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Available online at www.sciencedirect.com
ScienceDirect www.journals.elsevier.com/journal-of-environmental-sciences
Photocatalytic degradation of endocrine disruptor Bisphenol-A in the presence of prepared CexZn1 − xO nanocomposites under irradiation of sunlight Kamaraj M.1,⁎, Ranjith K.S.2 , Rajeshwari Sivaraj3 , Rajendra Kumar R.T.2 , Hasna Abdul Salam3 1. Department of Biotechnology, Dr.N.G.P. Arts and Science College, Coimbatore 641048, Tamil Nadu, India. E-mail: [email protected] 2. Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India 3. Department of Biotechnology, School of Life Sciences, Karpagam University, Coimbatore 641021, Tamil Nadu, India
AR TIC LE I N FO
ABS TR ACT
Article history:
Photocatalytic degradation of Bisphenol A (BPA), a representative endocrine disruptor
Received 29 December 2013
chemical, was carried out under irradiation of sunlight in the presence of CexZn1 − xO
Revised 30 June 2014
nanophotocatalyst. Cerium (Ce) ions were successfully incorporated into the bulk lattice
Accepted 2 July 2014
of ZnO by simple co-precipitation process. The CexZn1 − xO composite nanostructures
Available online 29 September 2014
exhibited higher photocatalytic efficiency than pure ZnO in the degradation of BPA under sunlight irradiation and nearly complete mineralization of BPA was achieved. The
Keywords:
degradation rate was strongly dependent on factors such as the size and structure of
Bisphenol A (BPA)
catalyst, doping material concentration, BPA concentration, catalyst load, irradiation time
ZnO
and pH levels. This work suggested that the CexZn1 − xO assisted photocatalytic degradation
CexZn1 − xO
is a versatile, economic, environmentally benign and efficient method for BPA removal in
Sunlight
the aqueous environment.
Co-precipitation
© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
Cerium
Introduction Wide varieties of toxic organic contaminants from industrial effluents often pose the impetus for fundamental and applied research into environmental areas (Shrivastava, 2010). Bisphenol A (BPA), common name for 2,2-bis(4-hydroxylphenyl)-propane, is widely used as a raw material for the production of polycarbonates, epoxy resins and various consumer products such as water bottles, medical devices, beverages, compact disks, food can linings, thermal paper, safety helmets, plastic windows, car parts, adhesives, protective coatings, powder paints and electronic devices (Staples et al., 1998; Kang et al., 2003). It is an endocrine disruptor that is released to the environment during manufacturing process or by leaching from the final consumer products (Kang et al., 2007).
Published by Elsevier B.V.
Various treatment techniques have been developed for BPA removal by using physical (Zeng et al., 2006), chemical (Fukahori et al., 2003) and biological procedures (Scully et al., 2006). In comparison to other treatments, photocatalytic degradation incorporated with light irradiation is of great importance owing to the ability to mineralize BPA (Tao et al., 2011). In recent years, ZnO has attracted much attention in degradation of various pollutants due to its high photosensitivity (Ohtomo et al., 1999). ZnO has wide band gap (3.37 eV), large excitation binding energy (60 MeV) and low threshold power for optical pumping and is thus considered a low cost alternative photocatalyst to TiO2 for degradation of organic compounds in aqueous solution (Daneshvar et al., 2004; Chen, 2007). Various methods are used to prepare ZnO nanoparticles, co-precipitation being one of them (Shen et al., 2008). It is a synthetic
⁎ Corresponding author. E-mail: [email protected] (M. Kamaraj).
http://dx.doi.org/10.1016/j.jes.2014.09.022 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
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method where particle size can be controlled easily at low process temperature (Chauhan et al., 2012). Excellent photo catalytic activity of ZnO for liquid pollutants has been reported (Li et al., 2010; Habibi and Askari, 2011), but little is known for degradation of volatile organic compounds (Liao et al., 2012). Generation and recombination of photo induced electrons and holes are critical in photocatalysis. Thus, doping with other metals improves the photochemical performance of semiconductor materials (Lei et al., 2009). In this study, cerium (Ce) was chosen as the metal dopant as Ce2+ ions create an impurity state below the conduction band (CB) edge of semiconductors (Bubendorff et al., 2006). CeO2 has a band gap of about 3 eV and may be an ideal material for visible light-emitting phosphors in display, high-power laser, and light-emitting diode (Lang et al., 2010; Jung et al., 2012). Sunlight is an abundantly available natural energy source, which can be utilized for irradiation (Nagaraja et al., 2012). Although cerium-doped TiO2 or CeO2/TiO2 composites have been reported in literature (Lamrani et al., 2007; Jung et al., 2012), Ce-doped ZnO nanocomposites' synthesis by co-precipitation method employed in this study, is a potential new approach for the development of an efficient photocatalyst for BPA degradation under sunlight irradiation.
1. Materials and methods 1.1. Chemicals and materials Cerium nitrate (Ce(NO3)3·6H2O), hexamine (C6H12N4), zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and solution ammonia 30% of analytical grade were purchased from HiMedia Laboratories Pvt Ltd, India. All solutions were prepared with distilled water (Nihon Millipore, Yonezawa, Japan). Bisphenol A (GC grade > 99%) was purchased from Sigma-Aldrich (India).
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JEM 2100, Peabody, USA). Optical properties were measured by UV–Vis spectrophotometer (JASCO, Easton, USA) (model UV2450—Shimadzu, Japan) and Photoluminescence (PL) spectra recorded on a Photoluminescence spectrophotometer (Perkin-Elmer LS-55, Massachusetts, USA). Fourier transform infrared (FTIR) spectra of the sample were analyzed using FTIR Spectrophoto meter (Avatar 370 spectrometer, Thermo Nicolet Corporation, Madison, USA).
1.3. Evaluation of photocatalytic degradation of BPA The photo catalytic degradation of BPA was carried out with four different ratios of Ce-doped ZnO nanostructures. Photocatalytic experiments were performed under sunlight at ambient temperature. The effect of the initial concentration of catalyst was evaluated by varying the catalyst concentration (10, 20, 30 and 40 mg) and BPA concentration (10, 20 and 30 mg/L). The effect of time varied between 1–8 hr. The effect of pH was investigated through pH adjustment by using HCl/NaOH in BPA solution. A blank study was carried out in the presence of sunlight without any catalyst. After treatment, particles and BPA solution were separated and centrifuged at 3000 r/min for 5 min. The concentration of BPA was determined by measuring the absorption intensity at its maximum absorbance wavelength of 276 nm, by using a UV–Vis spectrophotometer. The particle reusability capacity was analyzed by using the material for several photocatalytic experiments. All experiments were done in triplicates and the data were analyzed using Origin Pro 7.5 SRO software (Origin Lab Corporation, Northampton, USA).
