Underpotential Deposition Of Thallium Research Articles

Oxygen Reduction on Gold Nanocrystal Surfaces in Alkaline Electrolyte: Effects of Surface Proton Transfer

The surface atomic structures of metal catalysts play critical roles in determining the kinetics and mechanisms of catalytic reactions. Intrinsic correlations between surface structures and catalytic performances have been extensively studied for the bulk single-crystal surfaces. In oxygen reduction reaction (ORR), an essential reaction in energy conversion devices such as alkaline fuel cells, a complete four-electron (4e) ORR is preferred because energy conversion is twice as efficient compared with the partial two-electron (2e) ORR pathway. Adzic and co-workers pioneered the study of structural sensitivity of the ORR pathway on gold (Au) surfaces. They found that the ORR in alkaline electrolytes proceeds via a 2e reduction to hydroperoxide ions (O2H−) on the majority of facets including Au(111)[ 1 ], while the complete 4e reduction to hydroxide ions (OH−) occurs only in a limited potential region on Au(100)[ 2 ] and on the high-index Au(910) and Au(11,1,1) single crystal surfaces with a large fraction of the {100} sub-facet[ 3 ]. However, the reasons for such ORR behaviors on Au surfaces remain unclear. In this study, we synthesized three monofacet Au nanocrystals with well-defined shapes[ 4 ], including octahedra, cubes and truncated ditetragonal prisms (TDPs), which are enclosed by {111}, {100}, and {310} surfaces, respectively. The surface cleanness and high crystallinity of Au nanocrystals are confirmed by several structural characterization techniques, and by electrochemical characterizations including cyclic voltammetry (CV) and thallium underpotential deposition (Tl UPD). The Au octahedral and cubic nanocrystals demonstrate good resemblance of the ORR behaviors to those on their bulk single-crystal counterparts. Intriguingly, the TDP particles enclosed with twelve {310} facets catalyze the 4e ORR over the full potential range that has not been seen. Density functional theory (DFT) calculations were carried out for the ORR on the three Au surfaces without and with co-adsorbed water. The DFT-calculated free energy diagrams, in combination with experimental electrochemical results, elucidate how the surface proton transfers activate the facet- and potential-dependent 4e ORR on Au {100} and {310}. The new insight helps mechanism studies of facet-sensitive catalytic reactions, and provides guidance for designing nanocatalysts with optimal surface structures for various reactions. Acknowledgment The research was carried out at the Center for Functional Nanomaterials and the Chemistry Division of Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. We acknowledge the computational resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Morphological Transitions during Au Electrodeposition: From Porous Films to Compact Films and Nanowires

Gold electrodeposition was studied in a sulfite electrolyte to which micromolar concentrations of Tl2SO4 were added. Hysteresis and a regime of negative differential resistance (NDR) evident in electroanalytical measurements are correlated with deposit morphology and interpreted through measurements of thallium underpotential deposition (upd). Deposit morphologies range from specular surfaces to highly faceted dendrite-like grains of moderate aspect ratio and, for potentials within the NDR region, sub-50 nm diameter, high aspect ratio 110 oriented single crystal nanowires. The nanowires exhibit an epitaxial relationship to the substrate that permits one step fabrication of surfaces densely covered with high aspect ratio nanowires having controlled orientations. The NDR and nanowires are a consequence of the non-monotonic relationship between Tl coverage and growth velocity; at low coverage Tl accelerates Au deposition while at higher coverage it inhibits deposition. Immiscibility of the Tl and Au supports on-going surface segregation during area expansion that accompanies nanowire growth leading to greater dilution of the additive coverage and more rapid growth at the nanowire tips, while the sidewalls remain passivated by a saturated Tl coverage.

New insight on the behavior of the irreversible adsorption and underpotential deposition of thallium on platinum (111) and vicinal surfaces in acid electrolytes

We report, for the first time, the electrochemical behavior of thallium irreversibly adsorbed on Pt (111) and platinum stepped surfaces composed of (111) terraces and monoatomic steps. Similar to the case of thallium UPD, the voltammograms obtained after thallium irreversible adsorption present three characteristic features. After a careful analysis of the effect of the thallium concentration, the concentration and nature of the anion of the supporting electrolyte and the pH of the solution on these voltammetric features, we have been able to ascribe these processes to Tl/Tl+ oxidation and anion adsorption on the Tl-modified surface. In addition, the results obtained with stepped surfaces, indicate that some of the features are clearly associated to the presence of (111) surface domains, and thus they could be used for the quantification of these sites.

