Hydrated Orthorhombic/Hexagonal Mixed-Phase WO₃ Core–Shell Nanoribbons for Hole-Mediated Photocatalysis

Herein, we report the hydrothermal fabrication of hydrated hexagonal (h)/orthorhombic (o) mixed-phase (h-WO3/o-WO3·H2O) core–shell nanoribbons for the photodegradation of methylene blue (MB) under UV, visible, and monochromatic light excitations. The mixed-phase core–shell shows 2–3 times higher photocatalytic activity than the monoclinic (m)-WO3 and h-WO3 crystalline phases. The intercalated water, trap states, and prolonged carrier lifetimes play significant roles in the photocatalytic enhancement of the hydrated mixed phase. These developed nanosystems are rich in oxygen vacancies and show photoabsorption extended up to the near-infrared (NIR) region. Electrochemical spectroscopy revealed a reduced charge-transfer resistance in the mixed-phase compared with other crystalline phases. The underlying mechanism shows that the superiority of the mixed-phase core–shell over other crystalline phases is because of three factors. First, efficient carrier separation at the core–shell and mixed-phase junction; second, intercalated water from the reaction medium and the o-WO3·H2O phase; and third, electron trapping sites. The oxygen defects trap the photogenerated electrons and leave free holes. The tunnel structure of the mixed phase provides intercalation and easy diffusion of water molecules. The intercalated water molecules readily interact with the mobile holes to form hydroxyl radicals for activated photocatalysis. All of these factors contribute to the improved photocatalytic performance of the mixed-phase WO3 nanoribbons.