Letter pubs.acs.org/JPCL
Growth of Pt−Pd Nanoparticles Studied In Situ by HRTEM in a Liquid Cell Astrid De Clercq,†,‡ Walid Dachraoui,† Olivier Margeat,† Katrin Pelzer,‡ Claude R. Henry,† and Suzanne Giorgio*,† †
Aix Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France Aix Marseille Université, CNRS, MADIREL UMR 7246, 13397 Marseille, France
‡
ABSTRACT: The growth of Pt−Pd nanoparticles from organometallic precursors is studied in situ in real time by HRTEM in a graphene oxide liquid cell. The reduction of the metal precursors is induced by the electron beam. During the growth, the particles rearrange their internal structure to form faceted single crystals. The growth is compatible with the Lifshitz−Slyozov−Wagner (LSW) mechanism in the limiting case of a reactionlimited process. The same particles are also synthesized ex situ by using a chemical reducing agent and observed in HRTEM.
SECTION: Physical Processes in Nanomaterials and Nanostructures
P
The previous in situ TEM studies in liquid cells of the growth of monometallic particles have shown that growth occurs by addition of atoms and by coalescence. However, different power laws have been found for the growth rate, which gave rise to some contradictions in the interpretation. In the present work, we study the growth of Pt−Pd NPs in situ by TEM in a liquid cell in conditions where no coalescence occurs, which allows a clear interpretation of the growth rate. Furthermore, the use of a graphene liquid cell makes it possible to observe the internal structure of the particles during their growth. This technique can be used to study simultaneously the growth kinetics and the structure of individual NPs in a liquid environment. In this Letter, the growth of Pt−Pd NPs with oleylamine (OAm) ligands was studied in situ using a high-resolution liquid cell made by two membranes of graphene oxide, according to the technique described by Yuk et al.10 The particles obtained in the liquid cell could be compared to those prepared in solution in a reactor with a reducing agent, under argon atmosphere.22 These particles were observed after drying in conventional HRTEM, as shown in Figure 1. The Pt−Pd particles synthesized in a reactor are homogeneous in size and self-organized on the carbon substrate of the TEM grid (for more details, see the Experimental Methods section). The size histogram gives a homogeneous average diameter of 3.2 ± 0.8 nm. The NPs are well crystallized as single crystals or twins, as
t−Pd nanoalloys are mainly studied for their applications in catalysis, for example, for their high activity for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs),1−3 hydrogenation of aromatic hydrocarbons,4−6 or methane combustion in gas turbine combustors.7 Among all of the preparation techniques, solution-phase synthesis of nanocrystals is the best adapted to produce narrow size distributions and homogeneous compositions. Then, tuning the shape of metal nanoparticles (NPs) in solution is possible and mainly related to the choice of the organic ligands and metal precursors, that is, parameters affecting the nucleation and growth kinetics.8,9 However, the detailed mechanisms of nucleation and growth are still not completely clear. Since the introduction a few years ago of in situ environmental TEM using liquid cells,10 the studies of the nucleation and growth of metallic particles in solution have rapidly developed.11−21 The first studies were concerned with the diffusion and coalescence of Au11,13 and Pt.14 Later on, by using direct reduction by the electron beam of a metal precursor, the nucleation and growth by atom addition of Pd,15 Pt,12,16 PtFe,17,20 and Ag18,21 NPs were studied in real time. Most of the in situ studies in liquid use microfabricated silicon cells with thin silicon nitride windows.10 These cells have two main drawbacks, the thickness (10−100 nm) of the windows and the relatively high atomic number of the material window that limits the resolution in the images. To overcome these limitations, a new type of liquid cell has been developed that is based on liquid droplets trapped between two graphene layers.16 With this new type of liquid cell, it was possible to observe the growth of Pt NPs with atomic resolution.10 © 2014 American Chemical Society
Received: April 7, 2014 Accepted: June 3, 2014 Published: June 3, 2014 2126
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Figure 1. Overview of Pt−Pd nanoalloys dropped on an amorphous carbon film with the size histogram of the preparation and the HRTEM images of a single crystal and a twin, in insets.
