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J. Phys. Chem. C 2008, 112, 7284-7289
Charge Transfer across Self-Assembled Nanoscale Metal-Insulator-Metal Heterostructures Jianjun Zhao, Matthias Wasem, Christopher R. Bradbury, and David J. Fermı´n* Departement fu¨r Chemie und Biochemie, UniVersita¨t Bern, Freiestrasse 3, CH-3012 Berne, Switzerland ReceiVed: October 19, 2007; In Final Form: December 22, 2007
The kinetics of charge transfer across a metal-insulator-metal architecture is investigated by electrochemical impedance spectroscopy. The insulating component of the architecture is composed by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), polyelectrolyte multilayers, and a monolayer of 22 nm SiO2 nanoparticles. The charge transfer to the hexacyanoferrate couple is strongly hindered by the MUA monolayer. The blocking properties effectively vanish with the adsorption of a diluted monolayer of Au nanoparticles (19 nm). Atomic force microscopy and scanning electron microscopy analyses demonstrate that the Au nanoparticles are physically separated from the Au surface by the SiO2 monolayer. The strong electronic communication between the metal nanoparticles and the electrode is rationalized by a nonthermalized transport process involving redox species trapped in the multilayer assembly.
1. Introduction A variety of studies in the past 10 years have shown that the electronic coupling between nanostructures and metal surfaces can be manipulated by the nature of the linking units.1 The seminal work by Nichols and co-workers demonstrated that the conductance of viologen-containing molecules bridging Au nanoparticles and a metal electrode reaches a maximum when the electrode potential is set to the formal potential of the first reduction state of the viologen moiety.2 The decrease of the apparent transport barrier between the tip and the substrate was rationalized in terms of a resonant tunneling process. The first evidence of resonant tunneling observed under electrochemical control was reported by Tao.3 Other studies based on electron transport across a single molecule also show a decrease in the transport barrier when the Fermi level at the electrode coincides with the Fermi energy of the electrons in the molecule.4-6 Resonant transport processes attenuate the characteristic decay of the tunneling probability with distance. This is the rationale behind current developments in the design of molecular bridges with small HOMO-LUMO gaps to be used in molecular electronics.7 We have shown that charge transfer across a blocking selfassembled monolayer can be significantly accelerated upon adsorption of an ultrathin polyelectrolyte (PE) film and metal nanoparticles.8,9 Up to 3 orders of magnitude enhancement of the apparent electron-transfer rate constant has been observed in the presence of highly insulating monolayers and redox species with strong affinity to the polyelectrolyte film.10 This experimental evidence was rationalized in terms of nonthermalized charge transport between the metal nanoparticles and the electrode surface via trapped redox species. In this work, we shall demonstrate that this efficient electronic communication can be observed over distances longer than 20 nm as determined by a rigid nanoscopic spacer. The strategy involves the self-assembly of a metal-insulator-metal architecture at Au * To whom correspondence should be addressed. Current address: School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K. E-mail:
[email protected]. Phone: +44 117 9288981. Fax: +44 117 9250612.
surfaces incorporating SiO2 and Au nanoparticles as illustrated in Figure 1. The SiO2 particles act as a nanoscopic spacer to ensure that the metal nanoparticles are not in direct contact with the surface. The multilayer structure also features a selfassembled monolayer (SAM) at the Au surface to block the direct charge transfer from the redox probe. The results demonstrate that the enhancement of the charge transport dynamics in the presence of Au nanoparticles is not linked to defects in the ultrathin films. 2. Experimental Section Polycrystalline evaporated gold electrodes were used in the electrochemical experiments,8 while flame-annealed Arrandee gold substrates were used for the microscopic characterization of the assemblies. The sequential modification of the Au electrodes was begun with a SAM of 11-mercaptoundecanoic acid (MUA) obtained by immersion of the electrode in a 1 × 10-3 mol dm-3 ethanol solution of the thiol for more than 14 h. After thorough rinsing with ethanol and then ultrapure Milli-Q water, a polyelectrolyte multilayer incorporating poly-L-lysine (PLL) and poly-L-glutamic acid (PGA) was grown by sequential electrostatic layer-by-layer adsorption. This is achieved by the alternate dipping of the electrode in 1 g dm-3 of the corresponding polypeptide in aqueous solution (pH between 6 and 7) for 20 min. Three PLL-PGA bilayers were deposited terminating in a PLL layer. This ultrathin polyelectrolyte multilayer (PEM) film acts as a spacer between the SiO2 nanoparticles and the electrode surface. LUDOX TMA colloidal SiO2 nanoparticles (Sigma-Aldrich) with an average diameter of 22 nm were employed as received. Rather compact monolayers of the SiO2 nanoparticles were obtained by immersing the PLL-terminated electrode film in a 420 g dm-3 solution at pH 7.0 for 1 h. A further layer of PLL was adsorbed on top of the SiO2 assembly prior to the adsorption of the Au nanoparticles. The synthesis and assembly of the 19.2 ( 2.1 nm citrate-stabilized Au nanoparticles was performed following previously reported methods.8 To investigate the nanoparticle adsorption over large portions of the surface, multilayer structures were assembled on boron-
10.1021/jp7101644 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/15/2008
Charge Transfer across Metal-Insulator-Metal Layers
Figure 1. Schematic representation of the self-assembled metalinsulator-metal heterostructure at Au surfaces. The Au electrode is initially modified by a SAM of MUA, followed by a PEM featuring PLL and PGA. A two-dimensional monolayer of 22 nm SiO2 nanoparticles was electrostatically adsorbed on the PLL-terminated film. The heterostructure is completed by the adsorption of a single PLL layer and a diluted two-dimensional monolayer of citrate-stabilized Au nanoparticles (19 nm) on top of the SiO2 monolayer. For the sake of clarity, the image is not drawn to scale.
doped Si(111) wafers (ITME). The wafers were cleaned by sequential sonication in acetone, ethanol, and Milli-Q water. In this case, the PEM growth was initiated by direct adsorption of PLL onto the native silicon oxide layer. Scanning electron microscopy (SEM) studies of the assembly on the degenerate Si wafers provided high-quality images and a clear contrast between the Au and SiO2 nanoparticles. The structure of the multilayer assembly was probed by a JEOL JSM 6330F scanning electron microscope. This information was complemented by atomic force microscopy (AFM) employing a Molecular Imaging Pico LE in acoustic mode. The electrochemical responses of the modified Au electrodes were recorded in a single-compartment cell using an Autolab PGSTAT 30 featuring a frequency response analyzer module. A platinum foil of 0.34 cm2 and a silver-silver chloride (Ag/AgCl) electrode were used as counter and reference electrodes, respectively. All potentials in this paper are referred to the Ag/ AgCl electrode. Measurements were carried out in solutions containing 0.1 mol dm-3 Na2SO4 solution, 1 × 10-3 mol dm-3 K4Fe(CN)6 and 1 × 10-3 mol dm-3 K3Fe(CN)6. The impedance spectra were recorded at the equilibrium potential, Eeq ) 0.22 ( 0.01 V vs Ag/AgCl, in the frequency range of 30 kHz to 130 mHz and at a 10 mV rms amplitude. 3. Results and Discussion 3.1. Electrostatic Assembly of Two-Dimensional Layers of SiO2 and Au Nanoparticles on Si Wafers. The electrostatic adsorption of the SiO2 nanoparticles on the PEM films forms a homogeneously distributed monolayer as demonstrated by the SEM images in Figure 2A. This image was obtained after a Si wafer modified with a (PLL-PGA)3-PLL film was dipped in a colloidal solution of the SiO2 particles with a concentration
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Figure 2. Typical SEM microgaphs of a SiO2 monolayer adsorbed on a Si(111) wafer modified with a (PLL-PGA)3-PLL multilayer generated by electrostatic layer-by-layer adsorption (A, B). The SiO2 monolayer was formed by introducing the PLL-terminated surface into a 140 g dm-3 colloidal solution for 1 h. The particle number density was estimated to be ΓSiO2 ) (3.1 ( 0.8) × 1010 cm-2. SEM micrographs of the Au nanoassembly obtained after successive immersions of the SiO2-nanoparticle-terminated surface in a PLL solution and a citratestabilized Au nanoparticle (C, D). A particle number density of ΓAu ) (2.4 ( 0.4) × 1010 cm-2 was obtained after analysis of various areas of the electrode. The Au nanoparticles preferentially adsorb on the areas covered by the SiO2 nanoparticles.
of 140 g dm-3 for 60 min. Figure 2A clearly show that the SiO2 particles form a two-dimensional assembly on the PLLterminated film with a very low density of three-dimensional aggregates. Images obtained with higher magnification reveal that the assembly is formed of adsorbed single particles as well as small two-dimensional aggregates of two to five particles as illustrated in Figure 2B. Analysis of the images in various portions of the surface allowed estimation of an average particle number density of ΓSiO2 ) (3.1 ( 0.8) × 1010 cm-2. Considering that the SiO2 nanoparticles have an average diameter of 22 nm, the coverage of the SiO2 monolayer can be estimated on the order of 10%. The adsorption of the Au nanoparticles on the SiO2 monolayer modified by a layer of PLL generates a further diluted monolayer as shown in Figure 2C. The adsorption of the Au nanoparticles was performed by introducing the modified electrode in the as-prepared colloidal solution for 1 h. The SEM images show a rather limited number of nanoparticle aggreates at the surface. The image in Figure 2D further demonstrates that the overwhelming number of Au particles are adsorbed on top of the SiO2 nanostructures. Analysis of the SEM images indicates that the fraction of Au particles adsorbed on areas not covered by the SiO2 monolayer is below 5%. 3.2. SiO2/Au Nanoparticle Heterostructures at Au(111) Surfaces. In the case of Au surfaces, the construction of the multilayered heterostructures is started with a self-assembled monolayer of MUA. The acoustic AFM image displayed in Figure 3 shows that the topography of Au(111) terraces is not significantly affected by the adsorption of the MUA monolayer and the PEM film. Data reported elsewhere revealed that the roughness of the film is less than 1 nm and the thickness in the presence of the redox electrolyte is approximately 2.5 nm.10,11 As discussed further below, the MUA monolayer assembled at the Au surface establishes a strong barrier for the redox process
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Figure 3. AFM image (5 µm × 5 µm) of the Au surface after electrostatic adsorption of the PEM film. The 1 µm × 1 µm × 10 nm inset confirms the smooth properties of the ultrathin film.
involving the hexacyanoferrate couple. On the other hand, the redox couple exhibits a very strong affinity for the PEM film, leading to a swelling of the film.10 To increase the coverage of SiO2 as compared to that of the assemblies described in Figure 2, the concentration of the SiO2 colloid was increased by a factor of 3 while the same adsorption time was maintained. Under these conditions, the electrostatic adsorption of the SiO2 nanoparticles on the MUA-PEM film also generated a two-dimensional monolayer as exemplified in Figure 4A,B. The analysis of the AFM and SEM images revealed that ΓSiO2 ) (5.4 ( 0.4) × 1010 cm-2. Only a few isolated SiO2 nanoparticles can be seen adsorbed on top of the dense monolayer, while examination of large portions of the electrode did confirm that the SiO2 particles form a single monolayer. The surface coverage of the SiO2 particles appears somewhat smaller in the SEM image in comparison to the AFM image. The apparent difference in coverage arises from the convolution of the particle and tip shapes in the AFM measurement, which results in a broadening of the particle in the image plane. The subsequent adsorption of a PLL layer and the Au nanoparticles yielded a diluted monolayer of the metal particles as illustrated in Figure 4C,D. The adsorption of the Au nanoparticle was done by introducing the PLL-terminated film in the as-prepared Au colloidal solution for 1 h. The homogeneous coverage of the SiO2 particles allows distinguishing the topographic features associated with the Au nanoparticles in the flat sections of the electrode. Close examination of the AFM and SEM images strongly suggests that the majority of the Au nanoparticles are adsorbed on top of the SiO2 monolayer. The average number density of the Au nanoparticles was estimated to be ΓAu ) (1.9 ( 0.2) × 1010 cm-2. Furthermore, the images confirm that less than 5% of the Au nanoparticles are adsorbed in areas of the electrode not covered by the SiO2 nanoparticles. 3.3. Dynamic Electrochemical Responses of the SelfAssembled Heterostructure. The evolution of the voltammetric responses in the presence of 1 × 10-3 mol dm-3 Fe(CN)63-/4upon the various adsorption steps is illustrated in Figure 5. It can be seen that the faradaic process is strongly suppressed after the modification of polycrystalline Au electrodes with MUA
Zhao et al.
Figure 4. AFM (2 µm × 2 µm × 67 nm) (A) and SEM (scale bar 100 nm) (B) images of the SiO2 monolayer electrostatically adsorbed on Au(111) surfaces modified by the MUA monolayer and PGA-PLL film. A 420 g dm-3 concentration of SiO2 in the colloidal solution was used to increase the particle coverage with respect to the conditions described in Figure 2. AFM (2 µm × 2 µm × 75 nm) (C) and SEM (scale bar 100 nm) (D) images of the assembly after adsorption of a PLL film and citrate-stabilized Au nanoparticles. Images obtained from both techniques provided consistent values for ΓSiO2 ) (5.4 ( 0.4) × 1010 cm-2 and ΓAu ) (1.9 ( 0.2) × 1010 cm-2. As in the case of the electrostatic adsorption on Si(111) wafers, the Au nanoparticles exhibited a preferential adsorption on the areas covered by the SiO2 nanoparticles.
Figure 5. Cyclic voltammograms in the presence of 1 × 10-3 mol dm-3 ferro/ferricyanide as a function of the surface termination in the layer-by-layer assembly. The voltammetric behavior at 50 mV s-1 of the clean Au surface (Au) with a surface area of 0.071 cm2 is contrasted with that of the electrodes terminated in the polyelectrolyte film (PEM), 22 nm SiO2 nanoparticle monolayer (SiO2), and diluted 19 nm Au nanoparticle assemblies (Au(nano)).
and the PEM film. As demonstrated elsewhere, the hexacyanoferrate couple can effectively diffuse through the PEM film and the main barrier to the redox process originates from the MUA monolayer.8,10,12 The SiO2 layer does not alter the blocking nature of the ultrathin film in any substantial fashion. As
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Figure 6. Amplitude (A) and phase (B) of the impedance spectra obtained at the equilibrium potential (0.22 ( 0.01 V) for the various surface terminations. The fit of the phenomenological Randles circuit (C) to the experimental data is illustrated by the continuous line. The parameters Ru and W correspond to the uncompensated resistance and the Warburg diffusional impedance, respectively. The physical meaning of the phenomenological interfacial capacitance (Cint) and charge-transfer resistance (Rct) depends on the structure of the multilayer assembly.
discussed further below, the impedance responses show a minor decrease of the apparent charge-transfer resistance. Considering the substantial coverage of the SiO2 nanoparticle, it can be concluded that the electrostatic adsorption of these insulating particles does not introduce a substantial amount of defects in the MUA-PEM film. The electrostatic adsorption of the Au nanoparticle brings about a substantial increase in the faradaic current as seen in Figure 5. The peak current density and peak-to-peak potential difference approach the values observed for the clean Au electrode. This result clearly demonstrates that the Au nanoparticles induce a large increase in the phenomenological chargetransfer rate constant across the assembly. From the AFM and SEM images in Figure 4, it can be excluded that the current amplification arises from a direct physical contact between the Au nanoparticle and the metal electrode. It can be argued that up to 5% of the Au particles are adsorbed on the voids between the SiO2 particles; however, the extent of the current increase observed in Figure 5 indicates that the whole ensemble of Au nanoparticles participates in the electrochemical responses. This point is quantitatively demonstrated by the impedance analysis further below. The enhancement of the faradaic current upon the adsorption of the Au nanoparticles is clearly demonstrated by the impedance spectra at the equilibrium potential in Figure 6. For all the electrode modifications, the Bode plots are characterized by a single RC time constant at high frequencies. In the case of the clean Au and the Au-nanoparticle-terminated electrodes, a further element associated with diffusion of the redox species to the electrode is observed at frequencies below 10 Hz. These results show a 2-3 orders of magnitude decrease in the amplitude of the impedance upon adsorption of the Au nanoparticles. As mentioned previously, this substantial amplification of the electrochemical kinetics cannot be interpreted in terms of a direct physical contact between the metal nanoparticles and the electrode surface. The frequency-dependent amplitude (Figure 6A) and phase (Figure 6B) were analyzed by employing the Randles equivalent circuit shown in Figure 6C. The uncompensated resistance Ru and the Warburg component W correspond to the classical electrochemical impedance elements. The Warburg components for the Au-nanoparticle-terminated electrode and the clean Au surface were consistent with a diffusion coefficient Dferri/ferro ) (8 ( 5) × 10-6, taking a geometrical surface area of 0.071 cm-2.13 This value clearly indicates that the diffusional imped-
TABLE 1: Phenomenological Charge-Transfer Resistance (Rct) and Interfacial Capacitance (Cint) surface terminationa
Rct/Ω
Au PEM SiO2 Au(nano)
(2.39 ( 0.19) × 10 (2.04 ( 0.39) × 106 (6.52 ( 2.22) × 105 (2.92 ( 1.28) × 103
106Cintb/F cm-2 2
5.58 ( 1.25 1.61 ( 0.08 1.52 ( 0.31 0.97 ( 0.25
a “Au” corresponds to the clean Au electrode surface. All other abbreviations are defined in Figure 5. b Standard deviation estimated from at least four different samples. The geometrical area of the electrode was 0.071 cm2, and the concentration of the ferri/ferrocyanide couple was 1.0 × 10-3 mol dm-3.
ance is determined by the transport of the ions in the aqueous solution. In the case of the Au-nanoparticle-terminated electrode, the Warburg impedance indicates an overlap of the diffusion profiles generated around the nanoparticles.9 The fact that a planar diffusion profile is observed for the nanoparticle-terminated surface provides further evidence that the majority of the Au nanoparticles are involved in the charge-transfer process. The parameters Rct and Cint obtained from the fits of the impedance responses are summarized in Table 1. The data show a 4 orders of magnitude increase in Rct in the presence of the MUA-PEM ultrathin film with respect to the value for the clean Au surface. As also shown elsewhere, the blocking properties of the film arise from the MUA monolayer at the Au electrode.8,12 The redox species have a strong affinity for the PEM, and Rct is little affected by the number of polyelectrolyte films adsorbed in the multilayer.10 The same effect is evident upon the adsorption of the SiO2 monolayer, featuring a decrease of Rct by a factor of 3. This drop is inconsistent with concentration polarization effects of the redox probe at the nanoparticle-terminated surface. This phenomenon would manifest itself by an increase of the apparent Rct. The observed decrease in Rct could be rationalized in terms of a small density of defects in the multilayer assembly created by the adsorption of the insulating nanoparticles. From the partially blocked electrode theory (see eq 1), the magnitude in the drop of Rct would be correlated with less than 1% of the total particle number density of the SiO2 nanoparticles in the ensemble. Consequently, the SiO2 monolayer neither introduces an effective barrier to the transport of the redox probe into the film nor generates a substantial amount of defects in the structure of the multilayer.
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A fundamental change in the dynamics of the electrochemical responses is brought about by the adsorption of the Au nanoparticles. The Rct values in Table 1 reveal a 2 orders of magnitude drop in the presence of (1.9 ( 0.2) × 1010 particles cm-2. To control the number density of the Au nanoparticles in these experiments, a layer of PLL was electrostatically adsorbed on the SiO2-terminated surface. This PLL layer generated a minor decrease in the apparent Rct.14 The Aunanoparticle-terminated surface featured not only a significantly lower Rct but also a diffusional impedance value identical to that of the clean Au surface. Despite the significant change in the charge-transfer resistance, Cint appears little affected by the adsorption of the Au nanoparticles. The behavior of Cint in such complex multilayered materials is rather difficult to rationalize. As a first approximation, a strong electronic communication between the Au nanoparticles and the metal electrode is expected to manifest itself by an increase of Cint in proportion to the particle coverage. This has been observed in the presence of SAMs and ultrathin assembled films.10,15,16 On the other hand, electrically “isolated particles” will decrease the interfacial capacitance of the modified electrode. However, the strong decrease in Rct upon adsorption of the Au nanoparticles indicates an effective “wiring” to the Au electrode. Consequently, the small decrease in Cint could be related to the low coverage of the Au nanoparticles (
