Z. Tang

High Catalytic Activity of Nitrogen and Sulfur Co‐Doped Nanoporous Graphene in the Hydrogen Evolution Reaction

Chemical doping has been demonstrated to be an effective way to realize new functions of graphene as metal-free catalyst in energy-related electrochemical reactions. Although efficient catalysis for the oxygen reduction reaction (ORR) has been achieved with doped graphene, its performance in the hydrogen evolution reaction (HER) is rather poor. In this study we report that nitrogen and sulfur co-doping leads to high catalytic activity of nanoporous graphene in HER at low operating potential, comparable to the best Pt-free HER catalyst, 2D MoS2 . The interplay between the chemical dopants and geometric lattice defects of the nanoporous graphene plays the fundamental role in the superior HER catalysis.

Nanoporous Graphene with Single‐Atom Nickel Dopants: An Efficient and Stable Catalyst for Electrochemical Hydrogen Production

Single-atom nickel dopants anchored to three-dimensional nanoporous graphene can be used as catalysts of the hydrogen evolution reaction (HER) in acidic solutions. In contrast to conventional nickel-based catalysts and graphene, this material shows superior HER catalysis with a low overpotential of approximately 50 mV and a Tafel slope of 45 mV dec(-1) in 0.5 M H2SO4 solution, together with excellent cycling stability. Experimental and theoretical investigations suggest that the unusual catalytic performance of this catalyst is due to sp-d orbital charge transfer between the Ni dopants and the surrounding carbon atoms. The resultant local structure with empty C-Ni hybrid orbitals is catalytically active and electrochemically stable.

Versatile nanoporous bimetallic phosphides towards electrochemical water splitting

Alloying is an important approach to improving catalytic activities and realizing new functions of heterogeneous catalysts, which has extensively been employed in fabricating noble metal based bimetallic catalysts. However, it is technically unviable in the synthesis of alloyed transition metal compounds, which are emerging as important catalysts for water splitting, in a controllable manner using conventional wet chemical methods. Here we report nanoporous bimetallic (Co1−xFex)2P phosphides with controllable compositions and tuneable porosity, which are fabricated by the combination of metallurgical alloy design and electrochemical etching. By tailoring the Co/Fe ratios and nanoporosity, the bimetallic phosphides exhibit versatile catalytic activities towards HER and OER in acidic and basic electrolytes. As both the cathode and the anode of an electrolyser, nanoporous (Co0.52Fe0.48)2P shows an outstanding performance in water electrolysis, comparable to the commercial electrolyser with paired Pt/C and IrO2 catalysts.

Positron annihilation study of the interfacial defects in ZnO nanocrystals: Correlation with ferromagnetism

High purity ZnO nanopowders were pressed into pellets and annealed in air between 100 and 1200 °C. The crystal quality and grain size of the ZnO nanocrystals were investigated by x-ray diffraction 2θ scans. Annealing induces an increase in the grain size from 25 to 165 nm with temperature increasing from 400 to 1200 °C. Scanning electron microscopy and high-resolution transmission electron microscopy observations also confirm the grain growth during annealing. Positron annihilation measurements reveal vacancy defects including Zn vacancies, vacancy clusters, and voids in the grain boundary region. The voids show an easy recovery after annealing at 100–700 °C. However, Zn vacancies and vacancy clusters observed by positrons remain unchanged after annealing at temperatures below 500 °C and begin to recover at higher temperatures. After annealing at temperatures higher than 1000 °C, no positron trapping by the interfacial defects can be observed. Raman spectroscopy studies confirm the recovery of lattice dis...

Charge-Transfer Induced High Efficient Hydrogen Evolution of MoS2/graphene Cocatalyst.

The MoS2 and reduced graphite oxide (rGO) composite has attracted intensive attention due to its favorable performance as hydrogen evolution reaction (HER) catalyst, but still lacking is the theoretical understanding from a dynamic perspective regarding to the influence of electron transfer, as well as the connection between conductivity and the promoted HER performance. Based on the first-principles calculations, we here clearly reveal how an excess of negative charge density affects the variation of Gibbs free energy (ΔG) and the corresponding HER behavior. It is demonstrated that the electron plays a crucial role in the HER routine. To verify the theoretical analyses, the MoS2 and reduced graphite oxide (rGO) composite with well defined 3-dimensional configuration was synthesized via a facile one-step approach for the first time. The experimental data show that the HER performance have a direct link to the conductivity. These findings pave the way for a further developing of 2-dimension based composites for HER applications.

