Formation Of Oxygen Vacancy Defects Research Articles

Characteristics of 4-inch (100) oriented Mg-doped β-Ga2O3 bulk single crystals grown by a casting method

In this Letter, 4-inch diameter (100) magnesium-doped monoclinic gallium oxide (Mg:β-Ga2O3) bulk single crystals were grown by a casting method. The crystallographic structure, electrical, optical properties, and defects of Mg-doped β-Ga2O3 was thoroughly investigated by XRD rocking curves, AFM, non-contact resistivity measurements, XPS, optical transmission spectra, Raman spectroscopy, and defect-selective chemical etching processes. Comparing to the UID samples, the larger bandgap of 4.75 eV and high resistivity of 3.50×1012 Ω∙cm indicated Mg a proper dopant for fabricating semi-insulating β-Ga2O3 substrates which could be preferred for power devices. XPS spectra exhibited the ability of Mg2+ to inhibit the formation of oxygen vacancy defects in the crystals. Meanwhile, the high crystalline quality and low density of defects demonstrated the advantages of casting method in achieving large-size and high-quality β-Ga2O3 bulk single crystals with changeable resistivity. This result indicates a high potential to achieve high-quality and low-cost substrates which satisfy the requirement of high-performance power electronic devices.

Structural, optical, dielectric, and ferroelectric properties in [formula omitted] ceramics prepared by a solvothermal reflux approach

Chromium (Cr3+) doped BaTi1−xCrxO3 (x = 0.0, 0.03, 0.06, and 0.09) ceramics were prepared by the solvothermal reflux approach. X-ray diffraction analysis confirmed the development of tetragonal structures with P4mm symmetry in the prepared samples. The SEM analysis showed that Cr3+ doping resulted in a microstructure that was dense, uniform, and had smaller grains. Surface chemical information and oxidation states were studied using X-ray photoelectron spectroscopy. The diffuse reflectance spectrum confirmed the effect of Cr3+ content on the direct optical bandgap of BaTi1−xCrxO3 samples which shifts from 3.21 eV to 2.73 eV with increasing Cr3+ content. Dielectric permittivity was found to increase in the low-frequency region with increasing Cr3+ doping content up to x = 0.06, while it decreased in the high-frequency region compared to an undoped sample. The observed behavior of permittivity might be explained in terms of the formation of oxygen vacancy defects and their effects on the polarization dynamics of the material. Moreover, the addition of Cr3+ doping had a strong effect on the ferroelectric characteristics, as seen by the formation of a pinched hysteresis loop, resulting in the improvement of energy storage properties. Based on the obtained results, the sample with x = 0.06 exhibited an enhancement of about 3.3 times in energy discharge density and 26 % in storage efficiency compared to the undoped sample. This might be due to a large difference between the values of maximum and remnant polarizations, reduced tangent loss, and a smaller grain size, which makes this material attractive for use in energy storage devices.

The enhancement of large photochromic coloration contrast with rapid responsive behavior in Ba3MgSi2O8:Dy3+ ceramics for instant information displays

Designing photochromic systems with information displays has been seldom studied but is critical to the realization of instant hand-writable system. Previous instant hand-writable systems with inorganic photochromic materials limited to low color contrast. Herein, by controlling the sintering environment and ion doping concentration, an excellent light response is achieved by engineering the formation of oxygen vacancy defects in Ba3MgSi2O8:0.25%Dy3+, the maximum color contrast as high as 80.7%. A hand-rewritable system fabricated using these photochromic materials exhibits excellent writing and erasing reversibility in response to UV illumination and thermal stimulus. In addition, a large luminescence modulation of 86.2% is achieved due to the well overlap between the emission peak of Dy3+ and the absorption peak of Ba3MgSi2O8. The resulting insights will be useful to researchers in inducing oxygen vacancy defects in high-performance inorganic photochromic systems for superior functional design.

Near-Infrared Emission Material with Superlong Afterglow Performance and Its Multifunctional Applications.

A new near-infrared (NIR) emission material, BaSi1.5Ge2.5O9:Cr3+, with outstanding long afterglow properties is designed and synthesized successfully. Its structure, photoluminescence, and long afterglow emission features are studied in detail. Cr3+ occupies six-coordinated Ba2+ and Ge4+ sites in this system, which can emit characteristic broadband NIR light in the range of 700-1200 nm, and it has a long afterglow emission of more than 10 h. We calculated the band gap of the host at about 4.1 eV, the energy level distribution after Cr3+ doping, and the distribution of oxygen vacancies through density functional theory simulation, which theoretically proved the characteristic transition of Cr3+ and that the oxygen vacancies are crucial to form the defect energy levels. This is corroborated with the reflectance spectra, three-dimensional thermoluminescence spectra, and electron spin resonance (ESR) spectra. The oxygen-deficient environment favors the formation of oxygen vacancy defects and prevents the oxidation of Cr3+ to Cr6+, which are the key points for the luminescence and afterglow properties. A mechanistic model of defect formation and electron trapping and transport processes is successfully established. Finally, its multifunctional applications in information anticounterfeiting encryption, biological tissue penetration, and internal nondestructive testing and imaging are demonstrated.

Efficient and stable Pt/CaO-TiO2-Al2O3 for the catalytic dehydrogenation of cycloalkanes as an endothermic hydrocarbon fuel

The dehydrogenation of liquid hydrocarbon fuel is a potential endothermic reaction for the regenerative cooling of advanced aircraft. The high active and stable dehydrogenation catalyst plays a vital role in the catalytic heat exchange process. Herein, we show that a CaO-modified TiO2-Al2O3 supported Pt catalyst (Pt/CTA) is efficient and stable for the dehydrogenation of cycloalkanes (methylcyclohexane and decalin) under a supercritical condition. Adding an appropriate content of CaO (5 wt%) improves the catalyst durability. For the decalin dehydrogenation reaction (600 °C, 4 MPa, and WHSV = 208.8 h−1), the Pt/CTA maintains 27.9 wt% conversions at 600 min. For the methylcyclohexane dehydrogenation reaction (600 °C, 4 MPa, and WHSV = 188.4 h−1), the Pt/CTA maintains 42.6 wt% conversions at 600 min. The declines of cycloalkanes dehydrogenation conversion are < 7.0 % within 600 min, and no obvious sintering of Pt particles is observed. For a comparison, γ-Al2O3 supported Pt catalyst (Pt/A) exhibits poor catalytic activity and stability, and the conversion decreases to below 10 wt% at 180 min due to severe coke deposition and sintering of Pt particles over the catalysts. The characterization analyses demonstrate that the addition of CaO facilitates the formation of oxygen vacancy defects, thereby increasing the electron density of Pt particles (electronic effect) and promoting encapsulation of Pt particles (geometric effect). Furthermore, the addition of CaO reduces the acid amount and weakens the strength of acid sites over TiO2/Al2O3, inhibiting catalyst deactivation by mitigating coke formation. However, excessive CaO addition (10 wt%) rapidly decreases the surface area of the support, resulting in low dispersion and sintering of Pt particles, ultimately leading to poor catalytic activity and stability.

Surface engineering of hematite nanorods photoanode towards optimized photoelectrochemical water splitting

Rapid charge recombination in hematite (Fe2O3) during photoelectrochemical water splitting is a major obstacle to achieving high-efficiency photoelectrodes. Surface defect engineering is considered as a viable strategy for enhancing photoelectrochemical activity of oxide photoanodes. Herein, a one-dimensional (1D) defective γ-Fe2O3 nanorods (DFNRs) photoanode is prepared using solvothermal and high-temperature hydrogenation strategies. The as-prepared DFNRs possess superior visible-light absorption capacity and exhibit excellent photoelectrochemical performance (0.98 mA cm−2), with approximately three-fold higher photocurrent density than that of pristine Fe2O3 (FNRs, 0.32 mA cm−2). The enhanced activity of the DFNRs results from the moderate formation of oxygen vacancy defects, which promotes spatial charge separation and transfer at the DFNRs/electrolyte interface, as well as the 1D nanorod structure, which favors rapid charge transfer. The surface of γ-Fe2O3 with hydroxyl (OH) groups provides sufficient surface-active sites. This result suggests that surface-oxygen deficiency of γ-Fe2O3 can not only expand the light absorption range but also facilitating photo-generated charge carriers separation. This surface engineering strategy provides an alternative method for preparing stable and highly active metal oxide photoanodes for photoelectrochemical water splitting.

Self-floating biomass charcoal supported flower-like plasmon silver/carbon, nitrogen co-doped defective TiO2 as robust visible light photocatalysts

A self-floating biomass charcoal supported flower-like plasmon Ag/C, N co-doped defective TiO2 photocatalyst is fabricated by hydrothermal, calcination and photo-deposition strategies. Biomass charcoal, Ag nanoparticles, oxygen vacancy defects and C, N co-doping all favor to extend the photoresponse to visible light region. The biomass charcoal plays the role of doping and floating supporter, which can make it float on surface of water and effectively improve the collection of sunlight. The resultant plasmon Ag/C, N co-doped defective TiO2 photocatalyst exhibits excellent visible-light-driven photocatalytic performance, in which the photocatalytic degradation ratio of 2,4-dichlorophenol and hydrogen evolution are up to 95% and 1269 μmol h−1 g−1, respectively, several times higher than that of pristine TiO2. It can be ascribed to the synergistic effect of surface plasmon resonance (SPR) effect of Ag nanoparticles, the efficient C, N co-doping and formation of oxygen vacancy defects promotes spatial charge separation and enhances visible light absorption. In addition, the long-term stability of this novel self-floating photocatalyst implies the potential practical applications.

Tailoring Defects in Photocatalysts by Engineering Solvent Interactions for Highly Active and Responsive Color Switching

AbstractDesigning photochromic systems that exhibit tunable print/erase responses is seldom studied but critical to the implementation of rewritable paper. Previous systems printed information with photocatalysts limited to specific wavelengths and light sources and rarely considered how polymers used in film formation affect erasing of information. Herein, different visible/UV light responses are achieved by engineering the formation of oxygen vacancy defects in titanium dioxide/reduced graphene oxide composite photocatalysts. Defects are manipulated during synthesis by controlling the water concentration in a mixed solvent system, which leads to tunable photochromic response of redox dyes. Furthermore, because the accessibility of ambient oxygen directly impacts the kinetics of recoloration, tailoring the oxygen barrier properties of polyvinyl alcohol via modulating inherent polymer structure as well as the external environment, the recoloration can be further tuned to meet various application purposes. The resulting insight will assist fellow researchers in tailoring defects in photocatalyst systems for superior functional design.

Luminescence performance of yttrium-stabilized zirconia ceramics doped with Eu3+ ions fabricated by Spark Plasma Sintering technique

Yttrium stabilized zirconia (YSZ) ceramics doped with Eu3+ ions have been successfully fabricated by Spark Plasma Sintering (SPS) technique. The influence of the europium concentration and post-annealing process on the structural, optical, and luminescent properties of the ceramics has been studied. It is shown that an increase in the europium concentration from 0.1 to 3 wt% does not lead to significant changes in the transmission spectra. However, annealing in air atmosphere at temperature from 700 °C to 1300 °C significantly affects the transmission spectrum, as a possible consequenceofthe formation of oxygen vacancy defects. The analysis of the photoexcitation and photoluminescence spectra showed that the main excitation bands are determined by direct excitation of the 7F0 ground state of Eu3+ions to the higher 4f energy levels with further radiation transitionsfrom these states. Moreover, the europium ion in the obtained ceramics occupy low-symmetry sites without inversion center.The luminescence decay kineticsare described by a doubleexponential function with decay time τ1 ~ 20 ns and τ2~ 90 ns for intrinsic emission centers and millisecond (τ ~ 1.4 ms) for Eu3+emission, for all investigated ceramics. The luminescence spectra in nanosecond time region are characteristic for yttrium-stabilized zirconia and are caused by oxygen vacancies in the presence of heavy cations (Y3+ and Eu3+).

