Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.     

STM image of tungstosilicic acid molecular layer on highly oriented pyrolytic graphite

Molecular layer of tungstosilicic acid (H4SiW12O40) deposited on freshly-cleaved highly oriented pyrolytic graphite (HOPG) was observed by scanning tunneling microscopy (STM) in air at room temperature.The molecular dimension (11.5 A) of H4SiWi2O4o measured by STM is consistent with known crystallographic parameter.We also imaged the boundary of H4SiW12O40 molecular layer on HOPG showing that molecular layer of H4SiW12O40 was formed.It has been proved that individual tungstosilicic acid species is imaged.The probable reason for the formation of the molecular layer is also discussed.     

Nanoscale ordered structure of fullerenes on the surface of highly oriented pyrolytic graphite

The formation of an ordered arrangement of C₆₀ molecules as path-like structures on the surface of highly oriented pyrolytic graphite (HOPG) is reported for the first time with theoretical implementations. Fullerene nucleation and deposition from solutions with different concentrations of C₆₀ is performed under ambient conditions without electrochemical processes. Scanning tunneling microscopy (STM) is used to study the surface topography. The results reveal new aspects of fullerene deposition that can potentially aid in modeling with theoretical simulations.     

Arrays of double-decker porphyrins on highly oriented pyrolytic graphite

Three double-decker complexes of cerium(IV) were synthesized, which commonly have a 5,10,15,20-tetrakis(4-docosyloxyphenyl)porphyrin (C22OPP) moiety as one of the two tetrapyrrole rings. The three complexes-Ce(Pc)(C22OPP), Ce(C22OPP)2, and Ce(BPEPP)(C22OPP)-are distinguished by the other rings, which are Pc (=phthalocyanine), C22OPP, and BPEPP (=5,15-bis[4-(phenylethynyl)phenyl]porphyrin), respectively. The rate of inter-ring rotation of Ce(BPEPP)(C22OPP) was estimated to be approximately 3 s(-1) in solution at room temperature. These complexes assemble into ordered arrays at the interface of 1-phenyloctane and the highly oriented pyrolytic graphite surface, owing to the affinity of the long alkyl chains toward the surface, as revealed by means of scanning tunneling microscopy (STM) with molecular resolution. The shape of the upper ring is reflected in the STM image. Thus, Ce(Pc)(C22OPP), Ce(C22OPP)2, and Ce(BPEPP)(C22OPP) were observed as circular, square, and elliptic features, respectively. Possible molecular arrangements in the array of Ce(BPEPP)(C22OPP) are proposed by comparing STM images and molecular models. In the mixed arrays of Ce(BPEPP)(C22OPP) and H2(C22OPP), the double-decker complexes were distinguished by brighter features. Competitive adsorption experiments showed that the adsorption of Ce(BPEPP)(C22OPP) is less favorable than that of H2(C22OPP) by DeltaG(app) = 2.7 kJ mol(-1). Ce(BPEPP)(C22OPP) molecules appeared elliptic when placed within their own row, while they appeared isotropic when flanked by H2(C22OPP) molecules. Implications of the differences in the observed shapes to the inter-ring rotation are discussed.     

Molecular architecture of oligothiophene on a highly oriented pyrolytic graphite surface by employing hydrogen bondings

To achieve a controllable and predictable molecular architecture on a two-dimensional (2D) surface, a series of oligothiophenes with carboxylic groups and alkane chains were synthesized. The alkane chains and carboxylic groups, which can form hydrogen bonding, were intentionally designed in different positions of the oligothiophenes. The resulted molecular architectures by using the so-prepared oligothiophenes on a highly oriented pyrolytic graphite (HOPG) surface were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT). It is found that the hydrogen bonding plays an essential role in the formation of the ordered assemblies. A controlled 2D molecular assembly could be fabricated by using hydrogen bondings.     

Electrocatalytic properties of platinum anchored to the surface of highly oriented pyrolytic graphite

Nano-sized particles of platinum deposited on highly oriented pyrolytic graphite (HOPG) electrolessly (average sized = 3.4 nm) or electrochemically (d = 20 nm) and polycrystalline platinum are compared. The dispersed electrodes exhibit special features in the oxidation of ethylene glycol and adsorbed carbon monoxide. In particular, they reduce the overpotential of these processes. Possible reasons for the observed distinctions are discussed. The potential cycling leads to coalescence of nano-sized Pt particles on HOPG. As a result, their size distribution expands, and the distribution maximum shifts towards large sizes (d = 6.4 nm). This seriously complicates use of Pt/HOPG as a model electrode for investigating the size effect.     

Migration of Ag, Cd and Cu into highly oriented pyrolytic graphite and pyrolytic coated graphite

Migration of Cd, Cu and Ag from solution deposited samples of the respective nitrates into highly oriented pyrolytic graphite (HOPG) was studied using electrothermal atomic absorption spectroscopy (ETAAS) with platform vaporization. Metal migration was verified by removing the top layers of the HOPG platform after the sample had dried and performing the analysis using ETAAS. The results obtained suggest that the metals or their salts migrate into HOPG only when the sample solutions are deposited on those areas of the platform that have surface imperfections. The surface blemishes can be seen as tiny lines on the otherwise smooth surface of the HOPG platform. One possible driving force for the migration could be simple capillary action; however, additional information is needed to establish the true mechanism.

The effect of metal migration into graphite on atomic absorption profiles was also evaluated. This effect was studied by comparing the signals obtained after the sample had been deposited either on the imperfections or on the smooth areas of the HOPG platform. In addition, samples were atomized from both sides of a pyrolytic coated platform in which one of the sides had been roughened with an abrasive material to expose the electrographite. The main effect of metal migration on the absorption profiles seems to be an increased tailing of the back edge of the signals. This could suggest a secondary generation function limited by the rate of diffusion of the metal back to the substrate surface and subsequent vaporization.     

Nucleation and growth of cobalt nanostructures on highly oriented pyrolytic graphite

Cobalt in the form of three-dimensional (3D) hemispherical clusters (size approximately 10-30 nm) were observed to grow on pristine graphite surfaces via a Volmer-Weber growth mode. X-Ray photoelectron spectroscopy (XPS) reveals that these clusters are physisorbed on the surface. In the presence of minute surface contamination, the morphology of Co changes into a mixture of irregular and hemispherical three-dimensional islands. The formation of irregular islands appears to be mediated by the chemical interactions between Co and the surface contaminants as evidenced from analysis of the carbon pi-pi* transitions. Further analysis of size distribution of Co nanoclusters grown on pristine surfaces shows a critical nucleus size of i* = 1, i.e. a Co dimer forms the smallest stable cluster on a pristine graphite surface.     

Electrochemistry using self-assembled DNA monolayers on highly oriented pyrolytic graphite

Duplex DNA functionalized with pyrene has been utilized to fabricate DNA-modified electrodes on highly oriented pyrolytic graphite (HOPG). Films have been characterized using AFM and radioactive labeling as well as electrochemically. The data obtained are consistent with a close-packed structure in the film with helices oriented in a nearly upright orientation, as seen earlier with the fabrication of thiol-tethered duplexes on gold. Also as on gold, we observe the reduction of DNA-bound intercalators in a DNA-mediated reaction. The reduction of the intercalator is attenuated in the presence of the single-base mismatches, CA and GT, independent of the sequence composition of the oligonucleotide. This sensitivity to single-base mismatches is enhanced when methylene blue reduction is coupled in an electrocatalytic cycle with ferricyanide. The extended potential range afforded by the HOPG surface has allowed us also to investigate the electrochemistry of previously inaccessible metallointercalators, Ru(bpy)2dppz2+ and Os(phen)2dppz2+, at the DNA-modified HOPG surface. These results support the application of DNA-modified HOPG as a convenient and reproducible surface for electrochemical DNA sensors using DNA-mediated charge transport.     

Immobilization of metallothionein on highly oriented pyrolytic graphite for biosensor design

Immobilization of protein molecules on solid supports or surfaces in a controlled fashion is an important task for protein analysis at the solid/solution or solid/gas interface and biosensor fabrication. In this paper, the structure and biological activities of metallothionein (MT) layers immobilized on highly oriented pyrolytic graphite (HOPG) surfaces by means of two different strategies based on unspecific adsorption/chemisorption (MT‐HOPG system) and covalent binding (MT‐modified HOPG system) were studied respectively. The MT layers obtained by covalent binding to a previously functionalized HOPG surface are smooth and show a close‐packed ordered monolayer in contrast to those obtained by direct adsorption of the protein on substrate, which are disordered and relatively rough. Both adsorbed proteins exhibit reversible electron transfer at 0.25 V (Ag/AgCl) after immersion in CuSO₄ solution, whereas redox current of MT‐modified HOPG system is four times larger than that of MT‐HOPG system. Moreover, the MTs adsorbed on bare HOPG surfaces are obviously denatured. All the above results show that covalent binding strategies lead to high structural regularity and mechanical stability of the adsorbed protein molecules with a maintained biological activity, which is prospective for applications in immobilizing MT on a transducer for biosensor design. Copyright © 2009 John Wiley & Sons, Ltd.     

Scanning probe microscopic imaging of guanine on a highly oriented pyrolytic graphite electrode.

Guanine adsorbed onto a highly oriented pyrolytic graphite electrode was studied by MAC-Mode Atomic Force Microscopy (AFM), and the electrochemical behaviour of the guanine layer was investigated with Electrochemical AFM. Guanine adsorbs spontaneously, without forming a well-packed structure, into nucleation spots, which are stable with time and cover the surface uniformly and almost completely. The process of guanine adsorption and nucleation can be controlled and the effect of altering the exposure time and varying the potential was investigated.     

Imaging of highly oriented pyrolytic graphite corrosion accelerated by Pt particles

Carbon corrosion that is presumed to occur at the proton exchange membrane fuel cell (PEMFC) cathode was visualized by atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) observations using a fundamental model electrode. Platinum nanoparticles were deposited on a highly oriented pyrolytic graphite (HOPG) substrate as a model cathode catalyst, and its stability in an acid solution at a fixed potential was investigated. The formation of blisters on the surface of the model electrode was observed by AFM after it was kept at 1.0 V vs. RHE, especially at and around the Pt particles. FE-SEM observations using a backscattered electron detector revealed that Pt particles remained unchanged at their original positions after the formation of blisters.     

Self-assembly and odd-even effects of cis-unsaturated carboxylic acids on highly oriented pyrolytic graphite

The self-assembly of several cis-unsaturated carboxylic acids of the structure cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH on highly oriented pyrolytic graphite (HOPG) was studied. The impact of the interior cis-CH=CH group and the molecular chain length on their self-assembled structures was considered. Due to the cis conformation of the -HC=CH- group in the interior of these molecules, they display self-assembled structures significantly different from saturated acids with all-trans configurations. As an example of the class of molecules cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) (p=8, n=7), cis-CH3(CH2)7CH=CH(CH2)13COOH self-assembles into two kinds of enantiomer domains with opposite 2-D chirality. Due to the steric restriction of the interior cis-HC=CH group, all chains with acid groups are packed at the same side of a lamella, a head-to-head arrangement which is different from the head-to-tail packing of saturated all-trans acids. However, cis-CH3(CH2)7CH=CH(CH2)8COOH, considered as one example of the group cis-CH3(CH2)p-1CH=CH(CH2)2n-2COOH (p not equal 2n-1) (p=8, n=5), does not form any stable self-assembled domain, consistent with the molecular arrangement model. This difference in self-assembly behavior between cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) and cis-CH3(CH2)p-1CH=CHC2n-2COOH (p not equal 2n-1) shows an odd-even chain-length effect of cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH (p not equal m, m=2n or 2n-1). For another category of molecules, cis-unsaturated acids with equal numbers of all-trans carbon atoms on both sides of the cis-CH=CH group, cis-CH3(CH2)m-1CH=CH(CH2)m-1COOH (m=2n or 2n-1), display another odd-even effect. cis-CH3(CH2)7CH=CH(CH2)7COOH, one example of cis-CH3(CH2)2n-1-CH=CH(CH2)2n-1COOH (n=4), is predicted to form both an enantiomer and a nonchiral racemic structure, which is in accordance with the experimental observation of its self-assembled monolayer. However, cis-CH3(CH2)2n-2CH=CH(CH2)2n-2COOH does not form a stable self-assembled domain due to the same steric repulsion as that seen in the cis-CH3(CH2)7CH=CH(CH2)8COOH structure. These odd-even effects demonstrate that molecular self-assembly can be significantly tailored by slightly changing the molecular chain length.     

AFM observation of cation complexation of dibenzocrown ethers adsorbed on highly oriented pyrolytic graphite

The cation complexation behavior of dibenzocrown ethers adsorbed on highly oriented pyrolytic graphite substrates was investigated by means of atomic force microscopy using probe tips modified chemically with ammonium ion by silane coupling. The specific adhesion force based on the intermolecular force between dibenzocrown ether and ammonium ion was observed via force curve measurements in ethanol at the interface between the substrate and tip. The observed specific force decreased in the presence of the alkali metal ion in solution, indicating that the cation in solution interferes with the complexation of the crown ethers adsorbed on the substrate with the ammonium ion immobilized on the tip. The blocking effect of metal ions in solution on the observed force depended on the sizes of both the blocking cation and crown ether ring, suggesting that the surface-adsorbed dibenzocrown ethers possess a selective cation-complexing ability similar to that in their bulk state and that the adhesion force measurements using cation-modified tips allow evaluation of the cation-complexing ability of crown ethers under cation-competitive conditions.     

