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

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-alpha-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 center dot 6H(2)O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe2O3 also containing traces of beta-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 (versus NHE at pH = 7, at 1.0 mA cm(-2)) as eta = +0.43 V for Nano-Fe2O3, eta = +0.63 V for Sonic-Fe2O3, and eta = +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 O-2 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 mu mol h(-1) g(-1) for Bulk-Fe2O3, 381 mu mol h(-1) g(-1) for Sonic-Fe2O3 and 1072 mu mol h(-1) g(-1) for Nano-Fe2O3. Turnover numbers (TON = moles O-2/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.