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Growth mechanism and photoluminescence property of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method
Ultrasonics Sonochemistry 17 (2010) 560–565
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Growth mechanism and photoluminescence property of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method Priya Mishra *, Raghvendra S. Yadav, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211 002, India Physics Department, University of Allahabad, Allahabad 211 002, India
a r t i c l e
i n f o
Article history: Received 4 April 2009 Received in revised form 8 October 2009 Accepted 27 October 2009 Available online 30 October 2009 Keywords: Sonochemical method Nanostructure ZnO Photoluminescence
a b s t r a c t Flower-like ZnO nanostructures have been synthesized by starch-assisted sonochemical method and the effect of starch and ultrasound on the formation of ZnO nanostructure has been investigated. It is observed that starch and ultrasonic wave both plays a vital role on the growth of ZnO nanostructure. X-ray diffraction (XRD) pattern indicated that the synthesized flower-like ZnO nanostructures were hexagonal. FTIR spectrum confirms the presence of starch on the surface of flower-like ZnO nanostructure. The photoluminescence spectrum of flower-like ZnO nanostructure consists of band-edge emission at 393 nm as well as emission peaks due to defects. On the basis of structural information provided by Xray diffraction (XRD) and morphological information by Scanning Electron Microscopy (SEM), a growth mechanism is proposed for formation of flower-like ZnO nanostructures. Differential Scanning Calorimetry (DSC) of starch in liquid medium confirms that gelatinization is a two step process involving two phases. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures have attracted much interest due to their unique piezoelectric, semiconducting and catalytic properties and wide range of applications in sensors, optoelectronics, transducers and in medical sciences [1–8]. Various nanostructures of ZnO have been reported like nanotubes [9], nanowires [10], nanodisks [11], nanosprings [12], nanohelix [13], nanoplates and nanosheets [14]. ZnO being a II–VI semiconductor material has immense potential applications with band gap of 3.37 eV and large exciton binding energy of 60 meV [15]. Synthesis of nanostructures which are environmentally benign and have got their shape and size well controlled is the need of the hour as size, shape and dimensionality strongly affect the properties of nanomaterials. Many different approaches have been used for the generation of nanomaterials in order to obtain the required properties and structures [16]. Among them template assisted approaches are useful due to their ability to construct highly organised materials in a controllable manner [17,18]. Materials such as micelles [19], membranes [20], biopolymers [21,22] and even animal and plant tissues [23,24] have been used as templates. In this work, we have used sonochemical
method to synthesize flower-like ZnO nanostructure with using starch as a template. Sonochemical method has been known to form novel materials. Ultrasonic irradiation generates small bubbles in liquid medium and there is repeated formation, growth, and collapse of these bubbles in liquid medium this phenomenon is known as acoustic cavitation. An implosive collapse of cavitation bubbles by adiabatic compression results in very high temperature of 5000 K and high pressure [25]. Starch is one of the most important natural organic compounds abundant in nature. It is a biocompatible polymer and is known to form a wide range of inclusion complexes with several molecules [26]. It is made of repeating units of amylose and amylopectin with alpha 1,4 linkage between D-glucose units and its molecular formula is (C6H10O5)n. Starch is normally not soluble in aqueous solution of water, however, in presence of both water and heat it becomes gelatinized. Gelation of starch plays an important role in controlling morphology of nanoparticles.
2. Experimental section 2.1. Sample preparation
* Corresponding author. Present address: Nanophosphor Application Centre, University of Allahabad, Allahabad 211 002, India. Tel./fax: +91 0532 2460675. E-mail address: [email protected] (P. Mishra). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.10.017
All the chemicals used were of analytical grade and were used without further purification. The zinc nitrate, sodium hydroxide, starch were from E. Merck Ltd., Mumbai 400018, India. The procedure to prepare ZnO nanostructure is as follows: 0.025 g starch was
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(a) With starch and without ultrasonic
20
Intensity ( cps )
dissolved in 10 ml of double distilled water to prepare starch solution. 10 ml of 0.1 M aqueous solution of zinc acetate and 10 ml 1 M aqueous solution of sodium hydroxide were added to 25 ml alcohol followed by starch solution. The solution containing beaker was then immediately kept in sonication bath (33 K Hz, 350 W) at room temperature for 4 h. Initial temperature of reaction mixture was 27 °C however after 4 h temperature had risen to 57 °C. The white precipitate formed was collected by centrifugation and then thoroughly washed with water followed by alcohol to remove traces of starch and other impurities and then kept in vacuum oven for drying. The schematic illustration for synthesis of ZnO nanostructures by starch-assisted sonochemical method is shown in Fig. 1. In another set of experiment all the conditions were kept same however, ultrasonic waves were not used (the beaker was kept for stirring at room temperature of 27 °C and raised to 57 °C in 4 h) to compare and contrast the effect of ultrasonic waves on the morphology of ZnO. In the third set of experiment, keeping all the experimental conditions same as in first set of experiment but starch was not used to know the role played by starch on the morphology of ZnO.
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(b) With starch and with ultrasonic
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(c) Without starch and with ultrasonic
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Angle ( two theta ) Fig. 2. XRD pattern of ZnO nanostructure.
2.2. Instrumentation The crystal structure of ZnO nanoparticles were characterized by X-ray diffraction (XRD, Rigaku D/MAX-2200 H/PC, Cu Ka radiation). Scanning Electron Microscopy (SEM) images were taken on LEO Electron Microscopy Ltd., England and E-SEM images were taken on Quanta 200FEG (FEI company). Photoluminescence spectra were recorded on Perkin Elmer LS 55 spectrometer. FTIR spectrum study was carried out on a ABB FTLA 2000 FTIR spectrometer (Canada) in the wavenumber range of 4000–500 cm1 on pellets obtained by mixing the sample in KBr. DSC was done on a Perkin Elmer pyris diamond TGA/DTA/DSC 8.0 series (Perkin Elmer Corp., USA) The scanning rate was 10 °C/min. in the temperature range 35–85 °C. 3. Results and discussion 3.1. Structural study Fig. 2 shows the X-ray diffraction (XRD) pattern of ZnO nanostructure synthesized by sonochemical method with different experimental conditions (a) with starch and without ultrasonic,
(b) with starch and with ultrasonic, (c) without starch and with ultrasonic. It can be seen from the XRD pattern that in absence of ultrasonic wave and presence of starch, there is no formation of crystal structure of ZnO, although there is formation of ZnO crystal structure in presence of starch as well as ultrasonic wave. The diffraction peaks of ZnO nanostructure in presence of starch and ultrasonic wave can be indexed as ZnO with hexagonal phase (JCPDF Card File No. 36145). No characteristic peaks of impurities such as Zn, starch or zinc hydroxide are observed. This confirms that the obtained product contains pure ZnO nanostructure. The XRD pattern in absence of starch and in presence of ultrasonic wave also shows well formation of ZnO crystal structure. These results indicate that the ultrasonic wave played an important role in formation of ZnO crystal structure mainly in presence of starch. 3.2. Morphology study Fig. 3 shows the Scanning Electron Microscopy (SEM) image of ZnO nanostructure synthesized by sonochemical method with (A) 1 g starch, (B) 0.025 g starch and (C) in absence of starch. Fig. 3(A) shows flower-like ZnO nanostructure synthesized by sonochemical method in presence of 1 g starch using ultrasonic waves. Fig. 3(A) (i) shows ZnO nanoflower under low magnification while Fig. 3(A) (ii) shows ZnO nanoflower structure at high magnification. From Fig. 3 it is clear that there are many flat nanorods with tapering ends, are seen to arise from centre, it gives the appearance of a flower, their diameter are several hundred nanometers and length varies from several hundred nanometers to 1 lm and the average size of whole flowers is about 1.5 lm. From Fig. 3(B), it is also clear that there is again formation of flower-like ZnO nanostructure in presence of 0.025 g starch using ultrasonic waves. Fig. 3(C) shows the spherical-like ZnO nanostructure in absence of starch and in presence of ultrasonic waves. From these results, it is clear that starch played an important role in influencing the morphology of ZnO. 3.3. FTIR spectra study
Fig. 1. Schematic illustration for synthesis of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method.
Fig. 4 shows the FTIR spectra of flower-like ZnO nanostructure synthesized by sonochemical method in presence of 0.025 g starch. FTIR spectra shows the peaks at 900–1250 cm1 which corresponds to C–H stretching, and broad band starting at
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Fig. 3. SEM image of flower-like ZnO nanostructure synthesized by sonochemical method with (A) 1 g starch and (B) 0.025 g starch and (C) in absence of starch.
3500 cm1 corresponds to O–H bond. These bands are associated with starch. All the peaks show that starch is present at surface of
ZnO flower-like nanostructure. The peak at 1630 cm1 corresponds to O–H bending of absorbed water.
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Fig. 4. FTIR spectra of flower-like ZnO nanostructure synthesized by sonochemical method in presence of 0.025 g starch.
3.4. Differential scanning calorimetry (DSC) study DSC is a thermal analytical technique which measures the energy absorbed or emitted by a sample as a function of temperature. When a phase transition occurs, DSC provides a direct calorimetric measurement of the transition energy at the transition temperature by subjecting the sample and an inert reference material to identical temperature regimes in an environment heated or cooled at a controlled rate. DSC equipment can be used to determine phase-diagram, enthalpy, entropy and specific heat [27]. Erik Svensson and Ann-Charlotte Eliasson [28] reported that DSC data of starch has biphasic endotherm, similar result is also observed in our study. Fig. 5 shows DSC endotherm of starch in water as liquid medium. DSC endotherm of starch in water as liquid medium shows that gelatinization is a two stage process involving first plastization and hydration of amorphous parts of granules followed by melting of starch crystallites. Two phases are observed first from 35 °C to 47 °C which corresponds to hydration and plasticization and another phase from 47 °C to 82 °C which is for melting. Complete gelatinization of starch occurs before 85 °C as shown in endotherm (Fig. 5). 3.5. Growth mechanism It is observed that cavitation which occurs during ultrasonication plays key role in sonochemical synthesis of materials.
Fig. 5. DSC endotherm of starch in water as liquid medium.
During ultrasonic cavitation very high temperatures of about 5000 K, pressures of about 1800 atm [29–31] are reached followed by release of large amount of energy due to collapse of micro-bubbles. In the previous report [32], we have reported that the sonochemical bath, which we have used, there is not formation of hot-spot cavitation under ultrasonic irradiation and formed ZnO in presence of EDA was due to strong agitation effect of ultrasonic cavitation. Here, the formation of ZnO nanostructure in presence of starch is mainly due to strong agitation effect of ultrasonic cavitation. The conditions of strong agitation effect of ultrasonic cavitation favour simultaneous gelatinization of starch and formation of ZnO nanoparticles with gelatinized starch granules directing the growth of nanoparticles by binding on its surface. Starch is normally composed of amylose and amylopectin in a ratio of 30:70 or 20:80 with amylopectin found in larger amount. Due to continuous ultrasonication with strong agitation effect of ultrasonic cavitation phenomenon, starch granules swell, there is loss of both crystallinity and double helical order of amylopectin with the leaching of amylose into the reaction mixture [33–36]. Amylose has the ability to form coils therefore it can direct the ZnO growth units to take the shape of nanorods while still in the sonochemical bath. However when the reaction mixture was taken out of sonochemical bath cooling takes place and gel can form with free amylose molecules losing energy as temperature decreases and forming hydrogen bonds with each other as well with amylopectin entrapping ZnO nuclei in the process along with them and forming units of nanorods. These nanorods finally clump together with the help of amylose which normally has glue like property of holding starch gels together and finally giving shape of flower. The distinct advantage of using starch is that it avoids agglomeration of nanostructures. The experimental results show that in formation of flower-like ZnO nanostructure, the role of starch should be taken into consideration. In the absence of starch, there is agglomeration of ZnO spherical nanoparticles (Fig. 3C) and in presence of starch flower like morphology is obtained using sonochemical method (Fig. 3A and B). The mechanism of formation of ZnO nanostructures takes the consideration the ionic species formed from water molecules by absorption of ultrasound energy. The reaction steps taking place inside sonochemical bath can be summarised as follows:
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H2 O $ Hþ þ OH
ð1Þ
NaOH $ Naþ þ OH
ð2Þ
ZnðCH3 COOÞ2 $ Zn2þ þ 2CH3 COO
ð3Þ
Zn2þ þ 4OH $ ½ZnðOHÞ4 2
ð4Þ
2½ZnðOHÞ4 2 $ 2ZnO þ 4H2 O þ O2
ð5Þ
3.6. Photoluminescence study Fig. 7 illustrates the photoluminescence spectrum of flower-like ZnO nanostructure synthesized by starch-assisted sonochemical method at excitation wavelength 325 nm. Several emission bands, including band-edge emission at 393 nm (3.162 eV) and defect related ultraviolet emission at 416 nm (2.98 eV), blue emission at 442 nm (2.81 eV), blue-green emission 482 nm (2.57 eV) and green emission 526 nm (2.36 eV) were observed. It is commonly considered that band-edge emission at 393 nm should be attributed to the recombination of excitons [37]. In general, the visible emission in ZnO is attributed to different intrinsic defects such as oxygen vacancies (Vo), zinc vacancies (Vzn), oxygen
Eqs. (1)–(3) show formation of primary ions which have been formed by dissociation of water, sodium hydroxide and zinc acetate. Eq. (4) shows formation of [Zn(OH)4]2 which further decomposes to give ZnO spherical nanoparticles. In the meantime, the starch granules (in the presence of starch in the solution) undergo following process:
Starch granules
hydration of starch þ leaching of amylose
ð6Þ
ZnO nanorods
AmyloseðglueÞ þ ZnO nuclei
ð7Þ
Clumping of nanorods into flower
AmyloseðglueÞ þ ZnO nanorods
Eqs. (6)–(8) indicated that in presence of water and ultrasonic irradiation, starch undergoes gelatinization [33–36] process during which there is hydration of starch granules, leaching out of amylose and some granules get collapsed. After 4 h of ultrasonication temperature of solution which initially was 27 °C reaches 57 °C which favours gelatinization. Amylose which comes out has a gluing property it binds onto ZnO nuclei giving it shape of a nanorod as amylose itself has tendency to form coil. When reaction mixture gets cooled these amylose units along with amylopectin and collapsed starch granules form hydrogen bonds among them gluing nanorods together into a spherical structure with nanorods projecting out, giving the appearance of flower as represented in Fig. 6.
ð8Þ
interstitials (Oi), zinc interstitials (Zni) and oxygen antisites (Ozn). The origin of violet emission centred at 2.987 eV (416 nm) is ascribed to an electron transition from a deep donor level of neutral Zni to the valence band [38]. Therefore, the deep donor level of the Zni is suggested to locate at 0.39 eV below the conduction band in this study because the violet photoluminescence appears at 2.987 eV. A blue emission centred at 2.57 eV (482 nm) is due to a radiative transition of an electron from the deep donor level of Zni to an acceptor level of neutral Vzn [39]. In this study, it can be estimated that the acceptor level of Vzn locates at 0.41 eV above the valence band. Another blue emission was reported to appear at around 2.81 eV (442 nm) [40]. This emission may be related to surface defects of ZnO nanostructure, although the detailed
Ultrasonic waves
Amylose chain with some
Uncollapsed starch granules in solution
Uncollapsed granules.
ZnO nuclei
Amylose chains
Clumping of amylose chains on ZnO nuclei
On cooling
Clumped nanorods
Amylose chains
Flower-like morphology
Fig. 6. Diagramatic representation of flower-like ZnO nanostructure by starch-assisted sonochemical method.
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crystal structure. Photoluminescence spectrum of flower-like ZnO nanostructure revealed several emission bands, including bandedge emission at 393 nm (3.162 eV) and defect related ultraviolet emission at 416 nm (2.98 eV), blue emission at 442 nm (2.81 eV), blue-green emission 482 nm (2.57 eV) and green emission 526 nm (2.36 eV). In the end, the prepared flower-like ZnO nanostructures are pure and water soluble and there are lots of hydroxyl groups attached to nanostructures surface which in future can be functionalized. Acknowledgments The authors are very grateful and also wish to express their gratitude to all the scientific members of Nanophosphor Application Centre, University of Allahabad, Allahabad, India. This work was financially supported by DST, India. References
Fig. 7. Photoluminescence spectrum of flower-like and spherical-like ZnO nanostructures synthesized by sonochemical method in presence of 0.025 g starch and in absence of starch respectively at excitation 325 nm.
mechanism for blue emission at 442 nm has been not clarified. The green emission 526 nm (2.363 eV) are attributed to radiative transitions from the deep donor level of Zni to acceptor levels caused by singly ionized V Zn [41]. Because the peak positions of the green emission are 2.363 eV in this study, the acceptor level of the ionized V Zn locate at 0.62 eV above the valence band. However, spherical-like ZnO nanostructures has emission peak at 395 nm, 418 nm, 442 nm, 483 nm and 528 nm. It is noticeable that there is small shift in emission peak positions in spherical-like ZnO nanostructures synthesized in absence of starch as compared to flower-like ZnO nanostructures. The shift in emission peak position by starch as surfactant, indicate that the layer of starch result the change in morphology, size and defect content in ZnO nanostructures. However, recently, Y.L. Wu et al. [42] have shown no shift in U.V. emission peak position of ZnO nanoparticles using capping agents such as tetraethylorthosilicate, mercaptosuccinic acid, 3-mercaptopropyl trimethoxysilane, polyvinylpyrrolidone, 3-aminopropyl trimethoxysilane, the capping layers did not result in size changes or increased surface defects. It is also observed that the PL intensity of spherical-like ZnO nanoparticles synthesized in absence of starch has greater intensity as compared to flower-like ZnO nanostructures synthesized in presence of starch. The decrease in PL intensity in flower-like ZnO nanostructures is due to presence of surface absorbed starch on ZnO flower-like nanostructures, as it is confirmed from the FTIR study (Fig. 4). 4. Conclusion ZnO nanostructures viz. flower-like and spherical have been synthesized by sonochemical method with and without starch to investigate the influence of starch on morphology. In presence of starch and in absence of ultrasonic wave, there is no formation of ZnO crystal structure, although there is formation of crystal structure in presence of starch and ultrasonic wave, which confirmed that ultrasonic waves play an important role in formation of ZnO
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Growth mechanism and photoluminescence property of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method Priya Mishra *, Raghvendra S. Yadav, Avinash C. Pandey Nanophosphor Application Centre, University of Allahabad, Allahabad 211 002, India Physics Department, University of Allahabad, Allahabad 211 002, India
a r t i c l e
i n f o
Article history: Received 4 April 2009 Received in revised form 8 October 2009 Accepted 27 October 2009 Available online 30 October 2009 Keywords: Sonochemical method Nanostructure ZnO Photoluminescence
