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Starch assisted growth of dumbbell-shaped ZnO microstructures
Journal of Alloys and Compounds 646 (2015) 238e242
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Starch assisted growth of dumbbell-shaped ZnO microstructures V. Baranwal a, *, Abeer Zahra b, Prashant K. Singh a, Avinash C. Pandey a a b
Nanotechnology Application Centre, University of Allahabad, Allahabad 21002, India Department of Physics, Integral University, Lucknow 226026, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2015 Received in revised form 31 May 2015 Accepted 1 June 2015 Available online 10 June 2015
We present an experimental study on evolution of dumbbell-shaped ZnO microstructures. Structure, shape, size and optical properties were monitored by means of scanning electron microscopy, x-ray diffraction, and photoluminescence spectroscopy, respectively. Our results show that a crystalline phase of ZnO is formed. A uniform distribution of randomly oriented dumbbell-shaped ZnO microstructures is observed. Near band edge as well as deep level visible emissions confirmed that there are intrinsic defects present in the system. Emissions extending from UV region to visible region show that these microstructures are good quality optical material which can be used in photocatalytic field. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO micro-rods Dumbbell-shaped microstructures Starch assisted growth
1. Introduction ZnO is recognised as one of the most important material for applications in electronic and optoelectronic devices such as solar cells [1e4], blue light emitting diodes [5e7], piezoelectric transducers [8e10], chemical and gas sensors [11], bio-sensors [12], photo catalyst [13] etc. ZnO is a semiconductor material belonging to IIeVI group, has large excitation binding energy (60 meV) and direct wide band gap of 3.3 eV. Due to its remarkable physical properties and versatile applications, a substantial amount of efforts has been employed for synthesis, device fabrication, and performance improvement of ZnO nanostructures and microstructures for miniaturization of semiconductor devices. Controlling the size, morphology, and dimension of these ZnO crystals is a great challenge to realize the design of the novel functional devices. These parameters are the key element for determining the electronic and optical performance of ZnO as they can be modulated by varying the size and morphology. In recent years synthesis of many kind of nano and microstructures of ZnO with different shapes including wires [14], rods [15], tubes [16,17], needles [18], columns [19], towers [20], belts [21], helices [22], branches [23], combs [24], rings [25] have been of particular interest because of their promising applications in semiconducting devices and functional materials. For the fabrication of ZnO nano or microstructures mainly two type of approaches are pursued, top down and bottom up
* Corresponding author. E-mail address: [email protected] (V. Baranwal). http://dx.doi.org/10.1016/j.jallcom.2015.06.007 0925-8388/© 2015 Elsevier B.V. All rights reserved.
approach. Top down approach consists of deposition of films followed by restructuring by lithography and etching, ion implantation techniques. This approach is not very cost effective as well time consuming. Bottom up approach includes chemical synthesis, laser trapping, self-assembly, colloidal aggregation where small building blocks are produced and assembled into larger structures. A number of methods for the synthesis of ZnO nano- and micro structures have been established such as metal-organic chemical vapour deposition (MOCVD) [26], pulsed laser deposition [27], ion implantation [28], vapour transport process [29], molecular beam epitaxy [30], thermal evaporation [31] electrode deposition [32] vapoureliquidesolid [33,34] and flame transport synthesis [35e38]. Fabrication of a self-reporting ZnO tetrapod/elastomer composite material and substantial increase of the peeling strength between two non-adhesive polymers by the addition of concave tetrapodal ZnO crystals at the interface have been reported by Jin et al. [39]. Hydrothermal, solegel, co-precipitation techniques are widely used chemical synthesis root for ZnO nanostructures. ZnO nanorods were synthesized using aqueous solution method on Silicon substrate by Shraddha et al. for the phenol detection [40]. Zhao et al. has observed that flower like ZnO microstructures synthesized by sol gel assisted hydrothermal method were having superior photocatalytic activity as compared to other ZnO microstructures [13]. Large scale uniform dumbbell shaped ZnO nanorods with 300 nm diameter and length of 1.5 mm were prepared by Rajkumar et al. [41]. Facile hydrothermal method was used Wang et al. for the synthesis of dumbbell shaped ZnO microstructures [42]. Lian et al. prepared hexagonal ZnO micro-cups and rings via a template free hydrothermal synthetic method [43]. Bunches of ZnO
V. Baranwal et al. / Journal of Alloys and Compounds 646 (2015) 238e242
239
nanowires have been prepared by Raghvendra et al. by CTABassisted hydrothermal process [44]. Needle like ZnO nanostructure synthesized by organic free hydrothermal process and the mechanism is discussed in detail [18]. Li et al. presented the growth of two-dimensional ZnO nanoflakes on the stainless steel through low temperature hydrothermal process [45]. Dumbbell-shaped ZnO recently has received some attention due to its excellent gas sensing and optical properties [46]. In this paper our focus was to synthesize dumbbell-shaped ZnO microstructures with the assistance of surfactant starch via hydrothermal route. We found that this way of synthesis of microstructures is an effective, convenient, environment friendly, inexpensive and efficient route with low temperature processing and high yield. On the basis of results of XRD, SEM, PL spectra a growth mechanism is proposed for the formation of dumbbell-shaped ZnO microstructures.
