Control Of Surface Structure Research Articles

Controlled Surface Structure and Chemistry with Transition Metal Enrichment for Cathode Materials

Transition metals enrichment on the surface of layered nickel-cobalt-manganese oxide cathodes leads to the structural changes, hindering the facile lithium-ion transportation at the surface of the particles. These inhomogeneous metal distributions and structural evolutions are progressively proceeded with electrochemical cycling from the surface to the bulk, which causes to severe material degradation and short cycle life. In this study, we develop the new surface structure and chemistry on the layered cathode materials to enhance the battery performances by controlling the local concentration of transition metal. Our strategy is to improve the surface instabilities by either suppressing unwanted transition metal segregation or creating an ion conductive oxide phase or both on the specific crystallographic planes for the layered host materials. This is conducted by adding electrochemically inactive transition metals together with the active metals during the synthesis because that the transition metal elements are individually and preferentially enriched to the particular facets such as (20–2), (200), and (001) of the layered bulk. As a result, the unique surface layers are spontaneously formed along the surface planes within a few nanometer. The electrochemical characteristics shows that the newly created surface phases strongly influence on the high rate performance and long cycle life. This work is supported by the National Research Foundation of Korea (NRF-2011-C1AAA001-0030538).

A green method to reduce graphene oxide with carbonyl groups residual for enhanced electrochemical performance

Reduced graphene oxide nanosheets modified with rich carbonyl groups (C=O) was obtained through a facile dehydration process by making use of the synergistic effect between phosphoric acid and monosodium phosphate. The product exhibited a high specific capacitance of 413 F g−1 at the current density of 0.25 A g−1 mainly owing to the high percentage of retained C=O groups which provided reversible pseudocapacitance and favored good dispersion. Additionally, the symmetric supercapacitors based on the modified graphene nanosheets showed high specific capacitance (314.2 F g−1 at 1 A g−1) and superior electrochemical stability with only 5.8% loss of its initial specific capacitance after 10000 cycle tests (at the current densities of 5 A g−1). The present work proposed an environmentally friendly way in controlling surface structure of reduced graphene oxide which promises great potential of graphene-based electrode materials for high performance energy-storage devices.

Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides.

In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5 Ni4.5 S8 , a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis.

Control of periodic surface structures on silicon by combined temporal and polarization shaping of femtosecond laser pulses

We demonstrate the capability to exercise advanced control on the laser-induced periodic surface structures (LIPSS) on silicon by combining the effect of temporal shaping, via tuning the interpulse temporal delay between double femtosecond laser pulses, along with the independent manipulation of the polarization state of each of the individual pulses. For this, cross-polarized (CP) as well as counter-rotating (CR) double circularly polarized pulses have been utilized. The pulse duration was 40 fs and the central wavelength of 790 nm. The linearly polarized double pulses are generated by a modified Michelson interferometer allowing the temporal delay between the pulses to vary from Δτ = −80 ps to Δτ = +80 ps with an accuracy of 0.2 fs. We show the significance of fluence balance between the two pulse components and its interplay with the interpulse delay and with the order of arrival of the individually polarized pulse components of the double pulse sequence on the final surface morphology. For the case of CR pulses we found that when the pulses are temporally well separated the surface morphology attains no axial symmetry. But strikingly, when the two CP pulses temporally overlap, we demonstrate, for the first time in our knowledge, the detrimental effect that the phase delay has on the ripple orientation. Our results provide new insight showing that temporal pulse shaping in combination with polarization control gives a powerful tool for drastically controlling the surface nanostructure morphology.

Tunable active edge sites in PtSe2 films towards hydrogen evolution reaction

Layered transition-metal dichalcogenides (TMDCs) have received great interest due to their potential applications in many fields including electronics, optoelectronics, electrochemical hydrogen production and so on. Recent research effort on the development of effective hydrogen evolution reaction (HER) is to modulate the active edge sites through controlling surface structure at the atomic scale. Here we firstly demonstrate a facile strategy to synthesize large-area and edge-rich platinum diselenide (PtSe2) via selenization of Pt films by magnetron sputtering physical deposition method. The edge site density of the PtSe2 can be effectively controlled by tuning the thickness of Pt films. The HER activity of the PtSe2 can be enhanced significantly as the active edge site density increases. The maximum cathodic current density of 227mA/cm2 can be obtained through increasing the edge density, which well agrees with the density functional theory calculations. Our work provides a fundamental insight on the effect of active edge site density towards HER.

