Fabrication and evaluation of platinum/diamond composite electrodes for electrocatalysis: Preliminary studies of the oxygen-reduction reaction

Article abstract of DOI:10.1149/1.1524612

A catalytic electrode was prepared using a new electrically conducting and corrosion resistant carbon support material, boron-doped diamond. Fabrication of the composite electrode involves a three-step process: (i) continuous diamond thin-film deposition on a substrate, (ii) electrodeposition of Pt catalyst particles on the diamond surface, and (iii) short-term diamond deposition to entrap the metal particles into the surface microstructure. The process results in a conductive, morphologically, and microstructurally stable composite electrode containing metal particles of somewhat controlled composition, size, and catalytic activity. The metal catalyst particles were galvanostatically deposited from a K₂PtCl₆/HClO₄ solution, with the metal particle size (50-350 nm) and distribution (～10⁹ cm^-2) being controlled by adjusting the galvanostatic deposition and secondary diamond-growth conditions. For a 300 s Pt deposition time, the estimated loading was 75.8 μg/cm², assuming a 100% current efficiency. The composite electrode was extremely stable, both structurally and catalytically, during a 2 h polarization in 85% H₃PO₄ at 170°C and 0.1 A/cm². The electrode's catalytic activity was evaluated using the O₂ reduction reaction at room temperature in 0.1 M solutions of H₃PO₄, H₂SO₄, and HClO₄. The kinetic parameters (Tafel slope and exchange current density) were obtained by cyclic voltammetry and were found to be comparable to those for a polycrystalline Pt electrode in the same media. Tafel slopes of -63 to -80 mV/dec were observed at low overpotentials, with the lowest slope in HClO₄ and highest in H₃PO₄. The exchange current density ranged from 10^-12 to 10^-10 A/cm², and increased in the order of H₃PO₄ < H₂SO₄ < HClO₄. The potential advantages of the composite electrode, as compared with commercial sp² carbon electrodes, are (i) the corrosion resistance of the diamond support, resulting in highly stable reaction centers at high potentials, current densities, and temperatures, and (ii) the fact that all of the catalyst particles are strongly anchored at the film surface and are not contained inside pores.

Full text of DOI:10.1149/1.1524612

E24
Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
0013-4651/2002/150͑1͒/E24/9/$7.00 © The Electrochemical Society, Inc.
Fabrication and Evaluation of PlatinumÕDiamond Composite
Electrodes for Electrocatalysis
Preliminary Studies of the Oxygen-Reduction Reaction
*
**,z
Jian Wang and Greg M. Swain
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA
A catalytic electrode was prepared using a new electrically conducting and corrosion resistant carbon support material, boron-
doped diamond. Fabrication of the composite electrode involves a three-step process: ͑i͒ continuous diamond thin-film deposition
on a substrate, ͑ii͒ electrodeposition of Pt catalyst particles on the diamond surface, and ͑iii͒ short-term diamond deposition to
entrap the metal particles into the surface microstructure. The process results in a conductive, morphologically, and microstruc-
turally stable composite electrode containing metal particles of somewhat controlled composition, size, and catalytic activity. The
metal catalyst particles were galvanostatically deposited from a K2PtCl6 /HClO4 solution, with the metal particle size ͑50-350 nm͒
and distribution (ϳ109 cmϪ2) being controlled by adjusting the galvanostatic deposition and secondary diamond-growth condi-
tions. For a 300 s Pt deposition time, the estimated loading was 75.8 ␮g/cm2, assuming a 100% current efficiency. The composite
electrode was extremely stable, both structurally and catalytically, during a 2 h polarization in 85% H3PO4 at 170°C and
0.1 A/cm2. The electrode’s catalytic activity was evaluated using the O2 reduction reaction at room temperature in 0.1 M solutions
of H3PO4 , H2SO4 , and HClO4 . The kinetic parameters ͑Tafel slope and exchange current density͒ were obtained by cyclic
voltammetry and were found to be comparable to those for a polycrystalline Pt electrode in the same media. Tafel slopes of Ϫ63
to Ϫ80 mV/dec were observed at low overpotentials, with the lowest slope in HClO4 and highest in H3PO4 . The exchange current
density ranged from 10Ϫ12 to 10Ϫ10 A/cm2, and increased in the order of H3PO4
H2SO4
Ͻ
Ͻ
HClO4 . The potential advantages
of the composite electrode, as compared with commercial sp2 carbon electrodes, are ͑i͒ the corrosion resistance of the diamond
support, resulting in highly stable reaction centers at high potentials, current densities, and temperatures, and ͑ii͒ the fact that all
of the catalyst particles are strongly anchored at the film surface and are not contained inside pores.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1524612͔ All rights reserved.
Manuscript submitted April 12, 2002; revised manuscript received July 11, 2002. Available electronicallyNovember 21, 2002.
sp2-bonded carbon materials, diamond is highly resistant to electro-
chemical corrosion. For example, polycrystalline diamond can with-
stand anodic current densities of 0.1 A/cm2, or greater, for 12 h in
an acidic chloride media (E ϭ 3-4 V vs. SCE͒, without any struc-
tural degradation.8 The microstructure is also very stable during po-
larization in 0.1 M HNO3 ϩ 0.1 M NaF at 50°C, conditions that
lead to catastrophic failure of highly ordered pyrolytic graphite
͑HOPG͒, Grafoil™, and glassy carbon.9 This makes diamond an
ideal electrode for high-current density electrolysis, particularly un-
der demanding conditions ͑i.e., complex matrix, high-current den-
sity and potential, high temperature, extremes in pH, etc.͒.
Our group has previously demonstrated that electrically conduct-
ing diamond is a good electrocatalyst host/support.10,11 Nanometer-
sized Pt particles can be incorporated into the surface microstructure
of boron-doped microcrystalline diamond thin-films via a sequential
diamond growth/Pt magnetron sputtering/diamond growth fabrica-
tion procedure. The dispersed Pt particles are stabilized by the
growth of a very thin diamond film around their base and are in
good electrical communication with the current collecting substrate
through the boron-doped diamond support. While the catalyst par-
ticles are stably anchored, the magnetron sputtering approach af-
fords poor control over the particle size and distribution.
We report presently on another method for more controllably
fabricating Pt/diamond composite electrodes, in terms of the metal-
particle size and distribution. In this approach, the Pt particles are
galvanostatically electrodeposited from 1 mM K2PtCl4 ϩ 0.1 M
HClO4 at 0.5 mA/cm2 ͑geometric area͒. Control of the particle
size ͑30-500 nm͒ and distribution (ϳ108 to 109 particles/cm2) is
achieved through adjustments in the electrodeposition ͑current den-
sity and time͒ and secondary diamond growth conditions ͑C/H ratio,
pressure and time͒. Potentiodynamic deposition of the metal was
also investigated, and is briefly discussed herein, but was found to
be inferior to galvanostatic deposition. The morphology, microstruc-
ture, and catalytic activity of the composite electrodes were exam-
ined by atomic force microscopy ͑AFM͒, Raman spectroscopy, and
cyclic voltammetry ͑CV͒, respectively. The Pt-coated electrodes
were also characterized, before and after the secondary diamond
growth, by AFM, scanning electron microscopy ͑SEM͒, energy-
Electrodes consisting of supported metal catalysts are used in
electrosynthesis and electrochemical energy conversion devices
͑e.g., fuel cells͒. The metal catalysts are typically impregnated into
the porous structure of an sp2-bonded carbon support material. Typi-
cal carbon supports include chemically or physically activated car-
bon, carbon black, and graphitized carbons.1 The primary role of the
support is to provide a high surface area over which small metallic
particles can be dispersed and stabilized. The porous support should
also allow facile mass transport of reactants and products to and
from the active sites.2 Several properties of the support are critical,
porosity, pore size distribution, crush strength, surface chemistry,
and microstructural and morphological stability.1
A limitation of sp2-bonded carbon supports is their susceptibility
to corrosion and microstructural degradation during anodic polariza-
tion. One possible carbon electrochemical corrosion reaction is3
C ϩ 2H2O → CO2 ϩ 4Hϩ ϩ 4eϪ
The corrosion rate is strongly dependent on the electrode potential,
electrode microstructure, electrolyte composition, and pH. The rate
is also affected at a given potential by other factors, such as tem-
perature and vapor pressure.3 In general, high temperature, high
pressure, and high operating potentials result in an increased rate of
corrosion. Importantly, electrochemical corrosion produces micro-
structural degradation and surface chemical changes, which gener-
ally lead to lost catalytic activity or even catastrophic electrode
failure.
Conductive sp3-bonded diamond is a new type of carbon elec-
trode that has attracted much attention recently.4-7 Boron-doped dia-
mond thin-film electrodes possess excellent properties, such as elec-
trical conductivity and chemical inertness. But, two of the most
attractive features of diamond are its extreme corrosion resistance
and dimensional stability. Compared with more commonly used
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z E-mail: swain@cem.msu.edu
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Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
dispersive X-ray analysis, and CV. The resulting Pt/diamond com-
E25
posite electrodes have metal-catalyst particles that are well anchored
into the surface microstructure, all exposed at the surface, and are
extremely stable with unchanging catalytic activity even after 2 h of
anodic polarization in 85% H3PO4 at 170°C and 0.1 A/cm.11,12 An
acid etching experiment and cross-sectional SEM measurements
confirmed that the secondary diamond film grows around the base of
the metal deposits, stably anchoring them into the surface
microstructure.
Platinum is the best known electrocatalyst for the oxygen-
reduction reaction and it is widely used in electrochemical energy
conversion devices. Pt particles are generally dispersed onto a high
surface area conductive support, such as carbon powder or a porous
membrane, in order to obtain high electrocatalytic activity and op-
timized Pt utilization. The electrode kinetics for the oxygen-
reduction reaction have been extensively studied at Pt-supported
catalytic electrodes in both acidic and alkaline electrolytes.13-27 The
electrocatalytic activity of the Pt/diamond composite electrodes for
oxygen reduction was investigated in 0.1 M H3PO4 , H2SO4 , and
HClO4 at room temperature. Tafel slopes and exchange current den-
sities were determined as a function of the electrolyte composition
and
Pt
loading.
Experimental
Preparation of Pt/diamond composite electrodes.—Diamond
thin-films were deposited by microwave-assisted, chemical-vapor
deposition ͑CVD, 1.5 kW, 2.54 GHz, ASTeX, Inc., Lowell, MA͒ on
highly conducting p-Si͑100͒ substrates (ϳ10Ϫ3 ⍀ cm). The sub-
strates ͑0.1 cm thick ϫ 1 cm2 in area, Virginia Semiconductor, Inc.͒
were pretreated by rinsing in toluene, methylene chloride, acetone,
isopropanol, and methanol. They were then etched in concentrated
HF for 60 s, rinsed with ultrapure water, and air dried. The cleaned
substrates were then sonicated in a diamond powder/acetone slurry
͑0.1 ␮m diam, GE Superabrasives, Worthington, OH͒ for 20 min
followed by a rinse with clean acetone. The sonication scratches the
surface and leaves seed crystals (Ͼ108 particles/cm2), both of
which serve as nucleation sites during film growth. The seeded sub-
strates were then placed in the CVD reactor, and the system pressure
reduced to about 10 mTorr. Ultrahigh purity ͑99.999%͒ methane and
hydrogen were used as the source gases. The hydrogen flow rate was
200 sccm and the methane flow rate was 0.70 sccm during the
deposition. Prior to introducing methane, the substrates were heated
to the growth temperature in a hydrogen plasma for 10 min. The
plasma was formed with a system pressure of 40 Torr and a power
of 1000 W. The substrate temperature was approximately 850°C, as
measured using a disappearing filament optical pyrometer. The films
were doped using solid-state boron dopant sources, a boron diffu-
sion source ͑GS-126, BoronPlus™, Techniglas, Inc., Perrysburg,
OH͒ and a small piece of boron nitride ͑Goodfellow, Ltd., England͒.
Boron-dopant concentrations were estimated to be approximately
1019 to 1020 B/cm3 ͑300-3000 ppm B/C͒, based on boron nuclear
reaction analysis of other films deposited using similar conditions.
The apparent in-plane film resistivities ranged from 0.1 to 0.01 ⍀
cm, and were measured using a tungsten four-point probe ͑0.1 cm
probe spacing͒ with the diamond film attached to the conducting Si
substrate.
Figure 1. Fabrication process for the Pt/diamond composite electrodes.
through the solution during the deposition. This current, and all
others in the text, unless otherwise stated, is normalized to the geo-
metric area of the electrode (0.2 cm2). Four diamond films from the
same batch were examined, using deposition times of 100, 200, 300,
and 400 s each. Some experiments involved potentiodynamically
depositing the metal particles from the same solution while scanning
the potential from 0.8 to Ϫ0.3 V vs. Ag/AgCl at 50 mV/s. The
Pt-coated diamond film electrode is then placed back in the reactor
and boron-doped diamond deposited for an additional 3 h, using the
same conditions as described above. The secondary growth film
thickness is much less than 1 ␮m ͑on the order of 0.3 ␮m͒ and
entraps the metal particles, anchoring and stabilizing them within
the microstructure. Diamond will nucleate and grow on both the Pt
and the surrounding diamond matrix, however, the growth rate on
the diamond is much greater than on the Pt. Undoubtedly, some of
the smaller Pt particles can be completely covered during the pro-
cess, as has recently been confirmed in cross-sectional SEM mea-
surements, but if the conditions are optimized, then the complete
coverage of particles is minimized. Hydrocarbon deposits also accu-
mulate on the Pt surface, but these can be effectively removed by
potential cycling, as discussed below.
The three-step fabrication procedure for the composite electrodes
is shown diagrammatically in Fig. 1. A boron-doped diamond thin-
film was initially deposited on a Si͑100͒ substrate for 12 h. The film
thickness, at this stage, was about 4-6 ␮m. The diamond growth was
then stopped, and the substrate cooled to less than 300°C in the
presence of atomic hydrogen. After completely cooling to room tem-
perature, the coated substrate was removed from the reactor. Metal
particles were then electrodeposited on the surface. Galvanostatic
deposition was performed in 1 mM K2PtCl6 ϩ 0.1 M HClO4 at a
current density of 0.5 mA/cm2. Both diffusion and convection influ-
enced the magnitude of the current, as nitrogen was slowly bubbled
Film characterization.—The atomic force microscopy ͑AFM͒
was performed with a Nanoscope II instrument ͑Digital Instruments,
Santa Barbara, CA͒ in the contact mode. Pyramidal-shaped Si3N4
tips mounted on gold cantilevers ͑200 ␮m legs, 0.38 N/m spring
constant͒ were used to acquire topographical images in air. Raman
spectra were obtained at room temperature with a Chromex Raman
2000 system ͑Chromex, Inc., Albuquerque, NM͒ consisting of a
diode-pumped, frequency-doubled CW Nd:YAG ͑500 mW at 532
nm, Coherent͒, a Chromex 500is spectrometer ͑f/4, 600 grooves/mm
holographic grating͒, and a thermoelectrically cooled 1024 ϫ 256
charge coupled device ͑CCD͒ detector ͑Andor Tech., Ltd.͒. Spectra
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E26
Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
were collected with an incident power density of ca. 500 kW/cm2
͑100 mW at the sample and 5 ␮m diam spot size͒.
Electrochemical measurements.—The Pt electrodeposition and
cyclic voltammetry were performed in a single-compartment glass
cell, using a CYSY-2000 computer-controlled potentiostat ͑Cypress
Systems, Inc., Lawrence, KS͒. A Ag/AgCl ͑saturated KCl͒ electrode
was used as the reference and a large-area carbon rod served as the
counter electrode. The Pt/diamond composite working electrode was
pressed against the bottom of the glass cell, using an Al plate current
collector with the fluid being contained by a Viton® O-ring. Silver
paste was applied to the center portion on the back side of the Si
substrate, after cleaning. The composite electrodes were cleaned,
once mounted in the cell, by a thorough rinse with ultrapure water
(Ͼ17 M⍀ cm, Barnstead E-Pure͒, a 20 min soak in distilled isopro-
panol, followed by another thorough rinse with ultrapure water. The
electrolyte was deoxygenated with nitrogen ͑99.9%, BOC Gases͒ for
20 min prior to any electrochemical measurements, and the solution
was blanketed with the gas during the measurements. For the oxy-
gen reduction experiments, the solution was saturated with oxygen
gas ͑99.9%, BOC Gases͒ by bubbling for 30 min prior to any elec-
trochemical measurement.
Chemicals.—All chemicals were reagent grade quality or better
and used without additional purification. The acids, HClO4 ͑Ald-
rich͒, H2SO4 ͑Aldrich͒, and H3PO4 ͑Aldrich͒, were all of ultrahigh
purity ͑99.999%͒. The K2PtCl6 ͑Aldrich͒ was reagent grade quality.
All solutions were prepared with ultrapure water from a Barnstead
E-Pure purification system (Ͼ17 M⍀ cm).
Results and Discussion
Electrodeposition of Pt on diamond thin films.—The Pt elec-
trodeposition was performed by both potentiodynamic and galvano-
static electrolysis, and the results are compared herein. Figure 2A
shows cyclic voltammetric i-E curves obtained during the metal
deposition in 1 mM K2PtCl6 ϩ 0.1 M HClO4 . A total of twenty-
five potential sweeps were made, and the i-E curves for 1st, 5th,
10th, 20th, and 25th scans are presented. The potential sweep was
initiated at 800 mV and scanned in the negative direction to Ϫ300
mV at a scan rate of 50 mV/s. During the first cycle ͑innermost
curve͒, the cathodic current for the metal deposition begins to gradu-
ally increase at ca. 350 mV vs. Ag/AgCl, with the significant current
onset at 0 mV. Based on the redox reaction28
Figure 2. Successive potentiodynamic i-E curves for a boron-doped dia-
mond thin-film in 1 mM K2PtCl6 ϩ 0.1 M HClO4 ͑A͒. Arrows indicate the
current increase with increasing cycle number. Scan rate ϭ 50 mV/s. AFM
force mode image ͑air͒ of the diamond thin film after 25 potential cycles ͑B͒.
The white arrows indicate the position of some of the metal deposits.
PtCl24Ϫ ϩ 2eϪ ꢀ Pt ϩ 4ClϪ E0 ϭ 561 mV s. Ag/AgCl
v
This represents an overpotential of some 200 mV for the deposition
of Pt on diamond. A deposition overpotential has also been observed
for Ag and Hg on diamond,29,30 the cause of which has been attrib-
uted, in a very general way, to the inertness of hydrogen-terminated
diamond surface. The metal deposition likely proceeds by two irre-
versible reactions, corresponding to sequential two-electron trans-
fers (Pt4ϩ → Pt2ϩ → Pt or PtCl62Ϫ → PtCl24Ϫ → Pt), as has been
proposed for Pt electrodeposition on carbon fiber electrodes.31 The
reactions occur at potentials sufficiently close such that each step is
not well resolved at diamond ͑50 mV/s͒. The reduction peak at
Ϫ240 mV is attributed to the adsorption of hydrogen on the elec-
trodeposited Pt particles, a reaction that is occurring simultaneously
with the metal deposition. The current crossover seen during the
reverse sweep is a characteristic feature of metal nucleation and
growth.32 The voltammetric currents grow with cycle number ͑the
25th scan is the outermost curve͒ due to the progressive increase in
metal particle size and distribution. All voltammograms, after the
initial one, show the characteristic current features for Pt, Pt oxide
formation ͑700-1200 mV͒, oxygen evolution ͑1200 mV͒, Pt oxide
reduction ͑500 mV͒, hydrogen adsorption and desorption ͑0 to
Ϫ300 mV), and hydrogen evolution (Ϫ300 mV).
image features are influenced by the tip geometry, which is often the
case when imaging rough, polycrystalline diamond films with low
aspect ratio, pyramidally shaped tips. A well-faceted, polycrystalline
film morphology is inferred from the image with cubic, octahedral,
and cuboctahedral microcrystallites. This morphology was con-
firmed by SEM. Also present are the intercrystalline grain bound-
aries where the microcrystallites join together. The Pt deposits are
not uniformly distributed over the surface but rather are located
almost exclusively at grain boundaries and other visible, extended
defects ͑see arrows͒. The particle diameters range from 0.3 to 2 ␮m
with a distribution density of ca. 4 ϫ 107 cmϪ2 ͑geometric area͒,
although the shapes of the deposits are most strongly influenced by
the tip geometry. After 25 potential cycles, most of the surface is
still void of Pt, and many of the particles have diameters of up to ca.
2 ␮m. These observations, and others, suggest that the metal depo-
sition preferentially initiates at the grain boundaries, or other surface
defects, during the first cathodic scan, with a limited number of Pt
nuclei formed. 3D growth of the metal particles from these nuclei
then dominates during the successive potential sweeps.
Figure 2B shows a top-view, 10 ϫ 10 ␮m AFM image ͑force
mode, air͒ of the diamond surface after 25 potential cycles. The
A high degree of metal dispersion is desired when fabricating
catalytic electrodes. The deposition method, the deposition condi-
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Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
E27
Figure 3. AFM images ͑air͒ of boron-doped diamond thin-films after galvanostatic deposition of Pt from a solution of 1 mM K2PtCl6 ϩ 0.1 M HClO4 . The
deposition times are ͑A͒ 100, ͑B͒ 200, ͑C͒ 300, and ͑D͒ 400 s, respectively.
tions, and the form of the metal solute complex all play a role in
determining the size and distribution of the electrodeposited
particles.33 The potentiodynamic deposition produced particles with
large diameters and poor distribution. Therefore, the galvanostatic
deposition of Pt was also investigated. A current density of
0.5 mA/cm2 was determined to be optimal for the continuous depo-
sition, based on results from other experiments. During the deposi-
tion, the electrode potential ranged from Ϫ100 to Ϫ300 mV vs.
