Effects of temperature and atmosphere on the magnetic properties of Co-doped ZnO rods

Co-doped ZnO (Zn_0.95Co_0.05O) rods are fabricated by co-precipitation method at different temperatures and atmospheres. X-ray diffraction, Energy dispersive X-ray spectroscopy and Raman results indicate that the samples were crystalline with wurtzite structure and no metallic Co or other secondary phases were found. Raman results indicate that the Co-doped ZnO powders annealed at different temperatures have different oxygen vacancy concentrations. The oxygen vacancies play an important role in the magnetic origin for diluted magnetic semiconductors. At low oxygen vacancy concentration, room temperature ferromagnetism is presented in Co-doped ZnO rods, and the ferromagnetism increases with the increment of oxygen vacancy concentration. But at very high oxygen vacancy concentration, large paramagnetic or antiferromagnetic effects are observed in Co-doped ZnO rods due to the ferromagnetic-antiferromagnetic competition. In addition, the sample annealed in Ar gas has better magnetic properties than that annealed in air, which indicates that O₂ plays an important role. Therefore, the ferromagnetism is affected by the amounts of structural defects, which depend sensitively on atmosphere and annealing temperature.     

Effect of lattice strain on the oxygen vacancy formation and hydrogen adsorption at CeO2(111) surface

Using first-principles calculation, the effect of lattice strain on the oxygen vacancy formation at CeO₂(111) surface has been investigated. The tensile strain facilitates the oxygen vacancy formation at the surface and the compressive strain hinders the process. This is in part due to the strengthening or weakening of the surface Ce–O bond under the lattice strain. On the other hand, a more open surface with a larger lattice constant can better accommodate the larger Ce³⁺ and thus facilitate the structural relaxation of the reduced surface. The studies on the strain effect on the atomic hydrogen adsorption at the defect-free CeO₂(111) surface show that the adsorption strength monotonously increases with the increase of the lattice strain, further confirming the tunable surface chemical activity by lattice strain.     

氧空位对钴掺杂氧化锌半导体磁性能的影响

从实验和理论上阐述了氧空位对Co掺杂ZnO半导体磁性能的影响.采用磁控溅射法在不同的氧分压下制备了Zn_095Co_005O薄膜,研究了氧分压对薄膜磁性能的影响.实验结果表明,高真空条件下制备的Zn_095Co_005O薄膜具有室温铁磁性,提高氧分压后制备的薄膜铁磁性逐渐消失.第一性原理计算表明,在Co掺杂ZnO体系中引入氧空位有利于降低铁磁态的能量,铁磁态的稳定性与氧空位和Co之间的距离密切相关. 关键词： Co掺杂ZnO 稀磁半导体 第一性原理计算 氧空位缺陷     

