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Reduction and Stabilization of Silver Nanoparticles in Ethanol by Nonionic Surfactants Luis M. Liz-Marza´n* and Isabel Lado-Tourin˜o† Department of Pure and Applied Chemistry, Physical Chemistry Section, Vigo University, Apdo. 874, Vigo, Spain, and Department of Physical Chemistry, University of Santiago de Compostela, E-15706 Santiago de Compostela, Spain Received December 4, 1995. In Final Form: April 29, 1996X The reduction of Ag+ ions in ethanol when certain surfactants are also present in the solution is shown to take place, even in the total absence of light. The reduction process leads to dispersions of silver nanoparticles, which are stable for weeks, although they show a certain tendency to stick to glass walls. Several surfactants were tested, showing that there is a dependency on their nature and concentration. Nonionic ethoxylated surfactants proved to be the most effective of all the surfactants tested, the reduction then being due to the oxidation of oxyethylene groups. In any case, the yield is very low (around 1%). Colloidal stability is achieved by adsorption of surfactant molecules onto the particles, which permits easy transfer of the particles into nonpolar solvents. The reduction rate markedly depends in a first-order fashion on silver salt concentration and depends on temperature according to Arrhenius’s law. The optical properties of the obtained colloids are influenced by the surfactant molecules attached to the particle surface.
Introduction One of the main challenges in the preparation of metal colloids in the nanometer size range ("nanoparticles") is the possibility of solvent exchange. This should allow for dispersions of the particles in both polar and nonpolar solvents and avoid particle aggregation during the process. The advantages of such a method can be found in the applications of metal particles as catalysts1,2 (most organic reactions take place in nonpolar solvents) and in the study of the changes in surface chemistry depending on the surrounding medium and on the adsorption of different moieties.3-5 A variety of preparation methods can be found in the literature, such as radiation chemical reduction,6 chemical reduction in an aqueous medium with7 or without8-10 stabilizing polymers, chemical or photoreduction in reverse micelles,11,12 or thermal decomposition in organic solvents.13 From all this work it can be assessed that colloidal stability, particle size, and optical properties strongly depend on the specific method and experimental conditions followed. The stability of the particles when dry is still difficult to achieve, and that is why solvent exchange is no simple matter. Specific methods must be designed for each desired dispersing medium. * To whom correspondence should be addressed at Vigo University. † University of Santiago de Compostela. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) Andrews, M. P.; Ozin, G. A. J. Phys. Chem. 1986, 90, 2929. (2) Nakao, Y.; Kaeriyama, K. J.Colloid Interface Sci. 1989, 131, 186. (3) (a) Linnert, T.; Mulvaney, P.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 838. (b) Henglein, A.; Mulvaney, P.; Linnert, T. Faraday Discuss. Chem. Soc. 1991, 92, 31. (4) (a) Henglein, A.; Mulvaney, P.; Holzwarth, A.; Sosebee, T. E.; Fojtik, A. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 754. (b) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992, 96, 10419. (5) Mulvaney, P. Langmuir 1996, 12, 788. (6) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (7) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (8) Duff, D. G.; Baiker, A. Langmuir 1993, 9, 2301. (9) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjipanayis, G. C. Langmuir 1993, 9, 162. (10) Liz-Marza´n, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120. (11) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Chem. Phys. 1993, 98, 9933. (12) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (13) Esumi, K.; Tano, T.; Torigoe, K.; Meguro, K. Chem. Mater. 1990, 2, 564.
