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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Organic Nanostructures on Inorganic Ones: An Efficient Electrochromic Display by Design Suryakant Mishra,† Simran Lambora, Priyanka Yogi, Pankaj R. Sagdeo, and Rajesh Kumar* Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol 453552, India

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S Supporting Information *

ABSTRACT: Improved electrochromism has been reported from a hybrid nanoheterostructure-based array designed using transition-metal oxides and conducting polymers. An improvement in color contrast, coloration efficiency, and operating voltage makes these hybrid core−shell-type nanostructures (NSs) suitable for power efficient and reversible electrochromic applications showing switching between transparent and opaque states rather than resulting in colored/ bleached switching. Nanopetals (NPs) of nickel oxide have been used as the backbone to grow nanohemispheres (NHs) of polyaniline onto a fluorine-doped tin oxide electrode using a two-step synthesis methodology consisting of a hydrothermal method, followed by an electrodeposition method. The coaxial NPs/NHs core−shell arrays exhibit a better electrochromic performance compared to their individual constituents. Devices fabricated using these hybrid NSs show power efficient optical switching between transparent and opaque with fast response and a good cycle life of approximately 1500. The coloration efficiency of the fabricated device has been calculated to be more than 145 cm2/C and an optical modulation of more than 45%. KEYWORDS: nanostructures, transition-metal oxides, conducting polymers, nanopetals of nickel oxide, electrochromic

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INTRODUCTION A worldwide great investment, in terms of scientists’ time and money, in the field of nanoscience and nanotechnology has delivered its promises up to a great extent especially in the fields of optoelectronics, materials science, and energy.1−5 One of the most convincing reasons for this ongoing success story is related to application-specific (nano)material design. In the current scenario, new nanomaterials need to be designed by adopting a dual approach where these not only are used for potential applications directly in energy storage but also make other electronic devices more power-efficient. This approach will address the increasing demands by making good energy storage/generating devices as well as reducing the power consumed by other electronic devices. An electrochromic display is a very important electronic device where efforts are being made in developing novel materials to be used in powerefficient devices.6−8 Devices based on the electrochemical activity have been showing interesting behavior by controlling the nanoscale architecture of the active material.9,10 Among these, electrochromism exhibited by a fabricated electrode is one of the promising technologies, having various potential applications in the current era of advancement from automobile to smart buildings to display systems and many more.11,12 Various organic and inorganic nanostructures (NSs) and their combinations as gel forms6−8 along with conducting polymers (CPs) can be used as active materials to exhibit efficient electrochromic properties. Currently, CPs are in increasing demand because they are biocompatible, solution-processable, and inexpensive in nature. © XXXX American Chemical Society

CP NSs exhibit high electrical conductivity, large surface area, short path lengths for the ion transport, and superior electrochemical activity compared to their bulk counterparts, which make them suitable for various applications.13−16 Further, hybrid combinations of CPs with inorganic NSs show their combined properties and is found to be suitable for various applications.15,16 At the same time, tuning the nanoscale morphology proves to be very efficient in electrochemical-based application such as supercapacitors,17,18 electrochromic displays,19,20 water splitting,21−23 sensors,24−26 etc. Along with the general properties of these hybrid NSs, their tunable structures prove them to be outstanding functional segments or building blocks for various nanomaterial-based devices.27,28 The nanowire- or nanofiber-type shell of electrochromically active CP on transition-metal oxides (TMOs) shows a change in color as a result of electric bias. It would be interesting to see how the morphology of a polymer shell on a TMO affects the electrochromic properties. Although they are expected to show different behavior, it would be interesting to look for an inorganic/organic combination with the appropriate morphology that can result in transparent/opaque switching as a result of bias. This can be achieved even in the form of a device using these hybrid NSs grown on an electrode by appropriately using another electrode for biasing. Received: May 25, 2018 Accepted: June 7, 2018

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DOI: 10.1021/acsanm.8b00871 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 1. Schematic presentation of the two-step synthesis process of electrochromic electrode preparation combined with hydrothermal and electrodeposition methods.

The significance of growing these special kinds of hybrid NSs is to improve the performance of various parameters of the electrochromic device like the stability, color contrast ratio, and optical modulation. In some of the existing reports, it is observed that the combination of nickel and polyaniline (PANI) shows electrochromism with certain limitations. Xia et al.29 grew nickel oxide (NiO) NSs by a chemical bath method, resulting in a limited cycling performance, and most of the structures got peeled out from the electrode during the operation. Additionally, some reports show poor electrochromic behavior of cheeselike NiO NSs combined with PANI because of less surface area for the electrochemical activity, where the overall color contrast diminishes.30 The current work reports significant enhancement in the electrochromic switching by addressing the shortcomings of the already available electrochromic device paradigms through the nanomaterials’ design, which includes miniaturization, an inherent part of nanotechnology. Improvement in the device performance has been observed from typical hybrid core−shell NSs with novel morphology where rose-petal-like nanopetals of NiO (NiO NPs) are covered with PANI nanohemispheres (NHs) in a coaxial manner. The NiO NPs have been fabricated by a hydrothermal method on a conducting fluorine-doped tin oxide (FTO)-coated glass substrate, whereas CP NHs have been synthesized by an electrodeposition method. A new preparation recipe (as described in the Experimental Details section) has been adopted for the synthesis. A typical morphology of the hybrid core−shell NS shows improved optical modulation in the electrochromism compared to their parental materials. The combination of both the materials and their morphologies (NPs covered by NHs) on a conducting and transparent FTO substrate provides an excellent platform to enhance the electrochromism in the device form where switching between transparent and opaque states takes place rather than switching between two colored states, where the contrast is always a concern in addressing

which a power-efficient electrochromic device has been demonstrated.

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EXPERIMENTAL DETAILS

Two-step synthesis methodology has been used for the material fabrication. First, NiO NPs have been deposited on an FTO electrode using a hydrothermal technique, followed by electropolymerization of aniline to get the shell of PANI on it. A detailed recipe for fabricating a NiO NP array has been discussed elsewhere and is summarized in the Supporting Information (SI). In short, nickel nitrate with potassium persulfate as the oxidizing agent and a small amount of an ammonium solution was used to prepare a precursor solution. After a hydrothermal reactor was placed in an electric oven for 7 h at a temperature of 140 °C, it was cooled to environmental conditions. The film deposited on an FTO electrode was later rinsed with deionized (DI) water and dried in air with a nitrogen gun. After cleaning, the NiO NP film was annealed at 250 °C for 3 h, resulting in a uniform and well-aligned NP array on an FTO-coated glass. This NiO NP-deposited FTO substrate was used as an electrode for carrying out electropolymerization of aniline. A galvanostat was used with a three-electrode electrochemical system for electropolymerization of aniline. The NiO NP-deposited FTO electrode was used as a working electrode with platinum wire and Hg/HgCl as counter and reference electrodes, respectively, for PANI deposition on NiO. The electrolytic solution was prepared with 0.182 mL of aniline in 20 mL of DI water with 1 mL of perchloric acid. Before 0.01 mA of constant current was applied for 1 h for the polymerization of aniline, five 10-s pulses of 0.1 mA were applied to prepare the hemispherical-shaped NSs. After polymerization, the working electrode was taken out from the electrolyte solution, washed with ultrapure water, and dried under atmospheric conditions to obtain the finished electrode for various characterizations and later to be used in an electrochromic device. The whole process is summarized as a schematic representation in Figure 1, which also shows how NHs of a PANI shell cover a NP of NiO to result in a typical morphology of hybrid core−shell NSs. The microstructure of the film was investigated by X-ray diffraction (XRD; Rigaku SmartLab X-ray diffractometer using monochromatic Cu Kα radiation at λ = 1.54 Å), and the surface morphology was investigated by field-emission scanning electron microscopy (FESEM; Supra 55 Zeiss). Transmission electron microscopy (TEM) was carried out using a Tecnai G2 20 (FEI) S-Twin model, which is a 200 B


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Figure 2. Surface morphologies of NiO NPs as seen using (a) SEM and (b) AFM along with their magnified views in the insets, (c) combined PANI and NiO in the form NHs@NPs, and (d) EDX spectra of the same along with (e) TEM and (f) HRTEM images. The inset in part f shows the diffraction pattern of the present NHs.

Figure 3. (a) Raman spectra of electrochromic electrodes, (b) XRD patterns, (c) an X-ray photospectroscopy survey scan, and (d) a deep scan of Ni 2p and C 1s. sourcemeter unit was used for electrochromic characterization in an electrochemical shell as well as in the device form.

kV machine. Atomic force microscopy (AFM; Bruker Multi-Mode 8HR) was employed for the measurement and analysis of highresolution NSs, carried out using WSxM software.31 The absorption spectra have been recorded at room temperature using a UV−vis spectrophotometer (Agilent Cary 60). Raman spectra have been recorded using LABRAM HR-800 with an excitation wavelength of 488 nm. X-ray photoelectron spectrometry (XPS; ESCA System, SPECS GmbH) with Al Kα radiation (1486.6 eV) was used for measurements. A Keithley electrochemical workstation 2450-EC

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RESULTS AND DISCUSSION Structural and morphological studies of NiO NSs and PANIcoated NiO NS arrays have been carried out using FESEM, TEM, and AFM. Parts a and b of Figure 2 show SEM and AFM images, respectively, of NiO NSs grown in a petal-like shape, which can C


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Figure 4. CV measurement of the synthesis material using a SCE reference electrode: (a) PANI; (b) NiO NPs; (c) a combination of both; (d) showing the cyclic stability of the same.

be seen in the inset showing a magnified view of the same. The NSs, grown on a flat transparent conducting surface, are dense and uniform, which is essential for the electrochromic application, as was intended. Figure 2c shows the SEM image of electropolymerized PANI on the NiO NP array. The hemispherical-shaped look, resembling the schematic (Figure 1), was obtained because of the heteronuclear growth of PANI on NiO NPs. This unique kind of morphology has been obtained by using a special growth methodology, which is “initial current-pulse electrodeposition” followed by a constant current-mode deposition. The prepared NSs consist of nickel, carbon, oxygen, and tin, as is apparent from the corresponding peaks in an energy-dispersive X-ray (EDX) spectrum (Figure 2d), which reveals that NiO NPs and PANI NHs are present on the FTO electrode (source of Sn). Additionally, information about the size and shape of the grown NSs has been obtained using TEM. The TEM (Figure 2e) microstructures of PANI@NiO on a single NP shows a grown hemispherical-shaped PANI on the petal. The images in the inset have been recorded from the cross section of a petal, giving information about the width (white scale) and thickness of the grown PANI shell. The average width of the petals is around ∼20 nm, onto which a ∼7−10-nm-thick layer of PANI NHs is deposited. Figure 2f shows a high-resolution image of the same, giving information about NHs present on both sides of the petals’ wall. The size of the present NHs is around 14− 20 nm (diameter). Some other morphologies of NiO NPs and PANI grown on the NiO NPs and on some other substitutes are given in Figures S1−S3, along with a cross-sectional view of NiO NPs (Figure S4). Apart from the deposition parameter, the morphology of the PANI film also depends on the underlying substrate, which can be appreciated by looking at the SEM images shown in Figure S3. SEM images of the PANI film deposited on FTO, NiO NPs, and carbon-fiber substrates show different morphologies. The resulting morphology is due

to different nucleation as a result of the different surface energies involved on these substrates. The molecular composition and available bonding of PANI NH-covered NiO NSs have been characterized by Raman spectroscopy (Figure 3a) along with the Raman spectra of the constituents. Characteristic peaks corresponding to PANI can be seen at 1623 cm−1 (C−C stretching) and 1529 cm−1 (C−N vibration) in the black curve. Peaks at 827 and 415 cm−1 give information about the C−H bending, whereas other peaks at 1396, 1340, 1262, 1195, and 514 cm−1 appear because of benzene ring distortion (red curve). This spectrum can be compared with the Raman spectrum of NiO NPs (blue curve), where two Raman peaks are observed at 470 and 550 cm−1, corresponding to the Ni−O bending and stretching modes, respectively, which are the characteristic peaks of NiO.32 Figure 3b shows the XRD pattern from NiO NPs and PANI@NiO showing diffraction peaks, in the order of decreasing XRD peak intensities, at 43°, 37°, 63°, 76°, and 79°. The peak positions and their relative intensities are in good agreement with the face-centered-cubic structures of NiO NPs, revealing a crystalline nature of the NPs. In the presence of a PANI shell, XRD spectra show an additional broad hump (marked with an asterisk) around 23° due to the presence of carbon in the amorphous state (from PANI). XPS has been performed for analysis of the constituents and surface chemical compositions of NiO NPs. The XPS survey scan (Figure 3c) depicts the composition of nickel and oxygen with the substrate peak contribution from tin, which is consistent with the EDX results. Both characteristic Ni 2p peaks are observed at about 855.7 eV (2p3/2) and 873.4 eV (2p1/2) in the high-resolution scan (Figure 3d). The deconvoluted spectrum contains overall seven peaks, two stronger peaks along with their satellite (weak) peaks. The carbon peak in the inset appears from PANI-coated NiO NPs. The electrochemical properties of the above-mentioned electrodes have been studied by carrying out cyclic D


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Figure 5. (a) Experimental arrangement for carrying out in situ optical measurements by applying varying bias in a two-electrode system with different working electrodes as (b) NiO NPs, (c) PANI, and (d) PANI NHs@NiO NPs.

voltammetry (CV) in a typical three-electrode system at a scan rate of 50 mV/s, where a fabricated electrode was used as the working electrode and saturated calomel electrode (SCE) and platinum wire were used as the reference and counter electrodes, respectively. Figure 4a, the CV plot obtained during polymerization on aniline, gives information about the oxidation state/potential. During the forward scan of the given cycle, two major anodic peaks (A1 and A2) were observed, which corresponds to the three states of the aniline as follows: leucoemeraldine (LE), emeraldine salt, and pernigraniline.14 Details about these peaks and their corresponding redox processes are given in Figure S5. In the reverse scan, the corresponding counter redox peaks appear as C1 and C2 due to reduction (Figure 4a). The complete redox process is also summarized in the SI. The CV scan of NiO NPs alone shows only one redox couple, A3 and C3 (Figure 4b), corresponding to Ni2+ and Ni3+, respectively. On the other hand, the CV scan with PANI NHs@NiO NPs as the working electrode shows an improved performance in terms of the increased current density (Figure 4c) compared to their individual forms (Figure 4a,b). A color variation of the working electrode (the fabricated substrate) is also evident during the CV scan (Figure 4c) of the fabricated electrode, corresponding to a given redox peak and signifying that each state of the aniline shows a different color and is reversible. This can be seen more clearly in the short movie clip, recorded during the CV scan, provided in the SI. The stability of the fabricated electrode has been tested to check the electrochemical performance through multiple CV scans showing good cyclic stability up to 1000 cycles (Figure 4d). The improvement in the current density for the NHs@NPs electrode is due to the increased surface area of the active material available for interacting with the electrolyte. Overall, the combination of both materials shows improved coloration with viability of color tunability within a bias range of only 1 V.

The bias-induced color switching of the fabricated electrodes, characterized electrochemically above, has been validated using in situ optical absorption studies in two-electrode geometry. Figure 5a shows the experimental setup used for carrying out in situ optical measurements by UV−vis spectroscopy, where a quartz cuvette was used as the electrochemical cell with the necessary electrode arrangements. The electrolyte consists of 1 M KOH, with SCE and platinum wire used as the reference and counter electrodes, respectively. Figure 5b shows the potential-dependent absorption spectra of a bare NiO NP electrode placed inside the electrochemical cell. An overall increase in the absorption of all of the wavelengths in the whole spectral range (300− 1000 nm) can be seen that signifies changes in the contrast of the electrode from transparent to opaque rather than giving the appearance of a particular color. Figure 5c shows the electrochromic behavior of PANI alone, which changes color from transparent yellow to blue but remains transparent during the operation, signifying a lower contrast between the two states. In PANI, there are some intermediate redox states, initially in its reduced state LE shows a light-yellow color, and the intermediate oxidized state shows green color and finally gets converted to purple. The absorption spectrum of PANI alone deposited on an FTO electrode with different biases is given in Figure 5c, which shows two peaks at 350 nm (low absorbance) and 800 nm (high absorbance) in zero-bias condition. With increasing applied bias (between 0 and 0.5 V), higher absorbance peaks show a blue shift, further enhancing the absorbance. On the other hand, the low-absorbance peak did not show any significant shift in the peak position; however, a bias-induced decrease in the absorbance was very clear, as is evident from Figure 5c. The bias-induced diminishing of the low-intensity peak signifies a change of color from light yellow to sky blue, as can be seen visually in the inset image in Figure 4a. An appreciable, but poor, color E


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Figure 6. Schematic presentation of the various sequences followed during electrochromic device fabrication.

Figure 7. Device fabricated by the hybrid NS and its performance: (a) In situ UV−vis showing the change in the absorption spectra. The inset shows the measurement setup. (b) Absorption cyclic stability test for the absorption switching of the device with the operating potential. (c) Time response of one cycle. (d) Absorption cyclic stability with the corresponding current response. (e) Optical density and corresponding coloration efficiency calculation. (f) Coloration stability of the core−shell electrode in the ON state along with its constituents.

of a fabricated hybrid electrode consisting of PANI NHs@NiO NPs with an applied bias by keeping the electrode in the electrochemical cell. It can be observed from the UV−vis

contrast is observed from the PANI-alone electrode compared to NiO possibly because of the different surface morphologies between the two. Figure 5d shows the electrochromic behavior F


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square-wave pulses of 1 V (Figure 7b). The corresponding change in the absorbance (at 550 nm) is also shown in Figure 7b for multiple cycles taking place in 2500 s. The device appears to switch between 22% and 67% absorbance in its OFF and ON states, respectively, which gives around 45% optical modulation. The response time of the optical switching has been calculated from the time response of the absorbance switching (Figure 7b), with zoomed portion shown in Figure 7c predicting around a 0.75 s response for coloration as well as bleaching. Figure 7c also reveals that the average time for color switching is around 1.7 s for each cycle, which means that, in the time duration of 2500 s, around 1470 color switching cycles are present, as shown in Figure 7b. The amperometric response of the device, which gives information about the current flowing through the device during the operation, has been provided in Figure 7d. This has been used to estimate the corresponding charge flow (Q) while the bias-induced color switching takes place. Additionally, a comparison between Figures 4, 5, and 7 reveals that the CV curves of PANI@NiO are similar to that of PANI, whereas the absorption spectrum of PANI@NiO (when in the device) is similar to that of the NiO electrode. This observation can be understood as follows. The CV behaviors of PANI and NiO/PANI are similar because the exposed layer to the electrolyte is PANI in both cases, meaning that PANI shows more impact in the electrochemical cell because in this case ions may not penetrate significantly and react with PANI. It is worth mentioning here that, although they look similar in term of the shapes of the CV curve, the current density for NiO/PANI is more than that of PANI alone, which indicates that more active sites of the NSs are present, which improves the color contrast for the combined material. In the case of the device, the current flow geometry, where two FTO electrodes are present and the current flows across the two electrodes, is different in a way that the current has to flow between two FTO electrodes through the NiO/PANI layer with no electrolyte in between. This means that in the device both working and counter electrodes stick to each other and, in place of ions acting as sites, electrons will flow through the device and pass all along the whole layers present within the device. This enables increased participation of NiO NPs compared to PANI, which affects the overall absorbance spectra. Because the total geometry is different for the cases of the electrochromic electrode in an electrochemical cell and in the form of a device, minor variation in the absorption spectrum is not unexpected. The optical density (OD) and coloration efficiency (η) of the device, a measure of the device power consumption, have been calculated from its electrochromic performance, as shown in Figure 7e, through the following equations:

spectra (Figure 5d) that the bias-induced changes in the absorption spectra are much higher compared to their individual forms (Figure 5b,c) in the same potential window. The change in absorption is consistent with the actual image (shown in Figure 4c), which shows a switching from transparent (yellowish) to opaque (brownish) with some intermediate colors. Upon a comparison of the absorption spectra from NiO/PANI composite (Figure 5d) with the absorption spectra of its constituents (Figure 5b,c), one notices that, at 0 V (the films were in the transparent state), the absorbance value of the NiO/PANI film decreases (e.g., at 500 nm) compared to the pure PANI and NiO films. The likely reason for the same could be formation of the interface layer and variation of the refractive index, which, in turn, decreases the absorption in this particular range for the NiO/PANI film. Because the two constituents have different absorption spectral responses, the composite will have an absorbance similar to the superimposed individual responses. The actual spectra will also depend on the thickness of the layers and excitation wavelengths. To investigate the possibility of the fabricated electrode being used as an actual device, the electrode has been tested under device-like conditions by preparing a prototype electrochromic device in sandwich geometry. Steps involved in the device fabrication and operation have been given in Figure 6. To evaluate the electrochromic properties of the fabricated solid-state device, a NHs@NPs electrode was accompanied by another bare FTO counter electrode to enable bias application to the electrochromic electrode. The various steps involved in the electrochromic device fabrication are discussed in the SI along with the video clip recorded during the device operation. It is worth mentioning here that the device consists of two FTO electrodes, where the active electrode is composed of NiO/PANI nanocomposites prepared using hydrothermal and electrodeposition methods and the other FTO electrode is used solely for biasing the active electrochromic electrode, whereas while testing only the electrode (using an electrochemical cell) for electrochromic properties, a counter electrode is used so as to bias the electrochromic electrode. It is also important to notice that, during the device operation, removing a counter electrode (which was used for electrochemistry) means the device is in its open-circuit state. In the open circuit, the active electrode does not lose its colored state instantaneously; it takes around 4−5 min to get completely bleached, whereas in the short circuit (0 V across both electrode terminals), it shows rather quick changes in its colored state upon exposure to ambience. In situ bias-induced optical property variation, optical constants (optical density and coloration efficiency) measurement, cyclic repeatability, stability, and response time of the electrochromic device have been investigated and are shown in Figure 7. Various measurements have been used to calculate the color contrast ratio and chronoamperometric response from the fabricated device. A bias-induced change in the absorption spectra from the electrochromic device, in the bias window of 0−1 V, is shown in Figure 7a when measured in the arrangement shown in the corresponding inset. It is evident from Figure 7a that the absorbance of the device increases by more than 20 times for the whole visible-wavelength window. This indicates a reversible switching of the device between the transparent and opaque states. This can be seen very clearly in the movie clip available in the SI. The cyclability and reversibility of the device have been studied by applying

ij 1 yz OD(λ) = logjjj zzz j Aλ z k { CE(η) =

ΔOD(λ) Q

where Aλ is the absorbance for a given value of the wavelength (550 nm in the present case). By calculation of the change in the optical density (ΔD) in the ON and OFF states of the device, η can be obtained from the slope of the ΔOD versus Q plot, given in Figure 7e. CE(η), which should be high for a power-efficient electrochromic device, is estimated to be ∼147 G


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ACS Applied Nano Materials cm2/C for the present device. The value is comparatively higher than those of the other devices fabricated when either organic or inorganic electrochromic materials were used alone.20,29 It seems that a hybrid electrochromic device made using an inorganic/organic hybrid material is more efficient compared to the individual ones. Along with this, the stability test (Figure 7f) of the NiO/PANI electrode and its constituents, when used individually, reveals that the NiO/ PANI electrode has better electrochromic response. This response is not unexpected because the formation of a system with ultrathin NPs of NiO helps to reduce the oxidation potential of PANI because of modified surface dynamics, to make the ions diffusion easier, and to provide larger surface area for charge-transfer reactions, which, in turn, causes an improvement in the electrochromic performance.

Rajesh Kumar: 0000-0001-7977-986X Present Address †

S.M.: Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Department of Science and Technology, Government of India. We are thankful to Dr. Vinod Kumar for providing accessibility of the HRTEM facility. The authors are thankful for the SIC facility provided by IIT Indore and Kinny Pandey for his assistance. S.M. is also thankful to MHRD, Government of India, for providing fellowships. The authors acknowledge Prof. V. D. Vankar (IIT Delhi) and Dr. J. Jayabalan (RRCAT, Indore) for useful discussions and providing the AFM facility.

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CONCLUSIONS Hydrothermally grown NiO NPs, covered with electrodeposited PANI on an FTO electrode, show power-efficient electrochromic properties compared to the individual ones in an electrochemical cell as well as in the device form because of miniaturization of the material up to the range of nanoscale. In addition to reduced dimensionality, an overall increase in the surface area, due to coverage of thin PANI over fine NiO NPs, is the likely reason for the improvement in the overall electrochromic performance of the electrode. An electrochromic electrode having only PANI or NiO NPs shows poor color contrast in the colored and bleach states, which can be improved to ∼70% when PANI-covered NiO NPs are used. The PANI@NiO electrodes provide great adhesion stability because, in these hybrid materials, the inorganic NPs of NiO provide a very good template for polymer (PANI) growth. Eased electron transport through the inorganic/organic NS interface along with the higher surface area of PANI NSs increases ion diffusion. The beauty of the NiO/PANI core− shell electrochromic behavior is in its switching between transparent and opaque states rather than between colored and bleached states, as is evident from the bias-induced increased absorption of the whole visible spectrum with a minimum bias of only 1 V, resulting in an absorption modulation of more than 45% (for 550 nm) and a coloration efficiency of more than 145 cm2/C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00871. Various SEM images of NiO, PANI NSs, and a combination of both of them at different synthesis parameters and device fabrication steps (PDF) Movie clip showing color switching of the fabricated electrode in an electrochemical cell (AVI1) (AVI) Movie clip showing color switching of the fabricated electrode in device form (AVI2) (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suryakant Mishra: 0000-0002-9331-760X Pankaj R. Sagdeo: 0000-0002-2475-6676 H


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