Synthesis of anatase TiO2 nanotubes derived from a natural leucoxene mineral by the hydrothermal method
Introduction
TiO2 materials have been used for various applications, such as semiconductors, dye sensitized solar cells, water treatment, catalysis, and gas sensors [1], [2], [3], [4], [5], [6], [7], [8], [9], due to their exceptional properties, including chemical stability [10], biocompatibility [11], [12], high photocatalytic activity [10], [13], and cost effectiveness. TiO2 is known to exist in three natural polymorphs, rutile, anatase, and brookite, each exhibiting different properties. TiO2 can be synthesized into various nanostructures, such as nanoparticles, nanorods, nanowires, nanotubes, and mesoporous/nanoporous materials, by different preparation methods, including sol–gel, hydrothermal, solvothermal, and vapor deposition [14]. Each method has advantages and disadvantages depending on the application [15], [16], [17], [18], [19]. Nanotubes are of great interest due to their high surface-to-volume ratios and size-dependent properties [20]. The discovery of carbon nanotubes [21], with their variety of interesting properties, e.g., single-crystal characteristics, high surface-to-volume ratio, and good electron transport, has stimulated interest in the synthesis of nanotubular structures composed of other substances and chemical compounds.
TiO2 nanotubes can be synthesized by several preparation methods, such as electrochemical anodic oxidation, the template-assisted method, and the hydrothermal method. The hydrothermal method is regarded as a convenient, inexpensive, and environmentally innocuous method for producing high-quality TiO2 nanotubes [22]. In general, TiO2 nanotubes possess vast pore structures and high aspect ratios because of their unique tubular structure [22]. Kasuga et al. were the first researchers to report the production of TiO2 nanotubes through the hydrothermal method. Leucoxene is a natural source of high-titanium-content TiO2 (usually approximately 90%). The synthesis of TiO2 nanotubes from this natural mineral instead of a commercial TiO2 powder [23], [24], [25] by the hydrothermal method was investigated. The cost of natural leucoxene mineral used in this study is <US$1/kg, which is less expensive than commercial-grade TiO2 powder [26]. The hydrothermal method allows for TiO2 nanotubes to be directly synthesized without using a commercial TiO2 powder as the starting material. Generally, a commercial TiO2 powder can be prepared from a mineral by the chloride process. This process can reduce the cost of the synthesis of TiO2 nanotubes. In our previous study, nanofibers were prepared by a simple hydrothermal method using leucoxene as the starting material [27].
In this study, anatase TiO2 nanotubes were obtained by the hydrothermal method and subsequent heat treatment from natural leucoxene. The method afforded an increase in the BET specific surface area of the TiO2 nanotubes, which may improve the nanotubes photocatalytic activity.
Section snippets
Preparation of TiO2 powders from natural leucoxene
In this work, natural leucoxene was obtained from Sakorn Minerals, a mining company based in Prachuapkhirikhan Province, Thailand. The mineral was used as a starting material for the synthesis of TiO2 nanotubes because of its low cost and high TiO2 content. The chemical composition of natural leucoxene, as analyzed by the X-ray fluorescence technique (XRF), consists mainly of TiO2 (91.3%), Fe2O3 (4.01%), SiO2 (0.51%), and ZrO2 (1.49%). The mineral was ground into a powder by the ball-milling
As-synthesized nanotubes
The chemical compositions of the leucoxene mineral and the as-synthesized samples were analyzed by X-ray fluorescence, the results of which are shown in Table 1. After the hydrothermal process, the quantities of impurities such as Fe2O3, SiO2, and ZrO2 decreased as the TiO2 content increased from 91.3 to 92.8 wt%. This result may be due to the higher solubility of the impurities in the NaOH aqueous solution during the preparation process [33], [34]. The doping of transition metal ions such as Fe
Conclusion
In summary, anatase TiO2 nanotubes were synthesized by the hydrothermal method using natural leucoxene as the starting material. The BET surface area of the titanate nanotubes was ~103.80 m2/g. The photocatalytic activity of the anatase TiO2 sample was higher than the activities of titanate nanotubes, a commercial anatase TiO2 sample (JRC-01), and rutile TiO2.
Acknowledgments
This work was supported by the Office of the National Research Council of Thailand (NRCT), Sakorn Minerals Co., Ltd. (supplied the leucoxene mineral), King Mongkut׳s University of Technology Thonburi (KMUTT), and the Nanotechnology for Textile and Polymer Research Group (NanoTeP) of the Faculty of Engineering, Rajamangala University of Technology Thanyaburi (RMUTT) Thailand.
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