1.2. Catalysts preparation and characterization
2. Results and discussion
Ce-doped ZnO nanostructures were prepared by a simple co-precipitation method. In typical synthesis of ZnO nanostructures, 125 mmol/L of Zn(NO3)4·6H2O was dissolved in double distilled water and stirred for 5 min. Equal mole of hexamine was added drop wise in the prepared Zn(NO3)4·6H2O precursor and stirred vigorously for 5 min. For preparing CexZn1 − xO nanostructures, different ratios of cerium nitrate (X = 0.03, 0.06, 0.09) were taken with the Zn(NO3)4·6H2O precursor and dissolved in double distilled water under continuous stirring for 5 min. For controlling Zn(OH)2 formation, pH was maintained at 9 using aqueous ammonia solution and the growth precursor was maintained at 95°C for 3 hr. After being cooled to room temperature naturally, the precipitate, i.e., CexZn1 − xO, was collected by centrifugation, washed with deionized water and ethanol several times and dried at 150°C. Based on doping concentration (3%, 6% and 9%), samples were named as ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3 respectively. The crystalline structure, phase, purity and size of the particles were determined by X-ray diffraction (XRD) using X-ray diffractometer (D8 Advance, Bruker, Karlsruhe, Germany) (SEIFERT PTS 3003). Shape, size and microstructures of the particles were examined using scanning electron microscopy (ZEISS, Oberkochen, Germany) (SEM) (Model JSM 6390LV, JOEL, USA) and TEM images taken using Transmission scanning electron microscopy (JEOL
2.1. XRD analysis of catalysts Different mole presentable variations of Ce-doped ZnO nanostructures were synthesized by the simple co-precipitation method. Based on this, particles were assigned as ZnO, ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3, respectively. The powder X-ray diffraction patterns of the samples were obtained as shown in Fig. 1. The peaks were indexed and it showed hexagonal phase of zinc oxide when compared with the standard diffraction peaks of JCPDS card No. 89-0511 (Shim et al., 2001). The peaks located at angles (2θ) of 31.9°, 34.6° and 36.4° are attributed to (100), (002) and (101) planes of ZnO. Peak shift towards lower angle clearly indicated that the Ce atoms have successfully entered into the ZnO lattices. In a low concentration of impurity incorporation (Ce ions), Ce atom replaced few sites in the ZnO crystal structures and the peak corresponding to Ce was absent in XRD spectrum of ZnO·Ce1. Increasing the concentration of Ce(NO3)3·6H2O leads to the formation of more number of Ce ions in the ZnO crystal structure. In higher concentrations of Ce doping, XRD spectrum confirmed the formation of CeO2 along with the ZnO crystal structures and also clearly confirmed the formation of ZnO– CeO2 nanocomposites. The grain size for the peak (100) was calculated using the Debye–Scherrer formula (Anandan and Miyauchi, 2012) as 40.31, 10.35, 11.2 and 12.05 nm for ZnO, ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3, respectively, which was a
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Intensity (a.u.)
30
(110)
*
(102)
Ce
(100) (002) (101)
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images (Fig. 3b1, b4), showed size reduction and decayed flower morphology as Ce doping material increased. The sizes of the particles were reduced up to the range of 20 nm on increasing the Ce impurity concentration on the ZnO sites. Changes in surface and electronic properties provide an opportunity to control catalytic activity and selectivity (Bell, 2003). Influence of Ce2+ ions in the ZnO crystal may be responsible for the observed morphology and size reduction from micrometer to nanometer.
ZnO.Ce3
40
50
60
70 ZnO.Ce2
40
50
60
70
Ce * 30
ZnO.Ce1 30
40
50
60
70 ZnO
20
30
40
50 2θ (degree)
60
70
2.3. UV and PL analyses of catalysts
80
Fig. 1 – XRD pattern of the CexZn1 − xO nanostructures.
result of drastic reduction in the crystalline size of the Ce doped ZnO samples. Hence, Ce incorporation leads to an expansion of the ZnO lattice. Fig. 2 shows the variation of the hexagonal lattice parameters with Ce doping. The lattice parameters ‘c’ and ‘a’ increase on Ce doping and become almost constant for all doped samples. The lattice constant for the hexagonal structure is determined by Eq. (1): 1 0 2 2 2 4 @ h þ hk þ k A l þ 2 ¼ 2 2 3 a c d 1
ð1Þ
where, d is lattice spacing, a and c are dimensions of the crystal, and h, k, and l are miller indices of a particular diffraction plane.
2.2. SEM and TEM analyses of catalysts The SEM image of ZnO (Fig. 3a1) showed flower morphology having a size around ca, 2 μm. On inducing Ce as dopant, the flower-like morphology started to detract, and with an increase in the doping, the flower morphology completely detracted to form cluster of nanoparticle bunches with the size of 100 nm due to the formation of small CeO2 crystallites which disturbed the formation of isotropic ZnO nanostructures. The formation of Ce ions on the ZnO crystal structure, finally yielded CeZnO nanocomposite structures (Fig. 3a2, a3). TEM
The UV–Vis spectra (200–800 nm) of each Ce-induced ZnO nanostructures were recorded and the absorbance at selected wavelengths registered (Fig. 4). Due to incorporation of Ce in ZnO, there was a blue shift when compared with ZnO. At higher doping concentration of Ce, peak reduced in sharpness and became blunt due to the formation of CeO2/ZnO composite nature. Absorption spectrum appeared in the visible range due to the formation of defect states in Ce ions doped ZnO nanostructures. Photoluminescence (PL) spectra of ZnO and Ce-doped ZnO nanostructures were taken at the excitation wavelength of 350 nm (Fig. 5). Generally, ZnO shows four PL emissions (Ghosh et al., 2009) and in this study also, four emission peaks were observed. Sharp peak located at around 395 nm corresponds to the ultraviolet region caused by the recombination of free exciton (Djurišić and Leung, 2006). The broad visible emission band around 410–500 nm from pure ZnO nanoparticles can be attributed to a transition of a photo-generated electron from the conduction band to a deeply trapped holes (Van Dijken et al., 2000). Peak located at 468 nm (blue emission) might be attributed to intrinsic defects such as oxygen and zinc interstitials in ZnO (Umar et al., 2006). Broad peak located at 560 nm corresponded to green emission caused by the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy (Vanheusden et al., 1996). The change in peak shift of excitonic emission of CeZnO was an evidence for Ce incorporated into the ZnO lattice. On comparing the PL spectrum of ZnO with CeO2/ZnO composite, free excitonic emission was sharply suppressed, while oxygen defect green emission band was sharply enhanced. Broadening and change in the peak position of green emission can be
0.326 0.325 0.324
Lattice constant c (nm)
Lattice constant a (nm)
0.520 0.518
0.323 0.322
0.516
0.321 0.320
0.514 Lattice constant 'a' (nm) Lattice constant 'c' (nm)
0.512 ZnO
ZnO.Ce1
ZnO.Ce2
0.319 0.318
0.317 ZnO.Ce3
Fig. 2 – Variation of the hexagonal lattice parameters (a and c) with Ce concentration for CexZn1 − xO nanostructures.
