Abstract
This review article focuses on the mechanism of the non-aqueous fluorolytic sol gel-synthesis of nanoscopic metal fluorides and hydroxide fluorides. Based on MAS-NMR, XRD, WAXS and SAXS investigations in combination with computational calculations, it is shown that a stepwise replacement of alkoxide by F-ions takes place resulting in the formation of a large variety of metal alkoxide fluoride clusters, some of them being isolated and structurally characterised. It is shown that these nanoscopic metal fluorides obtained via this new synthesis approach exhibit distinctly different properties compared with their classically prepared homologues. Thus, extremely strong solid Lewis acids are available which give access to new catalytic reactions with sometimes unexpectedly high conversion degrees and selectivity. Even more interestingly, metal hydroxide fluorides can be obtained via this synthesis route that are not accessible via any other approach for which the hydroxide to fluoride ratios can be adjusted over a wide range. Optically fully transparent sols obtained in this way can be used for the first time to manufacture antireflective coatings, corundum ceramics with drastically improved properties as well as novel metal fluoride based organic-inorganic composites. The properties of these new fluoride based materials are presented and discussed in context with the above mentioned new fields of application.
Highlights
Nanomaterials chemistry has become an extremely important area of research over the past 20 years
Non-aqueous sol–gel approaches allow either the synthesis of novel materials, or in many cases give access to applications that are not possible via hydrolytic sol–gel routes. It is the intention of this review to provide an overview of a nonaqueous sol gel approach that gives for the first time direct access to novel nanoscaled metal fluorides
The authors are convinced that the fluorolytic sol– gel synthesis—like the classical hydrolytic sol–gel synthesis of metal oxides—is the most powerful approach for nanoscaled metal fluorides
Summary
Nanomaterials chemistry has become an extremely important area of research over the past 20 years. Non-aqueous sol–gel approaches allow either the synthesis of novel materials, or in many cases give access to applications that are not possible via hydrolytic sol–gel routes. Over the past few years a rapidly growing interest in nano metal fluorides has emerged from their attractive perspectives in application areas such as catalysis, optics, photonics, optical amplifiers, optoceramics, composite materials, biosensing, and biolabelling. This increasing interest in new nano fluoride based materials is clearly evidenced by the number of publications published in the past two decades. Most of them will be briefly mentioned in this review but special focus will be laid on the fluorolytic sol–gel approach
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