Covalent Organic Framework Nanoarchitectonics: Recent Advances for Precious Metal Recovery.

The recovery of precious metals (PMs) from secondary resources has garnered significant attention due to environmental and economic considerations. Covalent organic frameworks (COFs) have emerged as promising adsorbents for this purpose, owing to their tunable pore size, facile functionalization, exceptional chemical stability, and large specific surface area. This review provides an overview of the latest research progress in utilizing COFs to recover PMs. Firstly, the design and synthesis strategies of chemically stable COF-based materials, including pristine COFs, functionalized COFs, and COF-based composites, are delineated. Furthermore, the application of COFs in the recovery of gold, silver, and platinum group elements is delved into, emphasizing their high adsorption capacity and selectivity as well as recycling ability. Additionally, various interaction mechanisms between COFs and PM ions are analyzed. Finally, the current challenges faced by COFs in the field of PM recovery are discussed, and potential directions for future development are proposed, including enhancing the recyclability and reusability of COF materials and realizing the high recovery of PMs from actual acidic wastewater. With the targeted development of COF-based materials, the recovery of PMs can be realized more economically and efficiently in the future.

Effect of salinity on arsenic uptake, biotransformation, and time-dependent speciation pattern by Sargassum species.

The arsenic (As) content of seaweed has been extensively studied due to its toxicological concerns. As a primary producer, seaweed plays a vital role in the biochemical cycling of As in marine environments. Several studies have focused on the growth and behavior of seaweed under a salinity gradient; however, information related to the impact of salinity on As uptake, biotransformation mechanism, and time-dependent speciation patterns of these plants is limited. This study aimed to investigate the temporal effects of salinity on these factors in seaweed. Three seaweed species, Sargassum fusiforme, Sargassum thunbergii, and Sargassum horneri, were maintained in a 1% Provasoli-enriched seawater medium for 14 d under 5‰, 15‰, 25‰, and 34‰ salinities. The results revealed that the high salinity media promoted a rapid uptake of As by all three species. Arsenic accumulation inside the cell approached 100% within seven days of culture for S. thunbergii, irrespective of the salinity content of the media. In addition, As(V) biotransformation and release by S. fusiforme and S. thunbergii were time-dependent, while S. horneri released dimethylarsinic acid (DMAA) from day 3 of the culture. All seaweed species showed methylation of As(V) to DMAA during the culture period. Furthermore, S. thunbergii released DMAA when As(V) was completely depleted from the culture media, whereas the release by S. fusiforme and S. horneri was relatively earlier than that of S. thunbergii. S. horneri showed minimal tolerance to low salinity, as the cells revealed significant damage. Based on the results of this study, a conceptual model was developed that demonstrated the effects of salinity on As uptake and the biotransformation mechanism of seaweed.

Water Activation in Solar-Powered Vapor Generation.

Solar-powered vapor evaporation (SVG), based on the liquid-gas phase conversion concept using solar energy, has been given close attention as a promising technology to address the global water shortage. At molecular level, water molecules escaping from liquid water should overcome the attraction of the molecules on the liquid surface layer to evaporate. For this reason, it is better to reduce the energy required for evaporation by breaking a smaller number of hydrogen bonds or forming weak hydrogen bonds to ensure efficient and convenient vapor production. Many novel evaporator materials and effective water activation strategies have been proposed to stimulate rapid steam production and surpass the theoretical thermal limit. However, an in-depth understanding of the phase/enthalpy change process of water evaporation is unclear. In this review, a summary of theoretical analyses of vaporization enthalpy, general calculations, and characterization methods is provided. Various water activation mechanisms are also outlined to reduce evaporation enthalpy in evaporators. Moreover, unsolved issues associated with water activation are critically discussed to provide a direction for future research. Meanwhile, significant pioneering developments made in SVG are highlighted, hoping to provide a relatively entire chain for more scholars who are just stepping into this field.

Bridging the Gap between Zeolites and Dense Silica Polymorphs: Formation of All-Silica Zeolite with High Framework Density from Natural Layered Silicate Magadiite.

A silica zeolite (RWZ-1) with a very high framework density (FD) was synthesized from highly crystalline natural layered silicate magadiite, bridging the gap between the two research areas of zeolites and dense silica polymorphs. Magadiite was topotactically converted into a 3D framework through two-step heat treatment. The resulting structure had a 1D micropore system of channel-like cavities with an FD of 22.1 Si atoms/1000 Å3 . This value is higher than those of all other silica zeolites reported so far, approaching those of silica polymorphs (tridymite (22.6) and α-quartz (26.5)). RWZ-1 is a slight negative thermal expansion material with thermal properties approaching those of dense silica polymorphs. It contributes to the creation of a new field on microporous high-density silica/silicates. Synergistic interactions are expected between the micropores with molecular sieving properties and the dense layer-like building units with different topologies which provide thermal and mechanical stabilities.

Materials Space-Tectonics: Atomic-level Compositional and Spatial Control Methodologies for Synthesis of Future Materials.

Reactions occurring at surfaces and interfaces necessitate the creation of well-designed surface and interfacial structures. To achieve a combination of bulk material (i.e., framework) and void spaces, a meticulous process of "nano-architecting" of the available space is necessary. Conventional porous materials such as mesoporous silica, zeolites, and metal-organic frameworks lack advanced cooperative functionalities owing to their largely monotonous pore geometries and limited conductivities. To overcome these limitations and develop functional structures with surface-specific functions, the novel materials space-tectonics methodology has been proposed for future materials synthesis. This review summarizes recent examples of materials synthesis based on designing building blocks (i.e., tectons) and their hybridization, along with practical guidelines for implementing materials syntheses and state-of-the-art examples of practical applications. Lastly, the potential integration of materials space-tectonics with emerging technologies, such as materials informatics, is discussed.

Modulating Electronic Metal-Support Interactions to Boost Visible-Light-Driven Hydrolysis of Ammonia Borane: Nickel-Platinum Nanoparticles Supported on Phosphorus-Doped Titania.

Ammonia borane (AB) is a promising material for chemical H2 storage owing to its high H2 density (up to 19.6 wt %). However, the development of an efficient catalyst for driving H2 evolution through AB hydrolysis remains challenging. Therefore, a visible-light-driven strategy for generating H2 through AB hydrolysis was implemented in this study using Ni-Pt nanoparticles supported on phosphorus-doped TiO2 (Ni-Pt/P-TiO2 ) as photocatalysts. Through surface engineering, P-TiO2 was prepared by phytic-acid-assisted phosphorization and then employed as an ideal support for immobilizing Ni-Pt nanoparticles via a facile co-reduction strategy. Under visible-light irradiation at 283 K, Ni40 Pt60 /P-TiO2 exhibited improved recyclability and a high turnover frequency of 967.8 mol H 2 ${{_{{\rm H}{_{2}}}}}$ molPt -1 min-1 . Characterization experiments and density functional theory calculations indicated that the enhanced performance of Ni40 Pt60 /P-TiO2 originated from a combination of the Ni-Pt alloying effect, the Mott-Schottky junction at the metal-semiconductor interface, and strong metal-support interactions. These findings not only underscore the benefits of utilizing multipronged effects to construct highly active AB-hydrolyzing catalysts, but also pave a path toward designing high-performance catalysts by surface engineering to modulate the electronic metal-support interactions for other visible-light-induced reactions.

Nanoarchitectonics of Hollow Covalent Organic Frameworks: Synthesis and Applications.

Hollow covalent organic frameworks (COFs) have gained significant attention because of their specific properties, including enhanced surface-to-volume ratio, large surface area, hierarchical structure, highly ordered nanostructures, and excellent chemical stability. These intrinsic characteristics endow hollow COFs with fascinating physicochemical properties and make them highly attractive for widespread applications, such as catalysis, energy storage, drug delivery, therapy, sensing, and environmental remediation. This review focuses on the recent developments in the synthesis of hollow COFs and their derivatives. In addition, their practical applications in various fields are summarized. Finally, challenges and future opportunities in terms of their synthetic methodologies and practical applications are discussed. Hollow COFs are expected to play an important role in the future of materials science.

Synthesis of a Hybrid Composed of Anisotropic Niobate Layers Modified with MoC Nanoparticles.

The hybrid composed of anisotropic niobate layers modified with MoC nanoparticles is synthesized by multistep reactions. The stepwise interlayer reactions for layered hexaniobate induce selective surface modification at the alternate interlayers, and the following ultrasonication leads to the formation of double-layered nanosheets. The further liquid phase MoC deposition with the double-layered nanosheets leads to the decoration of MoC nanoparticles on the surfaces of the double-layered nanosheets. The new hybrid can be regarded as a stacking of the two layers with anisotropically modified nanoparticles. The relatively high temperature in the MoC synthesis causes partial leaching of the grafted phosphonate groups. The exposed surface of the niobate nanosheets due to the partial leaching may interact with MoC to succeed in the hybridization. The hybrid after heating exhibits photocatalytic activity, indicating that this hybridization method can be useful for hybrid synthesis of semiconductor nanosheets and co-catalyst nanoparticles toward photocatalytic application.

Phase Transfer of Inorganic Nanosheets in a Water/2-Butanone Biphasic System and Lateral Size Fractionation via Stepwise Extractions.

Lateral size fractionation of niobate nanosheets derived from K4Nb6O17·3H2O was achieved via phase transfer from the aqueous phase to the 2-butanone phase in a water/2-butanone biphasic system, in which tetra-n-dodecylammonium (TDDA+) bromide was used as a phase transfer reagent. Phase transfer of the nanosheets was observed when the TDDA+/[Nb6O17]4- molar ratios were 0.6 and 1.0, and the phase transfer ratios were 41 and 97%, respectively. FT-IR and thermogravimetry results showed that the extracted nanosheets contained TDDA+ ions. These results indicate that adsorption of TDDA+ likely induced an increase in the hydrophobicity of the nanosheet surface, leading to phase transfer. In the AFM image of the original nanosheets in the aqueous phase, their lateral sizes were in the range from several hundreds of nm to several tens of µm, while those of the nanosheets after phase transfer at a molar ratio of 0.6 were in the range from several hundreds of nm up to 2 µm, indicating that nanosheets with smaller lateral sizes were preferentially extracted into the 2-butanone phase. In addition, the phase transfer ratio of the fragmentated nanosheets with a much smaller lateral size distribution compared with the original nanosheets was 79% when the TDDA+/[Nb6O17]4- molar ratio was 0.6, indicating that phase transfer for the nanosheets with smaller lateral sizes proceeded efficiently. Following this extraction cycle, the nanosheets with a TDDA+/[Nb6O17]4- molar ratio of 0.6 remaining in the aqueous phase after extraction were extracted stepwise again through dilution of the aqueous phase with water and the addition of a fresh 2-butanone solution of tetra-n-dodecylammonium bromide to form a new biphasic system. The lateral sizes of the nanosheets increased as the extraction cycles were repeated. Completion of the three extraction cycles allowed formation of the three classes of the nanosheets with different lateral size ranges of 0.68 ± 0.5, 2.8 ± 1.9, and 6.6 ± 3.1 µm.

Cerium(III) Niobate Layered Perovskites: Abnormal Optical Absorption Modulations by Tuning of B-Site Composition and Perovskite Layer Charge Control.

The cerium-introduced layered perovskite of RbCeTa2O7 has exhibited a specific optical absorption due to metal-to-metal charge transfer transitions between Ce 4f and transition metal d-orbitals to show the unique pale-green coloration, which is different from conventional coloration mechanisms. To further extend the coloring state based on the same mechanism, in this work, a series of the [Ce(Ta,Nb)2O7]- layered perovskites, Rb1-xCsx[Ce(Ta1-xNbx)2O7] (x = 0∼1), with Nb substitutions in the perovskite units have been prepared and investigated in terms of those crystal structures and optical absorption mechanism. The Rietveld analysis using the XRD profile and EXAFS analyses well refined those structures as the Dion-Jacobson-type layered perovskite. The color of solid solutions gradually changed from pale-green to dark reddish-brown with increasing amount of substituted niobium. The unique coloring state change behavior of solid solutions from pale-green to dark reddish-brown depending on the amount of the substituted niobium is not observed in the other layered perovskite analogues (e.g., La and Pr analogues). The first-principles calculation based on the density functional theory method indicated that the band structural change should be a key factor for the coloration modulation. Furthermore, the redox ability through the charge modulation of the perovskite layer, which is a specific function of the cerium-based layered perovskite, was also investigated for the niobate [CeNb2O7]- perovskite layer, resulting in the anisotropic lattice changes similar to those of a Ta analogue with different structural changes in the stacking and in-plane directions. The accompanying change in electronic structure led to a clear modulation in optical absorption, yielding a drastic change in the coloring state from dark brown to yellow.

Utility of NaMoO3 F as a Precursor for Homogeneous Distribution of Cobalt Dopants in Molybdenum Oxynitrides.

Molybdenum nitrides and their related compounds have been focused as a catalyst for several reactions. Although the doping into molybdenum nitrides lead to the higher catalytic activity, the simultaneous control of the morphology, the crystallinity, and the dopant state in doped MoN cannot be easily achieved due to the limitation of the synthesis method. In this study, one of the mixed anion compounds, NaMoO3 F was used as a precursor for molybdenum oxynitrides with hexagonal MoN phase. This route led to the homogeneous distribution of cobalt in the molybdenum oxynitride compared with that obtained by the other method. The cobalt-doped molybdenum oxynitride from NaMoO3 F exhibited high oxygen reduction reaction catalytic activity due to the high distribution of cobalt in the crystal. This paper proposes that the mixed anion compounds can be a unique precursor for the other materials to expand the controllability of materials toward improvement of their activity.

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]- layers.

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite "charge-neutral perovskite layers" by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative reaction based on anionic [CeIIITa2O7]- layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [CeIIITa2O7]- layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7]- layers. Furthermore, the anionic [CeIIITa2O7]- layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the "charge-neutral" perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.

Improvement of the Oxygen Storage/Release Speed of YBaCo4O7+δ Synthesized by a Glycine-Complex Decomposition Method.

The present study explores the oxygen storage capacity of YBaCo4O7+δ prepared by a glycine-complex decomposition method. We reported for the first time that the YBaCo4O7+δ sample was successfully synthesized at such a low temperature of 800 °C by this method. The YBCO-800 N sample exhibited a faster oxygen absorption/desorption speed than that of high calcination temperature samples, and the time required for complete oxygen storage/release was 5 and 6 min at 360 °C, respectively. Moreover, the superior performance observed for this product in the temperature swing adsorption process makes it a promising candidate in oxygen production technologies. This research demonstrated that the glycine-complex decomposition method is an effective method for improving the oxygen storage property of YBaCo4O7+δ and provides a new insight into designing other novel oxygen storage materials.

Remarkable Effects of Lanthanide Substitution for the Y-Site on the Oxygen Storage/Release Performance of YMnO3+δ.

Lanthanide-substituted YMnO3+δ nanoparticles with the hexagonal phase, denoted as R0.25Y0.75MnO3+δ (R = Er, Dy, Tb, Gd, and Sm), have been successfully synthesized by the polymerized complex method. The substitutions did not largely affect the morphologies and specific surface area of the obtained R0.25Y0.75MnO3+δ nanoparticles. From the evaluation for the oxygen storage/release properties, the oxygen storage capacity (OSC) increased significantly by the Tb substitution, and the oxygen absorption/release rate strongly depended on the ion size of the substituted lanthanides. It was found that Tb4+ existed in Tb0.25Y0.75MnO3+δ after oxygen absorption, demonstrating that the remarkable increase in the OSC of the Tb-substituted sample was due to the oxidation of not only Mn3+ to Mn4+ but also Tb3+ to Tb4+. In addition, the unit cell volume increasing with the R ion size, which can lead to the promotion of the oxygen diffusion in the crystal structure, was the factor leading to the increase of the oxygen absorption rate. Especially, Sm0.25Y0.75MnO3+δ showed an excellent OSC of 3 + δ = 3.34 (the weight increase rate was 2.64 wt %) even under a rapid temperature swing rate of 20 °C/min.

Synthesis of NaMoO3F and Na5W3O9F5 with Morphological Controllability in Non-Aqueous Solvents.

NaMoO3F and Na5W3O9F5 were synthesized by solvothermal reaction of MoO3 and WO3, respectively, with NaF in nonaqueous solvents. These reactions were realized at low temperatures (150-200 °C) without the use of HF. This synthesis method is much more facile and safe procedure compared with general synthesis methods for oxyfluorides which includes hydrothermal reaction under a presence of HF or solid-state reaction at high temperatures in vacuum sealed tube or under high pressure. In the case of the reaction of MoO3 with NaF, the kind of solvent largely affected the obtained morphologies of NaMoO3F. The morphology in the case of acetonitrile as a solvent was rodlike with a micrometer-scale size, while that in the case of ethanol was polyhedral with a size of several hundred nanometers. In addition, the solvothermal reaction of WO3 with NaF led to the formation of Na5W3O9F5. Also, the difference of solvents for the solvothermal reaction affected the obtained particle sizes. The effect of the solvents on the morphologies of the obtained oxyfluorides probably resulted from the difference of the solubility of NaF and the subsequent dissolution ratio of MoO3 or WO3 in the used solvents. Our synthesis method can expand the applicability of oxyfluorides by providing a new phase and/or unique morphology.

Synthesis of zinc germanium oxynitride nanotube as a visible-light driven photocatalyst for NOx decomposition through ordered morphological transformation from Zn2GeO4 nanorod obtained by hydrothermal reaction.

Oxynitrides with narrow band gap are promising materials as visible-light sensitive photocatalysts, because introduction of nitrogen ions can negatively shift the position of valence band maximum of the corresponding oxides to negative side. (Zn1+xGe)(N2Ox) with wurtzite structure is one of the oxynitride materials. (Zn1+xGe)(N2Ox) with nanotube morphology was synthesized by nitridation of Zn2GeO4 nanorods at 800 °C for 6 h. During the nitridation process, the nanorod with smooth surface was transformed into nanotube with rough surface in spite of no template for formation of tube structure. The nanotube formation can be caused by ordered morphological transformation from Zn2GeO4 nanorod during the nitridation. (Zn1+xGe)(N2Ox) nanotube exhibited a large specific surface area due to its nanotube morphology and the ability to be responsive to visible light because of the narrow band gap of 2.76 eV. Compared to (Zn1+xGe)(N2Ox) synthesized by conventional solid state reaction, the optimized (Zn1+xGe)(N2Ox) nanotube possessed enhanced photocatalytic NOx decomposition activity under both ultraviolet and visible light irradiation.

Improvement of the O2 storage/release rate of YMnO3 nanoparticles synthesized by the polymerized complex method.

YMnO3 nanoparticles with a diameter of ca. 100 nm, which were synthesized by the polymerized complex method, exhibited a high O2 storage/release rate, because nanoparticle morphology increased the O2 accessible surface area of the material. This material with a high O2 storage/release rate is promising as a separator of O2 from air.

High yield post-thermal treatment of bulk graphitic carbon nitride with tunable band structure for enhanced deNOx photocatalysis.

The bulk graphitic carbon nitride (CN) suffers from low specific surface area, which limits its practical application for air purification. Here, we reported a facile post-thermal treatment to break bulk CN into nanosheets whose specific surface areas increased from 13.6 m2 g-1 to 68.0 m2 g-1. The yield of CN nanosheets reached up to 67%, and its photocatalytic decomposition of NOx activity was about 3.0 times higher than that of bulk CN. Moreover, the CN nanosheets obtained at 550 °C with higher specific surface area (113.9 m2 g-1) displayed lower photocatalytic activity than that obtained at 500 °C with lower specific surface area (68.0 m2 g-1), which was attributed to its lower valence band. This study illustrates that many factors including specific surface area and band structure could affect the performance of photocatalysts so that it is necessary to take account of various factors. Moreover, the facile and high yield thermal treatment provides the foundation for further large-scale industrial applications.

Preparation of (Zn1+xGe)(N2Ox) nanoparticles with enhanced NOx decomposition activity under visible light irradiation by nitridation of Zn2GeO4 nanoparticles designed precisely.

Quaternary zinc germanium oxynitride (Zn1+xGe)(N2Ox), a solid solution between ZnGeN2 and ZnO with a wurtzite structure, is one of the attractive photocatalysts under visible-light irradiation. In this study, the synthesis of (Zn1+xGe)(N2Ox) nanoparticles was achieved by the nitridation of Zn2GeO4 nanoparticles designed precisely to enhance their photocatalytic NOx decomposition activity under both UV and visible light irradiation. The obtained (Zn1+xGe)(N2Ox) nanoparticles exhibited a high specific surface area and visible light absorption induced by the narrow band gap of ca. 2.6-2.8 eV, both of which are reasons for the enhancement of photocatalytic activity. The oxide precursors with a nanoparticle morphology were prepared by a facile solvothermal method with various volumes of TEA (triethanolamine) as an additive. The relationships of nitridation time and TEA volume in the solvothermal reaction for the synthesis of the precursor with morphology, specific surface area, and photocatalytic NOx decomposition activity of the nitrided samples were investigated. The increase of active sites by the high surface area and the enhanced visible-light absorption ability as well as the defect amounts and states can be largely related to the excellent NOx decomposition activity of (Zn1+xGe)(N2Ox).

Formation of silicate nanoscrolls through solvothermal treatment of layered octosilicate intercalated with organoammonium ions.

We report silicate nanoscrolls composed of only SiO4 tetrahedra with crystalline walls for the first time in this study. The procedure consists of the intercalation of layered octosilicate with dioctadecyldimethylammonium bromide ((C18)2DMABr) and the subsequent solvothermal treatment of the intercalated material in heptane. The walls of the obtained nanoscrolls are crystalline, which originates from layer crystallinity in the pristine silicate. The direction of rolling up is fixed at the a- or b-axis of the silicate based on the electron diffraction patterns of the nanoscrolls. Desorption of (C18)2DMABr, which is present in addition to (C18)2DMA cations, from the interlayer during solvothermal treatment is likely related to the nanoscrolling process. Although the yield of nanoscrolls is low, these findings will lead to the re-estimation of many layered silicates intercalated with long-chain alkylammonium compounds as precursors for silicate nanoscrolls with crystalline walls.