Redox Replacement Research Articles

Self-assembly of well-ordered whisker-like manganese oxide arrays on carbon fiber paper and its application as electrode material for supercapacitors

Self-assembled well-ordered whisker-like manganese dioxide (MnO2) arrays on carbon fiber paper (MOWAs) were synthesized via a simple in situ redox replacement reaction between potassium permanganate (KMnO4) and carbon fiber paper (CFP) without any other oxidant or reductant addition. The CFP serves as not only a sacrificial reductant and converts aqueous permanganate (MnO4−) to insoluble MnO2 in this reaction, but also a substrate material and guarantees MnO2 deposition on the surface. The electrochemical properties were examined by cyclic voltammograms (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) in a three-electrode cell. According to the CV results, the ordered MOWAs yield high-capacitance performance with specific capacitance up to 274.1 F g−1 and excellent long cycle-life property with 95% of its specific capacitance kept after 5000 cycles at the current density of 0.1 A g−1. The high-performance hybrid composites result from a synergistic effect of large surface area and high degree of ordering of the ultrathin layer of MnO2 nanowhisker arrays, combined with the flexible CFP substrate and can offer great promise in large-scale energy storage device applications.

Electrochemical Atomic Layer Deposition (E-ALD) of Pt Nanofilms Using SLRR Cycles

This paper describes platinum nanofilm formation via the electrochemical form of atomic layer deposition (E-ALD), where the E-ALD cycles are based on surface limited redox replacement (SLRR) reaction. SLRR is where an atomic layer (AL) of a reactive (sacrificial) metal is exchanged for a more noble metal. In the present study both Cu and Pb AL were investigated as sacrificial atomic layers for replacement with both Pt(II) and Pt(IV) precursors. 25 E-ALD cycles were used to form Pt nanofilm deposits. Initial deposits contained an order of magnitude more Pt than expected, evidenced by a factor of 7 increase in surface roughness. Overly positive potentials achieved during the exchange promoted excess deposition and surface roughening. It is proposed that anionic Pt precursors adsorbed more strongly at high potentials, making them difficult to rinse from the cell. Those remaining adsorbed Pt anions are then reduced to Pto when the potential was shifted negative for deposition of the sacrificial element. The result was Pto formation at a large overpotential, which, contributed to excessive Pt deposits and roughening. Increased rinsing of the anionic Pt precursors from the cell eliminated the excess Pt deposition and roughening, resulting in the expected layer by layer growth of an ALD process.

All Electrochemical Fabrication of a Platinized Nanoporous Au Thin-Film Catalyst

In an effort to decrease the high cost associated with the design, testing, and production of electrocatalysts, a completely electrochemical scheme has been developed to deposit and platinize a nanoporous Au (NPG) based catalyst for formic acid oxidation. The proposed route enables synthesis of an alternative to the most established, nanoparticles based catalysts and addresses issues of the latter associated with either contamination inherent from the synthetic route or poor adhesion to the supporting electrode. The synthetic protocol includes as a first step, electrochemical codeposition of a Au((1-x))Ag(x) alloy in a thiosulfate based electrolyte followed by selective electrochemical dissolution (dealloying) of Ag as the less noble metal, that generates an ultrathin and preferably continuous porous structure featuring thickness of less than 20 nm. NPG is then functionalized with Pt (no thicker than 1 nm) by surface limited redox replacement (SLRR) of underpotentially deposited Pb layer to form Pt-NPG. SLRR ensures complete coverage of the surface with Pt, believed to spread evenly over the NPG matrix. Testing of the catalyst at a proof-of-concept level demonstrates its high catalytic activity toward formic acid oxidation. Current densities of 40-50 mA cm(-2) and mass activities of 1-3 A.mg(-1) (of combined Pt-Au catalyst) have been observed and the Pt-NPG thin films have lasted over 2600 cycles in standard formic acid oxidation testing.

