Fig. 2. Surfactant-stabilized Pd clusters on Au(l 11); surfactant, N(n-C8Hi7)4Br. Upper panel: average diameter c?stm = 4 nm. Lower panel: aveage diameter c?stm = 6.9 nm.

Fig. 3. HOPG oxidation in 0.1 M HC104 at 1.9 V RHE after 2, 4, 6 and 8 min (from left to right). Scan size 255 nm x 255 nm.

Similar model electrodes were prepared using highly oriented pyrolytic graphite (HOPG) as a substrate. On these electrodes, imaging of clusters was only possible when they were attached to defects, which are rather sparse on a freshly cleaved HOPG surface. There was some evidence that clusters were also present on the defect-free terraces, but these were apparently easily swept away by the tunneling tip [10]. Since the defect density was concluded to be crucial for such model electrodes, possibilities for its manipulation were investigated. It was found that electrochemical oxidation of the surface at high potentials increases the defect density. Since the respective process is controlled by the electrochemical potential and can be followed with STM in real time, as shown in Fig. 3, it allows direct interactive control on the surface morphology of the substrate.

Fig. 4. Pt clusters on preoxidized HOPG; metal core diameter djEu = 4 nm; surfactant, N(C4H9)4Br; sample prepared by electrophoretic deposition from a colloidal solution in THF at U= 5 V for 30 s.

On such defective surfaces the adhesion of particles is improved as compared with freshly cleaved graphite, but force interactions between tip and sample are still evident in the measurements. An example for a model electrode, which was prepared by electrophoretic deposition of a platinum colloid onto a defective HOPG surface, is shown in Fig. 4.

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