Electrochemical Deposition of Gold Metal Particles into 3‐Dimensional Polypyrrole Film Deposited on a Highly Oriented Pyrolytic Graphite

Electrochemical techniques and lateral friction microscopy (LFM) are exploited to characterize the deposition of gold metal particles onto the 3‐dimensional (3‐D) polypyrrole (PPy) film deposited on 2‐dimensional (2‐D) highly oriented pyrolytic graphite (HOPG) substrate surface in an aqueous solution involving 0.01 M pyrrole and 0.1 M LiClO₄? 3H₂O. Cyclic voltammetry is utilized to find the gold deposition potential onto the PPy film from 0.001 M KAu(CN)₂/KOH solution. The gold deposition potential is found to be in the range of ?1.2 V to ?1.4 V. Chronoamperometry is used to find out the nucleation and growth mechanism of gold metal particles onto PPy film. When the PPy film is thin, the mechanism follows the 3‐D instantaneous and moved towards 3‐D progressive as the film thickness increases. Considering the high resistance of thick PPy film and insulating and compact nature of the film at more cathodic potentials, it is suggested that the gold nuclei are formed first on the HOPG substrate surface, move to the PPy film surface and then distributed inside the PPy matrix. Since the friction of gold and the PPy film is different, the LFM is found to be an effective tool to see the distribution of gold particles in the domain boundaries of the PPy film.     

Polyelectrolyte-templated synthesis of bimetallic nanoparticles

Bimetallic nanoparticles (NPs) are known to exhibit enhanced optical and catalytic properties that can be optimized by tailoring NP composition, size, and morphology. Galvanic deposition of a second metal onto a primary metal NP template is a versatile method for fabricating bimetallic NPs using a scalable, solution-based synthesis. We demonstrate that the galvanic displacement reaction pathway can be controlled through appropriate surface modification of the NP template. To synthesize bimetallic Au-Ag NPs, we used colloidal Ag NPs modified by layer-by-layer (LBL) assembled polyelectrolyte layers to template the reduction of HAuCl(4). NPs terminated with positively and negatively charged polyelectrolytes yield highly contrasting morphologies and Au surface concentrations. We propose that these charged surface layers control galvanic charge transfer by controlling nucleation and diffusion at the deposition front. This surface-directed synthetic strategy can be advantageously used to tailor both overall NP morphology and Au surface concentrations.     

Physical vapor deposition of one-dimensional nanoparticle arrays on graphite: seeding the electrodeposition of gold nanowires

One-dimensional (1D) ensembles of 2-15 nm diameter gold nanoparticles were prepared using physical vapor deposition (PVD) on highly oriented pyrolytic graphite (HOPG) basal plane surfaces. These 1D Au nanoparticle ensembles (NPEs) were prepared by depositing gold (0.2-0.6 nm/s) at an equivalent thickness of 3-4 nm onto HOPG surfaces at 670-690 K. Under these conditions, vapor-deposited gold nucleated selectively at the linear step edge defects present on these HOPG surfaces with virtually no nucleation of gold particles on terraces. The number density of 2-15 nm diameter gold particles at step edges was 30-40 microm-1. These 1D NPEs were up to a millimeter in length and organized into parallel arrays on the HOPG surface, following the organization of step edges. Surprisingly, the deposition of more gold by PVD did not lead to the formation of continuous gold nanowires at step edges under the range of sample temperature or deposition flux we have investigated. Instead, these 1D Au NPEs were used as nucleation templates for the preparation by electrodeposition of gold nanowires. The electrodeposition of gold occurred selectively on PVD gold nanoparticles over the potential range from 700-640 mV vs SCE, and after optimization of the electrodeposition parameters continuous gold nanowires as small as 80-90 nm in diameter and several micrometers in length were obtained.     

Early stages of mercury electrodeposition on HOPG

The electrodeposition of mercury on highly oriented pyrolytic graphite (HOPG) from nitrate solution was studied using cyclic voltammetry, potentiostatic current transients, and in situ scanning tunneling microscopy (STM), in order to correlate the results of the kinetic nucleation with the deposit morphology. At relatively low overpotentials, the mercury deposition can be described by a model involving progressive nucleation on active sites and diffusion-controlled 3D growth without overlapping of diffusion zones. The deposition was initiated on step edges and surface defects forming 3D islands following the Volmer–Weber mechanism. The small number of atoms in the critical nucleus (n_k=1), together with the linear log J vs. η dependence, indicated that the nucleation process can be described by the atomistic model.     

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.     

Growth and reactions of SiOx/Si nanostructures on surface-templated molecule corrals

Surface-templated nanostructures on the highly oriented pyrolytic graphite (HOPG) basal plane were created by controlled Cs+- or Ga+)ion bombardment, followed by subsequent oxidation at high temperature, forming molecule corrals. The corrals were then used for template growth of SiOx/Si nanostructures. We demonstrate here that, for SiOx/Si nanostructures formed in controlled molecule corrals, the amount of silicon deposited on the surface is directly correlated with the corral density, making it possible to generate patterned SiOx/Si nanostructures on HOPG. Since the size, depth, position, and surface density of the nanostructures can be controlled on the HOPG, it is possible to produce surfaces with patterned or gradient functionalities for applications in fields such as biosensors, microelectronics, and biomaterials (e.g., neuron pathfinding). If desired, the SiOx structures can be reduced in size by etching in dilute HF, and further oxidation of the nanostructures is slow enough to provide plenty of time to functionalize them using ambient and solution reactions and to perform surface analysis. Organosilane monolayers on surface-templated SiOx/Si nanostructures were examined by X-ray photoelectron spectroscopy, time-of-flight secondary ion mas spectrometry, and atomic force microscopy. Silanes with long alkyl chains such as n-octadecyltrichlorosilane (C18) were found to both react on SiOx/Si nanostructures and to condense on the HOPG basal plane. Shorter-chain silanes, such as 11-bromoundicyltrimethoxysilane (C11) and 3-mercaptopropyltrimethoxysilane (C3) were found to react preferentially with SiOx/Si nanostructures, not HOPG. The SiOx/Si nanostructures were also found to be stable toward multiple chemical reactions. Selective modification of SiOx/Si nanostructures on the HOPG basal plane is thus achievable.     

Underpotential deposition and involved alloy formation of cadmium on silver particles modified HOPG substrates

Cadmium underpotential deposition (UPD) on Ag particles modified highly ordered pyrolytic graphite (HOPG) surfaces, and the involved alloy formation were studied by conventional electrochemical techniques. Voltammetric results indicated that the Cd UPD followed an adsorption behavior different from that observed for massive Ag electrodes and Ag particles supported on vitreous carbon. Nanometer-sized bimetallic Cd–Ag particles were characterized by ex situ atomic force microscopy (AFM). Initially, AFM images show Ag deposits of similar size distributed preferably on HOPG step edges. No remarkable morphological changes are observed on the surface after the subsequent Cd deposition, suggesting that the Cd particles are deposited selectively over the Ag crystals. From the analysis of desorption spectra, employing different polarization times, and density functional theory (DFT) calculations, the formation of a Cd–Ag surface alloy could be inferred.     

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.     

Studies on the electrochemical growth of (Per)2[Au(mnt)2

The first stages of the electrocrystallization of (Per)(2)[Au(mnt)(2)] salt from dichloromethane on gold, platinum, and highly orientated pyrolytic graphite (HOPG) were investigated by cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy in order to understand the determinant factors for nucleation and crystal growth. The crystal growth occurs from adsorbed films of dithiolate on gold or platinum and of perylene on HOPG, after homogeneous nucleation, and it is controlled by the low diffusion of the species toward the growing surface.     

Synthesis and acid functionalization of cerium oxide nanoparticles

Colloidal cerium oxide has been obtained by controlled oxidation of soluble Ce(III) salts in hydrothermal conditions. The homogeneous nucleation of CeO₂ through thermolysis of this oxidizing solution allows the formation of well dispersed colloidal particles. Under optimal conditions, well crystallized particles with an average size of 8 nm are obtained. The surface is terminated by acetate groups which can be substituted by grafting phosphonic acids or phosphoric acids. Particularly, the grafting of 2-carboxyethylphosphonic acid or phosphonoacetic acid increases the acidic character of the surface as observed by impedance spectroscopy.     

Growth of palladium nanoparticles on nanostructured highly ordered pyrolytic graphite

We report on the growth of palladium nanoparticles on the basal plane of as‐cleaved highly oriented pyrolytic graphite (HOPG) samples, and on CO₂ ion sputtered nanostructured HOPG surfaces. The morphology of Pd nanostructures grown at room temperature is investigated by scanning tunneling microscopy (STM). The STM observations indicate that the morphology of the Pd films is strongly dependent on the HOPG surface. Stabilized Pd particles only form on the sputtered surface, while ramified Pd particles decorate the clean HOPG terraces. The prestructuring of HOPG surface leads to a selective location of particles at the rim of the nanopits generated by the CO₂ ion sputtering and annealing of the surface. The correlation between size, form, density, spatial distribution of the Pd nanoparticles and the quantity of metal added on surface is discussed. We also describe trench channeling of graphite or graphene basal planes by means of Pd nanoparticles in an ambient environment. Copyright © 2014 John Wiley & Sons, Ltd.     

