Conventional Transition Metal Research Articles

Inelastic neutron scattering evidence for anomalous H–H distances in metal hydrides

Hydrogen-containing materials are of fundamental as well as technological interest. An outstanding question for both is the amount of hydrogen that can be incorporated in such materials, because that determines dramatically their physical properties such as electronic and crystalline structure. The number of hydrogen atoms in a metal is controlled by the interaction of hydrogens with the metal and by the hydrogen-hydrogen interactions. It is well established that the minimal possible hydrogen-hydrogen distances in conventional metal hydrides are around 2.1 Å under ambient conditions, although closer H-H distances are possible for materials under high pressure. We present inelastic neutron scattering measurements on hydrogen in [Formula: see text] showing nonexpected scattering at low-energy transfer. The analysis of the spectra reveals that these spectral features in part originate from hydrogen vibrations confined by neighboring hydrogen at distances as short as 1.6 Å. These distances are much smaller than those found in related hydrides, thereby violating the so-called Switendick criterion. The results have implications for the design and creation of hydrides with additional properties and applications.

Probing the versatility of metallo-electro hybrid catalysis: enabling access towards facile C–N bond formation

The development of versatile and mild methodologies for C-N bond construction has always been a hot topic of interest in synthetic organic chemistry. In recent years, electrochemistry has emerged as a promising green and sustainable environmentally benign approach to carry out these transformations under mild conditions utilizing electrons as oxidizing/reducing agents. The current state-of-the-art in combining electrocatalysis with transition metal catalysis has gained significant attention. This hybrid synthetic methodology has increasingly become a common tool and offers many potential advantages compared to direct electrolysis. This review comprehensively highlights recent developments in the merging of transition metal catalysis in electro-organic synthesis for the facile construction of C-N bonds. In this review major emphasis is given to mechanistic investigations and their synthetic applications of this hybrid catalysis.

A Water Stable, Near‐Zero‐Strain O3‐Layered Titanium‐Based Anode for Long Cycle Sodium‐Ion Battery

AbstractLayered transition metal (TM) oxides of the stoichiometry NaxMO2 (M = TM) have shown great promise in sodium‐ion batteries (SIBs); however, they are extremely sensitive to moisture. To date, most reported titanium‐based layered anodes exhibit a P2‐type structure. In contrast, O3‐type compounds are rarely investigated and their synthesis is challenging due to their higher percentage of unstable Ti3+ than the P2 type. Here, a pure phase and highly crystalline O3‐type Na0.73Li0.36Ti0.73O2 with high performance is successfully proposed in SIBs. This material delivers a reversible capacity of 108 mAh g−1 with a stable and safe potential of 0.75 V versus Na/Na+. In situ X‐ray diffraction reveals that this material does not undergo any phase transitions and exhibits a near‐zero volume change upon Na+ insertion/de‐insertion, which ensures exceptional long cycle life over 6000 cycles. Importantly, it is found that this O3‐Na0.73Li0.36Ti0.73O2 shows superior moisture stability, even when immersed into water, which are both elusive for conventional layered TM oxides in SIBs. It is believed that the small interlayer distance and high occupation of interlayer vacancy promise such unprecedented water stability.

Lithium-Ion Exchange Polymer Membranes Limit Migration of Dissolved Polysulfide Species in Li-S Batteries

