First-principles Study On The Rutile TiO₂（011）-2×1Surface

With the environment pollution and energy crisis becoming serious, the photocatalysistechnique of TiO2not only can deal with the air and water purification, but also can be usedfor the ultimate goal of producing renewable hydrogen fuels using photocatalysis fromwater. However, the common TiO2is an intrinsic wide band gap semiconductor (3.05eVfor rutile and3.20eV for anatase) and can absorb only accounts for~5%of the sunlight inthe ultraviolet region. Such fundamental limitation due to the wide band gap nature of TiO2made the material rather ineffective for the harvesting of solar light and limited thepractical applications under sunlight. To overcome the limited optical absorption of TiO2under sunlight, considerable studies of TiO2-based photocatalysts have been carried out inrecent years.Titanium dioxide is one of the most often studied photochemical materials and itschemical properties are controlled by the atomic scale structure of their surfaces.Understanding face-dependent photoactivity is of fundamental and practical interest, sincethe effectiveness of the solar-driven photocatalytic processes strongly depends on thecrystallographic surface orientations even for the same material. The recently proposedrutile TiO2(011)-2×1surface was reported with an ideal narrow band gap of only~2.10eV,matching it closely with the energy of visible light. At the same time, the rutileTiO2(011)-2×1surface provides us with the first opportunity to study a more complicatedoxide surface with a significant2×1reconstruction at the vacuum interface. However, themajority studies of the rutile TiO2(011)-2×1surface have focused on the experimentalworks. So, it is necessary to have a study of the rutile TiO2(011)-2×1surface performedwith first-principles density functional theory (DFT) calculations.For the complicated rutile TiO2(011)-2×1surface, the simulations with forming frombulk truncation to the2×1reconstruction have not been mentioned. In the present work, wesupplement the simulations of the rutile TiO2(011)-2×1surface by means of frst-principlesDFT calculations. On that basis, the calculated band structure shows a direct band gap at Гpoint, and the band gap of2.08eV reproduces the experimental measurement value~2.10eV very well. Not coincidentally, the calculated optical absorption edge at about2.01eVfurther clarifies the band gap narrowing and such an intrinsic band gap nature effectivelypushes the optical absorption edge into the visible light.On the other hand, the conventional DFT calculations generally fail to describesystems with localized d electrons and the origin of which could be associated with an inadequate treatment of the strong Coulomb repulsion between d electrons localized ontransition metal ions. For further analysis of the electronic and optical properties of therutile TiO2(011)-2×1surface and compared with those of the rutile TiO2(110) surface, wepresent a study concerning the effect of the on-site Coulomb interaction term U in DFT. Forthe rutile TiO2(110) surface, the conventional DFT calculations cannot give an obviousimprovement on the properties after the treatment of U approximation was considered.However, for the rutile TiO2(011)-2×1surface, the conventional DFT calculations couldreproduce the optimal agreement with the experiment, leading to the DFT+U calculationsgive an overestimation to the electronic and optical properties.In fact, hydroxyl groups and oxygen vacancies at TiO2surfaces play a key role in thecontrol and modification of surface properties and reactivities. So, it is necessary to have astudy of the hydroxylated and reduced (O-deficient) rutile TiO2(011)-2×1surfacesperformed with first-principles density functional theory calculations. For the H adsorptionand O vacancy on the rutile TiO2(011)-2×1surface, we investigated three different surfaceO sites. Based on the adsorption and formation energy calculations, we find that the top Ois an energetically preferential site for the adsorption of H atom or the formation of Ovacancy. The calculated electronic structures indicate that the energetically preferential Osite cannot create a band gap state, only the O vacancy at the side O site gives rise to aTi-3d like defect level at the edge of the conduction band. It is worth mentioning that allconsidered configurations of the H adsorption and O vacancy on the rutile TiO2(011)-2×1surface obviously enhance the optical absorptions in the areas of infrared, not just the rutileTiO2(011)-2×1surface only has a good absorption edge in the visible light region.