Self-Reducible Cu Nanoparticles for Conductive Inks - Industrial

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Self-Reducible Cu Nanoparticles for Conductive Inks Xiaofeng Dai, Wen Xu, Teng Zhang, and Tao Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04248 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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Industrial & Engineering Chemistry Research

Self-Reducible Cu Nanoparticles for Conductive Inks Xiaofeng Dai, Wen Xu, Teng Zhang, Tao Wang*

State Key Lab of Chemical Engineering, Department of Chemical Engineering

Tsinghua University, Beijing 10084, China

ABSTRACT: Copper nanoparticles (Cu NPs) are a potential material for conductive inks due to its low price, high conductivity and high electromigration resistance. However, Cu NPs are prone to be oxidized in air, which is lethal to conductivity. In this work, we report on the self-reduction of Cu NPs which are synthesized by the thermal decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilizing agent. Although the particle surface partially oxidized to Cu2O in air, the cuprous oxide could be reduced to copper during sintering, due to the release of H2 through the decomposition of OAM adsorbed on Cu NPs. This self-reduction ability without the help of additional reduction agent makes the Cu NPs be a promising material for conductive inks. The feasibility to formulate conductive ink with the prepared Cu NPs was demonstrated.

KEYWORDS: copper nanoparticles, conductive ink, self-reduction, copper formate, oleylamine

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1. INTRODUCTION With demands for more economic routes to manufacture of electronic devices, the increasing attention has been focused on the inkjet printing. Nanoparticles are considered to be highly useful for conductive inks. Currently, silver nanoparticles conductive inks were commercialized and applied for the fabrication of electronics such as conductive circuits,1 electrodes,2 radio frequency identification (RFID) tags3 and sensors.4 However, the high cost and electromigration of silver have limited the large-scale use of silver conductive inks. In contrast, copper nanoparticles (Cu NPs) can be considered as a potential material due to copper’s low price, high conductivity and high electromigration resistance.5 The resistivity of bulk copper is 1.72µΩ·cm, only slightly higher than that of bulk silver (1.59 µΩ·cm) .6 Unfortunately, it is hard to use Cu NPs for conductive inks directly, because they are prone to be oxidized in air, particularly as their size decreases below 20nm.7 Thus, the oxidation stability of Cu NPs is crucial for their use in conductive inks.8 Various protection methods have been implemented to inhibit the oxidation of copper. For example, capping molecules such as sodium dodecyl sulfate,9 PVP,10-12 CTAB13-15 and oleic acid8 were used to protect the Cu NPs from air. However, the effect of capping molecules as antioxidants is limited, and the oxidation was only diminished. Consequently, some researchers put forward creating a reductive atmosphere containing various carboxylic acids vapor such as formic acid or oxalic acid to reduce the surface oxidation of Cu NPs.16-20 Besides, Gu et al.21 used ammonia chloride (NH4Cl) to etch the surface oxide before sintering. Nevertheless, all of these methods 2


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have problems about the cost as well as safety. There are also some studies focusing on MOD (metal organic decomposition) copper inks.6, 22-28 Since the metal is already present in its oxidized form, they can overcome the oxidation of copper. However, the metal loadings of MOD inks are quite low. The typical metal loadings of MOD inks reach up to 30%, while the metal loadings in metal nanoparticles inks range from 20% to 70%.29 In addition, in contrast to metal nanoparticles inks, MOD inks are still in researching stage, and there is still no commercial product yet. Intense pulsed light (IPL) sintering was reported for the air sinter-able and low temperature-fabrication of Cu electrodes.27,

29-33

IPL sintering is essentially a thermal

technique which can sinter the metal conductive inks without damaging the polymer substrates. Due to the extreme short sintering time, there is no further oxidation occurred even in ambient condition IPL sintering process.32 However, IPL sintering is unable to eliminate the oxide which already existed in Cu NPs. Thus, extra reducing agent30,