Conventional lithium-ion (Li-ion) batteries dominate development efforts in electrical vehicles due to their high open circuit voltage, desirable power density, and long cycle life. Despite these advantages, state-of-the-art Li-ion batteries barely meet the energy requirement of EV-based power trains due low energy density (180-200 Wh/kg cell) and high cost. Lithium sulfur (Li-S) batteries are promising for EV applications because they address both these barriers. Li-S batteries exploit a redox couple described by the reaction below, which has an open circuit potential of 2.2 V with respect to Li+/Li: 16 Li+ + S8 + 16e-→ 8 Li2S Assuming full conversion, the theoretical energy density and specific capacity of Li-S batteries is 2500 Wh/kg S and 1675 mAh/g S, respectively.1 In addition, sulfur, the key active cathode material is significantly less expensive than conventional transition metal oxide cathodes used in Li-ion batteries. However, Li-S batteries have not gained acceptance for practical applications because of their poor cycle life. This limitation originates from polysulfide diffusion from the cathode through the electrolyte to the anode which causes parasitic side-reactions and limits capacity and cycle retention. Our work aims to overcome the challenges of polysulfide diffusion by incorporating lithium-ion exchange (LIE) ionomers as polysulfide blocking barriers in Li-S cells. When exposed to electrolyte solvent, these LIE membranes provide a physical barrier that limits polysulfide diffusion, while also delivering high Li+ ion conductivity through swelling in electrolyte solvent. The LIE ionomers can be applied in a variety of ways including as: binders in the electrode formulation, coatings applied directly to the electrode surface, and as freestanding polymer electrolyte membranes. We have also developed a porous, dimensionally stable membrane (DSM) which can be impregnated with LIE ionomers to improve mechanically stability by limiting in-plane (X-Y direction) swelling. This general approach of incorporating LIE ionomers into Li-S cells has achieved improvements in discharge capacity, cycle life, coulombic efficiency, and energy density.

Intriguing p-orbital magnetic semiconductors and carrier induced half-metallicity in surface oxygen-functionalized two-dimensional [formula omitted] ([formula omitted]) crystals

Two-dimensional (2D) ferromagnetic semiconductors with robust magnetism are urgently desired for nanoscale spintronics applications. However, it remains a challenge to realize them experimentally. In this work, we proposed intriguing 2D p-orbital ferromagnetic semiconductors X2NO2 (X=Ca,Sr) monolayer with 3μB magnet per unit under O surface termination using first-principles calculations. The Ca2NO2 monolayer is bipolar magnetic material (BMS) with spin-flip gap 0.24 eV, and the Sr2NO2 is half-semiconductor (HSC) with spin-flip gap 0.31 eV, which is large enough to prevent the spin-flip transition. The Curie temperature can reach to 206 and 239 K, respectively, due to the superexchange interaction between N- ions. The values are much higher than the boiling point of liquid nitrogen (77 K) and comparable to that of the reported ScCl monolayer. Moreover, the half metals are obtained via carrier doping for both the Ca2NO2 and Sr2NO2 monolayers. In addition, the half-metallic completely spin-polarized direction of BMS Ca2NO2 monolayer can be controlled by carrier doping type. Furthermore, the magnetism of X2NO2 monolayer is derived from p orbital, without the involvement of conventional transition metals or rare earth atoms. This is advantageous for high-speed and long-distance spin-polarization transport. These results suggest that the X2NO2 monolayer can develop spin field effect transistor for information processing and storage, and open opportunities for designing new ferromagnetic semiconductors.

Two-Dimensional Lateral Epitaxy of 2H (MoSe2)-1T' (ReSe2) Phases.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures have been proposed as potential candidates for a variety of applications like quantum computing, neuromorphic computing, solar cells, and flexible field effective transistors. The 2D TMDC heterostructures at the present stage face difficulties being implemented in these applications because of lack of large and sharp heterostructure interfaces. Herein, we address this problem via a CVD technique to grow thermodynamically stable heterostructure of 2H/1T' MoSe2-ReSe2 using conventional transition metal phase diagrams as a reference. We demonstrate how the thermodynamics of mixing in the MoReSe2 system during CVD growth dictates the formation of atomically sharp interfaces between MoSe2 and ReSe2, which can be confirmed by high-resolution scanning transmission electron microscopy imaging, revealing zigzag selenium-terminated interface between the epitaxial 2H and 1T' lattices. Our work provides useful insights for understanding the stability of 2D heterostructures and interfaces between chemically, structurally, and electronically different phases.

A Practical and Stereoselective In Situ NHC-Cobalt Catalytic System for Hydrogenation of Ketones and Aldehydes

Summary Homogeneous catalytic hydrogenation of carbonyl groups is a synthetically useful and widely applied organic transformation. Sustainable chemistry goals require replacing conventional noble transition metal catalysts for hydrogenation by earth-abundant base metals. Herein, we report how a practical in situ catalytic system generated by easily available pincer NHC precursors, CoCl2, and a base enabled efficient and high-yielding hydrogenation of a broad range of ketones and aldehydes (over 50 examples and a maximum turnover number [TON] of 2,610). This is the first example of NHC-Co-catalyzed hydrogenation of C=O bonds using flexible pincer NHC ligands consisting of a N-H substructure. Diastereodivergent hydrogenation of substituted cyclohexanone derivatives was also realized by fine-tuning of the steric bulk of pincer NHC ligands. Additionally, a bis(NHCs)-Co complex was successfully isolated and fully characterized, and it exhibits excellent catalytic activity that equals that of the in-situ-formed catalytic system.

