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Enhancement in the antibacterial efficiency of ZnO nanopowders by tuning the shape of the nanograins through fluorine doping
Superlattices and Microstructures 69 (2014) 17–28
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Enhancement in the antibacterial efficiency of ZnO nanopowders by tuning the shape of the nanograins through fluorine doping K. Ravichandran a,⇑, S. Snega a, N. Jabena Begum a, K. Swaminathan b, B. Sakthivel a, L. Rene Christena c, G. Chandramohan d, Shizuyasu Ochiai e a
P.G. & Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613 503, Tamil Nadu, India Department of Physics, RKM Vivekananda College (Autonomous), Chennai 600 004, Tamil Nadu, India c School of Chemical and Biotechnology, Shanmuga Arts, Science, Technology and Research Academy, Thanjavur 613 401, Tamil Nadu, India d P.G & Research Department of Chemistry, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613 503, Tamil Nadu, India e Department of Electrical Engineering, Aichi Institute of Technology, Toyota City 470-0392, Japan b
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 30 January 2014 Accepted 31 January 2014 Available online 8 February 2014 Keywords: Oxides Nanostructures Chemical synthesis X-ray diffraction Luminescence
a b s t r a c t Fluorine doped ZnO nanopowders were synthesized from starting solutions having different doping levels of F (0, 5, . . . , 20 at.%) using a simple soft chemical route and the effects of the doping level on the structural, optical, surface morphological and antibacterial properties were investigated. The XRD studies reveal that all the products have preferential orientation along the (1 0 1) plane. The PL studies show that all the samples exhibit strong visible emission with a peak at 425 nm. The enhancement in the visible emission indicates an increasing number of surface defects caused by the doping of F. The obtained FTIR spectra confirm the incorporation of F into ZnO lattice. From the SEM studies, it is observed that the ZnO nanowires formed at 10 at.% of F doping level exhibit excellent antibacterial activities. Antibacterial activity of F doped ZnO nanopowders against Staphylococcus aureus was found to be significantly higher than that against the Escherichia coli and Pseudomonas aeruginosa micro-organisms. All the physical properties were corroborated well with the findings related to antibacterial activity. Finally, we conclude that, the analysis of all
⇑ Corresponding author. Tel.: +91 4362 278602, +91 9443524180 (mobile); fax: +91 4374 239328. E-mail address: [email protected] (K. Ravichandran). http://dx.doi.org/10.1016/j.spmi.2014.01.020 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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the results shows that F doping level of 10 at.% is optimal in all respects and is suitable for antibacterial applications. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the recent decade, one of the most significant challenges faced by the world is the recurrence of infectious diseases and the bacterial contamination in all kinds of materials [1]. Therefore, several antibacterial agents are widely used in day-to-day life for the prevention of public health issues caused by the ubiquity of micro-organisms and their ability to establish themselves [2]. When antibacterial agents are used in the new packaging materials for health care and food applications, the most crucial parameters to be taken care of are low toxicity to human beings and high efficiency in controlling bacteria. The increasing use of inorganic antibacterial agents is of great interest because of their creditability towards safety and stability when compared with organic antibacterial agents. New nanostructured materials with antibacterial properties are the need of the day for preventing microbial growth because, the size, structure and surface properties of nanomaterials can improve the antimicrobial efficacy [3]. Several inorganic metal oxides such as TiO2, MgO, ZnO and CuO have gained increasing attention in recent years. Of these oxides, ZnO is a versatile and important semiconducting material with a band gap of 3.37 eV that could exhibit some special features like excellent chemical and thermal stability, high transparency and bio-compatibility. ZnO has attracted much attention from the industry, being suitable for many potential applications including opto-electronic devices [4], sensors [5], dye-sensitized solar cells [6], biodevices [7] and photocatalysts [8]. Moreover, various morphologies of ZnO like nanowires [9], nanorods [10], nanotubes [11], nanowhiskers [12] and nanoflowers [13] have been found to be useful in biomedical applications. The synthesis route plays an important role in controlling the particle size and the morphology. In order to control the particle size, several techniques have been used for the synthesis of ZnO nanopowders/nanoparticles such as sonochemical synthesis [14], hydrothermal method [15], combustion synthesis [16], sol–gel synthesis [17], polyol method [18] and simple soft chemical route [19,20]. Of these methods, soft chemical route offers several advantages. It is a very fascinating, facile and inexpensive method which is suitable to grow nanostructured materials in large scale [21]. Doping can play an essential role in controlling the size and shape of the ZnO particles and enhancing the physical and antibacterial properties. The physical properties of ZnO can be altered by doping with selective elements like Al, Mg, Mn, F and Ag [22–26]. In the present work, fluorine doped ZnO nanopowders with different F doping levels (0, 5, . . ., 20 at.%) have been synthesized using the soft chemical route and some of their physical properties along with the antibacterial activity have been investigated and reported. To the best of our knowledge, this is the first report on the antibacterial properties of F doped ZnO nanopowder. 2. Materials and methods 2.1. Synthesis process The fluorine doped ZnO nanopowders (ZnO:F) were synthesized with different F doping levels (0, 5, . . ., 20 at.%) using a soft chemical route. The starting solutions were prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)22H2O) in de-ionized water. Ammonium fluoride (NH4F) was used as the dopant precursor and the required amount of NaOH solution was added drop wise in the precursor solution till the pH value had reached 8. The obtained mixture was then magnetically stirred for 2 h at a temperature of 85 °C. After the completion of the stirring process, the precipitate formed was separated carefully by filtration and washed several times with a mixture of ethanol and water
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kept in the ratio of 1:3. The resultant product was then calcinated at 400 °C for 2 h to get the ZnO nanopowder in its final form. 2.2. Characterization of ZnO:F nanopowder The crystalline structure of the powders was analyzed using X-ray powder diffraction method (PANalytical-PW 340/60 X’pert PRO) with Cu Ka (k = 1.5406 Å) radiation. The luminescence spectra of the powders were observed using spectro-fluorometer (Jobin Yvon_FLUROLOG-FL3-11) with Xenon Lamp (450 W) as the excitation source of wavelength of 325 nm at room temperature. The Fourier transform infrared (FTIR) spectra were recorded using Perkin Elmer RX-I FTIR spectrophotometer. The surface morphological studies were made using scanning electron microscope (SEM–HITACHI S-3000 H). 2.3. Evaluation of antibacterial performance The antibacterial property of ZnO:F nanopowders were analyzed by their zone of inhibitions for Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus). An overnight culture of each organism was adjusted to an OD of 0.1 and swabbed onto Mueller Hilton agar plates. Using a cork borer, holes were punched on the agar, followed by addition of the stock solutions containing synthesized nanopowders in increasing volumes from 50 to 200 lL. These plates were incubated at 37 °C for 24 h and the zone of inhibitions was measured in diameter. 3. Results and discussion 3.1. Structural studies The XRD patterns of the pristine and fluorine doped ZnO nanopowders are shown in Fig. 1. From the Fig. 1, it is observed that, all samples have polycrystalline nature with a hexagonal wurtzite structure of ZnO. The strongest diffraction peak shows that the preferential orientation is along (1 0 1) plane. No metallic zinc or other oxide phases are detected which confirms the purity of the nanopowders. The resolution limit of X-ray diffraction technique is not so sufficient to detect the presence of fluorine compounds, if any, in the final product even when high concentration of fluorine (20 at.%) is used during the synthesis process [27].