A scanning tunneling microscope study of single platinum atoms,platinum dimers and trimers on highly-oriented pyrolytic graphite

We report a topographic study of platinum clusters on highly-oriented pyrolytic graphite (HOPG) using a scanning tunneling microscope operating in air. The particles were produced by evaporation of platinum onto the graphite-surface in high vacuum. The simultaneous finding of single platinum atoms, clusters and small particles on an otherwise clean and atomically flat surface shows that the platinum-HOPG surface interaction is strong enough to yield stable images of Pt atoms and yet is not strong enough to annihilate the Pt-Pt interaction. Small flat platinum clusters on HOPG can be imaged with atomic resolution of the cluster and the surrounding graphite lattice. We show the adsorption site distribution for the monomers. The Pt-dimers show a very broad bond length distribution on graphite with an average of 2.46 Å. We found two types of Pt-trimers, one which is almost linear and one of triangular form. The average nearest neighbour distance of the trimers is 2.61 Å.     

Incipient CdS thin film formation

The quality of a final thin film is essentially determined by the processes taking place at incipient CdS deposition, which in turn are strongly influenced by the physicochemical properties of the substrate and liquid in contact. SEM pictures of deposits formed through steady flow of a supersaturated (with respect to CdS) solution suggest that initially nuclei are continuously generated on the substrate and grow as discrete "surface" particles. With time, these particles tend to "coalesce" with neighboring ones, while new nuclei keep forming and growing, leading to the formation of a coherent film. There is evidence that similar growth patterns prevail in CdS deposition via the chemical bath deposition (CBD) process. Based on experimental observations, a simple model is developed, which is capable of predicting macroscopically determined film characteristics such as the temporal thickness evolution including the "induction period." Two cases of the growth pattern are examined theoretically; one based on instantaneous surface nucleation (due to its simplicity) and another with a constant surface nucleation rate, which appears to be closer to experimental observations.     

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.     

Electronic and geometric properties of Au nanoparticles on Highly Ordered Pyrolytic Graphite (HOPG) studied using X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM)

Au nanoparticles grown on mildly sputtered Highly Ordered Pyrolytic Graphite (HOPG) surfaces were studied using Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). The results were compared with those of Ag nanoparticles on the same substrate. By varying the defect densities of HOPG and the Au coverages, one can create Au nanoparticles in various sizes. At high Au coverages, the structures of the Au films significantly deviate from the ideal truncated octahedral form: the existence of many steps between different Au atomic layers can be observed, most likely due to a high activation barrier of the diffusion of Au atoms across the step edges. This implies that the particle growth at room temperature is strongly limited by kinetic factors. Hexagonal shapes of Au structures could be identified, indicating preferential growth of Au nanostructures along the (111) direction normal to the surface. In the case of Au, XPS studies reveal a weaker core level shift with decreasing particle size compared to the 3d level in similarly sized Ag particles. Also taking into account the Auger analysis of the Ag particles, the core level shifts of the metal nanoparticles on HOPG can be understood in terms of the metal/substrate charge transfer. Ag is (partially) positively charged, whereas Au negatively charged on HOPG. It is demonstrated that XPS can be a useful tool to study metal-support interactions, which plays an important role for heterogeneous catalysis, for example.     

ITO上电沉积Pd的成核机理及电催化性质

采用循环伏安技术和计时电流技术, 研究了ITO上电沉积Pd的过程, 发现Pd在ITO表面的电沉积是过电位成核且为不可逆的扩散控制过程; 根据Cottrell方程计算得到[PdCl₄]^2-的扩散系数为2.19×10^-5 cm²/s; 根据Scharifker的理论模型, 归一化处理电流-时间曲线, 与理论成核曲线对照, 判断Pd 的成核机理. 通过场发射扫描电子显微镜(FESEM)对Pd 的形貌进行分析, 讨论了沉积电位和沉积时间对Pd纳米粒子形貌的影响. 用X射线粉末衍射(XRD)对Pd纳米粒子进行结构分析, 并在0.5 mol/L H₂SO₄溶液中研究了其电化学性质及在碱性条件下乙醇分子的电催化性质.     

Influence of ultrathin poly-(3,4-ethylenedioxythiophene) (PEDOT) film supports on the electrodeposition and electrocatalytic activity of discrete platinum nanoparticles

Coating a carbon electrode surface, specifically highly oriented pyrolytic graphite (HOPG) with an ultrathin film of poly-(3,4-ethylenedioxythiophene), PEDOT, provides a support on which a high density of uniformly dispersed Pt nanoparticles (NPs) can readily be formed by electrodeposition. The NPs tend to be much smaller, have a higher surface coverage, better dispersion and show a much lower tendency to aggregate, than Pt NPs produced under identical electrochemical conditions on HOPG alone. The electrocatalytic activity of the NPs was investigated for methanol (MeOH) and formic acid (HCOOH) oxidation. Significantly, for similarly prepared particles, Pt NP-PEDOT arrays exhibited higher catalytic activity (in terms of current density, based on the Pt area), towards MeOH oxidation, by an order of magnitude, and towards HCOOH oxidation at high potentials, than Pt NPs supported on native HOPG. These findings can be rationalised in terms of the enhanced oxidation of adsorbed CO, a key reaction intermediate and a catalyst poison. This research provides strong evidence that employing conducting polymers, such as PEDOT, as a support substrate, can greatly improve particular catalytic reactions, allowing for better catalyst utilisation in fuel cell technology.     

纳米Sn-Co/石墨复合材料的制备、结构和电化学性能

以化学沉积法制备Sn-Co合金纳米粒子/石墨复合材料,XRD和Ram an光谱表征物相结构,SEM观察表面形貌.结果显示,500℃高温热处理的Sn-Co/石墨复合材料,其颗粒密集均匀地分散于石墨载体上.Sn-Co合金纳米粒子颗粒直径100 nm左右.Sn-Co/石墨电极具有较高的比容量和循环寿命,这可能是该合金纳米粒子与棒状石墨间的亲合能有效地阻止Sn的脱落,Sn-Co的颗粒间隙缓冲锂嵌脱过程的结构张力,防止合金的膨胀与粉化.     

Early Stages of CeO2 Thin-film Nucleation and Growth with Photo Irradiation

In this paper, the early stages of nucleation and photoirradiation growth of CeO₂ thin films have been studied. Cyclic voltammetry, chronoamperometry and scanning electron microscopy were used to analyze the nucleation process of CeO₂ thin films deposited on the anode with photo irradiation. Experimental results show that the anodic deposition process with photo illumination is controlled by diffusion. Compared with the dark state, photo illumination mainly contributed to increase the current density of the three-dimensional nucleation process, because photo illumination is helpful to create active sites and accelerate the nucleation progress on the surface that a thin ceria film has been formed. Two-dimensional nucleation process mainly exists within the initial 2 s, and then only three-dimensional instantaneous nucleation process continues, which may be the main reason why the thickness of the CeO₂ film can continue to grow with photo illumination but not in the dark state. Increasing the deposition overpotential can promote two-dimensional nucleation and growth rate, whilst when the potential exceeds 0.65 V, three-dimensional current density decreases. The li-miting factor at that time may be the diffusion rate of cerium ions in the solution towards the electrode substrate.     

Influence of ultrathin poly-(3,4-ethylenedioxythiophene) (PEDOT) film supports on the electrodeposition and electrocatalytic activity of discrete platinum nanoparticles

Coating a carbon electrode surface, specifically highly oriented pyrolytic graphite (HOPG) with an ultrathin film of poly-(3,4-ethylenedioxythiophene), PEDOT, provides a support on which a high density of uniformly dispersed Pt nanoparticles (NPs) can readily be formed by electrodeposition. The NPs tend to be much smaller, have a higher surface coverage, better dispersion and show a much lower tendency to aggregate, than Pt NPs produced under identical electrochemical conditions on HOPG alone. The electrocatalytic activity of the NPs was investigated for methanol (MeOH) and formic acid (HCOOH) oxidation. Significantly, for similarly prepared particles, Pt NP-PEDOT arrays exhibited higher catalytic activity (in terms of current density, based on the Pt area), towards MeOH oxidation, by an order of magnitude, and towards HCOOH oxidation at high potentials, than Pt NPs supported on native HOPG. These findings can be rationalised in terms of the enhanced oxidation of adsorbed CO, a key reaction intermediate and a catalyst poison. This research provides strong evidence that employing conducting polymers, such as PEDOT, as a support substrate, can greatly improve particular catalytic reactions, allowing for better catalyst utilisation in fuel cell technology.