Article pubs.acs.org/cm
Synthesis of Tin Catalyzed Silicon and Germanium Nanowires in a Solvent−Vapor System and Optimization of the Seed/Nanowire Interface for Dual Lithium Cycling Emma Mullane, Tadhg Kennedy, Hugh Geaney, Calum Dickinson, and Kevin M. Ryan* Materials and Surface Science Institute and Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland S Supporting Information *
ABSTRACT: Silicon and germanium nanowires are grown in high density directly from a tin layer evaporated on stainless steel. The nanowires are formed in low cost glassware apparatus using the vapor phase of a high boiling point organic solvent as the growth medium. HRTEM, DFSTEM, EELS, and EDX analysis show the NWs are single crystalline with predominant ⟨111⟩ growth directions. Investigation of the seed/nanowire interface shows that in the case of Si an amorphous carbon interlayer occurs that can be removed by modifying the growth conditions. Electrochemical data shows that both the tin metal catalyst and the semiconductor nanowire reversibly cycle with lithium when the interface between the crystalline phases of the metal and semiconductor is abrupt. The dually active nanowire arrays were shown to exhibit capacities greater than 1000 mAh g−1 after 50 charge/ discharge cycles. KEYWORDS: nanowire, silicon, germanium, tin catalyst, energy storage, lithium battery anodes g−1).3,20−23 The choice of catalyst for this application is also relevant with Au not just of low abundance but also leading to irreversible capacity losses for the anode.24,25 The catalyst material also adds to the weight of the anode thereby lowering the gravimetric capacity. The ideal catalyst would be of an abundant element that also cycles Li with Sn of particular interest given its high theoretical capacity (994 mAh g−1) for Li uptake.26 Sn catalyzed Si NWs have been formed using CVD27 and supercritical fluid methods,24 and while they have been investigated for use in Li storage applications, the Sn seed in these studies remained inactive for lithium cycling.24 We have recently developed a high boiling point solvent− vapor−growth (SVG) system as a low cost approach to high density NW growth. The method requires no addition of discrete nanoparticle seeds with NW growth occurring catalytically in the presence of a metal layer (Cu,9,28,29 In16) by in situ formation of metal particles. The NWs grow by either a vapor−liquid−solid (VLS) or a vapor−solid−solid (VSS) protocol, depending on the boiling point of the metal, allowing very high yields of NWs to be attained at low temperature. Additionally as the NWs can be grown directly from a metal
S
i and Ge nanowires (NWs) have attracted intense research interest due to their suitability as building blocks for a range of devices in the semiconductor, photovoltaic, and energy storage industries.1−3 The majority of the Si and Ge NW growth methods have focused on the use of Au as a catalyst material4−6 due to its well established eutectic behavior with both Si and Ge.4,5 In addition to its high cost and low abundance, Au has been found to be detrimental for electronic applications due to its propensity to act as an electron trap.7,8 As a consequence, an extensive range of transition metals have been investigated as alternative catalysts with growth protocols largely dictated by whether the seed remains solid or liquid at the reaction temperature.9−12 More recently, attention has turned to p-block metals with Bi,13 Ga,14 and In15,16 showing promise for high purity NW formation. While the major focus of initial studies was directed toward nanoelectronics, the emergence of viable applications for Si and Ge NWs for storage (Li-ion batteries) and conversion (photovoltaics) places a different set of demands on the synthetic protocols. These bulk applications ideally require low cost routes with high-density NW formation occurring directly from substrates of interest using abundant and low cost precursors.17−19 For example, as lithium battery anodes, NW diameter is of less significance, with Si or Ge NWs (up to 200 nm in diameter) showing resistance to pulverization during cycling allowing capacities of 3579 mAh g−1 (Si) and 1384 mAh g−1 (Ge) to be attained that are multiples of conventional materials (graphitic carbon 372 mAh © 2013 American Chemical Society
Received: January 29, 2013 Revised: March 19, 2013 Published: March 21, 2013 1816
dx.doi.org/10.1021/cm400367v | Chem. Mater. 2013, 25, 1816−1822
Chemistry of Materials
Article
