Journal of Colloid and Interface Science 438 (2015) 244–248

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Characterization of citrates on gold and silver nanoparticles Priastuti Wulandari a,b, Takeshi Nagahiro a, Nobuko Fukada c, Yasuo Kimura a, Michio Niwano a, Kaoru Tamada a,d,⇑ a

Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Physics, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung 40132, Indonesia c Advanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba 305-8561, Japan d Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812-8581, Japan b

a r t i c l e

i n f o

Article history: Received 12 August 2014 Accepted 28 September 2014 Available online 13 October 2014 Keywords: Vibration of adsorbed molecules Infrared spectroscopy Metal nanoparticles Carboxylate

a b s t r a c t In this paper, we report different coordinations of citrates on gold (AuNP) and silver (AgNP) nanoparticles, as determined using Fourier transform infrared spectroscopy (FTIR) and molecular orbital (MO) calculations. AuNPs and AgNPs are found to have completely different interactions with the carboxylate anchoring groups, as indicated by their unique asymmetric stretching vibrations in the FTIR spectra. The mas (COO) of citrate exhibits a high-frequency shift resulting from the formation of a unidentate coordination on AuNPs, whereas this vibration exhibits a low-frequency shift as a result of ionic bond formation on AgNPs, as predicted from the MO calculations of the corresponding metal complex salts. The enhancement in the IR signals when their vibration direction was perpendicular to the nanoparticle surface revealed the influence of localized surface plasmons excited on the metal nanoparticles. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Colloidal metals or metal nanoparticles (metal NPs) have been intensively investigated in recent years due to their unique electronic, chemical and optical properties, which differ from those of their bulk counterparts [1–4]. These small metal NPs with various functional groups have been reported to have potential applications in microelectronic devices, biosensors, catalysts, solar cells and other fields [5–10]. Capping ligands on metal NPs are quite important for surface passivation to prevent the aggregation and fusion of metal cores [11–18]. It is also well known that the physical and chemical properties of NPs, such as their dispersibilities in solvents, are controlled by the capping organic molecules [18]. Because the specific properties of metal NP architectures are determined not only by the properties of individual particles but also by their assembled structures, it is crucial to control the interactions and the gap distance between the particles through the design of an organic interface on the metal cores [19]. Citrate has been employed as a common reducing and stabilizing agent for colloidal Au dispersions, as reported by Frens in 1973 (Fig. 1) [12]. Following this report, the adsorption of citrate ⇑ Corresponding author at: Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashiku, Fukuoka 812-8581, Japan. E-mail address: [email protected] (K. Tamada). http://dx.doi.org/10.1016/j.jcis.2014.09.078 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

on Au has been carefully investigated in the field of surface science. Sandroff and Herschbach obtained direct evidence for the presence of citrate anions adsorbed on colloidal Au surfaces using surfaceenhanced Raman spectroscopy [20]. Biggs et al. measured the forces between a Au-coated colloidal silica sphere and a pure Au plate as a function of the citrate concentration using atomic force microscopy (AFM) [21]. Floate et al. investigated the specific adsorption of citrates from perchloric acid electrolytes onto Au (1 1 1) electrodes using in situ FTIR spectroscopy and proposed a model for the surface coordination of citrate [22]. Recently, Lin et al. succeeded in imaging the structure of citrate on Au (1 1 1) through the use of in situ scanning tunneling microscopy (STM) [23]. The study of carboxylates adsorbed on Ag has also become a current research interest for use as a stabilizer in the synthesis of AgNPs. Myristic acid and oleic acid are commonly used as capping molecules for the synthesis of AgNPs via thermal reduction [16–18]. The interaction between carboxylate and a flat Ag substrate has also been investigated. Chau and Porter found that perfluorocarboxylic acid symmetrically bonded with the flat Ag substrate with carboxylates as a bridging ligand [24]. Although many studies have been conducted to investigate the interactions of carboxylates with flat Au and Ag substrates, all of these studies focused on physisorbed molecules (weakly bound carboxylates, which can be rinsed out), i.e. the conformation and structure of the carboxylates in these reports are not required to be identical for the carboxylate ligands to strongly bind to the metal particles.

P. Wulandari et al. / Journal of Colloid and Interface Science 438 (2015) 244–248

In this study, we attempt to determine the coordinations of carboxylates in citrate-capped AuNPs and AgNPs using FTIR spectroscopy with consideration of the different metal cores. We also discuss the influence of localized surface plasmons excited on the metal cores on the vibration modes of the carboxylates.

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calculated using the Gaussian 98 program [33] under vacuum conditions at 298.2 K. The results were displayed as 3D images using the MolStudio software.

