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Pulsed Flow Near- and Supercritical Synthesis of Carbon Supported Pt Nanoparticles and In-situ Xray Diffraction Study of their Formation and Growth Jian-Li Mi, Henrik F. Clausen, Martin Bremholm, Patricia Hernandez-Fernandez, Jacob Becker, and Bo B. Iversen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5033817 • Publication Date (Web): 22 Dec 2014 Downloaded from http://pubs.acs.org on December 29, 2014

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Chemistry of Materials

Pulsed Flow Near- and Supercritical Synthesis of Carbon Supported Pt Nanoparticles and In-situ X-ray Diffraction Study of their Formation and Growth Jian-Li Mi,1,2 Henrik F. Clausen,3 Martin Bremholm,1 Mette S. Schmøkel,1 Patricia HernándezFernández,3 Jacob Becker1 and Bo B. Iversen1* 1

Center for Materials Crystallography, Department of Chemistry, Interdisciplinary Nanoscience

Center, Aarhus University, 8000 Aarhus C, Denmark. 2

Institute for Advanced Materials, School of Materials Science and Engineering, Jiangsu

University, Zhenjiang 212013, China 3

Danish Technological Institute, Nano- and Microtechnology, Gregersensvej 1, 2630 Taastrup,

Denmark * Corresponding author: E-mail: [email protected]

ABSTRACT: The formation and growth of carbon supported platinum nanoparticles in hightemperature, high-pressure ethanol solution have been studied by in situ synchrotron radiation powder X-ray diffraction (PXRD). Supercritical synthesis is shown to be an efficient way to prepare Pt nanoparticles, and the crystallite size of Pt nanoparticles is much smaller when formed with supporting carbon material compared with synthesis without carbon. On the basis of the time-resolved in situ PXRD data, a surface stress of 2.65 N/m is derived from the size dependence of the cell parameters. As proof of concept, carbon supported Pt nanoparticles were subsequently synthesized in a pulsed flow supercritical reactor, which offers complete control of reaction temperature, pressure and residence time. Well dispersed Pt nanoparticles decorated on the supporting carbon material can be obtained by adjusting the reaction conditions, and the

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electrocatalytic activity of the samples is explored. A mass activity of 0.1209 A/mgPt at a potential of 0.9 V is obtained for the products prepared at 400 °C for a residence time of 20 s. The pulsed flow supercritical method is a facile method to synthesize ligand-free carbon supported Pt nanoparticles with high electrocatalytic activity.

Keywords: supercritical synthesis, nanoparticles, platinum, electrocatalytic activity, in situ synchrotron powder X-ray diffraction

1. Introduction Platinum nanoparticles are of considerable interest for catalytic applications e.g. in fuel cells.1,2 It is generally accepted that the catalytic activity of metal nanoparticles depends on the morphology, size and size distribution of the particles.3 Colloidal synthesis methods, where surface stabilizers are necessary to stabilize nanoparticles during the synthesis, are particularly suitable for preparation of small nanoparticles with controlled morphologies and particle size.4 However, when the nanoparticles are deposited on the supporting materials the surface stabilizers are detrimental for the catalytic properties.5 It has been shown that the effect of stabilizers on the catalytic activity of metal nanoparticles significantly outweigh the particle size effect, and thus reports of size dependence of metal nanoparticles carrying various stabilizers should be interpreted with great care.6 In addition, the removal of these stabilizers introduces an additional process step, which typically involves thermal treatments that often causes problems such as aggregation, nanoparticle growth and a loss of monodispersity, which all alter the catalytic activity.7 Therefore, it remains a challenge to develop a facile, low-cost and surfactant-free approach for large-scale growth of metal nanoparticles with controlled particle size and size distribution.


