The morphology of CNTs is influenced by the catalyst particle size and density. Our experiment was thus designed to form and control the catalyst particles before CNT synthesis by NH3 gas pretreatment. A nickel catalyst layer was separated into well-defined discrete catalyst particles by etching of NH3 gas in high temperature. Also, the metal substrate in this work can be etched by NH3 gas. Figure presents FE-SEM images showing the morphology of the Ni catalyst nanoparticles according to the thickness of the Au buffer layer after the NH3 gas pretreatment. In the samples with no buffer layer and a 20-nm Au buffer layer, as shown in Figure , the surface of the substrate was very rough without noticeable particle formation, which is less favorable for CNT growth. Regarding these results, we have surmised that the absence of catalyst particles and the rough substrate surface after NH3 gas pretreatment are due to etching by NH3 gas and diffusion between the Ni catalyst layer and the Ti substrate. On the other hand, the nickel catalyst layer of the samples with Au buffer layers of 40 and 60 nm was transformed to nanoparticles, as shown in Figure . Also, the nanoparticles showed a uniform size distribution and have an average diameter of 100 nm. From the results of Figure , we confirmed that an Au layer over 40 nm plays an important role in the formation of catalyst particles through suppression of interdiffusion between the Ni layer and the Ti substrate and by protection of the Ti substrate from NH3 gas. To investigate the effects of the Au buffer layer in CNT synthesis, CNTs were grown on a Ni/Ti substrate (no Au buffer layer) and Ni/Au/Ti substrate.
FE-SEM images of Ni catalysts. FE-SEM images of Ni catalysts formed on (a) Ti substrate, (b) 20-nm Au buffer layer/Ti substrate (c) 40-nm Au buffer layer/Ti substrate, and (d) 60-nm Au buffer layer/Ti substrate.
For CNT synthesis, four samples shown in Figure were prepared. FE-SEM images of these samples after CNT growth are provided in Figure . Only random and sparse CNTs in the samples without and with a 20-nm Au buffer layer were obtained, as shown in Figure . On the other hand, the samples with 40- and 60-nm Au buffer layers present growth of vertically aligned CNTs, as shown in Figure . The results of Figure accord with that of Figure . The insets of Figures are low-magnified FE-SEM images. From these images, it is clear that the samples with a 60-nm Au layer have more vertically aligned CNTs. The CNTs in Figure have a diameter of about 80 to 120 nm and a length of about 5 to 10 μm.
FE-SEM images of CNTs. FE-SEM images of CNTs grown on (a) Ti substrate, (b) 20-nm Au buffer layer/Ti substrate (c) 40-nm Au buffer layer/Ti substrate, and (d) 60-nm Au buffer layer/Ti substrate.
Figure shows the Raman spectra of CNTs synthesized on samples with 40- and 60-nm Au buffer layers. The Raman spectra are observed to have two prominent peaks at approximately 1,363 cm-1 (noted as the D band) and approximately 1,604 cm-1 (noted as the G band) with an intensity ratio ID/IG of 0.99. From the D band, which has been ascribed to disorder-induced features due to the finite particle size effect for lattice distortion, the observation that the ID/IG ratio is about 1 leads us to conclude that the CNTs have a lower degree of structural defects.
Raman spectra of CNTs. Raman spectra of CNTs on (a) 40-nm Au buffer layer/Ti substrate and (b) 60-nm Au buffer layer/Ti substrate.
Figure shows the cycle performances of Li/CNT cells with a current of 50 μA and a cutoff voltage of 0.01 to 2.0 V. Figure present the electrochemical properties obtained using the two samples shown in Figure as an electrode of Li/CNT cells, respectively. The electrochemical properties in Figure are superior to those indicated in Figure . As seen in Figure , the first discharge capacity and discharge capacity after the 50th cycle were 210 and 80 μAh/cm2, respectively. A Li/CNT cell with CNTs on a 60-nm Au buffer layer exhibited a good charge/discharge behavior, good cycle performance, and slow capacity fading.
Cycle performances of Li/CNT cells with current of 50 μA. Cycle performances of Li/CNT cells with current of 50 μA on (a) Ti substrate and (b) 60-nm Au buffer layer/Ti substrate.