S0743-7463(95)01501-0 CCC: $12.00
We present here a simple method for the preparation of dispersions of silver nanoparticles in ethanol, with the particularity that the particles can be transferred into nonpolar solvents through a dry state. The method contains elements from other methods previously described in the literature. Barnickel et al.14 reported the reduction of several metals in microemulsions based on cationic and nonionic surfactants. In the case of nonionic surfactants, they observed that reduction of Ag and Au occurs even without the addition of a specific reductant. Recently, Monnoyer et al.15 also found the formation of Ag particles when synthesizing AgBr in AOT microemulsions. However, Petit et al.,12 Boutonnet et al.,16 and Kurihara et al.17 have not reported such phenomena during particle reduction in microemulsions of AOT12 and C12E517 (the same nonionic as in ref 14). On the other hand, surfactants themselves are known to play an important role in the stabilization of metallic colloids. Esumi et al. proposed the formation of metallic complexes and their reduction after transfer into nonpolar solvents,18-20 while Puvvada et al.21 described the formation of Pd particles in a bicontinuous phase through the polyol process. Larger surfactant molecules, like crown ethers, have also been used as stabilizers of metallic particles in dispersion.22,23 The method presented here combines the properties of (14) (a) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992, 148, 80. (b) Barnickel, P.; Wokaun, A. Mol. Phys. 1990, 69, 1. (15) Monnoyer, Ph.; Fonseca, A.; Nagy, J. B. Colloids Surf. 1995, 100, 233. (16) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloids Surf. 1982, 5, 209. (17) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (18) Meguro, K.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 341. (19) (a) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (b) Ishizuka, H.; Tano, T.; Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf. 1992, 63, 337. (20) Meguro, K.; Tano, T.; Torigoe, K.; Nakamura, H.; Esumi, K. Colloids Surf. 1988/89, 34, 381. (21) Puvvada, S.; Baral, S.; Chow, G. M.; Qadri, S. B.; Ratna, B. R. J. Am. Chem. Soc. 1994, 116, 2135. (22) Jao, T.; Beddard, G. S.; Tundo, P.; Fendler, J. H. J. Phys. Chem. 1981, 85, 1963. (23) Humphry-Baker, R.; Gra¨tzel, M.; Tundo, P.; Pelizzetti, E. Angew. Chem., Int. Ed. Engl. 1979, 18, 630.
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surfactants as reductants and as stabilizers preventing particle aggregation. We have furthermore simplified the environment where the reaction is to be performed, using ethanol as a solvent in which both surfactants and silver salts have a relatively high solubility rather than complex fluids like microemulsions. Ethanol has been previously used in methods where the formation of radicals is promoted by γ,24 UV,25 or ultrasound26 irradiation or by prolonged reflux.27 Even the spontaneous formation of silver particles in methanol under certain experimental conditions has been described.28 Experimental Section Materials. AgNO3 was from Panreac (Pure Grade), NaBH4 was from Merck, Aerosol-OT (sodium Bis(2-ethylhexyl) sulphosuccinate) was from Aldrich, Brij 92 (poly-(2)-oxyethylene oleyl ether) and Brij 72 (poly-(2)-oxyethylene stearyl ether) were provided by ICI, and Brij 97 (poly-(10)-oxyethylene oleyl ether) and Tween 80 (polyoxyethylene-(20)-sorbitan monooleate) were purchased from Sigma. It should be mentioned here that these are commercial surfactants, with a certain polydispersity in the length of the poly-oxyethylene chains. Ethyl alcohol was from Panreac (Pure Grade), and cyclohexane was from Aldrich. All the chemicals were used as received, except AOT, which was purified by repeated dissolution in ethanol and rotary evaporation to dryness. Particle Synthesis. Simple mixing of solutions of AgNO3 and of the chosen surfactant in ethanol leads to the slow formation of silver particles, as manifested by a gradual yellowish coloration of the solution. No stirring is necessary after initial shaking for homogenizing the solution. As will be shown below, the extent of the reaction depends on the kind and concentration of surfactant, while the time needed for its completion mainly depends on silver initial concentration and on temperature. Experimental Techniques. Transmission electron microscopy (TEM) was performed with a Philips CM12 microscope; particle size distributions were calculated by image analysis, always over more than 100 counts. UV-visible spectra were measured in 10 mm optical path length quartz cuvettes with a Hewlett Packard HP8472 spectrophotometer.
Results and Discussion Choice of Surfactant. Several surfactants were tested, which permitted us to find out that the nature and concentration of the surfactant are crucial parameters for both the yield of the reaction and the stability and optical properties of the particles. The anionic surfactant AOT was the first surfactant tested because it is widely used for particle synthesis in microemulsions.11,12,15 Upon mixture of the reactants, clear yellow dispersions were obtained, increasing the intensity of the coloration slowly with time. The dispersions were stable for some days, but slow particle aggregation took place afterward. One could think that the impurities usually present in this surfactant were responsible for the formation of the particles. To check this point, samples were prepared with several concentrations of the surfactant as received and with the same concentrations after the purification procedure described under Materials. The UV-visible spectra of the obtained dispersions are shown in Figure 1. The shape of the spectra coincides in each case, with a small shift of the maximum. In the case of purified AOT, the intensities are higher, probably due to the elimination of the water (24) Linnert, T.; Mulvaney, P.; Henglein, A.; Weller, H. J. Am. Chem. Soc. 1990, 112, 4567. (25) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K. J. Chem. Soc., Faraday Trans. 1991, 87, 1905. (26) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. J. Chem. Soc., Chem. Commun. 1993, 378. (27) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1979, A13, 727. (28) Huang, Z.-Y.; Mills, G.; Hajek, B. J. Phys. Chem. 1993, 97, 11542.
Figure 1. UV-visible spectra of silver sols obtained from ethanolic solutions of AgNO3 and AOT: (a) surfactant as received; (b) purified surfactant. The purification of the surfactant does not essentially influence the nature of the sols. [AgNO3] ) 0.01 M.
absorbed by the surfactant (which means that surfactant concentration is actually higher in every case). The commercial nonionic surfactant Brij 97 proved to be the most effective of the surfactants tried. No coloration was observed immediately after mixing the solutions, but the highest absorption intensity of all the surfactants tested (for [AgNO3] ) 0.01 M and the same surfactant concentration) was obtained after two days time. When its lower homolog Brij 92 or Brij 72 was used, almost no coloration of the dispersion was observed, and after a few days a gray precipitate was detected on the bottom of the container. This was a first indication that the oxyethylene groups were responsible for the reduction and stabilization processes. Another commercial nonionic surfactant was also tested, namely Tween 80. This surfactant yielded again stable colored dispersions. The light absorption by these dispersions was slightly lower than that by the silver/Brij 97 dispersions with the same surfactant and silver concentration (which however means half the oxyethylene group concentration), though the reaction proceeded faster. In every case, the amount of reduced silver (as indicated by the height of the plasmon absorption band) was roughly proportional to the concentration of surfactant, and very low yields were obtained in every case. This can be seen in Figure 1 for AOT, and in Figure 2 for Brij 97 and Tween 80. Barnickel et al.14 proposed that the oxo groups from the polyether can form hydroperoxides, reducing Ag+ to Ag0. These authors also found that their surfactant (with seven ethoxy groups per molecule) contained ∼2 × 10-3 moles of reducing equivalents per mole of surfactant. Though Brij 97 and Tween 80 have more ethoxy groups per molecule (10 and 20, respectively), the number of reducing equivalents should be of the same order, which explains the large stoichiometric [surfactant]/[Ag+] ratio found. The main practical consequence of this low yield was that high Ag+ concentrations had to be used to get dispersions suitable for characterization.
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Figure 3. Change in UV-visible spectrum due to solvent exchange from ethanol to cyclohexane.
Figure 2. UV-visible spectra of silver sols obtained from ethanolic solutions of AgNO3 and Brij 97 (upper) and of AgNO3 and Tween 80 (lower). [AgNO3] ) 0.01 M. The spectra of surfactant solutions with the same concentration were subtracted to eliminate the effect from scattering by micelles. Reaction time was 24 h in each case.
For both nonionic surfactants the dispersions remained basically stable for weeks. There was however an effect against this colloidal stability, namely adsorption on the walls of silica containers, especially for the higher concentrations. This effect is mostly observed for quartz (≈9% of the particles are lost after 1 day in a quartz cuvette, for a dispersion containing 0.1 M Brij 97), while it is hardly noticeable if the dispersions are stored in glass vials. This tendency to attach onto silica surfaces represented a serious problem for the spectroscopic study of the kinetics of particle formation. (After each measurement, the dispersion had to be removed from the cuvette, and this was cleaned with aqua regia to dissolve any remaining of silver attached to the quartz walls.) Advantage can be taken of this property, for the application of the particles to the preparation of thin films. This will be reported in a further study. It can already be noticed at this point that the position of the silver plasmon band is clearly shifted with respect to that of bare particles (around 400 nm). One should also notice that the shift is toward lower wavelengths for AOT but toward higher wavelengths for both Brij 97 and Tween 80. In the case of AOT, the position of the maximum (355 nm) suggests that very small silver particles/clusters are formed during the synthesis.3b Though such clusters have short lifetimes, the presence of a large excess of Ag+ can keep a noticeable concentration. Because very small yields and no successful transfer to nonpolar solvents were obtained with this surfactant, we do not go into more detail on the description of the obtained dispersions. Transfer into Nonpolar Solvents. The particles formed by reduction with Brij 97 can readily be transferred into organic, nonpolar solvents without affecting colloidal stability. To this aim, ethanol was completely removed by evaporation of a sample prepared with [AgNO3] ) 0.01 M, [Brij 97] ) 0.1 M which had been allowed to react for 4 days. A suspension of the nonaggregated particles in the liquid surfactant was obtained after the evaporation. When adding an organic solvent like cyclohexane, (most
of) the particles got redispersed, whilst the excess surfactant slowly formed agglomerates which could be allowed to sediment by themselves or be removed from the dispersion by filtration or brief centrifugation. The yellow dispersions obtained in this way were stable for months. From the absorbance at the maximum before and after the transfer (see Figure 3), we estimated that ca. 70% of the particles are redispersed, the rest remaining in the sedimented surfactant. A change in the position of the plasmon band due to the solvent exchange process was observed, which will be discussed below. In dispersions prepared with the same concentration of Tween 80 the solvent was also evaporated, but no redispersion was achieved in nonpolar solvents. Influence of Light. In order to study the influence of light on the synthesis, two identical sets of samples for each surfactant (Brij 97 and Tween 80) were prepared with the same AgNO3 concentration (0.01 M) and selected surfactant concentrations (between 0.01 and 0.1 M). The preparation of one set was performed without taking any care regarding the presence of light, whilst the other set was kept in complete darkness (injecting the surfactant solution in a blackened flask containing the AgNO3 solution and mixing by simple shaking). Two days after the preparation, the UV-visible spectrum of each dispersion was measured. The silver plasmon bands of the equivalent samples with and without light coincided in every case. However, an additional peak was measured in all the "dark" samples, which could not yet be identified. As already suggested by Barnickel et al., the redox process involved in the formation of the particles can be induced by both thermal and photochemical mechanisms. Our results suggest that the thermal mechanism is more important than the photochemical one. This is confirmed in the next section on kinetics by a dramatic increase in reaction rate when increasing temperature. Kinetics of Formation. The process of particle formation was monitored by UV-visible spectroscopy with different concentrations of AgNO3 and Brij 97. Figure 4 shows the time evolution of the absorbance at 400 nm. In the upper part of Figure 4 the dependency on initial surfactant concentration (at constant [Ag+] ) 0.01 M) can be observed, while the lower part shows the influence of initial Ag concentration (at constant [Brij 97] ) 0.01 M). It can be observed that the formation rate decreases with time. The position of the plasmon band (of its maximum) only slightly blue-shifts during the process. From Figure 4 it comes out that the starting concentration of surfactant determines the maximum extent that the reduction can reach in each case (as mentioned above, always much lower than the initial silver concentration). On the other hand, the concentration of silver salt is of
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Figure 5. Time evolution of the absorbance at λ ) 400 nm measured from ethanolic solutions of AgNO3 and Brij 97 at three selected temperatures. [AgNO3] ) [Brij 97] ) 0.01 M. The inset shows the Arrhenius-like change of rate constant with temperature.
Figure 4. Time evolution of the absorbance at λ ) 400 nm of ethanolic solutions of AgNO3 and Brij 97: (a) influence of surfactant concentration on the extent of the reduction; (b) influence of silver salt concentration. Table 1. First-Order Rate Constants Obtained from Fits by Eq 1 to the Time Evolution of the Absorbance at 400 nm during Particle Formation, with Varying Reactant Concentrations and Temperature [AgNO3] (mol/L)
[Brij 97 [(mol/L)
0.005 0.01 0.001 0.01 0.01 0.01
0.01 0.05 0.001 0.01 0.01
[Tween 80] (mol/L)
T (°C)
k (min-1 × 104)
A∞
0.01
25 25 25 40 50 25
4.57 9.6 5.9 34.0 144.0 18.2
0.225 1.325 0.246 0.240 0.188 0.250
relevance for the reaction rate. The solid lines in Figure 3 are fits by a first-order rate equation
At ) A∞(1 - e-kt)
(1)
where At is the absorbance at time t, A∞ the absorbance at a very long time, and k the first-order rate constant. The results of these fits are given in Table 1. In the cases with the lowest initial Ag+ concentration and the two lowest initial surfactant concentrations, the data were not accurate enough to perform meaningful fits. It should be noticed that other mechanisms (like particle number density and growth) can influence the absorbance, but this simplified model permits us to see the influence of reactant concentration. In Table 1, results are also given of the first-order fit for a synthesis with Tween 80 ([surfactant] ) [AgNO3] ) 0.01 M). The rate constant is about three times larger than that of the preparation with Brij 97 at the same temperature and with the same starting concentrations, though the long time absorbance is approximately the same. This shows that the larger amount of ethoxy groups per molecule does not noticeably improve the yield of the reaction, but the process proceeds faster. Apparently, the geometry of this surfactant hinders the reaction of some of the ethoxy groups with the silver ions. Whilst Brij 97
is formed by a long, single chain, Tween 80 contains three polyoxyethylene chains, so that they can sterically hinder each other. The influence of temperature on the reaction rate was studied for initial concentrations [Ag+] ) [Brij 97] ) 0.01 M. As shown in Figure 5, the reaction rate dramatically increased with temperature. However, no extra yield was obtained at higher temperature. Actually, it seems that the maximum intensity was lower when the reaction was performed at higher temperature. First-order rate constants were obtained for these reactions by means of eq 1. Such constants are given in Table 1 and plotted versus the inverse of the temperature in the inset of Figure 4. This plot shows that the rate constants follow Arrhenius’s law, expressed as
k ) Ae-Ea/RT
(2)
where A is the Arrhenius pre-exponential factor and Ea the activation energy. The activation energy calculated from the fit was on the order of 1 kJ/mol, which suggests that the process can easily be promoted by a thermal mechanism. This agrees with the conclusion given in the previous section. Electron Microscopy. The process of particle formation was also monitored by TEM, taking samples during the first 24 h and after one week (see Figure 6). The size distribution of each sample was calculated by image analysis, using at least 100 counts. No clear trend was observed, but larger particles seemed to be formed during the first steps of the reaction, increasing afterward the proportion of smaller clusters. Actually, in the sample measured one week after mixing the ethanolic solutions, the population of large particles was very small, most of the counts being in the range 3-10 nm. With respect to surfactant kind and concentration, the general observation was that smaller particles were formed for lower initial surfactant concentration. It was however difficult to establish clear differences between particles formed with Brij 97 and with Tween 80. Optical Properties. All the dispersions of silver particles prepared with nonionic surfactants showed plasmon bands peaked above 400 nm. The observed shift can in principle be due to the size and shape distribution of the particles, given that, as observed by TEM (see Figure 6), the particles are not spherical but rather have polyhedral shapes which can very well influence the optical properties of the dispersions. However, the presence of a relatively large concentration of Ag+ ions can induce their adsorption onto the surface of the particles, causing a red shift of the plasmon band.3b This and the adsorption
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On the other hand, the process of solvent exchange from ethanol to cyclohexane should lead, according to Mie’s theory,31 to a red shift of the plasmon band, due to the higher refractive index of cyclohexane (1.426) with respect to that of ethanol (1.361). What we observe instead is a small blue shift and a thinning of the band (see Figure 3). This is due to the insolubility of AgNO3 in cyclohexane, so that there are basically no adsorbed ions on the particles. This effect is opposite to that of solvent refractive index. The band position is however still at high wavelength, showing that indeed there is a remarkable effect of the adsorbed surfactant, which is still responsible for colloidal stability.
Figure 6. Electron micrographs and particle size distributions of silver particles formed by reduction/stabilization with Brij 97, 7.5 h (a) and 1 week (b) after mixing the reactants. [AgNO3] ) [Brij 97] ) 0.01 M.
of surfactant molecules onto the particle surface are the most likely reasons for the measured red shift of the band. The effect on the optical properties of the adsorption of different species onto the surface of metal particles has been recently reviewed by P. Mulvaney.5 This author points out that "sols prepared with different stabilizers often have quite different absorption spectra even though the particle size distributions appear similar". This sentence could readily be applied to our sols. The adsorption of iodide or sulfide ions29 or of stabilizers as gelatin or PVP30 onto silver sols leads to a marked red shift, with damping of the plasmon band. This effect is precisely the same as we observe here with the nonionic surfactants (Brij 97 and Tween 80), but we should still be cautious because of the high Ag+ concentration. (29) Strelow, F.; Henglein, A. J. Phys. Chem. 1995, 99, 11838. (30) (a) Berry, C. R.; Skillman, D. C. J. Appl. Phys. 1971, 42, 2818. (b) Skillman, D. C.; Berry, C. R. J. Chem. Phys. 1968, 48, 3297.
Conclusions Colloidal silver particles in the nanometer size range were synthesized in ethanol, by reduction of AgNO3 with nonionic surfactants. The main conclusion is that surfactants reduce silver ions to the neutral state through oxidation of oxyethylene groups into hydroperoxides. Surfactant molecules subsequently adsorb onto the surface of the particles, promoting steric stabilization. This adsorption permits also the transfer of the particles into nonpolar solvents through a dry state where no particle aggregation occurs. The [surfactant]/[metal] ratio has an important effect on the extent of the reduction: large surfactant concentrations are necessary for a complete reduction, which implies low yield when reasonable amounts of surfactant are used. On the other hand, the reaction rate is strongly influenced by silver salt concentration (in a first-orderlike fashion) and by temperature (following Arrhenius’s law). With regard to the optical properties of the dispersions, the silver plasmon absorption band is shifted to larger wavelengths, probably due to the presence of free silver ions and to surface modification by adsorbed surfactant molecules. Finally, we can point out that the use of stronger reductants on previously formed silver/surfactant dispersions can be used to get concentrated silver dispersions in ethanol, which is definitely difficult with other methods described in the literature. However, the dispersions are rather polydisperse, though the size distributions get narrower with time. This, together with the nonspherical shape, somehow limits the applications of the particles as models for the study of optical properties. Acknowledgment. This work has been partly supported by the Spanish Consellerı´a de Educacio´n e Ordenacio´n Universitaria, Xunta de Galicia, Project no. XUGA34701A95. LA951501E (31) (a) Bohren, C. F.; Huffman, D. F. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (b) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427.
