Optical studies of nano structured la-doped zn o prepared by combustion method

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Optical studies of nano structured la-doped zn o prepared by combustion method

  1. 1. Materials Science in Semiconductor Processing 15 (2012) 308–313 Contents lists available at SciVerse ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/msspOptical studies of nano-structured La-doped ZnO prepared bycombustion methodL. Arun Jose a, J. Mary Linet a, V. Sivasubramanian b, Akhilesh K. Arora c, C. Justin Raj d,T. Maiyalagan e, S. Jerome Das a,na Department of Physics, Loyola College, Chennai 600034, Indiab Light Scattering Studies Section, IGCAR, Kalpakkam 603102, Indiac Condensed Matter Physics Division, IGCAR, Kalpakkam 603102, Indiad Pusan National University, Jangjeon, Geumjeong, Busan 609 735, South Koreae School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639 798, Singaporea r t i c l e in f o abstractArticle history: Coral-shaped nano-structured zinc oxide (ZnO) was successfully synthesized and La-Received 4 August 2011 doped via a facile combustion process using glycine as a fuel. The auto-ignitionReceived in revised form (at $ 185 1C) of viscous reactants zinc nitrate and glycine resulted in ZnO powders.13 March 2012 Hexagonal wurtzite structure of pure and doped ZnO powder was confirmed by X-rayAccepted 14 March 2012Available online 21 April 2012 powder diffraction analysis. The transmission electron micrograph shows that the nano-structured ZnO is coral-shaped and possess maximal pore ( $ 10–50 nm pore size)Keywords: density in it and the grain size is approximately about 15 nm. Addition of dopantsDoping subsequently alters the structural and optical properties which were confirmed bySemiconducting II–VI materials UV–VIS studies.Nano-structures & 2012 Elsevier Ltd. All rights reserved.combustionX-ray diffraction spectraZinc compoundsRare earth compounds1. Introduction types of metallic ions in order to enhance the optical and conducting properties [10–14]. The exceptional interest Nano-structured metal oxide semiconductors are gain- on ZnO may be seen in the recent literatures. Theing attention due to their wide band-gap and related modified ZnO may be used as a base material for dilutedproperties [1]. Recent decades are witnessed with magnetic semiconductors [15–18], gas sensors [19],researchers paying much interest in synthesis and char- photocatalysts [20], field-effect transistors [21,22], light-acterization of II–VI group semiconducting materials at emitting materials [23–25], solar cells [26,27] and biolo-nano- [2] and bulk [3] levels. Zinc oxide (ZnO) is a widely gical systems (drug delivery, bio-imaging, etc.) [28,29]. Inexploited, due to its excellent physical and chemical the recent times, rare earth metal-doped ZnO (e.g., Tb, Er,properties. Numerous researchers proposed the solution Eu, Dy and Sm) has been broadly researched and concen-combustion method to synthesize simple and mixed trated on luminescence properties [24,30–33]. Lantha-metal oxides [4–9]. Normally ZnO is doped with different num (La)-doped ZnO nano-structures exhibit excellent photocatalytic activity and gas sensitivity [20,34–36]. n Nano-sized ZnO has been synthesized by the solution Corresponding author. Tel.: þ 91 44 2817 5662;fax: þ91 44 2817 5566. combustion method and there are no literature references E-mail addresses: sjeromedas2004@yahoo.com, for La-doped ZnO using this method. Current work is focusedjerome@loyolacollege.edu (S. Jerome Das). on investigating the result of La doping concentration on the1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mssp.2012.03.011
  2. 2. L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 309 Glycine (NH2CH2COOH) Zinc Nitrate (Zn(NO3)2•6H2O) Lanthanum Nitrate (La(NO3)2•6H2O) Mixed with (1-x): x molar ratio where x = 0.01, 0.02. 0.03 and 0.05 of (La(NO3)2•6H2O) Directly mixed with desired (1:09) molar ti Heating / Development of precursor Ignition / Combustion / Burning Synthesized Material (powder) Fig. 1. Procedural flow chart for preparation of ZnO with/without La-dopant.Fig. 2. TEM Images: (a) bright field, (b) dark field, (c) detailed view, (d) diffraction pattern and (e) EDS pattern of pure ZnO.
  3. 3. 310 L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313microstructure and optical properties of ZnO nano-structure when compared with urea (À2.98 kcal/g) or citric acidprepared by the combustion method. (À2.76 kcal/g) [36]. Lanthanum nitrate (La(NO3)2 Á 6H2O) is added to zinc nitrate with required molar ratio and glycine2. Experimental details is also added along with it, in a molar ratio of 0.9:1 (zinc nitrateþlanthanum nitrate:glycine) and stirred well for 1 h Distinct from usual thermal evaporation, ZnO nano- in 100 ml double distilled water. The obtained solution isstructures were prepared by the combustion method, which heated ($185 1C) till combustion reaction occurs. Proce-allows efficient synthesis of nano-size materials. This pro- dural flow chart diagram for the preparation of precursorscess involves a self-sustained reaction in homogeneous and the formation of nano-structures is shown in Fig. 1.solution of different oxidizers (e.g., metal nitrates) and fuels Crystallinity of pure ZnO and La-doped ZnO catalysts were(e.g., urea, glycine, citric acid, hydrazides). Depending on the analyzed by Philips CM 20 Transmission Electron Micro-type of precursors, and the suitable conditions for chemical scope which was operated between 20 and 200 kV. Com-reaction to take place, zinc nitrate (Zn(NO3)2 Á 6H2O) was position of the samples were analyzed by energy dispersivechosen as an oxidizer and glycine (NH2CH2COOH) as a fuel, X-ray spectroscopy (EDS) attached to the TEM instrument.since its combustion heat (À3.24 kcal/g) is more negative X-ray diffraction patterns of the synthesized samples were Fig. 3. TEM images: (a) bright field, (b) dark field, (c) detailed view, (d) diffraction pattern and (e) EDS pattern of 5 mol % La-doped ZnO.
  4. 4. L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 311recorded using PAN analytical X-ray diffractometer with Cu ˚Ka (1.5405 A) radiation in the scan range 2y between 301and 701 with a scan speed of 21/min. UV–VIS spectra of pureZnO and La-doped ZnO catalysts were recorded using VarianCARY 5E UV–VIS–NIR Spectrophotometer. The absorbancespectra were then recorded in the range 200–700 nm.Photoluminescence of pure ZnO and La-doped ZnO weremeasured by Jobin Yvon Fluorolog spectrofluorometer andthe results are discussed in detail.3. Results and discussion TEM analysis shows that the nano-structures whichhad been synthesized using combustion processing arecoral-shaped and porous as shown in Fig. 2. This shapemay be attributed to the thermal fluctuations whilesynthesizing the samples. Grain size is found to be$10–20 nm both in the case of pure and doped ZnO.Porous nature of the nano-structures significantly increasesas the La-dopant concentration increases as shown in Fig. 3. Fig. 4. Powder XRD spectra of samples pure–doped prepared at differentEach individual nano-structure is about 450–1000 nm mol percent of La.formed by tiny spherical ZnO nanoparticles. We can alsonotice that the pores are $ 10–50 nm in diameter whichconsiderably increase the surface to volume ratio. Selected even for the La-doped sample with a high La concentra-area diffraction patterns match very well with wurtzite tion, suggesting that lanthanum oxide is uniformly dis-ZnO in both pure and doped ZnO. EDS analysis shows that persed in the ZnO and no second phase such as La2O3 andsome La3 þ ions have been incorporated into the ZnO lattice La(OH)3 appears. It is evident that the introduction of Laby substituting zinc ions as shown in Fig. 3(e) and in ions does not alter the structure of ZnO and dopantTable 1. When La is present the composition of oxygen disperses homogeneously in the ZnO matrix as previouslyseems to be nearly constant. This may be due to the reported [37]. Using the Scherrer equations the crystalliteaddition of oxygen atoms in the La-doped ZnO which was sizes were estimated to be around 450 nm from the full-accommodated by the additional vacancy in the La3 þ ion. width at half-maximum (FWHM) of diffraction peaks. TheCopper peak in the EDS measurement originates from the diffraction pattern of ZnO is observed between the 2yTEM supporting carbon coated copper grid. values of 301 and 701. The peak intensities of doped ZnO XRD profiles of synthesized pure and doped materials increases with dopant concentration. Therefore, the crys-in appropriate ratio are shown in Fig. 4. The diffraction talline nature of ZnO nanostructure increases with La-peaks and their relative intensities match with the JCPDS dopant in the same manner as previously reported in thecard no. 36-1451. Hence the observed patterns can be case of Fe doped ZnO [38]. Doping of La ions restrains theclearly endorsed to the presence of hexagonal wurzite growth of ZnO grains and dopant with smaller ionicstructure. XRD peak of lanthanum oxide was not observed radius has a constructive effect on diffusivity which promotes orientation growth and good crystal [39]. The lattice parameters and the unit cell volume were deter-Table 1 mined using software program UnitCell method of TJBComposition of elements in La-doped ZnO samples. Holland & SAT Redfern [40]. The determined unit cell parameters, volume and c/a were plotted as a function of La concentration (mol%) Element weight (%) Atomic (%) La concentrations and are shown in Figs. 5 and 6 respec- 0 O 13.30 38.50 tively. The lattice constant gradually increases with Zn 86.70 61.50 increase in concentration of La3 þ ions. Consequently, cell 1 O 20.30 51.73 volume and c/a ratio changed, agreeing with the fact that La 04.34 01.27 ionic radii of La3 þ is higher than the Zn2 þ ion (0.106 nm Zn 75.36 47.00 for La and 0.074 nm for Zn) [41,42] but there is a small 2 O 19.60 51.32 variation in c-axis compared with the results of Chen et al. La 08.38 02.53 [37]. This distortion in the lattice parameters confirms the Zn 72.02 46.15 incorporation of La3 þ ions up to 5 mol% in ZnO wurzite 3 O 18.94 50.92 structure. La 12.15 03.76 UV–VIS spectrum shows that the absorbance is high Zn 68.91 45.32 below 380 nm for pure ZnO and as the La-dopant con- 5 O 18.30 50.80 centration increases the absorbance of ZnO decreases La 17.20 05.50 considerably below this region as shown in Fig. 7. The Zn 64.50 43.70 corresponding band gap values of pure and doped ZnO are
  5. 5. 312 L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313Fig. 5. Unit cell parameters a and c were plotted as a function of Laconcentration. Fig. 8. Calculated band gap of pure and La-doped ZnO.Fig. 6. Unit cell volume and c/a were plotted as a function of Laconcentration. Fig. 9. Room temperature PL emission spectra of ZnO with/without La-dopant. less compared with La-doped ZnO and absorbance increases with increase in dopant concentration. Photoluminescence (PL) spectra of La-doped ZnO nano-structures were measured with an excitation wave- length of 285 nm and is shown in Fig. 9. The intensity of PL emission is found to increase with increase in La- dopant, but the intensity of doped ZnO decreases in comparison with pure ZnO between 3.2 and 3.3 eV. The PL spectrum shows the La characteristic emission band at $ 2.9 eV and near UV emission between 3.27 and 3.30 eV. There is a shift in the emission spectra for pure and doped ZnO. This may be attributed due to the strain created in the crystal lattice to accommodate larger La atoms. Spectra in the range of 340–460 nm (2.7–3.6 eV) shows Fig. 7. UV–VIS spectra ZnO with/without dopant. that a violet peak at about 420 nm (2.95 eV) and the intensity of emission are found to be strongly reliant onpresented in Fig. 8. It can be clearly seen that the band gap the La concentration. Traps on the grain surface per unitof La-doped ZnO also increases gradually with increase in volume increases with the increase of specific surfaceLa concentration. After 380 nm, absorbance of pure ZnO is area. Cordaro et al. [43] assumed that interface traps lie in
  6. 6. L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 313the depletion regions and locate at the ZnO–ZnO grain [11] H. Huang, Y. Ou, S. Xu, G. Fang, M. Li, X. Zhao, Applied Surfaceboundaries when a polycrystalline varistor forms, and the Science 254 (2008) 2013–2016. [12] V. Zhitomirsky, E. Cetinorgu, R. Boxman, S. Goldsmith, Thin Solidlevel of interface trap was found to be about 0.33 eV Films 516 (2008) 5079–5086.below the conduction band edge. So violet emission is [13] T. Moriga, Y. Hayashi, K. Kondo, Y. Nishimura, K.-i. Murai,possibly attributed to the recombination centers linked I. Nakabayashi, H. Fukumoto, K. Tominaga, Journal of Vacuum Science and Technology A 22 (2004) 1705.with interface traps existing at the grain boundaries, and [14] S. Saha, V. Gupta, AIP Advances 1 (2011) 042112.radiative transition occurs between the level of interface [15] T.S. Herng, S.P. Lau, S.F. Yu, H.Y. Yang, K.S. Teng, J.S. 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