Bimetal Cufe Nanoparticles—Synthesis, Properties, and Applications

applied sciences

Review Bimetal CuFe Nanoparticles—Synthesis, Properties, and Applications

Zaneta Swiatkowska-Warkocka

Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 Krakow, Poland; [email protected]

Featured Application: This review article deals with the synthesis and applications of CuFe bimetallic nanoparti-cles. Although much research has been published on nanomaterials, over- all review articles on CuFe nanoparticles are lacking and this article presents the latest data on the synthesis, prop-erties and possible applications of CuFe nanoparticles.

Abstract: Bimetal CuFe (copper-iron) nanoparticles, which are based on the earth-abundant and inexpensive metals, have generated a great deal of interest in recent years. The possible modiﬁcation of the chemical and physical properties of these nanoparticles by changing their size, structure, and composition has contributed to the development of material science. At the same time, the strong tendency of these elements to oxidize under atmospheric conditions makes the synthesis of pure bimetallic CuFe nanoparticles still a great challenge. This review reports on different synthetic approaches to bimetallic CuFe nanoparticles and bimetallic CuFe nanoparticles supported on various materials (active carbide, carbide nanotubes, silica, graphite, cellulose, mesoporous carbide), their structure, physical, and chemical properties, as well as their utility as catalysts, including electrocatalysis and photocatalysis. Keywords: bimetal nanoparticles; copper-iron; magnetic properties; catalytic properties; synthesis; Citation: Swiatkowska-Warkocka, Z. core@shell; Janus structures; battery; photocatalysis; water treatment Bimetal CuFe Nanoparticles— Synthesis, Properties, and Applications. Appl. Sci. 2021, 11, 1978. https://doi.org/10.3390/ 1. Introduction app11051978 Bimetallic composite nanoparticles have generated great interest because the combination, at the nanoscale, of different metals can result in new or enhanced physicochemical Academic Editor: Andrea Atrei properties and vast potential applications in the areas of electronics, photonics, catalysis, and biomedicine [1–5]. Properties of obtained particles depend on and can be tailored Received: 29 December 2020 according to their architectures (e.g., core@shell or multishell structures, hollow structures, Accepted: 19 February 2021 Published: 24 February 2021 heterostructures, alloys), composition, size, and shape [1,6–10]. Copper (Cu) is a 3rd period transition metal and has some interesting physical and

Publisher’s Note: MDPI stays neutral chemical properties, such as catalytic activity, high electrical and thermal conductivity, with regard to jurisdictional claims in good ductility, malleability, and tensile strength. Due to the catalytic activity of copper published maps and institutional affil- nanoparticles, they find a number of applications, including gas-phase reactions, Ullmann iations. reactions, cross-coupling reactions, A3 coupling reactions, azide-alkyne cycloaddition, photocatalysis, and electrocatalysis [11]. An attribute of nanoparticles exhibiting magnetic properties is the ability to selectively attach functional particles and manipulate them using an external magnetic field produced by an electromagnet or permanent magnet [12]. Among them, iron (Fe) is a class of ferro- Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. magnetic materials with high magnetic moment density (218 emu/g) and is magnetically This article is an open access article soft. Iron nanoparticles in the size range below 20 nm are in the superparamagnetic regime, distributed under the terms and and their stable dispersions with high magnetic moment are predicted to have wide range conditions of the Creative Commons applications including data storage, environmental remediation, catalysis, and disease Attribution (CC BY) license (https:// diagnosis and therapy [13]. Nanoscale zero-valence iron (nZVI) is a strong reducing agent. creativecommons.org/licenses/by/ A large specific surface area and reactivity of these nanoparticles increase the efficiency of 4.0/). removing inorganic and organic pollutants as well as heavy metals. However, due to their

Appl. Sci. 2021, 11, 1978. https://doi.org/10.3390/app11051978 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1978 2 of 15

have wide range applications including data storage, environmental remediation, catalysis, and disease diagnosis and therapy [13]. Nanoscale zero-valence iron (nZVI) is a strong Appl. Sci. 2021, 11, 1978 2 of 15 reducing agent. A large specific surface area and reactivity of these nanoparticles increase the efficiency of removing inorganic and organic pollutants as well as heavy metals. How- ever, due to their small size and magnetic properties, they easily aggregate [14–17]. Be- tweensmall the size variety and magnetic of transition properties, metal macrocycle they easily complex aggregate catalysts [14–17]. with Between different the variety central of metaltransition atoms metal[14,15], macrocycle iron-based complex materials catalysts have been with identified different to central exhibit metal the best atoms activity [14,15 ], foriron-based ORR under materials various have metal been loadings identiﬁed [16– to23 exhibit]. the best activity for ORR under various metalOn loadingsaccount of [16 their–23]. properties, nanomaterials based on copper and iron can effectively replaceOn account rare ofand their expensive properties, noble nanomaterials-metal catalysts based commonly on copper employed and iron canin commer- effectively cialreplace chemical rare processes. and expensive However, noble-metal the synthesis catalysts and use commonly of both copper employed and iron in commercial nanopar- ticleschemical is still processes. a challenge However, due to the the high synthesis tendency and useof these of both materials copper andto oxidize iron nanoparticles under at- mosphericis still a challenge conditions. due Therefore, to the high complex tendency nanoparticles of these materials (e.g., to core@shell,oxidize under alloys) atmospheric have beenconditions. recently Therefore,adapted to complexovercome nanoparticles the instability (e.g., of copper core@shell, and iron alloys) nanoparticles have been recentlyin the presenceadapted of to oxyg overcomeen, water, the and instability several ofchemicals. copper and iron nanoparticles in the presence of oxygen,Since water,the fusion and of several the unique chemicals. properties of copper and iron in one single entity prom- Since the fusion of the unique properties of copper and iron in one single entity ises multifunctionality and potential applications, a lot of effort is put into preparing such promises multifunctionality and potential applications, a lot of effort is put into preparing nanoparticles containing Cu and Fe. This coupling may result in the formation of nano- such nanoparticles containing Cu and Fe. This coupling may result in the formation of composites with an extraordinary catalytic activity and ferromagnetic properties, which nanocomposites with an extraordinary catalytic activity and ferromagnetic properties, would allow for convenient separation of the nanocomposite catalyst from the reaction which would allow for convenient separation of the nanocomposite catalyst from the system in the applied magnetic field (Figure 1). reaction system in the applied magnetic ﬁeld (Figure1).

Figure 1. CuFe bimetallic nanoparticles, their synthesis, architecture, properties, and applications. Figure 1. CuFe bimetallic nanoparticles, their synthesis, architecture, properties, and applications. 2. Structure, Synthesis, and Properties of Bimetallic CuFe Nanoparticles 2. Structure, Synthesis, and Properties of Bimetallic CuFe Nanoparticles As already mentioned, bimetallic particles composed of two components are expected to displayAs already either mentioned, a combination bimetallic of the propertiesparticles composed associated of with two each componen materialts or are new ex- or pectedenhanced to display properties either and a combination capabilities dueof the to couplingproperties between associated two with different each materials. material or The newproperties or enhanced of bimetallic properties nanoparticles and capabilities can be due tuned to coupling by varying between the concentration two different of ma- their terials.constituent The properties elements, of their bimetallic architecture, nanoparticles shape, and can size. be tuned Although by varyi bimetallicng the nanoparticles concentra- tionconsist of their of constituent only two different elements, metals, their architecture, there are still shape, many and types size. ofAlthough possible bimetallic structures: nanoparticlesrandom and consist ordered of alloy, only Janus,two different and core@shell metals, (Figurethere are2)[ still5,24 many]. types of possible structures:The random CuFe bimetallic and ordered nanoparticles alloy, Janus, can and be core@sh orientedell in (Figure alloy, core@shell,2) [5,24]. Janus, and other architectures, depending mainly on their synthesis [25,26]. They are known to have face-centered cubic (fcc), body centered cubic (bcc), and hexagonal close-packed (hcp) crystal structures [25,26]. There are a lot of methods for designing various kinds of bimetallic nanoparticles; however, in the case of the CuFe system, both elements are unstable in the presence of air; therefore, using conventional methods is challenging.

2.1. Modeling As can be found in the literature, molecular dynamics simulations allow for the creation of a model and determine the properties before starting experiments or full-scale industrial production of metallic nanoparticles [27–32]. Molecular dynamics calculations were also used to investigate the CuFe system, including the inﬂuence of particle size, cooling rate, and Cu concentration on the morphology of the CuFe nanoparticles. Rojas- Nunez et al. studied the dependence of the morphologies of FeCu on different Fe and Cu concentrations and the appearance of possible defects in the stabilized NPs, as well Appl. Sci. 2021, 11, 1978 3 of 15

as the energy contributions to the bimetallic systems [25]. The researchers studied the Fe1−xCux systems with Cu concentration at 10%, 40%, and 70% (Figure3). They examined three Fe cores (DFe) with diameters 5, 7, and 9 nm with their respective Cu concentrations. At the low concentration of Cu (10%), the Fe core@shell is created. With increasing the Cu concentration to 40%, core@shell structures are observed for small (5 nm) Fe particles, while Janus structures are preferred for medium (7 nm) and big (9 nm) Fe particles. Seventy percent of Cu leads to segregated Janus (sub-clasters) particles. These simulations show that a segregated (Janus) structure becomes preferred over core@shell architecture when the Cu content increases. In addition, for low Cu concentration, the particles show a bcc phase, at 40% Cu, two phases exist, a bcc phase for Fe, and Cu is mainly ordered as an Appl. Sci. 2021, 11, 1978 fcc lattice, high Cu content shows a bcc lattice for Fe and an fcc lattice for Cu. Hexagonal 3 of 15 closed-packed (hcp) structures are nucleated only at the Fe−Cu interface because of twins and stacking faults.

Figure 2. 2. Types of of bimetallic bimetallic nanoparticles: nanoparticles: (a) random (a) random alloyed, alloyed, (b) ordered (b) ordered alloy, (c) sub-alloy, (c) sub-clusters/Janusclusters/Janus (d ()d core) core-shell,-shell, ((ee)) multi-shell multi-shell core-shell, core-shell, and (f and) multiple (f) multiple core materials core coatedmaterials by coated by a sin- a single shell material. Yellow and purple spheres represent two different kinds of metal atoms. gle shell material. Yellow and purple spheres represent two different kinds of metal atoms. Re- Reprinted from [5]. printed from [5]. Another example of molecular dynamics calculations used to investigate the CuFe systemThe was CuFe reported bimetallic by Kumar [nanoparticles26]. He showed thatcan the be FeCu oriented bimetallic in alloy, nanoparticles core@shell, Janus, and can be oriented in random alloy, core-shell, and Janus morphologies, depending on their othersize, and architectures, thermal processing depending (temperature mainly and coolingon their rate). synthesis At a very [ slow25,26 cooling]. They rate are known to have fac(0.1e K/ps),-centered clear interface cubic (fcc), Fe-Cu atombody is centeredcreated at a cubic temperature (bcc), of andT = 1200 hexagonal K and lower. close-packed (hcp) crystalAs the temperature structures decreases [25,26] further. below T = 1200 K, the Fe and Cu atoms crystallize in bcc andThere fcc crystalline are a lot structures, of methods respectively for designing (Figure4a). With various an increasing kinds coolingof bimetallic rate nanoparticles; (0.5 and 1 K/ps), the immiscibility between Fe and Cu atoms is not complete (Figure4b,c ), however,and patchy in nanoparticles the case of are the created. CuFe Atsystem, a cooling both rate elements of 0.5 K/ps, are Fe unstable and Cu atoms in the presence of air; therefore,crystalized inusing bcc and conventional fcc structures, methods respectively is challenging. (Figure4b), while at cooling rate of 1 K/ps, both Fe and Cu atoms have bcc structures (Figure4c). The semi-miscible phase of 2.1.Cu and Modeling Fe atoms is the result of very high cooling rates, 5 and 10 K/ps (Figure4d,e), which do not allow the atoms to crystallize neither in the fcc structure nor in the bcc structure. As can be found in the literature, molecular dynamics simulations allow for the creation of a model and determine the properties before starting experiments or full-scale industrial production of metallic nanoparticles [27–32]. Molecular dynamics calculations were also used to investigate the CuFe system, including the influence of particle size, cooling rate, and Cu concentration on the morphology of the CuFe nanoparticles. Rojas- Nunez et al. studied the dependence of the morphologies of FeCu on different Fe and Cu concentrations and the appearance of possible defects in the stabilized NPs, as well as the energy contributions to the bimetallic systems [25]. The researchers studied the Fe1−xCux systems with Cu concentration at 10%, 40%, and 70% (Figure 3). They examined three Fe cores (DFe) with diameters 5, 7, and 9 nm with their respective Cu concentrations. At the low concentration of Cu (10%), the Fe core@shell is created. With increasing the Cu concentration to 40%, core@shell structures are observed for small (5 nm) Fe particles, while Janus structures are preferred for medium (7 nm) and big (9 nm) Fe particles. Seventy percent of Cu leads to segregated Janus (sub-clasters) particles. These simulations show that a segregated (Janus) structure becomes preferred over core@shell architecture when the Cu content increases. In addition, for low Cu concentration, the particles show a bcc phase, at 40% Cu, two phases exist, a bcc phase for Fe, and Cu is mainly ordered as an fcc lattice, high Cu content shows a bcc lattice for Fe and an fcc lattice for Cu. Hexagonal closed-packed (hcp) structures are nucleated only at the Fe−Cu interface because of twins and stacking faults. Appl. Sci. 2021, 11, 1978 4 of 15 Appl. Sci. 2021, 11, 1978 4 of 15

