There has been much interest lately in the preparation of TiO2 in 2D morphologies due to its potential for improved properties for photocatalytic applications. Materials with layered (or lamellar) structures can be more easily prepared with platelet or sheet-like morphologies due to the natural bonding anisotropy in the intra- and interlayer directions, but anatase does not exhibit such a layered structure. Platelet-type nanocrystals of anatase have been successfully prepared using hydrothermal methods, but fluoride anions, typically from toxic reagents such as hydrofluoric acid, must be used as a structure-directing agent. Nanosheet-type structures with larger lateral dimensions have also been demonstrated by converting nanosheets of layered, lepidocrocite-type titanate compounds to anatase using hydrothermal or calcination treatments. However, this is a multi-step process that involves ion-exchange and intercalation of bulky ammonium cations, which may become strongly adsorbed on the nanosheets surfaces and interfere with their photocatalytic properites.
Here we report the preparation of TiO2 nanosheets by chemical conversion of TiS2 nanosheets. This approach is attractive, as the preparation of 2D nanosheets using chemical and electrochemical reduction methods has been demonstrated for a large number of layered, transition metal dichalcogenide materials and hence provides the opportunity for a large number of layered metal oxides through conversion of sulfide nanosheet starting materials. Additionally, using TiS2 as the precursor can enable sulfur-doped TiO2, which has a smaller bandgap than pure anatase and enhanced photocatalytic properties.
The synthesis of TiO2 nanosheets was achieved as follows: (1) The first step involves the reduction of bulk TiS2 material through electrochemical lithiation to form LixTiS2. The lithiation weakens the van der Waals interaction between the TiS2 interlayers and enables exfoliation via ultrasonication. The exfoliated TiS2 nanosheets are uniformly dispersed in water. (2) The as-obtained TiS2 nanosheet dispersion is oxidized using a hydrothermal treatment to obtain TiO2 nanosheets. Detailed structural characterization using atomic force microscopy, electron microscopy, and X-ray photoelectron spectroscopy was performed to understand the conversion process. It was found that the synthesized materials adopted the anatase structure and were polycrystalline, but the nanosheet morphology of the original TiS2 nanosheets was maintained. Photocatalytic tests using methylene blue as a model organic compound showed that the TiO2 nanosheets displayed good activity. The results show that this approach may be a potentially general method to obtain metal oxide nanosheets by conversion from the corresponding layered sulfide nanosheets.