EPR Study of TiO2 Based Nanotubes and NO2 Adsorption

TiO2 nanotubes were synthesized via reaction of TiO2 particles and NaOH solution. The average diameter of nanotubes was 15–20 nm and the active surface area increased by a factor of 13 in comparison to TiO2 powder. When a TiO2 nanotube material was exposed to NO2 gas a strong adsorption of gas on the nanotube surface was observed. A strong echo signal was detected in pulsed EPR experiments below 200 K. A field sweep echo experiments revealed two distinct signals: three‐lines at g=2, which are due to NO2 radical, and a broad line with singularities at g= 2.09 and g= 2.49. The origin of the broad signal is at the moment not yet clear. Spin lattice relaxation time measurements demonstrated that at temperatures above 50 K the reorientation dynamics of adsorbed NO2 molecules is thermally activated (Ea= 10 meV) while at low temperatures the molecules become nearly static on the EPR time scale. Upon extended heating at 60 °C in air the NO2 EPR signal almost completely disappeared.