Percentage dissociation of H2O on the surface of rutile (black) and cassiterite (orange) during the last 5 ps of the simulation. Cassiterite shows higher H2O dissociation percentage (∼60%) in comparison to rutile surface (∼25%). The fluctuations appearing in the dissociation percentage correspond to proton jump events. Cassiterite shows more than three times faster proton jump rate in comparison to rutile.
Density plots of oxygen positions in the L2 layer for (a) cassiterite and (b) rutile surfaces. The BO and TO corresponds to bridging oxygen and terminal oxygen on the surface, respectively. Comparison of the horizontal broken lines shows that the H2O on cassiterite is closer to the surface in comparison to rutile. Histograms (not shown) of the distances from BO and TO to oxygen in L2 H2O layer show that H2O is closer to the cassiterite surface by 0.06 Å compared to the same distance in rutile. This density plot has been averaged over last 5 ps of simulation time. In the density plot, red corresponds to higher density, while yellow corresponds to lower density.
H-bond lengths and respective O–H stretching band frequency for all, statistically significant, H-bonds on rutile (black) and cassiterite (orange) surface. The symbols are defined in the inset. M corresponds to the metal atom, i.e., Ti for rutile and Sn for cassiterite. The numbers 1, 2, or 3 correspond to the number of H-bonds accepted by the acceptor O atom. The bold O and H in these labels refer to the species participating in the H-bond. The vertical axis is the weighted mean of the stretching band of the H and the horizontal axis is the peak position of the histogram of corresponding H-bond length. The H-bonds on cassiterite surface (orange) are generally much stronger than on rutile surface (black) as shown by smaller H-bond length and corresponding weaker O–H stretching band frequency.
The energy difference between the 3a1 and the 1b2 valence states of H2O as a function of the distance between the H2O oxygen and the BO of rutile (black) and cassiterite (orange). The H2O molecule is oriented such that one of its Hs points toward a BO atom on the surface. The 3a1 state of H2O on cassiterite is around 0.3 eV more stabilized than on the rutile surface. This is an indication that the covalent component of this H-bond is stronger on cassiterite compared to rutile.
Average jump rate for proton transfers between different surface species (BO and TO) and H2O in the hydration layer (WO). Cassiterite shows a higher proton jump rate compared to rutile in all categories except for TO–TO jumps that are less probable in cassiterite due to its larger lattice constant.
Article metrics loading...
Full text loading...