A review of etching methods and applications of two-dimensional MXenes.

MXenes have been attracting much attention since their introduction due to their amazing properties such as unique structure, good hydrophilicity, metal-grade electrical conductivity, rich surface chemistry, low ionic diffusion resistance, and excellent mechanical strength. It is noteworthy that different synthesis methods have a great influence on the structure and properties of MXenes. In recent years, some modification strategies of MXenes with unique insights have been developed with the increasing research. In summary, this paper reviews and summarizes the recent research progress of MXenes from the perspective of preparation processes (including hydrofluoric acid direct etching, fluoride/concentrated acid hybrid etching, fluoride melt etching, electrochemical etching, alkali-assisted etching and Lewis acid etching strategies), which can provide valuable guidance for the preparation and application of high-performance MXenes-based materials.

Multiphoton emission of single CdZnSe/ZnS quantum dots coupled with plasmonic Au nanoparticles.

The fluorescence blinking and low multiphoton emission of quantum dots (QDs) have limited their application in lasing, light-emitting diodes, and so on. Coupling of single QDs to plasmonic nanostructures is an effective approach to control the photon properties. Here plasmon-exciton systems including Au nanoparticles and CdZnSe/ZnS QDs were investigated at the single particle level. With the modulation of the local electromagnetic field, the fluorescence intensity of single QDs is increased, accompanied by a significant suppression in blinking behavior, and the lifetime is shortened from 15 ns to 2 ns. Moreover, the second-order photon intensity correlation at zero lag time g2(0) of coupled single QDs is larger than 0.5, indicating an increased probability of multiphoton emission. The enhancement factors of radiative and nonradiative decay rates of QDs coupled with Au nanoparticles are calculated. The sharply increased radiative decay rate can be comparable to the nonradiative Auger rate, leading to dominated multiple exciton radiative recombination with PL intensity enhancement, suppressed blinking, lifetime shortening, and multiphoton emission. The results of the exciton decay dynamics and emission properties of single QDs in this work are helpful in exploring the mechanism of plasmon-exciton interaction and optoelectronic application of single QDs.

The optical properties and carrier mobility of MH3 (M = Co, Rh and Ir) monolayers.

A new series of two-dimensional transition metal hydrides MH3 (M = Co, Rh, Ir) are investigated using first principles calculations. Their electronic structures, phonon dispersion, optical absorptions, and carrier mobilities are obtained and discussed. Our results on the basis of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional reveal that CoH3, RhH3 and IrH3 are indirect semiconductors with band gaps of 2.54 eV, 1.80 eV and 1.82 eV, respectively. Moreover, MH3 monolayers show strong optical absorption in the visible and near-ultraviolet light regions. Under tensile strain, the band gaps decrease and the optical absorption is enhanced in the visible region. The obtained carrier mobilities are found to be anisotropic along the armchair and zigzag directions. The holes along the armchair are more easily transferred with high mobility. The strong optical absorption intensity and the relatively high carrier mobilities make MH3 monolayers (especially RhH3 and IrH3) potential candidates for applications in photovoltaics.

Self-assembling Ni(OH)2/α-Fe2O3 composites for pseudocapacitors with excellent electrochemical performance.

Novel two-dimensional (2D) nanostructured Ni(OH)2 nanosheet/α-Fe2O3 nanoplate composites (NFCs) were successfully synthesized by a simple two-step solvothermal method where the proportion of α-Fe2O3 nanoplates was found to be controllable. These composites achieved excellent performance in aqueous electrolyte due to the synergistic effect between Ni(OH)2 nanosheets and α-Fe2O3 nanoplates, such as high specific capacitance and long-term cycle stability. The obtained NFC8 possessed a maximum mass specific capacitance of 1745.33F g-1 at a current density of 2 A g-1, and with 84.28% retention after 3000 cycles galvanostatic charge/discharge at a high current density of 20 A g-1. The above results show that these novel 2D nanostructured Ni(OH)2/α-Fe2O3 composites are promising electrode materials for pseudocapacitors.

Ultrafast growth of carbon nanotubes on graphene for capacitive energy storage.

We have demonstrated a novel three-dimensional (3D) architecture of a graphene/carbon nanotube (G-CNT) hybrid synthesized at large scale within just 5 s via a simple microwave-heating method without the usage of any other conducting or expanding agent for the first time. The carbon composites obtained consist of evenly grown CNTs with an average diameter of about 15 nm on the surface of graphene nanosheets. The G-CNT hybrid exhibits enhanced electrochemical performance for both aqueous and organic supercapacitor devices. Particularly, the G-CNT electrodes demonstrate an enhanced specific capacitance of 361 F g(-1) at a current density of 1.1 A g(-1) in an aqueous electrolyte and a volumetric capacitance of 254 F cm(-3) in an organic electrolyte. They also display excellent cycle stability with nearly 91.2% of the initial capacitance retained after 10 000 charging-discharging cycles at a current density of 15 A g(-1). This demonstrates that the developed composites have potential applications in supercapacitors and other energy storage devices.

Freestanding polyaniline nanorods grown on graphene for highly capacitive energy storage.

Freestanding polyaniline (PANI) nanorods grown in situ on microwave-expanded graphene oxide (MEGO) sheets were prepared through a facile solution method. The morphological characterization indicates that large quantity of free-standing PANI nanorods with average diameter of 50 nm were uniformly deposited onto the double sides of the MEGO nanosheets to form a sandwich structure. The hybrid of PANI/MEGO (GPANI) exhibit high specific surface area and high electrical conductivity, compared with pristine PANI nanorods. When evaluated as electrodes for supercapacitors, the GPANI demonstrate high specific capacitance of 628 F g(-1) at a current density of 1.1 A g(-1), high-rate performance, and excellent cycle stability compared to individual component. Such excellent electrochemical performance should be attributed to the combined double-layer capacitance and pseudo -capacitance mechanisms from the MEGO sheets and PANI nanorods.

Construction of an n-Type Fluorinated ZnO Interfacial Phase for a Stable Anode of Aqueous Zinc-Ion Batteries.

Aqueous rechargeable zinc-ion batteries have become an ideal solution for the next generation of energy storage systems due to their low cost and high safety. However, the uncontrollable zinc dendrites and harmful side reactions of metal zinc anodes hinder the further development of aqueous zinc-ion batteries. In this work, the artificial fluoride zinc oxide (F-ZnO) interface phase is integrated in situ on the surface of zinc foil. The F-ZnO interface phase significantly inhibits the side reactions on the surface of the zinc electrode by reducing the direct contact between the electrolyte and the surface of the zinc foil. In addition, F-ZnO modified by a small amount of F doping shows enhanced conductivity and electron transport capacity, avoiding the accumulation of high concentration Zn2+ on the anode surface, and ultimately promoting the efficient nucleation and orderly deposition of a zinc anode. The cycle life of the symmetrical cell based on F-ZnO is as high as 2600 cycles at an area current density of 4 mA cm-2, which is much better than that of a commercial pure Zn electrode. The modified F-ZnO@Zn anode truly achieves the purpose of prolonging the anode's life.

Synthesis of centimeter-size two-dimensional hybrid perovskite single crystals with tunable, pure, and stable luminescence.

The environment-friendly synthesis and property modulation of two-dimensional organic-inorganic halide perovskite (2D OHP) single crystals with large sizes and high quality are important for the fabrication of optoelectric devices. In this work, plate-like and centimeter-size (BA)2Pb(BrxI1-x)4 (BA = n-butylammonium, x: 0-1) single crystals with high crystallinity were synthesized via the cooling crystallization method in a mixed HX (X: I, Br) acid aqueous solution. The synthesized samples were single-phase with homogenously distributed Br and I ions. The lattice structure and bandgap of (BA)2Pb(BrxI1-x)4 were both finely tuned through halide alloying. Pure photoluminescence with unitary wavelength was obtained in the mixed-halide samples compared to those of monohalides (BA)2PbI4 and (BA)2PbBr4. This is attributed to the structural homogeneity of the alloyed crystals. Moreover, the prepared (BA)2Pb(BrxI1-x)4 samples showed higher photo and thermal stability for a long duration even with ion migration. This study will be an important reference for the fabrication and property modulation of 2D OHP-based light-emitting and other optoelectric devices.

P-doped porous carbon derived from walnut shell for zinc ion hybrid capacitors.

Zinc ion hybrid capacitors (ZHCs) are expected to be candidates for large-scale energy storage products due to their high power density and large energy density. Due to their low cost and stability, carbon materials are generally the first choice for the cathode of ZHCs, but they face a challenge in the serious self-discharge behavior. Herein, zinc ion hybrid capacitors with high-performance are successfully assembled using a porous carbon cathode derived from low-cost p-doped waste biomass and a commercial zinc foil anode. The p-doped walnut shell ZHCs delivered a specific capacity of 158.9 mA h g-1 with an energy density of 127.1 W h kg-1 at a low current density. More importantly, the device had outstanding anti-self-discharge characteristics (retaining 77.98% of its specific capacity after a 72 h natural self-discharge test) and long-term cycle stability (retaining 88.2% of its initial specific capacity after 15 000 cycles at 7.5 A g-1). This work presents guidance and support for the design and optimization of electrode materials for zinc ion supercapacitors and next-generation aqueous zinc ion energy storage performance.

Free-Standing rGO-CNT Nanocomposites with Excellent Rate Capability and Cycling Stability for Na2SO4 Aqueous Electrolyte Supercapacitors.

Facing the increasing demand for various renewable energy storage devices and wearable and portable energy storage systems, the research on electrode materials with low costs and high energy densities has attracted great attention. Herein, free-standing rGO-CNT nanocomposites have been successfully synthesized by a facile hydrothermal method, in which the hierarchical porous network nanostructure is synergistically assembled by rGO nanosheets and CNT with interlaced network distribution. The rGO-CNT composite electrodes with synergistic enhancement of rGO and CNT exhibit high specific capacitance, excellent rate capability, exceptional conductivity and outstanding long-term cycling stability, especially for the optimal rGO-CNT30 electrode. Applied to a symmetric supercapacitor systems (SSS) assembled with an rGO-CNT30 electrode and with 1 M Na2SO4 aqueous solution as the electrolyte, the SSS possesses a high energy density of 12.29 W h kg-1 and an outstanding cycling stability, with 91.42% of initial specific capacitance after 18,000 cycles. Results from these electrochemical properties suggest that the rGO-CNT30 nanocomposite electrode is a promising candidate for the development of flexible and lightweight high-performance supercapacitors.

Synthesis of Two-Dimensional Sr-Doped LaNiO3 Nanosheets with Improved Electrochemical Performance for Energy Storage.

Designing a novel, efficient, and cost-effective nanostructure with the advantage of robust morphology and outstanding conductivity is highly promising for the electrode materials of high-performance electrochemical storage device. In this paper, a series of honeycombed perovskite-type Sr-doped LaNiO3 nanosheets with abundant porous structure were successfully synthesized by accurately controlling the Sr-doped content. The study showed that the optimal LSNO-0.4 (La0.6Sr0.4NiO3-δ) electrode exhibited excellent electrochemical performance, which showed a high capacity of 115.88 mAh g-1 at 0.6 A g-1. Furthermore, a hybrid supercapacitor device (LSNO//AC) based on LSNO-0.4 composites and activated carbon (AC) showed a high energy density of 17.94 W h kg-1, a high power density of 1600 W kg-1, and an outstanding long-term stability with 104.4% capacity retention after 16,000 cycles, showing an excellent electrochemical performance and a promising application as an electrode for energy storage.

Design and Synthesis of Zinc-Activated Cox Ni2-x P/Graphene Anode for High-Performance Zinc Ion Storage Device.

Zinc ion capacitors (ZICs) composed of capacitor-type cathodes and battery-type anodes have attracted widespread attention thanks to the huge potential in the next generation of low-cost energy-storage devices. It is a challenge to explore a universal anode for aqueous ZICs with high-efficiency energy-storage characteristics. In this work, the double-transition-metal composite Cox Ni2-x P/reduced graphene oxide (rGO) with sufficient electrochemical activity and charge-transfer kinetics was successfully synthesized. The Zn@CoNiP/rGO anode obtained by zinc-ion activation and a biomass-derived porous carbon cathode (PC) were assembled into an aqueous ZIC (CNP-ZIC) in 2 m ZnSO4 . Finally, the CNP-ZIC reveals excellent energy and power densities with a working potential range of 0.2-1.9 V. CNP-ZICs shows high capacitance of up to 356.6 F g-1 at 0.5 A g-1 (based on the mass of active material on the PC cathode), which is far superior to the performance of conventional asymmetric energy storage devices (CoNiP/rGO//PC and Co2 P/rGO//PC). The CNP-ZIC exhibits both a very high energy density of 143.14 Wh kg-1 and good cycling life (∼92.2 % retention after 10000 charge-discharge cycles at 7.5 A g-1 ). There is no doubt that this work provides a promising strategy for assembling novel zinc ion hybrid supercapacitors with high efficiency and stable output.

Enhancing the Electrochemical Properties of LaCoO3 by Sr-Doping, rGO-Compounding with Rational Design for Energy Storage Device.

Perovskite oxides, as a kind of functional materials, have been widely studied in recent years due to its unique physical, chemical, and electrical properties. Here, we successfully prepared perovskite-type LaCoO3 (LCOs) nanomaterials via an improved sol-gel method followed by calcination, and investigated the influence of calcination temperature and time on the morphology, structure, and electrochemical properties of LaCoO3 nanomaterials. Then, based on the optimal electrochemical performance of LCO-700-4 electrode sample, the newly synthesized nanocomposites of Sr-doping (LSCO-0.2) and rGO-compounding (rGO@LCO) through rational design exhibited a 1.45-fold and 2.03-fold enhancement in its specific capacitance (specific capacity). The rGO@LCO electrode with better electrochemical performances was further explored by assembling rGO@LCO//rGO asymmetric supercapacitor system (ASS) with aqueous electrolyte. The result showed that the ASS delivers a high energy density of 17.62 W h kg-1 and an excellent cyclic stability with 94.48% of initial capacitance after 10,000 cycles, which are good electrochemical performances among aqueous electrolytes for green and new efficient energy storage devices.

Electro-opto-mechano driven reversible multi-state memory devices based on photocurrent in Bi0.9Eu0.1FeO3/La0.67Sr0.33MnO3/PMN-PT heterostructures.

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro-opto-mechano-driven reversible multi-state memory device based on photocurrent in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructures. It is found that the short-circuit current density (J sc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric-optical-mechanism integrated devices based on the BiFeO3-based heterostructures.

High-performance solid-state supercapacitors based on graphene-ZnO hybrid nanocomposites.

In this paper, we report a facile low-cost synthesis of the graphene-ZnO hybrid nanocomposites for solid-state supercapacitors. Structural analysis revealed a homogeneous distribution of ZnO nanorods that are inserted in graphene nanosheets, forming a sandwiched architecture. The material exhibited a high specific capacitance of 156 F g-1 at a scan rate of 5 mV.s-1. The fabricated solid-state supercapacitor device using these graphene-ZnO hybrid nanocomposites exhibits good supercapacitive performance and long-term cycle stability. The improved supercapacitance property of these materials could be ascribed to the increased conductivity of ZnO and better utilization of graphene. These results demonstrate the potential of the graphene-ZnO hybrid nanocomposites as an electrode in high-performance supercapacitors.

SnO2Nanowire Arrays and Electrical Properties Synthesized by Fast Heating a Mixture of SnO2and CNTs Waste Soot.

SnO2nanowire arrays were synthesized by fast heating a mixture of SnO2and the carbon nanotubes waste soot by high-frequency induction heating. The resultant SnO2nanowires possess diameters from 50 to 100 nm and lengths up to tens of mircrometers. The field-effect transistors based on single SnO2nanowire exhibit that as-synthesized nanowires have better transistor performance in terms of transconductance and on/off ratio. This work demonstrates a simple technique to the growth of nanomaterials for application in future nanoelectronic devices.