Synthesis of ThO₂ nanostructures through a hydrothermal approach: influence of hexamethylenetetramine (HMTA) and sodium dodecyl sulfate (SDS)

Abstract

In this work, ThO₂ nanostructures with various morphologies are synthesized through a hydrothermal approach. The influences of some experimental parameters, such as the concentration of hexamethylenetetramine (HMTA), the amount of sodium dodecyl sulfate (SDS) and the hydrothermal temperature, on the synthesis and morphologies of ThO₂ nanostructures were systematically investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results showed that ThO₂ nanospheres with a particle size of 59 ± 9 nm could be synthesized at a molar ratio of Th⁴⁺/HMTA of 0.3 under the reaction temperature of 100 °C. Adding SDS could adjust the diameter of ThO₂ nanospheres from 240 to 71 nm while keeping other parameters constant. Furthermore, nanoparticles and the aggregated nanostructures of ThO₂ could be obtained by varying the concentration of HMTA and the amount of SDS. The possible formation mechanism of various ThO₂ nanostructures was also proposed.

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Introduction

Actinide oxide nanomaterials, which may serve as new building blocks for the design of innovative nuclear fuels,^1–3 have attracted much attention in recent years.^4–10 Among them, thorium dioxide, a very important inorganic material in the nuclear industry and catalysis, has been prepared using various methods.^11–16 For example, different morphologies of small ThO₂ nanocrystals, such as branched nanocrystals, nanodots and nanorods, have been synthesized through an organic-phase thermolysis method using thorium acetate and thorium acetylacetonate as the metal source in a mixture solution of oleic acid, oleylamine and octadecene.^17,18 This method has been extended to the synthesis of other actinide oxide nanocrystals, such as UO₂, NpO₂, PuO₂ and the mixing oxide of thorium/uranium nanocrystals.^19–21 ThO₂ nanotubes have also been fabricated using the so-called sol–gel method in a porous anodic aluminum oxide template.¹² Compared to the methods mentioned above, the hydrothermal technique occupies an important position on the synthesis of tailored metal oxide nanostructures.^22,23 By employing supercritical water as a reaction environment, ThO₂ nanoparticles with the size in the 5–31 nm range have been prepared.¹⁴ However, compared to the preparation of conventional metal oxide nanostructures, the morphology and size control on the synthesis of ThO₂ nanoparticles is less well investigated, thus further developments are still needed.

Surfactant based methods are usually considered for the synthesis of various nanostructures. The surfactants can be employed as a stabilizer, capping agent and template to control the size, anisotropic growth and agglomeration. Sodium dodecyl sulfate (SDS), as an anionic surfactant, has been used for synthesizing large number of inorganic materials with a tunable size and shape.^24–26 Hexamethylenetetramine (HMTA, (CH₂)₆N₄), which could release ammonia after heat treatment, has also been widely applied for the preparation of transition and main group metal oxide nanoparticles.^27–35 However, to the best of our knowledge, there are few literatures on the synthesis of ThO₂ nanostructures by adding HMTA and SDS under hydrothermal conditions with low temperature. Herein, we reported the investigation on the synthesis of various ThO₂ nanostructures through a hydrothermal approach by inducing HMTA and SDS. The effects of some experimental parameters such as the concentration of HMTA, the amount of SDS and the reaction temperature have been explored. Furthermore, the formation mechanism of ThO₂ nanostructures has also been proposed.

Experimental

All of the reagents were analytical grade and were used as received without further purification. In a typical procedure, 0.114 g (0.2 mmol) thorium nitrate pentahydrate (TNP) and 8.4 mg (0.06 mmol) hexamethylenetetramine (HMTA) was dissolved into 10 mL deionized water. The solution was stirred for 15 min before being transferred into a 15 mL Teflon-lined autoclave. Then the autoclave was sealed and maintained at 100 °C for 12 h, and cooled down to room temperature naturally. The white product which consists of ThO₂ nanospheres was centrifuged and washed with deionized water and alcohol three times, followed by drying at 60 °C in air. In order to investigate whether the SDS would affect the formation of ThO₂ nanospheres during the hydrothermal process, the same procedure was followed for the preparation of ThO₂ nanospheres by adding SDS. Other control experiments were carried out by adjusting some experiment parameters such as the concentration of HMTA, the amount of SDS and temperature, etc. Table 1 gives the synthesis conditions of the ThO₂ samples.

