Monolithic mesoporous silica glasses were synthesized. The presence of Cu2+ and Fe3+ cations during the synthesis of sol-gel precursors leads to different morphologies and pore sizes. The materials are characterized via IR and Raman scattering spectra to detect surface groups and -Si-O-Si- rings (i.e., 3–6 Si atoms) and morphology is examined through electron microscopy. N2 sorption isotherms reveal details of the mesoporous structure of the materials, which are endowed with significantly large surface areas and pore volumes. Vapor percolation occurs in these samples because of a void arrangement consisting of pore bulges delimited by narrower necks. The optical characterization shows the luminescence spectrum and thermoluminescent behavior subjected to successive exposures of beta particles.
Nowadays, a number of novel materials have been investigated, from organometallics, catalysis [
The modification of its surface to design silica materials provides a wide range of special applications. Their proper development allows studying the structural behavior at molecular level and their interactions. The sol-gel process [
FT-IR and surface-enhanced Raman scattering (SERS) spectroscopies have been employed as auxiliary techniques for the study of structure at the molecular level of the optical glasses based on silica [
The structural properties, particle size distribution, porous size, and surface area depend strongly of the condensation and hydrolysis rate of the gelation; therefore, the heat and/or use of doping metal must be tightly controlled to obtain an ordered structural arrangement.
The thermoluminescence phenomena occurred when a phosphor material is heated previously exposed to ionizing and nonionizing radiation. This is considered as an effective technique to understanding of the creation of defects generates a glow curve which is a graphical representation of light emission as a function of temperature and is related with trapping levels in the band gap, the main goal of analyzing trapping levels is to obtain several kinetic parameters [
In this work, a series of SiO2 xerogels, some of them doped with Cu2+ and Fe3+ ions, are synthesized by the sol-gel procedure. Porous glasses are created by precursor gels after thermal treatment at 500°C. The surface and pore morphology characteristics depicted by porous glasses are studied by surface-enhanced Raman scattering (SERS), TEM, and N2 sorption analyses. Some optical properties were measured as the behavior of the glow curve through the technique of thermally stimulated luminescence and luminescence spectrum response.
Mixtures of tetraethylorthorsilicate (TEOS) (98% Aldrich), triple distilled water and ethanol produced SiO2 gels. Doping cations came from either Cu(NO3)2·2.5H2O (Aldrich 99 wt.%) or Fe(NO3)2·9H2O (Baker 99 wt.%). Each sol synthesis was performed in a Nalgene flask at room temperature by dissolving the appropriate amount of TEOS in ethanol under a gentle stirring. Separately, suitable quantities of Cu2+ or Fe3+ nitrates were dissolved in water. The Cu2+ or Fe3+ solution was then poured inside the flask containing the TEOS mixture. The reaction system was subjected to stirring during 20 min; afterward 0.25 cm3 of concentrated HCl mixed with 0.25 cm3 of concentrated HF were added drop by drop while continuing the stirring for further 30 min. The resulting SiO2 sol was then poured into a Nalgene cylinder where gelation rapidly occurred. Each gel was dried at 110°C for 48 h and the resulting xerogel calcined at 500°C during 24 h then obtaining SiO2 cylindrical monoliths.
Molar TEOS: H2O: ethanol ratios of: 1 : 4 : 6 (MG1), 1 : 1 : 4 (MG2) and 1 : 3 : 6 (MG3) were chosen to produce an assortment of transparent monolithic glass materials. For all samples, the amounts of Cu(NO3)2·2.5H2O or Fe(NO3)2·9H2O employed for the synthesis of cation-doped xerogels corresponded to 10−3 and 6 × 10−4 mol per one mol of TEOS, respectively. Figure
MG1-Cu, MG2-Cu, MG3-Cu, and MG1-undoped at 110°C.
MG2-Fe and MG2-Cu at 500°C.
FT-IR silica glass spectra were measured on a Bruker Vector 33 spectrometer by means of the Diffuse Reflectance technique. Glasses were also analyzed by Raman scattering spectroscopy by mounting the cylindrical glass specimens transversally in the sampling compartment of a Senterra Bruker Raman scattering instrument with at laser 785 nm. Transmission electron microscopy (TEM) of powdery glass samples was performed in a JEOL JEM-1010 instrument.