Effect of defects and oxygen vacancies on the RTFM properties of pure and Gd-doped CeO2 nanomaterials through soft XAS

This work primarily is an investigation of the electronic structure properties of pure CeO2 and $${Ce}_{1-x}{Gd}_{x}{O}_{2}$$ (x = 0.02, 0.04, 0.06, 0.08 and 0.10) Nanoparticles using the soft X-ray absorption spectroscopy were used to explore defects and vacancies. For this purpose, pure CeO2 and $${Ce}_{1-x}{Gd}_{x}{O}_{2}$$ (x = 0.02, 0.04, 0.06, 0.08 and 0.10) nanomaterials were synthesized using the co-precipitation method. The XAS spectra at Ce M4,5, O K-edge and Gd M4,5 absorption edges clearly indicated a decrease in the valence state of Ce ions from Ce4+ to Ce3+ with the formation of oxygen vacancy defects upon incorporation of Gd3+ ions in CeO2 nanolattice. The results show meagre and sparse segregations of additive Gd+3 ions to form secondary phases in the samples. The deconvoluted Ce M4,5 peaks and O K-edge peaks clearly showed the existence of both oxidation states of Ce ions, Ce3+ and Ce4+,and also indicated the formation of oxygen vacancy defects in all the samples. The presence of Gd3+ oxidation state of Gd ions in Gd+3-doped CeO2 samples was investigated through Gd M4,5 edges. Furthermore, the origin of ferromagnetism in pure CeO2 and Gd3+-doped CeO2 samples is explained using F-centre exchange mechanism mediated by defects and oxygen vacancies.

One-step synthesis of oxygen vacancy-rich SnO2 quantum dots with ultrahigh visible-light photocatalytic activity

In this work, the oxygen vacancy-rich SnO2 quantum dots (QDs) have been synthesized via facile vapor hydrolysis method and employed as an efficient visible-light photocatalyst for MO degradation. The as-prepared oxygen vacancy-rich SnO2 QDs showed ultrahigh visible light photocatalytic activity for MO degradation (k = 1.1699 min-1), which was about 182.13 and 259.26 times as high as that of P25 TiO2 (k = 0.00642 min-1) and commercial SnO2 (k = 0.00451 min-1). Besides, the SnO2 QDs-160 photocatalyst had a bandgap of 2.29 eV, much narrower than that of commercial SnO2 (˜3.56 eV), mainly because of the formation of oxygen vacancy defects in the SnO2 QDs. The photocatalytic MO degradation under visible-light irradiation here was mainly attributed to the oxidation of the superoxide radicals and holes. This work also indicated that the as-prepared SnO2 QDs could serve as a promising visible-light photocatalyst for efficient environmental photocatalysis.

SnO2 Nanosheets for Selective Alkene Gas Sensing

SnO2 nanosheets with the (101) crystal face mainly exposed were prepared to investigate their sensing properties. The SnO2 nanosheets were ∼150 nm in size and well-dispersed on the sensor chip. The gas sensing properties of the SnO2 nanosheets were investigated for CH4, C2H4, C2H6, C3H6, C3H8, 1-C4H8, cis-2-C4H8, trans-2-C4H8, iso-C4H8, and n-C4H10 gases. Additionally, the relationship between the sensor signal response of the SnO2 nanosheets and the orbital energy of the target gas molecules was investigated. It was found that the sensor exhibited remarkable selectivity to alkene gases which have high HOMO energies. The presence of partially unoccupied oxygen sites was confirmed from crystal structure analysis. In addition, defects on the surface of the SnO2 nanosheets were certainly identified from the surface analysis. On the basis of these results, it was determined that the gas selectivity for alkene gases of the SnO2 gas sensor was improved by the formation of oxygen vacancy defects. This effect is ...

Synthesis of Bi2WO6 with gradient oxygen vacancies for highly photocatalytic NO oxidation and mechanism study

We successfully synthesized the Bi2WO6 (BWO) with gradient concentration of oxygen vacancies and exhibited the excellent photocatalytic NO oxidation activity. And the oxygen vacancy introduced in photocatalyst is a popular method to enhance the photocatalytic performance. The formation of oxygen vacancy defects in BWO is responsible for the tuning band structure and modifying surface chemical state to improve carriers separation efficiency, enlarge visible light absorption range and facilitate reactant activation, which is unambiguously verified by the UV–vis DRS, XPS, EPR and DFT calculations. Moreover, the role of surface oxygen vacancy defects and reaction mechanism of photocatalytic NO oxidation were discussed in detail by combing the in situ DRIFT and theoretical calculations. This work could be broadly used to the design and synthesize high efficient photocatalysts and pave a way to understand the reaction mechanism of NO oxidization.

Mesocrystalline Ta2O5 nanosheets supported Pd[sbnd]Pt nanoparticles for efficient photocatalytic hydrogen production

We successfully synthesized mesocrystalline Ta2O5 nanosheets supported bimetallic PdPt nanoparticles by the photo-reduction method. The as-prepared mesocrystalline Ta2O5 nanosheets in this work showed amazing visible-light absorption, mainly because of the formation of oxygen vacancy defects. And the as-prepared bimetallic PdPt/mesocrystalline Ta2O5 nanaosheets also showed highly enhanced UV–Vis light absorption and highly improved photocatalytic activity for hydrogen production in comparison to that of commercial Ta2O5, mesocrystalline Ta2O5 nanosheets, Pd/mesocrystalline Ta2O5 nanosheets and Pt/mesocrystalline Ta2O5 nanosheets. The highest photocatalytic hydrogen production rate of PdPt/mesocrystalline Ta2O5 nanaosheets was 21529.52 g−1 h−1, which was about 21.2 times of commercial Ta2O5, and the apparent quantum efficiency of PdPt/mesocrystalline Ta2O5 nanaosheets for hydrogen production was about 16.5% at 254 nm. The highly enhanced photocatalytic activity was mainly because of the significant roles of PdPt nanoparticles for accelerating the charge separation and transport upon illumination. The as-prepared PdPt/mesocrystalline Ta2O5 nanaosheets in this work could serve as an efficient photocatalyst for green energy production.

Size effects on electrical properties of chemically grown zinc oxide nanoparticles

In the present article, we study ZnO nanoparticles grown by cost effective sol–gel technique for various electrical properties. Structural studies performed by x-ray diffraction (XRD) revealed hexagonal unit cell phase with no observed impurities. Transmission electron microscopy (TEM) and particle size analyzer showed increased average particle size due to agglomeration effect with higher sintering. Dielectric constant (ε′) decreases with increase in frequency because of the disability of dipoles to follow higher electric field. With higher sintering, dielectric constant reduced owing to the important role of increased formation of oxygen vacancy defects. Universal dielectric response (UDR) was verified by straight line fitting of log (fε′) versus log (f) plots. All samples exhibit UDR behavior and with higher sintering more contribution from crystal cores. Impedance studies suggest an important role of boundary density while Cole–Cole (Z″ versus Z′) plots have been studied for the relaxation behavior of the samples. Average normalized change (ANC) in impedance has been studied for all the samples wherein boundaries play an important role. Frequency dependent electrical conductivity has been understood on the basis of Jonscher’s universal power law. Jonscher’s law fits suggest that conduction of charge carrier is possible in the context of correlated barrier hopping (CBH) mechanism for lower temperature sintered sample while for higher temperature sintered ZnO samples, Maxwell–Wagner (M–W) relaxation process has been determined.

Superstructure Ta2O5 mesocrystals derived from (NH4)2Ta2O3F6 mesocrystals with efficient photocatalytic activity.

Superstructured mesocrystalline Ta2O5 nanosheets were successfully prepared from mesocrystalline (NH4)2Ta2O3F6 nanorods by the annealing method for the first time. The as-prepared mesocrystalline Ta2O5 nanosheets in this work showed remarkable visible light absorption, mainly due to the formation of oxygen vacancy defects in the mesocrystalline Ta2O5 nanosheets, which was also confirmed by XPS spectra, Raman spectra and EPR spectra. Besides, the mesocrystalline Ta2O5 nanosheets showed a highly enhanced photocatalytic activity of 11 268.24 μmol g-1 h-1, about 3.95 times that of commercial Ta2O5. Moreover, the specific surface area of the mesocrystalline Ta2O5-800 nanosheets was 16.34 m2 g-1, about 5.32 times that of the commercial Ta2O5 (3.072 m2 g-1). The valence band XPS spectra indicated a strong oxidizing ability of the mesocrystalline Ta2O5 nanosheets in comparison to that of commercial Ta2O5. The formation of superstructured Ta2O5 mesocrystals generated long lifetime carriers and effective conduction pathways, which greatly enhanced the photocatalytic activity for hydrogen production.

