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Tin-based materials, characterized by their high specific capacities, show significant promise for use in the next generation of lithium-ion batteries (LIBs). However, the large volume change of Sn-based materials upon operation significantly compromises the battery performance. To solve this problem, a nanoporous Ni current collector with cone arrays is developed using a template-free method to support a Sn–Co alloy as an anode for LIBs. Interestingly, the Sn–Co alloy inherits the nanoporous cone array structure of the Ni current collector. Thus, the nanoporous and cone array-structured electrode provides abundant interspace to alleviate the mechanical stress of the Sn-based anode caused by volume changes and facilitate rapid mass transfer. Besides, the in situ generated inert Co dispersed in the electrode can further buffer the volume expansion of the Sn-based anode. Due to the synergistic effect, the electrode shows a high initial discharge/charge capacity of 1.34/0.64 mA h cm−2 and a reversible capacity of 0.89 mA h cm−2 with high coulombic efficiency (99.44%) even after 200 cycles at 1 mA cm−2. Moreover, the electrode also demonstrates excellent rate performance (0.55 mA h cm−2 at 8 mA cm−2). This work provides a novel design idea for the preparation of high-performance tin-based electrodes for next-generation lithium-ion batteries. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides exhibit various stacking polytypes and distinct electronic structures. In this study, first-principles calculations show that chalcogen enrichment induces the phase conversion from 2H to 1T′. Models for Se intercalation and substitution (MoSex and WSex with x > 2) were constructed for the 2H and three stacking polytypes of the 1T′ phase (1T′, 2M, and Td). Se intercalates and substitutes more favorably into the 1T′ polytypes than into the 2H phase. Intercalation in the 1T′ polytypes occurs by forming Se dimers, which is distinct from the Se monomer in the 2H phase. The relative stability predicts that as x increases, the 2H phase undergoes phase conversion preferentially into the 2M phase. On the experimental side, we synthesized Se-rich MoSe2 and WSe2 nanosheets using a colloidal reaction, and confirmed the stacking sequence of the 2M phase using atomic-resolution scanning transmission electron microscopy. The Se-rich 2M phase nanosheets exhibited enhanced electrocatalytic performance in the hydrogen evolution reaction (HER). Gibbs free energy calculations support that excess Se enhances the HER. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides have recently attracted much attention as excellent electrocatalysts for the hydrogen evolution reaction (HER). Herein, Re1−xMoxS2 alloy nanosheets in the entire composition range were synthesized using a hydrothermal reaction. High-resolution scanning transmission electron microscopy revealed anisotropic atomic distribution of the alloy phase, in which the Re and Mo atoms tend to segregate along a crystallographic axis. The phase transition occurs from the triclinic phase (1T′′) ReS2 to the monoclinic phase (1T′) MoS2 at 50% Mo. Re0.5Mo0.5S2 exhibited the highest electrocatalytic HER activity, which was characterized by a current density of 10 mA cm−2 at an overpotential of 98 mV (vs. RHE) and a Tafel slope of 54 mV dec−1 in 0.5 M H2SO4. Extensive calculations using spin-polarized density functional theory showed that the most energetically stable configuration consists of separated MoS2 and ReS2 domains along the b axis, and the 1T′′ → 1T′ phase transition at 50% Mo, which agrees with the experimental results. The Gibbs free energy along the HER pathway indicates that the best performance at Mo 50% is due to the formation of S–H or Mo–H (at S vacancies) on the MoS2 domain. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) MoS2 nanostructures have attracted much attention in recent years because of their excellent electrocatalytic activity toward the hydrogen evolution reaction (HER). Herein, we report unique 2D hybrid nanostructures of MoS2 and melamine synthesized via a one-step solvothermal process. Remarkably, few-layer metallic 1T′ phase MoS2 nanoflakes and orthorhombic phase melamine aggregate to form nanoplates. At a controlled concentration, the melamine molecules intercalated into the 1T′ phase MoS2 by forming charge-transfer complexes. The hybrid complexes with 7% intercalated melamine exhibited excellent performance for the catalytic HER, with a current of 10 mA cm−2 at 0.136 V (vs. RHE) and a Tafel slope of 37 mV dec−1. First-principles calculations showed that the intercalation of hydrogen-bonded melamine clusters could stabilize the 1T′ phase MoS2via substantial charge transfer. The activation barrier was calculated for the Volmer–Heyrovsky reactions, by identifying the active sites of the Volmer reaction as the basal S atoms above the hydrogen-bonded amine group of melamine. This rationalizes the dependence of the catalytic activity on the concentration of intercalated melamine. The present study highlights the opportunities for producing unique 2D hybrid complexes to enhance the HER catalytic activity by controlling the intercalating organic molecules. [ABSTRACT FROM AUTHOR]

We synthesized cobaltocene (7–20%)-intercalated WS2 nanosheets using a solvothermal process. The intercalation of cobaltocene between the expanded 1T′ phase WS2 layers was confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy. The intercalated complexes exhibited excellent performance for the catalytic hydrogen evolution reaction, with a Tafel slope of 40 mV dec−1 and a current density of 10 mA cm−2 at 0.17 V (vs. RHE). Spin-polarized density functional theory calculations showed that cobaltocene is intercalated with a fivefold symmetry parallel to the WS2 plane, driven by substantial charge transfer. Reaction pathway calculations suggest that the basal S atoms just above the Co atom are the active sites, and the activation barrier of the Heyrovsky reaction determines the catalytic activity. [ABSTRACT FROM AUTHOR]

