Highly Uniform Platinum Icosahedra Made by Hot Injection-Assisted

May 6, 2013 - The above results indicate that subtle changes of key reaction conditions make a big difference in the outcome for the formation of Pt n...
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Highly Uniform Platinum Icosahedra Made by Hot Injection-Assisted GRAILS Method Wei Zhou,†,‡ Jianbo Wu,† and Hong Yang*,† †

Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana−Champaign, 114 Roger Adams Laboratory, MC-712, 600 S. Mathews Avenue, Urbana, Illinois 61801, United States ‡ School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China S Supporting Information *

ABSTRACT: Highly uniform Pt icosahedral nanocrystals with an edge length of 8.8 nm were synthesized in nonhydrolytic systems using the hot injection-assisted GRAILS (gas reducing agent in liquid solution) method. The results show the key factors for the shape control include fast nucleation, kinetically controlled growth, and protection from oxidation by air. The effect of oxygen molecules on the Pt morphology was experimentally confirmed based on the study of shape evolution of icosahedral crystals upon exposure to oxygen gas. The Pt icosahedral catalysts obtained had an areaspecific activity of 0.83 mA/cm2 Pt, four times that of 0.20 mA/cm2 Pt for typical Pt/C catalysts, in an oxygen reduction reaction (ORR). KEYWORDS: Pt, nanocrystal, icosahedron, cube, carbon monoxide, oxygen reduction reaction, ORR

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are not stable due to the high overall surface energy, and the twin planes need to be stabilized through reconstruction to lower surface energy. This paper describes an optimized reaction system that satisfies the reaction conditions for making Pt icosahedral nanocrystals with high-level shape uniformity. We have identified the key factors that influence the formation and stability of icosahedra, thus experimental controls on a range of Pt nanostructures. Pt icosahedra were made from platinum acetylacetonate (Pt(acac)2) in dodecylamine (DDA) and with a small amount of oleic acid (OA) using a hot injection-assisted GRAILS (gas reducing agent in liquid solution) approach (see Supporting Information for details). CO was used as the reducing and cocapping agent and yttrium acetylacetonate (Y(acac)3) as the additive. Figure 1 shows the transmission electron microscope (TEM) micrographs and composition analysis of the icosahedral NPs. The uniform Pt icosahedra had an edge length of 8.8 ± 0.5 nm (Figure 1a). The population of icosahedral shape was about 95% based on the analysis of TEM micrographs at low magnifications (Figure S1). The twin boundaries were clearly observed in all the nanocrystals, and the high-resolution TEM (HRTEM) micrograph shows the characteristic feature with electron beam along 2-fold axis of icosahedral NPs (Figure 1b).3,4 The inset shows the ball model illustration of the atomic arrangement of a Pt icosahedron. The lattice spacing was determined to be ∼0.21 nm, slightly smaller

lthough polyhedral nanostructures, such as cube, tetrahedron, octahedron, cuboctahedron, and even icosahedron have been synthesized for several noble metals, uniform Pt icosahedra are rarely made.1,2 Only very recently, the Yan group reported the synthesis of Pt icosahedral nanoparticles (NPs) through a week-long reduction of a Pt(II) complex precursor.3 The Murray group also synthesized a series of Pt nanostructures including icosahedron in a nonaqueous reaction system.4 However, these icosahedra formed along with a considerable amount of branched particles or truncated cubes and needed to be separated from the byproduct. There is no comprehensive understanding so far on the factors that govern the formation of Pt icosahedral NPs. Pt icosahedral NPs do not form readily. Theoretical simulations and previous experimental data suggest Pt icosahedral clusters (Pt12 or Pt13 isomer, ∼0.75 nm) can exist, because they have the lowest energy.5 Simulation results further suggest that Pt with 22−56 atoms can still exist in the form of icosahedron based on its global energy minimum.6 The formation of multiply twinned icosahedral nanocrystals, however, is detrimental for the stability of Pt nanoparticles beyond this size due to the greatly increased strain energy caused by the twin defects;1 despite such crystals are bounded by the low surface energy {111} facets.7,8 It is suggested that through careful control of kinetics one may create conditions favored the formation of icosahedra.1 In such systems, nucleation often needs to be separated from growth in order to facilitate the uniform deposition of monomers on icosahedral seeds to form large NPs. Experimentally, slow reduction alone does not seem to be sufficient for the formation of icosahedral Pt NPs. While the reason is not clear, atoms at twin boundaries © 2013 American Chemical Society

Received: April 4, 2013 Revised: April 25, 2013 Published: May 6, 2013 2870

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Additional experiments were performed to better understand the formation of Pt icosahedral NPs (see the Supporting Information). In the GRAILS approach, CO gas is essential in controlling the formation of well-defined shapes. Without CO gas, branched structures of nanorod were formed (Figure 2a).

