Highly Oriented Pyrolytic Graphite Surface Research Articles

Molecular Characterization of Beef Liver Catalase by Scanning Tunneling Microscopy

Beef liver catalase molecules can stick tenaciously to the highly oriented pyrolytic graphite (HOPG) surface which has been activated by electrochemical anodization. The immobilized sample is stable enough for high resolution scanning tunneling microscope (STM) imaging. When the anodized conditions are controlled properly, the HOPG surface will be covered with a very thin oxide layer which can bind the protein molecules. Individual molecules of native beef liver catalase are directly observed in detail by STM, which shows an oval-shape structure with a waist. The dimensions of one catalase molecule in this study are estimated as 9.0 x 6.0x 2.0 nm(3), which are in good agreement with the known data obtained from X-ray analysis, except the height can not be exactly determined from STM. Electrochemical results confirm that the freshly adsorbed catalase molecules maintain their native structures with biological activities. However, the partly unfolding structure of catalase molecules is observed after the sample is stored for 15 days, this may be caused by the long-term interaction between catalase molecules and the anodized HOPG surface.

Cantilever flexure, adhesive\\attractive and lateral force measurements on highly-oriented pyrolytic graphite by scanning force microscopy

Pre-existing defects in the surface of highly-oriented pyrolytic graphite (HOPG) can be etched out by reaction with oxygen at high temperature. The result of this treatment is a population of one-monolayer-deep etch pits on the surface. An etch pit edge, possessing different chemical terminations than the basal plane of HOPG, may display unique interactions with any nanometer-scale material entity (viz. clusters and molecules) located in its vicinity. Since probe tips used in scanning force microscopy (SFM) can possess radii of curvature as small as 5–10 nm, it might be possible to measure the chemical uniqueness of etch pit edges with the help of SFM. In this paper, we consider possible instrumental artefacts which could hinder SPM studies of topography and friction on tailored HOPG surfaces for which there is a spatial variation in chemical properties. We conclude by discussing evidence for enhanced friction (≈100%) and adhesion (≈20%) at etch pit edges.

Anthraquinonedisulfonate Electrochemistry: A Comparison of Glassy Carbon, Hydrogenated Glassy Carbon, Highly Oriented Pyrolytic Graphite, and Diamond Electrodes

The electrochemistry of anthraquinone-2,6-disulfonate (2,6-AQDS) at glassy carbon (GC), hydrogenated glassy carbon (HGC), the basal plane of highly oriented pyrolytic graphite (HOPG), and boron-doped diamond was investigated by cyclic voltammetry and chronocoulometry. Quantitative determination of the surface coverage and qualitative assessment of the physisorption strength of 2,6-AQDS adsorption on each of these electrodes were done. The diamond and HGC surfaces are nonpolar and relatively oxygen-free, with the surface carbon atoms terminated by hydrogen. The polar 2,6-AQDS does not adsorb on these surfaces, and the electrolysis proceeds by a diffusion-controlled reaction. Conversely, the GC and HOPG surfaces are polar, with the exposed defect sites terminated by carbon-oxygen functionalities. 2,6-AQDS strongly physisorbs on both of these surfaces at near monolayer or greater coverages, such that the electrolysis proceeds through a surface-confined state. Less than 40% of the initial surface coverage can be removed by rinsing and solution replacement, reflective of strong physisorption. The results show the important role of the surface carbon-oxygen functionalities in promoting strong dipole-dipole and ion-dipole interactions with polar and ionic molecules such as 2,6-AQDS. The results also support the theory that diamond electrodes may be less subject to fouling by polar adsorbates, as compared to GC, leading to improved response stability in electroanalytical measurements. The relationship between the 2,6-AQDS surface coverage, the double-layer capacitance, and the heterogeneous electron-transfer rate constant for Fe(CN)(6)(3)(-)(/4)(-) for these four carbon electrodes is presented.

