Multiscale simulations of carbon nanotube nucleation and growth: electronic structure calculations

Several first-principles surface and bulk electronic structure calculations relating to the nucleation and growth of single-wall carbon nanotubes are described. Density-functional theory in various forms is used throughout. In the surface-related calculations, a 38-atom Ni cluster and several low-index Ni surfaces are investigated using pseudopotentials and plane-wave expansions. The energetic ordering of the sites for C atom adsorption is found to be the same, with the Ni(100) facet favored. The bulk diffusion coefficient of C in Ni as a function of cluster size and temperature is calculated from various molecular dynamics approaches. In another group of bulk-related calculations, Gaussian orbital basis sets are used to study a cluster or "flake" containing 14 C atoms. The flake is a segment of three hexagons from an "unrolled" carbon nanotube, with an armchair termination. The binding energies of C, Ni, Co, Fe, Cu, and Au atoms to it were calculated in an effort to gain insight into the mechanism for the high catalytic activity of Ni, Co, and Fe and the lack of it in Cu and Au. The binding energies of Cu and Au are about 1 eV less than those of the three catalytic elements. Similar methods are used to study the initial stages of nanotube growth within the context of classical nucleation theory. Finally, issues relating to the establishment of a fundamental catalytic mechanism are addressed.     

Interactions between transition metals and defective carbon nanotubes

Interactions between 3d transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) and (5,5) carbon nanotube (CNT) with a vacancy defect are quantitatively characterized using first-principles calculations. The binding energies between CNT and transition metals are found to be significantly enhanced when vacancy defects are introduced into the CNT. For the defective CNTs doped with Sc, Cr and Zn atoms, the structures of defective CNTs are found to be intact. The doping of Ti, Mn, Cu, Fe, Ni and Co alternates the structures of defective CNTs. Among all 3d transition metals, only the ferromagnetic metal atoms Fe, Co and Ni form bonds with carbon atoms of CNT, suggesting the important role of magnetic exchange interaction in the p–d hybridisation between carbons and transition-metal atoms. The results also indicate that the 3d transition-metal atoms acting as substitutional defects can substantially modify the electronic structure of CNT. It is suggested that these stable CNT-metal systems could become promising engineering materials in many fields such as CNT devices for various spintronics applications and CNT metal–matrix composites.     

Boosting the Loading of Metal Single Atoms via a Bioconcentration Strategy

Increasing the mass loading of transition metal single atoms coordinated with nitrogen in carbon‐based materials (M‐N‐C) is still challenging. Herein, inspired by the bioconcentration effect in the living body, a biochemistry strategy for the synthesis of Fe‐N‐C single atoms is demonstrated. Through introducing ferrous glycinate into the growth of fungus, the Fe atoms are bioconcentrated in hyphae. The highly dispersed Fe‐N‐C single atoms in hyphae‐derived carbon fibers (labeled as Fe‐N‐C SA/HCF) are prepared by the pyrolysis of Fe‐riched hyphae. In the bioconcentration process, the uptake of Fe ions by hyphae promotes the secretion of glutathione and ferritin, which provides additional coordination sites for Fe ions. Accordingly, the mass content of Fe in bioconcentrated Fe‐N‐C SA/HCF reaches 4.8%, which is 5.3 times larger than that of the sample prepared by the conventional pyrolysis process. The present bioconcentration strategy is further extended to the preparation of Co, Ni, and Mn single atoms. Owing to the high content of Fe‐N‐C single atoms, Fe‐N‐C SA/HCF shows the onset potential (E_onset) of 0.931 V versus reversible hydrogen electrode (RHE) and half‐wave potential (E_1/2) of 0.802 V versus RHE in oxygen reduction reaction measurements, which is comparable to the commercial Pt/C catalysts.     

Point defects and their clusters in bcc metals

Recently Doyama and Kogure have developed new EAM potentials of bcc metals (DK_EAM). The validity of DK_EAM potential of Fe and V is examined by calculating a thermal expansion coefficient, a phonon dispersion and properties of point defects. In Fe, a calculated thermal expansion coefficient is negative and its absolute value is much smaller than that of experimental data. The phonon dispersion relation shows fair agreements at several branch. The calculated density of state of phonon shows a lack of a peak at high frequency determined experimentally. In V, the thermal expansion coefficient is negative and the crystal volume decreases significantly during a molecular dynamics (MD) simulation at 1200 K. This means that the embedding energy and the repulsive pair potential are not calculated correctly at the position which deviates from the stable site of atoms. We calculate the property of point defects in Fe. The relaxation volume of vacancies in Fe is very small which leads to the void formation as a vacancy cluster grows to a larger cluster. A MD simulation shows that two atoms in interstitial clusters approach very closely. This suggests the repulsive pair potential in DK_EAM is weak. By combining the pair potential of DK_EAM with the classical pair potential, the DK_EAM is modified. The results are compared with the previous calculation and the experimental data.     

