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

  • Stefano Lettieri*
    Stefano Lettieri
    Institute of Applied Sciences and Intelligent Systems, National Research Council (ISASI-CNR), Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy
    *E-mail: [email protected]. Phone: +39-081676809 (S.L.).
  • Valentina Gargiulo
    Valentina Gargiulo
    Institute for Research on Combustion, National Research Council (IRC-CNR), Piazzale V. Tecchio 80, 80125 Napoli, Italy
  • Michela Alfè*
    Michela Alfè
    Institute for Research on Combustion, National Research Council (IRC-CNR), Piazzale V. Tecchio 80, 80125 Napoli, Italy
    *E-mail: [email protected]. Phone: +39-0817682247 (M.A.).
  • Matteo Amati
    Matteo Amati
    Elettra-Sincrotrone Trieste S.C.p.A di interesse nazionale, SS14 Km 163,5, I-34149 Trieste, Italy
    More by Matteo Amati
  • Patrick Zeller
    Patrick Zeller
    Elettra-Sincrotrone Trieste S.C.p.A di interesse nazionale, SS14 Km 163,5, I-34149 Trieste, Italy
  • Valentin-Adrian Maraloiu
    Valentin-Adrian Maraloiu
    National Institute of Materials Physics, 405A Atomistilor Street, 077125 Magurele, Romania
  • Fabio Borbone
    Fabio Borbone
    Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, Via Cintia 21, 80126 Napoli, Italy
  • Michele Pavone
    Michele Pavone
    Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, Via Cintia 21, 80126 Napoli, Italy
  • Ana B. Muñoz-García
    Ana B. Muñoz-García
    Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, Via Cintia 21, 80126 Napoli, Italy
  • , and 
  • Pasqualino Maddalena
    Pasqualino Maddalena
    Dipartimento di Fisica, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, Via Cintia 21, 80126 Napoli, Italy
Cite this: J. Phys. Chem. C 2020, 124, 6, 3564–3576
Publication Date (Web):January 19, 2020
https://doi.org/10.1021/acs.jpcc.9b08993
Copyright © 2020 American Chemical Society
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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.

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  • Pictures of the TiO2 powders before and after alcohol (EtOH, MeOH, PrOH) refluxing thermal treatment; F(λ) curves and the pictures of samples P25, P25EtOH(88), P25EtOH, Anat, AnatEtOH, AnatMeOH, and AnatPrOH; XRD spectra of the powders; nanoparticle size distribution obtained by HRTEM, XPS survey for P25EtOH; SPEM–XPS maps of the Ti 2p and O 1s photoemission signals for P25EtOH; computed PDOS of bulk TiO2 and of stoichiometric (101) TiO2; intensity spectrum of the lamp used for the VIS photodegradation of MB; and absorption spectra of MB aqueous solution in absence of TiO2 photocatalyst before and after 150 min of VIS illumination (PDF)

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