MoS2 Nanosheets: A Designed Structure with High Active Site Density for the Hydrogen Evolution Reaction

Molybdenum oxide/nickel molybdenum oxide heterostructures hybridized active platinum co-catalyst toward superb-efficiency water splitting catalysis

A new catalyst has been developed that utilizes molybdenum oxide (MoO3)/nickel molybdenum oxide (NiMoO4) heterostructured nanorods coupled with Pt ultrafine nanoparticles for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) toward industrial-grade water splitting. This catalyst has been synthesized using a versatile approach and has shown to perform better than noble-metals catalysts, such as Pt/C and RuO2, at industrial-grade current level (≥1000 mA·cm-2). When used simultaneously as a cathode and anode, the proposed material yields 10 mA·cm-2 at a remarkably small cell voltage of 1.55 V and has shown extraordinary durability for over 50 h. Density functional theory (DFT) calculations have proved that the combination of MoO3 and NiMoO4 creates a metallic heterostructure with outstanding charge transfer ability. The DFT calculations have also shown that the excellent chemical coupling effect between the MoO3/NiMoO4 and Pt synergistically optimize the charge transfer capability and Gibbs free energies of intermediate species, leading to remarkably speeding up the reaction kinetics of water electrolysis.

In-situ fabrication of self-supported cobalt molybdenum sulphide on carbon paper for bifunctional water electrocatalysis

The fabrication of highly efficient yet stable noble-metal-free bifunctional electrocatalysts that can simultaneously catalyse both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remains challenging. Herein, we employ the heterostructure coupling strategy, showcasing an aerosol-assisted chemical vapour deposition (AACVD) aided synthetic approach for the in-situ growth of cobalt molybdenum sulphide nanocomposites on carbon paper (CoMoS@CP) as a bifunctional electrocatalyst. The AACVD allows the rational incorporation of Co in the Mo-S binary structure, which modulates the morphology of CoMoS@CP, resulting in enhanced HER activity (ŋ10 = 171 mV in acidic and ŋ10 = 177 mV in alkaline conditions). Furthermore, the CoS2 species in the CoMoS@CP ternary structure extends the OER capability, yielding an ŋ100 of 455 mV in 1 M KOH. Lastly, we found that the synergistic effect of the Co-Mo-S interface elevates the bifunctional performance beyond binary counterparts, achieving a low cell voltage (1.70 V at 10 mA cm-2) in overall water splitting test and outstanding catalytic stability (∼90 % performance retention after 50-/30-h continuous operation at 10 and 100 mA cm-2, respectively). This work has opened up a new methodology for the controllable synthesis of self-supported transition metal-based electrocatalysts for applications in overall water splitting.

Selective activation of MoS2 grain boundaries for enhanced electrochemical activity

Molybdenum disulfide (MoS2) has emerged as a promising material for catalysis and sustainable energy conversion. However, the inertness of its basal plane to electrochemical reactions poses challenges to the utilization of wafer-scale MoS2 in electrocatalysis. To overcome this limitation, we present a technique that enhances the catalytic activity of continuous MoS2 by preferentially activating its buried grain boundaries (GBs). Through mild UV irradiation, a significant enhancement in GB activity was observed that approaches the values for MoS2 edges, as confirmed by a site-selective photo-deposition technique and micro-electrochemical hydrogen evolution reaction (HER) measurements. Combined spectroscopic characterization and ab-initio simulation demonstrates substitutional oxygen functionalization at the grain boundaries to be the origin of this selective catalytic enhancement by an order of magnitude. Our approach not only improves the density of active sites in MoS2 catalytic processes but yields a new photocatalytic conversion process. By exploiting the difference in electronic structure between activated GBs and the basal plane, homo-compositional junctions were realized that improve the photocatalytic synthesis of hydrogen by 47% and achieve performances beyond the capabilities of other catalytic sites.

Defective ZrSe2: a promising candidate for spintronics applications

The current study presents the electronic and magnetic properties of monolayer ZrSe2nanoribbons. The impact of various point defects in the form of Zr or Se vacancies, and their combinations, on the nanoribbon electronic and magnetic properties are investigated using density functional theory calculations in hydrogen-terminated zigzag and armchair ZrSe2nanoribbons. Although pristine ZrSe2is non-magnetic, all the defective ZrSe2structures exhibit ferromagnetic behavior. Our calculated results also show that the Zr and Se vacancy defects alter the total spin magnetic moment with D6Se,leading to a significant amount of 6.34µB in the zigzag nanoribbon, while the largest magnetic moment of 5.52µB is induced by D2Se-2in the armchair structure, with the spin density predominantly distributed around the Zr atoms near the defect sites. Further, the impact of defects on the performance of the ZrSe2nanoribbon-based devices is investigated. Our carrier transport calculations reveal spin-polarized current-voltage characteristics for both the zigzag and armchair devices, revealing negative differential resistance (NDR) feature. Moreover, the current level in the zigzag-based nanoribbon devices is ∼10 times higher than the armchair devices, while the peak-to-valley ratio is more pronounced in the armchair-based nanoribbon devices. It is also noted that defects increase the current level in the zigzag devices while they lead to multiple NDR peaks with rather negligible change in the current level in the armchair devices. Our results on the defective ZrSe2structures, as opposed to the pristine ones that are previously studied, provide insight into ZrSe2material and device properties as a promising nanomaterial for spintronics applications and can be considered as practical guidance to experimental work.

