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Electrodeposition fabrication of [email protected] core shell nanowire network for highly stable transparent conductive films
Journal Pre-proofs Electrodeposition fabrication of Cu@Ni core shell nanowire network for highly stable transparent conductive films He Zhang, Shang Wang, Yanhong Tian, Yubin Liu, Jiayue Wen, Yilong Huang, Chunjin Hang, Zhen Zheng, Chenxi Wang PII: DOI: Reference:
S1385-8947(20)30486-1 https://doi.org/10.1016/j.cej.2020.124495 CEJ 124495
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
30 December 2019 16 February 2020 17 February 2020
Please cite this article as: H. Zhang, S. Wang, Y. Tian, Y. Liu, J. Wen, Y. Huang, C. Hang, Z. Zheng, C. Wang, Electrodeposition fabrication of Cu@Ni core shell nanowire network for highly stable transparent conductive films, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124495
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Electrodeposition fabrication of Cu@Ni core shell nanowire network for highly stable transparent conductive films
He Zhanga,†, Shang Wanga,†, Yanhong Tiana,*, Yubin Liub, Jiayue Wena, Yilong Huanga, Chunjin Hang a, Zhen Zhenga, Chenxi Wanga
a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin,
China, 150001 b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, China, 150001
*Email: [email protected], Tel: +86-451-86418359
†These authors contributed equally to this work.
Abstract Copper nanowire (CuNW) is one of the most promising candidates for next-generation transparent conductive film (TCF). However, practical applications of CuNW are still limited by several drawbacks, including loose wire to wire junctions, poor resistance against oxidation, chemical and thermal damage. To concurrently address these urgent issues, highly conductive and stable TCF is prepared based on Cu@Ni core-shell NW networks by electrodeposition. The coated Ni shell can weld stacked NWs tightly, which decreases the sheet resistance from 513 to 15.8 Ohm/sq (at a transmittance of 88%), as well as improves the mechanical stability (1.03-fold resistance increase after 2000 cyclic bending). The Cu@Ni NW TCF exhibits excellent antioxidation performance after storing in atmospheric environment for 168 h. The passivation Ni shell also improvs the chemical and thermal stability. The Cu@Ni NW network keeps intact after 450 s H 2O2 corrosion and heating at 400 °C for 30 min. Moreover, a flexible transparent heater was fabricated based on these TCFs, which shows excellent uniform heating performance and high response speed. Keywords: Copper nanowires, Transparent conductive film, Core-shell, Stability
1. Introduction Over the past decades, transparent conductive film (TCF) has attracted increasing attentions because of their various applications, such as organic solar cells,[1-3] organic light-emitting diodes (OLEDs),[4-6] electrochromic devices[7-9] and transparent heaters.[10-12] Currently, the most commonly used TCF is indium tin oxide film.[13] Although ITO film exhibits satisfactory conductivity and transmittance, the usage of it in flexible device is restricted due to some fatal drawbacks, such as high brittleness, scarcity of indium, and the costly fabrication process.[14-16] Thus far, several materials, including conductive polymer, [17] graphene,[18, 19] carbon nanotubes,[20] metal grids[21] and metal nanowires[22] were proposed to replace ITO for flexible TCF. Among them, metal nanowires were considered as the most promising materials because of their superior conductivity, mechanical flexibility and solution-processability.[23-25] Silver nanowires TCF has been extensively investigated, which possesses comparable optical-electrical with ITO film.[26-28] However, silver is also expensive and easily fails down by electromigrating.[29] Luckily, the electrical conductivity of copper is similar to that of silver, while the price of copper is 100-times cheaper than silver.[30]
Therefore, CuNWs are considered to have the potential to fabricate TCFs with high-performance and low-cost.[31, 32] Nonetheless, several inherent shortcomings regarding CuNWs still need to be addressed for flexible devices. First, CuNW TCFs usually suffer from inadequate conductivity and poor mechanical flexibility because of loose wire-to-wire junctions.[33] The contact resistance and sliding during deformation make them difficult for practical usage. Various methods have been developed to decrease the contact resistance between NWs, such as thermal annealing,[34, 35] photo welding,[36, 37] laser sintering[38] and plasma treatment.[39] Although high performance CuNW TCF can be achieved by these methods, the conductivity of pristine CuNW TCF with extra protection degrades sharply even in atmospheric environment.[40] This is another problem to be solved, that is CuNWs always exhibit poor long-term stability against oxidation and chemical corrosion, which is crucial for some opto-electrical devices requiring fabrication process involving corrosive environment. For improving these stabilities of CuNW TCF, various protective layers were coated, such as Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS),[41] graphene.[42] Nevertheless, the junction between stacked nanowires is still loose. Besides, unselectively coating these low conductivity materials has a negative effect on the electrical and optical performance simultaneously. Wrapping CuNW with noble metals to form core shell structure by galvanic replacing method can selectively coated onto CuNW rather than blank area.[35, 43] However, the noble metals are also costly. Ma et al. successfully fabricated TCF with low cost using Cu@Ni core-shell NWs synthesized by onepot co-reduction of Cu2+ and Ni2+ captions method, but the electrical performance and mechanical stability are still inadequate due to the still loose junctions. Wiley et al. electrodeposited Ni shell onto
CuNW as a functional layer, which showed excellent water oxidation catalytic performance. However, for the application of CuNW TCF in heaters, OLEDs and other fields, the stabilities (resistance to chemical and thermal corrosion) of Cu@Ni core shell NW network are still highly needed to be studied. Hence, a convenient, efficient and cost-effective method which can address these problems concurrently is highly desirable. In this paper, a facile, fast, and low-cost method was introduced to enhance the conductivity and stability of Cu NW TCFs by electrodepositing Ni shell onto CuNW core. During the electrodeposition process, the reduced Ni atoms can fill in the gaps between stacked CuNWs and obtain firm junction, which can effectively improve the conductivity and mechanical stability. Besides, the coated Ni shell can also protect CuNW from oxidation and corrosion. Flexible transparent heaters based on these TCFs can yield uniform heating at low input voltage. This novel method could open the doors for fabrication for high-performance and reliable TCFs. 2. Experimental 2.1 Materials Copper chloride dehydrate, octadecylamine (ODA) and orthoboric acid were purchased from Tianjin Guangfu Chemical Reagent Co., Ltd. Glucose was purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. Ethanol, acetone, isopropanol, n-hexane, glacial acetic acid (GAA) and aqua ammonia were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. Nickel dichloride hexahydrate and ethylenediamine dihydrochloride were purchased from Tianjin Dongli District Tianda Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. 2.2 Electrodeposition fabrication of Cu@Ni NW TCFs
CuNWs were synthesized and purified by our pervious reported method.[44] The as-prepared CuNWs were dispersed in isopropanol with a concentration of 1 mg/mL. After that, CuNWs were filtered on cellulose acetate microfiltration membrane with a pore diameter of 0.45 μm. Then, CuNWs were transferred onto a polyethylene terephthalate (PET) film with a thickness of 0.15 mm by roller pressure. The obtained TCF was dried out by a hot plate at 60 ºC for 5 min. To temporarily increase conductivity, the TCF was immersed into GAA for 40 s to remove the surface organics and oxides. The plating bath consisted of a mix of 1M nickel dichloride, 0.5 M orthoboric acid and 0.5 M ethylenediamine dihydrochloride, and its PH value was adjusted to 4~5 by adding aqua ammonia. To ensure the complete dissolving of the solutes, the solution was stirred at 750 rpm for an hour by magnetic stirrer. The as-prepared CuNW TCF was set as cathode, and a pure Ni plate was used as anode. The electrodeposition process was typically carried out at a current density of 5 A/dm 2 for 10 s. Subsequently, the obtained Cu@Ni NW TCF was washed with deionized water and dried out by hair dryer. 2.3 Characterization The morphologies and microstructures of CuNWs before and after coating Ni shell were observed by a MERLIN Compact scanning electron microscope (SEM) and FEI Tecnai G2 F30 transmission electron microscope (TEM). The valence state of Cu element was analysed by a Thermo ESCALAB 250XI X-ray photoelectron spectrometer. The sheet resistance was tested by an MCP-T370 four-probe square resistance meter from Mitsubishi Chemical Corp. The transmittance was characterized using a UV1600PC UV/vis spectrophotometer from Shanghai Jinghua Technology. The changing of resistance during the bending tests and chemical corrosion tests were measured by a home-made system, which was consisted with a digital mustimeter (34401A, Keysight Technologies, American) and a DC
stabilized power supply (DPS-3005D, Zhaoxin, China). The thermal distribution of the flexible transparent heater was measured by an A-325 infrared camera system from FLIR Systems, Inc. 3 Results and discussion Fig.1 shows the scheme illustration of the fabricating process of Cu@Ni NW TCF. CuNWs were synthesized by a hydrothermal method and purified by an extraction method. Then, CuNWs were filtrated and transferred onto PET substrate to acquire a homogeneously distributed CuNW network. Due to the existence of surface oxides and organics, the TCF at this time is almost non-conductive. GAA treatment was utilized to remove these insulators and increase electrical conductivity. After immersing in GAA for only 40 s, the sheet resistance decreased to 513 Ohm/sq at a transmittance of 97 %. The electrodeposition process was carried at a two-electrode system with a current density of 5A/dm2. The as-prepared TCF was set as cathode and a Ni plate was employed as anode. The electrodeposition time was adjusted to obtain the TCF with different conductivity and transmittance. In a typical process, the time was chosen as 10 s to obtain appropriate sheet resistance and transmittance. Fig.2 a and b show the morphology of CuNW TCF before and after 10 s electrodeposition, respectively. Before electrodeposition, Cu NWs were randomly distributed onto PET substrate and formed a uniform conductive network. At this time, the junctions between NWs were weak physical contact driven by poor gravity and Van der Waals' force. Hence, the junction was not firm enough. Due to the large contact resistance between CuNWs, the electrical conductivity of the electrode was insufficient. A closer contact is highly expected to facilitate the transfer of electrons and decrease the contact resistance. After electrodeposition coating Ni shell, the upper CuNW was embedded into the bottom one at the contact spot, which was not only enlarge conductive area, but also firmly welded them together. The loose overlap was changed into strong welded joint. TEM and high resolution (HR)-
