High Fraction Synthesis of Two-Dimensional Nanoparticles through

CRYSTAL GROWTH & DESIGN

Polyhedral Gold Nanoplate: High Fraction Synthesis of Two-Dimensional Nanoparticles through Rapid Heating Process Jong-Hee Lee,† Kai Kamada,‡ Naoya Enomoto,‡ and Junichi Hojo*,‡ Material Laboratory, Corporate R&D Center, Samsung SDI, Gongsae-dong, Kiheung-gu, Yongin, Kyunggi-do 428-5, Japan, and Department of Applied Chemistry, Faculty of Engineering, Kyushu UniVersity, 744 Motooka, Nishi-ku, Fukuoka 819-039, Japan

2008 VOL. 8, NO. 8 2638–2645

ReceiVed March 1, 2007; ReVised Manuscript ReceiVed March 25, 2008

ABSTRACT: The morphology of gold nanoparticles was controlled with hydrogentetrachloroaurate (HAuCl4) and polyvinylpyrrolidon (PVP) through a polyol process using ethyleneglycol as solvent and reducing agent. The polyol process gave various particle morphologies: trihedron, tetrahedron, hexahedron, and sphere in the size range of 100-1000 nm. The polyhedral nanoplate fraction (PNF) was increased by injection of HAuCl4 and PVP to preheated ethyleneglycol. The synthesis method, so-called rapid heating process, provided a suitable crystal growth condition for formation of nanoplates because of rapid reduction of HAuCl4 to form gold nuclei and their oriented crystal growth under the affection of PVP at high temperature. The rapid heating process was found to straightforwardly obtain polyhedral gold nanoplates with an extremely small fraction of spherical particles. Particularly, trihedral gold nanoplates were obtained selectively in a short reaction time. Introduction Metal and semiconductor nanoparticles with well-controlled size and shape have a wide application range of catalytic, optical, bionic and electronic technologies.1–6 Gold particles with nanometer size and unique morphology are attractive materials that are available for catalyst, optical devices, bioengineering, and medical applications.7–9 Particularly, the unique properties of gold nanoparticles appear conspicuously in optical fields. For instance, when gold nanoparticles have morphologies of spheres, rods, and polyhedral plates, they show a complete difference in color.10–12 For more than a decade, most nanoscientists had concentrated solely on size reduction aiming at a quantum effect. Because of their efforts, various synthesis methods have been reported to selectively fabricate metal nanoparticles with desired sizes. But now, morphological studies have been undertaken extensivelybecausewell-crystallizedparticleshavenovelpeculiarities.13–17 Recently, it has been reported that nanometer-sized particles can be grown to zero, one, two, and three dimensions (0, 1, 2, and 3D) through various morphological control processes, including air-water interface,18 chemical vapor deposition,19,20 laser ablation,21 solution method.22,23 However, most morphological control methods have mainly focused on the fabrication of 1D nanoparticles such as rods and wires because of electric and thermal peculiarities on dimensionality.24–27 Conversely, studies to induce formation of 2D nanoparticles are still insufficient in comparison with 1D. The 2D nanoparticles such as nanodisks, nanoplates, and nanobelts have been expected to show novel abilities in optical peculiarity.28–30 Synthesis processes for metal nanoparticles with 2D growth are divided into two categories: photoinduced and liquid-thermal methods. The photoinduced method converts spherical nanoparticles to nanoprism through selective plasmon excitation and provides a method to fabricate nanoprisms with equilateral edges and monodispersibility. However, a longer irradiating time of about several tens of hours is required to induce nanoprisms.31,32

