Surface coordination layer passivates oxidation of copper

Abstract

Owing to its high thermal and electrical conductivities, its ductility and its overall non-toxicity1,2,3, copper is widely used in daily applications and in industry, particularly in anti-oxidation technologies. However, many widespread anti-oxidation techniques, such as alloying and electroplating1,2, often degrade some physical properties (for example, thermal and electrical conductivities and colour) and introduce harmful elements such as chromium and nickel. Although efforts have been made to develop surface passivation technologies using organic molecules, inorganic materials or carbon-based materials as oxidation inhibitors4,5,6,7,8,9,10,11,12, their large-scale application has had limited success. We have previously reported the solvothermal synthesis of highly air-stable copper nanosheets using formate as a reducing agent13. Here we report that a solvothermal treatment of copper in the presence of sodium formate leads to crystallographic reconstruction of the copper surface and formation of an ultrathin surface coordination layer. We reveal that the surface modification does not affect the electrical or thermal conductivities of the bulk copper, but introduces high oxidation resistance in air, salt spray and alkaline conditions. We also develop a rapid room-temperature electrochemical synthesis protocol, with the resulting materials demonstrating similarly strong passivation performance. We further improve the oxidation resistance of the copper surfaces by introducing alkanethiol ligands to coordinate with steps or defect sites that are not protected by the passivation layer. We demonstrate that the mild treatment conditions make this technology applicable to the preparation of air-stable copper materials in different forms, including foils, nanowires, nanoparticles and bulk pastes. We expect that the technology developed in this work will help to expand the industrial applications of copper.

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Fig. 1: Anti-corrosion properties of Cu foils after the formate treatment.
Fig. 2: STM and AFM imaging of formate-treated Cu.
Fig. 3: Importance of Cu(110) for effective passivation.
Fig. 4: Anti-corrosion strategy for stabilizing Cu NWs and room-temperature electrochemical anti-corrosion technique.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

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Acknowledgements

We acknowledge financial support from the National Key R&D Program of China (2017YFA0207302, 2017YFA0207303, 2016YFA0300901 and 2017YFA0205003), the National Natural Science Foundation of China (21890752, 21731005, 21721001, 11634001, 21725302, 11888101, 21905237 and 91845102), the China Postdoctoral Science Foundation (2017M612131), the Beijing Municipal Science & Technology Commission (Z191100007219005) and the Tencent Foundation through XPLORER PRIZE. Part of this research used Beamline 02B and Beamline 03U of the Shanghai Synchron Radiation Facility, which is supported by ME2 project under contract no. 11227902 from the National Natural Science Foundation of China. We thank H. Häkkinen for discussions, and Y. Cao and W. Song for providing the Cu–graphene sample. We also thank P. Cheng from Analytical Instrumentation Center, SPST, Shanghai Tech University for help with the XPS measurement.

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Contributions

N.Z. conceived and supervised the research project. J.P. synthesized and characterized the samples and investigated their anti-corrosion performance. G.F. supervised the DFT calculations and analysed the computational results together with B.C., who conducted the calculations. Y.J. supervised the STM and AFM measurements, which were carried out by Z.W., J.G., S.Y. and D.C.; Q. Zhang and L.G. contributed to the HRTEM characterizations of Cu-FA samples. S. Hao, A.F., H.X., S. Hong, and B.W. contributed to the sample preparations and characterizations. C.-J.L. supervised the SRET measurements. L.-S.Z. and Z.S. contributed to the TPD-MS measurements. Q. Zhou and Z.L. contributed to the XPS measurements. N.Z., G.F., Y.J., J.P., B.C., Z.W. and J.G. wrote and revised the manuscript.

Corresponding authors

Correspondence to Gang Fu or Ying Jiang or Nanfeng Zheng.

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The authors declare no competing interests.

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Peer review information Nature thanks C. Richard Catlow and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Anti-corrosion properties of Cu-FA.

