Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1135–1143

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Preparation and characterizations of SnO2 nanopowder andspectroscopic (FT-IR, FT-Raman, UV–Visible and NMR)analysis using HF and DFT calculations

1386-1425/$ - see front matter Crown Copyright � 2013 Published by Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.09.030

⇑ Corresponding author. Tel./fax: +91 4364 222264.E-mail address: ramalingam.physics@gmail.com (S. Ramalingam).

A. Ayeshamariam a, S. Ramalingam b,⇑, M. Bououdina c,d, M. Jayachandran e

a Department of Physics, Khadir Mohideen College, Adirampattinam, Tamilnadu, Indiab Department of Physics, A.V.C. College, Mayiladuthurai, Tamilnadu, Indiac Nanotechnology Centre, University of Bahrain, Bahraind Department of Physics, College of Science, University of Bahrain, Bahraine Central Electrochemical Research Institute, ECMS Division, Karaikudi, Tamilnadu, India

h i g h l i g h t s

� The pure and singe phase SnO2 Nanopowder is prepared by sol–gelcombustion route.� The vibrational pattern in nano phase

gets realigned and are shifted upcompared with bulk phase.� The HOMO–LUMO band gap (Kubo

gap) of compound is reduced to3.04 eV at 800 �C.� The Photoluminescence spectra

showed a peak shift with the changeof Kubo gap to 3.22 eV.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 July 2013Received in revised form 18 August 2013Accepted 1 September 2013Available online 13 September 2013

Keywords:SnO2

Bulk phase of SnO2

NMR and UV–Visible spectraPhotoluminescenceKubo gap

a b s t r a c t

In this work, pure and singe phase SnO2 Nano powder is successfully prepared by simple sol–gel combus-tion route. The photo luminescence and XRD measurements are made and compared the geometricalparameters with calculated values. The FT-IR and FT-Raman spectra are recorded and the fundamentalfrequencies are assigned. The optimized parameters and the frequencies are calculated using HF andDFT (LSDA, B3LYP and B3PW91) theory in bulk phase of SnO2 and are compared with its Nano phase.The vibrational frequency pattern in nano phase gets realigned and the frequencies are shifted up tohigher region of spectra when compared with bulk phase. The NMR and UV–Visible spectra are simulatedand analyzed. Transmittance studies showed that the HOMO–LUMO band gap (Kubo gap) is reduced from3.47 eV to 3.04 eV while it is heated up to 800 �C. The Photoluminescence spectra of SnO2 powder showeda peak shift towards lower energy side with the change of Kubo gap from 3.73 eV to 3.229 eV for as-pre-pared and heated up to 800 �C.

Crown Copyright � 2013 Published by Elsevier B.V. All rights reserved.

Introduction

Rapid developments in today’s electronic devices including flatpanel displays and touch panels are asking for highly sophisticated

components, with an ever increasing performance. The develop-ment of transparent conducting coatings as electrodes withcustomized characteristics like high electron work function, highelectron mobility and high transparency, hence, is becoming moreand more relevant [1]. To meet these forthcoming requirements,however, novel materials or improved coating techniques have tobe developed. One of these approaches is focusing on a combination

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of several coating methods, as for example, magnetron sputteringand sol–gel dip coating, to benefit from the advantages of bothtechniques [2]. The properties of such materials are fascinatingand have formed the subject of intense research in recent years[3,4]. These materials behave differently from bulk semiconductors.With decreasing particle size the band structure of the semiconduc-tor changes; the band gap increases and the edges of the bandssplits into discrete energy levels. These so-called quantum size ef-fects occur [5,6]. These quantum size effects have stimulated greatinterest in both basic and applied research.

Tin oxide, SnO2, is a well-known n-type semiconductor with awide band gap (Eg = 3.8 eV) and for its potential applications ingas sensors [7], dye-based solar cells, transparent conducting elec-trodes [8], and catalyst supports [9].

Research in the area of nano scale materials is motivated by thepossibility of processing and designing nano structured materialswith unique properties and important applications. Due to theirfinite small size and the high surface-to volume ratio, nano struc-tured materials often exhibit novel, and sometimes unusual prop-erties [10]. The optical, electrical, magnetic, thermal, and chemicalproperties depend to a large extent on the particle size and shapeof these materials [11]. Meanwhile the large number of surface andedge atoms provides active sites for catalyzing surface reactions. Inthe areas of optoelectronic devices such as transparent semicon-ductor electrodes in liquid crystal displays, light emitting transportconductors and solar–electrical energy converters and chemicaland gas sensitive semiconductor devices and nano electronics,the SnO2 and their composites are finding great interest and atten-tion in recent years [12].

