Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO2Core-Shell Nanostructures

© Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO2 nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO2 nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C3v, which implies that the Ti exists in two local symmetries (D2d and C3v) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO2 nanocrystals.11Nsciescopu

Precisely Constructing Orbital Coupling-Modulated Dual-Atom Fe Pair Sites for Synergistic CO2 Electroreduction

Electrochemical reduction of CO2 (CO2RR) provides an attractive pathway to achieve a carbon-neutral energy cycle. Single-atom catalysts (SAC) have shown unique potential in heterogeneous catalysis, but their structural simplicity prevents them from breaking linear scaling relationships. In this study, we develop a feasible strategy to precisely construct a series of electrocatalysts featuring well-defined single-atom and dual-site iron anchored on nitrogen-doped carbon matrix (Fe1–N–C and Fe2–N–C). The Fe2–N–C dual-atom electrocatalyst (DAC) achieves enhanced CO Faradaic efficiency above 80% in wider applied potential ranges along with higher turnover frequency (26,637 h–1) and better durability compared to SAC counterparts. Furthermore, based on in-depth experimental and theoretical analysis, the orbital coupling between the iron dual sites decreases the energy gap between antibonding and bonding states in *CO adsorption. This research presents new insights into the structure–performance relationship on CO2RR electrocatalysts at the atomic scale and extends the application of DACs for heterogeneous electrocatalysis and beyond.11Nsciescopu

Unprecedented electrocatalytic oxygen evolution performances by cobalt-incorporated molybdenum carbide microflowers with controlled charge re-distribution

Molybdenum carbide (MC) is a highly active electrocatalyst for the hydrogen evolution reaction (HER) due to its Pt-like d-band electronic density of states. Although a significant improvement in the HER activity was observed, much less effort has been made to realize the oxygen evolution reaction (OER) properties from MC. In this work, we propose controlled charge transfer from cobalt to MC, leading to an unprecedented OER performance. Electron transfer from Co to Mo was established by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) analyses. The optimized electrocatalyst, MoCo1.5C-700, showed a low overpotential of 232.5 mV to achieve a current density of 10 mA cm(-2) and a small Tafel slope of 61 mV dec(-1) for the OER in an alkaline medium. The electrocatalyst exhibited excellent durability for over 27 h at a current density of 10 mA cm(-2). Density functional theory (DFT) calculation proved favorable adsorption of the OER intermediates (*OH and *O) over Co-incorporated MC that facilitated the formation of *OOH, which led to an accelerated O-2 evolution. In addition, the electrocatalyst displayed superb HER performance, which was confirmed by an overpotential of 96.5 mV to attain a current density of 10 mA cm(-2) in 1.0 M KOH. We anticipate that our work will open up new ways to develop efficient and noble metal-free electrocatalysts for overall water splitting

Robust Room Temperature Ferromagnetism In Cobalt Doped Graphene by Precision Control of Metal Ion Hybridization

© 2022 Wiley-VCH GmbH.Graphene-based magnetic materials exhibit novel properties and promising applications in the development of next-generation spintronic devices. Modern synthesis techniques have paved the way to design precisely the local environments of metal atoms anchored onto a nitrogen-doped graphene matrix. Herein, it is demonstrated that grafting cobalt (Co) into the graphene lattice induces robust and stable room-temperature ferromagnetism. These comprehensive experiments and first-principles calculations unambiguously identify that the mechanism for this unusual ferromagnetism is π-d orbital hybridization between Co dxz and graphene pz orbitals. Here, it is found that the magnetic interactions of Co–carbon ions are mediated by the spin-polarized graphene pz orbitals, and room temperature ferromagnetism can be stabilized by electron doping. It is also found that the electronic structure near the Fermi level, which sets the nature of spin polarization of graphene pz bands, strongly depends on the local environment of the Co moiety. This is the crucial, previously missing, ingredient that enables control of the magnetism. Overall, these observations unambiguously reveal that engineering the atomic structure of metal-embedded graphene lattices through careful d to p orbital interactions opens a new window of opportunities for developing graphene-based spintronics devices.N

Predicting NO_<i>x</i> Catalysis by Quantifying Ce³⁺ from Surface and Lattice Oxygen

