- Email: [email protected]
Synthesis and characterisation of flower shaped Zinc Oxide nanostructures and its antimicrobial activity
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and characterisation of flower shaped Zinc Oxide nanostructures and its antimicrobial activity Kesarla Mohan Kumar a, Badal Kumar Mandal a,⇑, Etcherla Appala Naidu a, Madhulika Sinha a, Koppala Siva Kumar b, Pamanji Sreedhara Reddy b a b
Trace Elements Speciation Research Laboratory, Environmental and Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, TN, India Department of Physics, Sri Venkateswara University, Tirupati 517 502, AP, India
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
" A simple low temperature method
This is a simple method to synthesise flower shaped ZnO nanostructures. This method uses no structure directing agents. Thus obtained ZnO nanostructures showed good antimicrobial activity.
selectively for flower shaped ZnO nanostructures. " Only a metal precursor and a precipitating agent were used. " No other dispersing/structure directing agents were used. " ZnO nanostructures showed good antimicrobial activity.
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 5 November 2012 Accepted 6 November 2012 Available online 28 November 2012 Keywords: Flower shaped ZnO nanostructures Nanorods Antimicrobial activity Microbes
a b s t r a c t Flower shaped Zinc Oxide nanostructures was synthesized using a simple method without using any structure directing agents. Elemental analysis, crystalline nature, shape and size were examined using Powder X-ray Diffraction (XRD), scanning electron microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDAX). XRD revealed the formation of hexagonal ZnO nanostructures. SEM and TEM analyses revealed the formation of crystalline ZnO flowers in which a bunch of ZnO nanorods assembled together to form a leaf like structure followed by flower shaped ZnO nanostructures. Thus synthesised ZnO nanostructures showed good antimicrobial activity towards gram-positive bacteria Staphylococcus aureus as well as gram-negative bacteria Escherichia coli with a MIC/MBC of 25 mg/L. Ó 2012 Elsevier B.V. All rights reserved.
Introduction In manufacturing nanomaterials-based devices, the synthesis of controlled size and large yield of nanostructures (NSs) is a challenge to materials scientists. It is mandatory to study the
⇑ Corresponding author. Tel.: +91 416 220 2339; fax: +91 416 224 3092. E-mail addresses: [email protected], [email protected] (B.K. Mandal). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.11.025
physical and chemical properties of nanomaterials to ensure the crystalline nature and purity. Versatile applications such as power generators [1], field emission devices [2], gas sensors [3] and as ultraviolet lasers [4] make ZnO nanostructures (ZnNS) as promising and smart materials which catch the attention of researchers from all corners in recent times. Moreover, the incredible antibacterial activity of nanoZnO [5] makes them compatible in many cosmetic applications e.g. sunscreens and medicated creams [6–9].
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K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
Many growth techniques to synthesise ZnNS have been reported in the literature such as thermal decomposition [10], micro emulsion method [11], CVD [12] and sol–gel methods [13]. Usually the vapour phase methods use sophisticated instruments, vacuum and high temperatures which limit the nature of substrates used compared to aqueous medium based methods which are low-cost and simple. In the present study a simple low temperature method is developed to synthesise well dispersed flower shaped ZnNS using only a metal precursor and a precipitating agent. No other dispersing/structure directing agents were used. In this paper synthesis, mechanism of formation, characterisation and antimicrobial activity of flower shaped ZnNS are presented.
Experimental After many trials, a selective procedure to synthesise flower shaped nanostructures was achieved in the present study by the following procedure. Flower shaped ZnNS were prepared from 4 mM zinc acetate dihydrate and 20 mM of NaOH after dissolving in deionised water. Initially both the aqueous solutions were cooled in ice bath and then NaOH solution was added dropwise using a peristaltic pump to zinc acetate dihydrate solution with stirring at 550 rpm using a propeller. The resulting turbid solution was heated on a temperature controlled water bath at 75 °C for 30 min and white powders was settled at the bottom. The settled white powders was separated followed by washing with deionised water thrice and dried overnight in a dust free condition under room temperature.
Determination of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) The MIC and MBC were determined by taking nine sterile test tubes separately with 0.9 mL of sterile MH broth, and inoculated with the corresponding microorganism of concentration 108 CFU/mL of bacteria. Fresh bacterial culture was made by keeping the freshly inoculated bacterial sample in 5 mL sterilized broth for 6 h of incubation (to reach its Log phase) at 37 °C. 0.1 mL of the fresh culture was taken and added to the first test tube with sterile liquid medium and serial dilutions were performed for the other seven test tubes with extra two tubes as control. To each tube 10 lL of ZnNS dispersion was added and the tubes were incubated at 37 °C overnight. A drop of cultures from each tube was added on the specified marked plates. The plates were incubated for 18–24 h at 37 °C and the growth of colonies was monitored. Evaluation of Zone of Inhibition (ZOI): well diffusion method The bacterial lawn was prepared on sterile Muller–Hinton agar (1% Agar) plates by using a sterile cotton swab. 108 CFU/mL cultures of S. aureus and 106 CFU/mL culture of E. coli were used to make a lawn culture (by streaking culture using sterile cotton swabs). Wells of approx. 8 mm diameter were made. Flower shaped ZnNS of different concentrations were added inside each well. The plates were kept for incubation at 37 °C overnight and zone diameter were measured.
Characterisation
Results and discussion
The synthesized solid materials was subjected to Powder X-ray Diffraction (XRD) analysis using Bruker D8 Advance Diffractometer (Bruker AXS, Germany) with Cu Ka radiation (k = 1.54 Å). XRD pattern of ZnNS was recorded over a 2-theta range of 10–90°. Scanning Electron Microscope coupled with Energy Dispersive X-ray Spectroscopy (SEM–EDAX) analysis was done using a Carl Zeiss SEM Instrument attached to EVO MA 15 (Oxford Instrument). The solid samples were sprinkled on the adhesive carbon tape which was supported on the metallic disc. The sample surface images were taken at different magnifications. Simultaneously, EDAX spectrum was recorded at selected areas on the solid surface to obtain the elemental analysis. The morphology and structural properties of the fabricated ZnO nanostructures were characterized by High Resolution Transmission Electron Microscope (HR-TEM) (JEOL JEM 2100 HR-TEM) operated at an accelerating voltage of 200 kV. Sampling was done by dispersing ZnNS in ultrasonic bath and a drop of the dispersion was applied on Cu grid with Ultrathin Cu on holey C film and allowed to dry in vacuum.
The powder XRD pattern of the synthesised ZnNS was shown in Fig. 1 and all the peaks in the XRD diffractogram was indexed which matched to hexagonal ZnO (JCPDS Card No. 03-65-3411). The XRD pattern clearly showed the pure phase and no other peaks due to impurities were observed. The sharp and strong diffraction peaks revealed that the synthesised ZnNS were highly crystalline and pure. The lattice constants of the synthesised nanostructures were measured to be a = 3.2490 Å and c = 5.2066 Å which was in good agreement with the values of hexagonal ZnO of the space group P63mc. The SEM micrographs at different magnifications and EDAX spectrum of the flower shaped ZnNS were shown in Fig. 2. At lower magnifications, it is very clear that the formation of well dispersed ZnO nanoflowers. A keen observation of the same material at the
(1 0 1)
600
(2 0 3)
100
(1 0 4)
200
(2 0 0)
(1 0 2)
300
(1 0 3)
(1 1 0)
400
(2 0 2)
500
(1 1 2) (2 0 1) (0 0 4)
(1 0 0) (0 0 2)
NP-induced toxicity tests on selected bacteria were conducted in liquid and solid Muller Hinton media containing different concentrations of ZnNS (25–125 mg/L). For each experiment bacterial cultures were grown in ZnNS free liquid media keeping overnight at 37 oC in an incubator. The bactericidal experiments were carried out with gram negative bacteria Escherichia coli (strain ATCC 25922) and gram positive bacteria Staphylococcus aureus (strain ATCC 25923). The solid agar was made using both Muller Hinton Broth (MHB) and Agar to make 1% Agar for easy diffusion of ZnNS. Throughout this study, the same media was used for all strains. MHB broth was also used as a medium for preparing fresh cultures of organisms.
