Photochemical Properties, Composition, and Structure in Molecular Beam Epitaxy Grown Fe “Doped” and (Fe,N) Codoped Rutile TiO₂(110)

Abstract

We have investigated the surface photochemical properties of Fe "doped" and (Fe,N) codoped homoepitaxial rutile TiO 2 (110) films grown by plasma-assisted molecular beam epitaxy. Fe does not incorporate as an electronic dopant in the rutile lattice but rather segregates to the film surface. However, codeposition of Fe with N enhances the solubility of Fe, and DFT calculations suggest that codopant complex formation is the driving force behind the enhanced solubility. The codoped films, in which a few atomic percent of Ti (O) are replaced with Fe (N), exhibit significant disorder compared to undoped films grown under the same conditions, presumably due to dopant-induced strain. Codoping redshifts the rutile bandgap into the visible. However, the film surfaces are photochemically inert with respect to hole-mediated decomposition of adsorbed trimethyl acetate. The absence of photochemical activity may result from dopant-induced trap and/or recombination sites within the film. This study indicates that enhanced visible light absorptivity in TiO 2 does not necessarily result in visible light initiated surface photochemistry.

Structure of epitaxial (Fe,N) codoped rutile TiO2thin films by x-ray absorption

“…16 Briefly, Fe and Ti were evaporated from effusion cells, and activated oxygen and nitrogen were supplied from the same electron cyclotron resonance microwave plasma source at an N 2 :O 2 supply ratio of 3:1, maintaining a total chamber pressure of 2 × 10 −5 Torr. Films were deposited at 550…”

“…Two films are investigated in this study: Fe 0.02 Ti 0.98 O 2 , which showed surface segregation of Fe by angle-resolved x-ray photoelectron spectroscopy (XPS), and Fe 0.02 Ti 0.98 (O 0.98 N 0.02 ) 2 , which indicated no surface segregation of Fe. 16 X-ray absorption spectra for the O K-edge, N K-edge, Ti L-edge, and Fe L-edge were collected on beamline ID08 of the European Synchrotron Radiation Facility (ESRF); spectra were collected in total electron yield (TEY) mode to minimize signal from the underlying rutile substrate. The Ti K-edge spectra were collected on beamline ID12 in fluorescence yield (FY) mode.…”

“…15 To elucidate the mechanism of ferromagnetic ordering in these materials, careful and thorough materials characterization is necessary, particularly regarding the speciation and location of the transition metal dopants. Recently, we showed 16 indirect evidence of N substitution in (Fe,N) codoped rutile TiO 2 homoepitaxial thin films deposited by plasma-assisted molecular beam epitaxy (PAMBE). A combination of characterization techniques indicated that N was present as N 3− , which appeared to be substitutional in the lattice.…”

“…16 Analysis of epitaxial films provides substantial benefits compared to the study of nanoparticles to understand the fundamental role of dopants in a material, in particular due to the elimination of undercoordinated surface defects and surface-adsorbed species. The well-defined single crystal structure of epitaxial films also facilitates insightful XLD measurements, where the incident x-ray polarization is switched from parallel to perpendicular polarization relative to a given crystalline direction in the film; the difference in the XANES spectra arising from these signals is the XLD.…”

A brief review of co-doping

“…Figure 3 schematically shows the co-doped structure and the coupling mechanism. There are ongoing attempts to codope cation dopant with the mentioned anions for increasing the efficiencies of photochemical reactions, such as Co-C-S [161] and Fe-N [162], yet, theoretical studies of these complex co-doping schemes are limited. Furthermore, owing to the complex nature of photochemical reactions, the dopant can affect the surface texture and the reaction pathway [150,157,161], all of which complicate theoretical studies.…”