(Fe) Doped Zinc Oxide (ZnO) Nanoparticles

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*E-mail: sampac.2008@gmail.com; [email protected]. ABSTRACT ... Spectrochem Pvt. Ltd., Mumbai (India); Molecular weight 353.18) as dopant, .... OH• radical being very powerful oxidant attacks Zn- and Fe-acetylacetonate ...
Sonochemical Synthesis and Characterization of Iron (Fe) Doped Zinc Oxide (ZnO) Nanoparticles Anirban Roy1, Sobhan Ghosh2, Saikat Maitra3 and Sampa Chakrabarti1,4* 1

Department of Chemical Engineering, University of Calcutta, 92, Acharya P.C. Road, Kolkata–700 009, India 2 Managing Innovation, Sector 14, Faridabad–121 007, India 3 Government College of Engineering & Ceramic Technology, 73, Abinash Chandra Banerjee Lane, Kolkata–700 010, India 4 Centre for Research in Nanoscience & Nanotechnology, University of Calcutta, India *E-mail: [email protected]; [email protected]

ABSTRACT Fe-doped ZnO nanoparticles have been prepared by a green sonochemical method using zinc acetyl acetonate as the primary precursor and ferric acetyl acetonate as the doping agent. The synthesis was carried out at room temperature, atmospheric pressure and neutral pH. Surface morphology, crystal structure and elemental composition are analyzed by means of FESEM, XRD and EDX respectively. DLS was carried out to measure particle size distribution and uniformity of synthesized nanoparticles. Band gap energy was determined by UV-visible spectroscopy and it was considerably less (2.69 eV) than undoped ZnO (3.03 eV). EDX results confirmed the presence of iron and SQUID results showed magnetization. Hence doping made ZnO nanoparticles more active catalyst in visible region and imparted ferromagnetic properties. Keywords: Fe-doped ZnO Nanoparticles, Sonochemical Synthesis, Physical and Optical Properties.

INTRODUCTION Semiconductor nanoparticles have been widely investigated in the past two decades, particularly due to their size-dependent optical, electrical and magnetic properties (Özgür et al., 2005; Mandal et al., 2006) and thereby their use in field emission displays, photocatalysis, phosphors, spintronics or cosmetics (Marathe et al., 2006; Liu et al., 2005; Bhargava et al., 2002). ZnO is a low cost, biocompatible and environmentally safe semiconductor having a band gap of 3.37 eV and a large excitation binding energy of 60 meV (Pearton et al., 2003). For application, its physical properties should be engineered by doping with transition metal elements. ZnO doped with Transition Metals (TM) NANOSPECTRUM: A Current Scenario, pp. 55–61 (2015) S. Chakrabarti, P. Mukherjee, G. Khan, A. Adhikari, P. Patra & J. Bal (eds.) ISBN: 978-93-85926-06-8 • Published by Allied Publishers Pvt. Ltd.

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like Fe, Co, or Ni has favorable magnetic, optical and electronic properties required for spintronic materials and optoelectronic devices (Yilmaz et al., 2012). Iron is the most popular dopant for ZnO owing to its compatibility with Zn2+ ion as per Hume-Rothery rule, due to its abundance and low cost and its non-toxicity, which maintains the biocompatibility of ZnO particles (Smith, 1993). In recent years, several reports have been published on preparation of Fe-doped ZnO nanomaterials by thermal (Singhal et al., 2008), co-precipitation (Saleh et al., 2012) or spray pyrolysis (Salaken et al., 2013) methods. These methods employ high temperatures and complicated equipment or protocols. Therefore, a green and template-free method under mild conditions should be used. Ultrasonic irradiation method has been proved extremely useful in preparation of various nanomaterials (Gedanken, 2004; Yadav et al., 2008). With irradiation of ultrasound into a liquid, the formation, growth, and implosive collapse of bubbles resulting from acoustic cavitation create a very high temperature (up to 5000 K) and pressure (up to 1800 atm). Due to the high temperature and pressure produced during the cavitation, crystallization of semiconductors is facilitated. The process is simple and fast requiring no extreme conditions. Only few reports are there where doped-ZnO nanoparticles have been prepared sonochemically (Chakma et al., 2013; Khataee et al., 2015). Here we report the synthesis of Fe doped ZnO nanoparticles from organic precursors using ultrasonic irradiation method at room temperature. The structural characterizations were determined by analyzing X-Ray Diffraction (XRD) data and the optical characterization was done by UV-visible spectroscopy. DLS was performed to measure particle size distribution and uniformity of synthesized nanoparticles and EDX for confirming the presence of Fe into the prepared nanoparticles. SQUID data indicated magnetic properties.