Beltran et al. reported the structural and electronic properties ofthe TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems. Peri-odic quantum mechanical method with density functional theoryat the B3LYP level has been carried out. Relaxed surface energies,structural characteristics and electronic properties of the (110),(010), (101) and (00) low-index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 models are studied [13]. Yoichi Yamagu-chi et al., reported that density functional theory calculations with-in the generalized gradient approximation have been performedfor the interaction of oxygen with reduced M/SnO2(110) (M = Pd,Pt) surfaces [14].

However, so far, no work has been reported on the study (com-parison of Bulk and Nano phase) of preparation and characteriza-tion of pure and singe phase SnO2 Nano powder with HF and DFTcomputational calculation. In this work, the SnO2 nano powder issuccessfully prepared by sol–gel combustion method and is inves-tigated by FTIR, Laser Raman spectroscopy UV–Vis, and Photolumi-nescence measurements. Moreover, FT-IR and FT-Raman, NMR, UVand Visible spectra are recorded for bulk SnO2 for the comparison.The geometrical parameters and vibrational frequencies are calcu-lated using HF and DFT theory and their results are compared withthe experimental values.

Experimental methods

Synthesis of SnO2

Nanocrystalline SnO2 powders were prepared by sol–gel com-bustion process. Nitric acid was used in order to introduce oxidiz-ing ions, while urea was chosen as the fuel. Aqueous solutions ofpure metallic Sn (Malinckrodt), was dissolved in nitric acid (70%,Merck), and stoichiometric ratio of fuel urea (Merck) and deionisedwater were prepared. The solutions were heated under constantstirring at a temperature of about 90–100 �C in a Pyrex vesseland concentrated slowly without producing any precipitation untilit turned into a white, viscous gel. The portions of the gels were

heated at temperatures of about 350 �C, which suffered a strongself –propagating combustion reaction with the evolution of a largevolume of gases. The entire combustion process was over after fewseconds. The resulting yellow ashes were then calcined at100–800 �C for an hr in order to eliminate the carbonaceous resi-dues as shown in flowchart Fig. 6[15].

During the process of combustion using a fuel, carbon andhydrogen atoms combine with oxygen with simultaneous libera-tion of heat at a rapid rate. This energy is liberated due to the ‘‘rear-rangement of valency electrons’’ in these atoms, resulting in theformation of new compounds.

Fuelþ O2 ! Productsþ heat ð1Þ

According to Cooper et al. we can define the oxygen balance (OB) ofthe reaction, as defined in the field of propellants and explosives, asfollows:

OB% ¼ 100� AWoxygen

FWmixture2cO2 ð2Þ

where AW and FW are respectively the atomic weight of oxygenand the formula weight of the mixture, cO2 is the molar numberof oxygen.

Three modes of Combustion [16] can be classified depending onthe value of OB:

� Smoldering Combustion Synthesis (SCS) when OB > 0 (fuel –lean regime): an excess amount of oxygen in the reactant mix-ture is present and stifled the reaction. That’s why this mode ischaracterized by a slow and flameless reaction.

� Volume Combustion Synthesis: (VCS) when OB = 0 (stoichiome-try regime), a thermal explosion takes place and the reactionoccurred in the solution volume, resulting in the formation ofa homogeneous solid. The whole oxygen content of the metalnitrates can react completely in oxidizing the fuel.

� Self – propagating high-temperature Synthesis (SHS) whenOB < 0 (fuel – rich regime), the reaction is locally ignited andpropagate as a combustion wave in a self-sustained mannerthrough the reaction volume. In this regime, oxygen atmo-sphere is required for a complete combustion between fueland metal nitrates.

These three modes depend on the fuels/oxidizer ratio and deter-mine the reactivity of the reaction (explosive or moderate reac-tion). It is worth noting that the fuel to oxidizers ratio affectsalso the surface area of the resulting powder and then the particlesize of the final products.

