Solvent Interactions of Pd

Jun 12, 2013 - El Amine Mernissi Cherigui , Kadir Sentosun , Pieter Bouckenooge , Hans Vanrompay , Sara Bals , Herman Terryn , and Jon Ustarroz...
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Stability, Assembly, and Particle/Solvent Interactions of Pd Nanoparticles Electrodeposited from a Deep Eutectic Solvent Joshua A. Hammons,*,† Thibault Muselle,‡ Jon Ustarroz,‡ Maria Tzedaki,‡ Marc Raes,‡ Annick Hubin,‡ and Herman Terryn‡ †

X-ray Science Division, Argonne National Laboratory, 9700 S. Cass, Argonne, Illinois 60439, United States Department of Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium

‡

S Supporting Information *

ABSTRACT: Supported nanoparticle synthesis and assembly have application in a wide range of modern day applications. Key to the manipulation of the particle assembly is an understanding of the interaction between the particles and solvent. Here, we employ a comprehensive in situ approach, together with ex situ SEM imaging, to study supported palladium nanoparticles, electrodeposited from a 2:1 urea:choline Cl− DES. Using cyclic voltammetry, we confirm the expected adsorption of electroactive species onto the deposited particles. On the basis of our experimental results, we conclude that the electrodeposited nanoparticles assemble into 2-D superstructures, rich in adsorbed species. The abundance of these adsorbed species, within the superstructure, induces an anionic layer above them, which can be observed by ultrasmall-angle X-ray scattering (USAXS) as well as electrochemical impedance spectroscopy (EIS). The surface charge of the particles is, therefore, not neutralized locally, as is the case with traditional colloidal systems. We also show that these otherwise stable nanoparticles readily aggregate when the DES is removed. Thus, the stability of these particles is contingent upon the presence of the DES.

1. INTRODUCTION Supported palladium nanoparticles are promising materials for various technologies including fuel cells,1 catalysis,2 and sensors.3 Here we employ nanoparticle electrodeposition as the method of preparation. In general, electrodeposition is a simple and often cost-effective method to prepare supported nanoparticles, whereby dissolved metal cations are electrochemically reduced onto a substrate.4 Recently, nanoparticle electrodeposition from room temperature ionic liquids (RTIL)5 has been considered an attractive alternative to electrodeposition from traditional aqueous systems. One of the main attractions of ionic liquids is their potential to stabilize deposited nanoparticles,6−12 as the solvent and stabilizer are one and the same. This option makes the electrodeposition of supported nanoparticles from ionic liquids an exciting alternative to traditional aqueous electrodeposition. Type III deep eutectic solvents (DES) are considered a type of RTIL and are composed of a quaternary ammonium salt and a hydrogen bond donor, at their eutectic composition.13 Nanoparticle electrodeposition from DESs is relatively new and has proven to be an effective medium to deposit various shapes of Pt nanoparticles.14 In addition, DESs have also been shown to facilitate PbS15 and Au16 nanoparticle self-assembly. Another promising aspect of DESs is their potential to stabilize17 and assemble18 deposited Pd nanoparticles in the presence of quaternary ammonium salts. In addition, these solutions offer some practical advantages over ionic liquids, such as cost, © XXXX American Chemical Society

known toxicology, ease of preparation, and air/moisture stability.13 For these reasons, one of the earliest and most common DESs,19 2:1 urea:choline Cl−, was chosen as the electrodeposition solution here. To understand the unique advantages of nanoparticle selfassembly and stability in RTILs, an understanding of the particle/solvent interactions is keyspecifically, how they differ from a traditional aqueous solution. For example, in some cases the high concentration of adsorbing species can result in a complete protective layer around the particle.20−22 This is particularly important when one recognizes that the surface charge induced by the adsorbed species must be neutralized. In recent years, many authors have shown that ionic liquids tend to form a multilayer in the vicinity of a charged surface,23−26 as opposed to a simple double layer. Thus, the surface charge induced by the adsorbed species may be neutralized differently in DES than in aqueous systems. Specific to DESs, it has been shown that a correlation between the double-layer capacitance and the final deposit morphology has been observed for Zn deposition in different DESs.27 Thus, the mechanism of charge separation in DESs can be considered an important aspect of electrodeposition. In this study, both the Received: April 15, 2013 Revised: June 11, 2013

