Nonequilibrium Adsorption and Reorientation Dynamics of Molecules

Dec 26, 2012 - We further proposed a molecular mechanism involving the displacement of D289 and CH3CN molecules adsorbed on the electrode interface ...
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Nonequilibrium Adsorption and Reorientation Dynamics of Molecules at Electrode/Electrolyte Interfaces Probed via Real-Time Second Harmonic Generation Anan Liu,† Lu Lin,† Yuan Lin, and Yuan Guo* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Nonequilibrium adsorption and subsequent reorientation of organic molecules at electrode/electrolyte interfaces are important steps in electrochemical reactions and other interfacial processes, yet real-time quantitative characterization and monitoring of these processes, particularly for the reorientation step, are still challenging experimentally. Herein, we investigated the nonequilibrium adsorption process of 4-(4(diethylamino)styryl)-N-methyl-pyridinium iodide (D289) molecules from acetonitrile solution onto a polycrystalline platinum electrode surface using real-time second harmonic generation (SHG) in combination with the potential step method. The timedependent SHG curves exhibit two distinct regimes, which were interpreted with a twostep adsorption model consisting of a fast adsorption and a slow reorientation step for D289 on the surface. D289 was assumed to initially adsorb in an orientation perpendicular to the surface and then reorient to a parallel orientation. We derived a quantitative mathematical expression containing a biexponential function to fit the temporal SHG curves and obtain the rate constants for the two steps. The rate constants for fast adsorption and the slower reorientation processes show similar potentialdependent behavior: the rate decreases with an increase in the negative potential. We further proposed a molecular mechanism involving the displacement of D289 and CH3CN molecules adsorbed on the electrode interface to explain this potentialdependent behavior. On the basis of such analysis, we obtained a detailed picture of the adsorption of D289 molecules on the Pt electrode/CH3CN electrolyte, which consists of three consecutive steps: diffusion, adsorption, and reorientation. The results of this study may shed light on adsorption mechanisms at the electrode/electrolyte interface as well as at biological and other functional material interfaces.

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INTRODUCTION The adsorption of organic molecules at solid/liquid surfaces has been an extensively studied topic because it is related to a variety of interfacial processes such as heterogeneous catalysis, mineral flotation, oil recovery, interfacial self-assembly, and biological processes.1 The adsorption of organic molecules at the solid/liquid interface frequently presents a nonequilibrium adsorption state because of the slow adsorption kinetics.2−4 In addition, the nonequilibrium state is usually associated with slow adsorbate reorientation to reach orientational equilibrium.5 Consequently, investigations into reorientation dynamics, which are also referred to as the orientational evolution with time or orientation relaxation in the literature, are of fundamental significance for revealing the adsorption mechanism.5,3 It was revealed that the reorientation dynamics of molecules with large, flexible structures usually lasts for a long time, ranging from minutes to even days.6,7 In addition, this reorientation process has proven to play a crucial role in the formation of the well-ordered self-assembled monolayer structures5,8 and to stabilize adsorbed protein9,10 and polymer3,11−13 surfaces. Adsorption also has a pronounced effect on electrochemical reactions.14 It is known that an overall electrode reaction is composed of a series of steps.15 The adsorption process is one of the main steps and affects the main features of the ensuing © XXXX American Chemical Society

electron transfer reaction at the electrode surface. The adsorption process also follows a stepwise mechanism: diffusion of molecules to the interface, actual adsorption at the interface, and reorientation of the adsorbed molecules.16 The overall adsorption process is either governed by the diffusion step or the actual adsorption and the reorientation step, wherein the last step determines the adsorption configuration of the adsorbate on the surface. The orientation of the adsorbate on the electrode/electrolyte is thus one of the crucial issues in surface electrochemistry. It is important to note that, although investigations of adsorption at the electrode/electrolyte interface have been made intensively since the beginning of the last century, information about the orientation of adsorbates on the electrode surface derived from direct experimental measurements were relatively scarce before the advent of modern spectroscopic techniques.17 Modern advanced spectroscopic techniques are able to extract information about molecular orientation at solid/liquid interfaces at the molecular level.18 For instances, the flip-flop phenomena of water,19−22 acetonitrile,23 and the orientation of other organic molecules24−31 at electrode surfaces, have been Received: October 25, 2012 Revised: December 18, 2012

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electrode because it is a metallic electrode widely studied in electrochemistry and because it has a negligible SHG background signal compared to the large resonant SHG signal of D289 molecules. We found that the adsorption of D289 molecules at the Pt electrode/acetonitrile interface shows a two-step adsorption model consisting of a fast adsorption step from the bulk to the surface followed by a slow reorientation step in the adsorbed layer. Interestingly, quantitative analysis of the dynamics of the two steps indicates that both the adsorption rate and the reorientation dynamics depend on the applied potential. Furthermore, we proposed a molecular mechanism to interpret the potential dependence of the adsorption and reorientation dynamics. This work helps provide deeper insights into the nonequilibrium adsorption dynamics of organics at electrode/ solution and other important interfaces. To the best of our knowledge, our work represents the first time that the dependence of the reorientation dynamics on the potential has been reported.

