Article pubs.acs.org/Langmuir
Decoration of Graphitic Surfaces with Sn Nanoparticles through Surface Functionalization Using Diazonium Chemistry Gul Zeb,† Peter Gaskell,‡ Xuan Tuan Le,† Xingcheng Xiao,§ Thomas Szkopek,‡ and Marta Cerruti*,† †
Mining & Materials Engineering, McGill University, 3610 University Street, Montreal, Quebec H3A 0C5, Canada Electrical & Computer Engineering, McGill University, 3480 University Street, Montreal, Quebec H3A 0E9, Canada § General Motors R&D Center, 30500 Mound Road, Warren, Michigan 48090, United States ‡
S Supporting Information *
ABSTRACT: Composites of tin nanoparticles (Sn NP) and graphene are candidate materials for high capacity and mechanically stable negative electrodes in rechargeable Li ion batteries. A uniform dispersion of Sn NP with controlled size is necessary to obtain high electrochemical performance. We show that the nucleation of Sn particles on highly ordered pyrolitic graphite (HOPG) from solution can be controlled by functionalizing the HOPG surface by aryl groups prior to Sn deposition. On the contrary, we observe heterogeneous deposition of micrometer sized Sn islands on HOPG subjected to oxidation prior to deposition in the same conditions. We demonstrate that functional groups act as nucleation sites for Sn NP nucleation, and that homogeneous nucleation of small particles can be achieved by combining surface functionalization with diazonium chemistry and appropriate stabilizers in solution.
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INTRODUCTION Li ion batteries are the most widely used power source for portable electronics due to their very high gravimetric energy density (170−230 W h kg−1) and long cycle life (>500).1−3 Graphite is the most commonly used negative electrode for commercial Li ion batteries exhibiting a theoretical storage capacity of 372 mA h g−1. However, the demand for higher energy density, power density, and longer cycle life has encouraged research on improved electrode materials. It has been shown that the storage capacity of anodes can be significantly improved by using Sn based materials since the theoretical capacity of Sn is 994 mA h g−1.4,5 However, poor cycle life resulting from mechanical failure due to large volume change during lithiation and delithiation limits the application of their bulk anodes in commercial Li ion batteries.6 The composite of Sn or SnO2 nanoparticles (NP) with graphite or other carbon-based material such as graphene or carbon nanotubes has emerged as a candidate negative electrode for high energy density Li ion batteries.7−9 The composite of Sn or SnO2 NP with graphene nanosheets is of particular interest, since graphene itself shows good rate capability and excellent electric conductivity.8,10,11 The wrinkled structure of graphene sheets in the composite can accommodate the expansion of NP during lithiation, and helps in preventing NP agglomeration.12 The electrochemical performance of the nanocomposite negative electrodes, including charge capacity, rate capability, and cyclability, heavily depends on the mean particle size and dispersion of Sn or SnO2 NP.13,14 Accurate knowledge of the optimum size and dispersion of NP on graphene is thus a crucial factor which has to be explored. © 2012 American Chemical Society
Most methods for synthesizing composites of Sn or SnO2 NP with carbon-based materials can be classified into the following main categories. The first one is based on pyrolysis of SnCl4 onto graphite powder.15,16 Here the carbon-based material is mixed with the solution of SnCl4 and then heated above 400 °C under protective environment. Another method involves the attachment of presynthesized Sn or SnO2 NP onto a carbonbased material.12,17,18 Since the absence of an intimate contact between the carbon-based material and the NP is detrimental to the electrochemical performance of the composite,19 phenyl bridges have been proposed as a way to create covalent bonds between them.20−22 This method was tested on graphite, carbon nanotubes, and graphene to bind Si NP for Li ion battery application. Very stable capacity up to 35 cycles was obtained with this method for Si NP−carbon nanotube composites. Thus far, this method has not been applied to the binding of Sn or SnO2 NP to carbon-based materials. In the third category of synthesis methods, Sn or SnO2 NP are directly formed on carbon-based materials from solution. One of these methods involves the chemical reduction of Sn2+ or Sn4+ ions from precursor solutions of SnCl2 or SnCl4, to form Sn NP on carbon-based materials.8,23 Wang et al reported the deposition of 2−5 nm diameter Sn nanoparticles on graphene sheets with this method. Another method whereby SnO2 NP are obtained on graphene is based on a redox reaction where Sn2+ oxidizes to form SnO2 and graphene oxide reduces to form graphene.24−30 In situ conversion of graphene oxide to Received: May 28, 2012 Revised: August 6, 2012 Published: August 7, 2012 13042
dx.doi.org/10.1021/la302162c | Langmuir 2012, 28, 13042−13050
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Scheme 1. Mechanism of Grafting and Growth of Mixed Layers on HOPG
HOPG, respectively. The S-HOPG and A-HOPG samples were prepared using diazonium chemistry. Reviews of diazonium chemistry are available elsewhere.31−33 Sulfanilic acid (4-aminobenzene sulfonic acid, ACS reagent 99%, Sigma Aldrich) and para-phenylenediamine (ACS reagent 99%, Sigma Aldrich) were used as precursors for sulfophenyl and aminophenyl functionalization, respectively. Functionalization of HOPG through sulfophenyl groups involved preparing a solution of sulfanilic acid (0.05 M) in dilute HCl (0.5 M) with sodium nitrite (0.05 M) to achieve sulfophenyl diazonium cations (SO3H− C6H4−N2+) (Scheme 1(i)). A certain quantity of Fe powder (Alfa Aesar,
