Structure And Property Tuning Of Main Group-Transition Metal Composite Oxides Via Low-Temperature Reduction

The main group-transition metal composite oxides are rich in physical and chemical properties,many of which are strongly correlated electron systems,and contain abundant condensed-state physics such as Mott transition,spin polarization,giant magnetoresistance effect,and high-temperature superconductivity.In the fields such as electrical,magnetic and environmental sciences,the main group-transition metal composite oxides have a high research value and broad application prospects.Novel properties can be originated from various coordination structures formed by transition metal ions and oxygen ions,and also from the various d electron configurations.Therefore,through modification of oxidation states and crystal structures,novel physical properties can be discovered,and materials can be designed to meet the specific performance requirements in practical applications.At present,the high-temperature solid-state reaction is still the most common method for the synthesis of solid materials.However,this method can only obtain the most stable phase of thermodynamics,and only by lowering the reaction temperature can the kinetic control be achieved.Under low-temperature conditions,the lack of thermal activation energy makes it difficult to perform many solid-phase reactions,but the use of strong reducing agents makes reduction under low-temperature conditions possible.Via low-temperature reduction,one can create more oxygen vacancies in the oxide under the premise of keeping the structure from collapsing,so as to adjust the structure and the related properties.And sometimes,it even generates new phases.Focusing on the above issues,the specific work we conducted is as follows.In Chapter 1,we introduce the background knowledge involved in this paper,which mainly includes the strong correlation effect of transition metal oxides,the exchange interaction and the mechanism of photocatalysis.In addition,the recent advances in low-temperature reduction of the main group-transition metal composite oxides are reviewed.In Chapter 2,we studied the effect of oxygen content on structure and properties.Depending on the stability of the crystal structure and the nature of the transition metal ions,the amount of oxygen vacancies that can be accommodated by different materials varies widely.Taking SrMoO4-?,BaTiO3_? and SrFeO3_? as examples,we studied the effect of oxygen-vacancy content on the crystal structure and related properties.According to the oxygen-vacancy content from small to large,the material can undergo local structural changes,symmetry changes,and even the formation of a new crystal configuration,which can make a powerful modification on the optical,electrical and magnetic properties of materials.In Chapter 3,we study the electrical transport properties of spinel ferrite MgFe2O4-? reduced by metallic sodium.Usually MgFe2O4 is a good dielectric material.By using the low-temperature reduction method,when the oxygen vacancy content is increased to 1021/cm3,the material can be well conductive(?10-2 ?· m at room temperature).The well electrical conductivity can be attributed to the enhanced carrier concentration and the narrowed band gap.Quantitative fitting based on variable temperature resistivity found that its conduction mechanism satisfies the variable-range hopping model.Calculations based on density functional theory show that its carriers are highly spin-polarized.Oxygen vacancies can generate defect levels below the Hubbard band and the calculation results of the partial wave density shows that the defect energy levels mainly consist of spin-down Fe 3d orbitals.The rate of carrier polarization near the Fermi level is theoretically 100%.In Chapter 4,we study the doping effect of iron-site elements in SrFe2 02 with infinite-layer structure.In previous studies,the doping ions for Fe2+ were all equivalent states,such as Mn2+,Ru2+ and Co2+.In our work,SrFe1-xMoxO3-?(0?x?0.1)was used as a precursor to prepare Mo6+-doped SrFe2+ O2 by low-temperature reduction.Due to the large differences in ion reduction potentials,under appropriate reduction temperature,only the valence state of Fe element can be reduced,leading to the coexistence of Fe2+/Mo3+ in the final product.Due to the requirement of charge compensation,there are interlayer oxygen atoms in the structure,forming SrFe1-xMoxO2+?(0? x ? 0.1).The powder X-ray analysis showed that the crystallinity of the layered compound SrFe1-xMoxO2+? is highly anisotropic.With the increase of interlayer oxygen,the crystallinity in the in-plane direction remains unchanged,but that in the stacking direction is greatly reduced.This phenomenon usually occurs only in soft condensed matter such as high molecular liquid crystals,and is very rare in solid oxides.Distortion and dislocation were observed using high-resolution electron microscopy,indicating that the Fe-O-Fe formed extended network is highly robust but flexible.For the magnetic structure,these defects also destroy the antiferromagnetic order,resulting in the formation of parasitic ferromagnetism.We studied the parasitic ferromagnetism using variable-temperature magnetization measurements,hysteresis loops,and electron spin resonance,and concluded that this parasitic ferromagnetism are originated from several factors.Firstly,non-magnetic ions Mo6+(S=0)substitution of the magnetic ion Fe2+(S=2)can produce uncompensated magnetic moments.Secondly,structural distortions can lead to spin-canting.Thirdly,dislocations can give rise to geometrical frustration.In Chapter 5,we study the reduction and reoxidation of bismuth molybdate nanoplates with koechlinite structure,and their effects on the microstructure and photocatalytic properties.We found that the reduction(at?140 ?)and reoxidation(<250 ?)under low-temperature conditions can lead to the formation of a special structure Bi-MoO6-??Bi2MoO6 with crystallized core and disordered shell.The rate of degradation of phenol and methylene blue are increased by 5 and 10 times,respectively.The radical quenching experiment demonstrated that the photogenerated holes and OH radicals are the main active species for degradation.Studies have shown that oxygen vacancies can improve the light absorption and carrier conduction,while the disordered structure on the surface can inhibit the recombination of carriers,thereby enhancing the photocatalytic performance.In the last chapter of this dissertation,we summarized the whole work and provide an outlook of the future.