Redox Charging of Nanoparticle Thin Films in Ionic Liquids - The

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Redox Charging of Nanoparticle Thin Films in Ionic Liquids Wanzhen Li and Bin Su* Institute of Microanalytical Systems, Department of Chemistry, Zhejiang UniVersity, 310058 Hangzhou, People’s Republic of China ReceiVed: July 18, 2010; ReVised Manuscript ReceiVed: September 14, 2010

Thin films consisting of alkanethiolate protected gold nanoparticles (MPCs) immersed in room temperature ionic liquids (ILs) were investigated by electrochemistry. The anion-dependent oxidative charging of MPC films was observed in imidazolium based ILs, which is similar to that of anion-rectified/limited oxidative charging previously observed in aqueous media. The absence of reductive charging in imidazolium ILs is simply because of the inadequate hydrophobicity of the imidazolium cation. Replacing the imidazolium cation with more hydrophobic tetrahexylammonium, the reductive charging of MPC films was observed. The ionic dependence manifests the charging process to be an ion-coupled electron transfer event, with the oxidative/ reductive charging of MPCs at the electrode/film interface concomitant with the ionic partition at the film/IL interface. This system provides a diverse possibility of regulating the electronic properties of nanoparticle thin films by ionic/solvent functions, given that ILs are “designed solvents” with structures and functions easily tuned by ionic components (both anion and cation). Introduction Novel electronic properties and foreseen practical applications have stimulated enormous research activities toward particles in the nanosize regime, where quantum mechanical rules instead of the laws of classical physics are dominantly followed owing to the discrete number of electrons.1 Electrochemistry represents a valuable analytical tool for investigating the properties of discrete electrons in nanoparticles.2,3 For instance, ∼2 nm gold nanoparticles protected by alkanethiol molecules, so-called alkanethiolate monolayer-protected clusters (MPCs), behave like multivalent redox species manifesting a series of regularly spaced current peaks along the potential axis in voltammetry.3-8 In the pioneering work by Murray et al., this current form has been interpreted as the quantized charging of the tiny capacitance of MPCs with a concentric sphere model:3,7

()

CMPC ) 4πε0εd

r0 (r + d) d 0

(1)

where ε0 is the vacuum permittivity, εd the dielectric constant of the monolayer. The variables r0 and d are the metallic core radius and the alkanethiolate layer thickness, respectively. Equation 1 defines a subattofarad (aF ) 10-18 F) capacitance associated with a nanometer metallic core and a protecting monolayer, which is so small that electron addition to and removal from the core is discrete at room temperatures (∆V ) e/CMPC, here e is the electron charge). This model has successfully predicts CMPC in the correct order of magnitude,3,7 as well as the effects of the core size, the protecting ligand nature,9,10 and the temperature.11,12 Further considering effects of the medium factors (solvent, electrolyte ions), the concentric sphere model has been improved by including a spherical medium capacitance (Cm) outside the MPC sphere.8,12-15 Organizing MPCs on solid supports with two- and threedimensional structures has also become important in view of * To whom correspondence should be addressed. Tel: +86 571 88273496. Fax: +86 571 88273572. E-mail: [email protected].

applications in nanoelectronics and nanodevices. Various methodologies have been so far employed to construct MPC monolayer or multilayer films on different electrode surfaces.5,6,10,16-22 Although MPC films display charging behavior similar to that of dissolved MPCs, some novel and intriguing phenomena have been discovered, for instance the rectifying effect in aqueous media reported by Chen et al.10,17 The rectifying states that the oxidative charging of MPC films depends on the nature and concentration of the electrolyte anion,10,17 which has been primarily interpreted in terms of ion-pair formation involving oxidized MPCs and electrolyte anions, thereby MPC films being equivalent to modified electrodes with the difference in the MPC+-anion association constant accounting for the anion dependence.10,17 Quinn et al.18,19 have lately raised an “anionlimited” model, in which a solvation barrier at the interface between the hydrophobic MPC film and aqueous electrolyte was taken into account by transplanting the ideal of three-phase junction electrochemistry.23 The anion limiting figures that the charging of MPC films is an anion-coupled electron transfer process, involving concomitant charge transfer at two interfaces in series and thereby the anion dependence being pure Nernstian. Additionally, contrary to the oxidative charging at positive potentials, the reductive charging has not been observed until recently by using enough lipophilic cation.24 The mechanism of reductive charging is identical to that of oxidative charging, and the inadequate hydrophobicity of cation used accounts for the suppression or missing of reductive charging in prior work. Alternatively, it is well-known that room temperature ionic liquids (ILs) are “designed solvents” with structures and functions easily tuned by ionic components.25 So far, ILs have found applications in diverse fields of chemistry including organic and inorganic synthesis, biphasic catalysis, separation processes and electrochemistry, due to their unique chemical and physical properties, such as nonvolatile, nonflammable, low toxicity, noncorrosive and so on. In particular, ILs are attractive to electrochemists because they provide large potential windows and can act as both solvent and electrolyte, which basically motivates our present study. To the best of our knowledge, only

