Glucose-Derived Porous Carbon-Coated Silicon Nanowires as

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Glucose Derived Porous Carbon Coated Silicon Nanowires as Efficient Electrodes for Aqueous Micro-Supercapacitors Rami Reddy Devarapalli , Sabine Szunerits, Yannick Coffinier, Manjusha V. Shelke, and Rabah Boukherroub ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11240 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 11, 2016

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ACS Applied Materials & Interfaces

Glucose Derived Porous Carbon Coated Silicon Nanowires as Efficient Electrodes for Aqueous Micro-Supercapacitors

Rami Reddy Devarapallia, b, c, Sabine Szuneritsc, Yannick Coffinierc, Manjusha V. Shelkea, b, Rabah Boukherroubc*

a

Physical and Materials Chemistry Division, National chemical Laboratory, CSIR-NCL, Pune 411 008, India b

c

Academy of Scientific and Innovative Research (AcSIR), Chennai-600113,TN, India

Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Université Lille1, Avenue Poincaré – BP 60069, 59652 Villeneuve d’Ascq, France

ACS Paragon Plus Environment

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ABSTRACT In this study, we report on carbon coating of vertically aligned silicon nanowire (SiNWs) arrays via a simple hydrothermal process using glucose as carbon precursor. Using this process, a thin carbon layer is uniformly deposited on the SiNWs. Under optimized conditions, the coated SiNWs electrode material showed better electrochemical energy storage capacity as well as exceptional stability in aqueous system as compared to uncoated SiNWs. The as-measured capacitance reached 25.64 mF/cm2 with a good stability upto 25000 charging/discharging cycles in 1 M Na2SO4 aqueous solution.

KEYWORDS: Silicon nanowires; Carbon coating; Glucose precursor; Hydrothermal; Microsupercapacitors.


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With the advent of microelectronic devices such as backup power for computer memories, microelectromechanical systems (MEMS), remote sensors, etc., there is an increasing demand for the development of microchip-based electrochemical energy storage systems.1 Chip-based micro-supercapacitors (µ-SCs) gained special attention in this respect owing to their unique properties such as high power density, long life span, excellent reversibility, fast chargedischarge as well as high frequency response.2 Integration of high power, robust, micro-scale energy storage devices into miniaturized electronic devices is thus of outmost importance in modern technology. Silicon is the 2nd most earth abundant element and the basic material in microelectronics, thus it represents an attractive material for the construction of µ-SCs. Over the past years, silicon nanowires (SiNWs), grown by chemical vapor deposition (CVD), have attracted a great attention as µ-SCs electrodes, owing to their high surface area and quasi-ideal electrical double layer capacitive behavior.3-9 These studies have been performed in organic solvents, ionic liquids or organic solvent-ionic liquid mixtures to inhibit SiNWs corrosion. Thus, it is of high importance to develop strategies for extending the operation of SiNWs µ-SCs in aqueous electrolytes and overcome the stability issue. Next to the stability, SiNWs which can be produced in an easy manner displaying high surface area, 1D-electron transport nature and sufficient conductivity are particularly appealing as material for µ-SCs. Next to CVD approaches, metal-assisted chemical etching (MACE) represents an interesting alternative for the production of SiNWs arrays on crystalline Si substrate in a reproducible manner. The formed SiNW arrays display not only high surface area, but are also highly porous resulting in active surface area of 342 m2/g,10 several times larger than that of SiNWs prepared by physical (CVD, VLS) means (≈ 100 m2/g).11 These properties have directed much effort


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towards the synthesis of vertically aligned SiNWs on Si as electrode materials for µ-SCs. However, a major practical limitation of using SiNWs as electrode materials for µ-SCs is linked to the rapid oxidation of the silicon surface in aqueous electrolytes and dissolution in mild saline solutions. Post-coating SiNWs with thin films of highly capacitive materials allows improving the chemical stability of the wires. Deposition of ultra-thin porous carbon,12, 13 silicon carbide (SiC)14 or graphene nanosheets15 at elevated temperatures was performed. Additionally, some conductive polymers16, 17 and metal oxides18, 19 were studied for the same purpose. While coating of SiNWs with ultra-thin layers of carbon has its own advantages because of its high surface area as well as the electrical double layer charge storage behavior,20,

