Vibrational Sum Frequency Generation Spectroscopy of Surface Hydroxyls on Nickel Phyllosilicate Nanoscrolls

Phyllosilicate clays are layered structures with diverse nanoscale morphology depending on the composition. Size mismatch between the sheets can cause them to form nanoscrolls, a spiral structure with different inner and outer surface charges. The hydroxyls on the exposed surface of the nanoscrolls determine the adsorption properties and hydrophilicity of the surface. Vibrational sum frequency generation (VSFG) spectroscopy was applied to study the surface hydroxyls of nickel phyllosilicate (Ni₃Si₂O₅(OH)₄), revealing three distinct in-phase OH-stretch modes: 3642, 3645, and 3653 cm^-1. The relative signs of the peaks allow their assignment as "outward" and "inward" pointing hydroxyls on the opposite sides of the scrolled sheet, consistent with the crystal structure. Orientational analysis of polarization-selected VSFG spectra is consistent with a broad (140-164°) step-function distribution of the OH-stretch tilt angles, which we attribute to the uncompensated portion of the scroll rolled more than a whole number of full turns.