Coaxial RuO2–ITO Nanopillars for Transparent Supercapacitor

Article pubs.acs.org/Langmuir

Coaxial RuO2−ITO Nanopillars for Transparent Supercapacitor Application Ilhwan Ryu,† MinHo Yang,‡ Hyemin Kwon,† Hoo Keun Park,† Young Rag Do,† Sang Bok Lee,*,‡,§ and Sanggyu Yim*,† †

Department of Chemistry, Kookmin University, Seoul 136-702, South Korea Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea § Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States ‡

S Supporting Information *

ABSTRACT: Supercapacitive properties of ruthenium oxide (RuO2) nanoparticles electrodeposited onto the indium tin oxide (ITO) nanopillars were investigated. Compared to conventional planar current collectors, this coaxially nanostructured current collector−electrode system can provide increased contact for efficient charge transport, and the internanopillar spacing allows easy access of electrolyte ions. The morphological and electrochemical properties depended on the thickness of the RuO2 layers, i.e., the number of electrodeposition cycles. A maximum specific capacitance, Csp, of 1235 F/g at a scan rate of 50 mV/s was achieved for the 30-cycle deposited RuO2−ITO nanopillars. The other capacitive properties such as electrochemical reversibility and Csp retention at high scan rates also improved greatly.

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INTRODUCTION Transparent electronics is an emerging and promising technology for next-generation electronic systems and has recently received great attention due to its potential to make commercial impacts in consumer electronics.1−3 In this regards, the transparent electronic devices such as optical circuits, displays, solar cells, and touch screens as well as energy storage devices have been recently developed.4−7 However, significant challenges still remain for building fully transparent portable devices due to limitations with modern technology, especially the difficulty of fabricating transparent electrode for energy storage devices. Polytron Technologies, for example, has recently demonstrated transparent smartphones for nextgeneration display devices, but microSD card, microphone, and powering source are not transparent.8 Therefore, design and manufacture of transparent electrodes for batteries and/or supercapacitors play a key role in realizing fully transparent electronic devices. For most supercapacitors, coating and pattering a thin layer of nanostructures, such as carbon nanotubes, graphene, conducting polymers, metal oxides, and their composites, on transparent substrates have widely been used to achieve transparency.9−14 However, the randomly distributed electrode networks lengthen electron and/or ion pathway during the charge/discharge process, which could retard electron flow.15 In addition, a dilemma arises from the fact that the transparency of materials is inversely proportional to thickness, whereas the capacitance is directly proportional.16 To overcome © 2014 American Chemical Society

these problems, 1D arrays of ITO may be a promising candidate as a current collector due to its innate transparency (∼90% in visible region) and high electrical conductivity (∼104 S/cm).17,18 The ITO nanopillars can also act as a capacitive electrode to fabricate transparent supercapacitors by simply coating the nanostructures with capacitive materials (e.g., RuO2, MnO2, and Co3O4). In this work, we successfully fabricated standing ITO nanopillars as a current collector using a radio-frequency (RF) magnetron sputtering technique which is relatively simple and more favorable in terms of the scale-up and mass production.19,20 Hydrous RuO2 is selected as a model to evaluate capacitive properties of nanopillar-based transparent electrodes owing to its high specific capacitance (ca. 900−1400 F/g), excellent proton transport characteristics, and superior reversible redox transitions.21−23 A thin layer of RuO2·xH2O on ITO nanopillars was deposited by cyclic voltammetric deposition, which formed coaxial nanopillar structures.24 Then, their capacitive characteristics were investigated. Standing coaxial nanostructures facilitated charge transfer, ion diffusion, and electron propagation during electrochemical processes, which led to an improvement in the overall performance of supercapacitors. Received: November 19, 2013 Revised: January 26, 2014 Published: January 30, 2014 1704

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Scheme 1. Schematic Illustration of the Fabrication of Coaxial Current Collector (ITO) Core−Electrode (RuO2) Shell Nanopillars

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RESULTS AND DISCUSSION

Our strategy to fabricate coaxial RuO2−ITO nanopillars as a nanostructured transparent electrode was illustrated in Scheme 1. The standing ITO nanopillars were first grown via RFmagnetron sputtering method on ITO-coated glass substrate. Then, a thin layer of hydrous RuO2 was deposited on the surface of ITO nanopillars using a cyclic voltammetric deposition method. The loading amount of hydrous RuO2 was controlled by cycle number. For comparison, the hydrous RuO2 was also directly deposited onto bare ITO substrate under the same conditions. The cross-sectional FE-SEM image of ITO nanopillars is shown in Figure S1, which demonstrates that individually well-separated nanorods with a diameter of 46 nm and height of about 680 nm were almost vertically aligned on the flat ITO surface. More interestingly, the sheet resistance of the ITO nanopillars (8 Ω/□) was 3 times lower than that of the as-received ITO-coated glass substrate (25 Ω/□). To obtain coaxial RuO2−ITO nanopillars, hydrous RuO2 was deposited on ITO nanopillars using cyclic voltammetry performed in 0.01 M aqueous RuCl3·H2O solution with the potential range between −0.2 and 1.2 V (vs Ag/AgCl). The typical CV curves of RuO2 growth on ITO nanopillars are shown in Figure 1a. Both anodic and catholic currents gradually increased with consecutive CV scans, indicating the continuous growth of RuO2. The hydrous RuO2 could be deposited by oxidation of Ru3+ in plating solution and reduction of deposited hydroxyl Ru4+ to hydrous oxides on positive sweep and negative sweep of CVs, respectively.24 In Figure 1b, the deposit weights of RuO2 on ITO nanopillars and ITO films are plotted against deposition cycles, respectively. On the basis of Faraday’s law, the deposit weight of RuO2 is directly proportional to passed charge.25 It is seen that the both deposited RuO2 on ITO nanopillars and ITO films increase with increasing cycle number. However, oxide growth rate on ITO films decreases above cycle number 30. This result is attributed to a decrease in electrical charge passing through ITO films due to significant IR drop with increasing thickness of hydrous RuO2 and smaller exposed area of ITO films than ITO nanopillars to plating solution.24 X-ray photoelectron spectroscopy is frequently used as a complementary technique for assigning oxidation states and the stoichiometry of the oxides. Typical XPS spectrum of Ru 3d core-level electrons for hydrous RuO2 deposit on ITO nanopillars is shown in Figure 2. The peaks of Ru 3d2/5 binding energy at about 280.0, 280.7, and 281.4 eV corresponded well to the standard data at about Ru(0), RuO2, and hydrous RuO2, respectively.26 Note that the RuO2 deposit on ITO nanopillars by the CV method contained ruthenium hydroxides/oxides and a small amount of Ru(0). In

Figure 1. (a) Cyclic voltammetry growth of RuO2 on ITO films (black line) and ITO nanopillars (red line) between −0.2 and 1.2 V at a scan rate of 30 mV/s in 0.01 M RuCl3 at room temperature. (b) Weight of the RuO2 deposited on ITO nanopillars (■) and ITO films (▲) as a function of deposition cycle.

comparison with CV method for RuO2 deposit, the cathodic deposition of RuO2 on ITO nanopillars was also performed at −0.8 V. The major peak of Ru 3d2/5 is located at 280.0 eV and shifted toward lower binding energy than CV deposition, indicating that relative amounts of Ru(0) and oxyruthenium species increase (Figure S2). This implies that conventional CV deposited RuO2 can be better suited for supercapacitor applications than cathodically deposited RuO2 due to relatively higher amount of Ru(IV) species involved electrochemical redox reaction of RuO2.27 1705

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between the nanopillars was filled with nanoparticles and a number of nanopillars had coalesced. The electrochemical performance of the RuO2-deposited nanopillars with different RuO2 loading amounts was first investigated by CVs in 0.5 M H2SO4 aqueous electrolyte at a potential range from 0.3 to 0.8 V. Figure 4a describes the

Figure 2. XPS spectra of Ru 3d for RuO2 deposits on ITO nanopillars.

The morphology of coaxial RuO2−ITO nanopillars was examined by FE-SEM, and cross-sectional images are shown in Figure 3a−f. Cyclic voltammetric deposition led to a smooth,

Figure 3. FE-SEM images of RuO2 nanoparticles electrodeposited on the ITO nanopillars. The number of electrodeposition cycles are (a) 10, (b) 15, (c) 20, (d) 30, (e) 50, and (f) 100. Figure 4. (a) Representative cyclic voltammograms of the RuO2−ITO coaxial nanopillars measured in 0.5 M aqueous H2SO4 electrolyte. RuO2 layers were grown at various number of electrodeposition cycles. (b) Specific capacitance (■) and RuO2 thickness (▲) of the coaxial nanopillars as a function of the number of RuO2 electrodeposition cycles. (c) Transmittance spectra of the ITO film and ITO nanopillar before and after deposition of RuO2 for 30 cycle, respectively. Inset is photos of ITO film, RuO2 deposited-ITO film, ITO nanopillars, and RuO2 deposited-ITO nanopillars, which are arranged left to right.

uniform, and continuous deposition of RuO2 on the ITO nanopillars. The ITO nanopillars were almost completely covered with RuO2 nanoparticles after 15 cycles (Figure 3b). Additional deposits led to thicker, more uniform RuO2 layers, which seemed to start to aggregate (Figure 3c−e). After 100 cycles (Figure 3f), it was observed that most of the space 1706

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rectangular and symmetric CV curves of RuO2−ITO nanopillars with 10, 30, 50, and 100 consecutive cycles, indicating the reversibility of the redox transition of RuO2. Only one broad redox peak was observed in the investigated potential region. Current density and areal specific capacitance of the electrode depended strongly on the loading amount of RuO2. To evaluate the contribution of the active RuO2 to the electrochemical performance of the RuO2−ITO nanopillars, the specific capacitance of RuO2 was calculated after subtracting the charge of the bare ITO nanopillars in terms of the equation Csp,RuO2 = (QRuO2‑ITO − QITO)/(ΔVmRuO2).28 Here, QRuO2‑ITO and QITO are the voltammetric charge of the RuO2−ITO and the bare ITO nanopillars, respectively, and ΔV is the potential window. Figure 4b represents the dependency of the specific capacitance and thickness on the loading amount of RuO2. The specific capacitance of RuO2 increased from 10 to 30 cycles. However, continuous deposition of RuO2 over 40 cycles to further increase RuO2 loading led to a decrease in specific capacitance. The drop in capacitance can be caused by coalescence of the RuO2 nanoparticles, which blocks electrolyte accessibility to the space between pillars. The maximum specific capacitance for RuO2−ITO nanopillars after a 30-cycle RuO2 deposition was 1093 F/g, which can be explained by a uniform and desirable thickness of RuO2 for proton diffusion (maximal penetration distance of proton ≈20 nm).21 This value is as high as that for deposited-RuO2 on a conductive carbon surface with high surface area, such as mesoporous carbons, carbon nanofibers, carbon nanotubes, and graphenes.27−35 The excellent capacitive behavior of RuO2 is reasonably attributed to coaxial nanopillar structures with desirable thickness of RuO2, which not only facilitate charge transfer but also reduce ion transportation during the charge/discharge process.36,37 Figure 4c shows the transmittance spectra of the ITO film and ITO nanopillars before and after 30 cycles of RuO2 deposition, respectively. The transparency of the ITO nanopillars prepared via RF-magnetron sputtering method decreased to ∼60%, compared to the ITO film (∼90%), due to strong scattering of incident light among neighboring nanorods.38 Optical transmittance of both the ITO film and nanopillars decreased slightly after deposition of RuO2. However, the transparency of RuO2−ITO nanopillars is clearly visualized in the photo inset of Figure 4c. Furthermore, the optical properties of this electrode did not change after repetitive cycling was performed to evaluate its capacitive performance. The electrochemical characteristics of RuO2−ITO nanopillars were further investigated by CV and galvanostatic (GV) charge/discharge curves. Figure 5a shows the CV curves at various scan rates ranging from 10 to 200 mV/s. The CV curves reveal a good symmetric shape during forward and backward scans, which indicated a capacitive behavior of the RuO2−ITO nanopillars.21 The specific capacitance of RuO2−ITO nanopillars was calculated using the CV curves and plotted in Figure 5b. For comparison, the RuO2 nanoparticles were electrodeposited for 30 cycles on a planar ITO film, and their specific capacitance at various scan rates was also estimated by CV curves. While the specific capacitance of the RuO2−ITO film gradually decreased with increasing scan rate, it initially increased for the RuO2−ITO nanopillars until a maximum of 1235 F/g was reached at 50 mV/s, at which point the capacitance gradually decreased. The RuO2−ITO nanopillars exhibited an excellent rate capability with capacitance retention of 94.3%, while capacitance retention of the RuO2−ITO film

Figure 5. (a) Representative cyclic voltammograms for 30-cycle RuO2 deposited ITO nanopillars at various scan rates between 10 and 200 mV/s. (b) Corresponding specific capacitances of the RuO2 deposited for 30 cycles on the ITO nanopillars (■) and films (▲). (c) Longterm stability of 30-cycle RuO2 deposited ITO nanopillars at a scan rate of 100 mV/s.

was 43.8%. This large retention was probably due to easy and efficient access of both electrons and ions to the redox active surface, which originated from the 1D coaxial array of the RuO2−ITO nanopillars.35 GV charge/discharge curves of RuO2−ITO nanopillars performed in the potential window between 0.3 and 0.8 V at a constant current of 0.1, 0.5, and 1.0 mA/cm2 showed a capacitive behavior with almost symmetric charge/discharge curves (Figure S3). Moreover, the curves revealed a small deviation to linearity, indicating typical pseudocapacitive contribution during the charge/discharge process. The long-term stability of the RuO2−ITO nanopillars 1707

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was examined in 0.5 M H2SO4 aqueous electrolyte by continuous CV cycles at a scan rate of 100 mV/s (Figure 5c). The specific capacitance of RuO2−ITO nanopillars was retained at 75% of its initial capacitance after 4000 cycles. This excellent long-term stability can be attributed to the 1D nanoarray architecture and good electrical conductivity of the nanopillars, which make them a promising component for the fabrication of transparent energy storage devices.

electrode cell equipped with a platinum electrode and an Ag/AgCl electrode as counter and reference electrode, respectively. As-prepared RuO2-deposited nanopillars were used as a working electrode with geometric surface area of 1 × 1 cm.

CONCLUSIONS A nanostructured transparent electrode with high specific capacitance for supercapacitor was prepared by a relatively simple RF-magnetron sputtering and electrodeposition method. The as-prepared electrode showed high optical transparency and low sheet resistance. A smooth, uniform, and continuous layer of hydrous RuO2 nanoparticles were deposited on ITO nanopillars. The thickness of RuO2 was controlled by the number of CV cycles. Cyclic voltammetric studies evaluated the charge storage and capacitance retention properties of RuO2− ITO nanopillars. The highest specific capacitance of 1235 F/g at a scan rate of 50 mV/s was obtained for the 30-cycle RuO2− ITO nanopillars, which demonstrated the superior charge storage characteristics of that electrode. Capacitance retention at the scan rate ranging from 10 to 200 mV/s reached ∼94.3%. Moreover, the capacitance retention was ∼75% after 4000 cycles. The electrochemical results presented here demonstrate the benefits of transparent 1D nanopillars for applications of transparent supercapacitors.

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S Supporting Information *

SEM images and galvanostatic charge/discharge curves. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.Y.). *E-mail [email protected] (S.B.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant 2011-0030233) and another NRF grant funded by the Korea government (MEST) (No. 2011-0017449). S.B.L. was supported (work discussions and analysis of electrochemistry mechanism) as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001160.

EXPERIMENTAL SECTION

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Preparation of ITO Nanopillars. ITO-coated glass substrate (ITO thickness 140 nm, resistance 25 Ω/□) was cleaned by ultrasonication in acetone, ethanol, and deionized water for 10 min each and then dried with N2 gas, followed by annealing at 120 °C for 1 h in a vacuum chamber. The ITO nanopillars were grown on the substrate by RF-magnetron sputtering in Ar atmosphere under the following conditions: a base pressure of 2 × 10−6 Torr, working pressure of 7.8 × 10−3 Torr, and RF power of 30 W at substrate temperature of 500 °C for 1 h. Three In metal disks (d ∼ 3 mm) were placed on a 2-in. ITO target and used as a catalyst for the growth of ITO nanopillars. The atomic ratio of Sn metal in the target pellet was fixed at 10%. Fabrication of Nanopillar-Based Electrodes of RuO2 on ITO Nanopilllars. The electrodeposition of RuO2 was performed in a three-electrode electrochemical cell with ITO nanopillars or films as a working electrode (1 × 1 cm), a platinum plate (2.5 × 2.5 cm) as a counter electrode, and Ag/AgCl (in 3.0 M KCl) as a reference electrode. A cyclic voltammetric technique was used to deposit RuO2· xH2O nanoparticles onto the ITO nanopillars by cycling the potential between −0.2 and 1.2 V in 0.01 M aqueous RuCl3·H2O (SigmaAldrich Regent Plus) solution at a scan rate of 30 mV/s. In comparison, cathodic deposition of RuO2 on ITO nanopillars was perfomed at −0.8 V (Ag/AgCl) applied potential in identical experimental conditions. After deposition, the electrodes were annealed at 50 °C for 1 h to enhance the capacitive performance of RuO2. The weight of the deposited RuO2·xH2O nanoparticles was determined by a microbalance (Mettler AT21, 1 × 10−6 g) and quartz crystal microbalance (Stanford Research Systems QCM200). Characterization. The morphology and microstructures of the RuO2·xH2O deposited ITO nanopillars were characterized by field emission scanning electron microscope (FE-SEM, JEOL JSM-7410F, JEOL Ltd.). The optical transmittance was measured by a UV−vis spectrophotometer (Scinco S-3000). The electrochemical characteristics were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge using a WPG1000e potentiostat (WonATech). All electrochemical performances were carried out in a conventional three-

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