Towards The Design Of Fuctional Materials : Evaluation Based On Crystal Structure, Photocatalysis And Conductivity Measurements

Abstract

The thesis entitled “Towards the Design of Functional Materials: Evaluation based on Crystal Structure, Photocatalysis and Conductivity Measurements” consist of six chapters. A short introductory note outlines the basis of designing functional materials, different synthetic procedures, characterization techniques and properties such as photocatalysis and ionic conductivity. Chapter 1 describes the effect of Ti doping on photocatalytic activity in orthorhombic perovskite type LnVO3. All the compounds were synthesized by solid state method. Rietveld refinement with high resolution PXRD reveals that the substituent Ti occupies V site rather than Ln Site. Ti substituted compound showed higher photocatalytic activity than the unsubstituted compound and is comparable with that of commercial catalyst. These classes of compounds showed specific degradation towards chlorinated compounds. Chapter 2 discusses the solution combustion synthesis of γ(L)-Bi2MoO6 and its photocatalytic activity under solar radiation. The particle sizes were in the range 300–500 nm with a band gap of 2.51 eV. The degradation of wide variety of cationic and anionic dyes was investigated under solar radiation. Despite the low surface area (<1 m2/g), γ(L)-Bi2MoO6 showed higher photocatalytic activity under solar radiation due to its electronic and morphological properties. Chapter 3 presents a series of visible light photocatalyst M2Ce2O7, synthesized via solution combustion method and characterized by powder X-ray diffraction, solid-state UV-Visible diffuse reflectance spectra, SEM and TEM. The structure of Bi2Ce2O7 has been determined using laboratory as well as synchrotron PXRD. It crystallizes in a disordered F-type structure. The particle sizes are in the range 5–6 nm, band gaps lie within the range 1.7 to 3.2 eV. Bi2Ce2O7 shows high photocatalytic activity, comparable to the commercial Degussa P-25 TiO2 under solar radiation. Chapter 4 examines the effect of Bismuth substitution on crystal chemistry, photocatalysis and conductivity in Sr3V2O8, a variant of palmierite class. These compounds were synthesized by ceramic method and powder X-ray data reveals the limit of the Bi substitution in Sr3-xBi2x/3V2O8 is x=0.4. Single crystal study followed by careful difference Fourier analysis shows that Bi occupies a unique 18h position which is different than Sr1 and Sr2 position. The experimental band gap for Sr3V2O8 was calculated to be 3.45 eV and upon substitution band gap of the material decreases and reaches a value 3.15 eV for the composition x=0.4. Compound exhibits photocatalytic activity specifically towards anionic dyes. However, Bi Substitution leads to lower photocatalytic activity. Chapter 5 describes synthesis, structure, phase transition and ionic conductivity in scheelite type Li0.5Ce0.5MoO4. The compound was synthesized by ceramic method and single crystal study reveals that it crystallizes in the space group I41/a and exhibits conductivity of ~10-3 Ohm-1cm-1 at elevated temperature( 700 °C). It undergoes a first order phase transition around 510 °C. The nature of this transition has been evaluated by laboratory and synchrotron PXRD, DSC, dielectric spectroscopy and variable temperature Raman spectroscopy. The phase transition is shown to be characterized by an iso-structural phase transition which is first example in literature for temperature induced Cowley’s “Type Zero” phase transition. Chapter 6 discusses a new methodology for generating functional materials for fast ion conductors. Several varients of hydrated sodium cadmium bisulfate, Na2Cd2(SO4)3⋅3H2O, Na2Cd(SO4)2⋅2H2O and Na2Cd(SO4)2⋅4H2O have been synthesized and their thermal properties followed by phase transitions have been invesigated. Na2Cd2(SO4)3⋅3H2O (space group P3c). Na2Cd2(SO4)3⋅3H2O loses water completely when heated to 250 °C and transforms to a dehydrated phase (I⎯43d ) whose structure has been established using abinitio powder diffration techniques. Na2Cd(SO4)2⋅2H2O (P21/c) transforms to α−Na2Cd(SO4)2 (space group C2/c) on heating to 150 °C which is a known high ionic conductor. However, when α−Na2Cd(SO4)2 is heated to 570 °C followed by sudden quenching in liquid nitrogen, β−Na2Cd(SO4)2 (P21/c) is formed. β−Na2Cd(SO4)2 takes up water from the atmosphere and gets converted completely to the Kröhnkite type mineral. Further, β−Na2Cd(SO4)2 has a conductivity behavior comparable to α form up to 280 °C, the temperature required for the transformation of β to α form.