Suppression of Voltage Decay through Manganese Deactivation and Nickel Redox Buffering in High-Energy Layered Lithium-Rich Electrodes

Cobalt-free layered lithium-rich nickel manganese oxides, Li[LixNiyMn1−x−y]O2, are promising positive electrode materials for lithium rechargeable batteries because of their high energy density and low materials cost. Utilization of the oxygen anionic redox in this series of materials enables realization of a high capacity beyond that achieved via the conventional transition metal cationic redox when charging above 4.5 V vs. Li/Li+. However, substantial voltage decay is inevitable upon electrochemical cycling, which makes this class of materials less practical. The undesirable voltage decay has been proposed to be linked to irreversible structural rearrangement involving irreversible oxygen loss and cation migration. Herein, we demonstrate that the voltage decay of the electrode is correlated to the activation of Mn4+/Mn3+ redox and subsequent cation disordering, which is able to be remarkably suppressed via simple compositional tuning to induce the formation of Ni3+ in the pristine material. By implementing our new strategy, an alternative redox reaction involving the use of this pristine Ni3+ as a redox buffer, which has been designed to be widened from Ni3+/Ni4+ to Ni2+/Ni4+, subdued the Mn4+/Mn3+ reduction without compensation for the capacity in principle. Negligible change in the voltage profile of the modified lithium-rich nickel manganese oxide electrode is observed upon extended cycling, and manganese migration into the lithium layer is significantly suppressed. Based on these findings, we propose a general strategy to suppress the voltage decay of Mn-containing lithium-rich oxides to achieve long-lasting high energy density from this class of materials.

Tuning P2-Structured Cathode Material by Na-Site Mg Substitution for Na-Ion Batteries.

Most P2-type layered oxides suffer from multiple voltage plateaus, due to Na+/vacancy-order superstructures caused by strong interplay between Na-Na electrostatic interactions and charge ordering in the transition metal layers. Here, Mg ions are successfully introduced into Na sites in addition to the conventional transition metal sites in P2-type Na0.7[Mn0.6Ni0.4]O2 as new cathode materials for sodium-ion batteries. Mg ions in the Na layer serve as "pillars" to stabilize the layered structure, especially for high-voltage charging, meanwhile Mg ions in the transition metal layer can destroy charge ordering. More importantly, Mg ion occupation in both sodium and transition metal layers will be able to create "Na-O-Mg" and "Mg-O-Mg" configurations in layered structures, resulting in ionic O 2p character, which allocates these O 2p states on top of those interacting with transition metals in the O-valence band, thus promoting reversible oxygen redox. This innovative design contributes smooth voltage profiles and high structural stability. Na0.7Mg0.05[Mn0.6Ni0.2Mg0.15]O2 exhibits superior electrochemical performance, especially good capacity retention at high current rate under a high cutoff voltage (4.2 V). A new P2 phase is formed after charge, rather than an O2 phase for the unsubstituted material. Besides, multiple intermediate phases are observed during high-rate charging. Na-ion transport kinetics are mainly affected by elemental-related redox couples and structural reorganization. These findings will open new opportunities for designing and optimizing layer-structured cathodes for sodium-ion batteries.

Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold Nanoclusters.

