NANO LETTERS
Monodisperse Platinum Nanospheres with Adjustable Diameters from 10 to 100 nm: Synthesis and Distinct Optical Properties
2008 Vol. 8, No. 12 4588-4592
Nadja C. Bigall,† Thomas Ha¨rtling,‡ Markus Klose,† Paul Simon,§ Lukas M. Eng,‡ and Alexander Eychmu¨ller*,† Institut fu¨r Physikalische Chemie und Elektrochemie, TU Dresden, Bergstr. 66 b, 01062 Dresden, Germany Institut fu¨r Angewandte Photophysik, TU Dresden, George-Ba¨hr-Str. 1, 01069 Dresden, Germany Max-Planck-Institut fu¨r Chemische Physik fester Stoffe, No¨thnitzer Strasse 40, 01187 Dresden, Germany Received September 24, 2008; Revised Manuscript Received October 15, 2008
ABSTRACT We present a facile and reproducible method for synthesizing monodisperse platinum (Pt) spheres with sizes ranging from 10 to 100 nm in diameter and exceptionally small standard deviations of 3% for large spheres. The reaction takes place in aqueous solution using a multistep seed-mediated approach. The Pt nanospheres consist of several small crystallites resulting in a surface roughness of 5-10 nm. Extinction spectra are measured from particles dispersed in water and calculated for single particles which are found to be in excellent agreement. We obtain a linear correlation between the plasmon extinction maximum (from UV to the visible regions) and the particle diameter which might be of value for experimentalists in the field.
The optical properties of metal nanoparticles play a key role in the field of nanooptics, since they can support localized surface plasmons (LSP) with resonance wavelengths in the visible regime. In the spectral vicinity of this resonance, both the scattering cross section as well as the electric field surrounding the particle can reach very high values. Consequently, metal particles find applications for enhancing field-sensitive optical processes such as surface-enhanced Raman spectroscopy and fluorescence emission1 or their scattering properties are exploited, e.g., for environmental refractive index sensing.2 In order to achieve high sensitivities in these and many other applications, an exact knowledge and spectral control of the LSP resonance is necessary. Thus, intense research has been devoted to the fabrication of metal nanoparticles with tailored properties in size, shape, and material. However, in terms of LSP resonances, it is still difficult to cover the entire wavelength range interesting for nanooptics. Especially the short-wavelength region is difficult to reach with particles made from gold or silver, the two most commonly used materials in nanooptics. * Corresponding author,
[email protected]. † Institut fu ¨ r Physikalische Chemie and Elektrochemie, TU Dresden. ‡ Institut fu ¨ r Angewandte Photophysik, TU Dresden. § Max-Planck-Institut fu ¨ r Chemische Physik fester Stoffe. 10.1021/nl802901t CCC: $40.75 Published on Web 11/05/2008
2008 American Chemical Society
Platinum (Pt) is an interesting alternative, as Pt nanoparticles show resonances typically below 450 nm and exceed this value for large diameters only. Although their LSP curves are rather broad and field enhancement is lower than for instance for gold or silver,3 they are promising candidates for nanooptical applications in the UV region. Furthermore, there is a variety of other application fields specifically for Pt nanoparticles. Large and monodisperse high refractive index particles are needed, e.g., for photonic crystals, where Pt spheres will complement the already existing silver, palladium, bismuth, lead spheres and other monodisperse shapes.4 Also in catalysis, platinum nanocrystals play a key role due to their large surface area and chemical potential, which has been proven for several systems.5 The past decade has witnessed huge activities concerning Pt nanoparticle synthesis both in organic solvents6 and in aqueous solution.7 Recent publications treat shape control,8 like, e.g., tetrahedral, cubic, and tetrahexahedral nanoparticles, multipods, nanoflowers, nanospheres, nanowire networks, hollow spheres, or nanoshell tubes, and controlled aggregations of these.9 Also dendritic growth of small platinum nanoparticles has been observed.10 However, especially for spheres larger than 5 nm in diameter, an easy-
to-follow synthesis route to monodisperse platinum particles of controlled size at small standard deviation is still missing to date. Here we present a facile synthesis for platinum nanospheres ranging from 5 to 100 nm in diameter. The reaction is carried out in aqueous solution following a multistep seedmediated growth procedure. Individual Pt spheres consist of small crystallites ranging from a 3-8 nm diameter resulting in a peak-to-valley surface roughness ∆PP of 5-10 nm. Nevertheless, the overall diameter distributions of the outer diameters are very narrow with standard deviations of 3% for the larger spheres. Extinction spectra of water-dispersed nanospheres are gathered and compared to single-particle calculations. A linear correlation between the extinction maximum (from the UV to the visible regions) and the respective particle diameter is derived for both the measured and computed data, and excellent agreement is found between theory and experiment. (1) Synthesis of Small Platinum Seeds. Pt nanoparticle seeds of 5 nm in diameter were prepared according to Brown et al.11 Briefly, 36 mL of a 0.2% solution of chloroplatinic acid hexahydrate (Sigma-Aldrich, ACS reagent) was added to 464 mL of boiling deionized water. After 1 min, 11 mL of a solution containing 1% sodium citrate and 0.05% citric acid was added, followed half a minute later by a quick injection of 5.5 mL of a freshly prepared sodium borohydrate (0.08%) solution containing 1% sodium citrate and 0.05% citric acid. After 10 min, the product was cooled down to room temperature. (2) Synthesis of 29 nm Diameter Platinum Particles. To 29 mL of deionized water, 1 mL of the platinum seed solution was added at room temperature. A 0.045 mL portion of a 0.4 M chloroplatinic acid solution (VEB Bergbau- and Hu¨ttenkombinat “Albert Funk”, Hexachloroplatin(IV)-sa¨ure, reinst) was added, followed by the addition of 0.5 mL of a solution containing 1% sodium citrate and 1.25% L-ascorbic acid. Under stirring, the temperature was slowly increased to the boiling point (∼10 °C/min). The reaction time was 30 min in total. By varying the amount of chloroplatinic acid, we were able to synthesize particles with diameters from 10 to 30 nm in diameter (as obtained from transmission electron microscopy (TEM) measurements). (3) Synthesis of Large-Diameter Platinum Spheres. To obtain Pt spheres with diameters larger than 29 nm, the particles described above were used as seeds. By adding 4 mL of the 29 nm Pt particle solution to 26 mL of deionized water together with 0.045 mL of the chloroplatinic acid solution, followed by the addition of 0.5 mL of the solution containing 1% sodium citrate and 1.25% L-ascorbic acid and slowly increasing the temperature to the boiling point, we obtained spheres with diameter of 48 nm (diameter obtained from TEM measurements). The same procedure using 1 and 0.25 mL of seed solution in 29 mL of water resulted in spheres of 73 and 107 nm diameter, respectively. (4) Post-Treatment of the Particles. All nanoparticles were washed three times by precipitation in the centrifuge (Mini Spin (Eppendorf), operated at 1000-13400 rpm depending on the nanoparticle size), exchanging the superNano Lett., Vol. 8, No. 12, 2008
Figure 1. (a) Size distributions of four different batches of nanoparticles. (b) Transmission electron micrographs of Pt nanospheres with mean diameters of 29, 48, 73, and 107 nm, respectively.
natant against deionized water and redispersion of the nanoparticles. Then, extinction spectra were taken using a Cary 5000 spectrometer from 230 to 800 nm. Transmission electron microscopy (TEM) in the medium resolution regime and electron diffraction patterns of the nanospheres were performed using a FEI Tecnai 10 (LaB6-source) microscope. High-resolution TEM experiments were carried out with a field emission microscope, CM 200 FEG/ST-Lorentz (FEI company, Eindhoven, NL). Figure 1 displays the four different particle sizes as obtained directly from the above synthesis. The size distributions of particles with 29, 48, 73, and 107 nm diameter are depicted in Figure 1a. Remarkably, the standard deviations are very narrow and do not overlap. The distribution curves are extremely similar, revealing a standard deviation of 3% for the 107 nm particles, and also the 29 nm batch is still appreciably monodisperse with a 7% standard deviation. The transmission electron micrographs corresponding to each batch of particles are shown in Figure 1b. These images illustrate the uniformity of the spheres, and they also show the aforementioned surface roughness. 4589
Figure 2. (a) High-resolution micrograph of a 107 nm sized platinum sphere. Particles are electron transparent at their rim only, displaying the 2.27 Å (111) lattice planes. (b) Electron diffraction of a single platinum sphere. The strong 2.23 Å (11-1), 2.22 Å (1-1-1), and 1.96 Å (020) reflections appear as discrete reflection spots indicating [101] zone whereas the 1.18 Å (311) reflection is showing a polycrystalline orientation indicated by a diffuse ring. Literature data assign 2.27 Å to (111) and 1.96 Å to (020) reflections.12
A high-resolution TEM image of the rim region of a single 107 nm sphere is shown in Figure 2a. The surface roughness ∆PP of several nanometers stems from the dense packing of crystallites having a size of 3-10 nm. Nevertheless, Pt lattice planes are resolved up to the outer rim of the nanosphere and indicate a lattice constant of 2.27 Å ([111] direction) in good accordance with literature.12 From this, we conclude that the nanospheres entirely consist of several small platinum crystallites and possess no amorphous regions. An electron diffraction pattern of a single platinum sphere is depicted in Figure 2b. Besides the diffraction rings expected for a polycrystalline sphere, also discrete diffraction spots are observed. This indicates either a preferential direction of crystallite orientation or the existence of a larger crystal domain at the edges of the sphere, since the electron beam is unlikely to pass through the core of the sphere. The selected spherical electron beam aperture was purposely chosen to include one single platinum sphere for diffraction only. Next, we display in Figure 3a experimentally determined extinction spectra of the aqueous solutions containing spheres with mean diameters of 29 nm, 48 nm, 73 nm, and 107 nm. With increasing sphere diameter, the LSP resonance, i.e., the extinction peak, shifts to larger wavelengths from λ ) 248 nm for 29-nm-diameter particles, to λ ) 494 nm for the largest spheres (squares in Figure 3c). In order to gain further insight into the optical properties of these Pt particles, we calculated their extinction spectra. For the smallest particle size (29 nm), the computation was carried out using the electrostatic approximation, as the diameter of the particle is very small compared to the wavelength of the incident light.13 For larger spheres, the electrostatic model becomes more and more erroneous as retardation effects occur on the particles due to their increasing diameter. Therefore, we used the multiplemultipole (MMP) method for computing the extinction spectra of such particles.14 In a first approach, the particles were modeled as homogeneous polycrystalline platinum spheres, using the data of Weaver for the dielectric function εPt of the material.15 The 4590
resulting peak positions are represented by circles in Figure 3c. A comparison between these theoretical results with the experimental values reveals the distinct optical as well as structural properties of the particles described in this article. We find a very good agreement between the slope of the measured and the calculated curves. However, the computed values are clearly offset compared to the measured data. We attribute this effect to the crystallinity and roughness of the sphere surface, which leads to a dielectric function slightly different from the bulk material and thus faking a smaller effective particle radius. To account for this effect, we refined our modeling by treating the spheres as core-shell systems. The actual particle size measured by TEM was taken as the outer shell diameter. The peak-to-valley surface roughness ∆PP of 5 nm was taken into consideration by treating the outermost 5 nm of the particle as a shell with an effective dielectric function (see Supporting Information). This function was obtained by weighting the dielectric data εPt of platinum and εH2O of water, while the particle core again was treated as being pure polycrystalline platinum. With this approach, we obtained a very good agreement between the calculations for single particles and the experimental data measured for our Pt nanoparticle solutions assuming a ratio εPt:εH2O of 1:1 for the particle shell. The resulting extinction spectra are shown in Figure 3b, and the respective peak positions are depicted as triangles in Figure 3c. The linear dependence between particle diameter and LSP resonance wavelength is in good agreement with previous reports.16 In contrast to other materialssespecially gold and silversthis linearity holds even for small particle sizes and makes platinum a highly interesting material for sizedependent nano-optical experiments. For example, in conjunction with the monodispersity of the particle solutions, this dependence allows an easy optical characterization of the particle diameter by monitoring the extinction spectra during the fabrication process. In summary, a synthesis route in aqueous medium for monodisperse platinum spheres with diameters of 10-100 Nano Lett., Vol. 8, No. 12, 2008
3% for the larger spheres. The correlation of the extinction spectra with the mean diameter of the ensemble in solution was compared to the calculated correlation for single particles. We observe a linear size dependence of the spectral position (from UV to the visible region) of the extinction peak, i.e., the LSP resonance. Hence, the effective particle size can be determined very easily after each synthesis step. The synthesized Pt particles are highly promising candidates for plasmonic applications especially in the UV and blue wavelength regime. Such applications might benefit from further particle features specific for platinum. For example, the catalytic properties of the material will facilitate plasmon-assisted UV photochemistry. Since the size distribution of the particles is very narrow, the synthesized platinum particles are applicable both in single particle as well as in ensemble experiments. Acknowledgment. The authors thank Professor Dr. Hannes Lichte for TEM measurements, Ellen Kern for SEM work, and Dr. Dirk Dorfs, Rene´ Kullock, and Andreas Hille for stimulating discussions. This work has partially been funded by the Studienstiftung des Deutschen Volkes, as well as the STREP project PLEAS, the project STABILIGHT and the NoE PHOREMOST within framework program 6 of the European Union. Supporting Information Available: TEM micrographs of the platinum seeds and of the synthesized platinum spheres and a detailed theoretical description concerning the modeling of the extinction properties of these particles. This material is available free of charge via the Internet at http:// pubs.acs.org. References
Figure 3. (a) Experimentally determined extinction spectra of the platinum particles in aqueous solution. (b) Extinction peaks calculated for a core-shell platinum sphere. For details of the calculations, see text and Supporting Information. (c) Extinction peak positions of the experimental data (squares) and calculated values for massive platinum spheres (dots) as well as for core-shell particles (triangles). All lines depict linear fits to the respective data points. Note the good agreement between theory and experiment.
nm diameter was introduced following a multistep seedmediated growth approach. The platinum spheres exhibited a surface roughness of 5-10 nm and standard deviations of Nano Lett., Vol. 8, No. 12, 2008
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