Rapid, Facile Synthesis of Silver Nanostructure Using Hydrolyzable

May 18, 2009 - Due to hydroxyl groups and the dendritic macrostructure of tannin, it was used as both a reducer and stabilizer. The concentrations of ...
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Ind. Eng. Chem. Res. 2009, 48, 5686–5690

Rapid, Facile Synthesis of Silver Nanostructure Using Hydrolyzable Tannin ¨ zacar Emrah Bulut* and Mahmut O Department of Chemistry, Science & Arts Faculty, Sakarya UniVersity, 54187 Sakarya, Türkiye

A facial and distinct synthesis process of silver nanoparticles stabilized by tannin in aqueous medium is reported. Silver nitrate is solubilized in water as the source of silver ions, and tannin is solubilized in water as both a reducing agent and stabilizer matter. Due to hydroxyl groups and the dendritic macrostructure of tannin, it was used as both a reducer and stabilizer. The concentrations of tannin and of AgNO3 solution were changed to observe variation in particle size. However the role of the dendritic tannin molecule in forming silver nanoparticles was explained, and changes of the tannin molecule due to the redox reaction were determined by Fourier transform infrared (FTIR) measurements. The structure of synthesized silver nanoparticles has been investigated through scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis revealed the face-centered cubic (fcc) geometry of silver nanoparticles. Also a UV-vis spectrophotometer was used to obtain the absorption spectrum of silver nanoparticles. Introduction The recent development of nanoscience has opened up novel fundamental and applied frontiers in materials science and engineering. At the nanometer scale, the wavelike properties of electrons inside matter and atomic interactions are influenced by the size and shape (surface) of the material.1 When spherical metal particles are transformed to the nanoscale, surface plasmon resonances are strongly affected. Colloid chemists have recently made much progress in preparing monodispersed spherical nanocrystals via ambient temperature wet-chemical routes or organometallic methods.2 Changes in physicochemical properties (for example, melting points, magnetic, optic, and electronic properties) may be observed through a reduction in size, even without any compositional change.1 A variety of methods to prepare silver nanoparticles have been reported in the literature, including templated synthesis,2 silver mirror reaction,1,3 polyol process,4-7 sol-gel process8,9 reverse micelle,10-12 photochemical or radiation chemical reduction,13,14 chemical vapor deposition,15 sonochemical synthesis,16 electrophoresis,17,18 and electrochemical routes.19,20 The importance of silver nanoparticles for areas ranging from electron microscopy (contrast agents), analysis (chemical and biological sensors), electronics (single-electron transistors, electrical connects), materials (dyes, conductive coatings), fundamental research, printing techniques, and even catalysis (CO oxidation on Au/TiO2 composites) is significant.21-25 These applications require nanoparticles in the 2-100 nm size range that need to be surface derivatizable with hydrophobic and hydrophilic surfactants. Silver is a potential antibacterial agent and hence used as sterilizers and for removal of bacteria from drinking water. Silver ion kills microorganisms instantly by blocking the respiratory enzyme system while having no negative effect on human cells. The antibacterial activity of silver ions and their biological impact have been demonstrated in many works.26-28 Most of the synthetic methods reported to date rely heavily on the use of organic solvents and toxic reducing agents like sodium borohydride and N,N-dimethylformamide. All these chemicals are highly reactive and pose potential environmental and biological risks. With the increasing interest in minimiza* To whom correspondence should be addressed. Fax: 00. 90. 264. 295 59 50. E-mail:[email protected].

tion/elimination of waste and adoption of sustainable processes through green chemistry, the development of biological, biomimetic, and biochemical approaches is desirable. In earlier reports where natural polymers like starch and chitosan are reported to stabilize silver nanoparticles, separate reducing agents were used.29,30 Here, we use tannin to stabilize silver nanoparticles. Also, we used polyphenolic compounds at previous works.31,32 Tannins are extracted from plants hence it provides a “green” advantage and they are hydrophilic dendritic macromolecules consist of a large number of hydroxyl groups which reduce silver ions to metallic silver therefore any reducing agent was not used. Similarly, Huang et. al used carboxymethylated chitosan as both reducing agent for silver cation and stabilizing agent for silver nanoparticles.30 In hot conditions, Ag+ f Ag reaction occurs in aqueous solution in tannin, which is remarkably simple and mild, reminiscent of the classical Ag mirror reaction.4 In this study, we report an inexpensive, versatile, and very reproducible method for the large-scale synthesis of tanninprotected silver nanoparticles without using any reducing agent. To obtain optical absorption information of silver nanoparticles, UV-vis spectrophotometer is used. For investigating the morphology and size, we use a scanning electron microscopy (SEM) study on samples and show their SEM images. Also Fourier transform infrared (FTIR) measurements were recorded to clarify the modification on molecular structure of tannin. X-ray diffraction (XRD) is carried out to find out structure information of samples. Energy dispersive X-ray (EDX) patterns show the presence of silver in the samples. Experimental Details Silver nitrate was analytical grade. Tannin used in this study was obtained from AR-TU Kimya A.S¸. Salihli-Manisa-Tu¨rkiye. The tannin contents of valonia extracted in this study is determined 53.5% as hydrolyzable tannins according to the vanillin test,33 the Prussian blue test,33 and the 1-10 phenanthroline test.35,36 The Ag nanoparticles were synthesized using tannin as both reducing and stabilizing agent. A 0.01 M, 20 mL AgNO3 aqueous solution was added to 50 mL of distilled water, and 19 mg tannin was added to the mixture. The mixture was heated to 70-80 °C and stirred vigorously. After continuous

10.1021/ie801779f CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009

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Figure 2. Molecular structure of tannin (34).

Figure 1. SEM images of purified silver nanoparticles from 0.01 M AgNO3 with ratio of AgNO3/tannin ) 1/2.

heating at 70-80 °C for 30 min., it was allowed to cool to room temperature. Then, the centrifugated silver nanoparticles were dried. The UV-vis spectra of synthesized silver nanoparticles in the tannin solution were taken on a Shimadzu UV-2401PC spectrophotometer. The sample was placed on the cell of 1 cm path length quartz cuvette, with the incident light beam being perpendicular to the radial direction of the samples. The scanning range was from 200 to 800 nm with 0.5 nm resolution and background correction using tannin solution. FTIR measurements were recorded in solid phase using a Shimadzu IR Prestige-21 spectrophotometer with diamond attenuated total reflectance (ATR). The spectra were recorded from 1800 to 800 cm-1 (25 scans) on the sample. SEM experiments and EDX patterns were carried out on a JEOL JSM-6060 LV scanning electron microscope operated at 20 kV. Samples for SEM observation were prepared by purifying the nanoparticles from tannin solution. The powder X-ray diffraction was performed using a RIGAKU D max 2200 X-ray diffractometer with Cu KR (λ ) 0.154 nm). The diffracted intensities were recorded from 10 to 90, 2θ angles. The sample for XRD was supported on a glass substrate.

by Raveendran and co-workers29 with dendritic polyol molecules has demonstrated that large generation dendrimers did not yield particles with well-defined faces. Most of the particles formed from these dispersions were spherical in general. Surface enhanced activity in such molecules facilitates a surface enhanced Ag+ f Ag reaction, which can be expressed as38-40

In this model reaction, the Ag metal forms through an intermediate product of an Ag+-tannin complex. Oxidation of tannin through this reaction disrupts the Ag+-tannin complex structure. The Ag+ ions were converted to Ag atoms followed by coalescence, forming Ag clusters, and by cluster growth to yield Ag-particles. Ag atoms are stabilized from oxidative reactions and aggregations when capping in tannin molecules. In a simple case, the tannin- represents partially oxidized tannin as follows:41-43

Results and Discussion The SEM image of synthesized silver nanoparticles is given in Figure 1. Most of the particles are spherical. Linear as well as dendritic molecules have been successfully used for nanoparticle synthesis. Polyhydroxylated macromolecules present interesting dynamic supramolecular associations facilitated by inter- and intramolecular hydrogen bonding resulting in molecular level capsules, which can act as templates for nanoparticle growth. The high density of hydroxyl groups (Figure 2) in tannin molecules leads to extensive inter- and intramolecular hydrogen bonding, making them unique in their ability to form supramolecular assemblies. These supramolecular cages are dynamic in their structural attributes since intermolecular hydrogen bonds can act as dynamic gates for the controlled passage of ions as well as small nanoparticles into and out of the cages. By virtue of the high density of hydroxyl groups, the interior of these cages is polar and hydrophilic, making it possible to transport and reduce metal ions in the cages. Also, the hydroxyl groups help to passivate the surfaces of the silver metal nanoparticles. Previous work

The “OH” groups are just replaced by “O” groups, and the nascent hydrogen drives the Ag+ f Ag reduction. 2Ag+ + H2 f 2Ag + 2H+

(1)

Capping in tannin molecules divides the Ag+ f Ag reactions in small groups and, thus, controls morphology and topology in separated Ag particles. Figure 3 presents the FTIR spectra of tannin and the silver-tannin system. The band in the region of 3000-3500 cm-1 is characteristic of the sOH stretching of the phenol and methylol group of tannin. In all spectra, the small peaks in near the 2937 cm-1 are due to aromatic CsH stretching vibrations. The peaks at 1732 and 1716 cm-1 in the spectrum of tannin belongs to carboxyl-carbonyl groups. The absorption bands between 1600 and 1456 cm-1 are characteristic of the elongation of the aromatic sCdCs bonds. The peak at 1039 cm-1 in the spectrum of tannin is due to CsO stretching and CsH deformation. The band at 1315 and 1037 cm-1 in the spectrum

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Figure 3. FTIR spectra of hydrolyzable tannin: before (a) and after the synthesis of silver nanoparticles (0.1 M AgNO3 with ratio of AgNO3/tannin ) 1/2) (b).

of tannin belongs to phenol groups. The peak at 1174 cm-1 is due to phenolic sOH stretchings.35 The deformation vibrations of the CsH bond in the benzene rings also give small absorption bands in the 906-748 cm-1 range. When the spectra of tannin are compared with the spectra of silver-tannin, the peaks at 1732 and 1600 cm-1 belonging to the CdO of ketone and carboxyl and sCdCs are combined and broadened. This broad peak is located at 1714 cm-1. Also a small peak at 1651 cm-1 is attributed to the o-benzoquinone. The peak at 1600 cm-1 which is attributed to the sCdCs stretching vibrations of substituted benzene. The changes in intensity of bands of tannin-silver samples compared to those of the tannin sample indicate that the above surface functional groups have been occupied by silver particles during the process. The peak at 1315 cm-1 which may be belongs to the C-O stretching of benzene ring and phenolic -OH groups on tannin spectra shifted to the 1305 cm-1 on silver-tannin spectra. This shift may be caused by the formation of aromatic ketones during redox reaction. Also formation of ketone groups affected the phenolic -OH stretching at 1174 cm-1; therefore, the peak broadened and shifted to 1180 cm-1 with a shoulder. The bands in the regions of 906-748, 1039-1315, and 1456-1600 cm-1 with small peaks are combined due to the silver-tannin ion exchange reaction between some phenolic groups of tannin. A change is expected in the wide hydroxyl band 3500-3000 cm-1 region in the spectra of both tannin and silver-tannin after the ion exchange reactions between phenolic groups. But, these changes are observed at the 906-748, 1039-1315, and 1456-1600 cm-1 regions for the spectra of silver-tannin, because all of the -OH groups in the tannin do not participate in the ion exchange reactions between the tannin and silver ions. The phenolic groups participating in ion exchange reactions are located in the 1315-1037 and 1456-1600 cm-1 regions for tannin. The wide bands in the 3500-3000 cm-1 region belong to free hydroxyl groups (do not participate in reactions with metal ions) of tannin. Tannin molecules have debonded H-bonds of OH groups, which serve as the following:31 (i) a template for Ag+ f Ag reaction to occur over such surfaces (ii) a chemical reducer to render Ag+ f Ag reaction (iii) a surface stabilizer to preserve the sample of small Ag particles

