Hydrothermal synthesis of flower-like SnO₂ particles consisting of single-crystalline nanorods through crystal growth in the presence of poly(acrylic acid)

Abstract

We prepared crystalline SnO₂ particles from aqueous solutions of SnCl₄ containing poly(acrylic acid) (PAA) by hydrothermal treatment at 150 °C. The morphology of the SnO₂ particles depended on the concentration of PAA and the pH value of the precursor solutions. The flower-like SnO₂ particles of ca. 100 nm in diameter consisting of needle-like units ca. 50 nm in size were obtained from the solution of [SnCl₄·5H₂O] = 0.1 M and [PAA] = 0.02 M containing the acidic solvents of pH 0.35 via the hydrothermal treatment. The needle-like units were found to consist of nanorods of ca. 50 nm in length and ca. 5 nm in width. The flower-like SnO₂ particles were thought to be formed through the moderate nucleation and growth rates that were provided by the relatively high solubility of the Sn⁴⁺ species under the acidic conditions and the their coordination by PAA.

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1. Introduction

Rutile-type SnO₂, an n-type semiconductor with a wide band gap (E_g = 3.6 eV), is attracting much attention due to its wide range of applications, such as gas sensors,^1–3 electrode materials^4,5 and catalysts.⁶ Since the properties of such devices depend on the crystallinity, the crystallite size and shape, and the morphology of the secondary particles, the control of SnO₂ particles at the nanometer scale is important for practical applications. Recently, the crystal growth in aqueous solutions containing organic molecules is receiving attention as a synthetic route for nanostructured inorganic materials.^7–14 The adsorption of organic molecules on the surface of crystallites controls the crystal growth, which frequently leads to the formation of architectures consisting of nanometer-scaled crystalline units.

Previously, various types of SnO₂ particles have been prepared by the hydrothermal process in the presence of organic molecules, where the hydrothermal conditions accelerate the dissolution and precipitation and the organic molecules controls the nucleation and growth.^15–20 Guo et al. prepared nanorods consisting of SnO₂ crystallites using cetyltrimethyl ammonium bromide (CTAB) as a template.¹⁵ Du et al. reported the formation of SnO₂ hollow microspheres composed of nanoparticles from a precursor solution containing carbamide.¹⁶ Zu et al. proposed citric acid-assisted hydrothermal process for the preparation of SnO₂ nanoparticles.¹⁹ These suggest that the hydrothermal process with organic additives is an effective aqueous route for the fabrication of nanostructured SnO₂ particles.

We also reported the preparation of spherical SnO₂ particles of 1–3 μm in diameter consisting of nanocrystallites below 10 nm in size from aqueous solutions of SnCl₄ with citric acid.²¹ The coordination of Sn⁴⁺ ions by the carboxyl groups (–COO⁻) of citric acid provides the moderate nucleation rate, and consequently leads to the formation of the spherical shape of secondary particles. Such interactions between the Sn⁴⁺ ions and carboxyl groups is thought to be important for the nanostructural control of SnO₂ crystals. An organic polymer, poly(acrylic acid) (PAA), is another candidate as an organic agent that controls the nucleation and growth rates because of its many carboxyl groups, which are expected to interact strongly with Sn⁴⁺ ions. Cheng et al. used PAA as an additive, and prepared single-crystalline SnO₂ nanocones of ca. 1 μm in length and 100–500 nm in root size via a solvothermal process.²² Such nanocones, however, were formed through the self-assembly of SnO₂ colloids, not through the controlled growth of SnO₂ crystals. In other words, no attempts have been made so far on the control of nucleation and growth of SnO₂ crystals by PAA in aqueous solutions under hydrothermal conditions.

Here we address the preparation of nanostructured SnO₂ crystals in aqueous solutions under hydrothermal conditions where PAA controls their nucleation and growth. We employed SnCl₄ as the SnO₂ source, and HCl solutions as the acidic solvent. The acidic solvents were used because the nucleation and growth of SnO₂ crystals are often found in acidic aqueous solutions, where Sn⁴⁺ species are in supersaturated states.^21,23,24 Our strategy was to moderately supersaturate SnCl₄ solutions by the acidic solutions as well as by hydrothermal conditions, and to control the nucleation and growth of SnO₂ crystals by PAA. The morphology of the SnO₂ particles depended on the concentration of PAA and the pH value of the acidic solutions, and finally nanostructured SnO₂ particles were produced by optimizing these parameters.

2. Experimental

Hydrochloric acid solutions of pH 0.20–0.40 were prepared by diluting ca. 36.0 mass% hydrochroric acid (Wako Pure Chemical Industries, Osaka, Japan) with purified water. 0–0.08 g of PAA (M_W = 250 000) (35.0 wt% in water, Sigma-Aldrich, USA) and 1.4 g of SnCl₄·5H₂O (Wako Pure Chemical Industries, Osaka, Japan) were added and dissolved in 40 cm³ of the hydrochloric acid solutions under stirring at room temperature, resulting in transparent solutions of [PAA] = 0–0.02 M and [SnCl₄·5H₂O] = 0.1 M. The solutions thus obtained were hydrothermally treated at 150 °C in a Teflon-lined stainless steel autoclave (75 cm³, Flon Industry, Tokyo, Japan) for 24 h, resulting in the formation of white precipitates. The precipitates were washed with purified water and dried at 60 °C for 24 h.

The crystalline phases were identified by X-ray diffraction (XRD) measurements in ordinary 2θ/θ mode using an X-ray diffractometer (Model Rint 2550 V, Rigaku, Tokyo, Japan) with Cu-Kα radiation operated at 40 kV and 300 mA. The microstructure of the products was observed using a field-emission scanning electron microscope (FE-SEM) (Model JSM-6500F, JEOL, Tokyo, Japan) and a field-emission transmission electron microscope (FE-TEM) (JEM-2000EX, JEOL, Tokyo, Japan). The chemical compositions were determined by X-ray photoelectron spectroscopy (XPS) analyses (PHI 5000 Versa Probe, ULVAC-PHI, Kanagawa, Japan). Thermogravimetric and differential thermal analysis (TG-DTA) curves were obtained on the products at a heating rate of 10 °C min⁻¹ in flowing air using a thermal analyzer (Model ThermoPlus 2, Rigaku, Tokyo, Japan). Infrared (IR) absorption spectra were measured with a Fourier transform infrared (FTIR) spectrophotometer (FT/IR-410, Jasco, Tokyo, Japan), using the KBr method.

3. Results and discussion

The precursor solutions prepared in the present work had a fixed [SnCl₄·5H₂O] at 0.1 M and varying [PAA] values of 0–0.02 M, where HCl solutions of pH = 0.20–0.40 were used. All the precursor solutions were transparent irrespective of [PAA] and pH value of HCl solutions. Table 1 shows the precipitates obtained from the precursor solutions after the hydrothermal treatment at 150 °C for 24 h. The hydrothermal treatment resulted in formation of white precipitates from the solutions containing the acidic solvents of pH 0.35–0.40. On the other hand, the precipitation was suppressed by the addition of PAA in the solutions containing the acidic solvents below pH 0.30, while precipitates were formed from those without PAA. Residual chloride ions below 4 mol% were detected in all the precipitates by XPS analysis.

Table 1 Precipitates obtained from the precursor solutions after the hydrothermal treatment at 150 °C for 24 h

	Precipitation
pH of solvent	Without PAA	With PAA
0.20	White precipitate	No precipitate
0.30	White precipitate	No precipitate
0.35	White precipitate	White precipitate
0.40	White precipitate	White precipitate

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Fig. 1 shows the SEM images of the precipitates prepared by the hydrothermal treatment of the precursor solutions of [PAA] = 0–0.02 M containing the acidic solvents of pH 0.35–0.40. When the pH value of the acidic solvent was 0.35, nanoparticles below 50 nm in size were produced during the hydrothermal treatment, where the morphology of the secondary particles varied with the addition of PAA. Aggregates of nanoparticles below 50 nm in size were obtained from the solution of [PAA] = 0 M (Fig. 1a). Flower-like particles of ca. 100 nm in diameter consisting of needle-like units below 50 nm in size were produced from the solution of [PAA] = 0.02 M (Fig. 1b–c). When the acidic solvents of pH 0.40 were used, on the other hand, unshaped precipitates were obtained with and without PAA during the hydrothermal treatment (Fig. 1d).

Fig. 1 SEM images of the precipitates in the solutions via hydrothermal treatment at 150 °C. [PAA] and the solvent pH were 0 M and 0.35 (a), 0.02 M and 0.35 (b–c), and 0 M and 0.40 (d), respectively.

Fig. 2 shows the TEM images of the flower-like particles prepared from the solutions of [PAA] = 0.02 M containing the acidic solvents of pH 0.35. The needle-like units of the flower-like particles were found to consist of nanorods of ca. 50 nm in length and ca. 5 nm in width. The lattice fringes reveal that the nanorods were single-crystalline SnO₂ elongating parallel to the [110] direction, which means that the SnO₂ crystals grew preferentially in the [001] direction.

Fig. 2 TEM images of the precipitates prepared from the solution of [PAA] = 0.02 M containing the acidic solvent of pH 0.35 via hydrothermal treatment at 150 °C.

Fig. 3 shows the XRD patterns of the precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvents of pH 0.35. The diffraction peaks attributed to rutile-type SnO₂ were observed for the aggregates of nanoparticles ([PAA] = 0 M) and flower-like particles ([PAA] = 0.02 M), and the intensity of the peaks became lower with the addition of PAA. The crystallite sizes of the SnO₂ products, which were calculated with Scherrer's formula, are shown in Table 2. The size of the crystallites formed from the solution of [PAA] = 0.02 M was smaller than those of [PAA] = 0 M. Moreover, the ratio of the crystallite size in the [001] direction to that in the [110] direction (S_[001]/S_[110] shown in Table 2) increased with the addition of PAA. This fact suggests that the crystallites prepared with PAA are preferentially elongated in the [001] direction. Suppression of the growth of SnO₂ crystals is also revealed in the reduction in crystallite size given by the addition of PAA (Table 2).

Fig. 3 XRD patterns the precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvent of pH 0.35 via hydrothermal treatment at 150 °C.

Table 2 Crystallite sizes of SnO₂ precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvents of pH 0.35 calculated with Scherrer's formula

[PAA]/M	S [110] ^a/nm	S [001] ^a/nm	S [001]/S[110]
a S [110] and S[001] show the crystallite sizes calculated from the (110) and (002) peaks, respectively.
0	8	20	2.5
0.02	5	16	3.2

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The IR absorption spectra are shown in Fig. 4 for the precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvents of pH 0.35. In both cases, the bands were detected at around 500, 650, 1180, 1600, and 3400 cm⁻¹. The broad band at around 3400 cm⁻¹ is assigned to O–H groups including the stretching vibrations of terminal and bridged Sn–OH groups, and of hydrogen bonds between Sn–O and Sn–OH.²⁵ The band at 1600 cm⁻¹ is assigned to the bending vibrations of adsorbed H₂O, respectively. The broad band at around 1180 cm⁻¹ is assigned to the bending vibrations of terminal Sn–OH groups.^26,27 The broad bands at 500 and 650 cm⁻¹ may be the symmetric and asymmetric stretches of Sn–O–Sn bonds.^26,27 The bands at around 1250, 1380, 1450 and 1700 cm⁻¹ were detected in the spectrum of the precipitates obtained at [PAA] = 0.02 M. The bands at 1250 and 1700 cm⁻¹ are assigned to carboxylate groups that act as the monodentate ligands of metal ions, and the bands at 1380, 1450 cm⁻¹ are assigned to those act as the bidentate ligands.^28,29

Fig. 4 IR absorption spectra of the SnO₂ particles prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvent of pH 0.35 via hydrothermal treatment at 150 °C.

Fig. 5 shows the TG/DTA curves of the precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvents of pH 0.35. The first weight loss below 100 °C indicates the evaporation of H₂O. The weight loss at 200–500 °C shows the formation of SnO₂ from Sn(OH)₄.³⁰ An exothermic peak at around 340 °C for [PAA] = 0.02 M is attributed to the burning of PAA. Such an exothermic peak was not detected for [PAA] = 0 M. Table 3 shows the mass fraction of the components in the precipitates estimated from the TG curves. The contents of Sn(OH)₄ for the precipitates at [PAA] = 0 and 0.02 M was estimated to 25 and 23 wt%, respectively. The flower-like particles obtained at [PAA] = 0.02 M contained PAA at 6 wt%.

Fig. 5 TG-DTA curves of the SnO₂ particles prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvent of pH 0.35 via hydrothermal treatment at 150 °C.

Table 3 Mass fraction of the components in the precipitates prepared from the solutions of [PAA] = 0 and 0.02 M containing the acidic solvents of pH 0.35

	Mass fraction^a (wt%)
a The mass fraction was calculated on the assumption that Sn(OH)4 was produced at a constant ratio to SnO2 irrespective of the addition of PAA.
[PAA]/M	SnO2	Sn(OH)4	PAA
0	75	25	0
0.02	71	23	6

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In this work, SnO₂ precipitates were produced from SnCl₄ solutions containing PAA and the acidic solvent of pH 0.35–40 by the hydrothermal treatment at 150 °C, where the precipitates contained Sn(OH)₄ and PAA. Under acidic conditions, SnO₂ and Sn(OH)₄ are thought to be produced via the reaction of Sn⁴⁺ ions with H₂O, as follows;

Sn⁴⁺ + 2H₂O → SnO₂ + 4H⁺ (1)

Sn⁴⁺ + 4H₂O → Sn(OH)₄ + 4H⁺ (2)

where H⁺ ions are also produced during the precipitation of SnO₂ and Sn(OH)₄. In fact, we confirmed the decrease in the pH value of the solutions during the hydrothermal treatment. Here, SnO₂ nanoparticles were produced from the precursor solutions containing the acidic solvent of pH 0.35, where monodispersed flower-like particles consisting of nanorods of ca. 50 nm in length and ca. 5 nm in width were obtained by the addition of PAA (Fig. 1b–c). The formation of such monodispersed particles consisting of nanounits could be attributed to the moderate nucleation and growth rates of SnO₂ crystals that were provided by the relatively high solubility of Sn⁴⁺ species under the acidic conditions and their coordination by PAA. The high solubility of Sn⁴⁺ species under the acidic conditions and the coordination of Sn⁴⁺ ions by PAA suppressed the rapid nucleation of SnO₂ crystals, and consequently induced the formation of monodispersed secondary particles.

The presence of PAA in the SnO₂ secondary particles was suggested by the IR absorption spectra and TG/DTA curves (Fig. 4 and 5). PAA, which may be adsorbed on the crystallite surface parallel to the (110) planes, was deduced to suppress the growth of SnO₂ crystals perpendicular to the [001] direction, resulting in the formation of nanorods elongating in the [001] direction (Fig. 2b).

Fig. 6 shows the schematic illustration of the formation of the flower-like secondary particles. During the hydrothermal treatment at 150 °C, fine particles consisting of several SnO₂ crystallites were produced (Fig. 6a), and the crystallites grew to the nanorods elongating in the [001] direction in association with PAA (Fig. 6b). And then, several nanorods gathered for the reduction of the surface energy, and needle-like units were formed (Fig. 6c). Consequently, flower-like secondary particles consisting of nanorods were obtained.

Fig. 6 Schematic illustration of the formation of flower-like particles.

On the other hand, the morphology of the SnO₂ secondary particles changed to unshaped precipitates with increasing pH of the acidic solvent to 0.40 (Fig. 1c). The solubility of Sn⁴⁺ species in acidic solutions decreases with increasing pH of the solvents,³¹ resulting in the decrease in the nucleation rate of SnO₂ crystals. As a result, the rapid nucleation could lead to the formation of unshaped precipitates.

Previously, we reported the formation of SnO₂ nanorods of ca. 5 nm in width and ca. 20 nm in length from aqueous solutions of SnCl₄ containing citric acid ([SnCl₄] = 0.4 M, [citric acid] = 0.2 M). In that work, large secondary particles above 1 μm in diameter were formed because the high number of SnO₂ primary crystallites was supplied from the high concentration SnCl₄ solutions ([SnCl₄] = 0.4 M). On the other hand, in the present work, nano-meter scaled flower-like particles of ca. 100 nm in diameter were obtained from the low concentration SnCl₄ solutions containing PAA ([SnCl₄] = 0.1 M , [PAA] = 0 and 0.02 M). Here, we confirmed that the precipitation of SnO₂ crystals is suppressed by the addition of citric acid at such low [SnCl₄], which suggests that citric acid strongly chelates the Sn⁴⁺ ions compared with PAA. Therefore, the formation of nano-meter scaled secondary particles consisting of single-crystalline SnO₂ nanorods was deduced to require the low concentration of SnCl₄ and the relatively weak coordination of Sn⁴⁺ ions by PAA. However, the difference between the effect of PAA and citric acid on the nucleation and growth of SnO₂ crystals was not fully investigated in this work, and should be discussed under identical conditions in the near future.

4. Conclusion

We prepared crystalline SnO₂ particles from acidic aqueous solutions of SnCl₄ with and without PAA by hydrothermal treatment at 150 °C. The morphology of the secondary particles depended on the concentration of PAA and the pH value of the solvents. Flower-like SnO₂ secondary particles of ca. 100 nm in diameter consisting of needle-like units below 50 nm in size were obtained from the solution of [SnCl₄] = 0.1 M and [PAA] = 0.02 M containing the acidic solvents of pH 0.35. The needle-like units consisted of nanorods of ca. 50 nm in length and ca. 5 nm in width, where the nanorods were found to be single-crystalline SnO₂ elongating in the [001] direction. The flower-like particles consisting of nano-meter scaled crystalline units were thought to be formed through the moderate nucleation and growth rate that was provided the relatively high solubility of Sn⁴⁺ species of the acidic solvent and the coordination by PAA. Such nanostructure thus obtained holds great promise for the application to several devices such as gas sensors, electrode materials and catalysts because of the high crystallinity and porosity.

Acknowledgements

This work is financially supported by the Kansai University Research Grants: Grant-in-Aid for Encouragement of Scientists, 2010.

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