Figure 2. (a−g) Growth of Pt−Pd NPs in the liquid with time by reduction of the electron beam. The density of particles is stable after approximately 70 s of electron beam irradiation. The inset of g shows a faceted cluster, mostly limited by (111) and (001) faces.
electron intensity at the level of the sample corresponds to 1.5 × 104 A/m2. The reduction of the Pt and Pd precursors by the electron beam and subsequent nucleation and growth of Pt−Pd NPs can be seen in Figure 2a−g. These TEM images were taken from the same area during 180 s of beam exposure. During this time, no coalescence occurred. The time t = 0 s in Figure 2a represents the origin of the series of images. Before this time, adjustments were done in another area to minimize the irradiation effect in the area of interest. NPs start to be visible after 40 s of irradiation. However, the nucleation kinetics is too fast to be accurately measured. It is noticeable that the electron beam also induces changes of morphology and structure of the metal particles, as previously seen in conventional HRTEM23,24 and in environmental liquid
seen in the inset. EDS analysis showed that they are bimetallic, with the same composition (50−50%) as the initial ratio of metal precursors introduced inside of the reactor. Compared to NPs of pure Pt (3.1 ± 1.4 nm) or pure Pd (5.2 ± 1.0 nm) prepared using the same concentrations and with similar experimental parameters, the size of Pt−Pd NPs is closer to that of pure Pt NPs. However, among the pure Pt particles, a large proportion has an elongated shape due to coalescence. In situ observations in liquid have shown that coalescence of pure Pt NPs already occurs in solution.12,14,16 The solutions with precursors and surfactants have been encapsulated between two graphene oxide layers on top of a holey carbon grid and then directly observed by TEM with a current density of 150 pA/cm2. In these conditions, the 2127
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Figure 3. (a) Density of clusters in the liquid as a function of the irradiation time during the nucleation and growth process. (b) Cluster diameter as a function of the irradiation time during nucleation and growth process, for one isolated cluster.
HRTEM.16 As the clusters are free to move in the liquid, their orientation changes frequently during the growth, so that different lattice fringes frequently appear or disappear during the observations. The density of clusters was measured as a function of irradiation time and are represented in Figure 3a. The density increases until 70 s of observation in Figure 2a−c. After this time, no more nucleation is observed, which is represented with a stable density. The mean size increases slowly as a function of irradiation time until 80 s; however, the contrast increases strongly. In the case of the marked particle in Figure 2e, the cluster size during the growth process is represented in Figure 3b as a function of time (t). The same evolution was found in the case of a few other isolated clusters. After about 80 s, the cluster size is constant and close to the size measured in the assembly of NPs obtained in the chemical reactor (Figure 1). The size is dispersed between 2 and 4 nm. This dispersion is probably due to the absence of stirring in the graphene liquid cell. It appears that the particle size depends on t1/2, as previously reported for Ag NPs growing in a liquid cell.17 Figure 4 displays the time evolution for 180 s of a particle from another series of observation. We see that the contrast increases and that the crystalline order improves with time. Later on, the clusters become faceted. The resolution of the lattice in our conditions was better seen from images taken after 110 s. Most of the clusters are single crystals, although some have a twinned structure, as seen in Figure 5 taken at 500 (a), 530 (b), and 540 s (c). At this stage, nucleation and growth are finished. For both Figures 4 and 5, the size of the particles is almost constant, but from the lattice resolution, we see that they rotate and tilt and make translations at short distances. The final composition after complete evaporation of the liquid corresponds to the composition (50−50% in Pt and Pd) of the initial ratio of the precursors introduced in the liquid cell. The absence of coalescence of Pt−Pd nanoalloys during the growth is different from the mechanism observed in the case of pure Pt particles12,16 where nucleation and coalescence occur simultaneously. We also observed frequent coalescence events in our study with pure Pt. In the case of pure Pt, Zheng et al.12 observed that the growth rate follows a power law with time
Figure 4. Structure and morphology evolution with time of the same Pt−Pd NP in the liquid between 50 and 180 s.
Figure 5. Shape and structure of Pt−Pd NPs in liquid after a long time. Both single crystals and twins are observed.
