Sugar assisted evolution of mono- and bimetallic nanoparticles

Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 133–138

Sugar assisted evolution of mono- and bimetallic nanoparticles Sudipa Panigrahi, Subrata Kundu, Sujit Kumar Ghosh, Sudip Nath, Tarasankar Pal ∗ Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Received 1 November 2004; received in revised form 6 April 2005; accepted 14 April 2005 Available online 18 August 2005

Abstract Sugar assisted stable mono- and bimetallic nanoparticles were synthesized by wet chemical method following a general scheme. Proper manipulation of the reducing capabilities of different sugars has direct dependence on the particle size corroborating the shift of the absorbance peak positions. Under the experimental condition, the spherical nanoparticles were found to be ∼1, 3, 10 and 20 nm in diameter for gold, platinum, silver and palladium, respectively. Evaporation of the precursor solutions, led to the synthesis of smaller particles. Fructose has been found to be the best suited sugar for the synthesis of smallest particles and hence exploited for the evolution of Aucore –Agshell containing particles of ∼10 nm. © 2005 Elsevier B.V. All rights reserved. Keywords: Sugar; Nanoparticles; Solid support; Seed-mediated; Core-shell

1. Introduction Metal particles in the nanometer size regime shows characteristic size dependent properties and most dramatic size effects are observed for 1–10 nm diameters [1–5]. The enormous surface area-to-volume ratio of nanoparticles leads to excess surface free energy that is comparable to lattice energy, leading in turn to structural instabilities. A challenge in nanotechnology is to tailor (size and shape) the nanoparticles for tailor-made optical, electronic and electrical properties. Again, directed self-assembly of nanoparticles into specific structures can provide controlled fabrication of nanometer size building blocks with useful electronic, optical and magnetic properties [3–6]. Perfectly monodispersed metal nanoparticles are, of course ideal, but special properties are to be expected even if the ideality is not perfectly realized. Recent applications that take advantage of the size dependent properties of metal nanoparticles include catalysis [7,8], sensing [9–11], microelectronics [6,12–16] and in medicinal [17] applications. During the past few years, considerable interest has been paid to the preparation of bimetal∗

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lic nanoparticles. Bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic, optical and catalytic properties [18–20] compared to monometallics and have important biological applications in DNA sequencing [21], SERS [22], etc. Different chemical methods have been used for the synthesis of metallic nanoparticles, where the most common method involving the use of an excess of reducing agents such as sodium citrate [23] or NaBH4 [24]. Reduction of metallic salts in dry ethanol is one of the promising methods to synthesize Ag, Au, Pd and Cu nanoparticles [25]. Longenberger and Mills [26] found that metal colloids such as Au, Ag and Pd could be formed in air-saturated aqueous solutions of poly (ethylene glycol) (PEG). Corresponding mesityl derivatives is also a well-known precursors to synthesize Au, Ag and Cu nanoparticles [27]. In case of bimetallic nanoparticle synthesis, co-reduction is one of the most convenient methods [20,28,29]. Core-shell type bimetallic nanocomposites has been efficiently synthesized via laser [30] and UV [31] irradiation. In this article, we have reported a general method for the synthesis of different metal nanoparticles using commonly available sugars as reducing agents and extended the same theme for bimetallic core-shell particle generation. We have

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also addressed the effect of three different sugars as reducing agent on the sizes for different metal nanoparticles syntheses. This approach has several distinct features. (i) Sugars (glucose, fructose, and sucrose) are easily available to be used as reducing agents. (ii) Upon their exploitation no other stabilizing agent or capping agent is required to stabilize the nanoparticles. (iii) Sugars are very cheap and bio-friendly. (iv) Instead of keeping the nanoparticles in aqueous solution one can safely preserve the particles in a desiccators for months and can be redispersed in aqueous phase whenever required.

