Development of Site-Selective Nanoscale-Polymerization Method Based on Plasmon Induced Charge Separation

Abstract

Introduction Metal nanoparticles, such as gold and silver nanoparticles (AuNPs and AgNPs), have attracted much attention because they can generate strong enhanced electromagnetic fields in nanospaces around the metal nanoparticle surfaces by coupling with incident light in the wavelength region of visible to near-IR on the basis of localized surface plasmon resonance (LSPR). Hence, they can be applied to enhancement of both absorption and emission processes of the light-absorbing molecules and/or semiconductors placed in the nanospaces. Meanwhile, noble metal nanoparticles composited with n-type semiconductors, such as TiO2 and ZnO, exhibit plasmon induced charge separation (PICS).[1,2] When the light with resonant wavelength region irradiates the composition, the electrons of the metal nanoparticle are transferred to the n-type semiconductor resulting in reductive and oxidative reactions on the surfaces of the n-type semiconductor and the metal nanoparticles, respectively. This phenomenon has been utilized to unique applications, such as visible light responsive photocatalysts, photovoltaic cells, and sensing devices. In this study, we employed AuNP-TiO2 system so as to apply PICS as a site-selective nanoscale-polymerization method. Experimental A smooth TiO2 layer was obtained on a heat resistance glass plate by the spray pyrolysis technique, in which a 2-propanol solution containing 0.3 M titanium diisopropoxide bis(acetylacetonate).[3,4] Then, AuNPs were photocatalytically deposited onto the TiO2 substrate from a water/ethanol (v/v=3:2) solution containing 0.2 mM HAuCl4 for 15 min under UV light. Visible-light irradiation was carried out equipped with a UV sharp cut filter. Monochromatic light from a Xe lamp was also employed for action spectra measurements. (Fig. 1) Results and discussion First, we irradiated visible light to the AuNP-TiO2 substrate in an aqueous solution containing 10 mM pyrrole so that pyrrole was polymerized by anodic potential based on PICS at the interface region between AuNPs and TiO2. From UV-vis spectra before and after the irradiation, the absorbance of the substrate increased and the plasmon peak on the basis of AuNPs showed red-shift. X-ray photoelectron spectroscopic (XPS), Raman spectroscopic and scanning electron microscopic (SEM) measurements revealed that these changes of the spectra were caused by site-selective nanoscale-deposition of polypyrrole on the surfaces of AuNPs. An action spectrum of the increment of extinction intensity at 420 nm (π-π* of pyrrole) was well correlated with the extinction spectrum of AuNPs. These results verified that the polymerization should be proceeded via PICS.[5] We found that the polymerization method can also be applied to other monomers, such as phenol, 2,2’-bithiophene, and 3,4-ethylenedioxythiophene (EDOT).