Enhanced photocatalytic performance of non-stoichiometric WO3−x nanocrystals via near-infrared localized surface plasmon resonance

The use of non-stoichiometric metal oxides as plasmonic materials is an effective way to increase photocatalytic performance. This report shows that WO_3-x nanocrystals exhibit a localized surface plasmon resonance (LSPR) absorption band in the near-infrared (NIR) region and photocatalytic activity in methylene blue (MB) degradation. The LSPR absorption bands of the synthesized WO_3-x nanocrystals are simulated using the Mie-Gans theory, and their free carrier densities (N_e) are calculated. The findings show that the calcination process shifts the LSPR peak from 1213 to 1771 nm (a remarkable redshift at about 560 nm). In addition, the calcination process leads to the values of N_e as high as 2.03 × 10²² cm^-3, which is close to that of plasmonic noble metals, and thus the obtained WO_3-x nanostructures can be considered as quasi-metallic. Furthermore, the photocatalytic degradation mechanism of MB by the samples are discussed according to their photoluminescence emission, absorption spectra, predicted LSPR, and energy band diagrams and the main photocatalytic processes are proposed.