Two-dimensional materials and single-atom catalysts are two frontier research fields in catalysis. A new category of catalysts with the integration of both aspects has been rapidly developed in recent years, and significant advantages were established to make it an independent research field. In this Review, we will focus on the concept of two-dimensional materials confining single atoms for catalysis. The new electronic states via the integration lead to their mutual benefits in activity, that is, two-dimensional materials with unique geometric and electronic structures can modulate the catalytic performance of the confined single atoms, and in other cases the confined single atoms can in turn affect the intrinsic activity of two-dimensional materials. Three typical two-dimensional materials are mainly involved here, i.e., graphene, g-C3N4, and MoS2, and the confined single atoms include both metal and nonmetal atoms. First, we systematically introduce and discuss the classic synthesis methods, advanced characterization techniques, and various catalytic applications toward two-dimensional materials confining single-atom catalysts. Finally, the opportunities and challenges in this emerging field are featured on the basis of its current development.

Membranes with uniform, straight nanopores have important applications in diverse fields, but their application is limited by the lack of efficient producing methods with high controllability. In this work, we reported on an extremely simple and efficient strategy to produce such well-defined membranes. We demonstrated that neutral solvents were capable of annealing amphiphilic block copolymer (BCP) films of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) with thicknesses up to 600 nm to the perpendicular orientation within 1 min. Annealing in neutral solvents was also effective to the perpendicular alignment of block copolymers with very high molecular weights, e.g., 362 000 Da. Remarkably, simply by immersing the annealed BCP films in hot ethanol followed by drying in air, the originally dense BCP films were nondestructively converted into porous membranes containing highly ordered, straight nanopores traversing the entire thickness of the membrane (up to 1.1 μm). Grazing incident small-angle X-ray spectroscopy confirmed the hexagonal ordering of the nanopores over large areas. We found that the overflow of P2VP chains from their reservoir P2VP cylinders and the deformation of the PS matrix in the swelling process contributed to the transformation of the solid P2VP cylinders to empty straight pores. The pore diameters can be tuned by either changing the swelling temperatures or depositing thin layers of metal oxides on the preformed membranes via atomic layer deposition with a subnanometer accuracy. To demonstrate the application of the obtained porous membranes, we used them as templates and produced centimeter-scale arrays of aligned nanotubes of metal oxides with finely tunable wall thicknesses.

Block copolymers (BCPs) composed of two or more thermodynamically incompatible homopolymers self-assemble into periodic microdomains. Exposing self-assembled BCPs with solvents selective to one block causes a swelling of the domains composed of this block. Strong swelling in the confinement imposed by the matrix of the other glassy block leads to well-defined porous structures via morphology reconstruction. This confined swelling-induced pore-making process has emerged recently as a new strategy to produce porous materials due to synergic advantages that include extreme simplicity, high pore regularity, involvement of no chemical reactions, no weight loss, reversibility of the pore forming process, etc. The mechanism, kinetics, morphology, and governing parameters of the confined swelling-induced pore-making process in BCP thin films are discussed, and the main applications of nanoporous thin films in the fields of template synthesis, surface patterning, and guidance for the areal arrangements of nanomaterials and biomolecules are summarized. Recent, promising results of extending this mechanism to produce BCP nanofibers or nanotubes and bulk materials with well-defined porosity, which makes this strategy also attractive to researchers outside the nanocommunity, are also presented.

Nondestructive preparation of bicontinuous nanoporous metal membranes by replication of bicontinuous nanoporous polymeric membranes consisting of recoverable asymmetric block copolymers (BCPs) is reported. The BCP membranes are generated by swelling the minority domains of the BCP with selective solvents accompanied by reconstruction of the glassy matrix formed by the majority component (see figure).

