Double Transition Metal Research Articles

Computational prediction of Mo2@g-C6N6 monolayer as an efficient electrocatalyst for N2 reduction

Electrocatalytic nitrogen reduction reaction (NRR) is an environmentally friendly method for sustainable ammonia synthesis under ambient conditions. Searching for efficient NRR electrocatalysts with high activity and selectivity is currently urgent but remains great challenge. Herein, we systematically investigate the NRR catalytic activities of single and double transition metal atoms (TM = Fe, Co, Ni and Mo) anchored on g-C6N6 monolayers by performing first-principles calculation. Based on the stability, activity, and selectivity analysis, Mo2@g-C6N6 monolayer is screened out as the most promising candidate for NRR. Further exploration of the reaction mechanism demonstrates that the Mo dimer anchored on g-C6N6 can sufficiently activate and efficiently reduce the inert nitrogen molecule to ammonia through a preferred distal pathway with a particularly low limiting potential of -0.06 V. In addition, we find that Mo2@g-C6N6 has excellent NRR selectivity over the competing hydrogen evolution reaction, with the Faradaic efficiency being 100%. Our work not only predicts a kind of ideal NRR electrocatalyst but also encouraging more experimental and theoretical efforts to develop novel double-atom catalysts (DACs) for NRR.

Nb-based double transition metal silicides MAX-phase: A first-principle study

To explore the structural rationality and potential applications of the hypothetical chemically ordered double transition metal silicides MAX-phase Nb 2 ZrSiC 2 and Nb 2 TiSiC 2 , we used density functional theory to systematically study the crystal structure and related properties of them and compared with the case of Nb 3 SiC 2 . Through the calculations of elastic constant and phonon dispersive curves, the stability of these three ceramics were investigated. Results show they are stable in mechanics and lattice dynamics. Then, normalized bond lengths as function of pressure for these compounds were computed to discuss their bond stiffness. In addition, the calculations of the densities of states for them indicate that they are all metallic-like conductors. And then, the calculations of dielectric function and reflectivity for these ceramics were performed to explore their optical properties. The good reflectivity performance in the infrared and close infrared region means they are potential coating materials on spacecraft in the future. At the end of this work, the Debye temperatures of these three ceramics compounds were obtained by calculating their longitudinal, transverse and mean and velocities, and results indicate they all exhibit a relatively good high temperature resistance.

Mixed MXenes: Mo1.33CTz and Ti3C2Tz freestanding composite films for energy storage

MXenes are a class of 2D materials with outstanding properties, including high electronic conductivity, hydrophilicity, and high specific capacitance. In particular, Mo 1.33 CT z MXene has a high specific capacitance, whereas films of Ti 3 C 2 T z MXene possess high flexibility and high electronic conductivity. The fabrication of composite materials based on these two MXenes is therefore motivated, taking advantage of combining their good properties. In this article, we introduce a one-step approach to prepare composite MXene films using pristine Mo 1.33 CT z and Ti 3 C 2 T z MXenes. The composite films display superior flexibility and electronic conductivity, as well as high capacitance, up to 1380 F cm −3 (460 F g −1 ), in 1 M H 2 SO 4 . A capacitance retention of 96% is obtained after 17,000 cycles. In addition, the capacitance retentions are about 56% and 25% at scan rates of 200 mV s −1 and 1000 mV s −1 , respectively. A significant rise in the capacitance at high rates, 875 F cm −3 (282 F g −1 ) at a current density of 20 A g −1 , is achieved by using a 3 M H 2 SO 4 solution. The use of composite MXene as negative electrodes for asymmetric supercapacitor devices, as well as lithium-ion batteries, is also discussed. This work suggests new pathways for the use of MXene composites with double transition metals (Mo and Ti) in energy storage devices. • Simple, fast, and robust synthesis approach to fabricate mixed MXene films using Mo 1.33 CT z and Ti 3 C 2 T z pristine MXenes. • The mixed MXene films display good flexibility and high electronic conductivity (up to 140 S cm −1 ). • The mixed MXene films deliver a high capacitance (1380 F cm −3 ) and feature 96% retention after 17,000 cycles in 1 M H 2 SO 4 . • A significant rise in the rate performance is observed in 3 M H 2 SO 4 solution (875 F cm −3 at current density 20 A g −1 ). • Mixed MXene electrodes are employed as negative electrodes for asymmetric supercapacitor devices.

