Narrow Results

This study utilized XRD and Mössbauer spectroscopy to investigate the effects of copper ion doping on the structural properties and oxygen vacancies of ${\mathrm{\text{SrFeO}}}_{3-\delta }$ perovskite oxides. XRD analysis confirmed that all samples maintained a single-phase perovskite structure, with a decrease in lattice constants and contraction of unit cell volumes as copper doping increased. Mössbauer spectroscopy indicated that increased copper doping caused a transition in the valence state of iron ions from ${\mathrm{\text{Fe}}}^{3.5+}$ to ${\mathrm{\text{Fe}}}^{4+}$, along with significant distortions in the Fe–O bond environment, reflecting local structural changes. The introduction of ${\mathrm{\text{Cu}}}^{2+}$ and ${\mathrm{\text{Cu}}}^{3+}$ ions increased oxygen vacancies, disrupting charge balance and ion radius compatibility, which in turn exacerbated lattice distortions. Iodometric titration further corroborated the rise in oxygen vacancies with higher copper content, offering important theoretical insights for advancing ${\mathrm{\text{SrFeO}}}_{3-}$ based materials in energy conversion and storage applications.

The Pr-substituted YBa₂Cu₃O_7-δ(Y123) superconducting systems with the content of 0.0–1.0 have been systematically studied by positron lifetime and infrared absorption experiments. The results show that the short lifetime τ₁ decreases as a function of Pr-substitution x below x=0.6, but when above x=0.6, it increases inversely. The long lifetime τ₂ decreases as a function of Pr-substituted x. Based on the present results, we discuss that Pr⁴⁺ substitutes on Ba²⁺ ion and forms Pr_Ba defects. The variation of τ₁ is caused by the oxygen vacancies and τ₂ is probably affected by the absence of Cu due to Pr_Ba defects. The infrared absorption of Pr-substituted Y123 systems gives three variational modes, located at 560 cm^-1(A₁), 1435 cm^-1(A₂) and 1631 cm^-1(A₃) respectively. Here the A₃ mode shows that with Pr content increasing from x=0.1 to 1.0, the infrared absorption decreases and the peak tends to broaden. So, combining with the results of the positron lifetime spectra, we can argue that there always exists portion of Pr_Ba defects in the Pr-substituted systems, and the Pr⁴⁺ substituted into Ba²⁺ is also increasing with the Pr content increasing. When the Pr content is 0.6, the systems have a metal-insulator transition and Pr_Ba defects begin to domain the samples' microstructure.

A discussion of dc conduction mechanisms in thermally co-evaporated amorphous thin films of Al–In₂O₃–SnO₂–Al structure is presented. Composition (in molar %), film thickness, substrate temperature, and post deposition annealing have profound effects on the electrical properties of the films. The effects of temperature on the I–V characteristics and electrical conductivity of Al–In₂O₃–SnO₂–Al structure are also reported. The values of dielectric constants estimated by capacitance measurements suggest that high-field conduction mechanism is predominantly of Poole–Frenkel type. At low temperature and low field the electron hopping process dominates but at higher temperature the conduction takes place by transport in the extended states (free-band conduction). The transition from hopping to free band conduction is due to overlapping of localized levels and the free band. The increase in the formation of ionized donors with increase in temperature during electrical measurements indicates that electronic part of the conductivity is higher than the ionic part. The initial increase in conductivity with increase in Sn content in In₂O₃ lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in electrical conductivity above the critical Sn content (10 mol % SnO₂) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in electrical conductivity with film thickness is caused by the increase in free carriers density, which is generated by oxygen vacancy acting as two electrons donor. The increase in conductivity with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and formation of indium species of lower valence state so that the In³⁺ oxidation state may be changed to the In²⁺ oxidation state.

The ESR spectra of amorphous thin films of mixed oxides In₂O₃–SnO₂ system is presented. An initial increase in the intensity of ESR signal at g=1.994, and decrease in the intensity of the signal at g=1.98 with an increase in Sn content in In₂O₃ lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in the intensity of ESR signal at g=1.994 and an increase in the intensity of the signals at g=1.98 above the critical Sn content (10 mol% SnO₂) is caused by the defects formed by Sn atoms, which act as carrier traps rather than as electron donors. The increase in the intensity of ESR signal at g=1.89 with the increase in Sn content in mixed oxides In₂O₃–SnO₂ system is caused by the increase in the concentration of oxygen vacancies, which are generated in the In₂O₃ and SnO₂ lattices by thermal evaporation. The increase in the intensity of ESR signals at g=1.994, g=1.98 and g=1.89 with the increase in thickness is caused by the bulk properties of the films. The increase in the intensity of ESR signals at g=1.994, g=1.98 and g=1.89 with the increase in substrate temperatures is due to the increase in the concentration of oxygen vacancies, which are generated at higher substrate temperatures and to the diffusion of Sn atoms from interstitial locations into the In cation sites, which result in higher electron concentration. The decrease in the intensity of ESR signals at g=1.994, g=1.98 and g=1.89 with an increase in annealing temperature is attributed to the rearrangement of the atoms and to the removal of voids, which causes a decrease in the number of unsatisfied bonds and formation of indium and tin species of lower valence states. The new oxidation states, the In²⁺ and the Sn²⁺, formed due to the annealing of the samples can be attributed to the internal electron transfer from oxygen 2p to the In5s and Sn5s levels, both in In₂O₃ and SnO₂.

