Haiping Lin

A rhodium/silicon co-electrocatalyst design concept to surpass platinum hydrogen evolution activity at high overpotentials

Currently, platinum-based electrocatalysts show the best performance for hydrogen evolution. All hydrogen evolution reaction catalysts should however obey Sabatiers principle, that is, the adsorption energy of hydrogen to the catalyst surface should be neither too high nor too low to balance between hydrogen adsorption and desorption. To overcome the limitation of this principle, here we choose a composite (rhodium/silicon nanowire) catalyst, in which hydrogen adsorption occurs on rhodium with a large adsorption energy while hydrogen evolution occurs on silicon with a small adsorption energy. We show that the composite is stable with better hydrogen evolution activity than rhodium nanoparticles and even exceeding those of commercial platinum/carbon at high overpotentials. The results reveal that silicon plays a key role in the electrocatalysis. This work may thus open the door for the design and fabrication of electrocatalysts for high-efficiency electric energy to hydrogen energy conversion.

Stable and metallic borophene nanoribbons from first-principles calculations

Recently, borophene was reported to be produced on silver surfaces. We employed density functional theory and electronic transport calculations to investigate the stabilities, electronic structures and transport properties of borophene nanoribbons. The stability of a borophene nanoribbon increases with its width and only the lined-edged borophene nanoribbons are stable in the free-standing form. Such anisotropic stability dependence is ascribed to the large scale delocalization of π electrons along the boron rows. In particular, all line-edge borophene nanoribbons undergo edge reconstructions, in which the out-of-plane bulking edge atoms are reconstructed to form quasi planar edge structures. Such edge reconstructions have not yet been reported, which is critical for the formation of boron nanostructrues. Subsequent electronic transport calculations based on a non-equilibrium Green’s function indicated that line-edge borophene nanoribbons exhibit low-resistivity Ohmic conductance. Our results indicated that the line-edge borophene nanoribbons present remarkable properties (high thermal stabilities, Ohmic conductance with low electrical resistivity and good rigidities) and are promising for applications as one-dimensional electrical connections in compact nanoscale circuits.

Structural Variation in Surface-Supported Synthesis by Adjusting the Stoichiometric Ratio of the Reactants

Surface-supported coupling reactions between 1,3,5-tris(4-formylphenyl)benzene and aromatic amines have been investigated on Au(111) using scanning tunneling microscopy under ultra-high-vacuum conditions. Upon annealing to moderate temperatures, various products, involving the discrete oligomers and the surface covalent organic frameworks, are obtained through thermal-triggered on-surface chemical reactions. We conclude from the systematic experiments that the stoichiometric composition of the reactants is vital to the surface reaction products, which is rarely reported so far. With this knowledge, we have successfully prepared two-dimensional covalently bonded networks by optimizing the stoichiometric proportions of the reaction precursors.

B40 fullerene as a highly sensitive molecular device for NH3 detection at low bias: a first-principles study.

The adsorption of small molecules (NH3, N2, H2 and CH4) on all-boron fullerene B40 is investigated by density functional theory (DFT) and the non-equilibrium Greens function (NEGF) for its potential application in the field of single-molecular gas sensors. The high adsorption energies (-1.09 to -0.75 eV) of NH3 on different adsorption sites of the B40 surface indicate that NH3 strongly chemisorbs to B40. The charge transfer induced by the NH3 adsorption results in a modification of the density of states (DOS) of B40 near the Fermi level, and therefore changes its electronic transport properties. For all possible adsorption sites, the adsorption of NH3 exclusively leads to a decrease of the conductance of B40. Taking into consideration that the non-polar gas molecules (e.g. N2, H2 and CH4) are only physisorbed and show negligible effect on the conductance properties of B40, we would expect that B40 can be used as a single-molecular gas sensor to distinguish NH3 from non-polar gas molecules at low bias.

Induce magnetism into silicene by embedding transition-metal atoms

Embedding transition-metal (TM) atoms into nonmagnetic nanomaterials is an efficient way to induce magnetism. Using first-principles calculations, we systematically investigated the structural stability and magnetic properties of TM atoms from Sc to Zn embedded into silicene with single vacancy (SV) and double vacancies (DV). The binding energies for different TM atoms correlate with the TM d-shell electrons. Sc, Ti, and Co show the largest binding energies of as high as 6 eV, while Zn has the lowest binding energy of about 2 eV. The magnetic moment of silicene can be modulated by embedding TM atoms from V to Co, which mainly comes from the 3d orbitals of TM along with partly contributions from the neighboring Si atoms. Fe atom on SV and Mn atom on DV have the largest magnetic moment of more than 3 μB. In addition, we find that doping of N or C atoms on the vacancy site could greatly enhance the magnetism of the systems. Our results provide a promising approach to design silicene-based nanoelectronics and spintronics device.

Structures, mobility and electronic properties of point defects in arsenene, antimonene and an antimony arsenide alloy

Defects are unavoidable during the synthesis of materials, especially for two-dimensional (2D) nanomaterials. They are usually seen as detrimental to device properties, but sometimes bring about new beneficial effects. In order to clarify the influence of defects on the structural and electronic properties, we have performed first-principles calculations to systematically investigate the structural stability, mobility and electronic properties of typical point defects in 2D arsenene (h-As), antimonene (h-Sb) and antimony arsenide (h-AsSb), including the Stone–Wales defects, single vacancies (SVs), double vacancies (DVs) and adatoms. To provide visual guidance for experimental observations, scanning tunnelling microscopy (STM) images are simulated. Compared to defects in graphene and silicene, these defects form more easily with lower formation energies, and SVs can diffuse very quickly to the edges with a lower diffusion barrier of less than 1 eV. Monolayer arsenene, antimonene and antimony arsenide are indirect band gap semiconductors, and the defective structures significantly reduce the band gaps. Most of the SV and adatom defects carry magnetic moments due to the dangling bonds resulting from the absent or extra atom. Our present results have demonstrated that the point defects induce significant effects on the electronic properties of pristine arsenene, antimonene and the antimony arsenide alloy, which should be considered in their future applications.

Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and Their Electronic Structures

The inherent instability of CH3NH3PbX3 remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH3NH3PbX3 have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH3NH3PbX3 perovskite crystals and their effects on the stability of perovskite materials.

MoS2 supported single platinum atoms and their superior catalytic activity for CO oxidation: a density functional theory study

Late transition metals, such as Rh, Ir, Pd and Pt, have a strong tendency to form a square-planar 16-electron complex. Although this feature has been widely used in organometallics to develop homogeneous catalysts, a single-atom heterogeneous analogue has not yet been reported. In this work, we show that a 16-electron complex may act as an important transition state in the CO oxidation over a single Pt atom supported by a MoS2 monolayer (Pt/MoS2). The catalytic oxidation reaction prefers to start with the Langmuir–Hinshelwood (L–H) reaction, where the CO and O2 molecules are first co-adsorbed on the Pt atom, then cross a small barrier of 0.40 eV to form a square-planar 16-electron intermediate state, and subsequently the first CO2 is released. The activation barrier of the following Eley–Rideal (E–R) reaction is only 0.23 eV. The superior catalytic reactivity of the Pt/MoS2 surface can be explained by the quantum confinement effect of the Pt-5d orbitals and the stability of the square-planar 16-electron transition state. In addition, MoS2 may serve as a defect-free two dimensional anchoring substrate for Pt atomic adsorption. It provides not only a very large surface-to-volume ratio, but also a well-defined structure with a uniform distribution of anchoring points. The square-planar 16-electron intermediate state of the L–H reaction, together with the new MoS2 anchoring substrate, may provide a new opportunity for the design of single-atom catalysts on two-dimensional surfaces.

Heptazine-based graphitic carbon nitride as an effective hydrogen purification membrane

The purification of H2 from other gases (CH4, CO, CO2, N2, and H2O) is a vital step for its safe usage. By using first-principles calculations and molecular dynamics simulations, we find that the porous graphitic carbon nitride (g-C3N4) monolayer works as an efficient and highly selective hydrogen purification membrane. In the DFT calculations, the transition state theory is used to search the lowest diffusion barrier (0.55 eV) for H2 to go through the well-ordered intrinsic holes. Meanwhile, the excellent selectivity between H2 and other gases shows that the g-C3N4 nanosheet is specific for diffusion of H2. The MD simulations exhibit the whole dynamic purification process and confirm our previous DFT results. Our results indicate that the g-C3N4 nanosheet has great potential in separating H2 from undesirable gases.

Rh–Ag–Si ternary composites: highly active hydrogen evolution electrocatalysts over Pt–Ag–Si

Hydrogen production with the aid of electrocatalysis is a critical component for several developing clean-energy technologies. Such a renewable energy depends heavily on the choice of cheap and efficient catalysts for hydrogen evolution, which has still been a challenge until now. In this work, the theoretical calculation indicates that Rh–Ag–Si ternary catalysts exhibit more active hydrogen evolution performance than Pt–Ag–Si because the migration activation energies of H atoms from Rh(111) to Si are lower than those from Pt(111) to Si via the Ag surface. This simulation was confirmed by the experimental results: Rh–Ag/SiNW (or Pt–Ag/SiNW) catalysts were prepared by directly reducing Rh (or Pt) and Ag ions with Si–H bonds. The Rh–Ag/SiNW-2 with the optimal mass ratio of 2.3u2006:u200623.4u2006:u200674.3 (Rhu2006:u2006Agu2006:u2006Si) exhibited a lower Tafel slope (51 mV dec−1) and a larger exchange current density (87.1 × 10−6 A cm−2) than the Pt–Ag/SiNW. In addition, the mass activity of Rh–Ag/SiNW-2 at an overpotential of 0.2 V (11.5 mA μgRh−1) is 12.0, 5.0 and 3.3 fold higher than that of Rh–Ag (0.96 mA μgRh−1), Pt–Ag/SiNW (2.3 mA μgPt−1) and 40 wt% Pt/C (3.5 mA μgPt−1) catalysts, respectively. Moreover, the Rh–Ag/SiNW nanocatalysts had good stability in acidic media. The results presented herein may offer a novel and effective methodology for the designing of cost-efficient and environmentally friendly catalysts for electrochemical fields.