Fabrication of inverse opal molybdenum sulfide and its use as a catalyst for H2 evolution

Amorphous molybdenum sulfide (MoSx) and crystalline molybdenum disulfide (MoS2) are attractive noble-metal-free electrocatalysts for the H2 evolution reaction from water. Their actual activities depend on the quantity of active sites which are exposed to the electrolyte, which in turn, is influenced by their specific electrochemical surface area. Herein we report on the fabrication of regular inverse opal MoSx and MoS2 films by employing polystyrene nanoparticles with diameters in the range of 30-90 nm as hard templates. The use of these catalysts for the H2 evolution reaction in an acidic electrolyte solution is also presented. Impacts of the regular porous structure, the film thickness as well as the chemical nature of the catalyst (MoS2versus MoSx) are discussed. It shows a catalytically-effective-thickness of ca. 300 nm where the electrolyte can fully penetrate the catalyst macropores, thus all the catalytic active sites can be exposed to the electrolyte to achieve the maximal catalytic operation.

In Situ Growth of Nano-MoS2 on Graphite Substrates as Catalysts for Hydrogen Evolution Reaction

In order to synthesize a high-efficiency catalytic electrode for hydrogen evolution reactions, nano-MoS₂ was deposited in situ on the surface of graphite substrates via a one-step hydrothermal method. The effects of the reactant concentration on the microstructure and the electrocatalytic characteristics of the nano-MoS₂ catalyst layers were investigated in detail. The study results showed that nano-MoS₂ sheets with a thickness of about 10 nm were successfully deposited on the surface of the graphite substrates. The reactant concentration had an important effect on uniform distribution of the catalyst layers. A higher or lower reactant concentration was disadvantageous for the electrochemical performance of the nano-MoS₂ catalyst layers. The prepared electrode had the best electrocatalytic activity when the thiourea concentration was 0.10 mol·L^-1. The minimum hydrogen evolution reaction overpotential was 196 mV (j = 10 mV·cm^-2) and the corresponding Tafel slope was calculated to be 54.1 mV·dec^-1. Moreover, the prepared electrode had an excellent cycling stability, and the microstructure and the electrocatalytic properties of the electrode had almost no change after 2000 cycles. The results of the present study are helpful for developing low-cost and efficient electrode material for hydrogen evolution reactions.

Activating and optimizing the In-Plane interface of 1 T/2H MoS2 for efficient hydrogen evolution reaction

Implanting the octahedral phase (1 T) into the hexagonal phase (2H) of the molybdenum disulfide (MoS₂) matrix is considered one of the effective methods to enhance hydrogen evolution reaction (HER) performances of MoS₂. In this paper, hybrid 1 T/2H MoS₂ nanosheets array was successfully grown on conductive carbon cloth (1 T/2H MoS₂/CC) via facile hydrothermal method and the 1 T phase content in 1 T/2H MoS₂ is regulated to gradually increase from 0 % to 80 %. 1 T/2H MoS₂/CC with 75 % 1 T phase content exhibits optimal HER performances. The DFT calculation results show that S atoms in 1 T/2H MoS₂ interface exhibit the lowest hydrogen adsorption Gibbs free energies (ΔG_H*) compared with other sites. The improvement of HER performances are primarily attributed to activating the in-plane interface regions of the hybrid 1 T/2H MoS₂ nanosheets. Furthermore, the relationship between 1 T MoS₂ content in 1 T/2H MoS₂ and catalytic activity was simulated by a mathematical model, which shows that the catalytic activity presents a trend of increasing and then decreasing with the increase of 1 T phase content.

Experimental investigation of the electrochemical detection of sulfamethoxazole using copper oxide-MoS2 modified glassy carbon electrodes

An electrochemical sensor detection of sulfamethoxazole was performed using a copper oxide Molybdenum sulfide modified glassy carbon electrode using Molybdenum sulfide (CuO/MoS₂) functionalization. As part of the characterization process, materials were characterized via cyclic voltammetry (CV), Square wave voltammetry (SWV), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). To optimize the performance of the experiment, parameters like the scan rate and pH, the electrolytes study, the stability, the comparative study and repeatability were optimized. In comparison to CuO, MoS₂ and bare Glassy carbon electrode (GCE), an electrochemical sensor that incorporated CuO/MoS₂ exhibited exceptional electrochemical performance. CuO/MoS₂ modified electrodes showed a higher peak current for oxidation compared with bare, CuO and MoS₂ modified electrodes, which demonstrated enhanced electrochemical conductivity for detection of SMX by minimizing oxidation potential from +0.18 V to +0.10 V. In the range of 100-800 μl SMX concentrations, the peak current linearly correlated with the concentration of SMX. In the calibration plot, the modified electrode showed linearity under ideal circumstances for SMX concentrations starting at 0.3 μM. This study investigated the presence of SMX with a detection limit of 0.34 Pg/L. CuO/MoS₂ based electrochemical sensor, according to our analysis, are potentially useful in applications requiring the detection of trace amounts of SMX.

