Silver Nanodisk Growth by Surface Plasmon Enhanced Photoreduction of

The photoreduction of silver ions by citrate, catalyzed on silver seeds, is used to synthesize disk-shaped silver nanoparticles in solution. The reaction is characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), optical absorption spectroscopy, and by measuring the silver ion concentration during the reaction. The irradiation wavelength determines the final shape of these particles due to the shape dependence of the Ag plasmon spectrum. The quantum yield of this reaction has been calculated, and a growth mechanism is outlined. Introduction. Silver particles provide an ideal system for study of size and shape effects in the surface plasmon resonance; indeed, this sensitivity is a tool to monitor the shape of the particles during their synthesis via the optical extinction spectrum. Plasmon resonances concentrate an incident electromagnetic field via near-field enhancement; this antenna effect is the source of surface enhanced Raman scattering (SERS). The intense Ag resonance allows single molecule observation in SERS, 1-5 enables coupled Ag particles to form a subwavelength waveguide, 6 and enables sensitive colorimetric DNA screening. 7,8 Henglein9,10 has shown that the physical and chemical properties of finely divided, nm-sized Ag particles are strongly modified by the adsorption of nucleophilic species and that Ag particles catalyze thermal and optical electrochemical reactions. For example, core/shell metallic particles can be grown. 11 Indeed, adsorption of chemical species modifies the Fermi level of both the metal and the reactant, like a polarized nanoelectrode. Combining these effects, we now report controlled photochemical Ag particle growth from adsorbed Ag silver ions in the presence of citrate. If different shapes and sizes are present in a seed colloid, the particle with the largest plasmon absorption cross-section at the laser wavelength initially grows fastest. The reaction accelerates for those shapes whose plasmons move into resonance with the photochemical wavelength as growth occurs. This effect allows control of shape in the dominant photoproduct by choice of photochemical wavelength. There have been previous observations of reduced silver formation by irradiation of citrate and Ag ion, however, without control of shape or size. 12,13 Synthesis.An aqueous Ag seed colloid is synthesized by adding 0.5 mL of 25 mM ice-cold sodium borohydrate into a stirring solution of 20 mL of 10 mM citrate and 0.5 mL of 5 mM silver nitrate. This solution is stirred for 5 min and aged for 24 h. The growth solution is made by mixing 100 μL of seed colloid, 50μL of 10 mM citrate, 50μL of 5 mM silver nitrate, and 800μL of ultrapure water. Hence, the silver ion and the metallic silver concentrations are 2.5 × 10-4 M and 2.5 × 10-5 M, respectively. This solution is kept refrigerated in the dark. In a typical experiment, 200 μL of this solution is uniformly irradiated at 23 °C in a 4.3 mm wide optical cell with a linearly polarized argon laser beam. Growth and Characterization at 457 nm.TEM examination shows the seed colloid has round Ag particles of 8 nm average size. Growth solutions initially show the Ag seed resonance in the blue at 395 nm (Figure 1). Such solutions irradiated at 457 nm at 0.8 W/cm 2 change from pale yellow to purple within an hour. The dominant photochemical product Ag particles have a disklike or triangular shape; their average diameter, measured by TEM (Figure 2), is D ) 38 nm. Their average height has been measured by AFM as h ) 10.7 nm, corresponding to an aspect ratio D/h ) 3.57, which is in a good agreement with the aspect ratio measured by TEM of the few particles standing on the short side. As observed by electron diffraction, the majority of the disks are single crystal. This flat triangular shape already has been obtained by numerous syntheses using different experimental conditions with gold14 and silver 15-19 and seems to be related to a stacking fault constraint. 20 The corresponding absorption * Corresponding author. † REU student, present address New College of Florida. NANO LETTERS 2003 Vol. 3, No. 11 1611-1615 10.1021/nl034666d CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003 spectrum (Figure 1) exhibits two additional peaks at 338 and 540 nm corresponding to the transverse and longitudinal plasmon modes and an increase of the initial plasmon band at 400 nm. The absorption spectrum of such a particle in water can be easily calculated assuming a flat ellipsoid shape. 21 The calculated model exhibits two plasmon bands at 340 and 510 nm. The slight discrepancy in the long wavelength peak position is explained by the simplified shape of this model. According to calculations by Schatz et al., 17 the longitudinal plasmon mode of an exact triangular particle is, indeed, more red shifted. The growth process has been characterized by TEM (Figure 3), by AFM, by absorption spectroscopy (Figure 4B), and by measuring the silver nitrate concentration with an ion selective electrode (Figure 4A) at different irradiation times, up to 90 min. The silver concentration initially decreases linearly as a function of time, as: ∂[Ag+]/∂t ) K ) 2.09× 10-7mol s-1 which corresponds to a half-reaction time constant of 20 min. This linear behavior indicates that the reaction does not depend on the initial silver concentration in solution. The silver ions are in excess in the solution: the silver concentration on the nanoparticle surface is completely saturated and remains constant as long as the concentration is high enough. The quantum yield of this photoreaction has been calculated as where∂nAg+/∂t ) K is the rate of silver atoms reduction, deduced from the kinetic study, ∂nphoton a /∂t ) [Vhυ(1 10-DO)]/P is the rate of photon absorption, hυis the photon Figure 1. Plot of the absorption spectrum from a product disk solution (dashed line) and the reagent silver seed solution (solid line). Figure 2. (A) TEM picture of the silver nanodisks after 90 min of irradiation at 0.8 W/cm2 at λ ) 457 nm. (B) AFM picture of product particles. (C) height profile along the line in Figure 1B. Figure 3. From A to F, TEM pictures of the particles synthesized after t ) 5, 10, 15, 20, 50, and 90 min of irradiation. Figure 4. (A) Plot of the silver concentration (7.5 times diluted) vs the irradiation time. Solid line is a linear fit. Each point corresponds to a different diluted sample. (B) Plot of the absorption spectra aftert ) 5, 10, 15, 20, 50, and 90 min of irradiation. QY) ∂nAg+ ∂nphoton a ) ∂nAg+