Activation of the Solid Silica Layer of Aerosol-Based C/SiO₂ Particles for Preparation of Various Functional Multishelled Hollow Microspheres

Double-shelled C/SiO₂ hollow microspheres with an outer nanosheet-like silica shell and an inner carbon shell were reported. C/SiO₂ aerosol particles were synthesized first by a one-step rapid aerosol process. Then the solid silica layer of the aerosol particles was dissolved and regrown on the carbon surface to obtain novel C/SiO₂ double-shelled hollow microspheres. The new microspheres prepared by the facile approach possess high surface area and pore volume (226.3 m² g^–1, 0.51 cm³ g^–1) compared with the original aerosol particles (64.3 m² g^–1, 0.176 cm³ g^–1), providing its enhanced enzyme loading capacity. The nanosheet-like silica shell of the hollow microspheres favors the fixation of Au NPs (C/SiO₂/Au) and prevents them from growing and migrating at 500 °C. Novel C/C and C/Au/C (C/Pt/C) hollow microspheres were also prepared based on the hollow nanostructure. C/C microspheres (482.0 m² g^–1, 0.92 cm³ g^–1) were ideal electrode materials. In particular, the Au NPs embedded into the two carbon layers (C/Au/C, 431.2 m² g^–1, 0.774 cm³ g^–1) show a high catalytic activity and extremely chemical stability even at 850 °C. Moreover, C/SiO₂/Au, C/Au/C microspheres can be easily recycled and reused by an external magnetic field because of the presence of Fe₃O₄ species in the inner carbon shell. The synthetic route reported here is expected to simplify the fabrication process of double-shelled or yolk–shell microspheres, which usually entails multiple steps and a previously synthesized hard template. Such a capability can facilitate the preparation of various functional hollow microspheres by interfacial design

Preparation of Double-Shelled C/SiO₂ Hollow Spheres with Enhanced Adsorption Capacity

In this study, double-shelled C/SiO₂ hollow spheres with an outer hydrophilic silica shell and an inner hydrophobic carbon shell were initially prepared by activating a solid silica layer of C/SiO₂ aerosol particles. This low-cost preparation technique, which can easily be scaled up, includes a rapid aerosol process and a subsequent dissolution–regrowth process. The large surface area (226.3 m²/g), high pore volume (0.51 cm³/g), and high mechanical stability of the spheres benefit their high adsorption capacities for methylene blue (MB) and metal ions. The novel spheres show a high adsorption capacity of 171.2 mg/g for MB, which is higher than the adsorption capacity of single-shelled silica hollow spheres (150.0 mg/g). The adsorption efficiency of the hollow spheres remains higher than 95% after five cycles of regeneration. The saturation adsorption values of Pb²⁺ and Ag⁺ ions on the hollow spheres were found to be 216.5 and 283.1 mg/g, respectively, which are higher than the corresponding values of 189.8 and 213.4 mg/g on the single-shelled SiO₂ spheres. Moreover, the adsorption capacities of the five-times-recycled spheres for Pb²⁺ and Ag⁺ ions reached as high as ∼180 and ∼245 mg/g, respectively. These results reveal that the outer porous silica layer with a ζ-potential of −37.4 mV makes the main contribution to the excellent adsorption performance of the spheres. In addition to the contribution to the adsorption capacity of the double-shelled hollow spheres, the inner carbon layer plays a crucial role in supporting the outer silica shell and in improving the adsorption efficiency, mechanical stability, and recycling properties of the hollow spheres

Shape-Controlled Synthesis of Magnetic Iron Oxide@SiO₂–Au@C Particles with Core–Shell Nanostructures

The preparation of nonspherical magnetic core–shell nanostructures with uniform sizes still remains a challenge. In this study, magnetic iron oxide@SiO₂–Au@C particles with different shapes, such as pseduocube, ellipsoid, and peanut, were synthesized using hematite as templates and precursors of magnetic iron oxide. The as-obtained magnetic particles demonstrated uniform sizes, shapes, and well-designed core–shell nanostructures. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) analysis showed that the Au nanoparticles (AuNPs) of ∼6 nm were uniformly distributed between the silica and carbon layers. The embedding of the metal nanocrystals into the two different layers prevented the aggregation and reduced the loss of the metal nanocrystals during recycling. Catalytic performance of the peanut-like particles kept almost unchanged without a noticeable decrease in the reduction of 4-nitrophenol (4-NP) in 8 min even after 7 cycles, indicating excellent reusability of the particles. Moreover, the catalyst could be readily recycled magnetically after each reduction by an external magnetic field

Preparation of Magnetic Composite Hollow Microsphere and Its Adsorption Capacity for Basic Dyes

Magnetic microspheres with an Fe₃O₄ core and a SiO₂–TiO₂ hybrid shell were prepared by a surfactant-assisted aerosol process and subsequent etching treatment. The core–shell spheres with robust and chemically stable Ti–O–Si shells exhibit excellent adsorption performance toward basic dyes. The maximum adsorption capacities were obtained at 147 mg/g for methylene blue (MB) and 124.6 mg/g for basic fuchsin. MB with an initial concentration of 20 mg/L can be completely removed in 5 min at a dosage of 0.5 mg/L, and the equilibrium time is 90 min in the MB concentration range 20–250 mg/L. The adsorption kinetics follows the pseudo-second-order model. Furthermore, the dye saturated microspheres can be easily recycled by an external magnetic field and regenerated using 1–3 wt % NaOH aqueous solution. After six recycle runs, 98% of the adsorption capacity was still retained. The low-cost magnetic hollow spheres with good adsorption capacity are a promising candidate for water treatment

Multishelled Nickel–Cobalt Oxide Hollow Microspheres with Optimized Compositions and Shell Porosity for High-Performance Pseudocapacitors

Nickel–cobalt oxides/hydroxides have been considered as promising electrode materials for a high-performance supercapacitor. However, their energy density and cycle stability are still very poor at high current density. Moreover, there are few reports on the fabrication of mixed transition-metal oxides with multishelled hollow structures. Here, we demonstrate a new and flexible strategy for the preparation of hollow Ni–Co–O microspheres with optimized Ni/Co ratios, controlled shell porosity, shell numbers, and shell thickness. Owing to its high effective electrode area and electron transfer number (<i>n</i>^3/2 <i>A</i>), mesoporous shells, and fast electron/ion transfer, the triple-shelled Ni–Co_1.5–O electrode exhibits an ultrahigh capacitance (1884 F/g at 3A/g) and rate capability (77.7%, 3–30A/g). Moreover, the assembled sandwiched Ni–Co_1.5–O//RGO@Fe₃O₄ asymmetric supercapacitor (ACS) retains 79.4% of its initial capacitance after 10 000 cycles and shows a high energy density of 41.5 W h kg^–1 at 505 W kg^–1. Importantly, the ACS device delivers a high energy density of 22.8 W h kg^–1 even at 7600 W kg^–1, which is superior to most of the reported asymmetric capacitors. This study has provided a facile and general approach to fabricate Ni/Co mixed transition-metal oxides for energy storage

Morphology Control of TiO₂ Nanoparticle in Microemulsion and Its Photocatalytic Property

TiO₂ nanoparticles with controlled morphology and high photoactivity were prepared using a microemulsion-mediated hydrothermal method in this study, and the particles were characterized by means of TEM, XRD, BET, and BJH analysis. As the hydrothermal temperature is elevated, mean pore diameter, crystalline size, and crystallinity of the particles increase gradually, while the surface area decreases significantly, and the morphology changes from a spherical into a rod-like shape. The morphology transition mechanism of the TiO₂ crystal has been put forward based on a decrease in intensity of the microemulsion interface and an increase in collision efficiency between droplets with increasing the hydrothermal temperature. The photocatalytic activity of the TiO₂ particles synthesized at 120–200 °C is relatively low due to their weak crystallinity, though they have high surface area of 146–225 m²/g and small crystalline size of 6–10 nm. However, the TiO₂ samples prepared at 250–350 °C with low surface area (28–90 m²/g) exhibit high activity on the degradation of Rhodamine B (RhB), which is comparable or higher than that of the commercial P-25. The reason is ascribed to their high crystallinity that determines material activity in this temperature region. This study reveals that the effects of the surface area, crystallinity, and crystalline size on TiO₂ activity are interdependent, and the balance between these factors is important for improving the photoactivity of the catalyst

Rapidly Constructing Multiple AuPt Nanoalloy Yolk@Shell Hollow Particles in Ordered Mesoporous Silica Microspheres for Highly Efficient Catalysis

In this work, for the first time, AuPt alloy yolk@shell hollow nanoparticles (NPs) were constructed and simultaneously embedded into hollow interiors of a mesoporous silica microsphere based on a rapid aerosol process (AuPt@SiO₂). Resin nanospheres were utilized both as a hard template to create hollow interiors inside the mesoporous silica microspheres and as carriers to transport pregrown metal nanocrystals, AuPt alloy clusters, into the microspheres. Calcination removes the resin nanospheres and causes metal nanocrystals to embed into the hollow interiors of the silica microspheres. Due to the unique yolk@shell hollow structure of the AuPt nanoalloy, ordered mesopores (67 nm) in the silica support, the synergetic effect between the AuPt alloy and the high surface area and pore volume of the microspheres, the AuPt@SiO₂ spheres showed an excellent catalytic performance for styrene epoxidation with the conversion and selectivity of 85% and 87%, respectively. Notably, the novel catalyst showed a stable catalytic performance after five cycles of usage, suggesting the possible practical applications of the AuPt nanoalloy catalyst. In addition, the catalyst also exhibited a higher activity than the commercial Pt/C catalyst for the reduction reaction of 4-nitrophenol. The approach reported in this study could potentially be used to simplify the fabrication process of yolk@shell or hollow metal nanospheres, facilitating encapsulation of monometallic and multimetallic metal nanocrystals with various nanostructures and compositions into porous supports and thus guiding the design of catalysts with a special structure and high-performance