2D/1D MoS2 /TiO2 Heterostructure Photocatalyst with a Switchable CO2 Reduction Product

Regulating the transfer pathway of charge carriers in heterostructure photocatalysts is of great importance for selective CO₂ photoreduction. Herein, the charge transfer pathway and in turn the redox potential succeeded to regulate in 2D MoS₂ /1D TiO₂ heterostructure by varying the light wavelength range. Several in situ measurements and experiments confirm that charge transfer follows either an S-scheme mechanism under simulated solar irradiation or a heterojunction approach under visible light illumination, elucidating the switchable property of the MoS₂ /TiO₂ heterostructure. Replacing the simulated sunlight irradiation with the visible light illumination switches the photocatalytic CO₂ reduction product from CO to CH_4. ¹³ CO₂ isotope labeling confirms that CO₂ is the source of carbon for CH₄ and CO products. The photoelectrochemical H₂ generation further supports the switching property of MoS₂ /TiO₂ . Unlike previous studies, density functional theory calculations are used to investigate the band structure of Van der Waals MoS₂ /TiO₂ S scheme after contact, allowing to propose accurate charge transfer pathways, in which the theoretical results are well matched with the experimental results. This work opens the opportunity to develop photocatalysts with switchable charge transport and tunable redox potential for selective artificial photosynthesis.

Preparation and Performance Study of the Anodic Catalyst Layer via Doctor Blade Coating for PEM Water Electrolysis

The membrane electrode assembly (MEA) is the core component of proton exchange membrane (PEM) water electrolysis cell, which provides a place for water decomposition to generate hydrogen and oxygen. The microstructure, thickness, IrO₂ loading as well as the uniformity and quality of the anodic catalyst layer (ACL) have great influence on the performance of PEM water electrolysis cell. Aiming at providing an effective and low-cost fabrication method for MEA, the purpose of this work is to optimize the catalyst ink formulation and achieve the ink properties required to form an adherent and continuous layer with doctor blade coating method. The ink formulation (e.g., isopropanol/H₂O of solvents and solids content) were adjusted, and the doctor blade thickness was optimized. The porous structure and the thickness of the doctor blade coating ACL were further confirmed with the in-plane and the cross-sectional SEM analyses. Finally, the effect of the ink formulation and the doctor blade thickness of the ACL on the cell performance were characterized in a PEM electrolyzer under ambient pressure at 80 °C. Overall, the optimized doctor blade coating ACL showed comparable performance to that prepared with the spraying method. It is proved that the doctor blade coating is capable of high-uniformity coating.

Doughty-electronegative heteroatom-induced defective MoS2 for the hydrogen evolution reaction

Producing hydrogen through water electrolysis is one of the most promising green energy storage and conversion technologies for the long-term development of energy-related hydrogen technologies. MoS₂ is a very promising electrocatalyst which may replace precious metal catalysts for the hydrogen evolution reaction (HER). In this work, doughty-electronegative heteroatom defects (halogen atoms such as chlorine, fluorine, and nitrogen) were successfully introduced in MoS₂ by using a large-scale, green, and simple ball milling strategy to alter its electronic structure. The physicochemical properties (morphology, crystallization, chemical composition, and electronic structure) of the doughty-electronegative heteroatom-induced defective MoS₂ (N/Cl-MoS₂) were identified using SEM, TEM, Raman, XRD, and XPS. Furthermore, compared with bulk pristine MoS₂, the HER activity of N/Cl-MoS₂ significantly increased from 442 mV to 280 mV at a current of 10 mA cm^-2. Ball milling not only effectively reduced the size of the catalyst material, but also exposed more active sites. More importantly, the introduced doughty-electronegative heteroatom optimized the electronic structure of the catalyst. Therefore, the doughty-electronegative heteroatom induced by mechanical ball milling provides a useful reference for the large-scale production of green, efficient, and low-cost catalyst materials.

Au-TiO2/Ti Hybrid Coating as a Liquid and Gas Diffusion Layer with Improved Performance and Stability in Proton Exchange Membrane Water Electrolyzer

The liquid and gas diffusion layer is a key component of proton exchange membrane water electrolyzer (PEMWE), and its interfacial contact resistance (ICR) and corrosion resistance have a great impact on the performance and durability of PEMWE. In this work, a novel hybrid coating with Au contacts discontinuously embedded in a titanium oxidized layer was constructed on a Ti felt via facile electrochemical metallizing and followed by a pre-oxidization process. The physicochemical characterizations, such as scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction results confirmed that the distribution and morphology of the Au contacts could be regulated with the electrical pulse time, and a hybrid coating (Au-TiO₂/Ti) was eventually achieved after the long-term stability test under anode environment. At the compaction force of 140 N cm^-2, the ICR was reduced from 19.7 mΩ cm² of the P-Ti to 4.2 mΩ cm² of the Au-TiO₂/Ti. The corrosion current density at 1.8 V (RHE) is 0.689 μA cm^-2. Both the ICR and corrosion resistance results showed that the prepared protective coating could provide comparable ICR and corrosion resistance to a dense Au coating.

Stainless Steel-Supported Amorphous Nickel Phosphide/Nickel as an Electrocatalyst for Hydrogen Evolution Reaction

Recently, nickel phosphides (Ni-P) in an amorphous state have emerged as potential catalysts with high intrinsic activity and excellent electrochemical stability for hydrogen evolution reactions (HER). However, it still lacks a good strategy to prepare amorphous Ni-P with rich surface defects or structural boundaries, and it is also hard to construct a porous Ni-P layer with favorable electron transport and gas-liquid transport. Herein, an integrated porous electrode consisting of amorphous Ni-P and a Ni interlayer was successfully constructed on a 316L stainless steel felt (denoted as Ni-P/Ni-316L). The results demonstrated that the pH of the plating solution significantly affected the grain size, pore size and distribution, and roughness of the cell-like particle surface of the amorphous Ni-P layer. The Ni-P/Ni-316L prepared at pH = 3 presented the richest surface defects or structural boundaries as well as porous structure. As expected, the as-developed Ni-P/Ni-316L demonstrated the best kinetics, with η10 of 73 mV and a Tafel slope of ca. 52 mV dec-1 for the HER among all the electrocatalysts prepared at various pH values. Furthermore, the Ni-P/Ni-316L exhibited comparable electrocatalytic HER performance and better durability than the commercial Pt (20 wt%)/C in a real water electrolysis cell, indicating that the non-precious metal-based Ni-P/Ni-316L is promising for large-scale processing and practical use.

Synthesis, Physical Properties and Electrocatalytic Performance of Nickel Phosphides for Hydrogen Evolution Reaction of Water Electrolysis

Nickel phosphides have been investigated as an alternative to noble metals and have emerged as potential catalysts that can efficiently catalyze the hydrogen evolution reaction (HER). However, the impacts of facet morphology and crystal structure of the nickel phosphides on their catalytic reactivity have not been systematically investigated. Herein, nickel phosphides with different crystalline states were prepared through a facile calcination treatment. It was found that the calcination treatment had important effects on the phase compositions, morphologies, and crystallinity of nickel phosphides, which are closely related to their HER activity. Generally, the crystallized Ni-P catalysts exhibited faster kinetics than the amorphous Ni-P. In particular, the Ni-P 300 showed remarkable HER performance with η10 of ca. 65 mV, along with a very low Tafel slope of ca. 44 mV dec^-1 due to the increased catalytically active sites. Furthermore, the Ni-P 300 exhibited negligible decay during the 140 h galvanostatic electrolysis, showing better catalytic stability than the commercial Pt/C catalyst. Compared with the amorphous Ni-P, the boosted HER activity of the Ni-P 300 could benefit from the mixed nanocrystalline Ni₂P and Ni₃P, which could contribute to the H_ads adsorption/desorption abilities and helped provide more activity sites, promoting the HER performance.

Compression-induced crimping of boron nanotubes from borophenes: a DFT study

Several borophenes have been prepared successfully, but the synthesis of boron nanotubes is still very difficult. Our results suggest that the high flexibility of borophene in combination with van der Waals interactions makes it possible to coil boron nanotubes from rippled borophenes, which is confirmed by ab initio molecular dynamics simulations. The plane structures transform into rippled structures almost without any barrier under very small compression and weak perturbations like molecular adsorption. The compression energies of the rippled structures increase linearly and slowly with the increase of the compression. This suggests how the geometry of the borophene evolves with compression. Based on the evaluation of the free energy of hydrogen adsorption, a stronger compression suggests the improved hydrogen evolution performance of the borophene and even makes it better than Pt catalysts. Meanwhile, good hydrogen evolution performance is also suggested for boron nanotubes. Our results suggest a novel preparation method for boron nanotubes from borophenes and a possible way to improve their hydrogen evolution performance.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Regulation of the Sulfur Environment in Clusters to Construct a Mn-Sn2S6 Framework for Mercury Bonding

