Electrochemistry As A 17 Nanoscience
Electrochemistry as aNanoscienceby O. M. MagnussenThese are exciting times to becomean electrochemist. Nanoscience—and to a limited extent evennanotechnology—are here to stay andelectrochemistry plays an importantpart in it. How can we convey this tostudents, how can we win them asfuture researchers for this growing field,and how do we prepare them best forupcoming questions and challenges?This is the task faced by electrochemicaleducation today. It is a formidable taskand, as I must admit, my own answersto these questions are limited. A keypoint, however, seems the coalescenceof electrochemistry with other areas ofsurface and interface science, leading(together with other developments inchemistry, condensed matter physics,and materials science) to the currentfocus on nanoscale structures andphenomena. In the following, I try tosketch these developments and pointout some current and expected trends.To facilitate incorporation into practicalelectrochemical teaching I refer to recentreviews on the field rather than theoriginal literature whenever possible.The Development of ElectrochemicalSurface ScienceStimulated by the substantialprogress made in studies of surfacesunder ultrahigh vacuum conditions,new concepts and techniques started toemerge in interfacial electrochemistry inthe 1980s. These rapidly gained groundand have become a vital part of modernelectrochemical research. The mostimportant of these developments follow.The Electrochemical Society Interface Fall 2006Structurally defined electrodes.—Electrodes with well-defined andcharacterized surface structure are a keycomponent of every electrochemicalexperiment aiming at an understandingof atomic-scale interface structure andreactivity. Many types of such electrodes,specifically (but not exclusively)low-index and stepped single crystalelectrodes of noble and transition metals,can nowadays be prepared routinely withsurfaces that are (and stay) clean andsmooth on the atomic scale.In situ methods.—The toolkit oftoday’s electrochemists encompassesa variety of experimental methods fordirect in situ studies of electrochemicalinterfaces. Scanning probe microscopy,in particular scanning tunneling(STM),1,2 atomic force (AFM),1 andscanning electrochemical (SECM)microscopy, 3 can provide direct realspace images of electrode surfaces withidentical resolution as under ultrahighvacuum (UHV) conditions or dataon electrochemical reactivity on thesub-micrometer scale, respectively.Complementary data on the interfacestructure can be obtained bysynchrotron-based methods,4-7 such assurface X-ray scattering (SXS),4-7 X-rayabsorption spectroscopy (EXAFS),4,5,7 andX-ray standing wave (XSW),7 as well asspectroelectrochemical techniques, e.g.,Raman and Fourier transform infrared(FTIR) spectroscopy,7-9 second-harmonicgeneration (SHG),7,10 and sum-frequencygeneration (SFG).10Modern theoretical approaches.—Novelinsight also comes from studies byadvanced theoretical methods,11-13 rangingfrom density functional theory (DFT)calculations of adsorption geometriesand energies to molecular dynamicssimulations of the double layerstructure, quantum chemical modelsof ion and charge transfer, modelingof phase transitions in adsorbate layersby kinetic Monte Carlo simulations,and description of complex oscillatingreactions by nonlinear dynamics.Together, these experimental andtheoretical methods have started tounravel the complex microscopicstructure of electrochemical interfaceson the atomic scale. Particularlysuccessful examples are studies ofthe reconstruction or restructuring ofelectrode surfaces,14 of the surprisinglyrich potential-dependent phasebehavior of adsorbate layers, formed byanions,2,15,16 metal species,17 and organicspecies,2,16 and of the initial stages ofmetal electrodeposition/dissolutionas well as metal passivation.17,18 Thesestudies also encompass investigations ofthe corresponding structural changes ontime scales of several seconds to hours.Contributions of Electrochemistryto NanotechnologyOver the last decade severaldevelopments in electrochemistry havecontributed significantly to nanoscienceand nanotechnology. Most importantare the advances in nanoscalecharacterization of electrochemicalinterfaces described above, progress inelectrochemical processing methodsfor the formation of micro- andnanostructures, and the (re-)discoveryof electrochemical techniques andconcepts by the nanotechnologycommunity, in particular for studyingfunctional nanostructures. Morespecifically, important current trendsinclude the following.Nanostructuring by electrodepositionand etching.—Electrochemicaldeposition and dissolution haveproven to be powerful and versatiletools for the formation of structureswith lateral dimensions below 100nm on planar substrates. Examplescover a broad range, ranging fromindustrial applications, the arguablymost important of which is Cudamascene plating for ultralarge scaleintegrated (ULSI) microchips, to currentresearch, where nanoscale templates(e.g., pores, nanoparticles, naturalsurface heterogeneities, liquid crystals,and biological or supramolecularstructures) are employed to createordered patterns with dimensions downto a few nanometers. Furthermore,spatiotemporal pattern formationprocesses in electrochemical reactionscan be used to form well-definedmultilayers or pore structures (e.g., forphotonic crystals).23
Magnussen(continued from previous page)Nanostructuring by local probes.—Various schemes for local electrochemicalsurface restructuring by the tip of ascanning probe microscope have beendeveloped.19 They employ mechanicalinteractions between tip and sample(e.g., transfer of electrodepositedmaterial on the tip to the sample ormechanical removal of passivatinglayers), local changes of the solutioncomposition in the vicinity of the tip(typically induced by an electrochemicalreaction at the tip), local double layercharging by a conducting local probe(either by positioning the probe atdistances approaching the Debye lengthor by applying nanosecond pulses),or confinement of the electrolyte tonanosize droplets between tip andsample.Nanoelectronics.—One key issue incurrent efforts to construct electronicdevices based on single molecules is tocharacterize and understand electrontransfer between molecular speciesand metal electrodes, a subject whichis electrochemistry to the core. Notsurprisingly, electrochemical studiestherefore play an important role in thedevelopment of suitable systems formolecular electronics and are performedby many groups active in this area.(Editor’s note: This topic was featured inthe spring 2004 issue of Interface.)Nanoparticle synthesis.—Insightinto interfacial electrochemistry canalso guide the chemical synthesis ofnanoparticles by electroless depositionprocesses. The coverage and structure ofanionic and organic adsorbate layers atthe open circuit potential may differ fordifferent crystallographic orientations,which in turn can promote selectivegrowth of specific crystal planes. Forexample, the formation of Au nanorodsin cetyltrimethylammonium bromidesolution may be interpreted along theselines.20“Smart” nanomaterials.—Functionalnanostructured materials that can becontrolled by applying a potential orelectric current (e.g., electrochromiclayers) or, vice versa, provide electricsignals (e.g., sensors based onfunctionalized electrodes) are particularlyeasy to integrate into microelectronicsand therefore are attractive for manyapplications. This is already an importantarea of applied electrochemistry andmay be expected to grow in the next fewyears. Also a wide range of materials forelectrochemical energy technology maybe added to this category.This short list can give only a glimpseat the tremendous mutual influenceelectrochemistry and nanotechnologyhad on each other in recent years. Many24current trends in this area have beenhighlighted in past Interface issues, whichreflect the status of this very active fieldbetter than can be given in this briefoverview.Emerging Trends and FutureChallengesDispite these achievements, majorchallenges for electrochemical surfaceand nanoscience still lie ahead,which is good news for students ofelectrochemistry. Here is a personal viewon important contemporary questionsand tasks for the next decade(s) ofelectrochemical research.Understanding the double layer onthe atomic scale.—Contrary to thewell-characterized structure of solid metalelectrodes and ordered adsorbate layers,experimental data on the local order ofthe liquid in the near-interface regionare sparse and only partly consistent.Likewise, theoretical models are currentlynot capable of giving a full quantumchemical description of this complexinterface. A central question is the roleof the solvent, in particular water, whichcan have a decisive influence even onordered, specifically adsorbed ions in theinner part of the double layer.15 Howthe highly dynamic structure of solventand non-specifically adsorbed ions canbe described in detail, in which wayit depends on the potential or surfacecharge density, and how this affectselectrode reactions is of key importanceto electrochemical surface science. Toclarify these issues, advanced in situmethods, e.g., ultrafast spectroscopy,as well as complementary ab initiotheoretical approaches that allow largesystem calculations and include theelectrolyte and the surface charge mustbe developed.Dynamics at electrode surfaces.—Atomic-scale dynamic processes onelectrode surfaces, such as surfacediffusion, interactions betweenadsorbates, or the initial stagesof nucleation, are of considerableimportance to a wide variety ofFIG. 1. Examples of current research in nanoscale electrochemistry, illustrating central questions andchallenges. (a) Video-STM images of sulfide diffusion on Cu(100) electrodes in 0.01 M HCl solutionand resulting potential-dependent diffusion barriers, as an example for in situ dynamic studies.22 (b)Clarification of electrocatalytic reaction pathway by DFT calculations for the oxygen reaction on Pt(111)(courtesy of T. Jacob, after Ref. 28). (c) Studies of the interface structure in situ, during electrodeposition atdeposition rates of 30µA cm-2, by surface X-ray diffraction for Au on Au(100), showing the potential-dependent growth behavior.23 (d) Employing nanosecond voltage pulses for electrochemical machining with nanometer precision (courtesy of R. Schuster, from Ref. 29).The Electrochemical Society Interface Fall 2006
