Controlled Nitrogen Doping and Film Colorimetrics in Porous TiO2 Materials Using Plasma Processing

Nitrogen doping of TiO2 films (N:TiO2) has been shown to improve the visible-light sensitivity of TiO2, thereby increasing the performance of both photovoltaic and photocatalytic devices. Inductively coupled rf plasmas containing a wide range of nitrogen precursors were used to create nitrogen-doped TiO2 films. These treatments resulted in anatase-phased materials with as high as 34% nitrogen content. As monitored with high-resolution X-ray photoelectron spectroscopy spectra, the nitrogen binding environments within the films were controlled by varying the plasma processing conditions. XPS peak assignments for multiple N 1s binding environments were made based on high resolution Ti 2p and O 1s XPS spectra, Fourier transform infrared spectroscopy (FTIR) data, and literature N 1s XPS peak assignments. The N:TiO2 films produced via plasma treatments displayed colors ranging from gray to brown to blue to black, paralleling the N/Ti ratios of the films. Three possible mechanisms to explain the color changes in these materials are presented.