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Solvents such as methanol or ethanol are weak reducing agents but they are usually too weak to reduce metallic ions to elemental metals at room temperature. However, with increasing temperature they become efficient reducing agents. For example, Ni nanoparticles can be synthesized by reducing Ni(NO3)2 in supercritical methanol at 400 °C without other reducing agents.8 Ru nanoparticles have been prepared in autoclaves using solvothermal routes by reduction of RuCl3 in anhydrous ethanol or anhydrous methanol at 175 °C without other reducing agents.9 In a previous study we showed that Ru nanoparticles with both face-centered cubic structure and hexagonal close packed structure can be synthesized by using ethanol both as solvent and reducing agent.10 Synthesis of metal catalysts by reduction of metal salts using solvents such as ethanol, i.e. without other strong and toxic reducing agents, can be regarded as green chemistry. The problems of conventional solvothermal autoclave synthesis are the limited control of temperature and pressure as well as slow heating rates. A supercritical reactor which has high reaction temperature and pressure, as well as fast heating rates, provides supercritical solvents which can accelerate reactions which are not efficient at normal conditions. In addition, supercritical fluids have advantages such as high diffusivity and high solvation capacity, which make them attractive solvents in chemical processes.11-13 Thus, synthesis in supercritical or nearcritical solvents provides an efficient route for producing highly crystalline nanoparticles with a controllable particle size and a narrow size distribution even without the help of stabilizers. Moreover, supercritical flow reactors provide continuous synthesis which can be scaled up for commercial applications. The advantages of continuous flow supercritical synthesis have been demonstrated in the preparation of a wide range of nanostructured materials.14-22 Recently, there has been tremendous progress in the use of in situ studies to obtain insight into chemical reactions taking place in fluid medium.23-29 By using high intensity synchrotron radiation powder X-ray diffraction (SR-PXRD) and specially designed in situ reactors, it is


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possible to obtain unique information about the nanoparticle formation and growth processes in supercritical or near-critical solvents.30-38 In the present study, in situ SR-PXRD is applied to study the synthesis of carbon supported Pt nanoparticles. We also report a facile and fast supercritical synthesis route for the direct synthesis of carbon supported Pt nanoparticles using a pulsed flow reactor, where ethanol is used as both solvent and reductant without addition of any stabilizing surfactants. Furthermore, the electrocatalytic activity of the as prepared Pt nanoparticles in the oxygen reduction reaction (ORR) is reported. This reaction is very interesting because it takes place in the cathode of polymer electrolyte membrane fuel cells (PEMFCs).

2. Experimental section In situ SR-PXRD Direct formation and growth of Pt nanoparticles on Ketjenblack (KB) EC-600J carbon particles were studied by in situ SR-PXRD. The detailed description of the reactor used for in situ studies has been given by Becker et al.39 To obtain high quality PXRD, a relatively high concentration of precursor solution, 0.20 M of H2PtCl6·6H2O, was used. KB particles were dispersed in ethanol by sonication for 5 min leading to a KB suspension with the carbon concentration of about 3 g/L. The Pt precursor solution and the KB suspension were mixed with a volume ratio of 1:1, and a homogenous suspension was ensured by stirring prior to injection into the in situ reactor. Subsequently, the in situ reactor was sealed and pressurized with a HPLC pump, and heated to the desired temperature using a hot air flow. Experiments at temperatures of 200 and 400 °C under a pressure of 250 bar were performed at beamline I711, MAX-II, MAX-lab, Sweden, using monochromatic X-rays with a wavelength of 1.0009 Å. The data was collected on a Mar165 CCD detector and the time resolution was 5 seconds between each frame, of which 1 second was


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detector dead time for readout. The raw data images were integrated using FIT2D40 and then Rietveld refined using the FullProf program.41 The data were corrected for instrumental broadening using a LaB6 standard (NIST SRM 660a). The samples in the in situ study are named as “ISx”, where “IS” denotes the in situ study and “x” the synthesis temperature. Samples without carbon support were also performed for comparison and are named as “ISx*”.