TEM images were collected to further confirm the forming of strong junction, as demonstrated in Fig.2 c and d. The distinct edges of upper and bottom CuNWs along its length were observed at the pristine. However, for the Cu@Ni NWs achieved by electrodeposition, the boundaries of two crossed NWs were interrupted at contact spot, which suggested the Ni shell, formed by the reduced of Ni2+, effectively induced the welding between each other. As shown in inset image of Fig.2 d, it can be observed that the Ni atoms and Cu atoms have been tightly bonded at the junction, which proves that the electrodeposited Ni shell can transform the loose physical contact between NWs into strong metallurgical bonding. The welding mechanism between NWs was shown in Fig.2 e. During the electrodeposition process, Ni2+ in solution would get electrons and be reduced to Ni atoms as a result of current. Subsequently, Ni atoms crystallized and filled the gaps between stacked CuNWs and tightly connected them together. Because only CuNWs that are in contact with each other are electrically conductive, Ni layer will be selectively deposited onto CuNWs rather than the blank areas, and the Cu@Ni core shell NW network can be achieved. Energy dispersive X-ray spectroscopy (EDS) analysis was utilized to characterize the element elemental composition of the NW network, as shown in Fig.S1. In addition to the carbon, oxygen elements (caused by PET) and gold elements (caused by the deposited Au layer for the observation under SEM), only Cu element was observed in the pristine networks. After electrodeposition, obvious Ni peaks were present, indicating that the CuNW network was connected by thin Ni layer. The sheet resistance and transmittance of CuNW TCFs with different electrodeposition time are investigated, as demonstrated in Fig.3 a. In general, the electrodeposition increased but the transmittance decreased over electrodeposition time. Before electrodepositing Ni shell, the sheet resistance was 513 Ohm/sq at a transmittance of 97%. At this time, the junctions between CuNWs were
loose, as shown in the corresponding SEM image (Fig.3 b). After only 5 s electrodepositing time, the sheet resistance was down to 170 Ohm/sq, indicating the increasing of contact area between NWs. The transmittance was still up to 93.2 % due to insignificant increase in NW diameter (Fig.3 c). However, the current electrical conductivity was still unacceptable for practical usage. Further extending the electrodepositing time to 10 s, the sheet resistance reached 15.8 Ohm/sq at a transmittance of 88 %, which can meet the requirements of most devices for optoelectronic performance. It could be observed from Fig.3 d that firm junctions between NWs had formed and the surface of NWs were still smooth. When electrodepositing for 20 s, the sheet resistance showed a slight drop (7.9 Ohm/sq), because NWs had been tightly connected between each other previously. However, the transmittance sharply decreased to 68 %, which was caused by the continuous thickening of nanowires. Moreover, Ni thin film appeared at the blank area between the nanowires (Fig.3 d), which would exacerbate the loss of transmittance. Hence, 10 s was chosen as the optimized process parameter due to the excellent opticalelectrical performance. In the actual application process of flexible devices, mechanical deformation is inevitable. Hence, the mechanical stability of TCF is of great importance. NWs are easily moved between each other during the mechanical deformation process, resulting in the increase of electrical resistance. The bending cycle was performed to evaluate the mechanical stability of the CuNW TCF before and after coating Ni shell. The resistance was in situ monitored during the cyclic bending and releasing process, as shown in Fig.4. Compared with the relative resistance of the pristine CuNW TCF up to 1.98, the Cu@Ni NW TCF exhibited a slight increase (1.03), indicating its excellent performance against deformation. It was concluded that the mechanical stability was validly improved thanks to the formation of firm NWs junctions achieved by electrodeposition.
CuNW is easily oxidized by oxygen and water vapor even in atmospheric environment under room temperature and form copper oxides, which will degrade the electrical conductivity during the long-term service of TCF. In addition to improving the electrical performance of TCF, the coated Ni layer effectively prevented Cu NWs from oxidizing in the air, as indicated in Fig.5. For pristine CuNW TCF without extra protection, after placing in atmospheric environment for 12 h, the sheet resistance increased ten times. As a contrast, the conductivity of Cu@Ni NW TCF was still stable after placing in atmospheric environment for 168 h, showing excellent long-term stability. To further confirm the protective effect of Ni shell, the XPS was employed to characterize the surface chemical state of Cu element in two kinds of TCFs after storing in atmospheric environment for 168 h. The Cu 2p3/2 spectra of pristine CuNW network was fitted to several peaks, including the peak at 931.8 eV assigned to Cu or Cu2O, the peak at 933 eV (and a companion peak) assigned to CuO, the peak assigned to Cu(OH)2 at 933.9 eV, demonstrating the serve oxidation of CuNWs. However, for the Cu@Ni NW TCF, the peak of Cu(OH)2 almost disappeared and the and the peaks of CuO became weak. The results suggested that the electrodeposited Ni inhibitor can effectively protect CuNW from oxidation and improve the longterm stability of the TCF. Chemical stability is an important index for evaluating the performance of flexible electrodes, which determines the service stability of the TCF under corrosive environments. We assessed the chemical stability of the CuNW and Cu@NW TCF by dropping 15 wt. % H2O2 solution to simulate an extreme chemical corrosion environment, and the relative resistance change of TCF was monitored in real time, as shown in Fig.6 a. It can be observed that the relative resistance of the pristine CuNW network increased rapidly less than 70 s, suggesting the NW network has been damaged. However, the chemical stability of the TCF has been significantly improved after electrodepositing. Even after
