Surface Fluorinated Hematite for Energy, Stealth and Environmental Applications

Abstract

Iron group metal oxide, specially, hematite (α-Fe2O3) being highly abundant, low cost and non-toxic, material with a low bandgap of 2.1–2.2 eV has attracted immense attention in the recent past. It is widely used in a variety of applications such as lithium ion batteries, production of magnetic materials, visible light photocatalytic, water treatment, gas sensors, electromagnetic devices, paints and pigments, etc. However, there are certain drawbacks that limits the use of α-Fe2O3 in solar energy harvesting such as low mobility of carriers (< 0.1 cm2V-1s-1) high recombination rates, short diffusion length (2-4 nm), short excited state lifetime (< 10 ps), poor light absorption, improper band position for unassisted water splitting. Moreover, the nonmagnetic nature of α-Fe2O3 restricts for microwave application. There is lot of impetus in improving its properties by structural modification, variation in morphology and by doping. Surface fluorination is reported to increase surface electric conductivity and charge density. Surface property has significant impact on any electrochemical process as the transport of electron and hole occurs through surface. The recombination rate of photogenerated electrons and holes can greatly reduce by F− ions due to their strong electro-negativity. Actually, F− ion acts as electron-trapping sites on the surface to trap the photogenerated electrons. In this thesis work, a facile yet effective hydrothermal strategy has been developed for in-situ preparation of surface fluorinated α-Fe2O3 by using N-Chloromethyl-N′-fluorotriethylenediammoniumbis (tetrafluoroborate) or F-TEDA as a fluorinating agent that gives electrophilic fluoronium ion in solution as the reactive species. In order to understand the interaction and effect of fluorine with α-Fe2O3 we focused on the interplay between the fluorine content (varying F-TEDA concentration 0% to 40%) and resulting structural, optical, photoelectrochemical and magnetic properties together with study on its application in visible light photocatalysis, dye sensitized solar cells, microwave absorption and uranium removal from waste water. Reaction of F-TEDA with α-Fe2O3 precursor resulted in preferential growth of α-Fe2O3 along (110) orientation by ~ 35%, which is important for enhanced charge transport. On increasing fluorination, the dendritic leaf type α-Fe2O3 converts to a snow-flake type structure (F-TEDA-20%) anisotropically growing along the six directions. Therefore, F-TEDA acted as as a fluorinating as well as growth directing agent. However, at higher F-TEDA concentrations (above 30%), loosely held particulate aggregates are seen. The X-Ray Photoelectron Spectroscopy (XPS) suggest the maximum fluorinarion of α-Fe2O3 at 1.21 atomic % in 30% F-TEDA. Surface fluorinated α-Fe2O3 was investigated for application of solar water splitting, magnetic & microwave absorption and uranium removal from waste water. Screen-printed films of fluorinated α-Fe2O3 on FTO in photoelectrochemical cell exhibited an enhanced current density on illumination of ~100 W/m2. The increase in photoelectrochemical activity seems to be due to the combination of two factors namely preferentially growth of α-Fe2O3 along (110) direction resulting in improved charge transfer efficiency and reduced recombination losses due to presence of fluorine. A photoanode made by 30% fluorinated α-Fe2O3 in a ratio of 10:90 with respect to nano TiO2(P25) showed improved performance in dye sensitized solar cells with an increase in efficiency by ~16% in comparison to that of pure nano TiO2. Furthermore, upon fluorination with Selectfluor, the coercivity decreases and saturation magnetization increases systematically with increasing fluorine wt% due to transition from antiferromagnetic to ferromagnetic nature. Further, the material was evaluated for microwave absorption. The imaginary part of permeability (Imageand magnetic loss tangent value (Image) of surface fluorinated Fe2O3 are found to be superior to pristine α-Fe2O3. The maximum reflection loss in surface fluorinated Fe2O3 is observed at different frequencies as a function of sample’s thickness; for example, RL -18 dB at 7.5 GHz (t = 3.5 mm), -12 dB at 9 GHz (t = 3 mm), respectively. These characteristics make this material a potential candidate, in fact better than pristine α-Fe2O3, as radar absorbing material (RAM) both in C-band and X-bands. Fluorinated α-Fe2O3 was also studied for uranium removal from waste water. The adsorption capacity of F-α-Fe2O3 was affected by the pH, contact time and initial concentration of uranium. The optimal adsorption was achieved at around pH 5. The adsorption capacity of F-α-Fe2O3 was found excellent (Langmuir qm = 79 mg/g) as compared to pristine hematite/ hematite composite and the equilibrium achieved in 1 h time.