Zhongqin Yang

Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting.

The solar-driven water splitting process is highly attractive for alternative energy utilization, while developing efficient, earth-abundant, bifunctional catalysts for both oxygen evolution reaction and hydrogen evolution reaction has remained as a major challenge. Herein, we develop an ordered CoMnO@CN superlattice structure as an efficient bifunctional water-splitting electrocatalyst, in which uniform Co-Mn oxide (CoMnO) nanoparticles are coated with a thin, continuous nitrogen-doped carbon (CN) framework. The CoMnO nanoparticles enable optimized OER activity with effective electronic structure configuration, and the CN framework serves as an excellent HER catalyst. Importantly, the ordered superlattice structure is beneficial for enhanced reactive sites, efficient charge transfer, and structural stability. This bifunctional superlattice catalyst manifests optimized current densities and electrochemical stability in overall water splitting, outperforming most of the previously reported single- or bifunctional electrocatalysts. Combining with a silicon photovoltaic cell, this CoMnO@CN superlattice bifunctional catalyst enables unassisted solar water splitting continuously for ∼5 days with a solar-to-hydrogen conversion efficiency of ∼8.0%. Our discovery suggests that these transition metal oxide-based superlattices may serve as a unique structure modality for efficient bifunctional water splitting electrocatalysts with scale-up potentials.

Magnetism and perfect spin filtering effect in graphene nanoflakes.

Magnetic and spin-polarized transport properties in zigzag-edged graphene nanoflakes were investigated from first-principles calculations. Ferrimagnetic structure was found to be the ground state for triangular shaped graphene flakes. Magnetism is weakened by doping B or N atoms into the flakes, and it is enhanced if F atoms are doped in certain sublattices of the flakes. The magnetic properties can be rationalized by the behaviors of dopants as well as interactions between dopants and the host atoms. A perfect (100%) spin filtering effect was achieved for the pure or B doped graphene flake sandwiched between two gold electrodes. The orientation of the spin current is found to be flipped if the flake is doped with N, O, or F atoms. The orientation-tunable spin filtering effect is potentially useful in practical applications.

Bio‐Inspired Leaf‐Mimicking Nanosheet/Nanotube Heterostructure as a Highly Efficient Oxygen Evolution Catalyst

Plant leaves represent a unique 2D/1D heterostructure for enhanced surface reaction and efficient mass transport. Inspired by plant leaves, a 2D/1D CoOx heterostructure is developed that is composed of ultrathin CoOx nanosheets further assembled into a nanotube structure. This bio‐inspired architecture allows a highly active Co2+ electronic structure for an efficient oxygen evolution reaction (OER) at the atomic scale, ultrahigh surface area (371 m2 g−1) for interfacial electrochemical reaction at the nanoscale, and enhanced transport of charge and electrolyte over CoOx nanotube building blocks at the microscale. Consequently, this CoOx nanosheet/nanotube heterostructure demonstrates a record‐high OER performance based on cobalt compounds reported so far, with an onset potential of ≈1.46 V versus reversible hydrogen electrode (RHE), a current density of 51.2 mA cm−2 at 1.65 V versus RHE, and a Tafel slope of 75 mV dec−1. Using the CoOx nanosheet/nanotube catalyst and a Pt‐mesh, a full water splitting cell with a 1.5‐V battery is also demonstrated.

Abnormal electronic transport and negative differential resistance of graphene nanoribbons with defects

Electronic transport properties of zigzag graphene nanoribbons (GNRs) with two kinds of triangular defects are explored by using an ab-initio method. At a certain bias, the current of the GNR with an upward-triangle defect can be surprisingly larger than that of the perfect GNR due to the defect-induced symmetry breaking and more conductive channels. Dissimilarly, if the orientation of the triangle is changed rightward, the current is depressed much and shows negative differential resistance behavior. Our findings indicate that defect designs can be an efficient way to tune the electronic transport of GNR nanodevices.

Role of heating and current-induced forces in the stability of atomic wires

We investigate the role of local heating and forces on ions in the stability of current-carrying aluminum wires. For a given bias, we find that heating increases with wire length due to a redshift of the frequency spectrum. Nevertheless, the local temperature of the wire is relatively low for a wide range of biases provided good thermal contact exists between the wire and the bulk electrodes. On the contrary, current-induced forces increase substantially as a function of bias and reach bond-breaking values at about 1 V. These results suggest that local heating promotes low-bias instabilities if dissipation into the bulk electrodes is not efficient, while current-induced forces are mainly responsible for the wire breakup at large biases. We compare these results to experimental observations.

High-efficiency switching effect in porphyrin-ethyne-benzene conjugates

We have explored the electronic transport properties of porphyrin-ethyne-benzene conjugates using an ab initio method. The results indicate that these ethyne-bridged phenyl porphyrin molecules can be used as candidates for molecular switching devices. The coplanar conformation of phenyl and porphyrin moieties allows a far larger current than the perpendicular conformation due to the near vanishing overlap of the frontier molecular orbitals (π channels) in the porphyrin and phenyl parts in the latter. Higher current ratios of ON/OFF states can be obtained if amino or nitro substituent is placed at the position meta to the bridge connecting the π systems of the molecule. The substituent group affects the electronic state energy of the entire molecule in coplanar conformation, while only affecting the local part in perpendicular conformation. More complex ethyne-bridged diphenyl porphyrin molecules are found to yield more complex and interesting switching effects. Our results suggest that such molecular wires composed of appropriate π-conjugated molecules, can generally display perfect switching function and the efficiency can be tuned flexibly by adding certain substituent groups to the conjugates.

High-Energy Scale Revival and Giant Kink in the Dispersion of a Cuprate Superconductor

In the present photoemission study of a cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta, we discovered a large scale dispersion of the lowest band, which unexpectedly follows the band structure calculation very well. Similar behavior observed in blue bronze and the Mott insulator Ca2CuO2Cl2 suggests that the origin of hopping-dominated dispersion in an overdoped cuprate might be quite complicated. A giant kink in the dispersion is observed, and the complete self-energy containing all interaction information is extracted for a doped cuprate. These results recovered significant missing pieces in our current understanding of the electronic structure of cuprates.

Improving electronic transport of zigzag graphene nanoribbons by ordered doping of B or N atoms.

Using an ab initio method, we explored electronic structures and transport properties of zigzag graphene nanoribbons (ZGNRs) with ordered doping of B or N atoms. We find B or N atoms doping can increase significantly the conductance of the ZGNRs with an even number of zigzag chains due to additional conducting channels being induced and the breakdown of parity limitation. The higher the doping concentration, the larger the current amplification factor obtained. For the nanojunctions with one row B (or N) atoms, the current amplification factor can be larger when the doping position is near to the center, while for the junction with two rows, the trend is subtle due to the interactions between the two rows of B (or N) atoms. Negative differential resistive phenomena are found for the case of B doping at low concentrations and the case for N doping. The conductance of the ZGNR with odd numbers of zigzag chains can also be increased by doping of B or N atoms. More interestingly, the B or N doping can almost completely remove the even-odd effect on electronic transport of the ZGNRs. Our studies provide avenues to drastically improve the electronic transport of ZGNRs, helpful for graphene applications.

Spin-orbit splitting in graphene on metallic substrates.

Substrate-induced spin-orbit splitting in graphene on Ni, Au and Ag(111) is examined on the basis of density-functional theory. The Rashba splitting of π bands along the ΓM direction of the graphene surface Brillouin zone in graphene on Ni(111) is found to be very small (a few millielectronvolts), consistent with the experimental report of Rader et al. Instead, very strong Rashba splitting (near 100 meV) can be obtained for graphene with a certain stretch distortion on a Au substrate. It can be ascribed to the effective match in energy between the C 2p and Au 5d bands, obtained from the analysis of densities of states. The net charge transfer between the graphene and the substrates just affects the spin-orbit effect indirectly. The small spin-orbit splitting induced by the Ag substrates indicates that heavy metals do not always produce large SO splitting. Our findings provide important insights that are useful for understanding the metal-induced Rashba effect in graphene.

Quantum transport in alkane molecular wires: effects of binding modes and anchoring groups.

Effects of binding modes and anchoring groups on nonequilibrium electronic transport properties of alkane molecular wires are investigated from atomic first-principles based on density functional theory and nonequilibrium Greens function formalism. Four typical binding modes, top, bridge, hcp-hollow, and fcc-hollow, are considered at one of the two contacts. For wires with three different anchoring groups, dithiol, diamine, or dicarboxylic acid, the low bias conductances resulting from the four binding modes are all found to have either a high or a low value, well consistent with recent experimental observations. The trend can be rationalized by the behavior of electrode-induced gap states at small bias. When bias increases to higher values, states from the anchoring groups enter into the bias window and contribute significantly to the tunneling process so that transport properties become more complicated for the four binding modes. Other low bias behaviors including the values of the inverse length scale for tunneling characteristic, contact resistance, and the ratios of the high/low conductance values are also calculated and compared to experimental results. The conducting capabilities of the three anchoring groups are found to decrease from dithiol, diamine to dicarboxylic-acid, largely owing to a decrease in binding strength to the electrodes. Our results give a clear microscopic picture to the transport physics and provide reasonable qualitative explanations for the corresponding experimental data.