Underpotential Deposition Process Research Articles

Probe the Dynamic Adsorption and Phase Transition of Underpotential Deposition Processes at Electrode-Electrolyte Interfaces.

Electrochemical scanning tunneling microscopy (EC-STM) and electrochemical quartz crystal microbalance (E-QCM) techniques in combination with DFT calculations have been applied to reveal the static phase and the phase transition of copper underpotential deposition (UPD) on a gold electrode surface. EC-STM demonstrated, for the first time, the direct visualization of the disintegration of (√3 × √3)R30° copper UPD adlayer with coadsorbed SO42- while changing sample potential (ES) toward the redox Pa2/Pc2 peaks, which are associated with the phase transition between the Cu UPD (√3 × √3)R30° phase II and disordered randomly adsorbed phase III. DFT calculations show that SO42- binds via three oxygens to the bridge sites of the copper with sulfate being located directly above the copper vacancy in the (√3 × √3)R30° adlayer, whereas the remaining oxygen of the sulfate points away from the surface. E-QCM measurement of the change of the electric charge due to Cu UPD Faradaic processes, the change of the interfacial mass due to the adsorption and desorption of Cu(II) and SO42-, and the formation and stripping of UPD copper on the gold surface provide complementary information that validates the EC-STM and DFT results. This work demonstrated the advantage of using complementary in situ experimental techniques (E-QCM and EC-STM) combined with simulations to obtain an accurate and complete picture of the dynamic interfacial adsorption and UPD processes at the electrode/electrolyte interface.

Investigation of copper underpotential deposition on polycrystalline gold electrodes

A combination of cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry techniques has been used to study the underpotential deposition (UPD) of copper on polycrystalline gold (pc Au) electrodes. The experiments were conducted in an electrolyte composed of CuSO4 (5 mM) and H2SO4 (0.5 M). The results revealed that the concentration of copper (II) ions and the applied voltage exerted a synergistic effect on the position, shape, and range of spanning of the curves describing the redox potential and current density. The charge transfer resistance (Rct) corresponding to the UPD of Cu decreased monotonically with a decrease in the applied voltage, and the phase angle shifted toward high-frequency regions with a negative shift in the potential. The process of UPD of Cu on pc Au can be divided into three stages: Langmuir-type adsorption, growth of Cu monolayers on pc Au following the process of two-dimensional instantaneous nucleation, and the continuous two-dimensional nucleation of grains on the Cu layer. UPD on single crystal substrates and the single-layer and multi-layer structures associated with UPD have been extensively studied worldwide. However, the UPD of polycrystalline electrodes with various crystal faces, grain boundaries, and defects is complex, and the associated mechanism has rarely been studied. In recent years, the process of UPD has been widely used in various fields, such as fields associated with electrochemical sensing, fabrication of fuel cells, electrochemical CO2 reduction, and fabrication of solar cells.

On the Electrochemically Active Surface Area Determination of Electrodeposited Porous Cu 3D Nanostructures

Porous 3D Cu layers with the following average parameters: thickness ~35 µm, pore density ~4.0 × 106 cm−2, and pore sizes ~25 µm were electrodeposited from an acidic sulphate electrolyte, and the suitability of different electrochemically active surface area determination methods for characterising these electrodes was assessed. Structural characterisation of the samples was conducted using SEM and an optical profiler, while electrochemical measurements were performed using cyclic voltammetry and electrochemical impedance spectroscopy. The evaluation of electrochemically active surface area involved the underpotential deposition of Tl and Pb monolayers as well as double-layer capacitance measurements. The results obtained indicate that both methods yield similar results for non-porous Cu electrodes. However, for Cu 3D nanostructures, the evaluation mode significantly influences the results. Double-layer capacitance measurements show significantly higher values for the electrochemically active surface area compared to the underpotential deposition (UPD) technique. The complex spatial structure of the Cu 3D layer hinders the formation of a continuous monolayer during the UPD process, which is the principal reason for the observed differences.

Over- and underpotential deposition of copper from a deep eutectic solvent: Pt(1 1 1) single crystal versus polycrystalline Pt substrates

• Deposition/dissolution of Cu on Pt from ethaline occur in one-electron reactions. • A full (1x1) monolayer of Cu adatoms is formed on Pt(111) in ethaline. • No voltammetric signals of Cu UPD were observed for Pt(poly) in ethaline. • Surface structure determines the mechanism and rate of Cu OPD at the initial stages. • Cu OPD on Pt(111) and Pt(poly) in ethaline occurs via 2D and 3D growth, respectively. Deep eutectic solvents (DESs) attract ever greater interest as nonaqueous media for the electrodeposition of metals and alloys. This work investigates the initial stages of Cu electrodeposition from DES representing a mixture of choline chloride and ethylene glycol (ChCl:EG) with a (1:2) molar ratio that contains CuCl or CuCl 2 salts. These studies were performed by means of cyclic voltammetry and scanning probe microscopy on single-crystalline Pt(111) and polycrystalline Pt(poly) electrodes. Additional experiments on Cu deposition were carried out using an aqueous Cl - -rich solution (3 M NaCl + 0.01 M CuCl). The process of Cu underpotential deposition (UPD) on the surface of Pt(111) in DES is observed for the first time. It is proposed that a complete (1x1) monolayer of Cu adatoms is formed at potentials more positive than the onset of overpotential deposition (OPD). The comparison of results for Pt(111) and Pt(poly) in ChCl:EG (1:2) and in the aqueous 3 M NaCl solution confirms that the surface morphology and (co)adsorption of solution components play an important role in the Cu UPD process. In DES, as well as in the aqueous solutions containing CuCl, Cu OPD occurs via the reduction of Cu(I) ions. The Cu(I)/Cu(II) redox transition was observed at significantly more positive potentials. We conclude on the basis of voltammetry and in situ scanning tunneling microscopy (STM) data that the mechanism of Cu electrodeposition in DES is determined by the surface structure. STM data demonstrate the layer-by-layer growth of a two-dimensional Cu deposit on Pt(111), while the resulting Cu deposit on Pt(poly) is characterized by a grain-like morphology. We also demonstrate that the mechanism of Cu OPD affects the initial deposition rate under conditions of diffusion limitations.

Cathodic Zn underpotential deposition: an evitable degradation mechanism in aqueous zinc-ion batteries

Aqueous zinc-ion batteries (AZIBs) are promising for large-scale energy storage, but their development is plagued by inadequate cycle life. Here, for the first time, we reveal an unusual phenomenon of cathodic underpotential deposition (UPD) of Zn, which is highly irreversible and considered the origin of the inferior cycling stability of AZIBs. Combining experimental and theoretical simulation approaches, we propose that the UPD process agrees with a two-dimensional nucleation and growth model, following a thermodynamically feasible mechanism. Furthermore, the universality of Zn UPD is identified in systems, including VO2//Zn, TiO2//Zn, and SnO2//Zn. In practice, we propose and successfully implement removing cathodic Zn UPD and substantially mitigate the degradation of the battery by controlling the end-of-discharge voltage. This work provides new insights into AZIBs degradation and brings the cathodic UPD behavior of rechargeable batteries into the limelight.

In-Situ STM Study of Sodium Deposition Onto Au(111) in [Mppip][Tfsi]:Initial Stages, Nucleation, Cluster Growth, and Alloy Formation

The development of battery systems fabricated from abundant materials that meet the highest safety requirements is a major challenge, while the demand for efficient energy storage is continuously increasing.[1] Sodium metal batteries might be an adequate substitute for lithium-ion batteries. In comparison to lithium, sodium is more abundant, is a strong reducing agent, and has a less negative standard electrode potential.[2] Investigations of the deposition and the dissolution behavior of sodium are of utmost importance since dendritic growth is amongst the key factors limiting the lifetime and safety of sodium metal batteries.[3] In-situ scanning tunneling microscopy (STM) combined with cyclic voltammetry allows observing topographical changes in real-time and to determine potential regions of related electrochemical processes. Single-crystal surfaces are commonly used in fundamental studies of metal deposition due to their (i) well-defined structure, (ii) ease and reproducibility of preparation, and (iii) stability in the electrochemical environment.[4] However, metallic sodium is extremely reactive. In particular, Na reacts violently with water and molecules containing acidic hydrogen atoms, but even with commonly used battery electrolytes as organic carbonates.[5] Fortuitously, ionic liquids (ILs) are promising solvents in sodium metal batteries.[6] Due to their unique properties, they are appropriate for preparing electrolytes in investigations of the deposition and dissolution behaviour of metals. They are known for their relatively wide electrochemical stability windows for both sodium and lithium deposition, as well as for low diffusion rates, which facilitate real-time deposition studies.[7] In this work, the initial stages of sodium underpotential deposition (UPD) onto a Au(111) electrode surface from the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([MPPip][TFSI]) is investigated using in-situ STM and classical electrochemical measurements.Four subsequent stages in the UPD process of sodium on Au(111) can be discerned. (i) Nucleation of sodium starts around 1.1 V vs. Na/Na+ at the so-called elbows of the reconstructed Au(111) surface. (ii) Small mono-atomically high islands grow around the nuclei at a slightly more negative electrode potential of 1.0 V. (iii) The islands slowly coalesce into smooth layers after lowering the potential to 0.5 V (Figure 1), resulting in several clusters with defined steps. (iv) More islands grow on top of the existing layer and the preferred deposition mode changes from smooth layer formation to 3D-growth, resulting in cauliflower-like structures, especially at the step edges of the clusters. The deposition of several layers of sodium in the UPD regime can be explained by the alloy formation of sodium and sodiophilic gold.[8] It is worth mentioning that this deposition behaviour shows several similarities to that of lithium UPD on Au from ILs, including island formation and multiple layer growth.[7,9] So far, nucleation at the reconstruction elbows has not been reported for lithium, but it has already been observed during deposition of other metals such as nickel, cobalt, and palladium both for electrodeposition[10] and under UHV conditions.[11,12] References [1] B. L. Ellis, L. F. Nazar, Curr. Opin. Solid State Mater. Sci. 2012, 16, 168–177.[2] J. Zheng, S. Chen, W. Zhao, J. Song, M. H. Engelhard, J. Zhang, ACS Energy Lett. 2018, 3, 315–321.[3] Z. W. Seh, J. Sun, Y. Sun, Y. Cui, ACS Cent. Sci. 2015, 1, 449–455.[4] M. A. Schneeweiss, D. Kolb, Chemie unserer Zeit 2000, 34, 72–83.[5] K. Pfeifer, S. Arnold, J. Becherer, C. Das, J. Maibach, H. Ehrenberg, S. Dsoke, ChemSusChem 2019, 12, 3312–3319.[6] R. Wibowo, L. Aldous, E. I. Rogers, S. E. Ward Jones, R. G. Compton, J. Phys. Chem. C 2010, 114, 3618–3626.[7] C. A. Berger, M. U. Ceblin, T. Jacob, ChemElectroChem 2017, 4, 261–265.[8] S. Tang, Z. Qiu, X. Y. Wang, Y. Gu, X. G. Zhang, W. W. Wang, J. W. Yan, M. Sen Zheng, Q. F. Dong, B. W. Mao, Nano Energy 2018, 48, 101–106.[9] L. H. S. Gasparotto, N. Borisenko, N. Bocchi, S. Zein El Abedin, F. Endres, Phys. Chem. Chem. Phys. 2009, 11, 11140–11145.[10] M. Kleinert, H. F. Waibel, G. E. Engelmann, H. Martin, D. M. Kolb, Electrochim. Acta 2001, 46, 3129–3136.[11] D. D. Chambliss, R. J. Wilson, S. Chiang, Phys. Rev. Lett. 1991, 66, 1721–1724.[12] C. S. Casari, S. Foglio, F. Siviero, A. Li Bassi, M. Passoni, C. E. Bottani, Phys. Rev. B - Condens. Matter Mater. Phys. 2009, 79, 1–25. Figure 1

Atomic-Scale Friction Study: Underpotential Deposition (UPD) of Silver on I-Modified Au(111) in Aprotic Electrolyte

