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Electrochemical water splitting is a promising means to produce eco‐friendly hydrogen fuels. Inspired by the Mn4CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)‐based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn‐oxo moieties. We identify water oxidation intermediates on the surface of Mn3O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential‐dependent Raman peak was detected at 760 cm−1 and assigned to the active MnIV=O species generated during water oxidation. Isotope‐labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV=O intermediates in the Mn‐oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates. [ABSTRACT FROM AUTHOR]

As the demand for energy has dramatically increased in the past decade, electrochemical water splitting has been regarded as an attractive approach to produce renewable hydrogen energy. However, large overpotentials of oxygen‐evolving reaction (OER) is a key bottleneck for practical application. Thus, water‐oxidizing electrocatalysts with low cost and high efficiency should be developed. Here, 5 nm‐sized Mn3O4 nanoparticles (NPs) are synthesized by a hydrothermal method, which is appropriate for large‐scale production. To further improve their performance, various 3d transition metal elements are successfully doped in Mn3O4 NPs. Ni‐doped Mn3O4 NPs exhibit the highest efficiency among the Mn3O4 NPs doped with various elements. Based on structural analysis, the Ni‐doping process leads to the lattice distortion of their tetragonal spinel structure and it strongly correlates with the enhancement of OER activity. The overpotential at the current density of 10 mA cm–2 is 524 and 458 mV for pristine and 5 at% doped Mn3O4 NPs under neutral condition. The heteroatom‐doping process in sub‐10 nm‐sized nanocatalysts is expected to be a promising methodology to induce distorted structure related to active species. Thus, it can be effective to improve catalytic performance of various heterogeneous nano‐catalysts. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting is one of the ways to produce environmentally‐friendly hydrogen energy. Transition‐metal (TM)‐based catalysts have been attracting attention due to their low cost and abundance, but their insufficient activity still remains a challenge. Here, 4 nm Mn3O4 nanoparticles (NPs) are successfully synthesized and their electrochemical behavior is investigated. Using electrokinetic analyses, an identical water oxidizing mechanism is demonstrated between the 4 and 8 nm Mn3O4 NPs. In addition, it is confirmed that the overall increase in the active surface area is strongly correlated with the superb catalytic activity of the 4 nm Mn3O4 NPs. To further enhance the oxygen evolution reaction (OER) performance, Ni foam substrate is introduced to maximize the entire number of the NPs participating in OER. The 4 nm Mn3O4/Ni foam electrode exhibits outstanding electrocatalytic activity for OER with overpotential of 395 mV at a current density of 10 mA cm−2 under neutral conditions (0.5 m PBS, pH 7). [ABSTRACT FROM AUTHOR]

It has been reported that defects in molybdenum disulfide (MoS2) enable the hydrogen evolution reaction (HER). The most widely employed method of argon-plasma treatment for defect generation suffers from poor material stability and loss of conductivity. Here, we report a new method to synthesize highly polycrystalline molybdenum disulfide MoS2 bilayers with enhanced HER performance and material stability. This new method is based on metal organic chemical vapor deposition (MOCVD) followed by UV/ozone treatment to generate defects. The defect densities on MoS2 were identified by the increase in lifetime (∼76%) and intensity (∼15%) in photoluminescence (PL) as compared to those of pristine MoS2. Our fabrication and characterization methods can be further applied to optimize defect densities for catalytic effects in various transition metal dichalcogenide (TMDC) materials. [ABSTRACT FROM AUTHOR]

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand‐engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm−2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm−2) of BiVO4. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems. Structural formulas of Ca–ethylenediaminetetraacetic acid and BF4‐treated MnO/BiVO4 photoanodes show the hole transfer from BiVO4 to MnO and consequent photo‐electrochemical water splitting efficiency. [ABSTRACT FROM AUTHOR]

The band edge positions of semiconductors determine functionality in solar water splitting. In article number 1800727, Ki Tae Nam, Ho Won Jang, and co‐workers demonstrate cascade photoanodes with ligand engineered manganese oxide co‐catalyst nanoparticles to achieve enhanced photoelectrochemical performance. The proposed concept has considerable potential to explore a new efficient solar water splitting system. [ABSTRACT FROM AUTHOR]