Ionic-Ligand-Mediated Electrochemical Charging of Anionic Gold

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Ionic-Ligand-Mediated Electrochemical Charging of Anionic Gold Nanoparticle Films and Anionic-Cationic Gold Nanoparticle Bilayers Shannon W. Boettcher,† Martin Schierhorn,† Nicholas C. Strandwitz,‡ Mark C. Lonergan,§ and Galen D. Stucky*,†,‡ Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106, Materials Department, UniVersity of California, Santa Barbara, California 93106, and Department of Chemistry, The Materials Science Institute, UniVersity of Oregon, Eugene, Oregon 97403 ReceiVed: October 28, 2009; ReVised Manuscript ReceiVed: January 19, 2010

Gold nanoparticles ∼2 nm in diameter were synthesized with, on average, between 0 and ∼5.4 anionic thiols per particle. An electrochemical quartz-crystal microbalance was used to monitor the motion of ions and electrons during redox cycling (charging) of thin films of these nanoparticles. When the electrochemistry was performed using a polyanion electrolyte too large to penetrate the nanoparticle film, the degree of oxidation that was possible was found to be dictated by the average number of anionic ligands on the particle surface available for charge compensation. These anionic nanoparticle thin films were combined with previously reported/synthesized cationic nanoparticles into solution-processed nanoparticle film bilayers. We demonstrate using these bilayers that the control over charge compensation kinetics afforded by the use of a polyelectrolyte supporting electrolyte in conjunction with ionic surface functionalization allows for the selective charging of one layer of nanoparticles over the other and for the realization of structures consisting of oxidized and reduced nanoparticles in direct contact. 1. Introduction Ligand-stabilized inorganic nanoparticles (NPs) are an emerging class of electronically active, chemically tunable, and solution-processable materials that have been used for catalysis1 and to fabricate optical2 and chemical3 sensors, prototype transistors,4 memory devices,5 and photovoltaics.6 Electrochemical methods have proven valuable both for studying the electronic structure of nanoparticles7–9 as well as for controlling their electronic charge state for device applications.10–13 The continued development of such electrochemical techniques could be important for the use of NPs in a variety of electronic device structures. Small (diameter ∼ 2 nm), stable, thiol-capped gold nanoparticles (AuNPs, also known as monolayer protected clusters or MPCs) are especially well-suited for electrochemical study and charge-state manipulation.14 For sufficiently monodisperse samples, roughly equally spaced redox peaks corresponding to the sequential transfer of single electrons onto the particle cores can be observed for particles either in solution15 or attached to an electrode.13,16 The spacing of these peaks (∆V) is primarily governed by the attofarad-sized capacitance (C) of the nanoparticle

∆V ) e/C

(1)

where e is the charge of a single electron. Such AuNPs are easily synthesized via the Brust method17 and have ligand shells whose chemical structure and functionality can be tuned using thiol-gold linkage chemistry. * To whom correspondence should be addressed. E-mail: stucky@ chem.ucsb.edu. † Department of Chemistry and Biochemistry, University of California. ‡ Materials Department, University of California. § University of Oregon.

The manipulation of the electrochemical charging behavior of AuNP assemblies is important given the recent interest in integrating nanoparticles into solid-state devices.4,13,18,19 To electrochemically charge a nanoparticle film (i.e., change its redox state), counterions must exchange between the film and the electrolyte solution to maintain overall charge neutrality. For example, when a conventional nanoparticle film is oxidized, electrons are removed from the nanoparticle cores, and mobile anions migrate into the film from the electrolyte to compensate the electronic charge (Figure 1A). Such electrochemically charged nanoparticle assemblies are often stable, even after removal from the electrolyte and drying, as the associated counterions render the nanoparticle film overall neutral.13,20 Ionic charge compensation processes are an important aspect of understanding and controlling the electrochemical charging of nanoparticle thin films. For example, Chen and co-workers have investigated the effects of various counterions on the electrochemical charging of AuNP films in aqueous solution and found that, for certain conditions, the reductive charging current can be suppressed.21,22 Quinn and co-workers have explained such behavior in terms of the large ion-transfer energy between the aqueous electrolyte and hydrophobic nanoparticle film.23,24 These studies are important, as they provide a means to modulate the electrochemical/electronic properties of nanoparticle assemblies by controlling the counterion identity in addition to the electrode potential. Once a nanoparticle film has been electrochemically charged via the incorporation of electronic and ionic charges, the chemical nature and mobility of the counterions affect the subsequent electronic properties of the charged film. Mobile counterions could lead to limited stability in devices fabricated from charged nanoparticles because the charge states could change with time or under applied bias (due to the movement of both electronic and ionic charge carriers). For example, nanoparticle devices with nonuniform redox profiles, such as

10.1021/jp910308s  2010 American Chemical Society Published on Web 02/15/2010

Anionic AuNP Films and Anionic-Cationic AuNP Bilayers

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4169 generate samples with reduced and oxidized nanoparticles in direct contact.

Figure 1. Schematic illustrating how free and/or bound ions compensate electrochemically added charges. The nanoparticle films used in this study, represented by the light-brown shaded area, are ∼400 nm thick and, hence, consist of several hundred layers of gold particles (each ∼2 nm in diameter). For nonionic NP films (A), the application of a positive bias to the working electrode in the electrochemical cell (relative to the rest potential) results in the injection of positive charges (holes) into the particle cores. Mobile counteranions from the conventional small-molecule electrolyte migrate into the film to compensate the electronic charge. For anionic NP films (B), the positive charging can be coupled with cation departure when a polyanion-based supporting electrolyte, too large to penetrate the densely packed particle film, is used. The resulting particles are positively charged to a degree that is limited by the number of anionic ligands.

pn junctions fabricated from oxidized and reduced semiconductor nanoparticles, would lack long-term stability for this reason. Inspired by work involving ionic conjugated polymers,25–27 we have previously used chemically bound ionic ligands and polymer electrolyte solutions to directly control ion movement, and hence electrochemical charging, in solid-state nanoparticle assemblies.28 In that work, we demonstrated that, when electrochemical charging is performed in a polycation electrolyte whose large size prevents it from entering the film, the degree of reduction possible is dictated by the number of cationic ligands bound on the AuNP surface available for charge compensation. These methods allowed us to prepare reduced nanoparticles where the added electrons were precisely compensated by covalently bound cationic ligands. We suggested that the lack of mobile counterions present in such reduced NP films might make this “ionic-ligand-mediated” electrochemical charging method useful for electronic device fabrication. Herein, we extend upon our previous work in two important ways. First, we describe the use of anionic ligands and polyanion electrolytes to control the positive charge (hole) concentration on AuNP cores (Figure 1B). Using an electrochemical quartzcrystal microbalance (EQCM), we simultaneously tracked the movement of both ions and electrons during the charging processes. We demonstrate that the maximum number of holes injected into each nanoparticle is dictated by the surface coverage of anionic ligands when a polyanion supporting electrolyte is used. Second, we describe the fabrication and electrochemical charging of bilayers of cationic and anionic AuNPs and demonstrate several of the unique features of the ionic-ligand-mediated charging approach that may be applicable for device fabrication. We present evidence that suggests it is possible to selectively charge one film over the other and hence

2. Experimental Section 2.1. Materials. 2.1.1. HAuCl4 · 3H2O. Gold pellets (10 g, 99.99%) were boiled in a mixture of aq concentrated HCl (50 mL) and aq concentrated HNO3 (13 mL) for ∼1 h until completely dissolved.29 The resulting solution was then evaporated until an orange crystalline residue was formed. The mass of the collected product confirmed the presence of approximately three crystal waters. 2.1.2. Anionic Thiol Ligand, Tetraethylammonium 3-Mercaptopropanesulfonate (NEt4MPS). NEt4MPS was prepared by ion exchange of sodium 3-mercaptopropanesulfonate (Aldrich, 90%).Astrongcation-exchangeresin(50g,DowexT50WX8-100, Aldrich) was packed into a column and then charged by sequential washing with ∼3 M HCl (aq), ultrapure (18.2 MΩ) water, 40% (w/w) tetraethylammonium hydroxide (aq), and ultrapure water. Sodium 3-mercaptopropanesulfonate (5 g) dissolved in ∼100 mL of ultrapure water was then passed through the charged column. The water was removed under vacuum, and the product was then dried at 90 °C under vacuum (∼100 mTorr) for three days to yield a thick colorless oily residue, NEt4MPS. Proton NMR analysis suggested that the product was completely ion-exchanged and free from significant amounts of water. 2.1.3. Hexanethiol-Capped Particles (HT-AuNPs).30 A solution of HAuCl4 · 3H2O (1.57 g, 4.0 mmol) in 100 mL of water was added to tetraoctylammonium bromide (5.47 g, 10.0 mmol, Aldrich) in 300 mL of toluene and stirred until the gold salt was completely dissolved in the toluene phase. The water phase was then removed using a separatory funnel. A volume of 1-hexanethiol (1.68 mL, 12.0 mmol, Alfa Aesar) was added to the toluene phase, forming a white milky mixture, and the mixture was stirred for an additional 20 min. Under vigorous stirring, 40 mmol of NaBH4 (1.54 g, Aldrich), freshly dissolved in 40 mL of water, was rapidly introduced. The mixture was stirred for ∼24 h after which the aqueous phase was separated and the toluene phase reduced in volume to ∼30 mL under vacuum. Ethanol (300 mL) was added, and the solution was allowed to sit overnight. The resulting AuNP precipitate was collected on a fine fritted funnel. The particles were initially purified by two sequential dissolution-filtration-reprecipitation cycles. During the first cycle, the particles were dissolved in toluene, filtered through the fine fritted funnel previously used to collect them, and then reprecipitated with ethanol. In the second, the particles were dissolved in heptane, filtered through a 0.2 µm Teflon filter, and reprecipitated with ethanol. In each case, the particles were isolated by centrifugation. Typically, between 0.6 and 0.7 g of the HT-AuNPs were collected at this point. The HT-AuNPs were further purified using Soxhlet extraction in acetone for ∼24 h. Around 5% of the HT-AuNPs were soluble in the hot acetone and lost at this pointspresumably, these particles were smaller and charged due to interaction with tetraoctylammonium bromide. This treatment has been shown to ensure the removal of residual ionic tetraoctylammonium bromide surfactants on the particle surface.31 The particles were then dissolved in a heptane solution at a concentration of 200 mg mL-1. TEM measurements revealed that these particles have a size distribution typical for similarly sized AuNPs, 2.0 ( 0.4 nm. 2.1.4. Anionic Nanoparticles. AuNPs with a varied density of anionic ligands were synthesized from the HT-AuNPs via thiol-thiol ligand place-exchange reactions. HT-AuNPs (50 mg)

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were dissolved in 5 mL of chloroform and combined with the anionic thiol NEt4MPS (between 0 and 51.3 mg) dissolved in 0.2 mL of ethanol and 0.136 mL of 6-mercaptohexyl alcohol (MHA). The ethanol was essential to solubilize the anionic thiol, which was not soluble in chloroform alone. The sample was purged for 2 min with flowing Ar gas, sealed in a Teflon-capped glass centrifuge tube, and heated at 40 °C for ∼12 h in an oil bath. The molar ratio of MHA to HT-AuNP was kept at 1000: 1, while the amount of NEt4MPS was tuned to achieve the desired ionic ligand density (from 0 to 180 µmol). Following the reaction, the majority of the particles had precipitated from solution. Caution: MHA is a toxic and odiferous compound that should be handled only in a fume hood. Laboratory equipment that comes into contact with MHA should be treated with dilute bleach solution to oxidize the thiol functionality. The remaining solution was decanted from the precipitated particles. The particles were then washed three times with chloroform, dried under vacuum, dissolved in 0.5 mL of methanol, and filtered through a 0.2 µm PTFE filter (to remove a small amount of insoluble matter). These particles were precipitated via the addition of 3 mL of toluene and then 6 mL of heptanes. Next, the particles were washed again with chloroform, collected via centrifugation, and dried under vacuum. The yield of particles after ligand exchange was between 30 and 40 mg (60-80%). The particles were then dissolved in ∼0.75 mL of d6-dimethylsulfoxide, and NMR spectra were collected. The dimethylsulfoxide was then removed in vacuo, and the resulting particles were dissolved in 0.25 mL of methanol, filtered, and transferred to Teflon-sealed vials. The particle concentration was adjusted to ∼300 mg/mL by evaporating ca. one-half of the methanol with a gentle Ar gas stream. Although we refer to these particles as anionic AuNPs, overall, they are electrically neutral due to tetraethylammonium countercations. 2.1.5. Tetraethylammonium Polystyrene Sulfonate (NEt4PSS). A strong cation-exchange resin (50 g, Dowex T50WX8-100, Aldrich) was packed into a column and then charged by sequential washing with ∼3 M HCl (aq), ultrapure (18.2 MΩ) water, 40% (w/w) tetraethylammonium hydroxide (Aldrich, aq), and ultrapure water. Sodium poly(styrene sulfonate) (5 g, Scientific Polymer Products, MW ∼70 000) dissolved in ∼100 mL of 18.2 MΩ water was then passed through the charged column. The resulting NEt4PSS was dialyzed for ∼7 days using Spectra/Por 4 cellulose dialysis membranes and then dried under vacuum for 3 days at 100 °C. The dialysis step was performed to remove residual small ion pairs. 2.1.6. Tetraethylammonium Tetrafluoroborate (NEt4BF4). As-received NEt4BF4 (electrochemical grade, Aldrich) was dried for 24 h at 120 °C under vacuum and then transferred into an Ar-filled glovebox. 2.1.7. Cationic Nanoparticles and Polycation Electrolyte. The cationic polyelectrolyte, poly(N,N-dimethyl-3,5-dimethylene-piperidinium hexafluorophosphate) (PDDP-PF6), and the cationic AuNP samples were available from a previous study, and their preparation is described in detail there.28 2.2. Methods. 2.2.1. Nuclear Magnetic Resonance (NMR) Spectroscopy. A Bruker Avance 500 MHz NMR was used to characterize the relative ligand concentrations on the particle surface. Deuterated DMSO was used as a solvent. The ligand composition was analyzed by comparing the integrated intensity of the appropriate NMR resonances (Figure 2) 2.2.2. Transmission Electron Microscopy (TEM). TEM samples were prepared by dropping 2 µL of dilute AuNP

Boettcher et al.

