AVS 54th International Symposium
    Renewable Energy Science & Technology Topical Conference Thursday Sessions
       Session EN+SS+TF-ThM

Paper EN+SS+TF-ThM3
Lattice Site Location for N in Homoepitaxial N-doped TiO2 (110)

Thursday, October 18, 2007, 8:40 am, Room 602/603

Session: Surface Science Challenges for Solar Energy Conversion
Presenter: V. Shutthanandan, Pacific Northwest National Laboratory
Authors: V. Shutthanandan, Pacific Northwest National Laboratory
S.H. Cheung, Pacific Northwest National Laboratory
S. Thevuthasan, Pacific Northwest National Laboratory
P. Nachimuthu, Pacific Northwest National Laboratory
S.A. Chambers, Pacific Northwest National Laboratory
Correspondent: Click to Email

TiO2 is one of the most heavily studied materials for photocatalytic water splitting even though the optical absorption spectrum of TiO2 has poor overlap with the solar spectrum, and the e-/h+ pair recombination rate is high. Bandgap reduction is one approach to enhancing visible light absorption. N doping causes a redshift of the bandgap into the visible and visible-light-induced photochemistry has been observed in this material. We have grown TiO2-xNx rutile epitaxial films on rutile TiO2 (110) single crystal substrates using oxygen plasma assisted molecular beam epitaxy. The N concentration (x) was varied by careful control of the atomic fluxes. The N dopant site location was studied using nuclear reaction analysis (NRA) and Rutherford backscattering spectrometry (RBS) in channeling and random geometries. 14N(d,α)12C and 16O(d,p)17O nuclear reactions were used to identify the locations of N and O, respectively. NRA measurements in a channeling geometry for x = 0.04 and 0.05 clearly show that N substitutes for O in this concentration range. The x = 0.04 film shows a higher degree of N substitution (~98%) than the x = 0.05 film (~75%). Angular scans obtained around <110> for the x = 0.04 film exhibit a N angular half width that is slightly narrower (~0.05o) than that of host O. This narrowing is an indication that the N atoms are slightly displaced from the idealized anion lattice sites. The angular yield scan obtained for the x = 0.05 film exhibits a slightly larger angular half width for O, indicating that O positions are perturbed by N incorporation. In contrast, NRA and RBS measurements performed on the x = 0.12 film reveal that most of the N occupies random positions within the film, and glancing incidence XRD reveals limited Ti2N secondary phase formation. These results clearly demonstrate that the upper limit of N solid solubility in crystalline TiO2 rutile is ~3 at. % of the anions. Higher N concentrations can be incorporated by varying the growth conditions to facilitate defect formation, but the quality of the materials drops considerably and secondary phase formation occurs.