Reductive Desorption of Chlorine Adlattice

The oxidatively adsorbed adlayer of chlorine may be partially reduced to chloride at negative potentials, as shown by LEED and atomic emission spectroscopy (AES) studies [6]. The voltammetric signature associated with this process on Cu(lll) was recently reported [15]. Desorption begins at potentials more positive than that associated with heteroepitaxial nickel deposition and the structural ramifications of this reaction are of some importance to the growth of multilayered materials. Consequently, the evolution of this reaction on Cu(100) was examined in 10 mmol/1 HC1. A series of topographic images were collected while the potential was repeatedly stepped between -0.250 V and -0.650 V. As shown in Fig. 4, rapid movement of the surface steps along with the development of significant curvature is apparent immediately upon setting the potential to -0.650 V. Specifically, the steps migrate away from the <100> orientation and are somewhat frizzy.

Numerous attempts were made to obtain atomically resolved images of the terraces without success. The experiment was performed using both a constant imaging bias as well as a fixed tip potential with no apparent effect on the images obtained. This sequence of images demonstrates that partial disruption and/or desorption of the chloride adlattice at negative potentials leads to rapid movement and rearrangement of the highly kinked <100> metal steps. In UHV, surface steps on clean Cu(100) are aligned in the close-packed <110> direction [9]; thus, in the first approximation, the surface steps should relax towards this orientation upon desorption of chlorine. The frizziness of the steps may be

Fig. 4. A continous sequence of 50 nm x 50 nm images revealing the effects of stepping the potential between -0.25 and -0.65 V vs. Cu/Cu+ in 10 mmol/1 HC1 with £,ip = 0.050 V, 7t = 9 JiA and frame time of ~22 s The black horizontal line in images (d), (i), (p) and (t) correspond to the instant when the potential was stepped between the two values. A (V2 x V2)R45° chlorine adlattice is stable at -0.25 V whereas the adlattice is reductively disrupted at -0.65 V. For images (a) - (d) E =-0.25 V, (d) -(i) £=-0.65 V, (i) - (p) £=-0.25 V, (p) - (t) £—0.65 V and (t) - (x) £=-0.25 V. The rapid movement and development of step curvature in images (d) - (i) and (p) - (t) corresponds to the reductive disruption and desorption of the chloride adlattice. The images, which take -18 s to collect were obtained over a period of 8 min this reveals the remarkable effects of surface rearrangement associated with the adsorption/desorption process.

Fig. 4. A continous sequence of 50 nm x 50 nm images revealing the effects of stepping the potential between -0.25 and -0.65 V vs. Cu/Cu+ in 10 mmol/1 HC1 with £,ip = 0.050 V, 7t = 9 JiA and frame time of ~22 s The black horizontal line in images (d), (i), (p) and (t) correspond to the instant when the potential was stepped between the two values. A (V2 x V2)R45° chlorine adlattice is stable at -0.25 V whereas the adlattice is reductively disrupted at -0.65 V. For images (a) - (d) E =-0.25 V, (d) -(i) £=-0.65 V, (i) - (p) £=-0.25 V, (p) - (t) £—0.65 V and (t) - (x) £=-0.25 V. The rapid movement and development of step curvature in images (d) - (i) and (p) - (t) corresponds to the reductive disruption and desorption of the chloride adlattice. The images, which take -18 s to collect were obtained over a period of 8 min this reveals the remarkable effects of surface rearrangement associated with the adsorption/desorption process.

Fig. 4. A continous sequence of 50 nm x 50 nm images revealing the effects of stepping the potential between -0.25 and -0.65 V vs. Cu/Cu+ in 10 mmol/1 HC1 with £tip = 0.050 V, /t = 9 nA and frame time of -22 s The black horizontal line in images (d), (i), (p) and (t) correspond to the instant when the potential was stepped between the two values. A (V2 x "*/2)R45° chlorine adlattice is stable at -0.25 V whereas the adlattice is reductively disrupted at -0.65 V. For images (a) - (d) E =-0.25 V, (d) -(i) £=-0.65 V, (i) - (p) £=-0.25 V, (p) - (t) E— 0.65 V and (t) - (x) £—0.25 V. The rapid movement and development of step curvature in images (d) - (i) and (p) - (t) corresponds to the reductive disruption and desorption of the chloride adlattice. The images, which take ~18 s to collect were obtained over a period of 8 min this reveals the remarkable effects of surface rearrangement associated with the adsorption/desorption process.

Fig. 4. A continous sequence of 50 nm x 50 nm images revealing the effects of stepping the potential between -0.25 and -0.65 V vs. Cu/Cu+ in 10 mmol/1 HC1 with £tip = 0.050 V, /t = 9 nA and frame time of -22 s The black horizontal line in images (d), (i), (p) and (t) correspond to the instant when the potential was stepped between the two values. A (V2 x "*/2)R45° chlorine adlattice is stable at -0.25 V whereas the adlattice is reductively disrupted at -0.65 V. For images (a) - (d) E =-0.25 V, (d) -(i) £=-0.65 V, (i) - (p) £=-0.25 V, (p) - (t) E— 0.65 V and (t) - (x) £—0.25 V. The rapid movement and development of step curvature in images (d) - (i) and (p) - (t) corresponds to the reductive disruption and desorption of the chloride adlattice. The images, which take ~18 s to collect were obtained over a period of 8 min this reveals the remarkable effects of surface rearrangement associated with the adsorption/desorption process.

ascribed to the rapid movement of kinks and the curvature may correspond to higher activity of copper adatoms on the terraces. This explanation is congruent with the observation of thermally induced frizzy steps on Cu(100) in UHV [9]. When the potential is stepped back to -0.250 V an ordered chlorine adlattice is rapidly re-formed and the surface steps quickly move to adopt the anticipated <100> orientation. High-resolution images confirmed the (V2 x V2)R45° chlorine adlattice structure. Simultaneously, the nucleation of small islands on the terraces occurs, presumably due to a quenching of the elevated adatom density associated with the increased step curvature at negative potentials. The sequences of images in Fig. 4(i) - (o), and (t) - (x), reveal the reordering and coarsening of step structure.

An additional experiment was performed to investigate the role of chloride in the kinetics of the reordering and coarsening process. The (a/2 x V2)R45° chlorine adlayer was initially formed at -0.250 V and allowed to coarsen for at least one hour. The chloride solution was then replaced with 10 mmol/1 H2SO4 and the potential was stepped to -0.650 V. The surface steps became disordered with the reductive desorption of chlorine. However, when the potential was stepped back to -0.250 V the surface took a much longer time to reorder compared to the experiments performed in 10mmol/l HC1. This reflects the sharply diminished chloride concentration in the electrolyte. Nevertheless, the ordering process evolves, albeit more slowly, due to residual chloride present as contamination from the exchange of electrolytes.

0 0

Post a comment