© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 12, MARCH 20, 1997
LETTERS Tuning the Orientation of Molecules at Solid-Liquid Interfaces Using an Electrochemical Potential Gregory He† and Zhi Xu* Department of Chemistry and the Center for Molecular Electronics, UniVersity of MissourisSt. Louis, 8001 Natural Bridge Road, St. Louis, Missouri 63121 ReceiVed: NoVember 22, 1996; In Final Form: January 8, 1997X
Electrochemical potential induced orientation changes of polar molecules at the quartz-CH3CN interface have been studied using sum-frequency generation spectroscopy. Our result demonstrates the dramatic tuning effect of the electrochemical potential on the orientation of molecules in the electrical double-layer region. For D289+ cations with a large dipole moment (∼14 D), the average molecular orientation is found to change from 90° to about 34° when the electrochemical potential is changed from the potential of zero charge to -0.9 V. From the orientation data, the electrical field experienced by molecules in the electrical doublelayer region and the effective thickness of the electrical double layer have been extracted: E ≈ 5 × 106 V/cm and d ≈ 20 Å, respectively.
1. Introduction The interaction of an electric field with polar molecules in solution has long been an interesting subject because of its important role in the understanding of thermodynamic processes in solutions.1-3 When an electric field is applied, polar molecules in solution will experience a torque due to the electrostatic interaction and this torque will align the molecules in the direction of the electric field.4 This is shown by Figure 1 in which θ is the angle between the electric field and the dipole moment. At the room temperature, however, the collision among the molecules in solution will tend to make the orientation distribution random. The overall change in the molecular orientation by the electric field is the result of the balance between the electrostatic interaction and the thermal randomization. From statistical thermodynamics, the angular distribution function of polar molecules in the direction of the * Corresponding author. † Current address: Argonne National Laboratory, Building 200, Room D-185, 9700 S. Cass Avenue, Argonne, IL 60439. X Abstract published in AdVance ACS Abstracts, March 15, 1997.
S1089-5647(96)03893-X CCC: $14.00
Figure 1. Tuning effect of the electrostatic field on an electric dipole. q is the electric charge of the dipole, and θ is the angle between the direction of the electric field and the direction of the dipole moment.
electric field can be expressed as2
P(θ) ) NeAcos θ sin θ
(1)
in which N ) A/(eA - e-A) is the normalization factor, A ) µE/kT, µ is the dipole moment, and E is the applied electric field. To have a significant change in the overall orientation population, the interaction energy µE should have the same order of magnitude as the thermal energy kT. For a molecule with a © 1997 American Chemical Society
2102 J. Phys. Chem. B, Vol. 101, No. 12, 1997
Figure 2. Schematic drawing of the excitation geometry and the structure of the optical-electrochemical cell. The electrolyte solution was 5.0 × 10-2 M tetrabutylammonium tetrafluoroborate ([CH3(CH2)3]4NBF4) in HPLC-grade acetonitrile. The reference electrode was a saturated Ag/AgCl electrode, and the counter electrode was a Pt wire.
large dipole moment, e.g., 10 D, an electric field of 1.2 × 106 V cm-1 is needed in order to have A ) 1. This strong field is almost impossible to obtain by applying an electrostatic potential across the solution. Although it is very difficult to obtain the electric field needed in solution, it is quite straightforward to create such a strong field at solid-liquid interfaces. This has been a common practice in electrochemistry by adding strong electrolytes to polar solvents.5-9 In polar solvents, strong electrolytes will dissociate into cations and anions. When an electric field is applied across an electrolyte solution, the cations will move toward the electrode surface that is negatively charged, and anions will move toward the electrode surface that is positively charged. This results in a dramatic increase in the local ionic concentration at the two electrode surfaces, i.e., the formation of the electrical double layer. The thickness of the electrical double layer is only about 50-100 Å,5-9 and the electric field in the double-layer region is in the order or 106 V cm-1 or higher because the major voltage drop takes place in the double-layer region.5-9 This field should be strong enough to create a significant change in the orientation distribution of polar molecules at solid-liquid interfaces. The difficulty lies in, obviously, how to detect the change in the orientation of molecules in the electrical double-layer region. Traditional optical spectroscopic methods including IR, UVvis, Raman, and fluorescence are not interface specific. They cannot differentiate the signal generated in the electrical doublelayer region (∼100 Å) from that generated in the bulk solution. Vacuum-based surface analysis methods such as AES, LEED, XPS, and EELS cannot be applied because of the physical nature of the liquid phase involved. Thus the question remains: to what extent can the orientation of molecules in the electrical double-layer region be tuned with an electrochemical potential? 2. Experimental Section Because of the physical nature of the liquid phase, sumfrequency generation (SFG) is most suitable for detecting the change of the molecular orientation in the electrical doublelayer region. Sum-frequency generation is a nonlinear optical process that creates a high-frequency laser light from two lowfrequency laser excitations.10,11 The physics involved requires that the medium in which the nonlinear optical conversion takes place does not have inversion symmetry.10,11 This makes sumfrequency generation inherently sensitive to the interface because for most solid-liquid systems, only the interface meets this requirement. Studies have shown that only molecules and atoms within ∼100 Å on both sides of the interface can contribute to the signal.12 Figure 2 shows the structure of our opticalelectrochemical cell and the laser excitation geometry. The two laser excitations at 1064 and 532 nm were obtained from a
Letters
Figure 3. Structure of D289+ cation and the direction of the long molecular axis z′.
