Electrochemical and Corrosion Stability of Nanostructured Silicon by

Article pubs.acs.org/JPCC

Electrochemical and Corrosion Stability of Nanostructured Silicon by Graphene Coatings: Toward High Power Porous Silicon Supercapacitors Shahana Chatterjee,† Rachel Carter,† Landon Oakes,†,‡ William R. Erwin,§ Rizia Bardhan,‡,§ and Cary L. Pint*,‡,§ †

Department of Mechanical Engineering, ‡Interdisciplinary Materials Science Program, and §Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States S Supporting Information *

ABSTRACT: We demonstrate the electrochemical stability of nanostructured silicon in corrosive aqueous, organic, and ionic liquid media enabled by conformal few-layered graphene heterogeneous interfaces. We demonstrate direct gas-phase few-layered graphene passivation (d = 0.35 nm) at temperatures that preserve the structural integrity of the nanostructured silicon. This passivation technique is transferrable both to silicon nanoparticles (Si-NPs) as well as to electrochemically etched porous silicon (P-Si) materials. For Si-NPs, we find the graphene-passivated silicon to withstand physical corrosion in NaOH aqueous conditions where unpassivated Si-NPs spontaneously dissolve. For P-Si, we demonstrate electrochemical stability with widely different electrolytes, including NaOH, enabling these materials for electrochemical supercapacitors. This leads us to develop high-power on-chip porous silicon supercapacitors capable of up to 10 Wh/kg and 65 kW/kg energy and power densities, respectively, and 5 Wh/kg energy density at 35 kW/kgcomparable to many of the best high-power carbon-based supercapacitors. As surface reactivity wholly dictates the utilization of nanoscale silicon in diverse applications across electronics, energy storage, biological systems, energy conversion, and sensing, we emphasize direct formation of few-layered graphene on nanostructured silicon as a means to form heterogeneous on-chip interfaces that can maintain stability in even the most reactive of environments.

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studied since its discovery at Bell Laboratories in 1956. This form of nanostructured silicon is produced from bulk silicon utilizing standard electrochemical etch processes employable in large-scale manufacturing environments.21,22 The tunable onchip characteristics of porous silicon materials have made them ideal candidates for a broad range of applications including optoelectronic sensors for gases and biomolecules,24 photodetectors, light-emitting diodes,25 drug-delivery systems,26 solar cells,27 and energy storage devices.28 However, the inherent chemical and thermal instability of the porous silicon surface ultimately dictates the usability and performance of the porous silicon material in the device application. This has driven research efforts in areas focused on chemical stabilization primarily through room-temperature reactions such as hydrosilylation, electrochemical modification, or high-temperature thermal oxidation or carbonization routes.12−14,29−31 Utilizing thermal carbonization routes, studies have shown hydrocarbon films formed on porous silicon (P-Si) at temperatures below 500 °C and varying amounts of graphite and silicon carbide formed at higher temperatures that in some cases leave conductivity and photoluminescence properties in tact.12,32

anostructured silicon is a material with extraordinary promise for technological innovation in the future.1−3 Silicon is the second most abundant element in the earth’s crust, behind oxygen, and is already a central platform for modern electronics and solar industries. Furthermore, silicon at all length scales is biocompatible and biodegradable,4−7 can be highly doped to achieve low resistivities, and exhibits a low mass density enabling it to possess excellent specific properties in applications. However, as the size of silicon components in devices shrinks to nanometer length scales, the intense surface reactivity of silicon with its environment, especially in oxygencontaining environments,8 limits direct applicability into many devices or applications. This concept is responsible for the emergence of controlled passivation chemistries for nanoscale silicon surfaces using a wide range of approaches,9 including thermal gas-phase chemistries,10−14 solution-based chemistries,15,16 and electrochemical routes.17 Overall, the careful control of surface properties for silicon, ultimately forming chemically versatile heterogeneous templates, has been the foundation for the broad technological innovations attributed to silicon materials in the past few decades. Nanoscale silicon materials exist in diverse forms spanning nanoparticles and quantum dots, 18,19 nanowires, 20 and interconnected porous structures.21−23 Whereas the applications of different forms of nanostructured silicon overlap significantly, porous silicon has been the most extensively © 2014 American Chemical Society

