Reversible Switching of the Magnetic Orientation of Titanate Nanosheets by Photochemical Reduction and Autoxidation

Optical properties of aqueous colloidal dispersions of 2D electrolytes, if their aspect ratios are extra-large, can be determined by their orientation preferences. Recently, we reported that a colloidal dispersion of diamagnetic titanate(IV) nanosheets (Ti ^IV NSs), when placed in a magnetic field, is highly anisotropic because Ti ^IV NS anomalously orients its 2D plane orthogonal to the magnetic flux lines due to its large anisotropic magnetic susceptibility. Herein, we report a serendipitous finding that Ti ^IV NSs can be in situ photochemically reduced into a paramagnetic species (Ti ^IV/III NSs), so that their preference of magnetic orientation changes from orthogonal to parallel. This transition distinctly alters the structural anisotropy and therefore optical appearance of the colloidal dispersion in a magnetic field. We also found that Ti ^IV/III NSs is autoxidized back to Ti ^IV NSs under non-deaerated conditions. By using an elaborate setup, the dispersion of Ti ^IV NSs serves as an optical switch remotely operable by magnet and light.