U. Sandhya Shenoy

Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping.

Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb0.6 Sn0.4 Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb0.6 Sn0.4 Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb0.6 Sn0.4 Te increases p-type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb0.58 Sn0.40 Na0.02 Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications.

Novel RGO–ZnWO4–Fe3O4 nanocomposite as high performance visible light photocatalyst

A novel RGO–ZnWO4–Fe3O4 nanocomposite is synthesized by a microwave irradiation method and its catalytic activity for the photo degradation of Methylene Blue (MB) is investigated. The prepared nanocomposites are characterized by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), Raman spectroscopy, photoluminescence spectroscopy (PL) and UV-visible spectroscopy. The visible light photocatalytic activities of the prepared nanocomposites are investigated using a MB dye solution. It is noteworthy that RGO–ZnWO4–Fe3O4 nanocomposites exhibited relatively high photocatalytic activity compared to ZnWO4–RGO and pure ZnWO4 on MB in aqueous solution. This enhanced rate is due to the ability of the graphene in the RGO–ZnWO4–Fe3O4 composite to support carrier exploitation efficiently by tolerating the photo excited electron–hole pairs and thus encouraging oxidative degradation of the pollutants. This work could be extended to other organic pollutants as well and could provide new insights into ternary nanocomposites as high performance photocatalysts and their application in waste water treatment.

An enhanced Seebeck coefficient and high thermoelectric performance in p-type In and Mg co-doped Sn1−xPbxTe via the co-adjuvant effect of the resonance level and heavy hole valence band

Recently, tin telluride (SnTe) has drawn much attention as a potential candidate for thermoelectric power generation. Herein, we report the high thermoelectric performance in SnTe achieved through a two-step design (a) reduction in lattice thermal conductivity via solid solution alloying and (b) enhancement of the Seebeck coefficient (S) via the modification of the electronic structure through co-doping. First, we demonstrate that the introduction of Pb into the position of Sn in SnTe decreases the excess of p-type carrier concentration in SnTe. Notably, the Sn0.70Pb0.30Te sample exhibits a κlatt value of ∼0.67 W m−1 K−1 at 300 K, which is close to the theoretical minimum limit of the κlatt in SnTe, which results mainly from scattering of heat carrying phonons by solid solution point defects. Secondly, we achieve an S value of 121 μV K−1 at 300 K, which increases to ∼241 μV K−1 at 710 K for In and Mg co-doped Sn0.70Pb0.30Te, which is the highest Seebeck coefficient among all the state-of-the-art SnTe based materials known so far. Indium acts as a resonant dopant, leading to a remarkable enhancement in the Seebeck coefficient mainly near room temperature, whereas Mg doping enables the valence band convergence in Sn0.70Pb0.30Te, which is confirmed by density functional theory (DFT) calculations of its electronic structure. As a result of co-doping, a remarkable enhancement in the Seebeck coefficient over a wide range of temperatures is achieved due to the synergistic effect of resonance level formation and valence band convergence. Hence, we have achieved a maximum zT of 1 at 710 K for In and Mg co-doped Sn0.70Pb0.30Te. Notably, an average zT (zTavg) of ∼0.6 is achieved in the temperature range of 300–710 K for the Sn0.655Mg0.04In0.005Pb0.30Te sample.

Effect of potassium doping on electronic structure and thermoelectric properties of topological crystalline insulator

Topological crystalline insulator (TCI), Pb0.6Sn0.4Te, exhibits metallic surface states protected by crystal mirror symmetry with negligibly small band gap. Enhancement of its thermoelectric performances needs tuning of its electronic structure particularly through engineering of its band gap. While physical perturbations tune the electronic structure of TCI by breaking of the crystal mirror symmetry, chemical means such as doping have been more attractive recently as they result in better thermoelectric performance in TCIs. Here, we demonstrate that K doping in TCI, Pb0.6Sn0.4Te, breaks the crystal mirror symmetry locally and widens electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. K doping in Pb0.6Sn0.4Te increases p-type carrier concentration and suppresses the bipolar conduction via widening a band gap, which collectively boosts the thermoelectric figure of merit (ZT) to 1 at 708 K.

Synthesis of BaWO4/NRGO–g-C3N4 nanocomposites with excellent multifunctional catalytic performance via microwave approach

Novel barium tungstate/nitrogen-doped reduced graphene oxide-graphitic carbon nitride (BaWO4/NRGO-g-C3N4) nanocomposite has been synthesized by a simple one-pot microwave technique. The synthesized nanocomposites are well characterized by diffraction, microscopic and spectroscopic techniques to study its crystal structure, elemental composition, morphological features and optical properties. The material prepared is tested for its performance as an electrocatalyst, photocatalyst and reduction catalyst. The nanocomposite catalyzed the photodegradation of methylene blue (MB) dye in 120 min, reduction of 4-nitro phenol (4-NP) to 4-amino phenol (4-AP) in 60 s, showed an impressive Tafel slope of 62 mV/dec for hydrogen evolution reaction (HER). The observed results suggest that the nanocomposite acts as an efficient multifunctional catalyst. The reported approach provides fundamental insights which can be extended to other metal tungstate-based ternary composites for applications in the field of clean energy and environment in the future.