Nitrogen- and carbon-doped titanium dioxide thin films for solar hydrogen generation

Photoelectrochemical cells (PECs) are integrated devices that split water using only clean power from the sun, thus producing clean hydrogen fuel with no harmful carbon emissions. The search is on for optimal photoanode materials in PECs. Desired properties include adequate supply, non-toxicity, stability in aqueous solution, ability to split water, and ability to absorb most of the solar spectrum. Titanium dioxide satisfies all but this last criterion with a bandgap near 3 eV, only the ultraviolet (UV) range of the spectrum may be used. This thesis reports on attempts to improve the visible light activity of TiO2 by doping with the nonmetals nitrogen and carbon, in order to lower effective bandgap. We have synthesized sets of N- and C-containing TiO2 via a simple, easily controlled physical vapor deposition (PVD) process known as pulsed laser deposition (PLD). The dopants were introduced by means of a carrier gas: either nitrogen or methane. Atomic concentrations of dopant were found via X-ray photoelectron spectroscopy (XPS) to range from 0-4%. Pure TiO2 deposits in polycrystalline anatase form at 600°C nitrogen-doped films were also anatase, while carbon incorporation caused a shift to mixed anatase/rutile character. XPS scans showed changes in electronic structure with increased dopant concentration. In the case of nitrogen, a distinct Ti-N peak appeared, indicating N atoms substituting for O. Carbon-region scans only showed a Ti-C peak at high carbon concentrations. Valence band scans of both C-TiO2 and N-TiO2 showed distinct doping states developing just above the valence band. Nitrogen proved more adept at improving visible light activity with a quantum efficiency peak near 460 nm, and a UV-Visible absorption edge at 510 nm. No such visible light activity resulted from carbon-doping. It is theorized that the carbon states are too deep within the bandgap to significantly increase visible light activity, unlike nitrogen 1s states, which successfully overlap with valence band O2p states.