Phase Transformation and Charge Transfer in Heavily Iron Ion Doped Titanium Oxide and Oxynitride Nanocolloids

Abstract

Porous sol–gel generated TiO2 nanocolloids and their corresponding oxynitrides TiO2–xNx are investigated to evaluate the effects that accompany doping with iron (FeII) ions at high doping concentrations. The introduction of FeII at higher concentrations leads to an anatase-to-rutile conversion at room temperature for the seeded oxynitride nanocolloids and to a less crystalline state in the subsurface and bulk as suggested by Raman spectroscopy. Combinations of core level and valence band photoelectron spectroscopy are correlated with the results of density functional theory (DFT) calculations to demonstrate a facile charge transfer from FeII to TiIV, producing TiIII and FeIII, and subsequently the transformation of FeII + TiIII to FeIII + TiII. This process is associated with the detectable formation of Ti(III) and Ti(II) at the surface of the titania-based nanoparticles. With significant visible light absorption, the photocatalytic activities of the iron-seeded titania systems are comparable to that of the iron-doped oxynitride TiO2–xNx as a function of doping concentration. The observations reported herein suggest that the anatase-to-rutile phase transformation and the enhancement of electron transfer can control the visible-light catalytic activity within the doped nanoparticles to form FeII/FeIII-codoped TiO2 nanocolloids.