Sequential N2 Adsorption by the Nanopore Entrance Filling Scheme in Nanopores of Carbon Nanotubes.

The adsorption dynamics and mechanism of nitrogen molecules in 1-7 nm carbon nanotubes (CNTs) at 77 K were investigated by experiments and molecular dynamics simulations. The adsorbed nitrogen amount rapidly increased in 7 nm CNTs, while it gradually increased in 1 and 3 nm CNTs. The gradual increase in 3 nm CNTs was unexpected because of the presence of sufficient adsorption sites and the weak adsorption potential of nitrogen. The molecular dynamics simulations indicated that molecules were condensed in the entrance of nanopores after monolayer adsorption in 3 nm CNTs and monolayer and bilayer adsorption in 5 nm CNTs, called nanopore entrance filling. The proposed adsorption mechanism of nitrogen molecules in CNT nanopores is as follows: first, layer-by-layer adsorption occurs on monolayer sites, followed by preferential adsorption at the nanopore entrance. Consequently, preadsorbed molecules form a fluidic pore neck similar to an ink-bottle pore. Then, newly adsorbed molecules are condensed on the fluidic pore neck, and condensed molecules in the nanopore entrance finally move into the inner part of the nanopore. The proposed sequential adsorption mechanism via nanopore entrance filling without pore blocking starkly differs from micropore filling in micropores and layer-by-layer adsorption associated with capillary condensation in mesopores.

Size-dependent catalytic hydrogen production via methane decomposition and aromatization at a low-temperature using Co, Ni, Cu, Mo, and Ru nanometals.

Catalysts of methane decomposition to hydrogen and aromatization are inevitable for the development of natural gas applications. Metal catalysts have been developed to achieve highly efficient methane decomposition and aromatization under 1000 K using various substrates, such as zeolites and silica. Here, we performed a consecutive study on methane decomposition using Co-, Ni-, Cu-, Mo-, and Ru-based nanocatalysts in the bulk, on a SiO2 substrate, and in mesoporous SiO2. The crystallite sizes of the bulk nanocatalysts, and nanocatalysts on nonporous and mesoporous SiO2 were controlled to 80-85, 30-70, and 3-11 nm, respectively. The nanocatalysts on mesoporous SiO2 exhibited high activity on hydrogen and benzene productions via methane decomposition, owing to the nanosize effect of the nanocatalysts and adsorption potentials in the SiO2 mesopores. In particular, the Ni nanocatalysts on mesoporous SiO2 exhibited hydrogen production activity from 650 K, which was the lowest temperature, compared with those in previous reports on hydrogen production. In addition, the catalytic activity was maintained for over 15 h at 650 and 800 K with recyclability. The overoxidation of Ni species in the SiO2 mesopores might have promoted the transformation reaction of CH4 to CHx and prevented coking by the largeness of the SiO2 mesopores in comparison with microporous media.

Water Adsorption Control by Surface Nanostructures on Graphene-Related Materials by Grand Canonical Monte Carlo Simulations.

The interfaces of carbon materials play an important role in various technological and scientific research fields. Graphene is the fundamental unit of sp2 carbon allotropes, and the evaluation of the interfacial properties of graphene-related materials is thus essential to clarify the molecular mechanisms occurring at the interfaces. Ideally, graphene is exclusively composed of sp2 carbon atoms; however, some parts of graphene normally contain sp3 carbon atoms with oxygen functional groups, vacancy, and grain boundary defects, and these structural characteristics need to be considered to reveal the interfacial properties. Herein, we demonstrate the interfacial properties of graphene-related materials by analyzing the water adsorption properties of graphene, hydrogenated graphene (graphane), and partially oxidized graphene (named as graphoxide) using grand canonical Monte Carlo simulations. The hydrophobicity evaluated from the simulated water adsorption isotherms followed the order: graphane > graphene > graphoxide with 1% oxygen atomic ratio > graphoxide with 3% oxygen atomic ratio > graphoxide with 5% oxygen atomic ratio. The potential calculations between a single water molecule and graphoxides revealed that the presence of oxygen functional groups enhanced the hydrophilicity of graphoxide. This study also disclosed some differences between the hydrophobic interfaces of graphene and graphane, which have been rarely evaluated. Surprisingly, the hydrophobicity of graphane was higher than that of graphene despite the similar potential well depths between a water molecule and graphene/graphane. This was caused by the restriction of water orientation on graphane; water was preferentially adsorbed on the honeycomb center or concave sites in the initial adsorption, and it was hard to interact with neighboring water molecules. The different structures revealed for the graphene-related materials with nanoscale roughness played important roles in controlling the water vapor adsorption mechanism.

Piezoresistive and chemiresistive gas sensing by metal-free graphene layers.

Graphene is an ideal candidate to use in various applications as a component in semiconductor devices with excellent properties, such as its atomic thickness, optical transparency, chemical stability, and high electrical and thermal conductivities. The high gas sensitivities of graphene functionalized with metal, metal oxides, and other groups have been improved through intensive research. However, the development of a metal-free graphene gas sensor and clarification of its mechanism still remain a challenge. In this study, H2, CO2, NH3, and He gas sensing performances are demonstrated using two- to multilayered graphene, directly fabricated on a quartz substrate. The sheet resistances of more than 100 graphene layers were considerably changed from 3% to 6% by He gas injection, caused by its piezoresistive property. The anomalous resistance changes by piezoresistivity is a result of electron transfer path changes associated with graphene assemble structure changes by insertion of He gas between graphene crystal units and pressing graphene units. The sheet resistances of the synthesized graphene were found to dramatically change through physical adsorption and chemisorption. The chemisorption of NH3 gas on functional oxygen groups at graphene edges was responsible for the chemiresistive behavior of the material. The gas sensing and piezoresistive mechanisms of graphene determined in this work sheds light on the development of a graphene gas sensor.

Thermally Stimulated Light Reflection and Photoluminescence of BaTiO3.

Perovskites have been attracting attention because of their considerable luminescence properties. A conventional perovskite such as BaTiO3 has no intrinsic photoluminescence. Doping with rare metals, nanocrystallization, and addition of organometallic halides induce significant photoluminescence and photovoltages. Here, we report anomalous light reflection and photoluminescence of BaTiO3 on heating. Light absorption shifted from the near-ultraviolet region to the visible region on heating. The small emission peaks at around 400-500 nm disappeared and new peaks appeared above 800 nm; the quantum yields of these peaks were less than 1% and more than 7%, respectively.

The effect of different organic solvents on sodium ion storage in carbon nanopores.

In this study fully atomistic grand canonical Monte Carlo (GCMC) simulations have been employed to study the behaviour of an electrolyte salt (NaPF6) and different non-aqueous (organic) solvents in carbon nanopores, to reveal the structure and storage mechanism. Organic solutions of Na+ and PF6- ions at 1 M concentration were considered, based on the conditions in operational sodium ion batteries and supercapacitors. Three organic solvents with different properties were selected: ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The effects of solvents, pore size and surface charge were quantified by calculating the radial distribution functions and ionic density profiles. It is shown that the organic solvent properties and nanopore confinement can affect the structure of the organic electrolyte solution. For the pore size range (1-5 nm) investigated in this paper, the surface charge used in this study can alter the sodium ions but not the solvent structure inside the pore.

Supramolecular Polymerization of Supermacrocycles: Effect of Molecular Conformations on Kinetics and Morphology.

Intricately designed π-conjugated molecules containing interactive groups can be used to generate supramolecular polymers with outstanding structural and functional properties. To construct such supramolecular polymers, the non-covalent synthesis of supermacrocyclic monomers from relatively simple molecules represents an attractive strategy, although this has been rarely exploited. Here, we report the supramolecular polymerization of two barbiturate-naphthalene derivatives that circularly hexamerize by hydrogen bonding. The two molecules contain an aliphatic "wedge" unit with either an ether or ester linkage. This subtle difference is amplified into distinct features both in terms of the morphology of the supramolecular polymers and the polymerization process. The degrees of conformational freedom of the wedge unit determine the stacking of the supermacrocycles, as is evident from 2D X-ray diffraction analyses on the aligned fibers. The differences in stacking impart the supramolecular polymer fibers with different morphological features (cylindrical or helical), which are reflected in the properties of concentrated solutions (suspension or gel). The degrees of conformational freedom of the wedge unit also affect the polymerization kinetics, in which the more flexible ether linkage induces pathway complexity by the formation of off-pathway aggregates.

