Graphene nanoribbons (GNRs)—narrow stripes of graphene—have emerged as promising building blocks for nanoelectronic devices. Recent advances in bottom-up synthesis have allowed production of atomically well-defined armchair GNRs with different widths and doping. While all experimentally studied GNRs have exhibited wide bandgaps, theory predicts that every third armchair GNR (widths of N=3m+2, where m is an integer) should be nearly metallic with a very small bandgap. Here, we synthesize the narrowest possible GNR belonging to this family (five carbon atoms wide, N=5). We study the evolution of the electronic bandgap and orbital structure of GNR segments as a function of their length using low-temperature scanning tunnelling microscopy and density-functional theory calculations. Already GNRs with lengths of 5 nm reach almost metallic behaviour with ∼100 meV bandgap. Finally, we show that defects (kinks) in the GNRs do not strongly modify their electronic structure.

wileyonlinelibrary.comOur work is inspired by passive transport across cell membranes, which offer an intriguing example of regulating membrane permeability based on transmembrane hydrophilic/hydrophobic interactions. [ 10 ] The cell membrane has a lipid bilayer structure consisting of hydrophilic phosphate outer layers and hydrophobic hydrocarbon core layer. This structure allows spontaneous diffusion of hydrophobic molecules from the hydrophilic outer side across the hydrophobic core layer whereas hydrophilic polar molecules show reduced permeation. The cell membrane thus passively controls the permeation of molecular-sized entities, and it inspires us to apply hydrophilic/ hydrophobic membrane designs to rectify gating of liquids. Instead of a membrane of nanoscale thickness, we selected technically relevant porous membranes of submillimeter-thickness to construct hydrophobic/hydrophilic asymmetry across it. Taken water transport as an example, such membrane might preferentially allow water to penetrate from the hydrophobic side, but tend to hinder its penetration from the hydrophilic side. Accordingly, we prepare two types of hydrophilic/hydrophobic Janus membranes by facile vapor diffusion or plasma treatments and demonstrate directional gating of water droplets as well as continuous water fl ow in air-water system. More generally, our membranes show directional gating of droplets in oil-water systems with integrated selectivity for either oil or water. Such membranes possessing both selectivity and directionality in liquid gating represent a new concept of intelligent materials. We also demonstrate the construction of "Janus trapper" to collect water droplets from oil or oil droplets from water. Results and DiscussionIn order to demonstrate the tunability of gating properties in Janus membranes, two approaches were incorporated, i.e., (1) to hydrophobize selectively one side of an initially hydrophilic membrane, (2) to hydrophilize selectively one side of an initially hydrophobic membrane. These approaches led to a different hydrophilic/hydrophobic balance across the membranes, which allowed complementary liquid gating properties. In the fi rst approach, vapor of 1 H ,1 H ,2 H ,2 H -perfl uorooctyltrichlorosilane (POTS), which previously had been used to hydrophobize cellulose [ 11 ] and silica [ 12 ] aerogels, was allowed to topochemically react on one side of hydrophilic cotton fabric membrane ( Figure 1 a and Supporting Information, Figure S1). CottonThe ability to gate (i.e., allow or block) droplet and fl uid transport in a directional manner represents an important form of liquid manipulation and has tremendous application potential in fi elds involving intelligent liquid management. Inspired by passive transport across cell membranes which regulate permeability by transmembrane hydrophilic/hydrophobic interactions, macroscopic hydrophilic/hydrophobic Janus-type membranes are prepared by facile vapor diffusion or plasma treatments for liquid gating. The resultant Janus membrane shows directional wate...

The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. Platinum-based materials are the most active electrocatalysts for catalyzing HER, but reducing the use of Pt is required because of the high price and scarcity of Pt. Here, we achieve pseudo-atomic-scale dispersion of Pt, i.e. individual atoms or subnanometer clusters, on the sidewalls of single-walled carbon nanotubes (SWNTs) with a simple and readily upscalable electroplating deposition method. These SWNTs activated with an ultralow amount of Pt exhibit activity similar to that of commercial Pt/C with a notably higher (∼66–333-fold) Pt loading for catalyzing the HER under the acidic conditions required in proton exchange membrane technology. These catalysts resemble pseudo-atomic-scale Pt systems which are mainly composed of a few to tens of Pt atoms dispersed on the sidewalls of the SWNTs. The Pt loading is only 0.19–0.75 atom % at the electrode surface, and characteristic peaks for Pt cyclic voltammograms are undetectable. The atomic dispersion increases the portion of the surface active-atom sites, and therefore, notably lower Pt loading is needed to attain a high catalytic activity. Density functional theory (DFT) calculations suggest higher ability for SWNTs, in comparison to graphene, as a catalyst support for immobilizing Pt atoms, thus providing an atomic dispersion. Moreover, a high HER activity for the SWNTs activated with Pt atoms, similar to that of bulk Pt, is predicted.

Efficient hydrogen evolution reaction (HER) through effective and inexpensive electrocatalysts is a valuable approach for clean and renewable energy systems. Here, single-shell carbon-encapsulated iron nanoparticles (SCEINs) decorated on single-walled carbon nanotubes (SWNTs) are introduced as a novel highly active and durable non-noble-metal catalyst for the HER. This catalyst exhibits catalytic properties superior to previously studied nonprecious materials and comparable to those of platinum. The SCEIN/SWNT is synthesized by a novel fast and low-cost aerosol chemical vapor deposition method in a one-step synthesis. In SCEINs the single carbon layer does not prevent desired access of the reactants to the vicinity of the iron nanoparticles but protects the active metallic core from oxidation. This finding opens new avenues for utilizing active transition metals such as iron in a wide range of applications.

Chemical vapor deposition of a thin titanium dioxide (TiO 2 ) fi lm on lightweight native nanocellulose aerogels offers a novel type of functional material that shows photoswitching between water-superabsorbent and water-repellent states. Cellulose nanofi brils (diameters in the range of 5-20 nm) with native crystalline internal structures are topical due to their attractive mechanical properties, and they have become relevant for applications due to the recent progress in the methods of their preparation. Highly porous, nanocellulose aerogels are here fi rst formed by freeze-drying from the corresponding aqueous gels. Well-defi ned, nearly conformal TiO 2 coatings with thicknesses of about 7 nm are prepared by chemical vapor deposition on the aerogel skeleton. Weighing shows that such TiO 2 -coated aerogel specimens essentially do not absorb water upon immersion, which is also evidenced by a high contact angle for water of 140 ° on the surface. Upon UV illumination, they absorb water 16 times their own weight and show a vanishing contact angle on the surface, allowing them to be denoted as superabsorbents. Recovery of the original absorption and wetting properties occurs upon storage in the dark. That the cellulose nanofi brils spontaneously aggregate into porous sheets of different length scales during freeze-drying is relevant: in the water-repellent state they may stabilize air pockets, as evidenced by a high contact angle, in the superabsorbent state they facilitate rapid water-spreading into the aerogel cavities by capillary effects. The TiO 2 -coated nanocellulose aerogels also show photooxidative decomposition, i.e., photocatalytic activity, which, in combination with the porous structure, is interesting for applications such as water purifi cation. It is expected that the present dynamic, externally controlled, organic/inorganic aerogels will open technically relevant approaches for various applications.