Amphi-Luminescent MoS2 nanostructure for photocatalytic splitting of water and removal of Methylene Blue

Chemical dyes used in the textile industries are one of the major pollutants in water. Methylene blue (MB) is a commonly seen dye that creates hazardous health problems. In this article the photocatalytic degradation of MB by the nanocatalyst MoS2 (Nano-MoS2) and carbon dot (C Dots) incorporated MoS2 (Nano-CD-MoS2) is reported. The photocatalytic degradation of MB is analyzed based on the electron-hole recombination rate of the catalyst. Photoluminescence emission exhibited by the catalyst is used as a key indicator to probe the electron-hole recombination rate. Nano-MoS2 was synthesized hydrothermally at 180 0C for 8 h from ammonium tetra thiomolybdate (ATTM). C Dot was prepared following a green root from ash guard extract which later mixed with Nano-MoS2 and kept in an autoclave at a temperature 140 °C for 4 h to get Nano-CD-MoS2. The photoluminescence (PL) and photocatalytic behavior of Nano-MoS2 and Nano-CD-MoS2 and their application for water splitting and water purification are reported. The incorporation of graphene and artificial C Dot into MoS2 nanostructures are reported to increase the conductivity and active edge sites of MoS2 that enhances the photocatalytic action. Since green C Dots are eco-friendly and easily synthesizable than artificial C Dots, as a novel study, this article investigated the influence of green C Dots on the PL and photocatalytic performance of nanosized MoS2. Nano-MoS2 and Nano-CD-MoS2 exhibited both upconversion and downconversion PL; accordingly the nanostructures were termed as amphi-luminescent. The amphi-luminescence property widens the photon absorption range and hence enhances the catalytic degradation of dyes. Nano-MoS2 which exhibited lesser intensity of amphi-luminescence emission compared to Nano-CD-MoS2 showed better results in degradation of MB. C Dots may bind with the valence band electrons of MoS2, resulting in the reduction of dangling bonds. Dangling bonds can trap photo-induced excitons to hinder the rate of electron-hole recombination. So, fast electron-hole recombination occurs in Nano-CD-MoS2 than Nano-MoS2. Fast electron-hole recombination supports radiative electron-hole recombination while suppresses the non-radiative energy transfer of electrons and causes high PL intensity. However, according to the energy level diagram, Nano-MoS2 with minimal electron-hole recombination rate is more favorable for O2/O2-,.OH/ OH- and.OH/H2O reactions that facilitate MB degradation. Photocatalytic activity of catalysts were confirmed by measuring the photocurrent from a simple custom-made two-electrode water photolysis cell where the nanocatalysts were dispersed in electrolyte. Lead and steel rods were used as electrodes. Multimeter was used to measure current. Nano-MoS2 exhibited better performance with a maximum photocurrent of 141 µA. Influence of green C Dots in energy levels, PL and photocatalysis of MoS2 and mechanisms of PL and degradation of MB are thoroughly investigated in this article.

Unveiling the Radiative Electron-Hole Recombination of MoS2 Nanostructures at Extreme pH Conditions

Nanostructures of MoS2 are in wide research for optoelectronic, energy and biological applications. Opto-electronic and biological applications requires the tuning of photoluminescence properties of MoS2 nanostructures. In this article, nanosized MoS2 is hydrothermally synthesized, and photoluminescence at extreme pH conditions (pH 1 and 13) is examined. As the photoluminescence gives a key to probe the radiative electron-hole recombination, here, photoluminescence emissions are used as an indicator to suggest the pattern of electron-hole recombination in the material at extreme pH conditions. Raman spectroscopy, dynamic light scattering, Scanning electron microscopic image and energy dispersive x-ray analysis are done for material confirmation. At pH 1 and 13 as-synthesized nanostructured MoS2 exhibited both upconversion and downconversion photoluminescence. The intensity of photoluminescence is varied with respect to pH. Excitation-dependent photoluminescence mechanisms and preliminary understanding on the ratio of quantum yields and life span of excited state of as-synthesized nanostructured MoS2 are unveiled here.

Enhanced Upconversion and Downshifting emissions from Tb3+, Yb3+ co-doped CaZrO3 phosphor in presence of Li+ ions

This paper reports the enhanced green photoluminescence from Tb3+, Yb3+ co-doped CaZrO3 phosphor in the presence of Li+ ion synthesized through solid state reaction technique. The structural studies show an increase in the particle size and a shrink in crystal lattice due to Li+ co-doping in the phosphor. The phosphor sample emits intense green upconversion emission (UC) due to Tb3+ ions on excitation with 980 nm radiation which is further enhanced ~ 28 times on Li+ co-doping. The lifetime of 5D4 level of Tb3+ ion decreases in the presence of Li+ ions due to increase in asymmetry in crystal field. The downshifting (DS) emission intensity monitored on 378 and 487nm excitations is also enhanced in the presence of Li+ ions. Thus, the Tb3+, Yb3+, Li+ co-doped CaZrO3 phosphor can be a suitable candidate for UC solid state lighting.

Enhanced NIR downconversion luminescence by precipitating nano Ca5(PO4)3F crystals in Eu2+-Yb3+ co-doped glass

Eu(2+)-Yb(3+) co-doped transparent glass-ceramic containing nano-Ca5(PO4)3F (FAP) was prepared in reducing atmosphere. XRD and TEM analysis indicated that nano-FAP about 40 nm precipitated homogeneously in glass matrix after heat treatment. Confirmed by spectroscopy measurements, the crystal-like absorption and emission of Eu(2+) indicated the partition of Eu(2+) into FAP nanocrystals in glass ceramic. NIR emission due to the transition (2)F5/2→(2)F7/2 of Yb(3+) ions (about 980-1100 nm) was observed from glasses under ultraviolet excitation, ascribed to downconversion from Eu(2+) to Yb(3+), which can be enhanced by precipitating nano-FAP crystals. The results indicated that Eu(2+)-Yb(3+) co-doped glass-ceramic embedding with nano-FAP is a promising candidate as downconversion materials for enhancing conversion efficiency of solar cells.