Cyclodextrins as a Templating Agent in Solvent-Free Kneading- Based Syntheses of Nanosized SnO2 and ZnO

In recent years, in the ultrasensible sensor field, there has been a growing interest in diffusion of metal oxide-based semiconductors (MOS): they are suitable materials for sensors, combining high efficiency, fast response, stability, simple preparation, and low cost. In most applications (gas sensing, solar cells, and photocatalysis), MOS activity is strongly related to the specific surface area; to improve the properties of MOS, research is moving toward nanosized structures with controllable crystalline phases. There are many examples of synthetic methods that permit one to obtain ceramic nanoparticles, such as thermal and physical deposition, hydro/solvothermal processes, and more recently, electrospinning (ES) followed by thermal treatment. The addition of an oxide precursor to a polymeric solution and subsequent thermal treatment, above the thermal degradation of the polymer, allow one to ablate the polymeric matrix and, at the same time, to convert the precursor in the respective oxide. In this work, a new synthetic route of nanosized MOS is proposed, where the ES process, before calcination, is indeed replaced by the simple kneading of the precursors in the presence of an excess of cyclodextrins (CDs). The CD structure with the presence of a slightly apolar cavity and a hydrophilic external part determines the ability to establish specific interactions with various types of molecules through the formation of noncovalently bonded complexes, either in the solid phase or in aqueous solution. The possibility to exploit inclusion complexes of CDs and metal precursors to obtain nanostructured oxides is herein demonstrated. The inclusion complexes were prepared using zinc oxide (ZnAc) and tin(II) ethylexanoate (precursors of SnO2 and ZnO) and by exploiting a solvent-free approach based on the kneading of alpha-CD, beta-CD, and gamma-CD and precursors in a ball mill. The use of kneading overcomes all the limitations related to poorly soluble or insoluble compounds, permitting one to avoid the use of solvents and to speed up the preparation of the MOS precursor. The complexes after kneading were characterized by thermogravimetric analyses and then thermally treated to obtain oxides. After the synthesis, the so-obtained SnO2 and ZnO nanoparticles were characterized by high-resolution transmission electron microscopy and X-ray diffraction and by measuring the specific surface area by the Brunauer-Emmett-Teller method, putting in evidence of an influence of the CD size on the final oxide.