Abstract
To improve the understanding on CNT growth modes, the various processes, including thermal CVD, MP-CVD and ECR-CVD, have been used to deposit CNTs on nanoporous SBA-15 and Si wafer substrates with C2H2 and H2 as reaction gases. The experiments to vary process parameter of ΔT, defined as the vector quantities of temperature at catalyst top minus it at catalyst bottom, were carried out to demonstrate its effect on the CNT growth mode. The TEM and TGA analyses were used to characterize their growth modes and carbon yields of the processes. The results show that ΔT can be used to monitor the temperature gradient direction across the catalyst nanoparticle during the growth stage of CNTs. The results also indicate that the tip-growth CNTs, base-growth CNTs and onion-like carbon are generally fabricated under conditions of ΔT > 0, <0 and ~0, respectively. Our proposed growth mechanisms can be successfully adopted to explain why the base- and tip-growth CNTs are common in thermal CVD and plasma-enhanced CVD processes, respectively. Furthermore, our experiments have also successfully demonstrated the possibility to vary ΔT to obtain the desired growth mode of CNTs by thermal or plasma-enhanced CVD systems for different applications.
Highlights
The results indicate that the tip-growth carbon nanotubes (CNTs), base-growth CNTs and onion-like carbon are generally fabricated under conditions of DT [ 0, \0 and *0, respectively
The experiment results revealed that most of the CNTs deposited by plasma-enhanced chemical vapor deposition (CVD) and by thermal CVD generally grow in the tip-growth [12,13,14,15,16,17,18,19,20,21] and base-growth [22,23,24,25,26,27,28,29] modes, respectively
The results indicated the adhesion force between catalyst and substrate to be the main factor in CNT growth modes [11, 30]
Summary
Starting with Ijima’s [1] discovery of carbon nanotubes (CNTs), there has been continual discovery and investigation into a series of CNTs using various CVD systems [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]. About CNT growth modes, the adhesion force at catalyst/ substrate interface was first proposed by Bower’s group as one of the most important factors [11]. The present research includes specially designed experiments, testing different processes (thermal CVD, MP-CVD and ECR-CVD) and substrates and varying the possible process parameters to examine their effects on CNT growth modes. The CNTs were deposited on Co-coated substrates (i.e., mesoporous SBA-15 powders and Si wafer) by thermal CVD method with C2H2 and H2 as reaction gases. The main feature of this system is that the direction of temperature gradient on each catalyst can be manipulated
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