The stretching vibration of hydrogen adsorbed on epitaxial graphene studied by sum-frequency generation spectroscopy

Electrochemical Storage of Atomic Hydrogen on Single Layer Graphene

If hydrogen can be stored and carried safely at a high density, hydrogen-fuel cells offer effective solutions for vehicles. The stable chemisorption of atomic hydrogen on single layer graphene (SLG) seems a perfect solution in this regard, with a theoretical maximum storage capacity of 7.7 wt %. However, generating hydrogenated graphene from H₂ requires extreme temperatures and pressures. Alternatively, hydrogen adatoms can easily be produced under mild conditions by the electroreduction of protons in solid/liquid systems. Graphene is electrochemically inert for this reaction, but H-chemisorption on SLG can be carried out under mild conditions via a novel Pt-electrocatalyzed "spillover-surface diffusion-chemisorption" mechanism, as we demonstrate using dynamic electrochemistry and isotopic Raman spectroscopy. The apparent surface diffusion coefficient (∼10^-5 cm² s^-1), capacity (∼6.6 wt %, ∼85.7% surface coverage), and stability of hydrogen adatoms on SLG at room temperature and atmospheric pressure are significant, and they are perfectly suited for applications involving stored hydrogen atoms on graphene.

Characterisation, coverage, and orientation of functionalised graphene using sum-frequency generation spectroscopy

We report the unambiguous detection of phenyl groups covalently attached to functionalised graphene using non-linear spectroscopy. Sum-frequency generation was employed to probe graphene on a gold surface after chemical functionalisation using a benzene diazonium salt. We observe a distinct resonance at 3064 cm-1 which can clearly be assigned to an aromatic C-H stretch by comparison with a self-assembled monolayer on a gold substrate formed from benzenethiol. Not only does sum-frequency generation spectroscopy allow one to characterise functionalised graphene with higher sensitivity and much better specificity than many other spectroscopic techniques, but it also opens up the possibility to assess the coverage of graphene with functional groups, and to determine their orientation relative to the graphene surface.

Exciting H2 Molecules for Graphene Functionalization

Hydrogen functionalization of graphene by exposure to vibrationally excited H₂ molecules is investigated by combined scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H₂ molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H₂ dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H-H bond. The first reacting H₂ molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in reduced barriers for dissociative adsorption of subsequent H₂ molecules. This enables functionalization by H₂ molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface and additionally set the stage for a re-interpretation of previous experimental work involving elevated H₂ background gas pressures in the presence of hot filaments.

Selective Hydrogen Adsorption in Graphene Rotated Bilayers

The absorption energy of atomic hydrogen at rotated graphene bilayers is studied using ab initio methods based on the density functional theory including van der Waals interactions. We find that, due to the surface corrugation induced by the underneath rotated layer and the perturbation of the electronic density of states near the Fermi energy, the atoms with an almost AA stacking are the preferential ones for hydrogen chemisorption. The adsorption energy difference between different atoms can be as large as 80 meV. In addition, we find that, due to the logarithmic van Hove singularities in the electronic density of states at energies close to the Dirac point, the adsorption energy of either electron or hole doped samples is substantially increased. We also find that the adsorption energy increases with the decrease of the rotated angle between the layers. Finally, the large zero point energy of the C-H bond (∼0.3 eV) suggests adsorption and desorption of atomic hydrogen and deuterium should behave differently.

Unveiling the Mechanisms Leading to H2 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.

Optical Interference Enhances Nonlinear Spectroscopic Sensitivity: When Light Gives You Lemons, Model Lemonade

Optical interference effects can be a nuisance in spectroscopy, especially in nonlinear experiments in which multiple incoming and outgoing beams are present. Vibrational sum frequency generation is particularly susceptible to interference effects because it is often applied to planar, layered materials, driving many of its practitioners to great lengths to avoid signal generation from multiple interfaces. In this perspective, we take a positive view of this metaphorical "lemon" and demonstrate how optical interference can be used as a tool to extract subtle changes in interfacial vibrational spectra. Specifically, we use small frequency shifts at a buried interface in an organic field-effect transistor to determine the fractional charge per molecule during device operation. The transfer matrix approach to nonlinear signal modeling is general and readily applied to complex layered samples that are increasingly popular in modern studies. More importantly, we show that a failure to consider interference effects can lead to erroneous interpretations of nonlinear data.

Preparation of Graphene with Graphane Areas of Controlled Hydrogen Isotope Composition on Opposite Sides

Monolayer graphene was prepared on an Ir(111) substrate where it exhibits a 25 × 25 Å(2) moiré pattern. Molecular hydrogen was dosed first, allowing it to dissociate on open areas of the Ir substrate. The generated H atoms formed an intercalated reservoir that can bind to the graphene subsequently. Next, atomic hydrogen was dosed, which binds to the graphene sheet and also initiates the transfer of H from the Ir substrate to the graphene sheet. The opposite sides of the sheet can be hydrogenated with isotope selectivity, as a sequence of difference isotopes, H or D, can be chosen at will in the preparation procedure. Sum-frequency generation spectra prove that as consequence of the dosing sequence, C-H bonds are predominantly pointing toward the Ir substrate side when H2 is dosed first and alternatively toward the vacuum side when D2 is dosed first.

Controlling hydrogenation of graphene on Ir(111)

Combined fast X-ray photoelectron spectroscopy and density functional theory calculations reveal the presence of two types of hydrogen adsorbate structures at the graphene/Ir(111) interface, namely, graphane-like islands and hydrogen dimer structures. While the former give rise to a periodic pattern, dimers tend to destroy the periodicity. Our data reveal distinctive growth rates and stability of both types of structures, thereby allowing one to obtain well-defined patterns of hydrogen clusters. The ability to control and manipulate the formation and size of hydrogen structures on graphene facilitates tailoring of its properties for a wide range of applications by means of covalent functionalization.

Organic constituents on the surfaces of aerosol particles from Southern Finland, Amazonia, and California studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

Hydrocarbon on Carbon: Coherent Vibrational Spectroscopy of Toluene on Graphite

The ability to study the interactions of hydrocarbons on carbon surfaces is an integral step toward gaining a molecular level understanding of the chemical reactions and physical properties occurring on them. Here, we apply vibrational sum frequency generation (SFG) to determine the tilt angle of toluene, a common organic solvent, on millimeter-thick highly oriented pyrolytic graphite (HOPG). The combination of a time-delay technique, which results in the successful suppression of the nonresonant SFG response, and a null angle method is shown to overcome the "strong optical absorber" problem posed by macroscopically thick carbon samples and yields a molecular tilt angle of toluene in the range of 37° to 42° from the surface normal. The implications of this approach for determining the orientation of organic species adsorbed on carbon interfaces, which are important for energy-relevant processes, are discussed.

Effects of molecular adsorption on the electronic structure of single-layer graphene

The interaction of small molecules (CCl(4), CS(2), H(2)O, and acetone) with single-layer graphene (SLG) has been studied under steady-state conditions using infrared multiple-internal-reflection spectroscopy. Adsorption results in a broad and intense absorption band, spanning the ∼200 to 500 meV range, which is attributed to electronic excitation. This effect, which has not previously been reported for SLG, has been further investigated using dispersion-corrected density functional theory to model the adsorption of H(2)O on SLG supported on an SiO(2) substrate. However, the ideal and defect-free model does not reproduce the observed adsorption-induced electronic transition. This and other observations suggest that the effect is extrinsic, possibly the result of an adsorption-induced change in the in-plane strain, with important differences arising between species that form liquid-like layers under steady-state conditions and those that do not. Furthermore, the C-H stretching modes of CH(2) groups, incorporated in the SLG as defects, undergo nonadiabatic coupling to the electronic transition. This leads to pronounced antiresonance effects in the line shapes, which are analyzed quantitatively. These results are useful in understanding environmental effects on graphene electronic structure and in demonstrating the use of the vibrational spectroscopy of H-containing defects in characterizing SLG structure.