Pt Monolayer Creation on a Au Surface via an Underpotentially

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Pt Monolayer Creation on Au Surface via Underpotentially Deposited Cu Route Deyu Qu, Chan-Yong Jung, Chi-Woo J. Lee, and Kohei Uosaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09531 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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The Journal of Physical Chemistry

Pt Monolayer Creation on Au Surface via Underpotentially Deposited Cu Route Deyu Qu, 1,2† Chan-Yong Jung,1† Chi-Woo Lee1* and Kohei Uosaki3 1Department

of Advanced Materials Chemistry, College of Science and Technology, Korea University, 2511 Sejongro, Sejong 30019, Korea

2Department

of Chemistry, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan

University of Technology, 122 Luoshi Road, Wuhan, 430070 Hubei, PR China 3International

Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan

* To whom correspondence should be addressed. C.-W Lee: Phone: +82-44-860-1333. Fax: +82-44-867-6823. E-mail: [email protected]

Author contributions † D.Q. and C.-Y.J. equally contributed to this work.

Submitted to JPCC on 9/30/2018 Rev. 11/15/2018, 12/8/2018, 12/30/2018, 1/10/2019

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ABSTRACT: Kinetics of platinum monolayer formation via the redox replacement of an underpotentially deposited copper on Au(111) electrode surface by platinum tetrachloride complex (PtCl42-) was studied by voltammetry, X-ray photoelectron spectroscopy (XPS), electrochemical quartz crystal microbalance (EQCM), energy dispersive spectroscopy (EDS), and scanning tunneling microscopy (STM). The Pt 4f7/2 peak intensities of XPS and voltammetric responses of H adsorption/desorption increased as the incubation time increased in PtCl42- solutions. The resonance frequency of the Cu/Au QCM in 0.05 M H2SO4 solution was observed to quickly decrease within 1 min when it contacted with PtCl42- solutions, then it remained unchanged for hours. EDS data showed that Cu was not found at high PtCl42- solutions. Ex situ STM images revealed a largely uncovered Au(111) surface with mountain-like Pt nano particles at a replacement time of 10 min, and mostly covered Au(111) one with plain-like Pt nano particles of four hours. The average size of Pt particles decreased approximately logarithmically as a function of time, whereas the number of Pt particles increased. Thus, after replacement, Pt atoms were dynamic in hours, resulting in a flattened Pt monolayer. A mechanism that includes reverse Ostwald ripening is provided to rationalize the observations.


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1. INTRODUCTION Platinum has been widely used as catalysts in the reduction of molecular oxygen, oxidation of hydrogen, oxidation of alcohol and hydrogenation of unsaturated hydrocarbons.1–5 Deposition of a small amount of platinum on the metal surface will improve the catalytic and electrocatalytic properties of metal electrodes.6–10 Usually, for immobilization of a monolayer or submonolayer of an alien metal on a metal electrode surface, the underpotential deposition (UPD) technique can be used. However, in the case of platinum, its ions do not exhibit UPD behavior as other metals, such as Cu, Ag, and Pb. Previous studies demonstrated that platinum immobilization on Au(111) surface by electrochemical method from H2PtCl6 solution undergoes a layer by layer (2D) growth mode and a multilayer of Pt will be formed on the surface.11,12 However, Kolb and coworkers examined Pt deposition on Au(111) and Au(100) surfaces from K2PtCl4 acidic solution and reported that nucleation of Pt started at the step edges, and 3D clusters were formed at a low deposition rate.13 At high overpotentials, a higher coverage of Pt led to a cauliflower-like appearance. Nagahara et al.14 also examined electrochemically produced platinum on Au(111) from haloplatinate complexes. They reported that most Pt particles are 3.0 nm wide and 0.46 nm high.14 Those results indicated that it is difficult to obtain a platinum monolayer via direct electrochemical deposition. In 2001, Brankovic et al.15 presented a new method, which was called “galvanic replacement of a UPD layer” and is currently recognized as surface limited redox replacement (SLRR), to prepare a Pt monolayer/submonolayer on the surface of an electrode.16,17 In this approach, a Pt monolayer/submonolayer was formed on a gold surface via spontaneous redox replacement of a less noble metal monolayer, such as copper, which was preformed by UPD, in a Pt ionic solution. Weaver and coworkers applied it to form a Pt adlayer on gold with a high enhancement factor for their surface–enhanced Raman spectroscopy research.18 The research group of Buess-Herman used it to 3 ACS Paragon Plus Environment