MATERIALS AND METHODS Chemicals (a) Zinc acetyl acetonate, commonly expressed as Zn(acac)2, Zn(C5H7O2)2 (purchased from MP Biomedicals, LLC, France; Molecular weight 263.6082) as primary precursor, (b) Ferric acetyl acetonate, expressed as Fe(acac)3, Fe(C5H7O2)3 (purchased from Spectrochem Pvt. Ltd., Mumbai (India); Molecular weight 353.18) as dopant, (c) Ethanol C2H5OH and (d) single distilled water of pH 6.8 and conductivity 7–11 micromhos as solvent. All the reagents used in the present investigation, were of analytical grade. Preparation of Zinc Oxide Nanoparticles 0.95 mmol Zn(C5H7O2)2 and 0.05 mmol Fe(C5H7O2)3 were taken with 20 ml ethanol and shaken thoroughly until the solutes were dissolved completely. After that 210 ml of water was added and the mixture was subjected to sonication using 30±3 KHz frequency ultrasonic probe (Trans-O-Sonic, Model: D-120/P) for 40 seconds using 1 sec pulse mode. After sonication, the content became slightly turbid. This turbid solution was

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poured onto a petri dish and dried in a vacuum oven at 700–740 mm Hg vacuum at 65–100oC temperature to drive off the solvents. The dry powder obtained was repeatedly washed with a mixture of 1:1 distilled water and ethanol, centrifuged using a REMI R-24 research centrifuge at 8000–10000 rpm, followed by drying again under vacuum till constant weight and stored. The yield was approximately 30%.

Characterization of the Nanoparticles The nanoparticles were characterized by XRD, DLS, FESEM equipped with EDX, UV-Vis spectroscopy and SQUID-VSM magnetometer. XRD analysis was done with X-PERT-PRO Panalytical instrument diffractometer using Cu Kα (λ = 1.5406) at a scanning rate of 1°/min and generator voltage of 40 kV and current of 30 mA. The X-Ray Diffraction (XRD) patterns of the nanoparticles were verified by comparison with the JCPDS data (PDF No. 36–1451). Sizes of the nanoparticles have been calculated using the well known Scherrer equation (Patterson, 1939), Cell volume and lattice parameters were calculated by UnitCellWin software. Particle size distribution analysis was evaluated using a Zetasizer Nano-ZS90 System (Malvern Inc.). Optical characterization has been performed using a HITACHI U-4100 spectrophotometer with a photon wavelength range 200–800 nm. Size and surface morphology of ZnO was analyzed using FESEM (JEOL-SEM/Carl Zeiss, Germany, Supra 35VP). The samples for FESEM were prepared on glass slides. Elemental analysis of the samples was done by energy dispersive X-ray spectroscopy (EDX) using Oxford Link Isis (UK) instrument which was attached with the FESEM. MPMS SQUID VSM (Quantum design) instrument was used for analysis of magnetic properties of the nanoparticles. RESULTS AND DISCUSSIONS Particle Size Analysis by DLS The peak in the distribution diagram (Figure 1) obtained from DLS is observed at around 250 nm and most of the particles are in the range of 100–600 nm. Only one peak indicates monodispersity. The synthesis was carried out without any stabilizer or capping agent. Hence the crystals agglomerated and a large particle was observed here though the grain size obtained from XRD was about 33 nm.

Fig. 1: Particle Size Distribution of Fe Doped ZnO

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Powder X-Ray Diffraction Analysis The XRD pattern of the doped particles synthesized is displayed in Figure 2. The XRD pattern matches with the JCPDS card for ZnO (JCPDS 36-1451) and indicates a wurtzite structure. No visible secondary phases or impurity peaks were observed. This demonstrates that the dopant is well integrated into the lattice sites during the synthesis process. The lattice constants of Fe doped ZnO were a = b = 3.27078 Ǻ, c = 5.24840 Ǻ which was found to be slightly larger than those of pure ZnO (a = b = 3.249 Ǻ, c = 5.206 Ǻ). The unit cell volume calculated was 48.6250 (Ǻ)3. The estimated average crystallite size calculated by Scherrer equation was 33.67 nm.

