Reactive Sintering of Cu Nanoparticles at Ambient Conditions for Printed Electronics.

A new approach is presented to overcome the disadvantages of oxidation and harsh sintering conditions of Cu nanoparticle (Cu NP) conductive inks simultaneously. In this process, oleylamine (OAM) adsorbed on particles was effectively eliminated via the reactive desorption by formic acid in alcohols; meanwhile, Cu ion was generated on the surface. The desorption of OAM resulted in more severe surface oxidation of Cu NPs. The oxide (Cu2O) and Cu2+ distributed on the Cu NP surface could be reduced to Cu(0) by NaBH4 solution and take on the role of soldering flux to weld particles into a blocky structure. With the compact coalescence of particles without oxides, the resistivity of metal patterns could fall below 20 μΩ·cm and exhibit proper adhesion. Thanks to the sintering of Cu NPs at ambient conditions, the conductive patterns could be facilely formed on thermosensitive substrates. As the oxide state of Cu would be reduced during sintering, the partially oxidized Cu nanoparticles could be directly applied to conductive inks.

Introduction

Printed electronics are a set of electronic devices manufactured using large-scale and high-volume printing techniques. Owing to their capability of bypassing conventional high-priced and inflexible silicon-based electronics to manufacture a variety of devices on flexible substrates, printed electronics have attracted increasing attention in the field of electronic devices.[1] Various studies have illustrated that printed electronic technology is becoming an effective exploratory method to fabricate large-area and flexible electronic devices via patterning functional material-based inks.[2] In these inks, solution-processable conductive inks play an important role in preparing conductive circuits.

Recently, metallic nanomaterial conductive inks have been used as core materials with the implementation of printing techniques such as inkjet printing. For example, Ag nanomaterial has been widely applied in conductive inks due to its high conductivity and oxidation resistance. However, high cost and electromigration phenomenon hinder its widespread use. In contrast, Cu nanomaterial is considered as the most promising substitutive material due to its low price, high conductivity, and excellent electromigration resistance.[3,4] However, Cu nanomaterial inks are susceptible to oxidation in every step in applications such as storage, printing, and sintering,[5] which results in deterioration of their electrical conductivity and increase of the sintering temperature. To overcome the drawback of oxidation, core–shell nanomaterials with copper as the core were commonly prepared and applied.[6−8] Although the core–shell nanoparticles possess excellent oxidation resistance, the complex preparation could largely limit their applications.

Another challenge in using nanometallic inks is the need for a high-temperature postprinting process, which increases the cost and limits the choice of flexible substrates. Usually, bulky capping agents are used to protect nanomaterials from aggregation. Thus, a sintering step is necessary for nanomaterial coalescence and impurity removal.[9] For air-sensitive Cu nanoparticles, the sintering process should be conventionally conducted in an inert or reducing environment, which further complicates the process.[10,11] Moreover, high-temperature sintering (>200 °C) restricts the use of inks on thermosensitive substrates such as paper, plastic, and fabric.[12] To avoid heat damages of substrates, researchers have proposed chemical processes to sinter Ag or Cu@Ag nanoparticle inks at room temperature (RT) in air.[5,13−16] As for Cu nanoparticle inks, Lee et al. attempted to exclude the additional sintering process by employing Cu NPs treated with poly(VI-co-VTS) and obtained a room-temperature resistivity of 1.2 × 104 μΩ·cm for Cu patterns.[9] However, the resistivity is far from the conductive demand because of the lack of sintering. To the best of our knowledge, there are few reports on the formation of satisfactory conductive patterns from Cu nanoparticle inks at ambient conditions.

In this work, we present a new procedure to overcome both problems of oxidation and harsh sintering conditions of Cu nanoparticle conductive inks simultaneously. Reactive sintering at ambient conditions was achieved by a simple two-step process. To remove the capping agent from Cu nanoparticles, the patterns were first dipped in formic acid solution for a few seconds. Oleylamine (OAM) adsorbed on particles was removed quickly; meanwhile, Cu ion was generated on the surface. The effective OAM desorption triggers more severe surface oxidation of Cu NPs. The oxide (Cu2O) and Cu2+ on Cu NPs could be reduced to Cu(0) by NaBH4 solution and take on the role of soldering flux to weld particles into a blocky structure. With the compact coalescence of particles, the resistivity of the metal patterns could fall below 20 μΩ·cm and exhibit proper adhesion.

