Hui Pan

Electronic properties and lithium storage capacities of two-dimensional transition-metal nitride monolayers

Two-dimensional nanostructures have attracted increasing interest due to their fascinating properties and broad applications. In this study, we carry out first-principles studies on the electronic properties of MXenes and investigate their application in lithium-ion batteries. Herein, we focus on the transition-metal nitride monolayers Mi+1Ni; where M = Ta, Ti, and V; i = 1 and 2. We find that these monolayers are metallic and most of them show non-magnetic behavior, with the exception of the anti-ferromagnetic Ti2N and ferromagnetic Ti3N2. Our calculations show that Li atoms can be easily transported on the surfaces of the monolayers, with negligible barriers as low as 0.017 eV on Ti2N, because of low adsorption energy and long binding distance. We further show that these monolayers remain metallic under full Li-intercalation and have low open circuit voltages. In particular, the Ti2N monolayer shows the best performance with an open circuit voltage of 0.53 V and the highest specific capacity of 487 mA h g−1. Our calculations predict that transition-metal nitride monolayers may be applied in lithium ion batteries with improved performance.

Synergistic effect of 2D Ti2C and g-C3N4 for efficient photocatalytic hydrogen production

Photocatalytic water splitting is an environmentally friendly technique for hydrogen production. In this work, we report a novel photocatalyst consisting of two-dimensional (2D) titanium carbide (Ti2C) and graphitic carbon nitride (g-C3N4). We observe substantially enhanced water splitting activities due to the efficient synergistic interaction between Ti2C and g-C3N4. Optimal properties are achieved in the g-C3N4 with a loading of 0.4 wt% Ti2C with a hydrogen production rate of 47.5 μmol h−1, which is 14.4 times as much as that in the case using pure g-C3N4, and it even outperforms Pt-loaded g-C3N4. We further show that the Ti2C/g-C3N4 has high stability and good reproducibility. We expect that the Ti2C/g-C3N4 can be a photocatalyst for large scale applications because both Ti2C and g-C3N4 are low-cost, abundant, and nontoxic.

Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Ni3S2 nanowire arrays doped with vanadium(V) are directly grown on nickel foam by a facile one-step hydrothermal method. It is found that the doping can promote the formation of Ni3S2 nanowires at a low temperature. The doped nanowires show excellent electrocatalytic performance toward hydrogen evolution reaction (HER), and outperform pure Ni3S2 and other Ni3S2-based compounds. The stability test shows that the performance of V-doped Ni3S2 nanowires is improved and stabilized after thousands of linear sweep voltammetry test. The onset potential of V-doped Ni3S2 nanowire can be as low as 39 mV, which is comparable to platinum. The nanowire has an overpotential of 68 mV at 10 mA cm-2, a relatively low Tafel slope of 112 mV dec-1, good stability and high Faradaic efficiency. First-principles calculations show that the V-doping in Ni3S2 extremely enhances the free carrier density near the Fermi level, resulting in much improved catalytic activities. We expect that the doping can be an effective way to enhance the catalytic performance of metal disulfides in hydrogen evolution reaction and V-doped Ni3S2 nanowire is one of the most promising electrocatalysts for hydrogen production.

Ultra-high electrocatalytic activity of VS2 nanoflowers for efficient hydrogen evolution reaction

It is a great challenge to explore cheap, abundant and eco-friendly electrocatalysts for hydrogen evolution reaction (HER). Here, we report the fabrication of VS2 nanoflowers with 1T phase by a simple hydrothermal method and their electrocatalytic performance in the HER. We find that the VS2 nanoflowers show comparable HER performance to Pt in acids, including an ultra-low onset potential of 32 mV, a Tafel slope of 34 mV dec−1 which resembles that of Pt, and a small overpotential of 58 mV (54 mV for Pt) at a current density of 10 mA cm−2. High stability and almost 100% faradaic efficiency indicate the practical application of VS2 nanoflowers in the HER. Our first-principles calculations reveal that the thermoneutral Gibbs free energy of hydrogen adsorption on both the basal and edge sites of the 1T-VS2 monolayer can be achieved under certain hydrogen coverage and the monolayer shows good conductivity, which contribute to the impressive catalytic performance of VS2 nanoflowers. We expect that the VS2 nanostructures may be applicable in electrocatalysis with high efficiency.

Efficient coupling of a hierarchical V2O5@Ni3S2 hybrid nanoarray for pseudocapacitors and hydrogen production

Hierarchical materials are a favourite for energy storage/harvesting technologies, such as supercapacitors and electrically-driven hydrogen production. In this work, we report a robust and highly active nanomaterial for pseudocapacitors and hydrogen production by growing vanadium pentoxide on the surface of nickel sulfide (denoted as V2O5@Ni3S2). V2O5@Ni3S2 shows a capacitance of up to 854 F g−1 at a current density of 1 A g−1 and good rate capability. The capacitance can retain ca. 60% of the initial specific capacitance even after 1000 cycles at a current density of 1 A g−1. For the hydrogen evolution reaction (HER), the overpotential of V2O5@Ni3S2 is around 95 mV at 10 mA cm−2. The excellent electrochemical performance of the V2O5@Ni3S2 hybrid is attributed to the synergistic effect between vanadium pentoxide and nickel sulfide for pseudocapacitors, and the V2O5@Ni3S2 interface and V–S active bonding in the hierarchical structure for the HER. This work demonstrates an effective structure design for supercapacitors with high energy density and for efficient hydrogen production with high catalytic activity by incorporating metal oxide with metal sulfide.

