Understanding the “Activation” of the Nanocluster [HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y] for Low Temperature Growth of Carbon Nanotubes
Esquenazi, Gibran L.; Barron, Andrew R.
The molecular nanocluster HxPMo12O40⊂H4Mo72Fe30(O2CMe)15O254(H2O)98-y(EtOH)y(FeMoC), was the first molecular catalyst precursor (pro-catalyst) that promised controlled growth of carbon nanotubes (CNTs); however, temperatures in excess of ~ 900 °C or the addition of excess iron were required as catalyst promoters for CNT growth. To understand these disappointing results the “activation” of FeMoC for CNT growth was studied by systematic investigation of H2 gas concentration and growth temperature. The pathway for “activation” of FeMoC occurs through the sufficient reduction of both metal oxide components in the pro-catalyst. By ensuring pro-catalyst reduction prior to introduction of growth gases, we demonstrate for the first time, growth of CNTs at temperatures as low as 600 °C without the use of catalyst promoters using the single molecular precursor, FeMoC. To understand the role of catalyst promoters used in prior work, thermogravimetric analysis experiments were performed. The addition of an iron catalyst promoter is observed to play two key roles in the “activation” of FeMoC: (1) to replenish sublimated metal atoms, and (2) to reduce the reduction temperature required for reduction of FeMoC into an “active” catalyst. These results caution the conditions employed in many earlier studies for CNT growth, and create new possibilities for molecular pro-catalysts.