Helmholtz Layer Research Articles

Ion accumulation-induced capacitance elevation in a microporous graphene-based supercapacitor.

High-performance porous 3D graphene-based supercapacitors are one of the most promising and challenging directions for future energy technologies. Microporous graphene has been synthesized by the pyrolysis method. The fabricated lightweight graphene with a few layers (FLG) has an ultra-high surface area of 2266 m2 g−1 along with various-sized micropores. The defect-induced morphology and pore size distribution of the fabricated graphene are examined, and the results show that the micropores vary from 0.85 to 1.9 nm and the 1.02 nm pores contribute 30% of the total surface area. The electrochemical behaviour of the electrode fabricated using this graphene has been studied with various concentrations of the KOH electrolyte. The highest specific capacitance of the graphene electrode of 540 F g−1 (close to the theoretical value, ∼550 F g−1) can be achieved by using the 1 M KOH electrolyte. This high specific capacitance contribution involves the counter ion adsorption, co-ion desorption, and ion permutation mechanisms. The formation of a Helmholtz layer, as well as the diffusion of the electrolyte ions, confirms this phenomenon. The symmetrical solid-state supercapacitor fabricated with the graphene electrodes and PVA–KOH gel as the electrolyte exhibits excellent energy and power densities of 18 W h kg−1 and 10.2 kW kg−1, respectively. This supercapacitor also shows a superior 100% coulombic efficiency after 6000 cycles.

Nanostructures on the ZnSe Surface: Synthesis, Morphological and Photoluminescent Properties

Nanostructured zinc selenide has been obtained by electrochemical etching with an H2SO4:H2O:H2O5OH=4:1:1 solution used as the electrolyte. The experiment has indicated that the surface consists of two phases, namely the upper layer made up of a dense oxide film and a low-sized porous layer underneath, with a pore diameter of (30-80) nm and a thickness of interporous walls of (15-50) nm. The investigated dependence of surface porosity on the etching time allows us to explain the main stages of the crystal’s electrochemical dissolution during anodizing. The experiment has indicated the presence of three main stages, such as the formation of the Gouy and Helmholtz layers at the semiconductor/electrolyte segregation; pore formation at defect and oxide crystallite locations; spontaneous pore formation. The PL spectra of the samples under study have demonstrated three maxima. The emission band at 2.45 eV is attributable to the presence of oxides, the band at 2.78 EV can be accounted for the corresponding excitons while the band at 2.82 eV stems from quantum-dimensional effects. Chemical analysis of the samples has also indicated the presence of oxides on the surface of the nanostructure.

Modified Representations for the Close Evaluation Problem

When using boundary integral equation methods, we represent solutions of a linear partial differential equation as layer potentials. It is well-known that the approximation of layer potentials using quadrature rules suffer from poor resolution when evaluated closed to (but not on) the boundary. To address this challenge, we provide modified representations of the problem’s solution. Similar to Gauss’s law used to modify Laplace’s double-layer potential, we use modified representations of Laplace’s single-layer potential and Helmholtz layer potentials that avoid the close evaluation problem. Some techniques have been developed in the context of the representation formula or using interpolation techniques. We provide alternative modified representations of the layer potentials directly (or when only one density is at stake). Several numerical examples illustrate the efficiency of the technique in two and three dimensions.

Ultrathin space charge layer in hematite photoelectrodes: A theoretical investigation.

The space charge layer in hematite photoelectrodes has been analyzed by means of Poisson-Boltzmann equations, the Stern model, and density functional theory, in view of its application for photoelectrochemical water oxidation. The width of the space charge layer can be smaller than ∼10 Å under experimental conditions. In this regime, a substantial part of the potential drop takes place in the Helmholtz layer, leading to important corrections to the Mott-Schottky behavior of the space charge layer capacitance. These results shed light on an unexpected regime of high photoelectrocatalytic efficiency, different from the classical picture of the electrochemical interface of a semiconducting photocatalyst, which is also amenable to direct study by quantum-mechanical atomistic simulations. Density functional theory has been used to calculate the band bending (BB) in the space charge layer in atomistic models of pristine stoichiometric and hydroxylated surfaces. These surface terminations display BBs of 0.14 and 0.49eV, respectively, with an increasing width of the space charge layer, however still in the sub-nanometer regime. This work shows that, at high doping, the width of the space charge layer of a hematite photoelectrode can become comparable with interatomic distances.

