Octahedral morphology of NiO with (111) facet synthesized from the transformation of NiOHCl for the NOx detection and degradation: experiment and DFT calculation

Abstract

Nitrogen oxides (NO_x) are poisonous gases to humans and environment, and need to be monitored at an early stage. Accordingly, the design of facets on metal oxide semiconductors is an efficient approach to boost their gas sensing and photocatalytic performances due to the desirable active sites. However, in a rock-type structured NiO, a highly polar (111) exposed facets cannot be easily exposed due to their unfavorable thermodynamics. Herein, we demonstrate the synthesis of NiO with a dominant (111) facet from the transformation of NiOHCl with a layered structure. Among the crystal facets, NiO-Octa (111) exhibited the best NO_x gas sensing response (16.5%) to the 300 ppb level and deNO_x photocatalytic ability of over 50% under UV irradiation. DFT calculations revealed that the abundance of Ni atoms in the clean (111) surface layer allow the favorable adsorption of N adatoms, forming the Ni–N bond. The charge transfer occurring from NiO to the NO orbital was proven to be the cause for bond weakening and stretching from 1.1692 Å to 1.2231 Å, leading to NO_x molecular decomposition, consistent with experimental results.