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form a gold–supported platinum electrode to study oxygen reduction.19,20 This technique was also applied to form bimetallic nanoparticles to study H2 oxidation, O2 reduction and methanol oxidation.21–27 Platinum overlayers deposited on nanoporous Au or on a dendrimer–modified Au electrode via this UPD-redox replacement method were also reported.28,29 Kim et al.30 generated a Pt(111) nanofilm on a well-defined Au(111) surface by using electrochemical atomic layer epitaxy and iodine as a surfactant. By using UPD Pb as sacrificial atomic layer in an automated electrochemical flow cell system, they achieved layer–by–layer growth of Pt by controlling the stop potential and increasing the blank rinsing time.31 Dimitrov and coworkers reported successful epitaxial growth of Ag on Au(111) and Cu on Au(111) or Ag(111) substrate by applying SLRR and using UPD Pb as the sacrificial metal.32,33 They further developed a SLRR deposition method using a nonmetal UPD H system and the growth of a high quality Pt film on poly–crystalline Pt and Pd substrates. They also developed a protocol based on SLRR of UPD Pb layers to grow Pt films in different forms on the surfaces of polycrystalline as well as single crystalline gold.34,35 Vasiljevic and coworkers formed Pt ultrathin films on gold via SLRR using UPD Pb and Cu as the sacrificial layer and applied it towards CO and formic acid oxidation reactions.36 A recent review on SLRR approaches is available with the applied aspects considered.17 Because heterogeneous catalytic reactions are very selective depending on the surface conditions at the atomic level, it is necessary to study how the Pt layer is formed by Pt/Cu SLRR and what the properties of the resulting surface are in physical chemistry aspects. Although several kinetic models of SLRR of a UPD layer have been developed,16,37–39 to our knowledge, there has been no direct kinetic study on SLRR of Cu on a Au(111) substrate by PtCl42- complex.40 In this work, we describe the kinetics of replacement of a UPD Cu adlayer by PtCl42- complex and formation of a Pt nano-film on gold surface, as investigated by voltammetry, X-ray photoelectron spectroscopy (XPS), 4 ACS Paragon Plus Environment

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electrochemical quartz crystal microbalance (EQCM), energy dispersive spectroscopy (EDS) and scanning tunneling microscopy (STM). The effects of the replacement time and PtCl42- concentration were also considered. The results showed that Pt atoms were not homogenously deposited on the Au(111) surface after SLRR but they were dynamic in hours. A mechanism is provided to rationalize the observations.

2. EXPERIMANTAL SECTION 2.1. Methods The gold wire, which was 99.999% pure and purchased from Tanaka Precious Metal, was used and a Au(111) single crystal bead was generated through the Clavilier method.41 The Au(111) single crystal electrode was obtained by cutting and mechanically polishing of as-made gold single crystal bead and used in electrochemical measurements and XPS investigation. The electrochemical surface area (ECSA) of this Au(111) single crystal electrode was determined by the cathodic charge associated with the Au oxide reduction. In this study, the ECSA of Au (111) electrode was found to be 0.07 cm2. STM measurements were carried out on the (111) facet on a gold singlecrystal bead surface, which is atomically flat. Before each experiment, the Au single crystal electrodes were cleaned in the flame of hydrogen followed by cooling down in an argon atmosphere. A gold single crystal (Monocrystals Company, 99.99+%, 5mm diameter x 2 mm thickness) was also used for the EC-STM measurements in aqueous solutions. The electrode used in the EQCM measurements were the AT-cut quartz crystal at 9 MHz, which was coated with a Ti layer first and then sputtered gold on it. Before EQCM measurements, the electrodes were rinsed by concentrated sulfuric acid several times and cleaned with ethanol and de-ionized water. The gold-coated quartz crystal ECSA was measured as 0.196 cm2. The concentration of PtCl42- in solution was 1 mM unless otherwise specified. 5 ACS Paragon Plus Environment


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The electrochemical investigations were carried out in a conventional three-electrode cell. Ag/AgCl was used as reference electrode. Counter electrode was either a Pt wire or a Pt mesh. EG&G 283 potentiostat was used. Before each electrochemical experiment, bubbling Ar gas into the electrolyte solution for over 30 minutes was operated in order to remove oxygen. Experiments were conducted at ambient laboratory conditions. Two sets of instruments were used in the EQCM measurements. One set was an EG&G QCA-917 quartz crystal analyzer and an EG&G 283 potentiostat. Another one was Princeton Applied Research QCM 922 and a Princeton Applied Research Verstat 3 potentiostat. The frequency and mass changes in EQCM measurement are related by the Sauerbrey as follows.42

f = -2f02 m/A(qq )1/2

(1)

, where f0 is the quartz crystal resonance frequency when there is no mass loading, q is the shear modulus, A is the active piezoelectric area, m and f corresponded to the change in mass and frequency, respectively, and q is the quartz density. Since the f0, q, q and A are all constants in Eq. (1), it can be expressed as: m =  S ×f. In this study, 1.09 ng Hz-1 was used for S. The Teflon electrochemical cell and Viton o-rings were cleaned using a piranha solution (H2SO4 : H2O2 = 3 : 1) and rinsed with de-ionized water. XPS measurements were conducted by an X-ray photoelectron spectrometer (Rigakudenki XPS-7000 model) with Mg Kα radiation. The base pressure in the analysis chamber was 5 × 10-7 Pa. Wide scan measurements used a pass energy of 50-eV, an electron-beam power of 25-W, and a 0.8 eV resolution. Narrow-scans for the measurements of Cu 2p, Pt 4f and Au 4f XP spectra were accomplished by a pass energy of 15-eV, a 100-W electron beam power, and a 0.1 eV resolution. The Au 4f7/2 peak located at 84.0 eV was used to calibrate the other binding energies in this study. The sample electrodes were washed several times with deionized water and dried in air before the XPS measurements. STM and EC-STM images were taken from a Digital 6 ACS Paragon Plus Environment

