Photochromic organic–inorganic hybrid materials have attracted considerable attention owing to their potential application in photoactive devices, such as optical memories, windows, photochromic decorations, optical switches, filters or non-linear optics materials. The growing interest in this field has largely expanded the use of photochromic materials for the purpose of improving existing materials and exploring new photochromic hybrid systems. This tutorial review summarizes the design and preparation of photochromic hybrid materials, and particularly those based on the incorporation of organic molecules in organic–inorganic matrices by the sol–gel method. This is the most commonly used method for the preparation of these materials as it allows vitreous hybrid materials to be obtained at low temperatures, and controls the interaction between the organic molecule and its embedding matrix, and hence allows tailoring of the performance of the resulting devices.

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Photochromic organic-inorganic hybrid materials.

Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.

/pdf/review-of-sol-gel-thin-film-formation-1qb6rlkbtz.pdf

Review of sol-gel thin film formation

Structural characterization of noncrystalline solids and glasses using solid state NMR

https://ris.utwente.nl/ws/files/6646726/Gas2-92uhlhorn.pdf

Gas transport and separation with ceramic membranes. Part II: Synthesis and separation properties of microporous membranes

New tellurite glass: Optical properties

Glass formation occurs in the zinc tellurite system in the region of the eutectic (21 mol% ZnO) on the TeO2-rich side of the phase diagram. Glasses are characterized by a high refractive index which increases with TeO2 content. The glasses are transmitting from about 400 nm to about 6 μm with OH absorption bands at 3.3 and 4.4 μm. Short range order of the glasses was deduced with neutron diffraction, infrared and Raman spectroscopy by comparison with the spectra of the synthesized crystalline α-TeO2, Zn2Te3O8 and ZnTeO3. Glasses consist of disordered TeO4, TeO4, TeO3+ and TeO3 building units. The number of the TeO3+1 units is limited by ZnO addition. There is a relatively strong structural correlation between the glasses and the crystalline compound Zn2Te3O8 in accord with the phase diagram.

Glass formation, properties and structure of glasses in the TeO2ZnO system

Small- and wide-angle X-ray scattering (SAXS and WAXS, respectively) were used to examine in situ formation and evolution of SiO2-structures in alcoholic solutions of tetra-alkoxysilanes (TAOS) as a function of chemical conditions. In acid-catalyzed reactions with a molar ratio of ⩽rw ⩽ 51 forH 2O against TAOS primary particles with a diameter of ∼ 1 nm were always found. These particles aggregated to form secondary clusters. The resulting aggregated clusters can be described by means of the fractal concept, diffusion-limited or chemical-limited cluster-cluster aggregations being the dominant mechanisms of formation. When using an under-stoichiometric water content (rw = 1) the formation of smaller primary particles with a diameter of ∼ 0.5 nm is observed; these particles preferably aggregated to form linear particle chains. In alkaline-catalyzed reactions the SiO2-particles grew beyond the visual field of the measurement (rmax = 20 nm) and featured nearly smooth surfaces. As a result of WAXS examinations, it was found that the internal structure of the condensates is characterized by a well-defined topology (polyhedrons). The liquid environment exerts a force on the SiO2-particles and the polyhedron size varies with the composition of the solution.

WAXS- and SAXS-investigations of structure formation in alcoholic SiO2 solutions

Low-density x SiO 2 -(1 - x )Al 2 O 3 aerogels with x = 0, 0.3, 0.7, 1.0 (mole fractions) were prepared by CO 2 supercritical drying of alkogels formed from Si(OMe) 4 and Al(OBu s ) 3 by hydrolytic polycondensation under different chemical conditions. The resulting aerogels were structurally characterized by X-ray scattering at small and large angles as well as by electron diffraction and transmission electron microscopy. By investigating the dependence of linear shrinkage on the heat treatment it was found that the sintering of the aerogel is influenced by the presence of Al 2 O 3 in the network. Crystallization experiments were also performed up to 1200°C. The reduced height of the first sharp diffraction peak indicates less order in the SiO 2 network of the aerogel in comparison with amorphous SiO 2 prepared by other methods.

Structural characterization of SiO2Al2O3 aerogels

High-resolution 29SiNMR measurements in solids are performed to study the effect of DCCA (drying control chemical additive) formamide on the tempering of silica gel glass in the range of 200 to 900°C. The FtMASNMR spectra yield quantitative results and indicate only the existence of O3 (HOSi(-OSi≡)3] and Q4 [Si(OSi≡)4] groups in the limits of error. The increasing polycondensation is shown by an increasing content of Q4 groups which goes from about 75% at 200°C to about 95% at 900°C. Additionally CPMASNMR experiments give the qualitative evidence of small parts of Q2[(HO)2Si(-OSi≡)2] groups in all tempering steps.

29SiMASNMR investigation on silica glass prepared by the drying control chemical additives method

SiO2 aerogels with densities between 0.131 and 0.182 g/cm3 were prepared by CO2 supercritical drying of wet gels formed from differently concentrated TMOS solutions by hydrolytic polycondensation under basic conditions (pH ≈ 8). The formation and growth of SiO2 particles in the originally homogeneous starting solutions has been followed in situ by SAXS (small-angle X-ray scattering). A chemical limited particle growth takes place until a reduced time of t/tg ≈ 0.3, which slowed to the point of gelation and produced, on the average, ⋍ 11 nm sized particles for all alcogels. These partices form branched and interconnected chains determining the fractal structure of the aerogels. The internal structure of these particles is disturbed by incorporated alcoholic groups caused by the basic hydrolysis and the relatively soft regime of CO2 supercritical drying. In particular, the reduced height of the first sharp diffraction peak suggests that the middle-range order is less pronounced than in the corresponding SiO2 xerogel.

H. Bürger

Papers

Glass formation, properties and structure of glasses in the TeO2ZnO system

WAXS- and SAXS-investigations of structure formation in alcoholic SiO2 solutions

Structural characterization of SiO2Al2O3 aerogels

29SiMASNMR investigation on silica glass prepared by the drying control chemical additives method

Structural characterization of SiO2 aerogels