Hydrolysis of molybdenum and tungsten alkoxides: sols, powders and films
TL;DR: In this article, the properties of molybdenum and tungsten alkoxides of the two series MO(OEt)4 and MO2(OEts)2 (M = Mo, W) have been studied.
Abstract: Hydrolysis of molybdenum and tungsten alkoxides of the two series MO(OEt)4 and MO2(OEt)2 (M = Mo, W) has been studied. Hydrolysis products and the partially hydrolyzed solutions have been characterized by means of X-ray diffraction, IR, electron spectroscopy and small-angle X-ray scattering. Preparation of stable sols is reported and their structures are discussed in terms of a fractal model. While ‘tungsten solutions’ may be described as typical polymeric sols, ‘molybdenum solutions’ should be regarded as particulate sols. The reason for this difference has its origin in the structures of alkoxides and mechanisms of their hydrolysis. Sols were used for the preparation of electrochromic and photochromic films by spin-coating techniques. A coloration mechanism is discussed in terms of the percolation model. Some properties of the composite structures WO3TiO2, n-SiWO3 are also reported.
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263 citations
TL;DR: In this paper, the progress in all of these areas will be reviewed thoroughly, including the early stage of the research, the band structure, photochromic mechanism and the behavior of the oxide.
Abstract: Molybdenum oxide can exhibit pronounced photochromism and thus might act as an excellent photonic material for a number of technical applications. In the early stage of the research, the attention was focused mainly on the (band) structure, photochromic mechanism and the behavior of the oxide. Later, many investigations were carried out on the factors that might influence the photochromic performance. At the same time, the photochromic response has been extended from UV light to visible light. In this review, the progress in all of these areas will be reviewed thoroughly.
258 citations
TL;DR: In this article, a brief updated review of sol-gel-derived electrochromic films of different chemical systems is presented, where performances of selected films measured in electrochemical cells or in devices are discussed and degradation problems experienced by different authors enumerated.
235 citations
TL;DR: In this paper, a solution-sol-gel (SSG) method was used for aerogel preparation and their subsequent liberation from the solvent via critical-point drying or supercritical (or hypercritical) drying (SCD).
Abstract: Introduction Aerogels offer interesting opportunities for catalysis due to their unique morphological and chemical properties. These properties originate from their wet-chemical preparation by the solution-sol-gel (SSG) method and their subsequent liberation from the solvent via critical-point drying or supercritical (or hypercritical) drying (SCD). Due to the “structure-preserving” ability of SCD, the usually oxidic (or metallic) aerogels are solids of high porosity and specific surface area.
215 citations
TL;DR: In this article, a review of the chemistry behind the solar control coatings and chromic materials including ambient radiation and the ideal of a black-body object is presented, with experimental results and theoretical insight as well as production techniques and applications.
Abstract: The excessive use of heating systems in cold climates and air conditioning systems in hotter climates is resulting in the extensive use of electricity in order to maintain such systems. This in turn leads to the greater use of fossil fuels and higher emissions of carbon dioxide and other pollutant gases. The growing amount of carbon dioxide emissions is contributing to the problem of global warming, hence increasing the need for alternative technologies to heating and air conditioning systems. One such alternative is the production of thin films which can be used as window glazing coatings to construct ‘smart windows’. These windows have the greatest use within constant climates. In cold climates, windows with high solar transmittance and low thermal emittance are needed; this allows sunlight into the building to brighten the room but stops heat from escaping thus warming the room. In constantly hot climates, materials that are transparent in the visible region but reflective in the infrared, such as thin metallic coatings, can be used to ensure that the inside of the building remains cool. These solar control coatings, however, pose a problem in varying climates such as in northern and central Europe. For these cases, materials that have altering properties owing to external surroundings could be the solution. These ‘chromatic’ materials include several categories, such as photochromic glasses and polymers, thermochromic metal oxides and electrochromic materials. This review will begin with a discussion as to the chemistry behind the solar control coatings and chromic materials including ambient radiation and the ideal of a black-body object. It will then look in depth at each class of these chromic materials considering experimental results and theoretical insight as well as production techniques and applications.
120 citations
References
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Book•
01 Jan 1983
TL;DR: A survey of polyoxometalates can be found in this paper, where the authors survey the chemistry, structures, and applications of these species, as well as their applications in inorganic chemistry.
Abstract: Combined oxygen, in the form of water, metal oxides, silicates and other oxyions, accounts for about 50% by weight of the earth's crust. The chemistry of this most abundant element has two major aspects: that of water and aqueous solutions of electrolytes, and that of the solid state. The methodology and techniques appropriate for the study of these fields are sufficiently different that there have been very few points of contact between solution- and solid state chemists. One such contact, of great potential value, is provided by the extensive class of polyoxoanions formed by the transition metals of groups 5 and 6. As 'heteropoly and isopoly acids' these polyanions have been known and investigated for more than a century. The pre sent book is an attempt to survey the chemistry, structures, and applications of these species. Although the book forms part of a series in inorganic chemistry, the field of polyoxometalates deserves wider attention, for example, from organic chemists, especially those concerned with homogeneous and heterogeneous catalysis, and from biochemists, solid state- and materials scientists."
2,879 citations
TL;DR: In this paper, the authors reviewed the mechanisms of gel formation in silicate systems derived from metal alkoxides and proposed that the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster.
Abstract: The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.
511 citations
TL;DR: Using small-angle x-ray scattering, a simple model is developed which both is consistent with silicate chemistry and generates fractally rough structures in solution and in a porous solid made from the solution precursor.
Abstract: We report the growth and structure of fractally rough silicate particles in solution. Using small-angle x-ray scattering, we find fractal surfaces both in solution and in a porous solid made from the solution precursor. Finally, we develop a simple model which both is consistent with silicate chemistry and generates fractally rough structures.
153 citations
TL;DR: In this article, a model of silica network formation from silicon alkoxides is used as a model in driving relationships between physical and structural parameters, and supporting experimental evidence is presented.
Abstract: Oxide glasses, being thermoplastic polymers, should be subject to considerable property variations as a result of differences in the network structure. Melting point, viscosity, and strength are among the properties that are closely related to network connectivity and distribution of the network bonds in the polymer. Glass formation by melting, however, does not allow a significant variation in network parameters, except by compositional changes. Chemical polymerization, on the other hand, permits the introduction of structural variations into the polymer network, and provides preparation of modified oxide glasses without compositional alterations. Silica network formation from silicon alkoxides is used as a model in driving relationships between physical and structural parameters, and supporting experimental evidence is presented. Some general effects of these parameters in several glass systems are included.
143 citations
TL;DR: In this article, the fundamental chemistry of the sol-gel process is discussed and several examples based on this approach are presented for optical glass, structural ceramics and electromagnetic materials.
Abstract: The ultrastructural control of materials through sol-gel processes offers significant promise for the achievement of reliable performance in glass, glass-ceramics, ceramics and composites. This will be attainable only if the fundamental chemistry of the sol-gel process is understood. Several examples based on this approach will be presented for optical glass, structural ceramics and electromagnetic materials. New concepts, such as ceramic or glass molecular composites and optically active gels, will be discussed. It will be shown how these new concepts are derived from an understanding of polymer chemistry and chemical reactivity. Advanced sol-gel basic research directions and their prospects will be discussed.
132 citations