Stabilizing Ni2+ in Hollow Nano MOF/Polymetallic Phosphides Composites for Enhanced Electrochemical Performance in 3D-Printed Micro-Supercapacitors

Polymetallic phosphides exhibit favorable conductivities. A reasonable design of nano-metal-organic frame (MOF) composite morphologies and in situ introduction of polymetallic phosphides into the framework can effectively improve electrolyte penetration and rapid electron transfer. To address existing challenges, Ni, with a strong coordination ability with N, is introduced to partially replace Co in nano-Co-MOF composite. The hollow nanostructure is stabilized through CoNi bimetallic coordination and low-temperature controllable polymetallic phosphide generation rate. The Ni, Co, and P atoms, generated during reduction, effectively enhance electron transfer rate within the framework. X-ray absorption fine structure (XAFS) characterization results further confirm the existence of Ni-N, Ni-Ni, and Co-Co structures in the nanocomposite. The changes in each component during the charge-discharge process of the electrochemical reactions are investigated using in situ X-ray diffraction (XRD). Theoretical calculations further confirm that P can effectively improve conductivity. VZNPGC//MXene MSCs, constructed with active materials derived from the hollow nano MOF composites synthesized through the Ni2+ stabilization strategy, demonstrate a specific capacitance of 1184 mF cm-2, along with an energy density of 236.75 µWh cm-2 (power density of 0.14 mW cm-2). This approach introduces a new direction for the synthesis of highly conductive nano-MOF composites.

Fabrication of 3D Printed Hollow Microneedles by Digital Light Processing for the Buccal Delivery of Actives

In the present study, two different microneedle devices were produced using digital light processing (DLP). These devices hold promise as drug delivery systems to the buccal tissue as they increase the permeability of actives with molecular weights between 600 and 4000 Da. The attached reservoirs were designed and printed along with the arrays as a whole device. Light microscopy was used to quality control the printability of the designs, confirming that the actual dimensions are in agreement with the digital design. Non-destructive volume imaging by means of microfocus computed tomography was employed for dimensional and defect characterization of the DLP-printed devices, demonstrating the actual volumes of the reservoirs and the malformations that occurred during printing. The penetration test and finite element analysis showed that the maximum stress experienced by the needles during the insertion process (10 N) was below their ultimate compressive strength (240-310 N). Permeation studies showed the increased permeability of three model drugs when delivered with the MN devices. Size-exclusion chromatography validated the stability of all the actives throughout the permeability tests. The safety of these printed devices for buccal administration was confirmed by histological evaluation and cell viability studies using the TR146 cell line, which indicated no toxic effects.

Graphene Hollow Micropatterns via Capillarity-Driven Assembly for Drug Storage and Neural Cell Alignment

Electrical conductivity, cell-guided surface topology, and drug storage capacity of biomaterials are attractive properties for the repair and regeneration of anisotropic tissues with electrical sensitivity, such as nerves. However, designing and fabricating implantable biomaterials with all these functions remain challenging. Herein, we developed a freestanding graphene substrate with micropatterned surfaces by a simple templating method. Importantly, the raised surface micropatterns had an internal hollow structure. The morphology results showed that the template microgroove width and the graphene nanosheet size were important indicators of the formation of the hollow structures. Through real-time monitoring and theoretical analysis of the formation process, it was found that the main formation mechanism was the delamination and interlayer movement of the graphene nanosheets triggered by the evaporation-induced capillary force. Finally, we achieved the controlled release of loaded microparticles and promoted the orientation of rat dorsal root ganglion neurons by applying an electric field to the hollow micropatterns. This capillarity-induced self-assembly strategy paves the way for the development of high-performance graphene micropatterned films with a hollow structure that have potential for clinical application in the repair of nerve injury.

Nano-Cavities within Nano-Zeolites: The Influencing Factors of the Fabricating Process on Their Catalytic Activities

Titanium silicalite-1 (TS-1) is a milestone heterogeneous catalyst with single-atom tetrahedral titanium incorporated into silica framework for green oxidation reactions. Although TS-1 catalysts have been industrialized, the strategy of direct hydrothermal synthesis usually produces catalysts with low catalytic activities, which has still puzzled academic and industrial scientists. Post-treatment processes were widely chosen and were proven to be an essential process for the stable production of the high-activity zeolites with hollow structures. However, the reasons why post-treatment processes could improve catalytic activity are still not clear enough. Here, high-performance hollow TS-1 zeolites with nano-sized crystals and nano-sized cavities were synthesized via post-treatment of direct-synthesis nano-sized TS-1 zeolites. The influencing factors of the fabricating processes on their catalytic activities were investigated in detail, including the content of alkali metal ions, the state of titanium centers, hydrophilic/hydrophobic properties, and accessibility of micropores. The post-treatment processes could effectively solve these adverse effects to improve catalytic activity and to stabilize production. These findings contribute to the stable preparation of high-performance TS-1 catalysts in both factories and laboratories.

Layer-by-Layer Hollow Mesoporous Silica Nanoparticles with Tunable Degradation Profile

Silica nanoparticles (SNPs) have shown promise in biomedical applications such as drug delivery and imaging due to their versatile synthetic methods, tunable physicochemical properties, and ability to load both hydrophilic and hydrophobic cargo with high efficiency. To improve the utility of these nanostructures, there is a need to control the degradation profile relative to specific microenvironments. The design of such nanostructures for controlled combination drug delivery would benefit from minimizing degradation and cargo release in circulation while increasing intracellular biodegradation. Herein, we fabricated two types of layer-by-layer hollow mesoporous SNPs (HMSNPs) containing two and three layers with variations in disulfide precursor ratios. These disulfide bonds are redox-sensitive, resulting in a controllable degradation profile relative to the number of disulfide bonds present. Particles were characterized for morphology, size and size distribution, atomic composition, pore structure, and surface area. No difference was observed between in vitro cytotoxicity profiles of the fabricated nanoparticles at 24 h in the concentration range below 100 µg mL-1. The degradation profiles of particles were evaluated in simulated body fluid in the presence of glutathione. The results demonstrate that the composition and number of layers influence degradation rates, and particles containing a higher number of disulfide bridges were more responsive to enzymatic degradation. These results indicate the potential utility of layer-by-layer HMSNPs for delivery applications where tunable degradation is desired.

Liquid-embedded (bio)printing of alginate-free, standalone, ultrafine, and ultrathin-walled cannular structures

While there has been considerable success in the three-dimensional bioprinting of relatively large standalone filamentous tissues, the fabrication of solid fibers with ultrafine diameters or those cannular featuring ultrathin walls remains a particular challenge. Here, an enabling strategy for (bio)printing of solid and hollow fibers whose size ranges could be facilely adjusted across a broad spectrum, is reported, using an aqueous two-phase embedded (bio)printing approach combined with specially designed cross-linking and extrusion methods. The generation of standalone, alginate-free aqueous architectures using this aqueous two-phase strategy allowed freeform patterning of aqueous bioinks, such as those composed of gelatin methacryloyl, within the immiscible aqueous support bath of poly(ethylene oxide). Our (bio)printing strategy revealed the fabrication of standalone solid or cannular structures with diameters as small as approximately 3 or 40 μm, respectively, and wall thicknesses of hollow conduits down to as thin as <5 μm. With cellular functions also demonstrated, we anticipate the methodology to serve as a platform that may satisfy the needs for the different types of potential biomedical and other applications in the future, especially those pertaining to cannular tissues of ultrasmall diameters and ultrathin walls used toward regenerative medicine and tissue model engineering.

