Preparation of Concentrated Gold Nanoparticle Paste and

469

Original Technical Paper

J. Jpn. Soc. Colour Mater. (SHIKIZAI) , 76[14,469-475 (2003)

Preparation of Concentrated Gold Nanoparticle Paste

and Its Application to Paint Colorant

Hiroki KAMO*, Hideo ISHIBASHI* * and Toshikatsu KOBAYASHI*

Abstract

A novel preparation method of concentrated gold nanoparticle pastes was developed. A comb-shaped block

copolymer was used to prevent the mutual coagulation of gold particles. Certain kinds of amines were found to reduce

Au3+ ion at an industrially preferable and controllable rate. The maximum metal concentration in the paste was 20%

and the paste also contained 10% of the protective polymer. Gold nanoparticles having diameter of a few tens

nanometer were known to exhibit red color due to plasmon light absorption. The gold nanoparticle paste obtained

in this study was applied as a paint colorant and proved to show an aesthetic color and high weather durability.

Key- words : Gold nanoparticle, Concentrated paste, Paint colorant, High weather durability

1. Introduction

Metal nanoparticles are of interest in a variety of

field including electronics'), optics'), chemical catalysis')

and metal film patterning by ink jet printing. Nano-

sized gold particles are known to exhibit a clear and

stable red color due to light absorption around 520 nm

by its surface plasmon. This type of red coloring has

been utilized in a glass industry. Stained glass and high

class tableware such as Venetian glass are typical

examples. The authors intended to reproduce this color-

ing mechanism in paint films.

In order to attain sufficient coloring of paint films by

gold nanoparticles like glassware, we have to prepare a highly concentrated gold paste. This was because the

thickness of paint film is generally much thinner than

that of glassware. Our rough calculation required pro-

duction of a paste containing more than a few percents

of gold nanoparticles. Moreover, since the wavelength

of absorbed light depends on the particle size, uniform

particle size distribution was indispensable for clear color.

As for the preparation methods of nano-sized gold

particle, a lot of work has been reported. These

methods could be roughly classified into two categories.

One is the reduction method of Au3+ ion in liquid phase

in the presence of adequate protective agents against

mutual coagulation of gold particles. Citric acid has

often been used to reduce Au3+ ion and also to protect

the formed gold nanoparticles5-8). Sato et al. found that

SurfinolTM 465, a surfactant with acetylenic linkage in

the main chain, also behave as both a reductant and a

protective agent9). Esumi et al. utilized dendrimers as the protective agent for the reduction of Au3+ ionio---14)

by UV irradiation and by addition of NaBH4. The gold

nanoparticles obtained by this kind of method generally

exhibit rather clear color. However, no works reported

preparation at higher concentration more than 1%. This is because that the prevention of mutual coagula-

tion of gold nanoparticles becomes much more difficult

as the concentration increased.

The other category of preparation method is called

physical vaporization method'45-18). In this method,

gold ingot is placed in vacuum system, then heated and vaporized in inert gas such as Ar and He at reduced

pressure. The vaporized gold atoms come into collision each other and grow up to particles via clusters. The

formed particles are corrected by adequate methods

such as cold trap. Thus relatively higher concentration

of gold nanoparticles is attainable. The authors prelimi-

nary investigated the coloring properties of the gold

nanoparticles obtained by this method and found that

the color clearness was insufficient for a paint colorant.

Received June 1, 2003* Trade Use Paint , ** Innovative Technology Lab. Nippon

Paint Co., Ltd., 19-17, Ikeda-Naka, Neyagawa, 572-

8501 Japan

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470 Original Technical Paper SHIKIZAI

Wide distribution of the particle size may be the reason

for the dull color.

Since no methods were found to prepare a concen-

trated paste of nano-sized gold particles with sufficient

clear color for paint colorant, the authors decided to

develop a novel method. In order to insure the color

clearness, we adopted a reduction method in liquid

phase. Accordingly, a point of issue was how to prevent the mutual coagulation of gold nanoparticles at the

increased concentration. In paint industry, a specially

designed comb-shaped block copolymers were widely

used to stabilize the dispersed states of high perfor-

mance pigments'9'"). In this study, a similar type of the

polymer used in the pigment dispersion was adopted to

prevent the mutual coagulation of gold nanoparticles. In addition, fatty amines were found to reduce Au3+ ion.

