A Silicon Monoxide Lithium-Ion Battery Anode with Ultrahigh Areal Capacity

Silicon monoxide(SiO)is an attractive anode material for next-generation lithium-ion batteries for its ultra-high theoretical capacity of 2680 mAh g−1.The studies to date have been limited to electrodes with a rela-tively low mass loading(<3.5 mg cm^(−2)),which has seriously restricted the areal capacity and its potential in practical devices.Maximizing areal capacity with such high-capacity materials is critical for capitalizing their potential in practi-cal technologies.Herein,we report a monolithic three-dimensional(3D)large-sheet holey gra-phene framework/SiO(LHGF/SiO)composite for high-mass-loading electrode.By specifically using large-sheet holey graphene building blocks,we construct LHGF with super-elasticity and exceptional mechanical robustness,which is essential for accommodating the large volume change of SiO and ensuring the structure integrity even at ultrahigh mass loading.Additionally,the 3D porous graphene network structure in LHGF ensures excellent electron and ion transport.By systematically tailoring microstructure design,we show the LHGF/SiO anode with a mass loading of 44 mg cm^(−2)delivers a high areal capacity of 35.4 mAh cm^(−2)at a current of 8.8 mA cm^(−2)and retains a capacity of 10.6 mAh cm^(−2)at 17.6 mA cm^(−2),greatly exceeding those of the state-of-the-art commercial or research devices.Furthermore,we show an LHGF/SiO anode with an ultra-high mass loading of 94 mg cm^(−2)delivers an unprecedented areal capacity up to 140.8 mAh cm^(−2).The achievement of such high areal capacities marks a critical step toward realizing the full potential of high-capacity alloy-type electrode materials in practical lithium-ion batteries.

Improving hydrogen evolution reaction efficiency through lattice tuning

The intermittent nature of renewable energy sources sets a requirement for efficient energy storage to mitigate the conflict between energy supply and demand.Hydrogen is a promising choice for energy storage due to its high energy density.However,the conversion of electrical energy to chemical energy stored in hydrogen through water electrolysis suffers from low efficiency,and the electricity cost dominates the total cost of hydrogen production.Here,we report the study of improving the hydrogen evolution reaction activity of Pt-based catalysts by building a nanoscale surface NiO and Pt interface,further optimizing the performance via tuning the lattice parameter of the core of nanoparticles,which can be achieved by varying the dealloying annealing time.The optimized PtCuNi-O/C and PtNi-O/C catalysts are demonstrated to be one of the best catalysts,with a mass activity(MA)of 9.1 and 8.7 mA/μgPt,which is 9.9-fold and 9.5-fold of that of Pt/C,respectively.

Precision Control of Amphoteric Doping in Cu_(x)Bi_(2)Se_(3) Nanoplates

Copper-doped Bi_(2)Se_(3)(Cu_(x)Bi_(2)Se_(3))is of considerable interest for tailoring its electronic properties and inducing exotic charge correlations while retaining the unique Dirac surface states.However,the copper dopants in Cu_(x)Bi_(2)Se_(3) display complex electronic behaviors and may function as either electron donors or acceptors depending on their concentration and atomic sites within the Bi_(2)Se_(3) crystal lattice.Thus,a precise understanding and control of the doping concentration and sites is of both fundamental and practical significance.Herein,we report a solution-based one-pot synthesis of Cu_(x)Bi_(2)Se_(3) nanoplates with systematically tunable Cu doping concentrations and doping sites.Our studies reveal a gradual evolution from intercalative sites to substitutional sites with increasing Cu concentrations.The Cu atoms at intercalative sites function as electron donors while those at the substitutional sites function as electron acceptors,producing distinct effects on the electronic properties of the resulting materials.We further show that Cu_(0.18)Bi_(2)Se_(3) exhibits superconducting behavior,which is not present in Bi_(2)Se_(3),highlighting the essential role of Cu doping in tailoring exotic quantum properties.This study establishes an efficient methodology for precise synthesis of Cu_(x)Bi_(2)Se_(3) with tailored doping concentrations,doping sites,and electronic properties.