Figure 3. (a) Cluster structure as a function of size and Cu concentration. (Fe cores (DFe) have di- Figureameters 3.5, 7,( aand) Cluster 9 nm). The structure pink line asdepicts a function the core@shell of size to Janus and-like Cu stability concentration. transition from (Fe cores (DFe) have diametersthe continuous 5, 7, model. and 9Cases nm). I, TheII, and pink III are line shown depicts below. the Cross core@shell-sectional toviews Janus-like of FeCu particles stability transition from theencircled continuous at (a), showing model. elemental Cases I,composition II, and III (b are), and shown structural below. defects Cross-sectional (c). Reproduced viewswith of FeCu particles permission from ref. [25]. Copyright 2018 American Chemical Society. Appl. Sci. 2021, 11, 1978 encircled at (a), showing elemental composition (b), and structural defects (c).5 Reproduced of 15 with

permissionAnother from example ref. [ 25of ].molecular Copyright dynamics 2018 American calculations Chemical used to Society.investigate the CuFe system was reported by Kumar [26]. He showed that the FeCu bimetallic nanoparticles can be oriented in random alloy, core-shell, and Janus morphologies, depending on their size, and thermal processing (temperature and cooling rate). At a very slow cooling rate (0.1 K/ps), clear interface Fe-Cu atom is created at a temperature of T = 1200 K and lower. As the temperature decreases further below T = 1200 K, the Fe and Cu atoms crystallize in bcc and fcc crystalline structures, respectively (Figure 4a). With an increasing cooling rate (0.5 and 1 K/ps), the immiscibility between Fe and Cu atoms is not complete (Figure 4b,c), and patchy nanoparticles are created. At a cooling rate of 0.5 K/ps, Fe and Cu atoms crystalized in bcc and fcc structures, respectively (Figure 4b), while at cooling rate of 1 K/ps, both Fe and Cu atoms have bcc structures (Figure 4c). The semi-miscible phase of Cu and Fe atoms is the result of very high cooling rates, 5 and 10 K/ps (Figure 4d,e), which do not allow the atoms to crystallize neither in the fcc structure nor in the bcc structure.

Figure 4. Snapshots of FeCu bimetallic nanoparticles during cooling from T = 3000 to 10 K at vari- Figureous cooling 4. Snapshots rates: (a) 0.1 of K/ps, FeCu (b bimetallic) 0.5 K/ps, ( nanoparticlesc) 1 K/ps, (d) 5 K/ps, during and cooling(e) 10 K/ps from (Colors: T = 3000● Cu, to● 10 K at various coolingFe). The rates:rightmost (a) 0.1column K/ps, depicts (b) 0.5the K/ps,evolution (c) of 1 the K/ps, various (d) 5crystal K/ps, structures and (e) of 10 FeCu K/ps nanopar- (Colors: • Cu, • Fe). Theticles rightmost at 300 K (Colors: column ● fcc, depicts ● hcp, the and evolution ● bcc atoms). of the Red various arrows show crystal the structures progress of ofthe FeCu cooling nanoparticles at process. Reproduced with permission from ref. [26]. Copyright 2019 American Chemical Society. 300 K (Colors: • fcc, • hcp, and • bcc atoms). Red arrows show the progress of the cooling process. Reproduced2.2. Synthesis with and P permissionroperties from ref. [26]. Copyright 2019 American Chemical Society. There are two general approaches to the synthesis of nanoparticles, i.e., top-down methods and bottom-up methods. Top-down approach involves the breaking down of the bulk material into nanosized structures or particles. Crystal etching, ball milling and grinding are examples of this approach. Bottom-up approach refers to the build-up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by cluster. Chemical synthesis of nanoparticles is a typical example, but also self-assembly or nanopatterning can be classified as bottom-up techniques. Since Cu and Fe are immiscible in the whole range of composition in equilibrium, a solid solution of the FeCu system can be obtained only by non-equilibrium techniques, such as mechanical alloying [33–35] or vapor deposition [36].

2.2.1. Mechanical Milling CuFe bimetallic nanoparticles were synthesized by mechanical milling [37–39]. To- kada et al. [37] used FeCl3 and CuCl2 with Na to form FeCu nanoparticles by ball milling and studied their magnetic properties. The process was based on two reactions:

FeCl3 + 3Na → Fe + 3NaCl (1)

CuCl2 + 2Na → Cu + 2NaCl (2) The reactants were milled under Ar atmosphere up to 84 h. With the increase in ball milling duration from 3 h to 84 h, the particle size was found to increase from approximately 16 nm and 50 nm, respectively (Figure 5). They reported the bcc α-Fe and fcc Cu phases formation, and the formation of additional hexagonal Fe(OH)2 phase for the powders milled shorter than 24 h. They also observed that the coercivity (HC) value increased with the grinding time and reached a maximum value of 33.5 kA/m for 48 h of grinding, while further grinding resulted in a decrease in the HC value (Figure 5). Appl. Sci. 2021, 11, 1978 5 of 15

2.2. Synthesis and Properties There are two general approaches to the synthesis of nanoparticles, i.e., top-down methods and bottom-up methods. Top-down approach involves the breaking down of the bulk material into nanosized structures or particles. Crystal etching, ball milling and grinding are examples of this approach. Bottom-up approach refers to the build- up of a material from the bottom: atom-by-atom, molecule-by-molecule, or cluster-by cluster. Chemical synthesis of nanoparticles is a typical example, but also self-assembly or nanopatterning can be classiﬁed as bottom-up techniques. Since Cu and Fe are immiscible in the whole range of composition in equilibrium, a solid solution of the FeCu system can be obtained only by non-equilibrium techniques, such as mechanical alloying [33–35] or vapor deposition [36].

2.2.1. Mechanical Milling CuFe bimetallic nanoparticles were synthesized by mechanical milling [37–39]. Tokada et al. [37] used FeCl3 and CuCl2 with Na to form FeCu nanoparticles by ball milling and studied their magnetic properties. The process was based on two reactions:

FeCl3 + 3Na → Fe + 3NaCl (1)

CuCl2 + 2Na → Cu + 2NaCl (2) The reactants were milled under Ar atmosphere up to 84 h. With the increase in ball milling duration from 3 h to 84 h, the particle size was found to increase from approximately 16 nm and 50 nm, respectively (Figure5). They reported the bcc α-Fe and fcc Cu phases formation, and the formation of additional hexagonal Fe(OH)2 phase for the powders milled shorter than 24 h. They also observed that the coercivity (HC) value increased with Appl. Sci. 2021, 11, 1978 6 of 15 the grinding time and reached a maximum value of 33.5 kA/m for 48 h of grinding, while further grinding resulted in a decrease in the HC value (Figure5).

FigureFigure 5. Transmission 5. Transmission electron electron microscopy microscopy (TEM) (TEM) images images (a– (ac)– cof) ofthe the powders powders for for various various milling milling timestimes (18 (18 h, 48 h, h, 48 84 h, h, 84 respectively) h, respectively).. Particle Particle sizes sizesof the of powders the powders for various for various milling milling times ( timesd). (d). SaturationSaturation magnetization magnetization (MS (M) andS) andcoercivity coercivity (HC) (HvaluesC) values of the of as the-milled as-milled powders powders for various for various millingmilling times times (e) (.e Reproduced). Reproduced with with permission permission from from ref. ref. [37 [37]. Copyright]. Copyright 2002 2002 The The Japan Japan Institute Institute of of Metals and Materials.Materials.

