Pillar-Structured Ti3 C2 Tx MXene with Engineered Interlayer Spacing for High-Performance Magnesium Batteries

Two-dimentional (2D) Ti3 C2 Tx MXene has attracted significant attention in non-lithium-ion batteries due to its excellent electrical conductivity, high volumetric capacity, and ability to accommodate intercalants. Rechargeable magnesium batteries with Mg metal anodes are noted for their high theoretical energy density, potential safety, earth abundance, dendrite-free Mg2+ plating/stripping mechanism on the anode side, and low cost. Nevertheless, owing to the large polarity of divalent Mg2+ ions, the insertion of Mg2+ into the MXene layers suffers from sluggish kinetics, limiting the performance for storage of Mg2+ ions. Herein, a simple self-assembly strategy is demonstrated to achieve high magnesium ion storage capability with pillar-structured Ti3 C2 Tx MXene by intercalating a hyperbranched polyethylene ionomer containing quaternary ammonium ions. The ionomer intercalation/modification leads to the expansion of interlayer spacing of the MXene and, meanwhile, improves its affinity to low-polarity THF-based electrolyte. The delaminated ionomer-modified MXene shows significantly improved electrochemical performance as a cathode material for Mg batteries. It shows a promising cycling stability with a capacity retention of 86% after 400 cycles at 200 mA g-1 , as well as outstanding high-rate performance with a capacity of 110 mAh g-1 retained at 1,000 mA g-1 relative to 213 mAh g-1 at 20 mA g-1 .

Regulating Interlayer-Spacing of Vanadium Phosphates for High-Capacity and Long-Life Aqueous Iron-Ion Batteries

Although the research on aqueous batteries employing metal as the anode is still mainly focused on the aqueous zinc-ion battery, aqueous iron-ion batteries are considered as promising aqueous batteries owing to the lower cost, higher specific capacity, and better stability. However, the sluggish Fe2+ (de)intercalation leads to unsatisfactory specific capacity and poor electrochemical stability, which makes it difficult to find cathode materials with excellent electrochemical properties. Herein, phenylamine (PA)-intercalated VOPO4 materials with expanded interlayer spacing are synthesized and applied successfully in aqueous iron-ion batteries. Owing to enough diffusion space from the expanded interlayer, which can boost fast Fe2+ diffusion, the aqueous iron-ion battery shows a high specific capacity of 170 mAh g-1 at 0.2 A g-1 , excellent rate performance, and cycle stability (96.2% capacity retention after 2200 cycles). This work provides a new direction for cathode material design in the development of aqueous iron-ion batteries.

Scalable synthesis of N/S co-doped hard carbon microspheres as a high-performance anode material for sodium-ion batteries

Hard carbon is a promising anode material for sodium-ion batteries (SIBs) due to its abundance. However, it exhibits low reversible capacity and slow kinetics if inappropriate microstructural features are developed during synthesis. Herein, N/S co-doped phenolic resin-based hard carbon microspheres are prepared by a scalable strategy, and the electrochemical performance is assessed both in half cells and full cells. We demonstrate that the expanded interlayer spacing, the increased active sites, and the enhanced capacitive behavior result in the enhanced reversible capacity and promoted kinetics for Na+storage. The sample with appropriate doping amount exhibits an initial charge capacity of 536.8 mAh g-1at 50 mA g-1and maintains 445.9 mAh g-1after 1000 cycles at a current density of 1 A g-1in a Na-metal half cell. Coupled with a carbon-coated Na4Fe3(PO4)2P2O7(NFPP) cathode, the full cell exhibits a capacity of 92.5 mAh g-1after 90 cycles, with a capacity retention of 91.6%. This work provides a facile and scalable method for synthesizing high-performance hard carbon anode materials for SIBs.

Nacre-Inspired Strong MXene/Cellulose Fiber with Superior Supercapacitive Performance via Synergizing the Interfacial Bonding and Interlayer Spacing

MXene fibers are promising candidates for weaveable and wearable energy storage devices because of their good electrical conductivity and high theoretical capacitance. Herein, we propose a nacre-inspired strategy for simultaneously improving the mechanical strength, volumetric capacitance, and rate performance of MXene-based fibers through synergizing the interfacial interaction and interlayer spacing between Ti₃C₂T_X nanosheets. The optimized hybrid fibers (M-CMC-1.0%) with 99 wt % MXene loading exhibit an improved tensile strength of ∼81 MPa and a high specific capacitance of 885.0 F cm^-3 at 1 A cm^-3 together with an outstanding rate performance of 83.6% retention at 10 A cm^-3 (740.0 F cm^-3). As a consequence, the fiber supercapacitor (FSC) based on the M-CMC-1.0% hybrid delivers an output capacitance of 199.5 F cm^-3, a power density of 1186.9 mW cm^-3, and an energy density of 17.7 mWh cm^-3, respectively, implying its promising applications as portable energy storage devices for future wearable electronics.

Sodium Niobate with a Large Interlayer Spacing: A Fast-Charging, Long-Life, and Low-Temperature Friendly Lithium-Storage Material

Niobate Li⁺ -storage anode materials with shear ReO₃ crystal structures have attracted intensive attention due to their inherent safety and large capacities. However, they generally suffer from limited rate performance, cyclic stability, and temperature adaptability, which are rooted in their insufficient interlayer spacings. Here, sodium niobate (NaNb₁₃ O₃₃ ) micron-sized particles are developed as a new anode material owning the largest interlayer spacing among the known shear ReO₃ -type niobates. The large interlayer spacing of NaNb₁₃ O₃₃ enables very fast Li⁺ diffusivity, remarkably contributing to its superior rate performance with a 2500 to 125 mA g^-1 capacity percentage of 63.2%. Moreover, its large interlayer spacing increases the volume-accommodation capability during lithiation, allowing small unit-cell-volume variations (maximum 6.02%), which leads to its outstanding cyclic stability with 87.9% capacity retention after as long as 5000 cycles at 2500 mA g^-1 . Its cyclic stability is the best in the research field of niobate micron-sized particles, and comparable to that of "zero-strain" Li₄ Ti₅ O₁₂ . At a low temperature of -10 °C, it also exhibits high rate performance with a 1250 to 125 mA g^-1 capacity percentage of 65.6%, and even better cyclic stability with 105.4% capacity retention after 5000 cycles at 1250 mA g^-1 . These comprehensively good electrochemical results pave the way for the practical application of NaNb₁₃ O₃₃ in high-performance Li⁺ storage.

Rational Design of MXene Hollow Fiber Membranes for Gas Separations

One scalable and facile dip-coating approach was utilized to construct a thin CO₂-selection layer of Pebax/PEGDA-MXene on a hollow fiber PVDF substrate. An interlayer spacing of 3.59 Å was rationally designed and precisely controlled for the MXene stacks in the coated layer, allowing efficient separation of the CO₂ (3.3 Å) from N₂ (3.6 Å) and CH₄ (3.8 Å). In addition, CO₂-philic nanodomains in the separation layer were constructed by grafting PEGDA into MXene interlayers, which enhanced the CO₂ affinity through the MXene interlayers, while non-CO₂-philic nanodomains could promote CO₂ transport due to the low resistance. The membrane could exhibit optimal separation performance with a CO₂ permeance of 765.5 GPU, a CO₂/N₂ selectivity of 54.5, and a CO₂/CH₄ selectivity of 66.2, overcoming the 2008 Robeson upper bounds limitation. Overall, this facile approach endows a precise controlled molecular sieving MXene membrane for superior CO₂ separation, which could be applied for interlayer spacing control of other 2D materials during membrane construction.

Layered Metal Oxide Nanosheets with Enhanced Interlayer Space for Electrochemical Deionization

Electrochemical deionization is regarded as one of the promising water treatment technologies. Here, CoAl-layered metal oxide nanosheets intercalated by sodium dodecyl sulfate (SDS) with an enhanced interlayer spacing from 0.76 to 1.33 nm are synthesized and used as an anode. The enlarged interlayer spacing provides an enhanced ion-diffusion channel and improves the utilization of the interlayer electroactive sites, while heat treatment, transferring layered double hydroxides to layered metal oxides (LMOs), offers additional active oxidation reaction sites to facilitate the electro-sorption rate, contributing to the high salt adsorption capacity (31.78 mg g^-1 ) and average salt adsorption rate (3.75 mg g^-1  min^-1 ) at 1.2 V in 500 mg L^-1 NaCl solution. In addition, the excellent long-term cycling stability (92.9%) after 40 cycles proves the strong electronic interaction between SDS and the host layer, which is validated by density functional theory calculations later on. Moreover, the electro-sorption mechanism of LMOs that originated from the reconstruction of the layered structure based on the "memory effect" is revealed according to the X-ray photoelectron spectroscopy peak shifts of Co element. This strategy of expanding the interlayer spacing combined with heat treatment makes LMOs a competitive candidate for electrochemical water deionization.

Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp² C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH^- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and ∼3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

GO-Based Membranes for Desalination

Graphene oxide (GO), owing to its atomic thickness and tunable physicochemical properties, exhibits fascinating properties in membrane separation fields, especially in water treatment applications (due to unimpeded permeation of water through graphene-based membranes). Particularly, GO-based membranes used for desalination via pervaporation or nanofiltration have been widely investigated with respect to membrane design and preparation. However, the precise construction of transport pathways, facile fabrication of large-area GO-based membranes (GOMs), and robust stability in desalination applications are the main challenges restricting the industrial application of GOMs. This review summarizes the challenges and recent research and development of GOMs with respect to preparation methods, the regulation of GOM mass transfer pathways, desalination performance, and mass transport mechanisms. The review aims to provide an overview of the precise regulation methods of the horizontal and longitudinal mass transfer channels of GOMs, including GO reduction, interlayer cross-linking, intercalation with cations, polymers, or inorganic particles, etc., to clarify the relationship between the microstructure and desalination performance, which may provide some new insight regarding the structural design of high-performance GOMs. Based on the above analysis, the future and development of GOMs are proposed.

Biomass-Derived Hard Carbon with Interlayer Spacing Optimization toward Ultrastable Na-Ion Storage

Hard carbons as a kind of nongraphitized amorphous carbon have been recognized as potential anode materials for sodium-ion batteries (SIBs) due to its large interlayer spacing. However, the issues in terms of onerous synthetic procedure and elusive working mechanism remains critical bottlenecks for practical implement. Herein, we report a facile production of tubular hard carbon through direct carbonization of platanus flosses (FHC) for the first time. Through optimizing the pyrolysis temperatures, the FHC obtained at 1300 °C possesses a key balance between the interlayer spacing and surface area, which can maintain the substantial active sites as well as reduce the irreversible sodium storage. Accordingly, it can deliver a reversible capacity of 324.6 mAh g^-1 with a high initial Coulombic efficiency of 80%, superb rate property of 107.2 mAh g^-1 at 2 A g^-1, and long operating stability over 1000 cycles. Furthermore, the in situ Raman spectroscopic studies certify that sodium ions are stored in FHC following the "adsorption-insertion" mechanism. Our study could provide a promising route for large-scale development of the biomass-derived carbonaceous anodes for high-performance SIBs.

Interlayer Modulation of Layered Transition Metal Compounds for Energy Storage

Layered transition metal compounds are one of the most important electrode materials for high-performance electrochemical energy storage devices, such as batteries and supercapacitors. Charge storage in these materials can be achieved via intercalation of ions into the interlayer channels between the layer slabs. With the development of lithium-beyond batteries, larger carrier ions require optimized interlayer space for the unrestricted diffusion in the two-dimensional channels and effectively shielded electrostatic interaction between the slabs and interlayer ions. Therefore, interlayer modulation has become an efficient and promising approach to overcome the problems of sluggish kinetics, structural distortion, irreversible phase transition, dissolution of some transition metal elements, and air instability faced by these materials and thus enhance the overall electrochemical performance. In this review, we focus on the interlayer modulation of layered transition metal compounds for various batteries and supercapacitors. Merits of interlayer modulation on the charge storage procedures of charge transfer, ion diffusion, and structural transformation are first discussed, with emphasis on the state-of-art strategies of intercalation and doping with foreign species. Following the obtained insights, applications of modified layered electrode materials in various batteries and supercapacitors are summarized, which may guide the future development of high-performance and low-cost electrode materials for energy storage.

Graphene Oxide-Based Membranes Intercalated with an Aromatic Crosslinker for Low-Pressure Nanofiltration

Graphene oxide (GO), a carbonaceous 2D nanomaterial, has received significant interest as a next-generation membrane building block. To fabricate high-performance membranes, an effective strategy involves stacking GO nanosheets in laminated structures, thereby creating unique nanochannel galleries. One outstanding merit of laminar GO membranes is that their permselectivity is readily tunable by tailoring the size of the nanochannels. Here, a high-performance GO-based nanofiltration membrane was developed by intercalating an aromatic crosslinker, α,α/-dichloro-p-xylene (DCX), between the layers in laminated GO nanosheets. Owing to the formation of strong covalent bonds between the crosslinker and the GO, the resulting GO laminate membrane exhibited outstanding structural stability. Furthermore, due to the precisely controlled and enlarged interlayer spacing distance of the developed DCX-intercalated GO membrane, it achieved an over two-fold enhancement in water permeability (11 ± 2 LMH bar^-1) without sacrificing the rejection performance for divalent ions, contrary to the case with a pristine GO membrane.

Effect of Boron Doping on the Interlayer Spacing of Graphite

Boron-doped graphite was prepared by the heat treatment of coke using B₄C powder as a graphitization catalyst to investigate the effects of the substitutional boron atoms on the interlayer spacing of graphite. Boron atoms can be successfully incorporated into the lattice of graphite by heat treatment, resulting in a reduction in the interlayer spacing of graphite to a value close to that of ideal graphite (0.3354 nm). With an increase in the catalyst mass ratio, the content of substituted boron in the samples increased significantly, causing a decrease in the interlayer spacing of the boron-doped graphite. Density functional theory calculations suggested that the effects of the substitutional boron atoms on the interlayer spacing of the graphite may be attributed to the transfer of Π electrons between layers, the increase in the electrostatic surface potential of the carbon layer due to the electron-deficient nature of boron atoms, and Poisson contraction along the c-axis.

Engineering Interlayer Electron-Phonon Coupling in WS2/BN Heterostructures

In van der Waals (vdW) heterostructures, the interlayer electron-phonon coupling (EPC) provides one unique channel to nonlocally engineer these elementary particles. However, limited by the stringent occurrence conditions, the efficient engineering of interlayer EPC remains elusive. Here we report a multitier engineering of interlayer EPC in WS₂/boron nitride (BN) heterostructures, including isotope enrichments of BN substrates, temperature, and high-pressure tuning. The hyperfine isotope dependence of Raman intensities was unambiguously revealed. In combination with theoretical calculations, we anticipate that WS₂/BN supercells could induce Brillouin-zone-folded phonons that contribute to the interlayer coupling, leading to a complex nature of broad Raman peaks. We further demonstrate the significance of a previously unexplored parameter, the interlayer spacing. By varying the temperature and high pressure, we effectively manipulated the strengths of EPC with on/off capabilities, indicating critical thresholds of the layer-layer spacing for activating and strengthening interlayer EPC. Our findings provide new opportunities to engineer vdW heterostructures with controlled interlayer coupling.

Optimizing the Interlayer Spacing of Heteroatom-Doped Carbon Nanofibers toward Ultrahigh Potassium-Storage Performances

Precise control over the interlayer spacing for K⁺ intercalation is an effective approach to boost the potassium-storage performances in carbonaceous materials. Herein, we first found that the optimal interlayer spacing for K⁺ intercalation is around 0.38 nm for N, O codoped carbon nanofibers (NOCNs), displaying a reversible capacity of 627 mAh g^-1 at 0.1 A g^-1 after 200 cycles, excellent rate capability (123 mAh g^-1 at 20 A g^-1), and ultrastable cycling stability (262 mAh g^-1 at 5 A g^-1 after 10 000 cycles). Such good potassium-storage performances have never been reported in carbonaceous materials. The theoretical calculations and electrochemical studies reveal that the optimal interlayer spacing and N, O heteroatom-induced active sites work together to provide an intercalation-adsorption mechanism for storing K⁺ in carbonaceous materials. This work facilitates the understanding of the role of the critical interlayer spacing for K⁺ intercalation in carbonaceous materials.

l-Cysteine-Modified Graphene Oxide-Based Membrane for Chiral Selective Separation

A novel chiral separation membrane was fabricated by assembling l-cysteine (l-Cys)-modified graphene oxide sheets. l-Cys modification leads to an enantiomer separation membrane with an accessible interlayer spacing of 8 Å, which allows high solvent permeability. In the racemate separation experiments under isobaric conditions, the enantiomeric excess (ee) values of alanine (Ala), threonine (Thr), tyrosine (Tyr), and penicillamine (Pen) racemates in the permeation solution were 43.60, 44.11, 27.43, and 46.44%, respectively. In the racemate separation experiments under negative pressure, the separation performances of Ala, Thr, and Tyr were still maintained, and the enantiomeric excess (ee) values of the filtrate after separation were 56.80, 54.57, and 32.34%, respectively. These results indicate that the as-prepared GO-Cys membrane has a great practical value in the field of enantiomer separation.

Interferometric 4D-STEM for Lattice Distortion and Interlayer Spacing Measurements of Bilayer and Trilayer 2D Materials

Van der Waals materials composed of stacks of individual atomic layers have attracted considerable attention due to their exotic electronic properties that can be altered by, e.g., manipulating the twist angle of bilayer materials or the stacking sequence of trilayer materials. To fully understand and control the unique properties of these few-layer materials, a technique that can provide information about their local in-plane structural deformations, twist direction, and out-of-plane structure is needed. In principle, interference in overlap regions of Bragg disks originating from separate layers of a material encodes 3D information about the relative positions of atoms in the corresponding layers. Here, an interferometric 4D scanning transmission electron microscopy technique is described that utilizes this phenomenon to extract precise structural information from few-layer materials with nm-scale resolution. It is demonstrated how this technique enables measurement of local pm-scale in-plane lattice distortions as well as twist direction and average interlayer spacings in bilayer and trilayer graphene, and therefore provides a means to better understand the interplay between electronic properties and precise structural arrangements of few-layer 2D materials.

Graphene Oxides Derivatives Prepared by an Electrochemical Approach: Correlation between Structure and Properties

Graphene oxide (GO) can be defined as a single monolayer of graphite with oxygen-containing functionalities such as epoxides, alcohols, and carboxylic acids. It is an interesting alternative to graphene for many applications due to its exceptional properties and feasibility of functionalization. In this study, electrochemically exfoliated graphene oxides (EGOs) with different amounts of surface groups, hence level of oxidation, were prepared by an electrochemical two-stage approach using graphite as raw material. A complete characterization of the EGOs was carried out in order to correlate their surface topography, interlayer spacing, defect content, and specific surface area (SSA) with their electrical, thermal, and mechanical properties. It has been found that the SSA has a direct relationship with the d-spacing. The EGOs electrical resistance decreases with increasing SSA while rises with increasing the D/G band intensity ratio in the Raman spectra, hence the defect content. Their thermal stability under both nitrogen and dry air atmospheres depends on both their oxidation level and defect content. Their macroscopic mechanical properties, namely the Young’s modulus and tensile strength, are influenced by the defect content, while no correlation was found with their SSA or interlayer spacing. Young moduli values as high as 54 GPa have been measured, which corroborates that the developed method preserves the integrity of the graphene flakes. Understanding the structure-property relationships in these materials is useful for the design of modified GOs with controllable morphologies and properties for a wide range of applications in electrical/electronic devices.

Precisely Tunable Ion Sieving with an Al13-Ti3C2Tx Lamellar Membrane by Controlling Interlayer Spacing

Two-dimensional (2D) membranes exhibit exceptional properties in molecular separation and transport, which reveals their potential use in various applications. However, ion sieving with 2D membranes is severely restrained due to intercalation-induced swelling. Here, we describe how to efficiently stabilize the lamellar architecture using Keggin Al₁₃ polycations as pillars in a Ti₃C₂T_x membrane. More importantly, interlayer spacing can be easily adjusted with angstrom precision over a wide range (2.7-11.2 Å) to achieve selective and tunable ion sieving. A membrane with narrow d-spacing demonstrated enhanced selectivity for monovalent ions. When applied in a forward osmosis desalination process, this membrane exhibited high NaCl exclusion (99%) with a fast water flux (0.30 L m^-2 h^-1 bar^-1). A membrane with wide d-spacing showed notable selectivity, which was dependent on the cation valence. When it was applied to acid recovery from iron-based industrial wastewater, the membrane showed good H⁺/Fe²⁺ selectivity, which makes it a promising substitute for traditional polymeric membranes. Thus, we introduce a possible route to construct 2D membranes with appropriate structures to satisfy different ion-sieving requirements in diverse environment-, resource-, and energy-related applications.

Controlling the Structure of MoS2 Membranes via Covalent Functionalization with Molecular Spacers

Restacked two-dimensional (2D) materials represent a new class of membranes for water-ion separations. Understanding the interplay between the 2D membrane's structure and the constituent material's surface chemistry to its ion sieving properties is crucial for further membrane development. Here, we reveal, and tune via covalent functionalization, the structure of MoS₂-based membranes. We find features on both the ∼1 nm (interlayer spacing) and ∼100 nm (mesoporous voids between layers) length scales that evolve with the hydration level. The functional groups act as permanent molecular spacers, preventing local impermeability caused by irreversible restacking and promoting the uniform rehydration of the membrane. Molecular dynamics simulations show that the choice of functional group tunes the structure of water within the MoS₂ channel and consequently determines the hydrated interlayer spacing. We demonstrate that MoS₂ membranes functionalized with acetic acid have consistently ∼92% rejection of Na₂SO₄ with a flux of ∼1.5 lm^-2 hr^-1 bar^-1.

Efficient Raman Enhancement in Molybdenum Disulfide by Tuning the Interlayer Spacing

Two-dimensional nanomaterials, such as graphene and molybdenum disulfide (MoS₂), have recently attracted widespread attention as surface-enhanced Raman scattering (SERS) substrates. However, their SERS enhancement is of a smaller magnitude than that of noble metal nanomaterials, and therefore, the detection sensitivity still needs to be substantially improved for practical applications. Here, we present the first detailed studies on the effect of the (MoS₂) interlayer distances on the SERS intensity enhancement. We find that MoS₂ with smaller interlayer distances achieves an SERS enhancement factor as high as 5.31 × 10⁵, which is one of the highest enhancement factors to date among the two-dimensional nanomaterial SERS sensors. This remarkable SERS sensitivity is attributed to the highly efficient charge transfer from MoS₂ to probe molecules. The charge-transfer ability directly determines the variable quantity d_z² orbitals of Mo elements in the MoS₂-molecule system and then tunes the Raman intensity of probe molecules. Our work contributes to reveal the influence of MoS₂ interlayer spacing on SERS detection and to open a new way for designing a highly sensitive nonmetal SERS technology.

Correlating Interlayer Spacing and Separation Capability of Graphene Oxide Membranes in Organic Solvents

Membranes synthesized by stacking two-dimensional graphene oxide (GO) hold great promise for applications in organic solvent nanofiltration. However, the performance of a layer-stacked GO membrane in organic solvent nanofiltration can be significantly affected by its swelling and interlayer spacing, which have not been systematically characterized. In this study, the interlayer spacing of the layer-stacked GO membrane in different organic solvents was experimentally characterized by liquid-phase ellipsometry. To understand the swelling mechanism, the solubility parameters of GO were experimentally determined and used to mathematically predict the Hansen solubility distance between GO and solvents, which is found to be a good predictor for GO swelling and interlayer spacing. Solvents with a small solubility distance (e.g., dimethylformamide, N-methyl-2-pyrrolidone) tend to cause significant GO swelling, resulting in an interlayer spacing of up to 2.7 nm. Solvents with a solubility distance larger than 9.5 (e.g., ethanol, acetone, hexane, and toluene) only cause minor swelling and are thus able to maintain an interlayer spacing of around 1 nm. Correspondingly, GO membranes in solvents with a large solubility distance exhibit good separation performance, for example, rejection of more than 90% of the small organic dye molecules (e.g., rhodamine B and methylene blue) in ethanol and acetone. Additionally, solvents with a large solubility distance result in a high slip velocity in GO channels and thus high solvent flux through the GO membrane. In summary, the GO membrane performs better in solvents that are unlike GO, i.e., solvents with large solubility distance.

Tailorable Synthesis of Highly Oxidized Graphene Oxides via an Environmentally-Friendly Electrochemical Process

Graphene oxide (GO) is an attractive alternative to graphene for many applications due to its captivating optical, chemical, and electrical characteristics. In this work, GO powders with a different amount of surface groups were synthesized from graphite via an electrochemical two-stage process. Many synthesis conditions were tried to maximize the oxidation level, and comprehensive characterization of the resulting samples was carried out via elemental analysis, microscopies (TEM, SEM, AFM), X-ray diffraction, FT-IR and Raman spectroscopies as well as electrical resistance measurements. SEM and TEM images corroborate that the electrochemical process used herein preserves the integrity of the graphene flakes, enabling to obtain large, uniform and well exfoliated GO sheets. The GOs display a wide range of C/O ratios, determined by the voltage and time of each stage as well as the electrolyte concentration, and an unprecedented minimum C/O value was obtained for the optimal conditions. FT-IR evidences strong intermolecular interactions between neighbouring oxygenated groups. The intensity ratio of D/G bands in the Raman spectra is high for samples prepared using concentrated H2SO4 as an electrolyte, indicative of many defects. Furthermore, these GOs exhibit smaller interlayer spacing than that expected according to their oxygen content, which suggests predominant oxidation on the flake edges. Results point out that the electrical resistance is conditioned mostly by the interlayer distance and not simply by the C/O ratio. The tuning of the oxidation level is useful for the design of GOs with tailorable structural, electrical, optical, mechanical, and thermal properties.

A Pseudolayered MoS2 as Li-Ion Intercalation Host with Enhanced Rate Capability and Durability

As a popular strategy, interlayer expansion significantly improves the Li-ion diffusion kinetics in the MoS₂ host, while the large interlayer spacing weakens the van der Waals force between MoS₂ monolayers, thus harming its structural stability. Here, an oxygen-incorporated MoS₂ (O-MoS₂ )/graphene composite as a self-supported intercalation host of Li-ion is prepared. The composite delivers a specific capacity of 80 mAh g^-1 in only 36 s at a mass loading of 1 mg cm^-2 , and it can be cycled 3000 times (over 91% capacity retention) with a 5 mg cm^-2 loading at 2 A g^-1 . The O-MoS₂ exhibits a dominant 1T phase with an expanded layer spacing of 10.15 Å, leading to better Li-ion intercalation kinetics compared with pristine MoS₂ . Furthermore, ex situ X-ray diffraction tests indicate that O-MoS₂ sustains a stable structure in cycling compared with the gradual collapse of pristine MoS₂ , which suffers from excessive lattice breathing. Density functional theory calculations suggest that the MoO_x (OH)_y pillars in O-MoS₂ interlayers not only expand the layer spacing, but also tense the MoS₂ layers to avoid exfoliation in cycling. Therefore, the O-MoS₂ shows a pseudolayered structure, leading to remarkable durability besides the outstanding rate capability as a Li-ion intercalation host.

Controlling Interlayer Spacing of Graphene Oxide Membranes by External Pressure Regulation

Graphene oxide (GO) membranes have been attracting numerous attention due to their impressive performance in various applications, especially in water purification. However, because the swelling in water and polar organic solvents causes the increase of interlayer channels, GO membranes usually possess inferior rejection for subnanometer-sized molecules. How to control the transport channels of GO membranes at angstrom level is a significantly scientific and practical issue. Herein, a concept of external pressure regulation (EPR) is reported for restraining GO swelling and controlling its interlayer spacing precisely. Since anisotropic GO films only swell at vertical direction, the interlayer channels can be manipulated by externally unidirectional reverse force. Based on this concept, an EPR system with GO membranes is designed for water desalination by adjusting the external pressure that has high resolution. In cross-flow filtration, the compressed GO membranes show high KCl, NaCl, and CaCl₂ rejections of 94%, 97%, and 98%, respectively, accompanied by large water permeance up to 25 L m^-2 h^-1 under low feed pressure of 2 bar, despite the fact that the semi-free spatial swelling of ultrathin GO layer above the substrate pores can deteriorate salt rejection. Our work provides a straightforward physical strategy to adjust the interlayer spacing of the membranes fabricated by two-dimensional nanosheets for achieving desired filtration capacity.

Flat-Band Electronic Structure and Interlayer Spacing Influence in Rhombohedral Four-Layer Graphene

The stacking order of multilayer graphene significantly influences its electronic properties. The rhombohedral stacking sequence is predicted to introduce a flat band, which has high density of states and the enhanced Coulomb interaction between charge carriers, thus possibly resulting in superconductivity, fractional quantum Hall effect, and many other exotic phases of matter. In this work, we comprehensively study the effect of the stacking sequence and interlayer spacing on the electronic structure of four-layer graphene, which was grown on a high crystalline quality 3C-SiC(111) crystal. The number of graphene layers and coverage were determined by low energy electron microscopy. First-principles density functional theory calculations show distinctively different band structures for ABAB (Bernal), ABCA (rhombohedral), and ABCB (turbostratic) stacking sequences. By comparing with angle-resolved photoelectron spectroscopy data, we can verify the existence of a rhombohedral stacking sequence and a nearly dispersionless electronic band (flat band) near the Fermi level. Moreover, we find that the momentum width, bandgap, and curvature of the flat-band region can be tuned by the interlayer spacing, which plays an important role in superconductivity and many other exotic phases of matter.

Expanding Interlayer Spacing of Hard Carbon by Natural K+ Doping to Boost Na-Ion Storage

Heteroatom-doped carbon is an attractive material for anodes in lithium-/sodium-ion batteries as a replacement for traditional graphite anodes. However, the complex fabrication process and high cost limit practical applications of these carbon materials. Here, we report a low-cost, natural potassium-doped carbon material, which is directly carbonized from the coconut endocarp-a kind of high potassium-containing biomass material. The obtained carbon structure features an expanded d₍₀₀₂₎-spacing (0.4 nm) originating from the superhigh potassium content (6654 mg kg^-1). Because of the improvement on charge transfer kinetics and electrical properties, the potassium-doped carbon anode exhibits promising electrochemical performance in sodium-ion batteries, including high initial reversible capacity (314 mAh g^-1) and good cycle stability (289 mAh g^-1 after 200 cycles). Additionally, this work opens up a new approach for the design of heteroatom-doped carbon materials from the viewpoint of being naturally environmental friendly.

Interlayer-Spacing-Regulated VOPO4 Nanosheets with Fast Kinetics for High-Capacity and Durable Rechargeable Magnesium Batteries

Owing to the low-cost, safety, dendrite-free formation, and two-electron redox properties of magnesium (Mg), rechargeable Mg batteries are considered as promising next-generation secondary batteries with high specific capacity and energy density. However, the clumsy Mg²⁺ with high polarity inclines to sluggish Mg insertion/deinsertion, leading to inadequate reversible capacity and rate performance. Herein, 2D VOPO₄ nanosheets with expanded interlayer spacing (1.42 nm) are prepared and applied in rechargeable magnesium batteries for the first time. The interlayer expansion provides enough diffusion space for fast kinetics of MgCl⁺ ion flux with low polarization. Benefiting from the structural configuration, the Mg battery exhibits a remarkable reversible capacity of 310 mAh g^-1 at 50 mA g^-1 , excellent rate capability, and good cycling stability (192 mAh g^-1 at 100 mA g^-1 even after 500 cycles). In addition, density functional theory (DFT) computations are conducted to understand the electrode behavior with decreased MgCl⁺ migration energy barrier compared with Mg²⁺ . This approach, based on the regulation of interlayer distance to control cation insertion, represents a promising guideline for electrode material design on the development of advanced secondary multivalent-ion batteries.

Reticular V2O5·0.6H2O Xerogel as Cathode for Rechargeable Potassium Ion Batteries

Potassium ion batteries (KIBs), because of their low price, may exhibit advantages over lithium ion batteries as potential candidates for large-scale energy storage systems. However, owing to the large ionic radii of K-ions, it is challenging to find a suitable intercalation host for KIBs and thus the rechargeable KIB electrode materials are still largely unexplored. In this work, a reticular V₂O₅·0.6H₂O xerogel was synthesized via a hydrothermal process as a cathode material for rechargeable KIBs. Compared with the orthorhombic crystalline V₂O₅, the hydrated vanadium pentoxide (V₂O₅·0.6H₂O) exhibits the ability of accommodating larger alkali metal ions of K⁺ because of the enlarged layer space by hosting structural H₂O molecules in the interlayer. By intercalation of H₂O into the V₂O₅ layers, its potassium electrochemical activity is significantly improved. It exhibits an initial discharge capacity of ∼224.4 mA h g^-1 and a discharge capacity of ∼103.5 mA h g^-1 even after 500 discharge/charge cycles at a current density of 50 mA g^-1, which is much higher than that of the V₂O₅ electrode without structural water. Meanwhile, X-ray diffraction and X-ray photoelectron spectroscopy combined with energy dispersive spectroscopy techniques are carried out to investigate the potassiation/depotassiation process of the V₂O₅·0.6H₂O electrodes, which confirmed the potassium intercalation storage mechanisms of this hydrated material. The results demonstrate that the interlayer-spacing-enlarged V₂O₅·0.6H₂O is a promising cathode candidate for KIBs.

Understanding the Aqueous Stability and Filtration Capability of MoS2 Membranes

Membranes made of layer-stacked two-dimensional molybdenum disulfide (MoS₂) nanosheets have recently shown great promise for water filtration. At present, the reported water fluxes vary significantly, while the accountable structure and properties of MoS₂ nanochannels are largely unknown. This paper aims to mechanistically relate the performance of MoS₂ membranes to the size of their nanochannels in different hydration states. We discovered that fully hydrated MoS₂ membranes retained a 1.2 nm interlayer spacing (or 0.9 nm free spacing), leading to high water permeability and moderate-to-high ionic and molecular rejection. In comparison, completely dry MoS₂ membranes had a 0.62 nm interlayer spacing (or 0.3 nm free spacing) due to irreversible nanosheet restacking and were almost impermeable to water. Furthermore, we revealed that the interlayer spacing of MoS₂ membranes in aqueous solution is maintained by comparable van der Waals and hydration forces, thereby ensuring the aqueous stability of MoS₂ membranes without the need of cross-linking. In addition, we attributed the high water flux (30-250 L m^-2 h^-1 bar^-1) of MoS₂ membranes to the low hydraulic resistance of smooth, rigid MoS₂ nanochannels. We also concluded that compaction of MoS₂ membranes with a high pressure helps create a more neatly stacked nanostructure with minimum voids or looseness, leading to stable water flux and separation performance. Besides, this paper systematically compares MoS₂ membranes with the widely studied graphene oxide membranes to highlight the uniqueness and advantages of MoS₂ membranes for water-filtration applications.

Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms

Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a GO membrane in an aqueous environment was experimentally characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a d-spacing in liquid and well beyond the typical measurement limit of ∼2 nm. Molecular simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the d-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a d-spacing of 0.76 nm, and water molecules in the GO channel formed a semiordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its d-spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the d-spacing of a GO membrane in NaCl and Na₂SO₄ solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM.

Hierarchical α-Ni(OH)2 Composed of Ultrathin Nanosheets with Controlled Interlayer Distances and Their Enhanced Catalytic Performance

Hierarchical α-Ni(OH)₂ assembled of ultrathin nanosheets with the intercalation of diatomic alcohol molecules were synthesized via a facile one-step solvothermal process. The assembly structure avoided the agglomeration of ultrathin nanosheets while retaining their atomic-scale thickness and high surface area. The intercalation of the diatomic alcohol molecules into the transition-metal layers provided larger interlayer spacing and more exposed active sites, which guaranteed the high activity of the α-Ni(OH)₂. The as-obtained hierarchical α-Ni(OH)₂ exhibited excellent catalytic performance in the reduction of p-nitrophenol, with a maximum reaction rate constant (k) of 6.23 × 10^-3 s^-1 and a super high activity factor K (K = k/m) of 216.69 s^-1 g^-1. The layer spacing played the most important role in the reaction, and the catalytic efficiency increased greatly with the increase of the layer spacing of the α-Ni(OH)₂. This design concept and synthetic method can also be extended to the production of a wide variety of hierarchical catalysts for other reactions.

Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO2 Nanostructures for High-Rate Electrochemical Energy Storage

Two-dimensional (2D) layered materials with a high intercalation pseudocapacitance have long been investigated for Li⁺-ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li⁺ has been limited, although promising. The present study investigates intercalation/deintercalation behaviors of various metal ions in 2D layered MnO₂ with various interlayer distances, K-birnessite nanobelt (K-MnO₂), its protonated form (H-MnO₂), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as monovalent Li⁺, Na⁺, and K⁺ and divalent Mg²⁺, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO₂ of exfoliated nanosheets restacked with the largest interlayer spacing and a less compact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that the increased interlayer distance facilitates the fast diffusion of cations in layered MnO₂ hosts. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions.