Theoretical and Experimental Study of Anatase Nanotube Formation via Sodium Titanate Intermediates

Abstract

The initial stages of the formation of anatase nanotubes starting from TiO2 microparticles are studied theoretically at density-functional theory (DFT) level. Several formation mechanisms proposed in the literature are discussed. In the present study a mechanism is adapted that starts with NaOH adsorption on the anatase (101) surface. A phase transition from NaOH:anatase to sodium titanate is thermodynamically favorable but does not lead to the formation of sodium titanate nanotubes. Instead it is shown that anatase nanotubes with NaOH adsorbed on the inner surface are stabilized with respect to the unmodified 2D-periodic anatase surface structure. The structure and stability of selected intermediates of the nanotube formation process are investigated. In the experimental part we investigated the initial step of the nanotube formation and characterized the crystal structure of the as prepared titanate nanotubes.

Rotation-Assisted Hydrothermal Synthesis of Thermally Stable Multiwalled Titanate Nanotubes and Their Application to Selective Catalytic Reduction of NO with NH3

“…The obtained nanotubes had a diameter of 8 nm and a length of 100 nm, and the reproducibility by synthesis was also confirmed by several experiments. Many results have been reported on the synthetic conditions of TNT and the resulting structural characteristics following this paper. , On the basis of several characterizations, many researchers have agreed that the crystal structure of TNT is similar to that of layered trititanic acid (H 2 Ti 3 O 7 ). − Layered titanate nanosheets are exfoliated from anatase TiO 2 particles during hydrothermal treatment, and the scrolling of the sheets can form nanotubes . In general, the different values of free surface energy on each side of a nanosheet are the driving force of curving nanosheets into nanotubes .…”

Hydrothermal Transformation of Titanate Scrolled Nanosheets to Anatase over a Wide pH Range and Contribution of Triethanolamine and Oleic Acid to Control the Morphology

“…of Ti five-fold coordinated (Ti5c) is much more reactive than the thermodynamically more stable (101) facet presenting 50% of Ti5c and 50% of Ti six-fold coordinated Ti6c. [17][18][19][20][21][22] On the other hand, when NPs are expected to be very reactive, one cannot ignore their impact and their potential toxicity. [23][24][25][26][27][28][29][30][31][32] The latter depends not only on the chemical nature of the nanomaterial, but also on its size, morphology and surface chemistry.…”

“…This simple and costeffective hydrothermal method provides a large scale quantity of pure titania-based nanotubes with small diameter (9-10 nm) and lengths as high as several hundred nms and has been the subject of many efforts. 33,37,54 The different studies on the reactivity of these SNSs in various media 19,33,[55][56][57][58][59][60][61][62][63][64] show they can be effectively used as a cost-effective starting material as compared to titanium alkoxides for the controlled synthesis of targeted morphologies of TiO2.…”

“…The photoactivity of TiO 2 , is influenced not only by different properties such as the surface area, the crystallinity, the nanoparticle (NP) size, or the crystal structure , but also by the specific morphology of the particles, which determines which crystal facets are exposed, and then has a strong influence on the photocatalytic performance of the particles . On the one hand, both theoretical and experimental studies have shown that the (001) surface of the TiO 2 anatase truncated bipyramid that exhibits 100% of Ti fivefold coordinated (Ti5c) is much more reactive than the thermodynamically more stable (101) facet presenting 50% of Ti5c and 50% of Ti sixfold coordinated Ti6c. − On the other hand, when NPs are expected to be very reactive, one cannot ignore their impact and their potential toxicity. − The latter depends not only on the chemical nature of the nanomaterial but also on its size, morphology, and surface chemistry. ,,,− Therefore, for toxicology and other purposes, the synthesis of well-crystallized and nanostructured TiO 2 particles with tailored morphology is still the object of intense research and represents a current major challenge using either no capping agent or inorganic (F – ) or organic capping agents (amines, long chain organic acids). ,,− These agents that specifically adsorb on different facets can play a critical role in assisting the anisotropic growth of specific facets so that the NP develops in targeted directions and controlled morphology. Besides the numerous molecular titanium precursors , (alkoxides, halides, titanium bis­(ammoniumlactato) hydroxide, sulfate and oxysulfate, atranes, carboxylates) to grow TiO 2 NPs, the use of the so-called titania-based nanotubes was successfully tested by Nian et al in 2006 .…”

“…The synthesis of these titania-based nanotubes (TNTs), also referred to as scrolled nanosheets (SNSs), is based on the hydrothermal treatment of TiO 2 powder, at high temperatures in a highly concentrated NaOH solution without sacrificial templates and was first reported by Kasuga et al , in 1998. This simple and cost-effective hydrothermal method provides a large-scale quantity of pure titania-based nanotubes with small diameter (9–10 nm) and lengths as high as several hundred nanometers and has been the subject of many efforts. ,, The different studies on the reactivity of these SNSs in various media ,,− show they can be effectively used as a cost-effective starting material as compared to titanium alkoxides for the controlled synthesis of targeted morphologies of TiO 2 . We propose in this paper to use these SNSs in hydrothermal conditions under mild conditions (140 °C, 3 d) and scan the influence of parameters such the pH and the nature of the capping agents suggested by Sugimoto to direct NPs growth starting from molecular precursors .…”