Understanding the structure and structure-property relationship of atomic and ligated clusters is one of the central research tasks in the field of cluster research. In chemistry, empirical rules such as the polyhedral skeleton electron pair theory (PSEPT) approach had been outlined to account for skeleton structures of many main-group atomic and ligand-protected transition metal clusters. Nonetheless, because of the diversity of cluster structures and compositions, no uniform structural and electronic rule is available for various cluster compounds. Exploring new cluster structures and their evolution is a hot topic in the field of cluster research for both experiment and theory. In this Account, we introduce our recent progress in the theoretical exploration of structures and evolution patterns of a class of atomically precise thiolate-protected gold nanoclusters using density functional theory computations. Unlike the conventional ligand-protected transition metal compounds, the thiolate-protected gold clusters demonstrate novel metal core/ligand shell interfacial structures in which the Au m(SR) n clusters can be divided into an ordered Au(0) core and a group of oligomeric SR[Au(SR)] x ( x = 0, 1, 2, 3, ...) protection motifs. Guided by this "inherent structure rule", we have devised theoretical methods to rapidly explore cluster structures that do not necessarily require laborious global potential energy surface searches. The structural predictions of Au38(SR)24, Au24(SR)20, and Au44(SR)28 nanoclusters were completely or partially verified by the later X-ray crystallography studies. On the basis of the analysis of cluster structures determined by X-ray crystallography and theoretical prediction, a structural evolution diagram for the face-centered-cubic (fcc)-type Au m(SR) n clusters with m up to 92 has been preliminarily established. The structural evolution diagram indicates some basic structural and electronic evolution patterns of thiolate-protected gold nanoclusters. The fcc Au m(SR) n clusters show a genetic structural evolution pattern in which each step of cluster size increase results in the formation of another Au4 tetrahedron or Au3 triangle unit in the Au core, and every increase of a structural unit in the Au core leads to an increase of two electrons in the whole cluster. The unique one- or two-dimensional cluster size evolution, the isomerism of the Au-S framework, and the formation of a double-helical and cyclic tetrahedron network in the fcc Au m(SR) n clusters all can be addressed from this evolution pattern. The summarized cluster structural evolution diagrams enable us to further explore more stable cluster structures and understand their structure-electronic structure-property relationships.

Nano composite foam layer of CuO/graphene oxide (GO) for high performance supercapacitor

In this study, a facile and reproducible method has been developed to synthesize a nano composite foam layer of CuO/ graphene oxide (GO) intended for supercapacitor applications. GO particles were deposited on elecroplated CuO foam layers through electrophoretic deposition (EPD). The results showed that by varying EPD time (1, 3, 7 min), the degree of coverage of the foam layer with GO changes. This makes it feasible to manipulate the specific capacity of (Csp) CuO foam layers (238.3 F.g−1). It was shown that the specific capacity of the foam layer has been improved by amounts of 72%, 49% and 40% for EPD time deposition of 1, 3 and 7 min of GO, respectively.The results suggest a new versatile design to enhance super capacity and energy storage properties in conventional transition metal oxides. Morphology, phase structure, porosity and surface area of synthesized structures were investigated by field emission secondary electron microscopy (FE-SEM), X-ray diffraction (XRD) and BET (Brunauer– Emmett–Teller) approaches. Super capacity properties of the layers were from cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests.

Intercalation pseudocapacitance in flexible and self-standing V2O3 porous nanofibers for high-rate and ultra-stable K ion storage

Transition metal oxides possess eminently high capacity based on the conversion reaction by delivering multiple electrons. However, they usually demonstrate limited cycle life and inferior rate capability due to the pulverization and aggregation of active materials. V2O3 is considered to be an attractive electrode material due to its intrinsic tunnel structure provided by 3D V-V framework. Herein, a flexible and self-standing electrode of V2O3 nanoparticles embedded in porous N-doped carbon nanofibers (V2O3@PNCNFs) has been fabricated through a facile electrospinning method and subsequent thermal treatment. Electrochemical kinetic analysis, in situ XRD, ab initio molecular dynamics (AIMD) and density functional theory (DFT) calculation suggest that the potassium storage of V2O3 is dominated by intercalation pseudocapacitance and only up to 1 mol K+ can insert into V2O3 crystal. Different from the conversion reaction in conventional transition metal oxides, in the mechanism of intercalation pseudocapacitance, K+ intercalate into the tunnels of V2O3 accompanied by a faradaic charge-transfer with no crystallographic phase change. This mechanism combines short charging/discharging times with long cycle life, which is reported in PIBs for the first time. This work may open up a new avenue for further development of pseudocapacitive nanomaterials for high-rate and ultra-stable energy storage.

Dual-Ligand Synergistic Modulated Nico-Sulfhydroxides with High Activity and Stability As Oxygen Evolution Electrocatalysts

The most knotty problem of (electro)catalysis that arises ubiquitously is the notion that the more active catalysts for oxygen evolution reaction (OER) will simultaneously suffer from low resistance toward corrosion. In developing cost-effective stable materials for the oxygen evolution reaction with high activity, precisely tuning the structure of transition-metal materials in atomic level to obtain ideal chemical properties is urgent while difficult to achieve1. In this study, we propose a “dual ligand synergistic modulation” tactics to completely solve the painful compromise between the two conflicted indicators: stability and activity. By substituting S ligand into the adjustable and stable structure of layered hydroxides, which was selected as the start noumenon, we obtain a new class of NiCo2(Sx·OH2-x)y with single phase and homogenous composition. The elaborated designed dual-ligand catalyst ideally inherits the excellent activity of the conventional transition metal sulfides and the superior stability of the conventional transition metal hydroxides and even exceeds the predecessors. Impressively, the optimized NiCo2(SOH)x performs a small overpotential of 0.32 V at 10 mA cm-2, and shows no decay after 30h accelerated ageing even at a large current density of 100 mA cm-2, both of which have entered the top of all the transition-metal hydroxides and sulfides reported to date and exceeded the commercial RuO2 and Ir/C benchmarks. Theoretical calculations further disclose that the OH ligands on the surface of NiCo2(SOH)x could attracts electrons from the antibonding orbital of M-S bonds to M-O bonds, resulting in a strengthened binding energy between metal and S and thus enhancing the stability. Meanwhile, the synergy between S ligands and OH groups appropriately tunes the electronic structure of NiCo2(SOH)x and thus renders it with optimal binding energies for OER intermediates (*OH, *O, and *OOH), which is benefit for facilitate the O2 evolution proceeding. Besides, the varied magnetism induced by the modulation of catalysts’ electronic structure significantly influences the desorption action of paramagnetic O2 from the catalyst surface. The special non-magnetic NiCo2(SOH)x suffers the lowest O2 desorption resistance and proceeds the smoothest reaction kinetics pathway among NiCo2(SxOH2-x)y catalysts, thus owning the topmost OER activity. Figure 1. (a) The ultimate unity of OER performance of NiCo2(SxOH2-x)y. (b) The reaction steps and corresponding free energy diagram of the OER process. (c) OER polarization curves of Ni, RuO2, NiCo2S4, NiCo2(OH)x, and NiCo2(SOH)x and (d) Chronoamperometric curves of Ni, NiCo2(SOH)x and NiCo2S4 at a current density of 100mA/cm2. Key words: Oxygen evolution, electrocatalysis, dual-ligand modulation Reference: Lishan Peng, Jun Wang, Yao Nie, Kun Xiong, Yao Wang, Ling Zhang, Ke Chen, Wei Ding, Li Li, and Zidong Wei*. ACS Catal. , 2017, 7, 8184-8191. Figure 1

Oxidative Dehydrogenation on Nanocarbon: Insights into the Reaction Mechanism and Kinetics via in Situ Experimental Methods.

