A dual-carrier adsorbate-modulated surface conductance model better captures the thermal dependence of conductance in TiO 2 and MoO 3 powders than an inter-grain hopping model

Non-Arrhenius thermal dependence of surface conductance has previously been observed in the transition-metal oxides TiO2 and MoO3. Through the application of thermochemical modeling, kinetic modeling, and analysis of equivalent resistance networks, it is shown that a dual-charge-carrier model in which the adsorbate surface coverage is modulated by bi-Langmuir adsorption is better able to capture the thermal dependence of surface conductance in these materials than a model based on the hypothesis that conductance is governed by bottlenecks to charge hopping between grains. Adsorption energies predicted by the dual-charge-carrier model are in agreement with estimates of the same from published first-principles calculations. Particle-size dependence of the conductance likely arises from the increasing importance of surface processes to charge transport with decreasing particle size, not from an increase in the number of inter-particle contacts.