a b s t r a c t Flower-like ZnO nanostructures have been synthesized by starch-assisted sonochemical method and the effect of starch and ultrasound on the formation of ZnO nanostructure has been investigated. It is observed that starch and ultrasonic wave both plays a vital role on the growth of ZnO nanostructure. X-ray diffraction (XRD) pattern indicated that the synthesized flower-like ZnO nanostructures were hexagonal. FTIR spectrum confirms the presence of starch on the surface of flower-like ZnO nanostructure. The photoluminescence spectrum of flower-like ZnO nanostructure consists of band-edge emission at 393 nm as well as emission peaks due to defects. On the basis of structural information provided by Xray diffraction (XRD) and morphological information by Scanning Electron Microscopy (SEM), a growth mechanism is proposed for formation of flower-like ZnO nanostructures. Differential Scanning Calorimetry (DSC) of starch in liquid medium confirms that gelatinization is a two step process involving two phases. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures have attracted much interest due to their unique piezoelectric, semiconducting and catalytic properties and wide range of applications in sensors, optoelectronics, transducers and in medical sciences [1–8]. Various nanostructures of ZnO have been reported like nanotubes [9], nanowires [10], nanodisks [11], nanosprings [12], nanohelix [13], nanoplates and nanosheets [14]. ZnO being a II–VI semiconductor material has immense potential applications with band gap of 3.37 eV and large exciton binding energy of 60 meV [15]. Synthesis of nanostructures which are environmentally benign and have got their shape and size well controlled is the need of the hour as size, shape and dimensionality strongly affect the properties of nanomaterials. Many different approaches have been used for the generation of nanomaterials in order to obtain the required properties and structures [16]. Among them template assisted approaches are useful due to their ability to construct highly organised materials in a controllable manner [17,18]. Materials such as micelles [19], membranes [20], biopolymers [21,22] and even animal and plant tissues [23,24] have been used as templates. In this work, we have used sonochemical
method to synthesize flower-like ZnO nanostructure with using starch as a template. Sonochemical method has been known to form novel materials. Ultrasonic irradiation generates small bubbles in liquid medium and there is repeated formation, growth, and collapse of these bubbles in liquid medium this phenomenon is known as acoustic cavitation. An implosive collapse of cavitation bubbles by adiabatic compression results in very high temperature of 5000 K and high pressure [25]. Starch is one of the most important natural organic compounds abundant in nature. It is a biocompatible polymer and is known to form a wide range of inclusion complexes with several molecules [26]. It is made of repeating units of amylose and amylopectin with alpha 1,4 linkage between D-glucose units and its molecular formula is (C6H10O5)n. Starch is normally not soluble in aqueous solution of water, however, in presence of both water and heat it becomes gelatinized. Gelation of starch plays an important role in controlling morphology of nanoparticles.
2. Experimental section 2.1. Sample preparation
* Corresponding author. Present address: Nanophosphor Application Centre, University of Allahabad, Allahabad 211 002, India. Tel./fax: +91 0532 2460675. E-mail address: [email protected] (P. Mishra). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.10.017
All the chemicals used were of analytical grade and were used without further purification. The zinc nitrate, sodium hydroxide, starch were from E. Merck Ltd., Mumbai 400018, India. The procedure to prepare ZnO nanostructure is as follows: 0.025 g starch was
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(a) With starch and without ultrasonic
20
Intensity ( cps )
dissolved in 10 ml of double distilled water to prepare starch solution. 10 ml of 0.1 M aqueous solution of zinc acetate and 10 ml 1 M aqueous solution of sodium hydroxide were added to 25 ml alcohol followed by starch solution. The solution containing beaker was then immediately kept in sonication bath (33 K Hz, 350 W) at room temperature for 4 h. Initial temperature of reaction mixture was 27 °C however after 4 h temperature had risen to 57 °C. The white precipitate formed was collected by centrifugation and then thoroughly washed with water followed by alcohol to remove traces of starch and other impurities and then kept in vacuum oven for drying. The schematic illustration for synthesis of ZnO nanostructures by starch-assisted sonochemical method is shown in Fig. 1. In another set of experiment all the conditions were kept same however, ultrasonic waves were not used (the beaker was kept for stirring at room temperature of 27 °C and raised to 57 °C in 4 h) to compare and contrast the effect of ultrasonic waves on the morphology of ZnO. In the third set of experiment, keeping all the experimental conditions same as in first set of experiment but starch was not used to know the role played by starch on the morphology of ZnO.
