Temperature dependence of the absorption properties of silanol groups in amorphous : Are silanol groups organized in clusters?

We present a study on the vacuum-ultraviolet (VUV) and infrared (IR) absorption of silanol groups in amorphous silicon dioxide (a- SiO2) in the range of temperature from 4 to 300 K. The observed temperature induced modifications of IR and VUV absorption spectra are interpreted as due to a process of conversion of free into H-bonded silanol groups. The changes of the amplitude of the VUV absorption spectra are shown to be linearly correlated to the changes of the IR absorption of free Si–OH groups. This point together with the evidence that the shape of the Si–OH VUV absorption does not depend on temperature demonstrates that the different silanol group sub-species have different VUV absorption properties, being only the free Si–OH groups responsible for the near-edge VUV absorption. A rough estimation of the percentage of free Si–OH groups (∼43%) converted into H-bonded ones by cooling a- SiO2 down to 4 K is provided as well. A possible model for the interpretation of the observed experimental evidences is proposed: we suggest silanol groups to be arranged in clusters in the amorphous SiO2 network.