Uniformly citrate-assisted deposition of small-sized FeOOH on BiVO4 photoanode for efficient solar water oxidation

Nanoporous BiVO4 is a promising photoanode material for driving the photoelectrochemical (PEC) water splitting to hydrogen energy, whereas its performance is limited by the sluggish surface catalysis reaction. Herein, a facile citrate-assisted method was developed for growing uniform amorphous FeOOH overlayer on nanoporous BiVO4, which significantly improved the water oxidation kinetics and resulted in a remarkable photocurrent density of 3.33 mA cm−2 at 1.23 V (vs. RHE). The activity enhancement was even superior to the well-crystallized α-FeOOH and β-FeOOH modified BiVO4. The citrate serves as the bridging agent that not only optimizes the BiVO4/FeOOH interface facilitating the hole trapping/transport, but also inhibits the particle growth of FeOOH nanolayer and provides more oxygen vacancies to promote the water oxidation. We demonstrate that the uniform FeOOH overlayer acts as the hole storage layer and catalyst, which increases the potential drop in the Helmholtz layer and greatly facilitates the interfacial hole transfer. Moreover, the citrate-assisted modification method was proved to be a general strategy to improve the PEC performance of TiO2 and α-Fe2O3 photoanodes. Our work provides a simple and effective design strategy for constructing uniform semiconductor/electrocatalysts interface that can realize efficient solar-to-hydrogen conversion.

Parallel Combination of Inner Capacitance and Ionic Capacitance, Apparently Inconsistent with Stern’s Model

A double layer capacitance (DLC) has mainly been brought about in the Helmholtz layer rather than in the diffuse layer, as was demonstrated with the invariance of DLC to salt concentration, c, less than 0.5 M (M = mol dm−3). The DLC measured here increased with concentrations of KCl and HCl solutions as high as 1 M at a platinum electrode by the ac impedance method. It was represented as a sum of the Helmholtz capacitance and the ionic one which had 0.7 power of the concentrations. The simple addition implies that the Helmholtz contribution and the ionic one should be represented by a parallel combination rather than a series one such as in the Stern model. The disagreement of the experimental values of the DLC with the Gouy–Chapman theory at high concentrations has been conventionally attributed to the effects of packing of ions over their sizes. In this paper, a model of avoiding the packing was introduced, in which ions were distributed in the direction normal to the electrode in the balance of electric motive force and the thermal energy, keeping the uniform distribution on a plane projected to the electrode. The energy balance was taken by using the grand canonical ensemble in statistical mechanics. The ionic contribution had a linear relation with the applied voltage rather than exponential dependence. When a series combination was applied to the Helmholtz capacitance and the ionic one under the condition of difference between the locally anionic DLC and the cationic one, we obtained approximately a parallel combination of the two capacitances because either the anionic or the cationic DLC works predominantly.

Adsorption of bromide and iodide anions on a renewable liquid (Cd–Ga) electrode from acetonitrile solutions. Effect of (Cd–Ga)–solvent chemisorption interaction on the anion adsorption energy

The specific adsorption of bromide and iodide anions was studied at a (Cd–Ga)/acetonitrile (AN) interface from mixed solutions of [0.1 m M LiBr + 0.1 (1–m) M LiBF4] and [0.1 m M LiI + 0.1 (1–m) M LiBF4] at the following values of molar fraction m of the surface–active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The main experimental method was measurement of differential capacitance on a renewable liquid (Cd–Ga) electrode. The obtained experimental results can be described quantitatively by the Frumkin isotherm in a wide range of charge densities (σ). The adsorption parameters of halide anions (Hal−) were determined using regression analysis of two-dimensional pressure isotherms. It is shown that the adsorption energies (ΔGads) of Br− and I− at the (Cd–Ga)/AN interface practically coincide. It follows that the difference in the interaction energies of Br− and I− with (Cd–Ga) is equal to the difference of the solvation energies of these anions: (ΔG(Cd‐Ga)‐Br‐ − ΔG(Cd‐Ga)‐I‐)≈(ΔGAN‐Br‐ − ΔGAN‐I‐) and therefore ΔG(Cd-Ga)-I− < ΔG(Cd-Ga)-Br−. The adsorption parameters of Hal− obtained on the (Cd–Ga)/AN interface are compared with the similar data for the (Cd–Ga)/DMF interface. The adsorption energy of Br− and I− on a (Cd–Ga) electrode increases significantly in AN as compared to DMF. This result can be unambiguously explained by a significant decrease in the energy of chemisorption interaction between a (Cd–Ga) electrode and solvent molecules in the Helmholtz layer upon transition from DMF to AN. The ΔGads,σ-dependence obtained at the (Cd–Ga)/AN interface in the first approximation characterizes the change of the free energy of (Cd–Ga)–Hal− interaction with σ. It is shown that the slope of the ΔGads, σ-dependence decreases upon transition from the (Cd–Ga)/AN interface to the (Cd–Ga)/DMF interface due to a significant increase in the energy of the chemisorption interaction of (Cd–Ga) with DMF molecules in the Helmholtz layer during the transition from negative to positive values of charge density.

Study on the mechanism of hydrostatic pressure promoting electrochemical corrosion of pure iron in 3.5% NaCl solution

The mechanisms of hydrostatic pressure promoting the electrochemical corrosion of pure iron in 3.5% NaCl solution were studied using electrochemical measurements and molecular dynamics simulation at different hydrostatic pressures. The main results show that hydrostatic pressure amplifies ψ1 effect by changing the Fe2+ profile at the metal/solution interface and decreasing the Helmholtz layer's thickness. It is the determinant of the enhancement of the iron dissolution rate at hydrostatic pressure. On the contrary, the adsorption of Cl− at the iron surface does not play an important role in the promoted corrosion process by hydrostatic pressure.

Molecular origin of negative component of Helmholtz capacitance at electrified Pt(111)/water interface.

Electrified solid/liquid interfaces are the key to many physicochemical processes in a myriad of areas including electrochemistry and colloid science. With tremendous efforts devoted to this topic, it is unexpected that molecular-level understanding of electric double layers is still lacking. Particularly, it is perplexing why compact Helmholtz layers often show bell-shaped differential capacitances on metal electrodes, as this would suggest a negative capacitance in some layer of interface water. Here, we report state-of-the-art ab initio molecular dynamics simulations of electrified Pt(111)/water interfaces, aiming at unraveling the structure and capacitive behavior of interface water. Our calculation reproduces the bell-shaped differential Helmholtz capacitance and shows that the interface water follows the Frumkin adsorption isotherm when varying the electrode potential, leading to a peculiar negative capacitive response. Our work provides valuable insight into the structure and capacitance of interface water, which can help understand important processes in electrocatalysis and energy storage in supercapacitors.

Interpretation of photocurrent transients at semiconductor electrodes: Effects of band-edge unpinning

The transient photocurrent response of semiconductor electrodes to chopped illumination often shows spikes and overshoots that are usually interpreted as evidence that surface recombination is occurring. In the case of the high intensities used for light-driven water splitting, the interpretation is less straightforward since the electron transfer reactions are so slow that the minority carrier concentration at or near the surface increases to high values that modify the potential drop across the Helmholtz layer in the electrolyte, leading to ‘band edge unpinning’. In addition, changes in chemical composition of the surface or local changes in pH may also alter the potential distribution across the semiconductor/electrolyte junction. A quantitative theory of band edge unpinning due to minority carrier build up is presented, and numerical calculations of transient photocurrent responses are compared with experimental examples for n-type Fe2O3 and p-type lithium-doped CuO electrodes. It is shown that the apparently high reaction orders (up to third order) with respect to hole concentration reported for hematite photoanodes can be explained as arising from an acceleration of hole transfer by the increased voltage drop across the Helmholtz layer associated with band edge unpinning. The limitations of the band edge unpinning model are discussed considering additional effects associated with modification of the potential distribution brought about by light-induced changes in surface composition, surface dipoles and surface ionic charge.

Hydrogen at Electrochemical Interfaces

At the electrode–electrolyte interface, hydrogen shows peculiar behavior being influenced by the dipole layer called Helmholtz layer. For the understanding of this system, first-principles methods have been intensively developed and applied to the benchmarking interface like Pt(111)-water; however, controversy still exists even on the hydrogen adsorption although being the simplest and most-studied problem. After giving introductory explanation of the electrochemical interface and reaction, we briefly review the advance of the theory. It has been revealed that the typical density functional calculations are occasionally not accurate enough and need to advance the description of electron correlation and nuclear quantum effect. The advanced calculation shows a promise of elucidating some experimental data providing therewith further theoretical challenges.