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SAXS for 3 × 10−2 to 1 Å−1. However, in the experiments presented here, a sufficient overlap was not always obtained. In these cases, only the low-q USAXS data are shown. The custom-designed transmission cell was used to obtain the scattered intensity in situ. The sample cell is essentially the same as used previously,28 but with copper tubing that was used for temperature control, an NTC 100 kΩ insulated thermistor, a Ag/AgCl mini-reference electrode (eDAQ), and a Pt counter electrode; the temperature measured is considered accurate to ±0.5 °C, based on the noise collected during the measurement. The background scattering from the electrolyte, cell, and 0.18 mm thick glassy carbon was collected and subtracted from subsequent scattering data for each experiment. All subsequent data reduction was performed in the Irena package, available for Igor Pro.29 To deposit as many particles as possible, the approximate cathodic limit of the electrochemical window of the DES (at 32.5 °C) was used (ca. −1.8 V). A lower overpotential of −1.4 V was applied during the growth pulse to minimize the size dispersion of the particles.30 Because the viscosity and conductivity (and thus ion transport) of the DES are both strong functions of temperature,31 two different temperatures were used for each electrodeposition sequence: 32.5 and 44.5 °C. Upon completion of each experiment, each sample was washed with ethanol and water for SEM imaging using a JEOL JSM-7000F field emission gun scanning electron microscope, operated at an acceleration voltage of 20 kV. The impedance measurements were performed separately, using the same cell, conditions, and potentiostat (Ivium Compactsat) as was used at the synchrotron. These galvanostatic EIS measurements were made at OCP, using a root-mean-square amplitude of 50 nA. The measurements started 100 s after nucleation, as the OCP was found to change the most during this time. The most significant portions of the impedance spectra were found to occur at frequencies between 100 and 0.01 Hz. Using this frequency range, the measurement time was slightly less than the USAXS/pinhole SAXS acquisition time (∼20 min). Therefore, the USAXS and EIS measurements presented here were performed at roughly the same time immediately following each pulse.

charge separation induced from the Pd nanoparticles and their stability are studied in situ. The motivation of this study was to deposit stable Pd nanoparticles from the DES and to investigate how the DES interacts with these particles. Realizing the potential for interaction between the DES and the deposited nanoparticles, an in situ study is required. Here, we employ cyclic voltammetry (CV), synchrotron ultrasmall-angle X-ray scattering (USAXS), and electrochemical impedance spectroscopy (EIS) for a comprehensive characterization of the system. Furthermore, these in situ results are compared with ex situ SEM imaging.

2. EXPERIMENTAL SECTION The 2:1 (urea:choline) DES was prepared by recrystallizing choline chloride (Afla Aesar) and urea (Afla Aesar) in absolute ethanol, followed by vacuum drying. The DES solution was then prepared by mixing the two components, at a 2:1 ratio, and heating to ∼70 °C. Once clear, the 10 mM K2PdCl4 solution was prepared, at room temperature, and heated to 100 °C for 1 h before use. The glassy carbon foil (Hochtemperatur-Werkstoffe GmbH) was prepared by submersing the foil in a beaker of absolute ethanol and placed in an ultrasonic sink for 5 min. Following, the foil was rinsed and submerged in a beaker of Millipore water and placed in an ultrasonic sink for 5 min. Finally, the glassy carbon foil was placed in the sample cell with the counter electrode and taped, followed by cell assembly. The final solution was syringed into the transmission cell, where the scattered intensity was obtained using the ultrasmall-angle x-ray scattering (USAXS)/pinhole small-angle X-ray scattering (pinSAXS) setup at beamline 15-ID, Advanced Photon Source (APS). Using the setup shown in Figure 1, the sample cell was exposed to a 16.8 keV monochromatic X-ray beam. The

3. RESULTS AND ANALYSIS 3.1. Cyclic Voltammetry. With cyclic voltammetry, the electrochemical characteristics of the system can be observed. During the first cathodic scan, palladium reduction can be observed, followed by reduction of the solvent. The high cathodic currents, observed after the reduction of Pd2+, can be attributed to the adsorption and reduction of choline, as expected. During the anodic scan, the reduced species that are both adsorbed onto the Pd and dissolved are reoxidized, which results in two peaks characteristic of adsorbed species.32 The presence of adsorbed species is in agreement with results obtained by USAXS and EIS and shown in the following sections. 3.2. SEM Imaging. The resulting particle morphology from both temperatures is that of agglomerated nanoparticles, shown in Figures 3a and 3b. Qualitatively, larger particles (∼20 nm) are observed at 44.5 °C, compared to that observed at 32.5 °C (∼10 nm). However, the size distribution cannot be determined, accurately, from these images. The agglomerate size, on the other hand, can be quantified. A total of four images (available in the Supporting Information) were used to obtain the projected area of each agglomerate. The size distribution of

Figure 1. Illustration of the experimental setup used, highlighting all key components of the experiment.

scattered intensity was collected by both a Bronse-Hart camera setup (USAXS) and a pinhole SAXS setup that used a Pilatus 100k detector; this setup maximizes the signal-to-noise ratio at high q (3 × 10−2 to 1 Å−1) where the scattering signal is typically very weak. Using this setup, the intensity was measured at each q-value for 0.5 s at very low q to 2 s at higher q, where the scattering signal is typically very weak. The complete scattered intensity, I(q), was then obtained by combining the USAXS (10−4 to 6 × 10−2 Å−1) and the pinhole B

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Therefore, we conclude that nanoparticle aggregates are present, ex situ, with both primary particle size and agglomerate size larger at 44.5 °C. 3.3. Small-Angle X-ray Scattering. The scattered intensity contains information about the size, shape, and structure of any phase present after each electrodeposition pulse. Briefly, the scattered intensity is a function of the magnitude of the X-ray momentum transfer vector, q, which is related to the angle of measurement, θ, by the equation q = 4π

sin θ /2 λ

(1)

where λ is the X-ray wavelength (0.738 Å). Thus, by measuring the scattered intensity, as a function of q, one can determine the physical properties of a phase that is on the nanoscale (1 nm to 1 μm). In order to observe the scattered intensity from a nanosized phase, it must have a scattering length density (proportional to the electron density) that is different than its surrounding matrix. In these experiments, the surrounding matrix is the deep eutectic solvent, which is composed of organic compounds. Thus, a palladium phase (i.e., nanoparticles), within the DES, could be resolved with SAXS. Typically, the scaling of the scattered intensity can be used to determine the contrast and total scattering volume. However, since 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 in Figure 1 at 32.5 ± 0.5 °C.

the projected areas is related to the cross-sectional areas of the fluid phase observed by USAXS and is discussed further in the Discussion section. These size distributions are shown in Figures 3c and 3d. From this analysis, the distribution of the agglomerate sizes is approximately log-normal, with modes for the 32.5 and 44.5 °C samples at 200 and 800 nm2, respectively.

Figure 3. (a, b) SEM images of the same samples evaluated by USAXS, showing the presence of tightly packed particle aggregates. (c, d) Analyses of the aggregate area distribution on the glassy carbon surface. These results were obtained from a total of four SEM images (available in the Supporting Information). C

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where Fspheroid is the scattering amplitude from an oriented spheroid with an aspect ratio, AR, of 0.001 and radius Ri, Dv(Ri) is the volume distribution of particles of size, Ri, and H is the Fourier transform of the so-called “smoothing function” that would be convoluted with the ideal piecewise function to obtain the electron density gradient within the phase.35,36 For simplicity, the smoothing function here is taken to be a Gaussian,36 with a standard deviation of σg. The standard deviation, σc, and mean disk radius, Rl, of a log-normal distribution, Dv(Rl), was also fit to each USAXS data. From eq 2, the fluid phase(s) are not considered to have a preferred distance between them. While this may be true for most of the USAXS data, there is clearly some interference after nucleation at 44.5 °C, as evidenced by a peak intensity at very low q; however, this is not analyzed here. The resulting model fits are shown in Figure 4. The parameters obtained from these fits are shown in Table 1.

surface) is unknown here, no such calibration is possible. Thus, the scattered intensity is reported in arbitrary units. 3.3.1. Fluid Phase: USAXS. The smeared intensity, obtained by USAXS, from each experiment is shown in Figure 3. Applying the Guinier approximation to each of the I(q) curves indicates a scattering phase with a radius of gyration greater than 50 nm, which is too large to be associated with the primary particles, shown in Figures 3a and 3b. Qualitatively, the X-ray scattering in Figure 4 cannot be associated with the

Table 1. Parameters Obtained from the Fit of Eq 2 to the USAXS Data 32.5 32.5 44.5 44.5

agglomerates (Figures 3a and 3b) because the intensity decays well beyond the limit for a mass fractal (I = Cq−3);33 this argument is discussed in more detail in the Discussion section. For now, we consider that the low-q scattering in Figure 4 is not directly associated with the deposited particles or their structure. Curiously, the scattered intensity decays well beyond that of a smooth, well-defined surface (I = Cq−4), also known as a Porod decay.34 An intensity decay greater than a Porod decay can be attributed to a surface having an electron density gradient, as opposed to a piecewise function.35,36 Considering that the only change in the system, after the electrodeposition pulse, is the presence of supported nanoparticles on the surface, this phase is considered as an oriented disk (parallel to the surface). This model is supported by the EIS data and is discussed further in the Discussion. For now, scattering from an oriented 2-D phase is justified by recognizing that any influence the deposited particles have on the bulk is necessarily 2-D, since they are confined to the surface. The low-q scattering is therefore modeled as the scattered intensity from a fluid phase, which contains an electron density gradient by the equations

R̅ l (nm)

σc

σg

30 50 35 40

0.17