Carbon paramagnetic defects in silica sol-gel prepared materials

Silica obtained by annealing in air gels produced from the controlled hydrolysis and condensation of methacryloxypropyltriethoxysilane (MAPTES) exhibit Lorentzian and Gaussian EPR signals centred at g = 2.0037, similar to those found in coals. The number and widths of Lorentzian lines depend on the annealing temperature, and are attributed to graphitic clusters of different sizes showing spin exchange. These results agree with the estimated average dimensions of the clusters at the different annealing temperatures, obtained by correlating the carbon content with the number of spins. Inhomogeneous components have been identified by using the echo-detected EPR method and are attributed to smaller isolated clusters. These latter analysed by ESEEM and HYSCORE show strong signals at the free nuclear frequencies of C-13 and H-1 nuclei only, and hyperfine interactions of 5MHz with a C-13 nucleus.