Lead halide perovskite sensitized WSe_(2) photodiodes with ultrahigh open circuit voltages

Two-dimensional semiconductors(2DSCs)have attracted considerable interests for optoelectronic devices,but are often plagued by the difficulties in tailoring the charge doping type and poor optical absorption due to their atomically thin geometry.Herein,we report a methylammonium lead iodide perovskite(CH_(3)NH_(3)PbI_(3))/2DSC heterojunction device,in which the electric-field controllable ion migration in the perovskite layer is exploited to induce reversible electron-and hole-doping effects in the underlying monolayer tungsten diselenide(WSe_(2))to form a programmable p-n photodiode.At the same time,the CH_(3)NH_(3)PbI_(3) layer functions as a highly efficient sensitization layer to greatly boost the optical absorption and external quantum efficiency(EQE)of the resulting photodiode.By asymmetrically poling the perovskite layer,gold-contacted CH_(3)NH_(3)PbI_(3)/WSe_(2) devices show a switchable open circuit voltage up to 0.78 V,along with a high EQE of 84.3%.The integration of tunable graphene-contacts further improves the photodiode performance to achieve a highest open circuit voltage of 1.08 V and a maximum EQE of 91.3%,greatly exceeding those achieved previously in 2DSC lateral diodes.Our studies establish a non-invasive approach to switch optoelectronic functions and open up a new avenue toward high-performance reconfigurable optoelectronic devices from 2DSCs.

Van der Waals integration of artificial heterostructures and highorder superlattices

The recent blossom in 2D atomic crystals(2DACs)has ignited intense interest in a new type of bond-free van der Waals heterostructures(vdWHs),in which distinct material components are physically brought together within a vdW distance and held together by weak vdW interactions.Without direct chemical bonding between the constituent materials,the vdWHs negate the lattice matching requirements in typical epitaxially bonded heterostructures.Here we briefly summarize the key advances in the construction and fundamental investigation of versatile vdWHs from diverse 2DACs and beyond,and highlight a unique class of vdW superlattices(vdWSLs)consisting of alternating 2D atomic layers and/or self-assembled molecular layers,with tailored structural symmetry,electronic band modulation,interlayer coupling,and chirality.Lastly,we conclude with a brief outlook on the opportunities in exploring such artificial materials to unlock previously inaccessible physical limits and enable new device concepts beyond the reach of the existing materials.

One-step strategy to graphene/Ni（OH）2 composite hy- drogels as advanced three-dimensional supercapacitor electrode materials

Graphene-based three-dimensional （3D） macroscopic materials have recently attracted increasing interest by virtue of their exciting potential in electrochemical energy conversion and storage. Here we report a facile one-step strategy to prepare mechanically strong and electrically conductive graphene/Ni（OH）2 composite hydrogels with an interconnected porous network. The composite hydrogels were directly used as 3D supercapacitor electrode materials without adding any other binder or conductive additives. An optimized composite hydrogel containing -82 wt.% Ni（OH）2 exhibited a specific capacitance of -1,247 F/g at a scan rate of 5 mV/s and -785 F/g at 40 mV/s （-63% capacitance retention） with excellent cycling stability. The capacity of the 3D hydrogels greatly surpasses that of a physical mixture of graphene sheets and Ni（OH）2 nanoplates （-309 F/g at 40 mV/s）. The same strategy was also applied to fabricate graphene-carbon nanotube/Ni（OH）2 ternary composite hydrogels with further improved specific capacitances （-1,352 F/g at 5 mV/s） and rate capability （-66% capacitance retention at 40 mV/s）. Both composite hydrogels obtained here can deliver high energy densities （-43 and -47 Wh/kg, respectively） and power densities （-8 and -9 kW/kg, respectively）, making them attractive electrode materials for supercapacitor applications. This study opens a new pathway to the design and fabrication of functional 3D graphene composite materials, and can significantly impact broad areas including energy storage and beyond.

Chemical vapor deposition growth of monolayer MoSe2 nanosheets

The synthesis of two-dimensional （2D） layered materials with controllable thickness is of considerable interest for diverse applications. Here we report the first chemical vapor deposition growth of single- and few-layer MoSe2 nanosheets. By using Se and MoO3 as the chemical vapor supply, we demonstrate that highly crystalline MoSe2 can be directly grown on the 300 nm SiO2/Si substrates to form optically distinguishable single- and multi-layer nanosheets, typically in triangular shaped domains with edge lengths around 30 btm, which can merge into continuous thin films upon further growth. Micro-Raman spectroscopy and imaging was used to probe the thickness-dependent vibrational properties. Photoluminescence spectroscopy demonstrates that MoSe2 monolayers exhibit strong near band edge emission at 1.55 eV, while bilayers or multi-layers exhibit much weaker emission, indicating of the transition to a direct band gap semiconductor as the thickness is reduced to a monolayer.

