Doped Silica Xerogels Research Articles

Random Laser and Replica Symmetry Breaking in SiO2-Rhodamine 6G xerogel powder

Random laser (RL) based on Rhodamine 6G (Rh6G) doped silica xerogel, fabricated by a conventional sol-gel (SG) synthesis, was observed around 590 nm, in a large band typical from dye RLs. Different from other previous works, where the xerogel is just impregnated or infiltrated with a dye solution, here the Rh6G was added during the SG synthesis. The obtained material was grinded using a mortar and a pestle, and the resulting powder was carefully packed in a sample holder and pumped at 532 nm using a 6 ns pulsed laser. We used spectral and images measurements to perform statistical analysis and describe experimentally the Parisi replica breaking symmetry (RSB) phenomenon in a complex system. These results show that RSB obtained from image is a promising method for RL characterization. Indeed, by calculating the RSB maps, we demonstrate that the RL emission is not a homogenous process, depending on the scattering and gain properties of different regions.

Silica xerogel and iron doped silica xerogel synthesis in presence of drying control chemical additives

Silica xerogels were prepared by sol-gel method in presence of glycerol and oxalic acid as drying control chemical additivites (DCCAs). They were characterized with TGA, XRD, TEM, nitrogen adsorption/desorption isotherm, XPS, ICP, FTIR and UV–vis DR analyses. TGA results showed that the use of DCCAs provided a controlled release of water, thereby preventing structure collapse which was supported by nitrogen adsorption technique. DCCAs caused an increase in microporosity of xerogels, while some decreases in surface area and total pore volume values were occurred. Amorphous silica structure and silica density decreasing effect of DCCAs were observed from XRD patterns. While XPS, XRD and UV–vis DR analyses showed the presence of iron species in Fe2O3 form, they were found to settle as nanoparticles from TEM images. ICP results showed that iron incorporated to the xerogel structure as percent of 83. Iron loading caused significant increases in all textural properties. FTIR spectra exhibited bands representing the silica structure and DCCA-silica interaction.

Lanthanum-doped silica xerogels for the removal of fluorides from waters

The objective of this study was to determine the influence of different operational variables on fluoride (F−) removal from waters using lanthanum (La)-doped silica xerogels and the mechanisms involved in this process. Accordingly, four xerogels were synthesized, one acting as blank (X-B), two doped with LaCl3 and dried at different temperatures (X-LaCl and X-LaCl-M), and a fourth doped with La2O3 (X-LaO). The results show that fluorides are only removed when La-doped xerogels are utilized. In addition, X-LaCl yielded the highest adsorption capacity, removing 28.44% of the initial fluoride concentration at a solution pH of 7. Chemical characterization of materials confirmed that fluoride removal from waters is due to the precipitation of LaF3 on the surface of La-doped xerogels. The presence of dissolved organic matter on the aqueous solution also reduce the removal capacity of La xerogels. Finally, analysis of the influence of solution pH revealed that the adsorption capacity of all xerogels was highest at a solution pH of 7.

Influence of activator concentration on green-emitting Tb3+-doped materials derived by sol-gel method

In this study, visible-emitting Tb3+-doped silica xerogels and β-PbF2-based glass-ceramic materials with different concentration of optically active dopant have been investigated. After annealing process at 350°C, the formation of β-PbF2 crystal phase dispersed within silica host of studied samples was examined based on X-ray diffraction measurements. Luminescence characteristics were carried out by monitoring an excitation and emission spectra as well as analyzing luminescence decay kinetics from the 5D4 excited state of Tb3+ ions. Photoluminescence measurements have been performed using three different excitation wavelengths from the UV range: λexc=351nm, λexc=369nm and λexc=378nm, according to the 7F6→5L9, 7F6→5L10 and 7F6→5D3 optical transitions of Tb3+ions, respectively. In result, the well-resolved 5D4→7FJ (J =3–6) bands have been registered. For all prepared samples, a luminescence intensity of the 5D4→7F5 green band located at λem=543nm is significantly more prominent than that of the 5D4→7F6 blue peak with maximum at λem=488nm and the green-to-blue (G/B) ratio was calculated and analyzed. It was observed that the G/B-factor values were slightly increased with Tb3+ dopant content; moreover, the G/B values were strongly correlated with the symmetry of local environment around Tb3+ ions and significantly increased after heat-treatment. Additionally, the prolongation of Tb3+ luminescence lifetimes from the 5D4 excited state were observed for β-PbF2 glass-ceramic samples compared to precursor xerogel materials. Generally, obtained results clearly indicated the beneficial influence of heat-treatment process as well as increasing concentration of Tb3+ ions (up to 8.74 mass %) on luminescence properties of studied silica sol-gel phosphors.

