Showing papers by "Xiaolong Zou published in 2015"

Achieving Highly Efficient, Selective, and Stable CO2 Reduction on Nitrogen-Doped Carbon Nanotubes.

Abstract: The challenge in the electrosynthesis of fuels from CO2 is to achieve durable and active performance with cost-effective catalysts. Here, we report that carbon nanotubes (CNTs), doped with nitrogen to form resident electron-rich defects, can act as highly efficient and, more importantly, stable catalysts for the conversion of CO2 to CO. The unprecedented overpotential (-0.18 V) and selectivity (80%) observed on nitrogen-doped CNTs (NCNTs) are attributed to their unique features to facilitate the reaction, including (i) high electrical conductivity, (ii) preferable catalytic sites (pyridinic N defects), and (iii) low free energy for CO2 activation and high barrier for hydrogen evolution. Indeed, DFT calculations show a low free energy barrier for the potential-limiting step to form key intermediate COOH as well as strong binding energy of adsorbed COOH and weak binding energy for the adsorbed CO. The highest selective site toward CO production is pyridinic N, and the NCNT-based electrodes exhibit no degradation over 10 h of continuous operation, suggesting the structural stability of the electrode.

Boron- and Nitrogen-Substituted Graphene Nanoribbons as Efficient Catalysts for Oxygen Reduction Reaction

Abstract: We show that nanoribbons of boron- and nitrogen-substituted graphene can be used as efficient electrocatalysts for the oxygen reduction reaction (ORR). Optimally doped graphene nanoribbons made into three-dimensional porous constructs exhibit the highest onset and half-wave potentials among the reported metal-free catalysts for this reaction and show superior performance compared to commercial Pt/C catalyst. Furthermore, this catalyst possesses high kinetic current density and four-electron transfer pathway with low hydrogen peroxide yield during the reaction. First-principles calculations suggest that such excellent electrocatalytic properties originate from the abundant edges of boron- and nitrogen-codoped graphene nanoribbons, which significantly reduce the energy barriers of the rate-determining steps of the ORR reaction.

Electro-mechanical anisotropy of phosphorene

Abstract: The applied uniaxial stress can break the original symmetry of a material, providing an experimentally feasible way to alter material properties. Here, we explore the effects of uniaxial stress along an arbitrary direction on mechanical and electronic properties of phosphorene, showing the enhancement of inherent anisotropy. Basic physical quantities including Young's modulus, Poisson's ratio, band gap, and effective carrier masses under external stress are all computed from first principles using density functional theory, while the final results are presented in compact analytical forms.

An Open Canvas—2D Materials with Defects, Disorder, and Functionality

Abstract: ConspectusWhile some exceptional properties are unique to graphene only (its signature Dirac-cone gapless dispersion, carrier mobility, record strength), other features are common to other two-dimensional materials. The broader family “beyond graphene” offers greater choices to be explored and tailored for various applications. Transition metal dichalcogenides (TMDCs), hexagonal boron nitride (h-BN), and 2D layers of pure elements, like phosphorus or boron, can complement or even surpass graphene in many ways and uses, ranging from electronics and optoelectronics to catalysis and energy storage. Their availability greatly relies on chemical vapor deposition growth of large samples, which are highly polycrystalline and include interfaces such as edges, heterostructures, and grain boundaries, as well as dislocations and point defects. These imperfections do not always degrade the material properties, but they often bring new physics and even useful functionality. It turns particularly interesting in combina...

Grain Boundary Structures and Electronic Properties of Hexagonal Boron Nitride on Cu(111).

Abstract: Grain boundaries (GBs) of hexagonal boron nitride (h-BN) grown on Cu(111) were investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The first experimental evidence of the GBs composed of square-octagon pairs (4|8 GBs) was given, together with those containing pentagon-heptagon pairs (5|7 GBs). Two types of GBs were found to exhibit significantly different electronic properties, where the band gap of the 5|7 GB was dramatically decreased as compared with that of the 4|8 GB, consistent with our obtained result from density functional theory (DFT) calculations. Moreover, the present work may provide a possibility of tuning the inert electronic property of h-BN via grain boundary engineering.

Tunable Magnetism in Transition-Metal-Decorated Phosphorene

Abstract: We present a density functional theory study of 3d transition-metal (TM) atoms (Sc–Zn) adsorbed on a phosphorene sheet. We show that due to the existence of lone pair electrons on P atoms in phosphorene, all the TM atoms, except the closed-shell Zn atom, can bond strongly to the phosphorene with sizable binding energies. Moreover, the TM@phosphorene systems for TM from Sc to Co exhibit interesting magnetic properties, which arise from the exchange splitting of the TM 3d orbitals. We also find that strain is an effective way to control the magnetism of TM@phosphorene systems by tuning the interaction of the TM with phosphorene and, thus, the relative positions of in-gap TM 3d orbitals. In particular, a small biaxial strain could induce a magnetic transition from a low-spin to a high-spin state in phosphorene decorated by Sc, V, or Mn. These results clearly establish the potential for phosphorene utilization in innovative spintronic devices.

Two-dimensional boron–nitrogen–carbon monolayers with tunable direct band gaps

Abstract: The search for new candidate semiconductors with direct band gaps of ∼1.4 eV has attracted significant attention, especially among the two-dimensional (2D) materials, which have become potential candidates for next-generation optoelectronics. Herein, we systematically studied 2D Bx/2Nx/2C1−x (0 < x < 1) compounds in particular focus on the four stoichiometric Bx/2Nx/2C1−x (x = 2/3, 1/2, 2/5 and 1/3) using a recently developed global optimization method (CALYPSO) in conjunction with density functional theory. Furthermore, we examine more stoichiometries by the cluster expansion technique based on a hexagonal lattice. The results reveal that all monolayer Bx/2Nx/2C1−x stoichiometries adopt a planar honeycomb character and are dynamically stable. Remarkably, electronic structural calculations show that most of Bx/2Nx/2C1−x phases possess direct band gaps within the optical range, thereby they can potentially be used in high-efficiency conversion of solar energy to electric power, as well as in p–n junction photovoltaic modules. The present results also show that the band gaps of Bx/2Nx/2C1−x can be widely tuned within the optical range by changing the concentration of carbon, thus allowing the fast development of band gap engineered materials in optoelectronics. These new findings may enable new approaches to the design of microelectronic devices.

Selectively doping barlowite for quantum spin liquid: A first-principles study

Abstract: Barlowite ${\mathrm{Cu}}_{4}{(\mathrm{OH})}_{6}\mathrm{FBr}$ is a newly found mineral containing ${\mathrm{Cu}}^{2+}$ kagome planes. Despite similarities in many aspects to herbertsmithite ${\mathrm{Cu}}_{3}\mathrm{Zn}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$, the well-known quantum spin liquid (QSL) candidate, intrinsic barlowite turns out not to be a QSL, possibly due to the presence of ${\mathrm{Cu}}^{2+}$ ions in between kagome planes that induce interkagome magnetic interaction [Phys. Rev. Lett. 113, 227203 (2014)]. Using first-principles calculation, we systematically study the feasibility of selective substitution of the interkagome Cu ions with isovalent nonmagnetic ions as a function of ion concentration up to the stoichiometric limit. Unlike previous speculation of using larger dopants, such as ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Ca}}^{2+}$, we identify the most ideal stoichiometric doping elements to be Mg and Zn in forming ${\mathrm{Cu}}_{3}\mathrm{Mg}{(\mathrm{OH})}_{6}\mathrm{FBr}$ and ${\mathrm{Cu}}_{3}\mathrm{Zn}{(\mathrm{OH})}_{6}\mathrm{FBr}$ with the highest site selectivity and smallest lattice distortion. The equilibirium antisite disorder in Mg/Zn-doped barlowite is estimated to be one order of magnitude lower than that in herbertsmithite. The single-electron band structure and orbital component analysis show that the proposed selective doping effectively mitigates the difference between barlowite and herbertsmithite.

Environment-Controlled Dislocation Migration and Superplasticity in Monolayer MoS2.

Abstract: The two-dimensional (2D) transition metal dichalcogenides (TMDC, of generic formula MX2) monolayer displays the “triple-decker” structure with the chemical bond organization much more complex than in well-studied monatomic layers of graphene or boron nitride. Accordingly, the makeup of the dislocations in TMDC permits chemical variability, depending sensitively on the equilibrium with the environment. In particular, first-principles calculations show that dislocations state can be switched to highly mobile, profoundly changing the lattice relaxation and leading to superplastic behavior. With 2D MoS2 as an example, we construct full map for dislocation dynamics, at different chemical potentials, for both the M- and X-oriented dislocations. Depending on the structure of the migrating dislocation, two different dynamic mechanisms are revealed: either the direct rebonding (RB) mechanism where only a single metal atom shifts slightly, or generalized Stone–Wales (SWg) rotation in which several atoms undergo sig...

Enhanced thermoelectric figure of merit in thin GaAs nanowires.

Abstract: Combining density functional theory and the nonequilibrium Green's function method, we investigate the thermoelectric properties of thin GaAs nanowires (NWs). After identifying the most stable structures for GaAs NWs, either in wurtzite (wz) or zinc blende (zb) stacking, we present a systematic analysis on the thermoelectric properties of these NWs and their dependence on stacking type (wz or zb), size of NWs, and temperature. Although bulk GaAs is a well-known poor thermoelectric material, the thermoelectric figure of merit, ZT, is significantly enhanced in thin GaAs NWs. Typically, the room temperature ZT of a 1.1 nm-diameter GaAs NW reaches as high as 1.34, exhibiting more than 100-fold improvement over the bulk counterpart, which is attributed to both the reduced thermal conduction and enhanced power factor in thin NWs. Adopting their unique electronic characteristics, further enhancement is possible through surface engineering, for example, the introduction of surface roughness or dopants.