Rapid synthesis of nanocrystalline SnO 2 by a microwave-assisted combustion method

Transcription

1 Journal of Advanced Ceramics 014, 3(3): ISSN DOI: /s CN /TQ Research Article Rapid synthesis of nanocrystalline SnO by a microwave-assisted combustion method Lajapathi Chellappan NEHRU a, Chinnappanadar SANJEEVIRAJA b, a Department of Medical Physics, Bharathidasan University, Tiruchirappalli-60 04, India b Department of Physics, Alaappa Chettiar Collee of Enineerin & Technoloy, Karaikudi , India Received: December 1, 013; Revised: March 05, 014; Accepted: March 13, 014 The Author(s) 014. This article is published with open access at Sprinerlink.com Abstract: A facile and rapid microwave-assisted combustion method was used to synthesis nanocrystalline SnO powders, throuh dissolution of tin nitrate (as oxidant) and lycine (as fuel) as startin materials and water as solvent and then heatin the resultin solution in a microwave oven. The study suested that application of microwave heatin to produce the nanosize SnO was achieved in a few minutes. The structure and morpholoy of the as-prepared combustion products were investiated by means of powder X-ray diffraction (PXRD) and scannin electron microscopy (SEM). Fourier transform infrared spectroscopy (FTIR) and Raman spectra confirmed the formation of tetraonal rutile structure SnO, and the SEM results indicated the surface characteristic of the products. The as-prepared powders have larer band ap enery as 3.67 ev. Keywords: nanoparticles; tin oxide; lycine; stoichiometric; combustion; optical 1 Introduction Semiconductin nanoparticles have been extensively studied from both experimental and theoretical viewpoints because of their potential applications in solar enery conversion, photo catalysis and optoelectronics industry. Nanostructured semiconductors have unusual optical, electronic and chemical properties and wide potential uses as nanoscale devices. The desire for miniaturization is the drivin force behind nanoparticle synthesis efforts. Research on the preparation of oxide nanoparticles is an active area bein pursued all over the world. Tin oxide (SnO ) is an important n-type semiconductor Correspondin author. sanjeeviraja@rediffmail.com with lare band ap and hih sensitivity to various toxic or flammable ases, and has been used as one of the promisin materials for as sensors, optoelectronic devices and neative electrodes for lithium batteries [1]. Its ability to transmit visible liht or to absorb infrared makes SnO a very attractive material for liht conductive films and solar cell coatins []. For such applications, nanosize particles are required. Recent research shows that the properties of SnO depend reatly on product s crystallite size and specific surface area [3]. Compared with bulk SnO, nanosize SnO has especially ood properties and outstandin advantaes of low operatin temperature and hih sensitivity for as-sensin applications [4]. Many methods have been developed to synthesize SnO nanoparticles, includin evaporation of SnO or SnO powders at elevated temperatures [5], solvothermal and hydrothermal routes [6],

2 17 Journal of Advanced Ceramics 014, 3(3): homoeneous precipitation [7], microwave-assisted aqueous solution usin a kitchen oven [8] and sol el route [9]. Recent papers report on the microwave-assisted hydrothermal synthesis of nanocrystalline SnO and SnO powders [10,11]. Compared to the conventional methods, microwave synthesis has the advantaes of producin small and uniform particle size of metal oxides with hih purity owin to short reaction time. Microwave-assisted synthesis is a novel method to produce materials, since microwave heatin is an in situ mode of enery conversion and the microwave heatin process is fundamentally different from conventional heatin processes. Heat will be enerated internally within the material, instead of oriinatin from external sources. By means of this method, many functional materials and compounds with novel structures and properties have been obtained [1]. As for as the literature survey, microwave-assisted combustion synthesis of tin dioxide colloidal suspensions from aqueous solutions of tin nitrate in the presence of lycine is reported for the first time. This method is simple, and the combustion synthesis process is fast, mild, enery-efficient and environmentally friendly to produce SnO nanoparticles usin lycine as fuel. Experimental All the reaents used in the experiments were analytically pure, and were purchased from MERCK Company and used as received without further purification. Nanocrystalline SnO was synthesized by a microwave-assisted combustion synthesis process usin lycine as fuel. A schematic representation of the synthesis process used in the current study is raphically shown in Fi. 1. Stoichiometric amounts (based on the thermochemical concepts from propellant chemistry calculation of fuel-to-oxidant mixtures) of tin nitrate and lycine were dissolved in deionized water and poured into a quartz container. Further they were mixed well by manetic stirrin for 1 h, makin them almost as homoeneous mixture which was placed in a domestic microwave oven (BPL India Limited, Banalore, India, Model No. IFB, 17PG1S, microwave 700 W, input rane V, AC 50 Hz, microwave frequency.45 GHz). Initially, the solution boiled and underwent dehydration followed by decomposition with the evolution of lare Sn Sn(NO 3 ) HNO 3 + deionized water Homoeneous mixtures and stirrer Metal nitrates-fuels complex Microwave heatin for 10 min Combustion Fine SnO nanopowders obtained Analysis (XRD, SEM, TEM, FTIR, Raman and UV) Mixed throuh manetic stirrer and heatin at 90 for few minutes Add fuels (lycine) Transparent solution Fi. 1 Schematic representation of the synthesis process. amount of ases (such as CO, H O and N formed directly from the reaction between fuel and oxidant) with white fumes comin out from the exhaust openin provided on the top of the microwave oven. After the solution reached the point of spontaneous combustion, it bean burnin and released lots of heat, vaporized all the solution instantly and became foamy white solid powders. The synthesized SnO powders phase formation was identified by powder X-ray diffraction (PXRD) method usin X Pert PRO PANalytical diffractometer with nickel-filtered Cu Kα radiation (λ = nm) as source and operated at 40 kv and 30 ma. The sample was scanned in the θ anle ranin from 10 to 80 with minimum step size (0.05 ) and minimum scan step time ( s) mode. The observed peak positions were compared with the standard ICDD data, and Miller indices were assined to the Bra peaks. The structure of the as-prepared SnO powders was solved usin PXRD method with refinement by Rietveld method. Scannin electron microscopy (SEM) and composition analysis of the sample were obtained usin Hitachi S4800 scannin electron microscope equipped with an enery-dispersive X-ray spectrometer (EDX). SEM measurements were mounted on aluminum studs usin adhesive raphite

