(646f) Surface Chemistry of Ferroelectric Lithium Niobate

The goal of this project is to determine the magnitude of the effect of the ferroelectric poling direction on surface catalytic chemistry. Large differences in 2-propanol reaction on the positive and negative faces of LiNbO3 (0001) faces were observed in temperature programmed desorption (TPD) experiments. On the negative face, only molecular desorption was observed, predominantly in peaks at ~380 and ~500 K. In contrast, 2-propanol desorbed from the positive face in a single peak at ~600 K, and evidence that some of the adsorbed 2-propanol undergoes oxidative dehydrogenation to acetone was observed. The adsorption of acetic acid is also being studied. The results indicate that adsorption strength can be dramatically altered by changing the ferroelectric polarization and suggest that oxidation reactions can be turned on and off by switching the polarization direction.

Meanwhile, we found that the positive and negative faces of LiNbO3 (0001) appear remarkably similar in electron diffraction and spectroscopic measurements. Both surfaces showed (1x1) diffraction patterns and displayed no indications of a reconstruction, while the valence bands appeared identical in ultraviolet photoelectron spectra. Moreover, low energy ion scattering spectra were nearly indistinguishable, with both faces displaying spectra suggestive of surfaces predominantly covered by oxygen. X-ray photoemission spectroscopy showed identical Nb core levels indicative of Nb in the 5+ oxidation state. The only significant spectroscopic difference between the two faces was the appearance of a high binding energy shoulder on the O 1s peak of the negative face which may be due to hydroxyls on the negative face and is currently under investigation.