We investigate the photoconversion of aqueous 8 nm Ag nanocrystal seeds into 70 nm single crystal plate nanoprisms. The process relies on the excitation of Ag surface plasmons. The process requires dioxygen, and the transformation rate is first-order in seed concentration. Although citrate is necessary for the conversion, and is consumed, the transformation rate is independent of citrate concentration. We propose a mechanism that accounts for these features by coupling the oxidative etching of the seed and the subsequent photoreduction of aqueous Ag(+). The reduced Ag deposits onto a Ag prism of specific size that has a cathodic photovoltage resulting from plasmon "hot hole" citrate photo-oxidation. This photovoltage mechanism also explains recent experimental results involving single and dual wavelength irradiation and the core/shell synthesis of Ag layers on Au seeds.

The photoelectrochemistry of citrate-stabilized gold nanoparticles on an ITO electrode is studied at low light intensity. Weak cathodic photovoltage develops under visible gold plasmon irradiation. Photovoltage results from adsorbed citrate oxidation by "hot" holes in the irradiated Au. Under identical conditions, Ag nanocrystals give a stronger photovoltage than Au nanocrystals. The photocurrent is linear in light intensity, and a complete citrate monolayer forms on the Au particles above 10 -6 M aqueous concentration. At strongly oxidizing potentials, a photocurrent peak occurs which may be related to electron photoinjection from Au into the ITO substrate.

The plasmon-mediated photooxidation of citrate ions adsorbed on silver (Ag) nanoparticle−semiconductor electrodes is studied in a photoelectrochemical cell. Consistent with previous reports, a negative photovoltage and an anodic photocurrent arise from citrate photooxidation under weak visible light illumination. We measure the wavelength dependence of this reaction for three different types of Ag nanoparticles and find that both the photovoltage and photocurrent increase with photon energy over the visible spectral range. The electrode photoresponse does not closely track the localized surface plasmon resonance of the Ag nanoparticles. We also explore the role of the semiconductor substrate in this reaction, and we find a similar electrode photoresponse for several different substrates. The strong dependence of reaction rate on photon energy is consistent with a hot-carrier photochemical process where photoexcited hot holes generated in the Ag nanoparticles are responsible for the oxidation of adsorbed citrate.

No abstract