Spectroscopic studies of interfacial structures of CeO2-TiO2 mixed oxides

We have investigated the geometric and electronic structures of the cerium oxide (CeO₂)-titanium dioxide (TiO₂) mixed oxides with various Ce/TiO₂ weight ratios prepared by the sol-gel method in detail by means of X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS), Raman spectroscopy excited by 325 and 514.5 nm lasers, and scanning electron microscope (SEM). Existence of cerium effectively inhibits the phase transition of TiO₂ from the anatase phase to the rutile phase. XRD peaks of TiO₂ anatase attenuate continuously with the increasing amount of CeO₂ in the mixed oxide, but the XRD peaks of cubic CeO₂ appear only after the weight ratio of Ce/TiO₂ reaches 0.50. The average crystalline sizes of TiO₂ anatase and cubic CeO₂ in CeO₂-TiO₂ mixed oxides are smaller than those in the corresponding individual TiO₂ anatase and cubic CeO₂. Raman spectroscopy excited by the 514.5 nm laser detects CeO₂ after the weight ratio of Ce/TiO₂ reaches 0.70 whereas Raman spectroscopy excited by the 325 nm laser detects CeO₂ after the weight ratio of Ce/TiO₂ reaches 0.90. XPS results demonstrate that Ti exists in the form of Ti⁴⁺ in the CeO₂-TiO₂ mixed oxide. Ce is completely in the form of Ce³⁺ in the mixed oxides with a 0.05 weight ratio of Ce/TiO₂. With the increasing weight ratio of Ce/TiO₂, Ce⁴⁺ dominates. On basis of these results, we proposed that CeO₂ initially nucleates at the defects (oxygen vacancies) within TiO₂ anatase, forming an interface bridged with oxygen between CeO₂ and TiO₂ anatase. At the interface, Ce species cannot substitute Ti⁴⁺ in the lattice of TiO₂ anatase whereas Ti⁴⁺ can substitute Ce⁴⁺ in the lattice of cubic CeO₂. The decreasing concentration of oxygen vacancies, the Ti-O-Ce interface, and the decreasing average crystalline size of TiO₂ anatase act to inhibit the phase transformation of TiO₂ anatase. With the increasing amounts of CeO₂, the CeO₂ clusters continuously grow and form cubic CeO₂ nanocrystals. Spectroscopic results strongly demonstrate that the surface region of CeO₂-TiO₂ mixed oxide is enriched with TiO₂.     

Effect of Y2O3 addition on the conductivity and elastic modulus of (CeO2)1 − x(YO1.5)x

The conductivity and elastic modulus of (CeO₂)_1 − x(YO_1.5)_x for x values of 0.10, 0.15, 0.20, 0.30, and 0.40 were investigated by experiments and molecular dynamics simulations. The calculated conductivity exhibited a maximum value at approximately 15 mol% Y₂O₃; this trend agreed with that of the experimental results. In order to clarify the reason for the occurrence of the maximum conductivity, the paths for the transfer of oxygen vacancies were counted. The numerical result revealed that as the content of Y₂O₃ dopant increases, the number of paths for the transfer of oxygen vacancies decreases, whereas the number of oxygen vacancies for conductivity increases. Thus, the trade-off between the increase in the number of vacancy sites and the decrease in the vacancy transfer was considered to be the reason for the maximum conductivity occurring at the Y₂O₃ dopant content of approximately 15 mol%. The calculated elastic modulus also exhibited a minimum value at approximately 20 mol% Y₂O₃, which also agreed with the experimental results. It was shown that the Y–O–Y bonding energy increased with the increasing content of Y₂O₃ dopant. Thus, the trade-off between the increase in the number of vacancy sites and that in the Y–O–Y bonding energy was considered to be the reason for the minimum elastic modulus occurring at the Y₂O₃ dopant content of approximately 20 mol%.     

Room-temperature ferromagnetism in lightly Cr-doped ZnO nanoparticles

Zn_1−x Cr _x O (0≤x≤0.15) nanoparticles were synthesized by an auto-combustion method and characterized by x-ray diffraction and Raman scattering techniques. The solubility limit for Cr in ZnO was determined as x≈0.03. Room-temperature ferromagnetism (RT-FM) was observed in lightly Cr-doped ZnO nanoparticles with x=0.01 and 0.02. Raman scattering spectra of the lightly Cr-doped and Co-doped ZnO were studied and compared. The enhancement of both the magnetization and the intensity of Raman scattering peak associated with donor defects (Zn_i and/or V_O) and carriers indicates that light Cr doping in ZnO could be an effective way to achieve pronounced RT-FM and the ferromagnetism is closely related to the dopant-donor hybridization besides the ferromagnetic Cr–O–Cr superexchange interactions.     