Figure 1. (a) TEM micrograph of Pt icosahedra with an edge length of 8.8 nm. (b) HRTEM micrograph of a typical icosahedral nanocrystal (The inset is a ball−model illustration). (c) XRD pattern of Pt icosahedra. The standard diffraction (JCPDS No. 04-0802) is drawn using red lines. The inset shows the metal amount of Pt and Y elements (scale bar: 10 nm). (d) TEM micrograph of Pt icosahedra with an average edge length of 18 nm synthesized without using Y(acac)3.

than the bulk value for Pt (111) plane, which could be ascribed to the strain in this multiply twinned structure.9 The X-ray diffraction (XRD) pattern shows peaks with a small right shift, indicating the contraction of lattice distance, an observation agreed well with the HRTEM study. The relative intensity for the Pt (111) plane was higher than that for the standard fcc structured Pt (red lines, JCPDS No. 04-0802) (Figure 1c), further suggesting these icosahedra NPs are enclosed by {111} facets. Energy dispersive X-ray (EDX) elemental analysis on individual particles shows the Pt/Y atomic ratio was 98.41/ 1.59 (inset of Figure 1c), which closely agreed to the metal composition of Pt/Y (98.44/1.56) obtained using the particle ensemble (Figure S2). Electron energy loss spectroscopy (EELS) study shows Y M4,5 peaks appeared at 162 and 165 eV, respectively, indicating the dominate species is Y3+ ion (Figure S3).10 A control experiment of reacting Y(acac)3 with CO gas under the same reaction condition showed no formation of solid product. These results suggest that Y3+ ions were most likely adsorbed on the Pt surfaces, though it could not be ruled out that surface Y3+ species might diffuse into Pt nanocrystals. Surface-adsorbed metal species are known to have strong impacts on the shape control of Pt nanocrystals.11 Thus, Y(acac)3 functioned as a co-capping agent in this regard. Pt icosahedra with low relative population (∼50%) were obtained for the sample made under the same condition except in the absence of Y(acac)3 (Figure S4a). The rest was composed of cubes, overgrown icosahedra, and twinned rods. When the temperature of Pt(acac)2 solution in DDA and trace OA changed from 160 to 135 °C without Y(acac)3, Pt icosahedra with an edge length of ∼18 nm were obtained with shape selectivity of ∼50% (Figures 1d and S4b) (see the Supporting Information for the synthetic procedure).

Figure 2. TEM micrographs of Pt nanocrystals obtained under different conditions: (a) branched rods synthesized without CO gas, (b) single rod showing its single-crystalline structure grown along ⟨111⟩ direction, (c) cubes synthesized without hot injection, and (d) octapods made without degassing with argon.

HRTEM micrograph shows these nanorods were singlecrystalline and grew along ⟨111⟩ directions (Figure 2b). Similar rod-like Pt structures were observed in other nonaqueous phase syntheses.8,12 In the standard procedure for making uniform Pt icosahedra, a preheated Pt(acac)2 solution in the mixture of DDA and OA was injected to CO-saturated mixture of DDA, Y(acac)3, and DPE at 210 °C. However, if these two solutions were mixed at room temperature before transferring the flask to the oil bath at 210 °C, Pt cubes with a typical edge length of 12 nm were obtained (Figure 2c). In addition, if the reaction mixture was not degassed with argon to remove the air before the reactions, octapods formed from the cubes (Figure 2d). In all of these conditions, control over the shape uniformity was achievable. The above results indicate that subtle changes of key reaction conditions make a big difference in the outcome for the formation of Pt nanostructures. Figure 3 illustrates the mode of formation of these Pt nanocrystals and the key factors for the preparation of Pt icosahedra. In the case of Pt icosahedra, when the Pt(acac)2 solution was preheated at 160 °C and subsequently injected to the preheated and CO-saturated reaction mixture at 210 °C, the salt was quickly reduced and formed metal clusters after quickly reaching the supersaturation condition. The small clusters had the tendency to form twinned icosahedral seeds to meet the requirement of global energy minima under the fast 2871

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Figure 3. Mode of formation for various Pt nanocrystals based on the hot injection-based GRAILS method: (a) icosahedra, (b) hyper-branches, (c) cubes, and (d) octapods. Saturation with CO, rapid nucleation for the formation of twinned seeds, and CO-mediated growth were necessary for the formation of Pt icosahedra. The effect of O2 based on the Pt−O affinity is illustrated in part d. See the main text for details.