10.1021/jp1066764  2010 American Chemical Society Published on Web 10/01/2010

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Figure 1. ILs employed in this study.

a few works have reported redox charging of MPCs in ILs. Primary study on rectified oxidative charging of MPC films has been presented in a thesis work26 and by Murray et al. when studying the electrowetting of MPC films by an IL.21 More recent studies have demonstrated the possibility of both oxidative and reductive charging of MPC films in few ILs.22,27 The present work focuses on the ion-dependent quantized charging, either oxidative or both oxidative and reductive, by regulating structures of ILs, which points to an ion-coupled electron transfer mechanism. 2. Experimental Section 2.1. Materials and Reagents. All chemicals of analytical grade or higher were used as received. All of the aqueous solutions were prepared with ultrapure water (18.2 MΩ cm). Hydrogen tetrachloroaurate (HAuCl4, Au 47.8%), potassium hexafluoroborate (KPF6, g99.0%) and potassium perchlorate (KClO4, g99.5%) were bought from Aladdin. Lithium bis(trifluoromethylsulfonyl)amide (LiTf2N, >98.0%) and tetrahexylammonium chloride (THACl, >96.0%) were provided by ChengJie Chemicals. Toluene (g99.5%), 1,2-dichloroethane (DCE, g99.0%), acetonitrile (CH3CN, g99.0%) and ethanol (g99.7%) were ordered from Sinopharm. Tetraoctylammonium bromide (TOABr, 98%) and tetrabutylammonium perchlorate (TBAClO4, 99%) were provided by J&K Chemica. Sodium tetrahydroborate (NaBH4, 98%) and 1-hexanethiol (C6-SH, 97%) were bought from Alfa Aesar. The 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIm+BF4-), 1-n-butyl-3methylimidazolium hexafluorophosphate (BMIm+PF6-), 1-nbutyl-3-methylimidazolium perchlorate (BMIm+ClO4-) and 1-nbutyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIm+Tf2N-) were bought from LICP-CAS (Lanzhou, China). The 1-n-butyl-3-methylimidazolium hexafluoroantimonate (BMIm+SbF6-), 1-n-hexyl-3-methylimidazolium hexafluorophosphate (HMIm+PF6-) and 1-n-octyl-3-methylimidazolium hexafluorophosphate (OMIm+PF6-) were provided by ChengJie Chemicals. Tetrahexylammonium bis(trifluoromethylsulfonyl)amide (THA+Tf2N-) was prepared as previously reported.15 The structures of ILs are illustrated in Figure 1. 2.2. Synthesis and Characterization of MPCs. Hexanethiolate protected gold nanoparticles (MPCs) were prepared by the Brust reaction28 followed by extraction to yield particles with relatively small and uniform metallic cores.29 Briefly, 1.33 g

Li and Su of HAuCl4 in water was mixed with 1.11 g of TOABr in toluene under vigorous stirring; after AuCl4- was completely extracted into toluene, 1.8 mL C6-SH was added; the mixture was further stirred for 20 min, followed by the addition of 1.91 g of NaBH4 all at once at 0 °C; after 45 min, the water layer was removed with a separating funnel, and the toluene phase was rotovaped at e30 °C to obtain crude MPCs as black slurry. The slurry was dispersed in ethanol for 24 h, and MPCs were separated into ethanol-soluble and ethanol-insoluble fractions. The former fraction in the supernatant was carefully decanted, rotovaped, and washed with copious amounts of CH3CN and water; the ∼30 mg of this fraction was finally collected. The quality of ethanol-soluble MPCs was examined by transmission election microscopy (see Figure S-1 of the Supporting Information), which showed a relatively monodisperse fraction was obtained. The redox properties of MPCs synthesized were examined by cyclic and pulse voltammetric measurements in 0.10 M TBAClO4/DCE, where MPCs demonstrated successive and regularly spaced current waves along the potential axis. The fwhms of the first three DPV peaks are only a few mV larger than the theoretical value of ∼90 mV, indicating that the particles are highly monodisperse with an average size of 1.6 nm, as reported previously.29,30 The obtained average peak separation in a potential range of -0.4∼0.8 V is ∼0.253 V (see Figure S-2 of the Supporting Information). Moreover, the zero current lies in a potential between -0.2∼0 V, which is accordingly that of the MPCs in solutions.6,29 2.3. Electrochemistry Measurements. All of the electrochemical measurements were carried out on a CHI660D electrochemical workstation (CH-Instruments, Shanghai). MPC films were prepared by dropcasting 5 µL of 0.05 mM MPCs predissolved in DCE on a 3-mm diameter disk-shaped glass carbon (GC) electrode, which was transferred to electrolyte solutions after drying. A platinum wire was used as the counter electrode, and a silver/silver chloride (Ag/AgCl, saturated KCl) or a silver wire was used as aqueous reference electrode or a quasi-reference electrode in ILs. The square wave voltammetry (SWV) was performed with the following parameters: amplitude 0.05 V, frequency 10 Hz, increment potential 0.005 V. All of the experiments were performed at room temperatures (23 ( 2 °C). 3. Results and Discussion 3.1. Anion-Dependent Oxidative Charging in BMIm+ ILs. Figure 2a shows the cyclic voltammetry of an MPC film conducted in BMIm+BF4-. Apparently, four current waves were detected at positive potentials (more waVes can be obserVed at more positiVe potentials, and a small preceding peak is probably giVen by the little fraction of smaller size MPCs), whereas only a featureless current was displayed at negative potentials. This “current rectifier” type behavior is very much the same as that previously reported in aqueous electrolytes, where the oxidative charging of MPC films is concomitant with the ingress of electrolyte anion here BF4-. Morevoer, the current waves were rather stable, owing to the hydrophilic nature of BMIm+BF4and the lipophilicity of MPCs. The magnitude of oxidative charging current at each peaks is linearly dependent on the potential scan rate, as displayed in Figure 2b, suggesting the occurrence of successive surface oxidative of MPCs. From the linear dependence an average surface coverage of 3.76 × 10-6 mol m-2 can be estimated. A mechanistic illustration of this anion-coupled oxidative charging process is shown Figure 3, which defines the following reaction diagram:

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Figure 4. (a) CVs of MPC films in various BMIm+ ILs, scan rate 0.05 V s-1. (b) Corresponding SWVs with current normalized by the magnitude of first peak.

TABLE 1: Summary of ILs Physical Parameters and Charging Data

Figure 2. (a) CVs of an MPC film in BMIm+BF4- at various scan rates: 0.01, 0.02, 0.04, 0.05, 0.06, and 0.10 V s-1 from inner to outer. (b) Dependence of the anodic peak currents on the potential scan rate.

ILs

V1/V

Vint/V

r-/nma

εILb

Ceff/Mc

Λimp/ΛNMRd

BMIm+BF4BMIm+SbF6BMIm+ClO4BMIm+Tf2NBMIm+PF6HMIm+PF6OMIm+PF6-

0.34 0.25 0.18 0.15 0.14 0.31 0.38

0.129 0.135 0.129 0.153 0.142 0.143 0.138

0.259 0.307 0.269 0.381 0.296

11.7

5.30

0.68

11.6 11.4 8.9

4.31 3.42 4.80 4.12 3.56

0.66 0.61 0.64

a Calculated from the ionic volume.32 b From refs 32, 33. Estimated from density.34 d From ref31 and that of BMIm+ClO4- is an average value of BMIm+BF4- and BMIm+PF6-. c

Figure 3. Schematic model of ion-coupled electron charging/discharging of MPC films in ILs.

Electron transfer at the electrode/film interface for the first oxidative charging:

MPC0(film) f MPC+(film) + e-

(2)

Ion transfer at the film/IL interface:

A-(IL) f A-(film)

(3)

The overall reaction writes the Nernst equation:

E1/2 )

0
E+1/0

+

film 0
∆IL φA-

( )

0 RT cMPC RT + ln ln cAF 2 F

(4)

0′ is the formal potential of first oxidation in film that where E+1/0 can be akin to the first oxidation of free diffusing MPCs in a film 0
medium with similar permittivity. ∆IL φA- is the formal transfer potetnial of anion at the film/IL interface, c0MPC the molar concentration of MPCs in film, cA- the anion concentration, respectively.

