Adsorption of Water and Ammonia on Graphene: Evidence for Chemisorption from X-ray Absorption Spectra

Adsorption of Lanthanide Atoms on Graphene: Similar, Yet Different

Many theoretical studies address the interaction of different atoms with graphene; however, the relevant information on the adsorption of the lanthanide species remains limited and controversial, creating a gap in this important area of graphene chemistry and physics. By employing periodic density functional theory calculations, we provide the key theoretical information for the entire series from lanthanum to lutetium interacting with defect-free graphene, including the interaction strength and distances, charge and spin of the lanthanide atoms, and comparative features of the density of states. The central lanthanides Gd, Tb, and Dy exhibit the strongest bonding and shortest distances. The positive charge acquired by the lanthanide atoms varies insignificantly, with the exception of Yb and Lu with a filled 4f shell. The spin increases from La to Tb and then decreases sharply, achieving minimal values for Tm, Yb, and Lu. Interaction with graphene influences even the deeper 5s and 5p shells.

Metastable Brominated Nanodiamond Surface Enables Room Temperature and Catalysis-Free Amine Chemistry

Bromination of high-pressure, high-temperature (HPHT) nanodiamond (ND) surfaces has not been explored and can open new avenues for increased chemical reactivity and diamond lattice covalent bond formation. The large bond dissociation energy of the diamond lattice-oxygen bond is a challenge that prevents new bonds from forming, and most researchers simply use oxygen-terminated NDs (alcohols and acids) as reactive species. In this work, we transformed a tertiary-alcohol-rich ND surface to an amine surface with ∼50% surface coverage and was limited by the initial rate of bromination. We observed that alkyl bromide moieties are highly labile on HPHT NDs and are metastable as previously found using density functional theory. The strong leaving group properties of the alkyl bromide intermediate were found to form diamond-nitrogen bonds at room temperature and without catalysts. This robust pathway to activate a chemically inert ND surface broadens the modalities for surface termination, and the unique surface properties of brominated and aminated NDs are impactful to researchers for chemically tuning diamond for quantum sensing or biolabeling applications.

Computational modelling of ammonia addition on partially reduced graphene oxide flakes

Density functional theory is employed to model the chemisorption of ammonia on epoxy-containing polycyclic aromatic hydrocarbons (PAHs) and understand the reaction mechanism of ammonia addition on partially reduced graphene oxide flakes. Coronene (C₂₄H₁₂) and ovalene (C₃₂H₁₄) based four-epoxy group containing molecules are used to mimic the RGO surface properties. The reaction mechanism changing effect of a second ammonia molecule as well as explicit water molecules is considered. The proposed reaction mechanism consists of two steps: the migration of one epoxy group out of the modelled four-epoxy group formation to a thermodynamically less stable one and the nucleophilic addition of the ammonia molecule. The second step involves forming an amine group and reducing an epoxy group to a hydroxyl one. Interestingly, the forming amine group bonds to the carbon atom with the smallest bond order among the available ones and not necessarily to the carbon atom of the opening epoxy ring. Incorporating a second ammonia molecule has a negligible effect on the overall reaction mechanism, while in the presence of one water molecule, the reaction goes through a different pathway involving a trimolecular state during the nucleophilic addition. Including more than one water molecule or applying an implicit solvent model does not cause further changes in the reaction.

Few-layer graphene on nickel enabled sustainable dropwise condensation

Condensation is critical for a wide range of applications such as electrical power generation, distillation, natural gas processing, dehumidification and water harvest, and thermal management. Compared with "filmwise" mode of condensation (FWC) prevailing in industrial-scale systems, dropwise condensation (DWC) can provide an order of magnitude higher heat transfer rate owing to drastically reduced thermal resistance from the formation of discrete and mobile droplets. In the past, promoting DWC by controlling surface wetting has attracted wide attention, but DWC highly relies on non-wetting surfaces and only lasts days under practical conditions due to the poor reliability of coatings. Here, we developed nanostructured graphene coatings on nickel (Ni) substrates that we can control and enhance the nucleation of water droplets on graphene grain boundaries. Surprisingly, this enables DWC even under normal "wetting" conditions. This is contradictory to the widely accepted DWC mechanism. Moreover, the Ni-graphene surface enables exceptional long-term condensation from days to more than 3 years under practical or even more aggressive testing environments.

X-ray Photoionization Cross Section Spectra of Water and Ammonia Bonded on Polycyclic Aromatic Hydrocarbons: A Quantum Mechanical Interpretation to the Absorption Spectra on Graphene

A theoretical study of the K-shell total photoabsorption and photoionization cross section spectra of water and ammonia bonded to benzene (C₆H₆) and the polycyclic aromatic hydrocarbons (PAHs) naphthalene (C₁₀H₈), coronene (C₂₄H₁₂) and circumcoronene (C₅₄H₁₈) by van der Waals (vdW) forces is presented. The discretized electronic pseudospectra at the oxygen and nitrogen K-edges, covering the discrete and the continuum spectral regions, were obtained at the time-dependent density functional theory (TDDFT) level with dispersion correction. An analytic continuation procedure based on the Padé approximants was used in order to obtain the K-shell cross sections of the structures at the discrete and the continuum regions of the spectra. By examining the electronic spectra of water and ammonia bonded to coronene and circumcoronene, we observed that our results agree well with the experiments performed with graphene. This work provides a quantum mechanical interpretation to the NEXAFS experiments of water and ammonia adsorbed on graphene in terms of a physisorption model of these molecules by van der Waals forces.

