See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/249334931 Stability, Assembly, and Particle/Solvent Interactions of Pd Nanoparticles Electrodeposited from a Deep Eutectic Solvent ARTICLE in THE JOURNAL OF PHYSICAL CHEMISTRY C · JULY 2013 Impact Factor: 4.77 · DOI: 10.1021/jp403739y CITATIONS 11 READS 51 7 AUTHORS, INCLUDING: Joshua A Hammons Argonne National Laboratory 13 PUBLICATIONS 99 CITATIONS SEE PROFILE Jon Ustarroz Vrije Universiteit Brussel 21 PUBLICATIONS 128 CITATIONS SEE PROFILE Maria Tzedaki Vrije Universiteit Brussel 4 PUBLICATIONS 21 CITATIONS SEE PROFILE Herman Terryn Vrije Universiteit Brussel 391 PUBLICATIONS 4,337 CITATIONS SEE PROFILE Available from: Joshua A Hammons Retrieved on: 04 February 2016
1 Stability, Assembly, and Particle/Solvent Interactions of Pd2 Nanoparticles Electrodeposited from a Deep Eutectic Solvent3 Joshua A. Hammons,*,† Thibault Muselle,‡ Jon Ustarroz,‡ Maria Tzedaki,‡ Marc Raes,‡ Annick Hubin,‡
4 and Herman Terryn‡
5†X-ray Science Division, Argonne National Laboratory, 9700 S. Cass, Argonne, Illinois 60439, United States
6‡Department of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
7 *S Supporting Information
8 ABSTRACT: Supported nanoparticle synthesis and assembly9 have application in a wide range of modern day applications.10 Key to the manipulation of the particle assembly is an11 understanding of the interaction between the particles and12 solvent. Here, we employ a comprehensive in situ approach,13 together with ex situ SEM imaging, to study supported14 palladium nanoparticles, electrodeposited from a 2:1 urea:cho-15 line Cl− DES. Using cyclic voltammetry, we confirm the16 expected adsorption of electroactive species onto the17 deposited particles. On the basis of our experimental results,18 we conclude that the electrodeposited nanoparticles assemble19 into 2-D superstructures, rich in adsorbed species. The abundance of these adsorbed species, within the superstructure, induces20 an anionic layer above them, which can be observed by ultrasmall-angle X-ray scattering (USAXS) as well as electrochemical21 impedance spectroscopy (EIS). The surface charge of the particles is, therefore, not neutralized locally, as is the case with22 traditional colloidal systems. We also show that these otherwise stable nanoparticles readily aggregate when the DES is removed.23 Thus, the stability of these particles is contingent upon the presence of the DES.
1. INTRODUCTION
24 Supported palladium nanoparticles are promising materials for25 various technologies including fuel cells,1 catalysis,2 and26 sensors.3 Here we employ nanoparticle electrodeposition as27 the method of preparation. In general, electrodeposition is a28 simple and often cost-effective method to prepare supported29 nanoparticles, whereby dissolved metal cations are electro-30 chemically reduced onto a substrate.4 Recently, nanoparticle31 electrodeposition from room temperature ionic liquids (RTIL)5
32 has been considered an attractive alternative to electro-33 deposition from traditional aqueous systems. One of the34 main attractions of ionic liquids is their potential to stabilize35 deposited nanoparticles,6−12 as the solvent and stabilizer are36 one and the same. This option makes the electrodeposition of37 supported nanoparticles from ionic liquids an exciting38 alternative to traditional aqueous electrodeposition.39 Type III deep eutectic solvents (DES) are considered a type40 of RTIL and are composed of a quaternary ammonium salt and41 a hydrogen bond donor, at their eutectic composition.13
42 Nanoparticle electrodeposition from DESs is relatively new and43 has proven to be an effective medium to deposit various shapes44 of Pt nanoparticles.14 In addition, DESs have also been shown45 to facilitate PbS15 and Au16 nanoparticle self-assembly. Another46 promising aspect of DESs is their potential to stabilize17 and47 assemble18 deposited Pd nanoparticles in the presence of48 quaternary ammonium salts. In addition, these solutions offer49 some practical advantages over ionic liquids, such as cost,
50known toxicology, ease of preparation, and air/moisture51stability.13 For these reasons, one of the earliest and most52common DESs,19 2:1 urea:choline Cl−, was chosen as the53electrodeposition solution here.54To understand the unique advantages of nanoparticle self-55assembly and stability in RTILs, an understanding of the56particle/solvent interactions is keyspecifically, how they57differ from a traditional aqueous solution. For example, in58some cases the high concentration of adsorbing species can59result in a complete protective layer around the particle.20−22
60This is particularly important when one recognizes that the61surface charge induced by the adsorbed species must be62neutralized. In recent years, many authors have shown that63ionic liquids tend to form a multilayer in the vicinity of a64charged surface,23−26 as opposed to a simple double layer.65Thus, the surface charge induced by the adsorbed species may66be neutralized differently in DES than in aqueous systems.67Specific to DESs, it has been shown that a correlation between68the double-layer capacitance and the final deposit morphology69has been observed for Zn deposition in different DESs.27 Thus,70the mechanism of charge separation in DESs can be considered71an important aspect of electrodeposition. In this study, both the
