Hydrogen evolution descriptors of 55-atom PtNi nanoclusters and interaction with graphite

Density functional simulations have been performed for PtnNi55-nclusters (n=0,12,20,28,42,55) to investigate their catalytic properties for the hydrogen evolution reaction (HER). Starting from the icosahedralPt12Ni43, hydrogen adsorption energetics and electronicd-band descriptors indicate HER activity comparable to that of purePt55(distorted reduced core structure). The PtNi clusters accommodate a large number of adsorbed hydrogen before reaching a saturated coverage, corresponding to 3-4 H atoms per icosahedron facet (in total ∼70-80). The differential adsorption free energies are well within the window of|ΔGH|<0.1 eV which is considered optimal for HER. The electronic descriptors show similarities with the platinumd-band, although the uncovered PtNi clusters are magnetic. Increasing hydrogen coverage suppresses magnetism and depletes electron density, resulting in expansion of the PtNi clusters. For a single H atom, the adsorption free energy varies between -0.32 (Pt12Ni43) and -0.59 eV (Pt55). The most stable adsorption site is Pt-Pt bridge for Pt-rich compositions and a hollow site surrounded by three Ni for Pt-poor compositions. A hydrogen molecule dissociates spontaneously on the Pt-rich clusters. The above HER activity predictions can be extended to PtNi on carbon support as the interaction with a graphite model structure (w/o vacancy defect) results in minor changes in the cluster properties only. The cluster-surface interaction is the strongest forPt55due to its large facing facet and associated van der Waals forces.

Theoretical insight into the rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge induced by hydrogen adsorption under HDS reaction conditions

Density functional theory (DFT) calculations and an atomistic thermodynamic approach were used to study the geometric rearrangement of sulfur atoms on the Ni- and Cu-doped MoS2 S-edge upon hydrogen adsorption. Under HDS conditions, thermodynamically stable hydrogenated structures were identified as SH groups on the undoped S-edge with 100% sulfur coverage, on the Ni-doped S-edge with 50% sulfur coverage and on the Cu-doped S-edge with 25% sulfur coverage. It was found that the rearrangement of the S atoms is essential to reach the most stable state at the edge for the undoped and Ni-doped S-edge. Hydrogen adsorption on the Ni-doped S-edge leads to the greatest amount of S rearrangement (ΔERearrang = 0.93 eV/H2). Our results suggest that under the reaction conditions, the H2 dissociative adsorption process is strongly coupled to the rearrangement of the sulfur atoms. By examining the differential hydrogen adsorption energy on the most stable edge structures, we found a plausible explanation for the trend in the hydrogenation activity of the doped edges. Our results suggest that Ni enhances the hydrogenation activity of the S-edge by decreasing the S-H bond strength, while Cu poisons it by increasing the S-H bond strength.

Electrocatalytic study of the hydrogen evolution reaction on MoS2/BP and MoSSe/BP in acidic media

Molecular hydrogen (H2) production by the electrochemical hydrogen evolution reaction (HER) is being actively explored for non-precious metal-based electrocatalysts that are earth-abundant and low cost like MoS2. Although it is acid-stable, its applicability is limited by catalytically inactive basal planes, poor electrical transport and inefficient charge transfer at the interface. Therefore, the present work examines its bilayer van der Waals heterostructure (vdW HTS). The second constituent monolayer boron phosphide (BP) is advantageous as an electrode material owing to its chemical stability in both oxygen and water environments. Here, we have performed first-principles based calculations under the framework of density functional theory (DFT) for the HER in an electrochemical double layer model with the BP monolayer, MoS2/BP and MoSSe/BP vdW HTSs. The climbing image nudged elastic band method (CI-NEB) has been employed to determine the minimum energy pathways for Tafel and Heyrovsky reactions. The calculations reveal that the Tafel reaction shows no reaction barrier. Thereafter, for the Heyrovsky reaction, we obtained a low reaction barrier in the vdW HTSs as compared to that in the BP monolayer. Subsequently, we have observed no significant difference in the reaction profile of MoS2/BP and MoSSe/BP vdW HTSs in the case of 2 × 2 supercell configuration. However, in the case of 3 × 3 and 4 × 4 configurations, MoSSe/BP shows a feasible Heyrovsky reaction with no reaction barrier. The coverages with 1/4H+ concentration (conc.) deduced high coverage with low conc. and low coverage with high conc. to be apt for the HER via the Heyrovsky reaction path. Finally, on observing the activation barrier of the Heyrovsky pathway along with that of second H adsorption at the surface, the Heyrovsky path is expected to be favoured.