Results and Discussion

Reactive Sintering of Cu NPs

It is well known that amine can react with acid to produce ammonium salt or amide. Mazumder et al. found that the OAM-capped nanoparticles could be readily “cleaned” by acetic acid washing, resulting in the desorption of OAM.[17] In this work, we choose formic acid as a stronger acid to detach OAM more efficiently. The reaction of amide formation usually requires a long reaction time and high temperature or the help of a catalyst.[18] While the sintering process was realized at RT in less than 3 min, the formation of amide was unlikely to be the main reaction. Thus, it is believed that HCOOH reacted with OAM to obtain the ammonium salt during sintering, as shown in Scheme .

Scheme 1

Reaction of OAM and HCOOH

Due to the firm adhesion of the metal film on the substrate, the sintered metal coating could not be obtained in sufficient quantity for analysis. To clarify the mechanism of sintering, we conducted similar treatments on Cu NP powders. To confirm the removal of the OAM layer by HCOOH solution, the powders were immersed in 10 vol % HCOOH ethanol solution for 3 min and analyzed by thermogravimetric analysis (TGA) measurements. The release temperature of pure OAM is about 150 °C, whereas the OAM adsorbed on particles start to lose weight at around 200 °C.[10] Thus, dividing the mass loss above 200 °C by the mass at 200 °C was regarded as the amount of OAM adsorbed on particles. Table summarizes the amount of OAM adsorbed on particles after different treatments. It can be seen that the OAM residues dramatically decreased from 11.4 to 7.0% after a 3 min immersion in 10 vol % HCOOH ethanol solution. Then, on dipping in 3 wt % NaBH4 aqueous solution for 3 min, the OAM residues could be further reduced to 2.4%. When the solvent in step I was replaced with methanol, the decrease of OAM residues was more remarkable.

Table 1

OAM Residues on Particles Based on TGA Measurements from Figures S1–S3

immersion condition	solvent	untreated	alcohols	10 vol % HCOOH	10 vol % HCOOH + 3 wt % NaBH4
OAM residues (%)	ethanol	11.4	9.3	7.0	2.4
	methanol		8.8	2.2	1.0

Yi et al. considered that OAM on nanoparticles could act as a barrier network to inhibit the accessibility of formic acid on the particle surface due to the high polarity of HCOOH.[19] The permeation of HCOOH in CH2 chains of OAM is harder than monohydric alcohols. When the powders were immersed in pure HCOOH, the OAM residues remained at a high level at 11.1% (Table ). In contrast, the OAM residues were decreased to 9.3% after immersing in pure ethanol. These results again suggest that it is difficult for HCOOH to access the particle surface. Thus, the reasonable explanation is that OAM on particles is first dissolved by alcohols and is then captured by HCOOH to prevent OAM from readsorption. HCOOH and alcohols could mutually promote the removal of OAM adsorbed on particles. It was also found that the OAM residues on the powders treated by 20 vol % HCOOH ethanol solution were less than those on the powders treated by 10 and 30 vol %, as shown in Table . At lower HCOOH concentrations, OAM removed by ethanol could not be effectively reacted with HCOOH in time. The reaction rate of HCOOH with OAM is the rate-limiting step, resulting in a better removal efficiency of 20 vol % than 10 vol % HCOOH solutions. Conversely, a higher concentration of HCOOH would reduce the amount of ethanol and retard the OAM removal from the particle surface. Eventually, the increase of HCOOH concentration did not elevate the final removal efficiency, which is seen as a result of 30 vol % HCOOH solution treatment.

