Research Article www.acsami.org
Covalent Attachment of Ferrocene to Silicon Microwire Arrays Onkar S. Kang, Jared P. Bruce,† David E. Herbert,* and Michael S. Freund*,‡ Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada S Supporting Information *
ABSTRACT: A fully integrated, freestanding device for photoelectrochemical fuel generation will likely require covalent attachment of catalysts to the surface of the photoelectrodes. Ferrocene has been utilized in the past as a model system for molecular catalyst integration on planar silicon; however, the surface structure of highaspect ratio silicon microwires envisioned for a potential device presents potential challenges with respect to stability, characterization, and mass transport. Attachment of vinylferrocene to Clterminated surfaces of silicon microwires was performed thermally. By varying the reaction time, solutions of vinylferrocene in di-nbutyl ether were employed to control the extent of functionalization. X-ray photoelectron spectroscopy (XPS) and electrochemistry were used to estimate the density and surface coverage of the silicon microwire arrays with ferrocenyl groups, which could be controllably varied from 1.23 × 10−11 to 4.60 × 10−10 mol cm−2 or 1 to 30% of a monolayer. Subsequent backfill of the remaining Si−Cl sites with methyl groups produced ferrocenyl-terminated surfaces that showed unchanged cyclic volammograms following two months in air, under ambient conditions, and repeated electrochemical cycling. KEYWORDS: Si microwire arrays, vinylferrocene, surface functionalization, passivation, electrochemistry, surface coverage
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INTRODUCTION The global reliance on fossil fuels as a primary source of energy has led to a host of environmental challenges attributed to high and rising anthropogenic carbon dioxide emissions.1−6 To mitigate these and future challenges, a growing population with ever increasing energy needs will require a source of fuel that does not contribute to CO2 emissions. One proposed route to harness solar energy for on-demand use is by coupling light energy to the formation of a chemical fuel in an artificial photosynthetic device.3,5−10 Silicon is considered an appealing photoelectrode material for use in such devices due to its abundance, relatively low cost, and the maturity of current technology.11 A fully integrated, freestanding device may require attachment of molecular species capable of catalyzing fuel production to the surface of silicon. Understanding the chemistry and electrochemical properties of functionalized photoelectrodes is therefore a high priority,9,12−18 with controlled attachment protocols sought that do not adversely impact surface properties.19 In addition to catalyst attachment, preventing oxidation of silicon photoelectrode surfaces is also critical to device performance.20 Passivation of silicon surfaces via hydrosilylation reactions of H-terminated Si surfaces has been successfully employed,15,21 though functionalization is typically incomplete and the remaining unreacted surface can still be easily oxidized.22 Alternatively, a two-step halogenation/alkylation method has been reported to yield methyl terminated silicon surfaces with higher surface coverage that resist ambient oxidation for longer periods of time due to kinetically stable surface Si−C bonds.22−24 Complete methylation, however, © 2015 American Chemical Society
precludes subsequent facile functionalization, for example, with molecular catalysts.19 One approach being pursued to facilitate molecular attachment while preserving stability at modified Si surfaces is to generate mixed monolayers.19,25−27 A recent report described mixed methyl/vinylferrocene planar Si(111) surfaces that show excellent electron transfer properties while maintaining stability toward oxidation.19 Ferrocenyl groups (Fc) have long been used to model the attachment of redoxactive molecular species (such as electrocatalysts) on surfaces, due to stable redox states, fast electron transfer rates, low oxidation potentials, and well understood chemistry.15,19,28−31 While a variety of synthetic methodologies have been employed to append Fc to surfaces, with respect to planar silicon, the bulk of the work reported on ferrocene-terminated monolayers uses H-terminated silicon as a substrate.15,21 With H-terminated silicon, synthetic approaches include thermal, ultraviolet, radical, and white-light-initiated hydrosilylation processes.15,19,31−33 High aspect-ratio silicon microwire arrays fabricated by chemical vapor deposition (CVD) techniques have emerged as an attractive low-cost alternative to planar silicon as the photoabsorbing material.7,9,11,34 Silicon microwires offer a long axis to absorb maximum light and a shorter orthogonal distance in the radial direction of the wire to simplify charge carrier collection.6,7,35 In comparison to numerous reports on the functionalization of planar silicon,15,27,31 there are no studies on Received: August 21, 2015 Accepted: November 16, 2015 Published: November 16, 2015 26959
DOI: 10.1021/acsami.5b07814 ACS Appl. Mater. Interfaces 2015, 7, 26959−26967
Research Article
ACS Applied Materials & Interfaces Scheme 1. Preparation of a Mixed vFc/Me Monolayer on Silicon Microwire Arrays
microwires was collected in a vial by scraping the edge of an array substrate with a razor blade and dispersed in acetonitrile (∼15 μL). Small amounts of this suspension (∼1 μL) were deposited on a glass slide to perform electrical analysis of individual microwires. Ohmic contact was formed using 2 μm diameter tungsten probes (American Semiconductor) under a light microscope at 400× magnification. A parameter analyzer (Agilent B5200) was used to electrically characterize individual microwires. Resistance vs length curves were used to calculate the dopant density of the microwires. It should be noted that the scraped wires were used only for electrical measurements of individual microwires, and rest of the experiments (e.g., functionalization) were done on the microwire arrays. Functionalization of Microwire Arrays. Prior to further functionalization, all microwire arrays were subject to the following cleaning procedure: (a) 10% HFaq solution dip; (b) 15 min at 70 °C in RCA 1 (1:1:5 vol ratio of 30% NH4OHaq/30% H2O2(aq)/18 MΩ H2O); (c) 10% HFaq dip; (d) 15 min at 70 °C in RCA 2 (1:1:6 vol ratio of 27% HClaq/30% H2O2(aq)/18 MΩ H2O); (e) 10% HFaq dip; (f) 1 min in KOH (30 wt % solution in 18 MΩ H2O); (g) 15 min at 70 °C in RCA 2 (1:1:6 vol ratio of 27% HClaq/30% H2O2(aq)/18 MΩ H2O). The samples were subject to a final 10% HFaq dip to generate a H-terminated surface. For planar Si(111) samples (vide inf ra), steps a−e were followed by placing the substrate in an argon-purged ammonium fluoride solution for 6 min. These substrates were then transferred to a N2 filled glovebox (O2 level 99.9% anhydrous inhibitor free, Sigma-Aldrich). Vinylferrocene (vFc)43 terminated Si microwire arrays were produced by immersing Cl-terminated Si microwire arrays in 10 mM solutions of vinylferrocene (vFc; Sigma-Aldrich) in di-n-butyl ether at 120 °C in sealed pressure vessels (heavy wall cylindrical pressure vessels, Chemglass; VWR) for times ranging from 0.5 to 45 h (Si MW samples 2a−2g). The heating temperature was set below the boiling point of the solvent (142 °C). These partially functionalized surfaces were backfilled with MeMgCl (0.1 M in THF) at 65 °C for 3 h to produce the corresponding mixed vFc/Me microwires 3a−3g. The arrays were rinsed with THF and methanol and removed from the glovebox. All the functionalized arrays (2a−2g, 3a−3g) were rinsed sequentially with acetonitrile, methanol, and then water, prior to the characterization. For comparison, Cl-terminated planar Si(111) surfaces were also treated with vFc solutions following the same procedures used with the microwire arrays, in order to generate mixed monolayers of vFc/Cl (4 h immersion time in vFc solution) and vFc/ methyl (3 h immersion time in MeMgCl; surface 4). Methyl-terminated Si microwire arrays were prepared by immersing Cl-terminated Si microwire arrays in 1.0 M MeMgCl (diluted with
the functionalization and electrochemical characterization of silicon microwire arrays. Covalent attachment of Fc units to silicon microwire arrays was undertaken to build a model system for integration of surface-bound redox-active species and evaluate their stability toward ambient oxidation. Initial attempts to extend the approach of Lattimer et al., who applied vinylferrocene in the melt to functionalize Cl-terminated planar silicon19 resulted in high surface coverage of ferrocenyl groups (1.7 × 10−9 mol cm−2 or ∼106% of a monolayer) that suggested the presence of multiple layers of vFc on the surface of the wires. These wires also exhibited poor electron transfer behavior. Therefore, solution-based reaction conditions were pursued. Di-n-butyl ether has a high boiling point (142 °C) and has been used previously as a grafting medium for vFc attachment to Hterminated, planar silicon surfaces without evidence of deposition of multiple layers of vFc.33 The application of solution-based functionalization conditions to Cl-terminated silicon microwire arrays and subsequent methyl backfill19 to generate silicon microwire arrays with mixed Fc/Me monolayers is described herein. Surface attachment of Fc and Me groups was investigated using X-ray photoelectron spectroscopy (XPS), and surface coverage of Fc groups was quantified by electrochemistry. Fc-functionalized microwire arrays were tested for stability toward ambient oxidation by re-examining their electrochemistry after two months in air.
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EXPERIMENTAL SECTION
Fabrication of Microwire Arrays. Silicon microwire arrays were fabricated by chemical vapor deposition (CVD) using a vapor−liquid− solid (VLS) technique36−40 and following a well-established method.37,40 A Si(111) wafer (p+; ρ < 0.005 Ωcm) was used as a substrate for microwire growth. The wafer was first cleaned using Piranha solution (4:1; conc. H2SO4/H2O2), and an oxide layer of 500 nm was then grown using an oxidation furnace. The resulting substrate was photolithographically patterned by using HPR 504 photoresist and etched with 10:1 BOE solution to get 3-μm-diameter circular dots arranged in a square pattern with a 7 μm pitch. After etching, copper (4N, Kurt Leskar) was thermally evaporated and the photoresist removed by washing in acetone. Prior to microwire growth, substrates were annealed at 1000 °C in a custom built CVD reactor for 20 min under 1 atm of H2 (6.0 Semiconductor Purity) at a flow rate of 500 sccm. Microwire arrays were grown at 1000 °C for 20 min under 1 atm of H2 and SiCl4 (fiberoptic grade, Strem Chemicals) at flow rates of 490 and 10 sccm, respectively. BCl3 at a flow rate of 22 sccm was used as a source of boron to prepare p+-type (∼1018/cm3) microwires. SEM was used to observe the morphology of the synthesized wire arrays (Figure S1). The dopant density of the microwires was tested using a probe station technique described elsewhere.41,42 A small amount of 26960
DOI: 10.1021/acsami.5b07814 ACS Appl. Mater. Interfaces 2015, 7, 26959−26967
Research Article
ACS Applied Materials & Interfaces THF from 3.0 M MeMgCl, Sigma-Aldrich) at 65 °C for 3 h. The resulting Me−Si samples were rinsed with THF and methanol (99.8% anhydrous, Sigma-Aldrich) prior to being removed from the glovebox for analysis. Characterization of Functionalized Microwire Arrays. X-ray Photoelectron Spectroscopy. Surface elemental analysis was performed by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra DLD spectrometer (Kratos Analytical Ltd.) with monochromated Al Kα radiation (1486.6 eV) at 10 mA emission current and 15 kV anode voltage with a base pressure of 1 × 10−9 Torr. The photogenerated electrons were captured normal to the surface. Spectra were collected with a fixed analyzer transmission mode. Survey scans were collected with a pass energy of 160 eV, while high resolution scans for Si 2p, C 1s, Fe 2p, and Cl 2s regions were collected with a pass energy of 20 eV. Peak fitting was performed with CasaXPS software (Casa Software Ltd.) using a Shirley baseline and an 80% Gaussian/20% Lorenzian function for the peaks. All spectra were referenced to the aliphatic C 1s peak at 285 eV. The C 1s spectrum was fitted with three peaks. For the Si−C peak at 284 eV, fwhm was kept 200 redox cycles. This demonstrated the reproducibility and stability of a mixed vFc/Me monolayer silicon microwire array (3b) after more than 200 total electrochemical cycles. The long-term stability could be due to improved passivation toward oxidation of mixed vFc/Me functionalized microwire surfaces compared with vFc/Cl functionalized microwire surfaces.19,22,23
