Physico-chemical Nanomaterials Science

Sonochemistry and mechanochemistry as methods for activation and doping of tin dioxide photocatalyts
Samsonenko M.1, Khalameida S.1, Sydorchuk V.1, Starcheskyy V.2, Skubiszewska-Ziȩba J.3, Zakutevskyy O.1
1Institute for Sorption and Problems of Endoecology, NAS of Ukraine, Naumov Str.13, 03164 Kyiv Ukraine
E-mail:mashuna.08@gmail.com
2Institute of Chemistry and Chemical Technologies, L’viv National Polytechnic University S.Bandera Str. 12, L’viv Ukraine
3Faculty of Chemistry, Maria Curie-Skłodowska University, pl. Maria Curie-Skłodowska 3, 2‑031 Lublin, Poland

Tin dioxide as wide-gap semiconductor and promising materials for photocatalysts and sensors production needs to regulate its physical-chemical including electronic properties. Particularly, electronic characteristics of oxides (band gap, absorption of irradiation with different wavelength) can changed by means of introduction of intrinsic or extrinsic defects into their structure. Ultrasound and mechanochemical treatments (UST and MChT) based on solid dispersion processes are the effective ways to achieve of these goals. Due to intense mechanical action, the activation of the surface is observed even for low-dispersed oxides. We studied mechano- and sonochemical treatment of commercial well-crystallized non-porous tin dioxide powder with specific surface area SSA = 6 m2/g in different media including in the presence doping additives (oxides and salts of silver, zinc, copper, cobalt and chromium). The changes in some physical-chemical characteristics of SnO2 due to MChT and UST have been studied. Also, influence of these changes on photocatalytic activity of SnO2 in processes of safranin T degradation has been established. 
MChT and UST leads to increasing the SSA (dispersity) 1.5-2 times: to 10-14 m2/g. This corresponds to reduction in crystallites size from 47 to 14-23 nm. There is also formation of meso-macropous structure with total pore volume 0.1-0.2 cm3/g and mesopores diameter within 10-35 nm. These transformations in dispersity and porous structure are a prerequisite for improvement in photocatalytic properties of SnO2. Its electronic characteristics are another crucial factor. The latter also can vary in wide limits during MChT and UST. Thus, mechano- and sonochemical modifications of tin dioxide without doping additives lead to formation of defects in crystal structure (intrinsic defects) which was confirmed by XRD investigations. In turn, these defects promotes some narrowing the band gap, namely from 3.8 to 3.6 eV. As a result, photocatalytic activity of milled and sonicated samples under UV-illumination increases by 50-80% compared with initial powder for processes of dye pollutants degrada. It is important that tin dioxide acquired activity under visible irradiation due to the process of safranin T photosensitization. For example, the rate constant of dye photocatalytic degradation Kd reaches the values 2-3·10-5 s-1 depending on MChT and UST conditions. 
Mechano- and sonochemical treatment of SnO2 powder with compounds of transition metals gives a triple effect: increasing the specific surface area, introduction of intrinsic and extrinsic defects. The important net effect of all these transformations is a significant narrowing of band gap as well as increase in absorption of visible irradiation. As a result, maximal photocatalytic activity was obtained for powders doped via MChT and UST. The Kd value is within 3-7·10-5 s-1 for doped sonicated powders and milled at 300 rpm powders. This corresponds to degree of dye solution discoloration about 70-90% for 5 h.  At the same time, degree of mineralization, which is estimated as degree of total organic carbon reduction, reaches 60-75%.