The Manganite−Water Interface

Abstract

The properties of bulk manganite, its freshly cleaved (010) surface, and this surface exposed to water monolayers at both low and high coverage and to liquid water at 300 K, are examined using density-functional theory (DFT) by the PW91 density functional as well as using the new PM6 semiempirical electronic-structure method. The bonds between the (010) layers are calculated to be very weak, of average energy -2.7 kcal mol -1 , explaining the ease at which manganite surfaces cleave. Upon cleavage, a surface reconstruction is predicted that produces ferroelectrically ordered surface layers, and the surface manganese atoms are predicted to display different chemical properties depending on the nature of the oxygen atom to which they bind in the subsurface layer. Water is predicted to be only physisorbed to the surface, with this process acting to lift the surface reconstruction. At high water coverage, the differences between the two types of surface manganese atoms are also lost. Simulations at 300 K indicate that less than half of the surface manganese atoms coordinate to the fluid at 300 K while only two-thirds of the manganite oxygen atoms on the outside of the surface coordinate. No dominate liquid structure is found, suggesting that dielectric continuum models may be useful in understanding surface chemistry, but it is clear that water-water rather than water-surface interactions dominate the nature of the interface.

Influence of pH on Initial Concentration Effect of Arsenate Adsorption on TiO2 Surfaces: Thermodynamic, DFT, and EXAFS Interpretations

“…is the total Gibbs free energy of Ti-AsO 4 adsorption complex, G tot (arsenate) and G tot (TiO 2 ) are the total Gibbs free energy of arsenate molecule and TiO 2 cluster, respectively. The theoretical equilibrium adsorption constant K was calculated by equation 2 were conducted in a series of polypropylene tubes under pH 5.5, 6.2, and 7.0, respectively. Under a series of fixed total mass of arsenate, initial arsenate was added by different modes (i.e., 1-batch and 3-batch) to the suspension of TiO 2 (1.0 g/L, 30 mL).…”

“…2 The surface protonation-deprotonation process controlled by pH modifies the charge density and redox properties of mineral surface, and changes its reactivity and adsorption capacity. 1,2 Studies of sulfate at Al-and Fe-(hydr)oxide-H 2 O interface showed that the thermodynamic favorability of surface complexation was directly related to surface pH condition (surface proton density). 1,3 The dependence of oxyanions adsorption on pH created a complicated network of potential reaction pathways.…”

Studies on the reaction pathway of arsenate adsorption at water–TiO2 interfaces using density functional theory

“…3a). Such electronic structures indicated that the Ti atoms were the surface reactive sites to bond with water molecules and form the surface functional groups (i.e., hydrated surfaces) in aqueous solutions [11,31,39]. For the hydrated TiO 2 surfaces, the calculated surface Ti-O distances were 2.06 Å (Fig.…”

Theoretical Studies on the Structure, Optoelectronic and Photosensitizer Applications of NKX Class of Coumarin Dye Molecules

“…In this regard, various studies regarding the engineering of the photoanode interfaces in DSSCs were carried out. [31][32][33] Modification of the TiO 2 surface has been attempted inorder to reduce the back electron recombination and hence, to facilitate electron transfer process. The modifications mainly involved the insulation of the TiO 2 surface so as to form a energy barrier between the TiO 2 electrodes and electrolyte.…”