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Posted: Jul 05, 2012
Core-shell nanocrystals synthesized through surfactant-free technique
(Nanowerk News) Iranian researchers at Ferdowsi University of Mashhad devised a new scheme for preparation of core-shell nanocrystals, by combining the microemulsion and ultrasonication methods, which features moderate synthesis conditions and does not require any surfactants (see paper in Ultrasonics Sonochemistry: "Sono-synthesis of core–shell nanocrystal (CdS/TiO2) without surfactant ").
The properties of core-shell composites depend heavily on the materials constituting both the core and the shell parts. These compounds offer superiorities in terms of dimension, optical properties, electronic characteristics, etc. so that they have found various applications in photovoltaic cells, optical sensors and catalysis technology, just to mention a few areas.
As reported in a previous work available in the literature, a thin shell of a wide-band gap semiconductor has been deposited upon a small-band gap semiconducting core substrate through an innovative method. Despite its own advantages, the mentioned method is prone to yield non-uniform deposition of inorganic materials on small cores. Besides, the fabrication process demands high temperatures and long aging times as its other downsides.
In an attempt to refine and improve the aforementioned synthesis method, a number of researchers at the Ferdowsi University of Mashhad have come up with core-shell CdS/TiO2 nanocomposites via a surfactant-free approach.
The researchers initially prepared CdS nanoparticles by means of ultrasonication and microemulsification without adding any surfactants. The synthesized nanoparticles were then mixed with TiO2 under ultrasonic irradiation. The latter caused the formation of a thin layer of TiO2 over the CdS nanoparticles and resulted in their swelling. The formation of core-shell structures is due to the involved cavitation phenomenon which forces powerful and high-speed collisions of the nanoparticles. In addition, the ultrasonic waves control the condensation and hydrolysis of titanium tetra isopropoxide as well as shaping of the TiO2 shell.