The rate of this reaction was relatively low, which is

suitable for the industrial production. Combination of

the use of the amines for the reduction and the use of

the comb-shaped block copolymer for the protection

enabled the production of the concentrated paste

containing more than 20% of gold nanoparticles. Rough

information about this concentrated paste was

introduced previously21,22). In this report, precise experi-

mental condition and effect of amine species are discus-

sed.

2. Experimental

2.1 Materials

Tetrachloroauric (III) acid tetrahydrate (HAuC14

H2O) was purchased from Tanaka Kikinzoku Kogyo

and used without further purification.

A water-soluble polyacrylate-based comb-shaped

block copolymer (Polymer-W) was used for prepara-

tion of the aqueous gold nanoparticle pastes. Acid and

base amounts evaluated by potentiometric titration")

were 0.53 and 0.37 mmol g-1, respectively. A polyester -based comb-shaped block copolymer (Polymer-S)

was used for the solvent type pastes. Acid and base

amounts were 0.58 and 0.64 mmol g-', respectively.

Molecular weight of the both polymer was several tens

thousands. As schematically shown in Figure 1, Poly-

mer-W and Polymer-S possessed secondary or tertiary

amino groups in the main chain and were expected to

adsorb on the gold nanoparticles by these amino groups.

Reagent grades of 2-aminoethanol, 3-amino-l-

propanol, 2- (methylamino) ethanol, diethanolamine, 2-dimethylaminoethanol, N-methyldiethanolamine, N,N,

N',N'-tetramethylethylenediamine (up to here from

Kishida Chemical) , N, N-dimethylethylamine and N,N-diethylmethylamine (both from Tokyo Kasei Kogyo)

were used without further purification for reduction of

Au3+ ion.

Reagent grades of acetone and toluene (both from

Kishida Chemical) and ion exchanged water were used

as solvents for the reaction and dilution of the pastes

and paints.

2.2 Preparation of the aqueous pastes

Polymer-W (30 g) was dissolved in 0.1 M HAuC14

aqueous solution (1000 mL) in 2-L flask. The solution

was stirred at room temperature for 10 min. Predeter-

mined amount of amine was added to the solution,

which was kept stirred for 1 h. Residual ionic species

were removed from the mixture containing the gold

nanoparticle by electrodialyzer (Micro AcilyzerTM S-6,

Asahi Kasei) .

2.3 Preparation of the solvent type pastes

Polymer-S (12 g) was dissolved in acetone (400 mL)

in 1-L flask and 1 M HAuC14 aqueous solution (100 mL)

was added. The opaque mixture was stirred at room

temperature for 10 min. Predetermined amount of

amine was added to the mixture, which was kept stirred

for 1 h and left stand for 24 h. Settled dark red compos-

ite consisting of gold nanoparticle and Polymer-S was

collected, which was washed with water to remove

ionic species and dried under vacuum, then dissolved in

toluene.

2.4 Characterization of the pastes

2.4.1 Metal concentration

Solid content in the pastes was determined using

ordinary gravitational method, removing the solvents

by heating at 140°C for 20 min. Metal concentration in

the solid part of the pastes was evaluated by thermo-

gravimetric analyzer (TG-DTA 220, Seiko Instru-

Fig.1 Schematic illustration of the gold nanoparticle

stabilized by the comb-shaped block copolymer.

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色材, 76 〔12〕 (2003)

Preparation of Concentrated Gold Nanoparticle Paste and Its Application to Paint Colorant 471

ments) . Solid part of the pastes (ca. 10 mg) was heated

from room temperature to 500 °C at the rate of 5 C

min- Preliminary experiments had elucidated that

both Polymer-S and Polymer-W completely degrade at

250-450°C

2.4.2 Optical properties

Light absorption spectra were recorded by a spectro-

photometer (MCPD-1000, Otsuka Electronics) for 30000-time-diluted pastes. Ion exchanged water and

toluene were used for dilution of aqueous and solvent

type pastes, respectively.

2.5 Preparation of paint films and their prop-

erties

The solvent type gold nanoparticle paste was added

to a conventional automotive acrylic clear top coat

system to form colored clear top coats. The gold con-

centration in the solid contents was adjusted to 0.5 and

2 wt%. The clear coats containing the gold nanoparticle

were air-sprayed on a conventional silver-metallic base

coat containing aluminum flake and set for 15 min at

room temperature and then baked at 140°C for 20 min.

The aqueous-type gold nanoparticle paste was added

to a silver metallic automotive water-borne base coat

system to form silver-red colored base coats. The

concentration of the gold nanoparticle and aluminum

flake in the solid contents were 1.5 and 13.5 wt%,

respectively. The base coats were air-sprayed on the

primed steel panel and set for 15 min. Then a conven-tional clear top coat was over-sprayed and set for 15

min and baked at 140°C for 20 min. A flat and a car-

shaped steel panels were used as substrates.

The color appearance was visually inspected by color

designers of Nippon Paint. Also, angular dependency of

reflected light from the test panel containing gold

nanoparticle in the clear coat was measured. The reflec-

tion profile was compared with that without the gold

nanoparticles and that containing transparent type iron

oxide (120 nm) instead of the gold nanoparticles. Ac-

celerated weather durability was checked by Super

Xenon Weather Meter and evaluated by the color

differences (AL, Aa, Ob, AE) between before and after

1200-h exposure.

3. Results and discussions

3.1 Reduction of Au3+ ion by amines

For the confirmation of the reduction of Au3+ ion by

the addition of amines, 2-dimethylaminoethanol (50

mL) was mixed with 0.1 M HAuCl, solution (1000 mL)

in the absence of protective polymer. Immediately after

the mixing, the yellow color of Au3+ ion vanished, and

a few seconds later, red color appeared. Then it turned

blue via purple, and black sediment settled finally. The

collected black sediment was washed by water and

dried under vacuum at room temperature. X-ray dif-

fractive pattern of the black sediment was shown in

Figure 2 together with the standard diffractive pattern

of metal gold. The positions of the diffraction peaks

were completely coincided with those of standard metal

gold, indicating that Au3+ ion was reduced by the addition of 2-dimethylaminoethanol.

In a similar experiment in the presence of Polymer-

W (50 g) , a uniform dark red mixture was obtained.

The light absorption spectrum of the purified mixture is

shown in Figure 3. The peak position (530 nm) is simi-

lar to the reported value5'9) for the plasmon light absorp-

tion by gold nanoparticles having diameter of 15-20 nm.

TEM photograph of thus obtained particles in the

mixture is shown in Figure 4. Well dispersed particles

Fig.2 X-ray diffractive pattern of the black sediment obtained by the addition of 2-dimethylaminoeth-

anol to 0.1 M HAuCl, solution and the standard diffractive pattern of metal gold.

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with diameter around 15 nm were observed. Thus the

dark red mixture obtained in the presence of Polymer-

W was revealed to be a uniform and highly concen-

trated (ca. 2%) dispersion of gold nanoparticle.

The molar ratio of added amine to HAuC14 was

varied from 1 : 1 to 12 : 1 and the appearance of mix-

ture was investigated. As shown in Table 1, more than

1.5 times molar ratio was necessary to reduce Au' ion.

This result suggests that the lone pair of nitrogen atom

was responsible for the reduction of Au3+ ion. However,

1.5-3.5 times amine amount was not sufficient to yield

stable gold nanoparticle ; that is, only large black parti-

cles were obtained. Formation of stable gold nanoparti-

cle dispersion, indicated by dark clear red color, was

attained when at least 4 times amount of amine was

added.

Similar experiments about reduction of Ag+ ion and

formation of silver nanoparticles have revealed that 0.5

times amount of amine was necessary to reduce Ag+

ion24). This again suggests that the lone pair of nitrogen

atom of the amine was responsible for the reduction.

The experiments for Ag have also revealed that at least

2 times amine amount was required to yield stable

dispersion.

The coordination number of Au" and Ag+ are 4 and

2, respectively. Since the coordination number of Au'

and Ag+ coincides with the least amine amount to yield

stable dispersion of each nanoparticle, the formation of

an amine-Au" (Ag+) complex would contribute to the

preparation of the stable gold (silver) nanoparticle dispersion. This will be discussed again in the next

section.

The amine ratios more than 6 resulted in unstable

dispersions. The reason would be that the excess

amount of amine interfered with the adsorption of

polymer through substituting the aminogroups of Poly-mer-W on the gold surface.

