Heterostructures composed of two-dimensional (2D) nanosheets and zero-dimensional (0D) nanoparticles (NPs) have attracted increasing attention because of the synergy arising from the coupling interactions between the two mixed-dimensional components. Despite recent advances, it remains a challenge to fabricate 2D/0D heterostructures with clean and accessible surfaces, which is highly desirable for the diversity of catalytic, sensing, and energy storage applications. Herein, we report a generalized methodology that enables the facile assembly of sandwich-like 0D/2D/0D heterostructures with facilitated mass-transport channels and exposed surface active sites. A ligand-exchange strategy with HBF₄ is employed to strip off the surface-coating ligands of colloidal NPs, rendering them positively charged and dispersible in polar solvents. This allows subsequent electrostatic assembly of NPs with oppositely charged 2D nanosheets to afford sandwich-like 0D/2D/0D heterostructures. The barely covered surfaces and the advantageous architectures of such sandwich-like 0D/2D/0D heterostructures induce the desired synergistic effect, making them particularly suitable for electrochemical energy storage and conversion. We demonstrate this by employing MXene/NiFe₂O₄ and MXene/Fe₃O₄ heterostructures for high-performance electrocatalytic oxygen evolution and supercapacitors, respectively.

Carbon coating has been a routine strategy for improving the performance of Si-based anode materials for lithium-ion batteries. The ability to tailor the thickness, homogeneity and graphitization degree of carbon-coating layers is essential for addressing issues that hamper the real applications of Si anodes. Herein, we report the construction of two-dimensional (2D) assemblies of interconnected Si@graphitic carbon yolk-shell nanoparticles (2D-Si@gC) from commercial Si powders by exploiting oleic acid (OA). The OA molecules act as both the surface-coating ligands for facilitating 2D nanoparticle assembly and the precursor for forming uniform and conformal graphitic shells as thin as 4 nm. The as-prepared 2D-Si@gC with rationally designed void space exhibits excellent rate capability and cycling stability when used as anode materials for lithium-ion batteries, delivering a capacity of 1, 150 mAh·g^-1 at an ultrahigh current density of 10 A·g^-1 and maintaining a stabilized capacity of 1, 275 mAh·g^-1 after 200 cycles at 4 A·g^-1. The formation of yolk-shell nanoparticles confines the deposition of solid electrolyte interphase (SEI) onto the outer carbon shell, while simultaneously providing sufficient space for volumetric expansion of Si nanoparticles. These attributes effectively mitigate the thickness variations of the entire electrode during repeated lithiation and delithiation, which combined with the unique 2D architecture and interconnected graphitic carbon shells of 2D-Si@gC contributes to its superior rate capability and cycling performance.

Mesoporous carbons have been widely utilized as the sulfur host for lithium-sulfur (Li-S) batteries. The ability to engineer the porosity, wall thickness, and graphitization degree of the carbon host is essential for addressing issues that hamper commercialization of Li-S batteries, such as fast capacity decay and poor high-rate performance. In this work, highly ordered, ultrathin mesoporous graphitic-carbon frameworks (MGFs) having unique cage-like mesoporosity, derived from self-assembled Fe₃O₄ nanoparticle superlattices, are demonstrated to be an excellent host for encapsulating sulfur. The resulting S@MGFs exhibit high specific capacity (1, 446 mAh·g^-1 at 0.15 C), good rate capability (430 mAh·g^-1 at 6 C), and exceptional cycling stability (~0.049% capacity decay per cycle at 1 C) when used as Li-S cathodes. The superior electrochemical performance of the S@MGFs is attributed to the many unique and advantageous structural features of MGFs. In addition to the interconnected, ultrathin graphitic-carbon framework that ensures rapid electron and lithium-ion transport, the microporous openings between adjacent mesopores efficiently suppress the diffusion of polysulfides, leading to improved capacity retention even at high current densities.

Three-dimensional (3D) graphene has recently attracted enormous attention for electrochemical energy storage applications. However, current methods suffer from an inability to simultaneously control and engineer the porosity and morphology of the graphene frameworks. Here, we report the designed synthesis of ordered mesoporous graphene spheres (OMGSs) by transformation of self-assembled Fe₃O₄ nanocrystal superlattices. The resultant OMGSs have an ultrathin framework comprising few-layered graphene, with highly ordered and interconnected mesoporosity and a high surface area. These advantageous structural and textural features, in combination with the excellent electrical conductivity of the graphitic frameworks, render the OMGSs an ideal and general platform for creating hybrid materials that are well suited for use as composite electrodes in lithium-ion batteries (LIBs). As a proof-of-concept demonstration, SnO₂ and GeO₂ nanoparticles are incorporated into the OMGSs to afford SnO₂@OMGSs and GeO₂@OMGSs, respectively, both of which exhibit outstanding lithium storage properties when used as LIB anodes.