Sustainable and environmentally benign catalytic processes are vital for the future to supply the world population with clean energy and industrial products. The replacement of conventional metal or metal oxide catalysts with earth abundant and renewable nonmetallic materials has attracted considerable research interests in the field of catalysis and material science. The stable and efficient catalytic performance of nanocarbon materials was discovered at the end of last century, and these materials are considered as potential alternatives for conventional metal-based catalysts. With its rapid development in the past 20 years, the research field of carbon catalysis has been experiencing a smooth transition from the discovery of novel nanocarbon materials or related new reaction systems to the atomistic-level mechanistic understanding on the catalytic process and the subsequent rational design of the practical catalytic reaction systems. In this Account, we summarize the recent progress in the kinetic and mechanistic studies on nanocarbon catalyzed alkane oxidative dehydrogenation (ODH) reactions. The paper attempts to extract general concepts and basic regularities for carbon catalytic process directing us on the way for rational design of novel efficient metal-free catalysts. The nature of the active sites for ODH reactions has been revealed through microcalorimetric analysis, ambient pressure X-ray photoelectron spectroscopy (XPS) measurement, and in situ chemical titration strategies. The detailed kinetic analysis and in situ catalyst structure characterization suggests that carbon catalyzed ODH reactions involve the redox cycles of the ketonic carbonyl-hydroxyl pairs, and the key physicochemical parameters (activation energy, reaction order, and rate/equilibrium constants, etc.) of the carbon catalytic systems are proposed and compared with conventional transition metal oxide catalysts. The proposal of the intrinsic catalytic activity (TOF) provides the possibility for the fair comparisons of different nanocarbon catalysts and the consequent structure-function relation regularity. Surface modification and heteroatom doping are proved as the most effective strategies to adjust the catalytic property (activity and product selectivity etc.) of the nanocarbon catalysts. Nanocarbon is actually a proper candidate platform helping us to understand the classical catalytic reaction mechanism better, since there is no lattice oxygen and all the catalytic process happens on nanocarbon surface. This Account also exhibits the importance of the in situ structural characterizations for heterogeneous nanocarbon catalysis. The research strategy and methods proposed for carbon catalysts may also shed light on other complicated catalytic systems or fields concerning the applications of nonmetallic materials, such as energy storage and environment protection etc.

Magnetotransport properties of Cr1−δTe thin films with strong perpendicular magnetic anisotropy

P-type ferromagnetic Cr1-δTe thin films with the Curie temperature of 170K were epitaxially grown on GaAs substrate. Low-temperature magnetotransport study reveals that the film has a strong perpendicular magnetic anisotropy (PMA) and an anisotropic magnetoresistance (AMR) ratio up to 8.1%. Furthermore, reduced anomalous Hall effect is observed at low temperatures in Cr1-δTe, suggesting the possible crossover of the contribution to AHE from the intrinsic mechanism to extrinsic skew scattering. Distinctive from conventional transition metal ferromagnets, the AMR ratio is also greatly suppressed at low temperatures. Our work demonstrates that epitaxial Cr1-δTe films are interesting platforms for studying the physics underlying the strong PMA and large AMR.

Design of 2D Ferromagnets With High Exchange Bias in Fe‐MoS2 Heterostructure For Spintronic Applications

Fe‐MoS2 heterostructures are synthesized in simple hydrothermal method. Different concentrations of Fe precursor are used to grow different thickness of Fe thin sheets placing MoS2 as a template surface. Different characterization techniques are done to confirm the desired phases present in the samples. After that their magnetic characterization is carried out to measure its magnetic property. The hysteresis loop obtained is different from the conventional transition metal nanoparticles. Due to charge transfer from transition metal 3d orbital to 3p orbital of sulfur, the magnetic property of the as grown nanostructure is changed. Magnetic hysteresis loops reveal high amount of exchange bias which changes with temperature. At very low temperature ferromagnetic hysteresis loop is observed. With increasing temperature magnetic saturation dies away due to presence of antiferromagnetic interaction. This antiferromagnetic interaction comes due to charge transfer from Fe to MoS2. The transferred charges at MoS2 surface creates an induced spin moment which is in antiferromagnetic coupling with the Fe spins.

Design and Performance of Rechargeable Sodium Ion Batteries, and Symmetrical Li‐Ion Batteries with Supercapacitor‐Like Power Density Based upon Polyoxovanadates

AbstractThe polyanion Li7V15O36(CO3) is a nanosized molecular cluster (≈1 nm in size), that has the potential to form an open host framework with a higher surface‐to‐bulk ratio than conventional transition metal oxide electrode materials. Herein, practical rechargeable Na‐ion batteries and symmetric Li‐ion batteries are demonstrated based on the polyoxovanadate Li7V15O36(CO3). The vanadium centers in {V15O36(CO3)} do not all have the same VIV/V redox potentials, which permits symmetric devices to be created from this material that exhibit battery‐like energy density and supercapacitor‐like power density. An ultrahigh specific power of 51.5 kW kg−1 at 100 A g−1 and a specific energy of 125 W h kg−1 can be achieved, along with a long cycling life (>500 cycles). Moreover, electrochemical and theoretical studies reveal that {V15O36(CO3)} also allows the transport of large cations, like Na+, and that it can serve as the cathode material for rechargeable Na‐ion batteries with a high specific capacity of 240 mA h g−1 and a specific energy of 390 W h kg−1 for the full Na‐ion battery. Finally, the polyoxometalate material from these electrochemical energy storage devices can be easily extracted from spent electrodes by simple treatment with water, providing a potential route to recycling of the redox active material.

Domain wall motion in a nanoconstriction of Gd

We report the observation of a DC-current driven magnetic domain wall (DW) motion in an artificially created nanoconstriction (size ∼ 300 nm) in a microstrip of 4f ferromagnet Gd (film thickness, 40 nm), where beyond a critical current density, a sharp reduction in the resistance is observed which gives the DW resistance RW. This is similar to that seen in nanoconstrictions made in conventional 3d ferromagnets such as Ni and its alloys. The effect is most prominent at 100 K or below and becomes smaller as the temperature increases, and it vanishes as T→TC. The value of RW was used in conjunction with the measured anisotropic magnetoresistance to find the domain wall thickness (δ) and the anisotropy energy (EA) of the Gd film. The observation of this phenomenon in 4f metal Gd, which is distinct from itinerant conventional 3d transition metals, extends the phenomena to rare-earth ferromagnets. A low ferromagnetic transition temperature of Gd also allows us to study the phenomena and the evolution of δ and EA as a function of temperature.

Surface and Interface Engineering of Noble-Metal-Free Electrocatalysts for Efficient Energy Conversion Processes

Developing cost-effective and high-performance electrocatalysts for renewable energy conversion and storage is motivated by increasing concerns regarding global energy security and creating sustainable technologies dependent on inexpensive and abundant resources. Recent achievements in the design and synthesis of efficient non-precious-metal and even non-metal electrocatalysts make the replacement of noble metal counterparts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) with earth-abundant elements, for example, C, N, Fe, Mn, and Co, a realistic possibility. It has been found that surface atomic engineering (e.g., heteroatom-doping) and interface atomic or molecular engineering (e.g., interfacial bonding) can induce novel physicochemical properties and strong synergistic effects for electrocatalysts, providing new and efficient strategies to greatly enhance the catalytic activities. In this Account, we discuss recent progress in the design and fabrication of efficient electrocatalysts based on carbon materials, graphitic carbon nitride, and transition metal oxides or hydroxides for efficient ORR, OER, and HER through surface and interfacial atomic and molecular engineering. Atomic and molecular engineering of carbon materials through heteroatom doping with one or more elements of noticeably different electronegativities can maximally tailor their electronic structures and induce a synergistic effect to increase electrochemical activity. Nonetheless, the electrocatalytic performance of chemically modified carbonaceous materials remains inferior to that of their metallic counterparts, which is mainly due to the relatively limited amount of electrocatalytic active sites induced by heteroatom doping. Accordingly, coupling carbon substrates with other active electrocatalysts to produce composite structures can impart novel physicochemical properties, thereby boosting the electroactivity even further. Although the majority of carbon-based materials remain uncompetitive with state-of-the-art metal-based catalysts for the aforementioned catalytic processes, non-metal carbon hybrids have already shown performance that typically only conventional noble metals or transition metal materials can achieve. The idea of hybridized carbon-based catalysts possessing unique active surfaces and macro- or nanostructures is addressed herein. For metal-carbon couples, the incorporation of carbon can effectively compensate for the intrinsic deficiency in conductivity of the metallic components. Chemical modification of carbon frameworks, such as nitrogen doping, not only can change the electron-donor character, but also can introduce anchoring sites for immobilizing active metallic centers to form metal-nitrogen-carbon (M-N-C) species, which are thought to facilitate the electrocatalytic process. With thoughtful material design, control over the porosity of composites, the molecular architecture of active metal moieties and macromorphologies of the whole catalysts can be achieved, leading to a better understanding structure-activity relationships. We hope that we can offer new insight into material design, particularly the role of chemical composition and structural properties in electrochemical performance and reaction mechanisms.