Hydrogen deactivation of sputter-deposited platinum thin films on highly oriented pyrolytic graphite

Electrochemical techniques such as chronopotentiometry and a.c. impedance spectroscopy have been used successfully to characterise the behavior of sputter-deposited platinum on highly oriented pyrolytic graphite (HOPG) substrate under hydrogen evolution reaction (HER) in 1 M NaOH at 25 °C. Prior to the sputtering process, HOPG surfaces were etched up to 60 min and deposition time was varied up to 20 min. Under a constant applied current density, the rate of overpotential increase with time and its magnitude are strongly related to the experimental conditions during the electrode preparation. Deactivation during HER is attributed to a loss of sputtered platinum due to its poor adhesion to the substrate.

Submonolayer-Deposition of Mass-Selected Au+ and Au n + (n = 3 and 7) on HOPG and Amorphous Carbon

Ions of gold monomer and clusters emitted from a liquid metal ion source were mass-selected, and deposited on cleaved HOPG (highly oriented pyrolytic graphite) surfaces and on amorphous carbon thin films at room temperature with the impinging energy Ei from 0 to 500 eV. The coverage of deposited ions were 1/100 and 1/1000 monolayers on HOPG surfaces and 1/3 monolayers on carbon films. Scanning tunneling microscopy of the HOPG surfaces deposited with low impinging energy (Ei<50 eV) revealed that large clusters with diameters ranging from 2 to 5 nm and height of 1–2 layers were present instead of isolated monomers and original clusters. When Ei was higher than 100 eV, HOPG surfaces were damaged and only bumpy surfaces were observed by STM. Transmission electron microscopy of Au+-deposited carbon films showed the formation of clusters with diameter 0.5–20 nm, depending on the Ei and the time elapsed after deposition.

Interaction of Al clusters with the (0001) surface of highly oriented pyrolytic graphite

Interfacial reaction of Al clusters with the (0001) surface of highly oriented pyrolytic graphite (HOPG) was examined using soft X-ray photoelectron spectroscopy. Clusters were deposited at 300, 573 and 773 K. The most striking observation is that no chemical reaction occurs at any temperature. Reactions were only observed if the surface integrity was compromised. This may indicate that upon deposition of Al, there is little, if any, adatom-induced surface reconstruction on the HOPG surface. This result should resolve discrepancies that have arisen in previous experimental and theoretical studies.

COMPARATIVE STUDY OF THIOL FILMS ON C(0001) AND Au(111) SURFACES BY SCANNING PROBE MICROSCOPY

The structures resulting from 1-dodecanethiol, 1-butanethiol and 1,9-nonanedithiol films produced on highly oriented pyrolytic graphite (HOPG) and gold(111) have been comparatively studied by scanning probe microscopies. Molecular resolution images resulting from atomic force microscopy (AFM) and scanning tunneling microscopy (STM) of different thiol films show the formation of arrays of molecules parallel to the HOPG surface. The electrochemical response of the ferro-ferricyanide reaction was used to test the characteristics of electron transfer processes in thiol-covered HOPG as compared to the bare substrate. The decrease in the heterogeneous rate constant for the test reaction appears to be directly related to the degree of film thickness uniformity. For comparison, films with the same kind of thiols were produced on Au(111). Although the electrochemical characteristics of these films appear to be the same irrespective of the substrate nature, the structure of the films on Au(111) is different from that produced on HOPG.

Crystalline structure and orientation of gold clusters grown in preformed nanometer-sized pits

Gold clusters were produced by condensing evaporated gold in nanometer-sized preformed pits on the surface of highly oriented pyrolytic graphite (HOPG). The height of the clusters was 6.7 ± 0.7 nm as measured with scanning tunneling microscopy in ultrahigh vacuum, the lateral width was 10.1 ± 1.9 nm as determined with transmission electron microscopy (TEM). Using TEM for electron diffraction, we obtained information on the crystalline structure of the clusters. The intensity of the observed diffraction rings shows the preferential orientation of the clusters with the (111) plane of the gold lattice parallel to the (0001) surface of HOPG. This was compared to the diffraction pattern of gold clusters produced in the gas phase by inert-gas evaporation and deposited on a flat HOPG surface at room temperature as complete units which showed no preferential orientation. The directional alignment in the surface plane as it is described in the literature for larger gold crystallites grown on a flat HOPG surface is not observed for the nanometer-sized clusters grown in pits.

The Morphology of Platinum Black Electrodeposited on Highly Oriented Pyrolytic Graphite Studied with Scanning Electron Microscopy and Scanning Tunneling Microscopy

Scanning electron and scanning tunneling microscopies have been used to study the morphology of Pt films electrodeposited on highly oriented pyrolytic graphite (HOPG); deposition was from solutions containing K2PtCl6and Pb(C2H3O2)2in order to produce films of Pt black. The resulting films consisted of mixed regions of Pt black and Pt gray surrounding bare patches of HOPG. Microscopic analyses reveal that the gray areas contain Pt islands which form patterns consistent with deposition preferentially at defects on the electrode surface. These islands grow as large as 1 μm in diameter before coalescing. The black areas exhibit a greater density of Pt with deposition occurring both at defects and on the flat areas between them. Pt islands in the black areas grow to only ca. 0.2 μm before coalescing. The aggregates formed in the black region have a rougher morphology due to the smaller size which islands reach before they coalesce. The presence of both gray and black areas is attributed to two factors: the role of adsorbed Pb2+ions and the deposition rate gradients created by pockets of N2gas trapped on the HOPG surface.

Topographic and Energetic Heterogeneity Studies of Oxidized Graphite Surface by Scanning Tunneling Microscopy/Spectroscopy and Photoelectron Spectroscopy

Scanning tunneling microscopy and spectroscopy and X-ray photoelectron spectroscopy are used to study oxidation effects of nitric acid on a highly-oriented pyrolytic graphite surface. Various etching times at constant temperature are applied in order to create local binding sites on the surface without creating deep defects. A single and paired chains structure, different from pure graphite at atomic scale, is shown by scanning tunneling microscopy. This can be explained by the presence of oxygenate groups on the surface, revealed by X-ray photoelectron spectroscopy. Both scanning tunneling spectroscopy and X-ray photoelectron spectroscopy demonstrate the vanishing of a bands characteristic of sp2 graphite hybridization. This, in turn, can be explained by dehybridization related to new bondings of the graphite carbons in the oxygenate groups. An important result of area averaging spectroscopy is the observed energetic heterogeneity considered in terms of the changes of local electronic density of states of the oxidized surface. PACS numbers: 61.16.Ch, 71.20.—b

MANIPULATION OF GOLD SINGLE CRYSTAL ISLANDS- ON HOPG SURFACE WITH A STM TIP

It is demonstrated that nanometer-sized gold single crystal islands or even clusters of the islands,which were formed by deposition of gold onto an air-cleaved highly oriented pyrolytic graphite (HOPG) surface,can be manipulated on the surface in a controllable manner with the tip of a scanning tunneling microscope when the tunneling conductance is set to a large value. Since the tip does not disturb the islands if the tunneling conductance is small enough,even the islands are repeatedly scanned over by the tip, the results of the manipulation can thus be imaged with the same tip. The mechanism of the manipulation is suggested to be that at large tunneling conductance a metallic cohesive force may form between the tip and islands and may exceed the frictional force between the islands and the HOPG surface.

STM study of the organic semiconductor PTCDA on highly-oriented pyrolytic graphite

Monolayer and multilayer films of the archetype organic semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride are deposited at room temperature in ultrahigh vacuum on freshly cleaved highly-oriented pyrolytic graphite (HOPG) and analyzed by scanning tunneling microscopy (STM). STM images of monolayer films show a ‘herringbone’ structure with two molecules per unit cell and dimensions which are in good agreement with prior studies. High-resolution STM images acquired under a variety of bias voltages indicate an electronic inequivalence of the two molecules within the unit cell. Although the image of the most prominent of these molecules resembles a figure eight over a wide range of sample biases, a difference in shape between filled and empty states appears at the smallest bias voltages. These differences are compared to molecular orbital calculations for the highest occupied and lowest unoccupied molecular orbitals of an isolated molecule and found to correlate rather well. Moiré fringes are observed with much larger periodicity than those previously reported by Ludwig et al., and can be explained by the incommensurate nature of the overlayer, which grows quasi-epitaxially, constrained mostly by the symmetry of the substrate. The first reported results of STM on multilayer films is presented showing crystalline domains in multiples of 60°, the rotational symmetry of the HOPG (0001) surface. In most cases, edges of these domains are observed to preferentially align along the a-axis of the unit cell. Finally, given this evidence of stable tunneling with molecular resolution on monolayer and multilayer films, mechanisms for STM image contrast on PTCDA films are discussed.

