Selective etching of graphene edges by hydrogen plasma

We devised a controlled hydrogen plasma reaction at 300 °C to etch graphene and graphene nanoribbons (GNRs) selectively at the edges over the basal plane. Atomic force microscope imaging showed that the etching rates for single-layer and few-layer (≥2 layers) graphene are 0.27 ± 0.05 nm/min and 0.10 ± 0.03 nm/min, respectively. Meanwhile, Raman spectroscopic mapping revealed no D band in the planes of single-layer or few-layer graphene after the plasma reaction, suggesting selective etching at the graphene edges without introducing defects in the basal plane. We found that hydrogen plasma at lower temperature (room temperature) or a higher temperature (500 °C) could hydrogenate the basal plane or introduce defects in the basal plane. Using the hydrogen plasma reaction at the intermediate temperature (300 °C), we obtained narrow, presumably hydrogen terminated GNRs (sub-5 nm) by etching of wide GNRs derived from unzipping of multiwalled carbon nanotubes. Such GNRs exhibited semiconducting characteristics with high on/off ratios (～1000) in GNR field effect transistor devices at room temperature.     

Graphene Oxide Restricts Growth and Recrystallization of Ice Crystals

We show graphene oxide (GO) greatly suppresses the growth and recrystallization of ice crystals, and ice crystals display a hexagonal shape in the GO dispersion. Preferred adsorption of GO on the ice crystal surface in liquid water leads to curved ice crystal surface. Therefore, the growth of ice crystal is suppressed owing to the Gibbs–Thompson effect, that is, the curved surface lowers the freezing temperature. Molecular dynamics simulation analysis reveals that oxidized groups on the basal plane of GO form more hydrogen bonds with ice in comparison with liquid water because of the honeycomb hexagonal scaffold of graphene, giving a molecular‐level mechanism for controlling ice formation. Application of GO for cryopreservation shows that addition of only 0.01 wt % of GO to a culture medium greatly increases the motility (from 24.3 % to 71.3 %) of horse sperms. This work reports the control of growth of ice with GO, and opens a new avenue for the application of 2D materials.     

气相反应对CVD生长石墨烯的影响

化学气相沉积法(CVD)制备的石墨烯薄膜具有质量高、均匀性好、层数可控且可放大等优点,近年来受到了学术界和工业界的广泛关注。在高温CVD生长过程中,除衬底表面的反应外,气相反应同样会影响石墨烯的生长行为和薄膜质量。本文将综述气相反应对CVD生长石墨烯的影响：首先对CVD体系内的气相传质过程和气相反应进行了详细讨论;随后系统介绍了基于气相调控提高石墨烯的结晶性、洁净度、畴区尺寸、层数和生长速度的相关策略及其机理;最后对气相反应影响CVD生长石墨烯的规律进行总结,并展望了未来可能的发展方向。     

Defect‐Rich Graphene Nanomesh Produced by Thermal Exfoliation of Metal–Organic Frameworks for the Oxygen Reduction Reaction

Although graphene nanomesh is an attractive 2D carbon material, general synthetic routes to produce functional graphene nanomesh in large‐scale are complex and tedious. Herein, we elaborately design a simple two‐step dimensional reduction strategy for exploring nitrogen‐doped graphene nanomesh by thermal exfoliation of crystal‐ and shape‐modified metal‐organic frameworks (MOFs). MOF nanoleaves with 2D rather than 3D crystal structure are used as the precursor, which are further thermally unraveled into nitrogen‐doped graphene nanomesh by using metal chlorides as the exfoliators and etching agent. The nitrogen‐doped graphene nanomesh has a unique ultrathin two‐dimensional morphology, high porosity, rich and accessible nitrogen‐doped active sites, and defective graphene edges, contributing to an unprecedented catalytic activity for the oxygen reduction reaction (ORR) in acid electrolytes. This approach is suitable for scalable production.     