2. Experimental procedure 2.1. Chemicals The zinc acetate dihydrate (Zn(CH3COO)2$2H2O) (98%), potassium hydroxide (KOH) and starch were procured from Merck Limited, India. Starch is used as surfactant. All chemicals used were of analytical grade and used without any further purification.
2.2. Sample preparation In a typical hydrothermal synthesis process, aqueous solution of zinc acetate dihydrate was prepared under vigorous stirring. After 10 min stirring, 1 g of starch was added into the above solution. One solution was used without any surfactant. After the dissolution of starch, 0.2 M KOH aqueous solution was introduced into the above aqueous solutions, resulting in a white aqueous solution for which the pH value was maintained at 12.0. The two prepared solutions were transferred into stainless steel autoclave, sealed and maintained at temperature 110 C and 15 psi pressure for 12 h and then cooled down to room temperature naturally. The resulting solid products were centrifuged, washed with distilled water and absolute ethanol several time to remove the ions possibly remaining in the final products, and finally dried at 70 C in oven. Final white solid products were obtained.
2.3. Characterization The synthesized products were examined using scanning electron microscopy (SEM), images were taken on ZEISS EVO Series Scanning Electron Microscope Model EVO15 to find out the surface morphology and size. The crystalline structure of white products were characterized by X-ray diffraction (XRD) on Rigaku D/max2200 PC diffractometer operated at 40 kV/40 mA, using CuKa radiation with wavelength of 1.54 Å in the wide angle region from 25 to 75 . Photoluminescence (PL) spectroscopy studies were carried out using PerkineElmer LS55 spectrophotometer at room temperature with a Xe lamp used as the source of light. Initially PL excitation was done to get the wavelength for the maximum emission from the sample, which is ~325 nm. We have used 325 nm as an excitation wavelength for getting the emission from the sample.
Fig. 1. Scanning electron microscope images of sample A (no surfactant) with low magnification (a), high magnification (b); sample B (starch assisted) with low magnification (c) and high magnification (d).
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3. Results and discussion The morphology of prepared ZnO powder was characterized by scanning electron microscope. Fig. 1 shows the SEM images of sample A and sample B with different magnification. Fig. 1a and b shows the low magnification (500) and high magnification (5000) SEM images of sample A. The high resolution image (Fig. 1b) shows that the sample is typical hexagonal structure with size in the micron range. The as-prepared micro-rods were 6e12 mm in length and 1e3 mm in diameter. From the low magnification and high magnification images shown in Fig. 1c and d, it is clear that the prepared ZnO products are mainly composed of dumbbell-shaped microstructures. The length of these dumbbellshaped microstructures is about 5e13 mm, the diameters of the two ends and the middle part are about 2e5 mm and 1.5e6 mm, respectively. High magnification SEM images in Fig. 1d show that the ZnO dumbbells have a rough surface and spherical shape at both the ends. The spherical structure at both ends is a difference from the earlier reported dumbbell shaped ZnO microstructures. We propose that the formation of dumbbell-shaped ZnO microrods occurs when hexagonally grown micro-rods joined together. A schematic representation is shown in Fig. 2 to describe the proposed model. Initially randomly oriented hexagonally grown micro-rods formed. It is well known that the hexagonal ZnO crystal has both polar and nonpolar faces. Polar faces with surface dipoles are thermodynamically less stable than nonpolar faces; often undergo rearrangement to reduce surface energy. At the early stage of the reaction, ZnO2 2 ions are likely adsorbed on the polar face of the (0001) surface, resulting in the faster growth along the [0001] direction resulting into the formation of solid ZnO cones (Fig. 2a). After the completion of reaction the sharp edges or these cones of the two hexagonally grown micro-rods attract each other to form dumbbell shaped microstructure (Fig. 2b and c). Moreover after
Fig. 3. XRD pattern of Sample A (no surfactant) and Sample B (starch assisted), growth of ZnO by hydrothermal process.
insertion of starch in the reaction mechanism, starch will attach to the negative polar face of the (0001) surface resulting in spherical shape at the both ends of dumbbell-shaped ZnO microstructures (Fig. 2d). Fig. 3 shows the XRD pattern of ZnO product fabricated by without surfactant assisted (Sample A) as well as starch assisted (Sample B) hydrothermal process. The peaks appeared are in good agreement with JCPDS file number 36e1451 for ZnO and can be indexed as hexagonal wurtzite structure of ZnO with lattice parameter a ¼ 3.25 Å and c ¼ 5.21 Å. We have not observed any other diffraction peak which indicates that high quality ZnO phase
Fig. 2. Schematic diagram for the formation of dumbbell shaped ZnO microstructures.
V. Baranwal et al. / Journal of Alloys and Compounds 646 (2015) 238e242
Fig. 4. Photoluminescence spectra of sample A (ZnO grown hydrothermal process without surfactant) and Sample B (Starch assisted grown ZnO by hydrothermal process).
is formed. The high intensity and narrow width of the diffraction peaks exhibit an excellent crystalline structure. Photoluminescence measurement is an effective tool to study the electronic structure, optical and photochemical properties of semiconductor material, by which information about defects present in the system can be extracted. Fig. 4 illustrates the room temperature photoluminescence spectra of ZnO micro rods (sample A) and ZnO dumbbell shaped microstructures (sample B) grown by hydrothermal process excited by wavelength of 325 nm. The PL spectrum of both samples exhibits peaks centred at about 397, 421, 485, 529 nm. A strong near band-edge (UV) emission 397 nm (3.12 eV) and deep level visible emissions at 421 nm (2.94 eV, violet emission), 485 nm (2.55 eV, blueegreen band), and 529 nm (2.34 eV, green emission) are observed. UV-emission is band edge emission resulting from the recombination of free excitons. Comparing the band edge emission of ZnO nanoparticles a red shift is observed from 368 nm to 397 nm. This behaviour may be explained by the effect of increase in size of ZnO particles. In general redshift is observed when the particle size increase from a particular value. The visible emission was actually ascribed to various intrinsic defects produced during the synthesis of ZnO, which are mainly
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zinc vacancy, oxygen vacancy (Vo), zinc interstitials, oxygen interstitials, oxygen antisites (OZn). The origin of violet emission corresponding to 421 nm (2.94 eV) is due to electron transition from a deep donor level of neutral Zn interstitial to the valance band. A blueegreen band emission cantered at 485 nm (2.55 eV) may be correlated to a singly charged oxygen vacancy and a charge state of specific defect [47]. The origin of green emission at 529 nm (2.34 eV) is generally attributed to the singly ionized oxygen vacancy and the emission results from the radiative recombination of photogenerated hole with electron occupying the oxygen vacancy [48]. It has been earlier reported that hexagonal ZnO microcrystals exhibit whispering gallery modes (WGM) [49]. These additional resonance peaks near the broad green emission band may be due to the presence of WGM resonances. The schematic illustration of the band diagram for these emission peaks, ~397 nm, ~421 nm, ~485 nm, ~529 nm is shown in Fig. 5. In general as prepared dumbbell shaped ZnO microcrystals are very good optical material and may find applications in photocatalytic field. 4. Conclusion We have successfully synthesized ZnO micro rods as well as dumbbell-shaped ZnO microstructures by hydrothermal process. The phase of ZnO is hexagonal which is confirmed by XRD measurement. The size of dumbbell shaped ZnO microstructure synthesized by starch assisted hydrothermal process varied from 2 to 5 mm in diameter and 5e13 mm in length. The size of micro rods was 6e12 mm in length and 1e3 mm in diameter and both the end of these micro rods were hexagonal. Optical properties were studied by photoluminescence spectroscopy and UVevis absorption spectroscopy. A strong UV-emission as well as deep level visible emissions was observed in photoluminescence spectra. Visible emissions were attributed to the intrinsic defects present in the system. Acknowledgement We wish to acknowledge the support from the DST, India (SR/ NM/NS-87/2008) for financial support to NAC under Nano-mission scheme. One of the authors VB would like to acknowledge for the financial support from DST, India for HFIBF facility and Young Scientist project. The authors are thankful to Centre for interdisciplinary research facility, MNNIT, Allahabad, for SEM and PL measurements. References
Fig. 5. Schematic band diagram for the emission peaks from the ZnO particles.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Starch assisted growth of dumbbell-shaped ZnO microstructures V. Baranwal a, *, Abeer Zahra b, Prashant K. Singh a, Avinash C. Pandey a a b
Nanotechnology Application Centre, University of Allahabad, Allahabad 21002, India Department of Physics, Integral University, Lucknow 226026, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2015 Received in revised form 31 May 2015 Accepted 1 June 2015 Available online 10 June 2015
We present an experimental study on evolution of dumbbell-shaped ZnO microstructures. Structure, shape, size and optical properties were monitored by means of scanning electron microscopy, x-ray diffraction, and photoluminescence spectroscopy, respectively. Our results show that a crystalline phase of ZnO is formed. A uniform distribution of randomly oriented dumbbell-shaped ZnO microstructures is observed. Near band edge as well as deep level visible emissions confirmed that there are intrinsic defects present in the system. Emissions extending from UV region to visible region show that these microstructures are good quality optical material which can be used in photocatalytic field. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO micro-rods Dumbbell-shaped microstructures Starch assisted growth
1. Introduction ZnO is recognised as one of the most important material for applications in electronic and optoelectronic devices such as solar cells [1e4], blue light emitting diodes [5e7], piezoelectric transducers [8e10], chemical and gas sensors [11], bio-sensors [12], photo catalyst [13] etc. ZnO is a semiconductor material belonging to IIeVI group, has large excitation binding energy (60 meV) and direct wide band gap of 3.3 eV. Due to its remarkable physical properties and versatile applications, a substantial amount of efforts has been employed for synthesis, device fabrication, and performance improvement of ZnO nanostructures and microstructures for miniaturization of semiconductor devices. Controlling the size, morphology, and dimension of these ZnO crystals is a great challenge to realize the design of the novel functional devices. These parameters are the key element for determining the electronic and optical performance of ZnO as they can be modulated by varying the size and morphology. In recent years synthesis of many kind of nano and microstructures of ZnO with different shapes including wires [14], rods [15], tubes [16,17], needles [18], columns [19], towers [20], belts [21], helices [22], branches [23], combs [24], rings [25] have been of particular interest because of their promising applications in semiconducting devices and functional materials. For the fabrication of ZnO nano or microstructures mainly two type of approaches are pursued, top down and bottom up
* Corresponding author. E-mail address: [email protected] (V. Baranwal). http://dx.doi.org/10.1016/j.jallcom.2015.06.007 0925-8388/© 2015 Elsevier B.V. All rights reserved.
approach. Top down approach consists of deposition of films followed by restructuring by lithography and etching, ion implantation techniques. This approach is not very cost effective as well time consuming. Bottom up approach includes chemical synthesis, laser trapping, self-assembly, colloidal aggregation where small building blocks are produced and assembled into larger structures. A number of methods for the synthesis of ZnO nano- and micro structures have been established such as metal-organic chemical vapour deposition (MOCVD) [26], pulsed laser deposition [27], ion implantation [28], vapour transport process [29], molecular beam epitaxy [30], thermal evaporation [31] electrode deposition [32] vapoureliquidesolid [33,34] and flame transport synthesis [35e38]. Fabrication of a self-reporting ZnO tetrapod/elastomer composite material and substantial increase of the peeling strength between two non-adhesive polymers by the addition of concave tetrapodal ZnO crystals at the interface have been reported by Jin et al. [39]. Hydrothermal, solegel, co-precipitation techniques are widely used chemical synthesis root for ZnO nanostructures. ZnO nanorods were synthesized using aqueous solution method on Silicon substrate by Shraddha et al. for the phenol detection [40]. Zhao et al. has observed that flower like ZnO microstructures synthesized by sol gel assisted hydrothermal method were having superior photocatalytic activity as compared to other ZnO microstructures [13]. Large scale uniform dumbbell shaped ZnO nanorods with 300 nm diameter and length of 1.5 mm were prepared by Rajkumar et al. [41]. Facile hydrothermal method was used Wang et al. for the synthesis of dumbbell shaped ZnO microstructures [42]. Lian et al. prepared hexagonal ZnO micro-cups and rings via a template free hydrothermal synthetic method [43]. Bunches of ZnO
V. Baranwal et al. / Journal of Alloys and Compounds 646 (2015) 238e242
239
nanowires have been prepared by Raghvendra et al. by CTABassisted hydrothermal process [44]. Needle like ZnO nanostructure synthesized by organic free hydrothermal process and the mechanism is discussed in detail [18]. Li et al. presented the growth of two-dimensional ZnO nanoflakes on the stainless steel through low temperature hydrothermal process [45]. Dumbbell-shaped ZnO recently has received some attention due to its excellent gas sensing and optical properties [46]. In this paper our focus was to synthesize dumbbell-shaped ZnO microstructures with the assistance of surfactant starch via hydrothermal route. We found that this way of synthesis of microstructures is an effective, convenient, environment friendly, inexpensive and efficient route with low temperature processing and high yield. On the basis of results of XRD, SEM, PL spectra a growth mechanism is proposed for the formation of dumbbell-shaped ZnO microstructures.