Difference in structural and chemical properties of sol-gel spin coated Al doped TiO2, Y doped TiO2 and Gd doped TiO2 based on trivalent dopants.

In this research, pure titanium dioxide (TiO2) and doped TiO2 thin film layers were prepared using the spin coating method of titanium(iv) butoxide on a glass substrate from the sol–gel method and annealed at 500 °C. The effects on the structural and chemical properties of these thin films were then investigated. The metal doped TiO2 thin film which exists as trivalent electrons consists of aluminium (Al), yttrium (Y) and gadolinium (Gd). The anatase phase of the thin films was observed and it was found that the crystal size became smaller when the concentration of thin film increased. The grain size was found to be 0.487 to 13.925 nm. The types of surface morphologies of the thin films were nanoporous, with a little agglomeration and smaller nanoparticles corresponding to Al doped TiO2, Y doped TiO2 and Gd doped TiO2, respectively. The trivalent doping concentration of the thin films increased with a rising of thickness of the thin film. This can contribute to the defects that give advantages to the thin film when the mobility of the hole carriers is high and the electrons of Ti can move easily. Thus, Ti3+ existed as a defect state in the metal doped TiO2 thin film based on lattice distortion with a faster growth thin film that encouraged the formation of a higher level of oxygen vacancy defects.

High pressure structural study of samarium doped CeO2 oxygen vacancy conductor — Insight into the dopant concentration relationship to the strain effect in thin film ionic conductors

The bulk modulus of nanocrystalline, fluorite-structured samarium doped ceria, Sm0.2Ce0.8O1.9, has been investigated using synchrotron-based high-pressure X-ray diffraction technique. Experiments were carried out under both quasi-hydrostatic condition with silicon oil pressure transmitting medium (PTM) and nonhydrostatic conditions without PTM. The high pressure structural results indicate that the highly defected ionic conductor is stable up to 20GPa and has a lower bulk modulus than what has been reported for undoped-CeO2. The isothermal bulk modulus of Sm0.2Ce0.8O1.9 is ~150–190GPa compared to ~210–220GPa for CeO2. The collected data experimentally verifies the effect of Sm3+ dopant and oxygen vacancy defect formation on bulk modulus in doped CeO2. The effect of modulus on misfit dislocation formation and dopant ion segregation is discussed in relation to a fundamental understanding of the strain effect in this important family of fast ionic conductors, with potential application as oxygen vacancy conducting solid state electrolytes.

The acid–base and redox reactivity of CeO2 nanoparticles: Influence of the Hubbard U term in DFT + U studies

The interaction of small molecules with acid–base and redox centers in small Ce21O42 nanoparticles has been theoretically investigated using the DFT+U approach with the PW91 functional and U=0.2 and 4eV, in order to determine the influence of the U value on the trends observed in selected properties describing such interactions. CO adsorption at low coordinated Ce4+ Lewis acid centers, water adsorption and dissociation at acid–base pairs, formation of oxygen vacancy defects by removal of an oxygen atom from the system, and interaction of molecular O2 with such defects have been considered. The largest effect of the value of U is found for the description of the reduced Ce21O41 nanoparticle. In all other cases involving stoichiometric and oxidized Ce21O42 and Ce21O43 systems, the trends in the calculated adsorption and reaction energies, optimized geometries, charge distribution, and vibrational frequencies are quite similar at the three levels considered.

Surface-Dependence of Defect Chemistry of Nanostructured Ceria

The defect chemistry of reduced ceria nanoshapes was investigated using in situ Raman and FTIR spectroscopy. Octahedral- and rod-shaped particles behave similarly upon exposure to CO in terms of formation of anion Frenkel pair defects and oxygen vacancies. This similarity is attributed to the preferential exposure of (111) surfaces in both type of particles. Cube-shaped particles terminated with (100)-oriented surfaces exhibit very different defect behavior in CO, revealing formation of oxygen vacancy defects at the expense of existing anion Frenkel pairs. Octahedra and rods, prereduced in H2, can be further reduced with CO. In contrast, prereduced cubes can reactively adsorb CO, forming surface-bicarbonate, only via converting Frenkel-pair defects to oxygen vacancies, without any further net reduction of the ceria.