Elsevier

Electrochimica Acta

Volume 53, Issue 1, 20 November 2007, Pages 155-160
Electrochimica Acta

Enantiospecific electrodeposition of chiral CuO films on Cu(1 1 0) from aqueous Cu(II) tartrate and amino acid complexes

https://doi.org/10.1016/j.electacta.2007.01.040Get rights and content

Abstract

CuO films were anodically electrodeposited onto Cu(1 1 0) single crystals from alkaline solutions of Cu(II) complexed with tartrate, alanine, valine, and glycine. In all cases the films were epitaxial, exhibiting varying amounts of the chiral orientations (1 1 0) and (1¯1¯0). The enantiomeric excess (ee) of the chiral electrodeposition process was determined by X-ray pole figure analysis. The CuO films deposited from d-tartrate solution exhibited a (1 1 0) out-of-plane orientation with an ee of 61%, while films grown from l-tartrate exhibited a (1¯1¯0) orientation with an ee of 77%. The CuO film deposited from achiral meso-tartrate had essentially equal amounts of both chiral orientations. Epitaxial CuO films deposited from d-alanine and d-valine both gave (1¯1¯0) orientation with ee's of 40 and 65%, respectively, while CuO films deposited from l-alanine and l-valine gave (1 1 0) orientations with ee's of 39 and 67%, respectively. Films obtained from a glycine solution showed roughly equal amounts of both chiral orientations.

Introduction

Chemists are introduced to chemical handedness in organic chemistry with a discussion of sp3 carbon atoms bonded to four different substituents. Many of the molecules that make up living things are homochiral, with amino acids and sugars being the common example. In addition, the pharmaceutical industry relies heavily on enantiomerically pure materials, because one enantiomer often exhibits the desired effect while the other enantiomer is inactive or deleterious. Crystallographically, chiral materials are described by one of the eleven enantiomorphic space group pairs due to the presence of a screw axis within the unit cell. Quartz is an example of a covalent network solid that exists in two enantiomerically distinct forms.

Even those materials that do not crystallize with a chiral space group can be made to exhibit chirality. It has been demonstrated that certain high index (e.g., 643) faces of achiral fcc metals will exhibit chirality due to the presence of kink sites on the surface [1], [2], [3], [4], [5]. These types of chiral surfaces have been shown to be enantioselective toward the adsorption of chiral molecules [4] as well as the enantiospecific electrochemical oxidation of glucose [2], [3], [4], [5]. Alternatively, the adsorption of chiral organic molecules on single crystal surfaces has been shown to break the surface symmetry of the achiral substrate [6]. It has also recently been shown that supramolecular asymmetric structures of achiral molecules adsorbed on achiral substrates can be obtained from octadecanol self-assembled monolayers on highly ordered pyrolytic graphite [7]. For practical applications it would be desirable to expand the production of chiral surfaces to materials other than high index single crystal surfaces, which are difficult to manufacture, and adsorbed organic monolayers, which are not particularly robust.

Our approach to generating chiral films is through the electrodeposition of metal oxides. The electrodeposition process operates through three basic mechanisms. The first is the redox change method in which the oxidation state of a soluble metal cation is changed through direct oxidation or reduction at the electrode surface to an oxidation state in which it is insoluble. Examples of electrodeposition of oxides via this route include Fe3O4 [8], [9], AgO [10], PbO2 [11], Tl2O3 [12], [13], [14], [15], Cu2O [16], [17], [18], [19], [20], Pb8Tl5O24 [21], and LaMnO3 [22]. A second mechanism involves the electrochemical generation of base near the electrode surface through the irreversible reduction of water, nitrate ions, or dissolved oxygen. This mechanism has been utilized for the electrodeposition of thin films of CeO2 [12], [23], ZrO2 [24], and ZnO [25], [26]. A third mechanism involves the irreversible oxidation of a ligand used to stabilize a metal cation as in the present case where a solution of Cu(II) tartrate is oxidized to form CuO on the working electrode surface [27], [28], [29], [30]. Many of these electrodeposited films are epitaxial, and films of CuO have been deposited with chiral orientations.

The anodic electrodeposition of CuO has been demonstrated previously from alkaline solutions of Cu(II) complexed with a variety of amino acids [27] and tartrate [28], [29], [30]. The probable templating mechanism for the deposition proceeds via oxidation of the chiral ligand leading to precipitation of the insoluble Cu(II) as the oxide. We do not, at present, have detailed information on the orientation of tartrate ion or Cu(II) complexes of tartrate adsorbed on the electrode surfaces. Recently, we have shown that monoclinic CuO films with chiral orientations can be electrodeposited from enantiomerically pure Cu(II) complexes with tartrate on single crystal Au(0 0 1) [28] and Cu(1 1 1) [29]. Here we extend this work to the electrodeposition of chiral CuO films on Cu(1 1 0) single crystals from alkaline solutions of copper complexes with tartrate and the amino acids alanine, valine and glycine.

The surfaces of CuO that are chiral can be determined from symmetry considerations [31]. Chiral crystal surfaces lack mirror or glide plane symmetry. CuO has a monoclinic structure (space group C2/c), with a = 0.4685 nm, b = 0.3430 nm, c = 0.5139 nm, and β = 99.08°. The unique twofold axis for CuO is the b axis, and the mirror plane is perpendicular to the b axis. Achiral orientations, therefore, correspond to those planes parallel with the b axis (planes of the [0 1 0] zone). Achiral planes are those with k = 0, such as (1 0 0), (1 0 1), (0 0 1), and (7 0 9), and in the general case (h 0 l). Remaining planes with k  0, such as (0 1 0), (1 1 1), (0 1 1), and (0 9 0) are all chiral.

Section snippets

Electrochemical experiments

Electrochemical Experiments were carried out using an EG&G Princeton Applied Research (PAR) model 273A potentiostat/galvanostat. The cell consisted of a platinum counter electrode and a standard calomel reference electrode (SCE). Cu(1 1 0) single crystals (diameter 10 mm, thickness 2 mm) were purchased from Monocrystals Company (Medina, OH). A Cu wire fitted around the single crystal electrode served as an electrical contact to the working electrode and the working electrode was placed into the

Deposition from tartrate

Fig. 1 shows θ/2θ XRD scans for three films grown from Cu(II)-tartrate solutions on Cu(1 1 0). Fig. 1A shows the scan for the film obtained from the d-tartrate solution, Fig. 1B shows the scan obtained from l-tartrate, and Fig. 1C shows the scan obtained from meso-tartrate. Only the Cu(2 2 0) substrate peak is observed in each case. The θ/2θ patterns for the three films are essentially identical, with peaks observed at 32.4 and 67.9° 2θ. For each of the aforementioned films correct indexing of the

Conclusions

An interesting observation when comparing the orientations of films grown from tartrate versus amino acids is that the opposite enantiomorph is obtained when CuO films are grown from d-tartrate and d-amino acid and when the films are grown from l-tartrate and l-amino acid. The use of the d and l notation is an arbitrary assignment of configuration. Assignment of configuration for amino acids is determined by examining the Fischer projection with the carboxyl group on the top and the R group on

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