O 200 400 600

Fig. 9. In-situ AFM image of a YBa2Cu307.8 thin-film surface (c-axis oriented) under electrochemical conditions in the system YBa2Cu3O7-s/3.3xl0"3 M [CH3COCH=COCH3]2 Cu + CH3CN + 0.1 M C,<sH36C1N04 at T= 298 K, contact mode.

Surfaces of superconducting YBa2Cu307-5 thin films, which are epitaxially grown on SrTiC>3 single-crystal substrates, exhibit terraces separated by monatomic steps as shown in Fig. 9. The surface morphology is also suitable for nanostructuring by local Me deposition in the OPD range.

Local Cu OPD on Au(lll) is already successfully accomplished using a modified AFM technique with a special polarization routine [20]. First experiments have been started to use a similar technique for the nanostructuring of semiconductor and superconductor surfaces.

4 Conclusions

Investigations of UPD and OPD of metals leading to 2D and 3D Me phase formation are of great interest for electrochemical nanotechnology. Application of in-situ local probe techniques in this field gives new analytical information on an atomic level and offers possibilities for a defined nanostructuring of solid-state surfaces.

Acknowledgments. The authors thankfully acknowledge financial support of this work given by Deutsche Forschungsgemeinschaft (DFG), Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AIF), Bundesministerium für Wirtschaft (BMWi) and Fonds der Chemischen Industrie. The following coworkers contributed to the results presented: W. Obretenov, U. Schmidt, S. Vinzelberg, S. G. Garcia, D. Salinas, R. T. Pötzschke, A. Froese, C. Gervasi, and A. Baiy.

5 References

[ 1 ] G.Binnig, H.Rohrer, C.Gerber, E.Weibel, Phys. Rev. Lett. 49, 57 (1982).

[2] G.Binnig, H.Rohrer, IBM J. Res. Dev. 30, 355 (1986).

[3] G.Binnig, C.F.Quate, C.Gerber, Phys. Rev. Lett. 56, 930 (1986).

[4] 10 Years of STM, Ultramicroscopy 42-44 (1992).

[5] RJ.Behm, N.Garcia, H.Rohrer (Eds.),Scanning Tunneling Microscopy and Related Methods, NATO ASI Series E, Applied Sciences, Vol. 184, Kluwer Academic Publishers, Dordrecht, 1990.

[6] H.D.Abruna (Ed.), Electrochemical Interfaces: Modern Techniques for In-situ Interface, VCH, Weinheim, 1991.

[7] J.Lipkowski, P.N.Ross (Eds.), Structure of Electrified Interfaces, VCH, Weinheim, 1993.

[8] R.Wiesendanger, H.-J.Güntherodt (Eds.), Scanning Tunneling Microscopy n, Springer Series in Surface Sciences, Vol. 28, Springer-Verlag, Berlin, 1992.

[9] N.John Di Nardo, Nanoscale Characterization of Surfaces and Interfaces, VCH, Weinheim, 1994.

[10] E{-p = = - F <f>{l) where Ji{j} and f/ß denote the electrochemical and chemical potentials of electrons in phase j, respectively, and <P is the inner or Galvani potential of phase j which can be measured as electrode potential E vs. a reference electrode.

[11] E.Budevski, G.Staikov, W.J.Lorenz, Electrochemical Phase Formation and Growth - An Introduction to the Initial Stages of Metal Deposition, W-VCH, Weinheim, 1996.

[12] D.M.Kolb, A.S.Dakkouri, N.Batina, The Surface Structure of Gold Single-Crystal Electrodes, in Proc.of NATO Advanced Study Institute on Nanoscale Probes of the Solid/lLiquid Interface, Sophia Antipolis, France, July 10-20, 1993, H.Siegenthaler, A. Gewirth (Eds.), Kluwer Academic Publishers, Dordrecht, 1995, p.263.

[13] S.G.Garcia, Electrochemical and in-situ STM investigations in the system Au(M/)/Ag+, PhD Thesis, University of Bahia Bianca, Argentina, 1997;

S. Garcia, D. Salinas, C. Mayer, E. Schmidt, G. Staikov, and W. J. Lorenz, Electrochim. Acta, submitted.

[14] R.T.Pötzschke, C.A.Gervasi, S.Vinzelberg, G.Staikov, W.J.Lorenz, Electrochim. Acta 40, 1469(1995).

[15] S.G.Garcia, D.Salinas, C.Mayer, J.R. Vilche, H -J.Pauling, S.Vinzelberg, G.Staikov, W.J. Lorenz, Surface Sei. 316, 143 (1994).

