The effect of the post-annealing temperature on the nano-structure and energy band gap of SnO2 semiconducting oxide nano-particles synthesized by polymerizing–complexing sol–gel method

doi:10.1016/j.physb.2008.01.004

Physica B: Condensed Matter

Volume 403, Issues 13–16, 1 July 2008, Pages 2431-2437

https://doi.org/10.1016/j.physb.2008.01.004 Get rights and content

Abstract

Nano-crystalline SnO₂ particles have been synthesized by sol–gel process using a simple starting hydro-alcoholic solution consisting of SnCl₄, 5H₂O and citric acid as complexing and ethylene glycol as polymerization agents. The structural properties of the prepared tin oxide nano-powders annealed at different temperatures (300–700 °C) have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. The XRD patterns show SnO₂-cassiterite phase in the nano-powders, and size of crystals increases by increasing the annealing temperatures. The TEM images show nano-particles as clusters with size in the range of 5–25 nm. Electron diffraction pattern of nano-powders annealed at different temperatures shows a homogeneous distribution of spherical particles due to the effect of ethylene glycol as polymerizing agent in sol–gel process. The optical direct band gap values of SnO₂ nano-particles were calculated to be about 4.05–4.11 eV in the temperature range 300–700 °C by optical absorption measurements. These values exibit nearly a 0.5 eV blue shift from that of bulk SnO₂ (3.6 eV), which is related to size decrease of the particles and reaching to the quantum confinement limit of nano-particles.

Introduction

Tin oxide (SnO₂), with cassiterite structure, as bulk or thin films is a wide band gap n-type semiconductor (E_g=3.6–3.8 eV), known as one of the most widely used semiconducting oxides due to its chemical and mechanical stabilities. It has been widely studied over decades because of its most applications in various solid state and ceramic devices, sensors, optoelectronics and catalysis [1], [2], [3], [4], [5], [6]. Among the technical applications, the most important uses of SnO₂ are as gas sensors, bulk ceramics, glaze and pigments. On the other hand, the majority of chemical and physical properties of SnO₂ material depend on the number of structural parameters. For example, the sensing properties of SnO₂ sensors (sensitivity, selectivity and reproductively) critically depend on some features, mainly particle size and specific surface area. Therefore, preparation of primary powders as nono-particles increases specific surface area of the particles and hence improves the efficiency function of the sensor [1], [5], [6], [7]. Due to these modifications, application of nano-structured SnO₂ as an active material in gas sensor is well known.

Along with the different wet chemical methods of preparing nano-particle SnO₂, sol–gel [4], [6], [8], precipitation [9], [10], hydrothermal route [10], [11] and spray pyrolysis [12], [13] are well recognized. Recently, polymerizing–complexing (PC) combustion method, which is a modified Pechini process [14], [15], without any precipitation, has been used for preparation of semiconducting nano-particles, like SnO₂. This method consists of a variety of organic fuels such as citric acid, urea, hydrazine, EDTA and polyethylene glycol as complexing or polymerization agent [7], [16]. Indeed, PC method provides more homogeneous fine powder than any other techniques with immobilization of metal–chelate complexes and forming stable metal complexes via increasing the polymerization.

However, preparation of nano-particles of metal oxides at low cost in industrial scale is a challenge in material production. So, using the cheap materials, simple fabrication processes and suitable conditions of synthesis are the main requirement for this process. Therefore, the study of influence of various parameters such as initial solution combination, time and temperature of heat treatments, and type of complexing and polymerizing agents is very important for nano-powders production.

The main purpose of the present research is to study the post-annealing temperature effect on nano-particle size, nano-structure and energy band gap of SnO₂ powders synthesized by the PC sol–gel method.

Section snippets

Synthesis of SnO₂ nano-powders

SnO₂ nano-powder synthesization by the PC method is summarized in a flow chart shown in Fig. 1. First, the initial sol consisting of SnCl₄ and 5H₂O, H₂O and ethanol with equal weight percentage is prepared. Then, citrate acid and ethylene glycol subsequently added to this solution, and the resulting mixture was stirred and dissolved at 40 °C for 20 min until a completely clear solution was obtained. The obtained solution was refluxed at T=120 °C for 3 h. During refluxing the solution turned into a

Results and discussion

Table 1 shows the results of post-annealing of powders at different temperatures. As observed, by increasing the annealing temperature from 300 to 500 °C, a rapid mass loss of powders occurs due to evaporation and removing organic additives such as citrate acids and ethylene glycol. The most mass reduction occurs in T=500 °C and higher temperatures (Δw=76.6%). This indicates that all the organic additives are removed from powders by annealing at T=500 °C.

The XRD patterns of prepared SnO₂

Conclusions

Tin oxide nano-particles were synthesized successfully by the polymerizing–complexing sol–gel method. Based on XRD and TEM studies, using citrate acid as a complexing agent in the synthesis of SnO₂ nano-particles was effective to prevent the grain growth or agglomeration of particles, and the addition of ethylene glycol as polymerization agent to the precursor solution led to a homogeneous distribution of particles. The post-annealing results on nano-particles show that at high annealing

Acknowledgments

The authors would like to thank Mrs. M. Hassanzadeh from Solid State Physics Research Center, Damghan University of Basic Sciences for XRD analyses and Mrs. R. Pesyan from Central Research Laboratory of Ferdowsi University of Mashhad for TEM analysis.

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The effect of the post-annealing temperature on the nano-structure and energy band gap of SnO2 semiconducting oxide nano-particles synthesized by polymerizing–complexing sol–gel method

Abstract

Introduction

Section snippets

Synthesis of SnO2 nano-powders

Results and discussion

Conclusions

Acknowledgments

Mater. Lett.

Sensor. Actuat. B

Thin Solid Films

J. Eur. Ceram. Soc.

J. Solid State Chem.

J. Non-Cryst. Solids

MRS Bull.

Semi-conducting Transparent Thin Films

Solid State Ionics

The effect of the post-annealing temperature on the nano-structure and energy band gap of SnO₂ semiconducting oxide nano-particles synthesized by polymerizing–complexing sol–gel method

Synthesis of SnO₂ nano-powders