The precise synthesis of etched metal-organic frameworks and their applications in electrochemistry

Template method

Based on the nature of the templating materials, template-based synthesis strategies can be categorized into hard-templating and soft-templating approaches^[43,44]. Hard/soft templating etching removes embedded sacrificial template substances inside MOF crystals after framework growth. An ideal template should combine ease of preparation with controlled etchability, thereby enabling the precise preservation of the desired MOF architectures and morphological features.

Hard template strategies

Hard templates, including SiO₂, metals, and polymers, typically exhibit well-defined and tunable geometries that facilitate the formation of core-shell architectures. Subsequent removal of the core template allows the preservation of the shell’s original morphology^[45]. The fabrication of hollow MOFs via hard-templating generally involves three key steps: first, synthesis of a template with specific shape and dimensions; second, controlled growth of the MOF shell over the template surface; and finally, removal of the core to yield hollow structures^[46]. For instance, Wang et al. constructed nitrogen-doped carbon hollow microtubules (denoted as N-GCT) using ZnO nanorods as a sacrificial template in a ZnO@zeolitic imidazolate framework-8 (ZIF-8) system [Figure 2]^[47]. Their study revealed that the shell thickness could be precisely modulated by varying the reaction time or temperature. After etching with KOH, the resulting N-GCT retained a well-defined hollow architecture without structural collapse, and exhibited high specific surface area and pore volume, resulting in enhanced performance as a cathode material in lithium-sulfur batteries.

Figure 2. Schematic representation of the preparation of N-GCT; This figure is adapted from Ref.^[47]. Copyright@2022, Youke Publishing. DMF: N,N-dimethylformamide; ZIF: zeolitic imidazolate framework; N-GCT: nitrogen-doped carbon hollow microtubule.

Beyond conventional metal oxide templates, MOFs have been employed as sacrificial templates to fabricate hollow polyhedral architectures^[48,49]. The surfactant cetyltrimethylammonium bromide (CTAB) was critical in selectively inhibiting the growth of the (100) crystal facets, promoting the formation of a well-defined cubic architecture^[50]. This enabled the subsequent formation of ZIF-67/Co-Fe Prussian blue analog (PBA) multilayer nanoboxes. Finally, during annealing, the ZIF-67 template was etched away, resulting in hollow nanoboxes with maintained structural integrity.

Among various hard-template options, polystyrene (PS) spheres are widely utilized for constructing hollow MOFs and MOF-based composites owing to their mild synthesis and removal conditions^[51]. Carboxyl groups on the PS surface facilitate zinc ion coordination, promoting heterogeneous nucleation and growth of ZIF-8 on the template. Subsequent immersion selectively removes the PS core, yielding intact hollow microspheres^[52]. By precisely controlling the growth cycles, the ZIF-8 shell thickness can be finely tuned, enabling optimization of the material’s physicochemical and functional properties^[53]. Furthermore, the versatility of PS templates allows the engineering of more complex architectures, such as hollow double-shell MOF structures through iterative templating strategies.

Despite its utility in fabricating hollow MOFs, the hard-template method encounters several challenges during the etching process. These include difficulty in completely removing the template, limited structural stability of the resulting MOFs, imprecise control over pore size distribution, and overall procedural complexity^[54]. In contrast, soft-templating strategies offer notable advantages: milder template removal conditions, finer regulation of pore architecture, improved structural integrity of the MOFs, and a more streamlined synthesis process^[55]. These characteristics make soft-template etching particularly suitable for constructing MOFs with hierarchical porosity, well-defined hollow morphologies, and tailored functionality^[56].

Exchange of metal ions

A wide variety of MOFs with diverse compositions and structural features have been extensively investigated. Key strategies for tailoring MOF architectures include central metal ion exchange, ligand etching and substitution, defect engineering, template-guided assembly, and ligand functionalization. While extending the length of organic linkers can facilitate the formation of mesopores of varying sizes, it often compromises the framework stability due to reduced coordination connectivity^[65]. Among these approaches, etching and exchanging central metal ions represent one of the most straightforward routes to structural modification^[66]. Exchange of metal ions selectively dissolves or replaces intrinsic metal centers of the parent MOF skeleton via acid or competing metal ions. These processes not only alter the chemical composition but also enable precise control over morphology and crystalline orientation. Commonly employed metal ion etching techniques involve proton acid etching and competitive metal ion displacement, which typically target specific crystal facets or sites to create enlarged mesopores or customized morphological features^[67,68].

Acid etching serves as a highly effective route for constructing hierarchical porous MOF architectures. By selectively cleaving coordination bonds between metal nodes and organic ligands using proton acids, this method enables precise control over MOFs’ morphology^[69], facilitating the creation of hollow and highly porous structures. Tannic acid forms a protective coating on the external surface, while the released protons diffuse into the framework and protonate the N-donor sites of the 2-methylimidazolate linkers^[70]. This protonation decreases the electron-donating ability of the linkers and progressively weakens the Co-N coordination bonds, leading to preferential dissolution of the internal framework and the formation of hollow ZIF-67-derived structures. Liu et al. developed a synergistic protection etching strategy to fabricate hollow ZIF-67 polyhedra and carbon nanocages (Co@NCNs) displayed in Figure 3A, circumventing the limitations of conventional sacrificial templates and strong etchants^[71]. Tannic acid (TA), rich in hydroxyl groups, was coated on ZIF-67 surfaces. Protons released from TA diffused inward through pores, selectively breaking Co-N bonds and inducing core-selective etching, with shell thickness finely adjustable via etching duration^[72]. Liu et al. demonstrated multi-shell hollow MIL-101(Cr) with single-crystal shells through stepwise crystal growth and acetic acid etching [Figure 3B]^[73]. The differential stability between outer and inner layers allowed selective etching, with cavity size and shell thickness modulated by etching time and reactant concentration.

