Platinum Multipods

NANO LETTERS

Synthesis of Platinum Multipods: An Induced Anisotropic Growth

xxxx Vol. 0, No. 0 A-G

Xiaowei Teng† and Hong Yang*,†,‡ Department of Chemical Engineering and Laboratory for Laser Energetics, UniVersity of Rochester, Rochester, New York 14627 Received February 17, 2005; Revised Manuscript Received March 25, 2005

ABSTRACT This paper reports a highly effective synthesis of platinum multipods from platinum 2,4-pentanedionate in organic solvents. A trace amount of silver acetylacetonate is used to trigger the nucleation and the anisotropic growth of Pt nanocrystals. The morphologies of Pt multipods made include I- and V-shaped bipods, various types of tripods, and planar and three-dimensional (3D) tetrapods. The 3D Pt tetrapods can be well-defined, resembling those observed for II−VI semiconducting materials, such as CdS and CdSe. Control of morphology and the multipodto-sphere transitions under various conditions have been systematically studied. A mode of formation based on the induced kinetically controlled growth has been discussed.

Introduction. Development of nanometer scale branched nanocrystals of metal,1-4 metal oxide,5-8 and semiconducting materials9-14 is an important research subject that has attracted a great deal of attention in recent years. In addition to their intrinsic electronic, magnetic, photonic, and catalytic properties, these branched nanomaterials have been proposed to be used as building blocks for making complex nanostructures through self-assembly processes.15 Such structures are directly relevant to the fabrication of multiple-terminal nanodevices and as active components in new photovoltaic devices.16,17 Multipods such as 3D tetrapod nanocrystals can also have advantages in the generation of hierarchical nanostructured network over their counterparts of nanorods, nanowires, nanocubes, and nanoprisms. Although well-defined tetrapods of CdSe and CdS have been developed for several years,18,19 the metal multipod and hyper-branched nanostructures have only recently been explored intensively.1-4 So far the level of control in the anisotropic growth of branched metal nanocrystals cannot match that for the growth of well-defined tetrapods of cadmium chalcogenides (Se and S).10,11 This phenomenon may be due to the fact that most metals typically do not have polymorphism. The condition for branched growth based on the lattice match at the surfaces of two crystal types (such as zinc blende and wurtzite) in nanocrystals of cadmium chalcogenides can be hard to create for metal nanocrystals through a homogeneous nucleation and growth process, though it is possible. Only gold, platinum, and rhodium have been shown to form branched metal nano* Corresponding author: E-mail: [email protected], Telephone: (585) 275-2110; Fax: (585) 273-1348. † Department of Chemical Engineering. ‡ Laboratory for Laser Energetics. 10.1021/nl0503072 CCC: $30.25 Published on Web 00/00/0000

© xxxx American Chemical Society

crystals. Although the formation mechanism is not completely clear, it appears that use of proper capping reagents such as cetyltrimethylammonium bromide (CTAB) and poly(vinylpyrrolidone) (PVP) is very important, but it is only one of the prerequisites. The kinetic control of growth process is also critically important. The tested strategies for promoting the formation of metal multipods include homogeneous seeded growth4 and heterogeneous growth on metal plates.3 Xia and co-workers have also demonstrated that the addition of NaNO3 could promote the formation of Pt octapods and tetrapods in a polyol process.1 We noted that the length of the branches was rather short in these cases. The branches were either not well defined or in a cone shape rather than straight rods, which could be related to the growth direction. In this report, we present a synthetic method for making Pt multipods through an induced anisotropic growth process at temperatures below that for the homogeneous nucleation and growth of Pt nanoparticles from platinum 2,4-pentanedionate (Pt(acac)2). A small amount of silver acetylacetonate (Ag(acac)) was used to trigger the formation of branched Pt nanocrystal, as this silver precursor could readily decompose and be reduced to form colloidal metal clusters at < 200 °C. Experimental Section. In a typical synthesis, a mixture of 1, 2-hexadecanediol (Aldrich, 90%, 800 mg), Pt(acac)2 (Gelest Inc., 200 mg), diphenyl ether (Aldrich, 99%, 2 mL), hexadecylamine (HDA, Aldrich, 90%, 4 g), and 1,2adamantanecarboxylic acid (ACA, Aldrich, 99%, 180 mg) was added into a 25 mL, three-neck, round-bottom flask under argon protection. The reaction mixture was heated and held at the designed temperature using a glycerol bath. The PAGE EST: 6.2

temperature of this oil bath was monitored by Chemglass ETC temperature controller. A small amount of Ag(acac) (Aldrich, 99%, 3 mg) was then quickly added into the flask, which resulted in a rapid formation of gray suspension, indicating the formation of nanoparticles or multipods. The color of the reaction mixture quickly turned into black. The temperature could be maintained within ( 1 °C for the entire reaction time period of 60 min. Aliquots of samples (∼200 µL) were taken every minute during the first 10 min to monitor the growth kinetics. The concentrations of reagents were fixed for all the synthesis, while the reaction temperatures were varied from 160 °C to 210 °C; unless indicated otherwise. For those secondary injection experiments, a total amount of 2.5 mL of reactant mixture (100 mg of HDA, 100 mg of Pt(acac)2, 100 mg of ACA, and 100 mg of 1,2hexadecanediol in 1 mL of diphenyl ether) was preheated to 170 °C. Three equal aliquots (∼0.83 mL) of this mixture were injected into the reaction flask at 7, 9, and 12 min after the addition of Ag(acac). After the reaction, the nanoparticles were washed and separated using chloroform and ethanol. In a typical procedure, a designed amount of the product (200 µL) was dispersed in 800 µL chloroform in a small vial (2 mL), followed by the addition of 1 mL ethanol to induce the precipitation of nanoparticles. The precipitates were separated from the solvent mixtures by centrifuge at 5000 rpm for 5 min. Yellow-brownish supernatant was decanted and the black products were recovered by dispersing in 1 mL of chloroform. This process was followed by a second wash and separation using 1 mL of ethanol. The final products were stored in chloroform. Transmission electron microscopy (TEM) specimens were prepared by dispersing the samples of nanocrystal in chloroform (∼1 mg/mL) and drop-cast onto carbon-coated copper grids. An ultrahigh vacuum scanning transmission electron microscope (UHV-STEM, Cornell VG HB501) and a regular TEM (JEOL 2000 EX) were used to examine the size and shape of the nanoparticles. The UHV-STEM was also used to conduct the energy-dispersive X-ray (EDX) analysis and nanoelectron diffraction (ED) of individual nanoparticles. The electronic gun of this STEM system could be focused into a spot with a diameter of less than 1 nm. The lattice fringes of Pt multipods were examined on a Hitachi HD-2000 STEM operating in ultra-high resolution mode at 200 kV and 30 mA. Powder X-ray diffraction (PXRD) spectra were recorded on a Philips MPD diffractometer with a Cu KR X-ray source (λ ) 1.5405 Å) at a scan rate of 0.015 2θ/s. Details of the characterization methods could be found elsewhere.20, 21 Results and Discussion. The temperature for the formation of Pt nanoparticles from Pt(acac)2 via a homogeneous nucleation and growth process was g ∼210 °C using the reaction mixture described above. This reaction temperature was similar to those reported for such systems.22 It is known that various silver salts can be reduced to silver metal readily through different pathways. We chose silver salt as the reagent to induce nucleation and growth of Pt nanocrystals from Pt(acac)2 at temperatures below 210 °C. Ag(acac) was B

