Types Of Oxygen Vacancies Research Articles

Oxygen Polyvacancies as Conductive Filament in Zirconia: First Principle Simulation

ZrO2 is the high-κ dielectric perspective for nonvolatile memory applications [1]. Currently ZrO2 is investigated as potential active medium for the Resistive Random Access Memory [2]. One of the wide spread model of resistive effect based on the filament formation/dissociation. The simplest filament model is a chain of oxygen vacancies, i.e. oxygen polyvacancy. The study of oxygen vacancies chain formation possibility (energy benefit) in ZrO2and their ability to participate in conduction is of great theoretical and practical interest. The modeling was carried out in the ZrO2 cubic (c-), tetragonal (t-) and monoclinic (m-) phase. Simulations were performed in terms of the hybrid DFT using a B3LYP functional in the Quantum-ESPRESSO code. The 192-atom c-, t-ZrO2, and 96-atom m-ZrO2 supercells were used to simulate the oxygen polyvacancy. Each subsequent atom removal from the supercell is determined from the point of view of the minimum formation energy. This model reflects a gradual increase in the polyvacancy size. The trap thermal E th and optical E optionization energies are estimated by the formulas offered in [3]. The calculated band gap values of 5.7, 6.1 and 5.6 eV for c-, t- and m-ZrO2are consistent with the corresponding experimental values 5.1, 5.78, and 5.83 eV [4]. The 4- and 3-fold coordinated (4f and 3f) O vacancy formation energies, 6.11 and 6.17 eV, predict the 4f vacancies prevalence in the real structure. The Kohn–Sham levels of five VO charged states (q = -2, -1, 0, 1, 2) localized in the band gap indicate that any type of oxygen vacancies can capture both electrons and holes. The second added electron leads either to the singlet (V-2(s)) or a triplet (V-2(t)) defect state, and the latter is 0.06 eV lower in energy. The significant depth of negatively charged defect levels indicates a polaronic effect. The defect peaks width lower than 0.1 eV in TDOS spectra indicate the charge localization in space. The charge density spatial distribution of the excess charge also confirms the charge localization. The positive charge is distributed evenly between the nearest Zr atoms. An additional electron leads to a charge density distribution among two closest to each other Zr atoms, and one can say that there is a kind of Zr-Zr bond. The calculated values of E th can be interpreted as the energy gain when the conduction band bottom electron capture onto the defect. The positive E th values indicate that the charge capture to the O vacancy in m-ZrO2 is energetically favorable. Thus, an oxygen vacancy in the ZrO2 is the amphoteric defect, it can act as the electron and hole trap, and participate in charge transport. The calculated values are consisted with E th = 1.15 [5] for ZrO2, and E th = 1.25 eV, E opt=2.5 eV for the HfO2 [6]. For V0 and V+, a good agreement of the calculation and experiment is observed. The O polyvacancy atomic structure in c-, t - and m-ZrO2 (Fig. 1) shows that each subsequent vacancy forms near the already existing one, and no more than the 2 removed O atoms relate to any Zr atom. One can assume that this result is valid for amorphous ZrO2, and the external electric field determines the preferred direction of the vacancies chains growth. This is consistent with the filament model of resistive switching. For t-ZrO2, the energetically favorable O polyvacancy atomic structure is a chain along the [100] direction. Each subsequent removal of oxygen atom adds one filled level to the band gap. These levels are distributed on the band gap preferentially localized close to the monovacancy level. The oxygen polyvacancy growth in size yields a broad defective band in the energy spectrum of ZrO2 filled with electrons. Since, for the single oxygen vacancy, the extra electron forms something like a Zr-Zr bond, one can assume that ZrO2 can acquire conductivity, which is carried out through the channel of the Zr ions separating the vacancies in this polyvacancy chain. The work was supported by the Russian Science Foundation, grant #16-19-00002. The modeling was performed on a cluster of the Institute of Semiconductor Physics SB RAS. [1] X.D.Huang, R.P.Shi, P.T.Lai, Appl. Phys. Lett. 104, 162905 (2014). [2] C.-H.Lai, H.-W. Chen, C.-Y.Liu, Materials 9, 551 (2016). [3] D.Muñoz Ramo, J.L.Gavartin, A.L.Shluger, et al.Phys. Rev. B 75, 205336 (2007). [4] R.H.French, S.J.Glass, F.S.Ohuchi, et al.Phys. Rev. B 49, 5133 (1994). [5] G.Jegert, A.Kersch, W.Weinreich, et al.Appl. Phys. Lett. 96, 062113 (2010). [6] V.A.Gritsenko, T.V.Perevalov, D.R.Islamov, Physics Reports 613, 1 (2016). Figure 1

