Size-Selective Formation of Hexagonal Silver Nanoprisms in Silver Citrate Solution by Monochromatic-Visible-Light Irradiation

Abstract

Silver citrate solution was irradiated by monochromatic light from light-emitting diodes (LEDs). Under visible-light irradiation, a characteristic peak (Abs hex ) was observed in the absorption spectrum above a certain value of fluence (Φ thres ). The formation of hexagonal silver nanoprisms was confirmed by transmission electron microscopy observations; the size of these hexagons in the in-plane direction decreased with increasing E LED (nominal peak energy). The formation process was investigated from the fluence evolution of Abs hex, which was due to the localized surface plasmon resonance of the nanoprisms. With decreasing E LED, Φ thres showed an exponential increase and the growth rate of Abs hex beyond Φ thres (ΔAbs hex /ΔΦ*, Φ* = Φ – Φ thres ) showed a linear decrease. The ΔAbs hex /ΔΦ* vs E LED data indicated that ΔAbs hex /ΔΦ* became zero at E LED = 1.9 eV. Generally, the peak energy of Abs hex expected at Φ* = 0 was close to E LED . These results indicated that embryos were formed by visible-light irradiation below Φ thres, and some of them were converted into hexagonal silver nanoprisms. The conversion of embryos into nanoprisms is governed by a resonant process induced by the incident light. Further, the formation of nanoprisms is induced by visible-light irradiation with photon energy greater than 1.9 eV.