Time evolution of a sol–gel process monitored by Mn2+ EPR spectroscopy

The time evolution of the tetraethoxysilane (TEOS)-based sol–gel process was investigated by EPR spectroscopy of a Mn2+ spin probe. Analysis of the EPR spectra recorded at room temperature (RT) and liquid nitrogen temperature (77 K) showed that the values of the g-factor (for the central Mn2+ sextet) were found to be, within experimental error, identical during the sol–gel process evolution. However, the peak-to-peak half width, ΔBpp, of the spectral lines recorded at RT, the splitting of the forbidden hyperfine lines (and thus the zero-field splitting parameter, D) recorded at 77 K, and the averaged hyperfine splitting constant, Aav, at both temperatures, were found to change significantly during the various stages of the sol–gel process. These EPR spectral parameters allow us to monitor changes in the TEOS-based sol–gel reaction mixture during the liquid-to-solid-state transition. It was clearly demonstrated that the sol-to-gel transition as well as the hydration and dehydration process of gels could be successfully studied by Mn2+ EPR spectroscopy.