Pulsed flow supercritical synthesis Carbon supported Pt nanoparticles were synthesized by reducing the Pt precursor on KB particles using supercritical ethanol as both the solvent and reductant in a pulsed flow reactor. The custom designed pulsed flow apparatus is composed of several pulsed pumps, a heated reactor block, a pressure relief valve, a manometer and the temperature control and cooling system. A detailed technical description of the reactor has been reported previously.42 In short, the operation of heater, valve and pumps is automated using computer control, and a pulsed flow apparatus has high control of reaction conditions including reaction temperature, pressure and reaction residence time. In the synthesis of carbon supported Pt nanoparticles, H2PtCl6·6H2O ethanol solution with a concentration of 0.0135 M was used as metal precursor. The KB suspension (2.7 g/L) was prepared by dispersing KB particles in absolute ethanol by sonication for 5 min. The Pt precursor solution and the KB suspension were pumped simultaneously with same volume by two pumps into the preheated reactor. Thus, the Pt content is expected to be about 50 wt% in the resulting product. The reactants remained in the reactor for a fixed residence time (5 or 20 s in this study) and pure ethanol solvent was then pumped into the reactor by a third pump to push the product into the cooling zone and eventually through the outlet valve. Subsequently, new reactants were pumped into the reactor for continuous synthesis. The ratio of the pumping volumes of Pt precursor solution and KB suspension was kept at 1. All the synthesis parameters


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such as temperature, residence time, pumping rate, etc, are easily adjusted by computer control. All the experiments were performed at a pressure of 250 bar. The products were washed by distilled water and ethanol, and collected by centrifugation. The samples are named as “Px_y”, where “P” denotes the pulsed flow synthesis, “x” is the synthesis temperature in degrees Celsius, and “y” is the residence time in seconds.

Characterization and electrochemical measurements The products prepared by pulsed flow reactor were characterized using powder X-ray diffraction (PXRD) measured on a STOE powder diffractometer using Cu Kα1 radiation (λ = 1.5406 Å). The morphology of the carbon supported Pt nanoparticles was observed on a Zeiss XB-1540 FEG-SEM equipped with a multimode STEM detector operated at 30 kV acceleration voltage and a Philips CM20 transmission electron microscopy (TEM). The electrochemical experiments were performed in a standard three-compartment glass cell connected to a Bio-logic VMP3 potentiostat, which is controlled by a computer. The electrolyte, 0.1 M HClO4 (Fluka, Analytical), was prepared using Millipore water. The counter electrode was a platinum wire, and the reference was a Hg/Hg2SO4 electrode. The measurements were conducted at room temperature. All the potentials in the manuscript are quoted with respect to the RHE and corrected for Ohmic losses. Following each measurement, 0V vs RHE was established by carrying out the hydrogen oxidation and hydrogen evolution reaction in the same electrolyte. A glassy carbon disk (0.196 cm2) encased in a rotating disk electrode (RDE) was used as working electrode. The Pt nanoparticles were deposited onto the glassy carbon electrode by means of an ink (thin-layer method). The catalytic inks were prepared by mixing the proper amount of catalyst, Millipore water, ethanol (Fluka Analytical, ACS reagent), Nafion® ionomer


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(Dupont DE520, 5 wt%) and polyvinylpyrrolidone (PVP, Sigma-Aldrich). The final loading of the electrodes was around 12 µg cm-2. Once the working electrode was immersed into the electrochemical cell, its surface was activated by cycling the potential in N2-saturated electrolyte at 100 mV s-1 and 50 mV s-1 between 0.05 V and 1.0 V. The CO-stripping analyses were carried out afterwards in CO-free N2 purged solution, after adsorbing CO at 0.05 V for 5 min to establish the active surface area of the Pt nanoparticles. The CO stripping curves were recorded by cycling the potential until 1.0 V at 20 mV s-1. The active surface area was estimated using the area under the CO-stripping peak and assuming the CO linearly bound on Pt that provides a charge equivalence of 420 µC cm-2. The oxygen reduction reaction (ORR) activity was measured by cycling the potential between 0 V and 1.0 V, at 50 mV s-1 and 1600 rpm, after saturating the solution in O2. The kinetic current density for the ORR has been corrected for mass transport limitations, and the effect of the capacitance has been eliminated by background subtraction. The specific (mA cm-2Pt) and mass activity (A mg-1Pt) at 0.9 V were calculated from the anodic sweep of the ORR polarization curve.

