Long-Lasting Non-hydrogenated Dark Titanium Dioxide: Medium Vacuum Anneal for Enhanced Visible Activity of Modified Multiphase Photocatalysts Lütfiye Y. Ozer, [a] Harry Apostoleris, [b] Florent Ravaux, [b] Sergii I. Shylin, [c] Fikret Mamedov, [c] Andreas Lindblad, [d] Fredrik O. L. Johansson, [d] Matteo Chiesa, [b, e] Jacinto Sµ, [c, f] and Giovanni Palmisano* [a] Multiphase TiO 2 was stably modified by vacuum treatment for a dramatic improvement in visible-light absorption and photo- catalytic reactivity. The samples were made of rutile-brookite, bare or N doped, and were grown on reduced graphene oxide. The stable introduction of Ti 3 + species and oxygen vacancies resulting in mitigated electron–hole recombination was identi- fied as the main responsible factor, along with a change in sur- face charges. The absorbance of visible radiation is well known as the most critical drawback of TiO 2 , which is the most used photocatalyst with a band gap that falls in the range of 2.95 to 3.25 depend- ing on a number of factors, including crystal phases, particle size, morphology, and defects. Because of this, more than 90% of the solar spectrum is unusable for TiO 2 activation. [1] A number of methods are typically employed to enhance the visible absorption of and, thus, the total light harvested from TiO 2 catalysts. Great attention has been focused on doping TiO 2 with trace amounts of heteroatoms, [2] the surface sensitization of TiO 2 with organic dyes [3] or noble metals, [4] and the induction of defective states by annealing TiO 2 under a hy- drogen or ammonia atmosphere. [5] Generally, the introduction of Ti 3 + states is an effective method to induce visible absorp- tion of TiO 2 without introducing any heteroatom into its crystal framework. The generation of Ti 3 + states is, in most cases, a transitional phenomenon, and unless the catalyst is thermally treated under a reducing atmosphere by means of unsafe gases, such as ammonia and hydrogen, or starting from al- ready reduced precursors of TiO 2 , such as TiH 2 , [6a] the reduced states are entirely lost upon simple exposure to atmospheric oxygen for a sufficient time. [6b] Given the two-dimensional character of photocatalysis, sur- face states surely play a key role in determining the per- formance of the used semiconductors, and defects can both act as visible absorption centers and mitigate the rate of elec- tron–hole recombination. [7] Pioneering work in the field was published some seven years ago, [5b] accounting for the synthesis of highly active black TiO 2 obtained by the hydrogenation of TiO 2 . While the narrowed band gap of the catalyst allowed for a UV-to-near-in- frared absorption range, the hydrogenation of TiO 2 introduced intraband states that increased Ti 3 + abundancy and oxygen va- cancies, which were reported to boost the activity of this ma- terial relative to that of white TiO 2 . However, this study did not report on the stability of this semiconductor over the long term. In the present study, we investigate an unusual response of brookite-rutile samples, bare (labeled as TiO 2 ) or modified with nitrogen, and grown on reduced graphene oxide (N-TiO 2 /G), prepared through sol–gel and calcined at 450 8C, a method that was recently developed. [8, 9] Surprisingly, subsequent ther- mal annealing in medium vacuum (TAMV), that is, 0.1 Torr (1.00 Torr = 0.133 kPa), at 400 8C produced an irreversible and strong increase in the near-UV and visible absorption (l > 400 nm), which resulted in an apparent color change to dark gray, and diffuse reflectance in the visible range decreased drastically to half its initial value (Figure 1). This absorbance was only slightly mitigated after the powders were kept in open air for a few days, and afterwards, it did not vary signifi- cantly for at least 1 year, over which time the samples were ob- [a] L. Y. Ozer, Prof. G. Palmisano Department of Chemical Engineering Khalifa University of Science and Technology Masdar Institute Masdar City PO BOX 54224, Abu Dhabi (United Arab Emirates) E-mail : gpalmisano@masdar.ac.ae [b] H. Apostoleris, F. Ravaux, Prof. M. Chiesa Department of Mechanical Engineering Khalifa University of Science and Technology Masdar Institute Masdar City PO BOX 54224, Abu Dhabi (United Arab Emirates) [c] S. I. Shylin, Prof. F. Mamedov, Prof. J. Sµ Department of Chemistry—nsgtrçm Laboratory Uppsala University PO BOX 523, SE-751 20, Uppsala (Sweden) [d] Prof. A. Lindblad, F. O. L. Johansson Division of Molecular and Condensed Matter Physics Department of Physics and Astronomy Uppsala University PO BOX 516, SE-751 20, Uppsala (Sweden) [e] Prof. M. Chiesa Arctic Renewable Energy Center (ARC) Department of Physics and Technology The Arctic University of Norway (UiT) (Norway) [f] Prof. J. Sµ Institute of Physical Chemistry Polish Academy of Sciences Warsaw (Poland) Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/cctc.201800097. ChemCatChem 2018, 10, 2949 – 2954  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2949 Communications DOI: 10.1002/cctc.201800097