Investigation of Electron-Phonon Coupling in Epitaxial Silicene by In-situ Raman Spectroscopy

In this letter, we report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on Ag(111) surface was probed by an in-situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at edge sites in the silicene were identified. Detecting relationship between EPC and Dirac fermion through the Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.Comment: 15 pages, 4 figure

Indirect-direct band transformation of few-layer BiOCl under biaxial strain

Being a new two-dimensional layered compounds, the tunable indirect-direct band transformation of BiOCl with different layers can be realized by introducing the biaxial tensile or compressive strains. The band structure and stability of BiOCl with different layers are first researched to clarify the influence of layer numbers. A phase transformation of bilayer BiOCl and metallic characteristic for all are observed under large tensile and compressive strains, respectively. In addition, bond length, interlayer spacing, and band decomposed charge density are calculated to analyze the mechanism behind these phenomena. The results indicate that the band structure transformation is primarily related to the competitions between two kinds of intralayer and interlayer Bi-O bonds and hybridizations between atoms under strains

Microbial transformation of neomycin by a mutant of neomycin-producing Streptomyces fradiae

Utilizing a mutant of neomycin-producing Streptomyces fradiae mutagenized with neutron radiation, biotransformation of neomycin into modified compounds was studied. The biotransformation products were isolated by ion exchange chromatography and monitored by thin layer chromatography bioautography (TLCB). Antibacterial activity of biotransformation products against ten species of bacteria including four plant pathogens was tested qualitatively by TLCB and detected quantitatively by Oxford cup method. The minimal inhibitory concentration (MIC) of biotransformation products was tested by agar diffusion method. Three isolated transformation products had obvious antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Proteus vulgaris and Pseudomonas solanacarum. At the concentration of 50 μg/ml, the transformation product X had a similar antibacterial effect with neomycin but the transformation product Y and Z showed a decreased effect compared to neomycin except for P. vulgaris and P. solanacarum. However, the results from MIC analysis demonstrated that only the transformation product X maintained the same inhibitory effect with neomycin.Key words: Neomycin, biotransformation, Streptomyces fradiae, mutant, neutron radiation

Promoted Photocharge Separation in 2D Lateral Epitaxial Heterostructure for Visible‐Light‐Driven CO2 Photoreduction

Photocarrier recombination remains a big barrier for the improvement of solar energy conversion efficiency. For 2D materials, construction of heterostructures represents an efficient strategy to promote photoexcited carrier separation via an internal electric field at the heterointerface. However, due to the difficulty in seeking two components with suitable crystal lattice mismatch, most of the current 2D heterostructures are vertical heterostructures and the exploration of 2D lateral heterostructures is scarce and limited. Here, lateral epitaxial heterostructures of BiOCl @ Bi2O3 at the atomic level are fabricated via sonicating‐assisted etching of Cl in BiOCl. This unique lateral heterostructure expedites photoexcited charge separation and transportation through the internal electric field induced by chemical bonding at the lateral interface. As a result, the lateral BiOCl @ Bi2O3 heterostructure demonstrates superior CO2 photoreduction properties with a CO yield rate of about 30 µmol g−1 h−1 under visible light illumination. The strategy to fabricate lateral epitaxial heterostructures in this work is expected to provide inspiration for preparing other 2D lateral heterostructures used in optoelectronic devices, energy conversion, and storage fields

Synthesis of Au@Pd core/shell nanomaterials for high-performance ethanol oxidation reactions

This thesis summarizes my postgraduate research on the studies of Pd-based electrocatalysts for ethanol oxidation reactions, mainly on two directions: (1) the fabrication of a high-surface-area electrode, Pd coated Au nanowire (AuNW) forest on porous nickel foam substrate, to enhance the catalytic activity of ethanol oxidation reactions, and (2) cyclic voltammetry (CV) guided deposition of sub-monolayer Pd on Au surface with fine-tuning Pd thickness and real-time monitoring of the electrochemical properties. In Chapter Two, a modified procedure for the growth of AuNW forest on porous nickel foam substrate (previously only on oxide substrate), was proposed by changing the surface ligand from amino to cyano functionalized silane and applying the stirring condition. As a two-pronged strategy, the combination of AuNW and nickel foam could enhance the electrochemical active surface over 20 times, compared with Au nanoparticles decorated flat substrate. After the pulsed-current deposition of 1 nm Pd shell on Au, this flexible electrode showed significantly improved catalytic activity on the electro-oxidation of ethanol with high tolerance to hydroxyl and ethoxyl poisoning. The peak current density of as-prepared electrocatalyst was 3, 11 and 48 times higher than that of Pd coated bare nickel foam, Pd coated AuNW on fluorine doped tin oxide (FTO), and the state-of-the-art catalyst Pd on activated carbon (Pd/C), respectively. In Chapter Three, one convenient and effective deposition method for the coating of Pd overlayer on Au surface was realized by mimicking the primary cell in the 10 mM PdCl2 deposition solution. The deposited amount of Pd can be tuned by the concentration of Pd precursor. When the concentration of PdCl2 was reduced to 2.5 mM, the Au surface would be partially coated with Pd domains and the CV scan could facilitate the physical movement of Pd atoms to realize full coverage of the exposed Au surface. The mass-specific current of the Pd coated AuNW nickel foam electrode on ethanol oxidation can be enhanced to 6464 A/g, over two times higher than that of typical electrochemical deposited one. On the basis of a series of control experiments, the Pd movement mechanism was proposed that the Pd atoms may firstly get oxidized to free Pd ions in the electrolyte, which are then reduced back to cover the Au surface. In Chapter Four, to solidify our proposed Pd movement mechanism, solution form of Pd precursor was added into the system, replacing previous solid state Pd. With potential cycling in the 16 μM PdCl2, 1.0 M ethanol and 1.0 M NaOH electrolyte, sub-monolayer of Pd deposition on Au can be readily achieved. During CV scans, the onset potential and peak current density of backward peak reveals the binding affinity of –OH group and the catalytic activity, respectively. With such real-time monitoring, the thickness of the Pd overlayer can be continuously and precisely tuned by the number of CV cycles and directly correlates with the electrochemical properties.Doctor of Philosoph

