Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. ...Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni1–x Fe x OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe3+ in Ni1–x Fe x OOH occupies octahedral sites with unusually short Fe–O bond distances, induced by edge-sharing with surrounding NiO6 octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni1–x Fe x OOH are not active sites for the oxidation of water.
Structural symmetry-breaking plays a crucial role in determining the electronic band structures of two-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of ...graphene with electric fields on AB-stacked bilayers or stacked van der Waals heterostructures. In contrast, transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an additional degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external electric fields or, as theoretically proposed, with an asymmetric out-of-plane structural configuration. Here, we report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS
monolayer, we fully replace the top-layer S with Se atoms. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements.
Heteroatom-doped carbons have drawn increasing research interest as catalysts for various electrochemical reactions due to their unique electronic and surface structures. In particular, co-doping of ...carbon with boron and nitrogen has been shown to provide significant catalytic activity for oxygen reduction reaction (ORR). However, limited experimental work has been done to systematically study these materials, and much remains to be understood about the nature of the active site(s), particularly with regards to the factors underlying the activity enhancements of these boron–carbon–nitrogen (BCN) materials. Herein, we prepare several BCN materials experimentally with a facile and controlled synthesis method, and systematically study their electrochemical performance. We demonstrate the existence of h-BN domains embedded in the graphitic structures of these materials using X-ray spectroscopy. These synthesized structures yield higher activity and selectivity toward the 2e– ORR to H2O2 than structures with individual B or N doping. We further employ density functional theory calculations to understand the role of a variety of h-BN domains within the carbon lattice for the ORR and find that the interface between h-BN domains and graphene exhibits unique catalytic behavior that can preferentially drive the production of H2O2. To the best of our knowledge, this is the first example of h-BN domains in carbon identified as a novel system for the electrochemical production of H2O2.
It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring ...short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (Fe
Ru
) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of ångströms, 120-200 cm
frequency and ~100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.
The ability to manipulate quantum dot (QD) surfaces is foundational to their technological deployment. Surface manipulation of metal halide perovskite (MHP) QDs has proven particularly challenging in ...comparison to that of more established inorganic materials due to dynamic surface species and low material formation energy; most conventional methods of chemical manipulation targeted at the MHP QD surface will result in transformation or dissolution of the MHP crystal. In previous work, we have demonstrated record-efficiency QD solar cells (QDSCs) based on ligand-exchange procedures that electronically couple MHP QDs yet maintain their nanocrystalline size, which stabilizes the corner-sharing structure of the constituent PbI6 4– octahedra with optoelectronic properties optimal for solar energy conversion. In this work, we employ a variety of spectroscopic techniques to develop a molecular-level understanding of the MHP QD surface chemistry in this system. We individually target both the anionic (oleate) and cationic (oleylammonium) ligands. We find that atmospheric moisture aids the process by hydrolysis of methyl acetate to generate acetic acid and methanol. Acetic acid then replaces native oleate ligands to yield QD surface-bound acetate and free oleic acid. The native oleylammonium ligands remain throughout this film deposition process and are exchanged during a final treatment step employing smaller cationsnamely, formamidinium. This final treatment has a narrow processing window; initial treatment at this stage leads to a more strongly coupled QD regime followed by transformation into a bulk MHP film after longer treatment. These insights provide chemical understanding to the deposition of high-quality, electronically coupled MHP QD films that maintain both quantum confinement and their crystalline phase and attain high photovoltaic performance.
Inspired by the period-four oscillation in flash-induced oxygen evolution of photosystem II discovered by Joliot in 1969, Kok performed additional experiments and proposed a five-state kinetic model ...for photosynthetic oxygen evolution, known as Kok's S-state clock or cycle
. The model comprises four (meta)stable intermediates (S
, S
, S
and S
) and one transient S
state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn
CaO
) cluster in the oxygen-evolving complex
. This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q
at the acceptor side of PSII. Here, using serial femtosecond X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation at room temperature, we visualize all (meta)stable states of Kok's cycle as high-resolution structures (2.04-2.08 Å). In addition, we report structures of two transient states at 150 and 400 µs, revealing notable structural changes including the binding of one additional 'water', Ox, during the S
→S
state transition. Our results suggest that one water ligand to calcium (W3) is directly involved in substrate delivery. The binding of the additional oxygen Ox in the S
state between Ca and Mn1 supports O-O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O
release. Thus, our results exclude peroxo-bond formation in the S
state, and the nucleophilic attack of W3 onto W2 is unlikely.
Ru-based oxygen evolution reaction (OER) catalysts show significant promise for efficient water electrolysis, but rapid degradation poses a major challenge for commercial applications. In this work, ...we explore several Ru-based pyrochlores (A2Ru2O7, A = Y, Nd, Gd, Bi) as OER catalysts and demonstrate improved activity and stability of catalytic Ru sites relative to RuO2. Furthermore, we combine complementary experimental and theoretical analysis to understand how the A-site element impacts activity and stability under acidic OER conditions. Among the A2Ru2O7 studied herein, we find that a longer Ru–O bond and a weaker interaction of the Ru 4d and O 2p orbitals compared with RuO2 results in enhanced initial activity. We observe that the OER activity of the catalysts changes over time and is accompanied by both A-site and Ru dissolution at different relative rates depending on the identity of the A-site. Pourbaix diagrams constructed using density functional theory (DFT) calculations reveal a driving force for this experimentally observed dissolution, indicating that all compositions studied herein are thermodynamically unstable in acidic OER conditions. Theoretical activity predictions show consistent trends between A-site cation leaching and OER activity. These trends coupled with Bader charge analysis suggest that dissolution exposes highly oxidized Ru sites that exhibit enhanced activity. Overall, using the stability number (molO2 evolved/molRu dissolved) as a comparative metric, the A2Ru2O7 materials studied in this work show substantially greater stability than a standard RuO2 and commensurate stability to some Ir mixed metal oxides. Finally, the insights described herein provide a pathway to enhanced Ru catalyst activity and durability, ultimately improving the efficiency of water electrolyzers.
Hydrogen peroxide (H2O2), an important industrial chemical, is currently produced through an energy-intensive anthraquinone process that is limited to large-scale facilities. Small-scale ...decentralized electrochemical production of H2O2 via a two-electron oxygen reduction reaction (ORR) offers unique opportunities for sanitization applications and the purification of drinking water. The development of inexpensive, efficient, and selective catalysts for this reaction remains a challenge. Herein, we examine two different porous carbon-based electrocatalysts and show that they exhibit high selectivity for H2O2 under alkaline conditions. By rationally varying synthetic methods, we explore the effect of pore size on electrocatalytic performance. Furthermore, by means of density functional calculations, we point out the critical role of carbon defects. Our theory results show that the majority of defects in graphene are naturally selective for the two-electron reduction of O2 to H2O2, and we identify the types of defects with high activity.
To limit further rising levels in methane emissions from stationary and mobile sources and to enable promising technologies based on methane, the development of efficient combustion catalysts that ...completely oxidize CH4 to CO2 and H2O at low temperatures in the presence of high steam concentrations is required. Palladium is widely considered as one of the most promising materials for this reaction, and a better understanding of the factors affecting its activity and stability is crucial to design even more improved catalysts that efficiently utilize this precious metal. Here we report a study of the effect of three important variables (particle size, support, and reaction conditions including water) on the activity of supported Pd catalysts. We use uniform palladium nanocrystals as catalyst precursors to prepare a library of well-defined catalysts to systematically describe structure–property relationships with help from theory and in situ X-ray absorption spectroscopy. With this approach, we confirm that PdO is the most active phase and that small differences in reaction rates as a function of size are likely due to variations in the surface crystal structure. We further demonstrate that the support exerts a limited influence on the PdO activity, with inert (SiO2), acidic (Al2O3), and redox-active (Ce0.8Zr0.2O2) supports providing similar rates, while basic (MgO) supports show remarkably lower activity. Finally, we show that the introduction of steam leads to a considerable decrease in rates that is due to coverage effects, rather than structural and/or phase changes. Altogether, the data suggest that to further increase the activity and stability of Pd-based catalysts for methane combustion, increasing the surface area of supported PdO phases while avoiding strong adsorption of water on the catalytic surfaces is required. This study clarifies contrasting reports in the literature about the active phase and stability of Pd-based materials for methane combustion.
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