Figure 3. The schematic illustration for the synthetic procedure of (A) hollow Co@NCNs; This figure is adapted from Ref.^[71]. Copyright@2021, Wiley-VCH; (B) single-, double-, and triple-shelled hollow MIL-101; This figure is adapted from Ref.^[73]. Copyright@2017, Wiley-VCH; (C) tZIF-67 hollow nano-frame and the corresponding TEM images; This figure is adapted from Ref.^[75]. Copyright@2021, Wiley-VCH; (D) Schematic illustration of preparing Ni-Fe LDH nanocages with tunable shells. This figure is adapted from Ref.^[87]. Copyright@2021, Wiley-VCH. ZIF: Zeolitic imidazolate framework; TA: tannic acid; SSHM: Single-shelled hollow microcage; DSHM: Double-shelled hollow microcage; TSHM: Triple-shelled hollow microcage; DSNC: Double-shelled nanocage; SSNC: Single-shelled nanocage; MIL: Materials of the Institute Lavoisier; NCN: N-doped carbon nanosheets / Nitrogen-doped carbon network; LDH: layered double hydroxide.

The Ting group utilized phosphoric acid to selectively etch the metastable HKUST-1, generating a defect-rich porous copper-based MOF with hexagonal macroporosity while preserving crystallinity^[74]. Their experiments confirmed etching dependence on both time and acid concentration. As shown in Figure 3C, Cai et al. employed cyanuric acid (CA) to anisotropically etch ZIF-67 truncated rhombic dodecahedra^[75]. CA preferentially passivated the (100) facets, enabling inward excavation of the six equivalent crystal planes, ultimately yielding well-defined hollow nanoframes.

Beyond proton acid etching, the use of excess metal ions represents another prevalent strategy for modifying MOF structures. This approach operates on the principle of selective coordination differences, where the etching agent must exhibit a coordination affinity toward the organic ligand that is comparable to or greater than that of the native metal ions within the MOF framework. This method yielded a bimetallic MOF with high crystallinity and no detectable impurity phases. Zeng et al. developed a bifunctional etching strategy for ZIF-67 that avoids organic acids entirely^[76]. This dual-functional approach combines structural etching with in situ active-site loading. In a related study, Ulisso et al. employed a cationic etching method to transform ZIF-67 into hierarchical perovskite LaCoO₃ incorporated with spinel Co₃O₄ polycrystals^[77].

LDHs belong to a class of anionic layered materials characterized by brucite-like sheets in which metal cations occupy octahedral sites coordinated by hydroxyl groups^[78]. Owing to their tunable chemical composition, ion-exchange capability, high intrinsic stability, and rich redox properties, LDHs exhibit outstanding performance across various applications^[79]. Recently, metal ion etching has been widely adopted for synthesizing LDHs with hierarchical structures from MOF precursors, significantly enhancing the electrochemical utility of MOF-derived materials^[80,81]. Competitive metal-ion etching involves simultaneous dissolution of the original metal nodes and formation of new coordination environments. During the conversion of ZIF-67 or Co-ZIF-L into NiCo-based layered double hydroxides, incoming Ni²⁺ or Co²⁺ ions and hydrolyzed hydroxide species interact with the framework surface^[82]. The hydrolysis-induced weakening of the original Co-N coordination bonds promotes the release of imidazolate linkers and Co-containing species, while the incoming metal ions are subsequently re-coordinated with OH- to nucleate NiCo-LDH nanosheets. For example, Guan et al. constructed bimetallic NiCo-LDH polyhedra through Co²⁺ and Ni²⁺ etching of ZIF-67^[83]. The bimetallic system promotes the formation of numerous ultra-thin, vertically aligned LDH nanosheets assembled into hollow polyhedral superstructures in contrast to monometallic Co-LDH. This unique configuration maximizes interfacial exposure and facilitates guest-host interactions, thereby enhancing electrochemical activity. The Co-ZIF-L was first etched with Co²⁺ in a DMF-ethanol solution, followed by further Ni²⁺-assisted etching, yielding a material labeled CoNi-1 that demonstrated superior electrochemical performance^[84,85].

Furthermore, the shell architecture of hollow LDHs, including shell number and hierarchy, can be precisely tailored by modulating etching conditions^[86]. As demonstrated by Zhang et al., adjusting the ethanol-to-water ratio during etching allows fine control over reaction kinetics^[87]. With higher ethanol content, etching is slowed, leading to the formation of double-shelled Ni-Fe LDH nanocages (DSNCs), as illustrated in Figure 3D. In contrast, aqueous-rich conditions favor single-shelled structures. This involves etching-coupled dissolution and reprecipitation, in which a new active phase is generated during the etching process itself. The conversion of ZIF-67 or Ni-MOF into NiCo-LDH is a representative example^[88,89]. Incoming metal ions and hydrolyzed hydroxide species cleave the original metal-imidazolate coordination bonds while simultaneously nucleating LDH nanosheets. The increased capacitance or catalytic activity therefore arises from two coupled changes: the hollow or hierarchical morphology inherited from selective dissolution, and the formation of a redox-active NiCo-LDH phase containing Ni and Co centers capable of reversible valence transitions. In this situation, etching and phase transformation are chemically inseparable steps. It is more accurate to state that etching directs the reconstruction and spatial organization of the active LDH phase, rather than claiming that etching alone produces the electrochemical enhancement.

Building upon the complementary strengths of proton acid etching, which enables hollow structure formation and transition metal ion etching that allows crystal structure regulation, the integration of both strategies offers a powerful route for fabricating anisotropic materials with finely tunable composition and architecture^[90]. Liu et al. demonstrated this synergistic approach by simultaneously employing transition metal and protic acid etching to synthesize hollow carbon polyhedra^[91]. In their method, Ni²⁺ ions first etched ZIF-67 to form a NiCo@LDH intermediate, which was subsequently treated with boric acid (H₃BO₃) to selectively etch the framework, resulting in well-defined hollow structures with maintained polyhedra morphology^[91].

Exchange of ligands

Beyond metal ion etching, etching and exchange represent another effective approach for structural and compositional modulation of MOFs^[92]. Exchange of ligands replaces or strips partial original organic linkers by strong bases or competing coordinating molecules. This strategy readily yields hollow architectures and novel chemical functionalities. Alkaline agents or strongly coordinating ligands are typically employed as etchants to selectively remove or replace original linkers, thereby creating porous frameworks or inducing phase transformations^[93].

For example, Li et al. demonstrated alkaline etching of a copper-based MOF (Cu-MOF-74) to synthesize hollow copper sulfides with tunable morphologies. Ligand-oriented etching proceeds through the preferential cleavage or substitution of the original metal-linker coordination bonds by hydroxide ions, competing anions, or thermally generated reactive species. The alkaline transformation of Cu-MOF-74 illustrates this process. During KOH treatmen^[94], OH- coordinates with the Cu centers and promotes hydrolysis and dissociation of the original Cu-O bonds formed between Cu²⁺ and the carboxylate linkers. This gradual decoordination enables a pseudomorphic structural transformation while largely retaining the external particle morphology. Subsequent treatment with Na₂S replaces the original O-donor coordination environment with stronger Cu-S interactions, leading to further removal of the organic framework and the formation of hollow copper sulfides. Thus, the observed rod-to-cube-to-dodecahedron evolution reflects the balance between alkaline cleavage of Cu-O bonds, framework reconstruction, and sulfide-induced coordination substitution. Subsequent sulfidation yielded the corresponding hollow sulfide derivatives^[95]. Hydroxide ions generated from ammonia cleavage selectively removed the inner Co(CN)₆^3- ligands, resulting in structurally defined hollow frameworks. Hydroxide ions generated from NH₃·H₂O preferentially attack accessible metal-cyanide coordination units, weakening the Co-C≡N-Co bridges and promoting the dissolution of internal [Co(CN)₆]^3- units. Because the corners and edges possess lower coordination numbers and higher accessibility than the planar regions, ligand removal begins preferentially at these positions, producing hollow or open-frame intermediates. Subsequent chalcogenide-ion exchange reconstructs the residual metal centers into Cu-CoSe₂ nanoframes. At 650 °C, thiourea decomposition releases H₂S, which serves as a gaseous etchant enabling sulfide formation through reaction with Fe³⁺ nodes.

Zhang et al. further developed an ammonia-assisted in situ cation exchange method to fabricate dodecagonal N-doped carbon nanosheets (Pd-e-NiCo-PBA-C) [Figure 4]^[96]. Unlike conventional cubic hollow structures, this unique skeletal framework resulted from anisotropic etching initiated at the vertices of the precursor cube, evolving progressively into a cage-like architecture and ultimately a hollow morphology, exhibiting exceptional electrocatalytic performance in water splitting.

Figure 4. Schematic illustration of preparing Pd-e-NiCo-PBA-C; This figure is adapted from Ref.^[96]. Copyright@2021, licensed under CC BY 4.0. NiCo-PBA: NiCo Prussian blue analogue; Pd-e-NiCo-PBA: Pd-etched NiCo Prussian blue analogue; OER: oxygen evolution reaction; Pd-e-NiCo-PBA-C: Pd-etched NiCo Prussian blue analogue-derived carbon.

To better reveal the etching behavior on representative MOFs, a quantitative comparison is listed in Table 1.

Core-shell structure

Core-shell nanostructures constitute a significant class of nanomaterials characterized by a distinct core enveloped by a surrounding shell^[99]. Broadly speaking, they comprise ordered architectures formed through the encapsulation of one nanomaterial by another via chemical bonds or physical interactions, encompassing both conventional core-shell configurations and yolk-shell structures that incorporate an interstitial void between core and shell^[100].

In the context of MOF-based core-shell systems, structural design has evolved from relatively simple forms to increasingly complex architectures. The shell serves a critical protective role, shielding the core from the external environment and providing a confined space that enhances reaction stability. When the shell is composed of porous MOFs, it further facilitates molecular transport by offering additional pore channels, shortening diffusion pathways, promoting electrolyte infiltration, especially in hollow configurations and enhancing charge and mass transfer kinetics^[101,102]. Moreover, the synergistic interplay between core and shell components enables the integration of multifunctional properties, substantially broadening the application potential of these materials across catalytic, energy storage, sensing, and biomedical fields^[103].

