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Synthesis of stable ultra-small Cu nanoparticles for direct writing flexible electronics
Applied Surface Science 290 (2014) 240–245
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of stable ultra-small Cu nanoparticles for direct writing flexible electronics Wei Li a,b , Minfang Chen b,∗ a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 15 August 2013 Received in revised form 18 September 2013 Accepted 14 November 2013 Available online 21 November 2013 Keywords: Cu nanoparticle Ink Conductive pattern Oxidation Sintering
a b s t r a c t In this study, pure Cu nanoparticles (NPs) have been successfully synthesized and the Cu nano-ink was prepared for direct writing on photo paper using a roller pen. The tri-sodium citrate was used as initial reducing-cum-surfactant agent followed by hydrazine as a second massive reducing agent and cetyltrimethylammonium bromide (CTAB) as extra surfactant agent. From the XRD, TEM, and HRTEM analyses, the synthesized particles are confirmed to be Cu in spherical shape with sizes range of 2.5 ± 1.0 nm. By analyzing the FT-IR spectroscopy and TGA curves, it was found that the obtained particles capped with tri-sodium citrate and CTAB layers are stable to oxidation up to the temperature 228 ◦ C. The reduced size and enhanced air-stability of the Cu NPs result in an improved particle density upon sintering, which is mainly responsible for the increased conductivity of the Cu patterns. The resistivity of Cu patterns sintered in Ar at 160 ◦ C for 2 h is 7.2 ± 0.6 cm, which is 4.40 times the bulk Cu resistivity. The drawn Cu lines exhibited excellent integrity and good conductivity, which were experimentally tested. Moreover, a Cu electrode and a sample RFID antenna were successfully made. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The preparation of conductive patterns on flexible substrate is vital in the printed electronics and has been achieved for various applications, including flexible displays [1], flexible antennas [2], solar cells [3], transistors [4], sensors [5], batteries [6], etc. Traditionally, photolithography technology is widely used to produce very precise thin circuits in flexible electronics [7]. However, this method requires expensive facilities and involves etching, patterning, electroplating, etc., so it is an uneconomical, complicated and time consuming process. Therefore, many new manufacturing techniques for fabricating conductive patterns in flexible electronics are becoming more important and expected, such as inkjet printing [8], sputter coating [9], airbrush spraying [10]. However, these methods are either costly or some pollution and waste cannot be avoided. In this regard, the convenient and low-cost pen-onpaper approach, using a roller pen filled with conductive ink to directly write conductive patterns on paper substrate, is considered as a promising alternative technology [11,12]. Photo paper as a flexible substrate is newly explored to produce flexible patterns. This is because photo paper has a set of advantages over conventional flexible substrate (polyimide plastics) as follows:
∗ Corresponding author. Tel.: +86 22 60215845. E-mail addresses: [email protected], [email protected] (M. Chen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.057
(1) paper is ubiquitous and inexpensive; (2) a photo paper, which is porous and has a good graphomotor property as well as a low surface roughness, guarantees the flexibility of the substrate and excellent adsorption ability between the ink and the substrate; (3) paper can be trimmed, rolled, folded into 3D configurations or as one part of other devices, such as micro-portable analytical devices (mPADS). For these reasons, photo paper has attracted more and more attention. Russo [11] and Tai and Yang [12] reported the fabrication of conductive electronic arts and practical circuit on paper, respectively. Pen-on-paper approach requires conductive liquid phase materials, i.e., inks, to be printed through a pen head, from which the conductive patterns forms after sintering. Up to now, carbon [13], conductive polymers [14] or metal–organic complexes [15] as conductive ink materials have been used in the formation of conductive patterns or lines, but these materials typically exhibit low electrical conductivity and low resolution. In contrast, many metallic particles that possess high conductivity can be used as the alternative materials. Most recent studies have focused on noble metals such as gold and silver NPs [16–18]. However, their high costs limit their applications. Thus, copper is highly anticipated because it is highly conductive but much cheaper than silver and gold [19,20]. The main challenge to adopt copper NPs is that they are easily oxidized into either Cu2 O or CuO because the oxides phases are more thermodynamically stable than pure Cu. In order to obtain air stable copper particles, some previous papers have reported to synthesize
W. Li, M. Chen / Applied Surface Science 290 (2014) 240–245
Cu NPs in a glove box [21] or nitrogen atmosphere [22], but the copper oxide was easily formed when they exposed in air. As is well known, the presence of Cu2 O or CuO on Cu NPs surface is harmful to the pattern conductivity and results in increasing the sintering temperature of the pattern. In this regard, the synthesis of Cu NPs with anti-oxidation stability is a prerequisite to develop a highly conductive Cu pattern. In this paper, a facile and promising approach is used to fabricate a paper-based Cu conductive pattern for flexible electronics by direct writing. First, air stable Cu NPs were synthesized using CTAB as a capping polymer layer under tri-sodium citrate and massive hydrazine, from which a conductive ink applicable for direct writing was prepared. Second, a kind of roller pen as the writing implement was designed, and prepared by filling the Cu nano-ink into the pipe of the pen. Third, ordinary photo paper as the substrate guarantees the flexibility and fold ability of the Cu pattern on photo paper. The drawn Cu lines and electrode as well as prepared RFID antenna were conductive and flexible after sintering at 160 ◦ C for 2 h under Ar atmosphere.
