Structural analysis of nanostructured iron antimonate by experimental and quantum chemical simulation and its LPG sensing

Accepted Manuscript Title: Structural analysis of nanostructured iron antimonate by experimental and quantum chemical simulation and its LPG sensing Author: Satyendra Singh Vineet Gupta B.C. Yadav Poonam Tandon A.K. Singh PII: DOI: Reference:

S0925-4005(14)00050-1 http://dx.doi.org/doi:10.1016/j.snb.2014.01.033 SNB 16453

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

23-10-2013 6-1-2014 8-1-2014

Please cite this article as: S. Singh, V. Gupta, B.C. Yadav, P. Tandon, A.K. Singh, Structural analysis of nanostructured iron antimonate by experimental and quantum chemical simulation and its LPG sensing, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.01.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structural analysis of nanostructured iron antimonate by experimental and quantum chemical simulation and its LPG sensing Satyendra Singh1,2, Vineet Gupta1,3, B.C. Yadav*1,4, Poonam Tandon1 and A.K. Singh5 1

Department of Physics, University of Lucknow, Lucknow-226007, U.P., India 2

Department of Physics, University of Allahabad, Allahabad, U.P., India

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Department of Physics, Banaras Hindu University, Varanasi-221005, India 4

Department of Applied Physics, School for Physical Sciences, Babasaheb Bhimrao Ambedkar University, Lucknow-226025, U.P., India 5

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School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221005, India

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*Email address: [email protected], Mobile: +919450094590

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Abstract In this paper structural, electrical, magnetic as well as liquefied petroleum gas (LPG) sensing properties of the synthesized ultrafine iron antimony oxide along with quantum chemical

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simulation have been reported. A detailed study of the structural analysis is presented

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including a thorough Raman spectroscopy and infra-red investigations. A detailed vibrational analysis of iron antimony oxide was performed by ab-initio Hartree-Fock (HF) and density

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functional theory (DFT) employing B3LYP exchange correlation functional with LanL2DZ and 6-311++G(d,p) basis sets. The observed spectral patterns were compared and assigned

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with fundamental vibrational frequencies showing an overall excellent agreement. X-ray diffraction along with Rietveld analysis was used to confirm the crystal structure, space group

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and crystallite size. The estimated value of minimum crystallite size was found 2 nm and confirmed by Rietveld and Vibrational spectral analysis. Scanning electron microscopy,

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Elemental mapping and Energy dispersive X-ray analysis were applied for surface

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morphology, elemental distributions and compositions of the material respectively. The

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synthesized nanoparticles were used for the processing of gas-sensing device and the outstanding gas-sensing properties are accessible, proving the effectiveness of the whole process in advancing towards a new generation of gas-sensor. Keywords: Structural analysis; Quantum chemical simulation; LPG sensor; Surface morphology; Super-paramagnetism.   

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1. Introduction: Transition metal oxides exhibit an amazing spectrum of properties and applications [1-3]. Fe2O3 is an excellent example as it possesses interesting magnetic properties, and it has

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been used in gas sensing, pigments, photovoltaic solar cells, UV-photo detectors and even catalyst [4-9]. Nanostructuring of Fe2O3 can enhance the performance of this important

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functional material and provides unique properties that do not exist in its bulk form [9].

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Exceptional merits of nanostructured Fe2O3 compared to the bulk material include: (i) increased surface-to-volume ratio, which provides more surface area for both chemical and

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physical interactions; (ii) significantly altered surface energies that allow tuning and engineering of the material’s properties, as atomic species near the surface have different

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band structures than those embedded in the bulk; and (iii) quantum confinement effects, due to the inherently small size of nanostructured materials, that significantly influences charge

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transport, electronic band structure and optical properties.

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Many strategies have been used to improve the sensitivity and response/recovery times of the sensor, such as: using catalytic additives, increasing the operating temperature,

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using porous nanostructures, and extending the range of the measurement parameters [10-14]. More recently, porous structures have been used to enhance the response and recovery speeds by rapid gas diffusion through the pores. The response/recovery kinetics will be enhanced further by doping the catalysts to promote a surface reaction. The emphasis is currently being placed on the development of sensor materials for the detection of LPG that offer high sensitivity, short response and recovery times, superior reproducibility and selectivity. However, most of the sensors developed so far were kinetically slow with a limited sensitivity for the detection of LPG below its permissible level. Nowadays, lots of methodologies for the synthesis of nanomaterials are being exploited [12-14]. In this paper we apply a simple procedure for the synthesis at moderate

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temperatures and with short processing times. With the aim of increasing the sensitivity of the LPG sensor; here we added the antimony as a catalytic additive to the ferric oxide. Antimony acts as catalysis and increases the rate of reaction (adsorption/desorption) between

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the gas molecules and sensing surface. This improves the sensitivity, response and recovery times of the sensor.

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Iron antimony oxide is a material of major technological importance due to its applications as a catalyst in the selective oxidation of hydrocarbons [15]. It also possesses

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interesting magnetic properties as well. The FeSbO4 structure is a rutile-like framework with

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Fe and Sb cations distributed in the octahedral sites within the oxygen lattice [16]. From the point of view of catalytic properties, iron-antimonate belongs to a family of oxidation

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catalysts in which the oxygen is taken from the catalyst surface, which is then re-oxidised by the oxygen in the gas phase [17]. Therefore, in the present investigation a special attention

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was focused on FeSbO4 in order to obtain reliable gas sensors operable at room temperature.

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Tianshu et al. investigated the LPG sensing characteristics of FeSbO4 at higher operating temperatures and the sensitivity of the sensor was found considerably small and cumbersome

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to use [18]. Here our group have investigated the LPG sensing characteristics of FeSbO4 thin film at room temperature which is more reliable, stable and robust for commercial point of view.

In the present manuscript, the combination of semiconducting properties and catalytic

surface activity is most relevantly investigated which establishes the concept of changes in electrical resistance of FeSbO4, depending on the amount of surface bound oxygen atoms. It has been shown that the fabricated film has improved gas sensing performance having both high sensitivity and fast response. Sufficient stability and good reproducibility of the sensing performance have been achieved. The response and recovery of the sensor was measured from the electrical resistance changes upon the LPG injection and purging. Surface

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morphological, compositional and structural investigations were performed to understand the mechanism of the gas sensing properties. Surface modifications with catalytic metal nanoparticles and the use of oxides have been explored to increase the sensitivity of ferric

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oxide sensors to LPG. It has been reported that doping of antimony can modify the spacecharge layer thickness of the sensing film and hence the LPG sensing performance.

