Peter Jakes

Position of defects with respect to domain walls in Fe3+-doped Pb[Zr0.52Ti0.48]O3 piezoelectric ceramics

The position of (FeZr,Ti′−VO••)• defect complexes in Pb[Zr0.52Ti0.48]O3 (PZT) piezoelectric ceramics was investigated by means of electron paramagnetic resonance (EPR) spectroscopy. The method of analysis pursued to obtain information on the (FeZr,Ti′−VO••)• position is to compare the EPR spectra of Fe3+-doped PZT specimen at different states, i.e., a powder that is representative for a system with considerably reduced amount of non-180° domain walls and a sintered ceramic of identical composition but with markedly developed domain structure. By considering the local site symmetry for the Fe3+-functional center, indirect evidence is obtained that the (FeZr,Ti′−VO••)• defect complexes are located within domains and not at domain walls.

Zinc oxide derived from single source precursor chemistry under chimie douce conditions: formation pathway, defect chemistry and possible applications in thin film printing

A series of zinc complexes with oximate ligands is investigated for their suitability as precursors for zinc oxide in inkjet printing. The variation of hydrogen and alkyl groups in the side chains of the oximate framework (R1–ON–C2O2–R2) of the corresponding zinc complexes influences the decomposition temperature, and also important parameters such as solubility and wettability. Detailed investigations of the degradation mechanism reveal their behavior as excellent single source precursors for ZnO under very mild (chimie douce) conditions. Best results for the formation of zinc oxide thin films are obtained with solutions of [2-(methoxyimino)propanato]zinc in methoxyethanol. By calcincation well adherent (tensile strength of 1.95 (±0.95) MPa) nanocrystalline films of zincite are formed. This technique is applied for inkjet printing of ceramic layers on polyethylene-terephthalate thin films. Results of EPR spectroscopy studies on the ZnO nanoparticles are in accord with a core–shell model in which the grain particles of the core consist of vacancy centers which are electronically different from the surrounding shell of the ZnO nanoparticles.

Defect structure and materials “hardening” in Fe2O3-doped [Bi0.5Na0.5]TiO3 ferroelectrics

The effect of Fe2O3-doping on the defect structure and piezoelectric properties of [Bi0.5Na0.5]TiO3 ceramics is studied by means of electron paramagnetic resonance spectroscopy. The results show that the Fe3+ functional centers are incorporated at the perovskite B-site, and clearly can be attributed to the formation of (FeTi′–VO••)• defect complexes with charge compensating oxygen vacancies. Correspondingly, they act as acceptors which promotes materials “hardening.”

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium-Oxygen Battery.

Aprotic lithium-oxygen (Li-O2 ) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen ((1) Δg ) is formed upon Li2 O2 oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time- and voltage-resolved in operando EPR spectroscopy in a purpose-built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long-overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li-O2 cells.

Operando electron paramagnetic resonance spectroscopy – formation of mossy lithium on lithium anodes during charge–discharge cycling

The formation of mossy lithium and lithium dendrites so far prevents the use of lithium metal anodes in lithium ion batteries. To develop solutions for this problem (e.g., electrolyte additives), operando measurement techniques are required to monitor mossy lithium and dendrite formation during electrochemical cycling. Here we present a novel battery cell design that enables operando electron paramagnetic resonance (EPR) spectroscopy. It is shown that time-resolved operando EPR spectroscopy during electrochemical cycling of a lithium-metal/LiFePO4 (LFP) cell provides unique insights into the lithium plating/dissolution mechanisms, which are consistent with ex situ scanning electron microscopy (SEM) analysis. To demonstrate the viability of the operando EPR method, two cells using different electrolytes were studied. When using an electrolyte containing fluoroethylene carbonate (FEC) additive, a higher reversibility of the lithium anode and reduced formation of micro-structured (mossy/dendritic) lithium were observed.