Received: February 27, 2014 Revised: April 30, 2014 Published: April 30, 2014 10893

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wafers (0.01−0.02 Ω/cm) in a homemade cell using ∼15% HF in an ethanol/water mixture as the electrolyte at a current density of 45 mA/cm2. Following etching, P-Si was then rinsed with ethanol and hexanes and then dried under a nitrogen flow. This etch condition resulted in a porosity of ∼75%, confirmed by optical reflectivity measurements, and resulted in thicknesses of ∼4 μm as confirmed by SEM and optical imaging. B. Thermal Graphene Passivation. Thermal passivation was carried out in a home-built chemical vapor deposition apparatus using a Lindberg Blue 1″ tube furnace. Passivation was performed at atmospheric pressure where the active species to form the coating (C2H2) made up 0.8% of a 5:1 Ar/H2 reducing environment. To optimize conditions in forming passive heterogeneous silicon interfaces we took into account two varying parameters: (i) melting phenomena in nanostructured silicon materials that occur in inert environments at temperatures below 800 °C and (ii) the difference in carbon “quality” based on the temperature at which the carbon is deposited. Therefore, we designed temperature ramps over segments of 200 °C that allow nanostructured silicon stabilization at temperatures below melting but also inhibit poor quality carbonaceous species from being deposited on the silicon surface above and below optimum growth temperatures. In these ramps, the temperature was ramped up by 100 °C for 10 min and then another 100 °C for another 10 min, leading to a total 20 min growth period. We performed this both for silicon nanoparticles (Si-NPs, US Research Nanomaterials, ∼20−30 nm) and for electrochemically etched porous silicon. For Si-NPs, we loaded NPs into a ceramic boat that was placed in the center of the quartz tube, and for P-Si, we placed a cleaved wafer with P-Si etched on one side into the center of the tube furnace. We then placed the samples under vacuum, heated the furnace to the first temperature of the ramp process under a 1:5 H2/Ar mixture at atmosphere, introduced acetylene gas, and then carried out the ramp process. We found the ramp process to be important as low temperatures formed amorphous carbons that inhibited graphene coatings at higher temperatures, and higher temperatures without ramping resulted in thermal destruction of the silicon material through melting and highly disordered pyrolyzed coatings. Despite our utilization of the ramp process, we find good electrochemical characteristics from coatings applied at 750 °C as shown in the Supporting Information. C. Physical Corrosion Measurements. To assess the inhibition of physical corrosion in the passivated Si-NPs, we performed experiments where Si-NPs were immersed in aqueous conditions that lead to spontaneous dissolution, and we tracked the kinetics of dissolution using Raman spectroscopy. This was performed using 532 nm excitations with a Renishaw inVia Raman microscope and a timed collection sequence. D. Electrochemical Supercapacitor Devices. On-chip supercapacitor devices were fabricated from P-Si materials by utilizing thermal passivation routes carried out at different temperature ramps, in different electrolyte environments. This includes 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4, Sigma Alrich 99%), tetraethylammonium tetrafluoroborate (Sigma-Aldrich 98+%) dispersed in acetonitrile (TEATFB/AN, Sigma-Aldrich 99%), organic electrolyte, and 10 M NaOH (Alfa-Aesar) dissolved into water and diluted to 1 M concentration. Supercapacitors were fabricated using a Celgard 1180 separator sandwiched between two P-Si electrodes infiltrated with electrolyte. For ionic liquids, we utilized 1−

Other routes have focused on surface stabilization using hydrosilylation with alkenes, alkynes, and dienes in the presence of Lewis acids at room temperature or slightly elevated temperatures (85−115 °C).33−38 These routes have been demonstrated to improve oxidation resistance and preserve many of the inherent properties of the silicon material, giving promise to large-scale, solution-based passivation of silicon for broad applications. Whereas these passivation techniques have enabled broad application of nanoscale silicon in diverse applications, the challenge of producing corrosionresistant nanoscale Si for application in energy devices requires the passivation technique to be ultrathin, highly conformal, and capable of maintaining surface area of the initial material. In this spirit, silicon has significant promise as a transformational material for electrochemical energy storage applications.39,40 Whereas silicon has been extensively studied for its performance in Li-ion batteries,3,41,42 where cyclability is significantly enhanced by surface passivation routes,43−46 it has only recently been demonstrated as a competitive material for electrochemical supercapacitors.28 Recent studies have shown promise for the use of silicon nanostructures and SiCpassivated Si nanostructures in microsupercapacitors, with onchip power densities measured above 450 μW/cm2.47−51 Other studies have demonstrated the concept of porous silicon supercapacitors, except limited by the surface reactivity of silicon in standard electrolytes that inhibits a measure of capacitive performance.52,53 Whereas the use of silicon requires heterogeneous surface passivation to be stable in diverse electrolyte media, it provides a distinct advantage over carbon nanostructures in that it can be directly integrated into bulk platforms using existing semiconducting manufacturing processes. Nonetheless, carbon nanostructures have been highly studied for their application in electrochemical supercapacitors demonstrating storage capability of greater than 200 F/g in some cases.54−67 Despite this, commercial avenues still utilize activated carbons due to the cost of such high performance carbon nanomaterials, such as single-walled carbon nanotubes and single-layered graphene, which currently remains greater than the cost of platinum. In this context, the ability to utilize silicon as an efficient, manufacturable material for electrochemical energy storage could impact a broad scope of areas ranging from microsupercapacitors, to integrated energy storage, to grid-scale power delivery. In this work, we focus on a comprehensive study of the corrosion resistance and electrochemical stability of nanoscale silicon materials that are passivated with ultrathin carbonaceous coatings that we identify as few-layered graphene. Through optimization of catalytic growth conditions, we emphasize a route to passivate nanostructured silicon, including Si nanoparticles and porous silicon, with a thin enough coating to preserve the surface area but uniform enough to sustain stable electrochemical voltage cycling under conditions leading to spontaneous dissolution of unpassivated nanostructured silicon. We then demonstrate this material to be fabricated into supercapacitor electrodes that yield power densities comparable to or exceeding many of the best high-power carbon-based supercapacitors with true power densities achievable of up to 65 kW/kg and specifically 35 kW/kg at energy densities of 5 Wh/ kg.

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EXPERIMENTAL METHODS A. Porous Silicon Fabrication. Porous silicon (P-Si) was prepared by electrochemical etching of highly doped silicon 10894

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interface that exists between the crystalline surface of silicon and a carbonaceous material coating that is representative of the material obtained over growth temperatures between 600 and 900 °C. Whereas greater atomic disorder exists within the first few atomic layers of carbon at the C−Si interface, within a few layers the carbon atoms arrange in conformal graphene-like sheets that spread across the surface of the silicon material. These regions are apparent in the HR-TEM image shown in Figure 2b. FFT analysis of HR-TEM images (Supporting Information) indicates interplanar lattice spacing of few-layered graphene to be ∼0.35 nm which is consistent with graphene materials and improved compared to reduced graphene oxide materials (near and above 0.36 nm).70 Similar observations are made for coatings obtained with Si nanoparticles (20−30 nm in size), where similar SEM and TEM imaging is shown in Figure 2d,e. This emphasizes that the nanoscale properties of the silicon are responsible for driving the surface reactivity that enables the formation of few-layered graphene, and this is universal among forms of nanostructured silicon materials. Control experiments performed with bulk silicon wafers in identical conditions led to no carbon deposition on the surface of the silicon at temperatures at 900 °C and below, supporting that the nanoscale silicon is a catalyst for the formation of this material system (Supporting Information). To further emphasize the identity of the carbon and silicon materials, we performed high-angle annular dark-field imaging (HAADF) scanning TEM measurements of the coated porous silicon materials (Figure 2c). Evident from Figure 2c is the elemental presence of carbon and silicon in the conformally coated geometry represented in the HR-TEM images. EDX analysis of this material is also available in the Supporting Information. To correlate the characteristics of the passivation layer to the electrochemical and corrosion stability of passivated silicon nanostructures, we studied the corrosion properties of coatings applied at different temperatures from 300 to 1000 °C with a temperature ramp process consistently applied over a 200 °C window. Figure 3a shows Raman spectroscopic measurements of the materials formed in these experiments. At the lowest temperatures (300−500 °C), we observe the presence of amorphous carbon material emphasized by the single broad bump centered near 1500 cm−1. At 400−600 °C, we observe the emergence of distinct D and G peaks corresponding to sp3and sp2-hybridized carbons, respectively. As the temperature is further increased, the D peak increases slightly until the 700− 900 °C condition, where the D band becomes significantly larger than the G band. We explain the range of temperatures between 400 and 850 °C as the catalytic region, where the C2H2 molecules are thermally stable and the silicon catalyzes the formation of a graphene passivation layer. Above 850−900 °C, thermal instability of the C2H2 molecules leads to the deposition of poorer quality carbons, and above 950 °C noncatalytic pyrolysis occurs based on the significantly higher D band relative to the G band. The general mechanism that best fits with our electrochemical and imaging analysis of this catalytic growth process is illustrated in Figure 3b, generalizable broadly to silicon nanostructures. At low temperatures (75° in the highest temperature conditions that yield robust passivation layers. Notably, whereas the highest temperature passivation condition (700−900 °C) exhibits slightly improved corrosion stability, it also exhibits a poorer frequency response (i.e., lower frequency capacitive response, Figure 5c) that we attribute to the presence of passivation layers with higher defect content based on Raman spectroscopic analysis (Figure 3a). We further analyzed this result in the framework of a broader range of electrolytes, including both aqueous and organic-based media (Figure 6). Whereas ionic liquids are emerging as viable electrolytes for a variety of electrochemical systems, operation in aqueous and organic electrolytes covers a broad scope of chemical and biological environments that appeals to applications across areas of chemical and biological sensing, biodegradable systems, energy storage, and energy conversion. Our results indicate that graphene-based passivation layers enable transferrable operation in three diverse electrolytes spanning all of these environments. Shown in Figure 6a are cyclic voltammetry scans in a symmetric cell device (100 mV/s) using 1 M NaOH electrolyte. The first curve is reported after 50 cycles of equilibration, followed by a period of 12 h where the electrodes remain in the electrolyte, and the second curve (red) is shown after an additional 50 cycles. Whereas there is some change in the measured current near the edge of the

electrochemical window after lengthy exposure to electrolyte, the shape overall becomes more boxlike and ideal with approximately only 10% loss of average capacitance to a stable value of ∼6 F/g. This is further supported by measurements of the phase angle (Figure 6b) which emphasizes the more ideal capacitive behavior of the devices with extended exposure to the NaOH electrolyte. As NaOH will spontaneously dissolve any exposed nanostructured Si material (Figure 1), we attribute the improved capacitive properties to the removal of exposed, reactive Si to achieve optimized device performance. This yields a phase angle near ∼80° for passivated P-Si NaOH electrolytes after lengthy cycling and long-term electrolyte exposure and stable device performance in an electrolyte that spontaneously dissolves unpassivated P-Si. Furthermore, we observe similar electrochemical characteristics in organic electrolytes compared to ionic liquids, and we demonstrate this for tetraethylammonium tetrafluoroborate (TEATFB) electrolytes in acetonitrile (Figure 6c,d). For P-Si materials with complete passivation, we observe similar electrochemical windows (2.6 V) and performance in comparison to EMIBF4. Overall, between the studies performed in aqueous and organic media, we emphasize the versatility of the graphene passivation to protect the porous silicon materials and activate them for stable performance under voltage cycling in the most aggressive of chemical environments. Beyond electrochemical stability measurements, the broad utilization of silicon in electrochemical capacitors is ultimately dictated by the inhibition of electrochemical surface corrosion and the simultaneous capability to achieve high surface areas necessary to store ions. Similar to carbon nanostructures, which have been extensively utilized for supercapacitor applications, 10898

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Figure 7. Electrochemical supercapacitor performance of passivated P-Si. (a) Galvanostatic charge−discharge curves at 1.5 A/g over five successive cycles, (b) discharge curves from Galvanostatic cycling taken for devices utilizing different graphene passivation conditions, (c) Ragone plots showing true energy−power performance based on integrated discharge properties for P-Si samples with different passivation conditions (note, comparison to conventional 1/2CV2 calculations shown in Supporting Information), and (d) maximum power and energy capability, as determined from extrapolating curves in (c) for P-Si devices passivated at different temperature conditions.

where Ic is the charging current and V(t) is the discharge profile as a function of time. In the case where V(t) is a linear function, the energy density approximation of E = 1/2CV2 holds true. To find the power density, we calculate P = E/Δt, where Δt is the total discharge duration. As we demonstrate in the Supporting Information, even for ideally passivated materials that exhibit triangular discharge characteristics, we observe a factor of 2X larger energy and 3X larger power when using the conventional 1/2CV2 route to calculate the energy density. In this manner, Figure 7c shows Ragone plots, normalized to the measured active passivated P-Si mass, for the different temperatures utilized for graphene passivation taken at charging currents between 0.5 and 100 A/g. As the P-Si passivation becomes optimized, we observe the power capability and energy storage capability to consistently increase. To further emphasize the maximum power−energy behavior of these devices, we characterize the maximum true energy density and maximum true power density based on Figure 7c. We take the maximum power density to be the asymptotic limit at high charging currents from the y-axis of Figure 7c and the energy to be the similar asymptotic limit at low charging currents on the x-axis of Figure 7c, and these values are plotted in Figure 7d. Under ideal passivation conditions (650−850 °C), we measure energy densities near ∼10 Wh/kg and power densities up to ∼65 kW/ kg. Notably, these values are inflated to ∼20 Wh/kg and ∼220 kW/kg when using the standard 1/2CV2 approach. Whereas such values do not represent the true power and energy capability of our device, they enable us to compare to other studies emphasizing the notion that our devices exhibit among the best known power performance compared to other supercapacitor materials. Additionally, in accordance with EIS

silicon exhibits a native mass density that is low (2.65 vs 2.2 g/ cm3 for graphite). However, unlike carbon nanostructured materials which must be assembled into usable architectures, nanoporous silicon architectures can be directly integrated into the silicon employed in technological platforms across electronics, sensing, and solar devices. In this spirit, we perform testing of graphene-passivated porous silicon materials for electrochemical supercapacitors (Figure 7) using EMIBF4 ionic liquid electrolytes. After passivation, at charging currents of 1.5 A/g (Figure 6a,b), we observe good triangular Galvanostic charge−discharge behavior for devices charged to 2.7 V. Notably, as the passivation conditions are varied, we observe (i) large equivalent series resistances for P-Si materials that exhibit incomplete passivation, evidenced by the large voltage loss between charge and discharge processes, and (ii) greater capacitance for optimally passivated P-Si materials. To assess the storage capability of these materials, we calculated the energy density and power density from Galvanostatic measurements performed at different charging currents. Whereas it is conventional to use E = 1/2CV2 as a route to calculate the total energy density of supercapacitor, this technique is only accurate to elucidate the true energy stored when the discharge curve is perfectly linear. Even minor deviations to the linearity of the discharge curve can lead to significant overestimates of the energy density of between 2X and 10X greater than the true energy stored in the device. To overcome this, we calculated energy densities using the relation

E = Ic

∫0

t

V (t )dt 10899

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65 kW/kg and energy storage capability near 10 Wh/kg for these materials. As silicon is a material that is cheaply manufactured and chemically versatile, our work emphasizes atomically thin graphene passivation layers to enable nanoscale silicon as a platform for use in a range of biological and chemical environments. In particular, the notion of on-chip high power energy storage enables a broad platform to integrate energy storage into existing technological material frameworks, relevant for solar conversion, sensing, and electronics, as well as emerging platforms, such as biodegradable systems and implants. This builds upon the ability to controllably engineer the surface properties of silicona concept that builds upon a vast reservoir of knowledge that has enabled silicon to emerge as the most technologically important material between the 20th and 21st centuries despite its reactivity to normal ambient conditions.

analysis that emphasizes better high-frequency capacitive properties (Figure 5c), we observe improved power density for optimally passivated P-Si relative to the higher-temperature conditions that yield more sp3-hybridized carbon species. Whereas silicon has been an exciting material for the fabrication of high power microsupercapacitors, our results give promise to utilizing this approach for such a purpose as well. Normalizing the performance to the total footprint of the chip-based electrodes, we measure areal power capability of these devices as 160 mW/cm2 (∼583 mW/cm2 from energy calculation using 1/2CV2), which is ∼100X improved from current silicon-based microsupercapacitor device platforms. Finally, to emphasize the broad feasibility of passivated P-Si materials as supercapacitors, we also tested their energy−power performance in both organic-based (TEATFB/AN) electrolytes and aqueous NaOH electrolytes (Figure 8). We observe a

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ASSOCIATED CONTENT

S Supporting Information *

(i) EDX analysis of C−Si materials, (ii) FFT of an isolated graphene material showing d-spacing of 0.35 nm, (iii) Raman spectra of coatings synthesized on Si nanoparticles and P−Si at 750 °C for increasing periods of time, (iv) control measurements showing Raman spectra of bulk silicon wafers exposed to gas-phase graphene growth conditions, (v) detailed electrochemical analysis of coatings and supercapacitors fabricated at 750 °C for increasing periods of time, (vi) discussion of electrochemical stabilization of coatings synthesized at 500− 700 °C vs 650−850 °C in 1 M NaOH, and (vii) analysis of impact of energy density calculation using 1/2CV2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 8. Ragone plot showing the true power−energy performance for graphene-passivated P-Si devices (650−850 °C) in both 1 M NaOH and TEATFB-AN aqueous and organic electrolytes.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-615-322-3720.

similar true power−energy performance for TEATFB/AN electrolytes with optimally passivated P-Si (650−850 °C) that we attribute to the similar electrochemical window between the ionic liquid and TEATFB-AN electrolytes. However, due to the voltage window of aqueous-based electrolytes, we observe lower energy densities for these systems. Nonetheless, this work provides evidence that passivated P-Si materials exhibit competitive performance to commercial supercapacitors and can be broadly combined with a host of electrolytessome of which lead to the spontaneous dissolution when paired with unpassivated nanostructured silicon materials.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Sharon Weiss for use of facilities for porous silicon etching, Ethan Self for silicon nanoparticles, Yang Liu for insightful discussions regarding device measurement methodology, and Keith Share, Adam Cohn, and Andrew Westover for helpful discussions. This work was supported by NSF grant CMMI 1334269 and Vanderbilt start-up funds. TEM images were made possible by an instrument funded through NSF grant EPS 1004083. SEM and partial CVD materials fabrication parts of this research were conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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CONCLUSIONS We demonstrate here the ability to form robust passivation layers on nanostructured silicon materials using gas-phase thermal treatment to produce few-layered graphene materials. This forms heterogeneous silicon nanostructured materials that withstand corrosion under aggressive chemical environments, spanning aqueous, organic, and ionic liquid media, which can spontaneously dissolve unpassivated silicon nanostructures. We demonstrate both physical corrosion inhibition of passivated silicon nanoparticles based on combined mass loss measurements and optical spectroscopy and further demonstrate passivation of P-Si material architectures using EIS analysis. We then use this as a basis to develop electrochemical supercapacitors using ionic liquid, organic, and aqueous electrolytes, and we emphasize excellent power capability of

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