**2. Main synthesis methods**

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 oxy-

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

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‐

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

( )() ( )

and/ or - FeOOH oxidation - Fe O reduction

2 3

® ®

a

® ® (1)

g

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.

( )

3 4 2 3

Fe O controlled oxidation - Fe O

and/or O2- anions. In total, sixteen

hydroxides containing Fe(II) and/or Fe(III) cations and OH-

36 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

diphosphonic acid, and methoxyethoxyethoxyacetic acid (MEEA) [7].

maghemite is based on a multistep process (1):

 g

a

widespread material [4].

A number of currently used methods, shown below, are nowadays used for preparation of Fecontaining nanomaterials. At the same time, some well-known conventional *wet chemistry routes* have not been forgotten and are applied for nZVI synthesis [10] (for instance, by a borohydride reduction in laboratory scale) [11], Fe2O3 (sol-gel technique [12] or electrochemical deposition [13]), or Fe3O4 (urea- and NaOH-assisted hydrolysis of Fe3+ and Fe2+ salts and further *ultrasonic treatment* of FeO(OH)/Fe(OH)2). Particle sizes and morphologies of the formed nanomaterials, synthesized by distinct methods, can vary depending on the synthesis condi‐ tions. For example, the Fe3O4 nanoparticles [14], obtained by *radio frequency nitrogen plasma* technique, represent a regular spherical form, meanwhile the nanoparticles synthesized by wet chemistry synthesis were seen as well shaped cubic form. In case of the plasma prepared iron oxides, their size distribution was wider resulting small (25–80 nm) and larger (>100 nm) particles.

## **2.1. nZVI and Fe-M nanoalloys and core-shell nanostructures**

To produce nanopowders, the method of *electrical explosion of wire* was applied [15, 16], among other physical methods. Intermetallic phases can also be obtained by the *arc discharge* techni‐ que, for example Fe-Sn bimetallic nanoparticles [17]. Thus prepared nanoparticles have a core/ shell structure consisting of a SnO2 shell (5–10 nm in thickness) and a core containing poly‐ crystalline intermetallic compounds. The intermetallic compounds FeSn2 and Fe3Sn2 were shown to be generated; they coexist with the Sn phase as a single nanoparticle. *Microwave irradiation* (which is used to fabricate normally inorganic materials and composites, lesser organic- or organometallic-based materials), is an alternative method in comparison with classic heating of product precursors. Metal transformations in microwave field are extensively reviewed in a comprehensive recent book [18]. Elemental metal nanoparticles in various forms can also be fabricated using this method. For example, iron-based nanoparticles were obtained applying microwave–polyol route in ethyleneglycol at 100 and 150 °C with additives of polyvinyl pyrrolidone and dodecyl amine [19]. Also, pulsed excimer *laser radiation* (248 nm) was used to ablate a feedstock of permalloy (~2 μm, Ni 81%:Fe 19%) under both normal atmospheric conditions (in air) and in other gases, as well as under pressures [20]. α-Fe nanoparticles were also obtained with use of a modified *metal-membrane incorporation* method applying diffusing metal ions through a dialysis membrane [21] (the diffusion-time ≤15 min).

To get nZVI in laboratory conditions [22], the classic and usual synthesis technique is the reduction of Fe(II or III) salts using NaBH4, NaAlH4 or LiAlH4 as reductants. Thus, nZVI was synthesized (reaction 2) [23] in ethanol medium by the method of reduction of FeX*n* (X= Cl, OH, OR, CN, OCN, SCN) using sodium borohydride under atmospheric conditions [24].

$$2\text{FeCl}\_3 + 6\text{NaBH}\_4 + 18\text{H}\_2\text{O} \rightarrow 2\text{Fe}^0 + 6\text{NaCl} + 6\text{B(OH)}\_3 + 21\text{H}\_2\tag{2}$$

A patent [25] describes a route to metal nanoparticles by *thermal decomposition* of iron acetate Fe(OOCCH3)2, placed in a reaction vessel with a passivating solvent such as a glycol ether. Discussing the *pyrolysis* method, it should be noted the preparation of iron nanoparticles (embedded in a carbon matrix) from metal phthalocyanine as precursor [26] and carbonencapsulated iron nanoparticles (size 5–20 nm) *via* a picric acid-*detonation-induced pyrolysis* of ferrocene as precursor; this route has such peculiarities as self-heating and extremely fast process [27]. Also, the *"greener" techniques* [28, 29, 30, 31, 32] have been applied for nanoparticle fabrication. Use of plant extracts and other natural products on polyphenole basis in these syntheses as reductants and capping agents at the same time for obtaining nZVI and several other Fe-containing nanoparticles is intriguing [33] as well. For instance, the *herbal tea ex‐ tracts* were applied to reduce iron(III) chloride to elemental iron nanoparticles (50 nm) [34].

