Adsorption enhancement of nitrogen gas by atomically heterogeneous nanospace of boron nitride

In this study, porous boron nitride (p-BN) with hexagonal phase boron nitride (h-BN) pore walls was synthesized using high-temperature calcination. Negligible variation in pore-wall structure can be observed in powder X-ray diffraction (XRD) profiles and infrared (IR) spectra. However, a highly stable p-BN with a stable pore structure even at 973 K under the oxidative conditions is obtained when synthesized at higher than 1573 K under nitrogen gas flow. For p-BN, this stability is obtained by generating h-BN microcrystals. Nitrogen adsorption–desorption isotherms at 77 K provide type-IV features and typical adsorption–desorption hysteresis, which suggests micropore and mesopore formation. Moreover, adsorption–desorption isotherms of Ar at 87 K are measured and compared with those of nitrogen. The relative adsorbed amount of nitrogen (i.e., the amount of nitrogen normalized by that of Ar at each relative pressure or adsorption potential value) on p-BN is considerably larger than that on microporous carbon at low-pressure regions, which suggests the existence of strong adsorption sites on the p-BN surface. In fact, the relative number of adsorbed nitrogen molecules to that of Ar on p-BN is, at most, 150%–200% larger than that on microporous carbon for the same adsorption potential state. Furthermore, additional adsorption enhancement to nitrogen between P/P0 = 10−5 and 10−3 can be observed for p-BN treated at 1673 K, which suggests the uniformly adsorbed layer formation of nitrogen molecules in the vicinity of a basal planar surface. Thus, unlike typical nanoporous sp2 carbons, p-BN materials have the potential to enhance adsorption for certain gas species because of their unique surface state

New insights into the heat of adsorption of water, acetonitrile, and n-hexane in porous carbon with oxygen functional groups

Isosteric heat of adsorption is exquisitely sensitive to structural changes in carbon surfaces based on the energetic behavior of the interactions between adsorbates and carbon materials. We discuss the relationships between porous structures, oxygen functional groups, and heat of adsorption based on the behavior of the heat of adsorption of polar and non-polar fluids on porous carbon materials with oxygen functional groups. The porosity and functional groups of porous carbon materials were estimated from N2 adsorption isotherms at 77 K and temperature-programmed desorption. High-resolution adsorption isotherms of water, acetonitrile (polar fluid), and n-hexane (non-polar fluid) were measured on porous carbon materials with different pore size distributions and amounts of oxygen functional groups at various temperatures. The heats of adsorption were determined by applying the Clausius-Clapeyron equation to the adsorption isotherms. The heat of adsorption curves directly reflect the effects of interactions of fluid-oxygen functional groups,fluid-basal planes of pore walls, and fluid-fluid interfaces. In particular, the heat of adsorption curve of water is very sensitive to surface oxygen functional groups. This finding indicates the possibility of estimating the relative amounts of oxygen functional groups on porous carbon materials based on the amounts of water adsorbed at specific relative pressures

Rapid Synthesis and Charge?Discharge Properties of LiMnPO4 Nanocrystallite-embedded Porous Carbons

LiMnPO4 nanocrystallite-embedded porous carbons were successfully synthesized within a few minutes by a microwave-heating process. The nanocomposites showed higher charge?discharge capacity and better rate capability than bulk-LiMnPO4 particles synthesized in a similar manner without porous carbons

Charge?Discharge Property of Si and SiOx Nanoparticles Produced in Regulated Carbon Nanospace

Si and SiOx nanoparticle-embedded nanoporous carbons were successfully synthesized by reducing a SiO2 opal-carbon composite precursor with Mg, and their interstitial nanospace in carbon pores was tunable by partly etching the SiO2 particles in the precursor. It was revealed that dispersive loading of Si and SiOx nanoparticles in carbon nanospace is necessary to enhance the charge-discharge performance in addition to providing the interstitial nanospace as a buffer space for the volume change associated with Si-Li reactions

Selective probe of the morphology and local vibrations at carbon nanoasperities

We introduce a way to selectively probe local vibration modes at nanostructured asperities such as tips of carbon nanohorns. Our observations benefit from signal amplification in surface-enhanced Raman scattering (SERS) at sites near a silver surface. We observe nanohorn tip vibration modes in the range 200-500 cm(-1), which are obscured in regular Raman spectra. Ab initio density functional calculations assign modes in this frequency range to local vibrations at the nanohorn cap resembling the radial breathing mode of fullerenes. Careful interpretation of our SERS spectra indicates presence of caps with 5 or 6 pentagons, which are chemically the most active sites. Changes in the peak intensities and frequencies with time indicate that exposure to laser irradiation may cause structural rearrangements at the cap.ArticleJOURNAL OF CHEMICAL PHYSICS. 136(6):064505 (2012)journal articl

Design of nanoporous materials with optimal sorption capacity

Modern technological advances have enabled one to manufacture nanoporous materials with a prescribed pore structure. This raises a possibility of using controllable pore-scale parameters (e.g., pore size and connectivity) to design materials with desired macroscopic properties (e.g., diffusion coefficient and adsorption capacity). By relating these two scales, the homogenization theory (or other upscaling techniques) provides a means of guiding the experimental design. To demonstrate this approach, we consider a class of nanoporous materials whose pore space consists of nanotunnels interconnected by nanotube bridges. Such hierarchical nanoporous carbons with mesopores and micropores have shown high specific electric double layer capacitances and high rate capability in an organic electrolyte. We express the anisotropic diffusion coefficient and adsorption coefficient of such materials in terms of the tunnels\u27 properties (pore radius and inter-pore throat width) and their connectivity (spacing between the adjacent tunnels and nanotube-bridge density). Our analysis is applicable for solutes that undergo a non-equilibrium Langmuir adsorption reaction on the surfaces of fluid-filled pores, but other homogeneous and heterogeneous reactions can be handled in a similar fashion. The presented results can be used to guide the design of nanoporous materials with optimal permeability and sorption capacity

Selective probe of the morphology and local vibrations at carbon nanoasperities

We introduce a way to selectively probe local vibration modes at nanostructured asperities such astips of carbon nanohorns. Our observations benefit from signal amplification in surface-enhancedRaman scattering (SERS) at sites near a silver surface. We observe nanohorn tip vibration modes inthe range 200?500 cm?1, which are obscured in regular Raman spectra. Ab initio density functionalcalculations assign modes in this frequency range to local vibrations at the nanohorn cap resemblingthe radial breathing mode of fullerenes. Careful interpretation of our SERS spectra indicates presenceof caps with 5 or 6 pentagons, which are chemically the most active sites. Changes in the peakintensities and frequencies with time indicate that exposure to laser irradiation may cause structuralrearrangements at the cap

Conducting linear chains of sulphur inside carbon nanotubes

Despite extensive research for more than 200 years, the experimental isolation of monatomic sulphur chains, which are believed to exhibit a conducting character, has eluded scientists. Here we report the synthesis of a previously unobserved composite material of elemental sulphur, consisting of monatomic chains stabilized in the constraining volume of a carbon nanotube. This one-dimensional phase is confirmed by high-resolution transmission electron microscopy and synchrotron X-ray diffraction. Interestingly, these one-dimensional sulphur chains exhibit long domain sizes of up to 160 nm and high thermal stability (similar to 800 K). Synchrotron X-ray diffraction shows a sharp structural transition of the one-dimensional sulphur occurring at similar to 450-650 K. Our observations, and corresponding electronic structure and quantum transport calculations, indicate the conducting character of the one-dimensional sulphur chains under ambient pressure. This is in stark contrast to bulk sulphur that needs ultrahigh pressures exceeding similar to 90 GPa to become metallic.ArticleNATURE COMMUNICATIONS. 4:2162 (2013)journal articl

Patch-like, two dimensional WSe2-based hetero-structures activated by a healing catalyst for H2 photocatalytic generation

2D photoactive materials may offer interesting opportunities in photocatalytic devices since they combine strong light absorption and shortening of charge carriers’ diffusion path. Because of their high surface defect concentration and the formation of a majority of edge/plane vs plane/plane contacts between the anisotropic building blocks, surface defect passivation and improvement of charge carrier transport are critical for the large development of high surface area, 2D photo-catalysts. Here, we propose a hetero-structure nanoporous network with a patch-like coating as high performance 2D photo-catalysts. The hetero-structured building blocks are composed of a photo-active WSe2 nanoflake in direct contact with both a conducting rGO nanosheet and an ultrathin layer of healing catalyst. The resulting nanoporous film achieves a H2 evolution photocurrent density up to 5 mA cm−2 demonstrating that the patch-like hetero-structures represent an effective strategy to simultaneously improve hole collection, defect passivation and charge transfer. These hetero-structures made of an ultrathin healing catalyst layer represent promising building blocks for the bottom-up fabrication of high surface area photocathodes particularly for 2D photo-catalysts displaying high defect concentration

Enhanced charge?discharge properties of SnO2 nanocrystallites in confined carbon nanospace

Almost perfect embedding of SnO2 nanocrystallites in carbon nanopores was achieved by in situ synthesis using vaporized SnCl2 and silica opal-derived nanoporous carbons. The reversibility of SnO2-Sn conversion and Sn-Li alloying/de-alloying reactions was greatly enhanced by the confinement in regulated carbon nanospace