Three-dimensional graphene framework with ultra-high sulfur content for a robust lithium-sulfur battery

Lithium-sulfur batteries can deliver significantly higher specific capacity than standard lithium ion batteries, and represent the next generation of energy storage devices for both electric vehicles and mobile devices. However, the lithium-sulfur technology today is plagued with numerous challenges, including poor sulfur conductivity, large volumetric expansion, severe polysulfide shuttling and low sulfur utilization, which prevent its wide-spread adoption in the energy storage industry. Here we report a freestanding three-dimensional （3D） graphene frame- work for highly efficient loading of sulfur particles and creating a high capacity sulfur cathode. Using a one-pot synthesis method, we show a mechanically robust graphene-sulfur composite can be prepared with the highest sulfur weight content （90% sulfur） reported to date, and can be directly used as the sulfur cathode without additional binders or conductive additives. The graphene-sulfur composite features a highly interconnected graphene network ensuring excellent conductivity and a 3D porous structure allowing efficient ion transport and accommodating large volume expansion. Additionally, the 3D graphene framework can also function as an effective encapsulation layer to retard the polysulfide shuttling effect, thus enabling a highly robust sulfur cathode. Electrochemical studies show that such composite can deliver a highest capacity of 969 mAh-g-1, a record high number achieved for all sulfur cathodes reported to date when normalized by the total mass of the entire electrode. Our studies demonstrate that the 3D graphene framework represents an attractive scaffold material for a high performance lithium sulfur battery cathode, and could enable exciting opportunities for ultra-high capacity energy storage applications.

Chemical vapor deposition growth of single-crystalline cesium lead halide microplatelets and heterostructures for optoelectronic applications

Organic-inorganic hybrid halide perovskites, such as CH3NHgPbI3, have emerged as an exciting class of materials for solar photovoltaic applications; however, they are currently plagued by insufficient environmental stability. To solve this issue, all-inorganic halide perovskites have been developed and shown to exhibit significantly improved stability. Here, we report a single-step chemical vapor deposition growth of cesium lead halide （CsPbX3） microcrystals. Optical microscopy studies show that the resulting perovskite crystals predominantly adopt a square-platelet morphology. Powder X-ray diffraction （PXRD） studies of the resulting crystals demonstrate a highly crystalline nature, with CsPbC13, CsPbBr3, and CsPbI3 showing tetragonal, monoclinic, and orthorhombic phases, respectively. Scanning electron microscopy （SEM） and atomic force microscopy （AFM） studies show that the resulting platelets exhibit well-faceted structures with lateral dimensions of the order of 10-50 μm, thickness around 1 μm, and ultra-smooth surface, suggesting the absence of obvious grain boundaries and the single-crystalline nature of the individual microplatelets. Photoluminescence （PL） images and spectroscopic studies show a uniform and intense emission consistent with the expected band edge transition. Additionally, PL images show brighter emission around the edge of the platelets, demonstrating a wave-guiding effect in high-quality crystals. With a well-defined geometry and ultra-smooth surface, the square platelet structure can function as a whispering gallery mode cavity with a quality factor up to 2,863 to support laser emission at room temperature. Finally, we demonstrate that such microplatelets can be readily grown on a variety of substrates, including silicon, graphene, and other two-dimensional materials such as molybdenum disulfide, which can readily allow the construction of heterostructure optoelectronic devices, including a graphene/perovskite/ graphene vertically-stacked photodetector with photoresponsivity 〉 10^5 A/W. The extraordinary optical properties of CsPbX3 platelets, combined with their ability to be grown on diverse materials to form functional heterostructures, can lead to exciting opportunities for broad optoelectronic applications.

Highly-anisotropic optical and electrical properties in layered SnSe

Anisotropic materials are of considerable interest because of their unique combination of polarization- or direction-dependent electrical, optical, and thermoelectric properties. Low-symmetry two-dimensional （2D） materials formed by van der Waals stacking of covalently bonded atomic layers are inherently anisotropic. Layered SnSe exhibits a low degree of lattice symmetry, with a distorted NaC1 structure and an in-plane anisotropy. Here we report a systematic study of the in-plane anisotropic properties in layered SnSe, using angle-resolved Raman scattering, optical absorption, and electrical transport studies. The optical and electrical characterization was direction-dependent, and successfully identified the crystalline orientation in the layered SnSe. Furthermore, the dependence of Raman-intensity anisotropy on the SnSe flake thickness and the excitation wavelength were investigated by both experiments and theoretical calculations. Finally, the electrical transport studies demonstrated that few-layer SnSe field- effect transistors （FETs） have a large anisotropic ratio of carrier mobility （N 5.8） bet- ween the armchair and zigzag directions, which is a record high value reported for 2D anisotropic materials. The highly-anisotropic properties of layered SnSe indicate considerable promise for anisotropic optics, electronics, and optoelectronics.

Pt3Ag alloy wavy nanowires as highly effective electrocatalysts for ethanol oxidation reaction

Direct ethanol fuel cell(DEFC)has received tremendous research interests because of the more convenient storage and transportation of ethanol vs.compressed hydrogen.However,the electrocatalytic ethanol oxidation reaction typically requires precious metal catalysts and is plagued with relatively high over potential and low mass activity.Here we report the synthesis of Pt3Ag alloy wavy nanowires via a particle attachment mechanism in a facile solvothermal process.Transmission microscopy studies and elemental analyses show highly wavy nanowire structures with an average diameter of 4.6±1.0 nm and uniform Pt3Ag alloy formation.Electrocatalytic studies demonstrate that the resulting alloy nanowires can function as highly effective electrocatalysts for ethanol oxidation reactions(EOR)with ultrahigh specific activity of 28.0 mA/cm^2 and mass activity of 6.1 A/mg,far exceeding that of the commercial Pt/carbon samples(1.10 A/mg).The improved electrocatalytic activity may be partly attributed to partial electron transfer from Ag to Pt in the Pt3Ag alloy,which weakens CO binding and the CO poisoning effect.The one-dimensional nanowire morphology also contributes to favorable charge transport properties that are critical for extracting charge from catalytic active sites to external circuits.The chronoamperometry studies demonstrate considerably improved stability for long term operation compared with the commercial Pt/C samples,making the Pt3Ag wavy nanowires an attractive electrocatalyst for EOR.

Three-dimensional graphene membrane cathode for high energy density rechargeable lithium-air batteries in ambient conditions

Lithium-air batteries have attracted significant interest for applications in high energy density mobile power supplies, yet there are considerable challenges to the development of rechargeable Li-air batteries with stable cycling performance under ambient conditions. Here we report a three-dimensional （3D） hydrophobic graphene membrane as a moisture-resistive cathode for high performance Li-air batteries. The 3D graphene membrane features a highly interconnected graphene network for efficient charge transport, a highly porous structure for efficient diffusion of oxygen and electrolyte ions, a large specific surface area for high capacity storage of the insulating discharge product, and a network of highly tortuous hydrophobic channels for O2/H20 selectivity. These channels facilitate 02 ingression while retarding moisture diffusion and ensure excellent charge/ discharge cycling stability under ambient conditions. The membrane can thus enable robust Li-air batteries with exceptional performance, including a maximum cathode capacity that exceeds 5,700 mAh/g and excellent recharge cycling behavior （〉2,000 cycles at 140 mAh/g, and 〉100 cycles at 1,400 mAh/g）. The graphene membrane air cathode can deliver a lifetime capacity of 100,000-300,000 mAh/g, comparable to that of a typical lithium ion battery cathode. The stable operation of Li-air batteries with significantly improved single charge capacities and lifetime capacities comparable to those of Li-ion batteries may offer an attractive high energy density storage alternative for future mobile power supplies. These batteries may provide much longer battery lives and greatly reduced recharge frequency.

A field-effect approach to directly profiling the localized states in monolayer MoS2

A fundamental understanding of the charge transport mechanism in two-dimensional semiconductors(e.g., MoS2) is crucial for fully exploring their potential in electronic and optoelectronic devices. By using monolayer graphene as the barrier-free contact to MoS2, we show that the field-modulated conductivity can be used to probe the electronic structure of the localized states. A series of regularly distributed plateaus were observed in the gate-dependent transfer curves. Calculations based on the variable-range hopping theory indicate that such plateaus can be attributed to the discrete localized states near mobility edge. This method provides an effective approach to directly profiling the localized states in conduction channel with an ultrahigh resolution up to 1 meV.

Enhancement of oxygen reduction reaction activity by grain boundaries in platinum nanostructures

Systematic control of grain boundary densities in various platinum(Pt)nanostructures was achieved by specific peptide-assisted assembly and coagulation of nanocrystals.A positive quadratic correlation was observed between the oxygen reduction reaction(ORR)specific activities of the Pt nanostructures and the grain boundary densities on their surfaces.Compared to commercial Pt/C,the grain-boundary-rich strain-free Pt ultrathin nanoplates demonstrated a 15.5 times higher specific activity and a 13.7 times higher mass activity.Simulation studies suggested that the specific activity of ORR was proportional to the resident number and the resident time of oxygen on the catalyst surface,both of which correlate positively with grain boundary density,leading to improved ORR activities.

Ultrathin wavy Rh nanowires as highly effective electrocatalysts for methanol oxidation reaction with ultrahigh ECSA

Direct methanol fuel cells (DMFCs) have received tremendous research interests because of the facile storage of liquid methanol vs.hydrogen.However,the DMFC today is severely plagued by the poor kinetics and rather high overpotential in methanol oxidation reaction (MOR).Here we report the investigation of the ultrathin Rh wavy nanowires as a highly effective MOR electrocatalyst.We show that ultrathin wavy Rh nanowires can be robustly synthesized with 2-3 nm diameters.Electrochemical studies show a current peak at the potential of 0.61 V vs.reversible hydrogen electrode (RHE),considerably lower than that of Pt based catalysts (~ 0.8-0.9 V vs.RHE).Importantly,with ultrathin diameters and favorable charge transport,the Rh nanowires catalysts exhibit an ultrahigh electrochemically active surface area determined from CO-stripping (ECSAco) of 144.2 m2/g,far exceeding that of the commercial Rh black samples (20 m2/g).Together,the Rh nanowire catalysts deliver a mass activity of 722 mA/mg at 0.61 V,considerably higher than many previously reported electrocatalysts at the same potential.The chronoamperometry studies also demonstrate good stability and CO-tolerance compared with the Rh black control sample,making ultrathin Rh wavy nanowires an attractive electrocatalyst for MOR.

In situ development of highly concave and compositionconfined PtNi octahedra with high oxygen reduction reaction activity and durability

Controlled syntheses of PtNi metal nanocrystals with unique structures for catalyzing oxygen reduction reactions （ORRs） have attracted great interest. Here, we report the one-step synthesis of single-crystal PtNi octahedra with in situ-developed highly concave features and self-confined composition that are optimal for ORR. Detailed studies revealed that the Pt-rich seeding, subsequent Pt/Ni co-reduction, and Pt-Ni interfusion resulted in uniform single-crystal PtNi octahedra, and that the combination of Ni facet segregation and oxygen etching of a Ni-rich surface led to the concavity and confined Ni content. The concave PtNi nanocrystals exhibited much higher ORR performance than the commercially available Pt/C catalyst in terms of both specific activity （29.1 times higher） and mass activity （12.9 times higher） at 0.9 V （vs. reversible hydrogen electrode （RHE））. The performance was also higher than that of PtNi octahedra without concavity, confirming that the higher activity was closely related to its morphology. Moreover, the concave octahedra also exhibited remarkable stability in ORR （93% mass activity remained after 10,000 cycles between 0.6 and 1.1 V vs. RHE） owing to the passivation of the unstable sites.

