An in-situ STM investigation of the underpotential deposition of Ag on Au(111) electrodes

8.5

Journal of Electroanalytical Chemistry, 377 (1994) 85-90

An in-situ STM investigation of the underpotential of Ag on Au( 111) electrodes

deposition

S.G. Corcoran and G.S. Chakarova Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218 (USA)

K. Sieradzki Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-7531 (USA)

(Received 19 July 1993, in revised form 21 February 1994)

Abstract A detailed potentiodynamic and electrochemical scanning tunneling microscopy (ECSTM) study of the system Au(lll)/Ag++ HClO, is presented. The growth and dissolution dynamics of the first underpotentially deposited Ag monolayer were followed in situ with monolayer (ML) height resolution. For newly prepared Au substrates, the cyclic voltammetry consists of three distinct underpotential deposition (UPD) peaks, with the main peak located at 600 mV. The coulometry indicates that a coverage of 0.8 ML is obtained at an underpotential of 75 mV. For freshly deposited Au films, the formation of a complete Ag overlayer is shown by ECSTM to occur by a step growth mechanism at the 600 mV peak. No further morphology changes were observed during subsequent deposition between 550 and 100 mV indicating that the UPD peaks may correspond to different low density structures.

1. Introduction

Underpotential deposition (UPD), i.e. the deposition of a metal onto a foreign substrate at a potential positive to the equilibrium Nemstian potential, was studied extensively using electrochemical methods during the 1970s [1,2]. During the late 198Os, interest in the UPD phenomenon was renewed as a result of the appearance of the electrochemical scanning tunneling microscope (ECSTM) as a tool for probing the structure of the UPD adlayers on atomic length scales. Recently, the atomic structure of UPD overlayers has been observed in many systems: Cu on Au(ll1) [3,4], Cu on Pt(ll1) [51, Pb on Ag (hkf) [6,71, Ag on Pt(ll1) [8] and Ag on Au(ll1) 19,101. However, only a few studies have used monolayer (ML) height resolution to follow the growth morphology of the UPD adlayer [ll-131. In the Pb on Au0111 system, it was observed that the Pb adlayer formed by initial growth at Au step edges followed by Pb island nucleation on Au terraces and subsequent lateral growth [12]. The substantial roughening of the terraced Au surface after removal of the Pb monolayer was explained by the formation of a Pb + Au alloy in the surface layer 1131. 0022-0728/94/$7.00 SSDZ 0022-0728(94)03452-9

In 1949, Rogers et al. [14] discovered the UPD effect while studying the deposition of radioactive Ag on Au and Pt surfaces. Later, Propst [15] showed that the UPD of Ag on polycrystalline Au began at a potential 750 mV more positive than that predicted by the Nemst equation. Linear scanning voltammetry studies demonstrated that both the UPD peak position [16] and the charge passed [17] during the UPD process depended upon both the pretreatment of the electrode surface and the surface orientation [181. Applying the twin-electrode thin layer technique, which allows independent charge and coverage measurements, Schmidt and Stucki [19] showed that one full monolayer of silver was formed underpotentially on a polycrystalline Au electrode in H,SO,. They also found that small amounts of Cl- anions in the electrolyte strongly influenced the voltammetry of the UPD. Bruckenstein and coworkers [20-231 used the rotating ring-disk electrode technique to obtain quantitative electrochemical data, coverage isotherms and electrosorption valency for Ag UPD on polycrystalline Au in both H,SO, and HClO, electrolytes. In H,SO,, a complete Ag monolayer formed at an underpotential of 50 mV, and at least three UPD adlayers were 0 1994 - Elsevier Science S.A. All rights reserved

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S.G. Corcoran et al. / Unabpotential deposition of Ag on Au(llI)