CuFeCuFe nanocomposites nanocomposites were were obtained by by high-energy high-energy attrition attrition milling milling of Fe of nanopow- Fe na- nopowderder and cuprousand cuprous oxide oxide (n-Cu (n2O)-Cu in2O) hexane in hexane by Sharipova by Sharipova et al. [et38 ].al. The [38]. authors The authors studied studitheired mechanical their mechanical properties properties and found and found that FeCu that nanocomposite FeCu nanocomposite has a higher has a ductilityhigher ductilitycompared compared to nanostructured to nanostructured Fe (the Fe compressive (the compressive yield stress yield of stress FeCu of is FeCu 720 MPa, is 720 while MPa, for whileFe nanopowder for Fe nanopowder (30 nm) (30 is 1100nm) MPais 1100 [39 MPa]). [39]). Nanocomposites of CuFe/CNT (carbon nanotubes) were synthetized by two-step mechanical milling of a mixture of Cu, Fe, and CNT under argon atmosphere [39]. First, pure Fe and Cu powders in four various proportions were milled for 15 h. Next, the sample showing the maximum solubility of Fe into Cu, i.e., with 20% of Fe in the sample, was chosen, mixed with CNT, and milled for 15 h to synthesize nanocomposite samples. As a result, micron-sized agglomerates consisting of nanoparticles were created. The structural, magnetic, and electrical properties depend on the CNT addition. With the increasing of the CNT content, agglomerates’ size decreased, while coercivity, magnetization, and electrical resistivity of composite samples increased.

Nanocomposites of CuFe/CNT (carbon nanotubes) were synthetized by two-step mechanical milling of a mixture of Cu, Fe, and CNT under argon atmosphere [39]. First, pure Fe and Cu powders in four various proportions were milled for 15 h. Next, the sample showing the maximum solubility of Fe into Cu, i.e., with 20% of Fe in the sample, was chosen, mixed with CNT, and milled for 15 h to synthesize nanocomposite samples. As a result, micron-sized agglomerates consisting of nanoparticles were created. The structural, magnetic, and electrical properties depend on the CNT addition. With the increasing of the CNT content, agglomerates’ size decreased, while coercivity, magnetization, and electrical resistivity of composite samples increased.

2.2.2. Chemical Reduction FeCu nanoparticles have also been produced by chemical reduction [40–47]. FeCu nanoparticles were synthesized from chemical reduction of Cu and Fe dodecyl sulfate (Fe(DS)2 and Cu(DS)2) by sodium borohydride (NaBH4) and their magnetic properties were studied by Duxin et al. [40]. To prevent oxidation, the synthesis was carried out under a nitrogen atmosphere. The authors found single fcc phase alloys and superparamagnetic behavior above 10 K. The reduction of FeSO4·7H2O and CuSO4·5H2O aqueous mixture with sodium borohydride and stirring with the activated carbon was used for the synthesis of C/FeCu. Particles with the average size of 20 nm and a chain-like arrangement with Fe2O3 and Cu2O phases were reported [41]. A two-step reduction method with using FeSO4·7H2O stabilized by sodium citrate and CuSO4·5H2O with sodium borohydride under N2 protection were used by Feng et al. [42] for the synthesis of Cu doped Fe@Fe2O3 nanoparticles with average size of a dozen of nanometers and different weight ratio Fe/Cu (1:1, 1:2, 3:10, 1:5, 1:10, 1:20 by weight). The effectiveness of arsenic removal from water by these particles was tested (reported in Section3). In order to maintain or increase the stability of the particles, stabilizers such as chitosan (CS), green tea (GT), or carboxymethyl cellulose (CMC) are used. Jiang et al. [43] developed the bimetallic FeCu nanoparticles stabilized by chitosan (CS) with a diameter of 50–100 nm. They chose chitosan due to its biodegradability in the environment and the content of amine and hydroxyl groups, which should strongly bind to nZVI and effectively stabilize nanoparticles. They reported that the concentration of chitosan inﬂuences crystal nucleation and aggregation of FeCu particles, and higher chitosan amount results in a more uniform dispersion of FeCu nanoparticles in chitosan. The best amount of chitosan and Cu in CS-CuFe nanocomposites is 2 wt% and 3 wt%, respectively, both in terms of stabilization and catalytic properties. Green synthesis with green tea (GT) extracts as a reducing agent and stabilizer was used for the preparation of composite of GT and nZVICu nanoparticles with an average size of 60–120 nm [44]. FeCu alloy nanoparticles encapsulation in carbon (FeCu@C) by a combination of a chemical reduction of ferric nitrate Fe(NO3)3·9H2O and copper nitrate Cu(NO3)2·3H2O in alcohol solution and annealing carbonization (Figure6) was performed by Xue et al. [ 45]. The core-shell nanoparticles, with FeCu4 core and amorphous carbon shell, with particles size ranging from 24 nm to 73 nm, and coating thickness of approximately 5 nm were produced. The obtained material presented ultra-soft magnetic properties. FeCu nanoparticles of various composition (Fe:Cu = 8:2, 5:5, 2:8) prepared by the reduction of iron and copper nitrates were dispersed on the two various media, highly ordered “platelet” graphite nanoﬁbers GNF and amorphous silica [46]. Size of nanoparticles supported on “platelet” GNF is equal 13.1nm, 11.1 nm, 12.7 nm for Fe:Cu = 8:2, 5:5, 2:8, respectively and is twice as large on average as those supported in silica equal to 5.6 nm, 7.6 nm, 8.1 nm for Fe:Cu = 8:2, 5:5, 2:8, respectively. Comparison of catalytic activity and selectivity of those systems shows that the performance of the metal catalysts supported Appl. Sci. 2021, 11, 1978 7 of 15

In order to maintain or increase the stability of the particles, stabilizers such as chitosan (CS), green tea (GT), or carboxymethyl cellulose (CMC) are used. Jiang et al. [43] developed the bimetallic FeCu nanoparticles stabilized by chitosan (CS) with a diameter of 50–100 nm. They chose chitosan due to its biodegradability in the environment and the content of amine and hydroxyl groups, which should strongly bind to nZVI and effectively stabilize nanoparticles. They reported that the concentration of chitosan influences crystal nucleation and aggregation of FeCu particles, and higher chitosan amount results in a more uniform dispersion of FeCu nanoparticles in chitosan. The best amount of chitosan and Cu in CS-CuFe nanocomposites is 2 wt% and 3 wt%, respectively, both in terms of stabilization and catalytic properties. Green synthesis with green tea (GT) extracts as a reducing agent and stabilizer was used for the preparation of composite of GT and nZVICu nanoparticles with an average size of 60–120 nm [44]. FeCu alloy nanoparticles encapsulation in carbon (FeCu@C) by a combination of a chemical reduction of ferric nitrate Fe(NO3)3⋅9H2O and copper nitrate Cu(NO3)2⋅3H2O in alcohol solution and annealing carbonization (Figure 6) was performed by Xue et al. [45]. The core-shell nanoparticles, with FeCu4 core and amorphous carbon shell, with particles size ranging from 24 nm to 73 nm, and coating thickness of approximately 5 nm were produced. The obtained material presented ultra-soft magnetic properties. FeCu nanoparticles of various composition (Fe:Cu = 8:2, 5:5, 2:8) prepared by the reduction of iron and copper nitrates were dispersed on the two various media, highly ordered “platelet” graphite nanofibers GNF and amorphous silica [46]. Size of nanoparticles supported on “platelet” GNF is equal 13.1nm, 11.1 nm, 12.7 nm for Fe:Cu = 8:2, 5:5, 2:8, respectively and is twice as large on average as those supported in silica equal to 5.6 nm, 7.6 nm, 8.1 nm for Fe:Cu = 8:2, 5:5, 2:8, respectively. Comparison of catalytic activity and Appl. Sci. 2021, 11, 1978 selectivity of those systems shows that the performance of the metal7 of 15 catalysts supported on the graphite nanofibers was better than that on the silica system, what is connected with the nature of the interaction of nanoparticles with the support. on the graphite nanoﬁbers was better than that on the silica system, what is connected with the natureThe of CuFe the interaction bimetallic of nanoparticles nanoparticles with the with support. Cu/Fe molar ratios of 10.00, 3.00, 0.33 and 0.10The were CuFe prepared bimetallic nanoparticlesby a simultaneous with Cu/Fe reduction molar ratios of of cupric 10.00, 3.00, and 0.33 ferric and nitrates with sodium 0.10 were prepared by a simultaneous reduction of cupric and ferric nitrates with sodium borohydride (NaBH4) in ethylene glycol (EG) under Ar atmosphere by Xiao et al. [47,48]. borohydride (NaBH4) in ethylene glycol (EG) under Ar atmosphere by Xiao et al. [47,48]. TheThe nanoparticles nanoparticles were found were to found be spherical to be with spherical diameter with ranging diameter from 15 to ranging 20 nm and from 15 to 20 nm and containingcontaining FeCu FeCu4 and4 CuFe and2O CuFe4 phases.2O4 phases.

FigureFigure 6. XRD6. XRD results results of (a) nZVI of (a) (nano-scale nZVI ( zeronano valent-scale iron zero particles), valent (b) Feiron0.9Cu particles0.1, (c) Fe0.5),Cu b)0.5 Fe, 0.9Cu0.1, c) Fe0.5Cu0.5, andand (d) (d) Cu Cu NPs. NPs. Symbols: Symbols: (•) nZVI, (● (N)) nZVI Cu0,(, )(▲ cuprite,) Cu (0H, )(■ tenorite,) cuprite, and ((J▼) magnetite.) tenorite, Magnetic and (◄) magnetite. Mag- hysteresis of the nZVI, Fe Cu , Fe Cu , and Cu particles at room temperature. Reprinted netic hysteresis of the0.9 nZVI,0.1 Fe0.5 0.9Cu0.5 0.1, Fe0.5Cu0.5, and Cu particles at room temperature. Reprinted from [49]. Copyright 2018, with permission from Elsevier. from [49]. Copyright 2018, with permission from Elsevier. The same procedure was used by Sepúlveda et al. [49] for the synthesis of CuFe nanoparticlesThe same with differentprocedure amounts was of used Cu, 0.1, by and Sepúlveda 0.5 in the samples. et al. Structural,[49] for the crys- synthesis of CuFe na- tallographic, magnetic properties were studied (Figure6). A low amount of Cu in the noparticles with different amounts of Cu, 0.1, and 0.5 in the samples. Structural, crystal- particles (Fe0.9Cu0.1) favors the core@shell structure while higher amount of Cu (Fe0.5Cu0.5) preferslographic, the segregated magnetic (Janus) properties structure. The were authors studied found CuFe(Figure alloy 6). phase A accompaniedlow amount of Cu in the parti- by Fe3O4 phase in the case of Fe0.9Cu0.1, and CuO2 phase in the case of Fe0.5Cu0.5. The magnetic properties are associated with composition changes and with the increasing the Cu concertation, the coercivity decreases, going from 626 Oe for Fe0.9Cu0.1 to 53 Oe for Fe0.5Cu0.5. Synthesis of CuFe alloy NPs fully covered by graphene layers via high temperature pyrolysis is possible [50]. Using a copper phthalocyanine based precursor and an iron acetylacetonate led to CuFe nanoalloy in which most of the Fe atoms were surrounded by Cu atoms (Figure7). The main phase of the particles obtained is the CuFe alloy, but a small amount of iron carbide (Fe3C) phase is also observed. CuFe nanoparticles have also been produced by synthesizing the nZVI using the . reduction of ferric chloride (FeCl3 6H2O) by NaBH4 followed by planting Cu onto the surface of nZVI by redox reaction occurring between the Cu2+ and nZVI [51–54]. Synthesis of bimetallic FeCu nanoparticles with different Cu to Fe ratio of 1:20, 1:10, 1:6.7, and 1:5 (w/w) and average size of 45 nm, 59 nm, 72 nm, and 85 nm, respectively, was reported [52]. Cao et al. [53] proposed method for the synthesis of Fe@Cu core@shell structures with uniform shape and an average diameter of less than 20 nm using monodispersed carboxymethyl cellulose (CMC) as stabilizer (Figure8). At this point, it should be emphasized that the properties, and in particular the magnetic properties of the synthesized materials, depend not only on the size, shape, structure, but most of all on the composition of the synthesized materials. The presence of iron oxides in nanoparticles inﬂuences its magnetic properties and can change saturation Appl. Sci. 2021, 11, 1978 8 of 15

cles (Fe0.9Cu0.1) favors the core@shell structure while higher amount of Cu (Fe0.5Cu0.5) pre- fers the segregated (Janus) structure. The authors found CuFe alloy phase accompanied by Fe3O4 phase in the case of Fe0.9Cu0.1, and CuO2 phase in the case of Fe0.5Cu0.5. The magnetic properties are associated with composition changes and with the increasing the Cu concertation, the coercivity decreases, going from 626 Oe for Fe0.9Cu0.1 to 53 Oe for Fe0.5Cu0.5. Synthesis of CuFe alloy NPs fully covered by graphene layers via high temperature Appl. Sci. 2021, 11, 1978 pyrolysis is possible [50]. Using a copper phthalocyanine based precursor8 of and 15 an iron acetylacetonate led to CuFe nanoalloy in which most of the Fe atoms were surrounded by Cu atoms (Figure 7). The main phase of the particles obtained is the CuFe alloy, but a magnetization, coercivity, blocking temperature, and relaxation time, and thus may limit small amount of iron carbide (Fe3C) phase is also observed. their use [55].

Figure 7. Structural and elemental analysis of the CuFe alloy. (a) High-angle annular dark ﬁeld/STEM Appl. Sci. 2021, 11, 1978 Figure 7. Structural and elemental analysis of the CuFe alloy. (a) High-angle annular dark 9 of 15 image of CuFe alloy nanoparticles and EDS elemental mapping of (b) Cu and (d) Fe in the same area. field/STEM image of CuFe alloy nanoparticles and EDS elemental mapping of (b) Cu and (d) Fe in (c) EDS intensity proﬁles of yellow line in (a). Reproduced with permission from ref. [50]. Copyright the same2015 area. American (c) EDS Chemical intensity Society. profiles of yellow line in (a). Reproduced with permission from ref. [50]. Copyright 2015 American Chemical Society.

CuFe nanoparticles have also been produced by synthesizing the nZVI using the reduction of ferric chloride (FeCl3.6H2O) by NaBH4 followed by planting Cu onto the surface of nZVI by redox reaction occurring between the Cu2+ and nZVI [51–54]. Synthesis of bimetallic FeCu nanoparticles with different Cu to Fe ratio of 1:20, 1:10, 1:6.7, and 1:5 (w/w) and average size of 45 nm, 59 nm, 72 nm, and 85 nm, respectively, was reported [52]. Cao et al. [53] proposed method for the synthesis of Fe@Cu core@shell structures with uniform shape and an average diameter of less than 20 nm using monodispersed carboxymethyl cellulose (CMC) as stabilizer (Figure 8).

FigureFigure 8. 8. (a(a)) Transmission electron electron microscopy microscopy (TEM) images (TEM) of as-prepared images of carboxymethyl as-prepared cellulose carboxymethyl cellulose(CMC)-stabilized (CMC)-stabilized Fe@Cu Fe@Cu nanoparticles, nanoparticles, inset is TEM inset image is of TEM an individual image of nanoparticle. an individual (b) Size nanoparticle. (b) distribution of CMC-stabilized Fe@Cu nanoparticles. (c) The formation process of CMC-stabilized Size distribution of CMC-stabilized Fe@Cu nanoparticles. (c) The formation process of CMC-stabi- Fe@Cu nanoparticles. Reprinted from [53], Copyright 2011, with permission from Elsevier. lized Fe@Cu nanoparticles. Reprinted from [53], Copyright 2011, with permission from Elsevier.

At this point, it should be emphasized that the properties, and in particular the magnetic properties of the synthesized materials, depend not only on the size, shape, structure, but most of all on the composition of the synthesized materials. The presence of iron oxides in nanoparticles influences its magnetic properties and can change saturation magnetization, coercivity, blocking temperature, and relaxation time, and thus may limit their use [55].

3. Applications As it was shown, the structure depends mainly on the concentration of Cu and Fe in the particles, and mechanical, catalytical, and magnetic properties depend on the structure and composition, and also on the additions such as carbon, graphite, or silicon used for supporting the created nanoparticles. And these properties determinate the applications of bimetallic CuFe nanoparticles. The literature points to applications of CuFe systems in the areas of catalysis and electrochemistry [56–59], the FeCu nanoparticles are also used as electrocatalysts for energy storage batteries and as photocatalysts and adsorbents for wastewater or water treatment [43,44,47–54,60,61].

3.1. Oxygen Reduction Reaction (ORR) Energy storage and conversion are very important for the functioning of modern societies. Energy sources such as batteries and fuel cells are becoming the backbone of a conscious low-carbon economy. Vital for energy conversion, especially in the areas of fuel cells and metal-air batteries, is the oxygen reduction reaction (ORR) [62]. During the ORR process, the O2 molecules are reduced by electrons. They are very difficult to break electrochemically, because the bond O = O has an exceptionally strong bond energy of 498 kJ/mol. The energy barrier can be lowered by electrocatalysts that can activate and cleave bonds. ORR in an aqueous electrolyte occurs mainly by two pathways, one is the direct 4- electron reduction pathway from O2 to H2O, and the other is a 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O2 to superoxide (O2−) can also occur [62,63]. Since the introduction of the first fuel cell in 1842, platinum has been the most common catalytic material due to its properties, such as chemical inertness and properties in Appl. Sci. 2021, 11, 1978 9 of 15

3.1. Oxygen Reduction Reaction (ORR) Energy storage and conversion are very important for the functioning of modern societies. Energy sources such as batteries and fuel cells are becoming the backbone of a conscious low-carbon economy. Vital for energy conversion, especially in the areas of fuel cells and metal-air batteries, is the oxygen reduction reaction (ORR) [62]. During the ORR process, the O2 molecules are reduced by electrons. They are very difficult to break electrochemically, because the bond O = O has an exceptionally strong bond energy of 498 kJ/mol. The energy barrier can be lowered by electrocatalysts that can activate and cleave bonds. ORR in an aqueous electrolyte occurs mainly by two pathways, one is the direct 4-electron reduction pathway from O2 to H2O, and the other is a 2-electron reduction pathway from O2 to hydrogen peroxide (H2O2). In non-aqueous aprotic solvents and/or 2− in alkaline solutions, the 1-electron reduction pathway from O2 to superoxide (O ) can also occur [62,63]. Since the introduction of the first fuel cell in 1842, platinum has been the most common catalytic material due to its properties, such as chemical inertness and properties in the field of gas adsorption and dissociation. At the same time, due to platinum’s rarity and costliness, alternative catalytic materials are sought after. Nam et al. [50] show that FeCu bimetallic nanoparticles are very active and stable electrocatalysts for energy storage batteries. ORR activity for the CuFe alloy with graphitic carbon shells was comparable to that of Pt/C electrocatalysts, CuFe showed kinetic current density much higher than that for single Cu and Fe. The reduced “blocking effect” of active sites in the CuFe alloy may reduce the affinity of the hydroxyl groups controlling O2 adsorption for the alloy compared to single Cu or Fe catalysts. Direct pathway of oxygen reduction was deduced from the number of transferred electrons calculation (Figure9 ). To demonstrate the practical application of the nanoparticles, scientists created a zinc-air battery in which the air cathode was prepared from a mixture of active carbon CuFe@graphite, and synthetic fluoropolymer binder and the anode was prepared from Zn film. They showed that the CuFe catalyzed Zn-air cell can be used in cars, and that the battery can be recharged. They tested that an air electrode made of CuFe alloy worked for over 100 h without voltage loss, which shows the high stability of the CuFe alloy catalyst.

3.2. Catalysis It is known that the presence of toxic metals in water poses a serious threat to living organisms. Examples of the most dangerous toxic metal ions found in wastewater are Hg2+, Cr6+, As3+, Cd2+, Pb2+, Mn2+, Ni2+ [64,65]. They cause renal failure, lung impairment, lung cancer, dermatitis, dyspnea, and damage to the reproductive system, liver, and central nervous system [65,66]. In addition, the excess of the above-mentioned metals adversely affects agriculture and the environment [65,66]. CuFe nanoparticles are also examined as materials for the elimination of heavy metals—such as Cr or As—and metalloids from metallic ores in which Cu prevents the oxidation processes of corrosive metals [42,49]. Feng et al. [42] reported that for the efﬁciency of As3+ removal from smelting wastewater (wastewater produced from smelting, the extraction of metal from its ore by a process involving heating and melting), the copper content in Cu doped Fe@Fe2O3 nanoparticles is crucial and must not exceed 20%. Additionally, it was investigated how pH, temperature Appl. Sci. 2021, 11, 1978 10 of 15

the field of gas adsorption and dissociation. At the same time, due to platinum’s rarity and costliness, alternative catalytic materials are sought after. Nam et al. [50] show that FeCu bimetallic nanoparticles are very active and stable electrocatalysts for energy storage batteries. ORR activity for the CuFe alloy with graphitic carbon shells was comparable to that of Pt/C electrocatalysts, CuFe showed kinetic current density much higher than that for single Cu and Fe. The reduced “blocking effect” of ac- Appl. Sci. 2021, 11, 1978 10 of 15 tive sites in the CuFe alloy may reduce the affinity of the hydroxyl groups controlling O2 adsorption for the alloy compared to single Cu or Fe catalysts. Direct pathway of oxygen reduction was deduced from the number of transferred electrons calculation (Figure 9). 3+ 3+ To demonstrateand the initial concentration the practical of application As affect the of efﬁciency the nanoparticles, and rate of As scientistsremoval. created It has a zinc-air been shown that the maximum removal capacity of As3+ was achieved at a broad pH range battery in which the air cathode was prepared from a mixture of 3+ active carbon from 3 to 9, suggesting that Cu doped Fe@Fe2O3 particles have the potential to treat As in CuFe@graphite,most surface and and groundwater synthetic under fluoropolymer natural conditions. binder The and removal the capacityanode was increased prepared from Zn withfilm. the They temperature showed increase that the from CuFe 298 Kcatalyzed to 308 K, which Zn-air can cell be explained can be used by the in mobility cars, and that the batteryof the can ions be as recharged. the reaction They temperature tested increased.that an air The electrode adsorption made capacity of CuFe of obtained alloy worked for particles increased with an increase in the initial As3+ concentration, but with an increase overin 100 the initialh without As3+ concentration, voltage loss, the which removal shows rate was the almost high halved.stability of the CuFe alloy catalyst.

Appl. Sci. 2021, 11, 1978 11 of 15

broad pH range from 3 to 9, suggesting that Cu doped Fe@Fe2O3 particles have the potential to treat As3+ in most surface and groundwater under natural conditions. The removal capacity increased with the temperature increase from 298 K to 308 K, which can be ex- Figure 9. Electrocatalytic activity of the CuFe alloy. (a) Linear sweep voltammograms of CuFe, Cu, Figureplained 9. Electrocatalytic by the mobility activity of the of ions the CuFeas the alloy. reaction (a) Lineartemperature sweep increased.voltammograms The adsorption of CuFe, Cu, Fe, and Pt/C measured by a RRDE system. (b) Tafel plots of each catalyst. (c) Kinetic current at Fe, andcapacity Pt/C measuredof obtained by particles a RRDE system.increased (b )with Tafel an plots increase of each in thecatalyst. initial (c As) Kinetic3+ concentration, current at 0.9 0.9 V of each catalyst (except Fe in (b,c) due to low on-set potential of Fe). (d) Calculated number of but with an increase in the initial As3+ concentration, the removal rate was almost halved. V oftransferred each catalyst electrons (except of CuFe Fe andin ( Pt/C.b,c) due Reproduced to low withon-set permission potential from of ref.Fe). [50 (d].) Copyright Calculated 2015 number of transferredAmericanSepúlveda electrons Chemical et Society.of al. CuFe [49] and investigated Pt/C. Reproduced how the morphologywith permission of the from nanoparticles ref. [50]. Copyright and cop- 2015per American content Chemical influenced Society. the capacity, speed, and intensity of adsorption in As5+ removal. TheSepy foundúlveda that et al. the [49 ]low investigated content howof Cu the (Fe morphology0.9Cu0.1) favored of the nanoparticles the core@shell and copper structure and 5+ content inﬂuenced the capacity,5+ speed, and intensity of adsorption in As removal. They 3.2. increasedCatalysis the removal of As compared to Fe0.5Cu0.5 favoring the segregated (Janus) struc- foundture. thatThey the proposed low content the of Cumechanism (Fe0.9Cu0.1 of) favored As5+ removal the core@shell with structurethe CuFe and bimetal, increased indicating theIt removalis known of As that5+ compared the presence to Fe Cuof toxicfavoring metals the segregatedin water poses (Janus) a structure. serious They threat to living that Cu is responsible for the initial0.5 reduction0.5 of As5+ to As3+, and the removal of impurities proposed the mechanism of As5+ removal with the CuFe bimetal, indicating that Cu is organisms. Examples of the most dangerous toxic0 metal ions found in wastewater are is favored by the transfer of electrons5+ from3+ the Fe (nZVI) layer to the Cu oxide layer (Fig- Hg2+responsible, Cr6+, As for3+, theCd initial2+, Pb reduction2+, Mn2+, of Ni As2+ [to64 As,65]., andThey the removalcause renal of impurities failure, is favoredlung impairment, byure the 10). transfer of electrons from the Fe0(nZVI) layer to the Cu oxide layer (Figure 10). lung cancer, dermatitis, dyspnea, and damage to the reproductive system, liver, and central nervous system [65,66]. In addition, the excess of the above-mentioned metals adversely affects agriculture and the environment [65,66]. CuFe nanoparticles are also examined as materials for the elimination of heavy metals—such as Cr or As—and metalloids from metallic ores in which Cu prevents the oxidation processes of corrosive metals [42,49]. Feng et al. [42] reported that for the efficiency of As3+ removal from smelting wastewater (wastewater produced from smelting, the extraction of metal from its ore by

5+ a processFigureFigure 10. 10involvingProposed. Proposed mechanism heating mechanism ofand As of5+ melAsremoval tingremoval using), the using bimetallic copper bimetallic CuFe content materialsCuFe inmaterials asCu the doped adsorbent. as the [email protected] na- noparticlesReprintedReprinted withis with crucial permission permission and from must from [49]. not[ Copyright49]. exceed Copyright 2018, 20%. with2018, Additionally, permission with permission from Elsevier.it fromwas Elsevier.investigated how pH, temperature and the initial concentration of As3+ affect the efficiency and rate of As3+ re- It should also be noted that the modiﬁcation of NP CuFe surface with another material, moval. ItIt hasshould been also shown be noted that that the the maximum modification removal of NP capacityCuFe surface of As with3+ was another achieved mate- at a suchrial, assuc chitosanh as chitosan (CS), improved(CS), improved their adsorption their adsorption capacity capacity [43]. The [43 authors]. The showauthors that show that CS-FeCu has the highest Cr6+ removal efficiency in comparison with nZVI, FeCu, and CS- Fe. They reported that the highest stabilization and the reactivity of nZVI, because of the effect of surface coating, were obtained for 2 wt% of chitosan and 3 wt% of Cu in composites. Additionally, the maximum absorption capacities were achieved with pH in the range of 3–9, and CS-FeCu nanoparticles are active even when the initial concentrations of Cr6+ are high. To remove Cr6+ from water, the composite of GT and nZVICu was also tested. The initial pH, Cr6+ concentration, and reaction temperature have been shown to be important factors influencing the efficiency of Cr6+ removal (Figure 11). Cr6+ removal efficiency increases with a decrease in the initial pH and an increase in temperature [44].

As can be seen, to maximize the adsorption capacity of CuFe particles, particular attention should be paid to the choice of material for the formation of a heterojunction with CuFe nanoparticles or to modifying its surface, as well as the structure and composition Appl. Sci. 2021, 11, 1978 11 of 15

Figure 10. Proposed mechanism of As5+ removal using bimetallic CuFe materials as the adsorbent. Reprinted with permission from [49]. Copyright 2018, with permission from Elsevier. Appl. Sci. 2021, 11, 1978 11 of 15 It should also be noted that the modification of NP CuFe surface with another material, such as chitosan (CS), improved their adsorption capacity [43]. The authors show that CS-FeCu has the highest Cr6+ removal efficiency in comparison with nZVI, FeCu, and CS- CS-FeCu has the highest Cr6+ removal efficiency in comparison with nZVI, FeCu, and Fe.CS-Fe. They They reported reported that the that highest the highest stabilization stabilization and the and reactivity the reactivity of nZVI of, nZVI,because because of the effectof the of effect surface of surface coating, coating, were obtained were obtained for 2 wt% for of 2 wt%chitosan of chitosan and 3 wt% and of 3 Cu wt% in ofcompo- Cu in sites.composites. Additionally, Additionally, the maximum the maximum absorption absorption capacities capacities were were achieved achieved with with pH inpH the in rangethe range of 3 of–9, 3–9, and and CS CS-FeCu-FeCu nanoparticles nanoparticles are are active active even even when when the the initial initial concent concentrationsrations 6+ of Cr 6+ areare high. high. 6+ To remove Cr6+ fromfrom water, water, the composite of GT and nZVICunZVICu was alsoalso tested.tested. The initialinitial pH, Cr 6+ concentration,concentration, and and reaction reaction temperature temperature have have been shown to be important factors influencing influencing the the efficiency efficiency of of Cr Cr6+6+ removalremoval (Figure (Figure 11). 11 ).Cr Cr6+ removal6+ removal efficiency efficiency in- creasesincreases with with a decrease a decrease in inthe the initial initial pH pH and and an an increase increase in intemperature temperature [44 [44]. ].

Figure 11. The removal of Cr6+ byby GT GT-- nZVI nZVI and and GT GT-- nZVI nZVICuCu ( (aa)),, e effectffect of of pH pH ( b)),, and concentration ( c) on the removal of Cr6+. .R Reprintedeprinted from from [ [4444],], Copyright Copyright 2017, 2017, with with permission permission from from Elsevier. Elsevier.

As can can be be seen, seen, to to maximize maximize the the adsorption adsorption capacity capacity of ofCuFe CuFe particles, particles, particular particular at- tentionattention should should be be paid paid to to the the choice choice of of material material for for the the formation formation of of a a heterojunction heterojunction with CuFe nanopart nanoparticlesicles or to modifying its surface, as well as the structure and composition of created nanoparticles and nanocomposites. It would also be worth supplementing the research with the possibility of using CuFe to absorb other toxic metals, such as Pb2+ and Hg2+ or Cd2+. Bimetallic FeCu nanoparticles can be used to eliminate high concentrations of NO3−- N[51]. These pollutants are produced by agricultural runoff, landfill leachate, leaky septic tanks, municipal storm sewage, animal feed, and industrial waste [67,68] and pose a threat to human health [69]. The reduction of NO3−-N is the most effective while Cu content surrounding nZVI particles is 2.5% (w/w)[51]. The textile industry uses dyes that often pollute the water. Dyes are known to be mutagenic, carcinogenic, and toxic to humans and aquatic animals, and therefore adsorbents are needed to remove them [70–72]. CuFe and C/CuFe were used as adsorbents to remove indigo blue. Studies have shown that C/CuFe is more effective in removing indigo blue from aqueous solutions than CuFe [41]. CMC-stabilized Fe-Cu bimetal nanoparticles could effectively dechlorinate 1,2,4-trichlorobenzene (1,2,4-TCB) [53]. Cellulose supported Fe-Cu nanoparticles were also applied for reduction of nitroarenes to arylamines [54]. To increase the catalytic activity of the catalysts, porous materials such as mesoporous silica, activated carbon, mesoporous carbon and carbon nanotubes are also used. Compounds such as phenol, benzoic acid, bisphenol A, 2,4,6-trichlorophenol, imidacloprid, ketoprofen, methylene blue, and methyl orange have been effectively removed from water using CuFe nanoparticles dispersed on mesoporous carbon (CuFe-MC) [60]. The high specific surface area of mesoporous materials increases the absorption of pollutants. Wang et al. [60] indicated the features of a good catalyst: • the large surface area, and especially a mesoporous structure, favor the rapid diffusion of reagents and products. • large dispersion of bimetallic iron-copper nanoparticles in the mesoporous carbon matrix significantly increases the number of active centers, which increases absorption and discharge. • the synergistic effect of iron and copper favored the redox cycles Fe3+/Fe2+ and Appl. Sci. 2021, 11, 1978 12 of 15

• Cu2+/Cu+, increasing the catalytic activity. • the presence of mesoporous carbon which can also activate H2O2 to produce •OH. • An additional beneﬁt of using magnetic catalysts is that they can be easily separated by a magnet, which facilitates the removal of contaminants [60].

4. Conclusions As described in this review, the combination of copper and iron—two well-known and common materials—can provide multi-functional materials for numerous applications. In bulk, copper and iron are slightly soluble in each other, additionally they present a strong tendency to oxidation under atmospheric conditions; therefore, we paid special attention to the synthesis methods of bimetallic CuFe nanoparticles and bimetallic CuFe NPs supported on different materials and the structure of the obtained materials. By mechanical milling, reduction, or two-step reduction, CuFe nanoparticles with alloy, Janus, and core@shell structures were created. Often they were stabilized by carbon, silicon, or cellulose. Structure and properties depend on the size and composition. Despite the large number of experimental methodologies available, the challenge remains to evolve a simple, efficient, and reliable process for the synthesis of pure CuFe nanoparticles with a defined structure and without any oxide-like impurities. For the challenge of nanoparticle oxidation two techniques seem promising: a combination of a chemical reduction of ferric nitrate and copper nitrate in alcohol solution with annealing carbonization technique, and a high temperature pyrolysis of the copper precursor (chloro- phyllin) and the iron precursor (Fe(II) acetylacetonate). While the reduction of copper and iron sulfides with sodium borohydride, and reduction of cupric and ferric nitrates with sodium borohydride in ethylene glycol under Ar seem to be less effective. CuFe nanoparticles have a great application potential in energy storage, fuel cells, and pollution treatment. CuFe nanoparticles have a great potential for applications in energy storage, fuel cells, and water purification. Therefore, it is important to take advantage of their magnetic and catalytic properties, low cost, and abundance and move them from the laboratory to large-scale applications.

Funding: This research received no external funding. Data Availability Statement: Not applicable for a review paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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