Hydrogenation-controlled phase transition on two-dimensional transition metal dichalcogenides and their unique physical and catalytic properties.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been widely used from nanodevices to energy harvesting/storage because of their tunable physical and chemical properties. In this work, we systematically investigate the effects of hydrogenation on the structural, electronic, magnetic, and catalytic properties of 33 TMDs based on first-principles calculations. We find that the stable phases of TMD monolayers can transit from 1T to 2H phase or vice versa upon the hydrogenation. We show that the hydrogenation can switch their magnetic and electronic states accompanying with the phase transition. The hydrogenation can tune the magnetic states of TMDs among non-, ferro, para-, and antiferro-magnetism and their electronic states among semiconductor, metal, and half-metal. We further show that, out of 33 TMD monolayers, 2H-TiS2 has impressive catalytic ability comparable to Pt in hydrogen evolution reaction in a wide range of hydrogen coverages. Our findings would shed the light on the multi-functional applications of TMDs.

Exploring an effective oxygen reduction reaction catalyst via 4e− process based on waved-graphene

A novel pure and N-doped waved-graphene electro-catalyst with various wavelengths for oxygen reduction reaction (ORR) is explored in acid medium based on the first-principles calculations. The associative and dissociative mechanism are both explored from the O2 adsorption to the H2O formation step by step, from which the dissociative mechanism is likely the only way with an effective 4e− process. Furthermore, a very favorable and effective mechanism is proposed for the ORR process, which may provide a guideline on the design of new non-metal electro-catalysts.摘要本文基于第一性原理, 针对一种本征或氮掺杂的石墨烯在不同晶胞参数压缩比和在酸性条件下的氧气还原反应机理进行了探索. 文章假设了两种不同的反应机理, 并逐步分析了从氧气吸附到水分子形成的整个过程, 结果表明基于氧气分子解离机理的四电子过程是唯一有效的路径. 文章进一步提出了一种有效的氧气还原反应机理, 这为设计新的非金属电催化剂提供了新的思路.

Electronic, magnetic, catalytic, and electrochemical properties of two-dimensional Janus transition metal chalcogenides

Two dimensional (2D) nanomaterials have received increasing interest because of their unique properties for versatile applications. In this work, we present a first-principles study on a new family of 2D nanostructures, Janus transition metal chalcogenide MSX (M = Ti or V; and X = C, N, Si, or P) monolayers, for their multifunctional applications. In this work, we show that the Janus MSXs possess diverse electronic and magnetic properties, and can be semiconducting or metallic, and nonmagnetic or magnetic, depending on their composition. We find that a TiSC monolayer with a 1H phase (TiSC-1H) is suitable as a cathode for Li ion batteries and anode materials for Na and Mg ion batteries due to its high open circuit voltage (OCV) (2.121 eV) for Li, and low OCVs upon Na (0.676 eV) and Mg (1.044 eV) intercalation, respectively. Importantly, TiSC-1H shows fast charge/discharge rates, good cycling stability, and high storage density as electrode materials for rechargeable batteries because of low ion diffusion barriers, small volume expansion and high specific capacity. We further show that TiSP-1H has the best performance in the hydrogen evolution reaction due to both its catalytic activities on the surfaces and relatively low overpotentials upon hydrogenation. Our study demonstrates that the 2D Janus MSXs may find multifunctional applications in nanodevices, spintronics, catalysis, and electrochemical energy storage.

Exploring new two-dimensional monolayers: pentagonal transition metal borides/carbides (penta-TMB/Cs)

The development of two-dimensional (2D) materials with high conductivity and catalytic activity is important for the proposed hydrogen economy. Herein, we design a new family of 2D monolayers, pentagonal transition-metal borides/carbides (penta-TMBs and penta-TMCs), as electrocatalysts for the hydrogen evolution reaction (HER) on the basis of density functional theory (DFT). We find that all of the stable 2D penta-TMBs/TMCs are metallic, and 2D WB and HfC are ferromagnetic metals. We demonstrate that penta-TMBs and penta-TMCs show high catalytic performance for the HER. The Gibbs free energy for the adsorption of hydrogen atoms on catalyst surfaces (such as WB and ZrC) is close to the ideal value of zero electron volt (eV). Our findings highlight a new family of promising noble metal-free HER catalysts and provide new insight into the design of advanced 2D materials.

Fullerene/layered antiferromagnetic reconstructed spinterface: Subsurface layer dominates molecular orbitals’ spin-split and large induced magnetic moment

The interfaces between organic molecules and magnetic metals have gained increasing interest for both fundamental reasons and applications. Among them, the C60/layered antiferromagnetic (AFM) interfaces have been studied only for C60 bonded to the outermost ferromagnetic layer [S. L. Kawahara et al., Nano Lett. 12, 4558 (2012) and D. Li et al., Phys. Rev. B 93, 085425 (2016)]. Here, via density functional theory calculations combined with evidence from the literature, we demonstrate that C60 adsorption can reconstruct the layered-AFM Cr(001) surface at elevated annealing temperatures so that C60 bonds to both the outermost and the subsurface Cr layers in opposite spin directions. Surface reconstruction drastically changes the adsorbed molecule spintronic properties: (1) the spin-split p-d hybridization involves multi-orbitals of C60 and top two layers of Cr with opposite spin-polarization, (2) the subsurface Cr atom dominates the C60 electronic properties, and (3) the reconstruction induces a large magnetic moment of 0.58 μB in C60 as a synergistic effect of the top two Cr layers. The induced magnetic moment in C60 can be explained by the magnetic direct-exchange mechanism, which can be generalized to other C60/magnetic metal systems. Understanding these complex hybridization behaviors is a crucial step for molecular spintronic applications.