Preparation Of Visible Light Active Titanium Dioxide And The Synergistic Effect Between Oxygen Vacancies And Dopants Nitrogen Plus Metal

Nitrogen doping is one of the most efficient methods to extend light response of TiO₂into the visiblelight region in association with an expected enhancement of visible-light-responded photocatalytic activity.However, the origin of visible light photoactivity of N-doped TiO₂is still in debate. Four differentmechanisms have been proposed as below:(1) Band gap narrowing caused by mixing of N2p and O2porbitals, resulting in visible light response;(2) Localized midgap induced by doped-N, whose electrons canbe excited to jump to the conduction band by visible light;(3) Oxygen vacancy states below conductionband formed during nitrogen doping process;(4) A neutralization theory that suggests the synergistic effectbetween N2p localized midgap and oxygen vacancy states is responsible for the visible light photoactivity.Unfortunately, the role of the oxygen vacancy is usually not considered in the former two theories, whilehow the oxygen vacancy and doped-N work is not well elucidated in the latter two theories. Therefore, it isimperative to clarify the role of oxygen vacancy in enhancing visible light photoactivity and themechanisms by which the oxygen vacancy and doped-N work. The solution of these problems facilitates todesign and prepare highly visible-light-active photocatalyst.Therefore, we pay special attention to the effects of oxygen vacancy and dopants on the visible lightphotocatalytic activity, aiming at revealing the origin of visible light sensitization of N-doped TiO₂. Firstly,N-doped TiO₂is prepared by heat treatment of commercial P25-TiO₂in flowing NH3and the relationshipamong the doped-N, single-electron-trapped oxygen vacancy, optical absorbance and enhanced visible lightphotocatalytic activity is systematically studied. It is proposed that the origin of visible light sensitization ofN-doped TiO₂can be ascribed to the synergistic effect between oxygen vacancy and dopant nitrogen.Subsequently, three kinds of precursors, nanotubular titanic acid (denoted as NTA), raw P25-TiO₂andnovel-TiO₂, are separately used to prepare visible-light-active N-doped TiO₂samples by annealing inflowing NH3, aiming to reveal the determinative factors on visible light response. By comparing thephysicochemical properties of the three kinds of N-doped TiO₂samples with their visible lightphotocatalytic activity, the synergistic effect between oxygen vacancy and dopant nitrogen is proved andNTA precursor is pronounced to be one of the most promising materials for designing visible light activephotocatalyst. Furthermore, the characterization and mechanism of metal ion decorated TiO₂-xNxsamples hat based on the theory of N-doped TiO₂is systematically studied. In summary, four main conclusions aredrawn as below:1. N-doped TiO₂catalysts are prepared by nitridation of P25in NH3flowing under various temperaturesand the synergistic effect between the single-electron-trapped oxygen vacancy and doped-N in associationwith the origin of visible light sensitization of N-doped TiO₂is systematically studied. It is found that alarge amount of single-electron-trapped oxygen vacancies (denoted as SETOVs) generate during the dopingprocess, giving rise to a triplet ESR signal centered at g=2.004. The concentration of SETOVs is enhancedunder visible light irradiation as compared with that obtained in the dark, possibly because many freshadditional SETOVs generated under visible light irradiation. Therefore, it can be inferred that the oxygenvacancies can be divided into three categories: SETOVs with triplet ESR signal, dual-electrons-trappedoxygen vacancy and oxygen vacancy without electron, while the latter two will not produce ESR signals.The doped-N at interstitial site is directly combined with lattice oxygen or oxygen vacancy, showing abinding energy at400eV in XPS spectra. The origin of visible light photocatalytic activity is ascribed tothe synergetic effect between the formation of SETOVs in TiO₂matrix and the existence of doped-N on thesurface. Namlely, the formation of SETOVs results in visible light reponse, while doped-N plays a role inpreventing photoinduced electrons and holes from recombination. In other words, in