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Instrument NanoScope E and a Molecular Image PicoSPM. The Pt-Ir (8:2) wire, purchased from California Fine Wire Company or Tanaka Precious Metal, was used for preparing STM tips. After mechanical cut, the STM tips were wax coated in order to perform EC-STM measurements, 42 which was conducted with a homemade EC-STM cell. The constant current mode was used in the STM measurements. EDS measurements were performed by using an energy dispersive spectrometer (EDS, Bruker 410-M) at 5 kV. The voltammetry, EQCM, and STM experiments were performed using the cell inside a zippered bag in an Ar atmosphere under ambient laboratory conditions. 2.2. Materials CuSO4•5H2O, which was 99.999% pure, and K2PtCl4, which was 99.9+% pure, were provided by Aldrich. The suprapur® grade sulfuric acid and suprapur® grade perchloride acid were purchased from Wako Pure Chemicals. All chemicals used in this study were used as received. A Milli-Q purification system from Millipore was used to prepare de-ionized water.

3. RESULTS AND DISCUSSION 3.1. Pt Layer Formation on Au(111) by SLRR of UPD Cu by PtCl42- Complex. The procedure for Pt monolayer formation was similar to that provided in previous reports.15,17,40 At first, a Cu monolayer was underpotentially deposited on the surface of a Au(111) electrode. The corresponding cyclic voltammogram (CV) was presented in Figure 1. It was found that this recorded CV matched well with those reported in the literature.44–46 The two cathodic deposition peaks with peak potential at 0.255 and 0.085 V presented and associated with two different copper ad-structures shown in the STM images in Figure 1. A 3  3 R-30o honeycomb structure after the first deposition peak at 0.255 V and a (1  1) structure after the second deposition peak were observed.44–46 This clearly indicated that a Cu monolayer was produced on the surface of Au(111) electrode. After the copper adlayer with (1  1) structure was formed on Au(111) surface by holding the Au(111) electrode 7 ACS Paragon Plus Environment


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potential at + 0.02 V for 3 min in 0.05 M H2SO4 solution containing 5 mM CuSO4, this Cu/Au electrode was then washed by de-ionized water and transferred into a deaerated PtCl42- solution under the Ar atmosphere, where the following SLRR reaction may take place. Cu/Au + xs Pt(II) → Pt/Au + Cu(II) + Pt(II)

(2)

Here, xs Pt(II) on the left side is written to express that the SLRR reaction does not proceed completely unless Pt(II) is supplied in excess and Pt(II) on the right side denotes the amount of Pt(II) left, except for the Pt(0) plus Pt(II) adsorbed on the surface of the gold electrode after the SLRR (see below). Stoichiometrically, one copper atom will be replaced by one platinum atom. Therefore, ideally, a full monolayer of Pt with the (1  1) structure will be formed on the Au (111) surface, if platinum ions are reduced and deposited at the same location when Cu atoms are oxidized and leave the surface. To clarify this process, a kinetic study using XPS, CV, EQCM and STM is described.

Figure 1. Cyclic voltammogram at Au(111) electrode in a 0.05 M H2SO4 solution containing 5 mM CuSO4 at the scan rate of 20 mV s-1 (top). STM images (Ut = 50 mV, It = 100 pA) of Au(111) 8 ACS Paragon Plus Environment

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surface before and after Cu UPD (bottom).

3.2. XPS and Voltammetric Measurements. XPS and the interpretation of the Cu and Pt regions as a function of the replacement time in the PtCl42- complex solution have been reported in our previous publication in 2009, which is available as an open access Communication.40 The results revealed that no peaks in the Cu region were seen in the XPS data of the samples prepared for the replacement time greater than 10 minutes and at the same time that strong peaks in the Pt region were found in those at the same samples, suggesting that Eq. (2) completed in less than 10 minutes. Of note, the sample electrodes were washed several times in deionized water and dried in air before the XPS measurements. The ratio of Cu and Pt on the Au(111) surface at different exposure times in the PtCl42- complex solution was determined as follows. “CuUPD : Pt10min:Pt20min:Pt240min = 1 : 0.17 : 0.23 : 0.93” CuUPD refers to the relative amount of copper observed before SLRR (0 min of incubation), and Pt refers to platinum after SLRR (10, 20, and 240 min of incubation). To further investigate the surface structure of the electrode after Pt/Cu SLRR on Au(111) substrate with different incubation times, the UPD Cu technique was used.11 The underpotential deposition reaction of copper has been proved to be a good investigation tool to examine the order of the electrode surface. This is because that CV response for the UPD reaction is significantly affected by the electrode surface structure.11 To apply this technique to the present problem, the electrode was transferred to a Cu UPD characterization cell with 0.05 M H2SO4 solution and 5 mM CuSO4 after different exposure times to 1 mM PtCl42- for the SLRR experiments. The sample electrodes were washed several times with deionized water before the voltammetric measurements. The results are shown in Figure 2. After held at + 0.02 V for 3 min, the electrode potential was scanned back to + 9 ACS Paragon Plus Environment


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0.55 V. Two relatively sharp oxidative current peaks with peak potentials of + 0.15 V and + 0.27 V were obtained at the electrode within 5 min of exposure to the PtCl42- solution (Figure 2B). These two peaks are assigned to UPD copper stripped from Au (111) surface. Another broad oxidative current peak at approximately + 0.4 V was also observed due to the UPD Cu oxidative desorption from the polycrystalline platinum surface. This showed that polycrystalline Pt and the uncovered Au(111) surface co-existed after 5 min of incubation of the UPD Cu/Au(111) electrode in the PtCl42ion solution. When the immersion time was increased, the peaks corresponding to UPD Cu stripping from Au(111) became broad, and the current density of the UPD Cu stripping peak from the polycrystalline platinum increased. This shows that the coverage of platinum on Au(111) surface increased with SLRR replacement time.


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Figure 2. Linear sweep voltammogram of the Pt/Au electrode with different SLRR times recorded in the 0.05 M H2SO4 solution containing 5 mM CuSO4 at the scan rate of 20 mV s-1. (A) 0 minute, (B) 5 minutes, (C) 10 minutes, (D) 20 minutes and (E) 240 minutes.

The Pt/Au(111) electrodes with different SLRR times were also tested via CV measurements in a 0.05 M H2SO4 solution, and the results have been reported in our previous publication in 2009, which is available as an open access Communication.40 The sample electrode was treated in PtCl42complex solution for different incubation times, then removed from the incubation solution and washed with deionized water several times before each CV measurement. The CVs reported were all 11 ACS Paragon Plus Environment


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the first cycles recorded from 0.1 V towards the negative potential. The results showed that “the charges associated with hydrogen adsorption/desorption were 62, 107, 103, and 155 μC cm-2 for replacement times of 5, 10, 20 and 240 min, respectively, which are equal to 30%, 51%, 49% and 74% of the charge associated with the adsorption/desorption of one hydrogen full monolayer on a Pt(111) surface (210 μC cm-2).47” Therefore, there was still some uncovered Au surface even after 4 hours of incubation. The cause of this coverage dependence of Pt on the incubation time of Au(111) surface is different from the phenomena observed in the study of spontaneous deposition of Pt on Au(111).48,49 In the present work, the metallic platinum comes from redox replacement of the UPD copper that was predeposited on Au surface. In the prior studies, the metallic Pt formed via electrochemically reducing pre-adsorbed Pt ions on the Au surface. However, no electrochemical reduction was involved in the present work. The three different results (XPS, Voltammetry, and CV) provided above suggested that the expected Pt(111) surface did not form. Instead, the formation of polycrystalline Pt was observed. The deposited platinum atoms were not fixed but changing and took quite a long time to stabilize the platinum atoms on the gold surface, even though the process of Pt atoms replacing Cu atoms was fast and even though the copper atoms immediately left the surface. 3.3. EQCM. The process of UPD Cu on the gold substrate followed by Pt/Cu replacement was monitored via EQCM. Figure 3 shows the EQCM frequency and current response in a potential step experiment in a solution containing 5 mM CuSO4 and 0.05 M H2SO4. The potential was held at 0.55 V first, where there was no Cu deposition on Au. No current and frequency changes were observed. The potential then jumped to 0.02 V, where a full monolayer of Cu atoms was underpotentially deposited on the gold surface. A fast cathodic current transient associated with a sharp frequency decrease was observed. The EQCM frequency response closely followed the current transient. The 12 ACS Paragon Plus Environment

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integrated charge of this chronoamperogram was 445 C cm-2. This value was in good agreement with the theoretical one of 440 C cm-2, necessary for the deposition of a fully discharged monolayer of copper.44 The frequency change was found to be –28 Hz. Based on the Sauerbrey equation, this frequency change corresponded to 155 ng cm-2 (mUPDCu) of Cu atoms deposited on gold, which corresponded to the mass change per mole electron, abbreviated as mpe, of 33.6 g/mol electron. On the other hand, the theoretical value for a two-electron process of Cu UPD on a gold surface is found to be 31.7 (= 63.5/2) g/mol electron. Both values were so close, which verified the validity of using the Sauerbrey equation in this study.

Figure 3. The plots of time vs. current density (solid line) and of time vs. frequency change (open circle) at a gold covered quartz crystal electrode response to a potential step from 0.55 V to 0.02 V in 0.05 M H2SO4 solution containing 5 mM CuSO4.

