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

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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/mssp Optical studies of nano-structured La-doped ZnO prepared by combustion method L. 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,n a Department of Physics, Loyola College, Chennai 600034, India Light Scattering Studies Section, IGCAR, Kalpakkam 603102, India Condensed Matter Physics Division, IGCAR, Kalpakkam 603102, India d Pusan National University, Jangjeon, Geumjeong, Busan 609 735, South Korea e School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639 798, Singapore b c a r t i c l e in f o abstract Article history: Received 4 August 2011 Received in revised form 13 March 2012 Accepted 14 March 2012 Available online 21 April 2012 Coral-shaped nano-structured zinc oxide (ZnO) was successfully synthesized and Ladoped via a facile combustion process using glycine as a fuel. The auto-ignition (at $ 185 1C) of viscous reactants zinc nitrate and glycine resulted in ZnO powders. Hexagonal wurtzite structure of pure and doped ZnO powder was confirmed by X-ray 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) density in it and the grain size is approximately about 15 nm. Addition of dopants subsequently alters the structural and optical properties which were confirmed by UV–VIS studies. & 2012 Elsevier Ltd. All rights reserved. Keywords: Doping Semiconducting II–VI materials Nano-structures combustion X-ray diffraction spectra Zinc compounds Rare earth compounds 1. Introduction Nano-structured metal oxide semiconductors are gaining attention due to their wide band-gap and related properties [1]. Recent decades are witnessed with researchers paying much interest in synthesis and characterization of II–VI group semiconducting materials at nano- [2] and bulk [3] levels. Zinc oxide (ZnO) is a widely exploited, due to its excellent physical and chemical properties. Numerous researchers proposed the solution combustion method to synthesize simple and mixed metal oxides [4–9]. Normally ZnO is doped with different n Corresponding author. Tel.: þ 91 44 2817 5662; fax: þ91 44 2817 5566. E-mail addresses: sjeromedas2004@yahoo.com, jerome@loyolacollege.edu (S. Jerome Das). 1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.03.011 types of metallic ions in order to enhance the optical and conducting properties [10–14]. The exceptional interest on ZnO may be seen in the recent literatures. The modified ZnO may be used as a base material for diluted magnetic semiconductors [15–18], gas sensors [19], photocatalysts [20], field-effect transistors [21,22], lightemitting materials [23–25], solar cells [26,27] and biological systems (drug delivery, bio-imaging, etc.) [28,29]. In the recent times, rare earth metal-doped ZnO (e.g., Tb, Er, Eu, Dy and Sm) has been broadly researched and concentrated on luminescence properties [24,30–33]. Lanthanum (La)-doped ZnO nano-structures exhibit excellent photocatalytic activity and gas sensitivity [20,34–36]. Nano-sized ZnO has been synthesized by the solution combustion method and there are no literature references for La-doped ZnO using this method. Current work is focused on investigating the result of La doping concentration on the
  2. 2. L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 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. 309
  3. 3. 310 L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 microstructure and optical properties of ZnO nano-structure prepared by the combustion method. 2. Experimental details Distinct from usual thermal evaporation, ZnO nanostructures were prepared by the combustion method, which allows efficient synthesis of nano-size materials. This process involves a self-sustained reaction in homogeneous solution of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine, citric acid, hydrazides). Depending on the type of precursors, and the suitable conditions for chemical reaction to take place, zinc nitrate (Zn(NO3)2 Á 6H2O) was chosen as an oxidizer and glycine (NH2CH2COOH) as a fuel, since its combustion heat (À3.24 kcal/g) is more negative when compared with urea (À2.98 kcal/g) or citric acid (À2.76 kcal/g) [36]. Lanthanum nitrate (La(NO3)2 Á 6H2O) is added to zinc nitrate with required molar ratio and glycine is also added along with it, in a molar ratio of 0.9:1 (zinc nitrateþlanthanum nitrate:glycine) and stirred well for 1 h in 100 ml double distilled water. The obtained solution is heated ($185 1C) till combustion reaction occurs. Procedural flow chart diagram for the preparation of precursors and the formation of nano-structures is shown in Fig. 1. Crystallinity of pure ZnO and La-doped ZnO catalysts were analyzed by Philips CM 20 Transmission Electron Microscope which was operated between 20 and 200 kV. Composition of the samples were analyzed by energy dispersive X-ray spectroscopy (EDS) attached to the TEM instrument. 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) 308–313 311 recorded using PAN analytical X-ray diffractometer with Cu ˚ Ka (1.5405 A) radiation in the scan range 2y between 301 and 701 with a scan speed of 21/min. UV–VIS spectra of pure ZnO and La-doped ZnO catalysts were recorded using Varian CARY 5E UV–VIS–NIR Spectrophotometer. The absorbance spectra were then recorded in the range 200–700 nm. Photoluminescence of pure ZnO and La-doped ZnO were measured by Jobin Yvon Fluorolog spectrofluorometer and the results are discussed in detail. 3. Results and discussion TEM analysis shows that the nano-structures which had been synthesized using combustion processing are coral-shaped and porous as shown in Fig. 2. This shape may be attributed to the thermal fluctuations while synthesizing 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 increases as the La-dopant concentration increases as shown in Fig. 3. Each individual nano-structure is about 450–1000 nm formed by tiny spherical ZnO nanoparticles. We can also notice that the pores are $ 10–50 nm in diameter which considerably increase the surface to volume ratio. Selected area diffraction patterns match very well with wurtzite ZnO in both pure and doped ZnO. EDS analysis shows that some La3 þ ions have been incorporated into the ZnO lattice by substituting zinc ions as shown in Fig. 3(e) and in Table 1. When La is present the composition of oxygen seems to be nearly constant. This may be due to the addition of oxygen atoms in the La-doped ZnO which was accommodated by the additional vacancy in the La3 þ ion. Copper peak in the EDS measurement originates from the TEM supporting carbon coated copper grid. XRD profiles of synthesized pure and doped materials in appropriate ratio are shown in Fig. 4. The diffraction peaks and their relative intensities match with the JCPDS card no. 36-1451. Hence the observed patterns can be clearly endorsed to the presence of hexagonal wurzite structure. XRD peak of lanthanum oxide was not observed Table 1 Composition of elements in La-doped ZnO samples. La concentration (mol%) Element weight (%) Atomic (%) 0 O Zn 13.30 86.70 38.50 61.50 1 O La Zn 20.30 04.34 75.36 51.73 01.27 47.00 2 O La Zn 19.60 08.38 72.02 51.32 02.53 46.15 3 O La Zn 18.94 12.15 68.91 50.92 03.76 45.32 5 O La Zn 18.30 17.20 64.50 50.80 05.50 43.70 Fig. 4. Powder XRD spectra of samples pure–doped prepared at different mol percent of La. even for the La-doped sample with a high La concentration, suggesting that lanthanum oxide is uniformly dispersed in the ZnO and no second phase such as La2O3 and La(OH)3 appears. It is evident that the introduction of La ions does not alter the structure of ZnO and dopant disperses homogeneously in the ZnO matrix as previously reported [37]. Using the Scherrer equations the crystallite sizes were estimated to be around 450 nm from the fullwidth at half-maximum (FWHM) of diffraction peaks. The diffraction pattern of ZnO is observed between the 2y values of 301 and 701. The peak intensities of doped ZnO increases with dopant concentration. Therefore, the crystalline nature of ZnO nanostructure increases with Ladopant in the same manner as previously reported in the case of Fe doped ZnO [38]. Doping of La ions restrains the growth of ZnO grains and dopant with smaller ionic radius has a constructive effect on diffusivity which promotes orientation growth and good crystal [39]. The lattice parameters and the unit cell volume were determined using software program UnitCell method of TJB Holland & SAT Redfern [40]. The determined unit cell parameters, volume and c/a were plotted as a function of La concentrations and are shown in Figs. 5 and 6 respectively. The lattice constant gradually increases with increase in concentration of La3 þ ions. Consequently, cell volume and c/a ratio changed, agreeing with the fact that ionic radii of La3 þ is higher than the Zn2 þ ion (0.106 nm for La and 0.074 nm for Zn) [41,42] but there is a small variation in c-axis compared with the results of Chen et al. [37]. This distortion in the lattice parameters confirms the incorporation of La3 þ ions up to 5 mol% in ZnO wurzite structure. UV–VIS spectrum shows that the absorbance is high below 380 nm for pure ZnO and as the La-dopant concentration increases the absorbance of ZnO decreases considerably below this region as shown in Fig. 7. The 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) 308–313 Fig. 5. Unit cell parameters a and c were plotted as a function of La concentration. 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 La concentration. Fig. 9. Room temperature PL emission spectra of ZnO with/without La-dopant. Fig. 7. UV–VIS spectra ZnO with/without dopant. presented in Fig. 8. It can be clearly seen that the band gap of La-doped ZnO also increases gradually with increase in La concentration. After 380 nm, absorbance of pure ZnO is 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 wavelength of 285 nm and is shown in Fig. 9. The intensity of PL emission is found to increase with increase in Ladopant, 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 that a violet peak at about 420 nm (2.95 eV) and the intensity of emission are found to be strongly reliant on the La concentration. Traps on the grain surface per unit volume increases with the increase of specific surface 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 the depletion regions and locate at the ZnO–ZnO grain boundaries when a polycrystalline varistor forms, and the level of interface trap was found to be about 0.33 eV below the conduction band edge. So violet emission is possibly attributed to the recombination centers linked with interface traps existing at the grain boundaries, and radiative transition occurs between the level of interface traps and the valence band. 4. Conclusions La-doped ZnO was prepared by combustion processing; doping levels included undoped, 1, 2, 3 and 5 molar percentage. Significant transformation was observed upon different doping concentrations. Transmission electron micrograph shows an enhancement of pore density for doped ZnO. Lattice parameters and unit cell volume were determined from the XRD data and it confirms the entry of La-dopant inside ZnO crystal lattice by the increase in lattice constants. 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