Preparation of Terbium Hydroxide Nanowires by Hydrothermal

The preparation of Terbium hydroxide nanowires by hydrothermal method

The single-crystalline Tb(OH)3 with various morphologies was successfully prepared from Tb2O3 powder by adjusting the concentration of acetic acid under hydrothermal condition. The influence of the concentration of acetic acid and reaction temperature on the crystalline phase and morphologies of Tb(OH)3 products was discussed. The concentration of acetic acid plays a key role in determining the products morphology. Eventually, the mechanism of acetic acid on morphology of products was deeply discussed.

Keywords: Tb(OH)3, nanowires, spindle, rare earth, hydrothermal.

1. Introduction

Rare-earth elements are a group of 17 chemically similar metallic elements, including the 15 lanthanides, plus scandium and yttrium because of their similar chemical properties.

The oxides of rare-earth elements have been extensive used in past decades because of their optic, electric, magnetic, and catalytic properties, which are caused by their unique electronic structures and numerous transition modes involving the 4f shells of their ions. These properties strongly depend on the composition, morphology and dimensionality of products, which are sensitive to the bonding states of rare earth ions.

In recent years, many important materials have been prepared in the form of nanowires to generate some unexpected properties. Nanowires represent a class of quasi-one-dimensional materials, in which carrier motion is restricted in two directions so that they usually exhibit significant photochemical, physical, and electron-transport properties which differ from that of bulk or nanoparticle materials.

Recently, three dimensional (3D) nanostructured architectures have been explored for a new generation of advanced devices such as supercapacitors, fuel cells, and sensors owing to some improved properties originating from their nanobuilding blocks and the manners in which they are organized. Up to now, a wide variety of inorganic materials, including metal,6a metal oxide,6b–d hydrate,6e borate,6f molybdate,6g,h and tungstate,6i have been successfully prepared with complex 3D hierarchical shapes by the solution-phase chemical method, due to its low cost and potential advantage for large-scale production. However, exploration of reasonable synthetic methods for controlled construction of complex 3D architectures of other inorganic functional materials via a chemical self-assembly route is still an intensive and hot research topic.

In the controlled construction of self-assembly of 1D or 2D nanobuilding blocks into 3D novel nanoarchitectures, copolymers and surfactants always play important roles due to their directing functions during the aggregation process as well as their stabilizing effects in equilibrium systems. [Lu-1]

For example, who and who reported the

Who et al. have described systematically the …

However, there are only several report on the synthesis of

Among the family of rare earth compounds, the terbium oxide is the important functional rare earth material. It had been used as a promising candidate for ceramic pigments, catalysts, promoters and stabilizers in combustion catalysts, oxygen-storage components, and materials with higher electrical conductivity. In addition, terbium hydroxides are of great importance because rare earth oxides can be straight formed through dehydration from hydroxides.

To date, many terbium hydroxides particles have been synthesized via a hydrothermal route due to the advantages of high purity and good homogeneity, and the corresponding structured rare earth oxides were made by calcining the precursors. [Lu-1]

In the present work, we exploited a one-step hydrothermal route to prepare nano-scale terbium hydroxide with various morphologies. In addition, the mechanism of acetic acid on morphology of products was deeply discussed.

The reaction mechanism leading to the lutetium oxide precursor and the self-assembly process were discussed.

A possible formation mechanism for the morphology evolution of these microstructures was suggested, which was not reported before.

2. Experimental section:

2.1. Preparation of Tb(OH)3 precursors

All chemicals with analytical grade were purchased from Wako Pure Chemical Industries, Ltd., Japan, and used without further purification. A detailed description of the preparation process is as follows:

15 mL of acetic acid solution with various concentrations (0.001~0.2M) and 0.45 g of Tb2O₃were placed in a 25 mL Teflon-lined autoclave. The autoclave was sealed, heated in an electric oven to 200 °C at a heating rate of 5 °C/min, and maintained at 200 °C for 6 h with rotation for agitation. The autoclave was then cooled to room temperature via air quenching. The precipitate was collected using a centrifuge, washed with distilled water, and dried at room temperature. In order to investigate the mechanism, the reaction temperature and reaction time were appropriately changed. The pure water, sodium hydroxide and the other type of solutions were used as the solvent instead of acetic acid solution.

