Toward tailored functionality of titania nanotube arrays: Interpretation of the magnetic-structural correlations

Abstract

Ordered arrays of titania nanotubes (NTs) are considered as good candidates for photocatalytic applications including water splitting. Considering that the functionality of these nanostructures is influenced by their morphology, electronic and the crystallographic structure, fundamental understanding of these properties and their possible correlations can clarify the approaches toward enhanced photocatalytic efficiency. In this work, ordered arrays of titania NTs are synthesized electrochemically and are subjected to isochronal annealing treatments in various atmospheres (oxygen-rich, oxygen-deficient and reducing) to modify their morphology, crystal and electronic structure. Upon characterization of these NTs, direct correlations are found between the annealing atmosphere and the corresponding unit cell volume and the crystallite size. Furthermore, correlations between the NTs' structure and magnetic response are observed, revealing changes in the electronic structure such as the density of states, that are in turn relevant to the functional catalytic properties of titania.

TiO2 nanotube arrays for photocatalysis: Effects of crystallinity, local order, and electronic structure

“…Cross-sectional and pore-side SEM images of the as-anodized and annealed TiO 2 nanotube arrays are shown in Figure 1 XRD confirmed that the short and long TiO 2 nanotubes are amorphous in their asanodized states (figure not shown), consistent with previous reports 19,20,22,34 . The anatase structure develops upon annealing at 350 °C as evident by the XRD patterns in Figure 2.…”

“…In addition, the larger value of the e g peak separation, d, in the nanotubes annealed in reducing (H 2 ) and inert (Ar) gas atmospheres indicates a higher concentration of surface defects (Ti 3+ species or O vacancies) as compared to those annealed in the oxidizing environment (O 2 ). Reduction of Ti 4+ to lower oxidation states such as Ti 3+ is concurrent with the loss of lattice oxygen, as reported elsewhere for TiO 2 annealed in oxygen-deficient or reducing atmospheres, although for higher annealing temperatures such as 400 ºC to 650 ºC 1,22,29,30,46 . During such thermal treatment, two Ti 4+ ions transform into two Ti 3+ ions for charge compensation by receiving electrons freed from the removal of one lattice oxygen atom in stoichiometric TiO 2 lattice to create a doubly-charged oxygen vacancy, ̈.…”

“…tube length and diameter), electronic structure (such as band gap or local bonding environment) and their crystallinity, all of which may be modified via control of the synthesis parameters (e.g. electrolyte components and the applied voltage) [13][14][15][16] and post-synthesis annealing conditions (such as nature of the annealing gas, annealing rate and temperature range) 12,[17][18][19][20][21][22][23] . The enhanced photocatalytic capacity of TiO 2 nanotube arrays as compared to that of thin films and nanoparticles is attributed to the nanotubes' geometrically-ordered porous architecture, larger surface area and longer charge carrier lifetime 1, 2, 24-26 .…”

“…A higher concentration of local defects near the nanotube surface generated by annealing in a reducing gas can facilitate charge separation and may enhance the production of hydroxyl radicals. These hydroxyl radicals can contribute to catalytic activity of TiO 2 nanotubes by serving as reactive sites for degradation of organic compounds25,55,56 .Overall, the better catalytic performance of the H 2 -or Ar-annealed long nanotubes relative to the O 2 -annealed long nanotubes in the degradation of MO is attributed to the higher concentration of local surface defects (Ti 3+ or oxygen vacancies) that developed upon annealing at temperatures as low as 350 ºC, and the anticipated decrease in the electronic band gap, along with the presence of the anatase structure22,46,52,57 .…”

Effect of Annealing Atmosphere Induced Crystallite Size Changes on the Electrochemical Properties of TiO2 Nanotubes Arrays

“…The smallest crystallite size was observed for the H 2 -annealed TNAs sample. Hosseinpour et al [44,45] studied the annealing effect in different atmosphere and depicted that as lattice parameters altered the crystallite size also follow trend which is according to our observations. The specific capacity of TNAs annealed in different atmosphere was co-related with crystallite size (Fig.…”