Crystal habit of fcc metal particles controlled by electrode potential in solution

The crystal habit of fcc metal particles formed on an amorphous carbon film electrode in solution at different electrode potentials is discussed. The fcc metal particles have different crystallographic habits depending on applied electrode potential; that is, icosahedral and/or decahedral particles are formed at lower potentials, and fcc single-crystalline or polycrystalline particles at higher potentials. It was found that decahedra and icosahedra of Cu-Au alloy particles are formed in the potential region of underpotential deposition (UPD) of Cu at which only fcc Au single-crystalline particles and Au polycrystalline particles appear. This is attributed to the charge transfer from the UPD Cu ions to the Au overlayer of Cu-Au alloy particles. The formation of decahedral and icosahedral Cu-Au alloy particles depends on the composition of the Cu-Au alloy. On the basis of these results it was deduced that the contraction of the surface lattice of the growing particles is responsible for the formation of icosahedral and decahedral particles. Received: 25 February 1997 / Accepted: 21 April 1997     

CdTe electrodeposition on InP(100) via electrochemical atomic layer epitaxy (EC-ALE): studies using UHV-EC

The II-VI compound semiconductor CdTe was electrodeposited on InP(100) surfaces using electrochemical atomic layer epitaxy (EC-ALE). CdTe was deposited on a Te-modified InP(100) surface using this atomic layer by atomic layer methodology. The deposit started with formation of an atomic layer of Te on the InP(100) surface, as Cd was observed not to form an underpotential deposition (UPD) layer on InP(100), although it was found to UPD on Te atomic layers. On the In-terminated 'clean' InP(100) surface, Te was deposited at -0.80 V from a 0.1 mM solution of TeO2, resulting in formation of a Te atomic layer and some small amount of bulk Te. The excess bulk Te was then removed by reduction in blank solution at -0.90 V, leaving a Te atomic layer. Given the presences of the Te atomic layer, it was then possible to form an atomic layer of Cd by UPD at -0.58 V to complete the formation of a CdTe monolayer by EC-ALE. That cycle was then repeated to demonstrate the applicability of the cycle to the formation of CdTe nanofilms. Auger spectra recorded after the first three cycles of CdTe deposition on InP(100) were consistent with the layer-by-layer CdTe growth. It is interesting to note that Cd did not form a UPD deposit on the In-terminated InP(100) surface and only formed Cd clusters at an overpotential. This issue is probably related to the inability of the Cd and In to form a stable surface compound.     

Thermal stability of Ag–Au,Cu–Au,and Ag–Cu bimetallic nanoparticles supported on highly oriented pyrolytic graphite

The formation of Ag–Au, Cu–Au, and Ag–Cu bimetallic particles on the surface of highly oriented pyrolytic graphite was studied by X-ray photoelectron spectroscopy. Samples with the core–shell structure of particles were prepared by sequential thermal vacuum deposition. The thermal stability of the samples was studied over a wide range of temperatures (25-400°C) under ultrahigh-vacuum conditions. The heating of the samples to ~250°C leads to the formation of bimetallic alloy particles with a relatively uniform distribution of metals in the bulk. The thermal stability of the samples with respect to sintering depends on the nature of the supported metals. Thus, the Ag–Au particles exhibited the highest thermal resistance (~350°C) under ultrahigh-vacuum conditions, whereas the Ag–Cu particles agglomerated even at ~250°C.     

Underpotential deposition of thallium, lead, and cadmium at silver electrodes modified with self-assembled monolayers of (3-mercaptopropyl)trimethoxysilane

Investigation of the underpotential deposition (UPD) of three metals-Tl, Pb, and Cd-on Ag surfaces modified with self-assembled monolayers (SAMs) of (3-mercaptopropyl)trimethoxysilane (3MPT) is reported. On the basis of the observation of negative potential shifts for their UPD processes, Tl and Pb undergo UPD directly on the underlying Ag surface by insertion between the Ag-S bond. This process is proposed to occur by penetration of the 3MPT monolayer by hydrated metal ions through spaces in six-membered siloxane rings that form at the terminus of the 3MPT layer after hydrolysis and condensation. In contrast, Cd does not undergo similarly facile UPD at 3MPT-modified Ag electrodes due to a hydrated ion size too large to fit through these openings. The voltammetric evidence that suggests that the hydrated metal cation size, as described by the Stokes diameter, is the primary determinant of Ag electrode accessibility for UPD through the cross-linked 3MPT layer is further supported by molecular mechanics energy minimization computations of six-membered siloxane rings on each of the three low-index faces of Ag. Finally, the 3MPT monolayer is shown to be exceptionally stable to repeated UPD/stripping cycles of Tl and Pb in contrast to SAMs of similar thickness formed from normal alkanethiols.     

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.     

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X‐ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature‐programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub‐monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600–650 K, which is evidenced by core‐level shifts (CLSs) of the Ag(3d_5/2) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the CO_ad‐related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.     

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.     

Cu + Au alloy particles formed in the underpotential deposition region of copper in acid solutions

Cu + Au alloy particles electrodeposited on an amorphous carbon electrode at the underpotential region of Cu in both perchloric acid and sulfuric acid solutions were investigated by means of transmission electron microscopy. The fraction of Cu in the Cu + Au alloy particles grown in both acid solutions with a concentration of 1 mM Au ion increased while the underpotential deposition (UPD) potential was decreased. However, it was independent of the concentration of Cu ion in solution. It is inferred that the composition of the Cu + Au alloy particles is dependent on the UPD potential. The fraction of Cu in the Cu + Au alloy particles grown at around the reversible Nernst potential of Cu in 0.1 mM HAuCl₄ + 50 mM Cu(ClO₄)₂ containing perchloric acid solution was 505. This result suggests a layer-by-layer formation of the Cu + Au alloy particles. The fraction of Cu in the Cu + Au alloy particles formed in the presence of sulfate was lower than that in the perchloric acid solution as the UPD potential and the concentration of Cu ion were the same. This is attributed to an influence of coadsorbed sulfate ions.     

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.     

Lead underpotential deposition on Au(110)

The electrochemical underpotential deposition (UPD) of lead on Au(110) was investigated by XPS using a custom‐built ultrahigh vacuum apparatus containing a chamber for electrochemical studies. A two‐step deposition process for lead UPD was confirmed. A large increase in the surface concentration of oxygen was found in solutions containing lead. The presence of lead was detected on the gold surface at all potentials within the range investigated (?500 mV to 1500 mV vs. Ag/AgCl). Degradation of chlorine by x‐rays was observed. The change in surface components with potential was investigated and linked to models of UPD and oxidation. The initial random deposition of lead from solution led to surface disordering. Copyright © 2003 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.     

A general model of metal underpotential deposition in the presence of thiol-based additives based on an in situ STM study

Bis(3-sulfopropyl)disulfide (SPS) is a common additive in commercial copper electroplating baths. We have studied the influence of SPS on Cu underpotential deposition (UPD) on a Au(111) single crystal surface by means of cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). By combining our results with the results from the literature we propose a model that describes different stages of Cu UPD in the presence of SPS. Further analysis shows that our model is also applicable to a more general case of UPD of different metals, e.g. Cu and Ag, on a thiol-modified single-crystal surface, where the bond between the substrate and the thiol is adatom mediated. In addition, we have verified our model by in situ observation of the lifting of the Herringbone reconstruction on the Au(111) surface by Cu UPD.     

Initial stages of Ni–P electrodeposition: growth morphology and composition of deposits

The evolution of growth morphology and composition of deposits during the initial stages of Ni–P electrodeposition is studied using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Combined electrochemical and surface analytical measurements show that the deposition process starts at relatively low cathodic potentials by instantaneous formation and growth of hemispherical centres. The phosphorus content of deposits in the initial deposition stages is found to increase gradually with the deposition time. Additional electrochemical and XPS measurements, carried out on Ni substrates under same polarisation conditions in a Ni²⁺ ion free electrolyte solution, show the occurrence of a time dependent Ni–P surface alloy formation indicating a strong Ni–P interaction. It is suggested that the very early stages of Ni–P electrodeposition involve a primary instantaneous nucleation of Ni followed by a Ni–P alloy formation induced by the strong Ni–P interaction. AFM images show that in advanced deposition stages the coalescence of growing Ni–P centres leads to formation of larger growth mounds. The evolution of the resulting surface roughness is analysed on the basis of the so-called dynamic scaling concept. The estimated values for the roughness exponent and the growth exponent (α=1.07±0.05 and β=0.28±0.05) correspond to a model involving a smoothing of the growing surface driven by surface diffusion.     

Palladium nanostructures and nanoparticles from molecular precursors on highly ordered pyrolytic graphite

Nanostructures and nanoparticles of palladium assembled on highly ordered pyrolytic graphite (HOPG) by the adsorption of palladium molecular precursors (MPs), in dichloromethane solutions, have been prepared. Self-assemblies of palladium nanostructures on HOPG were characterized by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. In this work, palladium rings had a wide variety of sizes in the nanometer range, and the ring/tube structures were preserved after a reductive process in which palladium metallic nanoparticles were formed. Noncircular structures were observed at HOPG defects and atomic step sites, as well. It is proposed that the observed ring formation of the palladium molecular precursors on HOPG substrates is related to the functional groups in the MPs, van der Waals interactions between particles and between particle-substrate, as well as the wetting properties of the solvent. In the present work, we illustrate several examples of the formation and characterization of palladium complex tubes and the resulting palladium rings, via the reduction process.     

The effects of the underpotential and overpotential deposition of lead and thallium on silver on the Raman spectra of adsorbates

It is shown that the underpotential deposition (UPD) and dissolution of monolayers of Pb and Tl onto Ag surfaces roughened in a controlled oxidation-reduction cycle produces a Ag surface which shows diminished surface enhanced Raman scattering (DSERS). Significantly enhanced Raman spectra can still be obtained from electrodes covered by complete UPD and overpotential deposited (OPD) layers of the metals. Correct choice of electrolytes for the UPD of the metal reduces the loss of enhanced Raman scattering; chloride ions, constituents of many electrolytes used in the investigation of surface enhanced Raman scattering (SERS), are shown to be especially active in causing the loss of SERS.     

Electrodeposition of Au-Cd alloy nanostructures on Au(111)

This report concerns an in-situ scanning tunneling microscopy study of the initial stages in the formation of a Au-Cd alloy on the Au(111) herringbone reconstruction. Although Au-Cd nanoclusters of alloy have been observed in sulfate electrolyte by this group, alloy "nanowires" were observed to form preferentially in the hcp regions between the sets of "soliton" walls of the reconstruction only in the presence of chloride. The nanowires were formed at -0.55 V versus 3 M Ag/AgCl, corresponding to Cd underpotential deposition (upd). Upd is electrodeposition at a potential prior to that needed to deposit the bulk element.     

In situ and real-time surface differential diffraction (SDD) study of Cd underpotential deposition on Au(1 1 1)

In situ and real-time surface differential diffraction (SDD) has been used to study the underpotential deposition (UPD) of Cd on Au(1 1 1) in sulfuric acid media. Comparison of SDD results in sulfate electrolytes with and without the presence of Cd²⁺ ions reveals that the surface reconstruction associated with the sulfate adsorption and desorption dominates the structural effect. It is also found that the reconstructed gold surface is stable upon Cd UPD process. In the initial stages of UPD, Cd atoms bind to the surface in bridge sites. This is followed by an adlayer structure with Cd adsorption in threefold hollow sites before Au/Cd intermixing takes place.     

New insights on the Cd UPD on Au(111)

The Cd underpotential deposition (UPD) process on Au(111) was analyzed by means of combined electrochemical measurements and in situ scanning tunneling microscopy (STM). In the underpotential range 300?ΔE (mV) ?400, 2D Cd islands are formed on the fcc regions of the Au(111)‐(√3 × 22) reconstructed surface without lifting the reconstruction. At lower underpotentials, the 2D Cd islands grow and, simultaneously, new 2D islands nucleate and coalesce with the previous ones forming a complete condensed Cd monolayer (ML). STM images and long time polarization experiments performed at ΔE = 70 mV demonstrate the formation of an Au? Cd surface alloy. At ΔE = 10 mV, the formation of the complete Cd ML is accompanied by a significant Au? Cd surface alloying and the kinetic results reveal two different solid‐state diffusion processes. The first one, with a diffusion coefficient D₁ = 4 × 10^?17 cm² s^?1, could be ascribed to the mutual diffusion of Au and Cd atoms through a highly distorted (vacancy‐rich) Au? Cd alloy layer. The second and faster diffusion process (D₂ = 7 × 10^?16 cm² s^?1) is associated with the appearance of an additional peak in the anodic stripping curves and could be attributed to the formation of another Cd_zAu_x alloy phase. Copyright © 2008 John Wiley & Sons, Ltd.     

Decoration of Graphitic Surfaces with Sn Nanoparticles through Surface Functionalization Using Diazonium Chemistry

Composites of tin nanoparticles (Sn NP) and graphene are candidate materials for high capacity and mechanically stable negative electrodes in rechargeable Li ion batteries. A uniform dispersion of Sn NP with controlled size is necessary to obtain high electrochemical performance. We show that the nucleation of Sn particles on highly ordered pyrolitic graphite (HOPG) from solution can be controlled by functionalizing the HOPG surface by aryl groups prior to Sn deposition. On the contrary, we observe heterogeneous deposition of micrometer sized Sn islands on HOPG subjected to oxidation prior to deposition in the same conditions. We demonstrate that functional groups act as nucleation sites for Sn NP nucleation, and that homogeneous nucleation of small particles can be achieved by combining surface functionalization with diazonium chemistry and appropriate stabilizers in solution.     

Activation/Inhibition Effects during the Coelectrodeposition of PtAg Nanoparticles: Application for ORR in Alkaline Media

PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag⁺ ions predominates, thus an increased Ag content in the co‐deposit is accomplished. The mechanism for this anomalous co‐deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt²⁺ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface.