Figure 4: Weight gain and stoichiometry analysis of Ni oxides.

(a) Weight gain versus temperature of pristine Ni foils, 2- and 5 nm h-BN-coated Ni foils. Oxidation duration is 5 min in air. Inset shows the parabolic rate constant as a function of the inverse absolute temperature from 800 °C to 1,100 °C. (b) Weight gain versus oxidizing duration at 1,100 °C in 300 mT oxygen atmosphere. Inset represents the weight gain as a function of the square root of time. (c,d) Contours showing the depth profile of Ni oxidization profiles for durations from 10 to 60 min. The oxidations were performed at 1,100 °C and under 300 mT oxygen atmosphere. The colours indicate the oxidation level represented by the atomic concentration of oxygen over nickel from slight (blue) to heavy (yellow and red) oxidation. White dotted line is the contour lines indicating the boundaries of NiO.