Pseudocapacitive Ni‐Co‐Fe Hydroxides/N‐Doped Carbon Nanoplates‐Based Electrocatalyst for Efficient Oxygen Evolution

The oxygen evolution reaction (OER) catalytic activity of a transition metal oxides/hydroxides based electrocatalyst is related to its pseudocapacitance at potentials lower than the OER standard potential. Thus, a well‐defined pseudocapacitance could be a great supplement to boost OER. Herein, a highly pseudocapacitive Ni‐Fe‐Co hydroxides/N‐doped carbon nanoplates (NiCoFe‐NC)‐based electrocatalyst is synthesized using a facile one‐pot solvothermal approach. The NiCoFe‐NC has a great pseudocapacitive performance with 1849 F g^?1 specific capacitance and 31.5 Wh kg^?1 energy density. This material also exhibits an excellent OER catalytic activity comparable to the benchmark RuO₂ catalysts (an initiating overpotential of 160 mV and delivering 10 mA cm^?2 current density at 250 mV, with a Tafel slope of 31 mV dec^?1). The catalytic performance of the optimized NiCoFe‐NC catalyst could keep 24 h. X‐ray photoelectron spectroscopy, electrochemically active surface area, and other physicochemical and electrochemical analyses reveal that its great OER catalytic activity is ascribed to the Ni‐Co hydroxides with modular 2‐Dimensional layered structure, the synergistic interactions among the Fe(III) species and Ni, Co metal centers, and the improved hydrophily endowed by the incorporation of N‐doped carbon hydrogel. This work might provide a useful and general strategy to design and synthesize high‐performance metal (hydr)oxides OER electrocatalysts.     

过渡金属和磷共掺杂多孔碳作为氧气还原电催化剂

碳基材料作为非贵金属催化剂具有导电性能高、稳定性能好、价格低廉、环境友好等优点,在燃料电池阴极催化剂领域中引起了广泛的关注,尤其是过渡金属和异原子共掺杂能够显著提高碳材料的氧气还原催化活性。本文采用聚醚(F127)作为软模版,苯酚、甲醛作为碳源,四苯基溴化膦作为磷源,硝酸盐作为过渡金属来源,通过挥发溶剂自组装及高温煅烧过程制备了过渡金属(Co、Fe、Ni、Mn)和磷(P)共掺杂多孔碳材料(TM-P-C)。通过旋转环盘电极研究了TM-P-C在0.1 mol/L KOH电解液中的氧气还原电催化性能。研究结果表明：TM-P-C催化剂具有较高的氧化还原反应(ORR)电催化性能,其ORR活性为P-Co-C>P-NiC>P-Fe-C>P-Mn-C,其中P-Co-C的ORR电催化性能可与商业20wt%Pt/C催化剂相媲美,其电流密度与20wt%Pt/C催化剂的电流密度相当,与20wt%Pt/C仅存在66 mV的半波电位差,表现为ORR的4e–转移途径。制备的TM-P-C催化剂所具有的较高氧气还原电催化活性主要来自于过渡金属和P原子之间的协同作用。此外,TM-P-C催化剂表现出优异的...     

Instantaneous formation of metal and metal oxide nanoparticles on carbon nanotubes and graphene via solvent-free microwave heating

Microwave irradiation was shown to be an effective energy source for the rapid decomposition of organic metal salts (such as silver acetate) in a solid mixture with various carbon and noncarbon substrates under completely solvent-free conditions. The rapid and local Joule heating of microwave absorbing substrates (i.e., carbon-based) resulted in the instantaneous formation of metal and metal oxide nanoparticles on the substrate surfaces within seconds of microwave exposure. Other less absorbing substrates (such as hexagonal boron nitride) required longer exposure times for the salt decomposition to occur. Details of the effects of microwave reaction time, temperature, power, and other experimental parameters were investigated and discussed. The solvent-free microwave method was shown to be widely applicable to various organic metal salts with different substrates including single- and multiwalled carbon nanotubes, graphene, expanded graphite, hexagonal boron nitride and silica-alumina particles, forming substrate-supported metal (e.g., Ag, Au, Co, Ni, Pd, Pt) or metal oxide (e.g., Fe?O?, MnO, TiO?) nanoparticles in high yields within short duration of microwave irradiation. The method was also successfully applied to large structural substrates such as nanotube yarns, further suggesting its application potential and versatility. To demonstrate one potential application, we successfully used both carbon nanotube powder and yarn samples decorated with Ag nanoparticles prepared via the above method to improve data acquisition in surface enhanced Raman spectroscopy.     

