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

Abstract

Alpha-Fe 2 O 3 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 Fe 2 O 3 nanocrystals for photocatalytic water oxidation. Three morphologies of hematite were compared, including bulk-type-a-Fe 2 O 3 (Bulk-Fe 2 O 3 , 120 nm), ultrasonicated Bulk-Fe 2 O 3 (Sonic-Fe 2 O 3 , 44 nm), and synthetic Fe 2 O 3 (Nano-Fe 2 O 3 , 5.4 nm) obtained by hydrolysis of FeCl 3 $6H 2 O. According to X-ray diffraction, all phases were presented in the alpha structure type, with Nano-Fe 2 O 3 also containing traces of b-FeOOH. UV/Vis diffuse reflectance revealed an absorption edge near 600 nm (E G ¼ 2.06 eV) for all materials. Cyclic voltammetry gave the water oxidation overpotentials (versus NHE at pH ¼ 7, at 1.0 mA cm À2 ) as h ¼ +0.43 V for Nano-Fe 2 O 3 , h ¼ +0.63 V for Sonic-Fe 2 O 3 , and h ¼ +0.72 V for Bulk-Fe 2 O 3 . 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 AgNO 3 as sacrificial electron acceptor. The highest rates were obtained under UV/Vis (>250 nm) irradiation with 250 mmol h À1 g À1 for Bulk-Fe 2 O 3 , 381 mmol h À1 g À1 for Sonic-Fe 2 O 3 and 1072 mmol h À1 g À1 for Nano-Fe 2 O 3 . Turnover numbers (TON ¼ moles O 2 / moles Fe 2 O 3 ) were above unity for Nano-Fe 2 O 3 (1.13) and Sonic-Fe 2 O 3 (1.10) but not for Bulk-Fe 2 O 3 (0.49), showing that the nanoscale morphology was beneficial for catalytic activity.

Ir-phosphate cocatalyst for photoelectrochemical water oxidation using α-Fe2O3

“…These peaks are assigned to diffractions from (104), (110), (113), (024), (018), (300) and (220) planes of a-Fe 2 O 3 . 13 No other major peaks can be found indicating the phase purity of a-Fe 2 O 3 lm formed by the calcination of electrodeposited Fe. The XRD pattern for Ir-PiFe 2 O 3 ( Fig.…”

Nanoscale Effects in Water Splitting Photocatalysis

“…The effect of nanoscaling on the ability to extract short-lived minority carriers is also evident in colloidal Fe 2 O 3 dispersions, which catalyze the oxygen evolution reaction with aqueous 0.1 M AgNO 3 as the sacrificial electron acceptor [89]. Even though the performance of the system is limited by silver deposition on the Fe 2 O 3 nanocrystals (Fig.…”

“…7), a correlation between O 2 production rates and particle size is seen. This can be attributed to improved hole extraction in the smaller nanocrystals [89]. In principle, by reducing the nanocrystal size further, and by increasing the thermodynamic driving force for charge extraction, most metal oxide materials can be turned into photocatalysts.…”

“…Furthermore, the larger specific surface area of the particles promotes non-radiative and interfacial recombination rates. The effect of these parameters on photocatalytic activity and electron-hole lifetime are commonly acknowledged in the literature [89,[139][140][141][142][143], but quantitative studies on this topic are rare [65,79,[144][145][146][147][148].…”

Enhancement of Oxygen Evolution Activity of Ruddlesden-Popper-Type Strontium Ferrite by Stabilizing Fe4<sup>+</sup>

“…As a fourelectron transfer reaction, the O 2 evolution reaction (OER, 2H 2 O → O 2 + 4H + + 4e − ), which is a half-reaction of water splitting, suffers from sluggish kinetics owing to a large overpotential and has been considered to be the efficiencylimiting step of water splitting. Therefore, extensive research has been devoted to developing O 2 evolution catalysts [6]- [15]. Iridium oxide (IrO 2 ) and ruthenium oxide (RuO 2 ) are effective OER catalysts for solar water splitting as they exhibit high turnover frequencies under neutral pH conditions [6] [7] [8]; however, their high cost and scarcity render their use impractical for large-scale applications.…”