Analytic Modeling for Nanoscale Resistive Filament Variation in ReRAM With Stochastic Differential Equation

Nanoscale filament is the active area in oxygen vacancy type resistance random access memory (ReRAM), which is formed stochastically during electric test after being fabricated in a clean room. That is, the filament dimension cannot be controlled with a designed mask pattern. Here, we introduce a formula to describe the current cycle-to-cycle trajectories based on a stochastic differential equation (SDE) with microscopic structure parameters: filament dimension and oxygen vacancy concentration. Since ReRAM conduction follows hopping, the filament can be described by a random resistance network (RRN). The stochastic configuration of an RRN follows the Brownian motion, which is the key parameter in the diffusion of SDE. The formula provides a practically quantitative filament characterization method, which is verified by direct observation of the filament in actual devices. Based on the formula, we can predict ReRAM endurance with the given microscopic structure parameters.

High-temperature dielectric properties of BaZr 0.8 Y 0.2 O 3-δ ceramics

Abstract Ceramic samples of BaZr 0.8 Y 0.2 O 3-δ (BZY) were synthesized by solution combustion based on the glycine nitrate process. The dielectric properties were investigated in the temperature range of 300–870 K and frequency range of 10 2 –10 6 Hz. Two types of carriers, hydroxyl ions and oxygen vacancies, coexist in the material and lead to three sets of relaxations. The activation energies were calculated to be 0.50, 0.65, and 1.27 eV for low-, middle- and high-temperature relaxations, respectively. The low-temperature relaxation was ascribed to be a dipolar relaxation resulting from the OH O • -Y dipole. The middle- and high-temperature relaxations were argued to be Maxwell-Wagner relaxations associated with space charges caused by hopping oxygen vacancies blocked by grain boundaries and sample-electrode contacts, respectively.

Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2

High-κ hafnia/zirconia solid solutions Hf0.5Zr0.5O2 are promising for advanced microelectronics. It is known that the noncentrosymmetric orthorhombic (Pbc21) Hf0.5Zr0.5O2phase exhibits ferroelectric properties. This new ferroelectric material is of significant interest due to a potential possibility of replacement of traditional perovskite-based ferroelectrics in ferroelectric random access memory (FeRAM) [1]. In the present study the electronic structure of oxygen vacancies (VO) in o-Hf0.5Zr0.5O2 was investigated with quantum-chemical simulations using the Quantum-ESPRESSO code. The simulations are based on the density functional theory with B3LYP hybrid exchange-correlation functional. We use the periodic (96 atom) supercells model. The structure was obtained by the replacement of half of the Hf atom to Zr in the orthorhombic HfO2 cell [2]. Three types of VO with fourfold «HHZZ» and threefold coordinated «HHZ», «HZZ» were investigated. 20 nm thick orthorhombic Hf0.5Zr0.5O2 were synthesized by ALD technique with following rapid thermal annealing at 400 °C. X-ray photoelectron spectra was obtained using synchrotron radiation with a photon energy of 720 eV for the analysis of the chemical composition and 200 eV to record the spectrum of the valence band. Film stoichiometry was calculated as ratios of the integrated intensities of the main XPS lines of the metal atoms show the stoichiometry close to Hf0.5Zr0.5O2. The calculated band gap of o-Hf0.5Zr0.5O2 is 5.65 eV. The Kohn–Sham levels of five VO charged states (q = -2, -1, 0, 1, 2) localized in the band gap indicate that any type of oxygen vacancies can capture both electrons and holes. Note, that the oxygen vacancies with a trapped electron is a magnetic defect. The added electron localizes itself in an energy well due to strong neighboring atom relaxation (polaron effect). The values of thermal (E th) and optical (E opt) ionization energies were calculated for different charge states as described in [3]. It was found that the oxygen vacancy can act as an amphoteric localization center (trap) for charge carriers, i.e. it can capture both electrons and holes. The calculated E th and E opt values are in satisfactory agreement with the values of thermal (W t=1.25 eV) and optical (W opt=2.5 eV) trap energies, obtained from experiments on charge transport. Charge localization on the oxygen vacancy is confirmed by the spatial distribution of the electron density in VO and its first coordination sphere. The positive charge is distributed approximately evenly between the metal atoms nearest to VO, while the negative charge is distributed unevenly, and only a couple of atoms are bounded. The position of each subsequent atom for removing was found by examining of all the possible variants and determining the lowest defect formation energy. The oxygen atoms in o-Hf0.5Zr0.5O2, which are removed, have an HHZZ type and are placed at a significant distance from each other. The forming energy of triply coordinated VO (about 6.4 eV) is substantially higher than that for fourfold coordinated VO (about 6.1 eV). The formation of closely spaced VO (i.e. polyvacancy) in o-Hf0.5Zr0.5O2 is not profitable energetically. On the contrary, for the cubic, tetragonal and monoclinic HfO2 and ZrO2 formation energies of three- and fourfold coordinated VO is not so much different from each other, the it is energy profitable to form oxygen vacancies close to each other (there is some clustering of vacancies). Probably, this is the reason why the o-Hf0.5Zr0.5O2 film etching with Ar+ ions does not lead to metal enrichment, whereas the etching of amorphous HfO2 and ZrO2 leads to the generation high concentration of VO in the surface layer of the films. As a result of Hf0.5Zr0.5O2 etching with Ar+ ions, the XPS lines corresponding to Hf 4f and Zr 3d in the metallic state are not observed, and theoretically predicted XPS peak does not appear above the edge of the valence band. It is concluded that the original Hf0.5Zr0.5O2 films are not enriched with metal, and etching with Ar+ ions does not lead to depletion of oxygen. However, the experimental data do not exclude the oxygen vacancies presence in the Hf0.5Zr0.5O2 film, since XPS technique is insensitive to defects with density less than 0.1%. The work was supported by the Ministry of Education and Science of the Russian Federation, project #RFMEFI57514X0027 (Hf0.5Zr0.5O2 films synthesis), and by Russian Science Foundation, grant #14-19-00192 (XPS measurements, calculations). The modeling was performed on a cluster of the Institute of Semiconductor Physics SB RAS. [1] J.Müller, T.S.Böscke, U.Schröder, et al.Nano Lett. 12, 4318 (2012). [2] Q.Zeng, A.R.Oganov, A.O.Lyakhov, et al.Acta Cryst. C 70, 76 (2014). [3] D.Muñoz Ramo, J.L.Gavartin, A.L.Shluger et al. Phys. Rev. B 75, 205336 (2007).

Model for Stress Generation and Flow in Anodic Oxides

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, and other metals in baths that dissolve the oxide. Procedures to grow films with highly ordered and controllable arrangements of nanoscale pores have led to the extensive use of PAO films in nanofunctional devices. Experiments and calculations show that transport in the oxide occurs by electrical migration coupled with oxide flow driven by mechanical stress (1-3). Pores initiate by a morphological instability at the oxide-solution interface when the initially formed barrier oxide reaches a critical thickness (4). In another presentation at this meeting, we showed that this instability is preceded by the buildup of compressive stress due to localized electric field-induced electrolyte anion incorporation at the oxide-solution interface. The instability is modeled by competition between stabilizing oxide formation at the oxide-solution interface and destabilizing oxide flow driven by gradients of anion-induced surface stress. The model predicts important features of self-ordered PAO growth on many metals: the pore separation-voltage scaling ratio, the narrow ranges of anodizing efficiency, and the threshold electric field. Characteristics of anodic oxides that enable self-organized PAO formation include their ability to flow and to accumulate highly localized stress at interfaces due to electrochemical reactions. Here we report a new model for coupled interfacial reactions, electrochemical transport and mechanics of amorphous anodic oxides. In the model, ionic conduction occurs by high-field electrical migration of oxygen and metal vacancy type defects. In contrast with other point defect models, vacancies are created and destroyed homogeneously within the oxide by electric-field stimulated localized relaxations of the amorphous network. Vacancy creation expands the oxide network generating compressive stress, while vacancy destruction contracts the network producing tensile stress. Field-stimulated vacancy formation and removal affect the conductance of anodic films and hence may be probed by electrochemical impedance spectroscopy (EIS). Impedance measurements during steady-state Al anodizing in sulfuric and oxalic acids are presented. The spectra reveal inductive loops between 1 and 100 Hz as reported elsewhere (5). The characteristic time constant and resistance of the inductive loop are shown to be determined by the kinetic parameters of the vacancy formation/removal reaction. The values of these parameters are independent of anodizing current density or solution composition. The kinetic model for vacancy formation/removal identified by EIS was used to simulate evolution of stress distributions in the oxide during Al anodizing in phosphoric acid at a constant current density of 5 mA/cm2 (6). Calculations demonstrate that vacancies are continuously generated at the oxide-solution interface to accommodate incorporation of large electrolyte oxyanions. This process is shown to be quantitatively consistent with localized compressive stress buildup at this interface as revealed by in situ stress measurements, the precursor for oxide flow and pore formation. At the metal interface, vacancies produced by metal oxidation are destroyed continuously at the metal interface, consistent with the buildup of localized tensile stress found experimentally. Finally we discuss implications of vacancy formation/removal for oxide flow. It is argued that at sufficiently high stress levels, this reaction can account for displacement of oxide volume driven by stress gradients. Such a process is equivalent to vacancy creep and can explain the apparent viscous flow of anodic oxides reported in several papers (1-3,7). ACKNOWLEDGMENT This work was supported by the the National Science Foundation through NSF-CMMI-100748. REFERENCES 1. S. J. Garcia-Vergara et al., Electrochim. Acta. 52, 681 (2006). 2. J. E. Houser and K. R. Hebert, Nature Mater. 8, 415 (2009). 3. J. Oh and C. V. Thompson, Electrochim. Acta, 56, 4044 (2011). 4. K. R. Hebert et al., Nature Mater. 11, 162 (2012). 5. M. Curioni et al., Electrochim. Acta 55, 7044 (2010). 6. Ö. Ö. Çapraz, et al., Electrochim. Acta 167, 404 (2015). 7. D. H. Bradhurst and J. S. L. Leach, J. Electrochem. Soc., 113, 1245 (1966).

STEM-Cathodoluminescence of SnO2 nanowires and powders

For the first time, ultra-high spatial resolution STEM-cathodoluminescence has been applied to SnO2 nanowires and nanoparticles in order to study the spatial distribution of point defects that form deep levels in the band gap and often dictate their gas-sensor performance. SnO2 nanowires consistently had emission signals centered at 1.76, 1.99, and 2.45eV. Emission at 1.99eV was found to be more intense with the STEM probe at the edges of nanowires and in higher surface-area nanoparticles compared to emission most intense at 2.45eV with the probe in the center of a nanowire or particle. This result is contrary to recent studies proposing that both emission energies are associated with two different surface oxygen vacancy types. The ability to spatially map the luminescence of a single nanostructure also facilitated much less-ambiguous spectral deconvolution consistent across many particles which gives confidence in properly assigning the energies of each defect state. Individually-probed SnO2 nanowires were found to have much more consistent and reproducible defect states compared to commercial nanoparticles which has implications in stability as gas sensors and efforts to model interactions between analyte gases and the oxide surface.

Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2

The electronic structure of oxygen vacancies in noncentrosymmetric orthorhombic Hf0.5Zr0.5O2 was investigated with quantum chemical density functional theory simulations. The oxygen vacancies with different coordination in five charged states were investigated. The single electron levels of charged states is located in the band gap, as well as the values of trap thermal and optical ionization energies, indicate that any type of oxygen vacancies can capture both electrons and holes, i.e. they can act as an amphoteric localization center (trap) for charge carriers. The vacancy levels position in the band gap for an added electron is in excellent agreement with the experimental trap thermal ionization energy 1.25 eV for the amorphous Hf0.5Zr0.5O2 film. X-ray photoelectron spectroscopy revealed that high oxygen vacancies concentration generation by the Ar+ etching method is not appropriate for o-Hf0.5Zr0.5O2.

Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of γ-Monoclinic WO3

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.