Enhanced Magnetic Anisotropies of Single Transition-Metal Adatoms on a Defective MoS2 Monolayer

Single magnetic atoms absorbed on an atomically thin layer represent the ultimate limit of bit miniaturization for data storage. To approach the limit, a critical step is to find an appropriate material system with high chemical stability and large magnetic anisotropic energy. Here, on the basis of first-principles calculations and the spin-orbit coupling theory, it is elucidated that the transition-metal Mn and Fe atoms absorbed on disulfur vacancies of MoS2 monolayers are very promising candidates. It is analysed that these absorption systems are of not only high chemical stabilities but also much enhanced magnetic anisotropies and particularly the easy magnetization axis is changed from the in-plane one for Mn to the out-of-plane one for Fe by a symmetry-lowering Jahn-Teller distortion. The results point out a promising direction to achieve the ultimate goal of single adatomic magnets with utilizing the defective atomically thin layers.

Correlation between Chemical Dopants and Topological Defects in Catalytically Active Nanoporous Graphene.

The interplay between chemical dopants and topological defects plays a crucial role in electrocatalysis of doped graphene. By systematically tuning the curvatures, thereby the density of topological defects, of 3D nanoporous graphene, the intrinsic correlation of topological defects with chemical doping contents and dopant configurations is revealed, shining lights into the structural and chemical origins of HER activities of graphene.

Coral-Shaped MoS2 Decorated with Graphene Quantum Dots Performing as a Highly Active Electrocatalyst for Hydrogen Evolution Reaction

We report a new CVD method to prepare coral-shaped monolayer MoS2 with a large amount of exposed edge sites for catalyzing hydrogen evolution reaction. The electrocatalytic activities of the coral-shaped MoS2 can be further enhanced by electronic band engineering via decorated with graphene quantum dot (GQD) decoration. Generally, GQDs improve the electrical conductivity of the MoS2 electrocatalyst. First-principles calculations suggest that the coral MoS2@GQD is a zero-gap material. The high electric conductivity and pronounced catalytically active sites give the hybrid catalyst outstanding electrocatalytic performance with a small onset overpotential of 95 mV and a low Tafel slope of 40 mV/dec as well as excellent long-term electrocatalytic stability. The present work provides a potential way to design two-dimensional hydrogen evolution reaction (HER) electrocatalysts through controlling the shape and modulating the electric conductivity.

A paramagnetic neutral VAlON center in wurtzite AlN for spin qubit application

Spin-polarized electronic structures of VAlON centers consisting of an aluminum vacancy and a substitutional oxygen in AlN with different charge states are studied by first-principles calculations. It is observed that a paramagnetic neutral VAlON center is stable in p-type AlN. The defect center possesses a triplet ground state and a spin-conserved excited state with rather low excitation energy and its spin coherence time is in an order of second at T = 0 estimated by using a mean-field-based scheme. The results indicate that the neutral VAlON center is a promising candidate for spin coherent manipulation and qubit operation.Spin-polarized electronic structures of VAlON centers consisting of an aluminum vacancy and a substitutional oxygen in AlN with different charge states are studied by first-principles calculations. It is observed that a paramagnetic neutral VAlON center is stable in p-type AlN. The defect center possesses a triplet ground state and a spin-conserved excited state with rather low excitation energy and its spin coherence time is in an order of second at T = 0 estimated by using a mean-field-based scheme. The results indicate that the neutral VAlON center is a promising candidate for spin coherent manipulation and qubit operation.

First-principles studies of interlayer exchange coupling in (Ga, Mn)As-based diluted magnetic semiconductor multilayers

Interlayer exchange coupling (IEC) in a series model diluted magnetic semiconductor multilayer consisting of two magnetic (Ga, Cr)N layers separated by non-doped or Mg-doped GaN non-magnetic spacers has been studied by first-principles calculations. The effects of the spacer thickness and hole doping to the IEC were studied systematically. For the GaN spacers without Mg doping, the IEC between two magnetic (Ga, Cr)N layers is always ferromagnetic and is clarified as an intrinsic character of the Ruderman-Kittle-Kasuya-Yoshida interaction based on a two-band model for a gapped system. For the Mg-doped GaN spacers, the IEC is antiferromagnetic, and the antiferromagnetic IEC is stable with increasing the spacer thickness.