used in this work because of its good solubility in organic solvents, such as diphenyl ether. The evolution of Pt multipods and nanoparticles was followed by TEM. Figure 1 shows a series of TEM images of Pt nanoparticles obtained at 180 °C for reaction times ranging from 4.5 to 60 min after a tiny amount of Ag(acac) was added into the flask. At the initial stage, largely faceted rather than spherical nanocrystals began to appear at 4.5 min after adding the silver precursor, Figure 1a. Although some morphologies resembled those that were previously observed for Pt nanoparticles,23 the embryonic forms of various multipods began to develop. The population of multipods nevertheless was rather low at this stage. These nanocrystals grew rapidly into multipods within the next minute, which was indicated by the rapid change of color from brown to black. A large area of multipods with several different shapes was observed, Figure 1b. The various morphologies included triangular nanoprisms; I- and V-shaped bipods; regular, T- and Y-shaped tripods; and planar and 3D tetrapods. As the reaction continued, the growth along the branch directions became obvious, Figure 1c and 1d. The overall length-to-width aspect ratio of the branches in Figure 1c and 1d was visibly higher than those in Figure 1b, while the diameter of branches was relatively constant. There were no obvious spherical Pt nanoparticles. This observation was very different from the Au multipods formed on Ag plates, which contained ∼40% spherical nanoparticles.2 The rod-like shape of the branches was also different from those Pt tetrapods and octapods made in the polyol process through the addition of NaNO3 in that these tetrapods were fully developed.1 After reaction for ∼10 min, no distinct growth along the branch direction was observed; while the width of each branch began to increase, Figure 1e, followed by a continuous disappearance of multipods. The multipods turned into rice-shaped particles with the dissolution and growth process, Figure 1f and 1g. Upon 40 min after adding the silver precursor, multipods evolved into spherical nanoparticles, Figure 1h. The size distribution of nanoparticles became relatively monodisperse at the reaction time of 60 min and no further morphology change was observed, Figure 1i. This shape evolution suggested that the formation of multipods should be a kinetically controlled growth and the thermodynamically stable shape of nanocrystals should be spherical. Figure 2 showed PXRD and EDX spectra of Pt multipods. All the X-ray diffraction peaks could be assigned to facecentered cubic (fcc) platinum (space group: Fm3m). No diffraction signal from silver could be observed. EDX was used to analyze the chemical composition of individual multipods. The observed EDX peaks at 2.1, 9.4, and 11.1 keV could be assigned to Pt M, LR, and Lβ lines, respectively. The peaks at 8.0 and 8.9 keV could be assigned to the copper KR and Kβ lines, respectively, which were from the substrate. No silver signal could be detected. These data indicated that the multipods were made of metallic platinum. The structure details of multipods were analyzed by nanoelectron diffraction using the UHV-STEM. Figure 3 shows the dark and bright field TEM images of a V-shaped bipod, a regular 120° tripod, and a 3D tetrapod. As discussed Nano Lett.

Figure 1. TEM images of Pt particles synthesized at 180 °C for (a) 4.5, (b) 5.5, (c) 7.5, (d) 9.0, (e) 12, (f) 15, (g) 20, (h) 40, and (i) 60 min after Ag(acac) was added. All scale bars are 20 nm.

above, there were variations of bipods, tripods, and tetrapods. The two-branched structures could exist at different angles and subsequently lead to diverse forms, ranging from V-shaped bipods, Figure 2b, to almost straight I-shaped rods. For the tripods, different branches tended to have similar length but could also have a different aspect ratio for each branch in the same tripod. The angles between neighboring branches could also change from close to 180° in a T-shaped tripod to 120° or less. The ED images taken at the tip region of each branch of the tetrapod were shown as insets of Figure 3d. By calculating the length ratio from the center spots to the adjacent spots, we obtained the zone axes of the branches,24 which were [111] axes. The high-resolution TEM image shows the V-shaped Pt bipod is single crystalline, Figure 4. The measured d spacing corresponded to 2.2 Å (or 2.2 nm for 10 consecutive lattice fringes), which could be assigned to the (111) plane. These results indicated that the growth of the branches was along 〈110〉 directions. It has been reported that the formation of metallic multipods and controlled platonic shapes could be associated with the competitive growth between (111) and (100) planes.1,2,25 In the case of formation of gold tripods and planar tetrapods in aqueous solution using pre-synthesized silver plates as seeds,2 the growth was preferentially inhibited on (111) planes, and the particles ended up with cube shapes Nano Lett.

Figure 2. PXRD (top) and EDX (bottom) spectra of Pt multipods. The samples used were synthesized at 180 °C for 9 min after the addition of Ag(acac). C

Figure 3. (a) Dark and (b-d) bright field UHV-STEM images of Pt multipods synthesized at 180 °C. The insets show the nanodiffraction patterns used to determine the crystallographic direction (zone axis) of each branch. The 10 nm scale bars apply to all images.

Figure 4. High-resolution TEM image of a V-shaped Pt bipod showing the (111) lattice fringes. The labeled length is for 10 repeating fringes.

showing the (100) planes. On the other hand, when growth on (100) planes was inhibited, the resultant particles terminated by the (111) faces. When growth on both (111) and (100) planes was inhibited, (110) planes were dominant, which led to the formation of tripods. Under our synthetic conditions, the formation mechanism of Pt multipods appeared to agree with those for Au multipods. In the presence of surfactants, differences in the adsorption of surfactants on the various crystal planes led to the competitive growth. In our case, It seems that the branches grew preferentially along 〈110〉 directions. This could not be explained entirely by the steric hindrance in the case for typical Au and Pt nanoparticles.1-3,23,26-28 The formation of

Pt nanocrystals triggered by the trace amount of silver could play an important role in directing the preferential growth along a particular crystallographic direction. Although no silver could be detected in the Pt multipods, we noted no multipod formation without using silver precursors. In addition, the Ag seeds have been used in the synthesis of different metal particles.2,28-30 Addition of Ag(acac) into hot organic solvent led to the formation of silver clusters that promoted the Pt precursors decomposition and growth of Pt on the low-index planes. These Ag clusters could contain as few as four atoms.30 This seeded growth process allowed the Pt nanocrystal to form below the nucleation and growth temperature in the absence of silver species, which was experimentally determined to be about 210 °C in our reaction system. The growth kinetics could differ among the various low-index planes. Reaction temperature was another important parameter for the kinetically controlled process, thus one would expected it could affect the shape evolution of Pt nanocrystals. Figure 5 shows the typical TEM images of Pt nanoparticles obtained at various times between 3.5 and 20 min after Ag(acac) was added for the reactions conducted at 190 and 200 °C, respectively. The TEM data showed that the shape evolution from Pt multipods to spheres had a trend similar to those reactions conducted at 180 °C. The optimal reaction times for the formation of high population and well-defined multipods were different from those formed at 180 °C. Large populations of multipods appeared as early as 3.5 min after the silver precursor added at 190 °C. The multipods began to turn into spheres at ∼12 min. For those formed at 200

Figure 5. TEM images of Pt multipods and nanoparticles synthesized at (a-e) 190 and (f-j) 200 °C after the addition of Ag(acac) for given periods of times: (a, f) 3.5, (b, g) 5.5, (c, h) 7.5, (d, i) 12, and (e, j) 20 min. All scale bars are 20 nm. D

Nano Lett.

Figure 6. Effect of temperature on the shape of Pt nanoparticles and multipods. No Ag(acac) was added for reaction conducted at 210 °C, as particles formed spontaneously at this temperature.

°C, Pt multipods began to change their shapes as early as ∼7.5 min after the silver precursor was added. These results indicated that anisotropic growth disappeared at an earlier time with the increase of reaction temperature. A wide range of reaction times and temperatures was examined, and the results are summarized in Figure 6. The classification of shapes for each data point shown in this figure was based on a series of TEM studies. Three distinct regimes were observed for these reaction systems at the different temperatures, i.e., no particles, multipods, and spheres. No Pt particles formed for the entire reaction period up to 60 min, when the reaction temperatures were between 160 and 175 °C. Multipods were formed at 2.5 to 4.5 min after the addition of Ag(acac) for the reactions conducted between 180 and 200 °C. The multipods disappeared and turned into spheres after reacting for additional 10 to 15 min. The higher the reaction temperatures were, the earlier the Pt multipods formed and the sooner they evolved into spheres. We noted that the temperatures used to synthesize Pt particles were lower than that for the decomposition of Pt(acac)2 in the presence of HDA and ACA, which began at 210-220

°C.22 It was plausible that silver might work not only to induce the formation of nucleation seeds2,28-30 but also to function as catalysts to facilitate the formation of Pt particles below the decomposition temperature of Pt(acac)2. Recently, Song et al. reported the synthesis of platonic shapes of Pt nanoparticles with the addition of Ag precursors.28 Silver species such as Ag42+ could adsorb on (100) planes of Pt particles and promote the reduction of Pt salts preferentially on (100) planes that adsorbed these Ag species continuously. Further experiments are needed to determine if this mode of formation is reasonable in our system. The transitions from multipods to spherical nanoparticles could be interpreted by the Ostwald ripening.31 In these cases, there was a critical size at the equilibrium state.10,22,31 Particles that were smaller than the critical size dissolved while the large ones continued to grow. At the early stages after nucleation, the kinetic growth governed the shapes of Pt particles, resulting in multipod structures. When the monomer depleted, the ripening process became dominant. The branches of multipods began to dissolve, subsequently resulting in the formation of spherical particles at relatively large diameters. A thermodynamically controlled process should be favored when thermal energy is sufficiently high. Particles tend to minimize the surface energy through the transformation from the highly faceted multipods to spheres that possess lower specific surface area than multipods. The temperaturedependent transitions observed in our results could be interpreted by this formation mechanism. We further investigated if the addition of Pt(acac)2 at the various stages of the reaction could prolong the growth of multipods. A reaction mixture with total amount of 100 mg of Pt(acac)2 was injected into the reaction flask in three equal aliquots at 7, 9, and 12 min, respectively, after the addition of Ag(acac). To minimize the formation of new nuclei during the secondary injections, less than 40% of the existing reaction precursor was suggested in each addition.32 In our case, each aliquot was equal to ∼17% of the total amount

Figure 7. TEM images showing the shape evolution of Pt multipods and nanoparticles after multiple secondary injections of reaction mixtures at 180 °C. The TEM images were taken at (a) 7.0, (b) 8.5, (c) 11, (d) 15, (e) 20, (f) 30, (g) 40, and (h) 60 min, respectively. A mixture of HDA (100 mg), Pt(acac)2 (100 mg), ACA (100 mg), 1,2-hexadecanediol (100 mg), and diphenyl ether (1 mL) was preheated to 170 °C and injected into the reaction flask in three equal aliquots at 7, 9, and 12 min after the reaction, respectively. All scale bars are 20 nm. Nano Lett.

E

Figure 8. TEM images showing the effect of Pt(acac)2 amount on the formation of Pt multipods. The amounts of Pt(acac)2 used were (a) 50, (b) 100, and (c) 200 mg, respectively. All scale bars are 20 nm.

of Pt(acac)2 in the reaction flask prior to the secondary injection. Figure 7 shows the TEM images of Pt nanocrystals synthesized at 180 °C for the reaction time periods of 7 (prior to the first time of secondary injection), 8.5, 11, 15, 20, 30, 40, and 60 min, respectively. Our results indicated that the growth process of multipods could be extended using the multiple injection approach, and the transition time from multipods to spherical particles changed to 30 min after the initial addition of Ag(acac). Some multipods could still be observed, even when the reaction time reached 60 min. In comparison, without the multiple injection, the multipodto-sphere transition happened at ∼20 min after the initial addition of Ag(acac). The effect of the amount of Pt(acac)2 used on the shape evolution of particles was also investigated. Pt(acac)2 precursor at 50-, 100-, and 200-mg levels was chosen in these experiments. Reactions were conducted at 190 °C for 5 min after the addition of Ag(acac). Our TEM study shows that at the low precursor concentration, spherical Pt particles were predominant, Figure 8. At the intermediate precursor concentration, elongated nanoparticles and multipods with lowaspect ratio branches formed. We noted that the Pt nanocrystals formed using 100 mg Pt(acac)2 contained multipods that resembled some of the morphologies previously observed,1 suggesting that full growth of rod-like branched multipods may be achievable in the polyol process with the proper control of reactant molar ratio and reaction condition. Conclusion. Addition of a trace amount of Ag(acac) can trigger the nucleation and growth of Pt nanoparticles at temperatures below that for the homogeneous reaction. The relatively low reaction temperatures also favor the kinetically controlled anisotropic growth of platinum in the presence of surfactants. This induced growth leads to the formation of a high-population of Pt multipods. Unlike the cases reported previously, the Pt tetrapods made in this work are well-defined, resembling those observed for II-VI quantum dots of CdS and CdSe. We believe the approach developed in this work can be an effective method to make multipods of different metals under appropriate conditions. These Pt multipods may be used as new nanoscale building blocks for making complex functional nanostructures through selfassembly.15 Acknowledgment. This work is supported in part by U.S. NSF (CAREER Award, DMR-0449849 and SGER Grant, F

CTS-041722), the DOE through LLE (DE-FC03-92SF19460), and the subcontract of a MURI Grant from DoD (FA955004-1-0430). This work made use of the Shared Experimental Facilities at the Cornell Center for Materials research (CCMR) supported by NSF and the Hitachi HD-2000 STEM at the Centre for Nanostructure Imaging, University of Toronto, funded by Canada Foundation of Innovation and Ontario Innovation Trust. We are grateful to LLE for a Horton Fellowship (X.T.). We thank Drs. Mick Thomas (Cornell University) and Marc Mamak (University of Toronto) for running the high resolution TEM. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article. Note Added after ASAP Publication. Text phrase II-IV was corrected to II-VI in the Abstract and Conclusions. This paper was published ASAP on 4/9/05. The corrected version was reposted on 4/13/05. References (1) Herricks, T.; Chen, J. Y.; Xia, Y. N. Nano Lett. 2004, 4, 23672371. (2) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186-16187. (3) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Y. Nano Lett. 2004, 4, 327-330. (4) Hoefelmeyer, J. D.; Niesz, K.; Somorjai, G. A.; Tilley, T. D. Nano Lett. 2005, 5, 435-438. (5) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159-196. (6) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883-2885. (7) Cheng, Y.; Wang, Y. S.; Chen, D.; Bao, F. J. Phys. Chem. B 2005, 109, 794-798. (8) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728-4729. (9) Wang, D.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871-874. (10) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343-3353. (11) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382-385. (12) Grebinski, J. W.; Hull, K. L.; Zhang, J.; Kosel, T. H.; Kuno, M. Chem. Mater. 2004, 16, 5260-5272. (13) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244-11245. (14) Jun, Y. W.; Jung, Y. Y.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 615-619. (15) Wang, D. L.; Lieber, C. M. Nat. Mater. 2003, 2, 355-356. (16) Huang, Y.; Lieber, C. M. Pure Appl. Chem. 2004, 76, 2051-2068. (17) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425-2427. (18) Peng, X. G. AdV. Mater. 2003, 15, 459-463. (19) Lee, S. M.; Cho, S. N.; Cheon, J. AdV. Mater. 2003, 15, 441-444. (20) Teng, X. W.; Black, D.; Watkins, N. J.; Gao, Y. L.; Yang, H. Nano Lett. 2003, 3, 261-264. (21) Teng, X. W.; Yang, H. J. Am. Chem. Soc. 2003, 125, 14559-14563.

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