The influence of hydrothermal temperature on ^SnO₂ nanorod formation

In recent years, there has been a tremendous increase in research activity in the field of nanosized materials. This is because of interesting variations in the properties of materials in nanosized configuration as compared to their bulk counterpart, and hence their novel application potential. The size, shape and crystal structure of nanosized materials are important parameters that control their chemical, optical and electrical properties. Based on metal oxide materials for gas sensor applications, tin oxide (^SnO ₂) has been the most popular sensor material so far investigated and used in practice [1–3], and a great deal of research effort has been exerted to improve the gas-sensing properties of ^SnO ₂-based sensors. As the gas sensing properties of ^SnO ₂ materials are strongly dependent on their size and shape, it is obvious that the controlled synthesis of the nano/microstructure of ^SnO ₂ materials is very important [4].

Up to now, ^SnO ₂ materials with different morphologies have been fabricated by a few methods. Seong Hyeon Hong of Seoul National University has successfully prepared ^SnO ₂ nanoparticles by a magnetron sputtering method [5]. In addition, A Helwig of Corporate Research Centre, Germany, also fabricated ^SnO ₂ nanoparticles by a thermal oxidation method [6]. The experimental results show that diameters of ^SnO ₂ nanoparticles are around 10 nm to 20 nm and exhibit good sensitivity to LPG gas. The nanorods and nanowires of the ^SnO ₂ nanomaterial were synthesized by a vapor–liquid–solid approach [7–10]. Tin oxide nanowires have been grown on Au-coated Si substrates via a conventional thermal evaporation route using Sn powder. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) have been performed to investigate the structure of the products. The as-synthesized nanostructures have been found to have rutile structure. The diameters and the lengths of the ^SnO ₂ nanowires are in the ranges of 50–550 nm and 200–500 μ^m, respectively. ^SnO ₂ nanomaterial was obtained by another method, such as chemical vapor deposition (CVD) [11], infiltration technique or pulsed laser deposition method. However, these preparation methods usually require very high temperature and rigorous experimental conditions. In this paper, we report the preparation of tin dioxide nanorods by a hydrothermal soft-chemical process. By this method, we can easily control a variety of parameters, such as temperature, pressure, concentration of chemical species, solution concentration, pH and starting compounds. In addition, a possible growth mechanism of ^SnO ₂ nanorods is discussed.

Stannic acid gel was precipitated by mixing aqueous solutions of tin chloride ^SnCl ₄(0.2 ^M) and ammonia:

The resulting precipitate was thoroughly washed by repeating the procedures of suspending the gel into deionized water and collecting it back by filtration or centrifugation to remove ^{Cl –}. The wet gel thus obtained is named the untreated gel. The stannic acid gel (untreated gel) was suspended in water to obtain a suspension, after adjusting the pH of the suspension with ammonia. In this experiment, a pH of 10.5 was selected. This suspension is named the untreated sol. The above untreated sol was treated by a hydrothermal technique to make a more stable sol and better dispersion. The hydrothermal treatment was carried out in an autoclave at 200 °^C for 3 h. Usually, a transparent sol suspension (hereafter called the treated sol) was obtained after this treatment.

The above treated sol was added to 0.15 M NaOH solution with vigorous stirring to obtain an opaque solution. After that, 2 mmol of cetyltriethyl ammonium bromide (CTAB) was added into the above opaque solution, followed by heating to make the CTAB dissolve completely. Then the mixture was poured into a stainless teflon lined 200 ml autoclave and heated at 140 °^C for 16 h. After being cooled down to room temperature, the resulting white precipitate was collected by centrifugation, washed with ethanol several times and dried at 600 °^C in vacuum for 3 h. Thus ^SnO ₂ nanorods were obtained. Structural properties and surface morphologies of the as-prepared ^SnO ₂ nanorods were characterized by powder x-ray diffraction (XRD) and scanning electron microscopy (SEM). The diagram of preparation of ^SnO ₂ nanorods is shown in figure 1.

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The XRD pattern of ^SnO ₂ nanorods after calcinations at 600  ^{o C} for 3 h is shown in figure 2. All of the diffraction peaks can be perfectly indexed to the rutile structure of ^SnO ₂ with tetragonal lattice parameters a=4.7 Å and c=3.2 Å, which is consistent with the standard data file. No characteristic peaks of impurities, such as NaOH and cetyltrimethyl ammonium bromide (CTAB), were observed. This shows that the as fabricated ^SnO ₂ nanorods are a pure phase of rutile ^SnO ₂.

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Figure 3 showed typical SEM images of the ^SnO ₂ nanorods after annealing at 600 °^C for 3 h. The results showed that the diameters of the ^SnO ₂ nanorods are around 100 nm to 300 nm with lengths of several micrometers.

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In addition, it is noteworthy that varying the hydrothermal temperature during the synthesis led to a dramatic change in the diameter of the rods. We observed the size of the rods increase with the increase in hydrothermal temperature. If the reaction was carried out at a low temperature of 120 °^C, only irregular nanoparticles were obtained (figure 4(a)). The experimental results show that nanoparticles of ^SnO ₂ sol solution started to condense into large particles. ^SnO ₂ nanorods with diameters of 100–300 nm and lengths of several micrometers were formed when the temperature was kept at 140 °^C. Figure 4(b) shows the typical SEM images of ^SnO ₂ nanorods, which were derived from hydrothermal treatment at 140 °^C for 16 h. The products exhibited good morphology and high density. However, when increasing the hydrothermal temperature to 180 and 200 °^C, the result shows that the diameter of the ^SnO ₂ nanorods increases but the density of the product clearly decreases. When the hydrothermal temperature increased to 230 °^C, the bended foils and plates of ^SnO ₂ were produced (figure 4(d)). These results pave the way for further research to produce other morphologies of ^SnO ₂ material for different applications. A proposed growth mechanism of ^SnO ₂ nanorods can be explained in terms of chemical reactions and crystal growth, as follows. From the crystallization point of view, the synthesis of an oxide during an aqueous solution reaction is expected to experience a hydrolysis-condensation process.

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Growth of ^SnO ₂ nanorods occurs according to the reaction (1) and ^Sn(^OH)₄ is an amphoteric hydroxide, which can be dissolved in NaOH solution and form ^Sn(^OH)₆ ^2- anions:

The possible formation process for ^SnO ₂ nanorods under hydrothermal conditions can be represented as in figure 5.

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The surfactant CTAB is key to determining the morphology of the products. Because of the existence of the surfactant, the surface tension of the solution reduced, thus it was easy to carry out the reaction between ^Sn(^OH)₄ and NaOH. On the other hand, CTAB could be considered to influence the erosion process of tin dioxide and growth process of ^SnO ₂ by electrostatic and stereochemical effects (figure 5). As we know, CTAB is an ionic compound, which ionizes completely in water. The ^{CTAB +} cations condense into aggregates in which the ^Sn(^OH)₆ ^2– anions are intercalated in the interspaces between the head groups of CTAB to form ^{CTAB +}–^Sn(^OH)₆ ^2– ion pairs by electrostatic interaction. The ^{CTAB +}–^Sn(^OH)₆ ^2– ion pairs form a sandwich-like structure in water [13]. ^{CTAB +}–^Sn(^OH)₆ ^2- ion pairs are known as seeds of crystals. The nanorods' growth mechanism can be understood on the basis of oriented aggregation by polar forces. The primary seeds of crystals can transform into ^SnO ₂ nanorods by oriented aggregation. Jiggling seeds of crystals by the driving force may allow adjacent seeds of crystals to construct the low-energy structure, represented by coherent seeds of the crystal interface [12]. The experimental results show that the size of rods increases with the increase in hydrothermal temperature. This can be explained as follows. If the hydrothermal temperature increases, the reaction between ^Sn(^OH)₄ and NaOH will easily take place, the density of crystal seeds becomes larger and the size of the ^SnO ₂ nanorods increases.

^SnO ₂ nanorods with diameters of 100–300 nm and lengths of several micrometers have been successfully synthesized by a hydrothermal method. The effects of a hydrothermal temperature on the morphology and diameters of products were also investigated. In particular, when the hydrothermal temperature was increased to 230 °^C, the bended foils and plates of ^SnO ₂ were obtained. The success of preparing the bended foils and plates of ^SnO ₂ will promote research and preparation of gas sensors based on ^SnO ₂ materials. In addition, the XRD patterns revealed that the obtained products exhibit the tetragonal rutile structure of ^SnO ₂. Furthermore, a possible growth mechanism of ^SnO ₂ nanorods was discussed.

This work was supported by project code 06/2009/HD-DTDL.