Controlling the size of platinum nanoparticles prepared by cathodic corrosion

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Controlling the size of platinum nanoparticles prepared by cathodic corrosion A.I. Yanson a,∗ , P.V. Antonov a , Y.I. Yanson b , M.T.M. Koper a a b

Leiden Institute of Chemistry, Leiden University, Postbus 9502, 2300 RA Leiden, The Netherlands Leiden Institute of Physics, Leiden University, Postbus 9504, 2300 RA Leiden, The Netherlands

a r t i c l e

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Article history: Received 19 December 2012 Received in revised form 11 March 2013 Accepted 16 March 2013 Available online xxx Keywords: Cathodic corrosion Nanoparticle synthesis Size control CO oxidation

a b s t r a c t We report a method to control the size of platinum nanoparticles synthesized by cathodic corrosion. Using a 5 M solution of NaOH we show that current density during the synthesis determines the size of the resulting nanoparticles, producing an almost linear dependence for particle sizes between 6.5 and 12.5 nm in diameter. The catalytic electro-oxidation of carbon monoxide on these cathodic nanoparticles shows that not only the size, but possibly also their surface termination can be controlled. While smaller nanoparticles appear poly-oriented, larger ones exhibit CO oxidation activity characteristic of extended Pt(1 0 0) terraces. Comparing the results obtained in NaOH and KOH we conclude that the depletion in the diffusion layer close to the electrode is the leading factor influencing the nanoparticle size, and suggest further ways to control this new method of cathodic synthesis of nanoparticles. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Reductive precipitation is a relatively simple method to obtain a metal colloid from a solution of its salt. Since the process commonly follows the nucleation and growth pathway, the easiest way to control such reduction is by decreasing the salt concentration to the point when the nucleation becomes much faster compared to the diffusion-limited growth. Controlling the time of growth one can therefore tune the size of the particles. For applications in heterogeneous catalysis the surface area per gram of catalyst must be maximized, and therefore the size of particles must be reduced to a few nanometers. At the same time, catalytic activity and sometimes also selectivity has been shown to depend non-linearly on the particle size, warranting an investigation of size-dependent properties [1]. Clearly, in order to perform such studies, size-controlled methods of synthesis of nanoparticles are necessary. These have been achieved either by restricting the volume of reagents per particle by using micelles or porous media, or by limiting the maximum size up to which a particle can grow by using capping agents or other surface stabilizers [2–4]. Invariably, reduction of a cationic precursor lies at the heart of each of the commonly used methods. Here we demonstrate size controlled synthesis of nanoparticles using a method based on radically different chemistry. Instead of going via a cationic precursor route, the electrochemical synthesis

∗ Corresponding author. Tel.: +31 71 5274484. E-mail addresses: [email protected], [email protected] (A.I. Yanson).

of nanoparticles at extreme cathodic polarization proceeds via the formation of anionic metal species. As these are quickly oxidized by free water, metal particles are formed in the vicinity of the cathode [5]. Using the analogy with the cationic route, by varying the concentration of this anionic precursor in the nanoparticle formation layer, we should be able to influence the size of the resulting particles. Here we report that by systematically varying the current during cathodic synthesis such conditions can be realized, yielding a quasi-linear dependence between the current density and nanoparticle size in the range of 6.5–12 nm. Not only the size, but also the shape of the particles seems to be influenced, as verified by voltammetric studies of hydrogen adsorption and carbon monoxide oxidation on these nano-catalysts. At higher current densities the catalyst shows enhanced (1 0 0) surface termination, which could be used for steering the catalytic activity and selectivity toward desired products [6–9]. 2. Experimental For the cathodic synthesis of nanoparticles in 5.0 M NaOH (p.a. grade), a “marz” purity Pt wire, 0.13 mm diameter, was submerged by 2.5 mm in a 6 ml solution in MilliQ water (18.2 M cm). A platinum flag was used as a counter-electrode, placed inside an open-ended glass tube in order to separate the (gaseous) products of the anodic reaction. For the synthesis in KOH (p.a. grade), a glassy-carbon counter-electrode in an open-ended glass tube was used. Note that this choice of counter-electrode was arbitrary, both electrodes being unlikely to contaminate the solution with a significant concentration of reducible ionic species. An AC voltage of

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3. Results and discussion In Fig. 1a we present the measured XRD patterns of platinum nanoparticles synthesized at various currents. Although the synthesis was performed potentiostatically, previously we have shown that the system is under ohmic control, and therefore the current remains almost constant until the very last moments. The average value of the ac current during one half-period sampled at 20 s into the experiment was used to label the curves in Fig. 1 [2]. All curves show a sequence of lines characteristic of an fcc crystal structure, with their positions at values of 2 expected for pure Pt. One can observe a monotonous increase of the sharpness of the diffraction lines. As the major contribution to the broadening of diffraction lines comes from the finite number of atomic planes in each crystallite, we can qualitatively see that the average crystallite size increases with increased current density employed during electrochemical synthesis. A similar conclusion can be drawn from a complementary method of nanoparticle characterization, namely,

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Fig. 1. (a) XRD patterns of 200 ␮g of Pt nanoparticles, (b) anodic sweeps of cyclic voltammograms of 5 ␮g of Pt nanoparticles in 0.5 M H2 SO4 , scan rate 50 mV/s. Particles synthesized in 5 M NaOH at various ac currents.

“blank” cyclic voltammetry, the results of which are shown in Fig. 1b. Here the area under each curve between 0.05 and 0.45 V is representative of the charge transferred during the replacement of adsorbed hydrogen by sulfate [11]. This charge is commonly used to determine the electrochemically active area of a platinum electrode, using a coefficient of 210 ␮C/cm2 for a polycrystalline electrode [12], whose surface consists of areas with various crystal orientations. The apparent decrease of the area under the curves with increasing current density means a continuous decrease in the electroactive surface area, which for nanoparticle samples of constant mass indicates the increase of the average nanoparticle size, although here it should be kept in mind that other effects, such as particle agglomeration and variation in their surface termination, can influence the result. These results are further quantified in Fig. 2, where a monotonous increase of the particle size between 6 and 12 nm is plotted as a function of current density. While both methods show

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−10 to 10 V peak amplitude, provided by a non-inductive power amplifier, was applied between the electrodes, and both the current and voltage transients recorded until the corrosion of the wire was complete. Previously we have shown that nanoparticles can be produced from a platinum cathode without applying any anodic potential [2,5,10]. However, in order to detach nanoparticles from the cathode and disperse them in solution for subsequent characterization, especially in the case of low-amplitude polarization, symmetric AC voltage is required. This procedure was repeated four times in order to obtain 2.85 mg of Pt platinum nanoparticles per sample. The resulting black suspension was subsequently centrifuged, decanted and diluted with MilliQ water until the conductivity of the discarded liquid dropped below 1 ␮S/cm, measured with a Hanna Instruments HI4521 meter. The final concentration of the Pt nanoparticles suspension was always 1 mg/ml. Prior to use, the nanoparticles were re-dispersed by agitation in an ultrasonic bath for 2 min. It is interesting to note that the sedimentation time of freshly dispersed nanoparticles was clearly inversely proportional to their size. For the XRD measurements 200 ␮l of suspension (which corresponds to 200 ␮g of Pt) were drop-cast on a zero-background quartz XRD powder holder. The PAnalytical Xpert Pro diffractometer was used in the 2 configuration on the copper K␣1 X-ray wavelength ˚ Ten XRD patterns were collected and averaged for of 1.5406 A. each sample. The smoothly decaying background was subtracted manually to obtain a flat baseline for the peak fitting procedure. Subsequently, each diffraction line was fitted with a Lorentzian and their positions and full width at half maximum (FWHM) values were obtained from those fits. Electrochemical characterization was performed in a 0.5 M Merck UltraPurTM sulfuric acid solution purged with 5 N argon. Platinum foil was used as a counter electrode, and platinum/hydrogen electrode was used as a reference. For electrochemical characterization 5 ␮g of nanoparticles were drop-cast onto a polished gold electrode, with the Au blank voltammogram checked prior to each experiment. Cyclic voltammograms (CVs) were recorded at 50 mV/s between 0.05 and 0.7 V vs. RHE, scan rate was 50 mV/s unless noted otherwise. CO stripping experiments were performed in an identical setting, after verifying the negligible contribution of the bare gold electrode to the CO oxidation in the range 0.1–1.0 V vs. RHE. The electrode was kept at 0.1 V vs. RHE for 5 min during CO adsorption in a CO-saturated solution, and then for 15–20 min while purging this solution with argon. The latter time was sufficient to recover the true blank voltammogram after CO monolayer oxidation.

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Fig. 2. Average size determined from the 1st XRD line in Fig. 1 for nanoparticles synthesized in NaOH (squares) and KOH (triangles). For the former, size estimations from cyclic voltammetry are also shown for comparison (open squares). Solid squares and triangles are fit with an asymptotic exponential to guide the eye (solid and dashed line, respectively).

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a similar dependence, the absolute values vary by a factor of three. This is to be expected for bare metallic nanoparticles, as severe agglomeration during drop-cast deposition onto the electrode is expected and, indeed, observed. Since this is a random process, strongly dependent on the size and shape of the nanoparticles, a much larger scatter in the electrochemical data is also evident. In contrast to the electrochemical characterization, the XRD measurement is insensitive to particle agglomeration, and therefore yields a much more consistent and reproducible dataset. This result demonstrates that by choosing an appropriate current during synthesis one can selectively synthesize nanoparticles of a pre-defined size. A similar set of experiments, performed in a 5 M KOH solution, shows a comparable trend with nanoparticle size increasing with current density. Interestingly, the absolute size of nanoparticles obtained in this electrolyte is slightly larger than for NaOH for lower currents, and seems to saturate at about 11 nm for higher currents, while the NaOH series continues beyond 12 nm showing only slight saturation. Along with demonstrating the versatility of the method, this observation hints at possible extensions toward broadening the range of attainable particle sizes and intrinsic size-limiting effects. Comparing the results obtained in NaOH and KOH we suggest that the leading factor influencing the nanoparticle size is the depletion in the diffusion layer close to the electrode. Previously, we have shown that the size of nanoparticles is influenced by the concentration of the electrolyte. The complicated non-monotonous relationship obtained indicates that this influence is indirect [2]. Here by choosing a fixed electrolyte concentration we bypass this issue, and establish a direct relationship between the electrochemical current and the size of the resulting nanoparticles. By increasing this current, the bulk of which is carried by the hydrogen evolution reaction, we locally increase the pH and hence the concentration of the electrolyte in a thin layer directly at the electrode. This layer becomes depleted of free water, and according to the proposed mechanism of the formation of cathodic nanoparticles, the likelihood of the formation of solvated anionic platinum species, as well as their lifetime in solution, increase [5,13]. This increases the concentration of the building blocks for nanoparticles, and therefore their average size, with increasing current. At this point we must give careful consideration to the fact that XRD only provides the average value for crystallite size. If the actual size distribution was broad, contributions from both smaller and larger crystallites would be convoluted in the XRD line shapes. Ideally, one should employ TEM analysis of individual particle sizes. This is the method of choice for nanoparticles capped with organic molecules, which maintain their separation when dried on a TEM specimen. Ours, on the contrary, have a clean unprotected surface and therefore show a high degree of agglomeration [5], complicating the size distribution analysis. Even so, we have already shown that the size distribution is rather narrow [12]. For the purpose of this paper we address not only the width, but also the exact position of the diffraction lines. It has been well-established that in nanoparticles the lattice parameter starts to decrease from its bulk value as their size drops below ∼10 nm. Plotting the positions of diffraction line centers as a function of nanoparticle size we indeed observe this effect (Fig. 3). Using the systematic studies of lattice parameter on nanoparticle size [14,15] we can obtain the particle size which corresponds to a certain line shift. In this way both the position and the width of XRD lines can be used to estimate the size of nanoparticles. While the latter produces values between 6 and 13 nm (Fig. 2), the combination of Ref. [9] and Fig. 3 gives an overlapping range of 5–15 nm. This strongly suggests that the size distribution of nanoparticles is narrow enough and the size dependence presented in Fig. 2 is sufficiently well-defined. Another possibility which we must consider, however unlikely for metallic nanoparticles of such small sizes, is that of polycrystalline particles. Such structuring would lead to nanoparticles

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of larger diameters, while the size of individual crystallites would remain small enough to explain the observed XRD patterns. The mutual orientations of such crystallites within one particle should be quite different to prevent merging into one single crystal, and therefore the larger the particle, the rougher its surface would become. A closer look at Fig. 1b shows that this is not the case. While the voltammetry of smallest nanoparticles shows a poly-oriented surface termination, as the particles become larger, the surface order is enhanced. In particular, the features at 0.27 V corresponding to (1 0 0) steps and at 0.35 V corresponding to the extended (1 0 0) terraces, appear [11]. This strongly suggests that polycrystalline particles are not formed in our specimens. Invariance of XRD patterns toward sample heating up to ∼200 ◦ C corroborates this argument further. It is well known that for many oxidation reactions the catalytic properties of platinum nanoparticles are determined by their size and shape. Therefore we used the model reaction of electrooxidation of carbon monoxide to characterize our nanoparticles further. Voltammograms presented in Fig. 4 show oxidation of a single monolayer of CO adsorbed on nanoparticles of different sizes. The reaction itself is known to be limited by the availability of OH adsorption sites on a closely-packed CO-covered Pt surface [16–21]. This shifts the oxidation potential to much more positive values from the thermodynamic equilibrium, so that the reaction proceeds in stages with less ordered CO-covered regions oxidized at lower overpotentials [16,18,19]. The latter is a common explanation for the “pre-ignition” region observed between 0.4 and 0.6 V, while the well-ordered CO layer oxidizes with a sharp peak between 0.6 and 0.9 V, depending on the crystal orientation of the underlying platinum surface [16,18,19,22–27]. Both regions are shown

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synthesis, which was found to be the primary parameter, we achieve direct control over the size of the resulting nanoparticles. By employing complementary techniques of x-ray diffraction and electrochemical characterization we determine not only the size but also the catalytic activity of the particles toward the model reaction of CO oxidation. The extreme structure sensitivity of this reaction reveals that the shape of the nanoparticles is changing along with their size, increasing the amount of (1 0 0) surface sites at larger diameters. Comparison between synthesis in NaOH and KOH indicates possible pathways to further fine-tuning of the size selectivity.

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E vs. RHE / V Fig. 4. Stripping voltammograms of the pre-adsorbed CO monolayer on nanoparticles of various sizes. The pre-ignition region is enhanced in (a), while the evolution of the stripping double-peak is shown in (b). Part (c) shows complete sequential scans for 6.5, 8.5 and 12.4 nm nanoparticle samples. Potential sweep rate 50 mV/s.

in Fig. 4, and it can be seen that while no major changes are seen in the pre-ignition region, two overlapping peaks are resolved in the higher potential range, their total intensity gradually shifting toward higher oxidation potentials with increasing particle size. Size-dependent enhancement of catalytic activity of Pt nanoparticles for CO oxidation has been ascribed to the presence of defect sites, which are more likely to be found on larger particles, making them more active catalysts [28]. Here we find a seemingly opposite behavior: as the particles become larger, it becomes more difficult to oxidize adsorbed CO. This discrepancy can be resolved if we consider that the shape of the particle, and therefore the crystallographic facets exposed at the surface, can change with its size. As CO oxidation is a very structure sensitive reaction [22–25], we can infer the structure of our nanoparticles from the obtained voltammetric profiles. Comparing with [29] we conclude that the curves obtained for the largest particles reveal the presence of (1 0 0) surface domains (cf. corresponding blank voltammograms in Fig. 1). This explains why oxidation current shifts to higher potentials for larger nanoparticles, and opens an intriguing prospect of controlling the shape of nanoparticles synthesized by cathodic corrosion. In principle, just like in the case of hydrogen ad/desorption, electrical charge transferred during CO monolayer oxidation could be used to determine the active surface area, and thus the mean particle size. Extensive agglomeration, which randomly alters the electroactive area and is likely to be influenced by the shape of nanoparticles has, however, precluded us from obtaining a clear trend. Furthermore, a number of studies suggest that agglomeration may also influence the mechanism of carbon monoxide oxidation, with reacting OH and CO coming from different adjacent nanoparticles [30,31]. Ideally, one would want to study the size and shape dependence of catalytic properties on one single nanoparticle, and a recent advance in this direction, which opens the unprecedented possibility of directly relating structure and reactivity of one single nanoparticle, is a very welcome development [32]. 4. Conclusions In this study we present a new electrochemical method for size-controlled synthesis of platinum nanoparticles which is facile, green, uses no organics and should in principle be applicable to other metals. By controlling electrochemical current during

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