2. Experimental 2.1. Reagents and instruments All the reagents were of analytical reagent grade. Chloroauric acid (HAuCl4 ), silver nitrate (AgNO3 ), chloroplatinic acid (H2 PtCl6 ) and palladous chloride (PdCl2 ) [Aldrich] were used as received. Aqueous solutions (10−2 M) of all the salts were used as stock solution. Fructose, glucose and sucrose were purchased from S.D. Fine Chemicals, India. Milli-Q water was used throughout the experiment. All absorption spectra were recorded in a Shimadzu UV-160 spectrophotometer (Kyoto, Japan) taking the solutions in a 1 cm quartz cuvette. TEM measurements of the metal sols were performed in a Hitachi H-9000 NAR instrument on samples prepared by placing a drop of fresh metal sols on Cu grids precoated with carbon films, followed by solvent evaporation under vacuum. 2.2. Procedure for monometallic nanoparticles synthesis The metal nanoparticles (Au, Ag, Pd and Pt) were synthesized by dissolving 0.2 gm of a sugar (glucose, fructose and sucrose) in 3.9 mL water and 100 ␮L of the corresponding metal salt solutions (10−2 M) (HAuCl4 , AgNO3 , PdCl2 , and H2 PtCl6 were used for the respective nanoparticles preparation) were added into it. The solutions were heated on a hot water bath. After a particular time, the solution turned pink for gold, yellow for silver and for both platinum and palladium the solution turned black indicating the formation of the corresponding metal nanoparticles. The heating was continued until the solutions were evaporated to dryness. Now, 4 mL of water was again added to the container and sonicated for 30 min in order to redisperse the particles. 2.3. Procedure for Aucore –Agshell nanoparticle evolution To have a general consensus of the sugar reduction process smallest particles out of fructose reduction cases were exploited for the formation Aucore –Agshell type bimetallic nanoparticles. The Aucore –Agshell nanoparticles were prepared by using the following procedure. For the preparation of gold seeds (S), 150 ␮L of HAuCl4 (10−2 M) was diluted

to 4 mL in a 25 mL conical flask so that the final concentration of Au(III) ions remained 3.8 × 10−4 M. Then 0.2 gm of fructose was dissolved and heated on a hot water bath. The solution turned light yellow to pink after ∼3 min indicating the formation of gold nanoparticles. The heating was continued for 30 min for complete reduction of Au(III) ions. The solution was then allowed to stand for 30 min. These particles were then used as seed for the preparation of bimetallic nanoparticles in the next step. An aliquot of 400 ␮L of these particles was then diluted to 4.97 mL. To this solution, 30 ␮L of AgNO3 solution was added so that the final volume of the solution becomes 5 mL. The final concentrations of Au sol and Ag ions were adjusted to 3 × 10−5 and 9 × 10−5 M, respectively. Then the solution was kept for 20 min as an elapsed time. After that, 0.05 gm of fructose was dissolved in it and again heated on a hot water bath.

3. Results and discussion During the evolution of the metal nanoparticles we have recorded the UV–vis spectra of the metal sols at particular time intervals. For Au and Ag we monitored the progress of the reaction at the corresponding plasmon absorption peak positions and for Pd and Pt the reaction was followed from their featureless, monotonic increase in the UV–vis spectra. It was found that with progress of time the absorption bands for Au and Ag narrowed down and shifted continuously to the shorter wavelength regions with the successive increase in the absorbance values for sucrose and glucose reduction cases. Finally the λmax reaches constancy. Prolong heating caused slow decrease of the absorbance values because the metal particles adhered onto the glass surfaces of the container. After complete evaporation of the aqueous phase from the container metal nanoparticles were stored in a vacuum desiccator for months together. Again, the particles on sonication with water redispersed the metal nanoparticles to regenerate sol with blue shifting of the λmax [32]. The broad banded spectrum was obtained for all the sucrose reduction cases. However, for fructose and glucose cases the spectra for Au and Ag nanoparticle evolutions were rather sharp. The continuous blue shifting of the peaks for sucrose and glucose reductions were noticed and the blue shifting was very much pronounced and noticeable while these two sugars were involved. The UV–vis spectra for sucrose and glucose reduction cases are shown in Fig. 1 for gold nanoparticle evolution. Interestingly, the λmax remained constant from the very beginning while fructose-mediated reductions were conducted for the metal salts. The corresponding spectral profiles are given in Fig. 2 for gold nanoparticle synthesis. Therefore, we can conclude that if we use fructose we are able to produce nanoparticles of almost same size but glucose and sucrose on the other hand generate particles of variable sizes. Among all the three sugars, sucrose is a typical nonreducing sugar. Between glucose and fructose, glucose is a stronger reducing agent than fructose and the latter being a

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Fig. 3. The UV–vis spectra of the nanoparticles (Au, Ag, Pd and Pt) after redispersion. The metal particles were prepared using fructose as the reducing agent. Condition: [metal salt] = 2.5 × 10−4 M, [fructose] =2.8 × 10−1 M.

