Hydrofluoric Acid-Free Electroless Deposition of Metals on Silicon in

Mar 23, 2017 - Here, we show an HF-free process wherein metals such as Sb and Ag could be deposited onto electrodeposited silicon in ionic liquids. We...
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Hydrofluoric Acid-Free Electroless Deposition of Metals on Silicon in Ionic Liquids and Its Enhanced Performance in Lithium Storage Abhishek Lahiri,* Tianqi Lu, Niklas Behrens, Natalia Borisenko, Guozhu Li, and Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Straße 6, D-38678 Clausthal-Zellerfeld, Germany S Supporting Information *

ABSTRACT: Metal nanoparticles such as Au, Ag, Pt, and so forth have been deposited on silicon by electroless deposition in the presence of hydrofluoric acid (HF) for applications such as oxygen reduction reaction, surface-enhanced Raman spectroscopy, as well as for lithium ion batteries. Here, we show an HF-free process wherein metals such as Sb and Ag could be deposited onto electrodeposited silicon in ionic liquids. We further show that, compared to electrodeposited silicon, Sb-modified Si demonstrates a better performance for lithium storage. The present study opens a new paradigm for the electroless deposition technique in ionic liquids for developing and modifying functional materials. KEYWORDS: electroless deposition, ionic liquids, silicon, lithium-ion battery, in situ atomic force microscopy, quartz crystal microbalance, hydrofluoric acid-free

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electroless deposition technique to modify the silicon surface would be advantageous. In this communication, we show that electroless deposition of metals is also possible on silicon without HF. 1-Butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Py1,4]TFSA) containing SbCl3 or AgTFSA were used as the electrolytes for the electroless deposition process on Si. The feasibility for the electroless deposition to take place was evaluated by comparing the open circuit potentials (OCPs) of Si, Sb, and Ag in [Py1,4]TFSA. Panels a and b in Figure S1 compare the OCP of Si/Ag and Si/Sb in [Py1,4]TFSA, respectively. It is evident from Figure S1 that the OCP of the electrodeposited silicon starts at approximately −1.0 V vs Pt and increases to −0.8 V with time. Repeated experiments show that this value can vary by ±200 mV. In comparison, both Ag and Sb show lower OCPs of −0.1 and −0.78 V, respectively. Therefore, exposing silicon to a Ag- or Sb-containing ionic liquid should allow the direct deposition of these metals on silicon. Figure 1a compares the changes in the OCP upon exposing electrodeposited Si to 0.1 M AgTFSA and 0.1 M SbCl3 in [Py1,4]TFSA. An immediate decrease in OCP is seen when silicon is exposed to the metal-containing electrolyte. In both cases, an OCP change of approximately 500 mV took place in 10 min. The microstructure of electrodeposited silicon is shown in Figure 1b. Nanoparticles in the size regime of 200−300 nm are seen. After the electroless deposition of Ag on silicon, a change

ilicon nanostructures have stimulated extensive research in recent years because of their importance in electronic and photonic devices.1,2 They are increasingly being researched for devices such as batteries and solar cells,3,4 biological sensors,5 field effect transistors,6 and so forth. Silicon nanostructures have also shown a quantum confinement effect, which makes it particularly useful in optoelectronic devices.7 As silicon is biocompatible, their porous and nanotubular structure has been identified to be a useful drug delivery material.8 Doped silicon nanowires have also emerged as interesting plasmonic materials wherein the surface plasmon resonance can be modulated according to the doping concentration.9 This gives an advantage for increased solar light absorption over a wider range of wavelengths.10 Deposition of metals onto semiconductors is important in microelectronic industries. Various techniques have been used for the deposition of metals on silicon such as electrodeposition,11−13 electroless deposition in hydrofluoric acid (HF)-containing solution,13−16 sputtering,17,18 chemical vapor deposition (CVD),19,20 and evaporation.21−23 Among all these techniques, electroless deposition is a versatile technique that does not require additional energy. Furthermore, as silicon is a potential lithium-ion battery electrode, modifying the silicon electrode with noble metals by electroless deposition in the presence of HF has been shown to improve electronic conductivity as well as battery performance.24,25 Etching of silicon takes place in the presence of HF, which generates electrons and reduces the metal salts present in the electrolyte. As HF is a hazardous acid, other techniques are usually sought for metal deposition on silicon. The development of an HF-free © XXXX American Chemical Society

Received: January 27, 2017 Accepted: March 23, 2017 Published: March 23, 2017 A

DOI: 10.1021/acsami.7b01404 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) OCP change of electrodeposited silicon upon exposing it to 0.1 M AgTFSA and 0.1 M SbCl3 in [Py1,4]TFSA for 10 min. (b) Microstructure of electrodeposited Si. (c) SEM of electroless-deposited Ag on Si after 10 min. (d) SEM of electroless-deposited Sb on Si after 10 min.

eV is due to metallic Sb. The other peaks at 530 and 531 eV indicate the formation of some antimony oxide species.26 The Si 2p in Figure 2b shows the presence of elemental silicon at 99 eV along with the formation of SiOx and SiO2 at 102 and 104 eV, respectively. The peak at 105.5 eV can be related to some chemisorbed ionic liquid or decomposed product formed during sputtering. The XPS detailed spectra of Ag 3d5/2 and Si 2p after the electroless deposition process and sputtering are shown in Figure 2c and d, respectively. The detailed spectrum of Ag 3d5/2 in Figure 2c shows a dominant peak at 368.5 eV, which can be related to metallic silver. The peak at 369 eV is due to the presence of oxidized silver. Similar to the Si/Sb system, the Si 2p in Figure 2d shows the formation of SiOx and SiO2 at 102 and 104 eV, respectively. Furthermore, the peak at 105.5 eV related to chemisorbed species is also seen. To evaluate the reaction kinetics of the eletroless deposition process, we performed quartz crystal microbalance (QCM) experiments. Silicon was initially electrodeposited on the Au quartz crystal from 1 M SiCl4/[Py1,4]TFSA after which the silicon was washed in pure [Py1,4]TFSA. It was observed that a decrease of mass took place in pure [Py1,4]TFSA (Figure 2e), which indicates some corrosion of Si in the pure IL. Upon addition of 0.1 M SbCl3/[Py1,4]TFSA, a rapid increase in mass is observed (Figure 2e) until approximately 1000 s, after which the process plateaus. This indicates that a thin layer of Sb forms over the silicon, and the electroless deposition process slows down. This is in accordance with the SEM observed in Figure 1d, which shows complete coverage of the porous silicon deposit. The rate of deposition until 1000 s was found to be 2.7 ng sec−1, after which it decreases to 0.4 ng sec−1.

in the morphology is evident (Figure 1c), which shows a porous network-like structure. In comparison, after the electroless deposition of Sb on silicon, formation of a dense morphology is seen (Figure 1d). The energy-dispersive X-ray (EDX) spectrum of the electroless-deposited Ag on Si is shown in Figure S2a, which shows dominant peaks of Si, Ag, Au, and O. The Au peak comes from the substrate on which Si was electrodeposited. The presence of Si and Ag confirms that Ag has been deposited on Si by an electroless deposition process. The O peak is due to the exposure of the sample to air during transfer to the scanning electron microscope (SEM). The EDX in Figure S2b shows the electroless-deposited Sb on Si. Dominant peaks of Cu, Si, O, Cl, and Sb are seen. The Cu peak originate from the substrate on which Si was electrodeposited. The O and Cl originate from some surface oxidation during sample transfer to SEM and remaining trapped IL, respectively. The presence of Si and Sb confirms that the electroless deposition process has taken place. X-ray photoelectron spectroscopy (XPS) was used to confirm the electroless deposition process as well as to analyze the chemical composition along the depth of the deposit. The survey spectra of the electroless-deposited Ag and Sb before and after sputtering are shown in Figures S3 and S4, respectively. From both of the survey spectra, it is evident that Ag, Sb, and Si are present, which confirms the electroless deposition process. Moreover, Cl, S, C, and F are also present due to some residual ionic liquid likely present in the pores of the electrodeposit. The XPS detailed spectra of Sb 3d5/2 and Si 2p including Gauss-Lorentzian fits after sputtering are shown in Figure 2a and b. In the detailed spectra, there is an overlap of O 1s along with the Sb 3d5/2. The main peak of Sb 3d5/2 at 528.4 B

DOI: 10.1021/acsami.7b01404 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a, b) XPS detailed spectra with component fits for Sb 3d5/2 and Si 2p after electroless deposition of Sb on Si for 10 min. (c, d) XPS detailed spectra with component fits for Ag 3d5/2 and Si 2p after electroless deposition of Ag on Si for 10 min. (e) QCM of electroless deposition of 0.1 M SbCl3/[Py1,4]TFSA on electrodeposited Si. (f) QCM of electroless deposition of 0.1 M AgTFSA/[Py1,4]TFSA on electrodeposited Si.

Figure 3. (a) AFM image (2 μm × 2 μm) of the electrodeposited Si on polycrystalline Au. (b−f) In situ AFM deflection image snapshots of the electroless deposition of Ag from 1 mM AgTFSA-[Py1,4]TFSA at intervals of 3, 7.5, 18, 28, and 40 min.

The deposition rate for the first 1000 s was found to be 6.2 ng sec−1, which is more than 2 times faster than Sb deposition. After 1000 s, the deposition rate decreases to 1.8 ng sec−1,

Figure 2f shows the mass change during electroless deposition of Ag on Si on the Au quartz crystal. Compared to Sb deposition, the deposition of Ag takes place much faster. C

DOI: 10.1021/acsami.7b01404 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) CV of electrodeposited Si in 1 M LiTFSA-[Py1,4]TFSA. (b) CV of electrodeposited Si modified with Sb in 1 M LiTFSA-[Py1,4]TFSA. (c) Thirty charge−discharge cycles at a rate of 0.25 C. The first charge−discharge cycles were performed at 0.05 C to form a stable SEI layer.

which can be related to the formation of Ag film on Si as seen in Figure 1c. As there is one electron transfer in Ag compared to three electron transfers in Sb, a three times increase in deposition rate is ideally expected. An increase of more than 2 could be due to a difference in the reaction kinetics in the presence of two different anions in the metal salt. It has been shown that AgTFSA dissolves in the TFSA ionic liquid to form [Ag(TFSA) 3 ] 2− , 27 whereas for SbCl 3 , formation of [SbCl2(TFSA)2]− can be expected based on the results of AlCl3 speciation in TFSA ionic liquid.28 Therefore, the different species might have affected the kinetics of the electroless deposition process. In situ AFM was also used to follow the electroless deposition process in real time. Figure 3 shows the in situ AFM of the galvanic displacement process on electrodeposited Si in the presence of 1 mM AgTFSA-[Py1,4]TFSA at various time intervals. Initially, islands of 300−400 nm are seen (marked by circle in Figure 3a). After 3 and 7.5 min of the electroless deposition, a layer-by-layer growth of these islands is observed, which is marked by arrows (Figure 3b,c). After 18 min of the process (Figure 3d), new islands start to grow at the base of these large islands as indicated by arrows. These new islands then grow with time as seen in Figure 3e and f. The rate of deposition of Ag on Si was found to be 0.3 nm min−1 (Figure S5), which was determined by measuring the height change of the island marked with a circle (Figure 3a) with respect to the base of the island. All of the images were combined together to form a movie as shown in Movie S1, which clearly shows the layer-by-layer growth process. Similar

to Ag, in situ AFM was also performed to observe the deposition of Sb from 10 mM SbCl3-[Py1,4]TFSA. The in situ AFM snapshots are shown in Figure S6. Similar to Ag, a layerby layer growth was also observed. The rate of deposition was found to be 0.04 nm min−1 (Figure S7), which is more than 7times slower than Ag deposition. All of the images were combined together to form a movie as shown in Movie S2, which shows the layer-by-layer growth process. Finally, the electrodeposited Si and Sb-modified Si were tested as anodes for a lithium ion battery. Figure 4a shows the cyclic voltammetry (CV) of electrodeposited Si with 1 M lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) in [Py1,4]TFSA. In the first cycle, a reduction peak is observed at 1.3 V vs Li/Li+, which corresponds to the formation of a solidelectrolyte interphase (SEI) layer. The decrease in current at 0.15 V is related to the alloying of Li with Si. Correspondingly, a slight dealloying process is observed in the anodic scan at 0.5 V. In the 5th and 10th CV cycles, the alloying and dealloying processes are observed clearly at 0.4 and 0.5 V, respectively. In comparison, the Sb-modified Si shows a reduction peak corresponding to SEI formation at 1.1 V. The second reduction peak at 0.5 V can be related to the alloying of Li with Sb. The lithiation of silicon commences at 0.17 V. In the anodic regime, a dealloying peak at 1.2 V is seen and can be related to the delithiation of Sb. In the 5th and 10th cycles, reduction peaks at 0.73 and 0.36 V are observed that correspond to the lithiation process of Sb and Si, respectively. In the anodic regime, peaks at 0.51 and 1.15 V are observed that are due to delithiation process of Si and Sb, respectively. D

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Colloidally-Stable Monodisperse Silicon Nanocrystals. Adv. Mater. 2012, 24, 5890−5898. (2) Paul, D. J. Silicon Germanium Heterostructures in Electronics: The Present and the Future. Thin Solid Films 1998, 321, 172−180. (3) Hochbaum, A. I.; Yang, P. Semiconductor Nanowires for Energy Conversion. Chem. Rev. 2010, 110, 527−546. (4) Xie, J.; Yang, X.; Zhou, S.; Wang, D. Comparing One-and TwoDimensional Heteronanostructures as Silicon-Based Lithium Ion Battery Anode Materials. ACS Nano 2011, 5, 9225−9231. (5) Estrela, P.; Migliorato, P. Chemical and Biological Sensors Using Polycrystalline Silicon TFTs. J. Mater. Chem. 2007, 17, 219−224. (6) Härting, M.; Zhang, J.; Gamota, D. R.; Britton, D. T. Fully Printed Silicon Field Effect Transistors. Appl. Phys. Lett. 2009, 94, 193509. (7) Barbagiovanni, E. G.; Lockwood, D. J.; Simpson, P. J.; Goncharova, L. V. Quantum Confinement in Si and Ge Nanostructures. J. Appl. Phys. 2012, 111, 034307. (8) Martin, C. R.; Kohli, P. The Emerging Field of Nanotube Biotechnology. Nat. Rev. Drug Discovery 2003, 2, 29−37. (9) Chou, L.-W.; Filler, M. A. Engineering Multimodal Localized Surface Plasmon Resonances in Silicon Nanowires. Angew. Chem., Int. Ed. 2013, 52, 8079−8083. (10) Atwater, H. A.; Polman, A. Plasmonic for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (11) Osaka, T.; Kodera, A.; Misato, T.; Homma, T.; Okinaka, Y.; Yoshioka, O. Electrodeposition of Soft Gold from a Thiosulfate-Sulfite Bath for Electronics Application. J. Electrochem. Soc. 1997, 144, 3462− 3469. (12) Meulenkamp, E. A.; Peter, L. M. Mechanistic Aspects of the Electrodeposition of Stoichiometric CdTe on Semiconductor Substrates. J. Chem. Soc., Faraday Trans. 1996, 92, 4077−4082. (13) Oskam, G.; Long, J. G.; Natarajan, A.; Searson, P. C. Electrochemical Deposition of Metals onto Silicon. J. Phys. D: Appl. Phys. 1998, 31, 1927−1949. (14) Takano, N.; Hosoda, N.; Yamada, T.; Osaka, T. Mechanism of the Chemical Deposition of Nickel on Silicon Wafer in Aqueous Solution. J. Electrochem. Soc. 1999, 146, 1407−1411. (15) Miyake, H.; Ye, S.; Osawa, M. Electroless Deposition of Gold Thin Films on Silicon for Surface-Enhanced Infrared Spectroelectrochemistry. Electrochem. Commun. 2002, 4, 973−977. (16) Brejna, B. R.; Griffiths, P. R. Electroless Deposition of Silver onto Silicon as a Method of Preparation of Reproducible SurfaceEnhanced Raman Spectroscopy Substrates and Tip-Enhanced Raman Spectroscopy Tips. Appl. Spectrosc. 2010, 64, 493−499. (17) Lauder, A.; Myers, K. E.; Face, D. W. Thin-Film HighTemperature Superconductors for Advanced Communications and Electronics. Adv. Mater. 1998, 10, 1249−1254. (18) Furuya, A.; Baubet, C.; Yoshikawa, H.; Tanabe, T.; Hirono, S.; Yamamoto, M.; Tailhades, P.; Bouet, L.; Despax, C.; Rousset, A. Controlling Garnet Film Composition by Magnetic-Field-Controlled Radio Frequency Magnetron Sputtering. J. Appl. Phys. 2000, 87, 6776−6778. (19) Nazmul, A. M.; Shimizu, H.; Tanaka, M. Magneto-Optical Spectra of Epitaxial Ferromagnetic MnAs films grown on Si and GaAs Substrates. J. Appl. Phys. 2000, 87, 6791−6793. (20) Balog, M.; Schieber, M.; Patai, S.; Michman, M. Thin Films of Metal Oxides on Silicon by Chemical Vapor Deposition with Organometallic Compounds. I. J. Cryst. Growth 1972, 17, 298−301. (21) Koyanagi, H.; Hosaka, S.; Imura, R.; Shirai, M. Field Evaporation of Gold Atoms onto a Silicon dioxide Film by Using an Atomic Force Microscope. Appl. Phys. Lett. 1995, 67, 2609−2611. (22) Ye, X. R.; Wai, C. M.; Zhang, D.; Kranov, Y.; McIlroy, D. N.; Lin, Y.; Engelhard, M. Immersion Deposition of Metals Films on Silicon and Germanium Substrates in Supercritical Carbon Dioxide. Chem. Mater. 2003, 15, 83−91. (23) Jeon, C. W.; Kim, S. H. The Growth of GaN Film on Si (111) Substrate by an Ion-Beam Assisted Evaporation Process. Mater. Sci. Eng., B 1999, 57, 110−115.

Subsequently, the two electrodes were cycled galvanostatically in 1 M LiTFSA-[Py1,4]TFSA at a rate of 0.25 C. From Figure 4c, it is evident that the performance of Sb-modified Si shows higher storage capacity compared to that of electrodeposited silicon. The charge and discharge capacity for the first 10 cycles was found to be above 2000 mAh g−1 for Sbmodified Si compared to that of electrodeposited Si, which showed a capacity of 1500 mAh g−1 at the 10th cycle. A decrease in capacity to 1460 mAh g−1 was observed at the end of 30 cycles. In the case of silicon, after 30 cycles, a capacity of 1100 mAh g−1 was observed. On the contrary, the Coulombic efficiency of the charge/discharge processes was found to be only approximately 80%, which means that some lithium gets trapped during the lithiation process or that secondary reactions occur during cycling.29 However, compared to other results on electrodeposited Si with ionic liquid electrolyte, the results obtained here show significant progress. Schmuck et al.29 first showed that electrodeposited silicon showed a Coulombic efficiency of 50% with 1 M LiTFSA-[Py1,4]TFSA. Furthermore, Vlaic et al.30 showed a capacity of 630 mAh g−1 obtained on electrodeposited silicon with the same electrolyte. Thus, it is evident that the storage capacity values as well as Coulombic efficiency are much higher. In conclusion, we have shown an HF-free electroless deposition of metals on silicon from ionic liquids at room temperature. From SEM, EDX, and XPS analyses, it was confirmed that the electroless deposition of Ag and Sb took place on Si. From QCM measurements, it was observed that the deposition of Ag takes place more than 2-times faster than the deposition of Sb. In situ AFM showed a layer-by-layer growth of the metals on Si. Finally, the Sb-modified silicon was also tested as an LIB electrode, which showed a capacity of more than 1400 mAh g−1 for 30 cycles with an ionic liquid electrolyte.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01404. Experimental section, OCP measurements, EDX, in situ AFM and XPS spectra (PDF) AFM movie showing the layer-by-layer growth of Ag on Si (AVI) AFM movie showing the layer-by-layer growth of Sb on Si (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Abhishek Lahiri: 0000-0001-8264-9169 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Research Foundation (EN 370/25-1) for financial support REFERENCES

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DOI: 10.1021/acsami.7b01404 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX