Goli Nagaraju

PDMS-based Triboelectric and Transparent Nanogenerators with ZnO Nanorod Arrays

Vertically-grown ZnO nanorod arrays (NRAs) on indium tin oxide (ITO)-coated polyethylene terephthalate (PET), as a top electrode of nanogenerators, were investigated for the antireflective property as well as an efficient contact surface in bare polydimethysiloxane (PDMS)-based triboelectric nanogenerators. Compared to conventional ITO-coated PET (i.e., ITO/PET), the ZnO NRAs considerably suppressed the reflectance from 20 to 9.7% at wavelengths of 300-1100 nm, creating a highly transparent top electrode, as demonstrated by theoretical analysis. Also, the interval time between the peaks of generated output voltage under external pushing forces was significantly decreased from 1.84 to 0.19 s because the reduced contact area of the PDMS by discrete surfaces of the ZnO NRAs on ITO/PET causes a rapid sequence for triboelectric charge generation process including rubbing and separating. Therefore, the use of this top electrode enabled to operate the transparent PDMS-based triboelectric nanogenerator at high frequency of external pushing force. Under different external forces of 0.3-10 kgf, the output voltage and current were also characterized.

A facile one-step approach to hierarchically assembled core–shell-like MnO2@MnO2 nanoarchitectures on carbon fibers: An efficient and flexible electrode material to enhance energy storage

Hierarchical core–shell-like MnO2 nanostructures (NSs) were used to anchor MnO2 hexagonal nanoplate arrays (HNPAs) on carbon cloth (CC) fibers. The NSs were prepared by a novel one-step electrochemical deposition method. Under an external cathodic voltage of -2.0 V for 30 min, hierarchical core–shell-like MnO2-NS-decorated MnO2 HNPAs (MnO2 NSs@MnO2 HNPAs) were uniformly grown on CC with reliable adhesion. The phase purity and morphological properties of the samples were characterized by various physicochemical techniques. At a constant external cathodic voltage, growth of MnO2 NSs@MnO2 HNPAs on CC was carried for different time periods. When utilized as a flexible, robust, and binder-free electrode for pseudocapacitors, the hierarchical core–shell-like MnO2 NSs@MnO2 HNPAs on CC showed clearly enhanced electrochemical properties in 1 M Na2SO4 electrolyte solution. The results indicate that the MnO2 NSs@MnO2 HNPAs on CC have a maximum specific capacitance of 244.54 F/g at a current density of 0.5 A/g with excellent cycling stability compared to that of bare MnO2 HNPAs on CC (112.1 F/g at 0.5 A/g current density). We believe that the superior charge storage performance of the pseudocapacitive electrode can be mainly attributed to the hierarchical MnO2 NSs@MnO2 HNPAs building blocks that have a large specific surface area, offering additional electroactive sites for efficient electrochemical reactions. The facile and single-step approach to growth of hierarchical pseudocapacitive materials on textile based electrodes opens up the possibility for the fabrication of high-performance flexible energy storage devices.

Highly flexible conductive fabrics with hierarchically nanostructured amorphous nickel tungsten tetraoxide for enhanced electrochemical energy storage

Amorphous nickel tungsten tetraoxide (NiWO4) nanostructures (NSs) were successfully synthesized on a flexible conductive fabric (CF) using a facile onestep electrochemical deposition (ED) method. With an applied external cathodic voltage (–1.8 V for 15 min), the amorphous NiWO4 NSs with burl-like morphologies adhered well on the seed-coated CF substrate. The burl-like amorphous NiWO4 NSs on CF (NiWO4 NSs/CF) are employed as a flexible and binder-free electrode for pseudocapacitors, which exhibit remarkable electrochemical properties with high specific capacitance (1,190.2 F/g at 2 A/g), excellent cyclic stability (92% at 10 A/g), and good rate capability (765.7 F/g at 20 A/g) in 1 M KOH electrolyte solution. The superior electrochemical properties can be ascribed to the hierarchical structure and large specific surface area of the burl-like amorphous NiWO4 NSs/CF. This cost-effective facile method for the synthesis of metal tungsten tetraoxide nanomaterials on a flexible CF could be promising for advanced electronic and energy storage device applications.

Self-assembled hierarchical β-cobalt hydroxide nanostructures on conductive textiles by one-step electrochemical deposition

Three-dimensional (3D) hierarchical cabbage-like β-cobalt hydroxide (β-Co(OH)2) nanostructures were fabricated onto conducive textiles composed of copper-coated polyethylene terephthalate fibers by a simple one-step electrochemical deposition (ED) method. Under an external applied voltage of −0.75 V for 4 h, the hierarchical cabbage-like β-Co(OH)2 nanostructures were well decorated on the conductive textiles. During the ED process, they were self-assembled by the tangled layers of nanoplate (thickness of ~45–55 nm) building blocks and exhibited a cabbage-shaped architecture. The structure and morphology of the as-prepared hierarchical cabbage-like β-Co(OH)2 nanostructures were characterized. The growth mechanism of cabbage like β-Co(OH)2 nanostructures was also investigated by varying the growth time. Moreover, the feasibility test for supercapacitors by measuring the electrochemical properties of the as-prepared nanostructures was performed by cyclic voltammetry and galvanic charge–discharge measurements. The obtained results show that the cabbage-like β-Co(OH)2 nanostructures on the conductive textile substrate exhibited superior energy storage performance compared to the nanoplate morphology.

