Insulator film coating on top of walls in carbon nanowalls

抄録

Carbon nanowalls (CNWs) have a wall structure consisting of multilayered graphene that stands perpendicular to the substrate. CNWs have properties of a large surface area, high electrical conductivity, and chemical stability. Therefore, CNWs are expected to be used as an electrode material for electric double layer capacitor (EDLC). However, there are two problems in using CNW as an electrode material in EDLCs, because the top of the wall is sharp. The one is nonuniformity of the electric field. Electric field at the top of the wall is larger than that at the surface of the wall. The other is large hydrophobicity. CNWs have a large hydrophobicity due to the sharp top of the wall. Therefore, effective surface area of CNW electrode, because an electrolyte solution can not penetrate between the walls. The nonuniformity of the electric field and the large hydrophobicity would reduce the capacitance of EDLC using CNW electrode. It is expected to solve these two problems by coating the top of the walls with a hydrophobic insulator film such as SiO₂. In order to achieve that, a technique to coat only the top of the walls with insulator film. Therefore, in this work, we have studied the conditions to coat the top of the wall with oxide film in CNWs.

CNWs were fabricated on crystalline silicon substrates by hot-wire CVD using CH₄ and H₂. NiO films were deposited on the CNW samples by rf-magnetron sputtering with rf-power of 100 W and 150 W for 30 min. The line-scan profile of Ni along the cross-section of the NiO deposited CNW samples were measured using SEM-EDX.

Figure 1 shows the line profiles of Ni-Kα X-ray intensities for NiO-deposited CNW samples with rf-power of (a) 100 W and (b) 150 W. Here, The deposition rate of NiO films was 0.065 nm/sec and 0.072 nm/sec at rf-power of 100W and 150 W, respectively. In both samples, the signal was detected in whole CNW region. In the NiO-deposited CNW sample at rf-power of 100 W, the signal intensity was almost the constant. On the other hand, the signal intensity at the bottom of the wall (near the substrate) was larger than that at the top of the wall in the NiO-deposited CNW sample at rf-power of 150 W. These results suggest that the high deposition rate would be required in the coating only the top of wall.

Details will be reported at the presentation including other results.

This work was partially supported by JSPS KAKENHI Grant Number 23K03924.