Browsing by Subject "Defects in solids"

Now showing 1 - 2 of 2

Open Access
Elucidating the barriers on direct water splitting: key role of oxygen vacancy density and coordination over PbTiO3 and TiO2
(American Chemical Society, 2021-01-28) Ersen, M.; Ellialtıoğlu, Ş.; Gülseren, Oğuz; Uner, D.
In this work, using the state-of-the-art first-principles calculations based on density functional theory, we found that the concentration and coordination of surface oxygen vacancies with respect to each other were critical for the direct water-splitting reaction on the (001) surfaces of PbTiO3 and TiO2. For the water-splitting reaction to happen on TiO2-terminated surfaces, it is necessary to have two neighboring O vacancies acting as active sites that host two adsorbing water molecules. However, eventual dissociation of O–H bonds is possible only in the presence of an additional nearest-neighbor O vacancy. Unfortunately, this necessary third vacancy inhibits the formation of molecular hydrogen by trapping the dissociated H atoms on TiO2-terminated surfaces. Formation of up to three O vacancies is energetically less costly on both terminations of PbTiO3(001) surfaces compared with those on TiO2; the presence of Pb leads to weaker O bonds over these surfaces. Molecular hydrogen formation is more favorable on the PbO-terminated surface of PbTiO3, requiring only two neighboring oxygen vacancies. However, the hydrogen molecule is retained near the surface by weak van der Waals forces. Our study indicates two barriers leading to low productivity of direct water-splitting processes. First and foremost, there is an entropic barrier imposed by the requirement of at least two nearest-neighbor O vacancies, sterically hindering the process. Furthermore, there are also enthalpic barriers of formation on TiO2-terminated surfaces or removal of H2 molecules from the PbO-terminated surface. The requirement dictating three nearest-neighbor oxygen vacancies for hydrogen evolution is also consistent with the chemical intuition: The nearest neighbor of the formed hydrogen should be reduced enough to inhibit spontaneous oxidation under ambient conditions.
Open Access
Functional carbon and silicon monolayers in biphenylene network
(American Chemical Society, 2022-06-28) Gorkan, T.; Çallıoǧlu, Şafak; Demirci, S.; Aktürk, E.; Ciraci, S.
We investigated the effects of vacancy, void, substitutional impurity, isolated adsorption of selected adatoms, and their patterned coverage on the physical and chemical properties of metallic carbon and silicon monolayers in a biphenylene network. These monolayers can acquire diverse electronic and magnetic properties to become more functional depending on the repeating symmetry, size of the point defects, and on the type of adsorbed adatoms. While a carbon monovacancy attains a local magnetic moment, its void can display closed edge states with interesting physical effects. Adsorbed light-transition or rare-earth metal atoms attribute magnetism to these monolayers. The opening of a gap in the metallic density of states, which depends on the pattern and density of adsorbed hydrogen, oxygen, and carbon adatoms, can be used as the band gap engineering of these two-dimensional materials. The energy barriers against the passage of oxygen atoms through the centers of hexagon and octagon rings are investigated, and the coating of the active surfaces with carbon monolayers is exploited as a means of protection against oxidation. We showed that the repulsive forces exerting even at distant separations between two parallel, hydrogenated carbon monolayers in a biphenylene network can lead to the superlow friction features in their sliding motion. All these results obtained from the calculations using the density functional theory herald critical applications.