Solvothermal preparation of CaTiO₃ prism and CaTi₂O₄(OH)₂ nanosheet by a facile surfactant–free method

Weixia Dong, Gaoling Zhao, Bao Qifu, Gu Xingyong, Gaorong Han

Abstract: Calcium titanate (CaTiO₃) with prism-like-shaped morphology were synthesized by a simple solvorthermal process without any surfactants. It is found that NaOH concentration plays an important role in the formation of CaTiO₃ prisms. A rational mechanism is proposed to illustrate the growth of CaTiO₃ prisms. And the CaTi₂O₄(OH)₂ interlaminar structure is illustrated.

Keywords: solvorthermal preparation, surfactant-free, CaTiO₃ prism, CaTi₂O₄(OH)₂ interlaminar structure

Introduction

Due to its widespread potential applications in a lot fields, calcium titanium oxides have been considered as one of the most important inorganic materials. The most classic model for shape control is the Wulff facets argument or Gibbs-Curie-Wulff theorem, which suggests that the shape of a crystal depends on the relative specific surface energy of each face or facet of the crystal ¹. However, our experiment results show that this pure thermodynamic can not explain well. It can be the concentration of existing monomers by tuning NaOH concentration plays a key role for the evolution of the shapes and phases of the calcium titanium oxide crystals ². If the pH values can be well-controlled, the growth of crystals with different morphologies should be possible. As a result, the concentration of the remaining monomers after the nucleation process is dependent on the number of nuclei formed. To maintain the correct pH of the solvorthermal system, it is necessary to use alkaline or acidity mineralizers (i.e. pH adjusting agents). NaOH or HNO₃ are most convenient for this purpose. However, intermediates are often observed during the preparation of CaTiO₃. The presence of this impurity phase significantly affects the material’s properties and, thus, it should be avoided. Therefore, it is worthwhile to compare NaOH concentration that are necessary to obtain phase-pure caltium oxides ³.

In the present work, calcium tantium oxides was prepared by a solvorthermal method by tuning NaOH concentration. A rational mechanism is proposed to illustrate the growth of CaTiO₃ prisms. And the CaTi₂O₄(OH)₂ interlaminar structure is analysised by XRD, SEM and FT-IR .

Experimental procedure

Synthesis

The CaTiO₃ crystals were synthesized in an aqueous medium by a solvothermal route. In a typical synthesis, 0.01mol Ti(OC₄H₉)₄ (Sigma Aldrich, 99%) were mixed with molar ratio of water/enthonal=10/10. In particular, pH value of deionized water was adjusted 1 by adding HNO₃. Then added to 10 ml of a 1 M CaCl₂·5H₂O solution under vigorous stirring at room temperature. After the solution was stirred for 5 min, various NaOH concentration (0.005 M, 0.01 M, 1 M, 3 M, 5 M, 7 M) was added to adjust the pH. Subsequently, the autoclave was sealed and maintained at180 °C for 36 h, followed by natural cooling to room temperature. Afterward, the final products were centrifuged, washed with deionized water and absolute ethanol several times, and then dried at 80 °C for 15 h in air.

Characterization

The morphologies of the powders were investigated by field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The crystal phases of the products were characterized by X-ray diffraction (XRD, PANalytical X’Pert Pro, Holland), in a 2θ range from 10⁰ to 80⁰, using Cu-Ka radiation. UV–Vis absorption was measured by a TU-1901 spectrophotometer equipped with a reflectance attachment and BaSO₄ was used as the reference material. Infrared spectra of the samples were obtained using a Nicolet Nexus 470 Fourier transform infrared (FT-IR) spectrometer in the 400-4000 cm^–1 region by KBr pellet.

Results and discussion

We have systematically investigated the system in various NaOH concentrations while keeping the other reaction conditions unchanged, as shown in Fig. 1 and Fig. 2.

Fig. 1 XRD patterns of samples synthesized at different NaOH concentrations: (a) 0.005 M, (b) 0.01 M, (c) 1 M, (d) 3 M, (e) 5 M, (f) 7 M.

Fig. 1 shows the XRD patterns of the samples synthesized in various NaOH concentrations. When NaOH concentrations is 0.005 M, the sample is CaTi₂O₅ (JCPDS card 25-1450) phase. When NaOH concentrations is 0.01 M, the sample is pure CaTi₂O₄(OH)₂ (JCPDS card 39-0357) phase (Fig. 1(b)). When NaOH concentrations increases to 1 M, intensity of CaTi₂O₄(OH)₂ phase increases. Further increasing NaOH concentration to 3 M, CaTi₂O₄(OH)₂ phase disappears and CaTiO₃ (JCPDS card 42-0423) appears with a little trace of Ti₃O₅ and Ti₄O₇. When NaOH concentrations is above 5 M, pure CaTiO₃ is obtained and peak intenisity of CaTiO₃ further increases, which indicates well crystallized.

Fig. 2 FESEM images of the samples synthesized at different NaOH concentrations: (a) 0.005M, (b) 0.01M, (c) 1M, (d) 3M, (e) 5 M, (f) 7 M.

