(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.