700 Intensity (counts)
Bactericidal experiment
800
0 20
30
40
50
60
70
2 Fig. 1. XRD diffractogram of ZnO nanoflowers.
80
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K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
173
Fig. 2. SEM micrographs of ZnO nanoflowers at different magnifications (a and b) and EDX spectrum of ZnO nanoflowers (c).
little higher magnification (Fig. 2b) shows few leaf like structures (marked in dotted circles in Fig. 2b) apart from flower here and there, which help in building the mechanistic pathway for the formation of flower shaped ZnNS. At higher magnification (Fig. 2b) the formation of flowers by the combination of 8–10 leaf like petals was shown. The length of each petal was not exceeding 800 nm. The EDAX spectrum showed only the signal of O and Zn elements confirming the formation of pure ZnNS (Fig. 2c). High resolution-TEM micrographs at different magnifications and selected area electron diffraction (SAED) pattern of the synthesised flower shaped ZnNS were shown in Fig. S1. It is very clear from HRTEM micrograph that the flower shaped ZnNS consist of many leaf like petals and each petal was made of well-organized array of hexagonal ZnO nanorods. This observation may be again a key for demonstrating the growth mechanism. Fig. S1c shows the high resolution lattice image of flower shaped ZnNS and the interplanar d-spacing calculated from the image is d = 2.82 Å which matches correctly with the (1 0 0) plane d-spacing of hexagonal ZnO. Fig. S1d shows the corresponding SAED pattern which substantiates the crystalline nature of the product. Synthesized ZnNS showed remarkable antibacterial activity at concentrations as low as 25 mg/L. Also there was a specific change observed against those mentioned by Breyner et al. [14] and in the present study E. coli was less affected with ZnNS which might also attribute to the strains being used for the test suggesting strain specific effects of it. Fig. S2a and b shows well diffusion method for checking the antimicrobial activity of ZnNS against S. aureus and E. coli. The results obtained in this study show that concentration of 25 mg/L is the optimum concentration for particles to inhibit the growth of microorganisms in solid medium. The results of MIC/ MBC show growth of colony only in 1 108 CFU/mL concentration of S. aureus as well as E. coli in broth method against the lowest concentration of 25 mg/L for flower shaped ZnNS. This can be
attributed to the easy diffusion of ZnNS in liquid medium compared to solid agar. Fig. S3a and b shows the colony growth on the agar plate in the final stage of MIC/MBC study.
Growth mechanism of ZnO flower like structures Excessive addition of NaOH to zinc acetate dihydrate leads to the formation colourless solution of zincate ions as follows
Zn2þ þ 2OH ! ZnðOHÞ2
ZnðOHÞ2 þ 2OH !
ZnðOHÞ2 4
ð1Þ ð2Þ
On heating the solution containing zincate ions, the growth unit ZnðOHÞ2 slowly converts into ZnO and hydroxyl ions as shown 4 below ZnðOHÞ2 4 ! ZnO þ H2 O þ 2OH
ð3Þ
On heating the solution of zincate ions, the molecules start to rearrange into NS of hexagonal ZnO nanorods after growing along h0 0 0 1i direction. This phenomenon happened due to ZnO crystal structure was constructed gradually by OH ions and it acted as polar crystal whose axis was the C-axis with space group P63mc. When the molecules got saturated, the ZnO nuclei grew up to give rod shaped ZnO. Over time, these freshly forming nanorods deposit on the surface of formerly formed crystalline nanorods resulting leaf like structure first and number of such leafs came together in an ordered array which appeared to be as flower shaped ZnNS (Fig. 3). This thermodynamically obsessed natural progression yielded up due to larger flower shaped ZnNS were energetically stable than the smaller nanorods confirming the fact that the surface molecules of the particles are energetically not as much of stable than the ones in core.
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K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
Fig. 3. Plausible formation mechanism of ZnO nanoflowers.
Conclusion
References
A simple low temperature method to synthesise well dispersed flower shaped ZnO nanoflowers using only a metal precursor and a precipitating agent is demonstrated here. No other dispersing/ structure directing agents were used. The present study reveals that the flower shaped ZnNS showed good antimicrobial activity towards S. aureus as well as E. coli. It warrants further detailed studies to compare the antimicrobial activity of different shaped ZnNS towards microbes.
