Chemical diffusion in complex oxides with an emphasis on BaTiO3

Introducing dopants is an important way to tailor and improve electronic properties of transition metal oxides used as high-k dielectric thin films and resistance switching layers in leading memory technologies, such as dynamic and resistive random access memory (ReRAM). Ta2O5 has recently received increasing interest because Ta2O5-based ReRAM demonstrates high switching speed, long endurance, and low operating voltage. However, advances in optimizing device characteristics with dopants have been hindered by limited and contradictory experiments in this field. We report on a systematic study on how various metal dopants affect oxygen vacancy formation in crystalline and amorphous Ta2O5 from first principles. We find that isoelectronic dopants and weak n-type dopants have little impact on neutral vacancy formation energy and that p-type dopants can lower the formation energy significantly by introducing holes into the system. In contrast, n-type dopants have a deleterious effect and actually increase the formation energy for charged oxygen vacancies. Given the similar doping trend reported for other binary transition metal oxides, this doping trend should be universally valid for typical binary transition metal oxides. Based on this guideline, we propose that p-type dopants (Al, Hf, Zr, and Ti) can lower the forming/set voltage and improve retention properties of Ta2O5 ReRAM.

Eighteen binary metal oxides consisting of TiO2–MmOn, ZnO–MmOn and Al2O3–MmOn (MmOn: metal oxide) were prepared by the usual co-precipitation method, their acid amounts and strengths being determined by n-butylamine titration using various acid-base indicators. The acid strengths of fourteen of the tested binary oxides of molar ratio 1 : 1 were found to be remarkably higher than those of the component single oxides. High acid strengths were as follows: H0≤−8.2 for TiO2–SiO2, H0≤−5.6 for TiO2–Al2O3 and Al2O3–ZrO2 and H0≤−3 for TiO2–CdO, TiO2–SnO2 and ZnO–SiO2. The acid amounts of sixteen binary oxides were larger than those of the component oxides. The effect of the composition of binary oxides on acidity was examined for TiO2–Al2O3, ZnO–Al2O3 and Al2O3–ZrO2. The acidity maxima appearing for TiO2–Al2O3 and ZnO–Al2O3 were found to be of molar ratio\simeq9:1 and for Al2O3–ZrO2\simeq3:2. A fairly good correlation has been demonstrated between the observed highest acid strengths and the average electronegativities of metal ions of binary oxides.

Highly volatile dimethylamine vapour sensing studies using titanium-vanadium mixed oxide thin films

The experimental results for chemical and isoconcentration diffusion of Zn in AlSb, GaAs, and GaP have been interpreted using Longini's interstitial/substitutional model. Satisfactory agreement is found with the isoconcentration data if the substitutional Zn is a fully ionized monovalent acceptor and the interstitial is a monovalent donor. The characteristic features of chemical diffusion are explained in terms of a non‐equilibrium situation dominated by the rate of supply of vacancies on the group III sub‐lattice and the flux of interstitial Zn donors. Diffusion‐induced dislocations are assumed to play a major role in the supply of vacancies. Satisfactory qualitative agreement with the chemical diffusion data is found.

Major intrinsic causes of cathode degradation in Li-ion batteries include instability against irreversible phase transformations, for example, layered to spinel transformation in LiMO2 type cathodes, and dissolution of the redox-active transition metal ions into the electrolyte. Corrosive species are known to attack the cathode particles and accelerate transition metal dissolution, which often leads to a significant capacity loss upon cycling. Hydrofluoric acid (HF), for example, forms in the presence of only trace amount of water in the common LiPF6 based electrolytes. A strong correlation was observed between HF content in the electrolyte and transition metal loss for common battery cathode materials including the layered LiCoO2, spinel LiMn2O4 and similar cathodes. For LiMn2O4, in particular, disproportionation of surface Mn3+ to Mn2+ and Mn4+, and subsequent dissolution of Mn2+ into the electrolyte is triggered by the H+ ion (that is, acidic environments), and is a primary reason for capacity fade in this material. This dissolved Mn deposits at the anode surface and further contributes to degradation. Cathode–electrolyte reactions can further cause formation of a resistive solid–electrolyte interface, as a byproduct of the electrolyte breakdown. While alternative strategies such as doping, tailoring the particle morphology or core–shell structures have been suggested, a common approach to suppressing cathode degradation has been applying protective coatings on cathode particles. Stable binary oxides, such as Al2O3, MgO, ZnO, ZrO2, SiO2 and TiO2 may reduce the HF-content in the electrolyte, but their performance in suppressing the transition metal-loss from the cathode or the capacity fade can vary significantly. However, the complex nature of reactions between the cathode, coating and electrolyte prohibited the design of generic guidelines to find effective coatings beyond such simple binary oxides. Recently, we introduced a density functional theory (DFT)-based materials design approach considering the thermodynamic aspects of binary metal oxide cathode coatings, which reproduced the known effective coatings such as Al2O3, and predicted trivalent transition metal oxides as a promising class of under-explored cathode coatings. This framework was limited to only binary metal oxides, because the description of HF-reactivity and electrochemical stability of coatings were described by hypothesized reactions based on ‘chemical intuition’ (that is, reactions that had predefined forms) and could not be extended to other more complex materials. In recent years, high-throughput (HT) computational methods have significantly accelerated the search for new and better battery components. Here we introduce a comprehensive HT materials design framework to discover cathode coatings by combining the Open Quantum Materials Database (OQMD), a large collection of HT DFT calculations of ~500,000 inorganic materials, with reaction models to describe thermodynamic stability, electrochemical stability and HF-reactivity for any oxygen-bearing coating with non-intuitive, fully automated prediction of reaction products. With this framework, we design coatings with various functionalities geared towards specific battery chemistries; namely, (1) physical barriers for acid-free electrolytes, (2) HF-barriers for cathode particles fully covered with coatings and (3) HF-scavengers for particles with patchy coatings requiring active protection from HF-attack. We screen more than 130,000 oxygen-bearing materials (oxides and oxyanion compounds) available in the OQMD, and use multiobjective optimization methods, namely weighted-sum and rank aggregation, to select the best candidates for several distinct coating categories based on desired function of the coating (HF scavenger, physical barrier, or HF barrier) . We further show that coatings optimized for a particular cathode material (here, for layered- LiCoO2 and spinel-LiMn2O4) can be designed by incorporating the cathode material itself into the chemical space; that is, considering the cathode-coating reactivity and including the cathode in all chemical reactions of the framework. Screening more than 130,000 oxygen-bearing materials, we suggest physical and hydrofluoric-acid barrier coatings such as WO3, LiAl5O8 and ZrP2O7 and hydrofluoric-acid scavengers such as Sc2O3, Li2CaGeO4, LiBO2, Li3NbO4, Mg3(BO3)2 and Li2MgSiO4. Using a design strategy to find the thermodynamically optimal coatings for a cathode, we further present optimal hydrofluoric-acid scavengers such as Li2SrSiO4, Li2CaSiO4 and CaIn2O4 for the layered LiCoO2, and Li2GeO3, Li4NiTeO6 and Li2MnO3 for the spinel LiMn2O4 cathodes. These coating materials have the potential to prolong the cycle-life of Li-ion batteries and surpass the performance of common coatings based on conventional materials such as Al2O3, ZnO, MgO or ZrO2.

Core-shell structured Ni6MnO8@carbon nanotube hybrid as high-performance pseudocapacitive electrode material

Al-doped SrTiO 3: Part II, unusual thermodynamic factor and chemical diffusivity

Microwave-assisted synthesis of Mn3O4-Fe2O3/Fe3O4@rGO ternary hybrids and electrochemical performance for supercapacitor electrode

