NANO LETTERS
Ionic Ligand Mediated Electrochemical Charging of Gold Nanoparticle Assemblies
2008 Vol. 8, No. 10 3404-3408
Shannon W. Boettcher,† Sebastian A. Berg,§ Martin Schierhorn,† Nicholas C. Strandwitz,‡ Mark C. Lonergan,| and Galen D. Stucky*,†,‡ Department of Chemistry and Biochemistry and Materials Department, UniVersity of California, Santa Barbara, California 93106, Department of Chemistry, Mu¨nster UniVersity, Mu¨nster, Germany 48149, and Department of Chemistry, The Materials Science Institute, Oregon Nanoscience and Microtechnologies Institute, UniVersity of Oregon, Eugene, Oregon 97403 Received July 17, 2008; Revised Manuscript Received August 7, 2008
ABSTRACT We demonstrate that ionic surface functionalization is well-suited for controlling the electrochemical charging of nanoparticle assemblies. Gold nanoparticles ∼2 nm in diameter were functionalized with between 0 and ∼3.3 cationic thiols per particle and the coupled motion of ions and electrons during redox cycling (charging) was followed in situ using an electrochemical quartz-crystal microbalance. When the electrochemistry is performed using a polycation electrolyte too large to penetrate the nanoparticle film, the degree of reduction possible was found to be dictated by the number of cationic ligands on the particle surface available for charge compensation. This route to reduced particles might be useful for electronic device fabrication, since the negative electronic charge is precisely compensated by immobile cationic ligands.
Introduction. Assemblies of ligand-stabilized inorganic nanoparticles (NPs) are an emerging class of chemically tunable and solution-processable electronic materials.1-4 Electrochemistryswhereby the electronic charge on the nanoparticle cores is controlled by the application of a bias to a nanoparticle-covered electrode submerged in a suitable electrolyte solutionshas proven useful for controlling key NP properties including the “carrier” concentration and conductivity,5-7 chemical potential,8,9 and optical response.10,11 Electrochemical charging has been achieved with a variety of nanoparticle materials including both semiconducting5,6,12,13 and noble metal8,14,15 nanoparticles. For semiconducting particles, electrochemistry has been used to realize n- and p-doped materials,5,6 which are important because traditional doping methods that require atomic substitution are challenging to implement in NPs.16-18 Recently, we have shown that electrochemical control over the Fermi level of a solid assembly of gold nanoparticles in contact with a bulk * Correspondence should be addressed to G.D.S.,
[email protected]. † Department of Chemistry and Biochemistry, University of California, Santa Barbara. ‡ Materials Department, University of California, Santa Barbara. § Department of Chemistry, Münster University. | Department of Chemistry, The Materials Science Institute, Oregon Nanoscience and Microtechnologies Institute, University of Oregon. 10.1021/nl8021412 CCC: $40.75 Published on Web 08/30/2008
2008 American Chemical Society
semiconductor surface allows for the active tuning of the electronic structure of the nanoparticle-semiconductor interface.8 In order 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 nanoparticle film is reduced, electrons are injected in the nanoparticle cores and mobile cations migrate into the film from the electrolyte to compensate the electronic charge (Figure 1A). Such electrochemically charged nanoparticles can be stable, even after removal from the electrolyte and drying, as these associated counterions render the nanoparticle film overall neutral.8,9 Despite the importance of electrochemical methods for modulating the electronic properties of nanoparticle-based materials, little attention has been paid to manipulating these coupled, and often critical, ionic processes.19-21 For example, the kinetics of the ion incorporation into the nanoparticle film from the electrolyte depends strongly on ion size. Controlling the compensation kinetics using bulky ions provides a route to modulating the electrochemical charge injection. Once a nanoparticle film has been electrochemically charged (via the incorporation of electronic and ionic charges), the chemical nature and mobility of the counterions
Figure 1. Electrochemical reduction of inorganic nanoparticle films. 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 Au particles ∼2 nm in diameter. This schematic illustrates how electronic charging is inherently coupled to ion motion. For the nonionic particles (A), the application of a negative bias to the working electrode in the electrochemical cell results in the injection of electrons into the particle cores. Mobile countercations from the conventional small molecule electrolyte migrate into the film to compensate the electronic charge. For the cationic particles (B), electron injection is coupled with anion departure when a polycation supporting electrolyte that is too large to penetrate the densely packed particle film is used. The resulting particles are charged with electrons, the number of which is determined by the number of cationic ligands (which are immobile and whose coverage can be controlled synthetically).
affect the subsequent electronic properties of the charged film. Indeed, the mobility of electrochemically introduced counterions could be problematic because the NP charge states could change with time or under applied bias due to the movement of both electronic and ionic charge carriers. NP devices with heterogeneous redox profiles, such as pn junctions fabricated from oxidized and reduced semiconductor nanoparticles, would similarly lack stability. Herein, we present a method to control both the electrochemical charging of nanoparticle films, and the subsequent counterion mobility in the charged films, by exerting control over the ionic charge compensation process (Figure 1B). We synthesized a series of gold nanoparticle (AuNPs) samples with controlled numbers of covalently attached cationic groups and employed both small-molecule and polycation supporting electrolytes to study and manipulate their electrochemical charging. AuNPs were chosen for this investigation because they are easily synthesized,22 stable, amenable to diverse surface functionalization,23 and exhibit welldefined redox character due to quantized double-layer charging.14 Using an electrochemical quartz-crystal microbalance, we simultaneously tracked the movement of both ions and electrons during the charging (redox) processes. We demonstrate that the maximum number of electrons injected into each nanoparticle is dictated by the surface coverage of cationic ligands when the polycation supporting electrolyte is used. This route to reduced particles, whose electronic Nano Lett., Vol. 8, No. 10, 2008
charge is precisely compensated by chemically bound cationic charge, addresses the potential problems associated with the mobile counterions present in electrochemically charged NPs (stated above) and hence might be used to fabricate a wider array of electronic devices using electrochemistry. Although this work is performed with AuNPs, the general principles are applicable to NPs of other core compositions. Results and Discussion. Cationic AuNPs were synthesized from hexanethiolate-stabilized gold nanoparticles (HTAuNPs) via a thiol-thiol place-exchange reaction.23 The average number fraction of cationic ligands on each particle was controlled by the ratio of the cationic ligand, N,N,Ntrimethyl(11-mercaptoundecyl)ammonium tetrafluoroborate (TMUA),24 to the polar nonionic ligand, 1-mercaptohexanol (MHA) and characterized via 1H NMR (Table 1). This specific combination of nonionic and cationic thiols was selected so that the bulky cationic group on the TMUA was distant enough from the gold surface to avoid steric interference with the molecular packing of the base MHA layer. The relative concentrations of cationic ligands determined via NMR correspond to, on average, between 0 and ∼3.3 cationic ligands per particle, assuming ∼71 total ligands per particle.25 Transmission electron microscopy measurements suggest that the particle diameters (∼2.0 ( 0.5 nm) were unaffected by the ligand-exchange process. Although we refer to these particles as cationic AuNPs, as-synthesized they are electrically neutral due to tetrafluoroborate counteranions. These particles are soluble (>300 mg/mL) in common polar solvents (MeOH, EtOH, DMSO) and can be processed as uniform thin films over large areas (∼4 cm2) via spin-coating. A detailed description of all synthetic and experimental methods is given in the Supporting Information. To unambiguously demonstrate that the charging of nanoparticle solids can be governed by surface-attached ionic species, the coupled movement of electrons and ions was measured simultaneously using an electrochemical quartzcrystal microbalance (EQCM).26 A platinum-covered quartz crystal was coated with a thin film of AuNPs, briefly treated with a 1,9-nonanedithiol/ethyl acetate solution to lightly cross-link the AuNPs and prevent further solubility, and then used as the working electrode to sweep the potential of the AuNP film (ENP) in a typical three-electrode configuration.27 During the potential sweep, the resonance frequency of the quartz crystal was simultaneously recorded. Using impedance analysis, viscoelastic effects were determined to be insignificant (see Supporting Information). Consequently, shifts of resonance frequency were directly correlated to mass changes consistent with the rigid-film limit.26 With our experimental setup, and a sampling time of 0.5 s, a mass resolution of 30 ng cm-2 was achieved. The redox behavior of the cationic AuNPs was first studied utilizing the conventional small-molecule electrolyte, tetramethylammonium tetrafluoroborate (NMe4BF4), in which both anions and cations can cross the film-electrolyte boundary to participate in charge compensation. The cyclic voltammograms (CVs) for each AuNP film collected in NMe4BF4 consist of a series of redox waves whose spacing 3405
Table 1. Composition of AuNP Materials for Different Ligand-Exchange Conditionsa 103·[TMUA]/ [MHA] synthesis batch
synth.
ratiob
TMUA c (%) electrochem.