The fundamental absorption edge of SnO₂ amorphous thin films has been investigated. It has been observed that the optical energy gap decreases with the increase in film thickness, substrate temperature and post deposition annealing. The results are analysed by assuming optical absorption by non-direct transition. The decrease in optical band gap with increase in film thickness may be interpreted in terms of the incorporation of oxygen vacancies in the SnO₂ lattice. The decrease in optical energy due to the increase in substrate temperature may be ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperatures. The decrease in optical band gap due to annealing may be due to the formation of tin species of lower oxidation state.

${\mathrm{\text{SnO}}}_2$ porous nanotubes containing oxygen vacancies were prepared by electron spinning and H plasma treatment. The morphology and crystal structure of the samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The ethanol-sensing properties of the ${\mathrm{\text{SnO}}}_2$ sensor were tested. The results show that the samples treated with H plasma for 20 min have the best performance. Its working temperature is $230^{\circ }$C lower than $290^{\circ }$C of the original sample, with a sensitivity of 17 at 100 ppm, which is seven times higher than the original sample. It also shows good selectivity to some common interfering gases. This enhancement can be ascribed to the introduced oxygen vacancy. This work provides an efficient way to design high-performance gas sensor materials.

Resistive memories based on the resistive switching effect have promising application in the ultimate nonvolatile data memory field. This brief review focuses on the resistive switching phenomena in the perovskite oxide heterostructures, which originate from the modulation of the interface properties due to the movement of the oxygen vacancies and the ferroelectric polarization. Many recent experiments have been carried out to demonstrate the role of the oxygen vacancies by controlling the content of the oxygen vacancies in the oxide heterostructures with plenty of oxygen vacancies. The important role of the ferroelectric polarization was also carefully confirmed by analyzing the relationship between the current–voltage and polarization–voltage loops in the ferroelectric oxide heterostructures. The physical mechanisms have been revealed based on the developed numerical model.

The persistent luminescence performance of a novel color-tunable Tb${}^{\mathrm{\text{3}}+}$-doped La${}_{\mathrm{\text{3}}}$GaGe${}_{\mathrm{\text{5}}}$O${}_{\mathrm{\text{1}}6}$ phosphor has been modified by oxygen-deficiency control with an aim to explore the strategy to improve the persistent luminescence property. First-principles calculations were combined with thermoluminescence (TL) measurements to evaluate the role of oxygen vacancies in persistent luminescence of La${}_{\mathrm{\text{3}}}$GaGe${}_{\mathrm{\text{5}}}$O${}_{\mathrm{\text{1}}6}$:Tb${}^{\mathrm{\text{3}}+}$ persistent phosphor. Our results show that the oxygen vacancies that act as an electron trap center have a limited contribution to the persistent luminescence. The formation of the ring structure of La${}_{\mathrm{\text{3}}}$GaGe${}_{\mathrm{\text{5}}}$O${}_{\mathrm{\text{1}}6}$ crystal and the resulting localized trap levels are very different from that of other outstanding persistent phosphors, like Sr${}_{\mathrm{\text{2}}}$MgSi${}_{\mathrm{\text{2}}}$O${}_{\mathrm{\text{7}}}$, etc. Such differences are closely related to the inferior performance of La${}_{\mathrm{\text{3}}}$GaGe${}_{\mathrm{\text{5}}}$O${}_{\mathrm{\text{1}}6}$:Tb${}^{\mathrm{\text{3}}+}$ persistent phosphor.

In this work, ${\mathrm{\text{BiFe}}}_{1-x}{\mathrm{\text{Co}}}_x{\mathrm{\text{O}}}_3$ ($x$ = 0, 0.08) nanoparticles were synthesized by the solution method and their structural differences were studied. X-ray diffraction results show that the rhombohedral R3c space group and perovskite structures are detected in both samples, accompanied by an impurity phase. The (104) and (110) peaks merge when cobalt ions are doped. The decrease in lattice parameters indicates that the microstructure of the nanoparticles becomes gradually distorted. Mössbauer spectroscopy analysis at room temperature reveals an additional doublet due to the oxygen vacancies in ${\mathrm{\text{BiFe}}}_{0.92}{\mathrm{\text{Co}}}_{0.08}{\mathrm{\text{O}}}_3$. Hyperfine interactions, spatial spin-modulated structures and oxygen deficiencies around iron ions are also reflected in the observed spectra and variations in hyperfine parameters.

We synthesized lead-free ($1-x)$CaZrO₃–$x$SrTiO₃ solid-solution ceramics and studied their structure, dielectric and energy storage properties. X-ray diffraction and scanning electron microscope reveal the microstructure of the samples. A high temperature dielectric relaxation behavior at the temperature ranges of 200${}^{\circ }$C–550${}^{\circ }$C is found for $x=0.3$, 0.5 and 0.7 samples. We explore that the origin of high temperature dielectric relaxation behavior is related to the migration of oxygen vacancies by Arrhenius law and oxygen treatment experiment. Moreover, with the increase of SrTiO₃ content, the 0.3CaZrO₃–0.7SrTiO₃ exhibits high dielectric constant ($𝜀=3645$ at 1 kHz), low dielectric loss ($tan\delta =0.00081$ at 1 kHz), good energy density ($W_{\mathrm{\text{rec}}}=1.29$ J/cm${}^3)$ and high efficiency ($\eta =87.7$%) simultaneously, and the possible causes of energy storage behavior dependence are analyzed in detail.

Conductivity-frequency and capacitance-frequency characteristics of mixed oxides Al–In₂O₃–SnO₂–Al structure are examined to elicit any correlation with the conduction mechanisms most often observed in thin film work. The existence of Schottky barriers is believed to be due to a strong donor band in the insulator established during the vacuum evaporation when a layer of mixed oxides In₂O₃–SnO₂ system is sandwiched between two metal electrodes. Low values of activation energy at low temperatures indicate that the transport of the carriers between localized states is mainly due to electronic hopping over the barrier separating the two nearest neighbor sites. The increase in the formation of ionized donors with increase in temperature during electrical measurements indicates that electronic part of the conductivity is higher than the ionic part. The initial increase in conductivity with increase in Sn content in In₂O₃ lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in electrical conductivity above the critical Sn content (10 mol% SnO₂) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in electrical conductivity with film thickness is caused by the increase in free carriers density, which is generated by oxygen vacancy acting as two electron donor. The increase in conductivity with substrate and annealing temperatures is due to either the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and formation of indium species of lower oxidation state (In²⁺). Calculations of C and σ_ac from tan δ measurements suggest that there is some kind of space-charge polarization in the material, caused by the storage of carriers at the electrodes. Capacitance decreases not only with the rise of frequency but also with the lowering of temperature. At low temperatures the major contribution to capacitance arises from the ionic polarization, however, with the increase of temperature the contribution from orientation polarization would considerably increase. The decrease in capacitance with the increase in frequency may be attributed to interfacial polarization.

The IR spectra of thin amorphous films of mixed oxides In₂O₃–SnO₂ system have been studied in the spectral range 4000–400 cm^-1 by Fourier transform infrared technique. The effects of changes in composition, film thickness, substrate temperature and annealing on the IR absorption bands are observed. A shift in frequency and intensity of the bands with varying preparation parameters and an increase in ordering at higher annealing temperatures are observed in the infrared curves. Some new peaks appear, which reveal the presence of lower valency states both in In₂O₃ and SnO₂. The disappearance of bands at higher annealing temperatures is assigned to the removal of point defects in which oxygen plays an important role. The shift in band frequency with an increase in (i) the Sn content is attributed to the incorporation of Sn⁴⁺ ions in the In₂O₃ lattice, (ii) the film thickness is attributed to the large concentration of donor centers, (iii) the substrate temperature is attributed to the increase in diffusion of Sn atoms from interstitial locations into the In cation sites, and (iv) the annealing temperature is attributed to the oxygen vacancies and indium interstitials.

