Jingbin Han

Ordered poly(p-phenylene)/layered double hydroxide ultrathin films with blue luminescence by layer-by-layer assembly.

Lavender layers: A poly(p-phenylene) anionic derivate and exfoliated Mg-Al layered double hydroxide monolayers were assembled into ultrathin films with well-defined blue fluorescence (see picture; the numbers indicate the number of bilayers), long-range order, and high photostability. These films work as multiple quantum-well structures for valence electrons.

Three-dimensional WO3 nanostructures on carbon paper: photoelectrochemical property and visible light driven photocatalysis.

Three-dimensional (3D) WO(3) nanostructures were grown on carbon paper by a catalyst-free high temperature reactive vapor deposition process, which exhibit a good photoelectrochemical property and visible light driven photocatalytic performance.

Flexible CoAl LDH@PEDOT Core/Shell Nanoplatelet Array for High‐Performance Energy Storage

A CoAl-layered double hydroxide (LDH)@poly(3,4-ethylenedioxythiophene) (PEDOT) core/shell nanoplatelet array (NPA) is grown on a flexible Ni foil substrate as a high-performance pseudocapacitor. The LDH@PEDOT core/shell NPA shows a maximum specific capacitance of 649 F/g (based on the total mass) by cyclic voltammetry (scan rate: 2 mV/s) and 672 F/g by galvanostatic discharge (current density: 1 A/g). Furthermore, the hybrid NPA electrode also exhibits excellent rate capability with a specific energy of 39.4 Wh/kg at a current density of 40 A/g, as well as good long-term cycling stability (92.5% of its original capacitance is retained after 5000 cycles). These performances are superior to those of conventional supercapacitors and LDH NPA without the PEDOT coating. The largely enhanced pseudocapacitor behavior of the LDH@PEDOT NPA electrode is related to the synergistic effect of its individual components: the LDH nanoplatelet core provides abundant energy-storage capacity, while the highly conductive PEDOT shell and porous architecture facilitate the electron/mass transport in the redox reaction.

Erasable Nanoporous Antireflection Coatings Based on the Reconstruction Effect of Layered Double Hydroxides

Antireflection coatings play a pivotal role in photovoltaic and display devices and all kinds of optical lenses, because they can effectively reduce the intensity of reflected light at interfaces, which translates into increased transmission, improved contrast, reduced glare, and the elimination of ghost images. The principle of antireflection (AR) coatings is based on the destructive interference of reflected light from air–film and film–substrate interfaces. An ideal homogeneous single-layer AR coating satisfies the following conditions: 1) The thickness of the coating is l/4, where l is the wavelength of the incident light; and 2) nc = (nans) , where nc, na, and ns are the refractive indices of the coating, air, and the substrate, respectively. To satisfy these prerequisites for an effective AR coating on glass or plastics (na = 1 and ns = 1.5), nc should be 1.22. Although condition (1) can be easily met, condition (2) imposes a problem, as natural materials with such a low refractive index are either rare or expensive to obtain in thin-film form. One effective solution to this problem is to use appropriately designed porous materials, because the introduction of the nanopores can reduce the refractive index of the coatings and achieve the AR requirement. In recent years, many methods have been developed to obtain nanoporous film materials for use as AR coatings, including sol–gel processes, phase-separation, a sacrificial porogen approach, layer-by-layer (LBL) deposition of nanoparticle multilayers, plasma-enhanced chemical vapor deposition, and deposition of nanorods or nanowires. In particular, these nanoporous films with low refractive indices (nc) and thickness in the range 80–220 nm (about one-quarter of the range of visible wavelengths) can be applied as efficient antireflection coatings to improve the light transmission of transparent substrates in the visible wavelength range. However, only a few reports have focused on intelligent AR coatings. For example, Rubner and co-workers reported an intelligent multilayer polymer film with a nanoporous structure that could be erased and reconstructed by cycling solution pH, which endowed the film with reversible AR properties. However, polymer-based materials generally exhibit low optical and thermal stability and also environmental incompatibility, which restricts their long-term application. Layered double hydroxides (LDH) are layered anionic clays that are generally expressed by the formula [M1 xM 3+ x(OH)2](A n )x/n·mH2O, where M 2+ and M are diand trivalent metal cations, and A is a counteranion. The host structure consists of brucite-like layers of edgesharing M(OH)6 octahedra, and the partial substitution of M for M results in positively-charged host layers, which are balanced by the interlayer anions. One of the most exploited properties of LDH materials is the so called “reconstruction effect”: calcination of LDHs at moderate temperatures leads to the formation of mixed metal oxides (MMO) with a porous structure, and rehydration of MMO results in spontaneous structural reconstruction of the LDH. The fabrication of porous MMO materials using a sacrificial template method has been reported by Pr vot et al. Recently, Lu et al. reported the observation of colloidal LDH nanoparticle suspensions in aqueous solution with a positive zeta potential in the range 30–50 mV. This zeta potential indicates that the external surface of LDH nanoparticles is positively charged, even though most of the positive charges of the LDH host layers are neutralized by interlayer species. Therefore, colloidal LDH nanopaticles can be regarded as ideal building blocks without any surface modification for the assembly of functional multilayer films with negatively charged electrolytes by the electrostatic LBL technique. Herein, we present an erasable nanoporous AR coating based on the reconstruction effect of LDH materials. The precursor films were fabricated by assembly of LDH nanoparticles with the polyanion poly(sodium styrene-4-sulfonate), denoted as PSS, by an electrostatic LBL method, yielding the LDH/PSS multilayer films. Then nanoporous films with AR properties were obtained by calcination of LDH/PSS films by utilizing the so-called sacrificial porogen (pore generator) approach: The selective removal of the porogen (PSS and the interlayer anions of LDH) results in the formation of MMO films with a nanoporous structure (Scheme 1). This versatile process is particularly amenable to the creation of large-area uniform AR coatings on non-flat surfaces with precise control over thickness and optical properties. More significantly, after rehydration of the MMO films, the transformation to the non-porous LDH structure can be achieved by the structural “reconstruction effect” of LDH materials. By cycling the calcination–rehydration process, the AR properties of the coating can be [*] J. Han, Y. Dou, Prof. M. Wei, Prof. D. G. Evans, Prof. X. Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology Box 98, Beijing 100029 (P.R. China) Fax: (+ 86)10-6442-5385 E-mail: weimin@mail.buct.edu.cn

Layer-by-Layer Ultrathin Films of Azobenzene-Containing Polymer/Layered Double Hydroxides with Reversible Photoresponsive Behavior

We report the preparation of a reversible photoresponsive ultrathin film containing a photoactive azobenzene polymer poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO) and exfoliated layered double hydroxide (LDH) nanosheets using a layer-by-layer (LBL) self-assembly technique. Alternate irradiation with UV and visible light (lambda > 450 nm) results in a reversible switching between the trans isomer and the cis-rich photostationary state of the azobenzene group with concomitant significant changes in film color. Fluorescence polarization measurements indicated that the orientation of the azobenzene chromophores in the LDH matrix undergoes a reversible realignment during the photoisomerization processes. Photoisomerization induced by alternate UV and visible light irradiation was accompanied by reversible morphological changes of the LBL film, observable by atomic force microscopy (AFM). Molecular dynamics (MD) studies demonstrated that the LDH monolayers allow sufficient free space for the PAZO to undergo isomerization of its azobenzene chromophores. The reversible photoalignment of PAZO was also followed by MD simulations, and the results are in reasonable agreement with the experimental findings, further validating the configurational and orientational changes proposed for PAZO during the reversible photoprocess. It has been demonstrated that the host (LDH nanosheet)-guest (PAZO) interactions are key factors in determining the reversible photoresponsive performances of the film, since the comparative pristine PAZO film shows no such properties. Therefore, the incorporation of a photoactive moiety within the inorganic nanosheets of LDH by the LBL technique provides an attractive and feasible methodology for creating novel light-sensitive materials and devices with potential read-write capabilities.

Layer-by-layer assembly of layered double hydroxide/cobalt phthalocyanine ultrathin film and its application for sensors

This paper reports the preparation of cobalt phthalocyanine/layered double hydroxide ultrathin films (UTFs) through an electrostatic layer-by-layer (LBL) technique as well as its application in electrocatalysis for dopamine oxidation. UV-vis absorption and electrochemical impedance spectra indicate the uniform deposition of the LBL films. XRD measurements demonstrate the long-range ordered structure of organic/inorganic UTFs, with an average repeating distance of 1.89 nm. SEM images show that the film surface displays a continuous and uniform morphology, with the root-mean-square (rms) roughness of ∼6.4 nm revealed by AFM. The UTF modified ITO electrode exhibits significant electrocatalytic performance for the oxidation of dopamine which is related to the Co(II)/Co(III) couple in the (LDH/CoPcTs)n UTF. The dopamine biosensor shows rather high sensitivity, low detection limit and excellent anti-interference properties in the presence of ascorbic acid. Furthermore, compared with pristine organic multilayer (PDDA/CoPcTs)n modified electrodes, the (LDH/CoPcTs)nelectrodes show superior repeatability and long-term stability, due to the immobilization and dispersion of electroactive CoPcTs molecules by LDH nanosheets. Therefore, this work demonstrates a successful paradigm for the fabrication of electroactive species in an inorganic 2D matrix, which can be potentially used for practical applications in bioanalysis and biodetection.

Self-assembly of layered double hydroxide nanosheets/Au nanoparticles ultrathin films for enzyme-free electrocatalysis of glucose†

This paper reports the fabrication of layered double hydroxide nanosheets (LDH nanosheets)/Au nanoparticles (AuNPs) ultrathin films (UTFs) via the layer-by-layer (LBL) assembly technique, and their electrocatalytic performance for the oxidation of glucose was demonstrated. UV-vis absorption spectra show the uniform growth of the UTFs and the enhancement of interlayer plasmon coupling of AuNPs upon increasing deposition cycle. The XRD results indicate that the (LDH/AuNPs)n UTFs possess long-range order stacking in the normal direction of the substrate, with AuNPs accommodated between the LDH nanosheets as a monolayer arrangement. SEM, TEM and AFM images reveal a high dispersion of AuNPs on the surface of the LDH nanosheets without aggregation. The electrochemical behavior of the UTF modified fluorine-doped tin oxide (FTO) electrode was studied by cyclic voltammetry and electrochemical impedance spectroscopy. The (LDH/AuNPs)n UTF shows improved electron transfer kinetics, owing to the formation of electron tunneling junctions resulting from the interlayer plasmon coupling. This leads to new channels for facilitating electron transfer within the UTFs. In addition, the (LDH/AuNPs)8electrode displays significant electrocatalytic performance for glucose with a linear response range (50 μM–20 mM), low detection limit (10.8 μM), high sensitivity (343 μA mM−1 cm−2), good stability and reproducibility. Therefore, this work provides a feasible method to immobilize metal nanoparticles using the LDH nanosheet as a 2D matrix, which is promising for the development of enzyme-free sensors.

Biomimetic design and assembly of organic–inorganic composite films with simultaneously enhanced strength and toughness

Inorganic nanoplatelet reinforced polymer films were fabricated via alternate layer-by-layer assembly of layered double hydroxide (LDH) nanoplatelets with poly(vinyl alcohol) (PVA), which showed largely enhanced strength and good ductility simultaneously.

Magnetic-Field-Assisted Assembly of Layered Double Hydroxide/Metal Porphyrin Ultrathin Films and Their Application for Glucose Sensors

The ordered ultrathin films (UTFs) based on CoFe-LDH (layered double hydroxide) nanoplatelets and manganese porphyrin (Mn-TPPS) have been fabricated on ITO substrates via a magnetic-field-assisted (MFA) layer-by-layer (LBL) method and were demonstrated as an electrochemical sensor for glucose. The XRD pattern for the film indicates a long-range stacking order in the normal direction of the substrate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the MFA LDH/Mn-TPPS UTFs reveal a continuous and uniform surface morphology. Cyclic voltammetry, impedance spectroscopy, and chronoamperometry were used to evaluate the electrochemical performance of the film, and the results show that the MFA-0.5 (0.5 T magnetic field) CoFe-LDH/Mn-TPPS-modified electrode displays the strongest redox current peaks and fastest electron transfer process compared with those of MFA-0 (without magnetic-field) and MFA-0.15 (0.15 T magnetic field). Furthermore, the MFA-0.5 CoFe-LDH/Mn-TPPS exhibits remarkable electrocatalytic activity toward the oxidation of glucose with a linear response range (0.1-15 mM; R(2) = 0.999), low detection limit (0.79 μM) and high sensitivity (66.3 μA mM(-1) cm(-2)). In addition, the glucose sensor prepared by the MFA LBL method also shows good selectivity and reproducibility as well as resistance to poisoning in a chloride ion solution. Therefore, the novel strategy in this work creates new opportunities for the fabrication of nonenzyme sensors with prospective applications in practical detection.

Temperature-controlled electrochemical switch based on layered double hydroxide/poly(N-isopropylacrylamide) ultrathin films fabricated via layer-by-layer assembly.

In this paper we report the fabrication of layered double hydroxide (LDH) nanoparticles/poly(N-isopropylacrylamide) (pNIPAM) ultrathin films (UTFs) via the layer-by-layer assembly technique, and their switchable electrocatalytic performance in response to temperature stimuli was demonstrated. X-ray diffraction and UV-vis absorption spectroscopy indicate a periodic layered structure with uniform and regular growth of the (LDH/pNIPAM)(n) UTFs; an interaction based on hydrogen bonding between LDH nanoparticles and pNIPAM was confirmed by X-ray-photoelectron spectroscopy and Fourier transform infrared spectroscopy. Temperature-triggered cyclic voltammetry and electrochemical impedance spectroscopy switch for the UTFs was obtained between 20 and 40 °C, accompanied by reversible changes in surface topography and film thickness revealed by atomic force microscopy and ellipsometry, respectively. The electrochemical on-off property of the temperature-controlled (LDH/pNIPAM)(n) UTFs originates from the contraction-expansion configuration of pNIPAM with low-high electrochemical impedance. In addition, a switchable electrocatalytic behavior of the (LDH/pNIPAM)(n) UTFs toward the oxidation of glucose was observed, resulting from the temperature-controlled charge transfer rate. Therefore, this work provides a facile approach for the design and fabrication of a well-ordered command interface with a temperature-sensitive property, which can be potentially applied in electrochemical sensors and switching.