Nano-scale effects in electrochemistry

Recent Progress in Morphology-Tuned Nanomaterials for the Electrochemical Detection of Heavy Metals

Heavy metals are one of the most important classes of environmental pollutants which are toxic to living beings. Many efforts are made by scientists to fabricate better sensors for the identification and quantification of heavy metal ions (HMI) in water and food samples to ensure good health. Electrocatalysts have been demonstrated to play an important role in enhancing the sensitivity and selectivity of HMI detection in electrochemical sensors. In this review, we presented morphologically well-tuned nanomaterials used as efficient sensor materials. Based on the molecular dimensions, shapes, and orientation, nanomaterials can be classified into 0-D, 1-D, 2-D, and 3-D nanomaterials. Active surface areas with significant exposure of active sites and adsorption-desorption abilities are extensively varied with dimensionality, which in turn ultimately influence the sensing performance for HMI.

Recent Trends in the Improvement of the Electrochemical Response of Screen-Printed Electrodes by Their Modification with Shaped Metal Nanoparticles

Novel sensing technologies proposed must fulfill the demands of wastewater treatment plants, the food industry, and environmental control agencies: simple, fast, inexpensive, and reliable methodologies for onsite screening, monitoring, and analysis. These represent alternatives to conventional analytical methods (ICP-MS and LC-MS) that require expensive and non-portable instrumentation. This needs to be controlled by qualified technicians, resulting moreover in a long delay between sampling and high-cost analysis. Electrochemical analysis based on screen-printed electrodes (SPEs) represents an excellent miniaturized and portable alternative due to their disposable character, good reproducibility, and low-cost commercial availability. SPEs application is widely extended, which makes it important to design functionalization strategies to improve their analytical response. In this sense, different types of nanoparticles (NPs) have been used to enhance the electrochemical features of SPEs. NPs size (1–100 nm) provides them with unique optical, mechanical, electrical, and chemical properties that give the modified SPEs increased electrode surface area, increased mass-transport rate, and faster electron transfer. Recent progress in nanoscale material science has led to the creation of reproducible, customizable, and simple synthetic procedures to obtain a wide variety of shaped NPs. This mini-review attempts to present an overview of the enhancement of the electrochemical response of SPEs when NPs with different morphologies are used for their surface modification

Random alloy and intermetallic nanocatalysts in fuel cell reactions

Fuel cells that use small organic molecules or hydrogen as the anode fuel can power clean electric vehicles. From an experimental perspective, the possible fuel cells' electrocatalytic reaction mechanisms are obtained through in situ electrochemical spectroscopy techniques and density functional theory calculations, providing theoretical guidance for further development of novel nanocatalysts. As advanced nanocatalysts for fuel cells' electrochemical reactions, alloy nanomaterials have greatly improved electrocatalytic activity and stability and have attracted widespread attention. Enhanced electrocatalytic performance of alloy nanocatalysts could be closely related to the synergistic effects, such as electronic and strain effects. Depending on the arrangement of atoms, alloys can be classified into random alloy and intermetallic compounds (ordered structure). Intermetallic compounds generally have lower heats of formation and stronger heteroatomic bonding strength relative to the random alloy, resulting in high chemical and structural stability in either full pH solutions or electrochemical tests. Here, we summarize the latest advances and the structure-function relationship of noble metal alloy nanocatalysts, among which Pt-based catalysts are the main ones, as well as comprehensively understand why they significantly affect the electrocatalytic performance of fuel cells. Novel alloy nanocatalysts with a robust three-phase interface to achieve efficient charge and mass transfer can obtain desirable activity and stability in the electrochemical workstation tests, and is expected to acquire a higher power density on fuel cell test systems with harsh test conditions.