Spectral information

Bulk SnO2 is purchased from Sigma–Aldrich Chemicals, which isof spectroscopic grade and hence used for recording the spectra assuch without any further purification. SnO2 Nano powder is pre-pared by above said method. The FT-IR spectra of the compoundin both phases are recorded in Bruker IFS 66V spectrometer inthe range of 4000–400 cm�1. The spectral resolution is ±2 cm�1.FT-Raman spectra of same compound is also recorded in the sameinstrument with FRA 106 Raman module equipped with Nd:YAGlaser source operating at 1.064 lm line widths with 200 mWpower. The spectra are recorded in the range of 4000–100 cm�1

with scanning speed of 30 cm�1 min�1 of spectral width 2 cm�1.The frequencies of all sharp bands are accurate, i.e., ±1 cm�1.

Characterization techniques

As-prepared nano powder and bulk SnO2 were characterized bydifferent techniques. The structure and phase purity of the

Table 1Optimized geometrical parameters for Tin oxide computed at HF and DFT [LSDA, B3LYP and B3PW91] methods with 3-21G(d,p) basis sets.

Geometrical Parameters Methods Bulk phase Experimental valueNano phase

HF/3-21G(d,p) LSDA/3-21G(d,p) B3LYP/3-21G(d,p) B3PW91/3-21G(d,p)

Bond length (Å)Sn1–O2 1.841 1.852 1.865 1.858 2.0585Sn1–O3 1.841 1.852 1.865 1.858 2.0472

Bond angle (�)O2–Sn1–O3 179.94 173.34 172.53 173.72 180.0

A. Ayeshamariam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1135–1143 1137

powders are examined by powder X-ray diffraction (XRD) tech-nique using PAN alytical X’Pert diffractometer equipped with CuKa radiation of wavelength 1.5406 Å. UV–Vis absorption spectrom-eter Infrared spectra of the nano particles were recorded usingFourier transform infrared (FTIR) spectrometer (Shiraz) in therange of 4000–400 cm�1 with a resolution of 1 cm�1. Laser RamanSpectroscopy was carried out by Renishaw invia Laser Raman Spec-trometer. UV–Vis absorption measurements were carried out atroom temperature using (Shimadzu-2450). Varian Cary EclipseSpectrophotometer employing 15W Xe flash lamp was used forthe photoluminescence studies.

Computational methods

In the present work, HF and some of the hybrid methods; LSDA,B3LYP and B3PW91 are carried out using the basis set 3-21G(d,p).All these calculations are performed using GAUSSIAN 09W [17]program package on Pentium IV processor in personal computer.In DFT methods; Becke’s three parameter hybrids function com-bined with the Lee–Yang–Parr correlation function (B3LYP)[18,19], Becke’s three parameter exact exchange-function (B3)[20] combined with gradient-corrected correlational functional ofLee, Yang and Parr (LYP) [21,22] and Perdew and Wang (PW91)[23,24] predict the best results for molecular geometry and vibra-tional frequencies for moderately larger molecules. The calculated

Fig. 1. Molecular structure and MEP of SnO2.

frequencies are scaled down by suitable scaling factors to yield thecoherent with the observed frequencies.

The optimized molecular structure of the molecule is obtainedfrom Gaussian 09 and Gaussview program and is shown in Fig. 1.The comparative optimized structural parameters such as bondlength, bond angle and dihedral angle are presented in Table 1.The observed (FT-IR and FT-Raman) and calculated at HF and DFT(LSDA, B3LYP and B3PW91) with 3-21 basis set vibrational fre-quencies, vibrational assignments and the total energy distribution(TED) for B3PW91 of Tin Oxide are presented in Table 2. Experi-mental and simulated spectra of IR and Raman in both phasesare presented in the Figs. 2 and 3 respectively (see Table 3).

The total energy distribution (TED) calculations show the rela-tive contributions of the redundant internal coordinates to eachnormal vibrational mode of the molecule which enable numeri-cally to describe the character of each mode and are carried outby SQM method [25,26] using the output files created at the endof the frequency calculations. The TED calculations are performedby using PQS program [26].

Results and discussion

Molecular geometry

SnO2 posses a symmetry top molecular structure and belong toC2V point group symmetry. The optimized structure of the mole-cule is obtained from Gaussian 09 and Gaussview program and isshown in Fig. 1 with electrostatic potential surface map. The struc-ture optimization and zero point vibrational energy of the com-pound in HF, LSDA, B3LYP and B3PW91/3-21G(d,p) are 2.37, 2.49,2.41 and 2.44 kCal/mol, respectively. The present molecule hastwo symmetrical Sn–O bonds. The bond length of Sn–O in bulk is1.865 Å whereas the bond length of Sn–O in Nano phase is 2.058and 2.047 Å respectively. From these results of the two phases, itis observed that the Sn–O bond length of is 0.193 and 0.182 Ågreater in Nano phase than the bulk. Due to this bond length incre-ment, the property of present molecule is changed in Nano phase.The calculated bond angle of O–Sn–O is 172.53� in bulk and themolecular structure is rather symmetry top. But the angle O–Sn–O in ano phase is 180� precisely which means that the moleculeis linear tri atomic. The shape of the molecular structure is changedin Nano phase from bulk. So the present molecule SnO2 has at-tained a higher coordination and satisfied bonds in Nano phase.

Vibrational assignments

The molecule SnO2 belongs to C2V point group symmetry whichconsists of 3 atoms, so it has 4 normal vibrational modes. Out of 4fundamental vibrations of the molecule only three are observeddue to the suppression of vibration along bond axis and it can bedistributed as two stretching B2- asymmetric and A1 symmetricand one in plane bending A1 vibrations, i.e., Cvib = 2A1 + 1B2. Theharmonic vibrational frequencies (un-scaled and scaled) calculated

Table 2Observed and calculated vibrational frequencies at HF & DFT (LSDA/B3LYP/B3PW91) with 3-21G (d,p) basis set of Tin oxide.

S. No. FT-IR FT-Raman Calculated frequency (cm�1) [scaled] Vibrational assignments Species B3LYP 3-21G (d,p) TED

HF LSDA B3LYP B3PW91

3-21G (d,p) 3-21G (d,p) 3-21G (d,p) 3-21G (d,p)

Observed frequency(cm�1) bulk phase1 610vs – 627 624 625 617 (Sn–O)t Asym B2 (Sn–O)t 98%2 520vs – 576 582 574 574 (Sn–O)t Sym A1 (Sn–O)t 95%3 – 120w 120 104 119 119 (Sn–O)d A1 (Sn–O)d 95%

Unscaled values1 794 879 845 864 – – –2 778 786 765 779 – – –3 92 80 79 68 – – –

Observed frequency(cm�1) nano phase1 630vs 630m 627 624 625 617 (Sn–O)t Asym B2 (Sn–O)t 98%2 570vs 570m 576 582 574 574 (Sn–O)t Sym A1 (Sn–O)t 95%3 – 230w 120 104 119 119 (Sn–O)d A1 (Sn–O)d 95%

vs; very strong, s; strong, m; medium, w; weak, vw; very weak. t; stretching, d; in plane bending.

Fig. 2. Experimental and calculated FT-IR spectra of SnO2. Fig. 3. Experimental and calculated FTRaman spectra of SnO2.

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at HF, LSDA, B3LYP and B3PW91 levels using the double splitvalence basis set along with the diffuse and polarization functions,3-21G(d,p) and observed FT-IR and FT-Raman frequencies forvarious modes of vibrations have been presented in Table 2.

Sn–O vibrationsMany simple metal oxides with more than one oxygen atom

bound to a single metal atom usually absorb in the region 1020–970 cm�1 [27]. Owing to the Nano size effect, the Sn–O stretchingvibrations are found in the region 800–300 cm�1 [28–30]. TheSn–O stretching is usually observed around 670 and 560 cm�1

[31,32]. In this present case, the Sn–O asymmetric and symmetricstretching vibrations appeared with very strong intensity at 610and 520 cm�1 respectively in bulk phase whereas the same areobserved at 630 and 530 cm�1 respectively in Nano phase. Thedeformation is identified at 120 cm�1 in bulk and 230 cm�1 inNano phase. When compared with bulk, all the Sn–O vibrationsare shifted up to higher region of the spectra which is purely dueto the change of bulk Sn–O to pseudo (super atoms) Sn–O [33].

Electronic properties (HOMO–LUMO analysis)The calculations of the electronic structure of SnO2 are opti-

mized in singlet state. The low energy electronic excited states ofmolecule are calculated at the B3LYP/3-21G(d,p) level using theTD-DFT approach on the previously optimized ground-state geom-etry of the molecule. The calculations are performed with gasphase, DMSO and chloroform solvent effect. The calculated excita-tion energies, oscillator strength (f) and wavelength (k) andspectral assignments are given in Table 5. The major contributionsof the transitions are designated with the aid of SWizard program[34]. TD-DFT calculations predict three transitions in the nearultraviolet region for SnO2 molecule. The strong transitions at850.92 nm with an oscillator strength f = 0.0054 in gas phase, the843.08 nm with an oscillator strength f = 0.0054 in chloroformand the 833.14 nm with an oscillator strength f = 0.064 in DMSOare calculated and assigned to an r ? r* transition. As can be seen,the calculations performed at DMSO, gas phase and chloroform arevery close to each other. In view of calculated absorption spectra,the maximum absorption wavelength corresponds to the

Table 3Calculated IR intensity and Raman activity by HF/DFT (LSDA, B3LYP & B3PW91) with 3-21G(d,p) basis set.

S. No. Observed frequency cm�1 Methods

HF/3-21G (d,p) LSDA/3-21G(d,p) B3LYP/3-21G(d,p) B3PW91/3-21G(d,p)

IR intensity Raman activity IR Intensity Raman activity IR intensity Raman activity IR intensity Raman activity

1 630 0.00 74.82 13.27 0.148 12.83 0.256 15.50 0.1892 570 0.66 0.00 0.030 27.15 0.028 29.13 0.0248 30.173 120 46.87 0.00 12.40 0.088 16.74 0.138 18.61 0.103

Fig. 4. The atomic orbital compositions of the frontier molecular orbital for SnO2.

Fig. 5. Electrostatic potential surface map.

Fig. 6. Preparation flowchart of SnO2.

A. Ayeshamariam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1135–1143 1139

electronic transition from the HOMO to LUMO with 75% and fromthe HOMO to LUMO+1 with 22% contribution. The other wave-length, excitation energies, oscillator strength and calculated coun-terparts with major contributions can be seen in Table 5. In nanophase, the strong transitions at 303.94 nm with an oscillatorstrength f = 0.121 in gas phase. In nano phase the transition isshifted down with high oscillator strength which is due to the largeKubo gap d(band gap).

The frontier molecular orbitals play an important role in theelectric and optical properties. The HOMO represents the abilityto donate an electron, LUMO as an electron acceptor. The 3D plotsof the frontier orbitals, HOMO and LUMO for molecule are in gas,shown in Fig. 4. According to Fig. 4, LUMO is mainly localized overthe entire molecule and however HOMO is characterized by acharge distribution on all atoms except Sn atom. The HOMO?LU-MO transition implies an electron density transfer from Sn atom.The HOMO and LUMO energy is �3.7907 eV in gas phase. Energydifference between HOMO and LUMO orbital is called as Kubogap d that is an important stability for structures. The calculatedenergy gaps 3.7907 eV, but in nano phase Kubo gap is 4.079 eV.The increment of Kubo gap ensures the enhanced semiconductorproperty of the molecule. The Fig. 5 showed that the electrostaticpotential map of the present compound.

NMR analysisNMR spectroscopy is currently used for structure of organic

molecules as well as metal. The combined use of experimentaland computer simulation methods offer a powerful way to inter-pret and predict the structure of large molecules. The optimizedstructure of SnO2 is used to calculate the NMR spectra at the HFand DFT (B3LYP andB3PW91) methods with 3-21G(d,p) level using

Table 4Experimental and calculated 1H and 13C NMR chemical shifts (ppm) of SnO2.

Atom position Degeneracy HF B3LYP B3PW91

Sn 1 3139.25 1772.58 1949.45O 2 �6669.66 �6373.11 �6082.52

Nano phaseSn 1 2205.89O 2 �1756.37

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the GIAO method. The theoretical 1H and 13C NMR chemical shiftsof SnO2 have been compared with the experimental data as shownin Table 4. Chemical shifts are reported in ppm relative to TMS for1H and 13C NMR spectra. Taking into account that the range of 13CNMR chemical shifts for analogous organic molecules usually is>1000 ppm. The accuracy ensures reliable interpretation of spec-troscopic parameters. In the present work, 13C NMR chemicalshifts in the SnO2 are >1000 ppm, as they would be expected (inTable 4).

Sn atom has the most electronegative property than oxygenatom. The Sn atom which bonded to both the oxygen atom showstoo high chemical shifts. The values of the chemical shift of theB3LYP and B3PW91 are nearly equal and lower than HF value.The calculated chemical shift of the molecule in bulk phase is1772.58 ppm whereas in Nano phase 2205.89 ppm. From thisobservation it is clear that the diamagnetic shielding of SnO2 is alsoincreased in nano phase which results change in magnetic propertyof the compound.

Fig. 7. XRD analysis of SnO2 nano powder.

XRD analysis

XRD patterns for as-prepared and after annealing a tempera-tures from 100 to 800 �C for 2 h, are shown in Fig. 7. All the diffrac-tion peaks are assigned to the tetragonal crystalline phase of SnO2.The characteristics peaks positions are associated with (hkl) planes(110), (101), (200), (211), (220) (002) (310) and (301) respec-tively. The peak positions of as-prepared SnO2 phase agree wellwith the bulk and standard (SnO2) according to the JCPDS cardNo. 88-0287), as shown in Fig. 7. The crystallite size (D) was calcu-lated from line broadening analysis of the diffraction peaks usingthe Scherer equation. The estimated average crystallite size is inthe nanometer scale, about 10 nm for as-prepared and increasesup to 32 nm after annealing at 800 �C, which is the smallest crys-tallite size obtained so far through this route. The lattice constantvalues ‘a’ increases from 4.7461 to 4.7591 Å whereas ‘c’ decreasesfrom 3.2027 to 3.1763 Å. The results agree well with oxides nano-particles; usually they exhibit a lattice expansion with the reduc-tion of strain and dislocation density.

In addition to the fundamental Raman peaks of rutile SnO2, twoweak Raman peaks located at about 632 and 679 cm�1 are alsoobserved, whereas these two Raman peaks are not detected inthe rutile bulk SnO2 shown in Fig. 9. According to Granqvist andHultaker [35], these two weak Raman bands seem to correspondto IR-active Eu(3) TO and A2uLO (TO is the mode of the transverseoptical phonons, LO is the mode of the longitudinal optical pho-nons) modes, respectively. Generally, the relaxation of the k 1=4 0

Table 5Theoretical electronic absorption spectra of SnO2 (absorption wavelength k (nm), excitationDMSO, chloroform and gas phase.

DMSO Chloroform Gas

k (nm) E (eV) (f) k (nm) E (eV) (f) k (nm

833.14 1.4881 0.064 843.08 1.0746 0.0063 850.92

Nano phase [solid]303.94 1.716 0.121 –

a H: HOMO; L: LUMO.

selection rule is progressive when the rate of disorder increasesor the size decreases, and infrared (IR) modes can become weaklyactive when the structural changes induced by disorder and sizeeffects take place is shown in Fig. 8. And also, it is well known thatin an infinite perfect crystal only the phonons near the center ofthe Brillouin zone (BZ) contribute to the scattering of incident radi-ation due to the momentum conservation rule between phononsand incident light. As the crystallite is reduced to Nano scale, thephonon scattering will not be limited to the center of the Brillouinzone, and phonon dispersion near the center of Brillouin zone mustalso be considered. As a result, the symmetry-forbidden modes willbe observed, in addition to the shift and broadening of the first-or-der optical phonon.

UV–Vis–NIR analysis

The relationship between the absorption coefficient and opticalband gap can be expressed as,

aht ¼ A ðhtEgÞ1=n ð3Þ

where a is the absorption coefficient corresponding to frequency tand Eg is the band gap energy. The constant n depends on the natureof electronic transition. SnO2 possess direct allowed transition andhence n is equal to 2. The optical band gaps of SnO2 were deter-mined by extrapolating the linear portion of the curve from the plotof (aht)2 versus ht to aht = 0. For allowed transition (a)2 is plotted

energies E (eV) and oscillator strengths (f) using TD-DFT/B3LYP/3-21G(d,p) method in

Gas Assignment

) E (eV) (f) Major contributiona

1.4571 0.0054 H ? L (75%), H ? L+1 (22%) r ? r*

– – r ? r*

Fig. 8. FTIR Spectrum of SnO2 nano powder.

Fig. 9. FTRaman spectrum of SnO2 nano powder.

Fig. 10a. UV–Vis–NIR reflectance spectra of SnO2 as-prepared and annealed up to800oC.

Fig. 10b. UV–Vis–NIR direct allowed region of SnO2 as-prepared and annealed upto 800 �C.

Fig. 11. Photoluminescence spectra of SnO2 annealed up to 800 �C.

A. Ayeshamariam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1135–1143 1141

against energy (E = ht) to yield a straight line for direct transitionsas shown in Fig. 10a from which the band gap was found. In thisstudy, the obtained optical band gap values show a small variationbetween 3.476 and 3.048 eV, for as-prepared and samples heatedup to 800 �C, as shown in Table 6.

