conductive diamond thin-films in electrochemistry

Diamond and Related Materials 12(2003) 1940–1949

0925-9635/03/$ - see front matter� 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0925-9635Ž03.00260-7

Conductive diamond thin-films in electrochemistry

Matt Hupert, Alexander Muck, Jian Wang, Jason Stotter, Zuzana Cvackova, Shannon Haymond,Yoshiyuki Show, Greg M. Swain*

Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA

Abstract

Diamond electrodes offer superb properties for a variety of electrochemical technologies, properties that include: corrosionresistance, low background current, good responsiveness without pretreatment, resistance to fouling, and optical transparency.Electroanalysis, electrocatalysis, spectroelectrochemistry, and bioelectrochemistry are areas of electrochemistry in which diamondthin-films, both microcrystalline and nanocrystalline, are being successfully researched. A brief review is given herein of some ofour R&D efforts in each of these areas.� 2003 Elsevier B.V. All rights reserved.

Keywords: Diamond films; Electrochemistry; Dimensionally stable anodes; Electroanalysis; Electrocatalysis; Spectroelectrochemistry andbioelectrochemistry

1. Introduction

An emerging area in the field of diamond science andtechnology is electrochemistryw1,2x. Electrically con-ducting diamond thin-film electrodes, fabricated bychemical-vapor deposition(CVD), provide researcherswith a new material that meets the requirements for awide range of applications. There are two types ofsynthetic material being extensively researched: thin-film coatings of microcrystalline and nanocrystallinediamond.Many electrochemical measurements involve record-

ing an electrical signal(e.g. potential, current, or charge)associated with the oxidation or reduction of a redox-active analyte present in solution and relating this signalto the analyte concentration. The oxidation or reductionreaction occurs at the electrode–electrolyte solutioninterface, the so-called interfacial-reaction zone. Theelectron-transfer event is considered as tunneling of theelectron between electronic states on the electrode andstates on the reactant. The tunneling probability has theform of:

Tunneling probabilityA exp (ybx),

*Corresponding author. Tel.:q1-517-355-9715; fax:q1-517-353-1793.

E-mail address: [email protected](G.M. Swain).

wherex is the distance over which the electron is beingtransferred andb is a parameter that reflects the energybarrier height and the nature of the solventyelectrolytemedium between the statesw3x. Therefore, the electrode-reaction kinetics and mechanism are strongly influencedby the structure of this interface, particularly the physi-cochemical properties of the electrode material. In gen-eral, electrode properties, such as the surface cleanliness,microstructure, chemistry, and density of electronicstates(i.e. electrical conductivity), are important. Theextent to which any one of these parameters affects aredox reaction strongly depends upon the nature of theanalyte. This is a point often overlooked by researchersin the field. The electrode materials most often used inelectrochemical measurements are platinum, gold, andvarious forms of sp -bonded carbon(e.g. carbon fibers,2

glassy carbon, pyrolytic graphite). sp -bonded diamond3

is a new electrode material offering several advantagesfor electrochemical measurements, as compared to theseother carbon and metal electrodes. Several areas ofelectrochemistry in which diamond is being successfullyemployed include electroanalysis, electrocatalysis, spec-troelectrochemistry, and bioelectrochemistry. Herein, wereview some of our R&D efforts in each of these areas.

2. Diamond electrode architectures

Diamond thin-films can be produced synthetically byone of several established deposition protocols, the most

1941M. Hupert et al. / Diamond and Related Materials 12 (2003) 1940–1949

Fig. 1. SEM images of boron-doped(a and b) microcrystalline and(c and d) nanocrystalline diamond thin-films deposited on Si. A top viewand cross section image is shown for both film types. Scale bar for(a) is 2 mm and(b) is 1 mm.

popular being hot-filament and microwave-assistedCVD. Proper control over the source gas composition,system pressure, and substrate temperature allows dia-mond, instead of graphite or other sp -bonded carbon2

microstructures, to be grown preferentially and metast-ably. Metastable phases can form from precursors, if theactivation barriers to more stable phases are sufficientlyhigh. CVD diamond is produced under conditions wheregraphite is the more thermodynamically stable phase.However, both graphite and diamond are in deep poten-tial energy wells with a large activation energy barrierbetween them. Once diamond is formed, it remains astable phase with a negligible rate of transformationunless heated to high temperatures()1200 8C). Good-adhering diamond films can be deposited on severaldifferent substrates, the most common being silicon,molybdenum, tungsten, platinum, and quartz. Twoimportant considerations, when selecting a substrate, are(i) tolerance of the high deposition temperature(700–900 8C), and (ii) similarity in the thermal expansioncoefficient with that of diamond(1.1=10 K ). Twoy6 y1

