Crystal facet engineering has been proved as a dramatically efficient strategy to optimize the intrinsic catalytic property of catalysts. However, because of the difficulty in precise construction, facet engineering of two-dimensional metal−organic frameworks (2D MOFs) is rarely reported. Herein, we propose a new and facile coordination modulation method to realize the controllable synthesis of 2D NiFe-MOFs with a highly exposed (001) crystal facet. Specifically, acetate ions, which possess the same coordination groups with the participated terephthalic ligand, have been adopted into the MOF construction as the morphology and facet regulator. Through competitive coordination effect and ion-induced self-assembly, the fabricated NiFe-MOFs reveal a hierarchical microsphere architecture consisting of ultrathin nanosheets with thickness ∼5 nm. Further benefiting from the structural advantages, e.g., abundant active sites, highly exposed catalytic facet, and enhanced mass/charge transport, the hierarchical NiFe-MOF nanosheets exhibit an attractive overpotential of 240 mV at the current density of 10 mA cm −2 , a small Tafel slope of 73.44 mV dec −1 , and satisfactory stability for 16 h toward oxygen evolution reaction (OER). Besides, the increased exposure of the (001) facet will increase the proportion of high-valent nickel oxyhydroxides during OER, which will enhance the electrocatalytic performance. The universality of the modulation method is further verified by the successful synthesis of a series of 2D MOFs (Ni, Co, Cu, Zn, and corresponding bimetal MOFs). This method may serve as an important inspiration for the preparation and crystal facet engineering of 2D MOFs.

Carbon nitride has drawn widespread attention as a low-cost alternative to metal-based materials in the field of photocatalysis. However, the traditionally synthesized carbon nitrides always suffer a bulky architecture, which limits their intrinsic activities. Here, a cycloaddition reaction is proposed to synthesize a triazine-based precursor with implanted sodium and cyano groups, which are mostly retained in the resulting carbon nitride after the following polymerization. Incorporated sodium and cyano defects can not only tune the band structure of the carbon nitride but also provide more additive active sites. The optimized properties enable it an adorable photocatalytic hydrogen evolution rate of 1070 μmol h–1 g–1, varying by almost an order of magnitude from the pristine carbon nitride (79 μmol h–1 g–1). Moreover, a sequential self-assembly strategy has been adopted to further improve its architecture. As a consequence, a three-dimensional (3D) porous carbon nitride microtube cluster is constructed, indicating abundant exposed active sites and the faster separation of charge carriers. The corresponding photocatalytic hydrogen evolution rate is 1681 μmol h–1 g–1, which is very competitive compared with the reported pure carbon nitride photocatalysts. Briefly, this new approach may offer opportunities to fabricate task-specific carbon- and nitrogen-based materials from the molecular level.

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Mo atom in cubic Mo 2 N is normally 3-coordinated with N atoms, hence the 4d orbitals of the Mo sites are not fully occupied. [10,11] The residual unpaired 4d electrons endow the Mo 2 N with adorable adsorption capacity towards the water, especially preferable to the alkaline HER. Nevertheless, accompanying the nonstoichiometric configuration of the Mo 2 N, abundant interstitial vacancies are established in the skeleton as the intrinsic defects. [12] Unlike the positive functions brought from the engineering anion vacancies, such as sulfur vacancies in molybdenum disulfide [13,14] and oxygen vacancy in metal oxide, [15,16] the unpaired 4d orbitals above the interstitial vacancies in Mo 2 N is constant and concretely upraise the d band center. Therefore, more H 1s-d antibonding states are pulled above the E F , and strengthen the affinity of Mo towards generated H 2 . [11,[18][19][20] Even tremendous efforts have been devoted to balance the adsorption and release of generated H 2 on Mo 2 N, such as the heterojunction construction [21][22][23][24] and crystal orientation modulation, [25] the method directly aiming to modulate or eliminate the negative interstitial vacancy is still rarely reported.Tailoring the stoichiometric coefficient of the Mo 2 N seems to be a promising strategy from the view point of the atomic configuration. Along this line, Mo y N x , such as MoN and Mo 3 N 2 has been synthesized recently and proposed to be more efficient than the catalytic activity of the Mo 2 N in alkaline HER. [26][27][28] Unfortunately, restrained by the coordinatively saturated N atoms, the H/H 2 can only bind on the metal Mo atom in Mo x N y , [11] causing strong adsorption energy and higher overpotential than that of the platinum group metals. In this regard, implanting heteroatoms with lower 2p orbitals occupy into the interstitial vacancies should be a more promising path. Beyond accelerating the water adsorption and dissociation kinetics via lower the density of the empty d band, the 2p orbitals of the heteroatom may offer the optimal H binding site, simultaneously weakening the adsorption ability of the generated H 2 . Further according to the Hagg's rule, [10] carbon atom, which owns a similar atomic radius with N atom (R C = 0.77 Å, R N = 0.74 Å), and the appropriate radius ratio with Mo atom (R C /R Mo = 0.55, R N /R Mo = 0.53, both are in the range of 0.41-0.59, indicating the constructed interstitial compounds of a single lattice), is predicted to be the most pertinent heteroatom to fulfill the interstitial vacancies in Mo 2 N. However, since the common nitridation An interstitial vacancy on molybdenum nitride has been determined as a negative factor towards the alkaline hydrogen evolution reaction (HER) by reason of upraising the d orbitals of Mo. Nevertheless, investigations aiming to eliminate the vacancies are rarely reported. Here, an interstitial reconfiguration method for the design of stoichiometric molybdenum carbonitride (Mo 2 CN) is proposed, in which the vacancies are fulfilled by lattice carbon. ...