Assembly of different metal-organic framework (MOF) building blocks into hybrid MOF-on-MOF heterostructures is promising in chemistry and materials science, however the development of ternary MOF-on-MOF heterostructures with controllable architectural and compositional complexity is challenging. Here we report the synthesis of three types of ternary MOF-on-MOF heterostructures via a multiple selective assembly strategy. This strategy relies on the choice of one host MOF with more than one facet that can arrange the growth of a guest MOF, where the arrangement is site-selective without homogenous growth of guest MOF or homogenous coating of guest on host MOF. The growth of guest MOF on a selected site of host MOF in each step provides the opportunity to further vary the combinations of arrangements in multiple steps, leading to ternary MOF-on-MOF heterostructures with tunable complexity. The developed strategy paves the way towards the rational design of intricate and unprecedented MOF-based superstructures for various applications.

Construction of semiconducting metal-organic framework based heterostructures is emerging as a promising strategy for enhancing their photocatalytic performances. Here, tetrapod-like NH 2 -MIL-125@ZnS heterostructures with tunable heterojunction coverage and well-matched band structure are synthesized as photocatalysts. When used for photocatalytic O 2 reduction reaction, a heterojunction coverage of ≈45.1% leads to the best performance with a hydrogen peroxide production rate of ≈120 mm g −1 h −1 , outperforming control samples with either larger or smaller heterojunction coverages and most reported MOF based photocatalysts in similar reaction systems.

Constructing MOF‐on‐MOF heterojunction with elaborate charge transfer mechanism and interface is a promising strategy for improving the photocatalytic properties of MOFs. Herein, a Step‐scheme (S‐scheme) MIL‐125‐NH2@CoFe Prussian blue analogue (PBA) heterojunction is reported for the first time. The MOF‐on‐MOF heterostructure exhibits a sandwich‐like morphology with hollow CoFe PBA nanocages selectively assembled on the top‐down surfaces of MIL‐125‐NH2 nanocakes. Experimental findings and theoretical simulation results reveal the formation of internal electric field via interfacial TiOCo bonds at the heterojunction, providing driving force and atomic transportation highway for accelerating the S‐scheme charge transfer and enhancing the redox performance. Contributed further by the hollow sandwich‐like structures with increased active site exposure, the designed MOF‐on‐MOF heterojunction exhibits significantly enhanced photocatalytic activity for degradation of various organic pollutants. This study provides insights toward the rational design of semiconducting MOF‐based heterojunctions with improved properties.

Anisotropy plays a unique role in the structural regulation of metal-organic frameworks (MOFs) and their composites, especially at the micro-and nanoscale. However, there is a lack of a understanding of MOF micro/ nanoparticles (MNPs) from the perspective of morphological anisotropy. In this Minireview, recent advances in anisotropic MOF MNPs are summarized, with a focus on how morphological anisotropy leads to innovative structures and modulates properties. First, anisotropic pristine MOF MNPs with diverse morphologies are introduced and classified by their morphology-dependent and morphology-independent anisotropy. Secondly, the anisotropy-enabled site-selective higher-order construction of MOF-based materials is highlighted. Finally, challenges and prospects for anisotropic MOFs are discussed, aiming to provide inspiration for further developments in this interesting research field.

among which transitionmetal sulfides (TMSs) have received enormous interest. [10][11][12] The electrocatalytic performance of TMSs is mainly affected by their composition and architecture. [13][14][15] To adjust the composition, hetero-element doping (e.g., N, [16] O, [17] P, [18] Fe, [19] Cu [20] ) and nanoscale hybridization (e.g., carbon, [21] CeOx, [22] MoS 2 nanoparticles [23] ) have been used to prepare composite TMSs. Recently, titanium-based materials were reported as promising dopants to enhance the OER performances of metal oxides or hydroxides by tuning the electronic structure of active metal centers and improving the conductivity/stability. [24][25][26][27][28] However, the Ti doping strategy is rarely reported for improving the OER performance of TMSs. [29] Among various architectures, hollow structures have shown great promise as electrocatalysts due to high surface area, abundant active sites and shortened mass diffusion distance. [30][31][32][33][34][35][36] Most reported hollow structures exist as isolated particles, limiting the electron transfer between adjacent hollow structures. [37,38] To overcome this problem, assembly of hollow structures into hollow superstructure (HSS) is a newly emerged strategy, using interconnected hollow structures as building blocks to promote electron transfer. [39][40][41] For example, metalorganic frameworks (MOFs) have been utilized as precursors to fabricate individual hollow TMSs [17,22,[42][43][44][45][46] or TMS HSS assisted by additional templates (e.g., polymer fiber, colloid nanoparticles). [39,40,47] When compared with the individual hollow TMSs, the sophisticated TMS HSS exhibited enhanced performances in supercapacitors, photocatalysis, and lithium batteries, but rarely in OER applications. Moreover, current synthesis of TMSbased HSS mainly relied on one type of MOFs with a single metal center, limiting the compositional diversity. Ti-doped TMSs with hollow superstructures have not been reported to our knowledge.The intensive research of MOFs has led to a novel family of MOF-on-MOF hybrid materials conjugated by two or more distinct MOFs units. [48] Similar to single MOFs, MOF-on-MOF hybrids can be converted to carbon and metal oxides using heat treatment. [49][50][51][52] However, the development of MOF-on-MOF chemistry is still in its infancy, the conversion of MOF-on-MOF hybrids via sulfidation to composite metal sulfides remains largely unexplored. The competitive sulfidation of multiple metal centers is expected to provide delicate composition and Transition-metal sulfides (TMSs) are attractive oxygen evolution reaction (OER) electrocatalysts. Developing new strategies to improve their electrochemical performance of TMSs is of great significance. Herein, a unique pacman-like titanium-doped cobalt sulfide hollow superstructure (Ti-CoSx HSS) is fabricated as an OER electrocatalyst. Using a prearranged metalorganic framework (MOF)-on-MOF heterostructure as a precursor treated by one-pot sulfidation, a sequential structural conversion process leads t...