Subsurface Oxygen Vacancies Research Articles

Humic substances mediated superior photochemical pollutant conversion on defective TiO2 in environmentally relevant matrices: The key roles of oxygen vacancy in surface interactions, oxidant activation and radical generation

Ubiquitous humic substances usually exhibit strong interfering effects on target pollutant removal in advanced water purification. This work aims to develop a photochemical conversion system on the nonstoichiometric TiO2 for pollutant removal in environmentally relevant matrices. In this synergistic reaction system, the redox-reactive humic substances and defective oxygen vacancies can serve as the organic electron transfer mediator and the key surface reactive sites, respectively. This system achieves a superior pollutant degradation in real surface water at low oxidant concentrations. Reactive oxygen vacancies on the TiO2 surface and sub-surface are of considerable interest for this photochemical reaction system. By engineering defective oxygen vacancies on high-energy {001} polar facet, the surface and electronic interactions between tailored TiO2 and humic substances are greatly strengthened for the promoted electron transfer and oxidant activation. Rendered by the strong surface affinity and molecular activation, defective oxygen vacancies thermodynamically and dynamically promote reactive chain reactions for free radical formation, including the selective O2 reduction to ·O2− and the H2O2 activation to ·OH. Our findings take new insights into environmental geochemistry, and provide an effective strategy to in-situ boost the humic substances-mediated water purification without secondary pollution. Environmental implicationHumic substances are widely distributed in aquatic environment, thus playing important roles in environmental geochemistry. For example, humic substances can achieve good surface adsorption through electrostatic adsorption, ligand exchange and electronic interactions with typical TiO2 to form reactive ligand-metal charge transfer complexes for pollutant degradation. Inspired by the unique properties of surface and sub-surface oxygen vacancies, the defective TiO2 was designed to refine the humic substances-mediated photochemical reactions. A superior reactivity was measured for pollutant degradation. Our findings provide an effective strategy to boost naturally photochemical decontamination in environmentally relevant matrices.

Uncovering the Electronic State Interplay at Metal Oxide Electron Transport Layer/Nonfullerene Acceptor Interfaces in Stable Organic Photovoltaic Devices.

The implementation of sputter-deposited TiOx as an electron transport layer in nonfullerene acceptor-based organic photovoltaics has been shown to significantly increase the long-term stability of devices compared to conventional solution-processed ZnO due to a decreased photocatalytic activity of the sputtered TiOx. In this work, we utilize synchrotron-based photoemission and absorption spectroscopies to investigate the interface between the electron transport layer, TiOx prepared by magnetron sputtering, and the nonfullerene acceptor, ITIC, prepared in situ by spray deposition to study the electronic state interplay and defect states at this interface. This is used to unveil the mechanisms behind the decreased photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of the organic solar cell devices. The results have been compared to similar measurements on anatase TiOx since anatase TiOx is known to have a strong photocatalytic activity. We show that the deposition of ITIC on top of the sputter-deposited TiOx results in an oxidation of Ti3+ species in the TiOx and leads to the emergence of a new O 1s peak that can be attributed to the oxygen in ITIC. In addition, increasing the thickness of ITIC on TiOx leads to a shift in the O 1s and C 1s core levels toward higher binding energies, which is consistent with electron transfer at the interface. Resonant photoemission at the Ti L-edge shows that oxygen vacancies in sputtered TiOx lie mostly in the surface region, which contrasts the anatase TiOx where an equal distribution between surface and subsurface oxygen vacancies is observed. Furthermore, it is shown that the subsurface oxygen vacancies in sputtered TiOx are strongly reduced after ITIC deposition, which can reduce the photocatalytic activity of the oxide, while the oxygen vacancies in model anatase TiOx are not affected upon ITIC deposition. This difference can explain the inferior photocatalytic activity of the sputter-deposited TiOx and thus also the increased stability of devices with sputter-deposited TiOx used as an electron transport layer.

Investigation of the Reaction Mechanisms in La1-XSrxCo1-YFeyO3 Electrode by DFT Calculations

Solid Oxide Cells (SOCs) are electrochemical devices working at high temperatures for a clean hydrogen and electricity production in electrolysis and fuel cell modes, respectively. This technology has attracted a growing attention thanks to its high efficiency without the use of expensive catalysts. Although SOCs could play a key role in the energy transition, its commercialization at large scale is still limited due to performance and degradation issues. The state of the art oxygen electrode, La1-xSrxCo1-yFeyO3 (LSCF) exhibits a high electrochemical activity combined with a good mixed ionic and electronic conductivity. However, LSCF is prone to decompose under operation with a Strontium segregation at the surface reducing the cell performance over time [1].To date, the driving force of Sr segregation in LSCF and its impact on the cell performance is not precisely understood. However, to be able to discern these phenomena, it is still needed to better understand at atomic scale the mechanisms occurring at the electrode in the absence of Sr segregation. Therefore, we carried out a first study on the bulk properties of the parent material LSF [3] and LSCF using Density Functional Theory (DFT) calculations. Now we turn our attention to the surface properties of LSCF. In particular, the oxygen reduction reaction is decomposed in fuel cell mode into a sequence of elementary reactions. Thereby, a special focus lies on the identification of the rate determining steps limiting the overall reaction.To shade light into this, we modeled the A-site terminated (100) surface of LSCF using a symmetric slab. Our calculations revealed the existence of peroxide species at the surface arising from the reductive adsorption of a dioxygen molecule into a surface oxygen vacancy. Incorporation and dissociation of the peroxide requires a surface oxygen vacancy or sub-surface oxygen vacancy. To make this reaction happen, the peroxide and the vacancy have to encounter each other. Thereby it is still unclear weather the oxygen vacancy diffuses towards the peroxide, or vice-versa. Due to the particular orientation of the peroxide, which does not stand perpendicularly out of the (100) plane, but leans towards the direction between two A-site surface ions (cf. Fig. 1), four possible orientations of the peroxide exist. Thus, on-site rotations of the peroxide and its hopping to nearest neighbor sites are considered, as well as the diffusion of oxygen vacancies on surface and sub-surface. The Nudged Elastic Band (NEB) method is used to calculate the activation barriers. Diffusion coefficients are then determined using Transition State Theory (TST) combined with Kinetic Monte Carlo (KMC) calculations. These diffusion coefficients will be used as inputs to improve the existing elementary kinetic models [3] in order to investigate more precisely the performance and durability of SOCs under working conditions.[1] Chen, K., Jiang, S.P. Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties. Electrochem. Energ. Rev. 3, 730–765 (2020). https://doi.org/10.1007/s41918-020-00078-z[2] Hartmann, C., Geneste, G., Laurencin, J., Hole polarons in LaFeO3 and La1 − x Sr x FeO3 − δ : Stability, trapping, mobility, effect of Sr concentration, and oxygen vacancies. Accepted in Phys. Rev. B. [3] E. Effori, J. Laurencin, E. D. R. Silva, M. Hubert, T. David, M. Petitjean, G. Geneste, L. Dessemond, and E. Siebert, J.Electrochem. Soc. 168, 044520 (2021). Figure 1

Destruction and preservation of nonstoichiometric ZnCr oxide catalyst from machine-learning simulation

Structural destruction, for example via the solid disproportionation reaction (SDR), is one of the main causes of catalyst deactivation that often lacks atomic level understanding due to the strong coupling of reaction conditions and the long-time scale of the structural evolution. Here by using machine-learning-based atomic simulations we are able to clarify the detailed deactivation mechanism of the nonstoichiometric zinc-chromium oxide (ZnCrxOy) catalyst, an important industrial catalyst for syngas conversion. We show that the reductive reaction conditions first lead to the generation of surface and subsurface oxygen vacancies, and the subsequent cation migration via the replacement of subsurface Zn by surface Cr kills the catalyst activity by diminishing the active surface planer [CrO4] sites. The rate-determining step, the Zn/Cr cation migration, is exothermic towards forming stoichiometric ZnCr2O4, and has an accessible barrier (∼1.7 eV) under syngas conversion reaction conditions, indicating that the long-term deactivation is inevitable. By screening a set of additive elements, we demonstrate that Al element is an effective inhibitor to impede the formation of subsurface oxygen vacancy, the initial step of SDR. Our findings demonstrate the power of machine-learning-based atomic simulations in elucidating complex structural transformations and provide directions to mitigate ZnCrxOy catalyst deactivation.

Stoichiometric and reduced ceria surfaces: Atomic structure, energetics and electron localization

We examine the (111), (110) and (100) surfaces of the stoichiometric and reduced ceria (CeO2) using density functional theory (DFT) within LSDA + U. we explore the formation mechanism of oxygen vacancy sites on CeO2 (111), (110) and (100) surfaces and their stability near the ceria surface regions. Local ordering surface defects and excess electron localization were reported. Both surface and subsurface oxygen vacancies induce the electron localization of reduced CeO2 on all these three terminations of the surface, leading to the appearance of Ce3+ sites. In the case of CeO2 (111) surface, oxygen vacancy at the surface and subsurface forms Ce3+ at next nearest neighbor to the vacancy. This is consistent with the experimental finding that the ratios of these two types of oxygen vacancy are nearly the same. In the case of CeO2 (100) and (110) surfaces, Ce3+ is formed at the sites nearest to the oxygen vacancy sites. The reduced ceria expects to catalyze the dissociation of molecules on it surface. The dissociation is assisted by the oxidation of Ce3+ that generated on reduced ceria surfaces.

Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub-Ov) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub-Ov were prepared through precisely regulated spin-coating and calcination. Etching X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated Ov located at sub ~2-5 nm region. Mott-Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de-pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm-2 at 1.23 V vs. RHE were achieved on BiVO4, Bi2O3, TiO2 with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Subsurface Engineering Induced Fermi Level De‐pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting

AbstractPhotoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub‐Ov) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub‐Ovwere prepared through precisely regulated spin‐coating and calcination. Etching X‐ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated Ovlocated at sub ∼2–5 nm region. Mott–Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de‐pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm−2at 1.23 V vs. RHE were achieved on BiVO4, Bi2O3, TiO2with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

SnO2 grains with abundant surface oxygen vacancies for the Ultra-sensitive detection of NO2 at low temperature

N-type metal oxide semiconductors are widely used in the field of gas sensors, but achieving high sensor response at low temperatures with low concentrations is challenging. In this paper, we report a high-performance gas sensor for NO2. SnO2 grains with different sizes and oxygen vacancies were prepared by a simple hydrothermal method followed by high-temperature calcination, and the electronic and chemical properties of the SnO2 grains were significantly changed. The results demonstrate that the sensor prepared from SnO2 grains hydrothermally grown at 180 °C has the optimal gas sensing performance with a response up to 4358.8 for 1 ppm NO2 at a low temperature of 80 °C, a rapid response recovery (7 s), and a detection limit down to 9 ppb. Together with some series characterization, an empirical factor F between the sensor response and the influencing factors is proposed. In addition, through experimental and density generalization studies, the oxygen vacancies are distinguished into surface oxygen vacancies and subsurface oxygen vacancies. The dominance of the surface oxygen vacancies in dictating the gas response is demonstrated. The excellent sensor characteristics demonstrated and the elucidation of the gas-sensitive mechanism provide a clear framework for the preparation of high-performance gas sensors.

Re-examining the role of subsurface oxygen vacancies in the dissociation of H2O molecules on anatase TiO2

Titanium (IV) oxide is a reference photocatalytic material for a number of chemical processes of interest in the energy transition, including the reduction of water into hydrogen gas. Density functional theory calculations have previously revealed that the decomposition into hydrogen and hydroxyl ions could be catalyzed by a subsurface oxygen vacancy (VO). However, these theoretical investigations have been carried out with models considering only isolated water molecules. We investigate the role of the subsurface VO at different levels of coverage up to full monolayer (ML) coverage. We observe that migration of the vacancy concomitantly to water decomposition is mandatory to obtain an exothermic pathway. Furthermore, the migration of the VO only occurs in the presence of an under-coordinated surface oxygen atom. As coverage and decomposition increase, surface oxygen atoms are saturated which inhibit further migration of the subsurface VO; hence the VO shows no catalytic effect on the splitting of water into hydrogen and hydroxyl ions at full water coverage.

Interplay between H2S and Anatase TiO2(101) Surface: The Effect of Subsurface Oxygen Vacancy

Interplay between H₂S and Anatase TiO₂(101) Surface: The Effect of Subsurface Oxygen Vacancy

Combining reduction and oxidation pathways leads to efficient H2O2 production

In this issue of Chem Catalysis , Sun and colleagues demonstrate that stable, subsurface oxygen vacancies in titanium dioxide yield high H 2 O 2 selectivity in both the oxygen reduction reaction and water oxidation reaction. Coupling these two processes in an electrolyzer achieves a 134% faradic efficiency at a high production rate. In this issue of Chem Catalysis , Sun and colleagues demonstrate that stable, subsurface oxygen vacancies in titanium dioxide yield high H 2 O 2 selectivity in both the oxygen reduction reaction and water oxidation reaction. Coupling these two processes in an electrolyzer achieves a 134% faradic efficiency at a high production rate.

Elucidating the mechanism of the reverse water–gas shift reaction over Au/CeO2 catalysts using operando and transient spectroscopies

The reaction mechanism of the reverse water–gas shift reaction (rWGSR) over Au/ceria catalysts was investigated by monitoring the catalyst dynamics and reaction intermediates using operando and transient spectroscopies, as well as by DFT calculations. Combined operando Raman and UV-Vis spectroscopic analysis allows establishing a correlation between subsurface oxygen vacancies and catalytic activity. Comparison of different ceria support materials, i.e., polyhedra and polycrystalline sheets, reveals that the defect concentration is not rate-determinant. Using transient DRIFTS, we are able to identify individual steps of hydrogen dissociation on supported gold and to gain detailed insight into the reduction of CO2 via formate and carbonate formation. It is demonstrated that CO2 reduction is influenced by the surface pretreatment. Considering all spectroscopic findings, we propose an associative mechanism via carbonate and formate intermediates as the main route for the rWGSR over Au/ceria(111) catalysts, while a redox mechanism plays only a minor role.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2 surfaces

To study the dependence of the relative stability of surface (V A ) and subsurface (VB ) oxygen vacancies with the crystal facet of CeO2, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce3+ polarons, are facet- and concentration-dependent. At the higher vacancy concentration, the V A is more stable than the V B at the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both the V A and the VB have similar stability. The stability of the V A vacancies, compared to that of the V B , is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce3+−Ce3+ interactions.

D-Glucose Adsorption on the TiO2 Anatase (100) Surface: A Direct Comparison Between Cluster-Based and Periodic Approaches.

Titanium dioxide (TiO2) has been extensively studied as a suitable material for a wide range of fields including catalysis and sensing. For example, TiO2-based nanoparticles are active in the catalytic conversion of glucose into value-added chemicals, while the good biocompatibility of titania allows for its application in innovative biosensing devices for glucose detection. A key process for efficient and selective biosensors and catalysts is the interaction and binding mode between the analyte and the sensor/catalyst surface. The relevant features regard both the molecular recognition event and its effects on the nanoparticle electronic structure. In this work, we address both these features by combining two first-principles methods based on periodic boundary conditions and cluster approaches (CAs). While the former allows for the investigation of extended materials and surfaces, CAs focus only on a local region of the surface but allow for using hybrid functionals with low computational cost, leading to a highly accurate description of electronic properties. Moreover, the CA is suitable for the study of reaction mechanisms and charged systems, which can be cumbersome with PBC. Here, a direct and detailed comparison of the two computational methodologies is applied for the investigation of d-glucose on the TiO2 (100) anatase surface. As an alternative to the commonly used PBC calculations, the CA is successfully exploited to characterize the formation of surface and subsurface oxygen vacancies and to determine their decisive role in d-glucose adsorption. The results of such direct comparison allow for the selection of an efficient, finite-size structural model that is suitable for future investigations of biosensor electrocatalytic processes and biomass conversion catalysis.

Chemical Modification of Polaronic States in Anatase TiO2(101).

Two polymorphs of TiO2, anatase and rutile, are employed in photocatalytic applications. It is broadly accepted that anatase is the more catalytically active and subsequently finds wider commercial use. In this work, we focus on the Ti3+ polaronic states of anatase TiO2(101), which lie at ∼1.0 eV binding energy and are known to increase catalytic performance. Using UV-photoemission and two-photon photoemission spectroscopies, we demonstrate the capability to tune the excited state resonance of polarons by controlling the chemical environment. Anatase TiO2(101) contains subsurface polarons which undergo sub-band-gap photoexcitation to states ∼2.0 eV above the Fermi level. Formic acid adsorption dramatically influences the polaronic states, increasing the binding energy by ∼0.3 eV. Moreover, the photoexcitation oscillator strength changes significantly, resonating with states ∼3.0 eV above the Fermi level. We show that this behavior is likely due to the surface migration of subsurface oxygen vacancies.

Electrodeposition synthesis of 3D-NiO1-δ flowers grown on Ni foam monolithic catalysts for efficient catalytic ozonation of VOCs

NiO monolithic catalysts were synthesized by electrodeposition without additional Ni sources for the first time. The formation mechanism and morphology control of NiO grown on nickel foam were investigated. Among the different samples, fully developed 3D-NiO1-δ flowers anchored on nickel foam (3D-NiO1-δ/NF) presented excellent catalytic performance for ozonation of toluene (100% removal efficiency within 450 min, 9 times the breakthrough time of NiO film/NFs) due to its high specific surface area, rich oxygen vacancies (OVs), good redox ability and abundant empty space enclosed by the petals. The in-situ DRIFTS, ESR and DFT calculation results demonstrate that the bridge of two Ni around the subsurface OVs is the most active site, and the roles of OVs are to promote the adsorption and decomposition of ozone into surface O, •O2−, and •OH by chain reactions. This work shed a light on the design and preparation of monolithic catalysts for ozonation of VOCs.

Predominance of Subsurface and Bulk Oxygen Vacancies in Reduced Manganese Oxide

Oxygen vacancies (Vos) play a vital role in the physical and chemical properties of oxide materials and their applications. Because Vos are first produced on oxide surfaces, it has been often recognized that they are richer at the top surface than at the subsurface or bulk region. Here the depth distribution of Vos in reduced manganese oxide (MnOx) films has been studied by a combination of ion-sputtering treatment and variable-angle X-ray photoelectron spectroscopy measurement. The density of Vos on the top surface is determined to be ∼8% lower than that in the subsurface region after reduction treatment of the oxide in CO at 573 K. Density functional theory calculations show that lattice oxygen atoms in the bulk tend to migrate and fill into the Vos on the top surface with a barrier of 0.94 eV at most. The stability of Vos in the bulk is higher than that on the surface via a reduced Mn3O4 skeleton, which can be reflected by the energy of oxygen filling at the Vo.

Characterisation of oxygen defects and nitrogen impurities in TiO2 photocatalysts using variable-temperature X-ray powder diffraction

TiO2-based powder materials have been widely studied as efficient photocatalysts for water splitting due to their low cost, photo-responsivity, earthly abundance, chemical and thermal stability, etc. In particular, the recent breakthrough of nitrogen-doped TiO2, which enhances the presence of structural defects and dopant impurities at elevated temperatures, exhibits an impressive visible-light absorption for photocatalytic activity. Although their electronic and optical properties have been extensively studied, the structure-activity relationship and photocatalytic mechanism remain ambiguous. Herein, we report an in-depth structural study of rutile, anatase and mixed phases (commercial P25) with and without nitrogen-doping by variable-temperature synchrotron X-ray powder diffraction. We report that an unusual anisotropic thermal expansion of the anatase phase can reveal the intimate relationship between sub-surface oxygen vacancies, nitrogen-doping level and photocatalytic activity. For highly doped anatase, a new cubic titanium oxynitride phase is also identified which provides important information on the fundamental shift in absorption wavelength, leading to excellent photocatalysis using visible light.

Theoretical analysis of the adsorption of phosphoric acid and model phosphate monoesters on CeO2(111)

Ceria has been shown to catalyze the dephosphorylation of phosphate monoesters at ambient temperature. As a step toward understanding the reaction mechanism on ceria, first-principles density functional theory calculations in the GGA-PW91+U approach have been carried out to study the adsorption of phosphoric acid and two model phosphate monoesters, methyl phosphate and para-nitrophenyl phosphate, on CeO2(111). Consistent patterns are found in terms of energetic, structural, electronic, and vibrational properties upon adsorption. For the parent molecules, bonding of the phosphorous atom directly to a surface lattice oxygen atom (Olatt) is the only stable molecular adsorption state. Common entities that reduce Ce4+ to Ce3+ (e.g., co-adsorbed H atom, surface and subsurface oxygen vacancy) mildly perturb the P-Olatt bond, but Ce3+ by itself has only a negligible effect. Deprotonation is exothermic for all three phosphate species on CeO2(111), much more so if they are adsorbed in an oxygen vacancy via a phosphoryl O than on a stoichiometric surface. Infrared spectra have been simulated for the adsorbed states of the model phosphates for comparison with future experiments.

Simple Ethanol Refluxing Method for Production of Blue-Colored Titanium Dioxide with Oxygen Vacancies and Visible Light-Driven Photocatalytic Properties

We show that a simple ethanol (EtOH) refluxing treatment at mild temperature (120 °C) allows producing blue-colored and reduced titanium dioxide (TiO2–x) exhibiting improved visible-light (VIS) photocatalytic properties. The treatment causes an increase in the density of Ti(III) species and the appearance of two optical absorption features: a broad absorption band—responsible for the blue coloration—extending from the green region (∼2.3 eV) up to the near-infrared and a subgap absorption tail close to the band gap energy. The experimental results combined with a computation of the density of states via hybrid Hartree–Fock density functional support the hypothesis that the EtOH reflux treatment leads to formation of surface and subsurface oxygen (O) vacancies. We also show that the excitation-resolved photoluminescence technique allows a high-contrast detection of a subgap optical excitation band peaked at about 430 nm (∼2.9 eV), associated with anatase photoluminescence, whose intensity increases after the EtOH reflux treatment. This result gives a very direct support to the debated hypothesis identifying O vacancy states as the energy levels involved in the radiative transition of anatase TiO2. Improved photocatalytic degradation by the processed TiO2 under VIS illumination is demonstrated, and the possible mechanism involved in the formation of surface O vacancies is discussed. The method outlines a very simple, low-cost, and fast procedure to target the formation of O vacancies in the TiO2 surface region.