The SEM images of CNTs grown on Ni NPs without CuNPs @ a-C:H thin film and with Cu NPs @ a-C:H thin film contenting 5%, 40%, 75% Cu are shown in Figs. 1(a-d), respectively. By adding Cu NPs thin film the density of the CNTs is greatly enhanced. Furthermore, the average diameter of the grown CNTs increases by increasing of Cu content of thin film. The average diameter of the grown CNTs on the Ni-Cu@a-C:H with Cu content of 5%, 40%and 75% Cu are obtained about 100, 120 and 160 nm respectively. Figure.1(e) shows SEM image of carbon onion made on the Cu NPs @ a-C:H thin film without Ni NPs. As it is clear from this figure, MWCNTs are not formed on the Cu NPs @ a-C:Hand one can conclude that Cu by itself has no catalyst property in production of CNTs growth as was reported in reference [27].
Ni-Cu NPs @ a-C:H catalyst film were determined by RBS spectra, AFM images, UV-Visible spectroscopy. Cu, Ni, O, C and Si contents of the samples were obtained from RBS spectra.Figure.2shows RBS spectra for Ni NPs with Cu NPs @ a-C:H thin layer contenting 5%, 40% and 75% Cu as a function of the incident ions energy. The small step at 850 KeV is related to C nuclei. The steps at 650 and 1050 KeV is due to O and Si nuclei of amorphous SiO2 substrate respectively. The peak between 1450 and 1550KeV is attributed to Cu and Ni nuclei. The results of SIMN-RA software simulation of RBS spectra indicated that the different thin film with 5%, 40% and 75% Cu and their thickness is about 89, 124 and 113 nm respectively[28].The XRD profile show that the prepared Ni NPs and Cu NPs have fcc crystal structure.
The AFM images of nanoparticles on the surface (20 nm × 20 nm) of the thin films are shown in Figures.3(a–d) respectively. Atomic Force Microscopes (AFM) are a group of scanning probe microscopes that, in addition to being able to be imaged in a non-vacuum environment, can also take biological and non-inductive samples. These images can estimate the lateral size of the thin films on the sample surface. The AFM values of CNTs grown on Ni NPs without Cu NPs thin film and with Cu NPs thin film contenting 5%, 40%, 75% Cu are obtained about 7.2, 5.34, 6.04 and 11.16 nm respectively. The lateral size changes of nanoparticles on the surface of the thin films are shown in Figure. 4, where the lateral size of the nanoparticles is estimated using the AFM image. For Ni-Cu NPs thin films Most of the lateral changes are related to sample 75% Cu, which has a large leap, while it is changing for samples 5% and 40% Cu with low slope. The nanoparticles are almost spherical in shape and deforms from spherical state with increasing Cu content. Also, the lateral size changes of nanoparticles are increased on the surface of thin films and increase with increasing Cu content.
The optical density, Dopt or the absorbance is proportional to both the concentration of the absorbing material and the thickness of film samples. The optical density Dopt of thin filmscan be obtained from the following simple equation: Dopt = t[29, 30], where t is the thickness of the thin film which was calculated using DEKTAK 3 profilometer method and α is the absorption coefficient obtained from Lambert'sequation: = 2.303 A/ t, where A is the absorbent of the films[31].Figure. 5 illustrates the variation of the optical density(Dopt) with the incident photon energy,ℎ𝜈 (eV) for Ni NPs without Cu NPs @ a-C:H thin film and Ni-Cu NPs @ a-C:H thin films with different Cu content.By adding Cu NPs thin film,the opticaldensity for the prepared MWCNTs usingNi NPs with Cu NPs @ a-C:H containing 5%, 40%, 75% Cu enhanced with a monotonous slope. For Ni NPs without Cu NPs @ a-C:H thin film the opticalDensityis constant between 1.5 and 2.25 eV and increases with a slope very slowafter 2.25 eV.
Figures.6(a-d) show the height changes of nanoparticles on the surface relative to the axis for the prepared MWCNTs using Ni NPs without Cu NPs @ a-C:H thin layer, Ni NPs with Cu NPs @ a-C:H containing 5%, 40%, 75% Cu respectively. Since the scanning size on the surface of the layers is about 1 µm × 1 µm by the AFM, so the maximum numerical value on the X-axis is 1 µm.The height changes on the surface of the layers indicate that the layers have a phase change for typical sample Ni-Cu NPs @ a-C:Hwith 75% Cu compared to other samples, so that the layers have a phase change of about 6 nm and indicate that the layers in this case are, firstly, very smooth and, secondly, can have a phase change.
The Power Spectral Density (PSD) points of samples are extracted from 1 µ × 1 µ AFM images of Fig. 3. It can be seen that all the PSD points include of a high spatial frequency region. According to the Dynamical Scaling Theory, for a system of lateral size L, the relation P(k) and frequency k be written as: P(k) ∝, where is calculated as the slope of the log-log in PSD of high spatial frequency. The fractal dimension of Df by solving the slope of the log-log graphis obtained: Df = 4-. Figures.7 show the spectral density change of the spatial frequency layers for the prepared MWCNTs using Ni NPs without Cu NPs @ a-C:H thin layer, Ni NPs with Cu NPs @ a-C:H containing 5%, 40%, 75% Cu respectively. The spectral density power of all layers reflectsthe reverse flow changes,, especially in the high spatial frequency region, indicating the presence of fractal components in prominent topographies.Thus, the power spectrum method is sensitive to the size of the data set, frequency, and specific domain.This value determines the relative values of surface irregularities at different distance scales. By increasing of Cu content of Ni-Cu NPs @ a-C:H thin films, the slope of the spectral density power performance decreases, which may be due to the increase in the lateral size of the nanoparticles.The fractal dimension values of the thin films are shown in Figure. 8.It is obvious that the fractal dimension values depend on the amount of Cu content.The fractal dimension values of CNTs grown on Ni NPs without Cu NPs thin film and with Cu NPs thin film contenting 5%, 40%, 75% Cu are estimated about 5.6, 5.62, 5.7 and 5.53 nm respectively.Therefore, by adding of Cu NPs @ a-C:H with different contents up to 40% Cu the fractal dimension of the CNTs were enhanced and then over 40% Cu they decrease.
Figure.9 shows the bearing area versus height of thin films. It actually shows the amount of cavity (bottom curve) and single-layer (top curve) of the nanoparticles. For the prepared MWCNTs using Ni NPs without Cu NPs @ a-C:H thin film and Ni-Cu NPs @ a-C:H thin films with different Cu content, the cover coefficient of zero (cavity) is less than 10% and the single-layer content is about 95%, which is 90% isolated (between the cavity and the single-layer).The address layer did not have much effect on the amount of cavity and single-layer.