ABSTRACTThe overall goal of this research is to explore techniques for the development of novel binary magnetic oxide nanoclusters uniformly distributed within a polymer matrix. These CoFe2O4 nanoclusters were synthesized at room temperature, and are confined within the self-assembled nanoscale structure exhibited by block copolymers templates. The diblock copolymers were synthesized by ring opening metathesis polymerization of norbornene and norbornene trimethylsilane and the binary magnetic oxide was introduced through a nanoreaction scheme using wet chemical methods. Transmission electron micrographs of microtomed thin sections of these nanocomposites show that the metal oxide nanoclusters are ellipsoidal in shape and are uniformly distributed within the polymer matrix. A SQUID magnetometer was used to study the magnetic properties of the polymeric nanocomposites at applied fields up to 5 Tesla and at a temperature range between 300 °K and 5 °K. Mössbauer spectroscopy was used to study the structure of the nanoconfined metal oxide, and confirmed the synthesis of CoFe2O4 nanoclusters exhibiting an inverse spinel structure. This study provided a better understanding of the nucleation, growth and distribution of metal oxide nanoclusters within block copolymers and indicated ways to control the magnetic properties of polymeric based nanocomposite materials. The development of such binary metal oxide - block copolymer nanocomposites is targeting the functionalization of such nanostructures into magnetic device technologies.

Single-phase Ru1−x−yMnxCoyO2 nanoparticles as highly effective oxygen reduction electrocatalysts in alkaline media with enhanced stability and fuel-tolerance

Hybrid nanostructures made of porous binary transition metal oxides for high performance asymmetric supercapacitor application

Low-energy noble-gas ion scattering (LEIS) probes the outermost atomic layer of a material, but a quantitative compositional analysis of this layer is not straightforward. It is demonstrated that quantification by calibration can be done, assuming that ion fractions and shielding effects are the same for the reference sample and sample of interest. These assumptions are critically evaluated and LEIS experiments on binary alloys and metal oxides are presented that can partly verify these assumptions. The LEIS measurements of a Cu–Au alloy and CuO powder as a function of initial energy indicate the absence of matrix effects in the ion fractions after scattering from the metal atoms in these systems. In metal oxides, shielding of surface metal atoms by shadowing/blocking and ion neutralization by neighbouring atoms can significantly influence the quantification of the metal atom concentration and is determined by the local atomic arrangement as illustrated by LEIS experiments of CuO and ZnO samples.

To achieve high performance in perovskite solar cells (PSC), the hole-transporting layer (HTL) is very important. In this work, we introduce a novel copper-chromium binary metal oxide as the hole-transporting material for PSC application. This low temperature solution-process HTL enables high performance PSC. The energy levels and phase compositions of the binary metal oxide can be modulated with different copper acetylacetonate (y)/chromium acetylacetonate (z) volume ratios in the reacting solution precursors. X-ray diffraction and X-ray photoelectron spectroscopy show that both mono CuyO(z = 0) or CrzO(y = 0) and binary CuCrO2 and CuCr2O4 phases can be formed. The Cr-terminated polar CuCrO2 component leads to films with superior wettability and compatibility with perovskite precursor solution, while the formation of high coverage uniform perovskite films is favorable for device fabrication. Unlike copper oxides, the copper-chromium binary metal oxide films have wide band gaps of around 3.1 eV, as well as suitable energy levels for both electron-blocking and hole-transporting. As a result, PSCs with this low-temperature solution processed copper-chromium binary metal oxide HTL have achieved high PCE of 17.19% on glass and 15.53% on flexible PET substrates, which is a significant enhancement over mono-metal oxide based devices.

A novel quick and facile polymer-assisted microwave synthesis route to prepare mesoporous binary metal oxides α-Fe2O3 and α-Mn2O3 and spinel-type ferrites NiFe2O4 and ZnFe2O4 is presented, which can potentially be applied for many other mixed metal oxides. The presented synthesis only needs 15-30min, much shorter than conventional approaches for mesoporous materials. Thorough characterization of the materials is performed by Powder X-Ray Diffraction (PXRD), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDXS), nitrogen physisorption analysis, mercury intrusion porosimetry (MIP), diffuse reflectance infrared fourier transform (DRIFT) spectroscopy, UV-Vis-spectroscopy, X-Ray photoelectron spectroscopy (XPS), and scanning (SEM) as well as transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Furthermore, mesoporous α-Mn2O3 and NiFe2O4 are applied as electrocatalysts for electrocatalytic oxygen evolution in alkaline media, showing improved performance compared to nanoparticles or EISA-derived mesoporous NiFe2O4.

Studies on application of Fe based binary oxide nanoparticles for treatment of lead (Pb2+) contaminated water- A batch study