Magnetic Iron Oxide Nanoparticles in 10−40 nm Range: Composition in Terms of Magnetite/Maghemite Ratio and Effect on the Magnetic Properties

Abstract

Magnetic iron oxide nanoparticles in the 10-40 nm size range and with a reduced distribution in size have been synthesized under argon by using ammonium bases R 4 NOH (R = CH 3 , C 2 H 5 , C 3 H 7 ) and a hydrothermal treatment. The size is tuned owing to the base to iron ratio and to the length of the alkyl chain R. We precipitate first ferric hydroxides at pH 1.5-2, then ferrous hydroxide at pH 5.5-6. The rapid increase of pH up to basic pH leads to the formation of magnetic iron oxide particles of 12 nm. For [base] to [Fe] ratio above 3.5, a homogeneous growth occurs during further hydrothermal treatment at 250°C. The higher the quantity of base added and the longer the alkyl chain used, the smaller the particle size produced. For sizes above 20 nm, the Verwey transition at 120 K, characteristic of magnetite, is observed on the field cooling-zero field cooling magnetization curve. The nanoparticles can be described by a core-shell model, that is, a magnetite core surrounded by an oxidized layer close to maghemite. The fractional volume of maghemite increases as the particle size decreases so that below 20 nm, nanoparticles cannot be properly labeled as "magnetite".

Magnetically Recoverable Palladium(0) Nanocomposite Catalyst for Hydrogenation Reactions in Water

“…The values of the mean isomer, which give information on the valency state (d = 0.32 and 0.42 mm s À1 at 300 and 77 K, respectively), suggest the presence of rather exclusively Fe 3 + species and thus of maghemite form, according to previously reported works. [39] As already reported, [39] the obtained magnetic particles could not be labelled as magnetite, rather mainly as maghemite (g-Fe 2 O 3 ) as a result of the oxidation process of Fe 3 O 4 , which is facilitated by the very small particles that are characterized by a high surface/volume ratio.…”

On the ‘centre of gravity’ method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via57Fe Mössbauer spectroscopy

“…The linear correlation was fitted as: , of the room temperature isomer shift of pure maghemite and pure magnetite respectively [29]. Equation (13) may be rearranged to yield an expression for α as a function of the measured δ RT : [39] and da Costa et al [25], and with literature data on magnetite-maghemite solid solutions from Yang et al [51] and Gorski et al [33]. Note that the Gorski data in panel (d) was measured at 140 K, and that the solid line δ α 140 K ( ) has been derived from equation (17).…”

“…: a series of 10-40 nm core-shell magnetite/maghemite nanoparticles [39]; a series of 10-30 nm magnetite/maghemite nanoparticles, in which the individual nanoparticles were either magnetite or maghemite [25]; a series of 150-200 nm stoichiometric and cation-deficient magnetites prepared by chemical synthesis followed by heat treatment [51]; and a series of ca. 20 nm non-stoichiometric magnetites prepared by H 2 O 2 oxidation of stoichiometric magnetite nanoparticles [33].…”

“…The method is based on that proposed by da Costa et al [25]-a team including one of the current authors-who were inspired in part by the work of Santoyo Salazar et al [39], who, in discussing a Mössbauer study of core-shell magnetite-maghemite nanoparticles, had noted that 'the mean stoichiometry can be estimated from the mean isomer shift'. The underlying physical principle here is quite straightforward, viz.…”

Keeping Nanoparticles Fully Functional: Long‐Term Storage and Alteration of Magnetite

“…[18] Particle size has been shown to play an important role in changing the stoichiometric ratio, and also influences the magnetic properties. [19, 20]…”