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

Abstract

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 L2,3 and Mn L2,3 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 MnOx 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.