Colloidal synthesis of MoSe 2 nanonetworks and nanoflowers with efficient electrocatalytic hydrogen-evolution activity

Self-Assembled Preparation of Porous Nickel Phosphide Superparticles with Tunable Phase and Porosity for Efficient Hydrogen Evolution

Self-assembly of colloidal nanoparticles enables the easy building of assembly units into higher-order structures and the bottom-up preparation of functional materials. Nickel phosphides represent an important group of catalysts for hydrogen evolution reaction (HER) from water splitting. In this paper, the preparation of porous nickel phosphide superparticles and their HER efficiencies are reported. Ni and Ni2P nanoparticles are self-assembled into binary superparticles via an oil-in-water emulsion method. After annealing and acid etching, the as-prepared Ni-Ni2P binary superparticles change into porous nickel phosphide superparticles. The porosity and crystalline phase of the superparticles can be tuned by adjusting the ratio of Ni and Ni2P nanoparticles. The resulting porous superparticles are effective in driving HER under acidic conditions, and the modulation of porosity and phase further optimize the electrochemical performance. The prepared Ni3P porous superparticles not only possess a significantly enhanced specific surface area compared to solid Ni-Ni2P superparticles but also exhibit an excellent HER efficiency. The calculations based on the density functional theories show that the (110) crystal facet exhibits a relatively lower Gibbs free energy of hydrogen adsorption. This work provides a self-assembly approach for the construction of porous metal phosphide nanomaterials with tunable crystalline phase and porosity.

Two-dimensional layered MoSe2/graphene oxide (GO) nanohybrid coupled with the specific immune-recognition element for rapid detection of endosulfan

Endosulfan (En) is an organochlorine biocide (OCB), that ends up in the environment due to the enzymatic and microsomal activity even though it is not accumulated in living tissue. Endosulfan acts as an organic micro-pollutant which disrupts land as well as aquatic ecosystem. In the present study, we chemically modified endosulfan and conjugated it with a carrier protein to produce an immune response. The generated antibodies were tested for specificity against En, and characterized before further use. Transition Metal Chalcogenides (TMC) showed excellent optoelectrical potential due to its direct bandgap and distinct physical as well as chemical characteristics. Herein, we synthesized a novel nanohybrid using MoSe2 in combination with graphene oxide (GO) and characterized thoroughly. This was similar to graphene-based metal chalcogenides which were further used in this study to fabricate biosensor for the sensitive detection of En. The in-house developed antibodies (En-Ab) were coupled with the nanohybrid to make MoSe2/GO/En-Ab electrode. Fabricated electrode was tested for electrochemical parameters using differential pulse voltammetry (DPV). Working efficiency of the fabricated electrode i.e., limit of detection (LOD), was found to be 7.45 ppt. In conclusion, we hypothesized that the synthesized TMC nanohybrids could be employed for biosensing of endosulfan, and can likely be developed to test field samples.

Colloidal Control of Branching in Metal Chalcogenide Semiconductor Nanostructures

Colloidal syntheses of metal chalcogenides yield nanostructures of various 1D, 2D, and 3D nanocrystals (NCs), including branched nanostructures (BNSs) of nanoflowers, tetrapods, octopods, nanourchins, and more. Efforts are continuously being made to understand the branching mechanism in colloidally prepared metal chalcogenides for tailor-making them into various morphologies for dedicated applications in solar cells, light-emitting diodes, stress sensor devices, and near-infrared photodetectors. The vital role of precursors and ligands has widely been recognized in directing nanocrystal morphology during the colloidal synthesis of metal chalcogenide nanostructures. Moreover, a few basic branching mechanisms in nanocrystals have also been derived from decades-long observations of branching in NCs. This Perspective (a) accounts for the mediation of branching in In₂S₃, PbS, MoSe₂, WSe_2, and WS₂; (b) analyzes the underlying mechanisms; and (c) gives a future perspective toward better controlling the BNSs' morphologies and their impact on applications.

AuNPs@MoSe2 heterostructure as a highly efficient coreaction accelerator of electrocheluminescence for amplified immunosensing of DNA methylation