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6000 5000 4000 3000 2000 1000 0
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(b) With starch and with ultrasonic
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6000 5000 4000 3000 2000 1000 0
50
60
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(c) Without starch and with ultrasonic
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30
40
50
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Angle ( two theta ) Fig. 2. XRD pattern of ZnO nanostructure.
2.2. Instrumentation The crystal structure of ZnO nanoparticles were characterized by X-ray diffraction (XRD, Rigaku D/MAX-2200 H/PC, Cu Ka radiation). Scanning Electron Microscopy (SEM) images were taken on LEO Electron Microscopy Ltd., England and E-SEM images were taken on Quanta 200FEG (FEI company). Photoluminescence spectra were recorded on Perkin Elmer LS 55 spectrometer. FTIR spectrum study was carried out on a ABB FTLA 2000 FTIR spectrometer (Canada) in the wavenumber range of 4000–500 cm1 on pellets obtained by mixing the sample in KBr. DSC was done on a Perkin Elmer pyris diamond TGA/DTA/DSC 8.0 series (Perkin Elmer Corp., USA) The scanning rate was 10 °C/min. in the temperature range 35–85 °C. 3. Results and discussion 3.1. Structural study Fig. 2 shows the X-ray diffraction (XRD) pattern of ZnO nanostructure synthesized by sonochemical method with different experimental conditions (a) with starch and without ultrasonic,
(b) with starch and with ultrasonic, (c) without starch and with ultrasonic. It can be seen from the XRD pattern that in absence of ultrasonic wave and presence of starch, there is no formation of crystal structure of ZnO, although there is formation of ZnO crystal structure in presence of starch as well as ultrasonic wave. The diffraction peaks of ZnO nanostructure in presence of starch and ultrasonic wave can be indexed as ZnO with hexagonal phase (JCPDF Card File No. 36145). No characteristic peaks of impurities such as Zn, starch or zinc hydroxide are observed. This confirms that the obtained product contains pure ZnO nanostructure. The XRD pattern in absence of starch and in presence of ultrasonic wave also shows well formation of ZnO crystal structure. These results indicate that the ultrasonic wave played an important role in formation of ZnO crystal structure mainly in presence of starch. 3.2. Morphology study Fig. 3 shows the Scanning Electron Microscopy (SEM) image of ZnO nanostructure synthesized by sonochemical method with (A) 1 g starch, (B) 0.025 g starch and (C) in absence of starch. Fig. 3(A) shows flower-like ZnO nanostructure synthesized by sonochemical method in presence of 1 g starch using ultrasonic waves. Fig. 3(A) (i) shows ZnO nanoflower under low magnification while Fig. 3(A) (ii) shows ZnO nanoflower structure at high magnification. From Fig. 3 it is clear that there are many flat nanorods with tapering ends, are seen to arise from centre, it gives the appearance of a flower, their diameter are several hundred nanometers and length varies from several hundred nanometers to 1 lm and the average size of whole flowers is about 1.5 lm. From Fig. 3(B), it is also clear that there is again formation of flower-like ZnO nanostructure in presence of 0.025 g starch using ultrasonic waves. Fig. 3(C) shows the spherical-like ZnO nanostructure in absence of starch and in presence of ultrasonic waves. From these results, it is clear that starch played an important role in influencing the morphology of ZnO. 3.3. FTIR spectra study
Fig. 1. Schematic illustration for synthesis of flower-like ZnO nanostructures synthesized by starch-assisted sonochemical method.
Fig. 4 shows the FTIR spectra of flower-like ZnO nanostructure synthesized by sonochemical method in presence of 0.025 g starch. FTIR spectra shows the peaks at 900–1250 cm1 which corresponds to C–H stretching, and broad band starting at
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Fig. 3. SEM image of flower-like ZnO nanostructure synthesized by sonochemical method with (A) 1 g starch and (B) 0.025 g starch and (C) in absence of starch.
3500 cm1 corresponds to O–H bond. These bands are associated with starch. All the peaks show that starch is present at surface of
ZnO flower-like nanostructure. The peak at 1630 cm1 corresponds to O–H bending of absorbed water.
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Fig. 4. FTIR spectra of flower-like ZnO nanostructure synthesized by sonochemical method in presence of 0.025 g starch.
