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

Abstract

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 (<70 h) gelation time was observed at higher and lower CTA/TEOS molar ratios of 72 × 10-4 where as maximum (>180 h) gelation time was observed for CTA/TEOS molar ratio of 72 × 1024. While opaque and monolithic R6G doped SiO2 xerogels were obtained for <4.8 × 10-4 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 andobserved 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 × 1028 to 9.2 × 10-5. The effect of temperature on these sample was studied by varying the temperature from 50 to 300 8C and found that the R6G doped silica samples were stable up to 200 8C. 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