Cu-FA shows outstanding anti-corrosion properties while maintaining the excellent thermal and electrical conductivities of Cu, as revealed by qualitative and quantitative evaluations of the anti-corrosion performances of Cu foils before and after formate treatment. a, Optical and SEM images (left) and Raman spectra (right) of brass, bronze, Cu-G and Cu-BTA before and after corrosion in 0.1 M NaOH at 25 °C for 8 h. The Raman bands in the spectral ranges 170–230 cm−1, and 270–320 cm−1 and 590–680 cm−1 are attributed to Cu–O vibrations from Cu2O and CuO species68, respectively. The weak Raman bands in the range 564–600 cm−1 come from ZnO species69. The Raman band at ~535 cm−1 is assigned to the triazole ring bending mode70. b, Optical image (top), Raman spectra (middle) and XRD patterns (bottom) of the Cu foil and Cu-FA before and after corrosion in 0.1 M NaOH for 12 h. Raman bands centred at 298 cm−1 and 628 cm−1 come from CuO species. The XRD pattern of the Cu foil after corrosion reveals the formation of CuO. By contrast, Cu-FA remains almost the same after the 12-h corrosion as before the test. c, d, Electric (c) and thermal (d) conductivities of Cu, Cu-FA (prepared by Method I), brass and bronze foils before and after corrosion in 0.1 M NaOH at room temperature for 12 h. e, Microphotograph and corresponding Raman spectrum showing the corrosion of the Cu foils before treatment in a sodium formate–H2O mixture at 160 °C for 24 h. In the corrosion test, the foils were immersed in 0.1 M NaOH at 25 °C for 24 h. The two groups of Raman bands observed at (146, 217, 417, 532) cm−1 and (307, 628) cm−1 are assigned to Cu2O and CuO species, respectively68. f, Microphotograph and corresponding Raman spectra of Cu-FA before and after corrosion in 0.1 M NaOH at 25 °C for 24 h. (g) Cyclic voltammetry (CV) curves of bare Cu and Cu-FA in 0.1 M NaOH. Two anodic current peaks and two cathodic current peaks are observed for bare Cu, which can be assigned to the two Cu redox reactions. Whereas the anodic peaks at cell potentials of −0.30 V and −0.10 V are due to the forward reactions, the cathodic peaks at −0.40 V and −0.75 V are attributed to the reverse reactions. However, no oxidation peaks were observed for Cu-FA, indicating the substantial suppression of Cu oxidation. h, Tafel plots of bare Cu and Cu-FA in 0.1 M NaOH. The Tafel plot of bare Cu is in good accordance with the reference71. Although there is slight change in the peak potential, the corrosion current density of Cu-FA is 20 times lower than that of bare Cu. The slight shift in the Tafel plot of Cu-FA suggests that the transfer of oxygen from the bulk solution to the cathodic sites of Cu is inhibited by the formate treatment72. The polarization parameters of bare Cu and Cu-FA in 0.1 M NaOH are: Ecorr = −222 mV, Jcorr = 6.71 μA cm−2, corrosion rate 78.2 μm yr−1 (bare Cu); Ecorr = −213 mV, Jcorr = 0.33 μA cm−2, corrosion rate 3.89 μm yr−1 (Cu-FA); anti-corrosion factor 20.1. The anti-corrosion factor is defined as the ratio of the corrosion rate of the bare Cu foil to that of the modified Cu in 0.1 NaOH. i, Raman spectra of bare Cu and Cu-FA after electrochemical tests. The Raman bands at (149, 217) cm−1 and 636 cm−1 come from Cu2O and CuO species, respectively68. j, Nyquist impedance plots of bare Cu and Cu-FA at 0.1 V versus Ag/AgCl. At high frequency, the capacitive impedance of the electrode–electrolyte interface becomes more effective at shunting the charge-transfer resistance. Therefore, the charge-transfer resistance calculated from the impedance difference at lower and higher frequencies is used to qualitatively evaluate the corrosion rate. Compared with the bare Cu, a 14-fold increase in the charge-transfer resistance is observed on Cu-FA, indicating that the corrosion is indeed strongly inhibited by the FA modification. Z′ and Z″ are the real and imaginary parts of the impedance, respectively.

