Underpotential Deposition Of Lead Research Articles

Preparation of Tunable Cu−Ag Nanostructures by Electrodeposition in a Deep Eutectic Solvent

AbstractThe green transition requires new, clean, inexpensive, and sustainable strategies to prepare controllable bimetallic and multimetallic nanostructures. Cu−Ag nanostructures, for example, are promising bimetallic catalysts for different electrocatalytic reactions such as carbon monoxide and carbon dioxide reduction. In this work, we present the one‐step preparation method of electrodeposited Cu−Ag with tunable composition and morphology from choline chloride plus urea deep eutectic solvent (DES), a non‐toxic and green DES. We have assessed how different electrodeposition parameters affect the morphology and composition of our nanostructures. We combine electrochemical methods with ex‐situ scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS) to characterize the nanostructures. We have estimated the electrochemically active surface area (ECSA) and roughness factor (R) by lead underpotential deposition (UPD). The copper/silver ratio in the electrodeposited nanostructures is highly sensitive to the applied potential, bath composition, and loading. We observed that silver‐rich nanostructures were less adherent whereas the increase in copper content led to more stable and homogenous films with disperse rounded nanostructures with tiny spikes. These spikes were more stable when the deposition rate was fast enough and the molar ratio of Cu and Ag was no greater than approximately two to one.

Platinum-Grafted Twenty-Five Atom Gold Nanoclusters for Robust Hydrogen Evolution.

A robust hydrogen evolution is demonstrated from Au25 (PET)18 ]- nanoclusters (PET = 2-phenylethanethiol) grafted with minimal platinum atoms. The fabrication involves an electrochemical activation of nanoclusters by partial removal of thiols, without affecting the metallic core, which exposes Au-sites adsorbed with hydrogen and enables an electroless grafting of platinum. The exposed Au-sites feature the (111)-facet of the fcc-Au25 nanoclusters as assessed through lead underpotential deposition. The electrochemically activated nanoclusters (without Pt loading) show better electrocatalytic reactivity toward hydrogen evolution reaction than the pristine nanoclusters in an acidic medium. The platinum-grafted nanocluster outperformed with a lower overpotential of 0.117V vs RHE (RHE = Reversible Hydrogen Electrode) compared to electrochemically activated nanoclusters (0.353V vs RHE ) at 10mAcm-2 and is comparable with commercial Pt/C. The electrochemically activated nanoclusters show better reactivity at higher current density owing to the ease of hydrogen release from the active sites. The modified nanoclusters show unique supramolecular self-assembly characteristics as observed in electron microscopy and tomography due to the possible metallophilic interactions. These results suggest that the post-surface modification of nanoclusters will be an ideal tool to address the sustainable production of green hydrogen.

Time and Potential‐Resolved Comparison of Copper Disc and Copper Nanoparticles for Electrocatalytic Hydrogenation of Furfural

Herein, a comparative analysis of two case example catalysts for electrocatalytic hydrogenation (ECH) of furfural under acidic conditions, namely a copper polycrystalline disc and copper nanoparticles dispersed on carbon support, is performed. To gain a detailed insight on ECH trends, a task‐specific methodology is employed based on electrochemistry–mass spectrometry coupling, which enabled time‐ and potential‐resolved detection of volatile ECH products, i.e., 2‐methylfurane (2‐MF) and H2. In this way, the ability to elucidate potential‐dependent product distribution for the two catalysts, namely faradaic efficiency, is achieved. Accordingly, the nanoparticulate analog is significantly more active toward competitive hydrogen evolution reaction and 2‐MF production, whereas the polycrystalline sample is more selective toward furfuryl alcohol. The observed differences in ECH are ascribed to alterations in surface domains, which is supported by surface‐sensitive lead underpotential deposition characterization.

