Solution-phase epitaxial growth of noble metal nanostructures on dispersible single-layer molybdenum disulfide nanosheets

Tailoring the physicochemical properties of solution-processed transition metal dichalcogenides via molecular approaches

In this Feature Article we highlight the tremendous progress in solution-processed transition metal dichalcogenides and the molecular approaches employed to finely tune their physicochemical properties.

A flower-structured MoS2-decorated f-MWCNTs/ZnO hybrid nanocomposite-modified sensor for the selective electrochemical detection of vitamin C

We synthesized an MoS₂/f-MWCNTs/ZnO composite and successfully used it to prepare an electrochemical sensor for the selective detection of AA in blood serum samples.

Direct growth of oriented nanocrystals of gamma-iron on graphene oxide substrates. Detailed analysis of the factors affecting unexpected formation of the gamma-iron phase

Substrate-oriented nanocrystals of room-temperature-stable gamma-iron have been synthesized by a two-step impregnation/annealing method on a graphene oxide surface.

Synergistic Nanozymetic Activity of Hybrid Gold Bipyramid-Molybdenum Disulfide Core@Shell Nanostructures for Two-Photon Imaging and Anticancer Therapy

In recent years, the concept of combined therapy using gold hybrid nanomaterials has been broadly adopted to pioneer new anticancer treatments. However, their synergistic anticancer effects have yet to be thoroughly investigated. Herein,a hybrid gold nanobipyramid nanostructure coated with molybdenum disulfide (MoS₂) semiconductor (AuNBPs@MoS₂) was proposed as a smart nanozyme for anticancer therapy and two-photon bioimaging. The hybrid material showed dramatically enhanced localized surface plasmon resonance property under excitation owing to its anisotropic nature, coupled with the rich electron density in MoS_2, resulting in the superior in situ photogeneration of reactive oxidative species (ROS - ¹O₂, ^•OH). We demonstrated that the synergistic effect of enhanced photothermal conversion and generation of ROS could increase the anticancer effect of AuNBPs@MoS₂. Two-photon luminescence imaging confirmed that AuNBPs@MoS₂ was successfully internalized in cancer cells and that simultaneous anticancer treatments based on catalytic and photothermal therapy could be achieved. This study highlighted, for the first time, a novel approach of plasmon-mediated powerful anticancer therapy and imaging via the unprecedented combination of anisotropic AuNBPs and two-dimensional MoS₂ material.

Atomic-scale imaging of the ferrimagnetic/diamagnetic interface in Au-Fe3O4 nanodimers and correlated exchange-bias origin

Exchange-biased magnetic heterostructures have become one of the research frontiers due to their significance in enriching the fundamental knowledge in nanomagnetics and promising diverse applications in the information industry. However, the physical origin of their exchange bias effect is still controversial. A key reason for this is the lack of unequivocal observations of interface growth. In this work, we fill this gap by experimentally imaging the ferrimagnetic/diamagnetic interfaces of Au-Fe₃O₄ nanodimers at the atomic level. A different physical mechanism from the reported mechanisms is found based on the atomic-resolution observation of their interfacial structure and electronic states, which reveals that the antiferromagnetic and ferromagnetic interactions of the formed weak/strong ferrimagnetic bilayer are responsible for the intrinsic exchange-bias origin in Au-Fe₃O₄ nanodimers. The theoretical quantitative analysis of the exchange bias shift based on the observed interfacial occupation model agrees well with the experimental value for the exchange bias effect, strongly verifying the proposed exchange-bias mechanism.

Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2

Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by nonradiative energy dissipation, has greatly limited practical applications. To reveal the details for nonradiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS₂. With this method, the many-body effects, occupation effects, and phonon dynamics are clearly identified. In particular, thermalization of the MoS₂ lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within tens to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (∼9 ps) is mainly dissipated from the MoS₂ surfaces to external phonons.