Ab initio study of structure and magnetism of late transition metal oxide TMnOm clusters (TM = Fe, Co, Ni, n = 1, 2, m = 1-6)

We systematically investigate the structural and magnetic properties of late transition metal oxide clusters, TMnOm (TM = Fe, Co, Ni; n = 1, 2; m = 1-6) by using ab initio density functional theory approach. FenOm clusters prefer to adopt three dimensional configurations, while ConOm and NinOm clusters are apt to form planar structures. The O atoms are all atomic bonding to Fe atoms in the FenOm clusters, and are partly molecularly adsorbed to Co(Ni) in ConOm(NinOm) clusters, such as Co2O(5-6) (NiO3, and Ni2O(4-6)). The average binding energies per atom of TMnOm show a monotonous increase trend with the increase of O atoms for both n = 1 and 2 for TM = Fe, Co and Ni, and reach the peak at m = 4 for TM2Om and decrease a little bit afterwards. The odd-even magnetic oscillation is major trait with the peaks at odd and bottoms at even sizes for Fe2Om and Ni2Om (m = 2-6), respectively and large magnetic moments are found in Co2O3 and Co2O6.     

A Hydrangea‐Like Superstructure of Open Carbon Cages with Hierarchical Porosity and Highly Active Metal Sites

Carbon micro‐/nanocages have attracted great attention owing to their wide potential applications. Herein, a self‐templated strategy is presented for the synthesis of a hydrangea‐like superstructure of open carbon cages through morphology‐controlled thermal transformation of core@shell metal–organic frameworks (MOFs). Direct pyrolysis of core@shell zinc (Zn)@cobalt (Co)‐MOFs produces well‐defined open‐wall nitrogen‐doped carbon cages. By introducing guest iron (Fe) ions into the core@shell MOF precursor, the open carbon cages are self‐assembled into a hydrangea‐like 3D superstructure interconnected by carbon nanotubes, which are grown in situ on the Fe–Co alloy nanoparticles formed during the pyrolysis of Fe‐introduced Zn@Co‐MOFs. Taking advantage of such hierarchically porous superstructures with excellent accessibility, synergetic effects between the Fe and the Co, and the presence of catalytically active sites of both metal nanoparticles and metal–N_x species, this superstructure of open carbon cages exhibits efficient bifunctional catalysis for both oxygen evolution reaction and oxygen reduction reaction, achieving a great performance in Zn–air batteries.     

Probing the Irregular Lattice Strain‐Induced Electronic Structure Variations on Late Transition Metals for Boosting the Electrocatalyst Activity

Owing to the simplicity in practice and continuous fine‐tuning ability toward the binding strengths of adsorbates, the strain effect is intensively explored, especially focused on the modulation of catalytic activity in transition metal (TM) based electrocatalysts. Recently, more and more abnormal cases have been found that cannot be explained by the conventional simplified models. In this work, the strain effects in five late TMs, Fe, Co, Ni, Pd, and Pt are studied in‐depth regarding the facet engineering, the surface atom density, and the d‐band center. Interestingly, the irregular response of Fe lattice to the applied strain is identified, indicating the untapped potential of achieving the phase change by precise strain modulation. For the complicated high‐index facets, the surface atom density has become the pivotal factor in determining the surface stability and electroactivity, which identifies the potential of high entropy alloys (HEA) in electrocatalysis. The work supplies insightful understanding and significant references for future research in subtle modulation of electroactivity based on the precise facet engineering in the more complex facets and morphologies.     