Effect of 3.7 at% F doping on the atomic structure and reducibility of CeO2(111) surface: A first principles calculation

Density functional theory calculations was conducted to clarify effect of fluorination on the reducibility of CeO₂ polishing powder. The atomic structure and O vacancy formation energy of F-doped CeO₂(111) surface were systematically calculated, and electronic structure was also analyzed to understand the calculation results. It showed that the CeO_1.963(111) surface needs absorb 7.853 eV to be reduced further, so it is difficult to generate the second O vacancy (V₂). After fluorination, the first O vacancy (V₁) in CeO_1.926F_0.037(111) surface caused adjacent atoms to move significantly, and O vacancy formation energy decreased to 2.913 eV, which promoted the formation of V₂. Moreover, electronic structure calculation also showed that the interaction between O2p and Ce4f5d orbits was enhanced in reduced CeO_1.963(111) surface, which hindered the formation of V₂. The F2p orbit in CeO_1.926F_0.037(111) surface moved towards the low energy level, V₁ made the reduced surface stable and promoted the formation of V₂.     

Growth and scintillation properties of doped ZnWO4 crystals

A series of ZnWO₄ crystals with different purities of WO₃ and ZnO were grown by conventional Czochralski method and compared with Fe:ZnWO₄, Nb:ZnWO₄, Ag:ZnWO₄ and Ce:ZnWO₄ crystals grown by the same technology. Scintillation experiments revealed that the light yield of undoped ZnWO₄ crystals varied with the purities of raw materials WO₃ and ZnO. Raising the concentration of iron ions in ZnWO₄ crystals resulted in reduction of the light yield from the lighter appearance. On the contrary, the light yield was enhanced by doping Ag₂O, Nb₂O₅ and CeO₂, equivalent to that observed in ZnWO₄ crystals after oxygen-rich annealing at high temperature. The mechanisms of coloration and decolorization in doped ZnWO₄ crystals were analyzed. And the relationship between enhanced scintillation properties and the color center complex of iron impurity and oxygen vacancy in the crystals was discussed.     

A density functional theory study of formaldehyde adsorption and oxidation on CeO2(1 1 1) surface

The effects of different oxygen species and vacancies on the adsorption and oxidation of formaldehyde over CeO₂(1 1 1) surface were systematically investigated by using density functional theory (DFT) method. On the stoichiometric CeO₂(1 1 1) surface, the C-H bond rupture barriers of chemisorbed formaldehyde are much higher than that of formaldehyde desorption. On the reduced CeO₂(1 1 1) surface, the energy barriers of C-H bond ruptures are less than those on the stoichiometric CeO₂(1 1 1) surface. If the C-H bond rupture occurs, CO and H₂ form quickly with low energy barriers. When O₂ adsorbs on the reduced (1 1 1) surface (O₂/O_v species), the C-H bond rupture barriers of formaldehyde are greatly reduced in comparison with those on the stoichiometric CeO₂(1 1 1) surface. If O₂ adsorbs on oxygen vacancy at sub-layer surface, its oxidative roles on formaldehyde are much similar to that of O₂/O_v species.     

Determination of surface oxygen vacancy position in SnO2 nanocrystals by Raman spectroscopy

Raman spectra acquired from SnO₂ nanocrystals with different sizes show a size-independent Raman mode at ∼574 cm⁻¹. The intensity increases as the nanocrystal size decreases and this tendency is contrary to that of the normal bulk Raman modes. By considering the existence of oxygen vacancies at the nanocrystal surface, we adopt the density functional theory to calculate the Raman spectra with different oxygen vacancy positions and concentrations. The results clearly demonstrate that the in-plane oxygen vacancy is responsible for the 574 cm⁻¹ mode and the intensity enhancement is a result of the higher in-plane oxygen vacancy concentration.     

Selective CO oxidation in hydrogen-rich gas over CuO/CeO2 catalysts

A series of CuO/CeO₂ catalysts with different Cu-Ce compositions were synthesized by co-precipitation method and characterized by X-ray diffraction, H₂-TPR, CO-TPD, SEM and X-ray photoelectron spectroscopy (XPS) techniques. The effects of Cu-Ce composition and water vapor on the catalytic properties for the selective CO oxidation in the hydrogen-rich gas were investigated. The results indicated that CuO (10%)/CeO₂ catalyst remained the maximum CO conversion and selectivity at 140 and 160 °C, while the performance of CuO/CeO₂ catalysts deteriorated with the CuO molar ratio further increased. The interfacial CuO and CeO₂ interaction and synergistic effect enhanced the redox properties of CuO/CeO₂ catalyst and the highly dispersed copper species were proposed as the active sites for the selective CO oxidation. The blockage of catalytic active sites by absorbed water and the formation of CO-H₂O surface complexes reduced the activity of CuO (10%)/CeO₂ catalyst. The decreasing of surface lattice oxygen and absorbed oxygen species and the agglomeration of copper particles were the plausible interpretations for the deactivation of CuO (10%)/CeO₂ catalyst.     

Study of oxygen vacancy ordering in mullite at high temperatures

High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al₂O₃:SiO₂ ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature T_c > 3000°C.     

Distribution of oxygen vacancies and gadolinium dopants in ZrO2–CeO2 multi-layer films grown on α-Al2O3

Gdolinia doped ZrO₂ and CeO₂ multi-layer films were deposited on α-Al₂O₃ (0001) using oxygen-plasma-assisted molecular-beam epitaxy. Oxygen vacancies and Gd dopant distributions were investigated in these multi-layer films using X-ray diffraction (XRD), conventional and high-resolution transmission electron microscopy (HRTEM), annular dark-field imaging in scanning transmission electron microscopy (STEM), X-ray energy dispersive spectroscopy (EDS) elemental mapping and X-ray photoelectron spectroscopy (XPS) depth profiling. EDS and XPS depth profiling reveal that the Gd concentration in the ZrO₂ layer is lower than that in the CeO₂ layer. As a result, a higher oxygen vacancy concentration exists in the CeO₂ layers compared to that in the ZrO₂ layers. In addition, Gd is found to segregate only at the interfaces formed during the deposition of CeO₂ layers on ZrO₂ layers. On the other hand, the interfaces formed during the deposition of ZrO₂ layers on CeO₂ layers did not show any Gd segregation. The Gd segregation behavior at every other interface is believed to be associated with the low solubility of Gd in ZrO₂.     

Nonvolatile holographic recording in Ti, Fe Co-doped LiTaO3 crystal

Ti, Fe Co-doped LiTaO₃ (LT) crystals have been grown by the Czochralski method from the congruent melts. The absorption spectra of crystal were measured before and after ultraviolet illumination. Holograms have been recorded in doubly doped crystals with continuous-wave laser light by use of two-color method. The maximum value of refractive-index changes 7×10⁻⁵ is achieved.     

Study of ferromagnetism in Co-doped rutile powders and float-zone grown single crystals

Co-doped rutile samples in the form of both powders and bulk single crystals have been studied with particular emphasis on the dependence of their magnetic, compositional and structural properties upon the type of atmosphere used during their preparation. Both powders and single crystals were characterized using X-ray diffractometry and vibrating sample magnetometry, while the crystals were also studied using the X-ray Laue technique, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that an oxygen deficient environment during the preparation of Co-doped TiO₂ powders is crucial for the observation of room temperature ferromagnetism, while preparation in oxygen rich conditions destroys the ferromagnetism due to the formation of the paramagnetic second phase CoTiO₃. Floating zone growth of crystals under oxygen also led to the formation of material containing second phase CoTiO₃ that was paramagnetic at room temperature, while crystals grown under argon were ferromagnetic and contained Co-rich inclusions.     