Shell thickness and shell number are particularly important kinetic parameters. A thin porous shell shortens the solid-state ion-diffusion distance and provides rapid access to internal active sites, whereas an excessively thin shell may collapse during cycling or lose electrical continuity. Multishell structures provide a larger electrode-electrolyte interfacial area and multiple diffusion channels, but overly compact or poorly connected shells may increase ion-transport resistance.

Single-layer core-shell structures are commonly fabricated via self-templating methods, which enable precise control over dimensions and morphological uniformity^[104]. In contrast to conventional hard- or soft-templating approaches, self-templating not only ensures well-defined core-shell architectures but also allows tailored regulation over MOF composite properties. Among various synthetic routes, the one-pot method represents a straightforward and efficient strategy, wherein metal precursors and organic ligands are simultaneously introduced into a reaction system to achieve in situ core encapsulation and shell formation with narrow size distribution^[105,106]. Initially, polyvinylpyrrolidone (PVP)-stabilized CdS NPs were synthesized, followed by controlled growth of a ZIF-8 shell onto the CdS surface. By varying the concentration of the 2-methylimidazole precursor, the thickness of the ZIF-8 shell could be precisely tuned from 102 ± 5.4 nm down to 13.6 ± 2.5 nm^[107].

Magnetic Fe₃O₄ nanoparticles were sequentially immersed in ethanolic solutions containing the zirconium cluster precursor Zr₆O₄(OH)₄¹²⁺ and the organic linker 2-aminoterephthalic acid, with magnetic separation after each step, to achieve uniform MOF encapsulation. In contrast to conventional chemical etching, water etching offers an environmentally benign and cost-effective route for generating highly porous yolk-shell structures without aggressive reagents^[108].

From a structural perspective, MOF-derived core-shell architectures can be classified into two primary categories: single-shell and multi-shell systems. Unlike their single-shell counterparts, multi-shell heterostructures incorporate multiple concentric or hierarchical layers, offering enhanced compositional complexity and tailored functionality^[108]. These sophisticated architectures can be synthesized through self-templating strategies, often coupled with selective etching or ion-exchange processes. For example, Cao et al. fabricated hollow spherical structures starting from solid NiCo metal glycerate spheres^[109].

Alternatively, carbon-rich MOFs prepared via solvothermal methods exhibit exceptional metal cation permeability, enabling the construction of multi-shell hollow structures through post-synthetic modification. Guan et al. illustrated this approach by first synthesizing Mn-Co coordination polymer spheres through solvothermal reaction of Mn²⁺ and Co²⁺ with isophthalic acid (H₂IPA)^[110]. The resulting precursor was subsequently calcined to form multi-shell bimetallic oxide particles and then phosphatized using NaH₂PO₂ to yield multi-shell Mn-Co oxyphosphide, as depicted in Figure 5.

Figure 5. Schematic preparation of multishell mixed-metal oxyphosphide particle; This figure is adapted from Ref.^[110]. Copyright@2017, Wiley-VCH.

Hollow structure

Hollow nanostructured materials possess a range of advantageous properties, including high specific surface area for enhanced mass diffusion, abundant exposed active sites adaptable to diverse applications, and substantial internal void space enabling high loading capacity^[111]. Etched MOFs with hollow architectures further benefit from tunable surface chemistry, enlarged pore size, and elevated surface area^[112]. Notably, their hollow morphology facilitates rapid electrolyte penetration and efficient gas release compared to solid counterparts, contributing to their widespread use and high performance across numerous applications.

Two primary strategies are employed for fabricating hollow MOFs and their derivatives: template-assisted and post-processing methods. In the template-assisted approach, hollow structures are formed during MOF synthesis by growing the framework around a sacrificial template, such as silica or polystyrene spheres, followed by template removal. For instance, Yang et al. synthesized hollow porous carbon (HPC) by pyrolyzing ZIF-8 grown on PS templates, resulting in a porous shell with high surface area that significantly enhances catalytic efficiency^[113] [Figure 6A]. In contrast, Yu et al. adopted a post-processing strategy as shown in Figure 6B, constructing hollow ZIF-67 prismatic nanostructures through rapid ion exchange using Co(CH₃COO)₂(OH)₃ nanoprisms as sacrificial templates^[114]. Subsequent sulfidation transformed these into hollow CoS₂ particles that retained the prismatic morphology. The formation of the hollow interior is driven by the outward diffusion of Co²⁺ ions from the precursor nanoprisms and the inward migration of organic ligands, leading to the oriented growth of ZIF-67 polyhedra on the template surface and ultimately yielding well-defined hollow structures^[115].

Figure 6. Schematic illustration showing (A) the preparation process of the HPC; This figure is adapted from Ref.^[113]. Copyright@2019, Wiley-VCH; (B) Formation of CoS₂ nanobubble hollow prisms; This figure is adapted from Ref.^[114]. Copyright@2016, Wiley-VCH; (C) Structural evolution of ZIF-67. This figure is adapted from Ref.^[116]. Copyright@2015, Wiley-VCH. PS: Polystyrene; ZIF: zeolitic imidazolate framework; HPC: hollow porous carbon.

Subtle variations in reaction conditions can significantly influence the final morphology and composition of products. Yang et al. highlighted the role of solvent mediation in shaping bimetallic ZIF structures^[116], demonstrating that hollow Co-ZIF phases can be transformed from ZIF-67 seeds in methanol through Co²⁺ assistance (Figure 6C upper). This strategy was extended to the synthesis of hollow Zn/Co ZIF. The core shell ZIF-67@ZIF-8 intermediate further experienced Co²⁺-driven structural transformation in methanol. The gradual variation in Zn concentration suggests the reconstruction of the core shell architecture, finally producing hollow rhombohedral dodecahedra with a Co rich ZIF-67 core wrapped by a Zn containing ZIF-8 shell (Figure 6C below).

Hollow LDH nanostructures derived from MOFs have also emerged as promising materials for catalysis and energy storage. Their work revealed that the kinetics of ZIF-8 etching and LDH nucleation must be carefully balanced to achieve well-defined hollow structures, emphasizing the critical role of reaction rate matching in structural evolution.

The post-processing approach involves the formation of hollow structures through the modification of pre-synthesized MOFs. As an effective strategy, etching eliminates the need for template removal and offers distinct advantages over hard-templating methods, which often rely on particle agglomeration^[117]. In contrast, etching can be applied to single-crystalline MOFs, enabling precise control over cavity dimensions and shell thickness by modulating the etching extent. To achieve well-defined hollow structures via chemical etching, the integrity of the MOF shell must be maintained throughout the process^[118]. TA has been widely employed for this purpose, owing to its strong surface adhesion and metal-coordination capabilities, as noted in Section Exchange of metal ions^[119]. During etching, TA forms a protective layer on the MOF surface, preventing structural collapse, while the released protons diffuse inward to selectively remove the core. The molecular size of the etchant is critical: appropriately sized agents ensure uniform surface coverage and controlled etching, whereas mismatched sizes may lead to irregular pore formation or framework degradation^[120]. A low defect concentration generally increases the population of accessible metal centers without significantly disrupting the continuous framework. Missing-linker defects remove steric constraints around adjacent metal nodes and expose open coordination positions for the adsorption of H₂O, OH^-, H⁺, oxygen-containing intermediates, electrolyte ions, or sensing targets^[121]. These coordinatively unsaturated sites may also possess altered electron density because cleavage of a metal-ligand bond changes the local ligand field and metal oxidation environment. Consequently, sparse defects can increase the number of active sites and improve the intrinsic reactivity of neighboring metal centers. At an intermediate defect concentration, isolated vacancies begin to form interconnected defective regions and hierarchical pores^[122]. This defect level is often electrochemically favorable because it combines a high density of unsaturated metal sites with shortened ion-diffusion pathways and improved electrolyte penetration. Moderate etching can also promote charge redistribution, facilitate the formation of mixed-valence metal centers, and provide preferential nucleation sites for hydroxides, phosphides, sulfides, or metallic nanoparticles. Controlled formic acid modification changes the distribution and accessibility of unsaturated metal sites without completely destroying the crystalline phase. Formic acid, for instance, can modify MOF surfaces without altering their crystalline phase. It modulates the distribution and accessibility of unsaturated metal sites, thereby enhancing porosity, specific surface area, and charge transfer efficiency, which collectively improve electrochemical and catalytic performance^[123].

Excessive etching may cause vacancy coalescence, extensive metal-node dissolution, linker loss, pore-wall thinning, and partial amorphization or collapse of the framework. Although severe etching can produce a high geometric surface area, it may interrupt electron-transfer pathways, decrease mechanical stability, promote active-metal leaching, and generate poorly connected or inaccessible pores. An excessively high density of unsaturated sites may also bind reaction intermediates too strongly, hinder product desorption, or accelerate undesired electrolyte decomposition. Therefore, electrochemical activity is expected to exhibit an optimum with increasing defect concentration: activity initially increases because more accessible and electronically modified sites are generated, reaches a maximum when active-site density and transport efficiency are balanced, and subsequently decreases when structural and electronic continuity are substantially damaged. This non-monotonic relationship should be considered when comparing samples prepared using different etchant concentrations, reaction times, temperatures, or solvent compositions.

In addition to acid etching, metal ion etching was discussed in Section Exchange of metal ions. Another notable method is oxidative linker cleavage within single-crystalline MOFs, introduced by Xia et al.^[124]. The performance change can be more directly associated with etching-induced missing-linker or missing-node defects, coordinatively unsaturated metal sites, enlarged pores, and shortened diffusion pathways with no substantial phase transformation occurring.

Undoubtedly, while a variety of heterostructures can be derived from MOFs through etching, core-shell and hollow configurations remain among the most representative and widely studied^[125]. These two architectures offer distinct advantages: both significantly enhance the accessibility and exposure of active sites. Furthermore, the combination of intrinsic microporosity from the MOF framework and newly created mesoporosity from etching synergistically facilitates efficient mass transport within the structure^[126]. This hierarchical porosity markedly improves ion diffusion and charge transfer, contributing substantially to enhanced electrochemical performance.

To establish a meaningful defect-activity relationship, the defect descriptors should be correlated with both geometric and intrinsic electrochemical metrics. Electrochemically active surface area, double-layer capacitance, redox charge, and probe adsorption can estimate the number of accessible sites^[127], whereas turnover frequency or current normalized by the quantified active-site density can evaluate intrinsic activity per site. Electrochemical impedance spectroscopy^[128] can determine whether increasing defect concentration improves or impairs charge-transfer resistance. Combining these measurements with density functional theory calculations of charge density, density of states, adsorption free energies, and ion-migration barriers can distinguish whether the activity enhancement originates primarily from an increased number of active sites^[129], altered electronic structure, accelerated mass transport, or their cooperative effects. Accordingly, future studies should report etching parameters together with quantitative defect descriptors and site-normalized electrochemical performance, rather than relying solely on morphology, BET surface area, or geometric current density.

The beneficial effects of etching-induced defects must be balanced against the progressive loss of framework stability. Defect formation involves the cleavage of metal-linker coordination bonds that originally maintain the connectivity and mechanical integrity of the MOF lattice. At a relatively low defect concentration, isolated missing-linker defects and coordinatively unsaturated metal sites can be generated while the overall crystalline network remains intact^[130]. These defects increase the accessibility and chemical reactivity of metal centers without substantially disrupting long-range connectivity. As the etching extent increases, neighboring vacancies may merge into defect clusters, mesopores, and internal cavities, improving mass transport and exposing a larger fraction of the framework. However, continued removal of linkers or metal nodes progressively reduces the number of load-bearing coordination bonds, weakens the pore walls and shell, and increases the susceptibility of the material to hydrolysis, dissolution, mechanical deformation, and electrochemical reconstruction.

Accordingly, optimal etching should be defined as the condition that maximizes the density and accessibility of chemically active defects while maintaining sufficient metal-linker connectivity, shell integrity, electrical continuity, and resistance to leaching. Future studies should therefore report activity and stability as coupled optimization targets rather than treating defect creation as an exclusively beneficial process.

Electrolysis of water

Electrocatalytic water splitting is a promising method for converting electrical energy into chemical energy stored in hydrogen, offering a sustainable pathway for hydrogen production. During this process, the hydrogen evolution reaction (HER) takes place at the cathode, involving a two-electron transfer process that leads to the formation of H-H bonds^[131]. Simultaneously, the oxygen evolution reaction (OER) occurs at the anode, comprising a complex multi-step mechanism involving O-H bond cleavage and O-O bond formation through proton-coupled electron transfers^[132]. The intricate nature of OER results in inherently slow reaction kinetics, necessitating high overpotentials that limit the overall efficiency of water splitting and reduce the rate of oxygen generation. As a result, the OER half-reaction is widely regarded as the bottleneck in electrocatalytic water splitting^[133].

Phosphides exhibit remarkable properties such as high electrical conductivity, favorable electronic structure, and strong corrosion resistance, making them highly suitable for electrocatalytic applications. Specifically, cobalt phosphide (CoP) demonstrates high activity towards OER, attributed to the strong adsorption of oxygen intermediates on positively charged cobalt sites and facilitated O₂desorption by negatively charged phosphorus^[134]. He et al. developed carbon-doped metal phosphide nanocubes derived from ZIF-67 to enhance OER performance^[135]. Initially, ZIF-67 nanocubes were synthesized via a surface-mediated method, then transformed into Ni-Co LDH through etching, and finally converted into Ni-Co mixed metal phosphide (NiCoP and NiCoP/C) through phosphorylation using NaH₂PO₂ [Figure 7A]. Among these, the hollow NiCoP/C nanoboxes achieved an overpotential of 330 mV at 10 mA cm^-2 [Figure 7B], leveraging both the inherited high surface area and improved electrical conductivity from the MOF-derived hollow structure. These findings underscore the importance of morphological control in optimizing nanomaterial performance.

Figure 7. (A) Schematic illustration of the formation of NiCoP/C nano-boxes; (B) Polarization curves of Ni-Co LDH, NiCoP and NiCoP/C nano-boxes; (A and B) are reprinted with permission from Ref.^[135]. Copyright@2017, Wiley-VCH; (C) Schematic illustration of synthetic procedure for YHP-x; (D) LSV curves toward OER; (C and D) are reprinted with permission from Ref.^[137]. Copyright@2023, Springer Nature; (E) Schematic illustration of the CoMoP synthesis; (F) HER Polarization curves; (E and F) are reprinted with permission from Ref.^[139]. Copyright@2022. Licensed under CC BY 4.0. ZIF: Zeolitic imidazolate framework; LDH: layered double hydroxide; RHE: reversible hydrogen electrode; YHP: yolk-shell hollow polyhedra; MOF: metal-organic framework; LSV: linear sweep voltammetry; OER: oxygen evolution reaction; HER: hydrogen evolution reaction.

The newly formed inorganic phase commonly determines the intrinsic catalytic sites and principal redox chemistry, whereas etching controls the precursor morphology, elemental distribution, defect density, and accessibility of the final phase. The phosphide phase provides substantially higher electrical conductivity and different adsorption properties for oxygen-containing intermediates compared with the hydroxide precursor, while the carbon component further improves electron transport and suppresses particle aggregation. The hollow architecture produced through etching reduces ion- and gas-transport distances. Therefore, the final OER performance results from the cooperation of the NiCoP active phase, the conductive carbon matrix, and the etching-derived hollow morphology.

Beyond conventional acid etching, we also developed an electrochemical Lewis acid co-etching and electrochemical adsorption doping method to incorporate larger-radius Hf ions into Co-MOF^[136]. This approach introduced beneficial defects and markedly enhanced OER kinetics, as evidenced by a significant reduction in the Tafel slope. This strategy of doping high-valence metals offers a novel pathway for designing high-performance electrocatalysts. Given the challenging four-electron transfer processes in both OER and the oxygen reduction reaction (ORR), designing efficient and stable bifunctional catalysts is crucial for overall water splitting and regenerative fuel cells. Using a triply layered ZIF precursor^[137], we constructed yolk-shell hollow polyhedra (YHPs) embedded with ternary alloys and metal oxides through precisely controlled etching and pyrolysis [Figure 7C]. These YHP catalysts outperformed commercial benchmarks in both OER and ORR [Figure 7D], owing to the etching role of Lewis acid in forming yolk-shell configurations that provide abundant active sites and enhance mass and charge transfer.

As highlighted in previous sections, the development of bifunctional catalysts is critical for efficient electrocatalytic processes. While OER bifunctional catalysts have been widely explored, significant progress has also been made in designing HER/OER bifunctional catalysts for overall water splitting^[138]. For instance, Wang et al. synthesized ultrathin CoMoP nanosheets through ion etching and phosphorylation of ZIF-67 [Figure 7E]^[139]. Treatment with ammonium molybdate induced both structural etching and partial substitution of Co³⁺ by Mo⁶⁺, resulting in a porous, folded Mo-Co MOF intermediate. Subsequent phosphorylation produced bimetallic phosphide nanosheets that exhibited exceptional activity and stability for both HER and HER [Figure 7F].

Compared to their pristine counterparts, etched MOFs demonstrate superior and tunable functional properties^[140]. The quantitative comparison of representative etched MOF-derived electrocatalysts for water splitting is displayed in Table 2. Etching serves as a versatile and efficient strategy for tailoring MOF-derived materials, enabling the creation of well-defined pore systems and the introduction of heteroatoms without phase segregation^[141]. The incorporation of suitable foreign metal species further modulates the electronic structure and enhances electrical conductivity, leading to improved electrocatalytic performance. Overall, the strategic optimization of both structure and composition is indispensable for advancing the efficacy of MOF-based electrocatalysts^[142].

Supercapacitors

Supercapacitors represent a class of energy storage devices operating through Faradaic electrochemical processes, renowned for their long cycle life, rapid charge-discharge capability, and operational safety^[143]. The capacitance of their electrode materials originates from surface-mediated processes, including electrochemical adsorption/desorption of ions and reversible redox reactions at the electrode-electrolyte interface^[144].

Etched MOFs and their derivatives have emerged as high-performance electrode materials in supercapacitors, primarily due to their elevated specific surface area, abundant active sites, and tailored porosity resulting from etching, which collectively contribute to high specific capacitance^[145]. When MOFs are etched with metal ions, they can be transformed into ultrathin bimetallic hydroxide nanosheets with hollow architectures^[146]. The combination of layered morphology and hollow features accelerates ion diffusion, enabling efficient Faradaic redox reactions even at high current densities and thereby enhancing rate capability. This approach offers a generalized strategy for designing high-performance LDH-based energy materials. The evolution from bulk particles to ultrathin nanosheets or open nanocages also affects Faradaic kinetics^[147]. Thinner nanosheets reduce the distance required for electrolyte ions to reach redox-active metal centers and increase the proportion of edge and surface sites.

Hollow-structured nanomaterials have garnered significant research interest for their promising applications in energy storage due to their high surface area, efficient mass transport, and tunable porosity^[148,149]. For instance, Tang et al. synthesized hollow nano-polyhedra using ZIF-67 as a template, systematically investigating the critical roles of shell precipitation and template etching in nanocage formation^[150]. The resulting NiCo-LDH nanocages demonstrated excellent pseudocapacitive performance. In related work^[151], our group designed core-shell structures using macro-microporous ZIF-67 (M-ZIF-67) grown on PS templates as the core and LDH as the shell through facile ion etching [Figure 8A]. The optimized material, M-ZIF-67@LDH4, delivered a specific capacitance of 597.6 F g^-1 at 0.5 A g^-1 and maintained 92% retention at 3 A g^-1 [Figure 8B], underscoring the synergistic enhancement from its hierarchical porosity and LDH integration.

Figure 8. (A) The schematic diagram of the synthesis of M-ZIF-67@LDH4; (B) GCD curves of M-ZIF-67@LDH at current densities of 0.5-10 A g^-1; (A and B) are reprinted with permission from Ref.^[151]. Copyright@2021, Wiley-VCH. ZIF: Zeolitic imidazolate framework; PS: polystyrene; DMF: N,N-dimethylformamide; GCD: galvanostatic charge-discharge; LDH: layered double hydroxide.

The application of etched MOFs in supercapacitors has demonstrated considerable promise. By precisely tailoring the structure and properties of electrode materials, etching strategies have markedly improved key performance metrics including specific capacitance, rate capability, and cycling stability^[152]. Nevertheless, challenges such as limited electrical conductivity, long-term operational stability, and scalable production costs must be addressed to facilitate their practical implementation in commercial supercapacitors. Future research should focus on advancing novel etching methodologies, designing multifunctional composite architectures, and elucidating underlying reaction mechanisms^[153]. These efforts are expected to open new avenues for developing high-performance, economically viable supercapacitors.

Battery

Rechargeable batteries including lithium-ion, metal-air, and zinc-manganese systems are extensively utilized across numerous applications due to their high energy density and reversible operation^[154]. Etching MOFs has emerged as an efficient strategy for synthesizing advanced electrode materials with superior energy storage performance^[155]. The enhancements are primarily attributed to the following structural and functional advantages introduced by etching: (a) Hierarchically porous and hollow structures facilitate rapid ion and electrolyte transport, improving electrode-electrolyte contact; (b) Hollow architectures effectively accommodate volume changes during cycling, enhancing rate capability and long-term cyclability; (c) High specific surface area provides abundant electrochemically active sites, resulting in increased specific capacity.

Etched MOFs have been widely employed as electrodes in various battery systems. Here, we focus on lithium-based batteries, which represent a leading direction in energy storage research. In 2018, Lu et al. fabricated multi-shelled nanostructures with tunable shell numbers and evaluated their performance as anodes for lithium-ion batteries^[156]. As illustrated in Figure 9, simply by varying the amount of vanadium oxytriisopropoxide (VOT) during ion exchange with ZIF-67 nanocubes followed by calcination in air, they obtained nanoboxes with single, double, and triple shells. Electrochemical analysis revealed that the triple-shelled nanostructure delivered the most exceptional performance, a result directly attributable to its uniquely engineered architecture.

Figure 9. Schematic illustration of the formation of different shelled nanostructures; This figure is adapted from Ref.^[156]. Copyright@2018, Wiley-VCH. ZIF: Zeolitic imidazolate framework.

The confinement of active nanomaterials within porous carbon matrices has proven to be an effective strategy for enhancing the electrochemical properties of lithium-ion batteries^[157,158]. While carbon materials derived from MOFs hold great promise as electrodes, their inherently small pore sizes often limit accessibility to active sites during cycling^[159]. To address this limitation, the same research group developed hollow nanostructures designated as H-Co₃O₄@MCNBs two years later^[160]. These consist of hollow Co₃O₄ nanoparticles encapsulated within mesoporous carbon nanoboxes, fabricated through a combination of etching and a carefully controlled two-step thermal treatment. This distinctive structure and composition endow the H-Co₃O₄@MCNBs with remarkable lithium storage capabilities as an anode material, including high discharge/charge capacities and excellent Coulombic efficiency.

Lithium-sulfur (Li-S) batteries have attracted considerable attention for their high theoretical capacity and energy density^[161]. However, practical implementation is hindered by the insulating nature of sulfur, the shuttle effect of polysulfide intermediates, and substantial volume changes during cycling^[162]. Hollow frameworks offer a promising solution to accommodate volume expansion and enhance reaction kinetics^[163]. Wang et al. constructed hollow ZnSe-CoSe heterostructures with tunable cavity sizes through etching of ZIF-8@ZIF-67 precursors followed by gas-phase selenization^[164]. Furthermore, by integrating 45@ZnSe-CoSe with conductive MXene nanosheets, a high sulfur-loading cathode was achieved. The resulting battery exhibited exceptional performance, highlighting the effectiveness of this hollow heterostructure design in mitigating the challenges of Li-S batteries^[165].

As evidenced by recent advances, the application of etched MOFs represents a highly promising direction in battery research. Through precise structural and compositional regulation, etching techniques significantly enhance the electrochemical performance of MOF-based electrodes^[166]. By selectively removing parts of the framework, etching creates porous or hollow architectures that increase specific surface area and porosity, thereby promoting ion diffusion and improving active site utilization^[167].

Moreover, etched MOF structures exhibit improved mechanical stability, effectively mitigating volume changes during ion insertion and extraction. In the context of solid-state batteries, the porous networks formed via etching offer additional ion transport pathways, contributing to higher ionic conductivity of the electrolyte^[168,169]. These advantages collectively position etched MOFs and their derivatives as highly competitive materials for next-generation energy storage systems.

Electrochemical sensing

In addition to the above applications, MOFs are also used in electrochemical sensing and have demonstrated great potential. By regulating the structures and components, the sensitivity, selectivity and stability of the sensors based on etched MOFs and MOF derivatives have been significantly enhanced^[170,171]. Etching facilitates the creation of rich pores to facilitate the diffusion of materials and exposure of plenty of surface-active sites, enabling high sensing performance.

In the work of Zeng et al., they obtained hierarchical hollow MIL-101 cages simply via acid etching and employed them in the electrochemical sensor^[172]. Attributed to the huge specific surface area and adsorptive property, the hollow MIL-101 denoted high electrocatalytic activity and fantabulous voltammetric response towards the nitrofurazone oxidation. Apparently, compared with solid crystals, the design of hollow structures contributes to extraordinary electrocatalytic activity.

In summary, etching is a useful tool for tailoring the structure, composition, electronics, and physiochemistry of MOFs, and these beneficial changes help improve the performance of the target application. Future research directions include the development of novel etching techniques, the preparation of multifunctional composite materials, and in-depth studies on the mechanism of reaction, which should provide new ideas and methods for the design of high-performance electrochemical sensors.