Benign Synthesis of Black Microspheres of Anatase TiO2 with Paramagnetic Oxygen Vacancies through NH3 Treatment.

Coloured TiO2 is coveted for its ability to extract energy from the visible region of electromagnetic spectrum. Here a facile synthesis of black anatase titania microspheres (B-TiO2 ) through a two-step process is reported. In the first step, amorphous white TiO2 microspheres (W-TiO2 ) are obtained by hydrolysing titanium tetraisopropoxide by ammonia vapours in ethanol. In the second step, the W-TiO2 is thermally annealed at 500 °C to obtain B-TiO2 . The diffuse reflectance analysis showed that B-TiO2 absorbs across visible spectrum with absorption extending well into NIR region. Raman scattering together with EPR analysis showed compelling evidence of the existence of oxygen deficiency within the crystal in B-TiO2 that induces black colouration in the sample. The defects present in the black anatase sample were confirmed to be single-electron-trapped (or paramagnetic) oxygen vacancies (Vo ⋅) by XPS and EPR studies. The magnetic susceptibility studies showed existence of antiferromagnetic interactions between these unpaired electron spins.

Nano "Koosh Balls" of Mesoporous MnO2 : Improved Supercapacitor Performance through Superior Ion Transport.

Manganese dioxide nanomaterials with "Koosh-ball"-like morphology (MnO2 -KBs) as well as worm-like nanotubes (MnO2 -NWs) are obtained by employing Tween 20 as the reducing and structure-directing agent, and KMnO4 as a MnO2 precursor. Whereas the MnO2 -KBs are interconnected through tubular extensions, the MnO2 -NWs are largely disconnected. Both MnO2 -KBs and MnO2 -NWs have large BET surface areas (>200 m2 g-1 ), and are thermally robust up to 300 °C. Electrochemical studies reveal that the highest specific capacitance (Csp ) obtained for MnO2 -KBs (272 F g-1 ) is significantly higher than that of MnO2 -NWs (129 F g-1 ). The Csp values correlate well with the electroactive surface areas of the materials: MnO2 -KBs have a significantly higher electrolyte-accessible surface area. Electrochemical impedance spectroscopy (EIS) reveals a higher electron-transfer rate at the electrode/electrolyte interface for MnO2 -KBs than for MnO2 -NWs. The multiple tubular interconnections between individual MnO2 -KBs allow improved ion penetration and act as conduits for their propagation, shortening the diffusion distances of the ions from external electrolytes to the interior of the MnO2 framework. Thus, this work exemplifies the importance of interconnections for enhancing the electrochemical performance of nanomaterials employed for energy storage.

Chiral Magneto-Optical Properties of Supra-Assembled Fe3O4 Nanoparticles.

Research on the chiral magneto-optical properties of inorganic nanomaterials has enabled novel applications in advanced optical and electronic devices. However, the corresponding chiral magneto-optical responses have only been studied under strong magnetic fields of ≥1 T, which limits the wider application of these novel materials. In this paper, we report on the enhanced chiral magneto-optical activity of supra-assembled Fe3O4 magnetite nanoparticles in the visible range at weak magnetic fields of 1.5 mT. The spherical supra-assembled particles with a diameter of ∼90 nm prepared by solvothermal synthesis had single-crystal-like structures, which resulted from the oriented attachment of nanograins. They exhibited superparamagnetic behavior even with a relatively large supraparticle diameter that exceeded the size limit for superparamagnetism. This can be attributed to the small size of nanograins with a diameter of ∼12 nm that constitute the suprastructured particles. Magnetic circular dichroism (MCD) measurements at magnetic fields of 1.5 mT showed distinct chiral magneto-optical activity from charge transfer transitions of magnetite in the visible range. For the supraparticles with lower crystallinity, the MCD peaks in the 250-550 nm range assigned as the ligand-to-metal charge transfer (LMCT) and the inter-sublattice charge transfer (ISCT) show increased intensities in comparison to those with higher crystallinity samples. On the contrary, the higher crystallinity sample shows higher MCD intensities near 600-700 nm for the intervalence charge transfer (IVCT) transition. The differences in MCD responses can be attributed to the crystallinity determined by the reaction time, lattice distortion near grain boundaries of the constituent nanocrystals, and dipolar interactions in the supra-assembled structures.