Carbon Dioxide Reduction on Transition Metal Dichalcogenides with Ni and Cu Edge Doping: A Density-Functional Theory Study

Transition-metal dichalcogenides (TMDs) have promising properties for their use as catalysts of CO₂ reduction to methane via the Sabatier reaction. In this article we use density-functional theory calculations to gain insight into the energetics of this reaction for Mo/W-based and S/Se-based TMDs with non-, Ni- and Cu-doping. We show that sulfur-based TMDs with Ni/Cu doping exhibit better indicators for catalytic performance of the CO₂ reduction reaction than non-doped and doped TMDs without active sites. In addition, the role of the transition metal was found to a much smaller influence in the reaction than the role of the chalcogen and dopant atoms, which influence the bonding strength and type, respectively.

Impact of Hydrogen Coverage Trend on Methyl Formate Adsorption on MoS2 Surface: A First Principles Study

Adsorbates coverage plays a crucial role in a catalysis reaction. In hydrodeoxygenation (HDO), which involves high hydrogen pressure, hydrogen coverage on the surface may affect the adsorption of other adsorbates. The HDO is used in green diesel technology to produce clean and renewable energy from organic compounds. This motivates us to study the hydrogen coverage effect on methyl formate adsorption on MoS₂ as a model case of the actual HDO. We calculate the methyl formate adsorption energy as a function of hydrogen coverage using density functional theory (DFT) and then comprehensively analyze the physical origin of the results. We find that methyl formate can have several adsorption modes on the surface. The increased hydrogen coverage can stabilize or destabilize these adsorption modes. However, finally, it leads to convergence at high hydrogen coverage. We extrapolated the trend further and concluded that some adsorption modes might not exist at high hydrogen coverage, while others remain.

CuO-Modified PtSe2 Monolayer as a Promising Sensing Candidate toward C2H2 and C2H4 in Oil-Immersed Transformers: A Density Functional Theory Study

This work using the density functional theory simulates the strong potential of the CuO-decorated PtSe₂ (CuO-PtSe₂) monolayer as a recycle use C₂H₂ and C₂H₄ sensor in order to realize the arc discharge monitoring based on the nano-sensing method. Results indicate that CuO decoration causes strong n-type doping for the PtSe₂ monolayer with a binding force (E _b) of -2.49 eV, and the CuO-PtSe₂ monolayer exhibits strong chemisorption and electron-accepting properties in the two gas systems, with the adsorption energy (E _ad) and charge transfer (Q _T) obtained as -1.19 eV and 0.040 e for the C₂H₂ system and as -1.24 eV and 0.011 e for the C₂H₄ system, respectively. The density of states reveals the deformed electronic property of the CuO-PtSe₂ monolayer in gas adsorptions, and its sensing mechanism based on the change of electrical conductivity and the work function are uncovered. This work sheds light on the metal-oxide-decorated transition-metal dichalcogenides for gas sensor applications and would provide the guidance to explore novel sensing materials in many other fields as well.

Photocatalytic C–H activation for C–C/CN/C–S bond formation over CdS: effect of morphological regulation and S vacancies

CdS catalytic materials were utilized to fabricate C–C, CN and C–S bonds for drug intermediates or other value-added products through the high bond energy, low polarity and strong inertia C–H bonds activation.

Size-dependent trends in the hydrogen evolution activity and electronic structure of MoS2 nanotubes

