Mn²⁺ Substitutional Doping of TiO₂ Nanoribbons: A Three-Step Approach

An in situ doping approach was successfully employed for synthesis of Mn²⁺-doped sodium titanate nanoribbons, which were used as a precursor for preparation of TiO₂ nanoribbons with homogeneous distribution of Mn²⁺ ions. The comprehensive structural characterization using powder X-ray diffraction (XRD) and electron paramagnetic resonance (EPR) provided compelling evidence that the Mn²⁺ ion predominantly substitutes the Ti⁴⁺ ion at octahedral coordination sites in bulk. Measurements performed on individual nanoribbons using near edge X-ray absorption fine structure spectromicroscopy revealed that the strong alkaline environment required for the formation of sodium titanate nanoribbons did not affect the manganese oxidation state. In the next two steps, the ion exchange process in HCl­(aq) solution followed by the thermal treatment in air, lead to the formation of Mn²⁺ doped TiO₂ nanoribbons. Analysis of the manganese content by X-ray photoelectron spectroscopy of several TiO₂ nanoribbon samples calcined in the temperature range from 400 to 700 °C as well as analysis performed at the Ti L_2,3 and Mn L_2,3 edges with electron energy loss spectroscopy (EELS) showed that calcination at elevated temperatures induced the diffusion of manganese ions toward the nanoribbons’ surface. However, transformation of anatase nanoribbons to rutile nanoparticles, this process started at around 580 °C, was also accompanied by the partial oxidation of Mn²⁺ to Mn³⁺ and Mn⁴⁺. Manganese atoms that diffused to the TiO₂ surface preferentially formed MnO_<i>x</i> clusters as observed from characteristic electron paramagnetic resonance spectra and EELS measurements. In addition, the presence of Mn²⁺ reduced the beginning of phase transformation from anatase to rutile to near 120 °C