Jian Zhou

Development of non-noble-metal catalysts for hydrogen evolution reaction (HER) with both excellent activity and robust stability has remained a key challenge in the past decades. Herein, for the first time, N-doped carbon-wrapped cobalt nanoparticles supported on N-doped graphene nanosheets were prepared by a facile solvothermal procedure and subsequent calcination at controlled temperatures. The electrocatalytic activity for HER was examined in 0.5 M H2SO4. Electrochemical measurements showed a small overpotential of only −49 mV with a Tafel slope of 79.3 mV/dec. Theoretical calculations based on density functional theory showed that the catalytically active sites were due to carbon atoms promoted by the entrapped cobalt nanoparticles. The results may offer a new methodology for the preparation of effective catalysts for water splitting technology.

We propose a kind of topological quantum state of semimetals in the quasi-one-dimensional (1D) crystal family ${\text{Ba}MX}_{3}$ ($M\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{V}$, Nb, or Ta; $X\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}\mathrm{S}$ or Se) by using symmetry analysis and first-principles calculation. We find that in ${\mathrm{BaVS}}_{3}$ the valence and conduction bands are degenerate in the ${k}_{z}=\ensuremath{\pi}/c$ plane ($c$ is the lattice constant along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis) of the Brillouin zone (BZ). These nodal points form a node surface, and they are protected by a nonsymmorphic crystal symmetry consisting of a twofold rotation about the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis and a half-translation along the same $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ axis. The band degeneracy in the node surface is lifted in ${\mathrm{BaTaS}}_{3}$ by including strong spin-orbit coupling (SOC) of Ta. The node surface is reduced into 1D node lines along the high-symmetry paths ${k}_{x}=0$ and ${k}_{x}=\ifmmode\pm\else\textpm\fi{}\sqrt{3}{k}_{y}$ on the ${k}_{z}=\ensuremath{\pi}/c$ plane. These node lines are robust against SOC and guaranteed by the symmetries of the $P{6}_{3}/mmc$ space group. These node-line states are entirely different from previous proposals which are based on the accidental band touchings. We also propose a useful material design for realizing topological node-surface and node-line semimetals.

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this study, MoO2 nanobelts@nitrogen self-doped MoS2 nanosheets are produced by nitridation and sulfuration treatments of MoO3 nanobelts. The material structures are characterized by a variety of techniques including scanning electron microscopy, transmission electron microscopy, Raman scattering, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. It is found that because of nitrogen doping and the abundance of exposed active edges, the heterostructures exhibit high electronic conductivity, and more importantly, enhanced and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested in electrochemical studies. The onset potential is found to be only −156 mV (vs. RHE), which is 105 mV more positive than that of pure MoS2 under identical experimental conditions. The corresponding Tafel slope is estimated to be 47.5 mV dec−1, even slightly less than that of commercial 10 wt% Pt/C (49.8 mV dec−1), suggesting that the reaction dynamics is largely determined by the electrochemical desorption of hydrogen. This is accounted for by nitrogen doping that leads to an enhanced electronic conductivity of the heterostructures as well as a high density of spinning electron states around the N and Mo atoms in MoS2 nanosheets that are the active sites for HER, as manifested in density functional theory studies of a N-doped MoS2 monolayer.

The lattice structure and symmetry of two-dimensional (2D) layered materials are of key importance to their fundamental mechanical, thermal, electronic, and optical properties. Raman spectroscopy, as a convenient and nondestructive tool, however, has its limitations in identifying all symmetry allowing Raman modes and determining the corresponding crystal structure of 2D layered materials with high symmetry, such as graphene and $\mathrm{Mo}{\mathrm{S}}_{2}$. Due to the lower structural symmetry and extraordinary weak interlayer coupling of $\mathrm{Re}{\mathrm{S}}_{2}$, we successfully identify all 18 first-order Raman active modes for bulk and monolayer $\mathrm{Re}{\mathrm{S}}_{2}$. Without a van der Waals correction, our local density approximation (LDA) calculations successfully reproduce all the Raman modes. Our calculations also suggest no surface reconstruction effect and the absence of low frequency rigid-layer Raman modes below $100\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. Combining Raman spectroscopy and LDA thus provides a general approach for studying the vibrational and structural properties of 2D layered materials with lower symmetry.

Magneto-optical Kerr effect, normally found in magnetic materials with nonzero magnetization such as ferromagnets and ferrimagnets, has been known for more than a century. Here, using first-principles density functional theory, we demonstrate large magneto-optical Kerr effect in high-temperature noncollinear antiferromagnets ${\mathrm{Mn}}_{3}X\phantom{\rule{0.28em}{0ex}}(X=\mathrm{Rh},\phantom{\rule{0.28em}{0ex}}\mathrm{Ir},\phantom{\rule{0.28em}{0ex}}\mathrm{Pt})$, in contrast to usual wisdom. The calculated Kerr rotation angles are large, being comparable to that of transition-metal magnets such as bcc Fe. The large Kerr rotation angles and ellipticities are found to originate from the lifting of band double degeneracy due to the absence of spatial symmetry in the ${\mathrm{Mn}}_{3}X$ noncollinear antiferromagnets which together with the time-reversal symmetry would preserve the Kramers theorem. Our results indicate that ${\mathrm{Mn}}_{3}X$ would provide a rare material platform for exploration of subtle magneto-optical phenomena in noncollinear magnetic materials without net magnetization.