https://bib.irb.hr/datoteka/300775.Mossauer_FTIR_and_FE_SEM_investigation_of_iron_oxides_precipitated_from_FeSo4_solutions.pdf

Mössbauer, FT-IR and FE SEM investigation of iron oxides precipitated from FeSO4 solutions

The first-order metal-insulator phase transition in VO(2) is characterized by an ultrafast several-orders-of-magnitude change in electrical conductivity and optical transmittance, which makes this material an attractive candidate for the fabrication of optical limiting elements, thermochromic coatings, and Mott field-effect transistors. Here, we demonstrate that the phase-transition temperature and hysteresis can be tuned by scaling VO(2) to nanoscale dimensions. A simple hydrothermal protocol yields anisotropic free-standing single-crystalline VO(2) nanostructures with a phase-transition temperature depressed to as low as 32 degrees C from 67 degrees C in the bulk. The observations here point to the importance of carefully controlling the stoichiometry and dimensions of VO(2) nanostructures to tune the phase transition in this system.

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Depressed phase transition in solution-grown VO2 nanostructures.

The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid–solid phase transitions such as those involved in metal–insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution WxV1−xO2. Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal → insulator and insulator → metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk WxV1−xO2 of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W6+ doping in the WxV1−xO2 nanobelts.

Distinctive finite size effects on the phase diagram and metal–insulator transitions of tungsten-doped vanadium(IV) oxide

The ultrafast first-order metal–insulator phase transition in VO2 is characterized by a several orders of magnitude change in optical transmittance and electrical conductivity, which makes this material an attractive candidate for use in a wide variety of applications ranging from Mott field-effect transistors to thermochromic coatings and optical waveguides. However, relatively little progress has been achieved in fabricating VO2nanostructures exhibiting a well-defined metal–insulator transition despite the potential attractiveness of such structures as building blocks for novel nanoscale electronic devices. Herein, we present a solution-based approach for the synthesis of monoclinic single-crystalline VO2nanosheets exhibiting a well-defined metal–insulator phase transition by combining the facile hydrothermal reduction of V2O5, an inexpensive and abundant precursor, with an annealing process. The controlled hydrothermal reduction of bulk V2O5 using small molecule aliphatic alcohols and ketones as reducing agents yields metastable nanostructures of VO2(B) that can be readily transformed to monoclinic VO2 exhibiting a sharp metal–insulator phase transition via an annealing process. The morphology and size of the obtained VO2nanostructures are found to depend on the reaction time and concentration of the added structure-directing agent, providing substantial insight into the nanowire formation process.

VO2 nanosheets exhibiting a well-defined metal―insulator phase transition

We have previously reported Fe–Al–Cu catalysts prepared by the sol–gel method to exhibit superior water-gas shift performance compared to catalysts of similar composition prepared by precipitation/impregnation techniques. The difference in performance was ascribed to the formation of the γ-Fe 2 O 3 structure and a better incorporation and distribution of Cu in the iron oxide matrix. In the current study, the effect of the Cu loading level was examined for its influence on surface and structural properties, and in turn, catalytic performance. Catalysts with various Cu loading levels were evaluated at steady state at different reaction temperatures. In situ X-ray diffraction was used to monitor crystalline phases present in catalysts with different Cu loadings and phase evolution during reduction. The role of Cu loading in γ-Fe 2 O 3 structure formation was studied using X-ray diffraction and Mossbauer spectroscopy techniques.

Effect of Cu loading on the catalytic performance of Fe–Al–Cu for water-gas shift reaction

Phase transition behaviour of VO2

Optimum conditions for the preparation of tape recording quality γ-Fe2O3 by the thermal decomposition of ferrous oxalate dihydrate have been established. Formation of the intermediate Fe3O4 which is most important in forming γ-Fe2O3 takes place only in the presence of water vapour. Various stages of decomposition have been characterised by DTA, TG, DTG, and x-ray powder diffraction. The method for the preparation of acicular γ-Fe2O3 that matches very well with the commercial tape recording material has been developed.

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Preparation and characterisation of γ-Fe2O3 as tape recording material

Optimum conditions and experimental details for the formation of γ-Fe2O3 from goethite have been worked out. In another method, a cheap complexing medium of starch was employed for precipitating acicular ferrous oxalate, which on decomposition in nitrogen and subsequent oxidation yielded acicular γ-Fe2O3. On the basis of thermal decomposition in dry and moist nitrogen,dta, xrd, gc and thermodynamic arguments, the mechanism of decomposition was elucidated. New materials obtained by doping γ-Fe2O3 with 1–16 atomic percent magnesiu, cobalt, nickel and copper, were synthesised and characterized.

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K. Seshan

Papers

Phase transition behaviour of VO2

Preparation and characterisation of γ-Fe2O3 as tape recording material

Development of pure and doped gamma ferric oxide

Magnetic properties of the LaCoO3-LaMnO3 system