Realization of vertical metal semiconductor heterostructures via solution phase epitaxy

The creation of crystal phase heterostructures of transition metal chalcogenides, e.g., the 1T/2H heterostructures, has led to the formation of metal/semiconductor junctions with low potential barriers. Very differently, post-transition metal chalcogenides are semiconductors regardless of their phases. Herein, we report, based on experimental and simulation results, that alloying between 1T-SnS₂ and 1T-WS₂ induces a charge redistribution in Sn and W to realize metallic Sn_0.5W_0.5S₂ nanosheets. These nanosheets are epitaxially deposited on surfaces of semiconducting SnS₂ nanoplates to form vertical heterostructures. The ohmic-like contact formed at the Sn_0.5W_0.5S₂/SnS₂ heterointerface affords rapid transport of charge carriers, and allows for the fabrication of fast photodetectors. Such facile charge transfer, combined with a high surface affinity for acetone molecules, further enables their use as highly selective 100 ppb level acetone sensors. Our work suggests that combining compositional and structural control in solution-phase epitaxy holds promises for solution-processible thin-film optoelectronics and sensors.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

In Situ Synthesis of Gold Nanoparticles/Metal-Organic Gels Hybrids with Excellent Peroxidase-Like Activity for Sensitive Chemiluminescence Detection of Organophosphorus Pesticides

Until now, despite much progress in the study of metal-organic gels (MOGs), the modification of transition-metal containing MOGs with noble metal nanoparticles (NPs) is far from fully developed. Herein, iron-based MOGs nanosheet hybrids with gold NPs (AuNPs) immobilization were first synthesized by a facile in situ grown strategy at ambient conditions. It is found that the as-prepared AuNPs/MOGs (Fe) hybrids exhibited enhanced mimicking peroxidase-like activity, making them endowed with outstanding performance in chemiluminescence (CL) field in the presence of H₂O₂. The remarkable CL enhancement by AuNPs/MOGs (Fe) hybrids was attributed to the modification of AuNPs on MOGs (Fe) nanosheets, which could synergistically accelerate the CL reaction by speeding up the generation of OH^•, O₂^•-, and ¹O₂. Accordingly, a sensitive CL detection of organophosphorus pesticides was successfully achieved by the AuNPs/MOGs (Fe) hybrids CL enhancing system in the range of 5-800 nM with a detection limit of 1 nM. We envision that this highly active and novel enzyme mimetic catalyst can be applicable to other extended AuNPs/MOGs (Fe) hybrid-based CL systems for sensitive detection of various analytes.

In situ study of nucleation and growth dynamics of Au nanoparticles on MoS2 nanoflakes

Two-dimensional (2D) substrates decorated with metal nanoparticles offer new opportunities to achieve high-performance catalytic behavior. However, little is known on how the substrates control the nucleation and growth processes of the nanoparticles. This paper presents the visualization of dynamic nucleation and growth processes of gold nanoparticles on ultrathin MoS2 nanoflakes by in situ liquid-cell transmission electron microscopy (TEM). The galvanic displacement resulting in Au nuclei formation on MoS2 was observed in real time inside the liquid cell. We found that the growth mechanism of Au particles on pristine MoS2 is in between diffusion-limited and reaction-limited, possibly due to the presence of electrochemical Ostwald ripening. A larger size distribution and more orientation variation is observed for the Au particles along the MoS2 edge than on the interior. Differing from pristine MoS2, sulfur vacancies on MoS2 induce Au particle diffusion and coalescence during the growth process. Density functional theory (DFT) calculations show that the size difference is because the exposed molybdenum atoms at the edge with dangling bonds can strongly interact with Au atoms, whereas sulfur atoms on the MoS2 interior have no dangling bonds and weakly interact with gold atoms. In addition, S vacancies on MoS2 generate strong nucleation centers that can promote diffusion and coalescence of Au nanoparticles. The present work provides key insights into the role of 2D materials in controlling the size and orientation of noble metal nanoparticles vital to the design of next generation catalysts.

