Somnath Chanda Roy

KBiFe2O5 (KBFO) is an interesting multiferroic compound with the coexistence of ferroelectric and ferromagnetic phases with a narrow band gap. In this work, KBiFe2O5 (KBFO) is synthesized using a sol-gel technique, followed by calcination at 973 K. Rietveld refinement analysis of XRD data confirms the presence of monoclinic crystal structure with space group P2/c. The SEM micrograph of KBFO reflects the irregular shape of grains with slight agglomeration. The optical band gap measured by UV visible spectroscopy of KBFO is found to be 1.56 eV. The multiferroic behavior of KBFO has been confirmed by the electrical (dielectric) and magnetic (M-H loop) measurements. A weak ferromagnetic nature is observed due to canting of the magnetic moment of G-type anti-ferromagnetic ordering. The dielectric studies reveal the Maxwell-Wagner type dispersion and an anomaly near magnetic phase transition temperature. KBFO shows negative magneto-dielectric (MD) coupling as both ε and tan δ decreases with the increasing magnetic field. Impedance spectroscopy is performed to analyze the intrinsic and extrinsic effects in MD coupling. The Nyquist plot shows positive magneto-resistance and negative magneto-capacitance which represent strong MD coupling in KBFO. It also reveals that the bulk property (intrinsic effect) dominates over the extrinsic effect in the sample. These results affirm that KBFO is a new candidate for room-temperature multiferroicity with a suitable bandgap and strong MD coupling for photovoltaic applications.

We propose the design of low strained and energetically favourable mono and bilayer graphene overlayer on anatase TiO2 (001) surface and examined the electronic structure of the interface with the aid of first principle calculations. In the absence of hybridization between surface TiO2 and graphene states, dipolar fluctuations govern the minor charge transfer across the interface. As a result, both the substrate and the overlayer retain their pristine electronic structure. The interface with the monolayer graphene retains its gapless linear band dispersion irrespective of the induced epitaxial strain. The potential gradient opens up a few meV bandgap in the case of Bernal stacking and strengthens the interpenetration of the Dirac cones in the case of hexagonal stacking of the bilayer graphene. The difference between the macroscopic average potential of the TiO2 and graphene layer(s) in the heterostructure lies in the range 3–3.13 eV, which is very close to the TiO2 bandgap (~3.2 eV). Therefore, the proposed heterostructure will exhibit enhanced photo-induced charge transfer and the graphene component will serve as a visible light sensitizer.

TiO2-g-C3N4 composite material is fabricated by simple in situ thermal processes. From field emission scanning electrons spectroscopy (FESEM), it is observed that TiO2 nanoparticles are embedded on the surface of g-C3N4 (g-CN) sheets. The addition of g-CN with TiO2 reduces the bandgap from UV (3.10 eV) to visible (2.82 eV) light region, which helps to absorb more solar irradiation and shows higher photocurrent density for TiO2-g-CN compared to TiO2 and g-CN. Photoluminescence analysis reveals that the recombination rate of photogenerated charge carriers reduces, and impedance spectroscopy study shows charge transfer resistance decreases in TiO2-g-CN composite material. A type-II heterojunction between TiO2 and g-CN leads to the enhancement of the photoelectrochemical performance of the composite material.