Article pubs.acs.org/JPCC
Phase Transformation and Charge Transfer in Heavily Iron Ion Doped Titanium Oxide and Oxynitride Nanocolloids Lixia Sang,†,‡ James L. Gole,*,§ Junwei Wang,† Jonathan Brauer,⊥ Baodong Mao,† S. M. Prokes,*,¶ and Clemens Burda*,† †
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, 100124, China § Schools of Physics and Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332-0430, United States ⊥ Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama, 35487-0336, United States ¶ Code 6876, NRL, Washington, D.C. 20375, United States ‡
S Supporting Information *
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. TiO2−xNx, doped with high concentrations of cobalt (CoII) ion.12,13 Using Raman spectroscopy,12 we established that CoII ion seeding introduces a spinel-like cobalt oxide structure in both the TiO2 and the TiO2−xNx anatase nanocolloids. This led to the ready conversion of both the oxide and the oxynitride from the anatase to the rutile form at room temperature in contrast to temperatures in excess of 700 °C for the undoped titania nanocrystals.14 Here, we expand our studies to evaluate the effects of high-concentration doping with iron (FeII). We combine experimental X-ray photoelectron spectroscopy (XPS) and computational density functional theory (DFT) results to study possible Fe−Ti electron transfer and Raman spectroscopy to suggest similar anatase-to-rutile conversions in the FeII seeded systems. This analysis is relevant to surface catalysis since XPS is a surface-sensitive spectroscopy. The photoelectrons ejected from Ti with Al Kα irradiation have a mean free path of 1−2
1. INTRODUCTION Nanosized titanium dioxide (TiO2) materials have been the subject of great interest because they exhibit many modified electronic and optical properties as well as extensive applications, including photovoltaics, electrochromics, sensors, and photocatalysts.1−5 Many methods, such as gas condensation, sol−gel, titanium-alkoxide hydrolysis, and others, have been successfully developed to prepare TiO2 nanoparticles. Recent studies6−10 of sol−gel generated TiO2 with average particle size 10 nm suggest the importance of structural porosity as it influences and enhances the rate and efficiency of doping processes. The introduction of small concentrations (