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DISCUSSION The geometry of a microwire array is more complex (∼100μm-long wires aligned vertically in an array, with more than one crystal phase on the surface) compared with planar silicon (single crystal phase), so it was not obvious that the same functionalization protocols would be applicable to both high aspect-ratio microwires and planar surfaces. As described above, treating microwire arrays with neat vinylferrocene in the form of a melt gave high values of surface coverage measured electrochemically (1.7 × 10−9 mol cm−2 or ∼106% of a monolayer), which suggested the presence of multiple layers of vFc on the surface of the wires. These wires also exhibited poor electron transfer behavior (Figure S7). Moving from the melt to a solution of vFc in high boiling point di-n-butyl ether allowed for more controlled functionalization of silicon microwires and submonolayer surface coverage (1 cm2) with Au and Cu Catalysts. Appl. Phys. Lett. 2007, 91, 103110. (38) Strandwitz, N. C.; Turner-Evans, D. B.; Tamboli, A. C.; Chen, C. T.; Atwater, H. A.; Lewis, N. S. Photoelectrochemical Behavior of Planar and Microwire-Array Si|GaP Electrodes. Adv. Energy Mater. 2012, 2, 1109−1116. (39) Wagner, R. S.; Ellis, W. C. Vapor-Liquid-Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89−90. (40) Maiolo, J. R., III; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S. High Aspect Ratio Silicon Wire Array Photoelectrochemical Cells. J. Am. Chem. Soc. 2007, 129, 12346−12347. (41) Yahyaie, I.; McEleney, K.; Walter, M. G.; Oliver, D. R.; Thomson, D. J.; Freund, M. S.; Lewis, N. S. Characterization of the Electrical Properties of Individual p-Si Microwire/Polymer/n-Si Microwire Assemblies. J. Phys. Chem. C 2011, 115, 24945−24950. (42) Yahyaie, I.; Ardo, S.; Oliver, D. R.; Thomson, D. J.; Freund, M. S.; Lewis, N. S. Comparison between the Electrical Junction Properties of H-Terminated and Methyl-Terminated Individual Si Microwire/ Polymer Assemblies for Photoelectrochemical Fuel Production. Energy Environ. Sci. 2012, 5, 9789−9794. (43) Surface attachment of vinylferrocene via a Si−C bond presumably results in the formation of an ethylenylferrocene [−CH2CH2(η5-C5H4)Fe(η5-C5H5)] substituent on silicon. For consistency with the literature, the ferrocene-containing group is referred to herein simply as vFc. 26966
DOI: 10.1021/acsami.5b07814 ACS Appl. Mater. Interfaces 2015, 7, 26959−26967
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ACS Applied Materials & Interfaces (44) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applns, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (45) Abdulla, M. Synthesis and Characterisation of Ferrocenyl Monolayers on Silicon Surfaces. Ph.D. Thesis, Newcastle University, U.K., 2013. (46) Li, F.; Basile, V. M.; Pekarek, R. T.; Rose, M. J. Steric Spacing of Molecular Linkers on Passivated Si(111) Photoelectrodes. ACS Appl. Mater. Interfaces 2014, 6, 20557−20568. (47) Laviron, E. General Expression of the Linear Potential Sweep Voltammogram in the Case of Diffusionless Electrochemical Systems. J. Electroanal. Chem. Interfacial Electrochem. 1979, 101, 19−28. (48) Webb, L. J.; Lewis, N. S. Comparison of the Electrical Properties and Chemical Stability of Crystalline Silicon(111) Surfaces Alkylated Using Grignard Reagents or Olefins with Lewis Acid Catalysts. J. Phys. Chem. B 2003, 107, 5404−5412. (49) Grimm, R. L.; Bierman, M. J.; O’Leary, L. E.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. Comparison of the Photoelectrochemical Behavior of H-Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, OuterSphere Redox Couples in CH3CN. J. Phys. Chem. C 2012, 116, 23569−23576.
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DOI: 10.1021/acsami.5b07814 ACS Appl. Mater. Interfaces 2015, 7, 26959−26967
