# Wettability of CNW/ITO Micro Structure for Modification of Surface Hydrophilicity
## 3 Results and Discussion
We suppose that it might be affected by surface crystallization (orange quadrangle) as shown in FE-SEM (Figure 5b). In sputtering time of 15 min, surface crystallization was unexamined. While the ITO surface sputtered for 45 min onto pristine-CNW significantly exhibited surface crystallization. The OH bond around 533.3 eV changed by sputtering times with 15 min, 30 min, and 45 min was proportional to the sputtering time. It is summarized in Figure 5c, presenting a remarkably stepped plot than in pristine-CNW. Besides the structural modification identified by FE-SEM (Figures 2 and 3), we can infer that it may occur a variation in wettability by a chemical composition change. Figure 4: X-ray diffraction and Raman spectra: (**a**) X-ray diffraction of indium tin oxide with sputtering times; (**b**) Raman spectra of pristine-CNW and CNW/ITO micro-structure; (**c**) fitted G and D\({}^{\prime}\) band from Raman spectra. We conducted a surface-structure analysis through FE-SEM and chemical composition-based surface analysis with XPS followed by a wettability analysis for hydrophobicity and hydrophilicity using a water droplet method. As shown in Figure 6a, the pristine-CNW exhibits near hydrophobicity (low wettability, CA > 80\({}^{\circ}\)) as a high repellency towards water droplets. It is probably because the CNW composed of pure carbon is aromatic and non-polar causing difficulty with water interaction. Additionally, it may be contributed by the non-composition of oxygen-related groups on the pristine-CNW surface, and the air area located between the carbon nanowalls (pillars) hinders contact with the water droplet and substrate. The thin and narrow surface area at the top edge of the CNW with a nano-porous area results in low surface energy, which can cause almost hydrophobicity. While the CNW/ITO micro-structure by ITO sputtered onto pristine-CNW was investigated by showing hydrophilicity (high wettability, CAs < 50\({}^{\circ}\)) compared with pristine-CNW, which is similar to an oxidized surface. This hydrophilicity can be explained by a high surface energy proportional to the ITO surface area and reduced air area. Firstly, the lowest CA (11.23\({}^{\circ}\)) was obviously identified at a sputtering time of 45 min that means the widest ITO surface area in all specimens, being assumed the highest surface energy. In addition, the increase in surface energy caused by plasma in the sputtering step may be contributed to the reason for hydrophilicity. The plasma ion exhibit zig-zag movement in terms of nanowalls, is where the plasma-ion kinetic energy is reduced and absorbed [23]. As a result, the surface energy at the bottom is inevitably low because plasma ions are difficult to move to the bottom by its unique structure (Figure 6c). Hence, the surface-energy activation area in the CNW having a pillar-columnar structure is relatively higher at the top edge more than at the bottom. It is also associated with the Laplace pressure (P\({}_{\text{L}}\)) with surface energy, which occurred a low surface energy direction from high surface energy. In Figure 6d (left), P\({}_{\text{L}}\) is induced with an upward direction, and it created meniscus by forming a boundary surface with downward intrinsic pressure (P\({}_{\text{i}}\)), causing hydrophobicity [24]. In the case of CNW/ITO micro-structure, surface energy in proportion to surface area is increased by ITO deposited on CNW. It showed hydrophilicity because of the overlap \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Pristine-CNW** & **15 min** & **30 min** & **45 min** \\ \hline Oxygen & 2.12 & 51.77 & 54.49 & 57.33 \\ Carbon & 97.88 & 23.4 & 22.17 & 20.79 \\ In/Sn & - & 24.83 & 23.34 & 21.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The chemical components of all specimens. Figure 5: X-ray photoelectron spectra of pristine-CNW and ITO sputtered onto pristine-CNW with sputtering time: (**a**) O 1s result in XPS; (**b**) surface FE-SEM of ITO sputtered onto pristine-CNW; (**c**) OH bond value of ITO according to sputtering time.