Surface atomic structures of nickel plating films reacted with methane at high temperature

The surface structure of Ni plating films that were reacted with methane at 800°C was investigated using scanning electron microscopy and transmission electron microscopy (TEM). The surface of the Ni plating film was eroded by the reaction with methane, resulting in a nano‐roughened surface with a hexagonal close‐packed structure with a thickness of ~ 3 nm. The carbon layer deposited on the Ni plating film was evaluated by electron energy loss spectroscopy. An increase in the π + σ bonding energy and full width at half maximum of the π* peak was observed on moving toward the Ni plating film surface, indicating an increase in the hydrogenated or oxidized carbon atoms on the surface of the Ni plating film.

used for CDM experiments, [7][8][9] Ni plating thin films prepared on metal substrates are also interesting candidates for CDM. Fukuhara et al investigated the use of Ni plating films to decompose methanol. 10 However, to the best of our knowledge, CDM utilizing Ni plating films prepared on metal substrates has not been reported.
In our previous molecular dynamics study of the hightemperature reaction (1075 K) between Ni thin films and methane molecules, disordering of the surface of the Ni thin film was observed, along with the growth of hydrocarbon species on the surface of the Ni thin film. 11 Despite the intriguing question of how the surface of Ni catalysts changes due to the high-temperature Ni-methane reaction, atomic-scale observation of Ni thin films reacting with methane at high temperatures has not been reported to date.
The cross-sectional structural observation using a combination of high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) is a powerful method to directly characterize the structure and chemical properties of a material surface in atomic scale. In this study, we investigate the surface structural changes in Ni plating film catalysts with methane molecules based on the cross-sectional observations using HRTEM and EELS. The data demonstrate that the Ni plating surface is eroded by methane to form nano-roughened structures. For the first time, an atomistic view of the formation of the hexagonal close-packed (HCP) structure of Ni on the surface of the Ni plating film was successfully obtained. EELS measurements are demonstrated to be very effective for characterizing the atomic structure of the deposited carbon.
The results of the present study will be useful for investigating the mechanism of the reaction between Ni plating films and methane at high temperatures for the future design and application of Ni plating film catalysts.
2 | EXPERIMENTAL Figure 1 shows a schematic of the reaction system used for methane decomposition. Vertical tubular reactor 4 was made of SUS316 stainless steel with an inner diameter of 250 mm and a length of 1000 mm. Test pieces of Ni plating catalyst film with a thickness of 30 μm were prepared on permalloy substrates with a thickness of 1 mm and were subjected to reaction with methane at 800 C for 1 h in the reactor, where the flow rate of methane gas was set at 0.5 Nm 3 /h. The methane gas introduced into the reactor was heated using electric heater 6. The carbon produced by the reaction CH 4 ! C + 2H 2 2 was removed by filter 8, which was set before the hydrogen analyzer 9.
Cross-sectioned thin foil samples were prepared for both the as-   Although, as displayed in Figure Figure 3B illustrates a roughened structure with flaky carbon particles after the reaction with methane. Upon catalytic reaction with methane and Ni nanoparticles, the formation of fibrous carbon materials, such as carbon nanotubes (CNTs), was reported. [7][8][9] However, as displayed in Figure 3B, no formation of fibrous carbons on the Ni plating film was observed after the reaction with methane.