Regulation of oxygen vacancy types on SnO2 (110) surface by external strain

In tin dioxide nanostructures, oxygen vacancies (OVs) play an important role in their optical properties and thus regulation of both OV concentration and type via external strain is crucial to exploration of more applications. First-principle calculations of SnO2 (110) surface disclose that asymmetric deformations induced by external strain not only lead to its intrinsic surface elastic changes, but also result in different OV formation energy. In the absence of external strain, the energetically favorable oxygen vacancies(EFOV) appear in the bridging site of second layer. When -3.5% external strain is applied along y direction, the EFOV moves into plane site. This can be ascribed that the compressed deformation gives rise to redistribution of electronic wave function near OVs, therefore, formation of newly bond structures. Our results suggest that different type OVs in SnO2 surface can be controlled by strain engineering.

Cause of the appearance of oxygen vacancies in yttria-stabilized zirconia and its relation to 2.8 eV photoluminescence

When we implanted P+ or B+ ions into yttria-stabilized zirconia (YSZ), its crystallinity was degraded. Concurrently, the photoluminescence at around 2.8 eV originating from two types of oxygen vacancies with one or two captured electrons became weak, indicating a decrease in the number of oxygen vacancies. Oxygen vacancies appear in YSZ as a result of the replacement of Zr4+ by Y3+ in ZrO2. Therefore, it seems that the separation of YSZ into ZrO2 and Y2O3 induced by the ion implantation is responsible for the decrease in the number of oxygen vacancies. Moreover, the intensity of the 2.8 eV photoluminescence returns to the value before the ion implantation if the sample is annealed thermally after the implantation at temperatures higher than the crystallization temperature of YSZ. The reaction opposite to the above seems to be induced by the thermal annealing.

Insight for Formation of Oxygen Deficiency in Spinel-Type LiNi0.5Mn1.5O4-δ Using Ab Initio DFT Calculations

Spinel-type LiNi0.5Mn1.5O4 (LNMO) have attracted attention as cathode materials for lithium-ion batteries due to its high-voltage at ~ 4.7 V vs. Li+/Li. LNMO is considered to belong to two different crystal structures such that Ni/Mn ordered P4332 and disordered Fd-3m. Relationship between crystal structures and their electrochemical performances has been experimentally- and computationally- investigated, showing that disordered Fd-3m structure contains oxygen deficiency with reduction of Mn from Mn4+ to Mn3+. Although the presence of Mn3+ improve electronic conductivity, Jahn-Teller distortion and dissolution of Mn occurs. Phase transition from disordered Fd-3m to ordered P4332 occurs with diminish oxygen deficiency by annealing in oxygen atmosphere at 700 ̊C. Understanding the formation mechanism of the oxygen deficiency would be helpful to synthesis aimed crystal structure. However, computational studies with oxygen deficient model are limited so far. In this study, we evaluate the defect formation energies in oxygen deficient LNMO. The Vienna ab initio simulation package (VASP) was utilized with the generalized gradient approximation (GGA-PBEsol) + U and projector-augmented wave (PAW) methods. For the GGA + U calculations, the U values for the d-orbitals of Ni and Mn were set to 6.0 eV and 3.9 eV following relevant previous reports. An energy cutoff of 500 eV and a k-point mesh were chosen such that the product of the number of k-points and the number of atoms in the unit cell was greater than 1000. In the case of disordered Fd-3m structure, we used ‘uniform’ Ni/Mn distribution computationally reported by Lee et al., that is, the highest degree of local cation arrangements within random cation ordering. Coulombic energy was calculated for different 524 structures by Ewald method, the most stable Ni/Mn arrangement was consistent with ‘uniform’ distribution as mentioned above. The calculated lowest oxygen vacancy formation energies in P4332 and Fd-3m show 3.69 eV and 3.38 eV, respectively. Note that vacancy formation energy almost no difference in P4332 among different oxygen vacancy concentrations, whereas, in Fd-3m, vacancy formation energy decreased with increasing oxygen vacancy concentrations. This fact strongly suggests that Fd-3m structure promotes oxygen deficiency. In addition, not only oxygen vacancies type defect but also metal-excess type defect can be present as oxygen defect models. Formation energies for the system contains excess metals in P4332 and Fd-3m lattice show 2.98 eV and 2.58 eV, respectively. Formation energy for metal excess type in Fd-3m became 0.40 eV smaller than that in P4332, in agreement with experimental results that only Fd-3m contains oxygen defects. In addition, these values are lower than oxygen vacancy formation energies. These all finding indicates that metal-excess type oxygen deficiency can be more favorable than oxygen vacancy type oxygen deficiency for LNMO.

