Size‐Dependent Enhancement of Electrocatalytic Oxygen‐Reduction and Hydrogen‐Evolution Performance of MoS2 Particles

MoS₂ particles with different size distributions were prepared by simple ultrasonication of bulk MoS₂ followed by gradient centrifugation. Relative to the inert microscale MoS₂, nanoscale MoS₂ showed significantly improved catalytic activity toward the oxygen‐reduction reaction (ORR) and hydrogen‐evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four‐electron ORR process and an HER onset potential of ?0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size‐dependent effect of the ORR activity of MoS₂, and a four‐electron transfer route was found. The exposed abundant Mo edges of the MoS₂ nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS₂ catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS₂ nanoparticles in the fields of energy conversion and storage.     

Surfactant–Thermal Method to Synthesize a Novel Two‐Dimensional Oxochalcogenide

A new two‐dimensional (2D) oxosulfide, (N₂H₄)₂Mn₃Sb₄S₈(μ₃‐OH)₂ ( 1 ), has been successfully synthesized under surfactant–thermal conditions with hexadecyltributylphosphonium bromide as the surfactant. Compound 1 has a layered structure and contains a novel [Mn₃(μ₃‐OH)₂]_n chain along the b‐axis. The photocatalytic activity for compound 1 has been demonstrated under visible‐light irradiation and continuous H₂ evolution was observed. Our results indicate that surfactant–thermal synthesis could be a promising method for growing novel crystalline oxochalcogenides with interesting structures and properties.     

Dimeric [Mo2S12]2− Cluster: A Molecular Analogue of MoS2 Edges for Superior Hydrogen‐Evolution Electrocatalysis

Proton reduction is one of the most fundamental and important reactions in nature. MoS₂ edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS₂ edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein we report a dimeric molecular analogue [Mo₂S₁₂]^2?, as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo₂S₁₂]^2? is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo₂S₁₂]^2? exhibits a hydrogen adsorption free energy near zero (?0.05 eV). This work helps shed light on the rational design of HER catalysts and biomimetics of hydrogen‐evolving enzymes.     

Zirconium–Porphyrin‐Based Metal–Organic Framework Hollow Nanotubes for Immobilization of Noble‐Metal Single Atoms

Single atoms immobilized on metal–organic frameworks (MOFs) with unique nanostructures have drawn tremendous attention in the application of catalysis but remain a great challenge. Various single noble‐metal atoms have now been successfully anchored on the well‐defined anchoring sites of the zirconium porphyrin MOF hollow nanotubes, which are probed by aberration‐corrected scanning transmission electron microscopy and synchrotron‐radiation‐based X‐ray absorption fine‐structure spectroscopy. Owing to the hollow structure and excellent photoelectrochemical performance, the HNTM‐Ir/Pt exhibits outstanding catalytic activity in the visible‐light photocatalytic H₂ evolution via water splitting. The single atom immobilized on MOFs with hollow structures are expected to pave the way to expand the potential applications of MOFs.     

Aligned MoOx/MoS2 Core–Shell Nanotubular Structures with a High Density of Reactive Sites Based on Self‐Ordered Anodic Molybdenum Oxide Nanotubes

The present work demonstrates the self‐organized formation of anodic molybdenum oxide nanotube arrays. The amorphous tubes can be crystallized to MoO₂ or MoO₃ and be converted fully or partially into molybdenum sulfide. Vertically aligned MoO_x/MoS₂ nanotubes can be formed when, under optimized conditions, defined MoS₂ sheets form in a layer by layer arrangement that provide a high density of reactive stacking misalignments (defects). These core–shell nanotube arrays consist of a conductive suboxide core and a functional high defect density MoS₂ coating. Such structures are highly promising for applications in electrocatalysis (hydrogen evolution) or ion insertion devices.     

MoS2/WS2‐Graphene Composites through Thermal Decomposition of Tetrathiomolybdate/Tetrathiotungstate for Proton/Oxygen Electroreduction

MoS₂ and WS₂ have been prepared on a conductive graphene support by thermal reduction of tetrathiotungstate/tetrathiomolybdate and graphite oxide. Whereas the catalytic properties towards hydrogen evolution are strongly influenced by the Magnéli phases formed as a byproduct during the synthesis, the catalytic activity towards oxygen reduction of these composite materials is not affected by this phenomenon and these materials exhibit high catalytic activity towards this industrially important reaction.     

Hierarchical β‐Mo2C Nanotubes Organized by Ultrathin Nanosheets as a Highly Efficient Electrocatalyst for Hydrogen Production

Production of hydrogen by electrochemical water splitting has been hindered by the high cost of precious metal catalysts, such as Pt, for the hydrogen evolution reaction (HER). In this work, novel hierarchical β‐Mo₂C nanotubes constructed from porous nanosheets have been fabricated and investigated as a high‐performance and low‐cost electrocatalyst for HER. An unusual template‐engaged strategy has been utilized to controllably synthesize Mo‐polydopamine nanotubes, which are further converted into hierarchical β‐Mo₂C nanotubes by direct carburization at high temperature. Benefitting from several structural advantages including ultrafine primary nanocrystallites, large exposed surface, fast charge transfer, and unique tubular structure, the as‐prepared hierarchical β‐Mo₂C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions, as well as remarkable stability.     

