Polycrystalline Au Electrode Research Articles

Investigations on metal depositions and dissolutions with an improved EQCMB based on quartz crystal impedance measurements

With the experimental set-up frequency and damping changes of resonating quartz crystals in electrochemical experiments can be measured at a time resolution of about 500 ms. The damping effects accompanying metal depositions and dissolutions (Ag and Cu) on polycrystalline Au electrodes of 6 and 10 MHz AT quartzes were studied and discussed in terms of roughness and sliding effects.

Capacitance of a metal/liquid interface during anion adsorption: phase-selective measurement in the presence of d.c. voltage sweep and finite solution resistance

Anion adsorption plays a critical role in pitting corrosion of metals in aqueous media. Frequently, this adsorption occurs as a non-faradaic process, without any electron transfer across the interface. Such processes cannot be studied efficiently by using standard electrochemical methods such as linear sweep voltammetry (LSV). Phase-selective measurement of the interfacial differential capacitance is necessary in such cases. However, when accompanied by LSV, even these latter measurements can be affected severely by the uncompensated solution resistance. In this paper, we describe a relatively simple phase-selective impedance method where both the solution resistance and the differential capacitance can be measured in situ during the d.c. voltage scan of LSV. We apply this technique to study the adsorption properties of ClO4−, Cl− and SCN− on a polycrystalline Au electrode. The results indicate that this method can be standardized easily for the analysis of adsorption isotherms involving similar systems. Copyright © 1999 John Wiley & Sons, Ltd.

Electroreduction of nitrate ion to nitrite and ammonia on a gold electrode in acidic and basic sodium and cesium nitrate solutions

Abstract The electroreduction of NO 3 − to NO 2 − and NH 3 has been investigated using a polycrystalline Au electrode in 0.5 M NaNO 3 and CsNO 3 solutions with pH in the ranges 1.1–3.0 and 9.85–12.8. The reduction of NO 3 − does not occur significantly in the strongly acidic solutions with pH lower than 1.4 to 1.6: the current efficiency for the NO 3 − reduction is less than 5%. A further increase in pH significantly facilitates the NO 3 − reduction. In basic solutions the reduction of NO 3 − occurs predominantly: the current efficiency reaches the range 80 to 99%. In the case of NaNO 3 solutions the ratio of the current efficiencies for NH 3 and NO 2 − production, CE NH 3 /CE NO 2 − is nearly unity in the acidic solutions and 2 to 3 in basic solutions. On the other hand, in CsNO 3 solutions, the current efficiency of NO 2 − production becomes much higher and the CE NH 3 /CE NO 2 − is 1 to 0.5 at pH in the range 2.0–3.0 and 0.2 to 0.5 at pH in the range 10.0–12.7. Partially positively-charged Na or Cs adatoms underpotentially deposited (upd) on the Au electrode surface are believed to facilitate the adsorption of NO 3 − ions, resulting in the preferential reduction of NO 3 − .

Enhanced electrostatic interactions for selective and controllable permeation of ionic species at a monolayer of self-assembled dicationic nickel complex

Abstract Electrodic reactions of solution-dissolved ionic redox probes were carried out at a self-assembled monolayer (SAM) and mixed monolayers of {dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)-diethyl disulfide) perchlorate}, 1 , on polycrystalline Au electrodes. Electrostatic repulsive interaction between the dicationic nickel(II) redox center of the SAM of 1 and the solution-dissolved [Ru(NH 3 ) 6 ] 3+ complex inhibits this complex from undergoing electrodic reaction, e.g. the peak-to-peak separation (Δ E p ) of the cyclic voltammogram (CV) for the [Ru(NH 3 ) 6 ] 3+/2+ couple in a 20 mM NaNO 3 solution is more than 600 mV, which is much larger than that (60 mV) at the bare Au electrode. On the other hand, the redox reaction of electroactive anions, [W(CN) 8 ] 3−/4− and [Fe(CN) 6 ] 3−/4− redox couples, occurs at the SAM of 1 without any inhibition such as that for the [Ru(NH 3 ) 6 ] 3+/2+ couple. A reversible redox wave was observed for the [W(CN) 8 ] 3−/4− redox couple at the SAM of 1 with the Δ E p value of 69 mV; but at a bare Au electrode, a quasi-reversible redox wave with Δ E p =270 mV was obtained. Such different electrodic reactions of the highly positively and negatively charged redox probes at the SAM of 1 possessing a short alkyl chain (its carbon number is 2) were explained as being due solely to the electrostatic interactions between the SAM of 1 and the redox probes in solution. The results obtained here illustrate the selective and controllable permeation of electroactive cations and anions at the monolayer electrodes.