Interruption of Hydrogen Bonding Networks of Water in Carbon Nanotubes Due to Strong Hydration Shell Formation.

We present the structures of NaCl aqueous solution in carbon nanotubes with diameters of 1, 2, and 3 nm based on an analysis performed using X-ray diffraction and canonical ensemble Monte Carlo simulations. Anomalously longer nearest-neighbor distances were observed in the electrolyte for the 1-nm-diameter carbon nanotubes; in contrast, in the 2 and 3 nm carbon nanotubes, the nearest-neighbor distances were shorter than those in the bulk electrolyte. We also observed similar properties for water in carbon nanotubes, which was expected because the main component of the electrolyte was water. However, the nearest-neighbor distances of the electrolyte were longer than those of water in all of the carbon nanotubes; the difference was especially pronounced in the 2-nm-diameter carbon nanotubes. Thus, small numbers of ions affected the entire structure of the electrolyte in the nanopores of the carbon nanotubes. The formation of strong hydration shells between ions and water molecules considerably interrupted the hydrogen bonding between water molecules in the nanopores of the carbon nanotubes. The hydration shell had a diameter of approximately 1 nm, and hydration shells were thus adopted for the nanopores of the 2-nm-diameter carbon nanotubes, providing an explanation for the large difference in the nearest-neighbor distances between the electrolyte and water in these nanopores.

Fast Water Relaxation through One-Dimensional Channels by Rapid Energy Transfer.

Water in carbon nanotubes is surrounded by hydrophobic carbon surfaces and shows anomalous structural and fast transport properties. However, the dynamics of water in hydrophobic nanospaces is only phenomenologically understood. In this study, water dynamics in hydrophobic carbon nanotubes is evaluated based on water relaxation using nuclear magnetic resonance spectroscopy and molecular dynamics simulations. Extremely fast relaxation (0.001 s) of water confined in carbon nanotubes of 1 nm in diameter on average is observed; the relaxation times of water confined in carbon nanotubes with an average diameter of 2 nm (0.40 s) is similar to that of bulk water (0.44 s). The extremely fast relaxation time of water confined in carbon nanotubes with an average diameter of 1 nm is a result of frequent energy transfer between water and carbon surfaces. Water relaxation in carbon nanotubes of average diameter 2 nm is slow because of the limited number of collisions between water molecules. The dynamics of interfacial water can therefore be controlled by varying the size of the hydrophobic nanospace.

Double-Step Gate Phenomenon in CO2 Sorption of an Elastic Layer-Structured MOF.

A double-step CO2 sorption by [Cu(4,4'-bpy)2(BF4)2] (ELM-11) was observed during isothermal measurements at 195, 253, 273, and 298 K and was accompanied by interlayer expansion in the layered structure of ELM-11. The first step occurred in the range of the relative pressure (P/P0) from 10(-3) to 10(-2). The second step was observed at P/P0 ≈ 0.3 at the four temperatures. Structural changes in ELM-11 during the CO2 sorption process were examined by X-ray diffraction (XRD) measurements. The structural change for the first step was well understood from a detailed structural analysis, as reported previously. The XRD results showed further expansion of the layers during the second step as compared to the already expanded structure in the first step, and both steps were found to be caused by the gate phenomenon. The energy for the expansion of the layer structure was estimated from experimental and simulated data.

Changing water affinity from hydrophobic to hydrophilic in hydrophobic channels.

The behavior of water at hydrophobic interfaces can play a significant role in determining chemical reaction outcomes and physical properties. Carbon nanotubes and aluminophosphate materials have one-dimensional hydrophobic channels, which are entirely surrounded by hydrophobic interfaces. Unique water behavior was observed in such hydrophobic channels. In this article, changes in the water affinity in one-dimensional hydrophobic channels were assessed using water vapor adsorption isotherms at 303 K and grand canonical Monte Carlo simulations. Hydrophobic behavior of water adsorbed in channels wider than 3 nm was observed for both adsorption and desorption processes, owing to the hydrophobic environment. However, water showed hydrophilic properties in both adsorption and desorption processes in channels narrower than 1 nm. In intermediate-sized channels, the hydrophobic properties of water during the adsorption process were seen to transition to hydrophilic behavior during the desorption process. Hydrophilic properties in the narrow channels for both adsorption and desorption processes are a result of the relatively strong water-channel interactions (10-15 kJ mol(-1)). In the 2-3 nm channels, the water-channel interaction energy of 4-5 kJ mol(-1) was comparable to the thermal translational energy. The cohesive water interaction was approximately 35 kJ mol(-1), which was larger than the others. Thus, the water affinity change in the 2-3 nm channels for the adsorption and desorption processes was attributed to weak water-channel interactions and strong cohesive interactions. These results are inherently important to control the properties of water in hydrophobic environments.

Anomalously enhanced hydration of aqueous electrolyte solution in hydrophobic carbon nanotubes to maintain stability.

An understanding of the structure and behavior of electrolyte solutions in nanoenvironements is crucial not only for a wide variety of applications, but also for the development of physical, chemical, and biological processes. We demonstrate the structure and stability of electrolyte in carbon nanotubes using hybrid reverse Monte Carlo simulations of X-ray diffraction patterns. Hydrogen bonds between water are adequately formed in carbon nanotubes, although some hydrogen bonds are restricted by the interfaces of carbon nanotubes. The hydrogen bonding network of water in electrolyte in the carbon nanotubes is further weakened. On the other hand, formation of the ion hydration shell is significantly enhanced in the electrolyte in the carbon nanotubes in comparison to ion hydration in bulk electrolyte. The significant hydrogen bond and hydration shell formation are a result of gaining stability in the hydrophobic nanoenvironment.

Size-dependent water structures in carbon nanotubes.

Water surrounded by hydrophobic interfaces affects a variety of chemical reactions and biological activities. Carbon nanotubes (CNTs) can be used to investigate the behavior of water at hydrophobic interfaces. Here, we determined the fundamental unit of water by evaluating the ice-like cluster formation of water in the limited hydrophobic nanospaces of CNTs, using X-ray diffraction and molecular simulation analysis. The water in CNTs with a diameter of 1 nm had fewer hydrogen bonds than bulk water under ambient conditions. In CNTs with diameters of 2 and 3 nm, water formed nanoclusters even under ambient conditions, because of prolific hydrogen bonding; predominant ice-like cluster formation was induced in the 2-3 nm nanospaces. The results confirming the cluster formation in the CNTs also demonstrated that the critical cluster size was 0.8-3.4 nm. The fundamental cluster size was 0.8 nm; these results indicated that 0.8 nm clusters are the fundamental units of water assemblies.

Degradation of Graphene in High- and Low-Humidity Air, and Vacuum Conditions at 300-500 K.

Graphene is a fundamental unit of carbon materials and, thus, primary sp2-bonded carbon material. Graphene is, however, easily broken macroscopically despite high mechanical strength, although its natural degradation has rarely been considered. In this work, we evaluate the natural degradation of two-layer graphene in vacuo, in low-humidity air, and in high-humidity air at 300, 400, 450, and 500 K. Over 1000 days of degradation at 300 K, the graphene structure was highly maintained in vacuo, whereas the layer number of graphene tended to decrease in high- and low-humidity air. Water was slightly reacted/chemisorbed on graphene to form surface oxygen groups at 300 K. At 450 and 500 K, graphene was moderately volatilized in vacuo and was obviously oxidized in high- and low-humidity air. Surprisingly, the oxidation of graphene was more suppressed in the high-humidity air than in the low-humidity air, indicating that water worked as an anti-oxidizer of graphene by preventing the chemisorption of oxygen on the graphene surface.