frequency-doubled Nd:YAG laser (10 Hz). The SFG signal at 355 nm generated at the interface was collected by a PMT detector and analyzed by time-gating electronics. The details of the experimental apparatus can be found elsewhere.13 Second, an ideal electrode has to be identified for the observation of the effect of the electrochemical potential on the molecular orientation. The electrode should be optically flat, electrically conductive, and stable under the strong electric field in the electrical double-layer region. More importantly, the SFG signal from the substrate should be negligible. The latter requirement rules out most metal substrates because of the strong nonlinear optical response from free electrons at the metal surfaces. We selected optically flat, electrically conductive, and hydrogen-terminated Si(111) as our working electrode. The orientation of the Si(111) electrode was carefully aligned in the optical cell to make sure that both the surface normal and the [110] direction of the Si(111) electrode lie in the plane of incidence (within (0.5°), as shown in Figure 2. Our previous study has shown that the SFG signal from the Si(111) electrode can be canceled completely by employing this excitation geometry.13 As a result, only the SFG signal generated by interfacial molecules in the liquid side will be detected. In addition to the two requirements mentioned above, a proper molecular probe must be employed. The molecular probe should be stable in the electrical double-layer region. It should have a large dipole moment (∼10 D) so that the change in the orientation distribution will be significant. After many trials, we decided to use 4-(4-(diethylamino)styryl)-N-methylpyridinium iodide (D289) as our molecular probe. In a polar solvent such as CH3CN, D289 exists in the form of the D289+ cation and the I- anion. The D289+ cation has a large dipole moment (∼14 D)14 and is stable under reduction potentials up to -1.1 V.13 In our experiment, the reduction potential was always kept below -0.9 V. Figure 3 shows the structure of the D289+ cation. The molar absorptivity of D289+ at 532 nm, the wavelength of one of our laser excitations, is quite large, ) 13 500 M-1 cm-1. As a result, the SFG signal is resonantly enhanced.15,16 Typically, the SFG signal from D289+ cations at the interface is more than 100 times stronger than the SFG signal from the electrolyte and solvent molecules at the interface. 3. Results and Discussion Figure 4 shows the change of the p-polarized SFG signal as a function the electrochemical potential. The potential of zero charge (PZC) is used as the reference point (0.0 V).17 The concentration of D289 was 3.0 × 10-4 M. When the interface is excited by two p-polarized excitations, the p-polarized SFG SFG signal (IPPP ) increases dramatically with the applied potential. In contrast, when the interface is excited by two s-polarized SFG excitations, the p-polarized SFG signal (ISSP ) decreases slightly when the electrochemical potential changes from 0.0 to -0.9 V. The electric potential of the Si(111) electrode was controlled by a scanning potentiostat with a scanning rate of 5 mV s-1.
Letters
J. Phys. Chem. B, Vol. 101, No. 12, 1997 2103
Figure 4. Change of the p-polarized SFG responses as a function of the applied electrochemical potential for D289+ cations in the electrical double-layer region.
When a negative potential is applied to the Si(111) electrode, both the orientation and the density of D289+ cations at the interface will change. Consequently, the change in the magnitude of the SFG signal in Figure 4 is caused by the change in both the molecular orientation and the interfacial concentration SFG SFG /ISSP , that of D289+.18 It is the intensity ratio, F ) IPPP contains only the information on the molecular orientation.10,11,19 Previous studies indicate that the average molecular orientation is related to the intensity ratio by the following equation:11,19
F)
[
L2X 2 LX L2Z 2 cos R + sin RiK1 + K2 2 cos Ri cos Rt,SFG i 2 2 LY LY LY
]
2
(2)
In eq 2, |K1| ) 2(〈cos θ〉/〈cos3 θ〉 - 1)-1,20 θ is the angle between the long molecular axis of D289+ and the surface normal,20 Ri is the average angle of incidence of the two excitation beams,21 Rt is the average angle of refraction of the two excitation beams,21 LX, LY, and LZ are local field factors,22,23 and the factor K2 can be determined by an independent measurement.24 After the orientation factor 〈cos θ〉/〈cos3 θ〉 is determined from the experimentally measurable quantity F, the average molecular orientation can be extracted from the orientation distribution defined by eq 1. It is evident in Figure 4 that the intensity ratio F increases considerably with the electrochemical potential, indicating a significant change in the orientation distribution of D289+ cations at the interface. Figure 5 shows the change of the orientation factor, 〈cos θ〉/ 〈cos3 θ〉, as a function of the applied electrochemical potential. The experimental values (open circles) in Figure 5 are obtained by applying eq 2 to the data shown in Figure 4. The theoretical orientation factor is calculated using the following equation:25
〈cos θ〉
Figure 5. Change of the orientation factor 〈cos θ〉/〈cos3 θ〉 as a function of the electrochemical potential for D289+ cations in the electrical double-layer region. The experimental results are shown by open circles, and the theoretical calculation is shown by the solid line.
[
]
A2[A - tanh(A)] ) | theory 〈cos3 θ〉 A(A2 + 6) - 3(A2 + 2) tanh(A)
(3)
Note in eq 3 that A ) µE/kT is a function of the applied potential. Assuming that the electric field in the electrical double-layer region equals the applied voltage divided by the effective thickness of the electrical double layer, E ) V/d, eq 3 becomes
〈cos θ〉
(BV)2[BV - tanh(BV)] ) | (4) theory 〈cos3 θ〉 BV(B2V2 + 6) - 3(B2V2 + 2) tanh(BV) in which B ) µ/dkT should be independent of the applied
Figure 6. Change of the average molecular orientation 〈θ〉 as a function of the electrochemical potential for D289+ cations in the electrical double-layer region. The inset shows the direction of the surface normal (Z), the direction of the long molecular axis of D289+ (z′), the direction of the electric field (E), and the orientation angle (θ).
potential V.26 The solid curve in Figure 5 is a theoretical fit that is in excellent agreement with the experimental result. The fitting factor is B ≈ 6.5 V-1, from which the effective thickness of the electrical double layer for our electrochemical system can be obtained, d ≈ 20 Å. Therefore, when the applied potential reaches -0.9 V, the electric field in the double-layer region is about 5 × 106 V cm-1. After the orientation factor has been understood, the average orientation of D289+ cations at the interface under different electrochemical potential can be extracted. Figure 6 shows the result. At the PZC, the average population angle approaches 90°, corresponding to a random angular distribution.27 This is not surprising because D289+ cations at the interface do not experience any electric field at the PZC. When the electrochemical potential is changed from the PZC to -0.9 V, the average molecular orientation of D289+ changes from 90° to about 34°. This large tuning range, ∆θ ≈ 56°, indicates that the orientation of molecules at the solid-liquid interface can be controlled easily by applying an electrochemical potential. In conclusion, we have demonstrated that the orientation of polar molecules at solid-liquid interfaces is very sensitive to the applied electrochemical potential. The large tuning range (∆θ ≈ 56°) observed for D289+ cations agrees well with the prediction of the statistical thermodynamics model. Our results
2104 J. Phys. Chem. B, Vol. 101, No. 12, 1997 indicate that the electrochemical potential can be used as a powerful tool for the precision control of the orientation of polar molecules in the electrical double-layer region.
Letters law: ni sin Ri ) nt sin Rt. (22) The local field factors are defined by the following equations according to ref 23:
LY(Ri) ) (1 + rS) )
Acknowledgment. This work was supported by the Department of Energy (through the Center for Molecular Electronics at the University of MissourisSt. Louis), the Missouri Research Board, the Research Incentive Award from the University of MissourisSt. Louis, and the Monsanto Postdoctoral Fellowship. We also thank Dr. Wenyuan Lu at bioMe´rieux Vitek for many very useful discussions.
LZ(Ri) ) (1 + rP) )
LX(Ri) ) (1 - rP) )
References and Notes (1) Kielich, S. In Dielectric and Related Molecular Processes; Davies, M., et al., Eds., The Chemical Society: London, 1972; Vol. 1, Chapter 7. (2) Fredericq, E.; Houssier, C. Electric Dichroism and Electric Birefringence; Clarendon Press: Oxford, 1973. (3) Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; John Wiley & Sons: New York, 1954. (4) Sears, F. W.; Zemansky, M. W.; Young, H. D. UniVersity Physics, 7th ed.; Addison Wesley Publishing Co.: Reading, MA, 1987; pp 549550. (5) Bard, A. J.; Abrun˜a, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murry, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (6) Bard, A. J., Faulkner, L. R. Electrochemical Methods: Fundamental and Application; John Wiley & Sons, Inc.: New York, 1980. (7) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker, Inc.: New York, 1970. (8) Davies, C. W. Electrochemistry; George Newnes Limited: London, 1967. (9) Delahay, P. Double Layer and Electrode Kinetics; John Wiley & Sons, Inc.: New York, 1966. (10) Shen, Y. R. The Principle of Nonlinear Optics; Wiley: New York, 1985. (11) Bloembergen, N. Nonlinear Optics; Addison-Wesley Publishing Company, Inc.: Reading, MA, 1992. (12) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Phys. ReV. Lett. 1981, 46, 1010. (13) He, G.; Elking, M. D.; Xu, Z. Chem. Phys. Lett. 1996, 254, 184. (14) Duan, X. M.; Konami, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. J. Phys. Chem. 1996, 100, 17780. (15) Lundquist, P. M.; Yitzchaik, S.; Zhang, T. G.; Kanis, D. R; Ratner, M. A.; Marks, T. J. Appl. Phys. Lett. 1994, 64, 2194-2196. (16) Heinz, T. F.; Chen, C. K.; Ricard, D.; Shen, Y. R. Phys. ReV. Lett. 1982, 48, 478-481. (17) The PZC of our particular electrochemical system was determined SFG by measuring the p-polarized SFG response (IPPP ) of the interface as a function of the applied potential (without D289). The PZC corresponds to the minimum of the response. (18) We assume that the structure of D289+ cations does not change in the electrical double-layer region. (19) Bloembergen, N.; Pershan, P. S. Phys. ReV. 1962, 128, 606. (20) Li, J.; He, G.; Xu, Z. J. Phys. Chem., in press. (21) The angle of incidence was 33° for the 1064 nm excitation and was 30° for the 532 nm excitation. The average angle of incidence is about Ri ) 31°. The average angle of refraction can be obtained from Snell’s
ni cos Ri - nt cos Rt , ni cos Ri + nt cos Rt
rS )
2ni cos Ri (ni cos Ri + nt cos Rt) 2nt cos Ri (nt cos Ri + ni cos Rt) 2ni cos Rt (nt cos Ri + ni cos Rt) rP )
nt cos Ri - ni cos Rt nt cos Ri + ni cos Rt
(23) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (24) K2 can be determined from the following equation
K2 )
( )(
)( )( )
(2) nm,SFG sin Rm,SFG χXXZ LZ ) F(R)xη (2) nt,SFG sin Ri L χZYY Y
in which F(R) and η can be determined by the following equations according to ref 19:
η)
( )( )
SFG I45°-S AP , SFG A I S
F(R) )
SSP
(nr,SFG cos Rr,SFG + nt,SFG cos Rt,SFG) (nt,SFG cos Rr,SFG + nr,SFG cos Rt,SFG)
SFG In the above equations, I45°-S is the s-polarized SFG signal received by the detector when the two laser excitations are both polarized at 45° from the SFG plane of incidence, ISSP is the p-polarized SHG signal received by the detector when two s-polarized laser excitations are used, AP/AS is the ratio of the optical transmission of our detection system for p-polarized SFG signal to that for s-polarized SFG signal at 355 nm, nr,SFG is the refractive index of the SHG light in the liquid phase, nt,SFG is the refractive index of the SHG signal in Si(111), Rr,SFG is the angle of reflection of the SFG signal, and Rt,SFG is the angle of refraction of the SFG signal. (25) π 1 A 1 -A
N 〈cos θ〉 ) 〈cos3 θ〉 )
)
0
∫ P(θ) cos θ dθ ) AN (e π
2
0
∫ P(θ)(cos π
0
∫ P(θ) dθ ) A(e
3
θ) dθ )
A
-e )
+ e-A)[A - tanh(A)]
N A (e + e-A)[A(A2 + 6) A4 3(A2 + 2) tanh(A)]
(26) The interaction energy of an electrical dipole with the applied electrical field is µE cos θ, in which θ is the angle between the direction of the electrical field and the direction of the dipole moment. For D289+, the dipole moment is in the same direction of the long molecular axis and the electrical field in the electrical double-layer region is in the direction of the surface normal. (27) When the electric field vanishes, the orientation distribution function is reduced to P(θ) ) 1/2 sin θ, and the average population angle is
〈θ〉 )
∫
πθ
0
/2 sin θ dθ ) π/2
