Effects of Ag+ and Cu2+ Ions on the Shape of Triangular Gold Nanoparticles

Abstract

Triangular gold nanoplates were prepared by the tartaric acid reduction of HAuCl 4 in cationic surfactant solutions, in the absence and the presence of small amounts of AgNO 3 and CuSO 4 . With increasing concentration of AgNO 3 , the particle shape changes from triangular to spherical. In the presence of CuSO 4 , triangular plates with side lengths and tips that are longer and sharper, respectively, than those of the nanoplates prepared in the absence of CuSO 4 , are obtained.Metal nanoparticles show unique electronic and optical properties that are different from those of bulk metals, and their properties are dependent on their shapes and sizes.15 Silver and gold triangular nanoparticles (triangular nanoprisms and nanoplates) display interesting localized surface plasmon resonances. 615 The electromagnetic field is strongly enhanced around their tips. Furthermore, when two or more triangular particles meet, the enhancement is magnified and is expected to significantly affect spectral properties such as surface-enhanced Raman scattering 1618 Recently, we have reported the synthesis methods of gold triangular nanoplates. 31 The addition of Ag + ions, halide ions, and other chemicals are expected to control the shape and size of nanoparticles. In this paper, we want to report about the effects of adding AgNO 3 and CuSO 4 on the shape and size of gold nanoparticles prepared by the tartaric acid reduction method.The gold nanoparticles were prepared by modifications of the method reported elsewhere. 31 The solutions containing 0.5 mM (1 M = 1 mol dm ¹3 ) HAuCl 4 , 2.2 mM hexadecyltrimethylammonium chloride (HTACl) as a protective agent, and 60 mM tartaric acid (H 2 tart) as a reducing agent in the absence and presence of AgNO 3 and CuSO 4 were placed in an oil bath kept at 30°C. After 10 min, the bath temperature was increased to 70°C; subsequently, reactions took place for 13 min, after which the solutions were taken out of the bath and aged overnight. Transmission electron microscope (TEM) observations were performed using a Hitachi H-8100 TEM with an acceleration voltage of 200 kV. Extinction spectra were measured using a Hitachi U-2800 spectrophotometer.The extinction spectra for gold nanoparticles prepared in the absence and the presence of AgNO 3 and CuSO 4 are shown in Figure 1. The gold nanoparticles prepared in the absence of AgNO 3 and CuSO 4 show a peak at 550 nm and a tail extending up to 800 nm. Figure 2 shows the TEM images for triangular nanoplates having a side length of 54 nm. Each triangular nanoplate is a gold single crystal with the (111) main face.31 The adsorptions of Cl ¹ ions on the (111) facets were proposed to promote the growth of the {111} oriented triangular particles prepared by citrate reduction (Turkevich method) in the presence of KCl with a KCl/AuCl 4 ¹ molar ratio of 1. 23 In our case, adsorption of HTACl molecules on the (111) facets promotes the growth of the h111i oriented triangular nanoplates.In the presence of AgNO 3 , the extinction peak wavelengths for the obtained gold ...

Purposefully Designed Surfactants for Facile and Controllable Gold Colloidal Nanocrystal Synthesis

“…Finally, we investigated the addition of CuSO 4 to selectively prepare triangular AuNPs, as the shape control effects of Cu 2+ have been shown in a report from Takezaki et al 28 Briefly, 1, 5, and 10 mol % CuSO 4 relative to Au 3+ (colloids Q, R, and S, respectively) were added into the preheated surfactant solutions before the addition of HAuCl 4 , with the remainder of the synthesis being performed as described in our optimal protocol. The resulting UV–vis spectra ( Figure S18A ) show a monotonic red shift of the LSPR band as the relative concentration of CuSO 4 increases.…”