Template-free Sol Research Articles

Dark and UV-Enhanced Degradation of Dimethyl Methylphosphonate on Mesoporous CeO2 Aerogels

Our development of high surface-area mesoporous expressions of ceria (CeO2) is motivated by recent studies identifying CeO2 as one of the most reactive oxides for adsorbing and degrading toxic organophosphorus compounds, including chemical warfare simulants. We synthesize nanostructured CeO2 aerogels using a facile, scalable, and template-free sol–gel method to amplify the number of available and accessible surface sites for organophosphorus adsorption/degradation and then spectroscopically characterize the degradation of dimethyl methylphosphonate (DMMP) at the aerogel surface. The CeO2 aerogels retain high concentrations of surface-sited residual Cl from the chloride sol–gel precursors, which block DMMP binding sites and hinder the formation of surface hydroxyls (OH). We demonstrate that a simple alkaline soak removes surface-fouling Cl and creates a more OH-rich surface, thus unleashing the hydrolytic activity of the amplified CeO2 aerogel surfaces. Whereas the Cl-fouled CeO2 surface weakly and reversibly binds DMMP, the rinsed, OH-rich surface irreversibly binds DMMP and rapidly generates hydrolysis products. Exciting the CeO2 bandgap with UV light (390–400 nm) accelerates DMMP degradation and generates a higher proportion of mineralized POx products than the dark reaction. Our work marks the first-ever report of photodegradation of a chemical warfare simulant at CeO2 as well as the first report of mineralized POx formation at CeO2 at room temperature. The high surface-area CeO2 aerogels exhibit higher capacity than non-networked, nanoparticulate CeO2, thus highlighting the potential applicability of nanocrystalline CeO2 aerogels for protection against organophosphorus compounds.

Preparation of hydrophobic broadband antireflective SiO2 coating on flexible poly(methyl methacrylate) substrates

The double-layer broadband antireflective coatings is crucial for optical devices, e.g. plastic eyeglass lenses and the solar cell system. Among optical situation, the wide wavelength range of light is one of the most demanded. Here we designed a double-layer broadband antireflective (AR) coating with excellent average transmittance at visible region for poly (methyl methacrylate) (PMMA) using thin film design software. A template-free sol–gel process was developed to prepare the SiO2/PMHS coating with ultralow refractive index of 1.13 for top layer. By combining the colloidal silica and poly (methyl hydrogen) siloxane (PMHS), the coating with adjustable refractive index that vary between 1.13 and 1.37 was obtained. The formation mechanism of the ultralow refractive index was proposed. Meanwhile, using PMHS to partially substitute the hydrophilic groups in the coating, the hydrophobicity of the coatings was improved, and its static water contact angle was as high as 126°. Besides, the coating had excellent mechanical property and refractive index of 1.22–1.42 can be well prepared by combining base-catalyzed silica sol and acid-catalyzed silica sol together for bottom layer. Then, using the two raw materials base/acid -catalyzed mix SiO2 sol and the PMHS-SiO2 sol and optimizing the thickness, the prepared double-layer coatings with average transmittance of 99.30% in the 400nm to 800nm wavelength on PMMA substrate was successfully obtained. Those prepared hydrophobic broadband AR coatings represented excellent transmittance and had enormous potential application in the optical devices.

Template-free sol–gel synthesis of high surface area mesoporous silica based catalysts for esterification of di-carboxylic acids

High surface area mesoporous silica based catalysts have been prepared by a simple hydrolysis/sol–gel process without using any organic template and hydrothermal treatment. A controlled hydrolysis of ethyl silicate-40, an industrial bulk chemical, as a silica precursor, resulted in the formation of very high surface area (719 m 2 /g) mesoporous (pore size 67 Å and pore volume 1.19 cc/g) silica. The formation of mesoporous silica has been correlated with the polymeric nature of the ethyl silicate-40 silica precursor which on hydrolysis and further condensation forms long chain silica species which hinders the formation of a close condensed structure thus creating larger pores resulting in the formation of high surface mesoporous silica. Ethyl silicate-40 was used further for preparing a solid acid catalyst by supporting molybdenum oxide nanoparticles on mesoporous silica by a simple hydrolysis sol–gel synthesis procedure. The catalysts showed very high acidity as determined by NH 3 -TPD with the presence of Lewis as well as Brønsted acidity. These catalysts showed very high catalytic activity for esterification; a typical acid catalyzed organic transformation of various mono- and di-carboxylic acids with a range of alcohols. The in situ formed silicomolybdic acid heteropoly-anion species during the catalytic reactions were found to be catalytically active species for these reactions. Ethyl silicate-40, an industrial bulk silica precursor, has shown a good potential for its use as a silica precursor for the preparation of mesoporous silica based heterogeneous catalysts on a larger scale at a lower cost. Very high surface area (719 m 2 /g) mesoporous (pore size 67 Å and pore volume 1.19 cc/g) silica has been prepared by a simple sol–gel process using ethyl silicate-40 as a novel silca precursor without using any organic template and hydrothermal treatment. Molybdena supported on this high surface area silica has yielded a solid acid catalyst efficient for esterification of di-carboxylic acids with a range of alcohols.

