Electrical and dielectric properties of co-precipitated nanocrystalline tin oxide

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Abstract

Nanocrystalline tin oxide (SnO2) powders were synthesized by chemical co-precipitation method using stannic chloride pentahydrate (SnCl4·5H2O) precursor in aqueous medium. The influence of sintering temperatures on the crystalline structure, morphological, electrical, dielectric and XPS properties has been studied. X-ray diffraction study reveals that sintered powder which exhibits tetragonal crystal structure and both crystallinity as well as crystal size increase with increase in temperature. The nature of species of various absorption bonds viz. Sn–O, O–Sn–O and O–H involved in sintered SnO2 samples has been studied using FTIR technique. The morphological studies reveal randomly arranged grains with compact nature and grain size increases with sintering temperature. Measurements of electrical properties show relatively lower resistivity (≈102–103 Ω cm) and higher dielectric constant at 400 °C than other sintering temperatures. The compositional analysis and electronic behavior of SnO2 nanoparticles is studied using X-ray photoelectron spectroscopy. The symmetric spin orbit splitting of Sn 3d5/2 ground state and Sn 3d3/2 excited states is observed with sintering temperature while O 1s is recognized with O2 state.

Introduction

The nano-material forms the building blocks for new bottom-up approaches due to their intrinsic size dependent properties and resulting applications. Renewed interest has been developed in tin oxide (SnO2) due to its mechanical and chemical stability, environmental and good thermal properties [1], [2]. In the bulk form, SnO2 has a tetragonal structure (a = b = 4.737 Å and c = 3.186 Å), similar to the rutile structure with the wide energy gap Eg = 3.67 eV, an insulator and becomes n-type semiconductor when deposited in thin film form [3]. It plays an important role as transparent conductive oxide (TCO) material with remarkable photoelectrical properties such as solar battery, intelligent window, transparent electrode, electrochemical supercapacitors and low-e windows [4], [5], [6], [7], etc. Especially, SnO2 nanoparticles have been intensively studied for gas sensing [8], [9] applications not only because of their relatively low operating temperature, but also due to dual purpose of detecting reducing and oxidizing gases.

SnO2 with various nano- and microstructures, such as nanoparticles, nanorods, nanobelts, hollow microspheres, nanoflowers, nanoperticles, nanowhiskers nanowires and mesoporous structures [8], [10], [11], [12], [13] has been prepared successfully by different methods. Up to now various methods including molten-salt synthesis [14], chemical precipitation [8], sonochemical [15] carbothermal reduction [16], sol–gel [17], hydrothermal method [13], microwave technique [18], and r.f. sputtering [19], etc. have been developed to fabricate SnO2 nanostructures.

Xi et al. [8] have reported on the synthesis and characterization of large surface area SnO2 nanoparticles using ethanol-thermal method with urea and stannic chloride. They have dissolved both precursors in distilled water and heated at 90 °C for 5 h to promote the hydrolysis of SnCl4 and SnO2 formation. Phani et al. have studied the electrical properties of SnO2 based liquid petroleum gas (LPG) sensor prepared using tin chloride and ammonia solutions [20]. In order to increase the sensitivity they used noble metal like palladium as a sensitizer. Li et al. [12] studied the fabrication of high-quality rutile SnO2 nanowires by a vapor phase transport and condensation method assisted by carbothermal reduction. They found that many oxygen vacancies exist in the single-crystalline rutile SnO2 nanowires and can further influence the photoluminescence (PL) and dielectric property of the nanowires. The PL spectrum only exhibits a wide yellow emission centered at 570 nm and usual near band edge emission is not observed, which is ascribed to a large amount of ionized oxygen vacancies. The dielectric measurements indicate that the dielectric response of the SnO2 nanowires is significantly enhanced in the low-frequency range. Davar et al. [21] reported the synthesis of SnO2 nanoparticle using thermal decomposition and characterization of their physicochemical characterization. The SnO2 nanoparticle powder has been prepared using [bis(2-hydroxyacetophenato)tin(II)], [Sn(HAP)2]; as precursor. Transmission electron microscopy (TEM) analysis was demonstrated that SnO2 nanoparticle with an average diameter of about 14–22 nm. The novel precursor was thermal treated in solid-state reaction in different temperature, 350, 450, and 550 °C. Kong et al. [22] prepared nanocomposites of SnO2 and polythiophene (PTP) by the in situ chemical oxidative polymerization method. These nanocomposites were characterized by their physicochemical properties. The composites were used for gas sensing to methanol (MeOH), ethanol (EtOH), acetone, and NOx at different working temperature. It was found that PTP/SnO2 materials with different PTP mass percent (1%, 5%, 10%, 20% and 30%) could detect NOx with very higher selectivity and sensitivity at much lower working temperature than the reported SnO2. The sensing mechanism of PTP/SnO2 nanocomposites to NOx was presumed to be the effects of p–n heterojunction between PTP and SnO2. Dutta and De [23] synthesized nanostructured tin dioxide (SnO2) in the form of colloidal solution. Aniline monomer is polymerized in colloidal solution of SnO2 to prepare inorganic–organic hybrid nanocomposites. Optical band gap increases from 3.74 to 4.23 eV with increase of polyaniline concentration. The observed nonlinear current–voltage characteristics are satisfactorily explained using the Schottky type barriers. The temperature dependence of conductivity reveals three-dimensional Mott's hopping process. Xi et al. [24] studied the synthesis of Al–SnO2 nanoparticles by a co-precipitation route in water-in-oil microemulsion consisting of water, DBS (surfactant), 1-amyl-alcohol (assistant surfactant) and cyclohexane (oil phase). The results show that the particle size of Al–SnO2 is below 10 nm, while pure SnO2 is over 15 nm, which indicates that the introduction of Al can effectively prevent SnO2 from further growing up in the process of calcination. On the other hand, when the molar ratio of Al to Sn is 1:4 and the calcination temperature is 600 °C, the as-prepared Al–SnO2 nanoparticles have the lowest particle size in the experiments. In the photocatalytic degradation of phenol, the Al–SnO2 nanoparticles exhibit better activity than the pure SnO2 nanoparticles. To control crystallite size they have mixed aluminium silicate as grain growth inhibitor for SnO2. In a co-precipitation method, the tin oxide is obtained by adding ammonia directly to a solution containing tin cations. This method gives better control over the precipitate particle shape and size due to change of solution concentration and the localized introduction of the ammonia uniformly throughout the solution at the molecular levels.

In this paper we report the successful synthesis and characterization of nanocrystalline high surface area SnO2 powders using a simple chemical co-precipitation method [25]. The influence of sintering temperature on the structural, FTIR, electrical, morphological, and dielectrical and XPS properties is reported.

Section snippets

Preparation of samples

Tin oxide samples were synthesized using a chemical co-precipitation route with AR grade 0.5 M stannic chloride pentahydrate (SnCl4·5H2O). Only double distilled water was used as a solvent. The pH of solution was measured and then adjusted to neutral value by adding aqueous ammonia to preserve the hydroxide phase of tin. The resulted white gelatinous precipitate was filtered using Whatmann filter paper No. 17 and washed thoroughly until traces of Cl have been completely removed. It was further

X-ray diffraction studies

Fig. 1 shows the XRD patterns of the tin oxide samples sintered at different temperatures. The samples are polycrystalline and fit well with the tetragonal crystal structure with space group P42/mnm (1 3 6). The samples are analyzed by using JCPDS card No. 77-0447. The intensity and number of diffraction peaks are found to be influenced by sintering temperature. The sharpness and peak intensity increases with increase in sintering temperatures due to enhancement in the crystallinity. The decrease

Conclusions

The tetragonal tin oxide phase can be synthesized by using chemical co-precipitation technique. Powder sample shows randomly arranged irregular sized compact trapezoidal grains with spongy nature. The nature of species and oxide phase formation is confirmed through FTIR and XPS studies and the various absorption bands such as Sn–O, O–Sn–O and O–H corresponds to 619–673, 1020, 1630, 2923, 3400 cm−1, respectively. The decreasing behavior of dielectric constant with applied frequency is due to

Acknowledgements

The authors are thankful to University Grants Commission for their support through UGC-DRS-SAP IInd phase programme (2004-2009). The help rendered by R.C. Kambale during electrical measurements is highly acknowledged. AVM wish to thank the Department of Science and Technology (DST), New Delhi for awarding the BOYSCAST Fellowship (File No.SR/BY/P-02/2008) due to which he could provide the characterization facilities and could spent more time during the process of the manuscript.

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