Perspectives for in situ scanning tunnel microscopic imaging of metalloproteins at HOPG surfaces

We have investigated the behaviour of the four-copper fungal metalloenzyme laccase (MW ≈ 68kDa) at highly oriented pyrolytic graphite (HOPG) surfaces by ex situ and in situ STM. The four copper atoms are suited to stimulate long-range inelastic tunnel modes through the protein. The protein forms crystalline or amorphous structures of μm lateral extension during evaporation of aqueous laccase solution at low ionic strength. Individual molecular-size structures distinct from the HOPG background, and possibly arising from tip dislodging can also be imaged. The HOPG surface cracks at certain potentials on in situ potentiostatic control and releases nm size HOPG scrap bits. These are clearly different in shape from the ex situ imaged molecular-size structures. Laccase could not, however, be imaged by in situ STM, most likely due to structural incompatibility between the hydrophobic HOPG surface and the strongly negatively charged protein, and to high protein surface mobility.

Real-space investigation of initial growth process of hydrogenated amorphous silicon on a graphite substrate.

Initial growth of hydrogenated amorphous silicon (a-Si:H) from silane plasma on a cleaved surface of highly oriented pyrolytic graphite (HOPG) has been investigated by means of in situ x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). At a substrate temperature of 110 \ifmmode^\circ\else\textdegree\fi{}C, island nuclei were formed on atomically flat terraces as well as along the steps of the HOPG surface. At an increased substrate temperature of 230 \ifmmode^\circ\else\textdegree\fi{}C, the nucleation occurred only along the steps. Hydrogen plasma treatment of the HOPG surface prior to the a-Si:H deposition drastically changed the growth mode to a homogeneous coverage of the surface from the beginning of film deposition at both temperatures. In order to understand and analyze these results, we propose a mechanism, from analogy to epitaxial film growth, that consists of the adsorption, migration, and fixation (pinning) of precursors on the surface and of subsequent nucleation and growth of film. \textcopyright{} 1996 The American Physical Society.

Implementation of Electrochemically Synthesized Silver Nanocrystallites for the Preferential SERS Enhancement of Defect Modes on Thermally Etched Graphite Surfaces

Highly oriented pyrolytic graphite (HOPG) surfaces, on which atomically well-defined roughness has been introduced via high-temperature gasification reactions, are investigated by noncontact mode atomic force microscopy (NC-AFM) and Raman spectroscopy both before and after the electrochemical deposition of silver nanocrystallites on these surfaces. Exposure of freshly cleaved HOPG surfaces to an O(2)-rich ambient at 650 °C for a few minutes caused the formation of 1-monolayer-deep, circular etch pits on the HOPG basal plane surface. Silver nanocrystallites were electrochemically deposited onto these etched surfaces at two coverages: 0.5 mC cm(-)(2) (or 5 nmol of Ag(0) cm(-)(2)) and 2.4 mC cm(-)(2) (25 nmol of Ag(0) cm(-)(2)). At the lower coverage, NC-AFM images revealed that silver decorated only the circumference of the circular etch pits, forming a uniform annular ring with an apparent diameter of 200-250 Å and a height of ∼15 Å. At the higher silver coverage, an increase in the height but not the diameter of this annulus was observed, and additional silver nanostructures [Formula: see text] having dimensions of 300-350 Å diameter and 15 Å height [Formula: see text] were observed on atomically smooth regions of the graphite basal plane. The Raman spectroscopy of these surfaces was investigated and compared with spectra for nanocrystallite-modified but unetched HOPG basal plane surfaces and thermally etched surfaces on which no silver was deposited. For for thermally etched HOPG surfaces at either silver coverage, SERS-augmented Raman spectra were obtained in which defect modes of the graphite surface [Formula: see text] derived from "finite" graphite domains at the surface [Formula: see text] were strongly and preferentially enhanced. In addition, an enhanced band near 2900 cm(-)(1) was assigned to ν(OH) from carboxylate moieties present at step edges based on the basis of the observed pH dependence of the enhancement.