Three-layer tri-wavelength broadband antireflective coatings built from refractive indices controlled silica thin films

A three-layer tri-wavelength broadband antireflective (AR) coating with 100 % transmittance simultaneously at 1064, 532 and 355 nm was designed using TFCalc™ thin film design software. The refractive indices for the bottom, middle and top layers of the designed AR coating are 1.33, 1.21 and 1.10, respectively. To realize the designed AR coating, a template-free sol–gel process was proposed to regulate the refractive index of silica film from 1.09 to 1.44 by simple hybridization. Silica films with refractive index of 1.21–1.44 were prepared by the hybridization of acid-catalyzed silica and base-catalyzed silica, while organically modified silicates films with refractive index of 1.09–1.21 were prepared by the hybridization of tetraethoxysilane, methyltriethoxysilane and hexamethyldisilazane under base-catalyzed condition. These two hybridization methods could conveniently control the porosity, and hence the refractive index of films. Finally, the three-layer tri-wavelength broadband AR coating, which is very important in high-power laser systems, was realized with the transmittance of 99.9, 99.8 and 99.4 % at 1064, 532 and 355 nm, respectively. A three-layer tri-wavelength broadband antireflection coating with nearly 100 % transmittance simultaneously at 1064, 532 and 355 nm is designed by TFCalc™ thin film design software and prepared by a simple sol–gel process. A template-free sol–gel process was proposed to prepare silica films with controlled and ultra-low refractive index.

SiO2–ORMOSIL double-layered broadband antireflective coating for high-power laser system

Broadband antireflective (AR) coating for high-power laser system has been realized by depositing double-layered SiO2–ORMOSIL thin films onto fused silica substrates. The refractive indices of bottom and top layers are 1.29 and 1.13, respectively. The top layer was composed of an ORMOSIL thin film which was prepared by a template-free sol–gel process by hybridization of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS). The hybridization of TEOS and DDS decreased gradually the refractive indices of ORMOSIL thin films from 1.21 to 1.13 as the molar ratio of DDS to TEOS increased from 0.0 to 0.8. The bottom layer was prepared by hybridization of acid-catalyzed and base-catalyzed silica sols. Finally, the double-layer broadband AR coating with transmittance of 100.0 and 99.9 % at 1064 and 532 nm, respectively, was realized. The laser-induced damage threshold (LIDT) of double-layer double-wavelength AR coating is 48.8 J cm−2. This double-layer SiO2–ORMOSIL AR coating possessed excellent transmittance at simultaneously 1064 and 532 nm, good AR stability and high LIDT, and will find important application in high-power laser system.

Template-free sol–gel preparation of nanoporous ORMOSIL films with adjustable refractive index

A nanoporous organically modified silicate (ORMOSIL) film with adjustable refractive index from 1.21 to 1.10 was prepared at room temperature via a template-free sol–gel method, using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as co-precursors and hexamethylisilazane (HMDS) as surface modifier. Heating the ORMOSIL film, the refractive index of the ORMOSIL film can be further decreased to as low as 1.07. This adjustable refractive index thin film can find potential application in optical and microelectronic fields.

Synthesis of high specific surface area titania monolith by addition of poly (ethylene oxide)

The macro-mesoporously bimodal titania monolith with high surface area was synthesised via a template-free sol–gel route using titanium isopropoxide as titania source and poly(ethylene oxide) as an improver of the pore structure and specific surface area. The effects of poly(ethylene oxide) on the shape and specific surface area of the titania monolith were determined. The titania monolith was characterised by scanning electron microscope, Brunauer–Emmett–Teller method and ultraviolet diffuse reflectance spectra measurements. The obtained titania monolith shows excellent surface properties with macropore of 2 µm, mean pore size of 15·2 nm, Brunauer–Emmett–Teller surface area of 110·3 m2 g−1, cumulative pore volume of 0·32 mL g−1 and a narrow pore size distribution after 350°C calcination.

