Structural dependence of corrosion resistance of amorphous carbon films against nitric acid

To investigate the structural dependence of the corrosion resistance of amorphous carbon (a-C:H) films, three different types of a-C:H films etched by nitric acid were evaluated using a surface plasmon resonance (SPR) device with a multilayer structure consisting of an a-C:H layer on Ag. Two non-hydrogenated amorphous carbon (a-C) films and one hydrogenated a-C:H film were synthesized to estimate the effects of the sp²/sp³ ratio and hydrogenation, respectively. A flow cell for the introduction of nitric acid solution was placed on the amorphous carbon layer of the multilayer structure. A 0.3 mM nitric acid solution was used in the etching tests. The Kretschmann configuration was used for SPR measurement, and the SPR angle was determined as the angle with minimum reflectivity. The SPR angle decreased with increasing duration of nitric acid injection into the flow cell, indicating that the film was corroded by the nitric acid. The thickness of the films was calculated from the SPR angle. The rates of decrease in the thickness were 2.2, 0.8, and 1.6 nm/h for the a-C films with lower and higher sp² contents and the 17 at.% hydrogenated a-C:H film, respectively. Although the hydrogen content had little effect on the rate of change in the film thickness, the film thickness clearly decreased with decreasing sp²/sp³ ratio. These results indicate that the sp²/sp³ ratio is an important factor determining the chemical resistance to nitric acid solution.