Edge Facets Research Articles

Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction.

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to β-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals β-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate β-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared β-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Image Analysis Capabilities and Methodologies of Nb3Sn Rutherford Cables

The unique Rutherford cabling facility at the Berkeley Center for Magnet Technology, LBNL, has been leading the production of a range of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cables for a variety of magnet projects domestically and internationally. Cable fabrication is a critical step in accelerator magnet production: to ensure tight dimensional control of the multi-turn coils, fine tolerances must be kept on the cable width, thickness, keystone angle [1] with minimal degradation to the conductor. In addition to these dimensional measurements, we implemented an in-line image acquisition system that can monitor for critical defects such as cross-overs, as well as track the extent of strand deformation at the cable edges. At LBNL, we have collected a large dataset to establish the association between the cable edge facet dimension and the cross-sectional subelement damages. We thus can use image analysis on the facet as an integral and critical quick turn-around-time quality control monitor. Although such measurements are not direct and require a baseline for a given conductor and cable geometry combination, they are non-destructive. They can be measured across the entire length of the cable, which contrasts with critical current or residual resistance ratio measurements, which are only made at the point and tail of the cable and require a lengthy heat-treatment. Moreover, the high frequency of image acquisition coupled with Fast Fourier Transform analysis can give us insights into the key components of the cabling machine as they tend to produce periodic variations within the cable. Such analysis can help find potential faults and inform maintenance planning. This work describes our setup of in-line imaging of Rutherford cables during manufacture, the subsequent image analysis, and data processing. Several case studies illustrate typical usage of the system and lessons learned.

Engineering Lattice Planes of NiCo-LDH Ultrathin Sheets for Boosting Methanol/Ethanol Oxidation Performance.

Alcohol oxidation reactions are known to be significant in the advancement of sustainable, renewable energy sources. Searching for catalytic materials with powerful, reliable, and economic performance is of great importance. Due to their excellent intrinsic performance, outstanding stability, and inexpensiveness, ultrathin layered double hydroxides (LDHs) are considered to be competitive electrocatalysts. However, the electrocatalytic property of ultrathin LDHs is still confined by the predominant exposure of the (003) basal plane. Hence, we have engineered active edge facets in ultrathin NiCo-LDHs, which possess abundant oxygen vacancies (VO), by a facile one-step strategy. Experimental results show that NiCo-LDH-E synthesized in ethanol demonstrates an ultrathin structure, rich oxygen vacancies, and more active facets, exhibiting a higher electrochemical active area of 3.25 cm2, which is 1.18 times that of NiCo-LDH-W (2.75 cm2). In addition, the current density of NiCo-LDH-E in methanol and ethanol oxidation reactions could reach 159.5 and 136.3 mA cm-2, which are 2.8 and 1.7 times that of NiCo-LDH-W, respectively.

Insights into the Role of Graphitic Carbon Nitride as a Photobase in Proton-Coupled Electron Transfer in (sp3 )C-H Oxygenation of Oxazolidinones.

Graphitic carbon nitride (g-CN) is a transition metal free semiconductor that mediates a variety of photocatalytic reactions. Although photoinduced electron transfer is often postulated in the mechanism, proton-coupled electron transfer (PCET) is a more favorable pathway for substrates possessing X-H bonds. Upon excitation of an (sp2 )N-rich structure of g-CN with visible light, it behaves as a photobase-it undergoes reductive quenching accompanied by abstraction of a proton from a substrate. The results of modeling allowed us to identify active sites for PCET-the 'triangular pockets' on the edge facets of g-CN. We employ excited state PCET from the substrate to g-CN to selectively cleavethe endo-(sp3 )C-H bond in oxazolidine-2-ones followed by trapping the radical with O2 . This reaction affords 1,3-oxazolidine-2,4-diones. Measurement of the apparent pKa value and modeling suggest that g-CN excited state can cleave X-H bonds that are characterized by bond dissociation free energy (BDFE) ≈100 kcal mol-1 .

High-Discrimination Circular Polarization Detection Based on Dielectric-Metal-Hybrid Chiral Metamirror Integrated Quantum Well Infrared Photodetectors.

Circular polarization detection enables a wide range of applications. With the miniaturization of optoelectronic systems, integrated circular polarization detectors with native sensitivity to the spin state of light have become highly sought after. The key issues with this type of device are its low circular polarization extinction ratios (CPERs) and reduced responsivities. Metallic two-dimensional chiral metamaterials have been integrated with detection materials for filterless circular polarization detection. However, the CPERs of such devices are typically below five, and the light absorption in the detection materials is hardly enhanced and is even sometimes reduced. Here, we propose to sandwich multiple quantum wells between a dielectric two-dimensional chiral metamaterial and a metal grating to obtain both a high CPER and a photoresponse enhancement. The dielectric-metal-hybrid chiral metamirror integrated quantum well infrared photodetector (QWIP) exhibits a CPER as high as 100 in the long wave infrared range, exceeding all reported CPERs for integrated circular polarization detectors. The absorption efficiency of this device reaches 54%, which is 17 times higher than that of a standard 45° edge facet coupled device. The circular polarization discrimination is attributed to the interference between the principle-polarization radiation and the cross-polarization radiation of the chiral structure during multiple reflections and the structure-material double polarization selection. The enhanced absorption efficiency is due to the excitation of a surface plasmon polariton wave. The dielectric-metal-hybrid chiral mirror structure is compatible with QWIP focal plane arrays.

Porous edge confinement: High carrier potential and low activation energy barrier synergistically boosting the efficiency of selective photocatalytic CO2 conversion

Catalysts which allow directional transfer of photogenerated electrons to catalytic sites are crucial to efficient photocatalytic CO2 reduction. Herein, 2D BiOCl porous nanosheets (BOC-PNS) are prepared by triblock polymer (F127) assisted mechanical ball milling. The main exposed plane is (001) on the BOC-PNS surface and the porous structure increases the edge (110) facet. The (001)/(110) heterojunction enhances directional migration and separation of photogenerated carriers. In situ Raman scattering, in situ Fourier transform infrared spectroscopy, 3D FDTD simulation and theoretical calculations reveal that the BOC (001) plane is enriched with directionally migrating photogenerated electrons and provides the primary active sites to bridge adsorption-activated CO2 molecules consequently producing a smaller energy barrier for the intermediate product of *COOH. Weak CO adsorption on the BOC (001) plane further promotes CO2 reduction. Upon exposure to simulated sunlight, the CO yield of BOC-PNS is enhanced by the rich edge confinement effect reaching 28.2 μmol g−1 h−1, which is 2.1 and 2.8 times that of the BOC nanosheets (13.5 μmol g−1 h−1) and nanoplates (9.9 μmol g−1 h−1), respectively.

Detection band expansion by independently tunable double resonances in a long-wavelength dual-color QWIP.

Multi-resonance light coupling management is a promising way to expand the operating spectral ranges of optoelectronic devices. The classical strategies are either lack of independent tunability for each resonance or involved with complex fabrication. Here, we propose a new scheme for expanding the operating spectral range of an optoelectronic device through a dual-color active material integrated with a simple resonant waveguide structure. The TM waveguide mode and the SPP mode of the resonant waveguide structure are regulated to match the two active regions of the dual-color material both spectrally and spatially. Applying this scheme to a long-wavelength infrared quantum well photodetector, the absorption efficiencies at the two peak detection wavelengths of the dual-color quantum wells are both enhanced by more than 10 times compared with the case of a standard 45° edge facet coupled device with the same detection material. The simple light coupling structure is easy to accomplish and compatible with focal plane arrays. For thermal radiation detection, the absorption efficiency of the 300 K blackbody radiation by our dual-color detector is 83.8% higher than that by a single-color detector with the optimized structural parameters. Moreover, either polarization sensitive or polarization insensitive detection could be achieved in this dual-color infrared quantum well photodetector by using anisotropic or isotropic gratings.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties.

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

Narrow-band and peak responsivity enhanced metal microcavity quantum well infrared detector

The integration of narrow-band and spectral detection capabilities on pixel-level detectors is widely expected for compact infrared gas detection. This imposes great challenges on the detector performance, as the device response must precisely match with the gas absorption spectrum while also collecting enough signals in a limited spectral range to maintain high detection sensitivity. In this paper, a pixel-level narrow-band high quantum efficiency metal microcavity quantum well infrared photodetector (MC-QWIP) working around 10.6 μm is designed and fabricated. The device shows good narrow-band characteristics (200–550 nm) and high peak responsivity (at least eight times stronger than the reference device with 45° edge facet). The results of experiments and numerical simulations show that several different resonance modes with peak wavelengths close to the intrinsic detection wavelength can be obtained by changing the width of the microcavity. The response bandwidth of the device can be controlled by changing resonance modes, while the resonant wavelength can be fine-tuned by the width of the microcavity. This indicates that the MC-QWIP device has good prospects in narrow-band gas detection and narrow-band differential detection.

Metal‐Organic Frameworks‐Derived Nickel–Iron Oxyhydroxide with Highly Active Edge Sites for Electrochemical Oxygen Evolution

Accurate introduction of catalytic active sites to precise locations on the catalyst surface is a challenge in designing and synthesizing high‐efficiency catalysts. Herein, the α phase nickel–iron oxyhydroxide (α‐NiFeOxHy) rich of nickel active edge sites is electrochemically in situ generated from Fe‐square acid metal–organic framework precursor deposited on nickel‐containing electrode matrixes, which revealed superior oxygen evolution reaction performance signified by an overpotential of 167 mV to achieve a current density of 10 mA cm−2 in alkaline electrolytes. Notably, the as‐prepared metal oxyhydroxide exhibits long‐term electrochemical durability in 10 mA cm−2 for over 1080 h. By integrating the electrochemical evidence, Mössbauer spectroscopy, X‐Ray photoelectron spectroscopy, and density functional theory calculations, the nickel species enriched on the exposed edge facet of the as‐synthesized α‐NiFeOxHy are proposed to be the highly catalytic active site. This study provides an expedient and energy‐efficient approach to in situ electrochemical fabrication of high‐performance NiFeOxHy oxygen evolution reaction catalysts from metal‐organic frameworks.

2D SnSe Cathode Catalyst Featuring an Efficient Facet‐Dependent Selective Li2O2 Growth/Decomposition for Li–Oxygen Batteries

Abstract2D materials are attracting much attention in the field of cathode catalysts for lithium–oxygen batteries (LOBs) due to their layered structure, unique electronic properties, and high stability. However, different stacking layer structures trigger different catalytic capabilities in LOBs. In this work, tin selenide nanosheets with a black phosphorus‐like 2D structure are synthesized and used as the cathode catalyst for LOBs. SnSe nanosheets with exposed stack (200) facets and stack edge facets exhibit superior specific capacity over 20 783 mAh g−1 and ultralong cycle stability over 380 cycles at 500 mA g−1 in LOBs. This demonstrates that the growth of discharge products is mainly concentrated on the 2D surface (200) facets, rather than the stack edge facets. Experimental and theoretical studies reveal that the confined adsorption of Li2O2 on the stack edge facets of SnSe, due to the 2D layer structure and the unique electron distribution, restricts the growth of discharge products. The 2D surface facets of SnSe benefit for the formation and stabilization of LiO2 intermediates, leading to the efficient formation/decomposition of discharge products. The findings provide in‐depth insight into the elusive electrocatalytic mechanism for 2D layer‐structures materials in LOBs.

Facet-dependent catalytic activity of two-dimensional Ti3C2Tx MXene on hydrogen storage performance of MgH2

Two-dimensional Ti3C2Tx MXenes exposing different active facets are introduced into MgH2, and their catalytic effects are systematically investigated in depth through experimental and theoretical approaches. Excluding factors such as interlayer space, surface functional groups and experimental contingency, the exposed facets is considered to be the dominant factor for catalytic activity of Ti3C2Tx towards MgH2. More exposed edge facets of Ti3C2Tx displays higher catalytic activity than that with more exposed basal facets, which also leads to different rate-controlling steps of MgH2 in the de/hydrogenation process. The low work function, strong hydrogen affinity and high content of in situ metallic Ti for the edge facet contribute the high catalytic activity. This work will give insights into the structural design of two-dimensional Ti3C2Tx MXene used for enhancing the catalytic activity in various fields.

Optical coupling enhancement of multi-color terahertz quantum well detector

Multi-color terahertz (THz) detector has attracted much attention in various applications because of the ability to obtain more comprehensive information simultaneously. THz quantum well photodetectors (QWPs) have great advantages in realizing multi-color detection because of high speed, sensitivity, and mature technology. In this work, QWPs based on antenna coupled microcavity (AM-QWP) and etched antenna coupled microcavity (EAM-QWP) structures are proposed to realize multi-color THz detection. Thanks to the combination of the microcavity resonance and surface plasmon polariton mode, AM-QWP achieves a coupling efficiency of one order of magnitude higher than that of the conventional 45° edge facet coupler (45°-QWP) in multiple bands. The EAM-QWP only retains the active region where the effective photocurrent is generated so that the coupling light is highly localized in a small area, improving the optical coupling efficiency by two orders higher compared with 45°-QWP. It is theoretically estimated that the responsivity of AM-QWP and EAM-QWP at the temperature of 4 K is 9.6–24.0 A/W and 78.4–196.0 A/W while their noise equivalent power (NEP) is 5.4 × 10−4–1.1 × 10−3 pW/Hz1/2 and 1.7 × 10−5–3.5 × 10−5 pW/Hz1/2, and the specific detectivity is 4.4 × 1012–8.9 × 1012 and 6.9 × 1013–1.4 × 1014 cm Hz1/2/W, respectively. This work provides a guideline for the experimental realization of high-performance multi-color THz QWPs.

Numerical study of gas dynamics and heat transfer in matrix channels at various rib angles

The results of numerical simulation of the separation flow in matrix channels by the RANS method are presented. The simulation is performed at the Reynolds number Re = 12600, determined by the mass-average velocity and the height of the channel. The distribution of the local Nusselt number is obtained for various Reynolds numbers in the range of 5÷15⋅103 and several rib angles. It is shown that the temperature distribution on the surface is highly nonuniform; in particular, the maximum heat transfer value is observed near the upper edge facets, in the vicinity of which the greatest velocity gradient is observed.

Correlative operando microscopy of oxygen evolution electrocatalysts

Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1-3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline β-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an α-CoO2H1.5·0.5H2O-like structure-produced through hydroxide intercalation-in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted β-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.

Quantum well infrared detectors enhanced by faceted plasmonic cavities

Plasmonic cavities can prominently enhance the absorptance of quantum well infrared photodetectors (QWIPs). However, the total thickness of the semiconductor layers is usually in the deep subwavelength scale to ensure an intensified cavity mode, so the devices for the long wavelength infrared range usually contain a single quantum well. Although the relative enhancement is prominent, the absolute absorptance is not larger than common QWIPs and the ohmic loss is serious. Moreover, the ultra-thin semiconductor layer challenges the uniformity and the fabrication. Simply increasing the number of the QW stacks in a plasmonic cavity would probably ruin the critical coupling condition and result in an even worse situation. This work exhibits that creating real facets for the plasmonic cavities by etching away part of the semiconductors can enhance the light confinement capability and make the system with multiple quantum wells still close to a critical coupling status. Then, the quantum well absorptance is enhanced and the ohmic loss is suppressed. As experimentally demonstrated, the responsivity of a faceted plasmonic cavity integrated 4-stack QWIP at the wavelength around 15.5 μm is enhanced by 2.2 times with respect to an unfaceted one, and it is 12 times higher than that of a 45° edge facet coupled QWIP. In addition, the dark current is reduced by 32% due to the reduction of the photosensitive area. With the help of the enhanced light coupling, the specific detectivity D* initially increases with the number of QW stacks, reaches a peak around 3 stacks, and then drops back around 7 stacks to the similar level of the single stack structure.

HKUST-1-derived highly ordered Cu nanosheets with enriched edge sites, stepped (211) surfaces and (200) facets for effective electrochemical CO2 reduction

A novel electrode composed of Cu nanosheets constructed from nanoparticles was synthesized by in situ electrochemical derivation from the metal-organic framework (MOF) HKUST-1. The prepared derivative electrode (HE-Cu) exhibited higher Faradaic efficiency (FE, 56.0%) of electrochemical CO2 reduction (CO2R) compared with that of pristine Cu foil (p-Cu, 32.3%) at an overpotential of −1.03 V vs. a reversible hydrogen electrode (RHE). HE-Cu also exhibited lower onset potential of CO2R as well as inhibiting the H2 evolution reaction. Electrochemical measurements revealed that HE-Cu exhibited higher CO2 adsorption (1.58-fold) and a larger electrochemical active surface area (1.24-fold) compared with p-Cu. Physicochemical characterization and Tafel analysis showed that stepped Cu (211) surfaces, (200) facets and Cu edge atoms on HE-Cu contributed significantly to the enhanced CO2R activity and/or HCOOH and/or C2 product selectivity. The FEs of HCOOH and C2 products for HE-Cu increased 1.57-fold and 10.6-fold at an overpotential of −1.19 V vs. RHE compared with p-Cu. Although CH4 was produced on p-Cu, its formation was totally suppressed on HE-Cu due to the increase of edge sites and (200) facets. Our study demonstrates that electroreduction of MOFs is a promising method to prepare novel and stable electrochemical catalysts with unique surface structures. The fabricated derivative electrode not only promoted electrochemical CO2R activity but also exhibited high C2 product selectivity.

Heterostructure of ZnO Nanosheets/Zn with a Highly Enhanced Edge Surface for Efficient CO2 Electrochemical Reduction to CO.

Electrochemical reduction of CO2 to valuable chemicals or fuels is critical for closing the carbon cycle and preventing further deterioration of the environment. Here, we discover that by adopting the Zn foil as the substrate, a ZnO two-dimensional sheet array is in situ synthesized on the Zn foil by a facile hydrothermal method. The obtained ZnO sheet array/Zn foil exhibited an outstanding CO2 reduction performance to CO, which showed the highest Faraday efficiency of 85% for CO at -2.0 V (vs Ag/AgCl) with a current density of 11.5 mA/cm2 compared with the freestanding ZnO sheets and particles and excellent stability in the 0.1 M KHCO3 electrolyte. The in situ vertical ZnO sheet array exposed with abundant exposed (11̅00) edge facets can accelerate the electron transfer and improve the number of active sites, which leads to the enhanced reduction performance. Alongside, the density functional theory simulation indicated that the vertical-grown ZnO sheet array possesses lower Gibbs free energy for the CO2 activation, with a more exposed (11̅00) edge surface of ZnO.

Effect of Fluoroethylene Carbonate Additives on the Initial Formation of the Solid Electrolyte Interphase on an Oxygen-Functionalized Graphitic Anode in Lithium-Ion Batteries.

The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen-functionalized graphitic anodes to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen-functionalized graphitic (112̅0) edge facet through a nucleophilic attack on an ethylene carbon site (CE) of an EC molecule (S2 mechanism) is spontaneous during the initial charging process of LIBs. However, decomposition of EC through a nucleophilic attack on a carbonyl carbon (CC) site (S1 mechanism) results in alkoxide species regeneration that is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an S1 pathway, which does not promote alkoxide regeneration. Including FEC as an additive is thus able to suppress alkoxide regeneration and results in a smaller and thinner SEI layer that is more flexible toward lithium intercalation during the charging/discharging process. In addition, we find that the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and the LiF formation mechanism in the SEI.

Van Hove Singularities and Trap States in Two-Dimensional CdSe Nanoplatelets.

Semiconductor nanoplatelets, which offer a compelling combination of the flatness of two-dimensional semiconductors and the inherent richness brought about by colloidal nanostructure synthesis, form an ideal and general testbed to investigate fundamental physical effects related to the dimensionality of semiconductors. With low temperature scanning tunnelling spectroscopy and tight binding calculations, we investigate the conduction band density of states of individual CdSe nanoplatelets. We find an occurrence of peaks instead of the typical steplike function associated with a quantum well, that rule out a free in-plane electron motion, in agreement with the theoretical density of states. This finding, along with the detection of deep trap states located on the edge facets, which also restrict the electron motion, provides a detailed picture of the actual lateral confinement in quantum wells with finite length and width.