STM data on the passivation of nonnoble metals and alloys

Lev et al. [1] were the first authors to investigate the possibility of in-situ STM imaging of transitions induced by the electrochemical potential at non-noble metal surfaces. They studied nickel in sulfuric acid. At the rest potential and in the active region, the images were reproducible. Current versus tip-surface distance (I-d) curves showed the exponential decrease characteristic of the tunneling mechanism. When the potential was stepped in the passive region, erratic images were recorded under the same tunneling conditions. The I-d response was asymmetric and spread over large distances, which is not typical of tunneling. This effect was attributed to mechanical fractures of the tip with the passive layer. Surface deposition of a nickel sulfate film resulting from the extensive dissolution produced by the potential step may also have been responsible for the failure to record STM images of the passive film in this experiment.

Bhardwaj et al. [2] studied by in-situ real-time STM imaging the passivation of polycrystalline iron in borate buffer. They proceeded by alternating oxidation steps at increasing anodic potentials and reduction steps at cathodic potential. After reduction of the natural oxide at the cathodic potential, relatively flat surfaces were produced supposedly corresponding to the metal substrate. Upon oxidation at anodic potential, rougher surfaces were at first produced, with patches or clusters of nanometer dimensions. These patches were observed in the first image after the oxidation step, indicating instantaneous formation on the time scale of the data acquisition. Upon continuous imaging at the same oxidation potential, the surface was observed to smoothen. This was attributed to the completion of the passive layer by a fusing effect of the patches. When the potential was stepped back at the reduction value, surface roughening was first observed followed by smoothening within minutes of polarization. These topographic changes were attributed to the reduction of the passive film. The vertical dimensions of the patches produced at first by oxidation were found to increase from about 1 to about 4 nm when the anodic oxidation potential was increased. This suggested an increasing thickness of the passive film after completion for increasing values of the oxidation potential. This was supported by the increasing time periods necessary to reduce completely the passive film at cathodic potential. This study suggests that the growth process of the passive film by patches or islands fusing together is related to a nucleation, growth, and coalescence mechanism. The same authors performed a similar study on polycrystalline A1 in sodium hydroxide [3]. They also observed that reduction of the oxide formed at anodic potentials reproduced the original substrate surface, and that when the potential is increased in the positive direction an oxide begins to grow nonuniformly as small humps or patches of nanometer dimensions, which later fuse together after longer elapsed times or at more positive potentials. As in the case of iron, it is suggested that the formation of the passive film occurs via a nucleation, growth, and coalescence mechanism.

Ex-situ STM imaging has been applied in our group to the investigation of the passive films formed on Ni, Cr and Fe-22Cr in aqueous sulfuric acid solutions [4-8]. The study on Ni [4] was the first investigation of passive films with achievement of both lateral and vertical atomic resolution. In our surface science-oriented approach to passivation and corrosion, well-defined surfaces of single crystals are used. For Ni [4, 5], the (111) orientation was selected as a previous investigation had demonstrated the crystallinity of the passive film formed on this surface [9]. Much care was given to surface pretreaments. The samples were electrochemically polished and annealed at high temperature in a flow of pure hydrogen for several hours. These pretreaments were necessary to produce large atomic terraces of the substrate (of the order of tenths of micrometers). The passive films were produced in 0.05 M H2SO4 by potential steps from the corrosion potential value to three different values in the passive region (+550, +650 and +750 mV/RHE). STM analyses were performed in air, in which conditions the passive films were stable.

Modifications of the passivated Ni surfaces with respect to the nonpassivated one were recorded on two different lateral scales. On a mesoscopic scale of hundreds of nanometers, islands were observed. Their size was found to decrease and their density was found to increase with increasing passivation potential. Their shape varied from trigonal contours with ledges oriented along the main crystallographic directions of the

Fig. 1. STM images of the passive film formed on Ni(lll) in 0.05M H2S04 at +750 mV/RHE. The left image shows the stepped crystalline lattice corresponding to NiO. The right image shows the lattice recorded on some terraces consistent with NiO(lll). The unit cell and two point defects are marked.

substrate, to hexagonal contours still with ledges oriented along the main crystallographic directions, and finally to nonsymmetrical contours with nonoriented ledges after passivation at +550, +650 and +750 mV/RHE, respectively.

Fig. 1. STM images of the passive film formed on Ni(lll) in 0.05M H2S04 at +750 mV/RHE. The left image shows the stepped crystalline lattice corresponding to NiO. The right image shows the lattice recorded on some terraces consistent with NiO(lll). The unit cell and two point defects are marked.

These variations have been assigned to competition, dining the passivation treament, between metal dissolution and formation of the passive film. The roughening effect due to dissolution increases with potential and produces a higher density of islands with less oriented ledges. On these submicroscopic islands, a stepped crystalline lattice was imaged on the atomic scale. A typical image is shown in Fig. 1. The lattice parameters measured on the terraces of the stepped lattice correspond to the lattice parameters of the (111) orientation of NiO, the inner component of the passive film. These lattice parameters and the step density were found to be independent of the passivation potential. The density and height of the steps correspond to an average tilt of 8(±5)° between the surface of the film and the (111) orientation of the terraces. The resulting epitaxy relationships with the substrate are: NiO(433)//Ni(lll) with Ni0[0-U]//Ni[0-ll] and NiO(765)//Ni(lll) with NiO[l-21]//Ni[l-21]. The tilt is thought to result from a relaxation of the strained epitaxy due to a mismatch of 16% between the lattice parameters of the oxide film and those of the metal substrate and/or from a relaxation of the polar NiO(lll) terraces. A possible atomic model of the interface between a 10 nm thick NiO(l 11) layer tilted by 8° and the Ni(l 11) substrate is shown in Fig. 2. It illustrates different chemical terminations of the oxide film and possible

3-fold hollow sites

Ni 0 NI

0 0

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