Transparent Mesoporous Nanocomposite Films for Self‐Cleaning Applications

A versatile approach is studied for the elaboration of TiO₂ based photocatalytic coatings for self‐cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO₂ colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol–gel silica condensation around self‐organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO₂ particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO₂ loading (up to 50 %). Studies of photodegradation kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum‐yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the organic molecules and the surface of the TiO₂ crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.     

Template‐Assisted Synthesis of Mesoporous Magnetic Nanocomposite Particles

A new concept is proposed to synthesize mesoporous magnetic nanocomposite particles of great scientific and technological importance. Mesoporous silica coatings were created on micrometer‐sized magnetite (Fe₃O₄) particles using cetyltrimethylammonium chloride micelles as molecular templates. The characterization by transmission electron microscopy (TEM), nitrogen adsorption–desorption, diffuse‐reflectance Fourier‐transform infrared spectroscopy, and zeta‐potential measurements confirmed the deposition of mesoporous silica thin layers on the magnetite particles. The synthesized particles showed a drastic increase in specific surface area with an average pore size of 2.5 nm. The coating material showed a negligible effect on the saturation magnetization of the original particles that were fully protected by silica coatings. The synthesized mesoporous magnetic nanocomposite particles have a wide range of applications in toxin removal, waste remediation, catalysis, reactive sorbents, and biological cell separations.     

Shape‐ and Orientation‐Controlled Gold Nanoparticles Formed within Mesoporous Silica Nanofibers

Mesoporous silica nanofibers with longitudinal pore channels are synthesized in high yields using cetyltrimethylammonium bromide as the structure‐directing agent in hydrobromic acid solutions. These nanofibers are used as templates to prepare gold nanoparticles along the fiber axis. For the gold‐precursor‐loaded nanofibers that are not completely dried, nearly spherical gold nanoparticles are produced by hydrogen reduction. As the reduction temperature is lowered, the size of the gold nanoparticles decreases and the number density greatly increases, resulting in surface plasmon coupling between neighboring gold nanoparticles. For the gold‐precursor‐loaded nanofibers that undergo an additional drying process, ellipsoidal gold nanoparticles are obtained, with their major axes oriented along the direction of the pore channels. The major axes of ellipsoidal gold nanoparticles can be controlled to be oriented either parallel or perpendicular to the fiber axis by use of nanofibers with either longitudinal or circular pore channels, respectively. These gold‐nanoparticle‐embedded nanofibers can be expected to find interesting applications in the area of photonics and optoelectronics.     

Template‐Free Fabrication and Enhanced Photocatalytic Activity of Hierarchical Macro‐/Mesoporous Titania

Hierarchical macro‐/mesoporous titania is prepared without the addition of templates or auxiliary additives at room temperature by the simple dropwise addition of tetrabutyl titanate to pure water, and then calcined at various temperatures. The products are characterized by X‐ray diffraction, N₂‐adsorption–desorption analysis, scanning electron microscopy, and the corresponding photocatalytic activity is evaluated by measuring the photocatalytic oxidation of acetone in air. The results reveal that hierarchical macro‐/mesoporous structures of titania can spontaneously form by self‐assembly in alkoxide–water solutions in the absence of organic templates or auxiliary additives. The calcination temperature has a strong effect on the structures and photocatalytic activity of the prepared titania. At 300 °C, the calcined sample shows the highest photocatalytic activity. At 400 and 500 °C, the photocatalytic activity slightly decreases. When the calcination temperature is higher than 500 °C, the photocatalytic activity greatly decreases because of the destruction of the hierarchical macro‐/mesoporous structure of the titania and the drastic decrease of specific surface area. The hierarchically macro‐/mesostructured titania network with open and accessible pores is well‐preserved after calcination at 500 °C, indicating especially high thermal stability. The macroporous channel structures are even preserved after calcination at 800 °C. This hierarchical macro‐/mesostructured titania is significant because of its potential applications in photocatalysis, catalysis, solar‐cell, separation, and purification processes.     

Solventless Acid‐Free Synthesis of Mesostructured Titania: Nanovessels for Metal Complexes and Metal Nanoclusters

A new and highly reproducible method to obtain mesostructured titania materials is introduced in this contribution. The mesostructured titania is obtained by employing self‐assembled structures of non‐ionic alkyl‐poly(ethylene oxide) surfactants as templates. The materials are produced without additional solvents such as alcohols, or even water. Only the titanium(IV ) ethoxide and the surfactant (C₁₂EO₁₀) are needed. Water, in the form of that attached to the surfactant and from the atmosphere, induces growth of titania nanoclusters in the synthesis sol. It is indicated that these nanoclusters interact with the surfactant EO‐head groups to form a new titanotropic amphiphile. The new amphiphiles self‐assemble into titanium nanocluster–surfactant hybrid lyotropic phases, which are transformed to the final mesostructured materials by further condensation of the titania network. The titania materials can be obtained also with noble‐metal particles immobilized in the mesostructured framework. It is seen that when different metal salts are used as the metal precursors, different interactions with the titania walls are found. The materials are characterized by X‐ray diffraction (XRD), polarization optical microscopy (POM), transmission electron microscopy (TEM), UV‐vis spectroscopy, and micro‐Raman analysis.     

Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly

Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Highly Organized Mesoporous TiO2 Films with Controlled Crystallinity: A Li‐Insertion Study

A study of electrochemical Li insertion combined with structural and textural analysis enabled the identification and quantification of individual crystalline and amorphous phases in mesoporous TiO₂ films prepared by the evaporation‐induced self‐assembly procedure. It was found that the properties of the amphiphilic block copolymers used as templates, namely those of a novel poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) polymer (KLE) and commercial Pluronic P123 (HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H), decisively influence the physicochemical properties of the resulting films. The KLE‐templated films possess a 3D cubic mesoporous structure and are practically amorphous when calcined at temperatures below 450 °C, but treatment at 550–700 °C provides a pure‐phase (anatase), fully crystalline material with intact mesoporous architecture. The electrochemically determined fraction of crystalline anatase increases from 85 to 100 % for films calcined at 550 and 700 °C, respectively. In contrast, the films prepared using Pluronic P123, which also show a 3D cubic pore arrangement, exhibit almost 50 % crystallinity even at a calcination temperature of 400 °C, and their transformation into a fully crystalline material is accompanied by collapse of the mesoporous texture. Therefore, our study revealed the significance of using suitable block‐copolymer templates for the generation of mesoporous metal oxide films. Coupling of both electrochemical and X‐ray diffraction methods has shown to be highly advisable for the correct interpretation of structure properties, in particular the crystallinity, of such sol–gel derived films.     

Nanocrystalline Metal Oxides from the Injection of Metal Oxide Sols in Coordinating Solutions: Synthesis,Characterization, Thermal Stabilization,Device Processing,and Gas‐Sensing Properties

Metal oxide (SnO₂, TiO₂, In₂O₃, ZnO) sols are prepared by various sol–gel processes in such a way as to hinder the condensation reactions. The obtained sols are injected at 160 °C into a solution of tetradecene and dodecylamine, and kept under heating for different periods of time. Depending on the starting sol, variously crystallized oxide nanoparticles are obtained, whose phase compositions and chemical structure have been studied by X‐ray diffraction (XRD) and Fourier transform IR spectroscopy. The elimination of the organic residuals has been carried out by thermal treatment, and the thermal evolution of the nanoparticles has been studied by thermal analyses and Raman spectroscopy. High‐resolution transmission electron microscopy studies coupled with XRD measurements show that the thermal treatment does not markedly affect the particle size, which remains in the nanometer‐sized regime (from 3.5 to 8.5 nm, depending on the system), except in the case of ZnO. The thermally purified and stabilized powders, drop‐coated onto alumina substrates with pre‐deposited electrical contacts, have been tested as gas‐sensing devices, displaying outstanding sensing properties even at room temperature.     

Highly Dispersed Mixed Zirconia and Hafnia Nanoparticles in a Silica Matrix: First Example of a ZrO2–HfO2–SiO2 Ternary Oxide System

ZrO₂ and HfO₂ nanoparticles are homogeneously dispersed in SiO₂ matrices (supported film and bulk powders) by copolymerization of two oxozirconium and oxohafnium clusters (M₄O₂(OMc)₁₂, M = Zr, Hf; OMc = OC(O)–C(CH₃)?CH₂) with (methacryloxypropyl)trimethoxysilane (MAPTMS, (CH2?C(CH₃)C(O)O)–(CH₂)₃Si(OCH₃)₃). After calcination (at a temperature ≥800 °C), a silica matrix with homogeneously distributed MO₂ nanocrystallites is obtained. This route yields a spatially homogeneous dispersion of the metal precursors inside the silica matrix, which is maintained during calcination. The composition of the films and the powders is studied before and after calcination by using Fourier transform infrared (FTIR) analysis, X‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS). The local environment of the metal atoms in one of the calcined samples is investigated by using X‐ray Absorption Fine Structure (XAFS) spectroscopy. Through X‐ray diffraction (XRD) the crystallization of Hf and Zr oxides is seen at temperatures higher than those expected for the pure oxides, and transmission electron microscopy (TEM) shows the presence of well‐distributed and isolated crystalline oxide nanoparticles (5–10 nm).     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Gold‐Nanoparticle‐Doped TiO2 Semiconductor Thin Films: Optical Characterization

A simple procedure for creating titania sol–gel‐based semiconductor thin films is described. Gold nanoparticles are doped homogeneously into the precursor mixture and the particles are homogeneously distributed in the resultant films when prepared using spin‐coating. The effects of particle loading and annealing temperature on the optical properties of the resultant films are characterized. Ellipsometry, X‐ray diffraction, atomic force microscopy, and surface plasmon spectroscopy are used to monitor the crystallization and porosity changes during film synthesis.     