Multi-stacked PDMS-based triboelectric generators with conductive textile for efficient energy harvesting

We report triboelectric generators with multi-stacked layers of polydimethylsiloxane (PDMS) coated conductive textile (CT), and bare CT, fabricated by a simple and cost-effective methodology. Because PDMS is entirely covered over the upper and bottom surfaces of the CT substrate, both sides are utilized for the friction area of triboelectric charge generation, and the embedded CT can be used as an electrode. The bare CT, which is flexible and durable, also acts as a triboelectric material by rubbing the PDMS as well as an electrode. For a single-layer triboelectric generator, the averaged output voltage/current density of 8.12 V/25.77 nA cm−2 were observed under the external pushing forces of 3.5–4 kgf during footsteps test. The multi-stacked triboelectric generators were prepared by overlapping the PDMS coated CTs and bare CTs repeatedly. Under the same test conditions, the output voltage/current density of the triple-stacked device was considerably increased, up to 2.88/2.45 times, because the overlapped CT and PDMS layers could be uniformly pressed with the increased friction area. The external load dependent output power of the multi-stacked triboelectric generators was also investigated.

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.

UV-A and UV-B excitation region broadened novel green color-emitting CaGd2ZnO5:Tb3+ nanophosphors

Green color-emitting novel CaGd2ZnO5:Tb3+ (CGZO:Tb3+) nanophosphors were synthesized by a citrate sol–gel method. The structural and morphological properties were elucidated by X-ray diffraction and transmission electron microscope measurements. The photoluminescence properties of orthorhombic-phased CGZO:Tb3+ nanophosphors were studied as a function of Tb3+ ion concentration. The CGZO:Tb3+ nanophosphors revealed the enhanced broadband excitation between ultraviolet (UV)-B and UV-A regions. Under 317 nm excitation, even at dilute Tb3+ ion concentrations, only the emission transitions from 5D4 energy level were exhibited. This unusual behavior is due to the occurrence of nonradiative energy transfer via f–d transition rather than cross-relaxation process. The cathodoluminescence also showed similar behavior at low accelerating voltages. These luminescent powders are expected to find potential applications such as white light-emitting diodes and optical display systems.

La(OH)3:Eu3+ and La2O3:Eu3+ nanorod bundles: growth mechanism and luminescence properties

Oriented attachment assisted self-assembled three-dimensional (3D) flower-like La(OH)3:Eu3+ nanorod bundles were successfully synthesized by a facile wet-chemical method. Hexamethylenetetramine played an important role in the formation of the hexagonal phase of La(OH)3:Eu3+ with respect to the reaction time and its concentration. No other surfactants or capping agents were used. The calcination temperature did not show any influence on the morphological texture, and the La2O3:Eu3+ phase was obtained by a subsequent annealing process. The phase formation and morphological properties were confirmed by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The photoluminescence properties were studied for both the synthesized La(OH)3:Eu3+ and La2O3:Eu3+ samples, and also compared with that of the solid-state reaction based La2O3:Eu3+ phosphor. The 3D flower-like La2O3:Eu3+ nanorod bundles showed an intense red emission due to the hypersensitive 5D0 → 7F2 transition with good asymmetric ratio and chromaticity coordinates. Likewise, a systematic study of the cathodoluminescence (CL) properties was carried out in detail. Furthermore, to estimate the CL potentiality, the La2O3:Eu3+ phosphor was compared with a commercially available Y2O3:Eu3+ red phosphor.

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.

Ultrathin nickel hydroxide nanosheet arrays grafted biomass-derived honeycomb-like porous carbon with improved electrochemical performance as a supercapacitive material

Three-dimensional hierarchical honeycomb-like activated porous carbon pillared ultrathin Ni(OH)2 nanosheets (Ni(OH)2 NSs@HAPC) for use as supercapacitor materials were facilely synthesized. With an aid of pine cone flowers as a biomass source, HAPC conducting scaffolds were prepared by the alkali treatment and pyrolysis methods under an inert gas atmosphere. Subsequently, the Ni(OH)2 NSs were synthesized evenly on the surface of HAPC via a solvothermal method. The resulting HAPC and Ni(OH)2 NSs@HAPC composite materials offered free pathways for effective diffusion of electrolyte ions and fast transportation of electrons when employed as an electrode material. The Ni(OH)2 NSs@HAPC composite electrode exhibited excellent electrochemical properties including a relatively high specific capacitance (Csp) value of ~ 916.4 F/g at 1 A/g with good cycling stability compared to the pristine HAPC and Ni(OH)2 NSs electrodes. Such bio-friendly derived carbon-based materials with transition metal hydroxide/oxide composite materials could be a promising approach for high-performance energy storage devices because of their advantageous properties of cost effectiveness and easy availability.