Underpotential deposition and electrocatalysis

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.

Underpotential deposition (UPD) is important to modify the surface properties of nanocrystals. Here, the authors show the application of in situ electrochemical dark field spectroscopy in identifying the UPD processes of silver on different facets of gold nanocrystals at the single nanoparticle level.

Synthesis and characterization of size controlled alloy nanoparticles

Bimetallic and multimetallic alloy nanoparticles are emerging as a class of critical nanomaterials in electronic, optical and magnetic fields due to their unique physic-chemical properties. In particular, precise control of the nanoparticle size can endow them with broad versatility and high selectivity. This chapter reviews some tremendous achievements in the development of size controlled bimetallic and multimetallic alloy nanoparticles, with special emphasis on general preparation methods, characterization methodologies and instrumentation techniques. Some key factors and future perspectives on the development of size-controlled bimetallic and multimetallic alloy nanoparticles are also discussed.

Antimony deposition onto Au(111) and insertion of Mg

Magnesium-based secondary batteries have been regarded as a viable alternative to the immensely popular Li-ion systems owing to their high volumetric capacity. One of the largest challenges is the selection of Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg²⁺ compared with Li⁺. In this work, we prepared very thin films of Sb by electrodeposition on a Au(111) substrate. Monolayer and multilayer deposition (up to 20 monolayers) were characterized by cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Monolayer deposition results in a characteristic row structure; the monolayer is commensurate in one dimension, but not in the other. The row structure is to some extent maintained after deposition of further layers. After dissolution of the Sb multilayers the substrate is roughened on the atomic scale due to alloy formation, as demonstrated by CV and STM. Further multilayer deposition correspondingly leads to a rough deposit with protrusions of up to 3 nm. The cyclic voltammogram for Mg insertion/de-insertion from MgCl₂/AlCl₃/tetraglyme (MACC/TG) electrolyte into/from a Sb-modified electrode shows a positive shift (400 mV) of the onset potential of Mg deposition compared to that of a bare Au electrode. From the charge of the Mg deposition, we find that the ratio of Mg to Sb is 1:1, which is somewhat less than expected for the Mg₃Sb₂ alloy.

A Simple, Cost-Effective Sensor for Detecting Lead Ions in Water Using Under-Potential Deposited Bismuth Sub-Layer with Differential Pulse Voltammetry (DPV)

This research has developed a simple to use, cost effective sensor system for the detection of lead ions in tap water. An under-potential deposited bismuth sub-layer on a thin gold film based electrochemical sensor was designed, manufactured, and evaluated. Differential pulse voltammetry (DPV) measurement technique was employed in this detection. Tap water from the Cleveland, OH, USA regional water district was the test medium. Concentrations of lead ion in the range of 8 × 10⁻⁷ M to 5 × 10⁻⁴ M were evaluated, showing a good sensitivity over this concentration range. The calibration curve for the DPV measurements of lead ions in tap water showed excellent reproducibility with R² value of 0.970. This DPV detection system required 3–6 min to complete the detection measurement. A longer measurement time of 6 min was used for the lower lead ion concentration. The selectivity of this lead ion sensor was very good, and Fe III, Cu II, Ni II, and Mg II at a concentration level of 5 × 10⁻⁴ M did not interfere with the lead ion measurement.

Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals

During the past decade, electrocatalysis has attracted significant attention primarily due to the increased interest in the development of new generations of devices for electrochemical energy conversion. This has resulted in a progress in both fundamental understanding of the complex electrocatalytic systems and in the development of efficient synthetic schemes to tailor the surface precisely at the atomic level. One of the viable concepts in electrocatalysis is to optimise the activity through the direct engineering of the properties of the topmost layers of the surface, where the reactions take place, with monolayer and sub-monolayer amounts of metals. This forms (bi)metallic systems where the electronic structure of the active sites is optimised using the interplay between the nature and position of the atoms of solute metals at the surface. In this review, we focus on recent theoretical and experimental achievements in designing efficient (bi)metallic electrocatalysts with selective positioning of foreign atoms to form a variety of active catalytic sites at the electrode surface. We summarize recent results published in the literature and outline challenges for computational and experimental electrocatalysis to engineer active and selective catalysts using atomic layers.

Electrochemical degradation of trichloroacetic acid in aqueous media: influence of the electrode material

The electrochemical degradation of trichloroacetic acid (TCAA) in water has been analysed through voltammetric studies with a rotating disc electrode and controlled-potential bulk electrolyses. The influence of the mass-transport conditions and initial concentration of TCAA for titanium, stainless steel and carbon electrodes has been studied. It is shown that the electrochemical reduction of TCAA takes place prior to the massive hydrogen evolution in the potential window for all electrode materials studied. The current efficiency is high (> 18%) compared with those normally reported in the literature, and the fractional conversion is above 50% for all the electrodes studied. Only dichloroacetic acid (DCAA) and chloride anions were routinely detected as reduction products for any of the electrodes, and reasonable values of mass balance error were obtained. Of the three materials studied, the titanium cathode gave the best results.

Manipulation of Electrocatalytic Reaction Pathways through Surface Chemistry: In Situ Fourier Transform Infrared Spectroscopic Studies of 1,3-Butanediol Oxidation on a Pt Surface Modified with Sb and S Adatoms

Cyclic voltammetry and in situ Fourier transform infrared (FTIR) spectroscopy were employed to study the electrocatalytic properties of a Pt electrode modified with adatoms of antimony (Sb) or sulfur (S) for 1,3-butanediol (1,3-BD) oxidation. The results demonstrated the possibility of manipulating the reaction pathways involved in 1,3-BD oxidation through chemical modification of the Pt electrode surface. Both Sb and S adatoms (Sb(ad) and S(ad)) can inhibit the dissociative reaction of 1,3-BD into CO, which is the main source of self-poisoning in electrocatalysis of small organic molecules. On Pt electrodes modified with a high coverage of Sb(ad) (Pt/Sb(ad)) the onset oxidation potential of 1,3-BD has been significantly decreased, which is attributed to the fact that the oxidation of Sb(ad) occurs at lower potentials than that of the Pt surface. In situ FTIR results illustrated that, although at potentials below 0.5 V (vs a saturated calomel electrode), at which the Sb(ad) is stable on the Pt electrode surface, both carbonyl and CO2 species have been observed, the principal oxidation products of 1,3-BD are carbonyl species. Such results indicate that the reaction is mainly the dehydrogenation of 1,3-BD molecules. However, at potentials above 0.5 V the proportion of CO2 species in the oxidation products increases quickly, implying that the reaction has turned to the breakage of C-C bonds in 1,3-BD molecules and the subsequent oxidation of the cleaved fragments. In contrast with the cases of 1,3-BD oxidation on Pt and Pt/Sb(ad) electrodes, the reaction of 1,3-BD oxidation on a Pt electrode modified with S adatoms (Pt/S(ad)) is oriented completely to the production of carbonyl species when electrode potentials are below 0.9 V, though the reaction activity is relatively low. When the electrode potential is increased above 0.9 V, the intensity of the CO2 IR band in the FTIR spectra increases rapidly, corresponding to a fast oxidation of 1,3-BD on surface Pt sites recovered by the oxidation and desorption of S(ad) from the Pt surface.

Atomic Rearrangements during the Electrochemical Treatments of Au(111) Covered with Irreversibly Adsorbed Sb

A morphological variation of Au(111) covered with irreversibly adsorbed Sb was investigated with cyclic voltammetry and EC-STM. At open circuit potential (approximately 0.0 V vs a Ag/AgCl reference electrode), the oxygenated Sb layers were formed as an island on the wide terraces and a terrace at the step edges of Au(111). The ultimate morphology at the open circuit potential was a network adlayer with a (radical3 x radical3)R30 degrees atomic arrangement. When the oxygenated layer was reduced, the adsorption features, such as the island, shrunk or disappeared depending on their sizes. This modification was interpreted in terms of an alloy formation of Sb and Au. All of the Sb atoms, however, were not involved in the alloy formation, although the alloyed and unalloyed domains showed (radical3 x radical3)R30 degrees atomic structures with different brightness in EC-STM images. During oxidation of the reduced Sb layers, the alloyed and unalloyed domains of Sb behaved in a different way: the alloyed Sb was stripped to a soluble species to leave pits, while the unalloyed Sb became an oxygenated adspecies, which desorbed very slowly. A long oxidation led to a Au(111) covered with pits and islands of (1 x 1) without any adsorbed Sb.