32-33

or complicated equipment31 are still needed to resolve the oxidation

problem. In this study, we synthesized Cu NPs by the thermal decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilization agent. Although Cu2O was formed on the particle surface in air, the Cu2O could be eliminated in the sintering process without any additional reductive agent. We uncovered that the Cu2O was reduced by the released H2 through the decomposition of OAM adsorbed on Cu NPs. To our best knowledge, the reduction of Cu2O on the

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Cu NPs surface by the capping molecules (OAM) through sintering without additional reducing agent has not been examined so far. 2. EXPERIMENTAL SECTION 2.1 Chemicals and materials Copper formate tetrahydrate (98%) was purchased from Alfa Aesar. Oleylamine (OAM, 90%) was purchased from Aladdin. Paraffin liquid (CP, distillate temperature > 300°C) was purchased from Tianjin Guangfu Fine Chemicals Research Institution. Hexane (95%) and isopropyl alcohol (99.7%) were purchased from Xilong Scientific Company. Copper formate terahydrate was dried at 0.001MPa, 90°C for 12h to get anhydrated copper formate (Cuf). Other chemicals were used as received without further purification. 2.2 Cu NPs synthesis Cu NPs were synthesis by following procedure. First, 1.2g Cuf, OAM according to desired OAM:Cuf molar ratio and 80mL paraffin liquid were added to a 250mL three-neck flask, and stirred at 50oC until Cuf was fully dissolved. The formed deep blue solution was kept in the flask under pure N2 gas purging for 30min to remove oxygen. Then, the solution was heated to 170 oC and held at this temperature for 30min. Eventually, the red dark colloidal solution was formed and cooled rapidly using a water bath. The colloid was centrifuged at 10000 rpm for 30 min. The powders were washed three times with the solvent (hexane:isopropyl alcohol=1:1,

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v:v). Subsequently, the nanoparticles were obtained by drying under vacuum (0.001MPa) at 40°C for 12h. 2.3 Characterization The morphology and size distribution of the synthesized nanoparticles were determined by transmission electron microscopy (TEM, NITACHI H-7650B) analysis at 80Kv. TEM samples were prepared by dropping a hexane dispersion of the nanoparticles onto carbon-coated molybdenum grids. By counting 150 particles, the particle size distribution histogram, mean diameter and diameter variance were obtained. The crystalline structures of Cu NPs were identified by X-ray diffraction (XRD, D8 Advance, Bruker) using Cu Kɑ radiation(λ=1.5406Å). The chemical structures of Cu NPs were analyzed by Fourier transform infrared (FTIR) spectrometer (Tensor 27, Bruker). The surface and cross-sectional morphology of the film were investigated using field emission scanning electron microscopy (SEM, Hitachi, SU-8010) with EDX. EDX samples were prepared by dropping a hexane dispersion of the nanoparticles onto silicon wafer. Thus, some samples have the peaks of silicon in EDS spectra. Thermogravimetric analysis (TGA, Netzsch STA 409PC) was used to investigate the thermal decomposition behavior of neat Cuf, Cuf-OAM complexes and Cu NPs. The contents of OAM adsorbed on Cu NPs were determined from TGA data. To identify the decomposition products, TGA (Netzch X70) coupled with mass spectrometry (MS, Netzsch OMS 403 C) and FTIR spectrometry (VERTEX 70v) was

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applied. The transfer lines between the TGA and the MS and FTIR were heated to 200°C. 2.4 Conductive ink and patterning To demonstrate the feasibility of the Cu NPs for conductive inks, a model conductive ink with 30wt% Cu NPs was prepared using the synthesized Cu NPs with anhydrous toluene as the solvent. The formulated ink was injected into a pen tube for drawing patterns on the polyimide (PI) film. A straight line was drawn on the PI film to form a track and sintered by a tube furnace in N2 atmosphere. The sintering was conducted at 200°C, 250°C and 300°C for 1h. Intense pulsed light (Xenon, X-1100) was applied to sinter the track at ambient condition using energy of 7J/cm2 with 1pulse (2.5ms). A digital multimeter (UT58A) was used to measure the resistivity of the copper track. The resistivity of the formed track was given as: ߩ=

ோ஺

(1)

௟

where, R is the resistance; l is the length of the track; A is the cross section area calculated from the SEM image. 3. RESULTS AND DISCUSSION The advantages of low price, high conductivity and high electromigration resistance make Cu NPs be a potential material for conductive ink. However, the poor oxidation stability has badly blocked the application of Cu NPs ink. Presently, there is no satisfactory solution for this problem. In this work, we report on the self-reduction of Cu NPs which are synthesized by the thermal decomposition of copper formate (Cuf) 6


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using oleylamine (OAM) as the complexing ligand and stabilizing agent. Although the particle surface partially oxidized to Cu2O in air, the Cu2O could be reduced to copper during sintering. The OAM capped on Cu NPs can play the role of reductant for Cu2O through sintering without an additional reducing atmosphere. Such simple approach for solving the problem of Cu NPs oxidation was firstly presented in this paper. 3.1 Characterization of nanoparticles Figure 1 shows TEM images of particles synthesized with varying OAM:Cuf molar ratios, which indicate that the as-synthesized nanoparticles have spherical shape. The histograms in Figure 1 provide the particle size distribution corresponding to OAM:Cuf molar ratios of 2:1, 3:1, 4:1 and 5:1. It is well known that OAM can work as a protecting ligand in metal nanoparticles synthesis and control the particles diameter.34 Therefore, OAM takes the role of a stabilizer to make the particles be well dispersed without agglomeration. It can be seen that the particle size distribution of Cu NPs is highly dependent on the OAM:Cuf molar ratio. The smaller and more uniform Cu NPs were obtained at relatively higher OAM:Cuf molar ratio. The XRD patterns of the samples with different sizes were shown in Figure 2. All samples have the major characteristic for metallic Cu at 2θ values of 43.2°(Cu-111), 50.3°(Cu-200), 73.9°(Cu-220), and 37.1°(Cu2O-111) for Cu2O. The EDX data (Figure 3) provided the ratio of oxygen to copper for particles with different sizes, and indicated that more severe oxidation occurs on the surface of smaller particles. Due to

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the small size, large surface area and high surface atoms share, Cu NPs are easily oxidized when exposed in air.

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Figure 1. TEM images and size distribution of Cu NPs synthesized at 170°C for 30min. (A) and (a) OAM:Cuf=2:1; (B) and (b) OAM:Cuf=3:1; (C) and (c) OAM:Cuf=4:1; (D) and (d) OAM:Cuf=5:1. 9



Figure 2. XRD patterns of Cu NPs with different sizes.

Figure 3. EDX spectra of Cu nanoparticles with different sizes. (The silicon peaks were originated from the silicon wafer substrate for EDX samples.)

3.2 Tunable size of Cu nanoparticles The particle size of the copper particles is determined by the nucleation and growth mechanisms. Typically, a part of the Cu(II) is firstly reduced to zero valency Cu(0) 10


