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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
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 .
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.
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 . 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+ï¼Œleading 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 )ï¼Œthe absorption peak shifts to low wavelength, which is due to the coordinated water molecules via hydrogen bonding interaction to the CO32- of the interlamination . 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 CO32-, 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.
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.
The present work was supported by the National Natural Science Foundation (Grant No. 51262014 and 51172201).
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