High Performance LiMn₂O₄ Cathode Materials Grown with Epitaxial Layered Nanostructure for Li-Ion Batteries

Tremendous research works have been done to develop better cathode materials for a large scale battery to be used for electric vehicles (EVs). Spinel LiMn₂O₄ has been considered as the most promising cathode among the many candidates due to its advantages of high thermal stability, low cost, abundance, and environmental affinity. However, it still suffers from the surface dissolution of manganese in the electrolyte at elevated temperature, especially above 60 °C, which leads to a severe capacity fading. To overcome this barrier, we here report an imaginative material design; a novel heterostructure LiMn₂O₄ with epitaxially grown layered (<i>R</i>3̅<i>m</i>) surface phase. No defect was observed at the interface between the host spinel and layered surface phase, which provides an efficient path for the ionic and electronic mobility. In addition, the layered surface phase protects the host spinel from being directly exposed to the highly active electrolyte at 60 °C. The unique characteristics of the heterostructure LiMn₂O₄ phase exhibited a discharge capacity of 123 mAh g^–1 and retained 85% of its initial capacity at the elevated temperature (60 °C) after 100 cycles

Overestimation of Photoelectrochemical Hydrogen Evolution Reactivity Induced by Noble Metal Impurities Dissolved from Counter/Reference Electrodes

A three-electrode system is typically utilized in many voltammetry studies to understand the behavior of an analyte at the electrode/electrolyte interface. A bulk Pt piece is usually used as a counter electrode in such systems because of its high activity and stability in many electrochemical reactions. However, the dissolution of the Pt counter electrode led to growing concern about inaccurate evaluation of the inherent characteristics of the analyte. In the present study, we have demonstrated that strong interferences emerged from the conventional Pt counter and Ag/AgCl reference electrodes in the photoelectrochemical (PEC) hydrogen evolution reaction (HER) with a model photocathode of p-type silicon (p-Si). Under light illumination, the Pt counter electrode is polarized to as high as 1.6–2.0 VRHE, which leads to a non-negligible Pt dissolution from the oxidized surface, as monitored by operando inductively coupled plasma-mass spectrometry (ICP-MS). Postreaction spectroscopy and microscopy studies confirm the formation of Pt and Ag particles on p-Si photocathode, resulting in erroneous overestimation of the HER activity of p-Si. Various configurations of the three-electrode system, e.g., an H-type cell with a Nafion membrane, have been studied to find a suitable cell structure for prohibiting undesirable contamination of p-Si. Isolation of p-Si from the Pt counter and Ag/AgCl reference electrodes using the Nafion membrane effectively alleviates the contamination of p-Si but, toward the end, the metallic ions can be slowly deposited on p-Si by diffusion through the membrane. Consequently, this work highlights that the careful caution is necessary when the conventional Pt counter and Ag/AgCl reference electrodes are employed; it is recommended to use a graphite counter electrode and separate reference electrode to prevent artifacts originating from the dissolved Pt and Ag species during PEC cathode evaluations

Growth of Transition Metal Dichalcogenide Heterojunctions with Metal Oxides for Metal–Insulator–Semiconductor Capacitors

The coupling of transition metal dichalcogenides (TMDs) and other materials offers significant synergistic effects; however, the fabrication of artificial multiheterojunction (MHJ) TMDs is a significant challenge owing to complex processes, including layer-by-layer stacking and transfer of free-standing oxide layers. Herein, we developed a straightforward method using sequential pulsed laser deposition (PLD) to fabricate MHJ-TMD thin films. The artificially designed TMD-based (WSe2/MoS2) superlattice and TMD/oxide-based MHJ thin films were successfully synthesized on the centimeter-scale silicon-based substrate via an in situ PLD process. The PLD-grown MHJ-TMD films exhibited good uniformity, layer-by-layer stacking, and interlayer coupling between each TMD layer. Also, we fabricated MHJ-TMD films as a metal–semiconductor/insulator–metal device to confirm their potential as an electronic device. We believe that our technique will widen the scope of TMD applications in different fields

