A peculiar oxygen substitution reaction of 2D MoS₂

When you take a look at one of your older samples, usually two things happen: (1) nothing has changed, which does not provide you with new information, or (2) everything has changed, which is too difficult to interpret. However, on the rare occasions when the right amount of change occurred to the sample, you have a chance to understand what is going on. This was the fortunate case of our MoS₂ single layer sample sitting in a drawer for months. Since the basal plane of MoS₂ crystals is widely regarded as environmentally stable, we definitely expected the first scenario, namely that the sample will look just as we left it. However, when acquiring atomic resolution STM images, we noticed a significantly increased point defect density, as compared to freshly prepared samples. After one year a 10 x 10 nm² sample area displayed more than a hundred point defects as opposed to a couple of defects in the freshly exfoliated samples. Then it became entirely clear that the ambient exposure is capable of inducing new structural defects in the pristine basal plane of MoS₂ single layers. These point defects were identified as oxygen atoms replacing individual sulfur atoms in the MoS₂ crystal lattice. The resulting material was no longer MoS₂ but a new solid solution crystal with the formula of MoS_2-xO_x, which still preserved the original MoS₂ crystal structure. Previous attempts to synthesize the molybdenum oxy-sulfide phase resulted in amorphous or highly disordered structures. We have shown that 2D MoS₂ crystals can be chemically transformed into molybdenum oxy-sulfide, while fully preserving their long range crystalline order. In fact, this transformation spontaneously occurs upon ambient exposure on months-long time scale.The chemical transformation of MoS₂ under ambient exposure happens through an oxidation reaction upon which individual oxygen atoms one-by-one replace single S atoms of the MoS₂ lattice. 

H₂ evolution catalyzed by single oxygen hetero-atoms

The best catalyst for hydrogen evolution is platinum. However, the development of cheaper alternatives is highly desirable for industrial applications. Since MoS₂ is already intensely investigated as a promising catalyst, we have measured how the chemical doping of the MoS₂ basal plane by O atoms influences their catalytic activity. We found that the 2D MoS_2-xO_x samples display a substantially increased catalytic activity as compared to pure MoS₂. By directly imaging their atomic structure, we could clearly identify the O substitution sites as the source of the increased catalytic activity. However, it was not easy to understand how such O sites can be efficient in catalyzing H₂ evolution. The Gibbs free energy (G_H) is often used as a predictive parameter for the catalytic activity. It can be perceived as characterizing the adsorption strength of H atoms to the catalytic site. A value close to zero is desirable (neither too strong, nor too weak adsorption), as during the catalytic process H atoms have to both adsorb and desorp from the catalytic site. The pristine basal plane of MoS₂ has a G_H value of about 2eV, which tells us that H does not like to adsorb on it (hence its inactivity). We calculated the G_H of O substitution sites to be about 1 eV, which means that H species are much more likely to adsorb on O substitution sites, but still not likely enough to explain the observed catalytic efficiency. So we proposed an additional process that might facilitate the catalysis. Namely, the G_H value does not take into account the electrical charge potentially present on the catalytic site. It is known that O is more electronegative than sulfur. According to our calculations a significant surplus of negative charge (of about -0.4 e^-) is localized on the O substitution sites. This charge is only partially screened at the surface, providing an additional driving force to attract the positively charged H species from the electrolyte. Upon reduction (electron transfer), H becomes neutral, switching OFF this electrostatic force, this way facilitating its desorption. We propose that this process (together with the decreased G_H value) can explain the observed catalytic activity of single atomic O substitution sites in 2D MoS_2-xO_x crystals.

Our paper “Spontaneous doping of the basal plane of MoS2 single layers through oxygen substitution under ambient conditions” published in Nature Chemistry can be read from here.