72 charge separation induced from the Pd nanoparticles and their73 stability are studied in situ.74 The motivation of this study was to deposit stable Pd75 nanoparticles from the DES and to investigate how the DES76 interacts with these particles. Realizing the potential for77 interaction between the DES and the deposited nanoparticles,78 an in situ study is required. Here, we employ cyclic voltammetry79 (CV), synchrotron ultrasmall-angle X-ray scattering (USAXS),80 and electrochemical impedance spectroscopy (EIS) for a81 comprehensive characterization of the system. Furthermore,82 these in situ results are compared with ex situ SEM imaging.
2. EXPERIMENTAL SECTION83 The 2:1 (urea:choline) DES was prepared by recrystallizing84 choline chloride (Afla Aesar) and urea (Afla Aesar) in absolute85 ethanol, followed by vacuum drying. The DES solution was86 then prepared by mixing the two components, at a 2:1 ratio,87 and heating to ∼70 °C. Once clear, the 10 mM K2PdCl488 solution was prepared, at room temperature, and heated to 10089 °C for 1 h before use.90 The glassy carbon foil (Hochtemperatur-Werkstoffe GmbH)91 was prepared by submersing the foil in a beaker of absolute92 ethanol and placed in an ultrasonic sink for 5 min. Following,93 the foil was rinsed and submerged in a beaker of Millipore94 water and placed in an ultrasonic sink for 5 min. Finally, the95 glassy carbon foil was placed in the sample cell with the counter96 electrode and taped, followed by cell assembly. The final97 solution was syringed into the transmission cell, where the98 scattered intensity was obtained using the ultrasmall-angle x-ray99 scattering (USAXS)/pinhole small-angle X-ray scattering100 (pinSAXS) setup at beamline 15-ID, Advanced Photon Source101 (APS).
f1 102 Using the setup shown in Figure 1, the sample cell was103 exposed to a 16.8 keV monochromatic X-ray beam. The
104 scattered intensity was collected by both a Bronse-Hart camera105 setup (USAXS) and a pinhole SAXS setup that used a Pilatus106 100k detector; this setup maximizes the signal-to-noise ratio at107 high q (3 × 10−2 to 1 Å−1) where the scattering signal is108 typically very weak. Using this setup, the intensity was109 measured at each q-value for 0.5 s at very low q to 2 s at110 higher q, where the scattering signal is typically very weak. The111 complete scattered intensity, I(q), was then obtained by112 combining the USAXS (10−4 to 6 × 10−2 Å−1) and the pinhole
113SAXS for 3 × 10−2 to 1 Å−1. However, in the experiments114presented here, a sufficient overlap was not always obtained. In115these cases, only the low-q USAXS data are shown.116The custom-designed transmission cell was used to obtain117the scattered intensity in situ. The sample cell is essentially the118same as used previously,28 but with copper tubing that was used119for temperature control, an NTC 100 kΩ insulated thermistor,120a Ag/AgCl mini-reference electrode (eDAQ), and a Pt counter121electrode; the temperature measured is considered accurate to122±0.5 °C, based on the noise collected during the measurement.123The background scattering from the electrolyte, cell, and 0.18124mm thick glassy carbon was collected and subtracted from125subsequent scattering data for each experiment. All subsequent126data reduction was performed in the Irena package, available for127Igor Pro.29
128To deposit as many particles as possible, the approximate129cathodic limit of the electrochemical window of the DES (at13032.5 °C) was used (ca. −1.8 V). A lower overpotential of −1.4131V was applied during the growth pulse to minimize the size132dispersion of the particles.30 Because the viscosity and133conductivity (and thus ion transport) of the DES are both134strong functions of temperature,31 two different temperatures135were used for each electrodeposition sequence: 32.5 and 44.5136°C. Upon completion of each experiment, each sample was137washed with ethanol and water for SEM imaging using a JEOL138JSM-7000F field emission gun scanning electron microscope,139operated at an acceleration voltage of 20 kV.140The impedance measurements were performed separately,141using the same cell, conditions, and potentiostat (Ivium142Compactsat) as was used at the synchrotron. These143galvanostatic EIS measurements were made at OCP, using a144root-mean-square amplitude of 50 nA. The measurements145started 100 s after nucleation, as the OCP was found to change146the most during this time. The most significant portions of the147impedance spectra were found to occur at frequencies between148100 and 0.01 Hz. Using this frequency range, the measurement149time was slightly less than the USAXS/pinhole SAXS150acquisition time (∼20 min). Therefore, the USAXS and EIS151measurements presented here were performed at roughly the152same time immediately following each pulse.
3. RESULTS AND ANALYSIS1533.1. Cyclic Voltammetry. With cyclic voltammetry, the154electrochemical characteristics of the system can be observed.155During the first cathodic scan, palladium reduction can be156observed, followed by reduction of the solvent. The high157cathodic currents, observed after the reduction of Pd2+, can be158attributed to the adsorption and reduction of choline, as159expected. During the anodic scan, the reduced species that are160both adsorbed onto the Pd and dissolved are reoxidized, which161results in two peaks characteristic of adsorbed species.32 The162presence of adsorbed species is in agreement with results163obtained by USAXS and EIS and shown in the following164 f2sections.1653.2. SEM Imaging. The resulting particle morphology from166both temperatures is that of agglomerated nanoparticles, shown167 f3in Figures 3a and 3b. Qualitatively, larger particles (∼20 nm)168are observed at 44.5 °C, compared to that observed at 32.5 °C169(∼10 nm). However, the size distribution cannot be170determined, accurately, from these images. The agglomerate171size, on the other hand, can be quantified. A total of four images172(available in the Supporting Information) were used to obtain173the projected area of each agglomerate. The size distribution of
Figure 1. Illustration of the experimental setup used, highlighting allkey components of the experiment.
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174 the projected areas is related to the cross-sectional areas of the175 fluid phase observed by USAXS and is discussed further in the176 Discussion section. These size distributions are shown in177 Figures 3c and 3d. From this analysis, the distribution of the178 agglomerate sizes is approximately log-normal, with modes for179 the 32.5 and 44.5 °C samples at 200 and 800 nm2, respectively.
180Therefore, we conclude that nanoparticle aggregates are181present, ex situ, with both primary particle size and agglomerate182size larger at 44.5 °C.1833.3. Small-Angle X-ray Scattering. The scattered184intensity contains information about the size, shape, and185structure of any phase present after each electrodeposition186pulse. Briefly, the scattered intensity is a function of the187magnitude of the X-ray momentum transfer vector, q, which is188related to the angle of measurement, θ, by the equation
π θλ
=q 4sin /2
189(1)
190where λ is the X-ray wavelength (0.738 Å). Thus, by measuring191the scattered intensity, as a function of q, one can determine the192physical properties of a phase that is on the nanoscale (1 nm to1931 μm). In order to observe the scattered intensity from a194nanosized phase, it must have a scattering length density195(proportional to the electron density) that is different than its196surrounding matrix. In these experiments, the surrounding197matrix is the deep eutectic solvent, which is composed of198organic compounds. Thus, a palladium phase (i.e., nano-199particles), within the DES, could be resolved with SAXS.200Typically, the scaling of the scattered intensity can be used to201determine the contrast and total scattering volume. However,202since the thickness of the scattering phases (normal to the
Figure 2. Cyclic voltammograms of 10 mM K2PdCl4 in the DES (red)and blank DES (blue) that were performed in the sample cell shown inFigure 1 at 32.5 ± 0.5 °C.
Figure 3. (a, b) SEM images of the same samples evaluated by USAXS, showing the presence of tightly packed particle aggregates. (c, d) Analyses ofthe aggregate area distribution on the glassy carbon surface. These results were obtained from a total of four SEM images (available in the SupportingInformation).
