Nanocrystal production stability

Van Eerdenbrugh B, Van den Mooter G, Augustijns P (2008) Top-down production of drag nanocrystals nanosuspension stabilization, miniaturization and transfmniation into solid... 

Recently in the field of physics of semiconductors and materials science a great attention has been paid to formation and optical properties of semiconductor nanocrystals (quantum dots, QDs) dispersed in inorganic matrixes. An interest to glassy materials with QDs is associated with their unique physical properties and possibility to create elements of optoelectronic devices. Phase separation processes followed by crystallization are the basic in production of such materials. They result in formation of semiconductor nanocrystals stabilized within a glass matrix. The materials are advanced for various applications because of optical and thermal stability and possibility to control optical features through the technology of glass preparation and post-synthesis thermal treatment. 

Photocatalytic reduction of carbon dioxide on TiO-, nanocrystals embedded in Si02 matrices (Q-Ti02/Si0,) and bulk CdS particles with and without surface modifieation by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-Ti02/Si0, and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface. 

In this coimection, a cryochemical solid-phase synthesis of metal-polymer systems is of special importance. As a result of such a synthesis, metal clusters and organometallic assemblies formed at low temperatures are buried in a polymer environment, which offers possibilities to stabilize and study these products over a large temperature range. This method was first offered and described in reference 10. The thermal rearrangement of the initial low-temperature system is governed by relaxation processes in polymer matrix. In particular, the aggregation of metal atom clusters to form metal nanocrystals in cryochemically produced metal-polymer systems yields new nanocomposite materials with valuable properties. The study of the mechanism of cluster aggregation, which depends on the characteristics of the polymer matrix, will allow the nanocomposite structure to proceed in the needed direction. Thus, it becomes possible to determine the methods of cryochemical synthesis of metal-polymer materials with predetermined properties. 

Although the shaped nanocrystals have shown a high potential for the enhancement of catalytic activity, selectivity, and long-term stability, there are several issues that need to be resolved to expand their use to practical applications. The shaped nanocrystals are typically too large to effectively utilize expensive precious metals. The stability of the shaped nanocrystals should be carefully evaluated. The shape is often deformed too easily, especially at high temperatures where many practical gas-phase reactions occur. Shaped nanoparticles should also operate stably for long periods of time. Mass production of shaped nanocrystals is another important issue to be addressed properly. Indeed, much work on shaped nanoparticles is still needed to fundamentally understand their catalytic reactions and develop practical catalysts with better catalytic properties. 

While the microemulsion method has been widely applied to the production and stabilization of spherical metal particles with various sizes and compositions, shape control of noble metallic particles using this procedure has only been demonstrated in a handful of studies to date. Pileni and co-workers demonstrated that it is possible to control nanocrystal shape to some extent within microemulsions.Although the shape of the templates plays a role during the growth of the nanocrystals, these authors showed that the particle shape can be controlled even if the microscopic structure of the self-assembled surfactant system used as a template remains unchanged and that addition of salt to the templates can induce drastic changes in the particle shape. Recently, the same group also reported the synthesis of silver nanodisks in reverse micellar solution by reduction of Ag(AOT) with hydrazine, with various sizes that depended on the relative amount of hydrazine, but with constant aspect ratio. 

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