3. Results and discussion 2. Experimental Citrate-capped AuNPs were prepared in water by following the Frens method [12]. 90 mL of an aqueous HAuCl4 solution (0.05% by weight) was heated to its boiling temperature under reflux. Then, 10 mL of trisodium citrate (1% by weight) in an aqueous solution was rapidly injected into the solution. After 2 min of reaction time, the color of the reaction solution changed from yellow to deep purple, indicating the formation of AuNPs. The AuNP solution exhibited a homogenous red wine color after purification. The synthesis of citrate-capped AgNPs was conducted at room temperature by mixing 100 mL/0.1 mM of AgNO3 and trisodium citrate aqueous solutions under stirring at 570 rpm. The reaction was completed by adding a 10 mL/2 mM NaBH4 aqueous solution as a reductant. After 3 h of reaction time, the reaction mixture exhibited a yellow color, indicating the formation of AgNPs. The citrate-capped AuNPs and AgNPs were both purified more than 4 times to completely remove the free citrates in solution [25]. The purified solutions could be stored for several weeks at room temperature without aggregation. The conformations and coordinations of citrate on the AuNPs and AgNPs were confirmed by FTIR spectroscopy [26–31]. The FTIR measurements were conducted using the KBr method in transmission mode with a resolution of 8 cm1 and a total of 100 scans. The experimental AuNP and AgNP IR data were compared with the simulation results. The molecular orbital (MO) calculations were performed using the corresponding metal acetates as model compounds [32]. The optimized structures and vibration modes were

Fig. 1. Molecular structure of trisodium citrate (C6H5Na3O7).

The syntheses of citrate-capped AuNPs and AgNPs were confirmed by the surface plasmon absorbance bands at 525 nm for AuNPs and at 397 nm for AgNPs in aqueous solutions (Fig. 2). The transmission electron microscopy (TEM) data revealed that the diameters of the spherical NPs were 24 nm for AuNPs and 5 nm for AgNPs (data not shown) [25,34]. Fig. 3 presents the FTIR spectra of the citrate-capped AuNPs and AgNPs. Peaks for carboxylate asymmetric stretching (mas (COO)) and symmetric stretching (ms (COO)) from trisodium citrate appeared at 1591 cm1 and 1399 cm1, respectively. For citrate on the AuNPs, the mas (COO) peak was found to be largely highfrequency shift from the original peak position. In contrast, the mas (COO) appeared to be slightly low-frequency shift in the case of citrate on the AgNPs. The ms (COO) on the AuNPs and AgNPs are both low-frequency shift to 1382 cm1, and the peak intensity of ms (COO) on the AgNPs is significantly stronger compared to that on the AuNPs. Fig. 4 presents the comparison between the FTIR data and the MO calculation results of the corresponding metal carboxylates. In our previous study, we found that the coordinations of carboxylates on metallic NPs are reasonably predicted by MO calculations based on the corresponding metal carboxylate molecules [32]. The FTIR data of citrate on the AuNPs exhibited mas (COO) and ms (COO) stretching bands at 1638 cm1 and 1382 cm1, respectively. On the other hand, the MO calculations of CH3COOAu resulted in mas (COO) at 1603 cm1 and ms (COO) at 1421 cm1. The peak positions of both mas (COO) and ms (COO) were consistent between the experiment and the MO calculations. The peak intensities were also quite consistent. The optimized structure obtained from the MO calculation revealed covalent bond formation (unidentate), as shown in Fig. 4(a). The FTIR data of citrate on the AgNPs exhibited mas (COO) and ms (COO) stretching bands at 1585 cm1 and 1382 cm1, respectively, whereas the MO calculation of CH3COOAg revealed mas (COO) at 1429 cm1 and ms (COO) at 1382 cm1. The peak position of ms (COO) was in perfect agreement between the experimental data and the MO calculation; however, the peak position of mas (COO) differed by more than 150 cm1 between the experimental data and the MO calculation. The optimized structure obtained from the MO calculation consisted of an ionic bond between carboxylate and Ag atoms, as shown in Fig. 4(b). The

Fig. 2. UV–Vis spectra of citrate-capped AuNPs (a) and AgNPs (b) in aqueous solutions.

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P. Wulandari et al. / Journal of Colloid and Interface Science 438 (2015) 244–248

Fig. 5. Schematic illustration of the carboxylate stretching bands on AuNPs and AgNPs under the plasmon effect.

