Modification of Gold's Work Function Upon Adsorption of

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Modification of Gold’s Work Function Upon Adsorption of Mercaptobiphenylcarbonitrile: Experimental Evidence for a Theoretical Prediction Dieric Santos Abreu, Marcia L.A. Temperini, Héctor D. Abruña, and Izaura Cirino Nogueira Diógenes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12439 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Modification of Gold’s Work Function Upon Adsorption of Mercaptobiphenylcarbonitrile: Experimental Evidence for a Theoretical Prediction Dieric S. Abreu,‡ Marcia L. A. Temperini,§ Héctor D. Abruña,† Izaura C. N. Diógenes‡*

‡

Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Cx. Postal 6021,

Fortaleza, CE, Brasil, 60451-970. [email protected] §

Instituto de Química, Universidade de São Paulo, Cx. Postal 26077, São Paulo-SP, Brasil 05508-

000 †

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New

York, USA, 14853-1301

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Tuning the energy of the frontier orbitals of an adsorbed molecule in order to match the Fermi level of the electrode is a fundamental requirement for efficient charge injection in molecular electronic devices. In this manuscript we present electrochemical, impedimetric, spectroscopic and scanning electrochemical microscopy (SECM) data that were used to study the effects of the adsorption of 4′mercaptobiphenylcarbonitrile (HS2PCN) on the work function of gold. The adsorption process was studied and indicated the formation of a loosely packed self-assembled monolayer (SAM, ∆G ads = −43.3 kJ mol −1) following the immersion of the gold substrate in an ethanolic solution of HS2PCN. An increase in the immersion time resulted in the accumulation of negative charge density on the gold surface ascribed to the bonding dipoles resulting from the charge rearrangement at the metal/SAM interface that generates interfacial dipoles as a result of a charge transfer process. As a consequence, a modification of about 1.2 eV is estimated in the work function of the gold surface modified with HS2PCN. Electron transfer rate constants (k0), as measured via SECM, showed a strong dependence on the net charge of the redox probes, and increased on going from negatively (c.a. 1.14 x10−3 cm s−1) to positively charged species (> 1.0 cm s−1). Such behavior is ascribed to the polarity of the interface of the HS2PCN SAM on gold, which is negatively charged due to the electron withdrawing property of the nitrile fragment.


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Introduction As a concept, self-assembly has provided a platform for the deliberate modification of surfaces.1-4 In that context, the self-assembly of molecules on metallic surfaces1,3,5-8 has been of great importance due to their potential application in (opto)electronic devices, corrosion inhibition, molecular electronics, bioelectrochemistry, and others.6,9-12 Since adsorption phenomena affect the work function of metals (Φ M ), metallic surfaces have been deliberately modified so as to enable the control of surface properties.1,4 Knowledge of the molecular scale interactions between an adsorbed molecule and a metallic substrate is of fundamental importance, especially for processes that involve electron transfer/transport events. A series of theoretical papers published by Heimel et al.13-16 on the electronics of self-assembled monolayers (SAMs) formed on metallic substrates provides guidelines as the relevant aspects that should be addressed when considering the use of such systems in electronic devices. Accordingly, modification of the work function (∆Φ) and the alignment of the frontier molecular orbitals (MO) in relation to the electrode’s Fermi level (E F ) are highlighted as key parameters, with the former being connected to the Schottky barrier17,18 and the latter to the electron transport process.19,20 In evaluating metal-SAM systems, it is more appropriate to consider the effective surface work function of the metal with a chemisorbed molecule (Φ M−SAM ) which takes into account the interface dipole resulting from the bonding with the surface, and the molecular dipole. The most widely studied metal-SAM system is based on the immersion of gold substrates in solutions of thiol containing molecules, which generally results in the formation of robust SAMs due to the strong affinity between the sulfur and gold atoms.7,8,21,22 This strong chemical bond, in turn, produces interfaces classified as “ohmic-like” where the electrons move almost freely.4 In addition, within a given SAM, the intermolecular interactions as well as polarization effects are known to strongly affect the electron transport mechanisms. In this context and taking into account the fact that efficient electron (or hole) injection is only achieved if the work function of the metal matches the energy level of the LUMO (Lowest Unoccupied Molecular Orbital) or the HOMO (Highest Occupied Molecular Orbital) orbitals of the adsorbed molecules, tuning of Φ M has been proposed by using SAMs formed 3 ACS Paragon Plus Environment


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with polar molecules whose dipoles are deliberately chosen depending on the desired application. Theoretical reports13,15,23 have shown that densely packed SAMs of biphenylthiolate derivatives affect the work function of the Au(111) surface in a way dependent on the nature of the tail group. For example, the work function of Au(111) is expected to decrease upon the attachment of NH 2 as a tail group, whereas an increase is predicted when the tail group is the strong electron acceptor CN− moiety in SAMs formed with 4′-mercaptobiphenylaniline and 4′-mercaptobiphenylcarbonitrile on gold, respectively. In this work, results obtained by electrochemistry, EIS (electrochemical impedance spectroscopy), and SECM (scanning electrochemical microscopy) of SAMs formed by 4′mercaptobiphenylcarbonitrile (HS2PCN) on gold are presented with the aim of obtaining insights on the interfacial charge transfer process and, by correlating with theoretical data, to understand the influence of such process on the modification of the work function of gold. Experimental Section Chemicals Deionized water (18.2 MΩ cm at 25ºC, Milli-Q) was used to prepare all aqueous solutions. KF (99%), KCl (99+%), H 2 SO 4

