Ionic Conductivity Measurements—A Powerful Tool for Monitoring

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Ionic Conductivity Measurements – A Powerful Tool for Monitoring Polyol Reduction Reactions Hany A El-sayed, Veronika M. Burger, Melanie Miller, Klaus Wagenbauer, Manuel F. Wagenhofer, and Hubert Gasteiger Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03444 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Ionic Conductivity Measurements – A Powerful Tool for Monitoring Polyol Reduction Reactions Hany A. El-Sayeda,*, Veronika M. Burgera, Melanie Millera, Klaus Wagenbauerb, Manuel Wagenhoferc, Hubert A. Gasteigera a

Chair of Technical Electrochemistry, Technical University of Munich, Lichtenbergstraße 4, D-85748, Garching, Germany b

Walter Schottky Institut, Technical University of Munich, Am Coulombwall 4, D-85748, Garching, Germany

c

Chair of Technical Chemistry II, Technical University of Munich, Lichtenbergstraße 4, D-85748, Garching, Germany

*

Corresponding author: [email protected]

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Abstract: The reduction of metal precursors during the polyol synthesis of metal nanoparticles was monitored by ex-situ ionic conductivity measurements. Using commonly used platinum precursors (K2PtCl6, H2PtCl6, and K2PtCl4) as well as iridium and ruthenium precursors (IrCl3, RuCl3), we demonstrate that their reduction in ethylene glycol at elevated temperature is accompanied by a predictable change in ionic conductivity, enabling a precise quantification of the onset temperature for their reduction. This method also allows detecting the onset temperature for the further reaction of ethylene glycol with HCl produced by the reduction of chloride containing metal precursors (at ≈120 °C). Based on these findings, we show that the conversion of the metal precursor to reduced metal atoms/clusters can be precisely quantified, if the reaction occurs below 120 °C, which also enables a distinction between the stages of metal particle nucleation and growth. The latter is demonstrated by the reduction of H2PtCl6 in ethylene glycol, comparing ionic conductivity measurements with transmission electron microscopy (TEM) analysis. In summary, ionic conductivity measurements are a simple and straightforward tool to quantify the reduction kinetics of commonly used metal precursors in the polyol synthesis.

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Introduction: Metallic nanoparticles find widespread application in many areas of chemistry, physics, and materials science. Their physical and chemical properties, including electronic, optical, structural, and catalytic properties are affected by their size, shape, and size distribution.1 In the field of electrocatalysis for example, it is pivotal to understand the fundamental mechanisms that drive the formation of nanoparticles in order to tune their catalytic properties. This requires monitoring the progression of the nanoparticle synthesis reaction in a way by which the precursor reduction steps and the subsequent nucleation, growth, and sintering processes can be distinguished. The study of nucleation and growth mechanisms and kinetics has a long tradition in colloidal chemistry,2–6 and originates from the seminal study by LaMer and Dinegar7 who applied classical nucleation theory to the synthesis of monodispersed sulphur colloids in order to qualitatively describe the kinetics of nucleation and diffusional growth. The recent advances in preparing nanoparticles with well-defined properties by wet-chemical methods have resulted in an increased interest in gaining a more fundamental understanding of the nucleation and growth mechanisms.2 A commonly used wet-chemical reaction to prepare metallic nanoparticles is the so-called polyol method, in which typically diols (e.g., ethylene glycol) serve both as solvent and as reducing agent. To elucidate nanoparticle nucleation and growth mechanisms, different techniques which allow the monitoring of particle size changes have been applied, most commonly in-situ UVvisible spectroscopy combined with ex-situ transmission electron microscopy (TEM).8–11 Despite its ubiquitous use, it is limited to solvents which do not absorb in the UV-VIS region of interest12. In principle, more detailed insights can be obtained from in-situ TEM studies,13,14 but it is often difficult to avoid interference of the electron beam with nanoparticle formation steps,15 such as reduction and nucleation. A different set of experiments developed recently and enabling in-situ studies on nanoparticle nucleation and growth with a time resolution of milliseconds to seconds are synchrotron based Xray techniques.16–18 Thus, X-ray absorption spectroscopy (XAS) such as X-ray near edge structure (XANES)1,19 and extended X-ray absorption fine structure (EXAFS)20–23 have been used to monitor nanoparticle formation with precise atomic-level and local chemical state characterization. The main drawback of this technique is the interference of concomitantly produced X-ray radiolysis products from the solvents with the chemical reduction of metallic precursors24–26, so that the nanoparticle formation process is frequently affected by the analysis method. Thus, special care has to be taken in choosing the experimental conditions. In addition to XAS, synchrotron based small-angle X-ray scattering (SAXS) was used to monitor the nucleation of Pt nanoparticles from Pt-Sn complexes27 or to monitor Pt nanoparticles formation with atomicscale resolution across the entire length scale of the nanoparticle by means of pair distribution function (PDF) analysis,28 which had previously been shown to allow for the direct observation of nanoparticle formation mechanisms/kinetics.29 In situ small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS)/UV–vis absorption spectroscopy was successfully used to monitor the formation of Au nanoparticles and to extract kinetic data using sophisticated mathematical models.30 While many detailed insights have been gained by these methods, they are of limited availability and pose the risk of X-ray induced interference with the reaction, so 3 ACS Paragon Plus Environment

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that there is still a need for a simple and readily accessible technique to probe nanoparticles formation. The aim of this work is to demonstrate the possibility of using ionic conductivity measurements to monitor metal nanoparticle formation in a polyol based nanoparticle synthesis reaction, illustrated for a commonly used approach in which the reaction temperature is increased at constant rate from room temperature until the reaction is completed. These reactions are characterized by consuming and releasing ionic species during the course of metal precursor reduction, accompanied by a change in the ionic conductivity of the synthesis mixture as the reaction proceeds. Based on this, we will show that the onset and progression of precursor reduction can be quantified by an intriguingly simple method: i) collecting samples over the course of the reaction; ii) quenching them in an ice-bath to stop the reaction; and, iii) measuring their ionic conductivity from which the conversion of the precursor vs. reaction temperature can be obtained. We will show that this method not only allows to quantitatively monitor the progression of the precursor reduction reaction, but also allows to precisely detect the temperature at which the reduction reaction initiates (onset temperature), which was found to be precursor dependent. Combined with transmission electron microscopy, the profiles of ionic conductivity (IC) vs. temperature (IC-T profiles) can also be used to distinguish between nucleation and growth of metallic nanoparticles. Experimental Methods: Pt nanoparticles were synthesized using ethylene glycol (anhydrous, 99.8%, Sigma-Aldrich), EG, as both the solvent and reducing agent, without the use of any additional capping agents or sodium hydroxide. Three Pt precursors were used in this study; potassium tetrachloroplatinate (K2PtCl4, 99.99%), potassium hexachloroplatinate (K2PtCl6, 99.99%), and chloroplatinic acid hydrate (H2PtCl6.6H2O, 99.9%), all purchased from Sigma Aldrich. In addition to ethylene glycol (anhydrous, 99.8%), ruthenium chloride (RuCl3) and iridium chloride hydrate (IrCl3.xH2O, 99.9%) were also purchased from Sigma Aldrich. In a typical metal nanoparticle synthesis, the desired precursor amount is dissolved first in 20 ml of ethylene glycol at room temperature, and then transferred to a round-bottom flask to which another 60/80 ml of EG are added to bring the total volume to 80/100 ml, resulting in final precursor concentrations of 0.3, 0.6, and 1.2 mM. The freshly prepared solution is purged for 90 min with Ar under moderate stirring before heating it at a rate of 1 °C/min using a temperature controller (J-KEM Scientific, Model 310). For ionic conductivity measurements, 4 ml samples were collected at pre-defined temperatures using a 5 ml syringe, and subsequently placed in an 8 ml glass vial and rapidly quenched in an ice-bath to stop the reaction. Once all the samples were collected, they were removed from the ice-bath and warmed up to room temperature to measure their ionic conductivity using a conductivity meter (handylab LF 11, Schott Instruments) and a conductivity probe model (LF 1100 T+, SI Analytics). Samples for TEM analysis were prepared by depositing a few drops of the colloidal dispersion on Formvar-supported carbon-coated Cu400 TEM grids (Science Services, Munich, Germany). Imaging was performed using a Philips CM100 EM operated at 100 kV with a resolution of 0.5 nm. 4 ACS Paragon Plus Environment

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For combined gas chromatographic-mass spectrometric analysis, reaction samples were diluted with acetonitrile (≥99.5% AnalaR, VWR) in a 1:1 weight ratio and analyzed off-line in an Agilent 7890 gas chromatograph equipped with flame ionization detector (FID) and Agilent 5977A mass selective detector (MSD). The column was a non-polar Agilent 19091J-413 fused silica capillary column of dimensions 30 m × 320 µm × 0.25 µm with a stationary phase consisting of (5 % phenyl)-methylpolysiloxane The exit of the column was connected to a flow splitter, allowing for the simultaneous analysis via FID and MSD. The MSD was operated in electron ionization mode and set up to record the total ion current (TIC) in the range of m/z = 20– 150 with a frequency of 4 spectra/s. Extracted-ion chromatograms of the fragments at m/z = 49 and 80 were extracted from the TIC data after analysis. A solution of 1 wt% 2-chloroethanol (puriss. p.a., ≥99.0%, GC, Sigma-Aldrich) in ethylene glycol served as a reference sample. Reference mass spectra of pure substances were recorded in electron ionization mode and provided by the NIST Mass Spectrometry Data Center (2-chloroethanol: NIST#341749, ethylene glycol: NIST# 341866) .31

