Low-Valent Metal-Induced Lattice Swelling–Crystalline Shear Strategy for 2D Nonlamellar MOF Nanosheets

Abstract Two-dimensional (2D) metal–organic framework (MOF) nanosheets possess great potential in gas separation, photocatalysis, and other advanced applications, benefiting from their ultrathin morphology, high specific surface area, and abundant exposed active sites. However, rational synthesis of 2D nanosheets from nonlamellar high-valent metal MOFs remains challenging due to the lack of exploitable weak interlayer interactions. Herein, we report a versatile bottom-up strategy for “low-valent metal-induced lattice expansion–crystalline shear–competitive growth” to address this issue. Taking NaBH4-induced Ti(IV) reduction to Ti(III) as an example, the inherently longer bond length and lower bond energy of Ti(III)–O bonds induce NH2-MIL-125 unit cell expansion, preferential out-of-plane Ti–O bond cleavage, and rapid in-plane lateral competitive growth. This transforms NH2-MIL-125 from 3D particles (∼500 nm diameter, aspect ratio 2:1) to large-area ultrathin 2D nanosheets (∼20 μm lateral size, ∼35 nm thickness, an ultrahigh aspect ratio ∼600:1), the value of which is 35 times higher than the previous record. Notably, this surfactant- and layered-precursor-free approach is extendable to other nonlamellar Ti-MOFs. Extended X-ray absorption fine structure (EXAFS), integrated differential phase contrast STEM (iDPC-STEM), and other techniques verify the 2D structural evolution, oxygen vacancies, and unsaturated Ti sites. Practically, NH2-MIL-125 nanosheet-based MMMs exhibit 4.1-fold higher CO2/CH4 selectivity than their 3D counterparts, and the 2D nanosheets achieve a 3.3-fold higher CO production rate in visible-light CO2 photoreduction. This work breaks the paradigm of 2D MOF synthesis relying on layered structures, establishes an extensible methodology for the 2D transformation of nonlamellar high-valent metal MOFs, and advances their energy and environmental applications.