Study on the structural and electrocatalytic properties of Ba2+- and Eu3+-doped silica xerogels as sensory platforms

This work reports on the sol–gel synthesis of barium- and europium-doped silica xerogel and its use as an electrocatalytic sensor.

Characterization of Metal-Doped Methylated Microporous Silica for Molecular Separations

Novel silica xerogels are prepared and developed by sol-gel method in the present study. The preparation involves cobalt-doping within the organic templated silica matrices, where methyltriethoxysilane (MTES), which contains methyl groups as a covalently bonded organic template is used. The synthesis and surface properties of cobalt-doped methylated microporous silica xerogels with different MTES and cobalt content are revealed by surface and microstructural techniques, such as TGA, FTIR, X-ray and N2 adsorption measurements. The doping process enhances the thermal stability of the silica xerogels up to ~ 560 °C in oxidizing atmosphere. Besides, this process has no significant effect on the incorporation of the organic template within the silica matrix. As result of the promoted densification of the xerogels either by increasing MTES content and heat treatment, there is structural change of the silica xerogels such as decreasing the micropore volume and broadening of the pore size distribution. Heat treatment and increasing the cobalt oxide content from 5 to 10% weight ratio resulted in samples with approximately the same structural parameters. This suggests that the cobalt particles are homogeneously dispersed in the silica matrix. The novel silica xerogels exhibit trend toward micropores formation suggesting that these doped silica xerogels can be precursor materials for molecular sieve silica membranes applications. Two silica membranes, hydrophobic and cobalt-doped hydrophobic, are prepared and their performance is examined by the study of transport of He, H2 and N2. Preliminary results show that the microporous structure obtained in the unsupported cobalt-doped hydrophobic material are preserved after coating inside the tubular support.

Strategies for reducing dye aggregation in luminescent host-guest systems: Rhodamine 6G incorporated in new mesoporous sol-gel hosts

Aiming at the design of new luminescent host-guest materials with minimized aggregation effects, two classes of sol-gel derived mesoporous materials were explored as hosts for Rhodamine 6G (Rh6G) dye: The first consists of pure (SiO2) and phenyl-modified silica (Ph0.17SiO1.915) xerogels, prepared via sol-gel reaction using an ionic liquid as catalytic agent. The second consists of mesoporous sodium aluminosilicate glasses with Si to Al ratio in the range of 6 ≤ Si/Al ≤ 9. Characterization through high resolution solid state NMR proved the successful obtention of the designed host matrices. Following Rh6G-loading in various concentrations, the resulting materials were characterized by their luminescence and excitation spectra, excited state lifetimes, and quantum yields. The dye doped silica xerogels presented high quantum yield values (up to 87%), with no substantial decrease in efficiency with increasing dye concentration. At suitable Rh6G contents, all the final materials presented laser action under 532 nm excitation from a Q-switched frequency doubled Nd:YAG laser. The phenyl silicate sample presents the highest laser photostability with a half-life of 6560 pulses, under 2 mJ/pulse pump power, and 10 Hz repetition rate. The laser experiments provided further insights on the photodegradation mechanisms of such organic species.

A silica xerogel thin film based fluorescent sensor for pentachlorophenol rapid trace detection

Abstract We report a fluorescence approach to rapid detection of pentachlorophenol (PCP) by using 9-fluorenylmethanol (FMOC-OH) doped silica xerogel thin film (denoted as FMOC-OH@silica xerogel thin film). The FMOC-OH@silica xerogel thin film exhibits not only a very shorter discrimination time, but also with a high detection sensitivity to PCP with a lower limit down to 10 nM based on fluorescence quenching effect and a broad detection selectivity among diverse PCP analogues. The mechanism for fluorescence changes is mainly attributed to the fluorescence resonance energy transfer (FRET) between the analyte PCP and the fluorophore FMOC-OH in silica xerogel thin film, and the different sensitivities and selectivities to other PCP analogues are ascribed to different FRET efficiencies, pathways and fluorescence quantum yields. The FMOC-OH @silica xerogel thin film has potentials in detecting and monitoring trace PCP in the environment.