Ag/AgCl. Figure 3A-D shows top-view, 5 ϫ 5 ␮m AFM images
͑force mode, air͒ of diamond surfaces after deposition for 100, 200,
300 and 400 s, respectively. The nominal particle size, variances,
and distribution density for each deposition time are summarized in
Table I.
Table I. Pt particle size, distribution, hydrogen desorption charge, and roughness factor „RF… for the PtÕdiamond composite electrodes before
and after the secondary diamond growth.
Before
Distribution
After
Distribution
Particle
size
͑nm͒
Hydrogen
desorption
charge
RF
Particle
size
͑nm͒
Hydrogen
desorption
charge
RF
Surface
loss
͑%͒
density
density
Deposition
time
͑s͒
(109 cmϪ2
)
(108 cmϪ2
)
(mC/cm2)
(mC/cm2)
100
200
300
400
20-200
10-250
50-300
60-500
0.51
0.80
1.19
1.43
2.4
3.8
5.7
6.8
30-250
50-300
50-350
60-500
0.32
0.56
0.81
0.92
1.5
2.7
3.9
4.4
37.3
30.0
31.9
35.7
0.5 Ϯ 0.10
0.8 Ϯ 0.04
1.0 Ϯ 0.05
1.2 Ϯ 0.09
3.0 Ϯ 1.5
5.1 Ϯ 0.7
8.0 Ϯ 0.3
8.5 Ϯ 0.4
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Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
Figure 4. AFM images ͑air͒ of Pt-coated diamond thin-films ͑as indicated in Fig. 3͒ after a 3 h secondary diamond growth. The corresponding Pt deposition
times are ͑A͒ 100, ͑B͒ 200, ͑C͒ 300, and ͑D͒ 400 s, respectively.
The metal particles decorate the entire surface with much better
distribution than was achieved with the potentiodynamic method.
The nominal particle size is also smaller. Both are desirable trends
for a catalytic electrode. After 100 s of deposition ͑Fig. 3A͒, distin-
guishable Pt particles are uniformly dispersed over the diamond sur-
face, decorating both the microcrystallites and the grain boundaries.
The particle size ranges from 10-200 nm with an average particle
distribution of 0.5 ϫ 109 cmϪ2. The distribution was determined by
manually counting the particles in AFM images from multiple sites
on the surface. Slightly larger particles and a higher distribution are
observed after 200 s of deposition ͑Fig. 3B͒. The particle size ranges
from 20 to 250 nm, and the distribution increases to 0.9 ϫ 109
cmϪ2. A further increase in the distribution is seen after 300 s of
deposition ͑Fig. 3C͒. The particle size ranges from 50 to 300 nm and
the distribution is 2 ϫ 109 cmϪ2. Finally, somewhat larger particles
and a higher distribution are observed after 400 s of deposition ͑Fig.
3D͒. The particle size ranges from 60 to 400 nm and the distribution
is 4 ϫ 109 cmϪ2. The image features are consistent with a progres-
sive nucleation, and 3D growth mechanism in which new Pt nuclei
progressively form, as a function of time, while the existing nuclei
increase in size.32 Nuclei form in both the grain boundaries and on
the facet surfaces. This indicates that both regions are electrochemi-
cally active and support the flow of current.
Secondary diamond growth.—In general, electrodeposited Pt
metal particles weakly adhere to the hydrogen-terminated diamond
surface without a secondary diamond growth, as the particles can
easily be dislodged during hydrogen and oxygen gas evolution.2
Figure 4A-D shows 5 ϫ 5 ␮m AFM images ͑force mode, air͒ of
Pt-coated films after 3 h of additional diamond growth. The images
are of the same Pt-coated films shown in Fig. 3. Numerous particles
are randomly distributed over the surface on both the microcrystal-
lites and in the grain boundaries, particularly for 200, 300, and 400
s electrodeposition times. The nominal particle size is slightly in-
creased by the secondary diamond growth, ranging from 50 to 400
nm, but the distribution is lower, ϳ108 cmϪ2. The process of en-
trapping the particles with diamond slightly reduces the total active
Pt exposed. This is seen by comparing the hydrogen desorption
charge in Table I. There is a 30-37% loss in Pt activity, as measured
in the hydrogen desorption voltammetric charge between 0 and
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Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
E29
Figure 6. Cyclic voltammetric i-E curves for a Pt-coated diamond thin film
in 0.1 M HClO4 , before ͑solid line͒ and after ͑dashed line͒ a 3 h secondary
diamond growth. The Pt deposition time is 200 s. Scan rate ϭ 50 mV/s.
the deposition time ͑i.e., Pt loading͒, as the progressive nucleation
and growth leads to particles of varying size. Assuming a 100%
current efficiency, the mass of Pt deposited is calculated to be 25.2,
50.3, 75.8, and 100.1 ␮g/cm2 for the 100, 200, 300 and 400 s depo-
sition times, respectively. Thus, the hydrogen-desorption charge per
mass of Pt, after the secondary diamond growth, is calculated to be
12.6, 11.1, 10.7, and 9.2 ␮C/␮g, respectively. The ratio decreases
with increasing deposition time because of the increase in the nomi-
nal metal particle size. The mass loadings given above would cer-
tainly be upper limits, as the coulometric efficiency is probably less
than 100% due to competing hydrogen evolution.
Figure 5. SEM images of a Pt metal particle on the diamond surface ͑A͒ and
the corresponding energy dispersive X-ray analysis spectrum for the particle
͑B͒.
The voltammetric curve for the Pt-coated diamond film, prior to
the secondary growth ͑solid line͒, was obtained after the first scan,
and the shape remained unchanged during ten subsequent scans.
However, the voltammetric curve for the Pt-coated diamond film,
after the secondary diamond growth ͑Fig. 6, dashed line͒, had a
poorly shaped first scan, especially in the hydrogen adsorption/
desorption region. In fact, multiple scans were required for the char-
acteristic Pt features to develop. This observation is consistent with
the removal of surface contamination formed during the diamond
deposition. It is highly probable that the Pt surface is initially cov-
ered with adsorbed carbon and/or hydrocarbon moieties, and these
contaminants block surface sites for the hydrogen adsorption. The
contaminants are effectively removed by potential cycling between
the oxygen and hydrogen evolution regimes.10 Hence, potential cy-
cling is the normal protocol for cleaning and activating the Pt sur-
face after the secondary diamond film growth. The potential cycling
͑50-100 cycles͒ is performed in 0.1 M HClO4 at a scan rate of 50
mV/s between 1500 mV and Ϫ400 mV. The voltammogram shown
for the Pt-diamond composite electrode is for the 100th cycle. The
flat oxide-formation region at potentials between 700 and 1200 mV
indicates the oxidation of adsorbed contaminants is not occurring to
any appreciable extent.
There are, however, some notable differences between the two
voltammograms. The hydrogen adsorption/desorption peaks, the Pt
oxide formation/reduction waves, and the double-layer charging cur-
rent between 0 and 250 mV are slightly reduced after the secondary
diamond growth, indicating a reduction in the active Pt surface area.
An average of 34% of the active surface atoms are lost, based on the
reduction in the hydrogen desorption charge between Ϫ250 and 100
mV. This is summarized in Table I.
Several possible causes for the reduced Pt activity were system-
atically investigated. One possibility, actually confirmed in recent
cross-sectional SEM measurements, is the complete coverage of
some of the smaller particles by the secondary diamond layer. How
Ϫ300 mV. This is consistent with the secondary diamond growth
around the base of the metal deposits. The loss is further discussed
below.
SEM and semiquantitative X-ray fluorescence measurements
confirmed the deposits are Pt. Figure 5A shows an SEM image of
several diamond microcrystallites decorated with metal deposits.
The deposits occupy sites on both the facet surfaces and in the grain
boundaries, as is the case in the AFM images. The corresponding
energy dispersive X-ray analysis spectrum taken from one of the
deposits is shown in Fig. 5B. The C K␣ ͑0.277 keV͒, O K␣ ͑0.525
keV͒, and Pt M␣ ͑2.05 keV͒, L␣ ͑9.44 keV͒ emissions are observed.
The estimated atomic percentage of each element is 94.8, 1.7, and
3.5, respectively, yielding Pt/C and O/C atomic ratios of 0.037 and
0.018.
Basic electrochemical properties.—Figure 6 shows cyclic volta-
mmetric i-E curves for the Pt/diamond composite electrode before
͑solid line͒ and after ͑dashed line͒ the secondary diamond growth.
The curve shapes were unchanging with cycling and were obtained
in nitrogen-purged 0.1 M HClO4 at a scan rate of 50 mV/s. The
potential sweep was initiated at 200 mV and scanned in the positive
direction. All the characteristic current features of polycrystalline Pt
are present. Two reversible hydrogen adsorption/desorption peaks
are evident in the potential region from 0 to Ϫ300 mV. Integration
of the area under these peaks was used to estimate the electrochemi-
cally active surface area of the Pt particles.34 The roughness factor
(RF ϭ voltammetric charge density/210 ␮C/cm2) of the Pt/
diamond composite electrode can be calculated assuming a coulom-
bic charge of 210 ␮C/cm2 for hydrogen adsorption on an atomically
flat Pt͑111͒ surface.22 The results are presented in Table I. Both the
hydrogen desorption charge and the roughness factor increase with
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E30
Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
Figure 7. SEM images of Pt/diamond composite electrode ͑A͒ before and
͑B͒ after acid etching in aqua regia. The Pt deposition time is 200 s.
much active Pt is lost by this process is unknown at present. Another
possibility is particle aggregation on the diamond surface at the
deposition temperature of ca. 850°C. Still another possibility is gas-
ification or sputtering of the Pt by the atomic hydrogen present in
the plasma. To examine these latter two possibilities, an experiment
was performed in which another set of Pt-coated diamond films,
similar to those shown in Fig. 3, were placed into the CVD reactor.
The electrodes were then exposed to the conditions used for the
secondary diamond growth except that no CH4 was added. The ab-
sence of CH4 did not significantly affect the substrate temperature,
at least as determined by an optical pyrometer. Again, the morphol-
ogy was examined by AFM and the active surface area was deter-
mined from the hydrogen adsorption/desorption voltammetric
charge in 0.1 M HClO4 before and after a 3 h hydrogen plasma
treatment. No significant change in the particle size or voltammetric
charge for hydrogen desorption was observed ͑AFM images are not
shown here͒, at least for Pt particles with diameters from 20 to 300
nm. Therefore, the loss in active Pt is not due to particle aggrega-
tion, gasification, or sputtering. Voltammetric measurements indi-
cated that only 2-4%, not 30-37%, of the active Pt was lost during
the 3 h hydrogen plasma treatment. The melting and smoothing of
small particles may contribute to the slight activity loss. Although
the melting temperature of bulk Pt is 1772°C, nanometer-sized Pt
particles have a much lower melting temperature, as indicated by the
size-dependent melting temperature theory.35 As the size of the par-
ticle decreases, an increased proportion of atoms occupy surface or
interfacial sites. These atoms are more loosely bound than bulk at-
oms, which facilitates the melting.36 Pt particles with diameters of
less than 2 nm have a melting point less than 850°C, based on the
application of the theory. It should also be noted that segregated Pt
particles, rather than a continuous film, can be formed on different
substrates by high-temperature annealing in vacuum.37,38 The
breakup of an initially continuous or quasi-continuous Pt film into
individual Pt aggregates on a quartz substrate has been achieved by
vacuum heat-treatment at 600 K.37
Figure 8. Cyclic voltammetric i-E curves for ͑A͒ a Pt/diamond composite
electrode, ͑B͒ a clean Pt foil electrode, and ͑C͒ a bare, boron-doped diamond
thin film electrode in oxygen-saturated 0.1 M HClO4 . The Pt deposition time
for the composite electrode is 200 s. Apparent loading ϭ 50.3 ␮g/cm2. Scan
rate ϭ 50 mV/s.
ite electrode, before and after the acid etching, are presented in Fig.
7A and B. Pt particles with diameters of 50-300 nm are seen on the
surface, prior to the chemical treatment ͑Fig. 7A͒. Numerous pits are
observed in the diamond film after treatment. These pits are the
voids left after dissolving the metal and have diameters of 50 to 300
nm. This observation confirms that the diamond film does surround
the base of some of the metal particles, entrapping and anchoring
them in the surface microstructure. The depth of the pits depends on
the secondary diamond-growth time.
Stability of the Pt/diamond composite electrodes.—The short-
term dimensional stability and catalytic activity of the Pt/diamond
composite electrodes were investigated before and after anodic po-
larization in 85% phosphoric acid at 170°C and 0.1A/cm.11,12 Elec-
trochemical and AFM measurements revealed no loss in Pt activity
and no degradation of the diamond microstructure after 2 h of elec-
trolysis. There was no pitting or grain roughening of the diamond
support and no evidence for catalyst detachment due to any form of
carbon corrosion. It should be noted that sp2 carbon electrodes ͑e.g.,
HOPG, glassy carbon, Grafoil, activated carbon͒ corrode and de-
grade catastrophically during exposure to these or even milder an-
odization conditions.3,12
Electrocatalytic activity for oxygen reduction.—Figure 8A shows
a cyclic voltammetric i-E curve for an electrode in oxygen-saturated
0.1 M HClO4 . For comparison, the voltammetric responses for a
clean polycrystalline Pt foil and a bare diamond electrode are shown
in Fig. 8B and C, respectively. The potential sweep was initiated at
600 mV and scanned in the positive direction at a scan rate of 50
Confirmation that the secondary diamond growth causes much of
the active Pt loss by covering the base of the metal deposits was
next addressed. In the experiment, a Pt/diamond composite electrode
was etched in hot aqua regia ͓3 HCl:1 HNO3 ͑v/v͔͒ to dissolve the
Pt particles. The diamond lattice is quite stable in this solution and
undergoes no morphological alteration. SEM images of the compos-
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Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
E31
Plots of ␩ vs. log i for the composite electrode are shown as an inset
in Fig. 9. No correction for any mass-transfer effects was made in
the analysis of these data. The Tafel plots for all three electrolytes
are linear at low overpotentials, most likely being influenced at
higher overpotentials by mass transfer effects. The slopes are Ϫ63,
Ϫ69, and Ϫ80 mV/dec, respectively, for HClO4 , H2SO4 , and
H3PO4 . These results are similar to those reported by Kita et al.42
and Damjanovic et al.45,46 Mass-transfer corrected Tafel plots
for oxygen reduction at Pt in acid normally yields two slopes, ca.
Ϫ60 mV/dec at low overpotentials and Ϫ120 mV/dec at high
overpotentials.46 The low Tafel slope region corresponds to a poten-
tial regime where the oxygen reduction occurs on an oxide-covered
surface. Adsorbed ϪO and ϪOH species affect the adsorption of O2
and other reaction intermediates ͑Temkin conditions͒. The high
Tafel slope region (Ϫ120 mV/dec) corresponds to a potential region
where the reaction proceeds on an oxide-free surface ͑Langmuir
conditions͒. Due to the influence of mass-transfer effects, this higher
slope region is not observed in the present data. The slightly higher
Tafel slopes observed in H2SO4 and H3PO4 are likely attributable to
anion adsorption influencing the O2 chemsorption on the surface.42
The exchange currents can be determined by extrapolating the linear
Tafel region back to ␩ ϭ 0. Normalizing these currents to the elec-
trochemically active surface area of the Pt (0.78 cm2, as estimated
from the roughness factor͒, yields exchange current densities, i0 , of
1.9 ϫ 10Ϫ10 A cmϪ2 in HClO4 , 4.6 ϫ 10Ϫ11 A cmϪ2 in H2SO4 ,
and 7.1 ϫ 10Ϫ12 A cmϪ2 in H3PO4 . Thus, the reaction rate in-
Figure 9. Slow scan, linear sweep voltammograms for a Pt/diamond com-
posite electrode in oxygen saturated 0.1 M ͑A͒ HClO4 , ͑B͒ H2SO4 , and
͑C͒ H3PO4 . The Pt deposition time is 300 s. Apparent
loadingϭ75.8 ␮g/cm2. Scan rate ϭ 1 mV/s. The inset shows the corre-
sponding Tafel plots constructed from the rising portion of the i-E curves.
mV/s. The response for the Pt/diamond composite electrode re-
sembles that for the clean Pt foil. Oxygen is reduced to water by the
Pt catalyst according to the reaction39
Ͻ
Ͻ
HClO4 . This is a well-
creases in order of H3PO4
H2SO4
known phenomenon for smooth Pt surfaces and can be explained by
the different degrees of adsorption of phosphate/biphosphate,
sulfate/bisulfate, and perchlorate anions.42
O2 ϩ 4Hϩ ϩ 4eϪ ꢀ 2H2O
These preliminary results indicate that the kinetic parameters for
the oxygen-reduction reaction at Pt/diamond composite electrodes
are similar to the parameters for clean polycrystalline Pt.42 In addi-
tion, the diamond matrix exerts little influence on the oxygen-
reduction response.
The cathodic current commences at 400 mV and reaches a maxi-
mum of 215 ␮A or 1.1 mA/cm2 at ca. 250 mV. It should be noted
that the cathodic current is a sum of the oxygen reduction current,
the current due to Pt oxide reduction, and the double-layer charging
current. Clearly, the oxygen reduction current at 250 mV is due
solely to the activity of the Pt catalyst. This is evident by comparing
the cyclic voltammetric i-E curve for the bare diamond electrode.
This response is featureless from 1700 to Ϫ1000 mV with no reduc-
tion current at 250 mV. Diamond is known to have a large overpo-
tential for oxygen reduction in both acidic40 and basic media.41
Figure 9A-C shows slow scan linear sweep voltametric i-E
curves ͑1 mV/s͒ for a Pt/diamond composite electrode in oxygen-
saturated 0.1 M HClO4 , H2SO4 , and H3PO4 , respectively. The po-
larization curves have been corrected for the background current,
which was measured in nitrogen-purged solution. Peak-shaped
curves are seen with Erped varying with acid type. Values of 460, 390,
and 335 mV are seen for HClO4 , H2SO4 , and H3PO4 , respectively.
Conclusions
We report on a new, dimensionally stable Pt/diamond composite
electrode that is fabricated by using a sequential procedure of dia-
mond deposition, Pt electrochemical deposition, and diamond depo-
sition. Dispersed Pt particles are galvanostatically deposited onto a
boron-doped polycrystalline diamond thin-film surface and en-
trapped within the dimensionally stable microstructure by a subse-
quent diamond deposition. The second, short deposition serves to
anchor many of the metal particles into the diamond microstructure
by surrounding their base. The number of exposed particles, particle
size, and distribution can be controlled by adjusting the galvano-
static deposition time and secondary diamond-growth conditions.
Particle sizes are in the range of 10-300 nm with a distribution of
ϳ109 cmϪ2. This nominal particle size is still too large for a true
catalytic electrode, and work is ongoing to reduce the particle size to
the 10 nm range. About 33% of the active Pt surface is lost after the
secondary diamond deposition due to a combination of complete
coverage of some of the smaller particles and partial coverage of the
base of the larger particles. The catalytic activity of the composite
electrodes is extremely stable, as no microstructural alterations or
activity losses are observed during the 2 h of anodic polarization in
85% H3PO4 at 0.1 A/cm2 and 170°C.
The peak current, ipred
,
is Ϫ31.5, Ϫ29.5, and Ϫ28.0 A
(ϳ0.15 mA/cm2 or ϳ2.0 mA/mg Pt͒, respectively. The decrease in
the oxygen reduction current in the kinetically controlled region and
the negative shift of Erped can be explained by an anion adsorption
effect.42,43 H2POϪ4 has been reported to strongly adsorb on the Pt
surface in the potential region of the oxygen-reduction current onset.
H2POϪ4 adsorption can slow down the electrode reaction kinetics by
blocking sites for the initial oxygen chemsorption.44 Changes in the
double-layer structure, as a result of the adsorption, could also be
influential.44 The irped in all the acids increases linearly with the
square root of scan rate ͑50 to 200 mV/s͒, indicating the reaction
rate is limited by semi-infinite linear diffusion of oxygen to the
electrode surface.
The kinetic parameters for the oxygen-reduction reaction at the
composite electrode are similar to those for clean polycrystalline Pt.
Room-temperature Tafel slopes of Ϫ63 to Ϫ80 mV/dec are ob-
served at low overpotentials in 0.1 M HClO4 , H2SO4 , and H3PO4 ,
with the lowest slope in HClO4 and the highest in H3PO4 . The
exchange current density (10Ϫ12 to 10Ϫ10 A/cm2 active Pt͒ increases
The rising portion of the i-E curves in the kinetically controlled
region was analyzed and Tafel plots were constructed, based on the
equation
Ͻ
Ͻ
HClO4 . The maximum current den-
in order of H3PO4
H2SO4
log i ϭ log i ϩ ␣ nF/2.303 RT͒␩
sity for oxygen reduction is observed in 0.1 M HClO4 , during slow
͑
o
c
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E32
Journal of The Electrochemical Society, 150 ͑1͒ E24-E32 ͑2003͒
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Acknowledgments
This research was generously supported by the Department of
Energy, Office of Science ͑DE-FG03-95ER14577͒. The authors also
acknowledge the contributions of Dr. Koji Kobashi and Dr. Takeshi
Tachibana, of the Frontier Carbon Technology Project, with the
original diamond/Pt magnetron sputtering/diamond composite elec-
trodes.
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Michigan State University assisted in meeting the publication costs of
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