Unexpected Photoinduced Room Temperature Magnetization in Bi₂ WO₆ Nanosheets

The current investigation in magnetism in 2D materials offers new opportunities for studying spintronics at low dimensions. Here, reversible photoinduced room temperature magnetization in 2D Bi₂ WO₆ nanosheets is reported for the first time. Compared with the original state, the ultraviolet (UV)-illuminated Bi₂ WO₆ nanosheets show a yellow-green color change and significantly enhanced magnetic signals (saturated magnetization (M_s ) increased from 0.002 to 0.12 emu g^-1 ). X-ray photoelectron spectroscopy (XPS) results show unexpected W reduction (W⁶⁺ to W⁵⁺ /W4+) and Bi oxidation (Bi³⁺ to Bi⁵⁺ ) upon UV illumination for the Bi₂ WO₆ nanosheets, indicating a photoexcited Bi to W charge transfer. Density functional theory (DFT) calculations indicate spontaneous spin polarization of the Bi₂ WO₆ nanosheets in the excited metastable state. Meanwhile, thicker Bi₂ WO₆ nanoplates or nanoparticles show no enhanced magnetic signals upon UV illumination. UV illumination of the thin Bi₂ WO₆ nanosheets can induce the formation of internal electric field (polarization), leading to structural deformation/lattice distortion (photostriction). The photoexcited electrons are trapped in the WO₆ layers while the photogenerated holes are trapped in the Bi₂ O₂ layers, leading to spin polarization and enhance the magnetization. The research may bring some new insights in tuning the magnetic properties of 2D nanostructures.