Quantum Chemical Simulation of Carbon Nanotube Nucleation on Al₂O₃ Catalysts via CH₄ Chemical Vapor Deposition

Abstract

We present quantum chemical simulations demonstrating how single-walled carbon nanotubes (SWCNTs) form, or "nucleate", on the surface of Al2O3 nanoparticles during chemical vapor deposition (CVD) using CH4. SWCNT nucleation proceeds via the formation of extended polyyne chains that only interact with the catalyst surface at one or both ends. Consequently, SWCNT nucleation is not a surface-mediated process. We demonstrate that this unusual nucleation sequence is due to two factors. First, the π interaction between graphitic carbon and Al2O3 is extremely weak, such that graphitic carbon is expected to desorb at typical CVD temperatures. Second, hydrogen present at the catalyst surface actively passivates dangling carbon bonds, preventing a surface-mediated nucleation mechanism. The simulations reveal hydrogen's reactive chemical pathways during SWCNT nucleation and that the manner in which SWCNTs form on Al2O3 is fundamentally different from that observed using "traditional" transition metal catalysts.

Effect of ammonia on chemical vapour deposition and carbon nanotube nucleation mechanisms

“…We note here that SWCNT caps do not nucleate in trajectories 1-10 within the timescale employed here (300 ps) ( Figure S1). This is a direct consequence of the presence of hydrogen; removal of hydrogen would undoubtedly lead to cap nucleation and growth here, as it has done in a number of prior investigations of SWCNT cap nucleation, 59,60,82 polycyclic aromatic hydrocarbon growth 83 and fullerene formation. 84,85 Hydrogen plays a dual role during condensation of sp 2hybridized carbon networks.…”

“…Subsequently, SWCNT nucleation initially proceeds via polyyne chain growth. Previous simulations 59 have shown that CH and CH2 are the main carbon species responsible for driving polyyne chain formation, due to their spare carbon dangling bonds, i.e. spare valence electrons available for additional C-C bond formation.…”

“…[49][50][51][52][53][54] Our own group has employed density functional tight-binding (DFTB) approaches to study SWCNT nucleation and CVD in a range of conditions. [55][56][57][58][59] However, despite this wealth of literature, few investigations have considered the roles played by hydrogen, [58][59][60][61] undoubtedly present from feedstock decomposition, and other etchant additives, 45 during CVD nucleation and growth.…”

Chiral-Selective Carbon Nanotube Etching with Ammonia: A Quantum Chemical Investigation

“…Our hypothesis regarding chiral selective etching is that particular (n,m) SWCNTs are intrinsically more reactive with chemical etchants such as NH3 and NH3-based radical species (NH2, NH and H). We note that considering atomic hydrogen in this context also enables us to comment on the role of atomic hydrogen produced during catalytic CVD of hydrocarbon feedstock, [20][21][22][23][24] and the role of H2 as an additive, used extensively in SWCNT growth. [25][26][27] We test this hypothesis using model SWCNT caps; caps are chosen here, as opposed to extended SWCNT structures, since they are more reactive due to their higher structural curvature.…”

The influence of magnetic moment on carbon nanotube nucleation

“…At the molecular level, CNT nucleation and growth is an immensely complex process controlled by experimental parameters including temperature [17], catalyst type [18][19][20][21][22][23], catalyst particle size [24,25], the type of carbon feed stock and feedstock pressure [19,26,27], the presence of reducing agents [28] (notably hydrogen [23,[29][30][31][32][33][34]), and applied electric [35,36] and magnetic [37][38][39] fields. Understanding the relationships between these parameters and the CNT nucleation/growth mechanism has been the focus of the CNT growth community since the 1990s.…”