dropping H2O2 for 450 s, the relative resistance increased insignificantly. SEM images in Fig.6 b-d further confirmed these results. There is a clear boundary between the corroded area and the original area of the TCFs. Further observation of corroded area at high magnification revealed that CuNW network without protection has been dissolved, but Cu@Ni NW network are still intact even though PET substrate has been corroded. The tolerance of high temperature is another urgent issue to be addressed, because heating is an essential process for many flexible devices, such as heaters, OLEDs. However, CuNW is prone to fragment and transfer into discrete nanospheres when the temperature reached only 400 °C which is much below the melting point of bulk copper (1085 °C) due to the Rayleigh instability,[45] resulting in the loss of electrical conductivity. Here in, the pristine CuNW and Cu@Ni NW TCFs were both heated to 400 °C for 30 min, and the morphologies were observed in Fig.7. Obviously, pristine CuNW network has been destroyed, while Cu@Ni NW core shell structure showed negligible change. These results clearly illustrated that the introduction of Ni shell could improve the resistance of thermal corrosion. To demonstrate the potential of the fabricated TCFs in high-performance electronics, a flexible transparent heater based on a Cu@Ni NW TCF electrodeposited for 10 s was constructed. Uniform thermal distribution of the heater was shown in the infrared image (Fig.8 a) taken by a thermal camera at a temperature of 67.2 °C, which indicated that the electrodeposition method effectively joined the stacked CuNWs and prevented the overheating at local area. Stepwise input voltage was applied on the heater to test its performance, as shown in Fig.8 b. Rapid temperature response is observed as voltage steps, indicating its high sensitivity. The application of the heater in different occasions could be adjusted by the input voltage. When applying 5 V voltage, the temperature nearly increased to 47 °C,
which can meet the needs of electric blanket, anti-fog window, etc. Further increasing the input voltage to 10 V, the temperature nearly reached 120 °C, exhibiting its potential in water heating, hightemperature sterilizer field. On the other hand, stable high temperature output capability is a key requirement for high-performance in practical usage. The heater maintained stable temperature output even after prolonged service for 5 h, demonstrating its excellent long-term stability. Conclusions In this work, highly conductive and stable TCFs were successfully fabricated by electrodepositing a passivation Ni shell in air. Because Ag and Au atoms are selectively deposited onto NWs rather than blank area, the TCF exhibited a low sheet resistance (15.8 Ohm/sq) at a high transmittance (88 %). Superior mechanical stability was obtained due to the formation of firm joint between NWs. Moreover, the resistance of Cu@Ni TCF against oxidation was improved, which could stably work in atmospheric environment for a weak. High chemical and thermal stability were also realized after coating the protective Ni shell. A flexible transparent heater based on Cu@Ni NW TCF was successfully prepared, which showed excellent uniform, fast and continuous heating performance (nearly 120 °C at 10 V), indicating its potential applications in next-generation flexible electronics. Conflicts of interest. There are no conflicts of interest to declare. Acknowledgments The authors are grateful for the financial support from the National Key R&D Program of China (no. 2018YFB1307501) and National Natural Science Foundation of China (grant no. 51522503). References
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Figures and captions
Fig.1 Schematic illustration of the fabrication process of Cu@Ni NW TCF.
Fig. 2 SEM images of (a) CuNW network and (b) Cu@Ni NW network. The scale bars in the inset images are 200 nm. TEM image of a junction in (c) Cu NW network and (d) Cu@Ni NW network. The inset image is HR-TEM of Cu@Ni NW. The scale bar in the inset image is 5 nm. (e) Schematic illustration of the joining mechanism between NWs after electrodeposition.
Fig.3 (a) Sheet resistance and transmittance of the Cu@Ni NW network as a function of electrodeposition time. SEM image of Cu@Ni NW network after electrodeposition for (b) 0 s, (c) 5 s, (d) 10 s and (e) 20 s.
Fig.4 Relative resistance variation of pristine CuNW TCFs and CuNW@Ni TCFs as a function of bending cycles
Fig.5 (a) Relative resistance variation in the CuNW and Cu@Ni NW network after placing in atmospheric environment. Cu 2p3/2 XPS pattern in (b) CuNW network and (c) Cu@Ni NW network after placing in atmospheric environment for 168 h.
Fig.6 (a) Relative resistance variation in the CuNW and Cu@Ni NW network after H 2O2 corrosion. SEM images of (b) CuNW network and (d) Cu@Ni NW network after H2O2 corrosion. (c) and (e) are the relative high-resolution SEM images.
Fig.7 SEM images of (a) CuNW network and (d) Cu@Ni NW network after heating at 400 °C for 30 min. The scale bars in the inset images are 200 nm.
Fig.8 (a) Thermal distribution of the heater at 67.2 °C. (b) Temperature evolution of the heater upon stepwise voltage from 0 to 10 V. (c) Long-term working stability of the heater at 10 V.
Cu@Ni nanowire network was successfully fabricated by electrodeposition for highly stable transparent conductive films.
Firm junctions between stacked CuNWs were achieved by electrodeposition coating Ni shell.
The sheet resistance of Cu@Ni NW TCF decreased from 513 to 15.8 Ohm/sq at a transmittance of 88 %.
