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Chem. Mater. 2005, 17, 6042-6052
Corrosion at the Nanoscale: The Case of Silver Nanowires and Nanoparticles Jose Luis Elechiguerra,† Leticia Larios-Lopez,‡ Cui Liu,‡ Domingo Garcia-Gutierrez,‡ Alejandra Camacho-Bragado,‡ and Miguel Jose Yacaman*,‡ Department of Chemical Engineering and Texas Materials Institute, The UniVersity of Texas at Austin, 1 UniVersity Station, Austin, Texas 78712 ReceiVed July 14, 2005. ReVised Manuscript ReceiVed August 29, 2005
Metal nanostructures, such as nanoparticles and nanowires, have been proposed as building blocks for several applications in nanofabrication and nanoelectronics. However, even when atmospheric corrosion is common in metals, there is a lack of information about the stability of those nanostructures against such phenomenon. Therefore, we decided to study the atmospheric corrosion of silver nanowires and nanoparticles synthesized by the polyol method using poly(vinylpyrrolidone) (PVP) as the capping agent by different techniques, including transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). After synthesis and purification, the silver nanostructures were deposited on different substrates and exposed to laboratory air at ambient conditions. The structural changes in the samples were monitored by TEM as a function of time for a period of time of 24 weeks. Our results demonstrated that these silver nanostructures are susceptible to atmospheric corrosion and that, in most cases, a thin layer of silver sulfide nanocrystals is formed on their surfaces. The enhanced reactivity of regions with defects and dislocations could explain the observation that the corrosion rate of the nanowires is higher than the corrosion rate of the nanoparticles, since it is well-known that the structure of the nanowires synthesized by the polyol method is multitwinned, while most of the nanoparticles that remained after synthesis are single crystals. Additionally, part of the original sample of silver nanostructures was sulfidized using hydrogen sulfide (H2S) as corrodent gas. After performing XPS studies of this sample, we confirmed the presence of PVP on the surface of the sulfidized silver nanostructures. This result agrees with the observation that in the atmospherically corroded samples, even when in some cases the original silver nanostructure was completely corroded, the silver sulfide nanocrystals remained together adopting the shape of silver nanostructure. Finally, our results indicate that the corrosion at the nanoscale seems to be similar to that of the bulk silver.
1. Introduction In recent years, inorganic nanostructures have attracted growing interest due to their potential applications in catalysis,1 biological sensors,2 and nanoelectronics3 among others. As these materials have at least one of their dimensions between 1 and 100 nm, interesting properties arise due to phenomena such as quantum confinement and high surface-to-volume ratio.4 In the case of noble metal nanomaterials, the physicochemical properties are highly influenced by shape and size.1,5 For example, it is well-known that the optical absorption spectra of metal nanoparticles are dominated by surface plasmon resonances (SPR),6 the case of gold and silver being unique. Both have the proper density of free * To whom correspondence should be addressed. Phone: +512-232-9111. Fax: +512-471-7060. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Texas Materials Institute.
(1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Nam, J.-M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884. (3) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 10, 2455. (4) Di Ventra, M.; Evoy, S.; Heflin, J. R. Introduction to Nanoscale Science and Technology; Kluwer Academic Publishers: Boston, MA, 2004. (5) Liz-Marzan, L. M. Mater. Today 2004, 7, 26. (6) Mulvaney, P. Langmuir 1996, 12, 788.
electrons for their nanoparticles to possess SPR peaks in the visible region of the electromagnetic spectrum,7 which in addition to their large effective scattering cross section, makes them ideal candidates for molecular labeling.8 Recently, the absorption spectra of individual silver nanoparticles was correlated with their size and shape determined by transmission electron microscopy (TEM).9 The results indicate that spherical and roughly spherical nanoparticles absorb in the blue region of the spectrum, while decahedral nanoparticles and particles with triangular cross sections absorb in the green and red part of the spectrum, respectively. For each different morphology, the SPR peak wavelength increases with size. Thus, an exquisite control of size, composition, and morphology is highly desirable. Additionally, we recently found that the size of silver nanoparticles is important in other applications by demonstrating that silver nanoparticles undergo a size-dependent interaction with HIV1, with nanoparticles exclusively in the range of 1-10 nm attaching to the virus.10 Due to this interaction, silver nanoparticles inhibit the virus from binding to host cells. (7) Xia, Y.; Halas, N. MRS Bull. 2005, 30, 338. (8) Schultz, S.; Smith, D. R.; Mock, J. J.; Schultz, D. A. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 996. (9) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755.
10.1021/cm051532n CCC: $30.25 © 2005 American Chemical Society Published on Web 10/26/2005
Corrosion of SilVer Nanowires and Nanoparticles
Noble metal nanocrystals have been synthesized using a variety of methods, the solution phase techniques probably being the most employed ones.1 In solution phase synthesis of gold and silver nanoparticles the control of shape is a challenging task, since the structure of the original seeds plays a crucial role in the morphology of the final product.11 Multiple TEM studies on small (