Two-dimensional layered MoS2 has recently been considered as an excellent catalyst for the water-splitting hydrogen evolution reaction (HER). Herein, we synthesize 1T′ phase MoS2 that was intercalated with a series of alkylated p-phenylenediamines (PDs). The substituted N atoms produced S vacancies, leading to a composition of MoS2−2xNx (x = 0.1). The more abundant methyl groups induce a larger charge transfer, resulting in excellent HER performance: for tetramethyl PD, the overpotential is 0.15 V at 10 mA cm−2 with a Tafel slope of 35 mV dec−1. The catalytic activity of the complexes depends on the concentration of the intercalated molecules, showing an optimum at a concentration of 8 mol%. First-principles calculations showed that the intercalated complexes (1T′ phase) having N atom–S vacancy (N–VS) pairs are stabilized by a large charge transfer from the PD molecules that is enhanced by the methyl groups (i.e., 0.40e–0.84e per molecule at 6.25 mol% intercalation). The charge transfer increases the density of states at and just above the Fermi level, thereby increasing the electron concentration at low cathodic bias. The active sites for the Volmer reaction are found to be N atoms in the proximal N–VS pairs. The activation barrier for the Heyrovsky reaction becomes higher at higher concentrations of the intercalants, suggesting that the experimental HER performance is also kinetically controlled. [ABSTRACT FROM AUTHOR]

Two-dimensional layered structures have recently drawn worldwide attention because of their intriguing catalytic properties. Herein, we report 1T′-phase ammonium (A)- and methylammonium (MA)-intercalated MoS2 layered structures, synthesized by a one-step hydrothermal reaction. The MA-intercalated MoS2 exhibits excellent stability, as well as higher catalytic activity toward the hydrogen evolution reaction than the A-intercalated MoS2. First-principles calculations were used to assess the relative stabilities of the 2H and 1T′ phases, showing a trend of 2H → 1T′ phase conversion with increasing concentrations of A or MA. The calculations consistently predicted a higher stability of the MA-intercalated complex, due to reduced repulsion between the positively charged MA moieties, which underlies the excellent catalytic performance. [ABSTRACT FROM AUTHOR]

Based on a sophisticated crystal structure prediction method, we propose two-dimensional (2D) GeP2 in the tetragonal (T) phase never observed for other group IV–V compounds. The bulk of 2D GeP2 is more stable than both 2D orthogonal (O) and three-dimensional pyrite (P) phases that have been experimentally observed for group IV–V compounds. According to our calculations of phonon dispersion relations and elastic constants, as well as ab initio molecular dynamics simulation, monolayers of both the T and O phases (penta-GeP2 and O-GeP2, respectively) are dynamically, mechanically, and thermally stable. In addition, our HSE06 calculation shows that these monolayers are semiconductors with band gaps in the visible region. Among the various stacking patterns of their bilayers, specific ones are identified to be most stable, which are still semiconductors with band gaps redshifted in the visible region. Different from the case of their bulk, few-layers of O-GeP2 are more stable than those of penta-GeP2 up to a pentalayer. Furthermore, band offset with respect to the Fermi levels of appropriate half-reactions shows that both n-type few-layer penta-GeP2 and O-GeP2 can be useful in photocatalyzed CO2 splitting to CO as well as in photocatalyzed water splitting, specifically under acidic conditions. [ABSTRACT FROM AUTHOR]

The hydrothermal stability of metal organic frameworks (MOFs) is critical for their applications in hydrogen storage and other applications. The pKa1 values of the conjugated acids of ligands and natural bond orbital (NBO) charges of the coordinating atoms of 32 multicarboxylate ligands and 31 N-heterocyclic ligands applied in the MOF syntheses were calculated at 298.15 K with the B3LYP/6-31+G(d,p) level of calculations. Solvation Gibbs energies (∆Gsolv) in aqueous solution were obtained using SMD modeling. NBO charges were calculated based on optimized structures in the gas phase. We found that the calculated pK,1 values have a linear relationship with the experimental results of a few known bidentate ligands, but pKa2 values do not. We demonstrated that NBO charges of the coordinating atoms of the ligands, like pKa values of the conjugated acids of the ligands, can reflect the relative coordination abilities of the ligands to metal ions and the strength of the resulting M-L bonds. To the best of our knowledge, systematic calculations of the pKa values and NBO charges of multidentate ligands have not been reported. The calculated results can be easily expanded using this inexpensive calculation method. Some experimental results about water and thermal stability of MOF materials were successfully explained in terms of M-L bond strengths and polarity based on our calculated results. The results can be used to predict whether a targeted MOF structure would be water and thermally stable, predict the hydrothermal stability of known MOFs, design new hydrothermally stable MOFs and provide guidance for the synthetic procedures based on the calculated pKa values. The limitations of applying these values are also given. [ABSTRACT FROM AUTHOR]