Synthesis and sol–gel assembly of nanophosphors

Some recent works made in our group on inorganic nanophosphors are briefly reviewed in this paper. We first present the synthesis of highly concentrated semiconductor quantum dot colloids allowing the extension of the well-known oxide sol–gel process to chalcogenide compounds. Secondly, we show the synthesis and the chemical functionalization of lanthanide-doped insulator nanoparticles. In particular, the annealing process of these particles at high temperature leads to highly bright nanocrystals, which can be used as biological luminescent labels or for integration in transparent luminescent coatings. Finally, we consider luminescent transition metal clusters, which combine the inorganic structure of nanoparticles with the monodispersity and the easy functionalization of the organic molecules. Emphasis is put on the original thermochromic luminescence properties of copper iodide clusters trapped in siloxane-based films.     

A review of contamination-resistant antireflective sol–gel coatings

Sol–gel derived silica antireflective (AR) coatings have been widely used as the optical components for high peak power laser systems because of their excellent optical properties and high laser-induced damage thresholds. However, the sol–gel derived coatings have a high surface area that is more susceptible to be contaminated by absorption of trace amounts of water vapor and other volatile organic compounds from the environment. In this paper, the major approaches to fabricate contamination resistant sol–gel derived silica AR coatings have been extensively reviewed. Different approaches, including solution-phase and vapor-phase silanization, ammonia–water vapor treatment and fluorine modification have been discussed. The optical properties and laser-induced damage thresholds of modified coatings have also been evaluated. The improved sol–gel AR coatings have been shown to possess superior contamination resistance to work in vacuum systems compare to the traditional sol–gel AR coatings.     

Non-aqueous sol–gel preparation of carbon-supported nickel nanoparticles

This work addresses the novel non-aqueous sol–gel process preparation of carbon-supported nickel nanoparticles. In the sol–gel process, ethanol, nickel nitrate or nickel (П) acetylacetonate, and citric acid were used as solvent, source of metallic element, and chelating agent, respectively. Hexadecylamine (HDA), oleic acid and oleylamine were used as surfactants. The calcination process was performed under protecting Ar or N₂ flowing. Carbon supported nickel nanoparticles can be prepared by this sol–gel process. Moreover, no grain growth occurs in a temperature range of 200 K, meaning that the grain size of the nickel nanoparticles can be controlled in this sol–gel process. The nickel nanoparticles can display typical superparamagnetic behavior at room temperature when HDA has been used. This novel method is expected to have wide applications in the field of metallic nanoparticles.     

Synthesis and characterization of sol–gel alumina nanofibers

Abstract Alumina nanofibers of high aspect ratio with surface area of >300 m² g⁻¹ has been prepared successfully in bulk quantities by the sol–gel method. The synthesis parameters including the binary water–alcohol solvent system to aluminium isopropoxide ratio, pH, type of solvent and aging temperature affect the uniformity and formation of nanofibers. It is proposed that alumina nanofibers were formed by the curling of the nanosheets upon condensation after the hydrolysis. The phase evolution of alumina nanofibers from pseudoboehmite to α phase has been shown by XRD and FTIR. ²⁷Al NMR investigations show that the Al atoms are six and four coordinated. The morphology of the alumina nanofibers does not change much as the calcination temperature was increased. In addition, the average pore size increases and the BET surface area decreases as a function of calcination temperature. The thermal behavior of alumina nanofibers was investigated by TGA. Graphical Abstract      

Lactic acid based sol–gel process of Ag nanoparticles and crystalline phase control of Ni particles in aqueous sol–gel process

Nickel and silver particles were prepared by using sol–gel auto-combustion method under N₂ atmosphere where lactic acid was applied as chelating agent. The synthesis of nickel particles was carried out at various pH conditions (2–7), resulting in the face-centered-cubic or hexagonal-close-packed crystalline nickel phase. The morphology and structure of synthesized nickel particles and silver nanoparticles were characterized by X-ray diffraction, transmission electron microscope, energy dispersive X-Ray spectroscopy and differential scanning calorimetry-thermogravimetric analysis. The results show that the spherical Ag nanoparticles with diameters in the range of 18–27 nm and narrow size distribution can be obtained by this sol–gel process.     

Electron beam writing of epoxy based sol–gel materials

We report the use of an epoxy based hybrid sol–gel material as negative resist for electron beam lithography (EBL). The matrix has been prepared starting from 3-glycidoxypropyltrimethoxysilane as specific organic–inorganic precursor and the synthesis has been strictly controlled in order to preserve the epoxy ring and to obtain a proper inorganic cross-linking degree. The film has been exposed to an electron beam, inducing the polymerization of the organic part and generating the film hardening. Preliminary results of a resolution test on the synthesized epoxy based sol–gel material, performed with electron beam lithography, are presented. Structures below 300 nm were achieved. The direct nanopatterning of this hybrid sol–gel system simplify the nanofabrication process and can be exploited in the realization of photonic devices. A demonstration has been carried out doping the hybrid films with commercial Rhodamine 6G and reproducing an already tested laser structure.     

Synthesis of LiNiPO4 via citrate sol–gel route

Journal of Sol-Gel Science and Technology - In this paper, LiNiPO4 powders were prepared by sol gel method using citric acid as chelating agent. The article investigated the effect of annealing...     

Preparation and properties of sol–gel waterborne polyurethane adhesive

Waterborne polyurethane (WBPU) sol–gel adhesives were prepared through a prepolymer process followed by a sol–gel reaction of (3-aminopropyl)triethoxysilane (APTES). The terminal amine group of APTES reacted with the NCO group of the prepolymer, and the ethoxy group created Si–O–Si branching by hydrolysis and condensation reactions in water at the dispersion step. Water swelling (%), tensile strength and Young’s modulus of the synthesized WBPU sol–gel adhesives were improved by increasing APTES content. Synthesized WBPU sol–gel adhesives were used for bonding nylon fabrics. A significant improvement in adhesive strength was recorded, and the potential for good adhesive strength under water at moderately high temperature (up to 75 °C) was observed with 6.84 mol% APTES in WBPU sol–gel adhesives.     

Sensing abilities of materials prepared by sol–gel technology

The advantages of the sol–gel technology are undoubtedly simplicity and versatility. It enables to obtain for example oxides in the form of layers, powders, monoliths or fibers. These materials can be successfully applied for sensing purposes due to their properties such as transparency, porosity, and high surface areas. In this article, the basis of operation of mainly optical and semiconductor sensors are presented. A brief overview of various kinds of sensors is submitted. The utility of optical fibers and planar waveguides in these systems is discussed. The paper contains also some results obtained by the authors in the field of thin film-based sensors.     

Improved biocompatibility of protein encapsulation in sol–gel materials

By using the fluorescent dye 6-propionyl-2-(N,N-dimethylamino) naphthalene (PRODAN) to monitor methanol generated during tetramethyl orthosilicate polymerization we have optimised the encapsulation of protein in silica sol–gel monoliths with respect to completion of hydrolysis and distillation in order to remove methanol such that protein can be added without denaturation. A minimum of 24 h at +4 °C was found to be required before hydrolysis is complete and 3–5 min of vacuum distillation at 50 °C and 300 mbar needed to remove methanol before the gel is formed. The biocompatibility of a tetramethyl orthosilicate sol–gel monolith was demonstrated by preserving the trimer protein allophycocyanin (APC) in its native form for up to 500 h. This obviates the previously essential requirement of covalently binding the trimer together in order to prevent dissociation into monomers and has enabled observation of native APC trimer in a sol–gel pore for the first time down to the single molecule level using combined fluorescence spectroscopy and confocal microscopy. The higher stability afforded by the protocol we describe could impact on the application of sol–gel materials to single-molecule studies of wider bearing such as protein folding and aggregation.     

Synthesis of novel Schiff base doped sol–gel silicas

Sulfonamide Schiff bases were doped uniformly in silica sol–gels prepared from liquid precursors by a fast and easy way at room temperature and processed to form xerogels. Schiff bases are efficient chelating agents, bioactive and catalytically active compounds. The structures of the newly synthesized Schiff base doped xerogels were elucidated by their physical (morphology, surface area, porosity), spectral (FTIR) and analytical (CHNSO/Si) data. The powder X-ray diffraction studies were carried out to confirm the formation of single phase. Characterization confirmed that Schiff base molecules are entrapped inside the pores as well as physically bound onto the silica surface. All Schiff base doped xerogels are stable mesoporous materials showing hydrophilic properties. Loadings of Schiff bases from 0.10 to 0.23 g/g of xerogel were obtained resulting amorphous materials. The doping of Schiff bases with xerogel caused change in surface area, pore volume and pore diameter of xerogel without damaging the main framework of siliceous skeleton. Morphology and colour of xerogel was also changed after doping. The entrapment of Schiff bases in xerogel caused increase in their decomposition temperatures. The final Schiff base doped xerogels show remarkable thermal stability.     

Review of mullite synthesis routes by sol–gel method

The sol–gel method for the mullite synthesis is reviewed, with particular emphasis on the characterization of monophasic and diphasic gels at low, intermediate and high temperatures and the factors that influence the hydrolysis and condensation rate of the sol–gel process, which in turn determine the properties of the final material. A wide range of studies about mullite precursors synthesized via sol–gel is discussed here.     

Synthesis of polyferromethylsiloxane sorbents using a sol–gel method

Trivalent iron complexes which could be easily converted into materials are formed by the reaction of FeCl₃ with K₃[O₃SiMe] in a highly concentrated aqueous alkaline solution. The presence of the liquid glass as an additive and decreasing the pH results in solutions that give rise to homogeneous gels. Polycondensation proceeds very rapidly in the higher pH range (viz. pH 9–10) and substantially slower in an acidic medium (pH 2–3). Xerogels were obtained having microporous structure after treatment of polyferromethylsiloxane gels obtained from acidic medium or mesoporous structure when obtained from alkaline medium.     

Click approaches in sol–gel chemistry

The combination of the copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction with sol–gel processing enables the versatile preparation of sol–gel materials under different shapes with targeted functionalities through a diversity-oriented approach. In this account, the development of the CuAAC reaction under anhydrous conditions for the synthesis of sol–gel precursors and for the assembling of magnetic nanoparticles on self-assembled monolayers is related, as well as the use of the classical CuAAC methodologies for the functionalization of mesoporous silica nanoparticles and microdots arrays. Coupling CuAAC and Sol–Gel will result in simplified preparations of multifunctional materials with controlled morphologies.     

Utilization of sol–gel ceramics for the immobilization of living microorganisms

Two possible methods are described for using sol–gel technology to immobilize living microorganisms, either embedding the cells within thin silica layers, or using the technique of freeze-gelation to immobilize microorganisms within molded ceramic parts. The preparation and structure of both biocer variants are outlined, and examples are given for the activity and storage stability of embedded microorganisms. Silica layers were used to immobilize various bacteria and algae. Survival rates after storage are given for Pseudomonas fluorescence bacteria and for green algae such as H. pluvialis, C. vulgaris, B. braunii and N. limnetica embedded within thin transparent silica layers. The bioactivity of bacteria immobilized in freeze-gelation ceramics was investigated by monitoring glucose consumption for P. fluorescens NCIMB 11764, and phenol degradation for Rhodococcus ruber.     

High water solubility and sol–gel transition behavior of titania nanoparticles obtained by an in situ functionalization sol–gel process

Herein, addition reaction occurred between glycidol and partially hydrolyzed Ti⁴⁺ complexes provides a opportunity to obtain dry anatase nanopowder with high redispersity in water. This property is considered to be originated from the two OH groups located in the two ends of glycidol resulted chlorinated propandiol molecules. In aqueous solution, the two OH groups are respectively connected with particle surface and external free water by the formation of hydrogen bonds, resulting in high water redispersity of nanoparticles. Due to the much less amount of chlorinated propandiol molecules than adsorbed molecule water on particle, the wide space between organic molecules facilitates the mutual physical surface touch of individual particles to form hydrogen bond between them. A novel property is then obtained for surface modified titania nanoparticles, which is the gelation of redispered nanoparticles in aqueous solution.     

On the sol–gel preparation of different tungstates and molybdates

The preparation and characterization of the M′–M′′–O nitrate–tartrate (M′ = Ca, Ba, Gd and M′ = W, Mo) precursor gels synthesized by simple, inexpensive, and environmentally benign aqueous sol–gel method is reported. The obtained gels were studied by thermal (TG/DSC) analysis. TG/DSC measurements revealed the possible decomposition pathway of synthesized M′–M′′–O nitrate–tartrate gels. For the synthesis of different metal tungstates and molybdates, the precursor gels were calcined at different temperatures (650, 800, and 900 °C). According to the X-ray diffraction (XRD) analysis data, the crystalline compounds CaMo_1-x W _x O₄ doped with Ce³⁺ ions, BaMo_1-x W _x O₄ doped with Eu³⁺ ions and Gd₂Mo₃O₁₂ were obtained from nitrate–tartrate gels annealed at 650–900 °C temperatures. The XRD data confirmed that the fully crystalline single-phase powellite, scheelite, or Gd₂(MoO₄)₃ structures were formed already at 650 °C. Therefore, the suggested sol–gel method based on the complexation of metal ions with tartaric acid is suitable for the preparation of mixed tungstates–molybdates at relatively low temperature in comparison with solid-state synthesis.     

Synthesis of MCM-48 from silatrane via sol–gel process

High-quality cubic MCM-48 is successfully synthesized using a new silica source known as silatrane and cetyltrimethylammonium bromide (CTAB) as the structure-directing agent via sol–gel process. The effects of synthesis parameters, viz. crystallization temperature, crystallization time, surfactant concentration, quantity of NaOH, and silica source, on the product structure are investigated. The synthesized samples are characterized using X-ray diffractometer (XRD), N₂ adsorption–desorption isotherms, and electron microscopy. Optimally, this product is synthesized from samples crystallized at 140°C for 16 h with a CTAB/SiO₂ ratio of 0.3 and NaOH/SiO₂ ratio of 0.5. The XRD result exhibits a well-resolved pattern, corresponding to the Ia3d space group of MCM-48. The BET surface area of this product is as high as 1,300 m²/g with a narrow pore-size distribution of 2.86 nm. The scanning electron microscopic (SEM) images also show the truncated octahedral shape and well-ordered pore system of MCM-48 particles.     

Electrochemical oxidation of aniline in a silica sol–gel matrix

Electrochemical oxidation of aniline encapsulated in a silica solid electrolyte prepared by a sol–gel process yielded products that were dependent on the pore size. An acid-catalyzed process that used tetramethyl orthosilicate as the precursor and aniline as a dopant yielded the silica. When the aging time was limited to one day so that a mesoporous solid was obtained, the potentiodynamic oxidation of aniline at a carbon fiber electrode resulted in the formation of polyaniline. With aging times of 3–5 days, microporous silica was obtained. In this electrolyte, the formation of dimers and other oligomers was observed by cyclic voltammetry. Evidence for these products was the presence of a quasi-reversible redox couple at 0.2 V vs Ag/AgCl that was previously related to oligomeric aniline by Raman spectroscopy. The results supported the hypothesis that the pore structure of sol–gel electrolytes can influence the pathways of electrode reactions therein.