(99.999%), KH 2 PO 4

(99%) and K 2 HPO 4

(98%),

Hydroxymethylferrocene (FcMeOH, 97%) and [Ru(NH 3 ) 6 ]Cl 3 (98%), purchased from SigmaAldrich, K 4 [Fe(CN) 6 ].3H 2 O (98.5%) and K 3 [Fe(CN) 6 ] (99+%) from Acros Organics, KOH (Fisher Science, 85%) and H 2 O 2 (30%, Merck), were used as received. 4′-mercaptobiphenylcarbonitrile (HS2PCN - 98% Sigma-Aldrich) was purified by sublimation and stored under an inert atmosphere. Organic solvents (Merck and Aldrich) of spectroscopic grade were used as received. Apparatus and Methods Voltammetric and impedimetric measurements were performed in a three-compartment glass cell using an Autolab PGSTAT302N instrument (Echo Chemie, Utrecht, The Netherlands) equipped with an FRA2 module. Gold (geometric area = 0.07 cm2, Bioanalytical Systems), platinum mesh and a home-made Ag/AgCl (Sat. KCl) were used as working, auxiliary and reference electrodes, 4 ACS Paragon Plus Environment

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respectively. The supporting electrolyte was purged with nitrogen or argon for 20 min prior to experiments, and an inert atmosphere was maintained during all the experiments. Previous to its modification, the gold electrode was cleaned by immersion in “piranha” solution (3:1 concentrated H 2 SO 4 / 30%H 2 O 2 ; CAUTION: Piranha solution is a highly oxidizing mixture that reacts violently with organic compounds) followed by polishing with sandpaper using alumina slurries of different sizes: 1.0, 0.3 and 0.05µm. The polished electrode was sonicated for 10min in a mixture of deionized water and ethanol (50%) to remove any remaining alumina particles. An electrochemical procedure, i.e. reductive potential scan from −0.2 to −1.8 in 0.5 mol L−1 KOH followed by oxidative-reductive potential cycles (ORC) in 0.5 mol L−1 H 2 SO 4 was used to determine the active area of the gold electrode. The cleanness of the gold electrode was evaluated by comparison of the voltammetric profile obtained in a 0.5 mol L−1 H 2 SO 4 solution with the well-established profile for a clean gold surface.24 Finally, the mirror like bare gold electrode was immersed in a 1.0 mmol L−1 ethanol solution of HS2PCN at controlled temperature using a thermostatic bath. This procedure resulted in the formation of a robust SAM of HS2PCN on gold as will be discussed below. The impedimetric responses of the SAMs formed with HS2PCN on gold were obtained in two different electrolyte systems: (1) 0.5 mol L−1 KF aqueous solution free of redox species with the working electrode at its open circuit potential (OCP); (2) 0.5 mol L−1 KF aqueous solution containing 2.5 mmol L−1 [Fe(CN) 6 ]3−/4− redox probe, whose half-wave potential (E 1/2 ) was observed at 0.23 V vs Ag/AgCl. A sinusoidal ac potential of 20 mV (peak-to-peak amplitude) was superimposed on the applied potential and the impedance response was measured over the frequency range from 0.1 Hz to 30 kHz. For the impedance measurements, a gold electrode of 0.0314 cm2 of geometric area was used. The impedance data fitting to an equivalent circuit was performed by using the complex least squares software EIS Spectrum Analyser.25 Normal Raman and SERS (surface-enhanced Raman scattering) spectra were acquired using a Renishaw inVia Reflex spectrometer (controlled by WiRE 3.4 software) coupled to a Leica DM



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2500 M microscope equipped with a 60× water immersion objective (NA = 1.1) to focus the laser beam. Incident radiation of 785 nm (diode laser) was used at a spectral resolution of 1 cm−1. For the SERS spectra acquisition, a polycrystalline gold substrate of 1.0 cm2 geometric area was used as the working electrode. The gold SERS substrate was activated by applying successive ORCs via cyclic voltammetry in 0.1 mol L−1 KCl as electrolyte solution. The ex situ SERS spectra were obtained in air and without applied potential. Heterogeneous electron transfer kinetics were studied by using a scanning electrochemical microscope, model CH Model 900, from CH Instruments, Austin, TX. Such measurements were run by using an electrode configuration similar to those reported in the literature26 where a gold substrate is inserted from below a drilled Teflon cell with the SECM tip concentrically positioned at the top. An ultramicroelectrode (UME) constructed with a 25 µm Pt wire (99.99% Goodfellow Corp., Oakdale, PA) which was sealed in a glass tube using a pipet puller (Narishige Scientific Instrument Lab, Tokyo, Japan) was used as the SECM tip electrode.27 Back contact was made using galliumindium-eutectic and the tip was sharpened using sandpaper. Prior to each measurement, the tip was gently polished with 0.05 μm alumina on a polishing cloth (Buehler, Lake Bluff, IL), rinsed with water, sonicated for 10 min in water and cycled in 0.5 mol L−1 H 2 SO 4 until the typical clean cyclic voltammogram of polycrystalline platinum was obtained. The radius (a) of the Pt SECM tip was determined from the steady-state current (i T,∞ ) at the tip in a solution containing hydroxymethylferrocene (FcMeOH), according to i T,∞ = nFaDC, where n is the number of electrons transfered, F is the Faraday constant, D is the diffusion coefficient and C is the bulk concentration of the redox species (Figure S1 of the Supporting Information). Accordingly, a value of 12.9 µm was determined for the Pt radius and was confirmed by the charge consumed by the adsorption/desorption processes of H 2 on Pt as measured by cyclic voltammetry in 0.5 mol L−1 H 2 SO 4 . The value of RG (= rg/a = 10), which is the ratio of the radius of the glass sheath (rg) to the radius of the active area of the SECM tip (a), was calculated from the approach curves by plotting the tip current (i T ) as a function of the inter-electrode (d) distance in the negative feedback mode (insulating substrate; constant 6 ACS Paragon Plus Environment

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approach rate of 1µm s−1) in a solution containing FcMeOH 1.0 mmol L−1 and fitting to theoretical curves (Figure S1 of the Supporting Information).28 The approach curves were also used for determining the electron transfer rate constants of the redox mediators [Fe(CN) 6 ]4−, [Fe(CN) 6 ]3−, FcMeOH, and [Ru(NH 3 ) 6 ]3+. Geometry optimizations were performed using the Becke’s three-parameter density hybrid functional (B3LYP) with gradient corrected exchange correlation in conjunction with the Lee−Yang−Parr correlation functional.29-32 For calculations on the molecules attached to gold, the gen keyword approach was used for the gold atom to account for relativistic effects while for the rest of the atoms, Pople’s group basis set 6-311++G(d,p) was employed.33 Theoretical vibrational spectra were obtained using the B3LYP/6-311++G(d,p)/LANL2DZ basis set.34 A factor of 0.9689 was applied to scale down the calculated frequencies in order to take into account the effects of anharmonicity. Such approach has been previously shown by our group to yield reasonable agreement with experiments.35,36 Results and Discussion The immersion of a gold substrate in an ethanolic solution of 4′-mercaptobiphenylcarbonitrile (HS2PCN) modifies the surface as can be seen in the SERS spectrum presented in Figure 1. The normal Raman (NR) spectrum of HS2PCN in the solid state is also shown for comparative purposes. In addition, and in an effort to help in the assignment of the bands, simulated spectra were calculated for the HS2PCN molecule in the solid state and bound to a gold atom (dotted lines in Figure 1), and exhibited a reasonable correlation. By analogy to monosubstituted pyridines,37 the signal at 1101 cm−1 in the normal Raman spectrum of HS2PCN is assigned to the X-sensitive band due to the dependence of its intensity and frequency on the trans substituents of the aromatic ring.37,38 The relative intensification of this mode, as well as the observed downshift to 1085 cm−1 in the SERS spectrum, indicate that the adsorption to the gold surface occurs through the sulfur atom, as expected for the adsorption of sulfur containing compounds on gold.11,35,36,38-41 7 ACS Paragon Plus Environment


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Figure 1. Normal Raman (NR) spectrum of HS2PCN in the solid state (red solid line) and ex situ SERS spectrum (in air and without applied potential; blue solid line) of the gold electrode modified with HS2PCN (1h of immersion in 1.0 mmol L−1 HS2PCN ethanolic solution). Dotted lines are for the calculated spectra. Inset: successive SERS spectral acquisitions (from 1 to 60 min) and the dependence of the molecular tilt angle (ϕ) relative to the surface normal. λ 0 = 785 nm. In addition, an intensification in the whole spectrum was observed with increasing immersion time (inset in Figure 1) due to the increase in the magnitude of the dipole moment of vibrational modes near the gold surface. Such dependence on the immersion time is particularly evident in the X-sensitive band and suggests an increase in the monolayer packing with the molecular tilt angle (ϕ) going from ~90o at low surface coverage (t = 1 min) to ~38o at high surface coverage (t = 60 min), as typically observed for aromatic systems adsorbed on gold.42,32 This assignment is corroborated by the strong intensity diminution, in the SERS spectrum, of the band at 835 cm−1 which is ascribed to the out-of-plane CH wagging mode.44 This vibration is perpendicular to the phenyl rings so that the intensity decrease in the SERS spectrum indicates the perpendicular orientation of HS2PCN on the gold surface with the Au-S-C angle being close to 180o.44 Accounting for a near perpendicular orientation (ϕ = 38o), the existence of π interactions among the gold surface atoms and the sulfur


