Single Oxygen Vacancies Research Articles

Theoretical screening of single-atom catalysts (SACs) on Mo2TiC2O2 MXene for methane activation

Producing value-added chemicals and fuels from methane (CH4) under mild conditions efficiently utilizes this cheap and abundant feedstock, promoting economic growth, energy security, and environmental sustainability. However, the first CH bond activation is a significant challenge and requires high energy. Efficient catalysts have been sought for utilizing CH4 at low temperatures including emerging single-atom catalysts (SACs). In this work, we screened fourteen transition metals (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Pt) doped at a single oxygen vacancy in Mo2TiC2O2 (TMSA-Mo2TiC2O2 SACs) for methane activation using density functional theory (DFT) calculations. Our results reveal that methane adsorption is thermodynamically stable on all simulated TMSA-Mo2TiC2O2 SACs, with the adsorption energies (Eads) ranging from −0.92 to −0.40 eV. For the CH activation process, Ru-SAC exhibits the lowest activation barrier (Ea) of 0.22 eV. In summary, Ru-, Rh-, Co-, V-, Cr-, Ti-, and Pt-SACs demonstrate promising catalytic properties for methane activation, with Ea values below 1.0 eV and an exothermic nature. Our findings pave the way for the design and development of novel single-atom catalysts in MXene materials, applicable not only for methane activation but also for other alkane dehydrogenation processes.

Role of intrinsic point defects on electrocatalytic activity of a metal-free two-dimensional Polyaramid for enhanced hydrogen evolution reaction

Despite the current research aims at replacing expensive metals like Platinum with non-noble metals for producing hydrogen via hydrogen evolution reaction (HER), practical challenges like oxidation continue to be a significant concern. Our study aims at providing the metal-free alternative to such noble Pt-based catalysts. Taking care of the fact that the defects are inevitable during the material's growth process, we propose thermodynamically and thermally stable two-dimensional (2D) Polyaramid (2DPA) with possible intrinsic point defects as HER active materials with overpotentials comparable to that of Pt catalysts using density functional theory (DFT). Creating single carbon (VC), single nitrogen (VN), and single oxygen (VO) vacancies helps reducing its wide band gap upon creation of several defect states, thereby improving conductivity. VO-2DPA exhibits lowest ΔGH* value of −0.04 eV which is closest to thermoneutrality condition, i.e., ΔGH*≈0 while VN-2DPA is not found suitable for HER. The calculated low formation energies for generating point-vacancies under defect-poor growth conditions and the minimal bond length fluctuations within, serve as strong validation for the feasibility of their synthesis. Furthermore, underlying reaction mechanisms are also investigated for the HER process favouring the Heyrovsky reaction path over Tafel as the former requires lesser activation energy: 0.53 eV v. 0.86 eV for VC-2DPA and 0.34 eV v. 1.01 eV for VO-2DPA.

Quenching-induced oxygen vacancy engineering boosts photocatalytic activities of CaTiO3

Oxygen vacancies (VOs) play a pivotal role in promoting the photocatalytic activities of perovskite oxides. Herein, we proposed a simple and effective strategy of introducing both surface oxygen vacancies and bulk single electron trapped oxygen vacancies (SETOVs) into CaTiO3 by ethanol quenching. The purchased CaTiO3 was preheated at 800 °C and then the hot CaTiO3 was quenched in ethanol instantly. The ethanol-quenched QE-CaTiO3 delivers much improved photocatalytic activities towards both RhB degradation and hydrogen evolution under visible light irradiation by 6.5 and 65.2 times, respectively, in comparison with the pristine CaTiO3. Both the bulk SETOVs and surface VOs enhance light absorption, and the surface VOs serve as photocatalytic active sites. DFT calculations further reveal the narrowed band gap and accumulated charge density near the Fermi level in QE-CaTiO3, which help the visible light absorption, facilitate charge carriers generation and migration, and promote photocatalytic activities. Our findings provide an efficient and easily scalable approach to engineering oxygen vacancy defects in photocatalysts and other catalysts.

The role of oxygen vacancies in enhancement of photocatalysis and ferromagnetism of (Mn, Co) co-doped ZnO nanoparticles

(Mn, Co) co-doped ZnO nanoparticles (NPs) were synthesized using the sol-gel method. Rietveld refined XRD patterns indicated that the NPs exhibited hexagonal wurtzite structures. Co and Mn ions were successfully introduced into the ZnO NPs at different concentrations. Moreover, Raman peak at 520 cm−1 was related to the defects induced by doping with Mn and Co ions. Fitting the O 1s XPS spectra of the NPs showed that more oxygen vacancies formed with increasing Co and Mn doping. Because there were more oxygen vacancies, photocatalytic dye degradation of rhodamine B enhanced at higher Co and Mn concentrations. Photoluminescence spectra showed that the NPs containing more Mn and Co had more single charged oxygen vacancies. The experimental and theoretical results showed that the exchange interactions between the Mn2+-Co2+ ions mediated by singly charged oxygen vacancies was responsible for room temperature ferromagnetism in the (Mn, Co) co-doped ZnO NPs.

Chemical bonding and electronic properties along Group 13 metal oxides

ContextThe present work provides a systematic theoretical analysis of the nature of the chemical bond in Al2O3, Ga2O3, and In2O3 group 13 cubic crystal structure metal oxides. The influence of the functional in the resulting band gap is assessed. The topological analysis of the electron density provides unambiguous information about the degree of ionicity along the group which is linearly correlated with the band gap values and with the cost of forming a single oxygen vacancy. Overall, this study offers a comprehensive insight into the electronic structure of metal oxides and their interrelations. This will help researchers to harness information effectively, boosting the development of novel metal oxide catalysts or innovative methodologies for their preparation.MethodsPeriodic density functional theory was used to predict the atomic structure of the materials of interest. Structure optimization was carried out using the PBE functional, using a plane wave basis set and the PAW representation of the atomic cores, using the VASP code. Next, the electronic properties were computed by carrying out single point calculations employing PBE, PBE + U functionals using VASP and also with PBE and the hybrid HSE06 functionals using the FHI-AIMS software. For the hybrid HSE06, the impact of the screening parameter, ω, and mixing parameter, α, on the calculated band gap has also been assessed.

Electrocatalytic reduction of CO2 with enhanced C2 liquid products activity by the synergistic effect of Cu single atoms and oxygen vacancies

Electrochemical conversion of CO2 to high energy density multi-carbon liquid phase fuels such as ethanol offers a promising strategy to realize carbon neutrality. However, the selectivity of value-added C2 liquid products is still deemed unsatisfactory currently due to the high overpotential, poor selectivity, and the difficulty of the C–C coupling process. Herein, we report that Cu single atoms (SAs) on hydrogen reduced UIO66-NH2 (named Cu SAs/UIO-H2) achieve C2 liquid products Faraday efficiency (FE) of 58.62% and ethanol FE of 46.28% at a low potential of –0.66 V versus the reversible hydrogen electrode. The ethanol FE of Cu SAs/UIO-H2 is 9.61 times higher than UIO66-NH2. Moreover, the experimental results and theoretical calculations demonstrate that Cu SAs and oxygen vacancies (OVs) synergistically promote the generation of *HCCOH intermediate, thus accelerating the formation of ethanol. This work offers deeper understanding at the atomic scale for designing high-performance electrocatalysts for CO2 conversion to valuable liquid fuels.

