Structural characterization and effect of dehydration on the Ni-doped titanate nanotubes
Introduction
One dimensional titania have been extensively studied for photocatalysis, dye-sensitized solar cell, lithium ion batteries, hydrogen storage, and electrochemical capacitors [1], [2], [3], [4], [5] due to its large specific surface area, numerous surface defects and physicochemical potentials [6], [7], [8], [9]. Specially, there have been reports to apply titanate nanotubes for hydrogen storage, however, absorbing reaction occurred only at high temperature and/or low temperature (at −196 °C and over 120 °C) [10]. Bavykin et al. reported that the hydrogen could be intercalate between layers in the walls of TiO2 nanotubes forming host–guest compounds TiO2·xH2, where x ≤ 1.5 and decreases at higher temperatures [11]. Also, the TiO2 nanotubes by Lim et al. showed that about 2 wt% hydrogen could be reproducibly stored at room temperature and 6 MPa [10]. However, in spite of these efforts, much work on the characterizations of structural and hydrogen absorption of titanate nanotubes are required. Also, systematic study on dehydration of nanotubes and interlayer spacing has yet to be done. Most researchers reported that the crystal structures of the titanate nanotubes have A2Ti3O7, H2Ti4O9·H2O (A = Na and/or H), and lepidocrocite titanates with monoclinic crystal structure [12], [13], [14]. However, the corresponding of XRD patterns was not fully coincided with the crystal structure from JCPDS (Joint Committee on Power Diffraction Standards).
In our previous work, Ni-doped titanate nanotubes were synthesized by hydrothermal method using Ni-doped TiO2 rutile powders and demonstrated exact crystal structure of the nanotubes (Ni-doped H2Ti2O5·H2O) [15]. However, nanotubes had lots of hydrate in the crystal and on the surface which acted as a brake on hydrogen absorbing. In this work, Ni-doped titanate nanotubes were synthesized by hydrothermal method and simple firing and effect of dehydration on the Ni-doped titanate nanotubes for hydrogen storage was studied. And the crystal structures of synthesized nanotubes have been discussed. Also, the hydrogen absorption of nanotube was investigated by the conventional volumetric pressure–composition (P–C) isothermal method using an automated Sivert's type apparatus.
Section snippets
Experimental
The Ni-doped powders (0.8 g) and 15 mL of a 10 M NaOH aqueous solution were mixed with stirring for 1 h and placed in a Ni-lined stainless-steel autoclave at 120 °C for 24 h, and then cooled at room temperature. Next, 0.1 M HCl aqueous solution was added and washed repeatedly with distilled water until pH 7. Final powders were collected by the centrifugal separator (Oak redge tube) which operated at 15,000 rpm for 30 min. After then, the synthesized powders are heated at 450 and 600 °C for 2 h in to check
Results and discussion
Fig. 1 shows the XRD patterns of the Ni-doped titanate nanotubes (TNT) and fired Ni-doped titanate nanotubes with the reported crystal structure from JCPDS [9], [17], [18]. The Ni-doped TNT and fired TNT at 400 °C powders revealed characteristic peaks at around 2θ = 10°, 24°, and 28° which could be assigned to the diffraction of H2Ti2O5·H2O with peak broadening that was the result of nanometer size and bending of some atom planes of the tubes [11]. Also, Ni element peaks were not detected in the
Conclusion
Ni-doped titanate nanotube and Ni-doped nanotubes were synthesized by hydrothermal method and simple firing using rutile powders as starting materials. The hydrogen absorption of the nanotubes was investigated by the conventional volumetric pressure–composition (P–C) isothermal method using an automated Sivert's type apparatus.
The nanotubes compose of H2Ti2O5·H2O with outer and inner diameter of ∼10 and 6 nm, and the interlayer spacing is about 0.65–0.74 nm which were totally different from the
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