High-Dimensional Atomistic Neural Network Potential to Study the Alignment-Resolved O2 Scattering from Highly Oriented Pyrolytic Graphite.

Thermal reduction of PdCo molecular precursors may lead to the controlled production of nanoparticles on high surface area carbon supports that can be used as methanol tolerant oxygen reduction catalysts for direct methanol fuel cells (DMFC). Following this concept, a single molecular precursor source was used for the synthesis of bimetallic nanoparticles on highly oriented pyrolytic graphite (HOPG) and Vulcan (VC) carbon supports. Nanostructural formation of palladium-cobalt on highly ordered pyrolytic graphite (HOPG) was study by AFM, SEM and voltammetry. The relative humidity during precursor deposition was used to control the rings self-formation on HOPG surfaces. Palladium and palladium-cobalt nanoparticles were also formed on high surface area carbon support (Vulcan XC-72R) by thermal reduction and characterized by TEM. The Pd/VC and PdCo/VC nanoparticles were tested for the oxygen reduction reaction (ORR) with and without methanol. The Pd-based catalysts have ORR activity and high methanol tolerance.

Hyperthermal atomic oxygen beam-induced etching of HOPG (0001) studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy

We present ab initio molecular dynamics simulations based on the density functional theory to study the alignment-dependent scattering of O from highly oriented pyrolytic graphite (HOPG). Results are obtained for 200 meV O molecules with different alignments impinging with an angle of incidence measured from a surface normal of 22.5° on a thermalized (110 and 300 K) graphite surface. The choice of these initial conditions in our simulations is made to perform comparisons with recent experimental results on this system. Our results show that a lower number of molecules initially aligned normally to the surface is reflected in the scattering plane compared to the number of molecules initially aligned parallel to the surface. This is not due to an increase in the trapping of end-on-aligned molecules but due to the fact that a large amount of them are scattered out-of-plane. Additionally, we find that the final translational energy of end-on molecules is around 10% lower than that of side-on molecules. We show that this is due to a more efficient energy transfer from the translational degree of freedom to internal degrees of freedom for end-on molecules, and not due to a more efficient energy transfer to the surface. The results of our simulations are in overall agreement with the experimental observations regarding the alignment dependence of the in-plane scattering probabilities and the energy loss of the reflected molecules.

Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.

The overall performance of the polymer electrolyte membrane fuel cell (PEMFC) essentially depends on the catalytic activity of its core-part: the membrane electrode assembly (MEA). Palladium (Pd), one of the platinum group elements, is a potential substitute for Pt to be applied as the electrocatalyst, since it has more abundant reserves and a lower price compared to Pt. Pd is stable under extremely corrosive conditions similar to Pt.Nanosized metal particles perform considerably different as compared to the corresponding bulk materials. Furthermore, some reactions catalyzed by metal particles show a strong particle-size sensitivity. In previous work of our group, particle-size dependent activity of metal particles for electrocatalytic reaction was observed, such as Au nanoparticles supported on HOPG for oxygen reduction reaction (ORR) [1] and Pd nanoislands supported on Au(111) surface for hydrogen evolution reaction (HER) [2]. Beside the particle-size effects, support effects (e.g. ligand effect, strain effect) play important roles on electrocatalytic activity as well [3].In order to research the particle-size effects and support effects on the activity and stability of Pd nanoparticles, highly oriented pyrolytic graphite (HOPG) and nitrogen doped HOPG (N-HOPG) [4] were used in this work. Due to the weak interaction of HOPG with metal deposits, the substrate-related effects can be eliminated. N-HOPG is rich in electronegative nitrogen functional groups, and the roughness of N-HOPG is much higher than the untreated HOPG before the nitrogen implantation process. Pd nanoparticles were electrochemically deposited by double-pulse electrochemical deposition on both substrates. Pd nanoparticles are located mainly at the defects and step-edges on HOPG surface, but on the N-HOPG particles are distributed homogeneously. For ORR, the smaller particles have a negative particle-size effect. Pd nanoparticles supported on N-HOPG do not show enhanced activity for ORR. For HER, the case is much more difficult for the behavior of hydrogen absorption of Pd. The content of absorbed hydrogen atoms in Pd NPs is influenced by the particle size. The stability of Pd nanoparticles on HOPG is influenced not only by the particle size, but also by the pH value of the electrolyte. Pd particles on N-HOPG show poorer stability as compared to the pure HOPG insofar as they dissolved rapidly in potential cycling measurements.[1] Brülle, T.; Ju, W.; Niedermayr, P.; Denisenko, A.; Paschos, O.; Schneider, O.; Stimming, U., Molecules 2011, 16 (12), 10059-77.[2] Pandelov, S.; Stimming, U., Electrochimica Acta 2007, 52 (18), 5548-5555.[3] T. Bligaard, J.K. Nørskov, Electrochimica Acta, 52 (2007) 5512-5516.[4] Favaro, M.; Perini, L.; Agnoli, S.; Durante, C.; Granozzi, G.; Gennaro, A., Electrochimica Acta 2013, 88, 477-487.

Adsorption of synthetic homo- and hetero-oligodeoxynucleotides onto highly oriented pyrolytic graphite: Atomic force microscopy characterization

Ni nanoparticle evaporation onto highly oriented pyrolytic graphite (HOPG), with low and high surface defect densities, has been studied in situ by X-ray photoelectron spectroscopic (XPS) techniques, including binding energy shifts, Auger parameters, and Wagner plots. Ni nucleates at defect sites, whether initially present or those introduced by Ar+ bombardment, with the formation of spherical nanoparticles, which adhere strongly through Ni/HOPG back-bonding. The variation of the C 1s peak intensity with Ni coverage suggests that the photoelectron emission yield may be enhanced at lower Ni coverages, due to Ni nanoparticle-induced electron localization and work function reduction at the HOPG surface, which is evidence of such back-bonding.

Electrogenerated microscale bubbles that are confined at the electrode surface have already been extensively studied because of their significant influence on electrochemistry. In contrast, as far as we know, whether nanoscale bubbles exist on the electrode surface has not been experimentally confirmed yet. Here, we report the observation of electrochemically controlled formation and growth of hydrogen nanobubbles on bare highly oriented pyrolytic graphite (HOPG) surface via in-situ tapping mode atomic force microscopy (TMAFM). By using TMAFM imaging, we observed that electrochemically generated hydrogen gas led to the formation of nanobubbles at the HOPG surface. We then employed a combination of techniques, including phase imaging, ex-situ degassing, and tip perturbation, to confirm the gas origin of such observed nanobubbles. We further demonstrated that the formation and growth of nanobubbles could be well controlled by tuning either the applied voltage or the reaction time. Remarkably, we could also monitor the evolution process of nanobubbles, that is, formation, growth, coalescence, as well as the eventual release of merged microbubbles from the HOPG surface.

Molecular functionalization of graphitic surfaces with nanopatterned structures is regarded as one of the effective bottom-up techniques to tune their electronic properties towards electronics applications. Diazonium molecules have been often employed to covalently functionalize graphene and highly oriented pyrolytic graphite (HOPG) substrates. However, controlling the structure of the molecular adlayers is still challenging. In this contribution, we demonstrated an inconventional approach for covalent functionalization the HOPG surface by using mixture of 4-nitrobenzenediazonium (4-NBD) and 3,5-bis-tert-butylbenzenediazonium (3,5-TBD) molecules in which the former tends to polimezise and physisorb while the later chemically anchors on surface. The physisorbed features can be removed by washing with hot toluene and water. As a result, the HOPG surface is patterned in a quasi-periodic fashion. The efficiency of this development was verified by a combination of cyclic voltametry (CV) and atomic force microscopy (AFM) methods. This finding represents a convenient strategy for creating nanoconfined templates that might serve as nano-playgrounds for further supramolecular self-assembly and other on-surface reactions.

Sustainable process for functional group introduction onto HOPG by exposing [rad]OH and 1O2 using a radical vapor reactor (RVR) without any chemical reagents

We measured the local density of states of a quasi two-dimensional electron system (2DES) near defects, artificially created by Ar-ion sputtering, on surfaces of highly oriented pyrolytic graphite (HOPG) with scanning tunneling spectroscopy (STS) in high magnetic fields. At valley energies of the Landau level spectrum, we found two typical localized distributions of the 2DES depending on the defects. These are new types of distributions which are not observed in the previous STS work at the HOPG surface near a point defect [Y. Niimi, Phys. Rev. Lett. 97, 236804 (2006).10.1103/PhysRevLett.97.236804]. With increasing energy, we observed gradual transformation from the localized distributions to the extended ones as expected for the integer quantum Hall state. We show that the defect potential depth is responsible for the two localized distributions from comparison with theoretical calculations.

This reports provide an amorphous nanostructuralization technique on the surface modification in Highly Oriented Pyrolytic Graphite (HOPG) by using a femtosecond laser. We showed, for the first time, that the surface of HOPG is changed to the amorphous nanostructuralization graphite by using a femtosecond laser-driven compression technique. Our results also suggest that the HOPG surface is changed until the deeper area from the surface by the laser-driven shock wave. A single shot of a femtosecond laser beam (1.27 ∼ 1.33×1014 Wcm∼2 in intensity, with 2 mm-diameter, and 110 fs in pulse width) is irradiated under the vacuum ambience onto a 2 mm-thick of HOPG. The calculated impact pressures on a sample was 8.3 ∼ 8.7 GPa. Crystal structure in the HOPG were analyzed using a Raman spectroscopy and an X-ray diffraction, those analyzing depth from the surface were 50 nm and 350 μm, respectively.

Oxidative corrosion of the surface of highly oriented pyrolytic graphite (HOPG) and effects of platinum and oxygen on this corrosion were investigated at the atomic scale by using electrochemical scanning tunneling microscopy (EC-STM) to understand the degradation of the carbon supports in the cathodes of polymer electrolyte fuel cells. Morphological changes within several layers of the HOPG surface were observed under fuel-cell operating conditions. The EC-STM observations revealed that the corrosion occurred first at step edge (i.e., at defect sites on HOPG surfaces forming graphitic oxides) and then progressed toward terraces, indicating that the oxidation along the in-plane direction was fast and the basal plane was rather resistant to oxidative corrosion. We also found that platinum and oxygen at the HOPG surface strongly enhance the oxidative corrosion of HOPG. Oxygen species formed at platinum/carbon interface and oxygen atoms formed by dissociative adsorption would likely enhance the oxidation of graphitic-oxide intermediate species.

Nanofabrication on a Si surface by slow highly charged ion impact

Characterizing assemblies of non-aromatic organic molecules on carrier surfaces is still an underexplored field of nanomaterial design. Here we present a study of on-surface self-assembly for diamondoid ester compounds on a highly oriented pyrolytic graphite (HOPG) surface. Using a combination of experimental and computational tools, we have developed an approach to characterize such non-aromatic molecules that also do not possess long alkyl chains in their structures. AFM imaging visualized the formed on-surface domains and by using the semi-empirical quantum mechanical GFN2-xTB method we could identify the most stable on-surface orientations of the individual molecules, a non-trivial task for non-aromatic compounds. Our synergistic analysis revealed that intermolecular London dispersion interactions enable molecular chains formation, which then arrange into distinct domains of observable structural periodicity. Our findings not only shed light onto self-assembly behavior of diamondoid esters but also provide a more general approach for characterizing currently less explored non-aromatics on carrier surfaces.