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.

*thomas.moffat@nist.gov

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.

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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

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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

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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

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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

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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

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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

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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

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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).

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2. M. K. Debe, Nature 486, 43 (2012).

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Hoboken, NJ, 2009).

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Growth Far from Equilibrium (Springer Series in Surface Science, V42, New York,

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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).

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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

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17. G. Jerkiewicz, G. Vatankhah, S. Tanaka, J. Lessard, Langmuir 27, 4220 (2011).

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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).

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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).