(iv) a protective surface coating of a stable Ag-tannin surface interface to inhibit growth of rather big particles. The size of the particles is greatly influenced by experimental conditions. At higher temperatures and higher concentration of metal ions, the cluster growth is faster hence larger size particles are obtained. For the preparation of lower size particles, starting material concentration must be kept low. The mean size of the particle is about 70 nm when the precursor AgNO3 concentration is 0.1 M (Figure 4A). On the other hand, when the concentration of the starting material AgNO3 is 0.01 M, the particles size obtained is about 40 nm (Figure 4B). However, the same effect can be mentioned for tannin. Here, 70 nm sized particles were obtained by using an equal ratio (tannin/AgNO3 ) 1) with AgNO3 (Figure 4D). When we use a double amount of tannin and AgNO3 (tannin/AgNO3 ) 2), the particle size was increased to 50 nm (Figure 4C). Similar studies were carried out with carboxymethylated chitosan and PVP by Huang et al.30 and Tsuji et al.,5 respectively. Surface plasmon resonance is an important optical property of nanoparticles and is described as coherent fluctuations in electron density occurring at a free electron metal/dielectric interface. “Free electron” metals are those metals, which have a lone electron in a valence shell such as Au, Ag, Al, and Cu. The surface plasmon absorptions are responsible for the “red” and “yellow” colors of the gold and silver colloids, respectively. This shifts to higher wavelength when the size of the nanoparticle increases due to capping agents.26 The UV-vis absorption spectrum of this solution is given in Figure 5. The typical peak at 420 nm corresponds to the characteristic surface plasmon resonance of silver nanoparticles. Also, the plasmon band is symmetric, which indicates that the solution does not contain many aggregated particles. It is well-known that colloidal silver nanoparticles exhibit absorption at the wavelength from 390 to 420 nm due to Mie scattering. The plasmon bands are broad with an absorption tail in the longer wavelengths, which could be in principle due to the size distribution of the particles. Since the varying intensity of the plasmon resonance depends on the cluster size, the number of particles cannot be related linearly to the absorbance intensities. The concentration of silver nitrate and tannin do not produce any peak shift in the UV-vis spectrum, but the intensity of this peak increased in a nonlinear manner with the concentration of silver nitrate. Efficiencies of Ag+ ion and chitosan concentration as a stabilizer to the surface plasmon were investigated by Huang et al.30 In the XRD spectrum (Figure 6), the peaks observed at 2θ ) 38.281, 44.467, 64.615, 77.573, and 81.717 are assigned to diffractions from the (111), (200), (220), (311), and (222) planes of face-centered cubic (fcc) silver.31,32 The interplanar spacings obtained from the diffraction data are summarized in Table 1 and compared with ASTM data. These confirm that the particles are consisting only metallic silver. The broadening of these peaks is mostly due to the effect of nanosized particles. It is worth noting that the ratio between the intensities of the {111} and {200} diffraction peaks is much higher than the conventional value. This indicates that the nanoparticles are abundant in {111} surfaces and tend to lay with these planes parallel to the supporting substrate. Thus, the diffraction intensity of the {111} plane should be greatly enhanced compared to that of other planes. As analyzed with the EDX spectra from selected regions of the sample, the small particles as well as the derived clusters both comprise elemental Ag. For example, a typical EDX spectrum is given in Figure 7. The three main steps in the preparation of nanoparticles that should be evaluated from a green chemistry perspective are the

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Figure 4. SEM images of the products when using (A) 0.1 M AgNO3, (B) 0.01 M AgNO3, (C) tannin/AgNO3 ) 2, (D) tannin/AgNO3 ) 1: magnification of 25 000× and 40 000× at 20 kV.

