Oxygen Vacancy Engineering via High-Ionization-Potential Doping for Neuromorphic Visual Reservoir Computing

Oxygen vacancies in transition-metal oxides can facilitate photocatalysis activity critical for environmental remediation and solar energy conversion. Investigation of photocatalytic activity of TiO2 with different amounts of the oxygen vacancies can provide fundamental insights into the oxygen-vacancy-dependent reaction mechanisms. Here, the oxygen vacancies in TiO2 nanoparticles were investigated by the combination of transmission electron microscopy with electron energy loss spectra, x-ray near-edge structures, and x-ray absorption techniques. The results demonstrate that the oxygen vacancies mainly exist at the surface of TiO2 and the crystalline structure of TiO2 almost keeps no change during annealing under the different oxygen pressures at 500 °C. The exact quantity and distribution of the oxygen vacancies in TiO2 nanoparticles were obtained. The surface oxygen vacancies on TiO2 significantly enhance photocatalytic activity for Rhodamine B degradation and hydrogen generation. The formation of oxygen vacancies at the TiO2 surfaces is responsible for engineering the band gap of TiO2 and tailoring their electronic structures, and promoting light absorption and enhancement photocatalysis. This work could provide new insights into the understanding of the photocatalytic activity enhancement by the surface oxygen vacancies.

Oxygen vacancy concentration in nanoobjects of CeO2−δ: Effects of characteristic size, morphology, and temperature

Oxygen vacancy distributions and dynamics directly control the operation of solid-oxide fuel cells and are intrinsically coupled with magnetic, electronic and transport properties of oxides. For understanding the atomistic mechanisms involved during operation of the cell it is highly desirable to know the distribution of vacancies on the unit-cell scale. Here, we develop an approach for direct mapping of oxygen vacancy concentrations based on local lattice parameter measurements by scanning transmission electron microscopy. The concept of chemical expansivity is demonstrated to be applicable on the subunit-cell level: local stoichiometry variations produce local lattice expansion that can be quantified. This approach was successfully applied to lanthanum strontium cobaltite thin films epitaxially grown on substrates of different symmetry, where polarized neutron reflectometry revealed a strong difference in magnetic properties. The different vacancy content found in the two films suggests the change in oxygen chemical potential as a source of distinct magnetic properties, opening pathways for structural tuning of the vacancy concentrations and their gradients.

The crystal structure and magnetic and electric transport properties of polycrystalline La0.50D0.50MnO3−γmanganites (D=Ca, Sr) were studied experimentally depending on the concentration of oxygen vacancies. The La0.50Sr0.50MnO3−γ system of anion-deficient compositions was found to be stable and possess a perovskite structure only up to the γ=0.25 concentration of oxygen vacancies, whereas, for the La0.50Ca0.50MnO3−γ system, we were able to obtain samples with the concentrations of oxygen vacancies up to γ=0.50. The stoichiometric La0.50D0.50MnO3 (D=Ca, Sr) compositions had O-orthorhombic (Ca) and tetragonal (Sr) unit cells. The unit cell of the anion-deficient La0.50Sr0.50MnO3−γ manganites also became O-orthorhombic when the concentration of oxygen vacancies increased γτ;0.16). Oxygen deficiency in La0.50Sr0.50MnO3−γ first caused the transition from the antiferromagnetic to the ferromagnetic state γ∼0.06) and then to the spin glass state γ∼0.16). Supposedly, the oxygen vacancies in the reduced La0.50Sr0.50MnO3− γ samples with γ≥0.16 were disordered. The special feature of the La0.50Ca0.50MnO3−γ manganites was a nonuniform distribution of oxygen vacancies in the La0.50Ca0.50MnO2.75 phase. In the La0.50Ca0.50MnO2.50 phase, the type of oxygen vacancy ordering corresponded to that in Sr2Fe2O5, which led to antiferromagnetic ordering. The specific electric resistance of the La0.50D0.50MnO3−γ anion-deficient samples increased with increasing oxygen deficiency. The magnetoresistance of all samples gradually increased as a result of the transition to the magnetically ordered state. Supposedly, the La0.50Ca0.50MnO3−γ manganites in the range of oxygen vacancy concentrations 0.09≤γ≤0.50 had a mixed state and contained microdomains with different types of magnetic ordering. The experimentally observed properties can be interpreted based on the model of phase layering and the model of superexchange magnetic ordering.

Relationship between defect and strain in oxygen vacancy-engineered TiO2 towards photocatalytic H2 generation

Oxygen vacancy concentration in metal oxides is crucial to gas sensing performance. This study introduces a novel strategy of oxygen vacancies modulation on CoFe 2 O 4 , by two‐step controllable incorporation of TiO 2 @MXene and carbon. In the first step, MXene simultaneously acts as an incorporation agent and a titanium source to grow TiO 2 and regulate the oxygen vacancy concentration in CoFe 2 O 4 . The fabricated n‐n heterojunctions of TiO 2 /CoFe 2 O 4 further induced a gradient distribution of oxygen vacancies through energy band bending. And a second step, combining glucose as a carbon source, further increases the oxygen vacancy concentration in CoFe 2 O 4 by chemical reduction during a hydrothermal process, leading to the formation of a CoFe 2 O 4 ‐TiO 2 @MXene‐C nanostructured composite. Through controlling the glucose content, the ratio of Co 3+ to Co 2+ within the composite can be sequentially adjusted, which allows for the further regulation of oxygen vacancy concentration on the composite surface. The effectiveness of this two‐step incorporation is demonstrated through enhanced acetone sensing performance, providing valuable insights into the fabrication of high‐performance metal oxide‐based gas sensors via controlled oxygen vacancy modulation.