The electrical resistance of Cu@Ni NW network remained stable after storing in atmospheric environment for a week.
The Cu@Ni NW network could resist H2O2 corrosion up to 450 s.
The morphology of Cu@Ni NW network kept intact after heating at 400 °C for 30 min.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1385-8947(20)30486-1 https://doi.org/10.1016/j.cej.2020.124495 CEJ 124495
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
30 December 2019 16 February 2020 17 February 2020
Please cite this article as: H. Zhang, S. Wang, Y. Tian, Y. Liu, J. Wen, Y. Huang, C. Hang, Z. Zheng, C. Wang, Electrodeposition fabrication of Cu@Ni core shell nanowire network for highly stable transparent conductive films, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124495
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Electrodeposition fabrication of Cu@Ni core shell nanowire network for highly stable transparent conductive films
He Zhanga,†, Shang Wanga,†, Yanhong Tiana,*, Yubin Liub, Jiayue Wena, Yilong Huanga, Chunjin Hang a, Zhen Zhenga, Chenxi Wanga
a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin,
China, 150001 b
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin, China, 150001
*Email: [email protected], Tel: +86-451-86418359
†These authors contributed equally to this work.
Abstract Copper nanowire (CuNW) is one of the most promising candidates for next-generation transparent conductive film (TCF). However, practical applications of CuNW are still limited by several drawbacks, including loose wire to wire junctions, poor resistance against oxidation, chemical and thermal damage. To concurrently address these urgent issues, highly conductive and stable TCF is prepared based on Cu@Ni core-shell NW networks by electrodeposition. The coated Ni shell can weld stacked NWs tightly, which decreases the sheet resistance from 513 to 15.8 Ohm/sq (at a transmittance of 88%), as well as improves the mechanical stability (1.03-fold resistance increase after 2000 cyclic bending). The Cu@Ni NW TCF exhibits excellent antioxidation performance after storing in atmospheric environment for 168 h. The passivation Ni shell also improvs the chemical and thermal stability. The Cu@Ni NW network keeps intact after 450 s H 2O2 corrosion and heating at 400 °C for 30 min. Moreover, a flexible transparent heater was fabricated based on these TCFs, which shows excellent uniform heating performance and high response speed. Keywords: Copper nanowires, Transparent conductive film, Core-shell, Stability
1. Introduction Over the past decades, transparent conductive film (TCF) has attracted increasing attentions because of their various applications, such as organic solar cells,[1-3] organic light-emitting diodes (OLEDs),[4-6] electrochromic devices[7-9] and transparent heaters.[10-12] Currently, the most commonly used TCF is indium tin oxide film.[13] Although ITO film exhibits satisfactory conductivity and transmittance, the usage of it in flexible device is restricted due to some fatal drawbacks, such as high brittleness, scarcity of indium, and the costly fabrication process.[14-16] Thus far, several materials, including conductive polymer, [17] graphene,[18, 19] carbon nanotubes,[20] metal grids[21] and metal nanowires[22] were proposed to replace ITO for flexible TCF. Among them, metal nanowires were considered as the most promising materials because of their superior conductivity, mechanical flexibility and solution-processability.[23-25] Silver nanowires TCF has been extensively investigated, which possesses comparable optical-electrical with ITO film.[26-28] However, silver is also expensive and easily fails down by electromigrating.[29] Luckily, the electrical conductivity of copper is similar to that of silver, while the price of copper is 100-times cheaper than silver.[30]
Therefore, CuNWs are considered to have the potential to fabricate TCFs with high-performance and low-cost.[31, 32] Nonetheless, several inherent shortcomings regarding CuNWs still need to be addressed for flexible devices. First, CuNW TCFs usually suffer from inadequate conductivity and poor mechanical flexibility because of loose wire-to-wire junctions.[33] The contact resistance and sliding during deformation make them difficult for practical usage. Various methods have been developed to decrease the contact resistance between NWs, such as thermal annealing,[34, 35] photo welding,[36, 37] laser sintering[38] and plasma treatment.[39] Although high performance CuNW TCF can be achieved by these methods, the conductivity of pristine CuNW TCF with extra protection degrades sharply even in atmospheric environment.[40] This is another problem to be solved, that is CuNWs always exhibit poor long-term stability against oxidation and chemical corrosion, which is crucial for some opto-electrical devices requiring fabrication process involving corrosive environment. For improving these stabilities of CuNW TCF, various protective layers were coated, such as Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS),[41] graphene.[42] Nevertheless, the junction between stacked nanowires is still loose. Besides, unselectively coating these low conductivity materials has a negative effect on the electrical and optical performance simultaneously. Wrapping CuNW with noble metals to form core shell structure by galvanic replacing method can selectively coated onto CuNW rather than blank area.[35, 43] However, the noble metals are also costly. Ma et al. successfully fabricated TCF with low cost using Cu@Ni core-shell NWs synthesized by onepot co-reduction of Cu2+ and Ni2+ captions method, but the electrical performance and mechanical stability are still inadequate due to the still loose junctions. Wiley et al. electrodeposited Ni shell onto
CuNW as a functional layer, which showed excellent water oxidation catalytic performance. However, for the application of CuNW TCF in heaters, OLEDs and other fields, the stabilities (resistance to chemical and thermal corrosion) of Cu@Ni core shell NW network are still highly needed to be studied. Hence, a convenient, efficient and cost-effective method which can address these problems concurrently is highly desirable. In this paper, a facile, fast, and low-cost method was introduced to enhance the conductivity and stability of Cu NW TCFs by electrodepositing Ni shell onto CuNW core. During the electrodeposition process, the reduced Ni atoms can fill in the gaps between stacked CuNWs and obtain firm junction, which can effectively improve the conductivity and mechanical stability. Besides, the coated Ni shell can also protect CuNW from oxidation and corrosion. Flexible transparent heaters based on these TCFs can yield uniform heating at low input voltage. This novel method could open the doors for fabrication for high-performance and reliable TCFs. 2. Experimental 2.1 Materials Copper chloride dehydrate, octadecylamine (ODA) and orthoboric acid were purchased from Tianjin Guangfu Chemical Reagent Co., Ltd. Glucose was purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. Ethanol, acetone, isopropanol, n-hexane, glacial acetic acid (GAA) and aqua ammonia were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. Nickel dichloride hexahydrate and ethylenediamine dihydrochloride were purchased from Tianjin Dongli District Tianda Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. 2.2 Electrodeposition fabrication of Cu@Ni NW TCFs
CuNWs were synthesized and purified by our pervious reported method.[44] The as-prepared CuNWs were dispersed in isopropanol with a concentration of 1 mg/mL. After that, CuNWs were filtered on cellulose acetate microfiltration membrane with a pore diameter of 0.45 μm. Then, CuNWs were transferred onto a polyethylene terephthalate (PET) film with a thickness of 0.15 mm by roller pressure. The obtained TCF was dried out by a hot plate at 60 ºC for 5 min. To temporarily increase conductivity, the TCF was immersed into GAA for 40 s to remove the surface organics and oxides. The plating bath consisted of a mix of 1M nickel dichloride, 0.5 M orthoboric acid and 0.5 M ethylenediamine dihydrochloride, and its PH value was adjusted to 4~5 by adding aqua ammonia. To ensure the complete dissolving of the solutes, the solution was stirred at 750 rpm for an hour by magnetic stirrer. The as-prepared CuNW TCF was set as cathode, and a pure Ni plate was used as anode. The electrodeposition process was typically carried out at a current density of 5 A/dm 2 for 10 s. Subsequently, the obtained Cu@Ni NW TCF was washed with deionized water and dried out by hair dryer. 2.3 Characterization The morphologies and microstructures of CuNWs before and after coating Ni shell were observed by a MERLIN Compact scanning electron microscope (SEM) and FEI Tecnai G2 F30 transmission electron microscope (TEM). The valence state of Cu element was analysed by a Thermo ESCALAB 250XI X-ray photoelectron spectrometer. The sheet resistance was tested by an MCP-T370 four-probe square resistance meter from Mitsubishi Chemical Corp. The transmittance was characterized using a UV1600PC UV/vis spectrophotometer from Shanghai Jinghua Technology. The changing of resistance during the bending tests and chemical corrosion tests were measured by a home-made system, which was consisted with a digital mustimeter (34401A, Keysight Technologies, American) and a DC
stabilized power supply (DPS-3005D, Zhaoxin, China). The thermal distribution of the flexible transparent heater was measured by an A-325 infrared camera system from FLIR Systems, Inc. 3 Results and discussion Fig.1 shows the scheme illustration of the fabricating process of Cu@Ni NW TCF. CuNWs were synthesized by a hydrothermal method and purified by an extraction method. Then, CuNWs were filtrated and transferred onto PET substrate to acquire a homogeneously distributed CuNW network. Due to the existence of surface oxides and organics, the TCF at this time is almost non-conductive. GAA treatment was utilized to remove these insulators and increase electrical conductivity. After immersing in GAA for only 40 s, the sheet resistance decreased to 513 Ohm/sq at a transmittance of 97 %. The electrodeposition process was carried at a two-electrode system with a current density of 5A/dm2. The as-prepared TCF was set as cathode and a Ni plate was employed as anode. The electrodeposition time was adjusted to obtain the TCF with different conductivity and transmittance. In a typical process, the time was chosen as 10 s to obtain appropriate sheet resistance and transmittance. Fig.2 a and b show the morphology of CuNW TCF before and after 10 s electrodeposition, respectively. Before electrodeposition, Cu NWs were randomly distributed onto PET substrate and formed a uniform conductive network. At this time, the junctions between NWs were weak physical contact driven by poor gravity and Van der Waals' force. Hence, the junction was not firm enough. Due to the large contact resistance between CuNWs, the electrical conductivity of the electrode was insufficient. A closer contact is highly expected to facilitate the transfer of electrons and decrease the contact resistance. After electrodeposition coating Ni shell, the upper CuNW was embedded into the bottom one at the contact spot, which was not only enlarge conductive area, but also firmly welded them together. The loose overlap was changed into strong welded joint. TEM and high resolution (HR)-
TEM images were collected to further confirm the forming of strong junction, as demonstrated in Fig.2 c and d. The distinct edges of upper and bottom CuNWs along its length were observed at the pristine. However, for the Cu@Ni NWs achieved by electrodeposition, the boundaries of two crossed NWs were interrupted at contact spot, which suggested the Ni shell, formed by the reduced of Ni2+, effectively induced the welding between each other. As shown in inset image of Fig.2 d, it can be observed that the Ni atoms and Cu atoms have been tightly bonded at the junction, which proves that the electrodeposited Ni shell can transform the loose physical contact between NWs into strong metallurgical bonding. The welding mechanism between NWs was shown in Fig.2 e. During the electrodeposition process, Ni2+ in solution would get electrons and be reduced to Ni atoms as a result of current. Subsequently, Ni atoms crystallized and filled the gaps between stacked CuNWs and tightly connected them together. Because only CuNWs that are in contact with each other are electrically conductive, Ni layer will be selectively deposited onto CuNWs rather than the blank areas, and the Cu@Ni core shell NW network can be achieved. Energy dispersive X-ray spectroscopy (EDS) analysis was utilized to characterize the element elemental composition of the NW network, as shown in Fig.S1. In addition to the carbon, oxygen elements (caused by PET) and gold elements (caused by the deposited Au layer for the observation under SEM), only Cu element was observed in the pristine networks. After electrodeposition, obvious Ni peaks were present, indicating that the CuNW network was connected by thin Ni layer. The sheet resistance and transmittance of CuNW TCFs with different electrodeposition time are investigated, as demonstrated in Fig.3 a. In general, the electrodeposition increased but the transmittance decreased over electrodeposition time. Before electrodepositing Ni shell, the sheet resistance was 513 Ohm/sq at a transmittance of 97%. At this time, the junctions between CuNWs were
loose, as shown in the corresponding SEM image (Fig.3 b). After only 5 s electrodepositing time, the sheet resistance was down to 170 Ohm/sq, indicating the increasing of contact area between NWs. The transmittance was still up to 93.2 % due to insignificant increase in NW diameter (Fig.3 c). However, the current electrical conductivity was still unacceptable for practical usage. Further extending the electrodepositing time to 10 s, the sheet resistance reached 15.8 Ohm/sq at a transmittance of 88 %, which can meet the requirements of most devices for optoelectronic performance. It could be observed from Fig.3 d that firm junctions between NWs had formed and the surface of NWs were still smooth. When electrodepositing for 20 s, the sheet resistance showed a slight drop (7.9 Ohm/sq), because NWs had been tightly connected between each other previously. However, the transmittance sharply decreased to 68 %, which was caused by the continuous thickening of nanowires. Moreover, Ni thin film appeared at the blank area between the nanowires (Fig.3 d), which would exacerbate the loss of transmittance. Hence, 10 s was chosen as the optimized process parameter due to the excellent opticalelectrical performance. In the actual application process of flexible devices, mechanical deformation is inevitable. Hence, the mechanical stability of TCF is of great importance. NWs are easily moved between each other during the mechanical deformation process, resulting in the increase of electrical resistance. The bending cycle was performed to evaluate the mechanical stability of the CuNW TCF before and after coating Ni shell. The resistance was in situ monitored during the cyclic bending and releasing process, as shown in Fig.4. Compared with the relative resistance of the pristine CuNW TCF up to 1.98, the Cu@Ni NW TCF exhibited a slight increase (1.03), indicating its excellent performance against deformation. It was concluded that the mechanical stability was validly improved thanks to the formation of firm NWs junctions achieved by electrodeposition.
CuNW is easily oxidized by oxygen and water vapor even in atmospheric environment under room temperature and form copper oxides, which will degrade the electrical conductivity during the long-term service of TCF. In addition to improving the electrical performance of TCF, the coated Ni layer effectively prevented Cu NWs from oxidizing in the air, as indicated in Fig.5. For pristine CuNW TCF without extra protection, after placing in atmospheric environment for 12 h, the sheet resistance increased ten times. As a contrast, the conductivity of Cu@Ni NW TCF was still stable after placing in atmospheric environment for 168 h, showing excellent long-term stability. To further confirm the protective effect of Ni shell, the XPS was employed to characterize the surface chemical state of Cu element in two kinds of TCFs after storing in atmospheric environment for 168 h. The Cu 2p3/2 spectra of pristine CuNW network was fitted to several peaks, including the peak at 931.8 eV assigned to Cu or Cu2O, the peak at 933 eV (and a companion peak) assigned to CuO, the peak assigned to Cu(OH)2 at 933.9 eV, demonstrating the serve oxidation of CuNWs. However, for the Cu@Ni NW TCF, the peak of Cu(OH)2 almost disappeared and the and the peaks of CuO became weak. The results suggested that the electrodeposited Ni inhibitor can effectively protect CuNW from oxidation and improve the longterm stability of the TCF. Chemical stability is an important index for evaluating the performance of flexible electrodes, which determines the service stability of the TCF under corrosive environments. We assessed the chemical stability of the CuNW and Cu@NW TCF by dropping 15 wt. % H2O2 solution to simulate an extreme chemical corrosion environment, and the relative resistance change of TCF was monitored in real time, as shown in Fig.6 a. It can be observed that the relative resistance of the pristine CuNW network increased rapidly less than 70 s, suggesting the NW network has been damaged. However, the chemical stability of the TCF has been significantly improved after electrodepositing. Even after
dropping H2O2 for 450 s, the relative resistance increased insignificantly. SEM images in Fig.6 b-d further confirmed these results. There is a clear boundary between the corroded area and the original area of the TCFs. Further observation of corroded area at high magnification revealed that CuNW network without protection has been dissolved, but Cu@Ni NW network are still intact even though PET substrate has been corroded. The tolerance of high temperature is another urgent issue to be addressed, because heating is an essential process for many flexible devices, such as heaters, OLEDs. However, CuNW is prone to fragment and transfer into discrete nanospheres when the temperature reached only 400 °C which is much below the melting point of bulk copper (1085 °C) due to the Rayleigh instability,[45] resulting in the loss of electrical conductivity. Here in, the pristine CuNW and Cu@Ni NW TCFs were both heated to 400 °C for 30 min, and the morphologies were observed in Fig.7. Obviously, pristine CuNW network has been destroyed, while Cu@Ni NW core shell structure showed negligible change. These results clearly illustrated that the introduction of Ni shell could improve the resistance of thermal corrosion. To demonstrate the potential of the fabricated TCFs in high-performance electronics, a flexible transparent heater based on a Cu@Ni NW TCF electrodeposited for 10 s was constructed. Uniform thermal distribution of the heater was shown in the infrared image (Fig.8 a) taken by a thermal camera at a temperature of 67.2 °C, which indicated that the electrodeposition method effectively joined the stacked CuNWs and prevented the overheating at local area. Stepwise input voltage was applied on the heater to test its performance, as shown in Fig.8 b. Rapid temperature response is observed as voltage steps, indicating its high sensitivity. The application of the heater in different occasions could be adjusted by the input voltage. When applying 5 V voltage, the temperature nearly increased to 47 °C,
which can meet the needs of electric blanket, anti-fog window, etc. Further increasing the input voltage to 10 V, the temperature nearly reached 120 °C, exhibiting its potential in water heating, hightemperature sterilizer field. On the other hand, stable high temperature output capability is a key requirement for high-performance in practical usage. The heater maintained stable temperature output even after prolonged service for 5 h, demonstrating its excellent long-term stability. Conclusions In this work, highly conductive and stable TCFs were successfully fabricated by electrodepositing a passivation Ni shell in air. Because Ag and Au atoms are selectively deposited onto NWs rather than blank area, the TCF exhibited a low sheet resistance (15.8 Ohm/sq) at a high transmittance (88 %). Superior mechanical stability was obtained due to the formation of firm joint between NWs. Moreover, the resistance of Cu@Ni TCF against oxidation was improved, which could stably work in atmospheric environment for a weak. High chemical and thermal stability were also realized after coating the protective Ni shell. A flexible transparent heater based on Cu@Ni NW TCF was successfully prepared, which showed excellent uniform, fast and continuous heating performance (nearly 120 °C at 10 V), indicating its potential applications in next-generation flexible electronics. Conflicts of interest. There are no conflicts of interest to declare. Acknowledgments The authors are grateful for the financial support from the National Key R&D Program of China (no. 2018YFB1307501) and National Natural Science Foundation of China (grant no. 51522503). References
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Figures and captions
Fig.1 Schematic illustration of the fabrication process of Cu@Ni NW TCF.
Fig. 2 SEM images of (a) CuNW network and (b) Cu@Ni NW network. The scale bars in the inset images are 200 nm. TEM image of a junction in (c) Cu NW network and (d) Cu@Ni NW network. The inset image is HR-TEM of Cu@Ni NW. The scale bar in the inset image is 5 nm. (e) Schematic illustration of the joining mechanism between NWs after electrodeposition.
Fig.3 (a) Sheet resistance and transmittance of the Cu@Ni NW network as a function of electrodeposition time. SEM image of Cu@Ni NW network after electrodeposition for (b) 0 s, (c) 5 s, (d) 10 s and (e) 20 s.
Fig.4 Relative resistance variation of pristine CuNW TCFs and CuNW@Ni TCFs as a function of bending cycles
Fig.5 (a) Relative resistance variation in the CuNW and Cu@Ni NW network after placing in atmospheric environment. Cu 2p3/2 XPS pattern in (b) CuNW network and (c) Cu@Ni NW network after placing in atmospheric environment for 168 h.
Fig.6 (a) Relative resistance variation in the CuNW and Cu@Ni NW network after H 2O2 corrosion. SEM images of (b) CuNW network and (d) Cu@Ni NW network after H2O2 corrosion. (c) and (e) are the relative high-resolution SEM images.
Fig.7 SEM images of (a) CuNW network and (d) Cu@Ni NW network after heating at 400 °C for 30 min. The scale bars in the inset images are 200 nm.
Fig.8 (a) Thermal distribution of the heater at 67.2 °C. (b) Temperature evolution of the heater upon stepwise voltage from 0 to 10 V. (c) Long-term working stability of the heater at 10 V.
Cu@Ni nanowire network was successfully fabricated by electrodeposition for highly stable transparent conductive films.
Firm junctions between stacked CuNWs were achieved by electrodeposition coating Ni shell.
The sheet resistance of Cu@Ni NW TCF decreased from 513 to 15.8 Ohm/sq at a transmittance of 88 %.
The electrical resistance of Cu@Ni NW network remained stable after storing in atmospheric environment for a week.
The Cu@Ni NW network could resist H2O2 corrosion up to 450 s.
The morphology of Cu@Ni NW network kept intact after heating at 400 °C for 30 min.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: