Surface-defect induced modifications in the optical properties of α-MnO2 nanorods

•Alpha-MnO2 nanorods are prepared by chemical method.•Difference in surface defect density is achieved.•Characterized using XRD, Rietveld, XPS, EDS, HR-TEM, BET, UV–vis absorption spectroscopy and PL spectroscopy.•Explains the bandstructure modification due to Jahn–Teller distortions using crystal field theory.•Modification in the intensity of optical emissions related to defect levels validates the concept of surface defect induced tuning of optical properties. The science of defect engineering via surface tuning opens a new route to modify the inherent properties of nanomaterials for advanced functional and practical applications. In this work, two independent synthesis methods (hydrothermal and co-precipitation) are adopted to fabricate α-MnO2 nanorods with different defect structures so as to understand the effect of surface modifications on their optical properties. The crystal structure and morphology of samples are investigated with the aid of X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Atomic composition calculated from energy dispersive spectroscopy (EDS) confirms non-stoichiometry of the samples. The surface properties and chemical environment are thoroughly studied using X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. Bond angle variance and bond valence sum are determined to validate distortions in the basic MnO6 octahedron. The surface studies indicate that the concentration of Jahn–Teller manganese (III) (Mn3+) ion in the samples differ from each other which results in their distinct properties. Band structure modifications due to Jahn–Teller distortion are examined with the aid of ultraviolet–visible (UV) reflectance and photoluminescence (PL) studies. The dual peaks obtained in derivative spectrum conflict the current concept on the bandgap energy of MnO2. These studies suggest that emission lines from the samples can be strongly modified by selectively varying their defect density.