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atoms (or the phenyl rings) of the HS2PCN adsorbed molecules are precluded for longer immersion times (e.g. 1h of immersion). A CH bending mode and two bands assigned to the C=C stretching modes of both phenyl rings37,45 are observed at 1288 cm−1 and at 1596 and 1606 cm−1, respectively, in the normal Raman spectrum. In the SERS spectrum, there is an intensity decrease of the C=C stretching modes as well as a shift to 1587 and 1610 cm−1. The C≡N stretching mode45 is observed at 2226 and 2233 cm−1 in the normal Raman and SERS spectra, respectively. Assuming a gold-sulfur interaction, the intensity decrease of this band in the SERS spectrum is assigned to the distance to the surface while the shift to higher frequency is ascribed to electron delocalization towards the gold surface which decreases the population of the antibonding orbitals, thus increasing the bond order of the C≡N bonding. Similar behaviors have been observed for molecules containing the nitrile fragment, which are adsorbed on gold via the sulfur atom.35,39 The stability of SAMs formed with sulfur containing molecules on gold has been indirectly estimated through the measurement of the strength of the AuS bond by means of reductive desorption in alkaline media.9,46-50 Accordingly, a single wave is expected due to the following electrode reaction AuSR + e− → Au + SR− which occurs at E rd (reductive desorption potential). Thus, the more negative the value of E rd , the more stable the SAM. For the SAM formed with HS2PCN on gold, even for longer immersion times, a single wave was indeed at −1.2 V vs Ag/AgCl in 0.5 mol L−1 KOH. Figure 2 (A) shows the reductive process for the gold electrode after 1 h of immersion in the ethanolic solution of HS2PCN. Assuming a one-electron process (n =1), the area under the wave gives the reductive charge (Q rd ) which, in turn, allows the calculation of the surface coverage (Γ) based on the relation Γ = Q rd /nFA, where F is Faraday’s constant and A is the electrochemical active area. A surface coverage value of 1.8 x 10−10 mol cm−2 was calculated, which is consistent with the formation of a loosely packed, and thus not a densely packed, monolayer.



j/ µA cm-2

0

800

(A)

-3

(B)

400

-6

0

-9

Immersion time (s)

-400

-12 -15 -1.2

-0.8

-0.4

-800 -0.2 0.0 0.2 0.4 0.6

E/ V vs. Ag/AgCl 450

-ZIm, Ω cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(C)

300

0 10 40 220 520 1120 2020 3820 5620

150

0 0

300

600

900

ZRe, Ω cm2 Figure 2. (A) Linear sweep voltammetry of the modified gold electrode (1h of immersion in 1.0 mmol L−1 HS2PCN ethanolic solution) at 0.10 V s−1 in 0.5 mol L−1 KOH. (B) Cyclic voltammograms at 0.1 V s−1 and (C) Nyquist diagrams of bare and modified gold electrodes (after different immersion times) in 0.5 mol L−1 KF containing 2.5 mmol L−1 [Fe(CN) 6 ]3−/4− at 20 oC. Inset in panel (C): Equivalent electrical circuit. It has been recognized9,47,48,51 that the reductive desorption potential of SAMs formed with sulfur containing molecules on gold depends on several factors including: (i) chain length (related to the fractional drop of the applied potential through the SAM and permeability towards the electrolyte upon desorption); (ii) intermolecular interactions; (iii) surface crystallinity of the underlying gold substrate; and (iv) nature of the bonding between the sulfur binding group and the gold surface. For the latter factor, inductive effects as well as the chemical nature of the bonding are known to strongly affect the strength of the AuS bond. For example, while the desorption of the SAM formed with 4mercaptopyridine,41,48 which presents only σ interactions with the gold surface atoms, is observed at −0.56 V vs Ag/AgCl; the E rd value of the SAM formed with 1,4-dithiane on gold is observed51 at 10 ACS Paragon Plus Environment

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−0.87 V vs Ag/AgCl due to the additional π interactions. Regarding the inductive effects, the desorption of sulfur containing molecules coordinated to metal centers (e.g. [Ru(CN) 5 ]3−) that show π-back-bonding interactions occurs at more negative potentials relative to the non-coordinated molecules due to electron delocalization towards the AuS bond.9,40 For the molecules of HS2PCN adsorbed on gold surfaces, as indicated by the SERS spectrum, the perpendicular orientation precludes the existence of π interactions with the gold surface atoms. Also, no additional electron delocalization effects due to π-back-bonding interactions are expected; i.e. the HS2PCN molecules are not bounded to any metal center through the nitrile fragment. Therefore, the highly negative E rd value is ascribed, at least in part, to strong intermolecular lateral interactions, mostly π stacking. This conclusion is in accordance with previous reports on the study of phenylthiol-SAMs in which it was proposed that stronger intermolecular interactions in densely packed SAMs increase the stability and consequently shifts the desorption potential toward more negative values.47,50 Surface coverage values (Γ i ) obtained for different concentrations of HS2PCN in solution were used to determine the thermodynamic parameters of adsorption by using the Langmuir and Frumkin models. The Γ i values obtained were applied to the linearized Langmuir equation allowing for the calculation of the saturation surface coverage (Γ s = 6.62 x 10−10 mol cm−2) and the adsorption coefficient (β = 2.06 x 103 L mol −1). The best fit to the Frumkin isotherm with these data was achieved when a value of +1.66 was used for the interaction parameter, g, indicating attractive interactions between the adsorbed HS2PCN molecules. This result reinforces the assignment of strong intermolecular lateral interactions between the adjacent HS2PCN molecules. Figure S2 of the Supporting Information presents the Langmuir and Frumkin isotherms for the adsorption of HS2PCN on gold. Based on the equation ∆G ads = −RTln(a s β), where a s is the activity of the solvent and/or ions in solution, the value of the free energy of adsorption (∆G ads ) was determined to be −43.3 kJ mol−1, indicating a significant interaction. Also, this free energy value indicates a strong chemical bond suggesting the interface AuHS2PCN can present a ohmic-like contact.4 11 ACS Paragon Plus Environment


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The cyclic voltammograms presented in Figure 2 (B) show an increase in the difference between the peak potential values (∆E p ) and a decrease in the faradaic current assigned to the FeIII/II redox couple of the [Fe(CN) 6 ]4− probe molecules due to sluggish electron transfer kinetics after longer immersion times. These results indicate the increasing difficulty of the probe molecules to access the underlying gold surface with an increase in the immersion time of the gold substrate in the HS2PCN ethanolic solution. Such behavior is generally associated with an increase in the packing density of the monolayer at longer immersion times as can be concluded from the Nyquist diagrams shown in Figure 2 (C). Table 1 summarizes the dependence of the EIS parameters on the immersion time. Table 1. Electrochemical and EIS parameters dependence on the immersion time of the gold electrode in 1.0 mmol L−1 ethanolic solution of HS2PCN. Electrolyte medium: 0.5 mol L−1 KF containing 2.5 mmol L−1 [Fe(CN) 6 ]3−/4− at 20 oC. Immersion time ∆E p (mV)

∗ 𝑅𝑅𝐶𝐶𝐶𝐶 , Ω cm2

θ

k app x 103, cm s−1

0.0

70

4.14

0

25.3

10

71

7.09

0.415

14.8

40

73

7.19

0.424

14.5

220

112

65.9

0.937

1.59

520

225

2.12 x 102

0.980

0.494

1120

308

3.63 x 102

0.988

0.289

2020

352

5.01 x 102

0.992

0.209

3820

394

6.15 x 102

0.993

0.170

5620

396

6.57 x 102

0.994

0.159

(s)

∗ ∗ 𝜃𝜃 = 1 − [𝑅𝑅𝐶𝐶𝐶𝐶 − 𝑅𝑅𝐶𝐶𝐶𝐶 ],52 where 𝑅𝑅𝐶𝐶𝐶𝐶 and 𝑅𝑅𝐶𝐶𝐶𝐶 are the charge-transfer resistances of the bare and modified gold

electrodes, respectively, and 𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 = �RT�(𝐹𝐹 2

�. ∗ 𝑅𝑅𝐶𝐶𝐶𝐶 𝐶𝐶 ∗ )


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∗ The data presented in Table 1 clearly show an increase in the charge transfer resistance (𝑅𝑅𝐶𝐶𝐶𝐶 )

and fractional coverage (θ) with an increase in the immersion time of the gold substrate in the ethanolic solution of HS2PCN. Such results are indicative of the packing of the monolayer, as

corroborated by the strong decrease of the apparent electron charge transfer rate constant (k app ) on going from the bare surface (25.3 x 10−3 cm s−1) to the modified gold after c.a. 90 min of immersion (1.59 x 10−4 cm s−1). To further analyze the packing density of the SAM, impedimetric and voltammetric measurements were performed in KF aqueous solution as a function of the immersion time and without a redox species in solution. The voltammetric curves obtained for bare (i) and modified (ii) gold electrodes are presented in Figure 3. Plots of current density (j) vs scan rate (v), from which the differential capacitances of the bare and modified gold surfaces were determined, are also shown. (A) 25 V s

v, V s

0.50

(ii)

1.00

0

2.50

-600

10.00

5.00

15.00 25.00

-0.3

0.0

0.3

(i)

800

0.25

600

-1200 -0.6

(B)

1000

0.10

(i)

-1

-1

j, µA cm-2

1200

-2 j, µA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 R² = 0.9991

400

(ii)

200

R² = 0.9914

0

0.6

0

E, V vs. Ag/AgCl

5

10

15

v, V s

-1

20

25

Figure 3. (A) Cyclic voltammograms at different scan rates (values in the center) highlighting the curves obtained at 25 V s−1 for bare (red) and modified (black) electrodes. (B) Plots of current density (j at E = 0.0 V, in Figure 3 (A)) vs scan rate for bare (i) and (ii) modified (1 h of immersion) gold electrodes, respectively. Electrolyte conditions: 0.50 mol L-1 KF aqueous solution at 20 ºC. The double layer capacitances of the bare (C dl ) and modified (C m ) gold surfaces were calculated by cyclic voltammetry assuming a perfect capacitor model.53 The value of the total capacitance (C T ) is given by Equation (01):52,54 0 0 (1 − θ) + Cm CT = Cdl + Cm = Cdl ∗θ

ACS Paragon Plus Environment

(01) 13


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0 0 where θ is the fractional coverage and Cdl and Cm are, respectively, the capacitances of the bare gold

and the gold surface modified with a defect-free monolayer. From cyclic voltammetric measurements, 0 the value of Cm can be calculated according to Equation (02):53 𝑗𝑗𝑎𝑎 −𝑗𝑗𝑐𝑐 2

=

dE dt

Cm = vCm

(02)

where j a and j c are the anodic and cathodic current densities, respectively. For the studied system, C m 0 ≈ Cm after 1h of immersion when the maximum surface coverage is achieved (θ = 0.994, Table 1).

The calculated capacitances showed almost no dependence on potential as expected for a thin layer.

0 After calculating Cm (8.98 µF cm−2), the value of the dielectric constant of the monolayer (ε m = 10.1)

was determined according to Equation (03):

εm =

C0m ∗d

(03)

ε0

where ε 0 is the permittivity of free space (8.85 x 10−12 C2 N−1 m−2) and d is the monolayer thickness, which was estimated as 14Å from DFT and is consistent with values reported in the literature for 0 HS2PCN (L = 14.7 Å).55 In comparison to the SAMs formed on gold with thiophenol (Cm = 4.6 µF 0 cm−2 and ε m = 0.52), p-biphenylmercaptan (Cm = 4.5 µF cm−2 and ε m = 4.5) and p-terphenylmercaptan

0 0 (Cm = 3.4 µF cm−2 and ε m = 4.2),56 the relatively higher values found for HS2PCN (Cm = 8.98 µF

cm−2 and ε m = 10.1) reinforce the suggestion that differences in capacitance are associated to the dielectric behavior of the terminal groups.57 In fact, ε m gives a measure of the decrease in the dipole moment due to the depolarization effects.16 In this work, the molecular dipole of HS2PCN was calculated as 5.01 D for the non-adsorbed molecule, in agreement with values reported in the literature 0 for HS2PCN.23 From Cm and ε m data (Table S1 of the Supporting Information) obtained at different

immersion times, the values of the change of the effective work function of the gold substrate modified with HS2PCN (∆Φ M-SAM ) for different surface coverages were estimated according to the Helmholtz equation:59 𝑁𝑁 µ0 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶

∆ΦM−SAM = 𝑒𝑒 𝐴𝐴

𝜀𝜀0 𝜀𝜀𝑚𝑚

(04)


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where µ0 is the gas phase dipole and N/A is the number of molecules per surface area (see Equations S1 to S4 and Table S4 of the Supporting Information for details).

Figure 4 shows the dependence of ∆Φ M-SAM on the surface coverage and molecular tilt angle (ϕ) with respect to the surface normal.

Figure 4. Effective work function of the gold substrate modified with HS2PCN (∆Φ M-SAM ) as a function of surface coverage (Γ) and molecular tilt angle (ϕ) with respect to the surface normal. Results reported in the literature1,13,15,60 have shown that metal work functions are particularly sensitive to the molecular dipole of the adsorbed species, which can be tuned by attaching different terminal groups. Computational data on thiol SAMs having NH 2 and CN as terminal electrondonating and -withdrawing groups indicate, respectively, a decrease and increase in the work function of gold.23 For the SAM of HS2PCN on gold, the increase of the calculated work function, however, was higher than expected23 due to the high molecular dipole moment of the molecule. As a possible explanation, the authors considered the contribution from the thiol docking group, which was not taken into account in the calculations, and by the bonding dipoles resulting from the charge rearrangements at the metal/SAM interface upon the formation of the AuS bond.13,23 In fact, it is well-known that charge transfer processes from the metal to the adsorbed molecule or vice-versa can also generate an interfacial dipole which is strongly affected by the surface coverage.15,61 As 15 ACS Paragon Plus Environment


mentioned in the previous discussions, when increasing the immersion time, there is an increase in the surface coverage (Γ) that results in an increase of the dipole density and the decrease of the molecule tilt angle (ϕ) with respect to the surface normal. Figure 4 shows this trend, indeed, with an increase of work function of about 1.2 eV for the SAM formed after 1h of immersion in the solution of HS2PCN. Such work function modification is, however, about 1.45 eV lower than that theoretically calculated (2.65 eV).15 Such a difference could be ascribed to imperfections of the SAM formed with HS2PCN on a polycrystalline gold surface and also to the strong intermolecular interactions among the neighboring molecules (through π-stacking). These aspects could contribute to additional depolarization that could result in values of ∆Φ that are lower than those theoretically expected. The open circuit potential (OCP) in ohmic-like systems is determined by the HOMO and LUMO levels, respectively, of the acceptor and donor which are, in turn, determined by the frontier orbitals of the adsorbed molecules and the work function of the electrode.62 The dependence of the OCP parameter on the immersion time of the gold electrode in an ethanolic solution of HS2PCN, shown in Figure 5, illustrates its sensitivity to the surface characteristics, as there is no redox couple controlling the potential of the surface.53 (A)

(B)

0.15 0.10

-φ/ º

E, V vs Ag/AgCl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

0.05

80.0

Immersion time (s)

60.0

0 10 60 300 520 1500 3600

40.0

0.00

20.0

-0.05

0.0 0

30

60

90

120

1

10

100

Time, s

1k

10k 100k

f/ Hz

Figure 5. Plots of (A) potential vs time and (B) phase angle vs frequency (Bode diagrams) as function of the immersion time of the gold electrode in a 1.0 mmol L−1 ethanolic solution of HS2PCN. Electrolyte conditions: 0.50 mol L−1 KF aqueous solution at 20 ºC. 16 ACS Paragon Plus Environment

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As can be seen in Figure 5 (A), the OCP values (values at 120 s) decrease with an increase in the immersion time indicating the accumulation of negative charge on the surface. In addition, the Bode phase plots show phase angles (ϕ) from 60 to 72o within the frequency region 1 ≤ f ≤ 103 Hz indicating the existence of defects in the SAM, even at high fractional coverages (θ > 0.98). It is worth mentioning that two phase angles are observed (Figure S3 of the Supporting Information) for immersion times longer than 300s, suggesting a two-step adsorption process. As generally accepted for the adsorption of thiols on gold,8,63,64 such two-step processes can be reasonably ascribed to the fast formation of a AuS bond followed by self-organization that probably involves lateral diffusion on the surface and increases the packing density, thus decreasing the double-layer capacitance. Table 2 presents the relevant parameters as obtained from the potential vs time and Bode phase plots shown in Figure 5 for the modified surfaces in solution without redox probe molecules. Table 2. OCP values and impedimetric parameters obtained, respectively, from the potential vs time and Bode phase plots illustrated in Figure 5. Immersion time OCP (V)

ϕ (at 1Hz)

(s)

ν d (Hz) (experimental)

0.0

0.135

43.82

65.25

10

0.056

43.82

49.36

60

0.033

48.94

27.99

300

0.004

54.87

21.31

520

−0.022

59.71

9.12

1500

−0.045

63.22

6.89

3600

−0.047

66.72

2.99

The dielectric relaxation frequency (ν d ) is associated with the rate at which the SAM is charged by ions present in the electrolyte solution.65 In the present study, the decrease of ν d with the 17 ACS Paragon Plus Environment


Page 18 of 31

increase in the immersion time indicates lower charging rates due to the monolayer packing. Assuming that the change in the surface potential on the metal side (δM) remains constant, changes in the OCP is associated with the adsorbed molecules and can be considered as experimental evidence of the SAM-induced work function modification. It is well-known that the surface polarity, which depends on the distal end of the SAM (−CN moiety in this work), can affect the heterogeneous electron transfer (ET) rate constants (k0).66 To further investigate this issue, the electron transfer rate constants of redox species of different net charges were studied by SECM. In this work, the SECM was operated in the feedback mode in which the gold modified substrate was biased so that the substrate reaction is opposite to the diffusionlimited reaction at the SECM tip electrode as illustrated in Figure 6.

Figure 6. Schematic illustration showing the reactions occurring at the SECM tip and the gold substrate modified with HS2PCN during the acquisition of the SECM measurements. Considering the Butler-Volmer model, which establishes the exponential dependence of k0 on the overpotential (η = E − E eq ; where E eq = equilibrium potential) applied to the electrode,53 approach curves were obtained for different η values. By applying high enough overpotentials, one of the processes (forward, k f , or backward, k b ) can be neglected. Considering the forward reaction for the reduction, k b values were determined for the reducing complexes [Fe(CN) 6 ]4− and FcMeOH while 18 ACS Paragon Plus Environment

Page 19 of 31

the oxidizing species, [Fe(CN) 6 ]3− and [Ru(NH 3 ) 6 ]3+, allowed the determination of k f values upon fitting to theoretical curves proposed by Lefrou and Cornut.67 Figure 7 shows the approach curves obtained for the redox mediators [Fe(CN) 6 ]4−, [Fe(CN) 6 ]3−, [Ru(NH 3 ) 6 ]3+, and FcMeOH at different overpotentials applied to the gold substrate modified with HS2PCN. For the studied systems, η value was defined as E subs − E eq ; where E subs and E eq are, respectively, the potential applied to the gold modified substrate and the half-wave potential (E 1/2 ) as determined from the cyclic voltammogram of each redox mediator at one-half of the steady-state current of the SECM tip (Figure S4 of the Supporting Information).

IT = iT/iT,∞

η/mV

(A)

2.7

103kb/ cm s-1

1.8

η/mV

(B)

3.6

0.75 1.36 2.54 4.17 5.93 9.19 14.50

-57 -107 -157 -207 -257 -307 -457

90 120 170 220 270 320 370

2.4

0.9

103kf / cm s-1 4.4 5.7 12.3 16.1 22.8 31.6 40.6

1.2 Negative Feedback Positive Feedback

0.0

Negative Feedback Positive Feedback

0.0 0

1

2

3

4

0

1

2

6.0 η/mV

(C)

IT = iT/iT,∞

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


103kf / cm s-1

-190 160

4.5

η/mV

4.5

(D)

3.0 3.0 1.5

4 -1

3

10 kb/ cm s

-105 -155 -225 -455 -655

93.4 127.1

Negative Feedback Positive Feedback

3 19.3 23.3 28.7 37.9 122.7 Negative Feedback Positive Feedback

1.5

0.0

0.0 0

1

2

3

4

0

1

2

3

4

L = d/a

L = d/a

Figure 7. Approach curves of the SECM Pt tip towards the gold substrate modified with HS2PCN at different applied overpotentials in a 0.1 mol L−1 KF solution containing 1.0 mmol L−1 of the following redox mediators: [Fe(CN) 6 ]4− (A), [Fe(CN) 6 ]3− (B), [Ru(NH 3 ) 6 ]3+ (C), and FcMeOH (D). Open symbols are for experimental data, and solid lines are fitting theoretical curves.67 Pure negative and positive feedback responses are shown as black solid and dotted lines, respectively.



Page 20 of 31

For both negatively charged redox mediators a transition from negative to positive feedback response was observed at L < 1 with decreasing ([Fe(CN) 6 ]4−) and increasing ([Fe(CN) 6 ]3−) overpotentials indicating that theses species have finite electron transfer kinetics at the gold substrate modified with HS2PCN. For the positively charged redox mediators, on the other hand, positive feedback currents were seen at all applied overpotentials. This behavior indicates that the electron transfer kinetics for the latter species are faster than those observed for the negatively charged complexes. By plotting the values of k as a function of η (Figure S5 of the Supporting Information), 0 the values of the apparent rate constant, 𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 , are obtained from the y intercept giving a quantitative

0 determined in this work. measure of the electron transfer kinetics. Table 3 presents the values of 𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎

Table 3. Values of half-wave potentials (E 1/2 ), diffusion coefficients (D), and apparent rate constants 0 (𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 ) for different redox mediators (1.0 mmol L−1 in 0.1 mol L−1 KF) as determined by SECM in

the feedback mode using a gold electrode modified with HS2PCN as substrate. E 1/2 Redox Mediator

E Tip

V vs. Ag/AgCl (Cl− Std. )

D x 106 / cm2 s−1

0 𝑘𝑘𝑎𝑎𝑎𝑎𝑎𝑎 / cm s−1

[Fe(CN) 6 ]3−

+ 0.230

+ 0.1

7.6

1.14 x10−3

[Fe(CN) 6 ]4−

+ 0.257

+ 0.4

8.1

2.55 x10−2

FcMeOH

+ 0.260

+ 0.4

6.7

1.33 x10−2

[Ru(NH 3 ) 6 ]3+

− 0.140

− 0.30

8.4

1.42*

*For [Ru(NH 3 ) 6 ]3+, k0 value is, in fact, an observed rate constant (k obs ) since the approach curves were all the same despite of the applied overpotentials.

From the data displayed in Table 3, an increase in the rate constant values on going from negatively to positively charged species is clearly observed. Although lower k0 values are frequently reported for gold surfaces modified with mercaptan molecules due to the blocking effect,52, 68 in this work the electrostatic effect seems to be dominant, taking apart from the interface the negatively charged complexes and approaching the cationic complex. This rather simple explanation, however, does not explain the entire picture since one would expect a partial, if not a complete, blocking effect 20 ACS Paragon Plus Environment

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for the negatively charged species. Indeed, based only on electrostatic arguments, FcMeOH (k0 = 1.33 x 10−2 cm s−1) would have shown higher k0 value in comparison to the negatively charged [Fe(CN) 6 ]4− species (2.55 x 10−2 cm s−1). We hypothesize that hydrogen bonds are formed among FcMeOH and HS2PCN molecules thus making sluggish the electron transfer kinetics. Indeed, it was recently reported the influence of hydrogen bonding in the electron transfer kinetics of FcMeOH by using a thiolate SAM.69,70 The dependence of k0 values on the net charge of the redox mediators suggests the influence of the surface polarity which, for the HS2PCN SAM, results in negative charge density due to the withdrawing property of the nitrile fragment, as previously mentioned. Conclusions We have been shown in this work that the immersion of a gold substrate in an ethanolic solution of HS2PCN results in the spontaneous formation of a monolayer with a free energy of adsorption, ∆G ads = −43.3 kJ mol −1. As expected for sulfur containing compounds, the SERS spectra indicate that the adsorption occurs via the sulfur atom; a result that was corroborated by DFT calculations. Impedimetric and electrochemical results showed a dependence of the SAM packing density on the immersion time. The significant decreases observed in the double-layer capacitance, and apparent electron transfer rate constant, with an increase in the immersion time, were assigned to strong intermolecular lateral interactions that not only increased the stability but also the packing density of the SAM. A two-step adsorption mechanism was proposed for the formation of HS2PCN on gold based on the observation of two phase angles in the Bode plots. Such a two-step process was ascribed to the fast formation of AuS bond followed by self-organization that probably involves lateral diffusion on the surface and increases the packing density of the SAM. The electrochemical data presented in this work showed a decrease in the open circuit potential with an increase in the immersion time in solution of HS2PCN indicating the accumulation of negative charge density on the surface due to the bonding dipoles resulting from the charge rearrangements at the metal/SAM interface upon the formation of AuS bonds. This result is in accordance with theoretical calculations reported in the literature that state that the work function of 21 ACS Paragon Plus Environment


Page 22 of 31

the gold surface increases upon the adsorption of HS2PCN. The influence of the surface polarity of HS2PCN SAM on heterogeneous electron transfer reactions was studied by SECM using redox probes of different net charges. The values of the electron transfer rate constant (k0) showed a strong dependence on the net charge of the redox probes, increasing on going from negatively (c.a. 1.14 x10−3) to positively charged species (>1.0). Such behavior is ascribed to the surface polarity generated upon the formation of the HS2PCN SAM, which is negatively charged due to the withdrawing property of the nitrile fragment. Supporting Information. Voltammetric profile and approach curves for the SECM Pt tip (Figure S1); Langmuir and Frumkin isotherms for the adsorption isotherm of HS2PCN on gold (Figure S2); 0 Values of Cm and ε SAM obtained at different immersion times for the calculation of the effective work

function of the gold substrate modified with HS2PCN (Table S1); Bode plots for the adsorption process of HS2PCN on gold in function of the immersion time (Figure S3); Fitting parameters for the equivalent circuit analysis of the Bode plots (Tables S2 and S3); Values of ∆Φ M-SAM in function of

the molecular tilt angle (ϕ) and surface coverage (Γ i ) at different immersion times (Tables S4); Cyclic voltammograms of SECM Pt tip in solution containing the redox probes (Figure S4); Plots of lnk vs η (Figure S5).

Acknowledgements Diógenes, I. C. N. (# 304285/2014-5), Temperini, M. L. A (# 302792/2015-5), and Abreu, D. S. are thankful to CNPq, FAPESP 2012/13119-3, and FUNCAP (PR2-0101-00030.01.00/15/PRONEX) for the grants and financial support. The authors thank CENAPAD/CE for providing the computational resources.


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TOC Graphic


Modification of Gold's Work Function Upon Adsorption of

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