Barrier For Electron Transfer Research Articles

Insights on forming N,O-coordinated Cu single-atom catalysts for electrochemical reduction CO2 to methane

Single-atom catalysts (SACs) are promising candidates to catalyze electrochemical CO2 reduction (ECR) due to maximized atomic utilization. However, products are usually limited to CO instead of hydrocarbons or oxygenates due to unfavorable high energy barrier for further electron transfer on synthesized single atom catalytic sites. Here we report a novel partial-carbonization strategy to modify the electronic structures of center atoms on SACs for lowering the overall endothermic energy of key intermediates. A carbon-dots-based SAC margined with unique CuN2O2 sites was synthesized for the first time. The introduction of oxygen ligands brings remarkably high Faradaic efficiency (78%) and selectivity (99% of ECR products) for electrochemical converting CO2 to CH4 with current density of 40 mA·cm-2 in aqueous electrolytes, surpassing most reported SACs which stop at two-electron reduction. Theoretical calculations further revealed that the high selectivity and activity on CuN2O2 active sites are due to the proper elevated CH4 and H2 energy barrier and fine-tuned electronic structure of Cu active sites.

Grain-Boundary-Engineered La2CuO4 Perovskite Nanobamboos for Efficient CO2 Reduction Reaction.

Electroreduction of carbon dioxide (CO2RR) has been regarded as a promising approach to realize the production of useful fuels and to decrease greenhouse gas levels simultaneously, where high-efficiency catalysts are required. Herein, we report La2CuO4 nanobamboo (La2CuO4 NBs) perovskite with rich twin boundaries showing a high Faraday efficiency (FE) of 60% toward ethylene (C2H4), whereas bulk La2CuO4 exhibits a FECO of 91%. X-ray absorption spectroscopy (XAS) reveals that the Cu in La2CuO4 NBs is in the Cu2+ state, and no obvious change can be observed during the catalytic process, as monitored by in situ XAS. Density functional theory calculations reveal that the superior FEC2H4 of La2CuO4 NBs originates from the active (113) surfaces with intrinsic strain. The formation of gap states annihilates the electron transfer barrier of C-C coupling, resulting in the high FEC2H4. This work provides a new perspective for developing efficient perovskite catalysts via grain boundary engineering.

Synchronized ion and electron transfer in a blue T-Nb2O5-x with solid-solution-like process for fast and high volumetric charge storage

The practical application of supercapacitor is largely hindered by insufficient volumetric energy density. Herein, a pseudocapacitive material with high volumetric specific capacity is achieved by introducing oxygen deficiency into T-Nb2O5, which produces a blue T-Nb2O5-x material with simultaneously low barrier for ion and electron transfer. By carrying out over-discharge experiments combined with in-situ X-ray diffraction and transmission electron microscopy, we further revealed that a solid-solution like electrochemical process without substantial structural change and internal stress provides the structural foundation for the reversible electrochemical process. The blue T-Nb2O5-x could achieve a particle-level volumetric specific capacity of 907.9 mAh/cm3 at 10 mA/g, much higher than traditional pseudocapacitive materials that only work with their shallow surfaces. The intrinsic bulk lithium storage mechanism endows the material with an excellent electrode-level volumetric specific capacity of 141.0 mAh/cm3 within 1.2-minute discharge. An asymetric lithium-ion capacitor was assembled using active carbon as cathode to demonstrate its performance in practical device.

Secondary Metal Coordination Using a Tetranuclear Complex as Ligand Leading to Hexanuclear Complexes with Enhanced Thermal Barriers for Electron Transfer

Postsynthesis of the paramagnetic square-shaped complex {[(Tp*Me)Fe(μ-CN)2(CN)][Co(dmbpy)2]}2(BPh4)2·6MeCN·H2O [1, Tp*Me = tris(3,4,5-trimethylpyrazole)-borate; dmbpy = 4,4′-dimethyl-2,2′-bipyridin...

The spatially oriented redistribute of photogenerated carriers and photocatalytic hydrogen evolution mechanism research on polymeric carbon nitride Van der Waals homojunction

The weaker interlayer Van der Waals force of polymeric carbon nitride (PCN) increases the potential barrier of electron transfer between layers, which greatly inhibits the carrier separation efficiency of PCN. Herein, we designed and prepared Van der Waals homojunction of PCN (VHPCN), which the interlayer Van der Waals force forces were instead by Coulomb force. DFT calculated reveals that the equilibrium distance and the interface adhesion energy of VHPCN were significantly decreased and enhanced, respectively, under the function of Coulomb force. The equilibrium distance decreased from 3.27 Å to 2.49 Å and the interface adhesion energy enhanced from −0.35 eV to −1.78 eV. This structure not only the transfer distance of electrons at the interface is effectively decreased, but also the transfer barrier of electrons at the interface is reduced. Thus, the electrons/holes pairs in the VHPCN photocatalyst present the phenomenon of oriented interlayer transfer and ultrahigh carrier separation efficiency under the synergistic effect between the built-in electric field and the interlayer Coulomb force of the Van der Waals homojunction. The optimal VHPCN exhibited the photocatalytic H2 evolution rate and apparent quantum yield (AQY), which are 7.9 mmol·g−1·h−1 and 32.4% (>420 nm), respectively. This work put forward a promising strategy to construct novel Van der Waals homojunction within 2D materials for the desirable photogeneration carrier spatially distribution controlling, which could be generalized to other 2D materials for photocatalytic applications in the advanced energy and environmental field.

Solvent influence on non-adiabatic interfacial electron transfer at conductive oxide electrolyte interfaces.

The kinetics for interfacial electron transfer (ET) from a transparent conductive oxide (tin-doped indium oxide, ITO, Sn:In2O3) to molecular acceptors 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid, TPA, and [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+, RuP, positioned at variable distances within and beyond the electric double layer (EDL), were quantified in benzonitrile and methanol by nanosecond absorption spectroscopy as a function of the thermodynamic driving force, -ΔG°. Relevant ET parameters such as the rate constant, ket, reorganization energy, λ, and electronic coupling, Hab, were extracted from the kinetic data. Overall, ket increased as the distance between the molecular acceptor and the conductor decreased. For redox active molecules within the Helmholtz planes of the EDL, ket was nearly independent of -ΔG°, consistent with a negligibly small λ value. Rips-Jortner analysis revealed a non-adiabatic electron transfer mechanism consistent with Hab < 1 cm-1. The data indicate that the barrier for electron transfer is greatly diminished at the conductor-electrolyte interface.

Electrochemical investigation on adsorption kinetics of long chain alkylsilanes and influence of solvents on their self-assembly and electron transfer behavior on indium tin oxide

In situ adsorption kinetics of hexadecyltrimethoxy silane (HDTMS) and octadecyltrimethoxy silane (ODTMS) on indium tin oxide (ITO) surface at short time intervals were measured by electrochemical impedance (EIS) spectroscopy technique at constant frequency. The studies suggest that, self-assembly of silanes on ITO proceeds through two distinct adsorption steps, a fast first step followed by a slower second step. The inhibition of electron transfer by self-assembled silanes prepared in ethanol, toluene and hexane solvents were evaluated by cyclic voltammetry (CV) and EIS using [Fe(CN)6]3−/4− and [Ru(NH3)6]3+/2+ redox species. It is shown that, the quality of silane films are influenced by the polarity of solvent, chain length of silane and ITO substrate–solvent interactions. The effect of annealing on electron transfer barrier property of silane monolayers after their formation were studied. The pinhole characteristics of silane films on ITO prepared from various solvents under study were measured using EIS. This study demonstrates a strategy of producing an ultra-smooth and close-packed silane films on ITO surfaces which can have potential applications towards high performance organic electronics.

Kinetic Evidence That the Solvent Barrier for Electron Transfer Is Absent in the Electric Double Layer.