s-p orbital hybridization: a strategy for developing efficient photocatalysts with high carrier mobility

Photocatalysis has not only invigorated the field of energy conversion materials, but also is leading to bright prospects for application in the environmental purification field [1]. Akira Fujishima and Kenichi Honda [2] first reported photocatalytic water splitting on a TiO2 semiconductor electrode under ultraviolet (UV) light in 1972. In semiconductor photocatalysts, electrons are excited from valence band maximum (VBM) to conduction band minimum (CBM) under light irradiation, and then trigger the photocatalytic process [3]. Considering solar-light-driven photocatalysis, semiconductor photocatalysts should possess a narrow band gap and appropriate band positions [4]. It was also found that photoinduced charge generation, separation, and transportation determine activities of semiconductor photocatalysts. High mobility of charge carriers facilitates these processes, which can be achieved in the photocatalysts with highly dispersive bands, because their effective masses of charge carriers are small. Usually, the antibonding hybridization/coupling is predominantly responsible for the band dispersion, especially for oxides. For example, Sn-5s/O- 2p anti-bonding coupling in VBM of Sn2+ oxides, Cu-3d/O-2p anti-bonding coupling in VBM of Cu+ oxides and anti-bonding coupling in CBM of most of semiconductors [5-9]. Especially, s-p orbital hybridization is found to improve the performance of photocatalysts by affecting their band structures [10,11]

Magnetic field actuated manipulation and transfer of oil droplets on a stable underwater superoleophobic surface

The transport of fluids at functional interfaces, driven by the external stimuli, is well established. The lossless transport of oil-based fluids under water remains a challenge, however, due to their high stickiness towards the surface. Here, a superhydrophilic and underwater superoleophobic tri-phase water/oil/solid nanoarray surface has been designed and prepared. The unique tri-phase surface exhibits underwater superoleophobic properties with an extremely low stickiness towards oil-based fluids. The magnetic-field-driven manipulation and transport of oil-based magnetic fluids are demonstrated under water, which opens up a new pathway to design flexible and smart devices for the control and transfer of liquid droplets by using tri-phase systems

Role of Charge Density Wave in Monatomic Assembly in Transition Metal Dichalcogenides

The charge density wave (CDW) in transition metal dichalcogenides (TMDs) has drawn tremendous interest due to its potential for tailoring their surface electronic and chemical properties. Due to technical challenges, however, how the CDW could modulate the chemical behavior of TMDs is still not clear. Here, this work presents a study of applying the CDW of NbTe 2 , with a high transition temperature above room temperature, to generate the assembling adsorption of Sn adatoms on the surface. It is shown that highly ordered monatomic Sn adatoms with a quasi-1D structure can be obtained under regulation by the single-axis CDW of the substrate. In addition, the CDW modulated superlattices could in turn change the surface electronic properties from semimetallic to metallic. These results demonstrate an effective approach for tuning the surface chemical properties of TMDs by their CDWs, which could be applied in exploring them for various practical applications, such as heterogeneous catalysis, epitaxial growth of low-dimensional materials, and future nanoelectronics

Recent advances in two-dimensional van der Waals magnets

Two-dimensional (2D) magnets have evoked tremendous interest within the research community due to their fascinating features and novel mechanisms, as well as their potential applications in magnetic nanodevices. In this review, state-of-the-art research into the exploration of 2D magnets from the perspective of their magnetic interaction and order mechanisms is discussed. The properties of these magnets can be effectively modulated by varying the external parameters, such as the charge carrier doping, thickness effect, pressure and strain. The potential applications of heterostructures of these 2D magnets in terms of the interlayer coupling strength are reviewed, and the challenges and outlook for this field are proposed