¹⁵N Solid State NMR and EPR Characterization of N-Doped TiO₂ Photocatalysts

Abstract

The solid-state nuclear magnetic resonance (SSNMR) analysis and structural characterization of N-doped TiO2 nanoparticle and monolayer materials suitable for visible photocatalysis is reported. The SSNMR analysis of 15N-doped TiO2 synthesized using 15N-urea before calcination indicates formation of various amino functionalities of the type NH, NH2, NH3, and probably NH4 +, while the NMR spectrum of the yellow powder that results from high-temperature calcination shows that these nitrogen species oxidize to form nitrate. These and nonisotopically labeled materials were characterized also by X-ray diffraction, UV−vis, and electron paramagnetic resonance spectroscopy. Monitoring the oxidation of 1,2-13C-trichloroethylene (TCE), photochemical activity was confirmed by monitoring the production of CO2 by 13C NMR analysis. The powdered form of N-doped TiO2 proved to be a highly efficient visible light catalyst by oxidizing all of the TCE to chlorinated alkyl and acyl containing molecules along with CO2. The performance of a TiO2−N−TiO2/porous Vycor 7930 borosilicate glass monolayer catalyst was in turn evaluated by monitoring the photochemical oxidation of ethanol and acetone with UV−vis light, and it was found to perform better than TiO2-only monolayers. Direct nitridation of TiO2 powders and monolayers also was conducted to compare the 15N SSNMR and to discern if there is a nitridic bond in these materials. The SSNMR results provide more evidence supporting the hypothesis that the nitrogen atom in N-doped TiO2 is present in interstitial sites when N is in a highly oxidized state.

Nitrogen-Doped and CdSe Quantum-Dot-Sensitized Nanocrystalline TiO2 Films for Solar Energy Conversion Applications

“…27,28 Different techniques have been used to study N-doped TiO 2 crystallographically, including XPS, EPR, Raman spectroscopy, XRD, and absorption spectroscopy. [29][30][31] Theoretical studies have supported the visible absorption and the resulting yellowish color of TiO 2 /N thin films and powders. 32,33 Alternative techniques to increase the photoresponse besides doping include the utilization of tunable narrow band gap semiconductor nanoparticles or quantum dots (QDs) such as CdS, CdSe, and CdTe to sensitize wide band gap semiconductors such as the metal oxides, for example, TiO 2 and ZnO.…”

On the Mechanism of Urea‐Induced Titania Modification

“…The hydroxide precipitated with water is subsequently calcined at temperatures of 400-900 8C. The oxidation of 1,2-trichloroethylene [39,40] and acetaldehyde [40] was catalyzed by the resulting materials. Solvothermal treatment of aqueous TiCl 3 with urea in methanol at 190 8C afforded violet titania due to the presence of oxygen vacancies.…”

“…[12] Higher energies of 400-401, 402-403, and even 407-408 eV were proposed to correspond to N À N, N À O, or N À C groups, [10,11,37,41,42,49,51,52] chemisorbed N 2 , [38,63] or NO x (x < 2) and NO 2 À groups, [12] respectively. EPR and solid-state NMR spectra suggested the existence of interstitial NO 2 2À [9] or NO x species [39] and nitrate, [39] respectively. Both FTIR and solid-state NMR experiments indicated the presence of TiÀNH 2 groups.…”

Investigating Visible-Photocatalytic Activity of MoS2/TiO2 Heterostructure Thin Films at Various MoS2 Deposition Times

“…Impurity doping is one of the typical approaches [4][5][6][7][8]. However, doping can usually cause high recombination rate and the low carries mobility of photoexcited electron-hole pairs and make TiO 2 thermally unstable [9].…”