2D Transition‐Metal‐Dichalcogenide‐Nanosheet‐Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions

Hydrogen (H₂) is one of the most important clean and renewable energy sources for future energy sustainability. Nowadays, photocatalytic and electrocatalytic hydrogen evolution reactions (HERs) from water splitting are considered as two of the most efficient methods to convert sustainable energy to the clean energy carrier, H₂. Catalysts based on transition metal dichalcogenides (TMDs) are recognized as greatly promising substitutes for noble‐metal‐based catalysts for HER. The photocatalytic and electrocatalytic activities of TMD nanosheets for the HER can be further improved after hybridization with many kinds of nanomaterials, such as metals, oxides, sulfides, and carbon materials, through different methods including the in situ reduction method, the hot‐injection method, the heating‐up method, the hydro(solvo)thermal method, chemical vapor deposition (CVD), and thermal annealing. Here, recent progress in photocatalytic and electrocatalytic HERs using 2D TMD‐based composites as catalysts is discussed.     

Preparation of High‐Percentage 1T‐Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth‐abundant electrocatalysts to potentially replace precious platinum‐based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low‐density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high‐yield, large‐scale production of water‐dispersed, ultrasmall‐sized, high‐percentage 1T‐phase, single‐layer TMD nanodots with high‐density active edge sites and clean surface, including MoS₂, WS₂, MoSe₂, Mo_0.5W_0.5S₂, and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of ?140 mV at current density of 10 mA cm^?2, a Tafel slope of 40 mV dec^?1, and excellent long‐term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high‐density active edge sites, high‐percentage metallic 1T phase, alloying effect and basal‐plane Se‐vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging.     

Structure determination of chitosan-stabilized Pt and Pd based bimetallic nanoparticles by X-ray photoelectron spectroscopy and transmission electron microscopy

Chitosan (CTS)-stabilized bimetallic nanoparticles were prepared at room temperature (rt.) in aqueous solution. Palladium (Pd) and platinum (Pt) were selected as the first metals while iron (Fe) and nickel (Ni) functioned as the second metals. In order to obtain the noble metal core-transition metal shell structures, bimetallic nanoparticles were prepared in a two-step process: the preparation of mono noble metallic (Pd or Pt) nanoparticles and the deposition of transition metals (Fe or Ni) on the surface of the monometallic nanoparticles. The structures of the nanoparticles were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS results show that Pd and Pt exist mainly in zero valences. The presence of Fe and Ni in the bimetallic nanoparticles affects the binding energy of Pd and Pt. Moreover, the studies of O 1s spectra indicate the presence of Fe or Ni shells. The analyses of TEM micrographs give the particle size and size distributions while the high-resolution TEM (HRTEM) micrographs show the existence of noble metal core lattices. The results confirm the formation of noble metal core-transition metal shell structures.     

Development and test of a new catalytic converter for natural gas fuelled engine

This paper presents characteristics of a new catalytic converter (catco) to be used for natural gas fuelled engine. The catco were developed based on catalyst materials consisting of metal oxides such as titanium dioxide (TiO₂) and cobalt oxide (CoO) with wire mesh substrate. Both of the catalyst materials (such as TiO₂ and CoO) are inexpensive in comparison with conventional catalysts (noble metals) such as palladium or platinum. In addition, the noble metals such as platinum group metals are now identified as human health risk due to their rapid emissions in the environment from various resources like conventional catalytic converter, jewelers and other medical usages. It can be mentioned that the TiO₂/CoO based catalytic converter and a new natural gas engine such as compressed natural gas (CNG) direct injection (DI) engine were developed under a research collaboration program. The original engine manufacture catalytic conveter (OEM catco) was tested for comparison purposes. The OEM catco was based on noble metal catalyst with honeycomb ceramic substrate. It is experimentally found that the conversion efficiencies of TiO₂/CoO based catalytic converter are 93%, 89% and 82% for NO_x, CO and HC emissions respectively. It is calculated that the TiO₂/CoO based catalytic converter reduces 24%, 41% and 40% higher NO_x, CO and HC emissions in comparison to OEM catco respectively. The objective of this paper is to develop a low-cost three way catalytic converter to be used with the newly developed CNG-DI engine. Detailed review on catalytic converter, low-cost catalytic converter development characteristics and CNGDI engine test results have been presented with discussions.     