Oxygen vacancy in N-doped Cu2O crystals: A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

Synthesis and Optical Properties of SiO2-Coated CeO2 Nanoparticles

Silica (SiO₂)-coated ceria (CeO₂) nanoparticles were prepared using water-in-oil microemulsion. Polyoxyethylene (15) cetylether and cyclohexane were used as a surfactant and organic solvent. SiO₂-coated CeO₂ nanoparticles were obtained by hydrolysis of metal alkoxide (tetraethylorthosilicate, TEOS) in the solution containing CeO₂ precursor nanoparticles. The effects of CeO₂ sources (Ce metal salt) and CeO₂ particle-forming agents on the morphology of SiO₂–CeO₂ particles were investigated. Observation via transmission electron microscopy revealed that the type of particle-forming agent affected the nanoparticles' morphology and that CeO₂ nanoparticles were spherically coated with SiO₂ when using oxalic acid ((COOH)₂) as a particle-forming agent of CeO₂. Furthermore, the transmittance of the particles was high in the visible region (above 400 nm) and decreased in the ultraviolet region.     

Effects of oxygen vacancy location on the electronic structure and spin density of Co-doped rutile TiO2 dilute magnetic semiconductors

According to density functional theory （DFT） using the plane wave base and pseudo-potential, we investigate the effects of the specific location of oxygen vacancy （Vo） in a （Ti,Co）06 distorted octahedron on the spin density and magnetic properties of Co-doped rutile Ti02 dilute magnetic semiconductors. Our calculations suggest that the Vo location has a significant influence on the magnetic moment of individual Co cations. In the case where two Co atoms are separated far away from each other, when the Vo is located at the equatorial site of a Co-contained octahedron, the ground state of the two Co cations is d6（t3g↑, t23g ↓） without any magnetic moment. However, if the Vo is located at the apical site, these two Co sites have different ground states and magnetic moments. The spin densities are also observed to be modified by the exchange coupling between the Co cations and the location of Vo. Some positive spin polarization is induced around the adjacent O ions.     

The surface properties and the activities in catalytic wet air oxidation over CeO2-TiO2 catalysts

No-noble metal CeO₂-TiO₂ catalysts prepared by sol-gel method were developed and examined for catalytic wet air oxidation (CWAO) of acetic acid. The structure of the catalysts was measured by BET, SEM, XRD, XPS and DTA-TG. We investigated the effect of the interactions of Ce and Ti on the structure of CeO₂-TiO₂ catalysts. The mechanisms of the relationships between the different content of Ti and the activity of CeO₂-TiO₂ catalysts were discussed. The results showed that the average crystal size of CeO₂ decreased and the surface areas increased; the low valence of Ce³⁺ increase, and the chemisorbed oxygen slightly decreased with the increase of Ti content on the surface of CeO₂-TiO₂ catalysts. The order of the activity in CWAO of acetic acid followed: Ce/Ti 1/1 > Ce/Ti 3/1 > Ce/Ti 1/3 > Ce/Ti 5/1 > CeO₂ > TiO₂ > no catalyst. In CWAO of acetic acid, the optimal atomic ratio of Ce and Ti was 1, and the highest COD removal was over 64% at 230 °C, 5 MPa and 180 min reaction time over Ce/Ti 1/1 catalyst. The excellent activity and stability of CeO₂-TiO₂ catalysts was observed in our study.     

氧含量对CaCu3Ti4O12巨介电常数和介电过程的影响

本文研究了在真空、空气和氧气中烧结制备的三种 CaCu₃Ti₄O₁₂陶瓷材料的介电特性. 交流阻抗测量结果表明在10—300 K温度范围, 三种样品的介电温谱中均出现三个平台, 其电阻实部和电容虚部在相应温度出现损耗峰, 真空条件烧结的样品具有较高的介电平台和较明显的电阻实部与电容虚部峰值, 表明氧含量和氧空位对CaCu₃Ti₄O₁₂的介电性质具有重要影响, 介电温谱出现的三个平台分别源于晶粒、晶界及氧空位陷阱.温谱分析表明晶粒的激活能与烧结气氛有较大关系,氧空位引起的电子短程跳跃及跳跃产生的极化子是晶粒电导和电容的主要起源.氧空位陷阱的激活能基本与烧结气氛无关,约为0.46 eV. 氧空位对载流子的陷阱作用是CaCu₃Ti₄O₁₂ 低频高介电常数的重要起源.