Nanocomposites and nanomaterials

Quantum chemical study on the structure and transformations of hydrated forms of tin dioxide

O.V. Filonenko, A.G. Grebenyuk, V.V. Lobanov
Chuiko Institute of Surface Chemistry, NAS of Ukraine,
17 General Naumov Str., Kyiv 03164, Ukraine
E-mail: oksana.filonenko@isc.gov.ua

Nanoscale materials based on SnO2 in the form of nanoparticles, nanospheres, nanotubes, nanowires, nanobands are of great interest due to the combination of a number of their unique optical and electrophysical properties.
One of the most common methods of obtaining nanostructured forms of SnO2 is hydrothermal synthesis. Precursors in the production of nanoparticles of tin dioxide are its hydroxides and oxide hydroxides (so-called stannic acids), which are obtained by the action of an aqueous solution of ammonia on tin tetrachloride:

SnCl4 + 4 NH4OH → SnO2·nH2O↓ + 4 NH4Cl + (2 − n) H2O.

As a rule, there are two individual forms: - and -stannatic acids, which differ in composition, acidity and solubility. First, α-stannatic acid SnO2·nH2O (1 < n ≤ 2) precipitates from solutions of tin (IV) salts due to action of alkalis. Over time, the precipitate of α-stannatic acid is gradually converted to β-stannatic acid SnO2·nH2O (n < 1), losing water. Subsequent drying of the precipitate produces nanoparticles of SnO2. There is no unambiguous idea on the relationship between the structure and properties of these acids. The mechanisms of growth of the nanostructured forms of tin dioxide are not completely studied.
To find the regularities of the formation of these species, a systematic quantum chemical study was been carried out on the spatial structures and energy characteristics of the cluster models for nanoparticles of tin dioxide hydrated forms. The mechanisms of formation were studied of the simplest nanostructures containing up to four tin atoms from Sn(OH)4 units. Calculations of the properties of reagents, transition states, intermediates and reaction products were performed by the density functional method with exchange-correlation functional B3LYP and basis set 3-21G (d) using PC GAMESS software (FireFly 8.2.0).
It has been shown that Sn(OH)4 molecules tend to form two types of associates: hydrogen-bonded and stronger coordination; transitions between them lead to the formation of Sn-O-Sn and Sn-OH-Sn bridges. Structures with water molecules coordinated to Sn atoms can lose them quite easily, as evidenced by small values of activation energy and bonding energy (several kJ/mol), so forming intermediates with pentacoordinated Sn atoms; their further transformations lead to denser structures that have signs of crystallinity inherent in solid SnO2.