Translucency of Graphene to van der Waals Forces Applies to Atoms/Molecules with Different Polar Character

Graphene has been proposed to be either fully transparent to van der Waals interactions to the extent of allowing switching between hydrophobic and hydrophilic behavior, or partially transparent (translucent), yet there has been considerable debate on this topic, which is still ongoing. In a combined experimental and theoretical study we investigate the effects of different metal substrates on the adsorption energy of atomic (argon) and molecular (carbon monoxide) adsorbates on high-quality epitaxial graphene. We demonstrate that while the adsorption energy is certainly affected by the chemical composition of the supporting substrate and by the corrugation of the carbon lattice, the van der Waals interactions between adsorbates and the metal surfaces are partially screened by graphene. Our results indicate that the concept of graphene translucency, already introduced in the case of water droplets, is found to hold more generally also in the case of single polar molecules and atoms, which are apolar.

Gateless and reversible carrier density tunability in epitaxial graphene devices functionalized with chromium tricarbonyl

Monolayer epitaxial graphene (EG) has been shown to have clearly superior properties for the development of quantized Hall resistance (QHR) standards. One major difficulty with QHR devices based on EG is that their electrical properties drift slowly over time if the device is stored in air due to adsorption of atmospheric molecular dopants. The crucial parameter for device stability is the charge carrier density, which helps determine the magnetic flux density required for precise QHR measurements. This work presents one solution to this problem of instability in air by functionalizing the surface of EG devices with chromium tricarbonyl -Cr(CO)3. Observations of carrier density stability in air over the course of one year are reported, as well as the ability to tune the carrier density by annealing the devices. For low temperature annealing, the presence of Cr(CO)3 stabilizes the electrical properties and allows for the reversible tuning of the carrier density in millimeter-scale graphene devices close to the Dirac point. Precision measurements in the quantum Hall regime show no detrimental effect on the carrier mobility.

Water on Graphene-Coated TiO2: Role of Atomic Vacancies

Beyond two-dimensional (2D) materials, interfaces between 2D materials and underlying supports or 2D-coated metal or metal oxide nanoparticles exhibit excellent properties and promising applications. The hybrid interface between graphene and anatase TiO₂ shows great importance in photocatalytic, catalytic, and nanomedical applications due to the excellent and complementary properties of the two materials. Water, as a ubiquitous and essential element in practical conditions and in the human body, plays a significant role in the applications of graphene/TiO₂ composites for both electronic devices and nanomedicine. Carbon vacancies, as common defects in chemically prepared graphene, also need to be considered for the application of graphene-based materials. Therefore, the behavior of water on top and at the interface of defective graphene on anatase TiO₂ surface was systematically investigated by dispersion-corrected hybrid density functional calculations. The presence of the substrate only slightly enhances the on-top adsorption and reduces the on-top dissociation of water on defective graphene. However, at the interface, dissociated water is largely preferred compared with undissociated water on bare TiO₂ surface, showing a prominent cover effect. Reduced TiO₂ may further induce oxygen diffusion into the bulk. Our results are helpful to understand how the presence of water in the surrounding environment affects structural and electronic properties of the graphene/TiO₂ interface and thus its application in photocatalysis, electronic devices, and nanomedicine.

Liquid-Cell Electron Microscopy of Adsorbed Polymers

Individual macromolecules of polystyrene sulfonate and poly(ethylene oxide) are visualized with nanometer resolution using transmission electron microscopy (TEM) imaging of aqueous solutions with and without added salt, trapped in liquid pockets between creased graphene sheets. Successful imaging with 0.3 s per frame is enabled by the sluggish mobility of the adsorbed molecules. This study finds, validating others, that an advantage of this graphene liquid-cell approach is apparently to retard sample degradation from incident electrons, in addition to minimizing background scattering because graphene windows are atomically thin. Its new application here to polymers devoid of metal-ion labeling allows the projected sizes and conformational fluctuations of adsorbed molecules and adsorption-desorption events to be analyzed. Confirming the identification of the observed objects, this study reports statistical analysis of datasets of hundreds of images for times up to 100 s, with variation of the chemical makeup of the polymer, the molecular weight of the polymer, and the salt concentration. This observation of discrete polymer molecules in solution environment may be useful generally, as the findings are obtained using an ordinary TEM microscope, whose kind is available to many researchers routinely.