The thermodynamics of hydrogen evolution on MoS2 nanotubes is studied for the first time using periodic density functional theory calculations to obtain hydrogen adsorption free energies (ΔG Hads ) on pristine nanotubes and those with S-vacancy defects. Armchair and zigzag MoS2 nanotubes of different diameters, ranging from 12 to 22 Å, are examined. The H adsorption energy is observed to become more favourable (lower ΔG Hads ) as nanotube diameter decreases, with ΔG Hads values ranging from 1.82 to 1.39 eV on the pristine nanotubes, and from 0.03 to -0.30 eV at the nanotube S-vacancy defect sites. An ideal thermoneutral ΔG Hads value of nearly 0 eV is observed at the S-vacancy site on nanotubes around 20 to 22 Å in diameter. For the pristine nanotubes, density of states calculations reveal that electron transfer from S to Mo occurs during H adsorption, and the energy gap between these two states yields a highly reliable linear correlation with ΔG Hads , where a smaller gap leads to a more favourable hydrogen adsorption. For the S-vacancy defect site the H adsorption resembles that on a pure metallic surface, meaning that a traditional d-band centre model can be applied to explain the trends in ΔG Hads . A linear relation between the position of the Mo d-states and ΔG Hads is found, with d-states closer to the Fermi level leading to strong hydrogen adsorption. Overall this work highlights the relevance of MoS2 nanotubes as promising hydrogen evolution catalysts and explains trends in their activity using the energies of the electronic states involved in binding hydrogen.

Modulation effects of S vacancy and Mo edge on the adsorption and dissociation behaviors of toxic gas (H2S, SO2) molecules on the MoS2 monolayer

This study focuses on the modulation effects of S vacancy and Mo edges on the adsorption and dissociation behaviors of toxic gases (H2S and SO2) on MoS2 by first-principles calculations. Both molecules are found to chemisorb at the S vacancy (SV) and pristine Mo edge and physisorb at the Mo edge with a 50% sulfur coverage (Mo-50 edge). Among them, SO2 has larger adsorption energy than H2S on both S vacancy and pristine Mo edge, which is related to a more electronegative O than S atom and electronically rich for the pristine Mo edge. The defective states of MoS2 induced by SV can be removed by forming Mo-S, Mo-O and Mo-H bonds upon the adsorption of SO2 and the dissociation of H2S, which is applicable in designing MoS2 based nano-electronics devices in the future. The dissociations of H2S and SO2 on pristine Mo edges are found to be more favorable than those on S vacancies due to the catalytically active Mo4+ states at edge sites. H2S is found to dissociate on the Mo-50 edge more easily than SO2. The adsorptions and dissociations of toxic gas on MoS2 with SV or Mo edges suggest MoS2 is a potential candidate in detecting and removal of toxic gases.

Ultrasonication-assisted synthesis of 2D porous MoS2/GO nanocomposite catalysts as high-performance hydrodesulfurization catalysts of vacuum gasoil: Experimental and DFT study

In this study, a novel, simple, high yield, and scalable method is proposed to synthesize highly porous MoS₂/graphene oxide (M-GO) nanocomposites by reacting the GO and co-exfoliation of bulky MoS₂ in the presence of polyvinyl pyrrolidone (PVP) under different condition of ultrasonication. Also, the effect of ultrasonic output power on the particle size distribution of metal cluster on the surface of nanocatalysts is studied. It is found that the use of the ultrasonication method can reduce the particle size and increase the specific surface area of M-GO nanocomposite catalysts which leads to HDS activity is increased. These nanocomposite catalysts are characterized by XRD, Raman spectroscopy, SEM, STEM, HR-TEM, AFM, XPS, ICP, BET surface, TPR and TPD techniques. The effects of physicochemical properties of the M-GO nanocomposites on the hydrodesulfurization (HDS) reactions of vacuum gas oil (VGO) has been also studied. Catalytic activities of MoS₂-GO nanocomposite are investigated by different operating conditions. M9-GO nanocatalyst with high surface area (324 m²/g) and large pore size (110.3 Å), have the best catalytic performance (99.95%) compared with Co-Mo/γAl₂O₃ (97.91%). Density functional theory (DFT) calculations were also used to elucidate the HDS mechanism over the M-GO catalyst. It is found that the GO substrate can stabilize MoS₂ layers through weak van der Waals interactions between carbon atoms of the GO and S atoms of MoS₂. At both Mo- and S-edges, the direct desulfurization (DDS) is found as the main reaction pathway for the hydrodesulfurization of DBT molecules.

Controllable coordination of a phosphotungstic acid-modified carbon matrix for anchoring Pt species with different sizes: from single atoms and subnanoclusters to nanoparticles

Pt species with different sizes were uniformly dispersed on phosphotungstic acid-modified carbon, and Pt SAs_0.5/PTA-C exhibited outstanding catalytic performance.

Co-embedded sulfur vacant MoS2 monolayer as a promising catalyst for formaldehyde oxidation: a theoretical evaluation

In this work, we theoretically evaluated the complete catalytic oxidation of formaldehyde (HCHO) catalyzed by a cobalt embedded sulfur vacant MoS2 (COSv–MoS2) monolayer.

Air Pressure, Gas Exposure and Electron Beam Irradiation of 2D Transition Metal Dichalcogenides

In this study, we investigate the electrical transport properties of back-gated field-effect transistors in which the channel is realized with two-dimensional transition metal dichalcogenide nanosheets, namely palladium diselenide (PdSe2) and molybdenum disulfide (MoS2). The effects of the environment (pressure, gas type, electron beam irradiation) on the electrical properties are the subject of an intense experimental study that evidences how PdSe2-based devices can be reversibly tuned from a predominantly n-type conduction (under high vacuum) to a p-type conduction (at atmospheric pressure) by simply modifying the pressure. Similarly, we report that, in MoS2-based devices, the transport properties are affected by pressure and gas type. In particular, the observed hysteresis in the transfer characteristics is explained in terms of gas absorption on the MoS2 surface due to the presence of a large number of defects. Moreover, we demonstrate the monotonic (increasing) dependence of the width of the hysteresis on decreasing the gas adsorption energy. We also report the effects of electron beam irradiation on the transport properties of two-dimensional field-effect transistors, showing that low fluences of the order of few e-/nm2 are sufficient to cause appreciable modifications to the transport characteristics. Finally, we profit from our experimental setup, realized inside a scanning electron microscope and equipped with piezo-driven nanoprobes, to perform a field emission characterization of PdSe2 and MoS2 nanosheets at cathode–anode separation distances as small as 200 nm.

Revisiting the Active Sites at the MoS2/H2O Interface via Grand-Canonical DFT: The Role of Water Dissociation

MoS₂ is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains a subject of debate. By taking the electrochemcal potential explicitly into account using grand-canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we herein revisit the active sites of 2H-MoS₂. In addition to the well-known catalytically active edge sites, also specific point defects on the otherwise inert basal plane provide highly active sites for HER. Given that HER takes place in water, we also assess the reactivity of these active sites with respect to H₂O. The thermodynamics of proton reduction as a function of the electrochemical potential reveals that four edge sites and three basal plane defects feature thermodynamic overpotentials below 0.2 V. In contrast to current proposals, many of these active sites involve adsorbed OH. The results demonstrate that even though H₂O and OH block "active" sites, HER can also occur on these "blocked" sites, reducing protons on surface OH/H₂O entities. As a consequence, our results revise the active sites, highlighting the so far overlooked need to take the liquid component (H₂O) of the functional interface into account when considering the stability and activity of the various active sites.

Engineering interfacial structures to accelerate hydrogen evolution efficiency of MoS2 over a wide pH range

Developing low-cost electrocatalysts with outstanding electrochemical performance for water splitting over a wide pH range is urgently desired to meet the practical needs in different areas. Herein, a highly efficient hierarchical flower-like CoS₂@MoS₂ core-shell nanostructured electrocatalyst is fabricated by a two-step strategy, in which MoS₂ nanosheets with a layered structure are grown on the CoS₂ core supported on carbon paper (CP) and used as hydrogen evolution reaction (HER) electrocatalysts working in the whole pH range (0-14). Remarkably, benefiting from the interface-engineering in this 3D core-shell structure of the electrocatalyst, the optimum CoS₂@MoS₂/CP catalyst exhibits outstanding HER activity over a wide range of pH values and an overpotential of 69 mV in acidic solution, 145 mV in neutral solution and 82 mV in alkaline solution, respectively, to afford the standard current density of 10 mA cm^-2. Furthermore, it demonstrates superior stability under different pH conditions for at least 48 h. Density functional theory (DFT) calculations are performed to gain further insight into the effect of CoS₂@MoS₂ interfaces, revealing that the strong interfacial interaction between CoS₂ and MoS₂ dramatically reduces the Gibbs free energy of hydrogen adsorption and the energy barrier for water dissociation, thus enhancing the electrochemical HER activity in the whole pH range (0-14).

Synthesis of a Hybrid Nanostructure of ZnO-Decorated MoS2 by Atomic Layer Deposition

We introduce the synthesis of hybrid nanostructures comprised of ZnO nanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS₂ prepared by atomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio of the ZnO NCs can be systematically engineered by controlling both the number of ZnO ALD cycles and the properties of the MoS₂ substrates, which are prepared by sulfurizing ALD MoO₃. Analysis of the chemical composition combined with electron microscopy and synchrotron X-ray techniques as a function of the number of ZnO ALD cycles, together with the results of quantum chemical calculations, help elucidate the ZnO growth mechanism and its dependence on the properties of the MoS₂ substrate. The defect density and grain size of MoS₂ nanosheets are controlled by the sulfurization temperature of ALD MoO₃, and the ZnO NCs in turn nucleate selectively at defect sites on MoS₂ surface and enlarge with increasing ALD cycle numbers. At higher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase in areal coverage and NC size. Additionally, the geometry of the hybrid structures can be tuned by changing the dimensionality of the MoS₂, by employing vertical NWs of MoS₂ as the substrate for ALD ZnO NCs, which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected to find use in newly expanded applications, especially those such as sensors or photodevices based on a p-n heterojunction which relies on coupling transition-metal dichalcogenides with NCs.

Active site manipulation in MoS2 cluster electrocatalysts by transition metal doping

The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years, molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2-Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2-Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, which suggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant-vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2-Co than for MoS2-Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour.

Theoretical investigation on the hydrogen evolution reaction mechanism at MoS2 heterostructures: the essential role of the 1T/2H phase interface

DFT calculations have been performed to study the HER mechanism at 1T/2H MoS₂ heterostructures. The HER activity along the 1T/2H phase interface is comparable with those at the Mo-edge of 2H MoS₂ and the basal plane of 1T MoS₂.

New Insight on Hydrogen Evolution Reaction Activity of MoP2 from Theoretical Perspective

We systematically investigated the hydrogen evolution reaction (HER) of six facets of MoP2 based on the periodic density functional theory (DFT). The calculated values of Gibbs free energy of hydrogen adsorption (ΔGH) indicated that the (111) facet has a good HER activity for a large range of hydrogen coverages. The zigzagged patterns before 75% hydrogen coverage suggest a facilitation among Mo1, P1 and Mo2 sites, which are attributed to repeat occupancy sites of H atoms. From ab initial atomistic thermodynamics analysis of hydrogen coverage, we gained that the most stable coverage of hydrogen is 18.75% at 1 atm H2 and 298 K. Finally, the doping effects on HER activity were investigated and found that catalytic performance can be improved by substituting P with an S or N atom, as well as substituting the Mo atom with an Fe atom, respectively. We hope this work can provide new insights on further understanding of HER for MoP2 and give instructions for the experimental design and synthesis of transition metal phosphides (TMPs)-based high-performance catalysts.

Surface Morphology and Sulfur Reduction Pathways of MoS2 Mo Edges of the Monolayer and (100) and (103) Surfaces by Molecular Hydrogen: A DFT Study

We have performed a density functional theory study of the MoS₂ monolayer and the MoS₂ (100) and (103) surfaces in relation to the early stages of the hydrodesulfurization reaction. In many X-ray diffraction (XRD) results, the (103) surface exhibits a higher peak than the (100) surface, yet one of the most frequently occurring surface has not been studied extensively. By analyzing experimental studies, we conclude that the (103) surface of MoS₂ is the most frequently occurring edge surface when the sample size is thicker than ∼10-15 nm. Herein, we report the first comparison of reaction paths for the formation of a sulfur vacancy on the (103) surface of MoS₂, monolayer, and (100) surface of MoS₂. The reason for the occurence of the (103) surface in the XRD patterns has been established. We point out the similarity in the reaction barriers for the monolayer and (100) and (103) surfaces and discuss the reason for it. Moreover, we found a more energetically favorable step in the reaction pathway for the formation of a sulfur vacancy, which allowed us to refine the previously established pathway.

Surface Morphology and Sulfur Reduction Pathways of MoS2 Mo Edges of the Monolayer and (100) and (103) Surfaces by Molecular Hydrogen: A DFT Study

We have performed a density functional theory study of the MoS₂ monolayer and the MoS₂ (100) and (103) surfaces in relation to the early stages of the hydrodesulfurization reaction. In many X-ray diffraction (XRD) results, the (103) surface exhibits a higher peak than the (100) surface, yet one of the most frequently occurring surface has not been studied extensively. By analyzing experimental studies, we conclude that the (103) surface of MoS₂ is the most frequently occurring edge surface when the sample size is thicker than ∼10-15 nm. Herein, we report the first comparison of reaction paths for the formation of a sulfur vacancy on the (103) surface of MoS₂, monolayer, and (100) surface of MoS₂. The reason for the occurence of the (103) surface in the XRD patterns has been established. We point out the similarity in the reaction barriers for the monolayer and (100) and (103) surfaces and discuss the reason for it. Moreover, we found a more energetically favorable step in the reaction pathway for the formation of a sulfur vacancy, which allowed us to refine the previously established pathway.

Hydrogen interaction with a sulfur-vacancy-induced occupied defect state in the electronic band structure of MoS2

A combination study of photoemission spectroscopy and first-principles calculations reveals that a sufficiently high concentration (2.8–11.1%) of the V_S defect on the MoS₂ surface induces an occupied defect state in the electronic band structure, in addition to the in-gap defect states.

Earth-Abundant Electrocatalysts in Proton Exchange Membrane Electrolyzers

In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

Design of Boron Doped C2N-C3N Coplanar Conjugated Heterostructure for Efficient HER Electrocatalysis

Hydrogen evolution reaction (HER) via the electrocatalytic reduction of water on metal-free catalysts may become a promising method for a sustainable energy supply in the future. However, compared with noble metals or transition metals, the carbon-based metal-free electrocatalysts show poor activity. Here, a novel coplanar metal-free catalyst (C₂N-C₃N) was designed for the first time to achieve better efficiency for electron transfer and water reduction. Through the DFT calculations, we discovered that the unique coplanar C₂N-C₃N structure can promote the directional transfer of electrons from C₃N to C₂N under the drive of built-in electric potential in the π-conjugated plane. To achieve higher performance in HER, the single atom doping by the substitution of boron is carried out. Remarkably, after the boron is doped, the barrier in the Tafel step decreases from 2.35 eV to 0.86 eV. Our results indicate that the novel B-doped coplanar C₂N-C₃N structure is a promising metal-free catalyst for HER.

Hydrogen adsorption trends on Al-doped Ni2P surfaces for optimal catalyst design

Nanoparticles of nickel phosphide are promising materials to replace the currently used rare Pt-group metals at cathode-side electrodes in devices for electrochemical hydrogen production.

Hydrogen adsorption on doped MoS2 nanostructures

Electrochemical devices for efficient production of hydrogen as energy carrier rely still largely on rare platinum group metal catalysts. Chemically and structurally modified metal dichalcogenide MoS₂ is a promising substitute for these critical raw materials at the cathode side where the hydrogen evolution reaction takes place. For precise understanding of structure and hydrogen adsorption characteristics in chemically modified MoS₂ nanostructures, we perform comprehensive density functional theory calculations on transition metal (Fe, Co, Ni, Cu) doping at the experimentally relevant MoS₂ surfaces at substitutional Mo-sites. Clear benefits of doping the basal plane are found, whereas at the Mo- and S-edges complex modifications at the whole edge are observed. New insight into doping-enhanced activity is obtained and guidance is given for further experiments. We study a machine learning model to facilitate the screening of suitable structures and find a promising level of prediction accuracy with minimal structural input.

Surface phase diagram prediction from a minimal number of DFT calculations: redox-active adsorbates on zinc oxide

Density functional theory (DFT) is routinely used to calculate the adsorption energies of molecules on solid surfaces, which can be employed to derive surface phase diagrams. Such calculations become computationally expensive if the number of substrate atoms is large, which happens whenever the adsorbate coverage is small. Here, we propose an efficient method for calculating surface phase diagrams for redox-active adsorbates on semiconductors, that we apply to the important example of proton (H⁺) and hydride (H^-) adsorbates on a ZnO surface. We identify the leading cause for the coverage dependence of the adsorption energies to be the filling and depletion of the disperse substrate conduction band. From only four DFT calculations, coupled with an analysis of the substrate electronic band structure and changes in the electrostatic potential within the substrate upon adsorption, we derive a phenomenological model that well describes the coverage-dependent adsorption energies. Moreover, our model allows us to extrapolate to the "infinite" supercell limit, where additional H adsorption leads to an arbitrarily small increase of the surface coverage. With this tool we are able to derive a surface phase diagram containing structures with extremely small H coverages (<0.002 ML), that have so far been unattainable. We expect that such models can be applied to a wide range of semiconductor substrates and redox-active adsorbates.