the physicochemical and electrochemical properties of 100 and 500 nm diameter diamond powders...

doi: 10.1149/1.29583082008, Volume 155, Issue 10, Pages B1013-B1022.J. Electrochem. Soc.

Ayten Ay, Vernon M. Swope and Greg M. Swain Nanocrystalline Diamond500 nm Diameter Diamond Powders Coated with Boron-Doped The Physicochemical and Electrochemical Properties of 100 and

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Journal of The Electrochemical Society, 155 �10� B1013-B1022 �2008� B1013

The Physicochemical and Electrochemical Properties of 100and 500 nm Diameter Diamond Powders Coated withBoron-Doped Nanocrystalline DiamondAyten Ay, Vernon M. Swope, and Greg M. Swain*,z

Department of Chemistry and the Fraunhofer Center for Coatings and Laser Applications, Michigan StateUniversity, East Lansing, Michigan 48824-1322, USA

There is a need for advanced, corrosion-resistant electrocatalyst support materials for use in fuel cells. To this end, electricallyconducting diamond powder was prepared by depositing a layer of boron-doped nanocrystalline diamond on 100 and 500 nm diamdiamond powders. The doped layer was deposited by microwave plasma-assisted chemical vapor deposition using an Ar-richCH4/H2/Ar/B2H6 source gas mixture. After coating, the 100 nm doped diamond powder had a specific surface area of 27 m2/gand an electrical conductivity of 0.41 S/cm. The 500 nm doped diamond powder had a specific surface area of 8 m2/g and anelectrical conductivity of 0.59 S/cm after coating. The specific surface area of both powders decreased by ca. 50% after diamondcoating due mainly to particle–particle fusion. The electrical measurements provided conclusive evidence for a doped diamondoverlayer as the uncoated powders possessed no electrical conductivity. Furthermore, the fact that the electrical properties wereunaltered by acid washing confirmed that the conductivity arises from the doped diamond overlayer and not any adventitious sp2

carbon impurity on the particle surface, which is removed by such chemical treatment. Scanning electron microscopy images andRaman spectroscopy yielded further evidence in support of a nanocrystalline diamond overlayer. Both powders exhibited electro-chemical responses for Fe�CN�6

3−/4−, Ir�Cl�6−2/−3, and Fe+2/+3 that were comparable to typical responses seen for high-quality,

boron-doped nanocrystalline diamond thin-film electrodes. The electrochemical behavior of the powders was assessed using apipette electrode that housed the packed powder with no binder. The 100 nm doped diamond powder electrodes were moreplagued by ohmic resistance effects than were the 500 nm powder electrodes because of reduced particle contact. Importantly, itwas found that the doped diamond powder electrodes are dimensionally stable and corrosion-resistant during anodic polarizationat 1.4 V vs Ag/AgCl �1 h� in 0.5 M H2SO4 at 80°C. In contrast, glassy carbon powder polarized under identical conditionsunderwent significant microstructural degradation and corrosion.© 2008 The Electrochemical Society. �DOI: 10.1149/1.2958308� All rights reserved.

Manuscript submitted May 5, 2008; revised manuscript received June 23, 2008. Published August 8, 2008.

0013-4651/2008/155�10�/B1013/10/$23.00 © The Electrochemical Society

Research with high-surface-area carbon electrode materials hasincreased in recent years for a number of different applications:electroanalysis,1 electrocatalyst supports for fuel cells,2,3 storagematerials for batteries and electrochemical double-layer capacitors,4

and stationary phases for liquid chromatography.5-7 The carbon per-formance critically depends on the physical, chemical, and elec-tronic properties of the material. Boron-doped diamond electrodeshave attracted attention in recent years because of their outstandingphysiochemical, electronic, and electrochemical properties.8-14 Oneparticular property of relevance for electrocatalysis is the superbmicrostructural stability and corrosion resistance at anodic potentialsand high current densities.15-18 Diamond thin-film electrodes can beroutinely prepared by chemical vapor deposition �CVD�, are com-monly used in electrochemical studies, and are available commer-cially. Much of what is known about structure-function relationshipsat diamond electrodes has come from studies using thin-filmelectrodes.8-14,19-22 Unfortunately, these electrode geometries havelow surface area and do not lend themselves to use in electrochemi-cal energy storage and conversion.

High specific surface area forms ��100 m2/g� of diamond arecommercially available as powders and can be obtained with nomi-nal diameters from 10 �m down to 2 nm. However, none possessesinherent electrical conductivity. Any conductivity that exists in mostof these powders can be attributed to nondiamond sp2 carbon impu-rity present on the powder surface. Because of this, the electricalconductivity becomes highly dependent on any disruption of thissurface impurity phase, such as by electrochemical, chemical, orthermal oxidation. These surface phases are no more stable thanbulk-phase sp2 carbon materials. We recently reported on the over-coating of insulating diamond powder with a layer of boron-dopeddiamond.23,24 In this core-shell approach, a boron-doped microcrys-talline diamond layer was deposited on 8–12 �m diam diamondpowder particles using microwave CVD and a conventional CH4/H2source gas mixture. The electrical conductivity is controlled by the

* Electrochemical Society Active Member.z E-mail: [email protected]

address. Redistribu128.206.9.138Downloaded on 2013-03-06 to IP

doped-diamond layer �carrier concentration and mobility� ratherthan by some adventitious nondiamond sp2 carbon impurity phase.This was the first step in our development of a high-surface-area,electrically conducting form of diamond. In follow-up work, a rela-tively high surface area Toray carbon paper was modified with alayer of boron-doped nanocrystalline diamond.25 The coated paperexhibited better dimensional stability during anodic polarizationthan did the uncoated paper. Other work from our group has dem-onstrated that metal catalyst/diamond composite electrodes can beformed that exhibit good electrocatalytic activity and excellent di-mensional stability.26-28 The ultimate targets for an electrocatalystsupport are a specific surface area �100 m2/g and electrical con-ductivity �10 S/cm. In our earlier work, the specific surface areaand electrical conductivity of the doped diamond powder were be-low these targets: 2 m2/g and 1 S/cm.23 Electrically conductive dia-mond powder has also recently received attention by other research-ers as a possible electrocatalyst support material for fuel cells andelectroanalysis,29-32 and as an electrode for electrochemical double-layer capacitors.33 There have been other reports of powder elec-trodes prepared with high-surface-area nanodiamond particles.34-37

Boron-doped diamond powder has been prepared by crushing a free-standing boron-doped diamond film,29,30 and by surrounding deto-nation nanodiamond powder particles with a graphite shell.33 Therehave also been reports on the use of electrically insulating diamondpowders as catalyst supports for gas-phase chemical reactions.38-42

We report at present on the preparation of higher specific surfacearea ��50 m2/g� and electrically conducting ��1 S/cm� diamondpowders prepared by coating 500 and 100 nm diam particles with alayer of boron-doped nanocrystalline diamond. The nanocrystallinediamond was deposited from an Ar-rich source gas mixture consist-ing of 1% CH4, 5% H2, and 94% Ar with B2H6 used for borondoping. The physical, chemical, and electrochemical properties ofboron-doped nanocrystalline diamond thin films have been de-scribed elsewhere.10,43 Smooth, nanocrystalline diamond is formedin these Ar-rich mixtures because of the high rate of renucleationthat is achieved by virtue of the gas-phase composition and condi-tions. The Argonne group pioneered the development and applica-tion of this type of nanocrystalline diamond, and they have reported

www.esltbd.orgtion subject to ECS license or copyright; see

B1014 Journal of The Electrochemical Society, 155 �10� B1013-B1022 �2008�B1014

extensively on its preparation and properties.43-46 Because of thehigh renucleation rate, nanocrystalline diamond is a more practicalcoating for substrate powders than is diamond deposited fromH2-rich CH4/H2 source gas mixtures. The powders were character-ized before and after conductive diamond-deposition by scanningelectron microscopy �SEM�, Raman spectroscopy, X-ray diffraction,and Brunauer–Emmett–Teller �BET� surface area and electrical con-ductivity measurements. Additionally, the electrochemical responseof the powders for Fe�CN�6

−3/−4, IrCl6−2/−3, and Fe+3/+2 was evaluated

using a pipette electrode.34 Electrochemical characterization is notstraightforward when the electrode material is powder. Generally,some method for anchoring the particle in place is required. Forexample, composite electrodes can be prepared using a polymerbinder or other chemical, like paraffin oil.47-49 Polymeric binderstend to reduce the particle–particle contact, thereby increasing theelectrode resistance. Furthermore, the mixing, pressing, and heatingsteps needed for electrode preparation are laborious and time-consuming. This pipette electrode architecture enables electrochemi-cal characterization of the doped diamond powders without the needfor a binder.

Finally, the microstructural stability and corrosion resistance ofthe conductive diamond powder was evaluated during anodic polar-ization at 1.4 V vs Ag/AgCl and 80°C in 0.5 M H2SO4. This po-tential was selected because such values can be briefly experiencedin fuel cell stacks during start-up under H2-starved conditions. Un-der these conditions, H2 and O2 would exist at the anodesurface.50-53 Carbon corrosion �gasification� can occur under theseconditions that is significant enough to produce lost catalyst activityand increased ohmic resistance. For comparison, 3–4 �m diamglassy carbon �GC� powder was also tested. The powders wereevaluated by cyclic voltammetry and SEM before and after anodicpolarization. The key findings from this work are that �i� 100 and500 nm diam diamond powder particles can be coated with a layerof boron-doped nanocrystalline diamond to produce a relatively highspecific surface area material, �ii� electrical conductivity can be im-parted to the powders by coating with the doped diamond overlayer,�iii� the electrochemical properties of the nanocrystalline diamond-coated powders are comparable to the behavior of thin films of thematerial, and �iv� the nanocrystalline diamond exhibits excellentmicrostructural stability and corrosion resistance during anodic po-larization in acid.

Experimental

Boron-doped diamond growth.— The boron-doped diamondoverlayer was grown on insulating 100 and 500 nm diam diamondpowders �Tomei Diamond Co., Cedar Park, TX� by microwaveplasma-assisted CVD �1.5 kW ASTeX, Inc., Lowell, MA�. Figure 1illustrates the concept of overcoating electrically insulating diamondpowder with a conductive layer of doped diamond in a core-shellarrangement. This was accomplished by spreading approximately50 mg of insulating diamond powder on a silicon wafer in the CVD

Figure 1. �Color online� Illustration of the core-shell concept for coatingdiamond powders with a layer of boron-doped nanocrystalline diamond.


reactor. Nanocrystalline diamond was deposited from an Ar-richsource gas mixture in which the rate of renucleation greatly exceedsthe rate of crystal growth.10,44-46 The overcoating was depositedfrom a CH4/H2 + Ar source gas mixture consisting of 1% CH4, 5%H2, and 94% Ar, 10 ppm of B2H6 was added for boron doping. Themicrowave power was 800 W, the system pressure was 140 Torr,and the deposition time was 2 h. Because there was no independentcontrol of the substrate temperature �i.e., heating by the plasma�,variations in temperature existed across and within the powdersample. Postgrowth, the CH4 flow was stopped and the powdersample remained exposed to an H2/Ar plasma for �10 min. Afterthis, the power and pressure were slowly reduced over a few min-utes to cool the samples in the presence of atomic hydrogen to atemperature estimated to be below 400°C.

Diamond powder cleaning.— The diamond powder was pre-pared for deposition by first cleaning in aqua regia �3:1 HCl/HNO3�for 30 min and then in 30% hydrogen peroxide/H2O for 30 min,both at 60°C. Diamond powders often contain significant levels ofnondiamond sp2 carbon impurity on their surface, particularly in thiswork, due to the temperature variations. These chemical treatmentsoxidatively remove this impurity. Each cleaning step was followedby rinsing with ultrapure water, isopropyl alcohol, and acetone.Postdeposition, the powders were cleaned, as described above, withan additional step in a 1:1 hydrofluoric �48 wt %� and nitric acid�70 wt %� solution for 30 min at room temperature to remove anysilicon impurity.54 This cleaning is referred to as acid washing in thetext.

Raman spectroscopy and electron microscopy.— Raman spectrawere recorded in a backscattered collection geometry using a100 mW argon ion laser �Melles Griot CW� at 514.5 nm, an Olym-pus BH-2 microscope assembly, and a Spex 1250 spectrograph witha 600 grooves/mm holographic grating. The detector was a Sym-phony 2000 � 800 charge-coupled device �Horiba Jobin-Yvon�with a pixel size of 15 �m. All spectra were recorded at room tem-perature using an incident power density of approximately1.4 kW/cm2 �10 mW at the sample and 30 �m diam spot size� anda 45 s integration time. The spectrometer was calibrated using thefirst-order phonon peak for cubic diamond �a high-pressure, high-temperature-grown single-crystal sample� at 1332.6 cm−1. The pow-der morphology was probed by field-emission scanning electron mi-croscopy �JSM-6300F, JEOL, Ltd., Tokyo, Japan�. Some of the lessconducting diamond powders were sputter coated with Au to im-prove the image quality by reducing surface charging.

Specific surface area measurements.— The powder surface areawas determined by the BET method. Nitrogen adsorption isothermswere recorded at −196°C using a Micromeritics Tristar 3000 sorp-tometer following standard protocols. Approximately 300 mg pow-der sample was outgassed at 90°C and 10−6 Torr for a minimum of12 h prior to analysis. The sample, in vacuum, was exposed to dif-ferent amounts of N2 gas and the chamber pressure was measured.The greater the coverage of adsorbed gas, the lower the systempressure will be. An adsorption isotherm was generated by plotting1/v��P0/P� − 1� vs P/P0 �BET plot�. P and P0 are the equilibriumand the saturation pressures of nitrogen at the isotherm temperature,and v is the volume of gas adsorbed. Surface areas were calculatedfrom the linear part of the BET plot according to International Unionof Pure and Applied Chemistry �IUPAC� recommendations.55

Electrical conductivity.— The electrical resistance of the boron-doped diamond powder was determined by placing a fixed quantity�ca. 30 mg� of powder inside a vertical glass tube �area 0.11 cm2�.Two copper plates contacted the powder on both sides of the tube.One of these plates was small enough in diameter to fit inside theglass tube and was pressed against the powder using a 240 g weight.Contact-to-contact resistance was measured using an ohmmeter.Current–voltage curves were also generated by applying �6, �10,and �20 mA currents and recording the corresponding voltage


B1015Journal of The Electrochemical Society, 155 �10� B1013-B1022 �2008� B1015

drop. The conductivity �S/cm� was calculated from the measuredresistance, R��, and dividing by the powder thickness, l �cm�.

Electrochemical measurements.— Electrochemical characteriza-tion of the boron-doped diamond powders was performed with elec-trodes prepared by packing the powder in a plastic pipette tip �no.104 BioDot, Scientific Inc., Burton, MI�.34 The procedure involvedfirst melting the narrow end of the tube closed, and then 5–10 mg ofeither boron-doped diamond or GC powder �Sigradur G, HTWGmBH, Germany� was packed into the tube with a 1 mm diam cop-per metal rod. This metal rod also served as the current collector�see Fig. 2�. The powders were packed to a height of 6–7 mm withthe Cu wire embedded in the top of the powder. The Cu wire waspressed into the powder layer to form good ohmic contact. Theclosed end of the tip was then carefully cut open to expose thepowder with a geometric area of �0.002 cm2. The currents reportedherein were normalized to this geometric area. A CHI650a comput-erized potentiostat �CH Instruments, Inc., Austin, TX� was used tocarry out electrochemical measurements. A homemade Ag/AgClreference electrode �E = −65 mV vs a saturated calomel referenceelectrode� and a platinum wire counter electrode were used. Allsolutions were deoxygenated by purging with N2 for 20 min prior toa measurement, and the solution remained blanketed with the gasduring measurement.

Chemicals.— All chemicals were reagent-grade quality or betterand used without additional purification. All solutions were prepared

Figure 2. Diagram of the pipette electrode used for testing the electrochemi-cal properties of the conducting powders.


with ultrapure water from a Barnstead E-Pure purification system�18 M� cm�. Solutions �1 mM� of potassium ferrocyanide �Ald-rich�, potassium hexachloroiridate �Aldrich�, and ferric sulfate�Matheson Coleman and Bell� were prepared fresh daily. The sup-porting electrolytes were 1 M KCl �Fisher Scientific�, 0.5 M H2SO4�99.999%, Aldrich�, and 0.1 M HClO4 �99.999%, Aldrich�. Allglassware was cleaned by washing in an ethanol/KOH bath, analconox/ultrapure water bath, and rinsing with ultrapure water.

Results

Diamond powder morphology and microstructure.— The mor-phology of the powder before and after nanocrystalline diamonddeposition was investigated by SEM. Figure 3A shows an SEMimage of 500 nm diam powder prior to deposition. The uncoatedpowder particles are irregularly shaped with sharp, jagged edges.The facet surfaces, however, are relatively smooth. After deposition,the seed particles were covered by a diamond overlayer possessing anodular morphology, as seen in Fig. 3B. The jagged edges are nolonger present because of this nodular overlayer. This nodular mor-phology is characteristic of diamond grown from Ar-rich source gasmixtures and results from the high rate of renucleation duringgrowth.8,10,44-46 In other words, new nucleation events frequentlyhappen that limit any one of the nuclei from growing into a largecrystal. It appears in the image that much of the diamond powdersurface has been covered with the doped nanocrystalline diamondoverlayer. Particle–particle fusion, an inevitable occurrence with thisapproach, is evident in the three coalesced particles in the upper partof the image. This coalescence reduces the specific surface area butlikely improves the electrical conductivity through increases in thecarrier mobility.

