Surface interaction of WO3 nanocrystals with NH3. Role of the exposed crystal surfaces and porous structure in enhancing the electrical response

We report on the surface interaction between NH3 and WO3 nanoparticles having different exposed surfaces or different porous structure, to identify the relative importance of exposed crystal surfaces, porous architecture, and specific surface area in the oxide sensing properties. WO3 nanocrystals with tailored morphology and definite prominent surfaces were synthesized by hydrothermal reactions. In parallel, inverted opal macroporous WO3 films have been prepared by a one-step sol-gel procedure, and WO3 hierarchical layers have been obtained by an innovative one-step dual-templating strategy which leads to macropores and mesopores simultaneously. The performances of WO3 samples in NH3 sensing, indicate that high-energy surfaces result in a significant improvement of the electrical response. Enhanced porous structure and high surface area are not enough to produce high electrical response, while their synergistic combination with tailored crystal faceting appears effective. XPS survey performed on shape controlled WO3 nanocrystals demonstrated that, upon interaction with NH3, oxidized nitrogen atoms represent the prevalent species on the surface of rectangular (WO3-RE) nanocrystals with highly exposed high-energy {020} and {002} facets. Conversely, in the case of rectangular platelets (WO3-SS) and square platelets (WO3-RS) with very low surface area of high-energy surfaces, N-H surface groups are predominant. These results suggest that {020} and {002} crystal surfaces provide privileged reactive sites for ammonia oxidation and therefore they play a key role in driving the sensing properties of the WO3 layers.