Fig. 2: XRD Pattern of Fe Doped ZnO Nanoparticle

UV-Vis Spectroscopy Analysis The absorbance spectra for the Fe-ZnO sample was explored in the wavelength range 200–800 nm. As per the Tauc equation (Ziegler et al., 1981), band gap (Eg) of Fe-doped

Fig. 3: UV-Visible Spectra of Pure and Fe Doped ZnO Nanoparticles

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ZnO samples was found to be 2.69 eV. Undoped ZnO synthesized under the same experimental conditions were observed to have band gap energy of 3.03 eV. The most probable reason for decreasing bandgap is the development of different conducting phases as a result of interaction between zinc oxide and iron oxide. These developed phases are believed to create additional energy level between valence band and conduction band and can reduce the band gap. UV absorption spectra of doped and undoped nanoparticles are given in Figure 3.

Field Emission Scanning Electron Microscopy (FESEM) Analysis The morphology of Fe–ZnO nanoparticles is shown in Figure 4 (a) and (b). Most of the nanoparticles were flaky in shape. From Figure 4(b) some clustered agglomerates were visible on the flakes and this may be due to the high surface charge of the particles.

(a)

(b)

Fig. 4: SEM Micrographs of Fe Doped ZnO at (a) 50000 Times and (b) 160000 Times Magnification

EDX Analysis From the EDX result it could be concluded that Fe was successfully incorporated into the ZnO lattice. Atomic wt% compositions of Zn, Fe and O obtained from EDX analysis (figure not shown) are 43.23, 2.79 and 53.79 respectively. SQUID Analysis Magnetic properties of the particles are examined using a magnetic field of strength +10KOe to –10KOe in a SQUID instrument under room temperature. The doped nanoparticles exhibited ferro-paramagnetic behaviour. The coercive field (Hc) and remnant magnetization value for this was found to be approximately 40 Oe and 0.00107 emu/g. Mechanism of Formation In presence of ultrasound water molecules in the medium are dissociated into H• and OH• radicals. OH• radical being very powerful oxidant attacks Zn- and Fe-acetylacetonate

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molecules and Fe-doped ZnO crystals are synthesized. A possible scheme may be proposed as follows, the validation of which is under progress. 1. 2. 3. 4. 5.



• • •

CONCLUSION Iron doped zinc oxide nanoparticles have been synthesized from a mixture of organic precursors using ultrasound via a green route. Synthesized doped nanoparticles were observed to have less band-gap energy than the undoped particles; hence they are better photocatalyst in visible region. Ferromagnetism has been imparted by doping. Since no capping agent or stabilizers has been used, agglomeration was enhanced. The grain size calculated from XRD was 33.67 nm, but the agglomerated particle size was observed to be around 250 nm. FESEM shows evidence of agglomeration. Process parameters like precursor composition, pH, temperature and power of sonication should be optimized before utilizing the batch data to develop a continuous sonochemical process for synthesis of doped nanoparticles of specific characteristics. ACKNOWLEDGEMENT Prof. Arup Mukherjee for DLS; Dr. Sudipto Bandyopadhyay for SQUID; Prof. Dipankar Chattopadhyay for XRD. TEQIP-phase II, CU for fellowship to Anirban Roy. REFERENCES [1] Bhargava, R.N., Chhabra, V., Som, T., Ekimov, A. and Taskar, N. (2002). Quantum confined atoms of doped ZnO nanocrystals. Phys. Status Solidi., B 229, 897–901. [2] Chakma, S., Bhasarkar, J.B. and Moholkar, V.S. (2013). Preparation, characterization and application of sonochemically doped Fe3+ into ZnO nanoparticles. International Journal of Research in Engineering and Technology, 2, 177–183. [3] Gedanken, A. (2004). Using sonochemistry for the fabrication of nanomaterials. Ultrason. Sonochem., 11, 47–55. [4] Khataee, A., Karimi, A., Arefi-Oskoui, S., Soltani, R.D.C., Hanifehpour, Y., Soltani, B. and Woo Joo, S. (2015). Sonochemical synthesis of Pr-doped ZnO nanoparticles for sonocatalytic degradation of Acid Red 17. Ultrasonics Sonochemistry, 22, 371–381. [5] Liu, S., Liu, F. and Wang, Z. (2005). Relaxation of carriers in terbium-doped ZnO nanoparticles, Chem. Phys. Lett., 343, 489–492. [6] Mandal, S.K., Das, A.K., Debjani, K., Satpati, B. and Nath, T.K. (2006). Microstructural and magnetic properties of ZnO:TM (TM = Co, Mn) diluted magnetic semiconducting nanoparticles. J. Appl. Phys., 100, 104315. [7] Marathe, S.K., Koinkar, P.M., Ashtaputre, S.S., More, M.A., Gosavi, S.W., Joag, D.S. and Kulkarni, S.K. (2006). Efficient field emission from chemically grown inexpensive ZnO nanoparticles of different morphologies. Nanotechnology, 17, 1932–1936.

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