3 Journal of Advanced Ceramics 014, 3(3): tape and sputter coated with old before analysis. Infrared spectra were recorded on a Nicolet Avatar 360 FTIR spectrometer usin KBr pellets. Raman spectra were taken on a Renishaw model Nicolet FT-Raman 960 Spectrometer (Ar + laser, 480 nm). UV Vis (ultraviolet visible) absorption spectra were recorded on a Shimadzu UV-550 spectrophotometer. 3 Results and discussion The phase identification of the as-prepared SnO was determined by PXRD. Fiure shows the PXRD pattern of the as-prepared SnO. It is indicated as the diffraction planes (110), (101), (00), (11), (0), (00), (1), (11), (301), (0) and (31) in the pattern, which can be perfectly indexed to the tetraonal rutile structure. No diffraction peak correspondin to Sn and other impurities is observed in the pattern. The values of lattice parameters (a = Å and c = Å) were calculated by the PXRD data, which are in ood areement with the standard diffraction pattern ( ). Usin Scherrer s equation D 0.9 / ( cos ) (where D is the averae crystalline size; is the wavelenth of Cu Kα radiation; is the full width at half maximum (FWHM) of the diffraction peak; and θ is the Bra s anle), all the half peak widths are very broad, which indicates that the obtained crystallites are small in size, i.e., nanometer size. These results were confirmed by transmission electron microscopy (TEM) observation. The averae particle size of SnO is estimated as around 8 nm. The Rietveld refinement for the investiated SnO is summarized in Table 1. It shows that the refined lattice parameters of stibnite are in ood areement with the reported values. The results of Rietveld analysis demonstrate the tetraonal rutile structure as shown in Fi. 3. In eneral, R p (profile factor), R wp (weiht θ ( ) Fi. PXRD patterns of SnO synthesized by a microwave-assisted combustion method Table 1 Refined structure parameters of lycine fuel used SnO nanopowders Space roup P4 /mnm (136) a (Å) () Cell parameters (Rietveld) c (Å) (3) V (Å) (3) Crystal density (/cm 3 ) Atomic positional parameters x y z B (iso) Sn (a) O (4f) The structure was refined with the followin R factors: R exp (expected weiht profile factor, %) = 16.10, R wp (%) = 10.63, R p (%) = 10.36, R Bra (Bra factor, %) = 1.01, DW(Durban Watson statistic, %) = 0.13 and S(%) = θ ( ) Fi. 3 Observed (blue line), calculated (red line) and difference profiles (black line below) resultin from the adjustment of the de rayos X (DRX) curves of the Rietveld analysis for lycine fuel used SnO. differences between measured and calculated value) and S (ood averae factor) show decreasin trend with the increase of the weiht fraction of albite and suest that the pattern fittin is accurate. The Rietveld fittin of the SnO phase was performed on the tetraonal rutile structure with space roup P4 /mnm (136). It is formed by a tin atom at the center, surrounded by six oxyen atoms in the vertex. The tin atom is bounded to four oxyen atoms with the same bond lenth in the basal plane side and to another two apical oxyen atoms with another same bond lenth. Rietveld refinement is probably the method of choice for determinin accurate unit-cell parameters. The refinements of lattice parameters of these phases are a = Å, c = Å and V = Å 3 for all samples, while the calculated lattice parameters of SnO are slihtly hiher than the bulk values with respect to the expected ones. Fiure 4(a) shows the SEM microraph of the as-prepared SnO synthesized by a microwave-assisted combustion process. It can be observed that ultrafine SnO particles with nano sized dimension are interconnected, which shows stron alomeration accompanied with a lot of small spherically shaped particles. This alomerate actually consists of much smaller rains of about 10 0 nm in diameter. These morpholoy results can be attributed to the liberation