Nanomaterials: Electrochemical Properties and Application in Sensors

The unique properties of nanoparticles make them an extremely valuable modifying material, being used in electrochemical sensors. The features of nanoparticles affect the kinetics and thermodynamics of electrode processes of both nanoparticles and redox reactions occurring on their surface. The paper describes theoretical background and experimental studies of these processes. During the transition from macro- to micro- and nanostructures, the analytical characteristics of sensors modify. These features of metal nanoparticles are related to their size and energy effects, which affects the analytical characteristics of developed sensors. Modification of the macroelectrode with nanoparticles and other nanomaterials reduces the detection limit and improves the degree of sensitivity and selectivity of measurements. The use of nanoparticles as transducers, catalytic constituents, parts of electrochemical sensors for antioxidant detection, adsorbents, analyte transporters, and labels in electrochemical immunosensors and signal-generating elements is described.

Plasmonic response and SERS modulation in electrochemical applied potentials

We study the optical response of individual nm-wide plasmonic nanocavities using a nanoparticle-on-mirror design utilised as an electrode in an electrochemical cell. In this geometry Au nanoparticles are separated from a bulk Au film by an ultrathin molecular spacer, giving intense and stable Raman amplification of 100 molecules. Modulation of the plasmonic spectra and the SERS response is observed with an applied voltage under a variety of electrolytes. Different scenarios are discussed to untangle the various mechanisms that can be involved in the electronic interaction between NPs and electrode surfaces.

Free Electrons to Molecular Bonds and Back: Closing the Energetic Oxygen Reduction (ORR)-Oxygen Evolution (OER) Cycle Using Core-Shell Nanoelectrocatalysts

Nanomaterial science and electrocatalytic science have entered a successful "nanoelectrochemical" symbiosis, in which novel nanomaterials offer new frontiers for studies on electrocatalytic charge transfer, while electrocatalytic processes give meaning and often practical importance to novel nanomaterial concepts. Examples of this fruitful symbiosis are dealloyed core-shell nanoparticle electrocatalysts, which often exhibit enhanced kinetic charge transfer rates at greatly improved atom-efficiency. As such, they represent ideal electrocatalyst architectures for the acidic oxygen reduction reaction to water (ORR) and the acidic oxygen evolution reaction from water (OER) that require scarce Pt- and Ir-based catalysts. Together, these two reactions constitute the "O-cycle", a key elemental process loop in the field of electrochemical energy interconversion between electricity (free electrons) and molecular bonds (H₂O/O₂), realized in the combination of water electrolyzers and hydrogen/oxygen fuel cells. In this Account, we describe our recent efforts to design, synthesize, understand, and test noble metal-poor dealloyed Pt and Ir core-shell nanoparticles for deployment in acidic polymer electrolyte membrane (PEM) electrolyzers and PEM fuel cells. Spherical dealloyed Pt core-shell particles, derived from PtNi₃ precursor alloys, showed favorable ORR activity. More detailed size-activity correlation studies further revealed that the 6-8 nm diameter range is a most desirable initial particle size range in order to maximize the particle Ni content after ORR testing and to preserve performance stability. Similarly, dealloyed and oxidized IrO_x core-shell particles derived from Ni-rich Ir-Ni precursor particles proved highly efficient oxygen evolution reaction (OER) catalysts in acidic conditions. In addition to the noble metal savings in the particle cores, the Pt core-shell particles are believed to benefit in terms of their mass-based electrochemical kinetics from surface lattice strain effects that tune the adsorption energies and barriers of elementary steps. The molecular mechanism of the kinetic benefit of the dealloyed IrO_x particle needs more attention, but there is mounting evidence for ligand hole effects in defect-rich IrO_x shells that generate preactive oxygen centers.

Unconventional Electrochemistry in Micro-/Nanofluidic Systems

Electrochemistry is ideally suited to serve as a detection mechanism in miniaturized analysis systems. A significant hurdle can, however, be the implementation of reliable micrometer-scale reference electrodes. In this tutorial review, we introduce the principal challenges and discuss the approaches that have been employed to build suitable references. We then discuss several alternative strategies aimed at eliminating the reference electrode altogether, in particular two-electrode electrochemical cells, bipolar electrodes and chronopotentiometry.

Unravelling the hydrogen absorption process in Pd overlayers on a Au(111) surface

Shedding light on the mechanism of hydrogen absorption occurring in nano-structured materials using the power of modern computational chemistry.

Platinum–carbide interactions: core–shells for catalytic use

A selection of carbides were modelled using DFT to assess their suitability as core–shell components for the oxygen reduction reaction.

Ternary Pt-Ru-Ni catalytic layers for methanol electrooxidation prepared by electrodeposition and galvanic replacement

Ternary Pt-Ru-Ni deposits on glassy carbon substrates, Pt-Ru(Ni)/GC, have been formed by initial electrodeposition of Ni layers onto glassy carbon electrodes, followed by their partial exchange for Pt and Ru, upon their immersion into equimolar solutions containing complex ions of the precious metals. The overall morphology and composition of the deposits has been studied by SEM microscopy and EDS spectroscopy. Continuous but nodular films have been confirmed, with a Pt ÷ Ru ÷ Ni % bulk atomic composition ratio of 37 ÷ 12 ÷ 51 (and for binary Pt-Ni control systems of 47 ÷ 53). Fine topographical details as well as film thickness have been directly recorded using AFM microscopy. The composition of the outer layers as well as the interactions of the three metals present have been studied by XPS spectroscopy and a Pt ÷ Ru ÷ Ni % surface atomic composition ratio of 61 ÷ 12 ÷ 27 (and for binary Pt-Ni control systems of 85 ÷ 15) has been found, indicating the enrichment of the outer layers in Pt; a shift of the Pt binding energy peaks to higher values was only observed in the presence of Ru and points to an electronic effect of Ru on Pt. The surface electrochemistry of the thus prepared Pt-Ru(Ni)/GC and Pt(Ni)/GC electrodes in deaerated acid solutions (studied by cyclic voltammetry) proves the existence of a shell consisting exclusively of Pt-Ru or Pt. The activity of the Pt-Ru(Ni) deposits toward methanol oxidation (studied by slow potential sweep voltammetry) is higher from that of the Pt(Ni) deposit and of pure Pt; this enhancement is attributed both to the well-known Ru synergistic effect due to the presence of its oxides but also (based on the XPS findings) to a modification effect of Pt electronic properties.

Hydrogen adsorption on bimetallic PdAu(111) surface alloys: minimum adsorption ensemble, ligand and ensemble effects, and ensemble confinement

Microscopy and spectroscopy measurements together with periodic DFT calculations provide detailed insight into the adsorption behavior of hydrogen on disordered, but structurally well defined PdAu–Pd(111) surface alloys.

Electrodeposition of palladium onto a pyridine-terminated self-assembled monolayer

The electrodeposition of Pd onto self-assembled monolayers (SAMs) of 3-(4-pyridine-4-ylphenyl)propane-1-thiol on Au(111) has been investigated by scanning tunneling microscopy. Two schemes are compared: One involves an established two-step procedure where Pd(2+) ions are first coordinated to the pyridine moieties and subsequently reduced in Pd(2+)-free electrolyte. The second deposition routine involves electroreduction in an electrolyte containing low concentration of Pd(2+) which merges both steps and, thus, significantly simplifies metal deposition onto pyridine-terminated SAMs. Both strategies produce identical Pd nanoparticles (NPs) which exhibit a narrow size distribution and an apparent STM height of ∼2.4 nm. The observation of a Coulomb blockade and easy displacement of the nanoparticles in STM experiments evidence deposition on top of the SAM. The NPs are concluded to be essentially spherical. Growth of the NPs is found to be self-limiting since repeating the complexation-deposition cycle increases the density of the nanoparticles rather than their size but only close to full coverage. At high concentration of the Pd(2+) electrolyte, deposition on top of the SAM is impeded by a competitive mushroom-type growth.

