Morphological Evolution of Fe–Mo Bimetallic Catalysts for Diameter and Density Modulation of Vertically Aligned Carbon Nanotubes

Abstract

Fe−Mo bimetallic catalyst is a promising candidate for achieving efficient growth and fine structures of vertically aligned carbon nanotubes (VA-CNTs) by catalytic chemical vapor deposition (CVD). Understanding the surface morphological evolution of a bilayer metal film and accompanied changes in the resultant CNT structure is a key to modulating the VA-CNT growth. We correlate the growth rates and individual CNT structures with the Fe−Mo catalyst morphological evolution. At CVD conditions, the layered bimetallic films evolve into entirely different ensembles of polydispersed nanoparticles depending on the ratio and stacking order between Fe and Mo. Specifically, Fe-on-Mo type nanofilms develop to denser and smaller nanoparticles with more pronounced bimodality in size than do Mo-on-Fe type nanofilms, leading to better alignment and unique diameter distribution of resultant CNTs. Also, the decrease in the catalytic activity of Mo-rich Fe−Mo nanoparticles can contribute to the diameter, alignment, and density modulation of VA-CNTs.

Understanding the “Activation” of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y] for Low Temperature Growth of Carbon Nanotubes

“…Later, Edgar et al [18], Jeon et al [19], Goss et al [20], Peng et al [21], demonstrated CNT growth by employing growth temperatures in the range of 880-1010°C without the use of additional catalyst promoters. The excessive growth temperatures required with FeMoC appear to counter previous work with Fe-Mo catalyst systems demonstrating CNT growth at temperatures as low as 550°C [22][23][24]. It should be noted that both Fe and Mo have been shown to be efficient catalysts both independently and as mixed metal nanoparticles [25][26][27][28][29][30]; thus, FeMoC should be ''active'' for CNT growth, but under the prior growth conditions studied it is not [16][17][18][19][20][21].…”

A Forest of Sub-1.5-nm-wide Single-Walled Carbon Nanotubes over an Engineered Alumina Support

“…Other than the CVD conditions, researchers have long been interested in the catalyst engineering, given the fact that the CNT CVD is a heterogeneous catalysis process. One way of engineering the catalyst is to optimize the preparation method and tailor the composition such as mono- and bimetallic systems 24 25 26 . It is not only important to select the right catalyst material but also to prepare the catalyst properly in order to provide the desired nanoparticle morphology and stability.…”

Catalytic Growth of Carbon Nanotubes by Direct Liquid Injection CVD Using the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y]

“…FeMoC was synthesized and purified as previously reported [14]. In a typical synthesis, iron(II) chloride tetrahydrate (1.00 g, 5.03 mmol) was dissolved in Millipore water (75 mL) followed by the addition of sodium molybdate dihydrate (2.00 g, 8.27 mmol).…”

“…Additionally, FeMoC belongs to the Fe-Mo system that consists of high temperature alloys, likely to maintain their structure under CCVD conditions. The Fe-Mo system has also demonstrated excellent catalytic performance; ultra-long single walled carbon nanotubes (SWCNT) growth (>18.5 cm), growth of ultra-high-density SWCNTs (160 SWCNTs/μm 2 ), and SWCNT growth under variable temperatures (450-1000 °C) [12][13][14][15].…”