Oxide formation on Au(111) an in situ STM study

SOLID STATE ELSEVIER

Solid State Ionics 94 (1997)

IoNlcs

171-179

Oxide formation on Au( 111) An in situ STM study M.A. Schneeweiss, Department

of Electrochemisrryv,

D.M. Kolb”

University

of Ulm, 89069

Ulm, Germany

Abstract The anodic oxidation of a Au( 111) electrode in sulfuric acid was investigated by in situ scanning tunneling microscopy (STM) and compared to previous work in that field. The oxide formation was monitored and the topography of the oxide layer, as well as of the gold surface after oxidation-reduction cycles, was imaged. Keywords:

Gold oxide; STM: Corrosion;

Oxidation

1. Introduction The anodic oxidation of noble metal electrodes and the characterization of the oxide phase is a widely studied field. It is of relevance to the mechanism and kinetics of corrosion processes, as well as to the catalytic activities of oxide phases in the electrochemical oxidation of small organic molecules. The oxidation of noble metals in acidic media containing anions like HSO, or ClO, has been extensively studied, in regard to both the initial stages of the oxidation process and the growth of thicker oxide films. A comprehensive review of electrochemical oxide film formation on noble metals exemplified by the case of Au and Pt can be found in a recent article by Conway [l]. The initial stages of oxidation of monocrystalline Au and Pt electrodes have been found to consist of four consecutive steps, the first of which is the adsorption of anions from the solution. In the case of sulfate an ordered adlayer structure is formed in *Corresponding

author.

0167-2738/97/$17.00 01997 PII SO167-2738(96)00587-5

Elsevier Science B.V. All rights reserved

which the anions are in effect chemisorbed, undergoing partial or full charge transfer [2]. This adlayer determines the actual onset potential for oxidation because of its competition with the initial stages of OH electrosorption. This was demonstrated by Angerstein-Kozlowska et al. by creating conditions referred to as ‘anions on’ and ‘anions off’ [3]. In the latter case, the electrosorption of OH and 0 could be seen in the cyclic voltammogram. The second step comprises two-dimensional electrodeposition of OH or 0 species into the anion adlayer [2] and this is followed by a place exchange (‘turnover process’) between OH or 0 species and the metal surface atoms with simultaneous anion desorption [4,5]. Finally, the oxide film grows by a high-field type mechanism as proposed by Mott and Cabrera [6,7]. It is well established by means of conventional electrochemical methods [8,9,2] that gold oxidation and subsequent reduction leads to a roughening of smooth electrode surfaces. The first to investigate by in situ STM the effect of an oxidation-reduction-cycle (ORC) on Au( 111) in HClO, were Trevor et al. [lo]. They found that

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Kolb I Solid State tonics 94 (1997)

anodic sweep limits beyond the potential necessary to generate an oxide monolayer were required in order to observe monoatomic deep pits on flat terraces after completion of the ORC. They attributed the formation of pits to a place exchange during oxidation. Honbo et al. [I l] were able to show that the development of those pits takes place during reduction of the oxide rather than during its formation. The same conclusions for Ag( 111) electrodes were made earlier on the basis of reflectance measurements [ 121. Meanwhile, several STM studies [13-171 and one AFM study [ 181 on this subject have been performed. Gao et al. [14] found islands, as well as holes, after ORCs, while Vitus et al. [15] observed wormlike structures after ORCs on Au( 111) in HCIO,. Our group has reported that the morphology of a gold surface after oxide reduction depends on whether the reduction was achieved by a cathodic potential sweep (resulting in pits only) or a potential step (resulting in islands as well as pits) [17]. The oxide covered surface is generally described as rough. In a very preliminary study in situ images of an oxide covered Au( 100) electrode were presented by Nichols et al. [ 131. Oxide formation on the Au( 100) surface in perchloric acid was accompanied by a dramatic change in the image, which suddenly led to an apparently amorphous structure. At the usual sweep rates of several millivolts per second the turnover process sets in too fast to be monitored by STM, transforming the smooth surface into a completely roughened one during the course of a few scan lines. The present work however shows that it is possible to follow the growth of the ‘tumedover’ phase by applying extremely slow sweep rates. The topographic characteristics of the oxide phase are shown, and reversibility and irreversibility of the different oxidation processes illustrated for the system Au( 11 l)/H,SO,.

2. Experimental All experiments were performed with a Topometrix TMX 2010 using W and Pt-Ir tips. Tungsten tips were electrochemically etched from a 0.25 mm diameter wire in 2 M NaOH, whereas platinumiridium tips (Pt to Ir ratio 80:20) were etched in 3.4

171-179

M NaCN [19]. To minimize Faradaic currents at the tip-electrolyte interface, all tips were coated with Apiezon wax [20]. Tip and sample potentials were controlled independently from each other by means of a bipotentiostat. The electrolyte solutions (0.1 M H,SO, or 0.1 M HClO, with 1 mM Cu++) were prepared from H,SO, and HClO, (Merck, suprapure), CuSO, and Cu(ClO,), (Fluka, puriss. p.a.) and Milli-Q water. The gold samples consisted of 200 nm thick films evaporated onto ‘Robax’ glass (AF 45, Berliner Glas KG) on top of a 2 nm Cr layer for better adhesion. The samples were annealed in a hydrogen flame for 2 min to yield large, atomically flat (111) terraces [21]. A copper wire was used as a convenient, low-noise reference electrode, but all potentials are quoted with respect to SCE. All STM data were recorded in the constantcurrent mode. The STM images are displayed either as top-views with different shades of grey representing different heights (dark areas indicating low and light areas high parts of the surface) or 3D-plots based on shaded top views.

3. Results and discussion 3.1. The sulfate adlayer on Au(lll) Fig. 1 shows the well-known cyclic voltammogram of Au( 111) in 0.1 M sulfuric acid which was recorded at a sweep rate of 10 mV/s. The oxidation peaks are marked OAl to 4 according to the nomenclature of Conway and coworkers. The current-density scale of the double layer region has been expanded by a factor of ten for a close-up on the anion adsorption. The first peak in this region at about + 0.2 V vs. SCE is caused by the lifting of the (22 X &) reconstruction due to disordered sulfate adsorption [22], whereas the sharp spike just below 0.8 V has been assigned to the formation of an ordered adlayer [23]. An in situ STM-image of the Au( 111) surface recorded at a potential just positive of that spike (& = 3 nA; U, = 0.8 V) is shown in Fig. 2. The previously reported (&X &)R19.1” superstructure due to an ordered sulfate layer [24] is clearly seen. It has not been ascertained as yet whether the adsorbed layer consists of sulfate or bisulfate anions (Shi et al. [25], based on

M.A.

Schneeweiss,

D.M. Kolb

I Solid State Ionics

94 (1997)

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171-179

50.0

r!

E

0.0

.3

I

H

:

:

:

:

:

:

;

,:

:

:

1I \; I :I :: :I I, II II :: 8, 4, ::

:

-50.0

-

Au(lll)/O.lMH,SO,

I I I I -0.5

I

I

I I I

0.0

I

I

I

0.5

Fig. I. Cyclic voltammograms for Au( 111) in 0.1 M H,SO,. The current density enlarged scale to reveal details of the anion adsorption. Sweep rate: 10 mV/s.

chronocoulometric data, assume that the layer consists of sulfate). The term sulfate,’ as used in this context, does imply the possibility that the species might as well be bisulfate. There is still some disagreement about the actual sulfate coverage, as well as the origin of the satellite spots in the STM images. Some authors [24] claim that the weaker maxima are caused also by sulfate, which would imply a total coverage of 0.4. Results from IRAS measurements, on the other hand, yield a sulfate coverage of 0.2 [26], which led Edens et al. to assume that the satellite spots are caused by co-adsorbed hydronium cations. In their most recent

I I

I 1.0

I

I

I

I 1.5

in the double layer charging

region

is shown on an

report on IRAS measurements, Ito et al. [27] suggest water molecules to be the reason for the weaker maxima in the adlayer image. We mention in passing that the very same superstructure, namely (4 X J?)R19.1” with strong and weak maxima has been found also for Pt( 111) [28] and Rh( 111) [29] in sulfuric acid solutions. It thus appears that this structure is typical for sulfate adsorption at high coverages on the (111) surfaces of metals which interact strongly with the anion. In all three cases a current spike in the cyclic voltammogram announces the structure formation. In sulfuric acid electrolytes this highly packed

174

M.A. Schneeweiss. D.M. Kolb I Solid State lonics 94 (1997) I71 - 179

6.8nm x 6.8 nm Fig. 2. STM image of the sulfate adlayer on Au( 111) in 0. I M H,SO, at 0.8 V vs. SCE. I,=3 nA. E,,,=O V vs. SCE.

ordered adlayer is the starting point for the oxide formation on A~(11 l), which will be described in the following. 3.2. Reversible processes

and irreversible

oxidation

As mentioned above, oxidation and oxide formation on a gold surface can be described by four consecutive steps ([2] and literature cited therein). The first two processes, anion adsorption and partial decomposition and discharge of water, are reversible, and hence cycling into the corresponding potential range (peaks OAl-3; see Fig. 1) should have no effect on the integrity of the monocrystalline surface. As the coverage of coadsorbed OH and 0 species from water decomposition increases with potential, the resulting repulsive interactions between the AuOH dipoles eventually cause a turnover process, which produces a two atoms thick oxide layer. In electrolytes containing strongly adsorbing anions, such as sulfate, the turnover process can only take place simultaneously with anion desorption and replacement (i.e. replacement-turnover process; RTO [2]). For sulfuric acid electrolytes, the RTO process takes place mainly in the potential range of the OA4 peak. Reduction of the surface oxide causes

roughening of the smooth electrode; the replacement-turnover process is irreversible. Fig. 3 illustrates reversibility and irreversibility of the various stages of the surface oxidation process. The potential was scanned first from 0.8 to 1.16 V and back to 0.5 V, then from 0.8 to 1.3 Vand back to 0.5 V. Both sweeps were conducted with the STM imaging the same site. During the experiment, the tunneling voltage (rather than the tip potential) was held constant at 300 mV negative of sample potential to minimize tip effects. The sweep rate was 10 mV/s, one image took about one and a half minutes to record. Fig. 3a shows an STM image (83 nm X 83 nm) which was recorded while the electrode potential was simultaneously swept from 0.8 to 1.16 V and back; the potential and the resulting current are plotted along the y-axis of the STM-image, which acts like a time-scale. During the potential sweep, which covers the range of the OAl/2 peak, the surface morphology, as well as the imaging quality, remain practically unchanged. Fig. 3b shows the site directly after the return sweep at a potential of 0.5 V. No disruption of the atomically flat terraces has taken place. Fig. 3c shows again the same site during a potential sweep from 0.8 to 1.3 V and back to a potential value just negative of the reduction peak (the few white spots at the monoatomic high step are most likely due to impurities). Notably, the surface morphology has changed from smooth to rough only well inside the 0A4 peak; at this time scale (10 mV/s sweep rate), the characteristic features of an oxide covered Au( 1 11)-surface seem to evolve instantaneously. (Gao et al. [14] observed that steps became diffuse before surface roughening sets in, which could not be confirmed here.) After reduction (Fig. 3d) the surface is studded with monoatomic deep holes, but their counterparts, monoatomic high islands, are hardly found. 3.3. Onset of the RTO Fig. 3c proves that at a sweep rate of 10 mV/s the onset of the RTO process is instantaneous in respect to the time resolution of STM. To observe nucleation and growth of the oxide phase, either much faster image scan rates (leading to very poor image quality) or, preferably, very much slower potential sweep

M.A. Schneeweiss,

D.M. Kolh I Solid State Ionic.7 94 (1997) I71 - I79

83 nm x 83 MI

0.8V

175

1.16V

0.5 v

-imversihle

Fig. 3. STM images recorded during (a,c) and after (b,d) an ORC performed on Au( I1 I) in 0.1 M H2S0,. (a) STM image recorded during a potential sweep from 0.8 to I. 16 V and back to 0.5 V. (b) Resulting surface at 0.5 V. (c) STM image recorded during a potential sweep from 0.X to I.3 V and back to 0.5 V. (d) STM image at 0.5 V. Potential and current are shown as a function of time along the y-axis of the image. U, =O..? V. I, =2 nA

rates have to be employed. Fig. 4 shows six images ( 180 nm X 180 nm) out of a sequence recorded during a potential sweep at a rate of 0.13 mV/s. Each image took about 2 min to scan and covers a potential range of 15- 16 mV. Fig. 4a shows the Au( 111) surface imaged at 0.88 V. Atomically flat terraces are seen which at this potential are covered with an ordered sulfate adlayer. At this sweep rate, first small patches of roughened surface appear at potentials of about 1.O to 1.1 V, but start to grow only at a potential of about 1.15-1.20 V. The increasing roughening is shown in Fig. 4b-f. As this process mainly takes place in the potential range of OA4, the growing patches can be ascribed to the turned-over material forming at sites where anion desorption has occurred. Vitus et al. [I.51 saw propagation of darker (lowlying) patches at a high anodic potential, claiming this to be oxide. This structure was not observed here. If the images in Fig. 4 can be interpreted in purely

topographical terms, the roughened surface of Fig. 4f exhibits an r.m.s.-roughness of 0.1 nm. Details of the structure are shown in the following. 3.4. The oxide covered

surface

Despite many attempts, an ordered two-dimensional phase in the oxide range has not yet been observed. Instead, a seemingly amorphous phase consisting of small hillocks is always found. (Macdonald suggested that although anodically grown passive oxide films are usually considered to be crystalline, the density of point defects might in the extreme be so high that the overall structure appears amorphous (301.) The image in Fig. 5, which was acquired at 1.3 V at the end of a slow anodic sweep (0.17 mV/s), shows hillocks of an average 0.1 to 0.25 nm in height (note the difference scaling of xq’ and z which lets the surface appear rougher than it really is). The overall surface roughness agrees with that

176

M.A. Schneeweiss,

18Jnmx

0.X8

v

D.M. Kolb I Solid State Ionics 94 (1997) 171-179

185nm

6 min

1.2ov

43 min 42 s

1.23 v

47 min 40 s

I.lYV

41 min 46 s

Fig. 4. Au( 111)/0.1 M H,SO,. Images recorded during a slow potential sweep into the oxidation V. IT: 2 nA. Potential and time given for each image apply to the last scan line.

Onm Fig. 5. Au( 111)/O. 1 M H,SO,. nA.

STM image recorded in the oxidation

peak 0A4 (sweep rate: 0.13 mV/s). E,,,: 0

Onm

regime at 1.3 V after a slow anodic sweep (0.17 mV/s). 15,~: 0 V. IT: 2

M.A. Schneeweiss, D.M. Kolb I Solid State Ionics 94 (1997) I71 - I79

observed by Gao et al. [14] in HClO,, who reported a height of 0.2-0.3 nm and a width of 2-4 nm. These authors also stated that no long-range ordering was discernible. Vitus et al. [ 151 determined a surface roughness of 0.08-+0.02 nm. Nichols et al. [13] reported an average height of 0.2-0.3 nm for the roughened structure observed in the oxidation regime of the system Au(lOO)/HClO,. In the same system, Honbo et al. [l l] found small corrugations with heights less than 0.1 nm on the terraces in the region of the OA4 peak, after encountering spiky irregular structures in the range of OA3. Our observations correspond with those of these earlier reports. Apparently, no long-range ordered oxide structure is formed under these conditions. 3.5. Morphological

changes due to ORCs

As was reported earlier [17], gold surfaces exhibit different characteristics after oxide reduction, depending on whether the reduction was achieved by a potential sweep or step.

117

Fig. 6 shows areas of a Au( 111) electrode in 0.1 M H,SO,+ lo-“ M CuSO, before and during oxidation and after reduction by sweep (a-c) and step (d-e). The inset shows the very first moment of reduction during the cathodic sweep. Tip effects (see Section 4) could largely be excluded here. Fig. 6a-c shows three images taken from a sequence acquired during a slow potential sweep (0.5 mV/s) from a potential of 0.8 to 1.3 V and back to 0.6 V. One image took about 2 to 2.5 min to scan (depending on the time elapsed during drift correction) and covers 70 to 80 mV; the time and potential shown at each image refers to the last scanline (images were scanned downwards). Fig. 6a shows clean atomically flat terraces (covered with an ordered sulfate adlayer not visible at this resolution). Image (b), acquired in the oxidation range at the begin of the cathodic sweep shows the surface covered with what is the typical appearance of the oxide layer; namely small amorphous hillocks. After reduction (Fig. 6c), monoatomic deep pits have been formed, a common observation for an electrode surface after ORCs. The

t

Fig. 6. Au( II l)/O.l M H2S0,. For both sequences (a-c) and (d-f) the potential was slowly scanned from 0.8 to I .3 V (sweep rate 0.5 mV/s). In (c) reduction was achieved by slowly sweeping the potential back to 0.6 Vat 0.5 mV/s: the inset shows the surface at the moment of reduction. In (f) the potential was stepped back to 0.6 V. Potential and time given for each image apply to the last scan line. U, : 0.3 V. I, : 2 nA.

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D.M. Kolb I Solid State Ionics 94 (1997) 171-179

inset, however, shows that directly after reduction (see arrow), a few small islands have been formed which disappeared during the subsequent sweep into the double layer region. Fig. 6d-f illustrates the effect of stepping the potential from the oxidation range to 0.6 V, after a potential sweep with 0.5 mV/s from 0.8 to 1.3 V. Fig. 6d shows the virgin surface in the sulfate adlayer region, while Fig. 6e shows the same region in the oxidation range. The effect of reduction by way of a potential step is illustrated in Fig. 6f (the arrow marks the moment of stepping the potential down). Pits and a few islands are formed. This is in agreement with what was reported earlier [ 171. More islands are generated when the potential is held longer in the oxidation range [31].

0.50 v

0 min

170nmx

0.50 v

40 min

170nm

4. Tip effects Although STM is a superb tool for imaging surfaces, tip effects are frequent and have to be taken care of. STM results should always be checked for tip influences, e.g., by imaging a larger area after the final image of a series has been obtained, or even by checking on different sample sites. A striking example for a tip effect is given in Fig. 7. Fig. 7a and Fig. 7b show the same surface area at 0.5 V before and after 30 min oxidation at 1.36 V in 0.1 M HClO,, while the surface was scanned all the time. Reduction was achieved by a slow potential sweep (5 mV/s), which was expected to yield the well known monoatomic deep holes (see Fig. 3d). Instead, along with some holes, a channel-like structure was formed. However, imaging a larger area made evident that extended corrosion had taken place only in the region directly beneath the tip. How can this enhanced surface corrosion caused by the tip be explained? Cadle et al. [32] found in a ring-disk study of anodic dissolution that the cathodic reduction of an oxide-covered gold surface generates some soluble Au”‘, which implies that gold oxide exists on the electrode surface partly as Au,O,, some of which is dissolved into solution during reduction. The dissolved Au”’ was collected as Au0 at a platinum ring electrode held at various potentials (which incidently

Fig. 7. Au( 11 l)/O.l M HCIO,. STM images of a surface site recorded at 0.5 V (a) before and (b) following 30 min oxidation at 1.36 V during which the tip is continuously scanned. Expanding the scan range (c) reveals a strong tip effect: The channel-like structure is found only for the area on which the tip had scanned continuously. L’,: 0.3 V IT: 2 nA.

were all in the range of the tip potentials that we used in our STM experiments). Less gold was deposited at higher ring electrode potentials. Hence in addition to the turnover process which is a hallmark of the oxidation process there might be another cause for the formation of pits; they could be caused by gold dissolution and redeposition on the tip. In that case the tip would act as a very local drain for soluble gold species. This would explain the increased terrace erosion beneath the tip. Furthermore, a marked improvement in the imaging quality of the tip could often be observed after ORCs which could well be attributed to gold deposition onto the tip (similar to the improvement of tip performance by Cu deposition).

M.A.

Schneeweiss,

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I Solid State Ionic.7 94 (1997)

5. Summary

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179

1101D.J. Trevor, C.E.D. Chidsey and D.N. Loiacano, Phys. Rev. Lett. 62 (1989) 929.

An oxidation-reduction cycle has a destructive effect on a monocrystalline gold surface only when potentials are applied which are high enough to cause a turnover process. The onset of that process is too fast to be monitored by STM using conventional sweep rates, but it can be followed during an extremely slow anodic sweep. Rough patches appear on the surface at potentials below 0A4, but only grow significantly in the potential range of the socalled replacement-turnover process. There was no long-range order found in the oxide region. The topography of the oxide consists of small amorphous hillocks with an average height of 0.1 to 0.25 nm. Reduction by potential sweep causes islands as well as pits to form; the islands though disappear during the cathodic sweep due to electrochemical annealing [33]. Reduction by potential step causes these islands to be retained. STM seems to be an appropriate tool for in situ investigations of oxidation processes, but tip effects have to be taken into account. Results cannot always be taken at face value but have to be checked for tip influences.

[Ill H. Honbo, S. Sugawara and K. Itaya, Anal. Chem. 62 (1990) 2424.

Cl21 D.M. Kolb, in: Spectroelectrochemistry: [I31 [14J [I51 Cl61 (171

[I81 r191 rw [211 WI [231 P41 ~251

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