Influence of the synthesis route on the electrocatalytic performance for ORR of citrate-stabilized gold nanoparticles

Citrate-stabilized gold nanoparticles (AuNPs) were synthesized by two different citrate-based reduction methods, namely a one-pot wet chemical synthesis (Turkevich method) and seed-mediated growth (Bastús method). Although both types of AuNPs produce similar surface plasmon resonance bands and have a similar particle size and shape, the NPs obtained by the Turkevich method show significantly higher crystallinity. Static and dynamic electrochemical analysis using both types of AuNPs for the oxygen reduction reaction (ORR), under alkaline conditions, confirmed the significant influence of the synthesis route on the resulting onset potentials and maximum intensities in ORR electrocatalysis. These results were attributed to the higher percentage of Au(100) facets and the greater number of active sites on citrate-stabilized AuNPs obtained by the Turkevich method. Underpotential deposition of lead and electrochemical active surface area (ECSA) analysis further confirmed the importance of the crystalline facets in the performance of the AuNPs as electrocatalysts. This study demonstrates the influence of the synthesis route on the electrocatalytic activity of AuNPs, despite using the same starting reagents.

The Underpotential Deposition of Lead and its Effect on Carbon Monoxide Adsorption on Pt(111) Electrode

In situ scanning tunneling microscopy (STM) was used to examine the spatial structures of lead (Pb) adatom and carbon monoxide (CO) molecule adsorbed onto an ordered Pt(111) electrode in 0.1 M perchloric acid (HClO4) containing 1 mM Pb(ClO4)2 under potential control. Ordered Pb structures were not seen until the potential was made more negative to 0 V (vs Ag/AgCl), where a quasi-square array (√7 × √57) was seen. Shifting the potential more negative substantiated Pb deposition, which triggered restructuring of the Pb adlayer into (√21 × √21)R11° and (3 × 2√3)rect (θ = 0.62 and 0.67) at −0.1 V. Pb adatom reduced the poisoning effect of CO on the Pt(111) electrode by blocking the adsorption of CO and by forcing CO molecule to asymmetric sites. An ordered (√7 × 3)—Pb + CO structure was imaged at 0 V in 0.1 M HClO4 + 1 mM Pb(ClO4)2, from which the surface coverage and binding sites of CO molecule were inferred.

Electrochemical Surface Area Quantification, CO2 Reduction Performance and Stability Studies of Au and Au-Cu Aerogels

The efficient scale-up of CO2-reduction technologies is a pivotal step to facilitate intermittent energy storage and for closing the carbon cycle, and thus there is a strong need for catalysts with minimal side reactions and high specific surface areas. While the latter requirement is usually fulfilled by nanostructured catalysts, it is often combined with the use of carbon supports that improve the nanoparticles’ dispersion and concomitantly increases the catalysts’ electrochemical surface area (ECSA), but can also shift the product selectivity towards undesirable H2 due to the intrinsic H2-evolution activity of C-surfaces.1 This problem could be circumvented by employing unsupported aerogel catalysts, which are tridimensionally interconnected nanodomain networks (e.g., nano-wires or -particles) of highly porous materials with large surface areas.2 With this hypothesis, and motivated by the high activity and selectivity of Au for converting CO2 to CO3 and the high economic value of CO as a reduction product,4 in the first part of the talk we present our work employing Au aerogels with ≈ 5 nm web thickness for CO2 electroreduction. Additionally, polycrystalline Au and a commercial catalyst consisting of Au nanoparticles supported on a carbon black (20 % Au/C) were used as benchmarks for comparison of our results. The ECSAs of these materials were first quantified using the surface oxide reduction method, followed by more accurate copper underpotential deposition (Cu-UPD) measurements in a rotating disc electrode (RDE) configuration. Further, their CO2 reduction performance was assessed using an in-house developed, two compartment electrochemical cell coupled to an online gas chromatograph for detection of gaseous products and subsequently subjected to ion chromatography (IC) analysis for liquid product quantification.5 Finally, to test the structural stability of all Au nanocatalysts during electrochemical conditioning and CO2 electroreduction, identical location transmission electron microscopy (IL-TEM) was employed before and after these electrochemical processes, which were conducted directly on finder TEM grids.6 We show that Au aerogels exhibit a CO faradaic efficiency (FECO) of ≈ 97 % at ≈ − 0.48 VRHE in 0.5 M KHCO3, with suppressed H2 production as compared to Au/C across the entire potential range (refer Fig. 1a). An in-depth analysis of the partial H2 current densities of Au aerogel, Au/C and the carbon material alone proved the aforementioned H2 suppression to be an effect of Au nanoparticle size and shape, and not of the absence of carbon support in the aerogels.Complementarily, and inspired by the prospects to alter the selectivity trends as a result of surface d-band tuning upon alloying,7 and by reports of enhanced CO2-to-CO conversions observed for Au-Cu materials,8 the second part of the talk features our CO2 reduction results using Au3Cu and AuCu alloy aerogels. Firstly, the quantification of the electrochemical surface area was carried out by lead underpotential deposition (Pb-UPD) on Au-Cu aerogels with 4-6 nm web thickness. To establish a benchmark for the ECSA results, polycrystalline Au, Au aerogel, polycrystalline Cu and Cu aerogel were also subjected to Pb-UPD measurements. Similar to the Au aerogel, the alloys were next assessed for their CO2 reduction performance at various potentials and were compared with the baseline established by Au- and Cu-aerogels. Following this, IL-TEM and STEM/EDX were used to determine if the alloy aerogels’ nanostructure and local composition undergo drastic changes during the process of CO2 electroreduction. To investigate the effect of dissolution of surface Cu during conditioning on the CO2RR selectivity, additional CO2RR measurements were conducted without the conditioning step and with conditioning and CO2-reduction tests being performed in separate cells. Au3Cu and AuCu aerogels show a similar qualitative trend towards CO selectivity, with a peak FECO of ≈ 96% and ≈ 92% at ≈ − 0.48 VRHE (refer Fig. 1a) in 0.5 M KHCO3, respectively. However, AuCu aerogels exhibit higher CO partial current densities than Au3Cu and Au aerogels when normalized with respect to the Au loading of the respective catalysts.In summary, this contribution will present a detailed electrochemical study of the applicability of aerogel catalysts for CO2 reduction, including a comparison with well-documented polycrystalline and commercial catalysts and outlining the effects of alloying on the activity and stability of these materials. Figure 1