Classical capacitance studies have revealed that the first layer of water present at an aqueous metal-electrolyte interface has a dielectric constant less than 1/10th of that of bulk water. Modern theory indicates that the barrier for electron transfer will decrease substantially in this layer; yet, this important prediction has not been tested experimentally. Here, we report the interfacial electron transfer kinetics for molecules positioned at variable distances within the electric double layer of a transparent conductive oxide as a function of the Gibbs free energy change. The data indicate that the solvent reorganization is indeed near zero and increases to bulk values only when the molecules are positioned greater than 15 Å from the conductive electrode. Consistent with this conclusion, lateral intermolecular electron transfer, parallel to a semiconducting oxide electrode, was shown to be more rapid when the molecules were within the electric double layer. The results provide much needed feedback for theoretical studies and also indicate a huge kinetic advantage for aqueous electron transfer and redox catalysis that takes place proximate to a solid interface.

Synergistic Use of Pyridine and Selenophene in a Diketopyrrolopyrrole‐Based Conjugated Polymer Enhances the Electron Mobility in Organic Transistors

AbstractTo achieve semiconducting materials with high electron mobility in organic field‐effect transistors (OFETs), low‐lying energy levels (the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) and favorable molecular packing and ordering are two crucial factors. Here, it is reported that the incorporation of pyridine and selenophene into the backbone of a diketopyrrolopyrrole (DPP)‐based copolymer produces a high‐electron‐mobility semiconductor, PDPPy‐Se. Compared with analogous polymers based on other DPP derivatives and selenophene, PDPPy‐Se features a lower LUMO that can decrease the electron transfer barrier for more effective electron injection, and simultaneously a lower HOMO that, however, can increase the hole transfer barrier to suppress the hole injection. Combined with thermal annealing at 240 °C for thin film morphology optimization to achieve large‐scale crystallite domains with tight molecular packing for effective charge transport along the conducting channel, OFET devices fabricated with PDPPy‐Se exhibit an n‐type‐dominant performance with an electron mobility (μe) as high as 2.22 cm2 V−1 s−1 and a hole/electron mobility ratio (μh/μe) of 0.26. Overall, this study demonstrates a simple yet effective approach to boost the electron mobility in organic transistors by synergistic use of pyridine and selenophene in the backbone of a DPP‐based copolymer.

Effects of different exposed-facets on photocatalytic activity of BiOBr /Bi2Ti2O7 heterostructure and mechanism analysis

In this paper, the novel heterojunctions of Bi2Ti2O7 and BiOBr with different crystal facets ({001} and {010}) were synthesized by in-situ precipitation method under room temperature. The structure and properties of the materials were analyzed. The photodecomposition properties of the materials were studied by using rhodamine B (RhB) and ciprofloxacin (CIP) as targets. The possible mechanism of the difference of facet-engineering on photocatalytic degradation was studied. The XPS results indicated that the high-quality interfacial connection existed in Bi2Ti2O7/BiOBr heterostructure. It was found that the RhB and CIP degradation rates of Bi2Ti2O7/BiOBr–{010} photocatalyst are 2.05 and 1.82 times higher respectively than that of Bi2Ti2O7/BiOBr–{001} under visible light. This enhancement was mainly attributed that laminate BiOBr dominating {010} exposed-facet with higher the specific area (SBET) combined with spherical Bi2Ti2O7, which extended SBET and provided more active site; furthermore, the formation of the type-II band location and strong interfacial interaction vastly weaken the barrier of electron transfer, promoting the quantum efficiency. The synergistic effect of facet-engineering and composite structure were discussed in detail, providing significant reference to design highly active photocatalyst.

Graphite defect network constitutes a robust and polishable matrix: Ultralow catalyst loading and excellent electrocatalytic performance

An industrially attractive electrocatalyst needs to meet a series of criteria such as simple and scalable preparation, robust stability, as well as highly efficient catalytic activity. In this study, we demonstrate that a defect network present within high pure polycrystalline graphite can work as a wonderful matrix of electrocatalysts. The electrodes were prepared simply by drop-casting binder-free catalyst precursor solutions on the graphite surface. The precursor ions can infiltrate into the defect channels up to about 3.6 mm deep. Excellent electrocatalytic activity and robust stability were verified using hydrogen and oxygen evolution reactions with different electrocatalysts. The depth-dependent catalytic activities were measured and the depth-profile analyses were performed. The rigid defect network structure significantly improves structural integrity and robust stability and minimizes the electron transfer barrier. Such sustainable electrocatalysts fixed in a rigid defect network have the potential to provide all the features necessary for industrial applications.

