Scheme 1 displayed the synthetic procedures of Cu-doped PANI porous films and corresponding devices. The Cu-doped PANI porous films were obtained by in situ electrochemical deposition of aniline molecules in H2SO4 aqueous solution. The Au porous substrate layer has two functions: one is to provide conductive framework and external electrode for the electrode of IR electrochromic device; Second, the gold metal has a high reflectivity in the IR range, which can effectively reflect the IR energy through the working electrode of PANI films. When polymerized in acid dopant, the PANI begin to aggregate on the surface of Au/ PES porous membrane. Meanwhile, the color of the membrane surface changes from golden to yellowish green and to dark green as the polymerization charge increased. To reveal the composition information, the XRD patterns are collected. As shown in Fig. 1(a) and Fig. S1, all the samples present a wide peak centered at 18.5°, which could be from the amorphous structure of the PES substrate . As a comparison, the pure PANI film shows a diffraction peak at 2θ = 25.4°, indicating the semicrystalline nature of polyaniline and in consistent with the previous results . With the introduction of copper ions, the PANI signal peaks from Cu-doped PANI porous films become stronger and sharper. According to the previous reports , the sharpness of the diffraction peak usually relates to the mono-distributions of the periodicity between the polymer backbone chains. This suggests the enhancement of the molecular arrangement order in Cu-doped PANI porous films. Furthermore, it is found that two signal peaks from Cu3(SO4)2(OH)2•4H2O and (NH3)2Cu(NO3)2 exist in the prepared Cu-doped PANI porous films, as marked by the diamonds and clubs (PDF# Cu3(SO4)2(OH)2•4H2O and PDF# (NH3)2Cu(NO3)2). It is indicated the possible existence of Cu-N coordination bonds in Cu-doped PANI porous films, and the simultaneous doping of sulfuric and nitric acid. The signal peaks of (NH3)2Cu(NO3)2 from PANI-2 porous film is sharpest among the prepared PANI porous films (Fig. S1). This suggests the existing Cu3(SO4)2(OH)2•4H2O and (NH3)2Cu(NO3)2 may has a great impact on the subsequent electrochromic properties.
To analyze the chemical bonds in the PANI films, the Raman spectra were measured in the range of 100–2000 cm− 1. In Fig. 1b, all the copper-doped PANI porous films present the signals at the same positions. Specifically, the characteristic peaks observed at 1622 and 1572 cm− 1 are assigned to the C-C stretching vibration of the benzenoid ring and C = C stretching vibration of the quinoid ring, respectively. The peak at 1480 ,1313 and 1252 cm− 1 in the spectra are attributed to the C = N stretching vibration of the quinoid ring and the C-N stretching vibrations of the benzenoid ring and quinoid ring, respectively. The C-N•+ stretching vibration of more delocalized polaronic structures is represented by the peak at 1343 cm− 1, and the benzenoid ring deformation in polarons is suggested by the signal at 868 cm− 1 . Compared with the Raman spectrum of the PANI-0 porous film, the Cu-doped PANI porous films show a stronger and sharper absorption peak at 1343 cm− 1. As expectly, this reveals that a large number of polarons and bipolarons delocalized on the PANI chains with the introduction of copper ions. The peaks at 1198 cm− 1 and 812 cm− 1 represent the C-H in-plane bending vibration and out-plane bending vibration in the quinoid ring . Note that the intensity of the peak at 1198 cm− 1 increases with the introduction of copper ion, suggesting the increased oxidation degree of Cu-doped PANI porous films. The similar intensity of the PANI-1, PANI-2 and PANI-3 indicate the comparable REDOX components.
The geometric structure of the material is critical to optical response performance, since the ion transfer between electrode material and electrolyte plays a decisive role during the electrochemical redox process. Herein, the morphologies of PANI porous films were investigated by SEM images. As shown in Fig. S2, similar to that of the PES and Au/PES membranes with a linear porous network structure, pure PANI film presents a fibrous structure, which constructs a reticular membrane. The porous structure benefits to electrolyte infiltration and ion transmission. The surface morphology of the Cu-doped PANI porous films are exhibited in Fig. 2(a)-(d). With the addition of copper concentration, the diameter of the porous structure from Cu-doped PANI films becomes smaller resulting from the filling of smaller PANI nanoaprticles, even though the general frameworksremain consistent. Correspondingly, the surface morphologies of Cu-doped PANI porous films become more compact and rougher. Figure 2(e)-(h) show the cross sections of the prepared Cu-doped PANI films. Obviously, all the PANI layer with the thickness of 40 µm are tightly attached to the Au/PES porous membrane, and uniform voids can be found in the porous structures. The EDS mapping images of Cu-doped PANI film surface are shown in Fig. 3(a), (c)-(f)). The results show that the elements of carbon, oxygen, copper, and nitrogen are uniform distribution on surface of PANI-2 porous films. Compared to that in spectrum 4/6, the higher copper content in spectrum 5 indicates that the copper element uniformly distributed in the prepared films, rather than the local larger nanoparticles, as shown in Fig. 3(b).
To reveal the elements’ electronic state, XPS analysis were conducted. As shown in Fig. S3, all the Cu-doped PANI films show the presence of carbon, nitrogen, oxygen, sulfur and copper element (Fig S3). As a comparison, the copper signals are missing in PANI-0 films. As reported, the electron polarization in the PANI films occurs mainly on nitrogen atoms. Therefore, a fine analysis of the N 1s core level spectra was performed. As shown in Fig. 4(a)-(e), all the spectra from PANI films can be fitted into three major components located at 398.5, 399.5, 401.1 and 402.2 eV, which are attributed to the quinonoid imine (= N-), benzenoid amine (-NH-), protonated amine (-NH2+) and protonated imine (= NH+), respectively. Compared to PANI-0 porous film, the quinonoid imine (= N-) peak disappears in the XPS N 1s spectra from Cu-doped PANI films. This indicates that the copper ions doping induced the transformation of quinonoid imines in PANI films. The = N- structure in the Cu-doped PANI films is converted to a positively charged -N+•- structure, corresponding to the two peaks in the XPS spectra with binding energy greater than 400 eV (i.e. protonated amine and protonated imine). Specifically, with the increase of copper concentration from 0.005 M to 0.02 M, the percentage of the protonated amine and protonated imine increases from 50–66%, while the content of benzenoid amine decreases. It hints that the initial introduction of copper ions promotes the protonation, leading to the formation of protonated amine. When the concentration is further increased to 0.03 M, PANI-3 porous film occur a reverse trend, which the content of protonated amine and protonated imine decrease to 52%. Because the -N+•- structure is polaron structure in the PANI chain , the percentage of protonated amine (-NH2+) and protonated imine (= NH+) represent the number of polarons and bipolarons. As mentioned above, the formation and the elimination of polarons and bipolarons delocalized on the PANI chains are the direct and most critical factors in realizing excellent emissivity modulation performance. Thus, based on the maximum number of polarons and bipolarons, the PANI-2 porous film is expected to exhibit the best infrared electrochromic performance.
To analyze the chemical valence state of copper element and disclose its role on polarons, the high resolution Cu 2p spectra are exhibited in Fig. 5. Benefitting from the sensitivity of X-ray photoelectron energy spectra toward the element coordination environment, the copper signals from all the Cu-doped PANI films can be fitted into two components with the presence of two satellite peaks. The binding energies at 934.98 and 955.7 eV are originated from the characteristic peak of Cu 2p3/2 and Cu 2p1/2 from Cu(II) species, while the two peaks located at 933.3 and 952.5 eV correspond to the Cu 2p3/2 and Cu 2p1/2 from Cu(δ) species, respectively [30–32]. The Cuδ+ (+ 1 < δ < +2) with lower oxidation state could be from Cu-N coordination bonds, and the Cu(II) species come from the metal salt. These are in consistent with those from the above XRD. Such results indicate that the introduction of copper ions not only facilitate protonation of quinonoid imine, but also form metal-nitrogen coordination compounds in polymer films.
According to the above structural analysis, it is anticipated the obvious differences in infrared emission modulation on the Cu-doped PANI films prepared with different copper ions concentration. Cyclic voltammetric measurements were performed to preliminarily determine the electrochemical performance of the Cu-doped PANI porous films. As displayed in Fig. 6, the CV curves of Cu-doped PANI porous film present a pair of redox peaks corresponding to the state of emeraldine salt (ES) and leucoemeraldine (LE). The electrochemical active area under the CV curves is increasing with the increase of copper ions concentration. The PANI-2 porous film shows the largest electrochemical active area, suggesting the maximum amount of charge transfer and the potential optimal infrared electrochromic regulation performance. As reported, the IR electrochromic properties of PANI-based films are directly related to their existential states [33, 34]. Hence, the potentials of -0.25 V and 0.5 V were chosen on account of the films’ states completely transforming between LE and ES. Chronoamperometric (CA) experiment was conducted to investigate of the IR electrochromic performance of PANI-2 and PANI-0 porous films. As shown in Fig. 6(b), the color of the film materials obviously changed from yellow to atrovirens in the voltage positive conversion process. For the PANI-2 porous film, the response time, defined as the times required for achieving 95% change of the full current density, from coloring state to bleaching state (the bias voltage: -0.25 V) can be calculated to be 0.7 s. When the bias voltage of 0.5 V is applied, the response time is determined to be 1.5 s (Fig. S4(a)), much superior to that of pure PANI and previous reports [13, 14, 19, 35]. This can be reasonable by the reticular structure that provides more ion channels and larger reactive area, improving the electrochemical reaction rate. The faster response time of the PANI-2 porous film suggest a more sensitive device. Furthermore, the response time of the present PANI-2 porous film is respectively about 1.3 s and 2.7 s for bleaching state and the coloring state after 50 cycles durability test, as shown in Fig. 6b and Fig. S4(b). Therefore, the prepared PANI-2 films not only show the excellent response ability, but also the good stability.
To quantatively analyze and compare the IR emissivity change of PANI-based films, the Fourier transform infrared spectra were studied in the whole wavelength range from 2.5 to 25 µm and the values of ε are calculated according to Eqs. (1) and (2). Figure 7 presents the emittance curves of Cu-doped PANI porous films at potentials of -0.25 V and 0.5 V. The emittance curves at 0.5 V gradually rise with the copper ions concentration changing from 0.005 M to 0.02 M, while a decrease is observed for the further increasing to 0.03 M. This trend is in consistent with the number of polarons and bipolarons obtainedin XPS results. As discussed above, the copper ions can activate the part of the inactive PANI film. Therefore, the Cu-doped PANI films, especially for PANI-1 and PANI-2 films, have low IR emissivity at -0.25 V. Similar to the emittance curves, the ∆ε between the curves at -0.25 V to 0.5 V presents a change in volcanic patterns. The maximum ∆ε from the PANI-2 porous film is 0.35 in ranges of wavelength 8–12 µm. By contrast, the PANI-0, PANI-0.5, PANI-1 and PANI-3 only provides the ∆ε of 0.1, 0.12, 0.21and 0.06, respectively, as shown in Fig. 7(e). The decreased ∆ε of PANI-3may be caused by the dense microstructure of PANI-3 porous film, hindering the transmission of carriers [33, 36]. Therefore, the PANI-2 porous film exhibits the best infrared modulation performance in this work (∆ε in the wavelength ranges of 2.5 to 25 µm). It indicates that the evolution of the emittance curves largely depend on the copper ions concentration and microstructures of PANI on the porous films.
Based on the best Δε value, the PANI-2 porous film is selected as the functional layer to assemble IR electrochromic (EC) device. Figure 8(a)-(d) exhibits the digital photographs of the Cu-doped PANI IR EC device at different voltages. When the applied voltage is fixed at 0.3V, the device presents the color change from yellow to green. As the applied voltage increases to 0.5 V, the color of the device changes to dark green. When the applied voltage reaches 0.8 V, the device presents atrovirens. The results demonstrate that the Cu-doped PANI IR devices can tunably blend with green or yellow background under different voltages.
In order to explore the application potential of the Cu-doped PANI IR EC device in optical and thermal management, the IR emissivity of EC device was also investigated in the wavelength ranges of 3–5 µm and 8–12 µm at the different voltages. As shown in Fig. 8(e), the evolution of ε in two wavelength ranges shows a similar tendency. That is, the emissivity gradually increases along with the applied voltage. When the applied voltage reaches 0.5 V, the emissivity of device increases a little and tends to be stable as a result of the PANI layer in the ES state. Note that the present △ε of 0.32 in 8–12 µm is much larger than that (0.24) from the previous pure sulfuric acid-doped PANI device . In addition, the response time and stability of the device were also studied in Fig. 9(f). The coloring time of the device is about 5.74 s (1st cycle) and 6 s (50th cycle), while the fading time is about 3.77 s (1st cycle) and 5 s (50th cycle). Meanwhile, the current density of device still retains less than 0.3 mA cm− 2 loss after the 50 cycles. Therefore, the present IR electrochromic device exhibits the superior tenability and the good electrochromic stability.