Inner-Sphere Heterogeneous Electrode Reactions. Electrocatalysis and Photocatalysis: The Challenge

Probing Redox Reactions at the Nanoscale with Electrochemical Tip-Enhanced Raman Spectroscopy

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm(-1) band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm(-1) band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip-sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes.

Reaction Dynamics of Proton-Coupled Electron Transfer from Reduced ZnO Nanocrystals

The creation of systems that efficiently interconvert chemical and electrical energies will be aided by understanding proton-coupled electron transfers at solution-semiconductor interfaces. Steps in developing that understanding are described here through kinetic studies of reactions of photoreduced colloidal zinc oxide (ZnO) nanocrystals (NCs) with the nitroxyl radical TEMPO. These reactions proceed by proton-coupled electron transfer (PCET) to give the hydroxylamine TEMPOH. They occur on the submillisecond to seconds time scale, as monitored by stopped-flow optical spectroscopy. Under conditions of excess TEMPO, the reactions are multiexponential in character. One of the contributors to this multiexponential kinetics may be a distribution of reactive proton sites. A graphical overlay method shows the reaction to be first order in [TEMPO]. Different electron concentrations in otherwise identical NC samples were achieved by three different methods: differing photolysis times, premixing with an unphotolyzed sample, or prereaction with TEMPO. The reaction velocities were consistently higher for NCs with higher numbers of electrons. For instance, NCs with an average of 2.6 e(-)/NC reacted faster than otherwise identical samples containing ≤1 e(-)/NC. Surprisingly, NC samples with the same average number of electrons but prepared in different ways often had different reaction profiles. These results show that properties beyond electron content determine PCET reactivity of the particles.

Techniques and methodologies in modern electrocatalysis: evaluation of activity, selectivity and stability of catalytic materials

The development and optimisation of materials that promote electrochemical reactions have recently attracted attention mainly due to the challenge of sustainable provision of renewable energy in the future. The need for better understanding and control of electrode-electrolyte interfaces where these reactions take place, however, implies the continuous need for development of efficient analytical techniques and methodologies capable of providing detailed information about the performance of electrocatalysts, especially in situ, under real operational conditions of electrochemical systems. During the past decade, significant efforts in the fields of electrocatalysis and (electro)analytical chemistry have resulted in the evolution of new powerful methods and approaches providing ever deeper and unique insight into complex and dynamic catalytic systems. The combination of various electrochemical and non-electrochemical methods as well as the application of quantum chemistry calculations has become a viable modern approach in the field. The focus of this critical review is primarily set on discussion of the most recent cutting-edge achievements in the development of analytical techniques and methodologies designed to evaluate three key constituents of the performance of electrocatalysts, namely, activity, selectivity and stability. Possible directions and future challenges in the design and elaboration of analytical methods for electrocatalytic research are outlined.

Cleaning nanoelectrodes with air plasma

Unlike macroscopic and micrometer-sized solid electrodes whose surface can be reproducibly cleaned by mechanical polishing, cleaning the nanoelectrode surface is challenging because of its small size and extreme fragility. Even very gentle polishing typically changes the nanoelectrode size and geometry, thus, complicating the replication of nanoelectrochemical experiments. In this letter, we show the possibility of cleaning nanoelectrode surfaces nondestructively by using an air plasma cleaner. The effects of plasma cleaning have been investigated by atomic force microscopy (AFM) imaging, voltammetry, and scanning electrochemical microscopy (SECM). A related issue, the removal of an insoluble organic film from the nanoelectrode by plasma cleaning, is also discussed.

Edge overgrowth of spiral bimetallic hydroxides ultrathin-nanosheets for water oxidation

The structure of edges may dramatically influence the properties of nanomaterials, so the rational design or control over the structures of the edges is required. Here we synthesized spiral ultrathin-nanosheets with overgrown edges (SUNOE) of NiFe, CoNi and CoFe bimetallic hydroxides by governing the growth rates of different directions in screw dislocation driven growth (SDDG) in nonaqueous solvents. The driving force for the SDDG is supersaturation, which could be controlled by the concentration of the different precursors, thus achieving non-uniform structures of the edges and inner sheets. NiFe, CoNi and CoFe bimetallic hydroxides possess layered structures, in which overgrown edges may prevent them from re-stacking. The as prepared SUNOE all show good performance for the oxygen evolution reaction (OER) in the electrolysis of water, and the lowest onset potential was 1.45 V (vs. RHE) (the lowest potential when the current density reached 10 mA cm^-2 was 1.51 V (vs. RHE)).