In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation

The development of catalysts with earth-abundant elements for efficient oxygen evolution reactions is of paramount significance for clean and sustainable energy storage and conversion devices. Our group demonstrated recently that the electrochemical tuning of catalysts via lithium insertion and extraction has emerged as a powerful approach to improve catalytic activity. Here we report a novel in situ electrochemical oxidation tuning approach to develop a series of binary, ternary, and quaternary transition metal (e.g., Co, Ni, Fe) oxides from their corresponding sulfides as highly active catalysts for much enhanced water oxidation. The electrochemically tuned cobalt–nickel–iron oxides grown directly on the three-dimensional carbon fiber electrodes exhibit a low overpotential of 232 mV at current density of 10 mA cm^–2, small Tafel slope of 37.6 mV dec^–1, and exceptional long-term stability of electrolysis for over 100 h in 1 M KOH alkaline medium, superior to most non-noble oxygen evolution catalysts reported so far. The materials evolution associated with the electrochemical oxidation tuning is systematically investigated by various characterizations, manifesting that the improved activities are attributed to the significant grain size reduction and increase of surface area and electroactive sites. This work provides a promising strategy to develop electrocatalysts for large-scale water-splitting systems and many other applications

Growth of Multilayer WSe₂/Bi₂O₂Se Heterostructures for Photodetection without Lithography

Novel oxychalcogenides, such as Bi2O2Se, have many applications because of their interesting properties such as remarkable hall mobility, the presence of a bandgap, and high air stability. Among them, photodetectors based on Bi2O2Se are one of the best applicable devices. In addition, the Bi2O2Se heterostructure with other 2D materials can enhance the photoresponse of the device. In this study, we successfully fabricated the WSe2/Bi2O2Se heterostructure for photodetector application via in situ pulsed laser deposition. The band alignment of the as-grown WSe2/Bi2O2Se heterostructure was confirmed to be type II, which increases the photoresponse. Furthermore, the WSe2/Bi2O2Se photodetector exhibited an approximately 110% on/off ratio with a photoresponsivity of 0.96 mA/W even without using lithography for its fabrication

Daylight-Induced Metal–Insulator Transition in Ag-Decorated Vanadium Dioxide Nanorod Arrays

Metal–insulator transition (MIT) in strongly correlated electronic materials has enormous potential with scientific and technological impacts in future oxide nanoelectronic devices. Although photo-induced MIT can provide opportunities to extend the novel functionality of strongly correlated electronic materials, there have rarely been reports on it. Here, we report MIT provoked by visible–near-infrared light in Ag-decorated VO2 nanorod arrays (NRs) because of localized surface plasmon resonance (LSPR) and its application to broadband photodetectors. Our simulation results based on the finite-difference time-domain method show that the electric field resulting from LSPR can be generated at the interface between Ag nanoparticles and VO2 layers under vis NIR illumination. Using high-resolution transmission electronic microscopy and Raman spectroscopy, we observe the MIT and structural phase transition in the Ag-decorated VO2 NRs due to the LSPR effect. The optoelectronic measurements confirm that high, fast, and broad photoresponse of Ag-decorated VO2 NRs is attributed to photo-induced MIT due to LSPR. Our study will open up a new strategy to trigger MIT in strongly correlated electronic materials through functionalization with plasmonic nanoparticles and serve as a valuable proof of concept for next-generation optoelectronic devices with fast response, low power consumption, and high performance

Effect of Ceramic-Target Crystallinity on Metal-to-Insulator Transition of Epitaxial Rare-Earth Nickelate Films Grown by Pulsed Laser Deposition

We demonstrate the effect of the crystallinity of ceramic targets on the electronic properties of LaNiO3 (001) thin films epitaxially grown by pulsed laser deposition (PLD). We prepared two kinds of LaNiO3 targets with different crystallinity by manipulating calcination temperature (i.e., 300 and 1000 °C) in the solid state reaction for ceramic synthesis. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) experiments of the as-sintered LaNiO3 ceramic targets clearly show that the LaNiO3 target sintered after high-temperature (1000 °C, high crystallinity) calcination is more oxidized to Ni3+ with better crystallinity than the LaNiO3 target sintered after low-temperature (300 °C, poor crystallinity) calcination. Using these two LaNiO3 ceramics as PLD targets, we fabricated epitaxial LaNiO3/LaAlO3 (001) thin-film heterostructures to examine how target crystallinity affects the physical properties of LaNiO3 films. Intriguingly, the electrical transport properties of the as-grown LaNiO3 thin films are quite different depending on crystallinity of the LaNiO3 ceramic target used for film deposition. In conjunction with subsequent XPS analyses of our LaNiO3 thin films, it appears that LaNiO3 (001) films deposited from the high-temperature-calcined target with better crystallinity are less disproportionate in Ni charge valency with more Ni3+ oxidation states compared with LaNiO3 (001) films deposited from the low-temperature-calcined target with poor crystallinity. This difference in degree of charge disproportionation can induce a discrepancy in the metal-to-insulator transition temperature of ultrathin LaNiO3 (001) films and in their electrical conductance