When NaOH concentrations is 0.005 M, CaTi₂O₅ aggregated particles are obtained (Fig. 2(a)). When NaOH concentrations is 0.01 M, CaTi₂O₄(OH)₂ porous needlelike and floating irregular clouds-like particles are obtained (Fig. 2(b)). When the NaOH concentrations are 1 M, CaTi₂O₄(OH)₂ morpholgy is mainly composed of overlap leaves (Fig. 2(c)). When the NaOH concentrations are 3 M, nanosheets disspeared, and a lot of aggregated particles with a little trace of prisms(Fig. 2(d)). Whereas above 5 M, CaTiO₃ rectangular prisms were formed (Fig. 2 (e)-(f)).

On the basis of all the above observations, it is indicated that the presence of NaOH concentrtion in the solution is necessary for the formation of calcium titanium oxides. From the viewpoint of the chemical composition effect, the NaOH concentration may influence the combination of free Ca²⁺ and release Ti⁴⁺ ions of TiO₂ in the solvothermal process ^[4]. Due to forming a lower amounts of active OH^– ions and small amounts of TiO₂ soluble species in low NaOH concentration (0.005M, 0.01M), reactive Ca²⁺ and TiO₂ causes the reaction to be controlled by the transport of TiO₂ soluble species from hydroxide crystals to an interface bearing reactive Ca species. A shortage of TiO₂ soluble species near Ca²⁺ will halt the reaction to form CaTiO₃. Instead, owing to the small solubility of titanium dioxide in the acid conditions, CaTi₂O₅ forms. Increasing NaOH to 0.01M, the solvothermal processing accelerate the TiO₂ formation and promote TiO₂ to transform small amounts of Ti⁴⁺ ions ^[4-5], which are involved in a reaction with Ca²⁺，leading to the formation of CaTi₂O₄(OH)₂ crystallites. However, a large amount of Ti(OH)₄ was formed when NaOH concentration was increased to 1M, O-H group of TiO₆ octahedron free end decreases, the probability of Ca²⁺ into the lattice of the increase, the formation of tetragonal CaTiO₃ particles. With the increase of NaOH concentration to 5 M, i. e. in the high OH^– ion concentration, because of solvothermal synthesis of CaTiO₃ crystal defects and grain size effect, CaTiO₃ particles are formed. For the cubic phase CaTiO₃, (110) and (100) surfaces can exist at the same time, gamma (110) is slightly larger than the gamma (100) crystal^[3-4], which makes the tetragonal CaTiO₃ nanocrystals along (110) plane and (100) surface growth, so CaTiO₃ particles will give priority to the relatively low surface energy (100) surface growth, resulting in the formation of prism. Further increasing NaOH concentration, prism further Ostwald ripening, forming distinct edges and corners of CaTiO₃ prisms ^[3-4].

A detailed time study is obvious for the growth process of the CaTiO₃ prism in the case of 7 M NaOH. Unfortunately the experiments show the CaTiO₃ prism are quickly fromed due to the fast growth rate, which prevents the direct observations of its detailed growth process. Howerver, based on the morphology evolution (Fig. 2(a)-(f)) , the growth process of CaTiO₃ prism by the solvothermal process with 7 M NaOH is simply illustrated in Fig. 3.

Fig. 3 Schematic representation of the growing process of CaTiO₃ prism-like structures.

In previous researches, our work found that CaTi₂O₄(OH)₂ may have photocatalytic and electrochemical properties [5-6]. On view of the potential application, the structure of CaTi₂O₄(OH)₂ is what we want. Fig. 4 shows FT-IR spectra in the range of 400-4000 cm–1 of CaTi2O4(OH)2 sample. The peaks at 3425 cm^–1 can be attributed to the O-H stretching. Compared with free –OH ( 3600 cm^-1 )，the absorption peak shifts to low wavelength, which is due to the coordinated water molecules via hydrogen bonding interaction to the CO₃^2- of the interlamination ^[5]. The broad absorption band observed at 3200 cm^-1 is originated from the presence of hydroxyl groups of water ⁷. The absorption peak at 1538 cm^-1 attributes to H-O-H bending of the lattice water ⁸. The sharp absorption peak at 1357 cm^-1 attributes to C-O-C bending of carbonate ion. A band centered at 750 cm^-1, which is attributed to isolated tetrahedron TiO₄ stretching vibration. The absorption bands below 500 cm^-1, i. e. bands centered at 495 and 425 cm^-1 can be ascribed to Ca-Ti-O bending vibrations ⁸. From XRD and FT-IR results, the interlamination contains CO₃^2-, H₂O, isolated tetrahedron TiO₄ and –OH ions, Fig. 5 shows diagrammatic sketch of CaTi2O4(OH) 2 sample.

Fig. 4 FT-IR spectra in the range of 400-4000 cm–1 of CaTi2O4(OH)2 sample.

Fig. 5 Diagrammatic sketch of CaTi2O4(OH)2 sample.

Conclusion

In summary, we report here a simple solvothermal process for the formation of pure calcium tantium oxides without any surfactants. It is also found that the appropriate concentration of NaOH is vital for the formation of CaTiO₃ prism. A possible mechanism has been proposed to explain the formation of CaTiO₃ prism. And the CaTi2O4(OH) 2 interlaminar structure is illustrated.

Acknowledgement

The present work was supported by the National Natural Science Foundation (Grant No. 51262014 and 51172201).

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