Effect of oxygen defects blocking barriers on gadolinium doped ceria (GDC) electro-chemo-mechanical properties

Non-volatile resistance change under electric stimulation in oxides is a promising path to next generation memory devices. However, the underlying mechanisms are still not fully understood. We report here on the study of switching in Pr0.67Ca0.33MnO3-δ (PCMO) films sandwiched by noble metal Pt electrodes, where electrode oxidation can be excluded. In order to develop an understanding of the switching induced oxygen migration, its initial concentration is modified by post-annealing of the deposited PCMO films. The oxygen distribution is obtained by manganese valence determination using spatially resolved electron energy loss spectroscopy in scanning transmission electron microscopy mode. We observe correlations between virgin state resistance, resistive switching properties, oxygen vacancy distribution, and stress/strain state of the PCMO films and propose a simplified interface resistance model based on the measured valence distribution. It assumes a linear correlation of oxygen vacancy concentration with conductivity and a metal to insulator transition above a critical vacancy concentration threshold. Our results suggest that resistance changes can take place at both interfaces of symmetric devices and only requires small changes in oxygen vacancy concentration.

High-entropy perovskite oxides (HEPOs) have emerged as a promising class of materials for use in solid oxide electrolyzer cells (SOECs), particularly as oxygen electrodes, due to their excellent thermal and chemical stability. The concentration of oxygen vacancies in HEPOs directly impacts critical properties such as ionic conductivity, thermal expansion, and overall electrochemical behavior. Despite their importance, predicting how the oxygen vacancy concentration in HEPOs varies with five or more metal cation dopants and temperature remains elusive, as the net energy needed to remove a lattice oxygen is significantly complicated by variations from different metal cations and configurational entropy.In this work, we investigate how oxygen vacancy concentrations are affected by low-entropy (LE, 2–3 cations) and high-entropy (HE, 5+ cations) A-site compositions in perovskite oxides. We synthesized 14 compositions, inspired by the state-of-the-art SOEC material lanthanum strontium cobalt ferrite (LSCF), using solution combustion synthesis and measured changes in oxygen vacancy concentration between 500–1000°C via thermogravimetric analysis. Using established defect models, we estimated and compared the oxygen vacancy formation enthalpy (ΔH) and entropy (ΔS).Our results confirm the expected role of divalent A-site cations in facilitating oxygen vacancy formation. However, substantial variation was observed among HE samples with identical divalent cation concentrations, suggesting additional influencing factors. We identify the variance in A-site ionic radii as a key compositional descriptor that strongly correlates with oxygen vacancy thermodynamics. HE samples with larger A-site size variance consistently exhibited lower ΔH and ΔS of oxygen vacancy formation, as well as more linear oxygen vacancy growth with temperature compared to their LE counterparts.To investigate the origin of this behavior, we conducted high-throughput atomistic simulations using a machine-learned universal interatomic potential. We computed the energy required to form an oxygen vacancy across hundreds of oxygen sites in large supercells (1280 atoms) for each material. While the average vacancy formation energies were comparable between LE and HE samples, HE compositions exhibited a broader distribution of vacancy formation energies due to the local strain induced by size mismatches between A-site cations.We developed a statistical thermodynamic model to analyze how variations in formation energy impact a material's equilibrium oxygen vacancy concentration. By treating oxygen sites as a distribution of energy states, we derived analytical expressions for formation ΔH and ΔS as a function of vacancy energy variance. The results show that broadening the vacancy energy distribution leads to a decrease in both formation enthalpy and configurational entropy, consistent with experimental data. Notably, the model predicts a scaling relation between ΔHf and ΔSf, explaining the experimentally observed correlation between LE and HE samples.In summary, our combined experimental, computational, and theoretical approach reveals that A-site cation disorder—quantified by size variance—plays a crucial role in determining the thermodynamics of oxygen vacancy formation in HEPOs. This disorder induces variation in oxygen vacancy formation energies, which lowers the formation enthalpy (ΔH) and entropy (ΔS), leading to greater oxygen vacancy populations at lower temperatures. The findings establish A-site size variance as a practical and predictive design parameter for tuning defect behavior in complex oxides. This framework advances our understanding of defect chemistry in disordered systems and provides guidance for engineering functional materials for SOECs, catalysis, and beyond. Figure 1

We report on the analysis of oxygen vacancies (OVs) content and ordering of Sr2-x Ca x Fe2O6-δ perovskites and explain how OVs change the electrical conductivity and oxygen evolution catalytic activity of these compounds. The structure and OV content are tuned by controlling the A-site composition and the reaction atmosphere. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) are used to identify the crystal structures and to quantify bulk oxygen vacancy contents. Our analysis shows that OV content and crystal structure govern the electronic transport properties of the Sr2-x Ca x Fe2O6-δ system. The electrical conductivity of oxygen-deficient perovskites (ODPs) is significantly higher than those in brownmillerites (by at least 2 orders of magnitude). Seebeck coefficient measurements identified that the Sr-rich ODPs and brownmillerites are p-type semiconductors, while Ca-rich brownmillerites are either insulators (within the experimental temperature range) or p-type semiconductors at lower temperatures (<750 K). Electrical conductivity of p-type semiconductors (Sr-rich compounds) reduces with higher OV content, and in brownmillerites with x ≥ 1.25, a transition to n-type semiconductor is observed at temperatures above 750 K. Our analysis shows that the hole and electron concentrations are similar in these brownmillerites, indicating major contributions from ionic transport. Finally, we show how oxygen deficiency alters the electrical conductivity and catalytic activity of the Sr2-x Ca x Fe2O6-δ system in noncomplementary ways.

Wide band gap heterostructures can be accurately fabricated at room temperature to exhibit remarkable electrical properties that facilitate the transfer of electrons across the heterojunction while blocking the transfer of holes. The present investigation focuses on engineering the electronic structure of a TiO2 overlayer on a p-type doped Si(100) substrate by controlling the concentration of TiO2 oxygen vacancies. TiO2 films are deposited on p-Si(100) in an ultrahigh vacuum setup by evaporation of Ti atoms at a constant rate in a variable O2 atmosphere. The concentration of oxygen vacancies and consequently the degree of n-type doping of TiO2 is tuned by controlling the oxygen background pressure during the TiO2 formation. To investigate the electronic structure and the concentration of defects in the TiO2 layer as well as to characterize the TiO2/p-Si(100) band alignment we used photoelectron spectroscopy employing femtosecond extreme ultraviolet laser pulses produced via high harmonic generation. Furth...

Oxygen vacancy engineering in Fe doped akhtenskite-type MnO2 for low-temperature toluene oxidation

Dual role of oxygen vacancies in TiO2/Bi2O3 for enhanced carrier separation and PDS activation: A combined experimental and DFT study.

Oxygen vacancy engineering in metal oxide-based semiconductors has emerged as an important area of research for sensing applications, such as Surface-enhanced Raman scattering (SERS), gas sensing, etc. It has the potential to replace high-cost and unstable noble metal-based substrates in the near future. However, improving the SERS enhancement factor in semiconductor-based substrates remains a challenge. In the present study, we demonstrate that oxygen vacancy engineering in Niobium pentoxide (Nb2O5) enables ultrahigh SERS sensitivity. Oxygen vacancies were induced and manipulated in the Nb2O5 nanoparticles via a facile high-energy ball milling method and post-growth oxygen annealing. A high enhancement factor (EF) of 5.15 × 107 was obtained for the Methylene Blue (MeB) molecule on the oxygen-deficient substrate with the lowest detection limit of 10-8 M, which is 2 orders of magnitude lower than the pristine substrate. Through a careful analysis of the experimental data and theoretical calculations, we investigated the underlying mechanism behind the high EF in SERS and showed that the SERS performance is directly proportional to the oxygen vacancy concentration in the Nb2O5 nanoparticles. Density functional theory (DFT) calculation suggests a strong coupling of the vibronic states and an increased charge transfer (CT) efficiency in the Nb2O5-MeB complex mediated through the vacancy-induced trap states in the defective Nb2O5 structure. Finite element method (FEM)-based simulations revealed a field enhancement factor of ∼4.17 × 102 that contributed to the SERS EF, while the remaining is contributed to the oxygen vacancy-mediated charge transfer, i.e., a factor of ∼1.23 × 105 is due to the high CT efficiency, the highest among the reported values. We believe that these findings offer valuable insights into the fabrication of defect-tailored cost-effective semiconductor-based SERS substrates for ensuing applications, such as trace dye detection.

Designing a highly efficient, low-cost, sustainable electrocatalyst for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) through water splitting is a current challenge for renewable energy technologies. This work presents a modified sol-gel route to prepare metal-ion(s) doped cerium oxide nanostructures as an efficient electrocatalyst for overall water splitting. Nickle (Ni) and iron (Fe) co-doping impacts the morphology in cerium oxide resulting in 5 nm nanoparticles with a mesoporous-like microstructure. The high level 20 mol% (1 : 1 ratio) of Ni + Fe bimetal-ion(s) doped CeO2 shows excellent HER and OER activities compared to the monodoped Fe/Ni and pristine CeO2. The co-doped catalysts required a low overpotential of 104 mV and 380 mV for HER and OER, respectively, in 1 M KOH, at a current density of 10 mA cm-2. The Tafel slopes of 95 mV dec-1 and 65 mV dec-1 were measured for HER and OER with the same representative samples which demonstrated excellent stability even after continuous operation for 20 hours in the alkaline medium. The unique morphology, enhanced oxygen vacancy (Ov) content and the synergistic effects of dopants in CeO2 play essential roles in enhancing the activities of Ni + Fe doped samples.