Single-Atom Catalysts for CO2 Electroreduction with Significant Activity and Selectivity Improvements

Single-atom catalyst (SAC) has an electronic structure that is very different from its bulk counterparts, and has shown unexpectedly high specific activity with a significant reduction of noble metal usages although physical origins of such performance enhancements are still poorly understood. Herein, by means of density functional theory calculations, we for the first time investigate the great potential of single atom catalyst for CO2 electroreduction applications. In particular, we study a single transition metal atom anchored on the defective graphene with single or double vacancies, denoted as M@sv-Gr or M@dv-Gr, where M are various transition metals, as a CO2 reduction catalyst. Many SACs are indeed shown to be highly selective for CO2 reduction reaction over a competitive H2 evolution reaction due to a favorable adsorption of carboxyl or formate over hydrogen on the catalysts. On the basis of free energies, we found that the Pt@dv-Gr catalyst shows a remarkable reduction in the limiting potential for CH3OH production compared to any existing catalysts, synthesized or predicted (Figure 1). We focus on the Pt@dv-Gr to investigate the origin of improvement on the SACs compared to the transition metal. As shown in Figure 1, with the Pt@dv-Gr catalyst, all the intermediates are destabilized compared to those on Pt (211), but most importantly, the destabilization of *CO (0.98 eV) is much more noticeable than that of *CHO (0.42 eV), leading to a 0.49 V reduction in the limiting potential. We also observe that with SACs, the conventional scaling relation between the *CO binding and *CHO binding established for the bulk transition metal significantly deviates from linearity (Figure 2). We discuss features of SACs which contribute to the breakdown of scaling relation between *CO and *CHO, namely, the lack of atomic ensemble for adsorbates binding and the metal-support interactions that lead to the electronic structures conducive to the catalysis. Atomic Ensemble: The optimized geometries of the bare catalysts are shown for Pt@dv-Gr and Pt (211) in Figure 3. One can see that, for Pt (211), two surface Pt atoms are involved in the *CO bonding while only one Pt atom is bonding with *CHO, leading to a large destabilization in the relative free energies when going from *CO to *CHO. On the other hand, for Pt@dv-Gr, only one Pt atom is utilized for both *CO and *CHO binding, resulting in a much more moderate destabilization in relative free energies compared to Pt (211). Thus, the lack of Pt ensemble in the Pt@dv-Gr is responsible for a significantly weaker binding of *CO on the Pt@dv-Gr compared to the Pt (211) surface. Electronic Structure: The strong metal-support interaction affects the electronic structure of a metal atom in the SACs greatly. In Figure 4A, the Pt 5d density of states (DOS) in Pt@dv-Gr shows a significant orbital overlap with the C 2p orbitals of the graphene. The electron density isosurfaces (Figure 4B) illustrate that electron clouds of four carbon atoms surrounding the Pt atom are significantly hybridized with the Pt atom. The differential charge density map (Figure 4C) between the defective graphene and Pt@dv-Gr also suggests that the Pt atom is positively charged by electron transfer from the Pt atom to the defective graphene support. In this paper, we investigated the single atom catalysts (SACs) as a promising CO2 electroreduction catalyst using DFT calculations. The main findings of this work are as follows. (i) By comparing the free energies of the initial protonation steps for the CRR and HER, we found that all SACs can selectively reduce CO2 rather than producing H2. In particular, the predicted limiting potential for Pt@dv-Gr (-0.27 V) for CH3OH production is considerably less negative than the conventional transition metal catalysts. (ii) To understand the origin of the improvements in SACs, we investigated two aspects of the Pt@dv-Gr that affect the relative stability of *CO vs. *CHO. A one-fold bonding of *CO on Pt@dv-Gr due to a lack of atomic ensemble, as compared to the two-fold *CO bonding on the Pt (211) is responsible for the significant weakening of *CO binding on the Pt@dv-Gr. (iii) We investigated the electronic structure of Pt atom in the SAC to find the origin of the deviation of SACs from the conventional scaling relation of transition metals, which arises from the d-band center theory. We suggest that the strong electronic interaction between the d-orbital of metal atom and the p-orbital of graphene is responsible for the different behavior from the transition metal surfaces, as evidenced by the electron transfer and the overlap in the DOS. Figure 1