[1] X. Wang, J. Song, J. Liu, Z.L. Wang, Science 316 (2007) 102–105. [2] C.Y. Lee, S.Y. Li, P. Lin, T.-Y. Tseng, IEEE Trans. Nanotechnol. 5 (2006) 216–219. [3] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Appl. Phys. Lett. 84 (2004) 3654–3656. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897–1899. [5] K.H. Tam, A.B. Djurisic, C.M.N. Chan, Y.Y. Xi, C.W. Tse, Y.H. Leung, W.K. Chan, F.C.C. Leung, Thin Solid Films 516 (2008) 6167–6174. [6] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Appl. Phys. Lett. 86 (2005) 243503–243505. [7] J.J. Wu, G.R. Chen, H.H. Yang, C.H. Ku, J.Y. Lai, Appl. Phys. Lett. 90 (2007) 213109–213111. [8] S.F. Yu, C. Yuen, S.P. Lau, W.I. Park, G.C. Yi, Appl. Phys. Lett. 84 (2004) 3241– 3243. [9] N. Serpone, D. Dondi, A. Albini, Inorg. Chim. Acta 360 (2007) 794–802. [10] S. Music, A. Saric, S. Popovic, Ceram. Int. 36 (2010) 1117–1123. [11] J. Agrell, G. Germani, S.G. Jaras, M. Boutonnet, Appl. Catal. A 242 (2003) 233– 245. [12] C.L. Wu, L. Chang, H.G. Chen, C.W. Lin, T.F. Chang, Y.C. Chao, J.K. Yan, Thin Solid Films 498 (2006) 137–141. [13] S. Rani, P. Suri, P.K. Shishodia, R.M. Mehra, Sol. Energy Mater. Sol. Cells 92 (2008) 1639–1645. [14] R. Brayner, R. Ferrari-Iliou, N. Brivois, S. Djediat, M.F. Benedetti, F. Fievet, Nano Lett. 6 (2006) 866–870.
Acknowledgment Mr. KMK greatly acknowledges the help of VIT University, Vellore 632 014, India for the platform given to do this research.
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.2012.11.025.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis and characterisation of flower shaped Zinc Oxide nanostructures and its antimicrobial activity Kesarla Mohan Kumar a, Badal Kumar Mandal a,⇑, Etcherla Appala Naidu a, Madhulika Sinha a, Koppala Siva Kumar b, Pamanji Sreedhara Reddy b a b
Trace Elements Speciation Research Laboratory, Environmental and Analytical Chemistry Division, School of Advanced Sciences, VIT University, Vellore 632 014, TN, India Department of Physics, Sri Venkateswara University, Tirupati 517 502, AP, India
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
" A simple low temperature method
This is a simple method to synthesise flower shaped ZnO nanostructures. This method uses no structure directing agents. Thus obtained ZnO nanostructures showed good antimicrobial activity.
selectively for flower shaped ZnO nanostructures. " Only a metal precursor and a precipitating agent were used. " No other dispersing/structure directing agents were used. " ZnO nanostructures showed good antimicrobial activity.
a r t i c l e
i n f o
Article history: Received 3 May 2012 Received in revised form 5 November 2012 Accepted 6 November 2012 Available online 28 November 2012 Keywords: Flower shaped ZnO nanostructures Nanorods Antimicrobial activity Microbes
a b s t r a c t Flower shaped Zinc Oxide nanostructures was synthesized using a simple method without using any structure directing agents. Elemental analysis, crystalline nature, shape and size were examined using Powder X-ray Diffraction (XRD), scanning electron microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDAX). XRD revealed the formation of hexagonal ZnO nanostructures. SEM and TEM analyses revealed the formation of crystalline ZnO flowers in which a bunch of ZnO nanorods assembled together to form a leaf like structure followed by flower shaped ZnO nanostructures. Thus synthesised ZnO nanostructures showed good antimicrobial activity towards gram-positive bacteria Staphylococcus aureus as well as gram-negative bacteria Escherichia coli with a MIC/MBC of 25 mg/L. Ó 2012 Elsevier B.V. All rights reserved.
Introduction In manufacturing nanomaterials-based devices, the synthesis of controlled size and large yield of nanostructures (NSs) is a challenge to materials scientists. It is mandatory to study the
⇑ Corresponding author. Tel.: +91 416 220 2339; fax: +91 416 224 3092. E-mail addresses: [email protected], [email protected] (B.K. Mandal). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.11.025
physical and chemical properties of nanomaterials to ensure the crystalline nature and purity. Versatile applications such as power generators [1], field emission devices [2], gas sensors [3] and as ultraviolet lasers [4] make ZnO nanostructures (ZnNS) as promising and smart materials which catch the attention of researchers from all corners in recent times. Moreover, the incredible antibacterial activity of nanoZnO [5] makes them compatible in many cosmetic applications e.g. sunscreens and medicated creams [6–9].
172
K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
Many growth techniques to synthesise ZnNS have been reported in the literature such as thermal decomposition [10], micro emulsion method [11], CVD [12] and sol–gel methods [13]. Usually the vapour phase methods use sophisticated instruments, vacuum and high temperatures which limit the nature of substrates used compared to aqueous medium based methods which are low-cost and simple. In the present study a simple low temperature method is developed to synthesise well dispersed flower shaped ZnNS using only a metal precursor and a precipitating agent. No other dispersing/structure directing agents were used. In this paper synthesis, mechanism of formation, characterisation and antimicrobial activity of flower shaped ZnNS are presented.
Experimental After many trials, a selective procedure to synthesise flower shaped nanostructures was achieved in the present study by the following procedure. Flower shaped ZnNS were prepared from 4 mM zinc acetate dihydrate and 20 mM of NaOH after dissolving in deionised water. Initially both the aqueous solutions were cooled in ice bath and then NaOH solution was added dropwise using a peristaltic pump to zinc acetate dihydrate solution with stirring at 550 rpm using a propeller. The resulting turbid solution was heated on a temperature controlled water bath at 75 °C for 30 min and white powders was settled at the bottom. The settled white powders was separated followed by washing with deionised water thrice and dried overnight in a dust free condition under room temperature.
Determination of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) The MIC and MBC were determined by taking nine sterile test tubes separately with 0.9 mL of sterile MH broth, and inoculated with the corresponding microorganism of concentration 108 CFU/mL of bacteria. Fresh bacterial culture was made by keeping the freshly inoculated bacterial sample in 5 mL sterilized broth for 6 h of incubation (to reach its Log phase) at 37 °C. 0.1 mL of the fresh culture was taken and added to the first test tube with sterile liquid medium and serial dilutions were performed for the other seven test tubes with extra two tubes as control. To each tube 10 lL of ZnNS dispersion was added and the tubes were incubated at 37 °C overnight. A drop of cultures from each tube was added on the specified marked plates. The plates were incubated for 18–24 h at 37 °C and the growth of colonies was monitored. Evaluation of Zone of Inhibition (ZOI): well diffusion method The bacterial lawn was prepared on sterile Muller–Hinton agar (1% Agar) plates by using a sterile cotton swab. 108 CFU/mL cultures of S. aureus and 106 CFU/mL culture of E. coli were used to make a lawn culture (by streaking culture using sterile cotton swabs). Wells of approx. 8 mm diameter were made. Flower shaped ZnNS of different concentrations were added inside each well. The plates were kept for incubation at 37 °C overnight and zone diameter were measured.
Characterisation
Results and discussion
The synthesized solid materials was subjected to Powder X-ray Diffraction (XRD) analysis using Bruker D8 Advance Diffractometer (Bruker AXS, Germany) with Cu Ka radiation (k = 1.54 Å). XRD pattern of ZnNS was recorded over a 2-theta range of 10–90°. Scanning Electron Microscope coupled with Energy Dispersive X-ray Spectroscopy (SEM–EDAX) analysis was done using a Carl Zeiss SEM Instrument attached to EVO MA 15 (Oxford Instrument). The solid samples were sprinkled on the adhesive carbon tape which was supported on the metallic disc. The sample surface images were taken at different magnifications. Simultaneously, EDAX spectrum was recorded at selected areas on the solid surface to obtain the elemental analysis. The morphology and structural properties of the fabricated ZnO nanostructures were characterized by High Resolution Transmission Electron Microscope (HR-TEM) (JEOL JEM 2100 HR-TEM) operated at an accelerating voltage of 200 kV. Sampling was done by dispersing ZnNS in ultrasonic bath and a drop of the dispersion was applied on Cu grid with Ultrathin Cu on holey C film and allowed to dry in vacuum.
The powder XRD pattern of the synthesised ZnNS was shown in Fig. 1 and all the peaks in the XRD diffractogram was indexed which matched to hexagonal ZnO (JCPDS Card No. 03-65-3411). The XRD pattern clearly showed the pure phase and no other peaks due to impurities were observed. The sharp and strong diffraction peaks revealed that the synthesised ZnNS were highly crystalline and pure. The lattice constants of the synthesised nanostructures were measured to be a = 3.2490 Å and c = 5.2066 Å which was in good agreement with the values of hexagonal ZnO of the space group P63mc. The SEM micrographs at different magnifications and EDAX spectrum of the flower shaped ZnNS were shown in Fig. 2. At lower magnifications, it is very clear that the formation of well dispersed ZnO nanoflowers. A keen observation of the same material at the
(1 0 1)
600
(2 0 3)
100
(1 0 4)
200
(2 0 0)
(1 0 2)
300
(1 0 3)
(1 1 0)
400
(2 0 2)
500
(1 1 2) (2 0 1) (0 0 4)
(1 0 0) (0 0 2)
NP-induced toxicity tests on selected bacteria were conducted in liquid and solid Muller Hinton media containing different concentrations of ZnNS (25–125 mg/L). For each experiment bacterial cultures were grown in ZnNS free liquid media keeping overnight at 37 oC in an incubator. The bactericidal experiments were carried out with gram negative bacteria Escherichia coli (strain ATCC 25922) and gram positive bacteria Staphylococcus aureus (strain ATCC 25923). The solid agar was made using both Muller Hinton Broth (MHB) and Agar to make 1% Agar for easy diffusion of ZnNS. Throughout this study, the same media was used for all strains. MHB broth was also used as a medium for preparing fresh cultures of organisms.
700 Intensity (counts)
Bactericidal experiment
800
0 20
30
40
50
60
70
2 Fig. 1. XRD diffractogram of ZnO nanoflowers.
80
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K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
173
Fig. 2. SEM micrographs of ZnO nanoflowers at different magnifications (a and b) and EDX spectrum of ZnO nanoflowers (c).
little higher magnification (Fig. 2b) shows few leaf like structures (marked in dotted circles in Fig. 2b) apart from flower here and there, which help in building the mechanistic pathway for the formation of flower shaped ZnNS. At higher magnification (Fig. 2b) the formation of flowers by the combination of 8–10 leaf like petals was shown. The length of each petal was not exceeding 800 nm. The EDAX spectrum showed only the signal of O and Zn elements confirming the formation of pure ZnNS (Fig. 2c). High resolution-TEM micrographs at different magnifications and selected area electron diffraction (SAED) pattern of the synthesised flower shaped ZnNS were shown in Fig. S1. It is very clear from HRTEM micrograph that the flower shaped ZnNS consist of many leaf like petals and each petal was made of well-organized array of hexagonal ZnO nanorods. This observation may be again a key for demonstrating the growth mechanism. Fig. S1c shows the high resolution lattice image of flower shaped ZnNS and the interplanar d-spacing calculated from the image is d = 2.82 Å which matches correctly with the (1 0 0) plane d-spacing of hexagonal ZnO. Fig. S1d shows the corresponding SAED pattern which substantiates the crystalline nature of the product. Synthesized ZnNS showed remarkable antibacterial activity at concentrations as low as 25 mg/L. Also there was a specific change observed against those mentioned by Breyner et al. [14] and in the present study E. coli was less affected with ZnNS which might also attribute to the strains being used for the test suggesting strain specific effects of it. Fig. S2a and b shows well diffusion method for checking the antimicrobial activity of ZnNS against S. aureus and E. coli. The results obtained in this study show that concentration of 25 mg/L is the optimum concentration for particles to inhibit the growth of microorganisms in solid medium. The results of MIC/ MBC show growth of colony only in 1 108 CFU/mL concentration of S. aureus as well as E. coli in broth method against the lowest concentration of 25 mg/L for flower shaped ZnNS. This can be
attributed to the easy diffusion of ZnNS in liquid medium compared to solid agar. Fig. S3a and b shows the colony growth on the agar plate in the final stage of MIC/MBC study.