2.3. Characterization

Powder X-ray diffractions (XRD) were performed using a Rigaku RTP-300RC diffractometer operating at 40 kV and 100 mA with Cu K radiation () 1.54056 μm. The patterns were collected in the range of 10° to 70° with a 0.02° step and scanning speed of 4°/min. The micrographs of field emission scanning electron microscopy (FE-SEM) were obtained using a JEOL JSM-6500F electron microscope operating at 15 kV. The samples were heated in air at a ramp rate of 10 ºC/min. Fourier transform infrared (FT-IR) spectra were obtained using a Shimadzu FTIR-8200PC spectrophotometer at room temperature.

3. Result and discussion

3.1. Influence of acetic acid concentration on the phase and morphology of the obtained precursors

The chemical composition and crystal structure of the samples were firstly determined by XRD measurements. Fig. 1 shows the XRD patterns of as-prepared samples in the various concentrations of acetic acid solution. It is easy found that when the concentration of HAc was lower than 0.15M (fig.1a~e), the XRD patterns can be indexed to be a pure hexagonal phase of Tb(OH)3, in agreement with the reported data (JCPDS 83-2038) with lattice constants a=6.3150 Å and c=3.6030Å. With the enhancement of concentration of HAc, the intensity of main diffraction peak (100) gradually reduced, meanwhile the full width at half maximum (FWHM) increased gradually. It means that the crystallinity of products and crystalline grain were gradually decreased. It is important to note that the relative intensity ratio of (110) and (101) peaks was changed with the increse of concentration of CH3COOH, suggesting that the preferential growth along c-axis occurs with the increse of concentration of CH3COOH. When the concentration was further increased to 0.2 M, an unknown phase was formed. The XRD patters showed the compound have the layered structure according to the d value.

In order to determine the chemical composition of the unknown phase, the Infrared spectra was allowed to use. Fig.2 shows the FT-IR spectra for the as-prepared simples. The typical peaks of Tb(OH)3 products (fig2.a~e)were found at ca. 3610 and 670 cm-1. In accordance with the results in literature, these two bands can be associated with OH stretching and with Tb-OH bending modes in the hydroxide[æ-‡çŒ®]. The FTIR spectra show that these products are free of organic byproducts. The IR spectra of the unknown phase (fig2.f) show additional adsorption bands at 3380, 2924 , 2853,1568, 1443 and 1011 cm-1. The occurrence of a broad 3390 cm-1 band is attributed to residual traces of water in the sample. The bands at 2924 and 2853 cm-1 correspond to –CH3 stretching and –CH3 against stretching vibration, respectively. These bands are located between 1568 and 1443 cm-1, which are typical for stretching vibrations of carboxylate groups Va(COO-) and Vs(COO-), respectively[æ-‡çŒ®]. The band at 1011 cm-1 corresponds to Tb-OH bending vibration. On the basis of the IR spectra，the precursor material is assumed to contain metal acetate hydrate and metallic hydrate (Tb(CH3COO)X(OH)Y•H2O).

The SEM images of several typical samples with distinct morphologies are presented in Fig.3a-f. It is found that the concentration of acetic acid (HAc) plays a key role in determining the product morphology. Tb(OH)3 could be obtained at concentration between 0.001 to 0.15M. At low HAc concentration (fig.3a), the obtained product is composed of granular aggregates and it was difficult to distinguish each other. A slight increase in concentration to 0.01M (fig.3b), the morphology changed to microfibers of nanowires aggregated with diameter of 1.5 μm and length up to 5 μm. It is easy to find that these microfibers were composed of bundles of nanowires. The diameter of these nanowires ranges from 50 to 100 nanometers. As HAc concentration was further increased to 0.03M (fig.3c), the spindle-like structures ranging in diameter less than 2μm and length more than 10μm were obtained. When HAc concentration was added to 0.1 M (fig.3d), the length of the bundles of nanowires was further increase to more than 12μm along with the decrease of the diameter. At the same time, the diameter of the nanowires also gradually reduced. To further increase the concentration of HAc to 0.15M (fig.3e), the bundles of nanowires began to varying degrees of separation, some single nanowires began to form. These nanostructures are found to be in a wide scale of size, ranging in diameter from less than 12μm to more than 2 μm. Finally, when the concentration was further increased to 0.2 M (fig.3f), the SEM photograph also showed that this compound consisted of plate-like crystals, which is in agreement with the XRD result.

3.2．Effect of temperature

To determine the effect of temperature, the Tb2O3 with 15ml 0.067M CH3COOH solution were hydrothermally treated at 100, 160, 200, 220℃ for 6h. According to the XRD patterns, the pure phase of Tb(OH)3 was only obtained at hydrothermal temperature above 160℃, while only a small amount of Tb(OH)3 was formed at 100℃, and most of products proved to be raw material Tb2O3. With the increase of reaction temperature to above 160℃，the raw material Tb2O3 disappeared, and pure phase of Tb(OH)3 was obtained. When the temperature was increased to 220℃，the crystallinity of Tb(OH)3 was significantly increased.

The hydrothermal temperature has a great impact on the size of the terbium hydroxide precursors. Figure 5 shows typical SEM images of Tb(OH)3 in 0.067 mol/L HAc solution at various temperature. It can be seen that the a few microfiber-like structure of Tb(OH)3 was formed at low temperature of 100℃(Fig.5a). When the temperature reached to 160℃, the uniform microfibers of Tb(OH)3 nanowires with length of about 13 m were obtained(Fig.5b). With the enhancement of reaction temperature, the length of microfibers gradually increased, as well as the diameter of nanowires (inset in Fig.5b and Fig.5d). The more quantity of microfibers was formed at high reaction temperature than the low temperature. In addition these microfibers grew very slowly as the extension of reaction time at high temperature of 220℃. Details of results are as shown in table1.

That is because the reaction rate of the dissolving – recrystallization increases at high temperature, a large number of crystal nucleus has been quickly formed in the initial stage of reaction. In the case of no changing the total amount of raw materials, the smaller grain size has been formed under the condition of higher temperature. Meanwhile, the defect of products is less at high temperature than low temperature. Therefore the diameter of nanowires of microfibers at high temperature is bigger than at low temperature.

3.3. Effect of reaction

In order to understand the reaction process, the reaction time was changed from 0 to 24h, while the CH3COOH concentration and reaction temperature were fixed to 0.067M and 200℃, respectively, Figure 6 show the XRD results of samples received after 0 h (just when the oven reached 200°C at a heating ramp of 5°C /min), 0.5h, 2h, and 6h of hydrothermal reaction at 200 °C, respectively. According to the XRD patterns, when the oven reached 200 °C, the characteristic peaks of hexagonal Tb(OH)3 just began to appear in the XRD pattern, as shown in Figure 6a. Most of the characteristic peaks proved to be raw material Tb2O3. The pure phase Tb(OH)3 could be obtained after 30 min of hydrothermal reaction, and with the enhancement of reaction time, the intensity of main diffraction peak (100) gradually increased. It means that the crystallinity was gradually increased with the reaction time increased.

Figure 7 shows SEM images of samples received after 0 h (just when the oven reached 200 °C at a heating ramp of 5 °C /min), 0.5 h, 2 h, 6 h, and 24 h of hydrothermal reaction at 200 °C, respectively. When the oven reached 200 °C, the product is composed of starfish-like microstructure (Figure 7a). With the increased of time to 0.5h(fig.7c), the morphology changed to microfibers of nanowires aggregated with diameter of 1.5 μm and length up to 5 μm. It is easy to find that these microfibers were composed of bundles of nanowires. As time was further increased to 6h (fig.7d), the spindle-like structures ranging in diameter less than 1.5μm and length more than 10μm were obtained. Finally, when time was increased to 24h (fig.7e), the nanowires of the spindle-like microfibers began to split.

3.4 the effect of CH3COOH on morphology

According to the above phenomenon, CH3COOH plays a key role in determining the morphology of products. To determine the effect of CH3COOH on morphology, the pure water, sodium hydroxide, CH3COONa, HCl, oxalic acid and ascorbic acid (L-Ac) solution as the solvent instead of acetic acid.

From XRD patterns we can see, no matter what kind of solution was used as a solvent, the pure phase Tb(OH)3 could be obtained.

In 5M NaOH solution, the product was composed of nanorods aggregates. In pure water, nanoparticals and some mocrorods aggregates were obtained. (比较碱,æ°´,盐酸,硝酸,草酸,ç»´ä»-命c)

Figure 9. SEM images of products synthesized from 0.45g of Tb2O3 in (a)10M NaOH solution and (b) pure water at 200 oC for 6h.

4. Conclusion

We have used terbium oxide and different concentrations of acetic acid to synthesize the single-crystalline Tb(OH)3 with various morphologies by a hydrothermal method. The concentration of acetic acid, reaction temperature, reaction time and types of solvents strongly affect the morphology and size of products. The morphology of the Tb(OH)3 products changed from granular aggregates, to microfibers of nanowires aggregated, spindle of nanowires, eventually into nanowires with increasing concentration. The crystallinity of the Tb(OH)3 products can be increased by enhanced the reaction temperature. The method utilized in this study to fabricate the terbium hydroxide with tunable morphologies is general and could be extended to synthesize the other rare earth hydroxides by simple adjusting the concentration of acetic acid.