Table 2

OAM Residues on Particles after Immersing in Different Concentrations of HCOOH Ethanol Solution Based on TGA Measurements from Figure S4

HCOOH concentration	10 vol %	20 vol %	30 vol %	100 vol %
OAM residues (%)	7.0	4.0	5.1	11.1

Through the above treatment, most OAM adsorbed on particles could be detached. The OAM removal by HCOOH alcohol solutions was also confirmed by Fourier transform infrared (FT-IR) spectra of HCOOH immersion liquids, as shown in Figure . To concentrate the detached OAM, 10 vol % HCOOH ethanol immersion liquid was dried at 0.001 MPa and 50 °C to remove ethanol and excess HCOOH. The sample had the features of both OAM and copper formate (Cuf). The peak around 1729 cm–1 related to C=O of HCOOH was not detected, proving that HCOOH had been removed in the drying process. The peaks around 3200–3400 cm–1 belonged to the stretching vibration of N–H, and the peaks of 1600 and 795 cm–1 were the in-plane and out-of-plane bending vibrations of N–H. This suggests that OAM adsorbed on particles could be eliminated by HCOOH alcohol solutions. It should be noted that the bond observed at 832 cm–1 was attributed to Cu–O vibration, reflecting the ligand’s carboxyl group that formed the bond with the Cu(II) center in Cuf.[20,21] The double sharp peaks around 1575 and 1374 cm–1 represent the asymmetric and symmetric stretching vibrations of carboxylate, indicating the formation of Cuf. When the particles were immersed in HCOOH solution, a part of the Cu oxides on the particles would react with HCOOH to generate copper formate.

Figure 1

FT-IR profiles of OAM, 10 vol % HCOOH (EtOH) immersion liquid treated in vacuum, Cuf, and HCOOH.

With the lack of OAM as the capping agent, the Cu NPs were prone to further oxidation. The X-ray diffraction (XRD) patterns in Figure show that the intensities of Cu2O peaks become stronger after 10 vol % HCOOH ethanol immersion. Furthermore, the oxide of Cu could react with HCOOH to generate Cuf but is inadequate to be detected by XRD, whereas the dissolution of copper ions was confirmed by analyzing the immersion alcohol solution. Cu2+ was detected at 5.8 ppm in 10 vol % HCOOH ethanol immersion using an atomic adsorption spectrometer (AAS). To confirm that a part of Cu2+ remained on the pattern, the film was then immersed in deionized water (pH 7). Cu2+ was detected in the immersion water as 0.3 ppm. This means that not only Cu2O but also Cu2+ was present on the surface of Cu NPs, as schematically presented in Figure . Thus, Cu2+ could distribute among the particles’ surface, resulting in a strong interconnection of the particles. As seen from the transmission electron microscopy (TEM) images in Figure , the average diameter of the original particles was 11.9 nm (σ = 14.9%). However, the profile of the particles became ambiguous after HCOOH ethanol immersion, indicating the interconnection of particles, which may result from Cu2+ distribution.

Figure 2

XRD patterns of the particles: as-synthesized, 10 vol % HCOOH (EtOH) immersion, followed by 3 wt % NaBH4 solution immersion.

Figure 3

TEM images of the particles: (a) as-synthesized, (b) 10 vol % HCOOH (EtOH) immersion, (c) followed by 3 wt % NaBH4 solution immersion.

Subsequently, NaBH4 solution reduced the oxide (Cu2O) and Cu2+ to Cu(0) and led to the strong fusion among particles. The peaks of Cu2O disappeared after NaBH4 treatment in Figure . From the XRD spectra, the mean Cu crystal size of the particles was decreased from 7.3 to 5.9 nm after 10 vol % HCOOH (EtOH) immersion and increased to 14.2 nm through 3 wt % NaBH4 solution immersion. The crystal size reduction after 10 vol % HCOOH (EtOH) immersion was due to further oxidation of Cu, resulting in the decrease of Cu(0) atoms. Moreover, the oxide state of Cu on the surface could protect Cu(0) particles from agglomeration. On the contrary, NaBH4 solution immersion could reduce the oxide state of Cu and coalesce the particles, hence the crystal size increased to 14.2 nm. Most of the particles coalesced together and developed a blocky structure, as shown in Figure c. Because the pH of NaBH4 solution is high enough to avoid the dissolution loss of Cu2+, Cu2+ could act as soldering flux to weld particles. No Cu2+ could be detected in the NaBH4 immersion solution by AAS. With the coalescence of particles, OAM adsorbed on particles would be squeezed out eventually. As described in Table , on being dipped in 3 wt % NaBH4 solution for 3 min, the OAM residues could be further reduced. The mechanism of sintering is illustrated in Figure .

Figure 4

Schematic illustration of the chemical sintering mechanism.

Effect of Alcohols

Figure shows the resistivity of the metal layer treated by 10 vol % HCOOH alcohol solution, then by 0.75 wt % NaBH4 immersion for different time periods. On prolonging the dipping time, the resistivity dramatically decreased first and finally tended to be stable. It should be noted that the optimum resistivity was 24.38 μΩ·cm (ethanol as the solvent) and 16.93 μΩ·cm (methanol as the solvent). This means that the selection of the solvent in step I has a significant influence on the resistivity of Cu films. Methanol seems to have a positive effect. This result is consistent with that reported by Wakuda et al. They showed that the removal of alkyl amine from Ag nanoparticles by methanol was superior to that by ethanol; therefore, the resistivity of the Ag film dipped in methanol was better than that in ethanol.[22]

Figure 5

Final resistivity of the metal layer treated by HCOOH solution and followed by 0.75 wt % NaBH4 immersion for different time periods. (a) 10 vol % HCOOH in ethanol and (b) 10 vol % HCOOH in methanol.

To clarify the removal efficiency of OAM by different alcohols, Cu NP powders were dipped in methanol or ethanol for 3 min and then analyzed by TGA measurements. The results in Table showed that OAM residues were reduced from 11.4 to 8.8 and 9.3% by methanol and ethanol, respectively. Due to the superior removal efficiency, the OAM residues on Cu powders treated by 10 vol % HCOOH in methanol were far less than those on Cu powders treated by 10 vol % HCOOH in ethanol solution (2.2 vs 7.0%), as shown in Table . Benefiting by the higher removal efficiency in methanol, we obtained lower resistivity by dipping the film in 10 vol % HCOOH methanol.

Effect of HCOOH Concentration

As described previously, the Cu oxide could react with HCOOH to generate copper formate. The dissolution of Cu ion in alcohol solution would lead to the loss of copper. Thus, it is essential to explore the relationships between Cu loss and HCOOH concentration. The Cu ion content in the immersion liquid was used to measure the Cu loss, as shown in Figure . As the concentration of HCOOH increased, the Cu ion content increased, indicating the augmented Cu loss. Obviously, this is because the increasing concentration of the reactant (HCOOH) accelerated the formation rate of Cu ion. Moreover, the Cu loss in methanol solution was higher than that in ethanol under the same HCOOH concentration. This result seems related to the higher removal efficiency of OAM in methanol, which leads to the exposure of Cu particles.

Figure 6

Cu2+ content in HCOOH alcohol immersion liquid.

To quantify the effect of HCOOH concentration on resistivity, the coating films were immersed in HCOOH ethanol solutions (5, 10, and 20 vol %), then dipped in 0.75 wt % NaBH4 for different time periods. As shown in Figure a, at the concentration of 5 vol %, the resistivity was uneven and maintained at a high level due to the large OAM residues. Even when NaBH4 immersion time was prolonged to 15 min, the resistivity still remained at 84.38 μΩ·cm. At higher concentrations of 10 and 20 vol %, the resistivity at both concentrations significantly decreased to about 25 μΩ·cm in Figure b,c. This result illustrates that 10 vol % HCOOH in ethanol is enough to obtain low resistivity.

Figure 7

Final resistivity of the metal layer treated by HCOOH solution, followed by 0.75 wt % NaBH4 solution for different time periods. (a) 5 vol % HCOOH in ethanol, (b) 10 vol % HCOOH in ethanol, (c) 20 vol % HCOOH in ethanol, (d) 5 vol % HCOOH in methanol, and (e) 10 vol % HCOOH in methanol.

