Effect of ferrous ion concentration on the kinetics of radiation-induced iron-oxide nanoparticle formation and growth.

Magnetite nanoparticles were formed by γ-radiolysis of solutions containing different initial concentrations of FeSO4 without any other chemical additives. The particles formed in a given [Fe2+]0 had a narrow size distribution and the average size increased with [Fe2+]0. Five hour irradiation at 0.8 Gy s-1 produced an average size ranging from 23 ± 2 nm to 300 ± 40 nm in 0.1 mM or 10 mM [Fe2+]0 solutions, respectively. To ascertain the size-determining mechanism, the kinetics of γ-radiation-induced particle formation and growth were investigated by simultaneously analyzing the [H2(g)] in the headspace, the [FeII] and [FeIII] dispersed in solution, UV-Vis absorbances at 304 nm and 380 nm, and the pH of the solution. The particles formed were characterized by TEM imaging and various spectroscopic analyses. For a given [Fe2+]0 the time-dependent behaviours of different analyses collectively show three distinct kinetic stages of iron oxidation. The [Fe2+]0 affects the oxidation kinetics of different stages and hence, the oxidation yields and the size of particles formed after irradiation. The main processes which cause the observed kinetics and yields in the three stages are proposed.

Radiation-induced formation of Co3O4 nanoparticles from Co(2+)(aq): probing the kinetics using radical scavengers.

The effects of the Co(2+) content and different radical scavengers on the kinetics of γ-radiation-induced Co3O4 nanoparticle formation and growth were investigated. There are four distinct stages of particle formation with different oxidation rates. Scavengers and [Co(2+)]0 affect the oxidation kinetics in the different stages and consequently the final size of the particles formed. Radiolysis model calculations were performed to obtain the time-evolution of the concentrations of key oxidants and reductants, and the effect of scavengers on those concentrations. Based on the model results and experimental data a reaction mechanism for Co3O4 particle formation by γ-irradiation of solutions containing Co(2+)(aq) is proposed. The main cobalt oxidation reaction changes with time. Oxidation of Co(2+)(aq) to Co(3+)(aq) by radiolytically produced ˙OH occurs first in the solution phase. This is followed by spontaneous co-precipitation of mixed Co(II)/Co(III) hydroxide nucleate particles. Adsorption of Co(II)(ad) followed by surface oxidation of Co(II)(ad) to CoOOH(ad) by H2O2 grows particles with a solid CoOOH(s) phase. In parallel, the solid-state transformation of CoOOH(s) and Co(II)(ad) to form Co3O4(s) occurs.

A mechanistic model for oxide growth and dissolution during corrosion of Cr-containing alloys.

We have developed a corrosion model that can predict metal oxide growth and dissolution rates as a function of time for a range of solution conditions. Our model considers electrochemical reactions at the metal/oxide and oxide/solution interfaces, and the metal cation flux from the metal to the solution phase through a growing oxide layer, and formulates the key processes using classical chemical reaction rate or flux equations. The model imposes mass and charge balance and hence, is labeled as the Mass Charge Balance (MCB) model. Mass and charge balance dictate that at any given time the oxidation (or metal cation) flux must be equal to the sum of the oxide growth flux and the dissolution flux. For each redox reaction leading to the formation of a specific oxide, the metal oxidation flux is formulated using a modified Butler-Volmer equation with an oxide-thickness-dependent effective overpotential. The oxide growth and dissolution fluxes have a first-order dependence on the metal cation flux. The rate constant for oxide formation also follows an Arrhenius dependence on the potential drop across the oxide layer and hence decreases exponentially with oxide thickness. This model is able to predict the time-dependent potentiostatic corrosion behaviour of both pure iron, and Co-Cr and Fe-Ni-Cr alloys.

Gamma-radiolysis-assisted cobalt oxide nanoparticle formation.

The formation of Co(3)O(4) nano-scale colloid particles by gamma irradiation of CoSO(4) solutions was investigated. Solutions of 0.2-0.3 mM CoSO(4) at pH 6.0 and 10.6 (air-saturated and Ar-purged) were irradiated at an absorbed dose rate of 5.5 kGy h(-1). The resulting concentrations of H(2), H(2)O(2), Co(II) and Co(III) species in solution and the chemical composition and sizes of particles that were formed were measured as a function of irradiation time. Particle formation was observed only for initially air-saturated CoSO(4) solutions at pH 10.6. Analysis of the particle formation as a function of irradiation time shows that the particles evolve from Co(OH)(2) to CoOOH and then to Co(3)O(4). The radiolytic oxidation of Co(II) to Co(III) was completed in 100 min and the chemical composition of the final particles was identified as Co(3)O(4) by XPS, Raman and UV-Vis spectroscopy. Transmission electron microscopy (TEM) images show the final particles are approximately uniform in size, ranging from 8 to 20 nm. A mechanism is proposed to explain the particle formation. A key factor is the low solubility of Co(OH)(2) in air-saturated solutions at high pH. This mechanism for particle formation is compared with the mechanism previously reported for the radiolytic formation of γ-FeOOH nanoparticles.

Gamma-radiation induced formation of chromium oxide nanoparticles from dissolved dichromate.

The formation of chromium oxide nanoparticles by gamma radiolysis of Cr(VI) (CrO(4)(2-) or Cr(2)O(7)(2-)) solutions was investigated as a function of pH and initial Cr(VI) concentration by measuring [Cr(VI)], the particle concentration ([Cr(III)(col)]) and [H(2)], and by characterizing the particles using TEM, Raman, FTIR and XPS. The results show that Cr(VI) is easily reduced to Cr(III) by a homogeneous aqueous reaction with ˙e(aq)(-), but, due to the stability of Cr(III) colloids, the growth of the Cr(OH)(3) particles is very slow. As the particles grow the interior of the particle dehydrates to form Cr(2)O(3) while the outer layer remains hydrated. When most of the Cr(VI) that is initially present in the solution is converted to Cr(OH)(3) further redox reactions of chromium species occur on the particle surfaces. The redox system reaches a pseudo-equilibrium state due to cyclic reactions of Cr(III) with ˙OH and H(2)O(2), and reactions of Cr(VI) with ˙e(aq)(-) and H(2)O(2). The size distribution of the particles that are formed is controlled by these solution-solid interface reactions.

Iron oxyhydroxide colloid formation by gamma-radiolysis.

Gamma-irradiation of deaerated aqueous solutions containing FeSO(4) leads to the formation of uniform-sized colloidal particles of γ-FeOOH. At short irradiation times, or in solutions with a low initial [Fe(2+)](0), spherical particles with a size less than 10 nm are formed. These primary particles grow to form a dendritic structure upon longer irradiation, and the final size of the large particles is ∼60 nm with a very narrow size distribution. Further prolonged irradiation does not change the final particle size. The narrow size distribution is attributed to rapid homogeneous radiolytic oxidation of soluble Fe(2+) to relatively insoluble Fe(3+) hydroxides [Fe(H(2)O)(6-n)(OH)(n)](3-n) leading to particle nucleation by spontaneous condensation. These primary particles then grow into γ-FeOOH particles with a dendritic structure. The final size reached at long times is regulated by the steady-state redox conditions established during long-term irradiation at the aqueous-solid interface.