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1.
Nano Lett ; 20(2): 918-928, 2020 02 12.
Artigo em Inglês | MEDLINE | ID: mdl-31815484

RESUMO

We combined advanced TEM (HRTEM, HAADF, EELS) with solid-state (SS)MAS NMR and electroanalytical techniques (GITT, etc.) to understand the site-specific sodiation of selenium (Se) encapsulated in a nanoporous carbon host. The architecture employed is representative of a wide number of electrochemically stable and rate-capable Se-based sodium metal battery (SMB) cathodes. SSNMR demonstrates that during the first sodiation, the Se chains are progressively cut to form an amorphous mixture of polyselenides of varying lengths, with no evidence for discrete phase transitions during sodiation. It also shows that Se nearest the carbon pore surface is sodiated first, leading to the formation of a core-shell compositional profile. HRTEM indicates that the vast majority of the pore-confined Se is amorphous, with the only localized presence of nanocrystalline equilibrium Na2Se2 (hcp) and Na2Se (fcc). A nanoscale fracture of terminally sodiated Na-Se is observed by HAADF, with SSNMR, indicating a physical separation of some Se from the carbon host after the first cycle. GITT reveals a 3-fold increase in Na+ diffusivity at cycle 2, which may be explained by the creation of extra interfaces. These combined findings highlight the complex phenomenology of electrochemical phase transformations in nanoconfined materials, which may profoundly differ from their "free" counterparts.

2.
J Phys Condens Matter ; 31(6): 063001, 2019 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-30523939

RESUMO

There are a rapidly growing number of studies showing exciting new opportunities in the way confinement effects on surfaces affect the properties of materials and their chemistry. These effects have been observed recently under two-dimensional (2D) van der Waals materials such as a graphene and boron nitride and for the case of supported 2D-porous oxides, including silicates, aluminosilicates and zeolite nanosheets. This review summarizes the current state of the art in this area of research and how confinement effects in 2D systems relate to those found in 3D porous and layered materials. The focus of this review is put in 2D-materials with inherent porosity, such as 2D-porous oxides. An outlook is also given for the future of this exciting emerging area.

3.
J Phys Chem B ; 122(2): 472-478, 2018 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-28800394

RESUMO

The interaction of water vapor with a single crystal ZnO(101̅0) surface was investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments were performed at 0.3 and 0.07 Torr water vapor pressure at sample temperatures ranging from 750 to 295 K up to a maximum of 2% relative humidity (RH). Below 10-4 % RH the ZnO(101̅0) interface is covered with ∼0.25 monolayers of OH groups attributed to dissociation at nonstoichiometric defect sites. At ∼10-4 % RH there is a sharp onset in increased surface hydroxylation attributed to reaction at stoichiometric terrace sites. The surface saturates with an OH monolayer ∼0.26 nm thick and occurs in the absence of any observable molecularly bound water, suggesting the formation of a 1 × 1 dissociated monolayer structure. This is in stark contrast to ultrahigh vacuum experiments and molecular simulations that show the optimum structure is a 2 × 1 partially dissociated H2O/OH monolayer. The sharp onset to terrace site hydroxylation at ∼10-4 % RH for ZnO(101̅0) contrasts with APXPS observations for MgO(100) which show a sharp onset at 10-2 % RH. A surface thermodynamic analysis reveals that this shift to lower RH for ZnO(101̅0) compared to MgO(100) is due to a more favorable Gibbs free energy for terrace site hydroxylation.

4.
J Phys Chem B ; 122(2): 855-863, 2018 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-29091455

RESUMO

The role of oxygen in the activation of C-H bonds in methane on clean and oxygen-precovered Cu(111) and Cu2O(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and "moderate pressures" was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of C-H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipole-dipole attraction of O-H and Cu-CH3 species. The C-H bond activation barriers on Cu2O(111) surfaces are large due to the weak stabilization of H and CH3 fragments.

5.
Rev Sci Instrum ; 88(10): 105109, 2017 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-29092473

RESUMO

This paper describes the design and construction of a compact, "user-friendly" polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS) instrument at the Center for Functional Nanomaterials (CFN) of Brookhaven National Laboratory, which allows studying surfaces at pressures ranging from ultra-high vacuum to 100 Torr. Surface infrared spectroscopy is ideally suited for studying these processes as the vibrational frequencies of the IR chromophores are sensitive to the nature of the bonding environment on the surface. Relying on the surface selection rules, by modulating the polarization of incident light, it is possible to separate the contributions from the isotropic gas or solution phase, from the surface bound species. A spectral frequency range between 1000 cm-1 and 4000 cm-1 can be acquired. While typical spectra with a good signal to noise ratio can be obtained at elevated pressures of gases in ∼2 min at 4 cm-1 resolution, we have also acquired higher resolution spectra at 0.25 cm-1 with longer acquisition times. By way of verification, CO uptake on a heavily oxidized Ru(0001) sample was studied. As part of this test study, the presence of CO adsorbed on Ru bridge sites was confirmed, in agreement with previous ambient pressure X ray photoelectron spectroscopy studies. In terms of instrument performance, it was also determined that the gas phase contribution from CO could be completely removed even up to pressures close to 100 Torr. A second test study demonstrated the use of the technique for studying morphological properties of a spin coated polymer on a conductive surface. Note that this is a novel application of this technique. In this experiment, the polarization of incident light was modulated manually (vs. through a photoelastic modulator). It was demonstrated, in good agreement with the literature, that the polymer chains preferentially lie parallel with the surface. This PM-IRRAS system is small, modular, and easily reconfigurable. It also features a "vacuum suitcase" that allows for the integration of the PM-IRRAS system with the rest of the suite of instrumentation at our laboratory available to external users through the CFN user proposal system.

6.
Nat Commun ; 8: 16118, 2017 07 17.
Artigo em Inglês | MEDLINE | ID: mdl-28714478

RESUMO

The confinement of noble gases on nanostructured surfaces, in contrast to bulk materials, at non-cryogenic temperatures represents a formidable challenge. In this work, individual Ar atoms are trapped at 300 K in nano-cages consisting of (alumino)silicate hexagonal prisms forming a two-dimensional array on a planar surface. The trapping of Ar atoms is detected in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. The atoms remain in the cages upon heating to 400 K. The trapping and release of Ar is studied combining surface science methods and density functional theory calculations. While the frameworks stay intact with the inclusion of Ar atoms, the permeability of gasses (for example, CO) through them is significantly affected, making these structures also interesting candidates for tunable atomic and molecular sieves. These findings enable the study of individually confined noble gas atoms using surface science methods, opening up new opportunities for fundamental research.

7.
J Am Chem Soc ; 137(32): 10104-7, 2015 Aug 19.
Artigo em Inglês | MEDLINE | ID: mdl-26218072

RESUMO

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.

8.
Chemistry ; 20(30): 9176-83, 2014 Jul 21.
Artigo em Inglês | MEDLINE | ID: mdl-24990633

RESUMO

For the last 15 years, we have been studying the preparation and characterization of ordered silica films on metal supports. We review the efforts so far, and then discuss the specific case of a silica bilayer, which exists in a crystalline and a vitreous variety, and puts us into a position to investigate, for the first time, the real space structure (AFM/STM) of a two-dimensional glass and its properties. We show that pair correlation functions determined from the images of this two-dimensional glass are similar to those determined by X-ray and neutron scattering from three-dimensional glasses, if the appropriate sensitivity factors are taken into account. We are in a position, to verify, for the first time, a model of the vitreous silica structure proposed by William Zachariasen in 1932. Beyond this, the possibility to prepare the crystalline and the glassy structure on the same support allows us to study the crystal-glass phase transition in real space. We, finally, discuss possibilities to use silica films to start investigating related systems such as zeolites and clay films. We also mention hydroxylation of the silica films in order to adsorb metal atoms modeling heterogenized homogeneous catalysts.

9.
Angew Chem Int Ed Engl ; 53(26): 6705-9, 2014 Jun 23.
Artigo em Inglês | MEDLINE | ID: mdl-24839958

RESUMO

Rising atmospheric CO2 is expected to have negative effects on the global environment from its role in climate change and ocean acidification. Utilizing CO2 as a feedstock to make valuable chemicals is potentially more desirable than sequestration. A substantial reduction of CO2 levels requires a large-scale CO2 catalytic conversion process, which in turn requires the discovery of low-cost catalysts. Results from the current study demonstrate the feasibility of using the non-precious metal material molybdenum carbide (Mo2C) as an active and selective catalyst for CO2 conversion by H2.

10.
J Phys Chem Lett ; 5(20): 3577-81, 2014 Oct 16.
Artigo em Inglês | MEDLINE | ID: mdl-26278612

RESUMO

The structure of the 1-D oligomer chains that form on a Au(111) surface following adsorption of 1,4-phenylene diisocyanide (PDI) is explored using reflection-absorption infrared spectroscopy and scanning tunneling microscopy (STM). The experimental work is complemented by first-principles density functional theory calculations, which indicate that the previously proposed gold-PDI oligomer chains in which the PDI molecule bridged gold adatoms are thermodynamically stable. In addition, the calculated vibrational modes for this structure are in excellent agreement with the experimental infrared data. The linkage of the PDI units by gold adatoms is confirmed by comparing STM images collected as a function of tip bias with images for the calculated structure by the Bardeen method.

11.
Chem Commun (Camb) ; 49(14): 1422-4, 2013 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-23310932

RESUMO

It is demonstrated that 1,4-phenylene diisocyanide (PDI)-gold oligomers can spontaneously bridge between gold nanoparticles on mica, thereby providing a strategy for electrically interconnecting nanoelectrodes. The barrier height of the bridging oligomer is 0.10 ± 0.02 eV, within the range of previous single-molecule measurements of PDI.

12.
Langmuir ; 27(15): 9337-44, 2011 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-21671579

RESUMO

A thiophene-containing molecule attached to a scanning tunneling microscopy (STM) tip is used to transport gold atoms on a Au(111) surface. The molecule contains eight thiophene rings and therefore has sulfur atoms that are known to bind to gold atoms. Using a gold-coated tip, the molecules previously deposited on the surface bind to the lower-coordination gold atoms of the tip. When that tip is used to scan the surface, the still free thiophene rings (not all of the sulfur atoms bind to the tip) can attach to gold atoms from the surface and drag them along the scanning direction, depositing them either at the position where the tip changes its scanning direction or where the tip encounters an "up step", whichever event occurs first.


Assuntos
Ouro/química , Tiofenos/química , Microscopia de Tunelamento , Conformação Molecular , Tamanho da Partícula , Propriedades de Superfície
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