Doped Silica Gel Research Articles

Study on the optical limiting mechanism of metallo-phthalocyanine/silica gel glass composites

We report here the study of the optical limiting mechanism of phthalocyanine(H 2Pc) and different kinds of metallo-phthalocyanine(MPc) in silica gel glass matrix. Prepared by sol-gel technique, the H 2Pc or MPc doped silica gel glass composites were transparent and homogeneous with the required regular shape. The optical limiting behaviors of the composites were measured at 532 nm with 8 ns pulses. The results show that among three samples of H 2Pc, NiPc or PbPc doped composites, PbPc has the most obviously optical limiting effect owing to the heavy atom effect. Besides, PbPc has better optical limiting properties in solid-state silica gel glass matrix than in liquid-state DMF solution. Fluorescent ZnTSPc or GaPcCl doped composites have little optical limiting effect. The reasons are discussed from the view of excited physical process and the implied mechanism is given.

Pressure-induced changes on the optical properties and microstructure of silica-gel matrices doped with rhodamine 6G

Sol–gel method and high-pressure technique were associated to produce silica compacts with low porosity and doped with rhodamine 6G (R6G). Acid catalyzed solutions of tetraethyl orthosilicate mixed with dye previously dissolved in ethanol were used for the synthesis of doped silica gel. The monolith obtained was comminuted and the powder was compacted between 3.0 and 7.7 GPa, at room temperature, using a toroidal-type high-pressure chamber. Excitation–emission fluorescence spectroscopy was used to investigate the optical properties of R6G embedded in the closed pores of the silica matrix. Measurements of surface areas and pore size distribution for the powders and compacts, using the N2 isotherms were performed, showing a great reduction in surface area and porosity, after compaction. The dye entrapped in the compacted silica maintains its optical properties similar to that in ethanolic solutions, meaning that the dye is dispersed in a molecular level. Additionally, these samples are optically transparent, hard and resistant to dye leaching and to chemical attacks, being a promising material to be used in optical devices.

EPR studies on the dynamics of the structure of copper (II) doped silica gels

Detailed X-band EPR studies on 1.0 mol% copper oxide doped silica gels, oven dried as well as calcined at various temperatures up to 1000 °C have been carried out in the temperature range 10.4–300 K. Spin Hamiltonian and line-width parameters are determined by computer simulation of the observed spectra. Important information derived on the dynamics of the porous structure of Cu-doped silica gel in the light of the EPR parameters is as follows: (i) Three kinds of Cu 2+ species (including one Cu-pair) are present in the 100 °C oven dried silica gel. (ii) Two kinds of slow rotating isolated Cu 2+ species (one Cu 2+ pair) occur in the 400 °C calcined sample at room temperature. The Cu-pair species agglomerate at low temperatures (⩽200 K) to form CuO clusters which remain undetected apparently because of their large line-width. (iii) Only one kind of Cu-species is detected in the 700 °C calcined silica glass at all measuring temperatures. The Cu 2+-pair/CuO cluster spectrum is absent. (iv) The isotropic EPR spectrum due to rapidly rotating Cu 2+-species occurring in very large pores of silica glass calcined at 800 °C has been obtained at room temperature. (v) Pore collapse starts at 900 °C, and results in the observance of a very broad EPR line (∼5000 Oe) for 900 and 1000 °C calcined samples. This shows that the pore collapse and the formation of superparamagnetic nano-sized CuO particles in the silica glass matrix take place simultaneously.

Optical limiting properties of peripherally modified palladium phthalocyanines doped silica gel glass

We report here the study of different peripherally modified palladium phthalocyanine (PdPc) encapsulated in silica gel glass obtained by sol–gel technique. The resulted composites were transparent and homogeneous with required optical limiting properties. Trapping effect was checked by UV/Vis absorption spectra and pore structure of the composites. Optical limiting properties were measured at 532 nm with 8 ns pulses. The results show that all the PdPcs have better optical limiting effect in composite than in solution, among which, PdPc-C has the best optical limiting effect due to its largest peripheral substituent.

Vanadia–silica xerogels: optical and structural properties

Vanadium doped silica gels were prepared from tetraethyl orthosilicate and three different inorganic vanadium precursors with formal oxidation states of V 3+, V 4+ and V 5+ respectively. Optical and EPR studies were carried out on the dried gels to understand the changes in the oxidation state and coordination of vanadium in the doped silica gel matrix. The observed optical and EPR results provide very strong evidence to establish that irrespective of the starting material, vanadium is stabilized as vanadyl ion in the gel monoliths. EPR studies on the powdered samples corroborate the optical data on the gel samples and confirmed that the stabilized vanadyl ion is situated in a distorted octahedral geometry in these silica gels.

Laser-induced changes in optical properties of H 4TPP 2+ doped silica gel prepared by sol–gel method

Optical absorption and photoluminescence of diacid tetraphenylprophyrine, H 4TPP 2+, species in silica gel glass prepared by the sol–gel (SG) method were measured in the 350–800 nm range. Under continuous (CW) laser excitation the characteristics of thermal lensing (TL) were measured at room temperature, under resonant and non-resonant conditions. Our results are analyzed based on a theoretical model for CW laser-induced thermal lensing, under a Gaussian-beam approximation. The effects of the power input and the response time for TL are explained in terms of the coupling of the H 4TPP 2+ molecules to the local vibrations, for resonant excitations. Also, the observation of laser-induced permanent changes in the refractive index of our glass sample under resonant and non-resonant excitations is reported and discussed.

An X-ray and Neutron Reflection Study of Water Penetration into Fluorocarbon Doped Silica Gel Films

Specular neutron and X-ray reflection have been used to investigate the penetration of water into silica gel films. The effect on water penetration of incorporating fluoroalkysilane (FAS) into the gel has also been measured. Gel films were deposited onto float glass substrates and exposed to either liquid water or an atmosphere of known relative humidity (between 0% and 100%). It was shown that, irrespective of FAS content, water penetrates throughout the gel film, and deeper into the glass substrate if the sample has prolonged contact with water. The X-ray reflection measurements have shown there are no changes in the structure of the gel film on glass on exposure to a humid atmosphere. This result is verified by the neutron reflection data and suggests that as water penetrates into the surface it occupies in the gel network. These voids can be filled or emptied by changing the relative humidity. The neutron reflectivity measurements have also revealed additional information about the distribution of the water. For the pure silica gel, with no added FAS, there is a water-rich layer (∼30 A thick) at the gel surface and a significant amount of water (volume fraction up to 18%) penetrated throughout the gel film. Gel films made with moderate amounts of FAS show no evidence for a water-rich layer on the surface. In fact, the water density is effectively zero within ∼20 A of the gel's surface. This suggests that the fluorinated materialformed a hydrophobic monolayer near the surface of the gel. For gels with much higher levels of FAS, the film surface becomes very rough and a detailed interpretation is not possible.

Spectroscopic studies of Cu2+ ions in sol-gel derived silica matrix

The Cu2+ ion doped silica gel matrices in monolithic shape were prepared by hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The absorption, transmittance and fluorescence spectra of the gel matrices heat treated at different temperatures were monitored. The loss of water and hydroxyl group from silica network changes the optical properties of the Cu2+ ions in the host, noted by the change in colour of monolith and spectral characteristics. The pronounced blue shift observed (700–900 nm to 600–850 nm) for the broad band of the absorption spectra of the samples heated up to 700°C is attributed to the ligand field splitting and partial removal of hydroxyl group from the silica matrices. The results indicate broadband filtering effects of the samples in the wavelength region 400–600 nm. Absorption and fluorescence spectra of the glass matrices heated to 1000°C confirms the conversion of Cu2+ ion to Cu+ ion.

Structural and Optical Studies of Nano-structure Monolithic Silica-Gel Derived Glasses Containing Nd3+

The sol-gel process was applied for the preparation of neodymium (Nd 3+ ) doped silica gel derived glasses. Two different procedures have been studied using two different precursor materials, Tetramethoxy-silane (TMOS) and Tetraethoxy-silane (TEOS), giving silica gel and silica doped with Nd 3+ of type I and type II, respectively. Both type materials were subjected to the same heattreatment temperature. The normal transmission was measured using Fourier Transform Infrared Spectrometer FTIR, Raman analysis and X-ray diffraction (XRD), were carried out to determine the crystal structure of the prepared samples. The prepared samples change from amorphous to ∝-crystoballite phase at higher temperature regardless the two type materials. The crystallite size was found to be as small as 10.4 nm at 1400 o C for pure sample type II. The thermally treated sample microstructures were investigated using scanning and transmission electron microscope. Also DTA and TGA Characteristic techniques were used. The photoluminescence PL measurements for pure sample (type I) heat-treated at 1400 o C shows bands of maximum in the blue green spectral region at 535 and 546 nm accompanied with band of weak intensity at 590 nm. Doped sample (type I) heat-treated at 1400 o C shows new bigger, sharper and intense bands at 875, 889 and 900 nm that were interpreted on the light of 4F3/2 to 4I9/2 transitions of Nd 3+ ion. PL spectra are found to be sensitive to defects such as impurities and E’ centers.

Photoluminescence of terbium-organic complex in aerosol–gel-derived planar waveguides

This work deals with the low-temperature preparation of optically active silica-based materials. Tb3+-doped silica monolithic gels were elaborated at 40 °C. Tb3+–SSA-doped SiO2–TiO2 thin films were deposited by an original sol–gel approach, the aerosol–gel process, and heat-treated at 150 °C. Organic complexation of terbium ions was used to improve the active properties of doped silica gels and thin-film samples. Spectroscopic characterisations are reported for these samples. Photoluminescence increase by a factor two was observed for complexation by sulphosalicylic acid.

Enhanced fluorescence from Eu 3+-doped silica gels by adsorbed CdS nanoparticles

Cadmium sulfide (CdS) semiconductor colloids are used, for the first time, in assessing the optical properties of a lanthanide ion, europium (Eu 3+)-doped silica gels prepared by a sol–gel method. Adsorption and size-quantization of CdS nanoclusters enable a greater enhancement of fluorescence from the aforementioned gels.

Preferred Formation of Coplanar Inclined Fluorescent J-Dimers in Rhodamine 101 Doped Silica Gels

Rhodamine 101 (R101) doped silica gels were prepared through the sol−gel process with dye concentrations ranging from 4.0 × 10-6 to 6.4 × 10-4 M. Excitation and emission fluorescence spectra and lifetime measurements were recorded for R101 doped silica gels in the whole range of concentrations. The photophysical behavior of the gel-glasses doped with high concentrations of R101 resembles that of fluorescent J-dimers in a coplanar inclined configuration. The formation of fluorescent J-dimers in a coplanar inclined rather than in an oblique configuration is discussed in terms of the molecular structure of R101.

Preparation and optical properties of ZnS-microcrystals deposited in silica gels

ZnO doped silica gels were synthesized by the hydrolysis and condensation of complex solution tetraethyl orthosilicate (TEOS) and zinc acetate (Zn(Ac) 2) and heating at 500°C in the air. For conversion to ZnS doped gels, they were exposed to H 2S gas at appropriate temperatures. The crystal size of ZnS determined by the width of X-ray diffraction pattern and direct transmission electron micrograph (TEM) grows from 1 to 4 nm with an increase of reaction temperature with H 2S gas. The edge energies of optical absorption shifted to higher energy side compared with that of bulk crystal and reciprocally increased in proportional to the square of the crystal size. The peak energy of fluorescence spectra also shifted to lower energy side with an increase of reaction temperature exposing to the H 2S gas. Thus, the quantum size effect could be found for ZnS microcrystals in the gels.

Rhodamine Fluorescent Dimers Adsorbed on the Porous Surface of Silica Gels

Rhodamine 6G (R6G)-doped silica gels were prepared through the sol−gel process using dye concentrations ranging from 4.00 × 10-6 to 6.40 × 10-4 M. The adsorption of the R6G on the porous surface of the silica gels determined the formation of R6G fluorescent J-dimers as concentration increased, in concordance with the behavior reported for other xanthene molecules as rhodamine B (RB) and rhodamine 110 (R110) also doped in silica gels. The photophysical study of the R6G fluorescent dimers doped in silica gels was done through the recording of the excitation and emission fluorescence spectra as well as the measurement of the lifetime values. The exciton theory was used for the elucidation of the adsorption angle (θ), the angle between the monomer units (α), and the separation distance between the molecules (R) of the R6G fluorescent dimers. The angle between the monomer units (α) and the separation distance between the molecules (R) were also determined for RB and R110 fluorescent dimers doped in silica gels.

Photoacoustic study of the co-fluorescence effect of lanthanide ternary complexes in solid states

Photoacoustic (PA) spectra of Eu(TTA) 3·Phen and Eu 0.8Gd 0.2(TTA) 3·Phen (TTA: thenoyltrifluoroacetone; Phen: phenanthroline) complex powders and their doped silica gels are reported. The PA absorption of the ligand of Eu(TTA) 3·Phen is stronger than that of Eu 0.8Gd 0.2(TTA) 3·Phen for complex powders. For the PA intensity of the complexes in silica gels, the reverse is true. This indicates that the addition of Gd 3+ ion changes the relaxation processes of the europium ternary complex and that the relaxation processes of ternary complexes are different in the two kinds of solid state. The changes of the fluorescence spectra turn out to be complementary to the PA spectra. Eu 0.8Gd 0.2(TTA) 3·Phen was prepared by coprecipitation, the luminescence of Eu 3+ is enhanced by an intermolecular energy transfer process. In the silica gel doped with Eu 0.8Gd 0.2(TTA) 3·Phen, complex molecules are trapped in the pores and isolated from each other. The fluorescence intensity of Eu 3+ decreases with the replacement of Eu 3+ by Gd 3+ ion.

