Publication date: 15th May 2025
The WO3 NIR band has been alternately assigned to either a delocalized free electron-associated plasmon resonance or a localized polaronic electron-associated transition, two contradictory explanations. In this work, we found a photochemically driven semiconductor-to-metal phase transition in WO3 nanoparticles in solution. At low trapped electron density, the peak position of NIR band is independent of trapped electron density. Beyond a trapped electron density, 10^21/〖cm〗^3, the NIR peak blue-shifts towards the visible region, likely indicating the delocalization of the trapped electrons. The linear correlation between NIR peak position and the trapped electron concentration argues against the assignment of the NIR band to a plasmon resonance. In addition, the linear correlation between the trapped electron density and the corresponding NIR absorbance supports the polaron transition. We found that the critical trapped electron density, 0.95×10^21/〖cm〗^3 (~5.1 % of formula units), guarantees that one quarter of the surface tungsten sites are reduced to form a fully covered surface by polarons. Additional polaronic electrons, beyond that critical density, are mobile due to repulsive polaron-polaron interaction and yield the blue-shifting NIR signature. This work demonstrates that the NIR band of WO3 is a result of a polaron rather than a plasmon and challenges the textbook description of plasmonic metal oxide semiconductors.
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