When only the solvent was replaced with methanol, the resistivity could be reduced to less than 20 μΩ·cm, as shown in Figure d,e. The lowest resistivity reached 16.93 μΩ·cm, only 10 times the resistivity of bulk copper. Moreover, 5 vol % HCOOH in methanol was high enough to achieve a low resistivity of 19.40 μΩ·cm in Figure d. The resistivity of 20.23 μΩ·cm was obtained after 3 min immersion in 0.75 wt % NaBH4 solution. This is largely due to the high OAM removal efficiency of HCOOH methanol solution. As described above, the Cu loss in 5 vol % HCOOH methanol solution was close to that in 10 vol % HCOOH in ethanol solution, whereas the Cu films possessed a lower resistivity. Therefore, 5 vol % HCOOH methanol solution for step I is economic and efficient for reactive sintering.

Effect of NaBH4 Concentration

At the second step of sintering, the concentration of NaBH4 may influence the structure of the metal film and the final resistivity. Higher concentrations could guarantee sufficient reaction and thorough coalescence. The coatings were immersed in 10 vol % HCOOH ethanol solution then dipped into different concentration of NaBH4 for 3 min. With the increase of NaBH4 concentration, the resistivity decreased to 21.68 μΩ·cm at 3 wt % (Figure a). As shown in Figure b,c, most of the particles coalesced throughout the whole thickness of the coating and not only in the surface. Due to the small thickness (approximately 350 nm), the sintering agent could reach deep into the interior of the coating and developed a blocky connected structure. On further increasing the concentration to 4 wt %, the resistivity dramatically elevated to 87.71 μΩ·cm due to the severe damage of the metal film, as shown in Figure e. The broken film resulted from the vigorous reduction reaction that occurred as the NaBH4 concentration reached 4 wt %. However, when the film was treated by 3 wt % NaBH4, and even extended the immersion time to 40 min, it remained uniform without any damages, as shown in Figure f.

Figure 8

(a) Resistivity of the metal layer treated by 10 vol % HCOOH ethanol solution, followed by different NaBH4 concentrations for 3 min. (b, c) Scanning electron microscopy (SEM) images of Cu layer after immersing in 3 wt % NaBH4 for 3 min and (d–f) scanned images of metal films sintered with different concentrations of NaBH4 for 3 min.

As for 10 vol % HCOOH methanol solution, 3 wt % NaBH4 immersion caused film damage, as shown in Figure b. Only when the concentration of NaBH4 was below 1 wt %, the metal films could remain even in appearance. However, the resistivity in Figure a increased from 16.93 to 25.89 μΩ·cm as the concentration of NaBH4 increased from 0.75 to 1 wt %. This change indicates that internal damages of the film occurred with 1 wt % NaBH4 immersion. Thus, the films treated by 10 vol % HCOOH methanol solution were more fragile. Because of the lower OAM residues as described above, the oxidation of Cu was more severe and resulted in a fast reduction reaction in step II. Meanwhile, the reduction of OAM residues may also decrease the adhesion of particles.

Figure 9

(a) Resistivity of the metal layer treated by 10 vol % HCOOH methanol solution, followed by different NaBH4 concentrations for 9 min. (b) Scanned images of metal films sintered with different concentrations of NaBH4 for 9 min.

These results are encouraging because a Cu film with low resistivity (16.93 μΩ·cm) was obtained by a simple two-step sintering process at ambient conditions. Moreover, the metal pattern exhibited proper adhesion. Figure S5 shows the remaining coating structure after the peel-off adhesion test for sintered samples. The results show that upon peel-off almost all materials remain at the substrate.

