Co-Catalytic Solid-State Reduction Applied to Carbon Nanotube Growth

Abstract

We report on a new class of cocatalysts for the chemical vapor deposition of carbon nanotubes, where the cocomponent (Ta) acts as a solid-state reducing agent for the active catalyst (Fe). The cocatalytic FeTa system enables carbon nanotube growth without the need for a reducing gas atmosphere such as H2 or NH3. In situ X-ray photoelectron spectroscopy reveals that the tantalum (oxide) getters the oxygen from the iron (oxide) by a diffusive solid-state process, driven by the much larger affinity to oxygen of Ta compared to Fe. We suggest that this redox-based mechanism is applicable to a wide range of metal (oxide)/catalyst systems and relevant to rational catalyst design in general heterogeneous catalysis

High Quality Carbon Nanotubes on Conductive Substrates Grown at Low Temperatures

“…In the case of pure ballistic conduction and perfect metal‐CNT contact, each CNT should pose a quantum resistance of 6.45 kΩ in the bundle, resulting in the resistance values of 0.645 and 2.58 Ω for 10 × 10 nd 5 × 5 μm 2 vias, respectively, which are much smaller than our calculated values of 25 and 100 Ω. The origin of additional high resistance may be because of imperfect metal‐CNT contacts, diffusive conduction in the CNTs because of structural defects and oxidation of catalyst, as the sample was exposed to the atmosphere when transferred from sputter deposition system to the PTCVD system . Assuming diffusive conduction and perfect metal‐CNT contact, the resistance of a single CNT (250 kΩ in our case) is given from ref .…”

Co-catalytic Absorption Layers for Controlled Laser-Induced Chemical Vapor Deposition of Carbon Nanotubes

“…As-grown CNTs are multiwalled, similar to previous reports. 16,27,42 Reference samples without a Ta support layer did not give CNT growth under nonreducing conditions. This clearly confirms that Ta is required for catalyst activation and acts as the primary reducing agent for the Fe, 27 with no significant catalyst activation from the C 2 H 2 gas itself under these conditions.…”

“…We first use thermal CVD, i.e. conventional global substrate back heating, to investigate the influence of the Ta support layer thickness on the solid-state reduction 27 of the initially oxidized Fe catalyst (from transport/storage in ambient air), and hence the effectiveness of the layer stack for CNT CVD. Figure 2 compares thermal CNT CVD results for Fe(1 nm)/Ta(∼0− 130 nm) layer stacks for a nonreducing (∼10 −3 mbar vacuum; Figure 2a) and, as reference, for a reducing pretreatment atmosphere (100 sccm NH 3 at 1 mbar; Figure 2b).…”

“…27 Residual oxygen in the CVD growth chamber can cause reoxidation of the Fe catalyst (IV, Figure 6) and a direct gas phase oxidation of the Ta. Hence under oxygen-residual-rich low vacuum conditions (as used in this study, base pressure …”

Low-Temperature Growth of Carbon Nanotube Forests Consisting of Tubes with Narrow Inner Spacing Using Co/Al/Mo Catalyst on Conductive Supports

“…53 The same mechanism has been reported with Fe-Ta catalyst as the solid-state reduction. [54][55] The annealing in the presence of a reduction gas, as H 2 and/or NH 3 , enhances the fraction of metallic Co, which is active state to grow CNTs. A similar behaviour is also found in the analysis of the Mo 3d level.…”