An in situ and ex situ TEM study into the oxidation of titanium (IV) sulphide

Edmund Long, Sean O'Brien, Edward A. Lewis, Clive Downing, Clotilde S. Cucinotta, Stefano Sanvito, Sarah J. Haigh, Valeria Nicolosi, 2017

Image courtesy of npj 2D Materials and Applications

Abstract

Titanium (IV) sulphide (TiS2) is a layered transition metal dichalcogenide, which we exfoliate using liquid phase exfoliation. TiS2 is a candidate for being part of a range of future technologies. These applications are varied, and include supercapacitor and battery energy storage devices, catalytic substrates and the splitting of water. The driving force behind our interest was as a material for energy storage devices. Here we investigate a potential failure mechanism for such devices, namely oxidation and subsequent loss of sulphur. This degradation is important to understand, since these applications are highly property-dependent, and changes to the chemistry will result in changes in desired properties. Two approaches to study oxidisation were taken: ex situ oxidation by water and oxygen at room temperature and in situ oxidation by a 5% O2/Ar gas at elevated temperatures. Both sources of oxygen resulted in oxidation of the starting TiS2 flakes, with differing morphologies. Water produced amorphous oxide slowly growing in from the edge of the flakes. Oxygen gas at ≥375 °C produced crystalline oxide, with a range of structures due to oxidation initiating from various regions of the observed flakes.

Impact Statement

The degradation dynamics of TiS2 reveal that the flakes oxidise in water and atmosphere, pointing towards moisture as the key driving force. A team led by Valeria Nicolosi at Trinity College Dublin used advanced electron microscopy techniques to investigate the influence of different environments on the deterioration pathways of TiS2, a promising candidate for future energy storage applications. By comparing the effect of ex-situ oxidation by water and oxygen at room temperature, and in-situ oxidation at high temperatures, water was proven to effectively oxidise the TiS2 flakes from the edges thereby forming an amorphous oxide phase. Conversely, the degradation was found to proceed more slowly in atmosphere or vacuum conditions. These results suggest that TiS2 oxidation could be avoided in a desiccated environment that would prevent water molecules from dissociating in reactive ionic species.