Facile synthesis of highly conductive Ag/TiN nanofibers for cost-saving transparent electrodes

Abstract

By homogeneously incorporating Ag into a conductive TiN matrix, ultra high electrical conductivity of 1181 S cm⁻¹ was achieved in hybrid Ag/TiN nanofibers. In comparison to the state-of-the-art pure Ag nanowire transparent electrodes, the cost-saving Ag/TiN nanofiber network exhibited comparable optoelectronical performance. This newly developed material may serve as an alternative for cost-efficient transparent electrodes.

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A transparent electrode has a remarkable combination of high optical transmittance and high electrical conductivity. Motivated by the emerging smart display technology, transparent electrodes have attracted significant attention because of their tremendous potential in various applications such as liquid crystal displays (LCD), electroluminescent displays (ELD), light-emitting diodes (LED) and solar cells.^1–4 Since the first discovery of CdO that realized the integration of optical transparency and electrically conductive ability in a single material, metal oxides have been under the spotlight in the field of transparent electrodes.⁵ Up to now, tin-doped indium oxide (ITO) has extensively been used as the transparent electrode in electronic industry due to its excellent optoelectronical property.⁶ However, the increasing price of indium and inefficient ITO production technique will certainly limit its future large-scale utilization. In this context, introducing new materials and developing new production method is of critical importance to the development of transparent electrode.

During the past decades, much efforts have been endeavored to find new materials to replace ITO. Transparent electrodes assembled by conducting polymers,⁷ carbon nanotubes,⁸ and graphene⁹ have been reported. However, the property of conducting polymers was well below that of ITO transparent electrode, due to their limited electrical conductivity. The performance of the transparent electrode based on carbon nanotubes was much better than that of conducting polymers, but it was still lagged behind that of the ITO film. Besides, the complicated and uncontrollable procedures for fabricating carbon nanotubes inevitably leaded to sample-to-sample variation in their performance. Recently, graphene produced by chemical vapor deposition was also developed as candidate material for transparent electrode.¹⁰ Despite its promising performance, it is a great challenge to fabricate high quality graphene by a low-cost technical with high throughput.

Among the metals, silver is well known to own the highest electrical conductivity, but bulk silver is non-transparent. In an effort to achieve optical transparency in silver material, silver nanowires were successfully fabricated and subsequently assembled into transparent electrode with an outstanding opto-electronical achievement of 16 ohm sq⁻¹ at 95% transparency.¹¹ The performance of silver nanowire was comparable to, or even surpassed that of ITO. Whereas, as one type of the noble metals, the high price of silver will inevitably add to the costs of transparent electrode production. Therefore, reducing the dosage of silver while maintaining its high optoelectronical performance is difficult but is considerably important and desirable to develop silver-based transparent electrode.

Electrospinning – a simple, versatile and controllable method – has provided a platform for incorporating silver into metal oxide nanofibers. As previously reported, heterostructured Ag–ZnO nanofibers with Ag nanoparticles embedded in ZnO nanofiber matrix has been synthesized and achieved a high conductivity of 115 S cm⁻¹ with 50.0 at% Ag.¹² However, the conductivity of Ag–ZnO nanofibers was greatly limited by the insulating ZnO matrix. We supposed that the conductivity of Ag-containing heterostructured nanofibers might be further enhanced using electrically conductive material as the matrix instead of using insulating ZnO. Titanium nitride (TiN) is an inexpensive ceramic material not only possessing high chemical durability due to its covalent bond but also possessing high electrical conductivity due to its metallic bond.¹³ Bulk TiN material has a conductivity of approximately 10⁵ S cm⁻¹ even close to that of the metals. Concerning its low cost and high conductivity, TiN was chosen as the matrix. Therefore, our attention has been focused on a new heterostructure design-incorporating silver into TiN nanofibers.

Herein, for the first time, heterostructured Ag/TiN nanofibers were synthesized, and their electrical transport properties were investigated. With 30.0 at% Ag incorporation, the Ag/TiN nanofibers reached a conductivity as high as 1181 S cm⁻¹, increased by one order of magnitude compared to that of TiN nanofibers.¹³ Importantly, we assembled the Ag/TiN nanofibers into transparent electrode, achieving a low sheet resistance of ∼10 ohm sq⁻¹ at 84% transparency, which is comparable to the optoelectronical performance of Ag nanowires.

The electrospun precursor nanofibers were transformed into Ag/TiN nanofibers by two stage annealing processes. Since silver nitrate is chemical unstable, Ag nanoparticles can be directly formed by heating silver nitrate. Our previous study on Ag/BaTiO₃ verified that silver nitrate could be decomposed into Ag at a temperature of 300 °C.¹⁴ Based on this Ag formation mechanism, several studies have also successfully produced Ag/metal oxide nanocomposites such as Ag/ZnO and Ag/NiO.^12,15 Therefore, at the first stage, the precursor nanofibers were converted into Ag/TiO₂ nanofibers by calcination in air. At the second stage, the obtained Ag/TiO₂ nanofibers were heated in ammonia atmosphere to turn Ag/TiO₂ nanofibers into Ag/TiN nanofibers based on the following chemical reaction:

6TiO₂ + 8NH₃ → 6TiN + N₂↑ + 12H₂O↑ (1)

The morphology of the resultant nanofibers after nitridation at temperatures ranging from 700 to 1000 °C was characterized by scanning electron microscopy (SEM). As shown in Fig. 1, the resultant nanofibers with diameters ranging from 100 to 300 nm were still continuous. The length of them could even beyond a few hundred micrometers, which was highly favorable for the following assembly of Ag/TiN nanofibers into electronic devices. It is interesting to note that spherical particles emerged on the surface of the fiber and were distributed uniformly along the fiber length (Fig. 1a). With the nitridation temperature increasing from 700 to 1000 °C, the size of the emerging particles gradually increased to approximately three micrometers, accompanied with roughened surface of the fiber. The particles were composed of Ag, as verified by the following transmission electron microscopy analysis. The increase in the size of Ag particles with the increasing annealing temperature is analyzed as follows. During the calcination in ammonia, Ag particles that were formed in the first stage, grew up quickly and were then agglomerated into spheres, caused by their large specific surface area. These Ag spheres finally broke out of the TiN fiber and emerged on the fiber surface. With the annealing temperature increasing, the Ag spheres became larger due to the increase in heating up time. Therefore, the growth of spheres on the fiber surface was observed in the inset of Fig. 1. The rougher surface of the fiber after calcination resulted from the grain growth of TiN. To figure out the composition of the spherical particles, energy dispersive spectroscopy (EDS) was carried out. As shown in Fig. S2,† the EDS results suggest that these particles were mainly composed of Ag element.

Fig. 1 SEM images of the Ag/TiN nanofibers calcined at various temperatures ranging from 700 to 1000 °C; insets are the corresponding high resolution SEM images.

To gain a further insight into the microstructure of the resulting nanofibers, transmission electron microscopy (TEM) characterization was conducted. As observed in Fig. 2a, the black particle adhered firmly to the fiber.

Fig. 2 (a) TEM image of Ag/TiN nanofiber; (b) and (c) HRTEM image of the selected area in the fiber.

In the high-resolution TEM (HRTEM) image of the particle (Fig. 2b), the spacing d of the crystal lattice was calculated to be 2.35 Å, according well with the (111) crystal plane of metal Ag. Such large Ag particle might stem from the agglomeration of Ag grains located at the fiber surface that could rapidly grow under high-temperature heating without any restriction surrounded. In Fig. 2c, the calculated crystal lattices of the fiber were 2.43 Å and 2.11 Å, corresponding to (111) and (200) crystal planes of TiN, respectively. Thus, during the nitridation process, TiO₂ in the nanofibers could react with ammonia effectively even at 700 °C, leading to the formation of TiN phase. To check out whether the Ag existed insider the fiber, elemental mapping was taken by using an EDS system attached to the TEM instrument. Fig. 3 displays the EDS results for the Ag, Ti and N elements contained in the fiber. Clearly, a vast number of Ag crystalline grains were distributed uniformly among the TiN components. On the contrary to the large Ag particles emerging on the fiber surface, the presence of Ag grains in the fiber might be attributed to the TiN phases, which could surround Ag and ultimately prevent Ag grains from sharply growing. The distribution of Ag in the TiN matrix is expected to favorably improve the electrical conductivity of the hybrid nanofibers.

Fig. 3 (a) TEM image of the Ag/TiN nanofiber and element mapping of (b) Ag, (c) Ti, (d) N.

The phase constitution of the resultant nanofibers after nitridation at temperatures ranging from 700 to 1000 °C was also determined by X-ray diffraction (XRD). The obtained XRD patterns are shown in Fig. 4a, where diffraction angles at 36.7, 42.6, 61.8, 74.0° were indexed into TiN, and those at 38.1, 44.3 and 64.4° corresponded to metal Ag.

Fig. 4 (a) XRD patterns and (b) Raman spectra of Ag/TiN nanofibers calcined at various temperatures ranging from 700 to 1000 °C in ammonia atmosphere for 2 h.

This structure indication of the nanofibers was consistent with the TEM analysis. However, the (200) diffraction peak of TiN significantly shifted toward a smaller 2θ direction as the temperature rose. This finding was in a good agreement with the research on TiN, in which similar shifting of TiN diffraction peaks was observed and was ascribed to the increasing amount of N atom in the TiN lattice.¹⁰ Raman scattering has been a highly useful technique for assessing the material composition. Besides XRD, Raman scattering was further used to offer more detailed information on the nanofiber constitution. The Raman spectra of the resultant Ag/TiN nanofibers in the range of 100–800 cm⁻¹ were shown in Fig. 4b. On the basis of the literatures, the peaks at 260, 418, and 609 cm⁻¹ can be assigned to TiN.¹⁶ Unexpectedly, the peak at 155 cm⁻¹, a characteristic of TiO₂, was also noticed, indicating the slight existence of unreacted TiO₂ in the fibers.¹⁷ As the nitridation temperature increased, the intensity of the peak corresponding to TiO₂ significantly decreased, which suggests an enhanced degree of the reaction between TiO₂ and ammonia.

Note that the easily controllable electrospinning technique used here offers the advantage of assembling nanofibers into electronic devices. Thereby, by assembling Ag/TiN nanofiber-based device, the assessment of their electrical conductivity was facilely enabled. As depicted in Fig. 5, precursor nanofibers were firstly oriented through a modified electrospinning method as previously reported, and were then transferred to a Si/SiO₂ wafer.¹² After suffering heat treatment procedure, gold electrodes with a 500 μm channel were deposited on the wafer. A simple Ag/TiN nanofiber-based electrical device was thus obtained as shown in Fig. 5c. Even going through the whole device-assembling process, the resultant nanofiber remained intact as observed in Fig. 5b, which guaranteed the proceeding of the following electrical conductivity test.

Fig. 5 (a) Schematic illustration of the electronic device; (b) the optical micrograph of the aligned nanofiber; (c) the photograph of the electronic device.

The resulted I–V characteristics of Ag/TiN nanofibers nitrided at a wide temperature range from 700 to 1000 °C are shown in Fig. 6. Remarkably, the current through nanofibers at the same voltage continuously increased with nitridation temperature going up. On the basis of Raman spectra analysis, this increase in current was supposed to originate from the apparently reduction of TiO₂ residue in the resulting nanofibers, as the efficiency of reaction between TiO₂ and ammonia was improved at higher nitridation temperatures. The morphology of the Ag/TiN nanofibers may also exercise an influence on their properties, which need to be further studied. The electrical conductivity of these Ag/TiN nanofibers was calculated accordingly, and the results are presented as a function of nitridation temperature (Fig. 6c). When nitrided at 700, 800, 900 and 1000 °C, the Ag/TiN nanofibers had a respectable conductivity value of 103, 106, 385, and 1181 S cm⁻¹, respectively. In comparison to TiN nanofibers, the conductivity of Ag/TiN nanofibers was significantly enhanced by one order of magnitude, which can be attributable to the conductive chains formed by the existing Ag nanoparticles. It is worth to mention that the conductivity of Ag/TiN nanofibers was among the highest value of the currently developed Ag-contained composites as well as TiN-based composites (Table S1†).^12,13,18,19 Even more notably, the conductivity up to 1181 S cm⁻¹ achieved in Ag/TiN nanofibers greatly surpassed most of the state-of-the-art electrospun conductive nanofibers, such as conductive polymer nanofibers (10⁻⁸ to 10⁻¹ S cm⁻¹),^20–22 ITO nanofiber that has a reported value of 1 S cm⁻¹,²³ carbon nanofibers (10⁻⁷ to 10³ S cm⁻¹),^24,25 and Ag/ZnO nanofibers (115 S cm⁻¹ with 50 at% Ag addition).¹² In this case, this newly developed material with ultra-high conductivity is anticipated to offer an accessible alternative for the development of electronic devices.

Fig. 6 (a) and (b) I–V curve of the Ag/TiN nanofibers nitrided at different temperatures from 700 to 1000 °C; (c) electrical conductivity of the Ag/TiN nanofibers; (d) conductivity of the Ag/TiN nanofiber and the reported electrospun conductive nanofibers in the literatures.^{12,13,18,20–25}

Herein, we have assembled Ag/TiN nanofibers into networks and examined their performance as transparent electrode, as one example of the future potential applications for these highly conductive fibers. The Ag/TiN nanofiber networks was assembled on the surface of a quartz glass, as shown schematically in Fig. S3.† Even though both Ag and TiN bulk are traditionally opaque, the resulting nanofiber network devices exhibited a high-level transparency (the lower inset in Fig. 7a). As presented in Fig. 7a, the transparency of the device reached up to 84.6% in the visible spectral range, due to the plentiful hollow space among the fibers. Additionally, the corresponding sheet resistance of the network was tested using a four-probe method. The optoelectronic achievements of 18 ohm sq⁻¹ at 88% transparency and 10 ohm sq⁻¹ at 84% transparency were attainable. Such low sheet resistance reached was attributed to the high conductivity of the Ag/TiN nanofibers. The high transparency combined with the low sheet resistance achieved in Ag/TiN nanofiber network device enable it to be applicable as transparent electrodes. Particularly, the assembled Ag/TiN nanofiber-based transparent electrode exhibited superior performance compared with the currently available transparent electrodes built on other attractive materials, such as conducting polymers (30 ohm sq⁻¹ at 70% transparency),²⁶ carbon nanotubes (110 ohm sq⁻¹ at 80% transparency),²⁶ graphene (280 ohm sq⁻¹ at 80% transparency),¹⁰ and even comparable to metallic nanowires (16 ohm sq⁻¹ at 95% transparency).^8,11,27–29 This outstanding behavior boosts the application of Ag/TiN nanofibers as a new type of cost-saving transparent electrode.

Fig. 7 (a) Transparency of Ag/TiN transparent electrodes, inset is the corresponding optical photograph of the assembled transparent electrode device; (b) optoelectronical performance of the Ag/TiN nanofiber transparent electrode and the emerging materials in the literatures.^{6–8,13,26,27}

Conclusions

In summary, we have achieved considerable electrical conductivity in low-cost ceramic nanofibers with a novel heterostructure design. Ag/TiN hybrid nanofibers have been synthesized with a conductivity up to 1181 S cm⁻¹. We further assembled Ag/TiN nanofibers into transparent electrode, which reached 10 ohm sq⁻¹ at 84% transparency. This performance can be comparable to most of the currently available transparent electrode candidates. The achievement in this work is expected to stimulate several new directions toward searching cost-saving platform for transparent electrodes.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant no. 51402116). The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology for support.

References

1. N. Yamamoto, H. Makino, S. Osone, A. Ujihara, T. Ito, H. Hokari, T. Maruyama and T. Yamamoto, Thin Solid Films, 2012, 520, 4131.
2. S. Chen and H. Kwok, Org. Electron., 2012, 13, 31.
3. Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu and Q. Pei, Adv. Mater., 2011, 23, 664.
4. S. Maity and S. Sahare, Optik, 2016, 127, 5240.
5. B. G. Lewis and D. C. Paine, MRS Bull., 2000, 25, 22.
6. D. R. Carins, R. P. Witte II, D. K. Sparacin, S. M. Sachsman, D. C. Paine and G. P. Crawford, Appl. Phys. Lett., 2000, 76, 1425.
7. X. Zhang, J. Wu, J. Wang, J. Zhang, Q. Yang, Y. Fu and Z. Xie, Sol. Energy Mater. Sol. Cells, 2016, 144, 143.
8. V. C. Tung, L. Chen, M. J. Allen, J. K. Wassei, K. Nelson, R. B. Kaner and Y. Yang, Nano Lett., 2009, 9, 1949.
9. J. Wu, M. Agrawal, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen and P. Peumans, ACS Nano, 2010, 4, 43.
10. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. Ahn, P. Kim, J. Choi and B. H. Hong, Nature, 2009, 457, 706.
11. M. Song, J. Kim, S. Y. Yang and J. W. Kang, Thin Solid Films, 2014, 573, 14.
12. D. Lin, H. Wu, X. Qin and W. Pan, Appl. Phys. Lett., 2009, 95, 112104.
13. H. Li, W. Pan, W. Zhang, S. Huang and H. Wu, Adv. Funct. Mater., 2013, 23, 209.
14. H. Li, Y. Sun, W. Zhang and W. Pan, J. Alloys Compd., 2010, 508, 536.
15. H. Wu, D. Lin, R. Zhang and W. Pan, Chem. Mater., 2007, 19, 1895.
16. C. P. Constable, J. Yarwood and W. D. Munz, Surf. Coat. Technol., 1999, 116–119, 155.
17. H. Wang, Y. Li, X. Ba, L. Huang and Y. Yu, Appl. Surf. Sci., 2015, 345, 49.
18. P. Chen, H. Li, S. Hu, T. Zhou, Y. Yan and W. Pan, J. Mater. Chem. C, 2015, 3, 7272.
19. S. Huang, Z. Wen, X. Zhu and Z. Gu, Electrochem. Commun., 2004, 6, 1093.
20. I. S. Chronkis, S. Grapenson and A. Jakob, Polymer, 2006, 47, 1597.
21. T. S. Kang, S. W. Lee, J. Joo and J. Y. Lee, Synth. Met., 2005, 153, 61.
22. Y. Zhu, J. C. Zhang, Y. M. Zheng, Z. B. Huang, L. Feng and L. Jiang, Adv. Funct. Mater., 2006, 16, 568.
23. D. Lin, H. Wu, R. Zhang and W. Pan, Nanotechnology, 2007, 18, 465301.
24. J. S. Im, S. J. Kim, P. H. Kang and Y.-S. Lee, J. Ind. Eng. Chem., 2009, 15, 699.
25. B. Zhang, F. Kang, J.-M. Tarascon and J. K. Kim, Prog. Mater. Sci., 2016, 76, 319.
26. E. M. Doherty, S. De, P. E. Lyons, A. Shmeliov, P. N. Nirmalraj, V. Scardaci, J. Joimel, W. J. Blau, J. J. Boland and J. N. Coleman, Carbon, 2009, 47, 2466.
27. M. Kang and L. J. Guo, Adv. Mater., 2007, 19, 1391.
28. M. Song, D. S. You, K. Lim, S. Park, S. Jung, C. S. Kim, D. Kim, D. Kim, J. Kim, J. Park, Y. Kang, J. Heo, S. Jin, J. H. Park and J. W. Kang, Adv. Funct. Mater., 2013, 23, 4177.
29. A. Morag, V. Ezersky, N. Froumin, D. Mogilianskya and R. Jelinek, Chem. Commun., 2013, 49, 8552.

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Footnote

† Electronic supplementary information (ESI) available: Syntheses and characterization of the nanowires; supplementary figures. See DOI: 10.1039/c6ra19981j

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