Simin Xu

CoMn-layered double hydroxide nanowalls supported on carbon fibers for high-performance flexible energy storage devices

CoMn-layered double hydroxide (LDH) nanowalls were supported on flexible carbon fibers (CFs) via an in situ growth approach; the resulting CoMn-LDH/CF electrode delivers a high specific capacitance (1079 F g−1 at 2.1 A g−1 normalized to the weight of the active LDH material) with excellent rate capability even at high current densities (82.5% capacitance retention at 42.0 A g−1). A combined experimental and theoretical study reveals that the dramatic performance enhancement is mainly attributed to the homogeneous and ordered dispersion of metal units within the LDH framework, which enriches the redox reactions associated with charge storage by both Co and Mn. The hierarchical configuration further improves the exposure of active sites and enables a fast charge transfer to the electrode/electrolyte interface, with CFs serving as both the current collector and binderless electrode. In addition, a solid-state supercapacitor device with good flexibility was fabricated using the CoMn-LDH/CFs, which achieves a specific energy up to 126.1 W h kg−1 and a specific power of 65.6 kW kg−1. By virtue of rational design of the chemical composition and architecture, this work demonstrates a facile strategy for the fabrication of a hierarchical configuration based on CoMn-LDH nanowalls anchored to CFs, which can be potentially used in wearable and miniaturized devices for energy storage.

TiO2/graphene/NiFe-layered double hydroxide nanorod array photoanodes for efficient photoelectrochemical water splitting

The ever-increasing demand for renewable and clean power sources has triggered the development of novel materials for photoelectrochemical (PEC) water splitting, but how to improve the solar conversion efficiency remains a big challenge. In this work, we report a conceptual strategy in a ternary material system to simultaneously enhance the charge separation and water oxidation efficiency of photoanodes by introducing reduced graphite oxide (rGO) and NiFe-layered double hydroxide (LDH) on TiO2 nanorod arrays (NAs). An experimental–computational combination study reveals that rGO with a high work function and superior electron mobility accepts photogenerated electrons from TiO2 and enables fast electron transportation; while NiFe-LDH acts as a cocatalyst which accelerates the surface water oxidation reaction. This synergistic effect in this ternary TiO2/rGO/NiFe-LDH photoanode gives rise to a largely enhanced photoconversion efficiency (0.58% at 0.13 V) and photocurrent density (1.74 mA cm−2 at 0.6 V). It is worth mentioning that the photocurrent density of TiO2/rGO/NiFe-LDH, to the best of our knowledge, is superior to previously reported TiO2-based photoanodes in benign and neutral media. In addition, the method presented here can be extended to the preparation of other efficient photoanodes (WO3/rGO/NiFe-LDH and α-Fe2O3/rGO/NiFe-LDH) toward high level PEC performance.

Preparation and evaluation of well-defined hemocompatible layered double hydroxide-poly(sulfobetaine) nanohybrids

The ability to manipulate and control the surface properties of layered double hydroxide (LDH) nanoparticles is of crucial importance in the designing of LDH-based carriers of therapeutic agents. In this work, surface-initiated atom transfer radical polymerization (ATRP) of zwitterionic 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) is first employed to tailor the functionality of LDH surfaces in a well-controlled manner and produce a series of well-defined hemocompatible hybrids (termed as LDHPS). The blood compatibilities of the modified LDH nanoparticles were investigated using coagulation tests, complement activation, platelet activation, hemolysis assay, morphological changes of red blood cells, and cytotoxicity assay. The results confirmed that the P(DMAPS) grafting can substantially enhance the hemocompatibility of the LDH particles, and the LDHPS hybrids can be used as biomaterials without causing any hemolysis. With the versatility of surface-initiated ATRP and the excellent hemocompatibility of zwitterionic polymer chains, the LDH nanoparticles with desirable blood properties can be readily tailored to cater to various biomedical applications.

Fabrication of host–guest UV-blocking materials by intercalation of fluorescent anions into layered double hydroxides

Materials for blocking UV light play important roles in a variety of areas such as protecting the human skin and increasing the lifetime of polymers. In this work, a new type of host–guest UV-blocking material has been synthesized by the introduction of a fluorescent anion, 2-[2-[4-[2-(4-carboxyphenyl)vinyl]phenyl]vinyl]benzoate (CPBA), into the interlayer galleries of a ZnAl–NO3 layered double hydroxide (LDH) precursor by an anion-exchange method. The structure and the thermal and photostability of the intercalated ZnAl–CPBA-LDH were investigated by powder X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermogravimetry and differential thermal analysis (TG-DTA), fluorescence spectroscopy and UV-vis spectroscopy. The supramolecular layered host–guest structure of ZnAl–CPBA-LDH enables both physical shielding and absorption of UV light. Furthermore, in contrast to conventional UV blocking materials—which convert UV light into thermal energy—the CPBA anions in the LDH interlayer galleries convert UV light (in the range 250–380 nm) into lower energy fluorescence emission (λemmax = 430 nm), thus reducing the thermal aging of the polymer composite materials. Intercalation of the CPBA anions into the LDH host also markedly enhances the thermal stability of CPBA. In polypropylene (PP) aging performance tests, after adding 1–5 wt% ZnAl–CPBA-LDH to PP, the resistance to UV degradation of the resulting ZnAl–CPBA-LDH/PP composites is higher than that of pristine PP or a CPBA/PP composite. Therefore, this work provides a way to construct a new type of host–guest layered material for UV-blocking applications.

Understanding the thermal motion of the luminescent dyes in the dye–surfactant cointercalated ZnAl-layered double hydroxides: a molecular dynamics study

Previous work has demonstrated that cointercalation of luminescent dyes and surfactants into layered double hydroxides (LDHs) is an efficient approach to inhibit the aggregation of dye and therefore enhance its photoluminescence behavior. In this work, molecular dynamics simulations are performed on different ZnAl-LDHs cointercalated with dye (fluorescein or 1-anilinonaphthalene-8-sulfonate) and alkylsulfonate with different alkyl chain length (CnH2n+1SO3, n = 5, 6, 7, 10 and 12, respectively), together with dye–alkylsulfonate solutions for comparison. The structure, binding energy and the thermal motion characterized by the diffusion coefficient of each dye are analyzed. In the dye–alkylsulfonate/LDHs, the diffusion coefficient and the binding energy of the dye show a minimum when the dye is cointercalated with heptanesulfonate (HPS, n = 7), whose size is the closest to that of the dye. While in the case of dye–alkylsulfonate solutions, the diffusion coefficient and the binding energy vary monotonously with the increasing alkylsulfonate size. Furthermore, it is found that the increase of Al3+ content in LDH matrix in dye–HPS/LDHs is favorable for the restriction of the dye motion. These results indicate that the dye–alkylsulfonate/LDH system is more effective in restraining both the thermal motion and the aggregation of the dye than that of dye–alkylsulfonate solutions due to the confined microenvironment provided by the LDH matrix. Therefore, it is possible to inhibit the aggregation of the dye in dye/LDHs by two aspects: choosing a surfactant with a size close to that of the dye as the cointercalant and increasing the content of trivalent cations in the LDH matrix.

Remarkable oxygen barrier films based on a layered double hydroxide/chitosan hierarchical structure

A high performance gas barrier film was fabricated via alternate spin-coating of chitosan (CTS) and hierarchical layered double hydroxide (H-LDH). The H-LDH synthesized by a calcination–rehydration method shows a hierarchical structure with nanowalls aligned vertically on the LDH platelets, which were subsequently assembled in the CTS matrix, generating a hybrid film with excellent gas barrier properties. Compared with the (P-LDH/CTS)10 barrier film based on plate-like LDH (P-LDH), the (H-LDH/CTS)10 film exhibits significantly enhanced oxygen barrier properties with an oxygen transmission rate (OTR) below the detection limit of commercial instruments (<0.005 cm3 m−2 day−1 atm−1). The greatly improved performance of the (H-LDH/CTS)10 film is attributed to the tortuous diffusion path in hierarchical architecture space. Moreover, experimental results and theoretical calculations reveal the existence of the adhesive force between oxygen and H-LDH (adsorption energy = −2.46 eV), which further reduces the oxygen diffusion rate and thus promotes oxygen barrier properties. Therefore, this work provides a facile and cost-effective strategy to fabricate high gas barrier materials, which can serve as a promising candidate for food/pharmaceutical packaging and encapsulation of electronic devices.

Terbium doped ZnCr-layered double hydroxides with largely enhanced visible light photocatalytic performance

Recently, layered double hydroxides (LDHs) have emerged as highly active photocatalysts due to their unique structure, large specific surface area and semiconductor properties. However, the slow interfacial kinetics and fast charge recombination are the major obstacles which limit the performance of LDH-based photocatalysts. Here, we demonstrate the doping of rare earth ions into the host layer of LDHs to inhibit the charge recombination and increase the charge injection efficiency simultaneously. A series of terbium ion (Tb3+) doped ZnCr–LDHs (Tb-ZnCr–LDHs) have been successfully synthesized via a co-precipitation method, and their photocatalytic water splitting activities were evaluated under visible light irradiation. The sample with a Tb3+ doping content of 0.5% (molar ratio) shows optimal performance for oxygen evolution (1022 μmol h−1 g−1) among all these Tb-ZnCr–LDH materials. The photoluminescence and photoelectrochemistry measurements over the Tb-ZnCr–LDH samples prove effective separation of photo-induced charge carriers and high charge injection efficiency, compared with a pristine ZnCr–LDH. This strategy can be applied to modify other photocatalysts toward low-cost solar fuel generation systems.

Confined Synthesis of Carbon Nitride in a Layered Host Matrix with Unprecedented Solid-State Quantum Yield and Stability

Fluorescent carbon nanomaterials have drawn tremendous attention for their intriguing optical performances, but their employment in solid-state luminescent devices is rather limited as a result of aggregation-induced photoluminescence quenching. Herein, ultrathin carbon nitride (CN) is synthesized within the 2D confined region of layered double hydroxide (LDH) via triggering the interlayer condensation reaction of citric acid and urea. The resulting CN/LDH phosphor emits strong cyan light under UV-light irradiation with an absolute solid-state quantum yield (SSQY) of 95.9 ± 2.2%, which is, to the best of our knowledge, the highest value of carbon-based fluorescent materials ever reported. Furthermore, it exhibits a strong luminescence stability toward temperature, environmental pH, and photocorrosion. Both experimental studies and theoretical calculations reveal that the host-guest interactions between the rigid LDH matrix and interlayer carbon nitride give the predominant contribution to the unprecedented SSQY and stability. In addition, prospective applications of the CN/LDH material are demonstrated in both white light-emitting diodes and upconversion fluorescence imaging of cancer cells.

A chiroptical switch based on DNA/layered double hydroxide ultrathin films.

A highly oriented film was fabricated by layer-by-layer self-assembly of DNA and MgAl-layered double hydroxide nanosheets, and its application in chiroptical switch was demonstrated via intercalation and deintercalation of an achiral molecule into the DNA cavity. DNA molecules are prone to forming an ordered and dispersive state in the interlayer region of rigid layered double hydroxide (LDH) nanosheets as confirmed by scanning electron microscopy and atomic force microscopy. The induced chiroptical ultrathin film (UTF) is achieved via the intercalation of an achiral chromophore [5,10,15,20-tetrakis(4-N-methylpyridyl)porphine tetra(p-toluenesulfonate) (TMPyP)] into the spiral cavity of DNA stabilized in the LDH matrix [denoted as TMPyP-(DNA/LDH)20]. Fluorescence and circular dichroism spectroscopy are utilized to testify the intercalation of TMPyP into (DNA/LDH)20 UTF that involves two steps: the electrostatic binding of TMPyP onto the surface of (DNA/LDH)20 followed by intercalation into base pairs of DNA. In addition, the TMPyP-(DNA/LDH)20 UTF exhibits good reversibility and repeatability in induced optical chirality, based on the intercalation and deintercalation of TMPyP by alternate exposure to HCl and NH3/H2O vapor, which can be potentially used as a chiroptical switch in data storage.

Multi-dimensional, light-controlled switch of fluorescence resonance energy transfer based on orderly assembly of 0D dye@micro-micelles and 2D ultrathin-layered nanosheets

Fluorescence resonance energy transfer (FRET) systems have broad applications in visual detection, intelligent materials, and biological imaging, all of which favor the transmission of light through multiple dimensions and in diverse directions. Herein, we have demonstrated multi-dimensional (0D and 2D) FRET within a multi-layer ultrathin film (UTF) by employing a layer-by-layer (LBL) assembly technique. The anionic block copolymer micelle poly(tert-butyl acrylate-co-ethyl acrylate-co-methacrylic acid) (PTBEM) is chosen as a molecular carrier for the incorporation of bis(8-hydroxyquinolate) zinc (Znq2) and open-ring merocyanine (MC) (denoted as (Znq2/MC)@PTBEM). Alternatively, electrostatic assembly is performed with cationic layered double hydroxide (LDH) nanosheets (denoted as [(Znq2/MC)@PTBEM/LDH]n). This [(Znq2/MC)@PTBEM/LDH]n system offers a multi-dimensional propagation medium and ensures that the FRET donor and acceptor are located within their Förster radii in each direction. The system demonstrates a FRET process that can be switched via alternating ultraviolet/visible (UV/vis) irradiation, with tunable blue–green/red fluorescence, resulting in a FRET efficiency as high as 81.7%. It is expected that this assembly method, which uses 0D micelles on a 2D layered material, can be extended to other systems for further development of multi-dimensional FRET.