The Role of Oxygen in Carbon Nanotube Synthesis

Chemical vapor deposition (CVD) has been recognized as one of the most promising methods to produce carbon nanotubes (CNTs) industrially. Oxygen is important in CNT high-volume production, but few of the studies propose mechanistic details for how oxygen exerts these effects. Since reported optimization conditions to generate CNTs are based on empirical results, several gray areas still exist in the CNT growth mechanism. Uncovering the CNT growth mechanism, especially for the oxygen-related CNT synthesis, is necessary to promote CNT production with atomistic control.

Here, in order to separate gas and catalyst thermal effects and allow for uniform transformation of gas as it approaches substrate, a specifically designed CVD reactor was assembled for determining the CNT growth mechanism. Two typical and promising oxygen-related CVD processes (equimolar C2H2-CO2 reaction and water-assisted CNT growth) were studied to analyze the role of oxygen. The reported equimolar C2H2-CO2 reaction process could deposit CNTs on various substrates and at relatively low temperature. The water-assisted CVD process could obtain dense, high-purity, and millimeter-scale CNTs, which is promising for mass production of CNTs.

Firstly, native CO2 (12C CO2) and 13C-labeled CO2 were individually used as feedstock with C2H2 to grow CNTs. A statistical study using the Raman spectra of the yielded CNTs in both conditions indicated that the C in CO2 was not incorporated into CNTs. Based on this conclusion, an electron-pushing CNT growth mechanism was proposed. The role of oxygen in CO2 was indicated as grasping hydrogen atom in the raw CNT lattice.

Secondly, CNTs were synthesized with various H2O concentrations ranging from 10 parts per million (ppm) to 220 ppm. Quantitative study of the CNT outer diameters by TEM imaging indicated that the outer diameter tended to increase with H2O concentration (the average outer diameter ranges from 4.8 nm at 10 ppm H2O to 6.4 nm at 220 ppm H2O). Raman spectra revealed that the dominant CNTs went from SWCNTs at 10 ppm H2O to MWCNTs at high H2O concentration, consistent with the diameter increase trend. The formed catalyst might explain the CNT quality change. The AFM images of the catalyst demonstrated that the height, size and spacing of the iron nanoparticles on the substrate increased with water concentration. The alignment property was tested by SEM imaging. The yielded CNTs at 120 ppm H2O got the best alignment. The gas composition analysis results indicated that the H2O might promote the decomposition of the main carbon precursor. The oxygen in H2O might influence the catalyst activation and carbon precursor decomposition.

Through the CNT characterization and gas composition analysis, the role of oxygen could be categorized into three areas: 1) absorb hydrogen in immature CNT lattice; 2) influence catalyst formation; 3) promote carbon precursor decomposition.