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a1
a2
a3
a4
b1
b2
b3
b4
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Fig. 3 – SEM images (a) and TEM images (b) of the CexZn1 − xO nanocatalysts (1) ZnO, (2) ZnO·Ce1, (3) ZnO·Ce2, and (4) ZnO·Ce3.
attributed to Ce dopant introduced into the ZnO layer because in nanocrystallites, the deformation potential influences short range interaction (Bae et al., 2011).
2.4. Photocatalytic degradation of BPA Photocatalytic activity of synthesized Ce-induced ZnO nanostructures was evaluated by measuring the degradation of BPA. Experiments were performed under sunlight during summer season between 8 am and 4 pm during which fluctuation in the solar intensity is minimum (Nagaraja et al., 2012). The effect of initial particle concentration and BPA concentration was studied (Table 1). Maximum degradation was measured as 50% ZnO, 98% ZnO·Ce1, 84% ZnO·Ce2, and 70% ZnO·Ce3 for BPA (10 ppm) at 8 hr. In the same condition, only 3.4% degradation was observed in control samples. The reduction in the rate was constant when the catalyst concentration was increased above 40 mg/50 mL, which may be due to light scattering and reduction in light penetration
through solution. When the catalysts reached higher concentration, reaction was dominated by deactivation of activated molecules by collision with ground state molecules (Nagaveni et al., 2004; Toor et al., 2006). Number of studies has shown that the photocatalytic activity initially increased with catalyst loading and then decreased at high dosage because of light scattering and screening effects (Chen, 2007; Ahmed et al., 2011). When concentration of BPA increased, the solution becomes more intense and the path length of photons entering the solution decreases and only few photons reach the catalytic surface (Toor et al., 2006). It has been reported that the removal efficiency decreased from 98% to 67% with further increase in BPA concentration to 0.44 mmol/L (Tsai et al., 2009). At high concentration the available OHU− radicals are inadequate for degradation of pollutants, so the degradation rate decreases as the pollutant concentration increases (Bahnemann et al., 2007). Ce-doped ZnO (ZnO·Ce1) nanocatalysts showed better degradation of BPA compared with ZnO under sunlight. During
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ZnO
ZnO.Ce2
ZnO. Ce3 ZnO. Ce2
Zn Interestials
ZnO
PL intensity
Absorbance (a.u.)
ZnO.Ce1
ZnO. Ce1
300
400
Oxygen defects
ZnO.Ce3
500 600 Wavelength (nm)
700
800
Fig. 4 – UV–Vis absorption spectrum of CexZn1 − xO nanocatalysts.
photocatalysis, the adsorption of photon by zinc oxide leads to the promotion of an electron from the valence band to the conduction band and produce electron–hole pair (Prevot et al., 1999). These holes and electrons migrate to the surface and promote interfacial oxidation and reduction reactions with the substrate adsorbed on the surface of the semiconductor oxide. The separate mechanisms of BPA degradation during the photocatalytic degradation or ozonization have been previously reported (Gultekin and Ince, 2007). Photocatalytic degradation of BPA at different time intervals was examined under sunlight and it increased from 1 hr to 8 hr in all the four catalysts (Fig. 6); it is probably due to electron– hole formation in the photochemical reaction, which is mainly dependent on the light intensity to initiate the rate of photocatalyst (Cassano and Alfano, 2000). The catalyst absorbs more photons and produces more electron–hole pairs in the catalyst surface when absorbing more photos, which increases the hydroxyl radicals' concentration and consequently increases the degradation rate (Ahmed et al., 2011). The effect of light intensity on the solar photocatalytic degradation of BPA in water with TiO2 on sunny and cloudy days has been investigated (Kaneco et al., 2004). The UV source is expensive and hazardous while other light sources like tungsten light and mercury light which requires electricity are not cost-effective options. Therefore, the present study highlighted cheap photo energy (sunlight) as a potential natural energy source for irradiation of catalyst. Polluted water has wide range of pH values based on their origin, and pH plays a major role in the generation of hydroxyl radicals. Thus, understanding the effect of pH in the BPA degradation by Ce-induced ZnO catalysts was necessary. In the present study, degradation efficiency was observed in the order as follows: pH unadjusted (6.5) > pH 5 > pH 9 > pH 3 > pH 11 for BPA (10 mg/L) with the catalyst (30 mg/50 mL) under sunlight (Fig. 7). Highest degradation was obtained in pH 6.5, which subsequently reduced in high alkaline condition for all four catalysts. Similar pH influence on BPA degradation by CN–TiO2 under white LED irradiation has been previously observed (Wang and Lim, 2010). Kaneco et al (2004) have reported that, TiO2 nanostructure exhibited better photodegradation efficiency in the alkaline medium (pH = 10) due to the generation of dominant oxidizing species (hydroxyl free radical) in the
400
450
500 550 Wavelength (nm)
600
650
Fig. 5 – Photoluminescence (PL) spectra of CexZn1 − xO nanocatalysts.