Influence of adsorbed CdS nanoparticles on 5D 0→ 7F J emissions in Eu 3+-doped silica gel

In this paper, we report the enhanced fluorescence from Eu 3+-doped silica gels which were prepared by the sol–gel method and subsequently adsorbed with colloidal cadmium sulfide (CdS). Optical absorption, Raman, and photoluminescence studies were employed. The fluorescence intensities were compared for different gels with and without CdS particles by variation in the immersion time in CdS sol. The enhancement of fluorescence was observed for 24 h-immersed gel, dried at 50°C. Photoluminescence studies allowed a plausible mechanism that the enhanced fluorescence could be attributed to a surface electron-trap as an energy donor provided by CdS particle.

Remarkable Influence of Silver Islands on the Enhancement of Fluorescence from Eu3+ Ion-Doped Silica Gels

A simple methodology has been developed to enable a greater enhancement of fluorescence from europium (Eu3+)-doped silica gels containing adsorbed silver (Ag) islands. The procedure involves the preparation of Eu3+-doped silica gels by a sol−gel method and then immersion in a Ag sol for ca. 48 h to allow the adsorption of Ag islands on the pore surfaces of silica gels. UV−vis spectra show the characteristic surface plasmon resonance of the Ag at around 390 nm, and transmission electron microscopy (TEM) presents Ag islandlike particles on the surface of silica gels. The photoluminescence studies reveal the fact that the enhanced fluorescence originates from the local field enhancement around Eu3+ ions, caused by the electronic plasmon resonance of the Ag islands.

Electric double-layer capacitor using composites composed of phosphoric acid-doped silica gel and styrene–ethylene–butylene–styrene elastomer as a solid electrolyte

Solid state electric double-layer capacitors have been fabricated using a composite composed of H 3 PO 4 -doped silica gel and styrene–ethylene–butylene–styrene elastomer as the electrolyte. A composite hybridized with activated carbon powders (ACP) was the polarizable electrode. The cyclic voltammogram of the electric double-layer capacitor fabricated demonstrated that electric charge was stored in the electric double-layer at the interface between the polarizable electrode and the electrolyte. Resistance of the capacitor obtained from impedance plots in ambient air was smaller by three orders of magnitude than that of the capacitor in a dry Ar atmosphere. The drastic decrease in the resistance should be ascribed to the increase in the proton conductivity of the composite due to the water adsorption in H 3 PO 4 -doped silica gel in an ambient atmosphere. The value of capacitance of the capacitor in an ambient atmosphere was 10 F/(gram of total ACP), which was 10 times larger than that of the capacitor in a dry Ar atmosphere. The large capacitance of the capacitor in an ambient atmosphere is attributable to the formation of electric double-layer with large electric charge at the interface between the composite and ACP in the capacitor due to the water adsorption.

Optical properties of Pr3+ doped silica gel glasses obtained by sol–gel method

Thermally densified silica glasses doped with Pr3+ ions were prepared by sol–gel method. Three procedures of synthesis are described. Absorption, emission and excitation spectra of Pr3+ doped silica glasses obtained by sol–gel method are reported. We found on the basis of now presented results and those earlier reported that intensities of the absorption bands of Pr3+ change with concentration nonlinearly, especially in the range of 3H4→3P2; 3P0 and 1D2 transitions. This phenomenon could be caused by clustering, which leads to cooperative interactions. On the basis of the absorption spectra measurement, the energy level diagram was prepared which helps in the explanation of the emission properties of glasses synthesized by different techniques. The luminescence spectra were dependent on excitation wavelength. The spectra excited in high UV range exhibited, except emission in the red range corresponding to the 1D2→3H4 transition, a broad band emission with characteristic holes at the envelope corresponding to the 3P2, (3P1, 1I6) and 3P0 terms. Nature of this broad band emission is discussed. The observed holes result from reabsorption processes, which are controlled by concentration of active ions. Mechanism of their occurrence remains unknown. No emission from 3P0 level was observed for some kind of glasses, for another one the emission from 1D2 and 3P0 levels was detected.

Gelation of vanadium(V)oxide-triisopropoxide doped silica gels

Gelation of mixtures of vanadium(V)oxide-triisopropoxide, tetramethoxysilane, methanol and water has been studied. Catalyst type and concentration as well as vanadium content were varied over a wide range. Gels were also prepared with tetraethoxysilane, ethanol or isopropanol. For higher vanadium contents the use of ethanol or isopropanol as solvent caused precipitation while the use of methanol led to transparent sols. Due to the acidity of the vanadium oxide containing silica sol, gel times as long as four weeks were observed. Gel times are reduced if a basic catalyst is employed, although the content must exceed the vanadium content. Neutralization of the starting solution is closely correlated with a decolorization of the sol or gel. Aerogels were prepared using supercritical drying with respect to CO 2. UV/VIS/NIR-spectroscopy was employed to measure the optical transmission of vanadia-doped silica aerogels.