纳米金属粒子的表面等离子体光谱
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Langmuir 1996, 12, 788-800
Surface Plasmon Spectroscopy of Nanosized Metal Particles
Paul Mulvaney
Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia Received April 4, 1995. In Final Form: July 7, 1995X
Introduction Interest in the optical properties of colloidal metals dates back to Roman times. Nanosized gold particles were often used as colorants in glasses, and quite complex optical effects were created using metal particles.1 In the seventeenth century, “Purple of Cassius”, a colloid of heterocoagulated tin dioxide and gold particles, became a popular colorant in glasses.2 These early manifestations of the unusual colors displayed by metal particles prompted Faraday’s investigations into the colors of colloidal gold in the middle of the last century. Today his studies are generally considered to mark the foundations of modern colloid science.3 The formation of color centers and small colloidal metal particles in ionic matrices and glasses has remained an area of very active research,4-6 driven, in part, by the technical importance of the photographic process.7 However, colloid chemists have tended to neglect the study of metal particles in aqueous solution because of their complicated double layer structure, which is more amenable to direct electrochemical investigation. The more recent discovery that the surface plasmon absorption band can also provide information on the development of the band structure in metals8-11 has led to a plethora of studies on the size dependent optical properties of metal particles, particularly those of silver and gold,12-17 while advances in molecular beam techniques now enable
X Abstract published in Advance ACS Abstracts, December 15, 1995.
spectroscopic analysis of metal clusters to be carried out in vacuum.18,19 Although many of the optical effects associated with nanosized metal particles are now reasonably well understood, there are large discrepancies between the optical properties of metal sols prepared in water, particularly those of silver, and sols prepared in other matrices.6,20-27 In a recent review Kreibig noted that while much work has been done to isolate matrix effects and to determine the roles of defects, grain boundaries, crystallinity, and polydispersity on the optical properties of sols, little is known about the way specific sBiblioteka Baidurface chemical interactions may influence the absorption of light by small metal particles.28 These differences are attributed to unique double layer effects present at the metal-water interface. This review focuses on some of these surface chemical effects, and attempts to show how changes to the surface plasmon absorption band of aqueous metal colloids can be related to electrochemical processes occurring at metal particle surfaces. Simple models are proposed to explain some of these chemical changes within the Drude framework for surface plasmon absorption. 1. Light Absorption by Colloids In the presence of a dilute colloidal solution containing N particles per unit volume, the measured attenuation of light of intensity Io, over a pathlength d cm is given by
The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in the shell.
(15) von Fragstein, C.; Schoenes, F. J. Z. Phys. 1967, 198, 477. (16) Kreibig, U. Z. Phys. B: Condens. Matter Quanta 1978, 31, 39; J. Phys. (Paris) 1977, 38, C2-97. (17) Yanase, A.; Komiyama, H. Surf. Sci. 1991, 248, 11, 20. (18) Fallgren, H.; Martin T. P.; Chem. Phys. Lett. 1990, 168, 233. (19) (a) Tiggesbau ¨ mker, J.; Ko ¨ ller, L.; Meiwes-Broer, K.-H.; Liebsch, A. Phys. Rev. A 1993, 48, R1749. (b) Huffman, D. R. Adv. Phys. 1977, 26, 129. (20) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233, 922. (21) Berry, C. R.; Skillman, D. C. J. Appl. Phys. 1971, 42, 2818. (22) Miller, W. J.; Herz, A. H. In Colloid and Interface Science; Academic Press: New York, 1976; Vol. 4. (23) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (24) Henglein, A. J. Phys. Chem. 1979, 83, 2209. (25) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (26) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (27) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (28) Kreibig, U.; Genzel, U. Surf. Sci. 1985, 156, 678.
Langmuir 1996, 12, 788-800
Surface Plasmon Spectroscopy of Nanosized Metal Particles
Paul Mulvaney
Advanced Mineral Products Research Centre, School of Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia Received April 4, 1995. In Final Form: July 7, 1995X
Introduction Interest in the optical properties of colloidal metals dates back to Roman times. Nanosized gold particles were often used as colorants in glasses, and quite complex optical effects were created using metal particles.1 In the seventeenth century, “Purple of Cassius”, a colloid of heterocoagulated tin dioxide and gold particles, became a popular colorant in glasses.2 These early manifestations of the unusual colors displayed by metal particles prompted Faraday’s investigations into the colors of colloidal gold in the middle of the last century. Today his studies are generally considered to mark the foundations of modern colloid science.3 The formation of color centers and small colloidal metal particles in ionic matrices and glasses has remained an area of very active research,4-6 driven, in part, by the technical importance of the photographic process.7 However, colloid chemists have tended to neglect the study of metal particles in aqueous solution because of their complicated double layer structure, which is more amenable to direct electrochemical investigation. The more recent discovery that the surface plasmon absorption band can also provide information on the development of the band structure in metals8-11 has led to a plethora of studies on the size dependent optical properties of metal particles, particularly those of silver and gold,12-17 while advances in molecular beam techniques now enable
X Abstract published in Advance ACS Abstracts, December 15, 1995.
spectroscopic analysis of metal clusters to be carried out in vacuum.18,19 Although many of the optical effects associated with nanosized metal particles are now reasonably well understood, there are large discrepancies between the optical properties of metal sols prepared in water, particularly those of silver, and sols prepared in other matrices.6,20-27 In a recent review Kreibig noted that while much work has been done to isolate matrix effects and to determine the roles of defects, grain boundaries, crystallinity, and polydispersity on the optical properties of sols, little is known about the way specific sBiblioteka Baidurface chemical interactions may influence the absorption of light by small metal particles.28 These differences are attributed to unique double layer effects present at the metal-water interface. This review focuses on some of these surface chemical effects, and attempts to show how changes to the surface plasmon absorption band of aqueous metal colloids can be related to electrochemical processes occurring at metal particle surfaces. Simple models are proposed to explain some of these chemical changes within the Drude framework for surface plasmon absorption. 1. Light Absorption by Colloids In the presence of a dilute colloidal solution containing N particles per unit volume, the measured attenuation of light of intensity Io, over a pathlength d cm is given by
The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in the shell.
(15) von Fragstein, C.; Schoenes, F. J. Z. Phys. 1967, 198, 477. (16) Kreibig, U. Z. Phys. B: Condens. Matter Quanta 1978, 31, 39; J. Phys. (Paris) 1977, 38, C2-97. (17) Yanase, A.; Komiyama, H. Surf. Sci. 1991, 248, 11, 20. (18) Fallgren, H.; Martin T. P.; Chem. Phys. Lett. 1990, 168, 233. (19) (a) Tiggesbau ¨ mker, J.; Ko ¨ ller, L.; Meiwes-Broer, K.-H.; Liebsch, A. Phys. Rev. A 1993, 48, R1749. (b) Huffman, D. R. Adv. Phys. 1977, 26, 129. (20) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233, 922. (21) Berry, C. R.; Skillman, D. C. J. Appl. Phys. 1971, 42, 2818. (22) Miller, W. J.; Herz, A. H. In Colloid and Interface Science; Academic Press: New York, 1976; Vol. 4. (23) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sanders, J. V. J. Colloid Interface Sci. 1983, 93, 545. (24) Henglein, A. J. Phys. Chem. 1979, 83, 2209. (25) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (26) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (27) Linnert, T.; Mulvaney, P.; Henglein, A. J. Phys. Chem. 1993, 97, 679. (28) Kreibig, U.; Genzel, U. Surf. Sci. 1985, 156, 678.