with a 1/3 exponent, which was explained by the Lifshitz− Slyozov−Wagner (LSW) growth model for Ostwald ripening.25 The same exponent was also observed in the case of pure Ag NPs.21 For other studies of Ag NPs, a 1/8 exponent was observed at high beam current, while at low beam current, the exponent was 1/2.18 The 1/2 exponent corresponds to the case where the growth kinetics is limited by the rate of surface reduction of the precursor (reaction-limited growth).26 In the case of Pt,12,16 Pd,15 and PtFe17 particles, the growth occurred simultaneously by addition of atoms and by coalescence.12,16,17 2128
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However, as noticed by Woehl et al.,21 the LSW model is not valid when coalescence occurs. The coalescence is treated by the model of coagulation developed by Smoluchowski.27 Application of this model explains the evolution of the size distribution observed in the case of Ag.21 In the present case, no coalescence is observed during the growth of the Pt−Pd NPs, and the beam current is rather weak compared to that from experiments for pure Pt12 and corresponds to the low current range in the study of pure Ag.18 Therefore, we can conclude that we are in conditions of the LSW model when the growth is limited by the surface reaction, giving the exponent equal to 1/2. This was experimentally verified in the case of a few different NPs followed from the early stage of nucleation until their final size. The fact that no dynamic coalescence occurs is possibly due to the fact that the Pt−Pd particles nucleate on the graphene oxide windows and diffuse slowly. However, the NPs are not strongly bound on the graphene oxide windows because we see from the lattice resolution that the particles freely rotate, tilt, or slowly diffuse. The main advantage of using a graphene (or graphene oxide) liquid cell is that the contrast in the image of the NPs is very high, allowing the observation of the atomic lattice. It appears that at the early stage of growth, the contrast in the particles is weak, and no lattice fringes are seen. This suggests that at the beginning of the nucleation and growth process, the particles are disordered. Lattice fringes are seen in a few NPs starting from 70 s. After about 100 s, lattice fringes are observed in most of them, but defects like twins are frequently seen. Probably some particles are not favorably oriented relative to the electron beam, and therefore, no fringes appear. Later on, particles rotate or tilt and rearrange in order to become single crystals. However, the particles are not epitaxially oriented on graphene oxide, which can be explained by the presence of the OAm ligands on the surface of the NPs. A similar behavior was observed for pure Pt NPs when using a graphene cell.12 The easiness for the particles to rearrange their internal structure is due to the influence of the electron beam that induces atom displacements by nonthermal effects.10,23,28 In this study, the growth of Pt−Pd particles with OAm ligands was studied in situ in real time by HRTEM in a graphene oxide liquid cell, using the electron beam as a reducing agent. The nucleation ends after 70 s. Later on, the particles grow slowly by addition of monomers; no coalescence occurs. The growth rate of some particles, visible from the early stage of nucleation until their final size, follows a power law as a function of time with an exponent of 1/2. This is in agreement with the LSW model in the case where the growth is limited by surface reaction. Taking into account that in a collection of growing NPs, the density increases with time without coalescence, the LSW model is the best appropriated to describe the Pt−Pd kinetics of growth. At the beginning, the NPs are not well structured, and later on, they rearrange their internal structure to form faceted crystals. The growth mechanism without coalescence, which is different from the case of pure Pt, could be partially due to a segregation of Pd atoms at the surface, corresponding to a more stable configuration, as found from DFT calculations and Monte Carlo simulations.29,30
easier to prepare and to manipulate than graphene. The solution that was encapsulated between two graphene oxide layers (on top of holey carbon grids) was made by mixing 20 mg of palladium(II) acetylacetonate (Pd(acac)2), (99%, SigmaAldrich) and platinum(II) acetylacetonate (Pt(acac)2) (98%, Acros Organics) in a 2 mL mixture of o-dichlorobenzene with oleylamine (OAm) (technical grade, 70%, Aldrich) (9:1 volume ratio). Then, this liquid cell was directly observed by TEM. The wet chemical synthesis under an argon atmosphere of both mono- and bimetallic Pt−Pd NPs was realized according to the procedure recently developed by Liu et al.22 Pd(acac)2 and Pt(acac)2 dissolved in OAm were reduced with injection of morpholine borane MB (95%, Aldrich) at 60 °C. All solutions turned black rapidly and were heated to 90 °C for 30 min (Pd NPs) and up to 180 °C for another 30 min (for Pt and Pt−Pd NPs). For all TEM observations, a JEOL 3010 operated at 300 kV was used.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the Conseil Régional PACA for supporting the Ph.D. thesis of A.D.C. and the French National Research Agency (ANR) for supporting this work under the SANAM Project (N° ANR 2011-IS10-004-01).
