Simple Ethanol Refluxing Method for Production of Blue-Colored Titanium Dioxide with Oxygen Vacancies and Visible Light-Driven Photocatalytic Properties

Abstract

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2–x ) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti­(III) species and the appearance of two optical absorption features: a broad absorption bandresponsible for the blue colorationextending from the green region (∼2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree–Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (∼2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review

“…Follow-up analyses based on PL measurements characterized the grey anatase H 2 -treated samples with an additional light emission in the range of 400–450 nm [ 247 ], which corresponds to sub-bandgap states positioned 0.2–0.4 eV below the conduction band edge. Interestingly, such a conclusion is not only compatible with scheme proposed in Figure 14 (shallow states) to explain the sub-bandgap excitation of PL in anatase (see processes (f) and (g)), but also is supported by a work by Lettieri and coworkers, who found a “optical signature” of reduced anatase by exploring a different strategy for the production of visible light-activated reduced TiO 2 [ 205 ]. In this work, a simple ethanol refluxing treatment at mild temperatures of P25 nanoparticles was employed to produce “blue TiO 2 ” with improved visible light photocatalytic activity.…”

“…Samples with very high concentrations of oxygen vacancies appear as dark grey or even black as the result of a large and approximately wavelength-independent optical absorption in the visible spectrum. These samples are referred to in literature as “black TiO 2 ” [ 22 , 236 , 237 , 238 ] or as “grey” or otherwise “colored” version of TiO 2 (e.g., blue TiO 2 ) [ 205 , 239 , 240 ]. In all of these cases, the formation of defects which are not present in pristine TiO 2 nanoparticles leads to an additional optical absorption band (“additional” here meaning that is absent in stoichiometric TiO 2 ) and then to the sample coloration.…”

“…Additional procedures have been experimented to produce colored TiO 2 with photocatalytically-active intrinsic defects. Some examples include H + ion implantation [ 244 ], photodegradation of organic molecules on TiO 2 surface [ 205 , 248 , 249 ], different treatments with reducing agents [ 250 , 251 , 252 ] and others: due to the vastness of the topic, the reader is referred to specialized reviews [ 22 , 233 , 238 , 253 ] for additional information.…”

“…The new excitation band which appears for ethanol-treated samples with a peak excitation wavelength of about 420 nm is associated to an additional radiative recombination channel that involves sub-gap excitations of valence band electrons to shallow states characterizing the reduced TiO 2 , which then relax to deeper and luminescence-active electron traps. Figure 25 ( c , d ) are reprinted with permission from Reference [ 205 ] . Copyright 2020 American Chemical Society.…”

Efficient Photocatalytic Hydrogen Evolution over TiO2-X Mesoporous Spheres-ZnO Nanorods Heterojunction

“…Two different types of lattice images are obtained with d spaces of 0.350 and 0.260 nm, corresponding to the (101) plane of typical anatase TiO 2 and the (002) plane of hexagonal ZnO, respectively [ 34 ]. The hybrid photocatalyst has clear and continuous stripes, indicating that there is an intense mutual attraction between ZnO NRs and TiO 2-X due to suitable lattice parameters [ 35 ]. Furthermore, this also proves that the reduced TiO 2 nanocrystals are still highly crystalline, and the Ti 3+ introduced by the reduction does not make the crystal lattice disordered.…”