The Assessment of Cr(VI) Removal by Iron Oxide Nanosheets and Nanowires Synthesized by Thermal Oxidation of Iron in Water Vapour

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Synthesis of Hematite (α-Fe2O3) Nanostructures by Thermal Oxidation of Iron Sheet for Cr (VI) Adsorption

In this study, hematite (α-Fe2O3) nanostructures were synthesized via thermal oxidation of Fe sheet in dry air and in water vapor. SEM images show nanoblades and nanowires growing on the surface of the sheet. Samples synthesized in water vapor generally produced larger nanostructures while samples oxidized in higher temperatures formed taller and slender nanostructures. The α-Fe2O3nanostructures were used as adsorbent for Cr (VI) in acidic medium. Chromium removal was highest with the samples synthesized at 650°C in water vapor with 95% efficiency. Kinetic and thermodynamic studies revealed that the adsorption process strongly followed pseudo-second order kinetics model and is endothermic. The process also follows the Langmuir adsorption isotherm model, suggesting that the process is described by homogeneous, monolayer adsorption. Adsorption of Cr (VI) onto hematite may be attributed to the electrostatic reaction between the positively charged hematite adsorbent and negative chromium ion.

Hierarchically three-dimensional (3D) nanotubular sea urchin-shaped iron oxide and its application in heavy metal removal and solar-induced photocatalytic degradation

In this study, hierarchically three-dimensional (3D) nanotubular sea urchin-shaped iron oxide nanostructures (3D-Fe₂O₃) were synthesized by a facile and rapid ultrasound irradiation method. Additives, templates, inert gas atmosphere, pH regulation, and other complicated procedures were not required. Dense 3D-Fe₂O₃ with a relatively large Brunauer-Emmett-Teller (BET) surface area of 129.4 m²/g was synthesized within 23 min, and the BET surface area was further improved to 282.7 m²/g by a post heat-treatment process. In addition, this post processing led to phase changes from maghemite (γ phase) to hematite (α phase) Fe₂O₃. Subsequent characterization suggested that the growth mechanism of the 3D-Fe₂O₃ follows self-assembly and oriented attachment. The prepared 3D-Fe₂O₃ was applied to wastewater purification. Ultrasound-irradiated 3D-Fe₂O₃ can eliminate a As(V) and Cr(VI) from water with 25 times faster removal rate by using a one third smaller amount than commercial α-Fe₂O₃. This was attributed to the inter-particle pores and relatively positively charged surface of the nanostructure. In addition, post heat treatment on ultrasound-irradiated 3D-Fe₂O₃ significantly influenced the photocatalytic degradation of methylene blue and phenol, with a 25 times higher removal efficiency than that of commercial α-Fe₂O₃, because of both high BET surface area and good crystallization of the prepared samples.