Synthesis of TiO 2 nanosheet photocatalysts from exfoliation of TiS 2 and hydrothermal treatment
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TiO2 nanomaterials with platelet or nanosheet morphologies can offer improved properties for photocatalytic applications, but established methods to produce them typically require structuredirecting agents since anatase-phase TiO2 does not have a layered structure. In the present work, the preparation of TiO2 nanosheets by the chemical oxidation of TiS2 nanosheets is demonstrated. Electrochemical exfoliation of bulk TiS2 into TiS2 nanosheets, followed by the hydrothermal treatment at 180 °C for 14 h is performed. The results show that polycrystalline TiO2 nanosheets with the anatase structure are formed, and that the nanosheet morphology can still be maintained after the hydrothermal treatment. The TiO2 nanosheets show good photocatalytic activity for the degradation of methylene blue, but the performance is negatively affected by the residual carbon black that was needed in the TiS2 electrode to enable electrochemical exfoliation. These results show that conversion of TiS2 nanosheets to TiO2 nanosheets is a promising synthetic strategy but highlights how the interfacial properties of the obtained materials could be affected by ancillary components in the preparation method.
I. INTRODUCTION
There has been much interest lately in synthesizing two-dimensional (2D) materials for potential applications in optoelectronics, energy storage, and catalysis/ photocatalysis applications.1–4 Among the semiconducting metal oxides, TiO2 is the most widely used photocatalyst because of its favorable physicochemical properties, including its high chemical stability, nontoxicity, and low cost.5,6 There have been many recent efforts to prepare anatase TiO2 in 2D morphologies, as these nanostructures can offer many attractive properties such as high specific surface area, specific exposed facets, and a large fraction of unsaturated surface atoms.7,8 Materials with layered or lamellar bulk structures can be more easily formed into platelet or sheet-like morphologies due to the natural bonding anisotropy in the intra and interlayer directions, but the anatase phase of TiO2 does not exhibit such a layered structure. Platelettype 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 structure-directing agents.9,10 Nanosheet-type structures with larger lateral dimensions have also been demonstrated by converting nanosheets of layered, lepidocrocite-type titanate compounds to anatase a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.165
using hydrothermal or calcination treatments.11–14 While the conversion of these titanates to the anatase structure has been found to be topochemical in nature, the layered titanate nanosheets must be obtained by exfoliation using osmotic swelling, which is a multistep process involving ion-exchange and intercalation of bulky ammonium cations.15 Our previous work exploring exfoliationbased methods for obtaining energy storage materials in 2D
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