**1. Introduction**

## **1.1. General information on the iron-containing nanostructures**

Nanomaterials on iron basis mainly include zero-valent iron (ZVI and nZVI (nano zero-valent iron) are nowadays classic terms), iron-based nanoalloys or core-shell nanoparticles, iron(II and III) oxides, and ferrites, among others. Metallic iron is normally covered with iron(II) and iron(III) oxides [1]. The iron oxides (iron oxide nanoparticles are also referred in several reports to as superparamagnetic iron-oxide nanoparticles (SPIONs) although SPIONS have inducible

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magnetic properties) [2, 3], belong to the most technologically important oxides of transition metals. The collective term "iron oxides" is also used for oxides, hydroxides, and oxyhydroxides containing Fe(II) and/or Fe(III) cations and OH and/or O2- anions. In total, sixteen pure iron oxide phases, *i.e.,* oxides, hydroxides or oxy-hydroxides, are currently known. These compounds are Fe(OH)3, Fe(OH)2, Fe5HO8⋅4H2O, Fe3O4, FeO, five polymorphs of FeOOH and four of Fe2O3. In these oxide compounds, which are generally low soluble and possess brilliant colors, the iron is present in the form of Fe(III). The extremely important advantages of nanostructured iron, in comparison with other nanomaterials, are its relatively low toxicity and capacity to be biodegradable. This metal, in addition, is non-expensive and commonly widespread material [4].

Particle diameters of nZVI are normally in the range from 10 to 100 nm [5], exhibiting a classic core-shell structure. Their core contains metallic iron phase, meanwhile the oxidation products of zero-valent iron form mixed valent [*i.e.,* Fe(II) and Fe(III)] oxide shell. If stabilizers in excess are present, these core-shell nanoparticles could be protected against further oxidation [6]. Among such stabilizers, a series of organic compounds can be used for nZVI functionalization to stabilize nZVI aqueous dispersions, inhibiting strongly their further agglomeration. Such compounds can be used to satisfy this purpose, for example, PEG, polyacrylic acid, 4-butane‐ diphosphonic acid, and methoxyethoxyethoxyacetic acid (MEEA) [7].

The magnetite (Fe3O4) and maghemite (γ-Fe2O3) are of a particular interest talking about iron oxides (SPIONs). The magnetite structure corresponds to an inverse spinel ferrite. The oxygen ions are the part of a close-packed cubic lattice, containing the iron ions between two different interstices, tetrahedral sites (A), and octahedral sites (B). In a chemical point of view, the magnetite/maghemite can be represented by the following formula: Fe3+ [Fe2+1-y Fe3+1-y Fe3+1.67y▯0.33y]O4, where y=0 for pure magnetite and y=1 for pure maghemite (completely oxidized magnetite). From room temperature up to Curie temperature (Tc=860 K), the A sites are filled by Fe3+ ions and the B sites are filled by Fe3+ and Fe2+ ions in equal quantity. Although the lepidocrocite (γ-FeOOH) dehydration transforms into γ-Fe2O3, industrial fabrication of maghemite is based on a multistep process (1):

$$\begin{aligned} \text{( $a$  and/ or  $\gamma$ ) - FeOOH} &\text{(oxidation)} \rightarrow a \text{ -Fe}\_2\text{O}\_3 \text{(reduction)} \rightarrow\\ \rightarrow \text{Fe}\_3\text{O}\_4 &\text{(controlized oxidation)} \rightarrow \gamma \text{ -Fe}\_2\text{O}\_3 \end{aligned} \tag{1}$$

In addition to the nZVI and SPIONs, a variety of composite inorganic iron-based nanomaterials have been discovered, in particular core-shell Fe(or Fe*x*O*y*)/Au or more complex trimetallic nanoparticles such as (Fe60Co49)core/Aushell [8]. These nanoparticles were classified [9] on the basis of on their complexity levels: 1) the nanostructures based of an iron-containing material with magnetic properties *different from iron oxide*; 2) the nanostructures with a non-spherical morphology (*e.g*. hollow structure); 3) the nanostructures with multi-material composition, *i.e.* each of them is constructed ≥2 more domains of joined together different inorganic materials.
