Incorporation Of Oxygen Vacancies Research Articles

Transition‐Metal‐Doped TiO2 Nanorods with High‐Visible‐Light Response via a Simple Hydrothermal Method

Transition‐metal‐ion doping is an effective method to shorten bandgap and enhance visible‐light absorption of TiO2, but there are still some serious challenges for most doping technologies, such as nonuniform doping, severe lattice damages, metal atoms agglomeration, and inactivated impurities. Herein, a simple and universal hydrothermal method is developed to realize the effective transition‐metal ions doping of TiO2 (including Fe, Co, Mn, Cu, V, and Cr). The photoelectrochemical water splitting performances of Cr‐doped TiO2 nanorods with multifarious Cr ion valence states (Cr3+ or Cr6+ ions), various Cr‐ion concentrations, with or without oxygen vacancy activation are emphatically and systematically studied. Experiments and theoretical calculations demonstrate that the remarkably enhanced carrier separation and injection efficiencies are the key factors to improve the visible‐light photoactivity, benefiting from the homogeneous distribution of impurity atoms and the incorporation of oxygen vacancies. The Cr3+‐ion‐doped TiO2 nanorod arrays with oxygen vacancy activation (Cr3+/Ov–TiO2) displays an impressive incident photon‐to‐electron conversion efficiency of 5.4% at 450 nm visible light, far superior to the pure TiO2 of 0.03%. Furthermore, no obvious photocurrent decay is observed in the Cr3+/Ov–TiO2 photoanode after 12 h continuous light illumination.

A silver and manganese dioxide composite with oxygen vacancies as a high-performance cathode material for aqueous zinc-ion batteries.

Aqueous zinc ion batteries (AZIBs) are regarded as a promising alternative for energy storage due to their safety, cost-effectiveness and environmental friendliness. Manganese dioxide is considered a promising cathode material for energy storage because of its abundant reserves and high energy density. However, its inherent low electronic conductivity and limited cycling performance due to structural instability hinder its further development. Herein, a silver and manganese dioxide composite (Ag@MnO2) enriched with oxygen vacancies was prepared by a simple liquid-phase reduction method. The introduction of silver particles facilitates the improvement of electrical conductivity, and the incorporation of oxygen vacancies helps change the surface properties of manganese dioxide, providing additional active sites for ion transport, enhancing the overall electrochemical kinetics, and further improving the battery performance. As a result, the Ag@MnO2 cathode exhibits an astonishingly high capacity of 353 mAh g-1 at a current density of 0.1 A g-1 and a capacity retention of 78% after 1500 cycles. Additionally, electrochemical and structural analyses have revealed that the Ag@MnO2 cathode undergoes a reversible and stable process of H+ and Zn2+ insertion/extraction.

Efficient Hole Transfer from CdSe Quantum Dots Enabled by Oxygen-Deficient Polyoxovanadate-Alkoxide Clusters.

A limitation of the implementation of cadmium chalcogenide quantum dots (QDs) in charge transfer systems is the efficient removal of photogenerated holes. Rapid hole transfer has typically required the ex situ functionalization of hole acceptors with groups that can coordinate to the surface of the QD. In addition to being synthetically limiting, this strategy also necessitates a competitive binding equilibrium between the hole acceptor and native, solubilizing ligands on the nanocrystal. Here we show that the incorporation of oxygen vacancies into polyoxovanadate-alkoxide clusters improves hole transfer kinetics by promoting surface interactions between the metal oxide assembly and the QD. Investigating the reactivity of oxygen-deficient clusters with phosphonate-capped QDs reveals reversible complexation of the POV-alkoxide with a phosphonate ligand at the nanocrystal surface. These findings reveal a new method of facilitating QD-hole acceptor association that bypasses the restrictions of exchange interactions.

Vacancy effect on the electrocatalytic activity of LaMn1/2Co1/2O3-δ for hydrogen and oxygen evolution reactions.

Development of efficient electrocatalysts for water splitting can be a significant step toward green hydrogen generation. In this work, a remarkable enhancement of electrocatalytic properties is achieved through the incorporation of oxygen-vacancies in a perovskite oxide, while maintaining the same structural framework. The oxygen-deficient material La2MnCoO6-δ (LaMn0.5Co0.5O3-δ) is isostructural to the parent stoichiometric material, but shows drastically enhanced electrocatalytic properties for both half-reactions of water-splitting, namely hydrogen-evolution and oxygen-evolution reactions, due to the oxygen-vacancies.

Sn Dopants with Synergistic Oxygen Vacancies Boost CO2 Electroreduction on CuO Nanosheets to CO at Low Overpotential.

Using the electrochemical CO2 reduction reaction (CO2RR) with Cu-based electrocatalysts to achieve carbon-neutral cycles remains a significant challenge because of its low selectivity and poor stability. Modulating the surface electron distribution by defects engineering or doping can effectively improve CO2RR performance. Herein, we synthesize the electrocatalyst of Vo-CuO(Sn) nanosheets containing oxygen vacancies and Sn dopants for application in CO2RR-to-CO. Density functional theory calculations confirm that the incorporation of oxygen vacancies and Sn atoms substantially reduces the energy barrier for *COOH and *CO intermediate formation, which results in the high efficiency, low overpotential, and superior stability of the CO2RR to CO conversion. This electrocatalyst possesses a high Faraday efficiency (FE) of 99.9% for CO at a low overpotential of 420 mV and a partial current density of up to 35.22 mA cm-2 at -1.03 V versus reversible hydrogen electrode (RHE). The FECO of Vo-CuO(Sn) could retain over 95% within a wide potential area from -0.48 to -0.93 V versus RHE. Moreover, we obtain long-term stability for more than 180 h with only a slight decay in its activity. Therefore, this work provides an effective route for designing environmentally friendly electrocatalysts to improve the selectivity and stability of the CO2RR to CO conversion.

Oxygen vacancies enriched MOF-derived MnO/C hybrids for high-performance aqueous zinc ion battery

Manganese-based oxides are considered as a promising class of zinc storage materials. However, the dissolution problem of manganese-based oxides leads to a sharp decay of its capacity, hindering its further development. Herein, Mn-metal organic framework (Mn-MOF) derived manganese-based oxide hybrids are synthesized and developed to suppress the dissolution of manganese. The incorporation of oxygen vacancies can tune the electronic rearrangement and crystal microstructure via the different calcination atmospheres of air (Mn2O3) and Ar (MnO/C). Furthermore, the oxygen vacancy can effectively improve the conductivity, which promotes the reaction kinetics and electrochemical performance of manganese-based oxide hybrids. In addition, the storage mechanism of manganese-based oxide hybrids with oxygen vacancies on the insertion and de-intercalation of H+/Zn2+ is demonstrated. Oxygen vacancies enriched MnO/C hybrid cathode shows a high specific capacity of 336.8 mAh g−1 (0.1 A g−1) and the reversible capacity retention 73.8 % after 10,000 cycles at 1.0 A g−1, suggesting excellent long-cycle stability. Significantly, the solid-state zinc ions batteries (ZIBs) is assembled and exhibits a reversible capacity of 58.0 mAh g−1 at 1.0 A g−1. These results suggest that oxygen vacancies provide an effective strategy for exploring high-performance MOF-derived cathodes in aqueous batteries.

Fluoride-Assisted Preparation of Plasmonic Oxygen-Deficient MoO3−x Nanowires for Dual-Band Electrochromic Smart Windows

Electrochromic materials are vital to the development of dual-band smart windows, which enable the individual control of visible and near-infrared (NIR) light transmittance. In this paper, we propose a novel single-component MoO3−x nanowire fabricated using a simplified preparation method via a fluoride-assisted route. The incorporation of oxygen vacancies into MoO3−x nanowire in the presence of fluoride anions has not been attempted before. Spectroscopic measurements confirm enhanced ion mobility in the MoO3−x conduction band through the Mo6+ substitution of Mo5+ cations as the origin of localized surface plasmon resonance (LSPR). Oxygen vacancies greatly improve Li+ diffusion in the MoO3−x host while providing near-infrared selective modulation due to tunable LSPR absorbance in the NIR region. The MoO3−x nanowire demonstrates excellent dual-band electrochromic performance in terms of switching speed (12.4 s and 5.4 s for coloration and bleaching between 1.0 V and 3.5 V), coloration efficiency (232.8 cm2·C−1 at 1080 nm and 211.7 cm2·C−1 at 450 nm), and electrochemical stability (91.8% at 1080 nm after 1,000 cycles). This suggests that MoO3−x nanowire with oxygen vacancy is a promising new electrochromic material for dual-band smart windows.