photocatalytic process. But in the current study, better photodegradation efficiency was obtained near the neutral pH of medium and consequently, pH 6.5 was selected for optimal experimental conditions, to avoid the unnecessary chemical treatment including neutralization process. The reusable catalyst activity was performed for 10 cycles for un-doped ZnO and CexZn1 − xO nanostructures. Reusable capacity of the catalysts was similar up to 8 cycles, and then it decreased from 5% to 15%. CexZn1 − xO nanostructures showed maximum reusable efficiency compared to un-doped ZnO. Adsorption of pollutant molecules on the surface of the nanostructures during the photocatalytic experiment may be the reason for the decrease in degradation efficiency on increasing the number of recycling times. Photocatalytic ability of ZnO nanoparticles may be affected by surface area of particles, high carrier generation and the presence of dopant or foreign particle in the sample. Increase in the charge separation efficiency enhances the formation of both free radicals and active oxygen species (Kato et al., 2005). Most of the Table 1 – Effect of catalyst load and BPA concentration in photocatalytic degradation. Catalyst
BPA degradation (%) Catalyst load (mg/50 mL)
Control ZnO
ZnO·Ce1
ZnO·Ce2
ZnO·Ce3
– 10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40
BPA concentration 10 mg/L
20 mg/L
30 mg/L
3.4 7.8 18.89 49.31 49.82 25 51 97.03 97.79 19 40.42 74.98 84.1 16 35.52 55.03 69.8
2.1 6.11 11.24 37.93 38.02 19..24 34.12 67.03 67.88 13.32 25.57 40.63 40.85 12.28 22.40 32.93 33.01
1.3 5.6 9.6 28.29 29.30 16.66 27.43 35.94 36.17 10.74 20.36 33.30 33.96 9.23 18.48 30.36 31.66
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100
80 Degradation efficiency (%)
like XRD, SEM, TEM, UV and PL analyses. Ce-doped ZnO catalysts showed enhanced efficiency for photocatalytic degradation of BPA in comparison with pure ZnO catalysts. The degradation rate was strongly affected by several major factors, such as BPA concentration, catalyst load, exposure time and pH value. Furthermore, nearly complete removal of BPA can be achieved with Ce-doped ZnO (ZnO·Ce1) nanocatalysts because of its higher electron mobility and surface carrier defects than ZnO catalysts. It can be concluded that CexZn1 − xO nanocomposite is a highly active photo catalyst for the photo degradation of BPA in water under irradiation of sunlight, which might have a potential application in waste water treatment.
Control ZnO ZnO.Ce1 ZnO.Ce2 ZnO.Ce3
90
70 60 50 40 30 20 10 0
0
1
2
3
4 5 Time (hr)
6
8
7
Fig. 6 – Effect of time in photocatalytic degradation of BPA using CexZn1 − xO nanostructures under sunlight. semiconductors have poor activity when used alone, but the presence of a metal on the semiconductor markedly increases their efficiency. The role of the loaded material is to trap and subsequently transfer photo excited electron onto surface, thus decreasing the recombination of hole–electron pairs (Mirkhani et al., 2009). In the present study, order of degradation efficiency was observed as ZnO·Ce1 > ZnO·Ce2 > ZnO·Ce3 > ZnO. Structural studies clearly indicate that ZnO·Ce2 and ZnO·Ce3 were in CeO2/ZnO composite nature which reduced the activity species in catalytic degradation. In all conditions used in the study, better degradation rates were observed in ZnO·Ce1 (Ce doped ZnO) when compared with ZnO alone, possibly because of Ce being used as a loading material to enhance the high electrical mobility to the surface and doped ions can also act as charge trapping sites that may reduce electron–hole recombination which enhances the degradation efficiency.
3. Conclusions
Degradation efficiency (%)
ZnO and three different concentrations of Ce-doped ZnO nanocomposites were synthesized via co-precipitation method. Prepared catalysts were characterized by various techniques 100 90 80 70 60 50 40 30 20 10 3.0 2.5 2.0 1.5 1.0 0.5 0.0
2
Control ZnO ZnO.Ce1 ZnO.Ce2 ZnO.Ce3
3
4
5
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6 7 8 9 pH of the BPA solution
10
11
12
Fig. 7 – Effect of pH in photocatalytic degradation of BPA using CexZn1 − xO nanostructures under sunlight.
Acknowledgments We are thankful to Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore, Tamil Nadu, India and Karpagam University, Coimbatore, Tamil Nadu, India, for providing the necessary lab facilities for this work.
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Nagaveni, K., Sivalingam, G., Hegde, M.S., Madras, G., 2004. Solar photocatalytic degradation of dyes: high activity of combustion synthesized nano TiO2. Appl. Catal. B Environ. 48 (2), 83–93. Ohtomo, A., Kawasaki, M., Ohkubo, I., Koinuma, H., Yasuda, T., Segawa, Y., 1999. Structural and optical properties of ZnO/Mg0.2Zn0.8O superlattices. Appl. Phys. Lett. 75 (7), 980–982. Prevot, A.B., Vincenti, M., Bianciotto, A., Pramauro, E., 1999. Photocatalytic and photolytic transformation of chloramben in aqueous solutions. Appl. Catal. B Environ. 22 (2), 149–158. Scully, C., Collins, G., O'Flaherty, V., 2006. Anaerobic biological treatment of phenol at 9.5–15°C in an expanded granular sludge bed (EGSB)-based bioreactor. Water Res. 40 (20), 3737–3744. Shen, W.Z., Liu, Z.J., Wang, H., Liu, Y.H., Guo, Q.J., Zhang, Y.L., 2008. Photocatalytic degradation for methylene blue using zinc oxide prepared by codeposition and sol–gel methods. J. Hazard. Mater. 152 (1), 172–175. Shim, S.H., Duffy, T.S., Shen, G., 2001. Stability and structure of MgSiO3 perovskite to 2300-kilometer depth in earth's mantle. Science 293 (5539), 2437–2440. Shrivastava, V.S., 2010. Metallic and organic nanomaterials and their use in pollution control: a review. Arch. Appl. Sci. Res. 2 (6), 82–92. Staples, C.A., Dorn, P.B., Klecka, G.M., O'Block, S.T., Harris, L.R., 1998. A review of the environmental fate, effects, and exposures of bisphenol A. Chemosphere 36 (10), 2149–2173. Tao, H., Hao, S.Q., Chang, F., Wang, L., Zhang, Y., Cai, X.H., et al., 2011. Photodegradation of bisphenol A by titana nanoparticles in mesoporous MCM-41. Water Air Soil Pollut. 214 (1–4), 491–498. Toor, P., Verma, A., Jotshi, C.K., Bajpai, P.K., Singh, V., 2006. Photocatalytic degradation of Direct Yellow 12 dye using UV/TiO 2 in a shallow pond slurry reactor. Dyes Pigments 68 (1), 53–60. Tsai, W.T., Lee, M.K., Su, T.Y., Chang, Y.M., 2009. Photodegradation of bisphenol-A in a batch TiO2 suspension reactor. J. Hazard. Mater. 168 (1), 269–275. Umar, A., Ra, H.W., Jeong, J.P., Suh, E.K., Hahn, Y.B., 2006. Synthesis of ZnO nanowires on Si substrate by thermal evaporation method without catalyst: structural and optical properties. Korean J. Chem. Eng. 23 (3), 499–504. Van Dijken, A., Meulenkamp, E.A., Vanmaekelbergh, D., Andries, M., 2000. Influence of adsorbed oxygen on the emission properties of nanocrystalline ZnO particles. J. Phys. Chem. B 104 (18), 4355–4360. Vanheusden, K., Warren, W.L., Seager, C.H., Tallant, D.R., Voigt, J.A., Gnade, B.E., 1996. Mechanisms behind green photoluminescence in ZnO phosphor powders. J. Appl. Phys. 79 (10), 7983–7990. Wang, X., Lim, T.T., 2010. Solvothermal synthesis of C–N codoped TiO2 and photocatalytic evaluation for bisphenol A degradation using a visible-light irradiated LED photoreactor. Appl. Catal. B Environ. 100 (1–2), 355–364. Zeng, G.M., Zhang, C., Huang, G.H., Yu, J., Wang, Q., Li, J.B., 2006. Adsorption behavior of bisphenol A on sediments in Xiangjiang River, central-south China. Chemosphere 65 (9), 1490–1499.