Fig. 1. XRD patterns of undoped and Fluorine doped ZnO nanopowders.
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The intensity of (1 0 1) peak increases with the increase in the fluorine content up to 10 at.% and then it decreases with further doping which is an indication of the degradation in the crystalline quality of the sample beyond 10 at.% of F doping. This may be due to the interstitial incorporation of excess fluorine atoms into the ZnO lattice. Each of these interstitial F atoms absorbs a free electron due to its high electro-negativity which in turn results in an increase in the electrical resistivity. These interstitials also cause a large number of dislocations in the crystal lattice. From these results, it is believed that the solubility limit of F into the ZnO lattice is approximately 10 at.% for the preparation conditions adopted in the present work. The crystallite size (D) of the samples is calculated using the Scherrer’s formula [28],
D¼
0:9k b cos h
ð1Þ
where k is the wavelength of the X-ray used, b is the broadening of the diffraction peak at half of its maximum intensity (i.e. FWHM) and h is the Bragg’s angle. The obtained crystallite size varies from 30 to 62 nm (Table 1). The crystallite size increases with increase in the F doping level up to 10 at.%. Beyond 10 at.% of F, the crystallite size decreases due to the creation of defects and imperfections in the grains. The lattice constants ‘a’ and ‘c’ are calculated using the formula [29,30]
1 2
d
2
¼
2
2
4 ðh þ hk þ k Þ l þ 2 3 a2 c
ð2Þ
The calculated lattice constants ‘a’ and ‘c’ are very close to that of the standard values (JCPDS:36-1451) up to 10 at.% of F doping. These results show that the lattice constants are not affected much by the substitutional incorporation of F into the O sites. This is understandable as the ionic radius of F (1.33 Å) is approximately equal to that of O2 (1.32 Å). The volume of the unit cell (v) is estimated using the relation [31],
v¼
pffiffiffi 3 2 a c 2
ð3Þ
and the volume of a crystallite (V) is calculated using the relation,
V ¼ D3
ð4Þ
From the volume of the unit cell and the volume of a crystallite, we can estimate the number of unit cells per crystallite (Nu) using the equation,
Nu ¼ V=v
ð5Þ
The estimated values (Table 1) clearly showed that the number of unit cells per crystallite (Nu) increases gradually up to the F doping level of 10 at.% and beyond this doping level Nu decreases. This result indicates that the proper substitutional incorporation of Fions into the regular lattice sites of O2facilitate well, the grouping of larger number of unit cells to form a crystallite.
Table 1 Structural parameters of ZnO:F nanopowders.
a
F doping level (at.%)
Lattice constantsa (Å) a
c
0 5 10 15 20
3.247 3.249 3.247 3.243 3.244
5.205 5.206 5.204 5.190 5.193
Crystallite size (D) in nm
Volume of the unit cell (v) in Å3
Volume of a crystallite (V) 105 in nm3
Number of unit cells per crystallite (Nu) 106
Bond length (L) in Å
30 60 62 38 35
47.52 47.59 47.51 47.27 47.33
0.27 2.13 2.38 0.55 0.42
0.568 4.478 5.009 1.162 0.887
1.976 1.976 1.972 1.973 1.973
Standard values: a = 3.248 Å and c = 5.206 Å (JCPDS card no. 36-1451).
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The Zn–O bond length ‘L’ is calculated using the following relation [32].
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 ! u a2 1 L¼t þ u c2 2 3
ð6Þ
where u parameter is given by Ozqur et al. [33]
u¼
a2 þ 0:25 3c2
ð7Þ
The bond length values are given in Table 1. The volume of the unit cell and the Zn–O bond lengths do not change remarkably with the incorporation of F which is an expected result and a similar result is reported by Ilican et al. [32]. 3.2. Photoluminescence (PL) studies Photoluminescence analysis is a powerful tool to understand the nature of the defects of synthesized nanopowders. The photoluminescence spectra of undoped and fluorine doped ZnO nanopowders obtained in the range of 350–525 nm at room temperature under the excitation wavelength of 325 nm are shown in Fig. 2. The PL spectra show dominant peaks at 390, 425 and 468 nm and some weak peaks related to crystal defects in the region of 430–500 nm. It is obvious that there is an overall enhancement in the PL intensity with increase in the F doping level indicating the increase in the number of luminescence centers. The peak observed at 390 nm is associated with the near band edge (NBE) emission which is due to the transition of electrons from conduction band to valence band [34]. The position of NBE peak is blue shifted as F doping level increases up to 10 at.%. As this blue shift is associated with the increase in the carrier concentration, this result confirms the proper substitution of F ions on the O2 sites. It is interesting to observe that there is a gradual red shift in the NBE peak as the doping level is increased beyond 10 at.%. This result indicates a decrease in carrier concentration and it is a strong evidence for the interstitial incorporation of F in the ZnO lattice beyond the doping level of 10 at.%. The peaks positioned at wavelengths higher than 390 nm originate from different defects like Zni, VO and antisites in the crystal structure of ZnO [35]. The strong violet peak at 425 nm is attributed to the recombination of electrons in the zinc interstitials and holes in the valance band [36]. This violet peak at 425 nm is found to be strong compared to the NBE peak at 390 nm. This may be due to the surface defects, resulting from large surface area to volume ratio as well as by other imperfections at the boundaries [37]. The peak at 468 nm is associated with the blue emission that originates from the transition of electrons from the donor level of singly ionized oxygen vacancies to the valence band [38]. The presence of excess surface defects leads to the stronger blue emission (468 nm)
Fig. 2. Photoluminescence spectra of ZnO:F nanopowders.
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corresponding to the formation of hydroxyl radicals [39]. These hydroxyl radicals are also responsible for the inhibition of bacterial growth which will be discussed in the next section.
3.3. Antibacterial studies The antibacterial activity of ZnO:F nanopowders having different doping levels of F (0, 5, 10, 15 and 20 at.%) was estimated using zone of inhibition method. In this work, bacterial strains of Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus) were employed during the antibacterial test. From the Fig. 3, it is observed that doped ZnO samples remarkably prevented the growth of bacteria and formed well defined zones around the samples. Moreover, the
Fig. 3. Variation in zone of inhibition exhibited by ZnO:F nanopowders against (a) E. coli, (b) P. aeruginosa and (c) S. aureus as a function of F doping level for different volume of stock solution.
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Fig. 3 (continued)
zone of inhibition for each of the individual micro-organisms increases with the increase in the volume of the stock solution taken in the well. The antibacterial activity of the ZnO nanoparticles may be related to several mechanisms including the generation of reactive oxygen species (ROS) on the surface of the particles [40], release of Zn2+ ions from the ZnO samples and the penetration of these nanoparticles which contributes to the mechanical destruction of cell membrane [41]. The generation of ROS like OH, O 2 and H2O2 can be explained as follows: When the ZnO nanopowders are irradiated with light having higher photon energy or energy equal to the band gap, they cause transfer of electrons from the valence band to the conduction band of the product material. As a result, holes are generated in the valence band which can react with hydroxyl groups and absorbed water to create hydroxyl radicals (OH). The electrons in the conduction band can be trapped by the presence of O2 to produce superoxide radical anions ( O 2 ) which in turn can react with hydrogen ions to form HO2 radicals. The H2O2 can be generated by the combination of hydrogen ions and electrons [42].
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Fig. 3 (continued)
The hydroxyl radicals and superoxide anions are negatively charged and hence they cannot penetrate into the cell membrane, but they can cause fatal damage to the outer surface of the bacteria like proteins, DNA and lipids, whereas, the H2O2 can penetrate directly into the cell wall and kill the bacteria [43]. In the present work, the antibacterial activity against E. coli, P. aeruginosa, S. aureus micro-organisms increases gradually as the F doping level increases up to 10 at.% (Fig. 3). The increase in the zone of inhibition is caused by the substitution of F ions into the O2 sites, because each of this substitution creates one free electron in the lattice which in turn enhances the production of hydroxyl radicals as evidenced from the peak at 468 nm in the PL spectra. This trend is valid up to 10 at.% of F doping level only. Beyond 10 at.% of F doping, the scenario is different . At higher doping levels, the possibility of the incorporation of excess F ions into the O2 sites is less because they tend to occupy the interstitial positions of ZnO matrix instead of occupying the regular oxygen sites, and hence it does
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Fig. 4. Variation in zone of inhibition exhibited by ZnO:F nanopowder having optimal doping level (10 at.%) against E. coli, P. aeruginosa and S. aureus micro-organisms as a function of volume of stock solution.
not donate free electrons to the system. This could be the reason for the decrease in the zone of inhibition with the increase in F doping level. From the Fig 3, it is seen that the F doped ZnO nanopowder has stronger antibacterial activity on Gram positive than on Gram negative bacteria which can be explained as follows: Gram negative bacteria like E. coli and P. aeruginosa have a thin cell wall made of peptidoglycans and lipopolysaccharides. On the other hand, Gram positive bacteria like S. aureus have a thick cell wall consisting of a large number of mucopeptides, murein and lipoteichoic acids. Moreover, the antioxidant enzyme of S. aureus yields a stronger oxidant resistance [36]. Despite these two reasons, a greater inhibition response is observed for S. aureus when compared with E. coli and P. aeruginosa. This result can be explained as follows: as S. aureus membrane has a smaller negative charge than E. coli and P. aeruginosa membrane, it would allow the penetration of a greater number of negatively charged free radicals such as superoxide anions and hydroxyl radicals, into the cell membrane, causing severe damage and kill the cell of S. aureus at lower volume of stock solutions than those required to damage E. coli [44]. Thus, the F (10 at.%) doped ZnO nanopowder exhibit more antibacterial efficacy for S. aureus bacteria than E. coli and P. aeruginosa organisms (Fig. 4). 3.4. FTIR studies The Fig. 5 shows the FTIR spectra of pristine and doped ZnO nanopowders recorded in the range of 400–4000 cm1. The KBr pellet technique has been used to record the spectra. The peaks appearing in the range of 450–510 cm1 can be assigned to the stretching vibrations of Zn–O [45]. In FTIR spectra, generally the bands in the range of 1020–1042 cm1 are assigned to the O–O bond. Therefore, in the present work, the band observed at 1028 cm1 is assigned to O–O bond. It is important to mention here that as the F doping level increases, the position of this peak gradually shifts towards the lower wave number region up to 920 cm1 which confirms the increase in the incorporation of F in the ZnO lattice [46]. The band at 1800–1900 cm1 corresponds to the stretching vibration of C–O group [47]. The band at 2344 cm1–2359 cm1 is related to carbon dioxide [48]. Absence of any peaks related to symmetric and asymmetric C–H bonds usually located at around 2848 and 2922 cm1 confirms that the surfactant molecules are not strongly adsorbed on ZnO crystal surface [49]. The presence of wide band in the region of 3250–3550 cm1 is attributed to the stretching mode of hydroxyl group indicating the absorption of H2O on the surface of the sample [50]. 3.5. Surface morphological studies The Fig. 6 shows SEM images of the pristine and F doped ZnO nanopowders. The pristine ZnO nanopowder consists of very small spherical particles and it is interesting to see that with fluorine doping,
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Fig. 5. FTIR spectra of undoped and F doped ZnO nanopowders.
Fig. 6. SEM images of ZnO:F nanopowders.
the shape of the grains drastically changes to well defined hexagonal pellets showing a remarkably polished surface. When the doping level is increased to 10 at.%, the shape of the grains again changes, but this time, to linearly shaped entities which can be called nanowires or nanorods, their diameters measuring in 30–50 nm. When the doping level is increased beyond 10 at.%, the powder is found to comprise grains of mixed shapes including spheres, hexagons and wires. From the antibacterial studies discussed earlier and the observed SEM images, we can say that the wire shaped nanoparticles possess highest antibacterial efficacy over the other nanostructures.
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4. Conclusion A simple soft chemical route was employed for the synthesis of fluorine doped ZnO nanopowders. The structural studies through X-ray diffraction technique clearly establish the purity of the ZnO crystal surface as no phase other than the hexagonal wurtzite ZnO is observed. The blue shift in the NBE peak of the PL spectra confirms the proper substitutional incorporation of F ions into the O2 sites of the ZnO lattice. The presence of surface defects which act as the sources for the formation of hydroxyl radicals, responsible for the inhibition of antibacterial growth, is also confirmed by the PL results. The SEM images show that this ZnO nanopowders exhibit various nanostructures. The antibacterial studies reveal that the F doping level of 10 at.% is the most suited for antibacterial activities of ZnO:F nanopowders and this antibacterial activity is found to be significantly higher for S. aureus compared with E. coli and P. aeruginosa micro-organisms. Acknowledgement Financial support from the University Grants Commission of India through the Major Research Project (F. No. 40-28/2011(SR)) is gratefully acknowledged. References [1] U. Desselberger, J. Infect. 40 (2000) 3–15. [2] J.S. Kim, E. Kuk, K.N. Yu, J.H. Kim, S.J. Park, H.J. Lee, S.H. Kim, Y.K. Park, Y.H. Park, C.Y. Hwang, Y.K. Kim, Y.S. Lee, Nanomedicine 3 (2005) 95–101. [3] K.R. Raghupathi, R.T. Koodali, A.C. Manna, Langmuir 27 (2011) 4020–4028. [4] J. Liu, Z. Guo, F.L. Meng, Cryst. Growth Des. 9 (2009) 1716–1722. [5] K. Ravichandran, R. Mohan, N Jabena Begum, K. Swaminathan, C. Ravidhas, J. Phys. Chem. Solids 74 (2013) 1794–1801. [6] B. Sundaresan, A. Vasumathi, K. Ravichandran, P. Ravikumar, B. Sakthivel, Surf. Eng. 28 (2012) 323–328. [7] X.S. Tang, E. Shi, G. Choo, L. Li, J. Ding, J.M. Xue, Chem. Mater. Am. Chem. Soc. 22 (2010) 3383–3388. [8] H. Wang, C. Xie, J. Phys. Chem. Solids 69 (2008) 2440–2444. [9] M. Ladanov, P. Algarin-Amaris, P. Villalba, Y. Emirov, G. Matthews, S. Thomas, K. Ram, A. Kumar, J. Wang, J. Phys. Chem. Solids 74 (2013) 1578–1588. [10] S.H. Chen, J.X. Ji, Q. Lian, Y.L. Wen, H.B. Shen, N.Q. Jia, Nano Biomed. Eng. 2 (2010) 15–23. [11] K.W. Chae, Q. Zhang, J.S. Kim, Y.H. Jeong, G. Cao, J. Nanotechnol. 1 (2010) 128–134. [12] X.-Y. Ma, W.-D. Zhang, Polym. Degrad. Stab. 94 (2009) 1103–1109. [13] S. Chakraborty, A.K. Kole, P. Kumbhakar, Mater. Lett. 67 (2012) 362–364. [14] A. Khorsand Zak, W.H.abd. Majid, H.Z. Wang, Ramin Yousefi, A. Moradi Golsheikh, Z.F. Ren, Ultrason. Sonochem. 20 (2013) 395–400. [15] S.D. Gopal Ram, G. Ravi, M.R. Manikandan, T. Mahalingam, M. Anbu Kulandainathan, Superlatt. Microstruct. 50 (2011) 296–302. [16] D. Sharma, S. Sharma, B.S. Kaith, J. Rajput, M. Kahn, Appl. Surf. Sci. 257 (2011) 9661–9672. [17] A. Azam, F. Ahmed, N. Arshi, M. Chaman, A.H. Naqvi, J. Alloys Compd. 496 (2010) 399–402. [18] S. Lee, S. Jeong, D. Kim, S. Hwang, M. Jeon, J. Moon, Superlatt. Microstruct. 43 (2008) 330–339. [19] K. Saravanakumar, K. Ravichandran, J Mater Sci: Mater Electron. 23 (2012) 1462–1469. [20] K. Saravanakumar, B. Sakthivel, K. Ravichandran, Mater. Lett. 65 (2011) 2278–2280. [21] K. Saravanakumar, K. Ravichandran, R. Chandramohan, S. Gobalakrishnan, Murthy Chavali, Superlatt. Microstruct. 52 (2012) 528–540. [22] N. Jabena Begum, K. Ravichandran, J. Phys. Chem. Solids 74 (2013) 841–848. [23] S. Snega, K. Ravichandran, N. Jabena Begum, K. Thirumurugan, J. Mater. Sci.: Mater. Electron. 24 (2013) 135–141. [24] K. Rekha, M. Nirmala, M.G. Nair, A. Anukaliani, Phys. B 405 (2010) 3180–3185. [25] Yu-Zen Tsai, Na-Fu Wang, Chun-Lung Tsai, Thin Solid Films 518 (2010) 4955–4959. [26] P. Amornpitoksuk, S. Suwanboon, S. Sangkanu, A. Sukhoom, N. Muensit, J. Baltrusaitis, Powder Technol. 219 (2012) 158– 164. [27] O.G. Morales-Saavedra, L. Castaneda, Opt. Commun. 269 (2007) 370–377. [28] V. Senthamilselvi, K. Saravanakumar, N. Jabena Begum, R. Anandhi, A.T. Ravichandran, B. Sakthivel, K. Ravichandran, J Mater Sci: Mater Electron. 23 (2012) 302–308. [29] R. Anandhi, R. Mohan, K. Swaminathan, K. Ravichandran, Superlatt. Microstruct. 51 (2012) 680–689. [30] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, 1978. [31] A. Goswami, Thin Film Fund, New Age International (P) Limited, Publications, New Delhi, 2005. [32] S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanolu, Appl. Surf. Sci. 255 (2008) 2353–2359. [33] U. Ozqur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phy. 98 (2005). 041301-103. [34] Z. Pan, X. Tian, S. Wu, X. Yu, Z. Li, J. Deng, C. Xiao, G. Hu, Z. Wei, Appl. Surf. Sci. 265 (2013) 870–877. [35] L.V. Trandafilovic, D.K. Bozanic, S.D. Brankovic, A.S. Luyt, V. Djokovic, Carbohydr. Polym. 88 (2012) 263–269. [36] S. Zhao, Y. Zhou, K. Zhao, Z. Liu, P. Han, S. Wang, W. Xiang, Z. Chen, H. Lu, B. Cheng, G. Yang, Phys. B 373 (2006) 154–156.
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Enhancement in the antibacterial efficiency of ZnO nanopowders by tuning the shape of the nanograins through fluorine doping K. Ravichandran a,⇑, S. Snega a, N. Jabena Begum a, K. Swaminathan b, B. Sakthivel a, L. Rene Christena c, G. Chandramohan d, Shizuyasu Ochiai e a
P.G. & Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613 503, Tamil Nadu, India Department of Physics, RKM Vivekananda College (Autonomous), Chennai 600 004, Tamil Nadu, India c School of Chemical and Biotechnology, Shanmuga Arts, Science, Technology and Research Academy, Thanjavur 613 401, Tamil Nadu, India d P.G & Research Department of Chemistry, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur 613 503, Tamil Nadu, India e Department of Electrical Engineering, Aichi Institute of Technology, Toyota City 470-0392, Japan b
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 30 January 2014 Accepted 31 January 2014 Available online 8 February 2014 Keywords: Oxides Nanostructures Chemical synthesis X-ray diffraction Luminescence
a b s t r a c t Fluorine doped ZnO nanopowders were synthesized from starting solutions having different doping levels of F (0, 5, . . . , 20 at.%) using a simple soft chemical route and the effects of the doping level on the structural, optical, surface morphological and antibacterial properties were investigated. The XRD studies reveal that all the products have preferential orientation along the (1 0 1) plane. The PL studies show that all the samples exhibit strong visible emission with a peak at 425 nm. The enhancement in the visible emission indicates an increasing number of surface defects caused by the doping of F. The obtained FTIR spectra confirm the incorporation of F into ZnO lattice. From the SEM studies, it is observed that the ZnO nanowires formed at 10 at.% of F doping level exhibit excellent antibacterial activities. Antibacterial activity of F doped ZnO nanopowders against Staphylococcus aureus was found to be significantly higher than that against the Escherichia coli and Pseudomonas aeruginosa micro-organisms. All the physical properties were corroborated well with the findings related to antibacterial activity. Finally, we conclude that, the analysis of all
⇑ Corresponding author. Tel.: +91 4362 278602, +91 9443524180 (mobile); fax: +91 4374 239328. E-mail address: [email protected] (K. Ravichandran). http://dx.doi.org/10.1016/j.spmi.2014.01.020 0749-6036/Ó 2014 Elsevier Ltd. All rights reserved.
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the results shows that F doping level of 10 at.% is optimal in all respects and is suitable for antibacterial applications. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction In the recent decade, one of the most significant challenges faced by the world is the recurrence of infectious diseases and the bacterial contamination in all kinds of materials [1]. Therefore, several antibacterial agents are widely used in day-to-day life for the prevention of public health issues caused by the ubiquity of micro-organisms and their ability to establish themselves [2]. When antibacterial agents are used in the new packaging materials for health care and food applications, the most crucial parameters to be taken care of are low toxicity to human beings and high efficiency in controlling bacteria. The increasing use of inorganic antibacterial agents is of great interest because of their creditability towards safety and stability when compared with organic antibacterial agents. New nanostructured materials with antibacterial properties are the need of the day for preventing microbial growth because, the size, structure and surface properties of nanomaterials can improve the antimicrobial efficacy [3]. Several inorganic metal oxides such as TiO2, MgO, ZnO and CuO have gained increasing attention in recent years. Of these oxides, ZnO is a versatile and important semiconducting material with a band gap of 3.37 eV that could exhibit some special features like excellent chemical and thermal stability, high transparency and bio-compatibility. ZnO has attracted much attention from the industry, being suitable for many potential applications including opto-electronic devices [4], sensors [5], dye-sensitized solar cells [6], biodevices [7] and photocatalysts [8]. Moreover, various morphologies of ZnO like nanowires [9], nanorods [10], nanotubes [11], nanowhiskers [12] and nanoflowers [13] have been found to be useful in biomedical applications. The synthesis route plays an important role in controlling the particle size and the morphology. In order to control the particle size, several techniques have been used for the synthesis of ZnO nanopowders/nanoparticles such as sonochemical synthesis [14], hydrothermal method [15], combustion synthesis [16], sol–gel synthesis [17], polyol method [18] and simple soft chemical route [19,20]. Of these methods, soft chemical route offers several advantages. It is a very fascinating, facile and inexpensive method which is suitable to grow nanostructured materials in large scale [21]. Doping can play an essential role in controlling the size and shape of the ZnO particles and enhancing the physical and antibacterial properties. The physical properties of ZnO can be altered by doping with selective elements like Al, Mg, Mn, F and Ag [22–26]. In the present work, fluorine doped ZnO nanopowders with different F doping levels (0, 5, . . ., 20 at.%) have been synthesized using the soft chemical route and some of their physical properties along with the antibacterial activity have been investigated and reported. To the best of our knowledge, this is the first report on the antibacterial properties of F doped ZnO nanopowder. 2. Materials and methods 2.1. Synthesis process The fluorine doped ZnO nanopowders (ZnO:F) were synthesized with different F doping levels (0, 5, . . ., 20 at.%) using a soft chemical route. The starting solutions were prepared by dissolving zinc acetate dihydrate (Zn(CH3COO)22H2O) in de-ionized water. Ammonium fluoride (NH4F) was used as the dopant precursor and the required amount of NaOH solution was added drop wise in the precursor solution till the pH value had reached 8. The obtained mixture was then magnetically stirred for 2 h at a temperature of 85 °C. After the completion of the stirring process, the precipitate formed was separated carefully by filtration and washed several times with a mixture of ethanol and water
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kept in the ratio of 1:3. The resultant product was then calcinated at 400 °C for 2 h to get the ZnO nanopowder in its final form. 2.2. Characterization of ZnO:F nanopowder The crystalline structure of the powders was analyzed using X-ray powder diffraction method (PANalytical-PW 340/60 X’pert PRO) with Cu Ka (k = 1.5406 Å) radiation. The luminescence spectra of the powders were observed using spectro-fluorometer (Jobin Yvon_FLUROLOG-FL3-11) with Xenon Lamp (450 W) as the excitation source of wavelength of 325 nm at room temperature. The Fourier transform infrared (FTIR) spectra were recorded using Perkin Elmer RX-I FTIR spectrophotometer. The surface morphological studies were made using scanning electron microscope (SEM–HITACHI S-3000 H). 2.3. Evaluation of antibacterial performance The antibacterial property of ZnO:F nanopowders were analyzed by their zone of inhibitions for Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus). An overnight culture of each organism was adjusted to an OD of 0.1 and swabbed onto Mueller Hilton agar plates. Using a cork borer, holes were punched on the agar, followed by addition of the stock solutions containing synthesized nanopowders in increasing volumes from 50 to 200 lL. These plates were incubated at 37 °C for 24 h and the zone of inhibitions was measured in diameter. 3. Results and discussion 3.1. Structural studies The XRD patterns of the pristine and fluorine doped ZnO nanopowders are shown in Fig. 1. From the Fig. 1, it is observed that, all samples have polycrystalline nature with a hexagonal wurtzite structure of ZnO. The strongest diffraction peak shows that the preferential orientation is along (1 0 1) plane. No metallic zinc or other oxide phases are detected which confirms the purity of the nanopowders. The resolution limit of X-ray diffraction technique is not so sufficient to detect the presence of fluorine compounds, if any, in the final product even when high concentration of fluorine (20 at.%) is used during the synthesis process [27].
Fig. 1. XRD patterns of undoped and Fluorine doped ZnO nanopowders.
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The intensity of (1 0 1) peak increases with the increase in the fluorine content up to 10 at.% and then it decreases with further doping which is an indication of the degradation in the crystalline quality of the sample beyond 10 at.% of F doping. This may be due to the interstitial incorporation of excess fluorine atoms into the ZnO lattice. Each of these interstitial F atoms absorbs a free electron due to its high electro-negativity which in turn results in an increase in the electrical resistivity. These interstitials also cause a large number of dislocations in the crystal lattice. From these results, it is believed that the solubility limit of F into the ZnO lattice is approximately 10 at.% for the preparation conditions adopted in the present work. The crystallite size (D) of the samples is calculated using the Scherrer’s formula [28],
D¼
0:9k b cos h
ð1Þ
where k is the wavelength of the X-ray used, b is the broadening of the diffraction peak at half of its maximum intensity (i.e. FWHM) and h is the Bragg’s angle. The obtained crystallite size varies from 30 to 62 nm (Table 1). The crystallite size increases with increase in the F doping level up to 10 at.%. Beyond 10 at.% of F, the crystallite size decreases due to the creation of defects and imperfections in the grains. The lattice constants ‘a’ and ‘c’ are calculated using the formula [29,30]
1 2
d
2
¼
2
2
4 ðh þ hk þ k Þ l þ 2 3 a2 c
ð2Þ
The calculated lattice constants ‘a’ and ‘c’ are very close to that of the standard values (JCPDS:36-1451) up to 10 at.% of F doping. These results show that the lattice constants are not affected much by the substitutional incorporation of F into the O sites. This is understandable as the ionic radius of F (1.33 Å) is approximately equal to that of O2 (1.32 Å). The volume of the unit cell (v) is estimated using the relation [31],
v¼
pffiffiffi 3 2 a c 2
ð3Þ
and the volume of a crystallite (V) is calculated using the relation,
V ¼ D3
ð4Þ
From the volume of the unit cell and the volume of a crystallite, we can estimate the number of unit cells per crystallite (Nu) using the equation,
Nu ¼ V=v
ð5Þ
The estimated values (Table 1) clearly showed that the number of unit cells per crystallite (Nu) increases gradually up to the F doping level of 10 at.% and beyond this doping level Nu decreases. This result indicates that the proper substitutional incorporation of Fions into the regular lattice sites of O2facilitate well, the grouping of larger number of unit cells to form a crystallite.
Table 1 Structural parameters of ZnO:F nanopowders.
a
F doping level (at.%)
Lattice constantsa (Å) a
c
0 5 10 15 20
3.247 3.249 3.247 3.243 3.244
5.205 5.206 5.204 5.190 5.193
Crystallite size (D) in nm
Volume of the unit cell (v) in Å3
Volume of a crystallite (V) 105 in nm3
Number of unit cells per crystallite (Nu) 106
Bond length (L) in Å
30 60 62 38 35
47.52 47.59 47.51 47.27 47.33
0.27 2.13 2.38 0.55 0.42
0.568 4.478 5.009 1.162 0.887
1.976 1.976 1.972 1.973 1.973
Standard values: a = 3.248 Å and c = 5.206 Å (JCPDS card no. 36-1451).
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The Zn–O bond length ‘L’ is calculated using the following relation [32].
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 ! u a2 1 L¼t þ u c2 2 3
ð6Þ
where u parameter is given by Ozqur et al. [33]
u¼
a2 þ 0:25 3c2
ð7Þ
The bond length values are given in Table 1. The volume of the unit cell and the Zn–O bond lengths do not change remarkably with the incorporation of F which is an expected result and a similar result is reported by Ilican et al. [32]. 3.2. Photoluminescence (PL) studies Photoluminescence analysis is a powerful tool to understand the nature of the defects of synthesized nanopowders. The photoluminescence spectra of undoped and fluorine doped ZnO nanopowders obtained in the range of 350–525 nm at room temperature under the excitation wavelength of 325 nm are shown in Fig. 2. The PL spectra show dominant peaks at 390, 425 and 468 nm and some weak peaks related to crystal defects in the region of 430–500 nm. It is obvious that there is an overall enhancement in the PL intensity with increase in the F doping level indicating the increase in the number of luminescence centers. The peak observed at 390 nm is associated with the near band edge (NBE) emission which is due to the transition of electrons from conduction band to valence band [34]. The position of NBE peak is blue shifted as F doping level increases up to 10 at.%. As this blue shift is associated with the increase in the carrier concentration, this result confirms the proper substitution of F ions on the O2 sites. It is interesting to observe that there is a gradual red shift in the NBE peak as the doping level is increased beyond 10 at.%. This result indicates a decrease in carrier concentration and it is a strong evidence for the interstitial incorporation of F in the ZnO lattice beyond the doping level of 10 at.%. The peaks positioned at wavelengths higher than 390 nm originate from different defects like Zni, VO and antisites in the crystal structure of ZnO [35]. The strong violet peak at 425 nm is attributed to the recombination of electrons in the zinc interstitials and holes in the valance band [36]. This violet peak at 425 nm is found to be strong compared to the NBE peak at 390 nm. This may be due to the surface defects, resulting from large surface area to volume ratio as well as by other imperfections at the boundaries [37]. The peak at 468 nm is associated with the blue emission that originates from the transition of electrons from the donor level of singly ionized oxygen vacancies to the valence band [38]. The presence of excess surface defects leads to the stronger blue emission (468 nm)
Fig. 2. Photoluminescence spectra of ZnO:F nanopowders.
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corresponding to the formation of hydroxyl radicals [39]. These hydroxyl radicals are also responsible for the inhibition of bacterial growth which will be discussed in the next section.
3.3. Antibacterial studies The antibacterial activity of ZnO:F nanopowders having different doping levels of F (0, 5, 10, 15 and 20 at.%) was estimated using zone of inhibition method. In this work, bacterial strains of Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus) were employed during the antibacterial test. From the Fig. 3, it is observed that doped ZnO samples remarkably prevented the growth of bacteria and formed well defined zones around the samples. Moreover, the
Fig. 3. Variation in zone of inhibition exhibited by ZnO:F nanopowders against (a) E. coli, (b) P. aeruginosa and (c) S. aureus as a function of F doping level for different volume of stock solution.
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Fig. 3 (continued)
zone of inhibition for each of the individual micro-organisms increases with the increase in the volume of the stock solution taken in the well. The antibacterial activity of the ZnO nanoparticles may be related to several mechanisms including the generation of reactive oxygen species (ROS) on the surface of the particles [40], release of Zn2+ ions from the ZnO samples and the penetration of these nanoparticles which contributes to the mechanical destruction of cell membrane [41]. The generation of ROS like OH, O 2 and H2O2 can be explained as follows: When the ZnO nanopowders are irradiated with light having higher photon energy or energy equal to the band gap, they cause transfer of electrons from the valence band to the conduction band of the product material. As a result, holes are generated in the valence band which can react with hydroxyl groups and absorbed water to create hydroxyl radicals (OH). The electrons in the conduction band can be trapped by the presence of O2 to produce superoxide radical anions ( O 2 ) which in turn can react with hydrogen ions to form HO2 radicals. The H2O2 can be generated by the combination of hydrogen ions and electrons [42].
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Fig. 3 (continued)
The hydroxyl radicals and superoxide anions are negatively charged and hence they cannot penetrate into the cell membrane, but they can cause fatal damage to the outer surface of the bacteria like proteins, DNA and lipids, whereas, the H2O2 can penetrate directly into the cell wall and kill the bacteria [43]. In the present work, the antibacterial activity against E. coli, P. aeruginosa, S. aureus micro-organisms increases gradually as the F doping level increases up to 10 at.% (Fig. 3). The increase in the zone of inhibition is caused by the substitution of F ions into the O2 sites, because each of this substitution creates one free electron in the lattice which in turn enhances the production of hydroxyl radicals as evidenced from the peak at 468 nm in the PL spectra. This trend is valid up to 10 at.% of F doping level only. Beyond 10 at.% of F doping, the scenario is different . At higher doping levels, the possibility of the incorporation of excess F ions into the O2 sites is less because they tend to occupy the interstitial positions of ZnO matrix instead of occupying the regular oxygen sites, and hence it does
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Fig. 4. Variation in zone of inhibition exhibited by ZnO:F nanopowder having optimal doping level (10 at.%) against E. coli, P. aeruginosa and S. aureus micro-organisms as a function of volume of stock solution.
not donate free electrons to the system. This could be the reason for the decrease in the zone of inhibition with the increase in F doping level. From the Fig 3, it is seen that the F doped ZnO nanopowder has stronger antibacterial activity on Gram positive than on Gram negative bacteria which can be explained as follows: Gram negative bacteria like E. coli and P. aeruginosa have a thin cell wall made of peptidoglycans and lipopolysaccharides. On the other hand, Gram positive bacteria like S. aureus have a thick cell wall consisting of a large number of mucopeptides, murein and lipoteichoic acids. Moreover, the antioxidant enzyme of S. aureus yields a stronger oxidant resistance [36]. Despite these two reasons, a greater inhibition response is observed for S. aureus when compared with E. coli and P. aeruginosa. This result can be explained as follows: as S. aureus membrane has a smaller negative charge than E. coli and P. aeruginosa membrane, it would allow the penetration of a greater number of negatively charged free radicals such as superoxide anions and hydroxyl radicals, into the cell membrane, causing severe damage and kill the cell of S. aureus at lower volume of stock solutions than those required to damage E. coli [44]. Thus, the F (10 at.%) doped ZnO nanopowder exhibit more antibacterial efficacy for S. aureus bacteria than E. coli and P. aeruginosa organisms (Fig. 4). 3.4. FTIR studies The Fig. 5 shows the FTIR spectra of pristine and doped ZnO nanopowders recorded in the range of 400–4000 cm1. The KBr pellet technique has been used to record the spectra. The peaks appearing in the range of 450–510 cm1 can be assigned to the stretching vibrations of Zn–O [45]. In FTIR spectra, generally the bands in the range of 1020–1042 cm1 are assigned to the O–O bond. Therefore, in the present work, the band observed at 1028 cm1 is assigned to O–O bond. It is important to mention here that as the F doping level increases, the position of this peak gradually shifts towards the lower wave number region up to 920 cm1 which confirms the increase in the incorporation of F in the ZnO lattice [46]. The band at 1800–1900 cm1 corresponds to the stretching vibration of C–O group [47]. The band at 2344 cm1–2359 cm1 is related to carbon dioxide [48]. Absence of any peaks related to symmetric and asymmetric C–H bonds usually located at around 2848 and 2922 cm1 confirms that the surfactant molecules are not strongly adsorbed on ZnO crystal surface [49]. The presence of wide band in the region of 3250–3550 cm1 is attributed to the stretching mode of hydroxyl group indicating the absorption of H2O on the surface of the sample [50]. 3.5. Surface morphological studies The Fig. 6 shows SEM images of the pristine and F doped ZnO nanopowders. The pristine ZnO nanopowder consists of very small spherical particles and it is interesting to see that with fluorine doping,
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Fig. 5. FTIR spectra of undoped and F doped ZnO nanopowders.
Fig. 6. SEM images of ZnO:F nanopowders.
the shape of the grains drastically changes to well defined hexagonal pellets showing a remarkably polished surface. When the doping level is increased to 10 at.%, the shape of the grains again changes, but this time, to linearly shaped entities which can be called nanowires or nanorods, their diameters measuring in 30–50 nm. When the doping level is increased beyond 10 at.%, the powder is found to comprise grains of mixed shapes including spheres, hexagons and wires. From the antibacterial studies discussed earlier and the observed SEM images, we can say that the wire shaped nanoparticles possess highest antibacterial efficacy over the other nanostructures.
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4. Conclusion A simple soft chemical route was employed for the synthesis of fluorine doped ZnO nanopowders. The structural studies through X-ray diffraction technique clearly establish the purity of the ZnO crystal surface as no phase other than the hexagonal wurtzite ZnO is observed. The blue shift in the NBE peak of the PL spectra confirms the proper substitutional incorporation of F ions into the O2 sites of the ZnO lattice. The presence of surface defects which act as the sources for the formation of hydroxyl radicals, responsible for the inhibition of antibacterial growth, is also confirmed by the PL results. The SEM images show that this ZnO nanopowders exhibit various nanostructures. The antibacterial studies reveal that the F doping level of 10 at.% is the most suited for antibacterial activities of ZnO:F nanopowders and this antibacterial activity is found to be significantly higher for S. aureus compared with E. coli and P. aeruginosa micro-organisms. Acknowledgement Financial support from the University Grants Commission of India through the Major Research Project (F. No. 40-28/2011(SR)) is gratefully acknowledged. References [1] U. Desselberger, J. Infect. 40 (2000) 3–15. [2] J.S. Kim, E. Kuk, K.N. Yu, J.H. Kim, S.J. Park, H.J. Lee, S.H. Kim, Y.K. Park, Y.H. Park, C.Y. Hwang, Y.K. Kim, Y.S. Lee, Nanomedicine 3 (2005) 95–101. [3] K.R. Raghupathi, R.T. Koodali, A.C. Manna, Langmuir 27 (2011) 4020–4028. [4] J. Liu, Z. Guo, F.L. Meng, Cryst. Growth Des. 9 (2009) 1716–1722. [5] K. Ravichandran, R. Mohan, N Jabena Begum, K. Swaminathan, C. Ravidhas, J. Phys. Chem. Solids 74 (2013) 1794–1801. [6] B. Sundaresan, A. Vasumathi, K. Ravichandran, P. Ravikumar, B. Sakthivel, Surf. Eng. 28 (2012) 323–328. [7] X.S. Tang, E. Shi, G. Choo, L. Li, J. Ding, J.M. Xue, Chem. Mater. Am. Chem. Soc. 22 (2010) 3383–3388. [8] H. Wang, C. Xie, J. Phys. Chem. Solids 69 (2008) 2440–2444. [9] M. Ladanov, P. Algarin-Amaris, P. Villalba, Y. Emirov, G. Matthews, S. Thomas, K. Ram, A. Kumar, J. Wang, J. Phys. Chem. Solids 74 (2013) 1578–1588. [10] S.H. Chen, J.X. Ji, Q. Lian, Y.L. Wen, H.B. Shen, N.Q. Jia, Nano Biomed. Eng. 2 (2010) 15–23. [11] K.W. Chae, Q. Zhang, J.S. Kim, Y.H. Jeong, G. Cao, J. Nanotechnol. 1 (2010) 128–134. [12] X.-Y. Ma, W.-D. Zhang, Polym. Degrad. Stab. 94 (2009) 1103–1109. [13] S. Chakraborty, A.K. Kole, P. Kumbhakar, Mater. Lett. 67 (2012) 362–364. [14] A. Khorsand Zak, W.H.abd. Majid, H.Z. Wang, Ramin Yousefi, A. Moradi Golsheikh, Z.F. Ren, Ultrason. Sonochem. 20 (2013) 395–400. [15] S.D. Gopal Ram, G. Ravi, M.R. Manikandan, T. Mahalingam, M. Anbu Kulandainathan, Superlatt. Microstruct. 50 (2011) 296–302. [16] D. Sharma, S. Sharma, B.S. Kaith, J. Rajput, M. Kahn, Appl. Surf. Sci. 257 (2011) 9661–9672. [17] A. Azam, F. Ahmed, N. Arshi, M. Chaman, A.H. Naqvi, J. Alloys Compd. 496 (2010) 399–402. [18] S. Lee, S. Jeong, D. Kim, S. Hwang, M. Jeon, J. Moon, Superlatt. Microstruct. 43 (2008) 330–339. [19] K. Saravanakumar, K. Ravichandran, J Mater Sci: Mater Electron. 23 (2012) 1462–1469. [20] K. Saravanakumar, B. Sakthivel, K. Ravichandran, Mater. Lett. 65 (2011) 2278–2280. [21] K. Saravanakumar, K. Ravichandran, R. Chandramohan, S. Gobalakrishnan, Murthy Chavali, Superlatt. Microstruct. 52 (2012) 528–540. [22] N. Jabena Begum, K. Ravichandran, J. Phys. Chem. Solids 74 (2013) 841–848. [23] S. Snega, K. Ravichandran, N. Jabena Begum, K. Thirumurugan, J. Mater. Sci.: Mater. Electron. 24 (2013) 135–141. [24] K. Rekha, M. Nirmala, M.G. Nair, A. Anukaliani, Phys. B 405 (2010) 3180–3185. [25] Yu-Zen Tsai, Na-Fu Wang, Chun-Lung Tsai, Thin Solid Films 518 (2010) 4955–4959. [26] P. Amornpitoksuk, S. Suwanboon, S. Sangkanu, A. Sukhoom, N. Muensit, J. Baltrusaitis, Powder Technol. 219 (2012) 158– 164. [27] O.G. Morales-Saavedra, L. Castaneda, Opt. Commun. 269 (2007) 370–377. [28] V. Senthamilselvi, K. Saravanakumar, N. Jabena Begum, R. Anandhi, A.T. Ravichandran, B. Sakthivel, K. Ravichandran, J Mater Sci: Mater Electron. 23 (2012) 302–308. [29] R. Anandhi, R. Mohan, K. Swaminathan, K. Ravichandran, Superlatt. Microstruct. 51 (2012) 680–689. [30] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Company, 1978. [31] A. Goswami, Thin Film Fund, New Age International (P) Limited, Publications, New Delhi, 2005. [32] S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanolu, Appl. Surf. Sci. 255 (2008) 2353–2359. [33] U. Ozqur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoc, J. Appl. Phy. 98 (2005). 041301-103. [34] Z. Pan, X. Tian, S. Wu, X. Yu, Z. Li, J. Deng, C. Xiao, G. Hu, Z. Wei, Appl. Surf. Sci. 265 (2013) 870–877. [35] L.V. Trandafilovic, D.K. Bozanic, S.D. Brankovic, A.S. Luyt, V. Djokovic, Carbohydr. Polym. 88 (2012) 263–269. [36] S. Zhao, Y. Zhou, K. Zhao, Z. Liu, P. Han, S. Wang, W. Xiang, Z. Chen, H. Lu, B. Cheng, G. Yang, Phys. B 373 (2006) 154–156.
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