Modulating the Morphological Features of Hydrothermally Derived Nickel Cobaltite for Supercapacitor Application: Role of Precursor Anions.

Morphologically engineered porous electrodes show great promise in energy applications as they exhibit improved electrochemical activity owing to increased electrical conductivity, increased surface area, and a shorter path length for ion transport. Herein, the role of precursors (chlorides, acetates and nitrates) on the crystallinity and textural features of Nickel Cobaltite, obtained by the controlled precipitation through hydrothermal synthesis, is studied. The synthesis yielded urchin like structures with morphological variations in substructures based on the precursor anion types. The surface area values obtained for nickel cobaltite derived from the chloride (NCO-C), nitrate (NCO-N), and acetate precursors (NCO-A) were 30,110 and 115 m2 g-1 , confirming the influence of anions on the textural features. The time dependant electrolyte (2 M KOH) infiltration behaviour on the electrode surfaces based on contact angle measurements is invoked to correlate its morphological and textural attributes with the electrolyte transport kinetics. The electrochemical performances were derived from cyclic voltammetry, galvanostatic charge discharge analysis and impedance measurements. The electrolyte infiltration studies established a dependence on the precursor anion. NCO-A facilitated faster electrolyte infiltration time of 7800 ms compared to 16200 ms and 54,000 ms for NCO-N and NCO-C electrodes, respectively. Furthermore, NCO-A exhibited a greater specific capacitance of 802 F g-1 than NCO-N (500 F g-1 ) and NCO-C (342 F g-1 ). The morphology modulation coupled with optimal porosity led to conducive pathways for reversible electrolyte infiltration resulting in increased capacitive contribution in NCO-A. The study revealed that the size of intercalating anions exercised a significant impact on the morphological and electrochemical features, signifying the importance of synthetic approaches in determining the functional properties of electrode materials.

Assessing the role of plasma-engineered acceptor-like intra- and inter-grain boundaries of heterogeneous WS2-WO3 nanosheets for photocurrent characteristics.

High-temperature annealing in tungsten disulfide resulted in heterogeneous WS2-WO3 in which intra- (within WS2 and WO3) and inter- (between WS2 and WO3) grain boundaries were observed, which were highly critical for charge transport and recombination. The heterogeneous WS2-WO3 phase was evidenced by observing the coexistence of d-spacing values of 0.26 nm (WS2) and 0.37 nm (WO3) in transmission electron microscopic (TEM) studies. Further systematic high-resolution TEM studies elucidated that intra-grain boundaries separated crystallites within WS2 and WO3, while inter-grain boundaries separated WS2 from WO3. As WS2 and WO3 are both n-type, these defects are acceptor-like in the grain boundaries and they actively participate in the capture (trapping) process, which impedes charge transport characteristics in the heterogeneous WS2-WO3 films. Plasma treatment in the heterogeneous WS2-WO3 film, for 60 minutes using argon, energetically modulated the defects in the intra/inter-grain boundaries, as evidenced from detailed comparative photocurrent characteristics obtained individually in (i) pristine WS2, (ii) heterogeneous WS2-WO3 and (iii) Ar plasma-treated heterogeneous WS2-WO3 films under blue and green lasers, along with AM1.5 (1 sun) illumination. Detrimental roles (trapping/de-trapping and scattering) of grain boundary states on photoelectrons were seen to be significantly suppressed under the influence of plasma.

Plasma driven nano-morphological changes and photovoltaic performance in dye sensitized 2D-layered dual oxy-sulfide phase WS2 films.

The present work examined dye sensitized 2D layered tungsten disulfide (WS2) as a photo-anode in solar cells with no use of nanocrystalline metal oxides as electron acceptors such as titanium dioxide. It is observed that coating WS2 directly onto a fluorine doped tin oxide (FTO) transparent conductor, annealed at 530 °C, resulted in a mixed oxy-sulfide dual phase as confirmed by transmission electron microscopy and X-ray diffraction studies. Further studies on the surface morphology of the dual phase WS2-WO3 film showed a random distribution of platelets which further shaped into precisely regulated hexagonal platelets upon plasma treatment. High resolution transmission electron microscopic studies elucidated two different phases, WS2 and WO3, with d-spacing values of 0.26 nm and 0.37 nm, respectively. A well-defined grain boundary was also observed which separated the oxy-sulfide phase in the sample. The dual WS2-WO3 phase films showed optical absorption in the wavelength range of 350 nm-800 nm with a systematic increase in plasma exposure duration. Photovoltaic devices fabricated using the WS2-WO3 mixed phase photo-anodes resulted in 0.61% efficiency (η) which further was observed to be sensitive to the plasma exposure as it was observed that the 20 minute plasma treated sample increased the η value to 0.67%. Plasma treatment on the dual-phase samples orients and modifies the shapes of the platelets with a significant change in the surface which eventually influences the charge transport in resulting photovoltaic devices and thus the variation with respect to exposure duration.