Density functional theory study of the NO<sub>2</sub>-sensing mechanism on a WO<sub>3</sub> (0 0 1) surface: the role of surface oxygen vacancies in the formation of NO and NO<sub>3</sub>
<p>The trapping and detection of nitrogen oxide with tungsten trioxide has become a popular research topic in recent years. Knowledge of the complete reaction mechanism for nitrogen oxide adsorption is necessary to improve detector performance. In this work, we used density functional theory (DFT) calculations to study the adsorption characteristics and electron transfer of nitrogen dioxide on an oxygen-deficient monoclinic WO<sub>3</sub> (0 0 1) surface. We observed different reactions of NO<sub>2</sub> on slabs with different O- and WO-terminated WO<sub>3</sub> (0 0 1) surfaces with oxygen vacancies. Our calculations show that the bridging oxygen atom on an oxygen defect on an O-terminated WO<sub>3</sub> (0 0 1) surface is the active site where an NO<sub>2</sub> molecule is oxidised into nitrate and is adsorbed onto the surface. On a WO-terminated (0 0 1) surface, one of the oxygen atoms from the NO<sub>2</sub> molecule fills the oxygen vacancy, and the resulting NO fragment is adsorbed onto a W atom. Both of these adsorption models can cause an increase in the electrical resistance of WO<sub>3</sub>. We also calculated the adsorption energies of NO<sub>2</sub> on slabs with different oxygen-deficient WO<sub>3</sub> surfaces.</p