Multinuclear SSNMR, EPR, and XPS studies of anion-doped titanium dioxide photocatalysts

Enrique A Reyes, Purdue University

Abstract

Visible light-absorbing anion-doped TiO2 photocatalysts (TiO 2-xAx, A = B/F, C, N, or P) that have the potential for water splitting and photocatalytic applications, were synthesized via sol-gel methodology and characterized by solid state nuclear magnetic resonance (SSNMR), electron paramagnetic resonance (EPR), X-ray Photoelectron spectroscopy (XPS), and UV/Vis diffuse reflectance spectroscopy, and powder X-ray diffraction (XRD). The 15N SSNMR showed that TiO2-x 15Nx has various amino functionalities (NH, NH2, NH3 and probably NH4+) before calcination while the photoactive powder has highly oxidized nitrogen species in the form of nitrate and, based on EPR, an NO radical species. 11B magic angle spinning (MAS) NMR of B/F-TiO2 showed that the photochemically active material contains tetrahedrally coordinated BO4 units and 19F MAS indicates that fluorine is found as TiO5F. The UV/Vis spectrum shows the presence of intraband gap states likely responsible for its absorption of visible light (2.3 eV), while the indirect band gap transition remains unaltered (3.1 eV). Photocatalytic activity was evaluated for N-doped and B/F co-doped TiO2 in terms of the oxidation of 1,2-13C-trichloroethylene and methylene blue, respectively. The use of phosphorous as the doping species effectively extended TiO2 absorption into the visible range. The P oxyanion is present in the 5+ oxidation state based on XPS while the 31P SSNMR spectra showed Q0, Q1 and Q2 PO4 tetrahedra units. EPR analysis of B/F-TiO 2 and TiO2-xPx shows the creation of two Ti 3+ electron trapping sites having nearly axially symmetric g-tensors; furthermore, the latter material also has two oxygen anion radical species, O·- and O3- that are thermally stable. Lastly, TiO2-xCx materials analyzed by SSNMR showed signals corresponding to carbonate- or sp 2-type carbon species in addition to a complex mixture of point defects, electron and hole trapping centers as detected by EPR. In summary, anion doping of TiO2 leads to the formation of point defects facilitating the formation of Ti3+ electron traps and anion vacancies that work as phototrapping sites. The photocatalytic efficiency of these materials will greatly benefit from better synthetic approches to fine tune the position of the intraband gap states relative to the valence and conduction band edges, and to ensure material reproducibility and stability. Advanced materials analysis is described in this thesis can be used to guide and evaluate the synthetic objectives.

Degree

Ph.D.

Advisors

Raftery, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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