Electrocheluminescence analysis amplified by coreaction accelerators has experienced breakthrough in ultrasensitive detection of biomarkers. Herein, a highly efficient coreaction accelerator, two-dimensional layered MoSe₂ nanosheets loaded with gold nanoparticles (AuNPs@MoSe₂ heterostructure), is proposed to enhance the ECL efficiency of Ru(bpy)₃²⁺/tripropylamine (TPrA) system. The presence of AuNPs avoids the aggregation of MoSe₂ nanosheets, and improves the electrical conductivity of modified surface. The AuNPs@MoSe₂ modified electrode also provides a large area for loading of abundant capture probe. MoSe₂ as an electroactive substrate can remarkably accelerate the generation of TPrA^•+ radicals to react with electrooxidized Ru(bpy)₃²⁺, which achieves about 3.4-fold stronger ECL intensity. Thus, an enhanced ECL immunoassay method can be achieved after Ru(bpy)₃²⁺-doped silica nanoparticle labeled antibody (Ab₂-Ru@SiO₂) is captured to the modified electrode via immunological recognition. Using methylated DNA as a target, the immunosensor was prepared by binding capture DNA on AuNPs@MoSe₂ modified electrode to successively capture the target, anti-5-methylcytosine antibody (anti-5mC) and Ab₂-Ru@SiO₂. The proposed strategy could detect 0.26 fM 5 mC (3σ) with a detectable concentration range of 1.0 fM - 10 nM at methylated DNA. This immunosensor showed excellent selectivity, good stability and reproducibility, and acceptable recovery, indicating the broad prospects of the novel coreaction accelerator in clinical diagnosis.

Controlled CO labilization of tungsten carbonyl precursors for the low-temperature synthesis of tungsten diselenide nanocrystals

We report a low-temperature colloidal synthesis of WSe2 nanocrystals from tungsten hexacarbonyl and diphenyl diselenide in trioctylphosphine oxide (TOPO). We identify TOPO-substituted intermediates, W(CO)5TOPO and cis-W(CO)4(TOPO)2 by infrared spectroscopy. To confirm these assignments, we synthesize aryl analogues of phosphine-oxide-substituted intermediates, W(CO)5TPPO (synthesized previously, TPPO = triphenylphosphine oxide) and cis-W(CO)4(TPPO)2 and fac-W(CO)3(TPPO)3 (new structures reported herein). Ligation of the tungsten carbonyl by either the alkyl or aryl phosphine oxides results in facile labilization of the remaining CO, enabling low-temperature decomposition to nucleate WSe2 nanocrystals. The reactivity in phosphine oxides is contrasted with syntheses containing phosphine ligands, where substitution results in decreased CO labilization and higher temperatures are required to induce nanocrystal nucleation.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Impact of the number of surface-attached tungsten diselenide layers on cadmium sulfide nanorods on the charge transfer and photocatalytic hydrogen evolution rate

The selection of layered number and time-course destruction of layers may affect the charge transfer between 2D-to-1D heterostructure, making it possible to improve the efficiency of solar-to-hydrogen evolution. Herein, we demonstrate a simple, low-cost systematic protocol of 2D-WSe₂ nanolayer numbers ranging from 7 to 60 aiding the ultrasonication time-course. The resultant nanolayers were assembled on the surface of 1D-CdS nanorods, which demonstrated an improved surface shuttling property. Consequently, a drastic improvement in photocatalytic solar-driven hydrogen evolution was observed (103.5 mmol h^-1 g^-1) with seven-layered WSe₂ (few-layered WSe₂) attached on CdS nanorods surface. This enhanced photocatalytic performance is attributed to the selection of layers on CdS surface that expose abundant active sites; along with suitable energy levels, this can facilitate increased charge transfer leading to feasible photocatalytic reactions. Significantly, the present study proposes an efficient and sustainable process to produce hydrogen and demonstrates the potential of numbered WSe₂ nanosheets as a co-catalyst material.

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

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

Constructing a 2D/2D heterojunction of MoSe2/ZnIn2S4 nanosheets for enhanced photocatalytic hydrogen evolution

2D/2D MoSe₂/ZnIn₂S₄ heterojunctions exhibit high photocatalytic activity owing to MoSe₂ as a cocatalyst, which provides more active sites, reducing the overpotential and the activation energy for water reduction.

Evaluating the Effect of Varying the Metal Precursor in the Colloidal Synthesis of MoSe2 Nanomaterials and Their Application as Electrodes in the Hydrogen Evolution Reaction

Herein we report on the use of different metal precursors in the synthesis of MoSe₂ nanomaterials in order to control their morphology. The use of Mo(CO)₆ as the metal precursor resulted in the formation of wrinkled few-layer nanosheets, while the use of H₂MoO₄ as the metal precursor resulted in the formation of nanoflowers. To investigate the effect of the morphologies on their performance as catalysts in the hydrogen evolution reaction, electrochemical characterization was done using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrical impedance spectroscopy (EIS). The MoSe₂ nanoflowers were found to have superior electrochemical performance towards the hydrogen evolution reaction with a lower Tafel slope, on-set potential, and overpotential at 10 mA/cm² compared to the wrinkled few-layer nanosheets. This was found to be due to the higher effective electrochemical surface area of the nanoflowers compared to the nanosheets which suggests a higher number of exposed edge sites in the nanoflowers.