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Photocatalytic degradation of endocrine disruptor Bisphenol-A in the presence of prepared CexZn1 − xO nanocomposites under irradiation of sunlight Kamaraj M.1,⁎, Ranjith K.S.2 , Rajeshwari Sivaraj3 , Rajendra Kumar R.T.2 , Hasna Abdul Salam3 1. Department of Biotechnology, Dr.N.G.P. Arts and Science College, Coimbatore 641048, Tamil Nadu, India. E-mail: [email protected] 2. Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, Tamil Nadu, India 3. Department of Biotechnology, School of Life Sciences, Karpagam University, Coimbatore 641021, Tamil Nadu, India
AR TIC LE I N FO
ABS TR ACT
Article history:
Photocatalytic degradation of Bisphenol A (BPA), a representative endocrine disruptor
Received 29 December 2013
chemical, was carried out under irradiation of sunlight in the presence of CexZn1 − xO
Revised 30 June 2014
nanophotocatalyst. Cerium (Ce) ions were successfully incorporated into the bulk lattice
Accepted 2 July 2014
of ZnO by simple co-precipitation process. The CexZn1 − xO composite nanostructures
Available online 29 September 2014
exhibited higher photocatalytic efficiency than pure ZnO in the degradation of BPA under sunlight irradiation and nearly complete mineralization of BPA was achieved. The
Keywords:
degradation rate was strongly dependent on factors such as the size and structure of
Bisphenol A (BPA)
catalyst, doping material concentration, BPA concentration, catalyst load, irradiation time
ZnO
and pH levels. This work suggested that the CexZn1 − xO assisted photocatalytic degradation
CexZn1 − xO
is a versatile, economic, environmentally benign and efficient method for BPA removal in
Sunlight
the aqueous environment.
Co-precipitation
© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
Cerium
Introduction Wide varieties of toxic organic contaminants from industrial effluents often pose the impetus for fundamental and applied research into environmental areas (Shrivastava, 2010). Bisphenol A (BPA), common name for 2,2-bis(4-hydroxylphenyl)-propane, is widely used as a raw material for the production of polycarbonates, epoxy resins and various consumer products such as water bottles, medical devices, beverages, compact disks, food can linings, thermal paper, safety helmets, plastic windows, car parts, adhesives, protective coatings, powder paints and electronic devices (Staples et al., 1998; Kang et al., 2003). It is an endocrine disruptor that is released to the environment during manufacturing process or by leaching from the final consumer products (Kang et al., 2007).
Published by Elsevier B.V.
Various treatment techniques have been developed for BPA removal by using physical (Zeng et al., 2006), chemical (Fukahori et al., 2003) and biological procedures (Scully et al., 2006). In comparison to other treatments, photocatalytic degradation incorporated with light irradiation is of great importance owing to the ability to mineralize BPA (Tao et al., 2011). In recent years, ZnO has attracted much attention in degradation of various pollutants due to its high photosensitivity (Ohtomo et al., 1999). ZnO has wide band gap (3.37 eV), large excitation binding energy (60 MeV) and low threshold power for optical pumping and is thus considered a low cost alternative photocatalyst to TiO2 for degradation of organic compounds in aqueous solution (Daneshvar et al., 2004; Chen, 2007). Various methods are used to prepare ZnO nanoparticles, co-precipitation being one of them (Shen et al., 2008). It is a synthetic
⁎ Corresponding author. E-mail: [email protected] (M. Kamaraj).
http://dx.doi.org/10.1016/j.jes.2014.09.022 1001-0742/© 2014 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
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method where particle size can be controlled easily at low process temperature (Chauhan et al., 2012). Excellent photo catalytic activity of ZnO for liquid pollutants has been reported (Li et al., 2010; Habibi and Askari, 2011), but little is known for degradation of volatile organic compounds (Liao et al., 2012). Generation and recombination of photo induced electrons and holes are critical in photocatalysis. Thus, doping with other metals improves the photochemical performance of semiconductor materials (Lei et al., 2009). In this study, cerium (Ce) was chosen as the metal dopant as Ce2+ ions create an impurity state below the conduction band (CB) edge of semiconductors (Bubendorff et al., 2006). CeO2 has a band gap of about 3 eV and may be an ideal material for visible light-emitting phosphors in display, high-power laser, and light-emitting diode (Lang et al., 2010; Jung et al., 2012). Sunlight is an abundantly available natural energy source, which can be utilized for irradiation (Nagaraja et al., 2012). Although cerium-doped TiO2 or CeO2/TiO2 composites have been reported in literature (Lamrani et al., 2007; Jung et al., 2012), Ce-doped ZnO nanocomposites' synthesis by co-precipitation method employed in this study, is a potential new approach for the development of an efficient photocatalyst for BPA degradation under sunlight irradiation.
