Surface gas sensing kinetics of a WO3 nanowire sensor: Part 2—Reducing gases

The sensor response mechanism of a highly porous WO3 nanowire ensemble structure is investigated based on a study of the reaction kinetics of reducing gases of ammonia (NH3) and hydrogen (H2) on the material surface. The discussion is a continuation of a previous examination on the sensing of oxidizing gases with the identical sensor. The kinetic parameters are again derived from the time-dependent transients of the response-and-recovery sensing curves based on the conduction model using the simple depletion approximation at the semiconductor surfaces caused by the chemical reactions between the reducing gases and the ionosorbed oxygen on the surface. Sensing properties are systematically tested with variables of working temperature, nanowire diameters, and dilution environment (either dry air or nitrogen). Although the oxidizing gases directly adsorb onto the oxide surface, the reducing gases react with oxygen to produce water molecules. The escape rate of the water molecules from the surface can cause abnormal response curves in some conditions. Hydrogen is shown to react with the surface in various ways: reaction with oxygen, direct adsorption, and diffusion in the lattice.