It is more difficult to resolve morphological changes associatedwith nanocrystalline diamond coating on the smaller 100 nm diampowders. Images of the powder before and after coating are pre-sented in Fig. 3C and D. After coating, there is a considerable in-crease in the size of most of the seed particles with quite a bit ofvariability. Based on the variable particle size across the image,there are undoubtedly some regions where the rate of diamondgrowth is higher than others. Additionally, the larger particles alsoresult from extensive particle–particle fusion. In fact, particle coa-lescence was more apparent with the 100 nm than with the500 nm diam powder. This caused a reduction in the specific surfacearea from 53 to 26 m2/g �Table I�. A challenge with preparing con-ducting diamond powder via this core-shell approach is the balancebetween the growth time and conditions needed to achieve a near-complete coating of individual particles and the avoidance of sig-nificant particle fusion.

Figure 4 shows typical Raman spectra for a 100 nm diam dia-mond powder sample before and after a 2 h nanocrystalline dopeddiamond deposition. The spectrum for the coated 500 nm diam dia-mond powder had similar spectral features. The spectrum for theuncoated diamond powder consists of a single peak at 1332 cm−1

with a full width at half maximum �fwhm� of 12 cm−1. This is thefirst-order phonon mode for cubic diamond.56-58 There is little pho-toluminescence background and little scattering in the1500–1600 cm−1 region that is characteristic of sp2 and mixedsp2/sp3-bonded carbon impurity, when present. The fwhm is in-versely related to the phonon lifetime.58 The relatively narrow line-width for the powder compares favorably with that seen for a dia-mond standard and indicates that the powder particles are relativelydefect-free.

The spectrum for the coated powder is identical to that for nano-crystalline films deposited from Ar-rich source gas mixtures.59-64 Inaddition to the diamond peak at 1332 cm−1, which is largely buriedwithin more intense scattering in this region, the spectrum has 4additional peaks at 1150, 1350, 1450, and 1550 cm−1. Nanocrystal-line diamond grown from Ar-rich source gas mixtures is character-ized by 50–100 nm nodules consisting of individual diamond grains��15 nm� connected to one another by 2–4 atoms wide sp2-bonded



carbon at the grain boundaries.65-67 The high fraction of grainboundary gives rise to a relatively large number of sp2-bonded car-bon scattering centers. The Raman scattering cross section for thiscarbon is significantly larger than that for diamond so the spectrumis dominated by the grain boundary carbon.56-58,64,68 The one-phonon diamond line at 1333 cm−1 decreased in intensity and sig-nificantly broadened from 12 to 140 cm−1 as the film changed froma microcrystalline �0.5% CH4� to a nanocrystalline �1% CH4� mor-phology. The high grain boundary density in this film reduces thephonon lifetime, and this is manifested in an increased linewidth.58

Relative to the diamond line, the intensities of the 1150, 1470, 1550,and 1590 cm−1 peaks increased significantly for the nanocrystalline

Table I. Electrical conductivity and specific surface area of theuncoated and boron-doped diamond, and GC powders as grownand after acid washing.a

Powder sampleConductivity �S/cm�

n = 6BET �m2/g�

n = 3

100 nm powder uncoated 0 52.6 � 0.3100 nm powder coated 0.47 � 0.14 26.5 � 0.2100 nm powder coatedand acid washed

0.41 � 0.02 N/A

500 nm powder coated 0.59 � 0.05 8.0 � 0.4500 nm powder coatedand acid washed

0.45 � 0.04 N/A

GC 1.41 � 0.24 1–2Vulcan XC-72 1.87 � 0.19 250

a Values shown are means � standard deviations.


film. The peak at 1150 cm−1 is much more pronounced and it occursin tandem with the 1470 cm−1 peak.62-64 The peaks at 1550 and1590 cm−1 arise from the nondiamond sp2-bonded carbon atoms inthe grain boundaries.59-64 The origin of the 1150 and 1470 cm−1

Figure 4. Visible Raman spectra of 100 nm diam diamond powder beforeand after coating with a layer of boron-doped nanocrystalline diamond for2 h.

Figure 3. SEM images of 500 and100 nm diam diamond powder �A� and�C� before, and �B� and �D� after deposi-tion of a boron-doped nanocrystalline dia-mond coating for 2 h.



peaks has been the subject of some discussion in the literature.62-64

The 1150 cm−1 peak is often used as a diagnostic feature of nano-crystalline sp3-bonded diamond mainly because of its presence inthe spectrum.8,10,59-64 This assignment is based on the fact that thispeak is near a maximum in the vibrational density of states at ca.1200 cm−1.61-64 It is generally observed that the 1150 and1470 cm−1 peaks occur in tandem, with the 1470 cm−1 peak beingmore intense.62-64 The most seminal work understanding the originof these two peaks has come from Ferrari and Robertson, who con-cluded that the scattering at these two frequencies is not due to C–Csp3 vibrations but rather to scattering from trans-polyacetylene oli-gomers �C–C sp2 vibrations� of different conjugation lengths in thegrain boundaries.62,63 These oligomers have a mixture of sp2 and sp3

bonding and differ in length depending on the film quality. Thesevibrational modes are connected to the presence of hydrogen aspostgrowth annealing in a vacuum �desorption of hydrogen� causesthe vibrational modes to disappear. Assignment of these two peaksto C–C sp2 rather than C–C sp3 vibrations is based on the followingobservations: �i� the peak intensities decreased with increasing ex-citation energy, which is exactly the opposite of what is expected foran sp3 phase, �ii� no peaks at 1150 or 1470 cm−1 were seen fornanocrystalline diamond powders produced by detonation tech-niques, and �iii� no peak near 1150 cm−1 was seen in tetrahedralamorphous carbon films �ta-C� deposited with 80–90% sp3 carbon,as the scattering from the 10–20% sp2 carbon overshadows thatfrom the sp3 phase with visible excitation.

Specific surface area and electrical conductivity measure-ments.— The specific surface area of the 500 and 100 nm diampowders was determined by the BET method before and after boron-doped nanocrystalline diamond deposition. As shown in Table I,specific surface area of the smallest diameter diamond powder de-creased from a nominal value 53–27 m2/g after coating with dopednanocrystalline diamond. This is consistent with the SEM imageshown in Fig. 3D, which revealed extensive particle–particle fusionand a corresponding increase in the particle size. Specific surfaceareas of other powders are also shown, for comparison. For ex-ample, the coated 500 nm diam diamond powder had a nominalvalue of 8 m2/g, 3–4 �m diam GC powder had a nominal value of1–2 m2/g, and Vulcan XC-72 carbon powder had a value of250 m2/g.

In order to verify that a conductive layer formed over the dia-mond particles, electrical resistance measurements were made onthe dry powder and the calculated conductivities are presented inTable I. The uncoated diamond powder had no measurable conduc-tivity after it has been cleaned by acid washing. This is an importantpoint to mention. As-received diamond powder often possessessome electrical conductivity, but this conductivity arises because ofsp2 carbon impurity phases on the surface. Anything chemically orelectrochemically that influences the stability of this impurity phase�e.g., acid washing� will necessarily decrease the electrical conduc-tivity. In contrast, the electrical conductivity of the nanocrystallinediamond-coated powder arises from a distinctly different mecha-nism, the carrier concentration and carrier mobility within the dopeddiamond overlayer. After a 2 h nanocrystalline diamond deposition,the conductivity of the 500 and 100 nm diam powders increased tonominal values of 0.59 and 0.41 S/cm, respectively. Importantly, theconductivity was not significantly altered after acid washing, consis-tent with a chemically stable, doped diamond coating. Furthermore,this result proved that adventitious nondiamond sp2 carbon impuritythat might form on the diamond surface during deposition does notcontribute to the electrical properties. The electrical conductivitymeasurements are the most definitive data proving the existence of adoped diamond overlayer. For comparison, the nominal conductivi-ties of the GC and Vulcan XC-72 powders were determined to be1.4 and 1.9 S/cm, respectively.


Background cyclic voltammetric current density–potentialcurves.— Background cyclic voltammetric current density–potential�i-E� curves are important to record as they provide insight on thephysicochemical properties of conductive diamond powders. Thepotential limits, voltammetric features, and the magnitude of thebackground current are all sensitive to the surface microstructureand chemistry, electronic properties, and electric double-layer struc-ture. Figure 5 shows typical cyclic voltammetric background i-Ecurves in 1 M KCl for the 500 and 100 nm diam doped diamondpowder. The currents are normalized to the electrochemically activearea, as estimated from background cyclic voltammetric i-E curves.We tried to ensure maximum wetting of the powder by extendedpotential cycling between −0.5 and 1.2 V. During the initial phasesof the potential cycling period, the diamond powder surface be-comes more hydrophilic due to incorporation of surface carbon–oxygen functionalities. As a consequence, more of the powder sur-face slowly wets with the supporting electrolyte solution. Allmeasurements with the powders were not begun until the back-ground voltammetric current reached a constant level. The anodiccurrent at 0.8 V was measured �assumed to be all capacitive� and theelectrochemically active area was then calculated from ich = ACdl�.Cdl was assumed to be 5 �F/cm2.69 These stabilized curves wereobtained after a few cycles and remained unchanged with additionalcycling. This is consistent with a constant wetted area during themeasurements. Due to its electrically insulating nature, the curve forthe uncoated diamond powder exhibited zero current at all potentials�dotted line�. The background current for the 500 nm powder islower than that for the 100 nm diamond powder due to a reducedsurface area exposed to the solution. This results because of thelower specific surface area of the 500 nm diam powder and thegreater mass of the 100 nm diam powder used to prepare the pipetteelectrode. The background current for the 500 nm powder electrodeis low and featureless between −300 and 800 mV. At the anodiclimit, chlorine is presumably generated from the oxidation of chlo-ride. The origin of the cathodic current, which begins to flow at−0.3 V, is unknown. This current does not appear to be associatedwith the reduction of dissolved O2, as it is still present in the volta-mmograms after solution purging with N2. The apparent potentialwindow for the 100 nm powder is wider than that for the 500 nmpowder electrode, particularly at negative potentials, as some ca-thodic current begins to flow at −0.6 V that is due to an unknownreaction. The wider potential window is due, at least to some extent,

Figure 5. Background cyclic voltammetric i-E curves for acid washed 500and 100 nm diam doped diamond powders recorded using the pipette elec-trode. A comparison i-E for the insulating diamond powder �prior to nano-crystalline diamond deposition� is also shown. The curves were recorded in1 M KCl at 50 mV/s. The currents are normalized to the area of the elec-trode contacting the supporting electrolyte solution.



to ohmic resistance within the powder network. This is discussedfurther below. Evidence for this is the upward slope of the i-E curverelative to the zero current line.

Cyclic voltammetric response for different redox systems.— Figure6 shows a series of curves for Fe�CN�6

−3/−4 and IrCl6−2/−3 at pipette

electrodes prepared with coated diamond and GC powders. The cur-rents are normalized to the geometric area of the electrode�0.002 cm2�. The Fe�CN�6

3−/4− redox couple does not undergosimple electron transfer at carbon electrodes, as is commonlythought.10,11,19,43,70-74 The heterogeneous electron-transfer rate con-


stant is strongly influenced by the electrode surface cleanliness,electrolyte type, and concentration, as well as the electrode’s densityof electronic states near the formal potential of the couple. The rateconstant is relatively insensitive to the surface oxygen functional-ities on sp2 carbon electrodes as long as a thick oxide film is notpresent,70-72,75 however, it is very sensitive to the presence of sur-face oxygen on diamond electrodes.73,74 The heterogeneouselectron-transfer rate constant for IrCl6

2−/3− is relatively insensitive tothe surface microstructure, surface oxides, and adsorbed monolayerson sp2 carbon electrodes.70-72 It is primarily influenced by the elec-

Figure 6. Cyclic voltammetric i-E curvesfor 1 mM Fe�CN�6

3−/4− in 1 M KCl at �A�500 nm and �C� 100 nm diam doped dia-mond powder, and �E� GC powder elec-trodes. Cyclic voltammetric i-E curves for1 mM IrCl6

2−/3− in 1 M KCl at �B� 500 nmand �D� 100 nm doped diamond powderand �F� GC powder electrodes. Cyclic vol-tammetric i-E curves for 1 mM Fe3+/2+ in1 M HClO4 at �G� 500 nm doped dia-mond and �H� GC powder electrodes. Thecurves were recorded using the pipetteelectrode, and the currents are normalizedto the geometric area of the electrode,0.002 cm2.



trode’s �both sp2 and sp3 carbon� density of electronic states near theformal potential.8-11,43,70-72 This redox system was chosen to probethe electronic nature of the powder electrode, which is related todensity of charge carriers and their mobility. The boron-doping leveland homogeneity are very important determinants of fast charge-transfer kinetics. In Fig. 6A and B, curves are shown for these tworedox systems at 500 nm doped diamond powder electrodes. Well-defined, peak-shape voltammograms are seen for both redox sys-tems at all the scan rates. At the lowest scan rate, 30 mV/s, thenominal �Ep values were 76 and 81 mV, respectively �see Table II�.The oxidation peak current for Fe�CN�6

−4 and the reduction peakcurrent for IrCl6

−2 varied linearly with �scan rate�1/2 from 10 to300 mV/s �r2 � 0.98�, indicative of reaction rate limited by semi-infinite linear diffusion of the reactant.

In Fig. 6C and D, curves are shown for the two redox systems atthe 100 nm doped diamond powder. Well-defined, peak-shaped vol-tammograms are also seen for both redox systems at the differentscan rates. At the lowest scan rate tested, 30 mV/s, the nominal �Epvalues were 120 and 98 mV, respectively, for Fe�CN�6

−3/−4 andIrCl6

−2/−3. These same redox systems were tested at a GC powderelectrode, and nominal �Ep values of 76 and 67 mV were observedat 30 mV/s. Clearly, the 500 nm powder exhibits electrochemicalactivity similar to that of the GC powder. Furthermore, the electro-chemical activity for these two redox systems is similar to that seenfor thin films of nanocrystalline diamond.8,10,43 Larger peak split-tings are seen for the 100 nm powder, even though the electricalconductivity of the powder is similar to that for the 500 nm powder.This apparently arises because of ohmic resistance within the pow-der network in the pipette electrode. In fact, measurements as afunction of the concentration revealed an increasing �Ep for bothredox systems �30 mV/s� with increasing concentration. The largerpeak splitting results from increased ohmic potential loss �internalresistance �iR� loss�. Additional evidence for the ohmic resistance isthe sloping background voltammetric i-E curve seen in Fig. 5.

The electrode response for Fe+3/+2 was also tested with as-received and acid washed/hydrogen-terminated 500 nm doped dia-mond powder to study the effect of surface chemistry on the re-sponse. The McCreery group has shown that Fe+3/+2 electrontransfer at carbon electrodes is catalyzed by a specific chemicalinteraction with surface carbonyl functionalities.76 Therefore, theheterogeneous electron-transfer rate constant is highly sensitive tothe presence of oxides on sp2 and sp3 carbon electrodes.8,11,73,74,76

The �Ep for the as-grown powder was nominally 221 mV. Thispowder consists of both sp3 and sp2 carbon impurity due to the

Table II. Cyclic voltammetric peak potential separations „�Ep…

and peak current ratios „ipforwardÕip

reverse… for Fe„CN…6

3−Õ4−, IrCl62−Õ3−,

and Fe+3Õ+2 at 500 and 100 nm diam doped diamond powder andGC powder electrodes.a

Analyte Powder electrode �Ep ipforward/ip

reverse

Fe�CN�6−3/−4 500 nm 76 � 6 0.99

100 nm 120 � 14 0.95GC 76 � 4 1.00

IrCl6−2/−3 500 nm 81 � 4 0.99

100 nm 98 � 8 1.00GC 67 � 2 1.00

Fe+3/+2 500 nm as grown 221 � 30 0.97500 nm rehydrogenated 560 � 28 0.92

GC 121 � 89 0.98

a Unless stated, the powders were acid washed and rehydrogenatedafter diamond deposition. The curves were recorded using the pipetteelectrode. The supporting electrolyte for Fe�CN�6

3−/4− and IrCl62−/3−

was 1 M KCl, while the supporting electrolyte for Fe3+/2+ was 0.1 MHClO4. Data are shown for a scan rate of 30 mV/s with no iR cor-rection.


variation in temperature during deposition. It is supposed that somecarbon–oxygen functionalities existed at sites within the impurityphase. After acid washing and rehydrogenation, �Ep increased con-siderably to 560 mV �50 mV/s�. It was the same powder that wasused to obtain the voltammograms for Fe�CN�6

−3/−4 and IrCl6−2/−3 in

Fig. 6A and B. Therefore, the large increase in �Ep for Fe+3/+2 is notdue to ohmic resistance effects but rather to a loss in surfacecarbon–oxygen functionalities on the surface. The large �Ep for thisredox system is consistent with the sluggish kinetics typically seenfor this redox system at hydrogen-terminated nanocrystalline dia-mond thin-film electrodes.8,11,73,74

Morphological and microstructural stability testing.— One mo-tivation for investigating boron-doped diamond powders is their po-tential use as a dimensionally stable electrocatalyst support material.Figure 7 shows continuous amperometric current density–time �i-t�curves recorded for 100 and 500 nm doped diamond powders duringa constant potential polarization at 1.4 V vs Ag/AgCl in 0.5 MH2SO4. The polarization was performed for 1 h at 80°C. A curve forGC powder is also shown for comparison. The current is normalizedto the area contacted by the supporting electrolyte solution. Thecurves reveal that significantly less current density passes throughthe diamond powders as compared to the GC powder. The i-t pro-files for the two diamond powders are relatively smooth and devoidof temporal fluctuations. The current for the GC powder, on theother hand, is significantly greater and there are sizable temporalfluctuations seen. These fluctuations are due to dynamic increases inthe surface area of the powder as the particles morphologically de-grade and corrode. There is also a general increase in the currentstarting at about the 2400 s mark. This is consistent with an increasein the electrode area due to microstructural degradation and corro-sion. Integration of the i-t curves yields the oxidation charge passed.This charge results from oxidation of the powder, which involves acombination of surface carbon–oxygen functionality formation, mi-crostructural alteration, and corrosion �i.e., gasification of the car-bon�. The total charge passed was greater for the GC powder�0.4 C/cm2� than for the diamond powders �0.06 C/cm2�. This isconsistent with the diamond powders being less active than the GCpowder toward oxidation. Several processes could contribute to theoxidation current for carbon powders: intercalation and subsequentoxidation of the intercalation compound, and surface oxide forma-

Figure 7. Continuous amperometric i-t curves for 500 and 100 nm diamdoped diamond powders recorded during anodic polarization at 1.4 V vsAg/AgCl and 80°C for 1 h in 0.5 M H2SO4. A comparison curve for GCpowder is also shown. The curves were recorded using the pipette electrode.The currents are normalized to the area of the electrode contacting the sup-porting electrolyte solution.



tion at existing edge plane sites and at edge sites formed as a resultof the lattice strain caused by the oxide formation, gas evolution,and corrosion or gasification.

Although from these electrochemical data alone, one cannot de-termine if any morphological or microstructural degradation of thecarbon occurred, other electrochemical tests and SEM images can beused to gain more insight regarding this. Cyclic voltammetric i-Ecurves were recorded for the 500 and 100 nm doped diamond andGC powders in 0.5 M H2SO4 at 80°C before and after polarizationat 1.4 V. Representative curves for all three electrodes are presentedin Fig. 8A-C. For the 500 nm doped diamond powder, the cyclicvoltammograms are largely unchanged after the polarization �Fig.8A�. The currents are normalized to the electrochemically activearea. If the surface had been damaged by degradation and corrosion,then an increased background current would be expected due to anincrease in the surface area of the powder exposed to the solution. Itis also possible that if the degradation were severe enough, then thepowder particle–particle contact would be significantly compro-mised, leading to an increase in the electrode ohmic resistance. Thiswould be evidenced by a sloping voltammetric i-E curve rather thanone that is symmetric around the zero current line. The fact that thecurves are largely unchanged is good evidence for the absence ofsignificant structural degradation or corrosion. The diamond surfacegets converted from an H-terminated one to an O-terminated one.There is however, no microstructural change associated with oxida-tion of diamond, simply a change in surface chemistry. This is con-sistent with previous studies of the dimensional stability of diamondpowders and thin films supported on Si.23-28 The fact that the curvesare unchanged indicates that there is no significant increase in thewetted area of the electrode after polarization.

There is some minor change in the curve for the 100 nm dopeddiam diamond powder. First, the curve prior to polarization is slopeddue to a high ohmic resistance within the powder network. It issupposed that this is due to poor packing of the hard, irregularlyshaped particles as well as incomplete doped diamond coveragearound the particles. Achieving a uniform coverage of diamondaround the seed particles is more arduous the smaller the particlesize. The upward slope of the curve actually increases some afterpolarization, consistent with an increase in the ohmic resistance.

Figure 8. Cyclic voltammetric i-E curves for �A� 500 and �B� 100 nm diamdoped diamond powders, and �C� GC powder before � . . . . . . . � and after�_ _ _ � anodic polarization at 1.4 V vs Ag/AgCl in 0.5 M H2SO4 for 1 h.Scan rate 20 mV/s. Temperature 80°C. The current is normalized to the areacontacted by the supporting electrolyte solution.


Furthermore, unlike the curve for the 500 nm doped diamond pow-der, the curve for the 100 nm powder prior to polarization is char-acterized by small oxidation and reduction peaks between 0 and0.2 V. These peaks are often seen on disordered sp2 carbon materi-als �see the GC powder data in Fig. 8C� and result from redox-activecarbon–oxygen functional groups �e.g., hydroquinone/quinonecouple� formed at the edge plane sites.77-81 Such redox-active func-tional groups are not expected for diamond and are normally notobserved. Their presence here may mean that these small powderparticles have some sp2 carbon impurity exposed with reactive siteswhere these functional groups can readily form. The backgroundcurrent and charge slightly decrease after the polarization, and theredox peaks are still present with an amplitude that is about the sameas before the polarization. It is possible that the reduced backgroundcurrent seen is caused by the loss of some of the nondiamond carbonimpurity from the diamond powder surface in the forms of CO andor CO2.82

The cyclic voltammetric i-E curve for the GC powder was sig-nificantly changed after the anodic polarization �Fig. 8C�. Specifi-cally, the peak current and charge for the redox-active functionalgroups increased.15,19,25,81 The peak positions are largely unchanged.The increased peak amplitude results from the formation of a greaterfunctional group coverage because of microstructural degradationand corrosion, processes that produce new active sites for the func-tional group formation. The overall background current and chargealso increased after polarization, consistent with an increase in theexposed surface area due to microstructural damage.

SEM was used to confirm the presence or absence of morpho-logical and microstructural damage to the powders. Characteristicimages of the GC and 500 nm doped diamond powders, before andafter polarization, are presented in Fig. 9A-D. These powders weretaken from the portion of the sample exposed to the electrolytesolution during the 1.4 V polarization. Prior to polarization, spheri-cally smooth, GC particles are seen with diameters of 3–4 �m �Fig.9A�. After polarization, morphological and microstructural degrada-tion is seen �Fig. 9B�. Some particles were fractured and brokenapart during the polarization, which produced a greater surface area

Figure 9. SEM images of GC powder �A� before and �B� after polarizationat 1.4 V vs Ag/AgCl for 1 h in 0.5 M H2SO4. Temperature 80°C. Images arealso shown of 500 nm doped diamond powder �C� before and �D� after thesame anodic polarization. The polarization was performed using the pipetteelectrode.



and led to the formation of a greater number of redox-active carbon–oxygen functional groups. Not only does the polarization degradethe powder morphology and microstructure, but if allowed to pro-ceed long enough, it would result in significantly increased ohmicresistance through the powder network due to reduced particle con-nectivity. In contrast, there was no apparent change in the structureof the 500 nm doped diamond powder after polarization. Sharp, ir-regularly shaped crystals are apparent before and after polarization.The nominal diameters are the same, and there is no evidence forany particle roughening or pitting.

Discussion

The results demonstrate that relatively high surface area��50 m2/g� and electrically conducting ��1 S/cm� diamond pow-der can be formed by overcoating inexpensive diamond abrasive gritwith a layer of boron-doped nanocrystalline diamond. In this core-shell approach, one must empirically determine the growth condi-tions needed to achieve maximum homogeneity in the overlayerwhile avoiding significant particle–particle fusion, which reducesthe specific surface area. Our procedure for producing the electri-cally conducting diamond powder is not optimal but does enablematerials to be produced for proof-of-concept investigations. Newreactor designs are required for more rapid, efficient, and homoge-neous diamond coatings over individual seed particles. For instance,designs where the powders are suspended in or repeatedly get trans-ported through the plasma would be ideal for achieving a morehomogeneous coating.

The physicochemical properties of the carbon coating depend onpowder position in the plasma. In our approach, the powders werespread across a Si wafer with most but not all of the particles im-mersed in the plasma. The height of the particles differed so thetemperature and plasma density were variable across the powdersample. Independent substrate temperature control, rather than col-lisional heating in the plasma, would be ideal for achieving a moreuniform sample temperature and hence, a more phase-pure diamondcoating. Furthermore, particles buried in the center of the sampleinteract less with the reactive plasma species than do those particlesat the surface. This leads to inhomogeneity in the coverage and typeof carbon formed. Even with these current technological limitations,the results clearly demonstrate that electrically conducting diamondparticles can be produced by the core-shell approach and that theseparticles are more resistant to microstructural degradation and cor-rosion during anodic polarization than are GC powders.

The pipette electrode allows one to test the electrochemical prop-erties of the powders conveniently without the need for a binder.The electrical conductivity data presented in Table I indicate that thepacked powders are highly conducting, on par with GC and VulcanXC-72 sp2 carbon powders. A difficulty, however, with making alow-resistance electrode out of diamond is not the electrical conduc-tivity of the individual powder particles but rather the resistance thatresults from poor particle–particle contact when packed. The dia-mond particles are hard and irregularly shaped. Therefore, they donot pack efficiently to yield high particle–particle contact areas andcannot be compressed to increase the conductivity. The contact re-sistance, Rc, between particles is given by Eq. 1, and this limits theelectrical conductivity of packed powders. It is larger for smallerdiameter powders because the contact radius, b, becomes smallerbetween two spherical particles

Rc =

2b�1�

where is the specific resistivity �� cm� and b is the contact radius�cm�.83 Innovative strategies will be needed to achieve lower resis-tance powder electrodes with small diameter diamond powders, likepreparing electrodes with mixed diameter powders.

Reasonably good electrochemical behavior was exhibited byboth the 500 and 100 nm doped diamond powders toward severalaqueous redox systems. The data, particularly that for the 500 nm


doped diamond powder, were very comparable to data obtained forhigh-quality, boron-doped nanocrystalline diamond thin-film elec-trodes. The voltammetric data for the 100 nm doped diamond pow-der were more influenced by ohmic resistance than were the data forthe 500 nm powder. The main contributor to this resistance is theparticle–particle contact resistance.

Conclusion

The deposition of boron-doped nanocrystalline diamond layer onthe insulating 500 and 100 nm diamond powder via CVD and itselectrochemical, microstructural, and morphological characteriza-tion was reported. On the basis of these results, we can conclude that100 and 500 nm diam diamond powders are coated with a thin layerof boron-doped diamond. According to BET surface area measure-ments the seed diamond powder has higher surface area than thecoated diamond powder. Overcoating makes the particles larger dueto particle–particle fusion, which reduces the specific surface area.The relatively high electrical conductivity after acid washing provedthat the electronic properties of the diamond powder result from thedoped diamond overlayer and not from any adventitious nondia-mond carbon impurity phase. SEM images and Raman spectroscopyyielded further evidence in support of a nanocrystalline diamondoverlayer. Both powders exhibited electrochemical responses forFe�CN�6

3−/4−, Ir�Cl�6−2/−3, and Fe+2/+3 that were comparable to typical

responses seen for high-quality, boron-doped nanocrystalline dia-mond thin-film electrodes. The electrochemical behavior of the pow-ders was assessed using a pipette electrode that housed the packedpowder with no binder. The 100 nm doped diamond powder elec-trodes were more plagued by ohmic resistance effects than were the500 nm powder electrodes because of reduced particle contact. Im-portantly, it was found that the doped diamond powder electrodesare dimensionally stable and corrosion-resistant during anodic po-larization at 1.4 V vs Ag/AgCl �1 h� in 0.5 M H2SO4 at 80°C. Incontrast, GC powder polarized under identical conditions underwentsignificant microstructural degradation and corrosion.

Acknowledgments

The authors thank Dr. Doo Young Kim and Dr. Liang Guo fortheir input on the preparation and characterization of the powders.The research was generously supported by the Department of En-ergy, Office of Science �DE-FG03-95ER146577�. V.M.S. acknowl-edges financial support provided by the Fraunhofer Center for Coat-ings and Laser Applications.

Michigan State University assisted in meeting the publication costs ofthis article.

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the physicochemical and electrochemical properties of 100 and 500 nm diameter diamond powders...

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