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Results and Discussion: For the formation of metal nanoparticles via the polyol method, several metal precursors can be used, including metal acetates, carbonyls, chlorides, nitrates, sulfates or oxides.32,33 The chemical reduction of these metal precursors results in the consumption/release of ions in the polyol, thus changing the overall ionic conductivity of the synthesis mixture. The nature and concentration of ions during the course of the reaction changes due to the reduction of the metal precursor as well as due to the oxidation of ethylene glycol (C2H6O2), which serves both as solvent and reducing agent. While the oxidation of ethylene glycol (and other polyols) can proceed through several different steps, the most facile oxidation reaction is its oxidation to the mono-aldehyde (glycolaldehyde, C2H4O2):34,35 HOCH2-CH2OH → HOCH2-CHO + 2H+ + 2e-

[1]

Assuming this to be the predominant reaction of ethylene glycol (or any other polyol) in a waterfree environment, it follows that the donation of each electron for the reduction of the metal precursor releases one proton into the solution. Thus, when predicting the effect of the precursor reduction reaction on the ionic conductivity, one needs to consider the consumption of the metal ion precursor and the formation of protons. This is illustrated in Table 1 for the various precursors considered in this study, writing the complete redox reaction based on Reaction 1, thus providing the reactions from which conductivity changes over the course of metal precursor reduction can be predicted. While details will be discussed after presentation of the experimental results, the redox reactions shown in Table 1 already indicate a significant increase in conductivity upon metal precursor reduction, owing to the very high ionic conductivity of the protons resulting from the reaction. Table 1 Complete redox reactions for the reduction of various metal precursors to the corresponding metal (nanoparticles), assuming the simultaneous oxidation of ethylene glycol (C2H6O2) to glycolaldehyde (C2H4O2) based on Reaction 1. metal precursor 2+

proposed redox reaction

K2PtCl4 (Pt )

2K+ + [PtCl4]2- + C2H6O2 → Pt↓ + 2K+ + 4Cl- + 2H+ + C2H4O2

K2PtCl6 (Pt4+)

2K+ + [PtCl6]2- + 2C2H6O2 → Pt↓ + 2K+ + 6Cl- + 4H+ + 2C2H4O2

H2PtCl6 (Pt4+)

2H+ + [PtCl6]2- + 2C2H6O2 → Pt↓

+ 6Cl- + 6H+ + 2C2H4O2

IrCl3 (Ir3+)

Ir3+ + 3Cl- + 1.5C2H6O2

+ 3Cl- + 3H+ + 1.5C2H4O2

IrCl3 (Ru3+)

Ru3+ + 3Cl- + 1.5C2H6O2 → Ru↓

→ Ir↓

+ 3Cl- + 3H+ + 1.5C2H4O2

Figure 1a shows the setup used in a typical polyol synthesis of metal nanoparticles. As a proofof-concept that ionic conductivity measurements can be used to monitor the reduction reaction, Pt nanoparticles were synthesized by heating a solution of 0.3 mM K2PtCl4 in ethylene glycol (EG) at a heating rate of 1 °C/min under Ar atmosphere. Over the course of the heating ramp, samples were collected at various temperatures and rapidly quenched in an ice-bath to stop the reduction reaction. Figure 1c shows the samples collected and the change in color reflects the extent of platinum nanoparticle formation. The ionic conductivity of all samples was measured at room 6 ACS Paragon Plus Environment

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temperature and is presented in Figure 1b as a function of collection temperature, indicating a strong increase in ionic conductivity over the course of the reaction. As shown in Table 1 (first row), we hypothesize that the formation of Pt nanoparticles results in the release of four chloride ions and two protons for every consumed tetrachloroplatinate ion ([PtCl4]2-,) which, at least qualitatively, would explain the observed increase in ionic conductivity. (a)

(b)

(c)

Fig. 1 (a) Schematic of a typical polyol synthesis setup; (b) Ionic conductivity-temperature profile of 0.3 mM K2PtCl4 in pure EG (blue line) and of pure EG (black line) at a linear temperature ramp of 1 °C/min under Ar atmosphere; and, (c) associated samples used for conductivity measurements.

While we will prove later that the changes in the ionic conductivity indeed quantitatively reflect the conversion of the metal precursor to metallic nanoparticles, let us first examine the ionic conductivity-temperature (IC-T) profile obtained for 0.3 mM K2PtCl4 in EG (s. Figure 1b) under this premise. In this case, the IC-T profile can be divided into three stages. At Stage I (light-blue region in Figure 1b), no significant conductivity change is observed up to 40 °C, which indicates that the solution temperature is still not high enough to initiate the reduction of the metal 7 ACS Paragon Plus Environment

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precursor, also indicated by the absence of any substantial change in the color of the first three samples (s. Fig. 1c, marked by light-blue background). This type of induction period was also observed for the temperature-driven reduction of H2PtCl6 in polyvinylpyrrolidone (PVP) by means of transmission electron microscopy in combination with UV-VIS spectroscopy,11 and for H2PtCl6 in citrate containing solution or methanol by extended X-ray absorption fine structure (EXAFS).36,37 As the temperature is increased above 40 °C, the conductivity rapidly increases with temperature (s. light-yellow region, marked as Stage II), until it reaches a maximum value at around 95 °C. In this stage, the conductivity increases first with a moderate rate and then with a fast rate before reaching its maximum value. As attaining a conductivity maximum (or plateau) suggests that no further reduction is possible and that all the tetrachloroplatinate precursor has been consumed, it seems reasonable that no change in the conductivity is observed in Stage III (light-red region in Figure 1b). Now we will examine whether the observed conductivity increase upon the complete reduction of [PtCl4]2- is consistent with the reaction hypothesized in Table 1 (first row). This reaction would predict that the complete reduction of a 0.3 mM K2PtCl4 solution in ethylene glycol would result in a solution containing 0.6 mM K+, 1.2 mM Cl-, and 0.6 mM H+. To prove whether this assumption is valid, a solution composed of 0.6 mM HCl (using 37%wt. HCl) and 0.6 mM KCl in EG was prepared and its ionic conductivity was measured. A value of 24.3±0.7 µS cm-1 was obtained (represented by a blue star in Fig. 1b), which is within less than 10% of the maximum conductivity after the complete reduction of the 0.3 mM K2PtCl4. This quite reasonable agreement supports our initial hypothesis that the reaction proceeds according to the redox reaction given in Table 1 (first row) and that the conductivity plateau essentially marks the complete conversion of the metal precursor. Therefore, we believe that the solution conductivity at any temperature can be correlated essentially quantitatively to the remaining precursor concentration, thereby enabling kinetics studies on the rate of metal precursor reduction. This result demonstrates that ionic conductivity measurements can be used to probe the progress of a chemical reduction reaction as long as the reduction process is associated with a change in ionic conductivity. In order to show that the IC-T profiles are not only useful at low concentrations, IC-T profiles were obtained with higher K2PtCl4 concentrations (0.6 and 1.2 mM), a typical range of precursor concentration that is used in polyol synthesis. Figure 2a shows IC-T profiles of 0.3, 0.6, and 1.2 mM K2PtCl4 in EG, all of which are showing a very similar behavior. It is also worth to mention that the onset temperature of the tetrachloroplatinate ion reduction, marked in the IC-T profiles (s. green arrow in Figure 2a), is the same at all concentrations (i.e., ≈40 °C), as one would expect. As the reduction starts at the same temperature for all the concentrations, the end of Stage II, in which the reduction process is completed, depends on the precursor concentration. For instance, for 0.3 mM [PtCl4]2-, Stage II ends at 80 °C, while it ends at 90 and 100 °C for 0.6 and 1.2 mM, respectively. As an additional consistency check for the ionic conductivity measurements, one can examine whether the conductivity shows the expected proportionality to the metal precursor concentration. Figure 2b indeed shows that the conductivity prior to metal precursor reduction (black line) and at the conductivity maximum (i.e., upon complete precursor reduction) is directly proportional to the precursor concentration; furthermore, the ratio of the latter to the former is a constant value (blue line), as expected for a complete reduction reaction. Finally, based on the redox reaction in Table 1 (first row), the final KCl and 8 ACS Paragon Plus Environment

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HCl concentrations should be 0.6, 1.2, and 2.4 mM for the 0.3, 0.6, and 1.2 mM K2PtCl4 precursor concentrations, respectively, and the conductivities of solutions of ethylene glycol with these concentrations of KCl and HCl (see blue, black and red stars in Figure 2a) are again in reasonable agreement with the observed conductivity maximum.

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(a)

(b)

Fig. 2 (a) Ionic conductivity-temperature profiles of 0.3, 0.6, and 1.2 mM K2PtCl4 in ethylene glycol (1 °C/min under Ar atmosphere) and associated visual changes of the solution as a function of temperature, with blue, black, and red stars marking the conductivity of 0.6, 1.2, and 2.4 mM HCl and KCl in EG, respectively; (b) initial and maximum solution conductivities, with linear regression fits shown in the figure. The blue line shows the ratio of maximum conductivity/initial conductivity. 10 ACS Paragon Plus Environment

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Theoretically, once the reduction is completed, the conductivity should reach a maximum and should remain constant thereafter. Experimentally, however, for the higher precursor concentrations, this is not the case (s. black and red lines in Figure 2a) and the conductivity was observed to decrease above 120 °C. In order to investigate the reason for this decrease in conductivity, the IC-T profiles were obtained again for 0.3, 0.6, and 1.2 mM K2PtCl4, but up to a higher final temperature of 200 °C. Fig. 3 shows that the measured conductivities for all three precursor concentrations decrease significantly at higher temperatures, essentially approaching the initial conductivity values of the precursor solution. This decrease in conductivity at high temperature can only be explained if a substantial fraction of the ions in solution are being consumed by a follow-up reaction at temperatures above 120 °C. A possible reaction would be the reaction of HCl (formed during the reduction of K2PtCl4; s. first row of Table 1) with ethylene glycol at high temperatures, resulting in the formation of 2-chloroethanol38 according to: C2H6O2 + HCl → HOCH2-CH2Cl + H2O

[2]

The formation of other chloro-compounds is not excluded, but the formation of 2-chloroethanol may be more facile. Assuming that Reaction 2 is the main reaction taking place and that it proceeds to completion, the sum of the initial K2PtCl4 reduction reaction (first row in Table 1) and Reaction 2 would yield: 2K+ + [PtCl4]2- + 3C2H6O2 → Pt↓ + 2K+ + 2Cl- + C2H4O2 + 2HOCH2-CH2Cl + 2H2O

[3]

Considering that the conductivity of two Cl- ions should be very similar to that of the [PtCl4]2- ion, the formation of 2-chloroethanol at high temperature would result in a similar final conductivity as that of the initial solution of K2PtCl4 in ethylene glycol, consistent with what is observed in Figure 3.

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Fig. 3 Ionic conductivity-temperature profiles of 0.3, 0.6 and 1.2 mM K2PtCl4 in ethylene glycol up to 200 °C (1 °C/min under Ar atmosphere) and associated visual changes of the solution.

The postulated chloro-compound, thought to be responsible for ionic conductivity decay at high temperature, was identified as 2-chloroethanol via combined gas chromatographic-mass spectrometric analysis (GC-MS). For this purpose, a sample was collected at high temperature (180 °C) after the conductivity had decayed significantly and then rapidly quenched in an ice bath, followed by GC-MS analysis. Figure 4a shows the total ion current chromatogram of the reaction sample in comparison with pure ethylene glycol and ethylene glycol with added 1 wt% of 2-chloroethanol. (Note that all samples were diluted with acetonitrile prior to analysis to optimize the elution behavior.) The TIC chromatogram of the reaction sample shows signals of ethylene glycol and acetonitrile, as well as two extra signals of ethylene glycol oligomers, but no separate signal of 2-chloroethanol. However, also the chromatogram of the reference sample, containing 1 wt% of 2-chloroethanol, does not exhibit a separate signal because it is masked by the much more intensive ethylene glycol peak. To isolate the 2-chloroethanol signals from those of the sample matrix, we used extracted-ion chromatograms (EIC) of the fragments at m/z = 49 and 80 (Fig. 4c and 4d). The first fragment is characteristic for compounds with C-Cl bonds and the second for 2-chloroethanol specifically (molecular peak). Most importantly, both fragments are not present in ethylene glycol (Fig. 4b). By analyzing the EICs, we found one compound in the reaction sample that matches the 2-chloroethanol reference in both retention time and the 12 ACS Paragon Plus Environment

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relative distribution of the fragments: i.e., the area ratio of the EIC signals recorded at m/z = 49 and 80, which is 1.63 in the reference and 1.64 in the reaction sample. This finding strongly supports the proposed formation of 2-chloroethanol and the accompanying conductivity decay at high temperatures.

Fig. 4 Identification of 2-chloroethanol in high-temperature reaction sample. (a) Total ion current chromatograms of reaction sample, ethylene glycol, and ethylene gylcol with added 1 wt% of 2chloroethanol. (b) Reference mass spectra (electron ionization mode) of ethylene glycol and 2chloroethanol. Asterisks indicate the fragments used for identification. (c) Extracted-ion chromatograms (EIC) of the fragment at m/z = 49. (d) EIC of the fragment at m/z = 80.

In order to examine the validity of this hypothesis, IC-T profiles of 0.3, 0.6, and 1.3 mM chloroplatinic acid (H2PtCl6) were examined, for which one expects an initial reaction in which the only ions present after the reduction of H2PtCl6 are Cl- and H+ at equimolar concentrations (see third row of Table 1). In this case, if Reaction 2 proceeds quantitatively at high temperature, the overall reaction would be: 2H+ + [PtCl6]2- + 8C2H6O2 → Pt↓ + 2C2H4O2 + 6HOCH2-CH2Cl + 6H2O

[4]

Reaction 4 would predict that the conductivity should drop to essentially zero upon its completion, in excellent agreement with our experimental data shown in Figure 5. This provides significant evidence for the proposed origin of the observed conductivity decrease at high 13 ACS Paragon Plus Environment

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temperatures, namely the reaction of ethylene glycol with HCl produced during the reaction (via Reaction 2 or further follow-up reactions). The IC-T profiles shown in Figure 5 indicate an onset temperature for the reduction of H2PtCl6 of ≈80 °C, which is much higher than that observed for K2PtCl4 at ≈40 °C (s. Fig. 3). Another major difference between the reduction of H2PtCl6 and K2PtCl4 can be observed by the visual inspection of the reaction mixture with temperature: for K2PtCl4 reduction, a gradual change in color was observed during Stage II (i.e., during the rapid rise in conductivity), until a dark black mixture was obtained at ≈115 °C (s. Fig. 3); on the other hand, in the case of H2PtCl6, no color change was observed during Stage II, and a black color was formed rather suddenly at the end of Stage II at ≈120 °C (s. Fig. 5). The latter fact that a conductivity change is measured without a change of color suggests that it may be related to the formation of metal atoms and/or very small metal clusters, which will be investigated and discussed later. 200

1.2 mM H2PtCl6 0.6 mM H2PtCl6 0.3 mM H2PtCl6

Conductivity (µS cm-1)

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150

100

50

0 0

20

40

60

80

100 120

140 160

180 200

220

Temperature (°C)

Fig. 5 Ionic conductivity-temperature profiles of 0.3, 0.6, and 1.2 mM H2PtCl6 in ethylene glycol up to 200 °C (1 °C/min under Ar atmosphere) 14 and associated visual changes of the solution. ACS Paragon Plus Environment

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The delayed onset temperature for the reduction of H2PtCl6 (≈80 °C) compared to that of K2PtCl4 (≈40 °C) may be either due to the different platinum oxidation state (4+ for H2PtCl6 vs. 2+ for K2PtCl4) or the different pH of the reaction mixtures (pH≈3.5 for H2PtCl6/EG and ≈6.0 for K2PtCl4/EG). In order to distinguish between these two possibilities, the IC-T profile of 0.3 mM K2PtCl6 (oxidation state of 4+) was obtained and compared to those of K2PtCl4 and H2PtCl6, which is shown in Figure 6. Figure 6a clearly shows that the onset of the reduction reaction occurs earlier for the Pt2+ precursor (K2PtCl4; blue line), compared to the Pt4+ precursors (K2PtCl6 and H2PtCl6; black and red lines, respectively). The differences in the onset of the metal precursor reduction can be seen more clearly, when expressed in terms of the temperature dependent conversion of the metal precursors. The latter can be determined based on the proposed reactions in Table 1 (rows 1-3), which imply that the conductivity maximum corresponds to the complete conversion of the precursors (consistent with the conductivity of EG with the added amounts of HCl and KCl predicted by reactions in rows 1-3 in Table 1), as long as the follow-up reaction of the HCl product with EG (Reaction 2) does not occur to an appreciable extent (i.e., as long as a clearly pronounced high-temperature conductivity plateau can be observed, as is the case in Figure 6a). In this case, the conversion of the metal precursors, X, can be obtained by dividing the difference between the conductivity at any given time t, σt, and the initial conductivity, σi, by the difference between the final conductivity, σf, and the initial conductivity:  

 =  

[5]



The conversion of the various metal precursors based on Equation 5, using the data shown in Figure 6a in combination with Equation 5 is plotted in Figure 6b. From the latter, it can be clearly seen that the reduction of the K2PtCl6 precursor (black line) starts at ≈60 °C, which is ≈20 °C higher than that of K2PtCl4 (blue line), and ≈20 °C lower than that of H2PtCl6 (red line). This seems to indicate that in addition to the Pt oxidation state, the pH also plays a significant role in the reduction process, since the two Pt4+ precursors (K2PtCl6 and H2PtCl6) clearly display different onset reduction temperatures for their initial reduction. It is worth to mention here that the water content in the ethylene glycol was always measured before every experiment and was found to be between 400-600 ppm, thus the effect of water on the reduction reaction is constant and that the observed differences in the onset reduction temperature is attributed solely to the metal precursors.

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Let us now examine the origin of the onset temperature for the reduction of the metal precursors as a function of pH and Pt oxidation state. Figure 6b shows that the reduction of K2PtCl6 starts a little later than that of K2PtCl4 (≈60 °C vs. ≈40 °C), which is consistent with the differences in the standard reduction potential of the two species of 0.680 V and 0.755 V vs. SHE (s. Equations 6 and 7). The reduction of [PtCl6]2- likely results in the formation of [PtCl4]2-, which in turn gets instantaneously reduced to Pt as the temperature is already higher than the onset temperature for the reduction of [PtCl4]2-, so that only a single reduction step is observed in the IC-T profiles. [  ] + 2  ⇌ + 4     = 0.755  ( !)

[6]

[ # ] + 2  ⇌ [  ] + 2     = 0.680  ( !)

[7]

On the other hand, the lower onset temperature for the reduction of K2PtCl6 (black line in Fig. 6b) vs. H2PtCl6 (red line in Fig. 6b), both having the same Pt oxidation state, must be related to the different pH of these two solutions (pH≈3.5 for H2PtCl6/EG and pH≈6.5 for K2PtCl6/EG). While the redox potential of Reactions 6 and 7 is independent of pH, the redox potential for the oxidation of EG (s. Reaction 1) decreases with increasing pH, which means that the overall driving force for the Pt precursor reduction would be expected to increase with increasing pH. This is consistent with our observation that the onset temperature for the reduction of K2PtCl6 (pH≈6.5) is lower than that of H2PtCl6 (pH≈3.5). The above analysis strongly supports our proposed approach to utilize conductivity measurements during polyol synthesis in order to monitor the metal precursor reduction during polyol synthesis and to determine the onset temperature for reduction. This, in turn, makes it possible to use this technique in studying the kinetics of polyol reduction reactions, an area that so far only could be investigated by sophisticated techniques such as small-angle X-ray scattering (SAXS)39, X-ray absorption spectroscopy (XAS)40, and pair distribution function (PDF) analysis.29 While we demonstrated the viability of our technique for Pt precursors, we will now examine whether it can also be applied to investigate the polyol reduction of other metal precursors. Figure 7 shows the IC-T profiles for RuCl3 and IrCl3 precursors at a concentration of 0.3 mM, indicating that the onset temperature for their reduction can also be clearly discerned by conductivity measurements. However, in contrast to the Pt precursors, no conductivity plateau which would mark the completion of precursor reduction can be observed. As a matter of fact, the conductivity at full conversion is never reached, indicated by the fact that the expected final conductivity at full conversion estimated from the conductivity of EG with 0.9 mM HCl (predicted for a 0.3 mM IrCl3 or RuCl3 precursor concentration from the reactions in rows 4 and 5 of Table 1) is never reached (s. red star in Figure 7). This is attributed to the much slower reduction kinetics and higher onset temperature for IrCl3 and RuCl3, so that at the chosen temperature ramp of 1 °C/min the reduction of the precursors is not completed prior to the onset of the reaction of the formed HCl with EG (s. Reaction 2). This leads to an overlap between the increase of the ionic conductivity due to precursor reduction, and the decrease of the ionic conductivity due to EG reaction with produced HCl. In summary, while these results demonstrate that conductivity measurements can generally be used to monitor the reduction of metal precursors during polyol synthesis, a quantitative relationship between conductivity and metal 17 ACS Paragon Plus Environment

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precursor conversion (via Equation 5) can only be obtained if the reduction temperatures are below the onset temperature for solvent reduction. In that case, the IC-T profiles can be used study the kinetics of precursor reduction, which is currently under investigation in our group.

Fig. 7 Ionic conductivity-temperature profiles of 0.3 mM RuCl3 and 0.3 mM IrCl3 in ethylene glycol (1 °C/min under Ar atmosphere).

After having validated the feasibility of our approach to quantify the conversion of metal precursors during polyol synthesis by conductivity measurements (as well as its constraints), we will now utilize this method in combination with ex-situ TEM measurements to investigate the nucleation and growth of platinum particles from the commonly used H2PtCl6 precursor. The formation of metal nanoparticles generally takes place through metal precursor reduction forming metal atoms and clusters (referred to as nucleation), followed by particle growth. During the initial nucleation, discrete particle precursors form in the initially homogeneous solution, while during the subsequent growth step additional material deposits on these particles, resulting in an increase in the particle size until all the metal precursor has been consumed.39 At the onset of this work, we demonstrated that the IC-T profiles clearly show three distinct stages of metal nanoparticle formation (s. Figure 1b), but a deeper analysis of Stage II suggests that there are two different reaction phases occurring within that stage. This can be discerned in Fig. 6b (red line), showing a gradual increase of the ionic conductivity up to about 50% conversion of the H2PtCl6 precursor, followed by a comparably faster conversion until the precursor has been consumed. Based on this, one might hypothesize that the initially gradual increase in conversion can be associated to forming metal atoms and/or small clusters, followed by a fast growth process. In order to confirm this hypothesis, several samples drawn during the reduction of 0.6 mM H2PtCl6 were examined by TEM (marked by points A-E in Figure 8), and representative TEM images are shown along with the IC-T profile in Fig. 8. The samples which were collected prior to 50% conversion (i.e., half-way between the initial and the final conductivity), essentially no 18 ACS Paragon Plus Environment

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nanoparticles were observed via TEM microscopy, as is apparent from Figures 8A and 8B. This observation, which has also been made for other metal precursors such as K2PtCl4 (not shown here), suggests that the initial gradual increase in conductivity up to ≈50% conversion marks the initial precursor reduction to metal atoms and/or small clusters, which cannot be imaged at the restricted resolution of our TEM (≈0.5 nm). On the other hand, once the conversion exceeds ≈50%, TEM images indicate the formation of larger Pt nanoparticle (s. Figure 8C) with an average size of 2.5±0.4 nm (average and standard deviation based on the particle size distribution shown in Figure 8C), followed by a gradual growth to 2.65±0.4 nm (s. Figure 8D) and reaching a final value of 2.75±0.4 nm upon the complete conversion of the precursor (see Figure 8E). Progressing from point C at ≈85% conversion to point D at ≈100%, the average particle diameter increases by 0.15 nm, which would be consistent with the direct deposition of the 15% of unreduced precursor at point C on the particles imaged at point C (i.e., a diameter of 2.65 nm would be predicted if 15% of the precursor would deposit homogeneously on the 2.5 nm diameter particles shown in Figure 8C). Further growth between Figure 8D and 8E would then have to be due to particle sintering. In summary, according to these results, it can be concluded that the gradual increase in conductivity in Phase II is due to metal atoms and clusters formation and that the fast increase in conductivity is associated to particle growth that takes place right after the nucleation event. Another conclusion obtained from these figures is that the system goes from metal atoms formation to growth mode (through nucleation) as soon as ≈50% of the precursor has been consumed.

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Conclusion: We have demonstrated an extremely simple technique that can be used to quantify the extent of metal precursor reduction during polyol synthesis of metallic nanoparticles. The technique makes use of the fact that all reduction reactions in polyol result in a net change in conductivity, from which the conversion of the precursor can be determined unambiguously. Correlating the ionic conductivity to the reaction temperature resulted in what here we referred to as ionic conductivity-temperature (IC-T) profiles. The basic findings of this study are presented graphically in Fig. 9, where a typical IC-T profile is demonstrated along with the four different stages during polyol synthesis.

Fig. 9 Ionic conductivity-temperature profile of 0.6 mM H2PtCl6, as an exemplary profile, and corresponding stages precisely indicating the beginning and end of both nucleation and growth processes.

The IC-T profiles of various precursors demonstrated that all reduction reactions go through an induction period at which no reduction is observed (blue area), which indicates that the solution temperature is still not high enough to initiate the reduction of the metal precursor. Once a threshold temperature is obtained (dependent on the precursor), the reduction takes place (yellow area), as indicated by a gradual increase of the conductivity, whereby the transition between these two regimes, i.e., the onset temperature for reduction, can be determined quite precisely for different precursors as demonstrated for K2PtCl4, K2PtCl6, H2PtCl6, IrCl3, and RuCl3. 21 ACS Paragon Plus Environment

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Finally, our conductivity measurements in combination with ex-situ TEM also allowed to distinguish between the nucleation and the subsequent particle growth regime (green area), which to our knowledge has not yet been monitored by other studies, where generally the reduction step has been treated as a single step due to the lack of experimental ability to distinguish between nucleation and growth steps. Finally, conductivity measurements also serve as a simple tool to determine the onset of polyol decomposition (red area). In summary, we believe to have demonstrated that ionic conductivity measurements during polyol synthesis will be a valuable tool to study the kinetics of polyol synthesis reactions.

Acknowledgements:

The authors gratefully acknowledge the Fuel Cells and Hydrogen Joint Undertaking (FCH JU) for financial support of this work within the CATAPULT project (FCH JU, GA 325268). Thanks are extended to Vignesh Sureshwaran for helping with the experimental setup and for carrying out the very first set of exploratory experiments and for Dr. Ehab El Sawy (University of Calgary, Canada) and Michael Klughammer (Technical University of Munich) for fruitful scientific discussions.

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