21

the utilization of highly

expensive techniques like CVD might limit its widespread applicability. Herein, we describe a cheap and easy strategy for post-coating vertically aligned high surface area SiNWs. The process is based on glucose pyrolysis at relatively low temperature (≤ 600 °C) to form an activated carbon layer around the SiNWs. We will show that the carbon-coated SiNWs (C@SiNWs) electrode material exhibits excellent electrochemical energy storage capacity as well as exceptional stability in aqueous media. SiNWs of different lengths were prepared by MACE by varying the concentration of the HF/AgNO3 etching solution (see supporting information, Table S1),22 covered with a thin glucose film, pyrolyzed under hydrothermal conditions and annealed at 600 °C. Figure 1A-D displays cross-sectional scanning electron microscopy (SEM) images of C@SiNWs of 1, 6, 17 and 19 µm in length. The carbon layer is uniformly coated over the SiNWs irrespective of the SiNWs’ length. These images suggest in addition that the hydrothermal/annealing process does not induce any damage to the SiNWs structures. Comparing the high resolution image of SiNWs


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(Figure 1E) with that of C@SiNWs (Figure 1F) the porous nature of C@SiNWs, absent in the case of SiNWs, becomes evident.

Figure 1. Cross-sectional SEM images of C@SiNWs formed by varying the concentration of the etching solution (see Table S1) (A-D); High resolution cross-sectional images of SiNWs (E) and C@SiNWs (F).

Furthermore, transmission electron microscopy (TEM) images of C@SiNWs clearly show the presence of a uniform carbon layer around the SiNWs with a layer thickness of ≈20 nm (Fig. 2A, B). The corresponding SAED pattern (Inset, Fig. 2B) indicates the crystalline nature of C@SiNWs.


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Raman scattering measurements performed on SiNWs and C@SiNWs at room temperature, by using an Ar+ laser (λex = 514 nm) as excitation source, are depicted in Fig. 2C. The Raman spectrum of SiNWs exhibits a prominent peak at 520.3 cm-1, corresponding to the Si first order transverse optical phonon mode (TO). In addition to this, two other broad peaks at 304.9 and 941.7 cm-1 assigned to the scattering of two transverse acoustic (2TA) and two transverse optical (2TO) phonons, respectively, are observed. Coating the SiNWs with a carbon layer results in a decrease of the Si prominent TO peak intensity and its blue shift to 518.9 cm-1.

Figure 2: (A, B) TEM images of C@SiNWs (inset: corresponding SAED pattern); (C) Raman spectra of SiNWs (black) and C@SiNWs (red).


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Along with the silicon peaks, the presence of bands at 1347, 1524 and 1621 cm-1, attributed to the D-band (breathing modes of sp2 carbon atoms), G-band (bond stretching of sp2 carbon atoms) and D’-band (finite size graphite crystals and defects) are in line with the formation of a carbon layer on the SiNWs.23-25 The small band at 1415. cm-1 is assigned to the third order Raman peak of Si (Fig. 2C).26 Further confirmation of the formation of C@SiNWs is given by X-ray photoelectron spectroscopy (XPS) analysis. The survey scan of C@SiNWs (Fig. 3A) shows the presence of bands due to C1s, Si2p, Si2s and O1s, in accordance with the chemical composition of the material. The high resolution C1s peak (Fig. 3B) can be deconvoluted into three components attributed to C-C (sp2), C-H/C-C and C-O with binding energies of 283.5, 285.2 and 286.9 eV, respectively.

Figure 3. XPS analysis of C@SiNWs: (A) Survey scan, (B) C1s high resolution spectrum.

The electrochemical behavior of C@SiNWs 17 µm in length was evaluated by using cyclic voltammetry (CV) and chronopotentiometry (CP) in 1M Na2SO4 aqueous electrolyte. The CVs of the C@SiNWs electrodes at different scan rates exhibit the typical rectangular shape for electric double layer capacitor (EDLC) type materials (Fig. 4A). In addition, some redox behavior is


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observed at ≈0.45V vs. Ag/AgCl, which might be due to the small amount of functional groups present on the carbon layer. No SiNWs oxidation peak is observed, suggesting that the electrolyte is interacting only with the outer carbon layer and not with the core silicon.

Figure 4. (A) Cyclic voltammograms at different scan rates and (B) charge-discharge plots at different applied current densities of C@SiNWs (17 µm in length) in 1 M Na2SO4 aqueous solution; (C) Charge-discharge plots of the C@SiNWs of different lengths at a current density of 0.1 mA/cm2; (D) Stability of the C@SiNWs (17 µm) at an applied current density of 1 mA/cm2.

In order to calculate the charge storage capacitance of the C@SiNWs (17 µm), CP measurements at different applied current densities were performed. The corresponding charge-discharge plots are displayed in Fig. 4B and specific capacitance values of 25.6±0.2, 17.0±0.2, 8.1±0.2, 2.1±0.2 and 0.5±0.2 mF/cm2 at 0.1, 0.25, 0.5, 1 and 2 mA/cm2 were determined, respectively.


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The influence of the wire length on the charge-discharge plots was in addition investigated. From Fig. 4C, capacitance values of 6.1±0.2 mF/cm2 (1 µm), 8.9±0.2 mF/cm2 (6 µm), 25.6±0.2 mF/cm2 (17 µm) and 13.7±0.2 mF/cm2 (19 µm), were extracted for an applied current density of 0.1 mA/cm2. The capacitance values increased with wire length with a maximum at 17 µm; thereafter a significant decrease in capacitance values was observed. The increase in length and electrolyte interfacial area resulted in increased capacitance values. Increasing the SiNWs above 17 µm led to wire bending and thus probably a loss of vertical alignment making it more difficult to form uniform carbon layer on individual SiNWs. Instead carbon is covered on the bunch of the bent nanowires as observed in the SEM image of 19 µm long SiNWs (Fig. 1C), resulting in an overall decrease of interfacial area and capacitance. The specific capacitance measured from the charge-discharge plots for the 17 µm long C@SiNWs is apparently much higher; other supercapacitor parameters like power density and energy density have been measured at various applied current densities, and the corresponding Ragone plot is displayed in Fig. S1. At an applied current density of 0.1 mA/cm2, an energy density of 17.4 mJ/m2 along with a power density of 0.351 W/m2 are obtained. Rapid chargedischarge studies at a higher applied current density (1 mA/cm2) (Fig. 4D) indicates that ≈75% of the initial specific capacity is retained up to 25000 cycles, indicating the stability of the carbon coating, crucial for the efficient use of chip-based µ-SCs. Moreover, electrochemical impedance spectroscopic studies (EIS) studies were performed on the C@SiNWs (17 µm) in the frequency range from 7 Mhz to 100 mHz at open circuit potential, and the corresponding Nyquist plot is displayed in Fig.S2. The Nyquist plot is fitted with an equivalent circuit and the corresponding Randles equivalent circuit is shown in the inset of the Fig. S2. In the higher frequency region, a semicircle is observed and the diameter of


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the semicircle corresponds to charge transfer resistance (Rct) due to the faradic reactions between electrode and electrolyte solution. In the lower frequency region, a vertical plot is observed, indicating the capacitive behavior of the electrode material. The Bode plot is generated at lower frequency of 100 mHz and the measured phase angle was 81o, which is near to value of an ideal capacitor (90o) (Fig.S3). In conclusion, a cheap and easy chemical method for forming highly activated carbon layer on vertically aligned SiNWs through pyrolysis of glucose followed by annealing at high temperature was developed. Under optimized conditions, the carbon-coated SiNWs displayed a specific capacity as high as 25.6 mF/cm2 at an applied current density of 0.1 mA/cm2, which is significantly superior for silicon-based µ-SCs. The technique presented here has several advantages such as ease to perform, cost effective, and offers a plethora of possibilities for doping the carbon layer with various hetero-elements.

ASSOCIATED CONTENT Supporting Information contains the experimental details, characterization details and Nyquist plot. “This material is available free of charge via the Internet at http://pubs.acs.org.” Corresponding Author *Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), UMR CNRS 8520, Université Lille1, Avenue Poincaré – BP 60069, 59652 Villeneuve d’Ascq, France E-mail: [email protected]

ACKNOWLEDGMENT


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RR is thankful for CEFIPRA and Campus France for providing fellowship under RamanCharpak scheme. RB, SS and YC acknowledge financial support from the Centre National de la Recherche Scientifique (CNRS), Lille1 University and Nord Pas de Calais région. This work was also partly supported by the French RENATECH network.

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Glucose-Derived Porous Carbon-Coated Silicon Nanowires as

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