L. Krishna Bharat

Synthesis and luminescence properties of color-tunable Dy3+-activated CaWO4 phosphors

Color-tunable CaWO4:xDy3+ phosphors were prepared via a simple conventional solid-state reaction and their luminescent properties were investigated as a function of Dy3+ ion concentration under 258 and 353 nm excitations. A gradual enhancement in the emission intensity was observed with the increment of Dy3+ ion concentration, reaching its maximum value when x = 0.05. The main reason for the concentration quenching of Dy3+ emission in CaWO4 host lattice is due to the electric dipole-dipole interaction. The cathodoluminescence spectra, which were measured at different accelerating voltages and filament currents, were in consistent with the photoluminescence spectrum excited at 258 nm. Additionally, the emission color of CaWO4:xDy3+ phosphors can be suitably tuned from blue to green, and finally to yellow by the modulation of excitation wavelength and Dy3+ ion concentration. Ultimately, these color-tunable phosphors may have potential applications in the fields of miniature color displays.

Wearable Fabrics with Self-Branched Bimetallic Layered Double Hydroxide Coaxial Nanostructures for Hybrid Supercapacitors

We report a flexible battery-type electrode based on binder-free nickel cobalt layered double hydroxide nanosheets adhered to nickel cobalt layered double hydroxide nanoflake arrays on nickel fabric (NC LDH NFAs@NSs/Ni fabric) using facile and eco-friendly synthesis methods. Herein, we utilized discarded polyester fabric as a cost-effective substrate for in situ electroless deposition of Ni, which exhibited good flexibility, light weight, and high conductivity. Subsequently, the vertically aligned NC LDH NFAs were grown on Ni fabric by means of a hot-air oven-based method, and fluffy-like NC LDH NS branches are further decorated on NC LDH NFAs by a simple electrochemical deposition method. The as-prepared core-shell-like nanoarchitectures improve the specific surface area and electrochemical activity, which provides the ideal pathways for electrolyte diffusion and charge transportation. When the electrochemical performance was tested in 1 M KOH aqueous solution, the core-shell-like NC LDH NFAs@NSs/Ni fabric electrode liberated a maximum areal capacity of 536.96 μAh/cm2 at a current density of 2 mA/cm2 and excellent rate capability of 78.3% at 30 mA/cm2 (420.5 μAh/cm2) with a good cycling stability. Moreover, a fabric-based hybrid supercapacitor (SC) was assembled, which achieves a stable operational potential window of 1.6 V, a large areal capacitance of 1147.23 mF/cm2 at 3 mA/cm2, and a high energy density of 0.392 mWh/cm2 at a power density of 2.353 mW/cm2. Utilizing such high energy storage abilities and flexible properties, the fabricated hybrid SC operated the wearable digital watch and electric motor fan for real-time applications.

Designed construction of yolk–shell structured trimanganese tetraoxide nanospheres via polar solvent-assisted etching and biomass-derived activated porous carbon materials for high-performance asymmetric supercapacitors

Recently, yolk–shell structured electrode materials have attracted increasing interest in supercapacitors (SCs) due to their high surface area, good electrochemical activity and excellent mechanical stability towards superior energy storage performance. However, the synthesis strategies to prepare such yolk–shell structured materials without using chemical surfactants/solid templates are still inferior. Herein, a facile and cost-effective strategy to design yolk–shell structured trimanganese tetraoxide nanospheres (Mn3O4 NSs) with a distinctive core–void–shell configuration to use as an efficient positive electrode material in asymmetric SCs is demonstrated. Specifically, the yolk–shell structured Mn3O4 NSs were prepared by the inclusion of water droplets to the manganese precursor–isopropyl alcohol system, which facilitates the inside-out Ostwald ripening process to construct a yolk–shell-like configuration with porous properties. In aqueous electrolyte solution, the corresponding material exhibited a high specific capacitance (211.36 F g−1 at a current density of 0.5 A g−1), a good rate capability (79.4% at 10 A g−1) and an excellent cycling stability (92% after 2000 cycles) compared to its solid counterparts. Meanwhile, a low-cost material based on biomass-derived activated carbon with a honeycomb-like structure is also prepared using waste corrugated boxes, which exhibits a reliable electrochemical performance for use as a negative electrode material. Moreover, the fabricated asymmetric SC using both electrode materials offers a maximum potential window of 2 V with higher energy density (19.47 W h kg−1) and power density (2263.89 W kg−1) values, which can effectively power up commercial light-emitting diodes for practical applications.