Fig. 3. FTIR spectra of citrate on AuNPs and AgNPs at 1800–1300 cm1 in comparison with bulk compound (trisodium citrate).

difference in the peak position of mas (COO) between the experiment and the calculation is potentially due to environmental effects: the IR spectra were collected in KBr, but the MO calculations were performed under vacuum conditions. It is known that the IR data of ionic molecules are sensitive to their surroundings. For example, the data for trisodium citrate also showed different frequencies for mas (COO) between the experiment and the calculation [32]. This result is further evidence that citrate adsorbed on the AgNPs via ionic bonding. The peak intensities between the experiment and the MO calculations are quite different. Although the FTIR data revealed a strong and sharp ms (COO) band at 1382 cm1, the calculation result showed only a weak band similar to that on the AuNPs. The different band intensities of ms (COO) on AuNPs and AgNPs can be explained by the conformation and coordination of carboxylate under the plasmon effect, as schematically illustrated in Fig. 5. The unidentate coordination (anchoring by one oxygen) of citrate on AuNPs may cause the stretching of ms (COO) to be equal to or less than that of mas (COO), as shown in Fig. 5(a). In contrast, the strong ms (COO) band on AgNPs compared with mas (COO) can be explained by ionic bond formation (anchoring by two oxygens), as shown in Fig. 5(b), where the direction of ms (COO), which is perpendicular to the NP surface, is highly enhanced by the plasmon excitation. It is known that metal NPs excite strong electromagnetic fields on their surface, which influence the vibrational

frequencies. In our previous studies, the effect of surface plasmons on the IR spectra of capping thiol molecules was investigated [18]. When AgNPs were capped by isobutyl mercaptan ((CH3)2CCH2SH), the mas (CH3) and ms (CH2) with vibration modes perpendicular to the NP surface were largely enhanced. This phenomenon was only obvious for the case of short capping molecules, which are completely under the influence of localized surface plasmon fields on the particles. Fig. 6 shows wide-scan FTIR spectra (4000–500 cm1) of citrate on AuNPs and AgNPs in comparison with that of trisodium citrate. In the low-frequency region, it was observed that the peak intensities of ms (C–O) on AuNPs at 1261 and 1098 cm1 were very strong, whereas those peaks disappeared on AgNP. A similar trend was found for the COO bending mode (deformation) at 804 cm1. This result can also be reasonably explained by the conformation/coordination model shown in Fig. 5. The ionic bonding of citrate on AgNP by anchoring two oxygens (bridging) may cause the stretching of the C–O mode (parallel to the particle surface) and the bending of COO(d (COO)) to weaken. In other words, the enhanced signals on AuNP were further evidence for the unidentate coordination of citrate by anchoring only one oxygen atom. The above discussion was largely dependent on the coordination of citrate obtained from the MO calculations. Because we conducted the MO calculations using metal acetates as the model compound, the influence of the initial Au position on the energy optimization process was still a concern, especially for the calculation of CH3COOAu. The optimized molecular structure for CH3 COOAu, as shown in Fig. 4(a), was almost identical to the initial structure. There is a possibility that the energy reaches another minimum point if we start the calculation from the different initial molecular structures. We investigated this possibility as follows.

Fig. 4. Comparison of the experimental FTIR spectra for citrate on AuNPs (a) and AgNPs (b) and the MO calculation results.

P. Wulandari et al. / Journal of Colloid and Interface Science 438 (2015) 244–248

Fig. 6. Wide-scan FTIR spectra (4000–500 cm1) of citrate on AuNPs and AgNPs in comparison with bulk compound (trisodium citrate).

When we reached the most stable position (local minimum point) on the surface potential, a real number for the frequency must be obtained (Eq. (1)). If the obtained frequency is an imaginary number, the reached structure must be still in the transition state (saddle point) (Eq. (2)).

1 m¼ 2p

i m¼ 2p

sffiffiffiffi k

l sffiffiffiffi k

l

: real number

ð1Þ

: imaginary number

ð2Þ

In Eqs. (1) and (2), m is frequency, l is reduced mass, and k is known as the spring constant [35]. Thus, by checking whether the frequency is a real number or an imaginary number, we can determine whether the obtained result is on the local energy minimum or on the saddle point. The calculation was conducted using Au formate (HCOOAu) as a model compound to simplify the calculation. Consequently, when Au is located at the side-position (near one oxygen atom) as the initial structure and with a different distance, the optimization always reached the same structure with the unidentate coordination. On the other hand, when Au was located at the center of carboxylate as the initial structure,

Fig. 7. Vibrational frequencies of HCOOAu resulting from MO calculations, in which the negative number indicates the saddle point.

