Subsurface Oxygen Vacancies Research Articles

Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zirconium Doping

The effects of Zr doping on the stability of the CeO2(111) surface as a function of the dopant concentration and distribution, as well as on the relative stability of surface and subsurface oxygen vacancies, were studied by means of density functional theory (DFT+U) calculations. For a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O-Ce-O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. For the undoped CeO2(111) surface, the preference of subsurface vacancies with next-nearest neighbor (NNN) Ce3+ configuration has earlier been predicted. For the Zr-doped surface, the formation of vacancies was studied using a surface unit cell with 2x2 periodicity, and it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with NNN Ce3+, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO2(111) surface. Moreover, when Zr is located in TL2 the subsurface vacancy becomes by 0.10 eV less stable, compared to Zr in the TL1. The Ce3+ preference for the next-nearest neighbor cationic sites to both surface and subsurface vacancies at CeO2 (111), becomes more pronounced upon Zr doping. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects.

Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zirconium Doping

The effects of Zr doping on the stability of the CeO₂(111) surface as a function of the dopant concentration and distribution, as well as on the relative stability of surface and subsurface oxygen vacancies, were studied by means of density functional theory (DFT+U) calculations. For a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O–Ce–O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. For the undoped CeO₂(111) surface, the preference of subsurface vacancies with next-nearest neighbor (NNN) Ce³⁺ configuration has earlier been predicted. For the Zr-doped surface, the formation of vacancies was studied using a surface unit cell with 2 × 2 periodicity, and it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with NNN Ce³⁺, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO₂(111) surface. Moreover, when Zr is located in TL2 the subsurface vacancy becomes by 0.10 eV less stable, compared to Zr in the TL1. The Ce³⁺ preference for the next-nearest neighbor cationic sites to both surface and subsurface vacancies at CeO₂ (111), becomes more pronounced upon Zr doping. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects.

Understanding oxygen vacancies in disorder-engineered surface and subsurface of CaTiO3 nanosheets on photocatalytic hydrogen evolution

Hydrogenation-induced surface disorder on semiconductors has been proved an efficient strategy in photocatalysis, but identification and understanding of surface and subsurface disordered layers from molecular level viewpoints are still unclear. Herein, we fabricate efficient disorder-engineered CaTiO3 nanosheets photocatalysts and illustrate functions of surface and subsurface oxygen vacancies on photocatalytic hydrogen evolution. Our experimental and theoretical results reveal that subsurface oxygen vacancies can change energy band structure of CaTiO3 to form band tail states, improving charge separation; and surface oxygen vacancies can act as active centers to facilitate H2 formation. Both merits promote ∼49 times of the hydrogen evolution rate than pristine CaTiO3. In addition, we discover that improving charge separation with subsurface oxygen vacancies is more important than promoting surface reactions with surface oxygen vacancies in defect-engineered photocatalysts. Our work provides new insights into the hydrogenated disordered surface layer, as well as the rational design of photocatalysts with defect-engineering strategies.

The Structure of Oxygen Vacancies in the Near-Surface of Reduced CeO2 (111) Under Strain

Strain has been widely recognized as important for tuning the behavior of defects in metal oxides since properties such as defect configuration, electronic structure, excess charge localization, and local atomic distortions may be affected by surface strain. In CeO2, the most widely used promoter in three-way catalysts and solid state electrolyte in fuel cells, the behaviors of oxygen vacancies, and associated Ce3+ polarons are crucial in applications. Recent STM and AFM investigations as well as DFT-based calculations have indicated that in the near-surface of CeO2 (111), at low temperatures and vacancy concentrations, subsurface oxygen vacancies are more stable than surface ones, and the Ce3+ ions are next-nearest neighbors to both types of vacancies, which can be explained by the better ability of the system to relax the lattice strain induced by vacancy formation as well as by the excess charge localization. The results also revealed that the interaction between first-neighbor vacancies is repulsive. In this work, the relative stability of surface and subsurface oxygen vacancies at the CeO2 (111) surface under in-plane strain is investigated by means of DFT+U calculations. The tensile strain favors isolated surface vacancies with next nearest neighbor polarons, whereas isolated subsurface vacancies with nearest neighbor polarons are energetically favored under compressive strain. In addition, the formation of both surface and subsurface dimers is favored over having corresponding isolated species under compressive strain, which implies the possibility of controlling the formation of vacancy clusters using strain. In many applications, ceria is employed as a supported thin film or within a heterostructure in which ceria can be strained, and this study shows that strain can be a useful handle to tune properties of such materials.

Surface, Subsurface, and Bulk Oxygen Vacancies Quantified by Decoupling and Deconvolution of the Defect Structure of Redox-Active Nanoceria.

Oxygen vacancy concentrations are critical to the redox/photocatalytic performance of nanoceria, but their direct analysis is problematic under controlled atmospheres but essentially impossible under aqueous conditions. The present work provides three novel approaches to analyze these data from XPS data for the three main morphologies of nanoceria synthesized under aqueous conditions and tested using in vacuo analytical conditions. First, the total oxygen vacancy concentrations are decoupled quantitatively into surface-filled, subsurface-unfilled, and bulk values. Second, the relative surface areas are calculated for all exposed crystallographic planes. Third, XPS and redox performance data are deconvoluted according to the relative surface areas of these planes. Correlations based on two independent empirical results from volumetric surface XPS, combined with sequential deep XPS and independent EELS data, confirm that these approaches provide quantitative determinations of the different oxygen vacancy concentrations. Critically, the redox/photocatalytic performance depends not on the total oxygen vacancy concentration but on the concentration of the active sites on each plane in the form of subsurface-unfilled oxygen vacancies. This is verified by the pH-dependent performance, which can be increased significantly by exposing these vacancies to the surroundings. These approaches have significance to the design and engineering of semiconducting materials exposed to the environment.

Extremely Large Magnetic-Field-Effects on the Impedance Response of TiO2 Quantum Dots

Here, we report large magnetoresistance and magnetocapacitance response of undoped TiO2 quantum dots weighting the contribution of both grain and grain boundaries by means of impedance spectroscopy. We also performed a complete characterization of the TiO2 quantum dots (~5 nm) prepared by sol-gel via water vapor diffusion method, using X-ray diffraction, small angle X-ray scattering, transmission electron microscopy and Raman spectroscopy. In addition, we showed a complete theoretical study on the electronic properties of TiO2 surface and subsurface oxygen and titanium vacancies to shed some light in their electronic and magnetic properties. Based in our study, we can conclude that the presence of defects, mainly at the grain boundary of these undoped TiO2 quantum dots, could be responsible for the large positive magnetoresistance (+1200%) and negative magnetocapacitance (−115%) responses at low applied magnetic fields (1.8 kOe) and room temperature.

Structural variation of anatase (101) under near infrared irradiations monitored by sum-frequency surface phonon spectroscopy.

We probed the anatase (101) surface irradiated by near-infrared and infrared photons in different ambient gases by monitoring the surface lattice phonon mode using sum-frequency spectroscopy. We found that even under the irradiation of such low energy photons, the stability of surface oxygen vacancies, in comparison to sub-surface oxygen vacancies, can increase sensibly. The variation of this surface phonon mode is also in accordance with the photo-induced hydrophilicity of titanium oxide surfaces, which may provide the microscopic insight into this phenomenon.

Hybrid Functional Study of the Electro-oxidation of Water on Pristine and Defective Hematite (0001)

In this study, water oxidation on both pristine and hematite containing oxygen vacancies was investigated using hybrid functional methods. The onset potentials for R–Fe–Fe–(OH)3 and R–Fe–Fe–O3 surface terminations were determined. It is found that oxygen vacancy stabilizes O adsorption on both terminations. The electronic structure was studied in order to explain the obtained reaction energy landscape. The density of states (DOS), projected density of states (PDOS), and the projected Crystal Orbital Hamilton Population (pCOHP) of the bond between the O and surface Fe atom of both terminations were plotted in order to explain the stabilization caused by oxygen vacancy. The pCOHP plot clearly illustrates a change in bond strength and provides a direct explanation for the stabilization. The results show that the presence of subsurface oxygen vacancies increase the overpotential for electro-oxidation of water and that the lowest overpotential is found on oxygen-terminated hematite.

DFT Calculation about Oxygen Vacancy to Promote Adsorption of a CO Molecule on Single Au‐Supported Titanium Dioxide

The properties and behavior of a single Au atom supported anatase TiO2(001) surface are calculated using density functional theory (DFT) methods. The structures and energies of adsorbed single Au on an anatase TiO2(001) surface with surface oxygen vacancy, as well as subsurface oxygen vacancy, are systematically determined. Initially, the surface threefold coordinated oxygen vacancy adjacent to a single Au atom, rather than the surface two‐fold coordinated oxygen vacancy and subsurface threefold coordinated oxygen vacancy, results in more stability for anatase TiO2(001) surface with an Au adatom. Afterwards, the CO molecule is more strongly adsorbed on the single Au‐supported TiO2(001) surface with surface threefold coordinated oxygen vacancy when comparing to other structures. This is attributed to the more negatively charged Au single‐atom caused by a surface threefold coordinated oxygen vacancy presents concededly active for CO adsorption.

The effects of subsurface Ov and Tiint of anatase (1 0 1) surface on CO2 conversion: A first-principles study

Using the first-principles calculations, we explored the effects of intrinsic subsurface oxygen vacancy and Ti interstitial defects on the adsorption and reduction reaction of CO2 at anatase (1 0 1) surface. Both PBE + U and HSE06 results show that such intrinsic defects have strong selectivity for the CO2 binding to sites near the defects. For the most favorable site, the CO2 adsorption energy can be greatly increased by 0.06 – 0.2 eV relative to the perfect surface. This enhancement is greatly affected by the local structure distortion and the location of the excess electrons. Furthermore, the reduction reactions of CO2 into useful fuels such as HCOOH have also been explored. It is revealed that the energy barrier for the rate-limiting step of CO2 activation can be greatly lowered by the intrinsic defects. These results show the important effects of the defects on the CO2 adsorption and reduction processes.

Combining High Photocatalytic Activity and Stability via Subsurface Defects in TiO2

Surface defects consisting of oxygen vacancy in TiO2 provide sites with high photocatalytic activity, while their photocatalytic stability is often undermined because of the trapping of oxygen species. Here, we conducted a theoretic study to propose the use of subsurface oxygen vacancies in anatase TiO2(101) for photocatalytic water-splitting, aiming to combine both high activity and stability. This study demonstrates that subsurface defects expand light-harvesting ability of TiO2 to the visible light region and facilitate photogenerated charge separation, and these results were then verified by experiments. These vacancies also bestow high catalytic activity on sites of water adsorption and oxidation above them on the defect-free surface. Importantly, subsurface defects are inert toward reactive oxygen species, ensuring high stability for the nearby surface catalytic sites. These results demonstrate the role of subsurface oxygen vacancies in catalyzing oxygen evolution reaction, leading to a new design s...

First-principles calculations of water adsorption on perfect and defect WO3(0 0 1)

The interactions between water and semiconductor metal oxides are critical to photocatalytic reactions. In this work, the water adsorption on stoichiometric, non-stoichiometric and hydrogenated WO3(0 0 1) were investigated by first-principles calculations. The results show that the adsorption of the intact water molecule is more favorable than the adsorption of the dissociated ones on stoichiometric WO3(0 0 1) at different coverages. As the coverage of the water molecules on the surface increases, the molecular and dissociated states tend to co-exist on the stoichiometric WO3(0 0 1). In addition, the oxygen vacancy and hydrogen atom affect the electronic properties of WO3(0 0 1) by introducing the excess electrons, and the water adsorption could change distribution of excess electrons which are introduced by oxygen vacancy and hydrogen atoms. The water molecule can readily dissociate at the surface oxygen vacancy sites, while water molecules prefer remaining intact than being dissociated on the WO3(0 0 1) with sub-surface oxygen vacancy or hydrogen.

Activating Titania for Efficient Electrocatalysis by Vacancy Engineering

Pursuing efficient and low-cost electrocatalysts is crucial for the performance of water–alkali electrolyzers toward water splitting. Earth-abundant transition-metal oxides, in spite of their alluring performances in the oxygen evolution reaction, are thought to be inactive in the hydrogen evolution reaction in alkaline media. Here, we demonstrate that pure TiO2 single crystals, a typical transition-metal oxide, can be activated toward electrocatalytic hydrogen evolution reaction in alkaline media through engineering interfacial oxygen vacancies. Experimental and theoretical results indicate that subsurface oxygen vacancies and low-coordinated Ti ions (Ti3+) can enhance the electrical conductivity and promote electron transfer and hydrogen desorption, which activate reduced TiO2 single crystals in the hydrogen evolution reaction in alkaline media. This study offers a rational route for developing reduced transition-metal oxides for low-cost and highly active hydrogen evolution reaction catalysts, to realize overall water splitting in alkaline media.

Diffusion Kinetics of Gold and Copper Atoms on Pristine and Reduced Rutile TiO2 (110) Surfaces

Statistical mechanics and transition-state theory have been used to investigate the diffusion kinetics of gold and copper atoms on pristine and various reduced surfaces of rutile TiO2 (110). A DFT+U approach has been employed to calculate potential energy maps and to evaluate the required diffusion activation barriers. The role of the support reducibility has been examined on the adsorption properties (optimal structures, energetics, and spin polarization) and diffusion kinetics, especially for the reduced support presenting a single subsurface oxygen vacancy. This approach has allowed us to demonstrate key discrepancies between Au and Cu atoms and to sketch out a comparative scenario for the early-stage nucleation of Au and Cu nanoparticles on the various surface states of TiO2 (110).

Carboxylic Acid Group-Induced Oxygen Vacancy Migration on an Anatase (101) Surface.

Dye-sensitized solar cells (DSSCs) have aroused intensive interest for the replacement of conventional crystalline silicon solar cells. Through carboxylic acid groups, the dyes attach to the TiO2 anatase (101) surface, on which the subsurface oxygen vacancies (Vosubs) are predominant. The performance of DSSCs can be affected by the presence and positions of oxygen vacancies (Vos). By applying density functional theory calculations, we found that the adsorption of a carboxylic acid group-decorated dye molecule reverses the relative stability between the surface oxygen vacancy (Vosurf) and Vosub on the anatase (101) surface, which facilitates the migration of the Vo from the subsurface to the surface by overcoming an energy barrier of less than 0.16 eV, which is significantly lower than the 1.01 eV energy barrier on the clean surface. Further, ab initio molecular dynamics simulations indicate that the Vosub can easily migrate to the surface at room temperature. This dynamic interplay between the Vo of the anatase (101) surface and the carboxylic acid group would be important for future studies concerning the stability and photovoltaic efficiency of the solar cells.

Ce0.3Zr0.7O1.88N0.12 solid solution as a stable photocatalyst for visible light driven water splitting

The search of efficient and stable photocatalysts for the evolution of hydrogen from water using solar energy is of great importance for material science today. Limited by the relatively inferior stability of oxynitrides, the availability of these visible-light-response materials in photocatalysis is still far below what is expected. Here we report a novel oxynitride Ce0.3Zr0.7O1.88N0.12 as an efficient and stable H2-evolving photocatalyst under visible light irradiation, which can further enable overall water splitting by coupling with an O2-evolving photocatalyst via Z-scheme. Experimental and theoretical results together reveal that the origin of the excellent activity and stability of Ce0.3Zr0.7O1.88N0.12 photocatalyst can be attributed to the improved separation rate of photoexcited charge carriers by surface and sub-surface oxygen vacancies. The present study provides a strategy to engineer efficient and stable photocatalysts, and the oxynitride mentioned above could act as a promising candidate of H2-evolving photocatalyst for designing a prominent Z-scheme photocatalytic system.

Visible Emission in Nanocrystalline Rutile: Free Exciton and Water as Probes for Midgap States in Adsorption/Desorption Using “Conventional” and Synchronous Luminescence Spectroscopy

We studied absorption and emission transitions in the visible range through three distinct electronic midgap states in nanocrystalline rutile TiO2 using absorption spectroscopy, “conventional” photoluminescence (PL) spectroscopy, synchronous luminescence spectroscopy for the first time, and complementary methods. We have used free exciton in rutile as an intrinsic midgap state (obtained under the subbandgap photoexcitation at 430 nm) to assign luminescence bands of “green light” (ca. 520 nm) and “green–yellow light” (ca. 560 nm) at 25 °C. Further, we utilized water in the vapor phase as spectroscopic probe to learn about origin of visible emission in rutile. Visible photoluminescence is increased by factor ×20 after water desorption and reversibly decreased (quenched) upon water adsorption in air under ambient conditions. Preferential quenching of “green–yellow light” by adsorbed water occurs versus “green light” as originating from surface vs subsurface oxygen vacancies. Water “adsorption/desorption” was...

Effect of transition metal Fe adsorption on CeO2 (110) surface in the methane activation and oxygen vacancy formation: A density functional theory study

Methane activation and oxygen vacancy formation over transition metal Fe adsorption on CeO2 (110) are studied by using the method of density functional theory (DFT)+U method. A set of model configurations are generated by placing Fe at five surface sites, viz., O-top site, O-bridge site, Ce-bridge site, Ce-top and double oxygen-bridge sites. The study shows that the energetically most favorable configuration is Fe adsorption at the double oxygen-bridge site. Based on the calculated surface, subsurface and the second oxygen vacancies formation energy with (or without) Fe adsorption, it shows that the Fe adsorption is in favor of the surface, subsurface and second oxygen vacancies formation. For the surface and subsurface oxygen vacancy on the Fe/CeO2 (110) surface, the main factor responsible for lowering of Evac is that the adsorption induces structural distortions, whereas, for the second oxygen vacancy, half can be attributed to the large structural relaxation, half can be attributed to the electronic effects. After calculating and discussing about the CH4 activation on CeO2 (110) and Fe/CeO2 (110) surface with (or without) the surface or subsurface oxygen vacancies at the possible adsorption sites, the results show that when the CH4 adsorbed on the Fe/CeO2 (110) with the surface oxygen vacancy at the Ce1 and Ce2 sites, the CH4 decomposed into the CH(ads) and H(ads), its belongs to the chemical absorption, whereas, when the CH4 adsorbed on the other possible sites, the mentioned phenomenon is not occurred, its belongs to the physical absorption. This study reveals the correlation between surface reducibility and catalytic activity for methane oxidation on cerium-based materials, which might be beneficial in developing improved catalysts for methane combustion.

Interplay between Methanol and Anatase TiO2(101) Surface: The Effect of Subsurface Oxygen Vacancy

The dissociation of methanol plays a vital role in many reactions of C1 chemistry. The effect of defects on methanol dissociation is unclear. The adsorption and reactivity of methanol on the anatase TiO2(101) surface with or without a subsurface oxygen vacancy (VOsub) have been studied using first-principles calculations. The adsorbed methanol on the perfect surface prefers the molecular state with a dissociation energy barrier of 0.56 eV. Interestingly, the adsorption and reactivity of methanol are greatly promoted by VOsub. Upon methanol adsorption, the predominant VOsub on the clean surface becomes active, which is comparable to the surface oxygen vacancy (VOsur). The VOsub migrates from the subsurface to the surface layer with a low energy barrier of 0.17 eV, and then the adsorbed methanol molecule easily dissociates through a barrierless pathway facilitated by VOsur. Such results clearly reveal that the VOsub plays a vital role in the dissociation of methanol. These results may provide new insights i...

Effects of Defects on Photocatalytic Activity of Hydrogen-Treated Titanium Oxide Nanobelts

Previous studies have shown that hydrogen treatment leads to the formation of blue to black TiO2, which exhibits photocatalytic activity different from that of white pristine TiO2. However, the underlying mechanism remains poorly understood. Herein, density functional theory is combined with comprehensive analytical approaches such as X-ray absorption near edge structure spectroscopy and transient absorption spectroscopy to gain fundamental understanding of the correlation among the oxygen vacancy, electronic band structure, charge separation, charge carrier lifetime, reactive oxygen species (ROS) generation, and photocatalytic activity. The present work reveals that hydrogen treatment results in chemical reduction of TiO2, inducing surface and subsurface oxygen vacancies, which create shallow and deep sub-band gap Ti(III) states below the conduction band. This leads to a blue color but limited enhancement of visible light photocatalytic activity up to 440 nm at the cost of reduced ultraviolet photocataly...