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To the best of our knowledge the LPG sensing characteristics of iron antimonate thin film at room temperature has not been investigated as yet. In the present investigation, the

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response of the fabricated sensor was found much better than the earlier reported work on

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LPG sensor [19-30]. Thus, the present investigation shows a significant advancement towards the fabrication of a robust and trusts worthy LPG sensor operable at room temperature. Also

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we report the first combined experimental and theoretical study on molecular structure and vibrational properties of iron antimony oxide.

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2. Experimental and computational details:

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Iron-antimonate was synthesized by a sol-gel spin coating process. We have used

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ferric and antimony chlorides as starting materials. 0.1 M solution of each starting material was made by dissolving it into the required amount of absolute ethanol under constant stirring of 6 hrs at 50 ºC. Further, the above two gels were properly mixed and the obtained solution was refluxed at 50 °C for 6 hrs in a rotary vacuum evaporator. For the formation of the powder, the resulting solution was precipitated using a controlled drop wise addition of ammonium hydroxide under continuous stirring to achieve simultaneous precipitation of both iron and antimony hydroxides. The chemical co-precipitation reaction that took place is as follows: NH 4 OH Ethanolic solution of (FeCl3 + SbCl3 ) ⎯⎯⎯⎯⎯ → ppt. of Fe and Sb hydroxides dropwise addition

450 º C 2Sb(OH) 3 + 2Fe(OH) 3 + O 2 ⎯⎯⎯ → 2FeSbO 4 + 6H 2 O

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Thus the powder of iron-antimony oxide was formed. Thin film of iron antimony oxide was fabricated using precursor solution of iron and antimony chlorides onto an ultrasonically cleaned alumina substrate. The fabricated film was stabilized at room temperature for 6 hrs

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and then it was annealed at 450 ºC for 2 hrs. The annealing process converts the film as a sensing material. Silver contacts on both ends of the film were made for signal registration.

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The thickness of the film was found 300 nm measured by accurion variable angle spectroscopic ellipsometer (Nanofilm EP3 Imaging).

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For the gas sensing measurements, the sensing film with silver contacts was placed

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within a specially designed gas chamber having gas inlet and outlet knobs. The details of the sensing set up are described in our previous paper [31]. The gas chamber was exposed to

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LPG and the variations in electrical resistance of the sensing material with the time were recorded using a Keithley electrometer (Model 6514). The fabricated film displays

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outstanding gas-sensing performances at room temperature.

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Hartree-Fock [32] and density functional [33] calculations were carried out for FeSbO4 model (Fig. 1) reported by Grau-Crespo et al [34]. Density functional calculations

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were performed by applying the hybrid of Becke’s nonlocal three parameter exchange and correlation functional and the Lee-Yang-Parr correlation functional (B3LYP) [35-37]. The LanL2DZ basis set [38] for iron and antimony and 6-311++G(d,p) split valence-shell basis set augmented by ‘d’ and ‘p’ polarization functions as well as diffuse functions for oxygen were used [39-40]. The molecular geometry optimization and vibrational frequencies calculations for dimer of FeSbO4 model were performed and compared with experimental spectra. All these calculations were performed with Gaussian 09 program [41].

3. Results and discussion: 3.1 Surface morphological analysis:

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The surface morphology of the sensing film was analyzed by Scanning electron microscope (SEM, LEO-0430 Cambridge). Figs. 2 (a) and (b) show the surface morphology of sensing film before and after exposing to LPG, respectively. Representative SEM images

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show surface morphology of FeSbO4 film in an agglomerated form with no specific structure. The small particles are almost spherical in shape, but particles bigger than 100 nm show some

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facets. Fig. 2 (a) shows that the particles are mostly irregular in shape and some particles are found as agglomerated and leaving some spaces as pores. The pores serve as gas adsorption

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sites and gas sensitivity depends on the size and number of these pores. Since sensing surface

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has dangling bonds, therefore, surface can be chemically very reactive. Owing to this reactivity, sensing surface easily adsorbed LPG in contact with them and accompanying

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surface interaction. Therefore, after exposition of LPG, the spherical particles adsorbed LPG and swelled out; which has been shown in Fig. 2 (b). When the LPG was exposed to the

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surface of the film then the gas diffuses at the pores where reaction of LPG molecules with

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material surface occurs. Therefore, when the LPG is adsorbed on the surface of sensing film, it reduces the size of pores which can be clearly visualised in Fig. 2 (b).

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XRD suggests much smaller particle sizes than those observed in the SEM images.

This may be due to the fusion of a large number of small crystallites, exhibiting an internal composite structure, or the agglomeration of particles to form at random surface morphology. Proposed by the Gibbs-Thomson expression, the reduction of the size of FeSbO4 crystallites enhances the surface energy of the system, and this enhanced surface energy increases the rate of adsorption of LPG. Gas sensitivity is observed to have a strong dependency on the surface morphology [11, 13, 28]. 3.2 Elemental mapping and Energy dispersive X-ray analysis: Energy dispersive X-ray spectroscopy was used for identifying the elemental compositions of the synthesized material. Spectrum study reveals the presence of antimony,

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iron, oxygen, gold and carbon elements in the compound with 7.20, 20.47, 64.45, 0.99 and 6.89 atomic weight percentages respectively. Elemental mapping is shown by Fig. 3 in which Fig. 3 (a) shows the homogeneous distribution of various elements such as antimony, iron

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and oxygen while Figs. 3 (b), (c) and (d) show independent distribution for oxygen, iron and antimony, respectively.