Once a full monolayer of metallic Cu UPD was formed on the Au/quartz electrode, the Cu(II) solution was carefully replaced several times with a deaerated solution of 0.05 M H2SO4; then, a 2 13 ACS Paragon Plus Environment


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mM K2PtCl4 stock solution (final concentration 1 mM) was added to the cell where the Au/quartz electrode was located. The resonance frequency decreased by 70 Hz as shown in Figure 4. This indicates that Pt deposited on gold electrode surface will cause a 389 ng cm–2 (mPt/Cu replace) mass increase on the electrode after Pt atoms (Mm = 195) replace the Cu atoms (Mm = 63.5). The ratio of mPt/Cu replace over mUPDCu was found to be 2.51. However, the ratio of molar mass difference of Pt and Cu over that of Cu ((MmPt – MmCu)/MmCu) provides a value of 2.07. The observed experimental value (2.51) is larger than the theoretical one (2.07). This is because the PtCl42- ions adsorb on the surface of the electrode deposited with Pt and that the observed 389.3 ng cm– 2 mass increase involves the mass of the adsorbed platinum complex ions.13,14,39 A decrease in resonance frequency of 11 Hz was detected upon contact of the electrode of Pt/quartz or Au/quartz with the 1 mM K2PtCl4 solution. With the reduction of 11 Hz from the total change of 70 Hz, the experimental value of the ratio of mPt/Cu

replace

over mUPDCu was corrected to be 2.10, which is in very good agreement with the

theoretical value and, suggests that Pt atoms completely replaced UPD Cu atoms on the surface of the Au/quartz electrode under the present experimental conditions. Repeated EDS measurements of the resulting Pt/Au/quartz electrodes showed no trace of copper within the detection limits of the instrument. As the PtCl42- concentration decreased, the time required for SLRR became longer as evidenced by the delayed frequency response and decreased plateau frequency. Copper was found when 0.1 mM PtCl42- was used for SLRR, suggesting that the SLRR protocol must be carefully controlled.30,35,36


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Figure 4. Frequency shift (circle) fitted using Eq. 3 (solid line) for the process of Pt/Cu replacement. For clarity, only every 20th data point is shown.

The EQCM measurement of the frequency change caused by the exposure of the platinum complex ions to the UPD Cu provided a direct measure of the kinetic process of Pt/Cu replacement. As shown in Figure 4, the frequency shift (f) versus time curve shows an exponential decrease in the frequency. A Langmuir model was used to describe the Pt/Cu spontaneous redox replacement process. The observed rate constant (kobs) for this process was calculated by fitting the plot of the frequency shift against time using the following equation: f (t) = ftotal (1 - exp (-kobst))

(3)

, where f (t) is the frequency change at time t, ftotal is the total frequency shift (70 Hz), and kobs is the rate constant. The fits (solid line in Figure 4) using Eq. 3 adequately described the process of Pt/Cu SLRR with a rate constant of 0.10 s–1, which may suggest that bare Au sites, Cu or a mixture of the two could act as a first order complex reaction process as the bare Au surface sites do in the one–step elementary reaction of the Langmuir adsorption model. The apparent fit of Langmuir model 15 ACS Paragon Plus Environment


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to the present reaction system (2) may arise from the following: (i) the charge transfer for SLRR is fast (XPS and QCM), (ii) Pt atoms generated from the charge transfer reaction adsorbs at the electrode (XPS and QCM), (iii) adsorption of PtCl42- occurs at the surface of both Au and Pt electrodes (QCM), (iv) the adsorption of PtCl42- on the surface of either Au or Pt electrodes is not too strong in the sulfuric acid media (XPS and CV versus QCM), and (v) the concentration and adsorption strength of Cu(II) generated from UPD Cu on the Au surface via SLRR (< 0.1 μM) is negligible compared with those of PtCl42- (1 mM). Certainly, the apparent fit of the EQCM data does not confirm that every step involved in the SLRR satisfies the assumptions of the model because the surface redox reaction, adsorption of Pt atoms and PtCl42- ions, and rearrangement of adsorbed Pt atoms are involved in the entire SLRR process (see below). Importantly, the resonance frequency remained unchanged for hours after its initial decrease at the beginning of the SLRR process, although significant changes in the XPS and voltammograms were observed for the same period.

3.4. STM Study of the Replacement Process. STM was used to monitor the platinum film formation via the Pt/Cu SLRR. Figure 5 shows representative STM images of the Cu adlayer on Au (111) electrode surface with 10, 15, 20, 60, and 240 min of incubation in the K2PtCl4 solution. The images were taken after the SLRR Au electrode was

rinsed

with

the

platinum

complex

ion–free

acidic

solution

several

times.

Pt

nanoparticles/nanoclusters and the bare Au(111) surface are observable. The data show that a larger uncovered Au(111) surface with Pt nanoparticles that were 6 to 7 nm high was observed for the shortest incubation time.35 With an increase in incubation time, the coverage of Pt nanoparticles increased with decreasing nanoparticle height. Additionally, a Pt nano-cluster with a 0.5 to 0.7 nm height covering the surface was found in the image in Figure 5E after 240 min of incubation time. Figure 5F shows a close-up image of Figure 5E, which was similar to the one in a previous report.15 16 ACS Paragon Plus Environment

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An analysis of Figure 5 is shown in Figure 6, where the average size, height and number of Pt particles changed approximately logarithmically as a function of time. The spreading of the nanoparticles into even smaller particles is rather interesting because the reverse process is commonly observed as Ostwald ripening,50 which is related to a variation in size of particles. Ostwald ripening occurs because atoms on the surface of particles are more energetically unstable than those inside, dissolving into solution and depositing on larger particles, and shrinking the size of smaller particles while increasing that of larger particles. A change in the STM images was observed only when the samples were incubated for different times in the PtCl42- solution, not in the absence of PtCl42-. Figure 7 shows EC-STM images taken in 0.1 M H2SO4 solution for samples incubated in PtCl42- solution for 60 min and 240 min, respectively. Both EC-STM images look similar to the corresponding ex situ ones shown in Figure 5 and did not change after hours in the solution. Thus, the process reversed to Ostwald ripening which is often observed in particle growth dynamics is supported by the presence of the PtCl42- ions in the electrolyte solution and the presence of Pt particles on the surface. This may be the first example of reverse Ostwald ripening in certain conditions.



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Figure 5. STM images observed at the Pt/Au (111) surface under Ut = 100 mV and It = 100 pA incubated for (A) 10 min (B) 15 min (C) 20 min (D) 60 min and (E) 240 min after replacement of the Cu adlayer in the K2PtCl4 solution. (F) close-up image of (E). 300x300 nm2. The electrodes were rinsed with Pt ion–free acidic solution before the STM images were taken.

Figure 6. Average size (A) and number (B) of Pt particles as a function of replacement time in K2PtCl4 solution (Figure 5). Open circles in A represent particle height.


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Figure 7. EC-STM images of SLRR Pt/Au(111) in 0.1 M H2SO4. Samples obtained after (A) 60 min and (B) 240 min of replacement of the Cu adlayer in the K2PtCl4 solution. 250x250 nm2. Potential of Pt/Au(111) = - 0.38, Etip = - 0.16, and Itunneling = 0.05 nA.

3.5. Mechanistic Aspects. Table 1 summarizes the experimental results obtained in the present study. XPS and CV data indicate that the surface area of platinum increased as a function of incubation time and that the surface of Au(111) electrode was increasingly covered with platinum as produced by SLRR of UPD Cu/Au in the PtCl42- complex solution for times up to four hours. The QCM data suggest that SLRR itself occurred in less than 1 minute and that no further changes occurred that were related to the adsorption/desorption on the surface of the SLRR Pt/Au QCM electrode for the same time period. The STM data shows that the Pt nanoparticles changed into even smaller Pt particles as the incubation time increased or that Pt nanoparticles reorganized/restructured into a flattened state on the surface of the gold electrode.



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Table 1. Summary of the Experimental Results of the SLRR of UPD Cu/Au by PtCl42-a Time (min) 10 20 60 240 XPS Amount (cps) CV UPD H/Pt Coverage STM Height (nm) STM size (nm2) STM particle number STM total area (nm2) QCM (-∆f) a

0.17 0.30b, 0.51 6.2 19.20 212 4070 (0.05)c 70d

0.23 0.49 2.5 15.87 2084 33070 (0.37)c 70

1.4 11.75 4190 49230 (0.55)c 70

0.93 0.74 0.7 8.65 9614 83200 (0.92)c 70

1 mM PtCl42- in 0.05 M H2SO4. b Result at 5 min. c Covered fraction d Result at 1 min. Because the QCM showed no change in Pt weight after the initial reaction time or no change in

the compositions of PtCl4- and Pt, the amount of Pt metal on the surface of gold should have remained unchanged for four hours once it was created during the first stage of reaction. For the same four hours, the XPS result shows that the amount of Pt gradually increased, meaning that the Pt morphology changed. Additionally, the H UPD data show the increase in the number of electrochemically accessible Pt atoms on the surface of the gold electrode as a function of incubation time. As shown in Table 1, the change in the fraction of the surface covered by Pt corresponding to morphological changes of Pt formed or the change in the fraction of Pt atoms observed by XPS, CV, and STM can be quantitatively compared as 0.17 : 0.51(0.30) : 0.05 at 10 min, 0.23 : 0.49 : 0.37 at 20 min, and 0.93 : 0.74 : 0.92 at 240 min, respectively. Although three independent methods were used to probe the surface Pt atoms, the results clearly show that the Pt nanoparticles changed their morphological shapes or spread onto the surface as a function of time. There are differences in the methods used in the present investigation. Namely, STM can probe the surface atoms of Pt nano particles, mostly top only, at the atomic level for a given local area, 300 x 300 nm2 in the present case. XPS can detect the surface atoms of Pt nano particles, top and sides not all though, at the atomic level for a given local area, the diameter of ca. 100 μm in the present case. CV can measure the surface atoms of Pt nano particles, top, sides and sometimes 20 ACS Paragon Plus Environment

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bottom also, at the atomic level on the whole electrode surface. Thus, the data show that the surface of the gold electrode was covered with Pt atoms in the following decreasing order for the measurements: CV (0.51), XPS (0.17), and STM (0.05) at 10 min, where mountain-like Pt nanoparticles were distributed in different forms. However, the data show that the Au surface was almost fully (0.93 (XPS) and 0.92 (STM) fraction) covered with Pt except for in the CV (0.74) case. In the case of CV, the error in the measurement will be necessarily large (0.30, 0.51, 0.49, 0.74 for 5, 10, 20, and 240 min, respectively) because of the variation in size of the single crystalline Au(111) prepared by the Clavilier method, where the surface area of the gold electrode was measured to one significant figure of 0.07 cm2 as described in the EXPERIMENTAL SECTION.41 When the large differences and errors involved in the independent techniques employed are considered, the agreements among the results by the three methods are surprisingly good to conclude that mountain-like Pt nanoparticles created during the first stage of the SLRR reaction dispersed into a smoothed Pt monolayer at the atomic layer on the surface via a certain additional process (see below). The above observations on the SLRR process given by Eq. (2) in the present experimental conditions imply (i) that UPD Cu on Au was completely replaced by Pt atoms via an electron transfer reaction between surface Cu atoms and PtCl42- ions in solution (XPS, QCM and EDS), (ii) that the initial structures of Pt/Au consist of nanoparticles in island (STM), (iii) that changes in the surface structure occur at the Pt/Au (XPS, voltammetry, STM), (iv) that Pt nanoparticles of the initial structure spread into even smaller particles (STM), and (v) that the structural changes occurred only in the solution containing PtCl42- (STM). The observations of (i), (ii) and (iii) may be caused by the presence and adsorption of PtCl42- in the solution, and those of (iii), (iv) and (v) may be caused by the disproportionation reactions of PtCl42- adsorbed on the surface of Pt and 21 ACS Paragon Plus Environment


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Au at Pt/Au, Eqs. (4) and (5). 2PtCl42- = PtCl62- + Pt* + 2Cl-

(4)

PtCl62- + Pt + 2Cl- = 2PtCl42-

(5)

, where all the species are on the surface of the electrode. Pt* in Eq. (4) represents the platinum generated from the disproportionation reaction and Pt in Eq. (5) is the platinum on the surface of the Pt nanoparticles produced by SLRR, Eq. (2). PtCl62- + 2Cl- produced from the disproportionation reaction, Eq. (4), may be recycled to take a direct part in attacking and removing Pt atoms from the surface of the mountain-like Pt nanoparticles formed during the first stage of SLRR, Eq. 2, to regenerate PtCl42- ions. Thus, Eqs. (4) and (5) constitute the cycle process of redispersion of Pt nanoparticles or of reverse Ostwald ripening of Pt nanoparticles into even smaller Pt particles. The reaction rates of (4) and (5) may depend on the concentration of each species and appear to be very slow in the present experimental conditions as evidenced by our experimental data of XPS (indirect), voltammetry (indirect), CV (indirect), and STM (direct). The above reaction mechanism, including the reverse Ostwald ripening process, on the surface of the electrode is based on the observations of the unchanged resonance frequency of QCM data for hours after abrupt initial change due to SLRR itself. The reverse Ostwald ripening process does occur thermodynamically because the sufficient amount of surface atoms on the mountain-like Pt nanoparticles initially formed by SLRR itself works as the source for the reverse disproportionation reaction, Eq. (5), or drives the disproportionation reaction back to produce PtCl42- ions. Of note, no extra Pt atoms are required for the reverse Ostwald ripening process or redispersion of Pt nanoparticles to operate on the surface of the SLRR Pt/Au electrode with PtCl42- adsorbed until the surface is reorganized or flattened at the atomic level. Through the reverse Ostwald ripening process, SLRR Pt is converted or broken down into Pt atoms without any extra atoms involved. Based on this mechanism, the final flattened surface state of the 22 ACS Paragon Plus Environment

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SLRR Pt/Au electrode at the atomic level is expected to be produced eventually at a certain stage after a long time. One can manufacture a 2D Pt layer on the surface by starting any form of 3D Pt particles prepared via other methods if the 3D Pt particles formed are treated in PtCl42- solutions. The entire SLRR may be proposed to proceed as follows upon addition of K2PtCl4 into a solution of UPD Cu/Au. When the Pt ions are added to the cell containing UPD Cu/Au, an electron transfer reaction may quickly take place between the Cu atoms on the surface and Pt ions near the electrode surface. As soon as Cu atoms on the Au surface lose electrons and leave the surface, there will be competitive adsorption between the generated Pt atoms and PtCl42- complex ions on the surface of the Au electrode. Two processes may occur competitively as well as simultaneously but the adsorption of Pt ions may be neither weaker nor slower than that of Pt atoms. Some sites on the Au surface will be occupied by Pt ions and others by Pt atoms. Pt atoms appear to accumulate preferentially on Pt atoms rather than on top of Pt ions adsorbed on the surface of the Au electrode. The adsorption of Pt atoms on the Pt particles may be faster or more favorable than that of PtCl42ions. Thereby Pt atoms form large nanometric or mountain-like nanoparticles on the surface of gold. This may be the major picture in the beginning minute after the Pt ions are added to the solution where UPD Cu on the Au electrode is situated. Then, the disproportionation reaction between PtCl42ions adsorbed on the electrode surface may control the progress of the reaction. This disproportionation is believed to be slow in aqueous solution51,52 but may be facilitated between adsorbed PtCl42- ions. It has been reported that the oxidation of PtCl42- complexes to PtCl62- seemed to be catalyzed by deposited Pt on the Au surface.13 The disproportionation reaction on the surface may be supported by the equilibrium between the PtCl42- ions on the surface and those in homogeneous solution, and it may occur at slow but measurable rates in hours, during which the large nanometric Pt particles formed at the beginning of SLRR dynamically diffuse on the surface of the 23 ACS Paragon Plus Environment


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gold electrode. The question has arisen whether the apparent surface diffusion of Pt atoms can occur in the absence of PtCl42- ions in homogeneous solutions. When EC-STM was performed at the surface of SLRR Pt/Au(111) in the aqueous solution of electrolyte only, no change was observed in hours as shown in Figure 7. Thus, dynamic surface diffusion of Pt atoms via disproportionation of adsorbed PtCl42- ions requires PtCl42- ions in homogeneous solution. Slow but measurable generation of Pt particles via disproportionation of PtCl42- ions has been reported in ionic liquid media at room temperature.52 The reverse Ostwald ripening might be expected to occur in other systems of elements and molecules with their disproportionation reaction of two redox species.54,55 A schematic of the proposed rationale for the observations is shown in Figure 8.


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Figure 8. (a) Schematic of whole SLRR process of Cu/Au in PtCl42-. Red and gray balls represent Cu and Pt atoms, respectively. (b) Reverse Ostwald ripening of Pt particles (bottom step in (a)). SLRR Pt denotes the Pt nanoparticles produced by SLRR of Cu/Au in PtCl42-. Reactants and products of disproportionation reaction 2PtCl42- = PtCl62- + Pt* + 2Cl- are half bracketed on the right side, 25 ACS Paragon Plus Environment


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respectively. PtCl62- + 2Cl- (circled in the dashed line) is recycled to react with Pt (dashed circle) on the surface of the SLRR Pt to regenerate 2PtCl4-. SLRR Pt is converted into Pt atoms without any extra atoms involved.

Questions on how the Pt layer is formed by Pt/Cu SLRR and what the properties of the resulting layer are in physical chemistry context were studied using PtCl42- in sulfuric acid media. The results demonstrated that SLRR itself occurs in less than five minutes under the experimental conditions but the accompanying structural changes continue to occur in hours. One must define the structure and properties of the resulting Pt layer to use it for catalytic purposes. The proposed rationale of the observations of the SLRR of UPD Cu on Au is based on the two important facts of the adsorption of PtCl42- and the facilitated disproportionation between the PtCl42- ions adsorbed on the surface. Disproportionation reactions in homogeneous solutions are well documented in the literature.51–53 The present results are a good SLRR kinetic addition for preparing (fractional) monolayers of Pt atoms, especially using PtCl42- ions. Reverse Ostwald ripening of Pt particle growth dynamics can occur via the disproportionation of PtCl42- at Pt nanoparticles, and apparent mobility or dynamicity of Pt nanoparticles can be attributed to the disproportionation reaction to produce Pt atoms. Thus, apparent surface diffusion via a disproportionation reaction may be possible.

4. CONCLUSIONS A kinetic study of Pt monolayer creation via the redox replacement of an underpotentially deposited Cu monolayer on the surface of Au(111) was performed in sulfuric acid solutions of PtCl42by means of XPS, CV, QCM, EDS, and STM. XPS and CV data showed that the surface of Au(111) electrode was increasingly covered by platinum produced by SLRR of UPD Cu/Au in PtCl4226 ACS Paragon Plus Environment

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complex solutions with time elapsed up to four hours, while QCM data suggested that SLRR itself took place in less than a minute and that no further changes occurred related to the adsorption/desorption on the surface of SLRR Pt/Au QCM electrode for the same period. EDS data revealed that Cu was found on the surface of Au(111) when the replacement reaction was performed in low concentrations of PtCl42-, but that no trace of the element in high concentrations. STM data showed that Pt nano particles changed into even smaller Pt particles as the incubation time increased in PtCl42- complex solutions, implying that Pt nano particles were reorganized/restructured into the flattened state at the atomic level on the surface of gold electrode. Thus, the results demonstrated that the UPD Cu adlayer was totally replaced by platinum in 5 minutes or even shorter after being exposed to the solution containing Pt(II) complex ions. The deposited platinum atoms were proposed to undergo an atomic rearrangement via reverse Ostwald ripening of disproportionation of adsorbed Pt(II) ions after having replaced UPD copper atoms. The new physical insight of two-step process schematically depicted in Figure 8 to rationalize the results obtained from the kinetic study of SLRR of UPD Cu/Au by Pt in PtCl42- complex solutions will be a good addition to the general field of physical chemistry in that the process may be applied to generate 2D Pt particles by the PtCl42- treatment of 3D Pt particles fabricated by any methods, which may be applied to electrocatalysis, surface catalysis, and homogeneous catalysis involving precious platinum as catalyst. The reverse Ostwald ripening might be expected to occur in other systems of elements and molecules having their disproportionation reaction of two redox species. It may be expected to be applied to the area of nanomaterials and nanostructures beyond that of surfaces, interfaces and catalysis as a new physical process for material synthesis and deeper understanding of the dynamic processes involved.



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■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.-W.L). Author Contributions D.Q. and C.-Y.J. equally contributed to this work. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS Korea University financially supported this work. We appreciate Professor Katsuaki Shimazu and Dr. Takayu Masuda for the use of XPS facilities at Hokkaido University. We appreciate the reviewers for their thoughtful questions, suggestions and recommendations.


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TOC graphic


Pt Monolayer Creation on a Au Surface via an Underpotentially

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