general source gas mixtures are routinely used fordeposition, which produce diamond thin-films with dif-ferent morphologies. First, methaneyhydrogen gasmixtures can be used to produce microcrystalline dia-mond thin-films. This film, as seen in the scanningelectron micrographs shown in Fig. 1a,b, possesses awell-faceted, polycrystalline morphology with a nominalcrystallite size of;2 mm, or greater. The cross-sectionSEM reveals a columnar growth structure, which is thetypical morphology of films deposited from methaneyhydrogen source gas mixtures and reflects the van der

Drift growth mechanismw4x. Typical growth conditionsfor such films are a 0.3–1.0% methaneyhydrogen volu-metric ratio, 35–65 torr, and 700–9008C. The sourcegas mixture is activated by either a plasma(microwave-assisted CVD) or thermal energy(hot-filament CVD)to form reactive radical species in close proximity tothe substrate surface. These radical species chemisorbon the substrate surface and react to form sp -bonded3

diamond, through a complex nucleation and growthmechanismw5,6x. The surface atoms of the film areterminated by hydrogen, making the surface very hydro-phobic. Typical instantaneous water contact angles arein excess of 708, reflective of the hydrophobicity.Second, methaneyargon gas mixtures can be used to

produce nanocrystalline diamond thin-filmsw7,8x. Thisfilm, as seen in the scanning electron micrographs shownin Fig. 1c,d, possesses a smoother texture with a nominalfeature size of;100 nm or less. The cross-section SEMshows a smooth fracture surface rather than the columnargrowth, which suggests the nanocrystalline film doesnot grow from a few initial nuclei on the surface but,rather, grows with a continuously high renucleation ratew7–10x. The features are actually aggregates of individ-ual diamond grains, which are approximately 15 nm indiameter. Typical growth conditions for such films are a1.0% methaneyargon volumetric ratio, 100–150 torr,and 700–9008C. The surface atoms of the film are alsoterminated by hydrogen, making the surface hydropho-bic. The smooth, nanocrystalline morphology resultsfrom a very high rate of nucleation in the methaneyargon gas mixturew7–10x.

1942 M. Hupert et al. / Diamond and Related Materials 12 (2003) 1940–1949

Table 1Summary of heterogeneous electron transfer rate constants(25 8C)for some aqueous and non-aqueous redox analytes at hydrogen-ter-minated diamond thin-film electrodes

koapp

Fe(CN)y3yy46 0.01 to 0.1 cmys

Ru(NH )q3yq23 6 0.01 to 0.1 cmys

IrCly2yy36 0.01 to 0.1 cmys

Methyl viologen(MV yMV )q2 q• 0.01 to 0.1 cmys4-tert-butylcatechol 10 to 10 cmysy6 y4

Feq3yq2 10 to 10 cmysy6 y4

Ferrocene 0.01 to 0.1 cmys

In order to have sufficient electrical conductivity forelectrochemical measurements(-0.1V-cm), diamondfilms must be doped with boron at a concentration of1=10 cm or greater, as this is the most useful19 y3

dopant at present. This is most often accomplished byadding controlled amounts of diborane or trimethylboronto the source gas mixture, although achieving highlydoped films(G10 cm ) is very difficult using the20 y3

latter. Boron atoms substitutionally insert for some ofthe carbon atoms into the growing diamond lattice.These boron atoms function as electron acceptors, withan activation energy of 0.37 eV or less, depending onthe doping level, and, at room temperature, contributeto the formation of free-charge carriers(i.e. holes orelectron vacancies) w11x. The film’s electrical conductiv-ity is directly related to the carrier concentration and thecarrier mobility. Typical carrier concentrations are in therange of 10 to 10 cm with carrier mobilities18 20 y3

(holes) of 10–500 cmyV-s. Of course, the mobility is2

limited by the defect density within the film.

