Absorption spectra and photocurrent densities of Ag nanoparticle/TiO2 composite thin films with various amounts of Ag

To study the absorption spectra and photocurrent densities of metallic Ag nanoparticle/titania (Ag NP)/TiO2 composite thin films, COMP-Agn, with various amounts of Ag (10 mol% ≤n ≤80 mol%) were fabricated on a quartz glass substrate at 600 °C using the molecular precursor method. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex. Apart from a surface plasmon resonance (SPR) peak around 400 nm, additional wide-range absorption spread in the wide vis-region at wavelengths greater than 400 nm was observed in the composite thin films. The wide-range absorption is due to a tip–tip plasmon mode, intraparticle plasmonic coupling of tip and cavity resonances (LSPR). The absorption spectral patterns could be categorized into three types, depending on the Ag NP content. The photocurrent density of the TiO2 thin film and COMP-Agn was measured under natural potential by a conventional three-electrode method using a Ag plate as a counter electrode. The photocurrent densities of COMP-Agn were comparable to those of the three patterns categorized according to their absorption spectra. The cathodic photocurrent densities generated by COMP-Agn with Ag content greater than 45 mol% could be observed under both UV- and visible (vis-) light irradiation. The enhanced photocurrent density was observed till the Ag content was increased up to 70 mol%. Under dark conditions, cathodic current densities were 1/5–1/20 of those under vis-light irradiation probable due to chemical redox reactions that may occur to the system. On the basis of photoexcited electron transfer from Ag NPs to the TiO2 conduction band and the electrical conductivity of COMP-Agn, the excellent response to vis-light and major factors affecting the photoresponse and photocurrent polarity were clarified by LSPR.