EPR Investigations of G-C3N4/TiO2 Nanocomposites

Dana Dvoranová,Vlasta Brezová,Tatiana Giannakopoulou,Christos Trapalis,Milan Mazúr,Ilias Papailias

doi:10.3390/catal8020047

Abstract

The g-C3N4/TiO2 nanopowders prepared by the annealing of melamine and TiO2 P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of powders monitored at room temperature and 100 K showed the impact of the initial loading ratio of melamine/TiO2 on the character of paramagnetic centers observed. For the photocatalysts synthesized using a lower titania content, the paramagnetic signals characteristic for the g-C3N4/TiO2 nanocomposites were already found before exposure. The samples annealed using the higher TiO2 loading revealed the photoinduced generation of paramagnetic nitrogen bulk centers (g-tensor components g1 = 2.005, g2 = 2.004, g3 = 2.003 and hyperfine couplings from the nitrogen A1 = 0.23 mT, A2 = 0.44 mT, A3 = 3.23 mT) typical for N-doped TiO2. The ability of photocatalysts to generate reactive oxygen species (ROS) upon in situ UV or visible-light photoexcitation was tested in water or dimethyl sulfoxide by EPR spin trapping using 5,5-dimethyl 1-pyrroline N-oxide. The results obtained reflect the differences in photocatalyst nanostructures caused by the differing initial ratio of melamine/TiO2; the photocatalyst prepared by the high-temperature treatment of melamine/TiO2 wt. ratio of 1:3 revealed an adequate photoactivity in both spectral regions.

Highlights

Among the previously-studied potential photocatalysts, TiO2 polymorphs meet the criteria for large-scale applications
The concentration of photogenerated spin-adducts was evaluated from the double-integrated electron paramagnetic resonance (EPR) spectra based on the calibration curve obtained from the EPR spectra of tetramethylpiperidine N-oxyl (TEMPOL) solutions measured under strictly identical experimental conditions
The paramagnetic centers of pristine g-C3 N4 and g-C3 N4 /TiO2 nanopowders prepared by the annealing of various ratios of dry melamine and TiO2 P25 at 550 ◦ C were investigated using EPR

Summary

Introduction

Among the previously-studied potential photocatalysts, TiO2 polymorphs meet the criteria for large-scale applications. The energy band gap of g-C3 N4 enables VIS-light absorption, its photocatalytic activity is negatively affected by the limited surface area and rapid recombination processes of photogenerated charge carriers [16,19]. The efficiency of photoinduced reactions in the presence of semiconducting photocatalysts is determined by the light absorption coupled with the effective generation of charge carriers, their recombination processes, as well as suitable structure with the surface active sites enabling substrate adsorption [14]. The aim of our EPR study is to bring insight into the presence of trapped paramagnetic charge carriers or free radicals generated upon UV or VIS-light irradiation in g-C3 N4 /TiO2 nanopowders or dispersed systems in water or dimethyl sulfoxide (DMSO)

Results and Discussion

X‐band

Experimental

Materials and Methods

Conclusions