Neutron diffraction study of the Th(Co1-xFex)5alloys

We have studied the ordering of Fe atoms and the magnetic properties of Th(Co1-xFex)5alloys with the CaCu5- type structure. From the neutron diffraction analysis it is concluded that the larger Fe atoms prefer to occupy the site3gwhereas the smaller Co prefer to occupy the site 2c. The ordering between Fe and Co in these alloys is mainly determined by stearic considerations. The Curie temperatures are found to be sensitive to the type of heat treatment. Although the bulk anisotropy of the alloys up tox leq 0.7remains essentially unchanged, the coercivity decreases with increasing iron concentration. The possibility of disordered substitution of a Th atom by a pair of transition metal atoms and its role on the structural and magnetic properties is discussed.     

Co–Fe Alloy/N‐Doped Carbon Hollow Spheres Derived from Dual Metal–Organic Frameworks for Enhanced Electrocatalytic Oxygen Reduction

Metal–organic framework (MOF) composites have recently been considered as promising precursors to derive advanced metal/carbon‐based materials for various energy‐related applications. Here, a dual‐MOF‐assisted pyrolysis approach is developed to synthesize Co–Fe alloy@N‐doped carbon hollow spheres. Novel core–shell architectures consisting of polystyrene cores and Co‐based MOF composite shells encapsulated with discrete Fe‐based MOF nanocrystallites are first synthesized, followed by a thermal treatment to prepare hollow composite materials composed of Co–Fe alloy nanoparticles homogeneously distributed in porous N‐doped carbon nanoshells. Benefitting from the unique structure and composition, the as‐derived Co–Fe alloy@N‐doped carbon hollow spheres exhibit enhanced electrocatalytic performance for oxygen reduction reaction. The present approach expands the toolbox for design and preparation of advanced MOF‐derived functional materials for diverse applications.     

Local moment formation in substituted and cobalt-rich CoSi

The magnetic susceptibilities of (Co_1?xM_x)Si, where M is Fe, Ni, Ru and Rh, and Co_1+xSi are newly measured. Only Fe substituted and Co-rich CoSi are found to have a temperature dependent susceptibility. Combined with 59Co NMR studies the temperature dependent susceptibility is interpreted as due to the development of local moments on the Co atoms having at least one Fe atom in its nearest neighbor metal or one Co atom in its nearest neighbor silicon sites.     

A Novel Heterostructure Based on RuMo Nanoalloys and N-doped Carbon as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec⁻¹), and a high turnover frequency (3.57 H₂ s⁻¹) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon–metal heterostructures for highly efficient electrocatalysis.     

二维纳米材料MXene的研究进展

MXene是一类新型碳/氮化物二维纳米层状材料,一般是利用化学刻蚀的手段通过选择性刻蚀掉前驱体MAX相中的A原子层而得到.其通式可表示为Mn+1XnTx,其中M代表早期过渡族金属,X代表碳和/或氮,Tx代表MXene在刻蚀过程中产生的附着在其表面的官能团(-OH、-F、=O、等).采用一定的手段将多层MXene剥落,可获得类石墨烯形貌的单层MXene.MXene除了具备传统二维材料的性能外,还兼具良好的导电性、亲水性、透光性、柔韧性以及能量储存性能,在复合材料、润滑剂、环境污染治理、电池、电容器、催化、传感器、抗菌等领域具有潜在的应用价值.文章总结了MXene的制备、结构、性能和应用等方面的最新成果,并展望了其今后的研究方向.     

Probing interfacial properties of ferromagnetic/insulator bilayers with X-ray spectroscopies: Application to Fe,Co, Mn/MgO(0 0 1) interfaces

Electronic and magnetic properties of bcc Co, Fe and Mn(0 0 1) epitaxial monolayers in contact with a single-crystalline MgO(0 0 1) film were studied using X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) measurements. The XPS and XAS analysis clearly evidenced the weak hybridization between the MgO barrier and Fe or Co. On the contrary, a net oxidization of the Mn layer in contact with the MgO layer was observed. The magnetic properties were characterized by probing the XMCD signal of a unique atomic plane of transition metal in contact with MgO. The total magnetic moment per Co and Fe atoms were observed to increase compared to the bulk at the metal/oxide interface. Finally, Mn at the interface with MgO does not present any ferromagnetic behavior. This was assumed to be a consequence of the Mn oxidization.     

