TiO2–Co3O4 Core–Shell Nanorods: Bifunctional Role in Better

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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TiO2−Co3O4 Core−Shell Nanorods: Bifunctional Role in Better Energy Storage and Electrochromism Suryakant Mishra, Priyanka Yogi, P. R. Sagdeo, and Rajesh Kumar* Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology, Indore, Simrol 453552, India S Supporting Information *

ABSTRACT: A suitably designed heterostructured TiO2−Co3O4 core−shell nanorod array has been found to exhibit improved supercapacitive as well as electrochromic properties as compared to the nanowires of either of the oxides when used individually. The core−shell nanostructures have been grown on an FTO coated glass substrate by preparing TiO2 nanorods through hydrothermal reaction followed by growing a Co3O4 shell layer by electrodeposition. The core−shell electrode shows high specific and areal capacitance of ∼342 F/g and ∼140 mF/cm2 (at scan rate of 100 mV/s), respectively. Such an improvement in supercapacitive behavior, as compared to the behavior of the existing ones, is likely due to increased surface area and modified charge dynamics within the core−shell heterojunction. Additionally, these core−shells also exhibit stable and power efficient bias induced color change between transparent (sky blue) and opaque (dark brown) states with coloration efficiency of ∼91 cm2/C. Porous morphology and strong adhesion to the surface of transparent conducting glass electrode give rise to superior cyclic stability in both energy storage and electrochromic applications, which make these core−shell structures suitable candidates for future electronic devices. KEYWORDS: energy storage, electrochromism, core−shell, heterojunction, hydrothermal, electrodeposition

1. INTRODUCTION In the era of increasing demand for alternate energy/power sources, materials with the capability of energy storage must be explored to meet this challenge in the field of supercapacitors and batteries. In the global context, materials not only with better efficiency but also with multifunctional operations need to be identified to address future requirements. Nanomaterials, especially one-dimensional (1D) core−shell nanoarrays, have attracted great interest recently due to their enhanced physical and chemical properties as compared to those of their bulk counterparts.1−8 The core−shell structures often possess different electronic properties from either of the constituents which make them eligible for applications in areas of global interest like energy harvesting.9,10 While synthesis of such kinds of materials remains one of the issues to be addressed, the choice of materials to be used in combination (in the form of core−shell) is also a matter of discussion and is mainly application oriented. Developing controlled protocols for the synthesis of hetrostructured core−shell porous nanorods is essential for developing new economic materials for highperformance electronic applications. One of the most important aspects, when choosing materials for energy application, is to look for large active surface area and short diffusion path lengths for charge carriers (electrons and ions) which are essential criteria mainly for efficient electrochemical energy storage purposes.9 An extremely high active surface area can be achieved through miniaturization of materials where increased surface to volume ratio are obtained inherently by reducing the © XXXX American Chemical Society

material dimension. A nano core−shell structure can help address the latter requirement also by appropriately choosing the core−shell combination so that it results in a hetrojunction. The effective width will then be decided by the shell thickness or the effective junction width, whichever dominates.11 A smaller width may have an additional advantage in achieving high capacitance, and thus, such materials can serve as supercapacitors. This approach for obtaining higher capacitance is otherwise not possible by using a single nanomaterial even in the nanostructured form. Generally, two-step synthesis protocols are used for the fabrication of self-supported core−shell nanorods.1,3 The core− shell nanorods are prepared by constructing a freestanding nanorod core backbone followed by coating with the shell material. This method needs to combine different methods such as sputtering, hydrothermal technique, electrodeposition, etc. to prepare core−shell nanorod arrays. Various types of core−shell nanowire arrays whose core or shell materials consist of metals, oxides, carbon, hydroxides, semiconductors, and polymers like core−shell structures of semiconductor@ semiconductor, semiconductor@metal, and metal@polymer have been studied rigorously in the past.1,12 Transition metal oxides such as Co3O4, NiO, and TiO2 are scientifically and technologically important materials for applications in electroReceived: December 11, 2017 Accepted: January 26, 2018

A

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials chemical energy storage,12,13 chemical sensing,14,15 catalysis,16,17 and electrochromism.18,19 Well-defined core−shell nanostructure fabrication and the achievement of tunable functions remain the major challenges when it comes to the realization in the form of a device. In recent years some progress has been made for developing cost-effective and controllable heteronanostructures by simple methods.20 Still, no simple and highly efficient method exists to synthesize transition metal oxide core−shell nanostructure arrays. Nanostructure based applications such as energy storage with supercapacitors becoming increasingly popular work on the principle of redox reactions. The benefit of using a supercapacitor in energy conservation is its higher power density while a shorter charging time remains an additional advantage. Pseudocapacitors, another member in the family of capacitors, work on the principle of reversible redox reaction near the surface of an active electrode within the electrochemical cell.21 In addition to the energy storage applications, electrochromism is also one of the associated properties of a transition metal oxide (TMO)2,3 along with various other organic counterparts.22−24 Electrochromism is the reversible change of color of any substance induced by application of an electrical current or a potential difference. The color change is due to a change of chemical species between two redox states with distinguishable optical absorption/transmission spectra. Various inorganic TMOs such as WO3, Co3O4, MoO3, V2O5, NiO, and TiO2 can be incorporated in an electrochromic as well as supercapacitive energy storage device, making these an intelligent dual application material.25−30 A supercapacitor that changes its color during charging and discharging would be a smart energy efficient device to fulfill both requirements. The concept of combining electrochromicity and supercapacitance becomes possible due to conducting transparent glass electrodes as the replacement of a conventional substrate like nickel foam or carbon. In the present work, oxides of two metals belonging to the fourth period of the periodic table (Ti and Co) have been combined by growing a shell of the latter on the Ti core to explore its suitability in bifunctional applications in supercapacitive and electrochromic applications. A two-step hydrothermal process followed by an electrodeposition technique has been employed to synthesize such core−shell nanostructures, making a heterojunction between the two oxides. It has been shown that the synthesized nanostructures prove to be an efficient electrode for the above-mentioned dual application. This has been done by addressing issues associated with Co3O4, which is one of the least studied materials as far as its electrochromic properties are concerned. In the core−shell nanostructures, a hydrothermally grown TiO2 core provide the vertically aligned nanorods with high surface area, allowing the electrolyte to penetrate and shorten the ion diffusion length, which cannot be compromised in energy storage. Moreover, the TiO2 nanorods are able to reduce the refractive index and improve optical transparency, which are very important in electrochromic applications.

received from Sigma-Aldrich). After stirring the resultant solution for another 10−12 min, it was transferred to a Teflon-lined stainless steel autoclave of volume 50 mL. An FTO coated glass substrate of size 1 × 2 cm2, cleaned with a solution of acetone, methanol, isopropanol, and deionized (DI) water, was placed inside using an appropriate arrangement with the conducting side facing toward the bottom of the autoclave. The hydrothermal process was carried out at 150 °C for 9 h in an electric oven and then was allowed to cool down to room temperature under normal conditions. After being washed properly with DI water and drying in ambient air, a TiO2 nanorod array film was uniformly grown on the FTO glass substrate. 2.2. Synthesis of TiO2−Co3O4 Core−Shell Nanorod Array. A Co3O4 shell over a TiO2 core nanorod array was synthesized by electrodeposition methods.18 A self-sustainable TiO2 nanorod array fabricated on an FTO substrate, grown in the above step, was used as the working electrode, providing the scaffolding for the Co3O4 nanostructure’s facile electrodeposition. Electrolyte for electrodeposition was prepared by dissolving 0.81 g of Co(NO3)2 into 10 mL of DI water. The electrodeposition of Co3O4 was carried out using a threeelectrode chemical cell with Ag/AgCl as reference, and Pt foil was used as the counter electrode. The chronopotentiometry technique (CP) was used at the constant current of −0.3 mA, for the deposition. During the deposition, constant current was maintained for 5 min. Finally, the samples were washed with distilled water and annealed for 2 h at 200 °C inside the electric oven. 2.3. Characterization. The structural and surface morphological analyses have been done using X-ray diffraction (XRD, RIGAKU with Cu Kα), field emission scanning electron microscopy (FESEM, Zeiss), and transmission electron microscopy (TEM, FEI Tecnai). Compositional studies have been carried out by XPS and energy-dispersive Xray (EDX Oxford Instrument). Raman spectra were recorded on LABRAM HR-800 with an excitation wavelength of 488 nm. Diffuse reflectance spectra (DRS) and UV−vis absorption spectra have been recorded using using a spectrophotometer from Agilent (Cary-60). The cyclic voltammetry (CV) and chronoamperometry (CA) measurements have been carried out using a Keithley 2450ec electrochemical workstation in a three-electrode system with 1 M KOH electrolytic solution. A Pt foil and Hg/HgCl were used as counter and reference electrodes, respectively.

3. RESULTS AND DISCUSSION 3.1. Material Synthesis and Characterization. The two steps, involved in the synthesis of a core−shell nanostructure, are hydrothermal and electrodeposition processes where the latter is used to deposit the shell layer on the core backbone (TiO2 in the present case). First, The TiO2 nanorods have been grown on the FTO coated glass substrate as described above. The electrodeposition process deposits the Co(OH)2 precursor film as a result of the following electrochemical reactions:12 NO3− + H 2O + 2e− → NO2− + 2OH− (R1) Co2 + + 2OH− → Co(OH)2

(R2)

After eqs R1 and R2, it is expected that the cobalt based shell will get deposited on the single-crystalline TiO2 as schematically presented in Figure 1. The Co(OH)2 produced as a result of reaction R2 converts to Co3O4 after the annealing step as described in the above section. The appearance of the resultant sample was clean and homogeneous in texture which is an indication that the typical shell nanostructures have covered the entire surface of the TiO2 nanorod backbone. Morphological analyses of the nanostructures have been carried out using SEM as shown in Figure 2 showing SEM images of bare TiO2 and Co3O4@ TiO2 core−shell nanorods grown on the FTO substrate. The average diameter of TiO2 nanorods is around 55 nm (Figure 2b), and the length is less than 2 μm (Figure 2c) as is apparent from top-view and cross-sectional SEM images.

2. EXPERIMENTAL METHODS 2.1. Synthesis of SC-TiO2 Nanorod Array. Single-crystalline (SC) TiO2 nanorods were grown on the FTO coated glass substrate using a hydrothermal method with some modification as reported somewhere else.31 A 15 mL portion of concentrated HCl, diluted in an equal volume of deionized water, was stirred at room temperature for 10 min followed by addition of 400 μL of titanium n-butoxide (used as B

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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structures, energy-dispersive X-ray spectroscopy (EDX) was carried out which reveals heterostructures of titanium−cobalt oxides (Figure 2h). Surface morphologies, of some additional samples prepared with different current densities during electrodeposition of Co3O4, have been provided (Figure S1) in Supporting Information (SI) along with the morphology of TiO2 nanorods (Figure S2). For a better understanding of the microstructure of these structures, TEM has been carried out as shown in Figure 3a,b. The crystallinity of the prepared TiO2 and Co3O4@TiO2 nanorods is also visible from TEM images shown. Figure 3c shows an HRTEM image of TiO2 nanorods showing rods of a few tens of nanometers thin with single crystalline nature. The TiO2 NRs have even sides and rough top surface with single crystalline rutile phase grown in the (001) direction with the fringe width of 3.02 Å. Uniform lattice parameters illustrate the growth of the nanorods in a single plane, which is clearly depicted in the FFT image (obtained using ImageJ) consisting of light spots well-aligned in a single line. Figure 3d shows the HRTEM image of the core−shell nanostructures in which TiO2 nanorods have been covered with the Co3O4 shell. The HRTEM image of the core−shell structure clearly shows the two planes on the rods, depicting two different materials with different fringe size and orientation. In the case of the core− shell, the fringe size of the grown shell is around 2.41 Å. The corresponding FFT images of the same confirm that the material has fringes in two mutually perpendicular directions. Further structural analysis (Figure 4) of the fabricated material has been carried out using Raman scattering and X-ray diffraction (XRD). Three Raman peaks around 225 cm−1

Figure 1. Schematic illustration showing the synthesis of core (TiO2)/ shell (Co3O4) nanowires on FTO coated glass substrate.

After electrochemical deposition, TiO2 nanorods get coated with Co3O4 to make core−shell nanostructures of size (diameter) around 200 nm (Figure 2e) and length less than 2 μm (Figure 2f). Further, to develop more clarity on the heterogeneous nanostructure phases, elemental color mapping proves to be an important tool for core−shell nanostructure analysis. Figure 2g shows the color mapped images of both elements separately and in combination recreated for clarity. To confirm that constituents are present in the core−shell

Figure 2. SEM images of a TiO2 nanowire array in top view (a, b) and cross-sectional view (c), TiO2−Co3O4 core−shell nanowire array grown on FTO substrate in top view (d, e) and with tilt angle of 45° (f). (g, h) Energy-dispersive X-ray along with elemental mapping where Ti and Co are represented as red and green colors, respectively. C

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Figure 3. (a, b) TEM images of TiO2 nanorods and TiO2−Co3O4 core−shell nanostructures (c, d) showing an HRTEM image of the same and the corresponding FFT images.

Figure 4. Raman spectra (a), X-ray diffraction patterns (b), and Tauc plot obtained from diffuse reflectance spectroscopy with their corresponding band structures (c). X-ray photoemission consisting of survey scan (d), TiO2 deep scan (e), and Co3O4 deep scan (f, inset Co3O4 sputtered TiO2 scan).

(weak), 446 cm−1 (strong), and 610 cm−1 (strong) in Figure 4a from the TiO2 nanorod array represent the O−Ti−O bending and the Ti−O stretching modes and correspond to the TiO232 which is consistent with the microscopy data discussed above. The Raman spectrum from cobalt oxide (Figure 4a) shows five optical-phonon vibrational modes at 465 cm−1 (moderate), 503 cm−1 (moderate), 604 cm−1 (low), and 663 cm−1 (strong), whereas the Raman spectrum from the core−shell nanostructure shows a combination of TiO2 and Co3O4 modes as expected with some low-intensity Raman peaks of cobalt oxide invisible as they get merged inside the strong peaks of the TiO2 nanorod array. The XRD pattern (Figure 4b) reveals the

presence of crystalline TiO2 (JCPDS 88-1175) corresponding to indexed planes (⧫) which is in consonance with the HRTEM image (Figure 3c). XRD patterns from Co3O4 on the FTO electrode show some weak peaks (spade symbol) compared to the intense FTO peaks (heart symbol) which are due to electrodeposition of the material, whereas the core− shell nanorod array (Figure 4b top) also confirms the presence of their corresponding phases where Co3O4 is present in its spinel phase (JCPDS 42-1467). In order to investigate the band structure, UV−vis absorbance spectroscopy has been carried out, and the obtained Tauc plots are shown in Figure 4c. Bandgap energies of the D

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves of the TiO2−Co3O4 core−shell nanostructures at different scan rates, (b) glavanostatic charge−discharge curves of the core−shell and their parental materials on glassy carbon electrode, (c, d) same study of the core−shell array with various current densities on GCE and FTO electrodes, (e) areal capacitance of fabricated electrode as a function of scan rate, and (f) stability test of areal capacitance with multiple cycle CV curve at 50 mV/s.

TiO2 nanorods, the TiO2−Co3O4 core−shell nanorods, and the pure Co3O4 films were calculated to be 3.16, 1.86, and 2.21 eV, respectively (Figure 4c). It is evident here that the bandgap of the core−shell nanostructure lies below that of both of the materials, which shows that the core−shell structure affects the properties of both of the materials. The same observation is not very correct intuitionally but is consistent with the available literature3 and can be understood using the band diagram in the inset (Figure 4c). A lower bandgap of the core−shell structure deposited on an FTO conducting electrode indicates an improvement in the charge transfer and overall improvement in performance when employed for electrochemical applications as will be discussed below. The oxidation states and constituents of the grown core− shell were analyzed via high-resolution X-ray photospectroscopy (XPS) as shown in Figure 4d. The XPS measurement confirms the presence of TiO2 and Co3O4 phases. The XPS survey and deep scans of titanium (Ti) and cobalt (Co) are shown in Figure 4d−f, respectively. The survey scan confirms the present constituents of the film fabricated on FTO coated glass. Figure 4e shows a high-resolution scan of the 2p shell of titanium combined with three peaks where two major peaks at 459.0 and 464.5 eV correspond to Ti-2p3/2 and Ti-2p1/2, along with satellite peaks. In case of cobalt, the Co-2p region (Figure 4f) contains total four peaks in which two are sharp (high intensity) peaks and the remaining are satellite peaks. Major peaks correspond to 2p3/2 (780 eV) and 2p1/2 (796 eV). The cobalt oxidation states are highly related to the energy gaps between the Co 2p main peak and satellite peaks. Oxides, such as CoO, Co2O3, and Co3O4, exhibit Co-2p3/2 peaks at nearly similar positions of 780.3, 779.9, and 779.3 eV, respectively, due to the very similar Co-2p binding energies for both Co2+ and

Co3+, making it difficult to identify the oxidation state of Co ions from the binding energy alone. The strong satellite peaks and the splitting (energy difference) of the 2p1/2 and 2p3/2 orbital will be the deciding parameters for the oxidation state of the Co ion. The presence of weak shakeup satellites at 785.3 and 802.2 eV indicates that the Co atom is in the oxidation state of 3+. In addition, the measured Δ value is 15.4 eV. 3.2. Supercapacitive Behavior. For potential applications in supercapacitors, the electrochemical performance of the synthesized core−shell nanorod array has been studied using cyclic voltammetry (CV) and chronopotentiometry (charge/ discharge) measurements. A standard three-electrode electrochemical system was used for this purpose, where a core−shell deposited FTO glass substrate was used as the working electrode with Ag/AgCl and Pt-wire used as reference and counter electrodes, respectively, in 1 M KOH solutions in the potential range −0.4 to 0.4 V. Figure 5a shows the recorded CV curves of the Co3O4@TiO2 core−shell nanostructures grown on FTO coated conducting glass with different scan rates between 20 and 100 mV/s. The effective working area dipped into the electrolyte and the mass loading of the electrode are 0.35 cm2 and 0.242 mg cm−2, respectively. The curves obtained (Figure 5a) indicate capacitive behavior, which comes from the redox reactions between Co3+ and Co4+. Interestingly, the Co3O4 electrodeposited film, exposed to the electrolyte, exhibits two redox peak couples A1/C1 and A2/C2 (shown in Figure S3 in Supporting Information, SI). The first redox couple A1/C1 corresponds to the conversion between Co(II)/ Co(III) whereas A2 could occur at higher potential range with C2 present in the cathodic process representing the conversion of Co(III)/Co(IV). Cobalt oxide, when deposited on a TiO2 nanorod, does not show its first redox peak in the CV scan in E

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Figure 6. (a) Schematic illustration of electrochromic characterization of the fabricated electrode using UV−vis spectroscopy, (b) reflectivity spectra of electrode in its ON (with bias) and OFF states (without bias) and their corresponding actual images of the electrode, (c) in situ UV−vis showing ON−OFF cycling of reflectivity with corresponding bias of the electrode at 422 nm, (d) stability test with response time analysis, (e) and variation of the optical density (OD) of the electrode at 422 nm as a function of charge density.

(τ), mass loaded (m), and active area (A) can be obtained from this which provides information about the specific capacitance of the electrode material. Another parameter in the characterization of the supercapacitor, specific capacitance (Cs), is calculated by the following expression and is found to be 341.88 F/g:

Figure S3 in the Supporting Information. This interesting phenomenon is indicating that the cobalt oxide nanostructures, grown on the single-crystalline TiO2, do not possess many more active sites and ions entangled within the heterostructure which is a favorable condition to improve the overall supercapacitive behavior of the electrode. It is also noticeable that the Co3O4@TiO2 core−shell nanostructure fabricated electrode exhibits higher current densities compared to their individual forms, indicating that the heterostructured composite film has higher a electrochemical activity (Figure S3 in Supporting Information). A galvanostatic charge−discharge (CD) study of the fabricated core−shell nanostructures on glassy carbon electrode as well as the FTO electrode has been carried out. Figure 5b shows the superior behavior of core−shell nanostructures as compared to their parental materials. Figure 5c,d presenting the charging and discharging nature of the core−shell nanostructures on GCE and FTO substrate shows nearly similar characterization except for the IR drop part. The IR drop in the case of the FTO electrode is greater because of the FTO core−shell Schottky junction effect. On the other hand, the same core−shell nanostructure when peeled-off from the FTO surface shows a nearly negligible drop (Figure 5c) when tested using a GCE. Areal capacitance (Ca) of core−shell nanostructures, obtained from the CV curve at various scan rates using the following formulas, was found to be ∼173.4 mF/cm2 at the scan rate of 50 mV/s. Ca =

Cs =

I × Δt m × ΔV

where I is current densisty, Δt is dicharging time, m is loaded mass per unit area, and ΔV is potential range. Variation in areal capacitance as a function of scan rate (Figure 5e) shows a reciprocal variation between the two. Stability of the capacitive behavior is another important parameter in supercapacitive studies. The areal capacitance has been calculated and shows a minor variation (Figure 5f) in its value at the end of the 5000th cycle indicating a stable capacitive behavior. For more clarity, the CV curves used for the calculation of Ca corresponding to 1st to 100th scan and 5000th scan have been shown in the inset for Figure 5f. A negligible decrease (1%) in the area under the curve with some minor shape change of the CV curves further confirms the stability and robustness of the material. The possible origin of the above-mentioned improved supercapacitive behvior of core−shell structures that was observed, as evident from high aerial and specific capacitance values, can be understood as follows. Apart from the increased surface area that resulted due to low-dimensionality, a very small effective device width is also likely to play a role in giving the very high capacitance values as evident from the supercapacitive bahaviour (Figure 5) of the core−shell. It is known that a heterojunction at the TiO2−Co3O4 core−shell structure is formed14,15 which restricts the movement of

charge (Q ) area (A) × potential window (ΔV )

Here, Ca is areal capacitance, A is active surface area, and ΔV is potential range. The CD curve is important, as time constant F

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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analysis, coloration efficiency has been obtained from a change in the optical property of the electrodes using the following equation:

electrons by confining them near the core−shell junction. The electrons are quasitrapped near the junction and need energy for conduction and thus usually remain stagnant and maintain the charged state of the capacitor until it is discharged intentionally by connecting a load which prompts the trapped electrons to flow and to constitute the current through the circuit. Under this geometry, the capacitance will be dominated either by the thickness of the shell or the width of the junction (if the core−shell thickness is very high). In the present situation, the reduction in effective capacitor width due to very thin shell thickness (Figure 3) as well as increased effective surface area appear to be likely reasons for the many-fold improvement in the supercapacitivie behavior of the TiO2− Co3O4 core−shell electrode. 3.3. Working as Electrochromic Display. Oxides of titanium (TiO2) and cobalt (Co3O4) are known for their electrochromic properties by virtue of their redox state dependent absorption.33,34 Though studied extensively, both of these have serious demerits like poor cycle life when either of the two is used as the sole electrochromic agent. As an inorganic material, TiO2 shows required modulation in its optical properties at very high voltage33 whereas it is hard to control the redox reaction in Co3O4 during the electrochromic applications. The core−shell structures deposited on substrates fabricated here have been studied for potential use in electrochromism. This has been investigated by carrying out electrochemical measurements by using it as working electrode and platinum wire as the counter as well as reference electrode in a two-electrode system with 1 M KOH electrolyte solution. Figure 6a illustrates the setup used to carry out the electrochromic measurements. Detector#1 and detector#2 were used for transmittance and reflectance measurements, respectively. Figure 6b shows the change in the reflectance spectra of the working electrode (core−shell) as a result of applied bias of 1 V (black curve) as compared to the as-prepared electrode (red curve). In the OFF state, the sample reflects almost all colors out of which 410 nm (blue) and 550 nm (green) wavelengths showing more prominent reflections. As a consequence of this blue-green primary color mixture, transparent sky-blue (magenta) color is apparent from the electrochromic device in OFF state (inset image). When potential on the working electrode increase it starts getting brownish in color with almost no reflection and eventually becomes opaque as shown in the corresponding actual images. Inset images correspond to the actual photographs of the electrode in its bleached (OFF, without bias) and colored (ON, with bias) states. It is worth mentioning here that the electrochromic experiment has been done from the core−shell structures prior to the annealing step. The image, corresponding to the ON state has been captured by pulling out the electrode after turning the device ON. A video, showing the bias induced color switching, is available as Supporting Information. To understand the cyclic nature of the fabricated electrode we have done in situ measurement of the reflectivity changes with applied bias. Figure 6c shows bias induced switching between colored and bleached states of the electrode. Further stability of the electrochromism has been checked up to 1500 cycles, which shows that the core−shell nanostructure array is more stable compared to the individual forms of the materials (red curve in Figure 6d). Cyclic change in the reflectance spectra has been recorded corresponding to a 422.6 nm wavelength, which shows a prominent reflection (Figure 6b) with applied potential of 0.8 V. For quantitative

η=

change in reflectance (ΔR ) current density (J ) × time (t )

where ΔR is the change in reflectivity by application of potential and current flow through the circuit is denoted by J (current density). The time required during the switching of reflectance is signified by t. The coloration efficiency is obtained as ∼91 cm2/C, and the contrast ratio of ∼80% was calculated from reflectance spectra (Figure S5 in Supporting Information). The obtained coloration efficiency and supercapacitive properties have been compared with similar reports in Table S1 in Supporting Information. The above results reveal that the fabricated core−shell electrode shows very stable and power efficient (working at less than a volt bias) electrochromic properties as compared to those of Co oxide or TiO2 electrode alone. The advantage with core−shell nanorods is its good adhesion and the presence of vertically aligned TiO2 which work as a scaffold for the cobalt oxide shell for controlling the charge transportation between the conducting electrode and electrolyte. Additionally, high surface area of nanorods reduces the biasing potential required to change the color of the cobalt substance. On the application of bias, during the voltage sweep between 0 and 0.8 V, the outer layer of the nanostructures made up of cobalt start getting reduced by taking negative ions that are present in the electrolyte.

4. CONCLUSIONS A suitably designed nanorod array of TiO2−Co3O4 core−shell shows superior supercapacitive poroperties and electrochromism as a result of the typical nature of the core−shell junction and miniaturization. The core−shell nanorods, grown on FTO glass substrate by combining hydrothermal and electrodeposition methods, show a supercapacitive nature with a high areal capacitance of 240 mF/cm2 at a scan rate of 10 mV/ s. Additionally, a high specific capacitance of 344.88 F/g is also observed, clearly demonstrating improved energy storage with superior cyclic stability for more than 5000 cycles. The availability of a high surface area, due to a nanodimensional material with aligned rods, allows the electrolyte to penetrate and shorten the ion diffusion length to help improve energy storage. Another factor that increases the capacitance is the charge dynamics at the heterojunction formed between TiO2− Co3O4 at the nanoscale. The width of the junction, a few nanometers thin, then becomes the effective capacitor width that governs the capacitance value. The core−shell structures also show power efficiency and stable electrochromism with a reflectivity variation of ∼40% corresponding to a 422.6 nm wavelength, giving a coloration efficiency of ∼91 cm2/C. The color change between a sky blue transparent state and a dark brown opaque state takes place with application of a bias as low as 0.8 V and shows good cycle life of more than 1500 cycles. The fabricated active electrode composed with unique nanostructures is a promising bifunctional energy storage and electrochromic device, which may have a profound impact on everyday life in the future which makes way for the realization of energy saving self-sustained electronic devices. G

DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00254. Additional characterization data, including CV, SEM, EIS impedance graph, and comparison with previous reports (PDF) Video of the bias induced color switching (AVI)

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AUTHOR INFORMATION

Corresponding Author

*Email Id:- [email protected]. ORCID

Suryakant Mishra: 0000-0002-9331-760X P. R. Sagdeo: 0000-0002-2475-6676 Rajesh Kumar: 0000-0001-7977-986X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Science and Engineering Research Board, Department of Science and Technology, Government of India (Project numbers SB/FTP/ PS-024/2014 and SB/S2/CMP-012-2014). We are thankful to Prof. T. Pradeep and J. S. Mohanty (IIT Madars) for access to the HRTEM facility. S.M. is thankful to Dr. V. Sathe (IUCDAE-CSIR Indore) for Raman measurements. The authors are thankful to the SIC facility provided by IIT Indore and Mr. Kinny Pandey for his assistance. One of the authors (S.M.) is thankful to MHRD, Government of India, for providing fellowships. The authors are thankful to Prof. Alexander S. Krylov (Kirensky Institute of Physics, Russia), Dr. Shailendra K. Saxena (NINT, Canada), and Prof. V.D. Vankar (IIT Delhi, India) for useful discussions.

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DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsaem.7b00254 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX