Langmuir 1998, 14, 1021-1025
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Preparation, Characterization, and Surface Modification of Silver Particles Sudhir Kapoor Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Received June 4, 1997. In Final Form: October 22, 1997 Colloidal silver has been formed either by irradiation with 253.7 nm light or by chemical reduction of AgClO4 in the presence of protective agents such as poly(vinylpyrrolidone), carboxy methyl cellulose, and gelatin. The particles were characterized by their absorption maximum and transmission electron micrographs. Peptization effect was observed in the presence of photosensitive benzophenone. Surface modification studies of silver particles in the presence of various complexing agents, viz., N-(hydoxyethyl)ethylenediaminetriacetic acid, iminodiacetic acid, nitrilotriacetic acid, 2-mercaptobenzimidazole, benzotriazole, 5-aminotetrazole, imidazole, and sanazole were carried out and it was shown that the reactivity of silver particles increases in their presence. However, in the presence of stabilizers, due to competitve reactions, partial recovery of the surface plasmon absorption band was observed. The Fermi potential of surface modified silver particles seems to lie in the range of -0.40 ( 0.05 V.
Introduction Synthesis of nanoparticles is an interesting field in solid state chemistry.1 Recently, much interest has been shown in ultrafine metal particles because they have unique properties that are different from bulk metals in optical property, catalytic activity, magnetic property, and so on.2-11 Many studies have been reported in particular for silver, gold, and copper colloids. In the case of silver, gold, and semiconductors such as CdS, surface modified studies were carried out to see the effect of various groups present in capping molecules.12,13 Recently, new experimental approaches have been initiated for studying surface phenomenon which allow the investigators to characterize adsorbed molecules and microscopic properties of the interface as well as to determine mechanisms of the adsorbate-adsorbent interaction. For Ag it is very interesting to investigate the optical properties since it strongly absorbs in the visible region due to surface plasmon resonance.1-7 Metal colloids can be prepared by various methods such as chemical liquid deposition,14 photochemical reduction,4-10 and chemical reduction.15,16 In this study we describe the formation of metal particles by UV irradiation (1) Ozin G. A. Adv Mater. 1992, 4, 612. (2) (a) Mie, G.; Ann. Phys. 1908, 25, 377. (b) Wang, D. S.; Kerker, M.; Chew, H. Appl. Opt. 1990, 19, 2135. (3) Genzel, L.; Martin, T. P. Surf. Sci. 1973, 34, 33. (4) Ershov, B. G.; Henglein, A. J. Phys. Chem. 1993, 97, 3434. (5) Henglein, A.; Mulvaney, P.; Holtzworth, A.; Sosebee, T.; Fojtik, A. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 754. (6) Gutierrez, M.; Henglem, A. J. Phys. Chem. 1993, 97, 11368. (7) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (8) Yonezawa, Y.; Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1559. (9) Yonezawa, Y.; Sato, T.; Kuroda, S. J. Chem. Soc., Faraday Trans. 1 l991, 87, 1905. (10) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull. Chem. Soc. Jpn. 1994, 67, 3165. (11) Subramanian, S.; Nedelikovic, J. M.; Patel, R. C. J. Colloid Interface Sci. 1992, 150, 81. (12) Hayes, D.; Micic, O. I.; Nenadovic, M. T.; Swayambunathan, W.; Meisel, D. J. Phys. Chem. 1989, 93, 4603. (13) Fischer, C.; Henglein, A. J. Phys. Chem. 1989, 93, 5578. (14) Esumi, K.; Itakura, T.; Torigoe, K. Collids Surf. 1994, 82, 111. (15) Hirai, H.; Nakamura, Y.; Toshima, N. J. Macromol. Sci. Chem., A 1979, 13, 727. (16) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Phys. Chem. 1991, 95, 7448.
