Effect of Ferroelectric Poling on the Adsorption of 2-Propanol on LiNbO₃(0001)

The effect of ferroelectric poling on the surface chemistry of ferroelectric materials was studied through the adsorption and temperature-programmed desorption (TPD) of 2-propanol on the positively and negatively poled LiNbO 3 (0001) surfaces. Although only molecular adsorption and desorption were observed on both surfaces, the desorption peak temperature of 2-propanol from the positive surface was over 100 K higher than that from the negative surface, indicating that the polar molecule adsorbs more strongly on the positive surface. Furthermore, the 2-propanol desorption peak temperature depended unusually strongly on the heating rate, yielding apparent desorption pre-exponentials on the order of 10 s -1 for the positive surface and 10 -1 s -1 for the negative surface. These unusually low numbers could be associated with the pyroelectric behavior of LiNbO 3 creating a temperature-dependent surface dipole; interaction of the polar molecule with this surface dipole then causes the desorption activation energy to also depend on temperature. Fitting the data to a simple linear model between the desorption activation energy and temperature suggests that the desorption activation energy of 2-propanol from LiNbO 3 increases by about 0.25 kJ/mol-K as the temperature increases. Taking this effect into account, the data indicate an 11 kJ/mol higher desorption activation energy from the positive surface. The results also revealed a sensitivity of the adsorption/desorption to the morphology of the opposite face of the crystal.