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 combi- nation, 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, cataly- sis, 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 effec- tively 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 orare 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