Decorating Pt nanoparticles on oxygen vacancies incorporated hematite photoanode for enhanced water oxidation

Pt-treatment and oxygen vacancies incorporation as effective strategies had been extensively used to improve the photoelectrochemical (PEC) water splitting performance of hematite. Herein, by coupling Pt-decoration and oxygen vacancies incorporation through a facile solution route, an efficient hematite photoanode for PEC water splitting was fabricated. PEC measurements indicated that the photocurrent density was increased from 0.80 mA/cm2 at 1.23 V versus RHE for the pristine Fe2O3 film to 1.15 mA/cm2 at 1.23 V versus RHE for the co-modified sample. Based on various characterizations, this improvement could be attributed to the cumulative effect of Pt-decoration and oxygen vacancies incorporation, in which the incorporation of oxygen vacancies could increase the donor density and consequently promote the charge transport in the bulk hematite, while the excellent electrical conductivity of Pt was beneficial for facilitating the charge transfer across hematite/electrolyte interface. This study provides a feasible approach for the development of high-performance hematite photoanodes.

Metal-organic framework derived α-Fe2O3 nano-octahedron with oxygen vacancies for realizing outstanding energy storage performance

Transition metal oxide, α-Fe2O3, has attracted great interest as a kind of potential battery-type electrode material due to its high theoretical specific capacity. However, α-Fe2O3 still suffers from poor conductivity and large volume expansion during charge and discharge process. Herein, for the first time, the oxygen vacancies-modifying α-Fe2O3 are prepared via a novel two-step calcination of MIL-88-Fe (Fe-MOF) template. Note that the incorporation of oxygen vacancies into α-Fe2O3 nano-octahedron will promote faster charge storage kinetics and enables the α-Fe2O3 structure to be retained during the Li-ions. The oxygen vacancies-modifying α-Fe2O3 nano-octahedron exhibits high specific capacity (250.2 mAh g−1), and an activated carbon (AC)//α-Fe2O3 hybrid supercapacitor is constructed, delivering a high energy density of 63 W h kg−1 at a power density of 900 W kg−1. This finding underlines the importance of incorporating oxygen vacancies into battery-type electrode materials for promoting the practical application.

Oxygen vacancies enable the visible light photoactivity of chromium-implanted TiO2 nanowires

The introduction of oxygen vacancies activates the visible-light photoactivity of chromium-doped TiO 2 by enhancing the efficiency of charge separation and transfer. Although computational studies have demonstrated that metal ion doping can effectively narrow the bandgap of TiO 2 , the visible-light photoactivity of metal-doped TiO 2 photoanodes is still far from satisfactory. Herein, we report an effective strategy to activate the visible-light photoactivity of chromium-implanted TiO 2 via the incorporation of oxygen vacancies. The chromium-doped TiO 2 activated by oxygen vacancies (Cr-TiO 2 -vac) exhibited an incident photon-to-electron conversion efficiency (IPCE) of ~6.8% at 450 nm, which is one of the best values reported for metal-doped TiO 2 . Moreover, Cr-TiO 2 -vac showed no obvious photocurrent decay after 100 h under visible-light illumination.

Oxygen vacancy engineering of Bi2O2CO3 hierarchical microspheres for enhanced adsorption of Cd2+ ions and photocatalytic degradation of Rodamine B

Oxygen vacancies (OVs) can adjust surface and electronic structures of materials and behave as active sites, extend spectral responsive range, and promote the transfer and separation of photogenerated carriers. Therefore surface properties and photocatalytic activities of materials can be enhanced via the incorporation of OVs. However, the influences of OV concentration inside a material on its adsorption ability for heavy metal ions associated with surface and photocatalytic activities under different atmospheres are still unexplored. In this work, in order to obtain highly efficient adsorbents and photocatalysts, rose-like Bi2O2CO3 hierarchical microspheres with rich OVs (Bi2O2CO3-ROVs) and deficient OVs (Bi2O2CO3-DOVs) were synthesized through a facile hydrothermal route and H2O2 oxidation treatment, respectively. Bi2O2CO3-ROVs hierarchical microspheres showed 2.4-times larger adsorption capacity than Bi2O2CO3-DOVs counterparts. The former also showed significantly enhanced photocatalytic activity for Rodamine B (RhB) degradation in aqueous solution under air, N2, and Ar atmospheres than the latter, especially under anaerobic conditions. Our work demonstrated that OV engineering could provide a new strategy for enhancing adsorption capability and photocatalytic activity under both aerobic and anaerobic conditions of materials.

Electrochemically self-doped WO3/TiO2 nanotubes for photocatalytic degradation of volatile organic compounds

In this study, an electrochemically self-doped WO3/TiO2 nanotubes (R-WO3/TNTs) composite film was developed for the photocatalytic degradation of waste gas. The doping of oxygen vacancies (OVs) into the heterojunction of WO3/TNTs was conducted by a simple electrochemical approach, by which the quantity and distribution of OVs on surface as well as bulk were tailored with respect to the applied cathodic potential and the duration of the treatment. With an increase of applied cathodic potential as well as the duration, the quantity of OVs on WO3/TNTs was observed to be consistently raised, while those presented in the surface were raised initially and further showed a plateau trend as the duration extended at a fixed potential. The incorporation of OVs into WO3/TNTs enhanced the charge-transport resistance and reduced the electron-hole recombination, thereby showed an enhanced photocatalytic performance. The R-WO3/TNTs prepared by the electrochemical polarization process at −1.4 V (vs SCE) exhibited a 12 times higher photo-current density and obviously, an enhanced efficiency in the photocatalytic degradation of VOCs with a prolonging photostability compared to that of the pristine WO3/TNTs, under a simulated solar light irradiation. The single-time purification trial demonstrated the effectiveness of the R-WO3/TNTs composite in treating the VOCs for the practical application.

Effect of oxygen stoichiometry on the magnetization profiles and negative magnetization in LSMO thin films

The depth-dependent magnetization in thin film oxygen stoichiometric and oxygen-deficient La0.7Sr0.3MnO3 is investigated by using polarized neutron reflectivity and DC bulk magnetometry. The polarized neutron reflectivity results reveal that the stoichiometric sample shows enhanced interfacial magnetization relative to the rest of the film. The oxygen-deficient sample exhibits a reduced average magnetization from the optimized recipe. Both films show regions of suppressed magnetization at the surface regardless of the growth pressure. The oxygen stoichiometric film does not show an interfacial dead layer, whereas the oxygen-deficient film exhibits a dead layer whose thickness changes with temperature. At a low applied field, we observe striking differences in the depth dependence of the magnetic ordering, with the oxygen-deficient film exhibiting exchange spring behavior, while the stoichiometric film shows a constant magnetization direction across the film. These results suggest that the incorporation of oxygen vacancies during growth leads to an accumulation of vacancies at the interface, which is enhanced at higher temperature due to increased oxygen mobility, creating a region of reduced magnetism that couples to the rest of the film. These results offer insight into the complex behavior and role of oxygen vacancies in the magnetism of these systems. Additionally, the study reveals further details of the negative magnetization in La0.7Sr0.3MnO3 reported in previous studies, which are discussed here.

Role of Oxygen Vacancy in Copper-Cobalt Spinel Nanostructures for Electrochemical Energy-Storage

The incorporation of oxygen vacancies in metal oxides has been reported to remarkably increase their electrochemical performance in water splitting, photoluminescence, and electrochemical energy-storage. The oxygen vacancies are originated by removal of neutral oxygen atoms from the metal oxides, leaving excess electrons in the vacancies. The excess electrons offer high electrical conductivity, huge number of electroactive sites at or within the surface of metal oxides that could facilitate a adsorption/intercalation of anions (e.g., OH-) at or within the electrode surface and diffusion of ions and electrons within the electrode/electrolyte interface for efficient energy-storage. It has been reported that the spinels with the proper oxygen vacancies could exhibit higher capacitance values compared with that for perfect-crystal. The agglomerated nanostructures of the spinels, Cu2Co2O4 are often prepared by conventional solid-phase or solvothermal/hydrothermal methods, which require non-conductive binders (e.g., Nafion and polyvinylidene fluoride (PVDF)) to achieve strong and stable adhesion onto a substrate. In addition, the conventional synthetic method usually requires high-temperature sintering with complicated procedures, which creates morphological deformation and severe aggregation, decreasing the content of electroactive oxygen vacancies. Therefore, the development of a synthetic procedure is one of the most important tasks needed to get effective nanostructures of Cu2Co2O4 with controllable surface morphology, oxygen vacancies/electroactive sites, and electrical conductivity for the application in capacitors as effective electrode materials. In the present report, we directly grew a series of CuxCo3 - xO4 nanostructures with controllable surface morphology, oxygen vacancies/electroactive sites and electrical conductivity on a stainless steel substrate by a simple and straightforward electrochemical deposition process and applied them as electrodes for electrochemical charge-storage.

Ceria Promoted Cu-Ni/SiO2 Catalyst for Selective Hydrodeoxygenation of Vanillin.

A ceria (CeO2) promoted Cu-Ni bimetallic catalyst supported on SiO2 (Cu-Ni/CeO2-SiO2) was prepared and evaluated for catalytic hydrodeoxygenation (HDO) of vanillin. Silica supported monometallic Cu and Ni catalysts and bimetallic Cu-Ni catalyst (Cu/SiO2, Ni/SiO2, and Cu-Ni/SiO2), without a ceria promoter, were also synthesized and tested for the same application. The highest conversion of vanillin was achieved with the Cu-Ni/CeO2-SiO2 catalyst. Vanillyl alcohol was the sole product in the initial 2 h, followed by the formation of 2-methoxy-4-methylphenol, which was observed. Characterization of the synthesized catalysts revealed the presence of overlapping crystalline phases of CuO, NiO, and CeO2 on the Cu-Ni/CeO2-SiO2 surface. We extended our study to find out the results of using CeO2 as the support of the Cu-Ni bimetallic catalyst (Cu-Ni/CeO2). Partial incorporation of Cu and Ni cations into the ceria lattice took place, leading to the decrease of specific surface area and a concomitant compromise in the conversion. In the case of the Cu-Ni/CeO2-SiO2 catalyst, the higher conversion was accredited to the facile formation of Cu+ active centers by the synergistic interaction between Ce+4/Ce+3 and Cu+2/Cu+ redox couples and the incorporation of oxygen vacancies on the catalyst surface.

Oxygen vacancies confined in ultrathin nickel oxide nanosheets for enhanced electrocatalytic methanol oxidation

Owing to the relatively sluggish charge transport and reaction kinetics in nickel-based oxides, the electrocatalytic efficiency for methanol oxidation reaction (MOR) is still far below what is expected. Herein, ultrathin NiO nanosheets with abundant oxygen vacancies is put forward as an applicable catalytic model to get in-depth insights into the role that oxygen vacancies play in MOR process. Theoretical investigations reveal that the incorporation of oxygen vacancies can not only enhance the electrical conductivity of NiO crystals, but also reduce the adsorption energy of methanol molecules onto the active surface, both of which are more favorable for optimizing the overall electrocatalytic MOR performance. Thanks to the ultrathin two-dimensional (2D) features and remarkably modulated electronic structures, ultrathin NiO nanosheets with abundant oxygen vacancies exhibit a dramatically improved electrocatalytic activity for MOR, which is significantly higher than that of the sample with less oxygen vacancies and bulk NiO. This work opens a promising and feasible pathway for the design and synthesis of highly efficient non-noble MOR catalysts.

Effect of oxygen vacancies at the Fe/SrTiO3(001) interface: Schottky barrier and surface electron accumulation layer

We have investigated the interface formation at room temperature between Fe and ${\mathrm{TiO}}_{2}$-terminated ${\mathrm{SrTiO}}_{3}(001)$ surface using x-ray photoelectron spectroscopy. Oxygen vacancies within the ${\mathrm{SrTiO}}_{3}$ lattice in the first planes beneath the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ interface are induced by the Fe deposition. Through a detailed analysis of the Fe $2p$, Sr $3d$, and Ti $2p$ core-level line shapes we propose a quantitative description of the impact of the vacancies on the electronic properties of the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ system. While for an abrupt $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ junction the Schottky barrier height for electrons is expected to be about 1 eV, we find that the presence of oxygen vacancies leads to a much lower barrier height value of 0.05 eV. The deposition of a fraction of Fe monolayer also pushes the surface conduction band edge of the ${\mathrm{SrTiO}}_{3}$ below the Fermi level in favor of the formation of a surface electron accumulation layer. This change in the band bending stems from the incorporation of oxygen vacancies in the near-surface region of ${\mathrm{SrTiO}}_{3}(001)$. We deduce the conduction band profile as well as the carrier density in the accumulation layer as a function of the surface potential by solving the one-dimensional Poisson equation within the modified Thomas-Fermi approximation. Owing to the electric-field dependence of the dielectric permittivity, the ${\mathrm{SrTiO}}_{3}$ with oxygen vacancies at the surface shows original electronic properties. In particular, our simulations reveal that variations of a few percent of the vacancies concentration at the surface can cause changes of several tenths of an eV in the band bending that can lead to important lateral surface inhomogeneities for the potential. We also find through our modeling that the defect states density related to oxygen vacancies at the ${\mathrm{SrTiO}}_{3}$ surface cannot exceed, at room temperature, a critical value of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{13}/\mathrm{c}{\mathrm{m}}^{2}$.

