Biomimetic Mineralization Synthesis of Flower-Like Cobalt Selenide/Reduced Graphene Oxide for Improved Electrochemical Deionization.

Rationally and precisely tuning the composition and structure of materials is a viable strategy to improve electrochemical deionization (EDI) performances, which yet faces enormous challenges. Herein, an eco-friendly biomimetic mineralization synthetic strategy is developed to synthesize the flower-like cobalt selenide/reduced graphene oxide (Bio-CoSe2 /rGO) composites and used as advanced sodium ion adsorption electrodes. Benefiting from the slow and controllable reaction kinetics provided by the biomimetic mineralization process, the flower-like CoSe2 is uniformly constructed in the rGO, which is endowed with robust architecture, substantial adsorption sites and rapid charge/ion transport. The Bio-CoSe2 /rGO electrode yields the maximum salt adsorption capacity and salt adsorption rate of 56.3 mg g-1 and 5.6 mg g-1 min-1 respectively, and 92.5% capacity retention after 60 cycles. These results overmatch the pristine CoSe2 and irregular granular CoSe2 /rGO synthesized by a hydrothermal method, proving the structural superiority of the Bio-CoSe2 /rGO composites. Furthermore, the in-depth adsorption kinetics study indicates the chemisorption nature of sodium ion adsorption. The structures of the Bio-CoSe2 /rGO composites after long term EDI cycles are intensively studied to unveil the mechanism behind such superior EDI performances. This study offers one effective method for constructing advanced EDI electrodes, and enriches the application of the biomimetic mineralization synthetic strategy.

Lateral layered semiconductor multijunctions for novel electronic devices.

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Ultralight Flexible Electrodes of Nitrogen-Doped Carbon Macrotube Sponges for High-Performance Supercapacitors.

Light-weight and flexible supercapacitors with outstanding electrochemical performances are strongly desired in portable and wearable electronics. Here, ultralight nitrogen-doped carbon macrotube (N-CMT) sponges with 3D interconnected macroporous structures are fabricated and used as substrate to grow nickel ferrite (NiFe2 O4 ) nanoparticles by vapor diffusion-precipitation and in situ growth. This process effectively suppresses the agglomeration of NiFe2 O4 , enabling good interfacial contact between N-CMT sponges and NiFe2 O4 . More remarkably, the as-synthesized NiFe2 O4 /N-CMT composite sponges can be directly used as electrodes without additional processing that could cause agglomeration and reduction of active sites. Benefiting from the tubular structure and the synergetic effect of NiFe2 O4 and N-CMT, the NiFe2 O4 /N-CMT-2 exhibits a high specific capacitance of 715.4 F g-1 at a current density of 1 A g-1 , and 508.3 F g-1 at 10 A g-1 , with 90.9% of capacitance retention after 50 000 cycles at 1 A g-1 in an alkaline electrolyte. Furthermore, flexible supercapacitors are fabricated, yielding areal specific capacitances of 1397.4 and 1041.2 mF cm-2 at 0.5 and 8 mA cm-2 , respectively. They also exhibit exceptional cycling performance with capacitance retention of 92.9% at 1 mA cm-2 after 10 000 cycles under bending. This work paves a new way to develop flexible, light-weight, and high-performance energy storage devices.

Advanced Materials for Sodium-Ion Capacitors with Superior Energy-Power Properties: Progress and Perspectives.

Developing electrochemical energy storage devices with high energy-power densities, long cycling life, as well as low cost is of great significance. Sodium-ion capacitors (NICs), with Na+ as carriers, are composed of a high capacity battery-type electrode and a high rate capacitive electrode. However, unlike their lithium-ion analogues, the research on NICs is still in its infancy. Rational material designs still need to be developed to meet the increasing requirements for NICs with superior energy-power performance and low cost. In the past few years, various materials have been explored to develop NICs with the merits of superior electrochemical performance, low cost, good stability, and environmental friendliness. Here, the material design strategies for sodium-ion capacitors are summarized, with focus on cathode materials, anode materials, and electrolytes. The challenges and opportunities ahead for the future research on materials for NICs are also proposed.

Electrochemically triggered degradation of silicon membranes for smart on-demand transient electronic devices.

Transient electronics is an emerging technology that enables unique functional transformation or the physical disappearance of electronic devices, and is attracting increasing attention for potential applications in data secured hardware as an ultimate solution against data breaches. Developing smart triggered degradation modalities of silicon (Si) remain the key challenge to achieve advanced non-recoverable on-demand transient electronics. Here, we present a novel electrochemically triggered transience mechanism of Si by lithiation, allowing complete and controllable destruction of Si devices. The depth and microstructure of the lithiation-affected zone over time is investigated in detail and the results suggest a few hours of lithiation is sufficient to create microcracks and significantly promote lithium penetration. Finite element models are proposed to confirm the mechanism. Electrochemically triggered degradation of thin film Si ribbons and Si integrated circuit chips with metal-oxide-semiconductor field-effect transistors from a commercial 0.35 micrometer complementary metal-oxide-semiconductor technology node is performed to demonstrate the potential applications for commercial electronics. This work opens new opportunities for versatile triggered transience of Si-based devices for critical secured information systems and green consumer electronics.

Locally Induced Spin States on Graphene by Chemical Attachment of Boron Atoms.

Pristine graphene is known to be nonmagnetic due to its π-conjugated electron system. However, we find that localized magnetic moments can be generated by chemically attaching boron atoms to the graphene sheets. Such spin-polarized states are evidenced by the spin-split of the density of states (DOS) peaks near the Fermi level in scanning tunneling spectroscopy (STS). In the vicinity of several coadsorbed boron atoms, the Coulomb repulsion between multiple spins leads to antiferromagnetic coupling for the induced spin states in the graphene lattice, manifesting itself as an increment of spin-down state at specific regions. Experimental observations and interpretations are rationalized by extensive density functional theory (DFT) simulations.

High Areal Capacity Li-Ion Storage of Binder-Free Metal Vanadate/Carbon Hybrid Anode by Ion-Exchange Reaction.

Storing more energy in a limited device area is very challenging but crucial for the applications of flexible and wearable electronics. Metal vanadates have been regarded as a fascinating group of materials in many areas, especially in lithium-ion storage. However, there has not been a versatile strategy to synthesize flexible metal vanadate hybrid nanostructures as binder-free anodes for Li-ion batteries so far. A convenient and versatile synthesis of Mx Vy Ox+2.5y @carbon cloth (M = Mn, Co, Ni, Cu) composites is proposed here based on a two-step hydrothermal route. As-synthesized products demonstrate hierarchical proliferous structure, ranging from nanoparticles (0D), and nanobelts (1D) to a 3D interconnected network. The metal vanadate/carbon hybrid nanostructures exhibit excellent lithium storage capability, with a high areal specific capacity up to 5.9 mAh cm-2 (which equals to 1676.8 mAh g-1 ) at a current density of 200 mA g-1 . Moreover, the nature of good flexibility, mixed valence states, and ultrahigh mass loading density (over 3.5 mg cm-2 ) all guarantee their great potential in compact energy storage for future wearable devices and other related applications.

Ultrasensitive gas detection of large-area boron-doped graphene.

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Transition metal dichalcogenides and beyond: synthesis, properties, and applications of single- and few-layer nanosheets.

CONSPECTUS: In the wake of the discovery of the remarkable electronic and physical properties of graphene, a vibrant research area on two-dimensional (2D) layered materials has emerged during the past decade. Transition metal dichalcogenides (TMDs) represent an alternative group of 2D layered materials that differ from the semimetallic character of graphene. They exhibit diverse properties that depend on their composition and can be semiconductors (e.g., MoS2, WS2), semimetals (e.g., WTe2, TiSe2), true metals (e.g., NbS2, VSe2), and superconductors (e.g., NbSe2, TaS2). The properties of TMDs can also be tailored according to the crystalline structure and the number and stacking sequence of layers in their crystals and thin films. For example, 2H-MoS2 is semiconducting, whereas 1T-MoS2 is metallic. Bulk 2H-MoS2 possesses an indirect band gap, but when 2H-MoS2 is exfoliated into monolayers, it exhibits direct electronic and optical band gaps, which leads to enhanced photoluminescence. Therefore, it is important to learn to control the growth of 2D TMD structures in order to exploit their properties in energy conversion and storage, catalysis, sensing, memory devices, and other applications. In this Account, we first introduce the history and structural basics of TMDs. We then briefly introduce the Raman fingerprints of TMDs of different layer numbers. Then, we summarize our progress on the controlled synthesis of 2D layered materials using wet chemical approaches, chemical exfoliation, and chemical vapor deposition (CVD). It is now possible to control the number of layers when synthesizing these materials, and novel van der Waals heterostructures (e.g., MoS2/graphene, WSe2/graphene, hBN/graphene) have recently been successfully assembled. Finally, the unique optical, electrical, photovoltaic, and catalytic properties of few-layered TMDs are summarized and discussed. In particular, their enhanced photoluminescence (PL), photosensing, photovoltaic conversion, and hydrogen evolution reaction (HER) catalysis are discussed in detail. Finally, challenges along each direction are described. For instance, how to grow perfect single crystalline monolayer TMDs without the presence of grain boundaries and dislocations is still an open question. Moreover, the morphology and crystal structure control of few-layered TMDs still requires further research. For wet chemical approaches and chemical exfoliation methods, it is still a significant challenge to control the lateral growth of TMDs without expansion in the c-axis direction. In fact, there is plenty of room in the 2D world beyond graphene. We envisage that with increasing progress in the controlled synthesis of these systems the unusual properties of mono- and few-layered TMDs and TMD heterostructures will be unveiled.

Amorphization and Directional Crystallization of Metals Confined in Carbon Nanotubes Investigated by in Situ Transmission Electron Microscopy.

The hollow core of a carbon nanotube (CNT) provides a unique opportunity to explore the physics, chemistry, biology, and metallurgy of different materials confined in such nanospace. Here, we investigate the nonequilibrium metallurgical processes taking place inside CNTs by in situ transmission electron microscopy using CNTs as nanoscale resistively heated crucibles having encapsulated metal nanowires/crystals in their channels. Because of nanometer size of the system and intimate contact between the CNTs and confined metals, an efficient heat transfer and high cooling rates (∼10(13) K/s) were achieved as a result of a flash bias pulse followed by system natural quenching, leading to the formation of disordered amorphous-like structures in iron, cobalt, and gold. An intermediate state between crystalline and amorphous phases was discovered, revealing a memory effect of local short-to-medium range order during these phase transitions. Furthermore, subsequent directional crystallization of an amorphous iron nanowire formed by this method was realized under controlled Joule heating. High-density crystalline defects were generated during crystallization due to a confinement effect from the CNT and severe plastic deformation involved.

Extraordinary room-temperature photoluminescence in triangular WS2 monolayers.

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from those of their bulk counterparts. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL weakens with increasing number of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet's center. The structure and chemical composition of the platelet edges appear to be critical for PL enhancement.

Free-standing and flexible polyvinyl alcohol-sodium alginate-polypyrrole electrodes based on interpenetrating network hydrogels.

Flexible supercapacitors (FSCs) have attracted much attention due to their strong mechanical flexibility, wearability and portability, which greatly rely on the employed flexible electrodes. The conductive polymer hydrogels with excellent flexibility, processability and capacitive performance are one of the most promising candidates, which are still limited by their poor mechanical properties. Constructing robust interpenetrating polymer networks (IPN) is an effective approach to promote their mechanical properties. Herein, interpenetrating polyvinyl alcohol (PVA)-sodium alginate (SA)-polypyrrole (PPy) hydrogels are prepared by the freeze-thaw and in-situ polymerization method. The IPN structure composed of PVA and SA not only enhances the mechanical properties of hydrogels, but also provides substantial active sites for electrochemical reactions. Moreover, the hydrogen-bonding interaction between different components in the PVA-SA-PPy hydrogel boosts the charge/ion transfer. The optimal PVA-SA-PPy hydrogels show an elongation at break of 380 %, a tensile strength of 1.5 MPa, and a specific capacitance of 2646 mF cm-2 at 2 mA cm-2. The symmetric PVA-SA-PPy FSCs show an energy density of 96.7 µWh cm-2 at a power density of 999.9 µW cm-2, and the capacitance retention is 66.3 % after 10,000 cycles. These exceptional mechanical and electrochemical properties make the PVA-SA-PPy hydrogels a promising candidate for FSCs.

Reversible intercalation of hexagonal boron nitride with Brønsted acids.

Hexagonal boron nitride (h-BN) is an insulating compound that is structurally similar to graphite. Like graphene, single sheets of BN are atomically flat, and they are of current interest in few-layer hybrid devices, such as transistors and capacitors, that contain insulating components. While graphite and other layered compounds can be intercalated by redox reactions and then converted chemically to suspensions of single sheets, insulating BN is not susceptible to oxidative intercalation except by extremely strong oxidizing agents. We report that stage-1 intercalation compounds can be formed by simple thermal drying of h-BN in Brønsted acids H2SO4, H3PO4, and HClO4. X-ray photoelectron and vibrational spectra, as well as electronic structure and molecular dynamics calculations, demonstrate that noncovalent interactions of these oxyacids with the basic N atoms of the sheets drive the intercalation process.

Stabilizing non-iridium active sites by non-stoichiometric oxide for acidic water oxidation at high current density.

Stabilizing active sites of non-iridium-based oxygen evolution reaction (OER) electrocatalysts is crucial, but remains a big challenge for hydrogen production by acidic water splitting. Here, we report that non-stoichiometric Ti oxides (TiOx) can safeguard the Ru sites through structural-confinement and charge-redistribution, thereby extending the catalyst lifetime in acid by 10 orders of magnitude longer compared to that of the stoichiometric one (Ru/TiO2). By exploiting the redox interaction-engaged strategy, the in situ growth of TiOx on Ti foam and the loading of Ru nanoparticles are realized in one step. The as-synthesized binder-free Ru/TiOx catalyst exhibits low OER overpotentials of 174 and 265 mV at 10 and 500 mA cm-2, respectively. Experimental characterizations and theoretical calculations confirm that TiOx stabilizes the Ru active center, enabling operation at 10 mA cm-2 for over 37 days. This work opens an avenue of using non-stoichiometric compounds as stable and active materials for energy technologies.

Robust double-network polyvinyl alcohol-polypyrrole hydrogels as high-performance electrodes for flexible supercapacitors.

The growing demands of flexible and wearable electronic devices boost the rapid development of flexible supercapacitors (FSCs). Conductive hydrogels are considered to be one type of promising electrode materials for FSCs due to their good processability and electrochemical properties. However, the poor mechanical properties of conductive hydrogels hinder their practical applications. Building robust cross-linked network structures is a feasible way to enhance their mechanical properties. Herein, the double-network polyvinyl alcohol (PVA)-polypyrrole (PPy) conductive hydrogels are synthesized by the freeze-thaw and in-situ polymerization method. The double-network structure not only enhances mechanical properties of the hydrogels, but also promotes their electrolyte ion transport. The maximum elongation at break of the optimized PVA-PPy hydrogels can reach 156.4%, and the specific capacitance is 1718.7 mF cm-2 at 0.5 mA cm-2. Furthermore, the energy densities of the symmetrical PVA-PPy FSCs are 46.7 and 13.3 µWh cm-2 at power densities of 200.0 and 2000.0 µW cm-2. Such excellent electrochemical performances and mechanical properties make the synthesized PVA-PPy hydrogels a promising candidate for FSCs.

High mass loading and additive-free prussian blue analogue based flexible electrodes for Na-ion supercapacitors.

Supercapacitor electrodes often suffer from the low mass loading of active substances and the unsatisfactory ion/charge transport features due to the use of various additives. Exploring high mass loading and additive-free electrodes is of huge significance to develop advanced supercapacitors with commercial application prospects, which still remains challenging. Herein, high mass loading CoFe-prussian blue analogue (CoFe-PBA) electrodes are developed by a facile co-precipitation method using activated carbon cloth (ACC) as the flexible substrate. The homogeneous nanocube structure, large specific surface area (143.9 m2 g-1) and appropriate pore size distribution (3.4 nm) of the CoFe-PBA endow the as-prepared CoFe-PBA/ACC electrodes with low resistance and appealing ion diffusion characteristics. Typically, the high areal capacitance (1155.0 mF cm-2 at 0.5 mA cm-2) is obtained for high mass loading CoFe-PBA/ACC electrodes (9.7 mg cm-2). Furthermore, symmetrical flexible supercapacitors (FSCs) are constructed using CoFe-PBA/ACC electrodes and Na2SO4/polyving alcohol (Na2SO4/PVA) gel electrolyte, achieving superior stability (85.6% capacitance retention after 5,000 cycles), maximum energy density of 33.8 µWh cm-2 at 200.0 µW cm-2 and promising mechanical flexibility. This work is expected to offer inspirations for the development of high mass loading and additive-free electrodes for FSCs.

Ultrafast charge transfer in mixed-dimensional WO3-x nanowire/WSe2 heterostructures for attomolar-level molecular sensing.

