Hard Pd Nanorods in the Soft Surfactant Mixture of CTAB and

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Hard Pd Nanorods in the Soft Surfactant Mixture of CTAB and Pluronics: Seedless Synthesis and Their Self-Assembly Hyon-Min Song, and Jeffrey I. Zink Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00205 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Hard Pd Nanorods in the Soft Surfactant Mixture of CTAB and Pluronics: Seedless Synthesis and Their Self-Assembly Hyon-Min Song,†,* and Jeffrey I. Zink‡ †Department of Chemistry, Dong-A University, Busan 604-714, South Korea

‡Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095-1569

ABSTRACT Seedless synthesis of Pd nanorods and their self-assembly into the layered smectic ordering is described. Aqueous Pluronic triblock copolymers (14.3% - 35.7%) are used as a soft template along with cetyltrimethylammonium bromide (CTAB) for inducing one-dimensional growth of Pd nanorods. Pluronic triblock copolymers are probably the most used polymer surfactants, and they are composed of PEO(polyethylene oxide)-PPO(polypropylene oxide)-PEO triblocks. Neither pH adjustment, nor AgNO3, nor other additives such as polyvinyl pyrolidone, and ethylene glycol are required to obtain Pd nanorods. Sonochemical synthesis at 43 °C, followed by thermal annealing for 1 hr at 65 °C produces Pd nanorods with the aspect ratio from 3.1 (17.9%, Pluronic L-64) to 6.7 (35.7%, Pluronic P-123). Two-dimensional self-assembly of the nanarods is observed, and both nematic ordering between the mesogens and smectic ordering

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between the layers is identified. Micellar hydrophobic PPO with hydrated PEO coronas are known to self-assemble into many crystalline orders including cubic, hexagonal, lamellar, and inverse hexagonal mesophases, which extend into cylindrical micelles with increasing temperature. Relatively small size of Pluronic copolymers with regard to general polymers, but rather large size of their micelles and their tendency to organize into crystalline mesophases are thought to contribute to the anisotropic growth of Pd nanorods.

Introduction Palladium is one of the noble metals and has been thought an alternative to Pt materials. It is hardly oxidized once it is a metallic form at room temperature, while the oxidized form of Pd is available by the annealing of metallic Pd in air condition. It forms alloyed materials quite easily with diverse metals, particularly with 3d-5d late transition metals with face-centered-cubic (fcc) symmetry. It also serves as an important catalyst in C-C bond forming coupling reactions, as well as in the hydrogenation reactions due to its hydrogen hosting properties. Nowadays, nanosize Pd finds broader applications, extending to the acoustic wave sensing of H2 gas,1 and the nanomolar detection of cafferic acid.2 In one-dimensional nanomaterials, two-dimensional confined geometry with another bulky one dimension along the longitudinal direction finds applications in electronics and device fabrication. Optical extinctions are observed in gold and silver nanomaterials, while particularly interesting is the longitudinal plasmon resonance modes in gold nanorods. Sidewalls of these metal nanorods tend to be composed of relatively unstable {110} planes,3,4 and the defect sites with high index planes usually accompany.5 In the synthesis of Pd nanorods, seed-mediated approach has typically been utilized.6 Seedless synthesis, though, has been achieved in a few examples, one in a chemical solution method either using AgNO3, KI and HCl in cetyl


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trimethylammonium bromide (CTAB) at 90 °C,7 or using poly(vinyl pyrrolidone) (PVP), ethylene glycol (EG) and CTAB at 150 °C,8 and another as a hydrothermal autoclave method using PVP and NaI at 200 °C,9 and the final as an electrochemical method using standard three electrode cells in potentiostat.10 In the current study, one step seedless synthesis of Pd nanorods was achieved in a much milder condition in the surfactant mixture of CTAB and Pluronic triblock copolymers. Sonochemical method at 43 °C, followed by thermal annealing at 65 °C for 1 hr produces Pd nanorods. Pluronic copolymers consist of PEO(polyethylene oxide)PPO(polypropylene oxide)-PEO triblocks. Hydrophilic PEO and hydrophobic PPO form micelles, while the ratio and the size of each block decide the concentration and the temperature at which micelles are formed. Critical micelle concentrations of Pluronic copolymers are quite low, for example, from 5.3 x 10-3 M (L-35, Mn ~ 1,900) to 2.8 x 10-6 M (F-127, Mn ~ 12,600) at body temperature.11 At sufficiently high temperature, spherical micelles of Pluronic copolymers are known to integrate into body-centered cubic crystalline mesophases,12 and they further grow into worm-like micelles or cylindrical micelles.13 In fact, molecular dynamic simulations predict that micelles of Pluronic L-64 prefer anisotropic ellipsoidal shape, which helps to minimize repulsions at shorter distances.14 This prolate ellipsoidal structure of L-64 micelles is verified experimentally through neutron scattering techniques.15 The remarkable feature of Pluronic L-64 is that hexagonal phase is observed next to isotropic micelles with increasing concentrations.16 Cubic phase resides in a confined region between hexagonal and lamellar phases, and it is a bicontinuous phase, being hardly existing as an isolating cube. In addition, micelles of Pluronic F-127 are also known to exhibit cubic, hexagonal, and lamellar mesophases in the mixed solvent of water and butanol,17 or water and biocompatible solvents such as propylene carbonate and triacetin.18 With inorganic substance, they help to build porous structures with cubic symmetry.19