Growth mechanism of ZnO flower like structures Excessive addition of NaOH to zinc acetate dihydrate leads to the formation colourless solution of zincate ions as follows
Zn2þ þ 2OH ! ZnðOHÞ2
ZnðOHÞ2 þ 2OH !
ZnðOHÞ2 4
ð1Þ ð2Þ
On heating the solution containing zincate ions, the growth unit ZnðOHÞ2 slowly converts into ZnO and hydroxyl ions as shown 4 below ZnðOHÞ2 4 ! ZnO þ H2 O þ 2OH
ð3Þ
On heating the solution of zincate ions, the molecules start to rearrange into NS of hexagonal ZnO nanorods after growing along h0 0 0 1i direction. This phenomenon happened due to ZnO crystal structure was constructed gradually by OH ions and it acted as polar crystal whose axis was the C-axis with space group P63mc. When the molecules got saturated, the ZnO nuclei grew up to give rod shaped ZnO. Over time, these freshly forming nanorods deposit on the surface of formerly formed crystalline nanorods resulting leaf like structure first and number of such leafs came together in an ordered array which appeared to be as flower shaped ZnNS (Fig. 3). This thermodynamically obsessed natural progression yielded up due to larger flower shaped ZnNS were energetically stable than the smaller nanorods confirming the fact that the surface molecules of the particles are energetically not as much of stable than the ones in core.
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K. Mohan Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) 171–174
Fig. 3. Plausible formation mechanism of ZnO nanoflowers.
Conclusion
References
A simple low temperature method to synthesise well dispersed flower shaped ZnO nanoflowers using only a metal precursor and a precipitating agent is demonstrated here. No other dispersing/ structure directing agents were used. The present study reveals that the flower shaped ZnNS showed good antimicrobial activity towards S. aureus as well as E. coli. It warrants further detailed studies to compare the antimicrobial activity of different shaped ZnNS towards microbes.
[1] X. Wang, J. Song, J. Liu, Z.L. Wang, Science 316 (2007) 102–105. [2] C.Y. Lee, S.Y. Li, P. Lin, T.-Y. Tseng, IEEE Trans. Nanotechnol. 5 (2006) 216–219. [3] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Appl. Phys. Lett. 84 (2004) 3654–3656. [4] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897–1899. [5] K.H. Tam, A.B. Djurisic, C.M.N. Chan, Y.Y. Xi, C.W. Tse, Y.H. Leung, W.K. Chan, F.C.C. Leung, Thin Solid Films 516 (2008) 6167–6174. [6] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Appl. Phys. Lett. 86 (2005) 243503–243505. [7] J.J. Wu, G.R. Chen, H.H. Yang, C.H. Ku, J.Y. Lai, Appl. Phys. Lett. 90 (2007) 213109–213111. [8] S.F. Yu, C. Yuen, S.P. Lau, W.I. Park, G.C. Yi, Appl. Phys. Lett. 84 (2004) 3241– 3243. [9] N. Serpone, D. Dondi, A. Albini, Inorg. Chim. Acta 360 (2007) 794–802. [10] S. Music, A. Saric, S. Popovic, Ceram. Int. 36 (2010) 1117–1123. [11] J. Agrell, G. Germani, S.G. Jaras, M. Boutonnet, Appl. Catal. A 242 (2003) 233– 245. [12] C.L. Wu, L. Chang, H.G. Chen, C.W. Lin, T.F. Chang, Y.C. Chao, J.K. Yan, Thin Solid Films 498 (2006) 137–141. [13] S. Rani, P. Suri, P.K. Shishodia, R.M. Mehra, Sol. Energy Mater. Sol. Cells 92 (2008) 1639–1645. [14] R. Brayner, R. Ferrari-Iliou, N. Brivois, S. Djediat, M.F. Benedetti, F. Fievet, Nano Lett. 6 (2006) 866–870.
Acknowledgment Mr. KMK greatly acknowledges the help of VIT University, Vellore 632 014, India for the platform given to do this research.
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.2012.11.025.