Table 1 Experimental conditions for the preparation of various ThO₂ nanostructures^a

Sample number	The concentration of TNP/mM	Hydrothermal temperature (°C)	The concentration of HMTA/mM	The amount of SDS/g	The solution pH before the hydrothermal treatment	The solution pH after the hydrothermal treatment	Major products
a —: not measured.
1	20	100	6	0	2.80	1.94	ThO2 nanospheres
2		100	6	0.02	2.57	2.23
3		100	6	0.04	2.56	2.02
4		100	6	0.06	—	—
5		100	7	0	—	—	Smaller ThO2 nanospheres
6		100	8	0	—	—
7		100	9	0	—	—
8		100	10	0	—	2.15	Amorphous Th(OH)4 and smaller ThO2 nanospheres
9		100	12	0	—	—
10		100	14	0	—	2.80
11		100	20	0	—	—	Amorphous Th(OH)4 and small ThO2 nanoparticles
12		100	40	0	—	—	ThO2 nanoparticles
13		100	20	0.02	3.00	3.13	Assemblies of ThO2 nanoparticles
14		100	20	0.04	2.68	3.39
15		100	40	0.02	3.26	4.92
16		100	40	0.04	3.10	5.09
17		130	6	0	—	—	ThO2 nanospheres
18		160	6	0	—	—
19		190	6	0	—	—	ThO2 nanoparticles

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The X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) observations were performed with a Hitachi S-4800 scanning electron microscope. The precursors were characterized by Fourier transform infrared (FT-IR) spectroscopy on a Bruker Tensor 27 FT-IR spectrometer using the KBr pellet method. Thermogravimetry (TGA, TA Instruments, Q-500) was used to test the thermal stability of thorium dioxide nanospheres.

Results and discussion

The XRD pattern of the sample 1 is shown in Fig. 1. All the diffraction peaks can be indexed to cubic structure of ThO₂ (JCPDS 42-1462). No other diffraction peaks are detected, indicating the final product is of pure phase. The broaden peaks can be attributed to the presence of small-sized ThO₂ nanoparticles, indicating that the ThO₂ phase does not crystallize very well. The morphology of the product was further characterized by SEM. It can be clearly seen from Fig. 2 that these ThO₂ nanospheres with rough surfaces show a spherical morphology with an average diameter of 59 ± 9 nm (Fig. S1†). Magnified image reveals that these ThO₂ nanospheres consist of many tiny ThO₂ nanoparticles, which is consistent with the XRD pattern.

Fig. 1 XRD Pattern of the sample 1.

Fig. 2 SEM images of the sample 1. (b) is the magnified image of (a).

For the SDS-induced sample 2–4, the obtained XRD patterns exhibit similar diffraction peaks as Fig. 1, which could be also indexed to the pure ThO₂ phase (data not shown). However, it can be seen from the SEM images that the anionic surfactant SDS plays important roles in controlling the sizes of ThO₂ nanospheres. When 0.02 g of SDS was added into the initial solution, ThO₂ nanospheres with a diameter of 240 ± 30 nm were obtained (Fig. 3a and b and S2a†). As the amount of SDS increases to 0.04 and 0.06 g, the average size of ThO₂ nanospheres decreases to 138 ± 25 and 71 ± 16 nm, respectively (Fig. 3c–f, S2b and c†).

Fig. 3 SEM images of samples 2 (a and b), 3 (c and d) and 4 (e and f). b, d and f are the enlarged images of a, c and e, respectively.

The above as-synthesized products have also been subjected to infrared characterization (Fig. 4). For the sample 1 which is free of SDS (Fig. 4a), some peaks in the range of 1600–700 cm⁻¹ display characteristic absorptions of nitrate (1479, 1383 and 743 cm⁻¹) and carbonate (1572, 1043 and 840 cm⁻¹) anions, indicating that these two species are adsorbed on the ThO₂ nanospheres. The absorption around 500 cm⁻¹ can be assigned to Th–O vibrations.^36,37 However, for the sample 2 and 3, the C–H symmetric and asymmetric stretching vibration frequencies at 2926 and 2854 cm⁻¹ are clearly seen. Meanwhile, the peaks situated around 1070 cm⁻¹ are attributed to the sulfate group.^25,26 Hence, these FT-IR results demonstrate the interaction of SDS and ThO₂ in the SDS-induced samples (Fig. 4b and c).

Fig. 4 FT-IR spectrum of the sample 1 (a), 2 (b) and 3 (c).

Thermal analysis of the products is shown in Fig. 5 for sample 1 and 2. Thermal degradation of both samples showed three independent weight loss steps, which are related to the desorption of adsorbed water (70 °C), the decomposition of thorium oxynitrate intermediate (300 °C) and the pyrolysis of thorium carbonate (470 °C).³⁶ This is in good agreement with the FT-IR results, indicating the presence of nitrate ions and carbonate ions in the ThO₂ nanospheres. However, for the sample 2, the degradation peak of the SDS should appear at the temperature of 160–380 °C,³⁸ which is overlapped with the decomposition of thorium oxynitrate intermediate.

Fig. 5 Thermogravimetric weight loss curve (solid line) and its derivative (dashed line) of the sample 1 (a) and 2 (b).

