Luminiscent dye Rhodamine 6G doped monolithic and transparent TEOS silica xerogels and spectral properties

摘要

Sol–gel process was used for the preparation of Rhodamine 6G (R6G) doped silica xerogels, using tetraethylorthosilicate [TEOS, Si(OC₂H₅)₄] as the precursor for the silica network. Silica alcosol was prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (CTA) catalyst. The ethanolic R6G was added to the alcosol to trap R6G molecules inside the SiO₂ gel network during the gelation of the TEOS alcosol. The effect of CTA/TEOS molar ratio on the gelation time of the R6G doped TEOS alcosol, transparency and monolithicity of the R6G doped silica xerogel was studied by varying the CTA/TEOS molar ratio from 1.2×10^?4 to 180×10^?4 by keeping the molar ratios of TEOS:EtOH:H₂O:R6G constant at 1:5:7:9.2×10^?6, respectively. It was found that the minimum (?4 where as maximum (>180 h) gelation time was observed for CTA/TEOS molar ratio of 72×10^?4. While opaque and monolithic R6G doped SiO₂ xerogels were obtained for ?4 CTA/TEOS molar ratios, whereas cracked and transparent xerogels were obtained for >120×10^?4 molar ratios of CTA/TEOS. Transparent, homogeneous and monolithic samples were obtained between 4.8×10^?4 and 120×10^?4 of CTA/TEOS molar ratios. Leaching out property was studied by using water, methanol and ethanol solvents for the R6G doped SiO₂ xerogels of 9.2×10^?6 and 12×10^?4 of R6G/TEOS and CTA/TEOS molar ratios, respectively, and found that R6G molecules were trapped in the pores of the SiO₂ network. Bleaching out phenomena of the R6G doped SiO₂ xerogels was studied by focusing the high intensity light on some part of the samples for a period of 1 h and found that the pores were continuous in SiO₂ network. Visible spectra of R6G in water, ethanol, SiO₂ alcosol and xerogel were taken for 1.6×10^?4?M R6G and observed that there were two absorption peaks at 499 and 525 nm in the spectrum of R6G in water due to dimerization of R6G molecules and only one absorption peak at 530 nm in the spectra of ethanol, SiO₂ alcosol and xerogel because of monomerization of R6G molecules. Visible spectra of the R6G doped silica xerogels for varying R6G/TEOS molar ratios from 9.2×10^?8 to 9.2×10^?5 were taken and found the red shift (5–10 nm) with increasing the R6G/TEOS molar ratio from 9.2×10^?8 to 9.2×10^?5. The effect of temperature on these sample was studied by varying the temperature from 50 to 300 °C and found that the R6G doped silica samples were stable up to 200 °C. IR spectra were taken for pure R6G powder and R6G doped silica xerogels of 9.2×10^?8 and 9.2×10^?5 R6G/TEOS molar ratios and found that most of the peaks present in pure R6G powder spectrum were absent in the spectra of trapped R6G SiO₂ xerogels. This shows that, the SiO₂ network hinders the rotational and vibrational transitions of R6G when it is caged in the SiO₂ network. The peaks related to bending motion in R6G molecules were not disturbed by the SiO₂ network