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nanoparticles. Then, the metal cores are gradually accumulated by the newly reduced atoms. When the reduction rate of Cu(II) is faster than the consumption rate of copper atoms, the concentration of copper atoms is likely greater than the critical supersaturation level for longer time. If there is no stabilizer combining with the surface of Cu NPs, big aggregates are formed by the collision among particles. Therefore, one of the key factors to control Cu particle size is the stabilizer. It has been reported that amines can be adsorbed preferentially on the metal nanoparticles surface with the corresponding alkane chains ordered outward.34 OAM is known as a ligand that binds tightly to the metal nanoparticles surface.35 Therefore, the presence of OAM makes the particles be well dispersed without agglomeration. To give the evidence of OAM adsorbed on the Cu NPs, we analyzed the particle samples with TGA and FTIR. Figure 4 shows the FTIR spectra of OAM and Cu NPs corresponding to 5:1 OAM:Cuf. The strong peaks were observed at 2955, 2922 and 2851 cm-1 in the FTIR spectra attributed to the CH2 symmetric and asymmetric stretching. The peaks at 1543 and 1655 cm-1 were corresponded to NH2 bending vibration and C=C bending vibration, respectively. These results confirmed the surface adsorption of OAM on Cu NPs. Besides, it could be seen from TGA in Figure 5 that the particles from different OAM:Cuf molar ratios went through two stages as increasing temperature. The first stage is between 100-130oC due to the evaporation of adsorbed washing solvent from the Cu NPs surface. Subsequent mass loss occurred at 190-350oC, which was mainly attributed to the loss of OAM absorbed on the Cu NPs. As compared with the pure OAM, the release temperature of OAM adsorbed on 11



Cu NPs increased significantly by about 50oC. These results indicate that OAM binds tightly to the Cu NPs surface and provides a steric hindrance to prevent the Cu NPs from agglomeration. As the particle size decreased, the mass loss of the second stage gradually increased, that means the amount of capped OAM increases as the particle size decreases. Table 1 summarizes the mean diameters and size variances of Cu NPs corresponding to different OAM:Cuf molar ratios. The larger particles with broader size distribution could be caused by the aggregation of small nanoparticles during growth, due to the inadequate protection of each particle by the capping agent under the condition of a low stabilizer concentration.36 Thus, higher the OAM:Cuf ratio was, higher stabilizer concentration was, smaller the particle size and more narrow distribution were.

Figure 4. FTIR spectra of pure OAM and Cu NPs (d=9.4nm).

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Figure 5. TGA profiles of OAM and Cu NPs with different sizes at 10K/min under N2 atmosphere. Table 1. Mean diameter, diameter variance and absorbed OAM content of Cu NPs synthesized with different OAM:Cuf ratio. OAM:Cuf ratio

2:1

3:1

4:1

5:1

d(nm)

25.8

14.7

10.6

9.4

σ (%)

18.8

16.5

15.8

12.7

Adsorbed OAM (wt%)

4.07

5.85

8.15

11.21

3.3 Self-reduction of Cu NPs Figure 6 presented the XRD patterns of the Cu NPs (having different sizes and OAM contents) sintered using a tube furnace in a N2 atmosphere. The sintering was conducted at 150°C, 200°C, 250°C and 300°C for 1h. Interestingly, the peak related to Cu2O disappeared without any reducing agent as the sintering temperature elevated. It 13



was inferred that Cu2O was reduced to Cu by the OAM adsorbed on the particle surface at the elevated temperature. Sintered at 150°C, all samples contain a significant peak derived from Cu2O. When the sintering temperature was elevated to 200°C, the Cu2O peak disappeared for 25.8nm sample, while there still exists Cu2O peak for 14.7nm and 10.6nm samples. With the decrease of particle size, the content of oxygen increased as Figure 3 shown. This indicates that the higher sintering temperature is needed to completely reduce Cu2O, due to the severe oxidation of the small particles. Besides, the amount of OAM ligands did not affect the self-reducibility of Cu NPs. As the XRD pattern (Figure 6) shown, the Cu2O could be completely eliminated with the enough high sintering temperature. Thus, the amount of OAM adsorbed on particles is sufficient to completely reduce the Cu2O. These results are encouraging, because this property of self-reducing Cu2O is helpful to solve the oxidation problem of Cu NPs, especially useful for conductive ink. The XRD pattern in Figure 6 shows the intensities of Cu2O peaks become stronger as the sintering temperature increased to 150°C, especially for 14.7nm and 10.6nm samples. Since the sintering was conducted under N2 atmosphere, there was no possibility of Cu NPs further oxidation. EDX spectra of Cu NPs were recorded after 150°C sintering. The EDX spectra in Figure 7 showed the oxygen content did not increase, which indicates the oxidation of Cu NPs did not be intensified after 150°C sintering. Whereas the little decrease of oxygen content was caused by the slight self-reduced due to prolonged sintering at 150°C. The increase of Cu2O peak in XRD

