Photochemical Synthesis in Formamide and Room-Temperature

Photochemical Synthesis in Formamide and. Room-Temperature Coulomb Staircase Behavior of. Size-Controlled Gold Nanoparticles. M. Y. Han*,† and C. H...
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Langmuir 2000, 16, 362-367

Photochemical Synthesis in Formamide and Room-Temperature Coulomb Staircase Behavior of Size-Controlled Gold Nanoparticles M. Y. Han*,† and C. H. Quek‡ Department of Chemistry, Indiana University, Bloomington, Indiana 47405, and Department of Chemistry, National University of Singapore, Singapore 119260 Received April 8, 1999. In Final Form: August 31, 1999 An effective photochemical route is reported here for size-controlled synthesis of nonaqueous colloidal gold particles in a highly polar and viscous medium, formamide in the presence of poly(vinylpyrrolidone). The effective photoreduction of AuCl4- in the nonaqueous system is attributed to the higher degree of dissociation of AuCl4- in formamide and the direct reduction by the photogenerated free radicals from the coordinated formamide molecules. Evident photochemical formation of gold particles was not achieved in dimethylformamide because AuCl4- cannot be dissociated in dimethylformamide, having no amino group but an aldehyde group as in formamide. Under similar conditions, uniform gold particles were prepared much more easily in formamide than in water, and the photoreduction of AuCl4- was also carried out much more completely in formamide than in water. A Coulomb staircase was clearly observed at room temperature on the nonaqueously grown uniform gold particles of 12 nm, but this behavior did not occur on the aqueously grown gold particles of similar size. This new photochemical route is also very effective to produce other uniform transition-metal particles such as silver, palladium, and platinum.

Introduction The continuous interest in the nanosized transitionmetal particles has been driving by their unusual physical and chemical properties, which are quite different from those of the bulk materials.1-5 Intensive studies have been devoted to their uses as catalysts, ferrofluids, and sensors as well as their potential applications in a new generation of optical, electronic, and magnetic devices.2-5 These applications are closely related to their size- or shapedependent properties, such as size quantization, optical and electronic properties, surface effects, chemical reactivity, and self-assembled ability.1-7 Although many chemical routes are available for the production of sizecontrolled or shape-selective transition-metal particles in aqueous solutions,6,7 it is still desirable to design new synthetic methods for producing them in nonaqueous solutions, which could bring about some new features in the above-mentioned applications.8,9 The pioneering work of Burst, a two-phase chemical reduction method to † ‡

Indiana University. National University of Singapore.

(1) (a) Henglein, A. Chem. Rev. 1989, 89, 1861. (b) Schmid, G. Chem. Rev. 1992, 92, 1709. (c) Schmid, G. Clusters and Colloids; VCH: Weinheim, 1994. (2) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (3) (a) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Verlag: 1995. (b) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (4) (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (b) Fan, F. R. F.; Bard, A. J. Science 1997, 277, 1791. (c) Klein, D. L.; McEuen, P. L.; Katari, J. E.; Roth, R.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 68, 2574. (5) (a) Shi, J.; Gider, S.; Babcock, K.; Awschalom, D. D. Science 1996, 271, 937. (b) Hehn, M.; Ounadjela, K.; Bucher, J.-P.; Rousseaux, F.; Decanini, D.; Bartenlian, B.; Chappert, C. Science 1996, 272, 1782. (c) Suslick, K. S.; Fang, M.; Hyeon, T. J. Am. Chem. Soc. 1996, 118, 11960. (6) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924. (b) Curtis, A. C.; Duff, D. G.; Edwards, P. P.; Jefferson, D. A.; Johnson, B. F. G.; Kirkland, A. I.; Wallace, A. S. Angew. Chem., Int. Ed. Engl. 1988, 27, 1530. (7) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. Frens, G. Nature Phys. Sci. 1973, 241, 20.

nonaqueous transition-metal particles bearing a surface coating of thiol, has been widely used now,10-12 and the electrochemical synthesis of size-selective nanostructured palladium clusters in tetrahydrofuran is another successful method.9 Here we present an effective photochemical route of producing size-controlled spherical gold particles in a highly polar and viscous medium, formamide in the presence of poly(vinylpyrrolidone). A Coulomb staircase was clearly observed at room temperature on the resulting uniform gold particles of 12 nm prepared in the new nonaqueous system. However, the phenomenon did not occur on the gold particles of similar size prepared in aqueous solution.4 This photochemical route was also used to prepare other uniform transition-metal particles such as cubic palladium particles in the new nonaqueous system. Experimental Section Chloroauric acid (HAuCl4) and chloroplatinic acid (H2PtCl6‚ xH2O) were supplied by Sigma and Aldrich, respectively. Silver nitrate and palladium acetate were obtained from Aldrich. Poly(vinylpyrrolidone) (PVP, Mw 40 000) from Fluka and analytical reagent grade formamide from Merck were used as received. All glassware used in all preparations was scrupulously cleaned with chromic acid solution. Nearly monodisperse gold particles were photochemically prepared in the 6.0 × 10-4 M HAuCl4formamide solutions at a PVP/HAuCl4 molar ratio of 8/1, 5/1, and 3/5, separately. A certain quantity of PVP was first added into each of the freshly prepared HAuCl4-formamide solutions. (8) (a) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129. (b) Weaver, S.; Marignier, J. L.; Belloni, J.; Delcourt, M. O.; Chevalier, J. P. Nature 1985, 317, 344. (c) Yonezawa, Y.; Sato, T.; Kuroda, S. J. Chem. Soc., Faraday, Trans. 1991, 87, 1905. (9) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (10) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801. (11) (a) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao C. N. R. J. Phys. Chem. B 1997, 101, 9876. (b) Manna; A.; Kulkarni, B. D.; Bandyopadhyay, K.; Vijayamohanan, K. Chem. Mater. 1997, 9, 3032. (12) (a) Johnson, S. R.; Evans, S. D.; Brydson, R. Langmuir 1998, 14, 6639. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706.

10.1021/la990406t CCC: $19.00 © 2000 American Chemical Society Published on Web 10/09/1999

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After being mixed thoroughly and purged with nitrogen for 10 min, the colorless formamide solutions were irradiated with four 40 W low-pressure mercury lamps at room temperature until no further changes in the color of the solutions were observed. Under similar conditions, palladium and platinum particles were prepared in the 6.0 × 10-4 M H2PtCl6- and Pd(CH3CO2)2formamide solutions at an identical PVP/HAuCl4 molar ratio of 5/1, respectively, and silver particles was produced in the 6.0 × 10-4 M AgNO3-formamide solution at a PVP/HAuCl4 molar ratio of 3/5. As a comparison, gold particles were also produced in the aqueous solution of 6 × 10-4 M HAuCl4 at a PVP/HAuCl4 molar ratio of 5/1. Absorption spectra of gold colloids were recorded on a HP8452A diode array spectrophotometer. The samples for transmission electron microscopy (TEM) were prepared by depositing metal colloids on Formvar-coated copper grids followed by drying them in a desiccator. TEM micrographs were taken with a JEOL JEM100CXII electron microscope operated at 100 kV. Scanning tunneling microscopy (STM) imaging and current-voltage (IV) characteristics of gold particles were investigated by a multimode Topometrix TMX 2000 scanning probe microscope. The samples for STM were prepared by depositing gold colloids on the surfaces of highly oriented pyrolytic graphite (HOPG) substrates following by drying them in the air. A mechanically cut Pt-Ir tip was used for STM imaging and I-V spectroscopy. The STM tip-sample bias voltage and tunneling current used here were 200 mV and 1.0 nA, respectively. For registering I-V characteristics, an STM tip was situated above the chosen particle from an STM image recorded, and the I-V curve was then measured. A two-liquid system (water + 1-butanol + formamide) was used here to extract the nonaqueously grown gold particles from formamide into water because formamide has a higher solubility than water in butanol. When the liquid-liquid equilibrium was reached in the system after being shaken vigorously, relatively more formamide and relatively less water were extracted in the upper butanol layer, and PVP-stabilized gold particles were transferred into the lower water layer. Formamide in the water layer was extracted by using the partially miscible water-butanol mixture several times, and the gold particles were then dispersed in water alone, indicated by the colorless butanol layer and the deep magenta water layer. Simultaneously, the unreacted gold complexes residing in the nonaqueous gold colloid were mainly extracted into the upper butanol layer too. The formamidebutanol mixture was used to extract the unreacted gold complexes from the aqueously grown gold colloid. The amounts of the unreacted gold complexes in the gold colloids were analyzed with a Shimadzu AA-670 spectrophotometer.

Results and Discussion The effect of increasing the stabilizer/HAuCl4 molar ratio in decreasing the particle size in aqueous systems has been made well-known for a long time by numerous studies. However, it is still interesting to study it in nonaqueous media with different polarities, viscosities, and reducing abilities because some new features could be generated in the resulting size-controlled gold particles. The photochemical formation of size-controlled nonaqueous gold colloids was achieved here by changing the molar ratio of PVP/HAuCl4 in the 6.0 × 10-4 M HAuCl4formamide solutions. As shown in the TEM micrographs in Figure 1, the uniform gold particles ∼6, 12, and 18 nm in diameter were prepared in the formamide solutions at a PVP/HAuCl4 molar ratio of 8/1, 5/1, and 3/5, respectively. The particle size distributions of these nearly monodisperse gold particles are shown in Figure 2. The uniform cubic palladium particles ∼16 nm in diameter (size distribution from 14 to 18 nm) as displayed by the TEM in Figure 3 were prepared in the 6.0 × 10-4 M Pd(CH3CO2)2-formamide solution at a PVP/Pd(CH3CO2)2 molar ratio 5/1. The successful preparation of uniform silver (size distribution from 13 to 18 nm) and platinum (size distribution from 8 to 12 nm) particles under similar

Figure 1. TEM micrographs of the uniform gold particles prepared in the 6.0 × 10-4 M HAuCl4-formamide solutions at a PVP/HAuCl4 molar ratio of (a) 8/1, (b) 5/1, and (c) 3/5, respectively.

conditions further implies that this new photochemical route can be developed into an effective method of producing uniform transition-metal particles. Formamide is one of the standard compounds used as a solvent for various chemical or biochemical reactions. As its dielectric constant is much higher than that of water or alcohols, gold complexes dissolved in it can exhibit a much higher degree of dissociation.8,13 The ligand-to-metal (13) (a) Zamudio, W.; Carcia, A. M.; Baraona, R. Transition Met. Chem. 1995, 20, 518. (b) Belevantsev, V. I.; Kolonin, G. R.; Ryakhovskaya, S. K. Russ. J. Inorg. Chem. 1972, 17, 1303.

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Figure 3. TEM micrographs of the palladium particles prepared in the 6.0 × 10-4 M Pd(CH3CO2)2-formamide solution at a PVP/Pd(CH3CO2)2 molar ratio 5/1. The insert is the TEM micrograph of the palladium particles at a higher magnification.

Figure 2. Particle size distributions of the uniform gold particles prepared in the 6.0 × 10-4 M HAuCl4-formamide solutions at an identical PVP/HAuCl4 molar ratio of (a) 8/1, (b) 5/1, and (c) 3/5, respectively.

charge transfer (LMCT) band of AuCl4- can thus decay much faster by displacing Cl- from AuCl4- than that in water or alcohols.11,14 Figure 4 shows the spectral changes of AuCl4- in formamide and water with increasing time.15 The LMCT band at 322 nm disappeared completely in formamide within 1 min; however, the hydrolysis of AuCl4in water (or methanol) was much slower and a part of the band was still observed after 10 min. The slower reaction of AuCl4- with water or ethanol was also described by (14) (a) Siiman, O.; Hsu, W. P. J. Chem. Soc., Faraday trans. 1, 1986, 82, 851. (b) Quinn, M.; Mills G. J. Phys. Chem. 1994, 98, 9840. (c) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (15) The LMCT band of AuCl4- decayed very fast in formamide due to its higher solvating ability, and the time course of the absorption spectra was thus difficult to capture. As AuCl4- is not dissociated in dimethylformamide, a stock solution of AuCl4- was then prepared in it. A formamide solution of desirable concentration of AuCl4- was prepared by adding a small amount of the stock solution into a quartz cell. As shown in Figure 4, the clear dissociation process of AuCl4- was observed by immediately recording the absorption spectra of the formamide solution in the quartz cell.

Figure 4. The time course of absorption spectra of AuCl4- in formamide (solid lines) and aqueous (dashed line) solutions. The solid-line curves exhibit the fast dissociation process of AuCl4- in formamide. The dashed-line curve is the absorption spectrum of AuCl4- in water after 10 min.

Quinn and Torigoe et al.14 Therefore, the rapid dissociation process of AuCl4- in formamide can also be carried out much more completely than that in water or alcohols. A similar ligand-substitution reaction can also occur in alkylamine.16 However, the LMCT band remains unchanged in dimethylformamide. It is thus assumed that the amino group instead of the aldehyde group in formamide participates in the ligand-substitution reactions as the hydroxyl group does in the aqueous or alcohol solution. The ligand-substitution reaction is suggested to be as follows: AuCl4- + HCONH2 f HCONH2-AuCl3 + Cl-. Evident photochemical formation of gold particles was not achieved in dimethylformamide because AuCl4cannot be dissociated in dimethylformamide, having no primary amino group but an aldehyde group as in formamide. The higher solvating ability of formamide is thus very important for the effective formation of gold particles. (16) Nagel, Y.; Beck, W. Z. Anorg. Allg. Chem. 1985, 529, 57.

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The photolysis of AuCl4- in water can result in the formation of gold particles by eliminating chloro ligands from AuCl4-.17 A similar formation mechanism was also suggested by Itakura in the photochemical formation of gold particles in ethanol.8 The photoreduction of AuCl4- in ethanol can be greatly accelerated by benzoin, an effective photoinitiator. After 30-min irradiation of the 10-4 M HAuCl4-PVP-ethanol solutions, the surface plasmon band intensity of the resulting gold colloid was only half of that in the presence of benzoin. A similar case (to be discussed below) was also observed in the photochemical formation of gold colloids in the formamide and aqueous solutions of 6.0 × 10-4 M HAuCl4 at an identical PVP/HAuCl4 ratio of 5/1. The absorption intensity of the gold colloid in the formamide solution is again stronger than that in the aqueous solution. The effective formation of gold particles indicates that the formation mechanism in formamide is different than the one in water. The photolysis of HCOND2 in acetone gave (CH3)2C• OH rather than (CH3)2C•OD, together with a radical having no detectable coupling to protons.18 The radical structure was thus identified as OCNH2 (carbamoyl radical) by Livingston and Zeldes, which was originally thought to be HCO‚H.18 The effective formation of gold particles in formamide should be attributed to the direct reduction of the formamide-dissociated gold complexes by the photogenerated free radicals from the coordinated formamide molecules. The rapid photochemical formation of colloidal silver particles via the reduction by the acetone ketyl radicals ((CH3)2COH) was reported.8,19 The reduction reaction of silver ions proceeded as follows: (CH3)2COH + Ag+ f Ag0 + (CH3)2CO + H+. Experimental data in the previous reports also showed that the generation of carbamoyl radicals can be accomplished through hydrogen atom abstraction from formamide molecules by the photoactivated acetone molecules.18 We also believe that the hydrogen atoms can be photogenerated from the formamide-coordinated gold complexes and abstracted by them directly, and they can further reduce the gold complexes to gold just as the acetone ketyl radicals do in the reduction of silver ions to silver. The stabilizer PVP can reduce the growth and flocculation of gold particles via steric stabilization as it can be adsorbed onto the gold nuclei. Because of the higher degree of dissociation of AuCl4- in formamide, free radicals can be photogenerated directly from the formamide-coordinated gold complexes, and the direct reduction of AuCl4- by the free radicals can increase the nucleation rate of gold particles. The highly viscous formamide solution can prevent the reduced gold atoms from migrating to a nucleus before being nucleated itself via the lower rate of diffusion of AuCl4- in it. The higher polarity and viscosity of formamide besides the steric stabilization by PVP can effectively reduce the growth rate and enlarge the nucleation rate of gold particles, resulting in uniform gold nanoparticles.13,20 This is because only a wide size distribution of gold particles from 10 to 20 nm was achieved in the production of gold colloid in (17) (a) Yonezawa, Y.; Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1559. (b) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (18) (a) Livingston, R.; Zeldes, H. J. Chem. Phys. 1967, 47, 4173. (b) Hayon, E.; Ibata, T.; Lichtin, N. N.; Simic, M. J. Am. Chem. Soc. 1970, 92, 3898. (c) Elad, D.; Friedman, G. J. Chem. Soc. C 1970, 893. (d) Rokach, J.; Elad, D. J. Org. Chem. 1966, 31, 4210. (19) (a) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull. Chem. Soc. Jpn. 1994, 67, 3165. (b) Esumi, K.; Wakabayashi, M.; Torigoe, K. Colloids Surf. A 1996, 109, 55. (20) Satoh, N.; Hasegawa, H.; Tsujii, K. J. Phys. Chem. 1994, 98, 2143.

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Figure 5. The evolution of the absorbance at maximum absorption wavelength in the (a) formamide and (b) aqueous solutions of 6.0 × 10-4 M HAuCl4 at an identical PVP/HAuCl4 molar ratio of 5/1.

the aqueous solution under similar conditions. In comparison with the preparation of gold colloid in ethanol accelerated by the effective photoinitiator benzoin,8 the narrow size distribution of colloidal gold particles produced in formamide could arise from its more moderate reduction conditions. Figure 5 shows the evolution of the absorbance at maximum absorption wavelengths in the aqueous and formamide solutions of 6.0 × 10-4 M HAuCl4 at an identical PVP/HAuCl4 molar ratio of 5/1. The formation of gold particles in the aqueous solution was first evident within an irradiation time of 10 min, and the surface plasmon band of the resulting gold colloid became stronger with time and reached its maximum absorption after 30 min. The formation of gold particles in the formamide solution was also evident within 10 min. However, it took 100 min to reach the maximum absorption of the resulting nonaqueous gold colloid, and the maximum absorption intensity of the gold colloid is once again stronger than that of the gold colloid prepared in the aqueous solution. In addition to the much more complete photoreduction of AuCl4- in formamide than that in water, the difference in absorbance for the two gold colloids discussed below could also arise from the effects of different solvents and different particle size distributions in them. The two-liquid system water + 1-butanol + formamide was used here to extract the nonaqueously grown gold particles from formamide into water by using the partially miscible water-butanol mixture. After correcting the volume of the gold colloid in water to the original one in formamide, the absorbance of the gold colloid is still much higher as compared to that of the gold colloid prepared in the aqueous solution. It is known that the absorption intensity of gold particles increases with particle size up to a maximum at ∼80 nm.21 The aqueously grown gold colloid should have shown much stronger absorption than the nonaqueously grown gold colloid because of its larger mean size than the latter one. Therefore, the difference in absorbance for the two gold colloids does not arise from the effects of the different solvents or particle size distributions, and the photoreduction of AuCl4- must be carried out much more (21) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Phys. Chem. B 1993, 98, 9933.

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Figure 6. Absorption spectra of the final gold colloids prepared in the (a) formamide and (b) aqueous solutions of 6.0 × 10-4 M HAuCl4 at an identical PVP/HAuCl4 molar ratio of 5/1.

completely in the nonaqueous solution than in the aqueous solution. As shown in Figure 6, the surface plasmon band of nonaqueous gold colloid, shifted to longer wavelengths, arises from the higher refractive index of formamide in comparison with the one prepared in the aqueous solution.22 In the two-phase extractive processes for extracting gold particles from formamide into water several times, the unreacted gold complexes residing in the nonaqueous gold colloid were mainly extracted into the butanol layer simultaneously. A formamide-butanol mixture was used to extract the unreacted residues from the aqueously grown gold colloid. In the concentrated butanol layers, only a trace amount of gold from the nonaqueous gold colloid but an obvious amount of gold from the aqueous gold colloid were detected by an atomic absorption spectrophotometer. These results further show that there were much fewer unreacted gold complexes in the nonaqueously grown gold colloid than that in the aqueously grown gold colloid. As compared to the conventional chemical reduction routes, the photochemical reduction route used here can also avoid residing the unreacted reducing agent in the resulting gold colloid because it does not require the introduction of foreign reducing agent into formamide solution. For a single-electron-tunneling (SET) device, a small particle size is essential to meet the requirements of small device capacitance, and a medium free of charged impurities is also required.23 Room-temperature SET phenomena have been observed recently in several nanoscopic systems based on metal and semiconductor clusters with diameters of less than 10 nm.4 The significant challenge to incorporating single-electron devices into nanoscale electronic circuitry is still the sensitivity of SET currents to impurities, which may reside on or near the particle. A gold particle in an impurity-free environment could show an SET effect even at a relatively large size. The nonaqueously grown gold particles 12 nm in size as revealed by TEM in Figure 1 were thus used here for the (22) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (23) (a) Graber, H.; Devoret, M. H. Single Charge Tunneling, Coulomb Blockade Pheomena in Nanostructures; NATO ASI Series B, Vol. 294; Plenum: New York, 1992. (b) Korotkov, A. N. Molecular Electronics; Blackwell Science Ltd: Malden, MA, 1997.

Figure 7. I-V characteristics obtained on a nonaqueously grown 12 nm gold particle. The insert is the STM image of the chosen gold particle by using STM tip for recording the I-V characteristics.

study of SET effect because there were almost no unreacted gold complexes in the final gold colloid. Figure 7 shows the clear Coulomb staircase behavior at room temperature on the 12 nm gold particle supported by HOPG substrate. A series of spaced steps is superimposed with a step width of ∆V ≈ 90 mV, which by far exceeds the thermal energy of 26 mV at 300 K. The step width is closely associated with the total capacitance of the gold particle to its environment, C ≈ 1.8 × 10-18 F (C ≈ e/∆V). However, no SET effect was observed on the aqueously grown gold particle of similar size besides a nonohmic behavior, arising from the semiconducting properties of HOPG. This is because the unreacted gold complexes in the aqueously grown colloid resided on or near gold particles while being deposited on HOPG substrate.24 The nonaqueously grown gold particle chosen by using the STM tip is shown in the STM image in Figure 6 for recording I-V characteristics. The Coulomb staircase behavior was then recorded on the 15.6 nm gold particle. As compared with the TEM micrograph, the relatively large size of the gold particle in the STM image allows the determination of the thickness of the stabilizer layer, which was used first by Reetz.25 The surface-capped layer thickness of 1.8 nm is determined from the difference between the diameter measured by STM and that characterized by TEM. The 1.8 nm surface-capped stabilizer shell on the gold particle served as a single-electron tunnel junction in the SET experiment.26 The good nonconducting environment around the gold particle is the reason that the room-temperature SET effect occurred on the gold particle at a relatively large size. I-V curves were also recorded on several other uniform gold nanoparticles with the same diameter, and their SET effects are reproducible. (24) The Cl- from the reduction of AuCl4- in the nonaqueously or aqueously grown gold colloid can be removed in the form of HCl in acidic environment by drying a drop of the colloid on HOPG substrate. (25) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer N.; Vogel, R. Science 1995, 267, 367. (26) Yau, S. T.; Mulvaney, P.; Xu, W.; Spinks, G. M. Phys. Rev. B 1998, 57, R15124.

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Conclusion

was clearly observed at room temperature in the nonaqueously grow 12 nm gold particles. This photochemical route is also very effective to produce other uniform transition-metal particles.

Size-controlled gold colloids were prepared in formamide via the photoreduction of AuCl4- in the presence of poly(vinylpyrrolidone). The photoreduction of AuCl4- was carried out much more completely in the new nonaqueous system as compared to the aqueous solution. SET effect

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