International Journal of Nanoscience Vol. 10, Nos. 1 & 2 (2011) 253257 # .c World Scienti¯c Publishing Company DOI: 10.1142/S0219581X11007867
PHOTOCATALYTIC DEGRADATION OF ORGANIC DYE USING NANO ZnO R. SARAVANAN, H. SHANKAR, G. RAJASUDHA and A. STEPHEN* Department of Nuclear Physics, University of Madras Guindy Campus, Chennai-600025, India *stephen
[email protected] V. NARAYANAN Department of Inorganic Chemistry, University of Madras Guindy Campus, Chennai-600025, India
[email protected] Photocatalytic degradation of methylene blue was investigated using nanocrystalline ZnO prepared by chemical precipitation and thermal decomposition routes. In both preparation routes zinc acetate dihydrate was used as the precursor. In the case of precipitation route, sodium hydroxide was used as the precipitating agent at room temperature, but in thermal decomposition method the precursor was just calcined at 450 C for 30 min. The structural and morphological properties were studied by X-ray di®raction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM). The sample prepared by precipitation route exhibits higher photocatalytic activity than that by thermal decomposition method and the reasons for the enhanced photocatalytic activity have been discussed in detail. Keywords: Zinc oxide; chemical precipitation method; thermal decomposition method.
1. Introduction
chemical and physical properties, environmental stability, and low cost, as compared to other nanosized metal oxides. The biggest advantage of ZnO is that it absorbs a large fraction of the solar spectrum than TiO2 .1 ZnO has been used in various applications, e.g., gas sensor, catalyst, varistor, transparent conducting ¯lms, UV diode, piezoelectric transducers, and pigments.2 In this work, nano ZnO has been synthesized by two di®erent methods at uniform temperature. The prepared samples were characterized by various techniques. Further, the photocatalytic activity of nano ZnO was tested for the degradation of methylene blue (MB) in aqueous medium and the e±ciency of the catalyst prepared by two di®erent methods was compared.
In the modern world, the growing population with its increased needs has promoted many industrial developments. The development of textile industries has caused an increase in pollution, mainly contamination of surface and groundwater. One of the best ways to reduce the contamination of water is by photocatalytic treatment. Titanium dioxide (TiO2 ) is the most used photocatalyst for this purpose. However, zinc oxide (ZnO) has recently been receiving attention from researchers. The energy levels for the conduction and valence bands and the electron a±nity of ZnO are similar to those of TiO2 . ZnO is a promising semiconducting material for photocatalytic applications because of its unique
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2. Experimental 2.1.
Materials
Zinc acetate dihydrate (Rankem) and sodium hydroxide (Qualigens) used in the present study were of analytical reagent grade and used as received without further puri¯cation. MB was purchased from Aldrich chemicals. All aqueous solutions were prepared by using double distilled water.
2.2.
( ¼ 0:15406 nm). Fourier-transform IR spectra were measured on a BrukerTensor 27 Fourier transform IR spectrophotometer using the KBr pellet technique. The UVvis transmission spectra were obtained on a CARY 5E UV-VIS-NIR spectrophotometer. Scanning Electron Microscopy (SEM) was measured on JEOL, JSM-840A scanning electron microscopy and the photocatalytic activity was measured by PerkinElmer UVvis spectrophotometer RX1.
Procedures
2.2.1. Method 1: Chemical precipitation (CP)
2.4.
In this preparation, 2.195 g of zinc acetate dihydrate was dissolved in 200 mL of double distilled water under stirring, and then 0.05 M of NaOH (200 mL) was added into this solution. The resulting solution was stirred for another 2 h and ¯ltered. The obtained precipitation was washed several times with double distilled water and dried at room temperature for 7 days. The collected powder was annealed at 450 C for 30 min.
The photocatalytic activity was estimated by measuring the decomposition rate of MB in aqueous solution under UV light irradiation. The experiment was carried out in a 600 mL cylindrical vessel with an 8 W mercury vapor lamp (365 nm) placed at the axis of the vessel. The lamp was installed into a quartz glass tube to protect from direct contact with the aqueous solution. Reaction suspensions were prepared by adding required amount of photocatalyst into 500 mL of MB solution with an initial concentration of 3 10 5 moles/L. Prior to the photodegradation, the suspension was magnetically stirred under dark condition for 30 min to establish adsorptiondesorption equilibrium condition. The aqueous suspension containing MB and photocatalyst was irradiated with constant stirring. The analytical samples from the suspension were collected at equal intervals of time, centrifuged, and ¯ltered. The concentration of MB was analyzed by UVvis spectrometer at the wavelength of 664 nm. The photocatalytic e±ciency is calculated from the following expression ¼ ð1 C=C0 Þ, where C0 is concentration of MB before illumination and C is concentration of MB after a certain period of irradiation time.
2.2.2. Method 2: Thermal decomposition (TD) Nanorods of ZnO were synthesized by thermal decomposition method. The TGA result of zinc acetate dihydrate indicates an initial weight loss of 16% at 150 C, due to dehydration yielding anhydrous zinc acetate. As the temperature was further increased, 52% weight loss in the temperature range 151 C312 C indicated the decomposition of acetate and formation of zinc oxide. The 52% weight loss indicated the residual zinc oxide, which coincided with the calculated theoritical residual value of 46.5% for ZnO being the only residue. The 5.5% di®erence might be due to the sublimation of zinc acetate species or zinc organic composition such as Zn4 O(CH3 CO2 Þ6 .3,4 There was no further decomposition beyond 312 C. The process resulted in the synthesis of ZnO with the total weight loss being 68%. A known amount of zinc acetate dihydrate was taken in the mortar and it was ground for 1 h after which the powder was annealed in an alumina crucible at 450 C for 30 min in a mu®le furnace.