Silica xerogel films hybridized with carbon nanotubes by single step sol–gel processing

Synthesis of multi-walled carbon nanotubes (MWCNTs) doped silica xerogel films was reported in this work. A crucial step of introducing MWCNTs was achieved by functionalizing them by acid treatment to form stable and homogenous SiO2/MWCNTs sol. Scanning electron microscopy showed spherical particles in honeycomb network structure for undoped xerogel films whereas dispersion and wrapping of MWCNTs in silica matrix was observed for MWCNTs doped films. Various bond formations during the sol–gel process and surface modification were confirmed using Fourier transform infra-red and detailed study on the chemical bonding state of the films was carried out using X-ray photoelectron spectroscopy. Nanoindentation studies showed that the mechanical properties of MWCNTs doped xerogel film increase dramatically: higher modulus (E=2.127±0.095GPa) and hardness (H=0.035±0.017GPa) values than those of pristine xerogel film (E=0.234±0.058GPa, H=0.01±0.003GPa).

处理温度及CdS量子点对Eu3+掺杂SiO2干凝胶光致发光性质的影响

Silica xerogels doped with Eu3+ ions and co-doped with Eu3+/CdS nanoparticles were prepared through a two-step hydrolysis process.The effect of annealing temperature on photoluminescence properties of Eu3+-doped silica xerogel was investigated.The results showed that the photoluminescence intensity of Eu3+-doped silica xerogels was significantly dependent on the annealing temperature,which was closely related to the amount of the defect in xerogel network.The emission intensity increased with the annealing temperature from 40 ℃ to 300 ℃.But when the temperature exceeded 300 ℃,the intensity decreased with the annealing temperature.The study of the photoluminescence of co-doped xerogels showed that the intrinsic emissions due to 5D0→7FJ transitions of Eu3+ were observed,and the intensity was sensitive to the surface states of CdS particles and of the porous silica gel.

Eu3+ doped silica xerogel luminescent layer having antireflection and spectrum modifying properties suitable for solar cell applications

A spectrum modifying glassy luminescent layer with antireflection properties has been prepared with Eu3+ ions embedded in silica gel for solar cell applications. Preparation of such matrix by sol–gel process and change in luminescence properties in transforming from amorphous to crystalline phase are described. Luminescent Eu3+ species in amorphous silica matrix show intense red emission at 614 nm by absorbing UV/blue light. Amorphous environment with reduced symmetry for Eu3+ ions results in an unprecedented short decay time of 145 μs for 5D0–7F2 electric dipole transition. In the crystalline monoclinic phase of Eu2O3 nanoparticles, concentration quenching of Eu3+ species reduces luminescence output.

Preparation and thermal properties of samarium doped silica xerogels

Abstract Samarium doped silica xerogels with different content of Sm 3+ up to 0.5 Sm/Si are prepared using a sol–gel method based on acid-catalysed hydrolysation of tetraethylorthosilicate (TEOS) and gelation. The prepared xerogels are investigated by SEM, X-ray diffraction, TG/FTIR, differential scanning calorimetry (DSC) and thermogravimetry (TG). It is proved that triclinic microcrystallites of Sm(NO 3 ) 3 ·6H 2 O are formed at high Sm contents. Detailed thermal analysis up to 500 °C demonstrates a strong effect of Sm content on the thermal properties of doped xerogels. A peculiar sharp endotherm is observed in the vicinity of 80 °C which position, unexpectedly, shifts to higher temperatures by decreasing of the heating rate. Other two wide endotherms are observed depending on both Sm amount and heating rate. Two different activation energies related to these endotherms are evaluated for the lower- and higher temperature effects: E a = 38 kJ/mol, and E a = 210 kJ/mol, respectively. They characterize the corresponding processes of xerogels dehydratation and chemical decomposition of Sm(NO 3 ) 3 ·6H 2 O. This finding is also in accordance with TG/FTIR analysis of Sm-doped silica xerogels.