Nanoparticle–electrode collision processes: Investigating the contact time required for the diffusion-controlled monolayer underpotential deposition on impacting nanoparticles

Recent work on faradaic processes occurring during thermal nanoparticle–electrode collisions contrasts significantly from analogous research using ultrasonically-driven microparticles, where no faradaic signals were found. It is suggested that this might be explained by the differences in both particle size and contact time. To investigate this, we present results from adapted Monte Carlo random walk simulations. Using the underpotential deposition of thallium onto silver nanoparticles as a model system, it is found that an estimated minimum contact time of ca. 10−4s is required to deposit a complete monolayer (from a 10mM solution) onto a nanoparticle of radius 45nm.

Nanoparticle–Electrode Collision Processes: The Underpotential deposition of Thallium on Silver Nanoparticles in Aqueous Solution

The electrochemistry of collisions between metal nanoparticles (NPs) and electrode surfaces has been of recent interest with the development of anodic particle coulometry as a characterisation method. For the first time the underpotential deposition of metal ions from solution onto metal nanoparticles during collisions between the NPs and an inert electrode is reported.

Thallium underpotential deposition on silver nanoparticles: size-dependent adsorption behaviour

We report the size-dependent adsorption of thallium on silver nanoparticles (AgNPs) in the region of underpotential deposition; the phenomenon is observed for large nanoparticles, and bulk silver, but absent for nanoparticles below 50 nm diameter.

The contrasting behaviour of polycrystalline bulk gold and gold nanoparticle modified electrodes towards the underpotential deposition of thallium

The underpotential deposition (UPD) of thallium onto a polycrystalline gold macroelectrode or gold nanoparticles (10 ± 5 nm in diameter) supported on multiwalled carbon nanotubes (MWCNTs) is compared both in the presence and the absence of adsorbed chloride anions. On the polycrystalline gold macrodisc electrode the UPD of thallium is observed. However, on the 10 nm gold nanoparticles supported on MWCNTs no UPD of thallium is observed despite several control experiments which confirmed that the gold nanoparticles were indeed electroactive. This is compared to the reported behaviour of larger 30–60 nm diameter gold nanoparticles at which UPD of thallium is also found to occur and the possible reasons for this are discussed. This report provides an example where the properties of metal nanoparticles differ markedly from the bulk metal and the properties of the nanoparticles vary significantly with the size of the nanoparticles.

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.

Electrochemical Behavior of Thallium Underpotential Deposition on Pt (111) and Stepped Surfaces. Applications in the Study of Superficial Structure of Nanoparticles

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A study of the initial stages of the electrochemical deposition of thallium on copper, Part IV. The potential step results: Underpotential deposition on (111), (110) and (100) oriented copper single c

The underpotential deposition and dissolution of thallium onto carefully chemically polished single crystal copper (111), (110) and (100) electrode surfaces from acetate, sulphate and perchlorate solutions have been investigated using single and double potential step techniques. It appeared that the different anions used did not change the current-time response characteristics significantly. Current-time responses to the potential steps applied strongly resemble those observed in the case of thallium underpotential deposition on silver single crystals [1]. The characteristics of i-t transients obtained by single potential step suggest very fast 2D crystal growth processes taking place. Sharp linear voltammetry peaks, which are observed for both thallium and lead deposition on Cu(111) [3,8] and on Ag(111) [1,3], probably always indicate nucleative phase formation processes but the rate of the lattice growth step will vary from system to system. Comparison of the obtained results with those for lead underpotential deposition [8-13] on Cu(111), (110) and (100) suggests that, in the present case, the rate of the 2D lattice building process for both thallium underpotential monolayers formed on three copper single crystal surfaces examined is so fast that it becomes controlled by diffusive processes; this will probably be surface diffusion [24,25] initially and, at longer times planar bulk diffusion. Double pulse experiments did not help significantly in attempts to obtain i-t transients capable of providing data suitable for showing conclusively the occurrence of 2D crystal formation processes. A similar situation occurred also with silver substrates [1].