Size and Shape Dependence of Organo‐Interconnected Silsesquioxanes through Hydrolysis‐Condensation Reaction Conditions: Nanotubes,Spheres and Films

A tris‐silylated urea‐based compound, 1,3,5‐tris(triethoxysilylpropylureido)benzene has been synthesized. Hybrids materials with different shapes (films, spheres and tubes) are obtained upon base‐catalyzed hydrolytic condensation of this compound. A self‐templating process is demonstrated using electron microscopy (SEM and TEM) and X‐ray diffraction (SAXS and WAXS) techniques. We show the formation of nanometer size hollow tubes according to the size of the self‐templating crystalline precursor. The possibility for this self‐templating to occur is due to a favored dissolution of the self‐assembled molecules of the precursor in a preferential direction parallel to the cross‐section of the crystals.     

Synthesis and Assembly of Gold Nanoparticles in Quasi‐Linear Lysine–Keggin‐Ion Colloidal Particles

The formation of fiber‐like colloidal particles of the amino acid lysine complexed with Keggin ions is demonstrated. The lysine–phosphotungstic acid (PTA) colloidal particles act as excellent templates for the synthesis and assembly of gold nanoparticles wherein the lysine‐PTA complex acts as a UV‐switchable reducing agent for gold ions. This novel bio‐organic–inorganic template shows excellent potential as a regulated nanoreactor for application in programmed nanoparticle synthesis and assembly in a single step.     

Wavelength‐Dependent Photosensitivity in a Germanium‐Doped Sol–Gel Hybrid Material for Direct Photopatterning

Photosensitivity, as evident in permanent changes in refractive index and volume upon light exposure, is observed in a germanium‐doped methacrylate hybrid material (hybrimer) and found to depend on the wavelength of the UV light. Exposure to short‐wavelength UV illumination (220–260 nm) results in very high photosensitivity with changes in refractive index (Δn ≈ 0.0164) and film thickness (Δt ≈ –40 %) that are mainly a result of photopolymerization and Ge‐related densification. In contrast, the hybrimer is hardly photosensitive to light in the long UV‐wavelength range (350–390 nm). Direct photopatterning of a single circle on the hybrimer film creates a concave lens‐like topography upon illumination with UV light of short wavelength and a convex lens‐like one upon illumination with UV light of long wavelength.     

A Facile Sol–Gel Strategy for the Synthesis of Rod‐Shaped Nanocrystalline High‐Surface‐Area Lanthanum Phosphate Powders and Nanocoatings

Synthesis of rod‐shaped nanocrystalline lanthanum phosphate with an average length of 40 nm even after calcination at 400 °C has been realized through a room‐temperature aqueous sol–gel process. The sol is characterized by particle‐size, zeta‐potential, and viscosity measurements. Gelation of the sol is induced by ammonia. The lanthanum phosphate phase‐formation process is followed by thermal, Fourier‐transform IR, and X‐ray diffraction analysis. Transmission electron microscopy shows that the sol and gel particles have a rod‐shaped morphology and comparable particle sizes. Using the Scherrer equation a crystallite size of 11 nm is obtained for the gel powder calcined at 400 °C and Brunauer–Emmett–Teller (BET) nitrogen‐adsorption analysis showed a high specific surface area of 100 m² g^–1. Ammonia temperature‐programmed desorption measurements show that the density of Lewis acid sites is four times higher than ever reported in the case of lanthanum phosphates. The catalytic activity of the above sample is demonstrated by using it as a Lewis‐acid catalyst in an acetal‐formation reaction with a very good yield of 85 %. The sol is used to develop nanocoatings on a glass surface and the morphology of the coatings is investigated using atomic force microscopy and scanning electron microscopy. The microstructure of the coating confirmed the rod‐shaped nature of the sol particles. The coating was uniform with a thickness of about 55 nm.     

3D Hexagonal (R‐3m) Mesostructured Nanocrystalline Titania Thin Films: Synthesis and Characterization

A straightforward and reproducible synthesis of crack‐free large‐area thin films of 3D hexagonal (R‐3m) mesostructured nanocrystalline titania (meso‐nc‐TiO₂) using a Pluronic triblock copolymer (P123)/1‐butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high‐resolution scanning electron microscopy and scanning transmission electron microscopy), grazing‐incidence small‐angle X‐ray scattering, in situ high‐temperature X‐ray diffraction, and variable‐angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 °C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 °C. The accessible surface area of 3D hexagonal meso‐nc‐TiO₂ estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso‐nc‐TiO₂ at the same annealing temperature.     

Luminescence and Amplified Stimulated Emission in CdSe–ZnS‐Nanocrystal‐Doped TiO2 and ZrO2 Waveguides

A reproducible route for the preparation of high‐quality CdSe–ZnS‐doped titania and zirconia waveguides is presented. The optical properties of the resultant composite materials are found to be sensitive to the semiconducting properties of the host matrix. Titania‐based composites are seen to be inherently photounstable because of photoelectron injection into the bulk matrix and subsequent nanocrystal (NC) oxidation. In comparison, zirconia composites are significantly more robust with high photoluminescence (PL) retained for annealing temperatures up to 300 °C. Both titania and zirconia composite waveguides exhibit amplified stimulated emission (ASE); however only zirconia‐based waveguides exhibit long‐term photostability (loss of less than 30 % ASE intensity after more than 40 min continuous excitation). We conclude that the low electron affinity of zirconia and its inherent high refractive index makes it an ideal candidate for NC‐based optical waveguides.     

Designed Self‐Assembled β‐Sheet Peptide Fibrils as Templates for Silica Nanotubes

Sol–gel condensation of tetraethoxysilane in the presence of designed self‐assembled β‐sheet peptide fibril templates, followed by template extraction, yields hollow silica nanotubes. The nanotubes are hundreds of nanometers long and possess a central pore of ～ 3.5 nm, determined by the fibril template diameter. The effects of synthesis conditions have been investigated and the resultant silica materials characterized by various techniques. Silica nanostructures with various morphologies have been produced previously using supramolecular organic assemblies as templates. Hollow nano‐ or microtubes, which may have applications in separations, catalysis, nano‐optics, and ‐electronics have been of particular interest. Peptide‐based templates are especially interesting because of their relevance to biological silica microstructure formation. The new fibrillar peptide templates described here have the advantages of prescribed diameter, twist pitch, and handedness, which should impart chirality on the resulting silica nanotubes, providing control of the internal surface architecture by appropriate peptide design.     

Oligothia Dendrimers for the Formation of Gold Nanoparticles

The synthesis and characterization of oligothia dendrimers and their use for the formation of gold nanoparticles is described. The role played by these dendrimers in controlling the stability and size of the particles is discussed. It is shown that the generation of the dendrimers, as well as the position of the sulfur atoms in the dendritic structure (S₆G1 and S₉G1), influence the formation and reactivity of the nanoparticles. UV‐visible spectroscopy, ¹H nuclear magnetic resonance spectroscopy and high‐resolution transmission electron microscopy have been employed for the characterization of the nanoparticles. Furthermore, purification by Soxhlet extraction has been performed.     

Spin‐Coated Periodic Mesoporous Organosilica Thin Films—Towards a New Generation of Low‐Dielectric‐Constant Materials

Periodic mesoporous organosilica (PMO) thin films have been produced using an evaporation‐induced self‐assembly (EISA) spin‐coating procedure and a cationic surfactant template. The precursors are silsesquioxanes of the type (C₂H₅O)₃Si–R–Si(OC₂H₅)₃ or R′–[Si(OC₂H₅)₃]₃ with R = methene (–CH₂–), ethylene (–C₂H₂–), ethene (–C₂H₄–), 1,4‐phenylene (C₆H₄), and R′ = 1,3,5‐phenylene (C₆H₃). The surfactant is successfully removed by solvent extraction or calcination without any significant Si–C bond cleavage of the organic bridging groups R and R′ within the channel walls. The materials have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X‐ray diffraction (PXRD), and ²⁹Si and ¹³C magic‐angle spinning (MAS) NMR spectroscopy. The d‐spacing of the PMOs is found to be a function of R. Nanoindentation measurements reveal increased mechanical strength and stiffness for the PMOs with R = CH₂ and C₂H₄ compared to silica. Films with different organic‐group content have been prepared using mixtures of silsesquioxane and tetramethylorthosilicate (TMOS) precursors. The dielectric constant (k) is found to decrease with organic content, and values as low as 1.8 have been measured for films thermally treated to cause a “self‐hydrophobizing” bridging‐to‐terminal transformation of the methene to methyl groups with concomitant loss of silanols. Increasing the organic content and thermal treatment also increases the resistance to moisture adsorption in 60 and 80 %‐relative‐humidity (RH) environments. Methene PMO films treated at 500 °C are found to be practically unchanged after five days exposure to 80 % RH. These low dielectric constants, plus the good thermal and mechanical stability and the hydrophobicity suggest the potential utility of these films as low‐k layers in microelectronics.