The remarkable chemical activity of metal-sulfur clusters lies in their unique spatial configuration associated with the abundant unsaturated-coordination nature of sulfur sites. Yet, the manipulation of sulfur sites normally requires direct contact with other metal atoms, which inevitably changes the state of the coordinated sulfur. Herein, we facilely construct a Mn-Sn₂S₆ framework by regulating the sulfur environment of the [Sn₂S₆]^4- cluster with metal ions. Mn-Sn₂S₆ showed superior removal performance to gaseous elemental mercury (Hg⁰) at low temperatures (20-60 °C) and exhibited high resistance against SO₂. Moreover, Mn-Sn₂S₆ can completely remove liquid Hg²⁺ ions with low or high concentrations from acid wastewater. In addition, the adsorption capacities of Mn-Sn₂S₆ toward Hg⁰ and Hg²⁺ reached 21.05 and 413.3 mg/g, respectively. The results of physico-chemical characterizations revealed that compared with Cu²⁺, Co²⁺, and Fe²⁺, the moderate regulation of Mn²⁺ led to the special porous spherical structure of Mn-Sn₂S₆ with uniform element distribution, due to the difference of electrode potentials [E^θ(Mn²⁺/Mn) < E^θ(S/S^2-) < E^θ(Sn⁴⁺/Sn²⁺)]. The porous structure was beneficial to Hg⁰ and Hg²⁺ adsorption, and the presence of Mn⁴⁺/Mn³⁺ and S^1- promoted the oxidation of Hg⁰, resulting in stable HgS species. The constructed Mn-Sn₂S₆, thus, is a promising sorbent for both Hg⁰ ang Hg²⁺ removal and provides guidelines for cluster-based materials design and tuning.

Edge Rich Ultrathin Layered MoS2 Nanostructures for Superior Visible Light Photocatalytic Activity

Nanostructures of layered 2D materials have been proven one of the significant recent trends for visible-light-driven photocatalysis because of their unique morphology, effective optical adsorption, and rich active sites. Herein, we synthesized ultrathin-layered MoS₂ nanoflowers and nanosheets with rich active sites by using a facile hydrothermal technique. The photocatalytic performance of the as-synthesized MoS₂ nanoflowers (NF) and nanosheets (NS) were investigated for the photodegradation of MB (methylene blue), MG (malachite Green), and RhB (rhodamine B) dye under visible light irradiations. Ultrathin-layered nanoflowers showed faster degradation (96% in 150 min) in RhB under visible light irradiation, probably due to a large number of active sites and high available surface area. The kinetic study demonstrated that the first-order kinetic model best explained the process of photodegradation. The MoS₂ nanoflowers catalysts has similar catalytic performance after four consecutive cyclic performances, demonstrating their good stability. The results showed that the MoS₂ nanoflowers have outstanding visible-light-driven photocatalytic activity and could be an effective catalyst for industrial wastewater treatment.

A Simple Molten Salt Route to Crystalline β-MoB2 Nanosheets with High Activity for the Hydrogen Evolution Reaction

Molybdenum borides have been of interest due to their potential applications as electrocatalysts for the hydrogen evolution reaction (HER). As one of the common molybdenum borides, β-MoB₂ is deemed to exhibit low electrocatalytic activity due to the presence of puckered B layers, and improvement of its behavior is hindered by the lack of convenient synthetic methods. Herein, we report the synthesis of crystalline β-MoB₂ for the first time via the simple reaction of MoCl₃ and B at 850 °C in LiCl-KCl. The as-prepared β-MoB₂ sample was shown to exhibit a nanosheet structure with a large Brunauer-Emmett-Teller surface area of 48 m²/g. Such β-MoB₂ nanosheets exhibit promising HER activity in acidic medium with an overpotential of 187 mV at a current density of 10 mA/cm² and a Tafel slope of 49.3 mV/decade, which is much better than that of the known bulk β-MoB₂ and even close to the results of α-MoB₂ that is commonly recognized as the best molybdenum boride electrocatalyst for HER. The high HER activity of β-MoB₂ nanosheets results from the large surface area that allows more active sites to be exposed, which compensates for the disadvantage arising from the intrinsic structure of β-MoB₂.

Unravelling the Role of Strong Metal-Support Interactions in Boosting the Activity toward Hydrogen Evolution Reaction on Ir Nanoparticle/N-Doped Carbon Nanosheet Catalysts

Pt-based catalysts are commercial electrocatalysts for the hydrogen evolution reaction (HER), but their shortcomings of expensive and imperfect efficiency hinder their large-scale application. Here, we report an Ir-based HER catalyst supported by N-doped carbon nanosheets (Ir-NCNSs). The NCNSs, with a high surface area and unique atomic composition, enable Ir nanoparticles (NPs) to disperse at 2-3 nm and strongly coordinate to the Ir through Ir-N bonds, which exposes many active sites and strengthens their durability. The catalyst displays a low overpotential and a small Tafel slope of 46.3 mV at 10 mA cm^-2 and 52 mV dec^-1 in 0.5 M H₂SO₄, respectively. When used in 1.0 M KOH, Ir-NCNSs also show excellent electrocatalytic activity with a low overpotential of 125 mV at 10 mA cm^-2. The calculated results further suggest that Ir NPs and NCNSs have excellent selectivity for strong metal-support interactions, corresponding to a significant and stable HER characteristic. Our findings provide insight into the design of high-efficiency Ir-based HER catalysts.