Photoluminescence and FTIR studies of pure and rare earth doped silica xerogels and aerogels

The luminescence properties of silica xerogels and aerogels have been studied using PL spectroscopy. Both silica xerogels and aerogels exhibit photoluminescence in the visible region when excited with UV radiation. The luminescence observed in xerogels and aerogels is attributed due to defect centers. Silica aerogels exhibit better photoluminescence than xerogels due to the increase in defect sites. The incorporation of rare earth ions (La3+, Nd3+and Sm3+) enhanced the luminescence of both silica xerogels and aerogels. La3+ shows least and Nd3+ shows maximum enhancement. Increase in the concentration of Nd3+ resulted in an increase in the luminescence intensity. The change in the environment of the defect centers due to the incorporation of rare earth ions and the superposition of 4f-f transitions of rare earths are considered as the reasons for the luminescence enhancement.

Materials doping through sol–gel chemistry: a little something can make a big difference

Several examples of sol–gel preparation of doped materials are taken to illustrate the various situations where the doping elements are responsible for the main function of the material or govern its structure. Other examples are used to illustrate that sometimes unexpected effects can be observed like structural modification and the appearance of new properties. Rare earth doped scintillators demonstrate higher homogeneity for materials prepared via sol–gel chemistry when compared with classical solid state reaction. The XRD study of rare earth doped orthoborates shows that doping can affect the vaterite to calcite phase transition observed in these compounds. A Raman spectroscopic study has been performed on doped silica xerogels and it has been shown that doping ions can modify greatly the densification process in these amorphous materials. Finally, it has been evidenced that sol–gel chemistry allows the preparation of bioactive ceramics with enhanced properties. In particular Zn-doped HAP with anti inflammatory properties has been prepared and Sr-doped bioactive glasses have demonstrated superior in-vitro bioactivity as evidenced by PIXE-RBS study.

The nonlinear optical, magnetic, and Mössbauer spectral properties of some iron(III) doped silica xerogels

Iron(III) species dispersed in silica have been synthesized with a sol-gel process. The iron(III) was introduced as the acetylacetonate complex into a solution of tetraethoxysilane to yield, after evaporative drying, pellets or monoliths. Two gels were dried very slowly over a period of five months in order to prepare a defect free monolith useful for nonlinear optical studies. Z-scan experimental studies on the resulting, transparent, monolithic, doped solid revealed an optical Kerr effect, a third order nonlinear optical phenomenon showing a linear dependence of the refractive index on the irradiance with a nonlinear refractive index, n2, of −1.95 × 10−11 cm2/W. Magnetic susceptibility studies between 4.2 and 295 K revealed paramagnetic behavior with a Curie constant of 4.433 (emu/mol)K and a Weiss temperature of −7.1 K. Magnetization studies at 5 K and at applied fields of up to 4 T and Mossbauer spectral studies between 4.2 and 295 K revealed a 50:50 mixture of paramagnetic species and nanoparticles with an average particle radius of 1.3 ± 0.2 nm. A blocking temperature of 70 K and a magnetic anisotropy energy of 2.4 × 105 J/m3 are derived from the Mossbauer spectra.

Photoluminescence Properties of Silica Xerogels Co‐doped with Eu3+ Ions and CdS Nanoparticles

Silica xerogels doped with Eu3+ ions and co‐doped with Eu3+ ions and CdS nanoparticles were prepared using a two‐step hydrolysis process. The effect of temperature on photoluminescence properties of Eu3+‐doped silica xerogel was investigated. The results showed that the photoluminescence of Eu3+‐doped silica xerogel was significantly dependent on the temperature of heat treatment. The study of the photoluminescence of co‐doped xerogels showed that the defect emission of silica was weakened due to competition among defects, CdS nanoparticles, and Eu3+ ions.

Decontamination of Gaseous Acetaldehyde over CoOx-Loaded SiO2 Xerogels under Ambient, Dark Conditions

A series of CoO(x)-doped silica xerogels with various Co(2+) loadings (Co/Si = 0, 1, 2, 4, 6, and 10 mol %) has been prepared. All xerogels exhibit large (800-1050 m(2)/g) surface areas. Narrow pore size distributions with pore size maxima around 3 nm are characteristic for Co/Si = 1, 2, 4, 6, 10 samples. As-prepared CoO(x)/SiO(2) xerogels show high catalytic activity in the air oxidation of gaseous acetaldehyde at room temperature. Carbon dioxide and trace amounts of methane are the only products detected in the gas phase. Acetic acid, a less volatile product, resides on the surface of the xerogels but can slowly desorb. The formation of CO(2) begins after an induction period. The beginning of CO(2) production coincides with the conversion of Co(2+) incorporated in the SiO(2) framework into Co(3+). Thermogravimetry/gas chromatography/mass spectrometry analysis, UV-vis and FTIR spectroscopies, as well as kinetic measurements are employed for CoO(x)/SiO(2) catalyst characterization. A possible mechanism of the reaction is discussed.

