Existence of Ti²⁺ States on the Surface of Heavily Reduced SrTiO3 Nanocubes

Using advanced electron microscopy, we demonstrate the presence of Ti2+ on the {001} surfaces of heavily reduced strontium titanate nanocubes. While high-angle annular dark field images show a clear difference between the surfaces of the unreduced and reduced samples, electron energy loss spectroscopy detects the presence of Ti2+ on the surface of the reduced cubes. Conventional reduction only leads to the formation of Ti3+ and involves the use of high temperatures. Reduction is achieved at relatively lower temperatures in the solid state using sodium borohydride as the reducing agent. Coupled with first principles calculations, our findings provide insights into the optical properties of the samples and provide a convenient method to produce highly reduced surfaces that could demonstrate a range of exotic physical phenomena.