Synergistic-Effect-Controlled CoTe₂/Carbon Nanotube Hybrid Material for Efficient Water Oxidation

In anode, electrocatalytic water splitting involves oxygen evolution reaction (OER), which is a complex and sluggish reaction, and thus the efficiency to produce hydrogen is seriously limited by OER. We report that CoTe₂ exhibits optimized OER activity for the first time. Multiwalled carbon nanotube (MWCNT) is utilized to support CoTe₂ in generating a synergistic effect to enhance OER activity and improve stability by tuning different loading amounts of CoTe₂ on CNT. In 1.0 M KOH, bare CoTe₂ needed overpotential of 323 mV to produce 10 mA/cm² with Tafel slope of 85.1 mV/dec, but CoTe₂/carbon nanotube (CNT) with optimized loading amount of CoTe₂ required only 291 mV to produce10 mA/cm² with Tafel slope of 44.2 mV/dec. X-ray absorption near edge structure (XANES) was applied to prove that an electron transfer from e_g band of CoTe₂ to CNT caused a synergistic effect. This electron transfer modulated the bond strength of oxygen-related intermediate species on the surface of catalyst and optimized OER performance. In situ XANES was used to compare CoTe₂/CNT and pristine CoTe₂ during OER. It proved the transition state of CoOOH more easily existed by adding CNT in hybrid material during OER to enhance the efficiency of OER. Moreover, bare CoTe₂ is unstable under OER, but the CoTe₂/CNT hybrid materials exhibited improved and exceptional durability by time-dependent potentiostatic electrochemical measurement for 24 h and continuous cyclic voltammetry for 1000 times. Our result suggests that this new OER electrocatalyst for OER can be applied in various water-splitting devices and can promote hydrogen economy

Wide Range pH-Tolerable Silicon@Pyrite Cobalt Dichalcogenide Microwire Array Photoelectrodes for Solar Hydrogen Evolution

This study employed silicon@cobalt dichalcogenide microwires (MWs) as wide range pH-tolerable photocathode material for solar water splitting. Silicon microwire arrays were fabricated through lithography and dry etching technologies. Si@Co­(OH)₂ MWs were utilized as precursors to synthesize Si@CoX₂ (X = S or Se) photocathodes. Si@CoS₂ and Si@CoSe₂ MWs were subsequently prepared by thermal sulfidation and hydrothermal selenization reaction of Si@Co­(OH)₂, respectively. The CoX₂ outer shell served as cocatalyst to accelerate the kinetics of photogenerated electrons from the underlying Si MWs and reduce the recombination. Moreover, the CoX₂ layer completely deposited on the Si surface functioned as a passivation layer by decreasing the oxide formation on Si MWs during solar hydrogen evolution. Si@CoS₂ photocathode showed a photocurrent density of −3.22 mA cm^–2 at 0 V (vs RHE) in 0.5 M sulfuric acid electrolyte, and Si@CoSe₂ MWs revealed moderate photocurrent density of −2.55 mA cm^–2. However, Si@CoSe₂ presented high charge transfer efficiency in neutral and alkaline electrolytes. Continuous chronoamperometry in acid, neutral, and alkaline solutions was conducted at 0 V (vs RHE) to evaluate the photoelectrochemical durability of Si@CoX₂ MWs. Si@CoS₂ electrode showed no photoresponse after the chronoamperometry test because it was etched through the electrolyte. By contrast, the photocurrent density of Si@CoSe₂ MWs gradually increased to −5 mA cm^–2 after chronoamperometry characterization owing to the amorphous structure generation

Evolution of Visible Photocatalytic Properties of Cu-Doped CeO₂ Nanoparticles: Role of Cu²⁺-Mediated Oxygen Vacancies and the Mixed-Valence States of Ce Ions

We report the contribution of oxygen vacancies for enhancing the optical and visible photocatalytic properties of Cu-doped CeO₂ nanoparticles (NPs) synthesized through a low-temperature coprecipitation method. Doping Cu ions in the ceria lattice in different mole percentages, 0, 3, 5, 7, 9, and 15 wt %, results in enhancement of visible photocatalytic properties even under natural sunlight. Transmission electron microscopy and X-ray diffraction studies showcase the monodispersive nature of Cu-doped CeO₂ NPs in the size range of 3–7 nm with face-centered cubic structure. The Cu-based defect states induce a narrow band function in ceria nanostructures and influence the red shift in absorption with the Cu concentrations. Visible photocatalytic degradation of methylene blue was investigated in the presence of pure CeO₂ NPs, CuO NPs, and Cu-doped CeO₂ NPs. These studies revealed that the 7 wt % of Cu-doped CeO₂ NPs exhibit the degradation rates of 1.41 × 10^–2 and 1.12 × 10^–2 min^–1 under exposure to natural sunlight and visible light (Xe light source), respectively. This is nearly 23.5 and 1.61 times faster than the undoped CeO₂ and CuO NPs, respectively. The inclusion of more Cu²⁺ ions in the CeO₂ structure leads to the interaction and spatial distribution of oxygen vacancies with a Ce⁴⁺/Ce³⁺ ratio defect. This promotes the narrowing of the band function to the visible photocatalytic characteristics. Detailed investigations from X-ray absorption spectroscopy support the fact that the oxygen vacancies may strongly affect the valences of Ce ions in CeO₂, which improves the carrier mobility and visible response

X-ray Absorption Spectroscopic Study on Interfacial Electronic Properties of FeOOH/Reduced Graphene Oxide for Asymmetric Supercapacitors

[[abstract]]The effects of growth time and interface between the iron oxyhydroxide (FeOOH) and carbon materials (carbon nanotubes (CNT) and reduced graphene oxide (RGO)) to form an asymmetric supercapacitor was studied by X-ray absorption spectroscopy (XAS) and electrochemical measurements. FeOOH/CNT (FCNT) and FeOOH/RGO (FRGO) were successfully synthesized by a simple spontaneous redox reaction with FeCl3. The RGO functions as an ideal substrate, providing rich growth sites for FeOOH, and it is believed to facilitate the transport of electrons/ions across the electrode/electrolyte interface. FRGO has been identified as a supercapacitor and found to exhibit significantly greater capacitance than FCNT. To gain further insight into the effects of growth times and the interface of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO with various FeOOH growth times were elucidated by XAS. The difference between the surface electronic structures of CNT and RGO yields different nucleation and growth rates of FeOOH of FeOOH. RGO with excellent interface properties arises from a high degree of covalent functionalization, and/or defects make it favorable for FeOOH growth. FRGO is therefore a promising electrode material for use in the fabrication of asymmetric supercapacitors. In this work, coupled XAS and electrochemical measurements reveal the electronic structure of the interface between FeOOH and the carbon materials and the capacitance performance of asymmetric supercapacitors, which are very useful in the fields of nanomaterials and nanotechnology, especially for their applications in storing energy[[notice]]補正完