Abstract

Multimetallic alloys (MMAs) with various compositions enrich the materials library with increasing diversity and have received much attention in catalysis applications. However, precisely shaping MMAs in mesoporous nanostructures and mapping the distributions of multiple elements remain big challenge due to the different reduction kinetics of various metal precursors and the complexity of crystal growth. Here we design a one-pot wet-chemical reduction approach to synthesize core–shell motif PtPdRhRuCu mesoporous nanospheres (PtPdRhRuCu MMNs) using a diblock copolymer as the soft template. The PtPdRhRuCu MMNs feature adjustable compositions and exposed porous structures rich in highly entropic alloy sites. The formation processes of the mesoporous structures and the reduction and growth kinetics of different metal precursors of PtPdRhRuCu MMNs are revealed. The PtPdRhRuCu MMNs exhibit robust electrocatalytic hydrogen evolution reaction (HER) activities and low overpotentials of 10, 13, and 28 mV at a current density of 10 mA cm⁻² in alkaline (1.0 M KOH), acidic (0.5 M H₂SO₄), and neutral (1.0 M phosphate buffer solution (PBS)) electrolytes, respectively. The accelerated kinetics of the HER in PtPdRhRuCu MMNs are derived from multiple compositions with synergistic interactions among various metal sites and mesoporous structures with excellent mass/electron transportation characteristics.

Composite polymer electrolytes (CPEs) with high ionic conductivity and favorable electrolyte/electrode interfacial compatibility are promising alternatives to liquid electrolytes. However, severe parasitic reactions in the Li/electrolyte interface and the air-unstable inorganic fillers have hindered their industrial applications. Herein, surface-edge opposite charged Laponite (LAP) multilayer particles with high air stability were grafted with imidazole ionic liquid (IL-TFSI) to enhance the thermal, mechanical, and electrochemical performances of polyethylene oxide (PEO)-based CPEs. The electrostatic repulsion between multilayers of LAP-IL-TFSI enables them to be easily penetrated by PEO segments, resulting in a pronounced amorphous region in the PEO matrix. Therefore, the CPE-0.2LAP-IL-TFSI exhibits a high ionic conductivity of 1.5 × 10−3 S cm−1 and a high lithium-ion transference number of 0.53. Moreover, LAP-IL-TFSI ameliorates the chemistry of the solid electrolyte interphase, significantly suppressing the growth of lithium dendrites and extending the cycling life of symmetric Li cells to over 1000 h. As a result, the LiFePO4||CPE-0.2LAP-IL-TFSI||Li cell delivers an outstanding capacity retention of 80% after 500 cycles at 2C at 60 °C. CPE-LAP-IL-TFSI also shows good compatibility with high-voltage LiNi0.8Co0.1Mn0.1O2 cathodes.

To increase chemical reaction rates, general solutions include increasing the concentration/temperature and introducing catalysts. In this study, the rate constant of an electrophilic metal coordination reaction is accelerated 23-fold on the surface of layered aluminosilicate (LAS), where the reaction substrate (ligand molecule) induces dielectric polarization owing to the polar and anionic surface. According to the Arrhenius plot, the frequency factor (A) is increased by almost three orders of magnitude on the surface. This leads to the conclusion that the collision efficiency between the ligands and metal ions is enhanced on the surface due to the dielectric polarization. This is surprising because one side of the ligand is obscured by the surface, so the collision efficiency is expected to be decreased. This unique method to accelerate the chemical reaction is expected to expand the range of utilization of LASs, which are chemically inert, abundant, and environmentally friendly. The concept is also applicable to other metal oxides which have polar surfaces, which will be useful for various chemical reactions in the future.

A bottom-up approach for constructing large-area ultrathin nanolayers with meso-architectures and simultaneously patterning them on the required substrates is still facing a significant challenge thus far. In this work, polydopamine tightly adheres to substrate surfaces, then adsorbs and assembles amphipathic micelles to form a closely arranged monolayer. This surface modification enables the oriented growth of different precursors to prepare functional monolayer coating of various mesoporous materials with regular arrays and ultrathin thickness, including polymers (polypyrrole (PPy) and polyaniline (PANi), etc.) and metal oxides (tin oxide (SnO2), etc.). Furthermore, this process allows for the direct integration of material construction and device fabrication in one step. The resultant single-layer mesoporous PPy-based gas sensor device achieves an excellent sensing response to ammonia (concentration as low as 200 ppb), with an extremely rapid response time (4 s) and recovery time (13 s). This work provides a general control on engineering mesoscale materials and effective integration of material growth, structural control, and device fabrication for potential applications.

A uniform nanoframe structure derived from a Prussian blue analogue (PBA) with an internal cavity is successfully synthesized by sonochemical etching. The uniquely structured PBA nanoframes possess a three-dimensional open structure and high surface area, resulting in enhanced electrochemical properties for the oxygen evolution reaction as a model reaction.

The large-scale application of nanozymes remains a significant challenge owing to their unsatisfactory catalytic performances. Featuring a unique electronic structure and coordination environment, single-atom nanozymes provide great opportunities to vividly mimic the specific metal catalytic center of natural enzymes and achieve superior enzyme-like activity. In this study, the spin state engineering of Fe single-atom nanozymes (FeNC) is employed to enhance their peroxidase-like activity. Pd nanoclusters (PdNC) are introduced into FeNC, whose electron-withdrawing properties rearrange the spin electron occupation in Fe(ii) of FeNC-PdNC from low spin to medium spin, facilitating the heterolysis of H2O2 and timely desorption of H2O. The spin-rearranged FeNC-PdNC exhibits greater H2O2 activation activity and rapid reaction kinetics compared to those of FeNC. As a proof of concept, FeNC-PdNC is used in the immunosorbent assay for the colorimetric detection of prostate-specific antigen and achieves an ultralow detection limit of 0.38 pg mL−1. Our spin-state engineering strategy provides a fundamental understanding of the catalytic mechanism of nanozymes and facilitates the design of advanced enzyme mimics.

The organized assembly of nanoparticles into complex macroarchitectures opens up a promising pathway to create functional materials. Here, we demonstrate a scalable strategy to fabricate macroarchitectures with high compressibility and elasticity from hollow particle-based carbon nanofibers. This strategy causes zeolitic imidazolate framework (ZIF-8)-polyacrylonitrile nanofibers to assemble into centimetre-sized aerogels (ZIF-8/NFAs) with expected shapes and tunable functions on a large scale. On further carbonization of ZIF-8/NFAs, ZIF-8 nanoparticles are transformed into a hollow structure to form the carbon nanofiber aerogels (CNFAs). The resulting CNFAs integrate the properties of zero-dimensional hollow structures, one-dimensional nanofibers, and three-dimensional carbon aerogels, and exhibit a low density of 7.32 mg cm−3, high mechanical strength (rapid recovery from 80% strain), outstanding adsorption capacity, and excellent photo-thermal conversion potential. These results provide a platform for the future development of macroarchitectured assemblies from nanometres to centimetres and facilitate the design of multifunctional materials.

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

The synthesis of highly crystalline mesoporous materials is key to realizing high-performance chemical and biological sensors and optoelectronics. However, minimizing surface oxidation and enhancing the domain size without affecting the porous nanoarchitecture are daunting challenges. Herein, we report a hybrid technique that combines bottom-up electrochemical growth with top-down plasma treatment to produce mesoporous semiconductors with large crystalline domain sizes and excellent surface passivation. By passivating unsaturated bonds without incorporating any chemical or physical layers, these films show better stability and enhancement in the optoelectronic properties of mesoporous copper telluride (CuTe) with different pore diameters. These results provide exciting opportunities for the development of long-term, stable, and high-performance mesoporous semiconductor materials for future technologies.

A gas barrier film with moderate permeance due to capillary flow and high adhesion was made from only water and layered aluminosilicates. The surface adsorbed water acts as a polar binder between the layers of aluminosilicates which enhances gas barrier efficiency, especially toward less polar gases. Oxygen permeability was 4.91 × 10-16 mol m m-2 s-1 Pa-1 (23 °C, 60% relative humidity (RH), permeance: 4.31 × 10-12 mol m-2 s-1 Pa-1), which is 1/26th that of Kraft paper (at 25 °C, 65% RH) and 146 times that of polyvinylidene chloride films. This film with moderate gas permeability is suitable for the preservation of fresh produce requiring low level respiration after harvest. The film applied to the surface of apples preserved their freshness, presumably by blocking oxygen transfer and microorganisms. Furthermore, the high adhesion of this film is more expedient for blocking gases generated from produce because the film excludes gaps between the film and the produce, which is difficult for usual cling wraps. This film can also be used for cultivating produce instead of conventional pesticides because it reduces the emission of aromatic volatiles that attract pests. This sustainable film whose component is the same as the main component of soil has the potential to reduce food loss. In addition, the film from another smectite with larger lateral size was revealed to have lower gas permeability due to diffusion and not capillary flow. This journal is

Transition metal phosphides and oxides are heralded as inexpensive alternatives to precious metal catalysts for the electrochemical hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively. Combing both transition metal phosphides and oxides into one catalyst system can generate a bifunctional electrocatalyst for overall water splitting. Still, the synthesis of such a catalyst has always been very challenging. Herein, we report the synthesis of Cu3P-Cu2O Janus bifunctional catalysts into a N, P co-doped 3D hierarchically porous carbon framework (Cu3P-Cu2O/NPC). Simple carbonization of Cu2+-containing ion-exchange resins with KOH make hundred-gram scale production of this superior catalyst possible. The Janus Cu3P-Cu2O heterostructure within a N, P-doped hierarchically porous carbon framework provides increased mass transport, enhanced electrocatalytic activity, and promoted cycling durability. As an electrocatalyst, the designed Cu3P-Cu2O/NPC delivers superior bifunctional activity for both the HER and OER in 1 M KOH, including extremely low overpotentials (138 mV for HER and 286 mV for OER) to reach a current density of 10 mA cm−2 and small Tafel slopes (62.64 mV dec-1 for HER and 79.02 mV dec-1 for OER). More impressively, the Cu3P-Cu2O/NPC-assembled electrolyzer needs drive voltages of only approximately 1.57 and 1.81 V to achieve current densities of 10 and 50 mV cm−2, respectively, demonstrating a superior electrocatalytic activity compared to the state-of-the-art electrolyzer (Pt/C || RuO2/C), and exhibits ultrahigh stability over a week of continuous overall water splitting reaction. This work highlights the significance of nanoengineering on the production of highly efficient electrocatalysts and provides a promising bifunctional electrocatalyst for future industrial implementation.

Here, we present a new family of hierarchical porous hybrid materials as an innovative tool for ultrasensitive and selective sensing of enantiomeric drugs in complex biosamples via chiral surface-enhanced Raman spectroscopy (SERS). Hierarchical porous hybrid films were prepared by the combination of mesoporous plasmonic Au films and microporous homochiral metal-organic frameworks (HMOFs). The proposed hierarchical porous substrates enable extremely low limit of detection values (10−12 M) for pseudoephedrine in undiluted blood plasma due to dual enhancement mechanisms (physical enhancement by the mesoporous Au nanostructures and chemical enhancement by HMOF), chemical recognition by HMOF, and a discriminant function for bio-samples containing large biomolecules, such as blood components. We demonstrate the effect of each component (mesoporous Au and microporous AlaZnCl (HMOF)) on the analytical performance for sensing. The growth of AlaZnCl leads to an increase in the SERS signal (by around 17 times), while the use of mesoporous Au leads to an increase in the signal (by up to 40%). In the presence of a complex biomatrix (blood serum or plasma), the hybrid hierarchical porous substrate provides control over the transport of the molecules inside the pores and prevents blood protein infiltration, provoking competition with existing plasmonic materials at the limit of detection and enantioselectivity in the presence of a multicomponent biomatrix.

Hybrid materials constructed from a visible-light-absorbing semiconductor and a functional metal complex have attracted attention as efficient photocatalysts for CO2 reduction with high selectivity to a desired product. In this work, defect fluorite-type Ln-Ta oxynitrides LnTaO(x)N(y) (Ln = Nd, Sm, Gd, Tb, Dy and Ho) were examined as the semiconductor component in a hybrid photocatalyst system combined with known Ag nanoparticle promoter and binuclear ruthenium(II) complex (RuRu'). Among the LnTaO(x)N(y) examined, TbTaOxNy gave the highest performance for CO2 reduction under visible light (lambda > 400 nm), with a RuRu'-based turnover number of 18 and high selectivity to formate (>99%). Physicochemical analyses indicated that high crystallinity and more negative conduction band potential of LnTaO(x)N(y) with the absence of Ln-4f states in the band gap structure contributed to higher activity of the hybrid photocatalyst. (C) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Metal-organic frameworks (MOFs) are a versatile source of carbon nanoarchitectures in gas sensing applications (Torad et al., 2019). Herein, several types of nanoporous carbons (NPCs) have been prepared by in-situ carbothermal treatment of zeolitic imidazolate frameworks (ZIFs) under different inert atmospheres to achieve a highly sensitive discrimination of vaporized aromatic compounds. In this study, we demonstrate how different carbonization conditions under the flow of N2 or H2 gases affect the surface area and the degree of graphitization of the resulting NPCs polyhedrons, and their consequent effect on the sensing performance in terms of sensitivity and selectivity toward toxic volatile hydrocarbons. A growth of carbon nanotubes (CNTs) is observed on the surface of polyhedral NPCs after careful carbonization of ZIF crystals under H2 atmosphere. The fabricated quartz crystal microbalance (QCM) sensor with CNT-containing NPCs demonstrates increased sensitivity and selectivity towards toxic volatile aromatic hydrocarbons over the aliphatic analogues, suggesting the rich growth of hairy graphitic-like CNTs on the surface of carbon framework act as highly selective sensing antennae for vapor molecular discrimination of toxic aromatic hydrocarbons. Despite of increased selectivity towards volatile aromatic compounds, however, the surface area of CNT-rich NPCs derived from hybrid ZIFs and ZIF-67 is greatly sacrificed as compared to CNT-free NPCs from ZIF-8 polyhedron. In the case of Co-containing ZIF-67, the rich growth of hair-like CNTs, which is induced by the presence of Co, is observed during carbothermal reduction under a flow of H2 gas, thus allowing ultra-selective detection of aromatic hydrocarbons in the vapor phase, such as benzene (C6H6) and toluene (C6H5CH3) over their aliphatic analogue, c-hexane (c-C6H12) of same molecular mass, size and vapor pressure.

The exploration and design of high-performance sodium-ion insertion host materials is of great significance to the development of hybrid capacitive deionization (HCDI). NaTi2(PO4)3, abbreviated as NTP, is famous for its high theoretical sodium-ion storage performance, but due to the poor electrical conductivity, its desalination capacity has been largely limited. Herein we report the design and synthesis of NTP/MXene (NTP/M) nanohybrid by the transformation of Ti3C2 MXene under solvothermal conditions. Due to its improved electrical conductivity and enhanced sodium-insertion ability with the introduction of MXene, the NTP/M nanohybrid shows an extraordinary desalination performance including a maximum deionization rate of 29.6 mg g−1 min−1, an ultrahigh desalination capacity of 128.6 mg g−1 and a stable cycling desalination ability, suggesting the promising application for practical HCDI.

Mesoporous oxides are outstanding metal nanoparticle catalyst supports owing to their well-defined porous structures. Such mesoporous architectures not only prevent the aggregation of metal nanoparticles but also enhance their catalytic performance. Metal/metal oxide heterojunctions exhibit unique chemical and physical properties because of the surface reconstruction around the junction and electron transfer/interaction across the interface. This article reviews the methods used for synthesizing metal-supported hybrid nanostructures and their applications as catalysts for environmental remediation and sensors for detecting hazardous materials.

Nanostructured metal films and particles have been recently exploited as substrates for surface-enhanced Raman spectroscopy (SERS) studies because their intrinsic properties lead to substantial Raman scattering enhancement. Among metals, silver (Ag) is one of the efficient elements for SERS because of its potential for high Raman scattering enhancement and low molecular detection level. Substantial electric field enhancement takes place around the center of uniform mesopores as well as on the walls between the pores, leading to enhanced light scattering as well as SERS. However, aggregation and high surface energy, which are intrinsic to Ag structures, hamper the formation of mesoporous structures. In this study, we have successfully achieved the first synthesis of uniformly sized mesoporous silver films (MAgFs) using self-assembled polymeric micelles under controlled conditions. The synthetic conditions such as temperatures, applied potentials, and deposition times were fine-tuned to obtain the optimized mesoporous structure. The MAgF demonstrated a 10-9 M level of detection and 107 to 108 enhancement factor without any surface modification. The simple synthetic process, the potential of high performance, and a large detecting area of MAgFs enable us to realize the fabrication of a suitable SERS substrate.

Two-dimensional (2D) mesoporous heterostructures combining ultrathin nanosheet morphology, periodic porous surface structures, and diverse hybrid compositions have become increasingly important for renewable energy storage and electronics. However, it remains a great challenge to develop a universal method to prepare 2D mesoporous heterostructures. Herein, we report a composite-micelle-directed interfacial assembly method to synthesize heterostructures of an ultrathin 2D material covered with mesoporous monolayers assembled on both sides. To demonstrate the concept, we first fabricated a new sandwichlike carbon@MXene@carbon mesoporous heterostructure through the self-assembly of exfoliated MXene nanosheets and block copolymer F127/melamine-formaldehyde resin composite micelles and subsequent thermal treatment. Finally, we demonstrate that the carbon@MXene@carbon mesoporous heterostructured nanosheets manifest remarkably enhanced electrochemical performance as a cathode material for lithium–sulfur batteries.

This study investigated the adsorption of a non-ionic waterinsoluble organic molecule (meso-tetraphenylporphyrin, TPP) on a trioctahedral layered aluminosilicate (saponite, SP) in organic solvents in order to investigate properties of SP as an adsorbent for a wider variety of molecules, other than cationic or polar ones. The affinity of solvents for the layered aluminosilicate surface was an important factor for predicting the molecule's adsorption proprieties. Namely, solvents with low affinity for the aluminosilicate should be selected so that molecules can approach the surface, thus prompting adsorption of the molecules. Under these conditions, TPP was adsorbed to SP due to their basicity and acidity. The acid on SP adsorbing TPP was revealed to be Lewis acid. The weaker acid strength (H0, estimated to be 0.81.5) compared with that of Brønsted acids appeared to be more suitable for adsorbents because molecules are recovered efficiently from SP where their intramolecular charge distributions are shifted. The acidity of SP is discussed quantitatively in this study because this has not been done thoroughly in comparison with dioctahedral aluminosilicates such as montmorillonite, which are conventionally used as catalysts.

SrTiO3 is a well-studied n-type metal oxide based thermoelectric (TE) material. In this work, the first-principles calculation of La-doped SrTiO3 has been performed using the density functional theory. In addition, high TE properties of bulk SrTiO3 material have been achieved by introducing nanoscale porosity and optimizing carrier concentration by La doping. The X-ray diffraction, atomic resolution scanning transmission electron microscopy imaging, and energy-dispersive X-ray spectrometry results show that La has been doped successfully into the lattice. The scanning electron microscopy images confirm that all the samples have nearly similar nanoscale porosities. The significant enhancement of electrical conductivity over the broad temperature range has been observed through optimization of La doping. Additionally, the samples possess very low thermal conductivity, which is speculated because of the nanoscale porosity of the samples. Because of this dual mechanism of doping optimization and nanoscale porosity, there is a remarkable improvement in power factor, 1 mW/m2K from 650 to 800 K, and figure of merit, zT of 0.26 at 850 K, of the sample, 22 at. % La-doped SrTiO3.

Sensitization of a wide-gap oxide semiconductor with a visible-lightabsorbing dye has been studied for decades as a means of producing H-2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H-2 and O-2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3 -based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H-2 evolution component, the dye-based turnover number and frequency for H-2 evolution reached 4580 and 1960 h(-1), respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H-2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

Over the past decades, the development of porous materials has directly or indirectly affected industrial production methods. Metal-organic frameworks (MOFs) as an emerging class of porous materials exhibit some unique advantages, including controllable composition, a large surface area, high porosity, and so on. These attractive characteristics of MOFs have led to their potential applications in energy storage and conversion devices, drug delivery, adsorption and storage, sensors, and other areas. However, powdered MOFs have limited practical applications owing to poor processability, safety hazards from dust formation, and poor recyclability. In addition, the inherent micro/mesoporosities of MOFs also reduce the accessibility and diffusion kinetics for large molecules. To improve their processability for practical applications, MOFs are often deposited as MOF layers or films (i.e., MOF-coated composites) on supporting materials or are formed into 3D structured composites, such as aerogels and hydrogels. In this article, we review recent researches on these MOF composites, including their synthetic methods and potential applications in energy storage devices, heavy metal ion adsorption, and water purification. Finally, the future outlook and challenges associated with the large-scale fabrication of MOF-based composites for practical applications are discussed.

The Pb-V oxyhalide apatite compounds Pb5(VO4)3X (X=F, Cl, Br, I) were successfully synthesized using a facile solution method and studied with respect to their structural/optical characteristics and electronic band structures. UV-visible diffuse reflectance spectroscopy, electrochemical analysis and first-principles calculations showed that the synthesized apatites behaved as n-type semiconductors, with absorption bands in the UV-visible region that could be assigned to electron transitions from the valence band to a conduction band formed by hybridized V 3d and Pb 6p orbitals. Among the apatites examined, Pb5(VO4)3I had the smallest band gap of 2.7 eV, due to an obvious contribution of I 5p orbitals to the valence band maximum. Based on its visible light absorption capability, Pb5(VO4)3I generated a continuous anodic photocurrent under visible light (λ>420 nm) in a solution of 0.1 m NaI in acetonitrile.

Spinel-type Co-Al-Cr mixed oxides (CoAl2-xCrxO4) were developed as electrochemical water oxidation catalysts. CoAl2-xCrxO4 powders were synthesized by a sol-gel method, followed by calcination at elevated temperatures in air. Electrochemical water oxidation was conducted in a phosphate-buffered aqueous solution (pH 7.5) using CoAl2-xCrxO4-loaded fluorine-doped tin oxide (FTO) conductive glass support under potentiostatic control. The electrochemical water oxidation activity was enhanced by substitution of Cr3+ ions for B-site Al3+ in CoAl2O4 in all activity metrics (i.e., geometric, specific and mass activities). Among the Cr-substituted CoAl2O4 examined, the highest activity was obtained for CoAl1.6Cr0.4O4, which was stable at +1.80V (vs. RHE) for the generation of O-2 with a Faradaic efficiency of close to unity and with minimal impact of undesirable Cr3+ oxidation. Physicochemical analyses by means of transmission electron microscopy, X-ray absorption spectroscopy, and electrochemical impedance spectroscopy indicated that the key to achieve high geometric activity was a reduction in the resistance of the electrode by the use of high surface area CoAl2-xCrxO4, which was achieved by controlling the preparation conditions of CoAl2-xCrxO4, i.e., appropriate adjustment of the Cr concentration and the calcination temperature.

© 2019 Elsevier Ltd The effect of clay surface on the equilibrium between aldehyde and diol was examined by using 4-formyl-1-methylpyridin-1-ium chloride (MPy+) as a substrate. MPy+ exists as diol (DHMPy+) and aldehyde (FMPy+) in water and acetonitrile, respectively. It was turned out that FMPy+ was clearly observed in the presence of clay nanosheet even in water. This indicates that clay nanosheet surface acts as unique chemical reaction field, affecting the potential energy surface between FMPy+ and DHMPy+.

The nanostructure control of strong coupling has been less thoroughly investigated in organic semiconductor structures than in resonator structures. We achieved strong coupling by introducing a layered silicate between a silver nanodisk resonator and a porphyrin organic semiconductor, where the porphyrin nanostructure is monomeric and parallel to the silicate surface.

The synthesis of silver nanoparticles (AgNPs) within the interlayer space of transparent layered titania nanosheet (TNS) films is investigated. A considerable number of silver ions (approximate to 70% against the cation exchange capacity of the TNS) are intercalated in the TNS films using methyl-viologen-containing TNSs as a precursor. The silver ion (Ag+)-containing TNS films are treated with aqueous sodium tetrahydroborate (NaBH4), resulting in a gradual color change to bright blue. Various structural analyses clearly show that crystalline AgNPs are generated within the interlayer space of the TNSs. The NaBH4-treated films show intense and characteristic near-infrared (NIR) extinction spectra up to 1800 nm. The tability of the AgNPs within the TNS against oxygen and moisture is also investigated, and 96% and 82% of the AgNPs remain after standing in air for 1 month and 1 year, respectively. The NIR extinctions of the AgNP-containing TNS films are further extended by employing different preparation procedures, for example, using sintered TNS films as starting materials and irradiating the Ag+-containing TNSs with ultraviolet (UV) light. The obtained AgNP-containing TNS films exhibit photochemical activities in the production of hydrogen from ammonia borane under visible-light irradiation and the decomposition of nitrogen monoxide under UV-light irradiation.

The photocatalytic activity of SrTiO3 modified with Co3O4 nanoparticles for water oxidation under visible light (λ > 480 nm) was investigated with respect to the physicochemical properties of the SrTiO3 support. SrTiO3 was synthesized by a polymerized complex method or a hydrothermal method, followed by calcination in air at different temperatures in order to obtain SrTiO3 particles having different sizes. Co3O4 nanoparticles, which provide both visible light absorption and water oxidation centers, were loaded on the as-prepared SrTiO3 by an impregnation method using Co(NO3)2 as the precursor, followed by heating at 423 K in air. Decreasing the SrTiO3 particle size (that is, improving the crystallinity) enhanced the photocatalytic activity by promoting the formation of Co3O4 nanoparticles that provided optimal light absorption and catalytic sites. However, Co3O4 aggregation occurred on overly large SrTiO3 particles, leading to a decrease in activity. This study demonstrates the possibility of tuning the photocatalytic activity of a Co3O4-loaded wide-gap semiconductor for visible light water oxidation through the appropriate selection of the support material.

H1.07Ti1.73 square O-0.27(4) (HTO) showed largely enhanced photocatalytic activity upon mixing with a small amount of Au-loaded TiO2 (Au@P25) due to electron transfer from the excited HTO to Au@P25 at the particle interfaces. Equal amounts of Au@P25 and HTO/Au@P25 showed similar activities, despite the fact that the latter contained considerably less Au.

A chemically inert, insulating layered silicate (saponite; SP) and an iron(II)-based metallo-supramolecular complex polymer (polyFe) were combined via electrostatic attraction to improve the electrochromic properties of polyFe. Structural characterization indicated that polyFe was intercalated into the SP nanosheets. Interestingly, the redox potential of polyFe was lowered by combining it with SP, and the current was measurable despite the insulating nature of SP. X-ray photoelectron spectroscopy showed that the decrease in the redox potential observed in the SP-polyFe hybrid was caused by the electrostatic neutralization of the Fe cation in polyFe by the negative charge on SP. Electrochemical analyses indicated that electron transfer occurred through electron hopping across the SP-polyFe hybrid. Control experiments using a metal complex composed of Fe and two 2,2':6',2 ''-terpyridine ligands (terpyFe) showed that SP contributes to the effective electron hopping. This modulation of the electrochemical properties by the layered silicates could be applied to other electrochemical systems, including hybrids of the redox-active ionic species and ion-exchangeable adsorbents.

A new three-layer perovskite oxide with the Ruddlesden-Popper (R-P) phase, K2CaNaNb3O10, and its protonated form were synthesised and their photocatalytic performance was compared to that of KCa2Nb3O10 or the protonated form with the Dion-Jacobson (D-J) structure in terms of H-2 and O-2 evolution. K2CaNaNb3O10 exhibited a higher activity for O-2 evolution than KCa2Nb3O10 when IO3- was used as an electron acceptor. However, protonated KCa2Nb3O10 worked more efficiently than protonated K2CaNaNb3O10 when Fe3+ was used as an electron acceptor. In both cases, it is likely that the stronger affinity of water with the interlayer contributed to higher performance. The activity of the D-J material for H-2 evolution was much lower when 1-propanol was used as an electron donor than when methanol was used. In contrast, the R-P phase exhibited a similar activity regardless of the electron donor. These results indicate that the interlayer space acts as an oxidation site, so that better access of the electron donor to the interlayer is an important factor that enhances photocatalytic activity.

Photocatalytic reduction of CO2 to CO proceeded by visible light (lambda > 400 nm) using mesoporous graphitic carbon nitride (C3N4) coupled with a Ru-II-Re-I binuclear complex (RuRe) containing a photosensitizer and catalytic units. The selectivity to CO exceeded 90% during the initial stage. Photocatalytic reactions (including isotope tracer experiments) and electrochemical measurements revealed that the reaction proceeded according to a two-step photoexcitation of C3N4 and the Ru-II photosensitizer unit, that is, it followed the Z-Scheme mechanism. Modification of C3N4 with highly dispersed silica was found to improve the ability of C3N4 to accommodate RuRe, which enhanced the photocatalytic activity for CO production.

Layered metal oxides and their nanosheets are interesting materials as photocatalysts for water splitting. In particular, semiconductor nanosheets are attractive building blocks for synthesizing a photocatalytic material because of their high surface area and the wide variety of compositions available. The anisotropic feature of nanosheets, which have a thickness of ~ 1 nm and lateral dimensions ranging from several hundred nanometers to a few micrometer, is advantageous for heterogeneous photocatalysts, as the diffusion length of photogenerated electrons and holes to the surface is shortened, resulting in higher activity. In this chapter, recent progress of layered metal oxide and their nanosheets for application in photocatalytic water splitting made by our group is described.

Cobalt-based compounds, such as cobalt(II) hydroxide, are known to be good catalysts for water oxidation. Herein, we report that such cobalt species can also activate wide-band-gap semiconductors towards visible-light water oxidation. Rutile TiO2 powder, a well-known wide-band-gap semiconductor, was capable of harvesting visible light with wavelengths of up to 850 nm, and thus catalyzed water oxidation to produce molecular oxygen, when decorated with cobalt(II) hydroxide nanoclusters. To the best of our knowledge, this system constitutes the first example that a particulate photocatalytic material that is capable of water oxidation upon excitation by visible light can also operate at such long wavelengths, even when it is based on earth-abundant elements only.

In a mixed solvent of water and dimethylformamide (DMF), porphyrin molecules have two types of orientation, tilted and parallel, toward a surface of silicate nanosheet. In the solvent, tilted species have lower energy. The T-n <- T-1 absorption of porphyrin molecules adsorbed at the surface of the nanosheet in the mixed solvent was observed at five different temperatures. The decay curve was analyzed with an equation for transient absorption difference, describing the behavior of parallel and tilted adsorbed species in the ground state and excited state to determine the rate constants for the orientation change and the radiationless deactivation. The rate constants of the orientation change increased with the temperature. The activation energy and energy gap between parallel and tilted species were estimated by analyzing the temperature dependence of the rate constants. The energy gap obtained in this kinetic study was consistent with our thermodynamically obtained value previously reported.

Metal oxide nanosheets having a three-layer perovskite structure were studied as photocatalysts for water oxidation in the presence of IO3- as a reversible electron acceptor. This work examined the effects of the lateral dimensions and composition of the nanosheets as well as metal oxide co-catalysts deposited on the restacked nanosheets. Depositing metal oxides capable of promoting reduction reactions on the nanosheets were found to promote the water oxidation activity. In contrast, the lateral dimensions and the degree of crystallinity of the nanosheets had little effect on the activity. Experimental results demonstrated that the reduction of IO3- is the rate-limiting step in this reaction and that nanosheets with less distorted structures are advantageous with regard to increasing both light absorption and the mobility of photoexcited charge carriers.

Restacked nanosheets of Dion-Jacobson perovskite Ca2Nb2TaO10- were studied with respect to the structural features as photocatalysts for H-2 evolution from an aqueous methanol solution. The materials were prepared by the reaction of layered HCa2Nb2TaO10 with tetra-n-butylammonium hydroxide (TBA(+) OH-) at room temperature, followed by restacking with a proper restacking agent. According to structural characterization by means of X-ray diffraction, transmission electron microscopy, Raman spectroscopy, and UV-visible diffuse reflectance spectroscopy, the structural features and the degree of interlayer hydration of the restacked material depended on the restacking agent employed. The highest photocatalytic activity was obtained for the restacked nanosheets using HCl as the restacking agent. Results of structural characterizations and photocatalytic reactions suggested that the high activity resulted from not interlayer hydration but protonation, which is favorable for the oxidation reaction.

An yttrium-tantalum oxynitride having a band gap of 2.1 eV (absorbing visible light at < 580 nm) was applicable as a semiconductor component of a Z-scheme CO2 reduction system operable under visible light, in combination with a binuclear Ru(II) complex that has strong absorption in the visible region (< 600 nm). Excitation of this system with visible light under a CO2 atmosphere induced photocatalytic formation of formic acid with very high selectivity (> 99%).

A metal sphere-plane structure consisting of gold nanoparticles, p-methylbenzenethiol and a gold substrate was measured by vibrational sum frequency generation spectroscopy with four excitation wavelengths, 630, 680, 720, and 780 nm. The enhancement factors of Raman signals were estimated to be 250 and 104 for the 532 and 647 nm excitation. Contrastingly, we found that the enhancements of VSFG signals were much smaller, a factor of 5 at maximum. We speculate that the small enhancement factor of VSFG signals is due to the coherent nature of the VSFG process or the extinction of the infrared laser by the gold nanoparticles. (C) 2015 Elsevier B.V. All rights reserved.

Hybrid materials consisting of a ruthenium(II) polypyridyl complex and a Dion-Jacobson type perovskite oxide nanosheet were studied as photocatalysts for dye-sensitized H-2 evolution under visible light with respect to the energy structure of the hybrids. Three Ru(II) complexes, Ru-II{(4,4'-X-2-bpy)2(4,4'-(CH2PO3H2)(2)-bpy)} (X = H, CH3, CF3; bpy = 2,2'-bipyridine), were used as redox photosensitizers. HCa2-xSrxNb3O10 (0 = x = 2) and HCa2Nb3-yTayO10 (0 = y = 1.5) nanosheet aggregates, having a tunable conduction band potential (ECB), were employed as the building block. Nanosheets that possess more negative ECB value were found to be preferable for the dye-sensitized H-2 evolution, unless electron injection from the excited-state sensitizer to the conduction band of a nanosheet is hindered. Among the combinations tested, the highest activity was obtained when an HCa2Nb3O10 nanosheet was sensitized by RuII{(4,4'-(CH3)(2)-bpy)(2)(4,4'-(CH2PO3H2)(2)-bpy)}, exhibiting a maximum apparent quantum yield of ca. 10% at 460 nm and a turnover number of ca. 3800 (for 20 h). This study highlighted that it is possible to maximize the performance of dye-sensitized H-2 evolution on a sensitizer/semiconductor hybrid by refining the ECB value of a semiconductor and the oxidation potential of the excited state of a photosensitizer.

Perovskite nanosheets of HCa2-xSrxNb3O10 and HCa2Nb3-yTayO10 with controlled band-edge potentials were prepared. They worked as highly efficient heterogeneous photocatalysts for H-2 evolution from a water/methanol mixture under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa2Nb2TaO10 nanosheets, recording an apparent quantum yield of approximately 80% at 300 nm, which is the highest value for a nanosheet-based photocatalyst reported to date.

Photochemical energy transfer was examined on the flat clay surface. By the change of surrounding atmosphere, the molecular adsorption orientation angle of dyes can be modulated. It is turned out that the energy transfer efficiency between dyes can be controlled by the relative orientation change between dyes. The change of orientation factor and spectral overlap factor was the main factor to affect the energy transfer efficiency. This technique would be useful to construct the photo-functional materials such as chromic and light harvesting system. (C) 2014 Elsevier Ltd. All rights reserved.

The surface stability of metal nanoparticles was enhanced by adsorption of layered silicate (so-called "clay") on metal nanocubes whose surface was protected by a surfactant. The strong adsorption of the layered silicate to the metal nanocubes via the surfactant eliminated the need for using excess surfactant for metal nanoparticle dispersion. Furthermore, the surface of the negatively charged hybrids was demonstrated to be modifiable with cationic molecules.

The adsorption orientational behavior of dicationic porphyrins on clay surfaces in various solvents was examined. Addition of aprotic solvents to the aqueous solution containing the clay-porphyrin complex induced a large spectral shift to shorter wavelength. The blue shift turned out to be due to the orientation angle change of porphyrin, which leads to the relaxation of molecular flattening. From dichroic measurements on a waveguide, the orientation of the porphyrin was directly observed and found to be parallel to the clay surface in water and to be at a tilt angle of 68 with respect to the clay surface in DMF. A thermodynamic study of the porphyrin orientation on the clay surface was undertaken. This analysis showed that the parallel orientation of the porphyrin was stabilized mainly by the entropy term, and the tilted orientation was stabilized mainly by the enthalpy term. Furthermore, the effects of different organic solvents on porphyrin orientation on the clay surface were examined, and strong correlation between hydrogen bonding parameters and orientation change was found. © 2013 American Chemical Society.

We have been investigating complexes composed of nanolayered materials with anionic charges such as clay nanosheets and dye molecules such as cationic porphyrins. It was found that the structure of dye assembly on the layered materials can be effectively controlled by the use of electrostatic host-guest interaction. The intermolecular distance, the molecular orientation angle, the segregation/integration behavior, and the immobilization strength of the dyes can be controlled in the clay-dye complexes. The mechanism to control these structural factors has been discussed and was established as a size-matching effect. Unique photochemical reactions such as energy transfer through the use of this methodology have been examined. Almost 100% efficiency of the energy-transfer reaction was achieved in the clay-porphyrin complexes as a typical example for an artificial light-harvesting system. Control of the molecular orientation angle is found to be useful in regulating the energy-transfer efficiency and in preparing photofunctional materials exhibiting solvatochromic behavior. Through our study, clay minerals turned out to serve as protein-like media to control the molecular position, modify the properties of the molecule, and provide a unique environment for chemical reactions.

We report a facile seed-mediated method for the synthesis of monodisperse polyhedral gold nanoparticles, with systematic shape evolution from octahedral to trisoctahedral structures. The control over the particle growth process was achieved simply by changing the concentration of the reductant in the growth solution, in the presence of small spherical seed nanoparticles. By progressively increasing the concentration of the reductant used in the growth solution (ascorbic acid), while keeping the amount and type of added surfactant constant, the morphology of the gold nanoparticles was varied from octahedral to truncated octahedral, cuboctahedral, truncated cubic, cubic, and finally trisoctahedral structures. These nanoparticles were monodisperse in size, possessed similar volumes, and were naturally oriented so that their larger crystal planes were face down on quartz substrates when deposited from the solution. By adjusting the volume of gold seed nanoparticle solution added to a growth solution, the size of the simplest gold nanoparticles (with a highly symmetric cubic morphology) could be tuned from SO +/- 2.1 to 112 +/- 11 nm. When other seed nanoparticles were used, the size of the cubic Au nanoparticles reached 169 +/- 7.0 nm. The nanoparticle growth mechanism and the plasmonic properties of the resulting polyhedral nanoparticles are discussed in this paper.

An enhancing factor: The enhancement of the electric properties of a dye molecule (IR26) by indium-tin oxide nanoparticles (ITO NPs, see picture) has been shown by measuring the near-infrared two-photon-excited transient absorption spectra. The dye molecule was excited much more efficiently in the presence of an ITO NP layer. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Plasmonic nanoantenna arrays hold great promise for diffraction-unlimited light localization, confinement, and transport. Here, we report on linear plasmonic nanoantenna arrays composed of colloidal gold nanocubes precisely assembled using a nanomanipulation technique. In particular, we show the direct evidence of dark propagating modes In the plasmon coupling regime, allowing for transport of guided plasmon waves without far-field radiation losses. Additionally, we demonstrate the possibility of plasmon dispersion engineering in coupled gold nanocube chains. By assembling a nanocube chain with two sections of coupled nanocubes of different intercube separations, we are able to produce the effect of a band-pass nanofilter.

The magnetic field-driven orientation of microcrystals of six diamagnetic layered transition metal oxides (HLaNb2O7, HCa2Nb3O10 center dot 0.5H(2)O, KNaCa2Nb4O13, KTiTaO5, KTiNbO5, and H2.2K1.8Nb6O17 center dot nH(2)O) suspended in epoxy resins was studied by X-ray diffraction using permanent magnets producing a 0.8 T field. Although the degree of orientation, quantified as the Hermans order parameter, was strongly affected by the particle size distribution, in all cases microcrystals with similar to 1-2 mu m lateral dimensions were found to orient with the magnetic field vector in the layer plane. Control of the orientation of ionically conducting layered oxides is of interest for practical applications in batteries and fuel cells. The consistent direction of orientation of the lamellar oxides studied can be rationalized in the framework of a quantitative bond anisotropy model developed by Uyeda (Phys. Chem. Miner. 1993, 20, 77-80). The asymmetry of metal-oxygen bonding at the faces of the octahedral layers results in long and short M-O bonds perpendicular to the plane of the sheets. This distortion of the M-O octahedra, which is a structural feature of almost all layered materials that contain octahedral bonding frameworks, gives rise to the diamagnetic anisotropy and results in an easy axis or plane of magnetization in the plane of the sheets.

We present several methods for tuning the localized surface plasmon resonance (LSPR) wavelength of conductive nanoparticles (NPs) over the ultraviolet to near-IR (UV-NIR) region. The methods presented in this article include changing the size of the icosahedral Pd NPs (LSPR in the UV region), using in-phase and antiphase surface plasmon coupling with octahedral Au NPs (LSPR in the visible region), and altering the carrier density in indium tin oxide NPs (LSPR in the NIR region) without changing the particle shape.

Lamellar inorganic solids have interesting physical properties related to anisotropic transport of electrons and ions, host-guest chemistry, magnetism, and catalysis. Because these solids are held together in the stacking direction by relatively weak bonds, they are subject to intercalation and exfoliation reactions, as well as low-temperature topochemical reactions that convert one structure type into another. We have recently found that intercalation of cationic polymers into anionic layered hosts can invert the charge on the sheets, making the galleries hosts for anionic molecular guests and nanoparticles. Topochemical condensation reactions can be used to convert layered solids into three-dimensionally bonded materials with unusual crystal textures, magnetic, and electronic transport properties. The anisotropic bonding in diamagnetic layered compounds also leads to anisotropy in their magnetic susceptibility, such that they tend to align in strong magnetic fields with one of their in-plane axes parallel to the field direction. We are now studying this effect as a means of preparing oriented particle membranes of proton-conducting platelets, exfoliated sheets, and nanoscrolls for intermediate temperature fuel cells.

Photochemical energy transfer of non-aggregated cationic porphyrins on an anionic-type clay (Smecton SA) surface was investigated. The efficiency of energy transfer and excited-state quenching in the absence of energy transfer were evaluated at various loading levels of porphyrin on the clay surface and were found to be significantly affected by the loading level. As the latter increased, both energy transfer efficiency and excited-state quenching increased. Judging from the dependency of energy-transfer efficiency on the porphyrin loading level, a partially clustered structure, but without aggregation, of porphyrins on the clay surface is proposed.