3. Basic electrochemical properties of diamondelectrodes

Boron-doped microcrystalline and nanocrystalline dia-mond thin-films possess a number of important andpractical electrochemical properties, unequivocally dis-tinguishing them from other commonly used sp -bonded2

carbon electrodes, such as glassy carbon, pyrolyticgraphite, and carbon pastew1,2x. These properties are(i) a low and stable background current, leading toimproved signal-to-background(SBR) and signal-to-noise(SNR) ratios; (ii) a wide working potential win-dow in aqueous and non-aqueous media;(iii ) superbmicrostructural and morphological stability at high tem-peratures(e.g. 1808C) and current densities(e.g. 0.1–10 Aycm , 85% H PO); (iv) good responsiveness for2

3 4

several aqueous and nonaqueous redox analytes withoutany conventional pretreatment;(v) weak adsorption ofpolar molecules, leading to improved resistance to elec-trode deactivation and fouling;(vi) long-term responsestability (e.g. months during air exposure); and (vii)optical transparency in the UVyVis and IR regions ofthe electromagnetic spectrum, useful properties for spec-troelectrochemical measurements.Detailed cyclic voltammetric studies of a large number

of aqueous(e.g. Fe(CN) , Ru(NH ) , methyly3yy4 q2yq36 3 6

viologen, chlorpromazine, ascorbic acid, catechol, 4-methylcatechol, 4-tert-butylcatechol, dopamine,Fe , hydrazine and azide) and nonaqueous(e.g.q2yq3

ferrocene) redox systems have been performed usingboth types of electrodesw12–16x. For all the analytes atthe highest quality diamond films, whether they bemicrocrystalline or nanocrystalline,(i) the oxidationpeak currents vary linearly with the scan rate(r )1y2 2

0.99), reflective of a reaction rate limited by semi-

infinite linear diffusion, and(ii) the oxidation peakcurrents vary linearly with the concentration between 1mM and 1mM (r )0.99). Using these redox systems,2

with vastly differentE8 values, it has been shown thatmoderately(;10 cm ) to heavily (;10 cm )19 y3 21 y3

boron-doped films behave as a semimetal, rather than asemiconductor, at least from potentials ofq1.5 to –1.6V vs. SCE, and that the grain boundaries and nondia-mond sp -bonded carbon impurities are not the sole2

active sitesw12–16x. In other words, the boron-dopeddiamond is electrically conductive and active electro-chemically. A summary of the calculated and simulatedheterogeneous electron transfer rate constants, fromcyclic voltammetricDE –n trends, is presented in1y2

p

Table 1.The redox systems, Ru(NH ) , methyl viologen,q2yq3

3 6

chlorpromazine, and ferrocene, proceed by an outer-sphere electron-transfer mechanism, and the electrodekinetics are relatively insensitive to the physicochemicalproperties of diamond. Apparent heterogeneous electrontransfer rate constants,k between 0.01 and 0.1 cmyso

app,

are commonly observed for both microcrystalline andnanocrystalline filmsw12–16x Fe(CN) proceedsy3yy4

6

through a more inner-sphere electron-transfer pathway,and the electrode kinetics are highly sensitive to thesurface termination.k values ranging from 0.01 to 0.1o

app

cmys are commonly observed for clean, hydrogen-terminated films, but the rate constants decrease by overtwo orders of magnitude for oxygen-terminated filmsw17x. Fe is also of the inner-sphere type, withq2yq3

k being very sensitive to the surface carbon–oxygenoapp

functionalities(e.g. carbonyls), at least on glassy carbonw18x. These functional groups are absent on hydrogen-terminated diamond, and this is presumably the reasonk is low, in the range of 10 to 10 cmys. Theo y4 y6app

more complicated organic systems, dopmaine, 4-meth-ylcatechol, and 4-tert-butylcatechol, are inner-spheresystems withk values of 10 to 10 cmys ato y4 y6

app

hydrogen-terminated diamond. It is postulated that theslow kinetics for these polar aromatic analytes is causedby weak adsorption on the hydrogen-terminated dia-mond surface.


