H2O Nanosheets Research Articles

Interface-rich core-shell ammonium nickel cobalt phosphate for high-performance aqueous hybrid energy storage device without a depressed power density

Introducing diffusion-controlled battery materials to supercapacitors, can significantly enhance the energy density of supercapacitors, which however encounter depressed power density due to the intrinsic sluggish charge storage kinetics of battery materials. This problem can be efficiently solved by modifying the microstructure, and enable capacitive controlled charge storage mechanism in the battery materials. In this work, a novel interface-rich core-shell structure with (NH4)(Ni,Co)PO4·0.67 H2O nanosheets @ single crystal microplatelets (NH4)(Ni,Co)PO4·0.67 H2O (NCoNiP@NCoNiP) is constructed via a facile two-step hydrothermal method, taking advantage of etching induced Kirkendall effect and Ostwald ripening. This unique structure can enable the extrinsic pseudocapacitance by providing extra charges (e.g. holes, electrons or voids) on the interfaces, and realize synergy and fast charge storage. Specifically, a maximum specific capacity of 190.3 mAh g−1 and ultrahigh rate performance with capacity retention of 96.1% from 1 to 10 A g−1 in a three-electrode test. The kinetic analysis indicates that the electrochemical response of the hybrid battery-supercapacitor storage devices shows obvious characteristic of supercapacitor especially at high scanning rates. Simultaneously, the hybrid battery-supercapacitor devices based on NCoNiP@NCoNiP exhibits a high energy density of 44.5 Wh kg−1 at a power density of 150 W kg−1, which maintains 30 Wh kg−1 at high power density of 7.4 kW kg−1 with capacitance retention 77.5% after 7000 cycles. This work provides a novel strategy for the application of battery materials in high power devices, by enabling the capacitive charge storage mechanism of battery materials through nanostructure engineering.

One-pot synthesis of secondary amine via photoalkylation of nitroarenes with benzyl alcohol over Pd/monolayer H1.07Ti1.73O4·H2O nanosheets

The photoalkylation of nitroarenes with benzyl alcohols in one pot at room temperature and 1 atm N2 was achieved over the Pd/H1.07Ti1.73O4·H2O nanosheets. The sample shows efficient photocatalytic activity with high conversion of nitrobenzene (99%) and selectivity of secondary amine (85%). This flexible photocatalytic system is also applicable to other nitroarenes with high efficiency. Results of in situ FTIR, DRS, and in situ ESR revealed that the benzyl alcohol and nitrobenzene molecules can bind with the surface Lewis and Brønsted acid sites in the catalyst via the HO⋯Ti and NO2⋯HOTi species. The formation of surface coordination species results in not only the activation of reactant molecules via surface electron transfer, but also the expanded visible light absorption of the catalyst. Moreover, in situ ESR suggested that the surface coordination can also facilitate the formation of oxygen vacancies in catalysts, which can greatly promote the exposure of Lewis sites and enhance the activation of reactant molecules. Finally, a possible hydrogen transfer strategy over the sample is proposed on a molecular level.

In Situ Growth of Layered Bimetallic ZnCo Hydroxide Nanosheets for High-Performance All-Solid-State Pseudocapacitor.

Two-dimensional (2D) hydroxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Herein, we report ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets (thickness ∼30 nm) which crystallize in a layered structure and exhibit a high specific capacitance of 3946.5 F g-1 at 3 A g-1 for an electrochemical pseudocapacitor. ZnCo1.5(OH)4.5Cl0.5·0.45H2O was synthesized by a homogeneous precipitation method and spontaneously crystallized into 2D nanosheets in well-defined hexagonal morphology with crystal structure revealed by synchrotron X-ray powder diffraction data analysis. In situ growth of ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheet arrays on conductive Ni foam substrate was successfully realized. Asymmetric supercapacitors based on ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets @Ni foam// PVA, KOH//reduced graphene oxide exhibits a high energy density of 114.8 Wh kg-1 at an average power density of 643.8 W kg-1, which surpasses most of the reported all-solid-state supercapacitors based on carbonaceous materials, transition metal oxides/hydroxides, and MXenes. Furthermore, a supercapacitor constructed from ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets@PET substrate shows excellent flexibility and mechanical stability. This study provides layered bimetallic hydroxide nanosheets as promising electroactive materials for flexible, solid-state energy storage devices, presenting the best reported performance to date.

Electrochemical fabrication of interconnected tungsten bronze nanosheets for high performance supercapacitor

Interconnected H0.12WO3⋅H2O nanosheets with high electrochemical performances are fabricated on partial exfoliated graphite substrate (Ex-GF) by potential-limited pulse galvanostatic method (PLPG). The dead volume problem of bulk pesudocapacitive materials is addressed by the novel interconnected nanosheets structure, enabling a large specific capacitance of 5.95 F cm−2 (495.8 F g−1) at 2 mA cm−2. Merited from the fluent electrolyte penetration channels established by the plenty voids among nanosheets, as well as fast electron transportation in the electronic conductive tungsten bronze which is directly grown from graphite substrate, the obtained WO3/Ex-GF demonstrates excellent rate capability. The material can maintain 60.0% of its capacitance when the discharge current density increases from 2 to 100 mA cm−2. Moreover, WO3/Ex-GF doesn't show capacitance decay after 5000 galvanostatic charge-discharge cycles, displaying its super stability. Furthermore, a high performance asymmetric supercapacitor assembled by using WO3/Ex-GF and electrochemical fabricated MnO2/Ex-GF as negative and positive electrodes, respectively displays a high energy density of 2.88 mWh cm−3 at the power density of 11.1 mW cm−3, demonstrating its potential application for energy storage.

Crystal transformation of 2D tungstic acid H2WO4 to WO3 for enhanced photocatalytic water oxidation

New photocatalytic materials for stable reduction and/or oxidization of water by harvesting a wider range of visible light are indispensable to achieve high practical efficiency in artificial photosynthesis. In this work, we prepared 2D WO3·H2O and WO3 nanosheets by a one-pot hydrothermal method and sequent calcination, focusing on the effects of crystal transformation on band structure and photocatalytic performance for photocatalytic water oxidation in the presence of electron acceptors (Ag+) under simulated solar light irradiation. The as-prepared WO3 nanosheets exhibit enhanced rate of photocatalytic water oxidation, which is 6.3 and 3.6 times higher than that of WO3·H2O nanosheets and commercial WO3, respectively. It is demonstrated that the releasing of water molecules in the crystal phase of tungstic acid results in transformation of the crystal phase from orthorhombic WO3·H2O to monoclinic WO3, significantly improving the activity of photocatalytic water oxidation in the presence of Ag+ because the shift-up of conduction band of WO3 matches well with the electrode potential of Ag+/Ag(s), leading to efficient separation of photoinduced electrons and holes in pure WO3 nanosheets.

Fabrication of hierarchical MnMoO4·H2O@MnO2 core-shell nanosheet arrays on nickel foam as an advanced electrode for asymmetric supercapacitors

The hierarchical MnMoO4·H2O@MnO2 core-shell nanosheet arrays on nickel foam have been successfully fabricated by a mild two-step hydrothermal method. MnMoO4·H2O nanosheet arrays is synthesized by adding sodium molybdates solution into the manganese chloride solution containing a nickel foam, and followed by the first step hydrothermal treatment. The MnMoO4·H2O@MnO2 is fabricated by the second step hydrothermal treatment of above-step product in potassium permanganate solution. The core-shell nanosheet structure significantly increases the amount of active sites for redox reactions, shortens the diffusion pathway for ions, and facilitates the effective penetration of the electrolytes for energy storage application. The MnMoO4·H2O@MnO2 as electrode material reaches to a high specific capacitance of 3560.2 F g−1 at a current density 1 A g−1, as well as advantageous cycling stability (84.1% capacitance retention after 10,000 cycles). Furthermore, an asymmetric supercapacitor is also assembled, exhibiting an energy density of 45.6 Wh kg−1 at a power density of 507.3 W kg−1. The MnMoO4·H2O@MnO2 is a promising electrode material for supercapacitors.

Highly selective oxidation of furfuryl alcohol over monolayer titanate nanosheet under visible light irradiation

Monolayer H1.4Ti1.65O4·H2O nanosheet (denoted as NST) has been prepared as a photocatalyst for the oxidation of furfuryl alcohol (denoted as FA) to furaldehyde under visible light irradiation. The photocatalytic activity of NST is over 10 times higher than that of its layered counterpart. This enhanced activity may be attributed to the high percentage of exposed Lewis acid sites in NST. The in-situ FTIR result suggests that FA are efficiently chemosorbed on the exposed Lewis acid sites forming the surface coordination species via CO groups, resulting in the activation of FA. The surface coordination species would respond to visible light absorption. Furthermore, The XAFS result shows that the signals for TiO and Ti–Ti in NST are weaker dramatically and the signals positions are shifted by 0.03Å in the higher R direction as compared with those in layered counterpart, suggesting the more exposed Ti and O defects in NST. The oxygen molecules absorbed on these surface defects are activated forming O2− by photo-electrons under visible light irradiation. Finally, a possible mechanism has been proposed at a molecular level.

Photocatalytic hydrogen evolution over monolayer H1.07Ti1.73O4·H2O nanosheets: Roles of metal defects and greatly enhanced performances

Monolayer H1.07Ti1.73O4·H2O nanosheets with the thickness about 0.67nm were prepared and developed as an efficient photocatalyst for hydrogen evolution. The prepared sample exhibits greatly improved photocatalytic activity with more 10.5 times higher than its layered counterpart. The morphologies, microstructures, superficial properties and electronic structures of the sample were characterized by XRD, TEM, AFM, BET, and UV–vis DRS in detail. Moreover, EXAFS, FTIR, XPS and in-suit FTIR of D2O absorption results suggested that Ti vacancies result in the formation of abundant active O species around vacancies sites, which can be exposed fully in the monolayer nanosheets and bind with water molecules in the formation of surface coordination via hydrogen bonds. An efficient electron transition from nanosheets to surficial coordinated H2O molecules takes place. Finally, a synergistic effect between titanium vacancies and ultrathin 2D structure was proposed to elucidate that the enhanced photocatalytic performance over metal defects may be attributed to efficient exposure of active species and transition of photo-electrons from surface to H2O molecules.

3D mace-like hierarchical ZnO nanoarchitecture constructed with microrod bundles and porous single-crystalline nanosheets for acetone sensors with enhanced performances

3D mace-like hierarchical ZnO nanoarchitectures consisting of porous single-crystalline nanosheets as well as microrod bundles were synthesized by growing Zn4CO3(OH)6·H2O nanosheets on the surfaces of ZnO microrod bundles on the basis of a facile one-step solution method combined with a successive annealing treatment at 400°C. The investigation for the evolution mechanism of hierarchical ZnO nanoarchitectures shows that the reaction time, concentrations of reactants together with the surfactant can radically influence the compositions and morphologies of materials generated during the reactions. A potential mechanism is suggested to help in understanding the formation of the hierarchical nanoarchitectures based on the reaction time and the concentrations of reactants effect on the morphologies of the products. The chemical sensors made with 3D mace-like hierarchical ZnO nanoarchitectures reveals enhanced sensing performances with high responses, selectivity as well as stability to acetone vapor.

Fabrication and assembly of two-dimensional TiO2/WO3·H2O heterostructures with type II band alignment for enhanced photocatalytic performance

The recombination of photo-induced charges is one of the main issues to limit the large-scale applications in photocatalysis and photoelectrocatalysis. To improve the charge separation, we fabricate a novel type II 2D ultrathin TiO2/WO3·H2O heterostructures with the assistance of supercritical CO2 (SC CO2) in this work. The as-fabricated heterostructures possess high photocatalytic activity for the degradation of methyl orange(MO) and high photocurrent response under simulated solar light (AM 1.5). For the TiO2/WO3·H2O heterostructures, the MO solution could be degraded by 95.5% in 150min, and the photocurrent density reaches to 6.5μAcm−2, exhibiting a significant enhancement compared with pure TiO2 and WO3·H2O nanosheets.

Liquid-phase exfoliation of NH4Co0.4Ni0.6PO4·H2O for energy storage device

Liquid-phase exfoliation is an effective method to fabricate thin nanosheet materials with layered structure. In this paper, we successfully synthesized NH4Co0.4Ni0.6PO4·H2O nanosheets with the thickness of about tens of nanometers through liquid-phase exfoliation under ultrasonication. Specifically, the formamide solution was chosen as the exfoliation solvent due to it is capable of entering the interlayer of bulk materials and rapidly swelling between layers. Electrochemistry tests revealed that the optimized exfoliation time was 4 days. The asymmetric battery type energy storage device AC//NH4Co0.4Ni0.6PO4·H2O nanosheets showed an energy density of 10.8 Wh kg−1 at the power density of 703.1 W kg−1. More importantly, the long-term cycle stability measurements demonstrated that NH4Co0.4Ni0.6PO4·H2O nanosheets after exfoliation had an ultra-long cycling life that still retained 102.3% of specific capacitance even 10000 continuous charge/discharge cycles.

Refining waste hardmetals into tungsten oxide nanosheets via facile method

A new hydrothermal system has been designed to recycle waste WC–Co hardmetal with low cobalt (Co) content (3 %). In the solution system, nitric acid was designed to dissolve Co, H2O2 served as oxidant to accelerate the oxidation of the WC–Co hardmetals, and fluorine (F−) was designed to dissolve and recrystallize generated tungsten oxides, which were found to possess a layered structure using scanning electron microscopy and transmission electron microscopy. The obtained tungsten oxides were identified as WO3·0.33H2O by X-ray diffraction and their specific surface area was measured as 89.2 m2 g−1 via N2 adsorption–desorption techniques. The present layered structure tungsten oxides exhibited a promising capability for removing lead ion (Pb2+) and organic species, such as methyl blue. The adsorption model was found to be in agreement with Langmuir isotherm model. Given the facile synthesis procedure and promising properties of final products, this new approach should have great potential for refining some other waste hardmetals or tungsten products. A new hydrothermal system was designed to recycle waste hardmetal with low cobalt content. Through this method, waste hardmetal was refined into WO3·0.33H2O nanosheets which shows excellent adsorption capacities toward methylene blue and lead ion (Pb2+).

Design and synthesis of two-dimensional porous Fe-doped LiCoPO4 nano-plates as improved cathode for lithium ion batteries

Novel two-dimensional (2D) Fe-doped LiCoPO4 nano-plates with porous structure have been successfully fabricated using NH4CoPO4·H2O nanosheets as templates followed by Fe doping and high temperature annealing. The obtained Fe-doped LiCoPO4 nano-plates exhibit several merits in morphology and crystal structure, including well-crystallized feature, porous structure, numerous interconnected pathway, improved electric conductivity and good structural stabilization. All the advantages endow the nano-plates with enhanced electrochemical performance when they are used as cathode materials for lithium ion batteries (LIBs). In this research, high specific capacity, excellent cyclability and outstanding rate capability in electrochemical energy storage are presented. This synthetic strategy is simple, effective, and could be broadly applied in designed synthesis of other electrode materials for LIBs.

A one-step water based strategy for synthesizing hydrated vanadium pentoxide nanosheets from VO2(B) as free-standing electrodes for lithium battery applications

V2O5·0.55H2O nanosheets synthesizedviaa water based technique, and showing promising lithium-ion storage capacity.

Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts, and single crystalline ultra-long Na4Mn9O18 nanowires.

Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn2O3 by inserting Na+ ions. In particular, Mn2O3 nanoparticles were first transformed to 2 nm thin Na0.55Mn2O4·1.5H2O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation, and finally to sub-mm ultra-long single crystalline Na4Mn9O18 nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.

Enhanced toluene sensing performance of gold-functionalized WO3·H2O nanosheets

Au nanoparticle-functionalized composites have been proven to be promising materials for gas sensing applications. We successfully synthesized tungsten oxide (WO3·H2O) nanosheets by a facile and effective hydrothermal process and decorated Au nanoparticles on their surface for ultra-high sensitivity levels to toluene gas. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), Energy Dispersive X-ray Spectrometer (EDX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Gas-sensing measurements revealed superior toluene sensing properties of Au nanoparticle-functionalized WO3·H2O nanosheets (Au-WO3·H2O) compared with bare WO3·H2O. The maximum response value of Au-WO3 reached 50–100ppm toluene at 300°C, which was nearly four times as high as that of bare WO3. Meanwhile, Au-WO3·H2O exhibited the shorter response/recovery time, better selectivity and lower operating temperature to toluene gas than those of bare WO3·H2O. Also, the mechanism involved in improving the toluene sensing properties of WO3·H2O by Au nanoparticle-functionalization was discussed.

Synthesis and characterization of anatase TiO2 nanosheet arrays on FTO substrate

We have exploited a green approach to prepare layered titanate Na2-xHxTi2O5•H2O nanosheet arrays on FTO substrate by hydrothermal hydrolysis of titanium(IV) isopropoxide (TTIP) with aids of Na2EDTA and TEOA as co-coordination agents, which were then treated by HNO3 to replace Na+ by H+, followed by a calcination at 450 °C to topotactically transform into anatase TiO2 nanosheet arrays. SEM, TEM, XRD, and Raman spectroscopy have been employed to characterize the nanosheet films. The TiO2 nanosheet arrays were further applied as electron transport materials of CH3NH3PbI3 perovskite solar cells, achieving power conversion efficiency of 6.99%.

Vacancy associates-rich ultrathin nanosheets for high performance and flexible nonvolatile memory device.

On the road of innovation in modern information technology, resistive switching random access memory (RRAM) has been considered to be the best potential candidate to replace the conventional Si-based technologies. In fact, the key prerequisite of high storage density and low power consumption as well as flexibility for the tangible next generation of nonvolatile memories has stimulated extensive research into RRAM. Herein, we highlight an inorganic graphene analogue, ultrathin WO3·H2O nanosheets with only 2-3 nm thickness, as a promising material to construct a high performance and flexible RRAM device. The abundant vacancy associates in the ultrathin nanosheets, revealed by the positron annihilation spectra, act not only carrier reservoir to provide carriers but also capture center to trap the actived Cu(2+) for the formation of conductive filaments, which synergistically realize the resistive switching memory with low operating voltage (+1.0 V/-1.14 V) and large resistance ON/OFF ratio (>10(5)). This ultrathin-nanosheets-based RRAM device also shows long retention time (>10(5) s), good endurance (>5000 cycles), and excellent flexibility. The finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to an atomic level deep understanding of the underlying nature of the resistive switching behavior, which may serve as a guide to improve the performances and promote the rapid development of RRAM.

Interfacial Bonding Stabilizes Rhodium and Rhodium Oxide Nanoparticles on Layered Nb Oxide and Ta Oxide Supports

Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and α-Zr(HPO4)2·H2O. High surface area SiO2 and γ-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ΔH values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and γ-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports.

Zn5(OH)8Cl2·H2O-based quantum dots-sensitized solar cells: A common corrosion product enhances the performance of photoelectrochemical cells

Exploiting new and efficient wide-band gap semiconductors is an important way to improve the performance of photoelectrochemical cells (dye-sensitized solar cells (DSSCs) and quantum dots-sensitized solar cells (QDSCs)). In this paper, Zn5(OH)8Cl2·H2O semiconductor that is a common corrosion product for zinc-contained materials in the electrolytes containing Cl− ions is for the first time explored as wide-band gap semiconductor in the photoelectrode of photoelectrochemical cells. The QDSCs based on CdS QDs-sensitized Zn5(OH)8Cl2·H2O nanosheets achieve the promising power conversion efficiencies of 0.62% and 1.11% for Pt and Cu2S counter electrodes, respectively. Furthermore, combining Zn5(OH)8Cl2·H2O and ZnO to form ZnO/Zn5(OH)8Cl2·H2O composite photoelectrode greatly increases the conversion efficiencies of QDSCs to 1.39% and 1.92% for Pt and Cu2S counter electrodes, respectively, which are higher than those based on conventional ZnO (1.11% and 1.53% for Pt and Cu2S counter electrodes, respectively) or TiO2 porous films (1.07% for Pt counter electrode). Therefore, Zn5(OH)8Cl2·H2O has shown much promise as an efficient wide-band gap semiconductor in the photoelectrode of photoelectrochemical cells.