A microwave molecular solution based approach towards high-κ-tantalum(V)oxide nanoparticles: synthesis, dielectric properties and electron paramagnetic resonance spectroscopic studies of their defect chemistry.

Stable dispersions of tantalum oxide nanoparticles are accessible from solutions of tantalum(V) complexes with a mixed malonato and alkanolato ligand sphere in ethoxyethanol by microwave processing. The malonato ligand is cleaved during decomposition and acetic acid or acetic acid esters are formed as derived from in situ spectroscopic studies. The solubility of the tantalum precursor and the obtained particle size therefrom depend strongly on the type of alkanolato ligand moiety. Dispersions of the molecular complexes possess good film forming properties. Films with low surface roughness can be obtained by spincoating. These exhibited a dielectric constant of about 15 and disruptive strengths above 1.5 MV cm(-1). The electrical measurements indicate that the presence of moisture is detrimental with respect to the dielectric performance of the films. After removal of the solvent from the suspensions of the nanoparticles, the residue can be redispersed in aprotic solvents. The particles can be isolated therefrom by precipitation with pentane. XRD and HRTEM indicate that the material remains amorphous up to temperatures of 750 °C. XPS proved that only Ta2O5 is formed as lower oxidation states of Ta cannot be detected. A detailed EPR study allows us to gain insight into the surface defect chemistry. Multiple types of oxygen vacancies exist at the surface of the Ta2O5 particles which are influenced by additional calcination and annealing in a vacuum.

Existence of basal oxygen vacancies on the rutileTiO2(110)surface

On a rutile ${\mathrm{TiO}}_{2}\phantom{\rule{0.16em}{0ex}}(110)$ surface, vacancies at the bridging oxygen sites are thought to be major surface point defects and strongly influence the surface phenomena. Using systematic density functional theory (DFT) calculations, however, we show that vacancies at the basal oxygen sites have comparable formation energies to the bridging oxygen vacancies. The correction of self-interaction error for localized $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ states in DFT plays an important role for correctly describing the relative stability of the oxygen vacancies as well as the electronic structures. This new type of stable surface oxygen vacancy can be formed with a similar amount of the bridging oxygen vacancy in a relatively dilute condition, which may affect atom deposition and chemical reactions on the surface.

Interplay between External Strain and Oxygen Vacancies on Raman Spectra of SnO2

Comprehensive first-principle calculations on strained SnO2 crystal structure indicate that the formation energy of different types of oxygen vacancies depends on the external strain. Many novel Raman modes can be observed, their intensities and positions are strongly dependent on applied stain, which can be ascribed to crystal symmetry destruction by oxygen vacancies. Applied strain can compress/stretch distances between Sn and O atoms; therefore, Sn—O band vibration frequencies can be adjusted accordingly. Our calculated results disclose that the Raman spectra of SnO2 crystal structure with different types of oxygen vacancies are obviously different, which can be used to identify the oxygen vacancy types in strained SnO2 crystal structures.

Theory of oxidation/reduction-induced valence transformations of metal ion dopants in oxide crystals mediated by oxide-vacancy diffusion: II. Kinetic analysis

We consider theoretically valence transformations of doping metal ions in oxide crystals induced by oxidation and reduction obtained by changes in the ambient oxygen partial pressure. Three types of oxygen vacancies are assumed to mediate transformations: neutral, singly ionized, and doubly ionized. In the companion part I paper we provide thermodynamic analyses yielding concentration relations among the oxygen vacancy, metal ions, holes and electrons, as functions of the ambient oxygen pressure. In the present companion part II paper we provide time dependent concentration profiles of the various species and reaction rate profiles. The diffusion exhibits a complex behavior; under some conditions, it may be described by a constant diffusivity, and is symmetric with respect to oxidation and reduction. However, under a wide range of conditions, the ionic state changes are highly asymmetric with respect to oxidation and reduction. For example, in the case of a neutral vacancy, a very narrow reaction front may establish during reduction. In the inverse (oxidation) process, however, the different species' profiles are quite smooth.