Size‐Dependent Nonlinear Optical Properties of Atomically Thin Transition Metal Dichalcogenide Nanosheets

Size‐dependent nonlinear optical properties of modification‐free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS₂, WS₂, and NbSe₂. Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size‐dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z‐scan measurements indicate that the dispersion of MoS₂ and WS₂ nanosheets that are 50–60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe₂ nanosheets show no size‐dependent effects because of their metallic nature. The mechanism behind the size‐dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.     

Chloride Ion Mediated Synthesis of Metal/Semiconductor Hybrid Nanocrystals

A synthetic route to prepare metal–semiconductor hybrid nanoparticles is presented, along with the possibility to tune the ratio of primary to secondary nucleation and the morphology of the semiconductor material grown on the metal nanoparticle seeds. Gold and cobalt‐platinum nanoparticles are employed as metal seeds, on which CdS or CdSe is grown. Using transmission electron microscopy, absorption spectroscopy (UV–vis), and powder X‐ray diffraction as characterization techniques, a significant influence of chloride ions on the type of nucleation (that is, secondary or primary nucleation) as well as on the shape of the resulting heterostructures is observed. Partially replacing the commonly used cadmium precursor CdO by varying amounts of CdCl₂ opens access to rod‐like, multiarmed, flower‐like, and bullet‐like structures. The results suggest that neither pure CdO nor pure CdCl₂ as precursors but only a mixture of both make these structures obtainable. In this article, the influence of the chloride ion concentration during semiconductor growth on metal seeds is investigated in depth. The morphology of the resulting heterostructures is characterized carefully, and a growth mechanism is suggested. Furthermore, it is shown that this synthetic approach can be transferred to seeds of various metals such as platinum, gold, and cobalt platinum.     

Impact of H‐Doping on n‐Type TMD Channels for Low‐Temperature Band‐Like Transport

Band‐like transport behavior of H‐doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low‐temperature electrical measurements, where MoTe₂, WSe₂, and MoS₂ are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n‐type conduction and their mobility increases without losing on‐state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p‐ or n‐type. Density functional theory calculations show that H‐doped MoTe₂, WSe₂, and MoS₂ have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H‐doped TMD channels is metallic showing band‐like transport rather than thermal hopping. These results indicate that H‐doped TMD FETs are practically useful even at low‐temperature ranges.     

Highly Dynamic Nanocomposite Hydrogels Self‐Assembled by Metal Ion‐Ligand Coordination

Hydrogels are emerging biomaterials with desirable physicochemical characteristics. Doping of metal ions such as Ca²⁺, Mg²⁺, and Fe²⁺ provides the hydrogels with unique attributes, including bioactivity, conductivity, and tunability. Traditionally, this doping is achieved by the interaction between metal ions and corresponding ligands or the direct incorporation of as‐prepared metal‐based nanoparticles (NPs). However, these approaches rely on a complex and laborious preparation and are typically restricted to few selected ion species. Herein, by mixing aqueous solutions of ligands (bisphosphonates, BPs), polymer grafted with ligands, and metal ions, a series of self‐assembled metallic‐ion nanocomposite hydrogels that are stabilized by the in situ formed ligand‐metal ion (BP‐M) NPs are prepared. Owing to the universal coordination between BPs and multivalent metal ions, the strategy is highly versatile and can be generalized for a wide array of metal ions. Such hydrogels exhibit a wide spectrum of mechanical properties and remarkable dynamic properties, such as excellent injectability, rapid stress relaxation, efficient ion diffusion, and triggered disassembly for harvesting encapsulated cells. Meanwhile, the hydrogels can be conveniently coated or patterned onto the surface of metals via electrophoresis. This work presents a universal strategy to prepare designer nanocomposite materials with highly tunable and dynamic behaviors.     

