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Reversible Formation and Dissolution of Silver Nanoparticles in Aqueous Surfactant Media† Tarasankar Pal,* Tapan K. Sau, and Nikhil R. Jana Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Received August 22, 1996. In Final Form: December 11, 1996X Nanosize silver particles are formed in aqueous surfactant systems and have been found to be very reactive toward oxygen in the presence of borohydride. Purging of air (O2) or simple shaking leads to complete dissolution of silver particles, which is further regenerated on standing. Thus an oscillation between a yellow silver plasmon band and colorless solution is observed on periodic shaking. This oscillation continues as long as excess borohydride is present. Investigation shows that borohydride, which is a strong nucleophile, adsorbs on the surface of the silver particle and decreases the reduction potential of the silver particle so that it reacts rapidly with oxygen. Surfactant plays an extra role over particle stabilization. It removes the oxidized silver from the particle surface to the bulk and also retards the rate of reduction of AgI to Ag0.
Introduction Recently nanoparticle research has become the focus of intense research activity not only due to its unusual behavior compared to the bulk metal but for its wide spread applications in the practical world also. The aim of such investigations can be divided into two main groups. The first group constitutes the preparation and the studies of the particle itself.1-15 The second group comprises various experiments aimed at the development of advanced nanostructured materials for catalyst production, semiconductors, superconductors, photographic suspensions, supermagnets, magnetic protective coatings, ultramodern molecular devices, etc.16-22 The nanoscale particles possess a very large surfaceto-volume ratio, and consequently their properties are † Part of this work appeared in Book of Abstracts; National Meeting of the American Chemical Society, New Orleans, LA, 1996; American Chemical Society: Washington, DC, 1996. X Abstract published in Advance ACS Abstracts, February 1, 1997.
(1) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (2) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82. (3) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiat. Phys. Chem. 1989, 34, 605. (4) Mostafavi, M.; Keghouche, N.; Delcourt, M. O. Chem. Phys. Lett. 1990, 169, 81. (5) Platzer, O.; Amblard, J.; Marignier, J. L.; Belloni, J. J. Phys. Chem. 1992, 96, 2334. (6) Amblard, J.; Platzer, O.; Ridard, J.; Belloni, J. J. Phys. Chem. 1992, 96, 2341. (7) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (8) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (9) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman R. J. Chem. Soc., Chem. Commun. 1994, 801. (10) Chen. J. P.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C. J. Appl. Phys. 1994, 76, 6316. (11) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (12) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (13) Seshadri, R.; Subbanna, G. N.; Vijayakrishnan, V.; Kulkarni, G. U.; Ananthakrishna, G.; Rao, C. N. R. J. Phys. Chem. 1995, 99, 5639. (14) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (15) Duteil, A.; Schmid, G.; Meyer-Zaika, W. J. Chem. Soc., Chem. Commun. 1995, 31. (16) Fendler, J. H. Chem. Rev. 1987, 87, 877. (17) Henglein, A. Chem. Rev. 1989, 89, 1861. (18) Schmid, G. Chem. Rev. 1992, 92, 1709. (19) Kamat, P. V. Chem. Rev. 1993, 93, 267. (20) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (21) Hailstone, R. K. J. Phys. Chem. 1995, 99, 4414. (22) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Ormerod, R. M.; Lambert, R. M.; Schmid, G.; West, H. J. Phys. Chem. 1995, 99, 6096.