Figure 2. NMR analysis of the anionic nanoparticle ligand composition. Peaks associated with the MHA -OH group, the ionic counterion, N(CH2CH3)4+, and the hexanethiol -CH3 group were integrated to determine the average ligand composition and are marked as shown in the spectra. The broad peaks centered at ∼1.4 and 3.4 ppm result from the aliphatic chain of the MHA ligand. The signal from the -CH2units on the mercaptopropanesulfonate ligand is present only as a broad background due to the minimal motional freedom of -CH2- groups bound near the particle surface. The spectra are for different anionic samples A1-A9, with sample A1 (no anionic ligands) at the bottom to A9 (most anionic ligands) at the top.

solutions (using methanol and heptane as solvents for the cationic and hexanethiol-capped AuNPs, respectively) onto TEM grids with an ultrathin carbon support (Ted Pella). Electron micrographs were obtained using an FEI T20 microscope operating at 200 kV. Average particles sizes and size distributions (taken as one standard deviation) were obtained by measuring at least 250 particles (Table 1). The thiol-thiol placeexchange reactions did not affect the particle size distributions in a statistically relevant way. A representative TEM image is included in the Supporting Information. 2.2.3. Thin-Film Preparation. Approximately 400 nm thick AuNP films were deposited by dropping 5 µL of AuNPs in methanol (∼300 mg mL-1) onto either a Ti/Pt-coated glass slide or a quartz-crystal microbalance (QCM) crystal spinning at 2500 rpm. The thicknesses of the AuNP films were determined using a surface profilometer (Dektak 6M) to measure the trench depth where a small section of the film had been removed with a razor. Although film thickness varied from sample to sample (∼300 to 500 nm), the thickness variation for a single film was minimal over the electrode area ((20 nm). Film uniformity is important, as thickness variations will result in errors calculating the film mass from the QCM frequency shifts.32 The deposited films were slightly cross-linked to render them insoluble and increase their rigidity by soaking in a 1,9-nonanedithiol (Aldrich) solution in ethyl acetate (5 µL/mL) for 15-20 min. This cross-linking treatment did not significantly affect the ligand distribution (see the Supporting Information). 2.2.4. Electrochemical Methods. Electrochemistry experiments were performed in an Ar-purged, single-compartment, glass electrochemical cell using a PAR 273 potentiostat in the typical three-electrode configuration. A coiled platinum wire was used as a counter electrode and a silver wire in contact with 0.005 M AgNO3/0.1 M TBAPF6 in anhydrous acetonitrile separated by a Vycor glass frit (Bio-Analytical Systems) served as the reference electrode. Either 0.1 M NEt4PSS or 0.1 M NEt4BF4 in anhydrous acetonitrile (Aldrich) was prepared as described above and used as supporting electrolytes. Following electrochemical measurements, the redox potential of ferrocene (Fc) was measured using the same electrolyte/reference electrode combination to provide an internal standard (E°(Fc|Fc+) ) 0.31 V vs SCE).33 Reported electrode potentials are referenced to


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TABLE 1: Composition of Anionic AuNP Materials for Different Ligand-Exchange Conditionsa synthesis batch

synth. ratio A1 A2 A3 A4 A5 A6 A7 A8 A9

NEt4MPSc (%)

([NEt4MPS])/([MHA]) b

0 0.0075 0.015 0.025 0.04 0.06 0.09 0.13 0.18

electrochem. 0 1.7 ( 0.2 4.2 ( 0.3 6.5 ( 0.4 8.2 ( 0.5

MHA (%)

HT (%)

core diameter (nm)

NMPSd (no. per AuNP)

NMR

NMR

NMR

TEM

NMR

0 0.4 ( 0.1 1.0 ( 0.2 2.2 ( 0.3 3.2 ( 0.3 4.1 ( 0.4 5.6 ( 0.4 6.9 ( 0.5 7.5 ( 0.5

93 ( 2 92 ( 2 91 ( 2 92 ( 2 91 ( 2 91 ( 2 87 ( 2 86 ( 2 87 ( 2

6.6 ( 1.3 7.6 ( 1.5 8.1 ( 1.6 6.2 ( 1.2 5.7 ( 1.1 5.2 ( 1.0 7.5 ( 1.5 7.2 ( 1.4 5.2 ( 1.0

1.8 ( 0.4

0 0.29 ( 0.07 0.73 ( 0.14 1.6 ( 0.2 2.3 ( 0.2 2.9 ( 0.3 4.0 ( 0.3 4.9 ( 0.4 5.4 ( 0.4

1.9 ( 0.4 1.9 ( 0.5 1.9 ( 0.4 2.0 ( 0.4

a

NEt4MPS (tetraethylammonium mercaptopropanesulfonate), MHA (1-mercaptohexyl alcohol), and HT (1-hexanethiol) amounts are reported as the mole fraction of each component. b During the ligand exchange, the [MHA]/[HT-AuNP] ratio was kept at 1000:1 (136 µL of MHA per 50 mg of HT-AuNP). c The fraction of the cationic NEt4MPS was measured independently using NMR and electrochemistry (see text). d NMPS is the number of anionic ligands per particle and is calculated assuming that the average ∼2 nm particle carries ∼71 organic ligands.

the standard calomel electrode (SCE). To correct for film thickness variation, the reported electrochemical currents were normalized by the mass of the nanoparticle film measured with the QCM, as described below. For the bilayer structures, the reported voltammograms are normalized with respect to the thickness (measured using profilometry) and the area of each film, as the experiments were not performed using the QCM. 2.2.5. QCM Measurements. The QCM experiments were performed using a Stanford Research Instruments QCM100 oscillator circuit coupled with a PAR 273 potentiostat. Ti/Ptcoated 5 MHz AT-cut quartz crystals (Stanford Research Systems) with an electrochemically active area of 1.35 cm2 were used as the potentiostat working electrode. The resonance frequency was measured from the oscillator circuit using a National Instruments PCI-6601 frequency counter board, which enables a 2 Hz frequency measurement resolution using a 0.5 s acquisition time. It was found that, to ensure consistent oscillation characteristics, the potentiostat and QCM had to be secured to a common ground. The average areal mass of the nanoparticle film was calculated by measuring the frequency change upon deposition of the film and applying the Sauerbrey equation