Theory of oxidation/reduction-induced valence transformations of metal ion dopants in oxide crystals mediated by oxide-vacancy diffusion: I. Thermodynamic analysis

We consider theoretically valence transformations of doping metal ions in oxide crystals induced by oxidation and reduction obtained by changes in the ambient oxygen partial pressure. Three types of oxygen vacancies are assumed to mediate transformations: neutral, singly ionized, and doubly ionized. We provide thermodynamic equilibrium analyses, yielding concentration relations among the oxygen vacancy, metal ions, holes and electrons as functions of the ambient oxygen pressure. The results suggest that experimental study of different species concentrations at thermodynamic equilibrium as functions of pressure and temperature should allow assessment of various reversible reaction constants controlling the process. In the Part II companion paper, the kinetic (diffusion) characteristics are considered in detail.

ChemInform Abstract: Crystal Plane‐Dependent Surface Reactivity and Catalytic Property of Oxide Catalysts Studied with Oxide Nanocrystal Model Catalysts

AbstractReview: 139 refs.

Crystal Plane-Dependent Surface Reactivity and Catalytic Property of Oxide Catalysts Studied with Oxide Nanocrystal Model Catalysts

Oxides are widely used in heterogeneous catalysis, thus fundamental understanding of structure–activity relationships of oxide catalysts is of great importance for the improvement and design of efficient catalysts. Recently the synthesis of oxide nanocrystals with uniform and well-defined structures has achieved great progress, and these oxide nanocrystals consist of a novel type of model catalysts for oxide catalysts. Employing oxide nanocrystal model catalysts, crystal plane-dependent surface reactivity and catalytic property of oxide catalysts have been successfully revealed. The recent progress is reviewed in this article and two systems are highlighted. One is cuprous oxide whose crystal planes affect the surface reactivity and catalytic property dominantly by crystal plane-dependent surface composition and structure; the other is ceria whose crystal planes affect the surface reactivity and catalytic property dominantly by crystal plane-dependent concentration and type of oxygen vacancy.

Optical identification of oxygen vacancy types in SnO2 nanocrystals

The oxygen vacancies in spherical and cuboid SnO2 nanocrystals prepared by hydrothermal and laser ablation methods are investigated optically. Three oxygen-vacancy-related photoluminescence peaks at ∼430, ∼501, and ∼618 nm are observed, and Raman scattering and density functional calculation disclose that they originate from in-plane, sub-bridging, and bridging oxygen vacancies, respectively. This work reveals that the photoluminescence peaks together with the Raman modes can be used to identify the oxygen vacancy types in SnO2 nanostructures.

Nanocrystalline Ce1−xSmxO2−δ (x = 0.4) solid solutions: structural characterization versus CO oxidation

A nanocrystalline Ce–Sm–oxide solid solution, with an excellent redox property and remarkable oxygen storage/release capacity, has been synthesized by means of a simple and highly practicable coprecipitation method. To understand the thermal and textural stability, the synthesized catalyst was subjected to calcination at various temperatures (773–1073 K). Physicochemical characterization was achieved using XRD, HRTEM, BET surface area, Raman, ICP-OES, XPS, TG-TDA, UV-vis DRS, TPR, and FTIR techniques, and the catalytic performance was evaluated for the oxidation of CO. Coprecipitation of Ce4+ and Sm3+ ions through ultra-high dilute solutions provided the single phase Ce0.6Sm0.4O2−δ solid solution in the nanoscale range, as confirmed by XRD and TEM studies. Raman studies revealed two types of lattice defects, namely, oxygen vacancies and MO8 complex defects due to disparity in the oxidation state and ionic radius of Sm3+ and Ce4+, respectively. Calculations made from XPS atomic ratios (Ce/Sm) and Raman band intensity ratios (AD1/AF2g) indicated migration of Sm from the bulk to the surface at elevated temperatures that caused a negative effect on the oxygen vacancy concentration. The doping of Sm3+ into the ceria lattice effectively enhanced the reduction behaviour of ceria by shifting the surface and bulk reduction to lower temperatures. Remarkably, Sm-incorporation showed an optimistic influence on the oxygen storage ability and CO oxidation efficiency of ceria attributed to profound lattice defects and enhanced bulk oxygen mobility. The salient features of physicochemical characterization versus catalytic CO oxidation efficiency of Ce–Sm–oxide solid solutions have been elaborated in this article.