High Surface Area Iron Oxide Microspheres via Ultrasonic Spray Pyrolysis of Ferritin Core Analogues

The controlled synthesis and fabrication of nano- andmicrostructured materials has received much attention over the past decade owing to the unique chemical, electrical, magnetic, and optical properties of nanoscaled materials when compared to the bulk. Due to its low price and abundance, low toxicity, biodegradability, and high chemical stability, iron oxide is an especially promising material which has been studied for use in applications such as photocatalysis,1−6 heavy metal removal,7,8 sensing,9−11 and energy storage.4,10−14 Previously, high surface area, nanostructured iron oxides have been synthesized by using a template, such as polystyrene microspheres15 or copper nanowires,16 on which iron oxide or an iron oxide precursor is grown. These templates can be expensive and are typically destroyed upon removal. Forced hydrolysis of an iron precursor in the presence of surfactants can also lead to nano- or microspheres of iron oxide with varying surface areas.17,18 Iron oxide nanodiscs have been made using a controlled etching with oxalic acid.19 The most common method for producing iron oxide microspheres has been solvothermal.6,7,9,20−26 Some of these procedures require harsh chemicals or expensive surfactants, while other try to minimize the environmental impact of production. All, however, are batch processes that take many days in a high pressure vessel and generally require further workup; in addition, all of these methods are expensive (often prohib-itively) and are difficult to scale-up. Spray pyrolysis and similar aerosol techniques are well-known as scalable synthetic methodologies for the preparation of metal oxide materials from relatively inexpensive precur-sors.27−32 Porous particles with relatively high surface areas have been obtained with spray pyrolysis for carbon,33−40 silica,41,42 titania,43 alumina,44 Bi2WO6,