Tailoring Electron Flow in the Redox Reactions of CoCu2O3 Nanocatalyst with Surface Hydroxyls for Humidity-Enhanced Molecular Sensing

Oxidation chemistry requires understanding the electron flow within a redox cycle involving reactants, products, and reaction intermediates. In heterogeneous catalysis, the latter are tracked because they precede product formation. In chemoresistive gas sensors, however, adsorbates matter only if they are ionized and electronically coupled to the solid, in which case electrons are injected into (or withdrawn from) the semiconductor. Here we develop a framework for oxidation-driven sensing that separates ionized from dipolar surface populations, focusing on where the transferred electrons are allocated within the oxide. We apply this framework to elucidate the unusual promoter effect of humidity on metastable, nanocrystalline CoCu 2 O 3 chemoresistors. By combining operando work function analysis with in situ vibrational and X-ray spectroscopies, we follow electron redistribution during chemical sensing of different reducing molecules. Despite inhibiting lattice-oxygen chemistry, we find that water vapor improves the oxidative removal of carbonaceous dipoles and tailors electron injection and flow between Co and Cu sublattices, which amplifies conductivity changes and thus sensor performance. To date, humidity has been considered as detrimental to gas sensor performance, but our results outline strategies to turn it into a functional advantage, by tailoring intermediates speciation (ionized vs. dipolar) and electron allocation.