Tailored nanoparticles and wires of Sn, Ge and Pb inside carbon nanotubes

Abstract

We demonstrate three different synthesis procedures to fill carbon nanotubes with main group IV-elements in their metallic state. We demonstrated, that filling CNT with nanoparticles really demands an adjusted procedure. After finding the best filling method, we tuned the morphology of filling particles, degree of filling and sample purity, in terms of coating-to-filling-ratio, by variation of the reaction parameters. A mass fraction of up to 30% of a highly crystalline filling material and a pure outer nanotube surface at once, can address conditions for application. Using the template CNT, we prepared chemically stable, metallic nanoparticles with a well-defined size distribution and high rates of filling. Such materials are promising candidates for sensoric and energy storage applications.

Introduction

Materials, used in electronic or magnetic devices, are comprehensively optimized in their performance, after years of technological improvement. Structuring, advanced processing or optimization of the composition improve the material's properties. One can for example enhance the energy product of hard magnetic materials [1] or the specific capacity of batteries [2]. Yet, there is still a strong demand for further progress in such devices to meet requirements for latest applications.

A complementary approach to the development and design of novel materials is the downscaling, to sizes at nanometer-level, of systems, that are well-developed in their bulk state. Such a structuring of materials on the nanoscale can lead to enhanced properties compared to bulk. In this way, for example, the performance of thermoelectric materials was increased by 70% [3], [4]. When downscaled, properties arise from a drastically increased surface-to-volume-ratio and quantum confinement.

In this sense one can decrease e.g. the melting temperature with decreasing particle diameter [5], [6], or tune the electrical properties, and therefore, the band gap [7]. Also the ballistic transport in carbon nanotubes (CNT) is attributed to their small size [8]. Furthermore, nanoparticles may show a higher catalytic activity [9], [10] and magnetic properties like an altered coercive field [11].

In order to benefit from nanoscale materials, one has to address some challenges. Very small particles lack stability and suffer from dissolution, agglomeration or, due to their chemical activity, from oxidation. Since properties like the band gap depend strongly on particle size, narrow size distribution is desirable. Group-IV elements, like tin and germanium, are discussed to have high potentials in energy converting and Li-ion-battery materials. As anode materials, they possess high relative capacities with respect to their masses and especially in respect to their volume. Compared to graphite (372 mAh⋅g⁻¹) they show a up to three times higher relative capacity (Sn 993 mAh⋅g⁻¹), caused by high stoichiometric lithium uptake. The volume expansion leads to structural decomposition during charge-discharge-process, and hence, to poor cycling stability. A suitable strategy to overcome the structural instability, caused by cycling, is the downscaling of anode materials to nanoparticles or nanowires [12].

Until now, nano wires of Ge or Si for example, are synthesized by catalytic chemical vapor deposition processes [13]. There, size and distribution of catalyst particles on a substrate influences the wire synthesis. The appropriate catalyst is chosen only empirically and is later left as residue in the wires.

To produce stable and pure nanoobjects, one has to avoid agglomeration and to control chemical composition. In our work we present ways to synthesize nano objects from group-IV elements Sn, Ge and Pb by using CNT as template. These CNT serve as reaction container for the formation of nano objects, and later, as protective shell. Due to their shape and size, CNT posses a strong capillary activity. Suitable chemical compounds can enter the inner cavity of the tube by capillary action. The CNT, firstly, control filling-particle-homogeneity, and secondly, prevent those particles from agglomeration and oxidation.

The idea of filling CNT is nearly as old as their discovery by Iijima [14]. In 1993, Ajayan deposited lead particles on CNT and annealed them [15]. As a result, in a low fraction of tubes he could detect lead particles. Later Dujardin and Ebbesen presented some findings about the compounds that are able to enter the CNT and concluded a dependence from their wetting behavior [16], [17]. In the following years, extensive work has been done in order to fill the inner cavity of CNT, especially of multi-wall CNT (MWCNT). Lots of work, where catalyst residues from the chemical vapor synthesis, mainly iron, cobalt, nickel, copper and manganese are incorporated into the CNT have been published [18], [19], [20], [21], [22], [23]. There are many examples of particles in CNT, but most of them stem from a catalyst residue and are generated during the CNT synthesis. Besides filled MWCNT, the filling of single-wall [24] and double-wall CNT [25], [26], [27] has also been established. Filling pure, catalyst free CNT offers a richer spectrum of filling elements since their catalytic activity for CNT synthesis does not matter. The filling with Pd or Ru enables the use of these CNT in catalytically influencing reactions [28], [29]. Tsang first proposed a simple route to post-synthetically fill CNT with acidic solutions of nitrate salts [30]. Later, different methods to mobilize the filling material and transport it into the CNT have been applied [31], [32]. The idea of taking CNT as 1D template for nanowire synthesis was first proposed by Tessonnier [33], who managed to synthesize CoFe nanowires. Prem Kumar et al. [34] already reported about CNT filled with Sn. A closer look on XRD revealed the presence of mainly SnO₂ in their CNT.

We synthesized nanostructures of Sn, Ge and Pb in their elemental, non-oxidized state by using CNT as template. To optimize the synthesis we worked out three different approaches for generation of particles or wires. To tune particle size and shape we varied parameters of the filling reaction, like reaction time, reduction time and temperature, as well as concentration of filling solutions.