Formation Mechanisms of Pt and Pt3Gd Nanoparticles under

May 15, 2015 - Espen D. Bøjesen , Kirsten M. Ø. Jensen , Christoffer Tyrsted , Aref Mamakhel , Henrik L. Andersen , Hazel Reardon , Jacques Chevalier ...
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Formation Mechanisms of Pt and Pt3Gd Nanoparticles under Solvothermal Conditions: An in Situ Total X‑ray Scattering Study Dipankar Saha,† Espen D. Bøjesen,† Kirsten M. Ø. Jensen,‡ Ann-Christin Dippel,§ and Bo B. Iversen*,† †

Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark Department of Applied Mathematics and Applied Physics, Columbia University, New York, New York 10027, United States § Deutsches Elektronen-Synchrotron (DESY), D-22607 Hamburg, Germany ‡

S Supporting Information *

ABSTRACT: Platinum-based nanoparticles play a crucial role as catalysts, and solvothermal synthesis methods are attractive due to the excellent control of nanoparticle characteristics such as size, crystallinity, and morphology, which strongly affect the chemical and physical properties. Insight into the reaction mechanism leading to nanoparticle formation under solvothermal conditions generally remains elusive. This is mainly due to the experimental difficulties that lie in obtaining atomistic information on the nanoscale during the progression of the synthesis. Using in situ total X-ray scattering and pair distribution function (PDF) analysis with a time resolution of 1 s, we unravel the formation mechanisms of Pt and Pt3Gd nanoparticles under solvothermal conditions. We demonstrate that an octahedral Pt4+ platinic acid precursor complex is reduced in two steps. Both Pt and Pt3Gd nanocrystal formation proceeds via conversion of a square-planar Pt2+ complex to an unsaturated nanocluster (Pt0), which subsequently grows by a combination of aggregation and ripening. In contrast to Rietveld analysis of powder X-ray diffraction data, the PDF method is able to uniquely establish that the structure of the Pt−Gd alloy is Pt3Gd rather than PtGd.

1. INTRODUCTION The precious metal platinum has significant impact on our daily life. It is used in a multitude of fields ranging from jewelry, medicine, nitric acid manufacturing, silicones, dentistry, hard disks, glasses, jet engine fuel nozzles, to catalysis.1 Platinum nanoparticles (NPs) are particularly important as they are crucial components in many critically important catalytic reactions such as in automotive diesel exhaust or fuel cells.2−7 However, the use of platinum NPs in catalysis is challenging because the metal is very expensive and the resource scarce. The most direct route to decrease the Pt loading is by alloying it with other metals, and in fact alloying may enhance the catalytic efficiency.8,9 Pt-based binary alloys having formula Pt3X have been found to be superior to Pt in oxygen reduction in low-temperature polymer electrolyte membrane (PEM) fuel cells, borohydride oxidation, water activation, and methanol fuel cell performance.9−14 Further Pt5Gd has been found to show 5-fold increase in oxygen reduction reaction compared to pure Pt.15 Therefore, Pt alloys have gained tremendous attention. The main focus has been on the performance of the different alloys, while structural aspects have been less studied. It may be argued that Pt alloy structures are well documented in the literature,16 but as we show here it is in fact highly challenging to accurately establish both the local structure and the average crystal structure of nanoscale Pt based alloys. Structure−property correlation is crucial for understanding the underlying mechanism of these catalysts as the structure strongly affects the properties of the material.17 Although solvothermal methods have been widely applied to obtain Pt-based NPs, general insight into the reaction © XXXX American Chemical Society

mechanism of their formation is lacking. This is mainly due to the experimental difficulty in obtaining atomistic information on structures on the nanoscale, described as “the nanostructure problem”.18 During the past decade the growth (in contrast to formation) of nanocrystals under hydro- or solvothermal conditions has been widely studied using in situ powder X-ray diffraction (PXRD) methods.19 However, PXRD analysis only applies to crystalline materials. It provides no insight into the nature of the amorphous state before crystallization. If we are to truly design the structure and properties of nanocrystals, it is important to understand the chemical transformations leading to the first crystal nuclei, since they control the nature of the final nanocrystal. Atomistic insight into the nanoscale can be obtained by measuring total X-ray scattering from the sample and calculating the atomic pair distribution function (PDF).20,21 PDF analysis can provide structural information from gasses, liquids, amorphous materials, nanocrystalline, and disordered materials as well as crystalline structures. Recent technical advances in both hardware (availability of intense high-energy synchrotron radiation, large and fast detectors, development of in situ reactors22,23) and software (direct space structural modeling, efficient background subtraction24,25) have made it possible to measure and analyze temporally resolved total X-ray scattering data from dilute solvothermal reaction solutions. This has for the first time provided atomistic insight into the formation of nanocrystals all the way from precursor Received: April 8, 2015 Revised: May 14, 2015

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DOI: 10.1021/acs.jpcc.5b03394 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C solution to final nanocrystalline product.26−29 Once the precursor complexes have crystallized from solution, conventional PXRD methods can be used to elucidate the crystal structure as well as crystallite size and morphology. In this article we show that PDF analysis can be used to unearth the reaction mechanism as well as the reaction kinetics of NP formation and growth. Crystallographic analysis provides the average structure of a material, but it is sometimes difficult to distinguish very similar structures. Here we show that PDF analysis also can be used to clearly establish that the Pt−Gd alloy formed under the present conditions is Pt3Gd rather than PtGd as otherwise indicated by Rietveld analysis of PXRD data.

that at 3.5 Å arises from Cl−Cl distances in the octahedral complexes. No long-range order is present in the precursor solution.32 The solution was heated to 250 °C at 250 bar pressure to obtain Pt nanoparticles. The intensity of the peak at 2.36 Å decreases with time whereas the peak intensity at 2.77 Å increases, suggesting the breaking of Pt−Cl bonds and the formation of Pt−Pt bonds. However, in the first 10 s of heating no new peaks appear, indicating a change within the octahedral complex before the formation of Pt nanoparticles. PDF analysis also reveals that the final particle size was around 5 nm, and ex situ TEM images confirm the particle size (see Supporting Information). In order to have more insight into the first 10 s of the reaction after heating, Gaussian functions were used to fit the peaks in the PDF (Supporting Information Figure S2). In Figure 2 the area under the peaks at 2.33 and 2.77 Å is plotted

2. EXPERIMENTAL SECTION H2PtCl6·3H2O was dissolved in absolute ethanol to make a clear solution (0.5 M), which was used for the Pt nanoparticle synthesis. For the synthesis of intermetallic Pt−Gd, H2PtCl6· 3H2O and GdCl3·6H2O were used in 1:1 molar ratio and dissolved in EtOH. The concentration of the solution was kept at 0.5 M. Clear precursor solutions were used for the in situ experiments in a custom-made capillary reactor pressurized to 250 bar and heated to temperatures of 250 °C.22 The in situ total X-ray scattering experiments were carried out at beamline P02.1 at PETRA III, DESY, Germany, using a wavelength of 0.207 Å. The data were integrated using Fit2D.30 The integrated total scattering data were analyzed with the PDFgetX3 program.24 Prior to the Fourier transformation, the data were corrected for background scattering using measurements on ethanol in the same capillary at appropriate temperatures. The resulting PDFs were refined using PDFgui.25 Rietveld refinement was carried out using the FullProf Suite.31

Figure 2. (top) Time evolution of the normalized area under the PDF peaks for Pt−Cl and Pt−Pt bonds. (bottom) Changes in Pt−Cl bond distaces as a function of reaction time (initiation of heating was set to 0 s for plotting).

3. RESULTS AND DISCUSSION PDF analysis of the Pt nanoparticle synthesis from the ethanolic solution of H2PtCl6·3H2O reveals that an octahedral [PtCl6]2− complex exists in the solution. Figure 1 shows the evolution of the PDF with respect to time. The peak at 2.36 Å in the precursor PDF corresponds to the Pt−Cl bond, while

with respect to time along with the peak position, which represents the bond distances. In the [PtCl6]2− precursor complex, Pt remains in the +4 oxidation state, and the corresponding bond distance is at 2.33 Å.32 Initially the area under the Pt−Cl peak decreases rapidly before reaching a plateau 5 s after initiation of heating, indicating Pt−Cl bond breaking. At the same time the Pt−Cl bond distance decreases from 2.33 to 2.30 Å. The short 2.30 Å bond distance is typical for square-planar complexes in which Pt exists in the +2 oxidation state. Therefore, it can be inferred that within the first 5 s the octahedral [PtCl6]2− complex gets converted into a square-planar [PtCl4]2− complex with Pt4+ getting reduced to Pt2+.33 However, during this reduction no Pt−Pt bond distance is observed in the PDF. Thus, it can be concluded that all the Pt4+ ions get reduced to Pt2+ before further reduction of Pt2+ to Pt. After 10 s of heating, the area under the peak at 2.77 Å (Pt− Pt distances in Pt nanopaticles) begins to increase, indicating the formation of Pt nanoparticles. Different in situ experimental techniques have been applied to reveal the underlying mechanism and reaction dynamic in the nucleation and growth processes of Pt nanoparticle formation. These include smallangle X-ray scattering (SAXS),34,35 X-ray absorption fine structure (XAFS), 36 transmission electron microscopy (TEM),37−41 UV−vis,42 and hydrogen reduction.43 A similar two-step reduction mechanism was observed. However, the time scale involved in those studies is much larger (few hours) than the PDF analysis presented here (few seconds). The difficulties in the elucidation of particle formation kinetics from solution usually arise from the fact that these are fast reactions

Figure 1. Temporal evolution of the PDF during Pt nanoparticle formation. Heating was initiated after 20 s. B

DOI: 10.1021/acs.jpcc.5b03394 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C involving nucleation of only very few atoms. In situ total X-ray scattering and PDF studies open possibilities to study much faster reaction mechanisms with atomic scale resolution. The growth curve obtained from the PDF data for the Pt NPs can be fitted with a kinetic model by analyzing the area under the peak at 2.77 Å corresponding to Pt−Pt bond distances. The Avrami−Erofe’ev (AE) model of nucleation and growth was used to fit the data. However, a simple AE model was unable to fit the rising slope at the later time of the reaction (see Supporting Information Figure S3). Therefore, a modified AE equation was introduced to fit the data.44−49

