Letter pubs.acs.org/NanoLett
Generalized Redox-Responsive Assembly of Carbon-Sheathed Metallic and Semiconducting Nanowire Heterostructures Sinho Choi, Jieun Kim, Dae Yeon Hwang, Hyungmin Park, Jaegeon Ryu, Sang Kyu Kwak,* and Soojin Park* Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea S Supporting Information *
ABSTRACT: One-dimensional metallic/semiconducting materials have demonstrated as building blocks for various potential applications. Here, we report on a unique synthesis technique for redox-responsive assembled carbon-sheathed metal/semiconducting nanowire heterostructures that does not require a metal catalyst. In our approach, germanium nanowires are grown by the reduction of germanium oxide particles and subsequent self-catalytic growth during the thermal decomposition of natural gas, and simultaneously, carbon sheath layers are uniformly coated on the nanowire surface. This process is a simple, reproducible, size-controllable, and cost-effective process whereby most metal oxides can be transformed into metallic/semiconducting nanowires. Furthermore, the germanium nanowires exhibit stable chemical/thermal stability and outstanding electrochemical performance including a capacity retention of ∼96% after 1200 cycles at the 0.5−1C rate as lithium-ion battery anode. KEYWORDS: Metallic/semiconducting nanowire, carbon-sheathed coaxial nanowire, redox-responsive assembly, energy storage devices
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A second class of nanowire heterostructures that involves the synthesis of coaxial nanowires may represent a good solution to overcome these obstacles. Coaxial structures can be fabricated by coating nanowires with a conformal layer of a second material (i.e., organic monolayer or carbon).20 In particular, since GeNWs can be easily oxidized and contaminated due to their high surface chemical reactivity, the generation of coaxial nanowire could potentially increase the use of GeNWs in nanoelectronics and energy storage applications.21 To date, various coaxial semiconductor nanowires have been synthesized by catalytic growth techniques. Lieber’s group reported high quality Ge−Si core−sheath nanowires produced by the CVD method, in which the nanowire field-effect transistors outperformed their planar counterparts.21 As another example, Whang’s group reported on the catalytic growth of a GeNW core and a very thin amorphous carbon sheath via the VLS growth process.22 The resulting carbon-sheathed GeNWs showed excellent performance in field effect transistors even after high temperature annealing. Crocker’s group demonstrated single-step synthesis of GeNW encapsulated within carbon nanotubes by using the CVD method with a solid precursor, phenyltrimethylgermane, which provides both a carbon and Ge source.23 Also, Yu’s group synthesized alternating Ge/GeOx and carbon layer by a one-step controlled pyrolysis of organic−inorganic materials.24
emiconducting nanowires have demonstrated utility as building blocks for a rich variety of potential applications including field-effect transistors, light-emitting diodes, photodetectors, solar cells, and lithium-ion batteries.1−7 In particular, germanium nanowires (GeNWs), an especially attractive class of one-dimensional nanostructures, have received a great attention because of their large exciton Bohr radius (∼24.3 nm) and higher carrier mobility compared to silicon. Germanium nanowires have been synthesized with numerous methods such as solvothermal, supercritical fluidic, laser ablation, vapor transport, and chemical vapor deposition (CVD) technologies.8−13 However, metal-catalyzed growth by using a vapor−liquid−solid (VLS) mechanism is the most widely used method for synthesizing GeNWs at a relatively low synthetic temperature of 335 °C.13 Even though significant advances have been made in regards to the synthesis of GeNWs with controlled dimensions and electrical properties, there are still several challenges.14 First, the Ge precursor is highly toxic and flammable.15 Second, its synthetic yield is extremely low and may not be suitable for commercial production because of the limited amount of metal catalyst available.16 Third, during the VLS growth process, unintentional tapering takes place, which gives rise to adverse effects on the structural and electrical uniformities of the nanowires.17 Fourth, the high solubility of the semiconductor material in the metal catalyst can produce detrimental effects on the electrical properties of the nanowires.18 Fifth, thermally and chemically unstable germanium oxide (GeOx) can be formed on the surface of the as-synthesized GeNWs during sample storage when the materials are exposed to the atmosphere.19 © XXXX American Chemical Society
Received: November 3, 2015 Revised: December 31, 2015
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DOI: 10.1021/acs.nanolett.5b04476 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. (a) Schematic illustration showing the synthetic route of c-GeNW from reaction between decomposed C2H2 and GeO2. (b) SEM images showing the growth mechanism of GeNW as a function of the reaction time at 800 °C.