Extended Data Fig. 2 Comparison of anti-corrosion performance of Cu-FA with Cu foils with other corrosion inhibitors.

a, Electrochemical characterizations of Cu-FA foil and Cu foils treated with DT and BTA corrosion inhibitors. Whereas all measurements were carried out in 0.1 M NaOH, the Nyquist impedance and Bode plots were recorded at 0.1 V versus Ag/AgCl. The polarization parameters of Cu foils with different surface treatments in 0.1 M NaOH are: Ecorr = −212 mV, Jcorr = 5.12 μA cm−2, corrosion rate 59.7 μm yr−1, anti-corrosion factor 1.31 (Cu-BTA); Ecorr = −210 mV, Jcorr = 4.33 μA cm−2, corrosion rate 50.4 μm yr−1, anti-corrosion factor 1.55 (Cu-DT). For comparison, the anti-corrosion factor of Cu-FA in 0.1 M NaOH is 20.1. b, Tafel and Nyquist plots, SEM images and Raman spectra of the samples before and after the electrochemical measurements. The Nyquist plots were measured at 0.1 V versus Ag/AgCl in 1 M NaOH. The Raman bands at 297 cm−1 and 623 cm−1 come from CuO species68. Owing to the concentration difference, the Tafel plot of Cu-FA in 1 NaOH displays a minimum that is different from that in 0.1 M NaOH. The polarization parameters of Cu foils before and after the formate treatment in 1.0 M NaOH are: Ecorr = −281 mV, Jcorr = 59.7 μA cm−2, corrosion rate 695.8 μm yr−1 (bare Cu); Ecorr = −311 mV, Jcorr = 5.10 μA cm−2, corrosion rate 59.4 μm yr−1 (Cu-FA), anti-corrosion factor 11.7. The anti-corrosion factor is defined as the ratio of the corrosion rate bare Cu foil to that of modified Cu in 1 NaOH. c, SEM images of bare Cu and Cu-FA (prepared by Method II) foils after exposure to 30% H2O2 for 0, 5 and 15 min. d, Optical images of Cu-FA (prepared by Method II) and untreated Cu foils before and after heating at 160 °C in air for 1 h and corresponding Raman spectra after thermal treatment. e, Localized corrosion of Cu-FA monitored by SRET/STM. The three-dimensional corrosion current maps were recorded on scratched Cu-FA and bare Cu foils after immersion in 0.1 M NaOH for 5, 30 and 90 min, and SEM images before and after the SRET/STM test were taken to identify the oxidized area.

Extended Data Fig. 3 Effect of treatment time and crystallinity on the anti-corrosion performance of Cu.

a, Photographs and microphotographs of Cu-FA foils after corrosion in 0.1 M NaOH for 24 h as a function of treatment time. The foils were obtained with the formate treatment at 160 °C for different periods. b, XRD spectra of different Cu foil samples treated hydrothermally in the presence of formate in different conditions. Sample 1: Cu-FA foil (25 μm; Alfa Asear) treated at 160 °C for 0–12 h (Method II). Sample 2: Cu foil (25 μm; Alfa Asear) treated at 200 °C for 2–24 h (Method I). Sample 3: Cu foil (10 μm; Aladdin) treated at 200 °C for 0–4 h (Method I). c, Cross-sectional TEM and high-resolution STEM images of Cu-FA (I–II) and bare Cu foils (III–IV), using samples prepared by the focused-ion-beam technique. d, Cross-sectional TEM and high-resolution TEM (HRTEM) images of Cu-FA (1–3) and bare Cu foils (4–6), using samples prepared by microtoming.