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Considering this dynamic soft nature of Pluronic copolymers, extended soft templates in the presence of CTAB is thought to contribute to the anisotropic growth of Pd nanorods. The aspect ratio of the nanorods has dependence on the number average molecular weights (Mn) and the ratio between PO and EO of triblock copolymers, producing Pd nanorods with the aspect ratios from 3.1 to 6.7.

Experimental Section Pluronics (CAS 9003-11-6) with Mn of ~2,900 (Pluronic L-64), ~5,800 (Pluronic P-123), ~12,600 (Pluronic F-127), and ~14,600 (Pluronic F-108) were bought from Sigma-Aldrich. CTAB was bought from Sigma-Aldrich and 0.2 M solution was recrystallized twice. Rigaku Ultima Ⅳ diffractometer with CuKα radiation (λ = 1.54056 Å) in a θ–θ mode was used for the X-ray diffraction (XRD) measurement. For measuring optical extinction spectra (200 – 1200 nm), JASCO V-770 spectrophotometer with a 1 cm cell path length and with a UV/VIS bandwidth of 2.0 nm and NIR bandwidth of 8.0 nm was used. JAC ultrasonic 1505 (150 W, 40 KHz) was used for the sonication. Transmission electron microscopy (TEM) images were obtained with JEOL JEM 2010FX operating at 200 kV. SEM images were taken with JEOL JSM 6700F with an operating voltage of 5 kV. Azimuthal intensity profile was obtained by ImageJ software with the azimuthal average plugins. Preparation of Pd nanorods Pluronic copolymers were prepared as 17.9% aqueous (L-64 and F-108), 14.3% aqueous (F-127), or 35.7% aqueous (P-123) concentrations. For 17.9% aqueous solution, 2.5 g of Pluronics was dissolved in 11.5 mL of H2O, and for 14.3% aqueous solution, 5.0 g of Pluronic F-127 was dissolved in 30.0 mL of H2O. For 35.7% aqueous solution, 5.0 g of Pluronic P-123 was dissolved in 8.5 mL of H2O. For the synthesis of Pd nanorods with the aspect ratio of 3.1 and the dimension of 28.9 nm (± 4.2) x 86.9 nm (± 10.5), aqueous solution of


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Pluronic L-64 (3.0 mL, 17.9%) and CTAB (2.5 mL, 0.2 M) was mixed. H2PdCl4 (0.5 mL, 0.015 M) was added and the mixture was vortex-mixed for 15 sec, followed by the sonication at 23 °C for 10 min. Ascorbic acid (0.65 mL, 0.568 M) was added, and the mixture was vortex-mixed for 15 sec, followed by the sonication at 43 °C for 25 min. The mixture was placed in the heating oven without stirring or agitation for 1 h at 65 °C. Black precipitates were observed and they were separated by centrifuging at 2500 rpm for 3 min. For the synthesis of Pd nanorods with the aspect ratio of 3.1 and the dimension of 35.6 nm (± 5.7) x 111.2 nm (± 17.9), other conditions are the same, except H2PdCl4 (0.25 mL, 0.090 M) and ascorbic acid (1.2 mL, 0.568 M). For the synthesis of Pd nanorods with the aspect ratio of 4.3, experimental conditions are the same, except with the materials of F-127 (3.0 mL, 14.3%), CTAB (2.5 mL, 0.2 M), H2PdCl4 (1.5 mL, 0.015 M) and ascorbic acid (0.65 mL, 0.568 M) used. Pd nanorods with the aspect ratios of 6.7 and 5.2 were prepared with P-123 (17.9%) and F-108 (17.9%) respectively with the experimental conditions as is shown in Table 1.

Table 1. Experimental conditions for the synthesis of Pd nanorods and the dimensions of the products

Pluronic copolymers and their concentrations (wt % in H2O) L-64 (17.9%)

Concentration and the amount of H2PdCl4 0.090 M, 0.25 mL

Amount of ascorbic acid (0.568 M) 0.80 mL

L-64 (17.9%)

0.015 M, 0.50 mL

0.65 mL

F-127 (14.3%)

0.015 M, 1.5 mL

0.65 mL

P-123 (35.7%)

0.015 M, 1.5 mL

0.65 mL

F-108 (17.9%)

0.090 M, 0.50 mL

1.00 mL

Dimension of Pd nanorods (width x height) 35.6 nm (± 5.7) x 111.2 nm (± 17.9) 28.9 nm (± 4.2) x 86.9 nm (± 10.5) 26.0 nm (± 1.9) x 111.6 nm (± 20.9) 14.3 nm (± 2.2) x 95.5 nm (± 26.6) 18.6 nm (± 2.1) x 96.0 nm (± 15.6)

Aspect ratio of Pd nanorods 3.1 3.1 4.3 6.7 5.2


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Results and Discussion Pluronic copolymers are widely used as a polymer surfactant for the synthesis of mesoporous silica with various crystalline mesophases.20 Their biocompatibility also enables many applications in pharmaceutical areas, for example as the delivery vehicle for gene therapy.21 These Pluronics actively reduce noble metal cations, and the experiments without the addition of CTAB were conducted with L-64 (17.9% and 35.7%). Pluronic L-64 (3.0 mL) was mixed with H2PdCl4 (0.5 mL, 15 mM) and the color of the mixture changed immediately after the addition of H2PdCl4 from light brown to black. Sonication at 43 °C, followed by heating at 65 °C for 1 h produced dendritic Pd nanoparticles (Figure 1a and 1b). The average diameter of the spheres is 28.4(± 4.1) nm for L-64 (17.9%), and 14.3 (± 1.9) nm for L-64 (35.7%). Roughly, twice the concentration of L-64 from 17.9% to 35.7% produced nanoparticles with half of the sizes. In the binary phase diagram of L-64 and water, isotropic liquid crystalline lamellar phases dominate at 25 °C.16 Hydrated corona regions contain 10 – 20 water molecules per each ethylene glycol in EO region.22 It is also known that the dehydrated hydrophobic PPO cores contain 20% of aqueous solution, 23 which implies that the cores are not completely dry. The reduction of Pd(II) is thought to occur in the cores and the decreased water content both in the cores and around the coronas causes the deswollen state of micelles and produces smaller Pd nanoparticles. The experiment without Pluronics was also conducted. CTAB (2.5 mL) and H2PdCl4 (0.5 mL, 15 mM) were mixed and the mixture was sonicated at 43 °C for 10 min, followed by the addition of ascorbic acid (0.65 mL, 0.568 M). The mixture was again sonicated at 43 °C for 25 min, and was heated at 65 °C for 1 h. Products were the mixture of rods and spheres with edges (Figure 1c). It is in fact CTAB that retards the reduction of Pd(II) in the presence of Pluronics. Usually, higher concentration of CTAB favors the growth into direction and slows the deposition on