GeO2 films showed a peak corresponding to the Ge4+ binding energy (Supporting Information, Figure S1b). After chemical reduction for a short time (at 800 °C for 1 min), the surface of GeO2 film was partly converted to GeOx and synthesized cGeNWs (Supporting Information, Figure S1c). The growth mechanism of c-GeNWs was schematically illustrated (Figure 1a). An unsaturated hydro-carbon (here, acetylene (C2H2) gas) was thermally decomposed into hydrogen and carbon clusters at 800 °C. Initially, the hydrogen/carbon generated can easily reduce GeO2 to Ge nanoclusters because of its low activation energy. Simultaneously, excess carbon clusters produced by the decomposed gas were uniformly covered on the reduced Ge surface. Subsequently, c-GeNWs were grown via a self-catalytic growth mechanism. We monitored the growth process of GeNWs at 800 °C at various reaction times. First, the Ge nanoclusters made by the reduction of GeO2 were aggregated into a hemispherical shape at a reaction time of 10 s. Simultaneously, carbon clusters were knitted on the Ge surface due to the negligible carbon solubility in bulk Ge ( CO2 > H2O at all surfaces, where the reduction reactions were possible as indicated by negative Gibbs free energies and CO2 generation was more favorable and dominant. To check the formation of germanium carbide (GeC), we estimated the formation energy Ef of GeC. The formation energy of germanium carbide (GeC) was calculated by the relative energy of GeC with respect to Ge crystal and graphite from the following eq 1: Ef = [EGeC − (NGeEGe + NCEC)]/NGe
(1)
where NGe and NC are the number of germanium and carbon atoms in GeC model system, EGe and EC are the total energies per atom of bulk germanium crystal and graphite, and EGe is the total energy of GeC model system (Supporting Information, Figure S7). We have used graphite as a reference material for carbon atoms since it is stable and graphitic layer can be formed when carbon coating occurs. At all carbon compositions of interest, E f values became positive and larger as the composition of carbon increased. This indicates that GeC is thermodynamically unstable, thus not preferred. Stimulated by the hierarchical c-GeNW heterostructure, we explored its chemical/thermal stability and electrochemical properties as a lithium-ion battery anode. First, the chemical E
DOI: 10.1021/acs.nanolett.5b04476 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
rate was adjusted to keep the net flow rate constant at 20.0 sccm. The plasma was generated using a Dressler CESAR power supply at a power of RF 150W. A shutter was used to shield the substrate from any spurious droplets caused by arcing at the onset of plasma ignition. The shutter was opened upon achieving steady-state plasma. A 3-in. germanium sputter target (RNDKOREA, 99.999% purity) was used as the source material. The GeO2 films were deposited on prime grade, ptype (100) oxidized Si wafers (LG Siltron Inc., Korea). The wafers were placed on a rotating sample holder (30 rpm) located at a distance of 90 mm from the surface of the sputtering target. Synthesis of Carbon-Sheathed Ge Coaxial Nanowire Heterostructures. Commercially available GeO2 powders (99.99%, Germanium Corporation of America Inc.) were uniformly placed inside an alumina boat. In a typical process, the furnace was heated to 700−1000 °C with a ramping speed of 5 °C min−1 and kept at target temperature for 10 s to 60 min (Figure S1A). Subsequently, hydrocarbon gases (acetylene, ethylene) were introduced inside the furnace by controlling flow rate and exposure time at ambient pressure. Physical Characterization of c-GeNWs. Surface morphologies of GeO2 particles and c-GeNWs were characterized by field emission scanning electron microscopy (FE-SEM, FEI Verios 460 and Hitachi S-4800) operating at 10 kV and high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F) operating at 200 kV. Also, the microstructures of GeO2 and as-synthesized c-GeNWs were investigated by X-ray diffractometer (XRD, Bruker D8-Advance) operated at 3 kW using CuKα radiation. Raman spectra were obtained from a JASCO spectrometer (NRS-3000) operating at 532 nm to characterize the Ge and GeO2 phase in the Ge based materials. Electrochemical Test of c-GeNW Electrode. Electrochemical properties of Ge-based electrodes were evaluated using cointype half-cell (2016R) at 25 °C. The Ge-based electrodes were composed of Ge active materials, super-P carbon black, and poly(acrylic acid)/sodium carboxymethyl cellulose (1:1, w/w) binder in a weight ratio of 8:1:1. The electrolyte was 1.3 M LiPF6 with ethylene carbonate/diethyl carbonate (PANAX Starlyte, Korea, 3/7 (v/v)) including 10 wt % fluoroethylene carbonate (FEC) additive. The half-cells were tested galvanostatically between 0.01 and 1.5 V (versus Li/Li+) in the range of 0.05−5 C rates. The cell performance was examined using a cycle tester (WBCS 3000 battery systems, Wonatech).
stability of c-GeNWs was investigated with the use of a strong oxidant, H2O2 (Supporting Information, Figure S8). The cGeNWs showed significantly suppressed antioxidation properties, while pure Ge nanoparticles (GeNPs) were quickly transformed to GeO2 (Figure 4a,b). Second, the GeNPs were thermally oxidized at 550 °C for 2 h, while the c-GeNWs remained unchanged without changes in the original morphology (Supporting Information, Figure S9). These results demonstrate that the carbon layers can effectively protect the product from a chemical attack and side reactions by oxygen. Third, we tested the electrochemical properties of the c-GeNW anode. Galvanostatic discharge−charge cycling for the c-GeNWs (synthesized at 900 °C for 20 min) was performed. The first discharge and charge capacities of cGeNWs were 665 and 521 mAh g−1, respectively, at a rate of 0.05 C (Figure 4c). The c-GeNW electrodes exhibited high rate capabilities corresponding to a capacity retention of 40% even at a 5 C rate compared to the 0.2 C rate (Figure 4d). A more outstanding electrochemical property was the long-term cycling stability. The c-GeNW electrode exhibited a capacity retention of >99% at the 0.5 C rate after 300 cycles and subsequent retention of >96% after 900 cycles at the 1 C rate (Figure 4e). More interestingly, the synthetic process of carbon-sheathed metallic/semiconducting nanowires can be extended to several other metal oxides including SnO2, In2O3, and NiO. Even though these materials have different melting temperatures and activation energies for reduction, the hydrogen/carbon-assisted chemical reduction process led to the successful synthesis of carbon-sheathed metallic/semiconducting nanowires at different reaction temperatures (Supporting Information, Figure S10). In addition, we investigated the effect of other reactant gases (including H2, CH4, and C2H4) on chemical reduction of GeO2 particles. Among them, C2H4 gas showed successful synthesis of c-GeNWs, which are similar as those of C2H2 gas (Supporting Information, Figure S11). Conclusion. In summary, we have demonstrated a newly developed synthetic process for observation of acetylene oxidation with concomitant GeO2 reduction to form cGeNWs that employs a unique redox-responsive reaction. We proved the synthetic mechanism of the c-GeNWs by combining experimental results and DFT calculations. Our synthetic route for carbon-sheathed nanowires has the following advantages: (i) the simple redox-responsive assembly enables us to synthesize hierarchically assembled materials from inexpensive metal oxides at a large scale; (ii) our synthetic process can be generalized to other metal oxides; and (iii) the resulting cGeNWs exhibit chemical/thermal stability and outstanding electrochemical properties. This simple strategy may open up an effective way to make other metallic/semiconducting nanomaterials via one-step synthetic reactions through an environmentally benign and cost-effective approach. Materials and Methods. Fabrication of Germanium Oxide Films. GeO2 thin films were deposited within a stainless steel vacuum chamber that was evacuated to a pressure of below 1.5 × 10−6 Torr. Pumping was accomplished using a turbo-molecular pump in conjunction with a mechanical roughing pump. After the required pressure was reached, high-purity O2 (99.995%) and Ar (99.999%) were introduced into the chamber using mass flow controller. The working pressure, controlled via an automated gate valve assembly, was maintained at 30 mTorr. The O2 flow rate was varied between 0.0 and 20.0 sccm at increments of 5.0 sccm, and the Ar flow
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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/acs.nanolett.5b04476. Details of c-GeNWs growth, DFT calculation, growth of metal, and semiconducting; supporting figures (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Author Contributions
S.C., J.K., and D.Y.H. contributed equally to this work. Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acs.nanolett.5b04476 Nano Lett. XXXX, XXX, XXX−XXX
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(29) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley Publishing Company, Inc., 1978. (30) Lotty, O.; Hobbs, R.; O’Regan, C.; Hlina, J.; Marschner, C.; O’Dwyer, C.; Petkov, N.; Holmes, J. D. Chem. Mater. 2013, 25, 215− 222. (31) O’Regan, C.; Biswas, S.; O’Kelly, C.; Jung, S. J.; Boland, J. J.; Petkov, N.; Holmes, J. D. Chem. Mater. 2013, 25, 3096−3104. (32) Deringer, V. L.; Dronskowski, R. Chem. Sci. 2014, 5, 894−903.
ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (2015-01003143). S.K.K. acknowledges financial support from NRF-2014R1A5A1009799 and computational resources from UNIST-HPC and KISTI-PLSI.
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DOI: 10.1021/acs.nanolett.5b04476 Nano Lett. XXXX, XXX, XXX−XXX