Our work introduces a novel technique based on the magnetic response of Ce³⁺ and molecular oxygen adsorbed on the surface of nanoceria and ceria-based catalysts that quantifies the number and type of defects and demonstrates that this information is the missing link that finally enables predictive design of NO_<i>x</i> catalysis in ceria-based systems. The new insights into ceria catalysis are enabled by quantifying the above for different ceria nanoparticle shapes (i.e., surface terminations) and O₂ partial pressure. We used ceria nanorods, cubes, and spheres and evaluated them for catalytic reduction of NO by CO. We then demonstrated the quantitative prediction of the reactivity of nanomaterials via their magnetism in different atmospheric environments. We find that the observed enhancement of reactivity for ceria nanocubes and nanorods is not directly due to improved reactivity on those surface terminations but rather due to the increased ease of generating lattice defects in these materials. Finally, we demonstrate that the method is equally applicable to highly topical and industrially relevant ceria mixed oxides, using nanoscale alumina-supported ceria as a representative case–a most ill-defined catalyst. Because the total oxide surface is a mixture of active ceria and inactive support and ceria is not likely present as crystallographically well-defined phases, reactivity does not easily scale with surface area or a surface termination. The key parameter to design efficient NO reduction in ceria-based catalysts is knowing and controlling the surface localized excess Ce³⁺ ion areal density

Reversible Ligand Exchange in Atomically Dispersed Catalysts for Modulating the Activity and Selectivity of the Oxygen Reduction Reaction

Rational control of the coordination environment of atomically dispersed catalysts is pivotal to achieve desirable catalytic reactivity. We report the reversible control of coordination structure in atomically dispersed electrocatalysts via ligand exchange reactions to reversibly modulate their reactivity for oxygen reduction reaction (ORR). The CO-ligated atomically dispersed Rh catalyst exhibited ca. 30-fold higher ORR activity than the NHx-ligated catalyst, whereas the latter showed three times higher H2O2 selectivity than the former. Post-treatments of the catalysts with CO or NH3 allowed the reversible exchange of CO and NHx ligands, which reversibly tuned oxidation state of metal centers and their ORR activity and selectivity. DFT calculations revealed that more reduced oxidation state of CO-ligated Rh site could further stabilize the *OOH intermediate, facilitating the two- and four-electron pathway ORR. The reversible ligand exchange reactions were generalized to Ir- and Pt-based catalysts

One‐dimensional π-d conjugated coordination polymer for electrochromic energy storage device with exceptionally high performance

The rational design of previously unidentified materials that could realize excellent electrochemical‐controlled optical and charge storage properties simultaneously, are especially desirable and useful for fabricating smart multifunctional devices. Here, a facile synthesis of a 1D π–d conjugated coordination polymer (Ni‐BTA) is reported, consisting of metal (Ni)‐containing nodes and organic linkers (1,2,4,5‐benzenetetramine), which could be easily grown on various substrates via a scalable chemical bath deposition method. The resulting Ni‐BTA film exhibits superior performances for both electrochromic and energy storage functions, such as large optical modulation (61.3%), high coloration efficiency (223.6 cm2 C−1), and high gravimetric capacity (168.1 mAh g−1). In particular, the Ni‐BTA film can maintain its electrochemical recharge‐ability and electrochromic properties even after 10 000 electrochemical cycles demonstrating excellent durability. Moreover, a smart energy storage indicator is demonstrated in which the energy storage states can be visually recognized in real time. The excellent electrochromic and charge storage performances of Ni‐BTA films present a great promise for Ni‐BTA nanowires to be used as practical electrode materials in various applications such as electrochromic devices, energy storage cells, and multifunctional smart windows.National Research Foundation (NRF)Published versionG. C. and P.C. contributed equally to this work. This work was financially supported by the Competitive Research Programme under NRF‐CRP‐13‐2014‐02; NRF‐Investigatorship under NRF‐NRFI2016‐05; the Campus for Research Excellence and Technological Enterprise (CREATE) programme under the National Research Foundation, Prime Minister's Office, Singapore; the National Natural Science Foundation of China (51902086); the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF‐2018M1A2A2061998). The authors appreciate Dr. W. Liu, Dr. P. Buenconsejo (Facility for Analysis Characterisation Testing & Simulation, FACTS, Nanyang Technological University), Prof. N. Sharma, and Mr J. Wu (School of Chemistry, The University of New South Wales) for the valuable discussion and analysis of XRD results. Special thanks go to beamline staff at I15‐1 Diamond Light Source Synchrotron for completing the XPDF measurements rapidly and thoroughly under rapid access call EE21425‐1. And the authors also appreciate Dr, J. Wang, Dr. S. Li, Dr. K. Qian, Prof. Y. Mai, and Dr. G. Thangavel for the insightful discussions

Oxygen-Plasma-Treated Fe-N-C Catalysts with Dual Binding Sites for Enhanced Electrocatalytic Polysulfide Conversion in Lithium-Sulfur Batteries

Enhanced polysulfide conversion kinetics is essential for realizing lithium-sulfur batteries with high energy density and rate performance and promising cyclability. The modification of the local atomic structure of MNx active sites in single-atom M-N-C catalysts was proposed to improve their electrocatalytic activity for demanding reactions by fine-tuning the interaction with reaction intermediates. Here, we demonstrate that engineering the binding geometry of lithium polysulfides (LiPSs) by introducing dual binding sites improves the LiPS conversion kinetics. We use mild oxygen plasma treatment to introduce oxygen species into the Fe-N-C catalyst. The plasma-treated Fe-N-C (pFeNG) catalyst with dual sulfiphilic (mononuclear iron) and lithiophilic (oxygen) binding sites has a lower polysulfide decomposition energy, especially for Li2S redox, which is known to be the most sluggish process. The pFeNG cathode shows significant improvement, especially at high C rates (916.3 mA h g(-1) at 5C), with promising cycling performance.11Nsciescopu

Operando Fe Dissolution in Fe-N-C Electrocatalysts during Acidic Oxygen Reduction and Impact of Local pH Change

Atomic Fe in N-doped C (Fe-N-C) catalysts provide the most promising non-precious metal O2 reduction activity at the cathodes of proton exchange membrane fuel cells. However, one of the biggest remaining challenges to address towards their implementation in fuel cells is their limited durability. Fe demetallation has been suggested as the primary initial degradation mechanism. However, the fate of Fe under different operating conditions varies. Here, we monitor operando Fe dissolution of a highly porous and >50% FeNx electrochemical utilization Fe-N-C catalyst in 0.1 M HClO4, under O2 and Ar at different temperatures, in both flow cell and gas diffusion electrode (GDE) half-cell coupled to inductively coupled plasma mass spectrometry (ICP-MS). By combining these results with pre- and post-mortem analyses, we demonstrate that in the absence of oxygen, Fe cations diffuse away within the liquid phase. Conversely, at -15 mA cm-2geo and more negative O2 reduction currents, the Fe cations reprecipitate as Fe-oxides. We support our conclusions with a microkinetic model, revealing that the local pH in the catalyst layer predominantly accounts for the observed trend. Even at a moderate current density of -15 mA cm-2geo and under O2 at 25 oC, a significant H+ consumption and therefore pH increase (pH = 8-9) within the bulk Fe-N-C layer facilitate precipitation of Fe cations. This work provides a unified view on the Fe degradation mechanism for a model Fe-N-C in both high-throughput flow cell and practical operating GDE conditions, underscoring the crucial role of local pH in regulating the stability of the active sites

Direct Synthesis of an Unprecedented Stable Radical of Nickel(II) 3,5-Bis(dimedonyl)azadiisoindomethene with Strong and Narrow Near-Infrared Absorption at λ ∼ 1000 nm

An unprecedented stable neutral radical nickel­(II) complex of 3,5-bis­(dimedonyl)­azadiisoindomethene (<b>1</b>) was prepared by the direct reaction between 1,3-diiminoisoindoline and dimedone. A new radical complex <b>1</b> has an intense and narrow absorption at 1008 nm and can be reduced to a less stable anionic [<b>1</b>]⁻ with a typical aza­(dibenzo)­boron dipyrromethene (aza-BODIPY) UV−vis spectrum. Complex <b>1</b>, along with two other colored condensation reaction products <b>2</b> and <b>3</b>, was characterized by spectroscopy and X-ray crystallography, while the paramagnetic nature of <b>1</b> was probed by EPR and SQUID methods. Complex <b>1</b> forms dimers in the solid state with short (∼3.16 Å) Ni---Ni contacts. Redox data on <b>1</b> are indicative of a reversible reduction process in this complex; its magnetism suggests a <i>S</i> = ¹/₂ state with the spin density delocalized over the aza-BODIPY core. The experimental data <b>1</b> and [<b>1</b>]⁻ were correlated with the density functional theory (DFT) and time-dependent DFT calculations