Polarization-dependent methanol adsorption on lithium niobate Z-cut surfaces

The adsorption of single methanol molecules on the lithium niobate (0001) surface, commonly referred to as Z-cut, is investigated using first-principles calculations. It is found that the binding energy for molecular adsorption on the negative surface ($\sim$1 eV) is about twice as large as for the positive surface. This difference is related to different bond strengths rather than electrostatics. Depending on the reaction path, lithium extraction from the negative surface may occur. This leads to an additional energy lowering by a few tenths of an eV. Larger energy gains are realized by dissociative adsorption, which is an activated process, however.

Figure 1

Side view of the stable positive (a) (-Nb-O${}_3$-Li${}_2$ terminated) and negative (b) (O-Li- terminated) LN(0001) surface. White, gray, and small circles denote Li, Nb, and O atoms, respectively.

Figure 2

Schematic view of the eight different methanol starting orientations used in the PES calculation. See text.

Figure 3

Calculated potential energy surfaces for the adsorption of methanol on the positive (a) and negative (b) LN surface. The primitive surface unit cell used is indicated.

Figure 4

Top view of the relaxed bonding configuration A on the positive (a) and negative (b) LN surface (see text). The primitive surface unit cell is indicated.

Figure 5

Calculated charge-density difference (in $e$/Å${}^3$) of the methanol bonding configuration A on the positive (a) and negative (b) LN surface.

Figure 6

Top view (a) and side view (b) of the adsorption configuration D on the negative LN surface (see text). The surface extracted Li${}_D$ atom is marked.

Figure 7

Calculated energy vs molecule-surface distance (given with respect to the uppermost surface atom) for the adsorption configurations A, B, and C on the positive (a) and negative (b) LN surface. The energy of the state describing the isolated molecule and surface is set to zero.

Figure 8

Sketches of (intermediate) geometries of the reactions (a) CH${}_3$OH $\to$ H${}_2$COH+H, (b) CH${}_3$OH $\to$ H${}_2$O+CH${}_2$ on the positive LN surface, and (c) CH${}_3$OH $\to$ CH${}_3$+OH on the negative surface. The atom positions of the intermediate geometries are light. Only the moving hydrogen, the carbon, and the oxygen are shown.

Figure 9

Energetics of dissociative surface reactions of methanol on the positive [(a), (b)] or negative (c) LN surface. I, P, and D denote the state of the isolated molecule and surface, the interacting but intact (physisorbed), and the dissociated molecule on the surface.