4 174 Journal of Advanced Ceramics 014, 3(3): of a lare amount of ases durin a stron combustion reaction, i.e., the rapid release of lare volume of aseous products like H O vapor, CO and N, and nitrate ion roup acts as an initer durin combustion reaction which promotes the disinteration of the precursor, ivin nanocrystalline particles. It is helpful for the finest particle size formation. The alomerated particles have neck with their neihbors, and they form into spherical shapes intermitted with voids ensurin hih surface area. The size can not be exactly determined by the imae because they are very small. In spite of that, it is still found from the particle-like morpholoy. In addition, the EDX analysis (Fi. 4(b)) coupled with SEM confirms that the as-prepared nanocrystalline SnO is composed of tin and oxyen elements, indicatin that pure SnO nanoparticles are obtained. The particle size measured by SEM is close to the averae particle size obtained from XRD Rietveld method. The as-synthesized particles are shown in the TEM photoraph of nearly tetraonal rutile nanocrystalline SnO particles (Fi. 5). These sizes are well areed with the Rietveld refinement method. The correspondin selected area electron diffraction (SAED) imae is shown as inset in the TEM imae. The diffraction rins clearly confirm that tetraonal structure of SnO has been synthesized successfully. These SnO particle sizes based on XRD and Rietveld refinement results are in quite ood areement with each other and are further confirmed by the particle size estimated by TEM. The chemical structure of the as-prepared SnO was investiated by FTIR and Raman spectra. Fiure 6 shows the FTIR spectrum of the as-prepared nanocrystalline SnO. The absorption bands located in the spectral reion of cm 1 are caused by different Sn O or Sn=O stretchin modes. The absorption band located around ~460 cm 1 has been ascribed to the stretchin frequency of oxide roup (a) Fi. 5 TEM imae of the synthesized SnO nanoparticles. (b) Element wt% at% kev Fi. 4 (a) SEM microraph and (b) EDX spectrum of SnO powders synthesized by a microwave-assisted combustion method Wavenumber (cm 1 ) Fi. 6 FTIR spectrum of SnO powders.

5 Journal of Advanced Ceramics 014, 3(3): ( SnO ). The peaks observed at ~119 cm 1 in the FTIR spectra may be assined to a vibration of hydroxyl tin bonds of different oranic types of surface hydroxyl roups. A broad peak in the rane between 130 cm 1 and 1430 cm 1 demonstrates that a small amount of adsorbed water and NO 3 ions exist in the powder particles. A stron peak centered at ~355 cm 1 indicates that the tin complex transforms into tin oxide. It is known that Raman scatterin is very sensitive to the microstructure of nanocrystalline materials, and it is also used here to clarify the structure of the SnO nanopowders. As shown in Fi. 7, the first-order Raman spectrum of the as-prepared SnO shows the sharpest and stronest peaks at about cm 1, cm 1 and cm 1, which are in ood areement with those of tetraonal rutile structure of SnO with point roup D 4h [13], a typical Raman active branch of tetraonal rutile phase of nanocrystalline SnO oriinated from the non-polar optical phonon E (hih) mode [14]. The UV Vis spectrum of nanocrystalline SnO is shown in Fi. 8. The absorption peak shoulder onset Wavenumber (cm 1 ) Fi. 7 Raman spectrum of SnO powders h Wavelenth (nm) Fi. 8 UV Vis spectrum of SnO powders. (αh ) peaks are located at 340 nm correspondin to the band ap of 3.67 ev. Compared with the other methods (like sol el, co-precipitation, etc.), nanocrystalline SnO obtained by this microwave-assisted combustion method has larer band ap enery [13,14]. In eneral, the hexaonal wurtzite SnO structure has a direct band ap enery ( E ) for the SnO powders and can be determined by extrapolation to the zero absorption coefficient ( ) which is calculated from the followin equation:.303a (1) IC where A is the absorbance of the sample; is the density of SnO powders; C is the concentration of the particles; and I is the optical path lenth. The optical absorption coefficient ( ) of a semiconductor is close to the band ede which is estimated from the Tauc s relationship [15] as follows: ( h ) A( h E ) () where is the absorption coefficient; h is the Planck s constant; is the frequency of the incident photon; E is the optical direct band ap enery; and A is a constant. The exact value of the band ap is calculated by extrapolatin the straiht line portion of the ( h ) vs. h curve. Plottin ( h ) as a function of photon enery and extrapolatin the linear portion of the curve to the photon enery axis (absorption equal to zero) are shown in inset of Fi. 8. The dependence of particle size of the SnO powders can be determined experimentally from the band ap enery inferred from the optical absorption spectra, which is expressed from an effective mass model [16]. The quantum size effect in the SnO powders can be described from the followin equation: (nano) (bulk) h 1 E E 0.48 E RY (3) D me mh (nano) where E is the band ap enery of the synthesized SnO particles; E (bulk) is the band ap enery of the bulk material as 3.6 ev; h is the Planck s constant; D is the averae crystalline size calculated from PXRD results of SnO powders; the electron and hole effective masses are taken as me 0.99m0 and mh 0.34m0; and the bulk exciton bindin enery E RY is 130 mev. After simplification of the equation, (nano) (bulk) the band ap enery value is E E D. The optical band ap enery ( E ) was calculated usin effective mass equation and from

