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2007, 111, 14290-14292 Published on Web 09/11/2007
Detection of Surface Chirality by Electrolyte Electroreflectance Rotational Anisotropy A Ä ngel Cuesta* and Carlos Borra´ s† Instituto de Quı´mica Fı´sica “Rocasolano”, CSIC, C. Serrano 119, E-28006 Madrid, Spain ReceiVed: July 26, 2007; In Final Form: August 28, 2007
The ability of electrolyte electroreflectance rotational anisotropy (EERRA) to detect surface chirality is demonstrated by measuring the EERRA of Au(11 7 5)L and Au(11 7 5)D. Chiral surfaces do not contain any symmetry planes, and, hence, in their case χxy * χyx. Accordingly, the EERRA patterns, like the surfaces they originate from, do not contain any symmetry planes, and are, hence, chiral, the pattern corresponding to Au(11 7 5)L being the mirror image of the pattern corresponding to Au(11 7 5)D.
Introduction Surface chirality plays an important role in asymmetric chromatography,1 anisotropic crystal growth,2,3 chiral sensors,4-6 and asymmetric heterogeneous catalysis7-9 (the first heterogeneous enantioselective process was described in 197910). Twodimensional chirality can be achieved in two ways: adsorbing chiral molecules on a solid, nonchiral surface11-14 or with singlecrystal surfaces with an intrinsically chiral crystallographic orientation.15-21 Single-crystal surfaces of metals crystallizing in the fcc system are chiral if they contain terraces separated by monatomic kinked steps.16 In the case of metals crystallizing in the bcc system, even some surfaces composed of terraces separated by straight monatomic steps are chiral.22 The chirality of fcc metal surfaces with kinked steps has been indirectly confirmed from differences in the electrochemical response16,17 or the temperature-programmed desorption of chiral reactants.15,23 Obviously, the chirality of a surface can be detected using methods for structural determination, but these methods either require ultrahigh vacuum (UHV), as in lowenergy electron diffraction (LEED), or are local probes, like scanning probe microscopy. The measurement of circular dichroism in the photoelectron emission from a chiral surface using circularly polarized synchrotron radiation has been attempted,24 but it could not be demonstrated that the asymmetry observed in the photoemission spectra was due, at least partly, to the circular dichroism (in addition, the method is not very practical because it requires both UHV and synchrotron radiation). Optical methods could be an ideal tool for the detection of surface chirality because they can be used in situ with conventional light sources, and, furthermore, they do not require UHV. A recent example of the application of optical techniques is the detection of the chirality of gold single-crystal kinked surfaces by the rotational anisotropy of second-harmonic generation (SHG).25 Electroreflectance, a physical phenomenon consisting of the variation of the reflectance of an interface due to the application of an electric field, was first reported for semiconductors in * Corresponding author. E-mail:
[email protected]. † Permanent address: Departamento de Quı´mica, Universidad Simo ´n Bolı´var, Caracas, Venezuela.
10.1021/jp075895h CCC: $37.00
1965,26 and 1 year later for metals.27 The very small penetration depth of electric fields in metals (Thomas-Fermi length, ca. 0.05 nm for a typical metal) makes electroreflectance an extremely sensitive technique for the study of surface phenomena on metals. The measurement of electroreflectance was simplified enormously with the introduction by Cardona and co-workers of electrolyte electroreflectance (EER)28 in which the sample, immersed in an electrolyte, is the working electrode of an electrochemical cell, in order to take advantage of the huge electric fields inherent to electrode-electrolyte interfaces. Electrolyte electroreflectance rotational anisotropy (EERRA) was observed for the first time for Ag(1 1 0) by Furtak and Lynch,29,30 and has also been observed for the (1 1 0) faces of Cu31,32 and Au.32-34 Figure 1 shows the EERRA of Au(1 1 0), at λ ) 500 nm. As can be seen, when the electric field of the incident light oscillates parallel to the [1 0 0] direction (perpendicular to the troughs and the close-packed atomic rows) the intensity of the EER signal is nearly twice that found when the electric field is parallel to the [1 1 0] direction (parallel to the close-packed atomic rows), thus reflecting the surface anisotropy, with different interatomic distances along the two main crystallographic directions. Alternatively, (1 0 0) and (1 1 1) faces, with the same interatomic distances along the main crystallographic directions, show no EERRA.33 Since the optical behavior of a surface is given by its dielectric tensor (or equivalently, its first-order polarizability tensor, χ, because ) 0(1 + χ)), (1 0 0) and (1 1 1) surfaces can be considered as uniaxial, whereas (1 1 0) surfaces are biaxial, the components of χ having different values along the [1 1 0] and the [1 0 0] directions. Only the (1 0 0) and (1 1 1) facets of an fcc crystal are atomically flat, any other surface being either stepped or kinked.22 The (1 1 0) face is the surface with (1 1 1)-oriented terraces separated by (1 1 1)-oriented monatomic steps with the shortest possible terrace, the presence of steps being the reason that (1 1 0) surfaces of the fcc system are biaxial. Therefore, any stepped surface of an fcc metal will show EERRA, as demonstrated by Ko¨tz and Lewerenz.35 Similarly, kinked surfaces can also be expected to show EERRA, but, in this case, if the EERRA pattern reflects the surface symmetry as it does in the case of stepped surfaces (see Figure 1), then it will lack any symmetry planes, i.e., it will reflect the chirality © 2007 American Chemical Society
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J. Phys. Chem. C, Vol. 111, No. 39, 2007 14291
Figure 1. EERRA at λ ) 500 nm of Au(1 1 0), measured at 0.14 V vs SHE in 0.1 M H2SO4.
of kinked fcc surfaces. Here we demonstrate chiral recognition using EERRA for Au(11 7 5)D and Au(11 7 5)L. (We use here the notation proposed by Pratt et al. in ref 22. Please note that, as noted by Pratt et al., for fcc surfaces the D and L labels match the R and S labels, respectively, of the Attard convention17). Experimental Section The experiments have been performed in a homemade spectrometer. Light from a tungsten lamp went through a monochromator and was reflected from the electrode surface at an angle of incidence of 45°. Polaroid films were used to select s-polarized light (perpendicular to the plane of incidence, i.e., parallel to the surface) before entering the photomultiplier tube used as a detector. The electrode was rotated using a computer-controlled step motor engaged to it. A sine wave (∆V ) 50-200 mVrms, f ) 63 Hz) was superimposed on the electrode potential, which was controlled by a potentiostat, and the corresponding changes in the surface reflectance were recorded using a lock-in amplifier. The single-crystal electrodes (disks 2 mm thick and 10 mm in diameter, MaTecK) have a mark on their side indicating the [3 2 1] direction (see Figure 2), with respect to which the rotation angle was measured, and were flame-annealed before every experiment to clean the surface and restore surface order. The polar graphs in Figures 1 and 3 show the intensity of the EER signal (normalized reflectance variation for a potential modulation of 1 Vrms) as a function of the rotation angle. The vertical scales indicate the intensity of the signal at a given distance from the center of the polar graph. Please note that the value of the EER signal at the center of the graph is not zero. A platinum wire was used as a counter electrode, and a reversible hydrogen electrode (RHE) was used as a reference, although all of the potentials in the text are referred to the standard hydrogen electrode (SHE). Results and Discussion Figure 2 shows an stereogram projected on the (1 1 1) face of an fcc crystal and ball models corresponding to the (11 7 5)L and (11 7 5)D surfaces, whose position has been included in the stereographic projection. All of the surfaces lying within any of the stereographic triangles limited by the solid lines in the stereographic projection are kinked and share the same chirality, whereas the chiralities of neighboring triangles sharing an edge are opposite.22 Surfaces lying on the dotted lines bisecting the innermost stereographic triangles (similar lines could be drawn in all of the other triangles, although they have not been included in the figure) have kinks composed of steps of the same length at 120°, whereas surfaces on any side of the
Figure 2. Stereogram projected on the (1 1 1) face of an fcc crystal and ball models corresponding to the (11 7 5)L and (11 7 5)D surfaces. The position of the (11 7 5) planes has been indicated in the six innermost stereographic triangles of the stereographic projection.
Figure 3. EERRA at λ ) 500 nm of Au(11 7 5)D (left) and Au(11 7 5)L (right), measured at 0.14 V vs SHE in 0.1 M H2SO4.
dotted lines have kinks composed of steps with different lengths. Au(11 7 5) surfaces have kinks with (1 1 1)- and (1 0 0)-oriented monatomic steps, the latter being twice as long as the former, separating terraces approximately 2.5 atomic diameters wide. The kinks are parallel to the [3 2 1] direction. Figure 3 shows the EERRA of Au(11 7 5)D (left) and Au(11 7 5)L (right). The patterns, like the surfaces they originate from, do not contain any symmetry planes, and are, hence, chiral. Accordingly, the pattern corresponding to Au(11 7 5)L is the mirror image of the pattern corresponding to Au(11 7 5)D (the small difference in the lobe near the horizontal axis most likely being due to small differences in the exact orientation of each crystal), thus demonstrating the ability of EERRA to detect surface chirality. Interestingly, the angles between the lobes in the anisotropy patterns are ca. 120°, which corresponds to the angle between the two steps forming the kink.
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The asymmetric EERRA pattern of the chiral surfaces must originate from an asymmetric first-order surface susceptibility tensor, χ, which is a rank-2 tensor with nine elements. The firstorder response of a surface to incident light will be given by its first-order polarization, which, taking into account that we are using s-polarized light, with the electric field of the radiation parallel to the electrode surface, can be described for our twodimensional system by eq 1
( ) (
)( )
Px χxx χxy Ex Py ) 0 χyx χyy Ey
(1)
According to Neumann’s principle, the susceptibility tensors must remain invariant under all coordinate transformations reflecting symmetry operations characteristic of the system.36 Nonchiral surfaces contain a symmetry plane, and hence χxy ) χyx, and a coordinate system can be found such that χxy ) χyx ) 0. If, in addition, χxx ) χyy, then the surface shows no EERRA, as in the case of Au(1 1 1) and Au(1 0 0), whereas if χxx * χyy, as in the case of Au(1 1 0), then the surface will show a nonchiral EERRA (see Figure 1). Chiral surfaces contain no symmetry planes and, hence, in this case χxy * χyx, yielding a chiral EERRA pattern, as shown in Figure 3 for the two enantiomers of the Au(11 7 5) surface. Conclusions We have demonstrated the ability of EERRA to detect surface chirality. Like the rotational anisotropy of SHG, EERRA has the advantage of being, in the case of metal surfaces, an extremely surface-sensitive optical technique, which does not need UHV. An advantage of EERRA over SHG is that highpower pulsed lasers and a complex setup are not required, this making EERRA much simpler and cheaper. A disadvantage of EERRA, as compared to SHG, is that it can be used only with conductors or semiconductors, and that only the coinage metals (Cu, Ag, and Au) show an appreciable electroreflectance (the electroreflectance spectra of the rest of the metals being of low intensity and structureless), whereas SHG can be generated by any interface. Acknowledgment. Funding from the Spanish Ministry of Education and Science through Project CTQ2006-02109 is gratefully acknowledged. We thank Prof. C. Gutie´rrez for fruitful discussions, and Prof. A.U.A. Acun˜a for his kind lending of Polaroid films.
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