Electrodeposition of platinum on highly oriented pyrolytic graphite. Part I: electrochemical characterization

The electrochemical deposition of Pt on highly oriented pyrolytic graphite (HOPG) from H2PtCl6 solutions was investigated by cyclic voltammetry and chronoamperometry. The effects of deposition overpotential, H2PtCl6 concentration, supporting electrolyte, and anion additions on the deposition process were evaluated. Addition of chloride inhibits Pt deposition due to adsorption on the substrate and blocking of reduction sites, while SO4(2-) and ClO4- slightly promote Pt reduction. By comparing potentiostatic current-time transients with the Scharifker-Hills model, a transition from progressive to instantaneous nucleation was observed when increasing the deposition overpotential. Following addition of chloride anions the fit of experimental transients with the instantaneous nucleation mode improves, while the addition of SO4(2-) induces only small changes. Chloride anions strongly inhibit the reduction process, which is shifted in the cathodic direction. The above results indicate that the most appropriate conditions for growing Pt nanoparticles on HOPG with narrow size distribution are to use an H2PtCl6 solution with HCl as supporting electrolyte and to apply a high cathodic overpotential.     

Electroanalytical and spectroscopic characterization of poly(o-phenylenediamine) grown on highly oriented pyrolytic graphite

The polymerization of ortho-phenylenediamine (o-PD) on Highly Oriented Pyrolytic Graphite (HOPG) at different pH (1,3,5,7) was investigated by electroanalytical and spectroscopic methods. Cyclic voltammetry was used both to polymerize o-PD and to study the electroactivity of the resulting poly(ortho-phenylenediamine) (PPD) film. A redox couple associated to the PPD electroactivity, deeply influenced by the pH adopted during polymerization, was recorded. A correlation between this feature and the electrochemistry shown by the oligomers of o-PD, generated in solution during the polymer synthesis, was also found. A comparison between the absorption spectra, in the visible region, of the soluble oligomers and of the PPD films was also performed, suggesting that changes in both the polymer and the oligomer structure occur and are highly related to the polymerization pH. In particular, a higher degree of conjugation is exhibited by the PPD films electrosynthesised at lower pH and this likely explains the higher conductivity as well as the higher electroactivity shown by the material obtained in these conditions.     

Anion intercalation into highly oriented pyrolytic graphite studied by electrochemical atomic force microscopy

Knowledge of the dimensional changes occurring during electrochemical processes is fundamental for understanding of the electrochemical intercalation/insertion mechanism and for evaluation of potential application in electrochemical devices. We studied a highly oriented pyrolitic graphite (HOPG) electrode in perchloric acid, as a model to elucidate the mechanism of electrochemical anion intercalation in graphite. The aim of the work is the local and time dependent investigation of dimensional changes of the host material during electrochemical intercalation processes on the nanometer scale. We used atomic force microscopy (AFM), combined with cyclic voltammetry, as the in situ tool of analysis during intercalation and deintercalation of perchlorate anions. According to the AFM measurements, the HOPG interlayer spacing increases by 32% in agreement with the formation of stage IV of graphite intercalation compounds, when perchlorate anions intercalate. In addition, the local aspect of the process has been demonstrated by revealing coexisting regions with different kinetics for intercalation and deintercalation processes.     

Dynamic assembly of porphyrin wires trapped on a highly oriented pyrolitic graphite surface

Carefully designed porphyrin building blocks assemble through selective imidazole binding in various solvents to form linear multiporphyrin objects. From a dynamic mixture of monomers, dimers, and oligomers, linear objects were observed on a highly oriented pyrolitic graphite (HOPG) surface. On the surface, the objects' morphology clearly depended on the solvent used for deposition and was modified upon heating.     

Effect of pH and applied potential on the adsorption of DNA on highly oriented pyrolytic graphite electrodes. Atomic force microscopy surface characterisation

Single-stranded and double-stranded DNA electrochemical biosensors prepared by adsorption during 3 min on HOPG, with or without an applied potential, at pH 5.3 and 7.0, were characterised by MAC mode AFM. During adsorption DNA condenses on the substrate forming complex network films with pores exposing the HOPG surface. The thin films formed in pH 5.3 acetate buffer always presented a better coverage of the HOPG surface with DNA molecules than films formed in pH 7.0 phosphate buffer. The application of a positive potential of 300 mV during adsorption enhanced the robustness and stability of the DNA films with the formation of bigger network holes and a more condensed and compact self-assembled DNA lattice. The knowledge of the morphology of adsorbed DNA on electrode surfaces explains non-specific adsorption on the electrode surface and can be used to improve and develop DNA-electrochemical biosensors.     

Nucleation and growth mechanism for the electropolymerization of aniline on highly oriented pyrolytic graphite at higher potentials

In this work, the nucleation and growth mechanism for the electropolymerization of aniline was investigated at higher potentials on highly oriented pyrolytic graphite by potentiostatic current-time transient and atomic force microscopic (AFM) measurements. The electrochemical data fitted to the theoretical curves for the nucleation and growth suggest that electropolymerization of aniline follows the three-dimensional growth and progressive nucleation mechanism. These results were also compared with the results obtained at lower potentials. The results obtained from transient analysis, at higher potentials, were in good agreement with the results of AFM images. Electronic Publication