Photocatalytic water oxidation with suspended alpha-Fe2O3 particles-effects of nanoscaling

Abstract

Alpha-Fe₂O₃ is cheap and abundant, and has a visible light indirect (phonon assisted) band gap of 2.06 eV (600 nm) due to a d–d transition, and a direct band gap at 3.3 eV (375 nm), associated with the ligand to metal charge transfer process. Here we describe results on using freely dispersed Fe₂O₃ nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-α-Fe₂O₃ (Bulk-Fe₂O₃, 120 nm), ultrasonicated Bulk-Fe₂O₃ (Sonic-Fe₂O₃, 44 nm), and synthetic Fe₂O₃ (Nano-Fe₂O₃, 5.4 nm) obtained by hydrolysis of FeCl₃·6H₂O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe₂O₃ also containing traces of β-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (E_G = 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versusNHE at pH = 7, at 1.0 mA cm⁻²) as η = +0.43 V for Nano-Fe₂O₃, η = +0.63 V for Sonic-Fe₂O₃, and η = +0.72 V for Bulk-Fe₂O₃. Under UV and visible irradiation from a 300 W Xe-arc lamp, all three materials (5.6 mg) evolved O₂ from water with 20.0 mM aqueous AgNO₃ as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 μmol h⁻¹ g⁻¹ for Bulk-Fe₂O₃, 381 μmol h⁻¹ g⁻¹ for Sonic-Fe₂O₃ and 1072 μmol h⁻¹ g⁻¹ for Nano-Fe₂O₃. Turnover numbers (TON = moles O₂/moles Fe₂O₃) were above unity for Nano-Fe₂O₃ (1.13) and Sonic-Fe₂O₃ (1.10) but not for Bulk-Fe₂O₃ (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.