Structural, optical and magnetic properties of Fe-doped CeO2 samples probed using X-ray photoelectron spectroscopy

The present study reports the effect of Fe-doping on the structural, optical, magnetic and electronic properties of polycrystalline CeO2 (for 5 and 10% doping concentration of Fe-cation) samples synthesized by low-temperature solid-state reaction method. Rietveld refinement of the X-ray diffraction patterns establishes fluorite-type face-centred cubic structure of the Fe-doped CeO2 samples and also confirms successful incorporation of Fe ions in the CeO2 lattice. The UV–Vis–NIR absorption spectra displays reduce band gap energy with rising fluency of Fe-ions, which confirm red shifts in the Fe-doped CeO2 samples. The electronic structure of the pure CeO2 and Fe-doped CeO2 polycrystalline samples have been investigated by X-ray photoemission spectroscopy (XPS). The XPS spectra of Ce 3d reveals the reduction of Ce4+ to Ce3+ states Fe-doped CeO2 samples, which are well supported by the Fe 2p and O 1s spectra. Pure polycrystalline CeO2 displays diamagnetic behaviour at room temperature. Interestingly, 5% Fe-doped CeO2 sample displays S-shape hysteresis loop and establishes room temperature ferromagnetism, whereas, 10% Fe-doped CeO2 sample shows weak ferromagnetic behaviour. A decrement is observed in the magnetization on increasing the doping concentration. The possible reason for ferromagnetism in the Fe-doped CeO2 samples may be incorporation of oxygen vacancies, which are further discussed using F-centre exchange mechanism and double exchange interaction. These experimental findings offer potential opportunities for spintronics and optoelectronics applications by integrating them into device structures and evaluating their performance as a function of their material properties.

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes.

The incorporation of oxygen vacancies in anatase TiO2 has been studied as a promising way to accelerate the transport of electrons and Na+ ions, which is important for achieving excellent electrochemical properties for anatase TiO2. However, wittingly introducing oxygen vacancies in anatase TiO2 for sodium-ion anodes by a facile and effective method is still a challenge. In this work, we report an innovative method to introduce oxygen vacancies into the urchin-like N-doped carbon coated anatase TiO2 (NC-DTO) by a facile plasma treatment. The superiorities of the oxygen vacancies combined with the conductive N-doped carbon coating enable the obtained NC-DTO of greatly improved sodium storage performance. When served as the anode for sodium-ion batteries, the NC-DTO electrode shows superior electrochemical performance (capacity: 272 mA h g-1 at 0.25 C, capacity retention: 98.8% after 5000 cycles at 10 C, as well as ultrahigh capacity: 150 mA h g-1 at 15 C). Density functional theory calculations combined with experimental results suggest that considerably improved sodium storage performance of NC-DTO is due to the enhanced electronic conductivity from the N-doped carbon layer as well as narrowed band gap and lowered sodiation energy barrier from the introduction of oxygen vacancies. This work highlights that introducing oxygen vacancies into TiO2 by plasma is a promising method to enhance the electrochemical property of TiO2, which also can be applied to different metal oxides for energy storage devices.

Ni/ZrO2 Catalysts Synthesized via Urea Combustion Method for CO2 Methanation

A series of Ni/ZrO2 catalysts were synthesized by urea combustion method for CO2 methanation. The effects of zirconium precursors and urea dosage on the structure and catalytic performance of the catalysts were tested. Results showed that the Ni/ZrO2–O catalyst derived from zirconium oxynitrate hydrate exhibited better catalytic activity than the Ni/ZrO2 catalyst because of its higher Ni dispersion and smaller Ni particle size. In addition, the urea dosage significantly influenced the low-temperature activity of the catalysts by affecting the metal–support interaction, Ni dispersion, and Ni particle size. The Ni/ZrO2–O-0.4 catalyst with a urea-to-nitrate molar ratio of 0.4 exhibited the best catalytic activity owing to its moderate metal–support interaction, highest Ni dispersion, and smallest Ni particle size, achieving 69.2% CO2 conversion and 100% CH4 selectivity at 300 °C, 0.1 MPa, and a weight hour space velocity (WHSV) of 50,000 mL/(g·h). Moreover, the urea combustion method can lead to the entire phase transformation from monoclinic ZrO2 to tetragonal ZrO2 accompanied by the incorporation of oxygen vacancies in the ZrO2 lattice. This phenomenon can also be related to the high catalytic activity of the as-prepared catalysts.