∆f ) -Cf∆M

(2)

where ∆f and ∆M are the frequency and mass changes, respectively. For the QCM used in this study, the sensitivity factor, Cf, was found to be 66 Hz µg-1 cm2 from the well-defined electrochemical deposition of Ag. In other words, the resonance frequency of the crystal shifts 1 Hz for every 15 ng cm-2 of mass added to the film. The use of the Sauerbrey equation assumes the rigid film limit, which we have previously verified using impedance analysis to determine any changes in the crystal quality factor.28 During the EQCM experiments, the QCM resonance frequency was measured as the potential of the working electrode resonator was swept using the potentiostat. In order to directly compare the QCM data for different film samples, the following corrections were applied. First, the reference point, f 0, for the frequency change, ∆f ) f - f 0, was taken as the most reduced state of the film studied (ENP ) -0.9 V vs SCE), where, for each film, the number of added ions per unit mass will presumably be the same. The ∆f versus ENP data for each film were normalized by the total film mass by dividing by the resonance frequency shift measured when the film was first deposited, ∆ffilm, to obtain a relative mass change, ∆f/∆ffilm ) ∆M/Mfilm. The ∆M/Mfilm data for all of the samples were then

shifted in unison to set the minimum ∆M/Mfilm for the nonionic film to zero. This shift references the data to the uncharged nonionic film so that the relative addition or loss of ions is more readily apparent. These corrections are performed to enhance the clarity of presentation but do not change the overall shape of the QCM curves and hence do not affect the conclusions drawn from them. 3. Results and Discussion 3.1. Nanoparticle Synthesis and Ligand Exchange Dynamics. Anionic AuNPs were synthesized from hexanethiolatestabilized gold nanoparticles (HT-AuNPs) via a thiol-thiol place-exchange reaction.34 The average number fraction of cationic ligands on each particle was controlled by the ratio of the anionic ligand, tetraethylammonium 3-mercaptopropanesulfonate (NEt4MPS), to the polar nonionic ligand, 1-mercaptohexanol (MHA), and characterized via 1H NMR (Table 1). The tetraethylammonium counterion (NEt4+) imparted the solubility in organic solvents necessary for ligand exchange while still being small enough to be effectively shuttled in and out of the AuNP film during electrochemical charging (see more below). Tetramethylammonium and tetrabutylammonium were also investigated as counterions. The former did not provide the required solubility, and the later yielded NP films for which the cyclic voltammetry was strongly influenced by the large ion size. The relative surface coverages of anionic ligands (NMPS) determined via NMR correspond to, on average, between 0 (for sample A1) and ∼5.4 (for sample A9) anionic ligands per particle, assuming ∼71 total ligands.35 The final ratio of ionic to nonionic ligands on the AuNPs was not a linear function of the ratio of the two ligands in the ligand-exchange solution (Figure 3). At low anionic ligand concentrations, the affinity for the particle surface of the anionic and nonionic ligands appears roughly the same; that is, the ratio of anionic to nonionic ligands in the ligand exchange solution is similar to the final composition on the particle surface. However, at higher anionic ligand concentrations, proportionately fewer anionic ligands were incorporated onto the NP surface. For example, as the anionic ligand concentration in the exchange solution was doubled going from sample A7 to sample A9, the final number of anionic ligands on the particle surface only increased by ∼30%. The nonlinear relationship between the ligand concentrations in solution and on the particle surface depicted in Figure 3 might be attributed to the presence of different ligand-binding sites on the nanoparticle surface.36 Small gold nanoparticles are

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Figure 3. Average coverage of anionic ligands on the nanoparticle surface is presented as a function of the ratio of anionic (NEt4MPS) to nonionic (MHA) ligands during particle synthesis. The values were obtained from the integration of the NMR resonances and from the electrochemical charging current passed in the anionic polyelectrolyte. The samples are referred to in the text as A1-A9; sample A1 has no anionic ligands, and sample A9 has the most.

known to be faceted with structures such as truncated octahedra.37 Ligands bound to the gold atoms on the crystal faces will be sterically crowded compared with those bound at edge or corner sites. The anionic ligands are bulky compared with the nonionic alcoholic ligands. Therefore, the ionic ligands are expected to prefer less-crowded corner and edge sites. If this is the case, once the corner and edge sites are filled, additional binding of anionic ligands will be hindered because they must sit on the more-crowded nanocrystal faces. It is interesting to note that we did not observe this behavior in our previous work using cationic thiol ligands.28 This apparent difference could be explained by the much longer alkane chain on the cationic ligands (11 carbons) versus the anionic ligands (3 carbons), allowing the bulky cationic group (and counterion) to sit much further from the particle surface and, therefore, avoid steric interference with the neighboring, shorter (6 carbon), alcoholic ligands. Electrostatic repulsion between the charged ligands could also play a role in limiting the degree of ionic ligand coverage. 3.2. Electrochemical Charging of Anionic AuNP Films. To investigate the role of bound anionic ligands in charge compensation during electrochemical charging, the coupled movement of electrons and ions was measured using an electrochemical quartz-crystal microbalance (EQCM). A Ti/Ptcovered quartz crystal was coated with a thin film of AuNPs and then used as the working electrode to sweep the electrode potential of the AuNP film (ENP) in either a small molecule, NEt4BF4, or a polyanion, NEt4PSS, electrolyte. Shifts in the resonance frequency of the quartz crystal were simultaneously recorded and directly correlated to mass changes, assuming the rigid-film limit.32 3.2.1. Electrochemistry in the Small-Molecule Electrolyte, NEt4BF4. The electrochemical charging of the anionic AuNPs was first studied utilizing a traditional small-molecule electrolyte in which both anions and cations can cross the film-electrolyte boundary to participate in charge compensation. The cyclic voltammograms (CVs) for AuNP films A1, A3, A5, A7, and A9 (which span the anionic ligand surface coverage fraction of interest) collected in NEt4BF4 consist of a series of redox waves. The spacing of these waves (roughly 150 mV) is consistent with the attofarad-sized capacitance of the majority fraction of particles with diameters around 2 nm (Figure 4A), but somewhat smaller than that typically observed for the alkanethiol-capped particles prior to ligand exchange (∼190 mV).13,14 This difference could be explained by the higher effective dielectric

Boettcher et al. constant of the ionic and mercaptohexyl alcohol ligands compared with alkane thiol ligands.24,38,39 Additionally, shallow “valleys” of reduced current and increased peak spacing are observed in the voltammograms shown in Figure 4A. The magnitude of this effect is dependent on the degree of ionic functionalization of the particle; increasing numbers of ionic ligands leads to a decreasing depth of the current valleys and less variation in peak spacing. These observations are consistent with previous reports that indicate that the energetic cost of transporting ions across the film-electrolyte interface can significantly affect the voltammetry of AuNPs; in films already containing ions, such effects are expected to be reduced.24,38,39 The electrode potential at which, on average, the particle cores are neutral (ENP0, i.e., oxidation state of 0), can be estimated either from the current minima for the nonionic particles in NMe4BF4 (Figure 4A, curve A1) or from the electrode potential measured when a fresh film is first placed in the electrolyte. For nonionic particles, this potential is often referred to as the point of zero charge (PZC).16 We found ENP0 to be around -0.2 to -0.3 V versus the standard calomel electrode (SCE), which is comparable to that reported for similar-sized AuNPs.16,40 Furthermore, ENP0 did not seem to depend on the degree of ionic functionalization, which suggests that the ionic ligands alone do not affect the electrochemical potential of the AuNPs. At potentials more negative than ENP0, each particle core is reduced. To compensate the negative charge on the particle core, mobile NEt4+ cations migrate from the electrolyte into the film, and an increasing film mass with decreasing ENP is registered on the QCM (Figure 4B). As the anionic ligands, and their associated NEt4+ counterions, are not involved in charge compensation, the reductive portions of the current and mass traces collected for each NP sample are similar. When the nanoparticles are oxidized at electrode potentials more positive than ENP0, more complicated, coupled ion-electron motion is observed. For nonionic AuNP films, hole injection (oxidation) is coupled with anion (BF4-) transfer from the supporting electrolyte into the film (Figure 4A,B, curve A1). In contrast, anionic AuNP films first preferentially expel NEt4+ counterions, which originally balanced the charge on the NEt4MPS anionic ligand, before introducing additional BF4from the electrolyte. The preferential loss of cations from anionic films upon oxidation is readily discerned from the shape of the ∆M versus ENP plots (Figure 4B). Whereas sample A1 (no anionic ligands) shows an immediate mass gain as the potential of the film is driven more positive than ENP0, samples A3-A9 (with increasing anionic ligand coverage) exhibit increasing degrees of mass loss upon oxidation. The mass loss continues as the electrode potential is swept more positive until the supply of cations present in the film as counterions to the bound anionic ligands is exhausted. When the number of holes injected into the film becomes larger than the number of anionic ligands, additional injected holes must be compensated by the incorporation of anions from the electrolyte. Hence, the crossover point from mass loss to mass gain in Figure 4B is driven to increasingly positive ENP as the anionic ligand density in the AuNP film increases. The experimental observation that cations preferentially leave the film before additional anions enter the film can be rationalized in terms of the free-energy cost of transporting an ion from solution into the film23,24 (as opposed to ejecting an ion) as for the analogous cationic NP films.28 3.2.2. Electrochemistry in the Polyanion Electrolyte, NEt4PSS. The electrochemical oxidation process can be kinetically limited by controlling the sign of the ions available for



Figure 4. Panels A and B show EQCM data collected for each anionic nanoparticle film studied (samples A1, A3, A5, A7, and A9) in the small-molecule electrolyte, NEt4BF4. Panels C and D show identical measurements for the same set of films in the polyanion electrolyte, NEt4PSS. Voltammetry in NEt4PSS was collected prior to that in NEt4BF4 in order to prevent contamination with NEt4+ ions. Sample A1 has no ionic groups in the ligand shell, and samples A3-A9 have increasing numbers of ionic ligands (Table 1). Panels A and C show the cyclic voltammetry data (50 mV s-1) normalized to the film mass, and panels B and D show the simultaneously collected mass-change data normalized to account for differences in the original film mass. ENP is the electrode potential of the nanoparticle film as controlled by the applied voltage of the potentiostat. The point where these particles are electronically neutral, ENP0, is around -0.2 to -0.3 V vs SCE. The observation of increasing oxidation currents and relative mass losses for increasing anionic functionalization (curves for samples A3-A9) in the NEt4PSS electrolyte is strong evidence that the electrochemical oxidation of the AuNPs is kinetically controlled by the number of anionic ligands on the particle surface.

charge compensation using the polyanion supporting electrolyte, tetraethylammonium polystyrene sulfonate (NEt4PSS). As the large polyanion chains are unable to enter the densely packed AuNP film, EQCM measurements in NEt4PSS allow us to demonstrate that the maximum degree of oxidation is dictated by the average number of anionic ligands per particle. Voltammograms for the same set of AuNP films collected in NEt4PSS (Figure 4C) are markedly different from those collected in NEt4BF4 (Figure 4A). At potentials more negative than ENP0, electron injection into the particle core is coupled with cation incorporation (as in NEt4BF4), regardless of ionic ligand content. At potentials more positive than ENP0, electrochemical oxidation of nonionic particles (Figure 4C, curve A1) is largely prevented due to the lack of anions available to compensate the positive core charges. For the anionic AuNPs, increased oxidation currents and larger potential ranges over which mass loss occurs (due to cation departure) are observed for samples with increasing anionic ligand coverage (Figure 4C,D, curves A3-A9). These results show that the covalent binding of anionic ligands, in conjunction with the use of an anionic polyelectrolyte, is a useful tool to manipulate oxidation and ion-compensation processes in films of nanoparticle materials. As ENP of the anionic particles is swept to oxidative potentials in NEt4PSS, hole injection occurs only until the point at which the number of holes equals the number of anionic surface charges introduced synthetically. In other words, the positive charging process is self-limited. The fact that the extent of nanoparticle charging is limited by the number of anionic ligands is confirmed by comparing the integrated charge corresponding to particle oxidation (Qox) with the number of anionic ligands in the film calculated from the NMR peak integrations. The incremental increases in maximum integrated charge (Qmax) for the anionic

NPs (Figure 5C, curves A3-A9) over that for the nonionic NPs (Figure 5C, curve A1) were approximated as Qox for each film. Qox divided by Faraday’s constant (F) yields the number of moles of charge passed, which should equal the total number of anionic ligands in the film. This information, combined with the total mass of the film and the total ligand weight percentage in the film (∼19%, determined by thermal gravimetric analysis) was then used to estimate the mole fraction of anionic ligands in AuNP samples A3-A9. These values agree well with those determined using NMR (Table 1 and Figure 3). 3.2.3. Role of SolWent Molecules in the Charging Process. Analysis of the mass-charge relationships (Figure 5B,D) allows us to consider the role of solvent molecules in the charging process. As has been well-documented, neutral solvent molecules often accompany counterions during electrochemical charging processes in surface-immobilized films.24,41,42 Solvent incorporation can electrostatically stabilize ions in the film, which often has a dielectric constant lower than that of the electrolyte. For the charging of the cationic nanoparticles reported previously, the solvent contributes a significant mass in addition to the ion mass crossing the film boundary.28 During the charging of the anionic particles discussed in this report, the role of solvent is important, but less pronounced than that for the cationic particles. In NEt4PSS, the only ions able to cross the film-electrolyte boundary are the NEt4+ cations. For each electron added to (or removed from) the film, a corresponding NEt4+ must incorporate (or leave). In the simplest case, where solvent molecules do not accompany these cations, the slope of the mass-charge plot multiplied by Faraday’s constant should equal the molecular weight of the NEt4+. For Q < 1000 µC (i.e., for reductive charging or ENP < ENP0), this simple behavior is observed in both NEt4PSS and NEt4BF4 (Figure 5B).

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Figure 5. Panels (A) and (B) show integrated charge and QCM data collected for each nanoparticle film studied (samples A1, A3, A5, A7, and A9) in the small-molecule electrolyte, NEt4BF4. Panels (C) and (D) show identical measurements for the same set of films in the polyanion electrolyte, NEt4PSS. Panels (A) and (C) show the integrated charge passed into the film normalized to the film mass. The most reduced state is arbitrarily assigned an integrated charge of zero. By comparing differences in the maximum charge (Qmax) for sample A1 vs samples A3-A9, the anionic ligand coverage is estimated. Panels (B) and (D) show the absolute mass changes of the film vs the integrated charge. The derivative of the data in panels (B) and (D) yields the mass change of each film (due to the movement of ions and solvent) per injected electronic charge.

In this regime, the slope of the mass-charge curves yields a molecular weight of 140 ( 10 g/mol, in agreement with the molecular weight of the NEt4+ ion, 130 g/mol. As the voltage is swept further oxidative and additional charge is passed, the supply of NEt4+ions in the film is eventually exhausted. In the small-molecule electrolyte, oxidation occurs alongside the incorporation of BF4- anions. The slopes of the mass-charge curves in the linear regimes at large integrated charge (Figure 5B, right side of panel) yield molecular weights between 140 and 200 g/mol. These weights are much larger than that of the BF4- anion, 97 g/mol, signifying that one to two CH3CN molecules are involved in each ion transfer. The observation that the transfer of the BF4- anion involves solvent, whereas the transfer of the NEt4+ cation does not, is consistent with the idea that the NEt4+ cation is relatively large and hydrophobic, whereas the anion BF4- is small with a higher charge density. The difference in solvent contributions to the recorded mass-charge curves also explains the differences in the observed hysteresis between the oxidative and reductive branches (Figure 5B)sthe motion of the neutral solvent is slow compared with the electric-field-driven motion of the ions associated with the redox process. Because solvent motion is not associated with the reductive branch, little hysteresis in the mass-charge curves is observed there. As the supply of NEt4+ ions is exhausted during oxidation in the polyanion electrolyte, the mass-charge curves tend to show deviation from linearity (Figure 5D). The decreasing slope as the oxidation proceeds suggests that the total mass of the ion plus solvent compensating the added electronic charge also decreases. This behavior is increasingly evident in the anionic NP films with higher anionic ligand coverage, and it can be explained by a flux of solvent in the opposite direction as the flux of NEt4+ cations. When the anionic films are driven more positive than ENP0, they will acquire a positive electronic charge. The hydrophobic NEt4+ cations will leave the film to compensate

the injected holes. The small, and highly charged, sulfonate groups bound to the surface of the particles will be left behind. It is probable that CH3CN molecules migrate into the film to solvate the sulfonate groups as the NEt4+ cations leave. These mass fluxes effectively cancel in this regime. 3.3. Electrochemical Charging of Cationic|Anionic Nanoparticle Bilayers. One of the objectives of the experiments described herein is to demonstrate that the functionalization of NPs with chemically fixed ionic ligands is a useful technique for the fabrication of junctions between nanoparticles with different charging/redox states. A potentially interesting example of such a junction would be that consisting of a layer of oxidized anionic particles in direct contact with a layer of reduced cationic particles. Such a structure bears some analogy to traditional semiconductor pn junctions, with the chemically bound ionic ligands conceptually similar to the fixed, ionized dopant atoms in the semiconductor lattice. In principle, these oxidized/reduced bilayers would be stable as the lack of mobile counterions would prohibit a bulk redox reaction between the phases, despite drastically different electrochemical potentials. At the interface between the two phases, however, one expects the formation of a volume analogous to a depletion region over which the added electrons and holes recombine, leaving behind the bound ionic groups on the nanoparticle surface and hence an internal electric field. These structures would be particularly interesting if visible-light-absorbing semiconducting NPs were used instead of AuNPs since, in that case, the interfacial electric field in the engineered “depletion region” could be used to split excitons and collect carriers using drift (as opposed to the diffusion usually relied upon in solution-processed nanoparticle devices). Achieving such device structures requires the ability to positively charge one film of NPs, while negatively charging the other film, in a way compatible with the condition that they are in intimate contact. There have been several efforts in the organic electronics field aimed toward similar goals, for instance,



Figure 6. Cyclic voltammograms (CVs) of AuNP thin-film monolayer and bilayer samples collected at 50 mV/s. The currents were normalized by both the film thickness and the area. For the bilayer samples, the thickness of the top layer was used for normalization to aid direct comparison of the magnitude of the charging current with the single-layer films. Reported charging currents for the bilayers that are larger than those for the corresponding single layers indicate substantial charging of both layers at that electrode potential. The data curves are labeled with both a roman numeral (I-IV) to identify the physical structure of the sample and a letter (A-C) to distinguish in which electrolyte the curves were collected. In panel A, the polycation electrolyte was used, in panel B, the polyanion, and in panel C, the small-molecule electrolyte. In cases where the voltammetry changed significantly between subsequent sweeps, multiple CV sweep cycles are displayed for the same sample. Specifically, the first three CV sweeps for A-I, the first two sweeps for B-II, the first and sixth sweeps for B-III, and the first two sweeps for B-IV are shown as the CV response in these particular cases was sweep-number dependent. The start of these sweeps is indicated by an asterisk and the sweep direction by an arrow. The sweep-number-dependent response is due to charge trapping in the ionic particles, as discussed in the text.

using electrochemical disproportionation of conducting polymers26 or small molecules,43 as well as using in situ polymerization of an ionic monomer/conducting polymer mixture44 to define device structures with oppositely charged/doped components in direct contact. In this section, we report how control over both cationic and anionic functionalization combined with the judicious use of polymer supporting electrolytes can be used to selectively and/or sequentially charge anionic and cationic nanoparticle films. Four different AuNP thin-film samples (I-IV) were fabricated and tested electrochemically in different electrolytes. Each sample consists of one or two layers of AuNPs deposited on Ti/Pt-coated glass electrodes by spin-coating. After each spincoating step, the AuNPs were cross-linked in a solution of nonanedithiol in ethyl acetate (5 µL/mL) for 15 min to render them insoluble. Sample I is composed of a film (thickness ∼ 400 nm) of AuNPs with approximately five anionic ligands per particle (AuNP sample A8). Sample II is composed of a film (thickness ∼ 500 nm) of AnNPs with ∼3.5 cationic ligands per particle. Sample III is composed of a bottom layer of the same anionic AuNPs as in I and a top layer of the same cationic AuNPs as in II. Sample IV was similar, except that the order of film deposition was reversed; the cationic particles were deposited as the bottom film and the anionic particles as the top film. For the fabrication of the bilayer structures, the bottom nanoparticle film was cross-linked prior to depositing the top layer so that the bottom layer would not dissolve during the deposition of the top layer. The samples were then further cut into three pieces that could be individually tested in the different electrolytes. The electrochemical charging of these films in polycation, PDDP-PF6, polyanion, NEt4PSS, and small-molecule, NEt4BF4,

electrolytes (Figure 6) reveals a wealth of information regarding the role of bound ionic ligands in the electrochemical charging process, as will be discussed on a case-by-case basis in the following sections. 3.3.1. Mono- and Bilayer Voltammetry in the Polycation Electrolyte. The voltammetry of the different mono- and bilayer films was first investigated in the polycation electrolyte, PDDPPF6, where reduction processes are expected to be hindered by the lack of small cations in the electrolyte solution. The behavior of the cationic nanoparticle film in PDDP-PF6 (Figure 6, curve A-II) is as described before:28 the positive charging of the particle cores is compensated by the flux of small anions from the electrolyte, whereas negative charging is limited by the number of cationic groups bound to the particle surface because the large polycation cannot enter the film. The electrochemistry of the anionic AuNP film in PDDPPF6 (Figure 6, curve A-I) is more complex. During the first sweep, an irreversible oxidation current is observed. Subsequent sweeps show reversible charging/discharging, but significant current flows only at potentials more positive than ∼0.25 V versus SCE. The observation of irreversible oxidation current can be explained by the trapping of positive charges on the nanoparticle cores due to the departure of mobile cations from the AuNP film into the bulk of the electrolyte solution. This phenomenon has been previously observed in ionically functionalized conjugated polymers and termed “ion-stripping.”25 The proposed process is schematically depicted in Figure 7. The charge-trapping behavior exhibited by the anionic AuNP films in the polycation electrolyte may be useful for the construction of interfaces between oxidized and reduced nanoparticle samples. For example, the ion-compensation dynamics in PDDP-PF6 suggest that sweeping an uncharged anionic|cationic

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Figure 8. Schematic of selective electrochemistry on AuNP bilayers, corresponding to Figure 6, curve A-III. (A) Application of an oxidizing potential leads to the injection of positive charge onto the cores of both cationic and anionic AuNP films. The countercations in the anionic film are expelled into solution (and diffuse away) to help compensate the positive electronic charge. (B) Application of a negative bias causes electron injection in both films. However, due to the lack of mobile positive charges, electron injection must be compensated by the departure of mobile anions. The top film, therefore, acquires a negative electronic charge compensated by bound cationic ligands. The bottom film, however, remains positively charged due to the lack of ionic species to compensate. Figure 7. Schematic of the irreversible oxidative charging of an anionic AuNP thin film in the polycation electrolyte (PDDP-PF6), corresponding to curve A-I in Figure 6. (A) Application of an oxidizing potential to the anionic film leads to the injection of positive charge on the particle core. Countercations depart from the film to compensate the added electronic charge and eventually diffuse into the bulk of the electrolyte solution. (B) Positive charges added in excess of the anionic ligand content are compensated by anions from the electrolyte. (C) When the electrode potential is then swept negative, the AuNPs stay positively charged because there are no available cations to compensate the injection of negative charge into the film. The cations originally in the sample are no longer available for compensation as they have diffused into the bulk of the electrolyte. The injected positive charge thus remains trapped in the anionic AuNP layer, regardless of applied potential. (D) Upon repeating the cycle, charging current is only observed at sufficient potentials where oxidation is coupled with the introduction of anions from solution. Charging at these levels (i.e., above the potential where compensation is due to anionic ligands) is reversible, as expected.

bilayer in this polycation electrolyte, first to oxidative potentials and then to reductive potentials, will result in the irreversible oxidation of the bottom anionic film and then the selective reduction of the top cationic film. Evidence suggestive of such behavior is indeed observed in the cyclic voltammograms collected for the anionic|cationic bilayer in PDDP-PF6 (Figure 6, curve A-III). During the first sweep, a partially irreversible oxidation is observed. When the potential sweep is reversed, current associated with the reduction of the bilayer is observed. At ca. -1 V versus SCE, the reduction current diminishes, similar to that observed for the cationic thin-film sample in PDDP-PF6 (Figure 6, curve A-II). We interpret the voltammetry for the anionic|cationic bilayer in polycation electrolyte shown in Figure 6, curve A-III, as follows. During the initial positive sweep, the reversible current component is attributed to the oxidation of the top cationic AuNP film and the incorporation of anions from solution. The irreversible oxidative current on the first sweep between 0 and 0.5 V versus SCE is attributed to the positive charging of the bottom anionic film and departure of the mobile cations. The combination of these two processes is shown schematically in step A of Figure 8. When the potential sweep is reversed and a reductive potential is applied, the bottom anionic film remains positively charged due to the lack of cations in the polycation electrolyte that are able to penetrate the film. The reduction current observed corresponds to the negative charging of the

top cationic film and is limited by the number of cationic ligands attached to the particle surface, as shown in step B of Figure 8 and as discussed previously for the single-layer system.28 Therefore, the process of sweeping the anionic|cationic bilayer potential to + 0.5 V and then to - 1.0 V (vs SCE) should remove the mobile ions originally in the layers and leave the injected electronic charge in both layers compensated only by covalently bound ionic charge. Remarkably, the charging behavior of the cationic|anionic bilayer in PDDP-PF6 is strikingly different from that of the anionic|cationic bilayer in PDDP-PF6 (Figure 6, curve A-IV vs A-III). This difference, due only to the relative position of the films (top versus bottom), highlights the importance of ion transport for electrochemical charging. During the first sweep (not shown in Figure 6, curve A-IV), some irreversible oxidation current for the cationic|anionic bilayer is observed, which we assign to charge trapping in the anionic layer, as discussed before. All subsequent sweeps are similar. Oxidative current is only observed for potentials greater than ∼0.3 V versus SCE; upon reversal of the voltage sweep, the added positive charges are removed, but negative charging of the bottom cationic film is suppressed even at potentials as negative as -1 V versus SCE (Figure 6, curve A-IV). These results suggest that the top anionic film blocks ion transport to/from the bottom cationic film unless it is undergoing a redox process itself involving ion transport from solution (Figure 9). In other words, the top anionic film appears to act as a potential-sensitive ion gate. 3.3.2. Mono- and Bilayer Voltammetry in the Polyanion Electrolyte. To augment our understanding of the selective/ sequential electrochemical charging of bilayer structures, we also investigated the voltammetry of an analogous set of samples in the polyanion electrolyte, NEt4PSS. The charging behavior of the mono- and bilayer films in NEt4PSS is similar to that observed in the cationic electrolyte, except, here, oxidation processes are hindered by the lack of small anions in the polyelectrolyte solution. The anionic monolayer exhibits charging behavior in NEt4PSS (Figure 6, curve B-I), as observed previously (section 3.2). Reduction is associated with the incorporation of cations from the electrolyte. Oxidation is associated with (and eventually limited by) the departure of countercations from the anionic AuNP ligands. The cationic film swept in NEt4PSS (Figure 6, curve B-II) exhibits electrochemical behavior analogous to the anionic film


Figure 9. Schematic of proposed ion motion during electrochemical charging of AuNP bilayers, corresponding to the voltammogram shown in Figure 6, curve A-IV. (A) During the first oxidative sweep, complete oxidation of the bilayer occurs in concert with the ejection of mobile cations and the incorporation of counteranions. (B) Upon reversal of the sweep direction, a reduction peak at ∼0.5 V vs SCE is observed. The reduction is apparently limited by the lack of mobile cations in the electrolyte and the inability of remaining anions to escape from the bottom cationic AuNP film.

swept in PDDP-PF6 (Figure 6, curve A-I). During the first sweep, an irreversible reduction current is observed. Subsequent sweeps show reversible charging/discharging, but significant current flows only at potentials more negative than ca. -0.3 V versus SCE. We attribute the irreversible reduction current with negative charge trapping on the particle core. The charging is irreversible because the original anionic counterions diffuse away into the bulk of the electrolyte solution and the polyanion is unable to penetrate the film to allow reoxidation. Reduction at potentials greater than ca.-0.3 V versus SCE becomes reversible because it is coupled with the motion of countercations in/out of the film. The bilayers measured in NEt4PSS exhibit analogous, but opposite, electrochemical charging behavior to the bilayers measured in PDDP-PF6. For example, the cationic|anionic bilayer swept in NEt4PSS (Figure 6, curve B-IV), shows an irreversible reduction upon first driving to negative potentialss similar to the irreversible oxidation observed when the anionic|cationic bilayer was first swept to oxidative potentials in PDDP-PF6. When the potential sweep is reversed, current associated with the oxidation of the bilayer is observed. At ca. +0.6 V versus SCE, this oxidation current diminishes. These observations are interpreted as follows. During the first reductive sweep, negative charge is trapped in the bottom cationic film by the departure of the counteranions into the bulk polyanion electrolyte. As the potential sweep is reversed and swept positive, countercations are ejected from the top anionic film during the selective oxidation of that film. At ca. +0.6 V versus SCE, the supply of countercations is exhausted, thereby limiting the extent of oxidation. Thus, the voltammetry data lead us to believe that the resulting structure consists of a negatively charged, cationic bottom layer and a positively charged, anionic top layer. For the anionic|cationic bilayer samples swept in NEt4PSS, ion transport appears gated by the redox activity of the top cationic film (Figure 6, curve B-III). During the first sweep, some irreversible reduction current for the bilayer is observed, which we associate with charge trapping in the cationic layer, as discussed before. Subsequent sweeps are similarsreductive current is only observed for potentials greater than ca. -0.3 V versus SCE. Upon reversal of the voltage sweep, the added negative charges are removed, but oxidation of the bottom anionic film is suppressed even at potentials as positive as +0.6 V versus SCE. Thus, we conclude that redox of the bottom anionic film is blocked unless the top cationic film also undergoes redox (which is limited to potentials more negative


Figure 10. Schematic representation of the electrochemical charging of bilayer samples in NEt4BF4. The charging of the bilayer samples is significantly affected by ion motion dynamics in the top layer. (A) During the reduction of the anionic|cationic NP bilayer, initially, current response is only recorded for the negative charging of the top cationic sample (Figure 6, curve C-III). Apparently, the outward flux of the counteranions in the top film prevents the flux of cations into the bottom layer. (B) Once the supply of counteranions in the top film has been exhausted, rapid charging of the bottom layer results in a large cathodic peak. As the voltammetry is reversible (i.e., the total integrated charge for one cycle is negligible), and no peak is observed on the reverse sweep, the reductive charging must occur normally as a function of voltage (i.e., not kinetically limited by the ions). An analogous interpretation can be used to describe the charging behavior of the cationic|anionic bilayers (Figure 6, curve C-IV).

than -0.3 vs SCE). This ion-gating process is similar to that observed for the analogous cationic|anionic bilayer swept in PDDP-PF6 (Figure 6, curve A-IV, and Figure 9). 3.3.3. Mono- and Bilayer Voltammetry in the SmallMolecule Electrolyte. We also investigated the voltammetry of the mono- and bilayer samples in the small-molecule electrolyte, NEt4BF4. Anionic and cationic monolayer samples exhibit the same voltammetric response as discussed previouslysboth oxidation and reduction are accessible due to the large concentration of mobile anions and cations in the electrolyte solution (Figure 6, curves C-I and C-II) The voltammetry of the bilayer samples in the small-molecule electrolyte is indicative of charging behavior strongly influenced by the dynamic ion motion in/out of the layers. In the case where the cationic film is on top, the voltammogram (Figure 6, curve C-III) appears to be the superposition of the typical charging current from the cationic film, along with a large reductive peak near -1.0 V versus SCE. Likewise, in the case were the anionic film is on top, the voltammogram (Figure 6, curve C-IV) appears to be the superposition of the typical charging current from the anionic film, along with a large oxidative peak near +0.4 V versus SCE. The voltammograms are reversible. We attribute the peak in both cases to the rapid charging of the bottom film in the sample, gated by the ion motion through the top film, as is depicted in Figure 10. These apparent ion-gating effects seem conceptually similar to those observed in conducting polymers and in gold nanopore electrodes, in which case, the electrochemical potential controls the permeability of an ionic species.45,46 4. Conclusions We have demonstrated, using a variety of characterization techniques, the fabrication of electrochemically charged nanoparticle assemblies in which each electronic charge is compensated by a counterion covalently bound to the particle surface. The degree and type of electrochemical charging can be controlled by the density and sign (anionic or cationic) of the attached ionic ligands when the appropriate anionic or cationic supporting polyelectrolyte is used. To elucidate the range of useful features afforded by the ionic ligand functionalization, the voltammetry of anionic and cationic AuNPs was investigated in small-molecule, polycation, and polyanion electrolytes. “Ion-gating” and “ion-stripping” effects

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were observed in the small-molecule and polyelectrolytes, respectively, which were attributed to the presence of the covalently bound ionic groups within the NP film. Finally, we described the sequential electrochemical charging of individual layers in anionic|cationic NP bilayers to yield oxidized|reduced AuNP interfaces. Because the counterions are covalently bound to the surface of the charged particles, the ions should not diffuse over time or with the application of an electric field. In principle, the methods reported here should be applicable to NPs with other (e.g., semiconducting) core compositions. These features may make this “ionic-ligand-mediated” electrochemical charging method potentially useful for fabricating a wider variety of electronic devices from NP materials than currently possible. Acknowledgment. We thank Anna Ivanovskaya for helpful discussions and Eric McFarland for the loan of the QCM. This work was supported by the NSF under awards DMR-0805148 and DMR-0519489, the Air Force Research Laboratory under agreement FA8650-05-1-5041, and the U.S. Army Research Office via the Institute for Collaborative Biotechnologies through Grant No. DAAD19-03D-0004. We made use of the MRL central facilities supported by the MRSEC Program of the NSF under award DMR-05-20415. S.W.B. thanks U.C. Santa Barbara for a Chancellors Fellowship. Supporting Information Available: Transmission electron micrograph of AuNP samples and information regarding the effects of chemical cross-linking on the ionic ligand coverage. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zheng, N. F.; Stucky, G. D. J. Am. Chem. Soc. 2006, 128, 14278– 14280. (2) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Nature 2006, 442, 180–183. (3) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18– 52. (4) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86–89. (5) Ouyang, J. Y.; Chu, C. W.; Szmanda, C. R.; Ma, L. P.; Yang, Y. Nat. Mater. 2004, 3, 918–922. (6) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462–465. (7) Ding, Z. F.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293–1297. (8) Vanmaekelbergh, D.; Houtepen, A. J.; Kelly, J. J. Electrochim. Acta 2007, 53, 1140–1149. (9) Vanmaekelbergh, D.; Liljeroth, P. Chem. Soc. ReV. 2005, 34, 299– 312. (10) Yu, D.; Wang, C. J.; Guyot-Sionnest, P. Science 2003, 300, 1277– 1280. (11) Jha, P. P.; Guyot-Sionnest, P. J. Phys. Chem. C 2007, 111, 15440– 15445. (12) Shim, M.; Wang, C. J.; Guyot-Sionnest, P. J. Phys. Chem. B 2001, 105, 2369–2373.

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JP910308S

Ionic-Ligand-Mediated Electrochemical Charging of Anionic Gold

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