Covalent modular approach for dimension-controlled self-organization of perylene bisimide dyes.

Organogels: Dimerization of perylene bisimide dyes through an oligomethylene linker enabled the facile control over columnar and lamellar self-organized architectures by an odd/even effect with respect to the number of methylene groups. The difference in the self-organized architectures was shown to have an impact on their material morphologies, as well as charge-carrier mobilities (see scheme).

Rapid water transportation through narrow one-dimensional channels by restricted hydrogen bonds.

Water plays an important role in controlling chemical reactions and bioactivities. For example, water transportation through water channels in a biomembrane is a key factor in bioactivities. However, molecular-level mechanisms of water transportation are as yet unknown. Here, we investigate water transportation through narrow and wide one-dimensional (1D) channels on the basis of water-vapor adsorption rates and those determined by molecular dynamics simulations. We observed that water in narrow 1D channels was transported 3-5 times faster than that in wide 1D channels, although the narrow 1D channels provide fewer free nanospaces for water transportation. This rapid transportation is attributed to the formation of fewer hydrogen bonds between water molecules adsorbed in narrow 1D channels. The water-transportation mechanism provides the possibility of rapid communication through 1D channels and will be useful in controlling reactions and activities in water systems.

Energetic contribution to hydration shells in one-dimensional aqueous electrolyte solution by anomalous hydrogen bonds.

The hydration structure of NaCl aqueous solution was elucidated in carbon nanotubes (CNTs) on the basis of canonical ensemble Monte Carlo simulations. Hydration shells were preferentially formed even in narrow CNTs to gain stabilization energy, whereas hydrogen bonding between water molecules in such CNTs was sacrificed. Nanoscale-confined aqueous electrolyte solutions therefore prioritize hydration shell formation between ions and water rather than hydrogen-bond formation between water molecules.

Significant hydration shell formation instead of hydrogen bonds in nanoconfined aqueous electrolyte solutions.

Nanoscale confined electrolyte solutions are frequently observed, specifically in electrochemistry and biochemistry. However, the mechanism and structure of such electrolyte solutions are not well understood. We investigated the structure of aqueous electrolyte solutions in the internal nanospaces of single-walled carbon nanotubes, using synchrotron X-ray diffraction. The intermolecular distance between the water molecules in the electrolyte solution was increased because of anomalously strong hydration shell formation. Water correlation was further weakened at second-neighbor or longer distances. The anomalous hydrogen-bonding structure improves our understanding of electrolyte solutions in nanoenvironments.

Control over hierarchy levels in the self-assembly of stackable nanotoroids.

We report a precise control over the hierarchy levels in the outstanding self-organization process shown by chiral azobenzene dimer 1. This compound forms uniform toroidal nanostructures that can hierarchically organize into chiral nanotubes under the control by temperature, concentration, or light. The nanotubes further organized into supercoiled fibrils, which finally intertwined to form double helices with one-handed helical sense.

Formation of CO(x)-free H2 and cup-stacked carbon nanotubes over nano-Ni dispersed single wall carbon nanohorns.

Transitional metals (M) were dispersed on single-wall carbon nanohorns (M/SWCNHs, M = Fe, Co, Ni, Cu) by simple thermal treatment of the deposited metal nitrate without H(2) reduction. Nanometallic Ni particles on SWCNH were evidenced by high-resolution transmission electron microscopic observation and X-ray photoelectron spectroscopy. The nano-Ni dispersed on SWCNH showed the highest CH(4) decomposition activity; the activity of used transitional metals decreases in the order Ni ≫ Co > Fe ≫ Cu. On the other hand, the reaction rate over Ni/SWCNH was much larger than that over Ni/Al(2)O(3), and the former provided CO(x)-free H(2) and cup-stacked carbon nanotubes, while Ni/Al(2)O(3) produced CO(x) in addition to H(2). SWCNH was superior to Al(2)O(3) as the catalyst support of Ni for the CH(4) decomposition reaction.