth Metal (Nd and Gd) doped ZnO Nanostructures. Mechanics, Materials Science & Engineering, Vol 9.

doi:10.2412/mmse.89.80.76

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enhanced photocatalytic activity of rare earth metal (nd ...scanning electron microscopy (fe-sem)...

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