Nanomaterials can find a great number of applications in modern-day technology, ranging from household to on-board satellite electronics. Because of the increasing need for speed and miniaturization of electronic devices, nanotechnology has become a very active field of study [1] - [6]. Since the discovery of carbon nanotubes (CNTs) [7], owing to their extraordinary properties, researchers have taken much interest in exploring the immature feature and the potential applications of CNTs and also those of the related carbon nanomaterials [8] [9] [10]. Carbon nanocones (CNCs) are one of these nanomaterials. Because CNCs have structures similar to those of the CNTs and have properties much like CNTs, they have attracted much attention in recent years [11] [12]. At the same time, due to the rapid development silicon carbide SiC technology, the crystalline structure of SiC can be considered to consist of the close-packed stacking of double layers of Si and C atoms. Each C or Si atom is surrounded by four Si or C atoms in strong tetrahedral sp3 bonds. The distance between neighboring Si and C atoms is approximately 3.08 Å for all polytypes [13] [14]. The existence of SiC nanostructures that can exploit the superior physical-chemical properties of SiC [15] has been investigated and, in the last few years, many studies on SiC nanotubes have been reported in the literature [16] [17] [18]. On the contrary, little information on SiC nanocones SiCNCs is available [19]. This is unfortunate because it was proven that nanocones arrays for solar panels applications can be beneficial in terms of increased optical absorption coefficient [20]. Since space probes rely on photovoltaic power generation, solar panels based on SiCNCs arrays can be a valid alternative in terms of high-temperature, high-light intensity and high radiation conditions.

Hydrogen storage in carbon materials such as carbon nanotubes, graphite nanofibers, activated carbon and nanostructured graphite has been paid considerable attention [21] - [33]. The adsorption in carbon nanostructures seems to be the most promising method for storing hydrogen, especially on board vehicles. In particular, carbon nanotubes have been suggested as the most promising nanostructure due to its light mass density, cylindrical structure, high surface to volume ratio, as well as high degree of reactivity between the carbon and hydrogen [34] [35] [36]. Furthermore, it is well known that the adsorption of hydrogen by carbon nanotubes is temperature dependent, which can be effectively used for hydrogen uptake and release. Many experimental and theoretical efforts have been focused on hydrogen storage in carbon nanotubes [37] [38]. Recently, it has been suggested that hydrogen storage on carbon nanomaterial involves dissociation of molecular hydrogen followed by atomic hydrogen spillover. The dissociation of hydrogen takes place at transition metal sites, which are present due to residues of transition metals in the nanomaterial samples [39] [40] [41]. In this work we investigate SiCNCs with five disclination angles 60˚, 120˚, 180˚, 240˚ and 300˚. For 60˚, 180˚ and 300˚ disclination angles, there are four structures for each disclination angle of SiCNCs. However, for 120˚ and 240˚ disclination angles, there are only two structures for each disclination angle. Furthermore, the hydrogenation is done on three different sites, H^S1 (above the first neighbor atom of the apex atoms), H^S2 (above one atom of the apex atoms) and H^S3 (above one atom far from the apex atoms), in addition to the study of the effect of different sizes. The adsorption energy of hydrogen as a function of cone angle, cone size and hydrogenation site are investigated. Finally, the energy gaps (E_g), highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), density of states (DOS), and the surface reactivity of the hydrogenations SiCNCs are investigated.

Density functional theory (DFT) calculations have been performed using Becke’s three parameters hybrid exchange functional B3 [42] in combination with the correlation functional of Lee, Yang, and Parr (B3LYP) [43], and the 6 - 31 g standard basis set as implemented in the Gaussian 03W program [44]. All the calculations were performed by using Gauss View 4 molecular visualization program Package [45]. The samples of study are pure and mono-hydrogenated SiCNCs with five disclination angles, 60˚, 120˚, 180˚, 240˚ and 300˚. For every disclination angle, there are two structures with different sizes. For disclination angles 60˚, 180˚, and 300˚, there are two models M1 and M2 according the connected atoms edges. It can be either silicon atoms (named M1) or can be carbon atoms (named M2). For disclination angles 120˚ and 240˚, there is only one model, see Figure 1. The hydrogen atom can be adsorbed on silicon atom (named Type 1) or can be adsorbed on carbon atom (named Type 2). The hydrogenation is applied on three different sizes as shown in Figure 2. To avoid the dangling effects, the hydrogen atoms have been used to saturate the ending atoms for SiCNCs. All the atomic geometries of pure and mono-hydrogenated SiCNCs have been allowed to fully relaxation during the optimization processes. The adsorption energy of hydrogen are obtained as a function of cone angle, cone size and hydrogenation site.

We have performed the adsorption energy and electronic properties of SiCNCs using the DFT calculations. It is found that the adsorption energy is getting lower by increasing the size and disclination angle of mono-hydrogenated SiCNCs. The more lower adsorption energy is found to be −4.69 eV for Si₄₁N₄₉H₁₀-H^S2-M1-Type 2 with disclination angle 300˚. Also, the energy gaps for mono-hydrogenated SiCNCs when the hydrogen atom is adsorbed on carbon atom are always smaller than the energy gaps for mono-hydrogenated SiNCs when the hydrogen atom is adsorbed on silicon atom where the smallest energy gap is 0.01 eV for Si₄₁C₄₉H₁₀-H^S2-M1-Type 2 with disclination angle 300˚. Finally, the surface reactivity is found to be increased by hydrogenation where the highest surface reactivity is found to be (36.86 Debye) for Si₄₉N₄₁H₁₀-H^S2 (M2) when the hydrogenation is applied on the silicon atom (Type 1). Both the high surface area and the opportunity for SiCNCs consolidation are key attributes for hydrogen storage devices and lead to the design of light weight hydrogen storage systems with better hydrogen storage characteristics. Hence, more investigations of the capabilities of SiCNCs for the physicochemical reactions, such as surface interactions and hydrogen atom dissociation are needed.

The authors declare no conflicts of interest regarding the publication of this paper.

Al-Khateeb, M.A. and El-Barbary, A.A. (2020) Hydrogen Adsorption Mechanism of SiC Nanocones. Graphene, 9, 1-12. https://doi.org/10.4236/graphene.2020.91001