Holey graphene hydrogel with in-plane pores for high- performance capacitive desalination

Capacitive deionization is an attractive approach to water desalination and treatment. To achieve efficient capacitative desalination, rationally designed electrodes with high specific capacitances, conductivities, and stabilities are necessary. Here we report the construction of a three-dimensional （3D） holey graphene hydrogel （HGH）. This material contains abundant in-plane pores, offering efficient ion transport pathways. Furthermore, it forms a highly interconnected network of graphene sheets, providing efficient electron transport pathways, and its 3D hierarchical porous structure can provide a large specific surface area for the adsorption and storage of ions. Consequently, HGH serves as a binder-free electrode material with excellent electrical conductivity. Cyclic voltammetry （CV） measurements indicate that the optimized HGH can achieve specific capacitances of 358.4 F.g 1 in 6 M KOH solution and 148 F.g-1 in 0.5 M NaCl solution. Because of these high capacitances, HGH has a desalination capadty as high as 26.8 mg.g-1 （applied potential： 1.2 V; initial NaCI concentration： -5,000 mg.L-l）.

Two-dimensional van der Waals thin film transistors as active matrix for spatially resolved pressure sensing

The development of pressure sensor arrays capable of distinguishing the shape and texture details of objects is of considerable interest in the emerging fields of smart robots,prostheses,human-machine interfaces,and artificial intelligence(AI).Here we report an integrated pressure sensor array,by combining solution-processed two-dimensional(2D)MoS2 van der Waals(vdW)thin film transistor(TFT)active matrix and conductive micropyramidal pressure-sensitive rubber(PSR)electrodes made of polydimethylsiloxane/carbon nanotube composites,to achieve spatially revolved pressure mapping with excellent contrast and low power consumption.We demonstrate a 10×10 active matrix by using the 2D MoS2 vdW-TFTs with high on-off ratio>10^(6),minimal hysteresis,and excellent device-to-device uniformity.The combination of the vdW-TFT active matrix with the highly uniform micropyramidal PSR electrodes creates an integrated pressure sensing array for spatially resolved pressure mapping.This study demonstrates that the solution-processed 2D vdW-TFTs offer a solution for active-matrix control of pressure sensor arrays,and could be extended for other active-matrix arrays of electronic or optoelectronic devices.

Direct van der Waals epitaxial growth of 1D/2D Sb2Se3/WS2 mixed-dimensional p-n heterojunctions

The mixed-dimensional integration of two-dimensional (2D) materials with non-2D materials can give rise to prominent advances in performance or function.To date,the mixed-dimensional one-dimensional (1D)/2D heterostructures have been fabricated using various physical assembly approaches.However,direct epitaxial growth method which has notable advantages in preparing large-scale products and obtaining perfect interfaces is rarely investigated.Herein,we demonstrate for the first time the direct synthesis of the 1D/2D mixed-dimensional heterostructures by sequential vapor-phase growth of Sb2Se3 nanowires on WS2 monolayers.X-ray diffraction (XRD) pattern and Raman spectrum confirm the composition of the Sb2Se3/NS2 heterostructures.Transmission electron microscope (TEM) measurement demonstrates high quality of the heterostructures.Electrical transport characterization reveals that Sb2Se3 nanowire exhibits p-type characteristic and that WS2 monolayer exhibits n-type behavior,and that the p-n diode from 1D/2D mixed-dimensional Sb2Se3/WS2 heterostructure possesses obvious current rectification behavior.Optoelectronic measurements of the heterostructures show apparent photovoltaic response with an open-circuit voltage of 0.19 V,photoresponsivity of 1.51 A/W (Vds =5 V) and fast response time of less than 8 ms.The van der Waals epitaxial growth mode of Sb2Se3 nanowires on WS2 monolayers is verified by stripping the Sb2Se3 nanowire from the heterostructures using tape.Together,the direct van der Waals epitaxy opens a facile pathway to 1D/2D mixed-dimensional heterostructures for functional electronic and optoelectronic devices.

Pt-Ni alloy catalysts for highly selective anti-Markovnikov alkene hydrosilylation

Alkene hydrosilylation with homogeneous catalysts is an industrially important reaction for the preparation of organosilicon compounds such as silicone-based release coatings, surfactants, adhesives and molding products [1,2]. Moreover, alkene hydrosilylation is used for cross- linking of silicone polymers, and for coupling of silanes and siloxanes to organic polymers. For decades, noble metal based catalysts （e.g., Pt [3-7], Pd [8], Rh [9], etc. [10]） have been widely used in this dass of reactions. However, the use of precious metals catalysts with a re- latively low turn-over number （TON） represents a critical barrier to industrial applications.