Preparation and Catalytic Effect of Gold Nanoparticles in Water

The time course of UV−vis reveals a rapid decrease of the 290-nm CT band of AuCl4- upon addition of CS2, suggesting their interaction before reducti...
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J. Phys. Chem. B 1999, 103, 2862-2866

Preparation and Catalytic Effect of Gold Nanoparticles in Water Dissolving Carbon Disulfide Kanjiro Torigoe* and Kunio Esumi Department of Applied Chemistry, Institute of Colloid and Interface Science, Science UniVersity of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-8601 Japan ReceiVed: October 26, 1998; In Final Form: February 5, 1999

Stable gold nanoparticles of 2-nm diameter are prepared in water by chemical reduction of AuCl4- by NaBH4 in the presence of CS2 without adding any other stabilizers. The nanoparticles are proved to be Au and not Au sulfides by electron diffraction. In their UV-visible absorption spectra the surface plasmon (SP) band at around 520 nm is strongly damped, due to small particle size and possibly chemisorption of a CS2 derivative. The time course of UV-vis reveals a rapid decrease of the 290-nm CT band of AuCl4- upon addition of CS2, suggesting their interaction before reduction of Au3+. After the reduction of Au3+ has been completed, an intense band is built up at 330 nm which can be assigned to a reaction product of CS2 and NaBH4. Furthermore, it is found that this reaction is promoted by Au nanoparticles, although no change was observed for the SP band intensity. This catalytic effect is also found for citrate-capped Au hydrosols.

Introduction Sulfur-containing compounds are increasingly used for preparation of fine gold nanoparticles1-6 and formation of ordered assembly of the nanoparticles,3,5,7-10 as well as self-assembled monolayers (SAMs) on bulk gold surfaces.11-18 Physicochemical properties of these materials attract extensive interest not only for fundamental research such as the quantum size effect on the UV-vis absorption,19 nonlinear optic effects,20,21 and electron dynamics22,23 but also for many applications as microelectronic devices.7,24,25 Concerning the preparation of gold nanoparticles, alkanethiols with long alkyl chains (C12-C18) are frequently employed since the pioneering work by Brust et al.1,2 They synthesized gold nanoparticles of 1-3 nm in diameter in toluene by using dodecanethiol. In some detail, their method involves charge neutralization of AuCl4- in aqueous solution by addition of a cationic surfactant tetraoctylammonium bromide, followed by the phase transfer to toluene with dodecanethiol and the reduction to Au(0) by mixing with aqueous NaBH4. Due to high affinity of the thiol with the Au surface, these nanoparticles can be dried to a powder without coalescence and repeatedly dissolved in organic solvents such as toluene, chloroform, and pentane. Whetten et al.3 have prepared Au nanoparticles of 1.4-3.2 nm in diameter (75-800 atoms) by choosing alkanethiols with different alkyl chain lengths. Owing to the steric repulsion between the alkyl chain of thiols adsorbed on the Au surface, these nanoparticles are self-assembled to form a two-dimensionally close-packed structure upon evaporation of the solvent. In this system, the formation of thiol-derivatized Au nanoparticles is based on the solution-phase decomposition of a polymeric AuSR (R: alkyl) compound.4 On the other hand, dispersions of Au nanoparticles in water or hydrosols are also intriguing systems.26,27 However, alkanethiols with long alkyl chains are not appropriate for the aqueous dispersion, since they are hardly soluble in water. For these systems, some sulfur compounds having a relatively short alkyl chain and a (or some) hydrophilic group(s) may be effective. In this paper, we show that carbon disulfide (CS2) provides stable Au hydrosols without adding any other stabilizers

although CS2 is only slightly soluble in water. Moreover, the particle size can be decreased to 2 nm, which is comparable to the alkanethiol-derivatized Au nanoparticles in organic solvents or the Au hydrosols protected by a phosphorus compound.27 Optical properties of the CS2-derivatized Au nanoparticles and the stabilization mechanism are studied. In addition, a catalytic effect of the Au nanoparticles for the production of stabilizer from CS2 is discussed. Experimental Section Materials. Tetrachloroauric acid tetrahydrate (HAuCl4‚4H2O) was supplied by Tanaka Kikinzoku Kogyo Co. and used without further purification. Carbon disulfide (CS2) was pure grade from Wako Chemicals, and trace impurities were removed by agitating with mercury. The final purity was verified by a single peak in the gas chromatograph. Sodium borohydrate (NaBH4) was reagent grade from Wako and used as received. Water was deionized (resistivity > 1.8 MΩ cm) with a Milli-Q purification system. Procedure. To 19.8 cm3 of aqueous HAuCl4 (0.025-0.25 mmol dm-3), was added 6-24 mm3 of CS2 (5-20 mmol dm-3) with a Gilson micropipet, and the mixture was stirred until CS2 was completely dissolved.28 Then 0.2 cm3 of ice-chilled aqueous NaBH4 (350 mmol dm-3) was quickly added to the solution under vigorous stirring. Upon addition of the reductant, the color of the solution changed to yellowish brown for lower Au concentrations (e0.1 mmol dm-3). For higher Au concentrations (>0.1 mmol dm-3), the solution became a reddish dark color. Immediately a 2.5 cm3 portion was transferred to a quartz cell to measure the time course of UV-visible absorption spectra. The UV-visible spectra of the hydrosols were recorded for the wavelength range 190-820 nm at a 5 or 10-s interval at 30 °C on a Hewlett-Packard 8452A diode array spectrophotometer. As a reference, citrate-capped Au hydrosols were prepared by reduction with sodium citrate according to the method by Turkevich et al.29 The gold nanoparticles were characterized by transmission electron microscopy (TEM) and electron diffraction. The

10.1021/jp9841642 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/27/1999

Gold Nanoparticles

J. Phys. Chem. B, Vol. 103, No. 15, 1999 2863

Figure 1. UV-vis spectra of CS2-derivatized Au hydrosol before reduction (a, b) and after reduction (c) of Au3+: (a) 0.15 mmol dm-3 HAuCl4; (b) 15 mmol dm-3 CS2; (c) hydrosol prepared from 0.15 mmol dm-3 HAuCl4, 15 mmol dm-3 CS2, and 3.5 mmol dm-3 NaBH4.

Figure 2. UV-vis spectra of CS2-derivatized hydrosols for different Au concentrations. The spectra were recorded immediately after addition of NaBH4 ([CS2] ) 15 mmol dm-3; [NaBH4] ) 3.5 mmol dm-3; 30 °C.

samples were prepared by mounting a drop of the hydrosols on carbon-coated Cu grids and allowed to dry in the air. They were observed with a Hitachi H-9000 NAR operating at 200 kV and direct magnification of 100 000×. The electron diffraction was recorded at 1 m of camera length. Results and Discussion UV-visible absorption spectra of the component solutions were measured prior to the sol preparation. As shown in Figure 1, 0.15 mmol dm-3 HAuCl4 (a) shows a strong absorption band at λ ) 220 nm ( ) 14 700 dm3 mol-1 cm-1) and a shoulder at 290 nm ( ) 2100 dm3 mol-1 cm-1) due to charge transfer between the metal and chloro ligands.30 On the other hand, 15 mmol dm-3 CS2 (b) shows an absorption band at 208 nm ( ) 150 dm3 mol-1 cm-1). It can be seen also that both solutions do not absorb the light for λ > 400 nm. On the other hand, Figure 1c shows absorption spectra of the mixed solution at immediately after NaBH4 addition. It is found that the 220-nm band of AuCl4- vanishes, indicating that AuCl4- has completely been reduced. Instead, one can see an absorption band at 330 nm and, passing through a very weak shoulder at around 520 nm, a long tail extending to longer wavelengths. To obtain further information on the 330-nm band, UV-vis spectra of the CS2-derivatized hydrosols were compared in Figure 2 for different Au concentrations (0.025-0.25 mmol dm-3) immediately after the addition of NaBH4. It is obvious that the absorbance at 330 nm increases with increasing Au concentration. From this result, one may consider that the 330nm absorption band is associated to an Au derivative, although it does not belong to Au nanoparticles since the citrate-capped