NMR
MHA (%)
HT (%)
core diameter (nm)
NTMUAd (# per AuNP)
NMR
NMR
TEM
NMR
1 0 0 0 92.4 ( 1.7 7.6 ( 1.7 2.0 ( 0.5 0 2 1.5 0.76 ( 0.04 0.9 ( 0.3 91.5 ( 1.8 7.6 ( 1.7 1.9 ( 0.5 0.6 ( 0.2 3 3 2.1 ( 0.1 1.7 ( 0.4 90.5 ( 1.9 7.8 ( 1.8 2.0 ( 0.5 1.2 ( 0.3 4 4.5 3.5 ( 0.2 3.0 ( 0.6 89.3 ( 1.8 7.7 ( 1.5 1.9 ( 0.4 2.1 ( 0.4 5 6 4.4 ( 0.2 3.8 ( 0.7 88.7 ( 1.9 7.4 ( 1.6 2.0 ( 0.4 2.7 ( 0.5 6 7.5 5.8 ( 0.3 4.7 ( 0.8 89.0 ( 1.7 6.3 ( 1.4 2.1 ( 0.5 3.3 ( 0.6 a TMUA (N,N,N-trimethyl(11-mercaptoundecyl)ammonium tetrafluoroborate), MHA (1-mercaptohexyl alcohol), and HT (1-hexanethiol) amounts are reported as the mole fraction of each component. b During the ligand exchange, [MHA]/[HT-AuNP] ratio was kept at 1000:1 (136 µL MHA per 50 mg AuNP). c The fraction of the cationic TMUA was measured independently using NMR and electrochemistry (see text). d Calculated assuming the average ∼2 nm particle carries ∼71 organic ligands.
Figure 2. Electrochemical quartz crystal microbalance (EQCM) measurements of cationic AuNPs. Panels A and B show EQCM data collected for each nanoparticle film (samples 1-6) in the small-molecule electrolyte, 0.08 M NMe4BF4, and panels C and D show identical measurements for the same set of films in the polycation electrolyte, 0.1 M PDDP-PF6. Voltammetry in PDDP was collected prior to that in NMe4BF4 to prevent contamination with NMe4+ ions. Sample 1 has no ionic groups in the ligand shell and samples 2-6 have linearly increasing numbers of ionic ligands (Table 1). Panels A and C show cyclic voltammograms (50 mV s-1) normalized to the film mass, and panels C and D show the simultaneously collected mass-change data normalized to account for variations in the original film mass and solvent intercalation effects (the normalization procedure is described in the Supporting Information). ENP is the electrode potential of the nanoparticle film as controlled by the applied voltage of a potentiostat. The electrode potential at which the particle cores have zero electronic charge is about -0.2 V vs SCE. The observation of increasing reduction currents and relative mass losses for increasing cationic functionalization (curves 2-6) in the PDDP electrolyte is strong evidence that the electrochemical reduction of the AuNPs is dictated by the number of cationic ligands on the particle surface.
(∼190 mV) is dictated by the single-electron charging energy of the majority fraction of AuNP cores with diameters around 2 nm (Figure 2A).8,14,15 The potential at which, on average, the AuNP cores are neutral (oxidation state of 0), ENP0, can be estimated either from the current minima for the nonionic particles in NMe4BF4 (Figure 2A, curve 1) or from the open circuit voltage 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 charge15). We found that ENP0 was around -0.2 V versus the standard calomel electrode (SCE), which is comparable to that reported for similar-sized AuNPs.15,28 ENP0 did not depend significantly on the degree of ionic functionalization, which is consistent with the notion 3406
that the ionic ligands alone do not affect the electrochemical potential of the AuNPs. At electrode potentials more positive than ENP0, each particle core is oxidized and hence carries a net positive charge. In order to compensate the positive electronic charge on the particle core, mobile anions migrate from the electrolyte into the film and an increasing film mass with increasing ENP is registered on the QCM (Figure 2B). Because the cationic ligands (and their associated anions) are not involved in charge compensation during oxidation, these portions of the current and mass traces collected for each NP sample (with different degrees of cationic functionalization) are similar. Nano Lett., Vol. 8, No. 10, 2008
More complicated ionic charge compensation processes are observed when the cationic AuNPs are negatively charged (reduction). At potentials more negative than ENP0, electrons are injected into the AuNP core electronic states. For nonionic AuNP films, electron injection is coupled with cation (NMe4+) transfer from the supporting electrolyte (Figure 2A,B, curve 1). In contrast, cationic AuNP films preferentially expel available BF4- counterions originally balancing the charge on the TMUA cationic ligand before introducing additional NMe4+ from the electrolyte. The preferential loss of anions is readily discerned from the shape of the ∆M vs ENP plots (Figure 2B). Whereas sample 1 (no cationic ligands) shows an immediate mass gain as the potential of the film is driven more negative than ENP0 (reduction), samples 2-6 (with increasing cationic density) exhibit increasing degrees of mass loss upon reduction. The mass loss continues as the electrode potential is swept more negative until the supply of anions in the film, which were introduced synthetically as the counterions to the covalently bound cationic ligands, is exhausted. When the number of electrons injected into the film becomes larger than the number of cationic ligands, injected electrons must then be compensated by the incorporation of additional cations from the electrolyte; the crossover point from mass loss to mass gain in Figure 2B is driven to increasingly negative ENP as the cationic ligand density in the AuNP film increases. The experimental observation that anions preferentially leave the film before additional cations enter the film can be rationalized in terms of the free energy cost of transporting an ion from solution into the film.19 Similar behavior has been observed in polymeric systems.29 The electrochemical charging of the AuNP films is reversible by switching the sweep direction of the potential and returning to ENP0. Some hysteresis is apparent in the ∆M vs ENP curves, which is primarily due to accompanying neutral solvent molecules that lag the charged ions in traversing the film-electrolyte boundary.26 Precise control over the coupled ion-electron motion during electrochemical charging can be achieved by kinetically limiting the sign of the ions available for charge compensation using the polycation supporting electrolyte, poly(1,1-dimethyl-3,5-dimethylenepiperidinium hexafluorophosphate)/CH3CN (PDDP). Such polyelectrolyte-mediated electrochemistry has proven to be a useful tool to study doping mechanics and ion transport properties of conjugated and redox polymers,30-32 but it has not previously been applied to NP systems. Because the large polycation chains are unable to enter the densely packed AuNP film (the small anions can still compensate for positive charging), EQCM measurements in PDDP allow us to demonstrate that the maximum degree of reduction is dictated by the average number of cationic ligands per particle (Figure 1B). Voltammograms for the same set of AuNP films collected in PDDP (Figure 2C) are markedly different from those collected in NMe4BF4 (Figure 2A). At potentials more positive than ENP0, hole injection into the particle core is coupled with anion incorporation (as above) regardless of ionic ligand content. At potentials more negative than ENP0, Nano Lett., Vol. 8, No. 10, 2008