Thiacalix[4]arene derivatives as extractants for metal ions in aqueous solutions: Application to the selective facilitated transport of Ag(I)

The complexation abilities of different thiacalix[4]arene derivatives towards some rare earth metal ions, metallic pollutants, and noble metals have been investigated in liquid–liquid experiments. Thiacalix[4]arene dissolved in chloroform effectively extracts Pd(II) (in acidic chloride media) and also Ag(I), Cd(II), Sm(III) and Ce(III), all buffered at pH 6 or 8. The modification of this compound to form an amide derivative results in an effective extraction of noble metals, ranked according to Au(III) > Pd(II) > Pt(IV) > Ag(I). Moreover, a supported liquid membrane system for silver transport has been developed based on thiacalix[4]arene dissolved in NPOE, and parameters affecting its efficiency have been investigated, such as the stripping composition and the pH of the feed solution. Finally, the selectivity of the membrane system has been evaluated by using as feed sources mixtures of silver and other metal ions.     

Borocarbonitrides as Metal‐Free Catalysts for the Hydrogen Evolution Reaction

Hydrogen generation by water splitting is clearly a predominant and essential strategy to tackle the problems related to renewable energy. In this context, the discovery of proper catalysts for electrochemical and photochemical water splitting assumes great importance. There is also a serious intent to eliminate platinum and other noble metal catalysts. To replace Pt by a non‐metallic catalyst with desirable characteristics is a challenge. Borocarbonitrides, (B_xC_yN_z) which constitutes a new class of 2D material, offer great promise as non‐metallic catalysts because of the easy tunability of bandgap, surface area, and other electronic properties with variation in composition. Recently, B_xC_yN_z composites with excellent electrochemical and photochemical hydrogen generation activities have been found, especially noteworthy being the observation that B_xC_yN_z with a carbon‐rich composition or its nanocomposites with MoS₂ come close to Pt in electrocatalytic properties, showing equally good photochemical activity.     

A General Method for the Chemical Synthesis of Large‐Scale,Seamless Transition Metal Dichalcogenide Electronics

The capability to directly build atomically thin transition metal dichalcogenide (TMD) devices by chemical synthesis offers important opportunities to achieve large‐scale electronics and optoelectronics with seamless interfaces. Here, a general approach for the chemical synthesis of a variety of TMD (e.g., MoS₂, WS₂, and MoSe₂) device arrays over large areas is reported. During chemical vapor deposition, semiconducting TMD channels and metallic TMD/carbon nanotube (CNT) hybrid electrodes are simultaneously formed on CNT‐patterned substrate, and then coalesce into seamless devices. Chemically synthesized TMD devices exhibit attractive electrical and mechanical properties. It is demonstrated that chemically synthesized MoS₂–MoS₂/CNT devices have Ohmic contacts between MoS₂/CNT hybrid electrodes and MoS₂ channels. In addition, MoS₂–MoS₂/CNT devices show greatly enhanced mechanical stability and photoresponsivity compared with conventional gold‐contacted devices, which makes them suitable for flexible optoelectronics. Accordingly, a highly flexible pixel array based on chemically synthesized MoS₂–MoS₂/CNT photodetectors is applied for image sensing.     

Noble‐Metal‐Free Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) plays a vital role in many energy conversion and storage processes including electrochemical water splitting for the production of hydrogen and carbon dioxide reduction to value‐added chemicals. IrO₂ and RuO₂, known as the state‐of‐the‐art OER electrocatalysts, are severely limited by the high cost and low earth abundance of these noble metals. Developing noble‐metal‐free OER electrocatalysts with high performance has been in great demand. In this review, recent advances in the design and synthesis of noble‐metal‐free OER electrocatalysts including Ni, Co, Fe, Mn‐based hydroxides/oxyhydroxides, oxides, chalcogenides, nitrides, phosphides, and metal‐free compounds in alkaline, neutral as well as acidic electrolytes are summarized. Perspectives are also provided on the fabrication, evaluation of OER electrocatalysts and correlations between the structures of the electrocatalysts and their OER activities.     

Plasmon Modes Induced by Anisotropic Gap Opening in Au@Cu2O Nanorods

Integration of semiconductors with noble metals to form heteronanostructures can give rise to many interesting plasmonic and electronic properties. A number of such heteronanostructures have been demonstrated comprising noble metals and n‐type semiconductors, such as TiO₂, ZnO, SnO₂, Fe₃O₄, and CuO. In contrast, reports on heteronanostructures made of noble metals and p‐type semiconductors are scarce. Cu₂O is an unintentional p‐type semiconductor with unique properties. Here, the uniform coating of Cu₂O on two types of Au nanorods and systematic studies of the plasmonic properties of the resultant core–shell heteronanostructures are reported. One type of Au nanorods is prepared by seed‐mediated growth, and the other is obtained by oxidation of the as‐prepared Au nanorods. The (Au nanorod)@Cu₂O nanostructures produced from the as‐prepared nanorods exhibit two transverse plasmon peaks, whereas those derived from the oxidized nanorods display only one transverse plasmon peak. Through electrodynamic simulations the additional transverse plasmon peak is found to originate from a discontinuous gap formed at the side of the as‐prepared nanorods. The existence of the gap is verified and its formation mechanism is unraveled with additional experiments. The results will be useful for designing metal–semiconductor heteronanostructures with desired plasmonic properties and therefore also for exploring plasmon‐enhanced applications in photocatalysis, solar‐energy harvesting, and biotechnologies.     

pH‐Dependent Galvanic Replacement of Supported and Colloidal Cu2O Nanocrystals with Gold and Palladium

Galvanic replacement reactions (GRRs) on nanoparticles (NPs) are typically performed between two metals, i.e., a solid metal NP and a replacing salt solution of a more noble metal. The solution pH in GRRs is commonly considered an irrelevant parameter. Yet, the solution pH plays a major role in GRRs involving metal oxide NPs. Here, Cu₂O nanocrystals (NCs) are studied as galvanic replacement (GR) precursors, undergoing replacement by gold and palladium, with the resulting nanostructures showing a strong dependence on the pH of the replacing metal salt solution. GRRs are reported for the first time on supported (chemically deposited) oxide NCs and the results are compared with those obtained with corresponding colloidal systems. Control of the pH enables production of different nanostructures, from metal‐decorated Cu₂O NCs to uniformly coated Cu₂O‐in‐metal (Cu₂O@Me) core–shell nanoarchitectures. Improved metal nucleation efficiencies at low pHs are attributed to changes in the Cu₂O surface charge resulting from protonation of the oxide surface. GR followed by etching of the Cu₂O cores provides metal nanocages that collapse upon drying; the latter is prevented using a sol–gel silica overlayer stabilizing the metal nanocages. Metal‐replaced Cu₂O NCs and their corresponding stabilized nanostructures may be useful as photocatalysts, electrocatalysts, and nanosensors.     

Substrate effect of STM-induced luminescence from porphyrin molecules

We investigated scanning tunneling microscope (STM)-induced luminescence from Meso-tetrakis (3,5-di-tertiarybutyl-phenyl) porphyrin (H₂TBPP) thin films on various metal substrates (copper, platinum and silver) under ambient conditions. Molecular fluorescence similar to the corresponding photoluminescence was observed from the H₂TBPP/Ag and H₂TBPP/Pt, but not H₂TBPP/Cu, depending on the spectra of the surface plasmons of the substrates. The results support the mechanism that intense molecular fluorescence from porphyrin film on the noble metals is a result of enhancement of molecular excitation by substrate surface plasmons.     

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface‐Enhanced Raman Spectroscopy Substrates with High Sensitivity

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface‐enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO₂) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10^?8m and enhancement factor up to 2.1 × 10⁵, exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO₂ substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle‐based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV–vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO₂ nanosheets. The overall superior performance makes the MoO₂ nanosheets an ideal substrate for practical SERS applications.     