Efficient Optosensing of Hippuric Acid in the Undiluted Human Urine with Hydrophilic "Turn-On"-Type Fluorescent Hollow Molecularly Imprinted Polymer Microparticles

The development of complex biological sample-compatible fluorescent molecularly imprinted polymers (MIPs) with improved performances is highly important for their real-world bioanalytical and biomedical applications. Herein, we report on the first hydrophilic "turn-on"-type fluorescent hollow MIP microparticles capable of directly, highly selectively, and rapidly optosensing hippuric acid (HA) in the undiluted human urine samples. These fluorescent hollow MIP microparticles were readily obtained through first the synthesis of core-shell-corona-structured nitrobenzoxadiazole (NBD)-labeled hydrophilic fluorescent MIP microspheres by performing one-pot surface-initiated atom transfer radical polymerization on the preformed "living" silica particles and subsequent removal of their silica core via hydrofluoric acid etching. They showed "turn-on" fluorescence and high optosensing selectivity and sensitivity toward HA in the artificial urine (the limit of detection = 0.097 μM) as well as outstanding photostability and reusability. Particularly, they exhibited much more stable aqueous dispersion ability, significantly faster optosensing kinetics, and higher optosensing sensitivity than their solid counterparts. They were also directly used for quantifying HA in the undiluted human urine with good recoveries (96.0%-102.0%) and high accuracy (RSD ≤ 4.0%), even in the presence of several analogues of HA. Such fluorescent hollow MIP microparticles hold much promise for rapid and accurate HA detection in the clinical diagnostic field.

Hollow Nanooxidase Enhanced Phototherapy Against Solid Tumors

Although phototherapy has attracted extensive attention in antitumor field in recent years, its therapeutic effect is usually unsatisfactory because of the complexity and variability of the tumor microenvironment (TME). Herein, we report novel CoSn(OH)₆@CoOOH hollow carriers with oxidase properties that can enhance phototherapy. Hollow CoSn(OH)₆@CoOOH nanocubes (NCs) with a particle size of ∼160 nm were synthesized via a two-step process of coprecipitation and etching. These NCs can react with O₂ to generate singlet oxygen without hydrogen peroxide and consume glutathione, and their hollow structure can be utilized to carry drug molecules. After loading indocyanine green (ICG) and 1,2-bis(2-(4,5-dihydro-1H-imidazol-2-yl)propan-2-yl) diazene dihydrochloride (AIPH), the resulting nanosystem (HCIA) exhibited enhanced phototherapy effects through the catalytic activity of oxidase, production of alkyl radicals, and consumption of glutathione. Cell and mouse experiments showed that HCIA combined with near-infrared laser irradiation significantly inhibited the growth of 4T1 tumors. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that PI3K-Akt and MAPK signaling pathways were highly relevant to this therapeutic system. Such hollow NCs with oxidase activity have considerable potential for the design of multifunctional drug delivery vehicles for tumor therapy.

Preparation and In Vitro Bioactivity Study of a Novel Hollow Mesoporous Bioactive Glass Nanofiber Scaffold

In this study, a novel three-dimensional hollow mesoporous bioactive glass nanofiber scaffold has been synthesized with a template-assisted sol-gel method using bacterial cellulose (BC) as a template and nonionic triblock copolymer (P123) as a pore-directing agent, ethyl orthosilicate (TEOS), calcium nitrate tetrahydrate (CN), and triethyl phosphate (TEP) as glass precursors. Scanning and transmission electron microscopies, X-ray diffraction, nitrogen adsorption-desorption, and nuclear magnetic resonance method were applied to characterize the morphology, crystal structure, and chemical structure of the mesoporous bioactive glass nanofiber scaffold. Furthermore, the in vitro bioactivity and biocompatibility were also explored. The obtained scaffold depicted nanofiber-like morphology and interconnected three-dimensional network structure that replicated the BC template. The scaffold showed a large specific surface area (230.0 cm² g^-1) and pore volume (0.2 m³ g^-1). More importantly, the scaffold exhibited excellent apatite-forming ability and cellular biocompatibility. We believe that the hollow mesoporous bioactive glass nanofiber scaffold has great potential application in bone tissue regeneration.

Compressive Strength Properties Perpendicular to the Grain of Hollow Glue-Laminated Timber Elements

Timber is one of the fundamental materials of human civilization, it is very useful and ecologically acceptable in its natural environment, and it fits very well with modern trends in green construction. The paper presents innovative hollow glued laminated (GL) timber elements intended for log-house construction. Due to the lack of data on the behavior of the hollow timber section in compression perpendicular to the grain, the paper presented involves testing the compression strength of elliptical hollow cross-section glue-laminated timber specimens made of softwood and hardwood, as well as full cross-section glue-laminated softwood timber specimens. The experimental research was carried out on a total of 120 specimens. With the maximal reduction of 26% compared to the full cross-section, regardless of the type of wood and direction of load, the compression strength perpendicular to the grain of hollow specimens decreases by about 55% compared to the full cross-section, with the coefficient k_c,90 equal to 1.0. For load actions at the edge and the middle of the element, k_c,90 factors were obtained with a value closer to those obtained for full cross-section, which indicates the same phenomenology, regardless of cross-sectional weakening. At the same time, the factors in the stronger axis are lower by about 10%, and in the weaker axis by about 30% compared to those prescribed by the Eurocode. Experimental research was confirmed by FEM analysis. Comparative finite element analysis was performed in order to provide recommendations for future research and, consequently, to determine the optimal cross-section form of the hollow GL timber element. By removing the holes in the central part of the cross-section, the stress is reduced. The distance of the holes from the edges defines the local cracking. Finally, if the holes are present only in the central part of the element, the behavior of the element is more favorable.

Interlayer-Expanded Titanate Hierarchical Hollow Spheres Embedded in Carbon Nanofibers for Enhanced Na Storage

Layered titanates are of great potential for hybrid Na-ion capacitors (NICs). However, the poor conductivity and sluggish reaction kinetics are the critical issues for the practical applications of titanates. Herein, an approach to synthesize magnesium titanate hierarchical hollow spheres embedded in carbon nanofibers (denoted as MTO@C) by electrospinning coupled with interlayer engineering processes is reported. 3D conductive carbon framework helps to enhance the electronic conductivity for binder-free electrode, while the expanded interlayer spacing of titanate hierarchical hollow spheres via the incorporation of Mg²⁺ ions help to reduce the charge transfer resistance and expose more active sites for Na storage. The interconnected hollow spheres can effectively accommodate the volume expansion during the repeated cycles. The results have shown that the MTO@C electrode can deliver a high capacity of 136 mAh g^-1 at 1 A g^-1 with long lifespan. The assembled NIC device with MTO@C as anode and active carbon as cathode produces a high energy density of 110.3 Wh kg^-1 at 112 W kg-¹ and a high power density of 5380 W kg^-1 at 41.9 Wh kg^-1 , together with a high capacity retention of 80% after 5000 cycles.

Synthesis of Hollow Silica Nanotubes from Linear Metallosupramolecular Polyelectrolytes and Their Application as Catalyst Supports for Hydrogenation

In this study, we report a facile templating method to construct hollow silica nanotubes. In particular, bis-terpyridine is used to coordinate metal ions to form polymeric bis-terpyridine-metal complexes that further self-assemble into tubular organogels in the presence of cetyltrimethylammonium bromide. The following silica deposition on organogels and subsequent calcination give metal oxide-doped hollow silica nanotubes (M_xO_y-HSNs). Facilitated by the high coordination stability constants of bis-terpyridine with metal ions, a series of metal ions, such as Fe, Ni, Cu, and Zn, could be used to form polymeric bis-terpyridine-metal complexes. The supported Pt/Fe_xO_y-HSNs exhibit good catalytic performance for hydrogenation of a series of substituted nitrobenzenes, showing that the obtained M_xO_y-HSNs could be used as catalyst supports due to their good textural properties.

Comprehensive Characterization of PVDF Nanofibers at Macro- and Nanolevel

This study is focused on the characterization and investigation of polyvinylidene fluoride (PVDF) nanofibers from the point of view of macro- and nanometer level. The fibers were produced using electrostatic spinning process in air. Two types of fibers were produced since the collector speed (300 rpm and 2000 rpm) differed as the only one processing parameter. Differences in fiber's properties were studied by scanning electron microscopy (SEM) with cross-sections observation utilizing focused ion beam (FIB). The phase composition was determined by Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. The crystallinity was determined by differential scanning calorimetry (DSC), and chemical analysis of fiber's surfaces and bonding states were studied using X-ray photoelectron spectroscopy (XPS). Other methods, such as atomic force microscopy (AFM) and piezoelectric force microscopy (PFM), were employed to describe morphology and piezoelectric response of single fiber, respectively. Moreover, the wetting behavior (hydrophobicity or hydrophilicity) was also studied. It was found that collector speed significantly affects fibers alignment and wettability (directionally ordered fibers produced at 2000 rpm almost super-hydrophobic in comparison with disordered fibers spun at 300 rpm with hydrophilic behavior) as properties at macrolevel. However, it was confirmed that these differences at the macrolevel are closely connected and originate from nanolevel attributes. The study of single individual fibers revealed some protrusions on the fiber's surface, and fibers spun at 300 rpm had a core-shell design, while fibers spun at 2000 rpm were hollow.

Robust Amino-Functionalized Mesoporous Silica Hollow Spheres Templated by CO2 Bubbles

Hollow-structured mesoporous silica has wide applications in catalysis and drug delivery due to its high surface area, large hollow space, and short diffusion mesochannels. However, the synthesis of hollow structures usually requires sacrificial templates, leading to increased production costs and environmental problems. Here, for the first time, amino-functionalized mesoporous silica hollow spheres were synthesized by using CO₂ gaseous bubbles as templates. The assembly of anionic surfactants, co-structure directing agents, and inorganic silica precursors around CO₂ bubbles formed the mesoporous silica shells. The hollow silica spheres, 200–400 nm in size with 20–30 nm spherical shell thickness, had abundant amine groups on the surface of the mesopores, indicating excellent applications for CO₂ capture, Knoevenagel condensation reaction, and the controlled release of Drugs.

Behavioral Assessment and Evaluation of Innovative Hollow Glue-Laminated Timber Elements

Due to the growing need to preserve our planet and reduce carbon emissions during construction, the use of the only carbon-absorbing material, timber, is increasingly being imposed. In addition to the requirement of reducing emissions, there is a necessity for the shortest possible construction time and the minimum use of construction machinery, which has led to the development of prefabricated construction systems. This paper deals with the innovative, hollow, glue-laminated timber elements which are intended for modular construction. Comparing this new system with existing modular systems, the main features and behavior of the constitutive elements, i.e., the hollow, glue-laminated timber elements, are presented. Experimental and numerical analysis of the mechanical performance of the timber elements was carried out and a comparative analysis of the behavior of two different types of hollow timber elements was conducted. The finite element method was used to predict the behavior of this innovative structural system. The results are compared with the analytical procedure to provide a background for the development of standardized methods for the design of timber structures.

Metal-Organic Framework-Derived Hollow and Hierarchical Porous Multivariate Metal-Oxide Microspheres for Efficient Phosphoproteomics Analysis

Pre-enrichment of the biological samples is a crucial step in phosphoproteomics research. At present, metal-oxide affinity chromatography (MOAC) is one of the most recognized enrichment strategy. Therefore, the design and preparation of a MOAC-based affinity material with better enrichment properties will be of great significance for the phosphoproteomics study. In this work, we obtained a novel multivariate metal-oxide microsphere (NiFe₂O₄@C@TiO₂) with a hollow and hierarchical porous structure through pyrolysis of TiO_2-modified Fe/Ni-based metal-organic frameworks (MOFs). After pyrolysis, the carbon matrix derived from the MOFs provided support and porous properties. Meanwhile, multivariate metal oxides endowed the microspheres with an excellent magnetic response property and superior enrichment performance for phosphorylated biomolecules. The unique hollow and hierarchical porous structure greatly enhanced the diffusion of phosphorylated biomolecules. Therefore, the microspheres exhibited excellent enrichment performance for phosphorylated biomolecules: a large adsorption capacity (124 μmol g^-1), excellent selectivity (α-casein/BSA, 1:5000, m/m), perfect size-exclusion performance (α-casein digests/α-casein/BSA, 1:500:500), and extremely low detection limit (2 fmol). Furthermore, the microspheres showed excellent enrichment performance in a series of real biological samples, such as nonfat milk, serum, saliva, rat brain tissue, and plasma exosomes of patients with esophageal cancer, which further demonstrated its huge application potential in MS-based phosphoproteomics research.

Synthesis and electrochemical performance of hollow-structured NiO + Ni nanofibers wrapped by graphene as anodes for Li-ion batteries

Hollow-structured NiO + Ni nanofibers wrapped by graphene were designed and successfully fabricated via a simple method. First, solid NiO + Ni nanofibers were prepared by electrospinning followed by calcination. Here, a portion of the metallic Ni was retained to improve the electrochemical performance of NiO by adjusting the calcination temperature. Next, the nanofibers were thoroughly mixed with different amounts of graphene and calcinated once more to form hollow-structured NiO + Ni nanofibers with an extremely high specific surface via the reaction between graphene and NiO on the nanofiber surface and subsequent migration of NiO into the nanofibers. Results showed that the obtained hollow-structured NiO + Ni electrode demonstrates optimal electrochemical performance when the graphene content is controlled to 3 wt%. The first cycle discharge/charge specific capacity of the electrode peaked (1596/1181 mAh · g^-1) at 100 mA · g^-1, with a coulombic efficiency of approximately 74% (60% for 0 wt% graphene, 65% for 1 wt% graphene, and 51% for 4 wt% graphene). It also presented excellent cycling stability after 100 cycles at 100 mA · g^-1on account of its high retained discharge specific capacity (251 mAh · g^-1for 0 wt% graphene, 385 mAh · g^-1for 1 wt% graphene, 741 mAh · g^-1for 3 wt% graphene, and 367 mAh · g^-1for 4 wt% graphene). Moreover, the synthesized electrode possessed outstanding rate capability owing to its large average discharge specific capacity of approximately 546 mAh · g^-1(45 mAh · g^-1for 0 wt% graphene, 256 mAh · g^-1for 1 wt% graphene, and 174 mAh · g^-1for 4 wt% graphene) from 100 mA · g^-1to 2000 mA · g^-1. The observed improvement in electrochemical performance could be attributed to the increase in active sites and decrease in charge transport distance in the hollow-structured NiO + Ni nanofibers. Excessive introduction of graphene caused a sharp loss in electrochemical performance due to the agglomeration of graphene sheets on the nanofiber surfaces.

Synthesis of Double-Shell Hollow TiO2@ZIF-8 Nanoparticles With Enhanced Photocatalytic Activities

Synthesis of semiconductor-MOF heterostructure photocatalysts has attracted considerable attention for their thermal stability, controllable crystallinity, and enhanced photocatalytic activity. In this work, the hollow nanostructure of anatase TiO₂ was prepared by etching SiO₂ from core–shell SiO₂@TiO₂ nanoparticles. ZIF-8, one of the metal-organic frameworks (MOFs), was hybrid synthesized on the surface of hollow TiO₂ and formed double-shell hollow nanoparticles. The photocatalytic activity of the double-shell hollow TiO₂@ZIF-8 nanoparticles toward methylene blue (MB) under UV light irradiation was processed, and the highest photocatalytic efficiency of 99.1% was shown compared with TiO₂ and SiO₂@TiO₂ nanoparticles. This study suggests a promising approach to achieve an advanced photocatalytic performance toward dye degradation using MOFs for the surface engineering of semiconductors.

Fabrication of Folic Acid-Decorated Hollow ZIF-8/Au/CuS Nanocomposites for Enhanced and Selective Anticancer Therapy

Hollow nanomaterials have been used as an attractive platform for the integration of multiple bioactive components for effective anticancer therapy. Herein, we report a novel and facile strategy for the fabrication of hollow and monodispersed zeolitic imidazolate framework-8 (ZIF-8) by the self-template method with folic acid (FA) as a bioetcher. Gold nanocluster and folic acid were critical for the formation of the hollow ZIF-8 (thickness of 38 nm) during solvothermal synthesis. By integrating CuS nanoparticles (size of 4.9 nm), the resultant quadruple ZIF-8/Au/CuS/FA nanocomposites (denoted as FACZ) exhibited effective anticancer activities on FA receptor-positive MCF-7 and HepG-2 tumor cells but a weak killing effect on HCMEC/D3 cells. Folic acid molecules were conjugated to the external surface of FACZ, which simultaneously offered an excellent tumor-targeting ability and fluorescence imaging property. Although the photothermal therapy caused by CuS was not so obvious due to partial reduction, the nanosized FACZ after cellular uptake was able to release Cu(I) to enable chemodynamic therapy. This catalytically decomposed H₂O₂ to produce highly reactive oxygen species via the Fenton-like reaction as determined by the extracellular and intracellular hydroxyl radical. Our work offers a simple route for the fabrication of hollow ZIF-8 nanocomposite with active and selective anticancer activity. This is envisaged to have great potentials in biomedical applications.

Hollow Silica Particles: Recent Progress and Future Perspectives

Hollow silica particles (or mesoporous hollow silica particles) are sought after for applications across several fields, including drug delivery, battery anodes, catalysis, thermal insulation, and functional coatings. Significant progress has been made in hollow silica particle synthesis and several new methods are being explored to use these particles in real-world applications. This review article presents a brief and critical discussion of synthesis strategies, characterization techniques, and current and possible future applications of these particles.

Nanotwinned Structure-Dependent Photocatalytic Performances of the Multipod Frameworks of Cu7S4 Hollow Microcages

The 14-pods Cu₇S₄ hollow microcages wholly exposed with nanotwinned building blocks were successfully prepared by an ethanol-assisted sacrificial Cu₂O template approach. Its photocatalytic activity for the degradation of methylene blue (MB) was determined. The results suggest that the Cu₇S₄ microcages with nanotwinned building blocks possess higher catalytic activity than the Cu₇S₄ microcages without the nanotwinned structures, suggesting that the special nanotwinned components can improve the catalytic performance of the multipod framework. Further investigate reveals that the nanotwins inside the Cu₇S₄ microcages can facilite the transport of free charges, decrease the recombination of photoinduced electrons and holes, and elongate the lifetime of the electron–hole pairs. Our work will provide powerful evidence that the nanotwinned building blocks of the synthesized Cu₇S₄ microcages play a crucial role for the high catalytic activity.

Amorphous Fe-Ni-P-B-O Nanocages as Efficient Electrocatalysts for Oxygen Evolution Reaction

Electrocatalysts are one of the most important parts for oxygen evolution reaction (OER) to overcome the sluggish kinetics. Herein, amorphous Fe-Ni-P-B-O (FNPBO) nanocages as efficient OER catalysts are synthesized by a simple low-cost and scalable method at room temperature. The samples are chemically stable, in clear contrast to reported unstable or even pyrophoric boride samples. The Fe/Ni ratio of the FNPBO nanocages can be continuously adjusted to optimize the OER catalytic performance. The FNPBO nanocages composed of multicomponent elements can weaken the metal-metal bonds, thus rearranging the electron density around the catalytic metal atom centers and reducing the energy barrier for intermediate formation. Hence the optimized FNPBO (Fe_6.4Ni_16.1P_12.9B_4.3O_60.2) catalyst shows superior intrinsic electrocatalytic activity for OER. The low overpotential to afford the current density of 10 mA cm^-2 (236 mV), the small Tafel slope (39 mV dec^-1), and the high specific current density (26.44 mA cm^-2) at a given overpotential of 300 mV make a sharp contrast to state-of-the-art RuO₂ (327 mV, 136 mV dec^-1, and 0.028 mA cm^-2, respectively).

Dandelion-like Mn/Ni Co-doped CoO/C Hollow Microspheres with Oxygen Vacancies for Advanced Lithium Storage

Hollow structures have attracted great attention based on the advantage to accommodate volume expansion. However, template removal usually results in structure destruction. Herein, dandelion-like Mn/Ni co-doped CoO/C hollow microspheres (CMNC-10h) are synthesized via an Ostwald ripening process without templates. The high-angle annular dark field mapping images at the atomic level indicate the successful doping of Mn and Ni into CoO. Via an annular bright field image, oxygen vacancies induced by doping can be clearly observed. The residual two electrons in the oxygen vacancy site are highly delocalized, as confirmed by density functional theory calculations, effectively improving electrical conductivity. According to electron holography analysis, the dielectric polarization field in superficial regions of primary nanoparticles can facilitate insertion of Li⁺ ions into nanoparticles and thus enhance electrochemical kinetics. Combining those advantages, CMNC-10h demonstrates a high capacity of 1126 mAh g^-1 at 1 A g^-1 after 1000 cycles as anode material for a lithium-ion battery. Additionally, based on the strong adsorption toward polysulfide, the porous structure to accommodate sulfer/polysulfide, and the effects of oxygen vacancies to immobilize and catalyze polysulfide, CMNC-10h-S as cathode material for a lithium-sulfur battery also displays a high capacity of 642 mAh g^-1 after 500 cycles at 1 C.

Construction of Hierarchical Co-Fe Oxyphosphide Microtubes for Electrocatalytic Overall Water Splitting

Development of efficient electrocatalysts is a crucial requirement to build water splitting systems for the production of clean and sustainable fuels. This goal could be achieved by fine-tuning the composition and structure of the electrocatalytic materials. Here, a facile self-templated synthetic strategy is developed for the fabrication of hierarchical Co-Fe oxyphosphide microtubes (MTs). Fe-based metal-organic compound microrods are first synthesized as the self-sacrificing template. Afterward, the Fe-based precursors are converted into hierarchical Co-Fe layered double hydroxide MTs through a hydrothermal approach, which are then transformed into the hierarchical Co-Fe oxyphosphide MTs by a phosphidation treatment. Benefiting from the synergistic effect of the compositions and the advantages of the hierarchical hollow structure, the obtained electrocatalyst exhibits enhanced performance for overall water splitting.

Sequential Templating Approach: A Groundbreaking Strategy to Create Hollow Multishelled Structures

Thanks to their distinguished properties such as optimized specific surface area, low density, high loading capacity, and sequential matter transfer and storage, hollow multishelled structures (HoMSs) have attracted great interest from scientists in broad fields, including catalysis, drug delivery, solar cells, supercapacitors, lithium-ion batteries, electromagnetic wave absorption, and sensors. However, traditional synthesis methods such as soft-templating and hierarchical self-assembly methods can hardly realize the controllable synthesis of HoMSs, thus limiting their development and application. Here, the development process of HoMSs is first succinctly reviewed and the shortcomings of the traditional synthesis method are concluded. Subsequently, the sequential templating approach, which shows great generality for the synthesis of HoMSs with controllable composition and geometry configuration and exhibits remarkable effect on the scientific research field, is introduced. The basic material science and chemical reaction mechanism involved in the synthesis and manipulation of HoMSs using the sequential templating approach are then explained in detail. In addition, the effect of the geometric characteristics of HoMSs on their application properties is highlighted. Finally, the current challenges and future research directions of HoMSs are also suggested.

Mesoporous Organosilica Hollow Nanoparticles: Synthesis and Applications

Hollow periodic mesoporous organosilicas (PMOs) with molecularly homogeneous organic functional groups in the inorganic pore walls are attracting more and more attention due to the high surface areas, tunable pore sizes, low densities, large cavities in the center, permeable thin shells, and versatile organic-inorganic hybrid frameworks, which make them promising in a variety of applications including adsorption, catalysis, drug delivery, and nanotheranostics. Herein, recent advances in the synthesis of hollow PMO nanoparticles with various organic moieties are summarized, and the mechanism and new insights of synthesis approaches, including hard-core templating methods, liquid-interface assembly methods, and the interfacial reassembly and transformation strategy are discussed in-depth. Meanwhile, the design principles, properties, and synthetic strategies for some smart hollow architectures such as multishelled hollow PMOs, yolk-shell structured PMOs, and nonspherical hollow PMOs are discussed. Moreover, the typical applications of hollow PMO nanomaterials as nanoreactors for chemical transformations and nanoplatforms for biomedicine are summarized. Finally, the challenges and prospects for the future development of hollow PMOs are described.

Effect of Cation Substitution on the Gas-Sensing Performances of Ternary Spinel MCo2O4 (M = Mn, Ni, and Zn) Multishelled Hollow Twin Spheres

Advanced sensing materials are in high demand for sensitive, real-time, and continuous detection of gas molecules for gas sensors, which have been becoming an effective tool for environmental monitoring and disease diagnosis. Cobalt-containing spinel oxides are promising sensing materials for the gas-sensing reaction owing to their element abundance and remarkable activity. Structural and component properties can be modulated to optimize the sensing performances by substituting Co with other transition metals. Herein, a systematic study of spinel MCo₂O₄ oxides (M = Mn, Ni, and Zn) toward gas sensing is presented. Results show that ZnCo₂O₄ materials with a multishelled hollow twin-sphere structure obtained excellent sensing performances to formaldehyde and acetone at different temperatures. The replacement of Co with Zn in the lattice improves the oxygen-chemisorbing ability, which allows new opportunities to synthesize and design highly sensitive chemical sensors.

Strong Hollow Spherical La2NiO4 Photocatalytic Microreactor for Round-the-Clock Environmental Remediation

This work reports a moderate round-the-clock route to treating organic pollutants by utilizing a La₂NiO₄ hollow-sphere microreactor. A glycerol-assisted solvothermal route followed by an annealing process was applied for fabricating the catalyst. Both the physicochemical properties and the catalytic performance of the as-obtained microreactor for treating pollutants were discussed. The microreactor exhibited a strong ability to degrade phenol and anionic dyes in the absence of light irradiation, owing to its high surface area and positively charged surface. With the aid of visible-light irradiation, the degradation rate of the organic pollutants could be further accelerated due to the light multireflection in a hollow structure, which enhances the utilization of light. The present work indicates that the hollow-sphere La₂NiO₄ microreactor is effectively energy saving for environmental remediation.

Design of Heterostructured Hollow Photocatalysts for Solar-to-Chemical Energy Conversion

Direct conversion of solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues. Fabrication of highly active photocatalysts is of great significance for the practical applications of efficient solar-to-chemical energy conversion systems. Among various photocatalytic materials, semiconductor-based heterostructured photocatalysts with hollow features show distinct advantages. Recent research efforts on rational design of heterostructured hollow photocatalysts toward photocatalytic water splitting and CO₂ reduction are presented. First, both single-shelled and multishelled heterostructured photocatalysts are surveyed. Then, heterostructured hollow photocatalysts with tube-like and frame-like morphologies are discussed. It is intended that further innovative works on the material design of high-performance photocatalysts for solar energy utilization can be inspired.

The controlled synthesis of complex hollow nanostructures and prospective applications†

Functionality in nanoengineered materials has been usually explored on structural and chemical compositional aspects of matter that exist in such solid materials. It is well known that the absence of solid matter is also relevant and the existence of voids confined in the nanostructure of certain particles is no exception. Indeed, over the past decades, there has been great interest in exploring hollow nanostructured materials that besides the properties recognized in the dense particles also provide empty spaces, in the sense of condensed matter absence, as an additional functionality to be explored. As such, the chemical synthesis of hollow nanostructures has been driven not only for tailoring the size and shape of particles with well-defined chemical composition, but also to achieve control on the type of hollowness that characterize such materials. This review describes the state of the art on late developments concerning the chemical synthesis of hollow nanostructures, providing a number of examples of materials obtained by distinct strategies. It will be apparent by reading this progress report that the absence of solid matter determines the functionality of hollow nanomaterials for several technological applications.

Upconversion-Magnetic Carbon Sphere for Near Infrared Light-Triggered Bioimaging and Photothermal Therapy

Nanoparticle-based theranostics combines tumor imaging and cancer therapy in one platform, but the synthesis of theranostic agents is impeded by chemical groups on the surface and the size and morphology of the components. Strategies to construct a multifunctional platform for bioimaging and photothermal therapy (PTT) are urgently needed. A new upconversion-magnetic agent (FeCUPs) based on hollow carbon spheres, which is both a photothermal agent and a dual carrier of luminescent and magnetic nanoparticles, provides an effective approach for tumor elimination. Methods: The morphology of FeCUPs was characterized for the construction and size adjustment of the theranostic agent using transmission electron microscopy, high-resolution transmission electron microscopy, energy dispersive spectroscopy and high angle annular dark field scanning transmission electron microscopy. The distribution of FeCUPs was tracked under in-situ upconversion luminescence (UCL) imaging and magnetic resonance imaging (MRI) in vivo. Photothermal therapy was carried out on tumor-bearing mice, after which the toxicity of PTT was evaluated by a blood biochemistry test and histological section analysis. Results: Stable and uniform loading of luminescent nanocomposites on three-dimensional carbon materials is reported for the first time. Based on the mechanism of synthesis, the size of the hybrid particles was adjusted from micrometers to nanometers. External magnetic field-enhanced photothermal therapy with multi-model imaging was accomplished using FeCUPs. Moreover, no cancer recurrence was found during 14 days of recovery without PTT. Conclusions: Hollow carbon spheres, photothermal agents loaded with upconversion nanoparticles inside and magnetic nanoparticles outside were prepared for photothermal therapy. The aggregation of FeCUPs in tumors by the local magnetic field was verified by MRI and UCL imaging, and PTT was enhanced.

Metal-Organic Framework Template-Directed Fabrication of Well-Aligned Pentagon-like Hollow Transition-Metal Sulfides as the Anode and Cathode for High-Performance Asymmetric Supercapacitors

Given the exceptional specific surface area, geometry, and periodic porosity, transition-metal sulfides derived from crystalline metal-organic frameworks have spurred great interest in energy storage systems. Herein, employing a different sulfurization process, well-aligned NiCo₂S₄ and CoS₂ nanoarrays with a hollow/porous configuration derived from pentagon-like ZIF-67 are successfully designed and constructed on Ni foam. The hollow/porous structure grown on a conductive matrix can significantly improve electroactive sites, shorten charge/ion diffusion length, and enhance mass/electron transfer. Consequently, the obtained NiCo₂S₄ possesses an excellent specific capacitance of 939 C/g, a fast charge/discharge rate, and a favorable life span. An advanced asymmetrical supercapacitor is fabricated by engaging NiCo₂S₄ and CoS₂ as cathode and anode materials, respectively, with a well-separated potential window. The obtained device delivers an exceptional energy density of 55.8 W h/kg at 695.2 W/kg, which is highly considerable to the recent transition metal sulfide-based devices. This facile tactic could be employed to construct other electrode materials with superior electrochemical properties.

Preparation of Hollow Cu and CuO x Microspheres with a Hierarchical Structure for Heterogeneous Catalysis

Diffusion is one of the most critical factors which affect the performance of porous catalysts in heterogeneous reactions. Hollow spheres with a hierarchical structure could significantly improve the mass transfer in the spherical catalyst. Therefore, preparation of such kind of microspheres is an important work in the field of inorganic synthesis. Herein, we combine microfluidic technology and electroless deposition to prepare hollow Cu and CuO _x microspheres with a hierarchically porous structure. These microspheres have a controllable diameter (100-500 μm) and shell thickness (10-60 μm). Numerical simulation and experimental results indicate that the hollow structure is beneficial for the diffusion and utilization of the catalyst in heterogeneous reactions. The Cu and CuO _x microspheres were used to catalyze the hydrogenation and Fenton-like reactions in a flow reactor, respectively. The conversion of all reactants can reach more than 95%, and catalysts can maintain their reactivity in long reaction times. Thus, the strategy in the present research should apply in the construction of other porous catalysts with high performance.

Ultrastretchable Multilayered Fiber with a Hollow-Monolith Structure for High-Performance Strain Sensor

As a crucial component of data terminal acquisition devices, flexible strain sensor has shown promising applications in numerous fields, such as healthcare, bodynet, the intelligent traffic system, and the robotic system. For stretchable strain sensor, it remains a huge challenge to realize a fine balance of wide detection range and high sensitivity. Here, an electrically conductive carbon nanotube/thermoplastic polyurethane fiber with a multilayered, hollow, and monolith structure, accompanying high stretchability (up to 476% strain) and low density (about 0.46 g/cm³) is fabricated through a facile coaxial wet-spun assembly strategy. The as-prepared fibers with a designed independent sensitive zone and flexible supporting zone possess an ultralow percolation threshold (0.17 wt %) and a tunable size and structure. This structure endows the fiber with a good integration of adequate flexibility, suitable strength, and high elongation at break for wearable electronics. The fiber, which is then assembled as a strain sensor, realizes the perfect combination of the wide sensing range (>350% strain), high sensitivity (gauge factor (GF) = 166.7 at 350% strain), and excellent working durability (>10 000 cycles). Our sensor could also detect small compressing deformations (0.35% N^-1 at 0.025-50 N) by capturing the resistance change of the fiber with superior stability. The highly stretchable, light weight, and multilayered fiber with the designed hollow-monolith structure provides a new route for the preparation of high-performance wearable electronics.

Dual-Porosity Hollow Carbon Spheres with Tunable Through-Holes for Multi-Guest Delivery

Dual-porosity hollow carbon spheres (DPHCs) with small mesopores (2-4 nm) and large through-holes (20-30 nm) in shells were successfully synthesized using colloidal silica as the template, small silica nanoparticles as nanomasks, and nontoxic dopamine as the carbon precursor followed by post-carbonization and etching. The synthesized DPHCs were further oxidized to be hydrophilic and then used to simultaneously deliver the protein bovine serum albumin (21 × 4 × 14 nm³) and the small molecule doxorubicin (<1 nm), which exhibited a high loading capacity of 689.4 and 1421.2 mg/g, respectively. The release of these two guest molecules can be controlled independently under the stimuli of heat and acidity. In vitro and in vivo experiments also proved that the DPHCs are promising for the co-delivery of multiple cargoes of different sizes.

The Design and Synthesis of Hollow Micro-/Nanostructures: Present and Future Trends

Hollow micro-/nanostructures have attracted tremendous interest owing to their intriguing structure-induced physicochemical properties and great potential for widespread applications. With the development of modern synthetic methodology and analytical instruments, a rapid structural/compositional evolution of hollow structures from simple to complex has occurred in recent decades. Here, an updated overview of research progress made in the synthesis of hollow structures is provided. After an introduction of definition and classification, achievements in synthetic approaches for these delicate hollow architectures are presented in detail. According to formation mechanisms, these strategies can be categorized into four different types, including hard-templating, soft-templating, self-templated, and template-free methods. In particular, the rationales and emerging innovations in conventional templating syntheses are in focus. The development of burgeoning self-templating strategies based on controlled etching, outward diffusion, and heterogeneous contraction is also summarized. In addition, a brief overview of template-free methods and recent advances on combined mechanisms is provided. Notably, the strengths and weaknesses of each category are discussed in detail. In conclusion, a perspective on future trends in the research of hollow micro-/nanostructures is given.

Hollow TiNb2 O7 @C Spheres with Superior Rate Capability and Excellent Cycle Performance as Anode Material for Lithium-Ion Batteries

TiNb₂ O₇ , with high charge/discharge voltage (1.65 V), long life span, and excellent reversibility has become a new anode material for lithium ion batteries in recent years. However, the inherent poor electronic conductivity strictly limits its practical applications. Herein, we report for the first time hollow TiNb₂ O₇ @C spheres, with carbon spheres as sacrificial template. The special hollow structure can effectively relieve the volume expansion during the repeated process of charging/discharging. The conductivity of hollow TiNb₂ O₇ spheres can be greatly improved by the carbon layers uniformly coated outside the surface of TiNb₂ O₇ spheres. Besides, the particle aggregation can be conspicuously suppressed by the uniformly coated carbon layers. As a result, hollow TiNb₂ O₇ @C spheres show a prominent charge/discharge capacity of 282.6/283.8 mAh g^-1 after 100 cycles at 0.25 C (1 C=388 mA g^-1 ), revealing the excellent cycle performance and high reversible capacity. Meanwhile, the charge/discharge capacity of 157.5 and 157.9 mAh g^-1 can be maintained at 10 C after 10 cycles. The facial synthetic method can be extended to prepare various materials with volume expansion during repeated charging/discharging process.

Controlling the Internal Structures of Polymeric Microspheres via the Introduction of a Water-Soluble Organic Solvent

Polymeric microspheres with different internal structures have been widely used because of their characteristics in the structures. This paper reports a method of controlling the internal structures of polymeric microspheres via the introduction of a water-soluble organic solvent to the continuous phase in the foam phase preparation of porous polymeric microspheres. The introduction of a water-soluble organic solvent enables the control of polymeric microspheres' internal structures, from porous to hollow. Because a water-soluble organic solvent is introduced, the organic solvent may be diffused toward the interface because of the affinity between the organic solvent and the oil droplets, resulting an accumulation of organic solvent molecules at the interface to form an organic solvent layer. The presence of this layer may decrease the evaporation rate of the internal organic solvent in an oil droplet, which extends the time for the mingling of porogen droplets to form a few large pores or even an extremely large single pore inside. This method is also capable of altering the thickness of hollow microspheres' shells in a desired way, with improved efficiency, yield and the capacity for continuous use on an industrial scale.

Fabrication and Characterization of Nanoenergetic Hollow Spherical Hexanitrostibene (HNS) Derivatives

The spherization of nanoenergetic materials is the best way to improve the sensitivity and increase loading densities and detonation properties for weapons and ammunition, but the preparation of spherical nanoenergetic materials with high regularization, uniform size and monodispersity is still a challenge. In this paper, nanoenergetic hollow spherical hexanitrostibene (HNS) derivatives were fabricated via a one-pot copolymerization strategy, which is based on the reaction of HNS and piperazine in acetonitrile solution. Characterization results indicated the as-prepared reaction nanoenergetic products were HNS-derived oligomers, where a free radical copolymerization reaction process was inferred. The hollow sphere structure of the HNS derivatives was characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), and synchrotron radiation X-ray imaging technology. The properties of the nanoenergetic hollow spherical derivatives, including thermal decomposition and sensitivity are discussed in detail. Sensitivity studies showed that the nanoenergetic derivatives exhibited lower impact, friction and spark sensitivity than raw HNS. Thermogravimetric-differential scanning calorimeter (TG-DSC) results showed that continuous exothermic decomposition occurred in the whole temperature range, which indicated that nanoenergetic derivatives have a unique role in thermal applications. Therefore, nanoenergetic hollow spherical HNS derivatives could provide a new way to modify the properties of certain energetic compounds and fabricate spherical nanomaterials to improve the charge configuration.

Synthesis of Hierarchical Silica/Titania Hollow Nanoparticles and Their Enhanced Electroresponsive Activity

Wrinkled silica nanoparticle (WSN)-based hollow SiO₂/TiO₂ nanoparticles (W-HNPs) with hierarchically arrayed internal surfaces were prepared via the combination of sol-gel, TiO₂ coating, and etching of core template techniques. The hierarchical internal surface of W-HNPs was attained using WSNs as a core template. Compared with SiO₂ sphere-templated hollow SiO₂/TiO₂ nanoparticles (S-HNPs) with flat inner surfaces, W-HNPs displayed distinctive surface areas, TiO₂ loading amounts, and dielectric properties arising from the hierarchical internal surface. The unique properties of W-HNPs were further investigated as an electrorheological (ER) material. W-HNP-based ER fluids exhibited ca. 1.9-fold enhancement in the ER efficiency compared to that of S-HNP-based ER fluids. Such enhancement was attributed to the unique inner surface of W-HNPs, which effectively enhanced the polarizability by increasing the number of charge accumulation sites, and to the presence of the high-dielectric TiO₂. This study demonstrated the advantages, in terms of practical ER applications, of hollow nanomaterials having uniquely arrayed internal spaces.

Hollow Echinus-like PdCuCo Alloy for Superior Efficient Catalysis of Ethanol

Large-scale preparation of hollow echinus-like PdCuCo alloy nanostructures (HENSs) with a high surface area-to-volume ratio, rich active sites, and relatively efficient catalytic activity has attracted considerable research interest. Herein, we present an economic and facile approach to synthesize HENSs by galvanic exchange reactions using Co nanospheres as sacrificial templates. Moreover, the catalytic activity could be adjusted via changing the composition of the catalyst. The composition, morphology, and crystal structure of the as-obtained nanomaterials are characterized by various techniques, such as inductively coupled plasma atomic emission spectrometry, transmission electron microscopy, and X-ray diffraction. Electrochemical catalytic measurement results prove that the Pd₇₅Cu₈Co₃ catalyst obtained under optimal preparation conditions exhibits 10-fold higher activity for ethanol oxidation in comparison with the commercially available 20% Pd/C catalyst. The eminent performance of the Pd₇₅Cu₈Co₃ electrochemical catalyst could be ascribed to the peculiar echinus-like nanostructures.

A Simplified Approach to Fabricate a Hollow Ocular Prosthesis

The prosthetic rehabilitation of a large anophthalmic socket is always a challenge. It demands an equally sized ocular prosthesis to replace lost tissue volume. A conventional solid acrylic ocular prosthesis may be deterrent to retention and esthetics because of its weight, which can be reduced by eliminating the mass of material within it. This article attempts to present a simplified and accurate technique to fabricate a hollow ocular prosthesis using silicone putty cavity form. It is removed prior to final processing and serves to ensure appropriate uniform thickness of acrylic resin and optimal weight reduction of the definitive prosthesis without compromising mobility, esthetics, and structural integrity.

Design of Porous/Hollow Structured Ceria by Partial Thermal Decomposition of Ce-MOF and Selective Etching

Metal-organic frameworks (MOFs) have been widely used to prepare corresponding porous metal oxides via thermal treatment. However, high temperature treatment always leads to obtained metal oxides with a large crystallite size, thus decreasing their specific surface area. Different from the conventional complete thermal decomposition of MOFs, herein, using Ce-MOF as a demonstration, we choose partial thermal decomposition of MOF, followed by selective etching to prepare porous/hollow structured ceria because of the poor stability of Ce-MOF under acidic conditions. Compared with the ceria derived from complete thermal decomposition of Ce-MOF, the as-prepared ceria is demonstrated to be a good support for copper oxide species during the CO oxidation catalytic reaction. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and hydrogen temperature-programmed reduction (H₂-TPR) analysis revealed that the as-prepared ceria is favorable for strengthening the interaction between the ceria and loaded copper oxide species. This work is expected to open a new, simple avenue for the synthesis of metal oxides from MOFs via partial thermal decomposition.