Experimental and Numerical Studies on Gas Flow Through Silicon Microchannels

The present work investigates the extension of Navier–Stokes equations from slip-to-transition regimes with higher-order slip boundary condition. To achieve this, a slip model based on the second-order slip boundary condition was derived and a special procedure was developed to simulate slip models using FLUENT®. The boundary profile for both top and bottom walls was solved for each pressure ratio by the customized user-defined function and then passed to the FLUENT® solver. The flow characteristics in microchannels of various aspect ratios (a = H/W = 0.002, 0.01, and 0.1) by generating accurate and high-resolution experimental data along with the computational validation was studied. For that, microchannel system was fabricated in silicon wafers with controlled surface structure and each system has several identical microchannels of same dimensions in parallel and the processed wafer was bonded with a plane wafer. The increased flow rate reduced uncertainty substantially. The experiments were performed up to maximum outlet Knudsen number of 1.01 with nitrogen and the second-order slip coefficients were found to be C1 = 1.119–1.288 (TMAC = 0.944–0.874) and C2 = 0.34.

전자선 조사로 표면 구조가 제어된 활성탄소섬유의 전기화학적 특성 향상

전자선 조사로 표면 구조가 제어된 활성탄소섬유의 전기화학적 특성 향상

Octahedral PtNi nanoparticles with controlled surface structure and composition for oxygen reduction reaction

Controlling the surface structure and composition at the atomic level is an effective way to tune the catalytic properties of bimetallic catalysts. Herein, we demonstrate a generalized strategy to synthesize highly monodisperse, surfactant-free octahedral Pt x Ni1−x nanoparticles with tunable surface structure and composition. With increasing the Ni content in the bulk composition, the degree of concaveness of the octahedral Pt x Ni1−x nanoparticles increases. We systematically studied the correlation between their surface structure/composition and their observed oxygen reduction activity. Electrochemical studies have shown that all the octahedral Pt x Ni1−x nanoparticles exhibit enhanced oxygen reduction activity relative to the state-of-the-art commercial Pt/C catalyst. More importantly, we find that the surface structure and composition of the octahedral Pt x Ni1−x nanoparticles have significant effect on their oxygen reduction activity. Among the studied Pt x Ni1−x nanoparticles, the octahedral Pt1Ni1 nanoparticles with slight concaveness in its (111) facet show the highest activity. At 0.90 V vs. RHE, the Pt mass and specific activity of the octahedral Pt1Ni1 nanoparticles are 7.0 and 7.5-fold higher than that of commercial Pt/C catalyst, respectively. The present work not only provides a generalized strategy to synthesize highly monodisperse, surfactant-free octahedral Pt x Ni1−x nanoparticles with tunable surface structure and composition, but also provides insights to the structure-activity correlation.

Shaping Gold Nanocrystals in Dimethyl Sulfoxide: Toward Trapezohedral and Bipyramidal Nanocrystals Enclosed by {311} Facets.

The remarkable synthetically tunable structural, electronic, and optical properties of gold nanocrystals have attracted increasing interest and enabled multidisciplinary applications. Over the past decades, nearly all the possible fundamental shapes of faceted Au nanocrystals have been synthesized, except for only one missing-the trapezohedron enclosed by {hkk} facets. In this report, the unprecedented synthesis of trapezohedral Au nanocrystals with {311} crystal facets was realized. Dimethyl sulfoxide (DMSO) was discovered as a solvent for shaping Au nanocrystals with {311} crystal facets for the first time. Mechanistic studies, together with previous DFT and STM studies, attribute the unique role of DMSO to its ambidentate nature, where both sulfur and oxygen of DMSO can coordinate to gold surface, endowing its unique role in stabilizing high-index {311} facets through a "two center bonding" mode. The DMSO-based synthesis provides a new synthetic tool toward the synthesis of a series of unreported Au nanocrystals with new structures. In particular, a new type of gold bipyramids, the octagonal bipyramids, was first synthesized with additional plasmonic tunability while simultaneously retaining their {311} facets. The application of these new Au nanocrystals in surface-enhanced Raman scattering spectroscopy was investigated, and their shape-dependent performances were demonstrated. These results highlight the tremendous potential of using ambidentate molecules as shape- and surface-directing agents for metal nanocrystals and offer the promise of enabling new synthetic tools toward atomically precise control of surface structures of metal nanocrystals.

Surface structure-controlled synthesis of nanocrystals through supersaturation dependent growth strategy

Many physical and chemical properties of nanocrystals are closely related to their surface structures. Studies on the surface structure control of nanocrystals and related theories are hot and frontier research topics recently. In this review, we introduce the supersaturation control strategy and its application in the surface structure control of nanocrystals ranging from ionic, molecular, metallic, metal oxide, to metal-organic frameworks crystals. Deduced from thermodynamics, it can be found that the surface energy of nanocrystals is directly proportional to the supersaturation of growth units. The relationship between supersaturation and surface energy provides an important theoretical basis for the controlled synthesis of nanocrystals with specific surface structure.

Thermosensitive polymer controlled morphogenesis and phase discrimination of calcium carbonate.

Homogeneous aragonite flowers with controlled surface structures can be synthesized by using a thermosensitive polymer, i.e. poly (ethylene glycol)-poly(N-isopropyl acrylamide)-poly(acrylamido methyl propane sulfonate) (PEG-PNIPAM-PAMPS), as a crystal growth modifier in the mineralization of calcium carbonate.

Methodology of laser processing for precise control of surface micro-topology

Abstract Laser surface texturing of materials potentially offers precise control of surface structure and mechanical properties. This has a wide range of applications such as control of frictional forces, control of bond strength in interference fit joints, and production of antifouling surfaces. To achieve such texturing in a well-defined, useful manner, precise control of the applied laser processing parameters over a sizeable surface area is required. This paper presents the development of a method for creating highly repeatable and predetermined moire textured patterns on metallic samples via laser processing. While the method developed is broadly applicable to various materials and laser systems, in the example detailed here the surfaces of cylindrical stainless steel samples were processed with a pulsed CO 2 laser. The resulting modified surfaces contained texture geometries with pre-definable peak-to-peak widths, valley-to-peak heights, and texture directions. The method of achieving this theoretically and experimentally is detailed in this paper. The relationship between the laser processing parameters and resulting diameter increase was confirmed via Design of Experiment response surface methodology. Precise control of the laser textured cylindrical surface outer diameter and texture pattern are key factors in determining the potential suitability of this process for application to the production of interference fit fasteners. The effects of the laser processing parameters and topologies of the resulting re-solidified metal profile on the surfaces were assessed in detail with a focus on this application.

Local protrusions formed on Si(111) surface by surface melting and solidification under applied tensile stress

The surface structure and composition of Si(111) was modified, by heating it to 1300 °C in ultrahigh vacuum under an external tensile stress. A stress of approximately 1 GPa was applied, by pressing on the center of the rear side of the sample. This process produced two protrusions of approximately 100 μm in height, to the left and right of the center. Scanning Auger electron spectroscopy revealed Fe, Cr, Ni, and C impurities at the top of one protrusion, and C at the top of the other. These impurities likely diffused into the tops of the protrusions during heating, and segregated to the local surface during cooling when the protrusions formed. The protrusion formation mechanism is discussed. Their formation was related to non-uniform surface temperature, electromigration, piezoresistivity, freezing-point depression due to surface alloying with the impurities, and volume expansion during solidification from surface melting. These findings provide a perspective on controlling surface structures and compositions using heat and stress to induce self-assembly.

Pathway Selection in Wet-Chemical Reaction for Hierarchical High Surface Nano-Ceria

Ceria has been received great attention owing to its unique optical, electrical and magnetic properties that make it valuable material for solid oxide fuel cells, chemical mechanical polishing, gas sensors, UV-adsorbents and automotive three-way catalysts. We study to find the right pathway in low-cost synthesis processing for nano-ceria with tremendous high surface reactivity and redox/thermal stability. Wet chemistry needs to be designed in fastidious details in order to obtain the nano-ceria with anticipated properties by the control of hierarchical surface structure of the nano-ceria. Each cerias we make has been showed poor repeatability resulted from the final architectures of it, albeit, it is synthesized from the same reactant concentration. It would be the thermodynamic differences of the pathways lead to crucial distinction in structure of nano-ceria depending on reaction time, the moment concentration and the temperature. The Hierarchically well-constructed nano-ceria shows high thermal stability in consequence of sufficient space to avoid the agglomerating among the crystals could maintain its structure at high temperature. Thus, to determine the relation between the pathway and result thermodynamically is one of the crucial part of design the right wet-chemical reaction, in order to get the nano-ceria with high surface reactivity and the redox/thermal stability with good repeatability. Furthermore, hierarchical high surface nano-ceria could be used for electrochemical device operating in extreme condition. Figure 1

Review on formation mechanism of chemical reaction layer during chemical mechanical polishing of monocrystalline SiC and sapphire substrates

Chemical mechanical polishing/planarization (CMP) of wafers is routinely used as the last mechanical process to produce super-smooth or critically planarized surfaces by removing machining-induced surface/subsurface damages and at the same time reduce the surface roughness to the level nanometers or even sub-nanometers without sacrificing the overall flatness. Successful CMP of monocrystalline silicon carbide (SiC) and sapphire substrates—two major substrates besides silicon for fabrication of high-brightness light emission diode (LED) devices, and also two important optical materials for space applications—demands a deeper mechanistic understanding into the CMP material removal process of these two materials. One of crucial challenges is to reveal the mechanism of formation of the chemical reaction layer affected by chemical-mechanical interaction during CMP. This paper critically reviews the current status of relevant progresses, controversies and challenges. First, we reviewed the debate about the thermodynamic behavior of formation of the chemical reaction layer by carefully comparing the original experimental results and arguments by different groups, including numerous studies on the equilibrium of the reactions involved and the stable reaction products. Second, the inconsistencies on the reaction kinetic behavior of formation and removal of the reaction layer, i.e., the temperature dependent materials removal rates in terms of activation energy, were critically compared and re-analyzed by deriving the numerical values of activation energy from the existing literature. Third, the challenge in establishing the model CMP substrate systems was highlightened in order to stress the urgent requirement for a firm foundation without which the results of different labs were difficult to be critically compared. Last, problems of incomplete characterization of the structure and compositions of the reaction layer and substrates as well as the poor cross-lab repeatability of characterization results were discussed with examples. In the perspective part, we propose a few directions for possible breakthroughs in the area of the mechanisms of formation of the chemical reaction layer: (1) establishment of reliable model CMP substrate systems with controlled surface structure by taking into account a few key parameters including surface/ subsurface defect density, misorientation angle (off-cut angle) as well as crystallography planes exposed; (2) determination and control of the reaction rate to facilitate decoupling reaction and diffusion processes with the help of the frictional force microscopy technique. A multidisciplinary strategy for addressing the issues discussed above is thus strongly advocated through combining materials/surface science/chemistry related approaches—careful and detailed characterization using a variety of state-of-the-art analytical techniques, precise control of surface structure and defects for developing model CMP substrate systems, in-depth analyses of surface structure and compositions, chemical thermodynamic and kinetic studies on chemical reaction mechanisms, as well as microscopic frictional/wear tests and surface force measurements. This review will also have important implications for CMP processing of many more different types of materials other than sapphire and SiC, including but not limited to other promising LED substrates such as GaN and ZnO, optical materials for laser inertial confinement fusion applications such as hard-brittle fused silica and soft-brittle KDP crystals, as well as conventional and novel ceramics and crystals for strong lasers such as YAG and sesquioxides (Lu2O3, Y2O3).

Fabrication and Control of Fine Periodic Surface Structures by Short Pulsed Laser

Ultrashort-pulsed laser irradiation is a more efficient approach to the fabrication of fine surface structures than traditional processing methods. However, it has some problems: the equipment expenses usually increase as the pulse shortens, and the process principle has not been clarified completely, although the collisional relaxation time (CRT) is assumed to be a major factor. In this study, a 20-ps pulsed laser was employed to fabricate nanometer-sized periodic structures on a stainless steel alloy, SUS304. The pitch length of the fabricated fine periodic structures was similar to the laser wavelength, and the results suggested that periodic structures could be fabricated within a limited range of the laser fluence. In order to expand the effective fluence range (EFR) and to control the pitch length, laser irradiation was carried out with different workpiece temperatures and the laser wavelengths. In this way, CRT was extended and EFR was expanded by cooling the workpiece, and the pitch lengths were approximately equal to the laser wavelengths. As a result, two things were found: it is easier to fabricate the fine periodic structures by cooling the workpiece, and it is possible to control the pitch length of the fine periodic structures by changing the laser wavelength.

Epitaxial graphene on SiC formed by the surface structure control technique

The thermal decomposition of silicon carbide (SiC) is a promising method for producing wafer-scale single-crystal graphene. The optimal growth condition for high-mobility epitaxial graphene fabricated by infrared rapid thermal annealing is discussed in this paper. The surface structures, such as step–terrace and graphene coverage structures, on a non-off-axis SiC(0001) substrate were well controlled by varying the annealing time in a range below 10 min. The mobility of graphene grown at 1620 °C for 5 min in 100 Torr Ar ambient had a maximum value of 2089 cm2 V−1 s−1. We found that the causes of the mobility reduction were low graphene coverage, high sheet carrier density, and nonuniformity of the step structure.

Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels. The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects. However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production. In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method. With the increase of cooling rate, it is observed that the phase is transformed from austenite → grain boundary film-like alltromorph ferrite → Widmanstätten ferrite plates (or intragranular ferrite plates) → bainite+martensite. Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to be refined through the single γ → α → γ cycle. Even though, the reduction of area (RA) is improved drastically to over 70% in the third ductility trough, whereas the RA value is just < 40% for the conventional cooling. The reason can be deduced by eliminating grain boundary film-like ferrite, and carbonitrides were precipitated inside the grain, which simultaneously act as the nucleus of intragranular ferrite. Thus, the principle of SSC process was established, and based on heat transfer calculation, water flow adjustment was put forward to implement the intense secondary cooling, which is a meaningful exploration for reducing transverse cracking.

Kinetically Controlling Surface Structure to Construct Defect-Rich Intermetallic Nanocrystals: Effective and Stable Catalysts.

Kinetic control of surface defects is achieved, and cubic, concave cubic, and defect-rich cubic intermetallic Pt3 Sn nanocrystals are prepared for the electro-oxidation of formic acid. The generality of this kinetic approach is demonstrated by the fabrication of Pt-Mn nanocrystals with different surface defects. The defect-rich nanocrystals exhibit high catalytic activity and stability concurrently, indicating their potential application in fuel cells.

One-pot synthesis of Pd@Pt core–shell nanocrystals for electrocatalysis: control of crystal morphology with polyoxometalate

Pt-based bimetallic nanocrystals (NCs) are an important class of catalytic materials especially in the area of energy conversion due to their outstanding catalytic performance. The catalytic function of Pt-based bimetallic NCs toward a specific catalysis reaction can be optimized by tailoring their surface structure, which can be realized through the precise control of the NC morphology. In the present work, we developed a facile one-pot polyoxometalate (POM)-mediated synthesis approach for the synthesis of Pt-based bimetallic NCs with controlled morphologies and fine crystal structures, and investigated the influence of their morphology on their catalytic function. Pd@Pt core–shell NCs with well-defined morphologies and controlled surface structures could be prepared by the simultaneous reduction of Pt and Pd precursors in the presence of a typical Keggin-type POM (H3PMo12O40) and ascorbic acid (AA). During the formation of NCs, the POMs served not only as a stabilizing agent but also a reducing agent. Notably, the presence of the POMs as well as the relative concentration of AA to metal precursors could afford fine control over the growth mode of the Pt shells on the Pd cores, and thus the final morphology of the Pd@Pt NCs. The prepared POM-passivated Pd@Pt NCs outperformed Pd@Pt NCs synthesized without the POMs as well as a commercial Pt/C catalyst for the electrooxidation of methanol, and their catalytic activity and stability distinctly depended on their Pt shell structure. We envision that this strategy will pave the way for the rational design of NC-based catalyst systems.