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 νOH and of their first overtones 2νOH 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 xOH and the ratio of the fundamental/overtone extinction coefficients ϵ(νOH)/ϵ(2νOH) both increase for decreasing νOH 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 νOH mode significantly increases with hydrogen bonding, whereas that of 2νOH 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.