layer, they are anchored to the metal surface, making them ideal for application as Li-ion battery anode materials. Here we report the growth of high density Si and Ge NWs within the SVG system using Sn as the catalyst layer which is both elementally abundant and lithium active. The Sn catalyst layer is evaporated onto stainless steel (SS) substrates with growth occurring in high yield from the solvent−vapor system. The NWs are extensively examined by HRTEM, HRSEM, EELS, EDX, EBSD, and XRD analysis. For Si NWs, we show that the interface between the metal tip and the NW is tunable occurring either as an amorphous region (400 °C) that nucleate NWs via the VLS mechanism (Sn melting point, 231.9 °C). This Sn island formation circumvented the need for the addition of discrete catalyst seeds allowing the growth of highly dense Si or Ge NWs directly from the underlying growth substrate. Si NW Growth. Due to the high decomposition temperature of the phenylsilane (PS) precursor, reaction temperatures of 460 °C (for 1 h) were required for Si NW growth. Phenylsilane undergoes phenyl redistribution at elevated temperatures to supply the monomer for NW growth.30 A high magnification SEM image (taken at a 50° tilt) showing an example of Sn catalyzed Si NWs (105 ± 30 nm) grown on SS is shown in Figure 2a. The high density of Si NWs grown on the 1817
dx.doi.org/10.1021/cm400367v | Chem. Mater. 2013, 25, 1816−1822
Chemistry of Materials
Article
Figure 3. DFSTEM image of Sn seeded Si NW taken from the inset NW in i. EDX elemental maps for Sn (ii) and Si (iii) within the region highlighted in the orange rectangle are shown along with corresponding EELS maps for C (iv) and O (v). Inset vi is an overall EDX spectrum of the region.
Sn catalyst and Si NW. The presence of O was noted primarily next to the Si NW, which is consistent with the SiOx(x≤2) oxide covering typically noted for Si NWs exposed to ambient conditions.16 This amorphous region was found to be present for Sn catalyzed Si NWs regardless of reaction time, temperature, and NW diameter. The source of amorphous C noted in the growth system is likely due to partial decomposition of the organic solvent squalane at the elevated temperatures. However, we do not believe that the C is purely localized at this seed/NW interface, that it is during NW growth or postgrowth that C coats the Sn seeded Si NWs, and that the subsequent cooling and dewetting of the seed at the growth interface postreaction expose this C region. As the NW catalyst was obviously attached to the NW during growth, this exposed amorphous region between the NW and the catalyst may thus be due to a dewetting effect caused by slow cooling employed in the growth system coupled with the atypical wetting behavior of Sn as a low surface tension metal seed.33,35 By fast quenching the system above the melting temperature of the Sn seed, rapid solidification of the catalytic droplet takes place at the growth front and leads to the absence of an amorphous interfacial region. To verify this, the role of NW growth termination was investigated by rapidly quenching the NW growth reaction above the melting point of the Sn. Figure 4 shows TEM analysis of a NW taken from a quenched reaction. The HRTEM image in Figure 4a taken of the NW in Figure 4 (inset i) shows an example of the Si NW/ Sn catalyst interface with no visible gap between the catalyst and NW. The corresponding FFT pattern for the Si NW, Figure 4 (inset ii), is consistent with diamond cubic Si with a ⟨111⟩ growth direction. The annular DF-STEM image in Figure 4b taken from the same NW (on the same major zone axis) and overlaid EDX line profile indicate the presence of nonzero Si and Sn at the catalyst/NW interface in contrast to that seen in Figure 3c, suggesting that there is no gap between the NW and the catalyst. This is consistent with all the NWs formed using the rapid quenching approach (see Supporting Information, Figure S5 for additional examples). Ge NW Growth. Ge NWs were grown from the evaporated Sn layer at the lower reaction temperature of 430 °C due to the lower thermal stability of diphenylgermane (DPG) in comparison to PS with a typical reaction time of 10 min.
presence of twin defects (Figure 2c). Similar types of transverse twin faults have previously been noted for Si NWs grown from comparable low melting point metals (In15 and Ga31). It can also be seen that the area closest to the Sn catalyst is amorphous (due to the abrupt termination of crystalline fringes in the image) (Supporting Information, Figure S3). The composition of this interfacial region was analyzed using EELS and EDX analysis and will be discussed later in the text. This interface is quite different to those typically noted for the archetypal VLS NW catalyst Au, which characteristically possess a hemispherical catalyst tip in contact with the extruded NW.7 It is likely that the nature of Sn as a “type B” catalyst (which forms a eutectic alloy that is composed of