Spontaneous Formation of Noble‐ and Heavy‐Metal‐Free Alloyed Semiconductor Quantum Rods for Efficient Photocatalysis

Quasi‐1D cadmium chalcogenide quantum rods (QRs) are benchmark semiconductor materials that are combined with noble metals to constitute QR heterostructures for efficient photocatalysis. However, the high toxicity of cadmium and cost of noble metals are the main obstacles to their widespread use. Herein, a facile colloidal synthetic approach is reported that leads to the spontaneous formation of cadmium‐free alloyed ZnS_xSe_1?x QRs from polydisperse ZnSe nanowires by alkylthiol etching. The obtained non‐noble‐metal ZnS_xSe_1?x QRs can not only be directly adopted as efficient photocatalysts for water oxidation, showing a striking oxygen evolution capability of 3000 µmol g^?1 h^?1, but also be utilized to prepare QR‐sensitized TiO₂ photoanodes which present enhanced photo‐electrochemical (PEC) activity. Density functional theory (DFT) simulations reveal that alloyed ZnS_xSe_1?x QRs have highly active Zn sites on the (100) surface and reduced energy barrier for oxygen evolution, which in turn, are beneficial to their outstanding photocatalytic and PEC activities.     

Application of electrochemical methods to aqueous reprocessing of spent nuclear fuel

Published data demonstrating the possibilities of electrochemical methods as applied to solving particular problems arising in various steps of industrial reprocessing of spent nuclear fuel using the Purex process are summarized. Attention is given to stabilization of U, Np, and Pu in required valence states in aqueous solutions and two-phase systems in the course of Pu stripping in the step of its separation from U, to efficient dissolution of PuO₂, to isolation of noble metals from high-level liquid waste, and to breakdown of spent organic solvents and complexing agents. Progress in the industrial use of electrolysis, in search for new electrode materials, and in studies of the mechanisms of electrocatalytic oxidation processes is noted.     

Enhanced Photoelectrochemical Performance in Reduced Graphene Oxide/BiFeO3 Heterostructures

BiFeO₃ (BFO)‐based ferroelectrics have been proved to be visible‐light‐driven photoelectrodes for O₂ production. However, the hitherto reported photoelectrochemical performances remain inferior to meet the requirements for any applications. Besides, expensive noble metals (Ag, Au) are commonly required to achieve high photoelectric conversion efficiency. Here, the significant enhancements of photoelectrochemical performance is reported by fabricating a noble‐metal‐free reduced graphene oxide (RGO)/BFO composite film via a simple and cost‐effective solution process. The optimized RGO/BFO composite film exhibits a 600% improvement of the short‐circuit photocurrent density compared to that of the pristine BFO, and also outperforms the noble‐metal/BFO cells under the same reaction conditions. Furthermore, the incident photon‐to‐current efficiency of the optimized RGO/BFO sample shows threefold enhancement. This study delivers a facile and low‐cost approach to preparing 2D materials/ferroelectric heterostructures and offers a promising pathway to boost the performance of semiconducting ferroelectric photoelectrodes.     

Transition‐Metal‐Ions‐Induced Coalescence: Stitching Au Nanoclusters into Tubular Au‐Based Nanocomposites

Rod‐shaped assemblages of Au nanoclusters (AuNCs) can serve as self‐templating solid precursors to produce tubular Au‐based nanocomposites via the coalescence induced by transition metal ions. Specifically, when the AuNC assemblages react with transition metal ions with relatively high standard oxidation potentials such as Cu(II), Ag(I), Pd(II), and Au(III), a series of polycrystalline and ultrathin Au and Au_xM_y (where M = Cu, Ag, and Pd) alloy hollow nanorods (HNRs) can be obtained with further reduction; these metallic products are evaluated for electrooxidation of methanol. Alternatively, the above transition metal ions‐induced transformations can also be carried out after coating the AuNC assemblages with a layer of mesoporous SiO₂ (mSiO₂), giving rise to many mSiO₂‐coated Au‐based HNRs. Onto the formed AuPd_0.18 alloy HNRs, furthermore, a range of transition metal oxides such as TiO₂, Co₃O_4, and Cu₂O nanocrystals can be deposited easily to prepare metal oxide–AuPd_0.18 HNRs nanocomposites, which can be used as photocatalysts. Compared with those conventional galvanic replacement reactions, the controlled coalescence of AuNCs induced by transition metal ions provides a novel and efficient chemical approach with improved element efficiency to tubular Au‐based nanocomposites.