It was found that the concentrations of HMTA and SDS could greatly affect the formation process of ThO₂ nanospheres. A series of experiments were performed by changing the molar ratio of HMTA/TNP to evaluate the influence of the HMTA concentration in the absence of SDS (Table 1, sample 5–12). Fig. 6 shows SEM images of the samples prepared at HMTA/TNP molar ratio of 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 1 and 2, respectively. In all these cases, the other reaction conditions were kept constant. When the molar ratio of HMTA/TNP varied from 0.35 to 0.45 (sample 5–7), the size of ThO₂ nanospheres reduced to 55 ± 10, 47 ± 7 and 45 ± 12 nm, respectively (Fig. 6a–c and S3a–c†). When the molar ratio of HMTA/TNP exceeds 0.5, both the size and the yield of ThO₂ nanospheres were decreased (Fig. 6d–g). However, as shown in Fig. 6h, too much excess of HMTA in the solution (the ratio of HMTA/TNP is 2) could produce a high yield of tiny ThO₂ nanoparticles with an average size of 6 nm. This result is also similar to the previous report.¹⁴

Fig. 6 SEM images of samples prepared at different initial molar ratios of HMTA/TNP at 100 °C: (a) sample 5-0.35, (b) sample 6-0.4, (c) sample 7-0.45, (d) sample 8-0.5, (e) sample 9-0.6, (f) sample 10-0.7, (g) sample 11-1 and (h) sample 12-2. The inset of g showed the enlarged image.

The influence of the SDS concentration on the formation of ThO₂ nanoparticles under high concentration of HMTA was also studied. All of the samples (Table 1, sample 13–16) produced aggregated ThO₂ nanostructures which came from the disordered assembly of small ThO₂ particles (Fig. S4†). The strengthened peaks of alkyl C–H stretching vibration (2926 and 2854 cm⁻¹) and sulfate group (1062 cm⁻¹) in Fig. S5b and c† enclosed that more SDS molecules are incorporated in the aggregated ThO₂ nanostructures. Besides, adding SDS to the higher concentration of HMTA solutions would produce white bulk precipitate even before the hydrothermal treatment. This implies that SDS also played an important role in the formation of the aggregated ThO₂ nanostructures, since well dispersed tiny particles instead of bulk aggregated nanostructures were observed under the absence of SDS (Fig. 6g and h). Detailed discussion about the formation mechanism of various ThO₂ structures will be elucidated later.

Apart from the concentration of HMTA and SDS, the temperature of hydrothermal reaction can also alter the morphology of the final product. Fig. 7 shows the SEM images of the samples prepared at different temperatures under otherwise identical conditions (Table 1, sample 17–19). No product was obtained at 80 °C, suggesting the incomplete decomposition of HMTA. At 130 °C, ThO₂ nanospheres with a diameter of 45 ± 8 nm were generated (Fig. 7a and S6a†), which is smaller than the product obtained at 100 °C (Fig. 2). When the temperature rises to 160 °C, ThO₂ nanoparticles with similar diameters (45 ± 7 nm) and rough surfaces were obtained (Fig. 7b and c and S6b†). Only tiny ThO₂ nanoparticles were observed after the hydrothermal treatment at 190 °C (Fig. 7d).

Fig. 7 SEM images of samples prepared at different temperatures with a molar ratio of HMTA/TNP of 0.3: (a) sample 17-130 °C, (b and c) sample 18-160 °C, (d) sample 19-190 °C. (c) is the enlarged image of (b).

According to the above Experimental results, a probable mechanism of the formation of ThO₂ nanospheres, nanoparticles and the aggregated ThO₂ nanostructures can be expressed as follows.

Thorium ion, Th⁴⁺, is easily to hydrolysis in aqueous solution. Overall, amorphous thorium hydroxide were formed.^14,39

Th⁴⁺ + 4H₂O ⇌ Th(OH)₄(am) + 4H⁺ (1)

It has been reported that, at elevated temperatures, HMTA is unstable and decompose to release HCHO and NH₃.^31,32

(CH₂)₆N₄ + 6H₂O → 6HCHO + 4NH₃ (2)

While the formed NH₃ will react with H⁺, pushing the equilibrium (1) to the right side, forming more amorphous thorium hydroxide.

NH₃ + H⁺ → NH₄⁺ (3)

Finally, ThO₂ particles are produced under consecutive heating.

