1. Introduction
Chiral surfaces of solid materials could serve as a chiral catalyst, which plays an indispensable role in enantioselective processes of the pharmaceutical industry. An electrochemical study of chiral surface formation was reported on chiral electrodeposition of CuO by Switzer et al. [
1]. Magnetoelectrodeposition (MED; electrodeposition under magnetic fields) has potential to produce chiral surfaces on metal films [
2,
3,
4,
5,
6,
7]. When a magnetic field is imposed perpendicularly to a working electrode surface, two types of magnetohydrodynamic (MHD) flows are excited around the electrode [
8,
9]. One is a macroscopic vertical MHD flow near the electrode edge, the other is micro-MHD vortices around local bumps on the deposit surface, as shown in
Figure 1a. A lot of screw dislocations exist on the surfaces of copper electrodeposit films [
10]. The micro-MHD vortices affect the formation of right- and left-handed screw dislocations, leading to the surface chirality [
9].
In the micro-MHD effect, both clockwise and anticlockwise vortices must coexist on the electrode surface such that the adjoining flows never conflict with each other. Thus, the self-organized micro-MHD state is symmetric. Aogaki and Morimoto proposed two mechanisms to break the symmetry in the micro-MHD vortices [
9]. The first one is the influence of vertical MHD flow on the micro-MHD vortices. When the micro-MHD vortices are excited under the vertical MHD flow, the cyclonic micro-MHD vortices would be stable, and the anti-cyclonic vortices would become unstable [
11,
12], resulting in the different relative amounts of right- and left-handed chiral sites on the deposit surfaces. Our previous paper reported the validity of this mechanism by means of the MED with a tube wall around the electrode, where the surface chirality disappeared with suppression of vertical MHD flows [
13]. Another mechanism is a system rotation, which is the rotation of an electrolytic cell in magnetic fields. This is termed as rotational magnetoelectrodeposition (RMED) [
14]. The system rotation causes the precession of micro-MHD vortices through the Coriolis force (
Figure 1b), and induces asymmetric effects on the clockwise and anticlockwise vortices, leading to the surface chirality. The surface chirality is expected to depend on the combination of magnetic field strength and rotational frequency, and thus RMED can be expected to lead to a novel technique to control the surface chirality of metal films. In this study, we have conducted RMED of Cu films with various magnetic fields and rotational frequencies, and examined the surface chirality of such RMED films. Here we show the unique chiral behaviors of RMED films.
3. Results and Discussion
Figure 3 shows the voltammograms on {2T, 4Hz} film electrodes, which represent the RMED films prepared at 2 T and 4 Hz. The current increase around 0.6–0.7 V in all voltammograms corresponds to the oxidation currents of alanine molecule [
15], and the current difference between
l- and
d-alanines represents chiral behavior. In
Figure 3a, the currents of
d-alanine are greater than those of
l-alanine, thus the {+2T, CW4Hz} film exhibits
d-activity. Similarly, the {+2T, ACW4Hz} film exhibits
d-activity (
Figure 3b). On the contrary, the films of {−2T, CW4Hz} and {−2T, ACW4Hz} exhibit
l-activity (
Figure 3c,d). These results represent that the chiral signs of {2T, 4Hz} films depend on the magnetic field polarity.
On the other hand,
Figure 4 shows the results of {4T, 2Hz} films, where the {+4T, CW2Hz} and {−4T, CW2Hz} films exhibit
l-activity (
Figure 4a,c), and the {+4T, ACW2Hz} and {−4T, ACW2Hz} films exhibit
d-activity (
Figure 4b,d). These results represent that the chiral sign of {4T, 2Hz} films depend on the rotational direction. This means that the cell rotation allows control of the surface chirality.
Figure 5 shows the chiral behaviors of the RMED films prepared in various conditions of magnetic fields and rotational frequencies, where each table contains the chiral sign (L or D) and the
ee ratios (in parenthesis) corresponding to the magnetic field polarities and the rotational directions. These tables are divided to four types I–IV. Type I exhibits the chiral signs depending on the magnetic field polarity, as shown in
Figure 3. This indicates that the effect of vertical MHD flow is dominant in this type. The vertical MHD flow could make cyclonic micro-MHD vortices stable, and such stable micro-vortices contribute to the formation of chiral dislocations. The vertical MHD flow can be excited at the entrance of tube, then it could penetrate into the tube and arrive at the electrode surface [
13]. The direction of vertical MHD flow depends on the magnetic field polarity, thus the chiral behavior of type I is responsible for the effect of vertical MHD flow.
On the contrary, type II exhibits chiral signs depending on the rotational direction, as shown in
Figure 4. This represents that the rotational effect is dominant at this condition. The cell rotation causes the precession of micro-MHD vortices, and the precession motion should be asymmetric between the CW and ACW vortices (
Figure 1b), leading to the unbalance between right- and left-handed dislocations. Such a chiral pattern was observed at only one condition of {4T, 2Hz}.
Type III exhibits the chiral patterns of 3L+D and L+3D, indicating that both effects of vertical MHD flow and rotation are superimposed. When the vertical MHD flows survive in RMED processes, their effects could superimpose on the rotational effects. Such RMED conditions are most probable, and the six patterns are seen in
Figure 5. More detailed patterns (3L+D or L+3D) in type III would depend on the magnetic field and the rotational frequency, but it is not easy to find a rule in the chiral patterns.
On the other hand, type IV surprisingly exhibits only L-activity in the all four conditions in each table. This suggests the chiral symmetry breaking in certain conditions of RMED.
The types of chiral patterns in
Figure 5. are plotted in the map with coordinates of magnetic field versus rotational frequency, as shown in
Figure 6. The largest area in the middle of map is occupied by type III, which spreads from {low magnetic fields, low frequencies} to {high magnetic fields, high frequencies}. This is the most likely consequence. When the vertical MHD flows could penetrate into the tube, both the cell rotation and the vertical MHD flow could make considerable influence on the micro-MHD vortices. On the other hand, type I occupies the {low magnetic fields, high frequencies} area. The appearance of type I indicates that the cell rotation makes no influence on the micro-MHD vortices, as a result, the surface chirality is determined by the vertical MHD flows. This result suggests that the rotational frequency is too high to make interference on the micro-MHD vortices excited in this magnetic field area.
From the above consideration, it can be expected that type II occupies the {high magnetic fields, low frequencies} area. Actually, it is seen at {4T, 2Hz}. However, type IV surrounds type II and occupies most of this area. The appearance of type II indicates that the cell rotation makes strong interference on the micro-MHD vortices, as a result, it could determine the surface chirality.
Type IV exhibits only L-activity and this represents chiral symmetry breaking. A similar symmetry breaking was reported in the special conditions of MED. When the vertical MHD flow is active, the chiral sign should represent the odd dependence of magnetic field polarity, this can be written by
The breaking of such odd chirality was observed in the Cu MED with chloride additives [
16,
17], where the specific adsorption of chloride ions on the film surfaces causes the fluctuation in the self-organized state of micro-MHD vortices. The odd chirality breaking was also observed in the Cu MED at weak magnetic fields [
18], where the self-organized rigid state of micro-MHD vortices cannot be well developed. These facts imply that the large fluctuation in the micro-MHD vortices causes the breaking of odd chirality.
As for type IV in RMED, if the effect of cell rotation is too strong, the precession of micro-MHD vortices could be incoherent and cause the fluctuation in the self-organized state of micro-MHD vortices. Such large fluctuation could induce the breaking of odd chirality in the RMED films for not only the magnetic field polarity but also the rotational direction. This leads to type IV.
The breaking of odd chirality was observed in several cases such as the specific adsorption of chloride ions, weak magnetic fields, and the cell rotation. The common situation is the large fluctuation of micro-MHD vortices, which includes the spacial, size, and coherence fluctuation.
As for the
ee ratios in
Figure 5, it should be noted that some absolute values of
ee ratios are larger than 0.33. This means that the peak current of an active enantiomer is two times greater than that of another one. Such large |
ee| ratios are surprising results in comparison with those in MED with only vertical MHD flows (|
ee| < 0.1) [
16]. Thus, the RMED technique could be applicable to the synthetic methods of chiral catalysts.