Formation of Single-Layered Pt Islands on Au(111) Using Irreversible

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Formation of Single-Layered Pt Islands on Au(111) Using Irreversible Adsorption of Pt and Selective Adsorption of CO to Pt Jandee Kim,† Dongwan Shin,† Choong Kyun Rhee,*,† and Seong-Ho Yoon‡ †

Department of Chemistry, Chungnam National University, Daejeon 305-764, Korea Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan

‡

S Supporting Information *

ABSTRACT: This communication compares two different multiple deposition routes of Pt on Au(111), using irreversible adsorption of Pt precursor ions and selective adsorption of CO. A scanning tunneling microscopy study revealed that the conventional route, not utilizing CO, produced multiplelayered Pt cluster islands, while the CO route, employing CO, formed single-layered Pt islands exclusively. The role of CO selectively adsorbed on pre-existing Pt islands was to prevent additional irreversible adsorption of Pt precursor ions onto Pt islands. Cyclic voltammetric works disclosed that the CO and hydrogen coverages on single-layered Pt islands were higher than those on multiple-layered ones, and that the Pt islands on Au were more effective in adsorbing CO than hydrogen.

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INTRODUCTION Pt thin films at nanometer scales on metal substrates (especially Au surfaces) are important for the development of Pt-based electrocatalysts in fuel cell technology.1−5 The properties of the Pt thin films on a substrate differ from those of bulk Pt due to influences of the substrate metal, allowing the electrocatalytic behavior of Pt overlayers to be manipulated. Pt layers have been produced from the precursor ions of PtCl42− or PtCl62− by direct electrochemical deposition,6−9 galvanic replacement of the bulk layers or underpotentially deposited monolayers of less noble metals,10−15 and irreversible adsorption.16−18 The Pt layers produced using the various methods consisted of Pt clusters 2−10 nm wide and several atomic layers high, due to the high surface energy of Pt. To avoid formation of threedimensional Pt clusters (i.e., to prepare a smooth Pt layer), recently, a new electrochemical deposition method using adsorbed CO on Pt as a deposition-inhibiting agent was demonstrated.19 In this communication, we introduce a novel method to produce single-layered islands using multiple irreversible depositions of Pt precursor ions17 and selective adsorption of CO toward Pt (not Au). Figure 1 compares the basic principles operating in two different deposition procedures, termed conventional and CO routes, respectively. The two procedures start identically, with the first deposition step consisting of the irreversible adsorption of Pt precursors and the subsequent electrochemical reduction of adsorbed Pt precursors to form Pt islands, as depicted in Figure 1a.16 In the conventional route, these steps are repeated in a cyclic manner as many times to deposit additional Pt atoms on the Au sites not yet covered, as well as the pre-existing Pt islands (Figure 1, panels b and c). © 2014 American Chemical Society

Figure 1. Schematic illustration of conventional and CO routes. The yellow, green, gray, blue, and red balls represent Au and Pt atoms in the first−fourth layers of Pt deposits, respectively.

Eventually, multiple-layered Pt clusters cover the Au surface, as illustrated in Figure 1d. In the CO route, a CO adsorption step Received: January 5, 2014 Revised: March 3, 2014 Published: April 2, 2014 4203

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Figure 2. STM images of Pt deposits on Au(111). The upper panel represents the images along the conventional route after (a−e) first−fifth deposition cycles, respectively. The lower panel shows the images along CO route after (f−j) first−fifth deposition cycles, respectively. Image size: 500 nm.

twice, the number of small Pt islands randomly distributed over the surface increased (Figure 2b). The third deposition cycle induced a significant change: in Figure 2c, the aggregates of Pt islands were brighter and individual Pt islands were darker. As the deposition cycle continued further, the images of Figure 2 (panels d and e) became filled with Pt islands of fairly uniform contrast. Also, the increase in island density during the conventional route supported an increase in the amount of Pt deposited on the surface. The lower panel of Figure 2 presents the evolution of STM images along with the CO route. The image of Figure 2f, observed upon the first deposition cycle, was similar to that of Figure 2a. The second deposition cycle in the CO route did not generate a significant difference when comparing Figure 2 (panels b and g). However, repeating the deposition cycle three times induced a significant difference: Figure 2h is mostly filled with small Pt islands with some brighter ones. In the images obtained after the fourth and fifth depositions (Figure 2, panels i and j), the Au(111) surface was almost completely covered with Pt islands, whose contrast was not as developed compared with the conventional route. Therefore, the CO route was clearly distinctive from the conventional route in the evolution of the morphology of Pt islands deposited on Au(111). Figure 3a presents a line profile of the Pt islands observed after the third cycles of the CO route. The line profile along the arrow in the adjacent image clearly reveals that the heights of the single layers uncovered by higher layers were less than 0.1 nm, while that of the double-layered layer was around 0.6 nm. A close inspection of all the images in Figure 2 strongly supports that the heights of the uncovered single layers ranged from 0.07 to 0.09 nm. In contrast, the height of the doublelayered feature (∼0.6 nm) indicates that two atomic layers of Pt were on the Au surface. Stickney group reported that during galvanic replacement of Cu monolayer with Pt under the presence of iodine as a surfactant, the Pt layers of submonolayer regimes (∼0.08 nm high) signaled an alloy of Pt and Au, and that the Pt layer of full monolayer was on the surface of Au.11 Then, the line profiles in Figure 3a imply that the Pt atoms in uncovered single layers might be embedded into the Au

follows the electrochemical reduction of adsorbed Pt precursors in a deposition cycle to selectively adsorb on only Pt islands (Figure 1e), so that the incoming Pt precursor ions will solely contact the bare Au sites for adsorption (Figure 1f). The CO route may then ultimately cover the Au surface with a continuous Pt monolayer or discrete single-layered Pt islands (Figure 1g). Therefore, the morphologies of Pt deposits on Au surfaces prepared using the two routes would differ significantly.

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EXPERMENTAL SECTION

Experimental details are described in the Supporting Information. A brief description is as follows. Au electrodes of two types were employed in this work: Au single crystal beads for STM works and polished Au polycrystalline hemispheres for electrochemical measurements. The irreversible adsorption of Pt on the Au surfaces was performed by immersing Au electrodes in a 10−5 M H2PtCl4 solution in 0.05 M H2SO4 for 10 min without potential control. After thoroughly rinsing with water, the adlayer of Pt precursor was electrochemically reduced at 0 V (versus a Ag/AgCl electrode with [Cl−] = 1.0 M) in 0.05 M H2SO4 solution for 10 min. In the conventional route, the deposition cycle consisting of the irreversible adsorption followed by the electrochemical reduction was repeated. In the CO route, the deposition cycle following the first deposition cycle changed to a sequential operation of the adsorption of CO, the irreversible adsorption of additional Pt precursors, and the electrochemical reduction of additional Pt precursors. CO adsorption was performed by immersing the Au electrodes covered with reduced Pt deposits into a CO-saturated 0.05 M H2SO4 solution for 3 min at 0 V. For STM measurements, the Au single crystal bead with Pt deposits was integrated into an electrochemical STM cell for a (111) facet to be pointed to W tip, and imaging was performed in a clean 0.05 M H2SO4 solution at 0 V. In electrochemical experiments, a meniscus position was maintained to expose only the polished Au surfaces.

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RESULTS AND DISCUSSION Figure 2 compares STM images of the Pt deposits on Au(111) prepared using the conventional and CO routes. Figure 2 (panels a−e) were obtained as the number of deposition cycles using the conventional route increased. The first deposition resulted in an image randomly filled with small islands of Pt, as shown in Figure 2a. When the deposition cycle was repeated 4204

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Figure 4. (a) Typical cyclic voltammograms of Pt islands on Au prepared using conventional and CO routes in 0.05 M H2SO4 solution (scan rate: 50 mV/s): red traces are CO stripping voltammograms, and black ones are plain voltammograms. (b) Coverage variations of CO and hydrogen on Pt islands. Figure 3. (a) Line profile (left) along the arrow in the image (right) obtained after the third deposition cycle using the CO route. Variations in the area occupied by each layer in multiple-layered Pt islands: (b) conventional route and (c) CO route. The numbers in (b) and (c) correspond to stacked orders in Pt deposits. Image size: 100 nm.

deposition (below 0 V) took place sequentially. As shown in Figure S6 of the Supporting Information, the charges of the processes related to Pt (i.e., CO stripping, Pt reduction and hydrogen underpotential deposition) increased as the deposition proceeded, while that of Au reduction decreased. Figure 4b shows the variations in the coverages of adsorbed CO and hydrogen. Two distinctions are clear: the CO coverage was higher than the hydrogen coverage regardless of the deposition routes, and the CO and hydrogen coverages on the Pt islands prepared using the CO route were higher than those using the conventional route. Generally, the charges of CO and hydrogen reflect the number of electrochemically active Pt atoms, not the total number of deposited Pt atoms. If stoichiometries of CO and hydrogen adsorption are fixed on a certain type of Pt, thus, the coverage ratio of CO and hydrogen should be constant. For example, the CO/H coverage ratio of 0.75 on Pt(111)20 implies the presence of adsorbed CO on bridge sites. The CO/H coverage ratio observed in this work ranged from 3 to 1.5, indicating that the stoichiometry of hydrogen adsorption onto the Pt deposits on Au was relatively lower, perhaps due to the Au substrate not adsorbing hydrogen. On the other hand, the higher CO and hydrogen coverages on the Pt deposits prepared using the CO route would be understood based on two reasons: more available electrochemically active Pt atoms and/or higher adsorption abilities toward the two species. Although this work is not able to provide a clear answer, the investigated routes obviously induced the different electrochemical behavior regarding adsorption of CO and hydrogen. The experimental results presented so far demonstrate that the CO adsorbed on Pt surfaces was able to limit or prevent chemical reactions on Pt, particularly deposition of additional Pt. Specifically the adsorbed CO prevented the irreversible adsorption of Pt precursor ions on the pre-existing Pt islands to increase the population of uncovered single-layered Pt islands. This observation is consistent with a recently reported method preparing a smooth Pt monolayer on Au during an electrochemical deposition of PtCl42− in the presence of CO.19 Therefore, the adsorbed CO would be a powerful tool to manipulate morphologies of Pt deposits by inhibiting further chemical and electrochemical deposition processes. This particular strategy of adsorbed CO would open a new avenue

substrate as an alloy, while those in covered layers would be on the Au surface (see Figure S2b of the Supporting Information). Figure 3 (panels b and c) show variations in the area occupied by each Pt layer stacked in the Pt deposits as a function of the number of deposition cycles in the conventional and CO routes. Estimation of the occupied areas is illustrated in the Supporting Information. A clear difference between the two routes was that the populations of the layers above the first one were higher in the conventional route than in the CO route. In the CO route, indeed, the population of double-layered ones was much lower (i.e., ∼15% in the CO route versus ∼50% in the conventional route), and there was no island higher than the double-layered ones. These observations indicate that the adsorbed CO limited additional adsorption of Pt precursor ions on the pre-existing Pt islands and that only bare Au sites were available to the adsorbing Pt precursors. Thus, the CO route resulted in smoother surfaces (see Figure S5 of the Supporting Information.). The existence of double-layered islands along with the CO route, although at significantly lower levels, was ascribable to inevitable partial removal of the adsorbed CO molecules protecting Pt islands during transfer from the COsaturated solution to the Pt precursor solution through the air for the next deposition cycle. Figure 4a compares typical voltammograms demonstrating CO stripping, reduction of Au and Pt surface oxides, and hydrogen underpotential deposition on Pt islands on polycrystalline Au after the third cycles of the conventional and CO routes. All of the voltammograms along the two deposition routes are displayed in the Supporting Information. Two CO oxidation peaks at ∼0.6 and ∼0.8 V (in the red traces) were clearly discernible. As the deposition cycle was repeated, the majority of the adsorbed CO moved from the strongly bound CO to the weakly adsorbed one regardless of deposition routes. The voltammograms obtained in CO-free 0.05 M H2SO4 solution (black traces) obviously showed that in the cathodic scan, Au oxide reduction (peaked at ∼0.8 V), Pt oxide reduction (peaked at ∼0.3 V), and hydrogen underpotential 4205

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(8) Jin, C.; Sun, C.; Dong, R.; Chen, Z. Platinum Modification of Gold and Electrocatalytic Oxidation of Ethylene Glycol on Ptmodified Au Electrodes. Electrochim. Acta 2010, 56, 321−325. (9) Kondo, T.; Shibata, M.; Hayashi, N.; Fukumitsu, H.; Masuda, T. Resonance Surface X-ray Scattering Technique to Determine the Dtructure of Electrodeposited Pt Ultrathin Layers on Au(111) Surface. Electrochim. Acta 2010, 55, 8302−8306. (10) Sasaki, K.; Mo, Y.; Wang, J. X.; Balasubramanian, M.; Uribe, F.; McBreen, J.; Adzic, R. R. Pt Submonolayers on Metal NanoparticlesNovel Electrocatalysts for H2 Oxidation and O2 Reduction. Electrochim. Acta 2003, 48, 3841−3849. (11) Kim, Y.-G.; Kim, J. Y.; Vairavapandian, D.; Stickney, J. L. Platinum Nanofilm Formation by EC-ALE via Redox Replacement of UPD Copper: Studies Using In- Situ Scanning Tunneling Microscopy. J. Phys. Chem. B 2006, 110, 17998−18006. (12) Zhang, M. B. J.; Sasaki, K.; Nilekar, A. U.; Uribe, F.; Mavrikakis, M.; Adzic, R. R. Platinum Monolayer Electrocatalysts for Oxygen Reduction. Electrochim. Acta 2007, 52, 2257−2263. (13) Gokcen, D.; Bae, S.-E.; Brankovic, S. R. Reaction Kinetics of Metal Deposition via Surface Limited Red-ox Replacement of Underpotentially Deposited Metal Monolayers. Electrochim. Acta 2011, 56, 5545−5553. (14) Fayette, M.; Liu, Y.; Bertrand, D.; Nutariya, J.; Vasiljevic, N.; Dimitrov, N. From Au to Pt via Surface Limited Redox Replacement of Pb UPD in One-Cell Configuration. Langmuir 2011, 27, 5650− 5658. (15) Nutariya, J.; Fayette, M.; Dimitrov, N.; Vasiljevic, N. Growth of Pt by Surface Limited Redox Replacement of Underpotentially Deposited Hydrogen. Electrochim. Acta 2013, 112, 813−823. (16) Kim, J.; Jung, C.; Rhee, C. K.; Lim, T.-H. Electrocatalytic Oxidation of Formic Acid and Methanol on Pt Deposits on Au(111). Langmuir 2007, 23, 10831−10836. (17) Strbac, S.; Petrovic, S.; Vasilic, R.; Kovac, J.; Zalar, A.; Rakocevic, Z. Carbon Monoxide Oxidation on Au(111) Surface Decorated by Spontaneously Deposited Pt. Electrochim. Acta 2007, 53, 998−1005. (18) Kim, S.; Jung, C.; Kim, J.; Rhee, C. K.; Choi, S.-M.; Lim, T.-H. Modification of Au Nanoparticles Dispersed on Carbon Support Using Spontaneous Deposition of Pt toward Formic Acid Oxidation. Langmuir 2010, 26, 4497−4505. (19) Brimaud, S.; Behm, R. J. Electrodeposition of a Pt Monolayer Film: Using Kinetic Limitations for Atomic Layer Epitaxy. J. Am. Chem. Soc. 2013, 135, 11716−11719. (20) Jung, C.; Kim, J.; Rhee, C. K. Electrochemical Scanning Tunneling Microscopic Observation of the Pre-Oxidation Process of CO on Pt(111) Electrode Surface. Langmuir 2007, 23, 9495−9500.

to tune catalytic properties of Pt, especially in the form of Pt layers on metal substrates.

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CONCLUSION In summary, a novel method to produce uncovered singlelayered Pt islands on Au(111) utilizing irreversible adsorption of Pt and selective adsorption of CO on pre-existing Pt surface (CO route) was introduced. Compared to conventional multiple depositions without CO (conventional route), the populations of multiple-layered Pt islands were significantly reduced. In addition, the Au substrate reduced the hydrogen adsorption ability of Pt islands on Au (compared with bulk Pt), and the higher population of uncovered single-layered Pt islands correlated to higher CO and hydrogen coverages. The CO route would be an important method to manipulate the morphologies of Pt deposits on Au surfaces, thus their electrocatalytic properties. Currently, a research work including more details on Pt deposits prepared using the CO route and their electrocatalytic reactions is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details (Figure S1), estimation of the area occupied by each layer in multiple-layered Pt deposits (Figures S2−S5), and cyclic voltammograms of Pt islands on Au (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel:82-42-821-5483. Fax: 82-42821-8896. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2013R1A1A2007139).

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REFERENCES

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