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Fig. 1. Schematic illustration for the preparation of conductive patterns by writing on photo paper using a roller pen.
2. Experimental 2.1. Materials The cetyltrimethylammonium bromide (CTAB), diethylene glycol (DEG) were purchased from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. Copper sulfate pentahydrate (CuSO4 ·5H2 O), tri-sodium citrate (Na3 C6 H5 O7 ), hydrazine ((N2 H4 ·H2 O), ethanol, ethylene glycol (EG) and glycerol were all obtained from Chemical Reagent Company, Tianjin, China. All these agents were used without further purification. The distilled water was used in all experiments. 2.2. Preparation of Cu NPs and Cu nano-ink For a typical preparation of Cu NPs in ambient atmosphere, 0.35 g (0.96 mmol) of CTAB, 0.5 g (0.02 mmol) of CuSO4 ·5H2 O were added to a single-neck round-bottom flask containing 20 ml DEG and dissolved with magnetic stirring. Then 10 ml distilled water with 1.0 g (3.8 mmol) tri-sodium citrate added. The mixture solutions were heated to 80 ◦ C and 20 ml (1.6 M) hydrazine was successively into the solution at a rate of 9.6 ml/min. After reaction for 3 min, the solutions were cooled down to room temperature and the stirring was continued for an additional 2 h. The resulting particles were separated by centrifugation and washed with ethanol. To build a surface tension gradient between the solvents to acquire high quality Cu nano-ink, the obtained Cu NPs were redissolved into the mixture of water, ethanol, glycerol and ethylene glycol (their corresponding surface tension were 72.8, 22.4, 63.3, 48.5 mN/m, respectively) with a corresponding vol% of about 23.0:8.0:30.5:38.5. The Cu NPs were dispersed by ultrasonic treatment to obtain 35 wt% Cu nano-ink and used for drawing flexible printed patterns. 2.3. Preparation of Cu patterns The Cu patterns were created by directly writing on a flexible photo paper using a roller pen loaded with Cu ink. The schematic illustration is shown in Fig. 1. First, the empty pipe of the roller pen was washed and the photo paper is used as the flexible substrate without any other treatment. Second, a certain amount of Cu ink (35 wt%) was injected by a syringe. Third, the patterns we designed can be easily drawn by this roller pen on photo paper. Finally, the Cu
Fig. 2. X-ray diffraction patterns for Cu NPs: (a) as-prepared at 80 ◦ C and (b) after storing in a capped bottle under atmospheric condition for 6 months.
pattern (together with the photo paper) was sintered under argon atmosphere at 80–160 ◦ C for different time. 3. Characterizations The purity of the Cu NPs was investigated by X-ray powder diffraction (XRD, D/Max 2500pc, Rigaku, Japan) with CuKa radiation ˚ The morphology and size distribution of the parti( = 1.54059 A). cles were characterized by transmission electron microscopy (TEM, JEM-2100, Japan) at 200 kV. The dried Cu particles were grinded with KBr and compressed into a circle flake for FT-IR analysis. FTIR spectra were performed and recorded with a Fourier-transform infrared spectrophotometer (Nicolet 6700) between 4000 cm−1 and 400 cm−1 . Thermogravimetric study of copper NPs was performed with an NETZSCH STA 499 C thermal gravity analyzer. The particles were heated up to 600 ◦ C at a ramped temperature of 10 ◦ C min−1 in air. The resistivity of the written patterns was measured by a 4-point probe (CMT-SR200 N, Chang Min Co., Ltd., Korea). The morphology of the copper patterns after sintering was investigated by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) with a voltage of 2.0 kV. 4. Results and discussion 4.1. Characteristics of Cu nanoparticles Fig. 2 shows the XRD patterns of the synthesized copper NPs depending on storage conditions, which were first dried in a vacuum oven at 60 ◦ C for 3 h and then stored in a capped bottle under
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W. Li, M. Chen / Applied Surface Science 290 (2014) 240–245
Fig. 3. (a) TEM image of typical synthesized copper particles, inset: Cu nano-ink and (b) the particle size distribution obtained by image analysis. (c) HR-TEM image, inset: high-resolution image and corresponding FFT patterns (circle).
ambient conditions. For the as-synthesized particles, three main characteristic peaks at 2 = 43.3◦ , 50.4◦ , and 74.08◦ , corresponding to diffraction peaks of (1 1 1), (2 0 0), and (2 2 0) of face-centered cubic (fcc) Cu are presented. After the synthesized Cu particles are stored for 6 months, it also exhibits the similar XRD patterns without any copper oxides as a secondary phase, suggesting that the Cu NPs are relatively stable against oxidation. The anti-oxidation stability is believed to attribute to the high reducing ability of hydrazine and the presence of CTAB capping layer on the Cu surface. In the synthesis process, the formation of Cu2 O or CuO is actually prevented by excessful hydrazine. Moreover, CTAB layer on the surface of the Cu NPs protects Cu particles from oxidation acting as a diffusion barrier. Fig. 3a shows the TEM images of Cu particles as-synthesized. It exhibits that the uniform particles are spherical with a narrow size distribution of 2.5 ± 1.0 nm (Fig. 3b). The lattice fringes shown in Fig. 3c are visible with a spacing of about 0.2088 nm, which corresponds to the lattice spacing of the (1 1 1) planes of Cu. In the synthesis of Cu particles, tri-sodium citrate as a weak reducingcum-surfactant agent has two effects: one is to control the reducing power with strong reducing agent-hydrazine to form a fast nucleation and a slow growth period to prepare uniform nanoparticles, which is similar to the synthesis of Ag NPs [23]; and another is to help the CTAB to adsorb onto the particles promptly to inhibit their growth. The formulated dark red Cu nano-ink is shown in the insert of Fig. 3a. The Cu nano-ink, containing a solid content of 35%, is stable with keeping in a bottle over one month. The inks with smaller NPs size (<10 nm) should have good printed image quality and better print head reliability [24]. In addition, when particle sizes are reduced to nanoscale, sintering can occur at a lower temperature than the melting point of bulk material because specific surface area increases with decreasing the particle size [25]. Thus, the use of the Cu nano-ink with these ultra-small particles could allow us to develop highly conductive patterns at lower sintering temperature. The FTIR spectra of pure CTAB and CTAB-Cu are presented in Fig. 4. Compared with the pure CTAB spectrum, the peaks of the
Fig. 4. FT-IR spectra of pure CTAB and CTAB-Cu nanoparticles.
asym(C–H) of CH3 –N+ moiety at 1487, 1462 cm−1 combine into 1460 cm−1 and the CH3 –N+ bands at 961 cm−1 are split into three peaks at 1125 cm−1 , 1024 cm−1 , 841 cm−1 [26]. This confirms that the interaction between CTAB molecules and copper NPs is via their (CH3 )3 N+ - head group. Moreover, the newly split peaks at 1638 cm−1 and 1618 cm−1 might be the symmetric stretching of COO− of tri-sodium citrate [27]. Fig. 5 shows the TGA/DTA curve of Cu NPs. No exothermic peak or endothermic peak is detected at a temperature less than 228 ◦ C in the DTA curve, indicating that no remarkable oxidation of Cu NPs happens at this stage. Meanwhile a major mass loss (∼11 wt%) is found between room temperature and 228 ◦ C, which should be the decomposition of citrate sodium layer adsorbed on the Cu surfaces
W. Li, M. Chen / Applied Surface Science 290 (2014) 240–245
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Fig. 5. TGA-DTA results for CTAB-Cu nanoparticles.
Fig. 6. SEM images of the drawn lines after sintering for 120 min at: (a) 80 ◦ C, (b) 120 ◦ C and (c) 160 ◦ C.
[28,29]. After 228 ◦ C, the TGA curve begins to increase. It means that the Cu oxide forms, which results in weight gain of the sample. Based on the previous reports [30,31], the weight gain of the sample in TGA curve could be divided into three parts: the first stage from 228 ◦ C to 272 ◦ C is a fast weight gain, contributing to the reaction of Cu to CuOx (x – 0.67); the second stage from 272 ◦ C to 375 ◦ C is a slow weight gain, meaning the slow reaction of CuOx (x − 0.67) to CuOx (x − 1); the curve of the third stage from 375 ◦ C to 600 ◦ C is attributed to the reaction of CuOx (x − 1) to CuO. Moreover, in all these stages, the decomposition of CTAB is happening [32]. From the calculation of TG curve, the total mass of the sample increases to 5 wt%, which is lower than the theoretical mass gain of 20% (=(1 − 63.5/79.5) × 100; 63.5 and 79.5 are the molar mass of Cu and CuO, respectively). This result confirms that the weight gain by ongoing copper oxidation is superposed by still CTAB decomposition. Based on the analysis mentioned above, to guarantee anti-oxidation of the Cu NPs (≤228 ◦ C) and the flexibility of paper substrate (≤160 ◦ C), the Cu pattern together with the photo paper is sintered at 160 ◦ C. 5. Microstructures and conductivity of the Cu patterns Fig. 6 shows the microstructures of the written patterns sintered at different temperatures for 120 min. After sintering at 80 ◦ C, though the non-uniform copper NPs are connected each other, the connection is loose (Fig. 6a). With increasing the temperature to 120 ◦ C, the surface diffusion of unstable surface atoms becomes easier, more grains grows quickly as expense of the small particles, which improves the connection between the particles (Fig. 6b). When the sintering temperature increases to
Fig. 7. Resistivity variation of the drawn lines after sintering at 160 ◦ C for different time.
160 ◦ C, the connection between the particles is close integration and a dense structure forms (Fig. 6c). This result demonstrates that the patterns become much more densified after sintering, which greatly decreases the lower resistivity of the patterns. From the TGA analysis (Fig. 5), CTAB molecules capped on the Cu surfaces are not completely decomposed at 160 ◦ C, but the residual was small enough not to adversely affect the electrical conductivity [7]. To decrease the residual capping molecules, increasing sintering time is vital and inevitable. The resistivity curves of the written patterns sintered at 160 ◦ C for various time are shown in Fig. 7. The thickness and width of Cu line for evaluating electrical properties are 15 ± 0.5 m and 100 ± 2 m, respectively (Fig. 8a). The resistivity greatly decreases with increasing sintering time. When the sintering time is 120 min, the lowest resistivity is achieved
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5.1. The application of the Cu pattern To demonstrate the applicability of the mixed Cu nano-ink, a series of Cu lines or patterns are designed, as shown in Fig. 8. The Cu line made on the photo paper is shiny (Fig. 8a). To test the conductivity of the Cu line, a conductive wire is connected to the positive and negative electrodes of a cell (not shown here). Interestingly, when the conductive wire is connected to the written conductive line, a lamp can immediately be turned on, demonstrating that conduction path between the particles is established and the Cu line is conductive. Moreover, the Cu electrode designed with metallic brightness (Fig. 8b), which can be used in solar cells or triboelectronic nanogenerators, is flexible and stable in air. When the substrate is bent, the Cu film is stable on the substrate without detachment from it. An example of Cu RFID antenna made on the photo paper also show metallic brightness (Fig. 8c). These results have demonstrated that the Cu ink with nano-sized Cu NPs containing the mixed solvents is appropriate to be used as the conductive materials and the photo paper guarantees the flexibility and foldability of the substrate. Furthermore, the other conductive and flexible printed patterns or electronics, such as flexible displays, flexible solar cells, flexible transistors, flexible sensors, etc. can be fabricated at low sintering temperature using this simple way. Although paper-based conductive patterns have several limitations including mechanical/electrical fatigue, less thermal stability, and low strength, etc., the new kind of roller pen filled with the cheaper Cu nano-ink provides an inexpensive, facile and promising approach to prepare conductive patterns for portable applications where a pattern is required. These types of patterns may form the first step in the integration of electronics in materials that are present in all areas of modern society. 6. Conclusion In this paper, air stable Cu NPs with a diameter of 2.5 ± 1.0 nm were reduced by citrate tri-sodium and the massive hydrazine from Cu2+ and protected by CTAB molecules, from which a conductive ink applicable to writing was prepared. Cu patterns were fabricated on photo paper substrate using a roller pen filled with the Cu nano-ink. A series of Cu patterns were designed, such as lines, electrode and RFID antenna, etc. The lowest resistivity of Cu patterns sintered in Ar at 160 ◦ C for 2 h is 7.2 ± 0.6 cm, 4.40 times higher than the bulk Cu wire resistivity. Their conductivity was also tested by a lamp and they were flexibility when the substrate was bent. This is a simple and promising approach to prepare conductive patterns for portable applications where a pattern is required. Fig. 8. Applications of paper-based Cu conductive patterns drawn using a roller pen and sintered at 160 ◦ C for 2 h: (a) ordinary small bulb operation and (b) a Cu electrode on flexible photo paper by directly written; (c) an example of an RFID antenna.
to 7.2 ± 0.6 cm. Compared to bulk Cu wire (1.72 cm), the resistivity of the pattern is not much higher, presenting a better conductive property. Further sintering for 140 min or 160 min leads to a slightly higher resistivity, this might be caused by the damage of the photo paper, which decreases the adhesion stability of the pattern on the photo paper and increases the resistance. Based on the analyses mentioned above, both the smaller particles (2.5 ± 1.0 nm) and the purity of the Cu NPs favor the densification of Cu NPs at a lower sintering temperature (160 ◦ C), compared with the particles with size 45 ± 8 nm and obvious densification begins at temperature of 300 ◦ C [33]. Furthermore, the sintering condition in Ar is also an important factor because it helps to avoid rapid oxidation of the Cu films in sintering process.
Acknowledgments The authors acknowledge the financial support for this work from the National Nature Science Foundation of China (No. 51071108), Tianjin Key subject for Materials Physics and Chemistry, Key Laboratory of Display Materials & Photoelectric Devices, and Tianjin Key Lab for Photoelectric Materials & Devices. References [1] Y. Chen, J. Au, P. Kazlas, A. Ritenour, H. Gates, M. McCreary, Flexible activematrix electronic ink display, Nature 423 (2003) 136–137. [2] K. Woo, D. Kim, J.S. Kim, S. Lim, J. Moon, Ink-jet printing of Cu–Ag-based highly conductive tracks on a transparent substrate, Langmuir 25 (2009) 429–433. [3] K. Otte, L. Makhova, A. Braun, I. Konovalov, Flexible Cu(In,Ga)Se2 thin-film solar cells for space application, Thin Solid Films 511 (2006) 613–622. [4] H. Okimoto, T. Takenobu, K. Yanagi, Y. Miyata, H. Shimotani, H. Kataura, Y. Iwasa, Tunable carbon nanotube thin-film transistors produced exclusively via inkjet printing, Advanced Materials 22 (2010) 3981–3986. [5] C.T. Wang, K.Y. Huang, D.T.W. Lin, W.C. Liao, H.W. Lin, Y.C. Hu, A flexible proximity sensor fully fabricated by inkjet printing, Sensors 10 (2010) 5054–5062.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of stable ultra-small Cu nanoparticles for direct writing flexible electronics Wei Li a,b , Minfang Chen b,∗ a b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China
a r t i c l e
i n f o
Article history: Received 15 August 2013 Received in revised form 18 September 2013 Accepted 14 November 2013 Available online 21 November 2013 Keywords: Cu nanoparticle Ink Conductive pattern Oxidation Sintering
a b s t r a c t In this study, pure Cu nanoparticles (NPs) have been successfully synthesized and the Cu nano-ink was prepared for direct writing on photo paper using a roller pen. The tri-sodium citrate was used as initial reducing-cum-surfactant agent followed by hydrazine as a second massive reducing agent and cetyltrimethylammonium bromide (CTAB) as extra surfactant agent. From the XRD, TEM, and HRTEM analyses, the synthesized particles are confirmed to be Cu in spherical shape with sizes range of 2.5 ± 1.0 nm. By analyzing the FT-IR spectroscopy and TGA curves, it was found that the obtained particles capped with tri-sodium citrate and CTAB layers are stable to oxidation up to the temperature 228 ◦ C. The reduced size and enhanced air-stability of the Cu NPs result in an improved particle density upon sintering, which is mainly responsible for the increased conductivity of the Cu patterns. The resistivity of Cu patterns sintered in Ar at 160 ◦ C for 2 h is 7.2 ± 0.6 cm, which is 4.40 times the bulk Cu resistivity. The drawn Cu lines exhibited excellent integrity and good conductivity, which were experimentally tested. Moreover, a Cu electrode and a sample RFID antenna were successfully made. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The preparation of conductive patterns on flexible substrate is vital in the printed electronics and has been achieved for various applications, including flexible displays [1], flexible antennas [2], solar cells [3], transistors [4], sensors [5], batteries [6], etc. Traditionally, photolithography technology is widely used to produce very precise thin circuits in flexible electronics [7]. However, this method requires expensive facilities and involves etching, patterning, electroplating, etc., so it is an uneconomical, complicated and time consuming process. Therefore, many new manufacturing techniques for fabricating conductive patterns in flexible electronics are becoming more important and expected, such as inkjet printing [8], sputter coating [9], airbrush spraying [10]. However, these methods are either costly or some pollution and waste cannot be avoided. In this regard, the convenient and low-cost pen-onpaper approach, using a roller pen filled with conductive ink to directly write conductive patterns on paper substrate, is considered as a promising alternative technology [11,12]. Photo paper as a flexible substrate is newly explored to produce flexible patterns. This is because photo paper has a set of advantages over conventional flexible substrate (polyimide plastics) as follows:
∗ Corresponding author. Tel.: +86 22 60215845. E-mail addresses: [email protected], [email protected] (M. Chen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.057
(1) paper is ubiquitous and inexpensive; (2) a photo paper, which is porous and has a good graphomotor property as well as a low surface roughness, guarantees the flexibility of the substrate and excellent adsorption ability between the ink and the substrate; (3) paper can be trimmed, rolled, folded into 3D configurations or as one part of other devices, such as micro-portable analytical devices (mPADS). For these reasons, photo paper has attracted more and more attention. Russo [11] and Tai and Yang [12] reported the fabrication of conductive electronic arts and practical circuit on paper, respectively. Pen-on-paper approach requires conductive liquid phase materials, i.e., inks, to be printed through a pen head, from which the conductive patterns forms after sintering. Up to now, carbon [13], conductive polymers [14] or metal–organic complexes [15] as conductive ink materials have been used in the formation of conductive patterns or lines, but these materials typically exhibit low electrical conductivity and low resolution. In contrast, many metallic particles that possess high conductivity can be used as the alternative materials. Most recent studies have focused on noble metals such as gold and silver NPs [16–18]. However, their high costs limit their applications. Thus, copper is highly anticipated because it is highly conductive but much cheaper than silver and gold [19,20]. The main challenge to adopt copper NPs is that they are easily oxidized into either Cu2 O or CuO because the oxides phases are more thermodynamically stable than pure Cu. In order to obtain air stable copper particles, some previous papers have reported to synthesize
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Cu NPs in a glove box [21] or nitrogen atmosphere [22], but the copper oxide was easily formed when they exposed in air. As is well known, the presence of Cu2 O or CuO on Cu NPs surface is harmful to the pattern conductivity and results in increasing the sintering temperature of the pattern. In this regard, the synthesis of Cu NPs with anti-oxidation stability is a prerequisite to develop a highly conductive Cu pattern. In this paper, a facile and promising approach is used to fabricate a paper-based Cu conductive pattern for flexible electronics by direct writing. First, air stable Cu NPs were synthesized using CTAB as a capping polymer layer under tri-sodium citrate and massive hydrazine, from which a conductive ink applicable for direct writing was prepared. Second, a kind of roller pen as the writing implement was designed, and prepared by filling the Cu nano-ink into the pipe of the pen. Third, ordinary photo paper as the substrate guarantees the flexibility and fold ability of the Cu pattern on photo paper. The drawn Cu lines and electrode as well as prepared RFID antenna were conductive and flexible after sintering at 160 ◦ C for 2 h under Ar atmosphere.
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Fig. 1. Schematic illustration for the preparation of conductive patterns by writing on photo paper using a roller pen.
2. Experimental 2.1. Materials The cetyltrimethylammonium bromide (CTAB), diethylene glycol (DEG) were purchased from Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. Copper sulfate pentahydrate (CuSO4 ·5H2 O), tri-sodium citrate (Na3 C6 H5 O7 ), hydrazine ((N2 H4 ·H2 O), ethanol, ethylene glycol (EG) and glycerol were all obtained from Chemical Reagent Company, Tianjin, China. All these agents were used without further purification. The distilled water was used in all experiments. 2.2. Preparation of Cu NPs and Cu nano-ink For a typical preparation of Cu NPs in ambient atmosphere, 0.35 g (0.96 mmol) of CTAB, 0.5 g (0.02 mmol) of CuSO4 ·5H2 O were added to a single-neck round-bottom flask containing 20 ml DEG and dissolved with magnetic stirring. Then 10 ml distilled water with 1.0 g (3.8 mmol) tri-sodium citrate added. The mixture solutions were heated to 80 ◦ C and 20 ml (1.6 M) hydrazine was successively into the solution at a rate of 9.6 ml/min. After reaction for 3 min, the solutions were cooled down to room temperature and the stirring was continued for an additional 2 h. The resulting particles were separated by centrifugation and washed with ethanol. To build a surface tension gradient between the solvents to acquire high quality Cu nano-ink, the obtained Cu NPs were redissolved into the mixture of water, ethanol, glycerol and ethylene glycol (their corresponding surface tension were 72.8, 22.4, 63.3, 48.5 mN/m, respectively) with a corresponding vol% of about 23.0:8.0:30.5:38.5. The Cu NPs were dispersed by ultrasonic treatment to obtain 35 wt% Cu nano-ink and used for drawing flexible printed patterns. 2.3. Preparation of Cu patterns The Cu patterns were created by directly writing on a flexible photo paper using a roller pen loaded with Cu ink. The schematic illustration is shown in Fig. 1. First, the empty pipe of the roller pen was washed and the photo paper is used as the flexible substrate without any other treatment. Second, a certain amount of Cu ink (35 wt%) was injected by a syringe. Third, the patterns we designed can be easily drawn by this roller pen on photo paper. Finally, the Cu
Fig. 2. X-ray diffraction patterns for Cu NPs: (a) as-prepared at 80 ◦ C and (b) after storing in a capped bottle under atmospheric condition for 6 months.
pattern (together with the photo paper) was sintered under argon atmosphere at 80–160 ◦ C for different time. 3. Characterizations The purity of the Cu NPs was investigated by X-ray powder diffraction (XRD, D/Max 2500pc, Rigaku, Japan) with CuKa radiation ˚ The morphology and size distribution of the parti( = 1.54059 A). cles were characterized by transmission electron microscopy (TEM, JEM-2100, Japan) at 200 kV. The dried Cu particles were grinded with KBr and compressed into a circle flake for FT-IR analysis. FTIR spectra were performed and recorded with a Fourier-transform infrared spectrophotometer (Nicolet 6700) between 4000 cm−1 and 400 cm−1 . Thermogravimetric study of copper NPs was performed with an NETZSCH STA 499 C thermal gravity analyzer. The particles were heated up to 600 ◦ C at a ramped temperature of 10 ◦ C min−1 in air. The resistivity of the written patterns was measured by a 4-point probe (CMT-SR200 N, Chang Min Co., Ltd., Korea). The morphology of the copper patterns after sintering was investigated by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) with a voltage of 2.0 kV. 4. Results and discussion 4.1. Characteristics of Cu nanoparticles Fig. 2 shows the XRD patterns of the synthesized copper NPs depending on storage conditions, which were first dried in a vacuum oven at 60 ◦ C for 3 h and then stored in a capped bottle under
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Fig. 3. (a) TEM image of typical synthesized copper particles, inset: Cu nano-ink and (b) the particle size distribution obtained by image analysis. (c) HR-TEM image, inset: high-resolution image and corresponding FFT patterns (circle).
ambient conditions. For the as-synthesized particles, three main characteristic peaks at 2 = 43.3◦ , 50.4◦ , and 74.08◦ , corresponding to diffraction peaks of (1 1 1), (2 0 0), and (2 2 0) of face-centered cubic (fcc) Cu are presented. After the synthesized Cu particles are stored for 6 months, it also exhibits the similar XRD patterns without any copper oxides as a secondary phase, suggesting that the Cu NPs are relatively stable against oxidation. The anti-oxidation stability is believed to attribute to the high reducing ability of hydrazine and the presence of CTAB capping layer on the Cu surface. In the synthesis process, the formation of Cu2 O or CuO is actually prevented by excessful hydrazine. Moreover, CTAB layer on the surface of the Cu NPs protects Cu particles from oxidation acting as a diffusion barrier. Fig. 3a shows the TEM images of Cu particles as-synthesized. It exhibits that the uniform particles are spherical with a narrow size distribution of 2.5 ± 1.0 nm (Fig. 3b). The lattice fringes shown in Fig. 3c are visible with a spacing of about 0.2088 nm, which corresponds to the lattice spacing of the (1 1 1) planes of Cu. In the synthesis of Cu particles, tri-sodium citrate as a weak reducingcum-surfactant agent has two effects: one is to control the reducing power with strong reducing agent-hydrazine to form a fast nucleation and a slow growth period to prepare uniform nanoparticles, which is similar to the synthesis of Ag NPs [23]; and another is to help the CTAB to adsorb onto the particles promptly to inhibit their growth. The formulated dark red Cu nano-ink is shown in the insert of Fig. 3a. The Cu nano-ink, containing a solid content of 35%, is stable with keeping in a bottle over one month. The inks with smaller NPs size (<10 nm) should have good printed image quality and better print head reliability [24]. In addition, when particle sizes are reduced to nanoscale, sintering can occur at a lower temperature than the melting point of bulk material because specific surface area increases with decreasing the particle size [25]. Thus, the use of the Cu nano-ink with these ultra-small particles could allow us to develop highly conductive patterns at lower sintering temperature. The FTIR spectra of pure CTAB and CTAB-Cu are presented in Fig. 4. Compared with the pure CTAB spectrum, the peaks of the
Fig. 4. FT-IR spectra of pure CTAB and CTAB-Cu nanoparticles.
asym(C–H) of CH3 –N+ moiety at 1487, 1462 cm−1 combine into 1460 cm−1 and the CH3 –N+ bands at 961 cm−1 are split into three peaks at 1125 cm−1 , 1024 cm−1 , 841 cm−1 [26]. This confirms that the interaction between CTAB molecules and copper NPs is via their (CH3 )3 N+ - head group. Moreover, the newly split peaks at 1638 cm−1 and 1618 cm−1 might be the symmetric stretching of COO− of tri-sodium citrate [27]. Fig. 5 shows the TGA/DTA curve of Cu NPs. No exothermic peak or endothermic peak is detected at a temperature less than 228 ◦ C in the DTA curve, indicating that no remarkable oxidation of Cu NPs happens at this stage. Meanwhile a major mass loss (∼11 wt%) is found between room temperature and 228 ◦ C, which should be the decomposition of citrate sodium layer adsorbed on the Cu surfaces
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Fig. 5. TGA-DTA results for CTAB-Cu nanoparticles.
Fig. 6. SEM images of the drawn lines after sintering for 120 min at: (a) 80 ◦ C, (b) 120 ◦ C and (c) 160 ◦ C.
[28,29]. After 228 ◦ C, the TGA curve begins to increase. It means that the Cu oxide forms, which results in weight gain of the sample. Based on the previous reports [30,31], the weight gain of the sample in TGA curve could be divided into three parts: the first stage from 228 ◦ C to 272 ◦ C is a fast weight gain, contributing to the reaction of Cu to CuOx (x – 0.67); the second stage from 272 ◦ C to 375 ◦ C is a slow weight gain, meaning the slow reaction of CuOx (x − 0.67) to CuOx (x − 1); the curve of the third stage from 375 ◦ C to 600 ◦ C is attributed to the reaction of CuOx (x − 1) to CuO. Moreover, in all these stages, the decomposition of CTAB is happening [32]. From the calculation of TG curve, the total mass of the sample increases to 5 wt%, which is lower than the theoretical mass gain of 20% (=(1 − 63.5/79.5) × 100; 63.5 and 79.5 are the molar mass of Cu and CuO, respectively). This result confirms that the weight gain by ongoing copper oxidation is superposed by still CTAB decomposition. Based on the analysis mentioned above, to guarantee anti-oxidation of the Cu NPs (≤228 ◦ C) and the flexibility of paper substrate (≤160 ◦ C), the Cu pattern together with the photo paper is sintered at 160 ◦ C. 5. Microstructures and conductivity of the Cu patterns Fig. 6 shows the microstructures of the written patterns sintered at different temperatures for 120 min. After sintering at 80 ◦ C, though the non-uniform copper NPs are connected each other, the connection is loose (Fig. 6a). With increasing the temperature to 120 ◦ C, the surface diffusion of unstable surface atoms becomes easier, more grains grows quickly as expense of the small particles, which improves the connection between the particles (Fig. 6b). When the sintering temperature increases to
Fig. 7. Resistivity variation of the drawn lines after sintering at 160 ◦ C for different time.
160 ◦ C, the connection between the particles is close integration and a dense structure forms (Fig. 6c). This result demonstrates that the patterns become much more densified after sintering, which greatly decreases the lower resistivity of the patterns. From the TGA analysis (Fig. 5), CTAB molecules capped on the Cu surfaces are not completely decomposed at 160 ◦ C, but the residual was small enough not to adversely affect the electrical conductivity [7]. To decrease the residual capping molecules, increasing sintering time is vital and inevitable. The resistivity curves of the written patterns sintered at 160 ◦ C for various time are shown in Fig. 7. The thickness and width of Cu line for evaluating electrical properties are 15 ± 0.5 m and 100 ± 2 m, respectively (Fig. 8a). The resistivity greatly decreases with increasing sintering time. When the sintering time is 120 min, the lowest resistivity is achieved
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5.1. The application of the Cu pattern To demonstrate the applicability of the mixed Cu nano-ink, a series of Cu lines or patterns are designed, as shown in Fig. 8. The Cu line made on the photo paper is shiny (Fig. 8a). To test the conductivity of the Cu line, a conductive wire is connected to the positive and negative electrodes of a cell (not shown here). Interestingly, when the conductive wire is connected to the written conductive line, a lamp can immediately be turned on, demonstrating that conduction path between the particles is established and the Cu line is conductive. Moreover, the Cu electrode designed with metallic brightness (Fig. 8b), which can be used in solar cells or triboelectronic nanogenerators, is flexible and stable in air. When the substrate is bent, the Cu film is stable on the substrate without detachment from it. An example of Cu RFID antenna made on the photo paper also show metallic brightness (Fig. 8c). These results have demonstrated that the Cu ink with nano-sized Cu NPs containing the mixed solvents is appropriate to be used as the conductive materials and the photo paper guarantees the flexibility and foldability of the substrate. Furthermore, the other conductive and flexible printed patterns or electronics, such as flexible displays, flexible solar cells, flexible transistors, flexible sensors, etc. can be fabricated at low sintering temperature using this simple way. Although paper-based conductive patterns have several limitations including mechanical/electrical fatigue, less thermal stability, and low strength, etc., the new kind of roller pen filled with the cheaper Cu nano-ink provides an inexpensive, facile and promising approach to prepare conductive patterns for portable applications where a pattern is required. These types of patterns may form the first step in the integration of electronics in materials that are present in all areas of modern society. 6. Conclusion In this paper, air stable Cu NPs with a diameter of 2.5 ± 1.0 nm were reduced by citrate tri-sodium and the massive hydrazine from Cu2+ and protected by CTAB molecules, from which a conductive ink applicable to writing was prepared. Cu patterns were fabricated on photo paper substrate using a roller pen filled with the Cu nano-ink. A series of Cu patterns were designed, such as lines, electrode and RFID antenna, etc. The lowest resistivity of Cu patterns sintered in Ar at 160 ◦ C for 2 h is 7.2 ± 0.6 cm, 4.40 times higher than the bulk Cu wire resistivity. Their conductivity was also tested by a lamp and they were flexibility when the substrate was bent. This is a simple and promising approach to prepare conductive patterns for portable applications where a pattern is required. Fig. 8. Applications of paper-based Cu conductive patterns drawn using a roller pen and sintered at 160 ◦ C for 2 h: (a) ordinary small bulb operation and (b) a Cu electrode on flexible photo paper by directly written; (c) an example of an RFID antenna.
to 7.2 ± 0.6 cm. Compared to bulk Cu wire (1.72 cm), the resistivity of the pattern is not much higher, presenting a better conductive property. Further sintering for 140 min or 160 min leads to a slightly higher resistivity, this might be caused by the damage of the photo paper, which decreases the adhesion stability of the pattern on the photo paper and increases the resistance. Based on the analyses mentioned above, both the smaller particles (2.5 ± 1.0 nm) and the purity of the Cu NPs favor the densification of Cu NPs at a lower sintering temperature (160 ◦ C), compared with the particles with size 45 ± 8 nm and obvious densification begins at temperature of 300 ◦ C [33]. Furthermore, the sintering condition in Ar is also an important factor because it helps to avoid rapid oxidation of the Cu films in sintering process.
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