[16] S.Vinzelberg, Elektrochemische 2D und 3D Phasenbildung aus atomarer Sicht -Rastertunnelmikroskopische und -tunnelspektroskopische Untersuchungen in den Modellsystemen Au(M/)/Ag+ und Au(M/)/Pb2+. PhD Theses, Universität Karlsruhe, 1995.

[17] U.Schmidt, S.Vinzelberg, G.Staikov, Surface Sei. 348, 261 (1996).

[18] W.Obretenov, U.Schmidt, W.J.Lorenz, G.Staikov, E.Budevski, D.Carnal, U.Müller, H. Siegenthaler, E.Schmidt, J. Electrochem. Soc. 140, 692 (1993).

[19] R.T.Pötzschke, Nanostrukturierung elektronenleitender Festkörperoberflächen, PhD Thesis, University of Karlsruhe, 1997, in preparation.

[20] A.Froese, Elektrochemisches Phasengrenzverhalten von Supraleitern, PhD Thesis, University of Karlsruhe, 1996.

Electrochemical Nanotechnology

In-situ Local Probe Techniques at Electrochemical Interfaces Edited by W. J. Lorenz and W. Plieth © WILEY-VCH Verlag GmbH, 1998

Imaging of Electrochemical Processes and Biological Macromolecular Adsorbates by in-situ Scanning Tunneling Microscopy

Jens E.T. Andersen, Jens Ulstrup, Per Möller


1 Introduction 28

2 Imaging electrochemistry by in-situ scanning tunneling microscopy

(in-situ STM/electrochemical STM) 29

2.1 Pure gold surfaces in electrolyte solutions 29

2.2 Metal deposition and metal dissolution 31

3 STM of adsorbed metalloproteins 37

3.1 In-situ STM patterns of cytochrome c and other metalloproteins 3 8

3.2 In-situ tunneling through metalloproteins as a three-center multiphonon electron transfer process 40

4 References 42

Summary. In-situ scanning tunneling microscopy (STM) has provided intriguing new information about electrocrystallization, corrosion, and the surface dynamics of metall proteins.

Repetive cycles of copper deposition on gold have been found to lead to surface alloy formation where nucleation occurs during the first cycle, followed by growth of the alloy phase in subsequent cycles. Bulk metal crystallites nucleate and grow on top of the alloyed surface at cathodic overpotentials. The entire process can be followed in time and the surface morphology mapped while cyclic voltammograms are simultaneously recorded. This has led to a new understanding of surface atom mobility, mechanisms of electrociystallization, and electrosorption. It has also been shown that the potentials of copper deposition and dissolution on gold are separated by exactly 59 mV. This is not reflected in the cyclic voltammograms but indicates that copper electrodeposition is indeed a single-electron process such as predicted by the Bockris-Mattson model.

In other investigations in-situ STM imaging has shown that the single-center metalloproteins cytochrome c (Fe) and azurin (Cu) are strongly adsorbed on gold at low ionics strength. In contrast, the multicenter copper oxidase laccase, surprisingly, appears to be weakly adsorbed on pyrolytic graphite in spite of good electrochemistry. Cytochrome c (cyt c) and azurin assemble in flat aggregates, organized in a heterophase, and corresponding in lateral size («50 nm x 50 nm) to about 100 molecules. The organized aggregates evolve in time, and pinholes open and split. Further investigation of cyt c has revealed smaller structures, of the size of individual molecules, between the larger aggregates. This holds interesting perspectives for in-situ characterization of protein structure and dynamics on solid surfaces.

A theory of in-situ STM of large adsorbates, based on inelastic tunneling, strong electronic vibrational coupling, and molecular electron transfer theory, has been developed.

1 Introduction

By introduction of electrochemical control of both the tip and the working electrode, conventional STM has been developed to image surfaces in-situ, i.e. with both surface and tip in contact with electrolyte [1-4]. Key factors in high-quality images are potentiostatic control [1,2] and tip coating [1-5]. The tip in contact with the electrolyte acts as an additional working electrode. The system therefore needs independent potentiostatic control of the two working electrodes with respect to a common reference electrode. This is most frequently achieved by a bipotentiostat [1-4, 7] but other methods are also encountered [3]. Faradaic current densities at the tip or working electrode may then be kept within pre-determined limits. The uncoated tip in contact with electrolyte usually results in a Faradaic current density which exceeds the tunnel current by orders of magnitude [8] and prevents meaningful imaging. The Faradaic current at the tip is, however, minimized by coating the tip with an insulating film [9, 10]. Our in-situ STM research has followed two lines. One addresses electrochemical metal deposition, the other the structural surface organization and functional mechanisms of adsorbed metall proteins.

2 Imaging Electrochemistry by in-situ Scanning Tunneling microscopy (in-situ STM/electrochemical STM)

In contrast to conventional microscopy, in-situ STM affects the electrochemical process imaged. Although the tip is well coated and only small currents (nA) are conducted, the tip is still important. A linear relation between the tip potential and the electrochemical potential for copper deposition on polycrystalline gold was found recently [11], The potential was swept to a point where the surface was just covered by copper as observed by in-situ STM, i.e., the potentials (-Eon, £tip) for the onset of copper electrodeposition were registered. Similarly, the potentials (£c>ff, £tip) where the bare surface was recovered during copper dissolution were registered. Both potential relations follow a linear behaviour in a wide range of tip potentials (Fig. 1). The parallel lines are separated by 59 mV, which indicates that copper electro-deposition/dissolution is a one-electron process [11]. An implication of this is that cyclic voltammetry cannot directly relate voltammetry features to in-situ STM images. The procedure is rather to record in-situ STM at several tip potentials and extrapolate all the data to zero tip potential in comparison with voltammetry. Alternatively, the tip potential should be kept at the value zero.

2.1 Pure Gold surfaces in electrolyte solutions

Atomic structures on crystalline surfaces in electrolytes have been identified [1-8]. It is essential that the surface is kept under electrochemical control [3-8]. Figure 2 shows images of a crystalline gold surface prepared by flame annealing [12] and instantly subjected to potentiostatic control (E = 180 mV vs. Cu2+/Cu). Individual gold atoms are readily identified (Fig. 2). Imaging may be maintained for many hours provided that the potential is kept at a sufficiently large anodic value to prevent underpotential deposition (UPD) [13] but not so large that surface oxidation begins [14]. Davenport et al. [14] have shown that a worm-like structure of gold oxide appears on a crystalline gold surface oxidized electrochemically. By comparing in-situ STM images with electrochemical data it was evident that monolayers of gold oxide were formed at anodic potentials and that a slow reduction of the gold oxide at cathodic potentials recovered the bare surface. The gold surface, however, is not oxidized solely by electrochemical methods.

fit to OFF

-fit to ON

fit to OFF

-fit to ON

Fig.l. The electrochemical potential of copper electrodeposition ( -A. , £0N) and of copper electrodissolution (■, £0ff) on a gold polyciystalline working electrode as imaged by in-situ STM. The separation between the two straight lines fitted to the data are parallel and separated by 59 ± 2 mV. Electrolyte: 0.01 M CuS04 and 0.01 M H2S04 in Millipore water.

Fig.2. Gold crystalline surface imaged at atomic resolution by in-situ STM in 0.01 M CuS04 and 0.05 M H2S04 in Millipore water with E = 200 mV and£t = -110 mV (filtered), d = 0.02 nm, It = -3 nA.

Figure 3 illustrates surface oxidation by contact with ambient atmosphere for 24 h, i.e., ex-situ. Figure 3 is recorded by installing the oxidized sample in the in-situ STM system, keeping it under slightly acidic conditions and anodic potentials. Under these conditions the atomic surface structure is visible (Fig. 3, center) together with a few layers of gold oxide (Fig. 3, upper left). A growing oxide layer is observed to the lower right of Fig. 3 as evidenced by blurring of the atomic structure.

Figure 3 illustrates surface oxidation by contact with ambient atmosphere for 24 h, i.e., ex-situ. Figure 3 is recorded by installing the oxidized sample in the in-situ STM system, keeping it under slightly acidic conditions and anodic potentials. Under these conditions the atomic surface structure is visible (Fig. 3, center) together with a few layers of gold oxide (Fig. 3, upper left). A growing oxide layer is observed to the lower right of Fig. 3 as evidenced by blurring of the atomic structure.

Fig. 3. Gold crystalline surface oxidized ex-situ at ambient pressure for 24 h. Two layers of gold oxide are seen to the upper left, and to the lower right the gold was only partially oxidized. Along the mid-diagonal, rows of individual atoms are observed. In-situ STM images, electrolyte: 0.01 M CuS04 and 0.05 H2S04 in Millipore water, E = 400 mV, Et = -91 mV, d = 0.5 nm, /, = 1.2 nA.

2.2 Metal deposition and metal dissolution

In-situ STM offers unique high resolution of the dynamics of atoms and molecules at surfaces in solution [15]. The technique applies, however, also to bulk phenomena at lower magnification in the whole range from optical microscopic to atomic resolution [16-18]. At lower magnifications, e.g., with image dimensions in the micron range, the results may be compared with results obtained by methods such as ex-situ electron diffraction [6], Intermediate resolution has provided much new information about electrochemical mechanisms of bulk electrocrystallization and -dissolution. The mechamism of growth during metal electrocrystallization is classically divided into three major categories: layer-by layer growth (Frank-van der Merwe) [19], monolayer

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