Iron and iron oxide particle growth in porous Vycor glass; correlation with optical and magnetic properties

Abstract

The growth of elemental iron and iron oxide particles derived from the photolysis of Fe(CO) 5 adsorbed onto porous Vycor glass has been characterized and correlated with the optical and magnetic properties of the particles. Photolysis does not result in particle growth per se. Photolysis causes extensive diffusion of the precursor, which leads to a high surface coverage in the outer volumes of the glass and the formation of small, ⩽1 nm in diameter particles consisting of an elemental iron core with an Fe 2O 3 coating. Particle growth occurs during the subsequent heating with the majority of growth occurring at temperatures of ⩽700 °C. The resultant particle size in the consolidated glass is equivalent to initial pore size in the glass, 10±1 nm, and the inter-particle spacing equivalent to the correlation length of the porous glass, 24±1 nm implying particle size and spacing are defined by the morphology of the porous glass. The 10±1 nm diameter particles consist of an ≈8 nm diameter Fe 0 core surrounded by 1 nm thick α-Fe 2O 3 cladding. Particle growth is accompanied by a decline in optical transparency and a conversion from superparamagnetism to ferromagnetism with the particles exhibiting a large coercivity. The latter is attributed to an exchange anisotropy between the Fe 0 core and the oxide which becomes distributed throughout the body of the 10±1 nm diameter particles.