Tuning electronic structure of RuO2 by single atom Zn and oxygen vacancies to boost oxygen evolution reaction in acidic medium

The poor stability of RuO2 electrocatalysts has been the primary obstacles for their practical application in polymer electrolyte membrane electrolyzers. To dramatically enhance the durability of RuO2 to construct activity-stability trade-off model is full of significance but challenging. Herein, a single atom Zn stabilized RuO2 with enriched oxygen vacancies (SA Zn-RuO2) is developed as a promising alternative to iridium oxide for acidic oxygen evolution reaction (OER). Compared with commercial RuO2, the enhanced Ru–O bond strength of SA Zn-RuO2 by forming Zn-O-Ru local structure motif is favorable to stabilize surface Ru, while the electrons transferred from Zn single atoms to adjacent Ru atoms protects the Ru active sites from overoxidation. Simultaneously, the optimized surrounding electronic structure of Ru sites in SA Zn-RuO2 decreases the adsorption energies of OER intermediates to reduce the reaction barrier. As a result, the representative SA Zn-RuO2 exhibits a low overpotential of 210 mV to achieve 10 mA cm−2 and a greatly enhanced durability than commercial RuO2. This work provides a promising dual-engineering strategy by coupling single atom doping and vacancy for the tradeoff of high activity and catalytic stability toward acidic OER.

Promoting CO2 and H2O activation on O-vacancy regulated In-Ti dual-sites for enhanced CH4 photo-production

Engineering the specific active sites of photocatalysts for simultaneously promoting CO2 and H2O activation is important to achieve the efficient conversion of CO2 to hydrocarbon with H2O as a proton source under sunlight. Herein, we delicately design the In/TiO2-VO photocatalyst by engineering In single atoms (SAs) and oxygen vacancies (VOs) on porous TiO2. The relation between structure and performance of the photocatalyst is clarified by both experimental and theoretical analyses at the atomic levels. The In/TiO2-VO photocatalyst furnish a high CH4 production rate up to 35.49 μmol g−1 h−1 with a high selectivity of 91.3% under simulated sunlight, while only CO is sluggishly generated on TiO2-VO. The combination of in situ spectroscopic analyses with theoretical calculations reveal that the VO sites accelerate H2O dissociation and increase proton feeding for CO2 reduction. Furthermore, the VO regulated In-Ti dual sites enable the formation of a stable adsorption conformation of In-C-O-Ti intermediate, which is responsible for the highly selective reduction of CO2 to CH4. This work demonstrates a new strategy for the development of effective photocatalysts by coupling metal SA sites with the adjacent metal sites of support to synergistically enhance the activity and selectivity of CO2 photoreduction.

Tunable electronic structure and magnetic characteristics of ZnO monolayer via vacancy defects, and domain/atomic doping

Two-dimensional ZnO monolayers have attracted great attention due to the potential applications in nano-optoelectronic and spintronic devices. However, tuning a sizeable band gap or magnetic properties is a significant challenge to the actual applications. Herein, the effects of monovacancy, divacancy, and voids defects, as well as domain/atomic doping on the electronic properties and magnetic characteristics of ZnO monolayer are investigated by spin-polarized density functional theory. The results demonstrate that single oxygen vacancy could realize the tunablity of bandgap and Zn vacancy could lead the transition between the nonmagnetic state and magnetic state. Remarkably, ZnO monolayers with different divacancies demonstrate magnetic or nonmagnetic characteristic. Zn divacancy produces the magnetic states due to the unpaired O atom, while O divacancy and Zn-O pair vacancy lead no spin polarization due to the occupied states. Void defects also can modify the magnetic properties of ZnO monolayer with the maximum magnetic moment of 1.258 μB. VI-IIA domain doping induces the nonmagnetic states, and widens or narrows the energy gap of ZnO monolayer. Additionally, ZnO monolayers with the doping of the selected atomic Rh and Ru exhibit the magnetic behavior with the total magnetic moment of 2.770 μB, and 3.678 μB, respectively. The total moments are mainly contributed by the dopants and their nearest O atoms. Our study demonstrates that the electronic properties and magnetic properties of ZnO monolayer could be effectively controlled by vacancy, divacancy, and voids as well as domain/atomic doping, being suitable for applications in optoelectronic and spintronic devices.

