EPR studies on the dynamics of the structure of copper (II) doped silica gels

Detailed X-band EPR studies on 1.0 mol% copper oxide doped silica gels, oven dried as well as calcined at various temperatures up to 1000 °C have been carried out in the temperature range 10.4–300 K. Spin Hamiltonian and line-width parameters are determined by computer simulation of the observed spectra. Important information derived on the dynamics of the porous structure of Cu-doped silica gel in the light of the EPR parameters is as follows: (i) Three kinds of Cu2+ species (including one Cu-pair) are present in the 100 °C oven dried silica gel. (ii) Two kinds of slow rotating isolated Cu2+ species (one Cu2+ pair) occur in the 400 °C calcined sample at room temperature. The Cu-pair species agglomerate at low temperatures (⩽200 K) to form CuO clusters which remain undetected apparently because of their large line-width. (iii) Only one kind of Cu-species is detected in the 700 °C calcined silica glass at all measuring temperatures. The Cu2+-pair/CuO cluster spectrum is absent. (iv) The isotropic EPR spectrum due to rapidly rotating Cu2+-species occurring in very large pores of silica glass calcined at 800 °C has been obtained at room temperature. (v) Pore collapse starts at 900 °C, and results in the observance of a very broad EPR line (∼5000 Oe) for 900 and 1000 °C calcined samples. This shows that the pore collapse and the formation of superparamagnetic nano-sized CuO particles in the silica glass matrix take place simultaneously.