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pattern of samples sintered at 150oC might be caused by the crystallization of amorphous Cu2O due to sintering treatment.37

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Figure 6. XRD patterns of Cu NPs sintered at 150°C, 200°C, 250°C and 300°C. (a) d=25.8nm, OAM wt%=4.07%; (b) d=14.7nm, OAM wt%=5.85%; (c) d=10.6nm, OAM wt%=8.15%.

Figure 7. EDX spectra of Cu nanoparticles sintered at 150°C.（a）14.7nm; (b) 10.6nm. (The silicon peaks were originated from the silicon wafer substrate for EDX samples.)

To understand the self-reduction of Cu NPs, the decomposition mechanism of OAM need to be investigated. Currently, copper has been widely recognized as an active catalyst in extensive reactions.38-45 Thus, copper may play a role in catalyzing the reduction reaction. However, it is hard to obtain pure Cu NPs without oxidation to analyze the decomposition mechanism of adsorbed OAM. Whereas the thermal decomposition of Cuf-OAM complex is under N2 atmosphere, Cu2O could not be formed. Therefore, we firstly explored the decomposition of Cuf-OAM in N2 to confirm that the Cu NPs catalyze OAM decomposition. Figure 8 presents the thermogravimetric analysis (TGA) plots of Cuf, OAM and Cuf-OAM complex sample. It can be clearly seen that the Cuf-OAM complex mass loss takes place in two stages: the first between 110-150°C and the second between 16


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150-350°C. These two dramatic mass losses can be attributed first to the HCOOdecomposition and simultaneously reduction Cu(II) to Cu(0), followed by a mass loss of OAM. The TGA of neat OAM showed that the mass loss of OAM took place between 150-300°C, which was consistent with the second mass loss of the Cuf-OAM complex. Furthermore, it was found that the first mass loss is only due to decomposition of HCOO-, which is consistent well with the theoretical content of HCOO- in complex sample (12.3wt% vs 12.1wt%). Conclusively, the existence of OAM promoted the decomposition of Cuf, and decreased its complete decomposition temperature from 240°C to 150°C.

Figure 8. TGA profiles of Cuf, OAM and Cuf-OAM complex (OAM:Cuf=2:1) at 10K/min under N2 atmosphere.

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The thermal decomposition products of Cuf-OAM complex (OAM:Cuf=2:1) were identified using TG-MS-FTIR as a function of temperature (shown in Figure 9(a), (b)). The MS and FTIR spectra show that the main gaseous products are CO2 (MS: m/z=44, 28, 12. FTIR: peaks near 2354cm-1) and H2 (MS: m/z=2) in the first mass loss stage. No OAM fragment emerges concurrently in this stage. The generation of CO2 and H2 during 100-150oC indicated that the complete decomposition temperature of Cuf was decreased to 150oC. Peaks related to C-H stretching vibration (2862 and 2930cm-1) appeared at 270oC, representing the evaporation of OAM or its fragments. It should be noted that a significant peak of H2 (MS: m/z=2) emerged at 170-295oC as Figure 9(a) shown, while this peak (H2: m/z=2) was absent in the measurement for pure OAM (Figure 9(c)) during the whole heating process. It was inferred that H2 significantly releases at the temperature more than 170°C due to the OAM decomposition catalyzed by Cu NPs. No significant H2 release was detected at 150°C by MS in Figure 9(a). That is why the self-reduction of Cu2O was slight during 150°C sintering as Figure 7 shown. According to the argumentation as above, the thermal decomposition mechanism of Cuf-OAM complex was proposed as Schemes 1 & 2. In the first stage (Scheme 1), Cu(II) is reduced to Cu(0) at 110-150oC, while the formate is decomposed to release CO2 and H2, as well as OAM is dissociated from the complex. In the second stage (Scheme 2), the OAM is decomposed to release H2 owing to the catalysis of Cu NPs at temperature more than 150oC. For the complexes of Cuf with short alkyl chain amines, the deocompostion of Cuf and the elimination of amine occur simultaneously.6 18


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Therefore, there is no stage in which the short alkyl chain amine was decomposed to

generate H2 by catalysis of Cu NPs. Obviously, the thermal decomposition mechanism of Cuf-OAM is quite diferent with that of the complex of Cuf with a short alkyl chain amine. Scheme 1. Decompostion of Cuf-OAM complex.

Scheme 2. Catalyzed decomposition of OAM.

As mentioned above, the Cu NPs could catalyze OAM decomposition to release H2 at temperature more than 150°C. Therefore, the Cu2O in the particle surface, which was produced by the oxidation of Cu NPs in air during the post-treatment, could be reduced to Cu by H2 when it was heated to the suitable temperature even in an inert atmosphere. The disappearance of Cu2O peak in the XRD spectra of the samples sintered at different temperatures (Figure 6(a)) is consistent to appear of the H2 peak in TGA-MS (Figure 9(a)). To further understand the reduction of Cu2O, we conducted a TG-MS measurement to analyze Cu NPs (d=25.8nm and OAM wt%=4.07%) as Figure 9(d) shown. The H2O (MS: m/z=18) was detected, while no H2 emerged in the whole heating process. The position of H2O peak (at 200-325°C) for TGA-MS of the Cu NPs lags behind that of H2 peak for the Cuf-OAM complex (at 170-295oC). These results suggest that the H2O is generated by H2 reducing Cu2O. There is a MS peak of 19



H2 and no MS peak of H2O in Figure 9(a), while a completely opposite result is shown in Figure 9(d). Because Cu2O could not be formed during the thermal decomposition of Cuf-OAM complex in the N2 atmosphere, the produced H2 could not be consumed to form H2O. It could be concluded that H2 from the decomposition OAM on the Cu NPs surface was consumed as the reducing agent to eliminate the Cu2O, as Scheme 3 shown. Scheme 3. Reduction of Cu2O by H2 from OAM decomposition.

Figure 9. TG-MS-FTIR analysis. (a) MS for Cuf-OAM complex (OAM:Cuf =2:1); (b) FTIR for Cuf-OAM complex (OAM:Cuf =2:1); (c) MS for OAM; (d) MS for Cu NPs (d=25.8nm, OAM wt%=4.07%)

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3.4 Feasibility of formulating conductive ink Owing to nanoparticles’ small size and high surface area, the melting point of metal nanoparticles could be greatly lower than that of bulk metals. That means the smaller nanoparticles could be sintered at lower temperatures to obtain the material with low electric resistance. Besides, the particles should not clog the printer’s nozzle so that their maximum diameter should be less than one-hundredth of the nozzle diameter.46 The smaller particles will also benefit the stability of conductive inks in avoiding sedimentation. Thus, the Cu NPs sample with size 9.4nm seems to be more suitable among the synthesized particles. However, it is much difficult to separate such small particles from the solution. Ultimately, we chose 10.6nm sample to prepare the conductive ink. Figure 10 shows the surface morphologies of copper film from the formulated Cu nanoparticle conductive ink using 10.6nm Cu NPs which were capped with 8.15wt% OAM. The resistivity were measured to be 388.6µΩ·cm and 84.2µΩ·cm for the films sintered at 250 oC and 300 oC, respectively. For the sample sintered at 200 oC, the resistivity was too high to be measured. According to SEM images of Figure 10(a), the Cu NPs were not agglomerated after sintering at 200 oC, due to the existence of surface oxide layer47 as shown in Figure 6(c). As the sintering temperature increased to 250 oC, the surface oxide layer was eliminated (as shown in Figure 6(c)) and the Cu nanoparticles started agglomerating (Figure 10(b)). Sintered at 300oC, the particles become coarser (Figure 10(c)), and the resistivity is decreased significantly to 84.2µΩ·cm. 21