1. Materials and methods 1.1. Chemicals and materials Cerium nitrate (Ce(NO3)3·6H2O), hexamine (C6H12N4), zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and solution ammonia 30% of analytical grade were purchased from HiMedia Laboratories Pvt Ltd, India. All solutions were prepared with distilled water (Nihon Millipore, Yonezawa, Japan). Bisphenol A (GC grade > 99%) was purchased from Sigma-Aldrich (India).
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JEM 2100, Peabody, USA). Optical properties were measured by UV–Vis spectrophotometer (JASCO, Easton, USA) (model UV2450—Shimadzu, Japan) and Photoluminescence (PL) spectra recorded on a Photoluminescence spectrophotometer (Perkin-Elmer LS-55, Massachusetts, USA). Fourier transform infrared (FTIR) spectra of the sample were analyzed using FTIR Spectrophoto meter (Avatar 370 spectrometer, Thermo Nicolet Corporation, Madison, USA).
1.3. Evaluation of photocatalytic degradation of BPA The photo catalytic degradation of BPA was carried out with four different ratios of Ce-doped ZnO nanostructures. Photocatalytic experiments were performed under sunlight at ambient temperature. The effect of the initial concentration of catalyst was evaluated by varying the catalyst concentration (10, 20, 30 and 40 mg) and BPA concentration (10, 20 and 30 mg/L). The effect of time varied between 1–8 hr. The effect of pH was investigated through pH adjustment by using HCl/NaOH in BPA solution. A blank study was carried out in the presence of sunlight without any catalyst. After treatment, particles and BPA solution were separated and centrifuged at 3000 r/min for 5 min. The concentration of BPA was determined by measuring the absorption intensity at its maximum absorbance wavelength of 276 nm, by using a UV–Vis spectrophotometer. The particle reusability capacity was analyzed by using the material for several photocatalytic experiments. All experiments were done in triplicates and the data were analyzed using Origin Pro 7.5 SRO software (Origin Lab Corporation, Northampton, USA).
1.2. Catalysts preparation and characterization
2. Results and discussion
Ce-doped ZnO nanostructures were prepared by a simple co-precipitation method. In typical synthesis of ZnO nanostructures, 125 mmol/L of Zn(NO3)4·6H2O was dissolved in double distilled water and stirred for 5 min. Equal mole of hexamine was added drop wise in the prepared Zn(NO3)4·6H2O precursor and stirred vigorously for 5 min. For preparing CexZn1 − xO nanostructures, different ratios of cerium nitrate (X = 0.03, 0.06, 0.09) were taken with the Zn(NO3)4·6H2O precursor and dissolved in double distilled water under continuous stirring for 5 min. For controlling Zn(OH)2 formation, pH was maintained at 9 using aqueous ammonia solution and the growth precursor was maintained at 95°C for 3 hr. After being cooled to room temperature naturally, the precipitate, i.e., CexZn1 − xO, was collected by centrifugation, washed with deionized water and ethanol several times and dried at 150°C. Based on doping concentration (3%, 6% and 9%), samples were named as ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3 respectively. The crystalline structure, phase, purity and size of the particles were determined by X-ray diffraction (XRD) using X-ray diffractometer (D8 Advance, Bruker, Karlsruhe, Germany) (SEIFERT PTS 3003). Shape, size and microstructures of the particles were examined using scanning electron microscopy (ZEISS, Oberkochen, Germany) (SEM) (Model JSM 6390LV, JOEL, USA) and TEM images taken using Transmission scanning electron microscopy (JEOL
2.1. XRD analysis of catalysts Different mole presentable variations of Ce-doped ZnO nanostructures were synthesized by the simple co-precipitation method. Based on this, particles were assigned as ZnO, ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3, respectively. The powder X-ray diffraction patterns of the samples were obtained as shown in Fig. 1. The peaks were indexed and it showed hexagonal phase of zinc oxide when compared with the standard diffraction peaks of JCPDS card No. 89-0511 (Shim et al., 2001). The peaks located at angles (2θ) of 31.9°, 34.6° and 36.4° are attributed to (100), (002) and (101) planes of ZnO. Peak shift towards lower angle clearly indicated that the Ce atoms have successfully entered into the ZnO lattices. In a low concentration of impurity incorporation (Ce ions), Ce atom replaced few sites in the ZnO crystal structures and the peak corresponding to Ce was absent in XRD spectrum of ZnO·Ce1. Increasing the concentration of Ce(NO3)3·6H2O leads to the formation of more number of Ce ions in the ZnO crystal structure. In higher concentrations of Ce doping, XRD spectrum confirmed the formation of CeO2 along with the ZnO crystal structures and also clearly confirmed the formation of ZnO– CeO2 nanocomposites. The grain size for the peak (100) was calculated using the Debye–Scherrer formula (Anandan and Miyauchi, 2012) as 40.31, 10.35, 11.2 and 12.05 nm for ZnO, ZnO·Ce1, ZnO·Ce2 and ZnO·Ce3, respectively, which was a
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Intensity (a.u.)
30
(110)
*
(102)
Ce
(100) (002) (101)
J O U R NA L OF EN V I RO N M EN T A L S CI EN C ES 2 6 ( 20 1 4 ) 2 3 62–2 3 6 8
images (Fig. 3b1, b4), showed size reduction and decayed flower morphology as Ce doping material increased. The sizes of the particles were reduced up to the range of 20 nm on increasing the Ce impurity concentration on the ZnO sites. Changes in surface and electronic properties provide an opportunity to control catalytic activity and selectivity (Bell, 2003). Influence of Ce2+ ions in the ZnO crystal may be responsible for the observed morphology and size reduction from micrometer to nanometer.
ZnO.Ce3
40
50
60
70 ZnO.Ce2
40
50
60
70
Ce * 30
ZnO.Ce1 30
40
50
60
70 ZnO
20
30
40
50 2θ (degree)
60
70
2.3. UV and PL analyses of catalysts
80
Fig. 1 – XRD pattern of the CexZn1 − xO nanostructures.
result of drastic reduction in the crystalline size of the Ce doped ZnO samples. Hence, Ce incorporation leads to an expansion of the ZnO lattice. Fig. 2 shows the variation of the hexagonal lattice parameters with Ce doping. The lattice parameters ‘c’ and ‘a’ increase on Ce doping and become almost constant for all doped samples. The lattice constant for the hexagonal structure is determined by Eq. (1): 1 0 2 2 2 4 @ h þ hk þ k A l þ 2 ¼ 2 2 3 a c d 1
ð1Þ
where, d is lattice spacing, a and c are dimensions of the crystal, and h, k, and l are miller indices of a particular diffraction plane.
2.2. SEM and TEM analyses of catalysts The SEM image of ZnO (Fig. 3a1) showed flower morphology having a size around ca, 2 μm. On inducing Ce as dopant, the flower-like morphology started to detract, and with an increase in the doping, the flower morphology completely detracted to form cluster of nanoparticle bunches with the size of 100 nm due to the formation of small CeO2 crystallites which disturbed the formation of isotropic ZnO nanostructures. The formation of Ce ions on the ZnO crystal structure, finally yielded CeZnO nanocomposite structures (Fig. 3a2, a3). TEM
The UV–Vis spectra (200–800 nm) of each Ce-induced ZnO nanostructures were recorded and the absorbance at selected wavelengths registered (Fig. 4). Due to incorporation of Ce in ZnO, there was a blue shift when compared with ZnO. At higher doping concentration of Ce, peak reduced in sharpness and became blunt due to the formation of CeO2/ZnO composite nature. Absorption spectrum appeared in the visible range due to the formation of defect states in Ce ions doped ZnO nanostructures. Photoluminescence (PL) spectra of ZnO and Ce-doped ZnO nanostructures were taken at the excitation wavelength of 350 nm (Fig. 5). Generally, ZnO shows four PL emissions (Ghosh et al., 2009) and in this study also, four emission peaks were observed. Sharp peak located at around 395 nm corresponds to the ultraviolet region caused by the recombination of free exciton (Djurišić and Leung, 2006). The broad visible emission band around 410–500 nm from pure ZnO nanoparticles can be attributed to a transition of a photo-generated electron from the conduction band to a deeply trapped holes (Van Dijken et al., 2000). Peak located at 468 nm (blue emission) might be attributed to intrinsic defects such as oxygen and zinc interstitials in ZnO (Umar et al., 2006). Broad peak located at 560 nm corresponded to green emission caused by the radiative recombination of a photo-generated hole with an electron occupying the oxygen vacancy (Vanheusden et al., 1996). The change in peak shift of excitonic emission of CeZnO was an evidence for Ce incorporated into the ZnO lattice. On comparing the PL spectrum of ZnO with CeO2/ZnO composite, free excitonic emission was sharply suppressed, while oxygen defect green emission band was sharply enhanced. Broadening and change in the peak position of green emission can be
0.326 0.325 0.324
Lattice constant c (nm)
Lattice constant a (nm)
0.520 0.518
0.323 0.322
0.516
0.321 0.320
0.514 Lattice constant 'a' (nm) Lattice constant 'c' (nm)
0.512 ZnO
ZnO.Ce1
ZnO.Ce2
0.319 0.318
0.317 ZnO.Ce3
Fig. 2 – Variation of the hexagonal lattice parameters (a and c) with Ce concentration for CexZn1 − xO nanostructures.
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a1
a2
a3
a4
b1
b2
b3
b4
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Fig. 3 – SEM images (a) and TEM images (b) of the CexZn1 − xO nanocatalysts (1) ZnO, (2) ZnO·Ce1, (3) ZnO·Ce2, and (4) ZnO·Ce3.
attributed to Ce dopant introduced into the ZnO layer because in nanocrystallites, the deformation potential influences short range interaction (Bae et al., 2011).
2.4. Photocatalytic degradation of BPA Photocatalytic activity of synthesized Ce-induced ZnO nanostructures was evaluated by measuring the degradation of BPA. Experiments were performed under sunlight during summer season between 8 am and 4 pm during which fluctuation in the solar intensity is minimum (Nagaraja et al., 2012). The effect of initial particle concentration and BPA concentration was studied (Table 1). Maximum degradation was measured as 50% ZnO, 98% ZnO·Ce1, 84% ZnO·Ce2, and 70% ZnO·Ce3 for BPA (10 ppm) at 8 hr. In the same condition, only 3.4% degradation was observed in control samples. The reduction in the rate was constant when the catalyst concentration was increased above 40 mg/50 mL, which may be due to light scattering and reduction in light penetration
through solution. When the catalysts reached higher concentration, reaction was dominated by deactivation of activated molecules by collision with ground state molecules (Nagaveni et al., 2004; Toor et al., 2006). Number of studies has shown that the photocatalytic activity initially increased with catalyst loading and then decreased at high dosage because of light scattering and screening effects (Chen, 2007; Ahmed et al., 2011). When concentration of BPA increased, the solution becomes more intense and the path length of photons entering the solution decreases and only few photons reach the catalytic surface (Toor et al., 2006). It has been reported that the removal efficiency decreased from 98% to 67% with further increase in BPA concentration to 0.44 mmol/L (Tsai et al., 2009). At high concentration the available OHU− radicals are inadequate for degradation of pollutants, so the degradation rate decreases as the pollutant concentration increases (Bahnemann et al., 2007). Ce-doped ZnO (ZnO·Ce1) nanocatalysts showed better degradation of BPA compared with ZnO under sunlight. During
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ZnO
ZnO.Ce2
ZnO. Ce3 ZnO. Ce2
Zn Interestials
ZnO
PL intensity
Absorbance (a.u.)
ZnO.Ce1
ZnO. Ce1
300
400
Oxygen defects
ZnO.Ce3
500 600 Wavelength (nm)
700
800
Fig. 4 – UV–Vis absorption spectrum of CexZn1 − xO nanocatalysts.
photocatalysis, the adsorption of photon by zinc oxide leads to the promotion of an electron from the valence band to the conduction band and produce electron–hole pair (Prevot et al., 1999). These holes and electrons migrate to the surface and promote interfacial oxidation and reduction reactions with the substrate adsorbed on the surface of the semiconductor oxide. The separate mechanisms of BPA degradation during the photocatalytic degradation or ozonization have been previously reported (Gultekin and Ince, 2007). Photocatalytic degradation of BPA at different time intervals was examined under sunlight and it increased from 1 hr to 8 hr in all the four catalysts (Fig. 6); it is probably due to electron– hole formation in the photochemical reaction, which is mainly dependent on the light intensity to initiate the rate of photocatalyst (Cassano and Alfano, 2000). The catalyst absorbs more photons and produces more electron–hole pairs in the catalyst surface when absorbing more photos, which increases the hydroxyl radicals' concentration and consequently increases the degradation rate (Ahmed et al., 2011). The effect of light intensity on the solar photocatalytic degradation of BPA in water with TiO2 on sunny and cloudy days has been investigated (Kaneco et al., 2004). The UV source is expensive and hazardous while other light sources like tungsten light and mercury light which requires electricity are not cost-effective options. Therefore, the present study highlighted cheap photo energy (sunlight) as a potential natural energy source for irradiation of catalyst. Polluted water has wide range of pH values based on their origin, and pH plays a major role in the generation of hydroxyl radicals. Thus, understanding the effect of pH in the BPA degradation by Ce-induced ZnO catalysts was necessary. In the present study, degradation efficiency was observed in the order as follows: pH unadjusted (6.5) > pH 5 > pH 9 > pH 3 > pH 11 for BPA (10 mg/L) with the catalyst (30 mg/50 mL) under sunlight (Fig. 7). Highest degradation was obtained in pH 6.5, which subsequently reduced in high alkaline condition for all four catalysts. Similar pH influence on BPA degradation by CN–TiO2 under white LED irradiation has been previously observed (Wang and Lim, 2010). Kaneco et al (2004) have reported that, TiO2 nanostructure exhibited better photodegradation efficiency in the alkaline medium (pH = 10) due to the generation of dominant oxidizing species (hydroxyl free radical) in the
400
450
500 550 Wavelength (nm)
600
650
Fig. 5 – Photoluminescence (PL) spectra of CexZn1 − xO nanocatalysts.
photocatalytic process. But in the current study, better photodegradation efficiency was obtained near the neutral pH of medium and consequently, pH 6.5 was selected for optimal experimental conditions, to avoid the unnecessary chemical treatment including neutralization process. The reusable catalyst activity was performed for 10 cycles for un-doped ZnO and CexZn1 − xO nanostructures. Reusable capacity of the catalysts was similar up to 8 cycles, and then it decreased from 5% to 15%. CexZn1 − xO nanostructures showed maximum reusable efficiency compared to un-doped ZnO. Adsorption of pollutant molecules on the surface of the nanostructures during the photocatalytic experiment may be the reason for the decrease in degradation efficiency on increasing the number of recycling times. Photocatalytic ability of ZnO nanoparticles may be affected by surface area of particles, high carrier generation and the presence of dopant or foreign particle in the sample. Increase in the charge separation efficiency enhances the formation of both free radicals and active oxygen species (Kato et al., 2005). Most of the Table 1 – Effect of catalyst load and BPA concentration in photocatalytic degradation. Catalyst
BPA degradation (%) Catalyst load (mg/50 mL)
Control ZnO
ZnO·Ce1
ZnO·Ce2
ZnO·Ce3
– 10 20 30 40 10 20 30 40 10 20 30 40 10 20 30 40
BPA concentration 10 mg/L
20 mg/L
30 mg/L
3.4 7.8 18.89 49.31 49.82 25 51 97.03 97.79 19 40.42 74.98 84.1 16 35.52 55.03 69.8
2.1 6.11 11.24 37.93 38.02 19..24 34.12 67.03 67.88 13.32 25.57 40.63 40.85 12.28 22.40 32.93 33.01
1.3 5.6 9.6 28.29 29.30 16.66 27.43 35.94 36.17 10.74 20.36 33.30 33.96 9.23 18.48 30.36 31.66
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100
80 Degradation efficiency (%)
like XRD, SEM, TEM, UV and PL analyses. Ce-doped ZnO catalysts showed enhanced efficiency for photocatalytic degradation of BPA in comparison with pure ZnO catalysts. The degradation rate was strongly affected by several major factors, such as BPA concentration, catalyst load, exposure time and pH value. Furthermore, nearly complete removal of BPA can be achieved with Ce-doped ZnO (ZnO·Ce1) nanocatalysts because of its higher electron mobility and surface carrier defects than ZnO catalysts. It can be concluded that CexZn1 − xO nanocomposite is a highly active photo catalyst for the photo degradation of BPA in water under irradiation of sunlight, which might have a potential application in waste water treatment.
Control ZnO ZnO.Ce1 ZnO.Ce2 ZnO.Ce3
90
70 60 50 40 30 20 10 0
0
1
2
3
4 5 Time (hr)
6
8
7
Fig. 6 – Effect of time in photocatalytic degradation of BPA using CexZn1 − xO nanostructures under sunlight. semiconductors have poor activity when used alone, but the presence of a metal on the semiconductor markedly increases their efficiency. The role of the loaded material is to trap and subsequently transfer photo excited electron onto surface, thus decreasing the recombination of hole–electron pairs (Mirkhani et al., 2009). In the present study, order of degradation efficiency was observed as ZnO·Ce1 > ZnO·Ce2 > ZnO·Ce3 > ZnO. Structural studies clearly indicate that ZnO·Ce2 and ZnO·Ce3 were in CeO2/ZnO composite nature which reduced the activity species in catalytic degradation. In all conditions used in the study, better degradation rates were observed in ZnO·Ce1 (Ce doped ZnO) when compared with ZnO alone, possibly because of Ce being used as a loading material to enhance the high electrical mobility to the surface and doped ions can also act as charge trapping sites that may reduce electron–hole recombination which enhances the degradation efficiency.
3. Conclusions
Degradation efficiency (%)
ZnO and three different concentrations of Ce-doped ZnO nanocomposites were synthesized via co-precipitation method. Prepared catalysts were characterized by various techniques 100 90 80 70 60 50 40 30 20 10 3.0 2.5 2.0 1.5 1.0 0.5 0.0
2
Control ZnO ZnO.Ce1 ZnO.Ce2 ZnO.Ce3
3
4
5
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6 7 8 9 pH of the BPA solution
10
11
12
Fig. 7 – Effect of pH in photocatalytic degradation of BPA using CexZn1 − xO nanostructures under sunlight.
Acknowledgments We are thankful to Advanced Materials and Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore, Tamil Nadu, India and Karpagam University, Coimbatore, Tamil Nadu, India, for providing the necessary lab facilities for this work.
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