Formation of Copper Nickel Bimetallic Nanoalloy Film Using Precursor Inks

Precursor (Metal-organic decomposition (MOD)) inks are used to fabricate 2D and 3D printed conductive structures directly onto a substrate. By formulating a nanoalloy structure containing multiple metals, the opportunity to modify chemical and physical properties exists. In this paper, a copper-nickel bimetallic nanoalloy film was fabricated by mixing copper and nickel precursor inks and sintering them in vacuum. The individual elemental inks were formulated and characterized using SEM, EDS, and XRD. During thermal processing, elemental copper forms first and is followed by the formation of bimetallic copper-nickel alloy. The encapsulation of the underlying copper by the nickel-rich alloy provides excellent oxidation resistance. No change in film resistance was observed after the film was exposed to an oxygen plasma. Nanoalloy films printed using reactive metallic inks have a variety of important applications involving local control of alloy composition. Examples include facile formation of layered nanostructures, and electrical conductivity with oxidative stability.


Introduction
Functional printing has evolved from fabricating parts using a single material, to make multifunctional components using two or more materials. There is tremendous interest in functional printing, where different printing techniques [1] are used to make devices such as antennas [2] [3] [4] [5], electrical circuit components [6] [7] [8] [9] [10], and sensors [11] [12] [13] [14]. Metallic inks are often used to fabricate 2D and 3D structures to conduct heat and electricity. Numerous examples of printing of copper [15]- [20] and silver [21]- [26] inks on solid and flexible substrates have been demonstrated in the literature. These inks are either metal nanoparticle or precursor (metal-organic decomposition (MOD) inks. The opportunity to tune the physical and chemical properties of a material could be achieved by mixing several metal inks together to create an alloy. A nanoalloy is a solid state solution containing atoms or molecules of two or more metals [27]. Elements which are immiscible in the bulk form or have a large miscibility gap can be mixed at the nanoscale to produce properties that differ from those of the individual bulk materials [28] [29]. The physical and chemical properties of nanoalloys can be enhanced by varying their structure, composition and particle size to provide properties having applications in biomedical devices, electronics, engineering, and catalysis [30] [31] [32] [33]. It is generally easier to control the structure of nanoalloys using bottom-up synthesis where single atoms or molecules are assembled to form a nanostructure [34]. When using precursor inks, different structures are possible depending on the decomposition profiles, solid loading, and reactivity of individual inks. If the decomposition profiles are identical, then a well-mixed structure is possible. If the decomposition profiles are different, it is possible to synthesize a variety of different structures, for example core-shell structures [35] [36] [37].
In recent years, copper nickel nanoalloys have gained considerable interest in the research community due to the conductivity of copper and the corrosion resistance of nickel. For this reason, they are potential candidates as electrodes in corrosive environments. CuNi alloys are used in marine applications [38], solid oxide fuel cells [39], glucose sensors [40], hydrogenation of refined soybean oil [32], photodegradation of organic dyes [41], and in hyperthermia therapy [42]. Different methods have been used to fabricate CuNi nanoalloy particles. Chemical methods include electrochemistry [40], hydrothermal reduction [41] [43], microemulsions [44] [45], mechanical alloying [30] [46], reduction of polyols [30] [47] or salts [48] [49], and solution combustion [50] [51]. Synthesizing such nanoalloy particles is often accompanied by undesirable contamination in the final composition. For example, impurities from the metal salts may not be completely removed by repeated washing and drying of the nanoparticles. Oxidation is also a problem, requiring special care during washing, filtering and drying of the nanoalloy particles. Printing a precursor nanoalloy ink and directly sintering in situ on the substrate will eliminate some of the problems encountered in the synthesis and use of premade nanoparticles.
The printing of functional devices using nanoalloys is still at a relatively early stage and has not been widely explored. Inks containing nanoalloy particles can be printed using a wide variety of printing techniques to create customized parts

Characterization
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) To study the effect of oxygen exposure on resistance, the sintered film was exposed to an O 2 plasma using a SurFxAtomflo TM 400 atmospheric plasma system. This system generates plasma using a helium-oxygen gas mixture to create an oxidizing environment. The plasma head was mounted on a linear stage to move in X and Y direction. The Z height between the sample and the head was kept constant at 10 mm. For each pass, the power of the plasma was set to 100 W. Helium and oxygen gas flow rates of 15.00 l/min and 0.30 l/min were used.

Inkjet Printing
The inks were printed onto a substrate using a Fuji Dimatix inkjet printer (DMP

Copper and Nickel Ink Characterization
The copper and nickel precursor (MOD) inks contain the metal formate salts complexed by ethylenediamine. Formate ions were used because of their low molecular weight and volatility [53]. Furthermore, the decomposition of formate is accompanied by the release of carbon dioxide and molecular hydrogen, (Equation (1) & Equation (2)), the latter of which generates a reducing atmosphere, thus limiting the amount of oxidation of the reduced metals [36].      nucleation and crystal growth [54]. Small particles can coalesce into larger particles. XRD analysis confirmed the presence of elemental copper and nickel ( Figure 4). Kα1 peak positions were obtained from Pearson type VII peak decomposition [55]. Precise lattice parameter was determined from a plot of lattice parameter versus cos 2 θ/sinθ [56]. The experimental lattice parameters for copper and nickel were found to be 3.6145 Å and 3.5242 Å respectively. These values are in good agreement with the lattice parameter for pure copper (3.614 Å, PDF# 00-004-0836) and pure nickel (3.524 Å, PDF# 00-004-0850). Figure 5 shows the differential thermal analysis for copper and nickel inks heated up to 300˚C at 10˚C/min. The reduction of copper and nickel to their elemental states occurs in four stages. The first stage is same for both the inks and includes evaporation of ethylene glycol and water as indicated by the endothermic region below 140˚C. The second stage includes reduction of the metal complex as indicated by the exothermic range up to 155˚C for copper and around 200˚C for nickel. The exothermic ranges for the metals match the TA-MS   ( Figure 2). Nucleation takes place in the third stage. This is indicated by endothermic range from ~160˚C -180˚C for Cu, and ~195˚C -230˚C for Ni. Nucleation can be affected by impurities present in the ink and/or substrate. Impurities may inhibit or accelerate the rate of the reactions. The fourth stage is the crystal growth indicated by exothermic ranges > ~180˚C for Cu and ~230˚C for Ni.  reaction result in copper-nickel nanoalloy particles. These nanoalloy particles subsequently sinter into a continuous film.

Alloy Characterization
In order to observe the endothermic and exothermic events for the formation of bimetallic film, DTA was carried out for Cu, Ni and CuNi MOD inks at 2˚C/min ( Figure 7). As shown above for 10˚C/min, the reduction of the metal complex to elemental metal was observed in four stages (S1 -S4). The first stage (S1, solvent evaporation) was the same for all inks. The copper complex reduces at a lower temperature than nickel due to its lower reduction potential. For the CuNi alloy ink, copper reduces first and provides nucleation sites for nickel which reduces later when the temperature is increased. The endothermic peak for the nickel reduction takes place at a lower temperature suggesting that the presence of copper nanoparticles may catalyze the reaction. DTA data suggests the presence of two phases which are copper and copper nickel alloy. The resulting alloy films have a layered structure with copper reducing first at the substrate and a non-uniform distribution of Cu and Ni atoms (CuNi) alloy on top of the reduced copper.
The EDS elemental mapping (Figure 8(A)) shows no segregation of copper and nickel (within the measurement resolution). The x-ray diffraction pattern in Figure 8(C) shows peaks for both a copper phase and a copper-nickel alloy phase. This corroborates the results obtained from the DTA where copper reduced first and the copper-nickel alloy deposited on the reduced copper. The lattice parameter obtained for the copper peak was 3.6143 Å, and from the copper-nickel alloy peak was 3.5318 Å respectively. Using Vegard's law, the composition of the alloy phase was estimated to be Cu 8 Ni 92 .   same area shown in Figure 9(A). The d spacing from electron diffraction data was used to calculate the lattice constant and then was averaged over all four lattice constants. The lattice constants obtained from the TEM analysis were not consistent with either a pure copper or pure nickel phase, indicating an alloy formation. Other unresolved ring patterns may be due to impurities, amorphous materials, or formation of small amounts of metal oxides (NiO, CuO or NiCuO).  To study the effect of oxygen exposure on resistance, the sample was printed with copper, nickel and alloy MOD ink on a glass substrate using an inkjet printing technique. The resistance was measured across the sintered sample after every ten passes of the oxygen plasma. Figure 10 shows the change in relative resistance of the printed film with exposure. As expected, the relative resistance of the sintered copper increased due to oxidization as the number of passes were

Conclusion
A new method to easily form copper-nickel nanoalloys directly onto a substrate has been shown. The copper and nickel precursor inks were formulated using metal formate salts, ethylenediamine, and ethylene glycol. The reduction process of copper, nickel and combined solutions was studied in detail. XRD analysis of the sintered alloy shows the presence of two phases, copper and bimetallic cop-