Conversion of TiO2 Nanotubes into BaTiO3
Electron Microscopy of Rod-Like BaTiO3 Nanostructures for Energy Harvesting
- Conversion of TiO2 Nanotubes into BaTiO3 - Electron Microscopy of Rod-Like BaTiO3 Nanostructures for Energy Harvesting
- Fig. 1: FESEM images of (a) TiO2 NT array before the hydrothermal treatment and after hydrothermal treatment for (b) 2 h at 150°C and (c) 24 h at 200°C.
- Fig. 2: Bright field TEM images of (a) TiO2 NT array before the hydrothermal treatment and after hydrothermal treatment for (b) 2 h at 150°C and (c) 24 h at 200°C.
- Fig. 3: Raman spectra of amorphous TiO2 and BaTiO3 nanostructures.
Due to their excellent electro-mechanical energy conversion capability also at the nanoscale, BaTiO3 nanostructure arrays are gaining a great interest for energy harvesting applications. The systematic study on the hydrothermal conversion of TiO2 nanotubes obtained by anodic oxidation allowed the fabrication of rod-like nanostructured BaTiO3 arrays with promising ferroelectric properties. Their compositional homogeneity, crystalline phase and size, of importance for their application, are discussed.
Piezoelectric nanostructures have attracted tremendous attention in the field of nanotechnology for energy applications since they possess excellent electromechanical energy conversion capability, which remains constant at reduced scale. This scalability can result in the ability of the piezoelectric nanostructures to be used in a miniaturized alternative power source due to their potential for energy production and storage. Due to environmental concerns over the toxicity of lead based materials, lead-free ferroelectrics, including BaTiO3, have gained importance and scientific interest.
The behavior of nanosized ferroelectric oxides depends on the compositional homogeneity, crystalline phase and size of the final nanostructures. Thus, there is a need for a well-controlled synthesis route for nanostructured piezoelectric materials. Several synthesis methods, as hydrothermal synthesis, sol-gel deposition in templating structures and electrospinning  have been explored to prepare such nanostructured materials.
A promising and interesting approach to obtain 1D nanostructures is the hydrothermal conversion of 1D nanostructured precursors. The main advantage of this technique is that BaTiO3 is directly synthesized in its tetragonal ferroelectric phase , without the need of any further thermal treatment.In this work we report on the different degree of the conversion of TiO2 into BaTiO3. The study is based on some selected samples prepared within a larger systematic study on the hydrothermal conversion of TiO2 nanotubes obtained by anodic oxidation , which allowed the fabrication of rod-like nanostructured BaTiO3 arrays with promising properties.
Materials and Methods
Three samples have been selected to be highlighted as representative and important for understanding the degree of the conversion process.
The first is an amorphous TiO2 NT array fabricated by anodic oxidation, which is the starting material for further conversion. The second and third are BaTiO3 tube-like arrays fabricated by hydrothermal conversion of TiO2 NT arrays, using 0.1M barium acetate and 0.25 or 1M potassium hydroxide as described in details elsewhere . The samples were hydrothermally treated for 2 hours at 150°C (BaTiO3_A) and 24 hours at 200°C (BaTiO3_B).
The morphological and the compositional characterization of nanostructured materials were performed by Field Emission Scanning Electron Microscopy (FESEM, Zeiss Dual Beam Auriga) and Transmission Electron Microscopy (TEM, FEI Tecnai F20ST) operating at 200 kV. Electron transparent TEM specimens in cross-section were prepared from the parent sample using Focused Ion Beam (FIB, Zeiss Dual Beam Auriga) operated at 30kV. A final cleaning step using a FIB voltage of 2kV was also performed. Raman spectra of the nanostructured BaTiO3 were acquired using a Renishaw Invia micro-Raman spectrometer, with a laser excitation wavelength of 514 nm and a laser spot size of 20 µm.
In figure 1 the FESEM micrographs of the investigated samples are reported. The as grown TiO2 NTs (figure 1a) are highly ordered and vertically aligned, exhibiting a very smooth wall and open top . The external diameter is in the range 100-130 nm, while the inner diameter is around 70 nm. The sample converted for 2h at 150°C (sample BaTiO3_A, figure 1b) preserved the NTs original shape; however a transformation of the smooth walls to the nanocrystalline form and a partial occlusion of the inner channel is observed. This conversion to the nanocrystalline form is mainly observed in the top part of the sample, where the contact with the basic solution is higher. In fact, during the conversion, the NT walls undergo a dissolution-recrystallization process that strongly affects the starting morphology . BaTiO3_B (fig. 1c) shows a more complete conversion, reaching the bottom of the structure, which is evidenced by the presence of the nanocrystalline structure also down there. The proof of this complete conversion is much clearer when looking at the head picture, where in the left the amorphous tubes (TiO2) are shown, while in the right a polycrystalline rod-like structure (BaTiO3_B) is visible.
Bright Field TEM was used to investigate in detail the conversion process. Images of the top part of the investigated samples are shown in figure 2, bottom parts are shown in the insets. The as-fabricated amorphous TiO2 (fig. 2a) doesn't show any crystalline contrast and smooth walls are observed, which confirms the FESEM observation. The bottom of the tubes is visible in the inset, showing the same characteristics of the top. This part of the sample is particularly significant as for its application in energy harvesting it is important that all the nanostructures are converted along the overall thickness.
The BaTiO3_A sample (fig. 2b) shows nanometric BaTiO3 crystals with a broad range of sizes within the first micron of the material, while the rest of the sample still exhibits the nanotubular shape, evidencing that conversion conditions haven't been effective enough to complete the reaction on the entire sample. These crystals have been partially clogging the TiO2 NT structure just beneath. Thus, it is more difficult for the fresh solution to come in contact with the inner tube walls during the hydrothermal treatment . Beneath the first converted layer, the walls of the tubes are well defined and some crystalline contrast is observed, indicating that the conversion process has been initiated also in the rest of the sample, but to lesser degree than in the top. Finally, the inset in figure 2b shows the bottom of the sample, showing that the conversion is not completed because there is no evidence of crystalline material.
Longer reaction time and higher temperature proved to be important for a successful reaction. The TEM micrograph of BaTiO3_B (fig. 2c) confirms the complete conversion attested by the presence of the crystals with a more homogeneous shape (spherical) and diameter of 70 nm approximately. In addition, the vertical orientation and the rod-like shape microstructure, is still preserved. In this case the bottom of the sample exhibits crystalline contrast (inset in figure) confirming the complete conversion of the TiO2 layer into BaTiO3.
Raman analysis has been performed to investigate the crystallographic phase of the obtained BaTiO3. The spectrum of amorphous TiO2 shows broad bands at 200, 450 and 610 cm-1 , as expected . Cubic phase of the BaTiO3 is not Raman active, however generally broad bands at 250 and 520 cm-1 related to local disorder are present . The tetragonal phase is characterized by 250, 306, 520 and 720 cm-1 bands . In the spectra of both BaTiO3 samples, vibrational bands of BaTiO3 at 250, 520 and 720 cm-1 can be recognized. However, only the BaTiO3_B spectrum shows a well defined sharp peak at 306 cm-1, which is a "fingerprint" of the tetragonal phase . Its lack in the BaTiO3_ A spectrum suggests that this sample is composed by a mixture of cubic and tetragonal phases .
In summary, we have employed FESEM, Bright Field TEM and Raman measurements to determine the degree of the conversion of TiO2 into BaTiO3 under different conversion conditions. The morphology and crystallographic structure of the material have been analyzed. There is an evidence of the tetragonal phase of the final nanorod-like structure with preservation of the vertical alignment.
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Dr. Katarzyna Bejtka
Dr. Angelica Monica Chiodoni
Dr. José Alejandro Munoz Tabares
Dr. Nadia Garino
Dr. Marzia Quaglio
Prof. Candido F. Pirri, Ph.D.
Istituto Italiano di Tecnologia IIT@POLITO
Center for Space Human Robotics
Dr. Andrea Lamberti
DISAT, Politecnico di Torino, Italy