Nucleation, aggregative growth and detachment of metal nanoparticles during electrodeposition at electrode surfaces

The nucleation and growth of metal nanoparticles (NPs) on surfaces is of considerable interest with regard to creating functional interfaces with myriad applications. Yet, key features of these processes remain elusive and are undergoing revision. Here, the mechanism of the electrodeposition of silver on basal plane highly oriented pyrolytic graphite (HOPG) is investigated as a model system at a wide range of length scales, spanning electrochemical measurements from the macroscale to the nanoscale using scanning electrochemical cell microscopy (SECCM), a pipette-based approach. The macroscale measurements show that the nucleation process cannot be modelled as either truly instantaneous or progressive, and that step edge sites of HOPG do not play a dominant role in nucleation events compared to the HOPG basal plane, as has been widely proposed. Moreover, nucleation numbers extracted from electrochemical analysis do not match those determined by atomic force microscopy (AFM). The high time and spatial resolution of the nanoscale pipette set-up reveals individual nucleation and growth events at the graphite basal surface that are resolved and analysed in detail. Based on these results, corroborated with complementary microscopy measurements, we propose that a nucleation-aggregative growth-detachment mechanism is an important feature of the electrodeposition of silver NPs on HOPG. These findings have major implications for NP electrodeposition and for understanding electrochemical processes at graphitic materials generally.

A practical guide to using boron doped diamond in electrochemical research

This article serves as a guide to those working with boron doped diamond electrodes, especially the first time user. It outlines the key material properties required when interpretating electrochemical data and provides a summary of experimental approaches to determining electrode quality.

NiCo2S4 sub-micron spheres: an efficient non-precious metal bifunctional electrocatalyst

Urchin-like NiCo2S4 sub-micron spheres integrated with nano-sized and micro-sized structures, which were synthesized via a facile one-pot method, deliver efficient electrocatalytic activities for oxygen reduction and evolution reactions. The excellent electrocatalytic property of NiCo2S4 sub-micron spheres is originated from their unique urchin-like microstructure, composition and d-electronic configurations of the transition metal ions.

Characterisation of Complex Electrode Processes using Simultaneous Impedance Spectroscopy and Electrochemical Nanogravimetric Measurements

The methodology and illustrative examples of application are presented for a technique that simultaneously combines electrochemical impedance spectroscopy (EIS) and nanogravimetric measurements; the latter are implemented using a so-called electrochemical quartz crystal nanobalance (EQCN). The combination of EIS and EQCN provides a powerful method for the characterisation of many complex processes at electrochemical interfaces. This method gives in one relatively simple experiment more detailed information than is available from conventional electrochemical techniques. The combined measurements can be performed either as a function of time, at a constant electrode potential, or under potentiodynamic conditions, as a function of the electrode potential. Herein, we show how this can be applied to enable more accurate investigation of processes that occur at boundaries between electrodes and electrolytes. The application examples range from eletrocatalysis, in which evaluation of a catalyst is performed simultaneously with its formation, and the intercalation and electrodeposition of thin metal films to in situ characterisation of non-electroactive self-assembled monolayers during their formation.

Heterogeneous electron transfer at nanoscopic electrodes: importance of electronic structures and electric double layers

Recent insights into the nanoscopic electrode size and structure effects on heterogeneous ET kinetics are presented.

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.

Activity descriptor identification for oxygen reduction on nonprecious electrocatalysts: linking surface science to coordination chemistry

Developing nonprecious group metal based electrocatalysts for oxygen reduction is crucial for the commercial success of environmentally friendly energy conversion devices such as fuel cells and metal-air batteries. Despite recent progress, elegant bottom-up synthesis of nonprecious electrocatalysts (typically Fe-N(x)/C) is unavailable due to lack of fundamental understanding of molecular governing factors. Here, we elucidate the mechanistic origin of oxygen reduction on pyrolyzed nonprecious catalysts and identify an activity descriptor based on principles of surface science and coordination chemistry. A linear relationship, depicting the ascending portion of a volcano curve, is established between oxygen-reduction turnover number and the Lewis basicity of graphitic carbon support (accessed via C 1s photoemission spectroscopy). Tuning electron donating/withdrawing capability of the carbon basal plane, conferred upon it by the delocalized π-electrons, (i) causes a downshift of e(g)-orbitals (d(z(2))) thereby anodically shifting the metal ion's redox potential and (ii) optimizes the bond strength between the metal ion and adsorbed reaction intermediates thereby maximizing oxygen-reduction activity.

Pseudo-single-crystal electrochemistry on polycrystalline electrodes: visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing 'pseudo-single-crystal' electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe(2+/3+) couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe(2+) oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale.

Carbon-layer-protected cuprous oxide nanowire arrays for efficient water reduction

In this work, we propose a solution-based carbon precursor coating and subsequent carbonization strategy to form a thin protective carbon layer on unstable semiconductor nanostructures as a solution to the commonly occurring photocorrosion problem of many semiconductors. A proof-of-concept is provided by using glucose as the carbon precursor to form a protective carbon coating onto cuprous oxide (Cu₂O) nanowire arrays which were synthesized from copper mesh. The carbon-layer-protected Cu₂O nanowire arrays exhibited remarkably improved photostability as well as considerably enhanced photocurrent density. The Cu₂O nanowire arrays coated with a carbon layer of 20 nm thickness were found to give an optimal water splitting performance, producing a photocurrent density of -3.95 mA cm⁻² and an optimal photocathode efficiency of 0.56% under illumination of AM 1.5G (100 mW cm⁻²). This is the highest value ever reported for a Cu₂O-based electrode coated with a metal/co-catalyst-free protective layer. The photostability, measured as the percentage of the photocurrent density at the end of 20 min measurement period relative to that at the beginning of the measurement, improved from 12.6% on the bare, nonprotected Cu₂O nanowire arrays to 80.7% on the continuous carbon coating protected ones, more than a 6-fold increase. We believe that the facile strategy presented in this work is a general approach that can address the stability issue of many nonstable photoelectrodes and thus has the potential to make a meaningful contribution in the general field of energy conversion.

Titanium and zinc oxide nanoparticles are proton-coupled electron transfer agents

Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e(-)/H(+) coupling.

Electrochemistry at carbon nanotube forests: sidewalls and closed ends allow fast electron transfer

The electrochemical properties of the closed ends and sidewalls of pristine carbon nanotube forests are investigated directly using a nanopipet electrochemical cell. Both are shown to promote fast electron transfer, without any activation or processing of the carbon nanotube material required, in contrast to the current model in the literature.

Nanoelectrodes: recent advances and new directions

This article reviews recent work involving the development and application of nanoelectrodes in electrochemistry and related areas. We first discuss common analytical methods for characterizing the size, shape, and quality of nanoelectrodes, including electron microscopy, steady-state cyclic voltammetry, scanning electrochemical microscopy, and surface modification. We then emphasize recent developments in fabrication techniques that have led to structurally well-defined nanoelectrodes. We highlight recent advances in the application of nanoelectrodes in important analytical chemistry areas, such as single-molecule studies, single-nanoparticle electrochemistry, and measurements of neurotransmitters from single neuronal cells.

Fundamental Mechanistic Understanding of Electrocatalysis of Oxygen Reduction on Pt and Non-Pt Surfaces: Acid versus Alkaline Media

Complex electrochemical reactions such as Oxygen Reduction Reaction (ORR) involving multi-electron transfer is an electrocatalytic inner-sphere electron transfer process that exhibit strong dependence on the nature of the electrode surface. This criterion (along with required stability in acidic electrolytes) has largely limited ORR catalysts to the platinum-based surfaces. New evidence in alkaline media, discussed here, throws light on the involvement of surface-independent outer-sphere electron transfer component in the overall electrocatalytic process. This surface non-specificity gives rise to the possibility of using a wide-range of non-noble metal surfaces as electrode materials for ORR in alkaline media. However, this outer-sphere process predominantly leads only to peroxide intermediate as the final product. The importance of promoting the electrocatalytic inner-sphere electron transfer by facilitation of direct adsorption of molecular oxygen on the active site is emphasized by using pyrolyzed metal porphyrins as electrocatalysts. A comparison of ORR reaction mechanisms between acidic and alkaline conditions is elucidated here. The primary advantage of performing ORR in alkaline media is found to be the enhanced activation of the peroxide intermediate on the active site that enables the complete four-electron transfer. ORR reaction schemes involving both outer- and inner-sphere electron transfer mechanisms are proposed.