Friction in general is one of the main sources of energy loss, and therefore finding ways for better controlling friction is of uttermost importance. Atomic-scale friction is measured using electrochemical lateral force microscopy (EC-LFM) where the AFM tip slides on the surface of electrode under electrochemical control. It helps us to understand interfacial and tribological behavior on nanoscale. In ionic liquids (ILs) including solvate ILs, the friction as function of normal load has been studied on HOPG and Au(111), which shows that the structure of ions is a decisive factor for the friction behavior [1-4]. Our interest was focused on the adsorption of foreign metals, ranging from (sub)monolayer (underpotential deposition (UPD)) to multilayer (bulk) deposition and the adsorption of organic adsorbates, and its influence on friction as function of potential on single crystal electrodes in aqueous electrolyte[5-7].The underpotential deposition (UPD) of silver had often been studied on gold in aqueous electrolyte since the lattice constants of silver and gold are fairly identical (Å, Å) [8-12]. Previous results clearly show that the Ag UPD process is very sensitive to the presence of other adsorbates (e.g. SO4 2-, ClO4 -). Especially, the Ag UPD on iodine covered surface of Au(111) shows completely different atomic structures after the 1st, 2nd, and 3rd Ag UPD compared to the results on Au(111) [9, 13].In this study, we investigate the interfacial properties on I-modified Au(111) during the Ag UPD in aprotic electrolyte. We observed two sets of Ag UPD peaks in cyclic voltammetry (CV). The charge density for these peaks is about 216 µC/cm² close to the theoretical charge density for the monolayer (222 µC/cm²). We observed the iodine structure at low normal load (FN < 20nN), which shows that the iodine adlayer on Au(111) and Ag monolayer forms a (˅3×˅3)R30°. It indicates that the iodine adlayer is stable during the Ag UPD. At high normal load, the AFM tip penetrates into iodine layer and reveals the structure of Au(111) or Ag monolayer depending on the potential. It is remarkable that there is no irreversible wear during the penetration of AFM tip meaning that the iodine structure appears again with decreasing normal loads. At high normal load where the tip penetrates into iodine layer and interacts with substrates (Au and Ag), the friction on Ag monolayer is higher than on Au(111). Interestingly, even though the cyclic voltammetry (CV) shows that the amount of water in the electrolyte has minor influence, the friction increases with an increase in water content on the I/Ag/Au(111).[1] H. Li, M.W. Rutland, R. Atkin, Physical Chemistry Chemical Physics 15 (2013) 14616-14623.[2] H. Li, M.W. Rutland, M. Watanabe, R. Atkin, Faraday discussions 199 (2017) 311-322.[3] O.Y. Fajardo, F. Bresme, A.A. Kornyshev, M. Urbakh, Scientific Reports 5 (2015) 6.[4] A. Elbourne, J. Sweeney, G.B. Webber, E.J. Wanless, G.G. Warr, M.W. Rutland, R. Atkin, Chem. Commun. 49 (2013) 6797-6799.[5] S. Iqbal, S. Wezisla, N. Podgaynyy, H. Baltruschat, Electrochimica Acta 186 (2015) 427-435.[6] M. Nielinger, H. Baltruschat, Physical Chemistry Chemical Physics 9 (2007) 3965-3969.[7] N. Podgaynyy, S. Iqbal, H. Baltruschat, Surface Science 631 (2015) 67-72.[8] T. Hachiya, K. Itaya, Ultramicroscopy 42 (1992) 445-452.[9] K. Ogaki, K. Itaya, Electrochimica acta 40 (1995) 1249-1257.[10] M. Espladiu, M. Schneeweiss, D. Kolb, Physical Chemistry Chemical Physics 1 (1999) 4847-4854.[11] C.H. Chen, S.M. Vesecky, A.A. Gewirth, Journal of the American Chemical Society 114 (1992) 451-458.[12] P. Mrozek, Y.-e. Sung, M. Han, M. Gamboa-Aldeco, A. Wieckowski, C.-h. Chen, A.A. Gewirth, Electrochimica acta 40 (1995) 17-28.[13] S. Sugita, T. Abe, K. Itaya, The Journal of Physical Chemistry 97 (1993) 8780-8785.

Epitaxial Electrocrystallization of Magnesium via Synergy of Magnesiophilic Interface, Lattice Matching, and Electrostatic Confinement.

Rechargeable magnesium batteries are particularly advantageous for renewable energy storage systems. However, the inhomogeneous Mg electrodeposits greatly shorten their cycle life under practical conditions. Herein, the epitaxial electrocrystallization of Mg on a three-dimensional magnesiophilic host is implemented via the synergy of a magnesiophilic interface, lattice matching, and electrostatic confinement effects. The vertically aligned nickel hydroxide nanosheet arrays grown on carbon cloth (abbreviated as "Ni(OH)2@CC") have been delicately designed, which satisfy the essential prerequisite of a low lattice geometrical misfit with Mg (about 2.8%) to realize epitaxial electrocrystallization. Simultaneously, the ionic crystal nature of Ni(OH)2 displays a periodic and hillock-like electrostatic potential field over its exposed facets, which can precisely capture and confine the reduced Mg0 species onto the local electron-enriched sites at the atomic level. The Ni(OH)2@CC substrate undergoes sequential Mg-ion intercalation, underpotential deposition, and electrocrystallization processes, during which the uniform, lamellar Mg electrodeposits with a locked crystallographic orientation are formed. Under practical conditions (10 mA cm-2 and 10 mAh cm-2), the Ni(OH)2@CC substrate exhibits stable Mg stripping/plating cycle performances over 600 h, 2 orders of magnitude longer than those of the pristine copper foil and carbon cloth substrates.

A Finite-Element Model for Underpotential Deposition of Ce(III) on an Active Aluminum Electrode in LiCl–KCl Melts

Underpotential deposition on an active Al cathode plays an increasingly important role in pyroprocessing of used nuclear fuel, but most of the developed models are applied to simulate underpotential deposition at atomic level without considering electron transfer process which is a critical step in electrochemical reactions. In this work, a novel finite-element model for the underpotential deposition of Ce(III) in LiCl–KCl melts is developed with the consideration of Ce(III) activity in the Al electrode and the electron transfer process which is described by Butler-Volmer equation. This model was applied to investigate cyclic voltammetry(CV), square wave voltammetry(SWV) and electrodeposition behaviors of Ce(III) on an Al/Mo electrode. Additionally, the effect of temperature and electrode surface area on the electrodeposition thickness was investigated. Simulated CV and SWV curves are obtained and compared with our pervious experimental data. The results also provide the distribution diagrams of current density, electrostatic potential, Ce(III) concentration and electrodeposition thickness. Furthermore, the electrodeposition thickness is found to be linearly proportional to temperature and the inverse of cathode’s area, respectively. This work proposes a new pathway for the further study of underpotential deposition process.

Investigation of the Photoinduced Deposition Mechanism and Interfacial Properties Related to the Photoelectrochemical Performance of Pb/Cu–Au Bimetallic Nanocrystals

Modifying TiO2 with noble metal nanocrystals via photodeposition is an efficient strategy for improving its photoelectrochemical performance. The photodeposition process is still an open question and needs to be further clarified. Herein, we selectively deposited Pb or Cu on Au nanocrystals supported by the branched TiO2 nanostructure to prepare the Au@metal/semiconductor photocatalyst. The results revealed the dependence of the photodeposition mechanism on the type of electrolyte. In an acetic acid–acetate system Pb deposition on Au undergoes both underpotential and overpotential deposition processes, while Cu deposition on Au only displays overpotential deposition behavior. In a nitric acid–nitrate system, however, only underpotential deposition occurs during Pb deposition on Au. Surface microstructure analysis shows that the Cu–Au bimetallic nanocrystals form a typical core/shell structure, while Pb–Au has a Janus structure. The lower ideality factor of Cu/Au–TiO2 than that of Pb/Au–TiO2 indicates an ideal photoelectrode/electrolyte junction for photoelectrochemical reaction, further enhancing carrier transport with a short recombination lifetime of 0.899 ms. Finally, the evaluation of photoelectrochemical performance further confirmed Cu/Au–TiO2 (Au loading amount of about 0.55 mass%) as the optimal photocatalyst, with an efficiency as high as 0.29% or about 1.93 times that of Au–TiO2 (0.15%).

On the thermodynamics of hydrogen adsorption over Pt(111) in 0.05M NaOH.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads ◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

Impact of Nickel Ions on the Oxygen Reduction Reaction Kinetics of Pt and on Oxygen Diffusion through Ionomer Thin Films

The effects of dissolved nickel on the oxygen reduction reaction (ORR) kinetics and oxygen transport properties of perfluorosulfonic acid (PFSA) thin films were investigated using rotating disk electrode (RDE) measurements of ORR on a PFSA-coated platinum electrode. The electrochemical characterization in 0.1 M perchloric acid (HClO4) with and without added Ni2+ quantitatively measured the impact of ionic interactions between the Ni2+ cations and sulfonate (SO3 −) anions on oxygen transport through the PFSA thin film. Cyclic voltammetry (CV) curves in deaerated electrolyte showed that Ni2+ cations diffusing through the PFSA thin film interact with the Pt surface altering the hydrogen underpotential deposition and stripping processes and decreasing ORR kinetics. The RDE limiting current results point to reduced permeability of oxygen through PFSA-Ni2+ compared to PFSA-H+. The results indicate that transition metals leached from Pt alloy catalyst may be detrimental not only to the intrinsic ORR kinetics of the PEFC cathode catalyst through loss of the ORR-enhancing transition metal, but may also inhibit the diffusion of oxygen to the catalytic sites and poison the ORR.

A strongly interactive adatom/substrate interface for dendrite-free and high-rate Li metal anodes

Lithium (Li) metal is considered as one of the most promising anode materials to build next-generation high-energy–density batteries. Nonetheless, dendritic Li deposition has dramatically hindered the practical applications of Li metal batteries (LMBs). Uniformizing Li deposition is a prerequisite to achieve safe and practical LMBs. Herein, an underpotential deposition (UPD) process is first proposed to alter the kinetic and uniformity of Li deposition morphology. Based on the strong interaction between the Li adatoms and manganese (Mn) based substrate, a competition between the UPD and bulk Li deposition is observed, on which the predominance of the UPD scenario tends to uniformize Li nucleation and deposition by the surface coverage of Li monolayers at potentials that are more positive than the Nernst potential of Li metal. Inspired by this process, an advanced hybrid Mn-graphene oxide structure is developed for Li protection, not only enabling dendrite-free Li anodes for high-capacity and -current density cycling, but also improving the interfacial kinetic of Li metal anodes at subzero temperatures, showing potential applicability in low temperature conditions.

Electrochemically Assisted Deposition of Nanoporous Silica Membranes on Gold Electrodes: Effect of 3‐Mercaptopropyl(trimethoxysilane) “Molecular Glue” on Film Formation, Permeability and Metal Underpotential Deposition

AbstractGold electrodes have been covered with vertically aligned mesoporous silica thin films by electro‐assisted self‐assembly. To ensure uniform surface coverage and good adhesion of the membrane to the gold surface, it was necessary to use a (3‐mercaptopropyl)trimethoxysilane (MPTMS) reagent to act as a “molecular glue” thanks to its thiol functionality, which is able to bind to the gold substrate and to its alkoxysilane moieties enabling condensation with the silica material. The MPTMS layer had, however, a significant effect on film permeability to redox species [Ru(NH3)63+ and ferrocenedimethanol (Fc(MeOH)2)]. MPTMS adsorption should be long enough to ensure proper adhesion of the film but not too long so as to avoid surface blocking or film defects; an optimal time window of 60–600 s soaking in 0.1 mM MPTMS in ethanol was evidenced. In addition, Cu underpotential deposition (UPD) experiments revealed that the silica membrane significantly affected the UPD process, suggesting some barrier effect of the porous membrane, but the interface Au/MPTMS/silica is not sharp and allowed metal UPD between the gold substrate and the silica thin film.

Observing atomic layer electrodeposition on single nanocrystals surface by dark field spectroscopy

Underpotential deposition offers a predominant way to tailor the electronic structure of the catalytic surface at the atomic level, which is key to engineering materials with a high activity for (electro)catalysis. However, it remains challenging to precisely control and directly probe the underpotential deposition of a (sub)monolayer of atoms on nanoparticle surfaces. In this work, we in situ observe silver electrodeposited on gold nanocrystals surface from sub-monolayer to one monolayer by designing a highly sensitive electrochemical dark field scattering setup. The spectral variation is used to reconstruct the optical “cyclic voltammogram” of every single nanocrystal for understanding the underpotential deposition process on nanocrystals, which cannot be achieved by any other methods but are essential for creating novel nanomaterials.

Electrochemical Design of Nanoporous Gold with Tunable Porosity Using Lead Underpotential Deposition

Nanoporous gold (NPG) is a metamaterial with unique physicochemical, mechanical, and plasmonic properties of interest for various applications in catalysis and electrocatalysis, energy conversion and storage, (bio)sensing, actuation, and surface-enhanced Raman scattering.1-3 A monolithic structure with the nanoscales size of ligaments and rich surface electrochemistry3 make the NPG materials promising for many energy applications such highly active electrocatalysts for energy conversion, and three-dimensional substrates for energy storage in supercapacitors and lithium ion batteries. The length scale of NPG porosity (ligaments & open pores) and its effect on the properties is an essential aspect for such a broad range of applications. The size of porosity can range from a few nanometres to several microns and can be controlled by parent alloy composition, the method of dealloying (chemical, electrochemical) as well as the temperature of dealloying and annealing. Using the electrochemical surface processes to change and fine-tune size, uniformity and hierarchy of the ligament network has been attracting attention as a possible tool for manipulation and design of new and improved NPG functionalities 4, 5 Here we present an all-electrochemical approach to creating NPG thin films with different size of porosity from AgxAu1-x alloys. AgxAu1-x alloys of various compositions were electrodeposited at the constant potential from a thiosulfate-based solution by controlling the potential and concentration of Ag+ and Au+ ions in solution6. Following the structural (SEM, AFM) and composition (XPS) characterisation of the electrodeposited alloys, the selective dissolution of Ag under electrochemical control was used to create NPG with different size of porosity (5 - 15 nm). A process of Pb monolayer underpotential deposition (UPD) has been shown to be an excellent analytical tool to measure the surface area of as created NPG structures7. In this work, we demonstrate that this UPD process featuring surface stress driven surface allying can also be used as an electrochemical tool to shape and change NPG structure. The in-situ STM and electrochemical studies on Au(111) have shown that Pb ML formation is accompanied by significant Au surface restructuring. The top-surface atoms get displaced by alloying at low Pb ML coverage, followed by dealloying at high Pb-coverage creating a highly mobile Au ad-island and vacancy islands that with repeated cycling can lead to a significant surface morphology changes8. We observed that repeated potential cycling of Pb UPD on NPG resulted in ‘coarsening’ of the nanoporous structure with the effect similar to a so called ‘electrochemical annealing’. Combining the electrochemical and structural characterisation were followed the evolution of porosity and showed that changes of the surface area and ligament/pore size could be controlled by the range of the potentials, scan rate and a number of cycles applied.

(Invited) Dilute Cu Alloying in Au Nanostructures for Highly Selective and Long-Term Stability of CO2 Reduction

Electrochemical CO2 reduction can produce value-added chemical or fuels in moderate reaction environment. For efficient CO2 reduction to a target product with high selectivity, vast research works have been carried out over a few decades. Particularly, CO2 conversion performance using Au and Ag based catalysts for CO production has reached to economically viable level according to recent techno-economic analysis [1]. In various routes to improve catalytic property, metal alloys are shown effective by tailoring intermediate binding energy of CO2 reduction reaction (CO2RR). For example, bimetallic AuCu alloys, such as AuCu, AuCu3 and Au3Cu, have shown synergetic effects which lead to higher catalytic activities (i.e. overpotential, stability) although CO selectivity is still lower than state-of-art records (over 95%) [2-4]. Herein, we demonstrate the dilute Cu alloying in Au nanostructure for highly selective CO production with long-term stability. To synthesize nanostructured Au with controlled Cu alloying, we firstly formed Cu/Au bilayers by employing the electrochemical deposition (ED) and underpotential deposition (UPD) of Cu on 200-nm-thick Au thin films on Si substrates. ED and UPD processes were conducted at applied potentials of –0.1 and 0.3 V (vs. Ag/AgCl), respectively, in 50 mM H2SO4 electrolyte with 50 mM CuSO4. We carried out electrodepositon for 10s and 60s while underpotentially deposited Cu sample were only prepared for 60s. The Cu-covered Au surfaces were then electrochemically treated for nanostructuring with the same method previously reported by our research group [5]. Specifically, we electrochemically oxidized the Cu/Au bilayers at 2.5 V (vs. RHE) for 40 minutes in 0.2 M KHCO3 solution (pH = 8.5). Then electro-reduction was sequentially followed under constant current density of –0.5 mA cm–2 for about 10 minutes in the same solution. We denote the prepared samples by Nano-ED-CuAu-10s, -60s and Nano-UPD-CuAu in accordance with the type of Cu deposition processes. Additionally, nanostructured Au without Cu was prepared for the comparison. As a result of the electrochemical treatment, nanoporous structures were formed for all samples. The heights of the nanostructures were 200, 215, 225 and 240 nm for Nano-Au, Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s. XPS investigations clearly show the appearance of Cu 2p peak and slight shift of Au 4f peak position to the higher binding energy (~0.1 eV) when Cu was incorporated in Au nanostructures. In addition, the surface concentration of Cu were found to be 2, 11 and 13% for Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s, respectively. Our nanostructured CuAu and Au exhibit low CO2RR overpotential and high selectivity for CO in CO2 saturated 0.2 M KHCO3. For instance, the selectivity of CO2RR for all electrodes is around 50 – 70% (Nano-Au : 47%, Nano-UPD-CuAu : 65% Nano-ED-CuAu-10s : 69%, Nano-ED-CuAu-60s : 57%) at low overpotential (–0.34 V (vs. RHE)). It is noted that bare Au thin film shows almost no CO production. At higher applied potential (–0.59 V), the CO selectivity increases over 90% and notably, Nano-UPD-CuAu shows almost unity CO production (~99.5%) with nearly completely suppressing H2 evolution. In addition, Cu-Au nanostructures show the remarkably improved durability as compared to nanostructured Au catalysts. Whereas the CO selectivity at –0.49 V for Nano-Au drops from 92% at 30 minutes to below 80% in about 4.5 hours, Nano-UPD-CuAu retains the same performance for about 7 hours. Surprisingly, Nano-ED-CuAu samples are stable for 12 hours showing the selective CO production over 80%. These results indicate that dilute Cu-Au alloy nanostructures can be effective to improve stability with highly selective CO production. Additionally, in the presentation, we will introduce an in-situ tailoring technique to re-active the degraded Au catalysts by utilizing Cu impurities in electrolyte during CO2 electroreduction process for robust and efficient CO2 reduction. [1] Jouny et al., Ind. Eng. Chem. Res. 57, 2165 (2018) [2] Kim et al., Nat. Commun. 5, 4948 (2014) [3] Kim et al., Appl. Catal. B, 213, 211 (2017) [4] Kim et al., ACS Energy Lett., 3, 2144 (2018) [5] Kim et al., J. Mater. Chem. A., 6, 5119 (2018)

Initial stages of silver electrodeposition on single crystal electrodes from ionic liquids

We present a comprehensive study on the initial stages of silver electrodeposition on Pt (111), Au (111), and Au (100) single crystal surfaces in low-viscosity ionic liquids (ILs) containing dicyanamide anions: 1-butyl-1-methylpyrrolidinium dicyanamide [BMP][DCA] and 1-butyl-3-methylimidazolium dicyanamide [BMIm][DCA]. Electrochemical methods in combination with in situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) are employed to explore the Ag underpotential (upd) and overpotential (opd) deposition processes as well as the stability of the single crystal electrode surfaces in the absence of Ag+ ions. The substrate material is shown to significantly affect the mechanism of Ag deposit nucleation and growth in the ILs. While no Ag upd is detected on Pt (111), the formation of a Ag upd monolayer on a Au (111) electrode in both ILs is clearly visualized by in situ STM. The Ag adlayer formation on the Au electrodes in the underpotential regime facilitates Ag opd, which starts on Au (111) and Au (100) at much less negative potentials than on Pt (111). There is an excellent agreement between the electrochemical (voltammetry and chronoamperometry), AFM and STM data, demonstrating the nucleation and growth of individual Ag crystallites on Pt (111) according to the Volmer–Weber mechanism and the layer–by–layer growth of Ag deposit on Au (111) and Au (100). Only at high overpotentials, the Ag growth on the gold electrodes switches to the Stranski-Krastanov mode involving the appearance of 3D crystallites.

Scale‐Up Process of Core@Shell Monolayer Catalyst without Active Potential Control through Electroless Underpotential Deposition Galvanic Replacement

AbstractCore@shell monolayer catalysts have dual advantages in electrocatalysis, cost of the catalyst is minimized by depositing expensive metal as a monolayer shell and the catalytic activity can be enhanced by selecting an appropriate core metal. In this communication, a novel chemically assisted underpotential deposition process to fabricate core@shell monolayer nanoparticles in bulk scale has been introduced. Ag(shell)@Pd(core)/C nanoparticles have been prepared by this electroless underpotential deposition method. The thickness of the Ag shell has been calculated as one atomic layer from XPS analysis. Core‐shell structure of nanoparticles has been confirmed by STEM characterization. Further, Pt(shell)@Pd(core)/C nanoparticles have been engineered from Ag@Pd/C nanoparticles by Galvanic replacement of Ag by Pt. This Pt@Pd/C electrocatalyst has been tested for formic acid oxidation.

A phenomenological theory of electrosorption

A phenomenological theory of electrosorption is presented based upon notions of local charge and local electronic polarisability (Friedel oscillations). The strong link between the extent and nature of ionic and molecular adsorption at an electrode surface and the uv-absorption properties displayed by such molecular entities in an aqueous solution is emphasised. The theory may be used to predict, in certain cases the extent of ionic adsorption at the potential of zero charge (PZC) and, also, to rationalise recent results pertaining to metal underpotential deposition (UPD) processes on single crystal electrodes.