P. K. Giri

Bio: P. K. Giri is an academic researcher from Indian Institute of Technology Guwahati. The author has contributed to research in topics: Photoluminescence & Raman spectroscopy. The author has an hindex of 38, co-authored 158 publications receiving 4528 citations. Previous affiliations of P. K. Giri include Shiv Nadar University & Kobe University.

Evidence of oxygen vacancy induced room temperature ferromagnetism in solvothermally synthesized undoped TiO2 nanoribbons.

Abstract: 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.

Correlation between microstructure and optical properties of ZnO nanoparticles synthesized by ball milling

Abstract: Zinc oxide (ZnO) nanoparticles (NPs) in the size range ∼7–35 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...

High temperature ferromagnetism and optical properties of Co doped ZnO nanoparticles

Abstract: 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 wurtzite-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 a...

Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis

Abstract: The present work focuses on understanding the heterojunction formation of graphene quantum dots (GQDs) and oxygen deficient TiO2 nanoparticle hybrid system and its enhanced photocatalytic activity under visible light illumination. We explain the formation of TiO2-GQD heterojunction through the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional (epoxy) groups in GQDs possibly via C O Ti bonds. Our FTIR, XPS and Raman results lend support to the proposed mechanism of heterojunction formation. In the TiO2/GQD hybrid, the Raman Eg(1) peak of anatase TiO2 is blue shifted indicating the strong interaction between the GQD and TiO2. The heterojunction formation was simulated through the density functional theory (DFT) calculation to obtain the optical spectrum on the hybrid between oxygen deficient TiO2 and oxygen functionalized GQDs. Interestingly, the calculated results for the hybrid structure show strong optical absorption in the visible to near infrared region, which is in close agreement with the experimental results. The TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation, as revealed from the steady state and time resolved photoluminescence studies. Interestingly, the photoluminescence intensity of the TiO2-GQD heterojunction was partially quenched indicating the electron transfer from GQDs to TiO2. The degradation rate constant (first order) for TiO2-GQD hybrid is 5.2 times higher than that of the TiO2. Free radical scavenger test revealed that OH radical played a major role in MB degradation as compared to O 2 ° − radical. These results are significant for the development of metal free catalysts based on carbon nano-materials for ensuing optoelectronic, energy and environmental applications.

Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability?

Abstract: As a fascinating conjugated polymer, graphitic carbon nitride (g-C3N4) has become a new research hotspot and drawn broad interdisciplinary attention as a metal-free and visible-light-responsive photocatalyst in the arena of solar energy conversion and environmental remediation. This is due to its appealing electronic band structure, high physicochemical stability, and “earth-abundant” nature. This critical review summarizes a panorama of the latest progress related to the design and construction of pristine g-C3N4 and g-C3N4-based nanocomposites, including (1) nanoarchitecture design of bare g-C3N4, such as hard and soft templating approaches, supramolecular preorganization assembly, exfoliation, and template-free synthesis routes, (2) functionalization of g-C3N4 at an atomic level (elemental doping) and molecular level (copolymerization), and (3) modification of g-C3N4 with well-matched energy levels of another semiconductor or a metal as a cocatalyst to form heterojunction nanostructures. The constructi...