Conversely, the liquid-thermal method is capable to prepare 2D nanoparticles in a shorter time than the photoinduced one, by about several tens of minutes.33 However, large fraction and shape-uniformity of metal nanoparticles with 2D growth are still difficult to obtain. In this method, metal salts and stabilizers were added to a reductant solution at room temperature before heating. This could not be avoided from the formation of spherical particles because nucleation and crystal growth simultaneously and continuously proceed in the course of raising the temperature. Consequently, morphology and size of the formed particles were irregular, although polyhedral plates were partly obtained by selective adsorption of protecting agents on a crystal plane. In the liquid-thermal method, particle shapes are very complicated. The 0D (spheres) and 1D (rods, wires) nanostructures have been attained on a higher level. However, 2D nanostructures still meet difficulties in the fabrication process. Thereupon, the development of efficient liquid-thermal method is necessary for fabrication of 2D nanoparticles with uniform morphology and monodispersibility through a simple process. In morphological control to 1, 2, and 3-dimensional nanoparticles, spherical particles are the major hindrance to their peculiarity. Therefore, the efficient and simple control methods are required to avoid the formation of spherical particles. Herein, an efficient liquid-thermal method is reported to obtain a large fraction of 2D nanoparticles with uniform shape through a rapid heating process. Polyhedral gold nanoplates were prepared straightforwardly in a very short time by injection of hydrogentetrachloroaurate (HAuCl4) and polyvinylpyrrolidone (PVP) into hot ethyleneglycol. This study focuses on (i) fabrication of gold nanoparticles with high polyhedral nanoplate fraction and (ii) mechanism of morphological change under various reaction conditions. The tendencies of change in fraction and growth of polyhedral gold plates were studied under various experimental conditions such as concentration of HAuCl4, weight ratio of PVP/HAuCl4, and reaction temperature and time. Experimental Section

* Corresponding author. Tel: 81-92-802-2859. Fax: 81-92-802-2860. E-mail: [email protected]. † Samsung SDI. ‡ Kyushu University.

Synthesis of Gold Particles. HAuCl4 · 4H2O and PVP (MW 40 000) were used as metallic precursor and protecting agent, respectively. Ethyleneglycol was utilized as solvent and reducing agent. First, HAuCl4

10.1021/cg0702075 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

Polyhedral Gold Nanoplate

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Figure 1. (a-c) SEM, (d) TEM, and (e) SAED images of the product obtained at 0.1 M HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 for 20 min at 155 °C.

Figure 2. XRD pattern of the nanoplates (see Figure 1). and PVP were separately dissolved in ethyleneglycol (1 mL each). Ethyleneglycol (5 mL) was preheated to an adequate temperature (135∼165 °C). The HAuCl4 and PVP solutions were added all at once into the preheated ethyleneglycol. The reacting solution was kept at a set temperature and stirred for 10-120 min. The color of the solution was changed from colorless to blue, and then to reddish brown after several minutes. The products were washed several times with ethanol and the precipitates were redispersed in ethanol for characterization. Characterization. X-ray diffraction analysis was performed by Rigaku (miniflex) using Cu KR radiation (Ni filter) at 30 kV and 15 mA. Scanning electron microscope (SEM) images were obtained by Hitachi S-5200 at 5.0 kV. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) were obtained by Hitachi H-7000 at 100 kV. UV-vis absorption spectra were obtained by Hitachi U-3300 using a quartz cuvette.

Results and Discussion Characterization of Polyhedral Gold Nanoplates. Figure 13 show the analysis results of the product obtained at 0.1 M

Figure 3. UV-vis absorption spectra of HAuCl4 solution and colloidal gold nanoplates obtained under the conditions in Figure 1.

HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 for 20 min at 155 °C. The SEM images of the product are given at low and high magnifications in images a and b in Figure 1. The morphologies of the particles were polyhedral plates such as trihedral and hexahedral plates. The other morphologies scarcely formed. All of the polyhedral plates had a sharp edge and grew to 2D nanoparticles. To obtain more precise information for the nanoplates, the thickness of nanoplate was observed by tilting the plate as shown in Figure 1c. The polyhedral plate was ca. 360 nm in diameter and ca. 60 nm in thickness. The typical morphology was trihedral under the reaction condition in Figure 1. Figure 1e exhibits the SAED pattern of trihedral nanoplate shown in Figure 1d. The hexagonal symmetry of the diffraction spots indicates that the nanoplate is a singlecrystalline gold. The hexagonal symmetry of these pattern spots indicates that these nanoplates are single crystals bounded mainly by (111) facets. The powder XRD analysis was performed to obtain the further information of crystal structure.

2640 Crystal Growth & Design, Vol. 8, No. 8, 2008

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Figure 4. SEM images of various morphologies of gold particles obtained at different PVP/HAuCl4 ratios (0.1 M HAuCl4 for 20 min at 155 °C): (a) 0.8, (b) 1.1, and (c) 2.

As shown in Figure 2, the XRD peaks appeared at (111), (200), (220), and (311) indices of gold with a face-centered cubic (fcc) structure (JCPDS card no. 04-0783). The (111) peak was strong compared to bulk gold. This result indicates that the gold nanoplates were dominated by (111) facet. According to some reports on 2D nanoparticles, the nanoplates, nanoprisms, and nanodisks of gold and silver show the strong intensity at the (111) index, and the other lattice planes show the weak intensities in agreement with the present result.31,34 Figure 3 exhibits the UV-vis absorption spectra of HAuCl4 solution before and after reaction. The HAuCl4 solution at room temperature had a yellow tint, and its absorption spectrum displayed a sharp band at a short wavelength of about 320 nm. After the reaction, the absorption band around 320 nm diminished and the produced polyhedral nanoplates induced a different absorption spectrum at 600 nm over with broad absorption, changing the color to reddish brown. In general, when major products are spherical gold nanoparticles, the sharp band appears around 550 nm by plasmon resonance.34 The absorption band blue-shifts with reduction of the particle size,35 whereas the absorption band apparently red-shifts with broad adsorption by agglomeration of nanoparticles. According to some reports on 2 D nanoparticles of gold and silver, in particular nanoprisms,

Figure 5. SEM images of various morphologies of gold particles obtained at different HAuCl4 concentrations (PVP/HAuCl4 )1.5 for 20 min at 155 °C): (a) 0.01, (b) 0.03, (c) 0.05, and (d) 0.15 M.

the absorption spectra normally exhibit 2 or 4 peaks because of mutually different dipole resonance.28,36,37 In the present sample, however, the absorption spectrum of the polyhedral gold nanoplates appeared broadly with a similar tendency to agglomerated gold nanospheres. This may be due to the formation of large polyhedral plates greater than ca. 300 nm.38,39 Achievement of High PNF Value. Figures 4–7 show the SEM images of gold particles obtained under different conditions. Depending on the reaction conditions, various morphologies appeared: trihedral, tetrahedral, and hexahedral plates and spherical particles in the size range from 100 nm to greater than



Figure 6. SEM images of various morphologies of gold particles obtained at different reaction temperature (0.1 M HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 for 20 min); (a) 135, (b) 145 and (c) 165 °C.

10 µm. The fractions of different morphologies were calculated on the number base by observation of more than 300 particles on the SEM images. The size of particle was determined for the edge length of trihedral and tetrahedral plates, the maximum diagonal of hexahedral plates and the diameter of spherical particles. Figure 8 shows the fractions of different morphologies obtained at 0.1 M HAuCl4 and PVP/HAuCl4 ) 1.5 for 20 min at 155 °C. Under these conditions, trihedral, tetrahedral, and hexahedral nanoplates were formed with average sizes of 280, 390, and 310 nm, respectively although a small amount of nanospheres was included with an average size of 120 nm. The polyhedral nanoplate fraction (PNF) reached the highest value of 96% totally for trihedral, tetrahedral, and hexahedral nanoplates. The major product was trihedral plate with a fraction of 72%. Hexagonal and tetragonal plates formed at totally 24% in fraction. The fraction of nanospheres was very small. In comparison, the PNFs from other liquid-thermal methods are fairly small as mostly 20-40% and 70% at maximum, judging from SEM or TEM images reported so far.28,29,31–33 As confirmed from the above results, the rapid heating process in the present study was effective to fabricate gold nanoplates with a high fraction compared to the conventional heating process.

Figure 7. SEM images of various morphologies of gold particles obtained at different reaction times (0.1 M HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 at 155 °C): (a) 10, (b) 40, (c) 60, and (d) 120 min.

The effects of experimental conditions on the PNF are presented in detail below. Effect of PVP/HAuCl4 Weight Ratio. Figure 9 shows the morphological fraction (a) and average size of gold particles (b) at different weight ratios of PVP/HAuCl4 (0.8-2). When the PVP weight ratio was 0.8-1.5, the major product was trihedral plates. The fraction of trihedral plates increased from 51 to 72% with an increase in PVP weight ratio. The fraction of hexahedral plates ranged from 14 to 22% and the fraction of tetrahedral plates was small. The fraction of nanospheres


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Figure 8. Morphological fraction of gold particles obtained under the conditions in Figure 1. The average particle sizes are written in the figure.

Figure 10. Morphological fraction and average particle size at different concentrations of HAuCl4 (PVP/HAuCl4 ) 1.5 for 20 min at 155 °C).

Figure 9. Morphological fraction and average particle size at different weight ratios of PVP/HAuCl4 (0.1 M HAuCl4 for 20 min at 155 °C).

decreased from 25 to 3% with increasing PVP weight ratio. On the other hand, when the PVP weight ratio was 2, the fraction of trihedral plates significantly decreased, whereas the fraction of nanosheres increased. The sizes of trihedral, tetrahedral, and hexahedral nanoplates were in the range of 130-320, 160-360,

Figure 11. UV-vis absorption spectra of gold particles with different concentrations of HAuCl4 (PVP/HAuCl4 ) 1.5 for 20 min at 155 °C).

and 250-580 nm, respectively. There was a tendency for the hexahedral nanoplates to be larger than trihedral and tetrahedral ones. The sizes of the polyhedral plates increased with an increase in PVP weight ratio from 0.8 to 1.1 but decreased with a further increase in the PVP weight ratio. The size of



Figure 12. Morphological fraction and average particle size at different reaction temperatures (0.1 M HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 for 20 min).

Figure 13. Morphological fraction and average particle size at different reaction times (0.1 M HAuCl4 and weight ratio of PVP/HAuCl4 ) 1.5 at 155 °C).

nanospheres was 150-200 nm and decreased with an increase in the PVP weight ratio. As described above, the PVP addition significantly enhanced the formation of polyhedral nanoplates, expecially trihedral nanoplates, with retardation of nanosphere formation. However, the excessive PVP addition was not effective. Although the sizes of the polyhedral nanoplates increased by PVP addition, a large amount of PVP retarded particle growth. Influence of HAuCl4 Concentration. Figure 10 shows the morphological fraction (a) and average size of gold particles (b) at different HAuCl4 concentrations (0.01-0.15 M), where the PVP/HAuCl4 weight ratio was kept at 1.5. When HAuCl4 was added at 0.01 M, nanospheres were only formed with a 160 nm size. By increasing HAuCl4 concentration to 0.03 and 0.05 M, spherical particles grew closely to 1 µm, and polyhedral plates appeared with a size ranging from 500 nm to 1 µm. With the increase in HAuCl4 concentration, the fraction of spherical particles decreased, whereas the fraction of polyhedral plates increased. The high fraction of trihedral plates was noteworthy at 0.1 M HAuCl4. However, the selective formation of trihedral plates was not observed at larger HAuCl4 concentrations. It should be noted that the size of gold particles decreased at 0.1 M HAuCl4 and above for both polyhedral plates and spherical particles. These results suggest that polyhedral plates, especially the trihedral one, tend to form on the nanosized level.

Figure 11 shows the variation in the UV-vis absorption spectrum of gold particles with HAuCl4 concentration. When the concentration of HAuCl4 was 0.01 M, a sharp absorption band appeared at 546.6 nm due to the formation of nanospheres. With an increase of HAuCl4 concentration, the band was broadened and red-shifted, indicating the formation of polyhedral gold nanoplates. Dependence on Reaction Temperature and Time. Figure 12 shows the morphological fraction (a) and average size of gold particles (b) at different temperatures. When the reaction temperature was 135 °C, large gold particles formed with a size over micrometer level in shapes of trihedral, tetrahedral and hexahedral plates and spherical particles. The particle size decreased with an elevation of temperature, reaching nanosize at 155 °C and above. The fraction of trihedral plates increased with raising temperature from 135 to 155 °C. The fraction of hexahedral plates increased at 145 °C but decreased at higher temperatures. The fractions of tetrahedral plates and spherical particles also decreased with increasing temperature. The high fraction of trihedral plateds at 155 °C on the nanosized level was noteworthy but the fraction of trihedral nanoplates decreased at 165 °C with the increase in the fraction of nanospheres. Figure 13 shows the morphological fraction (a) and average size of gold particles (b) at different reaction times. At times of 20 min and less, the particle was sized on the nanometer level. The particle size increased remarkably over micrometer level


with time. The fraction of trihedral plates was high in the early stage but decreased at 40 min and above. On the other hand, the fraction of hexahedral plates and spherical particles increased. After 120 min, large polyhedral plates tended to be fractured because of an external stress during stirring. In this case, there was a large amount of precipitate in a reaction vessel. Discussion on Formation of Polyhedral Nanoplates. In the present reaction system, gold particles form by reduction of HAuCl4. The particle size is determined by the rates of nucleation and crystal growth. After the precipitation, the dissolution-reprecipitation process, the so-called Ostwald ripening, should be concerned, during which small particles are dissolved and digested by large particles, leading to particle growth with different morphologies. PVP played a critical role to form gold nanoplates. PVP molecules may interact selectively with different facets of gold crystals. According to the morphological anisotropy of gold particles with fcc crystal structure, for example, trihedral nanoplate shown in Figure 1, PVP molecules seem to interact strongly with (111) facet and limit the crystal growth in (111) direction. The weak interaction of PVP molecules with the other facets leads to fast growth to the anisotropical directions. A similar mechanism is assumed for tetrahedral and hexahedral plates. The increase in the fraction of trihedral plates by addition of PVP was noteworthy, as seen in Figure 8. The size of polyhedral plates increased with the increase in PVP weight ratio at small quantities because of the selective formation of polyhedral plates. The particle size was reduced by the addition of a large quantity of PVP. This phenomenon means that the adsorption of many PVP molecules retards the crystal growth. When the PVP content is large, PVP molecules should be adsorbed on the whole surface of growing particles. Therefore, the anisotropic growth of trihedral plates was retarded, leading to spherical growth of gold particles. Morphologies and fractions of gold plates were also affected by HAuCl4 concentration, reaction temperature and time. When HAuCl4 concentration was low, spherical particles mainly formed, and the polyhedral plates appeared with an increase in HAuCl4 concentration. The increase of particle size with increasing HAuCl4 concentration means that the crystal growth was more enhanced compared to the nucleation rate. When the HAuCl4 concentration was more increased, the particle size decreased, whereas the fractions of polyhedral plates increased. However, the selectivity of trihedral plate was reduced at higher HAuCl4 concentration. Because the PVP/HAuCl4 weight ratio was kept constant in the present work, the PVP content became large with increasing HAuCl4 concentration. The large quantity of PVP seems to reduce the particle size and enhance the selective formation of polyhedral plates in the similar manner to the PVP/HAuCl4 weight ratio. The elevation of temperature decreased the particle size. This means that the nucleation was more stimulated. When the particle size was large, the fraction of polyhedral plates was not large. The trihedral plate tended to selectively form on nanometer size at high temperatures. As seen in the time dependence of morphological fraction and particle size, the trihedral plates mainly formed in the early stage of reaction. Because HAuCl4 diminished in the early stage, the change in size and morphology with time may be caused by the Ostwald ripening. According to the experimental results, the trihedral nanoplates were digested with time to form large spherical particles and hexahedral plates. As shown in the above results, the trihedral nanoplates are thought to be a metastable form under the presence of PVP. To

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increase the selectivity of trihedral nanoplates, the required conditions are the appropriate amount of PVP, short reaction time, and high temperature. The high temperature is needed to form nanoparticles. However, the adsorption of PVP is weakened, leading to the decrease in the fraction of trihedral nanoplates at too high a temperature. Conclusions It was found that injection method of HAuCl4 and PVP solutions can directly affect morphology and uniformity of gold nanoparticles. High PNF was obtained straight-forwardly in very short reaction time in the rapid heating process. Long reaction time and complex procedure were not needed to fabricate nanoplates such as trihedral plate. Anisotropic growth is induced through selective adsorption due to appropriate control of amount between HAuCl4 and PVP under rapid heating. Different reducing rates of HAuCl4 can be induced by controlling the reaction temperature. Under these conditions, the adjustment of the concentration of HAuCl4 and PVP is critical for reaching high contents of nanoplates. Acknowledgment. The present work is supported by a Grant for the 21st century COE program from the Ministry of Education, Culture, Sports and Technology of Japan.

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High Fraction Synthesis of Two-Dimensional Nanoparticles through

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