Carboxylated viologens mixed with the bromic acid (0.1 mM)

The counts-anions of carboxylated viologens are bromide anions ([V − (C − COOH) ] − 2Br ) rather than chloride anions. Therefore, the chloride anion layer will be replaced by the bromide anion layer with increasing the viologen concentration on the Cu(100) surface. According to section 4.4, viologen molecules directly absorb on the Br/Cu(100)-c(2×2) surface to form the stripe pattern without crossing the middle phase transition of the metastable phases. In order to clearly confirm different influence of the chloride and the bromide anion layers, an experiment of the viologens mixed with the bromic acid (0.1 mM) on the Cu(100) surface is carried out. The 0.1 mM concentration of the viologen solution is prepared by viologens mixed with a 10 mM bromic acid, and hence all solutions are no chloride anions. The anion layer will thus be constructed only by bromide anions. The serial STM images of the experiment dependent on the potential evolution are shown in Figure 4-8. Before the potential reached -125 mV, the dot array is kept on the Br/Cu(100)-c(2×2) surface and then the metastable phases also are not found as shown in Figure 4-8(a) and (b). At -140 mV of Figure 4-8(c) and (d), viologen molecules directly absorb on the surface and then immediately form the stripe pattern.

At more negative potential -218 mV, the stripe pattern transfers to the closed stacking stripe pattern as shown in Figure 4-8(e). At the potential where the HER occurs (i.e.

-420 mV), the stripe pattern can still be observed on the surface as indicated by the black arrows of Figure 4-8(f). Here, the bromide anion layer plays an important role to make the stripe pattern stable on the surface at this potential because the anion layer of the bromide desorbs only at more negative potential then that of chloride as indicated by corresponding onset potentials of the HER with the pure anion layers absorbed.^145,147 The onset potentials for the chloride and bromide anions desorption from the Cu(100) surface are in the vicinity of -350 and -530 mV versus the RHE, respectively. However, the strong HER at more negative potentials than -530 mV makes the surface structure become very disordered, but even at -530 mV the stripe pattern remains to be found as shown by the white arrow of Figure 4-8(g). Until -600 mV, the dimer phase also cannot be observed on the surface. From section 3.10, it is known that the dimer phase is observed only on the pure Cu(100)-1×1 surface. The bromide anion layer can stay on the pure Cu(100)-1×1 surface at very negative potential. As the bromide anion layer desorbs from the surface, the HER becomes violent and makes a hard situation for viologen molecules to absorb on the pure Cu(100)-1×1 surface to form the dimer phase. According to the lack of the metastable phases, a dimer phase, and the chloride desorption, the bromide anion layer is confirmed on the Cu(100) surface at a high viologen concentration.

The anion layer plays a crucial role to influence the formation of the metastable phases by comparison of the chloride and the bromide anion layers. Such effect can be explained by the degree of polarizability of the adsorbed Br and Cl on the Cu(100) surface according to corresponding electronegativity values. The larger polarizability of the adsorbed Br has a greater charge transfer from the Br to the copper than the -Cl- in a constant coverage condition such as the same surface structure c(2×2) within the monolayer feature on the Cu(100) surface.¹⁶⁵ According to the relation of the polarizability proportional to the electric field. The diagrams are built in Figure 4-9(a) and (b). Because the bromide anion layer (a) has greater polarizability, the charge distribution is near the Cu(100) surface compared to the chloride anion layer (b). Therefore, the more ionic character of chloride anions under the electrochemical conditions results in more electrostatic attraction to overcome the intermolecular repulsion between the dications of the viologens, and then the bilayer stacking is formed at the dot array phase (c). Because the bilayer stacking is formed at the dot array, the surface molecule density is enough to make the dot array phase (the lower surface molecule density) transfer to the metastable phases (the higher surface molecule density). Otherwise, phase transition from the dot array phase to the stripe pattern does not cross the metastable phases and then the stripe pattern is formed by directly absorbing on the surface, liked the Br/Cu(100)-c(2 × 2) case.

Figure 4-8. Series of STM images of carboxylated viologens in mixed hydrobromic acid (0.1 mM) on the Cu(100) surface dependent on the potential evolution. (a)-(h):

Ubias = 320 mV, It = 0.2 nA, and 57.62×57.62 nm². The long black arrows indicate the potential evolution. The corresponding potential is marked in the individual image.

4.6 Summary

In summary, the two topics of the bilayer formation and the anion layer effect are discussing in chapter 4. By the experiments with the dilute viologen solution on the Cu(100) surface and 0.1 mM viologens on HOPG, the bilayer feature is confirmed and the initial absorption step of the bilayer formation is observed between the dot array and the metastable phases. The bilayer formation of the stripe pattern remains stable until transferring to the monolayer of the closed stacking stripe pattern on the Cl/Cu(100) surface or the HER on HOPG. Because the stripe pattern is a bilayer formation, the closed stacking stripe pattern only can be found at more negative potential due to reduce the surface molecules mobility. Otherwise, only the stripe pattern exists similar to DBV^ without the bilayer formation on the Cu(100) surface at low viologen concentration (0.1 mM).

It is known that the counter anions of dicarboxylated viologens are bromide anions. In order to avoid confusing the chloride with the bromide anion layers on the Cu(100) surface, the experiments with higher viologen concentration (1.0 mM) and a pure bromide solution without any chlorine anions on the Cu(100) surfaces are carried out.

In these cases the lack of the metastable phases, a dimer phase, and the chloride desorption is attributed to the bromine anion layer on the Cu(100) surface. Here, the interaction strength between the anions and the substrate plays an important role to determine which anions construct the anion layer rather than the concentration of the bromide/chloride anions at this case (10 mM chloride anions: 1.0 mM bromide anions). Therefore, bromide anions will replace chloride anions to form the bromide

Cl anion case Figure 4-9 The diagrams of (a) and (b) are the bromide and chloride anion layers on the Cu(100) surfaces. (c) is the bilayer stacking.

anion layer on the Cu(100) surface as shown in the section 4.4 of “High concentration carboxylated viologen (1 mM) on the Cu(100) surface”. However, for the case (10 mM chloride anions: 0.1 mM bromide anions), the concentration of the

bromide/chloride anions is the major factor according to the Fick's First Law. The driving force from bulk space to the surface is proportional to the concentration gradient (∂ Co/∂xi) and hence the anion layer is contributed by chloride anions as shown in chapter 3.

The more ionic character of the chloride anion layer which shows that the new dot array layer is able to absorb on top of the original dot array layer compared to the bromide anion layer. Therefore, the high surface molecules density makes the metastable phases formed from the dot array. The bilayer formation is due to the intermolecular interactions of the hydrogen bonding of N-H according to the fact of the multilayer growth behavior and the bilayer formation on HOPG.

Chapter 5. Conclusion

The redox chemistry of dicarboxylated viologens is studied by electronchemical scanning tunneling microscopy combination with cyclic voltammetry focusing on the variety of phase transitions accompanied with the reduction of the dicationic viologens reducing to the radical viologens. A 0.1 mM dicarboxylated viologens mixed with 10 mM HCl on the Cu(100) surface show major six phases in the sequence of a dot array (and oblique phase), the metastable phases (type A, B, and C), a stripe pattern (four types), the closed stacking stripe pattern, a chloride desorption, and the dimer phase dependent on the working potential between the copper dissolution and the HER. The dot array and the oblique row phase are found for the dicationic state and the other phases are observed for the radical state of the viologens.

In the dicationic state, viologen molecules prefer a face-on configuration on the surface due to electrostatic attention between the molecules and the anion layer. For the radical state, the enhanced π − π stacking interaction between the bipyridinium cores results in the π − π stacking formation of a stripe pattern, the closed stacking stripe pattern, and a dimer phase. The best explanation for the surface rearrangements of the viologens is due to the surface space limitations and intermolecular interactions.

The bilayer feature is confirmed in the potential range between the onset of phase transition from the dot array to the metastable phases and the stripe pattern. The complex stripe phases of the stripe pattern (four kinds) and the closed stacking stripe pattern are due to long alkyl chains stacking in limit surface space.

As the viologen concentration increases to 1.0 mV, the bromide anion layer replaces the chloride anion layer on the Cu(100) surface. The high concentration results in the multilayer growth behavior of the stripe pattern in the layer-plus-island mode (Stranski-Krastanow mode) due to the closed stacking stripe pattern formed and occupied on the growth sites of the multilayer. The multilayer growth is observed for the first time by the STM due to the intermolecular interactions of the hydrogen bounding of N-H between the nitrogen atoms and carboxylic acid groups exiting.

Further, the anion layers can influence the observed metastable phases or not, even though the chloride and the bromide anion layer have the same c(2×2) surface structure on the Cu(100)-1×1 surface. For the chloride anion layer, phase transition from the dot array to the stripe pattern has to step over the metastable phases but not for the bromine anion layer. The reason is that the large ionic character of the chloride anion layer overcomes the intermolecular repulsion interaction between the dicationic the viologens which makes new dot array layer absorb on top of the original dot array layer to from the bilayer, which supported enough surface molecule density to transfer to the metastable phases. For the bromine anion layer case, phase transition from the dot array phase to the stripe pattern without crossing the metastable phases is due to

the insufficient surface molecule density on the surface. Finally, a “phase diagram” of

Table 5.1. List of six phases corresponding to three different viologen concentrations on the Cu(100) surface and 0.1 mM viologens on a HOPG. The hydrogen bonding of N-H between the nitrogen atom and carboxylic acid group is the intermolecular interaction to construct the phase network.