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Block copolymers (BCPs) self-assemble into ordered arrays of nanoscopic domains, the nature of which depends on the constituents of the BCPs and their molecular architecture.[1] BCPs have been exploited as precursors for nanoporous materials [2] and as templates for the rational design of nanoscopic architectures with periods from below 10 nm up to the 100-nm-range in thin-film configurations.[3] Whereas molds containing arrays of aligned cylindrical nanochannels with hard confining walls, such as self-ordered anodic aluminum oxide (AAO), [4] have been used to fabricate one-dimensional nanostructures from a plethora of materials, [5] the self-assembly of BCPs in cylindrical confinement has only recently emerged as an access to nanorods exhibiting ordered nanoscopic fine structures.[6] Sol-gels containing precursors for inorganic scaffold materials and BCPs as structuredirecting soft templates were infiltrated into AAO. Subsequent gelation and calcination yielded nanorods consisting of various inorganic oxides and amorphous carbon exhibiting ordered mesoporous fine structures. [7] However, direct infiltration of microphase-separated BCP melts into AAO [8] has received much less attention, mainly because of a lack of obvious applications of the solid BCP nanorods thus obtained. Only recently, BCP nanorods have been converted into mesoporous polymeric nanorods by degrading sacrificial blocks [9] and by selective swelling.[10]BCPs confined to AAO with pore diameters about one order of magnitude larger than their characteristic bulk periods have been reported to retain bulk-like morphologies, such as cylinders oriented along the pore axes and concentric-cylindrical lamellae.Nanoscopic domain structures substantially different from those obtained in the absence of geometric constraints form if BCPs self-assemble within cylindrical pores having diameters of the same order of magnitude than their characteristic periods. [11][12][13] Wu et al. synthesized silica nanorods containing helical and circular-cylindrical ''stacked-doughnuts'' mesopore structures by gelation and calcination of sol-gels containing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) and tetraethyl orthosilicate infiltrated into AAO with pore diameters below 70 nm, [11] and prepared metal replicas of helical mesopores by electrochemical deposition.[14] However, only little effort has been directed to the experimental study of BCP melts selfassembling under strong cylindrical confinement, [12] even though the formation of a broad variety of unprecedented confinementinduced nanoscopic domain structures has been predicted. [15] Thus, only a small range of the potentially accessible morphologies has been realized, and their exploitation for the generation of functional nanostructures has not yet been addressed.Here, we report the fabrication of hierarchical one-dimensional semiconductor nanostructures containing structural motifs, such as helices and stacked doughnuts, that mimic unconventional, confinement-induced nanoscopic...

Engineering the topography of thin block copolymer (BCP) films by surface reconstruction associated with selective swelling of one of the blocks has been investigated intensively. Here we show that swelling-induced structural transitions in nanorods consisting of amphiphilic BCPs involve pronounced reshaping of the nonswollen glassy domains in the course of the transition from the equilibrium morphology of the molten BCP in cylindrical confinement to that of the BCP dissolved in the swelling agent. The reconstruction process can be quenched to retain intermediate nonequilibrium morphologies. The collapse of the swollen chains upon drying yields polymeric nanorods exhibiting complex nanoscopic architectures characterized by a variety of mesopore structures and surface topographies, including channels along the nanorods, bunches of partially interconnected strands, and strings of spheres. The complex BCP nanorods thus obtained can be used as soft templates for the rational arrangement of metal nanoparticles.

Covalent organic frameworks (COFs) are penetrated with uniform and ordered nanopores, implying their great potential in molecular/ion separations. As an imine-linked, stable COF, TpPa-1 is receiving tremendous interest for molecular sieving membranes. Theoretically, atomically thin TpPa-1 monolayers exhibit extremely high water permeance but unfortunately no rejection to ions because of its large pore size (∼1.58 nm). The COF monolayers tend to stack to form laminated multilayers, but how this stacking influences water transport and ion rejections remains unknown. Herein, we investigate the transport behavior of water and salt ions through multilayered TpPa-1 COFs by nonequilibrium molecular dynamics simulations. By analyzing both the interfacial and interior resistance for water transport, we reveal that with rising stacking number of COF multilayers exhibit increasing ion rejections at the expense of water permeance. More importantly, stacking in the offset eclipsed fashion significantly reduces the equivalent pore size of COF multilayers to 0.89 nm, and ion rejection is correspondingly increased. Remarkably, 25 COF monolayers stacked in this fashion give 100% MgCl2 rejection, whereas water permeance remains 1 to 2 orders of magnitude higher than that of commercial nanofiltration membranes. This work demonstrates the rational design of fast membranes for desalination by tailoring stacking number and fashion of the COF monolayers.