Gold nanoparticle patterning on titanium dioxide thin films by hydrophilic and hydrophobic interactions effect on band gap

Thin films of patterned, thiol capped Au nanoparticle (NP) over layer on titanium dioxide (TiO
2
) matrix (Au-TO-Ov), prepared by a unique surfactant assisted 2D self-assembling technique with molecular level control, in conjuncture with sol-gel route showed significant decrease in optical band gap and enhanced crystallinity compared to sol-gel prepared pristine TiO
2
. Combined results of scanning electron microscopy, UV-Vis spectroscopy, Raman spectroscopy and near edge x-ray absorption fine structure spectroscopy indicated that decrease in optical band gap was associated with changes in local electronic structure of Au-TO films. No such changes were observed for the film with AuNP as underlayer (Au-TO-Un). Dependence of structure and band gap, on order of Au NP layer is attributed to difference in hydrophilic and hydrophobic interactions occurring at AuNP-TiO
2
interface. Unlike spin coated Au NPs on TiO
2
, which resulted in Au NP agglomeration, or in-situ doping of Au NPs in titanium dioxide precursor by sol-gel method, which indicated a slight increase in band gap, Langmuir-Schaefer method caused significant changes in local structure and hence band gap of the system brought about by these weak Van der Waal's interactions. This cheaper, scalable technique of surfactant aided deposition of 2D layers of Au NPs on semiconducting oxides may be used for development of multilayered structures with promising light harvesting and unidirectional energy transfer (LUET) applications.