Developing efficient noble-metal-free surface-enhanced Raman scattering (SERS) substrates and unveiling the underlying mechanism is crucial for ultrasensitive molecular sensing. Herein, we report a facile synthesis of mixed-dimensional heterostructures via oxygen plasma treatments of two-dimensional (2D) materials. As a proof-of-concept, 1D/2D WO3-x/WSe2 heterostructures with good controllability and reproducibility are synthesized, in which 1D WO3-x nanowire patterns are laterally arranged along the three-fold symmetric directions of 2D WSe2. The WO3-x/WSe2 heterostructures exhibited high molecular sensitivity, with a limit of detection of 5 × 10-18 M and an enhancement factor of 5.0 × 1011 for methylene blue molecules, even in mixed solutions. We associate the ultrasensitive performance to the efficient charge transfer induced by the unique structures of 1D WO3-x nanowires and the effective interlayer coupling of the heterostructures. We observed a charge transfer timescale of around 1.0 picosecond via ultrafast transient spectroscopy. Our work provides an alternative strategy for the synthesis of 1D nanostructures from 2D materials and offers insights on the role of ultrafast charge transfer mechanisms in plasmon-free SERS-based molecular sensing.

The role of defects and doping in 2D graphene sheets and 1D nanoribbons.

Defects are usually seen as imperfections in materials that could significantly degrade their performance. However, at the nanoscale, defects could be extremely useful since they could be exploited to generate novel, innovative and useful materials and devices. Graphene and graphene nanoribbons are no exception. This review therefore tries to categorize defects, emphasize their importance, introduce the common routes to study and identify them and to propose new ways to construct novel devices based on 'defective' graphene-like materials. In particular, we will discuss defects in graphene-like systems including (a) structural (sp(2)-like) defects, (b) topological (sp(2)-like) defects, (c) doping or functionalization (sp(2)- and sp(3)-like) defects and (d) vacancies/edge type defects (non-sp(2)-like). It will be demonstrated that defects play a key role in graphene physicochemical properties and could even be critical to generate biocompatible materials. There are numerous challenges in this emerging field, and we intend to provide a stimulating account which could trigger new science and technological developments based on defective graphene-like materials that could be introduced into other atomic layered materials, such as BN, MoS(2) and WS(2), not discussed in this review.

Preparation of highly oxidized nitrogen-doped carbon nanotubes.

A method for the preparation of highly oxidized nitrogen-doped carbon nanotubes (N-CNTs) from KMnO(4) + H(2)SO(4) solution is described. The atomic ratio of C/O in oxidized N-CNTs is as low as 1.2. The x-ray photoelectron spectroscopy results show that about 75% of the carbon atoms are oxidized and bound to oxygen-containing functional groups. The oxidation reaction mainly occurs at the outer sidewalls, which destroys the graphene stack to an sp(3)-rich structure and helps to preserve the tubular structure of the inner N-CNTs. The oxidized N-CNTs show an energy gap of ~2.1 eV.

Mo2C-MoO2 Heterostructure Quantum Dots for Enhanced Electrocatalytic Nitrogen Reduction to Ammonia.

The electrocatalytic nitrogen reduction reaction (NRR) has been regarded as a promising strategy for producing ammonia (NH3) at ambient conditions. However, the development of the NRR is severely hindered by the difficult adsorption and activation of N2 on the catalyst surface and the competitive hydrogen evolution reaction (HER) due to the lack of efficient NRR electrocatalysts. Herein, Mo2C-MoO2 heterostructure quantum dots embedded in reduced graphene oxide (RGO) are proposed as efficient catalysts for the electrocatalytic NRR. The ultrasmall size of the quantum dot is beneficial for exposing the active sites for the NRR, and the synergetic effect of Mo2C and MoO2 can promote N2 adsorption and activation and suppress the competitive HER simultaneously. As a result, a well-balanced NRR performance is achieved with a high NH3 yield rate of 13.94 ± 0.39 µg h-1 mg-1 at -0.15 V vs RHE and a high Faradaic efficiency of 12.72 ± 0.58% at -0.1 V vs RHE. Density functional theory (DFT) calculations reveal that the Mo2C (001) surface has a strong N2 adsorption energy of -1.47 eV with the side-on configuration, and the N≡N bond length is elongated to 1.254 Å, indicating the enhanced N2 adsorption and activation on the Mo2C surface. On the other hand, the low ΔGH* for HER over the MoO2 (-111) surface demonstrates the impeded HER process for MoO2. This work may provide effective catalyst-design strategies for enhancing the electrocatalytic NRR performance of Mo-based materials.