Electrochemically controlled self-assembled monolayers characterized with molecular and sub-molecular resolution

Self-assembled organization of functional molecules on solid surfaces has developed into a powerful and sophisticated tool for surface chemistry and nanotechnology. A number of reviews on the topic have been available since the mid 1990s. This perspective article aims to focus on recent development in the investigations of electronic structures and assembling dynamics of electrochemically controlled self-assembled monolayers (SAMs) of thiol containing molecules on gold surfaces. A brief introduction is first given and particularly illustrated by a Table summarizing the molecules studied, the surface lattice structures and the experimental operating conditions. This is followed by discussion of two major high-resolution experimental methods, scanning tunnelling microscopy (STM) and single-crystal electrochemistry. In Section 3, we briefly address choice of supporting electrolytes and substrate surfaces, and their effects on the SAM structures. Section 4 constitutes the major body of the article by offering some details of recent studies for the selected cases, including in situ monitoring of assembling dynamics, molecular electronic structures, and the key external factors determining the SAM packing. In Section 5, we give examples of what can be offered by theoretical computations for the detailed understanding of the SAM electronic structures revealed by STM images. A brief summary of the current applications of SAMs in wiring metalloproteins, design and fabrication of sensors, and single-molecule electronics is described in Section 6. In the final two sections (7 and 8), we discuss the current status in understanding of electronic structures and properties of SAMs in electrochemical environments and what could be expected for future perspectives.

STM, SECPM, AFM and Electrochemistry on Single Crystalline Surfaces

Scanning probe microscopy (SPM) techniques have had a great impact on research fields of surface science and nanotechnology during the last decades. They are used to investigate surfaces with scanning ranges between several 100 μm down to atomic resolution. Depending on experimental conditions, and the interaction forces between probe and sample, different SPM techniques allow mapping of different surface properties. In this work, scanning tunneling microscopy (STM) in air and under electrochemical conditions (EC-STM), atomic force microscopy (AFM) in air and scanning electrochemical potential microscopy (SECPM) under electrochemical conditions, were used to study different single crystalline surfaces in electrochemistry. Especially SECPM offers potentially new insights into the solid-liquid interface by providing the possibility to image the potential distribution of the surface, with a resolution that is comparable to STM. In electrocatalysis, nanostructured catalysts supported on different electrode materials often show behavior different from their bulk electrodes. This was experimentally and theoretically shown for several combinations and recently on Pt on Au(111) towards fuel cell relevant reactions. For these investigations single crystals often provide accurate and well defined reference and support systems. We will show heteroepitaxially grown Ru, Ir and Rh single crystalline surface films and bulk Au single crystals with different orientations under electrochemical conditions. Image studies from all three different SPM methods will be presented and compared to electrochemical data obtained by cyclic voltammetry in acidic media. The quality of the single crystalline supports will be verified by the SPM images and the cyclic voltammograms. Furthermore, an outlook will be presented on how such supports can be used in electrocatalytic studies.

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

Scanning electrochemical microscopy (SECM) has developed into a very versatile tool for the investigation of solid-liquid, liquid-liquid and liquid-gas interfaces. The arrangement of an ultramicroelectrode (UME) in close proximity to the interface under study allows the application of a large variety of different experimental schemes. The most important have been named feedback mode, generation-collection mode, redox competition mode and direct mode. Quantitative descriptions are available for the UME signal, depending on different sample properties and experimental variables. Therefore, SECM has been established as an indispensible tool in many areas of fundamental electrochemical research. Currently, it also spreads as an important new method to solve more applied problems, in which inhomogeneous current distributions are typically observed on different length scales. Prominent examples include devices for electrochemical energy conversion such as fuel cells and batteries as well as localized corrosion phenomena. However, the direct local investigation of such systems is often impossible. Instead, suitable reaction schemes, sample environments, model samples and even new operation modes have to be introduced in order to obtain results that are relevant to the practical application. This review outlines and compares the theoretical basis of the different SECM working modes and reviews the application in the area of electrochemical energy conversion and localized corrosion with a special emphasis on the problems encountered when working with practical samples.