Fig. 2. Cyclic voltammetrici–E curves for boron-doped microcrystalline and nanocrystalline diamond thin-films deposited on Si in(a and b) 1M KCl and (c and d) 1 mM Fe(CN) q1 M KCl. Scan rates0.1 Vys.y3yy4

6

Cyclic voltammetry is a commonly used electrochem-ical technique in which the potential applied to theworking electrode is changed linearly, in time, from aninitial value to a switching value, and back to the initialvalue, with the resulting current being measured. Thereare two kinds of current—background and Faradaic. Thebackground current results from the charging of theelectric double layer, electrochemical reactions on theelectrode surface, and the electrolysis of any adventitioussolution impurities. The Faradaic current is the one ofanalytical interest and is associated with the redoxactivity of the solution analytew3x. Background cyclicvoltammetrici–E curves for boron-doped microcrystal-line and nanocrystalline diamond thin-film electrodes in1 M KCl (0.1 Vys) are shown in Fig. 2a,b. Featurelessresponses with low background currents are seen forboth types of diamond. The background current for thenanocrystalline film is slightly larger than that for themicrocrystalline film, presumably due to a higher dopinglevel, but both are about a factor of 10 lower than forglassy carbon. The lower background currents lead toimproved SBR in electroanalytical measurements. Thecurves also reveal the wide working potential windowcharacteristic of both types of diamond—;3.1 V.

Fig. 2c,d show cyclic voltammetrici–E curves for 1mM Fe(CN) q1 M KCl at boron-doped micro-y3yy4

6

crystalline and nanocrystalline diamond thin-films,respectively. The scan rate is 0.1 Vys. Well-defined,symmetric curves are seen for both electrodes with peakseparations,DE , of 65–75 mV.E is 270 mV vs.p py2

AgyAgCl and i yi is 1 for both electrodes. The lowox redp p

DE reflects relatively rapid electrode-reaction kinetics.p

The k of this particular redox analyte is highlyoapp

sensitive to the physical and chemical properties of thediamond surfacew17x. The fact that the kinetics arerapid, even though the electrodes were exposed to thelaboratory atmosphere and no pretreatment was applied,attests to the electrodes’ responsiveness and resistanceto fouling.Fig. 3 shows a typical cyclic voltammetrici–E curve

for 1 mM ferrocene in acetonitrile containing 0.1 MTBAClO at a microcrystalline diamond thin-filmw16x.4

The scan rate was 0.1 Vys. The background voltam-mogram at the same scan rate is also presented, forcomparison. A well-defined response is seen at thisuntreated diamond electrode with a peak potential sep-aration,DE , of 74 mV. TheE is 0.440 V vs. Agp py2


Fig. 3. Cyclic voltammetrici–E curve for a boron-doped microcrys-talline diamond thin-film deposited on Si in 1 mM ferroceneq0.1 MTBAClO . Scan rates0.1 Vys.4

QRE andi yi is 1. The background current, at theox redp p

same scan rate, is very low, leading to a large SBR.

4. Factors affecting the electrochemical response

Several factors can affect the diamond electroderesponse, including the surface cleanliness, the dopinglevel and type, the presence of non-diamond sp carbon2

impurities, and the surface termination. Surface cleanli-ness is an important parameter influencing the respon-siveness of all electrodes, diamond and metals alike. Ina very general way, adsorbed contaminants can eitherblock specific surface sites, thus inhibiting surface-sensitive redox reactions, or increase the electron-tun-neling distance for redox analytes, thereby lowering theprobability of tunneling(i.e. the rate of electron trans-fer). The hydrogen-terminated surface is, generally, themost active and gives the most reproducible response.It is not as susceptible to contamination(air or solution-borne contaminants), as other electrodes are, because ofthe hydrophobic surface and the absence ofp electrons.Hydrogen-terminated diamond thin-film electrodes canbe effectively cleaned, when necessary, by simply soak-ing in ultraclean(i.e. distilled) isopropanol for a periodof 15–20 minw19x. Our normal protocol for preparingelectrodes after deposition initially involves chemicalcleaning for 30 min each in warm(i) 3:1 HNO yHCl3

(vyv) and (ii) 30% H O yH O (vyv), followed by2 2 2

rehydrogenation in a hydrogen microwave plasmaw12x.

5. Electroanalytical applications

CVD diamond provides electrochemists with a newtype of carbon electrode that meets the requirements ofresponsiveness, conductivity, and stability for a wide

range of applications. Diamond offers advantages overother electrodes, especially sp -bonded carbon elec-2

trodes, in terms of linear-dynamic range, limit of detec-tion, response time, response precision, and responsestability.The growing importance and utility of these materials

is reflected by the fact that there are now at least fourcommercial suppliers of diamond electrodes. Using elec-trochemical methods of analysis, flow-injection analysiswith amperometric detection(FIA-EC), ion chromatog-raphy with amperometric detection(IC-EC), and high-performance liquid chromatography with amperometricdetection(LC-EC), our group, and others, have dem-onstrated superior electrode performance for diamond,as compared with glassy carbon(GC), for the detectionof azide w14,20x, metal ionsw14,21x, nitrite w14x, dopa-mine w14,22,23x, chlorpromazinew14x, hydrazine, bio-genic aliphatic polyaminesw24–26x, NADH w27x, uricacid w28x, histamine and serotoninw29x, and carbamatepesticidesw30x.One of the first demonstrations of diamond’s useful-

ness in electroanalysis was the oxidative detection ofazide anion in aqueous mediaw20x. Sodium azide iswidely used commercially and formerly was used as aninflator in automotive airbags. Azide anion is highlytoxic and presents a health hazard at relatively modestlevels, so there is a need for sensitive and stableanalytical methods for its detection. Industries producingor using azide generally have tight controls on the anionlevels the water discharge. Moreover, as an ever-increas-ing number of automobiles containing azide-based infla-tors in their airbags are retired to salvage yards, theincidence of ground-water contamination is moreprobable.Microcrystalline diamond provides a sensitive, repro-

ducible, and stable voltammetric response for the elec-trooxidation of azide. A well-defined oxidation peak,E , is observed in the cyclic voltammogram(pH 7.2oxp

phosphate buffer) at 1100 mV vs. AgyAgCl with abackground-corrected peak current,i , of 88 mA (seeox

p

Fig in ref. w20x). Most importantly, the oxidation currentfor azide, at this positive potential, is recorded on a lowand unchanging background signal. In contrast, the azideoxidation current for glassy carbon is recorded on alarge and rising background current. The wide workingpotential window for diamond allowed the measurementof the azide oxidation current without interference fromthe oxygen evolution current(solvent electrolysis), instark contrast to what was observed for glassy carbon.Also, there is no microstructural or morphological alter-ations of diamond at the azide oxidation potential, sothe response stability is superb.Azide anion was also detected by flow-injection

analysis with electrochemical detection(FIA-EC) (i.e.amperometric mode). Table 2 shows a comparison ofdata for diamond and glassy carbonw20x. Clearly,


Table 2Summary of FIA-EC data for azide at diamond and glassy carbonw20x

Diamond Glassy Carbon

Dynamic range(mM) 0.30–3300 1.0–3300Sensitivity(nAymM) 33"5 36"7Detection limit(nM) SyNs3 8"8 (0.3 ppb) 50"20 (2.1 ppb)Reproducibility(% RSD) 0.5–5 6–20Stability (response loss 5% 50%over 12 h)

diamond outperforms glassy carbon, in terms of linear-dynamic range, limit of detection, response precision,and response stability.Another electroanalytical application of microcrystal-

line diamond is the detection of carcinogenic pyrenederivatives w31x. 1-Nitropyrene (1-NP) is the majornitroarene observed in diesel exhaust and one of thefrequent products of atmospheric reactions of polycyclicaromatic hydrocarbons(PAHs) with nitrous oxidesw32x.It is a direct acting mutagen capable of generatingmetabolic species(in vivo), which bind to DNA andare themselves mutagenicw33x. These include com-pounds such as 1-aminopyrene(1-AP) and 1-hydroxy-pyrene(1-OHP) w33x. Therefore, great attention is paidto their monitoring in the environment.Hydrodynamic voltammograms, using flow-injection

analysis with amperometric detection, were recorded atmicrocrystalline diamond for 50mM concentrations of1-AP, 1-OHP, and 1-NP in CH OHq0.1 M acetate3

buffer, pH 4.5(20:80). 1-OHP and 1-AP were oxidized,starting at potentials approximately 300 mV vs. AgyAgCl, and a maximum current was reached near 900mV. The reduction of 1-NP began approximatelyy900mV and a maximum was reached neary1500 mV.The electrochemical detection of the analytes, coupled

with reversed-phase liquid chromatography, was inves-tigated as a function of the organic modifier and pH.The optimum mobile-phase composition was 80%(vyv) acetate buffer(20 mM, pH 4.5) and 20% methanolat a 0.8 mlymin flow rate. Both UVyVis absorption andelectrochemical detection were compared. Diamond out-performed both glassy carbon and HOPG, in terms oflinear-dynamic range, limit of detection, response pre-cision, and response stability. Typical detection figuresof merit for 1-AP and 1-OHP at diamond were,(i) alinear-dynamic range from 0.2–200mM, (ii) a limit ofdetection of 0.3mM (20 ml inj., SyN G3), (iii ) aresponse variability of approximately 2% over 14 injec-tions, and(iv) a response stability of better than 91%,during 8 h of daily operation over a 7-day period.

6. Electrocatalysis

Two technologies requiring dimensionally stable cat-alytic electrodes are fuel cells and electrolytic reactors

for water disinfection and decontamination. In particular,there is a need to develop advanced catalyst-supportmaterials that can stably function—that is, withoutmicrostructural or morphological degradation or lostcatalytic activity—at the elevated temperatures(150–200 8C) in aggressive chemical environments. sp -2

bonded carbon is the most-often-used support material,with three forms normally employed—activated carbon,carbon black, and graphitic materials. A limitation ofthese supports is their susceptibility to corrosion, in thepresence of oxygen or in aggressive chemical environ-ments, at elevated temperature. Support corrosion causeslost catalytic activity, due to catalyst detachment andaggregation, catalyst fouling due to the production ofCO as a gasification product, and general mechanicalfailure of the electrode. For example, in alkaline media(e.g. fuel cell), carbon supports can degradeycorrodeaccording to the following reaction:

y 2y yCq6OH ™CO q3H Oq4e3 2

Not only does this reaction damage the carbon micro-structure, but the formation of carbonates is destructiveto the electrolyte and causes electrode foulingw34x. Thedegradation process is less damaging structurally; but,equally catastrophic, catalytically, are the microstructuralchanges that occur due to exfoliation and oxidation—changes that occur over a wide potential range in bothacid and base. These changes affect the carbon–metalparticle contact, extent of aggregation, and the accessi-bility of the catalyst particles in the support pores(so-called hidden catalyst effect).We have found that boron-doped microcrystalline

diamond thin-films are superb catalyst supportyhostmaterials because of high electrical conductivity(;0.01V-cm), dimensional stability, and the fact that catalystparticles can be stably anchored into the surface micro-structure during deposition. Nanometer-sized(10–500nm) particles of Pt, for example, can be incorporatedinto diamond by a sequential diamond depositionymetalelectrodepositionydiamond deposition procedurew35–37x. The process results in a conductive, dimensionallystable carbon electrode containing Pt particles of some-what-controlled composition, size, and catalytic activity.Fig. 4a shows atomic force micrographs of a compositeelectrode, which reveal the Pt particles decorating thesurface. The Pt particles range in diameter from 30 to300 nm with a distribution of approximately 2=109

cm . Apparent loadings range from 25 to 100mgyy2

cm . The advantages of the composite electrode, com-2

pared to commercial Pt-impregnated sp carbon2

electrodes, are(i) the dimensional stability of the dia-mond support, leading to highly stable reaction centers,even at high-current densities(1–10 Aycm ) and tem-2

peratures(150–2008C); and(ii) the fact that all of themetal catalyst is available at the electrode surface and


Fig. 4. (a) AFM images of a Ptydiamond composite electrode deposited on Si. An image of the diamond surface without any electrodepositedPt is also shown, for comparison.(b) Cyclic voltammetrici–E curve for a Ptydiamond composite electrode in contact with oxygen-saturated 0.1M HClO . Scan rates0.050 Vys. The background voltammetric curve for a bare diamond electrode is also shown, for comparison.4

not inside pores. These electrodes exhibit good perform-ance for the oxidation of methanol and the reduction ofoxygen, two important fuel-cell reactionsw35–37x. Fig.4b shows a cyclic voltammetrici–E curve for a Ptydiamond composite electrode in contact with O -satu-2

rated 0.1 M HClO . Ani–E curve for bare diamond is4

also shown, for comparison. An oxygen-reduction peakcurrent of approximately 200mA (1 mAycm ) is seen2

at 260 mV. Clearly, the oxygen-reduction current is dueto the catalytic activity of the incorporated Pt particles,as no reduction current is observed for the bare diamondelectrode, even at potentials as negative asy750 mV.These composite electrodes are being prepared on high

surface area metal meshes for use in H –O fuel cells2 2

and water electrolyzers.

7. Spectroelectrochemistry

Spectroelectrochemical measurements involve thecombination of electrochemistry and spectroscopy tostudy electrochemical reactionsw38x. The simplestexperiment is to pass a beam of electromagnetic radia-tion through an optically transparent electrode(OTE)and to measure the changes in the optical properties(e.g. absorbance) of a chemical entity consumed orproduced in an electrochemical reaction. Electrically


Fig. 5. Absorption-time curves at 252 and 285 nm recorded for anoptically transparent diamond disk electrode during the oxidation of1 mM ferroceneq0.5 M TBAClO . The potential was stepped from4

0 to 700 and back to 0 mV vs. AgQRE.

conducting diamond is a new type of optically transpar-ent electrode that has only recently begun to be studiedw39–41x. The material has a wide optical windowranging from the near-UV to the far-IR. Depending onthe defect density, chemical composition, doping level,thickness, grain size, etc., certain portions of the spec-trum are opaque, but regions in the visible(300–900nm) and the far-infrared(-1500 cm ) are transparent,y1

even for boron-doped films. Moreover, the optical prop-erties can be manipulated through adjustments in thefilm deposition conditions. The wide optical window,coupled with the interesting electrochemical properties,make diamond a viable, new OTE.Diamond OTEs offer a number of advantages over

traditional materials, such as indium-doped tin oxide(ITO): (i) stability in acidic media and chlorinatedorganic solvents,(ii) the ability to withstand cathodicpolarization,(iii ) a reasonably well-defined and stablesurface chemistry,(iv) transparency in the UV, visibleand IR regions of the electromagetic spectrum,(v) awide working potential window in excess of 3 V inaqueous media, and(vi) a low and stable backgroundcurrent, which leads to improved SBR in electroanalyt-ical measurements. These materials hold great promise,for example, in bioelectrochemical research, due to thelow background current, wide working potential win-dow, and broad range of optical transparency.Our initial work in this area involved the use of a

free-standing(0.38 mm thick and 8 mm in diameter)diamond disc that was mechanically polished to a 7 nmrms roughness over a 10mm linear distance, and boron-doped(0.05% ByC in the reactant gas mixture) w39x.The electrode had a short wavelength cut-off of approx-imately 225 nm, which is the indirect bandgap of thematerial, and transmitted light out to at least 1000 nm(15–30%). The electrode was used to oxidize ferrocy-anide to ferricyanide and the absorbance change asso-ciated with the formation of the oxidized product(l s420 nm) was spectroscopically monitored. Themax

electrode was also used to reduce methyl viologen(MV ) to the cation radical(MV ) and the neutralq2 q•

(MV ). The depletion of MV (l s257 nm) and0 q2max

formation of MV (l s398 and 605 nm) wereq•max

spectroscopically monitored.Fig. 5 shows absorbance-time profiles recorded during

a spectroelectrochemical measurement of 1 mM ferro-ceneq0.5 M TBAClO yCH CN, using the diamond4 3

disk. The absorbance data were recorded in a thin-layerspectroelectrochemical cell, during slow-scan cyclic vol-tammetry(2 mVys) from 0 to 700 and back to 0 mV.As the potential becomes more positive, the oxidationof ferrocene to ferricenium occurs and the absorbancesat 252 and 285 nm increase. The two absorbances arelikely related to charge-transfer process between themetal center and the cyclopentadienyl ring. A maximumabsorbance is reached once all of the ferrocene in the

thin-layer cell has been oxidized to ferricenium ion. Ofcourse, the absorbance signal decreases when ferricen-ium is reduced back to ferrocene during the reversescan. The absorbance presented is an absolute value,uncorrected for the background absorbance. Additional-ly, IR spectroelectrochemical measurements were made,and a shifting absorbance feature from 823 to 857cm upon oxidation was clearly observed.y1

Martin and Morrison described the application of aconducting diamond film as a transparent electrode forin situ attenuated total-reflectance infrared spectroscopyw40x. The electrode consisted of a polycrystalline dia-mond film (4–6mm) deposited on a silicon wafer(50mm). The electrode was pressed onto a ZnSe ATRcrystal with the silicon side in contact with the crystal,and the diamond surface in contact with an electrolytesolution. The authors used IR to determine the types offunctional groups formed during electrochemical polar-ization. Two bands developed during anodic polarizationin H SO at 3240 cm (O–H stretch) and 1100 cmy1 y1

2 4

(C–O stretch). The features changed with the polariza-tion potential and the time at a particular potential(i.e.charged passed).More recently, our group has investigated the optical

and electrochemical properties of optically transparent,boron-doped diamond thin-films, deposited on quartzw41x. The OTE(300–800 nm, 40–50% transmittance)was characterized by cyclic voltammetry, atomic forcemicroscopy, and UV–Vis absorption spectrophotometry.The film was deposited for 1 h, using a 0.5% CyH ratioat 45 torr and 600 W of microwave power. A high rateof nucleation was achieved by mechanically scratchingthe quartz. This pretreatment led to the formation of athin, continuous film containing nanometer-sized grains


Fig. 6. Cyclic voltammetrici–E curves for a boron-doped microcrys-talline diamond thin-film deposited on Si in 100mM horse heart cyto-chromecq50 mM NaCl and 1 mM Tris buffer, pH 7.0. Scan rates0.020 Vys.

of diamond (i.e. reduction of scattering losses). Thefilm’s electrochemical response was evaluated withRu(NH ) q1 M KCl, Fe(CN) q1 M KCl,q3yq2 y3yy4

3 6 6

and chlorpromazineq10 mM HClO . The film exhibited4

a low voltammetric background current and a stable andan active voltammetric response for these redox systems.The optical transmittance of the polycrystalline film inthe visible region was 35–50%, while in a thin-layerspectroelectrochemical cell, and was fairly constant overthe range from 250 to 800 nm. The optical and electricalproperties were extremely stable during potentiodynamiccycling in aggressive chemical environments, exposureto chlorinated organic solvents, and exposure to temper-ature extremes. The spectroelectrochemical response forchlorpromazine was monitored in the transmissionmode. A stable, Nernstian response was observed withlinear-dynamic range from 20 to 1000mM (r )0.99)2

and a limit of detection of 0.5mM (SNRs3).

8. Bioelectrochemistry

Good-quality, hydrogen-terminated, microcrystallineand nanocrystalline diamond thin-film electrodes exhibitproperties well suited for protein electrochemical studies,such as:(i) a low and stable background current over awide potential range,(ii) a wide working potentialwindow, and(iii ) a resistance to fouling, due to weakadsorption of polar molecules on the nonpolar surface.The electrodes, both microcrystalline and nanocrystal-line, have been used to study the direct electron transferof horse-heart cytochromec w42x. Fig. 6 shows a typicalcyclic voltammetrici–E curve for 100mM cytochromec in 50 mM NaClq1 mM Tris HCl buffer, pH 7.0 atan untreated microcrystalline diamond. The scan rate

was 20 mVys.DE is 105 mV,E is 73 mV (268 mVp py2

vs. NHE), and i yi is 1. The peak currents changedox redp p

linearly with the concentration, and, importantly, therewas no electrode fouling. The heterogeneous electrontransfer rate constant was estimated to be 1.1=10y3

("0.2) cmys. This is an interesting result, consideringthat most reported measurements of cytochromec elec-trochemical kinetics have indicated the necessity of ahydrophilic, negatively charged, and oxygen-rich elec-trode surface. The background voltammetric curve,measured after exposure to the protein solution, is alsoshown, for comparison. The background current is gen-erally low over the entire potential range, as is charac-teristic of diamond, except at negative potentials wherea cathodic current begins to flow. We believe that thiscurrent is associated with the reduction of residualdissolved oxygen—a reaction that is catalyzed by theadventitious adsorption of some of the protein mole-cules. In particular, exposed Fe centers could be thecatalytic sites, as the catalytic reduction of oxygen byvarious metal macrocycles, such as Fe porphyrins, iswell known w43x.

9. Conclusions

Diamond offers significant advantages over otherelectrodes, in particular, sp -bonded carbon electrodes,2

in terms of linear-dynamic range, limit of detection,response time, response precision, and response stability.A number of electrochemical technologies could benefitfrom the use of this electrode material: electroanalysis,electrocatalysis, spectroelectrochemistry, and bioelectro-chemistry. Some important areas of future work includegrowth and characterization of diamond films depositedon fiberous substrates of 10mm diameter, or less, andhigh surface area metal meshes; chemical modificationof diamond surfaces to control adsorption and electron-transfer kinetics; and patterning of electrically conduc-tive diamond electrodes into microarray geometries.

Acknowledgments

Financial support for the research was provided bygrants from the National Science Foundation(CHE-9983676), Department of Energy (DE-FG03-95ER14577), and Department of Agriculture(NRICGP2001-35102-10045). The free-standing, optically trans-parent diamond disc electrode was generously providedby Dr James Butler(NRL).

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