Defect Engineering of MoS2 and Its Impacts on Electrocatalytic and Photocatalytic Behavior in Hydrogen Evolution Reactions

Molybdenum disulfide (MoS₂) has been regarded as a favorable photocatalytic co‐catalyst and efficient hydrogen evolution reaction (HER) electrocatalyst alternative to expensive noble‐metals catalysts, owing to earth‐abundance, proper band gap, high surface area, and fast electron transfer ability. In order to achieve a higher catalytic efficiency, defects strategies such as phase engineering and vacancy introduction are considered as promising methods for natural 2H‐MoS₂ to increase its active sites and promote electron transfer rate. In this study, we report a new two‐step defect engineering process to generate vacancies‐rich hybrid‐phase MoS₂ and to introduce Ru particles at the same time, which includes hydrothermal reaction and a subsequent hydrogen reduction. Compositional and structural properties of the synthesized defects‐rich MoS₂ are investigated by XRD, XPS, XAFS and Raman measurements, and the electrochemical hydrogen evolution reaction performance, as well as photocatalytic hydrogen evolution performance in the ammonia borane dehydrogenation are evaluated. Both catalytic activities are boosted with the increase of defects concentrations in MoS₂, which ascertains that the defects engineering is a promising route to promote catalytic performance of MoS₂.     

A Titanium Metal–Organic Framework with Visible‐Light‐Responsive Photocatalytic Activity

The one‐step synthesis and characterization of a new and robust titanium‐based metal–organic framework, ACM‐1 , is reported. In this structure, which is based on infinite Ti?O chains and 4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayl) tetrabenzoic acid as a photosensitizer ligand, the combination of highly mobile photogenerated electrons and a strong hole localization at the organic linker results in large charge‐separation lifetimes. The suitable energies for band gap and conduction band minimum (CBM) offer great potential for a wide range of photocatalytic reactions, from hydrogen evolution to the selective oxidation of organic substrates.     

Facile Formation of Redox‐Active Totally Organic Nanoparticles in Water by In Situ Reduction of Organic Precursors Stabilized through Aromatic–Aromatic Interactions by Aromatic Polyelectrolytes

The formation of redox‐active, totally organic nanoparticles in water is achieved following a strategy similar to that used to form metal nanoparticles. It is based on two fundamental concepts: i) complexation through aromatic–aromatic interactions of a water‐soluble precursor aromatic molecule with polyelectrolytes bearing complementary charged aromatic rings, and ii) reduction of the precursor molecule to achieve stabilized nanoparticles. Thus, formazan nanoparticles are synthesized by reduction of a tetrazolium salt with ascorbic acid using polyelectrolytes bearing benzene sulfonate residues of high linear aromatic density, but cannot be formed in the presence of nonaromatic polyelectrolytes. The red colored nanoparticles are efficiently encapsulated in calcium alginate beads, showing macroscopic homogeneity. Bleaching kinetics with chlorine show linear rates on the order of tenths of milli­meters per minute. A linear behavior of the dependence of the rate of bleaching on the chlorine concentration is found, showing the potential of the nanoparticles for chlorine sensing.

     

Molybdenum Carbide‐Oxide Heterostructures: In Situ Surface Reconfiguration toward Efficient Electrocatalytic Hydrogen Evolution

Heterostructured Mo₂C‐MoO_x on carbon cloth (Mo₂C‐MoO_x/CC), as a model of easily oxidized electrocatalysts under ambient conditions, is investigated to uncover surface reconfiguration during the hydrogen evolution reaction (HER). Raman spectroscopy combined with electrochemical tests demonstrates that the Mo^VI oxides on the surface are in situ reduced to Mo^IV, accomplishing promoted HER in acidic condition. As indicated by density functional theoretical calculations, the in situ reduced surface with terminal Mo=O moieties can effectively bring the negative ΔG_H* on bare Mo₂C close to a thermodynamic neutral value, addressing difficult H* desorption toward fast HER kinetics. The optimized Mo₂C‐MoO_x/CC only requires a low overpotential (η₁₀) of 60 mV at ?10 mA cm^?2 in 1.0 m HClO₄, outperforming Mo₂C/CC and most non‐precious electrocatalysts. In situ surface reconfiguration are shown on W₂C‐WO_x, highlighting the significance to boost various metal‐carbides and to identify active sites.     

Enhanced Photocatalytic Hydrogen‐Production Performance of Graphene–ZnxCd1−xS Composites by Using an Organic S Source

In response to the increasing concerns over energy and environmental sustainability, photocatalytic water‐splitting technology has attracted broad attention for its application in directly converting solar energy to valuable hydrogen (H₂) energy. In this study, high‐efficiency visible‐light‐driven photocatalytic H₂ production without the assistance of precious‐metal cocatalysts was achieved on graphene–Zn_xCd_1?xS composites with controlled compositions. The graphene‐Zn_xCd_1?xS composites were for the first time fabricated by a one‐step hydrothermal method with thiourea as an organic S source. It was found that thiourea facilitates heterogeneous nucleation of Zn_xCd_1?xS and in situ growth of Zn_xCd_1?xS nanoparticles on graphene nanosheets. Such a scenario results in abundant and intimate interfacial contact between graphene and Zn_xCd_1?xS nanoparticles, efficient transfer of the photogenerated charge carriers, and enhanced photocatalytic activity for H₂ production. The highest H₂‐production rate of 1.06 mmol h^?1 g^?1 was achieved on a graphene–Zn_0.5Cd_0.5S composite photocatalyst with a graphene content of 0.5 wt %, and the apparent quantum efficiency was 19.8 % at 420 nm. In comparison, the graphene–Zn_xCd_1?xS composite photocatalyst prepared by using an inorganic S source such as Na₂S exhibited much lower activity for photocatalytic H₂ production. In this case, homogeneous nucleation of Zn_xCd_1?xS becomes predominant and results in insufficient and loose contact with the graphene backbone through weak van der Waals forces and a large particle size. This study highlights the significance of the choice of S source in the design and fabrication of advanced graphene‐based sulfide photocatalytic materials with enhanced activity for photocatalytic H₂ production.     

ZnCr Layered Double Hydroxide (LDH) Nanosheets Assisted Formation of Hierarchical Flower‐Like CdZnS@LDH Microstructures with Improved Visible‐Light‐Driven H2 Production

The development of new semiconductor photocatalysts toward splitting water has supplied a promising way to obtain sustainable and clean hydrogen energy. Herein, CdZnS@layered double hydroxide (LDH) composites with a hierarchical flower‐like microstructure have been fabricated with the aid of ZnCr–LDH nanosheets as templates. XRD, SEM and HRTEM show that the ZnCr–LDH nanosheets are uniformly dispersed within the composites. The surface of the hierarchical structures is rough and composed of numerous nanocrystals of CdZnS. The HRTEM images indicate that the surface of CdZnS nanocrystals is mainly composed of the (111) plane. Moreover, the visible‐light‐driven H₂ production performance of the CdZnS in the presence and absence of ZnCr–LDH nanosheets has been measured. The results show that ZnCr–LDH nanosheets play an important role in the hierarchical morphology and photocatalytic activity of the as‐prepared samples. In the water‐splitting process, the visible‐light‐driven H₂‐production rate of hierarchical flower‐like CdZnS@LDH is 4.03 times and nearly 10 times higher than that of pristine CdZnS microsphere and pure commercial CdS, respectively. Therefore, this work not only achieves enhanced catalytic performance of the CdZnS by the introduction of ZnCr–LDH nanosheets, but also supplies an insight into the relationship between the hierarchical morphology and the semiconductor photocatalytic activity.     

Hierarchical Integration of Photosensitizing Metal–Organic Frameworks and Nickel‐Containing Polyoxometalates for Efficient Visible‐Light‐Driven Hydrogen Evolution

Metal–organic frameworks (MOFs) provide a tunable platform for hierarchically integrating multiple components to effect synergistic functions that cannot be achieved in solution. Here we report the encapsulation of a Ni‐containing polyoxometalate (POM) [Ni₄(H₂O)₂(PW₉O₃₄)₂]^10? ( Ni₄P₂ ) into two highly stable and porous phosphorescent MOFs. The proximity of Ni₄P₂ to multiple photosensitizers in Ni₄P₂ @MOF allows for facile multi‐electron transfer to enable efficient visible‐light‐driven hydrogen evolution reaction (HER) with turnover numbers as high as 1476. Photophysical and electrochemical studies established the oxidative quenching of the excited photosensitizer by Ni₄P₂ as the initiating step of HER and explained the drastic catalytic activity difference of the two POM@MOFs. Our work shows that POM@MOF assemblies not only provide a tunable platform for designing highly effective photocatalytic HER catalysts but also facilitate detailed mechanistic understanding of HER processes.     

Gold Nanoclusters Grown on MoS2 Nanosheets by Pulsed Laser Deposition: An Enhanced Hydrogen Evolution Reaction

Au nanoparticles were decorated on a 2H MoS₂ surface to form an Au/MoS₂ composite by pulse laser deposition. Improved HER activity of Au/MoS₂ is evidenced by a positively shifted overpotential (−77 mV) at a current density of −10 mA cm⁻² compared with pure MoS₂ nanosheets. Experimental evidence shows that the interface between Au and MoS₂ provides more sites to combine protons to form an active H atom. The density functional theory calculations found that new Au active sites on the Au and MoS₂ interface with improved conductivity of the whole system are essential for enhancing HER activity of Au/MoS₂.