The effect of ferroelectric polarization on the interaction of water and methanol with the surface of LiNbO 3(0 0 0 1)

Abstract

Water and methanol temperature programmed desorption (TPD) measurements were performed on the positive ( c +) and negative ( c −) surfaces of poled ferroelectric lithium niobate (LiNbO 3) single crystals. The results indicate that the molecule–surface interactions are both coverage and polarization-dependent. From a comparison of the TPD spectra for the positive and negative surfaces, it is shown that the desorption temperatures of water and methanol are consistently lower on the negative surface by 15 K and 20 K, respectively. The TPD spectra were simulated using the Polanyi–Wigner equation with a coverage-dependent energy term. These calculations show that the polarization dependence of the desorption temperature is due to a difference in the zero-coverage desorption energies on the two surfaces equal to a few kJ per mole. The mechanism for the polarization effect is explored with in situ pyroelectric voltage measurements, which indicate that a surface voltage of ±2 mV develops in the LiNbO 3(0 0 0 1) samples during TPD measurements. The magnitude of the pyroelectric-induced surface charge is heating rate dependent.