Single oxygen vacancies of (TiO₂)₃₅as a prototype reduced nanoparticle: implication for photocatalytic activity

Abstract

Titanium dioxide (TiO2), as a semiconductor metal oxide, has been one of the most popular materials studied in the field of photocatalysis. In the present study, the properties of single oxygen vacancies of (TiO2)35, a prototype of anatase nanoparticle, were investigated by DFT calculations. (TiO2)35 is the minimum sized model (~ 2 nm) as bipyramidal nanoparticle with anatase phase and eight {101} facets. All the available oxygen vacancies at various sites according to position, coordination number, and distance from the center atom were examined. The geometric, energetic and electronic properties of the reduced TiO2 clusters were analyzed by hybrid DFT functionals with different Hartree-Fock exchange ratio (0, 12.5 and 25 %). It was found that the structure of pristine (TiO2)35 is somewhat different from bulk lattice with relatively high surface to volume ratio. Moreover, the particular high (three)-coordinated oxygen atom is energetically most favorable for oxygen vacancy formation from nanoparticle mainly due to substantially high relaxation energy. TiO2 nanoparticle has low oxygen vacancy formation energy and narrow band gap by defect state, and can be utilized as an efficient photocatalyst material.

Visible‐Light Photocatalytic Activity of Ionic Liquid TiO2 Spheres: Effect of the Ionic Liquid's Anion Structure

“…As the data gathered in Table point to a crucial role of bromide anions, whereas the photoreactivity if TiO 2 could be related to crystallographic defects and oxygen vacancies, a computational model employing an oxygen vacancy and bromide anion was built to explore the mechanism explaining the photoreactivity of TiO 2 under visible irradiation. Furthermore, the results shown in Table unequivocally demonstrate that the O 2 .− radical anion forms in the primary photochemical reaction.…”

“…Both latter species do not form a stable complex in solution but may interact with TiO 2 to form a well‐defined O 2 ⋅⋅⋅Br − dimer on a TiO 2 vacancy. To demonstrate how the formation of such a complex influences the electronic absorption of the system, we employed a vacancy modeled at the DFT level for a prototype of an anatase particle, Ti 35 O 70 . This vacancy along with the spin density originating from two excess electrons that remain in the particle once an oxygen vacancy is created was calculated at the PM6 level for the geometry obtained by Kim et al .…”

“…For this model, a constrained optimization was performed. Namely, the Ti 2 O 6 fragment of a particle was fixed (a stiff structure of particle does not deform because of the formation of a surface complex with oxygen and bromide anion) at the geometry obtained by Illas et al . whereas all degrees of freedom related to Br − and O 2 were fully optimized at the PBE0/6‐311+G(3df) level and the PCM model of water.…”

“…The geometry of anatase oxygen vacancy, Ti 35 O 69 , optimized at the PBE0 level was taken from Kim et al. (referred there as E2‐2) . First, the bromide anion and oxygen molecule were added to a vacancy and their positions were pre‐optimized at the PM6 level while keeping the geometry of vacancy fixed.…”

Highly Active TiO2 Microspheres Formation in the Presence of Ethylammonium Nitrate Ionic Liquid

“…The generated reactive oxygen species (•OH and O 2 • − ) are crucial reagents in the photodegradation of pollutants [16,17,24].Notwithstanding, the direct relationship between the structure of the ILs and the size/morphology of the nano-and microparticles of the semiconductors, such as the TiO 2 photocatalysts, still remains ambiguous. Up until now, the following factors had been reported as predominant in effectuating the structure, and thus the activity of said particles, as follows: (1) IL anion type (the number of atoms it is composed o growth; apart from this, π-π stacking of imidazolium cations promotes the ILs role as templating agents); (3) cation-anion interaction energy where the frailer, the weaker the cation-anion interactions, the firmer the capping on growing the TiO 2 particles and the more efficient the inhibition/hampering of the unfavorable Ostwald ripening process; and (4) the type of overall interactions (π-π, van der Waals, coulomb and electrostatic forces, and hydrogen bonding) [15,16,21,25].Furthermore, the proposed fons et orgio of the influences on the TiO 2 photoactivity are tenable, as follows: (1) doping with N, C, and F elements after ILs thermal decomposition; (2) directly sensitizing the TiO 2 particles; (3) affecting the transfer of photo-generated charges through the bulk of particles; and (4) favoring oxygen vacancies and Ti 3+ species formation during synthesis [26][27][28]. However, an up to date kinetics of the formation of the TiO 2 particles during ionic liquid-assisted synthesis has not been presented and discussed.In this regard, in this study, the TiO 2 particles were synthesized solvothermally, with the assistance of the selected ionic liquid, ethylammonium nitrate [EAN] [NO 3 ], which is one of the earliest reported in the literature of protic ionic liquid [29,30].Apart from prevailing in studies on the topics of the synthesis process and characterization of the IL-assisted TiO 2 microparticles [22,[30][31][32], we focused on evaluating the functional properties of the obtained micro-particles, namely, their photoactivity.…”