2. Experimental procedure 2.1. Chemicals The zinc acetate dihydrate (Zn(CH3COO)2$2H2O) (98%), potassium hydroxide (KOH) and starch were procured from Merck Limited, India. Starch is used as surfactant. All chemicals used were of analytical grade and used without any further purification.
2.2. Sample preparation In a typical hydrothermal synthesis process, aqueous solution of zinc acetate dihydrate was prepared under vigorous stirring. After 10 min stirring, 1 g of starch was added into the above solution. One solution was used without any surfactant. After the dissolution of starch, 0.2 M KOH aqueous solution was introduced into the above aqueous solutions, resulting in a white aqueous solution for which the pH value was maintained at 12.0. The two prepared solutions were transferred into stainless steel autoclave, sealed and maintained at temperature 110 C and 15 psi pressure for 12 h and then cooled down to room temperature naturally. The resulting solid products were centrifuged, washed with distilled water and absolute ethanol several time to remove the ions possibly remaining in the final products, and finally dried at 70 C in oven. Final white solid products were obtained.
2.3. Characterization The synthesized products were examined using scanning electron microscopy (SEM), images were taken on ZEISS EVO Series Scanning Electron Microscope Model EVO15 to find out the surface morphology and size. The crystalline structure of white products were characterized by X-ray diffraction (XRD) on Rigaku D/max2200 PC diffractometer operated at 40 kV/40 mA, using CuKa radiation with wavelength of 1.54 Å in the wide angle region from 25 to 75 . Photoluminescence (PL) spectroscopy studies were carried out using PerkineElmer LS55 spectrophotometer at room temperature with a Xe lamp used as the source of light. Initially PL excitation was done to get the wavelength for the maximum emission from the sample, which is ~325 nm. We have used 325 nm as an excitation wavelength for getting the emission from the sample.
Fig. 1. Scanning electron microscope images of sample A (no surfactant) with low magnification (a), high magnification (b); sample B (starch assisted) with low magnification (c) and high magnification (d).
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3. Results and discussion The morphology of prepared ZnO powder was characterized by scanning electron microscope. Fig. 1 shows the SEM images of sample A and sample B with different magnification. Fig. 1a and b shows the low magnification (500) and high magnification (5000) SEM images of sample A. The high resolution image (Fig. 1b) shows that the sample is typical hexagonal structure with size in the micron range. The as-prepared micro-rods were 6e12 mm in length and 1e3 mm in diameter. From the low magnification and high magnification images shown in Fig. 1c and d, it is clear that the prepared ZnO products are mainly composed of dumbbell-shaped microstructures. The length of these dumbbellshaped microstructures is about 5e13 mm, the diameters of the two ends and the middle part are about 2e5 mm and 1.5e6 mm, respectively. High magnification SEM images in Fig. 1d show that the ZnO dumbbells have a rough surface and spherical shape at both the ends. The spherical structure at both ends is a difference from the earlier reported dumbbell shaped ZnO microstructures. We propose that the formation of dumbbell-shaped ZnO microrods occurs when hexagonally grown micro-rods joined together. A schematic representation is shown in Fig. 2 to describe the proposed model. Initially randomly oriented hexagonally grown micro-rods formed. It is well known that the hexagonal ZnO crystal has both polar and nonpolar faces. Polar faces with surface dipoles are thermodynamically less stable than nonpolar faces; often undergo rearrangement to reduce surface energy. At the early stage of the reaction, ZnO2 2 ions are likely adsorbed on the polar face of the (0001) surface, resulting in the faster growth along the [0001] direction resulting into the formation of solid ZnO cones (Fig. 2a). After the completion of reaction the sharp edges or these cones of the two hexagonally grown micro-rods attract each other to form dumbbell shaped microstructure (Fig. 2b and c). Moreover after
Fig. 3. XRD pattern of Sample A (no surfactant) and Sample B (starch assisted), growth of ZnO by hydrothermal process.