Radiotracer study of thallium underpotential deposition on a polycrystalline gold electrode in alkaline solutions

Abstract Surface concentration of thallium atoms at the polycrystalline gold electrode in 0.1 M KOH supporting electrolyte containing 1×10 −5 M and 1×10 −4 M Tl + ions was measured by the radioactive labeling ( 204 Tl) method. Kinetics of thallium electrodeposition, anodic dissolution, and underpotential deposition (upd) as well as the equilibrium surface concentration of thallium atoms in the potential region of thallium upd were determined from the radiotracer data. The potential dependence of the equilibrium surface concentration obtained from the radiometric method was compared with that calculated from the cyclic voltammetry. At potentials close to the Nernst potential for the Tl/Tl(I) couple, the surface concentration of thallium atoms is close to the close-packed thallium monolayer capacity. In the potential region from the Nernst potential to the main upd peak, the thallium surface concentration decreases monotonically with the electrode potential. However, at more positive potentials, the surface concentration does not drop to zero, but goes through a minimum. Around this minimum, the surface concentration is apparently determined both by the upd of thallium atoms and electrostatic adsorption of thallium cations induced by the specific adsorption of hydroxyl anions.

Effect of anions on the underpotential deposition of thallium(I) on polycrystalline silver

Underpotential deposition (upd) of thallium(I) on a polycrystalline silver surface in solutions containing specifically adsorbing anions (sulphate and chloride) has been studied by voltammetry and the electrochemical quartz crystal microbalance (EQCM). The specific adsorption of thallium cations on the silver surface at potentials positive with respect to the pzc appears to be enhanced by the specific adsorption of anions. This leads to ion pair formation between the specifically adsorbed thallium cation and anion even at potentials positive with respect to the upd region. The decrease in the upd shift is explained by the fact that additional energy is required to break the bond between the formed pair. The electrode mass change measured by EQCM is smaller due to the discharge of thallium cations from the adsorbed ion pairs. The agreement between voltammetric and EQCM data is observed in the potential region close to the bulk thallium deposition where the adsorption of anions is weak and, consequently, there is little formation of ion pairs. The effect of sulphate and chloride anions on the thallium upd process is in accordance with their adsorbing strength. It seems very likely that this scheme can be applied when other upd processes in strongly adsorbing anions containing solutions are considered.

Effect of thallium ions on the adsorption of cyanide-containing species from cyanide and dicyanoaurate solutions on a polycrystalline gold electrode

Abstract The effect of thallium(I) ions on the adsorption of CN-containing species on a gold electrode from alkaline cyanide and dicyanoaurate solutions was studied by a radioactive labelling method. In cyanide solutions, adsorption of CN-containing species decreases linearly with the coverage of thallium adatoms in the potential range of thallium underpotential deposition. No adsorption of CN-containing species was observed on the bulk thallium layer formed below the Nernst potential for the couple Tl/Tl+. In dicyanoaurate solutions, the effect of thallium ions on the adsorption of CN-containing species depends strongly on dicyanoaurate and thallium bulk concentrations. At the lowest dicyanoaurate concentration (1 mM) and the highest thallium concentration (0.1 mM) investigated, the effect of thallium ions on the adsorption of CN-containing species is similar to that in cyanide solutions. However, at the highest dicyanoaurate concentration (5 mM) and the lowest thallium concentration (0.01 mM) investigated this effect is significantly lower than in the first case. The observed effects of thallium ions both on the absorption of CN-containing species from dicyanoaurate solutions and on the kinetics of dicyanoaurate reduction on gold are discussed in terms of the thallium surface coverage on a gold electrodeposit.

In-situ photoacoustic investigation of thallium underpotential deposition on polycrystalline copper electrodes

Photoacoustic spectrometry (PAS) was used as an ancillary in-situ technique for the investigation of underpotential deposition (UPD) of thallium on polycrystalline copper. The PAS signal generated by 605 nm radiation was found to increase upon deposition of Tl on Cu. Since underpotential deposition is generally a monolayer event, the limit of detection obtained for this technique was estimated to be 0.05 of a monolayer. The PAS results support the conclusion that the second UPD peak is due to a second discrete monolayer rather than a filling-in of vacant sites in the first monolayer. The use of PAS to study the thallium system at pH 12 confirmed that removal of Cu2O from the electrode surface occurred at a sufficient rate that it did not interfere with the UPD of thallium.

The influence of solution composition on lead, cadmium and thallium underpotential deposition on (111) oriented silver single crystal surfaces

The influence of solution composition on the process of lead, cadmium and thallium UPD on (111) oriented silver single crystal surfaces has been investigated by using linear sweep voltammetry and differential capacity measurements. It is shown that the Y'-E curves can be used qualitatively as a criterion for the occurrence of partial charge transfer reactions in the double layer region and the commencement of the faradaic reaction of UPD. The C-E curves obtained in the presence and in the absence of depositing metal cations can give useful information about the nature of species present in the solution and adsorbed onto the substrate surface. It is also demonstrated that the presence of different complexes of cations investigated and the kinetics of their reduction can be responsible for a change in the UPD voltammograms.

Phase measurements of optical second harmonic generation on Au(111) during thallium underpotential deposition

Phase measurements are used for the first time to monitor the resonant contribution from an adsorbate to the anisotropic second harmonic response from a metal electrode. Changes in the phase of the optical second harmonic response from Au(111) are measured during electrochemical deposition of thallium. The phase of the signal at coverages of one and two monolayers of thallium relative to the adsorbate free surface is delayed by 180° and 258°, respectively. Possible sources for the observed nonlinearities are discussed.

A comparative study of the underpotential deposition of two monovalent cations, Tl + and H +, on Pt (111) as a way to the interpretation of the unusual adsorption states

The underpotential deposition (UPD) of thallium and hydrogen on Pt (111) in the presence of specific adsorption of sulphuric acid anions is compared by using voltammetry. Common voltammetric features are observed. It is shown that repeated cycles of thallium adsorption-desorption do not induce any surface structural change of the Pt (111) oriented electrode. The effect of thallium concentration on the potential of the thallium adsorption states is studied in the same way as the effect of pH. From the experimental results it may be deduced that thallium UPD at Pt (111) is an equilibrium adsorption-desorption process. The various adsorption states are quantitatively in agreement with the existence of 2 × 2, 3 0.5 × 3 0.5 R 30° and hexagonal compact overlayers. The unusual adsorption states of hydrogen are replicated on the voltammogram of thallium; the sharpest voltammetric peak is explained as a sudden change in the adsorption isotherm of the electroactive species (Tl + or H +) due to a surface phase transition of the anion specifically adsorbed in the inner layer.

The adsorption and underpotential deposition of thallium on gold in 0.5 M potassium chloride

The rotating ring-disk electrode technique has been used to study the adsorption of Tl(I) and Tl(0) on gold in slightly acidified 0.5 M KCl solution. The processes which occur, starting at the most anodic potential sustainable without appreciable supporting electrolyte (and gold electrode) oxidation are: +0.7 V (vs. SCE) to −0.2 V, adsorption of an ionic thallium species containing a formal oxidation charge of +1, Tl(I)ads; E< −0.2 V, (1) reduction of Tl(I)ads to underpotentially deposited Tl(0), (2) reduction of Tl(I) in solution to underpotentially deposited Tl(0). The surface coverage of Tl(0) at the Nernst potential is 150 μC per real cm2. As the electrode potential is scanned to anodic potentials, underpotentially deposited Tl(0) is oxidized to Tl(I), part of which dissolves in solution, and part of which remains as Tl(I)ads. In 0.11 mM Tl(I) solutions, the maximum coverage for Tl(I)ads is about 50 μC per real cm2.

The adsorption and underpotential deposition of thallium on gold in 0.5 M potassium chloride

The rotating ring-disk electrode technique has been used to study the adsorption of Tl(I) and Tl(0) on gold in slightly acidified 0.5 M KCl solution. The processes which occur, starting at the most anodic potential sustainable without appreciable supporting electrolyte (and gold electrode) oxidation are: +0.7 V (vs. SCE) to −0.2 V, adsorption of an ionic thallium species containing a formal oxidation charge of +1, Tl(I) ads; E< −0.2 V, (1) reduction of Tl(I) ads to underpotentially deposited Tl(0), (2) reduction of Tl(I) in solution to underpotentially deposited Tl(0). The surface coverage of Tl(0) at the Nernst potential is 150 μC per real cm 2. As the electrode potential is scanned to anodic potentials, underpotentially deposited Tl(0) is oxidized to Tl(I), part of which dissolves in solution, and part of which remains as Tl(I) ads. In 0.11 m M Tl(I) solutions, the maximum coverage for Tl(I) ads is about 50 μC per real cm 2.

Optical and electrochemical studies of the underpotential deposition of metals Part I. Thallium deposition on single crystal silver electrodes

The underpotential deposition of thallium on single crystal silver cathodes, orientations 100, 110 and 111, was investigated optically and voltammetrically. Careful chemical polishing of the silver surfaces was necessary to reveal the fine structure on the voltammograms reproducibly. The deposition process for the first monolayer was shown to be the formation of a layer of adsorbed atoms initially, followed by a phase transformation to form a crystal plane by two-dimensional nucleation and growth. The extent of the initial adsorption was related to the number of favourable sites on each crystal face such that the adsorbed layer formed a superlattice. The influence of the substrate structure was seen on the formation of the second thallium monolayer. The formation of this layer also involved adsorption followed by transformation into a crystal plane.