Figure 5. UV-vis absorption spectrum of purified silver nanoparticles prepared with 0.1 M AgNO3 and a double ratio of tannin (tannin/AgNO3 ) 2).

choice of the solvent medium used for synthesis, the choice of an environmentally benign reducing agent, and the choice of a nontoxic material for the stabilization of nanoparticles. Also, the binding interaction between tannin and silver nanoparticles is relatively weak compared to the interaction between the nanoparticles and typical thiol-based protecting groups. This implies that the protection should be easily reversible, enabling the separation of these nanoparticles. Conclusion In summary, this study has demonstrated that silver nanoparticles could be prepared by a simply carrying out reductionoxidation reaction using hydrolyzable tannin. This reaction is similar to the polyol and alcoholic reduction methods. However owing to the fact that the polyol method is a nonaqueous route,

Figure 6. XRD pattern of synthesized and purified silver nanoparticles. 0.01 M AgNO3 and a double ratio of tannin. Table 1. Comparison of Interplanar Spacings dhkl Obtained from the X-ray Diffraction Pattern with ASTM Values observed dhkl (nm)

ASTM silver (nm)

0.2349 0.2035 0.1441 0.1229 0.1177

0.2425 0.2000 0.1443 0.1212 0.1170

organic solvents must be used. On the other hand, both the reducing agent and stabilizer matter need to be used in the alcoholic reduction method. Compared to these two methods mentioned above, we have used neither organic solvent nor reducing agent. Instead of these reagents, just tannin was used. The macrostructure of hydrolyzable tannin provided a stabilizing effect on the particles. Also many -OH groups on tannin reduced the Ag+ to Ag0. Thus, silver nanoparticles with 40-70

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Figure 7. EDX spectrum of silver nanoparticles (tannin/AgNO3 ) 2 with 0.01 M AgNO3).

nm average sizes were prepared. As a result, the tannin molecules form a protection layer on the surface of the particles to endow the particles with good stability in water. The stability of as-synthesized colloidal particles is so excellent as to be preserved for a few months without precipitation. Thus, the silver colloids prepared by this method have extensive application prospects such as use as catalytic materials, antibacterial materials, and so on. Also tannin is an environmental friendly chemical which therefore includes a “green chemistry” effect. Concentrations of tannin and AgNO3 solutions play important role in the sizes of the particles. While we used AgNO3 aqueous solution at low concentration as a precursor, we obtained smaller particles. On the other hand, when a higher amount of tannin was used, smaller sized particles were prepared. Acknowledgment We are grateful for financial support from the Scientific Research Projects Commission of Sakarya University (Project number: 2007 02 04 003). Supporting Information Available: Detailed XRD pattern, SEM images, and EDS characterization results (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Qu, L.; Dai, L. J. Phys. Chem. B 2005, 109, 13985. (2) Hao, E.; Kelly, K. L.; Hupp, J. T.; Schatz, G. C. J. Am. Chem. Soc. 2002, 124, 15182. (3) Yu, D.; Yam, V. W. W. J. Phys. Chem. B 2005, 109, 5497. (4) Gautam, A.; Singh, G. P.; Ram, S. Synth. Met. 2007, 157, 5. (5) Tsuji, M.; Nishizawa, Y.; Matsumoto, K.; Miyamae, N.; Tsuji, T.; Zhang, X. Colloids Surf., A 2007, 293, 185. (6) Slistan-Grijalvaa, A.; Herrera-Urbinab, R.; Rivas-Silvac, J. F.; Avalos-Borjad, M.; Castillon-Barrazad, F. F.; Posada-Amarillas, A. Physica E 2005, 25, 438. (7) Lin-Bao, L.; Shu-Hong, Y.; Hai-Sheng, Q.; Tao, Z. J. Am. Chem. Soc. 2005, 127, 2822. (8) Raffia, M.; Akhter, J. I.; Hasan, M. M. Mater. Chem. Phys. 2006, 99, 405.

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ReceiVed for reView November 20, 2008 ReVised manuscript receiVed March 6, 2009 Accepted March 11, 2009 IE801779F