Surface fluorination of α-Fe2O3 using selectfluor for enhancement in photoelectrochemical properties

•Fluorination of Fe2O3 is carried out by using Selectfluor (F-TEDA) as an electrophilic source of fluorine.•F-TEDA acts as fluorinating as well as shape directing agent for growth of Fe2O3 resulting in an increase in 110 orientation.•A detailed XPS study is performed to estimate the fluorine present on the surface.•F-Fe2O3 exhibits a systematic decrease in band gap and increase in their photoelectrochemical properties.

Fluorinated α-Fe2O3 nanostructures are synthesized via a facile hydrothermal route using Selectfluor™ (F-TEDA) as a fluorinating as well as growth directing agent. The addition of incrementally increasing amount of F-TEDA to Fe precursor under hydrothermal conditions resulted in preferential growth of α-Fe2O3 along (110) orientation with respect to (104) direction by ~ 35%, the former being important for enhanced charge transport. On increasing fluorination, the heirarchical dendritic-type α-Fe2O3 changes to a snow-flake type structure (F-TEDA-20%) anisotropically growing along the six directions however, at higher F-TEDA concentrations (≥ 30%), loosely held particulate aggregates are seen to be formed. The X-Ray Photoelectron Spectroscopy (XPS) suggest the maximum fluorinarion of α-Fe2O3 at 1.21 at% in 30% F-TEDA. Further, optical absorption studies reveal reduction in optical band gap from 2.10 eV in case of pristine to 1.95 eV for fluorinated α-Fe2O3. A photoanode made by taking 20% fluorinated α-Fe2O3 in a ratio of 10:90 with respect to TiO2 (P-25) showed improved performance in dye sensitized solar cells with an increase in efficiency by ~16% in comparision to that of pristine Fe2O3 and TiO2. Furthermore, anode consisting of thin films of fluorinated α-Fe2O3 on FTO also exhibit 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 preferential growth of α-Fe2O3 along (110) direction resulting in an improved charge transfer efficiency and reduced recombination losses due to the presence of fluorine.

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