N2 sorption isotherms at 76 K of powdery samples were measured in a Quantachrome Autosorb 1 instrument. The N2 and He gases were UHP grade (Praxair, 99.99%). All samples were outgassed at 200°C overnight prior to the sorption run.
TL measurements and beta irradiation were performed in a Risø TL/OSL model TL/OSL-DA-15 unit equipped with a 90Sr beta source 3.33 Gy min−1 dose rate. The TL signal was integrated from room temperature up to 650 K under N2 atmosphere using a heating rate of 1°C s−1. The luminescence spectra were obtained prior to the UV excitation by using a Perkin Elmer spectrofluorometer model LS50-B at room temperature.
Figure
FT-IR spectra of MG1, MG2 -Cu doped, and MG2-Fe doped.
In the case of FT-IR spectra of Cu-doped silica glasses (Figure
Observing the Cu-O and Fe-O groups for the doped monolithic glasses in Figure
FT-IR spectra at 700–400 cm−1 interval, for MG3-Cu doped and MG3-Fe doped.
The Raman scattering spectra of MG3, MG3-Cu, and MG3-Fe calcined samples are shown in Figure
Raman scattering spectra of MG3, MG3-Cu, and MG3-Fe.
Raman scattering spectra at 80–700 cm−1 of MG3-Cu and MG3-Fe doped.
Table
Textural properties of porous glasses.
Sample | MG1 | MG1-Fe | MG1-Cu | MG2 | MG2-Fe | MG2-Cu | MG3 | MG3-Fe | MG3-Cu |
---|---|---|---|---|---|---|---|---|---|
449 | 352 | 680 | 766 | 134 | 711 | 298 | 970 | 629 | |
440 | 322 | 609 | 706 | 181 | 605 | 3630 | 1908 | 1749 | |
4.7 | 3.4 | 6.0 | 4.8 | 3.5 | 3.7 | 4.8 | 6.0 | 6.8 |
The N2 sorption isotherms and pore-size distributions (PSD) of three SiO2 glasses one Cu-doped (MG1-Cu) and two cation-undoped (MG1 and MG2), are shown in Figures
N2 sorption on (a) MG3-Cu.
N2 sorption on (a) MG1 undoped.
N2 sorption on (a) MG2 undoped.
An interesting characteristic of porous glasses consists of the appreciable overlap between the PSD functions obtained from the ABC and DBC isotherms. The DBC-PSD result depicts a mode value higher than the ABC-PSD outcome in all cases; this means that a pore-blocking phenomenon is taking place along the DBC.
Another interesting feature of all PSD distributions consists in pointing out that the DBC percolation peak protrudes above the ABC curve by about one third of its total area (compare the shaded peak area versus the crosshatched area in Figure
TEM photographs (Figure
TEM micrographs of (a) MG3-Cu and (b) MG3-Fe.
The TL response induced by an irradiation dose of 400 Gy in SiO2 glasses with impurities: MG2-Fe, MG3-Cu, and MG3-Fe, is presented in Figure
TL glow curves of SiO2 glasses with different doping concentrations irradiated using a 400 Gy dose.
Reproducibility of the TL glow of silica glasses after being expose 5 times to beta particles for 100 Gy dose each.
TL glow curves of MG2-Fe at different doses in the interval 100–800 Gy.
TL glow curves of MG3-Cu obtained by exposing to beta irradiation for different doses: 100–800 Gy.
TL glow curves of MG3-Fe at different doses in the interval 100–800 Gy.
Figure
Excitation and emission spectra of MG2-Fe, MG3-Cu, and MG3-Fe taken at room temperature.
The sol-gel process assisted by supercritical drying is an efficient method to fabricate highly porous monolithic silica materials. A variety of porous glasses of outstanding surface areas and mesopore volumes can be synthesized by the sol-gel procedure. Thermal treatment of precursory gels produces mesoporous substrates free of micropores and of well-defined pore volumes. The effect of doping cations consists in decreasing the pore sizes of the resultant glasses.
The authors would like to thank M. A. Lourdes Palma Tirado for technical support by TEM, M. en C. Alicia Del Real López for technical support by EDX analysis, M. en C. Carmen Peza Ledesma by help in textural properties measurement, and M. en C. Guillermo Vázquez Sánchez for the processing photographs.