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203 surface) is unknown here, no such calibration is possible. Thus,204 the scattered intensity is reported in arbitrary units.205 3.3.1. Fluid Phase: USAXS. The smeared intensity, obtained206 by USAXS, from each experiment is shown in Figure 3.207 Applying the Guinier approximation to each of the I(q) curves208 indicates a scattering phase with a radius of gyration greater209 than 50 nm, which is too large to be associated with the primary210 particles, shown in Figures 3a and 3b. Qualitatively, the X-ray
f4 211 scattering in Figure 4 cannot be associated with the
212 agglomerates (Figures 3a and 3b) because the intensity decays213 well beyond the limit for a mass fractal (I = Cq−3);33 this214 argument is discussed in more detail in the Discussion section.215 For now, we consider that the low-q scattering in Figure 4 is216 not directly associated with the deposited particles or their217 structure.218 Curiously, the scattered intensity decays well beyond that of219 a smooth, well-defined surface (I = Cq−4), also known as a220 Porod decay.34 An intensity decay greater than a Porod decay221 can be attributed to a surface having an electron density222 gradient, as opposed to a piecewise function.35,36 Considering223 that the only change in the system, after the electrodeposition224 pulse, is the presence of supported nanoparticles on the surface,225 this phase is considered as an oriented disk (parallel to the226 surface). This model is supported by the EIS data and is227 discussed further in the Discussion. For now, scattering from an228 oriented 2-D phase is justified by recognizing that any influence229 the deposited particles have on the bulk is necessarily 2-D, since230 they are confined to the surface. The low-q scattering is231 therefore modeled as the scattered intensity from a fluid phase,232 which contains an electron density gradient by the equations
∑ σ= ΔI q F q R AR H q R D R R R( ) ( , , ) ( , , ) ( )l li g i v lispheroid2 2
233 (2)
σ = σ−H q R( , , ) eg liR q( )li g
2
234 (3)
235where Fspheroid is the scattering amplitude from an oriented236spheroid with an aspect ratio, AR, of 0.001 and radius Ri, Dv(Ri)237is the volume distribution of particles of size, Ri, and H is the238Fourier transform of the so-called “smoothing function” that239would be convoluted with the ideal piecewise function to240obtain the electron density gradient within the phase.35,36 For241simplicity, the smoothing function here is taken to be a242Gaussian,36 with a standard deviation of σg. The standard243deviation, σc, and mean disk radius, Rl, of a log-normal244distribution, Dv(Rl), was also fit to each USAXS data. From eq2452, the fluid phase(s) are not considered to have a preferred246distance between them. While this may be true for most of the247USAXS data, there is clearly some interference after nucleation248at 44.5 °C, as evidenced by a peak intensity at very low q;249however, this is not analyzed here. The resulting model fits are250shown in Figure 4. The parameters obtained from these fits are251 t1shown in Table 1.
252The model fits of eqs 2 and 3 to the USAXS data results in253model intensities that have the same general shape as the raw254data. In all cases, an electron density gradient is present255throughout most of the phase. Although assuming a log-normal256distribution results in reasonable fits, we note that much better257fits can be obtained using the method of maximum entropy;258this is discussed in more detail in the following section.259Nonetheless, eqs 2 and 3 are considered to accurately represent260the scattering phase at low q since the model intensity has the261same general shape as the data.2623.3.2. Stable Particles and the Fluid Phase: USAXS/Pinhole263SAXS. As mentioned previously, the complete USAXS/pinhole264SAXS data were only obtained after both pulses at 44.5 °C,265allowing for entire I(q) curve to be modeled. From the SEM266image in Figure 3b, particles of approximately 20 nm can be267observed after electrodeposition at 44.5 °C. Considering that268adsorption is observed in Figure 2 and the absence of269agglomerate scattering in Figure 4, the high q region of the270combined USAXS/pinhole SAXS data is modeled as stable271spheres. To account for adsorbed species, the form factor for272spheres with attached Gaussian chains is used.37 From a273contrast standpoint, there is little difference in the electron274density between choline and the DES. However, because the275particles occupy a significant portion of the surface (∼20% by276SEM), the volume average electron density would be much277higher than that of the bulk DES. This would otherwise278enhance the contrast of the adsorbed species, which is the279difference between its own electron density and the volume280average. Accounting for particle interference, the resulting281equation to be fit to the entire combined USAXS data is
∑= +
Δ
I q I q S q R v F q R V R
D R R
( ) ( ) ( , , ) [ ( , ) ( )
( ) ]
l pHS HS HS2
i
v i 282(4)
283where Il(q) is defined in eq 2, Fp(q,R) and V(R) are the form284factor and volume of spheres with attached Gaussian chains of
Figure 4. USAXS data (markers) with the corresponding model fits(solid lines) of eq 2 to each data set. A Porod decay, proportional toq−4, is shown as a dashed line to show that all of the surface scatteringcannot be considered from a smooth, well-defined surface.
Table 1. Parameters Obtained from the Fit of Eq 2 to theUSAXS Data
R̅l (nm) σc σg
32.5 °C nucleation 30 0.17 0.932.5 °C growth 50 <0.01 1.044.5 °C nucleation 35 0.27 0.744.5 °C growth 40 0.42 0.7
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285 size, R, SHS(q,RHS,vHS) is the hard sphere structure factor of286 impenetrable spheres38 of radius RHS and volume fraction vHS,287 and Dv(R) is the volume distribution of the particles. In order288 to obtain the best fit, the low q region was fit separately, using289 the method of maximum entropy,39 assuming a σg value of 0.7.290 From the model fit of eq 4 to the entire USAXS/pinhole291 SAXS data, we can obtain the mean particle size, R̅p, hard292 sphere interaction (∼ distance between particles), RHS, hard293 sphere volume fraction, vHS, the radius of gyration of the294 adsorbed species, Rg, number of adsorbed molecules attached to295 each particle, Nc, and the contrast between adsorbed species
t2 296 and the DES, Δρc. These parameters are summarized in Tablet2 297 2. As expected, there is little contrast between the adsorbed
298 species and the surrounding matrix, which is predominantly299 composed of the DES. The mean particle diameter (16 nm) is300 also in good agreement with those observed by SEM imaging301 (∼20 nm). Finally, the stable nanoparticle model (eq 4) is302 consistent with the observation of particle interference at303 ∼0.015 Å. Therefore, we consider the model fit shown in304 Figure 4 as a representative model for the system.305 3.4. Electrochemical Impedance Spectroscopy. From
f5 306 the fit of eq 4 (Figure 5), we concluded that stable
307 nanoparticles are present on the glassy carbon surface, in situ,308 after the electrodeposition sequence. However, the source of309 the low-q scattering phase remains ambiguous, to this point,310 and has only been modeled as an oriented, 2-D fluid phase that311 contains a gradient boundary. To provide further insight into312 this phase (Figure 4), electrochemical impedance spectroscopy313 (EIS) was employed. The impedance response before electro-314 deposition is consistent with a linear response observed
315previously for DES, at frequencies greater than 2 Hz, which316can be modeled as a resistance and CPE in series.40 From317 f6Figure 6, the impedance collected after the nucleation pulse
318significantly decreases well below that obtained from the bare319surface. From this we can conclude that the presence of surface320nanoparticles results in a significant change in the surface321electrochemistry.322From the CV (Figure 2) and USAXS/pinhole SAXS (Figure3234), we conclude the presence of electroactive, adsorbed species324onto the Pd nanoparticles. This adsorption can be accounted325for in the impedance spectra, using an equivalent circuit.41 In326the impedance data, reported here, the adsorption impedance327becomes relevant only at very low frequencies (<0.1 Hz). Thus,328only a simplified parallel circuit41 that represents the adsorption329impedance is used. This parallel circuit consists of a capacitor330and Warburg element and approximates the adsorption331impedance at either very low or very high frequencies.41 This332adsorption impedance is considered to be in series with a333parallel circuit, which consists of a constant phase element334(CPE) and a charge transfer resistance (Rct) and the solution335resistance, Rs. For simplicity, the impedance circuit from the336bare surface40 is neglected, as it is much higher than that337obtained after electrodeposition and does not contribute338significantly. The complete equivalent circuit is shown in339Figure 6 and results in the fit equation
ω
ω
= + + −
+ − +
α −
−
Z R R Q
C Z
(1/ ( 1 ) )
( 1 1/ )s t
a w
1
1340(5)
ωω
= −−
Z Rs
stanh(( 1 ) )
( 1 )w d
1/2
1/2341(6)
342where ω is the angular frequency, Q has units of sα Ω−1, and α343is an exponent that approaches one for a narrow distribution of344resistances, Rt.
42 The model impedance (eq 5) fits well to the345impedance data, with the exception of very low frequencies346where the data noise is highest, resulting in high uncertainty347from the adsorption components. From these model fits, we348 t3can obtain the values of Q, Rf, and α with minimal uncertainty
Table 2. Summary of the Results from the Fit of Eq 4 to theCombined USAXS/Pinhole SAXS Data Shown in Figure 5a
R̅p (nm) RHS (nm) vHS Rg (nm) Δρc (cm−2)
44.5 °C 8 15 0.4 1.5 0.32aThese results are specific to the nanoparticles themselves.
Figure 5. A log−log plot of the combined USAXS data obtained afterthe double pulse electrodeposition sequence together with the modelfit intensity (solid line). The resulting size distributions, obtained fromthe model fit, are shown in the insets.
Figure 6. Bode plot of the impedance modulus (top) and phase(bottom) of the impedance data (crosses) and model fits of eq 5(lines). The postnucleation (light blue and light red) are shown withthe postgrowth (dark blue and dark red) for 32.5 and 44.5 °C,respectively.
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t3 349 and are shown in Table 3. While the other parameters in eq 5350 must be included to obtain a model fit that is consistent with
351 the presence of adsorbed species (Figure 2) and a solution352 resistance, their uncertainty is very high and are not discussed353 further; these values are available in the Supporting354 Information. Therefore, we conclude that after electro-355 deposition of Pd nanoparticles the expected impedance356 response from adsorbed species can be accounted for. In357 addition to this impedance, a polydisperse RC circuit is358 observed in the impedance spectra.
4. DISCUSSION359 4.1. Particle Stability. From the sample electrodeposited at360 44.5 °C, we were able to resolve the size of the deposited361 particles. Based on the model fit of the high q-region shown in362 Table 2, the 8 nm particles interact with a hard sphere radius of363 15 nm. This means that the deposited particles do not touch364 and cannot, therefore, be contained within an aggregate.365 Further evidence that supports the absence of aggregates, in366 situ, is provided by the difference between aggregate scattering367 and what is actually obtained.368 From Figure 3b, one observes an approximate aggregate size369 of 100 nm, which would correspond to a Guinier knee at370 approximately q = 0.006 Å−1. This is clearly not observed in the371 USAXS data, as the intensity decays according to q−4. However,372 because there is a broad aggregate size distribution (Figure 3d),373 a distribution in aggregate sizes should be considered. The374 aggregate size distribution can be estimated by performing a375 model fit, using the appropriate form factor,33 to the very low q376 region (0.0005 Å−1 < q < 0.005 Å−1). In this form factor, a377 maximum fractal dimension of 3 was used to show that it is378 physically impossible to fit a fractal aggregate model to the data;379 fractal dimensions of less than 3 result in even further380 deviations from the experimental scattered intensity. The
f7 381 resulting scattered intensity from the fractal aggregates (Figuref7 382 7) is very different from what is actually obtained. Therefore,
383 we conclude that the Pd nanoparticles are stabilized in situ and384 that the low-q scattering cannot be attributed to the aggregates385 observed by SEM (Figure 7). The same argument can also be386 made for the sample electrodeposited at 32.5 °C, whereby the387 low-q scattering cannot be modeled as fractal aggregates.388 Therefore, we conclude that the deposited particles do not exist389 in the aggregated state, in situ. However, these particles become390 unstable once the DES is removed resulting into the aggregates391 observed by SEM (Figure 3a,b).392 4.2. Ionic Layer. 4.2.1. Ensemble−Ionic Layer System.393 Because no aggregated particles exist in situ, the source of the394 low-q scattering in Figure 4 remains ambiguous. Rather than395 modeling the low-q scattering as aggregated particles, the
396scattering was modeled as a phase that contains an electron397density gradient at its boundary.35 In the context of the current398system, this phase is considered a fluid because the only other399solid phases (Pd and glassy carbon) are already accounted for400in the scattering. Thus, the low-q scattering must be from near401interfacial phases, within the solvent. Therefore, the presence of402this phase must affect the electrochemical response (deter-403mined by EIS).404From the fit of eq 5 to the impedance spectra (Figure 6), a405polydisperse RC system and solvent adsorption are both406observed. Because the adsorption impedance is accounted for407in the high-q region in Figure 7, the polydisperse RC circuit is408therefore associated with the low-q scattering. Physically, this409polydisperse RC circuit is a system of in-plane capacitors and410resistors that have a distribution of resistances;42 in this circuit,411the distribution of resistances is accounted for by the α value,412which approaches 1 for a monodisperse system.42 Therefore,413the fluid phase observed by USAXS must consist of opposing414ions, resulting in the charge separation in the polydisperse RC415circuit.416These ion layers are finite in size (Table 1) and do not span417across the entire surface. Their size (Table 1) is very similar to418the size of the particle aggregates, which indicates that the ion419layers are related to the size of the particle groups, which exist420as stable nanoparticles in situ. Considering the ubiquitous421nature of quaternary ammonium adsorption onto Pd2 (also422observed in Figure 2), the particle surfaces will have a net423positive charge that must be neutralized. Consequently, the424opposing ion layers must have a negative charge.425We propose that the stabilized particles exist within426otherwise 2-D ensembles that are rich in adsorbed species.427The resulting net positive charge of the ensembles then induces428an anionic layer above it. In the proposed system, the charge429separation between the ensemble and ionic layers would be430considered as a capacitor. Presumably, some charge transfer431through the ensemble−ion layer is also expected, resulting in a432parallel system of capacitors and resistors in parallel. This in-433plane distribution of capacitors and resistors is consistent with
Table 3. Parameters Associated with the Ensemble DoubleLayers Obtained from the Fit of Eq 5 to the EIS Data
sampleQ × 10−6
(sα Ω−1)
Q relerror(%) α
α relerror(%)
Rt(kΩ)
Rt relerror(%)
32.5 °Cnucleation
3.1 16 0.95 6 95 16
32.5 °Cgrowth
3.7 3 0.94 1 120 1
44.5 °Cnucleation
3.5 3 0.94 1 85 2
44.5 °Cgrowth
4.6 7 0.95 2 130 3
Figure 7. A log−log plot of the scattered intensity from the sampleelectrodeposited at 44.5 °C with the modeled scattered intensitiesfrom the deposited particles (gray line) and ion layers (red line). Thescattered intensity from otherwise aggregated particles (observed bySEM) is also shown (black dashed line).
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434 eq 5.42 Therefore, the proposed system is consistent with both435 the EIS data shown in Figure 6 and the USAXS data, where the436 low-q scattering is fit to a system of oriented disks parallel to437 the surface.438 4.2.2. Composition. Though the composition of the ionic439 layers was not measured directly, their electron density should440 be sufficiently different from the DES in order to provide a441 sufficient scattering signal. Also, the ions within this layer442 should possess a negative charge. Therefore, we expect the443 composition of the ionic layer to be metal anion complexes444 (e.g., PdClx
2−x), which have also been reported for Ag,43 Zn,27
445 and Cu,44 dissolved in DESs. We also note that it is possible446 more than a single layer exists above the particle ensembles, as447 has been observed in other room temperature ionic liquids.25
448 Regardless of the exact composition of the ionic layer, the449 concentration gradient of ions within it will depend on the450 electric field. In the proposed system, the electric field would451 depend on the charge distribution within the particle ensemble452 as well as in the opposing ion layer. In the particle ensemble,453 the net charge must diffuse to zero near the ensemble boundary454 where no Pd particles exist. The presence of this charge455 gradient would certainly result in an electric field above the456 ensemble by Poisson’s equation. Therefore, a concentration457 gradient of ions, at the ionic layer boundary, would necessarily458 exist and is observed by USAXS.459 Values of σg near one indicate that a concentration gradient is460 present throughout most of the ionic layer. This gradient is461 necessarily monotonic; otherwise, the distance between462 fluctuations would have been resolved. Because the charge463 distribution in the ion layer would be directly related to the464 charge distribution within the ensemble, we conclude that the465 latter cannot contain large fluctuations either. This conclusion466 means that the surface charge on the particles, within the467 ensemble, is not neutralized locally. Instead, charge neutraliza-468 tion is achieved by the opposing ion layer that is observed by469 both USAXS and EIS.470 4.3. In Situ vs Ex Situ. When the DES is removed, the471 particles readily aggregate (Figure 3). The results obtained by472 the complete fit of eq 4 can be compared to the results473 obtained by SEM for comparison. First, the projected area of474 these aggregates, Aagg, should be related to the cross-section475 area of the ionic layer, Al, by the equation
≈ = =A A v piR 2000 nml lagg HS2 2
476 (7)
477 remembering that vHS would be the volume fraction of particles478 within the ensemble, in situ, which is only available at 44.5 °C.479 The value obtained from eq 7 is significantly larger than the480 log-normal mean (800 nm2) obtained by SEM (Figure 3d).481 This deviation is due to some 3-D aggregation, as evidenced by482 the presence of some particles on top of others in Figure 3b.483 The average number of particles, Np, within an ensemble, can484 also be calculated by the equation
= ≅N V v V/ 8p p eHS485 (8)
486 where Vp is the volume of the mean spherical particle and Ve is487 the volume of a disk with the same radius of the ionic layer and488 thickness of the mean particle diameter. Equation 8 is also in489 general agreement with the aggregates observed by SEM;490 although the exact number of particles in each is difficult to491 extract, the particle number in each is on the order of 10.492 Therefore, the size of the ionic layers obtained by USAXS does493 correlate well with the aggregates, observed by SEM.
4944.3. Impedance vs USAXS. From the fit of eq 5, the values495of Q and Rt from the ensemble double layers are obtained.496Physically, the value of Rt represents the charge transfer497resistance through the ion-layer ensemble; however, there is498insufficient data to report a quantitative relationship between499this value and the size of the ion layers. The Q value, on the500other hand, can qualitatively related to the ion-layer size501because it is directly proportional to the capacitance. Both Q502and Rl increase with growth and temperature. To a first503approximation, the overall capacitance from these layers is504proportional to Rl
2, which is in qualitative agreement with the505values shown in Tables 1 and 3. Therefore, we consider the506general trend in the Q values obtained by EIS to be consistent507with the values of Rl obtained by USAXS based on the508relationship between the overall capacitance and the total509surface area of the ion layers.5104.4. Particle Stabilization. 4.4.1. DES vs Aqueous. The511ensemble double layers that are observed indicate that charge512neutralization is, as expected, significantly different in the DES,513compared to an aqueous system. Specifically, the charge on the514deposited particles is not neutralized locally, as commonly the515case in aqueous systems.45 Instead, the charge, induced from516adsorbed species, is neutralized in a well-defined layer above the517ensemble. Here, the term “well-defined” means that there is no518electron density fluctuation within the ionic layer; otherwise,519the distance between fluctuations would be observed by520USAXS. Therefore, we conclude that particle charge neutraliza-521tion in this DES system is different from that observed in a522typical aqueous system.5234.4.2. Stabilizing Species. Finally, the size of the molecules524adsorbed to the Pd particles, found by the fit of eq 4, is much525larger than that which is possible for choline. Considering the526bond lengths on choline, an Rg of 1.5 nm is about an order of527magnitude more than what is possible based on the bond528lengths in choline. At the moment, it is unclear whether this is529due to structuring of the solvent at the particle surface40 or the530formation of larger quaternary ammonium salts during choline531reduction, which is believed to produce carbon radicals.46
532However, the speciation at the particle surface and within the533ensemble cannot be determined from these experiments. We534can conclude, however, that the adsorbed species at the particle535surface are not choline alone.
5. CONCLUSIONS536Using an in situ approach, we were able to show that the DES537stabilizes electrodeposited Pd nanoparticles. These particles are538assembled into a 2-D ensemble, rich in adsorbed species. As539evidenced from combined USAXS/pinhole SAXS, the particles540are stabilized by adsorbed species much larger than choline.541Using USAXS/pinSAXS, SEM, and EIS, we conclude that an542ionic layer(s) exists above ensembles of particles, as a result of543the charge induced by the adsorbed species within the544ensemble. The presence of these ionic layers was observed at545two different temperatures after each electrodeposition pulse.546These layers necessarily contain an electron density gradient at547their boundary instead of a well-defined piecewise function548commonly observed in solid phases.549This charge neutralization is different from an aqueous550system, where the surface charge is neutralized locally. These551stable ensembles of particles become unstable when the DES is552removed and the sample is washed with ethanol and water.553Thus, the stability of these particles is contingent on the554presence of the DES.
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555 ■ ASSOCIATED CONTENT
556 *S Supporting Information557 Additional SEM images and parameters obtained from the558 adsorption circuit in the EIS. This material is available free of559 charge via the Internet at http://pubs.acs.org.
563 Notes564 The authors declare no competing financial interest.
565 ■ ACKNOWLEDGMENTS
566 We greatly acknowledge NWO/FWO Vlaanderen for the567 provision of a travel grant and help and support of Dr. Jan568 Ilavsky and support staff at ID-15, Advanced Photon Source,569 Chicago, IL. ChemMatCARS Sector 15 is principally supported570 by the National Science Foundation/Department of Energy571 under Grant NSF/CHE-0822838. Use of the Advanced Photon572 Source was supported by the U.S. Department of Energy,573 Office of Science, Office of Basic Energy Sciences, under574 Contract DE-AC02-06CH11357.
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