Electrochemical STM measurements
A.V. Rudneva, A.V. Khlynovb
a - Research Scientist, Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia,
b - Development Engineer, NT-MDT Company, Zelenograd, Russia
The scanning tunnelling microscope (STM) is a powerful instrument allowing performing in situ measurements in electrochemical media and combining classical electrochemical procedures with simultaneous controlling of the state of electrode surface by STM imaging. We present two illustrative examples of STM application for studies of electrochemical processes in an electrolyte solution.
We used the scanning probe microscope NTEGRA (production of the NT-MDT Company, Zelenograd, Russia, www.ntmdt.com) for STM imaging and the computer-controlled bipotentiostat furnished with the NTEGRA microscope for electrochemical measurements in cell shown in Fig. 1. The STM studies were performed with usage of tungsten tips.
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a) side view
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b) view from above
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Fig. 1. Electrochemical cell for STM and AFM measurements: 1 – base; 2 – cell housing; 3 – clips; 4 – electrodes; 5 – contacts; 6 – grounding socket; 7 and 8 – inlet and outlet for feeding gas.
1. In situ STM observation of ultrathin cobalt film electrodeposition.
Introduction.
Because of their magnetic properties, cobalt thin layers and clusters which are fabricated electrochemically on non-magnetic surfaces, represent a technologically important system. The size dependence of the magnetic properties which, for example, leads to a transition from ferromagnetic behaviour in the case of bulk Co to a superparamagnetic behaviour for small isolated clusters, has focussed the interest on the initial stages of deposition [1]. A recent study has shown that Cu/Co/Au(111) layers, electrodeposited from a sulfate + chloride solution, exhibit enhanced perpendicular magnetic anisotropy (PMA), due to magnetoelastic effects at the Cu/Au interface [2].
The initial stages of Co deposition onto Au(111) electrode have been investigated by cyclic voltammetry, potentiostatic transients, and in situ scanning tunnelling microscopy.
Experimental.
As working electrodes we used commercial samples: gold evaporated onto borosilicate glass (gold layer thickness is usually 200 to 300 nm). Before each experiment the electrodes were flame-annealed with a Bunsen burner in order to obtain clean and structurally well-defined surfaces. The reference electrode was Ag/AgCl, counter electrode was Pt wire. All values of potentials are given in a scale of Ag/AgCl electrode. Cobalt electrodeposition was performing from 10 mM K2SO4 + 1 mM KCl + 1 mM H2SO4 + 1 mM CoSO4 (pH≈4) electrolyte which was prepared with use of Milli-Q water and ultrapure reagents.
Results.
Fig. 2 shows the cyclic voltammogram (CV) of Co deposition and dissolution. In cathodic scan first peak at -0.65 V corresponds to hydrogen evolution [2] on gold surface and second peak at -0.92 V corresponds to cobalt deposition. In anodic scan peak at -0.30 V is attributed to Co dissolution [2].
Fig. 2. CV of Au(111)/glass in the Co2+ solution. Potential sweep rate is 50 mV s-1. Blue arrows indicate direction of potential sweep.
Au(111) electrode surface contains of wide (111) terraces separated monatomic height steps (Fig. 3A). During STM scanning we applied Esam = -0.8 V (this moment corresponds to blue dotted line in Fig. 3B). Co deposition starts at the end of the scan (Fig. 3B). In next scan gold surface is covered by thin Co layer (Fig. 3C).
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Fig. 3. In situ STM observation of cobalt deposition on gold. A – wide (111) terraces of gold at Esam = -0.4 V. B, C – Co deposition at Esam = -0.8 V. Dotted lines indicate the moment of application of the overpotential. The vertical arrows indicate the y-scanning direction.
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B
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D
E
Fig. 4. In situ STM observation of Co deposition on Au(111)/glass substrate. Co deposition was performed at FB off , at Esam=-0.87 V for 15 s, Etip=-0.55 V. After that the potentials were corrected (see text). A – gold surface at Esam=-0.38 V, Etip=-0.30 V, B, C - Co deposit, D – vertical profile of X cross section of image C (see blue solid line in image C), E was recorded after addition of copper (a few drops of 1 mM CuSO4 + 50 mM H2SO4) and it shows Cu deposit on Co. Additive explanations are given in text.
Thus the microscope NTEGRA (NT MDT) allows us to obtain fine quality images directly during metal deposition on scanned electrode surface. But in this case a shielding of a part of the surface by STM tip takes place, and metal deposition can be hindered on the surface area under the tip.
In next experiments our goal was to obtain a sandwich structure Cu/Co/Au(111) on gold electrode (Fig. 4A). Cobalt was deposited on gold surface at Esam = -0.87 V for 15 s (during this period of time Etip = -0.55 V). During Co deposition Feedback loop (FB) was turned off (in this case the tip is retracted from electrode surface on ~2 µm) to prevent a shielding of surface by tip. After that the potentials of the sample and the tip were corrected to Esam=-0.63 V and Etip=-0.40 V, respectively. In these conditions (Esam=-0.63 V) no dissolution and deposition of cobalt occurs (Fig. 4B). The numbers in the images refer to the Nth atomic plane of the deposited layer (see image C). In the case of cobalt, the deposition starts with fast nucleation of a biatomic layer followed by a layer by layer growth [3]. The 28 Å hexagonal Moiré pattern resolved on the first bilayer of cobalt suggests the presence of some tensile strains at the Co/Au interface [2]. On top of bilayer one can see islets of 3rd Co monolayer and on top of 3rd one – islets of 4th monolayer. Image D shows vertical profile of X cross section of image C. One can see more clearly the height distribution (Z-scale) along scan line. There are four levels of heights, which correspond to gold surface, top of bilayer, 3rd and 4th layers.
In 5 mins after correction of sample and tip potentials (images B and C) we added into the cell a few drops of 1 mM CuSO4 + 50 mM H2SO4 solution (at turned off feedback loop). Cathodic current increased dramatically because of copper electrodeposition. Image E shows Cu deposit on cobalt. Thus the sandwich structure Cu/Co/Au(111) was obtained at the full control of the electrode surface state by STM NTEGRA.
2. STM study of Cu electrosorption on Au(111).
Introduction.
The study of the specificities of the coadsorption of adatoms, water and anions as well as the detailed mechanisms of the formation and growth of new phase nuclei under conditions of the competitive adsorption is of great fundamental importance. Underpotential deposition (UPD) of copper (in other words, Cu electrosorption) on Au(111) in sulfate solution is an excellent candidate for theoretical modeling and so it is widely studied. We used STM NTEGRA to obtain good quality atomic resolution of Au(111) surface during Cu UPD.
Experemental.
As working electrodes we used flame-annealed Au(111)/glass samples. The reference electrode was Cu/Cu2+, counter electrode was Pt wire. All values of potentials are given in a scale of Cu/Cu2+ electrode. Copper UPD was performing from 50 mM H2SO4 + 1 mM CuSO4 electrolyte which was prepared with use of Milli-Q water and ultrapure reagents.
Results.
The first Cu UPD stage on the Au(111) terraces in a copper sulfate solution is the formation of a co-adsorption lattice of copper adatoms with the sulfate anions (√3х√3)R30º (2/3 Cu monolayer and 1/3 monolayer of sulfate anions). The cathodic and anodic peaks at Esam=0.15-0.30 V (Fig. 5) correspond to the formation and destruction of this co-adsorption lattice [4]. At Esam=0.1 V full Cu monolayer is formed (second stage of Cu UPD).
Fig. 6 presents in situ STM images of Au(111) electrode recorded at sample potential Esam=0.155 V (between the peaks corresponding to first and second stages of Cu UPD, it is marked by arrow in Fig. 5). One can see atomic resolution of (√3х√3)R30º lattice by in situ STM (Fig. 6, sulfate anions are seen).
Fig. 5. Cyclic voltammogram of Au(111) electrode in the copper sulfate solution. Potential sweep rate is
20 mV s-1.
Fig. 6. STM images of Au(111) in copper sulfate solution at Esam=0.155 V.
Negative shifting the sample potential leads to the transition of (√3х√3)R30º phase into a pseudomorphic Cu(1x1) monolayer. Cu adatoms replace gradually sulfate anions from electrode surface. This process can be observed from STM images in Fig. 7. It can be seen that Cu islets are formed along step (light areas in images).
It is known from literature that steps are more preferential sites for Cu deposition than sites on terraces. With usage of STM NTEGRA we could illustrate this phenomenon very easily.
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Fig. 7. STM images of Au(111) in copper sulfate solution at Esam, V: 0.155 (A), 0.130 (B), 0.110 (C), 0.090 (D).
References.
1. M. Kleinert, H. F. Waibel, G. E. Engelmann, H. Martin and D. M. Kolb, Electrochimica Acta. 46 (2001) 3129.
2. L. Cagnon, T. Devolder, R. Cortes, A. Morrone, J. E. Schmidt, C. Chappert and P. Allongue. Physical Review B 63 (2001) 104419.
3. A. Gundel, L. Cagnon, C. Gomes, A. Morrone, J. Schmidt and P. Allongue, Physical Chemistry Chemical Physics 3 (2001) 3330.
4. M.H. Holzle, V. Zwing and D.M. Kolb. Electrochim.Acta 40 (1995) 1237.
