Usage of atomic-force microscopy in the studies of electrochemical processes

A.V. Rudnev^a, A.V. Khlynov^b
a - Research Scientist, Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia,
^b - Development Engineer, NT-MDT Company, Zelenograd, Russia

Atomic-force microscope (AFM) can be efficiently used in the studies of different electrochemical processes. AFM imaging is a very informative research method and it can be useful both in theoretical and in applied investigations. Here we present two examples of AFM application performed both in-situ (AFM imaging in solution during electrochemical processes) and in ex-situ (AFM imaging in air after manipulations with a sample in electrochemical cell) procedures.
The study of sample surfaces was performed by a scanning probe microscope Solver Pro EC (production of the NT-MDT Company, Zelenograd, Russia, www.ntmdt.com) in the configuration of an atomic force microscope. The images were obtained in semicontact mode in air and contact mode in solution.

1. AFM study of influence of electrode surface structure on the morphology of electrochemical deposits.

Introduction.
Metal electrodeposition as a method of synthesis of micro- and nanoparticles on foreign substrates as well as adsorptive and catalytic properties of nanoobjects is widely studied. Mechanisms of these processes and properties of nano-sized objects are of fundamental importance and are a basis for further development of up-to-date technologies in the fields of microelectronics and electrocatalisis. Using of single crystals of different orientation as a substrate allows synthesizing epitaxial clusters of definite structure. This study has illustrated that scanning probe microscopy is eminently suitable method for control of the structure of obtained deposits. The influence of surface structure on morphology of deposits was shown as well.
Experimental.
The preparation of electrodes was performed using Clavilier’s technique describedin Refs. [1,2]. At making of electrodes, firstly a teardrop-shaped single crystal is formed by melting (Fig. 1a), then it is orientated and cut to obtain hemisphere (Fig. 1b). Thereafter obtained face is polished and heat-treated. In this study we used single crystals with basal faces: Pt(111), Pt(100), Pt(110) (Fig. 2).
Solutions were prepared from high purity reactants (Merck p.a.) using ultrapure water (18 Mohm Millipore Milli-Q). Electrochemical measurements and electrodeposition were conducted in the glass cell of 50 ml volume with aid of the computer-controlled bipotentiostat furnished with the Solver Pro EC microscope (NT-MDT). High-purity argon was employed to deaerate the solutions; during electrochemical experiments the inert gas was passed over the electrolyte surface. The experimental procedure was described in detail in Refs. [3,4].

(a)           (b)
Fig. 1. (a) Image of single crystal with face-centered cubic (fcc) lattice. (b) The photograph of single crystal Pt electrode

            
Fig. 2. Basal faces (111), (110), and (100) of single crystal with fcc lattice.

Cyclic voltammograms for different Pt single crystals in supporting solution of 0.5 M H₂SO₄ (Fig. 3) are according to literature data [2]. They characterize the electrodes with well ordered surface structure and indicate the purity of the electrochemical system. Copper deposits on Pt electrodes were obtained in 0.5 M H₂SO₄ + 0.01 M CuSO₄ solution by applying of overpotential. Then electrode was rinsed by water, dried and installed at Solver Pro EC microscopic stage for AFM imaging.

Fig. 3. Cyclic voltammograms for Pt single crystals in solution of 0.5 M H₂SO₄ at 100 mV s-1.

Atomic structure of basal faces are shown in Fig. 4a. For (111) face (the closest-packed one) atoms form regular triangles (or hexagons), while for (100) face - squares. For these structures the formation of
 

Fig. 4. (a) Atomic structure of basal faces (111), (100), and (110). (b)-(e) AFM-images of Cu deposits on Pt single crystal faces obtained in solution of 0.5 M H₂SO₄ + 0.01 M CuSO₄ at overpotential η=50 mV for 50 (b) and 500 seconds (c-e).

crystallites of triangular (or hexagonal) and square shapes, respectively, is expected during deposition. (110) face presents stepped surface, where atom rows alternate with furrows. For this structure the growth of crystallite of rectangular shape elongated along the steps (atom rows) is expected.
To obtain epitaxial island deposit we used a quite low overpotential (η=50 mV). Under these conditions a small number of Cu nuclei forms on electrode surface. Fig. 4b presents Cu deposits obtained for a short period of time (50 s). The shape of crystallites is expected to correlate with crystallographic face orientation (Fig. 4b, hexagons, squares, and rectangles for Pt(111), Pt(100), and Pt(110), respectively). In the case of longer deposition the growth of crystallites occurs in directions defined surface structure (Figs. 4c-4e), and the crystallites can coalesce.
It should be note that Solver Pro EC microscope (NT-MDT) allows getting images of very high quality (all presented images are unfiltered). Using of semicontact mode of atomic-force microscope does not lead to the destruction or any changes in the surface deposit structure as in this case the force of pressure of the probe on scanned surface is minimized (see detailed description of this mode in website www.ntmdt.com). Scanning of the same place of sample surface for a long time results in several similar images, which indicates a very good reproducibility of results obtained with device Solver Pro EC.
Thus, with Solver Pro EC microscope from NT-MDT one can exercise total control of the morphology of deposits obtained at different electrochemical regimes on various structure substrates. In present study we recorded AFM-images of the Cu deposits of different roughness. The quality of images drawn by Solver Pro EC microscope is close to perfect even though the height of individual crystallite was more than 500 nm.

2. Study of steel corrosion in acidic medium by atomic force microscopy.
Introduction.
The process of metal corrosion is studied by means of numerous methods. However the atomic force microscopy allows observing rusting of metal directly during the corrosion process (in situ study).

Fig. 5. Dependence of metal dissolution rate (anodic current) on electrode potential.

Fig. 6. AFM observation of corrosion process of steel in 0.5 M H₂SO₄ solution.

Experimental.
The plates from stainless steel (12 % Cr, 17 % Ni, 10 % Ti) were used as working samples. The corrosive media were 0.5 М H₂SO₄ and 1 M HCl solutions.
AFM and electrochemical measurements were conducted in the cell “mp4ec” (NT MDT). The control of the electrode potential was carried out with aid of the computer-controlled bipotentiostat furnished with the Solver Pro EC microscope (NT-MDT). The auxiliary and quasi-reversible reference electrodes were Pt wires (minor shift of the reference electrode potential is possible in the course of measurements).
Results.
The polarization curve of anodic dissolution of steel is shown in Fig. 5. One can recognize the following regions – active dissolution of steel (E < 0 V), passive state (0 V < E < 0.9 V) and disturbance of passive state (E > 0.9 V).
AFM observations of corrosion process of steel were conducted at E = 0.5 V, i.e. in the region of passive state (Fig. 6). As one can see in Fig. 6, corrosion occurs on the overall area of the surface. In the first hour the formation of the sites of dimple corrosion (diameter of holes considerably exceeds their depth). Then the pitting corrosion (as expected for metal predisposed to passivation [5]) starts to occur on steel sample surface.
In Fig. 7 AFM images obtained in water before and after corrosion for 2 hours in solutions of sulfuric and hydrochloric acid are presented. The alterations in morphology and roughness of steel surface are more demonstrably seen from vertical profiles shown below the respective images (Fig. 7). In two hours of corrosion process of steel in sulfuric acid solution, the pitting depth reached half a micron (black pits in Fig. 7b). In hydrochloric acid solution no potential region of passive state of steel was found. Corrosion process was characterized by a high rate of steel dissolution and anodic currents were considerably higher than those in sulfuric acid solution. On the sample undergone corrosion in hydrochloric acid solution, no pitting (deep pits) is observed (Fig. 7c, 7d), as the pitting is typical for metal and alloys situated in passive state [5].

References.
1. J. Clavilier, D. Armand, S.G. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267.
2. J. Clavilier, Interfacial Electrochemistry. Theory, Experimental, and Applications. Ed. A. Wieckowski, N.Y.: Marcel Dekker, Inc., 1999.
3.A.V. Rudnev, E.B. Molodkina, A.I. Danilov, Yu.M. Polukarov, J. Feliu, Rus. J. Electrochem. 42 (2006) 381.
4.A.I. Danilov, E.B. Molodkina, A.V. Rudnev, Yu.M. Polukarov, J. Feliu, Electrochim. Acta 50 (2005) 5032.
5. Semenova I.V., Florianovich G.M., Khoroshilov A.V. Corrosion and corrosion protection (Korrozia I zatshita ot korrozii). Ed. I.V. Semenova – M.: FIZMATLIT. 2002. P. 336.

BEFORE                                                                          AFTER

 (a)         (b)

   BEFORE                                                                      AFTER

  (c)          (d)
Fig. 7. AFM images obtained in distilled water before and after corrosion in (a, b) 0.5 М H₂SO₄ and (c, d) 1 M HCl. Under AFM images respective vertical profiles.