Figure 3. TEM micrographs, size distributions, and electron diffraction patterns of CS2-derivatized Au nanoparticles (a-c) and citrate-capped ones (d-f).

one has no peak at this wavelength.29 In fact, however, it was proved that this band could be assigned to a reaction product from CS2 and NaBH4, as discussed below. On the other hand, the small shoulder at around 520 nm for CS2-derivatized hydrosols makes a clear contrast to a distinct absorption maximum at this wavelength for the citrate-capped one. Since we know that the 520-nm band corresponds to the surface plasma resonance wavelength of metallic gold nanoparticles, there are at least three possibilities for explaining the flat structure of the 520-nm band for CS2-derivatized hydrosols: The first possibility is that the nanoparticles are not Au but one of gold sulfides (e.g. Au2S, Au2S3, and AuS), as it is known that CS2 reacts with various metals at high temperature (3001000 °C) to afford their sulfide.31 For metal sulfides, the light absorption is due to excitons, and hence, a resonance absorption by surface plasmon which is characteristic of the metal is not observed. The second possibility is the size effect, although the particles are Au metal. With decreasing size, the metal nanoparticles become less metallic and more close to covalent crystals where the electrons are bound to the ionic core. The third possibility is chemisorption of CS2 or its derivative whereby the SP band of Au metal particles is strongly damped. In Figure 3 TEM micrographs, size distributions, and electron diffraction (ED) patterns are compared for CS2-derivatized hydrosols (a-c) and citrate-capped ones (d-f) prepared from the same HAuCl4 concentration (0.1 mmol dm-3). It is evident from (a) and (d) that the CS2-derivatized hydrosols are much

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Figure 4. Evolution of UV-vis spectra by interaction of AuCl4- and CS2 (full line) immediately after addition of CS2 to HAuCl4 solution (dashed line) after 30 min ([HAuCl4] ) 0.1 mmol dm-3; [CS2] ) 15 mmol dm-3).

smaller than citrate-capped one. The average diameter of CS2derivatized nanoparticles is 2.2 nm (b), while 15.4 nm for citratecapped ones (e). In both samples, the ED patterns (c, f) show concentric circles (Debye-Scherrer rings) due to random orientation of crystal planes for an ensemble of the particles. In the broadness of the diffraction rings, however, a remarkable difference lies between the two samples. Sharp rings are found for the citrate-capped particles (c), in contrast to very broad ones for the CS2-derivatized counterpart (f). This difference is presumably due to the different particle sizes. Since electron diffraction is analogous to X-ray powder diffraction for which Scherrer’s equation holds, it would be also true for ED that the broadness is inversely proportional to the particle size and that smaller CS2-derivatized particles gave broader diffraction rings. On the other hand, it is important to note that apart from the broadness the two patterns can be superimposed on each other with no additional or missing rings, which indicates that they are the same species. As the radii of diffraction rings for the citrate-capped nanoparticles are in good agreement with those for Au foil, the CS2-derivatized ones are identified to be Au. Formation of a gold sulfide (Au2S, Au2S3, or AuS) would give diffraction rings with radii different from those for Au metal,32 but it can be ruled out from the results of several measurements. Therefore, the second of aforementioned possibilities (quantum size effect of gold nanoparticles) is at least one of the reasons for the dampening of 520-nm band for CS2-derivatized hydrosols and the first one (formation of gold sulfides) is not relevant. This speculation can be supported by the optical spectra of thioland phosphorus-derivatized Au nanoparticles of less than 2 nm in diameter.4,19,27 Compared with the first two possibilities, the third one (effect of chemisorption) is less obvious. However, it is clear from Figure 3a that CS2 or its derivative is strongly adsorbing on the Au surface and protects from coagulation of the particles. Moreover, it was found that the addition of CS2 to the previously prepared Au sol had no effect on the UV-vis spectra; thus the chemisorption effect of CS2 on the 520-nm SP band of CS2derivatized Au sol is negligible. Then it is important to identify the CS2-derived species since it is very likely to be a stabilizer for Au nanoparticles. For this purpose, interaction of CS2 with Au compounds before and after the reduction of Au3+ was investigated by UVvis absorption spectroscopy. As shown in Figure 4, immediately after addition of CS2 (a), the HAuCl4 solution shows a distinct shoulder at 290 nm due to charge transfer in AuCl4-.30 However, it was smoothed out after stirring for 30 min (b). Closer investigation revealed that the absorbance markedly decreased within 1 min, followed by a slow decrease until 15 min, and

Torigoe and Esumi

Figure 5. Evolution of UV-visible spectra after reduction of Au3+ by NaBH4 ([HAuCl4] ) 0.1 mmol dm-3; [CS2] ) 15 mmol dm-3; [NaBH4] ) 3.5 mmol dm-3; 30 °C).

Figure 6. Dependence of the reaction rate on CS2 concentration ([HAuCl4] ) 0.035 mmol dm-3; [NaBH4] ) 3.5 mmol dm-3).

leveled off at longer time. This evolution indicates an interaction between AuCl4- and CS2. It is likely that due to this interaction the reduction of Au3+ by NaBH4 proceeds in a controlled manner leading to small particles. However, this interaction is not responsible for the 330-nm band observed after reduction of Au3+. On the other hand, Figure 5 shows the time course spectra of the samples after reduction of Au3+. It is found that the 330-nm band develops with time, which indicates that some reaction is taking place. In contrast, the absorbance at 520 nm does not change at all. At first glance, it may appear that this result contradicts with the aforementioned one (Figure 2) that the absorbance at 330 nm increases with increasing Au concentration. However, if the Au nanoparticles work as catalyst for a reaction of CS2 and the product is responsible for the 330nm band, then these two results are consistent with each other. To verify this speculation, we have studied first the effect of CS2 and NaBH4 concentrations on the reaction rate by measuring the time course of the absorbance at 330 nm. As shown in Figures 6 and 7, the initial increase in the absorbance increases with independently increasing CS2 and NaBH4 concentrations. However, no reaction occurs in the absence of NaBH4. These results implies that the reaction involves both CS2 and BH4-. Second, the catalytic effect of the Au nanoparticles was investigated from the evolution of absorbance at 330 nm for different Au concentrations. As shown in Figure 8, the reaction can take place in the absence of Au nanoparticles but the initial rate increases with increasing Au concentration. Yet a possible pH effect cannot be ruled out from this result alone, since the pH slightly decreases from 10.08 to 9.65 with increasing HAuCl4 concentration from 0.025 to 0.15 mmol dm-3. However, an experiment in buffer solution (NaHCO3-NaOH) proved that the pH effect was negligible in this range. Furthermore, a comparison of the reaction rate for the same concentration (0.10

Gold Nanoparticles

Figure 7. Dependence of the reaction rate on NaBH4 concentration ([HAuCl4] ) 0.05 mmol dm-3; [CS2] ) 15 mmol dm-3).

