We report on the oxygen vacancy induced ferromagnetism (FM) at and above room temperature in undoped TiO₂ nanoporous nanoribbons synthesized by a solvothermal route. The origin of FM in as-synthesized and vacuum annealed undoped nanoribbons grown for different reaction durations followed by calcinations was investigated by several experimental tools. X-Ray diffraction pattern and micro-Raman studies reveal the TiO₂(B), TiO₂(B)-anatase, and anatase–rutile mixed phases of TiO₂ structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal nanoribbons with uniform pore distribution and nanopits/nanobricks formed on the surface. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and a clear ferromagnetic hysteresis loop, both of which dramatically enhanced after vacuum annealing. Direct evidence of oxygen vacancies and related Ti³⁺ in the as-prepared and vacuum annealed TiO₂ samples are provided through X-ray photoelectron spectroscopy analysis. Micro-Raman, infrared absorption and optical absorption spectroscopic analyses further support our conclusion. The observed room temperature FM in undoped TiO₂ nanoribbons is quantitatively analyzed and explained through a model involving bound magnetic polarons (BMP), which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d¹) of Ti³⁺ ion. Our analysis interestingly shows that the calculated BMP concentration scales linearly with concentration of oxygen vacancies and provides a stronger footing for exploiting defect engineered ferromagnetism in undoped TiO₂ nanostructures. The development of such highly porous TiO₂ nanoribbons constitutes an important step towards realizing improved visible light photocatalytic and photovoltaic applications of this novel material.

Zinc oxide (ZnO) nanoparticles (NPs) in the size range $\sim 7–35\text{nm}$ are synthesized by ball-milling technique, and microstructural and optical properties of the NPs are studied using varieties of techniques. Results from ball-milled NPs are compared with those of the commercially available ZnO nanopowder. X-ray diffraction pattern of the milled NPs indicates lattice strain in the NPs. High-resolution transmission electron microscopy analysis reveal severe lattice distortion and reduction in lattice spacing in some of the NPs. Optical absorption spectra of milled NPs show enhanced absorption peaked at 368 nm, which is blueshifted with reference to starting ZnO powder. Room-temperature photoluminescence spectra show five peaks consisting of ultraviolet and visible bands, and relative intensity of these peaks drastically changes with increasing milling time. Raman spectra of milled powders show redshift and broadening of the Raman modes of ZnO, and a new Raman mode evolve in the milled NPs. A correlation between the microstructure and optical properties of ZnO NPs is made on the basis of these results. Our results clearly demonstrate that commercially available ZnO nanopowders do not exhibit nanosize effects due to relatively large size of the ZnO NPs. Implications of these results are discussed.

We investigate the formation mechanism of graphene quantum dots (GQDs) from a graphene oxide (GO) precursor and study the inter-conversion of edge states during thermal annealing of GQDs through various microscopic and spectroscopic tools. Monitoring the early stages of growth of GQDs reveals that the in-plane epoxy (C–O–C) functional groups attached at the defect sites essentially cut the oxidized GOs into small pieces of GQDs during the hydrothermal reaction. By conducting a series of controlled annealing experiments under oxygen and hydrogen gas environments, we monitor and quantify the inter-conversion of edge states and functional groups in GQDs from Raman and photoluminescence (PL) studies. The deconvolution of the Raman spectrum allowed us to monitor the evolution of new Raman bands at ∼1260 and ∼1438 cm⁻¹, which are assigned to the edge functional groups. Through the fitting of the PL spectra and a quantitative analysis of integrated intensities of PL, we unambiguously assign the blue PL emission bands at ∼407 and ∼440 nm to zigzag and armchair free edge states in GQDs, respectively, for the first time. On the other hand, the green emission bands at ∼490 and ∼530 nm are attributed to COOH/C–OH and CO/C–O edge functional groups, respectively. Our conclusions are corroborated by XPS and FTIR spectroscopic analyses of the as-grown and annealed GQDs.

We report on the occurrence of high temperature ferromagnetism (FM) in ZnO nanoparticles (NPs) doped with Co-atoms. ZnO NPs of two different initial sizes are doped with 3% and 5% Co using ball milling and FM is studied at room temperature and above. X-ray diffraction and high-resolution transmission electron microscopy analysis confirm the absence of metallic Co clusters or any other phase different from würtzite-type ZnO. UV-visible absorption studies show change in band structure and photoluminescence studies show green emission band at 520 nm indicating incorporation of Co-atoms and presence of oxygen vacancy defects, respectively in ZnO lattice. Micro-Raman studies of doped samples shows defect related additional bands at 547 and 574 cm−1. XRD and Raman spectra provide clear evidence for strain in the doped ZnO NPs. The field dependence of magnetization (M-H curve) measured at room temperature exhibits the clear FM with saturation magnetization (Ms) and coercive field (Hc) of the order of 3–7 emu/g and 260 Oe, respectively. Temperature dependence of magnetization (M-T) measurement shows sharp ferromagnetic to paramagnetic transition with a high Curie temperature (Tc) of ∼800 K for 3% Co doped ZnO NPs. It is found that doping at 5% and higher concentration does not exhibit a proper magnetic transition. We attempt to fit the observed FM data with the bound magnetic polaron (BMP) model involving localized carriers and magnetic cations. However, calculated concentration of the BMPs is well below the typical percolation threshold in ZnO. We believe that observed high temperature FM is primarily mediated by defects in the strained NPs. ZnO NPs of lower initial size show enhanced FM that may be attributed to size dependent doping effect.