3.2 Effect of amine species

Amines in section 2.1 were used to prepare the gold

nanoparticle according to procedure described in sec-

tion 2.2. Time from the addition of the amines to the

point that the mixture began to show red color was recorded and summarized in Table 2. Molar ratio of the

amine and HAuC14 was 5 : 1.

Tertiary amines showed the highest formation rate,

followed by secondary amines. Primary amines did not

yield the gold nanoparticles. Generally, reaction rates of amino compounds lie in the order primary > secon-

dary > tertiary. The observed order in this study was

opposite. This result also can be explained by the

assumption of the contribution of amine-Au" complex.

Possible reaction mechanism is as follows. Amines

coordinate to Au' first. Stability of the amine-coor-

dinated Au' complex depend on the class of amine ;

that is, stability will be in the order primary > secon-

dary > tertiary. Unstable tertiary amine-Au3+ complex

degraded easily and Au3+ fell to more stable Au atom

Fig. 3 The light absorption spectrum of the gold nanopar-ticle pastes (30000 times dilution).

Fig. 4 TEM photograph of the gold nanoparticle.

Table 1 Effect of amine/HAuC14 ratio to the formation of gold nanoparticle

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色材, 76 〔12〕 (2003)


subtracting electrons from environment. Exact electron

donor is not identified at the moment. Since primary

amine-Au3+ complex were stable and the reaction did

not proceed further. Disappearance of yellow color of

Au3+ was thought to be an indication of the amine-Au3+

complex formation.

In order to strengthen the above assumption, we used

N,N,N',N'- tetramethylethylenediamine (TMED) ,

known as a strong chelating compound, to form the

gold nanoparticles. Though TMED classified as tertiary amine, the time taken to yield the red colored mixture

was 700 s (Table 2) , similar to those of secondary

amines. This is because that the TMED-Au3+ complex

was more stable than those of ordinary tertiary amines

and prolonged time was necessary to degrade.

3.3 Preparation of concentrated pastes

From the above study, molar ratio of amine to

HAuC14 was set to 5 : 1 in the procedure described in

sections 2.2 and 2.3. Aqueous pastes were concentrated

by removing water using a rotary evaporator. Final

composition of the aqueous paste was gold nanoparticle

(16%) , Polymer-W (20%) , and water (64%) . The

solvent type paste consisted of the gold nanoparticle

(20%) , Polymer S (10%) , and toluene (70%) . A part of

Polymer-W and Polymer-S was seemed to be lost in

the purification process.

3.4 Properties as paint colorant

A car-shaped panel painted with the colored clear top

coat containing the gold nanoparticle (2 wt%) on a

silver base coat (aluminum concentration ; 13.5 wt%)

is shown in Figure 5. The color difference between the

illuminated and shaded areas were much larger than the

paint system with the conventionally pigmented colored clear top coats. This type of appearance was judged to

be highly aesthetic by the designers.

The angular dependence of light reflection by the

above coating painted on a flat panel is shown in Figure

6. Also in this figure, those by top-coatings with an

ordinary noncolored clear paint and with a colored

clear paint containing a transparent type iron oxide (5

wt%) are shown. A decrease from the highlighted

region (around 45•‹) toward -70•‹ (the shaded region)

can be seen for the noncolored clear coat. A large

difference in the reflection strength between at the

highlighted and at the shaded regions is created by the

orientation of aluminum flake in the base coat and this

effect is called flip-flop effect. For the noncolored clear

top coating, a relatively large flip-flop effect can be

seen. The coloring of top coat by the gold nanoparticle

enhanced the effect. On the other hand, coloring by the

ordinary transparent type iron oxide pigment resulted

in an increase in the shaded region, spoiling the flip-flop

effect.

This was because the particle size of the iron oxide

was much larger than gold nanoparticle. Even though it

was called transparent type, still the particle size was

120 nm, which is ten times larger than the gold nanopar-

ticles. Light scattering from particulate components in

the coating contributes to the strength of reflected light

and plays definitive role at the diffusion region. The

light-scattering properties of particles largely depend

on their size and show maximum value when the size is

Table 2 Effect of the amine species on the formation rate of the gold nanoparticles

Fig. 5 The car-shaped panel painted by the colored top coating containing the gold nanoparticle (Painted on a conventional silver base coating).

(See color photo on the first page.)

