Practical cluster models for a layered β-NiOOH material

Due to the high oxygen evolution reaction (OER) activity, stability, and abundance of NiOx materials, they are found to be promising catalysts, competitive with expensive metal oxides such as IrO2 and RuO2. From a theoretical point of view, studies reported in the literature so far are mostly based on density functional theory using periodic slab models for the bulk and surface of βNiOOH, one of the active NiOx phases. However, cluster models are a valid method to investigate many aspects about structure, charge carrier transport properties, and OER activity of β-NiOOH. Hence, here we present new cluster models for the surface of β-NiOOH, where the oxygen atoms are bonded to Mg effective core potentials (ECPs) mimicking neighboring atom cores. This cluster embedding procedure is superior to saturating the cluster with hydrogen atoms, and to using other atomic ECPs for β-NiOOH.We find that layered materials such as β-NiOOH are more vulnerable to geometrical rupture and therefore a cluster approach requires additional care in choosing the embedding approach. We evaluated the models by using them to calculate the energy required for water adsorption and deprotonation, which are essential ingredients for OER. Specifically, our results agree with previous slab models that the first deprotonation reaction step requires a large amount of energy. In addition, we find that water and hydroxyl groups have high adsorption energy and therefore the first deprotonation step is limiting the reaction efficiency

An ultra-high gain and efficient amplifier based on Raman amplification in plasma

Raman amplification arising from the excitation of a density echelon in plasma could lead to amplifiers that significantly exceed current power limits of conventional laser media. Here we show that 1-100 J pump pulses can amplify picojoule seed pulses to nearly joule level. The extremely high gain also leads to significant amplification of backscattered radiation from "noise", arising from stochastic plasma fluctuations that competes with externally injected seed pulses, which are amplified to similar levels at the highest pump energies. The pump energy is scattered into the seed at an oblique angle with 14 J sr(-1), and net gains of more than eight orders of magnitude. The maximum gain coefficient, of 180 cm(-1), exceeds high-power solid-state amplifying media by orders of magnitude. The observation of a minimum of 640 J sr(-1) directly backscattered from noise, corresponding to approximate to 10% of the pump energy in the observation solid angle, implies potential overall efficiencies greater than 10%

Electronic Properties of Pure and Fe-Doped β-Ni(OH)2: New Insights Using Density Functional Theory with a Cluster Approach

NiOx has recently emerged as a robust catalyst with high catalytic activity for water oxidation reaction. Despite extensive studies, the origin of the high oxygen evolution reaction activity upon Fe doping is not fully solved, even for one of its simplest phases, β-Ni(OH)2. We present here density functional theory calculations using for the first time a cluster approach to revisit the electronic structure of pure and Fe-doped β-Ni(OH)2. First, our findings agree with a recent hypothesis that the band gap of the pure case reduces upon Fe doping. Second, in agreement with earlier calculations, we find that the highest occupied state consists of O and Ni states in pure and Fe-doped β-Ni(OH)2. However, the lowest unoccupied orbitals are Ni and O for pure β-Ni(OH)2 and mainly Fe for Fe-doped β-Ni(OH)2. We argue that the two different states for the highest occupied state and for the lowest unoccupied state of Fe-doped β-Ni(OH)2 may lead to low electron-hole recombination. Third, the delocalized nature of the band edge states that may be associated with high mobility is not damaged after Fe doping. These electronic effects could be some of the reasons experiments show that doping β-Ni(OH)2 with Fe enhances overall efficiency

Three fundamental questions on one of our best water oxidation catalysts: a critical perspective

Nickel oxyhydroxide (NiOOH) is considered to be one of the best-known catalysts for the water oxidation reaction. Recently, progress has been made in pushing the limits of water splitting efficiency by incorporating NiOOH in photo-electrochemical cell architectures. Despite these cutting-edge advances, some basic questions have yet been fully answered. This perspective highlights the three most critical questions that are considered to be the very first step for any theoretical investigation. We suggest possible ways to answer these questions from a theoretician’s perspective. Progress toward this direction is expected to shed light on the origin of NiOOH’s success