and chemical reduction. We have focused on the effect of benzophenone (BP) and the surface modification of silver particles with imidazole, sanazole, N-(hydoxyethyl)ethylenediaminetriacetic acid (HEDTA), iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), 2-mercaptobenzimidazole, benzothiazole, and 5-aminotetrazole. The effect of conditions on the adsorption of complexing agents on silver particles is also discussed. Experimental Section Materials. Silver perchlorate (Sigma), benzophenone (Aldrich), poly(vinylpyrrolidone) (PVP) (Sigma, Mw ) 10 000), carboxymethyl cellulose (CMC, Apex Chemicals), gelatin (BDH), imidazole (Aldrich), HEDTA (Sigma), IDA (Sigma), NTA (Sigma), 2-mercaptobenzimidazole (Fluka), benzotriazole, and 5-aminotetrazole (Aldrich) were used as received. Sanazole [(N-2′(methoxyethyl)-2-(3′-nitro-1′-trizolyl)acetamide] was a generous gift from Professor V. T. Kagiya, Kyoto, Japan. Solutions were prepared with nanopure water (conductivity ) 0.06 µS). Spectrophotochemical measurements were carried out on Hitachi-330 UV-VIS spectrophotometer. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of the colloidal solution on a copper grid coated with a thin amorphous carbon film, and the samples were dried and kept under vacuum in a desiccator before putting them in specimen holder. The TEM characterization was carried out using a JEOL JEM-2000FX electron microscope. Preparation of Metal Colloids Using the Photochemical Method. An aerated aqueous or N2 saturated solution of metal salt (0.25 mmol dm-3) containing PVP (0.5%, w/v) or gelatin (0.5%, w/v) or CMC (0.5%, w/v) and benzophenone (2.0 × 10-4 mol dm-3) was placed in a rectangular quartz vessel of 1 cm × 1 cm × 5 cm in size. A 200 W low-pressure Hg lamp (Rayonet, Photochemical Reactor) was used as the light source for 253.7 nm UV light irradiation at ambient temperature. The cell was placed in the reactor and a 4-4.5 mL solution was put in it for photolysis. The incident photon numbers of 253.7 nm light (determined by a tris(oxalato) ferrate (111) actinometer were 5.035 × 1015 cm2 s-1. Preparation of Metal Colloids Using NaBH4 as a Reducing Agent. The silver sol was prepared by reduction of Ag+ ions using NaBH4 as reported in literature.17 To a 100 mL N2 saturated solution of AgNO3 (1.0 × 10-4 mol dm-3) a 10 mg sample of NaBH4 was added. The solution was shaken vigorously, and a clear yellow solution was obtained. The pH of the solution was 9.3 due to the hydrolysis of excess NaBH4. (17) Vukovic, V. V.; Nedelikovic, J. M. Langmuir 1993, 9, 980.
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1022 Langmuir, Vol. 14, No. 5, 1998
Kapoor
carboxylate ion to Ag+. The reaction mechanism would be the following
Figure 1. Change in the absorption spectra of aerated silver perchlorate (2.5 × 10-4 mol dm-3) with UV irradiation time: (a) in 0.5% (w/v) PVP; (b) in 0.5% (w/v) gelatin; (c) in 0.5% (w/v) CMC. Number on spectrum indicates the time of irradiation. Path length of cell was 3 mm.
Results and Discussion The variation in the absorbance of the solution containing AgClO4 in the presence of various stabilizers, viz., PVP, gelatin, or CMC, with irradiation time is shown in Figure 1. In the absence of BP, a very broad band was observed in the case of CMC (Figure 1c) while in case of gelatin a distinct peak of colloidal silver was centered at 410 nm (Figure 1b). On the other hand, in the case of PVP, the peak shifted to 400 nm Figure 1a). The induction period for the generation of the colloidal silver absorption band was observed only in the case of PVP. The TEM image of the sample taken from the solution phase shows a mixture of colloidal particles and agglomerates. In the case of gelatin, spherical particles of size 6-20 nm were observed. Similar behavior was observed in the case of PVP. However, when CMC was used as a stabilizer, mixtures of colloidal particles and agglomerates with irregular shapes in the size range of 20-100 nm were observed. These observations are not inconsistent with the absorption spectra of colloidal solutions observed in Figure 1. Irradiation of AgClO4 with 253.7 nm light induced photooxidation of water by excited Ag+, resulting in the formation of Ag atoms, Ag0. Subsequent agglomeration processes of Ag0 produce colloidal silver (Ag0)n.
Ag+ + H2O -Df Ag0 + H+ + OH•
(1)
nAg0 f (Ag0)n
(2)
Effect of CMC. The photolysis of silver-CMC solution by 253.7 nm light is initiated by charge transfer from the
RCO-2Ag+ -Df RCO2• + Ag0
(3)
RCO2• + H2O f RCO2H, R•, OH•, CO2
(4)
nAg0 f (Ag0)n
(5)
Effect of PVP and Gelatin. In the case of PVP, Ag particles interact with the excited CdO*, which reduces Ag+ to Ag. In gelatin Ag+ interacts through amide linkages.18,19 We have also carried out photolysis of the AgClO4 solution in the presence of BP and various stabilizers. Before studying the formation of Ag colloids by UV irradiation using BP, it is vital to investigate how BP molecules or its concentration changes with UV irradiation in both the presence and absence of Ag salt solutions. It was observed that the photochemical formation of colloidal Ag was accompanied by the photobleaching of BP. The amount of BP remaining in the solution can be estimated spectrophotometrically from the absorption peak at the wavelength of the π-π* transition of BP. A molar extinction of 1.4 × 104 dm3 mol-1 cm-1 is employed for calculating the yields of BP. A quantitative analysis of the spectral changes by taking the intensity with the molar extinction coefficient of BP leads to the conclusion that the photoreaction of BP proceeds at a faster rate in the absence of Ag salt solution because in its presence some intensity of light is also absorbed by the Ag salt solution. The changes in the absorption spectra and the absorbance at λmax with irradiation are shown in Figure 2. The distinct peak of colloidal silver was observed for all solutions after photolysis. An interesting aspect is that no induction period was observed for solutions containing BP, and therefore it indicates the role played by BP. The TEM image of the colloidal silver shows particles in the range of 5-20 nm. The experimental results suggest that the BP* causes peptization of colloidal silver and agglomerates in solution. The results are similar as reported in literature for the Ag/SDS/acetone system.9 Development of the sharp band in silver salt solution containing BP (Figure 2) is explained in terms of the formation of *BP. In solutions containing BP, 253.7 nm light is absorbed by both Ag+ and BP independently. Excited Ag+ causes the photooxidation of water as explained earlier and excited BP leads to the following reactions
(C6H5)2CO -Df (C6H5)2CO
(6)
(C6H5)2CO + RH f (C6H5)2COH• + R•
(7)
where RH denotes the stabilizer molecule and R• an alkyl radical formed from stabilizer respectively. It can be assumed that the BP ketyl radical (C6H5)2COH• (BPK) reduces silver ions and silver ion clusters. Subsequent agglomeration processes of silver atoms and clusters produced colloidal silver.10 The presence of stabilizer prevents the colloidal silver particles from coalescing with each other. (18) Huang, H. H.; Hi, X P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 409. (19) Rose, P. I. In The theory of the photographic process, 4th ed.; James, T. H., Ed.; MacMillan Publishing Co.: New York 1977; pp 5167.
Surface Modification of Silver Particles
Figure 2. Change in the absorption spectra of aerated silver perchlorate (2.5 × 10-4 mol dm-3) containing benzophenone (BP) (2.0 × 10-4 mol dm-3) with UV irradiation time: (a) in 0.5% (w/v) PVP; (b) in 0.5% (w/v) gelatin; (c) in 0.5% (w/v) CMC. Number on spectrum indicates the time of irradiation. Path length of cell was 3 mm.
It is known that in the case of silver particles the UVvis absorption spectrum is very sensitive to their formation.2 An absorption peak at around 400 nm observed generally is attributed to the surface plasmon excitation of silver particles. Mie2a and Wang2b have carried out theoretical studies on the dependence of UV absorption maximum on the size of metal particles. It has been suggested that the absorption peak shifts toward longer wavelengths as particles become bigger.20 The aggregation of colloidal silver particles causes a decrease in the intensity of the peak and also results in a long tail at the higher wavelength side of the peak. Dipolar absorption is dominant for particles