the absence of eitherSETOVs in TiO₂matrix or doped-N on the surface, N-doped TiO₂will not show visible light photocatalyticactivity; and the higher the SETOVs concentration is, the better the visible light photocatalytic activity willbe.2. Not only as a connecting link between the preceding and the following but also in order to prove thesynergistic effect between oxygen vacancy and dopant nitrogen, three kinds of precursors are separatelyused to prepare visible light active N-doped TiO₂samples. Both the visible light absorption andphotocatalytic activity of the three kinds of N-doped TiO₂samples are found to be proportional to theconcentration of SETOVs, well conforming to what are summarized above. According to the DRS results,the band gap energy structure model of N-doped TiO₂is proposed. It is calculated that an intra-bandinduced by SETOVs is located at0.57eV below conduction band and2.34eV above valence band, with aband gap Eg=0.19eV resulting in visible light response which varies with varying concentration ofSETOVs. Given that the formation of SETOVs induces an intra-band, it is proposed that electrons are initially excited from valence band to intra-band and then jumped to the conduction band under visible lightirradiation. As a result, the photoinduced electrons may jump back from conduction band to intra-band,accompanying with a predicted charge transfer state that contributes to the visible light photocatalyticactivity:V_o+N→V_o+N^-(I)N^-+O₂→N+O₂^-(II)Where V_o represents for dual-electrons-trapped oxygen vacancy and V_o equivalents to SETOV. Thedoped-N plays a role in preventing photoinduced electrons and holes from recombination by cutting off theroute that the electrons jump back to valence band, resulting in enhanced visible light photocatalyticactivity. The N-doped TiO₂samples obtained by annealing of NTA as the precursor in flowing NH3exhibited the highest visible light photocatalytic activity, implying that NTA is one of the most promisingmaterials for designing visible light active photocatalyst.3. Au/TiO_2-xN_xsamples are prepared via a facile one-pot route and they showed much better visible lightperformance than Au/TiO₂or TiO_2-xN_xsamples. An additional absorption band from550⁶50nm index togold surface plasmon resonance is observed in DRS spectra for Au/TiO₂, while this peak for Au/TiO₂-xNxsamples is very weak. The binding energy values of Au4f7/2are83.1and83.4eV for Au/TiO₂and TiO₂-xNxsamples, respectively, lower than84.0eV of metallic Au. As to Au/TiO₂samples, the electrons can transferfrom SETOVs to Au6s obitals, resulting in lower Au4f binding energy. Accompanying with the Nincorporation, an electron transfer from Au6s obital toward the N2p level can be expected, resulting in thebinding energy shifting from83.1eV for Au/TiO₂to83.4eV for Au/TiO₂-xNxsamples. The higher visiblelight photocatalytic activity of Au/TiO₂-xNxsamples is attributed to the synergetic effect among doped-N,modified-Au and oxygen vacancy. Namely, nitrogen doping favors the formation of oxygen vacancy andincreases the Au-surface adhesion energy, while the existence of both Au and oxygen vacancy results ineasily nitrogen doping into TiO₂.4. Noble metal ions Pd2+and Pt4+or transition metal ions Cu2+and Ni2+modified TiO₂-xNxsamples(denoted as M/TiO₂-xNx) are also prepared separately according to the same method mentioned in the thirdportion. It is found that there has no direct relationship between the valence state or ionic radius of metalion and Anatase/Rutile phase transformation or visible light photocatalytic activity with respect to M/TiO₂-xNxsamples. The visible light response of M/TiO₂-xNxis attributed to the formation of SETOVsduring the preparation process, while dopant nitrogen plus modified metal contribute not only to improvethe visible light absorption but also suppress the recombination of photogenerated electrons and holes. Inone word, the enhanced separation efficiency of photogenerated electron–hole pairs attributed to thesynergistic effect among oxygen vacancies and N dopant as well as modified metal particulates on thesurface of TiO₂jointly account for the increased visible light photocatalytic performance of M/TiO₂-xNxsamples. The visible light photocatalytic mechanism of M/TiO₂-xNxis proposed and discussed in detail.