In situ Scanning Tunneling Microscopy of the Electrochemical Deposition of Ag on Graphite

A scanning tunneling microscope (STM) has been constructed to observe the surface change of a working electrode during the electrochemical reactions in electrolyte solution. Both potentials of STM tip and substrate with respect to a reference electrode can be independently controlled by simply combining with a commercially available potentiostat and the STM. These types of apparatus are used to study the electrochemical deposition of Ag on a highly oriented pyrolytic graphite (HOPG) substrate in HClO 4 solution. Before Ag deposition and after stripping the Ag film, atomic images of HOPG surface are obtained. The STM images of Ag deposit produced by cyclic voltammetric and potential pulse methods are observed. The observed STM images support the conclusion that the electrodeposition occurs according to the island growth mechanism.

STM analysis of triosmium carbonyl cluster adsorption at HOPG

Abstract The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os 3 (CO) 11 (NCCH 3 )) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a =1.4 nm and b =1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.

Electrochemical Scanning Tunneling Microscopy Observation of Highly Oriented Pyrolytic Graphite Surface Reactions in an Ethylene Carbonate-Based Electrolyte Solution

In order to elucidate surface reactions on graphite negative electrodes of secondary lithium ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/ethylene carbonate−diethyl carbonate (1:1 by volume) were observed under polarization by electrochemical scanning tunneling microscopy. A step edge on the basal plane of HOPG was treated as a model of the edge plane of HOPG. When the sample potential was stepped to 1.1 V vs Li/Li+, two kinds of hill-like structure of ca. 10 Å height appeared on the HOPG surface. The first hill was formed far apart from a step edge and was almost unchanged with time. The second hill was formed in the vicinity of the step and was spread out with time. The formation of the second hill caused the exfoliation of graphite layers. The observed height of the hills was comparable to the values of the increment of the interlayer spacing for ternary graphite intercalation compounds of alkali metal with solvent molecules prepared by a solution method. It was considered that the intercalation of solvated lithium ion is responsible for the formation of the hills and that this process corresponds to the initial stage of the solvent decomposition and subsequent film formation processes.

Effects of anodic oxidation on the surface structure of highly oriented pyrolytic graphite revealed by in situ electrochemical scanning tunnelling microscopy in H 2SO 4 solution

Effects of the potential of anodic oxidation and of potential cycling on the surface structure of a highly oriented pyrolytic graphite (HOPG) electrode were observed by in situ electrochemical scanning tunnelling microscopy (ECSTM) in dilute H 2SO 4 solution with atomic resolution. With potential cycling between −0.1 V and 1.8 V vs. Ag/AgCl (sat. KC1), some atoms on the top layer of HOPG protrude out of the base plane, and the graphite lattice of these protrusions is still intact but is strained and expanded. With further potential cycling, some protrusions coalesced and some grew larger, and an anomalous superperiodic feature was observed (spacing 90 Å with a rotation 30 ° relative to atomic corrugations) which superimposed on the atomic corrugation of HOPG. On the topmost of these protrusions, some atoms form oxides and others are still resolved by the ECSTM image. With potential cycling between −0.1 V and +2.0 V vs. Ag/AgCl (sat. KC1), damage to freshly cleaved HOPG surface is more serious and fast, some ridges are observed, the atomic structure of the HOPG surface is partially and then completely damaged due to the formation of oxide. We also found that anodic oxidation occurred nonuniformly on the surface of HOPG near defects during potential cycling.

Characterization by a.c. impedance spectroscopy of highly oriented pyrolytic graphite surfaces modified by argon plasma etching

The a.c. impedance technique has been used to characterize the surface of a highly oriented pyrolytic graphite (HOPG) basal plane after exposure to an argon plasma. HOPG surfaces show considerable roughening for plasma etching times of up to 60 min as evidenced by scanning electron microscopy. Later, a.c. impedance measurements were carried out in alkaline aqueous solutions to characterize the hydrogen-evolution reaction and the data were analysed using the constant-phase element model for etching times of up to 1 h. They showed that the surface roughness factor increases almost linearly with the etching time.