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mostly governed by the surface states. At the same time, their presence in solution builds a house of complexity. The solution becomes microscopically inhomogeneous. Regions of solid-liquid interface come into play. Geometric effect and the asymmetry in the intermolecular forces experienced by the molecules at the interface give rise to gradual variation in the density and dielectric properties of the liquid. These may significantly influence the chemical reactivity and selectivity of the nanoparticle and of the species localized at such an interface compared to bulk liquid.23 Thus the physicochemical properties of such a nanoparticle system are the sum total of surface/interface effects. Important and sometimes unusual chemical phenomena occur at such a surface/interface region. The chemical activity of the nanostructured particles and the behavior of the chemically active species in these environments are at a very first but promising beginning.6,24-27 It requires more attention on both theoretical and experimental aspects to get insight into such microscopically heterogeneous systems. In this report we present the reactivity of surfactantstabilized silver nanoparticles in the presence of borohydride ion. The interconversion between silver ion and silver particle, i.e., oscillation between a colorless solution and yellow dispersion, occurs periodically by purging of air (O2)/shaking in open atmosphere. Our choice of silver metal is based on several factors. Firstly, it is the most frequently studied cheapest noble metal in this field. Secondly, it finds topical importance in the practical world, viz., photography,21,28,29 catalysis,30 surface enhanced Raman spectroscopy,31-33 chemical (23) McConell, B. L.; Williams, K. C.; Daniel, J. L.; Stanton, J. H.; Irby, B. N. Dugger, D. L.; Madtman, R. W. J. Phys. Chem. 1964, 68, 2941. (24) Henglein, A. Chem. Phys. Lett. 1989, 154, 473. (25) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (26) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (27) Strelow, F.; Henglein, A. J. Phys. Chem. 1995, 99, 11834. (28) Hamilton, J. F. The Theory of the Photographic Process; James, T. H., Ed.; Macmillon: New York, 1977. (29) Lewis, M.; Tarlov, M. J. Am. Chem. Soc. 1995, 117, 9574. (30) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (31) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton. T. M. J Phys. Chem. 1995, 99, 9466. (32) Nabiev, I.; Baranov, A.; Chourpa, I.; Beljebbar, A.; Sockalingum, G. D.; Manfait, M. J. Phys. Chem. 1995, 99, 1608. (33) Vl kova, B.; Gu, X. J.; Tsai, D. P.; Moskovits, M. J. Phys. Chem. 1996, 100, 3169.
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Table 1. Characteristics of Silver Particles in the Presence of Sufactants stabilizer surfactanta
concn of stabilizer (mol dm-3)
plasmon band λmax (nm)
band width at half height (nm)
av particle size from TEM (nm)
stability
CTAB (0.9 × 10-3) SDS (8.1 × 10-3) TX-100 (0.2 × 10-3)
2.0 × 10-3 2.0 × 10-3 2.5 × 10-4
410 392 394
60 70 100
65 80 55
>30 days ∼5 days ∼24 h
a
Value in parentheses represents critical micellar concentration, cmc, in mol dm-3.
analysis,34 etc. Thirdly, it has a narrow intense plasmon absorption band in the visible region which is very much susceptible to surface/interface effects.7 Experimental Section (a) Sample Preparation. Nanoscale silver particles were prepared by reduction of AgNO3 (BDH) solution with NaBH4 in the presence and absence of surfactant. The redox reaction can be written as 2AgNO3 + 2NaBH4 + 6H2O ) 2Ag + 2NaNO3 + 2H3BO3 + 7H2. Three types of surfactants were used, namely, cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium dodecyl sulfate, SDS) and neutral (poly(oxyethylene) isooctylphenyl ether, TX-100), all were purchased from Aldrich Chemical Co., USA. All other reagents were of AR grade. In most cases AgNO3 concentration was 10-4 mol dm-3 and surfactant concentration was varied from 10-6 to 10-1 mol dm-3. The usual mole ratio of NaBH4 to AgNO3 was 6. However, when a large excess of NaBH4 was used, this ratio was varied from 10 to 500. (b) Instrumentation. UV-visible and fluorescence measurements were done in a Shimadzu UV-160 spectrophotometer and Perkin-Elmer LS 50B spectrofluorometer respectively. Size of the silver particle was determined using a transmission electron microscope (TEM). Samples were prepared by placing a drop of the dispersed solution on a carbon-coated standard Cu-grid. The sample grids were examined by Phillips CM12 TEM operating at 80 kV. The X-ray diffraction (XRD) line broadening method has also been used to measure average particle size. Solid samples were prepared after evaporating the solvent from the dispersed solution and examined in a Phillips PW 1727 X-ray generator and PW 1710 diffractometer control. Results (1) Nature of Silver Plasmon Band in the Presence of Surfactants. Preparation of stable silver particles in a very small concentration of CTAB (respective critical micellar concentration, cmc) no such precipitate is observed. Detailed investigation reveals that reversibility is observed only near and above the cmc of CTAB and SDS. In the presence of conventional complexing agents for AgI like NH3, CN-, EDTA etc., instead of SDS and CTAB, the plasmon band does not vanish at all. The usual chemical test for AgI, i.e., addition of NaCl, NaBr, NaI, or Na2CrO4, does not give any precipitate at completely bleached condition due to the presence of surfactant. However, indirect proof for the presence of AgI at bleached condition comes from fluorescence quenching effects of the 1-aminonaphthalene probe.35 It has been observed that AgI quenched the fluorescence intensity of
the probe only by ∼10%. When the particles were formed in situ by the reduction with borohydride, a further quenching up to ∼70% was observed. The effect is identical if the same concentration of silver particles is added from outside. A similar type of quenching effect by silver particles in solution has been reported recently by Ren et al.36 After complete bleaching of the dispersion the fluorescence intensity enhanced again and matched with 10% decreased spectra. This suggests the formation of AgI under the bleached condition. 3. Kinetic Measurements. To understand the exact role of surfactants, the rate of particle formation was studied in the presence and absence of surfactants. Rate was monitored in the respective plasmon band maxima. The formation of silver particles from a solution of AgNO3 or suspended AgBr/Ag2O is almost instantaneous, and addition of TX-100 does not alter the rate. However, the rate of particle formation is comparatively slower in CTAB and SDS and the rate decreases with the increase in surfactant concentration. The rate of reaction increases if O2 is purged out from the medium by N2, but for higher concentrations of CTAB/SDS (>10-2 mol dm-3) the rate remains the same in N2 as well as O2 atmosphere. 4. Size Distribution of Ag Particles. A TEM study was done for surfactant-stabilized Ag particles to obtain particle size distribution and average particle size. This study shows that most of the particles are agglomerated, presumably due to the presence of surfactants. So the histogram for size distribution was prepared by measuring individual particle size, which is reasonably non-agglomerated. In each case ∼100 particles were considered for preparation of the distribution curve. Figure 2 shows the particle size distribution in three different types of surfactants. Average particle size was also calculated for each case (Table 1). A comparison between parts c and d of Figure 2 shows that after completion of one cycle of reversibility, the smaller particles become more populated. (35) Pal, T.; Jana, N. R.; Sau, T. Radiat. Phys. Chem. 1997, 49, 127. (36) Zang, L.; Liu, C. Y.; Ren, X. M. J. Chem. Soc., Chem. Commun. 1995, 447.