2.3.
Characterizations
X-ray di®raction (XRD) was done on a Rich Siefert 3000 di®ractometer using CuK1 radiation
Photocatalytic testing
3. Results and Discussion The powder XRD patterns of ZnO samples synthesized by the two di®erent methods are shown in Figs. 1(a) and 1(b). For the CP sample all the diffraction peaks are indexed as the high-purity hexagonal ZnO with lattice constant values a ¼ 3:2450 0:009 Å and c ¼ 5:1913 0:002 Å, which match well with the standard data (JCPDS card No. 79-0205). The di®raction peaks of the TD sample indicate that the product has high-purity
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(a)
Fig. 1.
XRD patterns of (a) CP and (b) TD.
hexagonal ZnO wurtzite structure, which also match well with the standard data (JCPDS card No. 79-0208) and lattice parameter values are a ¼ 3:2450 0:002 Å and c ¼ 5:2387 0:002 Å. The crystallite size (D) was calculated from the major di®raction peak of the base of (101) using the Scherrer formula (2) D ¼ K= cos , where K is a constant or shape factor (0.9), is the X-ray wavelength used in XRD (CuK1 ¼ 1:5406 Å), is the Bragg angle, and is the full width half maxima for major di®raction peak. The size of the crystallite was about 22.8 and 30.2 nm for CP and TD samples, respectively. SEM of ZnO powder is shown in Figs. 2(a) and 2(b) at the scale value of 100 nm. The sample obtained in CP method is found to be uniform nanosphere of few nanometers arranged in an orderly fashion, whereas the samples from TD method are irregular nanorods in nature with an average diameter of 30 nm and lengths of 500 nm. Thus, SEM micrographs evidently proved the formation of ZnO nanospheres and nanorods. The characteristic functional groups, formation, and shape were investigated via FTIR spectra. The IR spectra of CP and TD samples are given in Figs. 3(a) and 3(b). Both samples show almost the broad absorption bands at 3428 cm 1 , which correspond to the OH stretching vibrations that might be due to the presence of water in ZnO. The absorption bands at 1622 cm 1 , 1605 cm 1 , 940 cm 1 , and 875 cm 1 in both the samples were due to the residual carboxylate anions which might remain adsorbed on the surface of ZnO. Figure 4 shows that for the spherical particles (CP) the IR band was
(b)
Fig. 2.
SEM images of (a) CP and (b) TD.
obtained at 447 cm 1 and for the nanorods the stretching of ZnO nanorods appeared at 546 cm 1 . This not only helps in the characterization but also in the con¯rmation of the formation of spherical and
Fig. 3.
FTIR spectra of (a) CP and (b) TD.
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4. Photocatalytic Degradation of MB Under UV Light Illumination In the present study, the prepared samples were tested in the photodegaradation of MB decoloration. The solution mixture contains ZnO nanoparticles, and MB was irradiated by UV light radiation for various time intervals to evaluate the photocatalytic performance of CP and TD samples. Figures 6(a) and 6(b) show the decolartion of MB
Fig. 4. FTIR spectra of low wave number range (a) CP and (b) TD.
rod-shaped ZnO particles which can be proved from the litearture values of the IR spectra having the range 680300 cm 1 for ZnO.5 The optical properties of ZnO were determined from absorption measurement. Figures 5(a) and 5(b) show the absorption spectra of ZnO for CP and TD samples, respectively. It is clearly observed that both samples exhibit the absorption of UV light very strongly and the result suggested that the photocatalytic activity might be good under UV light irradiation.
(a)
(b)
Fig. 5.
UVvis absorption spectra of (a) CP and (b) TD.
Fig. 6. Optical absorption spectra of MB: (a) CP and (b) TD for di®erent exposure times under UV light illumination.
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sample is higher than the irregular nanorods of TD sample.
5. Conclusions
Fig. 7. The photocatalytic degradation of MB under UV light illumination (a) CP, and (b) TD.
by ZnO catalyst under UV light irradiation for di®erent time intervals. The disappearance of the band at 664 nm indicates that most of the MB has been photodegraded by ZnO within 2 h time. The enhanced photocatalytic activity of CP sample for MB degradation is due to more uniform size of ZnO nanosphere particle with lower aggregation of crystallites as compared with TD sample. The e±ciency of CP samples was found to be 97:64% 1:127% under UV irradiation as against 83:64% 2:048% for the TD sample. The degradation curve for MB is shown in Fig. 7. The catalytic e±ciency of CP sample is higher due to the smaller crystallite size 22.8 nm. The decrease in crystallite size also increases the surface area and thereby increases the number of active sites. Decrease in crystallite size also increases the photocatalytic e±ciency.6 This shows that the photodegradation activity is higher for smaller crystalline size of CP method. The second important factor is morphology: because of the uniform nanospherical morphology the activity of CP
This study reports the synthesis of high-purity nanocrystalline ZnO by precipitation and thermal decomposition methods. Both methods are simple and of low cost compared with other methods. The prepared of ZnO powders di®ering in size and morphology in the two methods were evindently shown by XRD, FTIR, and SEM results. The UVlight photocatalytic activities were studied on the basis of MB degradation. Maximum photocatalytic e±ciency of 97.64% was achieved with ZnO nanospheres. The spherical shaped catalyst shows higher photocatalytic activity due to small crystalline size and large surface area compared to rod shaped ones.
Acknowledgment One of the author (RS) is grateful to the University of Madras for the ¯nancial assistance in the form of fellowship (URF).
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