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 mn 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 (E-a= 10 meV) while at low temperatures the molecules become nearly static on the EPR time scale. Upon extended heating at 60 degreesC in air the NO2 EPR signal almost completely disappeared.