Double Transition Metal Carbides MXenes (D-MXenes) as Promising Electrocatalysts for Hydrogen Reduction Reaction: Ab Initio Calculations.

Double-transition-metal MXenes (D-MXenes) have been widely pursued in the advancement of the renewable energy storage technology in recent years. In this work, the hydrogen evolution reaction (HER) catalytic mechanism of several oxygen-terminated D-MXenes with the chemical formula of M′2M″C2O2 (M′ = Mo, Cr; M″ = Ti, V, Nb, Ta) is theoretically studied. For comparison, the corresponding monometallic MXenes (M-MXenes, M′3C2O2) are fairly compared by means of the density functional theory calculations. Based on our theoretical results, the HER performance of M-MXenes can be improved by constructing a “sandwich-like” ordered D-MXene configuration. Moreover, the HER performance of Mo-based D-MXenes (Mo2M″C2O2) is superior to that of Cr-based D-MXenes (Cr2M″C2O2), which highlights that the HER activity of Mo2VC2O2 and Mo2NbC2O2 is better than that of Pt(111). This work not only unravels the HER mechanism of D-MXenes (M′2M″C2O2) but also paves the way in designing emergent MXene-based HER electrocatalysts with high efficiency.

3D TiO2/ZnO hybrid framework: Stable host for lithium metal anodes

Lithium (Li) metal is a promising anode material for high energy density lithium-ion batteries. In this work, we develop a 3D hierarchical double transition metal oxide-framework (TiO2/ZnO) that allows the infusion of molten Li into the framework. The TiO2/ZnO framework consists of lithiophobic TiO2 nanotube arrays grown via anodic oxidation and lithiophilic ZnO nanoparticles prepared by thermal decomposition of hydroxide, which acts as host of molten Li forming integrated trilayer TiO2/ZnO/Li. The as-prepared TiO2/ZnO/Li anode exhibits prominent electrochemical performance with a small hysteresis of less than 45 mV beyond 1000 cycle (at 0.5 mA cm−2). The full cell with trilayer TiO2/ZnO/Li anodes coupled with lithium iron phosphate shows outstanding electrochemical performance with a steady Coulombic efficiency and a stable cycle capacity of 133 mA h g−1 at 0.5C after 300 cycles, much higher than 8 mA h g−1 obtained under the same condition but replacing the trilayer TiO2/ZnO/Li anode with bare Li. The unique TiO2/ZnO framework can not only provide lithiophilic base to accommodate Li, but also afford rapid transfer paths for electrons and ions with low local current, which can effectively inhibit the growth of Li dendrite.

Single, double, and triple transition metal atoms embedded in defective V3C2O2 for nitrogen reduction reaction: A DFT study

Although numerous theoretical studies on single-atom catalysts (SACs) for an efficient electrochemical nitrogen reduction reaction (NRR) have been implemented to compare their catalytic performance. However, a rigid screening criterion excludes plenty of SACs with moderate catalytic activity. To overcome this limitation, we electronically modify the d-band structure of MXenes-based SACs to achieve higher catalytic performance. Here, multiple-transition metal atoms (TMn, TM = Cr, Mo, W, n = 1–3) embedded in defective V3C2O2 are systematically investigated. First-principles calculations reveal that with the addition of foreign Mo and W atoms, the d-band centers of TMn@V3C2O2 are negatively shifted, and more electron-deficient sites are created. Consequently, Mo2@V3C2O2, Mo3@V3C2O2, W2@V3C2O2, and W3@V3C2O2 effectively inhibit the competitive hydrogen evolution reaction with favorable limiting potentials of −0.31, −0.33, −0.34, and −0.31 V, respectively. Our findings provide a new avenue of efficient atomic catalysts toward NH3 production while offering a reliable strategy for tuning the NRR performance of the MXenes-based SACs.

