Solid-Phase Photochemical Growth of Composition-Variable Au-Ag Alloy Nanoparticles in AgBr Crystal

Silver-silver halides (Ag-AgX, X = Cl, Br, I) have emerged as a new type of visible-light photocatalyst for solar-to-chemical transformations. The key to improving the activity of the plasmonic photocatalyts is the compatibility of local electric field enhancement (LEFE) and effective utilization of the sunlight as the energy source. A Ag nanoparticle (NP) possesses an intense LEFE effect, while the absorption peak is situated near the blue edge of the visible region. On the other hand, the Au NP has an absorption matching well the solar spectrum, but the LEFE is much smaller than that of a Ag NP. The optical property of a Au-x-Ag1-x alloy NP varies between those of Ag and Au NPs depending on the composition x, and thus, Au-x-Ag1-x alloy NP-incorporated AgX (Au-x-Ag1-x@AgX) can be a promising plasmonic photocatalyst. At the first step, gold ion-doped AgBr NPs are formed on mesoporous TiO2 film by the successive ionic layer adsorption and reaction (SILAR) method. At the second step, UV-light irradiation (lambda > 320 nm) of the sample in methanol yields Au-x-Ag1-x alloy particles having diameter of similar to 5 nm in the interior of AgBr with crystallite size of similar to 50 nm. The peak wavelength for the localized surface plasmon resonance can be tuned in the range between 500 and 600 nm through the alloy composition. On the basis of the experimental and density functional theory calculation results, we propose a plausible reaction mechanism.