6 176 Journal of Advanced Ceramics 014, 3(3): Tauc s relation as 3.67 ev and 3.71 ev, respectively. Hence the optical band ap enery value appears slihtly lower than the calculated band ap enery value (effective mass model) due to a tiht-bindin model used in the experimental data (optical absorption spectra). 4 Conclusions Nanocrystalline SnO has been successfully synthesized by a microwave-assisted combustion method usin lycine as fuel. PXRD, FTIR and Raman spectra confirm the formation of a tetraonal rutile phase. The crystal structural parameters are in ood areement with the literature values. Besides, there is no peak other than SnO in the synthesized powders. FTIR and Raman spectra confirm the compound formation. SEM microraphs show the uniform particles present in the powders. EDX analyses show that the stoichiometric formation of SnO nanopowders. From absorption spectrum, the band ap enery of the material is estimated as 3.67 ev which is comparable with the bulk value. Open Access: This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the oriinal author(s) and the source are credited. References [1] Chen D, Gao L. Facile synthesis of sinle-crystal tin oxide nanorods with tunable dimensions via hydrothermal process. Chem Phys Lett 004, 398: [] Michel E, Stuera D, Chaumont D. Microwave flash synthesis of tin dioxide sols from tin chloride aqueous solutions. J Mater Sci Lett 001, 0: [3] Jian L, Sun G, Zhou Z, et al. Size-controllable synthesis of monodispersed SnO nanoparticles and application in electrocatalysts. J Phys Chem B 005, 109: [4] Yan H, Hu Y, Tan A, et al. Synthesis of tin oxide nanoparticles by mechanochemical reaction. J Alloys Compd 004, 363: [5] Pan ZW, Dai ZR, Wan ZL. Nanobelts of semiconductin oxides. Science 001, 91: [6] Baik NS, Sakai G, Miura N, et al. Preparation of stabilized nanosized tin oxide particles by hydrothermal treatment. J Am Ceram Soc 000, 83: [7] Son KC, Kan Y. Preparation of hih surface area tin oxide powders by a homoeneous precipitation method. Mater Lett 000, 4: [8] Zhu J-J, Zhu J-M, Liao X-H, et al. Rapid synthesis of nanocrystalline SnO powders by microwave heatin method. Mater Lett 00, 53: [9] Wu D-S, Han C-Y, Wan S-Y, et al. Microwave-assisted solution synthesis of SnO nanocrystallites. Mater Lett 00, 53: [10] Pires FI, Joanni E, Savu R, et al. Microwave-assisted hydrothermal synthesis of nanocrystalline SnO powders. Mater Lett 008, 6: [11] Krishna M, Komarneni S. Conventional- vs microwave-hydrothermal synthesis of tin oxide, SnO nanoparticles. Ceram Int 009, 35: [1] Nehru LC, Swaminathan V, Sanjeeviraja C. Rapid synthesis of nanocrystalline ZnO by a microwaveassisted combustion method. Powder Technol 01, 6: [13] Ristić M, Ivanda M, Popović S, et al. Dependence of nanocrystalline SnO particle size on synthesis route. J Non-Cryst Solids 00, 303: [14] Wu J-J, Liu S-C. Catalyst-free rowth and characterization of ZnO nanorods. J Phys Chem B 00, 106: [15] Tauc J. Amorphous and Liquid Semiconductor. New York: Plenum Press, [16] Brus LE. Structure and electronic states of quantum semiconductor crystallites. Nanostruct Mater 199, 1: