Nanoporous Shell Research Articles

Hierarchical conductive metal-organic framework films enabling efficient interfacial mass transfer

Heterogeneous reactions associated with porous solid films are ubiquitous and play an important role in both nature and industrial processes. However, due to the no-slip boundary condition in pressure-driven flows, the interfacial mass transfer between the porous solid surface and the environment is largely limited to slow molecular diffusion, which severely hinders the enhancement of heterogeneous reaction kinetics. Herein, we report a hierarchical-structure-accelerated interfacial dynamic strategy to improve interfacial gas transfer on hierarchical conductive metal-organic framework (c-MOF) films. Hierarchical c-MOF films are synthesized via the in-situ transformation of insulating MOF film precursors using π-conjugated ligands and comprise both a nanoporous shell and hollow inner voids. The introduction of hollow structures in the c-MOF films enables an increase of gas permeability, thus enhancing the motion velocity of gas molecules toward the c-MOF film surface, which is more than 8.0-fold higher than that of bulk-type film. The c-MOF film-based chemiresistive sensor exhibits a faster response towards ammonia than other reported chemiresistive ammonia sensors at room temperature and a response speed 10 times faster than that of the bulk-type film.

Understanding the microstructure of a core–shell anode catalyst layer for polymer electrolyte water electrolysis

Reducing precious metal loading in the anodic catalyst layer (CL) is indispensable for lowering capital costs and enabling the widespread adoption of polymer electrolyte water electrolysis. This work presents the first three-dimensional reconstruction of a TiO2-supported IrO2 based core shell CL (3 mgIrO2/cm2), using high-resolution X-ray ptychographic tomography at cryogenic temperature of 90 K. The high data quality and phase sensitivity of the technique have allowed the reconstruction of all four phases namely pore space, IrO2, TiO2 support matrix and the ionomer network, the latter of which has proven to be a challenge in the past. Results show that the IrO2 forms thin nanoporous shells around the TiO2 particles and that the ionomer has a non-uniform thickness and partially covers the catalyst. The TiO2 particles do not form a percolating network while all other phases have high connectivity. The analysis of the CL ionic and electronic conductivity shows that for a dry CL, the ionic conductivity is orders of magnitudes lower than the electronic conductivity. Varying the electronic conductivity of the support phase by simulations, reveals that the conductivity of the support does not have a considerable impact on the overall CL electrical conductivity.

(Invited) Three-Dimensional Investigation of the Anodic Catalyst Layer in Polymer Electrolyte Water Electrolysis

Reducing precious metal loading in the anodic catalyst layer (CL) is indispensable for lowering capital costs and enabling the widespread adoption of polymer electrolyte water electrolysis (PEWE). This work presents the first three-dimensional reconstruction of a state-of-the-art TiO2-supported IrO2 based core shell catalyst layer (3 mgIrO2/cm2), using high-resolution X-ray ptychographic tomography at a cryogenic temperature of 90 K. The ptychographic tomography was performed at the cSAXS beamline of the Swiss Light Source (PSI). The high data quality and phase sensitivity of the technique have allowed the reconstruction of all four phases namely pore space, IrO2, TiO2 support matrix and the ionomer network, the latter of which has proven to be a challenge in the past. Results show that the IrO2 forms thin nanoporous shells around the TiO2 particles and that the ionomer has a non-uniform thickness and partially covers the catalyst. The TiO2 particles do not form a percolating network while all other phases have high connectivity. The analysis of the CL ionic and electronic conductivity shows that for a dry CL, the ionic conductivity is orders of magnitudes lower than the electronic conductivity. Varying the electronic conductivity of the support phase by simulations reveals that the conductivity of the support does not have a considerable impact on the overall CL electrical conductivity. Figure 1

Micro interfacial Effect: The stability of catalytic activity under gas–liquid reaction pressure changes

The conversion of CO2 usually requires high temperature, high pressure and long reaction time, due to its chemical stability and gas–liquid mass transfer resistance. Based on the micro interfacial effect, the nano-micro interface reaction units (NMRUs) were designed and prepared for the efficient conversion of CO2 under mild conditions. NMRUs are made up of the micron-sized droplet inside and the nano-porous shell, which can significantly increase the gas–liquid contact area and thus intensify mass transfer from gas to liquid. Compared with the gas–liquid reaction system (G-L) in the traditional autoclave, the prominent intensification on CO2 cycloaddition by NMRUs is achieved at 25℃ and 0.1 MPa, with the equilibrium time shortened by more than a half. Noticeably, the influence of pressure on the gas–liquid reaction in NMRUs was nearly negligible, which was explained by the interface mass transfer mathematical model. The strengthening mechanism of NMRUs was further explored with the kinetic simulation.

High Catalytic Performance of Silver‐Doped Gold Nanocages for Methanol Electrooxidation

AbstractHollow or porous noble metal nanocrystals with large specific electrochemical surface area and stability hold stimulating technological applications in fuel cell catalysis. Here, a simple galvanic replacement reaction synthesis of Ag‐doped Au nanocages with a tunable Ag/Au atomic ratio is developed. Without surface ligands being used in the synthesis, the obtained AuAg nanocages with hollow cavity and opened nanoporous shell possess clean surface and enhanced electrocatalytic activity. In the electrocatalysis of the methanol oxidation reaction (MOR), the obtained AuAg nanocages possess high electrocatalysis properties and show morphology and composition‐dependent electrocatalytic property. The optimized AuAg cubic nanocages that have 5% Ag exhibit excellent electrocatalytic activity for MOR, and this is attributed to the unique geometric structure and the well‐controlled Ag content. The hollow and nanoporous structure can effectively promote the mass transfer and atomic utilization during the catalytic process for MOR. Density functional theory calculations indicate that introducing of a small amount of Ag facilitates methanol adsorption to the surface of catalysts and promotes the break of the OH bond during the MOR process, thus accordingly enhances the catalytic activity.

Engineering Caged Microbubbles for Controlled Acoustic Cavitation and Pressure Sensing

Acoustic microbubbles (MBs) are an important class of biomaterials that play an increasingly prominent role in advanced applications such as contrast and super-resolution imaging, sonoporation, drug delivery, microrobotics, and biosensors. The ability to control and regulate acoustic cavitation of MBs by ultrasound (US) pulses is fundamental to achieve these applications. However, most MBs are coated with a soft shell that may undergo alteration (e.g., rupture or dissolution) under insonification and result in unintended acoustic responses. This work introduces a system of stable polymeric caged MBs in which the gas core is encapsulated within a rigid but nanoporous shell, so that their acoustic response is regulated by both shell compressibility and metastructure (i.e., porosity), thus permitting high control over their cavitation behaviors via pulse manipulation. These caged MBs are fabricated via a method of interfacial nanoprecipitation. Fabrication parameters can be varied to manipulate shell elasticity and porosity and subsequently control a wide range of acoustic properties such as tuning resonance frequency, controlling modes of nonlinear cavitation. Furthermore, the cavitation of caged MBs can be manipulated to develop tunable acoustic pressure sensors. These caged MBs offer insight of the acoustic–material relationship to design acoustic biomaterials for US-guided diagnostic and therapeutic technologies.

Superhydrophobic magnetic hollow carbon microspheres with hierarchical micro/nano-structure for ultrafast and highly-efficient multitasking oil-water separation

Nowadays, developing separation materials for oil-water mixture and even emulsion have drawn extensive interest, with increasing oily wastewater pollution. Among kinds of separation materials, hydrophobic magnetic particles can rapidly and high-efficiently separate oil from either oil-water mixtures or emulsions. However, they not only commonly suffer from complicated preparation procedures and poor resistance to harsh conditions, but also are almost unable to satisfy the demand of multitasking separations towards oil-water mixtures and micro/nanoemulsions with surfactant. Herein, a facile, simple and scalable approach is introduced to prepare superhydrophobic magnetic hollow carbon microspheres (SMHCMs) with micro/nano hierarchical surface. Thanks to the special wettability properties, micro/nano hierarchical surface, nano-porous shell and hollow cores, the as-prepared SMHCMs show an unprecedented absorption capacity of over 10 g/g for oil-water mixtures within an absorption time of 60 s as well as a superior separation efficiency of near 99% for surfactant-free and surfactant-stabilized micro/nanoemulsions at an extremely short time of as low as 10 s. In addition, the SMHCMs also possess stable performance, good reusability, durability and resistance to harsh environments (e.g., acidic, alkaline and high-saline). This work proposes a new prospect for designing advanced materials applied to the fast and highly-efficient oil-water separation.

Mulberry‐Like Core‐Shell Structured C@MnO2 as Electrode Material for Li–Ion Batteries and Pseudo‐Capacitors

AbstractIn present work, mulberry‐like core‐shell structured C@MnO2 was synthesized as electrodes for Li‐ion batteries and pseudo‐capacitors. Benefiting from the nanoporous shell and conductive carbon core that improve the ion transference and conductivity, the C@MnO2 delivers excellent electrochemical performance in terms of a stable lithium storage capacity of 553 mAh g−1 after 250 cycles under 0.5 A g−1 and a high capacitance up to 274.44F g−1 under 1 A g−1 in 6 M KOH and the capacity retention is 90.2% after 5000 cycles. Such electrochemical properties combined with the facile synthesis route make C@MnO2 promising electrode materials for energy storage.

Nitrogen-doped hollow carbon spheres as highly effective multifunctional electrocatalysts for fuel cells, Zn–air batteries, and water-splitting electrolyzers

Clean energy conversion technologies require low-cost multifunctional catalysts. In this work, nitrogen-doped hollow carbon spheres with nanoporous shells (NHCS–W) are prepared by using the ionic liquid 1-butyl-3-methylimidazolium chloride as a source of both carbon and nitrogen. A systematic investigation of the electrocatalytic performance and durability of NHCS–W reveals excellent performance and high stabilities for the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction in alkaline electrolytes. This multifunctionality facilitates promising practical applicability for NHCS–W. An alkaline exchange membrane fuel cell (AEMFC) with NHCS–W as the cathode electrocatalyst has a peak power density of 109 mW cm−2, which is 7 mW cm−2 higher than that of an AEMFC with Pt/C (20%) as the cathode electrocatalyst. By using NHCS–W as an air electrode, two rechargeable Zn–air batteries in series can power a 5 mm red light-emitting diode (~2.2 V). Moreover, a cell voltage of only 1.61 V is needed to achieve a current density of 10 mA cm−2 in a water-splitting electrolyzer employing NHCS–W as both the cathode and anode catalysts. This work reveals the potential of ionic liquid precursors for constructing nitrogen-doped carbon-based materials for a wide variety of electrocatalytic applications.

Enhanced Hydrogenation Performance over Hollow Structured Co-CoOx@N-C Capsules.

It is desirable to design nonprecious metal nanocatalysts with high stability and catalytic performance for fine chemicals production. Here, a method is reported for the preparation of cobalt metal and cobalt oxide cores confined within nanoporous nitrogen‐doped hollow carbon capsules. Core–shell structured Zn/Co‐ZIF@polymer materials are fabricated through a facile coating polymer strategy on the surface of zeolitic imidazolate frameworks (ZIF). A series of hollow carbon capsules with cobalt metal and cobalt oxide are derived from a facile confined pyrolysis of Zn/Co‐ZIF@polymer. The hollow Co‐CoOx@N‐C capsules can prevent sintering and agglomeration of the cobalt nanoparticles and the nanoporous shell allows for efficient mass transport. The specific surface area and Co particle size are optimized through finely tuning the original Zn content in ZIF particles, thus enhancing overall catalytic activity. The yolk–shell structured Zn4Co1Ox@carbon hollow capsules are shown to be a highly active and selective catalyst (selectivity >99%) for hydrogenation of nitrobenzene to aniline. Furthermore, Zn4Co1Ox@carbon hollow particles show superior catalytic stability, and no deactivation after 8 cycles of reaction. The hollow Co‐CoOx@N‐C capsules may shed light on a green and sustainable catalytic process for fine chemicals production.

On scale-dependent vibration of circular cylindrical nanoporous metal foam shells

In this paper, free vibrations of cylindrical nanoshells made of nanoporous metal foam are investigated for the first time. Based on the modified couple stress theory and Love’s thin shell theory, the governing equations of the present system are derived by using Hamilton’s principle. Two types of nanoporosity distribution are considered in the construction of the nanoporous shells. Then, the Navier method and Galerkin method are utilized to solve natural frequencies of the nanoporous shells under different boundary conditions. Afterwards, a detailed parametric study is conducted. Results show that the nanoporosity type, the material length scale parameter, the porosity coefficient, the length-to-radius ratio, and the radius-to-thickness ratio play important role on the free vibrations of nanoporous shells. To check the validity of the present analysis, the results are compared with those in previous studies for the special cases.

Porous Core–Shell CuCo2S4 Nanospheres as Anode Material for Enhanced Lithium‐Ion Batteries

Porous core-shell CuCo2 S4 nanospheres that exhibit a large specific surface area, sufficient inner space, and a nanoporous shell were synthesized through a facile solvothermal method. The diameter of the core-shell CuCo2 S4 nanospheres is approximately 800 nm" the radius of the core is about 265 nm and the thickness of the shell are approximately 45 nm, respectively. On the basis of the experimental results, the formation mechanism of the core-shell structure is also discussed. These CuCo2 S4 nanospheres show excellent Li storage performance when used as anode material for lithium-ion batteries. This material delivers high reversible capacity of 773.7 mA h g-1 after 1000 cycles at a current density of 1 A g-1 and displays a stable capacity of 358.4 mA h g-1 after 1000 cycles even at a higher current density of 10 A g-1 . The excellent Li storage performance, in terms of high reversible capacity, cycling performance, and rate capability, can be attributed to the synergistic effects of both the core and shell during Li+ ion insertion/extraction processes.

Ultrasound-assisted fabrication of metal nano-porous shells across polymer beads and their catalytic activity for reduction of 4-nitrophenol

Metal nano-porous architectures are a novel class of nanomaterials which has been applied in the fields of catalysis, sensing and gas storage because of their high surface-to-volume ratio, high mechanical strength and long-range ordered architectures. A commonly-used synthetic strategies to achieve architectures with high precision and diverse porosity design is the seed-and-growth method. In this work, using a dual-frequency sequential sonication approach, we have demonstrated a sonochemical-assisted one-pot seeding with a successive shell growth synthetic strategy for mesoporous metal deposition via a gold (Au) nanoparticle and poly(styrene) beads system. A uniform coating of gold nanoparticle seeds with dense surface coverage was formed by first employing 300 kHz ultrasound irradiation while the nano-porous shell growth was then performed under 1 MHz ultrasonic frequency. The precise control over the process conditions and parameters allowed for the design of well-defined shell thicknesses and surface roughness and area. The catalytic property of the MNMs was evaluated for the degradation of 4-nitrophenol and a high catalytic activity was achieved for the most porous gold structures, suggesting synergistic effects between the architecture of the nanomaterials and their surface reactivity.

Hollow Rh nanoparticles with nanoporous shell as efficient electrocatalyst for hydrogen evolution reaction

Studies on highly effective electrocatalysts for producing hydrogen via electrochemical water splitting is of great significance due to the increasing demand of clean and sustainable hydrogen energy economy. In this work, hollow RhCo nanoparticles with an average diameter of 21 nm were fabricated using the galvanic replacement method in a water-ice bath. Electrochemical evaluation showed that the as-synthesized electrocatalyst exhibited better performance for hydrogen evolution reaction (HER) under acid condition, with a lower overpotential of 28.1 mV at 10 mA cm−2 and a lower Tafel slope of 24 mV dec−1, in comparison with commercial Rh/C. The better HER performance of the as-prepared hollow Rh nanoparticles could be attributed to the larger surface area than commercial Rh nanoparticles, which could provide more active sites and therefore improve the electrocatalytic performance.

Hollow carbon spheres with nanoporous shells and tailored chemical interfaces as sulfur host for long cycle life of lithium sulfur batteries

Lithium sulfur batteries suffer from poor cycle life mainly due to the shuttle effect of dissolved polysulfides and volume expansion of electrode. Herein, we report hollow carbon spheres with nanoporous shells and tailored chemical interfaces (PHCS) via annealing precursors under hydrogen contained inert gas as sulfur host to realize ultralong cycle life of lithium sulfur battery. The obtained PHCS has a larger pore volume, a higher specific surface area and less oxygen content compared to the conventional treated HCS. Benefiting from the combination of the unique pore structure and chemical interfaces which induce stronger polysulfides adsorption ability and less unwanted side reactions, which were confirmed by the ultraviolet–visible (UV–vis) spectroscopy test and XPS measurement, the obtained PHCS/S cathode demonstrates excellent rate performance and ultralong cycle life. It delivers a high discharge capacity of 806.3 mAhg−1 at 4C rate and an ultralong cycle life with a low capacity decay rate of 0.037% after 1500 cycles at 2C rate. Moreover, it also supports high sulfur content (82%) and high sulfur loading (7.2 mg cm−2) with good electrochemical performance. The EIS analysis and post-mortem SEM measurements further confirm that the PHCS/S cathode electrode has more stable resistance and morphology characteristics, and the corresponding lithium anode suffers from less structural destruction and volume expansion, which may lead to the excellent battery performance.

Growing a hydrophilic nanoporous shell on a hydrophobic catalyst interface for aqueous reactions with high reaction efficiency and in situ catalyst recycling

We developed a strategy to construct a catalyst with a hydrophobic catalytic interface covered by a hydrophilic, nanoporous shell, which displays high catalytic efficiency and recyclability in aqueous reactions.

Assessment of in vivo systemic toxicity and biodistribution of iron-doped silica nanoshells

Silica nanoparticles are an emerging class of biomaterials which may be used as diagnostic and therapeutic tools for biomedical applications. In particular, hollow silica nanoshells are attractive due to their hollow core. Approximately 70% of a 500 nm nanoshell is hollow, therefore more particles can be administered on a mg/kg basis compared to solid nanoparticles. Additionally, their nanoporous shell permits influx/efflux of gases and small molecules. Since the size, shape, and composition of a nanoparticle can dramatically alter its toxicity and biodistribution, the toxicology of these nanomaterials was assessed. A single dose toxicity study was performed in vivo to assess the toxicity of 500 nm iron-doped silica nanoshells at clinically relevant doses of 10-20 mg/kg. This study showed that only a trace amount of silica was detected in the body 10 weeks post-administration. The hematology, biochemistry and pathological results show that the nanoshells exhibit no acute or chronic toxicity in mice.

Oxygen delivery using engineered microparticles

A continuous supply of oxygen to tissues is vital to life and interruptions in its delivery are poorly tolerated. The treatment of low-blood oxygen tensions requires restoration of functional airways and lungs. Unfortunately, severe oxygen deprivation carries a high mortality rate and can make otherwise-survivable illnesses unsurvivable. Thus, an effective and rapid treatment for hypoxemia would be revolutionary. The i.v. injection of oxygen bubbles has recently emerged as a potential strategy to rapidly raise arterial oxygen tensions. In this report, we describe the fabrication of a polymer-based intravascular oxygen delivery agent. Polymer hollow microparticles (PHMs) are thin-walled, hollow polymer microcapsules with tunable nanoporous shells. We show that PHMs are easily charged with oxygen gas and that they release their oxygen payload only when exposed to desaturated blood. We demonstrate that oxygen release from PHMs is diffusion-controlled, that they deliver approximately five times more oxygen gas than human red blood cells (per gram), and that they are safe and effective when injected in vivo. Finally, we show that PHMs can be stored at room temperature under dry ambient conditions for at least 2 mo without any effect on particle size distribution or gas carrying capacity.

Mesoporous hollow carbon spheres for lithium-sulfur batteries: distribution of sulfur and electrochemical performance.

Hollow carbon spheres (HCS) with a nanoporous shell are promising for the use in lithium–sulfur batteries because of the large internal void offering space for sulfur and polysulfide storage and confinement. However, there is an ongoing discussion whether the cavity is accessible for sulfur. Yet no valid proof of cavity filling has been presented, mostly due to application of unsuitable high-vacuum methods for the analysis of sulfur distribution. Here we describe the distribution of sulfur in hollow carbon spheres by powder X-ray diffraction and Raman spectroscopy along with results from scanning electron microscopy and nitrogen physisorption. The results of these methods lead to the conclusion that the cavity is not accessible for sulfur infiltration. Nevertheless, HCS/sulfur composite cathodes with areal sulfur loadings of 2.0 mg·cm−2 were investigated electrochemically, showing stable cycling performance with specific capacities of about 500 mAh·g−1 based on the mass of sulfur over 500 cycles.

High-performance supercapacitor based on nanocages with nanoporous thin-walled graphitic shells

Thin-walled graphitic nanocages (GNCs) made up from highly nanoporous shells have been prepared by removing nitrogen-doping (N-doping) template. N-doping template is carboxyl groups and nitro-groups, which have formed from oxidation of N-doped structure of GNCs by acid-treatment. After partially removing N-doping template by annealing in vacuum, nanopores are created in the shells of the thin-walled GNCs. Thus, their specific surface area (1100 vs. 700 m2 g−1) and specific capacitance (370 vs. 300 F g−1 at current density of 0.1 A g−1) are sharply raised. With current density increasing to 1 A g−1, the capacitance of the GNCs is still high (230 F g−1), indicating such GNCs is a superb electrode material of supercapacitor.