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3.3 X-ray diffraction analysis:

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 The XRD pattern of the synthesized material was recorded by X-ray diffractometer

(X-Pert PRO PANalytical) and is shown in Fig. 4 (a). All the diffraction peaks that appear in

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the pattern are identified and indexed from the known patterns of the standard powder X-ray diffraction data. The synthesized material exhibits four diffraction peaks at (110), (101),

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(211) and (301) planes. These XRD peaks correspond to FeSbO4. No peaks of α-Fe2O3 or Sb2O3 are detected. Thus XRD pattern reveals that Fe2O3 and Sb2O3 phase disappear

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completely at 450 ºC and formed FeSbO4 compound completely. Single phase FeSbO4

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obtained at 450 ºC might be due to the strong contact between antimony and iron atoms and

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the possibly high diffusion rate. The XRD reflections assigned to the FeSbO4 appear too broad. The crystallites of FeSbO4 laid in the range 2-4 nm, estimated by employing DebyeScherrer’s formula. The material has tetragonal crystal structure. The calculated lattice parameters were a = b = 4.5820 Å and c = 3.0854 Å. These tally quite well with the lattice parameters of iron-antimonate given in JCPDS data card (a = b = 4.6351 Å and c = 3.0734 Å). This shows that the material consists of tetragonal FeSbO4 crystal structure. Rietveld profile analysis of experimental XRD pattern provides information about a material’s space group and structural parameters. Rietveld profile analysis and the Scherrer’s formula were employed to determine the structure and average crystallite size. The analysis of XRD data and the Rietveld fit for P 42/m n m space group is shown in Fig. 4 (b). The minimum crystallite size was found ~ 2 nm from this analysis which is in well agreement

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with Scherer’s result. We can see from the figure that first peak is much broader than the second one and it is difficult to fit the both. The cell parameters are also different from that reported for bulk sample values. It can be due to size/shape and strain effects or due to a

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different crystal structure as well. 3.4 UV-visible absorption spectroscopy:

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UV-visible absorption was done by UV-visible spectrophotometer (Varian, Carry-50

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Bio). Variations in the absorption with the wavelength were recorded. The absorption data was further used for analyzing optical band gap energy (Eg) using the formula for optical

K ( hν-E g )

n/2

hν

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α =

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absorption of a semiconductor:

Where ‘α’ is the absorption coefficient, ‘K’ is a constant, ‘Eg’ the optical band gap and ‘n’ is

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the number equal to 1 for a direct band gap and 4 for an indirect band gap material. The plot

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of (αhν)2 versus hν was used for estimating the value of direct band gap energy of FeSbO4 by extrapolating the linear part of the curve to zero absorption and is shown by Fig. 5. The

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estimated value of the band gap was found 3.8 eV. The extremely small size of nanoparticles will result in quantum confinement, leading to a blue shift in the absorption spectrum [42]. 3.5 Differential scanning calorimetry analysis: Thermal behaviour of the as-synthesized material was investigated by Differential

scanning calorimetry (Shimadzu), in a nitrogen flux (40 ml/min) and with a heating rate of 10 ºC/min. DSC curve of as synthesized powder is shown in Fig. 6. The curve shows three peaks at 56, 179 and 271 °C. These peaks may correspond to the dehydration of the sample, formation of Fe2O3 and Sb2O3, respectively. The first strong endothermic peak at 56 °C indicates the removal of physisorbed water and water of crystallization of some of hydrated ferric oxide. The second peak at 179 °C may correspond to the complete departure of water 9   

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of crystallization of hydrated ferric oxide yielding formation of Fe2O3. The third peak at 271 °C refers to the iron antimony hydroxide and formation of its oxide. The next broad

3.6 Study of FT-IR and Raman spectra of iron antimonate:

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endothermic peak starting from 300 °C may corresponds to the formation of FeSbO4.

The iron antimony oxide was also investigated by Raman and infrared spectroscopy.

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For the recording of FT-IR spectra, sample was prepared by pressing the iron antimonate

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powder mixed with KBr in a weight ratio 1:100 by hydraulic press into the pellet form. Spectrum was recorded using a Bruker Tensor 27 FT-IR spectrometer using 250 signal

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averaged scans at a resolution of 4 cm-1.

We have presented an experimental and theoretical investigation of the vibrational

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properties of the iron antimony oxide. The Raman and infrared spectra of iron antimony oxide were measured for the first time and thus serve as fingerprints in structural analysis.

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The geometry optimizations of isolated monomer and dimer of iron antimony oxide were

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studied by comparing Hartree-Fock (HF) and Density functional theory (DFT). The infrared

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and Raman spectra of FeSbO4 as a model molecule were also simulated in the harmonic approximation. All prominent peaks of the Raman and infrared spectra of iron antimony oxide coincide with calculated frequencies and assigned to the normal modes of the iron antimony oxide molecule. The spectra of iron antimony oxide are interpreted for the first time in terms of DFT; furthermore the DFT force fields calculated using the B3LYP functional yields infrared and Raman spectra in good agreement with experiments. The B3LYP/LANL2DZ//6-311++G(d,p) calculations predict nearly the same geometry for iron antimony oxide, the optimized geometries reveal that the four-membered rings of FeSbO4 display similar Fe-O and Sb-O bond lengths. Table 1 summarizes the selected optimized geometrical parameters for iron antimony oxide using HF and DFT. Size of the single crystal of iron antimony oxide can be about the three times of the Sb12 and O9 atomic distance in the 10   

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dimer of FeSbO4. The three times of the atomic distance between Sb12 and O9 yields 17.508 Å using DFT calculations, which agrees with the experimental crystallite size. DFT calculations yield Raman scattering amplitudes which cannot be taken directly to

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be the Raman intensities. The Raman scattering cross section, ∂σ/∂Ω, which are proportional to Raman intensity may be calculated from Raman scattering amplitude and predicted wave

∂Ω

j

⎛2 π = ⎜⎜ ⎝ 45

4

⎛ ⎜ (ν 0 ⎞⎜ ⎟⎟⎜ ⎠⎜ ⎜ 1 − exp ⎝

⎞ ⎟ −ν j ) ⎟⎛ h ⎟ ⎜⎜ 2 ν hc − ⎡ j ⎤ ⎟ ⎝ 8π c ν ⎢ ⎥⎟ kT ⎣ ⎦⎠ 4

⎞ ⎟S ⎟ ⎠

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∂σ

4

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numbers for each normal mode using the relationship [43-44]:

j

j

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Where Sj and υj are the scattering activities and the predicted wave numbers, respectively of the jth normal mode, υ0 is the wave number of Raman excitation line, T is the temperature and

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h, c and k are universal constants. The calculated and experimental FT-IR and Raman spectra of iron antimony oxide are compared in Fig. 7 (a) and (b) respectively. Both Raman and IR

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spectra were simulated using line shape of Lorentzian curves type with 8 cm-1 FWHM (the full width at half-maximum) for each peak. The Raman intensities obtained using this

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relationship match quite nicely with the experimentally observed intensities. The selected harmonic vibrational bands using DFT with experimental Raman and infrared bands are assigned in Table 2 for FeSbO4 dimer. In the infrared experimental spectra 825-890 cm-1 region is poorly resolved and one

broad peak at 854 cm–1 with shoulders appears. The infrared Fe-O stretching frequencies fall into the 867-775 cm–1 and bending into the 740-622 cm–1 regions. In case of DFT calculated infrared and Raman spectra the peaks due to Sb-O-Sb (741 cm–1), Fe-O-Sb (701 cm–1) and OFe-O (652 cm–1) bending can be found. Notice that, Sb-O-Sb bending modes fall around 741 cm–1, in addition to the peak at 763 cm–1, attributed to Sb-O stretching, to other very weak peaks at 714, 700 and 684 cm–1 are found and assigned to the modes with the closest

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calculated values. The Raman spectrum of iron antimony oxide has an intense band at 746 cm-1 assigned to a Sb-O stretching vibration associated to the DFT at 762 cm-1. The Raman band is calculated at 418 cm-1 using DFT that we correlate with the experimental band at 404

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cm-1 and assign to the O3-Sb11 torsion and O6-Fe1-O8 bending mode on the basis of its theoretical eigen-vector using gauss view program. In Raman spectra, the components of the

mode with the O10-Fe4 torsion and Fe1-O3-Sb11 bending mode.

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mode have very weak intensity in 490 cm–1, and are attributed to the Sb12-O5,7 wagging

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The frontier highest occupied molecular orbital (HOMO) and lowest unoccupied

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molecular orbital (LUMO) of the monomer of FeSbO4 model using DFT are plotted in Fig 8. The HOMO-LUMO transition is clearly accompanied by significant charge transfer between

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the metal and molecule. The energy gap between HOMO and LUMO is 2.65 eV. 3.7 Magnetic properties:

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Magnetization studies were conducted to determine the magnetic nature of the iron

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antimony oxide at room temperature. Magnetic properties of iron antimonate were measured

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at room temperature (298 K) using Vibrating Sample Magnetometer (VSM Lakeshore 7410) and is shown in Fig. 9. It demonstrates that the magnetization decreasing to zero when the applied field is removed which reveals superparamagnetic nature. Superparamagnetism is a unique phenomenon exhibited in some nanostructured materials that would otherwise show paramagmetic behavior above a critical size. Nanostructured superparamagnetics can act as single-domains and are therefore small enough to overcome energy barriers that would prevent alignment in the direction of the field at room temperature. As a result, SPMs have an increased susceptibility and do not require very high magnetic fields or low temperatures to attain saturation. Because these materials require the presence of a field to align their magnetic moments and magnetize, they lose most or all of their magnetization once the field is removed. The iron antimony oxide nanoparticles with average crystallite size ~ 3 nm have

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almost no remenant magnetization at zero magnetic field strength, as can be seen in Fig. 9, which is an indication of superparamagnetism. The smallest crystallites (2 nm) are apparently single-domain particles, which also exhibit superparamagnetism. This implies that the

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magnetic domain size of these particles is approximately or perhaps slightly larger than 2 nm. In superparamagnetic materials, responsiveness to an applied magnetic field without retaining

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any magnetism after removal of the magnetic field is observed. This behaviour is an important property for magnetic targeting carriers. In fact, the difference between

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ferromagnetism and superparamagnetism fabricates in the particle size. Literature data imply

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that below a critical size, the particles show the character of superparamagnetism. When decreasing the size of magnetic particles, they change from multi domain to single domain. If

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the single-domain particles become small enough, the magnetic moment in the domain fluctuates in direction, due to thermal agitation which leads to super paramagnetism.

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3.8 Gas sensing properties of iron-antimonate film:

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We have investigated the LPG sensing properties of iron-antimonate film. Variation

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in the resistance with the time for different concentrations of LPG were recorded and shown in Fig. 10 (a), which exhibits that resistance increases sharply at initial stage of exposure. Later it increases slowly and then decreases up to its initial value (Ra) for further range of time. The variations in sensitivity of the sensing film with concentration of LPG are shown in Fig. 10 (b). It reveals that sensitivity increases linearly for 1-2 vol.% of LPG, and after that it increases slowly. The low concentration implies a lower surface coverage of gas molecules, resulting in a lower surface reaction between the surface adsorbed oxygen species and the gas molecules. The increase in LPG concentration increases the surface reaction due to a large surface coverage. Further on increasing the concentration of LPG, the surface reaction does not increase and eventually saturation takes place. The linearity of sensitivity for the LPG (≤ 2 vol.%) suggests that the FeSbO4 sensing film can be reliably used to monitor the LPG over 13   

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this range of concentration. The value of maximum sensitivity was 2.9 MΩ/second for 5 vol.% of LPG. It is easy for the gas molecule to adsorb on the surface of the sensing film and to diffuse through the pores. With increase in the concentration of the LPG, the interaction

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between the gas molecules and the film surface enhances due to which the sensitivity of the sensor increases. Average sensor response for the film as a function of LPG concentration

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was calculated which shows that as the concentration of LPG increases, the average sensor response increases. The maximum average sensor response was 89.7 for 5 vol.% of LPG as

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maximum variation in resistance was observed for 5 vol.%. This high value of sensor

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response may be attributed to adsorption of LPG and reaction between LPG and the adsorbed oxygen species. The values of the response and recovery times of the sensor were ~ 120 and

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200 seconds respectively. Fig. 10 (c) shows the reproducibility curve of the sensing film after one month of the fabrication of the film. It was found that after one month, it performs 98%

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of its initial performance. Thus the sensor showed a high stability.

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Basic mechanism involved in the gas sensor is adsorption and desorption of the target gas. The resistance of the film was reported to increase with exposition of LPG, due to an

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increase in the volume of the grain boundaries, which contributes to more trapping and scattering of free charge carriers. If the size of the crystallites is smaller than the electron mean free path, grain boundary scattering dominates and hence the electrical resistance increases. In addition, lattice strain and crystal distortions can also affect the motion of charge, causing an increase in the resistance of the film. Improvements in the sensing properties can be expected for several reasons. Owing to

the high surface-to volume ratio in an aggregate of nanoparticles, a significant increase in the number of adsorption sites for gaseous molecules may take place, with the adsorption phenomena clearly recognized as fundamental in determining the gas-sensing mechanisms. Further, the reactions of the gaseous species to be detected are generally assumed to take

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place with adsorbed oxygen species on the oxide surface, leading to a modulation of the charge-depletion layer previously induced by the oxygen adsorption. With nanosized particles, such a depletion layer may extend through regions whose size is comparable with

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the particle size. The reaction with the gaseous molecule will then affect a large region of the oxide nanoparticles, inducing the large sensor responses possibly coupled with the fast

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response and recovery times. When the reaction of the gaseous molecules takes place directly with the oxide surface, a nanostructured material may offer particularly strained surfaces and

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hence highly active surface sites that could beneficially influence the reaction rates with the

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LPG. The increased reactivity of the nanostructured materials at room temperatures may result in an increase in the sensitivity, with a decrease of required power consumption and a

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beneficial effect on sensor stability and lifetime.

Hydrocarbon gets dissociated on the surface of the film before reacting with adsorbed

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oxygen species. Since dominant constituents of LPG are butane and propane thereby

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reactions pertaining to the components must be considered. For butane and propane, the activation of the C-H bond is the first crucial step in all oxidation reactions. Once the first

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bond is broken, sequential reactions to carbon dioxide and water are relatively facile. The proposed reaction is given as:

C n H 2n +2 + O −2 ⎯ ⎯ → C n H 2n O + H 2 O + e −

Where, CnH2n+2 represent the various hydrocarbons. When the LPG reacts with the surface oxygen ions then the combustion products such as water depart and a potential barrier to charge transport would be developed i.e., this mechanism involves the displacement of adsorbed oxygen species by formation of the water. The formation of barrier is due to the reduction in the concentration of conduction carriers and thereby, increases the resistance of the sensing material with time. As the concentration of the gas inside the chamber increases, the rate of the formation of such product increases and potential barrier to charge transport

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becomes strong which has stopped the further formation constituting the resistance constant. Antimony acts as a catalyst for dissociation of oxygen over the film surface, and thus, enhances the spillover of oxygen species over the film surface which will enhance the

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sensitivity of the sensor. 3.9 Electrical properties of iron antimony oxide:

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Variations in electrical resistance of FeSbO4 film with temperature were observed. In

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the lower temperature range, FeSbO4 has high resistance. But, with increasing temperature; its resistance became small and smaller. The change in temperature will alter the resistance

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because both the charge of the surface species ( O2 , O −2 , O − or O 2− ) as well as their coverage can be altered in this process. Fig. 11 shows the Arrhenius plot for the sensing film. By

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measuring the slope of Arrhenius plot of a linear zone, we have calculated the activation energy. Its value was found 0.49 eV. As the interaction probability of sensing material with

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LPG is given by the Boltzmann factor exp(-Ea/kT). Therefore for the higher probability of

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interaction to be occurring, Ea should be least for the room temperature LPG sensors. Thus,

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the small value of the activation energy (0.49 eV) of the sensing film is a significant factor for the detection of LPG at room temperature. Noble metal catalyst such as antimony which coated on the oxide surface are found to improve the sensing capabilities and tends to promote chemical reactions by reducing the activation energy between the oxide and the target gas.

4. Conclusion

A new strategy to produce LPG sensor by combining the gas-accessible sensing layers and catalytic surface additives has been reported. The inclusion of antimony to iron improved the surface-to-volume ratio, allowing more surface area available for reactions, hence producing superior sensing properties. Additionally, the tetragonal FeSbO4 phase structure

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enhances the diffusion of charge carriers with respect to oxygen vacancies within the system, which greatly improves the response time. Rietveld analysis confirmed the minimum crystallite size of FeSbO4 as 2 nm. The maximum sensitivity of film was 2.9 MΩ/second,

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which is ~ 12 times greater than that of pellet. The maximum percentage sensor response of the film was 8970. The sensing characteristic of thin film sensor was 98% reproducible after

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one month. Thus, this study demonstrates the possibility of utilizing nanostructured FeSbO4

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thin film for the detection of LPG at room temperature.

Acknowledgement

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Satyendra Singh is thankful to Council of Scientific and Industrial Research (CSIR), India for senior research fellowship vide award no. 09/107(0331)/2008-EMR. Vineet Gupta

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acknowledges the financial support provided by the University Grants Commission, India

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under the U.G.C.-Dr. D.S. Kothari Postdoctoral Fellowship.

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References: [1] X. Gou, G. Wang, J. Park, H. Liu, Y. Juan, Monodisperse hematite porous nanospheres: synthesis, characterization, and applications for gas sensors, Nanotech. 19 (2008) 125606-

ip t

125612. [2] S. Sun, H. Zeng, Size-controlled synthesis of magnetic nanoparticles, J. Am. Chem. Soc.

cr

124 (2002) 8204-8205.

us

[3] S. Chakrabarti, S.K. Mandal, S. Chaudhuri, Cobalt doped γ- Fe2O3 nanoparticles:

an

synthesis and magnetic properties, Nanotech. 16 (2005) 506-511.

[4] J. Chatterjee, Y. Haik, C.J. Chen, Size dependent magnetic properties of iron oxide

M

nanoparticles, J. Magn. Magn. Mater. 257 (2003) 113-118.

[5] H.T. Sun, C. Cantalini, M. Faccio, M. Pelino, NO2 gas sensitivity of sol-gel-derived α-

te

d

Fe2O3 thin films, Thin Solid Films 269 (1995) 97-101. [6] H. Lihua, L. Qiang, Z. Hui, Y. Lijun, G. Shan, Z. Jinggui, Sol-gel route to pseudo cubic

Ac ce p

shaped α-Fe2O3 alcohol sensor: preparation and characterization, Sens. Actuators B 107 (2005) 915-920.

[7] N.K. Chaudhari, J.S. Yu, Size control synthesis of uniform β-FeOOH to high coercive field porous magnetic α-Fe2O3 nanorods, J. Phys. Chem. C 112 (2008) 19957-19962.

[8] Z. Jing, Fabrication and gas sensing properties of Ni-doped gamma-Fe2O3 by anhydrous solvent method, Mater. Lett. 60 (2006) 3315-3318. [9] P. Sun, L. You, D. Wang, Y. Sun, J. Ma, G. Lu, Synthesis and gas sensing properties of bundle-like α-Fe2O3 nanorods, Sens. Actuators B 156 (2011) 368-374.

18   

Page 18 of 40

[10] S. Polarz, A. Roy, M. Lehmann, M. Driess, F.E. Kruis, A. Hoffmann, P. Zimmer, Structure-property-function relationships in nanoscale oxide sensors: a case study based on zinc oxide, Adv. Funct. Mater. 17 (2007) 1385-1391. J.H. Liu, X.J. Huang, S.H. Yu,

ip t

[11] X. Chen, Z. Guo, W.H. Xu, H.B. Yao, M.Q. Li,

Templating synthesis of SnO2 nanotubes loaded with Ag2O nanoparticles and their

cr

enhanced gas sensing properties, Adv. Funct. Mater. 21 (2011) 2049-2056.

[12] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and

us

hierarchical assembly of hollow Cu2O microspheres with nanocrystals-composed porous

an

multi shell and their gas-sensing properties, Adv. Funct. Mater. 17 (2007) 2766-2771. [13] L. Jia, W. Cai, Micro/Nanostructured ordered porous films and their structurally induced

M

control of the gas sensing performances, Adv. Funct. Mater. 20 (2010) 3765-3773. [14] S.C. Navale, S.W. Gosavi, I.S. Mulla, Controlled synthesis of ZnO from nanospheres to

d

micro-rods and its gas sensing studies, Talanta 75 (2008) 1315-1319.

te

[15] P. Nag, S. Banerjee, Y. Lee, A. Bumajdad, P.S. Devi, Sonochemical synthesis and

Ac ce p

properties of nanoparticles of FeSbO4, Inorg. Chem. 51 (2012) 844-850. [16] R.G. Crespo, I.P.R. Moreira, F. Illas, N.H. Leeuw, C.R.A. Catlow, The effect of coordination on the properties of oxygen vacancies in FeSbO4, J. Mater. Chem. 16 (2006) 1943-1949.

[17] G.W. Keulks, M.Y. Lo, An investigation of the kinetics and mechanism over iron antimony oxide, J. Phy. Chem. 90 (1986) 4768-4775.

[18] Z. Tianshu, P. Hing, FeSbO4 semiconductor ceramics: a new material for sensing liquidpetroleum gas, J. Mater. Sci.: Mater. Electron. 10 (1999) 509-518.

19   

Page 19 of 40

[19] B.C. Yadav, A. Yadav, S. Singh, K. Singh, Nanocrystalline zinc titanate synthesized via physicochemical route and its application as liquefied petroleum gas sensor, Sens. Actuators B 177 (2013) 605-611.

ip t

[20] D.S. Dhawale, R.R. Salunkhe, U.M. Patil, K.V. Gurav, A.M. More, C.D. Lokhande, Room temperature liquefied petroleum gas (LPG) sensor based on p-polyaniline/n-TiO2

cr

heterojunction, Sens. Actuators B 134 (2008) 988-992.

[21] B.C. Yadav, A. Yadav, T. Shukla, S. Singh, Solid state titania based gas sensor for

us

liquefied petroleum gas detection at room temperature, Bull. Mater. Sci. 34 (2011) 1639-

an

1644.

[22] R.B. Kamble, V.L. Mathe, Nanocrystalline nickel ferrite thick film as an efficient gas

M

sensor at room temperature, Sens. Actuators B 131 (2008) 205-209. [23] B.C. Yadav, S. Singh, A. Yadav, T. Shukla, Experimental investigations on nano-sized

d

ferric oxide and its LPG sensing, Inter. J. Nanoscience 10 (2011) 135-139.

te

[24] B.C. Yadav, S. Singh, A. Yadav, Nanonails structured ferric oxide thick film as room temperature liquefied petroleum gas (LPG) sensor, App. Surf. Sci. 257 (2011) 1960-1966.

Ac ce p

[25] S. Singh, B.C. Yadav, R. Prakash, B. Bajaj, J.R. lee, Synthesis of nanorods and mixed shaped copper ferrite and their applications as liquefied petroleum gas sensor, App. Surf. Sci. 257 (2011) 10763-10770.

[26] S. Singh, H. Kaur, V.N. Singh, K. Jain, T.D. Senguttuvan, Highly sensitive and pulse-like response toward ethanol of Nb doped TiO2 nanorods based gas sensors, Sens. Actuators B 171-172 (2012) 899-906.

[27] S.S. Joshi, T.P. Gujar, V.R. Shinde, C.D. Lokhande, Fabrication of n-CdTe/p-polyaniline heterojunction-based room temperature LPG sensor, Sens. Actuators B 132 (2008) 349– 355.

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[28] S. Singh, A. Singh, B.C. Yadav, P.K. Dwivedi, Fabrication of nanobeads structured perovskite type neodymium iron oxide film: its structural, optical, electrical and LPG sensing investigations, Sens. Actuators B 177 (2013) 730-739.

ip t

[29] A.M. More, J.L. Gunjakar, C.D. Lokhande, Liquefied petroleum gas (LPG) sensor properties of interconnected web-like structured sprayed TiO2 films, Sens. Actuators B

cr

129 (2008) 671-677.

[30] S. Singh, B.C. Yadav, V.D. Gupta, P.K. Dwivedi, Investigation on effects of surface

us

morphologies on response of LPG sensor based on nanostructured copper ferrite system,

an

Mater. Res. Bull. 47 (2012) 3538-3547.

[31] T. Shukla, B.C. Yadav, P. Tandon, Synthesis of nanostructured cobalt titanate and its

M

application as liquefied petroleum gas sensor at room temperature, Sens. Lett. 9 (2011) 533-540.

d

[32] J.C. Slater, A simplification of the Hartree-Fock method, Phys. Rev. 81 (1951) 385-390.

te

[33] P. Hohenberg, W. Kohn, Inhomogeneous electron gas, Phys Rev. B 136 (1964) 864-871. [34] R.G. Crespo, N.H. Leeuw, C.R.A. Catlow, Distribution of cations in FeSbO4: a computer

Ac ce p

modeling study, Chem. Mater. 16 (2004) 1954-1960.

[35] C.T. Lee, W.T. Yang, P.G. Parr, Development of the colle-salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785-789.

[36] R.G. Parr, W. Yang, Density functional theory of atoms and molecules; Oxford University Press: New York (1989).

[37] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys. 98 (1993) 5648-5652. [38] P.J. Hay, W.R. Wadt, Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals, J. Chem. Phys. 82 (1985) 299-310.

21   

Page 21 of 40

[39] G.A. Petersson, M.A. Allaham, A complete basis set model chemistry. II. Open shell systems and the total energies of the first row atoms, J. Chem. Phys. 94 (1991) 60816090.

ip t

[40] G.A. Petersson, A. Bennett, T.G. Tensfeldt, M.A. Allaham, W.A. Shirley, J. Mantzaris, A complete basis set model chemistry. I. The total energies of closed shell atoms and

cr

hydrides of the first row elements, J. Chem. Phys. 89 (1988) 2193-2218.

[41] Gaussian 09, Revision A.1, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria,

us

M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H.

an

Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.

M

Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov,

R.

d

Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J.

te

Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R.

Ac ce p

Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc., Wallingford CT, 2009.

[42] D.P. Singh, J. Singh, P.R. Mishra, R.S. Tiwari, O.N. Srivastava, Synthesis, characterization

and

application

of

semiconducting

oxide

(Cu2O

and

ZnO)

nanostructures, Bull. Mater. Sci. 31 (2008) 319-325.

[43] G.A. Guirgis, P. Klaboe, S. Shen, D.L. Powell, A. Gruodis, V. Aleksa, C.J. Nielsen, J. Tao, C. Zheng, J.R. Durig, Spectra and structure of silicon-containing compounds XXXVI- Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignment of ethyldibromosilane, J. Raman. Spectrosc. 34 (2003) 322-336.

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[44] P.L. Polavarapu, Ab initio vibrational Raman and Raman optical activity spectra, J. Phys. Chem. 94 (1990) 8106-8112.

Ac ce p

te

d

M

an

us

cr

ip t

 

23   

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Table 1: Calculated geometrical parameters of iron antimony oxide using Hartree-Fock and

HF/LANL2DZ// 6311++G(d,p)

B3LYP/LANL2DZ//

Fe1-O3

1.9039

1.7147

Fe1-O6

1.9980

Fe1-O8

1.9889

Fe4-O2

1.8433

Sb12-O2

1.8654

us

Density functional theory.

Fe4-O10

1.9040

1.8311

Sb11-O10

1.8960

1.9752

6-311++G(d,p)

ip t

Parameters

1.6695

an

M

Bond angles (˚)

102.9

d

Fe4-O2-Sb11

1.6695 1.8240

1.9872

98.8

109.4

104.5

O9-Sb11-O10

105.0

109.8

Fe1-O3-Sb11

141.6

126.5

76.9

83.9

Sb12-O2-Fe4-O10

180.0

180.0

Fe1-O3-Sb11-O2

0.0

0.0

Fe1-O3-Sb11-O9

180.0

180.0

Fe4-O10-Sb11-O9

180.0

180.0

Ac ce p

te

Fe4-O10-Sb11

cr

Bond lengths (Ả)

O2-Fe4-O10

Dihedral angles (˚)

Table 2: Comparison of calculated and experimental vibrational wavenumbers obtained in iron antimony oxide.

24   

Page 24 of 40

Calculated  (B3LYP/LANL2DZ//

Experimental Raman

Infrared

861

867, 854

855

R νasym(Fe1-O)

833

820, 810, 804

827

R νasym(Fe1-O)

788

790, 775

785

746

763

762

---

740

741

---

714, 700, 684

701

649

653, 644, 622

652

607

607, 594, 582, 555

490

---

404

---

289

---

221

---

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6-311++G(d,p))

R νsym(Fe1-O)

cr

ν (Sb11-O9)

δ(Sb12-O2-Sb11) + ν(Fe1-O3)

us

an

te

d

M

578

δ(Fe4-O10-Sb11) + δ(Sb12O2-Sb11) + ν(Fe1-O3)

δ(O10-Fe4-O2) + δ(Fe4-O2Sb12)

τ(Fe4-O2) + δ(O10-Fe4-O2) + δ(Fe4-O2-Sb12)

480

ω( Sb12-O5,7) + τ(O10-Fe4) + δ(Fe1-O3-Sb11)

418

τ(O3-Sb11) + δ(O6-Fe1-O8)

287

τ(Sb12-O2) + oop(Sb11-O9)

236, 213

ρ(Fe1-O6,8) + ρ(Sb12-O5,7) + τ(Fe4-O2)

Ac ce p

a

Assignmenta

Proposed assignment for vibrational normal modes. Types of vibration: υ, stretching; δ, deformation; oop, out-of-plane bending; ω, wagging; γ, twisting; ρ, rocking; τ, torsion.

25   

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Table Caption Table 1: Calculated geometrical parameters of iron antimony oxide using Hartree-Fock and Density functional theory.

ip t

Table 2: Comparison of calculated and experimental vibrational wavenumbers obtained in

cr

iron antimony oxide.

 

Ac ce p

te

d

M

an

 

us

 

26   

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Author Biographies Satyendra Singh has received his B.Sc., M.Sc. and Ph.D. degrees in 2005, 2007 and 2013 respectively from University of Lucknow, Lucknow, India. Currently he is a D.S.

ip t

Kothari Post Doctoral Fellow in Department of Physics, University of Allahabad, Allahabad.

oxides and their Applications in Gas and Humidity Sensors”.

cr

His research topic is “Synthesis and Characterization of Nanostructured Semiconducting

us

Vineet Gupta received his Ph.D. degree in Physics in 2010 from University of Lucknow, India. He moved to the Institut UTINAM-UMR CNRS, France in 2010 and after in

an

the end of 2011 he joined the Banaras Hindu University, India for postdoctoral research. He worked as research fellow in Laboratoire de Spectrochimie Infrarouge et Raman du CNRS,

M

France in 2007 and 2008 on the modeling of biomolecules. He was the guest scientist in Max Planck Institute for Polymer Research, Germany in 2012 and Philipps-Universität Marburg,

te

nanomaterials.

d

Geramny in 2012 and 2013. He is currently working on the synthesis and spectroscopy of

Dr. B. C. Yadav has received his Ph.D. degree in 2001 from Department of Physics,

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University of Lucknow, India. Currently he is an Associate Professor/Coordinator of Department of Applied Physics, School for Physical Sciences in the Babasaheb Bhimrao Ambedkar University, Lucknow. He is recipient of prestigious Young Scientist Award-2005 instituted by the State Council of Science and Technology. Also Dr. Yadav was selected in 2011 for Brain Pool International Fellowship of South Korea. He has published more than eighty research/review papers in reputed international journals. His current interests of research includes the synthesis of metal oxides nanoparticles, metallopolymers, etc., characterizations and their applications as sensors.

Dr. Poonam Tandon is Professor of Physics (specialization in experimental solid state physics) at University of Lucknow, India. She is presently working on synthesis,

27   

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characterization and various applications of nanomaterials. She has outstanding contribution in field of Macromolecular Science covering natural, synthetic and biopolymeric systems.

Prof. Tandon is recipient of prestigious Young Scientist Award-2002, nominated as Fellow

ip t

in IUPAC 2010, Alexander von Humboldt 1999, Germany and Regular Associateship of ICTP, Trieste, Italy, 2005-2011.

cr

Akhilesh Kumar Singh is the Assistant Professor of Materials Science at School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu

us

University), India. His research interest includes Structural Phase Transitions in Perovskites,

an

Nanomaterials, Smart Materials, CMR Manganites, Relaxor Ferroelectrics, Synthesis and characterization of Advanced Ceramics. He has investigated crystal structure of several

M

piezoelectric solid solutions using Rietveld structural analysis of powder X-ray, neutron and synchrotron X-ray, diffraction data. His group has developed several new ferroelectric,

d

piezoelectric solid solutions and has investigated their crystal structures. He has received best

Ac ce p

 

te

poster paper award of MRSI in 2004 and 2006.

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Figure Caption Fig. 1 (a): Molecular model of iron antimonate. Fig. 1 (b): DFT optimized geometry of dimer of iron antimonate.

ip t

Fig. 2 (a): SEM image of the film before exposing to LPG. Fig. 2 (b): SEM image of the film after exposing to LPG.

cr

Fig. 3: Elemental mapping for iron antimonate.

us

Fig. 4 (a): XRD pattern of FeSbO4. Fig. 4 (b): Rietveld fit for iron antimonate.

an

Fig. 5: Tauc plot for optical band gap. Fig. 6: DSC curve of the as synthesized material.

M

Fig. 7: Experimental and calculated (a) infrared absorbance, (b) Raman spectra of FeSbO4. Fig. 8: Frontier orbitals and energy gap (Eg) of iron antimonite by DFT (B3LYP/

d

LANL2DZ//6-311++G(d,p)) method.

te

Fig. 9: Magnetic curve of iron antimony oxide. Fig. 10 (a): Variations in resistance of sensing film with time for different vol.% of LPG.

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Fig. 10 (b): Sensitivity of sensor with different vol.% of LPG. Fig. 10 (c): Reproducibility curve for sensing film after one month. Fig. 11: Arrhenius plot of iron antimonate film.

29   

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Research Highlights FeSbO4 having minimum crystallite size 2 nm was synthesized by sol-gel process.

ip t

Rietveld method and vibrational spectroscopy confirmed size and other properties.

cr

SEM, XRD, UV, DSC, FT-IR and Raman spectra characterized the material. First combined experimental and theoretical study on vibrational properties.

an

us

Significant advancement towards the fabrication of robust gas-sensor.

Ac ce p

te

d

M

 

31   

Page 30 of 40

Figure(s)

M

an

us

cr

ip t

Figures

Ac ce p

te

d

Fig. 1 (a)

Fig. 1 (b)

1

Page 31 of 40

ip t cr us an M

Ac ce p

te

d

Fig. 2 (a)

Fig. 2 (b) 2

Page 32 of 40

ip t cr us an M 120

110

100

211

80 60

301

Intensity (a.u.)

Ac ce p

140

101

te

d

Fig. 3

40 20 0

10

20

30

40

50

60

70

80

o

Position (2 ) Fig. 4 (a) 3

Page 33 of 40

120

Obs Cal diff Bragg Peaks

80

ip t

60 40 20

cr

Intensity (a.u.)

100

0

us

-20 -40 10

20

30

40

50

60

70

80



an

Position (2 )

M

a= 4.677(6) 01 c= 3.215(3)

1500

6

-2

* 10 cm eV

2

Ac ce p

2000

te

2500

d

Fig. 4 (b)

(h  )

2

1000

500

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Energy (eV) Fig. 5 4

Page 34 of 40

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-2.5

cr

Heat flow (a.u.)

-2.0

0

100

200

an

us

-3.0

300

400

500



M

Temperature ( C)

Ac ce p

te

d

Fig. 6

Fig. 7 (a) 5

Page 35 of 40

ip t cr us an M

Ac ce p

te

d

Fig. 7 (b)

Fig. 8 6

Page 36 of 40

FeSbO4 0.2

-5

3

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298 K, +2.4 10 cm /g

0.1

cr

0.0 -0.1

us

Magnetisation (emu/g)

0.3

-0.2

-10000

an

-0.3 -5000

0

5000

10000

M

Magnetic field (Oe)

d

Fig. 9

te

1 vol.%

1200

3 vol.%

Ac ce p

1000

2 vol.%

4 vol.% 5 vol.%

R (M)

800 600 400 200 0

0

300

600

900

1200

1500

Time (Seconds) Fig. 10 (a) 7

Page 37 of 40

3

ip t

2.5

cr

2

1

0

1

2

3

us

1.5

an

Sensitivity (M/second)

3.5

4

5

6

M

LPG Concentration (in vol.%)

te

d

Fig. 10 (b)

1200

R

Ac ce p

1000

R (rep.)

R (M)

800

600 400 200 0

0

300

600

900

1200

1500

Time (Seconds) Fig. 10 (c) 8

Page 38 of 40

19.5 19

ip t

18.5

cr

ln R

18 17.5

us

17

an

16.5 16 1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

M

1000/T (K -1 )

Ac ce p

te

d

Fig. 11

9

Page 39 of 40

Graphical Abstract (for review)

Graphical Abstract

1 vol.% 2 vol.%

1200

3 vol.%

1000

4 vol.%

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5 vol.%

cr

600 400

200 0 300

600

900

an

0

us

R (M)

800

1200

1500

Ac

ce pt

ed

M

Time (Seconds)

Page 40 of 40