Mid- and Near-Infrared Study of Hydroxyl Groups at a Silica Surface: H-Bond Effect

This paper studies the IR absorption of the OH stretching modes nu OH and of their first overtones 2 nu OH for silanol groups at silica surfaces. Heat treatment of silica induces a condensation of silanol groups. The subsequent decrease in silanol concentration results in strong modification of the spectral profiles. The spectra are interpreted in terms of distributions of OH oscillators. The mechanical anharmonicity coefficient x(OH) and the ratio of the fundamental/overtone extinction coefficients epsilon(nu OH)/epsilon(2 nu OH) both increase for decreasing nu OH wavenumber decreases, i.e., as the strength of the H-bond increases. The decadic integrated intensities are plotted against silanol concentrations. The intensity of the nu OH mode significantly increases with hydrogen bonding, whereas that of 2 nu OH slightly decreases. As a consequence, compared to the fundamental mode, the first overtone provides a better picture of the distribution of OH states. Analysis of the infrared spectra also demonstrates the presence of cooperative bonding between silanol groups at the silica surface.