Near-Infrared Laser-Induced Photothermal Coloration in WO3·H2O Nanoflakes

The structure and chromic properties of WO3·H2O nanoflakes have been investigated in situ by Raman spectroscopy and optical transmission under 785-nm laser irradiation with different powers. At a small power below 48 mW, the characteristic stretching vibrations of orthorhombic WO3·H2O at ν1 ≈ 650 cm-1 (W6+−O) and ν2 ≈ 947 cm-1 (terminal W6+−O) are observed, and their respective peak intensities, along with optical transmission, decay monotonically with irradiation time. At high-power irradiation (>48 mW), the Raman peaks at ν1 and ν2 decrease rapidly, and two new W6+−O stretching vibrations at ν3 ≈ 710 cm-1 and ν4 ≈ 805 cm-1, ascribed to monoclinic WO3, appear. At the same time, a photoinduced color change occurs rapidly, accompanied by a subsequent slow oxidation process, as reflected in optical transmission. The change of the Raman peaks' intensities with the irradiation time is consistent with the change of the optical transmission. Heat transfer analysis and the annealing experiment show that the color change is ascribed to photoinduced thermochromism, and a photothermal coloration reaction mechanism is proposed to explain quantitatively this near-infrared laser-induced effect.