Carbon nanotube spiderweb promoted growth of hierarchical transition metal dichalcogenide nanostructures for seamless devices

Hierarchical transition metal dichalcogenide (h-TMDC) nanostructures with abundant active edge sites and good electrical conductivity hold great promise for numerous applications. Here, we report a general method for the chemical synthesis of a series of large-area, free-standing h-TMDC films and their devices by using carbon nanotube (CNT) spiderwebs as both growth promoters and electrical/mechanical reinforcement networks. Our approach allows the seamless integration of h-TMDC nanostructures with abundant active edge sites and CNT networks with good electrical conductivity and mechanical flexibility. As a proof of concept, h-MoSe₂/CNT hybrid films with CNT contacts have been chemically synthesized and applied as flexible electrocatalytic devices for hydrogen evolution reaction (HER). Owing to the seamless connection between the CNT contacts and the electroactive h-TMDC/CNT nanostructures, the flexible electrocatalytic devices exhibited excellent mechanical stability and maintained stable electrocatalytic performance under cyclic bendings. Our method can be readily extended to the large-scale production of various h-TMDC/CNT hybrid films and their seamless devices.

MoSe2-Ni3Se4 Hybrid Nanoelectrocatalysts and Their Enhanced Electrocatalytic Activity for Hydrogen Evolution Reaction

Combining MoSe₂ with other transition metal dichalcogenides to form a hybrid nanostructure is an effective route to enhance the electrocatalytic activities for hydrogen evolution reaction (HER). In this study, MoSe₂-Ni₃Se₄ hybrid nanoelectrocatalysts with a flower-like morphology are synthesized by a seed-induced solution approach. Instead of independently nucleating to form separate nanocrystals, the Ni₃Se₄ component tends to nucleate and grow on the surfaces of ultrathin nanoflakes of MoSe₂ to form a hybrid nanostructure. MoSe₂-Ni₃Se₄ hybrid nanoelectrocatalysts with different Mo:Ni ratios are prepared and their HER catalytic activities are compared. The results show that the HER activities are affected by the Mo:Ni ratios. In comparison with pure MoSe₂, the MoSe₂-Ni₃Se₄ hybrid nanoelectrocatalysts having a Mo:Ni molar ratio of 2:1 exhibit enhanced HER properties with an overpotential of 203 mV at 10 mA/cm² and a Tafel slope of 57 mV per decade. Improved conductivity and increased turnover frequencies (TOFs) are also observed for the MoSe₂-Ni₃Se₄ hybrid samples.

Continuous phase regulation of MoSe2 from 2H to 1T for the optimization of peroxidase-like catalysis

Simultaneous and synergistic modulation of the crystal phase and disorder in MoSe₂ to dramatically enhance their peroxidase-like activity.

Photoelectrochemical Hydrogen Evolution and CO2 Reduction over MoS2/Si and MoSe2/Si Nanostructures by Combined Photoelectrochemical Deposition and Rapid-Thermal Annealing Process

Diverse methods have been employed to synthesize MoS2 and MoSe2 catalyst systems. Herein, a combined photoelectrochemical (PEC) deposition and rapid-thermal annealing process has first been employed to fabricate MoS2 and MoSe2 thin films on Si substrates. The newly developed transition-metal dichalcogenides were characterized by scanning electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. PEC hydrogen evolution reaction (HER) was demonstrated in an acidic condition to show a PEC catalytic performance order of MoOx/Si < MoS2/Si << MoSe2/Si under the visible light-on condition. The HER activity (4.5 mA/cm2 at −1.0 V vs Ag/AgCl) of MoSe2/Si was increased by 4.8× compared with that under the dark condition. For CO2 reduction, the PEC activity was observed to be in the order of MoS2/Si < MoOx/Si << MoSe2/Si under the visible light-on condition. The reduction activity (0.127 mA/cm2) of MoSe2/Si was increased by 9.3× compared with that under the dark condition. The combined electrochemical deposition and rapid-thermal annealing method could be a very useful method for fabricating a thin film state catalytic system perusing hydrogen production and CO2 energy conversion.

MoSe₂-GO/rGO Composite Catalyst for Hydrogen Evolution Reaction

There has been considerable research to engineer composites of transition metal dichalcogenides with other materials to improve their catalytic performance. In this work, we present a modified solution-processed method for the formation of molybdenum selenide (MoSe₂) nanosheets and a facile method of structuring composites with graphene oxide (GO) or reduced graphene oxide (rGO) at different ratios to prevent aggregation of the MoSe₂ nanosheets and hence improve their electrocatalytic hydrogen evolution reaction performance. The prepared GO, rGO, and MoSe₂ nanosheets were characterized by X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The electrocatalytic performance results showed that the pure MoSe₂ nanosheets exhibited a somewhat high Tafel slope of 80 mV/dec, whereas the MoSe₂-GO and MoSe₂-rGO composites showed lower Tafel slopes of 57 and 67 mV/dec at ratios of 6:4 and 4:6, respectively. We attribute the improved catalytic effects to the better contact and faster carrier transfer between the edge of MoSe₂ and the electrode due to the addition of GO or rGO.

Phase Modulation of (1T-2H)-MoSe2/TiC-C Shell/Core Arrays via Nitrogen Doping for Highly Efficient Hydrogen Evolution Reaction

Tailoring molybdenum selenide electrocatalysts with tunable phase and morphology is of great importance for advancement of hydrogen evolution reaction (HER). In this work, phase- and morphology-modulated N-doped MoSe2 /TiC-C shell/core arrays through a facile hydrothermal and postannealing treatment strategy are reported. Highly conductive TiC-C nanorod arrays serve as the backbone for MoSe2 nanosheets to form high-quality MoSe2 /TiC-C shell/core arrays. Impressively, continuous phase modulation of MoSe2 is realized on the MoSe2 /TiC-C arrays. Except for the pure 1T-MoSe2 and 2H-MoSe2 , mixed (1T-2H)-MoSe2 nanosheets are achieved in the N-MoSe2 by N doping and demonstrated by spherical aberration electron microscope. Plausible mechanism of phase transformation and different doping sites of N atom are proposed via theoretical calculation. The much smaller energy barrier, longer HSe bond length, and diminished bandgap endow N-MoSe2 /TiC-C arrays with substantially superior HER performance compared to 1T and 2H phase counterparts. Impressively, the designed N-MoSe2 /TiC-C arrays exhibit a low overpotential of 137 mV at a large current density of 100 mA cm-2 , and a small Tafel slope of 32 mV dec-1 . Our results pave the way to unravel the enhancement mechanism of HER on 2D transition metal dichalcogenides by N doping.

Facile Synthesis of Molybdenum Diselenide Layers for High-Performance Hydrogen Evolution Electrocatalysts

A cost-effective solution-based synthesis route to grow MoSe₂ thin films with vertically aligned atomic layers, thereby maximally exposing the edge sites on the film surface as well as enhancing charge transport to the electrode, is demonstrated for hydrogen evolution reaction. The surface morphologies of thin films are investigated by scanning electron microscopy and atomic force microscopy, and transmission electron microscopy analyses confirm the formation of the vertically aligned layered structure of MoSe₂ in those films, with supporting evidences obtained by Raman. Additionally, their optical and compositional properties are investigated by photoluminescence and X-ray photoelectron spectroscopy, and their electrical properties are evaluated using bottom-gate field-effect transistors. The resultant pristine MoSe₂ thin film exhibited low overpotential of 88 mV (at 10 mA·cm^-2) and a noticeably high exchange current density of 0.845 mA·cm^-2 with excellent stability, which is superior to most of other reported MoS₂ or MoSe₂-based catalysts, even without any other strategies such as doping, phase transformation, and integration with other materials.

Hierarchical ZnIn2 S4 /MoSe2 Nanoarchitectures for Efficient Noble-Metal-Free Photocatalytic Hydrogen Evolution under Visible Light

A highly efficient visible-light-driven photocatalyst is urgently necessary for photocatalytic hydrogen generation through water splitting. Herein, ZnIn₂ S₄ hierarchical architectures assembled as ultrathin nanosheets were synthesized by a facile one-pot polyol approach. Subsequently, the two-dimensional-network-like MoSe₂ was successfully hybridized with ZnIn₂ S₄ by taking advantage of their analogous intrinsic layered morphologies. The noble-metal-free ZnIn₂ S₄ /MoSe₂ heterostructures show enhanced photocatalytic H₂ evolution compared to pure ZnIn₂ S₄ . It is noteworthy that the optimum nanocomposite of ZnIn₂ S₄ /2 % MoSe₂ photocatalyst displays a high H₂ generation rate of 2228 μmol g^-1  h^-1 and an apparent quantum yield (AQY) of 21.39 % at 420 nm. This study presents an unprecedented ZnIn₂ S₄ /MoSe₂ metal-sulfide-metal-selenide hybrid system for H₂ evolution. Importantly, the present efficient hybridization strategy reveals the potential of hierarchical nanoarchitectures for a multitude of energy storage and solar energy conversion applications.