Adsorption-Mediated Electrochemical Atomic Layer Deposition of Gold

Electrochemical atomic layer deposition (e-ALD) is an effective method for achieving atomic monolayer-by-monolayer deposition of metals. In this presentation, we will discuss the mechanistic events underlying gold (Au) e-ALD. The e-ALD process involves lead underpotential deposition (PbUPD) followed by spontaneous redox replacement (SLRR) of PbUPD by Au. During Au e-ALD, a unique three-stage mass transient is observed. Using voltammetry, electrochemical quartz crystal microgravimetry (e-QCM), and chronoamperometry, we present an in-depth study of the characteristics of this mass transient during Au e-ALD. Adsorption of Au+3-ligand complex(es) (Au-L) on the Au surface is shown to play a key role in the Au-ALD process. The adsorbed Au-L species are reduced in stage I while a PbUPD adlayer is formed. The SLRR of PbUPD adlayer by nobler Au occurs in stage II, followed by the re-adsorption of the Au-L species in stage III. A quantitative analysis of the deposit mass under different conditions allows estimation of the average molecular weight of the adsorbed Au-L complexes. Ramifications of such adsorption-mediated deposition to thickness uniformity, conformality and film roughness will be discussed. Figure 1

Electrochemical Surface Area Quantification, CO2 Reduction Performance and Stability Studies of Au and Au-Cu Aerogels