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However, the average particle size does not change appreciably. Inspection of each particle shows that the surfactant layer surrounds the particles. The width of the layer increases for aged particles. TEM was done for completely bleached Ag particles. Particles could also be seen in these cases, although they were not very distinct, and they did not have any characteristic crystalline pattern like Ag particles. This suggests that probably AgBr/Ag2O has been formed under bleached condition. TEM study was supplemented by XRD line broadening technique and average particle sizes were 46-56 nm. The presence of a thick surfactant layer is thought to be responsible for the larger diameter assessed by TEM. Discussion During dissolution the yellow plasmon band vanishes gradually. Throughout the bleaching process, the solution remains transparent without developing any turbidity. It indicates that the bleaching of the solution is not due to the agglomeration of particles. It was authenticated further from an inspection of the UV-vis spectra (Figure 1). The bands remain symmetrical without showing any sign of scattering. So the discoloration is caused either by oxidation of silver particles or by complete damping25 of plasmon band. The necessity of oxygen in the bleaching process indicates that the discoloration is due to the oxidative dissolution and not due to damping because damping is an oxygen independent phenomenon. Florescence study using 1-aminonaphthalene also corroborates this fact. If the bleached condition was due to damping of the plasmon band, then it would decrease the field enhancement factor, further decreasing the fluorescence which is already quenched by silver particles.37 On the contrary, fluorescence increases under bleached condition indicating that AgI is formed during bleaching. It is well-known that adsorption of nucleophile onto the particle surface increases the Fermi level of silver particle due to its donation of electron density to the particle.7 Similarly withdrawal of electron density from the particle surface by an electrophile lowers the Fermi level.7 Experimentally we observed the phenomenon as blue and red shift of the plasmon band, respectively. The observed red shift, associated with the addition of surfactant stabilizers, is due to the displacement of nucleophiles (i.e., anions adsorbed on the particle surface) by the surfactants from the surface. The increase in sharpness and peak height of the plasmon band with the addition of surfactant is due to narrower distribution of particle size and decrease in agglomeration of particles. With the gradual addition of sodium borohydride to the surfactant-stabilized particle dispersion, the band position blue shifts and finally becomes constant. The blue shift is caused by displacement of surfactant by borohydride, which is a strong nucleophile. This blue shift proves that BH4- ions get adsorbed on the particle surface as was observed in the case of deposition of excess electrons to the silver particle.25 During dissolution, the plasmon band λmax first shifts to longer wavelength (Figure 1) with a broadening of spectra and then blue shift occurs gradually until it vanishes completely. The usual red shift is due to the withdrawal of electron density from the surface and/or due to the increase in particle size. As band width is inversely proportional to the particle size,7 an increase in particle size should lead to the narrowing in band width, which is contrary to the observed broadening in the present case. Thus the red shift is attributed to the removal of (37) Weitz, D. A.; Garoff, S.; Gersten, J. I.; Nitzan, A. J. Chem. Phys. 1983, 78, 5324.
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electron density from the particle surface. As oxygen is necessary for the dissolution, it is reasonable that oxygen removes the electron density from the surface. Hence there is the possibility of oxidation of surface silver atoms. Such oxidation seems feasible from the redox potential point of view. It is well-known that redox potential of silver decreases with the decrease in particle size and upon adsorption of nucleophile on particle surfaces.3,7 The average particle size is ∼60-70 nm (diameter) in this case whose reduction potential is expected to be lower than the conventional value, i.e., 0.799 V. This value is further decreased by the adsorption of borohydride onto the particle surface. The more the electron donation by nucleophile, the more negative is the shift in the redox potential value. Consequently, a large negative shift in the redox potential of the silver nanoparticle can be expected from the strong electron injection capability of the highly reducing BH4- ion. Thus adsorption of BH4ion makes the nanoparticles susceptible toward oxidation by oxygen (the reduction potential of O2/OH- is 0.4 V). The redox reaction can be written as 4Ag + O2 + 2H2O ) 4Ag+ + 4OH-. Similar types of oxidation of Ag particles is also observed by Henglein et al.26 where oxidizing systems were CN-/methylviologen, SH-/nitrobenzene, SH-/nitropyridinium oxide, and CN-/K3Fe(CN)6 and the reduction potentials are -0.44, -0.40, -0.20, and +0.36 V, respectively, for the final electron acceptors. Initial oxidation of surface silver atoms causes a red shifted, broad plasmon band and lowering of absorbance values. This fact is observed in all the three types of surfactant systems. A similar observation was noted while AgI ion was introduced from outside to the dispersed silver particle. Further shaking does not change the plasmon band for TX-100, but for CTAB and SDS it gradually decreases. This suggests that the oxidized surface silver atoms dissolve away and enter into the bulk in CTAB and SDS media but not in TX-100. This is quite obvious, as AgBr and Ag2O, which are probable products of surface oxidation, are soluble in CTAB and SDS but not in TX-100. The ionic equilibria for the formation of AgBr/Ag2O are as follows: Ag + Br- ) AgBr, 2Ag + 2OH- ) Ag2O + H2O. With the removal of the oxidized surface silver atoms, the effective particle size diminishes with decrease in total number of silver atoms. These effects lead to the broadening of the spectra and lowering of the absorbance value. Similarly, the initial appearance of a broad plasmon band in longer wavelength region (which is more prominent for higher CTAB/SDS concentration) in the regeneration step can be explained with the consideration of small silver particles which are associated with surface AgI layer. The observed oscillation indicates that during shaking the rate of particle formation from AgI by borohydride is comparatively slow, whereas oxygen reacts very fast with silver particles to produce AgI ion. On standing the oxygen concentration gradually decreases. Once the oxygen concentration falls below a critical value, the borohydride reaction with AgI ion becomes predominant, leading to the formation of particles again. Common complexing agents like NH3, CN-, EDTA, etc., are very effective in removing the oxidized surface silver atoms, but instantaneous reduction of their AgI complex by borohydride inhibits observing the oscillatory phenomena. So the slower reduction rate of AgI by borohydride in the presence of CTAB/SDS plays a very crucial role in this oscillatory reaction, and further investigations are needed to understand the mechanism of this slow reduction process. At this stage it may be presumed that (i) AgBr/Ag2O, the possible products of oxidation, form sol which are protected from borohydride by the protective
Formation and Dissolution of Ag Nanoparticles
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layer of CTAB/SDS or (ii) compartmentalization of the reduction reaction to form silver particles from AgI, due to the presence of surfactant.16,38 Aged Ag particles do not bleach in the presence of excess borohydride and so do not show reversibility, because of the formation of very thick surfactant layer at the particle surface. This thick layer inhibits borohydride adsorption at the particle surface, and so no bleaching of particles. Bleaching always requires shaking; however from the second cycle onward it needs comparatively less shaking, which indicates that bleaching becomes more efficient after the first cycle. This fact can be explained by the increased population of comparatively smaller particles after the first cycle (Figure 2c, d), which are expected to be more reactive. Again for the freshly produced particles, surfactant layers will be very thin or may be absent even. Hence dissolution becomes easier. Conclusion We have developed a novel oscillating system where an oscillation reaction occurs between AgI and silver nanoparticle in solution with periodic shaking/purging of air (O2). The presence of borohydride, oxygen, and surfactant is essential for conversion of silver particle to AgI, whereas borohydride and surfactant are essential for reconversion into silver particle. Borohydride has two roles. Firstly, it increases the reactivity of silver particle toward oxidation by oxygen and, secondly, it acts as reducing agent for AgI. On the other hand, surfactant has 3-fold effects. Firstly, it acts as particle stablizer. Secondly, it dissolves away the AgI from the particle surface to the bulk. Thirdly, it retards the rate of particle formation from AgI. The observed reversibility is summarized in Figure 3 and the following are the predominating reactions for shaking and standing. Shaking. (i) O2 concentration in water increases. (ii) BH4- gets adsorbed on a particle surface by removing adsorbed surfactant. (iii) Surface atoms of particles become oxidized by O2. (iv) Surfactant removes the (38) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975.
Figure 3. Mechanism of reversible formation and dissolution of silver nanoparticle.
oxidized surface silver atoms to the bulk water. (v) Steps iii and iv continue until dissolution is complete. Standing. (i) Borohydride starts reducing the AgI. (ii) Individual atoms aggregate to form small particles. (iii) Dissolved oxygen reacts with silver particle. (iv) After the consumption of O2, the silver particles becomes predominant due to the absence of the competitive oxidation reaction. (v) Remaining bulk AgI ions adsorb onto the surface of the newly formed particles. (vi) Borohydride reduces the adsorbed AgI ion. (vii) Processes v and vi continue until AgI is present in solution. This system, we hope, will be an efficient catalytic medium for reduction of >CdO to >CH-OH by borohydride. Acknowledgment. We thank Professor P. G. Mukunda and Dr. B. S. Murty for their help in TEM studies. N.R.J. thanks CSIR, India, for financial support. LA960834O
