Download 9.5. Combined Methods: Electrochemical

9. Chemical, Photochemical and Electrochemical Modification of Diamond
9. Chemical, Photochemical and Electrochemical Modification of Diamond
Donald A. Tryk, Takeshi Kondo and Akira Fujishima
9.1. Introduction
The attractive characteristics of electrodes based on conductive
diamond films have led a number of research groups around the
world to use these electrodes for electroanalytical applications. As
a way to extend the analytical capabilities of diamond electrodes,
researchers have also become interested in chemically modifying
the diamond surface. One of the principal motivations is to impart
selectivity for analytical purposes. Closely associated with this is
the
desire
to
impart
electrocatalytic
activity
for
specific
electrochemical reactions, making use of diamond as a highly
robust support. One of the more interesting recent applications of
the modified diamond surface is the fabrication of DNA arrays.
The reports that have appeared thus far can be classified into
the
following
categories:
1)
chemical
modification;
2)
photochemical modification; 3) electrochemical modification; 4) ion
implantation techniques; and 5) combined methods, for example,
electrochemical modification followed by chemical modification. All
of
these
methods
have
their
respective
advantages
and
disadvantages, which we will examine in this chapter.
173
Another way to group the published reports on the covalent
modification of the diamond surface is as follows: 1) conversion of
the hydrogen termination to oxygen, chlorine or fluorine; 2)
cycloaddition reactions of alkenes with the bare diamond surface,
either after high temperature vacuum annealing or during UV
illumination, resulting in carbon-carbon bonds; 3) reactions with
functional group-specific reagents, such as those that react with
either hydroxyl groups or carbonyl groups; and 4) radical reactions
with reagents, directly without activation, or after either
electrochemical or photochemical activation .
In general, the features that are desired for the modification of
the diamond surface include the following:
Chemical or electrochemical selectivity for a particular species
Chemical inertness
Chemical stability
Electrochemical stability
Mechanical robustness
Electrical contact (specifically for electrochemical applications)
Ability to be patterned, particularly at the micrometer to
nanometer scales
Speed of modification
9.2. Chemical Modification Methods
Some of the chemical modification methods have been investigated
for many years, even though the analytical aspects were not
envisaged initially. The simplest technique involves the treatment
with an oxidizing acid solution such as nitric acid or chromic acid.
174
9. Chemical, Photochemical and Electrochemical Modification of Diamond
This type of treatment can convert the hydrogen termination to
oxygen termination. An additional benefit is that possible nondiamond carbon impurities, as well as metallic impurities, can be
removed from the surface in this way. The chemical oxidation of
diamond is closely related to electrochemical oxidation, discussed
later, and which is also discussed in Chapters 8 and 10.
The carbon-oxygen surface functional groups that are
produced via chemical oxidation, irrespective of the details of the
reaction, include carbonyl and ether groups, which can form
predominantly on the diamond (100) surface, and hydroxyl groups,
which can form predominantly on the (111) surface [1-12]. It has
also been found that hydroxyl groups can be stabilized on the (100)
surface, particularly if they are hydrogen-bonded to each other [5].
Thus, on this surface, the three principal types of functional groups
can exist in various proportions, depending on the coverage; this
uncertainty exists even if the (100) surface is crystallographically
perfect.
In many cases, chemical oxidation has been used as a
standard preparation technique for certain types of experimental
measurements, particularly those that involve semiconducting
properties, because it removes hydrogen from the surface and/or
subsurface, which can impart metallic conductivity [13].
There
have been several reports in which the electronic properties of
hydrogen-terminated vs. oxygen-terminated diamond have been
compared [1, 3].
Briefly, the carbon-oxygen surface functional groups that are
produced possess a strong dipole, in which the negative end points
outward from the surface [14, 15]. Depending on the details of the
175
surface crystal structure and the coverage with various functional
groups, this dipole can be as large as ~3.6 eV (carbonyl group) or
2.6 eV (ether group), which is enough to affect the placement of the
energy band edges with respect to the vacuum level (Evac), i.e.,
pulling the conduction band (CB) edge below Evac. The dipole can
also affect the electrochemical behavior significantly, leading to a
sizable repulsion of anions, with an accompanying decrease in the
electron transfer (ET) rate (see section on electrochemical
oxidation).
Oxidation can also be carried out via gas-phase reaction with
various forms of oxygen, including molecular oxygen, singlet
(molecular) oxygen, and atomic oxygen.
The reaction with
molecular oxygen has been studied extensively. It begins at around
500C and leads to the formation of carbonyl, ether, hydroxyl and
carboxylic acid groups, but there can also be significant
graphitization [2, 11]. The reaction with atomic oxygen takes place
without thermal activation [1].
Initial oxidation can be used in conjunction with a subsequent
chemical modification step. This is very similar to the approach
discussed later for initial electrochemical oxidation followed by
chemical modification. For example, Ushizawa et al. started with
an oxidized diamond powder surface, containing carboxylic acid
groups, and, via the acid chloride, made use of an esterification
reaction with hydroxyl groups on ribose moieties attached to the
DNA strands (16). Wenmackers et al. used this approach to attach
DNA strands to diamond films (17). In another example, Krysinski
et al. chemically oxidized a polycrystalline diamond film to produce
hydroxyl groups, converted these to acid chlorides, and then used
176
9. Chemical, Photochemical and Electrochemical Modification of Diamond
esterification to attach aminopyrene moieties (18). In connection
with the latter work, there is an apparent discrepancy with other
work, in that the surface coverage of oxygen is stated to be quite
low. This is a question that requires further examination, because,
for most purposes, it is desirable to maximize the surface coverage
of both the oxygen-containing surface groups and the subsequently
attached moieties.
Halogenation reactions have also been studied for many years
(19-31). Freedman found that molecular fluorine and chlorine do
not react with the diamond surface without activation, whereas
atomic fluorine and chlorine do react.
The coverage of fluorine
after treatment with atomic fluorine was about 0.75 of a monolayer,
and this was stable at temperatures up to 700 K. The stabilities of
the halogenated surfaces are experimentally less than predicted
theoretically, as pointed out by Hukka et al. (23).
Fluorination (22) and chlorination (21) of diamond powders
were carried out by Ando et al. without thermal activation for
fluorine and with thermal activation for chlorine. Comparing the
work with diamond films with that for powders shows that there
are definite differences in reactivity.
This is to be expected,
because the surfaces of nanoparticles (or even microparticles)
present a variety of crystallographic planes, as well as edges and
corners.
A more efficient means of fluorinating the diamond surface is
the plasma. Several groups have used CF4 as a fluorine source (27,
29-31). The electrochemical behavior of the resulting surface has
been examined by these same groups. In an early report, there was
no significant effect on the voltammetric background of the
177
fluorination compared to the as-deposited, hydrogen-terminated
surface (27). However, recently, there has been renewed interest in
this topic with the finding that heavily fluorinated surfaces provide
a 5-V potential working range (29).
At present, there is no
explanation of the difference in potential window between the
earlier and later reports, but the degree of fluorination may have
been greater in the latter.
The electrochemical behavior of the
heavily fluorinated surfaces is interesting, because the rates of
various redox reactions are affected quite differently from each
other. For example, hydrogen evolution is shifted by about two
volts, and the rate of ferrocyanide oxidation is decreased by three
orders of magnitude, but the rates of reactions involving several
aquo complexes are only decreased by factors of around five (30).
Thus, it appears that the ET is sensitive to the intimate details of
the approach of the redox species to the diamond surface.
The halogenated diamond surface, specifically, the chlorinated
surface, can be used for further chemical modification, for example,
to produce amine-covered or thiol-covered surfaces. This approach
has been developed for diamond powders (24), as well as for films;
the latter will be described in more detail later, in the section on
photochemical modification.
The chemical reactions of the halogens with diamond are
usually thermally activated in order to produce significant
quantities of the halogen atoms, e.g., chlorine atoms. This is a
recurring theme: the very low reactivity of the diamond surface
often requires that reactions be initiated by radicals, as halogen
atoms are. As already mentioned, the plasma is an efficient means
of generating radicals.
