TiO₂ Nanotubes Synthesis with Dominant {001} Exposed Facets for Efficient Photocatalytic and Photoelectrochemical Water Oxidation Applications

Abstract

The photoreactivity of TiO2 nanotube arrays (TiNT) is mainly related to the light absorption and photogenerated charge carriers separation efficiencies. Recently, engineering of the crystal facets has gained increasing attention because of its exceptional ability to extend the photoresponse toward visible light and to separate photogenerated charges. Here, TiNT with high crystallinity and percentage of exposed {001} facets was synthesized using a two-step anodization process in ethylene glycol based electrolyte containing 2 wt% of water and 0.27 M NH4F. Decreasing the potential of the second anodization (from 50 to 20 V) resulted in an increase in crystallinity, crystallite size, and percentage of exposed {001} facets. Because trap states limit the photoreactivity performance of TiNT by facilitating the recombination of photogenerated electron/hole pairs, correlation between crystallinity, preferential crystalline orientation, gap states characteristics (density and location), and photoreactivity performance was investigated. TiNT with a high percentage of {001} exposed facets, consisting mainly of shallow gap states with low density, significantly improve photoelectrochemical water oxidation and photocatalytic efficiencies.

Promising Supercapacitive and Photocatalytic Performances of TiO2 Nanotubes Loaded with Graphene: Insight on the Quantitative Chatacterisation by EIS

“…The origin of the different emission peaks can be found in previous work. 40 The introduction of graphene in TiNT results in a decrease of the PL intensity, demonstrating a significant reduction of the defects density and charge carrier recombination rate and so an enhanced charge separation of electron hole pairs when compared to TiNT. A lower PL emission intensity was depicted for PGr/TiNT, which may be attributed to its lower defects density and so lower photogenerated electron-hole recombination.…”

Recent Advances in TiO2‐based Photoanodes for Photoelectrochemical Water Splitting

“…Indeed, TiO 2 NTs do not show significant enhancement in the carrier transport properties probably due to the random orientation of the grains along different planes in the polycrystalline tubes and grain boundary effects [84,85] . Thus, it is necessary to further improve the ordering and crystallinity of TiO 2 nanotubes, which is commonly realized by a multi‐step approach [86–89] . For example, John et al.…”

“…[84,85] Thus, it is necessary to further improve the ordering and crystallinity of TiO 2 nanotubes, which is commonly realized by a multi-step approach. [86][87][88][89] For example, John et al synthesized single crystal like TiO 2 NTs with preferential orientation along the [001] direction with two-step technique. They introduced zinc doping into amorphous TiO 2 NTs to minimize the (001) surface energy, resulting in preferential orientation of grains along the [001] direction (Figure 7a-c).…”