We present a systematic study of metal−organic honeycomb lattices assembled from simple ditopic molecular bricks and Co atoms on Ag(111). This approach enables us to fabricate size-and shape-controlled open nanomeshes with pore dimensions up to 5.7 nm. The networks are thermally robust while extending over µm 2 large areas as single domains. They are shape resistant in the presence of further deposited materials and represent templates to organize guest species and realize molecular rotary systems.

Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure–activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO2 photocatalysts for hydrogen production such as (i) defects formation in reduced TiO2, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.

Self-assembly techniques allow for the fabrication of highly organized architectures with atomiclevel precision. Here, we report on molecular-level scanning tunneling microscopy observations demonstrating the supramolecular engineering of complex, regular, and long-range ordered periodic networks on a surface atomic lattice using simple linear molecular bricks. The length variation of the employed de novo synthesized linear dicarbonitrile polyphenyl molecules translates to distinct changes of the bonding motifs that lead to hierarchic order phenomena and unexpected changes of the surface tessellations. The achieved 2D organic networks range from a close-packed chevron pattern via a rhombic network to a hitherto unobserved supramolecular chiral kagomé lattice.

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201908505.Single-atom (SA) catalysis is a novel frontline in the catalysis field due to the often drastically enhanced specific activity and selectivity of many catalytic reactions. Here, an atomic-scale defect engineering approach to form and control traps for platinum SA sites as co-catalyst for photocatalytic H 2 generation is described. Thin sputtered TiO 2 layers are used as a model photocatalyst, and compared to the more frequently used (001) anatase sheets. To form stable SA platinum, the TiO 2 layers are reduced in Ar/H 2 under different conditions (leading to different but defined Ti 3+ -O v surface defects), followed by immersion in a dilute hexachloroplatinic acid solution. HAADF-STEM results show that only on the thin-film substrate can the density of SA sites be successfully controlled by the degree of reduction by annealing. An optimized SA-Pt decoration can enhance the normalized photocatalytic activity of a TiO 2 sputtered sample by 150 times in comparison to a conventional platinum-nanoparticle-decorated TiO 2 surface. HAADF-STEM, XPS, and EPR investigation jointly confirm the atomic nature of the decorated Pt on TiO 2 . Importantly, the density of the relevant surface exposed defect centers-thus the density of Pt-SA sites, which play the key role in photocatalytic activity-can be precisely optimized.Single-atom (SA) or single-site catalysis (SACs) has over the past years become an increasingly fascinating topic in the catalysis field. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] SACs have allowed new approaches in heterogeneous catalysis, [12,13] minimized the use of precious metals, [14]

Coordination-based supramolecular chemistry, [1] with its characteristic control of the self-assembly process and intrinsic defect tolerance, has been proven to be a very efficient synthetic tool to fabricate metallosupramolecular networks of well-defined topology in one, two, and three dimensions. [2][3][4][5][6][7][8] This strategy was recently applied to low dimensions by assembling regular molecular architectures from organic molecules and transition-metal centers directly on solid surfaces. [9, 10] A variety of surface-supported molecular network structures has been made accessible by the general application of a surface-assisted metal-coordination method to metal centers and aromatic polycarboxylic acids on metal surfaces. [9] As valid for supramolecular structures in general, the structures of the two-dimensional metal-organic coordination networks (2D-MOCNs) formed are predetermined by the properties of the ligands (e.g., donor atoms and their spatial arrangement, steric crowding) and the electronic characteristics of the metal ions (e.g., involved orbitals, ionization energies). However, under 2D conditions, the realization of a given coordination algorithm might be altered by the presence of a metal substrate, which results in deviating coordination geometries for the same metal-ligand coupling in comparison to the 3D situation (e.g., in the bulk phase). Such deviation can be attributed to charge transfer or screening effects and the strict 2D confinement of ligands and metal centers imposed by the substrate, which substantially influences the characteristics of the metal-to-ligand bonding within the 2D coordination network. [10e]

SACs) (see also reviews [11][12][13] ). SACs could offer ultimate atom economy and make every active site accessible, like homogeneous catalysts but being recyclable, which is a subject of paramount importance. [14] Major challenges in the field though encompass: i) the development of materials with precise functionalities for robust metal ion binding and ii) metal cooperativity in heterometallic and mixed-valence SACs, as identified in the recent topical perspective. [12] Meeting the first challenge could facilitate higher metal contents avoiding clustering and leaching upon reaction and catalyst recycling. This is also a prerequisite for the second challenge (metal-metal cooperation), since low metal content translates into large intermetallic distances. [6] Cooperation between two metal ions linked by a single-frame ligand has shown enormous potential in homogeneous catalysis. [15] For example, biocatalysts (metalloenzymes) use binuclear [16] and mixed-valence metal centers [17] for effective catalysis. Therefore, the development of heterogeneous catalysts with cooperativity between metal centers, keeping all the salient features of SACs, could offer a platform for the development of the next generation of catalysts.Graphene-based 2D materials have contributed to the development of SACs, [10,[12][13][14][18][19][20][21][22][23][24][25][26][27] in which metal ions are tetracoordinated in porphyrinic-like vacancies. Although only low contents of metal atoms can be achieved (up to ≈1 wt%), [10,12,14,18,[22][23][24][25][26] Single-atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valencestates, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed-valence SAC performs flawlessly in two O 2 -mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high-value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu(I) is the essential part in the reaction mechanism. The developed strategy opens the door to a broad portfolio of other SACs via their coordination to various functional groups of graphene, as demonstrated by successful entrapment of Fe III /Fe II single atoms to carboxy-graphene. Single-Atom CatalysisThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201900323.

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The bulk properties of glasses and amorphous materials have been studied widely, but the determination of their structural details at the molecular level is hindered by the lack of long-range order. Recently, two-dimensional, supramolecular random networks were assembled on surfaces, and the identification of elementary structural motifs and defects has provided insights into the intriguing nature of disordered materials. So far, however, such networks have been obtained with homomolecular hydrogen-bonded systems of limited stability. Here we explore robust, disordered coordination networks that incorporate transition-metal centres. Cobalt atoms were co-deposited on metal surfaces with a ditopic linker that is nonlinear, prochiral (deconvoluted in three stereoisomers on two-dimensional confinement) and bears terminal carbonitrile groups. In situ scanning tunnelling microscopy revealed the formation of a set of coordination nodes of similar energy that drives a divergent assembly scenario. The expressed string formation and bifurcation motifs result in a random reticulation of the entire surface.