Kinetics of Metal Deposition via Surface-Limited Redox Replacement Reaction

In this work, we examined the kinetics of metal deposition via surface-limited redox replacement reaction (SLRR). The model system was Pt submonolayer deposition on Au(111) via redox replacement of Pb and Cu underpotentially deposited (UPD) monolayers (MLs) on Au(111). The deposition kinetics were studied through the formalism of the comprehensive analytical model developed to fit open circuit potential (OCP) transients from our deposition experiments. Reaction half time, reaction order, and reaction rate constant were extracted and discussed within the frame of their application to the design and control of deposition experiments. Through the scope of the analytical model, different aspects of SLRR (i.e., the nature of the UPD metal monolayer participating in the redox reaction, the transport limitation, and the role of the anions/electrolyte) were investigated in terms of their effects on the reaction kinetics.

Architecture-Dependent Surface Chemistry for Pt Monolayers on Carbon-Supported Au

Pt monolayers were grown by surface-limited redox replacement (SLRR) on two types of Au nanostructures. The Au nanostructures were fabricated electrochemically on carbon fiber paper (CFP) by either potentiostatic deposition (PSD) or potential square wave deposition (PSWD). The morphology of the Au/CFP heterostructures, examined using scanning electron microscopy (SEM), was found to depend on the type of Au growth method employed. The properties of the Pt deposit, as studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and cyclic voltammetry (CV), were found to depend strongly on the morphology of the support. Specifically, it was found that smaller Au morphologies led to a higher degree of cationicity in the resulting Pt deposit, with Pt(4+) and Pt(2+) species being identified using XPS and XAS. For fuel-cell catalysts, the resistance of ultrathin catalyst deposits to surface area loss through dissolution, poisoning, and agglomeration is critical. This study shows that an equivalent of two monolayers (ML) is the low-loading limit of Pt on Au. At 1 ML or below, the Pt film decreases in activity and durability very rapidly due to presence of cationic Pt.

Boosting the performance of Pt electro-catalysts toward formic acid electro-oxidation by depositing sub-monolayer Au clusters

CO poisoning is the main obstacle to the application of Pt nanoparticles as anode catalysts in direct formic acid fuel cells (DFAFCs). Significant types of Pt alloys have been investigated, which often demonstrate evidently improved catalytic performance governed by difference mechanisms. By using a well-known electrochemical technique of under potential deposition and in situ redox replacement, sub-monolayer Au clusters are deposited onto Pt nanoparticle surfaces in a highly controlled manner, generating a unique surface alloy structure. Under optimum conditions, the modified Pt nanoparticles can exhibit greatly enhanced specific activity (up to 23-fold increase) at potential of −0.2V vs. MSE toward formic acid electro-oxidation (FAEO). Interestingly, the mass specific activity can also be improved by a factor of 2.3 at potential of −0.35V vs. MSE although significant amount of surface Pt atoms are covered by the overlayer Au clusters. The much enhanced catalytic activity can be ascribed to a Pt surface ensemble effect, which induces change of the reaction path. Moreover, the sub-monolayer Au coating on the surface also contributes to the enhanced catalyst durability by inhibiting the Pt oxidation. These results show great potential to rationally design more active and stable nanocatalysts by modifying the Pt surface with otherwise inactive materials.

Pt Deposition by Surface Limited Redox Replacement of H-UPD

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A Comparative Study of Au Grown by Pb UPD Assisted Methods (Surface Limited Redox Replacement, Surfactant Mediated Growth) and Overpotential Deposition

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Cobalt Monolayer Islands on Ag(111) for ORR Catalysis

The design of a catalyst for one of the most important electrocatalytic reactions, the oxygen reduction reaction (ORR), was done following the most recent guidelines of theoretical studies on this topic. Aim of this work was to achieve a synergic effect of two different metals acting on different steps of the ORR. The catalytic activity of Ag, already known and characterized, was enhanced by the presence of a monolayer of cobalt subdivided into nanosized islands. To obtain such a controlled nanostructure, a novel method utilizing self-assembled monolayers (SAMs) as templates was employed. In a recent study, we were able to perform a confined electrodeposition of cobalt onto Ag(111) in a template formed by selectively desorbing a short-chain thiol (3-mercaptopropionic acid, MPA) from binary SAMs using 1-dodecanthiols (DDT). This method allows for an excellent control of the morphology of the deposit by varying the molar ratio of the two thiols. Because cobalt does not deposit on silver at an underpotential, the alternative approach of surface limited redox replacement (SLRR) was used. This method, recently developed by Adžić et al., consists of the use of a monolayer of a third metal, which can be deposited at an underpotential, as a template for the spontaneous deposition of a more noble metal. Herein, we choose zinc as template for the deposition of cobalt. Ag(111) crystals were covered by monolayer islands consisting of cobalt, with the surface atomic ratios ranging from 12 to 39% for cobalt. The catalytic activity of such samples towards ORR was evaluated and the best improvement in activity was found to be that of the sample with a cobalt percentage of approximately 30% with respect to the bare silver, which is in good agreement with theoretical hypotheses.

From Au to Pt via Surface Limited Redox Replacement of Pb UPD in One-Cell Configuration

This work is aimed at developing a protocol based on surface limited redox replacement (SLRR) of underpotentially deposited (UPD) Pb layers for the growth of epitaxial and continuous Pt thin films on polycrystalline and single crystalline Au surfaces. Different from previously reported papers using SLRR in multiple immersion or flow cell setups, this work explores the one-cell configuration setup as an alternative to improve the efficiency and quality of the growth. Open circuit chronopotentiometry and quartz-crystal microbalance experiments demonstrate steady displacement kinetics and a yield that is higher than the stoichiometric Pt(II)-Pb exchange ratio (1:1). This high yield is attributed to oxidative adsorption of OH(ad) taking place on Pt along with the displacement process. Also, ex situ scanning tunneling microscopy surface characterization reveals after the first replacement event the formation of a dense Pt cluster network that homogenously covers the Au surface. The Pt films grow homogenously with no significant changes in the cluster distribution and surface roughness observed up to 10 successive replacement events. X-ray diffraction analysis shows distinct (111) crystallographic orientation of thicker Pt films deposited on (111) textured Au thin films. Coarse energy dispersive spectroscopy measurements and finer X-ray photoelectron spectroscopy suggest at least 4 atom % Pb incorporating into the Pt layer compared to 13 atom % alloyed Cu when the growth is carried out by SLRR of Cu UPD.

Reaction kinetics of metal deposition via surface limited red-ox replacement of underpotentially deposited metal monolayers

The study of the kinetics of metal deposition via surface limited red-ox replacement of underpotentially deposited metal monolayers is presented. The model system was Pt submonolayer deposition on Au(1 1 1) via red-ox replacement of Pb and Cu UPD monolayers on Au(1 1 1). The kinetics of a single replacement reaction was studied using the formalism of the comprehensive analytical model developed to fit the open circuit potential transients from deposition experiments. The practical reaction kinetics parameters like reaction half life, reaction order and reaction rate constant are determined and discussed with their relevance to design and control of deposition experiments. The effects of transport limitation and the role of the anions/electrolyte on deposition kinetics are investigated and their significance to design of effective deposition process is discussed.

Formation of continuous platinum layer on top of an organic monolayer by electrochemical deposition followed by electroless deposition

A metal–molecule–metal sandwich structure is constructed to wire the molecule to external circuits. A continuous mono-atomic thick metallic layer of platinum is formed on top of a 1,4-benzenedimethanethiol (BDMT) self-assembled monolayer (SAM) on a Au(1 1 1) substrate through electrochemical deposition followed by electroless deposition (ELD). At first, about 1/3 monolayer of Pt is formed on top of the BDMT SAM by electrochemical reduction of pre-adsorbed PtCl 4 2 - complex, then the coverage of Pt layer is increased by ELD. Two approaches for ELD of Pt are examined. In the first approach, the pre-formed sub-monolayer of Pt is used as a nucleation center for ELD of Pt by reducing Pt ions in a solution containing a reducing agent. X-ray photoelectron spectroscopy (XPS), angle resolved X-ray photoelectron spectroscopy (AR-XPS) and electrochemical measurements show that the coverage of Pt increases with the increase of plating time but the metallic Pt penetrates through organic film and deposits between SAM and Au(1 1 1) substrate if the plating time is too long. In the second approach, the electrochemically formed sub-monolayer of Pt is used not only as a nucleation center but also as a catalyst for the selective ELD of Cu. Electrochemical measurements as well as AR-XPS after a spontaneous redox replacement of Cu by Pt confirm that a full layer of Pt is formed on top of the BDMT SAM and no platinum is found underneath the SAM. The latter is a new method for a construction of a metal–molecule–metal sandwich structure, which allows wiring molecules to external circuits.

Growth of Metal Multilayer Structures by Surface Limited Redox Replacement and Surfactant Mediation

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From Au to Pt via Surface Limited Redox Replacement of Pb UPD in One Pot Configuration

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Pt Nano-Structures Formed through Surface Limited Redox Replacement Reaction with Cu Adlayer on Au(111) Surfaces

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Preparation of ‘Sandwich-Like’ Au/Pt Composite Multilayer Films for Methanol Electrooxidation

Pt/Au composite monolayer films were fabricated by combining interfacial assembly and under-potential deposition (UPD) with redox replacement. Based on the Pt/Au composite monolayers, an organic linker-free method was proposed for the fabrication of sandwich-like Pt/Au composite multilayer films: (Pt/Au)n, Ptm/Au, and (Pt3/Au)k (n, m, or k represents the layer number). Electron microscopy was used to characterize the morphologies of the Au monolayer films and the Pt/Au composite multilayer films. For each type of composite multilayer films, a common characteristic was that the effective electroactive areas increased with an increase in the layer number. Additionally, the electrocatalytic activities of the composite multilayer films for methanol electrooxidation are systematically discussed by examining the catalytic current densities and its tolerance toward carbonaceous species. For the same series of composite multilayer films (Pt/Au)3, Pt3/Au, and (Pt3/Au)2 showed a higher catalytic current density than bulk Pt (Ptbulk). Among the three composite multilayer films, (Pt/Au)3 showed the best catalytic performance in terms of the current density and tolerance toward carbonaceous species. The tolerance of (Pt/Au)3 to carbonaceous species was found to be better than that of the commercial Pt/C catalyst. This better electrocatalytic activity may be attributed to the maximum synergistic effect between Au and Pt, which depends on the Pt:Au atomic ratio and also the arrangement of Pt and Au nanoparticles. (Article)

Stoichiometry of Pt Submonolayer Deposition via Surface-Limited Redox Replacement Reaction

The work exploring the stoichiometry of Pt deposition via surface-limited redox replacement of the underpotentially deposited (UPD) Cu monolayer on Au(111) is presented. The Cu UPD monolayer is formed from solution, whereas the Pt deposition via surface-limited redox replacement reaction is carried out in solution at open-circuit potential. Our results indicate that the Pt submonolayers have two-dimensional morphology and linear dependence of their coverage on the amount (coverage) of the replaced Cu UPD monolayers. Our analysis shows that the oxidation state of Cu during redox replacement reaction is , suggesting that four Cu UPD adatoms are replaced by each deposited Pt adatom. This work stresses the general importance of the anions, determining the stoichiometry of metal deposition reaction via surface-limited redox replacement of the UPD monolayers.

Electrochemical quartz crystal microbalance study on Au-supported Pt adlayers for electrocatalytic oxidation of methanol in alkaline solution

Underpotential deposition (UPD) of Cu on an Au electrode followed by redox replacement reaction (RRR) of CuUPD with a Pt source (H2PtCl6 or K2PtCl4) yielded Au-supported Pt adlayers (for short, Pt(CuUPD-Pt4+)n/Au for H2PtCl6, or Pt(CuUPD-Pt2+)n/Au for K2PtCl4, where n denotes the number of UPD-redox replacement cycles). The electrochemical quartz crystal microbalance (EQCM) technique was used for the first time to quantitatively study the fabricated electrodes and estimate their mass-normalized specific electrocatalytic activity (SECA) for methanol oxidation in alkaline solution. In comparison with Pt(CuUPD-Pt2+)n/Au, Pt(CuUPD-Pt4+)n/Au exhibited a higher electrocatalytic activity, and the maximum SECA was obtained to be as high as 35.7 mA μg−1 at Pt(CuUPD-Pt4+)3/Au. The layer-by-layer architecture of Pt atoms on Au is briefly discussed based on the EQCM-revealed redox replacement efficiency, and the calculated distribution percentages of bare Au sites agree with the experimental results deduced from the charge under the AuO x -reduction peaks. The EQCM is highly recommended as an efficient technique to quantitatively examine various electrode-supported catalyst adlayers, and the highly efficient catalyst adlayers of noble metals are promising in electrocatalysis relevant to biological, energy and environmental sciences and technologies.

Carbon paper supported Pt/Au catalysts prepared via Cu underpotential deposition-redox replacement and investigation of their electrocatalytic activity for methanol oxidation and oxygen reduction reactions

Through a simple and rapid method, carbon papers (CPs) were coated with Au and the resulting Au/CP substrates were used for the preparation of Pt/Au/CP by Cu underpotential deposition (Cu UPD) and redox replacement technique. A series of Pt n /Au/CP catalysts (where n = number of UPD-redox replacement cycles) were synthesized and their electrochemical properties for methanol oxidation reaction (MOR), and oxygen reduction reaction (ORR) were investigated by electrochemical measurements. The Pt n /Au/CP electrodes show higher electrocatalytic activity and enhanced poison tolerance for the MOR as compared to a commercial Pt/C on CP (Pt/C/CP). The highest mass specific activity and Pt utilization efficiency for MOR was observed on Pt 1/Au/CP with a thickness close to a monatomic Pt layer. Chronoamperometric tests in methanol solution revealed that Pt n /Au/CPs have much higher CO tolerance compared to Pt/C/CP. Among the Pt n /Au/CPs, CO tolerance decreases with increasing the amount of deposited Pt, indicating that the exposed Au atoms in close proximity to Pt plays a positive role against CO poisoning. Compared with the Pt/C/CP, all the Pt n /Au/CP electrodes show more positive onset potentials and lower overpotentials for ORR. For instance, the onset potential of ORR is 150 mV more positive and the overpotential is ∼140 mV lower on Pt 4/Au/CP with respect to Pt/C/CP.

E-ALD of Cu Nanofilms on Ru/Ta Wafers Using Surface Limited Redox Replacement

This paper describes the formation of Cu nanofilms on a Ru/Ta-coated wafer using electrochemical atomic layer deposition (E-ALD). The initial steps involved cleaning and oxide removal from the Ru/Ta substrate using ultrahigh vacuum electrochemical system. Auger spectroscopy was used to follow the relative amounts of oxygen, Ru, and Cu on the wafer: as-received, after electrochemical treatment, after ion bombardment, and after Cu deposition. An automated flow cell electrodeposition system was employed to grow Cu nanofilms with up to 200 cycles using surface limited redox replacement (SLRR) reaction. The open-circuit potential was used to follow the exchange of Pb for Cu in the SLRR reaction. Electron probe microanalysis was used to determine the homogeneity of the Cu films. The use of a complexing agent (citrate) greatly improved the homogeneity. Different concentrations of citrate were investigated, containing 2 or 4 mM citrate. Atomic force microscopy images of the Cu films showed the ingress morphology to be the same as the egress when 4 mM citrate was used. A prominent Cu(111) peak was displayed in the X-ray diffraction pattern for 200 cycles of Cu grown on the Ru/Ta-coated wafer.