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EXPERIMENTAL METHODS For the in situ TEM observations, we used a liquid cell similar to those described in ref 16, but in the present case, graphene oxide was used instead of pure graphene. Graphene oxide is 2129
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dx.doi.org/10.1021/jz500690a | J. Phys. Chem. Lett. 2014, 5, 2126−2130
Growth of Pt−Pd Nanoparticles Studied In Situ by HRTEM in a Liquid Cell Astrid De Clercq,†,‡ Walid Dachraoui,† Olivier Margeat,† Katrin Pelzer,‡ Claude R. Henry,† and Suzanne Giorgio*,† †
Aix Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France Aix Marseille Université, CNRS, MADIREL UMR 7246, 13397 Marseille, France
‡
ABSTRACT: The growth of Pt−Pd nanoparticles from organometallic precursors is studied in situ in real time by HRTEM in a graphene oxide liquid cell. The reduction of the metal precursors is induced by the electron beam. During the growth, the particles rearrange their internal structure to form faceted single crystals. The growth is compatible with the Lifshitz−Slyozov−Wagner (LSW) mechanism in the limiting case of a reactionlimited process. The same particles are also synthesized ex situ by using a chemical reducing agent and observed in HRTEM.
SECTION: Physical Processes in Nanomaterials and Nanostructures
P
The previous in situ TEM studies in liquid cells of the growth of monometallic particles have shown that growth occurs by addition of atoms and by coalescence. However, different power laws have been found for the growth rate, which gave rise to some contradictions in the interpretation. In the present work, we study the growth of Pt−Pd NPs in situ by TEM in a liquid cell in conditions where no coalescence occurs, which allows a clear interpretation of the growth rate. Furthermore, the use of a graphene liquid cell makes it possible to observe the internal structure of the particles during their growth. This technique can be used to study simultaneously the growth kinetics and the structure of individual NPs in a liquid environment. In this Letter, the growth of Pt−Pd NPs with oleylamine (OAm) ligands was studied in situ using a high-resolution liquid cell made by two membranes of graphene oxide, according to the technique described by Yuk et al.10 The particles obtained in the liquid cell could be compared to those prepared in solution in a reactor with a reducing agent, under argon atmosphere.22 These particles were observed after drying in conventional HRTEM, as shown in Figure 1. The Pt−Pd particles synthesized in a reactor are homogeneous in size and self-organized on the carbon substrate of the TEM grid (for more details, see the Experimental Methods section). The size histogram gives a homogeneous average diameter of 3.2 ± 0.8 nm. The NPs are well crystallized as single crystals or twins, as
t−Pd nanoalloys are mainly studied for their applications in catalysis, for example, for their high activity for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs),1−3 hydrogenation of aromatic hydrocarbons,4−6 or methane combustion in gas turbine combustors.7 Among all of the preparation techniques, solution-phase synthesis of nanocrystals is the best adapted to produce narrow size distributions and homogeneous compositions. Then, tuning the shape of metal nanoparticles (NPs) in solution is possible and mainly related to the choice of the organic ligands and metal precursors, that is, parameters affecting the nucleation and growth kinetics.8,9 However, the detailed mechanisms of nucleation and growth are still not completely clear. Since the introduction a few years ago of in situ environmental TEM using liquid cells,10 the studies of the nucleation and growth of metallic particles in solution have rapidly developed.11−21 The first studies were concerned with the diffusion and coalescence of Au11,13 and Pt.14 Later on, by using direct reduction by the electron beam of a metal precursor, the nucleation and growth by atom addition of Pd,15 Pt,12,16 PtFe,17,20 and Ag18,21 NPs were studied in real time. Most of the in situ studies in liquid use microfabricated silicon cells with thin silicon nitride windows.10 These cells have two main drawbacks, the thickness (10−100 nm) of the windows and the relatively high atomic number of the material window that limits the resolution in the images. To overcome these limitations, a new type of liquid cell has been developed that is based on liquid droplets trapped between two graphene layers.16 With this new type of liquid cell, it was possible to observe the growth of Pt NPs with atomic resolution.10 © 2014 American Chemical Society
Received: April 7, 2014 Accepted: June 3, 2014 Published: June 3, 2014 2126
dx.doi.org/10.1021/jz500690a | J. Phys. Chem. Lett. 2014, 5, 2126−2130
The Journal of Physical Chemistry Letters
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Figure 1. Overview of Pt−Pd nanoalloys dropped on an amorphous carbon film with the size histogram of the preparation and the HRTEM images of a single crystal and a twin, in insets.