178
As we shall see later, radicals can be
9. Chemical, Photochemical and Electrochemical Modification of Diamond
generated photochemically. There has also been a sustained effort
to make use of a solution-phase, ambient temperature approach to
initiate radical reactions (32-38). This work has involved various
types of organic peroxides as radical initiators.
These workers
have succeeded in attaching several different organic compounds,
such as carboxylic acids, to the surfaces of diamond powders.
Another solution-phase approach has been examined, with the
use of sulfuryl chloride, a nucleophilic reagent (25). In this work,
the surfaces of diamond powders were chlorinated and butylated.
Next, we will treat cycloaddition reactions of alkenes with the
bare diamond surface, after high temperature vacuum annealing,
which results in the formation of carbon-carbon bonds (39-41). For
example, if the diamond (100) surface is heated in vacuum to
1000C, hydrogen desorbs, leaving surface C-C dimers. These have
appreciable double-bond character and can react with alkenes
under conditions appropriate for the Diels-Alder cycloaddition
reaction. Either the [2+2] or the [2+4] product can be formed, with
the latter being the energetically favored pathway.
This type of
modification can also be carried out photochemically, and this
approach is the one that is used more commonly, as discussed in
the next section.
9.3. Photochemical Modification Methods
There are two principal types of photochemical modification
techniques: 1) cycloaddition reactions of alkenes with the bare
diamond surface under UV illumination, resulting in carbon-carbon
179
bonds; and 2) radical reactions with reagents that are activated
photochemically.
As mentioned in the previous section, alkenes react with C-C
dimers on the clean diamond (100) surface, which are produced
during high temperature treatment in vacuum. This reaction can
also be activated photochemically.
This approach can be used to
attach alkyl chains that are terminated with carboxylic acid or
primary amine groups, for example, which are useful for further
functionalization (42). These groups must be protected during the
UV illumination and then subsequently deprotected. Hamers and
coworkers have used this technique to attach DNA strands to the
diamond surface, and they found that the stability of the
attachment is excellent, much better than that to other surfaces,
such as silicon or gold (43-45).
UV illumination can be used to activate radical-type reactions,
for example, chlorination, as first shown by Miller and Brown (46).
They
also
showed
that
the
chlorinated
surface
can
be
photochemically converted to an amine-covered surface (28, 46) and
to a thiol-covered surface, the latter being accomplished also
directly from the hydrogen-terminated surface (28).
9.4. Electrochemical Modification Methods
Electrochemical
modification
methods
include
1)
anodic
polarization in aqueous acid or base; and 2) radical reactions with
reagents that are activated electrochemically.
Both of these
approaches can provide a surface that can be further functionalized.
In addition, the electrochemical approach leads to the possibility of
180
9. Chemical, Photochemical and Electrochemical Modification of Diamond
patterning the surface down to the nanometer scale through the
use of scanning electrochemical
microscopy (SECM) or of
conductive atomic force microscopy (CAFM).
One of the motivations for using electrochemical oxidation,
compared to chemical oxidation, is that the oxidizing power can be
immediately controlled over a wide potential range.
Other
motivations, compared to plasma oxidation, are that the process is
simple to implement and, since it does not involve high kinetic
energy, leads to negligible surface damage.
In addition, the
amount of charge that is passed in the oxidation process can be
monitored. The electrochemical oxidation approach has been
studied in detail by the Angus group (27), by the Fujishima group
(47-55), by the Swain group (56, 57), as well as others (58-60).
As with chemical oxidation, electrochemical oxidation of the
polycrystalline surface produces a mixture of several types of
carbon-oxygen functional groups, which can reasonably be expected
to include the following: carbonyl, ether and hydroxyl on the (100)
surface and principally hydroxyl on the (111) surface, based on the
previously cited surface characterization and theoretical studies.
The
presence
of
the
carbonyl
group
has
been
confirmed
unambiguously by work with polycrystalline samples (52) and on
single-crystal-like homoepitaxial samples (61). The presence of the
hydroxyl group has also been confirmed unambiguously by work
with polycrystalline samples (54) and homoepitaxial samples (58,
61).
Just as in the case of fluorination, the oxygenation of the
surface, due to the presence of the strong dipoles mentioned
already, leads to a variety of effects on different redox couples. ET
181
to anions, such as the members of the ferro/ferricyanide redox
couple, is often slowed down considerably, compared to the
hydrogen-terminated surface, while it is either speeded up or there
is little effect for cations (50, 55-57, 62). For neutral compounds,
the effects are subtler, probably involving dipole-dipole interactions
as well as other types of interactions.
The selective inhibition of ET due to electrochemical preoxidation can lead to enhanced selectivity in the analytical
determination of components of mixtures.
One example is the
determination of dopamine (DA) in the presence of ascorbic acid
(AA) (47, 48), which is important for patients with Parkinson’s
disease and the determination of uric acid (UA) in the presence of
AA (53). Unfortunately, the determination of DA in the presence of
AA at oxidized diamond can only be carried out successfully at low
pH values (0-2), where DA is protonated and thus positively
charged. Thus, in vivo analysis is not possible. In the case of UA,
this is not a drawback, and electrochemical sensors for UA in urine
have been developed.
Surface dipole-related effects can be accentuated if there are
insufficient charge carriers near the diamond surface, either due to
a somewhat low intrinsic boron doping level (<1019 cm-3) or to a
passivation of the existing boron dopant. The latter can occur as a
result of hydrogen donors compensating boron acceptors. In either
case, the relatively small number of charge carriers cannot support
a normal level of ET to species that are at a distance from the
electrode surface, particularly those that are sensitive to the
number of charge carriers or the density of states (DOS) at the
Fermi level, such as ferrocyanide.
182
9. Chemical, Photochemical and Electrochemical Modification of Diamond
There are other examples in which analytical determinations
have been enhanced with the use of the electrochemically oxidized
surface: 1) the determination of chlorophenols (63); and the
determination of sulfur-containing organic compounds (64, 65). In
the latter case, the negatively charged surface attracts the cationic
disulfide compound. In both cases, the use of the oxidized surface
is convenient, because electrochemical oxidation is actually used
periodically to clean the surface, which can slowly become fouled
with polymeric oxidation products of the analytes.
The fouling
process proceeds much more slowly than it does on glassy carbon,
but it can still occur. These topics will be treated in greater detail
in subsequent chapters (12 and 15).
The electrochemical oxidation approach has also been carried
out at the nanometer scale on homoepitaxial films, through the use
of conductive AFM (51, 66-69). With this technique, although there
is not a conventional electrochemical cell involved, there is a small
amount of water present from the ambient air, and this condenses
at the gold-coated AFM tip.
If the tip is biased negative with
respect to the diamond surface, by 2 to 3 volts, oxidation of the
diamond surface occurs.
The actual presence of carbon-oxygen
functional groups has not been confirmed yet, due to the extremely
small scale of the modification, however. The principal effect is
that the conductivity of the homoepitaxial film is effectively
decreased, by several orders of magnitude.
In the work of the
Fujishima group (51, 69, 70), the films were boron-doped, in the
1019 cm-3 region; in work of the Kawarada group, the films were
undoped crystals, but these contained hydrogen on or near the
surface, due to treatment in a hydrogen plasma (67, 68, 71-73). In
183
both cases, the CAFM polarization serves to remove hydrogen,
leading to extremely low conductivity. In the case of the borondoped films, the conductivity may also be quite low, due to the
passivation phenomenon mentioned earlier.
In any case, the
principal application of this technique at present is the fabrication
of nanoscale electronic devices, as reported by the Kawarada group
(68). In the future, it may be quite interesting to try to expand the
range of electronic devices that can be fabricated.
The other major electrochemical modification approach has
been that in which aromatic diazonium salts in an electrolyte
solution are reduced at a diamond electrode; this leads to the
formation of an aryl radical, which can then attach to the diamond
surface (74). This work is based on a series of papers in which the
same technique was applied to the surface modification of glassy
carbon and highly ordered pyrolytic graphite (HOPG) (75-78). This
approach may also be quite fruitful for the covalent modification of
diamond surfaces, if the attachment is as robust as it is on glassy
carbon surfaces.
9.5. Combined Methods: Electrochemical/
Chemical methods
Although there are other examples of combined methods, such
as photochemical–chemical, which have already been mentioned
briefly, as in the case of the photochemical–chemical attachment of
DNA strands, we shall focus on that of electrochemical–chemical
method. This combined approach has been used recently to attach
a protein to the diamond surface (59), using first electrochemical
184
9. Chemical, Photochemical and Electrochemical Modification of Diamond
oxidation to produce hydroxyl groups, followed by esterification to
attach biotin, with subsequent attachment of streptavidin. Even
though these workers used electrochemical oxidation, they found
that chemical oxidation with singlet oxygen was more effective in
achieving high hydroxyl coverage.
We shall continue by discussing in greater detail a recent
study, in which we made use of electrochemical oxidation of singlecrystal-like homoepitaxial films and then used specific reagents to
bind to the resulting carbonyl and hydroxyl groups (61). In this
study, we found that the diamond (100) surface contains a mixture
of carbonyl and hydroxyl groups, while the (111) surface contains
mainly hydroxyl groups. The presence of other types of groups has
not examined.
In
previous
modification
of
work,
carbonyl
2,4-dinitrophenylhydrazine
(C=O)
groups
(52)
(DNPH)
and
3-
aminopropyltriethoxysilane (APTES) modification of hydroxyl
(-OH) groups (54) were carried out on oxidized polycrystalline
diamond electrodes.
The latter is particularly important for
possible applications, because the terminal amino group can be
used to form a covalent bond, e.g., via amidization, with a variety of
functional species, including examples already mentioned, such as
DNA and proteins. In this study, the surface functional groups
generated by anodic treatment on (100) and (111) homoepitaxial
single-crystal boron-doped diamond electrodes were first examined
with X-ray photoelectron spectroscopy (XPS). Subsequently, DNPH
and APTES modifications of the anodically treated surfaces were
carried out and were characterized again with XPS, as well as with
cyclic voltammetry and electrochemical impedance measurements.
185
The original work can be consulted for the experimental
details, which will be given here briefly. The single-crystal-like
boron-doped homoepitaxial diamond films were prepared by
microwave plasma-assisted chemical vapor deposition on synthetic
high pressure-high temperature single-crystal substrates (3-4° offaxis polished). Hydrogen-terminated surfaces were prepared from
as-deposited or oxidized surfaces by heating to 900 °C in hydrogen
ambient for 1 h.
For the anodic treatment, a single positive
potential sweep (0 to +3 V vs. Ag/AgCl), with sweep rate 100 mV s -1
in 0.1 M sulfuric acid, was used unless otherwise noted.
The
procedures used for the surface modification with DNPH and
APTES were the same as those in described in the previous
references.
First, the XPS spectra for the oxidized surfaces will be
presented. Figure 9.1 shows the C 1s spectra for the hydrogenterminated, anodically treated and oxygen-plasma treated (100)
and (111) surfaces. For the hydrogen-terminated surfaces (a and b),
the main peak is assigned to carbon in the diamond bulk; its
binding energy was found to be 285.0 ± 0.2 eV, and this was used
as the binding energy shift (BES) reference. The small peaks found
at ca. +1.6 and +1.8 eV BES for the (100) and (111) surfaces are
similar to those that have been assigned to carbon bonded to
multiple hydrogens (79) or to subsurface hydrogen (80). There is
negligible oxygen present on the hydrogen-terminated surfaces.
For the oxidized surfaces (both anodic and plasma treatment),
shoulder-like peaks at ca. +2 eV appeared (c-f). These peaks are
assigned to carbon singly bonded to oxygen (11, 81, 82). For the
anodically treated (100) diamond surface, another shoulder-like
186
9. Chemical, Photochemical and Electrochemical Modification of Diamond
peak, at ca. +4 eV BES, was observed (c). This peak is assigned to
doubly bonded carbon-oxygen (11, 83).
b
(100)
Intensity / cps
Intensity / cps
a
Binding Energy Shift / eV
Binding Energy Shift / eV
d
(100)
Intensity / cps
Intensity / cps
c
Binding Energy Shift / eV
(111)
Binding Energy Shift / eV
f
(100)
Binding Energy Shift / eV
Intensity / cps
e
Intensity / cps
(111)
(111)
Binding Energy Shift / eV
Fig. 9.1. XPS C 1s spectra for (a, b) hydrogen-terminated, (c, d)
anodically-treated and (e, f) oxygen-plasma treated (100) and (111)
single-crystal diamond thin films.
187
For the anodically oxidized (100) surface (c), both singly and
doubly bonded C-O are present, whereas, for the (111) surface (d),
only singly bonded oxygen appears to be present. As already
mentioned above, the possible oxygen-containing surface functional
groups for diamond (100) include the hydroxyl group (C-OH), the
ether structure (C-O-C) and the carbonyl (C=O) group (4-7, 9). The
hydroxyl group is the only one that could reasonably exist on the
unreconstructed (111) surface (10, 84).
The results for the XPS C 1s peak fittings for the
homoepitaxial surfaces are summarized in Table 9.1, together with
previously reported results for the polycrystalline surfaces. It can
be concluded from these results that the polycrystalline surfaces
consisted of a slightly greater amount of (111) than (100) faces for
the first electrode and just the reverse for the second electrode.
From the areas of the O 1s and C 1s peaks in the XPS survey
spectra (not shown), corresponding to Fig. 9.1, the overall O/C
ratios have been estimated; these are summarized in Table 9.2.
For the (100) surface, the O/C ratios tend to be in the 0.1 to 0.2
range, while those for (111) tend to be around 0.2. The latter value
is probably rather close to a full monolayer, based on the following
argument:
the sampling depth is expected to lead to an
enhancement in the carbon spectrum of between a factor of 3.0 and
a factor of 8.8, depending on the specific model used, (85) for the
present conditions (Mg K radiation, X-ray incidence angle, 45), so
that we have chosen a compromise value of five.
188
9. Chemical, Photochemical and Electrochemical Modification of Diamond
Table 9.1. Atomic concentration on modified single-crystal diamond
surface estimated from XPS analysis.
N/C ratio at DNPH-modified
N/O ratio at APTES-
surface
modified surface
(100)
0.015-0.025
0.16
(111)
negligible
0.45
Table 9.2. Estimated ratio of surface functional groups to the total
surface oxygen on anodically-treated single-crystal diamond surfaces.
C=O
C-OH
poly
0.05a
0.6b
(100)
0.19
0.48
(111)
negligible
~1
Notsu et al., J. Electroanal. Chem., 492 (2000) 31;
Electrochem. Solid-State Lett., 4 (2001) H1.
a
b
Notsu et al.,
For oxygen-plasma treated surfaces (e and f), a shoulder at ca.
+2 eV was observed for both the (100) and (111) single-crystal
diamond surfaces, with no evidence of a peak at +4 eV on the (100)
surface.
This result suggests that the higher kinetic energy
involved with the oxygen plasma treatment somehow leads to a
surface on which there is little carbonyl coverage. Direct evidence
for the effect of the plasma treatment is found in the significantly
enlarged shoulders at -1.8 to -2.0 eV; for spectra a-d, the peaks in
this position can be assigned to carbon dimers (79) or to crystal
defects (82). For spectra d and f, the most reasonable assignment is
surface graphite (79), which can be produced by the energetic
oxygen atoms.
189
In the original paper, linear sweep voltammograms (LSV) are
shown for the hydrogen-terminated (100), (111) single-crystal and
polycrystalline diamond electrodes in 0.1 M sulfuric acid. The
potentials found for the anodic peaks were ca. +1.5 V for the (100)
surface and +1.65 vs. Ag/AgCl for (111) surface. In addition, for the
polycrystalline surface, both peaks were observed in a single LSV.
At +1.8 V vs. Ag/AgCl, current for oxygen evolution reaction began
to be seen. These anodic peaks could be seen in the first positivegoing potential sweep but not in further sweeps. These peaks were
assigned to the formation of oxygen-containing groups on the
electrode surface.
These results were compared to those reported by Martin et al.
(27) Differences in the LSV behavior for the (100) and (111)
surfaces were also reported by these authors. However, they
observed anodic currents during the first sweep that were on the
order of 1 to 2 µA cm-2, whereas those observed in the later work
were on the order of 100-200 µA cm-2 (61). At present, there is no
clear explanation for this large difference in the oxidation currents.
Based on an integration of the experimental currents and
comparison with the theoretically calculated ones, it can be
concluded that the LSV curves correspond to conversion of the
hydrogen-terminated surfaces to nearly fully oxygen-terminated
surfaces. For the theoretical estimates, we made the assumption
that the (100) surface was initially (21): H, and the (111) surface
was initially (11):H. Furthermore, as a result of anodic oxidation,
we assumed that every carbon atom on the (100) surface produces a
carbonyl (C=O) group, and every carbon on the (111) surface
produces a hydroxyl (C-OH) group. Thus, the anodic oxidation of
190
9. Chemical, Photochemical and Electrochemical Modification of Diamond
the (100) (21):H surface to a (11):O surface would require three
electrons per oxygen atom, yielding 7.64  10-4 C cm-2, based on a
spacing of 2.508 Å between surface carbon atoms. For one (100)
sample (A), the oxidation charge was less than this value, but for
sample B, the charge was slightly greater, but still in good
agreement. For the (111) (11):H surface, oxidation to (11):OH
requires two electrons per OH group, yielding a value of 7.84  10-4
C cm-2, (2.508 Å spacing). In this case also, the integrated charge
was slightly greater, but still in good agreement.
30000
Intensity / cps
25000
A
C 1s
20000
15000
O 1s
N 1s
10000
5000
0
1000
800
600
400
200
0
Intensity / cps
20000
15000
B
C 1s
O 1s
10000
5000
0
1000
800
600
400
200
0
Binding energy / eV
Fig. 9.2. XPS survey spectra for DNPH-modified single-crystal
diamond surfaces: (A) (100) and (B) (111) homoepitaxial boron-doped
diamond films.
191
9500
10500
Intensity / cps
Intensity / cps
10000
A
9000
8500
8000
7500
405
403
401
399
397
10000
B
9500
9000
8500
8000
405
395
403
Intensity / cps
Intensity / cps
397
395
397
395
10000
11000
C
10000
9500
9000
8500
405
399
Binding energy / eV
Binding energy / eV
10500
401
403
401
399
397
395
9500
D
9000
8500
8000
7500
405
Binding energy / eV
403
401
399
Binding energy / eV
Fig. 9.3. XPS N1s spectra for single-crystal diamond surfaces: (A, B)
before and (C, D) after DNPH-modification; (A, C) (100) and (B, D)
(111) homoepitaxial boron-doped diamond films.
The XPS survey spectra obtained after the DNPH modification
are shown in Fig. 9.2.
In spectrum a (100), there is a small N 1s
peak, whereas, in spectrum b (111), there is no discernable peak.
The N 1s region is shown in detail in Fig. 9.3; from these results, it
is rather clear that the (100) surface was partially modified with
DNPH, indicating the presence of carbonyl groups, but the (111)
surface was not, indicating their absence. The N/C atomic ratio for
the DNPH-treated single-crystal diamond (100) surface was
estimated to be 0.015-0.025 (Table 9.3).
These values can be
compared to values that would be obtained if DNPH were at full
192
9. Chemical, Photochemical and Electrochemical Modification of Diamond
coverage. For a vertical orientation, the DNPH molecule would
occupy approximately 30 Å2, compared to the area occupied by one
carbon atom on the (100) surface (6.29 Å2), so that one DNPH
would cover 4.77 surface carbons (see Fig. 9.4). Assuming some
disorder, one could assume that a limiting coverage might be one
DNPH per 10 surface carbons. The N/C ratio for this situation
should be 0.071, taking into account the number of carbon and
nitrogen atoms (6 and 4, respectively) for an attached DNPH
molecule and the fact that several layers of carbon atoms
underlying the surface are also being sampled (so that the carbon
signal is enhanced by a factor of four to five) (52, 85). Thus, the
experimentally obtained N/C ratios would correspond to 21-35% of
“full” coverage. This level of coverage seems reasonable, since the
surface oxygens on the anodically treated (100) face are expected
to include a mixture of carbonyl, hydroxyl and ether groups. In fact,
the
experimental
XPS
results
(Fig.
9.1c)
indicated
that
approximately 19% of the surface oxygens (at essentially one full
monolayer) were carbonyl groups for the unmodified anodically
treated (100) surface.
On the other hand, the N/C ratios
correspond to only one DNPH to every 30-50 surface carbons, which
means that only 10-17% of the carbonyl groups reacted with DNPH.
Table 3.
Atomic concentration ratios on modified single-crystal
diamond surfaces estimated from XPS analysis.
Surface
(100)
(111)
N/C ratio (DNPHN/O ratio (APTESmodified)
modified)
0.015-0.025
0.16
negligible
0.45
193
top
side
Fig. 9.4. Representation of the attachment of DNPH moieties to the
diamond (100) surface.
194
9. Chemical, Photochemical and Electrochemical Modification of Diamond
The APTES coverage can also be estimated from the atomic
concentrations obtained by XPS, shown in Fig. 9.5. For a diamond
surface fully covered by APTES, the N/O atomic ratio would be 0.33,
assuming one APTES molecule reacts with three hydroxyl groups
on the surface (see Fig. 9.6). In the present study, the N/O ratios
for the (100) and (111) APTES-modified diamond surface were
estimated to be 0.16 and 0.45, respectively. Given the uncertainty
in the results, they indicate that the coverage of hydroxyl on the
(100) surface is around 50%, while that on the (111) surface is
around 100%. It was also seen in the C 1s spectrum for anodically
treated (100) diamond (Fig. 9.1c) that the surface could contain
well over 50% hydroxyl groups.
40000
35000
Intensity / cps
Intensity / cps
40000
A
30000
25000
20000
405
403
401
399
397
35000
30000
25000
20000
405
395
B
403
C
25000
20000
15000
405
399
397
395
35000
Intensity / cps
Intensity / cps
35000
30000
401
Biding energy / eV
Biding energy / eV
403
401
399
Biding energy / eV
397
395
30000
D
25000
20000
15000
405
403
401
399
397
395
Biding energy / eV
Fig. 9.5. XPS N1s spectra for single-crystal diamond surfaces: (A, B)
before and (C, D) after APTES-modification; (A, C) (100) and (B, D)
(111) homoepitaxial boron-doped diamond films.
195
Fig. 9.6. Representation of the attachment of APTES moieties to the
diamond (111) surface.
Taken together with the earlier results for polycrystalline
diamond electrodes (54), the APTES coverage versus oxygen on the
diamond surface exhibits a trend of (111) > polycrystalline (N/O =
0.15—0.25) > (100).
This is quite reasonable, because the
polycrystalline surface contains a mixture of the two crystal faces.
From the results of the covalent surface modification of the singlecrystal diamond surfaces, we can conclude that anodically treated
diamond (100) surfaces consist of a mixture of carbonyl and
hydroxyl groups, although the presence of ether groups is not ruled
out. On the diamond (111) surface, the hydroxyl group appears to
be present at around 100%.
We should emphasize that the ether structure (C-O-C) is also
possible on the (100) diamond surface, given the predominance of
singly bonded oxygen on this surface indicated by the C 1s
196
9. Chemical, Photochemical and Electrochemical Modification of Diamond
spectrum. In fact, even though most of the work that has appeared
concerning the oxidation of the (100) surface has discussed mainly
ether and carbonyl functionalities (4, 6, 7, 9), it has been shown
theoretically that a hydroxyl-covered diamond (100) surface can be
greatly stabilized if these groups exist side-by-side on C-C dimers
and can undergo hydrogen bonding.(5)
We now turn to the electrochemical behavior.
Figure 9.7
shows cyclic voltammograms (CVs) obtained for the Fe2+/3+ redox
couple
at
subsequently
hydrogen-terminated,
DNPH-modified
(100)
anodically
and
(111)
treated,
and
single-crystal
diamond electrode surfaces. It has been reported that the electrontransfer rate for this redox couple tends to be affected by surface
termination (50, 86). For polycrystalline diamond electrodes, the
electron-transfer rate is larger at oxygen-terminated surfaces than
at hydrogen-terminated ones. In the present case, the electrontransfer rate became greater after anodic treatment of the
electrode surfaces, as evidenced by decreases in the CV anodiccathodic peak separations (Ep) (Fig. 9.8).
Following DNPH
treatment, the Ep value obtained for the (100) electrode surface
was found to be greater than that for the corresponding anodically
treated surface, but, in contrast, that for the (111) surface showed
almost no change. These results support the presence of carbonyl
groups on the (100) surface and their absence on the (111) surface.
For glassy carbon electrodes, it has been reported that
carbonyl groups on the surface, possibly in the form of quinones,
can act as catalysts for the Fe2+/3+ couple (50, 52, 86). Given the
fact that graphitic carbon is essentially not present on the diamond
surfaces being studied, there could be few quinone groups.
197
Current density / A cm-2
a
Potential / V vs. Ag/AgCl
Current density / A cm-2
b
Potential / V vs. Ag/AgCl
Fig. 9.7. CVs for 1 mM Fe(ClO4)2 in 0.1 M H2SO4 at (a) (100) and (b)
(111) single-crystal diamond electrodes;
dotted line, hydrogenterminated; dashed line, anodically-treated; and solid line, DNPHmodified after anodic treatment.
On the oxidized surface of polycrystalline diamond, carbonyl
groups can certainly exist, but they are of an aliphatic type, which
behave quite differently from the quinone type. Indeed, this type of
198
9. Chemical, Photochemical and Electrochemical Modification of Diamond
carbonyl also appears to accelerate the ET for the Fe2+/3+ couple, but
this is due to the previously mentioned attraction of the positively
charged ions to the negatively charge electrode surface. The DNPH
molecule reacts selectively with surface carbonyl groups, effectively
blocking them.
Therefore, for electrode surfaces containing
carbonyl groups, DNPH treatment should cause a decrease in the
ET rate for the Fe2+/3+ couple, whereas, for surfaces containing no
carbonyl groups, this treatment should not produce any effect.
Thus, we can conclude that the anodically treated (100) diamond
surface does include a significant coverage of carbonyl groups, but
that the anodically treated (111) surfaces contain essentially no
carbonyl groups. This conclusion is consistent with the XPS results.
Ep / mV
■ (100)
● (111)
Hydrogenterminated
Anodically
-treated
DNPHmodified
Fig. 9.8. Variation of peak separations (Ep) of CVs in Fig. 9.7.
For the APTES modification, the surfaces were characterized
by recording the CVs for the Fe(CN)63-/4- redox couple (Fig. 9.9).
From the variation of Ep, it was found that the ET behavior was
nearly reversible at the hydrogen-terminated diamond electrode
199
surfaces, but, at the corresponding oxygen-terminated surfaces, it
became much more irreversible, i.e., with a tendency opposite to
that for Fe2+/3+. This is also likely to be due to an electrostatic
effect, in which the surface dipoles of the C-O functional groups
tend to repel the negative charges of both members of the redox
couple (50, 55, 87).
200
Current density / A cm-2
9. Chemical, Photochemical and Electrochemical Modification of Diamond
a
Current density / A cm-2
Potential / V vs. Ag/AgCl
b
Current density / A cm-2
Potential / V vs. Ag/AgCl
c
Potential / V vs. Ag/AgCl
Fig. 9.8. CVs for 1 mM K3Fe(CN)6 in 0.1 M H2SO4 at (a) polycrystalline,
(b) (100) and (c) (111) single-crystal diamond electrodes; dotted line,
hydrogen-terminated; dashed line, anodically-treated; and solid line,
APTES-modified after anodic treatment.
It should pointed out that the shapes of the CVs for the
anodically-treated single-crystal electrodes in Fig. 9.9 were
201
asymmetric,
i.e.,
the
cathodic
peak
currents
were
either
significantly smaller than the anodic peak currents, on the (111)
surface (c), or could not be observed at all, on the (100) surface (b).
This behavior may be due to semiconductor-type behavior. As
mentioned above, the surface dipole effects are accentuated in
cases in which the carrier concentrations fall below the 1019 cm-3
level.
We have also shown that the acceptor density near the
surface for as-deposited, hydrogen-terminated diamond electrodes
can be much higher than that expected from the boron doping level,
and can decrease to a density lower than the doping level after
surface oxidation (62). The differences in the electrical properties
are also reflected in the ET behavior on the electrode surface. Thus,
in the case of the hydrogen-terminated (100) single-crystal
diamond electrode, the ET for Fe(CN)63-/4- (Fig. 9.9b) showed
reversible character, which may be due to higher accepter density
in the near-surface region than that expected based on the doping
level.
Similar behavior has been reported for lightly-doped
polycrystalline diamond electrodes.(55).
It should also be noted that the voltammetric behavior for the
Fe2+/3+ redox couple (Fig. 9.7) did not exhibit significant asymmetry.
This difference is probably due to the fact that this redox couple is
not as sensitive to the number of charge carriers as is the
ferro/ferricyanide couple.
For
the
oxidized
and
subsequently
APTES-treated
polycrystalline (Fig. 9.9a) and (111) single-crystal diamond
electrodes (Fig. 9.9c), the CV shapes for the ferro/ferricyanide
couple became almost the same as those for the respective
hydrogen-terminated surfaces. The result for the APTES-modified
202
9. Chemical, Photochemical and Electrochemical Modification of Diamond
polycrystalline diamond electrode is similar to that previously
reported by Notsu et al. (54). In that paper, it was proposed that
the protonated amino group of the APTES moiety was responsible
for electrostatically attracting the anionic redox species. For the
diamond (100) surface (Fig. 9.9b), the shape of the CV for the
APTES-modified surface did not return to that for the hydrogenterminated surface, but the anodic peak potential shifted
significantly negative of that for the anodically treated surface.
The fact that it did not return to the initial behavior is probably
due to the fact that the doping level for the (100) epitaxial film was
lower than those for the polycrystalline and (111) films (55, 62).
In
the
original
paper,
electrochemical
AC
impedance
measurements were also carried out, and these results also tend to
support those obtained in the cyclic voltammetry. Thus, in this
study, by use of XPS in conjunction with chemical surface
modification techniques, new information was obtained on the
surface functional groups generated on oxidized diamond surfaces.
In particular, it was found that the variation of oxygen-containing
surface functional groups strongly depends on the exposed crystal
face, as well as on the type of oxidative treatment, e.g., anodic or
oxygen plasma. To understand the dependence of the coverages of
oxygen-containing surface functional groups on treatment and
crystal face is especially important, for two reasons: one is that the
oxygen termination can include various types of surface functional
groups, and thus a variety of functional surfaces can be created by
utilizing these groups.
The other is that diamond electrode
surfaces typically become oxidized during practical use as an
electrode material, because, as we have seen, only a single positive-
203
going potential sweep may be enough to completely oxidize the
diamond.
9.6. Metal and Metal Oxides on Diamond Surfaces
As a separate topic, we now discuss metals and metal oxides
deposited on diamond surfaces. The topic of metals on diamond has
been studied for a number of years from the standpoint of
semiconductor devices (3, 88-105). Many of these papers discuss
the effect of hydrogen vs. oxygen termination and the resulting
surface dipoles.
As mentioned above, this work has received
attention over the years due to the great interest in electron
emission.
However,
electrochemical
our
interest
here
characteristics.
is
more
The
related
often-cited
to
the
intrinsic
electrochemical characteristics of diamond, including extreme
stability, low background current and large potential working
range, are also advantages for the fundamental study of the
electrochemical
characteristics
of
solid
materials.
The
electroanalytical applications of electrochemical metal deposition
will be treated in Chapter 16. The materials of interest in the
present chapter are those with applications as battery electrodes,
capacitor electrodes and electrocatalysts. There may even be cases
in which diamond can play a role as a practical support material.
We begin with electrocatalysts. Platinum has been deposited
on diamond surfaces electrolytically and then examined with
various techniques (106-109). The Pt deposit has been found to be
quite stable on the polycrystalline diamond surface during
204
9. Chemical, Photochemical and Electrochemical Modification of Diamond
potential cycling, more so than on conventional carbon or graphite
substrates.
This is most likely due to stability of the diamond
substrate itself, particularly at highly positive potentials, at which
graphitic carbon undergoes irreversible oxidation.
Other metals that have been of interest include copper (110)
and nickel (111).
These have been studied due to their
electrocatalytic activity for glucose oxidation.
The copper was
deposited electrochemically, whereas the nickel was deposited via
ion implantation.
The behavior of these metals in the form of
nanoparticles on diamond is highly attractive in terms of glucose
determination, because the current for glucose oxidation is
increased dramatically without increasing the background current
substantially.
There has been some interest in the possibility that diamond
electrodes could be intercalated or inserted with lithium, because
there are still problems associated with graphitic materials in
terms of stability. Li et al. showed that there is essentially no
“underpotential deposition” of lithium on homoepitaxial diamond
(111) surfaces under highly controlled conditions, particularly the
absence of sp2 carbon-containing grain boundaries (112). In work
of Pleskov and coworkers, they also concluded that there is
neglibible Li intercalation into diamond nanoparticles (113).
However, there is recent work that does suggest that significant
insertion of Li into a diamond film grown on carbon is possible
(114).
One of the first metal oxides to be examined electrochemically
on a diamond substrate was ruthenium dioxide (115, 116). This
material is important both for electrochemical capacitor and
205
electrocatalytic applications (chlorine evolution). Another example
is cobalt hydrous oxide, which has catalytic activity for oxygen
evolution (117). A very recent example is lead dioxide (118). A
metal oxide (V2O3) has also been supported on particulate diamond
as a catalyst for an organic gas-phase reaction (119).
9.7. Conclusions
The highly attractive characteristics of conductive diamond
electrodes continue to bring attention.
Recently, it has also
become clear that the possibilities for chemical modification can
take advantage of these fundamental characteristics and build
upon them, imparting novel selectivity and catalytic activity. The
possible applications are as diverse as those for all other types of
electrodes, because diamond can be a useful substrate for all.
Moreover, all of the modification techniques that have been worked
out over the past several years for other surfaces, such as carbon
and gold, can now be transplanted onto diamond. Now is just the
beginning of the blossoming of this field.
Acknowledgements
The authors would like to express their appreciation to the
students and researchers who have participated in this research.
The APTES figure was drawn by Mr. Hiroyuki Ito, and the DNPH
206
9. Chemical, Photochemical and Electrochemical Modification of Diamond
figure was drawn by Mr. Shinsuke Aoshima, both of the Tokyo
Science University.
The research has been supported by the
Ministry of Education, Science, Sports and Culture of Japan, the
National Aeronautics and Space Administration, the U.S. Army
Office of Research, and the National Science Foundation.
References
1.
R. E. Thomas, R. A. Rudder, and R. J. Markunas, J. Vac. Sci.
Technol. A 10, 2451 (1992).
2.
T. Ando, M. Ishii, M. Kamo, and Y. Sato, J. Chem. Soc., Farad.
Trans. 89, 1783 (1993).
3.
J. van der Weide and R. J. Nemanich, Phys. Rev. B 49, 13629
(1994).
4.
J. L. Whitten, P. Cremaschi, R. E. Thomas, R. A. Rudder, and R. J.
Markunas, Appl. Surf. Sci. 75, 45 (1994).
5.
S. Skokov, B. Weiner, and M. Frenklach, Phys. Rev. B 55, 1895
(1997).
6.
M. Z. Hossain, T. Kubo, T. Aruga, N. Takagi, T. Tsuno, N.
Fujimori, and M. Nishijuma, Surf. Sci. 436, 63 (1999).
7.
P. E. Pehrsson and T. W. Mercer, Surf. Sci. 460, 49 (2000).
8.
P. E. Pehrsson and T. E. Mercer, Surf. Sci. 460, 74 (2000).
9.
H. Tamura, H. Zhou, K. Sugisako, Y. Yokoi, S. Takami, M. Kubo,
K. Teraishi, A. Miyamoto, A. Imamura, M. N.-Gamo, and T. Ando,
Phys. Rev. B 61, 11025 (2000).
10. K. P. Loh, X. N. Xie, S. W. Wang, and J. C. Zheng, J. Phys. Chem.
B 106, 5230 (2002).
207
11. P. John, N. Polwart, C. E. Troupe, and J. I. B. Wilson, J. Amer.
Chem. Soc. 125, 6600 (2003).
12. A. Laikhtman, A. Lafosse, Y. L. Coat, R. Azria, and A. Hoffman, J.
Chem. Phys. 119, 1794 (2003).
13. Y. V. Pleskov, Russ. Chem. Rev. 68, 381 (1999).
14. M. J. Rutter and J. Robertson, Phys. Rev. B 57, 9241 (1998).
15. J. Robertson and M. J. Rutter, Diamond Rel. Mater. 7, 620 (1998).
16. K. Ushizawa, Y. Sato, T. Mitsumori, T. Machinami, T. Ueda, and
T. Ando, Chem. Phys. Lett. 351, 105 (2002).
17. S. Wenmackers, K. Haenen, M. Nesladek, P. Wagner, L. Michiels,
M. vandeVen, and M. Ameloot, Phys. Status Solidi A 199, 44
(2003).
18. P. Krysinski, Y. Show, J. Stotter, and G. J. Blanchard, J. Am.
Chem. Soc. 125, 12726 (2003).
19. A. Freedman and C. D. Stinespring, Appl. Phys. Lett. 57, 1194
(1990).
20. A. Freedman, J. Appl. Phys. 75, 3112 (1994).
21. T. Y. Ando, Kazuo; Suehara, Shigeru; Kamo, Matsukazu; Sato,
Yoichiro; Shimosaki, Shinji; Nishitani-Gamo, Mikka, J. Chin.
Chem. Soc. (Taipei) 42, 285 (1995).
22. T. Ando, K. Yamamoto, M. Kamo, Y. Sato, Y. Takamatsu, S.
Kawasaki, F. Okino, and H. Touhara, J. Chem. Soc., Farad. Trans.
91, 3209 (1995).
23. T. I. Hukka, T. A. Pakkanen, and M. P. D'Evelyn, J. Am. Chem.
Soc. 99, 4710 (1995).
24. T. Ando, Nishitani-Gamo, M. Rawles, R. E., K. Yamamoto, M.
Kamo, and Y. Sato, Diamond Rel. Mater. 5, 1136 (1996).
208
9. Chemical, Photochemical and Electrochemical Modification of Diamond
25. Y. Ikeda, T. Saito, K. Kusakabe, S. Morooka, H. Maeda, Y.
Taniguchi, and Y. Fujiwara, Diamond Rel. Mater. 7, 830 (1998).
26. B. K. Ohtani, Young-Ho; Yano, Takashi; Hashimoto, Kazuhito;
Fujishima, Akira; Uosaki, Kohei, Chem. Lett., 953 (1998).
27. H. B. Martin, A. Argoitia, J. C. Angus, and U. Landau, J.
Electrochem. Soc. 146, 2959 (1999).
28. J. B. Miller, Surf. Sci. 439, 21 (1999).
29. S. Ferro and A. DeBattisti, Anal. Chem. 75, 7040 (2003).
30. S. Ferro and A. DeBattisti, J. Phys. Chem. B 107, 7567 (2003).
31. G.
Sine,
L.
Ouattara,
M.
Panizza,
and
C.
Comninellis,
Electrochem. Solid-State Lett. 6, D9 (2003).
32. T. H. Tsubota, Osamu; Ida, Shintaro; Nagaoka, Shoji; Nagata,
Masanori; Matsumoto, Yasumichi, Diamond Rel. Mater. 11, 1360
(2002).
33. T. Tsubota, O. Hirabayashi, S. Ida, S. Nagaoka, M. Nagata, and Y.
Matsumoto, Diamond Rel. Mater. 11, 1374 (2002).
34. T. H. Tsubota, Osamu; Ida, Shintaro; Nagaoka, Shoji; Nagata,
Masanori; Matsumoto, Yasumichi, Phys. Chem. Chem. Phys. 4,
806 (2002).
35. T. I. Tsubota, Shintaro; Hirabayashi, Osamu; Nagaoka, Shoji;
Nagata, Masanori; Matsumoto, Yasumichi, Phys. Chem. Chem.
Phys. 4, 3881 (2002).
36. T. T. Tsubota, Shunsuke; Ida, Shintaro; Nagata, Masanori;
Matsumoto, Yasumichi, Phys. Chem. Chem. Phys. 5, 1474 (2003).
37. S. Ida, T. Tsubota, S. Tanii, M. Nagata, and Y. Matsumoto,
Langmuir 19, 9693 (2003).
209
38. S. T. Ida, Toshiki; Hirabayashi, Osamu; Nagata, Masanori;
Matsumoto, Yasumichi; Fujishima, Akira, Diamond Rel. Mater.
12, 601 (2003).
39. D. R. Fitzgerald and D. J. Doren, J. Am. Chem. Soc. 122, 12334
(2000).
40. J. S. Hovis, S. K. Coulter, R. J. Hamers, M. P. D'Evelyn, J. N.
Russell, Jr., and J. E. Butler, J. Am. Chem. Soc. 122, 732 (2000).
41. G. T. Wang, S. F. Bent, J. N. Russell, Jr., and J. E. Butler, J.
Amer. Chem. Soc. 122, 744 (2000).
42. T. Strother, T. Knickerbocker, J. N. Russell, Jr., J. E. Butler, L. M.
Smith, and R. J. Hamers, Langmuir 18, 968 (2002).
43. W. Yang, O. Auciello, J. E. Butler, W. Cai, J. A. Carlisle, J. E.
Gerbi, D. M. Gruen, T. Knickerbocker, T. L. Lasseter, J. John N.
Russell, L. M. Smith, and R. J. Hamers, Nature Mat. 1, 253 (2002).
44. T. Knickerbocker, T. Strother, M. P. Schwartz, J. N. Russell, Jr., J.
Butler, L. M. Smith, and R. J. Hamers, Langmuir 19, 1938 (2003).
45. W. Yang, J. E. Butler, J. N. Russell, Jr., and R. J. Hamers,
Langmuir 20, 6778 (2004).
46. J. B. Miller and D. W. Brown, Langmuir 12, 5809 (1996).
47. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk, and A. Fujishima,
Electrochem. Solid-State Lett. 2, 522 (1999).
48. E. Popa, H. Notsu, T. Miwa, D. A. Tryk, and A. Fujishima,
Electrochem. Solid-State Lett. 2, 49 (1999).
49. I. Yagi, K. Tsunozaki, D. A. Tryk, and A. Fujishima, Electrochem.
Solid-State Lett. 9, 457 (1999).
50. I. Yagi, H. Notsu, T. Kondo, D. A. Tryk, and A. Fujishima, J.
Electroanal. Chem. 473, 173 (1999).
210
9. Chemical, Photochemical and Electrochemical Modification of Diamond
51. M. Yanagisawa, H. Tai, I. Yagi, D. A. Tryk, A. Fujishima, and L.
Jiang, in "Sixth International Symposium on Diamond Materials,
Extended Abstracts, Joint Meeting of the Electrochemical Society
and the Electrochemical Society of Japan". The Electrochemical
Society, Honolulu, Hawaii, 1999.
52. H. Notsu, I. Yagi, T. Tatsuma, D. A. Tryk, and A. Fujishima, J.
Electroanal. Chem. 492, 31 (2000).
53. E. Popa, Y. Kubota, D. A. Tryk, and A. Fujishima, Anal. Chem. 72,
1724 (2000).
54. H. Notsu, T. Fukuzawa, T. Tatsuma, D. A. Tryk, and A. Fujishima,
Electrochem. Solid State Lett. 4, H1 (2001).
55. D. A. Tryk, K. Tsunozaki, T. N. Rao, and A. Fujishima, Diamond
Rel. Mater. 10 (2001).
56. M. C. Granger and G. M. Swain, J. Electrochem. Soc. 146, 4551
(1999).
57. M. C. Granger, M. Witek, J. Xu, J. Wang, M. Hupert, A. Hanks, M.
D. Koppang, J. E. Butler, G. Lucazeau, M. Mermoux, J. W. Strojek,
and G. M. Swain, Anal. Chem. 72, 3793 (2000).
58. H. B. Martin and J. Philip W. Morrison, Electrochem. Solid-State
Lett. 4, E17 (2001).
59. D. Delabouglise, B. Marcus, M. Mermoux, P. Bouvier, J. ChaneTune, J.-P. Petit, P. Mailley, and T. Livache, Chem. Commun.,
2698 (2003).
60. W. C. Poh, K. P. Loh, W. D. Zhang, S. Triparthy, J.-S. Ye, and F.-S.
Sheu, Langmuir 20, 5484 (2004).
61. T. Kondo, K. Honda, D. A. Tryk, and A. Fujishima, J. Electrochem.
Soc., in press (2004).
211
62. T. Kondo, D. A. Tryk, and A. Fujishima, Electrochim. Acta 48,
2739 (2003).
63. C. Terashima, T. N. Rao, B. V. Sarada, D. A. Tryk, and A.
Fujishima, Anal. Chem. 74, 895 (2002).
64. O. S. Chailapakul, W.; Sarada, B. V.; Terashima, C.; Rao, Tata N.;
Tryk, D. A.; Fujishima, A.;, Analyst (Cambridge, U.K.) 127, 1164
(2002).
65. C. Terashima, T. N. Rao, B. V. Sarada, Y. Kubota, and A.
Fujishima, Anal. Chem. 75, 1564 (2003).
66. We should note that the nanolithographic AFM modification of
diamond was apparently discovered independently and nearly
simultaneously by the Fujishima and Kawarada groups.
67. M. Tachiki, T. Fukuda, K. Sugata, H. Seo, H. Umezawa, and H.
Kawarada, Jpn. J. Appl. Phys. 39, 4631 (2000).
68. M. Tachiki, H. Seo, T. Banno, Y. Sumikawa, H. Umezawa, and H.
Kawarada, Appl. Phys. Lett. 81, 2854 (2002).
69. T. Kondo, M. Yanagisawa, L. Jiang, D. A. Tryk, and A. Fujishima,
Diamond Rel. Mater. 11, 1788–1796 (2002).
70. M. Yanagisawa, L. Jiang, D. A. Tryk, K. Hashimoto, and A.
Fujishima, Diamond Rel. Mater. 8, 2059 (1999).
71. M. Tachiki, T. Fukuda, K. Sugata, H. Seo, H. Umezawa, and H.
Kawarada, in "Third International Symposium on the Control of
Semiconductor Interfaces", p. 578, Karuizawa, Japan, 1999.
72. H. Seo, M. Tachiki, T. Banno, Y. Sumikawa, H. Umezawa, and H.
Kawarada, Jpn. J. Appl. Phys., Part I 41, 4980 (2002).
73. K. Sugata, M. Tachiki, T. Fukuda, H. Seo, and H. Kawarada, Jpn.
J. Appl. Phys., Part I 41, 4983 (2002).
212
9. Chemical, Photochemical and Electrochemical Modification of Diamond
74. T.-C. Kuo, R. L. McCreery, and G. M. Swain, Electrochem. Solid-
State Lett. 2, 288 (1999).
75. M. Delamar, R. Hitmi, J. Pinson, and J. M. Saveant, J. Am. Chem.
Soc. 114, 5883 (1992).
76. C. Bourdillon, M. Delamar, C. Demaille, R. Hitmi, J. Moiroux, and
J. Pinson, J. Electroanal. Chem. 336, 113 (1992).
77. P. Allongue, M. Delamar, B. Desbat, O. Fagebaume, R. Hitmi, J.
Pinson, and J.-M. Saveant, J. Am. Chem. Soc. 119, 201 (1997).
78. C. Saby, B. Ortiz, G. Y. Champagne, and D. Belanger, Langmuir
13, 6805 (1997).
79. R. Graupner, F. Maier, J. Ristein, L. Ley, and C. Jung, Phys. Rev.
B 57, 12397 (1998).
80. K. Bobrov, A. Mayne, G. Comtet, G. Dujardin, L. Hellner, and A.
Hoffman, Phys. Rev. ) 68, 195416 (2003).
81. T. N. Rao, D. A. Tryk, K. Hashimoto, and A. Fujishima, J.
Electrochem. Soc. 146, 6870 (1999).
82. C. H. Goeting, F. Marken, A. Gutierrez-Sosa, R. G. Compton, and
J. S. Foord, Diamond Relat. Mater. 9, 390 (2000).
83. C. D. Wagner, A. V. Naumkin, A. Kraut-Vass, J. W. Allison, C. J.
Powell, and J. R. Rumble, Jr.; U. S. Department of Commerce,
2000.
84. F. K. de Theije, M. F. Reedijk, J. Arsic, W. J. P. van Enckevort,
and E. Vlieg, Phys. Rev. B 64, 085 (2001).
85. C. J. Powell and A. Jablonski, in "NIST Standard Reference
Database 82". National Institute of Standards and Technology,
Gaithersburg, MD, 2000.
86. P. Chen, M. A. Fryling, and R. L. McCreery, Anal. Chem. 67, 3115
(1995).
213
87. T. Kondo, Y. Einaga, B. V. Sarada, T. N. Rao, D. A. Tryk, and A.
Fujishima, J. Electrochem. Soc. 149, E179 (2002).
88. G. S. Gildenblat, S. A. Grot, C. R. Wronski, A. R. Badzian, T.
Badzian, and R. Messier, Appl. Phys. Lett. 53, 586 (1988).
89. M. C. Hicks, C. R. Wronski, S. A. Grot, G. S. Gildenblat, A. R.
Badzian, T. Badzian, and R. Messier, J. Appl. Phys. 65, 2139
(1989).
90. S. A. Grot, G. S. Gildenblatt, C. W. Hatfield, C. R. Wronski, A. R.
Badzian, T. Badzian, and R. Messier, IEEE Electron Dev. Lett. 11,
100 (1990).
91. G. S. Gildenblat, S. A. Grot, and A. R. Badzian, Proc. IEEE 79,
647 (1991).
92. Y. Mori, H. Kawarada, and A. Hiraki, Appl. Phys. Lett. 58, 940
(1991).
93. K. Miyata, D. L. Dreifus, and K. Kobashi, Appl. Phys. Lett. 60,
480 (1992).
94. T. Tachibana, J. T. Glass, and R. J. Nemanich, J. Appl. Phys. 73,
835 (1993).
95. M. Aoki and H. Kawarada, Jpn. J. Appl. Phys. 33, L708 (1994).
96. H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, M. Kamo,
Y. Sato, and M. Iida, Appl. Phys. Lett. 67, 3596 (1995).
97. H. Kawarada, M. Itoh, and A. Hokazono, Jpn. J. Appl. Phys. 35,
L1165 (1996).
98. P. K. Baumann, S. P. Bozeman, B. L. Ward, and R. J. Nemanich,
Diamond Rel. Mater. 6, 398 (1997).
99. P. K. Baumann and R. J. Nemanich, in "The 24th Conference on
the Physics and Chemistry of Semiconductor Interfaces", Vol. 15,
214
9. Chemical, Photochemical and Electrochemical Modification of Diamond
p. 1236. AVS, Research Triangle Park, North Carolina (USA),
1997.
100. K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi,
and K. Kajimura, J. Appl. Phys. 81, 744 (1997).
101. P. K. Baumann and R. J. Nemanich, Diamond Rel. Mater 7, 612
(1998).
102. P. K. Baumann and R. J. Nemanich, J. Appl. Phys. 83, 2072
(1998).
103. A. Deneuville, C.R. Acad. Sci., Ser. IV, 81 (2000).
104. R. Zeisel, C. E. Nebel, and M. Stutzmann, Diamond Relat. Mater.
9, 413 (2000).
105. J. E. Gerbi, O. Auciello, J. Birrell, D. M. Gruen, B. W. Alphenaar,
and J. A. Carlisle, Appl. Phys. Lett. 83, 2001 (2003).
106.J. Wang, G. M. Swain, T. Tachibana, and K. Kobashi, Electrochem.
Solid-State Lett. 3, 286 (2000).
107. K. Honda, M. Yoshimura, T. N. Rao, D. A. Tryk, A. Fujishima, K.
Yasui, Y. Sakamoto, K. Nishio, and H. Masuda, J. Electroanal.
Chem. 514, 35 (2001).
108. O. Enea, B. Riedo, and G. Dietler, Nano Letters 2, 241 (2002).
109. J. Wang and G. M. Swain, Electrochem. Solid-State Lett. 5, E4
(2002).
110.R. Uchikado, T. N. Rao, D. A. Tryk, and A. Fujishima, Chem. Lett.,
144 (2001).
111. K. Ohnishi, Y. Einaga, H. Notsu, C. Terashima, T. N. Rao, S.-G.
Park, and A. Fujishima, Electrochem. Solid-State Lett. 5, D1
(2002).
112. L.-F. Li, D. Totir, B. Miller, G. Chottiner, A. Argoitia, J. C. Angus,
and D. Scherson, J. Am. Chem. Soc.; 119, 7875 (1997).
215
113. T. L. Kulova, Y. E. Evstefeeva, Y. V. Pleskov, A. M. Skundin, V. G.
Ral’chenko, S. B. Korchagina, and S. K. Gordeev, Phys. Solid
State 46, 726–728 (2004).
114. N. G. Ferreira, L. L. Mendonca, V. J. T. Airoldi, and J. M. Rosolen,
Diamond Rel. Mater. 12, 596–600 (2003).
115. A. DeBattisti, S. Ferro, and M. D. Colle, J. Phys. Chem. B 105,
1679 (2001).
116. S. Ferro and A. DeBattisti, J. Phys. Chem. B 106, 2249 (2002).
117. N. Spataru, C. Terashima, K. Tokuhiro, I. Sutanto, D. A. Tryk, S.M. Park, and A. Fujishima, J. Electrochem. Soc. 150, E337 (2003).
118. M. E. Hyde, R. M. J. Jacobs, and R. G. Compton, J. Phys. Chem. B
108, 6381 (2004).
119. K. Okumura, K. Nakagawa, T. Shimamura, N. Ikenaga, M.
Nishitani-Gamo, T. Ando, T. Kobayashi, and T. Suzuki, J. Phys.
Chem. B 107, 13419 (2003).
216