electrochemical reactions, rangingfrom electrocatalysis to two- or threedimensional phase formation processes,but are much less understood thantheir counterparts at solid-vacuuminterfaces. Only in recent years, powerfulin situ methods, e.g., scanning probemicroscopy with high time resolution,have become available and have startedto unravel the effect of the species insolution and of the high electric field inthe double layer on these processes.21,22An example of our own work in thisarea, a video-STM study of sulfidediffusion on Cu(100) electrodes in HClsolution, is shown in Fig. 1a.22 It revealsa dramatic influence of the potential onthe surface diffusion rate, which can berationalized by electrostatic contributionsto the activation energy for sulfidehopping diffusion. Not only the atomicscale dynamics itself, but also its linkto dynamic behavior on mesoscopicand macroscopic length scales is ofinterest, in particular in the formationof spatiotemporal patterns on electrodesurfaces.Identifying and controlling activesites.—The electrochemical reactivityof the different types of surface sites,present on a real electrode, can differvastly. Identifying those sites thatdominate the electrochemical behaviorand ascertaining the microscopicorigin of their reactivity is anotherHoly Grail of surface-science orientedelectrochemistry. Although previousstudies could determine active sites forselected cases, often revealing a decisiveinfluence of the (potential-dependent)structure of adsorbate layers,15 a trueunderstanding, based on the full reactionpathway, is still missing for any system.Ultimately, the objective is to describeelectrochemical reactivity ab initio asa property of the electrode materialand the electrochemical environment,linked to electronic structure of the solidsurface, the adsorbates in the doublelayer, and the reacting electroactivespecies. Obviously, progress in this areais not only of academic interest, butalso of great practical importance forthe rational design and optimization ofelectrode materials and electrochemicalprocessing methods. For example,precise knowledge of the mechanismsby which organic additives influencethe ion transfer reaction and surfacetransport of metal adatoms could guidethe development of plating baths fornanotechnology applications. Anothermajor area is electrocatalyis (see theexample in Fig. 1b), where questionsregarding active sites and reactionpathways have been central for a longtime and still many open questionsregarding the structure-reactivityrelationship abound.The Electrochemical Society Interface Fall 2006Bridging the current density gap. –Similar to the situation in heterogeneouscatalysis, where the conditions of modelstudies differ from those in reactors bya wide pressure gap, electrochemicalsurface science studies are primarilyperformed in the double layer regime orat current densities that are several ordersof magnitude lower than those employedin corresponding industrial applications,such as galvanic plating. In both casesthe interface structure under real-worldconditions may be significantly alteredby the reaction, e.g., involving thepresence of oxides, salt layers, or byproducts of the reaction. To developa technologically useful knowledge,the complex interdependence betweenatomic-scale surface structure and theelectrochemical reaction (i.e., the growthbehavior in electroplating) must beclarified in situ under realistic reactionconditions, which include high currentdensities, substantial mass transport inthe electrolyte, a wider potential andtemperature range, and more complexelectrolytes. Attempts to adapt modernin situ methods to these requirements areunder way (Fig. 1c) 23 and will continueto be an important challenge in thefuture. A related problem is bridgingthe materials gap, i.e., advancingto high-resolution in situ studies ofmore complex electrodes (e.g., alloys,nanomaterials) or electrode processes(e.g., polycrystalline or nanoscaletemplated growth).Electrochemistry at short length and timescales.—The breakdown of macroscopicconcepts in the nanometer range is thebasis of all nanoscience. In the contextof electrochemistry this translates tothe question of how electrochemicalreactivity is changed when the electrodeor the electrolyte is confined tonanometer dimensions. This includes,for example, the correlation of structureand reactivity of nanoparticles, a subjectof intense research in electrocatalyis.Another, currently largely unexploredexample concerns the transport andreactivity of electroactive species innanoscale cavities, which may offerexciting new science, according tothe results from structural biologyon transport through ion and waterchannels. Furthermore, the continuingminiaturization of ultramicroelectrodesopens up new time scales forelectrochemical studies. Inversely,ultrafast current pulses are offeringnew opportunities for electrochemicalmachining on the nanoscale (Fig. 1d).Technological applications ofelectrochemical nanoscience.—Althoughan impressive range of electrochemicalprocesses for nanostructure formationhave been demonstrated already, onlya few of them have made it out of thelaboratory. Many practical problems,viz., reliability, long-term stability, andcompatibility with existing productionmethods, must be addressed, specificallyfor methods based on nanoscaletemplates or self-ordering.Required Skills and Implementationin EducationAll these areas will provide ampleopportunities for basic and appliedscience, i.e., jobs for scientists trainedin electrochemistry. To prepare studentsadequately for these upcomingchallenges is the task of modernelectrochemical education, which bringsus back to the main issue at hand. Thequestion of how this will (or should)affect electrochemical education is ahighly subjective one, depending on theteacher as well as the audience. My ownperspective is affected by having spentand taught the last ten years at institutesof physical chemistry and condensedmatter physics. Electrochemicaleducation for students with differentbackgrounds may place the emphasison slightly different aspects. However,in conversations with colleagues that,like me, have started teaching interfacialelectrochemistry in the last few years,the following points were unanimouslyviewed as central.Solid working knowledge of quintessentialelectrochemical concepts.—Above all, ourteaching must provide expertise in thecore areas of electrochemistry, specificallyin charge transfer processes at interfaces.Basic knowledge on how to describeand measure electrode kinetics, i.e., thetraditional electrochemical curriculum,is and will remain an essentialprerequisite for future contributions ofelectrochemistry to nanoscience, despitethe current emphasis on structural,microscopic issues. However, due to theselinks to nanoscience, electrochemistry isnot exclusively the domain of physicaland analytical chemists anymore, butalso becomes relevant to condensedmatter physicists and material scientists.This has some important consequences.First, the change in audience mustbe reflected in the way we motivateand didactically approach these coretopics, i.e., the classical lore has tobe “reinterpreted.” For example, inmy experience, students of solid-statephysics usually find electrochemicalredox reactions uninteresting, but maywillingly embrace lectures on this topic,i.e., on electron transfer between solidsand molecules, if they can recognize theconnection to molecular electronics.Second, electrochemical educationshould not be confined to its traditionalplace in introductory courses on physicalor analytical chemistry anymore. The(continued on next page)25
1. Introduction to interfaces2. Liquids and liquid interfaces3. Structure of solid surfaces4. Simple solid-liquid interfaces5. Interface potentials6. Electrochemical interfaces7. Adsorption8. Interface processes9. Electron transfer at interfaces10. Growth processes11. Heteroepitaxy12. Interface-dominated materialselectrostatic conditions at interfaceelectrochemical equilibriacell potentialsabsolute electrode potentialthree-electrode configurationdouble-layer theorymicroscopic electrode surface structureadsorption at electrodessurface dipole moment and partial chargephase transitions in adlayersButler-Volmer equationexamples of electrochemical reactionsMarcus and Gerischer theoryelectrodepositionnucleation and growthFIG. 2. Integration of electrochemical topics in a course on interface science for third year physicsmajors. About one-half of the lectures are based predominantly or to a significant extenton interfacial electrochemistry.Magnussen(continued from previous page)current challenge is to include teachingof central electrochemical conceptsin programs on nanoscience andnanotechnology, at present emergingin most science departments. This willnot come by itself; it is the task of theelectrochemical community to ensurethat electrochemical knowledge isincorporated into these new courses andECS should promote and guide suchefforts. Within undergraduate educationthis will hardly exceed some basic ideas,even in a specialized course, whichmakes it even more important to strivefor a modern, accessible presentationthat is integrated into the more generalconceptual framework of nanoscience. Toillustrate these points, my own attemptat this, a course of interface science for(mostly) third year physics students,is outlined in Fig. 2. As shown in theright part of the figure, quintessentialelectrochemical concepts are embeddedin (at least) half of the lectures of thiscourse.Concepts
of electrochemistry with other areas of surface and interface science, leading (together with other developments in chemistry, condensed matter physics, and materials science) to the current focus on nanoscale structures and phenomena. In the following, I try to sketch these developments and point out some current and expected trends.
About Electrochemistry Electrochemistry has been the official journal of The Electrochemical Society of Japan (ECSJ) since 1999. Electrochemistry succeeds the journal Denki Kagaku (Japanese word for ‘Electrochemistry’) which was first published in 1933 with the aim of contributing to science and industry through electrochemistry.
Sep 30, 2003 · Nanotechnology Identity, resources, community and focus development in three different contexts: The role of Nanoscience and Nanotechnology at Pacific Northwest National Laboratory The PNNL and University of Washington - Joint Institute of Nanoscience Linking regional activities –The Northwest Nanoscience
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