Metal organic frameworks as sorption media for volatile and semi-volatile organic compounds at ambient conditions

In this research, we investigated the sorptive behavior of a mixture of 14 volatile and semi-volatile organic compounds (four aromatic hydrocarbons (benzene, toluene, p-xylene, and styrene), six C-2-C-5 volatile fatty acids (VFAs), two phenols, and two indoles) against three metal-organic frameworks (MOFs), i.e., MOF-5, Eu-MOF, and MOF-199 at 5 to 10 mPa VOC partial pressures (25 degrees C). The selected MOFs exhibited the strongest affinity for semi-volatile (polar) VOC molecules (skatole), whereas the weakest affinity toward was volatile (non-polar) VOC molecules (i.e., benzene). Our experimental results were also supported through simulation analysis in which polar molecules were bound most strongly to MOF-199, reflecting the presence of strong interactions of Cu2+ with polar VOCs. In addition, the performance of selected MOFs was compared to three well-known commercial sorbents (Tenax TA, Carbopack X, and Carboxen 1000) under the same conditions. The estimated equilibrium adsorption capacity (mg.g(-1)) for the all target VOCs was in the order of; MOF-199 (71.7) > Carboxen-1000 (68.4) > Eu-MOF (27.9) > Carbopack X (24.3) > MOF-5 (12.7) > Tenax TA (10.6). Hopefully, outcome of this study are expected to open a new corridor to expand the practical application of MOFs for the treatment diverse VOC mixtures.This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science, and Technology (MEST) (No. 2009-0093848). E Kwon also acknowledges the support made by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (No. 2914RA1A004893). The third author thanks SERB-DST, New Delhi for 'Young Scientist-Start up Research Grant (YSS/2015/001440)

Sn-loss effect in a Sn-implanted a-SiO2 host-matrix after thermal annealing: A combined XPS, PL, and DFT study

Amorphous a-SiO2 host-matrices were implanted with Sn-ions with and without posterior thermal tempering at 900 degrees C for 1 h in ambient air. X-ray photoelectron spectroscopy analysis (XPS core-levels, XPS valence band mapping), photoluminescence (PL) probing, and density functional calculations (DFT) were employed to enable a detailed electronic structure characterization of these samples. It was experimentally established that the process of Sn-embedding into the a-SiO2 host occurs following two dissimilar trends: the Sn4+ -�� SOi(4+) substitution in a-SiO2:Sn (without tempering), and Sn-metal clustering as interstitials in a-SiO2:Sn (900 degrees C, 1 h). Both trends were modeled using calculated formation energies and partial densities of states (PDOS) as well as valence band (VB) simulations, which yielded evidence that substitutional defect generation occurs with the help of ion-implantation stimulated translocation of the host-atoms from their stoichiometric positions to the interstitial void. Experimental and theoretical data obtained coincide in terms of the reported Sn-loss effect in a-SiO2:Sn (900 degrees C, 1 h) due to thermally-induced electronic host-structure re-arraignment, which manifests as backward host-atoms translocation into stoichiometric positions and the posterior formation of Sn-metal clusters. (C) 2016 Elsevier B.V. All rights reserved.The preparation of a-SiO<INF>2</INF> samples, ion-implantation treatment, and photoluminescence measurements were supported by the Russian Foundation for Basic Research (Projects RFBR Nos. 13-08-00568 and 13-02-91333), the Act 211 of the Government of the Russian Federation (Contract No. 02.A03.21.0006), and the Government Assignment of the Russian Ministry of Education and Science (3.1016.2014/K). The XPS measurements were supported by the Russian Science Foundation (Project No. 14-22-00004)

The influence of chemical reactivity of surface defects on ambient-stable InSe-based nanodevices

We demonstrate that, in contrast to most two-dimensional materials, ultrathin flakes of InSe are stable under ambient conditions. Despite their ambient stability, InSe-based nanodevices show an environmental p-type doping, suppressed by capping InSe with hexagonal boron nitride. By means of transport experiments, density functional theory and vibrational spectroscopy, we attribute the p-type doping assumed by uncapped InSe under an ambient atmosphere to the decomposition of water at Se vacancies. We have estimated the site-dependent adsorption energy of O-2, N-2, H2O, CO and CO2 on InSe. A stable adsorption is found only for the case of H2O, with a charge transfer of only 0.01 electrons per water molecule.AP and GC thank Fabio Vito for technical support. The work at the UC Riverside was supported, in part, by SRC and DARPA through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) and by the Emerging Frontiers of Research Initiative (EFRI) 2-DARE project NSF 005400

Toward the Effective Exploitation of Topological Phases of Matter in Catalysis: Chemical Reactions at the Surfaces of NbAs and TaAs Weyl Semimetals

By means of density functional theory and experiments, surface chemical reactivity of single crystals of NbAs and TaAs Weyl semimetals is studied. Weyl semimetals exhibit outstanding reactivity toward simple molecules (oxygen, carbon monoxide, and water), with several active sites available for surface chemical reactions (adsorption, decomposition, formation of reaction products, recombination of decomposition fragments). When different chemical species are adsorbed on Weyl semimetals, strong lateral interactions between coadsorbed species occur, evidenced by CO-promoted water decomposition at room temperature. The resulting OH groups react with CO to form HCOO, which is an intermediate species in water-gas shift reaction. These findings unambiguously demonstrate that Weyl semimetals could be effectively used in catalysis, whereas their employment in nanoelectronics or plasmonics is complicated by the poor ambient stability, due to the rapid surface oxidation, inevitably occurring unless protective capping layers are used.L.W. acknowledges support from the Youth Innovation Promotion Association (CAS), the State Key Program for Basic Research of China (No. 2017YFA0305500), and National Natural Science Foundation of China (Nos. 61405230 and 61675222)

Unveiling the Mechanisms Leading to H-2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.A.P. and G.C. thank Fabio Vito for technical support. A.P. and R.L. thank Elettra Sincrotrone Trieste S.C.p.A. for financial support. R.L. acknowledges the support by MIUR through the program "Progetto Premiale 2012" - Project ABNANOTECH. S.A., M.C., and G.G. acknowledge Italian MIUR through the national grant Futuro in Ricerca 2012 RBFR128BEC "Beyond graphene: tailored C-layers for novel catalytic materials and green chemistry" and by the University of Padova funded project: CPDA128318/12 "Study of the catalytic activity of complex graphene nanoarchitectures from ideal to real conditions". D.F. acknowledges European Union; (FP7): Theme NMP.2012.1.4-3 grant no. 309672. Calculations are partially supported by the Ministry of Education and Science of the Russian Federation, project no. 16.1751.2014/K

Mixed Substitution in P-Doped Anatase TiO2 Probed by XPS and DFT

The investigation of the electronic structure of P-ion implanted TiO2 thin films (E=30keV, D=1x10(17)cm(-2)) with anatase structure is performed by X-ray photoelectron spectroscopy (XPS) measurements (core levels and valence bands) and first-principles density functional theory (DFT) calculations. It is found that the XPS P 2p-spectra reveal the presence of two signals at 134.2 and 130.3eV which can be attributed to the formation of P-O (with P5+ ions) and P-Ti (with P3- ions) bonds, respectively. This means that both cationic (PTi) and anionic (PO) substitution take place in P-ion implanted anatase thin films. This conclusion is confirmed by DFT calculations which show that the XPS valence band structure of P:TiO2 can be reproduced only under mixed substitution. The presence of two kinds of phosphorus ions (P5+ and P3-) in ion-implanted TiO2 can be useful for developing new multifunctional advanced materials with configurable properties for a wide range of applications.Authors thank D.A. Zatsepin for help in XPS measurements. The CPA calculations were supported by Russian Science Foundation (Project 14-22-00004). Ion-implantation stimulated synthesis of the samples and XPS measurements was made under support of the Act 211 of the Government of Russian Federation (Agreement No. 02.A03.21.0006)

Evidencing Interfacial Charge Transfer in 2D CdS/2D MXene Schottky Heterojunctions toward High‐Efficiency Photocatalytic Hydrogen Production

Photocatalytic water splitting by heterojunction nanostructures is considered as one of the most favorable pathways for direct solar-to-hydrogen conversion. High-efficiency solar hydrogen production demands an effective separation of charge carriers and their rapid transport to the interface, whereas the charge-transfer pathway in heterojunction photocatalysts is largely elusive. Herein, 2D CdS/2D MXene Schottky heterojunctions are synthesized via a sequence of electrostatic self-assembly process and solvothermal method. The composite photocatalysts exhibit highly efficient and robust hydrogen-evolving performance, far superior than the pristine CdS nanosheets. Furthermore, density functional theory (DFT) calculations are adopted to unveil the charge-transport pathway. It is revealed that an intimate Schottky contact is constructed between CdS and MXene, which further steers the formation of charge flow and expedites the charge migration from CdS to MXene, thus suppressing the recombination of photogenerated charge carriers and boosting the photocatalytic activity for hydrogen evolution.</p

Evidencing Interfacial Charge Transfer in 2D CdS/2D MXene Schottky Heterojunctions toward High‐Efficiency Photocatalytic Hydrogen Production

Photocatalytic water splitting by heterojunction nanostructures is considered as one of the most favorable pathways for direct solar-to-hydrogen conversion. High-efficiency solar hydrogen production demands an effective separation of charge carriers and their rapid transport to the interface, whereas the charge-transfer pathway in heterojunction photocatalysts is largely elusive. Herein, 2D CdS/2D MXene Schottky heterojunctions are synthesized via a sequence of electrostatic self-assembly process and solvothermal method. The composite photocatalysts exhibit highly efficient and robust hydrogen-evolving performance, far superior than the pristine CdS nanosheets. Furthermore, density functional theory (DFT) calculations are adopted to unveil the charge-transport pathway. It is revealed that an intimate Schottky contact is constructed between CdS and MXene, which further steers the formation of charge flow and expedites the charge migration from CdS to MXene, thus suppressing the recombination of photogenerated charge carriers and boosting the photocatalytic activity for hydrogen evolution.</p

Between scylla and charybdis: hydrophobic graphene-guided water diffusion on hydrophilic substrates.

The structure of water confined in nanometer-sized cavities is important because, at this scale, a large fraction of hydrogen bonds can be perturbed by interaction with the confining walls. Unusual fluidity properties can thus be expected in the narrow pores, leading to new phenomena like the enhanced fluidity reported in carbon nanotubes. Crystalline mica and amorphous silicon dioxide are hydrophilic substrates that strongly adsorb water. Graphene, on the other hand, interacts weakly with water. This presents the question as to what determines the structure and diffusivity of water when intercalated between hydrophilic substrates and hydrophobic graphene. Using atomic force microscopy, we have found that while the hydrophilic substrates determine the structure of water near its surface, graphene guides its diffusion, favouring growth of intercalated water domains along the C-C bond zigzag direction. Molecular dynamics and density functional calculations are provided to help understand the highly anisotropic water stripe patterns observed