Investigations on sol–gel process and structural characterization of SiO2-P2O5 powders

The purpose of the study is to investigate the influence of the precursors, pH of the solution and temperature on the gelation and structure evolution of the samples from the SiO2-P2O5 system. Tetraethoxysilane (TEOS) was used as precursor for SiO2 and triethylphosphate (TEP) or phosphoric acid for P2O5, together with water as reagent for hydrolysis reaction and ethylic alcohol as solvent. The pH of the sols was modified by adding hydrochloric acid, in the case of TEP and by adding ammonia, in the case of H3PO4. The samples have been prepared starting from P2O5/SiO2 = 1/10 and 1/5 molar ratio, H2O/TEOS = 1; 2; 3 mass ratios and C2H5OH/TEOS = 1 mass ratio. We prepared silico-phosphate samples in the 1.5–5 pH domain and we observed that in all the cases, the lowest gelation time was found in the 3.5–4.5 pH range. We found that for the same pH value samples prepared with H3PO4 had a lower gelation time (few days) by comparison with the samples prepared with TEP (weeks), explainable by the low rate of the hydrolysis and condensation reactions of TEP. When the amount of water was increased, the gelation time increased in the case of samples prepared with H3PO4 and it was not significantly changed in the case of the samples prepared with TEP. The increasing of the solution temperature up to 40–41 °C yielded a decreasing of the gelation time (hours), especially for the samples prepared with H3PO4 by comparison with those prepared using TEP. In all the cases, the increased amount of water resulted in an increasing of the gelation time, even the temperature was raised. FTIR and Raman spectroscopy characterization aimed at getting information about the structural changes in the case of the samples dried in air and also for those heated at 100 °C, 300 °C, 600 °C and 900 °C. Vibration modes specific for SiOEt, SiOH, hydrogen bonds, H2O and combined vibrations have been observed, which are in agreement with those revealed in literature data. 31P and 29Si MAS NMR spectra gave interesting information about first surrounding of P and Si ions meaning the type and proportion of Q species and their evolution starting from the room temperature up to 900 °C.