# Copper Nanowires for Transparent Electrodes: Properties, Challenges and Applications
## 3 Oxidation Prevention  
As already mentioned above, the major challenge facing Cu-based nanomaterials, including NWs, is oxidation, both as native oxide and as oxide forming over time in ambient conditions, both negatively affecting conductivity. While annealing in forming gas (5% H\({}_{2}\) and 95% N\({}_{2}\)) is a typical way to remove the native oxide, other strategies are to be developed to protect from oxidation after synthesis [20]. For example, a UV treatment, followed by acid bath, was proposed to remove organic residues from the NW surface with  
Figure 3: SEM image of curved Cu NWs. Reprinted with permission from Ref. [38]. © 2021 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Figure 2: Haze vs. transmittance plot. Reprinted with permission from Ref. [31]. Copyright 2015 American Chemical Society. subsequent performance improvement [41], though no information on the performance over time was given. A popular method for oxidation protection is making core-shell structures with other materials, for example with Nickel [20; 42; 43]. This was first reported by [PERSON] et al. who showed that for pristine Cu NW, when kept at 85 \({}^{\circ}\)C, \(Rs\) increased by two orders of magnitude within five days, while a layer of Ni kept \(Rs\) flat for at least up to 30 days (Figure 4a) [20]. The authors also reported that the presence of Ni significantly increased \(Rs\). However, there is a clear advantage with the stability of the NWs (Figure 4b) [20]. Similar results were obtained by [PERSON] et al. who obtained Cu NW films deposited onto glass substrates dipped in acetic acid first to remove native oxide and then in an electroless plating Ni solution to form Cu-Ni core-shell NWs [42]. The same group later reported the synthesis of Cu-Ag core shell NWs using the same electroless plating Ag deposition [43]. The same acetic acid treatment could be repeated several times after reoxidation [34]. Unlike the case of Ni, Ag-coated Cu NWs did not show a decrease in optical and electrical performance. [PERSON] et al. also reported Cu-Ni core-shell NWs, with findings similar to those reported above [44]. [PERSON] et al. reported the epitaxial growth of 1-2 nm layers of Au onto Cu NWs, reporting a dramatic improvement of stability under high temperature (80 \({}^{\circ}\)C) and humidity (80%) [45]. Other than metals, oxides can also be viable protective coatings onto Cu NWs. For example, [PERSON] et al. deposited metals like Zn, Sn and In by electrodeposition, followed by their oxidation, by hydrogen peroxide, into the corresponding transparent oxides [46]. A ZnO layer deposited by atomic layer deposition was used by [PERSON] et al. to improve the performance of transparent thin film heaters (see applications sections for more details) [47]. An Al-doped ZnO coating on the Cu NW was also proposed by [PERSON] et al. to convey ambient stability to the Cu NW network [48]. They also reported a lactic acid treatment to remove native oxide and other organic residues with no annealing. The use of graphene and its related materials was also reported as a viable strategy to improve ambient stability of Cu NWs. [PERSON] et al. reported hybrid thin films of Cu NWs and reduced graphene oxide (RGO) [49]. They showed that while pure RGO thin films have \(Rs\) about two orders of magnitude higher than Cu NW films (Figure 5a), when combined together the \(Rs\) is decreased by about 2\(\times\) from that of Cu NW films (Figure 5b). This is likely because the presence of RGO sheets lowers the junction resistance of Cu NWs and provides additional conduction paths. More importantly, \(Rs\) of such films is flat for at least 72 h at 60 \({}^{\circ}\)C (Figure 5c) [49]. [PERSON] et al. reported Cu NWs wrapped with RGO with a slightly improved electrical properties and stability over 200 days [50]. Among the RGO-Cu NW films, those with lower \(T\) are the most stable, while those with higher \(T\) are more susceptible to oxidation with almost 2\(\times\)\(Rs\) increase over 200 days. Core-shell  
Figure 4: Performance of Cu-Ni core shell NWs.