The DC conduction mechanisms in metal–insulator–metal sandwich structure based on amorphous thin films of SnO₂ have been studied in the thickness range 100–400 nm, in the substrate temperature range 293–543 K, and in the annealing temperature range 473–773 K, and the results are discussed in terms of current theory. It is observed that at low field and low temperature the conduction mechanism is found to obey the hopping model, at higher temperature the conduction takes place by transport in the extended states but at high field the main barrier lowering effect is associated with localized centers. The increase in electrical conductivity with film thickness is caused by the oxygen vacancies and defects which generate carriers in the films. The increase in electrical conductivity due to an increase in substrate temperature is ascribed to the increasing concentration of ionized donors and hoping of electrons between metal ions in two different valence states. The formation of tin species of lower valence states and doubly ionized oxygen vacancies are thought to be responsible for the increase in electrical conductivity at higher annealing temperature.

Cobalt-doped ZnO films were grown on the glass substrates using sol–gel/spin-coating technique to investigate the effect of annealing on the structural and magnetic properties. The X-ray diffraction (XRD) patterns of the Co-doped ZnO films are dominated by the (002) peak, suggesting an up-standing array of ZnO structure hexagonal (wurtzite) with a good crystalline quality, however, the secondary phases of Co₃O₄ and Co are present in the samples. With the annealing temperature increased, the secondary phases tend to disappear completely and the intensity of the (002) peak increased, indicating a high crystallinity of the samples. For the ZnO majority phase, the lattice constant ($c)$ decreases (from 5.232 $\mathrm{\text{Å}}$ to 5.224 $\mathrm{\text{Å}}$), while the crystallite size increases (from 22.040nm to 24.018nm) as the annealing temperature varies from 380^∘C to 600^∘C. Significant changes in the dislocation density ($\delta )$, strain $({\epsilon }_c)$ and stress $({\sigma }_c)$ of the Co-doped ZnO films were also observed, by increasing the annealing temperature. All samples display a ferromagnetic behavior with variations in the saturation magnetization ($M_s=1.31\times 10^{-5}$, $1.19\times 10^{-5}$ and $1.15\times 10^{-5}$ emu/cm${}^3)$ and coercive field ($H_c=82$, 104 and 75 Oe) for the temperatures of 380^∘C, 500^∘C and 600^∘C, respectively. The magnetic behavior of Co-doped ZnO films confirms the exchange interaction between the local spin moments produced by the oxygen vacancy. In addition, the ferromagnetic existence of the samples (380^∘C, 500^∘C and 600^∘C) can be attributed to certain nanoparticles or to the binding of Co${}^{+2}$ ions at the Zn${}^{+2}$ location in the ZnO lattice. Finally, it appears that the ferromagnetism at room temperature found in these films, is consistent with endogenous defects (oxygen vacancies) and magnetic ions insertion along the same lines.

In this paper, the chromium ions luminescence in Cr₂O₃/Al₂O₃ nanocomposites has been studied. No photoluminescence from the nanocomposite was observed at room temperature, while at liquid nitrogen temperature the two broad bands were detected at 497 and 665nm caused by $F$- and $F_2$-type oxygen vacancies and Cr${}^{3+}$ and Mn${}^{4+}$impurity centers. The photoluminescence band observed at 616nm is attributed to Cr${}^{6+}$ ions. The excitation of Cr${}^{6+}$ ions occurs at 265 and 375nm, and their formation is the result of Cr${}^{3+}$–Cr${}^{6+}$ redox processes on the surface of Cr₂O₃ crystallites at the annealing temperatures of 400–$900^{\circ }$C.

ZnO nanoparticles have been prepared by wet chemical method. The properties of the synthesized nanostructures are studied using X-ray diffraction, Transmission Electron Microscopy (TEM), Photoluminescence (PL), Ultraviolet-Visible absorption, Laser Raman and Fourier transform Infrared (FTIR) spectroscopy. The thermal decomposition is analyzed by Thermogravimetric (TG) and Differential Thermal Analysis (DTA). The influence of annealing on structural and optical properties of ZnO nanoparticles has been systematically investigated. The PL results demonstrated that the visible emission at 565 nm is associated with the combination of oxygen vacancies and OH group attached at the surface of ZnO nanoparticles. OH group is lessened from the surface of ZnO nanoparticles with annealing temperature and a blue-shift in visible emission peak is observed at 800°C annealing temperature.

In this work, reduced graphene oxide/CeO₂ nanocomposites (RGO/CeO₂) with two different RGO contents were synthesized using a facile one-step hydrothermal method, and the microwave absorption properties of RGO/CeO₂ were investigated for the first time. Morphology and structure analysis shows that the CeO₂ nanoparticles are uniformly dispersed on the RGO sheets with average size of 15nm. The as-prepared RGO/CeO₂ exhibits excellent microwave absorbability. An optimal reflection loss (RL) of $-$32dB is found at 17GHz with a coating layer thickness of 1.5mm. The product with a coating layer thickness of only 2.0mm shows a bandwidth of 4.3GHz, corresponding to RL at $-$10dB (90% of electromagnetic wave absorption). Compared with pristine RGO or pure CeO₂ nanoparticles, the reported nanocomposites achieved both wider and stronger wave absorption in the frequency range of 2–18GHz. The enhanced microwave absorption properties are attributed to the conductive loss and dielectric loss mainly caused by the higher oxygen vacancy concentration of CeO₂ in RGO/CeO₂, which is demonstrated by X-ray photoelectron spectroscopy. Moreover, multiple interfacial polarizations occurring in the multi-interfaces between CeO₂ and RGO sheets may be beneficial to microwave absorption. RGO/CeO₂ could be used as an attractive candidate for the new type of microwave absorptive materials.

We have fabricated BiFeO₃ thin film deposited on Pt/Ti/SiO2/Si substrates by the chemical solution deposition method. The effects of annealing temperature on the thin film structure, resistance switching (RS) properties, conduction mechanisms are investigated. It exhibits improved RS window with high ON/OFF ratio ($\sim$10⁴) for the sample annealed at 650${}^{\circ }$C. XPS characterization indicates that cation ratio of Fe${}^{2+}$/Fe${}^{3+}$ is increased with increasing annealing temperature. Crystal lattice distortion generated by Fe${}^{2+}$ cations, along with oxygen vacancies, commonly contribute to opening the RS window and the increment of conductive filaments. The film’s conduction mechanisms under different annealing temperatures are fully discussed. The RS properties of this system can be effectively improved by increasing the annealing temperature, which is crucial prerequisite for future applications of BFO-based thin film device in resistance random access memory.

Oxygen defects of nanoflower TiO₂ photo-catalyst was fabricated at the presence of hydrogen at different temperatures (100–600^∘C) and the concentrations of oxygen defects were firstly quantitatively analyzed by hydrogen programmed temperature reduction techniques (H₂-TPR). Total oxygen defect concentration and surface oxygen defect concentration were consistent with XPS and EPR results, respectively. Even at the hydrogen thermal temperature of 600^∘C, the shape of TiO₂ was still kept as nanoflower structure as characterized by SEM. However, the rutile and anatase coexist in the composition of crystal phase when hydrogen reduction temperature of the TiO₂ catalyst reached 400^∘C to 600^∘C as proved by Raman and XRD results. TiO₂ sample with oxygen defects shows excellent photo-catalytic activity for degradation of Direct Blue 78(DB) regardless of ultraviolet light (the maximum degradation rate achieved within 100min was 93.27%) or visible light (the maximum degradation rate achieved within 100min was 88.25%). The photo-catalytic activity seems to be highly correlated with the surface oxygen defects of TiO₂ catalyst. With surface oxygen-defect concentrations increase, the degradation ability on DB was significantly enhanced, while bulk oxygen defects had negligible effect on the photo-catalytic activity. The enhanced photo-catalytic performance of TiO₂ with a fixed amount of oxygen defects was attributed to the strong capturing capability of the photo-generated electrons. In addition, the surface defects could also improve the photo-catalytic reaction efficiency.

Ti₂${\text{Nb}}_{10}$${\text{O}}_{29}$ (TNO) is considered as a potential anode material due to its high capacity/power density and reliable safety. However, its poor electronic conductivity restricts its rate performance, which is important for its application in electric vehicles (EVs). In this study, we fabricated a hybrid of Ti₂${\text{Nb}}_{10}$${\text{O}}_{29-x}$/holey-reduced graphene oxide (TNO_x/HRGO) by a two-step method. In the structure of TNO_x/HRGO, TNO_x microspheres with oxygen vacancies are wrapped by gossamer-like HRGO. The oxygen vacancies of TNO_x and the high conductivity of HRGO can effectively enhance the electronic conductivity of the TNO_x/HRGO hybrid, and the HRGO holes are beneficial for the transmission of lithium-ion (${\text{Li}}^{+}$). The synergy effect of above features improves the rate performance of the TNO_x/HRGO hybrid. In addition, the existence of HRGO can buffer volume expansion during the insertion processes of ${\text{Li}}^{+}$, which can improve cyclic stability of the TNO_x/HRGO hybrid. Consequently, the TNO_x/HRGO electrode has excellent lithium-ion storage capacity, with high-rate performance (242mAh/g at 10^∘C, 225mAh/g at 20^∘C and 173mAh/g at 40^∘C) and excellent cyclic stability (98.0% capacity retention after 300 cycles at 10^∘C). This work reveals that TNO_x/HRGO can be a potential anode material for high-rate-performance lithium-ion storage.