Topological Transformations of Core-Shell Precursors to Hierarchically Hollow Assemblages of Copper Silicate Nanotubes

Functional hollow materials have attracted extensive research attention due to their promising prospects for catalysis. Herein, we report an alternative synthesis of hierarchically hollow structured materials directly from core-shell structured templates, based on confined chemical reactions between the solid matter of a core and shell under hydrothermal conditions. More specifically, we have developed a novel and facile strategy to transform core-shell structured Cu₂O@mSiO₂ (m = mesoporous) to tubular copper silicate assemblages (TCSA). Depending on the original shapes of Cu₂O, TCSA can be tailored as spherical or cubic assemblages with stacking copper silicate nanotubes (inner diameter: 4.5 nm, thickness: 0.8 nm, length: ca. 96 nm) in the shell. Moreover, by utilizing the residual reductive Cu(I) (ca. 10 at% of total surface copper) on TCSA support, in situ generations of Pd nanoparticles (∼4.5 nm) and Au nanoparticles (∼5.8 nm) were successfully achieved based on the spontaneous galvanic replacement reactions. Two integrated nanocatalysts (viz., Pd/TCSA and Au/TCSA) have been prepared with this approach. As an example, Pd/TCSA exhibits excellent activity and recyclability for Suzuki-Miyaura cross-coupling reactions.

Controlling the Deposition of Pd on Au Nanocages: Outer Surface Only versus Both Outer and Inner Surfaces

When a metal precursor is reduced in the presence of Au nanocages with a hollow interior and porous walls, in principle the resultant metal atoms can be deposited onto both the outer and inner surfaces or just the outer surface. Here we demonstrate that these two different scenarios of metal deposition can be deterministically achieved by controlling the reduction kinetics of the precursor. Specifically, if PdCl₄^2- is employed as the precursor, its fast reduction kinetics favors the solution reduction pathway, in which the resultant Pd atoms are deposited only onto the outer surface for the generation of Au@Pd double-shelled nanocages. When the precursor is switched to PdBr₄^2- to slow down the reduction, the precursor can readily diffuse into the interior of the Au nanocages prior to its reduction to elemental Pd. As such, both the outer and inner surfaces of the nanocages become coated with Pd for the generation of Pd@Au@Pd triple-shelled nanocages. This study not only offers a new synthetic approach to metal nanocages with diverse compositions and structures but also demonstrates the necessity of controlling the relative rates of reduction and bulk diffusion of a metal precursor when nanostructures with a hollow interior and porous walls are used for seed-mediated growth.

Confining the Nucleation of Pt to In Situ Form (Pt-Enriched Cage)@CeO2 Core@Shell Nanostructure as Excellent Catalysts for Hydrogenation Reactions

Ultrathin (Pt-enriched cage)@CeO₂ core@shell nanostructures are successfully fabricated via a facile hard-template method. It is found that the usage of Pd@Ag@CeO₂ bi-metallic core@shell nanostructure as the hard template plays an important role in avoiding the independent nucleation of Pt metal during the galvanic replacement process between K₂ PtCl₄ and Ag components. This unique core@shell samples show extraordinary activity and selectivity for the cinnamaldehyde hydrogenation reaction. It can achieve over 95% conversion with 87% selectivity of hydrocinnamaldehyde in 5 h under 1 atm H₂ pressure. It is considered that such high catalytic performance could be attributed to the densely CeO₂ -coated core@shell hybrid form as well as the ultrathin nature of the Pt-enriched cage.

Hollow Nitrogen-Doped Carbon Spheres with Fe3O4 Nanoparticles Encapsulated as a Highly Active Oxygen-Reduction Catalyst

The development of nonprecious electrocatalyst with low cost and high efficiency for the oxygen reduction reaction (ORR) is a main challenge for electrochemical energy technology. In this work, a hierarchical hollow core-shell structured N-doped carbon spheres (N-HSCS), in which Fe₃O₄ nanoparticles are encapsulated (Fe₃O₄/N-HCSC) has been successfully prepared. The Fe₃O₄/N-HCSC electrocatalyst exhibits a remarkable catalytic performance toward ORR. The porous hollow core-shell structure and synergistic effect between Fe₃O₄ and protective nitrogen-doped graphitic layers are mainly responsible for such an excellent ORR catalytic property and stability. This work demonstrates a promising strategy of nanostructure-engineering to the future design and preparation of highly efficient non-noble metal electrocatalysts.

Microfluidics-mediated self-template synthesis of anisotropic hollow ellipsoidal mesoporous silica nanomaterials

Herein, a facile strategy was firstly developed to synthesize ellipsoidal mesoporous silica nanomaterials (MSNs) with well-ordered parallel channels along the short axis. A miniaturized microfluidic device with spiral-shaped channels was then chosen as a straightforward and general platform to produce the corresponding hollow counterparts of MSNs under mild conditions. Such reaction process carried out in a microfluidic system was further demonstrated to be more rapid and efficient than conventional batch method under equivalent experimental conditions. The evolution of hollow structure can be well-tuned by flow rates (i.e., etching time), providing new paradigm for rational design and engineering of anisotropic nanostructures.

One-Step Synthesis of Cagelike Hollow Silica Spheres with Large Through-Holes for Macromolecule Delivery

A facile, one-step method to prepare cagelike hollow silica nanospheres with large through-holes (HSNLs) using a lysozyme-assisted O/W miniemulsion technique is presented. The tetraethoxysilane (TEOS)-xylene mixture forms oil droplets which are stabilized by the cationic surfactant cetyltrimethylammonium bromide (CTAB), cosurfactant hexadecane (HD), and protein lysozyme. HSNLs (with diameter of 300-460 nm) with large through-holes (10-30 nm) were obtained directly after ultrasonic treatment and aging. Lysozyme can not only stabilize the oil/water interface, assist the hydrolysis of TEOS, and interact with silica particles to assemble into silica-lysozyme clusters but also contribute to the formation of through-holes due to its hydrophilicity variation at different pH conditions. A possible new mechanism called the interface desorption method is proposed to explain the formation of the through-holes. To confirm the effectiveness of large through-holes in delivering large molecules, bovine serum albumin (BSA, 21 × 4 × 14 nm³) was chosen as a model guest molecule; HSNLs showed much higher loading capacity compared with common hollow mesoporous silica nanospheres (HMSNs). The release of BSA can be well controlled by wrapping HSNLs with a heat-sensitive phase change material (1-tetradecanol). Cell toxicity was also conducted with a Cell Counting Kit-8 (CCK-8) assay to roughly evaluate the feasibility of HSNLs in biomedical applications.

A "Solid Dual-Ions-Transformation" Route to S,N Co-Doped Carbon Nanotubes as Highly Efficient "Metal-Free" Catalysts for Organic Reactions

A green and cost-effective method, named the "solid dual-ions-transformation reaction", is reported to achieve the goal of successfully doping S and N into hollow carbon nanotubes. No harmful raw material is needed during the whole synthesis. Importantly, in the catalytic test of chemical reduction of p-nitrophenol by NaBH₄ , the as-obtained S,N co-doped hollow carbon nanotubes exhibit ultrahigh catalytic performances.