To boost the valorization of electrochemical CO2 reduction technologies, there is a strong need for active catalysts with minimal side reactions and high specific surface areas. While the latter requirement is usually fulfilled by nanostructured catalysts, it is often realized by using porous carbon supports which improve the dispersion of nanoparticles at the cost of shifting the product selectivity towards undesirable H2.1 This problem could be circumvented by employing unsupported aerogel catalysts, which are tridimensionally interconnected nanodomain networks (e.g., nano-wires or -particles) of highly porous materials with large surface areas.2 With this consideration, and motivated by the high activity and selectivity of Au for converting CO2 to CO3 and the high economic value of CO as a reduction product,4 in the first part of the talk we will present our efforts on employing Au aerogels for CO2 electroreduction. Specifically, Au aerogels with a web thickness of ≈ 5 nm were electrochemically investigated, along with polycrystalline Au and a commercial catalyst consisting of Au nanoparticles supported on a carbon black (20 % Au/C) that provided a benchmark for comparison of our results. The electrochemical surface areas (ECSAs) of these materials were first quantified using the surface oxide reduction method, followed by more accurate copper underpotential deposition (Cu-UPD) measurements in aqueous H2SO4 electrolyte in a rotating disc electrode (RDE) configuration. Further, their CO2 reduction performance was assessed using an in-house developed, two compartment electrochemical cell, coupled to an online gas chromatograph for detection of gaseous products and subsequently subjected to ion chromatography (IC) analysis for liquid product quantification. Finally, to test the structural stability of all Au nanocatalysts during electrochemical conditioning and CO2 electroreduction, identical location transmission electron microscopy (IL-TEM) was employed before and after the electrochemical processes, which were conducted directly on finder TEM grids.5 We show that Au aerogels exhibit a CO faradaic efficiency (FECO) of ≈ 97% at ≈ -0.55 VRHE in 0.1 M NaHCO3, with suppressed H2 production as compared to 20% Au/C across entire potential range.Complementarily, and inspired by the prospects to alter the selectivity trends as a result of surface d-band tuning upon alloying,6 and by reports of enhanced CO2-to-CO conversion observed for Au-Cu materials,7 the second part of the talk focusses on our on-going research on CO2 reduction using Au3Cu and AuCu alloy aerogels. Firstly, the quantification of the electrochemically available surface area was carried out by lead underpotential deposition (Pb-UPD) on Au-Cu aerogels with 4-6 nm web thickness. To establish a benchmark for the ECSA results, polycrystalline Au, Au aerogel and polycrystalline Cu were also subjected to Pb-UPD measurements. Similar to the Au aerogel, the alloys were next assessed for their CO2 reduction performance at various potentials and were compared with the baseline established by Au aerogels. Following this, IL-TEM was used to determine if the alloy aerogel nanostructure undergoes drastic changes during the process of CO2 electroreduction. Au3Cu aerogels show a shift in CO selectivity to lower CO2 reduction overpotentials, with a peak FECO of ≈ 97% at ≈ -0.4 VRHE in 0.5 M KHCO3.In summary, this contribution will present a detailed electrochemical study of the applicability of aerogel catalysts for CO2 reduction, including a comparison with well-documented polycrystalline and commercial catalysts and outlining the effects of alloying on the activity and stability of these materials. References O.A. Baturina et al., ACS Catalysis 2014, 4, 3682.B. Cai et al., Adv. Energy Mater. 2013, 3, 839.Y. Hori et al., Electrochim. Acta 1994, 39, 1833.J. Durst et al., CHIMIA 2015, 69, 769.K. Schlögl et al., J. Electroanal. Chem. 2011, 662, 355.K. Liu et al., ACS applied materials & interfaces, 2019, 11, 16546-16555.D. Kim et al., Nat. comm. 2014, 5, 1-8.

Preparation of high surface area Cu‐Au bimetallic nanostructured materials by co‐electrodeposition in a deep eutectic solvent

Our sustainable future requires finding new, affordable and green routes to prepare nanostructured materials used in renewable energy conversion. In this work we present an electrodeposition method in a deep eutectic solvent (DES) to prepare bimetallic high surface area nanostructures of Cu and Au with tunable structure and composition. The metal electrodeposition performed in choline chloride within a urea deep eutectic solvent allows us to tailor the size, morphology and elemental composition of the deposits. We combine electrochemical methods with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) to characterize the electrodeposited nanostructured materials. We assess the increase of the electroactive surface area through the analysis of the lead underpotential deposition (UPD) on the prepared films. Integrated Pb UPD charge values of ca. 1600–4000 μC/cm2 for the prepared Cu-Au films have been calculated, suggesting a 5–14 fold increase of the active surface area compared to flat surfaces of polycrystalline Cu or Au. Our work reports a versatile and environmentally friendly route for the electrodeposition of Cu-Au bimetallic nanostructures in a DES. The combination of a tailored morphology and composition with the high active surface area of the nanostructured materials show that electrodeposition in DES is promising for the development of multimetallic electrocatalysts.

Surface characterization of copper electrocatalysts by lead underpotential deposition

Cu electrodes are model catalysts for a wide variety of interesting energy conversion reactions. Structure sensitivity typically plays an important role in the mechanism of different electrocatalytic reactions. However, quantifying the presence of different domains on Cu electrocatalysts remains a challenge. Herein, we evaluate the lead (Pb) underpotential deposition (UPD) on well-defined Cu electrode surfaces in presence of chloride, as a tool to analyse the surface state and determine the presence and distribution of domains on Cu catalysts. A polycrystalline Cu electrode was specifically modified by applying different electrochemical pre-treatments. Then, its surface ordering was tested by Pb UPD. Results were also compared with well-defined Cu single crystalline electrodes, to estimate the ratio between 〈1 0 0〉, 〈1 1 1〉 facets and defects on the pre-treated Cu electrode. The voltammetric profile of Pb UPD is strongly sensitive to the changes in the structure of the Cu electrode, allowing to monitor the appearance of new defects and domains. We highlight that Pb UPD can be used to provide the electrochemical fingerprint of Cu catalysts and decouple the contribution of different facets and defects in the surface structure.

Electrochemical assessment of high active area of cobalt deposited in deep eutectic solvent

• Green electrodeposition of nanostructured Co films using deep eutectic solvent (DES). • Influence of substrate and potential conditions on the morphology and composition of Co films. • Use of outer sphere electron transfer redox pairs to assess the increase of the active area of Co films. • The validity of 1,4-benzoquinone and hexaammineruthenium(III) redox pairs probes is tested. • Lead underpotential deposition on Co films covered by gold to quantify the surface active area. Preparation of nanostructured materials with highly efficient active area is crucial for advancement in fields of electrocatalytic conversion of energy, energy storage, or biomedicine. In this regard, there are two main challenges in the electrochemical preparation of high-extended surface area of the nanostructures: 1) establishing the optimal potential conditions to enhance the active area while reducing the size of the prepared material and, 2) simultaneously, finding a practice and simple in situ methodology to qualitatively test the increase in the active surface area. Herein, we investigate the cobalt electrodeposition in choline chloride plus urea deep eutectic solvent (DES) and the effect of the DES in the increase of the active surface area of deposited Co films. In situ classical electrochemical techniques, in combination with ex situ characterization techniques, were employed to establish the best potential conditions to obtain metallic Co films with high active surface area. Then, the increase in active area was tested by using different voltammetric methodologies. The increase in intensity of the outer sphere electron transfer was tested as a tool to analyze the superficial change after cobalt deposition. Voltammetric response in organic medium using benzoquinone on Co films and in an aqueous medium using Ru(NH 3 ) 6 , after covering the Co films with gold via galvanic displacement, was assessed. Later, as alternative method, the active area of the freshly deposited Co films, after superficial modification with Au, was tested by lead underpotential deposition. Lead underpotential deposition on gold is a surface-structure process that indirectly allows the estimation of the increment in active area of prepared Co films.

(Invited) Gold Nanoparticles for Key Reactions in Electrochemical Energy Conversion

Glucose oxidation (GOR) and oxygen reduction (ORR) are two intensively studied reactions for their concern in the development of sustainable energy sources. The oxidation of carbon monoxide (CO), considered as a poisoning reactant in organics conversion, is also a key reaction to be understood in electrocatalysis. Efficient catalysts should be designed for this purpose. Moreover, due to the surface reactions involved, the control of the catalysts' shape, size and structure is required to understand their activity and improve their stability. The outstanding properties exhibited by gold at nanoscale lead to various applications, including electrocatalysis which depends on the nanoparticles (NPs) surface structure and morphology [1]. Gold nanospheres (AuNSs) and pentatwinned gold nanorods (AuPTW NRs) were prepared by a revisited Turkevich synthesis and seed mediated methods respectively. Their surfaces were probed by the lead underpotential deposition (updPb) and the reduction of 4-mercaptobenzoic acid to reveal, respectively, the low and high index facets. The (100) facets of AuNSs are considered to be the active sites for glucose and CO oxidation in alkaline medium, while a AuPTW NRs@Pd core-shell material markedly promotes the ORR activity. Figure 1

Electrochemical modification and tuning Ni/Ni(OH)2–Ag heterogeneous interface for efficient electrocatalytic hydrogen and oxygen evolution reactions

A facile and effective method, electrochemical modification and tuning was adopted to prepare the Ni/Ni(OH)2–Ag catalysts with plentiful heterogeneous interfaces. These catalysts performed a synergistic catalytic effect. The catalytic activities towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were enhanced by appropriate modification of Ni/Ni(OH)2 at Ag electrode surface. Interestingly, the HER activity was further significantly enhanced after electrochemical tuning compared with that of before. Particularly for Ni(10 min)/Ag electrode surface, the overpotential of HER at 10 mA cm−2 (E10mAcm−2) was drastically decreased from −0.372 V to −0.148 V, which is merely 13 mV difference to the value obtained on Pt electrode surface (−0.135 V). Furthermore, the optimized catalyst also exhibited superior catalytic performance towards OER, thus strongly suggesting that it possesses excellent HER and OER bifunctional electrocatalytic activity and stability in alkaline solution. All the results from X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectroscopy (EDS) mappings and lead-underpotential deposition (Pb-UPD) indicated that more Ag species appeared on the Ni/Ni(OH)2 modified Ag electrode surface and combined with Ni species to form a huge amount of the Ni/Ni(OH)2–Ag heterogeneous interfaces after electrochemical tuning process. It could be the crucial reason for the increase of the number of heterogeneous interfaces.

Codeposition of Pd and Pb and electrocatalytic properties of their composite

A mixed electrolytic deposit (e.d.) PdPb with 33 ± 4 at.% Pb is obtained on the gold electrode from 5 mM PdCl2 + 0.7 mM Pb(ClO4)2 + 0.1 M HClO4 under potentiostatic conditions in the region of lead underpotential deposition (+100 mV vs. RHE). The deposit is characterized by SEM, EDX, XPS, CVA and ICP- AES methods before and after polarization measurements. The problem of thermodynamic instability of the deposit is discussed. It is shown that cycling of the potential of e.d. PdPb in the interval of 0.1–1.25 V results in preferential dissolution of Pb; however, no dense Pd shell is formed. The codeposition of Pb with Pd increases the degree of dispersion and the defectiveness of the deposit. The possible reasons for the lower specific activity of e.d. PdPb in formic acid oxidation reaction (FAOR) as compared with e.d. Pd are considered. The high solubility of Pd from e.d. PdPb as compared with e.d. Pd is observed. The latter effect cannot be explained by only the increased surface area of the Pd component; it is assumed that the defectiveness of the deposit structure is also responsible for this effect.

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.

Communication—Electrochemical Sensor Concept for the Detection of Lead Contamination in Water Utilizing Lead Underpotential Deposition

A novel electrochemical sensor concept, which enables detection of ppb-levels of Pb2+ in water, is outlined. The sensor works on the principle of underpotential deposition of Pb onto a Cu electrode followed by measurement of the hydrogen evolution reaction (HER) current on the Pbupd-modified electrode surface. The degree of suppression of the HER current is correlated to the Pbupd coverage, which in turn depends on the Pb2+ concentration in solution. Feasibility of achieving a detection limit of 10 ppb Pb2+ in aerated DI water and tap water is demonstrated and the sensing response time is analyzed using diffusion-reaction calculations.

Underpotential deposition of lead on quasi-spherical and faceted gold nanoparticles

The underpotential deposition of lead was studied on spherical and faceted gold nanoparticles. On 21.7nm spherical nanoparticles, a single stripping peak from Au(111) face was observed at −0.2V, whereas on 82nm faceted gold nanoparticles the peak split in to two at −0.2V and −0.18V vs Ag/AgCl (3M KCl). The roughening of the Au(111) terraces by electrochemical treatment leads to the suppression of the spike. It appears that the extent of surface crystallinity of nanoparticles causes the observed differences in the upd-stripping behaviour. The splitting of the Pb-upd stripping peak from Au(111) face can be used as an indication for the surface crystallinity of gold nanoparticles.

A Carbon-Free Ag–Co3O4 Composite as a Bifunctional Catalyst for Oxygen Reduction and Evolution: Spectroscopic, Microscopic and Electrochemical Characterization

A key challenge for rechargeable metal–air batteries is the development of a cost-effective bifunctional catalyst for both oxygen evolution (OER) and reduction (ORR) reactions. Here, we took the advantages of high OER activity of Co3O4 spinel and high ORR activity of Ag to develop a carbon-free oxygen electrode, e.g., for Li–air batteries. The optimized Ag + Co3O4 catalyst was further characterized and exhibited a good bifunctional activity in alkaline media. From rotating ring-disk electrode results, the mixed Ag + Co3O4 catalyst revealed significantly lower (∼320 mV) overpotential for ORR than single Co3O4, and a slightly lower overpotential than pure Ag. A four-electron pathway was also elucidated. The OER activity of the mixed catalyst is 1.5-fold compared to pure Co3O4, although the Co3O4 loading is only 10%, suggesting a large synergistic effect. The potential difference between OER and ORR (i.e., the sum of the overpotentials at 1 mA cm−2) is ca. 0.85 V, which is comparable to noble metal based catalysts. To better understand the origin of this synergism, an XPS analysis was performed, demonstrating that only after oxidation of the mixed catalyst, Co3O4 was reduced to Co(OH)2 at potentials of the ORR, probably due to the presence of Ag+. This redox switching, which was not observed for pure Co3O4, is a probable explanation for the increased catalytic activity. The morphology and the electrochemically active surface area of Ag on the surface were examined by electron microscopy and lead-underpotential deposition, respectively. These results also show that when 88% of the Ag surface is blocked by Co3O4 particles, the residual 12% free Ag sites altogether have a higher activity for ORR than the (100%) pure Ag surface, i.e., the activity per Ag site is increased by more than a factor of 10. The combination of low cost and high performance endows this catalyst as a promising candidate for energy devices, and the present synergistic effect opens a new track for high activity.

Lead underpotential deposition for the surface characterization of silver ad-atom modified gold electrocatalysts for glucose oxidation

The surface composition of Ag–Au electrocatalysts plays an important role in enhancing their activity for glucose oxidation. In this work, the surface of a polycrystalline Au electrode was modified through underpotential deposition of Ag (Agupd) and the surface composition of the resulting Agupd-modified Au catalysts was characterized using an ex situ electrochemical technique involving underpotentially deposited lead (Pbupd). Recognizing that Ag and Au surfaces exhibit distinct Pbupd charge densities, the Pbupd charge density associated with a Ag ad-atom modified Au electrode surface is a representation of its surface composition. This enables quantification of the Ag and Au fractional surface coverages, which are in good agreement with those predicted based on the Agupd charge density passed during Agupd–Au surface preparation. The dependence of the glucose oxidation kinetics on the surface composition of the Agupd–Au catalyst was investigated using voltammetry. Optimal catalytic activity was provided by catalysts with 56% Agupd coverage. Our investigation, while providing a reliable analytical technique (using Pbupd) for compositional characterization of heterogeneous catalyst surfaces, supports the surface composition dependence of the activity of Agupd–Au catalysts toward glucose oxidation.

Size-Dependent Electrocatalytic Activity of Free Gold Nanoparticles for the Glucose Oxidation Reaction.

Understanding the fundamental relationship between the size and the structure of electrode materials is essential to design catalysts and enhance their activity. Therefore, spherical gold nanoparticles (GNSs) with a mean diameter from 4 to 15 nm were synthesized. UV/Vis spectroscopy, transmission electron microscopy, and under-potential deposition of lead (UPDPb ) were used to determine the morphology, size, and surface crystallographic structure of the GNSs. The UPDPb revealed that their crystallographic facets are affected by their size and the growth process. The catalytic properties of these GNSs toward glucose electrooxidation were studied by cyclic voltammetry, taking into account the scan rate and temperature effects. The results clearly show the size-dependent electrocatalytic activity for glucose oxidation reactions that are controlled by diffusion. Small GNSs with an average size of 4.2 nm exhibited high catalytic activity. This drastic increase in activity results from the high specific area and reactivity of the surface electrons induced by their small size. The reaction mechanism was investigated by in situ Fourier transform infrared reflectance spectroscopy. Gluconolactone and gluconate were identified as the intermediate and the final reaction product, respectively, of the glucose electrooxidation.