Colloidal suspensions of goethite (alfa-FeOOH) nanorods: effect of synthesis parameters on nanorods morphology and optical properties under magnetic field

Goethite colloids are known mineral liquid crystals and due to their peculiar magnetic properties, the optical properties of nanorod colloids can be tuned under suitably oriented magnetic field. In this work we show that it is possible to tune the transmittance of polarized light, mainly in the NIR range, by properly applying a magnetic field. This property could open new applications for these colloids such as on Smart Windows or Laser-Induced Thermotherapy. Goethite (?-FeOOH) nanorods have been synthetized using two different methods, by the aging of ferrihydrite suspensions obtained through co-precipitation in different conditions, or by fast conversion of ferrihydrite through ultrasonic irradiation. The synthesis parameters influence the morphology of nanoparticles and the optical properties of nanorods colloidal suspensions in water. The nanopowders have been characterized by Powder X-Ray Diffraction and Scanning Electron Microscopy and the colloidal suspensions by Dynamic Light Scattering and ?-potential measurements. Transmittance of colloids in the UV-Vis-NIR range has been determined under different magnetic field intensities and directions with respect to light polarization.