J. Phys. Chem. B, Vol. 103, No. 15, 1999 2865 concentration from 0.1 to 0.25 mmol dm-3. During the induction time, the reaction proceeds in the same rate as or a smaller rate than that for the blank sample (4 mmol dm-3 sodium citrate), and then suddenly the rate starts to increase. These results can be interpreted as follows. In the beginning the Au surfaces are covered with citrate ion which prevents adsorption of CS2 and BH4- by an ionic barrier; hence, the reaction of CS2 and NaBH4 would occur only in the bulk phase. However, as the citrate ions gradually desorb and are replaced by CS2 having a higher affinity for Au surface, the reaction is promoted by the catalytic effect of Au nanoparticles. In this system, the NaBH4 participates in two different reactions. One is the reduction of Au3+, and the other is the reaction with CS2. For the reduction of Au3+, the reaction formula is given as follows:33

4Au3+ + 12OH- + 3BH4- f 4Au + 3B(OH2)2- + 6H2O (1) 4Au3+ + 12OH- + 3B(OH2)2- f 4Au + 3BO2- + 6H2O (2)

Figure 8. Time course of the absorbance at 330 nm for different Au concentrations ([CS2] ) 15 mmol dm-3; [NaBH4] ) 3.5 mmol dm-3; [HAuCl4] ) (a) 0, (b) 0.025, (c) 0.035, (d) 0.05, (e) 0.10, (f) 0.125, (g) 0.15 mmol dm-3).

In these reactions, both the BH4- itself and its oxide B(OH2)2can reduce Au3+ to Au0. Concerning the reaction product of CS2 and BH4- in aqueous solution, we have not identified it yet; nor, to our knowledge, has it been reported. However, in THF, acetonitrile, and DMF, it has been studied by 1H and 11B NMR.34 It revealed that the reaction proceeded by four steps. The first step is addition of BH4- and CS2.

BH4- + CS2 f -H3BSCHdS:

(3)

The product reacts with itself to provide a dimer having an eightmembered ring conformation. Further, the reaction is followed by addition of CS2 and another dimerization, and finally tetrakis(dithiomethylene)borate anion [B(SCH2S)4]5- is produced. The overall reaction is given as follows:

5NaBH4 + 4CS2 f Na5[B(SCH2S)4] + 2B2H6

Figure 9. Catalytic effect of citrate-capped Au hydrosols for the reaction of CS2 in the presence of NaBH4 ([sodium citrate] ) 4 mmol dm-3; [CS2] ) 15 mmol dm-3; [NaBH4] ) 3.5 mmol dm-3; [HAuCl4] ) (a) 0, (b) 0.10, (c) 0.15, (d) 0.20, (e) 0.25 mmol dm-3).

mmol dm-3) of HAuCl4 (pH 4.18 before addition of NaBH4) and HCl (pH 4.15) verified that the reaction was promoted in the presence of Au nanoparticles. Therefore, a catalytic action of the Au nanoparticles for the reaction of CS2 was demonstrated. Finally, we have investigated whether the reaction could proceed on a Au surface other than CS2-derivatized ones. For an example, CS2 and NaBH4 were added to the citrate-capped Au hydrosols prepared beforehand and the absorbance at 330 nm was followed for 30 min. Prior to the experiment, we had confirmed that the addition of NaBH4 alone brings about no spectral change; thus, all Au3+ ions were reduced to Au. It is clear from Figure 9 that the kinetics for citrate-capped Au sols is different from that for CS2-derivatized ones in showing an induction time for the former system. The induction time decreases from about 9 to 1 min at 30 °C with increasing Au

(4)

This anion [B(SCH2S)4]5- has a cross structure with a boron atom as the center. Its five negative charges are shared between the center boron atom and four terminal sulfur atoms. If the same reaction is taking place for aqueous solution, then this final product or the intermediates are responsible for stabilization of Au nanoparticles. However, it must be confirmed that the above reaction takes place before the NaBH4 undergoes hydrolysis as follows:

NaBH4 + H2O f Na[BH3OH] + H2

(5)

Na[BH3OH] + 3H2O f Na[B(OH)4] + 3H2

(6)

Further studies are thus required to the identification of the product. Identification by Raman spectroscopy and 11B NMR is underway. Conclusions Gold nanoparticles of 2-nm diameter have been prepared by chemical reduction of HAuCl4 with NaBH4 in the presence of CS2. These Au hydrosols showed a very weak SP absorption band at λ ) 520 nm. The electron diffraction revealed diffuse Debye-Scherrer rings corresponding to Au metal. No additional rings were found; thus, the possible formation of Au sulfides

2866 J. Phys. Chem. B, Vol. 103, No. 15, 1999 (Au2S, Au2S3, and AuS) can be ruled out. Interaction between AuCl4- and CS2 was suggested from disappearance of the 290nm band of AuCl4-, whereby the reduction and crystal growth of Au particles may be controlled. On the other hand, after completion of the reduction of Au3+, an absorption band was built up at 330 nm. This absorption band was assigned to be a reaction product of CS2 and NaBH4, although it has not been identified yet. Furthermore, it was found that the reaction was promoted by a catalytic effect of Au nanoparticles at ambient temperature. It is rather surprising that Au nanoparticles catalyze the reaction at ambient temperature, but an extremely high affinity of the CS2 for the Au surface is presumably related to this phenomenon. References and Notes (1) Brust, M.; Walker, M.; Betthell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (3) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428. (4) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (5) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (6) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono. J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (7) Andres, R. P.; Bielefield, J. B.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (8) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (9) Luedtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323. (10) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (12) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097.

Torigoe and Esumi (13) Tarlov, M. J. Langmuir 1992, 8, 80. (14) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993, 9, 1517. (15) Ehler, T. T.; Malmberg, N.; Noe, L. J. J. Phys. Chem. B 1997, 101, 1268. (16) Badia, A.; Cuccia, L.; Demers, L.; Morin, F. G.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (17) Delamarche, E.; Hoole, A. C. F.; Michel, B.; Wilkes, S.; Despont, M.; Welland, M. E.; Biebuyck, H. J. Phys. Chem. B 1997, 101, 9263. (18) Horton, R. C., Jr.; Herne, T. M.; Myles, D. C. J. Am. Chem. Soc. 1997, 119, 12980. (19) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutie´rrez-Wing, C.; Ascensio, J.; Jose-Yacama´n, M. J. J. Phys. Chem. B 1997, 101, 7885. (20) Heilweil, E. J.; Hochstrasse, R. M. J. Chem. Phys. 1985, 82, 4762. (21) Bloemer, M. J.; Haus, J. W.; Ashley, P. R. J. Opt. Soc. Am. B 1990, 7, 790. (22) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713. (23) Feltstein, M. J.; Keating, C. D.; Liau, Y.-H.; Natan, M. J.; Scherer, N. F. J. Am. Chem. Soc. 1997, 119, 6638. (24) (a) Scho¨n, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101. (b) Scho¨n, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (25) Devoret, M. H.; Esteve, D.; Urbina, C. Nature 1992, 360, 547. (26) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Callis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634. (27) Duff, D. G.; Baiker, A.; Edwards, P. P. J. Chem. Soc., Chem. Commun. 1993, 96. (28) The solubility of CS2 in water is 20.4 mmol dm-3 at 30 °C. See: Gmelin Handbook of Inorganic Chemistry, 8th ed.; Springer: Berlin, 1977; Kohlenstoff, Vol. D4; p 193. (29) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 51. (30) These values are significantly lower than those in dilute HCl solutions. See: Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th ed.; Springer: Berlin, 1992; Gold, Suppl. Vol. B1, p 220. (31) Gmelin Handbook of Inorganic Chemistry, 8th ed.; Springer: Berlin, 1977; Kohlenstoff, Vol. D4, p 185. (32) Zhou, H. S.; Honma, I.; Komiyama, H.; Haus, J. W. Phys. ReV. B 1994, 50, 12052. (33) Khain, V. Zh. Neorg. Khim. 1983, 28, 2482. (34) Diamantikos, W.; Heinelmann, H.; Rath, E.; Binder, H. Z. Anorg. Allg. Chem. 1984, 517, 111.