Changes in the Electrical Charge Accumulation Ability of Nanoporous Activated Carbon under Ultrasonic Radiation Exposure

AbstractThe influence of ultrasonic radiation in the cavitation regime on the structural and electrochemical properties of activated carbon was studied. It was established, as the result of the X‐ray diffraction and nitrogen adsorption/desorption analysis, that the ultrasound treatment causes substantial changes in the atomic and porous structure of the samples. The ultrasound treatment resulted in an increase of the graphitization degree of the carbon material and thus, in an increase of its density. The specific surface area and specific volume of the investigated samples decreased after the ultrasound treatment. The impedance dependencies for the supercapacitors made either from the initial and the modified carbon have been analyzed. The equivalent electrical circuits modelling the Nyquist diagrams have been constructed. De Levie impedance model, modified by series connection of parallel RC‐circuit, was used. It has been shown that ultrasonic radiation changes the Fermi level position by shifts to the energy region with high states density of delocalised electrons. This is the reason for unblocking of the Helmholtz layer capacitance due to the increase in the capacitance of the layer of the space charge region in the carbon material. It was shown that ultrasound treatment allows also controlling successfully the admixture and native defects distribution, existing on material surface and which are responsible for the surface electron states formation.

Interfacial Analysis of Anatase TiO2 in KOH Solution by Molecular Dynamics Simulations and Photoelectrochemical Experiments.

Hydrogen can be produced by photoelectrochemical (PEC) water splitting using a KOH solution as an electrolyte and TiO2 as a photoanode. In this work, we fabricated anatase TiO2 nanoring/nanotube arrays and TiO2 nanotube arrays using an anodic oxidation method, confirmed by field-emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD), and then conducted the photoelectrochemical experiments with 1 M KOH and Na2SO4 solutions. The bias voltage, small impedance, negative flat-band potential, large capacitance, and depletion layer width in the anatase TiO2–KOH system were observed, leading to the stable and large photocurrent density. To understand the effects of KOH on the interface properties of TiO2/H2O, the electric double layers of anatase TiO2(001), (100), (101)/KOH interfaces were further investigated by calculating the ion–surface interaction with molecular dynamics simulations. It is noted that the number density of potassium ions has the same trend as that of oxygen atoms due to the layering effect in liquids and the strongest ionic hydration of K+ on anatase (101) is observed by analyzing the radial distribution function and coordination number. In addition, the electrostatic characteristics along the TiO2/KOH interfaces were analyzed based on the Grahame double layer model. The potential drops in the outer Helmholtz layer of anatase (001), (100), and (101) surfaces are 1.08, 0.26, and 0.51 V, respectively. Compared with TiO2–H2O systems, the larger potential drops in the TiO2–KOH system explain the phenomenon that KOH solute contributes substantially to a chemical bias in PEC reactions. At the same time, we estimated the depletion layer widths of anatase TiO2(001), (100), and (101) surfaces as 37.48, 173.25, and 64.49 Å, respectively, which are of similar magnitude to the experimental results. Anatase TiO2(100) with the widest depletion layer is suggested in photocatalysis. These works provide a clear understanding of interfacial behavior of KOH on anatase TiO2 from a microscale, which can be used to explain the promotion effect of the KOH electrolyte and guide the design of TiO2 nanocrystals in the PEC system.

Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion

We report a comparison of the photocatalytic performance of WSe2-on-MoSe2 and MoSe2-on-WSe2 heterostructures. While built-in electric fields exist in these heterostructures on the order of 100 kV/cm due to band offsets between these two materials, the photocatalytic performance (i.e., photocurrent) is independent of the stacking order of the two materials. Solving Poisson’s equation under these conditions, we find that the built-in electric field produced in the heterostructure is at least 1 order of magnitude smaller than that produced in the electrochemical double layer (i.e., Helmholtz layer). Mott–Schottky measurements indicate that transition metal dichalcogenides (TMDCs) on ITO electrodes have similar capacitance to that of bare ITO, providing further evidence that the interfacial electric fields produced in the solid state heterostructure are negligible compared to the fields generated by the ions in solution. The photocatalytic performance of these heterostructures provided the largest relative enhancement in the heterojunction region under 920 and 785 nm irradiation compared with 532 and 633 nm wavelength excitation. Here, the 920 nm (1.35 eV) photons lie below the band gaps and produce very little photocurrent in the constituent monolayer materials but resonantly excite the interlayer optical transition in the heterostructure, producing a 5-fold enhancement in the measured photocurrent.

A high sensitive single luminophore ratiometric electrochemiluminescence immunosensor in combined with anodic stripping voltammetry

Herein, we report a high sensitive ratiometric electrochemiluminescence (ECL) method for carcinoembryonic antigen (CEA) determination. Porous g-C3N4 nanosheets were immobilized on the surface of conductive polymer film doped with carbon nanotubes on glassy carbon electrode and used as ECL luminophore. Signal amplification was achieved by using CuS nanoparticles conjugated second antibody. After the immune recognition, the tag of CuS nanoparticles was dissolved and Cu(II) was electrochemically reduced to Cu on the surface of modified electrode at the potential of - 0.3 V (vs. Ag/AgCl), enhancing greatly the sensitivity. The cathodic ECL of g-C3N4 generated with K2S2O8 as the co-reactant was firstly measured and used as the reference signal in the ratiometric measurement. Afterwards, the anodic ECL of g-C3N4 was measured as the sensing signal with triethanolamine as the co-reactant. During the anodic ECL process, the pre-deposited Cu was stripped to Cu(II), which can quench the ECL signal with higher efficiency due to the short distance of g-C3N4 and Cu(II) within the Helmholtz layer on the electrode surface. In this ratiometric method, the amount of Cu(II) was determined to quantify the concentration of CEA in the range of 1 fg mL−1 to 0.1 ng mL−1, with the limit of detection of 0.3 fg mL−1, which is lower than that of 3 fg mL−1 in signal-on ECL immunesensor using g-C3N4 as the tag. The combination of biometallization and anodic stripping voltammetry with ratiometric ECL can enhance significantly the sensitivity and reliability in immunosensors.

(Invited) The Morphological Stability of Passive Oxide Films

Understanding the kinetics of passive oxide formation and breakdown has been an ongoing problem for corrosion scientists for several decades. A model for the formation of a passive oxide film on a metal that is an extension of the Point Defect Model (PDM) will be presented. The potential description of the PDM is replaced with a boundary value problem that ensures Gauss' law is satisfied at the oxide-solution interface by considering the presence of the Helmholtz layer in solution. Our model predicts the observed linear variation of the steady-state film thickness with anode potential, and an increasing steady-state film thickness with increasing pH. A linear stability analysis of the moving metal-oxide and oxide-solution interfaces shows that, depending on the parameters, the passive film may be unstable to morphological perturbations of the film interfaces, leading to non-planar films and potentially the formation of a pit in the oxide. This also implies that one-dimensional models of oxide growth, that assume planar interfaces, can be inapplicable in broad classes of corrosion processes. The analysis shows that a morphological instability exists if the oxide dissolution mechanism is such that an increasing Helmholtz layer potential drop leads to an increasing dissolution rate. The instability behavior is consistent with the literature on breakdown of passivity in the presence of chloride ions. The theory provides insights on the initiation of passivity breakdown leading to pitting corrosion and the role that interfacial energy plays in determining stability.

The effects of electrolyte on the capacitive behavior of nanostructured molybdenum oxides

AbstractBACKGROUNDNanostructured metal oxides have shown capabilities in the application of energy conversion and storage. Among a variety of metal oxide semiconducting materials, molybdenum oxide recently became a promising candidate as an electrochemical supercapacitor for storing electrical energy. Supercapacitors, which are storage devices for electrical energy, benefit from the separation of opposite charges in electrochemical double‐layer capacitors and charge transfer as a function of electrochemical potential in pseudocapacitors.RESULTSA thin film of nanostructured molybdenum oxide was prepared using potentiodynamic electrodeposition on the surface of a stainless‐steel electrode. Its electrochemical capacitive behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). To investigate the effects of the electrolyte ion type, two solutions were investigated in a comparative way: 0.005 mol L–1 H2SO4 and 0.005 mol L–1 H2SO4 + 0.095 mol L–1 Na2SO4 mixture where each solution has the same initial concentrations of hydronium ions but with different ionic strengths. The EIS studies gave rise to the specific capacitances of 340 and 160 F g–1 for H2SO4 and mixed solutions, respectively. An appropriated electrical circuit was used to interpret these EIS results.CONCLUSIONSThe present studies show that the electrolyte cations contribute to the capacitive behavior of the nanostructured molybdenum oxide in different ways. Sodium ions only participate in the process of charging the double layer from the outer Helmholtz and diffusion layers, whereas hydronium ions contribute to the Faradaic process of electrosorption by penetrating into the inner Helmholtz layer. © 2019 Society of Chemical Industry

The mechanism of concentric HfO2/Co3O4/TiO2 nanotubes investigated by intensity modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) for photoelectrochemical activity

In this work, we describe the preparation and photoelectrochemical (PEC) properties of concentric HfO2/Co3O4/TiO2 nanotubes (NTs). From a methodological point of view, we demonstrated, for the first time, that intensity modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) efficiently complement and can be highly valuable to optimize PEC water-splitting devices. We verified that the CSC and CH values (the capacitance of space charge region and Helmholtz layer, derived from a phenomenological equivalent circuits model, ECM) are of similar magnitude and that the CH can be affected by applied bias, the nature of the catalytic material and the passivation layer. The calculated CH/(CH + CSC) are in agreement with the high frequency intercept (HFI) derived by the IMPS experimental data.The charge transfer efficiencies (ղtr) were obtained from the IMPS and ECM data: the ղtr values for HfO2/Co3O4/TiO2-NTs increased to 80.4% (0 V vs Ag/AgCl) compared to those in Co3O4/TiO2-NTs (56.9%,0 V vs Ag/AgCl) and in TiO2-NT (25.6%, 0 V vs Ag/AgCl).We proved that the excellent PEC properties of HfO2/Co3O4/TiO2-NTs are due to i) the band alignment between small bad gap of Co3O4 and TiO2 semiconductors, ii) increased hole transfer efficiency ղtr caused by excellent catalytic properties of Co3O4 and iii) the passivation effect on surface defects of a HfO2 layer with appropriate thickness.

Critical One-Dimensional Absorption-Desorption with Long-Ranged Interaction**Supported by the National Natural Science Foundation of China under Grant No 11274393, the National Basic Research Program of China under Grant No 2013CB933601, and the National Key Research and Development Program of China under Grant No 2016YFA0202001.

An absorption-desorption model with long-ranged interaction is simulated by the dynamic Monte Carlo method. The dynamic process has an inert phase and an active phase that is controlled by the absorption rate. In the active phase, the number of vacancies increases with time exponentially, while in the inert phase the vacant sites will be occupied by adsorbates rapidly. At the critical absorption rate, both the number of vacancies and the time-depending active probability exhibit power-law behavior. We determine the critical absorption rate and the scaling exponents of the power-laws. The effect of the interaction range of desorption on the critical exponents is investigated. In the short-ranged interaction limit, the critical exponents of Schlögl’s first model are recovered. The model may describe the stability of the inner Helmholtz layer, an essential component of the electrochemical double-layer capacitor at a nanowire.

Quantum-Enhanced Hydrogen Evolution Reactions in a Monolayer Crystal

We have demonstrated the Fermi-level dependent catalysis performance in hydrogen evolution reactions 1-2. To go one step further, there is still plenty of room for catalysts to improve their performance if we can modulate the electronic structure without altering the chemical composition. Two-dimensional materials, which have shown unusual layer-dependent electronic structures and the corresponding quantum effects, are an excellent choice to verify this hypothesis. Here, 2H-MoS2 is chosen as an example due to its high activity for hydrogen evolution reactions. It is well known that the in-plane bandgap of monolayer MoS2 (~1.85 eV) is larger than that of the multilayer (~1.30 eV), resulting in inferior conductivity and presumably poorer electrocatalysis activity. Surprisingly, we reveal here that the monolayer 2H-MoS2 electrocatalyst possesses a small overpotential of 30 mV at 70 mA/cm2 and a Tafel slope of 34 mV/dec, considerably superior to that of the multilayer 2H-MoS2 (550 mV at 70 mA/cm2; Tafel slope: 103 mV/dec). Our studies suggest a relationship among the electronic structures, material thickness, and the Helmholtz layer. This work highlights opportunities for developing superior catalysts based on the unusual electronic structures of atomically thin materials. Reference: Mengyu Yan; Xuelei Pan; Peiyao Wang; Fei Chen; Liang He; Gengping Jiang; Junhui Wang; Jefferson Z Liu; Xu Xu; Xiaobin Liao, Field-effect tuned adsorption dynamics of VSe2 nanosheets for enhanced hydrogen evolution reaction. Nano Letters 2017, 17 (7), 4109-4115.Junhui Wang; Mengyu Yan; Kangning Zhao; Xiaobin Liao; Peiyao Wang; Xuelei Pan; Wei Yang; Liqiang Mai, Field Effect Enhanced Hydrogen Evolution Reaction of MoS2 Nanosheets. Advanced Materials 2017, 29 (7).