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract

Tailoring physical properties of WS2 nanosheets by defects control

The controllable growth of high-quality transition metal dichalcogenides (TMDs) is crucial for their device applications, which rely on the atomic and quantitative understanding of the growth mechanism of TMDs. In this work, we propose a comprehensive picture of the growth of WS₂ nanosheets via Monte Carlo simulation, and an extension of diffusion-limited growth under transition state theory is developed to describe heteroepitaxy growth of WS₂. Theoretical results are in good agreement with the results of chemical vapor deposition that growth temperature dominates growth processes leading to samples with various densities of vacancy defects. The vacancy defects modify the photoluminescence and ferromagnetic behavior. Our work provides a pathway toward realizing controllable physical properties in 2D materials.

Hybrid structure of PbS QDs and vertically-few-layer MoS2 nanosheets array for broadband photodetector

A novel three-dimensional (3D) vertically-few-layer MoS2 (V-MoS2) nanosheets- zero-dimensional PbS quantum dots (QDs) hybrid structure based broadband photodetector was fabricated, and its photoelectric performance was investigated in detail. We synthesized the V-MoS2 nanosheets by chemical vapor deposition, using the TiO2 layer as the induced layer, and proposed a possible growth mechanism. The use of the TiO2 induction layer successfully changed the growth direction of MoS2 from parallel to vertical. The prepared V-MoS2 nanosheets have a large specific surface area, abundantly exposed edges and excellent light absorption capacity. The V-MoS2 nanosheets detector was then fabricated and investigated, which exhibits a high sensitivity for 635 nm light, a fast response time and an excellent photoelectric response. The V-MoS2 nanosheets with a height of approximately 1 μm successfully broke the light absorption limit caused by the atomic thickness. Finally, we fabricated the PbS QDs/V-MoS2 nanosheets hybrid detector and demonstrated their potential for high-performance broadband photodetectors. The response wavelength of the hybrid detector extends from the visible band to the near-infrared band. The responsivity of the hybrid detector reaches 1.46 A W-1 under 1450 nm illumination. The combination of 3D MoS2 nanosheets and QDs further improves the performance of MoS2-based photodetector devices. We believe that the proposed zero-dimensional QDs and 3D vertical nanosheets hybrid structure broadband photodetector provides a promising way for the next-generation optoelectronic devices.

Visible light-driven oxidative coupling of dibenzylamine and substituted anilines with a 2D WSe2 nanomesh material

A novel 2D WSe2 nanomesh material was synthesized with a 3D SBA-15 mesoporous material via a nanocasting strategy. The formation of the 2D sheet-like nanomesh structure of WSe2 inside a 3D confined pore space is mainly attributed to the synergistic effect arising from the crystal self-limitation growth caused by the layered crystal structure of the WSe2 material and to the space-limitation effect coming from the unique pore structure of the SBA-15 template. The 2D WSe2 nanomesh material possesses extremely high exposure of crystal layer edges, making it an excellent photocatalyst. It shows good visible light-driven photocatalytic performance in oxidative coupling of dibenzylamine and 2-amino/hydroxy/mercaptoanilines to prepare a group of heterocyclic compounds, including benzimidazoles, benzoxazoles and benzothiazoles with oxygen as the sole oxidant. A gram-scale experiment was also carried out to exhibit the scope of this method.

1D/2D Heterostructures as Ultrathin Catalysts for Hydrogen Evolution Reaction

2D MoS₂ has emerged as a promising alternative to Pt-based catalysts for hydrogen evolution reaction (HER) due to its low cost and earth abundance. However, insufficient active sites of basal plane and poor conductivity become the foremost factors restricting the catalytic performance of MoS₂ . Here, a facile strategy is presented to enhance the HER performance of MoS₂ by converting its 2D structure into 1D/2D heterostructures of Mo₆ Te₆ /MoS_{2(1- x )} Te_{2 x} by the in situ tellurization. As-prepared 1D/2D heterostructures exhibit excellent HER performance with the Tafel slope of ≈56 mV dec^-1 (only one-third of that for pristine MoS₂ ). The enhanced HER catalytic activity is attributed to more Te/S vacancies introduced by tellurization, which serve as the active sites as suggested by theoretical calculations. Besides, the formation of highly conductive well-aligned quasi-1D Mo₆ Te₆ nanobelts facilitate charge transport in HER. Previous work provides a facile approach to construct mixed dimensional materials, and opens up a new avenue to the properties modulation of 2D transition metal chalcogenides.

The lightest solid meets the lightest gas: an overview of carbon aerogels and their composites for hydrogen related applications

Hydrogen, a renewable and recyclable energy, has been regarded as the best solution for global energy supply in the 21^st century. Hydrogen production, hydrogen storage and hydrogen sensing are three most important aspects for hydrogen economy. Interestingly, the lightest solid, carbon aerogels (CAs), has found wide applications in these aspects due to its unique characteristics including large specific surface area, hierarchical porous structure, high electrical conductivity, superb chemical stability, and low fabrication cost. Herein, various fabrication strategies of CAs are presented, and their applications in the three most important aspects are comprehensively reviewed. In addition, the challenges and prospects are also discussed. In the light of the recent progress in CAs for hydrogen-related applications, this review provides a comprehensive assessment on materials selection, synthesis, hydrogen adsorption characteristics of CAs and catalytic activity of CA-supported nanocatalysts, offering a strategic guide to build a close connection between CAs and hydrogen economy.

Substrate Impact on the Structure and Electrocatalyst Properties of Molybdenum Disulfide for HER from Water

It is expected that utilization of molybdenum disulfide (MoS2)-based nanostructured electrocatalysts might replace the Pt-group electrodes most effectively applied for hydrogen evolution reaction from water. Therefore, in the past two decades, various approaches have been reported for fabrication of nanostructured MoS2-based catalysts, but their applications in practice are still missing due to lower activity and stability. We envisaged that the knowledge about the peculiarities of MoS2 nanoplatelets attachment to various conductive substrates by hydrothermal processing could be helpful for fabrication of more active and stable working electrodes. Therefore, in this study, the hydrothermal syntheses at the Mo, Ti, Al, anodized Ti, and hydrothermally designed titanium suboxide substrates were performed; the electrodes obtained were characterized; and hydrogen evolution reaction (HER) activity was tested. In this way, MoS2-based HER catalyst possessing a surprising stability and a low Tafel slope was designed via attachment of nanoplatelet-shaped MoS2 species to the nanotube-shaped anatase-TiO2 surface.

Interpretable molecular models for molybdenum disulfide and insight into selective peptide recognition

Molybdenum disulfide (MoS2) is a layered material with outstanding electrical and optical properties. Numerous studies evaluate the performance in sensors, catalysts, batteries, and composites that can benefit from guidance by simulations in all-atom resolution. However, molecular simulations remain difficult due to lack of reliable models. We introduce an interpretable force field for MoS2 with record performance that reproduces structural, interfacial, and mechanical properties in 0.1% to 5% agreement with experiments. The model overcomes structural instability, deviations in interfacial and mechanical properties by several 100%, and empirical fitting protocols in earlier models. It is compatible with several force fields for molecular dynamics simulation, including the interface force field (IFF), CVFF, DREIDING, PCFF, COMPASS, CHARMM, AMBER, and OPLS-AA. The parameters capture polar covalent bonding, X-ray structure, cleavage energy, infrared spectra, bending stability, bulk modulus, Young's modulus, and contact angles with polar and nonpolar solvents. We utilized the models to uncover the binding mechanism of peptides to the MoS2 basal plane. The binding strength of several 7mer and 8mer peptides scales linearly with surface contact and replacement of surface-bound water molecules, and is tunable in a wide range from -86 to -6 kcal mol-1. The binding selectivity is multifactorial, including major contributions by van-der-Waals coordination and charge matching of certain side groups, orientation of hydrophilic side chains towards water, and conformation flexibility. We explain the relative attraction and role of the 20 amino acids using computational and experimental data. The force field can be used to screen and interpret the assembly of MoS2-based nanomaterials and electrolyte interfaces up to a billion atoms with high accuracy, including multiscale simulations from the quantum scale to the microscale.

Exploring the synergistic effect of NixSn2xS4x thiospinel with MWCNTs for enhanced performance in dye-sensitized solar cells, the hydrogen evolution reaction, and supercapacitors

Trifunctional nickel tin sulfide (NixSn2xS4x) with a thiospinel-like structure composited with multiwalled carbon nanotubes (MWCNTs) (M-NixSn2xS4x) was synthesized by a facile method. The unit cell arrangement of the prepared composite was studied by density functional theory, and the theoretical interpretation satisfactorily inferred the presence of a synergistic effect between the thiospinel and MWCNTs. The high metallic conductivity and superior electrocatalytic activity of the M-NixSn2xS4x composite endow it with diverse applications. The composite shows promise as a counter electrode for dye-sensitized solar cells (efficiency of 4.67% for fluorine-doped indium tin oxide compared to 5.23% for platinum); an efficient catalyst for the hydrogen evolution reaction with good cycling stability and a low overpotential of -41 mV at a cathode current density of 10 mA cm2 and a Tafel slope of 43 mV dec-1 on a graphite sheet electrode; and an impressive capacitance material on a graphite sheet electrode alternative to expensive current collectors such as Ni foam, with a specific capacitance value of 1200 F g-1 at a current density of 1 A g-1 and a long life span of 92.6% that is retained for up to 15 000 cycles.

1% defect enriches MoS2 quantum dot: catalysis and blue luminescence

Defects in solids are typically recognized as unfavorable, leading to degradation of the structure and properties of the material. However, defects occasionally provide extraordinary benefits as the active sites of catalysts and chemical reactions, and can result in the creation of new electronic states. In particular, a low-dimensional material can become a defect-rich material due to the unique ratio of surface area to volume, giving many dangling bonds. Herein, we report the rapid (20 min) synthesis of MoS₂ quantum dots (QDs) with a diameter of 4 nm at room temperature using nanosecond pulsed laser ablation in a binary solvent. The MoS₂ QDs are crystalline particles composed of 3-5 layers and contain sulfur vacancies at an atomic concentration of 1% acting as a functional defect. The MoS₂ QDs exhibit excellent electrocatalytic performance (Tafel slope = 49 mV dec^-1) for the hydrogen evolution reaction and high quantum yield blue photoluminescence with a large Stokes shift.

Improvement of HER activity for MoS2: insight into the effect and mechanism of phosphorus post-doping

The phosphorus-doped MoS₂ nanosheets were prepared by a post-doping method and exhibited good catalytic activity for hydrogen evolution reaction.

Hydrogen evolution reaction from bare and surface-functionalized few-layered MoS2 nanosheets in acidic and alkaline electrolytes

Hydrogen is considered as an ideal and sustainable energy carrier because of its high energy density and carbon-free combustion. Electrochemical water splitting is the only solution for uninterrupted, scalable, and sustainable production of hydrogen without carbon emission. However, a large-scale hydrogen production through electrochemical water splitting depends on the availability of earth-abundant electrocatalysts and a suitable electrolyte medium. In this article, we demonstrate that hydrogen evolution reaction (HER) performance of electrocatalytic materials can be controlled by their surface functionalization and selection of a suitable electrolyte solution. Here, we report syntheses of few-layered MoS₂ nanosheets, NiO nanoparticles (NPs), and multiwalled carbon nanotubes (MWCNTs) using scalable production methods from earth-abundant materials. Magnetic measurements of as-produced electrocatalyst materials demonstrate that MoS₂ nanoflakes are diamagnetic, whereas surface-functionalized MoS₂ and its composite with carbon nanotubes have strong ferromagnetism. The HER performance of the few-layered pristine MoS₂ nanoflakes, MoS₂/NiO NPs, and MoS₂/NiO NPs/MWCNT nanocomposite electrocatalysts are studied in acidic and alkaline media. For bare MoS₂, the values of overpotential (η₁₀) in alkaline and acidic media are 0.45 and 0.54 V, respectively. Similarly, the values of current density at 0.5 V overpotential are 27 and 6.2 mA/cm² in alkaline and acidic media, respectively. The surface functionalization acts adversely in the both alkaline and acidic media. MoS₂ nanosheets functionalized with NiO NPs also demonstrated excellent performance for oxygen evolution reaction with anodic current of ~60 mA/cm² and Tafel slope of 78 mVdec^-1 in alkaline medium.

Heterojunction Photoanode of Atomic-Layer-Deposited MoS2 on Single-Crystalline CdS Nanorod Arrays

Cadmium sulfide (CdS) is a semiconducting absorber for photoelectrochemical (PEC) hydrogen production with suitable electronic band structures. However, it suffers from severe photocorrosion and rapid charge recombination during the desired PEC reactions. Herein, we describe the identification of the optimal junction thickness of CdS/MoS₂ core/sheath heterojunction nanostructures by employing atomic layer deposition (ALD) techniques. ALD-grown MoS₂ sheath layers with different thicknesses were realized on single-crystalline CdS nanorod (NR) arrays on transparent conducting oxide substrates. We further monitored the resulting solar H₂ evolution performance with our heterojunction photoanodes. The results showed that the junction thickness of MoS₂ plays a key role in the reduction of photocorrosion and the enhanced photocurrent density by optimizing the charge separation. A better saturation photocurrent (∼46%) was obtained with the 7 nm-thick MoS₂@CdS NRs than that with the bare CdS NRs. Moreover, the external quantum efficiency was increased twofold over that of the pristine CdS NRs. The ALD-grown MoS₂@CdS heterojunction structures provides an efficient and versatile platform for hydrogen production when combining ALD-grown MoS₂ with ideal semiconducting absorbers.

A Hydrothermal-Assisted Ball Milling Approach for Scalable Production of High-Quality Functionalized MoS2 Nanosheets for Polymer Nanocomposites

The most known analogue of graphene, molybdenum disulfide (MoS₂) nanosheet, has recently captured great interest because it can present properties beyond graphene in several high technological applications. Nonetheless, the lack of a feasible, sustainable, and scalable approach, in which synthesizing and functionalization of 2H-MoS₂ nanosheets occur simultaneously, is still a challenge. Herein, a hydrothermal treatment has been utilised to reduce the effect of breaking mechanisms on the lateral size of produced nanosheets during the ball milling process. It was demonstrated that the hydrothermal pre-treatment led to the initial intercalation of an organic molecule such as 4,4′-diaminodiphenyl sulfone (DDS) within the stacked MoS₂ sheets. Such a phenomenon can promote the horizontal shear forces and cause sliding and peeling mechanisms to be the dominated ones during low energy ball milling. Such combined methods can result in the production of 2H functionalized MoS₂ nanosheets. The resultant few layers showed an average lateral dimension of more than 640 nm with the thickness as low as ~6 nm and a surface area as high as ~121.8 m²/g. These features of the synthesised MoS₂ nanosheets, alongside their functional groups, can result in fully harnessing the reinforcing potential of MoS₂ nanosheets for improvement of mechanical properties in different types of polymeric matrices.