(548d) N2/Ar-Plasma Assisted Nitrogen Doping of Ordered Mesoporous TiO2 Thin Films for Water Splitting Photocatalysis

TiO₂ has attracted much attention due to many advantageous properties including its low cost, high availability, chemical stability, and excellent opto-electronic properties. These unique properties have enabled titania to be utilized in a wide range of applications including photocatalysts, photovoltaics and photoconductors for energy production and environmental remediation. Despite many attractive features of TiO₂, one critical challenge is the innate inability of TiO₂ to absorb visible light. In this study, this shortcoming is addressed by doping cubic ordered mesoporous TiO₂ thin films (N-TiO₂) using nitrogen/argon (N₂/Ar) plasmas. This represents a rapid, scalable approach to high surface, controlled nanostructure thin films for photocatalytic applications. For this purpose, cubic ordered mesoporous TiO₂ thin films were prepared by a surfactant templated sol-gel method using TiCl₄ as precursor and triblock copolymer Pluronic F-127 as the template. Subsequently, the calcined TiO₂ films were treated with N₂/Ar plasma under controlled conditions of reaction gas pressures, microwave power, and plasma exposure duration. The primary variable studied here for N-TiO₂ films is the duration of plasma exposure (0-210 min). After doping, the films were confirmed to have an accessible, well-ordered cubic mesopore structure by SEM, TEM, STEM and XRD analyses. The content of nitrogen in the films increases with plasma exposure duration, up to over 3% atomic N. X-ray photoelectron spectroscopic (XPS) analyses and UV-vis absorbance spectra of N-TiO₂ films indicate that the incorporated N atoms reduce the band gap of TiO₂ and thus enhance the absorption of visible light. Finally, the visible-light photocatalytic activity of N-TiO₂ films was determined from the photocatalytic degradation of methylene blue under illumination with a visible-light LED (455 nm wavelength). The N-TiO₂ films prepared with 150 min plasma treatment show the best photocatalytic activity, with a first-order methylene blue decolorization rate coefficient 6 times greater than that of undoped TiO₂ films. N-TiO₂ films processed under further optimized conditions also showed about 80 times greater photocurrent in visible-light photoelectrochemical water splitting than undoped TiO₂ films. Insights from electrochemical impedance spectroscopy into the electronic structure and conductivity of the N-TiO₂ films will be discussed.