Click here to load reader
Upload
nm53f
View
29
Download
0
Tags:
Embed Size (px)
Citation preview
1
Self-terminating growth of Pt by electrochemical deposition
Yihua Liu, Dincer Gokcen, Ugo Bertocci and Thomas. P. Moffat* Material Measurement Laboratory
National Institute of Standards and Technology Gaithersburg, Maryland 20899
Abstract A self-terminating rapid electrodeposition process for controlled growth of Pt
monolayer films from a K2PtCl4-NaCl electrolyte has been developed that is tantamount
to wet atomic layer deposition (ALD). Despite the deposition overpotential being in
excess of 1 V, Pt deposition is quenched at potentials just negative of proton reduction by
an alteration of the double layer structure induced by a saturated surface coverage of
underpotential deposited hydrogen, (Hupd). The surface is reactivated for Pt deposition by
stepping the potential to more positive values where Hupd is oxidized and fresh sites for
adsorption of PtCl42- become available. Periodic pulsing of the potential enables
sequential deposition of two dimensional (2-D) Pt layers to fabricate films of desired
thickness relevant to a range of advanced technologies.
One sentence summary: An unanticipated process for atomic layer deposition of Pt is
detailed whereby potential control of adsorbed H enables sequential deposition of metal
monolayers from aqueous solutions.
2
Pt is a key constituent in a wide range of heterogeneous catalysts, but its high cost
constrains development of important alternative energy conversion systems such as low
temperature fuel cells (1-3). A variety of strategies are being explored to enhance catalyst
performance and minimize Pt loadings. These range from alloying to nanoscale
engineering of core-shell and related architectures that typically involve spontaneous
processes such as dealloying and segregation to form Pt-rich surface layers (4, 5). The
deposition of 2-D Pt layers, that are also of interest in thin film electronics and magnetic
materials, is non-trivial due to the step-edge barrier to interlayer transport that results in
roughening or 3-D mound formation (6). In situ scanning tunnel microscopy (STM) of Pt
electrodeposition at moderate overpotentials reveals that metal nucleation and growth on
Au proceeds by formation of 3-D clusters at defect sites on single crystal surfaces (7). At
small overpotentials, X-ray scattering indicates that smooth Pt monolayers can be
electrodeposited on Au (111) although a long growth time of 2000 s is required (8).
Voltammetric studies show a potential dependent transition between 2-D island versus 3-
D multilayer growth although it is only possible to obtain a partial Pt monolayer coverage
in the 2-D growth regime (9). To circumvent these difficulties, surface limited place
exchange reactions are being explored. For instance galvanic displacement of an
underpotential deposited (upd) metal monolayer, typically Cu, occurs by the desired Pt
group metal with the exchange resulting in a sub-monolayer coverage of the noble metal
(10, 11). The process can be repeated to form multiple layers using a variant known as
electrochemical atomic layer epitaxy (ECALE) (12). The multistep process typically
requires an exchange of electrolytes and some care to control (or avoid) the trapping of
the less noble metal as a minor alloying constituent within the film. The reversible nature
3
of many upd reactions makes it difficult to control deposition processes especially when
considering sub-nanometer scale films. Robust additive fabrication schemes are
facilitated by irreversible processes analogous to vapor phase deposition of thin films at
low temperatures, although kinetic factors often constrain the quality of the resulting
films.
Prior analytical studies of Pt deposition have largely limited the applied potential
to values positive of Hupd and proton reduction. One intriguing exception is Pt deposition
from a pH 10, Pt(NH3)2(H2O)22+ - NaHPO4 electrolyte, where inhibition of the reaction
was evident as the potential was scanned into the Hupd region, although the magnitude and
thus significance of the effect was not examined (13). Herein, we show that formation of
a saturated Hupd layer exerts a remarkable quenching or self terminating effect on Pt
deposition, restricting it to a high coverage of 2-D Pt islands. When repeated, by using a
pulsed potential waveform to periodically oxidize the Hupd layer, sequential deposition of
discrete Pt layers can be achieved. The process is thus analogous to ALD but with a rapid
potential cycle replacing the time consuming displacement and replacement of the
ambient reactant.
This report focuses on Pt deposition experiments performed at room temperature
in aqueous solutions consisting of 0.5 mol/L NaCl and 0.003 mol/L K2PtCl4 with pH
values ranging between 2.5 and 4. Beyond this particular electrolyte, self-terminating Pt
deposition was observed over a wide range of pH and Cl- concentrations and was not
dependent on the oxidation state (2+, 4+) of the Pt halide precursors.
To isolate the partial current associated with only the growth process, an
electrochemical quartz crystal microbalance (EQCM) was used to track Pt deposition on
4
an Au electrode as the potential was swept in the negative direction. Voltammetry in Fig.
1a shows the onset of Pt deposition at 0.25 VSSCE followed by a substantial current rise to
a maximum at -0.32 VSSCE that is close to diffusion limited PtCl42- reduction. Beyond the
peak, the deposition rate decreases smoothly as the mass transfer boundary layer
thickness expands. A sharp drop in the current occurs as the potential moves negative of
-0.5 VSSCE, eventually reaching a minimum near -0.7 VSSCE followed by an increase due
to hydrogen evolution from water. The gravimetrically determined (EQCM) metal
deposition rate reveals that the sharp drop below -0.5 VSSCE corresponds to complete
quenching of metal deposition. This remarkable self-termination or passivation process
occurs despite the large applied overpotential (> 1 V) available for driving the deposition
reaction.
The gravimetric data is used to reconstruct the partial voltammogram for Pt
deposition – a two electrons process. Good agreement with the measured voltammogram
indicates the current efficiency of Pt deposition is close to 100 % as the potential is swept
toward the diffusion limited value. As the current peak is approached, an apparent loss in
efficiency is observed, due to non-uniform deposition that develops as the PtCl42-
depletion gradient sets up a convective flow field that spans the electrode. In contrast to
the EQCM, voltammetry with a rotating disk electrode (RDE) provides uniform mass
transport that yields a more symmetric peak (Fig. 1b). The contribution of the proton
reduction reaction is isolated by performing voltammetry in the absence of the Pt
complex. Merging of the respective voltammograms at negative potentials indicates that
quenching of the metal deposition reaction is coincident with the onset of the H2
evolution reaction. The overlap of the diffusion limited proton reduction current also
5
indicates the absence of significant homogeneous reaction between the generated H2 and
PtCl42-, excluding this as an explanation for the quenching of the Pt deposition reaction.
The two electron reduction of PtCl42- to Pt is not expected to depend on pH, and
the onset of significant Pt deposition from PtCl42- at 0.0 VSSCE shown in Fig. 1d supports
this contention. In contrast, sharp acceleration of the deposition rate below -0.2 VSSCE is
clearly pH dependent. This correlates with the onset of Hupd evident in PtCl42--free
voltammetry (Fig. 1c). Chronocoulometry studies indicate that the transition between a
halide and a hydrogen covered Pt surface occurs in the same regime (14). The metal
deposition rate increases with Hupd coverage reaching a peak value that is independent of
pH, while the peak potential shifts by -0.059 V/pH reflecting the importance of H surface
chemistry in controlling the Pt deposition process. The onset of proton reduction in the
absence of PtCl42-, marked by the dotted line in Fig. 1b, occurs at the essentially the same
potential. Thus, the peak deposition rate occurs at the hydrogen reversible potential.
Moving to more negative potentials, the metal deposition rate declines rapidly and within
0.1 V of its peak value the current merges with that attributable solely to diffusion limited
proton reduction, indicating complete quenching of the Pt deposition reaction.
Importantly, transient studies of Hads on Pt indicate that the coverage does not reach
saturation at the reversible hydrogen potential but rather occurs 0.1 V below the
reversible value (15). This is precisely the potential regime where the metal deposition
reaction is fully quenched. Cyclic voltammetry reveals that the passivation process is
reversible with reactivation coincident with the onset of Hupd oxidation (Fig. 1S). Self-
termination of the metal deposition reaction arises from perturbation of the double layer
structure that accompanies Hads saturation of the Pt surface. Recent theoretical work
6
indicates that the water structure adjacent to a hydrogen covered Pt(111) surface is
significantly altered with the centroid of the O atoms within the first water layer being
displaced by more than 0.1 nm from the metal surface as the water-water interactions in
the first layer become stronger (16). In a related development, an EQCM study of Pt in
sulfuric acid has identified a “potential of minimal mass” near the reversible potential of
hydrogen reactions (17). The gravimetric measurements reflect the impact of Hupd on the
adjacent water structure that leads to a minimum in coupling between the electrode and
electrolyte, consistent with the recent theoretical result. In addition to Hupd perturbation of
the water structure, the quenching of metal deposition reaction occurs at potentials
negative of the Pt point of zero charge (pzc) where anions would have been desorbed (14).
The above combination exerts a remarkable effect whereby PtCl42- reduction is
completely quenched while diffusion limited proton reduction continues unabated.
Self-terminating Pt deposition was also examined under potentiostatic conditions.
Optical micrographs of a selection of films after 500 s deposition at various potentials are
shown as inserts in Fig. 1a. Only the lower half of Au-coated Si(100) wafer was
immersed in solution with differences in reflectivity and color indicate the anomalous
dependence of deposition on potential; specifically a 33 nm thick Pt film was deposited
at -0.4 VSSCE while a nearly invisible much thinner layer was grown at -0.8 VSSCE.
X-ray photoelectron spectroscopy (XPS) was used to further quantify the
composition and thickness of Pt grown as a function of deposition time and potential on
(111) textured Au. For films deposited at -0.8 VSSCE, a representative spectrum with the
4f doublets for the metallic states of Au and Pt is shown in Fig. 2 (insert). The ratio of the
Pt and Au peak areas was used to calculate the Pt thickness assuming it forms a uniform
7
overlayer (18). For deposition times up to 1000 s, the measured thickness varies between
0.21 nm and 0.25 nm, congruent with the deposition of a Pt monolayer with a thickness
comparable to the (111) d-spacing of Pt. Monolayer formation is complete within the first
second of stepping the potential to -0.8 VSSCE and the absence of further growth confirms
the self-terminating nature of the deposition reaction. Beyond 1000 s, an additional
increment of Pt deposition is evident. Inspection of the surface with scanning electron
microscopy revealed a sparse coverage of spherically shaped Pt particles on the surface
attributable to H2 induced precipitation, a process requiring some heterogeneity and
extended incubation to nucleate. Particle formation can be avoided by using shorter
deposition times.
Scanning tunneling microscopy was used to directly observe the Pt overlayer
morphology (Fig. 3). Analysis was facilitated by using a flame annealed Au (111) surface
with isolated surface steps, 0.24+/-0.02 nm in height, that serve as fiduciary markers (Fig.
3a). Pt deposition results in three distinct levels of contrast that reflect the surface height
with the lowest level being the original Au terraces (Fig. 3b). The same three-level
structure is observed independently of deposition time up to 500 s (Fig. 3c). The middle
contrast level corresponds to a high density of Pt islands that cover ~ 85 % of the Au
surface with a step height of ~ 0.24 nm consistent with XPS results. Inspection using a
higher rendering contrast reveals a ~10 % coverage of a second layer of small Pt islands
with a step height ranging between 0.23 nm to 0.26 nm (Fig. 3d). Step positions
associated with the flame annealed substrate are preserved with negligible expansion or
overgrowth of the 2-D Pt islands occurring beyond the original step edge. The lateral
span of the Pt islands lies in the range of 2.02+/-0.38 nm corresponding to an area of
8
4.23+/-1.97 nm2. Incipient coalescence of the islands is constrained by surrounding (dark)
narrow channels, 2.1+/-0.25 nm wide, that account for the remaining Pt-free portion of
the first layer. The reentrant channels correspond to open Au terrace sites that are
surrounded by adjacent Pt islands in what amounts to a huge increase in step density
relative to the original substrate, the net geometric or electronic effect of which is to
block further Pt deposition. The chemical nature of the inter-island region is assayed by
exploiting the distinctive voltammetry of Pt and Au with respect to Hupd and oxide
formation and reduction as detailed in Fig. 2S
Similar three level Pt overlayers have been observed for monolayer films
produced by molecular beam epitaxy (MBE) deposition at 0.05 ML/min (19). Pt-Au
intermixing driven by the decrease in surface energy that accompanies Au surface
segregation was evident. In the present work, Pt monolayer formation is effectively
complete within 1 s giving a growth rate three orders of magnitude greater than the MBE-
STM study. Exchange of the deposited Pt with the underlying Au substrate is expected to
be less developed although intermixing and possible chemical contrast is evident on
limited sections of the surface that are correlated with the original faulted geometry of the
partially reconstructed Au surface. Upon lifting of the reconstruction, the excess Au
atoms expelled mark the original fault location as linear one dimensional surface defects
in the Pt overlayer (Fig. 3e). Simplifying, a schematic of the self-terminating Pt
deposition process in Fig. 3f indicates that the Hupd accompanying incremental expansion
of the 2-D Pt islands serves to hinder the development of a second Pt layer, presumably
by perturbation the overlying water structure.16 This rapid process results in a much
9
higher island coverage than has been obtained by other methods such as galvanic
exchange reactions.
As the saturated Hupd coverage is the agent of termination, reactivation for further
Pt deposition is possible by removing the upd layer by sweeping or stepping the potential
to positive values, e.g. > +0.2 VSSCE, where negligible Pt deposition occurs. Sequential
pulsing between +0.4 VSSCE and -0.8 VSSCE enables Pt monolayer deposition to be
controlled in a digital manner. EQCM was used to track the mass gain showing two net
increments per cycle (Fig. 4a). The mass gain is attributed to a combination Pt deposition
(486 ng/cm2 for a monolayer of Pt(111)), anion adsorption and desorption (41 ng/cm2 for
7 x1014 Cl- ion /cm2, 117 ng/cm2 for a 0.14 fractional coverage of PtCl42-) (7, 20) and
coupling to other double layer components such as water. The anionic mass increments
are expected to be asymmetric for the first cycle on the Au surface but once it is covered
subsequent cycles only involve Pt surface chemistry. After correcting for the
electroactive surface area of the Au electrode (Areal/Ageometric=1.2 derived from reductive
desorption of Au oxide in perchloric acid) the net mass gain for each cycle indicates that
close to a pseudomorphic layer of Pt is deposited. XPS analysis of Pt films grown for
various deposition cycles gives remarkably good agreement with EQCM data (Fig. 4b).
The ability to rapidly manipulate potential and double layer structure, as opposed to
exchange of reactants, offers simplicity, substantially improved process efficiency, and
far greater process speed than other surface limited deposition methods.
References
1. F. T. Wagner, R. Lakshmanan, M. F. Mathias, J. Phys. Chem. Lett. 1, 2204 (2010).
10
2. M. K. Debe, Nature 486, 43 (2012).
3. M. T. M. Koper, Ed., Fuel Cell Catalysis, A Surface Science Approach (Wiley & Sons,
Hoboken, NJ, 2009).
4. V. R. Stamenkovic, et al. Nat. Mater. 6, 241 (2007).
5. R. R. Adzic, et al. Top. Catal. 46, 249 (2007).
6. T. Michely, J. Krug, Islands, Mounds and Atoms, Patterns and Processes in Crystal
Growth Far from Equilibrium (Springer Series in Surface Science, V42, New York,
2003).
7. H. F. Waibel, M. Kleinert, L. A. Kibler, D. M. Kolb, Electrochim. Acta 47, 1461
(2002).
8. T. Kondo, et al. Electrochim. Acta 55, 8302 (2010).
9. I. Bakos, S. Szabo, T. Pajkossy, J. Solid State Electrochem. 15, 2453 (2011).
10. S. R. Brankovic, J. X. Wang, R. R. Adzic, Surf. Sci. 474, L173 (2001).
11. D. Gokcen, S.-E. Bae, S. R. Brankovic, Electrochim. Acta 56, 5545 (2011).
12. B. W. Gregory, J. L. Stickney, J. Electroanal. Chem. 300, 543 (1991).
13. A. J. Gregory, W. Levason, R. E. Noftle, R. Le Penven, D. Pletcher, J. Electroanal.
Chem. 399, 105 (1995).
14. N. Garcia-Araez, V. Climent, E. Herrero, J. Feliu, J. Lipkowski, J. Electroanal. Chem.
582, 76 (2005).
15. D. Strmcnik, D. Tripkovic, D. van der Vliet, V. Stamenkovic, N. M. Markovic,
Electrochem. Commun. 10, 1602 (2008).
16. T. Roman, A. Groβ, “Structure of water layers on hydrogen-covered Pt electrodes”
Cataly. Today, available on line July 12, in press
11
17. G. Jerkiewicz, G. Vatankhah, S. Tanaka, J. Lessard, Langmuir 27, 4220 (2011).
18. P. J. Cumpson, M. P. Seah, Surf. Interface Anal. 25, 430 (1997).
19. M. O. Pedersen, et al. Surf. Sci. 426, 395 (1999).
20. Y. Nagahara, et al. J. Phys. Chem B. 108, 3224 (2004).
Figure 1. Gravimetric and voltammetric measurements (2 mV/s) of Pt deposition from a
NaCl-PtCl42- solution using either (a) a static EQCM or (b) (c) (d) an Au RDE (400 rpm).
The inserts in (a) are optical images of Pt films grown on 1 cm wide Au-coated Si(100)
wafers for 500 s at the indicated potentials. (b), (c), and (d) present the effect of pH on
PtCl42- reduction and the background reactions associated with the supporting electrolyte.
Background reactions were examined using a Pt RDE.
Figure 2. XPS derived thickness (red squares) of Pt films as a function of deposition time
at -0.8 VSSCE on Au-coated Si wafers from a pH=4 solution. The Au and Pt lines
correspond to the (111) d-spacing of the respective bulk metals.
Figure 3. (a) STM images of representative Au(111) surface with monoatomic steps. (b-
c) 2D Pt layers obtained after (b) 5 and (c) 500 second deposition at -0.8 VSSCE. (d) High
contrast image of 2-D Pt layer morphology on Au(111). (e) Linear defects in Pt layer
associated with lifting of the reconstructed Au substrate. (f) A schematic of Hupd
terminated Pt deposition on Au(111).
12
Figure 4. Sequential deposition of Pt monoatomic layers by pulsed deposition in a pH 4
solution. (a) Mass change accompanying each pulse. (b) EQCM mass increase is
converted to thickness and compared with XPS measurements. XPS analysis of the
EQCM specimen (◊)and a series of Pt films deposited on Au-coated Si wafers.
13
Figure 1
14
Figure 2
15
Figure3
16
Figure 4
Supplementary Information
Self-terminating growth of Pt by electrochemical deposition
Yihua Liu, Dincer Gokcen, Ugo Bertocci and Thomas. P. Moffat Material Measurement Laboratory
National Institute of Standards and Technology Gaithersburg, Maryland 20899
Material and methods
Kinetic study of PtCl42- reduction
Linear scan voltammetry (LSV) at a sweep rate of 2 mV/s was employed to study
the reduction kinetics of PtCl42- from solutions consisting of 0.50 mol/L NaCl and 0.003
mol/L K2PtCl4. The influence of pH, adjusted by HClO4 and NaOH additions, was
examined in a closed electrochemical cell filled with a Ar-saturated 100 ml PtCl42-
solution. A Pt plate counter electrode was held in a glass tube filled with 0.50 mol/L
NaClO4 connected to the PtCl42- electrolyte through a fine glass frit. The reference
electrode was a sodium chloride saturated calomel electrode (SSCE). The working
electrode was either a mechanically polished Au rotating disk electrode (RDE) spun at
400 rpm or a static Au-coated quartz crystal electrode. The background activity, i.e.
proton reduction, on a Pt RDE was determined using in 0.5 mol/L NaCl solutions at
various pH values. All solutions were prepared from analytical-grade chemicals
dissolving in 18 MΩ⋅cm water. The same grade of water was used for rinsing and
cleaning activities. All glassware was cleaned by soaking for one hour in aqua regia
followed by extensive rinsing.
Growth of Pt monolayer and multilayer
The potential pulse method outlined in the main text was utilized to grow Pt
monolayer and multilayer films from a pH 4 PtCl42- solution. Au-coated Si(100) wafer
fragments were used for XPS studies while an annealed Au (111) single crystal surfaces
was used for STM characterizations. The Au-coated Si (100) surfaces were prepared by
e-beam evaporation of a 150 nm Au layer on a 5 nm Ti seeded polished Si (100) surface.
Immediately before use the Au-coated Si (100) wafers were soaked for a minute in a
piranha* solution made of 3/1 volume ratio of concentrated H2SO4 (70%) and H2O2
(30%). The Au (111) surfaces were prepared using Clavilier’s flame annealing technique
the Au (111) surfaces were annealed using H2 flame for 10 minutes and then cooled to
room temperature gradually (21). Followed the electrodeposition, the deposits were
promptly rinsed with 18 MΩ water, dried in a stream of N2, and subjected to the
characterizations immediately.
• Warning: Piranha solution should be handled with caution: most probably when it has been mixed with significant quantities of oxidizable organic materials detonation may occur. Likewise, working solution should not be sealed from atmosphere due to gas evolution. Accordingly used solution should be properly disposed with appropriate care.
XPS
The XPS (Kratos AXIS Ultra DLD) (22) was operated at a base pressure of 3 x
10-10 torr using a monochromated AlKα source. The Casa XPS program was used in
evaluating the peak areas of Au 4f and Pt 4f spectra using Shirley’s algorithm for
background subtraction. The peak area ratio was used to calculate the thickness of the
overlayer, d, after accounting for the elemental sensitivity factors (si) (sAu = 6.25, sPt =
5.575) and the attenuation length of the photoelectrons in the Pt overlayer (λAL =1.252
nm) (18).
⎥⎦
⎤⎢⎣
⎡+=
AuAu
PtPtAL sI
sId//1lncosθλ [1]
For reference purposes the 111 d-spacing for bulk Pt (ao=0.39240 nm) is 0.227 nm, which
is slightly less than 0.235 nm of Au (ao=0.40786 nm).
STM
STM tips were made of etching Pt/Ir (90:10) wires in 2:1 CaCl2:H2O solution at
25V-AC potential. Etched STM tips were rinsed with water and acetone. All high-
resolution STM measurements were performed using a Digital Instruments Nanoscope III
controller with an A-type scanner at constant current mode (Itip<5nA). Plane-fit STM
images were not subjected to any other filtering options. Images were analyzed using
both Nanoscope and WSXM software (23). Average step height values, lateral
dimensions (both x and y directions) and sizes of the Pt islands are computed using
autonomous technique provided by the software for various images recorded at the
different regions of the sample surface. Standard deviation values (+/-) are quoted with
average values to reflect variances observed in the different images.
EQCM
The EQCM experiment was performed using AT cut quartz crystals coated with a
150 nm Au layer and an adhesion layer of 5 nm Ti (Maxtek). The same cleaning
procedure was followed as described for Au-coated Si(100) surfaces. The electrode was
maintained under potential control for the duration of the experiment. Specifically, the
experiments began with a 80 mL Ar-saturated 0.5 mol/L NaCl pH 4 solution with the
potential set to 0.400 VSSCE. After stability was established a 1 mL aliquot of
concentrated K2PtCl4 solution was added to give a final PtCl42- concentration of 0.003
mol/L after being homogenized by magnetic stirring. Once the signal drift became
insignificant, the potential pulses were applied.
Supplementary results
Reversibility of the self terminating deposition process
Figure 1S. Cyclic voltammetry reveals the reversible nature of suppressed and reactivated
Pt deposition from a pH 3.5 solution of 0.5 mol/L NaCl + 0.003 mol/L K2PtCl4 (400 rpm,
2 mV/s).
Voltammetric examination of Pt overlayer on Au
The chemical nature of the inter-island region is assayed by exploiting the
distinctive voltammetry of Pt and Au with respect to Hupd and oxide formation and
reduction. In 0.1 mol/L HClO4 Hupd features are evident at 0.050 VRHE ≤ E ≤ 0.400 VRHE.
The wave shape is consistent with that for Pt(111) although the magnitude 108 μC/cm2
+/- 5 is less than 146 μC/cm2 due to finite size effects (Fig. 2S).24 This is also similar to
the Hupd results observed for Pt rich Pt1-xAux surface alloys grown on Pt(111).25
Oxidation of the surface shows two distinct reduction waves for Pt oxide at 0.67
VRHE and Au oxide at 1.14 VRHE, with the former being more pronounced. The peak
potential for the Au oxide reduction is shifted to more negative values compared to pure
Au due to finite size effects. With due consideration of the background current for a fully
consolidated Pt deposit, the charge associated with the Au oxide formation and reduction
on the monolayer Pt film electrode corresponds to ~ 11 % of the Au substrate being
accessible to the electrolyte. This is in reasonable agreement with the STM coverage
determination.
Figure 2S. Cyclic voltammetry shows Hupd as well as oxide formation and reduction in
0.1 mol/L HClO4 on Au-coated Si surfaces before and after the growth of a Pt monolayer.
References
21. J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107, 205 (1980)
22. Identification of commercial products in this paper was done to specify the
experimental procedure. In no case does this imply endorsement or recommendation by
the National Institute of Standards and technology.
23. I. Horcas, et al. Rev. Sci. Instrum. 78, 013705 (2007) .
24. M.T.M. Koper, Electrochimica Acta, 56, 10645 (2011)
25. A. Bergbreiter, O. B. Alves and H.E. Hoster, Chem Phys Chem, 11, 1505 (2010).