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.

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.

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].

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.

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

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.

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 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.

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.

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.

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.

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.