Hybrid Control Mechanism of Crystal Morphology Modification for Ternary Solution Treatment via Membrane Assisted Crystallization

Herein, the hybrid control mechanism of the crystal morphology modification for the treatment of a classic industrial ternary solution system (NaCl–EG–H₂O) via membrane assisted crystallization was demonstrated. Solution concentration and component diffusion played the hybrid role on determining the polymorphic outcome of the crystal products. Metastable zone width under various operation temperatures and solution composition was simulated and validated by experimental results. The impact of the dominating growth mechanism (diffusion controlled growth or polynuclear growth) on the crystal morphology was also investigated. An optimized operation route aimed to simultaneously improve the crystal morphology and the separation effect was then developed based on the hybrid control mechanism. The solvent loss decreased from 4.8 to 1.2 wt %. Benefitting from the improved crystal morphology, the operative duration of corresponding downstream was also shortened. Advanced membrane assisted crystallization is a promising technology toward targeted crystal morphology for high-end solid products

Scalable SPAN Membrane Cathode with High Conductivity and Hierarchically Porous Framework for Enhanced Ion Transfer and Cycling Stability in Li–S Batteries

Lithium sulfur batteries with a high energy density of 2600 Wh kg–1 and theoretical specific capacity of 1675 mAh g–1 have been regarded as the most promising candidate for the next generation of high-energy storage devices. However, their commercial application is hindered by the undesirable troubles of rapid capacity fading, insulation of the products (Li2S/Li2S2), volume expansion, and low mass loading. Herein, a three-dimensional holey CNT/sulfurized polyacrylonitrile (CNT@SPAN) freestanding cathode has been fabricated by a one-step phase inversion method, followed by sulfurization without any binders and current collectors. The unique porous framework design with SPAN in situ encapsulating CNT can effectively facilitate the transportation of ions and electrons, and endure the volume expansion of sulfur during the reaction process. Simultaneously, by combining electrochemical impedance analysis and frontier molecular orbit theory, the initial activation mechanism of the Li-SPAN battery was explored. In the initial state of cell activation, Li+ occupies the carbon skeleton continuously and irreversibly, which enhances the conductivity of the composites. This work refreshes the current performance of Li-SPAN batteries with a maximum areal capacity of 10.21 mAh cm–2 at an ultrahigh mass loading of 7.5 mg cm–2, and an excellent rate capacity of 761.7 mAh g–1 at 4 C, which provides a promising method to make Li–S batteries to meet the requirements of commercial application

Highly Active Nanoreactors: Patchlike or Thick Ni Coating on Pt Nanoparticles Based on Confined Catalysis

Catalyst-containing nanoreactors have attracted considerable attention for specific applications. Here, we initially report preparation of PtNi@SiO₂ hollow microspheres based on confined catalysis. The previous encapsulation of dispersed Pt nanoparticles (NPs) in hollow silica microspheres ensures the formation of Pt@Ni coreshell NPs inside the silica porous shell. Thus, the Pt NPs not only catalyze the reduction of Ni ions but also direct Ni deposition on the Pt cores to obtain Pt@Ni core–shell catalyst. It is worthy to point out that this synthetic approach helps to form a patchlike or thick Ni coating on Pt cores by controlling the penetration time of Ni ions from the bulk solution into the SiO₂ microspheres (0.5, 1, 2, or 4 h). Notably, the Pt@Ni core–shell NPs with a patch-like Ni layer on Pt cores (0.5 and 1 h) show a higher H₂ generation rate of 1221–1475 H₂ mL min^–1 g^–1_cat than the Pt@Ni NPs with a thick Ni layer (2 and 4 h, 920–1183 H₂ mL min^–1 g^–1_cat), and much higher than that of pure Pt NPs (224 H₂ mL min^–1 g^–1_cat). In addition, the catalyst possesses good stability and recyclability for H₂ generation. The Pt@Ni core–shell NPs confined inside silica nanocapsules, with well-defined compositions and morphologies, high H₂ generation rate, and recyclability, should be an ideal catalyst for specific applications in liquid phase reaction