| RESULTS AND DISCUSSION
Conversely, Wei et al 12 Figure 8A also shows that the surface of the Ni plating film was eroded and roughened by reaction with methane. As mentioned above, the formation of a nano-roughened surface should enhance the catalytic activity of CDM.
The FFT pattern in Figure 8B corresponding to circled area 1 was indexed to the HCP structure of Ni according to refs. [14,15]. Table 1 compares the interplanar spacings measured using the FFT spots in Figure 8B and those calculated using the lattice constants of idealized HCP-Ni with a = 0.248 nm and c = 0.406 nm, as reported in ref. [14].
The measured interplanar spacings were close to the calculated values. HCP-Ni is known to be an infrequently observed metastable structure. 16 The FFT pattern in Figure 8C corresponding to circled area 2 is indexed to the FCC structure of Ni. Table 2 compares the interplanar spacings measured using the FFT spots in Figure 8C and those calculated using the FCC-Ni lattice constant a = 0.35238 nm from ref. [17]. The measured values were close to the calculated values, confirming the FCC structure of area 2.
The above observations show that the surface crystal structure of the Ni plating film in contact with the carbon layer in Figure 8 was disordered and transformed from the bulk FCC structure of Ni to the HCP structure. The measured thickness of the Ni-HCP structure was approximately 3 nm. Figure 9 depicts X-ray diffraction profiles of the Ni plating films before and after the reaction with methane. Although Figure 9A,B confirms the FCC structure of the Ni plating films, the transformation of the structure from FCC to HCP was not detectable owing to the weak diffraction intensity of the extremely thin layers in the HCP structure. The direct cross-sectional HRTEM image displayed in Figure 8 is very effective in indicating the surface atomic structure of Ni plating films. Figure 10C depicts the carbon the K-edge spectra obtained for the rectangular regions illustrated in Figure 10A,B. The carbon K-edge signals from Region 2 are negligibly weaker than those from Region 1, as indicated in Figure 10C. However, the integrated carbon K-edge signals (5.5 Â 10 5 ) in Figure 10D are comparable to the integrated Ni L 2 and L 3 signals (3.5 Â 10 5 ) in Figure 10E.  curve fitting using the Gaussian function and the baseline as shown in Figure 12B. The peak of the π + σ plasmon and the FWHM of the π * peak became more intense toward the surface of the Ni plating film, as shown in Figure 12C,D. The FWHM of the π * peak was exceptionally high for slab 9, where a graphitic shoulder peak was observed, as indicated by the arrow in Figure 12B. 21 This shows that the FWHM value sensitively reflects the local structural change in carbon.
It has been reported that the fraction of sp 3 bonding of carbon increases linearly with an increase in the π + σ plasmon energy of carbon in amorphous carbon and diamond-like carbon. 22,23 Hence, the increase in the π + σ plasmon energy indicates that carbon atoms with sp 3 bonds are more abundant near the Ni plating film surface.
On the other hand, the increase in the FWHM value of the π * peak indicates a decrease in the number of π bonds toward the Ni plating film surface, 19,20 which is consistent with the aforementioned increase in the sp 3 bonds toward the Ni plating film surface.
The EDS profile in Figure 2 shows an oxygen peak. Although the oxygen atoms could be detected as adsorbed atoms on the specimen surface, the above analysis of the EELS spectra indicates the formation of C-O bonds, which is consistent with the detection of oxygen atoms by the EDS analysis. The broadening of the π * peak is caused by the formation of phenol-type species, carbon atoms of aliphatic chain molecules, carboxyl groups, and so forth. [19][20][21] Because CDM produces hydrogen atoms, the deposited carbon layer must contain various carbon species, as indicated above. The oxygen atoms in the reacted Ni plating film surface possibly originated from the atmosphere of the reactor containing impurity oxygen atoms.

| CONCLUSIONS
The present research can be summarized as follows: 1. The surface of the Ni plating film was eroded by the reaction with methane at 800 C, and nanosized roughened structures were formed on the surface of the Ni plating film.
2. The surface of the Ni plating film was transformed into an HCP structure through reaction with methane at 800 C.
3. The EELS measurements showed an increase in the π + σ bonding energy and FWHM of the π * peak toward the Ni plating film surface, indicating an increase in hydrogenated or oxidized carbon atoms on the Ni plating film surface.

ACKNOWLEDGMENTS
This study is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). A part of this study was supported by NIMS microstructural characterization platform as a program of F I G U R E 1 2 (A) Low-and (B) high-loss EELS spectra obtained from the slabs numbered in Figure 11. (C) Energy of π + σ plasmon peak and (d) FWHM of π* peak plotted as a function of slab number "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant Number JPMXP09A20NM0120, and Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).