Abstract
The atom-scale characterisation of interfaces between transition metal oxides and liquid water is fundamental to our mechanistic understanding of diverse phenomena ranging from crystal growth to biogeochemical transformations to solar fuel production. Here we report on the results of large-scale hybrid density functional theory-based molecular dynamics simulations for the hematite(001)-liquid water interface. A specific focus is placed on understanding how different terminations of the same surface influence surface solvation. We find that the two dominant terminations for the hematite(001) surface exhibit strong differences both in terms of the active species formed on the surface and the strength of surface solvation. According to present simulations, we find that charged oxyanions (-O−) and doubly protonated oxygens (-OH) can be formed on the iron terminated layer via autoionization of neutral -OH groups. No such charged species are found for the oxygen terminated surface. In addition, the missing iron sublayer in the iron terminated surface strongly influences the solvation structure, which becomes less well ordered in the vicinity of the interface. These pronounced differences are likely to affect the reactivity of the two surface terminations, and in particular the energetics of excess charge carriers at the surface.
Highlights
Interfaces between transition metal oxides and liquid water have attracted considerable attention in the research community due to their importance in e.g. atmospheric science [1], catalysis [2], energy storage [3], colloid chemistry [4], energy harvesting [5, 6], and artificial photosynthesis
For the interpretation of crystal truncation rod (CTR) experiments, it is helpful to have information on an atomistic level, since the fit of the model to the experimental data involves a large number of degrees of freedom [6, 9, 10, 42]
The central oxygen layer of the symmetrically layered hematite slab has been used as point of reference for measuring distances between the layers
Summary
Interfaces between transition metal oxides and liquid water have attracted considerable attention in the research community due to their importance in e.g. atmospheric science [1], catalysis [2], energy storage [3], colloid chemistry [4], energy harvesting [5, 6], and artificial photosynthesis. Classical molecular dynamics simulations have been employed for modelling of oxide/water systems for some time and they gave insightful information mostly on structural properties [14]. As in our previous investigation on the oxygen terminated surface, we use DFTMD employing the hybrid density functional HSE06 with the fraction of exact exchange reduced to 12%. This functional was shown to reproduce the antiferrosymmetric spin pattern and other electronic properties such as band gaps and band ordering very well [18, 27]. Our simulations suggest that in addition to neutral (-OH) terminations, oxy-anions (-O−) and doubly protonated terminal oxygen atoms (-OH+2 ) co-exist on the iron surface at the point of zero potential charge
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