3. Results and discussion In situ study Continuous flow synthesis experiments have shown that ethanol becomes an effective reducing agent above 180 °C for the synthesis of Pt nanoparticles without carbon support (see Figure S1 in the Supporting Information), and the present in situ experiments are performed at temperatures equal to or above 200 °C. The formation and growth of carbon supported Pt nanoparticles were studied in situ by SR-PXRD at reaction temperatures of 200, 250, 300 and 400 °C (sample labeled as IS200, IS250, IS300 and IS400). The unsupported sample IS250* was studied for


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comparison. The time evolution of the SR-PXRD patterns (Figure S2) show that Pt particles can be obtained at all investigated reaction temperatures, indicating that ethanol is an effective reducing agent to prepare Pt nanoparticles at high temperatures possibly aided by the catalytic properties of the Pt precursors and Pt nanoparticles. Crystalline Pt nanoparticles start to form after 2 min for IS200, while Pt is obtained after only a few seconds for the other syntheses where the reaction temperature is at or above 250 °C. The diffraction peaks of (111) and (200) become sharper with reaction time for the carbon supported samples, whereas they are almost unchanged for the unsupported sample. For clarity, Figure 1 displays the time evolution of normalized intensity (height) of peak (111) for all the samples. It can be seen that the intensity of diffraction peak (111) grows with reaction time for all the carbon supported Pt samples even after the reaction of several minutes, while for the unsupported Pt IS250* the diffraction intensity increases rapidly and then remains unchanged. This shows that limited ripening occurs in the absence of the carbon support, whereas ripening is taking place for the samples with carbon support.

Figure 1. Time evolution of normalized height of peak (111) for IS200, IS250, IS300, IS400 and IS250*. An offset of 0.5 is used between each curve.


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In order to get detailed information about the formation and growth of Pt nanoparticles, the in situ SR-PXRD patterns were analyzed using the Rietveld method implemented in the FullProf program. The scale factor, unit cell parameter and a Lorentzian peak shape parameter were refined.31 The particle size is calculated from the peak shape parameter. Figure 2 shows the refinement of the SR-PXRD pattern for Pt nanoparticles IS300 at a reaction time of 10 min. There is good agreement between the calculated and the observed patterns. Table 1 displays selected refinement parameters for IS200, IS250, IS300 and IS400 at 10 min.

Figure 2. Observed (red), calculated (black), and difference patterns (blue) of a selected in situ SR-PXRD data of IS300 at the reaction time of 10 min.

Table 1. Refined parameters of the SR-PXRD data for IS200, IS250 and IS300 after heating 10 min. Sample

IS200

IS250

IS300

IS400

Data points

568

569

623

623

Refined parameters

3

3

3

3

Reflections

2

2

2

2

RWP (%)

6.4

3.0

4.7

10.8

RI (%)

22.1

22.4

17.3

11.0


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RF (%)

11.5

13.8

9.6

7.0

a (Å)

3.8842(2)

3.8872(6)

3.8943(2)

3.9025(2)

Particle size (nm)

6.3

8.9

15.0

24.7

The in situ data allow us to follow the particle growth process during the reaction. Figure 3a shows the time evolution of particle sizes for different samples. For the carbon supported Pt nanoparticles, the particle size increases with reaction temperature. Particle sizes of 6.3 nm, 8.9 nm, 15.0 nm and 24.7 nm are obtained for samples IS200, IS250, IS300 and IS400, respectively, at a reaction time of 10 min. For all the carbon supported samples, the particle grows quickly during the initial period and then the growth slows down upon further heating due to Ostwald ripening. Note that the particle size of IS300 is almost identical to IS250 during the first 4 min of the reaction time, which could be due to problems with the temperature control leading to a lower actual temperature than 300 °C. After the temperature recovers, the particle of IS300 grows significantly. As shown in Figure 3a, the size of the unsupported sample IS250* is larger than that of IS250. This could be explained by the fact that the supporting carbon material introduces an abundance of nucleation sites. The supporting carbon material is therefore favorable for the synthesis of metal nanoparticles with controlled particle size and size distribution without the necessity of adding any organic surfactants. As a result, the particle sizes are smaller for samples with carbon support due to the heterogeneous nucleation even though Ostwald ripening is also observed. On the other hand the particle size is relatively larger for the sample without carbon support, and here ripening is not evident. The relative amount of Pt product can be estimated from the normalized scale factors as shown Figure 3b. The normalized scale factor for the IS200 sample increases slowly until 8 min, which indicates that the formation of Pt nanoparticles is very slow at 200 °C. When the reaction


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temperature reaches 250 °C or more, the nucleation step becomes very fast, and the reactant is consumed completely in seconds for sample IS250, IS300 and IS400. As a result, temperatures of 250 °C or above, are needed for the efficient synthesis of carbon supported Pt nanoparticles using ethanol as both solvent and reducing agent. Figure 3c shows the cell parameters as a function of the reaction time for different carbon supported Pt samples. It is notable that the cell parameter of Pt nanoparticles increases with reaction time for all samples, that is to say, the cell parameter decreases with decreasing size of the Pt nanoparticles. It has been previously been reported that the unit cell parameters of carbon supported Pt nanoparticles with diameters ranging from 2 to 28 nm have a nonlinear decrease of unit cell parameter with decreasing particle size down to 2 nm.43 In the present study, the in situ experiments give credible data and provide valuable knowledge in understanding the size dependence of structural information compared to ex situ experiments. A single controlled in situ experiment yields a large mass of data all collected in a single coherent series of measurements using an unperturbed setup and the exact same reaction condition ensures good correspondence between data points, whereas multiple ex situ experiments can sometimes be questioned as to whether they are comparable with each other. On the other hand in the present study, a direct comparison of the unit cell parameters obtained in different in situ experiments requires some caution due to potential systematic error. In addition, only two diffraction peaks are used which make it difficult to determine the accurate absolute values of the cell parameters. As a result, it is necessary to deduce the offsets of the cell parameters among different in situ experiments. For example, we use the cell parameter of 3.8908 Å as a standard value for IS300 and IS400 when the calculated particle size is 18 nm for both samples. The standard value of 3.8908 Å at 18 nm is then used as a calibration for the cell parameters for both samples. Similarly, all the four sets of cell parameters for the in situ experiments can be calibrated to compensate for any offsets


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between the experiments. Figure 3d plots the cell parameters as a function of reciprocal crystallite diameter 1/D for all the four in situ data series after the calibration. It is well established that the cell parameters of most metallic nanoparticles decrease with decreasing size which can be interpreted in terms of the surface stress.44 This gives a linear relationship between the cell parameter a and reciprocal size 1/D. Thus, the cell parameter of the bulk material (a0) can be deduced to be 3.8939 Å at 1/D = 0. The bulk cell parameter is relatively lower than previously reported bulk unit cells of around 3.92 Å,45 and this may be attributed to the systematic error (e.g. in wavelength or detector distance). However, here we only focus on the relative changes of the cell parameters for the in situ results, and the absolute value is not relevant to the discussion. It has been suggested that for spherical particles with a cubic structure, the surface stress, f, can be calculated according to the following equation46: f =−

3 ∆a D 4 a0 K

(1)

where ∆a = a0 − a is the change in cell parameter due to the surface stress, D is particle size, and the compressibility K of the bulk material is the inverse of the bulk modulus. By linear fitting of ∆a/a0 vs 1/D as shown in the inset of Figure 3d, and using the bulk modulus of 256 GPa for Pt,47 the surface stress f of 2.65 N/m is obtained. The value is very close to the previous reported result of 2.57 N/m.48


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Figure 3. Time evolution of (a) particle sizes, (b) normalized scale factors and (c) cell parameters for different samples. (d) Relative change in unit cell parameter, ∆a/a, as a function of reciprocal crystallite size, 1/D.

Pulsed flow supercritical synthesis of Pt nanoparticles As a proof of concept, a pulsed flow supercritical one-step synthesis method was developed to directly synthesize carbon supported Pt nanoparticles. From the in situ results, the formation of Pt nanoparticles is observed to be quite slow when the temperature is below 250 °C, and therefore the synthesis was carried out at temperatures above 250 °C. Figure 4 shows the PXRD patterns of carbon supported Pt samples prepared at varied synthesis temperatures and different residence time. Average particle sizes of 8.5, 7.2, 6.5 and 6.3 nm were obtained for samples


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P250_5, P250_20, P400_5 and P400_20, respectively, through fitting of the diffraction peaks. From the in situ experiments, we get the particle sizes of 6.1, 6.3, 5.6, 10.1 nm for IS250_5, IS250_20, IS400_5 and IS400_20 (“5” and “20” denote the residence time in seconds), respectively. Note that even though the precursor concentrations are much larger for the in situ experiments, the particle sizes are at the same magnitude for the in situ and ex situ results. From the in situ results, it can be seen that for a residence time below 20 s, the reaction is still in the nucleation stage where the crystal size is far from reaching the equilibrium value for the carbon supported samples. Thus, at this stage the particle size is mainly controlled by the effective nucleation sites rather than ripening. The pulsed flow supercritical method results in small particle sizes at high reaction temperatures, and this indicates that the carbon supports provide effective nucleation sites at high reaction temperatures. This is further substantiated by the samples synthesized without carbon supports using a continuous flow synthesis method, where larger particle sizes of 11 and 27 nm at 250 and 350 °C, respectively, are observed due to the lack of nucleation sites (Figure S1).

Figure 4. PXRD of carbon supported Pt nanoparticles prepared by one step synthesis using pulsed flow supercritical method.


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Figure 5 shows representative STEM images of the carbon supported Pt nanoparticles of P250_05, P250_20, P400_5, and P400_20 prepared by the pulsed flow supercritical method. The Pt particles are severely aggregated in the sample P250_05 and P250_20, which were prepared at the relatively low temperature of 250 °C. When the reaction temperature is increased to 400 °C, the carbon supports in samples P400_5 and P400_20 are decorated with small Pt nanoparticles which are almost monodisperse. This agrees very well with the above discussion of the PXRD results that carbon supports provides more effective nucleation sites at high reaction temperatures. The results show that high temperature supercritical conditions are necessary to prepare carbon supported catalysts with monodisperse Pt nanoparticles of controllable particle size.

Figure 5. STEM images of the carbon supported Pt nanoparticles prepared by pulsed flow supercritical method. (a) P250_05, (b) P250_20, (c) P400_5, and (d) P400_20.


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For unsupported Pt nanoparticles, both the PXRD data and TEM image (Figure S1) show similar particle size of about 11 nm for a sample prepared in a continuous flow synthesis reactor at 250 °C. This agrees well with the in situ experiments where a particle size of 13 nm is obtained for IS250* at a residence time of 2s. Figure 6a and 6b show the TEM images of sample P250_20 and P400_20, respectively, to visually demonstrate the typical Pt nanoparticles that were decorated on the supporting carbon. Corresponding size distributions are shown in Figure 6c and 6d. If we focus on the nanoparticles formed on the carbon supports, the Pt particles are larger at high reaction temperatures, suggesting the typical phenomenon that generally nanoparticles are larger at higher synthesis temperatures. However, there are only very few Pt nanoparticles attached on the carbon supports for P250_20, which further suggests that the heterogeneous nucleation is not sufficient at low temperature and that most Pt nanoparticles are formed by homogeneous nucleation resulting in a larger crystal size. Furthermore, these nanoparticles tend to form large aggregates as shown in Figure 5b. The average particle size calculated from PXRD of 7.2 nm for the Pt particles that decorate the carbon support is much larger than the 3.9 nm obtained from TEM. This corroborates that the large average Pt particle size from PXRD for sample P250_20 is due to the lack of effective nucleation sites at low temperature. At high temperature there are well dispersed Pt nanoparticles decorated on the carbon black (P400_20), and this will result in a large surface area to improve the catalytic properties. The particle size of P400_20 of 5.2 nm measured from the TEM is close to the PXRD result of 6.3 nm indicating less aggregation of Pt particles which also agrees with the STEM studies. More effective nucleation sites are introduced at high reaction temperature which may be attributed to the carbon supports being better dispersed in the supercritical ethanol at high temperature than at low temperature. As a result, the effective heterogeneous nucleation not only provides small particle sizes but also results in less aggregation, which are both important factors


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when preparing effective Pt catalysts. As the catalytic properties on oxygen reduction reaction are mainly attributed to the Pt nanoparticle size and the extent of dispersion, one could expect better catalytic performance for the samples that were synthesized at 400 °C. Table 2 lists the electrochemical properties of samples P250_20 and P400_20. Not just the active surface area, but also the ORR mass activity of P400_20 is higher compared to P250_20. The mass activity of P400_20 is 0.1209 A/mgPt at potential of 0.9 V which is about twice as large as P250_20.

Figure 6. TEM images of Pt nanoparticles (a) P250_20, (b) sample P400_20, and corresponding size distributions of (c) P250_20, (d) P400_20.


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Table 2. Active surface area and ORR activity of Pt/C nanoparticles for samples P250_20 and P400_20 at 0.9 V. The particle sizes are calculated from the PXRD data.

P250_20

Pt particle size (nm) 7.2

Active surface area (m2 g-1Pt) 13.0

Specific activity (mA cm-2Pt) 0.5181

Mass activity (A mg-1Pt) 0.0672

P400_20

6.3

44

0.2756

0.1209

Sample

4. Conclusions Besides abundant information about the formation and growth process of nanoparticles in solvothermal synthesis, in situ SR-PXRD experiments also provides valuable knowledge about the size dependence of the structures of nanoparticles. A surface stress of 2.65 N/m is determined for Pt particles from the in situ data. Synthesis by supercritical ethanol method is a facile and efficient way to prepare Pt nanoparticles without using other stabilizer or reducing agent. It is shown that carbon support is important to control the particle size and stabilize the nanoparticles since high temperature deposition on the carbon support introduce an abundance of nucleation sites and reduces agglomeration of the nanoparticles. Due to the complete control of reaction temperature, pressure and residence time, as well as fast heating rate, the pulsed flow supercritical synthesis is a promising method to prepare ligand-free carbon supported Pt nanoparticles with high electrocatalytic activity. The electrocatalytic measurements show that the product has a promising activity when prepared at 400 °C for a residence time of 20 s.

Supporting Information Available. Continuous flow synthesis of Pt nanoparticles without carbon supports, the time evolutions of SR-PXRD of IS200, IS250, IS300 and IS250*. This information is available free of charge via the Internet at http://pubs.acs.org/.


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Acknowledgments This work was supported by the Danish National Research Foundation (Center for Materials Crystallography, DNRF93), and the Danish Research Council for Nature and Universe (DanScatt). Work at Jiangsu University was supported by the National Natural Science Foundation of China (No. 51401089), the Scientific Research Foundation of Jiangsu University (No. 1291220029) and the Natural Science Foundation of Jiangsu Province (No. BK20140552). The authors are grateful for the beamtime obtained at the beamline I711, MAX-lab synchrotron radiation source, Lund University, Sweden, and Jeppe Christensen is thanked for assistance during measurements.

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