Figure 2. (a−g) Growth of Pt−Pd NPs in the liquid with time by reduction of the electron beam. The density of particles is stable after approximately 70 s of electron beam irradiation. The inset of g shows a faceted cluster, mostly limited by (111) and (001) faces.
electron intensity at the level of the sample corresponds to 1.5 × 104 A/m2. The reduction of the Pt and Pd precursors by the electron beam and subsequent nucleation and growth of Pt−Pd NPs can be seen in Figure 2a−g. These TEM images were taken from the same area during 180 s of beam exposure. During this time, no coalescence occurred. The time t = 0 s in Figure 2a represents the origin of the series of images. Before this time, adjustments were done in another area to minimize the irradiation effect in the area of interest. NPs start to be visible after 40 s of irradiation. However, the nucleation kinetics is too fast to be accurately measured. It is noticeable that the electron beam also induces changes of morphology and structure of the metal particles, as previously seen in conventional HRTEM23,24 and in environmental liquid
seen in the inset. EDS analysis showed that they are bimetallic, with the same composition (50−50%) as the initial ratio of metal precursors introduced inside of the reactor. Compared to NPs of pure Pt (3.1 ± 1.4 nm) or pure Pd (5.2 ± 1.0 nm) prepared using the same concentrations and with similar experimental parameters, the size of Pt−Pd NPs is closer to that of pure Pt NPs. However, among the pure Pt particles, a large proportion has an elongated shape due to coalescence. In situ observations in liquid have shown that coalescence of pure Pt NPs already occurs in solution.12,14,16 The solutions with precursors and surfactants have been encapsulated between two graphene oxide layers on top of a holey carbon grid and then directly observed by TEM with a current density of 150 pA/cm2. In these conditions, the 2127
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Figure 3. (a) Density of clusters in the liquid as a function of the irradiation time during the nucleation and growth process. (b) Cluster diameter as a function of the irradiation time during nucleation and growth process, for one isolated cluster.
HRTEM.16 As the clusters are free to move in the liquid, their orientation changes frequently during the growth, so that different lattice fringes frequently appear or disappear during the observations. The density of clusters was measured as a function of irradiation time and are represented in Figure 3a. The density increases until 70 s of observation in Figure 2a−c. After this time, no more nucleation is observed, which is represented with a stable density. The mean size increases slowly as a function of irradiation time until 80 s; however, the contrast increases strongly. In the case of the marked particle in Figure 2e, the cluster size during the growth process is represented in Figure 3b as a function of time (t). The same evolution was found in the case of a few other isolated clusters. After about 80 s, the cluster size is constant and close to the size measured in the assembly of NPs obtained in the chemical reactor (Figure 1). The size is dispersed between 2 and 4 nm. This dispersion is probably due to the absence of stirring in the graphene liquid cell. It appears that the particle size depends on t1/2, as previously reported for Ag NPs growing in a liquid cell.17 Figure 4 displays the time evolution for 180 s of a particle from another series of observation. We see that the contrast increases and that the crystalline order improves with time. Later on, the clusters become faceted. The resolution of the lattice in our conditions was better seen from images taken after 110 s. Most of the clusters are single crystals, although some have a twinned structure, as seen in Figure 5 taken at 500 (a), 530 (b), and 540 s (c). At this stage, nucleation and growth are finished. For both Figures 4 and 5, the size of the particles is almost constant, but from the lattice resolution, we see that they rotate and tilt and make translations at short distances. The final composition after complete evaporation of the liquid corresponds to the composition (50−50% in Pt and Pd) of the initial ratio of the precursors introduced in the liquid cell. The absence of coalescence of Pt−Pd nanoalloys during the growth is different from the mechanism observed in the case of pure Pt particles12,16 where nucleation and coalescence occur simultaneously. We also observed frequent coalescence events in our study with pure Pt. In the case of pure Pt, Zheng et al.12 observed that the growth rate follows a power law with time
Figure 4. Structure and morphology evolution with time of the same Pt−Pd NP in the liquid between 50 and 180 s.
Figure 5. Shape and structure of Pt−Pd NPs in liquid after a long time. Both single crystals and twins are observed.
with a 1/3 exponent, which was explained by the Lifshitz− Slyozov−Wagner (LSW) growth model for Ostwald ripening.25 The same exponent was also observed in the case of pure Ag NPs.21 For other studies of Ag NPs, a 1/8 exponent was observed at high beam current, while at low beam current, the exponent was 1/2.18 The 1/2 exponent corresponds to the case where the growth kinetics is limited by the rate of surface reduction of the precursor (reaction-limited growth).26 In the case of Pt,12,16 Pd,15 and PtFe17 particles, the growth occurred simultaneously by addition of atoms and by coalescence.12,16,17 2128
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However, as noticed by Woehl et al.,21 the LSW model is not valid when coalescence occurs. The coalescence is treated by the model of coagulation developed by Smoluchowski.27 Application of this model explains the evolution of the size distribution observed in the case of Ag.21 In the present case, no coalescence is observed during the growth of the Pt−Pd NPs, and the beam current is rather weak compared to that from experiments for pure Pt12 and corresponds to the low current range in the study of pure Ag.18 Therefore, we can conclude that we are in conditions of the LSW model when the growth is limited by the surface reaction, giving the exponent equal to 1/2. This was experimentally verified in the case of a few different NPs followed from the early stage of nucleation until their final size. The fact that no dynamic coalescence occurs is possibly due to the fact that the Pt−Pd particles nucleate on the graphene oxide windows and diffuse slowly. However, the NPs are not strongly bound on the graphene oxide windows because we see from the lattice resolution that the particles freely rotate, tilt, or slowly diffuse. The main advantage of using a graphene (or graphene oxide) liquid cell is that the contrast in the image of the NPs is very high, allowing the observation of the atomic lattice. It appears that at the early stage of growth, the contrast in the particles is weak, and no lattice fringes are seen. This suggests that at the beginning of the nucleation and growth process, the particles are disordered. Lattice fringes are seen in a few NPs starting from 70 s. After about 100 s, lattice fringes are observed in most of them, but defects like twins are frequently seen. Probably some particles are not favorably oriented relative to the electron beam, and therefore, no fringes appear. Later on, particles rotate or tilt and rearrange in order to become single crystals. However, the particles are not epitaxially oriented on graphene oxide, which can be explained by the presence of the OAm ligands on the surface of the NPs. A similar behavior was observed for pure Pt NPs when using a graphene cell.12 The easiness for the particles to rearrange their internal structure is due to the influence of the electron beam that induces atom displacements by nonthermal effects.10,23,28 In this study, the growth of Pt−Pd particles with OAm ligands was studied in situ in real time by HRTEM in a graphene oxide liquid cell, using the electron beam as a reducing agent. The nucleation ends after 70 s. Later on, the particles grow slowly by addition of monomers; no coalescence occurs. The growth rate of some particles, visible from the early stage of nucleation until their final size, follows a power law as a function of time with an exponent of 1/2. This is in agreement with the LSW model in the case where the growth is limited by surface reaction. Taking into account that in a collection of growing NPs, the density increases with time without coalescence, the LSW model is the best appropriated to describe the Pt−Pd kinetics of growth. At the beginning, the NPs are not well structured, and later on, they rearrange their internal structure to form faceted crystals. The growth mechanism without coalescence, which is different from the case of pure Pt, could be partially due to a segregation of Pd atoms at the surface, corresponding to a more stable configuration, as found from DFT calculations and Monte Carlo simulations.29,30
easier to prepare and to manipulate than graphene. The solution that was encapsulated between two graphene oxide layers (on top of holey carbon grids) was made by mixing 20 mg of palladium(II) acetylacetonate (Pd(acac)2), (99%, SigmaAldrich) and platinum(II) acetylacetonate (Pt(acac)2) (98%, Acros Organics) in a 2 mL mixture of o-dichlorobenzene with oleylamine (OAm) (technical grade, 70%, Aldrich) (9:1 volume ratio). Then, this liquid cell was directly observed by TEM. The wet chemical synthesis under an argon atmosphere of both mono- and bimetallic Pt−Pd NPs was realized according to the procedure recently developed by Liu et al.22 Pd(acac)2 and Pt(acac)2 dissolved in OAm were reduced with injection of morpholine borane MB (95%, Aldrich) at 60 °C. All solutions turned black rapidly and were heated to 90 °C for 30 min (Pd NPs) and up to 180 °C for another 30 min (for Pt and Pt−Pd NPs). For all TEM observations, a JEOL 3010 operated at 300 kV was used.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the Conseil Régional PACA for supporting the Ph.D. thesis of A.D.C. and the French National Research Agency (ANR) for supporting this work under the SANAM Project (N° ANR 2011-IS10-004-01).
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EXPERIMENTAL METHODS For the in situ TEM observations, we used a liquid cell similar to those described in ref 16, but in the present case, graphene oxide was used instead of pure graphene. Graphene oxide is 2129
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