`Nitrogen doped' C60 dimers (N@C60–C60)

Abstract The formation of C60 dimers with a nitrogen atom in one of the C60 cages is studied. The dimers are produced by ball milling of a mixture of N@C60 with C60 and a suitable additive. The products are purified by high pressure liquid chromatography (HPLC) and identified by UV/Vis and IR spectroscopy. Electron paramagnetic resonance (EPR) measurements show that nitrogen remains in C60 during the dimerization and keeps its atomic configuration. A slight deformation of the electron shell reflecting the distortion of the cage is observed. The optimization of the dimer formation is described.

Interactions of defect complexes and domain walls in CuO-doped ferroelectric (K,Na)NbO3

“Lead-free” piezoelectric sodium potassium niobate has been studied with respect to its defect structure when doping with CuO. The results indicate that two kinds of mutually compensating charged defect complexes are formed, ( Cu ′ ′ ′ Nb − V O • • ) ′ and ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • . Concerning the interplay of these defect complexes with the piezoelectric materials properties, the trimeric ( V O • • − Cu ′ ′ ′ Nb − V O • • ) • defect complex primarily has an elastic dipole moment and thus is proposed to impact the electromechanical properties, whereas the dimeric ( Cu ′ ′ ′ Nb − V O • • ) ′ defect possesses an electric dipole moment in addition to an elastic distortion. Both types of defect complexes can impede domain-wall motion and may contribute to ferroelectric “hardening.”

Defect structure and formation of defect complexes in Cu2+-modified metal oxides derived from a spin-Hamiltonian parameter analysis

The nearest neighbour oxygen octahedron about copper functional centres in metal oxides has been systematically studied by means of electron paramagnetic resonance (EPR) spectroscopy. In particular, the determined g ∥,zz and spin-Hamiltonian parameters were analysed, finding linear dependences as a function of chemical bonding and local distortion of the oxygen octahedron. Moreover, through the introduction of a dimensionless coordination parameter ξ, different defect structures with respect to the number of coordinated oxygen vacancies may be distinguished. This allows for a distinct assignment of defect complexes between the copper functional centre with one or two oxygen vacancies.

Purification and optical spectroscopy of N@C60

Using multi-step high performance liquid chromatography (HPLC) it was possible to prepare a chromatographically pure sample of N@C60. Invoking EPR spectroscopy of solid samples prepared after each HPLC step, the increase of relative N@C60 concentration could unambiguously be determined. The UV/Vis spectrum of N@C60 is indistinguishable within experimental error from that of C60, confirming negligible coupling between nitrogen in its atomic ground state and C60 cage molecular wave functions. The observed large dipolar width of the EPR spectrum of the pure paramagnetic compound indicates weak spin exchange between neighboring paramagnetic centers which is further proof for a very small spin transfer from the encapsulated atom to the fullerene cage.

Unraveling the Degradation Process of LiNi0.8Co0.15Al0.05O2 Electrodes in Commercial Lithium Ion Batteries by Electronic Structure Investigations

The degradation of LiNi0.8Co0.15Al0.05O2 (LNCAO) is reflected by the electrochemical performance in the fatigued state and correlated with the redox behavior of these cathodes. The detailed electrochemical performance of these samples is investigated by galvanostatic and voltammetric cycling as well as with the galvanostatic intermittent titration technique (GITT). Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to investigate the oxidation state of all three materials at the Ni L2,3, O K, and Co L2,3 edges at five different states of charge. Surface and more bulklike properties are distinguished by total electron yield (TEY) and fluorescence yield (FY) measurements. The electrochemical investigations revealed that the changes in the cell performance of the differently aged materials can be explained by considering the reaction kinetics of the intercalation/deintercalation process. The failure of the redox process of oxygen and nickel at low voltages leads to a significant decrease of the reaction rates in the fatigued cathodes. The accompanied cyclic voltammogram (CV) peaks appear as two peaks because of the local minimum of the reaction rate, although it is one peak in the CV of the calendarically aged LNCAO. The absence of the oxidation/reduction process at low voltages can be traced back to changes in the surface morphology (formation of a NiO-like structure). Further consequences of these material changes are overpotentials, which lead to capacity losses of up to 30% (cycled with a C/3 rate).