Band gap engineering of TiO2 by Mn doping and the effect of p-TNT: Mn/n-MnO2 heterojunction on photocatalytic applications

The present study reports the achievement of improved photocatalytic performance of Titanium dioxide nanotubes (TNTs) by manganese doping and the subsequent formation of a p-TNT: Mn/n-MnO2 heterojunction. The nanotubes and junctions are prepared by simple and cost-effective electrochemical anodization and doping techniques. The influence of doping and junction formation on the structural, optical, morphological and electrical properties of TNTs are analysed in detail. Energy dispersive x-ray mapping together with X-ray photoelectron spectroscopy is used to confirm the composition of the samples, while atomic force microscopy, field emission scanning electron microscopy and transmission electron microscopy are used for morphological assessment. The TNT/MnO2 junction shows a photocatalytic degradation efficiency of 98.6% with good cyclic stability for Rhodomine B dye. Manganese doping tailors the optical band gap of TNT from similar to 3.04 eV to similar to 2.73 eV enabling the absorption of visible photons for carrier production and induces p-type conductivity in the sample. While valence band photoemission spectra give insight into the Fermi level positions of doped and undoped samples and confirm the p type conductivity of the latter, the photoluminiescence measurements give an idea regarding the defect states. The reduction in the band gap of TNT on Mn doping along with the formation of an n-MnO2 layer with a band gap similar to 1.50 eV on its top play a crucial role in the improvement of the photocatalytic performance of the pn- heterojunction device.