Article pubs.acs.org/JPCC
Potential of Zero Charge and Its Temperature Derivative for Au(111) Electrode|Alkanethiol SAM|1.0 M Aqueous Electrolyte Solution Interfaces: Impact of Electrolyte Solution Ionic Strength and Its Effect on the Structure of the Modified Electrode|Electrolyte Solution Interface John F. Smalley* Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973-5000, United States S Supporting Information *
ABSTRACT: We demonstrate how small and rapid temperature perturbations (produced by the indirect laser-induced temperature jump (ILIT) technique) of solid metal electrode|electrolyte solution interfaces may be used to determine the potential of zero (total) charge (Epzc) and its temperature derivative
dEpzc
( ) of Au(111) dT
electrode surfaces modified by alkanethiol self-assembled monolayers in contact with high ionic strength (i.e., 1.0 M) aqueous electrolyte solutions. The Epzc’s measured for two different types of SAMs (made from either HS(CH2)n−1CH3 (5 ≤ n ≤ 12, Epzc = −(0.99 ± 0.12) V vs SSCE) or HS(CH2)nOH (3 ≤ n ≤ 16, Epzc = (0.46 ± 0.22) V vs SSCE)) are considerably different than those measured previously at much lower electrolyte solution ionic strengths. For mixed monolayers made from both HS(CH2)n−1CH3 and HS(CH2)nFc (where Fc refers to ferrocene), the difference in Epzc decreases as a function of the surface concentration of the Fc moiety (i.e., [Fc]), and it completely disappears at a surprisingly small [Fc] (∼4.0 × 10−11 mol cm−2). These observations for the Au(111)|hydrophobic (neat and mixed) SAM|aqueous electrolyte solution interfaces, along with the surface potentials (gSml(dip)) evaluated for the contacting electrolyte solution surfaces of these interfaces, are consistent with a structure for the water molecule components of these surfaces where there is a net orientation of the dipoles of these molecules. Accordingly, the negative (oxygen) ends of these molecules point toward the SAM surface. The positive values of gSml(dip) evaluated for hydrophilic SAM (e.g., made from HS(CH2)nOH)|aqueous electrolyte solution interfaces) also indicate that the structure of these interfaces is similar to that of the hydrophobic interfaces. However, gSml(dip) decreases with increasing ionic strength for the hydrophilic interfaces, while it increases with increasing ionic strength for the hydrophobic interfaces. The data (and calculations) reported in the present work and other studies of hydrophobic (and hydrophilic)|aqueous solution interfaces are as yet insufficient to support a complete explanation for the effects of ionic strength observed in the present study. Nevertheless, an dE
analysis based upon the value of pzc (= (0.51 ± 0.12) mV/K, essentially the same for SAMs made from both HS(CH2)n−1CH3 dT and HS(CH2)nOH), determined in the present study provides a further indication that upon formation of the SAM there is a partial charge transfer of electrons from the relevant gold atoms on the Au(111) surface to the sulfur atoms of the alkanethiols.
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INTRODUCTION Knowledge of the structure, properties, and performance in various functions of chemically modified electrode surfaces is quite important for the further development of a wide variety of technological applications.1−5 Electronic devices based upon organic chemical compounds, specifically organic light-emitting diodes, are primary examples of these applications.5−12 The selfassembly of organized organic compound monolayer films (i.e., self-assembled monolayers or SAMs) onto both metal and semiconductor surfaces has been shown to effect the properties needed for these electronic devices.2−4,12−20 For example, the electronic structure of the organic molecule|metal (or semiconductor) contact is frequently the factor that limits the © XXXX American Chemical Society
charge-carrier injection rate in organic light-emitting diodes (i.e., because this electronic structure induces a large, electronic energy barrier at the metal (or semiconductor)|molecule interface).6−12 Accordingly, because SAMs modify the electronic structures of these interfaces, they also modify their energy barriers8 and thereby the work functions of the metal (or semiconductor) components associated with these interfaces. In electrochemistry, the relationship between the work function of a metallic electrode in contact with a specific electrolyte Received: October 31, 2016 Revised: April 3, 2017 Published: April 6, 2017 A
DOI: 10.1021/acs.jpcc.6b10954 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Schematic of a Au electrode|alkanethiol SAM|electrolyte solution interface at a potential (E) very positive of Epzc. The dark green ovals attached to orange rectangles represent the alkanethiols that produce an inner layer component of the electrochemical double layer.32 The light green ovaloid forms represent a layer of solvent molecules that produce an outer layer component of the electrochemical double layer.32 The red ovals and the blue circles represent the cation and anion components of the electrolyte solution. The arrows represent the dipoles that induce the components (δχAu ml Au−S S (and its components δχ⊥thiol, δχAu thiol, and δχthiol ) as well as −gml(dip)) of the surface dipole potential (see the Supporting Information). The inner Helmholtz and outer Helmholtz planes (IHP and OHP) associated with this interface are represented by pink and cyan lines.
solution and the (electrochemical) potential of zero charge (Epzc) of this electrode/electrolyte solution combination has been known for quite some time.21−23 Measurement of the Epzc of a metal electrode coated with a SAM24−31 therefore provides information on the work function of the modified metal surface and the electronic structure of the metal|SAM interface.11,12,15,16,18−20 However, there is an additional interface consisting of the surface of the SAM in contact with the electrolyte solution as well as the electrolyte solution itself; see Figure 1. Since the average orientation of the solvent molecule dipoles of the electrolyte solution at the SAM|electrolyte solution interface should be quite different from that in the bulk of this solution,20 the surface potential associated with this interface also contributes to Epzc. The potential of zero charge is therefore an essential determinant of the structure of the electrochemical double layer.22,23 Additionally, since an Epzc is determined by the arrangement of the ions and dipoles that comprise a double layer, it has an impact on the kinetics of any interfacial charge-transfer reaction that might take place across this double layer. Measurements of the Epzc of chemically modified surfaces (such as SAMs) therefore provide information that is of great technological importance. For example, these measurements and the information on interfacial reactions that they supply are of consequence in biosensor33 and molecular electronics technologies.34 Double layer properties (including Epzc) are also relevant to electrical energy storage technologies such as batteries, electrochemical capacitors, and hybrids of batteries and electrochemical capacitors.35−37
The rapid change in the temperature of (solid metal) electrode|electrolyte solution interfaces effected by the indirect laser-induced temperature jump (ILIT) technique was originally intended as the perturbation for a relaxation method38 for the study of the kinetics of fast interfacial charge (e.g., electron) transfer reactions. However, the initial (i.e., before any charge transfer has taken place across the relevant interface) change in the open-circuit potential (denoted as A) of an electrode| electrolyte solution interface in response to the ILIT temperature perturbation is a function of Epzc, its temperature derivative dEpzc
( ), as well as other properties of the interface. For interfaces dT
that do not contain ions adsorbed on the surface of the electrode (i.e., ideally polarizable and (electron transfer) inactive interfaces such as those effected by the Au electrodes coated with SAMs investigated in the present study), the measured Epzc is the potential of zero total charge.22 As described in refs 39 and 40, the ILIT-induced initial change in the open-circuit potential (i.e., A) for such interfaces is defined by ⎛ d ln[C T] ⎞⎤ A 1 ⎡ dEpzc = ⎢ − (Ei − Epzc)⎜ ⎟⎥ + bSoret ⎝ d T ⎠⎦ ΔTeq G ⎣ dT (1)
where Ei (measured versus the same reference electrode that determines the value of Epzc) is the potential of the (working) electrode just before the ILIT laser pulse, CT (F/cm2) is the total integral specific capacitance of the electrode|electrolyte solution interface, and bSoret is a coefficient that describes the Soret potential41 caused by the temperature difference between the electrode and the bulk of the electrolyte solution. (A glossary of B
DOI: 10.1021/acs.jpcc.6b10954 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C terms is in the Supporting Information.) The parameter ΔTeq in eq 1 is the change in the temperature of the electrode surface| electrolyte solution interface that would be produced by the heat induced by the absorbed laser energy if none of this absorbed heat were lost to either the dielectric “backing”39 of the electrode or the bulk of the electrolyte solution. Note that in ILIT the temperature perturbation is the result of an “indirect” excitation of the back side of the electrode. In other words, the laser pulse is directed onto the back side of the electrode and is then partially absorbed by the Ti “glue” that attached the Au electrode to its quartz disk backing (see the Experimental Section below and
If
d ln[C T] dT
are also not functions of potential, then a plot
)
⎛ dEpzc ⎞⎛ d ln[C T] ⎞−1 Ei,pzr ′ = Epzc + ⎜ ⎟⎜ ⎟ ⎝ d T ⎠⎝ d T ⎠
(4)
The linearity of these plots (of eq 3) establishes that the values of both Epzc and Epzc and
dEpzc dT 29
dEpzc dT
are independent of potential. If the values of
are also both independent of the thickness of the
monolayer and the value of d ln[C T] is a function of the thickness
(2)
dT
of the monolayer, then a plot of Ei,pzr′ versus ( d ln[C T] )−1 (i.e.,
For alkanethiol-modified surfaces of the Au electrodes in contact with aqueous electrolyte solutions that are equivalent to the interfaces examined in the present study, data reported in
dT
dEpzc
and an intercept that eq 4) will be linear with a slope of dT determines Epzc. In this paper, we describe the determination of both Epzc and
dEpzc
refs 24−29 establishes that dE = 0 over a substantial range of potential (including the potentials investigated in the present study (vide infra)). Additionally, cyclic voltammograms such as those reported in ref 3 and Figure 2 below (as well as
dEpzc
for alkanethiol monolayers comprised of normal alkanedT thiols (i.e., HS(CH2)n−1CH3) and ω-hydroxyalkanethiols (i.e., HS(CH2)nOH) on Au(111). These determinations were accomplished through analyses (using eqs 3 and 4) of the potential dependence (for various values of n) of the parameter A resulting from the ILIT temperature perturbations of each of the different Au(111)|monolayer|electrolyte solution interfaces. The aqueous electrolyte solutions employed in the study described in this paper all had an ionic strength of 1.0M. This ionic strength is very much higher than those employed in previous studies24−29 of the Epzc of alkanethiol monolayers in contact with aqueous electrolyte solutions. All these previous studies report considerably different values of Epzc (i.e., more positive for the normal alkanethiols and more negative for the ωhydroxythiols) than those determined in the present study. We also find that this difference in Epzc (between that measured here for the normal alkanethiols and those measured (using other techniques)24−29 at much lower ionic strengths, i.e., ≤0.10 M) disappears upon the addition of a small amount (
