Enhancement of nano titanium dioxide coatings by fullerene and polyhydroxy fullerene in the photocatalytic degradation of the herbicide mesotrione

The surface modification of commercial TiO2 Hombikat (TiO2) using nanoparticles of fullerene C60 with tetrahydrofuran (THF-nC60), as well as fullerenol C60(OH)24 nanoparticles (FNP) was investigated in this study. Characterization of THF-nC60, FNP, TiO2, TiO2/THF-nC60, and TiO2/FNP was studied by using DES, ELS, TEM, SEM, DRS and BET measurements and their photoactivity has been examined on the mesotrione degradation under simulated sunlight. It was found that FNP in self-assembled nanocomposite TiO2/FNP increased negatively charge, as well as catalytic surface of TiO2. In addition, TiO2/FNP exhibits a shift of band gap energy to lower values compared to TiO2 and TiO2/THF-nC60. BET surface area has not showed significant differences among catalysts. Furthermore, it was found that the highest photoactivity was obtained for TiO2/FNP system. Besides, influence of different concentrations of electron acceptors (H2O2 and KBrO3), as well as scavengers on the kinetics of mesotrione removal in aqueous solution with/without TiO2 and FNP under simulated sunlight was investigated. Namely, addition of mentioned electron acceptors has resulted in higher mesotrione degradation efficiency compared to O2 alone. Besides, in the first period substrate degradation probably takes place via hydroxyl radicals and after 60 min of irradiation the reaction mechanism proceeds mainly via holes. The most efficient system for mesotrione degradation and mineralization were TiO2/7 mM KBrO3 and TiO2/7 mM KBrO3/40 μl FNP, respectively.


Nanoparticles characterization
Dynamic light scattering (DLS) was used for the determination of hydrodynamic size, and electrophoretic light scattering (ELS) for measurements of the surface charge (zeta potential, ζ) of analyzed samples. The measurements were conducted on a Zetasizer Nano ZS instrument (Malvern Instruments Inc, UK). All DLS analysis was done in triplicate, and the zeta potential measurements were performed in duplicate. The results were presented as mean values of obtained results for all repetitions.
Transmission electron microscopy (TEM) analyses were conducted on the JEM 1400 microscope with an accelerating voltage of 120 kV on copper grid 300 mesh.
The samples for scanning electron microscopy (SEM) measurements were prepared on grid size 300 mesh × 83 μm pitch, copper, and analyses were performed on instrument JEOL JSM 6460 LV.
The absorbance () of the TiO 2 , TiO 2 /THF-nC 60 and TiO 2 /FNP suspensions was measured by UV-VIS spectrophotometer Evolution 600, Thermo Scientific in the range between 240 nm and 840 nm with the step of 1 nm and speed of 10 nm min − 1 . Demineralized water was used as reference. Brunauer-Emmett-Teller (BET) surface area analysis of the measured samples was performed using a gas adsorption porosimeter Surfer, Thermo Scientific following a standard procedure and by using liquid nitrogen.

Synthesis of THF-nC 60, FNP, TiO 2 /THF-nC 60 and TiO 2 /FNP nanoparticles
A solution of THF-nC 60 was obtained by intensive stirring of 250 mL of THF and 6.25 mg of fullerene C 60 in a nitrogen atmosphere, in the dark, at a temperature of 22 °C for two days.
A saturated solution of fullerene C 60 in THF was filtered through a 100 nm filter. Then, 250 mL of DDW pH-adjusted at 5 was added into the filtered C 60 solution and stirred. The excess of THF was evaporated by bubbling with N 2 at a temperature of 40 °C in the dark. A stable solution of fullerene C 60 nanoparticles in water was obtained, with residual amounts of the nanoparticles in the THF. The nanoparticles THF-nC 60 is pale yellow in color.
A solution of FNP was synthesized from a polybromine derivative C 60 Br 24 , obtained in a catalytic reaction of fullerene C 60 in Br 2 with FeBr 3 as the catalyst, by complete substitution of bromine with hydroxyl groups (Djordjevic et al., 1998;Mirkov et al., 2004). The obtained brown powder of FNP was stored in the dark. Band gap energies (E g ) of the samples was determined using the Tauc's plot (Tauc, 1968).

Characterization of THF-nC 60 , FNP, TiO 2 , TiO 2 /THF-nC 60 , and TiO 2 /FNP nanoparticles
The method is based on the fact that the absorption of the light is dependent on the band gap energy of the absorbing material (Kubelka-Munk theory) (Kubelka, 1948;Kubelka and Munk, 1931). A relation between  and E g is given by López and Gómez (2012): with the h being a Planck's constant,  the frequency (linked to the measured wavelength by  = c/, c is the speed of light in vacuum) and C an energy-independent constant. The factor n depends on the transition type and it was assumed to be a direct allowed (n = 2). The band gap energy can be determined from a plot of the modified absorbance