A mild aqueous synthesis of ligand-free copper nanoparticles for low temperature sintering nanopastes with nickel salt assistance

An organic ligand-free aqueous-phase synthesis of copper (Cu) nanoparticles (NPs) under an air atmosphere was successfully achieved by reducing copper(II) oxide particles with a leaf-like shape in the presence of Ni salts at room temperature. The resulting Cu NPs with a mean particle diameter of ca. 150 nm exhibited low-temperature sintering properties due to their polycrystalline internal structure and ligand-free surface. These Cu NPs were applied to obtain Cu NP-based nanopastes with low-temperature sintering properties, and the resistivities of the obtained Cu electrodes after annealing at 150 °C and 200 °C for 30 min were 64 μΩ∙cm and 27 μΩ∙cm, respectively. The bonding strength between oxygen-free Cu plates prepared using the Cu NP-based nanopastes reached 32 MPa after pressure-less sintering at 260 °C for 30 min under a nitrogen atmosphere. The developed manufacturing processes using the developed Cu nanopastes could provide sustainable and low-CO2-emission approaches to obtain Cu electrodes on flexible films and high-strength bonding between metal plates as die-attach materials for power devices under energy- and resource-saving conditions.

Printed electronics (PE) technology has attracted a great deal of attention as one of the most promising manufacturing techniques for items such as electronic, mobile, and wearable future devices under low energy consumption and low carbon dioxide emission conditions; this technology is appealing because the electronic circuits and semiconductor layers in the devices are directly fabricated on the device substrates through a successive printing process under atmospheric conditions [1][2][3] . In PE technology, nano-and micron-sized metal particle inks and pastes are widely used to prepare conductive patterns on electronic circuit boards [4][5][6][7][8] . In particular, silver particle-based inks and pastes are the most representative and useful due to their advantageous characteristics, such as low resistivity, low-temperature sinterability, and high antioxidation ability 9 . Recently, these advantages have been applied in the combination of PE and integrated circuit (IC) production technologies, namely, flexible hybrid electronics (FHE) technology [10][11][12] . This technology enables us to fabricate flexible devices on plastic films with low thermal resistivity through an on-demand and eco-friendly process. To date, various types of future devices, such as wearable devices, have been developed and demonstrated based on market needs. Silver particle-based pastes have also recently received considerable attention as die-attach materials for the fabrication of next-generation SiC-and GaN-based power devices 13,14 . The larger bandgaps of SiC-and GaN-based power semiconductor materials than the corresponding Si-based materials enable device operation above 200 °C, which could permit miniaturization of the power modules. Furthermore, the high energy efficiency of hightemperature-driven power semiconductors is also quite suitable for the electrification of vehicles and is desirable for a future carbon-neutral society. As conventional die-attach materials, solder alloys have been widely used for Si-based device manufacturing. However, the low melting point and low thermal conductivity of solder alloys 15 are becoming potential problems for die-attach materials from a viewpoint of application to high-temperaturedriven SiC-and GaN-based power semiconductors 16 . To overcome these problems, low-temperature-sintered silver particle-based pastes with high thermal stability and high thermal conductivity have attracted much attention as promising candidates for die-attach materials applicable to SiC-and GaN-based device manufacturing.

Results and discussion
Preparation of CuO particles with a leaf-like shape for Cu NP synthesis. In liquid-phase synthetic systems of inorganic NPs based on nucleation followed by particle growth, the particle size can be controlled by complexation of the precursors with organic ligands because the initial nucleation number determining the final mean particle size obeys the solubility product of the resulting complexes in the system 40 . However, another method should be considered under organic ligand-free systems. In our previous studies, monodispersed copper(I) oxide (Cu 2 O) particles were successfully obtained by using CuO solid particles with a leaf-like morphology as the precursor in the presence of gelatine as an anti-aggregation agent 41 . The leaf-like shape with a wide specific surface area accelerates the dissolution rate of the CuO particles into the solvent; supersaturation and rapid nucleation are reached in a short period, resulting in highly monodispersed Cu 2 O particles. Kobayashi et al. also reported the synthesis of Cu particles using CuO solid particles as a raw material in cetyltrimethylammonium bromide (CTAB) aqueous solution 42 . In this case, a decrease in the mean particle diameter of the resulting Cu NPs was observed due to decreasing the particle size of the CuO precursors. This behaviour is also www.nature.com/scientificreports/ probably due to acceleration of the dissolution rate of the CuO precursors into the solvent by the size-dependent increase in the specific surface area of the CuO precursors. These previous studies suggested that utilization of solid precursors is a promising technique for ligand-free liquid-phase syntheses of NPs because controlling the dissolution rate of the solid precursors by controlling their shape and size leads to precise size control of purpose-designed NPs. In the present study, we focused on CuO solid particles 42 with a leaf-like shape and therefore a large specific surface area as a raw material for the ligand-free synthesis of Cu NPs. The purpose-designed CuO particles were prepared as follows: initially, an aqueous light blue-coloured suspension of copper(II) hydroxides, which was obtained by mixing 5.0 L of 0.40 M Cu(NO 3 ) 2 and 5.0 L of 0.80 M NaOH aqueous solutions at room temperature, was aged at 40 °C for 8 h under atmospheric conditions. The solid particles thus obtained were filtered, washed with water, and dried at 120 °C prior to use for the present Cu NP synthesis. Figure 1 summarizes the characterization results for the solid particles used as the precursor for the preparation of Cu NPs. From the high-resolution transmission electron microscopy (HR-TEM) images shown in Fig. 1a,b, the particles consist of leaf-like shapes with a rough surface and internal voids. Similar polycrystalline images are also seen in the high-angle annular dark field scanning TEM (HAADF-STEM) images (Fig. 1c,d). The corresponding field emission scanning electron microscopy (FE-SEM) image in Fig. 1e reveals that the leafshaped particles have a major axis of 420 ± 89 nm, a minor axis of 245 ± 66 nm, and a thickness of several tens of nm. Small protrusions with a size of several tens of nm were observed on the surfaces of the particles in the SEM image. From the X-ray diffraction (XRD) pattern shown in Fig. 1f, all the diffraction peaks could be assigned to the formation of a single CuO crystal phase (JCPDS: 041-0254). The average crystallite size of the particles was calculated as 15 nm using Scherrer's equation 43 (Scherrer constant: 1.33), which was in good agreement with the primary particle size on the surface of the CuO particles in Fig. 1f. The results suggested that the CuO particles basically consist of a polycrystalline crystal structure. The inset in Fig. 1b presents a Fourier transform (FT) image of Fig. 1b. Highly oriented diffraction spots attributed to a single crystalline structure are visible in the FT images. These results mean that CuO particles with a polycrystalline structure were obtained through epitaxial growth of nuclei on the surface to produce primary particles with a crystallite size of ca. 15 nm. The specific surface area of the resulting polycrystalline particles was 22 m 2 /g, as determined by N 2 adsorption measurements.
Mild aqueous-phase synthesis of Cu NPs under organic ligand-free conditions by nickel salt assistance. As mentioned above, the addition of anti-aggregation agents such as gelatine 41 and CTAB 42 is a practical way to obtain highly monodispersed NPs. To establish an organic ligand-free liquid-phase synthetic www.nature.com/scientificreports/ method, we applied water-soluble base transition metal salts during particle growth. The representative procedure for the preparation of Cu NPs in the presence of base transition metal salts is as follows: Initially, CuO particles (6.12 mmol, 0.487 g) with a leaf-like shape, NiCl 2 ·6H 2 O (0.61 mmol, 0.148 g), and ion-exchanged water (20 mL) were mixed in a beaker (50 mL) with stirring (250 rpm) at room temperature under an air atmosphere. Then, an aqueous solution of hydrazine monohydrate (4.85 mol/L, 5.0 mL) was added in one portion to the mixture in the beaker with stirring (300 rpm) at room temperature. The resulting mixture was stirred for 2 h under the same conditions. The brown-coloured solids were collected by filtration using a cellulose acetate membrane filter with a pore size of 0.45 µm. The resulting solids were washed with ion-exchanged water until the conductivity of the filtrate became lower than 0.1 mS/cm. Finally, the collected solids were washed three times with denatured alcohol by centrifugation (10,000 G, 10 min) and dried under a reduced atmosphere to obtain Cu NPs. Further details of the synthetic procedure are summarized in the Methods section. [CuO] = 0.10). Both highly crystalline large particles with flat crystal planes (particle size: ca. 1 µm) and small NPs with an irregular shape (mean particle size: 144 ± 59 nm) were obtained when Cu NP synthesis was carried out without the use of Ni salts ( Fig. 2a-i). In contrast, no highly crystalline particles were seen in the solid particles prepared in the presence of NiCl 2 ( Fig. 2a-ii), which are abbreviated as C N . This result suggests that NiCl 2 has a positive effect on preventing the formation of highly crystalline large particles during the synthesis of Cu NPs starting from CuO. The effects of Ni(NO 3 ) 2 and NiSO 4 on Cu NP synthesis were lower than that of NiCl 2 because some highly crystalline large particles were clearly seen in Fig. 2a-iii,iv. The average particle sizes of the NPs, except for the large particles with flat crystal planes, obtained with NiCl 2 , Ni(NO 3 ) 2 , and NiSO 4 were 156 ± 48 nm, 209 ± 45 nm and 173 ± 77 nm, respectively. The particle size was determined by counting more than 200 NPs in the corresponding SEM images. Figure 2b shows XRD patterns of as-prepared particles in the absence or presence of nickel salts. The main diffraction peaks at 43.34°, 50.48°, and 74.14° could be assigned to the formation of a Cu metal phase as the main phase in the solid particles. The crystallite sizes of the Cu NPs obtained in the presence of NiCl 2 , Ni(NO 3 ) 2 , and NiSO 4 were calculated as 28 nm, 31 nm, and 39 nm, respectively 43 . The crystallite size was much smaller than the average particle size, indicating that the Cu NPs have a polycrystalline structure. Broad diffraction peaks due to a Cu 2 O phase in Fig. 2b suggest partial surface oxidation of the Cu NPs during purification. Similar behaviour was also observed in our previous study 30 . As shown in Fig. 2b-iii,iv, the formation of nickel hydrazine solid complexes was observed with the use of Ni(NO 3 ) 2 and NiSO 4 . Precipitation of such solid complexes is the most plausible reason why the effects of Ni(NO 3 ) 2 and NiSO 4 on the prevention of highly crystalline large particles are lower than that of NiCl 2 in the preparation of C N . Next, we also tested the effect of the concentration of NiCl 2 on Cu NP synthesis.     Fig. 3a-i-vi, the evolution of highly crystalline large particles with flat crystal planes, similar to those seen in Fig. 1a-i, was inhibited by the usage of metal chlorides. The addition of metal salts provably prevents uniform crystal growth, allowing the formation of highly crystalline Cu particles in the growth solutions. XRD measurements revealed that all the solid particles had a Cu metal phase as the main phase (Fig. 3b). The crystallite sizes of the Cu NPs in Fig. 3b-i-v were calculated as 30.8 nm, 38.8 nm, 43.9 nm, 25.3 nm, and 64.7 nm, respectively 43 . The crystallite sizes were smaller than the mean particle sizes and indicated polycrystalline structures for all particles. From the XRD profiles in Fig. 3b-ii,iii,v, ZnO, γ-FeOOH, and an unknown phase, respectively, contaminated the solid phases. Furthermore, amorphous-like precipitates attributable to titanium hydroxides are clearly seen in Fig. 3a-iv. Formation of these solid by-products in Cu NPs results in an increase in the resistivity of the material. When PdCl 2 was used for the synthesis, a palladium metal phase was clearly observed (Fig. 3bvi). The NPs a single nanometre in size in Fig. 3a-vi are considered palladium metal NPs. The higher redox potential of palladium than copper resulted in the formation of both palladium and copper metal NPs. These results suggested that the addition of metal chlorides such as NiCl 2 and CoCl 2 has a positive effect on obtaining Cu NPs for the synthesis of ligand-free Cu NPs applicable for Cu NP-based nanopastes with low-temperature sintering ability.

Effect of metal salt species on Cu NP synthesis.
Effects of reducing agents on Cu NP synthesis. Figure 4a Fig. 4a-v, suggesting that direct reduction from CuO particles with a leaf-like shape to corresponding Cu particles proceeded by using NaBH 4 . From the XRD profile in Fig. 4b-v, the crystallite size of the Cu particles was 15 nm, which further supported the direct reduction of the CuO particles because the crystallite size of the CuO particles was also 15 nm. This behaviour is probably due to the higher reduction ability of NaBH 4 than N 2 H 4 . When AA was used as the reducing agent, Cu NPs with a cubic shape were obtained as the main product (Fig. 4vi). The crys- www.nature.com/scientificreports/ tallite size of 91.3 nm, calculated by using the XRD profile shown in Fig. 4b-vi, is similar to the corresponding mean particle size of the Cu nanocubes. It can be considered that the lower reduction ability of AA than N 2 H 4 leads to the formation of single-crystalline Cu nanocubes by slow and uniform particle growth in the solvent. As described above for the optimization of the reaction conditions, we could conclude that NiCl 2 and N 2 H 4 are the most suitable compounds for Ni salt-assisted Cu NP synthesis for the development of Cu NP-based nanopastes with low-temperature sintering properties. Figure 5a shows thermogravimetric (TG) profiles of C N and our previously prepared Cu NPs using nitrilotriacetic acid (C P ) 30 . Interestingly, the weight loss of C N up to 300 °C was only 0.19 wt%. Upon further heating, a 0.38 wt% weight loss was observed at approximately 310 °C for C N, which is due to decomposition of a Cu 2 O phase on the surface of C N . In contrast, the weight loss of C P up to 300 °C was 2.9 wt%. The loss is due to degradation of organic residues on the surface of C P . Such degradation has a potential problem to lead to gas generation in the preparation of Cu electrodes and adhesion of IC chips with the usage of Cu NP-based nanopastes. From this point of view, the present C N obtained under Ni salt-assisted ligand-free conditions has a large potential for application in Cu NP-based nanopastes with minimal gas evolution. Figure 5b shows the observed internal structures and surface states of C N obtained by TEM and HAADF-STEM equipped with an energy dispersive X-ray spectroscopy (EDS) system. The poly-  www.nature.com/scientificreports/ crystalline crystal structure of C N is clearly seen in the TEM image in Fig. 5b-i. The HR-TEM image suggests the existence of a rough shell structure on the surface of C N with a thickness of ca. 3 nm (Fig. 5b-ii). From the XRD profile of C N in Fig. 2b-ii, C N contains a Cu 2 O phase. This result means that the shell consists of a Cu 2 O phase due to the partial surface oxidation of C N during the purification process. Figure 5b-iii is the HAADF-STEM image of C N . The corresponding EDS mapping patterns of copper, nickel, and oxygen and the merged image are seen in Fig. 5b-iv-vii, respectively. These images suggest that Ni atoms are incorporated and uniformly distributed in C N and oxygen atoms are distributed on the surface of C N to form the Cu 2 O shell layer. X-ray photoelectron spectroscopy (XPS) revealed that Ni species on the surface of Cu particles exist mainly as oxides. The polycrystalline core-shell structure of C N provides a low sintering temperature, high oxidation resistance, and long-term stability.

Characterization of the surface and internal structures of C N .
Sintering behaviour of Cu NP-based pastes. Figure  Preparation of copper electrodes on flexible films from Cu NP-based nanopastes. Figure 7a,b show photographs of Cu electrodes on a polyethylene naphthalate (PEN) film and a polyethylene terephthalate (PET) film, respectively, prepared by using C N NP-based nanopastes. The sintering temperatures for Fig. 7a,b were 150 °C and 200 °C, respectively. Details for the preparation are summarized in the Methods section. No cracks were seen after repeated bending of the Cu electrode printed flexible films, and a simple light emission diode (LED) device on a PEN film was readily obtained. Figure 7c shows the results of stability testing of the Cu electrodes prepared by sintering at 150 °C, 180 °C, 200 °C, and 260 °C under a N 2 atmosphere for 30 min. After preparation of the Cu electrodes on a PEN film, the electrodes were left under atmospheric conditions at a temperature of 24 ± 1 °C and humidity of 44 ± 1% for the stability test. Regardless of the sintering temperature, the resistance of the Cu electrodes remained almost unchanged for more than one month.

Conclusions
In the present study, we have established an organic ligand-free aqueous-phase synthesis of Cu NPs that are applicable for use in Cu NP-based nanopastes with low-temperature sintering properties. Here, the addition of NiCl 2 played a critical role in reducing CuO solid particles with a leaf-like shape by hydrazine to obtain C N from an aqueous phase under ambient atmospheric conditions. Interestingly, the weight loss of C N due to organic residues on its surface was only 0.19 wt%, as determined from TG-DTA measurements. The results suggested that C N with an organic ligand-free surface obtained by Ni salt assistance is a promising material for the development of Cu NP-based nanopastes with minimal gas evolution during Cu electrode fabrication and Cu-Cu adhesion. Such gas formation is a serious problem in PE and FHE technologies and leads to the generation of cracks and voids, which reduce the quality and durability of the products. Due to the unique polycrystalline core-shell structure of

Preparation of Cu NP-based nanopastes.
For the preparation of the C N NP-based nanopastes, an ethanol slurry of C N (70 wt%) was synthesized at a scale 20 times larger than that described above (the stirring time after the addition of the aqueous solution of hydrazine monohydrate was 1 h). Then, the resulting slurry was dried in a reduced-pressure atmosphere, and the obtained solid particles were ground with an agate mortar in a N 2 atmosphere and sieved to 635 mesh. Next, C N (2.00 g) and triethanolamine (0.35 g) were mixed in a N 2 atmosphere using a rotation-revolution mixer (ARE-310, THINKY CORPORATION) and sieved to 635 mesh. Finally, 0.081 g methanolic solution of 3-glycidoxypropyltrimethoxysilane (63 wt%) was added to the mixture (1.00 g) to obtain the C N -based nanopastes (79 wt% C N ). Preparation of copper-bonded bodies and shear strength measurements. Flat Cu plates with a clean surface were prepared by rotary polishing oxygen-free Cu plates (i-ject Co., Ltd., 3 × 3 mm and 5 × 5 mm; t = 1 mm) with a manual polishing machine (grit size: # 4000). Then, the resulting Cu plate (5 × 5 mm) on which the Cu NP-based pastes (1 mm square) were printed was prepared by screen printing using a metal mask (Tokyo Process Service Co., Ltd.). Then, the 3 × 3 mm plate was placed on the printed nanopaste, and the paste was sandwiched between the Cu plates. www.nature.com/scientificreports/ sure-less conditions with flowing N 2 (1.3 L/min) for adhesion. The shear strength of the resulting Cu-bonded bodies was measured and calculated by a bond tester (Condor Sigma, XYZTEC) at a shear speed of 50 μm/s. Characterization equipment. X-ray diffraction (XRD) measurements were performed on a Rigaku Intelligent X-ray diffraction SmartLab system equipped with a PILATUS3 R 100 K detector using CuKα radiation (40 kV, 40 mA) to determine the crystal phase and measure crystallite size. Scanning electron microscopy (SEM) observations were carried out using a HITACHI SU 7000 with an acceleration voltage of 5 kV. TEM observations were performed using an FEI TITAN 80-300 instrument at 200 kV to obtain HR-TEM and high-angle annular dark field scanning TEM (HAADF-STEM) images. The electrical conductivity of the copper electrodes was measured by a Mitsubishi Chemical Analytech Loresta-GX MCP-T 700 instrument with a four-point probe method. Thermogravimetric analysis (TGA) of the Cu NPs was carried out by a Bruker Japan TG-DTA2000SA at a heating rate of 5 °C/min under atmospheric conditions.