190−210 °C).4 Under the slow nucleation condition, the solution turned gray about 5−10 min after the reaction flask (at room temperature) was moved into the oil bath at 210 °C. Single crystal seeds in truncated octahedral or cuboctahedral shapes formed (Figure 3c).16,17 The growth was mediated by CO gas and preferably along one low-index direction. In this system, the growth rate along ⟨111⟩ direction was faster than that along ⟨100⟩ direction, resulting in the formation of cubes.1,2,19,20 Pt and Pt alloy nanocubes were synthesized using the CO-based GRAILS method before.4,13 In addition, oxygen gas needs to be removed from the reaction mixture as it is detrimental for the formation of seeds with twin planes and the growth of icosahedra, similar to those situations where the oxidative etching (O2/Cl−) took place.1,21 Without degassing with argon, the twinned seeds were unstable due to the dissolved oxygen in the solution, and only the singlecrystal seeds survived.16 The change in the seed structure and preferred adsorption on (111) facets led to the overgrowth of octapods (Figure 3d). In this system, the main interaction occurred between surface Pt and adsorbed O atoms; that is, the surface Pt atoms could interact with unshared electron pairs of oxygen, as illustrated in Figure 3d.22 This interaction could change the deposition of monomers; thus the growth of Pt crystals. Among the low-indexed planes, the (111) surface has the highest density of surface atoms23 and thus is most sensitive to oxygen and easiest for deposition of monomers among the low-indexed surfaces. This structural difference should be the main contributing factor on why most of the Pt nanocrystals grew along the ⟨111⟩ direction.1,2,12,16,17 Besides oxygen, organic ligands and ionic additives could also induce preferred growth of Pt nanocrystals along ⟨111⟩ directions.16,17,24 The experimental results indicate that the O2-mediated reaction pathway prevailed over the effect of CO gas, leading to the formation of octapods (Figure 3d).

nucleation process.6 Indeed, under this reaction condition, the solution turned into a gray color within 1 min after the injection of salt precursors. In the GRAILS approach, CO gas acts not only as a reducing agent, but also a structurecontrolling agent through its highly selective interactions with different Pt planes.13,14 Our data indicate that the adsorbed CO, together with Y(acac)3 and organic capping ligands, greatly affected the reaction kinetics.11,15 Under the flow of CO gas, deposition of monomers on all surfaces was feasible without suffering from the oxidative reactions at the twin boundary regions that are typically highly reactive; thus icosahedral nanocrystals formed (Figure 3a). A control experiment with shortened reaction time was performed to test the process of growth. The mixture was kept under the same conditions for only 5 min instead of 30 min after injection, and icosahedra with edge length of ∼5.5 nm were obtained (Figure S5a). This edge length is noticeably smaller than those formed under the aforementioned synthetic procedures. In this case, rapid nucleation for twinned icosahedra seeds and kinetically controlled growth process are required for the formation of Pt icosahedra, which is the result of synergistic effects from CO, and other mediating agents, such as Y(acac)3 and long-chain amines. The clusters with a twinned structure grew into single-crystal seeds through a slow nucleation without CO gas, as observed previously.16 TEM studies show that the seeds in Figure 3b were composed of most single-crystal seeds, though there were still a small number of twinned seeds (Figure S5b). In the absence of CO gas monomers deposited on the seeds along a few crystallographic directions to form the hyper-branched structures (Figure 3b). Similar structures have been observed before.12,17,18 Burst nucleation was necessary for the formation of highly uniform Pt icosahedral seeds, which was achieved using the temperature jump or hot injection approach (from 160 °C to 2872

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protecting the reaction mixture from O2 (or air) is important in the synthesis of icosahedra. With the exposure to O2 gas, a variety of branched Pt structures were obtained from the mixtures of Pt(acac)2 and DDA at different temperatures (Figure S8). A structural feature of icosahedron is its surfaces enclosed by the {111} facets. This is important for applications in electrocatalysis in that the extended Pt (111) surface was reported to be nine times more active than the highly active commercial Pt/C catalysts toward the oxygen reduction reaction (ORR).26 The electrocatalytic properties of icosahedra, cubes (Figure 2c), and 3 nm Pt NPs on carbon (Figure S9) were studied using a three-electrode system. Figure 5 shows the

We performed additional experiments to study the effect of O2 gas on the shape evolution of icosahedra (see the Supporting Information for experimental detail). Figure 4

Figure 4. TEM micrographs of Pt icosahedra made by bubbling O2 gas at 210 °C for (a) 15 and (b) 30 min, respectively. (c) HRTEM micrograph showing twinned crystal morphology after reconstruction and overgrowth of icosahedron. (d) Morphological evolution for icosahedron and twinned rods upon exposure to O2.

Figure 5. ORR polarization curves for icosahedra (blue), cubes (red), and commercial (black) Pt catalysts. The upper inset shows their corresponding CV curves. The inset at the bottom shows their specific-area activities at 0.9 V.

shows the change in morphology of Pt icosahedra upon exposure to O2 gas in the solution after their formation under the typical CO-based GRAILS condition. After being exposed to O2 at 210 °C for 15 min, many of the icosahedral NPs turned into other morphologies (Figure 4a). At 30 min, most of the icosahedra changed their shape with only a small population preserved the morphology (Figure 4b).25 TEM micrograph shows the formation of a low-symmetry morphology from an individual icosahedron (Figure 4c). The multiply twinned structure was clearly observed after the reconstruction and overgrowth (also see Figure S6). The d-spacing was 0.22 nm, corresponding to the (111) planes. In the same synthesis, several other related morphologies could be observed. Figure 4d shows the evolution of two kinds of twinned structures. Both multiply twinned icosahedra and nanorods were found to turn into branched structures.17 HRTEM study confirmed the twinned structure in the nanorod (Figure S7). These observations clearly demonstrate the strong effect on shape control by introducing O2 gas. The surface atoms, especially those at the twin boundaries, were active and might migrate to relatively stable sites. These atoms are opted to reconstruct under the influence of O2, resulting in the formation of stable single-crystal cubes. The experimental results indicate the stability upon exposure to O2 followed the order: cube > twinned rod > icosahedron. After exposure to O2 for 15 min, most icosahedra changed their morphology, and the population of cubes and twinned rods increased. When the exposure time increased to 30 min, short branches formed on the twinned rods. If the reaction prolonged to 60 min, the reconstruction and overgrowth of cubes were observed, leading to the formation of octapod-like morphology (Caution: sudden boiling may occur at this step).4,16 Thus,

polarization and cyclic voltammetry (CV) curves of Pt icosahedra/C, cubes/C, and Pt/C reference. As shown in the CV curves, the main peak at 0.25 V for cube was due to the hydrogen adsorption on the (100) facets of Pt. The icosahedral Pt/C has a featureless box-like broad hydrogen adsorption peak on (111) surface between 0.1 and 0.3 V as well as OH adsorption of (111) surface at 0.7 V.26,27 Both of them indicated the icosahedron has the (111)-dominated surfaces. The on-set potentials for the ORR polarization curves follow the order of icosahedra > cubes > Pt reference (commercial). The specific surface activity reached 0.83 ± 0.14 mA/cm2Pt for Pt icosahedra and 0.35 ± 0.06 mA/cm2Pt for Pt cubes. These results indicate that the intrinsic ORR activity of (111) surface capped Pt icosahedra was about 4 times that of Pt/C reference catalyst (0.20 ± 0.03 mA/cm2Pt), and about half of the value of vacuum-generated, extended single-crystal (111) surface of Pt.26 Recent modeling and experimentally data suggest that the twinning in NPs of Pt and some Pt alloys help in shifting the binding energy of reacting intermediates due to the surface and strain effect of NPs.26,28,29 Thus, the muchimproved ORR activity for icosahedral NP catalysts could be attributed to the {111} facet nature and existence of twin planes in this kind of NP electrocatalysts. To verify the stability of the Pt icosahedra/C catalysts during cycling, samples were collected and examined using TEM in O2 atmosphere for 2500 CV cycles (Figure S10). The icosahedral morphology of most of nanoparticles remained or slightly changed, showing good shape stability. In conclusion, highly uniform Pt icosahedra were synthesized using the hot injection-assisted GRAILS approach. Rapid nucleation, kinetically controlled growth, and protection from 2873

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O2 are critical for the preparation of Pt icosahedra at high population. The synergetic effect of CO, additives and longchain amine was clearly demonstrated. By adjusting these key parameters, Pt hyper-branched rods, cubes, and octapods were also obtained. A mode of formation for different Pt nanocrystals including icosahedra was established based on the experimental data. In addition, favored growth along ⟨111⟩ direction in Pt nanocrystals is explained based on the role of different interactions between O and low-indexed Pt planes. The much improved area-specific ORR activity of icosahedral Pt/C catalysts over the commercial Pt/C catalysts can be attributed largely to the (111) facet and twinning-assisted, strained surface lattice of icosahedral nanocrystals. Such mechanistic understanding is valuable for the design of advanced, highperformance metal catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, TEM micrographs, EDS pattern, EELS spectrum for an individual icosahedron, and HRTEM micrographs of twinned rods. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 This work was supported by the NSF (CHE-1213926) and National Natural Science Foundation of China (51102005). The authors thank Dr. Yonghai Yue, Qian Zhang in School of Chemistry and Environment, Beihang University for EELS and EDS tests.

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REFERENCES

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