Extended Data Fig. 4 Identification of the surface coordinating species on Cu-FA.

a, STM images of Cu-FA foils (prepared by Method II) and untreated Cu foils after annealing for 4 h in UHV at 150 °C or 300 °C. A step height of 0.258 nm, a diatomic step height of Cu(110), was readily observed after annealing at 150 °C, although the high-resolution STM images were obtained only after annealing at 300 °C. In comparison, no ordered structure was observed on untreated Cu foils with different annealing conditions. b, STM images showing the successful formation of the c(6 × 2) superstructure on single-crystal Cu(110) treated with sodium formate solution followed by annealing at 150 °C, which is similar to the temperature used in the formate treatment to create effective passivation. The presence of dark depressions (highlighted by yellow arrows) may arise from interstitial O2− species of the hydrated c(6 × 2) surface structure. c, Structure models of the paddle-wheel dinuclear Cu(ii) formate complex and the Cu(110) surface co-passivated by [Cu(μ-HCOO)(OH)2]2 and O2−. d, FTIR, Raman and TPD-MS spectra of the Cu-FA foils (prepared by Method II) after annealing under the same conditions as those used for STM imaging and re-exposure to air. The inset in the Raman panel is the optical photograph of the annealed sample in air. The presence of a broad infrared absorption band at 3,378 cm-1 clearly suggests the presence of abundant -OH groups on the surface. The Raman spectrum was obtained by using Au@SiO2 SHINERS particles to enhance the Raman signals (524 cm−1 for Cu-O, 1,404 and 2,920 cm−1 for the C–H vibration (νC–H) on formate)73,74. Both control Raman spectra of the SHINERS particles and Cu-FA foils are given for comparison. The TPD-MS profiles clearly show the release of H2O and HCOO species from Cu-FA in a wide range of temperatures. The inset in the TPD-MS spectra shows the relative ionization intensities of the detected species. e, Tafel curves of Cu-FA before and after annealing. f, Linear sweep voltammetry of Cu-FA after annealing, obtained with a scan rate of 2 mV s−1 from −1.2 V to 0 V (versus Ag/AgCl) in 0.1 M NaHCO3 solution (pH 8). The Cu(ii) and Cu(i) species were detected with redox potentials in good accordance with reported values52. g, XPS spectra of bare Cu and Cu-FA foils after annealing under the same conditions as those used for STM/AFM characterizations. Once exposed to air, the bare Cu foil displays obvious peaks (935.1, 940.8 and 944.1 eV) corresponding to the oxidized Cu species. In comparison, the Cu 2p XPS (full-width at half-maximum, FWHM = 0.9 eV) and Cu LMM Auger spectra of annealed samples of bare Cu (without air exposure) and Cu-FA foils are almost identical and display peaks of metallic Cu reported in the literature75,76. These data clearly suggest the presence of a non-detectable amount of oxidized Cu species on annealed Cu-FA. Whereas the presence of OH is clearly observed in the O 1s XPS spectrum of the unannealed Cu-FA foil, the annealed Cu-FA displays O 1s XPS signals from O from the formate (532.3 eV, FWHM = 1.7 eV) and O2− on Cu (530.4 eV, FWHM = 1.2 eV) in a ratio close to 1. No obvious OH signal is identified on the annealed sample. In comparison, the bare Cu foil after air exposure displays a major O 1s XPS signal corresponding to OH. The annealed Cu foil shows O 1s XPS signals at 530.9 (FWHM = 1.2 eV), 531.9 (FWHM = 1.7 eV) and 533.4 (FWHM = 1.3 eV), corresponding to O2−, OH and O from the carbonate, respectively77. The C 1s spectra of both annealed Cu and Cu-FA foils show the dominant presence of carbon contamination. Source data

Extended Data Fig. 5 The importance of Cu(110) for the anti-corrosion properties.

a, STM topographies of the single-crystal Cu(110)-c(6 × 2) sample after exposure to air and then annealing at 120 °C. The zoom-in STM image shows distortion and darker depressions in the Cu(110)-c(6 × 2) superstructure, suggesting the occurrence of a hydration process during air exposure. b, STM image of the single-crystal Cu(110)-c(6 × 2) sample after air exposure followed by annealing at 300 °C. The regeneration of the dehydrated c(6 × 2) structure without dark depressions was confirmed. c, Structure models showing the adsorption of O2 and Cl on clean Cu(110) (I, II) and FA-modified Cu(110) (III, IV). O2 is easily dissociated on clean Cu(110) to form adsorbed O species. The Bader charge of Cu atoms on the modified Cu(110) and reference systems was as follows: (1) modified Cu(110): surface Cu +0.97 to +1.0, subsurface Cu +0.34 to +0.53, bulk Cu 0 to +0.14; (2) reference systems: bulk CuO +0.99, bulk Cu2O +0.57, Cu(110) +0.01 to −0.02. d, XRD patterns of Cu(100), Cu(111) and Cu(110) single crystals. e, XRD patterns of scratched Cu(111) single crystal treated with formate at 160 °C for 0–60 h. If the surface was not scratched, no change on the XRD pattern was detected even for treatment time of up to 60 h. f, Micrographs and Raman spectra of Cu(110), Cu(100) and Cu(111) single-crystal samples treated by an aqueous solution of formate at 100 °C for 1 h, 10 h and 10 h, respectively. The two groups of Raman bands at (146, 217, 416, 535) cm−1 and 635 cm−1 are attributed to the Cu2O and CuO species, respectively68.

Extended Data Fig. 6 Enhanced anti-corrosion performance in harsh conditions by introducing alkanethiol.

a, Structure model of Cu-FA with alkanethiol bound to the step sites, and experimental evidence of the successful introduction of 1-DT ligands. Whereas the change of contact angle of the water droplets suggests the change of surface hydrophobicity, TPD-MS confirms the presence of formate and DT on Cu-FA/DT. The Cu-FA foils were prepared by Method III. b, Electrochemical measurements (CV, Tafel, Nyquist impedance and Bode plots) confirming the enhanced anti-corrosion properties of Cu-FA after the DT treatment. All measurements were carried out in 0.1 M NaOH solution. The Nyquist impedance and Bode plots were measured at 0.1 V versus Ag/AgCl. The polarization parameters of Cu-FA/DT foils in 0.1 M NaOH and 1 M NaOH are: Ecorr = −258 mV, Jcorr = 0.067 μA cm−2, corrosion rate 0.755 μm yr−1, anti-corrosion enhancement 103.6 (0.1 M NaOH); Ecorr = −331 mV, Jcorr = 0.38 μA cm−2, corrosion rate 4.40 μm yr−1, anti-corrosion factor 158.1 (1 M NaOH). The anti-corrosion factor is defined as the ratio of the corrosion rate of the bare Cu foil to that of the modified Cu in the same NaOH solutions. c, Comparison of optical images and Raman spectra for the evaluation of the enhanced anti-corrosion of Cu-FA (prepared by Method III) against salt spray after introducing DT. Before the measurements, Cu, Cu-FA and Cu-FA/DT foil samples were subjected to a 5% NaCl salt spray test at 47 °C for 24 h. The Raman bands around (149, 217) cm−1 and 640 cm−1 are attributed to Cu2O and CuO species, respectively68. d, Comparison of optical imaging data that reveals the enhanced anti-corrosion of Cu-FA/DT against Na2S. Before recording the images, both Cu-FA and Cu-FA/DT foils were exposed to Na2S solutions with concentrations ranging from 0.1 mM to 1 M for 1 h. Compared to Cu-FA, Cu-FA/DT could survive and keep its copper colour in a much higher concentration of Na2S. e, SEM images and Raman spectra of Cu-FA and Cu-FA/DT after treatment in 10 mM Na2S for 1 h and 5 h, respectively. The Raman bands around 296 cm−1 and 624 cm−1 indicate the formation of oxidized and sulfurized Cu species. f, SEM images of Cu, Cu-FA and Cu-FA/DT foils after seawater inundation at room temperature for 30 days. g, Photographs showing enhanced anti-corrosion of Cu-FA/DT in H2O2. The photographs of the Cu-FA/DT, Cu-FA and Cu foils were taken after they were immersed in 30% H2O2 for different intervals.

Extended Data Fig. 7 Outstanding anti-corrosion performances regardless of the shape and size of Cu materials.

a, SEM images of Cu and Cu-FA (prepared by Method II) wires before and after treatment in 0.1 M NaOH at 60 °C for 60 h and in 3.5 wt% NaCl at 60 °C for 24 h (top), temperature-dependent resistances of Cu and Cu-FA wires (length 10 cm, diameter 1 mm) after heating in air for 24 h (bottom left) and resistance changes of Cu and Cu-FA (prepared by Method II) wires (length 10 cm, diameter 1 mm) after ageing in 0.1 M NaOH at 60 °C for different time periods (bottom right). The data were averaged from three independent measurements. Error bars reflect the standard errors. b, SEM images of the untreated Cu wire and the Cu-FA wire before and after heating at 160 °C in air for 24 h. c, Microphotographs of Cu, Cu-DT, Cu-FA (prepared by Method III) and Cu-FA/DT meshes after a 96-h salt spray test, and their corresponding Raman spectra and relative electric conductivities before (grey) and after (red) the salt spray test. d, Optical photographs of Cu-FA/DT and Cu-FA meshes (prepared by Method III) after immersion in Na2S solutions of different concentrations for 5 h, and microphotographs of Cu–FA/DT and Cu-FA meshes after 6 h of ageing in 10 mM Na2S. e, Robust anti-corrosion performance of Cu-FA/DT tubes under wet mechanical conditions; photographs of the outer and inner walls of Cu-FA/DT and bare Cu tubes (inner diameter 1.6 cm) after the wet mechanical test with flowing 3.5% NaCl solutions (flow rate of up to 1,400 l h−1, 10–30 °C) for 96 h and 12 h, respectively, and their corresponding Raman spectra. f, Robust anti-corrosion performance of Cu-FA/DT foils under wet mechanical conditions; photographs of the wet mechanical test set-up, with salty water (3.5% NaCl, 1% Na2CO3, 1% Na2SO4 and 0.1% NaOH) flowing through the foils, comparison photographs of Cu-FA/DT, bare Cu and patinated Cu foils before and after the wet mechanical tests (72, 12 and 12 h, respectively) and their corresponding Raman spectra after the tests. The polarization parameters of different Cu foils in 0.1 M NaOH solution were measured as follows: Ecorr = −222 mV, Jcorr = 6.71 μA cm−2, corrosion rate 78.2 μm yr−1 (bare Cu); Ecorr = −212 mV, Jcorr = 4.65 μA cm−2, corrosion rate 54.2 μm yr−1, anti-corrosion enhancement 1.44 (patinated Cu); Ecorr = −258 mV, Jcorr = 0.067 μA cm−2, corrosion rate 0.755 μm yr−1, anti-corrosion enhancement 103.6 (Cu-FA/DT). The anti-corrosion enhancement is defined as the ratio of the corrosion rate of the bare Cu foil to that of modified Cu. The patinated Cu foil was prepared by exposing bare Cu foils to 3.5% Na2CO3 salt spray for 60 h.

Extended Data Fig. 8 Application of the anti-corrosion technique to Cu nanomaterials.

a, SEM images of fresh Cu NWs and of untreated Cu NWs exposed to air for 5 days, showing the easy oxidation of untreated Cu NWs. b, XRD patterns of Cu NWs (prepared by Method III), Cu NWs-FA and Cu NWs-FA/DT after a 12-h corrosion test in 1 M NaOH (left), photographs of their water suspensions before and after being stored at room temperature in air for 90 days (top right) and their Raman spectra after storage in air for 5 and 30 days (bottom right). c, TEM and HRTEM images of Cu NWs and Cu NWs-FA. The insets display the corresponding selected-area electron diffraction patterns. d, TEM and HRTEM images of Cu NWs, Cu NWs-FA and Cu NWs-FA/DT before and after 24-h corrosion tests in 1 M NaOH. Typical lattice fringes are labelled in the HRTEM images. e, SEM images of Cu NWs, Cu NWs-FA and Cu NWs-FA/DT after heating at 80 °C for 24 h. f, Performance of passivated Cu NWs in transparent electrodes. The transmittance is shown as a function of the sheet resistance of transparent electrodes made from Cu NWs with different surface modifications. The inset shows a photograph of a transparent electrode with Cu NWs on PET. Together with the resistance change of the Cu NWs-FA/DT film during a bending test with sputtered indium tin oxide (ITO), the resistance changes of transparent electrodes made from untreated Cu NWs, Cu NWs-FA, Cu NWs/DT and Cu NWs-FA/DT after heating at 80 °C in air for 24 h are also provided for comparison. The data were averaged from three independent measurements. Error bars represent the standard errors. g, SEM images of Cu NWs, Cu NWs-FA and Cu NWs-FA/DT before and after a 24-h salt spray test. h, Change of the square resistance of Cu NPs and Cu NPs-FA (prepared by Method II) films after storage at ambient conditions for different periods. The measurements were carried out on films made from Cu NPs and Cu NPs-FA with a thickness of 100 μm. The insets show photographs of Cu NPs and Cu NPs-FA powders after a 14-day exposure to air at room temperature.

Extended Data Fig. 9 Application of anti-oxidation Cu materials to radio-frequency identification, and room-temperature electrochemical technique for preparing anti-corrosive Cu materials.

a, Photograph of kilogram-scale anti-corrosion Cu paste. b, Photograph of radio-frequency identification (RFID) tag antenna. This antenna was designed for ultrahigh-frequency RFID on a cloth substrate (the copper paste was printed on the cloth substrate by screen printing). The working frequency is 915 MHz, and its performance is equivalent to that of similar commercial aluminium-etching antennas. The recognition distance can reach 20.3 m. c, Reflection coefficient S11 of the antenna shown in b, measured by Vector network analyser ZNB8. The reflection coefficient S11 is the most important parameter of the antenna; a smaller S11 implies lower energy loss of the tag antenna in the process of receiving the signal from the card reader, that is, higher energy utilization. These data verify that the antenna works at 915 MHz and also show that the S11 value of the antenna at this frequency can reach −41.5 dB (a very low value), which is equivalent to S11 values reported in the literature78. This suggests that our copper paste can also be used to create high-efficiency and high-performance antennas. d, Stability of the electrical conductivity of our Cu-FA/DT-paste-based RFID tag antenna compared to that of a paste of untreated Cu. For copper-paste-based RFID films, the square resistance was acquired and S = 1/(sheet resistance × thickness of the foils or films). e, CV curve of Cu foil immersed in 2% HCOONa (pH 8–10) recorded at a scan rate of 10 mV s−1, and chronoamperometric curve with reduction potential −0.7 V (versus SCE) applied after an anodic sweep from −0.80 V to 0.20 V (versus SCE; 10 mV s−1). f, Photographs of Cu-FA(EC) and Cu foils before and after a 12-h corrosion test in 0.1 M NaOH. g, SEM images and Raman spectra of Cu-FA(EC) and bare Cu foils after the corrosion test in NaOH. h, Microphotographs and Raman spectra of Cu-FA(EC)/DT, Cu-FA(EC) and Cu foils after immersion in 50 mM Na2S solution for 10 min. i, Schematic illustration of the roll-to-roll setup used to produce Cu-FA(EC). RE, reference electrode; CE, counter electrode; WE, working electrode. An oxidation potential of 0.1 V versus SCE was applied for the generation of surface Cu(ii) when the Cu foil was moving from left to right, and an oxidation potential of −0.7 V versus SCE was used when the foil was moving back from right to left for the surface reconstruction.

Extended Data Table 1 Performance of Cu NWs-FA/DT (prepared by Method II) as transparent conductive electrodes compared with previously reported work

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Peng, J., Chen, B., Wang, Z. et al. Surface coordination layer passivates oxidation of copper. Nature 586, 390–394 (2020). https://doi.org/10.1038/s41586-020-2783-x

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