Ba3(PO4)2 hierarchical structures: synthesis, growth mechanism and luminescence properties

Europium ions-activated flower-like Ba3(PO4)2 phosphor samples with assembled nanoplates were prepared by a facile synthesis route. X-ray diffraction patterns confirmed the single crystal structure with a rhombohedral phase. The synthesis mechanism of these hierarchical structures was explained from scanning electron microscope images taken at different growth times. Transmission electron microscopy was performed to further examine their crystallinity and the structural properties were also studied by Fourier transform infrared spectroscopy analysis. The optical properties were investigated by analyzing the photoluminescence (PL) excitation and emission spectra of the samples. The PL emission spectra showed red emission of trivalent europium (Eu3+) ions along with blue emission due to the reduction of Eu3+ to its divalent (i.e., Eu2+) state. The cathodoluminescence spectra of the samples were almost similar with the PL spectra without any acute changes.

Rare-earth free self-luminescent Ca 2 KZn 2 (VO 4 ) 3 phosphors for intense white light-emitting diodes

The commercially available white-light-emitting diodes (WLEDs) are made with a combination of blue LEDs and yellow phosphors. These types of WLEDs lack certain properties which make them meagerly applicable for general illumination and flat panel displays. The solution for such problem is to use near-ultraviolet (NUV) chips as an excitation source because of their high excitation efficiency and good spectral distribution. Therefore, there is an active search for new phosphor materials which can be effectively excited within the NUV wavelength range (350–420 nm). In this work, novel rare-earth free self-luminescent Ca2KZn2(VO4)3 phosphors were synthesized by a citrate assisted sol-gel method at low calcination temperatures. Optical properties, internal quantum efficiency and thermal stability as well as morphology and crystal structure of Ca2KZn2(VO4)3 phosphors for their application to NUV-based WLEDs were studied. The crystal structure and phase formation were confirmed with XRD patterns and Rietveld refinement. The optical properties of these phosphor materials which can change the NUV excitation into visible yellow-green emissions were studied. The synthesized phosphors were then coated onto the surface of a NUV chip along with a blue phosphor (LiCaPO4:Eu2+) to get brighter WLEDs with a color rendering index of 94.8 and a correlated color temperature of 8549 K.

Preparation of Eu3+ ions activated Ca2La8(SiO4)6O2 oxyapatite nanophosphors through two-step surfactant-free method and their optical and electrical properties

Eu3+ ions activated Ca2La8(SiO4)6O2 (CLSO):Eu3+ nanophosphor samples were synthesized by a mixed solvothermal and hydrothermal method. The samples were carefully studied using various characterization techniques. The XRD patterns of CLSO:Eu3+ and CLSO confirmed that the samples were crystallized in hexagonal phase with a space group of P63/m (176). The morphology of the nanoparticles was studied by varying the reaction parameters such as growth, temperature and time. The photoluminescence (PL) excitation and PL emission spectra exhibited the typical Eu3+ bands in the wavelength range of 200-550 nm and 400-750 nm, respectively. The intensity of the [Formula: see text] electric dipole (ED) transition peak was strong in the PL emission spectrum which imparts the red color when observed under ultraviolet light. The ED transition peak intensity increased when the sample was calcined at an elevated temperature of 700 °C, indicating improved asymmetry ratio and good chromaticity coordinates. The electrical properties of the prepared materials were studied by spin-coating the powder dispersed solutions on the silica substrate. The output current values were also measured for the CLSO nanoparticles prepared under different growth conditions. These results showed the advantages of CLSO nanoparticles for their application in optics and feasibility in nanoelectronic and energy harvesting devices.

Controlled synthesis of yttrium gallium garnet spherical nanostructures modified by silver oxide nanoparticles for enhanced photocatalytic properties

Spherical yttrium gallium garnet (Y3Ga5O12) samples were prepared by a single-step hydrothermal synthesis method, and these particles were coated with silver oxide (Ag2O) nanoparticles by following a facile wet chemical approach at ambient temperature. The structural, morphological, vibrational and optical properties of the as-prepared and Ag2O modified Y3Ga5O12 samples were analyzed. From Rietveld refinement of X-ray diffraction patterns, the crystal structure of the Y3Ga5O12 sample was refined. The morphological properties confirmed that the particles were spherical and their surfaces were covered with Ag2O nanoparticles. The Ag2O modified Y3Ga5O12 sample exhibited a relatively enhanced photocatalytic activity compared to the pure Y3Ga5O12 sample in Rhodamine B solution, suggesting that the introduction of Ag2O broadens the absorption of light and controls the photogenerated carrier recombination in Y3Ga5O12. These results provide a clear perspective on the synthesis of hybrid photocatalyst materials which have several ways of photogenerated carrier migration.

Red and green colors emitting spherical-shaped calcium molybdate nanophosphors for enhanced latent fingerprint detection

We report the synthesis of spherical-shaped rare-earth (Eu3+ and Tb3+) ions doped CaMoO4 nanoparticles in double solvents (IPA and H2O) with the help of autoclave. The X-ray diffraction patterns well match with the standard values and confirm the crystallization in a tetragonal phase with an I41/a (88) space group. The luminescence spectra exhibit the strong red and green emissions from Eu3+ and Tb3+ ions doped samples, respectively. The X-ray photoelectron spectroscopy results show the oxidation states of all the elements present in the sample. The temperature-dependent luminescence spectra reveal the stability of Eu3+ and Tb3+ ions doped samples. The red- and green-emitting Eu3+ and Tb3+ ions doped CaMoO4 samples were used for detection and enhancement of latent fingerprints which are the common evidences found at crime scenes. The enhanced latent fingerprints obtained on different surfaces have high contrast with low background interference. The minute details of the fingerprint which are useful for individualization are clearly observed with the help of these nanopowders.

Enhancing the output performance of hybrid nanogenerators based on Al-doped BaTiO3 composite films: a self-powered utility system for portable electronics

Enhancing the output performance of nanogenerators using composite films consisting of a piezoelectric material embedded into polymers has gained much attention over the last few years. Such composite films can provide a high surface charge density and dielectric permittivity, which can further efficiently enhance the performance of nanogenerators. We, for the first time, employed aluminum (Al)-doped barium titanate (BaTiO3; ABTO) particles to enhance the performance of nanogenerators. These ABTO particles were synthesized via a solid-state technique, and the effect of Al dopant concentration on their crystallinity and ferroelectric properties was systematically investigated. However, the BTO particles with 2% Al dopant concentration exhibited a high remnant polarization and piezoelectric coefficient, and they were further employed to efficiently enhance the output performance of the hybrid piezo/triboelectric nanogenerators. For this, these ABTO particles were first mixed with polydimethylsiloxane (PDMS) to prepare a composite film. Next, the ABTO/PDMS composite film was employed as a piezoelectric material and triboelectric material of the hybrid nanogenerator (HNG) and exhibited a high output performance owing to their synergetic effects. In addition, the influence of the surface roughness of the composite film on the performance of the HNG was also investigated and optimized. Consequently, the HNG device with the rough surface ABTO/PDMS composite film exhibited maximal open-circuit voltage, short-circuit current, and power density values of ∼945 V, ∼59.8 μA, and ∼42.4 W m−2, respectively. For practical device application, the stable and high electrical power generated from the HNG device was employed to light several light-emitting diodes and power portable electronic devices.

Fallen leaves derived honeycomb-like porous carbon as a metal-free and low-cost counter electrode for dye-sensitized solar cells with excellent tri-iodide reduction

Utilizing carbon-based counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) have received much attention in recent times, owing to their low cost, good electrochemical activity, natural abundance and eco-friendly nature. Herein, we have facilely prepared quince leaves derived porous carbon (QLPC) using fallen quince leaves (QLs) and it was used as a cost-effective CE for the fabrication of DSSCs. By means of alkali treatment and pyrolysis process (at different temperatures of 700, 800 and 900 °C), the QLs powder undergoes chemical activation and carbonization, which results in a honeycomb-like QLPC with abundant micro/mesopores and large surface area. Simple and straightforward coating of QLPC samples onto fluorine doped tin oxide glass substrates led to improved electrocatalytic activity and good tri-iodide reduction in DSSCs. When the DSSCs were illuminated under 1 sun condition (AM 1.5; 100 mW cm-2), the device assembled with QLPC-based CE (prepared at 800 °C) showed a higher current density of ∼14.99 mA/cm2 and power conversion efficiency of ∼5.52% among the other QLPC-based CEs, which are comparable with the platinum-based CE in DSSCs. This facile process for the preparation of biomass derived carbon-based CE provides an alternative to the noble metal-free CE in DSSCs.