Fig. 1. The evolution of UV–vis spectra during the formation of gold nanoparticles using (A) glucose and (B) sucrose as the reducing agent. Condition: [HAuCl4 ] = 2.5 × 10−4 M, [glucose] = 2.8 × 10−1 M, [sucrose] = 2.8 × 10−1 M, temperature: 70 ◦ C.

ketohexose cannot be oxidized by Br2 water. In the experimental solutions, sucrose hydrolyses to equimolar mixture of d-glucose and d-fructose. Then the hydrolyzed products reduce the metal salts. Hence, it takes longer reduction time. Therefore, the resultant particle sizes became quite large. We

Fig. 2. The evolution of UV–vis spectra during the formation of gold nanoparticles using fructose as the reducing agent. Condition: [HAuCl4 ] = 2.5 × 10−4 M, [fructose] = 2.8 × 10−1 M, temperature: 70 ◦ C.

could not synthesize any silver or palladium particles using sucrose as the reductant even in the lower pH range. Between glucose and fructose since glucose is a stronger reducing agent it gives initially smaller particles. The smaller particles do not generate sufficient repulsive force for colloid stability and coalescence was reasonably rapid [33]. In normal DLVO (Derjaguin–Landau–Verwey–Overbeek) system [34], such particles cannot repeptise, and coalescence is irreversible. Therefore, we ultimately obtained the particles with larger diameter for glucose reduction cases though they are smaller than the particles obtained for sucrose cases (authenticated by absorption studies as described earlier). However, when we use fructose as reductant the λmax remained constant throughout. So we conclude that the particles are almost of same size and since fructose is a weaker reducing agent than glucose, the initial particle diameter was larger than those obtained from glucose. For the larger particles, the barrier to coalescence is large, which means colloid particles will experience weak flocculation forces [33]. Wiese and Healy also pointed out that the critical coagulation concentration of electrolyte needed to induce particle aggregation decreases rapidly as the particle size decreases because of the smaller electrostatic repulsion [35]. The UV–vis spectra of the metal particles (Au, Ag, Pd and Pt) prepared by fructose reduction are shown in Fig. 3. We obtained the characteristic surface plasmon excitation (due to oscillation of conduction band electrons) for gold and silver with λmax 514 and 404 nm, respectively. The spectra for Pd and Pt were without any characteristic absorption maxima. During reduction of the metal salts (the reduction potentials of the metal salts are given in Table 1) to the corresponding metal nanoparticles all the sugars are oxidized to the corresponding acids. To offset the van der Waals forces responsible for particle coalescence, carboxylic acids (here the oxidation products of the sugars) are often employed to generate a negative surface charge density. Self assembled

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Fig. 4. (a) The TEM images of the metal nanoparticles (Au, Ag, Pd and Pt) after redispersion of the metal particles. Condition: same as in Fig. 3. (b) The diameter histogram of metal nanoparticles (Au, Ag, Pd and Pt) after redispersion. Condition: same as in Fig. 3.

S. Panigrahi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 264 (2005) 133–138 Table 1 The reduction potentials (V) of the metal salts Au(III)/Au(0) Ag(I)/Ag(0) Pt(IV)/Pt(0) Pd(II)/Pd(0)

1.498 0.799 0.705 0.951

carboxylic acids ensure dense coating on the metal surfaces and therefore stabilized them [33]. TEM measurements were done for all the samples, wherefrom we observed that smallest particles are evolved from fructose reduction cases. The TEM images of all the particles formed by fructose reduction are shown in Fig. 4a. Non-agglomerated spherical nanoparticles with quite narrow size distribution was observed. From this analysis the particles with diameter ∼1, 3, 10 and 20 nm was found for gold, platinum, silver and palladium respectively for fructose reduction. From TEM measurement we see that the particle size in solution (before evaporation) was larger than those obtained after successive evaporation and redispersion. It was also found that the particle size was smallest in fructose cases then comes glucose and largest in sucrose. The particle distribution histograms are given in Fig. 4b. During the evolution of the bimetallic nanoparticles from the monometallic one we have recorded their UV–vis spectra at different stages. Gold particles were synthesized separately by fructose reduction method and were used as seed for the evolution of core-shell type bimetallics. The seed particles show a reddish pink colored sol having plasmon absorption band at 554 nm. The volume of the solution was maintained to ∼5 mL in the conical flask throughout the course of particle formation by successive addition of water in portion. The evolution of plasmon absorption band in successive stages of formation of core-shell structure from Au seed is shown in Fig. 5. In the intermediate stages of bimetallic nanoparticle formation, the UV–vis spectra exhibits two plasmon

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bands at 422 and 534 nm, characteristic of the partial coverage of Au(0) particles by Ag shell. It is important to realize that one cannot attribute a particular to the surface plasmon excitation to the single metal phase only. With progressive heating the plasmon absorption for the gold particle gradually becomes blue shifted and the plasmon absorption for silver is successively red shifted. After about ∼4 h of heating the two plasmon absorption bands merge together into a single plasmon band at 432 nm. Apparently, the spectrum seems to be the silver plasmon band, but it was nothing but the core-shell structure of the particles that reflects the true silver plasmon band as if there was no gold in the medium. Therefore, Au nanoparticles are now completely covered by Ag. Successive heating of the solution resulted in gradual thickening of the silver layer onto the gold nanoparticle. Prolonged heating resulted the appearance of the successive red-shifted band and finally fixed at 443 nm and the appearance of the golden brown solution indicating the thickening of the shell layer of the silver particles on the gold seeds, i.e., Aucore Agshell . TEM analysis reveals that the size of the gold seed and Aucore –Agshell particles was ∼6 and 10 nm, respectively. The TEM images of the monometallic (Auseed ) and bimetallic (Aucore –Agshell ) nanoparticles are shown in Fig. 6. The coreshell structure of the bimetallic was again authenticated by EDX. Further confirmation of the core-shell (Aucore –Agshell ) structure was authenticated from the well-known cyanide dissolution of the Ag layers under ambient condition. The successive changes (decrease and shift) of the absorbance of the plasmon band at 443 nm (due to the surface oscillation of the electron gas, whereas discrete electronic excitation due to interband transition occurs at shorter wavelength for Ag in the UV as well as visible spectra) during dissolution by cyanide also lend support to the core-shell structure. The cyanide dependent but step wise dissolution of the core-shell structures depends on the following facts: (i) addition of cyanide ions in portion; (ii) concentration effect; (iii) reduction potentials of the metals (Au3+ /Au system 1.498 V, Ag+ /Ag system 0.799 V).

Fig. 5. The evolution of plasmon absorption band during the formation of Aucore –Agshell structure: (a) gold seed, (b) gold seed after addition of AgNO3 , (c) at the intermediate stage of Aucore –Agshell particle formation and (d) finally formed Aucore –Agshell particles. Condition: [Auseed ] = 3 × 10−5 M, [AgNO3 ] = 9 × 10−5 M, [fructose] = 0.05 M.

From the cyanide dissolution study it was found that the shell (Ag layer) first dissolves slowly and thus the peak due to the Aucore –Agshell gradually red shifted. On continuous shaking in presence of oxygen the yellow colored solution (Aucore –Agshell ) turned pink due to the step wise removal of silver layers. Finally, bare gold nanoparticles reappeared with the plasmon peak for gold nanoparticles at 545 nm via a double hump curve (cf. Fig. 5). Addition of excess cyanide ions now dissolves the gold nanoparticles and finally the solution becomes colorless. This study confirms that the gold layer was completely covered by silver nanoparticles, i.e., there was Aucore –Agshell type bimetallic nanoparticles. The core-shell structure dissolved in cyanide

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Acknowledgements For financial support authors are thankful to IUC, Mumbai, DST, New Delhi and CSIR, New Delhi.

References

Fig. 6. TEM images of the (A) gold seed and (B) bimetallic Aucore –Agshell particles. Condition: same as in Fig. 5.

[32], and the kinetics of the dissolution is discussed elsewhere [36]. 4. Conclusion In summary, we have demonstrated the syntheses of monometallic as well as bimetallic nanoclusters, forming a mixture at the atomic level via a general methodology introduced for the first time. Different sugars have been successfully employed as reducing as well as stabilizing agent for the nanoparticle evolution. The isolated particles were spherical in shape with a tight size distribution. Fructose has been found to be the best suited reducing agent over other sugars. The sugar assisted evolution becomes eventful even for the seed mediated synthesis of Aucore –Agshell type bimetallic nanoparticles. Currently, a detailed extension of the study is being undertaken to synthesize different types of bimetallic as well as shape transformed nanoparticles.

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