Solvothermal synthesis of grain size controlled nanocrystalline SnO₂

XU Gang(徐 刚), XU Sheng-ming(徐盛明), CHEN Song-zhe(陈崧哲), WU Fang(吴 芳), LI Lin-yan(李林艳)

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

Received 20 April 2006; accepted 30 June 2006

Abstract: Uniform nanocrystalline SnO₂ with different particle sizes was prepared by a solvothermal method using SnCl₄×5H₂O as the starting material and ethanol as the solvent in the presence of different concentrations of HCl or KOH. The as-prepared powders were characterized by X-ray diffraction(XRD), transmission electron microscopy(TEM), thermo-gravimetry and differential thermal analysis(TG-DTA), and nitrogen BET measurements. With increasing alkalinity of the precursor solution, the crystallite size of the as-prepared SnO₂ nanocrystals decreases from 7.1 nm to 3.2 nm, while the specific surface area of the as-prepared powders increases from 101 m²/g to 195 m²/g. When calcined at 500 ℃ for 2 h in air, the crystallite size of the as-calcined powders increases slightly (12-5.9 nm) and the specific surface area of the as-calcined powders is 42-63 m²/g.

Key words: SnO₂; solvothermal synthesis, nanocrystalline particle

1 Introduction

In recent years, nanocrystalline materials with size-dependent optical, electronic and magnetic properties, due to the reduction of grain size in the nanometer range and/or due to the large fraction of atoms in the grain boundary regions, have attracted increasing interest. Tin dioxide (SnO₂) is a wide-bandgap semiconductor (with a room temperature bandgap of about 3.6 eV) and has received intensive interest for its applications in solar cells[1], catalysis[2], and transparent electrodes[3], especially in combustible and toxic gas detection devices (gas sensors)[4]. For most of these applications, using nanocrystalline SnO₂ with controlled grain size and surface area is regarded as a promising way to fabricate the catalytic and electronic devices (such as gas sensors) with superior performance.

So far, many techniques including sol-gel[5], anodic oxidation[6], thermal oxidation[7], physical/chemical vapor deposition[8], tin salt decomposition[9], and hydrothermal approaches[10] have been developed to fabricate ultrafine SnO₂ powders. However, it is still difficult to control the crystallite size of nanocrystalline SnO₂ powders precisely. In this study, we reported a facile solvothermal treatment to prepare nanocrystalline SnO₂ with different particle sizes. The morphology, structure, and calcination behavior of the as-prepared SnO₂ powders were also investigated.

2.1 Synthesis

All chemicals were analytically pure. For the synthesis of SnO₂ nanocrystals, SnCl₄×5H₂O was used as the starting material to prepare six precursor mixtures in the presence of different concentrations of HCl or KOH in ethanol: 1) 0.10 mol/L HCl (denoted as S1); 2) 0.05 mol/L HCl (S2); 3) 0.05 mol/L KOH (S3); 4) 0.10 mol/L KOH (S4); 5) 0.15 mol/L KOH (S5); and 6) 0.20 mol/L KOH (S6). The above six precursors were transferred into Teflon-lined stainless steel autoclaves, and subjected to solvothermal treatment at 180 ℃ for 24 h. The obtained precipitates were carefully washed with water and ethanol repeatedly, and dried in a vacuum oven at about 60 ℃. The as-prepared powders were then calcined in a muffle furnace at 500-800 ℃ for 2 h.

2.2 Characterization methods

The crystal structures were identified by a powder X-ray diffractometer (XRD, Rigaku D/max-2000, Tokyo,Japan), employing Cu K_a radiation (l=0.154 18 nm). The transmission electron microscopy(TEM) measurements were conducted employing a JEOL 200CX electronic microscope working at 200 kV. The BET specific surface area was measured by nitrogen physisorption at 77.5 K using an ASAP 2010 analyzer (Micromeritics Co Ltd, Norcross, USA). The combined thermo-gravimetry and differential thermal analysis (TG-DTA, SDT2960, Thermal Analysis) were performed at a heating rate of 10 ℃/min from room temperature to 1 000 ℃, using α-Al₂O₃ as a reference.

3 Results and discussion

Fig.1 shows the typical TEM image of the as-prepared SnO₂ powder (sample S4). The primary particles of the as-prepared SnO₂ nanocrystallites are highly uniform and well-dispersed, showing no definite and tight agglomeration with a crystallite size of less than 5 nm. The selected area electron diffraction(SAED) pattern, shown in the inset of Fig.1, confirms the formation of tetragonal SnO₂ nanocrystals for the as-prepared powders.

Fig.2 depicts the typical combined TG-DTA curves

Fig.1 Typical TEM image of as-prepared SnO₂ powder (S4) and corresponding SAED pattern

Fig.2 Typical combined TG-DTA curves of as-prepared SnO₂ powders

of the as-prepared SnO₂ powder (sample S4), indicating a two-step mass loss process on the TG curve. The first mass loss beginning from 25 to 200 ℃ can be attributed to the liberation of physically adsorbed water and ethanol molecules, and the second one between 200 and 400 ℃ can be ascribed to the elimination of residuals through oxidation-reduction reaction. However, no obvious exothermic (exo) peak for the transition from amorphous to crystalline at 400-500 ℃ is detected[11], indicating that the hydrolytic products have already been crystallized well.

Fig.3 shows the XRD patterns of the as-prepared SnO₂ powders. It is observed from Fig.3 that the main diffraction peaks are broadened, indicating the formation of nanocrystals under solvothermal treatment at 180 ℃, and can be indexed as tetragonal SnO₂ (JCPDS 41-1445, P42/mnm). When increasing alkalinity of the precursor solution from 0.1 mol/L HCl (S1) to 0.2 mol/L KOH (S6), the intensity of the diffraction peaks become weaker and more broadened, suggesting smaller crystals formed under alkaline condition. When being calcined at 500 ℃ for 2 h in air, as noted from Fig.4, the intensities of the diffraction peaks of rutile SnO₂ happen to be stronger and more sharpened, indicating that the as-prepared nanocrystals become well-crystallized at that calcination temperature.

Fig.3 XRD patterns of as-prepared SnO₂ powders: (a) 0.10 mol/L HCl; (b) 0.05 mol/L HCl; (c) 0.05 mol/L KOH; (d) 0.10 mol/L KOH; (e) 0.15 mol/L KOH; (f) 0.20 mol/L KOH

The average grain size D of both the as-prepared and the as-calcined SnO₂ powder was estimated according to the Scherrer equation[12]:

                                 (1)

where  q  is the diffraction angle of the (110) peak of

Table 1 Grain size D and BET specific surface area S_BET of SnO₂ powders

Fig.4 XRD patterns of SnO₂ powders calcined at 500 ℃ for  2 h

the tetragonal phase, b is the full width at half maximum (FWHM) of the (110) peak (in radian), which is calibrated from high purity silicon. All results are collected in Table 1, and the dependence of crystallite size on the acidity and alkalinity is elucidated. It is noted that the mean grain size reduces gradually with the increase of the alkalinity. The grain size of the as-prepared samples in the presence of HCl media is relatively larger, and it is 7.1 nm and 5.9 nm for samples S1 and S2, respectively. When using KOH media, the grain size of the as-prepared samples is much smaller, from 4.9 nm for S3 to 3.7 nm for S6. After calcining at 500 ℃ for 2 h, the grain size increases slightly from 11.7 nm for S1 to 8.6 nm for S6.

The BET specific surface area, S_BET, of the nano- crystalline SnO₂ powders is also summarized in Table 1. The variation sequence of the S_BET is opposite to that of the grain size, i.e. the S_BET of the as-prepared samples increases monotonously from 101 m²/g for S1 to 195 m²/g for S6. This is in accordance with the fact that smaller particle can provide larger specific surface area. When calcined at 500 ℃ for 2 h, the as-calcined samples still remain high specific surface area from 42 m²/g for S1 to 63 m²/g for S6. When calcined at 800 ℃ for 2 h, the S_BET of the as-calcined samples inclines to be the same, which is 7-13 m²/g. Thus, the particle size and the specific surface area can be controlled by simply modifying the acidity/alkalinity of the precursor solution, and the study on the essential of this influential factor is in progress.

4 Summary

The grain size controlled nanocrystalline SnO₂ powders were successfully prepared by a facile solvothermal synthetic approach. The concentration of the acid or alkali has a direct influence on the grain size of the nanocrystalline powders, i.e. with decreasing the acidity and increasing the alkalinity of the precursors, the grain size and the specific surface area of the nanocrystals become smaller and larger, respectively. The well- dispersed, uniform, and well-crystallized powders are favorable for gas-sensing and catalysis applications. The as-developed solvothermal crystalliz- ing process can be extended to prepare other oxide nanoparticles.

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(Edited by PENG Chao-qun)

Foundation item: Project(50274045) supported by the National Natural Science Foundation of China; Project supported by Hubei Key Laboratory of Novel Reactor and Green Chemical Technology(NRGCT)

Corresponding author: XU Sheng-ming; Tel: +86-10-89796082; Fax: +86-10-62773585; E-mail: smxu@tsinghua.edu.cn