Further increase of sintering temperature could ensure higher nanoparticles compactness, while the flexible substrates may be damaged. Thus, intense pulsed light (IPL), which could selectively sinter the nanoparticles but remained the PI film undamaged, is a suitable alternative. Estimated by the lumped mass method,32 only absorbed 0.02 J/cm2, the temperature of a 200 nm thick copper pattern could reach up to 300oC. As the Figure 10(d) shown, the actual thickness of the copper pattern is less than 200 nm. Thus, it is easy to increase the temperature of our copper pattern more than 300oC by IPL. An intense pulsed light (IPL, Xenon, X-1100) was applied to sinter the track at ambient condition using energy of 7J/cm2 with 1pulse (2.5ms). The resistivity of the line from IPL sintering is 25.3µΩ·cm, which is 14.7 times of bulk copper. As Figure 10(d) shown, the particles were fully sintered by IPL and formed a connected structure without obvious grainy. In order to identify whether the self-reduced reaction occurred during the IPL sintering, we measured the oxygen contents by EDX analysis. The results were shown in Figure 11. It was observed that the oxygen content decreased from 8.7% to 3.4% after IPL sintering, which indicated the Cu NPs were also self-reduced. However, the air environment may lead to the dissipation of the reducing atmosphere. Thus, compared to 300°C thermal sintering, IPL sintering produces a higher oxygen content. Even so, the Cu content in the track from IPL sintering was sufficient to produce the low electric resistivity. This result is encouraging because it has sufficiently evidenced the feasibility of formulating conductive ink by using the synthesized Cu nanoparticles. 22


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Figure 10. SEM images of samples sintered at (a) 200 oC, 1h; (b) 250 oC, 1h; (c) 300oC, 1h; (d) intense pulsed light(7J/cm2 with a 2.5ms pulse).

Figure 11. EDX spectra of Cu nanoparticles sintered at 300 oC and IPL. (The silicon peaks were originated from the silicon wafer substrate for EDX samples .)

4. CONCLUSIONS 23



In this work, we firstly demonstrated the self-reduction of Cu NPs, which were synthesized by the thermal decomposition of copper formate (Cuf) using oleylamine (OAM) as the complexing ligand and stabilizing agent. During the thermal decomposition of Cuf-OAM complex, the HCOO- will be decomposed and simultaneously reduces Cu2+ to Cu0 at 110-150oC, followed the evaporation and decomposition of OAM at temperature more than 150oC. At the temperature higher than 150oC, the Cu NPs catalyze the OAM decomposition to generate H2. Although the particle surface was partially oxidized to Cu2O due to exposing Cu NPs in air, the Cu2O could be reduced to copper during sintering at elevated temperature. In the sintering process, H2 from the adsorbed OAM could be consumed as the reducing agent to eliminate the Cu2O and form H2O. This self-reduction of Cu2O to Cu(0) without the help of an additive reducing agent makes the synthesized Cu NPs be a promising material for conductive inks. The feasibility of synthesized Cu NPs for conductive ink application was verified by forming a Cu pattern with a resistivity of 25.3µΩ·cm.

ASSOCIATED CONTENT

Corresponding Author *E-mail: [email protected]

Author contributions The manuscript was written through contributions of all authors. All authors have 24


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given approval to the final version of the manuscript. Notes There are no conflicts to declare.

ACKNOWLEDGMENT

The authors acknowledge gratefully the National Natural Science Foundations of China (Grant No. 21776161) ABBREVIATIONS Cu NPs, Copper nanoparticles; Cuf, copper formate; OAM, oleylamine; IPL, intense pulsed light; PI, polyimide.

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