observed near the Nemstian potential. In HClO,, a coverage of 0.9 monolayers (ML) was observed at 50 mV under-potential, and there was no mention of a second UPD adlayer. Coverage isotherms obtained at showed that the elecvarious Ag+ concentrations trosorption valency was equal to unity, i.e., there was complete charge transfer. Bruckenstein and coworkers also demonstrated that the practice of subtracting background currents from cyclic voltammograms is often without theoretical justification. The structure of the UPD monolayer and the peak positions of the cyclic voltammetry for Ag on A&11) thin films has been shown by electrochemical atomic force microscopy to depend upon the nature of the electrolyte [91. An open-lattice structure with a packing density of 65% f 10% was observed at 445 mV in 0.1 M HClO,. In this paper, we present a detailed electrochemical (potentiodynamic) and ECSTM study of the system Au(lll)/Ag++ HClO, with the aim of viewing the growth-dissolution dynamics of the first Ag UPD adlayer. A system such as Ag on Au serves as a model to observe the dynamics of monolayer formation because alloying does not occur in the UPD region [9,24], and a nearly unstrained epitaxial monolayer is formed owing to the very small lattice mismatch (approximately 0.19%). An HClO, electrolyte is used because the ClO; ion adsorbs only weakly on the Au substrate compared with SOi- [25,26], thus minimizing electrolyte influences on the growth of the UPD layer.

electrodes

had an exposed working area of ca. 0.29 cm* and a volume of ca. 0.25 cm’. The experiments were performed in an electrolyte composed of lop3 M AgClO, (Aldrich Gold Label Reagents) and 0.1 M HClO, (G.F. Smith double distilled). The effect of Ag+ concentration on the UPD was also investigated. Solutions were prepared with triple-distilled and Millipore-Q (18 Ma cm) purified water. The reference electrode consisted of two parts separated by a Vycor frit. The upper part was a Ag wire immersed in a 10e3 M Ag++ 0.1 M HClO, electrolyte. The lower part contained the solution used in the electrochemical cell. A Teflon tube was used to connect the lower part to the electrochemical cell. A junction potential of approximately + 10 mV was measured for the reference electrode. A platinum wire was used as a counter-electrode. The CV curves were stored on a Lecroy 9410 digital oscilloscope. All potentials for the CV are reported with respect to the Ag I 10m3 M Ag+ equilibrium potential. In the potential range investigated, it was unnecessary to deaerate the electrolyte. The STM studies were performed using a Digital Instruments Nanoscope II electrochemical scanning tunneling microscope in 10M4 M Ag++ 0.1 M HClO, electrolyte. Prior to each experiment, an STM tip was prepared by etching an 80 : 20 Pt + Ir wire in a CaCl, solution and isolating with Apiezon wax. All potentials for the ECSTM are given with respect to a Ag wire immersed directly into the electrolyte. 3. Results and discussion

2. Experimental 3.1. Electrochemistry

Au films, 2000 A thick, with exclusively (111) texture were prepared by magnetron sputtering at 380°C onto freshly cleaved mica substrates. The Au substrates were used within 1 h of removal from the vacuum system. Glassware was cleaned in a standard Na,Cr,O, + H,SO, cleaning solution at 60°C followed by rinsing in tapwater for a minimum of 1 h followed by multiple rinsing in triple-distilled and Millipore-Q (18 Ma cm) water. All parts of the electrochemical cell were cleaned immediately before each experiment by rinsing (several times) sequentially in acetone, ethanol, dichloromethane, triple-distilled and Millipore-Q (18 M0 cm) water, NaOH (5 wt.%), triple-distilled and Millipore-Q (18 M0 cm) water, 1: 2 H,O, (30%) + H,SO,, triple-distilled and Millipore-Q (18 MO cm> water, 0.1 M HClO,, and triple-distilled and Millipore-Q (18 MR cm) water. The cyclic voltammetry (CV) was carried out with the Nanoscope II electrochemical cell (to allow comparison with the STM results) and a Bioanalytical Systems CV-27 potentiostat. The electrochemical cell

Oxide reduction charge measurements, following the Burshtein minimum method [271, indicate a typical roughness factor for the Au thin-film electrodes of approximately 1.3. All electrochemical data were corrected for a roughness factor of 1.3. 3.1.1. System:Au(111)/10-3 MAg++ 0.1 MHCIO, (sweep rate, 1 mVs_‘) In order to avoid Au oxidation, an upper potential limit of 630 mV was used in all UPD experiments to be presented. A typical cyclic voltammogram for the UPD of Ag on Au is shown in Fig. 1. Three distinct adsorption peaks are observed at 600, 552 and 510 mV and one broad peak appears at 140 mV. The corresponding desorption peaks are at 608 mV, 578 mV, 518 mV and 140 mV respectively. The main UPD peak is clearly shown to be at 600 mV. The voltammetry is reproducible over many cycles for a period of a few hours, indicating that irreversible alloying processes are not occurring in the UPD region as also reported by others [9,241. The position and relative heights of the three

