Hydrothermal Processing of Hydrogen Titanate/Anatase-Titania Nanotubes and Their Application as Strong Dye-Adsorbents

Abstract

The nanotubes of pure hydrogen titanate and anatase-titania have been synthesized via hydrothermal treatment of as-received anatase-titania particles. The formation mechanism of anatase-titania nanotubes via hydrothermal has been discussed in detail in view of the finger-prints produced by characterizing the intermediate and end products using various microscopic and spectroscopic techniques such as scanning electron microscope, high-resolution transmission electron microscope, X-ray diffraction, Brunauer, Emmett, and Teller specific surface-area measurement, Fourier transform infrared spectroscope, diffuse reflectance, photoluminescence, thermal gravimetric and differential thermal analyses. The obtained results strongly support the rollup mechanism, involving multiple nanosheets, for the formation of anatase-titania nanotubes with the formation of different intermediate hydrothermal products having various morphologies such as sodium titanate having aggregated rectangular block-like structures, hydrogen sodium titanate and pure hydrogen titanate having highly aggregated unresolved fine-structures containing nanotubes, and finally, the pure anatase-TiO2 nanotubes. It is demonstrated that, during the hydrothermal treatment, the nanotubes of pure hydrogen titanate are formed first coinciding with the stable solution-pH during washing, indicating the completion of ion-exchange process, and a drastic increase in the specific surface-area of the hydrothermal product. The anatase-titania nanotubes are then derived from the pure hydrogen titanate nanotubes via thermal treatment. The use of pure hydrogen titanate and anatase-titania nanotubes for an organic textile dye-removal, from an aqueous solution under the dark condition, via surface-adsorption mechanism has been demonstrated. It is shown that, the specific surface-area and the surface-charge govern the maximum dye-absorption capacity of the anatase-TiO2 nanotubes under the dark condition.

Processing fly ash stabilized hydrogen titanate nano-sheets for industrial dye-removal application

“…This has been mainly observed during the hydrothermal treatment of free-standing anatase-TiO 2 powder [3,4]. Interestingly, the morphology of final hydrothermal product observed here is not nanotubes, but consists of highly aggregated micro-spheres, Fig.…”

“…For the latter method, various dyeadsorbents have been reported in the literature [2]. Recently, the nanotubes of hydrogen titanate (H 2 Ti n O 2n+1 ) and anatase-titania (TiO 2 ) processed via hydrothermal technique have received more attention due to their strong dye-adsorption property as a result of their higher specific surface-area and ability to exchange the cations in an aqueous solution under the dark-condition [3][4][5].…”

Hydrothermal synthesis of highly crystalline nanotubes/nanoplates of pure and silver-doped anatase-titania using acid-catalyst-modified sol–gel precursors

“…It is, thus, unambiguously confirmed that when the nanocrystalline anatase-TiO 2 processed via the CH 3 COOH-modified solgel method is used as a precursor during the hydrothermal processing, the final hydrothermal products (nanotubes/ nanoplates) also have the highly crystalline anatasestructure irrespective of the amount of Ag-doping. Since several investigations have already reported the formation of nanotubes of hydrogen titanate when the anatase-TiO 2 is used as a precursor [31][32][33][34]47], the formation of nanotubes/nanoplates of anatase-TiO 2 is striking and attributed here to the effect of precursor-modification due to its processing via the CH 3 COOH-modified sol-gel method.…”

“…However, as described below during the hydrothermal processing, since the mechanism of formation of nanotubes/nanoplates involves an exfoliation of Na 2 Ti 3 O 7 nanosheets, the entrapped-CH 3 COO -anions are likely to be exposed on the surface of these nanosheets which then play a major role during the first washing-cycle resulting in the operation of as-proposed double ionexchange mechanism. The mechanism of formation of hydrogen titanate nanotubes/nanoplates via the hydrothermal technique using an unmodified anatase-TiO 2 precursor has been described in detail elsewhere by us [31,45] and others [43,44] which is also shown here in the Scheme 1a. In short, the hydrothermal treatment of anatase-TiO 2 under the autoclave conditions, in a highly alkaline solution, results in the formation of blocks of sodium titanate (Na 2 Ti 3 O 7 ) phase which has a layered-structure.…”

“…However, there are no reports in the literature which demonstrate the processing of pure and doped highly crystalline anatase-TiO 2 nanotubes/nanoplates via the hydrothermal method using the acid-catalyst modified sol-gel precursors which can eliminate the thermal treatment. In our previous investigations [31][32][33][34], the pure nanocrystalline anatase-TiO 2 was used as a precursor which invariably transforms to the nanotubes of hydrogen titanate when subjected to the hydrothermal and subsequent washing-treatments. In view of this, the first major objective of the present investigation is to introduce a systematic procedure for demonstrating the effect of prior precursor-modification using an acid-catalyst on the development of highly crystalline pure and Ag-doped nanotubes/nanoplates of anatase-TiO 2 (instead of those of hydrogen titanante) via the hydrothermal method without the involvement of thermal treatment; while, the second major objective of the present investigation is to propose a possible mechanism for obtaining this new phase under the given processing conditions (that is, with the use of an acid-catalyst-modified sol-gel precursor and without the involvement of thermal treatment).…”