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1/2 of the wavelength. The size of the gold nanoparticle

was about 1/40 of the wavelength and its light scatter-

ing was negligibly small. However, for the iron oxide

particle, the light scattering could not be ignored because its particle size was more than 1/6 of the

wavelength.

Results of weather durability test for the coatings

containing gold nanoparticle are summarized in Table

3. All the panels tested showed color change (AE) of

less than 1. Thus the gold nanoparticles obtained in this

study were proved to possess a high durability.

References

1) M. Oda : Hyomen Gijutsu, 47, 910 (1996).2) S. Ogawa, Y. Hayashi, N. Kobayashi, T. Tokiza-

ki and A. Nakamura : Jpn. J. Appi. Phys., 33, Part 2, L 331 (1994).

3) M. Haruta : Gendai Kagaku, No. 5, 42 (1998).4) T. Oguchi, K. Suganami, T. Nanke and T.

Kobayashi : Int. Conf. Digital Printing Technol.,

(2003) , to be published.

5) J. Turkevich, P. C. Stevenson and J. Hiller : Trans. Faraday Soc., 11, 55 (1951).

6) M. K. Chow, C. F. Zukoski : J. Colloid Interface Sci., 165, 97 (1994).

7) L. M. Liz-Marzan, M. Giersig and P. Mulvaney : Langmuir, 12, 4329 (1996).

8) Y. Mori, Y. Murao and Y. Miyake : Funtaikouga-kukaishi, 33, 199 (1996).

9) S. Sato, N. Asai and M. Yonese : Colloid Polym. Sci., 274, 889 (1996).

10) K. Esumi, A. Suzuki, N. Aihara, K. Usui and K. Torigoe : Langmuir, 14, 3157 (1998).

11) K. Esumi, T. Hosoya, A. Suzuki and K. Tor-igoe : Langmuir, 16, 2978 (2000).

12) K. Esumi, T. Hosoya, A. Suzuki and K. Tor-igoe : J. Colloid Interface Sci., 229, 303 (2000).

13) K. Esumi, A. Kameo, A. Suzuki and K. Torigoe : Colloids Surf, A, 189 (1-3), 155 (2001).

14) K. Esumi, S. Sarashina and T. Yoshimura : J.

Jpn. Soc. Colour Material, 75, 513 (2002).15) K. Kimura and S. Bandow : Bull. Chem. Soc.

Jpn., 56, 3578 (1983).16) B. M. Sergeev, I. A. Gromchenko and G. B. Ser-

geev : Moscow Univ. Chem. Bull., 49, 38 (1994).

Fig. 6 Angular dependencies of the reflected light from;-------

: the colored top coating containing the gold nanoparticle.-------

: the colored top coating containing the transparent type iron oxide.-------

: the noncolored clear top coating.

Table 3 Weather durability of coatings containing the gold nanoparticles

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色材, 76 〔12〕 (2003)


17) R. Whyman : Gold Bull., 24, 11 (1996).18) K. Kimura : Hyomen, 34, 143 (1996).19) H. L. Jakubauskas : J. Coat. Tech., 58, 71 (1986).20) A. C. D. Cowley : J. Oil Colour Chem. Assoc., 70,

207 (1987).21) T. Kobayashi and H. Kamo : Kagaku To Kogyo,

53, 909 (2000).

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24) H. Ishibashi, H. Kamo and T. Kobayashi : to be submitted.

金ナノ粒子濃厚ペーストの調製と塗料用色材としての応用

加茂比呂毅*・石橋秀夫**・小林敏勝**

*日本ペイント株式会社汎用塗料事業本部開発部,**R&D本部創造技術研究所大阪府寝屋川市池田中町19-17 (〒572ー8501)

要旨

濃厚な金ナノ粒子ペーストの新規な調製方法を開発した。特定の櫛型ブロック共重合体を保護剤として用いて,金粒子問の凝

集を抑制した。また,特定のアミンを用いることにより,工業的に制御しやすい速度でAu3+イオンを還元できることを見いだ

した。ペースト中の最高金属濃度は20%であり,さらに10%の保護ポリマーを含有している。粒子径が数10nmの金ナノ粒子

はプラズモン光吸収により赤色を呈することが知られている。本研究ではこれを塗料用色材として用いた場合,美しい外観と高

度な耐候性を併せ持つことを見いだした。

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Documents

Preparation of Concentrated Gold Nanoparticle Paste and