investigated by means of these spectroscopic techniques. However, the previous work was limited in that the orientation was only considered as a function of the electrode potential under adsorption equilibrium conditions. In other words, the main concern in these studies was modulation of the molecular orientation using an applied electrode potential, rather than investigation of the reorientation dynamics. While several studies have addressed adsorption dynamics at the electrode/ electrolyte interface, these studies focused mainly on the surface concentration excess of the adsorbate as a function of time and failed to involve the reorientation dynamics. Therefore, many fundamental questions with respect to the adsorption of organic molecules on electrode/electrolyte interfaces still remain to be answered. The questions are the following: Can small organic molecule reorientation upon adsorption at the interface be observed experimentally? How fast does the reorientation occur and what role does it play in the overall adsorption? How do surface charge and interfacial adsorbate− substrate, adsorbate−solvent, and solvent−substrate interactions affect the reorientation dynamics? In this article, we attempt to answer these questions using real-time second-order nonlinear optical spectroscopy. Second-order nonlinear optical spectroscopies, in particular, real-time second harmonic generation (SHG) or real-time sum frequency generation (SFG), have proven to be powerful tools to study interfacial dynamics because of their unique interfacial sensitivity. These methods have been applied to investigate dynamic adsorption processes of interfacial molecules at air/ water interfaces or at biological membranes.32−35 These techniques can provide knowledge of molecular number density variations and orientational variations of interfacial molecules as a function of time, and from this data, the mechanistic aspects of the interfacial molecular processes can be ascertained. Using real-time SHG, we aim to gain insight into the nonequilibrium adsorption dynamics of organic molecules at electrode/ electrolyte interfaces. In this article, we investigated the nonequilibrium adsorption and reorientation dynamics of 4-(4-(diethylamino)styryl)-Nmethylpyridinium iodide (D289) at the Pt electrode/ acetonitrile interface using real-time SHG combined with potential step techniques. The D289 molecule is a dye molecule (Chart 1) and could be used as a fluorescence agent in

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THEORETICAL BACKGROUND The basic theory of SHG has been well described in previous literature.41−43 Only some necessary background for understanding the data analysis in the present report is briefly introduced here. The SHG signal intensity is directly related to the effective nonlinear susceptibility, χ(2) eff , by (2) 2 I(2ω) ∝ |χeff |

In an electrochemical system,

(1)

χ(2) eff

can be described by

44

(2) (2) (2) (2) χeff = χsub + χads + χint

(2)

where χ(2) sub is the nonlinear susceptibility of the metal substrate, χ(2) ads is the inherent nonlinear susceptibility of the adsorbate, and χ(2) int is the interaction between the substrate and the adsorbate. At the 800 nm fundamental wavelength used in this work, D289 exhibits a large SHG response owing to resonant enhancement.38,45 Thus, it is reasonable to ignore the χ(2) sub and (2) χ(2) terms because the adsorbate term χ dominates the SHG int ads response. In the reflection geometry, effective nonlinear susceptibilities for different polarization combinations of the incoming and outgoing beams have been well derived for the isotropic interface. Since all of our SHG measurements are performed with the P-in/P-out arrangement in this experiment, only the 41 expression for χ(2) eff,PP is given here:

Chart 1. Molecular Structure of D289a

(2) (2) χeff,PP = Lzz(2ω)(Lyy(ω))2 sin γ cos2 γ ·χD289, zxx

a

+

(2) − 2Lxx(2ω)Lzz(ω)Lxx (ω) sin γ cos2 γ ·χD289, xzx

−

D289 molecules dissociate into D289 cations and I anions in acetonitrile solvent.

(2) + Lzz(2ω)(Lzz(ω))2 sin 3 γ ·χD289, zzz

(3)

In eq 3, L(ω) and L(2ω) are the Fresnel factors for the fundamental and harmonic beams, respectively. γ is the incident angle of the fundamental beam with respect to the surface normal. χ(2) D289,ijk is the nonvanishing macroscopic susceptibility elements. The subindices refer to laboratory coordinates (x, y, z). The macroscopic susceptibility elements χ(2) D289,ijk, in turn, are a function of the microscopic hyperpolarizability and the surface concentration.43 For the rod-like D289 molecule, the molecular hyperpolarizability can be assumed to have only one dominant element, βccc, with c along the long axial of the molecule. We thus have

fluorescent microscopic mapping experiments.36 It also serves as a model molecule in studies of adsorption at the liquid/ zeolite.37 More importantly for us, the D289 molecule has an intense SHG response at 800 nm because of resonant enhancement38 and can be readily probed by our laser system. Additionally, D289 has a large dipole moment along its long dominant axis39 (∼14 Debye) and can be regarded as a rod-like molecule, which is favorable for studying electrochemical adsorption with applied potentials and subsequent analysis of SHG intensity data.40 We chose to use Pt as the working B

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(2) (2) χD289, = Ns ,D289βccc ⟨cos3 θ ⟩ zzz

1 (2) Ns ,D289βccc (⟨cos θ ⟩ − ⟨cos3 θ ⟩) 2 1 (2) = Ns ,D289βccc (⟨cos θ ⟩ − ⟨cos3 θ ⟩) 2

(2) χD289, = zxx (2) χD289, xzx

(4)

where θ is the orientational angle between the dominant D289 axis with respect to the surface normal. The operator ⟨ ⟩ denotes the orientational distribution average, and Ns,D289 is the surface number density of adsorbed D289 molecules. Here, the orientation distribution was approximated by a δ distribution. Substitution of eq 4 into eq 3 yields (2) χeff,PP = Ns ,D289 cos θ(A sin 2 θ + B cos2 θ )

(5)

with A=

1 sin γ cos2 γ(Lzz(2ω)(Lyy(ω))2 − 2Lxx(2ω)Lzz(ω) 2 Lxx(ω)) B = sin 3 γLzz (2ω)(Lzz (ω))2

We can see from eq 5 that any change in the SHG at PP polarization can be ascribed to changes in orientation or surface number density of D289.

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EXPERIMENTAL SECTION Chemicals and Solutions. For all of the SHG and electrochemical experiments, 80 μM D289 solutions were prepared by directly dissolving D289 (>98%, Fluka) in HPLC grade acetonitrile (Sigma Aldrich) without further purification. D289 molecules dissociate into D289+ cations and I− anions in acetonitrile solvent. Tetrabutylammonium tetrafluoroborate ([CH3(CH2)3]4NBF4) (>98%, Sigma Aldrich) was used as the supporting electrolyte, with the concentration maintained at 50 mM in all solutions. Electrochemical Experiments. A home-built electrochemical cell was designed to facilitate both optical and electrochemical measurements. As illustrated in Figure 1, the cell is a standard three-electrode system equipped with a Pt wire as the counter electrode and an Ag+/Ag (10 mM/L AgNO3) electrode as the reference electrode. The working electrode was fabricated with a 99.99% Pt rod, which was polished with 50 nm alumina particles and ultrasonically cleaned in subsequent baths of acetone, ethanol, and water. A 20 mm diameter, 2.5 mm thick fused quartz plate was used as an optical window. The spectroscopic experiments are performed on a thin layer of 5 to 10 μm electrolyte solution trapped between the fused quartz window and the working electrode.23,46 Because of the dilute D289 concentration, the solution was prepared in a circularly flowed system in order to avoid any possible solute depletion caused by adsorption. The potential of the working electrode was controlled by a potentiostat (Princeton Applied Research, Model 263A), and all the potentials in this article are referenced to the Ag+/Ag electrode. The potentials, currents, and SHG signals are recorded simultaneously. The capacitance measurements were conducted using a Solartron SI 1287 and Solartron 1255B as the frequency response analyzer and electrochemical interface, respectively. Impedance measurements were done at a constant frequency (1500 Hz) by scanning the electrode potential from the

Figure 1. Schematic diagram of the home-built electrochemical cell. It is equipped with a three-electrode system, an optical window, and a circuit for the flowing solution. The working electrode, reference electrode, and counter electrode are polycrystalline Pt, Ag+/Ag, and Pt wire, respectively.

positive to negative direction at a 10 mV/s scan rate, and the ac potential (10 mV peak-to-peak amplitude) was superimposed on the dc potential. The capacitance value, Cd, was directly obtained from the imaginary impedance component, Z″, using the equation Cd = −1/2πf Z″, where f is the frequency.15 SHG Experimental Section. The setup is typical for reflected-geometry SHG measurements.41,47 A broadband tunable mode-locked femtosecond Ti:Sapphire laser system (Tsunami 3960C, Spectra-Physics) with high-repetition rate (82 MHz) and short-pulse width (80 fs) was used. The polarization of the 800 nm fundamental laser beam was controlled by a half-wave plate, and the polarized light was focused around a 30−50 μm diameter spot at the solid/liquid interface consisting of the polycrystalline Pt working electrode and acetonitrile solution at an incident angle of 70° from the surface normal. The polarization of SHG signal was controlled by a polarizer. All experiments were performed with the P-in/Pout polarization combination. Typically, the laser power is set to lower than 150 mW in order to avoid any photon damage of the electrode surface. A short-wavelength pass filter and a monochromator serve to remove the fundamental beam from the SHG signal. The SHG signal in the reflected direction was detected with a high-gain photomultiplier (R585, Hamamatsu) and a photon counter (SR400, Stanford). The data acquisition was programmed and controlled with a personal computer in C

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order to record the SHG signal and the electrical current intensity simultaneously.

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RESULTS AND DISCUSSION Cyclic Voltammetry. Cyclic voltammetry experiments were carried out in order to determine the potential range in which there are no electrochemical reactions. Cyclic voltammograms of the polycrystalline Pt electrode in electrolyte CH3CN solutions containing 80 μM D289 are displayed in the Supporting Information (Figure S1). The potential was initially set at −0.40 V, which is close to the open circuit potential (−0.45 V), swept in the negative direction to −1.20 V with a scan rate of 50 mV/s, and then the sweep was reversed. The current is capacitive in the −1.20 V < E < −0.20 V potential region, and any redox reactions can be ruled out in this region. Differential Capacitance Measurements. Electrical impedance spectroscopy measurements were also performed in order to determine the potential of zero charge (pzc) of the electrode.15 Figure 2a presents the equivalent circuit used to

Figure 3. SHG signal with the PP polarization combination as a function of potential for acetonitrile solutions with (blue circles) or without 80 μM D298+ (red squares). The inset is a zoomed-in view of the SHG signal with the PP polarization combination as a function of potential for the neat acetonitrile solution.

and the pzc has been well discussed in previous literature.48−52 For the Pt/CH3CN interface, the SHG signal from the interface is nonresonant, so the SHG variation with applied potential is dominated by electric-field-induced second harmonic generation (EFISH).53 The EFISH contribution can be written as an (3) additional term, P(2) E (2ω) = χ :EdcE(ω)E(ω). It leads to a parabola-like potential-dependent SHG profile with a minimum close to the pzc value. However, in the presence of D289, the potential-dependent SHG exhibits an asymmetric profile on different sides of the pzc value (circles in Figure 3). The SHG signal remains small at potentials positive of pzc, but the onset of a drastic increase in the SHG signal was observed at around −0.80 V, which is slightly negative to the measured pzc, −0.74 V. By comparing the SHG signals of the D289 solution and the background CH3CN, we can readily conclude that D289 shows little adsorption when the electrode is positively charged, but adsorbs intensively when the electrode is negatively charged. This observation can be rationalized by the fact that D289 dissociates into D289+ cations and I− anions in CH3CN solution, and electrostatic interactions between the electrode surface and D289+ cations dominate the adsorption of D289. Real-Time SHG under Potential Step. The potentialdependent SHG measurements have qualitatively revealed the adsorption behavior of D289+ at the polycrystalline Pt electrode surface on the different sides of the pzc value. At potentials positive of pzc, D289+ cations are found to barely adsorb on the electrode, but intensive adsorption was observed at potentials negative of pzc. This potential-dependent adsorption behavior allows us to observe the dynamics of the nonequilibrium adsorption of D289+ by directly monitoring changes in the SHG signal after a potential step from positive to negative with respect to pzc. The detailed procedure of the experiment is as follows: first, the electrode potential was kept at E1 (−0.40 V), which is close to the open circuit potential. The SHG signal at this potential, E1, was as small as CH3CN solution background level, and the surface was considered to be free of the adsorbed D289+. Second, the potential was stepped to a desired value E2 within the double-layer region, and the temporal evolution of the SHG from the surface was recorded with a time step of 0.1 s. Figure 4 shows the time-dependent SHG curves after the electrode potential was stepped to −0.80, −0.85, −0.90, −0.95, −1.00, −1.05, and −1.10 V, respectively, which were negative of pzc. Intriguingly, we observed a bimodal behavior of the temporal SHG evolution. The signal initially increases fast with time, then decays slowly to a plateau. We are next going to develop a

Figure 2. (a) Equivalent circuit used to model the electrochemical impedance spectroscopy data. (b) Capacitance vs potential for the polycrystalline Pt electrode in acetonitrile solution with (hollow squares) or without (solid circles) 80 μM D298. The minimum in the capacitance curves indicate the potential of zero charge, −0.76 V vs Ag+/Ag for neat CH3CN solution and −0.74 V vs Ag+/Ag for 80 μM D298 solution.

model the electrochemical impedance spectroscopy data, and Figure 2b presents the capacitance Cd(E) for the Pt electrode interface in contact with the electrolyte CH3CN solution with or without 80 mM D289 as a function of potential. The minima in the potential dependent capacitances, −0.76 V for background CH3CN and −0.74 V for D289, correspond to the pzc of each solution. At potentials negative of the pzc value, the electrode surface is negatively charged, while at potentials positive of pzc, the electrode surface is positively charged.14 Organic substances, especially in their charged ionic forms, can display different adsorption behavior at different sides of the pzc. The pzc determination will thus facilitate further discussions of the SHG spectroscopy data in the following sections. Potential Dependent SHG. Figure 3 shows the SHG signal for the CH3CN electrolyte solution with (circles) or without (squares) 80 mM D289 recorded as the potential was linearly scanned from 0 to −1.40 V with a scan rate of 1 mV/s. The inset in Figure 3 is the potential-dependent SHG of the background CH3CN solution, which exhibits an evident minimum at around −0.82 V, close to the corresponding pzc, −0.76 V. The relation between the minimum in the SHG curve D

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orientation with respect to the electrode surface so that they can produce a large PP SHG signal. In this case, we consider that the dipole of D289+ are aligned by the interfacial electric field and adsorb with its long axis orientated in a relatively vertical arrangement along the electrode surface normal with the methyl group attached to the pyridine ring pointing toward the electrode surface. We denote this adsorption configuration as D289+ads,⊥. As to the signal decrease stage from S2 to S3, in principle, it may arise from several possible reasons: the decrease in the molecular number density originated from redox reaction, desorption, or the reorientation of D289+ads,⊥. The possibility of the redox reaction has been ruled out by careful examination of the cyclic voltammogram. However, the possibility that the dramatic decrease in SHG signal was resulted from desorption could also be excluded because the applied potential and other experimental conditions remain unchanged in the whole experimental process. As a result, we consider that the gradual decrease in SHG signal (from S2 to S3) might be most probably attributed to a reorientation process of D289+ads,⊥. In other words, information on the reorientation dynamics of D289+ads,⊥ on electrode interface can be extracted from the time-dependent SHG curves. It is worth noting that the SHG signal at S3 is so small that it is close to the background signal level, indicating that once the reorientation achieves the final equilibrium adsorption state, the D289+ have a very small contribution to the SHG signal. The implication of SHG signal at S3 in the orientation angle of D289+ is examined in detail below. It has been suggested by previous studies that many aromatic organic molecules, such as pyridine,26,28,57,58 bipyridine,27 pyrazine,59 and benzonitrile,24,25,60,61 would adsorb parallel to the electrode surface at negative potentials because the large conjugated, planar structures of these molecules enable them to act as electron acceptors through π−π interactions with the negatively charged surface. We thus consider the adsorbed D289+ cations would undergo a reorientation process from vertical orientation to parallel orientation, forming π−π bonds with the Pt electrode surface. To verify the validity of the reorientation assumption, we further simulated the PP SHG signal as a function of the tilted orientation of D289+. Since the orientation distribution was not known as a prior, the simulation was done with delta distribution and with Gaussian distribution.62 The results of the simulation indicate that the PP SHG signal decays rapidly with the increasing orientation angle θ, regardless of the distribution model or the orientation widths. When θ exceeds around 60°, the SHG response of the adsorbed D289+ becomes negligible as compared to that of the vertical configuration (θ = 0°) (Figure 6). In light of the previous electrochemical studies together with the simulation, we inferred that the reorientation of D289+ads,⊥ to a parallel + orientation, denoted as D289ads∥ , could be a reasonable interpretation of the loss in SHG signal (S2 to S3 in Figure 5). So far, we have interpreted the SHG signal variation with time in terms of the two-step physical model: adsorption and, consequently, reorientation. We will then give a quantitative analysis of the two rate constants in the following subsection. The adsorption of D289+ onto Pt surface can be expressed by following consecutive steps:

Figure 4. Time-dependent SHG signal with the PP polarization combination for a polycrystalline Pt electrode contacting 80 μM D298 solution after the potential stepped from −0.4 V to −0.80, −0.85, −0.90, −0.95, −1.00, −1.05, and −1.10 V vs Ag+/Ag. These stepped potentials are negative of the pzc.

physical model to interpret these interesting features displayed in the temporal SHG curves. Physical Model. We now apply the two-step adsorption model consisting of a fast adsorption and a slow reorientation step to interpret the time-dependent SHG curve in Figure 4. This model has been applied in a number of studies concerning the nonequilibrium adsorption of various substances on solid/ liquid interfaces.6,7,54−56 For clarity, we present the SHG evolution of E2 = −0.80 V as a representative example in Figure 5. The variation of SHG signal from S0 to S3 in the temporal

Figure 5. Time-dependent SHG signal with the PP polarization combination for a polycrystalline Pt electrode contacting the 80 μM D298 solution after the potential step from −0.40 V to −0.80 V vs Ag+/Ag. The SHG signal initially rises fast from S0 to S2 after the potential change. Following that, a slow decay from S2 to S3 ensues after the maximum value is reached. The black solid line is the fitting result by a biexponential function, which is derived on the basis of the two-step kinetics model we proposed. The detailed expression for the function is described by eq 7.

SHG curves is attributed to two different physical processes. The fast rise in the SHG signal (S0 to S2) results from the adsorption of D289+ cations, driven by the electrostatic interactions between the negatively charged surface and the positively charged D289+ cations. In addition, as we discussed in the section of theoretical background, the initial adsorption configuration of D289+ is expected to have a large net

kad

k re

fast adsorption

slow reorientation

D289+sol ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ D289+ads, ⊥ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ D289+ads,

(6)

with D289+sol denoting the D289+ cations in the bulk solution, and kad and kre representing the rate constants for adsorption E

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Figure 6. Simulation of PP SHG signal as a function of the orientation angle with Gaussian distribution and delta-distribution, respectively. All the PP SHG signal curves exhibit decreasing trends with the increasing orientation angle, regardless of the different distribution model or the different orientation widths, and the SHG signal becomes negligible when the orientation angle exceeds around 60°.

and reorientation, respectively. Bearing in mind that D289+ads,⊥ is the only species responsible for the SHG signal variation, we derived a biexponential function to describe the time evolution of SHG intensity I2ω(t) ⎡ k ′Γ (0 s) −k ′t ⎤2 I2ω(t ) = ⎢ ad 0 (e ad − e−kret )⎥ + C ⎣ k re − kad′ ⎦

Figure 7. Potential dependence of the rate constants for the (a) adsorption and (b) reorientation processes for D289+ cations on the Pt electrode surface. The decreasing trends for both rate constants imply an increasing energy barrier for both steps as the potential becomes more negative. The solid lines are just guides for the eyes.

(7)

where kad ′ = kadC0 is the apparent adsorption rate constant, C0 is the bulk concentration of D289 (80 μM), and Γ0(0 s) is the + maximally attainable concentration of D289ads,⊥ at the corresponding potential. C is a constant related to the SHG contribution from the substrate and solvent. The detailed derivation of eq 7 was presented in the Supporting Information. A representative example of the fitting for the case of E2 = −0.80 V is shown in Figure 5, where the solid line from S1 to S3 is the fitting result with eq 7.63 The fitting parameters, including kad ′ , kre, and Γ0(0 s) at each potential are listed in Table 1.

Both the interactions are expected to be enhanced when increasing the negative charge density on the electrode surface. However, the increase in the driving forces apparently cannot explain the decrease in the rate constants with the increasing negative potential. In the following, we propose a molecular mechanism to interpret this potential dependence in terms of displacement reactions between D289+ and adsorbed solvent molecules. It has been well established that adsorption on the electrode surface is dependent on the displacement reaction between the adsorbate and the adsorbed solvent.14,16,64 In the present case, D289+ adsorption can be described as

Table 1. Adsorption Parameters of D289 on the Pt Electrode Surface at Various Potentials electrode potential E2 (V vs Ag+/Ag) −0.80 −0.90 −0.95 −1.00 −1.05 −1.10 −1.15

apparent adsorption rate ′ constant kad (s−1) 0.391 0.164 0.142 0.073 0.070 0.066 0.043

± ± ± ± ± ± ±

0.032 0.007 0.005 0.002 0.002 0.001 0.001

reorientation rate constant kre (s−1)

maximum surface concentration of D289+ads,⊥ (au)

± ± ± ± ± ± ±

8.1 ± 0.1 12.3 ± 0.2 16.4 ± 0.2 24.2 ± 0.3 25.1 ± 0.3 24.9 ± 0.2 22.1 ± 0.1

0.0148 0.0144 0.0123 0.0118 0.0102 0.0055 0.0027

0.0007 0.0005 0.0003 0.0003 0.0003 0.0001 0.0001

D289+sol + n1CH3CNads = D289+ads, ⊥ + n1CH3CNsol

(8)

and D289+ads, ⊥ + n2CH3CNads = D289+ads, + n2CH3CNsol

(9)

65,66

where n1 and n2, termed size factors, are defined as the number of CH3CN molecules displaced by one D289 molecule in the adsorption and reorientation step, respectively. Earlier experimental and theoretical studies suggested that CH3CN chemisorbed on the Pt surface with its CH3 group pointing toward the electrode surface at the relatively negatively charged surface and exhibits strong layering and orientational ordering.23,67,68 Evidently, the adsorption and subsequent reorientation process has to overcome energetic barriers for removal of the adsorbed CH3CN molecules according to the replacement mechanics. This energy barrier depends mainly on the adsorption energy of CH3CN on the Pt surface, which is related to the applied electrode potential. Theoretical studies of the potential dependence of the CH3CN adsorption energy on a Pt surface demonstrated that the adsorption energy of CH3CN becomes more negative with increasing negative

Potential Dependence of Rate Constants. The apparent adsorption rate constant kad ′ and the reorientation rate constant kre are plotted as a function of applied potential in Figure 7. It is seen that both k′ad and kre decrease monotonically when the potential is negatively swept and were reduced by an order of magnitude as the potential changed from −0.80 V to −1.05 V. As we have mentioned in the description of our physical model, the driving forces for the adsorption and reorientation processes are the electrostatic interactions between the D289+ and negative surface charges and π−π interactions, respectively. F

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Figure 8. Adsorption mechanism of D289+ cations on a Pt electrode after the potential is stepped from positive to negative with respect to the pzc. The plots from a to d represent the mass transport, the fast adsorption, and the slow reorientation processes occurring for D289+ cations at the electrode surface, respectively. In the fast adsorption step, one adsorbed solvent CH3CN molecule is displaced by each D289+ with a vertical orientation with respect to the electrode surface. After the fast adsorption process, the slow reorientation process ensues and more CH3CN molecules are displaced to accommodate the parallel orientated D289+.

potential.68 Increasing the negative surface potential therefore gives rise to an increase in the energetic barrier for both the adsorption and reorientation processes, which is consistent with our observation that the rate constants decrease with increasing negative potential. However, the energy barrier for the displacement reaction also depends on the number of displaced CH3CN molecules. As more CH3CN molecules are displaced, the energy barrier increases. From a geometric viewpoint, the parallel D289+ configuration must displace more absorbed CH3CN molecules than in the perpendicular configuration, which may be responsible for the slow reorientation dynamics. At this stage, the displacement mechanism provides a consistent interpretation of the potential dependence of the rate constants, as well as of the slow dynamics of the reorientation process. On the basis of the discussions above, we can give a full picture of the adsorption of D289+ onto the Pt electrode surface as illustrated in Figure 8. Figure 8a represents the state of the electrode surface at E1 = −0.40 V. D289+ cations are randomly oriented in the bulk solution and barely adsorb on the electrode. Figure 8b presents the mass transport process of a D289+ cation from the bulk to be adjacent to the electrode surface. The dipole direction is aligned vertically by the electric field at negative potentials, preparing for the adsorption step. Figure 8c shows the displacement in the adsorption step. This vertical adsorption configuration is a precursor to the consequent slow reorientation process. Figure 8d depicts the slow reorientation process, during which additional CH3CN molecules are displaced by one D289+ads,∥. We conclude that the overall adsorption of D289+ proceeds as a nonequilibrium process and is governed by an interplay of kinetic and thermodynamic factors. At the initial stage of the adsorption,

D289+ads,⊥ was the dominant species on the electrode surface because it is the kinetically favored product; however, these adsorbed D289+ molecules would eventually reorient to the parallel orientation because this configuration is a more thermodynamically stable state. Thus, we have presented a new picture of organic molecule adsorption on the electrode surface using the D289 molecule as an example. Adsorption Free Energy of D289+ads,⊥. We now closely examine the fast adsorption process of using the timedependent SHG signal at different stepped potentials (4). We found that the rate constants decrease as the potential becomes more negative in this fast adsorption process; however, we also found that the maximum point of the temporal SHG curves becomes higher with more negative potentials. This indicates that the potential tends to affect the adsorption equilibrium, and from this data, we can extract further thermodynamic information about the fast adsorption process. Nine is a plot of the maximum adsorption concentration of D289+ads,⊥, Γ0(0 s), obtained by fitting the time-dependent SHG curves with eq 7, as a function of the potential with respect to the pzc, regarded as the adsorption isotherm of D289+ads,⊥. The adsorption isotherm can be directly described by the Stern equation, a modified version of the Langmuir adsorption equation in electrochemistry in which the adsorption free energy is described as a sum of chemical and electrostatic contributions.1 For the present case, it gives the expression Γ0 = ≈ G

Γ maxKCD289 CCH3CN + KC D289 Γ maxC D289 C D289 + 19.3M ·exp[(ΔGchem + bF ΔE)/RT ]

(10)

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where Γmax is the maximum adsorption concentration of D289+ads,⊥ on the polycrystalline Pt surface, CCH3CN is the bulk concentration of the CH3CN solvent, and CD289 is the bulk concentration of D289. The electrostatic contribution to the adsorption free energy is approximated to be proportional to the potential difference from the pzc, which gives the form of bFΔE, where F is the Faraday constant, ΔE is the potential with respect to the pzc, and b is the fraction of the applied potential felt by D289+ads,⊥.15,69 As presented in Figure 9, fitting the

also shows that adsorbed solvent molecules play a governing role in the adsorption and reorientation dynamics. Furthermore, we also calculated the Gibbs free energy of the adsorption step within the framework of the Stern model. The results indicate that the initial adsorption of D289 has weak chemisorption character. Our finding adds significant contributions to the understanding of electrochemical dynamics and in areas closely related to solid/liquid adsorption.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammograms, the deduction of theoretical modeling, and derivation of eq 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Graduate University of the Chinese Academy of Sciences, Beijing 100049, China. Notes

Figure 9. Surface concentration of D289+ads,⊥ vs potential on Pt electrode in terms of the second-order susceptibility, c(2) D289. The solid line is a fit to the Stern adsorption model described by eq 10, which yields a value of −24.8 ± 1.7 kJ/mol for the chemical contribution in adsorption free energy of D289ads,⊥ on polycrystalline Pt, and the electrostatic contribution in the adsorption energy is 0.50FΔE.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Professors Wang Hongfei and Gao Yiqin for helpful discussions. We also gratefully acknowledge the support of the Natural Science Foundation of China (NSFC No. 21073199, No. 91027042, and No. 21227802).

adsorption isotherm of D289+ads,⊥ with eq 10 yields a value of −24.8 ± 1.7 kJ/mol for the chemical adsorption free energy on polycrystalline at the pzc (−0.74 V), which shows weak chemisorption character. This value also falls in the reasonable range of the adsorption free energy for organics on metal electrodes.27,70 The electrostatic contribution to the adsorption energy was determined to be 0.50FΔE.

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REFERENCES

(1) Adamson, A. W.; Gast, A. P. Physcical Chemistry of Surfaces, 6th ed.; John Wiley & Sons, Inc.: New York, 1997. (2) Schrodle, S.; Richmond, G. L. J. Am. Chem. Soc. 2008, 130, 5072−5085. (3) Douglas, J. F.; Johnson, H. E.; Granick, S. Science 1993, 262, 2010−2012. (4) Fuerstenau, D. W. J. Colloid Interface Sci. 2002, 256, 79−90. (5) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145−1148. (6) Polizzi, M. A.; Plocinik, R. M.; Simpson, G. J. J. Am. Chem. Soc. 2004, 126, 5001−5007. (7) Rotella, C.; Napolitano, S.; Vandendriessche, S.; Valev, V. K.; Verbiest, T.; Larkowska, M.; Kucharski, S.; Wubbenhorst, M. Langmuir 2011, 27, 13533−13538. (8) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855−861. (9) Roach, P.; Farrar, D.; Perry, C. C. J. Am. Chem. Soc. 2005, 127, 8168−8173. (10) Nakanishi, K.; Sakiyama, T.; Imamura, K. J. Biosci. Bioeng. 2001, 91, 233−244. (11) Linse, P.; Kallrot, N. Macromolecules 2010, 43, 2054−2068. (12) Kallrot, N.; Dahlqvist, M.; Linse, P. Macromolecules 2009, 42, 3641−3649. (13) Fu, Z. L.; Santore, M. Macromolecules 1999, 32, 1939−1948. (14) Bockris, J. O. M. Modern Electrochemistry; Kluwer Academic Publishers: New York, 2002. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamental and Applications, 2nd ed.; John Wiley & Sons, Inc: New York, 2001. (16) Gileadi, E. Electrosorption; Plenum Press: New York, 1967. (17) Bockris, J. O.; Khan, S. U. M. Surface Electrochemistry: A Molecular Level Approach; Plenum Press: New York, 1993. (18) Zaera, F. Chem. Rev. 2012, 112, 2920−2986. (19) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444−446.

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CONCLUSIONS This work investigated the nonequilibrium adsorption and reorientation dynamics of molecules at electrode/electrolyte interfaces using real-time second harmonic generation in combination with the potential step method. We found that the adsorption of D289 from bulk solution onto Pt electrodes follows a two-step dynamics model consisting of a fast adsorption and a slow reorientation step, and we quantitatively determined the rate constants for the two steps at various electrode potentials. Both of the two rate constants strongly depend on the potential. To interpret the potential dependence of the rate constants, we proposed a molecular mechanism in terms of the displacement between D289 and the adsorbed solvent molecules on the electrode interface. On the basis of these considerations, we provided a new comprehensive understanding of the adsorption of D289 onto the Pt electrode surface. Initially, D289 cations are randomly oriented in the bulk solution, and few adsorb on the electrode. When the applied potential is more negative than the pzc, D289 cations transport from the bulk to the electrode surface with their dipole direction aligned vertically by the electric field and then displace adsorbed CH 3 CN molecules on the surface. Subsequently, the D289 cation surface adsorbates undergo a slow reorientation process. At the same time, more CH3CN molecules are displaced by each reorienting D289. This picture H

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(20) Osawa, M.; Tsushima, M.; Mogami, H.; Samjeske, G.; Yamakata, A. J. Phys. Chem. C 2008, 112, 4248−4256. (21) Garcia-Araez, N.; Climent, V.; Feliu, J. M. J. Am. Chem. Soc. 2008, 130, 3824−3833. (22) Schultz, Z. D.; Shaw, S. K.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 15916−15922. (23) Baldelli, S.; Mailhot, G.; Ross, P.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 2001, 105, 654−662. (24) Chen, A. C.; Richer, J.; Roscoe, S. G.; Lipkowski, J. Langmuir 1997, 13, 4737−4747. (25) Yao, J. L.; Yuan, Y. X.; Fan, X. M.; Ren, B.; Go, R. A.; Tian, Z. Q. J. Phys. Chem. B 2008, 624, 129−133. (26) Nanbu, N.; Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Phys. Chem. B 1999, 470, 136−143. (27) Yang, D. F.; Bizzotto, D.; Lipkowski, J.; Pettinger, B.; Mirwald, S. J. Phys. Chem. 1994, 98, 7083−7089. (28) Henglein, F.; Lipkowski, J.; Kolb, D. M. J. Electroanal. Chem. 1991, 303, 245−253. (29) Pettinger, B.; Mirwald, S.; Lipkowski, J. Ber. Bunsenges. Phys. Chem. 1993, 97, 395−398. (30) Higgins, D. A.; Naujok, R. R.; Corn, R. M. Chem. Phys. Lett. 1993, 213, 485−490. (31) Campbell, D. J.; Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1990, 94, 3681−3689. (32) Liu, J.; Conboy, J. C. J. Am. Chem. Soc. 2004, 126, 8376−8377. (33) Fu, L.; Ma, G.; Yan, E. C. Y. J. Am. Chem. Soc. 2010, 132, 5405− 5412. (34) Fu, L.; Liu, J.; Yan, E. C. Y. J. Am. Chem. Soc. 2011, 133, 8094− 8097. (35) Ye, S. J.; Li, H. C.; Wei, F.; Jasensky, J.; Boughton, A. P.; Yang, P.; Chen, Z. J. Am. Chem. Soc. 2012, 134, 6237−6243. (36) Roeffaers, M. B. J.; Ameloot, R.; Baruah, M.; Uji-i, H.; Bulut, M.; De Cremer, G.; Muller, U.; Jacobs, P. A.; Hofkens, J.; Sels, B. F.; et al. J. Am. Chem. Soc. 2008, 130, 5763−5772. (37) Van der Veen, M. A.; Valev, V. K.; Verbiest, T.; De Vos, D. E. Langmuir 2009, 25, 4256−4261. (38) Heinz, T. F.; Chen, C. K.; Ricard, D.; Shen, Y. R. Phys. Rev. Lett. 1982, 48, 478−481. (39) Duan, X. M.; Konami, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Nakanishi, H. J. Phys. Chem. 1996, 100, 17780−17785. (40) He, G.; Xu, Z. J. Phys. Chem. B 1997, 101, 2101−2104. (41) Rao, Y.; Tao, Y. S.; Wang, H. F. J. Chem. Phys. 2003, 119, 5226− 5236. (42) Shen, Y. R. Annu. Rev. Phys. Chem. 1989, 40, 327−350. (43) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. Rev. B 1999, 59, 12632−12640. (44) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107−125. (45) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Langmuir 1992, 8, 1994−2000. (46) Baldelli, S.; Mailhot, G.; Ross, P. N.; Somorjai, G. A. J. Am. Chem. Soc. 2001, 123, 7697−7702. (47) Zhang, W. K.; Zheng, D. S.; Xu, Y. Y.; Bian, H. T.; Guo, Y.; Wang, H. F. J. Chem. Phys. 2005, 123, 224713−224723. (48) Guyotsionnest, P.; Tadjeddine, A. J. Chem. Phys. 1990, 92, 734− 738. (49) Richmond, G. L. Langmuir 1986, 2, 132−139. (50) Rojhantalab, H. M.; Richmond, G. L. J. Chem. Phys. 1989, 93, 3269−3275. (51) Corn, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. J. Opt. Soc. Am. B 1984, 1, 446−446. (52) Corn, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. Chem. Phys. Lett. 1984, 106, 30−35. (53) Lee, C. H.; Chang, R. K.; Bloembergen, N. Phys. Rev. Lett. 1967, 18, 167−170. (54) Mishina, E.; Yu, Q. K.; Tamura, T.; Sakaguchi, H.; Karantonis, A.; Nakabayashi, S. Surf. Sci. 2003, 544, 269−276. (55) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155−7164.

(56) Mao, Y. A.; Wei, W. Z.; Zhang, J. Z.; Peng, H.; Wu, L. Microchem. J. 2001, 70, 133−142. (57) Andreasen, G.; Vela, M. E.; Salvarezza, R. C; Arvia, A. J. J. Electrocuted Chem. 1999, 467, 230−237. (58) Hamelin, A.; Morin, S.; Richer, J.; Lipkowski, J. J. Electrocuted Chem. 1991, 304, 195−209. (59) Iannelli, A.; Merza, J.; Lipkowski, J. J. Phys. Chem. B 1994, 376, 49−57. (60) Richer, J. R.; Chen, A.; Lipkowski, J. Electrochim. Acta 1998, 44, 1037−1052. (61) Richer, J.; Iannelli, A.; Lipkowski, J. J. Phys. Chem. B 1992, 324, 339−358. (62) Simpson, G. J.; Rowlen, K. L. J. Am. Chem. Soc. 1999, 727, 2635−2636. (63) As presented in Figure 5, the fitting of the time-dependent SHG curve with eq 7 produced the solid curve from S1 to S3, where S1 and S3 have the same SHG intensity. The signal difference between S0 and S1, ΔIS1−S0, was directly incorporated into the constant term C of eq 7 in the fitting. There were several possible interpretatons of ΔIS1−S0. First, only one dominant hyperpolarizability element βccc was considered in the derivation of eq 7, where as the out-plane hyperpolarizability elements that we neglected, although very small as compared to βccc, might make a small but detectable SHG signal for the parallel-oriented D289+. Second, the electrode surface was not perfectly flat as we assumed in the physical model, so that the parallelly adsorbed D289+ were not in absolutely horizontal direction. Third, the change in electrode potential gives rise to a difference in the electric field induced SHG of the substrate and the adsorbed species. On the basis of the above considerations, fitting from S1 to S3 was adequate to obtain the information of adsorption and reorientation dynamics. (64) Swinkels, D. A. J.; Bockris, J. O. J. Electrochem. Soc. 1964, 777, 736−743. (65) Bockris, J. O.; Green, M.; Swinkels, D. A. J. J. Electrochem. Soc. 1964, 777, 743−748. (66) Crispin, X.; Lazzaroni, R.; Geskin, V.; Baute, N.; Dubois, P.; Jerome, R.; Bredas, J. L. J. Am. Chem. Soc. 1999, 121, 176−187. (67) Feng, G.; Huang, J. S.; Sumpter, B. G.; Meunier, V.; Qiao, R. Phys. Chem. Chem. Phys. 2010, 12, 5468−5479. (68) Markovits, A.; Minot, C. Catal. Lett. 2003, 91, 225−234. (69) Higgins, D. A.; Corn, R. M. J. Phys. Chem. 1993, 97, 489−93. (70) Wopschall, R. H.; Shain, I. Anal. Chem. 1967, 39, 1527−1534.

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