Active Edge Research Articles

Insight for Designing Mass-Efficient Metal-Oxide-Supported Heterogeneous Catalyst from the Identification of the Catalytically Active Edge Sites Using Isotopically Labeled 13CO and 18O2

Insight for Designing Mass-Efficient Metal-Oxide-Supported Heterogeneous Catalyst from the Identification of the Catalytically Active Edge Sites Using Isotopically Labeled ¹³CO and ¹⁸O₂

Decoration of X2C nanoparticles on CdS nanostructures for highly efficient photocatalytic wastewater treatment under visible light

It is necessary to design and fabricate effective and stable heterostructures to enhance the photocatalyst performance. In this research, we developed stable metal carbide (Mo2C and W2C) embedded cadmium sulfide (CdS) hybrid nanostructure photocatalysts to increase the light adsorption capacity and effectively enhance the photocatalytic activity.A simple hydrothermal technique was employed to effectively fabricate the CdS and CdS:X2C (X = Mo or W) hybrid photocatalyst. Microscopy studies demonstrated the surface modification of the hybrid structure due to the X2C incorporation with CdS. X-ray photoelectron spectroscopy, X-ray diffraction, and energy dispersive analysis measurements were performed to characterize the hybrid structure materialization. The photocatalytic properties of CdS and CdS:X2C hybrid structures were evaluated by degrading Rhodamine B (RhB) under visible light. The polygonal shaped CdS:X2Cexhibited excellent photocatalytic activities. The highly porous structure of CdS:X2C indicated that manyexposed active facets were available due to the embedding of petal-shaped X2C, which enabled the effective adsorption and transference of RhB organic molecules to the active edges. Moreover, the hierarchical structure of CdS:X2Ccorresponded to a high light-absorption efficiency due to the increased length of light trails and enhanced interfacial shifting of the valence state electrons, which reduced the optical band-gap.

Electrochemically exfoliated Ni-doped MoS2 nanosheets for highly efficient hydrogen evolution and Zn-H2O battery

Thanks to tunable physical and chemical properties, two-dimensional (2D) materials have received intensive interest, endowing their excellent electrocatalytic performances for applications in energy conversion. However, their catalytic activities are largely determined by poor adsorption energy and limited active edge sites. Herein, a one-step electrochemical exfoliation strategy was developed to fabricate 2D Ni-doped MoS2 nanosheets (Ni-EX-MoS2) with a lateral size of ∼500 nm and thickness of ∼3.5 nm. Profiting from high electrical conductivity and abundant exposing active sites, Ni-EX-MoS2 catalyst displayed an admirable performance for electrochemical hydrogen evolution reaction (HER) with a low overpotential of 145 mV at 10 mA/cm2 as well as a small Tafel slope of 89 mV/dec in alkaline media, which are superior to those of the most reported MoS2-based electrocatalysts. The formed Ni species with tuning electronic structure played a crucial role as primary active center of Ni-EX-MoS2, as well as the forming stable 1T/2H phase MoS2 interface demonstrated a synergistic effect on electrocatalytic HER performance. Further, Ni-EX-MoS2 was employed as a cathode electrode for alkaline Zn-H2O battery, which displayed a high power density of 3.3 mW/cm2 with excellent stability. This work will provide a simple and effective guideline for design of electrochemically exfoliated transition metal-doped MoS2 nanosheets to inspire their practical applications in energy catalytic and storage.

A bubble-assisted strategy to prepare porous ultrathin carbon nitride for highly-active photocatalytic hydrogen production

Porous and ultrathin polymetric carbon nitride has recently emerged as a promising photocatalyst. However, the complex synthesis strategy of porous ultrathin carbon nitride (PTCN) impedes its further application in photocatalytic water splitting. It is a challenge to find a facile and powerful synthesis strategy of porous ultrathin polymetric carbon nitride. In this work, we demonstrate a bubble-assisted strategy for porous ultrathin polymetric carbon nitride. The thermal polymerization process under the blowing effect of NH4Cl endows PTCN with the porous and ultrathin structure. The larger accessible specific surface area (SSA) provides abundant active edges and sites, decreasing the diffusion distance of reactants and carriers. Compared to the bulk carbon nitride (bulk CN), PTCN possesses higher SSA, a broader bandgap, and improved photogenerated carriers transfer efficiency. In addition, the calculation of Gibbs free energy manifests the improved hydrogen absorption ability. As a result, a remarkable hydrogen evolution rate at 22.0 mmol h−1 g−1 is achieved, which is 6 times higher than that of bulk carbon nitride.

The effect of temperature on water desorption and oxide formation in MoS2 coatings and its impact on tribological properties

Molybdenum disulfide (MoS2) is used extensively in aerospace applications, where liquid lubricants are not suitable. MoS2 has excellent tribological properties in vacuum, however, further research is required to fully understand the mechanisms taking place when exposed to varying temperatures in ambient atmosphere due to storage and on-ground testing. MoS2 coatings were deposited on AISI 440C steel substrates, friction and wear tests were conducted at increasing temperatures, up to 75 °C, determining the effect of temperature on wear mechanisms in air. It was found that the detected surface oxides do not impact tribological properties in MoS2 at temperatures around 75 °C. The water molecules at room temperature hinder the alignment of MoS2 layers leaving some active edge sites exposed to contacting surfaces, leading to high friction and wear. Desorption of water molecules promote parallel alignment of layers, reducing shear strength, friction and wear.

The effect of avoiding known infected neighbors on the persistence of a recurring infection process

We study a generalization of the classical contact process (SIS epidemic model) on a directed graph G. Our model is a continuous-time interacting particle system in which at every time, each vertex is either healthy or infected, and each oriented edge is either active or inactive. Infected vertices become healthy at rate 1 and pass the infection along each active outgoing edge at rate λ. At rate α, healthy individuals deactivate each incoming edge from their infected neighbors, and an inactive edge becomes active again as soon as its tail vertex becomes healthy. When α=0, this model is the same as the classical contact process on a static graph. We study the persistence time of this epidemic model on the lattice Z, the n-cycle Zn, and the n-star graph. We show that on Z, for every α>0, there is a phase transition in λ between almost sure extinction and positive probability of indefinite survival; on Zn we show that there is a phase transition between poly-logarithmic and exponential survival time as the size of the graph increases. On the star graph, we show that the survival time is nΔ+o(1) for an explicit function Δ(α,λ) whenever α>0 and λ>0. In the cases of Z and Zn, our results qualitatively match what has been shown for the classical contact process, while in the case of the star graph, the classical contact process exhibits exponential survival for all λ>0, which is qualitatively different from our result. This model presents a challenge because, unlike the classical contact process, it has not been shown to be monotonic in the infection parameter λ or the initial infected set.

Hierarchical S-modified Cu porous nanoflakes for efficient CO2 electroreduction to formate.

Electrochemical reduction of CO2 into liquid fuels is a promising approach to achieving a carbon-neutral energy cycle but remains a great challenge due to the lack of efficient catalysts. Here, the hierarchical architectures assembled by ultrathin and porous S-modified Cu nanoflakes (Cu-S NFs) are designed and constructed as an efficient electrocatalyst for CO2 conversion to formate with high partial current density. Specifically, when integrated into a gas diffusion electrode in a flow cell, Cu-S NFs are capable of delivering the ultrahigh formate current density up to 404.1 mA cm-2 with a selectivity of 89.8%. Electrochemical tests and theoretical calculations indicate that the superior performance of the designed catalysts may be attributed to the unique structure, which can provide abundant active sites, fast charge transfer, and highly active edge sites.

Realization of Oriented and Nanoporous Bismuth Chalcogenide Layers via Topochemical Heteroepitaxy for Flexible Gas Sensors.

Most van der Waals two-dimensional (2D) materials without surface dangling bonds show limited surface activities except for their edge sites. Ultrathin Bi2Se3, a topological insulator that behaves metal-like under ambient conditions, has been overlooked on its surface activities. Herein, through a topochemical conversion process, ultrathin nanoporous Bi2Se3 layers were epitaxially deposited on BiOCl nanosheets with strong electronic coupling, leading to hybrid electronic states with further bandgap narrowing. Such oriented nanoporous Bi2Se3 layers possessed largely exposed active edge sites, along with improved surface roughness and film forming ability even on inkjet-printed flexible electrodes. Superior room-temperature NO2 sensing performance was achieved compared to other 2D materials under bent conditions. Our work demonstrates that creating nanoscale features in 2D materials through topochemical heteroepitaxy is promising to achieve both favorable electronic properties and surface activity toward practical applications.

A Conductive Dinuclear Cuprous Complex Mimicking the Active Edge Site of the Copper(100)/(111) Plane for Selective Electroreduction of CO2 to C2H4 at Industrial Current Density.

Inorganic solids are a kind of important catalysts, and their activities usually come from sparse active sites, which are structurally different from inactive bulk. Therefore, the rational optimization of activity depends on studying these active sites. Copper is a widely used catalyst and is expected to be a promising catalyst for the electroreduction of CO2 to C2H4. Here, we report a conductive dinuclear cuprous complex with a short Cu···Cu contact for the electroreduction of CO2 to C2H4. By using 1H-[1,10]phenanthrolin-2-one and Cu(I) ions, a dinuclear cuprous complex [Cu2(ophen)2] (Cuophen) with a remarkable conductivity (3.9 × 10-4 S m-1) and a short intramolecular Cu···Cu contact (2.62 Å) was obtained. Such a short Cu···Cu contact is close to the distance of 2.54 Å between 2 adjacent Cu atoms in the edge of the copper(100)/(111) plane. Detailed examination of Cuophen revealed a high activity for the electroreduction of CO2 to C2H4 with a Faradaic efficiency of 55(1)% and a current density of 580 mA cm-2, and no obvious degradation was observed over 50 h of continuous operation. Comparing the properties and mechanisms of Cuophen and 2 other copper complexes with different Cu···Cu distances, we found that the shorter Cu···Cu distance is conducive not only for a *CO species to bridge 2 copper ions into a more stable intermediate transition state but also for C-C coupling.

Accelerating Surface Photoreactions Using MoS2-FeS2 Nanoadsorbents: Photoreduction of Cr(VI) to Cr(III) and Photodegradation of Methylene Blue.

Nanocubic MoS2-FeS2, as a photocatalyst, was synthesized with high catalytic active edges and high specific surface areas with the capability of absorbing visible light. The results showed that the photocatalytic efficiency of nanocubic MoS2-FeS2 for adsorption/degradation of methylene blue (MB) as well as the reduction of Cr(VI) was high. The adsorption process was found to follow a kinetic model of a pseudo-second-order kind (Qe.cal = 464 mg g-1) along with an isotherm described by the Langmuir model with Qe.cal = 340 mg g-1. The photodegradation process was achieved by holes. It was found that the photodegradation rate constant of MB by MoS2-FeS2 (0.203 min-1) was about 22 times higher than that of MoS2 (0.0091 min-1). The percent apparent quantum yield for photoreduction of Cr(VI) to Cr(III) using MoS2-FeS2 (5.7%) was about 33 times higher than utilizing MoS2 (0.1709%). Therefore, the synergistically prolonged visible-light harvesting as well as the photocarrier diffusion length proved that MoS2-FeS2 nanotubes can effectively be utilized in environmental pollutant's remediation.

Ultrasonically derived WSe2 nanostructure embedded MXene hybrid composites for supercapacitors and hydrogen evolution reactions

Two-dimensional (2D) transition metal chalcogenides (TMDCs) and carbide have validated boundless prospective as multi-functional constituents for high performance energy storing and/or conversion devices. However, pure TMDCs exhibit poor performance for electrochemical applications due to low electrical conductivity, scarce electrochemically active edges and inadequate cycling stability. Therefore, this paper fabricated MXene/WSe2 hybrids with strong interfacial interactions and conductivities using a one-step chemical reaction as electrodes for supercapacitors and hydrogen evolution. Fabricated supercapacitors achieved high specific capacitance = 840 F g−1 at 2 A g−1, with symmetric capacitance = 246 F g−1 at 2 A g−1 for MXene/WSe2 hybrids. Hydrogen evolution achieved low overpotentials = 76 and 62 mV to drive 10 mA cm−2 current with small Tafel slopes = 78 and 84 mV.dec−1 in acid and base media, respectively. 2D and 2D hybrid WSe2 nanoparticle composite embedded MXene scaffolds achieved excellent electron/ion intercalation owing to its distinctive 2D-layered structure, increasing interlayer spacing and retaining large electrode/electrolyte contact to enhance efficiency. Two-dimensional TMDCs/MXene composites were verified as potentially efficient electrode materials for energy storing and exchange uses.

Distributed computation and reconfiguration in actively dynamic networks

We study here systems of distributed entities that can actively modify their communication network. This gives rise to distributed algorithms that apart from communication can also exploit network reconfiguration to carry out a given task. Also, the distributed task itself may now require a global reconfiguration from a given initial network G_s to a target network G_f from a desirable family of networks. To formally capture costs associated with creating and maintaining connections, we define three edge-complexity measures: the total edge activations, the maximum activated edges per round, and the maximum activated degree of a node. We give (poly)log(n) time algorithms for the task of transforming any G_s into a G_f of diameter (poly)log(n), while minimizing the edge-complexity. Our main lower bound shows that varOmega (n) total edge activations and varOmega (n/log n) activations per round must be paid by any algorithm (even centralized) that achieves an optimum of varTheta (log n) rounds. We give three distributed algorithms for our general task. The first runs in O(log n) time, with at most 2n active edges per round, a total of O(nlog n) edge activations, a maximum degree n-1, and a target network of diameter 2. The second achieves bounded degree by paying an additional logarithmic factor in time and in total edge activations. It gives a target network of diameter O(log n) and uses O(n) active edges per round. Our third algorithm shows that if we slightly increase the maximum degree to polylog(n) then we can achieve o(log ^2 n) running time.

Colloidal Synthesis of MoSe2/WSe2 Heterostructure Nanoflowers via Two-Step Growth.

The ability to control the active edge sites of transition metal dichalcogenides (TMDs) is crucial for modulating their chemical activity for various electrochemical applications, including hydrogen evolution reactions. In this study, we demonstrate a colloidal synthetic method to prepare core-shell-like heterostructures composed of MoSe2 and WSe2 via a two-step sequential growth. By overgrowing WSe2 on the surface of preexisting MoSe2 nanosheet edges, MoSe2-core/WSe2-shell heterostructures were successfully obtained. Systematic comparisons of the secondary growth time and sequential order of growth suggest that the low synthetic temperature conditions allow the stable overgrowth of shells rich in WSe2 on top of the core of MoSe2 with low Gibbs formation energy. The electrochemical analysis confirms that the catalytic activity correlates to the core-shell composition variation. Our results propose a new strategy to control the edge site activity of TMD materials prepared by colloidal synthesis, which is applicable to diverse electrochemical applications.

Prevalence, Circumstances, and Risk Factors of Falls Among Community Dwelling Members of University of the Third Age.

Objective: Study aimed to identify the prevalence of falls and associated contributory factors among older Malaysians.Methods: A cross sectional study among community dwelling older adults aged 50 years and above. Self-administered questionnaires on history of falls in the previous 12 months, physical assessment and computerized and clinical measures of balance were assessed on a single occasion.Results: Forty nine (31.0%) participants fell, with 4.4% reported having multiple falls within the previous 12 months. Slips were the most prevalent cause of falls, accounting for 49% of falls. More than half (54.5%) of falls occurred in the afternoon while participants walked inside the home (32.7%), outside home (30.6%), and 36.7% were in community areas. More than half of respondents were identified as having turning instability. Step Test, turn sway, depression, physical activity level and edge contrast sensitivity were significantly worse for fallers (p < 0.05). Multiple logistic regression analysis showed that turning performance, visual acuity and back pain were significantly associated with falls risk, accounting for 72% of the variance of risk factors for falls among studied population.Conclusion: Falls are common among community dwelling older Malaysians. The findings provide information of falls and falls risk factors among community dwelling older adults in Malaysia. Future intervention studies should target locally identified falls risk factors. This study has highlighted the importance of instability during turning as an important fall risk factor.

Active edge effect of pothole-rich WO3 nanosheets for enhancing dimethyl trisulfide gas sensing performance

The gas sensing mechanism of metal oxides has always been the research focus in the field of gas sensors. Herein, a novel gas sensing mechanism based on the active edge effect was proposed to account for the difference in gas sensing performance between pothole-rich WO3 and non-porous WO3 nanosheets. The gas sensor based on pothole-rich WO3 exhibited a higher response (9.5) than non-porous WO3 (6.8) when exposed to 500 ppb C2H6S3, which could be ascribed to the active edge sites on the pothole-rich WO3 nanosheets. DFT calculations and XPS confirmed that the edge sites showed higher adsorption energy and more charge transfer for C2H6S3gas, and W-S bond was formed at the edge site due to the strong chemical adsorption.

Ultrasensitive biochemical sensors based on controllably grown films of high-density edge-rich multilayer WS2 islands

Preferentially exposed active edges are advantageous in the application of two-dimensional materials (2DMs) in high-performance biochemical sensors. Despite current progresses, controllable large-scale production of edge-rich 2DM films for practical applications remains a challenge. In this work, we report the controllable single-step growth of wafer-scale edge-rich WS2 films of high-density nanoscale multilayer WS2 islands and their application in sensitive, low detection limit biochemical sensors. The abundant edge active sites are shown to be crucial to their superior biochemical sensing performance. Investigations reveal that high-density edge S-active sites enable the highly sensitive attomolar-level Hg2+ detection as direct target probes via selective Hg-S bonding. Furthermore, 1 fM level selective detection of target viral nucleic acids (RdRp, ORF1ad) are achieved via functionalization of active edge sites with cDNA receptors. Our investigations provide a comprehensive method for application of similar edge-rich 2DM films in high-performance sensors for broad ranges of analytes.

Graphene films on MoS2/SiO2/Si substrate for current density performance

Photoelectrochemical (PEC) water splitting, that could directly turn sunlight into hydrogen energy, is one of the effective ways for utilising solar energy. Two-dimensional (2D) graphene and transition metal dichalcogenides (TMDCs) have become effective materials for the advancement of photocatalyst due to their different thermal, electrical, optical, and mechanical properties. 2D graphene can be used as a beneficial support or substrate for catalyst, which not only prevents the catalyst from accumulating but also provides more active sites for the catalytic reaction. As a typical layer-structured of TMDC, MoS2 is considered to be a good semiconductor for PEC water splitting due to its highly active edge sites for effective photocatalytic activity. In this study, chemical vapor deposition was used to grow graphene sheets, which were then transferred onto vertical MoS2 nanoflakes on a SiO2/Si substrate using standard transfer process. Raman spectrum confirmed the presence of the graphene and MoS2 on SiO2/Si. The other morphologies and microstructures have been determined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). Current density performed using PEC measurement system indicates that the light current density under the illumination of sunlight on synthesized graphene/MoS2/SiO2/Si is higher compared to the dark current density. The enhanced in current density can be ascribed to the improved light absorption, strong light matter interaction and the capable charge separation of the heterostructure. Thus, graphene/MoS2/SiO2/Si has capability as a photocatalyst in photoelectrochemical water splitting.

Low loaded MoS2/Carbon cloth as a highly efficient electrocatalyst for hydrogen evolution reaction

Active edge sites of MoS2 nanosheets exhibit promising futures for hydrogen evolution reaction (HER), comparable with remarkable performances of highly cost platinum. However, 3D structures of MoS2 suffer from a lack of high mobility and unexposed active sites which lower the electrocatalytic activity. In this study, we show that there is a balance between increasing the active sites on the one hand and managing the charge transfer to facilitate the reaction on the other hand, and achieving this balance increases the efficiency of the electrocatalyst tremendously. For this purpose, we directly attached exfoliated MoS2 nanosheets onto carbon cloth (CC) substrate as a 3D network of conductive fibers via electrophoretic deposition at different applied voltages and deposition times, without adding any binder. This strategy gives rise to superior exposure of active sites while still maintaining good charge transferability over the whole 3D structure. Thus, a trace amount of loaded catalyst is enough to reach the benchmark current density of 10 mA/cm2 towards H2 production with a low overpotential of 137 mV vs. RHE. The Stability of the optimum structures under continuous operation was examined up to 50 min and further confirmed with 500 successive cyclic voltammetry (CV) sweeps. To understand the adsorption nature of hydrogen, density functional theory (DFT) was employed for the MoS2/graphene in both cases of pristine and defected MoS2. The results of hydrogen adsorption free energy calculations revealed that H adsorption on the S site is the most stable adsorption configuration and with increasing the MoS2 thickness, the MoS2/graphene activity towards hydrogen evolution decreases. A similar trend is also observed for the defected MoS2/graphene composite up to two-layer MoS2 and the activity remains the same for three-layer MoS2. The experimentally observed charge transfer into the MoS2 upon adsorption of the hydrogen atom and water molecule is also confirmed by our DFT calculations.

The Effect of Fe Dopant Location in Co(Fe)OOHx Nanoparticles for the Oxygen Evolution Reaction.

The addition of iron (Fe) can in certain cases have a strong positive effect on the activity of cobalt and nickel oxide nanoparticles in the electrocatalytic oxygen evolution reaction (OER). The reported optimal Fe dopant concentrations are, however, inconsistent, and the origin of the increased activity due to Fe dopants in mixed oxides has not been identified so far. Here, we combine density functional theory calculations, scanning tunneling microscopy, and OER activity measurements on atomically defined Fe-doped Co oxyhydroxide nanoparticles supported on a gold surface to establish the link between the activity and the Fe distribution and concentration within the oxyhydroxide phase. We find that addition of Fe results in distinct effects depending on its location on edge or basal plane sites of the oxyhydroxide nanoparticles, resulting in a nonlinear OER activity as a function of Fe content. Fe atom substitution itself does not lead to intrinsically more active OER sites than the best Co sites. Instead, the sensitivity to Fe promoter content is explained by the strong preference for Fe to locate on the most active edge sites of oxyhydroxide nanoparticles, which for low Fe concentrations stabilizes the particles but in higher concentrations leads to a shell structure with less active Fe on all edge positions. The optimal Fe content thereby becomes dependent on nanoparticle size. Our findings demonstrate that synthesis strategies that adjust not only the Fe concentration in mixed oxides but also its distribution within a catalyst nanoparticle can lead to enhanced OER performance.

Alumina abrasive wheel wear in ultrasonic vibration-assisted creep-feed grinding of Inconel 718 nickel-based superalloy

Blade root with a special fir-tree shape is a critical connection part in an aero-engine. This part has high requirements on the surface finish and geometrical accuracy. At present, the tangential ultrasonic vibration-assisted profile grinding (UVAPG) is an effective method for machining the blade roots. During UVAPG, the wheel wear degrades the surface finish and geometrical accuracy. However, the characteristics of the wheel wear in UVAPG are not fully revealed. This study proposes an analytical wear model by considering the intermittent grinding behavior of a grinding edge, the number of active grinding edges, and the effects of wheel profile on the tool wear. The model is experimentally demonstrated to be applicable to predicting the wear volume in UVAPG with a maximum error of 4.2 %. Based on the model, the tangential ultrasonic vibration increases the average unchanged chip thickness and the number of active grinding edges. UVAPG causes an approximately 22 % increase in wear volume at the steady wear stage compared with the conventional grinding. The wear speeds vary at different regions of a profile wheel. The convex regions represent higher wear speeds than the concave regions on the wheel.