When annealing temperature is increased from 100 to 800 �C,the band gap decreases due to mainly two factors: (i) decrease ofcarrier concentration and (ii) un-filling (empty state) of the elec-tronic states near the bottom of conduction band of SnO2 lattice.When annealing temperature increases, the grain sizes are largerthereby the density of grain boundary decreases and consequentlythe number of trapped carriers is less which makes the availabilityof higher amount of carriers for conduction. Such a variation in

Table 6UV–Vis band gap values and (PL) band gap values of SnO2 as-prepared and annealed up to800 �C.

SnO2 ASP 100 �C 200 �C 300 �C 400 �C 500 �C 600 �C 700 �C 800 �C

UV–Vis–NIR Eg (eV) 3.476 3.444 3.345 3.311 3.245 3.179 3.098 3.080 3.048(PL) Eg (eV) 3.735 3.69 3.679 3.625 3.615 3.298 3.289 3.255 3.229

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carrier concentration leads to a modification in the optical bandgap of degenerate semiconducting material which is related tothe Burstein-Moss effect [36] and represented as:

Eg � Ego ¼ DEgBM ¼h

2m�ð3p2neÞ

2=3 ð4Þ

where Ego is the intrinsic (un-doped) band gap, m⁄ is the electroneffective mass, ne number of carrier concentration.

The decrease of band gap for the SnO2 nanoparticles afterannealing at temperatures in the range 100–800 �C may be attrib-uted to some oxygen deficiency which is also based on Burstein-Moss shift. This can be due to the fact that the carrier concentrationof these SnO2 nanoparticles gets reduced because of the creation ofacceptor type non-stoichiometric defects produced with increasingtemperature which may act as compensation centers. Informationconcerning optical reflectance is important in evaluating the opti-cal performance of conducting oxide particles. An optical reflectionspectrum of pure SnO2 particles is shown in Figs. 10a and 10b andgap values of SnO2 powders annealed at different annealingtemperature are shown in Fig. 10b. It can be noticed that theoptical reflection decreases may be due to subsurface scattering,particularly true for reflections from surroundings and broadeneddecrease in reflection of nano particles may be due to the rough-ness of the surface of the nano particles.

The ripples observed in these spectra results from the interfer-ence light, since they show waveform that are characteristic of theinterference light [37]. Higher annealing temperature led to asteeper optical reflection curve, which indicates a better largercrystallinity of the particles and lower carrier density near theband edge. Furthermore, there was a shift in the absorption edgeto shorter wavelength with increasing annealing temperature,which was due to the Burstein Moss shift [38].

Photoluminescence analysis

Photoluminescence spectrum is convenient to investigate thestructure and defect or impurity levels [39]. Fig. 11, shows theroom temperature PL spectrum for the SnO2 as-prepared andannealed up to 800 �C samples. The excitation wavelength usedfor the PL measurements is 210 nm. PL spectra show a strong UVemission peak for all SnO2 samples and their corresponding bandgap values are given in Table 6. In polycrystalline and nano-crystal-line oxides, oxygen vacancies are the most common defects, whichact as radiative centers in photoluminescence emission processes.These peaks might have risen from the luminescence center of Sninterstitials present in SnO2 nano particles [40].

Conclusion

The size of SnO2 particles can be tuned from 10 to 32 nm byheating at different temperatures. The UV absorption of SnO2 nanocrystals occurs at 350 nm, indicating the existence of a weak quan-tum confinement effect. PL emissions of as-prepared SnO2 nanocrystals centered in the range of 330–370 nm and were originatedfrom various oxygen vacancies. It is interesting to note here thatthe amount of reduction of thermal conductivity in nano sizedSnO2 from bulk SnO2 can be associated with the reduction of oxy-gen vacancy within nano phase. The optimized parameters of SnO2

are calculated at the HF and DFT (LSDA, B3LYP andB3PW91) meth-ods with 3-21G(d,p) level and NMR spectra simulated using theGIAO method. From the molecular geometry, it is found thatSnO2 is symmetry top molecule and belong to C2V point groupsymmetry. From the molecular vibrations it is clear that, whencompared with bulk, all the Sn–O vibrations are shifted up to high-er region of the spectra which is purely due to the change of bulkSn–O to pseudo (super atoms) Sn–O.

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