Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and enhancement of photocatalytic activities of ZnO by silver nanoparticles Rupali S. Patil a, Mangesh R. Kokate a, Dipak V. Shinde a,b, Sanjay S. Kolekar a,⇑, Sung H. Han b a b
Department of Chemistry, Shivaji University, Kolhapur 416 004, MS, India Inorganic Nanomaterial Laboratories, Hanyang University, Seoul, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis of Ag–ZnO composite in
Mechanism elaborating photocatalytic activity.
ethanol–water mixture is reported. Method is simple, novel, non-toxic and cost effective without use of surfactant. The composite showed high photocatalytic activity than bare ZnO towards Rh. B. The results of XRD, DRS, PL, TEM and PSD are good agreement.
hv Ag
ZnO
3.2 eV H2O OH.
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 9 September 2013 Accepted 26 September 2013 Available online 19 October 2013 Keywords: ZnO Ag–ZnO composite Photocatalytic activity Rh B
a b s t r a c t Herein, we reports synthesis, characterization and photocatalytic degradation of Rhodamine B under natural sunlight using zinc oxide and Ag–ZnO composite. Zinc oxide nanoparticles were prepared by simple wet chemical method using ethanol–water mixture. Ag–ZnO composite was prepared in two steps by dispersing synthesized ZnO in silver nitrate solution and subsequently reducing it with Ocimum tenuiflorum leaves extract as bioreducing agent. The synthesized bare zinc oxide and Ag–ZnO composite was characterized by various techniques like XRD, DRS, FE-SEM, TEM, SAED, PSD, Zeta potentials, etc. Zinc oxide being wide band gap material can absorbs UV light from solar spectrum which is only 5% so is not efficient material for dye degradation under sunlight. The absorption of visible light was increased by preparing the Ag–ZnO composite. The enhancement in photocatalytic activities of Ag–ZnO composite was observed than bare ZnO. This enhancement is due to shift of absorption edge of ZnO in visible region and decrease in band gap. Ó 2013 Published by Elsevier B.V.
Introduction Nanotechnology has become one of the most interesting areas of scientific research in the past few years. The nanotechnology is receiving much attention in material research, including synthesis, characterization and applications of nano-meter sized metals, ⇑ Corresponding author. E-mail address: [email protected] (S.S. Kolekar). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.09.116
oxides, semiconductors and ceramics [1]. Zinc oxide (ZnO) is a ntype semiconductor with band gap of 3.37 eV and has large excitons binding energy of about 60 meV at room temperature. Thus it provides prospectus of practical lasers with thresholds even at high temperature [2] and important applications in electronics, optics, optoelectronics, lasers, and light-emitting diodes [3,4], as a phosphor in field emissive displays [5,6] and in other cathodoluminescent devices [7]. The piezoelectric and pyroelectric properties of
114
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
ZnO make it a great candidate for sensors, transducers, energy generators, and photocatalysts for hydrogen production. Nanocomposite materials involving metals and semiconductors as building blocks are especially attractive, as they provide the opportunity to modulate optical phenomena including harvesting, emission, and concentration of electromagnetic radiation [8–10]. These features are of practical relevance for the optimization of photovoltaic and photocatalytic systems and for the development of optoelectronic and sensing devices [11–14]. Although several successful synthetic routes exist for M@TiO2 [12,13,15], the production of M@ZnO nanocomposites is still challenging [16]. With the steady and fast growing field of nanoscience and nanotechnology, nanostructured zinc oxide (ZnO) has become a promising photocatalyst for its high catalytic activity, low cost, and environmental friendliness [17–20]. Major limitation to achieve high photocatalytic efficiency in the ZnO nanostructure systems is the quick recombination of charge carriers. The design and modification of ZnO photocatalysts with high sensitivity and reactivity has attracted much attention. It was found that the photocatalytic performance can be greatly improved by developing ZnO-based heterostructures or composites [21–24]. The prospect of developing novel photocatalysts of nanosilver hybridized with ZnO boosts relevant research because of the increase in the rate of electrontransfer process by silver nanoparticles [25,26]. Nanoparticles of Ag and ZnO are also used industrially for several purposes [27]. We herein report the synthesis of zinc oxide nanoparticles in ethanol–water mixture without use of any surfactant or template. The silver zinc oxide composite was prepared by loading silver nanoparticles on zinc oxide and subsequently reducing it using natural bioreducing agent, Ocimum tenuiflorum leaf extract [28]. Dye effluents from various industries are becoming a serious environmental problem because of their unacceptable color, high chemical oxygen demand and resistance to chemical, photochemical and biological degradation. So the degradation of such dye solution is very necessary. In this study the photocatalytic activity of bare zinc oxide and silver zinc oxide composite was tested for Rh B dye and enhancement in photocatalytic activity was observed for Ag–ZnO. Experimental Materials The chemicals used such as zinc acetate, silver nitrate, ethanol and potassium hydroxide were of AR grade, purchased from Merck, India and used without further purification. All solutions were prepared in double distilled water. Synthesis of zinc oxide nanoparticles Zinc oxide nanoparticles were synthesized by using ethanol– water mixture as a solvent. In actual synthesis, 0.2 M zinc acetate was dissolved in 80:20 ethanol–water mixtures. The temperature of reaction mixture was kept to 50 °C and the reaction mixture was stirred for 1 h. Then, 5 M KOH was added with constant stirring until the pH of resulting solution becomes 10, at this pH the gel precipitates out. The gel was stirred for 5 h for uniform distribution of particles. The gelatinous precipitate was centrifuged, and then washed with double distilled water for three times to remove unreacted acetate salt. Finally, it was washed with acetone and the powder was dried at 80 °C. The series of samples were prepared by varying the composition of ethanol-water system from 20:80, 40:60, 60:40 and 80:20. It is observed that, for all systems phase formation takes place. However, it is found that there is no control over particle size, only 80:20 ethanol–water mixture shows narrow particle size distribution.
Preparation of plant extract The leaves of Ocimum tenuiflorum were obtained from botanical garden, Shivaji University, Kolhapur. The obtained leaves were washed thoroughly several times with deionised water. 5 g of leaves were weighed, boiled for 5 min in 100 mL deionised water and the extract was filtered through Whatman filter paper No.-1. The filtered extract was stored in refrigerator at 4 °C and was used for the synthesis of silver zinc oxide composite. Synthesis of silver zinc oxide composite One gram of zinc oxide powder of particle size 25 nm was weighed and dispersed in 10 mL 0.01 M aqueous solution of silver nitrate. The whole solution was sonicated for 20 min to get uniform dispersion. The solution was then stirred for 1 h and then 5 mL leaf extract was extracted and further stirred for 1 h. The whole material was centrifuged and washed three times by distilled water and two times by ethanol to ensure the removal of unbounded silver. The series of reactions were carried out at different silver nitrate concentration as 0.01, 0.03 M and 0.05 M. The appropriate loading was observed for 0.03 M concentration of silver. All these samples were studied for dye degradation. The sample with 0.03 M silver concentration showed maximum activity. So this sample was characterized by various techniques. Powder X-ray diffraction (XRD) analysis was carried out by using Cu Ka radiations (k = 1.54 Å) on Bruker AXS D8. Field emission scanning electron microscopy (FE-SEM) image was obtained by JSM-6301F equipped with EDAX. The TEM observations were carried out with a JEOL TEM 2010 microscope. For TEM characterization, the powder sample was dispersed in methanol by ultrasonication for 20 min and loaded on copper grid. Diffuse reflectance UV–visible analysis was carried out with Variance Cary UV–visible NIR spectrophotometer (model-5000). Photoluminescence (PL) measurements were carried out at room temperature using 285 nm wavelengths as the excitation wavelength and emission was observed from 350 to 500 nm with spectroflurimeter JASCO, F. P. 750, Japan with a Xe lamp as the excitation source. Particle size of synthesized silver loaded zinc oxide nanoparticles were analyzed through photon correlation spectroscopy. Light scattering measurements were carried out at 90° on a photon correlation spectrometer (PCS) – Zetasizer 3000 HAS equipped with a digital autocorrelation from Malvern Instrument UK. Measurement of photocatalytic activity The evaluation of photocatalytic activity of as prepared samples for the photocatalytic degradation of Rhodamine B aqueous solution was performed at room temperature by measuring both adsorption and degradation. Experiments were done as follows: (1) 100 mL 20 ppm Rh B + 01. gm ZnO ? kept in sunlight with stirring. (2) 100 mL 20 ppm Rh B + 01. gm ZnO ? kept in dark with stirring. (3) 100 mL 20 ppm Rh B ? kept in sunlight with stirring. The same experiments were performed for silver zinc oxide composite. After irradiation from 0 h to 5 h, 3 mL of Rh B solution from flask was collected centrifuged and the remaining dye concentration was determined using a UV–visible spectrophotometer. Results and discussion The reaction taking place during synthesis is given by following equation:
115
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
1.2
Step II-ZnOðpowderÞ þ AgNO3 þ leaf extract ! Silver zinc oxide composite
1.0
Ag-ZnO composite
0.8 Absorbance (a.u.)
The synthesis of ZnO nanoparticles was done by taking advantage of ethanol–water mixture without use of any surfactant or template. Actually, ethanol from ethanol–water mixture increases the saturation index of precursor and driving force of crystallization giving rise to pure crystalline phase at low temperature without further annealing. Fig. 1 shows XRD pattern of bare zinc oxide silver–zinc oxide composite. The observed diffraction peaks of the pure ZnO can be indexed to hexagonal wurtzite structure. The high intensity of (1 0 1) peak suggests that the growth of nanoparticles has taken place along the easy direction of crystallization of material. The concentration of silver in Ag–ZnO composite is very small so XRD does not show the peaks of silver. Only the small hump was observed at 38.8° indicating [1 1 1] peak in composite. The average crystallite size for bare zinc oxide and Ag–ZnO composite was determined according to Scherrer formula, and was found to be 31 nm and 30 nm respectively. To know the morphologies of the as synthesized ZnO and Ag–ZnO composite the FE-SEM images were taken. FE-SEM image of bare ZnO and Ag–ZnO composite was presented in electronic supplementary information (ESI-1). The morphology was irregular less or more spherical or elliptical in nature. The grain size from the FE-SEM was found to be 40 nm and 45 nm respectively. FE-SEM images also reveal that the particles were uniformly distributed. The closer inspection of ZnO and Ag–ZnO was done with TEM as shown in ESI-2. The TEM images of bare ZnO showed formation of nanosheet which look quit aggregated in FE-SEM but clearly observed in TEM. The average age length of each sheet is 25 nm. The Ag–ZnO composite was highly porous less or more spherical or elliptical clearly observed here. The formation of porous morphology is one of the reasons for the enhancement in photocatalytic activity of Ag–ZnO composite. The EDS of bare ZnO showed Zn and O in 22% and 78% respectively. The Ag–ZnO samples with different loading were tested for EDS but 2.03% loading was appropriate for efficient dye degradation. The EDS spectra of Ag–ZnO optimized composite was provided in ESI-3. The DRS spectra of bare ZnO and Ag–ZnO was obtained in absorbance mode, bare ZnO showed absorption in UV-region of solar spectrum while Ag–ZnO showed absorption of visible region. Fig. 2 showed DRS spectra of bare ZnO and Ag–ZnO composite. DRS of composite were clearly indicated in inset of DRS. This inset
Intensity (a.u.)
Step I-ZnðCH3 COOÞ2 þ 2KOH ! ZnOðpowderÞ þ 2CH3 COOK þ H2 O
0.6 0.4 0.2
1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 200 300 400 500 600 700 800
Wavelength (nm)
0.0 200
ZnO 300
400
500
600
700
800
Wavelength (nm) Fig. 2. DRS spectra of ZnO and Ag–ZnO, the insect clearly showing plasmonic emission of Ag nanoparticles from of Ag–ZnO composite.
indicates absorption of total UV–visible region along with plasmonic emission of Ag nanoparticles. This is the one of the reason for the enhancement in photocatalytic activity of composite. The inset spectra shows the clear difference in absorbance in bare ZnO and Ag–ZnO composite, due to the silver composite the absorption of composite increase almost four times that of bare ZnO. Since the defect chemistry of oxide ceramics is of great importance for potential applications in electronic devices, photoluminescence (PL) spectra of bare ZnO and Ag–ZnO were studied. As expected for nanoscale zinc oxide powder, a sharp signal in the ultraviolet (UV) range (exciton emission) and a further broader signal, which might consist of several contributions in the visible region were observed, where by the latter is related to the defect states [29–31]. The photoluminescence (PL) property of the ZnO is one of the most interesting and important properties that have been intensively investigated recently [32]. PL emission spectra have been widely used to understand the fate of electron–hole pairs in semiconductor particles [33]. Fig. 3 depicts room temperature photoluminescence emission spectra of synthesized zinc oxide and Ag–ZnO composite. The emission spectrum of the excitation at 325 nm gives two peaks at 401 nm and 470 nm. It has been shown that the visible emission from nanocrystalline ZnO particles was due to a transition of a photo-generated electron from the conduction band to a deeply
5000 4000 3000 ZnO
2000 Ag [111]
Intensity (a.u.)
6000
1000 0 20
30
40
Ag-ZnO
50
60
70
80
Angle (2θ) Fig. 1. X-ray diffraction pattern of bare ZnO and Ag–ZnO composite.
Emission Intensity (a.u.)
700 7000
600 Bare ZnO
500 400 300
Ag-ZnO
200 100 0 400
450
500
Wavelength (nm) Fig. 3. Room temperature PL spectra of bare ZnO and Ag–ZnO composite.
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
Efficiencyð%Þ ¼
A0 A 100 A0
where A0 represents the initial absorbance of the Rh B solution, A is the absorbance after irradiation. The mechanism of dye degradation was illustrated as below,
0.7
t= 0 hr
0.6
Absorbance (a.u)
trapped hole. In ZnO nanoparticles along-with the visible green emission, UV excitonic emission is also reported. Therefore, for efficient visible emission a step must be involved in which the photo-generated hole is trapped efficiently somewhere in the particle. The rate of this hole trapping must be much faster than the radiative recombination rate of the exciton emission. As compared to the bare ZnO, loading Ag on ZnO leads to a decrease of the visible emissions indicating decrease in defects produced in the crystal lattice, suggesting that the metallic Ag was deposited on the defect sites. The Ag–ZnO composite showed emission spectrum of the excitation at 285 nm was observed from 350 to 500 nm gives single peak at 410 nm, which is the shoulder of the broad peak. Thus Ag–ZnO composite is good photocatalyst because in a photocatalytic process, the separation and recombination of photoinduced electron and hole are competitive pathways, and photocatalytic activity is effective when the recombination is prevented. The extent of recombination can be probed by the intensity of photoluminescence. The PSD of bare ZnO and Ag–ZnO was provided in ESI-4. Bare ZnO showed PSD in between 20 nm and 35 nm and Ag–ZnO showed PSD in between 35 nm and 55 nm. The zeta potentials of bare ZnO and Ag–ZnO composite was taken to know the charge on the particles. Bare ZnO and Ag–ZnO showed 4.96 mV and 20.2 mV respectively. The increase zeta potential of Ag–ZnO composite indicates increase in stability of composite in water. The plots of zeta potential were provided in ESI-5. The photocatalytic activities of both bare zinc oxide and Ag–ZnO composites were tested for degradation of Rh B in natural sunlight emission. The photocatalytic degradation efficiency of Rh B was calculated according to the following equation:
t= 1 hr
0.5
t=2 hrs
0.4
t=3 hrs t=4 hrs
0.3
t=5 hrs t=6 hrs
0.2
t=7 hrs t=8 hrs
0.1 0.0 -0.1
480 500 520 540 560 580 600 620 640
Wavelength (nm) Fig. 4a. Absorption changes of Rhodamine B obtained at different time intervals using ZnO catalyst.
1.2
Absorbance (a.u.)
116
t=0
0.9 t=1 hr
0.6
t=2 hrs t=3 hrs
0.3
t=4 hrs t=5 hrs
þ
ZnO þ hc ! ZnOðh þ e Þ
0.0 400
450
þ
hvb þ H2 O ! HO þ Hþ
HO þ dye ! dyeox ! intermediate ! CO2 þ H2 O
500
550
600
650
Wavelength (nm) Fig. 4b. Absorption changes of Rhodamine B obtained at different time intervals using Ag–ZnO catalyst.
The photocatalytic activity is supressed when the photo-induced electrons are recombined with holes as follows:
0.9
þ
hvb þ e ! heat
0.8 0.7 0.6
C/C 0
One blank experiment was performed to confirm degradation is photocatalytic degradation or adsorption. The absorbance changes at same experimental condition without using sunlight (in dark) were done. The table from ESI-6 showed there is no absorbance change in absorbance was observed indicating that this is photocatalytic degradation. Figs. 4a and 4b indicates the absorption changes in Rh B concentration after irradiation of sunlight using ZnO and Ag–ZnO composite. About 80% decrease in the concentration of Rh B was decreased within 8 h ZnO and 85% for Ag–ZnO within 5 h of sunlight irradiation. This enhancement in photocatalytic activity is due to absorption of maximum visible light and faster generation of electron hole pairs. Silver also helps for the charge separation. The plot of C/C0 with time is decrease in concentration of dye for bare ZnO and Ag–ZnO with respect to time is presented in Fig. 5. This indicates Ag–ZnO is suitable catalyst for the degradation of Rh B. This enhancement in dye degradation was explained as:
0.5 0.4 0.3
ZnO Ag-ZnO
0.2 0.1 1
2
3
4
5
6
7
Time (hrs) Fig. 5. Plot of C/C0 with respect to time of ZnO and Ag–ZnO.
8
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
(1) The DRS results reveal that, bare ZnO being a UV absorber can absorbs only UV light from solar spectrum which is only 5%. On the other hand Ag–ZnO composite can absorbs both UV–visible spectrums, so there is more formation and separation of electron–hole (chare) pairs takes place. (2) From the PL results of bare ZnO showing two peaks indicate presence of defects and these defect sites provides recombination centers and can suppress the photocatalytic activity. On the other hand PL of Ag–ZnO showed quenching in the intensity and only one emission peak, indicating decrease in recombination centers. (3) The presence of silver in Ag–ZnO composite helps for the separation of charges generated so photocatalytic activity is enhanced. Conclusion In this study, we have studied synthesis of ZnO and Ag–ZnO composite. The ZnO was synthesized in ethanol water mixture it helps to form pure phase at room temperature. The Ag–ZnO composite was synthesized by dispersing ZnO in silver nitrate and subsequently reducing it using Ocimum tenuiflorum leaf extract. The photocatalytic degradation of Rh B was studied for both bare ZnO and Ag–ZnO composite. The small loading of silver on zinc oxide enhances its photocatalytic largely, because absorption increased in visible region, decrease in defects state due to silver and silver helps in separation of charges generated. Hence the composite serves as an efficient photocatalyst for degradation of Rh B. Acknowledgements Miss. Rupali S. Patil is grateful to DST-INSPIRE for providing financial assistance and also we acknowledge DST-FIST and UGC-SAP programmes, Department of Chemistry, Shivaji University, Kolhapur for providing research facilities. We also thanks to Mr. Nagnath G. Patil for help in TEM measurement. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.116.
117
References [1] S.I. Bozhko, A.N. Chaika, G.A. Emelcheyko, V.M. Masalov, A.M. Ionov, A.N. Gruzintsev, G.M. Mikhailov, B.K. Medvedev, Appl. Surf. Sci. 34 (2004) 93–101. [2] C. Weitao, D. Weimin, J. Lumin. 124 (2007) 260–264. [3] C. Jagadish, S.J. Pearton (Eds.), Zinc oxide bulk, Thin Films and Nanostructures, Elsevier, NewYork, 2006. [4] Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, S.J. Pearton, J.R. LaRoche, Mater. Sci. Eng. R 47 (2004) 1–47. [5] J. Zhou, N.S. Xu, Z.L. Wang, Adv. Mater. 18 (2006) 2432–2435. [6] Z. Wang, Nanowires and Nanobelts vol. I Metal and Semiconductor Nanowires and vol. II Nanowires and Nanobelts of Functional Oxide, Kluwer Academic Publisher, New York, 2003. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [8] Y.B. Zheng, B. Kiraly, S. Cheunkar, T.J. Huang, P.S. Weiss, Nano Lett. 1 (2011) 2061–2065. [9] Y. Fedutik, V. Temnov, U. Woggon, E. Ustinovich, M. Artemyev, J. Am. Chem. Soc. 129 (2007) 14939–14945. [10] A.O. Govorov, G.W. Bryant, W. Zhang, T. Skeini, J. Lee, N.A. Kotov, J.M. Slocik, R.R. Naik, Nano Lett. 6 (2006) 984–994. [11] Q. Xiang, G. Meng, Y. Zhang, J. Xu, P. Xu, Q. Pan, W. Yu, Sens. Actuators B 143 (2010) 635–640. [12] T. Hirakawa, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 3928–3934. [13] P.V. Kamat, J. Phys. Chem. C 111 (2007) 488–494. [14] K.K. Haldar, T. Sen, A. Patra, J. Phys. Chem. C 112 (2008) 11650–11656. [15] N. Zhang, S. Liu, X. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 9136–9145. [16] P. Li, Z. Wei, T. Wu, Q. Peng, Y. Li, J. Am. Chem. Soc. 133 (2011) 5660–5663. [17] D. Li, H. Haneda, Chemosphere 51 (2003) 129–137. [18] J.L. Yang, S.J. An, W.I. Park, G.C. Yi, W. Choi, Adv. Mater. 16 (2004) 1661–1668. [19] S. Chakrabarti, B.K. Dutta, J. Hazard. Mater. 112 (2004) 269–278. [20] B. Pal, M. Sharon, Mater. Chem. Phys. 76 (2002) 82–87. [21] Y.J. He, X.Y. Yu, T.L. Li, L.Y. Yan, B.L. Yang, Powder Technol. 166 (2006) 72–76. [22] C. Kim, S.J. Doha, S.G. Leea, S.J. Leea, H.Y. Kima, Appl. Catal. A 330 (2007) 127– 133. [23] C. Wang, X.M. Wang, B.Q. Xu, J.C. Zhao, B.X. Mai, P.A. Peng, G.Y. Sheng, J.M. Fu, J. Photochem. Photobiol. A 168 (2004) 47–52. [24] M.L. Zhang, T.C. An, X.H. Hu, C. Wang, G.Y. Sheng, J.M. Fu, Appl. Catal. A 260 (2004) 115–222. [25] S. Bhattacharyya, A. Gedanken, J. Phys. Chem. C 112 (2008) 659–665. [26] Y.H. Zheng, L.R. Zheng, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, Inorg. Chem. 46 (2007) 6980–6986. [27] P. Gajjar, B. Pettee, D.W. Britt, W. Huang, W. Johnson, A.J. Anderson, J. Biol. Eng. 3 (2009) 1183–1187. [28] R.S. Patil, M.R. Kokate, S.S. Kolekar, Spectrochim. Acta Part A 91 (2012) 234– 238. [29] D. Behera, B.S. Acharya, J. Lumin. 128 (2008) 1577–1586. [30] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 86 (2000) 87–94. [31] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 90 (2000) 123–128. [32] L.Z. Pei, H.S. Zhao, W. Tan, H.Y. Yu, Y.W. Chen, Q. Feng Zhang, Mater. Charact. 60 (2009) 1063–1067. [33] Q. Xiao, J. Zhang, C. Xiao, X. Tan, Mater. Sci. Eng. B 142 (2007) 121–125.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and enhancement of photocatalytic activities of ZnO by silver nanoparticles Rupali S. Patil a, Mangesh R. Kokate a, Dipak V. Shinde a,b, Sanjay S. Kolekar a,⇑, Sung H. Han b a b
Department of Chemistry, Shivaji University, Kolhapur 416 004, MS, India Inorganic Nanomaterial Laboratories, Hanyang University, Seoul, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Synthesis of Ag–ZnO composite in
Mechanism elaborating photocatalytic activity.
ethanol–water mixture is reported. Method is simple, novel, non-toxic and cost effective without use of surfactant. The composite showed high photocatalytic activity than bare ZnO towards Rh. B. The results of XRD, DRS, PL, TEM and PSD are good agreement.
hv Ag
ZnO
3.2 eV H2O OH.
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 9 September 2013 Accepted 26 September 2013 Available online 19 October 2013 Keywords: ZnO Ag–ZnO composite Photocatalytic activity Rh B
a b s t r a c t Herein, we reports synthesis, characterization and photocatalytic degradation of Rhodamine B under natural sunlight using zinc oxide and Ag–ZnO composite. Zinc oxide nanoparticles were prepared by simple wet chemical method using ethanol–water mixture. Ag–ZnO composite was prepared in two steps by dispersing synthesized ZnO in silver nitrate solution and subsequently reducing it with Ocimum tenuiflorum leaves extract as bioreducing agent. The synthesized bare zinc oxide and Ag–ZnO composite was characterized by various techniques like XRD, DRS, FE-SEM, TEM, SAED, PSD, Zeta potentials, etc. Zinc oxide being wide band gap material can absorbs UV light from solar spectrum which is only 5% so is not efficient material for dye degradation under sunlight. The absorption of visible light was increased by preparing the Ag–ZnO composite. The enhancement in photocatalytic activities of Ag–ZnO composite was observed than bare ZnO. This enhancement is due to shift of absorption edge of ZnO in visible region and decrease in band gap. Ó 2013 Published by Elsevier B.V.
Introduction Nanotechnology has become one of the most interesting areas of scientific research in the past few years. The nanotechnology is receiving much attention in material research, including synthesis, characterization and applications of nano-meter sized metals, ⇑ Corresponding author. E-mail address: [email protected] (S.S. Kolekar). 1386-1425/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.saa.2013.09.116
oxides, semiconductors and ceramics [1]. Zinc oxide (ZnO) is a ntype semiconductor with band gap of 3.37 eV and has large excitons binding energy of about 60 meV at room temperature. Thus it provides prospectus of practical lasers with thresholds even at high temperature [2] and important applications in electronics, optics, optoelectronics, lasers, and light-emitting diodes [3,4], as a phosphor in field emissive displays [5,6] and in other cathodoluminescent devices [7]. The piezoelectric and pyroelectric properties of
114
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
ZnO make it a great candidate for sensors, transducers, energy generators, and photocatalysts for hydrogen production. Nanocomposite materials involving metals and semiconductors as building blocks are especially attractive, as they provide the opportunity to modulate optical phenomena including harvesting, emission, and concentration of electromagnetic radiation [8–10]. These features are of practical relevance for the optimization of photovoltaic and photocatalytic systems and for the development of optoelectronic and sensing devices [11–14]. Although several successful synthetic routes exist for M@TiO2 [12,13,15], the production of M@ZnO nanocomposites is still challenging [16]. With the steady and fast growing field of nanoscience and nanotechnology, nanostructured zinc oxide (ZnO) has become a promising photocatalyst for its high catalytic activity, low cost, and environmental friendliness [17–20]. Major limitation to achieve high photocatalytic efficiency in the ZnO nanostructure systems is the quick recombination of charge carriers. The design and modification of ZnO photocatalysts with high sensitivity and reactivity has attracted much attention. It was found that the photocatalytic performance can be greatly improved by developing ZnO-based heterostructures or composites [21–24]. The prospect of developing novel photocatalysts of nanosilver hybridized with ZnO boosts relevant research because of the increase in the rate of electrontransfer process by silver nanoparticles [25,26]. Nanoparticles of Ag and ZnO are also used industrially for several purposes [27]. We herein report the synthesis of zinc oxide nanoparticles in ethanol–water mixture without use of any surfactant or template. The silver zinc oxide composite was prepared by loading silver nanoparticles on zinc oxide and subsequently reducing it using natural bioreducing agent, Ocimum tenuiflorum leaf extract [28]. Dye effluents from various industries are becoming a serious environmental problem because of their unacceptable color, high chemical oxygen demand and resistance to chemical, photochemical and biological degradation. So the degradation of such dye solution is very necessary. In this study the photocatalytic activity of bare zinc oxide and silver zinc oxide composite was tested for Rh B dye and enhancement in photocatalytic activity was observed for Ag–ZnO. Experimental Materials The chemicals used such as zinc acetate, silver nitrate, ethanol and potassium hydroxide were of AR grade, purchased from Merck, India and used without further purification. All solutions were prepared in double distilled water. Synthesis of zinc oxide nanoparticles Zinc oxide nanoparticles were synthesized by using ethanol– water mixture as a solvent. In actual synthesis, 0.2 M zinc acetate was dissolved in 80:20 ethanol–water mixtures. The temperature of reaction mixture was kept to 50 °C and the reaction mixture was stirred for 1 h. Then, 5 M KOH was added with constant stirring until the pH of resulting solution becomes 10, at this pH the gel precipitates out. The gel was stirred for 5 h for uniform distribution of particles. The gelatinous precipitate was centrifuged, and then washed with double distilled water for three times to remove unreacted acetate salt. Finally, it was washed with acetone and the powder was dried at 80 °C. The series of samples were prepared by varying the composition of ethanol-water system from 20:80, 40:60, 60:40 and 80:20. It is observed that, for all systems phase formation takes place. However, it is found that there is no control over particle size, only 80:20 ethanol–water mixture shows narrow particle size distribution.