A theoretical investigation of quantum spin Hall state in ordered M′2M″2C3 MXenes (M′ = V, Nb, Ta and M″ = Ti, Zr, Hf)

MXenes have attracted lots of attention because of the potential applications in electronic devices and energy storage. A variety of transition metals in MXenes give rise to distinct properties and trigger more interests. Depending on the exfoliation processes from the MAX phase, the surfaces of MXenes can be terminated by O, F, Cl, OH groups. Theoretical calculations reveal that the electronic properties of MXenes can be tuned by different surface terminations. For example, some F and O terminated MXenes are predicted to be topological insulators with the quantum spin hall states. In OH terminated MXene multilayers, the image potential states are close to the Fermi level. The energies of these states are sensitive to the interlayer distances. Consequently, the topology of the energy bands can be modulated. Here, based on the density functional theory, we study the electronic structures of the ordered double transition metal MXenes M′2M″2C3T2 (where M′ = V, Nb, Ta, M″ = Ti, Zr, Hf and T = F, Cl). We propose that these materials are topologically nontrivial insulators or semimetals. The topological index is 1 and the presence of the conducting helical edge states is demonstrated. Their dynamical stabilities are confirmed by the phonon spectra. We expect that our prediction can facilitate the future application of MXenes as the topological insulating devices.

Computational screening study of double transition metal carbonitrides M\u20322M\u2033CNO2-MXene as catalysts for hydrogen evolution reaction

Two-dimensional (2D) transition metal carbonitrides (MXene) have attracted growing interest in electrocatalytic hydrogen production due to its structural and electronic properties. In this work, the hydrogen evolution reaction (HER) activity of all 64 O-terminated ordered double transition metal carbonitrides in the form of M′2M″CNO2 (M′ = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta; M″ = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta) has been investigated by well-defined density functional theory (DFT) calculations. The results indicate that there are 11 M′2M″CNO2-MXene candidates whose HER performance is superior to that of Pt. Moreover, according to the stability screening, it is proved that Ti2NbCNO2, Mo2TiCNO2, and Ti2VCNO2 are more stable than other candidates. Especially, Ti2NbCNO2 have the potential to be perfect HER catalyst with the small Gibbs free energies of hydrogen adsorption (ΔGH) value of 0.02 eV, abundant catalytic sites on the C-side, and better stability. This work paves the way on designing excellent HER catalyst candidates based on M′2M″CNO2-MXenes.

A theoretical investigation of topological phase modulation in carbide MXenes: Role of image potential states

MXenes have triggered great interests because of their potential applications. Depending on the exfoliation, MXenes can be terminated by O, F and OH groups. The properties of MXenes can be designed. Some F and O-terminated MXenes are predicted to be topological insulators (TIs). In OH-MXenes, the work-functions are significantly reduced. The image potential states (IPS), whose energies can be modulated by the interlayer distances, appear near the Fermi level. Based on the density functional theory, we study the electronic structures of the ordered double transition metal carbides multilayers M′2M”2C3(OH)2 (where M’ = V, Nb, Ta and M” = Ti, Zr, Hf) and investigate the role played by the IPS in the determination of the energy bands topology. For example, V2Ti2C3(OH)2 monolayer is a trivial semimetal. However, the layered V2Ti2C3(OH)2 becomes a strong TI by reducing the interlayer distance. In other words, the topological properties can be modulated. A novel energy band conversion involving IPS is proposed. Although the successful synthesis of M′2M”2C3(OH)2 MXenes faces plenty of challenges, the revealing topological transition mechanism will be valid for other materials whose IPS are near the Fermi levels. We expect that our prediction can advance the applications of low work-function multilayers as the controllable TI devices.