Conclusions

In summary, a process for sintering Cu NPs at ambient conditions was proposed. The sintering was triggered by the removal of the capping agent and “soldering” through reduction. Oleylamine (OAM) adsorbed on Cu NPs was effectively eliminated by the reactive desorption by formic acid in alcohols. Meanwhile, more severe surface oxidation occurred on Cu NPs and Cu ion was generated. Then, NaBH4 aqueous solution was applied to reduce the oxidation layer that contained Cu2+ and Cu2O. The oxide (Cu2O) and Cu2+ on the surface of particles could act as soldering flux to weld particles into a blocky structure. With the coalescence of particles, OAM adsorbed on particles would be squeezed out eventually. The resistivity of the metal patterns could fall below 20 μΩ·cm and exhibit proper adhesion.

Based on these findings, sintering can be achieved at ambient conditions, enabling the formation of conductive patterns on various thermosensitive substrates. More importantly, the oxidation of Cu nanoparticles would not hinder the application of Cu NP inks and could even act as soldering flux to coalescence particles. Furthermore, low-cost, continuous fabrication processes could be realized using the above simple sintering method.

Experimental Section

Chemicals and Materials

Copper formate tetrahydrate (98%) was purchased from Alfa Aesar. Oleylamine (OAM, 90%) was bought from Aladdin. Paraffin liquid (CP, distilate temperature >300 °C) was from Tianjin Guangfu Fine Chemicals Research Institution. Hexane (95%), isopropyl alcohol (99.7%), and NaOH (96%) were obtained from Xilong Scientific Company. Methanol (99.8%) was bought from Tong Guang Fine Chemicals Company. Formic acid (98%) and ethanol (99.7%) was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. NaBH4 (98%) and octane (99%) were supplied by Tianjin Fuchen Chemical Reagents Factory.

Copper formate terahydrate was dried at 0.001 MPa, 90 °C for 12 h to get anhydrous copper formate (Cuf). Other chemicals were used as received without further purification.

Characterization

The morphology of particles was investigated by transmission electron microscopy (TEM) analysis (HT-7700, Hitachi and JEM2010, JEOL), and the size distribution of particles was obtained by measuring the diameter of more than 100 nanoparticles. The chemical structures of the immersion liquid were analyzed by a Fourier transform infrared (FT-IR) spectrometer (Tensor 27, Bruker). Additionally, the crystalline structures of particles were identified by X-ray diffraction (XRD, D8 Advance, Bruker) using Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA, STA 409PC, Netzsch) was used to investigate the thermal decomposition behavior of organics at 10 K/min. The sheet resistance of the metal layer was measured using a 4-point probe (RTS-9, 4 Probes Tech). The surface and cross-sectional morphology of Cu films were observed using field emission scanning electron microscopy (SEM, SU-8010, Hitachi). Furthermore, the volume resistivity of the metal pattern was calculated from the sheet resistance together with the metal layer thickness determined by SEM. The Cu(II) ion concentrations in the immersion solution were analyzed by an atomic adsorption spectrometer (AAS, NITACHI 172-8035).

Cu NP Synthesis and Conductive Ink Formulation

Cu NPs were synthesized by the decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilizing agent as described previously.[10] Cuf (1.2 g) and OAM (9.3 g) were dissolved in 80 mL of paraffin liquid at 50 °C, and the air was removed from the solution by bubbling with N2. The solution was heated at 170 °C for 30 min under N2 and cooled with a water bath. Then, the colloid was centrifuged at 10 000 rpm for 30 min to separate the Cu NPs. The Cu NPs were washed once with the solvent (hexane/isopropyl alcohol = 1:1, v/v) before formulating the conductive inks. Finally, 30 wt % Cu NPs were dispersed in octane using an ultrasonic bath for 2 h to prepare the conductive ink.

Reactive Sintering at Ambient Conditions

The sintering process consists of two steps. The formulated ink was coated onto a poly(ethylene terephthalate) (PET) film using an applicator (25 μm, AICE Inc.). In the first step (step I), the coated PET films were immersed in formic acid solution (5, 10, 20 vol %) for 20 s and rinsed with the solvent (methanol or ethanol). The above operations were repeated three times to increase the coating thickness. For the second step (step II), NaBH4 was dissolved in pH 12 NaOH solution according to different concentrations (1–4 wt %). The films from step I were then immersed into the NaBH4 solution for 0–15 min and washed with deionized water.