Single-atom Au atomically trapped in WO3−x catalyst for selective demethoxylation of lignin-derived guaiacols

Utilizing the reducibility of oxygen vacancies and the low hydrogenation of single metal atoms, Au atoms automatically trapped in WO3−x. The resultant catalyst was characterized by many technologies to analyze its structure and morphology and exhibited high activity in the demethoxylation of 4-propylguaiacol, which was attributed to the synergistic effect between single Au atom and oxygen vacancy. The presence of Au enhanced the hydrogen dissociation but its single atom had a low hydrogenation activity, resulting in the total monophenols selectivity and yield of 98.3% and 94.2% at 225 °C, respectively. This Au/WO3−x catalyst also exhibited high stability: both the demethoxylation activity and structure were almost unchanged after 5 cycle runs. This preparation method was suit to fabricate other analogous catalysts such as Pt/WO3−x-R, Pd/WO3−x-R, Ir/WO3−x-R and Ru/WO3−x-R with high demethoxylation activity. In addition, other lignin-derived guaiacols were also successfully converted into the corresponded monophenols on Au/WO3−x catalyst. This study applied single atom catalyst into the demethoxylation of guaiacols, which provided a universal method to fabricate single metal atom trapped in WO3−x catalysts towards upgrading lignin-derived bio-oil into value-added chemicals.

Light Emission from Single Oxygen Vacancies in Cu2O Films Probed with Scanning Tunneling Microscopy.

Global photoluminescence (PL) and spatially resolved scanning tunneling microscopy (STM) luminescence are compared for thick Cu2O films grown on Au(111). While the PL data reveal two peaks at 750 and 850 nm, assigned to radiative electron decays via localized gap states induced by O vacancies, a wide-band emission between 700 and 950 nm is observed in STM luminescence. The latter is compatible with cavity plasmons stimulated by inelastic electron tunneling and contains no spectral signature of the Cu2O defects. The STM luminescence is nonetheless controlled by O vacancies that provide inelastic excitation channels for the cavity plasmons. In fact, the emission yield sharply peaks at 2.2 V sample bias, when tip electrons are resonantly injected into O defect states and recombine with holes at the valence-band top via plasmon stimulation. The spatially confined emission centers detected in photon maps of the Cu2O films are therefore assigned to excitation channels mediated by single or few O vacancies in the oxide matrix.

Multi-peak emission of In2O3 induced by oxygen vacancy aggregation

Oxygen vacancy is crucial to the optical properties in In2O3, however, the single oxygen vacancy model fails to explain the observed multi-peak emission in the experiment. Herein, we have theoretically investigated the diversity of oxygen vacancy distribution, revealing the relationship between the defect configurations and the optical properties. Combining the first-principles calculations and bayesian regularized artificial neural networks, we demonstrate that the structural stability can be remarkably enhanced by multi-oxygen vacancy aggregation, which will evolve with the defect concentration and temperature. Notably, our results indicate that the single oxygen vacancy will induce the emission peaks centered at 1.35 eV, while multi-peak emission near 2.35 eV will be attributed to the distribution of aggregated double oxygen vacancies. Our findings provide a comprehensive understanding of multi-peak emission observed in In2O3, and the rules of the vacancy distribution may be extended for other metal oxides to modulate the optical properties in practice.

Computational studies of Ag5 atomic quantum clusters deposited on anatase and rutile TiO2 surfaces

Aiming at boosting the photocatalytic activities of rutile and anatase TiO2 surfaces, an in-depth investigation of stoichiometric and reduced rutile TiO2 (110) and anatase TiO2 (101) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. In this study, density functional theory (DFT) plus a Hubbard correction (U) is implemented to explore the geometric and electronic properties of such systems. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a fivefold coordinated (Ti5c) atom or a sixfold coordinated (Ti6c) atom, indicating improved surface activity. Depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further by introducing a single oxygen vacancy into the studied surfaces, which means that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficient photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (101) surface, the energy required to form an oxygen vacancy is lower than that of rutile TiO2 (110). Moreover, the adsorption of both bipyramidal and trapezoidal Ag5 AQCs on both TiO2 surfaces generally leads to significant distortion of the clusters, which accounts for the significant reduction in the total energy as compared to the pristine TiO2. This detailed investigation provides insight into new mechanisms for enhancing photocatalytic efficiency of both rutile TiO2 (110) and anatase TiO2 (101) surfaces.

Carrier trapping centers in a two-dimensional silica bilayer: Strongly localized shallow gap states and resonances induced by oxygen vacancies

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_{2}$ bilayer (2D-${\mathrm{SiO}}_{2}$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_{2}$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_{2}$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_{2}$.

Pair vacancy defects in β-Ga2O3 crystal: Ab initio study

Despit many studies dedicated to the defects in β-Ga2O3, information about formation processes of complex “donor-acceptor” defects in β-Ga2O3 and their energetic characteristics is still very scarce. Meanwhile, complex defects, such as pair vacancies, are often indicated as electrically active centers that can play the role of acceptor defects. We have carried out comparative ab initio study of formation energies, as well as optical and thermodynamic transition levels of single and pair vacancies in β-Ga2O. It was confirmed that single gallium and oxygen vacancies are deep acceptors and deep donors, respectively. In this case, the optical transition levels of single gallium and oxygen vacancies are located in such a way that electrons can easily pass from donors to acceptors. Unlike single vacancies, a pair vacancy has a neutral state due to the location of the acceptor levels above the donor ones. However, if pair vacancies were thermally excited, the transition levels are shifted to ∼2.0 eV above the top of the valence band, at which the recombination of electrons and holes become possible, as is observed in the case of single vacancies.

Lowering C−C coupling barriers for efficient electrochemical CO2 reduction to C2H4 by jointly engineering single Bi atoms and oxygen vacancies on CuO

Electrochemical CO2 reduction (ECR) to commodity chemicals offers a promising way for mitigating the greenhouse effect and driving the transition from fossil-fuel dependence to a sustainable economy. To this end, the design and development of active and robust electrocatalysts is key. Here we report for the first time that incorporation of bismuth (Bi) single atoms on defective CuO could significantly enhance the ECR reaction to C2H4 by decreasing C–C coupling barriers. The overpotential for C2H4 production is lowered by at least 50 mV, along with a two-fold improvement in the faradaic efficiency, reaching 60% at 400 mA cm−2. The high performance is maintained even after 20 h of consecutive electrolysis. Control experiments along with density functional theory calculations suggest that the joint incorporation of Bi and oxygen vacancies greatly promotes CO2 adsorption and lowers C–C coupling energy barriers, thereby improving the C2H4 selectivity.

Computational Study of Zn Single-Atom Catalysts on In2O3 Nanomaterials for Direct Synthesis of Acetic Acid from CH4 and CO2

The direct conversion of methane and carbon dioxide will be a promising way to alleviate the greenhouse gas effect and reduce the dependency on traditional fossil fuels. In this study, the catalytic transformation of methane and carbon dioxide to acetic acid with a 100% atomic economy effect over the zinc single-atom catalyst supported on In2O3 nanomaterial with surface oxygen vacancy (Zn1/In2O3–x) was investigated by using density functional theory calculations, including dissociative adsorption of methane, C–C bond coupling, acetate species protonation, and desorption of acetic acid. The results indicate that the Zn single atom is beneficial to the dissociation of methane and the stabilization of methyl by forming the Zn–CH3 species. Besides, carbon dioxide can be adsorbed on the surface of the catalyst and activated further owing to the presence of surface oxygen vacancy. The facile C–C coupling of co-adsorbed methyl and carbon dioxide is realized by the synergistic mode combining the Zn single atom and surface oxygen vacancy following the Langmuir–Hinshelwood mechanism, in which a Zn–C–C moiety is formed in the transition state. The subsequently formed acetate species is bonded with Zn strongly and undergoes a protonation process to form acetic acid which also interacts strongly with the Zn single atom, resulting in desorption of acetic acid being the rate-determining step of the whole reaction. The insights in this work would be useful to clarify the reaction mechanism of producing acetic acid from methane and carbon dioxide over the Zn1/In2O3–x single-atom catalyst, which could provide a sufficient way to design efficient catalysts for methane and carbon dioxide conversion.

Synergy of Pd atoms and oxygen vacancies on In2O3 for methane conversion under visible light

Methane (CH4) oxidation to high value chemicals under mild conditions through photocatalysis is a sustainable and appealing pathway, nevertheless confronting the critical issues regarding both conversion and selectivity. Herein, under visible irradiation (420 nm), the synergy of palladium (Pd) atom cocatalyst and oxygen vacancies (OVs) on In2O3 nanorods enables superior photocatalytic CH4 activation by O2. The optimized catalyst reaches ca. 100 μmol h−1 of C1 oxygenates, with a selectivity of primary products (CH3OH and CH3OOH) up to 82.5%. Mechanism investigation elucidates that such superior photocatalysis is induced by the dedicated function of Pd single atoms and oxygen vacancies on boosting hole and electron transfer, respectively. O2 is proven to be the only oxygen source for CH3OH production, while H2O acts as the promoter for efficient CH4 activation through ·OH production and facilitates product desorption as indicated by DFT modeling. This work thus provides new understandings on simultaneous regulation of both activity and selectivity by the synergy of single atom cocatalysts and oxygen vacancies.

Reversible dehydrogenation and rehydrogenation of cyclohexane and methylcyclohexane by single-site platinum catalyst

Developing highly efficient and reversible hydrogenation-dehydrogenation catalysts shows great promise for hydrogen storage technologies with highly desirable economic and ecological benefits. Herein, we show that reaction sites consisting of single Pt atoms and neighboring oxygen vacancies (VO) can be prepared on CeO2 (Pt1/CeO2) with unique catalytic properties for the reversible dehydrogenation and rehydrogenation of large molecules such as cyclohexane and methylcyclohexane. Specifically, we find that the dehydrogenation rate of cyclohexane and methylcyclohexane on such sites can reach values above 32,000 molH2 molPt−1 h−1, which is 309 times higher than that of conventional supported Pt nanoparticles. Combining of DRIFTS, AP-XPS, EXAFS, and DFT calculations, we show that the Pt1/CeO2 catalyst exhibits a super-synergistic effect between the catalytic Pt atom and its support, involving redox coupling between Pt and Ce ions, enabling adsorption, activation and reaction of large molecules with sufficient versatility to drive abstraction/addition of hydrogen without requiring multiple reaction sites.

Single atom and defect engineering of CuO for efficient electrochemical reduction of CO2 to C2H4

AbstractElectrochemical CO2 transformation to high‐value ethylene (C2H4) at high currents and efficiencies is desired and yet remains a grand challenge. We show for the first time that coupling single Sb atoms and oxygen vacancies of CuO enable synergistic electrocatalytic reduction of CO2 to C2H4 at low overpotentials. Highly dispersed Sb atoms occupying metal substitutional sites of CuO are synthesized under mild conditions. The overall CO2 reduction faradaic efficiency (FE) reaches 89.3 ± 1.1% with an FE toward C2H4 exceeding 58.4% at a high‐current density of 500 mA/cm2. Addition of the p‐block metal is found to induce transformation of CuO from flakes to nanoribbons rich in nanoholes and oxygen vacancies, greatly enhancing CO2 adsorption and activation while suppressing hydrogen evolution. Further density functional theory calculations with in situ X‐ray diffraction reveal that combining Sb sites and oxygen vacancies prominently lessen the dimerization energy of adsorbed CO intermediate, thus boosting the conversion of CO2 to produce C2H4. This study provides a new perspective for promoting selective C–C coupling for electrochemical CO2 reduction.