X-ray Absorption Spectroscopy Study On The Oxidation State Of Ge And Ga Compounds

Oxidation state of an element is the vital property of the compound,which is related to the electron density of an atom,and can be used to evaluate the structure,the reactivity of molecules and the process of a reaction.For example,the assessment of the oxidation state of a given atom in a battery electrode during the charge and discharge process will help to predict the reaction mechanism.Therefore,it is of great importance to accurately determine the oxidation state of the element in a given compound.The characterization carried out by synthetic chemists is commonly focused on the structure of the molecules,such as Single-Crystal X-ray diffraction(SC-XRD)and Nuclear Magnetic Resonance(NMR).In order to predict the reactivity,people often rely on the empirical knowledge on the oxidation number or valence number.However,for novel complexes,it is not always accurate and can be misleading sometimes.Thus,there is a pressing need for developing experimental techniques which readily provide insights into the oxidation states of the atoms of interest.X-ray Absorption Spectroscopy(XAS)can be used to probe the density of the conduction band(lowest unoccupied electronic states)and is a direct measure of oxidation states.It's so powerful that can be applied to most of the elements in the periodic table and it is suitable for gas,liquid and solid samples even the concentration is quite low.The oxidation states of Ge and Ga were examined by XAS in this thesis.This thesis contains three parts of work.The first part studies the transition of compositions of GeOx after H2 annealing by XAS and XRD.A nominal GeOx(x ? 2)compound contains mixtures of Ge,Ge suboxides,and GeO2,and is promising for Li battery anode.GeOx was prepared by the chemical reduction of GeO2 according to the literature,then half of the sample was exposed to air for three days(air-exposed GeOx),while the rest was kept in the glove box(fresh GeOx).XAS and XRD were utilized to study the transformation after the samples were annealed in H2 at different temperatures.Freshly prepared GeOx is found consisting of Ge clusters and GeOx(x closes to 1),both in amorphous forms.Such GeOx undergoes disproportionation when annealed in a reducing environment.The amount of amorphous Ge increases,and finally crystallizes into metallic Ge,with GeOx presents at the grain boundaries.While after air exposure,the Ge clusters are replaced by a GeO2-like structure.The GeOx(x closes to 2)can be slowly reduced by H2 under mild temperature and there is a complete conversion of GeOx to Ge once the temperature reaches 700 ?.This study will help to find the GeOx composition as an anode material with best performance.The second part of the thesis studies the oxidation states of a few Ga compounds with multiple cores.It is demonstrated that XAS can be used to evaluate the molecule containing element with multiple oxidation states.The oxidation states of several Ga complexes with multiple Ga cores were assessed by experimental XAS spectra together with FDMNES and SIESTA calculation.[Ga(prismand)][GaCl4]and Ga[GaCl4]contain features of a mixed valent complex[Ga(?)/(?)],while the Ga in[Ga2Cl2(crypt-222)][OTf]2 and Ga2Cl4(1,4-dioxane)2,[Ga3Cl4(crypt-222)]+ was assigned to Ga(II).The third part of the thesis studied the oxidation state of Ge(NHC)3,GeNHC-Cl2,GeNHC-Br2,GeNHC-I2 and GeNHC-OTf by XAS and found that all were Ge(?)compounds.The peak and edge shift to higher energy when the ligand is more electronegative.