Template-free synthesis of hierarchical layer-stacking CeO2 nanostructure with enhanced catalytic oxidation activity

A template-free sol–gel chelating method was developed to synthesize a hierarchical layer-stacking CeO2 nanostructure. The morphologies of the as-prepared CeO2 were spindle-like and flower-like particles which were both assembled by many porous nano-flakes. The basic unit, nano-flake, was organized by numerous nanoparticles with a size of 10nm while the porous structure was formed. The as-prepared CeO2 exhibited a high surface area of 171.6m2g−1, high pore volume of 0.31cm−3g−1 and narrow pore size of 4.0nm. Compared to the commercial CeO2, this novel structure showed much higher activity for CO oxidation which can be ascribed to its higher surface area, small crystal size and novel hierarchical porous structure.

Synthesis of porous ZnO/TiO2 thin films with superhydrophilicity and photocatalytic activity via a template-free sol–gel method

Superhydrophilicity of TiO2 thin film under natural conditions is of great importance for antifogging and self-cleaning applications of glass products. Transparent superhydrophilic porous ZnO/TiO2 composite films were prepared on glass substrates using a template-free sol–gel method. The results indicated that the porous ZnO (10%)/TiO2 composite film possesses superhydrophilicity with water contact angle less than 5° without UV irradiation. Surface chemical composition of the films was characterized by X-ray photoelectron spectroscopy (XPS). The morphologies of the porous films were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The porous morphology, crystal structure and surface chemical composition were significantly affected by ZnO addition. The natural superhydrophilic performance was attributed to the synergistic effect of the porous structure and surface hydroxyl groups. Photocatalytic activities of the ZnO/TiO2 composites were also evaluated using methyl orange (MO) as a model pollutant, with 78.1% of MO degraded by ZnO (10%)/TiO2, which was higher than 49.3% obtained from pure TiO2.

α-Fe2O3 /(PVA + PEG) Nanocomposite films; synthesis, optical, and dielectric characterizations

Hematite (α-Fe2O3) nanorods with an average diameter of 40 nm were prepared using a template-free sol–gel method. These nanorods then mixed with polyvinyl alcohol (PVA)/polyethylene glycol (PEG) blend at concentrations of 0.0, 0.5, and 1.0 wt.%. The transmittance percentage (T%) of the films showed a decrease from 80.26 to 33.24 %. The direct optical band gap also decreased from 5.28 to 4.83 eV whereas the refractive index significantly increased with increasing the hematite content. The dielectric measurements were performed in the temperature range 303–413 K and frequency range 30 kHz–3.0 MHz. According to the temperature dependence of the dielectric constant (e’), αa-relaxation peaks observed in all films and assigned to the micro-Brownian motion of the polymer blend chains. The behavior of the ac conductivity, σac (f), of the nanocomposite films indicated that the homogenous distribution of α-Fe2O3 nanorods allows the formation of conductive three-dimensional networks throughout the nanocomposite film. Also, indicated that the correlated barrier hopping is the most suitable conduction mechanism.

Formation of Pyrene Excimers in Mesoporous Ormosil Thin Films for Visual Detection of Nitro-explosives

We report the preparation of mesoporous thin films with bright pyrene excimer emission and their application in visual and rapid detection of nitroaromatic explosive vapors. The fluorescent films were produced by physically encapsulating pyrene molecules in the organically modified silica (ormosil) networks which were prepared via a facile template-free sol-gel method. Formation and stability of pyrene excimer emission were investigated in both porous and nonporous ormosil thin films. Excimer emission was significantly brighter and excimer formation ability was more stable in porous films compared to nonporous films. Rapid and selective quenching was observed in the excimer emission against vapors of nitroaromatic molecules; trinitrotoluene (TNT), dinitrotoluene (DNT), and nitrobenzene (NB). Fluorescence quenching of the films can be easily observed under UV light, enabling the naked-eye detection of nitro-explosives. Furthermore, excimer emission signal can be recovered after quenching and the films can be reused at least five times.

Polymerization-induced phase separation in the preparation of macroporous TiO2/SiO2 thin films

Macroporous TiO 2 /SiO 2 (TS) composite thin films deposited on glass substrates were successfully prepared through a versatile template-free sol–gel approach based on the hydrolysis and polycondensation of tetrabutyl titanate and tetraethyl orthosilicate. A theoretic explanation for the formation of porous morphology was discussed in relation to the polymerization-induced phase separation and the concurrent sol–gel transition. The mechanism of phase separation was attributed to the spinodal decomposition. The field emission scanning electron microscope (FESEM) results indicate that the surface morphologies of TS thin films strongly depend on the amount of SiO 2 sols (mole fraction) and aging time before spin-coating. A systemic method of long-range pore size tailoring for macroporous TS composite thin film was achieved by adjusting mole ratio of TiO 2 to SiO 2 and the aging time.

The effect of metal ion dopants (V3+, Cr3+, Fe3+, Mn2+, Ce3+) and their concentration on the morphology and the texture of doped γ-alumina

Transition-metal-doped γ-alumina with various dopant concentrations was prepared by straightforward template-free sol–gel method. The presence of dopant metal ions, generally, enhanced the gel formation and their behavior was dependent on the nature of the metal ion and its concentration. Certain ions, especially Fe3+, resulted in rapid formation of a transparent gel upon hydrolysis. The prepared doped γ-alumina powders weakly crystalline at low dopant ion concentrations, 2%, and became completely amorphous at a concentration of 10%. The morphology of the particles was also dependent on the dopant concentration. While dopant concentration of 2% resulted in nano-particles with significant amount of inter-particle mesopores, 10% concentration led to significant aggregation into larger particles. The prepared doped γ-Al2O3 as well as the undoped γ-Al2O3 showed high surface areas and pore volumes which were largely dependent on the nature of the dopant metal ions and on their concentrations. While composites with low dopant concentration, 2%, exhibited surface areas and pore volumes comparable to those of undoped γ-alumina, a considerable decrease was associated with higher concentrations. The changes in textural properties were referred to the evident enhanced sintering associated with high dopant concentrations.

Preparation of Titania Monolith Column and Application in Determination of Benzoic Acid by HILIC

The titania monolith column has been synthesized through a template-free sol–gel route, and a simple and reliable method for the determination of benzoic acid by hydrophilic interaction liquid chromatography using the prepared titania monolith has been developed. The influences of acetonitrile, acetate buffer and buffer pH on the retention of benzoic acid were investigated. Benzoic acid in carbonated drinks and fruit beverages samples were determined within 5 min and quantitative analysis was carried out by external standard method with a correlation coefficient (R2) of 0.9984. The relative standard deviation was 0.91 % and the recovery ranged from 92.5 to 101.3 %. The proposed method is suitable for the analysis of benzoic acid in beverage samples.

3D porous LiFePO4/graphene hybrid cathodes with enhanced performance for Li-ion batteries

Three-dimensional porous self-assembled LiFePO4/graphene (LFP/G) composite was successfully fabricated using a facile template-free sol–gel approach. Graphene nanosheets were incorporated into the porous hierarchical network homogenously, which greatly enhances the electrical conductivity and efficient use of the LiFePO4 (LFP), leading to an outstanding electrochemical performance of the hybrid cathodes. The obtained LFP/G composite has a reversible capacity of 146mAhg−1 at 17mAg−1 after 100 cycles, which is more than 1.4 times higher than that of porous LFP (104mAhg−1). Moreover, the porous LFP/G composite also exhibits a desirable tolerance to varied charge/discharge current densities.

A template-free synthesis and structural characterization of hierarchically nano-structured lithium-titanium-oxide films

A template-free sol–gel method for the synthesis of hierarchically nano-structured lithium-titanium-oxide films is proposed involving a p-type doping of titanium isopropoxide sols with lithium. The structures of the films are analyzed by scanning electron microscopy, X-ray powder diffraction and Raman spectroscopy. Different phases can be accessed by controlling the annealing temperature. Supersaturated anatase (Ti, Li)O2 is formed at 400 °C, whereas anatase and/or rutile interspersed with Li2TiO3 and Li4Ti5O12 are observed between 450 °C and 800 °C. The nanostructure formation is explained by a dissolution–crystallization–self-assembly mechanism.

Mesoporous alumina prepared from waste aluminum cans and used as catalytic support for toluene oxidation

Mesoporous aluminas with high specific surface area were synthesized successfully via a template-free sol–gel method from aluminum isopropoxide (AIP) and aluminum hydroxide produced from waste aluminum cans. After calcinations at 500°C, the synthesized mesoporous aluminas were in gamma phase. The BET surface areas of the mesoporous aluminas obtained from synthesized AIP (427–556m2·g−1) were higher than that prepared from aluminum hydroxide (421m2·g−1). The synthesized alumina products have worm-like pores without long-range ordered framework. Liquid phase oxidation of toluene catalyzed by NiO supported on these mesoporous alumina has been carried out at room temperature.

Catalytic conversion of chloromethane to methanol and dimethyl ether over mesoporous γ-alumina

Mesoporous γ-alumina, mp-γ-Al 2O 3, possessing surface area around 385 m 2/g and total pore volume of 2.0 cm 3/g was prepared via template-free sol–gel synthesis. The catalytic activity of the prepared alumina for the conversion of chloromethane to dimethyl ether, DME, in the presence of water or methanol was studied in the temperature range of 170–450 °C employing FTIR spectroscopy. In the absence of water and methanol, the fresh surface of mp-γ-Al 2O 3 showed 100% conversion of chloromethane to DME at temperatures between 250 and 350 °C. However, rapid deactivation of the catalyst resulted in a sharp decrease in the conversion to < 5% within a few minutes of reaction. The catalytic activity was noticeably enhanced by adding water vapor to the gas feed resulting in higher conversions to DME and methanol. The catalytic activity and DME selectivity were further enhanced in the presence of methanol instead of water. In the temperature range of 200–300 °C, complete conversions were obtained at the beginning of reactions before they declined to values between 31 and 45% depending on the reaction temperature. It was proposed that the surface hydroxyl groups are the active sites where chloromethane molecules dissociatively adsorb forming adsorbed methoxy ion intermediates. The adsorption of molecular methanol regenerates the surface hydroxyl groups and enhances the formation of adsorbed methoxy ions on the surface which react further with chloromethane or methanol molecules to produce more DME.

Titanium-doped alumina for catalytic dehydration of methanol to dimethyl ether at relatively low temperatures

Mesoporous Al–Ti oxide composites with molar %Ti of 3, 5, 10, and 20 as well as pure γ-alumina were prepared using a template-free sol–gel method in the absence of a catalyst. The prepared composites were characterized by powder XRD, FTIR spectroscopy and N 2 adsorption for BET surface area and porosity measurements. The composites and the pure alumina possessed relatively high surface areas, 350–410 m 2/g, and high porosities after calcination at 500 °C. FTIR spectroscopy was employed to study the products of the catalytic dehydration of methanol to dimethyl ether, DME, over the prepared catalysts at reaction temperatures between 180 and 300 °C. Compared with pure γ-alumina, the Ti-modified alumina with %Ti < 10 showed higher catalytic activity in the methanol dehydration and better selectivity to DME. Composites with %Ti of 3 and 5 showed the highest activity at relatively lower temperatures than the other catalysts where they showed their highest activity at 190 and 200 °C, respectively. The activity of all studied catalysts slightly decreased as the temperature was raised to 300 °C and dropped considerably when the temperature was decreased to 180 °C. However, the activity of Al–Ti-3 dropped only slightly at both temperatures. The selectivity to DME was dependent on the reaction temperature where 100% DME selectivity was obtained at temperatures ⩽220 °C and as the temperature was raised to 300 °C, some CH 4 and CO 2 formed on the account of DME.

A template-free sol–gel technique for controlled growth of ZnO nanorod arrays

The growth of ZnO nanorod arrays via a template-free sol–gel process was investigated. The nanorod is single-crystalline wurtzite structure with [0 0 0 1] growth direction determined by the transmission electron microscope. The aligned ZnO arrays were obtained directly on the glass substrates by adjusting the temperatures and the withdrawal speeds, without seed-layer or template assistant. A thicker oriented ZnO nanorod arrays was obtained at proper experimental conditions by adding dip-coating layers. Room temperature photoluminescence spectrum exhibits an intensive UV emission with a weak broad green emission as well as a blue double-peak emission located at 451 and 468 nm, respectively. Further investigation results show that the difference in the alignment of nanorods ascribes to the different orientations of the nanoparticles-packed film formed prior to nanorods on the substrate. Well ordered ZnO nanorods are formed from this film with good c-axis orientation. Our study is expected to pave a way for direct growth of oriented nanorods by low-cost solution approaches.