EPR spectroscopic investigation of the sol–gel transition in modified tetraethylorthosilicate gels

Tetraethyl orthosilicate (TEOS)-based sol–gel process monitored by EPR spectroscopy with VO(II) cations as a spin-probe

“…The line shape and spectral parameters of the abovementioned multicomponent 77 K VO(II) EPR spectra are very similar to those previously reported [57][58][59][60][61][62][63][64][65]75] for VO(II) spin-probes, as present in silica gels and xerogels. Figure 2B shows that no substantial changes occur in the ''frozen-solution'' VO(II) EPR spectra of the low-viscous and high-viscous solutions in stages (a) and (b), although the peak-to-peak line width (DB pp ) was found to increase slightly.…”

“…In this case, the dry-gel hydration, sample (h), gives very similar VO(II) EPR spectra (and likewise their spectral parameters) to those seen in both the stage (c) and the closed capillary (z). The spin Hamiltonian parameter values all agree well with those reported in the literature for similar systems [57][58][59][60][61][62][63][64][65].…”

“…Even when air-dried, the gels still contained some liquid fraction, albeit that its effective viscosity was higher than that of a free aqueous solution. When the samples were frozen at 77 K, the molecular motion of the hydrated cation complexes became immobilised on the EPR spectroscopic timescale [55][56][57][58][59][60][61][62][63][64][65].…”

“…At low VO(II) concentrations (ca 1 mmol or less), the X-band EPR spectra recorded from borate, phosphate and silicate glasses [3,4,7] and also those from the sol-gel process products [57][58][59][60][61][62][63][64][65] are rich in hyperfine structure from the 51 V isotope (I = 7/2, natural abundance 99.76 %). VO(II) cations are especially useful as spin-probes, since they consistently give relative sharp EPR spectra from solutions recorded both at RT and when frozen to 77 K [71].…”

“…It is well documented that Cu(II) and Mn(II) ions are very sensitive as spin-probes to monitor the structural changes that occur in silica xerogels during the process of calcination, or in general, the properties of the sol-gel process products, and numerous EPR experiments, have been devised for this purpose [5,. However, there have been far fewer systematic EPR studies made of Cu(II) and Mn(II) cations with which to explore the earlier stages of the silica sol-gel process [55][56][57][58][59], while VO(II) ions have been employed relatively rarely to investigate the silica sol-gel process at all [57][58][59][60][61][62][63][64][65].…”