The Overlapped Electron‐Cloud Model for Electron Transfer in Contact Electrification

AbstractContact electrification (CE) is one of the oldest topics in physics, which has been discussed for more than 2600 years. Recently, an overlapped electron‐cloud (OEC) model was proposed by Wang to explain all types of CE phenomena for general materials in which a deep overlapping of electron clouds belonging to two atoms results in a lowered potential barrier for electron transfer from one to the other by applying a compressive force, which is simply referred to as Wang transition for CE. Here, the degree of electron‐cloud overlap between two atoms is controlled by using tapping mode atomic force microscopy. A temperature difference and electric field are applied between atoms of two surfaces. It is found that electron transfer only occurs when the contact tip and sample interact in the repulsive‐force region, which corresponds to a strong overlap of the electron clouds. Such a fact even preserves if the tip is at a higher temperature than the sample for 120 K. Alternatively, by applying a bias, electron tunneling would occur when the tip is in the attractive‐force region within which normal electron transfer would not occur. These studies solidify the overlapped electron‐cloud model first proposed by Wang. Further, the temperature and bias effects on the CE are explained based on a modified OEC model.

Increased activity in the oxygen evolution reaction by Fe4+-induced hole states in perovskite La1−xSrxFeO3

Hole for faster OER: The hole state induced by Fe4+ promotes the OER process. It reduces the energy barrier for electron transfer at the interface and facilitates a faster electron transfer from reaction intermediates to the catalyst.

Electron Transfer Reorganization Energies in the Electrode-Electrolyte Double Layer.

The total reorganization energy, λ, for interfacial electron transfer, ET, from a conductive electrode to redox-active molecules at fixed positions within the electric double layer, EDL, has been determined experimentally. Conductive indium-tin-oxide (ITO, In2O3:Sn) mesoporous films were functionalized with 4-[N,N-di(p-tolyl)-amino]benzylphosphonic acid (TPA) and/or [RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+ (RuP), where bpy is 2,2'-bipyridine. The small inner-sphere reorganizations, λi, for RuIII/IIP and TPA+/0 make them excellent probes of outer-sphere reorganization energy, λo, as λi ≪ λo such that λ = λi + λo ≈ λo. Consecutive layer-by-layer addition of ZrIV-bridged methylenediphosphonic acid enabled positioning at distances from 4 to 27 Å from the ITO. Excited-state injection into the ITO by RuP* generated ITO(e-)|RuIIIP. For ITO cofunctionalized with TPA and RuP, subnanosecond lateral ET yielded ITO(e-)|TPA+. The kinetics for ET from ITO to RuIIIP or TPA+ were quantified spectroscopically as a function of applied potential (Eapp) and hence driving force, -ΔG°. Marcus-Gerischer analysis of this data provided λ. Significantly, λo was near zero at close electrode proximity, λ = 0.11 eV at a distance of ∼4 Å, as manifest by kinetics largely insensitive to Eapp. In agreement with dielectric continuum theory, λ increased to values expected in CH3CN solution when the molecule was positioned at a distance of ∼27 Å (λ = 0.94 eV). The data reveal small intrinsic barriers for electron transfer proximate to conductive interfaces, which is an exploitable behavior in solar energy conversion and other applications that utilize transparent conductive oxides to accept or deliver electrons.

UV/VIS spectroelectrochemistry with 3D printed electrodes

Recent years have witnessed a boom in applying 3D printing technologies to manufacture customized prototypes in various fields of science. In electrochemistry, fused deposition modelling (FDM) 3D printing employing composite filaments based on thermoplastic materials and conductive allotropes of carbon enabled rapid, routine, inexpensive and operationally safe fabrication of conductive electrodes. Nevertheless, results of cyclovoltammetric measurements reported in the literature indicate that 3D printed electrodes give rise to considerable intrinsic kinetic barriers for electron transfer through the electrode/electrolyte interface. In this work we employ FDM-based 3D printing followed by a simple anodic activation procedure to manufacture electrodes from commercially available composites of polylactic acid (PLA) and carbon nanotubes (CNTs). Employing cyclic voltammetry with ruthenium(III) acetylacetonate as the electroactive probe we demonstrate that the previously reported kinetic barrier is almost completely removed upon the activation process. We apply such devised procedure to manufacture electrodes with optical windows allowing UV/VIS absorption spectroscopic detection of electrogenerated products. We are thus the first to perform a UV/VIS absorption spectroelectrochemical experiment employing 3D printed optically transparent working electrodes.

Electrochemiluminescent immunoassay for the lung cancer biomarker CYFRA21-1 using MoOx quantum dots.

Molybdenum oxide quantum dots (MoOx QDs) were synthesized by a one-pot method and used as a versatile probe in an electrochemiluminescent (ECL) immunoassay of the non-small cell lung cancer biomarker cytokeratin 19 fragment21-1 (CYFRA21-1) as a model analyte. The MoOx QDs exhibited stable and strong cathodic green ECL, with an emission peak at 535nm, in the presence of K2S2O8 within the potential range of -2.0 to 0V. On exposure toCYFRA21-1, theECL decreases because of the immunoreaction between CYFRA21-1 and its antibody which generates a barrier for electron transfer. The determination of CYFRA21-1 with favorable analytical performances was successfully realized under the optimal conditions. ECL decreases linearly in the 1pgmL-1 to 350ngmL-1 CYFRA21-1 concentration range, and the detection is as low as 0.3pgmL-1. Excellent recoveries from CYFRA21-1-spiked human serum indicate that the assay can be operated under physiological conditions. Graphical abstractSchematic representation of the fabrication of molybdenum oxide quantum dots (MoOx QDs) and the electrochemiluminescent (ECL) immunoassay based on the use of theMoOx QDs ECL probe for cytokeratin 19 fragment21-1 (CYFRA21-1).

Polarizability of the Active Site in Enzymatic Catalysis: Cytochrome c.

Anisotropic polarizability of the heme in cytochrome c is found to be a major factor in suppressing the activation barrier of protein electron transfer (catalytic effect). Polarizability couples to the electric field of protein and water to enhance fluctuations of the electron-transfer energy gap and the corresponding variance reorganization energy λvar. The reorganization energy observable by kinetic measurements λr = (λSt)2/λvar is composed of λvar and the Stokes-shift reorganization energy λSt. It is lowered compared to the usually reported λSt due to polarizability of the active site leading to λvar > λSt. The coupling of electrostatic protein-water fluctuations to the polarizable active site is accounted for here by empirical valence-bond diagonalization of the active-site Hamiltonian along the simulation trajectory. We show that recent simulations employing this technique, which failed to find the effect of polarizability on electron-transfer kinetics, were erroneous in neglecting the diagonal dipole moments in the Hamiltonian matrix and failing to rotate the electric field produced by the protein-water medium into the molecular frame of the active site. We find that anisotropy of the tensor of polarizability difference in the two oxidation states of the heme matches anisotropy of the second-rank tensor constructed from the electric field at the active site. Exposure of the heme to water from only one side carries significant catalytic function, directly leading to the field anisotropy and the corresponding depression of the activation barrier.