Å. It is also seen that there is no appearance of new peaks in the PDF after 10 s of heating, which indicates no formation of new bonds. However, the peak area at 2.36 Å decreases very rapidly during the first 10 s of heating (Figure 4c). Therefore, it may be inferred that Pt−Cl/Gd−O bond breaking is occurring during the initial period of heating. After 10 s a new peak is observed at 2.77 Å, which can be assigned to Pt−Pt, Pt−Gd, or Gd−Gd bond distances. Similar behavior was observed in crystallization of pure Pt NPs from the ethanol solution of H2PtCl6·3H2O (see above). As the reaction progresses, the intensity of the peak at 2.77 Å increases whereas the peak intensity at 2.36 Å decreases. The peak at 1.70 Å in the PDF does not correspond to any bond length in the structures described above, but it is likely stemming from a Pt−O bond. The normal Pt−O distance is 1.97 Å, but shorter Pt−O bond distances have been observed in thin films of Pt, when the surface of the film is partially oxidized.50,51 It may be argued that the Pt−Gd alloy surface is oxidized due to the presence of lattice water in the starting material, and indeed the peak at 1.70 Å can be modeled with a PtO2 structure in which Pt is tetrahedrally coordinated by oxygen. Although this model accounts for the position of the peak at 1.70 Å, the intensity of the peak could not be fitted until the occupancies of Pt and O were fixed at 0.5. This implies that only part of the surface is oxidized. It is noteworthy that there is no peak observed at 3.42 Å, which corresponds to the Pt−Pt distance in the PtO2 model (see Supporting Information, Figure S4). Thus, the surface layer is limited to only one layer of oxygen. In other words, the surface oxygen is only bonded to Pt atoms which are inside the alloy nanoparticle. It has been found that these surface bonded oxygen atoms enhance the catalytic acitvity of the Pt nanoparticles.51,52 It is indeed remarkable that the PDF data are able to detect a partial surface layer of oxygen on very small nanoparticles. A representative data set obtained 1200 s (20 min) after initiation of the reaction was subjected to both PDF and PXRD analysis to determine the precise crystal structure of the Pt−Gd nanoparticles. The PXRD data can be indexed in cubic space group Fm-3m with a cell parameter of a = 4.0160(1). Since the starting composition of the solution was in 1:1 molar ratio of Pt and Gd, and the cell parameter for pure Pt is 3.912(3),53 it may be argued that Gd is substituted in the Pt lattice. Full pattern Rietveld refinement was carried out with two different structural models (Figure 5). The first model is for Pt0.5Gd0.5, where Pt and Gd atoms are associated with the 4a site having an occupancy of 1/2. Isotropic thermal parameters were refined, and the final Rp value was 0.0175. In a second model the crystal structure of Pt3Gd was used (cubic space group Pm3m), and this resulted in a final Rp value of 0.0245. In Pt3Gd, the Gd atom is at the origin (0 0 0), whereas Pt occupies the face center (0 1/2 1/2). As shown in the Supporting Information (Figure S5), the simulated PXRD patterns for these two structures are very similar, and it is therefore very difficult to elucidate the correct structure of the Pt−Gd NPs. In fact, one may conclude that for NP systems with very broad peaks and weak scattering it is probably ambiguous to correctly assign the crystal structure based on PXRD data only. For comparison, PDF analysis was carried out using the same two crystal structure models (Figure 6). The resulting Rw for Pt0.5Gd0.5 structure is 0.54, whereas for Pt3Gd it is 3 times lower at 0.18. This is a highly significant difference in the fit, and the PDF data therefore allows us to conlude that the Pt−Gd NPs crystallize in space group Pm-3m with the Pt3Gd structure. As a

⎡ ⎤ n t − tOR Y = [1 − e−(kt ) ] + ⎢ ⎥k OR − 2 w ( t − t ) OR ⎦ ⎣1 + e

In this model the first term corresponds to the nucleation and aggregative particle growth, while the second term accounts for Ostwald ripening. At a given time t, k is the rate parameter for the nucleation and n is the dimentionality of the reaction. tOR and kOR are the time of the onset of Ostwald ripening and a rate parameter. The start of the heating was set to 0 s for the fitting of the data, and as shown in Figure 3 the modified AE model

Figure 3. PDF peak area for the Pt−Pt bond fitted with a modified AE model (see text).

provides an excellent fit to the Pt−Pt peak area data giving the parameters k = 2.43 × 10−2 s−1, n = 4.3, kOR = 5.4 × 10−3 s−1, and tOR = 40 s. The fitted model suggests that Pt nucleates continuously followed by aggregative particle growth and subsequent Ostwald ripening, which is consistent with previous literature on the Pt nucleation mechanism.36 This demonstrates that the PDF data can be used to probe the initial growth mechanism of NPs even at a time scale of seconds. In order to synthesize Pt−Gd alloy nanoparticles H2PtCl6· 3H2O and GdCl3·6H2O (1:1 molar ration) were dissolved in EtOH and then heated under high-pressure conditions using the same in situ reactor as for the Pt NPs. Figure 4a shows the evolution of the PDF with respect to time at 250 °C obtained for the dissolved precursor solution. The heater was switched on at t = 30 s. The reaction mechanism seems to be closely related to that of pure Pt NPs. In the ethanol solution both [PtCl6]2− and [GdCl2O6]− complexes exist, and they can be modeled with the PDF data (Figure 4b). The peak at 2.36 Å corresponds to both the Pt−Cl and Gd−O bonds while the broader feature at ca. 3.4 Å corresponds to ligand−ligand correlations. It is evident that there is no long-range ordering in the precursor solution as there is no correlation seen beyond 4 C

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Figure 4. (a) Evolution of PDF with respect to time for Pt-Gd NPs. Heating was initiated after 30 s. (b) PDF for the precursor solution fitted model consisting of [PtCl6]2− and [GdCl2O6]− ions. (c) Peak area at 2.36 Å (Pt−Cl/Gd−O) and 2.77 Å (Pt/Gd−Pt/Gd) as a function of time. Initiation of heating was set to 0 s for this plot.

Figure 5. Rietveld refinements using structural models of Pt0.5Gd0.5 (top) and Pt3Gd (bottom).

Figure 6. PDF refinements using structural models of Pt0.5Gd0.5 (top) and Pt3Gd (bottom).

particle size was found to be 4.9(1) nm after PDF fitting. The structures of Pt and Pt3Gd are quite similar (Figure 7), and the difference in bond distances between Pt3Gd and Pt are small and beyond the resolution limit of the current experiment.

check of this conclusion, the data were also fitted using only a Pt structure, and this also resulted in a much higher Rw of 0.41. Thus, in the present NPs, Gd is at the origin and Pt at the face center positions giving the Pt3Gd stoichiometry. The final D

DOI: 10.1021/acs.jpcc.5b03394 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Danish National Research Foundation (DNRF93) and Danscatt. We thank DESY, Germany, a member of the Helmholtz Association (HGF), for beamtime.

(1) Stepanov, A.; Golubev, A.; Nikitin, S.; Osin, Y. A Review on the Fabrication and Properties of Platinum Nanoparticles. Rev. Adv. Mater. Sci. 2014, 38, 160−175. (2) Yu, W.; Porosoff, M. D.; Chen, J. G. Review of Pt-Based Bimetallic Catalysis: From Model Surfaces To Supported Catalysts. Chem. Rev. 2012, 112, 5780−5817. (3) Zhang, S.; Yuan, X.-Z.; Hin, J. N. C.; Wang, H.; Friedrich, K. A.; Schulze, M. A Review of Platinum-Based Catalyst Layer Degradation in Proton Exchange Membrane Fuel Cells. J. Power Sources 2009, 194, 588−600. (4) Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.; Tessema, M. M. Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis. J. Am. Chem. Soc. 2012, 134, 8535−8542. (5) Wu, J.; Yang, H. Platinum-Based Oxygen Reduction Electrocatalysts. Acc. Chem. Res. 2013, 46, 1848−1857. (6) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. Anisotropic Metal Nanoparticles: Synthesis, Assembly, and Optical Applications. J. Phys. Chem. B 2005, 109, 13857−13870. (7) Wang, Y.-J.; Wilkinson, D. P.; Zhang, J. Noncarbon Support Materials for Polymer Electrolyte Membrane Fuel Cell Electrocatalysts. Chem. Rev. 2011, 111, 7625−7651. (8) Stephens, I. E. L.; Bondarenko, A. S.; Gronbjerg, U.; Rossmeisl, J.; Chorkendorff, I. Understanding the Electrocatalysis of Oxygen Reduction on Platinum and Its Alloys. Energy Environ. Sci. 2012, 5, 6744−6762. (9) Anderson, A. B.; Grantscharova, E.; Seong, S. Systematic Theoretical Study of Alloys of Platinum for Enhanced Methanol Fuel Cell Performance. J. Electrochem. Soc. 1996, 143, 2075−2082. (10) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556. (11) Gyenge, E.; Atwan, M.; Northwood, D. Electrocatalysis of Borohydride Oxidation on Colloidal Pt And Pt-Alloys (Pt-Ir, Pt-Ni, and Pt-Au) and Application for Direct Borohydride Fuel Cell Anodes. J. Electrochem. Soc. 2006, 153, A150−A158. (12) Teliska, M.; Murthi, V. S.; Mukerjee, S.; Ramaker, D. E. Correlation of Water Activation, Surface Properties, and Oxygen Reduction Reactivity of Supported Pt−M/C Bimetallic Electrocatalysts Using XAS. J. Electrochem. Soc. 2005, 152, A2159−A2169. (13) Cui, C.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional Segregation in Shaped Pt Alloy Nanoparticles and Their Structural Behaviour during Electrocatalysis. Nat. Mater. 2013, 12, 765−771. (14) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. Role of Structural and Electronic Properties of Pt And Pt Alloys on Electrocatalysis of Oxygen Reduction an In Situ XANES and EXAFS Investigation. J. Electrochem. Soc. 1995, 142, 1409−1422. (15) Escudero-Escribano, M.; Verdaguer-Casadevall, A.; Malacrida, P.; Grønbjerg, U.; Knudsen, B. P.; Jepsen, A. K.; Rossmeisl, J.; Stephens, I. E.; Chorkendorff, I. Pt5Gd as a Highly Active and Stable Catalyst for Oxygen Electroreduction. J. Am. Chem. Soc. 2012, 134, 16476−16479. (16) http://www1.asminternational.org/asmenterprise/apd/: Alloy Phase Diagram Database. (17) Kim, H. The Role of the Local Structure in Electronic Properties of Various Materials; ProQuest: Ann Arbor, MI, 2007. (18) Billinge, S. J. L. The Nanostructure Problem. Physics 2010, 3, 25.

Figure 7. Crystal structures of Pt3Gd and Pt.

However, the coordination number of Pt in Pt3Gd is different than in the case of Pt. In the pure Pt structure each Pt is coordinated by 12 Pt while in Pt3Gd some Pt are coordinated by 6 Pt and some by 6 Gd. This results in the difference in the intensity of the PDF peak as it depends on the type of atom pairs. The difference curve (Figure 6a) in the PDF fit clearly shows that the intensity does not match properly in the case of pure Pt whereas it matches when the Pt3Gd structure is fitted (Figure 6b).

4. CONCLUSION In the present work PDF analysis is used to determine the reaction mechanisms for nucleation and the growth of Pt and Pt3Gd nanoparticles. In both cases an octahedral Pt4+ platinic acid precursor complex is first reduced to a square-planar Pt2+ complex before further reduction allows formation of metallic nanoparticles. Once the pristine NPs are formed it is possible to follow the growth of the NPs using the PDF peak area of the Pt−Pt bond. In contrast to previous studies of Pt NP growth, it is quite remakarble that the PDF data can probe growth mechanisms under solvothermal conditions at a time scale of seconds. It is suggested that crystal growth takes place as a combination of aggregation and ripening. The PDF data furthermore show that a partial layer of oxygen exists on the surface of the NPs. Because of the similarity between the Pt0.5Gd0.5 and Pt3Gd crystal structures, it is not possible based only on Bragg diffraction data (PXRD) to assign a crystal structure to the Pt−Gd alloy NPs. In fact, if strictly taking the lowest residual from Rietveld refinements (Rp = 0.0175 for Pt0.5Gd0.5 versus 0.0245 for Pt3Gd), one would assign the structure to Pt0.5Gd0.5. On the other hand, the PDF data very clearly distinguish between the two structure and show beyond doubt that the present NPs have the Pt3Gd crystal structure.

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ASSOCIATED CONTENT

S Supporting Information *

Refinement parameters for the final phase, TEM images, Gaussian peak fits, simple AE growth model, simulated PXRD and PDF patterns, time-resolved PDF patterns. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03394.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.B.I.). Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.jpcc.5b03394 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b03394 J. Phys. Chem. C XXXX, XXX, XXX−XXX