Formation of Thermally Stable, High-Areal-Density, and Small-Diameter Catalyst Nanoparticles via Intermittent Sputtering Deposition for the High-Density Growth of Carbon Nanotubes.

Hirofumi Koji,Hiroshi Furuta,Akimitsu Hatta,Yuji Kusumoto

doi:10.3390/nano12030365

Abstract

We report the formation of thermally stable catalyst nanoparticles via intermittent sputtering deposition to prevent the agglomeration of the nanoparticles during thermal chemical vapor deposition (CVD) and for the high-density growth of carbon nanotubes (CNTs). The preparation of high-areal-density and small-diameter catalyst nanoparticles on substrates for the high-density growth of CNTs is still a challenging issue because surface diffusion and Ostwald ripening of the nanoparticles induce agglomeration, which results in the low-density growth of large-diameter CNTs during high-temperature thermal CVD. Enhancing the adhesion of nanoparticles or suppressing their diffusion on the substrate to retain a small particle diameter is desirable for the preparation of thermally stable, high-areal-density, and small-diameter catalyst nanoparticles. The intermittent sputtering method was employed to deposit Ni and Fe metal nanoparticles on a substrate for the synthesis of high-areal-density CNTs for Fe nanoparticle catalyst films. The metal particles deposited via intermittent sputtering with an interval time of over 30 s maintained their areal densities and diameters during the thermal CVD process in a vacuum for CNT synthesis. An interval of over 30 s was expected to oxidize the metal particles, which resulted in thermal stability during the CVD process. The intermittent sputtering method is thus a candidate process for the preparation of thermally stable catalyst films for the growth of a high density of long CNTs, which can be combined with the present CNT production process.

Highlights

Introduction iationsThe physical properties of carbon nanotubes (CNTs), including their optical properties, mechanical stiffness, electrical conductivity, and thermal conductivity, are known to be derived directly from their structural features of chirality, diameter, length, layer numbers, defects, and areal density [1,2,3]
We have reported high-density CNT growth on a multi-layered Ni/Fe catalyst [25,26], in which a thin Ni (0.8–1.6 nm thick) over-coating layer could prevent the agglomeration of iron particles at high temperatures before the introduction of the carbon gas source
We investigate the control of size and areal density of monometallic catalysts using the intermittent sputtering method for the synthesis of high-density and small-diameter carbon nanotubes

Summary

Introduction

The physical properties of carbon nanotubes (CNTs), including their optical properties, mechanical stiffness, electrical conductivity, and thermal conductivity, are known to be derived directly from their structural features of chirality, diameter, length, layer numbers, defects, and areal density [1,2,3]. The chirality of single-walled CNTs (SWNTs) can be successfully controlled by pretreatment of the catalysts [4], such as Co-W bi-metal catalyst [5], and cloning of nanocarbon segments using organic synthesis, which has been reported in review articles [6,7]. Color variations of SWNTs have been successfully controlled using chiral selective SWNTs [8,9]. The optical properties and thermal conductivity of CNT forests are highly dependent on the areal density of the CNT forest.

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Conclusion