Room temperature processed high mobility W-doped In2O3 electrodes coated via in-line arc plasma ion plating for flexible OLEDs and quantum dots LEDs

We fabricated W-doped In₂O₃ (IWO) films at room temperature on a flexible PET substrate using an in-line arc plasma ion plating system for application as flexible transparent conducting electrodes (FTCEs) in flexible organic light emitting diodes (OLEDs) and quantum dots light emitting diodes (QDLEDs). Due to the high-energy flux of the sublimated ions generated from the plasma region, the IWO films showed a well-developed crystalline structure with a low sheet resistance of 36.39 Ohm/square and an optical transmittance of 94.6% even though they were prepared at room temperature. The low sheet resistance of the IWO film processed at room temperature is attributed to the high mobility (59 cm²/V-s) in the well-developed crystalline structure of the ion-plated IWO film and screening effect of W dopants. In addition, the better adhesion of the ion-plated IWO film on the PET substrate led to small critical outer and inner bending radii of 6 and 3 mm, respectively, against substrate bending. Due to the low sheet resistance, high optical transmittance, better crystallinity, better adhesion, and outstanding flexibility of the ion-plated IWO films, the flexible OLEDs and QDLEDs with the IWO electrodes showed better performances than flexible OLEDs and QDLEDs with sputtered flexible ITO anodes. This indicates that in-line arc plasma ion plating is a promising large area coating technique to realize room temperature processed high-quality FTCEs for flexible OLEDs and QDLEDs.

Nitrogen-rich 1T'-MoS2 layered nanostructures using alkyl amines for high catalytic performance toward hydrogen evolution

The imminent global energy crisis and current environmental issues have stimulated considerable research on high-performance catalysts for sustainable hydrogen energy generation. Two-dimensional layered MoS2 has recently drawn worldwide attention because of its excellent catalytic properties for the hydrogen evolution reaction (HER). In the present work, we prepared nitrogen (N)-rich 1T' (distorted 1T) phase MoS2 layered nanostructures using different alkyl amines with 1-4 nitrogen atoms (methylamine, ethylenediamine, diethylenetriamine, and triethylenetetramine) as intercalants. The amine molecules intercalate at 10 atomic%, and simultaneously supply the N atoms that substitute the S atoms to produce the N-doped MoS2, whose composition is MoS2(1-x)Nx, where x = 0.1-0.26. MoS2 prepared with amines having more N atoms has enhanced catalytic HER performance: a Tafel slope of 36 mV dec-1 and 10 mA cm-2 at -160 mV (vs. RHE). First-principles calculations showed that the amine intercalation and N doping increase the density of states near the Fermi level in a narrow range and bring about an effective overlap of the dz2(Mo), pz(S), and pz(N) states. These factors in turn increase the carrier (electron) concentration and mobility for improved HER. The calculation also predicted that the most active site is S vacancies. The present work illustrates how the HER catalytic performance of 1T' phase MoS2 can be effectively controlled by the amine molecules.

Roll-to-roll sputtered and patterned Cu2-x O/Cu/Cu2-x O multilayer grid electrode for flexible smart windows

We fabricated cost-effective Cu2-x O/Cu/Cu2-x O multilayer grid electrodes using roll-to-roll (RTR) sputtering and patterning processes for use as transparent and flexible electrodes in flexible smart windows. To optimize the patterned Cu2-x O/Cu/Cu2-x O multilayer grid, the electrical and optical properties of the Cu2-x O/Cu/Cu2-x O multilayer grid electrodes were investigated as a function of grid width and pitch, which directly influence the filling factor of the grid. At the optimized grid width of 16 and pitch of 600 μm, the Cu2-x O/Cu/Cu2-x O multilayer grid had a sheet resistance of 7.17 Ohm per square and an optical transmittance of 87.6%. In addition, the mechanical properties of the optimized Cu2-x O/Cu/Cu2-x O multilayer grid electrode was compared to those of brittle ITO electrodes to demonstrate its outstanding flexibility. To show the potential of the Cu2-x O/Cu/Cu2-x O multilayer grid for smart windows, we fabricated a flexible and transparent thin film heater (TFH) and a flexible electrochromic (EC) device, which are key components of smart windows. The low saturation voltage of the Cu2-x O/Cu/Cu2-x O grid-based TFH and the fast on-off performance of the Cu2-x O/Cu/Cu2-x O grid-based EC device indicates that the RTR-processed Cu2-x O/Cu/Cu2-x O multilayer grid is promising as a low-cost and large-area flexible transparent electrode for high-performance smart windows.

Solution-Phase Synthesis of Platinum Nanoparticle-Decorated Metal-Organic Framework Hybrid Nanomaterials as Biomimetic Nanoenzymes for Biosensing Applications

The synthesis of nanomaterials with specific properties and functions as biomimetic nanoenzymes has attracted extensive attention in the past decades due to their great potential to substitute natural enzymes. Herein, a facile and simple method for the preparation of platinum nanoparticle (PtNP)-decorated two-dimensional metal-organic framework (MOF) nanocomposites was developed. A ligand with heme-like structure, Fe(III) tetra(4-carboxyphenyl)porphine chloride (TCPP(Fe)), was applied to synthesize MOF nanosheets (denoted as Cu-TCPP(Fe) nanosheets) in high yield. Ultrathin Cu-TCPP(Fe) nanosheets with thickness less than 10 nm were used as a novel template for the growth of ultrasmall and uniform PtNPs. Significantly, the obtained hybrid nanomaterials (PtNPs/Cu-TCPP(Fe) hybrid nanosheets) exhibit enhanced peroxidase-like activity compared to PtNPs, Cu-TCPP(Fe) nanosheets, and the physical mixture of both due to the synergistic effect. On account of the excellent peroxidase-like activity of PtNPs/Cu-TCPP(Fe) hybrid nanosheets, we established a colorimetric method for sensitive and rapid detection of hydrogen peroxide. Furthermore, by combining with glucose oxidase, a cascade colorimetric method was established to further detect glucose with excellent sensitivity and selectivity.

Size and Electronic Modulation of Iridium Nanoparticles on Nitrogen-Functionalized Carbon toward Advanced Electrocatalysts for Alkaline Water Splitting

Developing efficient catalytic materials for electrochemical water splitting is important. Herein, uniformly dispersed and size-controllable iridium (Ir) nanoparticles (NPs) were prepared using a nitrogen-functionalized carbon as the support (Ir/CN). We found that nitrogen functionalization can simultaneously modulate the size of Ir NPs to substantially enhance the catalytically active sites and adjust the electronic structure of Ir, thereby promoting electrocatalytic activity for water splitting. Consequently, the as-synthesized Ir/CN shows excellent electrocatalytic performance with overpotentials of 12 and 265 mV for hydrogen and oxygen evolution reactions in basic medium, respectively. These findings may pave the way for designing and synthesizing other similar materials as efficient catalysts for electrochemical water splitting.

Two-Dimensional Metal Nanomaterials: Synthesis, Properties, and Applications

As one unique group of two-dimensional (2D) nanomaterials, 2D metal nanomaterials have drawn increasing attention owing to their intriguing physiochemical properties and broad range of promising applications. In this Review, we briefly introduce the general synthetic strategies applied to 2D metal nanomaterials, followed by describing in detail the various synthetic methods classified in two categories, i.e. bottom-up methods and top-down methods. After introducing the unique physical and chemical properties of 2D metal nanomaterials, the potential applications of 2D metal nanomaterials in catalysis, surface enhanced Raman scattering, sensing, bioimaging, solar cells, and photothermal therapy are discussed in detail. Finally, the challenges and opportunities in this promising research area are proposed.

Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures

The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

BSA-caged metal clusters to exfoliate MoS2 nanosheets towards their hybridized functionalization

Herein, we develop a facile exfoliation and in situ functionalization strategy to produce hybridized Au/MoS2 nanostructures comprised of size-controlled gold nanoparticles (Au NPs) and ultrathin MoS2 nanosheets by using bovine serum albumin (BSA)-caged Au25 clusters as both exfoliating and functionalizing agents. As revealed, BSA molecules are strongly adsorbed on MoS2via their hydrophobic interaction, and this drives the expansion of the BSA molecules that initially protect Au25 cores at pH 4, leading to the effective exfoliation of MoS2 nanosheets together with the epitaxial growth of Au25 cores into 5 nm-sized Au NPs on MoS2 nanosheets due to their reduced surface protection. Upon the addition of H2O2, the resulting Au NPs can further grow to achieve a controlled size from 5 to 30 nm with an increase of the reaction time. It is demonstrated that the hybridized Au/MoS2 nanosheets exhibit a better performance in the photocatalytic degradation of substrates compared to the individual components or their mixture. Moreover, the hybridized Ag/MoS2, Au/WO3 and Au/graphene nanosheets are further produced by the usage of BSA-caged Ag and Au clusters, respectively. Overall, this work reports the first utilization of protein-caged metal clusters for the exfoliation and hybridized functionalization of 2D materials, and this brings more opportunities to exploit unusual properties of hybridized 2D materials for novel applications.

Prediction of Isoelectric Point of Manganese and Cobalt Lamellar Oxides: Application to Controlled Synthesis of Mixed Oxides

To design novel layered materials, bottom-up strategy is very promising. It consists of (1) synthesizing various layered oxides, (2) exfoliating them, then (3) restacking them in a controlled way. The last step is based on electrostatic interactions between different layered oxides and is difficult to control. The aim of this study is to facilitate this step by predicting the isoelectric point (IEP) of exfoliated materials. The Multisite Complexation model (MUSIC) was used for this objective and was shown to be able to predict IEP from the mean oxidation state of the metal in the (hydr)oxides, as the main parameter. Moreover, the effect of exfoliation on IEP has also been calculated. Starting from platelets with a high basal surface area over total surface area, we show that the exfoliation process has no impact on calculated IEP value, as verified with experiments. Moreover, the restacked materials containing different monometallic (hydr)oxide layers also have an IEP consistent with values calculated with the model. This study proves that MUSIC model is a useful tool to predict IEP of various complex metal oxides and hydroxides.

Experimentally Validated Structures of Supported Metal Nanoclusters on MoS2

In nanometer clusters (NCs), each atom counts. It is the specific arrangement of these atoms that determines the unique size-dependent functionalities of the NCs and hence their applications. Here, we employ a self-consistent, combined theoretical and experimental approach to determine atom-by-atom the structures of supported Pt NCs on MoS₂. The atomic structures are predicted using a genetic algorithm utilizing atomistic force fields and density functional theory, which are then validated using aberration-corrected scanning transmission electron microscopy. We find that relatively small clusters grow with (111) orientation such that Pt[11̅0] is parallel to MoS₂[100], which is different from predictions based on lattice-match for thin-film epitaxy. Other 4d and 5d transition metals show similar behavior. The underpinning of this growth mode is the tendency of the NCs to maximize the metal-sulfur interactions rather than to minimize lattice strain.

MoS2 Quantum Dots@TiO2 Nanotube Arrays: An Extended-Spectrum-Driven Photocatalyst for Solar Hydrogen Evolution

TiO₂ nanotube arrays (TiO₂ NTAs) decorated with molybdenum disulfide quantum dots (MoS₂ QDs) were synthesized by a facile electrodeposition method and used as a composite photocatalyst. MoS₂ QDs/TiO₂ NTAs showed enhanced photocatalytic activity compared with pristine TiO₂ NTAs for solar light-promoted H₂ evolution without adding any sacrificial agents or cocatalysts. The photocatalytic activity was influenced by the amount of MoS₂ QDs coated on TiO₂ NTAs. The optimal composition showed excellent photocatalytic activity, achieving H₂ evolution rates of 31.36, 5.29, and 1.67 μmol cm^-2  h^-1 corresponding to ultraviolet (UV, λ<420 nm), visible (Vis, λ≥420 nm), and near-infrared (NIR, λ>760) illumination, respectively. The improved photocatalytic activity was attributed to the decreased bandgap and the surface plasmonic properties of MoS₂ QDs/TiO₂ NTAs, which promoted electron-hole pair separation and the absorption capacity for Vis and NIR light. This study presents a facile approach for fabricating MoS₂ QDs/TiO₂ NTA heterostructures for efficient photocatalytic H₂ evolution, which will facilitate the development of designing new photocatalysts for environment and energy applications.

Novel structured transition metal dichalcogenide nanosheets

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their unique properties and great potential in a wide range of applications. Great efforts have been devoted to the preparation of novel-structured TMD nanosheets by engineering their intrinsic structures at the atomic scale. Until now, a lot of new-structured TMD nanosheets, such as vacancy-containing TMDs, heteroatom-doped TMDs, TMD alloys, 1T'/1T phase and in-plane TMD crystal-phase heterostructures, TMD heterostructures and Janus TMD nanosheets, have been prepared. These materials exhibit unique properties and hold great promise in various applications, including electronics/optoelectronics, thermoelectrics, catalysis, energy storage and conversion and biomedicine. This review focuses on the most recent important discoveries in the preparation, characterization and application of these new-structured ultrathin 2D layered TMDs.

Directed Assembly of Hybrid Nanomaterials and Nanocomposites

Hybrid nanomaterials are molecular or colloidal-level combinations of organic and inorganic materials, or otherwise strongly dissimilar materials. They are often, though not exclusively, anisotropic in shape. A canonical example is an inorganic nanorod or nanosheet sheathed in, or decorated by, a polymeric or other organic material, where both the inorganic and organic components are important for the properties of the system. Hybrid nanomaterials and nanocomposites have generated strong interest for a broad range of applications due to their functional properties. Generating macroscopic assemblies of hybrid nanomaterials and nanomaterials in nanocomposites with controlled orientation and placement by directed assembly is important for realizing such applications. Here, a survey of critical issues and themes in directed assembly of hybrid nanomaterials and nanocomposites is provided, highlighting recent efforts in this field with particular emphasis on scalable methods.

Pt decorated MoS2 nanoflakes for ultrasensitive resistive humidity sensor

In this work, we report the fabrication of a low power, humidity sensor where platinum nanoparticles (NPs) decorated few-layered molybdenum disulphide (MoS2) nanoflakes have been used as the sensing layer. A mixed solvent was used to exfoliate the nanoflakes from the bulk powder. Then the Pt/MoS2 composites were prepared by reducing Pt NPs from chloroplatinic acid hexahydrate using a novel reduction technique using sulphide salt. The successful reduction and composite preparation were confirmed using various material characterization tools like scanning electron microscopy, atomic force microscopy, transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy and UV-visible spectroscopy. The humidity sensors were prepared by drop-coating the Pt-decorated MoS2 on gold interdigitated electrodes and then exposed to various levels of relative humidity (RH). Composites with different weight ratios of Pt were tested and the best response was shown by the Pt/MoS2 (0.25:1) sample with a record high response of ∼4000 times at 85% RH. The response and recovery times were ∼92 s and ∼154 s respectively with repeatable behaviour. The sensor performance was found to be stable when tested over a few months. The underlying sensing mechanisms along with detailed characterization of the various composites have been discussed.

Tunable Pt-MoS x Hybrid Catalysts for Hydrogen Evolution

Platinum (Pt)-based materials are inevitably among the best-performing electrocatalysts for hydrogen evolution reaction (HER). MoS₂ was suggested to be a potent HER catalyst to replace Pt in this reaction by theoretical modeling; however, in practice, this dream remains elusive. Here we show a facile one-pot bottom-up synthesis of Pt-MoS _x composites using electrochemical reduction in an electrolytic bath of Pt precursor and ammonium tetrathiomolybdate under ambient conditions. By modifying the millimolar concentration of Pt precursors, composites of different surface elemental composition are fabricated; specifically, Pt_1.8MoS₂, Pt_0.1MoS_2.5, Pt_0.2MoS_0.6, and Pt_0.3MoS_0.8. All electrodeposited Pt-MoS _x hybrids showcase low overpotentials and small Tafel slopes that outperform MoS₂ as an electrocatalyst. Tantamount to electrodeposited Pt, the rate-limiting process in the HER mechanism is determined to be the Heyrovsky desorption across Pt-MoS _x hybrids and starkly swings from the rate-determining Volmer adsorption step in MoS₂. The Pt-MoS _x composites are equipped with catalytic performance that closely mirrors that of electrodeposited Pt, in particular the HER kinetics for Pt_1.8MoS₂ and Pt_0.1MoS_2.5. This work advocates electrosynthesis as a cost-effective method for catalyst design and fabrication of competent composite materials for water splitting applications.

Self-Template Synthesis of Co-Se-S-O Hierarchical Nanotubes as Efficient Electrocatalysts for Oxygen Evolution under Alkaline and Neutral Conditions

We develop a facile self-template synthetic method to construct hierarchical Co-Se-S-O (CoSe _xS_{2- x}@Co(OH)₂) nanotubes on a carbon cloth as a self-standing electrode for electrocatalytic oxygen evolution reaction (OER). In the synthetic process, separate selenization and sulfurization on the Co(OH)F precursor in different solvents have played an important role in constructing CoSe _xS_{2- x} (Co-Se-S) hierarchical nanotubes, which was further transformed into the nanotube-like Co-Se-S-O via an in situ electrochemical oxidation process. The Co-Se-S-O obtained by the Kirkendall effect through two stepwise anion-exchange reactions represents the first quaternary Co-Se-S-O nanotube array, which dramatically enhances its surface area and conductivity. Further, it only requires low overpotentials of 230 and 480 mV to achieve a 10 mA cm^-2 current density. The OER performance of Co-Se-S-O is much more efficient than that of its monochalcogenide counterparts, as well as the commercial benchmark catalyst IrO₂.

Tunable Nonvolatile Memory Behaviors of PCBM-MoS2 2D Nanocomposites through Surface Deposition Ratio Control

Efficient preparation of single-layer two-dimensional (2D) transition metal dichalcogenides, especially molybdenum disulfide (MoS₂), offers readily available 2D surface in nanoscale to template various materials to form nanocomposites with van der Waals heterostructures (vdWHs), opening up a new dimension for the design of functional electronic and optoelectronic materials and devices. Here, we report the tunable memory properties of the facilely prepared [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)-MoS₂ nanocomposites in a conventional diode device structure, where the vdWHs dominate the electric characteristics of the devices for various memory behaviors depending on different surface deposition ratios of PCBM on MoS₂ nanosheets. Both nonvolatile WORM and flash memory devices have been realized using the new developed PCBM-MoS₂ 2D composites. Specially, the flash characteristic devices show rewritable resistive switching with low switching voltages (∼2 V), high current on/off ratios (∼3 × 10²), and superior electrical bistability (>10⁴ s). This research, through successfully allocating massive vdWHs on the MoS₂ surface for organic/inorganic 2D nanocomposites, illustrates the great potential of 2D vdWHs in rectifying the electronic properties for high-performance memory devices and paves a way for the design of promising 2D nanocomposites with electronically active vdWHs for advanced device applications.

Nanoceria-Supported Ruthenium(0) Nanoparticles: Highly Active and Stable Catalysts for Hydrogen Evolution from Water

Ruthenium(0) nanoparticles supported on nanoceria (Ru⁰/CeO₂) were prepared by reduction of Ru³⁺ ions on the surface of ceria using aqueous solution of NaBH₄. The Ru⁰/CeO₂ samples were characterized by advanced analytical tools and employed as electrocatalysts on the glassy carbon electrode (GCE) in hydrogen evolution from water. The GCE, modified by Ru⁰/CeO₂ (1.86 wt % Ru), provides an incredible electrocatalytic activity with a high exchange current density of 0.67 mA·cm^-2, low overpotential of 47 mV at j = 10 mA·cm^-2, and small Tafel slope of 41 mV·dec^-1. Moreover, this modified GCE provides an unprecedented long-term stability without changing the onset potential (33 mV) even after 10 000 scans in acidic water splitting at room temperature. The hydrogen gas, evolved during the water splitting using the Ru⁰/CeO₂ (1.86 wt % Ru) electrocatalyst, was also collected. The amount of the evolved H₂ gas matches well with the calculated value, which indicates the achievement of nearly 100% Faradaic efficiency.

Interface Engineering of Anchored Ultrathin TiO2/MoS2 Heterolayers for Highly-Efficient Electrochemical Hydrogen Production

An efficient self-standing hydrogen evolution electrode was prepared by in situ growth of stacked ultrathin TiO₂/MoS₂ heterolayers on carbon paper (CP@TiO₂@MoS₂). Owing to the high overall conductivity, large electrochemical surface area and abundant active sites, this novel electrode exhibits an excellent performance for hydrogen evolution reaction (HER). Remarkably, the composite electrode shows a low Tafel slope of 41.7 mV/dec, and an ultrahigh cathodic current density of 550 mA/cm² at a very low overpotential of 0.25 V. This work presents a new universal strategy for the construction of effective, durable, scalable, and inexpensive electrodes that can be extended to other electrocatalytic systems.

