Framework confinement of multi-metals within silica hollow spheres by one-pot synthesis process.

The control incorporation of metals in silica hollow spheres (SHSs) may bring new functions to silica mesoporous structures for applications including catalysis, sensing, molecular delivery, adsorption filtration, and storage. However, the strategies for incorporating metals, whether through pre-loading in the hollow interior or post-encapsulation in the mesoporous shell, still face challenges in achieving quantitative doping of various metals and preventing metal aggregation or channel blockage during usage. In this study, we explored the doping of different metals into silica hollow spheres based on the dissolution-regrowth process of silica. The process may promote the formation of more structural defects and functional silanol groups, which could facilitate the fixation of metals in the silica networks. With this simple and efficient approach, we successfully achieved the integration of ten diverse metal species into silica hollow sphere (SHS). Various single-metal, dual-metal, triple-metal, and quadruple-metal doped SHSs have been prepared, with the doped metals being stable and homogeneously dispersed in the structure. Based on the structural characterizations, we analyzed the influence of metal types on the morphology features of SHSs. The synergistic effects of multi-metals on the catalysis applications were also studied and compared.

Significance of this work: The control incorporation of metals in silica hollow spheres (SHSs) may bring new functions to silica mesoporous structures for applications including catalysis, sensing, molecular delivery, adsorption filtration, and storage. The incorporation of metals within SHSs is always either at the interior core or in the porous shells. The former method mainly utilizes metal nanoparticles as the core and regulates the synthesis of outer porous silica shells. The latter is primarily driven by the capillary force or intermolecular interactions with surface ligands to facilitate the post-loading of metal species in porous silica structures. The main problems associated with metal-doped SHSs include 1) controlled loading of different metals with a homogeneous distribution; 2) fixation of metal species in the structures to prevent aggregation during usage, particularly at high temperatures; 3) pore channel blockage after metal loading, which may hinder the loading of other external molecules. In this work, we developed the dissolution-regrowth of silica strategy for integrating various metals in porous SHSs (M@SHSs) by a one-pot hydrothermal process without using any anchoring molecules. Unlike other sol-gel formations, the growth rate of silica in this process is greatly reduced. It thus may bring more possibilities to introduce external metals within the silica frameworks instead of in the porous channels. By regulating the addition of metal salts in the silica nanoparticles dispersions, we have successfully synthesized stable and highly homogeneous single-metal, dual-metal, triple-metal, and quadruplemetal doped SHSs. Based on the structural characterizations, we analyzed the influence of metal types on the morphology features of SHSs. The synergistic effects of multi-metals on the catalysis applications were also studied and compared. Our results offer a facile and effective strategy for preparing multi-metals as nano-catalysts. Through proper design of the doped metals in SHSs, the structures should find more applications in catalysis, drug delivery, and adsorption with unique and enhanced properties.

Fence-Type Molecular Electrocatalysts for High-Performance Lithium-Sulfur Batteries.

Improving the slow redox kinetics of sulfur species and shuttling issues of soluble intermediates induced from the multiphase sulfur redox reactions are crucial factors for developing the next-generation high-energy-density lithium-sulfur (Li-S) batteries. In this study, we successfully constructed a novel molecular electrocatalyst through in situ polymerization of bis(3,4-dibromobenzene)-18-crown-6 (BD18C6) with polysulfide anions on the cathode interface. The crown ether (CE)-based polymer acts as a spatial "fence" to precisely control the unique redox characteristics of sulfur species, which could confine sulfur substance within its interior and interact with lithium polysulfides (LiPSs) to optimize the reaction barrier of sulfur species. The "fence" structure and the double-sided Li+ penetrability of the CE molecule may also prevent the CE catalytic sites from being covered by sulfur during cycling. This new fence-type electrocatalyst mitigates the "shuttle effect", enhances the redox activity of sulfur species, and promotes the formation of three-dimensional stacked lithium sulfide (Li2S) simultaneously. It thus enables lithium-sulfur batteries to exhibit superior rate performance and cycle stability, which may also inspire development facing analogous multiphase electrochemical energy-efficient conversion process.

Coordinated regulation of phosphorus/nitrogen doping in fullerene-derived hollow carbon spheres and their synergistic effect for the oxygen reduction reaction.

Fullerene-derived carbons have been demonstrated as effective electrode materials for electrocatalytic reactions. The heteroatoms in the carbon matrix are essential to enhance their electrocatalytic performance but are still challenging for effective doping strategies and understanding their synergistic effect. Herein, we regulate the phosphorus/nitrogen (P/N) doping in the carbon structure based on the control mixing of pyritic acid (PA) with the assembled diamine-C60 hollow spheres (N@FHS). After pyrolysis, the carbon spheres are shown to have a homogenous distribution of N and P (NP@CHS). The structural and molecular analysis reveals that the doping of P may facilitate the formation of graphitic N in the carbon framework. When used as electrocatalysts for the oxygen reduction reaction (ORR), NP@CHSs exhibit superior oxygen reduction reaction (ORR) performance in contrast to those of fullerene-derived carbon with single N doping and the commercial Pt/C (20 wt%) catalyst. Density functional theory (DFT) studies indicate that P/N-doping promotes the charge transfer in the carbon structure owing to its strong electronegativity. The enhanced ORR activity should be mainly due to the P- and N-coordinated neighboring C sites with the defective fullerene pentagon ring.