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the optimization process could not reach the local minimum point but rather stopped at the transition state (saddle point), as indicated by the negative frequency (155 cm1). The structure at the saddle point and the vibrational frequency data are shown in Fig. 7. Because the calculation stopped at the saddle point, we attempted to increase the calculation temperature from 298.2 K to 400 K to provide more energy to the atoms. However, the calculation still stopped at the saddle point, i.e. the increasing temperature at experimental ambient was not sufficient to bring it to the local minimum point. Another concern in the MO calculation is the possibility of forming a bridging coordination on AuNPs. To confirm this possibility, we calculated the optimized structures with two Au ions binding one acetate molecule as the initial structure. Here, the optimization process stopped at the saddle point during the middle of the calculation, showing negative frequencies, as shown in Fig. 8. The unstable Au–carboxylate at the bridging position can be explained as follows: when one oxygen atom in carboxylate (COO) forms a bond with a Au atom, another oxygen atom becomes less reactive. As a result, Au does not have a stable position at the bridging position but rather has a unidentate coordination. The coordination of carboxylate was found to be completely different on Au and Ag, unlike thiol (–SH) derivatives that form quite identical self-assembled monolayers by chemisorption on both Au and Ag through chemisorption. There are several historical reports discussing the influence of metal atoms on the carboxylate asymmetric stretching frequency (mas (COO)). For example, Theimer et al. claimed that the radius of the metal ion is the main factor for determining the frequency [36], whereas Kagarise found that the electronegativity of the metal is crucial [37]. Ellis and Pyszora reported that the COO stretching frequency is a complicated function of the mass, radius and electronegativity of the metal [38]. In the noble metal group (Au, Ag and Cu), Au exhibits quite different physical properties from the other metals. Au has a large ionic radius and high electronegativity compared with Ag and Cu, as listed in Table 1. The high electronegativity of Au may be the reason for the slight charge separation between oxygen in molecules and results in the formation of covalent (unidentate) bonding. The large ionic radius may be another reason that Au atoms cannot lie at the center of carboxylate.

Fig. 8. Influence of the initial structure on the MO calculations: two Au atoms at the bridging position (a) or one Au atom at the unidentate position (b).

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Table 1 Comparison of physical properties of noble metal. Physical properties

Ionic radius (1+)/pm Electronegativitya (Pauling) Coordination

Noble metals Cu

Ag

Au

77 1.90

115 1.93

137 2.31

Ionic

a

Unidentate

Additional information of electronegativity values from several elements: Na = 0.9; K = 0.8; Mg = 1.2; Ca = 1.0; O = 3.5; S = 2.5; C = 2.5.

4. Conclusions In this study, the conformations and coordinations of citrate on AuNPs and AgNPs were precisely studied using FTIR spectroscopy. We found that citrate interacts with AuNPs and AgNPs in different manners. For the asymmetric stretching vibration mode, citrate on AuNPs exhibits a high-frequency shift, whereas that on AgNPs exhibits a low-frequency shift. The intensities of the stretching and bending modes also varied on the AuNPs and AgNPs under the influence of the local plasmon field. The MO calculations revealed that these results could be reasonably interpreted by the different conformations and coordinations of citrate on AuNPs and AgNPs, where citrate forms an ionic bond with Ag and a unidentate bond with Au. Although further investigation is necessary, if the simple MO calculation of metal complex salt compounds (e.g. metal acetate in this study) helps to determine the conformations and coordinations of adsorbed organic molecules on inorganic materials, the design of new functional hybrid materials, e.g. for dye-sensitized solar cell applications, may become considerably easier in future studies [39–42]. Acknowledgment This work was supported by JSPS KAKENHI Grant Number 26246005. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.09.078. References [1] K.L. Kelly, E. Coronado, L.L. Zha, G.C. Schatz, J. Phys. Chem. B 107 (2003) 668– 677. [2] M.P. Pileni, J. Phys. Chem. C 111 (26) (2007) 9019–9038. [3] R.P. Andres, T. Bein, M. Dorogi, S. Feng, J.J. Henderson, C.P. Kubiak, W. Mahoney, R.G. Osifchin, R. Reinfenberger, Science 272 (1996) 1323–1325. [4] M. Haruta, Catal. Today 36 (1997) 153–166. [5] P. Banerjee, D. Conklin, S. Nanayakkara, T.H. Park, M.J. Therien, D.A. Bonnell, ACS Nano 4 (2010) 1019–1025. [6] M. Yamamoto, T. Terui, K. Imazu, K. Tamada, T. Sakano, K. Matsuda, H. Ishii, Y. Noguchi, Appl. Phys. Lett. 101 (2) (2012) 023103. [7] M.S. Han, A.K.R. Lytton-Jean, B.K. Oh, J. Heo, C.A. Mirkin, Angew. Chem. Int. Ed. Engl. 45 (11) (2006) 1807–1810.

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Characterization of citrates on gold and silver nanoparticles.

In this paper, we report different coordinations of citrates on gold (AuNP) and silver (AgNP) nanoparticles, as determined using Fourier transform inf...
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