Controlled synthesis of monodispersed ZnSe microspheres for enhanced photo-catalytic application and its corroboration using density functional theory.

The present work reports enhanced photocatalytic performance of highly crystalline, monodisperse ZnSe microspheres, synthesized by the size-selective, ETDA-assisted hydrothermal method. Systematic studies on time-dependent reaction kinetics and growth parameters indicate dependency of morphology and crystal structure on the volume % of EDTA and dependency of size on the volume % of hydrazine hydrate for ZnSe microspheres. X-ray diffraction studies confirm highly crystalline cubic zinc blende crystal structure with crystallite size in the range of 10-15 nm. Diffuse reflectance spectra show blue shift having a broad absorption peak between 415 and 425 nm, with a band gap of ∼2.6 eV from the K-M plot. Photoluminescence spectra show higher ratio of near band edge emission to deep level emission, confirming the decrease in defect related emission and depicting the higher crystallinity of ZnSe. Raman spectroscopy also confirms the crystalline and pure nature of ZnSe microspheres, from the observation of a high intensity dominant peak at 248 cm-1, attributed to longitudinal optical phonon scattering. Morphological analysis using FE-SEM and HRTEM shows monodispersed microspheres having size ∼2.5 µm, made up of small ZnSe nanocrystals with ∼10 nm size and with an interplanar spacing of ∼0.32 nm, corresponding to zinc blende ZnSe(111) planes. Brunauer-Emmett-Teller analysis indicates type-IV adsorption-isotherms and hysteresis loops, confirming the presence of mesopores on the surface of ZnSe microspheres controlling the diffusion rate of the catalyst. The degradation rate constant for methylene-blue using first-order reaction kinetics confirms improvement in photocatalytic activity by a factor of 7 to 13 times higher than that of bulk ZnSe, which is attributed to the controlled, well-defined morphology of spherical microstructures made up of small-sized ZnSe nanocrystals. Density functional theory based calculations support the preferential adsorption of EDTA at the Se site of the ZnSe(111) surface with an energy of -1.90 eV. The electronic-structure plot demonstrates semiconducting behaviour with a direct band gap of ∼1.51 eV. First-principles calculations confirm enhancement in the photocatalytic water splitting activity of the ZnSe(111) surface via adsorption of intermediates. The improvement in dye degradation can be attributed to the enhanced oxidation process through the formation of intermediates such as O* (-3.13 eV) and HO* (-2.57 eV) at the Se site of the ZnSe surface.

Magnetic properties of 2D nickel nanostrips: structure dependent magnetism and Stoner criterion.

We have investigated different geometries of two-dimensional (2D) infinite length Ni nanowires of increasing width using spin density functional theory calculations. Our simulations demonstrate that the parallelogram motif is the most stable and structures that incorporate the parallelogram motif are more stable as compared to rectangular structures. The wires are conducting and the conductance channels increase with increasing width. The wires have a non-linear behavior in the ballistic anisotropic magnetoresistance ratios (BAMR) with respect to the magnetization directions. All 2D nanowires as well as Ni (1 1 1) and Ni (1 0 0) monolayer investigated are ferromagnetic under the Stoner criterion and exhibit enhanced magnetic moments as compared to bulk Ni and the respective Ni monolayers. The easy axis for all nickel nanowires under investigation is observed to be along the wire axis. The double rectangular nanowire exhibits a magnetic anomaly with a smaller magnetic moment when compared to Ni (1 0 0) monolayer and is the only structure with an easy axis perpendicular to the wire axis. The Stoner parameter which has been known to be structure independent in bulk and surfaces is found to vary with the structure and the width of the nanowires. The less stable rectangular and rhombus shaped nanowires have a higher ferromagnetic strength than parallelogram shaped nanowires.