In situ growth of 0D silica nanospheres on 2D molybdenum disulfide nanosheets: Towards reducing fire hazards of epoxy resin

This report described a facile process for the preparation of 2D/0D MoS₂-SiO₂ hybrids using a simple in situ growth method, with the purpose of promoting the dispersion of MoS₂ in polymer matrices and improving the properties of polymer materials. FTIR, XPS, TGA and TEM measurements were performed to characterize the structure and morphology of the synthesized hybrids which were then introduced into epoxy to reduce flammability. The hybrids dispersed well in the epoxy matrix. No obvious agglomerations were observed. In comparison with those of neat epoxy, the incorporation of a low loading of MoS₂-SiO₂ hybrids resulted in significant decrements in heat release rate, total heat release and volume of toxic effluents released during combustion, which indicated that the fire hazards of epoxy composites were strongly reduced. The good dispersion, labyrinth barrier effect and the catalytic effect of MoS₂-SiO₂ hybrids on char formation may contribute to the observed decrease in the flammability of epoxy resin.

The adsorption and diffusion behavior of noble metal adatoms (Pd, Pt, Cu, Ag and Au) on a MoS2 monolayer: a first-principles study

First-principles calculations based on periodic density functional theory (DFT) have been used to investigate the geometries, electronic structures, magnetic properties and diffusion behaviors of different noble metal adatoms (Pd, Pt, Cu, Ag and Au) on MoS₂ monolayers. The results demonstrate that these adatoms can chemically adsorb on MoS₂ monolayers. The band gaps of MoS₂ monolayers with a Pd or Pt atom adsorbed are reduced owing to impurity states that emerge simultaneously within the gap region of the pristine MoS₂ monolayer. The unpaired electrons in MoS₂ monolayers with a Cu, Ag or Au atom adsorbed are spin polarized, resulting in total magnetic moments of 1.0 μ_B per supercell, which is caused by the strong hybridization between the metal adatoms and surrounding Mo or S atoms. Long-range antiferromagnetic (AFM) coupling has been observed between group IB metal adatoms. Due to charge transfer between adatoms and the MoS₂ host, the work functions were modulated upon adsorption of noble metals. In addition, the diffusion behaviors of noble metal adatoms on the MoS₂ monolayer suggest that Cu, Pd and Pt atoms favor the formation of a metal nanotemplate on the MoS₂ monolayer, and Ag and Au are likely to form isolated particles in the initial growth stage. These findings may provide useful guidance to extend the potential applications of MoS₂ in low-dimensional nanoelectronic and spintronic devices.

Two-Dimensional Holey Nanoarchitectures Created by Confined Self-Assembly of Nanoparticles via Block Copolymers: From Synthesis to Energy Storage Property

Advances in liquid-phase exfoliation and surfactant-directed anisotropic growth of two-dimensional (2D) nanosheets have enabled their rapid development. However, it remains challenging to develop assembly strategies that lead to the construction of 2D nanomaterials with well-defined geometry and functional nanoarchitectures that are tailored to specific applications. Here we report a facile self-assembly method leading to the controlled synthesis of 2D transition metal oxide (TMO) nanosheets containing a high density of holes. We utilize graphene oxide sheets as a sacrificial template and Pluronic copolymers as surfactants. By using ZnFe₂O₄ (ZFO) nanoparticles as a model material, we demonstrate that by tuning the molecular weight of the Pluronic copolymers we can incorporate the ZFO particles and tune the size of the holes in the sheets. The resulting 2D ZFO nanosheets offer synergistic characteristics including increased electrochemically active surface areas, shortened ion diffusion paths, and strong inherent mechanical properties, leading to enhanced lithium-ion storage properties. Postcycling characterization confirms that the samples maintain structural integrity after electrochemical cycling. Our findings demonstrate that this template-assisted self-assembly method is a useful bottom-up route for controlled synthesis of 2D nanoarchitectures, and these holey 2D nanoarchitectures are promising for improving the electrochemical performance of next-generation lithium-ion batteries.