In addition, a variety of general physico-chemical methods have been applied for the produc‐ tion of as *Fe-containing bi- and polymetallic alloys* as core-shell nanostructures. For instance, high entropy Nd-Fe-Co-Ni-Mn alloy nanofilms were prepared [35] by *electrodeposition* at r.t. After preliminary preparation of alumina nanotemplates, Fe, Fe-Ni, and Fe-Pd nanowires were successfully electrodeposited within their porous structure. Also, the Fe-Pt nanocrystalline magnetic films (200 nm of thickness) with planar texture were obtained with use of *magnetron sputtering* and crystalline annealing in magnetic field [36].

## **2.2. Supported and coated iron nanoparticles**

A number of publications are devoted to *carbon-supported* ZVI nanomaterials [37]. This type of protective carbon-cage encapsulation of iron nanoparticles can result hybrid core-shell nanomaterials with unique properties [38]. This way, carbon encapsulated iron core-shell nanoparticles (15–40 nm in size) were obtained *via* confined *arc plasma method* [39]. Resulting nanoparticles possessed a clear core-shell structure. The core (16 nm in diameter) of the par‐ ticles corresponded to a BCC iron structure, and the shell (thickness 6–8 nm) was shown to be disorder carbon phase. A closely related *arc discharge technique* [40] is also frequently used for obtaining a variety of nanomaterials, in particular the iron containing ones. For instance, a simple, inexpensive and one-step arc discharge synthesis technique to prepare metal-con‐ taining carbon nanocapsules in aqueous solution is known [41]. It was established that iron nanoparticles can be *in situ* encapsulated in carbon shells when the arc discharge process was carried out in aqueous solutions of FeSO4.

*Combustion synthesis* of iron oxide/iron-coated carbons such as cellulose fiber, anthracite, and activated carbon was reported a classic microwave oven with inverter technology [42]. The size of the iron oxide/iron nanoparticle-coated samples were determined to be in the range of 50–400 nm. It was found that iron oxide/iron nanoparticles exist in 4 main phases: γ-Fe2O3, α-Fe2O3, Fe3O4, and Fe, some of them had significant arsenic adsorption. In addition, carboncoated iron nanoparticles with well-developed quasi-spherical shape were prepared with Fe(NO3)3⋅9H2O and starch as carbon source [43, 44]. This is an efficient approach for the mass production of nanocage structures under mild conditions, which needs to be further explored for preparing various carbon coated metal nanomaterials. *Radiation methods* are being applied more widely last 10–15 years, in that case for preparation of Fe-carbon-supported nanostruc‐ tures. This way, 2 types of amorphous carbon films (15 at.% iron containing film) were deposited onto Si substrates by a sputtering method and further exposed to an electron flow, where the energy and dose rate were much smaller compared to the electron beam in a TEM. In this case, graphitic structures were observed in amorphous matrix at temperatures up to 450 K. It was established that the graphitization progressed more intensively during the electron irradiation than in annealing at 773 K. This was attributed to thermal and catalytic effects, strongly related to grain growth of metal clusters.

## **2.3. Free and supported iron oxides and ferrites**

A patent [25] describes a route to metal nanoparticles by *thermal decomposition* of iron acetate Fe(OOCCH3)2, placed in a reaction vessel with a passivating solvent such as a glycol ether. Discussing the *pyrolysis* method, it should be noted the preparation of iron nanoparticles (embedded in a carbon matrix) from metal phthalocyanine as precursor [26] and carbonencapsulated iron nanoparticles (size 5–20 nm) *via* a picric acid-*detonation-induced pyrolysis* of ferrocene as precursor; this route has such peculiarities as self-heating and extremely fast process [27]. Also, the *"greener" techniques* [28, 29, 30, 31, 32] have been applied for nanoparticle fabrication. Use of plant extracts and other natural products on polyphenole basis in these syntheses as reductants and capping agents at the same time for obtaining nZVI and several other Fe-containing nanoparticles is intriguing [33] as well. For instance, the *herbal tea ex‐ tracts* were applied to reduce iron(III) chloride to elemental iron nanoparticles (50 nm) [34].

In addition, a variety of general physico-chemical methods have been applied for the produc‐ tion of as *Fe-containing bi- and polymetallic alloys* as core-shell nanostructures. For instance, high entropy Nd-Fe-Co-Ni-Mn alloy nanofilms were prepared [35] by *electrodeposition* at r.t. After preliminary preparation of alumina nanotemplates, Fe, Fe-Ni, and Fe-Pd nanowires were successfully electrodeposited within their porous structure. Also, the Fe-Pt nanocrystalline magnetic films (200 nm of thickness) with planar texture were obtained with use of *magnetron*

A number of publications are devoted to *carbon-supported* ZVI nanomaterials [37]. This type of protective carbon-cage encapsulation of iron nanoparticles can result hybrid core-shell nanomaterials with unique properties [38]. This way, carbon encapsulated iron core-shell nanoparticles (15–40 nm in size) were obtained *via* confined *arc plasma method* [39]. Resulting nanoparticles possessed a clear core-shell structure. The core (16 nm in diameter) of the par‐ ticles corresponded to a BCC iron structure, and the shell (thickness 6–8 nm) was shown to be disorder carbon phase. A closely related *arc discharge technique* [40] is also frequently used for obtaining a variety of nanomaterials, in particular the iron containing ones. For instance, a simple, inexpensive and one-step arc discharge synthesis technique to prepare metal-con‐ taining carbon nanocapsules in aqueous solution is known [41]. It was established that iron nanoparticles can be *in situ* encapsulated in carbon shells when the arc discharge process

*Combustion synthesis* of iron oxide/iron-coated carbons such as cellulose fiber, anthracite, and activated carbon was reported a classic microwave oven with inverter technology [42]. The size of the iron oxide/iron nanoparticle-coated samples were determined to be in the range of 50–400 nm. It was found that iron oxide/iron nanoparticles exist in 4 main phases: γ-Fe2O3, α-Fe2O3, Fe3O4, and Fe, some of them had significant arsenic adsorption. In addition, carboncoated iron nanoparticles with well-developed quasi-spherical shape were prepared with Fe(NO3)3⋅9H2O and starch as carbon source [43, 44]. This is an efficient approach for the mass production of nanocage structures under mild conditions, which needs to be further explored for preparing various carbon coated metal nanomaterials. *Radiation methods* are being applied more widely last 10–15 years, in that case for preparation of Fe-carbon-supported nanostruc‐

*sputtering* and crystalline annealing in magnetic field [36].

38 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**2.2. Supported and coated iron nanoparticles**

was carried out in aqueous solutions of FeSO4.

Zeolites and closely related supporting materials represent an ideal basis for iron oxide nanocomposites. This way, the zeolite loading with nanoiron oxide by a simple chemical process was described [45]. Final crystallite sizes of the doped nanomaterials were in the range of 4–6 nm. It was shown that the zeolites become to have magnetic properties after being doped with nanoiron oxide. Mesoporous nanocomposites "iron oxide/silicate" Fe2O3-SBA-15 (SBA-15 is an abbreviation for hexagonally ordered mesoporous silica) with iron loadings of 1.2–35.8 wt.% were prepared hydrothermally [46]. It was revealed that these composites contain welldispersed iron oxide nanoclusters in the walls of ordered mesoporous silica and high surface area. Certain number of composite nanomaterials based on Fe3O4 is known, for instance core/ shell Fe3O4 coated gold nanoparticles (diameter 50–100 nm) [47]. Their possible formation mechanism was proposed as follows: pH-sensitive polymer owing to a shrunken or stretched structure of polyethyleneimine (PEI), led to the aggregation of the Fe3O4-gold seed nanopar‐ ticles, then gold reduces onto the surface of Fe3O4-gold seed nanoparticles. It was concluded that these core/shell multifunction nanomaterials will not only have external magnetic separation by the core of Fe3O4 but also detect the large biological molecules using the shell of gold. In addition, iron phthalocyanine prepolymer/Fe3O4 nano hybrid magnetic material [48] can be applied as high temperature-resistant polymer magnetic composite material. At last, ferrites having different sizes, from ultrasmall (2 nm) to 50 nm, can be fabricated by distinct techniques [49] mainly co-precipitation method (CPM), sometimes without using any capping agents/surfactants.
