Carbon paramagnetic defects in silica sol-gel prepared materials

Abstract

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 ¹³C and ¹H nuclei only, and hyperfine interactions of 5 MHz with a ¹³C nucleus.

Keywords:

• EPR
• Sol-gel
• Silica
• Defect
• Carbon

Acknowledgement

This work was supported in part by the Italian Ministry of University and Research via the PRIN programme 2005 ‘Architetture molecolari organizzate su matrici inorganiche ed ibride’. Andrea Zattin is gratefully acknowledged for his help in the synthesis of the materials.