Structure and stability of cytosine adlayers on Au(111): an in-situ STM study

JC~JRNAL

ELSEVIER

Joumal of Electroanalytical Chemistry 404 (1996) 215- 226

Structure and stability of cytosine adlayers on Au(111): an in-situ STM study Th. Wandlowski a,*, D. Lampner b, S.M. Lindsay b a Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany b Department of Physics arm Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA

Received 19 April 1995; in revised form 3 July 1995

Abstract

The formation, structure and stability of cytosine adlayers on Au(111) in aqueous solution has been studied by current-potential, capacitance-potential, transient- and in-situ scanning tunneling microscopy (STM) measurements. In addition to adsorption of cytosine on gold at low coverages and negative potentials (state I), we found a complicated reorientational transition region (state II) and a densely packed two-dimensional condensed adlayer at rather positive potentials (state III). The stability of the various adlayers shifts towards more positive potentials with decreasing pH. The kinetics of dissolution of the "chemisorbed" adlayer III was studied with current vs. time transients employing a potential-step technique. The experimental curves were analyzed by comparison with a model based on hole nucleation and growth in combination with a parallel Langmuir-type desorption process. The structure and stability of state III was characterized employing in-situ STM. We found highly ordered domains and derived a unit cell with the following dimensions: a = 7.3 + 0.3 ,~, b = 8.7 +__0.3 ,~ and 3' = 50°+ 5°. The proposed packing model assumes the coordination of the N(3) ring nitrogen of cytosine with gold atoms. Finally, we followed the dissolution of the condensed cytosine adlayer using in-situ STM: two pathways of layer disintegration seem to exist. Keywords: Electroadsorption; Scanning tunnelling microscopy

1. Introduction

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been proven to be powerful realspace local probe techniques for studying the structure and stability of organic monomolecular films at well-defined surfaces [1-6]. Successful experiments with these methods require the formation of highly ordered and rather immobile adlayers, which modify the interaction between tip and substrate. Ex-situ studies of such adlayers have concentrated on the following two phenomena: (1) chemisorption of molecules via the formation of a covalent bond with the underlying substrate (naphthalene on Pt(l 11) [7], alkylthiols on Au(111) [8,9]); (2) Physisorption of molecules which self-assemble (attractive lateral interactions) into ordered patterns upon contact with rather inert substrates (liquid crystals of the

* Corresponding author. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 01)22-0728(95)04235-0

type 4-n-alkyl-4'-cyanobiphenyl [1,5], Langmuir-Blodgett films of cadmium arachidate [10] or D N A bases [11,12] on HOPG). These phenomena can also be investigated in solution, i.e in-situ. Pan et al. [13] have used AFM to study structure and dynamics of chemisorbed octylthiol adlayers on A u ( l l 1). The structure of various adlayers, which are formed by physisorption, has also been successfully characterized by in-situ STM studies in liquids. Examples are experiments in which long-chain n-alkanes and their derivatives were adsorbed at isoctanelHOPG [14] or phenyloctaneIHOPG [15,16] interfaces. The mobility of the individual molecules was reduced considerably by attractive lateral van der Waals forces, which caused the spontaneous formation of close-packed herringbone-like adlayer patterns. Performing the in-situ experiments in an electrochemical environment offers the following advantage: the formation and structure of two-dimensional (2D) condensed organic monolayers on electrical conducting substrates can be controlled and modified as required by the applied

216

Th. Wandlowski et al./ Journal of Electroam~lytical Chemistry 404 (1996) 215-226

electrode (substrate) potential [18-20]. Srinivasan and coworkers [21-23] and Tao and Shi [6] showed in four pioneering STM papers that, in situ and under potential control, the purine bases adenine and guanine form ordered adlayers at the HOPG [aqueous electrolyte interface. The images observed were interpreted using models involving the formation of hydrogen bonds between neighboring planar oriented molecules. In addition, Tao and Shi [6] discuss the first results of the dynamics of the potential-induced phase transition between a "gaseous-like" guanine adlayer and a 2D condensed "solid-like" film. The process is controlled by nucleation and growth via direct incorporation of admolecules. Problems with artifacts of the measurements observable on graphite surfaces [24] motivated Tao et al. [25] to explore the application of Au(111) as a substrate for in-situ STM and AFM studies with adenine, thymine, guanine and cytosine. They found that all these DNA bases form ordered adlayers in the absence (spontaneously) and also in the presence of strict control of the substrate potential. This study has been complemented by several electrochemical papers which illustrate convincingly with capacitance or current vs. potential measurements that 2D condensed organic films indeed exist at single-crystal electrodes with well-defined surface crystallographic orientations [26-35]. Relevant examples are thymine on Cd(0001) [30] and Ag(111) [34], uridine on Au(l I 1) [32], and uracil on Au(111), Au(100)-hex [34], Ag(111) and Ag(100) [35]. The documented ability to form 2D condensed adlayers electrochemically at defined solid electrodes and the in-situ application of scanning probe techniques (STM and AFM) to monitor not only their steady-state structures, but also dynamic changes, offers exciting prospects for explaining the detailed interplay of molecule-molecule and molecule-substrate forces on a scale previously unknown. Spatially averaging electrochemical techniques provide thermodynamic data and phenomenological kinetic models of film formation a n d / o r dissolution [18-20,36] which can now be compared with in-situ and real-space real-time structural information at a molecular level. In this article we focus on the adsorption and phase formation of cytosine on Au(111). Electrochemical capacitance and current vs. potential or time measurements are combined with steady-state and dynamic STM investigations of the adlayer structure under strict potential control. We aim to illustrate with this example the power and problems of combining average-scale macroscopic and local microscopic information. In addition, our work is motivated by the specific properties of cytosine adsorbed at the mercury [electrolyte interface and the correlation which obviously exists between phase formation at the mercury lelectrolyte interface and at the Au(hkl)lelectrolyte or Ag(hkl) [electrolyte interfaces [20,31-36]. Cytosine is known to form a 2D condensed film as a result of base-stacking between protonated and neutral molecules a n d / o r the formation of a hydrogen-bonded network at

rather negative potentials, e.g. E < - 1.000 V / S C E [3739]. Another type of cytosine adsorption was reported by Pale~ek and coworkers [40,41]. They discovered the existence of a very stable and sparingly soluble anodic film at E > _ - 0 . 1 0 0 V, which is produced by direct chemical interaction between cytosine and the mercury electrode. H

\/

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pKI(N3 ) = 4.5 PK2(NI,C20) = 12.2 Dipole moment p = 7.6 D (1 D = 3.3356 X 10 -30 cm)

H

Dipole angle a = 102 ° [42]

2. Experimental

2.1. Electrochemistry The electrochemical studies were performed with a cylindrical Au(ll 1) electrode of diameter 4 mm. Before each experiment the electrode was annealed in a Bunsen burner flame at slight red heat for 2 min. After a short period of cooling in air, the electrode was quenched in Milli-Q water and then transferred to the electrochemical cell. Contact with the electrolyte was established under potential control (at E = - 0 . 8 0 0 V / S C E ) using the dipping technique [43]. The solutions were prepared from Milli-Q water, K C I O 4 or NaCIO 4 (Fluka puriss., p.a.) and various additions of twice-recrystallized cytosine (Sigma p.a.). They were deaerated with 5 N nitrogen prior to each experiment. The counter-electrode was a platinum wire and a saturated calomel electrode was used as the reference. All experiments were performed at a controlled temperature (usually 20.0 + 0.5°C). The current measurements (i-E, j - t ) and capacitance measurements (C-E, C-t) were performed using standard electrochemical equipment [33].

2.2. Scanning tunneling microscopy The STM consisted of an electrochemical scanning head and cell constructed in the laboratory which were interfaced with a Nanoscope II controller and a work station. The Au(111) substrates were prepared by evaporating gold onto freshly cleaved mica surfaces [44] followed by careful annealing in a hydrogen flame. The STM tips were made from Pt0.8 + Ir0.2 coated with Apiezon wax and carefully tested for leakage. We used a silver quasi-reference electrode. The electrode stability, which was usually 10 mV constant within, was routinely checked with a Ag [AgC1 [KCl(sat.) or a Hg [Hg2CI2 IKCl(sat) electrode.

Th. Wandlowski et aL / Journal of Electroanalytical Chemistry 404 (1996) 215-226

All potentials in this work are cited versus the saturated calomel electrode (SCE). The counter-electrode was a single platinum wire. The STM cell compartments were cleaned with caroic acid overnight and then soaked in hot Barnstead Nanopure water, which was changed regularly, for several hours. This procedure improved the quality of the STM images considerably and minimized the effect of artifacts due to the presence of impurities. The STM cell was filled under potential control (E = - 0 . 1 0 V / S C E ) with 100 Izl 0.1 M NaCIO4 solution with or without cytosine. Furthermore, the cell was surrounded by a compartment filled with 5N nitrogen in order to reduce exposure to oxygen. Typical tunneling conditions were 1T = 0.1 nA in constant-current mode and a bias voltage of 0.020 V (tip vs. gold substrate).

3. Results and discussion 3.1. Adsorption o f cytosine on Au(111)

Fig. 1 shows a typical cyclic voltammogram and a set of capacitance vs. potential curves for 3 mM cytosine + 100 mM KC104 (pH 5.6) on Au(111). Three different adsorp-

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Fig. 1. (A) Current-potential and (B) capacitance-potential curves for Au(lll) in 0.1 MKCIO4 in the absence ( . . . . . . ) and presence ( ) of 3 mM cytosine at 20°C. The scan rate was 10 mV s -~. The capacitance curves in the presence of cytosine are composed of two scans starting at -0.200 V in opposite directions. The solid symbols correspond to the initial values of the frequency-normalizedadmittance when stepping the potential from either -0.700 V (0) or 0.500 V ( • ) towards various final potentials. The steady-state values of the transients (t ~ ~, cf. Fig. 2) are shown by the open symbols.They almostcoincide for both step regimes. The adsorption states are labelled I, II and 1II.

217

tion states, marked I, II and III, can be distinguished. All voltammetric and capacitance curves merge with those of the supporting electrolyte at rather negative potentials, indicating complete desorption of the organic species. Region I is assigned to molecules randomly adsorbed at the electrode. We observe typical adsorption/desorption maxima at negative potentials (here around - 0 . 6 0 0 V/SCE). Studies over wide ranges of concentration (0-50 mM cytosine) and temperature (1-30°C) did not give any indication of a phase transition between the "dilute" adlayer I and a 2D condensed physisorbed film, as was found for uracil on Au(111) [33]. The asymmetry of I can be referred to the potential dependence of the interfacial protonation equilibrium [39]. Broad peaks in the voltammogram as well as in the capacitance curve are present at potentials positive of region I (region II). They indicate a rather complex process, which involves in part the reorientation of the cytosine molecule on the electrode. The charge under this pair of peaks is about 65 IxC cm -2. The capacitance curve shows a pronounced hysteresis in region II ( - 0 . 2 0 V < E < 0.20 V). At potentials above 0.20 V there is a region of low capacitance (10 ixF cm -2) denoted region III. This region can be assigned to a chemisorbed phase of cytosine molecules, which shows great stability even at very positive potentials. The value of the saturation capacitance is independent of concentration and temperature, and is rather similar to data reported for A u ( l l l ) covered with a monolayer of chloride, bromide or iodide [45], or uracil [33]. A preliminary X-ray diffraction study showed that the reconstructed A u ( l l l ) - ( p ×~/3) phase should be kinetically stabilized by the presence of cytosine up to E = 0.500 V [46]. The potential sweep experiments do not provide sufficient information about the establishment of interfacial equilibrium structures. Therefore we performed potentialstep experiments, choosing the initial potential Ei as either - 0 . 7 0 0 V or at 0.500 V and stepping to various final potentials (Fig. 2). The initial and final values of the frequency-normalized out-of-phase component of the interfacial admittance are plotted in Fig. 1 for both step directions. Comparing the potential sweep and the step experiments, we can conclude that states I and IIl represent equilibrium structures which are established rather quickly. Complications arise as a result of the kinetically controlled transformations I ~ II and III ~ II. Further experiments showed that these features are present almost independently of the cytosine concentration. The effect of pH on the adsorption of cytosine is illustrated in Fig. 3. The pH was varied by adding HC10, to a solution of 100 mM KC104 and cytosine (the study was restricted to pH < 7.0 in order to avoid interference from O H - adsorption [47]). The actual value was always monitored in situ. We did not use buffer solutions in order to minimize strong effects due to the coadsorption of anions such as chloride, phosphate, citrate etc. Referring to Fig. 3, we can see that all three adsorption regions are

Th. Wamtlowski et al./Journal of Electroanalytical Chemistry 404 (1996) 215-226

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present although their stability range is shifted towards more positive potentials. The negative potential limit of our measurements (region I) is determined by the onset of hydrogen reduction which is catalyzed by the presence of cytosine. The charge associated with the broad voltammet6.0

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ric peak in region 1i (Fig. 3) is almost pH independent and amounts on average to 60 IxC cm -2 for 2.0 _< pH < 7.0. At the same time the capacitance in region II increases considerably, which may reflect the formation of a strongly chemisorbed layer as shown by comparison with the adsorption of halides [45]. The pronounced hysteresis inherent in the transitions I ~ II and III ~ II at pH 5.6 decreases with pH. These observations are paralleled by a change in the degree of bulk protonation of cytosine in the N(3) position from approximately 10% at pH 5.5 to 100% at pH 2 (pKI(N3)= 4.5 [43]), which causes a significant modification of the magnitude and direction of the dipole moment of the free base. The preferred protonation-deprotonation sites, which also determine the character of intermolecular hydrogen bonding, are distinctly different for cytosine (pKI(N3)= 4.5, p K 2 ( N 1 / C 2 0 = 12.2)) and uracil (pK](O4)=0.5, p K 2 ( N 1 / C 2 0 or N 3 / C 4 0 ) > 9) [43]. This fact may be supported by our observation of a compact film for uracil (here in the form of a "capacitance pit") [33] but not for cytosine adsorbed on Au(111) between region I and the complex orientational transition II. However, we observed chemisorbed adlayers for both molecules at rather positive potentials. This correlates with the existence of the same structural elements N1 and C20 which, under our experimental conditions, are chemically identical entities. Saenger and others [42] have pointed out that the lone electron pairs of the base nitrogen and the hetero-oxygens of the pyrimidines can coordinate with transition metal ions like Hg z+, Ag + and Au 3+ or form direct metal bonds. These findings may help to gain some insight into the structure of the chemisorbed region III.

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The properties of the chemisorbed adlayer of cytosine on Au(111) are almost independent of cytosine concentration and temperature. Decreasing pH shifts region III towards more positive potentials but does not change the saturation capacitance. The stability of region III points to similarities with the anodically formed, often monomolecular, films between mercury and several pyrimidine bases [40,41] or 5,5'-substituted barbiturates [48]. The formation and dissolution of this type of condensed adlayer has been described using models based on nucleation and growth mechanisms [48]. The complex nature of transition region II (cf. Fig. 2(A)) and the role of the interfacial protonation has hampered meaningful electrochemical studies of the formation of the chemisorbed state III at our present level of knowledge. We focused on the kinetics of film dissolution and studied the 3 mM cytosine +0.1 M KC104 system using a single-potential-step technique. The initial potential was E i = 0.500 V / S C E (waiting time 2 min) and we stepped towards more negative values just below the shoulder on the reverse scan of the voltammogram in Fig. 1. Typical

219

Th. Wandlowski et al. / Journal of Electroanalytical Chemistry 404 (1996) 215-226 80

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Fig. 4. Current-time transients of the dissolution of the "chemisorbed" cytosine layer (region I11) on Au(111). The solution contained 3 mM cytosine-~-0.1 M KCIOn and T= 20°C. Initial and final potentials are indicated on the figure. The waiting time at Ei was 2 min. The solid circles represent transients calculated using Eq. (1) and the parameters given in Table 1. transients are shown in Fig. 4. We observed rising j - t transients with an initial exponential decay. This result and its comparison with the work at the mercury ]electrolyte interface mentioned above motivated us to test the application of nucleation and growth mechanisms. We note at this stage that Bosco and Rangarajan [49] have demonstrated the mathematical isomorphism of models describing the nucleation of an ordered layer (formation) and the nucleation of " h o l e s " in a condensed structure. The time constant of the initial decay is far too large for pure double-layer charging. Recalling the non-ideal structure of the Au(111) surface, we assign this segment of the transient to a Langmuir-type desorption process that takes place at microscopic surface inhomogeneities. A similar explanation has recently been proposed for the dissolution of a copper layer underpotentially deposited on Au(l 11) from sulfuric acid [50]. The hole-nucleation and growth process for the film dissolution is treated within the framework of the well-known Bewick-Fleischmann-Thirsk model [51] and is considered to be the main reaction path that runs parallel to the Langmuir-type desorption. According to this, the total current density, assuming nucleation according to a power law and linear growth, is described by

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k 3 is a coefficient which combines the rates of nucleation and growth, k 2 reflects the dimensionality and the nature of the hole-nucleation process, qh, corresponds to the total charge involved in the hole nucleation, k 5 reflects the rate coefficient of the desorption on defects and q~es is the corresponding charge. With k 2 = 2 or 3 the first term in Eq. (1) reduces to the well-known case of instantaneous or progressive nucleation, provided that we assume a constant growth rate. The parameters of Eq. (1) have been fitted to the experimental transients shown in Fig. 4, employing a non-linear leastsquares procedure which is based on a LevenbergMarquardt routine [52]. The reasonable agreement between theory (points) and experiment (solid lines), as shown in Fig. 4, supports the view that the dissolution of the chemisorbed cytosine film on Au(11 1) can be described by a model which combines a Langmuir-type desorption step with a hole-nucleation and growth mechanism. Models based on the nucleation of holes have also been applied to other completely different systems including the reduction of oxide films (e.g. ZnO on zinc amalgam [53]) and the dielectric breakdown of bilayer membranes [54]. The parameters obtained by the fitting procedure are summarized in Table 1. The parameter k 2 is approximately 2 within the potential range studied. These results indicate an instantaneous hole nucleation process provided that the growth law is linear. No rising transients were obtained for Ef > 0.020 V. This potential can be considered as a critical threshold for the stability of the chemisorbed cytosine film under our present experimental conditions. The potential dependence of the rate parameters kj, k3, k 4 and k 5 are plotted in Fig. 5. The In k i vs. E relations are linear, but the individual slopes are different. The values of In k i increase towards more negative potentials. It is now possible to calculate the charge qhn due to the hole-nucleation and growth process and the charge qdes due to the Langmuir-like desorption using Eqs. (1)-(3). The values of qhn and qde~ increase slightly with decreasing final potential, but their ratio is almost constant with q h n / q ~ s - 0.83/0.17. The complexity of the overall pro-

Table 1 Fitted parameters of the experimental dissolution transients of state III for 3 mM cytosine + 0.1 M KC104 (Eq. (1) and Fig. 4) - E/mV kl/l.zA c m - 2 S- I k2 k3/s-k~ k4/izA cm-2 ks~s- i

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40 50 60 70 80 90

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2.12 + 0.01 2.12 __+0.0l 2.20 __+0.01 2.07 +__0.01 2.08+_0.01 2.10+_0.01

2.12 __+0.01 2.94 + 0.01 4.28 __+0.01 6.47 __+0.04 8.93__+0.06 14.4 +__0.1

6.4 + 0.2 5.2 __+ 0.3 1.2 __+ 0.4 8.8 __+ 3.0 135 +- 5 216 __+13

10.5 + 0.1 13.2 +- 0.2 14.7 + 0.1 29.5 __+0.6 36.2__+0.8 47.1+__ 1.6

Th. Wandlowski et al. / Journal of Electroanalytical Chemistry 404 (1996) 215-226

220

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cess is also illustrated by the fact that the transition time (as monitored by the position of the maximum of the rising part of the j - t curves) increases with the waiting time at Ei and with more positive values of the initial potentials. Further, it decreases with increasing temperature. In general we notice that the time-scale of the j - t dissolution transients is much shorter than the time necessary to achieve a steady-state interfacial capacitance when stepping from III to II. We shall interpret this situation in terms of the formation of a metastable adlayer just after the dissolution of the chemisorbed structure III, which undergoes further relaxation. This process is convoluted with slow potential-dependent modifications of the substrate structure [46].

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In order to characterize our starting conditions we show in Fig. 6 the STM image of a freshly prepared Au(111) electrode in 0.1 M NaC10 4 at E = 0.05 V / S C E . The

Fig. 6. STM image of a freshly annealed Au(l 11) electrode in 0.1 M NaCIO4 at E = 0.05 V/SCE showing the initial thermallyinduced reconstruction. The high resolutionimage (5 nm× 5 nm) illustratesthe hexagonal arrangementof the individual gold atoms. Tunnelingconditions:tip bias Vt, 15 mV; tunnelingcurrent Jr, 1 hA. The inset (20 nm X 20 nm) shows the typical double row structure of the ( p × ~/3) reconstruction.

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Th. Wandlowski et al. / Journal of Electroanalytical Chemistry 404 (1996) 215-226

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Fig. 7. (A) ln-situ STM image (19 nm× 19 nm) of cytosine adsorbed under potential control on Au(111) at E = 0.200 V/SCE (state Ill). The composition of the solution was 3 mM cytosine + 0.1 M NaCIO4. Tunneling conditions: Vt = 0.02 V; Jt = 0.1 nA; scan rate, 8.67 Hz. (B) Typical line scans in the directions indicated in (A).

figure shows an atomic resolution image which illustrates the hexagonal packing of the gold atoms. The inset shows a larger substrate area, with typical pairwise arranged corrugation lines of the Au(111)-(p x ~/3) structure. The periodicity is 6.4 nm and the corrugation height is about 0.02 nm. The terraces cover areas of up to 400 nm X 400

nm. The reconstructed surface remains fairly stable up to 0.400 V under our experimental conditions. Addition of cytosine causes the spontaneous formation of an organized adlayer at gold substrates [25]. In order to avoid uncontrolled substrate and adsorbate relaxations we performed most of our STM experiments under strict

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Fig. 8. (A) Unfiltered and (B) filtered high resolution image (4 nm× 4 nm) of the cytosine adlayer of one of the domains shown in Fig. 7(A). Tunneling conditions, Vt = 0.02 V; Jt = 0.11 nA; scan rate, 19.5 Hz. The unit cell is shown in (B).

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Th. Wandlowski et al. / Journal of Electroanalytical Chemistry 404 (1996) 215-226

Fig. 9. Proposed packing structure of cytosine on Au(111) at positive charge densities. potential control. W e were able to obtain stable STM images of an ordered cytosine structure for substrate potentials E > 0.100 V. This potential region corresponds to state III, as defined in Section 3.1. Fig. 7(A) shows a typical unfiltered in-situ S T M image for 3 m M cytosine adsorbed on A u ( l l l ) at 0.200 V. We observe several domains, each characterized by a set of parallel stripes with a periodicity of about 5.7 .~. Rather disordered features exist at the domain boundaries. Two line scans (directions indicated in Fig. 7(A)) illustrate these findings in more detail (Fig. 7(B)). They reveal that the corrugation height of the stripes is approximately 0.6 ,&. The size of the domains varies, and the angle between adjacent patches is usually greater then 120 ° when comparing different spots on the substrate surface or samples. At present we cannot explain the size and the angle between the different domains observed. W e also notice that the image shown in

Fig. 7(A) is fairly stable in the potential range 0.100 V < E < 0.500 V (for at least 3 h under favorable drift conditions). Disintegration of the structure occurs at more negative potentials (cf. Section 3.4). Slowly scanning the potential towards E > 0.500 V results in the formation of bright spots, often close to domain boundaries, which are probably associated with lifting of the substrate reconstruction. W e should mention at this point that the region of stability of the reconstructed phase is similar to that obtained recently in an in-situ surface X-ray scattering study of uracil adsorption on Au(111) in 50 m M KCIO 4 [46]. Finally, decreasing the pH of the bulk solution shifts the stability of the observed structure towards more positive potentials but has no effect on the image contrast. A high resolution image o f one of the individual domains shows that each stripe consists of periodically arranged blobs. Figs. 8(A) and 8(B) illustrate the original

Fig. 10. Sequence of STM images which illustrate the time dependence of the dissolution of an ordered cytosine adlayer on Au(111) after changing the substrate potential from 0.200 V/SCE to 0.025 V: (A) t = 0, E = 0.200 V; (B) t = 260 s, (C) t = 452 s; (D) t = 742 s; (E) t = 1041 s; ( F ) t = 1338 s; (G) t = 1842 s; (H) t = 2214 s. The substrate potential for images (B)-(H) was 0.025 V. The scale is indicated in (A). Other imaging conditions are the same as in Fig. 7.

Th. Wandlowski et al./ Journal of Electroanalytical Chemistry 404 (1996) 215-226

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224

Th. Wandlowski et al. / Journal o f Electroanalytical Chemistry 404 (1996) 215-226

raw and the high-pass filtered data. The unit cell can be obtained by Fourier analysis. The measured lattice constants are a = 7.3 + 0.3 ,~, b = 8.7 + 0.3 ,~ and y = 50 ° + 5 °"

Attempts to derive a packing model of cytosine on Au(l 11) in state III need to take the following into consideration (1) State III is also stable at relatively low adsorbate concentrations ( < 10 -5 M), " h i g h " temperatures (at least up to 70°C) and in a broad potential region, which is unusual for a classical physisorbed structure. (2) State III also exists at low pH (e.g. pH 2.0, Fig. 3). (3) Cytosine crystallizes as the monohydrate from aqueous solution and forms hydrogen-bonded ribbons [55]. Anhydrous cytosine forms hydrogen-bonded tetramers which are the building blocks of an infinite 2D network [56]. (4) No stacking is found in some crystal structures of protonated pyrimidine bases, suggesting that charged pyrimidines tend to unstack [43]. (5) Cytosine coordinates with transition metal ions such as Ag +, Au 3+ or Hg 2+ in the N3 position. Only lower four- or five-fold coordination numbers have been found owing to steric restrictions [43]. Electrochemical and structural data for the adsorption of uracil and some of its derivatives on Au(hkl) [33], Ag(hkl) [35] and mercury [40,41,57] illustrate rather convincingly that 2D condensation due to the formation of a hydrogenbonded adlayer usually occurs around zero charge potential or at even more negative values. The so-called "capacitance pits" are characteristic features of this type of phase formation. We did not observe them in the present system. Referring to point (4) and the fact that state III is stable even in an acidic medium, we can also exclude the structure-determining role of stacking interactions. The idea of adsorbate-substrate interfacial coordination has already been used successfully to interpret the adsorption of pyridine and pyrazine at a positively charged Au(111) surface [58]. Both these substances have structural similarities to cytosine. Taking into consideration the molecular dimensions of cytosine [55] and of the elementary Cell derived from results similar to those shown in Figs. 8(A) and 8(B), we propose the packing structure shown in Fig. 9 for cytosine adsorbed on Au(111) at E > 0.100 V [59]. The dimensions of the elementary cell are a = 7.63 A, b = 8.65 ,~ and y = 41 °. The spacing between the neighboring rows is 5.7 A. The model accounts for the N(3)-Au coordination and reproduces fairly well the experimentally determined unit cell parameters and the row spacing. Deviations with respect to the magnitude of the unit cell angle may be due to experimental uncertainties. Further insight into the packing structure of cytosine on Au(111) will be obtained by comparative studies of various of its derivatives, which are in progress. Our present results are quite different from data reported previously [25]. The two main reasons are probably

the strict application of potential control in this study and the use of a much more stringent and elaborate cleaning procedure, including flame annealing of the gold substrates.

3.4. Kinetics of film dissolution (STM) The ordered cytosine layer on A u ( l l l ) was formed at 0.200 V/SCE. Slowly scanning or stepping the potential to 0.025 V (or more negative values) causes significant dynamic changes of the image. A typical series of STM images which illustrate the dissolution of the ordered cytosine layer is shown in Fig. 10. Fig. 10(A) corresponds to the initial situation at 0.200 V. Fig. 10(B) shows the first indications of a new disordered region at the boundary of two domains (upper right segment of the image). As seen in the following images (time-scale, about 30 min), dissolution starts favorably at domain boundaries but soon covers much larger patches. Within one domain the dissolution front proceeds almost parallel to the direction of the corrugation stripes. The estimated rate of procession (growth) is around 0.1 ,~ s - i . Different dissolution zones merge until the observation area is completely covered (Fig. 10(H)). The position of the domain boundary in the lower right segment of the image also shows that thermal drift does not significantly disturb the kinetic features shown. Another interesting observation is the change in contrast of the disordered region with time. Although only a few dark and bright spots are developed in Fig. 10(C), more than half of the area imaged is covered with these features in Fig. 10(H). Line scans and Fourier analysis do not reveal any detailed structure. However, the observed features are still very different from those typical of the Au(ll 1) surface in the absence of cytosine at E < 0.100 V, which clearly indicates that cytosine is still adsorbed. Finally, we notice that it is possible to recover the ordered cytosine structure when stepping the potential back towards more positive values. Comparison with the electrochemical model of dissolution shows that dissolution takes place not only at defects (existing disordered regions) but also within ordered adlayer domains. Two different pathways were introduced in Section 3.2 when modelling the experimentally observed (macroscopic) j - t transients of dissolution of the chemisorbed cytosine film. Despite this qualitative agreement, it is still rather complicated to derive the exact kinetic mechanism unambigously because we still have no direct measure of the specific proximity effects between tip and substrate/adlayer.

4. Summary and conclusions Cytosine molecules in 0.1 M KC104 or NaCIO 4 adsorb on the Au(111) surface both spontaneously and under strict potential control. We found three different adsorption re-

Th. Wandlowski et al. / Journal of Electroanalytical Chemistry 404 (1996) 215-226

gions: "dilute" disordered phase (state I), a complicated reorientational transition region (state II) and a chemisorbed adlayer (state III). Changing the pH shifts the stability range of the three different adsorption states towards more positive potentials. The dissolution of the chemisorbed layer was studied in detail using a potential-step technique. Analysis of our data showed that the experimental transients can be described by a model based on a hole-nucleation and growth mechanism combined with Langmuir-type dissolution on defects. In-situ STM was used to investigate the various adlayers of cytosine. Reproducible structural data were reported for the chemisorbed state III. We obtained molecular resolution images in the potential range 0.100 V < E < 0.500 V. The dimensions of the unit cell are a = 7.3 + 0.3 ,~, b = 8.7 _+ 0.3 A and y = 50 ° + 5 °. The proposed structural model assumes the existence of an array of cytosine molecules which are coordinated with gold atoms of the underlying (reconstructed) lattice via the N3 position. Dynamic STM experiments on the film dissolution (state III) revealed the existence of two different mechanisms involving grain boundaries and also entire domains. Dissolution within a single domain seems to follow a linear growth law with an estimated rate of about 0.1 A s -1 "

The combined electrochemical and STM study of cytosine-adlayers on A u ( l l l ) reported here provides an encouraging approach to understanding the formation/dissolution and structural characteristics of organic adlayers at metal electrodes in a much more direct and detailed fashion.

Acknowledgements TW would like to thank the Deutsche Forschungsgemeinschaft for support through a Heisenberg Fellowship. The work at Arizona State University was supported by NIH grant 5R21HG00818-01A1 and ONR grant N 0001490-J-1455. Stimulating discussions with Professor D.M. Kolb are gratefully acknowledged.

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