Enhancing electrochemical performance of electrode material via combining defect and heterojunction engineering for supercapacitors

The poor conductivity and deficient active sites of transition metal oxides lead to low energy density of supercapacitors, which limits their wide application. In this work, double transition metal oxide heterojunctions with oxygen vacancy (Vo-ZnO/CoO) nanowires are prepared by effective hydrothermal and thermal treatments. The formation of the heterojunction results in the redistribution of interface charge between ZnO and CoO, generating an internal electric field to accelerate the electron transport. Meanwhile, oxygen vacancies can enhance the redox reaction activity to further improve the electrochemical kinetics of the electrode material. Therefore, the prepared Vo-ZnO/CoO can provide a superior specific capacity of 845 C g−1 (1 A g−1). An asymmetric supercapacitor with the Vo-ZnO/CoO as positive electrode shows a higher energy density of 51.6 Wh kg−1 when the power density reaches 799.9 W kg−1. This work proposes a synergistic combination of defect and heterojunction engineering to improve the electrochemical properties of materials, providing an important guidance for material design in energy-storage field.

The Activation and Reduction of N2 by Single/Double‐Atom Electrocatalysts: A First‐Principle Study

AbstractDeveloping low‐cost and highly efficient electrocatalysts for nitrogen reduction reaction (NRR) is one of the most attractive challenges. In this work, we systematically investigated the catalytic properties of the single atom catalysts (SAC) and the double atom catalysts (DAC) for NRR via First‐Principle calculations. The SAC and DAC were formed by single and double transition metal (TM) atoms anchored on N‐doped graphene monolayers (TMxN3x‐graphene, x=1, 2; TM=Mo, Fe, Co, Ni and Cu). It is shown that both the SAC and the DAC have exhibited high catalytic properties for NRR. Moreover, when N2 is absorbed by its side‐on sites, SAC even performs better than DAC. And in five kinds of TM atoms, Fe‐complexes exhibit significantly improved catalytic properties. Especially, Fe1N3‐graphene has the best catalytic activity for NRR with the lowest free energy change of 0.04 eV in the rate determining step (RDS). This work not only proposes the differences of SAC and DAC but also provides a new kind of single atom catalysts (SAC) for NNR.

First-principles prediction of enhanced thermoelectric properties of double transition metal MXenes: Ti3−xMoxC2T2;(x=0.5,1,1.5,2,2.5,T=−OH/−O/−F)

Employing first-principles electronic structure calculations, we investigate the stability, electronic, and magnetic properties of ordered double transition metal-based two-dimensional maxene (MXene) compounds, ${\mathrm{Ti}}_{3\ensuremath{-}x}{\mathrm{Mo}}_{x}{\mathrm{C}}_{2}\phantom{\rule{0.28em}{0ex}}(x=0.5,1,1.5,2,2.5)$. Both unpassivated and compounds passivated with -OH, -O, or -F are considered. Based on this investigation, three narrow gap semiconducting compounds are identified, namely, ${\mathrm{Ti}}_{2}{\mathrm{MoC}}_{2}{\mathrm{F}}_{2},\phantom{\rule{0.28em}{0ex}}{\mathrm{TiMo}}_{2}{\mathrm{C}}_{2}{\mathrm{F}}_{2}$, and ${\mathrm{TiMo}}_{2}{\mathrm{C}}_{2}{(\mathrm{OH})}_{2}$ for subsequent study of thermoelectric properties within the formalism of Boltzmann transport. Our paper highlights the potential of ${\mathrm{Ti}}_{2}{\mathrm{MoC}}_{2}{\mathrm{F}}_{2}$ as a thermoelectric material with a ZT value higher than 1 within a large temperature range of 300--800 K upon $p$-type carrier doping. The ZT value of $p$-type ${\mathrm{Ti}}_{2}{\mathrm{MoC}}_{2}{\mathrm{F}}_{2}$ is found to reach a high value of 3.1 and an efficiency of $\ensuremath{\sim}27%$ at 800 K. Our exercise should motivate experimental study of yet-to-be synthesized double transition metal MXene compound, ${\mathrm{Ti}}_{2}{\mathrm{MoC}}_{2}$.

