A pioneering work regarding the growth of CuNWs has been reported by Xie’s group [45]; in their work, CuNWs were synthesized through a hydrothermal method using glucose as a green reducing agent and HDA as a capping agent. As shown in Fig. 1a, glucose can be oxidized to glucono delta-lactone, reducing the copper ions to form copper nanoparticles via aggregation [46]. During the reaction, HDA binds preferentially onto the (100) facets of Cu and thereby enables Cu to grow nanowires along the orientation of 110. Figure 1c shows FESEM images of the pristine CuNWs, c-rGO/CuNWs, and h-rGO/CuNWs films. The high aspect ratio of CuNWs, with an average diameter of 57.78 ± 9.18 nm and a length of > 10 μm, allows them to form continuous conducting paths for electron transfer and multiple empty spaces for light transmittance. After the CuNWs films were dipped into a solution containing the c-rGO dispersion, the CuNWs films were covered with c-rGO. However, the wrinkled c-rGO layers reveal imperfect covering on CuNWs due to the difficulty of dispersion as well as the aggregation of rGO in nature [23]. In contrast, the h-rGO layers exhibit good coverage on CuNWs film compared with the c-rGO/CuNWs film (the SEM image with low magnification of h-rGO/CuNWs is shown in Additional file 1: Fig. S1). As mentioned, h-rGO/CuNW films were derived by dipping CuNWs films in a GO dispersion and subsequent annealing under a hydrogen atmosphere. Various functional groups on the GO surface enable great dispersity, facilitating tight and uniform coverage onto CuNWs films; thereby, after the hydrogen reduction process, h-rGO can act as an air barrier for better oxidation resistance.

Before filtration and transferring processes, the CuNWs dispersion was treated with lactic acid to etch away the adsorbed HDA and surface copper oxide/hydroxide according to the following equations [47]:

$$mathrm{Cu}{left(mathrm{OH}right)}_{2}+2left[{mathrm{CH}}_{3}mathrm{CH}left(mathrm{OH}right)mathrm{COOH}right]to mathrm{Cu}{[{mathrm{CH}}_{3}mathrm{CH}(mathrm{OH})mathrm{COO}]}_{2}+{mathrm{H}}_{2}mathrm{O}$$


$$mathrm{CuO}+2left[{mathrm{CH}}_{3}mathrm{CH}left(mathrm{OH}right)mathrm{COOH}right]to mathrm{Cu}{[{mathrm{CH}}_{3}mathrm{CH}(mathrm{OH})mathrm{COO}]}_{2}+{mathrm{H}}_{2}mathrm{O}$$


Therefore, lactic acid can react with copper oxide/hydroxide and form copper lactate, which is soluble in solvent and can be readily washed away. The XRD results confirm that the copper oxide/hydroxide was removed by lactic acid treatment (Fig. 2a). After lactic acid treatment, three obvious peaks at 43.5°, 50.7°, and 74.48° attributed to the copper crystal plane of (111), (200), and (220), respectively, are observed, indicating that CuNWs have a face-centered cubic structure (JCPDS 03-1018). Moreover, other peaks arising from the presence of copper oxide/hydroxides disappear, confirming that the lactic acid treatment resulted in high-purity CuNWs. In Fig. 2b, the Raman spectra also support the hypothesis that the oxide/hydroxide and HDA surfaces were removed after lactic acid treatment. The characteristic Raman peaks at approximately 218/523/623 cm−1 and 580 cm−1 are attributed to the Cu2O and CuO phases, respectively [48, 49]. Cu(OH)2 shows a weak characteristic peak at about 490 cm−1 [49]. In addition, a visible peak at ~ 180 cm−1 along with a wide and weak peak at 340 cm−1 could be assigned to acoustical motions of alkane chains in HDA [50]. All the above-mentioned characteristic bands in the pristine CuNWs disappeared after lactic acid treatment, indicating the removal of HDA and copper oxide/hydroxide; hence, electrons can transport between CuNWs network with less resistance.

Fig. 2
figure 2

a XRD patterns of the CuNWs before and after lactic acid treatment. The intensive peaks correspond to the (111), (200), and (220) crystal planes of the face-centered cubic structure of Cu, and no extra signals appear after the lactic acid treatment, indicating the high purity of the CuNWs. b Raman spectra of CuNWs before and after lactic acid treatment. After treatment with lactic acid, the surface HDA and oxides/hydroxides were removed

As depicted in Fig. 3a, the XRD pattern of GO reveals a dominant peak of the graphitic (002) plane at 2θ = 10.4° [51], corresponding to a layer d-spacing of 0.85 nm, which is shifted to 23.6° (d-spacing = 0.38 nm) and 23.4° (d-spacing = 0.39 nm) after chemical reduction by NaBH4 and hydrogen-annealing reduction, respectively. The disappearance of the broad (002) diffraction peak and a decrease in interlayer distance indicate restacking of the rGO sheets due to the removal of oxygen-containing functional groups during the reduction. Furthermore, the (111), (200) and (220) diffraction peaks attributed to the pure Cu phase were detected (as depicted by the brown star signs in Fig. 3a) in c-rGO/CuNWs and h-rGO/CuNWs films, implying that coating with c-rGO or h-rGO insignificantly affects the structure of the CuNWs.

Fig. 3
figure 3

a XRD patterns of GO, c-rGO/CuNWs, and h-rGO/CuNWs films. The graphitic peak (002) shifts to higher angles after chemical reduction and hydrogen-annealing reduction, indicating the restacking of rGO sheets. The copper crystal planes show the rGO-coated CuNWs films. b Raman spectra of c-rGO and c-rGO/CuNWs films. c Raman spectra of h-rGO and h-rGO/CuNWs films

Raman spectroscopy was used to examine the structure of GO as well as the coating of rGO layers on CuNWs (as summarized in Additional file 1: Table S1). In Additional file 1: Fig. S2, GO exhibits two prominent peaks; the D band indicates disorder of the graphitic structure or defects, and the G bands refer to sp2-bonded carbon atoms, which appear at 1351 and 1603 cm−1, respectively. The characteristic peaks obtained from Raman spectroscopy are consistent with a previously reported article regarding carbon materials [52]. Herein, the intensity ratio of the D and G bands (ID/IG ratio) increases from 0.99 for GO to 1.19 and 1.18 for c-rGO and h-rGO, respectively. The increase in defects caused by the removal of the oxygen-containing functional groups on GO hence verifies the formation of rGO. In addition, the weak 2D and D + G bands at 2600–3000 cm−1, arising from the sp2 domains and the π band in the graphitic electronic structure, can be used to define the graphene layers by peak shift and peak shape [53]. Based upon the peak positions in c-rGO and h-rGO, the 2D and D + G bands are approximately 2695 cm−1 and 2920 cm−1, respectively, confirming the disordered structure of multilayered graphene (2–10 layers) [54]. Comparing c-rGO with h-rGO, the Raman results of the c-rGO/CuNWs and h-rGO/CuNWs films, depicted in Fig. 3b and c, respectively, show the same ID/IG ratio, suggesting that the degrees of reduction for c-rGO and h-rGO are not affected after coating on the CuNWs film. Notably, the upshift of the G band from 1593(c-rGO/CuNWs) to 1998 cm−1 (c-rGO) can be observed; likewise, the upshift of the G band from 1590 cm−1 (h-rGO/CuNWs) to 1600 cm−1 (h-rGO) is also detected. The higher upshifting of the G band indicates a stronger p-type doping effect of h-rGO/CuNWs, which probably originated from a strong interaction existing in the rGO–Cu interface. The interfacial interaction may result from the chemical bonding between Cu and functional groups on GO, leading to not only a high interfacial adhesion but also an enlarged work function difference at the interface between rGO and Cu, which is beneficial for charge transfer across the rGO–Cu interface [55, 56]; therefore, after the H2-annealing reduction, a facilitated charge transfer process can be expected in h-rGO/CuNWs network, which will be beneficial for TCF applications.

Herein, we also studied the optoelectronic properties (T at λ = 550 nm) as well as the resistance to atmospheric oxidation of the pristine CuNWs, c-rGO/CuNWs, and h-rGO/CuNWs TCFs. As depicted in Fig. 4(1a), both T and Rsh of CuNWs film decrease as the CuNWs density increases, where the density was designed by controlling the vacuum filtration amount of the CuNWs solution. The optoelectronic properties of Rsh and T change from 869.7 Ω/sq and 96.6% to values of 6.8 Ω/sq and 74.9%, respectively, as the amount of the CuNWs solution increases from 3 to 12 ml. Moreover, there is a turning point at Rsh = 22.0 Ω/sq and T = 89.2% when 6 ml of the CuNWs solution was used, indicating a desirable optoelectronic performance with low Rsh and high T; therefore, 6 ml of the CuNWs solution was adopted in the subsequent studies. The oxidation resistance of the CuNWs TCFs was investigated by measuring the change in Rsh after different exposure durations to ambient atmosphere. In Fig. 4(1b), owing to the ease of oxidation of the CuNWs film, Rsh increases dramatically as the CuNWs density is low (3 and 4 ml CuNWs solution), implying that the sparse electrical conductive path is easily destroyed by oxidation. It is also noted that the CuNWs film with high density (> 8 ml CuNWs solution) shows good electrical stability because the dense CuNWs network can serve as a barrier to protect the CuNWs located at the bottom from oxidation, providing a conductive path for the electrical percolation threshold. However, the sacrifice of transmittance (T < 80%) makes the CuNWs films impracticable for optoelectronic applications.

Fig. 4
figure 4

Schematic illustration of (1) CuNWs, (2) c-rGO/CuNWs films, and (3) h-rGO/CuNWs films. Column a optical transmittance versus sheet resistance and b variations of the sheet resistance versus exposed time

Therefore, a transparent gas barrier to prevent oxidation of the CuNWs film is imperative. In this regard, the CuNWs films (6 ml CuNWs solution) were coated with c-rGO and h-rGO, and their optoelectronic performance and oxidation resistance were investigated. As shown in Figs. 4(2a) and (3a), both the c-rGO/CuNWs and the h-rGO/CuNWs films exhibited the same trend of increasing Rsh and decreasing T as the number of dipping cycles increased. Although an increase in the rGO layers reduces the local resistance, the overall sheet resistance of the TCF is dominated by the CuNWs; therefore, excessive rGO layers suppress electron transport in the rGO/CuNWs films. It is worth noting that the Rsh value of the c-rGO/CuNWs film with high dipping cycles (> 5 times) shows a dramatic increase after 20 days, as shown in Fig. 4(2b), which could be attributed to crevice corrosion induced by the low oxygen concentration in the gap between wrinkled c-rGO and CuNWs [57]. Copper oxidation is an electrochemical reaction involving the transport of electrons to the cathode. The primary anodic and cathodic reactions of copper oxidation are described by Eqs. (3) and (4), respectively [58]:

$$mathrm{Cu }to {mathrm{Cu}}^{2+}+2{mathrm{e}}^{-}$$


$$frac{1}{2}{mathrm{O}}_{2}+{mathrm{H}}_{2}mathrm{O}+2{mathrm{e}}^{-}to 2{mathrm{OH}}^{-}$$


As the dipping cycle increases, the nonuniform c-rGO distribution and non-adhesion c-rGO layers result in various open spaces at c-rGO/CuNWs surface, as shown in Fig. 5a and b. Because the crevices between c-rGO and CuNWs merely have extremely low oxygen concentrations, it leads to local anodic oxidation, while cathodic reduction occurs at the rest of the material, which results in accelerated corrosion. In contrast, increasing dipping cycles enable h-rGO layers to uniform cover the overall CuNWs film (Fig. 5c and d). Additional file 1: Fig S3a and S3b show the AFM image of h-rGO/CuNWs film and the line profile along the blue dashed lines in the AFM image, respectively. The h-rGO layers can cover the overall CuNWs surface with surface roughness average (Ra) and root mean square roughness (Rq) of 5.4 nm and 7.2 nm, respectively. The step height between two r-GO sheets is approximately 3.4 nm, indicating a three-layered rGO structure. In addition, the h-rGO/CuNWs TCF with 5 dipping cycles shows long-term durability after 30 days, which only results in a slight increase in Rsh from 25.1 to 42.2 Ω/sq while retaining an acceptable transmittance of 85.9%. The stable atmospheric oxidation resistance of h-rGO/CuNWs TCFs demonstrates that h-rGO can uniformly cover the CuNWs and play a role in atmospheric corrosion protection.

Fig. 5
figure 5

FESEM images of c-rGO/CuNWs with a 1 dipping cycle and b 5 dipping cycles. The open spaces between nonuniform c-rGO layers result in accelerated corrosion of CuNWs. FESEM images of h-rGO/CuNWs with c 1 dipping cycle and d 5 dipping cycles. The h-rGO/CuNWs with more dipping cycles shows large-area and uniform coverage of h-rGO layers on CuNWs surface

We conducted dynamic bending tests to examine the flexibility of the CuNWs and h-rGO/CuNWs TCFs. In Fig. 6a, no obvious increase in sheet resistance of the h-rGO/CuNWs TCF was detected as the sample was bent to a radius of 5.0 mm. Even though the radius of curvature was further decreased to 1.6 mm, the Rsh of h-rGO/CuNW TCFs showed only a negligible increase of 0.6 Ω/sq, implying the excellent flexibility of the h-rGO/CuNWs TCFs compared with ITO. Moreover, the CuNWs and the h-rGO/CuNWs TCFs were bent for 2,500 cycles under a bending radius of 5.3 mm to test the electromechanical stabilities; as shown in Fig. 6b, both the CuNWs and the h-rGO/CuNWs TCFs showed good flexibility with ΔRsh values of 2.0 and 1.0 Ω/sq, respectively. The superior flexibility of the h-rGO/CuNWs TCFs is contributed by the tight adhesion between the h-rGO and CuNWs, which stabilizes the CuNWs on the substrate.

Fig. 6
figure 6

a Sheet resistance as a function of the radius of curvature of the h-rGO/CuNWs film. b Variation in the sheet resistance of the CuNWs and h-rGO/CuNWs films on the PET substrate as a function of bending cycles. The films were bent to a radius of curvature of 5.3 mm, as shown in the inset

The FOM value is generally used to evaluate the optimal compromise between the optical transmittance and electrical conductivity of materials. The FOM can be calculated according to the equations shown as follows [59,60,61]:

$$T = left( {{ }1 + { }frac{188.5}{{R_{{{text{sh}}}} }}frac{{sigma_{{{text{OP}}}} }}{{sigma_{{{text{DC}}}} }}} right)^{ – 2}$$


$${text{FOM}} = { }frac{{sigma_{{{text{OP}}}} }}{{sigma_{{{text{DC}}}} }} = frac{188.5}{{R_{{{text{sh}}}} left( {frac{1}{sqrt T } – 1} right)}}$$


where T is the transmittance, ({R}_{mathrm{sh}}) is the sheet resistance, σOP is the optical conductivity and σDC is the direct current conductivity. The higher FOM values indicate lower sheet resistance at a given transmittance. To provide a holistic optoelectronic performance, we summarize the transmittance, sheet resistance, and FOM value of selected state-of-the-art flexible TCFs, as shown in Fig. 6a and Additional file 1: Table S1 in the Supporting Information [1, 2, 14,15,16, 18, 19, 28, 29, 39, 40, 59,60,61,62,63,64,65,66,67,68,69,70]. The optoelectronic performance of h-rGO/CuNWs TCF achieves Rsh = 18.2 Ω/sq and T = 86.9% with an FOM value of 142.8, which is competitive with most TCFs, including graphene, CNTs, PEDOT:PSS, CuNWs, and ITO. Although the AgNWs-based TCFs can reach higher FOM values due to their intrinsic electrical conductivity, the expensive price of silver restricts their industrial applications. Herein, we calculated the cost of the preparation of the CuNWs and AgNWs based on this study and our previous work [19], and the results clearly show that the CuNWs are 86.8% cheaper, mainly due to the lower cost of solvent and metal precursor, as shown in Fig. 6b. In addition, the aqueous process of preparing CuNWs is much more environmentally friendly than that of AgNWs using ethylene glycol as a solution, where toxicity and flammability are the most concerning issues [71]. The results show that the fabricated h-rGO/CuNWs TCF film possesses exceptional optoelectronic properties with high flexibility (Fig. 7), atmospheric oxidation resistance, and cost-effectiveness.

Fig. 7
figure 7

a Optoelectronic performances of selected state-of-the-art flexible TCFs. b Price analysis of preparation of the CuNWs and AgNWs. The total price is divided into four parts (metal precursor, capping agent, solvent, salt and other chemicals), which clearly show that the fabrication of CuNWs is more cost-effective

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