Regulation of electron-hole recombination kinetics on uniform metal-semiconductor nanostructures for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution has garnered considerable attention as a potential technology for the conversion of solar energy to chemical energy to replace fossil fuels with the development of hydrogen energy infrastructure. Semiconductors have been intensively studied as photocatalysts due to their tunable bandgap, eco-friendly reaction mechanism, photochemical stability, and ease of reusability. To achieve highly efficient photocatalysts, regulation of exctions, which are photoinduced electrons and holes in photocatalysts, is necessary. Semiconductor nanoparticles have been applied in this purpose because of their confined exciton pathways and differentiated catalytic characteristics depending on their size, shape, and morphology. In addition, metal cocatalysts have been decorated with semiconductor nanoparticles because the metal cocatalyst not only provides efficient shuttling of photoinduced electrons and proper reaction sites for the hydrogen evolution but also controls exciton pathways via fast electron transfer kinetics from semiconductor to metal. This research update reviews recent advances in representative metal-semiconductor hybrid nanostructures of core-shell and tipped nanorods for photocatalysts with a focus on the exciton pathways. The metal at semiconductor core-shell nanostructures has shown extraordinary photocatalytic stability via passivation of the metal by a semiconductor. In photocatalytic hydrogen evolution, the semiconductor shell hinders electron transfer to water. Hence, various core-shell related metal-semiconductor nanostructures such as yolk-shell, core-island shell, and double shell hollow structures have been proposed in efforts to overcome the electron transfer barrier to water. Metal tipped nanorods are another versatile nanostructure to control and monitor exciton pathways. The correlation between exciton pathways and photocatalytic efficiencies was demonstrated by monitoring metal tipped semiconductor nanorods with different composition, morphology, and surface structure. The insights reported here suggest a rational and versatile design strategy of metal-semiconductor hybrid nanostructures for developing highly efficient photocatalysts for hydrogen evolution.

Defect Engineering of Palladium-Tin Nanowires Enables Efficient Electrocatalysts for Fuel Cell Reactions.

The defect engineering of noble metal nanostructures is of vital importance because it can provide an additional yet advanced tier to further boost catalysis, especially for one-dimensional (1D) noble metal nanostructures with a high surface to bulk ratio and more importantly the ability to engineer the defect along the longitudinal direction of the 1D nanostructures. Herein, for the first time, we report that the defect in 1D noble metal nanostructures is a largely unrevealed yet essential factor in achieving highly active and stable electrocatalysts toward fuel cell reactions. The detailed electrocatalytic results show that the Pd-Sn nanowires (NWs) exhibit interesting defect-dependent performance, in which the defect-rich Pd4Sn wavy NWs display the highest activity and durability for both the methanol oxidation reaction (MOR) and the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations reveal that a large number of surface vacancies/agglomerated voids are the driving forces for forming surface grain boundaries (GBs) within Pd4Sn WNWs. These electronic active GB regions are the key factors in preserving the number of Pd0 sites, which are critical for minimizing the intrinsic site-to-site electron-transfer barriers. Through this defect engineering, the Pd4Sn WNWs ultimately yield highly efficient alkaline ORR and MOR. The present work highlights the importance of defect engineering in boosting the performance of electrocatalysts for potentially practical fuel cells and energy applications.

Improving Amorphous Carbon Anodes for Na Ion Batteries by Surface Treatment of a Presodiated Electrode with Al2O3.

Disordered carbons are promising anode materials for sodium ion batteries. However, a major drawback of these materials is their low coulombic efficiency in the first cycles, which indicates parasitic reactions. Such reactions can be suppressed by alumina coating on the surface of the anodic materials; more ions are then available for electrochemical activity, and less electrolyte solution is lost. On the other hand, some pores and surface edge sites are passivated by the coating and are no longer available for reversible reaction with sodium ions; hence, their contribution is eliminated, leading to reduction in specific capacity. We show herein that electrochemical insertion of sodium ions into carbon anodes prior to alumina coating has a double positive effect on anode perfomances, meaning preventing passivation and maintaining high specific capacity. We show that the artificial layer still prevented parasitic reactions, while the pores and surface edge sites retained electrochemical activity. The capacity values were thus restored and even became higher as a result of preventing the development of a surface layer. Ultraviolet photoelectron spectroscopy measurements assessed the energetic states of the electrodes and confirmed that the alumina coating forms a barrier for interfacial electron transfer from the electrode to the solution at any polarization stage.