Th(OH)₄(am) → ThO₂(cr) + 2H₂O (4)

For the sample 1, some amorphous thorium hydroxide nuclei were formed under the decomposition of HMTA. Noted that the amount of HMTA is insufficient to precipitate all of the Th⁴⁺ ions, a considerable amount of Th⁴⁺ ions are still in the solution. These Th⁴⁺ ions tend to hydrolysis at the as-formed amorphous thorium hydroxide nuclei, making the Th(OH)₄ nuclei growth bigger. Meanwhile, small crystallized ThO₂ particles were formed in the amorphous Th(OH)₄ under consecutive heating. Finally, ThO₂ nanospheres were formed after the conversion of Th(OH)₄ nanospheres (Fig. 2). Slightly increase the concentration of HMTA (sample 5–7) will lead to forming more amorphous thorium hydroxide nuclei and less amount of Th⁴⁺. This would reduce the growing speed of Th(OH)₄ nuclei and produced smaller ThO₂ spheres at the end (Fig. 6a–c). Although the solution is still strong acidic after the hydrothermal treatment, ThO₂ nanospheres can still be observed since crystalline ThO₂ is insoluble than amorphous Th(OH)₄ under the acidic conditions.⁴⁰

However, when the ratio of HMTA/TNP exceeds 0.5 but less than 2 (sample 8–11), the amorphous thorium hydroxide nuclei become much more and much smaller. Some of them may exist as some polynuclear species of thorium(IV).^39,40 These polynuclear species might be stable under the corresponding solution pH and the temperature. So the growth of Th(OH)₄ nuclei is much restricted, hence the formation of ThO₂ particles are also limited (Fig. 6d–g). Meanwhile, a large amount of smaller gel-like amorphous Th(OH)₄ nuclei and the polynuclear species are still remaining in the solution, leading to the decrease in the yield of ThO₂ nanospheres. When the HMTA is excess (sample 12), Th⁴⁺ ion is depleted after the formation of Th(OH)₄ nuclei due to the higher solution pH. Thus, these small nuclei cannot grow bigger and finally transferred to tiny ThO₂ nanoparticles (Fig. 6h).

As an anionic surfactant, SDS is known to form cylindrical micelles above the critical micelle concentration (CMC). Because the SDS concentration used in our experiments was higher than the CMC (CMC = 0.9 mmol L⁻¹), most of SDS existed as self-assemblies, which may serves as a scaffold for the formation of various structures.^24,25 For the sample 2, since the anionic head group in the SDS will attract Th⁴⁺ ions to the surface of micelles, the micelle can provide nucleation site of Th(OH)₄, making the nucleation step easier than the SDS-free sample. After the nucleation step, the Th(OH)₄ nuclei can grow bigger and transferred to ThO₂. Since the isoelectric point of ThO₂ is about 6–11,⁴¹ the surface of ThO₂ particles will be positively charged, and is still easy to adsorb anionic SDS. These adsorbed SDS molecules can greatly enhance the agglomeration and growth of the ThO₂ nuclei, thus larger size of ThO₂ nanospheres can be obtained (Fig. 3a and b). However, when the concentration of SDS goes higher, more Th(OH)₄ nuclei will form, producing a smaller size of Th(OH)₄ and ThO₂ nanospheres (sample 3 and 4, Fig. 3c–f).

When adding more HMTA to the SDS-induced sample, white precipitate will come into being before the hydrothermal treatment (sample 13–16). This bulk precipitate may consist of the complex of thorium and SDS, which is easy to precipitate from the solution at higher pH. The Th(OH)₄ nuclei were formed on this solid precipitate in situ during the decomposition of HMTA. After the conversion of Th(OH)₄ to ThO₂, the aggregated ThO₂ nanostructures, which is comprised of the disordered assembly of ThO₂ small particles, were formed (Fig. S4†).

Conclusions

In all, the influences of HMTA and SDS in the synthesis of ThO₂ nanostructures through a hydrothermal approach have been investigated. By varying the concentration of HMTA and SDS, ThO₂ nanospheres, nanoparticles and the aggregated ThO₂ nanostructures are successfully synthesized by employing a facile hydrothermal method. Especially, the reaction at 100 °C with a HMTA/TNP molar ratio of 0.3 results in the formation of ThO₂ nanospheres with a diameter of 59 ± 9 nm. The size of ThO₂ nanospheres can be adjusted from 240 to 71 nm by inducing the SDS while keep the HMTA/TNP molar ratio of 0.3. Such results can provide useful information on the control synthesis of ThO₂ nanospheres and help understanding the formation mechanism of ThO₂ nanostructures.

Acknowledgements

We gratefully acknowledge the financial support by the Natural Science Foundation of China (Grants 91326202, 11305184, 11275219, and 11405186) and the “Strategic Priority Research program” of the Chinese Academy of Sciences (Grant XDA030104).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The size histograms of the sample 1 (Fig. S1), 2–4 (Fig. S2), 5–7 (Fig. S3), 17 and 18 (Fig. S6); the SEM images of sample 13–16 (Fig. S4); the FT-IR spectra of sample 13 and 15 (Fig. S5). See DOI: 10.1039/c4ra07466a

‡ Ran Zhao and Lin Wang contributed equally to this work.

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