A General Method for Transition Metal Single Atoms Anchored on Honeycomb-Like Nitrogen-Doped Carbon Nanosheets

Excavating and developing highly efficient and cost-effective nonnoble metal single-atom catalysts for electrocatalytic reactions is of paramount significance but still in its infancy. Herein, reported is a general NaCl template-assisted strategy for rationally designing and preparing a series of isolated transition metal single atoms (Fe/Co/Ni) anchored on honeycomb-like nitrogen-doped carbon matrix (M₁-HNC-T₁-T₂, M = Fe/Co/Ni, T₁ = 500 °C, T₂ = 850 °C). The resulting M₁-HNC-500-850 with M-N₄ active sites exhibits superior capability for oxygen reduction reaction (ORR) with the half-wave potential order of Fe₁-HNC-500-850 > Co₁-HNC-500-850 > Ni₁-HNC-500-850, in which Fe₁-HNC-500-850 shows better performance than commercial Pt/C. Density functional theory calculations reveal a choice strategy that the strong p–d-coupled spatial charge separation results the Fe-N₄ effectively merges active electrons for elevating d-band activity in a van-Hove singularity like character. This essentially generalizes an optimal electronic exchange-and-transfer (ExT) capability for boosting sluggish alkaline ORR activity. This work not only presents a universal strategy for preparing single-atom electrocatalyst to accelerate the kinetics of cathodic ORR but also provides an insight into the relationship between the electronic structure and the electrocatalytical activity.     

All-electron DFT modeling of SWCNT growth on iron catalysts from carbon monoxide feedstock

Electronic and geometrical structures of Fe4Cn(CO)m (n + m < or = 6) and their singly negatively and positively charged ions are computed using density functional theory with generalized gradient approximation. Isomers with CO bonded directly to the cluster iron atoms and bonded to a carbon atom chemisorbed on the cluster surface are optimized for the Fe4C2CO, Fe4C2(CO)2, Fe4C3CO, and Fe4C4CO series. Optimizations of a large number of differently shaped Fe4C4, Fe4C5, and Fe4C6 clusters are performed to find trends in preferable arrangement of carbon atoms, in particular, to determine the relative energetics of structures with single C atoms versus those with C2 dimers or C3 trimers. The computed total energies are used to estimate the energetics of the Boudouard disproportionation reactions Fe4Cn(CO)m + 00-->Fe4Cn+1(CO)m-1 + CO2.     

Fabricating Single‐Atom Catalysts from Chelating Metal in Open Frameworks

In the present study, a highly efficient strategy is reported using open framework platforms with abundant chelating ligands to fabricate a series of stable metal single‐atom catalysts (SACs). Here, the metal ions are initially anchored onto the active bipyridine sites through postsynthetic modification, followed by pyrolysis and acid leaching. The resulting single metal atoms are uniformly distributed on a nitrogen‐doped carbon (N‐C) matrix. Interestingly, each metal atom is found to be coordinated with five N atoms, in contrast to the average coordination number of four as previously reported. The as‐prepared Fe SAC/N‐C catalyst exhibits excellent oxygen reduction reaction (ORR) activity (with a half‐wave potential of 0.89 V), outstanding stability, and good methanol tolerance. The density functional calculations reveal that the coordinated pyridine can favorably modulate the interaction strength of oxygen on the Fe ion and thus improve the ORR activity. More importantly, it is demonstrated that this strategy can be successfully extended to the preparation of other transition metal SACs, simply by altering the metal precursors used in the metalation step.     

Organic Small Molecule Activates Transition Metal Foam for Efficient Oxygen Evolution Reaction

Developing low-cost, highly efficient, and durable electrocatalysts for oxygen evolution reaction (OER) is essential for the practical application of electrochemical water splitting. Herein, it is discovered that organic small molecule (hexabromobenzene, HBB) can activate commercial transition metal (Ni, Fe, and NiFe) foam by directly evolving metal nanomeshes embedded in graphene-like films (M-NM@G) through a facile Br-induced solid-phase migration process. Systematic investigations indicate that HBB can conformally generate graphene-like network on bulk metal foam substrate via the cleavage of C Br bonds and the formation of CC linkage. Simultaneously, the cleaved C Br fragments can efficiently extract metal atoms from bulk substrate, in situ producing transition metal nanomeshes embedded in the graphene-like films. As a result, such functional nanostructure can serve as an efficient OER electrocatalyst with a low overpotential and excellent long-term stability. Specifically, the overpotential at 100 mA cm⁻² is only 208 mV for NiFe-NM@G, ranking the top-tier OER electrocatalysts. This work demonstrates an intriguing general strategy for directly transforming bulk transition metals into nanostructured functional electrocatalysts via the interaction with organic small molecules, opening up opportunities for bridging the application of organic small molecules in energy technologies.