A key indication of eq 4 is the dependence of onset charging film 0
potential on the ∆IL φA-. Figure 4 illustrates the anion dependence of oxidative charging of MPC films in BMIm+ group ILs. In each ILs multiple well-defined current waves with peak current linearly dependent on the scan rate (see more CVs in Supporting Information, Figures S3-S6) positioned at positive potentials was observed, whereas only featureless current was displayed at negative potentials. This form is very similar to that observed in aqueous media with corresponding alkaline salts (Figure S-7 of the Supporting Information). A most clear characteristic of Figure 4 is the shift of oxidative charging onset potential (V1) by an order of PF6- < Tf2N- < ClO4- < SbF6- < BF4-, which is basically the same as that in aqueous media yet with a discrepancy on the sequence of Tf2N- and PF6-. In water, the charging onset with Tf2N- is before with PF6-. Considering the ion-limited model,18,19 the onset charging potential is in principle determined by the anion hydrophobicity and concentration. On the basis of the Born solvation model and nearly the same permittivity of BMIm+PF6- and BMIm+Tf2N-, Tf2Nshould have a smaller barrier than PF6- and thus a lower onset charging potential. However, the molecular concentration of BMIm+PF6- is 1.4× higher than BMIm+Tf2N- (Table 1) yet a close ionicity (expressed by a ratio Λimp/ΛNMR expressed with Λimp and ΛNMR being the molecular conductivity values measured by impedance and NMR, which represents the percentage of free diffusing ions contributing to the ionic conduction of an ILs and can be considered as a measure of the IL ionicit).31 Thus, the order of onset charging potential in ILs reflects the counterbalance between hydrophobicity and ionic concentration. It should be mentioned that here a satisfactory stability of silver reference electrode is assumed, as well as in the following measurements. In terms of Figure 3 and eq 4, the junction at the MPC film/IL interface functions as a thermodynamic barrier, which can be equivalent to an interface between two immiscible liquids. Thus the onset oxidative charging is determined by the anion salvation

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Li and Su

Figure 5. E1/2 against the reciprocal of anion radius using eq 7.

in the IL relative to the MPC film, which can be estimated in terms of simple Born ionic solvation model:

0′, ILffilm ∆Gion )-

(

NAz2e2 1 1 8πε0rion εIL εfilm

)

(5)

where NA is the Avogadro constant, z the anion charge, e the elementary charge, ε0 the vacuum dielectric constant, rion the anion radius, εIL the relative permittivity of IL, and εfilm the relative permittivity of film, respectively. Equation 5 can be expressed in the voltage scale as follows:

film 0’ ∆IL φion ) -

0′, ILffilm ∆Gion

(6)

zF

Therefore, by combining the ion-limited model and the Born solvation model, namely eqs 4 and 6, the dependence of onset charging potential on the anion radius and the concentration follows:

E1/2 )

0
E+1/0

(

)

( )

0 NAze2 1 1 RT cMPC + + ln 8πFε0rion εIL εfilm F 2 RT ln cA- (7) F

After being corrected by the ionic concentration (estimated as Ceff × Λimp/ΛNMR), the half wave potential displays a linear correspondence with the reciprocal of anion radius for PF6-, Tf2N- and ClO4- (Figure 5). This fact supports the mechanism of anion-coupled electron charging of MPC films, namely the oxidation of MPCs at the electrode/film interface is concomitant with the anion transfer at the film/IL interface,18,19 as displayed in Figure 3. SbF6- was not included since its Ceff and Λimp/ ΛNMR are unknown. BF4- deviates seriously from the slope, which might be interpreted by its particular hydrophilic nature in the group. Another concern is the variation of charging potential interval (Vint) with the anion size (see Table 1 and Figure S-8 of the Supporting Information). Typically, the smaller the anion is, the larger the Vint becomes. Similar phonomena have been reported for solution dissolved MPCs, and it is most probably associated with the anion penetration to the protecting monolayer of MPCs.10,12,14,17,20 The ions penetration will essentially induce the increase of effective dielectric constant and the decrease of effective thickness of the monolayer, leading to a larger MPC capacitance and therefore a smaller charging voltage in terms of eq 1.

Figure 6. (a) CVs of MPC films in BMIm+PF6- (black), HMIm+PF6(red) and OMIm+PF6- (blue) with blank CVs of a bare glass carbon electrode compared as dotted lines, scan rate 0.05 V s-1. (b) Corresponding SWVs with current normalized by the magnitude of maximum peak.

