Unexpected Photoinduced Room Temperature Magnetization in Bi2WO6 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 Bi2WO6 nanosheets is reported for the first time. Compared with the original state, the ultraviolet (UV)-illuminated Bi2WO6 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 Bi2WO6 nanosheets, indicating a photoexcited Bi to W charge transfer. Density functional theory (DFT) calculations indicate spontaneous spin polarization of the Bi2WO6 nanosheets in the excited metastable state. Meanwhile, thicker Bi2WO6 nanoplates or nanoparticles show no enhanced magnetic signals upon UV illumination. UV illumination of the thin Bi2WO6 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 Bi2O2 layers, leading to spin polarization and enhance the magnetization. The research may bring some new insights in tuning the magnetic properties of 2D nanostructures.