In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

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Mesocrystals: Inorganic superstructures made by highly parallel crystallization and controlled alignment

2.3. Amorphous Minerals 4354 3. Biological Routes to Controlling Morphology 4354 3.1. General Mechanisms 4356 3.1.1. Soluble and Insoluble Organic Molecules 4356 3.1.2. Control over Crystal Polymorph 4357 3.1.3. Control over Crystal Orientation 4359 3.2. Single-Crystal Biominerals 4359 3.2.1. Organic and Inorganic Soluble Additives 4360 3.2.2. Templating of Single-Crystal Morphologies 4361 3.3. Polycrystalline Biominerals 4366 3.3.1. Nacre Formation in Mollusks 4366 3.3.2. ForaminiferasA Biogenic Mesocrystal 4368 3.4. Amorphous Biominerals 4370 3.4.1. Silicification in Diatoms 4370 3.4.2. In Vitro Studies of Silicification in Diatoms 4371 4. Bioinspired Routes to Controlling Crystal Morphologies 4371

Controlling Mineral Morphologies and Structures in Biological and Synthetic Systems

TiO2 thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH4)2TiF6 aqueous solution upon addition of boric acid (H3BO3) and calcined at various temperatures. The as-prepared films were characterized with thermogravimetry (TG), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), UV−Visible spectrophotometry (UV−Vis), scanning electron microscopy (SEM), photoluminescence spectra (PL), and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methyl orange aqueous solution. It was found that the as-prepared TiO2 thin films contained not only Ti and O elements, but also a small amount of F, N, and Si elements. The F and N came from the precursor solution, and the amount of F decreased with increasing calcination temperature. Two sources of Si were identified. One was from the SiF62- ions, which were formed by a reaction between the treatment solution and quartz ...

The Effect of Calcination Temperature on the Surface Microstructure and Photocatalytic Activity of TiO2 Thin Films Prepared by Liquid Phase Deposition

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as “non-classical crystallization”, including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

Oriented attachment and mesocrystals: Non-classical crystallization mechanisms based on nanoparticle assembly

Calcite particles are grown in a collagenous matrix using a counter-diffusion arrangement. Although microstructural analysis revealed a heterogeneous intergrowth of organic and inorganic phases within the particles, the composite growth is rather a consequence of local chemical environment that is not specific to the gelatin gel. However, using an artificial poly-acrylamide hydrogel results in a very different growth morphology. The addition of poly-aspartate to the pore solution in either gelatin or poly-acrylamide hydrogel appears to overcompensate the physical properties of the organic matrix, leading to morphologies independent of which hydrogel is used. Our results stress the importance of noncollageneous proteins within a physical growth environment.

Organic−Inorganic Hybrid Structure of Calcite Crystalline Assemblies Grown in a Gelatin Hydrogel Matrix: Relevance to Biomineralization

Biomimetic nucleation and growth of CaCO3 in hydrogels incorporating carboxylate groups.

Calcite aggregates are mineralized in an organic poly-acrylamide hydrogel using a counter diffusion arrangement. The particles obtained show a characteristic pseudo-octahedral morphology, which is unexpected for calcite crystals. Scanning and transmission electron microscopy reveal a microstructure composed of individual highly aligned calcite crystallites. Although the aggregates consist of independent crystallites, the X-ray diffraction patterns suggest calcite single crystals. By analogy with some biominerals, the inorganic assembly is intergrown with an organic hydrogel network. A specific model is proposed for growth of the aggregate.

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Biomimetic control of crystal assembly by growth in an organic hydrogel network

Hollow TiO2-composite spheres in the size range of 50–150 µm were prepared by liquid phase deposition on template assemblies of self-organised PS latex particles. These assemblies spontaneously formed on the surface of octanol droplets in an oil-in-water emulsion, the overall process resembling the stages of natural biomineralisation. The TiO2–polymer composite capsules were converted into highly porous inorganic spheres by dissolution of the PS template particles and calcination at various temperatures. Removal of the organic template was monitored by DTA/TGA and He-pycnometry. The microstructure of the TiO2 films was characterised by nitrogen sorption and XRD measurements. The calcination temperature dependence of the photocatalytic activity of the hollow spheres was assessed by UV induced degradation of DCA. High photocatalytic activity was observed after calcination at 200 °C due to a high specific surface area of the film and the formation of micropores and mesopores in the size range <4 nm. Calcination at higher temperatures leads to the formation of larger mesopores and a drop in the photocatalytic activity due to a decrease in specific surface area and the disappearance of pores <4 nm. A major fraction of the deposited film consists of crystalline TiO2 in anatase modification. Further crystallisation and crystal growth at higher calcination temperatures has only minor influence on the photocatalytic activity.

Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised Pickering emulsions

Sol–gel processing is a powerful tool to prepare antireflective (AR) coatings on optical surfaces. In this paper the different strategies to obtain antireflective properties are reviewed: porous λ/4 layers, multilayer interference-type films and index-gradient materials such as “moth eye” structures. The processing of the respective films is described and evaluated; references to respective commercial products on glass substrates are given. AR coatings may have a particularly high importance for transparent ceramics as their index of refraction is significantly higher than that of common glass types. Reflective losses therefore are higher which is especially unpleasant for materials with a yet improvable intrinsic transparency. Recent studies indicate that specific porous λ/4 layers may exhibit pronounced anti-soiling features. Laboratory experiments as well as outdoor exposure tests were used to demonstrate the dust-repellant properties.

Peer Löbmann

Papers

Organic−Inorganic Hybrid Structure of Calcite Crystalline Assemblies Grown in a Gelatin Hydrogel Matrix: Relevance to Biomineralization

Biomimetic nucleation and growth of CaCO3 in hydrogels incorporating carboxylate groups.

Biomimetic control of crystal assembly by growth in an organic hydrogel network

Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised Pickering emulsions

Antireflective coatings prepared by sol–gel processing: Principles and applications