3.2. Cation-Dependent Charging in PF6- ILs. If the ionlimited model, namely the ion-coupled electron charging mechanism, is correct, then the absence of reductive charging at negative potentials is simply because the ingress of BMIm+ requires a higher extent of negative electrochemical polarization. Namely, BMIm+ is not lipophilic enough and one has to apply a very negative potential to observe the transfer of BMIm+ from IL to MPC film, and therefore initiate the reductive charging. With curiosity about the reductive charging, we increased the chain length in imidazolium to improve the lipophilicity of ILs. As compared in Figure 6a, still only oxidative charging coupled by PF6- transfer was observed in 1-n-hexyl-3-methylimidazolium (HMIm+) PF6- and 1-n-octyl-3-methylimidazolium (OMIm+) PF6-. A plateau current was observed at very negative potentials, which is however most probably attributed to the impurities in ILs. As clearly demonstrated by the SWVs in Figure 6b, the onset charging potential shifted positively with increasing the chain length, which does not follow the intuitive hydrophobicity order. This might be also explained by the difference in the effective PF6- concentration. First, the molecular concentration decreases with increasing the imidazolium chain length (Table 1). Although values of Λimp/ΛNMR for HMIm+PF6- and OMIm+PF6- are not available, their magnitudes should follow the same order of those of imidazolium Tf2N- ILs, whose Λimp/ΛNMR as reported previously decreases with increasing the imidazolium chain length due to enhanced intermolecular inductive forces.31 Another possible reaction maybe the difference in dielectric constant. As shown in Table 1, the dielectric constant of HMIm+PF6- is slightly smaller than that of BMIm+PF6-, thereby a more positive charging onset is expected with the former according to eq 7. However, referring to a prior work by Wandlowski et al. in which they observed both oxidative and reductive charging in HMIm+ tris(pentafluoroethyl)trifluorophosphate (FEP-),22,27 we only detected the oxidative charging in HMIm+PF6-. The difference is probably due to the highly lipophilic nature of FEP-, which is however not available for us to prove. Indeed, as reported by Quinn et al. in aqueous media, the oxidative charging of MPC films can

Redox Charging of Nanoparticle Thin Films

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18107 Acknowledgment. This work is supported by the FundamentalResearchFundsfortheCentralUniversities(2009QNA3012, 2010QNA3012) and the new-faculty startup support of ZJU. Supporting Information Available: TEM image and more voltammetry data. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. (a) CVs of an MPC film (red) and a bare glass carbon electrode (black) in THA+Tf2N-, the potential sweep started from middle to positive at a scan rate of 0.05 V s-1. (b) Separate SWVs of an MPC film (red) and a bare glass carbon electrode (black) in the positive and negative potential regimes.

be observed throughout the available potential window with a very hydrophobic anion.19 In order to observe the reductive charging, experiments were done in tetrahexylammonium (THA+) Tf2N- considering the bulky and lipophilic nature of THA+. As shown in Figure 7, parallel oxidative and reductive charging was detected, which is very much the same with previously reported electrochemical responses in some organic solvents. The difference is here that a big middle potential gap >0.50 V between the first oxidative can the first reductive charging process was observed. This is something like the potential gap developed in the case of even smaller MPCs.3 However, the oxidative charging waves look ugly and the charging current decayed with potential scanning cycles, because of desorption of MPCs from the film to THA+Tf2N- given its low polarity. Nevertheless, it amply proves the scenario of electron charging the MPC films tightly coupled by ionic partition at the film/IL boundary as shown in Figure 3. The absence of reductive charging in imidazolium ILs is simply due to the inadequate lipophilicity of imidazolium cations. 4. Conclusions The anion-rectified oxidative charging of MPC films was observed in imidazolium based ILs, which is similar to that previously observed in the aqueous media. Furthermore, the charging of MPC films, either oxidative or both oxidative and reductive, can be selectively controlled in ILs by regulating the ionic constituents (both anion and cation). The ionic dependence manifests the charging process to be an ion-coupled electron transfer event, which suggests that the absence of reductive charging in imidazolium ILs is simply because of the inadequate hydrophobicity of imidazolium cation. This system provides a diverse possibility of regulating the electronic properties of nanoparticle thin films by ionic/solvent functions, given that ILs are “designed solvents” with structures and functions easily tuned by ionic components.

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