S.G. Corcoran et al. / Underpotential deposition of Ag on Au(II1)

distinct UPD peaks are also highly reproducible for samples with similar roughness. In previous investigations of this system, only one UPD peak has been observed above 500 mV. However, there is a discrepancy about the location of this peak. The peak position was observed by Chen et al. [9] at approximately 500 mV for A&11) thin films and by Pauling and Jiittner [24] at 600 mV for Au(ll1) single crystals. Our results are consistent with those of Pauling and Jiittner except that, owing to our lower scan rate (1 mV s-i vs. 10 mV s-i), we observe two additional minor peaks at 510 and 552 mV. All UPD peaks display Nernst behavior and shift approximately 60 mV for each decade change in electrolyte Ag+ concentration. It is interesting to point out that we observed an increase in the 510 mV peak height along with a decrease in the 600 mV peak height for samples with increased surface roughness. The coverage calculated from the Ag deposition and stripping waves of Fig. 1 along with the results of Swathirajan and Bruckenstein [23] for the equilibrium coverage obtained from rotating ring-disk electrode experiments on polycrystalline Au are shown in Fig. 2. We calculated the coverage using a charge of 222 PC cmm2 corresponding to a coverage of 1.0 ML for the Au(ll1) surface and a roughness factor of 1.3. It was not necessary to correct for double-layer charging and changes in the potential of zero charge of the electrode at the applied sweep rate [23]. As shown in this figure, our data are in good agreement with the equilibrium data of Swathirajan and Bruckenstein. These data further confirm the increased sensitivity in our experiment

2 1.5

f 0.5 :O B 8 E 0.5 E 6

-1

87

electrodes

1

0.8

0.2

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential/v Fig. 2. Coverage versus underpotential plot of Ag UF’D on A&11) deposition in a low3 M Ag+ +O.l M HClO, electrolyte: wave; - - - stripping wave; + equilibrium coverage data [23].

compared with that of Pauling and Jiittner [24] whose charge measurements only accounted for half the Ag UPD monolayer. A coverage of 0.7 ML was reached at an underpotential of 75 mV. A coverage of 0.8 ML was obtained for the stripping wave if the potential was held at a starting value of 75 mV for a duration of 10 min. No increase in the coverage was observed for longer holding times of up to 45 min. The onset of a second Ag UPD adlayer was observed by both coulometry and in-situ STM at an underpotential of 40 mV [28]. This is the first report of a second Ag UPD adlayer in this system. Figure 3 shows the effect of sample aging on the relative UPD peak heights. Three Au samples were prepared from the same sputtering. One sample was used within 1 h of removal from the vacuum system (curve A) and is also shown in Fig. 1. Curves B and C are from samples which were stored in a desiccator for periods of 4 days and 11 days respectively. With aging of the Au substrate, the peak at 500 mV increased in height while the peak at 600 mV disappeared. In-situ STM shows that the UPD overlayer on aged Au forms at 500 mV (in agreement with the coulometry) and nucleates at impurity sites on the Au surface [29].

-1.5 -2

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Potential/V Fig. 1. Cyclic voltammogram of Ag UPD on A~(1111 in 10m3 M Ag+ +O.l M HCIO, electrolyte. The scan rate was 1 mV s-l.

3.2. Scanning tunneling microscopy Figure 4 is a typical STM image at monolayer height resolution of a Au(ll1) thin-film surface in air. Au steps of approximately 0.24 nm in height are separated by terraces of a few 100 nm in width. Atomic resolution

S.G. Corcoran et al. / Underpotential deposition of Ag on Au(lll)

88

electrodes

Fig. 3. Effect of Au(ll1) aging on the voltammetry of Ag UPD in a 10W3 M Ag++O.l M HCIO, electrolyte. The scan rate was 1 mV s-t.

of the approximately

an interatomic

of

M. HClO,

5 shows the morphological

details that devel-

oped on

5. Sequence of in-situ of on Auflll) in lob4 M Ag’+O.l HCIO, electrolyte during cyclic voltammogram between 120 and 625 mV at 0.5 mV ‘. The in (D)-(F) of step in (J) is 160 is given as min:sec. mV and s frame-t. (18 exposures).

Fig. 4. = 1.0 nA. Bias voltage

of a Au(ll1) thin nm 200 mV.

in air

between 120 and 625 mV at 0.5 mV s-l. A Ag Au terrace at 300 in this is located in the of the is located at the of the by atomic of height 2 ML nm

S.G. Corcoran et al. / Underpotential deposition of Ag on Au(lII) electrodes

Fig. 6. Enlargement of Fig. 5(R) showing steps of height 2 ML (0.46 nm).

shown in Fig. 6). Small Ag clusters (of height 1 ML) approximately 4-8 nm in diameter are also observed in Figs. 6(A) and 6(B). As the potential was increased from 300 mV no morphological changes occurred at the monolayer height resolution scale until approximately 460 mV, at which time the small (4-8 nm) silver clusters started dissolving (Fig. 6(B)). Once these clusters dissolved, no further changes in the morphology were observed until 550 mV at which time step dissolution began as shown by the arrows in Figs. 5(D)-5(F). Dissolution of Ag from terrace sites began above 610 mV, as is evident from the instability of the STM images in Figs. 5(E)-5(I). On sweeping the potential back, silver deposition occurred quickly, forming an almost complete (low density) monolayer by 550 mV (Fig. 5(M)) re P ro d ucing the morphology of Fig. 5((A). The initial stages of the silver deposition are shown in Figs. 5(J)-5(L). Four atomic layers are observed in Fig. 5(J) as shown by the arrows. The first and third layers experienced step growth, reproducing the morphology of their underlying Au terrace, second and fourth layers respectively. Thus, at 587 mV (Fig. 5(L)) a double atomic step was formed. Between 587 and 520 mV, minor step growth was observed (Figs. 5(M)-5(O)). Further sweeping of the potential from 500 mV down to 120 mV resulted in no observable morphological changes (Figs. 5(P)-5(R)). However, we did notice an increase in the resolution of the image as the potential was lowered to 120 mV. After holding the potential at 625 mV for approximately 3.5 min, the upper terrace shown in Fig. 5(I) became poorly resolved. However, this terrace became well resolved in the early stages of Ag deposition (Fig. 5(J)). We speculate that in this small region there was a

89

mixing of Ag and Au. Therefore the poor resolution of the upper terrace shown in Fig. 50) was a result of injecting a high density of vacancies into this terrace during the Ag stripping. These vacancies were quickly annihilated during deposition. However, the majority of terraces, as shown in these images, did not exhibit mixing as expected from the stability of the voltammetry. Some mixing also occurred at terrace edges, as shown by the alteration in the terrace edge morphology in Fig. 5(R) compared with Fig. 5(A). It is important to point out that the mixing shown in these STM images is restricted to small localized regions on the Au surface and that this is different from what would be expected for systems that display surface alloying. For example, in the Pb/Au system, where alloying is known to occur, Hanson and Green [13] observed substantial roughening of the Au surface after a single monolayer of Pb was plated and subsequently stripped. The STM images show a completed Ag overlayer on the Au surface at 550 mV (Fig. 5(M)) while the coulometry (Fig. 2) indicates that this layer can be only 35% dense. This coverage corresponds to the deposition associated with the first peak observed in the cyclic voltammetry at 600 mV (Fig. 1). No morphological changes at the monolayer height resolution scale were observed for the deposition of silver associated with the remaining peaks at 552, 510 and 140 mV. However, we did observe increases in the spatial resolution of the image with corresponding decreases in the potential. We speculate from this contradiction that four different atomic packing configurations exist for the Ag monolayer, three low density structures corresponding to each of the first three peaks (600,552 and 510 mV> and a close-packed structure after the 140 mV peak. At a potential of 445 mV, the coulometry predicts an atomic Ag layer with 53% density. This is comparable with the dense (65% f 10%) atomic structure observed at 445 mV by Chen et al. [9] using in-situ atomic force microscopy. Atomic resolution studies are in progress throughout the entire underpotential region to establish the nature of the UPD peaks. 4. Conclusions CV shows three distinct UPD peaks with the main peak occurring at 600 mV. Coulometry indicates that a coverage of 0.8 ML is obtained at an underpotential of 75 mV. The formation of the Ag monolayer is shown by in-situ STM to occur by a step growth mechanism at the 600 mV peak. Further decreasing the potential produces no morphological changes at the monolayer height resolution scale, suggesting that the UPD peaks correspond to different low density structures. Future investigations using in-plane atomic resolution are needed throughout the UPD potential range.

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S.G. Corcoran et al. / Underpotential deposition ofAg on Au(ll1) electrodes

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