Voltammetric and rotating ring-disk studies of underpotential deposition of Ag and Cu on polycrystalline Au electrodes in aqueous H 2SO 4

The underpotential deposition (UPD) of Cu and Ag from sulphuric acid solutions was studied on polycrystalline Au surfaces by cyclic voltammetry at stationary and rotating ring-disk electrodes. Since the voltammetric profiles also include the surface oxide/reduction contributions, the charges were also calculated by means of the collection technique of the rotating ring-disk system. The charge values obtained from cyclic voltammetry at stationary and rotating ring-disk electrodes were 405 and 386 μC cm −2, respectively, for the oxidation of a complete monolayer of UPD Cu. The proximity of these values suggests epitaxial deposition in a two-electron reduction process. On the other hand, the charge values found for UPD Ag were 108 and 115 μC cm −2, which corresponds to the amount required for the oxidation of only half a monolayer of Ag. Since the atomic radii of the three atoms involved (Cu, Ag and Au) are very similar, the differences in the deposition models were associated with the co-adsorption of anions (sulphate or hydroxide ion) which hinders the formation of a full monolayer of Ag on polycrystalline Au surface.

Electrochemical deposition of the first Cd monolayer on polycrystalline Pt and Au electrodes: an Upd study

The underpotential deposition of Cd on polycrystalline Pt and Au was studied by voltammetry at stationary and rotating ring-disc electrodes. On Pt, the Cd ads dissolution peaks overlap those related to the oxidation of Hads, thus hindering the precise evaluation of desorption charges. A model proposed to calculate such charges from voltammetry at stationary electrodes revealed a value of 285 muC cm-2 for the monolayer dissolution, which corresponds to a coverage of 90% with Cd ads presenting an electrosorption valence of 0.5. Rotating ring-disc experiments fully confirmed such values. The misfit between atomic radii of Cd and Pt justifies the less-than-100% coverage. On the other hand, on Au, the absence of Hads simplifies the procedure for determination of dissolution charges for the Cd monolayer. Here, a value of only 41 muC cm-2 was calculated, which corresponds to a maximum coverage of 15%, with the electrosorption valence of 0.5. The results obtained in the collecting experiments with the rotating electrode are in complete agreement with those values.

Phases of Underpotentially Deposited Hg on Au(111): An in Situ Surface X-ray Diffraction Study

We report on an in situ surface X-ray diffraction study of the underpotential deposition (UPD) of mercury on Au(111). We have observed three UPD phases present at potentials prior to bulk mercury deposition. These phases consist of two well-ordered intermediate states and what appears to be either a fully discharged two-dimensional liquid Hg layer or a monolayer of an amorphous Hg-Au alloy. Both ordered intermediate phases have hexagonal structures with lattice vectors that are rotated 30{degree} from those of the Au(111) substrate. The first phase (phase I), present at a potential of +0.68 V, was only observed on fresh flame-annealed Au(111) electrodes and appears to be an open incommensurate structure with a lattice constant of 3.86 {+-} 0.03 A. This phase appears to be metastable since it changes to a second ordered phase (phase II) after a certain time. The second phase has a more compact lattice with a = 3.34 {+-} 0.01 A and appears to be a commensurate 2x2 structure with 2/3 of the Hg atoms at threefold hollow sites and 1/3 on atop sites. Similar to the first one, this phase is also metastable and can be transformed to a final, fully discharged, state of a two-dimensionalmore » liquid Hg layer or an amorphous Hg-Au alloy. The entire Hg UPD process, from Hg{sup 2+} to the fully discharged metallic Hg layer, agrees well with a multistep mechanism based on previous electrochemical kinetic studies on polycrystalline Au electrodes. 31 refs., 10 figs., 2 tabs.« less

Adsorption and electrooxidation of ethylene on Au surfaces

The electrochemical behavior of ethylene electrooxidation on preferentially oriented Au surfaces is investigated in 1 M H2 SO4 aqueous solution at 25°C. A symmetric square wave periodic potential signal is applied to polycrystalline Au electrodes to obtain (111)-, (110)-, and (100)-type oriented surfaces, which are characterized by cyclic voltammograms and SEM micrographs. It is found that ethylene adsorbs at potentials just below the potential of zero charge of the Au/1 M H2 SO4 interface, and two residues are detected in an anodic stripping experiment. The adsorbed ethylene voltammetric electrooxidation, and the bulk ethylene anodic polarization curves are measured at the differently oriented Au surfaces.

Electro-oxidation of propene on gold in acid solution studied by DEMS and FTIRS

The oxidation of propene in aqueous HClO 4 solution was studied on polycrystalline Au electrodes using differential electrochemical mass spectrometry (DEMS) and in-situ Fourier transform IR spectroscopy (FTIRS). Experimental results show that the interaction of propene with the electrode|electrolyte interface is characterized by a weak adsorption since no chemisorbed intermediates could be found. Different reaction pathways were observed during the electro-oxidation of propene. Acetone was identified as the main product of the intact molecule chain. The detection of ethanal and carbon dioxide indicates fission of a CC bond in a reaction pathway parallel to propene electro-oxidation.

Electrochemical reactions of ethene on polycrystalline Au electrodes in acid solution studied by differential electrochemical mass spectrometry and isotope labelling

Abstract The electro-oxidation and electroreduction of ethene on polycrystalline Au were studied by means of differential electrochemical mass spectrometry (DEMS). It was found that the electroreduction of ethene in 0.1 M HClO4 + H2O yielded ethane whereas in DClO4 + D2O deuterated ethane C2H4D2 was detetected. The electro-oxidation of C2H4 was accompanied by a large signal for m z = 44 in the mass spectrometer owing to the formation of both CO2 and acetaldehyde. Using C2D4 in H2O + HClO4 the production of CO2 and ethanal was distinguished. Experiments with C2H4 in DClO4 + D2O demonstrated that all hydrogen atoms in ethanal belong to the initial C2H4 molecule, thus indicating an internal rearrangement during ethene electro-oxidation.

Adsorption and oxidation of acetylene and ethylene on gold electrodes

A comparative study was made on the reactivity of acetylene and ethylene in acids (0.05 M H 2SO 4 and 0.1 M HClO 4) on polycrystalline Au electrodes. On-line mass spectrometry (DEMS) and in-situ Fourier Transform IR spectroscopy were used to identify reaction products. It was found that the interaction of the reactant with the electrode | electrolyte interface depends strongly on the electronic structure of the hydrocarbon. Acetylene forms a chemisorbed intermediate in a wide potential range above the potential of zero charge. Only CO 2 was observed during oxidation of both adsorbed and dissolved acetylene. On the other hand, ethylene interacts more weakly with the Au surface and no strongly bonded adsorbate was found. Acetaldehyde, acetic acid and CO 2 were detected as the main oxidation products of dissolved ethylene.

Thin-layer electrochemical studies of the oxidative underpotential deposition of sulfur and its application to the electrochemical atomic layer epitaxy deposition of CdS

The oxidative underpotential deposition (UPD) of S from a sulfide solution has been demonstrated and investigated using a polycrystalline Au electrode in a thin-layer electrochemical cell. UPD S appears to form spontaneously when a clean Au electrode is exposed to the sulfide solution. Subsequent oxidation of the sulfide to bulk sulfur is quantitative at potentials above −0.6 V vs. Ag/AgCl/1 M NaCl. This adsorbed S is quantitatively converted to sulfate in acidic electrolytes at potentials above 0.7 V. Oxidation of adsorbed S in a base solution also appears to form sulfates at potentials above 0.3 V; however, the process was considerably slower than that in an acid. The use of oxidative S UPD in the deposition of CdS using the method of electrochemical atomic layer epitaxy (ECALE) is also reported. An ECALE cycle was developed using a 2.5 mM Na2S solution alternated with a 10 mM CdSO4 solution. Both S and Cd were deposited at −0.6 V. Studies of deposits resulting from up to five ECALE cycles produced coverages of 0.45 monolayers of Cd and S per cycle, where 0.5 monolayers would have been ideal. In addition, the dependence of the deposit coverage on the Cd deposition potential was examined; this displayed the classic “S” shape expected for a surface-area-limited process.

Adsorption of benzoic acid on a polycrystalline gold electrode

Adsorption of benzoic acid on a polycrystalline Au electrode, obtained by electroplating of gold, has been studied in 0.1 M HClO 4 using cyclic voltammetry and radiotracer technique. Adsorption has been found to take place in the entire range of studied potentials, from 0.05 to 1.75 V ( rhe), with the surface concentration of the adsorbate exceeding 5 × 10 14 molecules cm −2 at saturation. Desorption into a clean supporting electrolyte is small and extremely slow. On the other hand, surface/bulk exchange of benzoic acid is much faster, attesting to the dynamic equilibrium between the adsorbed and solution species. Adsorption data and model calculations strongly indicate that two different orientations of the adsorbed molecules are present on the surface. Flat (parallel to the surface) orientation dominates at less positive potentials while the vertical (perpendicular to the surface) orientation dominates at more positive potentials. Regardless of orientation, benzoic acid adsorption on gold falls into the chemisorption category. General behavior of the system bears close resemblance to the adsorption of benzoic acid on platinum that was reported earlier in Zelenay and Sobkowski, Electrochim. Acta 29, 1715 (1984).

“In-situ” Quartz Crystal Microbalance Responses and Electrochemistry Regarding Langmuir–Blodgett Film of Viologen Polyion Complex on the Electrode Surface

Abstract The electrochemical behavior and the response of the quartz crystal microbalance(QCM) for the Langmuir–Blodgett(LB) viologen film have been examined. LB monolayers employed are prepared by electrostatically fixing the viologen monolayer containing the alkyl chain moiety on the poly(potassium 1-sulfatoethylene). The number of monolayers accumulated on the polycrystalline Au electrode of a piezoelectric quartz crystal was controlled by changing the general vertical dipping times. The QCM frequency response for the first reduction scan of viologen dication (V2+) to monocation radical (V+•) shows extremely large frequency increase, qualitatively indicative of mass loss, viscoelasticity decrease, or mechanical stress decrease during the reduction. A frequency increase during the second reduction scan is also observed, although not as large as for the first scan. The frequencies increase continuously with increasing number of scans and the steady-state responses can only be obtained after many repetitive potential scans. The results of the conductance spectra measurements of the quartz crystal/LB film composite resonator are interpreted as break-in and/or an increase of aggregation of the orientated structure during potential scans. The cyclic voltammetric wave obtained in the potential range 0 to −0.7 V vs. sodium chloride saturated calomel electrode has the characteristics of a simple, one electron transfer between V2+ and V+•, being chemically reversible process. The subsequent cyclic voltammogram responses obtained after the first cycle of potential scan are nearly identical for some hours, in contrast to the responses of the QCM. It is shown that structural change of the organized film dose not have a significant influence on the shape of the electrochemical response but does have an effect on the frequency response of the QCM.

Preliminary Studies of GaAs Deposition on Au(100), (110), and (111) Surfaces by Electrochemical Atomic Layer Epitaxy

The development of a new method for epitaxial growth of compound semiconductors is briefly described: electrochemical atomic layer epitaxy (ECALE). ECALE is based on the successive underpotential deposition (UPD) of atomic layers of different elements to form a compound. Preliminary studies of the ECALE deposition of were performed in a thin‐layer electrochemical cell with a polycrystalline Au electrode. Potentials required for oxidative UPD of As and reductive UPD of Ga were evaluated with different solution concentrations and pH's. Subsequent studies were performed in an ultrahigh vacuum surface analysis instrument interfaced to an electrochemical cell. A gold single crystal supporting the three low‐index planes was used in these studies. Auger electron spectroscopy (AES) and coulometry were used for surface composition analysis, while low energy electron diffraction (LEED) was used for structural analysis. Arsenic electrodeposition from solutions resulted in a succession of LEED patterns as the coverage increased on each face. Low coverage structures, corresponding to less than one monolayer of As, were observed only on the (100) and (110) faces: a at coverage and a at coverage. Oxidative UPD of As from was observed only on the (100) and (110) faces, resulting in low coverage ordered structures analogous to those formed during deposition from at low As coverages. Ga UPD from solutions was observed on all three faces, although it resulted in disordered layers of Ga oxide upon removal of the substrate from solution, due to partial oxidation of the Ga in contact with water in the absence of potential control. UPD of Ga on As‐covered surfaces ( and ) resulted in stoichiometric coverages of Ga and As on both. Structures displaying and LEED patterns were formed on the (100) and (110) faces, respectively. The stability of in aqueous solutions was evaluated. Potentials vs.pH plots were calculated for several concentrations of dissolved Ga and As species.

The electrochemical desorption of n-alkanethiol monolayers from polycrystalline Au and Ag electrodes

Monolayers formed at Au and Ag surfaces by the spontaneous adsorption of n-alkanethiols were characterized voltammetrically to examine the chemistry of the bound thiol head group. Electrode reactions that correspond to the oxidative- and reductive-desorption of the adsorbed n-alkanethiol monolayer are reported for the first time. Analysis of the data indicates that upon adsorption at both Au and Ag surfaces the hydrogen of the thiol group is lost and the sulfur atom is oxidized by one electron. Based on the charge required for the reductive-desorption of the monolayer, the surface coverage of the oxidized n-alkanethiol species is 9.3×10 −10 mol/cm 2 and 7.0 ×10 −10 mol/cm 2 on Au and Ag, respectively. The value of the surface coverage at Au is slightly greater than that expected for a closest-packed overlayer commensurate with a Au(111) substrate, whereas the value at Ag is somewhat less than that expected for layers commensurate at any of the low index Ag surface planes. The low apparent coverage observed for Ag electrodes is attributed to a portion of the monolayer being electroinactive at accessible applied voltages. As a probe of the mechanism for monolayer formation, films that were deposited on Ag and Au surfaces from solutions containing sodium n-octadecanethiolate were also characterized. Examination of the resulting layers with infrared reflection spectroscopy and optical ellipsometry indicates that only at Ag substrates were the films deposited from the thiolate solution complete and structurally similar to monolayers formed from n-octadecanethiol solutions. Based on this observation, it is postulated that the thiol hydrogen of the n-alkanethiol molecule participates in the reduction reaction that is concomitant to the thiol oxidation during adsorption on Au, whereas adsorption on Ag proceeds through the reduction of the Ag(I) surface species of the native oxide. A model for the electric double layer at the monolayer-coated-electrode/solution interface is also suggested based on the observed chain-length dependence of both the capacitance and the potential for the reductive desorption.

Ultraviolet photoelectron spectroscopy (UPS) of anodic oxide films on Au, Pt, Ru and Ru 0.5Ir 0.5 alloy

Anodically formed oxide films on polycrystalline Au, Pt and Ru electrodes were investigated using ultraviolet photoelectron spectroscopy (UPS) after removal from the electrolyte (0.1M HClO 4) under potential control and transfer to the UHV chamber. The UPS valence-band spectra after oxide formation clearly show a significant reduction of the density of states at the Fermi level to virtually zero for Au and Pt. For Ru the metallic properties of the oxide are reflected by a finite density of electronic states at E F. Rinsing of the emersed electrodes with water removes counter ions on the surface, which give rise to additional emission features overlapping with the O 2p band of the oxide. The results indicate that the oxide on Au is a semiconductor (n-type) with the valence-band edge located 1.3 eV below E F. The oxide on Pt is either semimetallic or semiconducting with a very small band gap (0.2 eV). As a case study, the oxide formation on Ru 0.5Ir 0.5 has been monitored using UPS. As indicated by the position of the t 2g band, the UPS results show a transition from ruthenium oxide to iridium oxide formation upon increasing anodic potentials. Our investigation clearly demonstrates that UPS studies on emersed electrodes provide valuable information about the electronic properties of anodically grown oxide layers.

Thin-layer electrochemical studies of the underpotential deposition of cadmium and tellurium on polycrystalline Au, Pt and Cu electrodes

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Cd and Te on polycrystalline Au, Pt, and Cu substrates have been performed. These studies were done in order to investigate the initial stages of the electrodeposition of CdTe. Tellurium deposition on Cu electrodes is very rapid at potentials between hydrogen evolution and Cu dissolution; as a result, the amount of electrodeposited Te cannot be suitably controlled with potential. Subsequent removal of Te on Cu also proved difficult by standard electrochemical cleaning procedures. Tellurium UPD and stripping on Pt occurred simultaneously with Pt oxide reduction and Pt oxidation, respectively. In addition, cadmium UPD on Pt is ill-defined, resulting in three peaks overlapping with the hydrogen wave voltammetry. It is presumed that the hydrogen waves are suppressed by the deposited Cd, although this is not definite. The most informative results were obtained with Au because of its broad double-layer window, which is free of complications from surface-specific faradaic reactions. Deposition of Te from TeO 2 solutions on Au was shown to require 3.9 ± 0.1 electrons per deposited Te. A substantial loss of electrodeposited Te was observed when the potential was cycled into the oxidation region on both the Au and Pt electrodes. Quantitation of the charge on Au indicated this loss was the result of oxidation of the Te(IV) to a Te(VI) compound which is difficult to reduce back to Te(IV) prior to hydrogen evolution on Au or Pt. At potentials between Au oxidation and Te UPD, the soluble Te species, HTeO + 2, was shown to adsorb reversibly on the Au surface. In studies of the alternated deposition of Cd and Te, initially deposited Cd is displaced by Te at potentials positive of −0.35 V. Subsequent UPD of Cd on the Te-covered Au resulted in one Cd atom for each deposited Te.

ChemInform Abstract: Electrochemical in situ Investigations on Polycrystalline Gold Electrodes with Oscillating Quartz Crystals: The Electrosorption Valency and Heterogeneity Constant of Halide and Sulfate Adlayers.

AbstractThe electrochemical formation and decay of halide (Cl‐, Br‐) and sulfate adlayers on polycrystalline Au electrodes are studied by the quartz microbalance method and the corresponding current and capacitance changes due to potential cycling are recorded.

An investigation of the oxidation of formaldehyde on noble metal electrodes in alkaline solutions by electrochemically modulated infrared spectroscopy (emirs)

In-situ spectroscopy was used to detect adsorbed species formed during the oxidation of formaldehyde at polycrystalline Au, Pt, Rh and Ir electrodes in alkaline solution. At the gold electrode, the spectra showed the oxidation of the gem-diol form of formaldehyde and the formation of an adsorbed formate species. At platinum, rhodium and iridium, adsorbed CO was observed in the hydrogen adsorption region where the oxidation of formaldehyde is strongly inhibited on these metals. Alternative routes for producing the poison are discussed.