Effect of catalysts and co-precursors on the photoluminiscence of malachite green dye doped TMOS silica xerogels

Malachite green (MG) laser dye doped silica matrix was prepared by sol–gel processing by keeping the molar ratio of tetramethylorthosilicate [TMOS,Si(OCH 3) 4]:methanol:catalytic water:methanolic MG constant at 1:4:5:7 × 10 −4:3 × 10 −6, respectively and varying the catalyst. The effect of acidic and basic catalysts, MG/TMOS molar ratio and co-precursors on the optical properties of the MG doped silica xerogels has been studied. Colourless transparent samples were obtained with strong acids, [hydrochloric (HCl)], [nitric acid (HNO 3)], strong base [sodiumhydroxide (NaOH)] and strong acid–weak base [ammoniumhydroxide (NH 4OH)] mixture and whereas turbid, opaque and light blue coloured samples were obtained with weak base. However, transparent, monolithic and blue coloured MG doped silica xerogels with longitivity of 6 months were obtained with weak acids [acetic acid (CH 3COOH, ACT), citricacid (C 6H 8O 7·H 2O, CTA)]. The MG/TMOS molar ratio was varied from 3 × 10 −8 to 3 × 10 −5 (10 −6 to 10 −3 M of MG) and the absorbance and luminiscence of the MG doped silica xerogels increased with increase in the MG/TMOS molar ratio to 1 × 10 −5 and remained constant for further increase of MG/TMOS molar ratio. Various co-precursors used such as methyltrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES), phenyltrimethoxysilane (PTMS), phenyltriethoxysilane (PTES), dimethyldodecyl chlorosilane (DDCS) and dimethyloctadecylmethoxysilane (DOMS) and the absorbance and luminescence of the MG doped silica xerogels increased with addition of MTMS, ETMS, MTES, ETES, PTMS and PTES but decreased with DDCS and DOMS.

Optical limiting effects of multi-walled carbon nanotubes suspension and silica xerogel composite

Nanostructured composite of multi-walled carbon nanotubes (MWNTs) doped silica xerogel matrix was prepared by sol–gel technique. The successful encapsulation of MWNTs was checked by linear transmittance spectra and scanning electrical microscope images which show that MWNTs were well dispersed in the matrix. The optical limiting responses of MWNTs suspension and xerogel composite were observed at both 532 and 1064 nm with nanosecond laser pulses and shown to be good optical limiters. The optical limiting properties of MWNTs are maintained or even improved after being introduced into solid state matrix. The implying mechanism of nonlinear absorption other than the presently accepted nonlinear scattering is proposed and discussed.

Dimerization of metallophthalocyanines in inorganic matrix

Metallophthalocyanine (MPc) and its composites have excellent photo-electricity properties. Here we report the study of MPc doped silica xerogel matrix obtained by sol–gel technique. Because the issue of dimerization is vital in determining the optical properties of dye doped sol–gel materials and MPc offers an excellent opportunity to characterize dimers from monomers because of their identified absorption bands, we studied the dimerization of different kinds of MPc doped silica composites using electronic absorption spectra. Changes in the absorption spectra were correlated with various stages of the sol–gel process. The results show that in the composite MPc molecules concurrence in forms of monomer and dimer, and dimerization is bad during the latter stages of drying because of the rapid decrease of remaining solvent and its ethanol/water ratio. To overcome this problem, a kind of dissociating agent Cremophor EL is added in order to protect the Zinc tetrasulfo-phthalocyanine molecules from dimerization. In the other hand, a newly developed in situ synthesis technique was successfully introduced to synthesize MPc during the sol–gel process. It is found that the in situ synthesized MPc only exist in the form of monomer in the composites.