Charge Transport Phenomena in Heterojunction Photocatalysts: The WO₃/TiO₂ System as an Archetypical Model

Recent studies have demonstrated the high efficiency through which nanostructured core-shell WO₃/TiO₂ (WT) heterojunctions can photocatalytically degrade model organic pollutants (stearic acid, QE ≈ 18% @ λ = 365 nm), and as such, has varied potential environmental and antimicrobial applications. The key motivation herein is to connect theoretical calculations of charge transport phenomena, with experimental measures of charge carrier behavior using transient absorption spectroscopy (TAS), to develop a fundamental understanding of how such WT heterojunctions achieve high photocatalytic efficiency (in comparison to standalone WO₃ and TiO₂ photocatalysts). This work reveals an order of magnitude enhancement in electron and hole recombination lifetimes, respectively located in the TiO₂ and WO₃ sides, when an optimally designed WT heterojunction photocatalyst operates under UV excitation. This observation is further supported by our computationally captured details of conduction band and valence band processes, identified as (i) dominant electron transfer from WO₃ to TiO₂ via the diffusion of excess electrons; and (ii) dominant hole transfer from TiO₂ to WO₃ via thermionic emission over the valence band edge. Simultaneously, our combined theoretical and experimental study offers a time-resolved understanding of what occurs on the micro- to milliseconds (μs-ms) time scale in this archetypical photocatalytic heterojunction. At the microsecond time scale, a portion of the accumulated holes in WO₃ contribute to the depopulation of W⁵⁺ polaronic states, whereas the remaining accumulated holes in WO₃ are separated from adjacent electrons in TiO₂ up to 3 ms after photoexcitation. The presence of these exceptionally long-lived photogenerated carriers, dynamically separated by the WT heterojunction, is the origin of the superior photocatalytic efficiency displayed by this system (in the degradation of stearic acid). Consequently, our combined computational and experimental approach delivers a robust understanding of the direction of charge separation along with critical time-resolved insights into the evolution of charge transport phenomena in this model heterojunction photocatalyst.