reduction of nonionic particles (Figure 2C, curve 1) is largely prevented due to the lack of cations available to compensate the negative core charges (some current is observed, which decays to near zero at more negative potentials and which can be primarily attributed to particles near the film surface). For the cationic AuNPs, increased reduction currents and larger ranges over which mass loss occurs (due to anion departure) are observed at potentials more negative than ENP0 for samples with increasing cationic ligand concentrations (Figure 2C,D, curves 2-6). The results presented in this work demonstrate that the covalent binding of ionic ligands, in conjunction with the use of an appropriate polyelectrolyte, is a powerful tool to manipulate the electrochemical charging of AuNP solids. As ENP of the cationic particles is swept to reductive potentials in the polycation electrolyte, electron injection occurs only until the point at which the number of injected electrons equals the number of cationic surface charges introduced synthetically (that is, the charging process is self-limited). This relationship can be confirmed by comparing the integrated charge associated with particle reduction (Qred) with the number of cationic ligands in the film determined from the NMR integrations. First, the absolute value of the current was integrated over a complete voltage sweep (Figure 2C) and divided by 2 (to account for the integration of both forward and reverse sweeps) to attain Qtot. The incremental increases in Qtot for curves 2-6 over that for curve 1 (nonionic) can be approximated as Qred for each film. The total ligand weight percentage in the particles (19 ( 1% determined by thermigravimetric analysis), along with Qred, was then used to calculate the mole fraction of cationic ligands in AuNP samples 2-6. These values compare well with those determined using NMR (Table 1), verifying the validity of our analysis. On the basis of the approximate NP film density (∼3.5 g/cm3),33 samples 2-6, in their most reduced states, have an added electron concentration equal to the cationic ligand concentration, that ranges from ∼2 × 1019 to ∼2 × 1020 cm-3. Conclusion. This work demonstrates the importance/utility of controlling ionic processes during the electrochemical charging of NP assemblies. We have shown that, when mobile cations from the electrolyte are not available for charge compensation, the degree of electrochemical reduction possible for a nanoparticle film is dictated by the surface coverage of cationic ligands. In other words, the range of accessible electrochemical potentials is restricted by the cationic ligand density. Preliminary results suggest that the analogous self-limited electrochemical oxidation of anionicfunctionalized AuNPs in a polyanion electrolyte proceeds in a similar fashion. In the context of device applications,8,28,34,35 these electronically charged, ionic ligand compensated AuNPs might be interesting due to the lack of free counterions that could move in the presence of an applied bias. Furthermore, AuNP samples with different numbers of ionic ligands could be used to fabricate devices which contain charged AuNP assemblies with different average charge states (i.e., different electrochemical potentials) in direct contact. 3407
If this ionic ligand mediated charging methodology were extended to semiconducting nanoparticles, it may be possible to combine positively (p-type) and negatively (n-type) charged particles into bilayer solution-processed pn junction devices. At the interface of the p and n materials, the added electronic charges will recombine over the “depletion region” leaving behind only the covalently bound anionic and cationic ligands, respectively.36 The resulting interfacial electric field in the engineered depletion region might be used to split excitons and collect carriers using drift (as opposed to the diffusion usually relied upon in solution-processed devices), which might allow the exploitation of the novel features associated with NPs, such as multiple exciton generation.37 Such depletion region formation would not be possible in NP materials using conventional electrochemical doping/charging, as the mobile counterions would simply move to screen the resulting field. We are currently working to extend these methods to electrochemically stable semiconducting nanoparticles, and investigating electrochemical methods to fabricate junctions between oppositely charged nanoparticle assemblies. Beyond the demonstrated use in nanoparticle materials, ionic ligand mediated electrochemical charging methods may have implications for the control of carrier concentrations in variety of inorganic nanostructures whose small volumes make it difficult to introduce impurities using conventional techniques.38,39 For example, ionic functionalization of a semiconducting nanowire or ultrathin film, followed with appropriate biasing under pure solvent to remove the mobile cations, should result in an electronic charge density determined by the degree of ionic charge compensation available on the surface. Acknowledgment. We thank Anna Ivanovskaya for helpful discussion and Eric McFarland for the loan of the QCM. This work was supported by the NSF under awards DMR0233728, DMR-0805148, and DMR-0519489, the Air Force Research Laboratory under agreement FA8650-05-1-5041, the U.S. Army Research Office via the Institute for Collaborative Biotechnologies through grant DAAD19-03D0004, and made use of the MRL central facilities supported by the MRSEC Program of the NSF under award DMR-0520415. S.W.B thanks UC Santa Barbara for a Chancellors Fellowship. Supporting Information Available: Procedures for ligand and nanoparticle synthesis, nanoparticle characterization (NMR, TEM, UV-vis), electrolyte purification and preparation, thin film preparation, electrochemical and EQCM methods/corrections/analysis. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Nature 2006, 442 (7099), 180– 183.
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NL8021412
Nano Lett., Vol. 8, No. 10, 2008