Preparation of plant extract The leaves of Ocimum tenuiflorum were obtained from botanical garden, Shivaji University, Kolhapur. The obtained leaves were washed thoroughly several times with deionised water. 5 g of leaves were weighed, boiled for 5 min in 100 mL deionised water and the extract was filtered through Whatman filter paper No.-1. The filtered extract was stored in refrigerator at 4 °C and was used for the synthesis of silver zinc oxide composite. Synthesis of silver zinc oxide composite One gram of zinc oxide powder of particle size 25 nm was weighed and dispersed in 10 mL 0.01 M aqueous solution of silver nitrate. The whole solution was sonicated for 20 min to get uniform dispersion. The solution was then stirred for 1 h and then 5 mL leaf extract was extracted and further stirred for 1 h. The whole material was centrifuged and washed three times by distilled water and two times by ethanol to ensure the removal of unbounded silver. The series of reactions were carried out at different silver nitrate concentration as 0.01, 0.03 M and 0.05 M. The appropriate loading was observed for 0.03 M concentration of silver. All these samples were studied for dye degradation. The sample with 0.03 M silver concentration showed maximum activity. So this sample was characterized by various techniques. Powder X-ray diffraction (XRD) analysis was carried out by using Cu Ka radiations (k = 1.54 Å) on Bruker AXS D8. Field emission scanning electron microscopy (FE-SEM) image was obtained by JSM-6301F equipped with EDAX. The TEM observations were carried out with a JEOL TEM 2010 microscope. For TEM characterization, the powder sample was dispersed in methanol by ultrasonication for 20 min and loaded on copper grid. Diffuse reflectance UV–visible analysis was carried out with Variance Cary UV–visible NIR spectrophotometer (model-5000). Photoluminescence (PL) measurements were carried out at room temperature using 285 nm wavelengths as the excitation wavelength and emission was observed from 350 to 500 nm with spectroflurimeter JASCO, F. P. 750, Japan with a Xe lamp as the excitation source. Particle size of synthesized silver loaded zinc oxide nanoparticles were analyzed through photon correlation spectroscopy. Light scattering measurements were carried out at 90° on a photon correlation spectrometer (PCS) – Zetasizer 3000 HAS equipped with a digital autocorrelation from Malvern Instrument UK. Measurement of photocatalytic activity The evaluation of photocatalytic activity of as prepared samples for the photocatalytic degradation of Rhodamine B aqueous solution was performed at room temperature by measuring both adsorption and degradation. Experiments were done as follows: (1) 100 mL 20 ppm Rh B + 01. gm ZnO ? kept in sunlight with stirring. (2) 100 mL 20 ppm Rh B + 01. gm ZnO ? kept in dark with stirring. (3) 100 mL 20 ppm Rh B ? kept in sunlight with stirring. The same experiments were performed for silver zinc oxide composite. After irradiation from 0 h to 5 h, 3 mL of Rh B solution from flask was collected centrifuged and the remaining dye concentration was determined using a UV–visible spectrophotometer. Results and discussion The reaction taking place during synthesis is given by following equation:
115
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
1.2
Step II-ZnOðpowderÞ þ AgNO3 þ leaf extract ! Silver zinc oxide composite
1.0
Ag-ZnO composite
0.8 Absorbance (a.u.)
The synthesis of ZnO nanoparticles was done by taking advantage of ethanol–water mixture without use of any surfactant or template. Actually, ethanol from ethanol–water mixture increases the saturation index of precursor and driving force of crystallization giving rise to pure crystalline phase at low temperature without further annealing. Fig. 1 shows XRD pattern of bare zinc oxide silver–zinc oxide composite. The observed diffraction peaks of the pure ZnO can be indexed to hexagonal wurtzite structure. The high intensity of (1 0 1) peak suggests that the growth of nanoparticles has taken place along the easy direction of crystallization of material. The concentration of silver in Ag–ZnO composite is very small so XRD does not show the peaks of silver. Only the small hump was observed at 38.8° indicating [1 1 1] peak in composite. The average crystallite size for bare zinc oxide and Ag–ZnO composite was determined according to Scherrer formula, and was found to be 31 nm and 30 nm respectively. To know the morphologies of the as synthesized ZnO and Ag–ZnO composite the FE-SEM images were taken. FE-SEM image of bare ZnO and Ag–ZnO composite was presented in electronic supplementary information (ESI-1). The morphology was irregular less or more spherical or elliptical in nature. The grain size from the FE-SEM was found to be 40 nm and 45 nm respectively. FE-SEM images also reveal that the particles were uniformly distributed. The closer inspection of ZnO and Ag–ZnO was done with TEM as shown in ESI-2. The TEM images of bare ZnO showed formation of nanosheet which look quit aggregated in FE-SEM but clearly observed in TEM. The average age length of each sheet is 25 nm. The Ag–ZnO composite was highly porous less or more spherical or elliptical clearly observed here. The formation of porous morphology is one of the reasons for the enhancement in photocatalytic activity of Ag–ZnO composite. The EDS of bare ZnO showed Zn and O in 22% and 78% respectively. The Ag–ZnO samples with different loading were tested for EDS but 2.03% loading was appropriate for efficient dye degradation. The EDS spectra of Ag–ZnO optimized composite was provided in ESI-3. The DRS spectra of bare ZnO and Ag–ZnO was obtained in absorbance mode, bare ZnO showed absorption in UV-region of solar spectrum while Ag–ZnO showed absorption of visible region. Fig. 2 showed DRS spectra of bare ZnO and Ag–ZnO composite. DRS of composite were clearly indicated in inset of DRS. This inset
Intensity (a.u.)
Step I-ZnðCH3 COOÞ2 þ 2KOH ! ZnOðpowderÞ þ 2CH3 COOK þ H2 O
0.6 0.4 0.2
1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 200 300 400 500 600 700 800
Wavelength (nm)
0.0 200
ZnO 300
400
500
600
700
800
Wavelength (nm) Fig. 2. DRS spectra of ZnO and Ag–ZnO, the insect clearly showing plasmonic emission of Ag nanoparticles from of Ag–ZnO composite.
indicates absorption of total UV–visible region along with plasmonic emission of Ag nanoparticles. This is the one of the reason for the enhancement in photocatalytic activity of composite. The inset spectra shows the clear difference in absorbance in bare ZnO and Ag–ZnO composite, due to the silver composite the absorption of composite increase almost four times that of bare ZnO. Since the defect chemistry of oxide ceramics is of great importance for potential applications in electronic devices, photoluminescence (PL) spectra of bare ZnO and Ag–ZnO were studied. As expected for nanoscale zinc oxide powder, a sharp signal in the ultraviolet (UV) range (exciton emission) and a further broader signal, which might consist of several contributions in the visible region were observed, where by the latter is related to the defect states [29–31]. The photoluminescence (PL) property of the ZnO is one of the most interesting and important properties that have been intensively investigated recently [32]. PL emission spectra have been widely used to understand the fate of electron–hole pairs in semiconductor particles [33]. Fig. 3 depicts room temperature photoluminescence emission spectra of synthesized zinc oxide and Ag–ZnO composite. The emission spectrum of the excitation at 325 nm gives two peaks at 401 nm and 470 nm. It has been shown that the visible emission from nanocrystalline ZnO particles was due to a transition of a photo-generated electron from the conduction band to a deeply
5000 4000 3000 ZnO
2000 Ag [111]
Intensity (a.u.)
6000
1000 0 20
30
40
Ag-ZnO
50
60
70
80
Angle (2θ) Fig. 1. X-ray diffraction pattern of bare ZnO and Ag–ZnO composite.
Emission Intensity (a.u.)
700 7000
600 Bare ZnO
500 400 300
Ag-ZnO
200 100 0 400
450
500
Wavelength (nm) Fig. 3. Room temperature PL spectra of bare ZnO and Ag–ZnO composite.
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
Efficiencyð%Þ ¼
A0 A 100 A0
where A0 represents the initial absorbance of the Rh B solution, A is the absorbance after irradiation. The mechanism of dye degradation was illustrated as below,
0.7
t= 0 hr
0.6
Absorbance (a.u)
trapped hole. In ZnO nanoparticles along-with the visible green emission, UV excitonic emission is also reported. Therefore, for efficient visible emission a step must be involved in which the photo-generated hole is trapped efficiently somewhere in the particle. The rate of this hole trapping must be much faster than the radiative recombination rate of the exciton emission. As compared to the bare ZnO, loading Ag on ZnO leads to a decrease of the visible emissions indicating decrease in defects produced in the crystal lattice, suggesting that the metallic Ag was deposited on the defect sites. The Ag–ZnO composite showed emission spectrum of the excitation at 285 nm was observed from 350 to 500 nm gives single peak at 410 nm, which is the shoulder of the broad peak. Thus Ag–ZnO composite is good photocatalyst because in a photocatalytic process, the separation and recombination of photoinduced electron and hole are competitive pathways, and photocatalytic activity is effective when the recombination is prevented. The extent of recombination can be probed by the intensity of photoluminescence. The PSD of bare ZnO and Ag–ZnO was provided in ESI-4. Bare ZnO showed PSD in between 20 nm and 35 nm and Ag–ZnO showed PSD in between 35 nm and 55 nm. The zeta potentials of bare ZnO and Ag–ZnO composite was taken to know the charge on the particles. Bare ZnO and Ag–ZnO showed 4.96 mV and 20.2 mV respectively. The increase zeta potential of Ag–ZnO composite indicates increase in stability of composite in water. The plots of zeta potential were provided in ESI-5. The photocatalytic activities of both bare zinc oxide and Ag–ZnO composites were tested for degradation of Rh B in natural sunlight emission. The photocatalytic degradation efficiency of Rh B was calculated according to the following equation:
t= 1 hr
0.5
t=2 hrs
0.4
t=3 hrs t=4 hrs
0.3
t=5 hrs t=6 hrs
0.2
t=7 hrs t=8 hrs
0.1 0.0 -0.1
480 500 520 540 560 580 600 620 640
Wavelength (nm) Fig. 4a. Absorption changes of Rhodamine B obtained at different time intervals using ZnO catalyst.
1.2
Absorbance (a.u.)
116
t=0
0.9 t=1 hr
0.6
t=2 hrs t=3 hrs
0.3
t=4 hrs t=5 hrs
þ
ZnO þ hc ! ZnOðh þ e Þ
0.0 400
450
þ
hvb þ H2 O ! HO þ Hþ
HO þ dye ! dyeox ! intermediate ! CO2 þ H2 O
500
550
600
650
Wavelength (nm) Fig. 4b. Absorption changes of Rhodamine B obtained at different time intervals using Ag–ZnO catalyst.
The photocatalytic activity is supressed when the photo-induced electrons are recombined with holes as follows:
0.9
þ
hvb þ e ! heat
0.8 0.7 0.6
C/C 0
One blank experiment was performed to confirm degradation is photocatalytic degradation or adsorption. The absorbance changes at same experimental condition without using sunlight (in dark) were done. The table from ESI-6 showed there is no absorbance change in absorbance was observed indicating that this is photocatalytic degradation. Figs. 4a and 4b indicates the absorption changes in Rh B concentration after irradiation of sunlight using ZnO and Ag–ZnO composite. About 80% decrease in the concentration of Rh B was decreased within 8 h ZnO and 85% for Ag–ZnO within 5 h of sunlight irradiation. This enhancement in photocatalytic activity is due to absorption of maximum visible light and faster generation of electron hole pairs. Silver also helps for the charge separation. The plot of C/C0 with time is decrease in concentration of dye for bare ZnO and Ag–ZnO with respect to time is presented in Fig. 5. This indicates Ag–ZnO is suitable catalyst for the degradation of Rh B. This enhancement in dye degradation was explained as:
0.5 0.4 0.3
ZnO Ag-ZnO
0.2 0.1 1
2
3
4
5
6
7
Time (hrs) Fig. 5. Plot of C/C0 with respect to time of ZnO and Ag–ZnO.
8
R.S. Patil et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 113–117
(1) The DRS results reveal that, bare ZnO being a UV absorber can absorbs only UV light from solar spectrum which is only 5%. On the other hand Ag–ZnO composite can absorbs both UV–visible spectrums, so there is more formation and separation of electron–hole (chare) pairs takes place. (2) From the PL results of bare ZnO showing two peaks indicate presence of defects and these defect sites provides recombination centers and can suppress the photocatalytic activity. On the other hand PL of Ag–ZnO showed quenching in the intensity and only one emission peak, indicating decrease in recombination centers. (3) The presence of silver in Ag–ZnO composite helps for the separation of charges generated so photocatalytic activity is enhanced. Conclusion In this study, we have studied synthesis of ZnO and Ag–ZnO composite. The ZnO was synthesized in ethanol water mixture it helps to form pure phase at room temperature. The Ag–ZnO composite was synthesized by dispersing ZnO in silver nitrate and subsequently reducing it using Ocimum tenuiflorum leaf extract. The photocatalytic degradation of Rh B was studied for both bare ZnO and Ag–ZnO composite. The small loading of silver on zinc oxide enhances its photocatalytic largely, because absorption increased in visible region, decrease in defects state due to silver and silver helps in separation of charges generated. Hence the composite serves as an efficient photocatalyst for degradation of Rh B. Acknowledgements Miss. Rupali S. Patil is grateful to DST-INSPIRE for providing financial assistance and also we acknowledge DST-FIST and UGC-SAP programmes, Department of Chemistry, Shivaji University, Kolhapur for providing research facilities. We also thanks to Mr. Nagnath G. Patil for help in TEM measurement. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.116.
117
References [1] S.I. Bozhko, A.N. Chaika, G.A. Emelcheyko, V.M. Masalov, A.M. Ionov, A.N. Gruzintsev, G.M. Mikhailov, B.K. Medvedev, Appl. Surf. Sci. 34 (2004) 93–101. [2] C. Weitao, D. Weimin, J. Lumin. 124 (2007) 260–264. [3] C. Jagadish, S.J. Pearton (Eds.), Zinc oxide bulk, Thin Films and Nanostructures, Elsevier, NewYork, 2006. [4] Y.W. Heo, D.P. Norton, L.C. Tien, Y. Kwon, B.S. Kang, F. Ren, S.J. Pearton, J.R. LaRoche, Mater. Sci. Eng. R 47 (2004) 1–47. [5] J. Zhou, N.S. Xu, Z.L. Wang, Adv. Mater. 18 (2006) 2432–2435. [6] Z. Wang, Nanowires and Nanobelts vol. I Metal and Semiconductor Nanowires and vol. II Nanowires and Nanobelts of Functional Oxide, Kluwer Academic Publisher, New York, 2003. [7] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947–1949. [8] Y.B. Zheng, B. Kiraly, S. Cheunkar, T.J. Huang, P.S. Weiss, Nano Lett. 1 (2011) 2061–2065. [9] Y. Fedutik, V. Temnov, U. Woggon, E. Ustinovich, M. Artemyev, J. Am. Chem. Soc. 129 (2007) 14939–14945. [10] A.O. Govorov, G.W. Bryant, W. Zhang, T. Skeini, J. Lee, N.A. Kotov, J.M. Slocik, R.R. Naik, Nano Lett. 6 (2006) 984–994. [11] Q. Xiang, G. Meng, Y. Zhang, J. Xu, P. Xu, Q. Pan, W. Yu, Sens. Actuators B 143 (2010) 635–640. [12] T. Hirakawa, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 3928–3934. [13] P.V. Kamat, J. Phys. Chem. C 111 (2007) 488–494. [14] K.K. Haldar, T. Sen, A. Patra, J. Phys. Chem. C 112 (2008) 11650–11656. [15] N. Zhang, S. Liu, X. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 9136–9145. [16] P. Li, Z. Wei, T. Wu, Q. Peng, Y. Li, J. Am. Chem. Soc. 133 (2011) 5660–5663. [17] D. Li, H. Haneda, Chemosphere 51 (2003) 129–137. [18] J.L. Yang, S.J. An, W.I. Park, G.C. Yi, W. Choi, Adv. Mater. 16 (2004) 1661–1668. [19] S. Chakrabarti, B.K. Dutta, J. Hazard. Mater. 112 (2004) 269–278. [20] B. Pal, M. Sharon, Mater. Chem. Phys. 76 (2002) 82–87. [21] Y.J. He, X.Y. Yu, T.L. Li, L.Y. Yan, B.L. Yang, Powder Technol. 166 (2006) 72–76. [22] C. Kim, S.J. Doha, S.G. Leea, S.J. Leea, H.Y. Kima, Appl. Catal. A 330 (2007) 127– 133. [23] C. Wang, X.M. Wang, B.Q. Xu, J.C. Zhao, B.X. Mai, P.A. Peng, G.Y. Sheng, J.M. Fu, J. Photochem. Photobiol. A 168 (2004) 47–52. [24] M.L. Zhang, T.C. An, X.H. Hu, C. Wang, G.Y. Sheng, J.M. Fu, Appl. Catal. A 260 (2004) 115–222. [25] S. Bhattacharyya, A. Gedanken, J. Phys. Chem. C 112 (2008) 659–665. [26] Y.H. Zheng, L.R. Zheng, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, Inorg. Chem. 46 (2007) 6980–6986. [27] P. Gajjar, B. Pettee, D.W. Britt, W. Huang, W. Johnson, A.J. Anderson, J. Biol. Eng. 3 (2009) 1183–1187. [28] R.S. Patil, M.R. Kokate, S.S. Kolekar, Spectrochim. Acta Part A 91 (2012) 234– 238. [29] D. Behera, B.S. Acharya, J. Lumin. 128 (2008) 1577–1586. [30] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 86 (2000) 87–94. [31] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 90 (2000) 123–128. [32] L.Z. Pei, H.S. Zhao, W. Tan, H.Y. Yu, Y.W. Chen, Q. Feng Zhang, Mater. Charact. 60 (2009) 1063–1067. [33] Q. Xiao, J. Zhang, C. Xiao, X. Tan, Mater. Sci. Eng. B 142 (2007) 121–125.