3.4. Differential scanning calorimetry (DSC) study DSC is a thermal analytical technique which measures the energy absorbed or emitted by a sample as a function of temperature. When a phase transition occurs, DSC provides a direct calorimetric measurement of the transition energy at the transition temperature by subjecting the sample and an inert reference material to identical temperature regimes in an environment heated or cooled at a controlled rate. DSC equipment can be used to determine phase-diagram, enthalpy, entropy and specific heat [27]. Erik Svensson and Ann-Charlotte Eliasson [28] reported that DSC data of starch has biphasic endotherm, similar result is also observed in our study. Fig. 5 shows DSC endotherm of starch in water as liquid medium. DSC endotherm of starch in water as liquid medium shows that gelatinization is a two stage process involving first plastization and hydration of amorphous parts of granules followed by melting of starch crystallites. Two phases are observed first from 35 °C to 47 °C which corresponds to hydration and plasticization and another phase from 47 °C to 82 °C which is for melting. Complete gelatinization of starch occurs before 85 °C as shown in endotherm (Fig. 5). 3.5. Growth mechanism It is observed that cavitation which occurs during ultrasonication plays key role in sonochemical synthesis of materials.
Fig. 5. DSC endotherm of starch in water as liquid medium.
During ultrasonic cavitation very high temperatures of about 5000 K, pressures of about 1800 atm [29–31] are reached followed by release of large amount of energy due to collapse of micro-bubbles. In the previous report [32], we have reported that the sonochemical bath, which we have used, there is not formation of hot-spot cavitation under ultrasonic irradiation and formed ZnO in presence of EDA was due to strong agitation effect of ultrasonic cavitation. Here, the formation of ZnO nanostructure in presence of starch is mainly due to strong agitation effect of ultrasonic cavitation. The conditions of strong agitation effect of ultrasonic cavitation favour simultaneous gelatinization of starch and formation of ZnO nanoparticles with gelatinized starch granules directing the growth of nanoparticles by binding on its surface. Starch is normally composed of amylose and amylopectin in a ratio of 30:70 or 20:80 with amylopectin found in larger amount. Due to continuous ultrasonication with strong agitation effect of ultrasonic cavitation phenomenon, starch granules swell, there is loss of both crystallinity and double helical order of amylopectin with the leaching of amylose into the reaction mixture [33–36]. Amylose has the ability to form coils therefore it can direct the ZnO growth units to take the shape of nanorods while still in the sonochemical bath. However when the reaction mixture was taken out of sonochemical bath cooling takes place and gel can form with free amylose molecules losing energy as temperature decreases and forming hydrogen bonds with each other as well with amylopectin entrapping ZnO nuclei in the process along with them and forming units of nanorods. These nanorods finally clump together with the help of amylose which normally has glue like property of holding starch gels together and finally giving shape of flower. The distinct advantage of using starch is that it avoids agglomeration of nanostructures. The experimental results show that in formation of flower-like ZnO nanostructure, the role of starch should be taken into consideration. In the absence of starch, there is agglomeration of ZnO spherical nanoparticles (Fig. 3C) and in presence of starch flower like morphology is obtained using sonochemical method (Fig. 3A and B). The mechanism of formation of ZnO nanostructures takes the consideration the ionic species formed from water molecules by absorption of ultrasound energy. The reaction steps taking place inside sonochemical bath can be summarised as follows:
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H2 O $ Hþ þ OH
ð1Þ
NaOH $ Naþ þ OH
ð2Þ
ZnðCH3 COOÞ2 $ Zn2þ þ 2CH3 COO
ð3Þ
Zn2þ þ 4OH $ ½ZnðOHÞ4 2
ð4Þ
2½ZnðOHÞ4 2 $ 2ZnO þ 4H2 O þ O2
ð5Þ
3.6. Photoluminescence study Fig. 7 illustrates the photoluminescence spectrum of flower-like ZnO nanostructure synthesized by starch-assisted sonochemical method at excitation wavelength 325 nm. Several emission bands, including band-edge emission at 393 nm (3.162 eV) and defect related ultraviolet emission at 416 nm (2.98 eV), blue emission at 442 nm (2.81 eV), blue-green emission 482 nm (2.57 eV) and green emission 526 nm (2.36 eV) were observed. It is commonly considered that band-edge emission at 393 nm should be attributed to the recombination of excitons [37]. In general, the visible emission in ZnO is attributed to different intrinsic defects such as oxygen vacancies (Vo), zinc vacancies (Vzn), oxygen
Eqs. (1)–(3) show formation of primary ions which have been formed by dissociation of water, sodium hydroxide and zinc acetate. Eq. (4) shows formation of [Zn(OH)4]2 which further decomposes to give ZnO spherical nanoparticles. In the meantime, the starch granules (in the presence of starch in the solution) undergo following process:
Starch granules
hydration of starch þ leaching of amylose
ð6Þ
ZnO nanorods
AmyloseðglueÞ þ ZnO nuclei
ð7Þ
Clumping of nanorods into flower
AmyloseðglueÞ þ ZnO nanorods
Eqs. (6)–(8) indicated that in presence of water and ultrasonic irradiation, starch undergoes gelatinization [33–36] process during which there is hydration of starch granules, leaching out of amylose and some granules get collapsed. After 4 h of ultrasonication temperature of solution which initially was 27 °C reaches 57 °C which favours gelatinization. Amylose which comes out has a gluing property it binds onto ZnO nuclei giving it shape of a nanorod as amylose itself has tendency to form coil. When reaction mixture gets cooled these amylose units along with amylopectin and collapsed starch granules form hydrogen bonds among them gluing nanorods together into a spherical structure with nanorods projecting out, giving the appearance of flower as represented in Fig. 6.
ð8Þ
interstitials (Oi), zinc interstitials (Zni) and oxygen antisites (Ozn). The origin of violet emission centred at 2.987 eV (416 nm) is ascribed to an electron transition from a deep donor level of neutral Zni to the valence band [38]. Therefore, the deep donor level of the Zni is suggested to locate at 0.39 eV below the conduction band in this study because the violet photoluminescence appears at 2.987 eV. A blue emission centred at 2.57 eV (482 nm) is due to a radiative transition of an electron from the deep donor level of Zni to an acceptor level of neutral Vzn [39]. In this study, it can be estimated that the acceptor level of Vzn locates at 0.41 eV above the valence band. Another blue emission was reported to appear at around 2.81 eV (442 nm) [40]. This emission may be related to surface defects of ZnO nanostructure, although the detailed
Ultrasonic waves
Amylose chain with some
Uncollapsed starch granules in solution
Uncollapsed granules.
ZnO nuclei
Amylose chains
Clumping of amylose chains on ZnO nuclei
On cooling
Clumped nanorods
Amylose chains
Flower-like morphology
Fig. 6. Diagramatic representation of flower-like ZnO nanostructure by starch-assisted sonochemical method.
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crystal structure. Photoluminescence spectrum of flower-like ZnO nanostructure revealed several emission bands, including bandedge emission at 393 nm (3.162 eV) and defect related ultraviolet emission at 416 nm (2.98 eV), blue emission at 442 nm (2.81 eV), blue-green emission 482 nm (2.57 eV) and green emission 526 nm (2.36 eV). In the end, the prepared flower-like ZnO nanostructures are pure and water soluble and there are lots of hydroxyl groups attached to nanostructures surface which in future can be functionalized. Acknowledgments The authors are very grateful and also wish to express their gratitude to all the scientific members of Nanophosphor Application Centre, University of Allahabad, Allahabad, India. This work was financially supported by DST, India. References
Fig. 7. Photoluminescence spectrum of flower-like and spherical-like ZnO nanostructures synthesized by sonochemical method in presence of 0.025 g starch and in absence of starch respectively at excitation 325 nm.
mechanism for blue emission at 442 nm has been not clarified. The green emission 526 nm (2.363 eV) are attributed to radiative transitions from the deep donor level of Zni to acceptor levels caused by singly ionized V Zn [41]. Because the peak positions of the green emission are 2.363 eV in this study, the acceptor level of the ionized V Zn locate at 0.62 eV above the valence band. However, spherical-like ZnO nanostructures has emission peak at 395 nm, 418 nm, 442 nm, 483 nm and 528 nm. It is noticeable that there is small shift in emission peak positions in spherical-like ZnO nanostructures synthesized in absence of starch as compared to flower-like ZnO nanostructures. The shift in emission peak position by starch as surfactant, indicate that the layer of starch result the change in morphology, size and defect content in ZnO nanostructures. However, recently, Y.L. Wu et al. [42] have shown no shift in U.V. emission peak position of ZnO nanoparticles using capping agents such as tetraethylorthosilicate, mercaptosuccinic acid, 3-mercaptopropyl trimethoxysilane, polyvinylpyrrolidone, 3-aminopropyl trimethoxysilane, the capping layers did not result in size changes or increased surface defects. It is also observed that the PL intensity of spherical-like ZnO nanoparticles synthesized in absence of starch has greater intensity as compared to flower-like ZnO nanostructures synthesized in presence of starch. The decrease in PL intensity in flower-like ZnO nanostructures is due to presence of surface absorbed starch on ZnO flower-like nanostructures, as it is confirmed from the FTIR study (Fig. 4). 4. Conclusion ZnO nanostructures viz. flower-like and spherical have been synthesized by sonochemical method with and without starch to investigate the influence of starch on morphology. In presence of starch and in absence of ultrasonic wave, there is no formation of ZnO crystal structure, although there is formation of crystal structure in presence of starch and ultrasonic wave, which confirmed that ultrasonic waves play an important role in formation of ZnO
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