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{111} planes. This produces nanomaterials with dominant {111} planes such as octahedrons.24 Lower concentration of CTAB typically produces cubes with {100} planes because it assists deposition on {111} planes. In the optical absorbance spectra in Figure 1d, peaks were observed at 282 nm (L-64, 35.7%), 287 nm (CTAB), and 291 nm (L-64, 17.9%). Broad tails in L-64 (35.7%) is due to the dehydrated micelles around Pd nanoparticles, which increases the refractive indices and consequently increases the dielectric constant of the surrounding medium. Pd nanorods in Figure 2 were prepared with Pluronic L-64 (3.0 mL, 17.9%), CTAB (2.5 mL, 0.2 M), H2PdCl4 (0.25 mL, 90 mM) and ascorbic acid (0.80 mL, 0.568 M). The nanorods contain well-defined facets with sharp edges, and the average size of Pd nanorods in Figure 2 is 35.6 nm (±5.7) x 111.2 nm (±17.9) (width x height) with the aspect ratio of 3.1. Electron diffraction patterns of the rods indicate that they are composed of two zones of [111] and [011], which is broadly observed in penta-twinned nanorods.4.10 Pd nanorods grow along [110] direction, and the growth is analogous to the seed-mediated growth of gold nanorods,4 in which the twinned seed determines the structures with the extrinsic assistance from CTAB surfactant. Pd nanorods in Figure 3 were prepared with 3.0 mL of Pluronic L-64 (17.9%) and the reduced concentration of H2PdCl4 (0.5 mL, 15 mM) compared to those in Figure 2. The size of the rods decreased to 28.9 nm (±4.2) x 86.9 nm (±10.5) (width x height) with the aspect ratio of 3.1. Electron diffraction pattern in Figure 3d indicates two zones of or with the rectangular pattern of and the square pattern of . This pattern is also analogous to the pattern of gold nanorods, which were produced by penta-twinned seeds.4 Twin structures, though in Pd nanorods, are hardly identified, but the electron diffraction pattern of two zone axes imply that they are not single crystalline Pd structures. Two superimposing nanorods with similar orientation produced Moiré fringes (Figure 4). The growth of the nanorods proceeds into


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direction, and the zone axis is or . With the zone axis of [112], the alignment of the lattice fringes follows Figure 4b. The planes of {111} are placed parallel to the growth direction of , and this is the usual observation in the body of gold nanorods. However, this alignment of the lattice fringes does not produce the Moiré fringes in Figure 4a. With the zone axis of [001], lattice alignment in Figure 4c generates the pattern of Moiré fringes in Figure 4a. Even considering the measurement error, the angle between two superimposed nanorods is between 1.7° ~ 2.3°, and in this small angle, Moiré fringes occur near perpendicular to the lattice fringes. Moiré fringes are observed when the distance between gratings is different while they are in the same orientation, or when there is a pure rotation while the distance between the gratings is the same. In Pd nanomaterials, Au-Pd,24 Pd-Ni,25 and Pd-Ag26 core-shell nanocubes produce Moiré fringes, in which the lattice planes are usually observed with zones in cubic shapes. In the case of Pd nanorods where only the rotation of lattice fringes takes place, the distance between Moiré fringes can be obtained by the schematic drawing in Figure 4d.27 The side length a is given by a = d/sinα, and the distance between Moiré fringes, D, is given by D = a cos(α/2). For Pd nanorods, d200 = 1.93 Å and α = 2.0° gives the distance between Moiré fringes as 55.3 Å. Self-assembly of Pd nanorods which were obtained with H2PdCl4 (0.5 mL, 15 mM) and L-64 (17.9% aqueous) was observed in SEM images (Figure 5). Both nematic and smecting orderings of individual mesogens are observed. Nematic ordering directs the average orientational axis and is parallel to this axis, and smectic ordering is perpendicular to the direction of the flow of nanorods.28 The distance between mesogenic units is 36 nm and the distance between layers is 91 nm, which were measured in SEM image in Figure 5a. Two-dimensional FFT pattern also indicates both nematic and smectic orderings (Figure 5d). In general, the distance from the


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origin in FFT pattern is inversely proportional to the distance in real space. In FFT pattern in Figure 5d, the spots indicating 1/a derive from the distance between the mesogenic units and show the orientational ordering of the individual nanorods. Several spots of 1/b, 2/b, and 3/b which are perpendicular to 1/a, originate from the distance between the layers and indicate the positional ordering of the nanorods. The nanorods with both nematic and smectic orderings are not typically observed, but similar ordering is found in CdSe nanorods which were assembled by the oversaturation of colloidal solution,29 and in amorphous silica nanorods, the growth of which is initiated in the presence of emulsion droplets.30 This type of self-assembly often leads to the superlattices of nanorods, such as vertically free-standing semiconducting superlattices31 and gold nanorod superlattices assembled by gemini surfactant.32 Azimuthal intensity profile can be obtained from the FFT pattern (Figure 5e), and the intensity distribution of smectic (2/b) and nematic (1/a) orderings indicates that the peaks from both nematic and smectic orderings are observed with the peak from the nematic ordering stronger. Longer nanorods with the average aspect ratio of 4.3 (± 1.0) were obtained with Pluronic F127 (14.3%) (Figure 6), and some vertically aligned nanorods were also observed (Figure 6b). The molecular structure of F-127 is EO98PO67EO98. The ratio between EO and PO is an important parameter to determine the micellar structure of block copolymers.33, 34 The increase of PO units increases hydrophobicity, and PO units tend to segregate themselves from EO units. It is observed in this study that L-64 (EO13PO30EO13) produced rods with an aspect ratio of 3.1 and F-127 produced rods with an aspect ratio of 4.3. The reduction of Pd(II) and the growth into Pd nanoparticles supposedly occur in the highly hydrated coronas surrounded by hydrophobic PO groups. The coordination of europium nitrate in the presence of P-123 occurs within the coronas in inverse hexagonal phase of P-123.35 At sufficiently high concentrations, both L-64 and F-127


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are known to produce anisotropic micelles or anisotropic liquid crystalline mesophases in an aqueous solution or in the solvent mixture of water and butanol. Experiments to obtain Pd nanorods with higher aspect ratios were tried, and Pd nanorods with aspect ratios of 6.7 and 5.2 were obtained using Pluronic P-123 (35.7%) and F-108 (17.9%) respectively (Figure 7). There are a lot of spherical particles and short rods in the synthesis of Pd nanorods with the aspect ratio of 6.7 (Figures 7a and 7b), though relatively homogeneous nanorods were obtained in the synthesis of Pd nanorods with the aspect ratio of 5.2 (Figures 7c and 7d). When lower concentration (15.7% aqueous) of Pluronic P-123 was used, isotropically grown nanoparticles were obtained as the major products (Supporting Information Figure S6). The molecular structure of P-123 is EO19PO69EO19. Hydrated coronas in 35.7% aqueous solution is thought deswelled, where the growth of Pd nanoparticles occurs in a constrained region. The comparison of the size of spherical Pd nanoparticles in Figure 7a (14.4 ± 3.9 nm) and Figure S6 (28.9 ± 4.2 nm) also indicates the deswelled hydrated coronas in 35.7% aqueous solution of P-123. In addition, the average size of spherical nanoparticles in Figure 7a is consistent with the average width of Pd nanorods In L-64, however, high concentration of L-64 (43.9%) produces spherical nanoparticles with smaller sizes (17.3 ± 3.3 nm, Supporting Information Figure S7). Higher concentration of Pluronics causes deswelling and dehydration of the hydrated coronas. In sufficiently large EO and PO groups such as P-123 (EO19PO69EO19), anisotropic nanorods can overcome such a deswelled state, while in small EO groups such as L-64 (EO13PO30EO13), the growth is isotropic with the size of Pd nanoparticles smaller in the dehydrated state. Optical spectra were measured for these Pd nanorods (Figure 8). Extinction coefficients of Pd nanorods can be simulated by Mie scattering method, which is later refined by Gans. The simplified equations for obtaining the extinction coefficients of small metallic nanomaterials


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from Gans theory are well documented in the study of Link and El-Sayed.36 We adopted the refractive indices of bulk metallic Pd from the experimental results of Johnson and Christy,37 and using the corrected depolarization factors from the literature,38 the extinction coefficients of Pd nanorods were obtained using eq. 5 of the study of Link and El-Sayed.36 Extinction coefficients are dependent on the aspect ratios of Pd nanorods, and with increasing aspect ratios in the surrounding medium of water, the longitudinal plasmon resonance is red-shifted and transverse resonance mode is blue-shifted (Figure 8a). The dielectric constant of the surrounding medium also affects the optical extinctions, and with the increase of dielectric constants, the extinction coefficients of Pd nanorods with the aspect ratio of 3.1 become larger, while in Pd nanorods with the aspect ratio of 5.2, red-shift of the resonance modes as well as the increase of extinction coefficients are observed by simulation (Figure 8c). In the absorbance spectra of Pd nanorods obtained in this study, slight blue-shift was observed with increasing aspect ratios from 292 nm (aspect ratio 3.1) to 284 nm (aspect ratios 6.7). Though low intensity, longitudinal resonance modes are observed at 417 nm (aspect ratio 4.3), 617 nm (aspect ratio 6.7), and 796 nm (aspect ratio 5.2) of Pd nanorods. At room temperature after cooling of the reaction mixture, XRD patterns of Pd nanorods after washing indicate layer-by-layer structures of the surfactant mixture of CTAB and Pluronics. XRD pattern of the crystalized CTAB is added in the bottom of the Figure 9. The shift of CTAB peaks and the additional peaks presumably stemming from another lamellar ordering are observed. With the low molecular weight such as Pluronic L-64 (aspect ratio 3.1), excess washing removes quite easily the surfactant mixture of CTAB and Pluronics, but with the high molecular weight such as F-108 (aspect ratio 5.2), the peaks from the surfactants govern XRD patterns. Pd nanorods with the aspect ratio of 6.7 includes broad Pd (111) plane at 40.6° due to


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the contribution from small size Pd nanoparticles (14.4 nm). CTAB and Pluronics are not simply mixed, but they are known to produce supramolecular assemblies, in which hydrophobic CTAB alkyl chains mix with hydrophobic cores of Pluronics and the ammonium head groups reside in the interface between cores and coronas.39,40 Usually the hydrophobicity of Pluronics increases with increasing temperature by the tendency of water to be segregated from the hydrophobic cores, and if the reduction of Pd(II) does not occur in the presence of Pluronics at room temperature, it is highly probable that the reduction does not occur at the elevated temperature. However, the addition of CTAB, which is an ionic surfactant, increases the hydrophilicity of the solution, and it compromises the hydrophobic environment at the elevated temperature. In addition, the increase of the molecular weight of Pluronics also increases the hydrophobicity. With the large number of methyl groups in Pluronics, alkyl chains of CTAB are likely to be involved in the micellar self-assembly, which causes relatively strong XRD peaks of the surfactants even after excess washing of the products. Polymer surfactants freely move in aqueous solution, but once in the micellar form, their movement is restrained, although the formation of micelles is believed entropy-driven process.41At sufficiently high temperature with the concentration of Pluronic copolymers higher than the critical micelle concentrations, micelles are self-assembled into the crystalline mesophases, producing soft solid structure.12 It is also known that the shape transition from spherical micelles to rods occurs at the elevated temperature.42 The size of micelles from F-127 is known to be 68 – 70 nm,43 measured during their encapsulation of hydrophobic drugs such as ibuprofen, aspirin and erythromycin. The concentrations of Pluronic L-64 (17.9%), F-127 (14.3%), P-123 (35.7%), and F-108 (17.9%) are 62.1 mM, 11.3 mM, 61.5 mM, and 12.2 mM respectively. These are sufficiently higher than the critical micelle concentrations of L-64 (0.48


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mM at body temperature11 and 5.17 mM at 30 °C44), F-127 (2.8 x 10-3 mM at body temperature11 and 3.17 mM at 19.5 °C44), P-123 (4.4 x 10-3 mM at body temperature11 and 0.313 mM at 19.5 °C44), and F-108 (2.2 x 10-2 mM at body temperature11 and 3.08 mM at 25 °C44), so that the micelles are formed at the sonication temperature of 43 °C. Increasing the temperature to 65 °C is thought to make crystalline mesophases, which act as the soft template for the growth of Pd nanorods. The process of being crystalline mesophases is thermally reversible, and lowering temperature dissolves these clusters through inverse melting.12 The growth of Pd nanorods is thought to occur in hexagonal or inverse hexagonal mesophases at the elevated temperature. In normal hexagonal phase, the increase of water content does not alter interfacial area significantly, while in the reverse hexagonal phase the size of the hydrated coronas increases significantly with increasing water molecules.45 In Pluronic F-127, relatively large size of EO groups does not allow inverse hexagonal phase,46 but inverse hexagonal phases in Pluronic L-64 and P-123 are prevalent. In inverse hexagonal phases, hydrated coronas determine the size of Pd nanoparticles, as shown in the different sizes of Pd nanoparticles in two concentrations of P-123 (15.9%, 35. 7%) However, hydrophobic PO cores in hexagonal phase are known not exclusive and contain water and EO segments, in which the growth of Pd nanorods can also occur.

Conclusion Pd nanorods with the aspect ratios from 3.1 to 6.7 were prepared in a seedless approach with the surfactant mixture of CTAB and Pluronic copolymers. At sufficiently high temperature, micelles of Pluronic copolymers are known to be monodisperse with the hydrodynamic radius of each micelle being almost invariant,47 which leads to crystalline mesophases. This thermally reversible process also causes inverse melting with the disruption of the crystalline phases when


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the temperature is lowered. These two features are utilized to obtain Pd nanorods. In addition, these Pd nanorods are easily self-assembled without any designed set-up for solvent evaporation. Both nematic and smectic orderings are observed during this self-assembly. With regard to noble metal nanoparticles, Pluronic copolymers have been used as a surfactant for mostly spherical shapes, and hardly as an influencing factor for controlling the shapes of those materials. The seedless method described in this study is thought useful, considering that various types of Pluronics with different ratios of each block could be used for obtaining specific types of Pd nanoparticles. In addition, Pd(II) has lower standard reduction potential (0.915 V) than Pt(II), Au(III), and Ir(III). With the proper Pluronics, seedless synthesis could be applied by slowing the nucleation and growth. The remaining question is the study of dynamics and structures of these crystalline mesophases which are supposed to be the templates for inducing one-dimensional growth.

ASSOCIATED CONTENT Supporting Information. Additional TEM and SEM images of Pd nanorods are available free of charge at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Tel: +82-51-200-7257; Fax: +82-51-200-7259 Notes The authors declare no competing financial interest. Funding Sources


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The authors acknowledge the support from Dong-A University.

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Figure 1. TEM images of Pd nanoparticles obtained with (a) Pluronic L-64 (17.9%), (b) Pluronic L-64 (35.7%), and (c) CTAB (0.2 M). (d) Optical absorbance spectra of Pd nanoparticles obtained with (red) Pluronic L-64 (17.9%), (green) Pluronic L-64 (35.7%), and (blue) CTAB (0.2 M).


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Figure 2. (a),(b),(c) TEM images and (d) electron diffraction pattern of Pd nanorods prepared with H2PdCl4 (0.25 mL, 90 mM) and Pluronic L-64 (17.9% aqueous).


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Figure 3. (a),(b),(c) TEM images with (d) electron diffraction pattern of Pd nanorods prepared with H2PdCl4 (0.50 mL, 15 mM) and Pluronic L-64 (17.9% aqueous).


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Figure 4. (a) TEM image of two superimposed Pd nanorods prepared with H2PdCl4 (0.50 mL, 15 mM) and Pluronic L-64. (b),(c) Schematic drawings of the atomic arrangement with zone axis and zone axis, respectively. (d) Illustration of the method for measuring the distance between Moiré fringes. (e) Simulation of the alignment of lattice fringes according to the pattern of Moiré fringes and the schematic drawings.


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Figure 5. (a),(b),(c) SEM images of the self-assembled Pd nanorods which were obtained with H2PdCl4 (0.50 mL, 15 mM) and Pluronic L-64 (17.9%). The insets in (b) and (c) are the FFT patterns of the respective self-assembly. (d) FFT pattern of the self-assembly in Figure 5a. (e) Azimuthal intensity profiles of 1/a (red) and 2/b (black) in Figure 5d. (f) Schematic drawing of the alignment of self-assembled Pd nanorods according to the FFT pattern in Figure 5d.


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Figure 6. (a),(b) SEM images and (c) TEM image of Pd nanorods which were obtained with H2PdCl4 (1.5 mL, 0.015 M) and F-127 (14.3% aqueous).


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Figure 7. (a) TEM and (b) SEM images of Pd nanorods prepared with H2PdCl4 (1.5 mL, 15 mM) and Pluronic P-123 (35.7% aqueous). (c) TEM and (d) SEM images of Pd nanorods prepared with H2PdCl4 (0.50 mL, 90 mM) and Pluronic F-108 (17.9% aqueous).


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Figure 8. (a) Simulated extinction coefficients of Pd nanorods depending on the aspect ratios while the dielectric constant of the surrounding medium is set at 1.77. (b) Simulated extinction coefficients of Pd nanorods depending on the dielectric constant of the surrounding medium while the aspect ratio is set at 3.1. (c) Simulated extinction coefficients of Pd nanorods depending on the dielectric constant of the surrounding medium while the aspect ratio is set at 5.2. (d) Experimental absorbance spectra of Pd nanorods which were obtained in this study.


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two-theta (degrees) Figure 9. XRD patterns of Pd nanorods with the aspect ratios of 3.1, 4.3, 5.2, and 6.7 from top to bottom, respectively. Pd nanorods with the aspect ratios of 3.1 were prepared with H2PdCl4 (0.25 mL, 90 mM). XRD pattern of the crystallized CTAB was added as a reference.


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Table of Contents Graphic and Synopsis


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Hard Pd Nanorods in the Soft Surfactant Mixture of CTAB and

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