Intrinsic property and catalytic performance of single and double metal atoms incorporated g-C3N4 for O2 activation: A DFT insight

The intrinsic properties and catalytic performances of single- and double-transition metals on graphitic carbon nitride, TMn@g-C3N4 (n = 1,2), toward the O2 activation were investigated by DFT calculation. The 3d-TM atoms are firmly trapped inside g-C3N4 which prevents the metal clustering and shows high thermodynamic stability. The dimetal-dioxygen adsorption configuration of the O2/TM2@g-C3N4 promotes electron transfer from catalyst to the adsorbed O2, which improves their catalytic performances over the O2/TM@g-C3N4. We observed the two different electron transfer mechanisms for O2 activation on TMn@g-C3N4, in which the double-metal acts as an electron donor while the single-metal acts as the bridge for electron transfer from the substrate to the adsorbed O2. Remarkably, the catalytic performance of the TMn@g-C3N4 for O2 dissociation has a strong correlation with the three factors, (i) the charge gained on adsorbed O2, (ii) the O2 adsorption energy, and (iii) the O-O distance. The Fe2@g-C3N4 as a low-cost and non-precious metal catalyst shows the best catalytic performance with the lowest activation energy barrier of 0.26 eV for O2 activation, and therefore, is predicted as a potential catalyst for O2 consuming reactions. Our finding provides useful information for further design and development of high efficient few-atom catalysts based 2D-carbon materials.

Double transition metal atoms anchored on Graphdiyne as promising catalyst for electrochemical nitrogen reduction reaction

Ammonia synthesis by electrochemical nitrogen reduction technique is an attractive alternative to traditional Haber-Bosch process. Currently, development of an efficient and effective electrocatalyst is one of the remaining key challenges. In this work, density functional theory (DFT) computations were systematically employed on double transition metal atoms (Fe, Co, Ni, Cu and Mo) anchored Graphdiyne (GDY) for nitrogen reduction reaction (NRR). The Co-Ni heteronuclear complex and Mo-Mo homonuclear complex showed the highest NRR activity while demonstrating synergistic effect of double atomic catalytic sites towards the promising NRR activity.

Effect of Double Transition Metal Salt Catalyst on Fushun Oil Shale Pyrolysis.

Shale ash (SA) as the carrier, the ratio of Cu to Ni in the Cu-Ni transition metal salt being, respectively, 1 : 0, 2 : 1, 1 : 1, 1 : 2, 0 : 1, the double transition metal salt catalyst (CumNin/SA) was prepared to explore the effect of such catalysts on the pyrolysis behavior and characteristics of Fushun OS. The research results show that the temperature (Tmax) corresponding to the maximum weight loss rate decreased by 12.9°C, 4.0°C, and 3.6°C; and the apparent activation energy decreased by 35.2%, 33.9%, and 29.6%, respectively, after adding catalysts Cu0Ni1/SA in pyrolysis. The addition of Cu0Ni1/SA and Cu2Ni1/SA further improves the shale oil (SO) yield of 3.5% and 3.1%, respectively. Cu0Ni1/SA produces more aromatic hydrocarbons, which, however, weakens the stability of SO and is of toxicity in use. After analyzing the pyrolysis product—semicoke (SC) and SO—with ATR-FTIR and GC-MS methods, CumNin/SA promotes the secondary cracking and aromatization of OS pyrolysis, increasing the content of the compound of olefins and aromatics in SO, and hastening the decomposition of long-chain aliphatic hydrocarbons to short-chain aliphatic hydrocarbons.

Engineering magnetic anisotropy and exchange couplings in double transition metal MXenes via surface defects

Two-dimensional (2D) materials have been experimentally proven to manifest almost all types of material properties observed in bulk materials. However, 2D magnetism was elusive until recently. In this work, we used an approach that synergistically uses density functional theory, and Monte Carlo methods to investigate the magnetic and electronic properties of magnetic double transition metal MXene alloys (Hf2MnC2O2 and Hf2VC2O2) by exploiting realistic surface terminations via creating surface defects including oxygen vacancies and H adatoms. We found that introducing surface oxygen vacancies or hydrogen adatoms is able to modify the electronic structures, magnetic anisotropies, and exchange couplings. Depending on the defect concentration, a ferromagnetic half-metallic state can be realized for both Hf2VC2O2 and Hf2MnC2O2. Bare Hf2VC2O2 exhibits easy-axis anisotropy, whereas bare Hf2MnC2O2 exhibits easy-plane anisotropy; however, defects can change the latter to easy-axis anisotropy, which is preferable for spintronics applications. The considered defects were found to modify the magnetic anisotropy by as much as 300%. Defects also produce an inhomogeneous pattern of exchange couplings, which can further enhance the Curie temperature. In particular, Hf2MnC2O2H0.22 was predicted to have a Curie temperature of about 171 K due to a combination of easy-axis anisotropy and a connected network of enhanced exchange couplings. Our calculations suggest a route toward engineering exchange couplings and magnetic anisotropy to improve magnetic properties.