insertion of starch in the reaction mechanism, starch will attach to the negative polar face of the (0001) surface resulting in spherical shape at the both ends of dumbbell-shaped ZnO microstructures (Fig. 2d). Fig. 3 shows the XRD pattern of ZnO product fabricated by without surfactant assisted (Sample A) as well as starch assisted (Sample B) hydrothermal process. The peaks appeared are in good agreement with JCPDS file number 36e1451 for ZnO and can be indexed as hexagonal wurtzite structure of ZnO with lattice parameter a ¼ 3.25 Å and c ¼ 5.21 Å. We have not observed any other diffraction peak which indicates that high quality ZnO phase
Fig. 2. Schematic diagram for the formation of dumbbell shaped ZnO microstructures.
V. Baranwal et al. / Journal of Alloys and Compounds 646 (2015) 238e242
Fig. 4. Photoluminescence spectra of sample A (ZnO grown hydrothermal process without surfactant) and Sample B (Starch assisted grown ZnO by hydrothermal process).
is formed. The high intensity and narrow width of the diffraction peaks exhibit an excellent crystalline structure. Photoluminescence measurement is an effective tool to study the electronic structure, optical and photochemical properties of semiconductor material, by which information about defects present in the system can be extracted. Fig. 4 illustrates the room temperature photoluminescence spectra of ZnO micro rods (sample A) and ZnO dumbbell shaped microstructures (sample B) grown by hydrothermal process excited by wavelength of 325 nm. The PL spectrum of both samples exhibits peaks centred at about 397, 421, 485, 529 nm. A strong near band-edge (UV) emission 397 nm (3.12 eV) and deep level visible emissions at 421 nm (2.94 eV, violet emission), 485 nm (2.55 eV, blueegreen band), and 529 nm (2.34 eV, green emission) are observed. UV-emission is band edge emission resulting from the recombination of free excitons. Comparing the band edge emission of ZnO nanoparticles a red shift is observed from 368 nm to 397 nm. This behaviour may be explained by the effect of increase in size of ZnO particles. In general redshift is observed when the particle size increase from a particular value. The visible emission was actually ascribed to various intrinsic defects produced during the synthesis of ZnO, which are mainly
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zinc vacancy, oxygen vacancy (Vo), zinc interstitials, oxygen interstitials, oxygen antisites (OZn). The origin of violet emission corresponding to 421 nm (2.94 eV) is due to electron transition from a deep donor level of neutral Zn interstitial to the valance band. A blueegreen band emission cantered at 485 nm (2.55 eV) may be correlated to a singly charged oxygen vacancy and a charge state of specific defect [47]. The origin of green emission at 529 nm (2.34 eV) is generally attributed to the singly ionized oxygen vacancy and the emission results from the radiative recombination of photogenerated hole with electron occupying the oxygen vacancy [48]. It has been earlier reported that hexagonal ZnO microcrystals exhibit whispering gallery modes (WGM) [49]. These additional resonance peaks near the broad green emission band may be due to the presence of WGM resonances. The schematic illustration of the band diagram for these emission peaks, ~397 nm, ~421 nm, ~485 nm, ~529 nm is shown in Fig. 5. In general as prepared dumbbell shaped ZnO microcrystals are very good optical material and may find applications in photocatalytic field. 4. Conclusion We have successfully synthesized ZnO micro rods as well as dumbbell-shaped ZnO microstructures by hydrothermal process. The phase of ZnO is hexagonal which is confirmed by XRD measurement. The size of dumbbell shaped ZnO microstructure synthesized by starch assisted hydrothermal process varied from 2 to 5 mm in diameter and 5e13 mm in length. The size of micro rods was 6e12 mm in length and 1e3 mm in diameter and both the end of these micro rods were hexagonal. Optical properties were studied by photoluminescence spectroscopy and UVevis absorption spectroscopy. A strong UV-emission as well as deep level visible emissions was observed in photoluminescence spectra. Visible emissions were attributed to the intrinsic defects present in the system. Acknowledgement We wish to acknowledge the support from the DST, India (SR/ NM/NS-87/2008) for financial support to NAC under Nano-mission scheme. One of the authors VB would like to acknowledge for the financial support from DST, India for HFIBF facility and Young Scientist project. The authors are thankful to Centre for interdisciplinary research facility, MNNIT, Allahabad, for SEM and PL measurements. References
Fig. 5. Schematic band diagram for the emission peaks from the ZnO particles.
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