Modeling the optimum conditions for the formation of defect-free CVD graphene on copper melt

The nucleation and growth of nuclei of graphene (graphene islets) on the surfaces of copper melts during catalytic CVD, i.e., the catalytic decomposition of a gas-phase carbon support, is considered. It is shown that on a copper melt the optimum combination of conditions for the preservation of islets with almost perfect hexagonal shape and the necessary conditions of the CVD-process are reached at the same time. The average distance between the islets and the dimensionless parameter that determines changes in the shape of islets is calculated. The maximum rate of decomposition of the carbon support at which this parameter simultaneously promotes the growth of defect-free islets and the maximum possible rate of growth of the graphene monolayer is determined.     

金属纳米结构的氧化刻蚀调控与催化性能优化

金属纳米结构的可控合成,对其性能优化和高效应用至为关键．氧化刻蚀作为金属纳米晶可控合成中的新兴有效调控手段之一,受到越来越多的关注．本文以本课题组近期的研究工作为例,说明了氧化刻蚀对金属纳米晶的形貌、尺寸、结构及组成等合成参数的有效调控作用．由此总结认为,在金属纳米晶可控合成的一般过程,尤其是成核和生长过程中,氧化刻蚀的本质是有效调控“两个速率”和“两个力学”,即减缓原子的生成速率与晶种的形成速率、选择性接受反应热力学和反应动力学的控制作用．我们将通过氧化刻蚀法调控合成得到的具有独特结构的Pd,Pt纳米晶,用于氧活化和电催化这两个重要的催化体系,获得了理想的催化结果,表明氧化刻蚀在金属纳米晶的功能改性和应用拓展方面,具有令人称奇的广阔应用前景．     

Density Functional Theory Study of Aspirin Adsorption on BCN Sheets and their Hydrogen Evolution Reaction Activity: a Comparative Study with Graphene and Hexagonal Boron Nitride

We explored the aspirin adsorption and their hydrogen evolution reaction (HER) activity in waste water of borocarbonitride sheets. Our results indicate that BCN sheets considered here show HER activity and exhibit superior performance regarding adsorption of aspirin in waste water in comparison with graphene and hexagonal boron nitride (h-BN). The drug molecule (aspirin) possesses a strong affinity to BCN, with the order of binding energy on following the order BCN∼h-BN>graphene. Upon drug adsorption, the band gap of h-BN is found to be reduced by up to 33 %, whereas the bandgaps of graphene and BCN remain unaltered that makes BCN a potential candidate for HER in waste water.     

Intercalation-etching of graphene on Pt(111) in H_2 and O_2 observed by in-situ low energy electron microscopy

Graphene layers are often exposed to gaseous environments in their synthesis and application processes, and interactions of graphene surfaces with molecules particularly H_2 and O_2 are of great importance in their physico-chemical properties. In this work, etching of graphene overlayers on Pt(111) in H_2 and O_2 atmospheres were investigated by in-situ low energy electron microscopy. Significant graphene etching was observed in 10~(-5) Torr H_2 above 1023 K, which occurs simultaneously at graphene island edges and interiors with a determined reaction barrier at 5.7 eV. The similar etching phenomena were found in 10.7 Torr O_2 above 973 K, while only island edges were reacted between 823 and 923 K. We suggest that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms while intercalation-etching is attributed to etching at the interiors at high temperatures. The different findings with etching in O_2 and H_2 depend on competitive adsorption, desorption, and diffusion processes of O and H atoms on Pt surface, as well as intercalation at the graphene/Pt interface.     

Hydrogen Plasmas Processing of Graphene Surfaces

To assist the development of plasma processes to pattern graphene in a controlled way, interactions between hydrogen plasma species (H, H⁺, H₂ ⁺) and various types of graphene surfaces (monolayer, nanoribbons, multilayer) are investigated using atomic-scale simulations. It is shown that only “hot” H particles (i.e., with a kinetic energy greater than ~0.4 eV at 300 K) can adsorb on the basal plane of surface-clean graphene while adsorption is barrierless on free edges or vacancies. Surface reaction probabilities (reflection, adsorption, penetration) are found to strongly vary with the incident species energy, which allows to determine specific energy ranges (or process windows) for different types of H₂ plasma treatment: lateral etching of graphene nanoribbons (GNRs), cleaning of graphene surfaces or vertical etching of multilayer graphene (MLG) stacks. Molecular dynamics simulations of GNRs trimming in downstream H₂ plasmas allow to understand the mechanism which governs the anisotropic etching of ribbons and explains the absence of line-edge roughness on their edges. Interactions between low-energy (5–25 eV) H _x ⁺ (x = 1, 2) ions with MLG are also investigated. Ion-induced damage (hydrogenation of successive graphene sheets, creation of vacancies) and etching of the MLG stack are found to vary with the ion energy, the ion fluence and the ion composition.     

Low temperature growth of highly nitrogen-doped single crystal graphene arrays by chemical vapor deposition

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.     

Strengthened graphene oxide/diazoresin multilayer composites from layer-by-layer assembly and cross-linking

The layer-by-layer assembly of graphene oxide and diazoresin is carried out via the electrostatic and hydrogen bond interactions on planar substrates and colloidal templates. The successful planar and spherical growth of multilayer could be investigated by UV-vis spectrophotometry and scanning electron microscopy, respectively. Subsequent UV irradiation or heating would convert the ionic bonds and hydrogen bonds to covalent bands, which significantly improves the stability of the multilayer composite against solvent etching. For the cross-linked core-shell composites, the template cores could be removed by dissolution and hollow microspheres are obtained.     

六角形氧化锌超晶格粒子的控制制备

本文通过蒸发微乳液体系中的溶剂得到了六角形的氧化锌亚微米粒子，其具有超晶格结构。所得产物用红外(FT-IR)和透射电镜(TEM)进行了表征，并进行了热重分析(TGA)。通过监测反应过程，研究了该粒子的形成机制。实验观察到约7nm的纳米氧化锌粒子聚集成亚微米的球形超晶格粒子，该球形粒子随溶剂蒸发进行了自组装，并由于界面相互作用转换成六角形的超晶格粒子。     

Fabrication of small-sized silver NPs/graphene sheets for high-quality surface-enhanced Raman scattering

In this paper, small-sized and highly dispersed Ag nanoparticles (NPs) supported on graphene nanosheets are fabricated via a strategy for etching a copper template with Ag(+). Firstly, big-sized Cu NPs are supported on graphene, and then the small-sized and highly dispersed Ag NPs are supported on graphene by replacement reaction, mainly making use of graphene passing electrons between Cu and Ag(+). The graphene used in the experiment is prepared by in situ self-generating template and has good dispersion, excellent crystallinity and little defects. Thus, in the process of Ag/graphene synthesis, there is no any intervention of surfactant, which ensures that SERS activity sites have not been passivated. And, the little defects of graphene benefit the excellent conductivity of graphene and ensured the replacement reaction between Cu and Ag(+). The obtained material exhibits significant high-quality and distinctive SERS activity. Especially, a serial new peak of p-aminothiophenol (PATP) is observed, this is suggested two reasons: one is "surface geometry" of the PATP on small-sized Ag NPs and another is the charge-transfer between Ag and graphene.     

Formation of Twinned Graphene Polycrystals

Liquid metals have been widely used as substrates to grow graphene and other 2D materials. On a homogeneous and isotropic liquid surface, a polycrystalline 2D material is formed by coalescence of many randomly nucleated single‐crystal islands, and as a result, the domains in a polycrystal are expected to be randomly aligned. Here, we report the unexpected finding that only 30°‐twinned graphene polycrystals are grown on a liquid Cu surface. Atomic simulations confirm that the unique domain alignment in graphene polycrystals is due to the free rotation of graphene islands on the liquid Cu surface and the highly stable 30°‐grain boundaries in graphene. In‐depth analysis predicts 30 types of possible 30°‐twinned graphene polycrystals and 27 of them are observed. The revealed formation mechanism of graphene polycrystals on a liquid Cu surface deepens our fundamental understanding on polycrystal growth and could serve as a guideline for the controlled synthesis of 2D materials.     

Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations

Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.     

Growth Mechanism of Graphene on Graphene Films Grown by Chemical Vapor Deposition

We report an approach for the synthesis of mono‐ or bilayer graphene films by atmospheric‐pressure chemical vapor deposition that can achieve a low defect density through control over the growth time. Different heating ramp rates were found to lead to variation in the smoothness and grain size of the Cu foil substrate, which directly influenced the density of the graphene domains. The rough Cu surface induced by rapid heating creates a high density of graphene domains in the initial stage, ultimately resulting in a graphene film with a high defect density due to an increased overlap between domains. Conversely, a slow heating rate resulted in a smooth and flat Cu surface, thereby lowering the density of the initial graphene domains and ensuring a uniform monolayer film. From this, we demonstrate that the growth mechanism of graphene on existing graphene films is dependent on the density of the initial graphene domains, which is affected by the heating ramp rate.     

Shape tunable two−dimensional ligand−free cesium antimony chloride perovskites

Organic ligands play a key role in determining the shape and stability of the perovskite nanocrystals (NCs). However, the ligands often create poor stability and defects through imperfect attachment, in addition to the post synthesis detachment. We developed a novel route to synthesize the ligand?free ambient stable two?dimensional (2D) cesium antimony chloride (Cs₃Sb₂Cl₉) NCs. First, hexagonal shape NCs are synthesized through a fast one?step reaction at room temperature using a reprecipitation method. The shape of hexagonal NCs is further tuned into well?defined 2D plates through a solid-state temperature-driven crystal phase transition. In?situ variable temperature X?ray diffraction and differential scanning calorimetry cycles probe temperature-sensitive metastability and irreversibility of trigonal to orthorhombic crystallographic phase transition. Rietveld analyses quantify volume fractions and coherently diffracting crystallite domains that promote the growth of the two crystal phases. Both the hexagonal NCs and plates show ambient structural stability for over months. The proposed formation mechanism can guide to improve synthetic methods to realize ligand?free shape-controlled perovskite NCs.     

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.     

Electrostatic Maxwell stress model of the shapes of condensed phase domains in monolayers at the air-water interface

The formation mechanism of the shapes of condensed phase domains in monolayers at the air-water interface was investigated taking into account the surface pressure, line tension, and electrostatic energy due to the spontaneous polarization generated in normal and in-plane direction. By deriving the shape equation of monolayer domains as the mechanical balance at the domain boundary, we found that the electrostatic energy contributes to the shape equation as electrostatic Maxwell stress. Development of a cusp from condensed phase domains of fatty acid monolayers, which has been experimentally observed, was analyzed by the shape equation. It was found that the development of a cusp originated from the strong Maxwell stress, which was induced by the non-uniform orientational distribution in the fatty acid domain, and that cusped shapes gave a minimum of the free energy of the domain. It demonstrates that the shape equation with Maxwell stress, which is derived in the present study, is useful to study the formation mechanism of the shapes of condensed phase domains in monolayers.     

Appearance and disappearance of dendritic and chiral patterns in domains of Langmuir monolayers observed with Brewster angle microscopy

Investigations on the aggregation behavior and morphology of Langmuir films of enantiomeric (L) and racemic (DL) N-acyl amino acids on pure aqueous as well as metal cation containing subphases were carried out at the mesoscale level with the help of Brewster angle microscopy (BAM). In the case of N-hexadecanoyl alanine on a pure aqueous subphase at 298 K the L-enantiomer forms crystal platelets, while the irregular fractal-like shape of the domains of the racemic mixture can be explained by a diffusion limited aggregation (DLA) growth mechanism. At 303 K the L-enantiomer shows a dendritic growth pattern, which leads to explicitly chiral domain shapes that correspond with the chirality of the film-forming molecules and for which hydrogen bridges as directed attractive forces are assumed to be responsible. The compression of the L-enantiomer on a zinc ion containing subphase is accompanied by a remarkable metamorphosis of the condensed structure. Starting from torus-like domains they were at first converted into strongly wound S-shaped domains, finally turning into a seahorse-like appearance. The origin of these chiral shapes can be explained on the basis of an electrostatic growth model. The enantiomer of N-hexadecanoyl alanine methyl ester shows three different asymmetric dendritic growth patterns. The domains of the racemic mixture are dendritic too, but in contrast they are symmetric and have a notably low branching density. On a pure aqueous subphase the L-enantiomer of N-octadecanoyl valine exhibits dendritic growth as well, but the overall outer shape of the domains is not explicitly chiral.