Structural, electronic and optical properties of two-dimensional (M2/3Y1/3)2CO2 (M = Mo,W) iMXene

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are continuously growing in terms of both crystalline and composition varieties. They have received significant attention in science and technology. The new members of MXenes with in-plane ordered double transition metals have been named as iMXenes. In this study, we have investigated the electronic structures and optical properties of 2D (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 iMXene monolayers, using a set of density functional theory calculations. We found that the (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 2CO2 are semiconductors with indirect bandgaps of 0.477 eV and 0.655 eV, respectively. To investigate the optical properties, we calculated the absorption spectrum and reflectivity percentage of these structures along x and z directions using the real and imaginary parts of the dielectric function. It is observed that the real and imaginary parts of their dielectric functions possess many peaks in the energy region of less than 3.1 eV. Interestingly, they show high absorption in the visible and UV regions, implying the potential applications of these semiconducting iMXenes in solar cells and optical nanodevices.

Theoretical insights into the performance of single and double transition metal atoms doped on N-graphenes for N2 electroreduction

Single- and double-atom catalysts are normally with high activity and selectivity in N2 electroreduction. However, the properties of impacting their catalytic performances in N2 reduction are still unclear. In order to gain insights into the factors that influence their performances, we have theoretically studied N2 activation and reduction on eight catalysts, including two single-atom catalysts with Mn/Fe supported on nitrogen doped graphenes (N-graphenes), and six double-atom catalysts in which Mn and Fe atoms form three non-bonded centers (Mn⋯Mn, Fe⋯Fe and Mn⋯Fe) and three bonded centers (Mn-Mn, Fe-Fe and Mn-Fe) on N-graphenes. Our calculational results indicate that the two single-atom catalysts and the three non-bonded double-atom catalysts can’t efficiently activate N2 or convert it into NH3, whereas the bonded double-atom catalysts can not only efficiently activate but also convert N2 at low overpotentials. Especially, the bonded Mn-Fe catalyst is found to be the most efficient catalyst due to its very lower overpotential (0.08 V) for N2 reduction reaction among the eight catalysts. Moreover, the charge analysis revealed that the electron-donating capacities and the synergistic effects of the two bonded metal atoms are both responsible for the enhanced catalytic performances.

Double transition-metal MXenes: Atomistic design of two-dimensional carbides and nitrides

Abstract

Improved photocatalytic activity of Ni2P/NiCo-LDH composites via a Co–P bond charge transfer channel to degrade tetracycline under visible light

A series of novel non-precious-metal-based composite photocatalysts was developed using layered double hydroxide (NiCo-LDH) and transition metal phosphide (Ni2P) to explore the performance of high-efficiency photocatalytic composite materials containing no precious metals. The developed Ni2P/NiCo-LDH composites displayed an enhanced photocatalytic performance under visible-light illumination. Further researches confirmed that a unique Co–P bond, which led to an increased light absorption, was formed in the composites. Importantly, this unique Co–P bond reduced the interface transfer resistance of the Ni2P/NiCo-LDH composites, enhanced their charge separation efficiency, and formed a rapid charge transport channel between Ni2P and NiCo-LDH in the composites to provide improved photocatalytic activity. Additionally, a mechanism was proposed for the further exploration of non-precious metal-based composites. The study provides novel insight into the research on the use of non-precious metal materials and NiCo-LDH in the field of photocatalysis.