Introdution

Gallium (II) sulphide (GaS) is a III-VI layered semiconductor, which has recently been exfoliated using liquid-phase exfoliation. As a wide-gap semiconductor it has potential in a range of applications including photodetectors, non-linear optics, optoelectronics, Li ion battery anodes, but also as a catalyst for hydrogen evolution. However, early devices fabricated using GaS stopped functioning after a few weeks. Upon close inspection it was noted that the edges of exfoliated flakes were seen to have oxidised very shortly after exfoliation, and become amorphous, but the cause of device failure was unclear. 

Methods

A combination of ab initio Density Functional Theory (DFT) calculations and High Resolution Scanning Transmission Electron Microscopy (HRSTEM) has been used to study the initiation and progression of oxidation from two potential species: O₂ and H₂O – the latter of particular importance due to potential effects on hydrogen evolution. DFT was implemented in the CP2K code, modelling GaS as monolayer nanoribbons with various edge terminations to explore potential reaction sites and pathways.

Experimentally, GaS was exfoliated in IPA using ultrasonication, resulting in flakes several layers thick. The dispersion was drop-cast onto Au TEM grids, and then aged in two environments: ambient and under de-ionised water, to explore the effect we have observed stark contrasts between flakes exposed to air (Figure 1) and those exposed to water (Figure 2). During ageing, they were investigated with HRSTEM combining high spatial resolution imaging with pixel-by-pixel energy dispersed X-ray (EDX) mapping.

Results

O₂ seems to react with the edges of the flake, substituting into sulphur sites, resulting in a loss of crystalline structure. However this reaction, while initially exothermic at all edges, ceases progressing the oxidation after the first few nm of the edge leaving the rest of the flake pristine, even after over 100 days.

Exposure to DI H₂O, on the other hand, resulted in significant changes after only a couple of weeks. The resulting HAADF contrast was mottled, with small crystallite regions remaining. The brighter regions (corresponding to thicker regions) contain higher concentrations of sulphur, but seem localised more towards edges, although overall there is an increase in the amount of sulphur relative to gallium. This suggests that GaS undergoes a reaction with water resulting in the loss of gallium from the flakes, and our theory is that remaining sulphur redeposits at the step edges. 

Figures:

Figure 1. A) HAADF STEM image of GaS aged in air for 106 days, B) EDX maps of O-K, S-K and Ga-K

Figure 2. A) HAADF STEM image of GaS aged in DI water for 21 days, B) EDX maps of S-K and Ga-K (O-K has been excluded for clarity, but was detected over the entire mapped region)

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EMC Abstracts - https://emc-proceedings.com/abstract/the-oxidation-of-gallium-ii-sulphide/