Hydrogen Adsorption Mechanism of SiC Nanocones

Due to rapid depletion of fossil energy sources and increasing the environ-mental pollution through high fossil energy consumption, an alternative re-newable and clean energy carrier as hydrogen is requested more investigations in order to get the optimal request by DOE. In this study, a deepest study on SiC nanocones is done including both of the geometrical and electronic properties of all possible five different disclination angles as a function of size using density functional (DFT) calculations at the B3LYP/6-31g level of theory. Then the hydrogen adsorption mechanism is investigated 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). Our calculations show that the most candidate SiC nanocone structure for hydrogen storage is Si 41 N 49 H 10 -HS 2 -M1-Type 2 with disclination angle 300˚. In addition, our results indicate that the hydrogen adsorption induced the energy gap to decrease. Hence, these results indicate that the SiCNCs can be considered as a good candidate for hydrogen storage.

Graphene 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 ef-

Methods
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].  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.

Surface Reactivity
The surface reactivity of pure and hydrogenated SiCNCs at different disclination angels n × 60 (n = 1 to 5) and different sizes are studied. The dipole moments of pure SiCNCs are calculated and are used as indicator for the surface reactivity [46] [47], see Table 1. It is known that the high values of the dipole moments indicate the high surface reactivity and vice versa. From Table 1, the surface reactivity of SiCNCs is increased by increasing the cone size. When we increase the cone size, the rate of a chemical reaction can be raised due to the increasing surface area of nanocone. Hence, when the surface area of nanocone is increased more hydrogen atoms are expected to be adsorbed on the surface indicating the high surface reactivity. Also, it is found that the Si 49 C 41 H 10 (M2) with cone angle 300˚ possesses the highest surface reactivity (34.98 Debye) and the smallest surface reactivity is for Si 34 C 36 H 20 (M1) with cone angle 60˚ (2.93 Debye). In other words, by increasing the number of silicon atoms and by decreasing the number of carbon atoms, the surface reactivity of SiCNCs is increased. The surface reactivity of mono-hydrogenated SiCNCs at three different hydrogenation 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) for both types (Type 1 and Type 2) are studied and are listed in Table 2 and Table 3. One can notice that the surface reactivity of mono-hydrogenated SiCNCs are increased by increasing the cone size and by increasing the disclination angle. Also, the Si 49 C 41 H 10 -H S2 (M2-Type 1) with cone angle 300˚ is found to be possess the highest surface reactivity of 36.86 Debye, and the smallest surface

Adsorption Energy
The adsorption energy of one hydrogen atom on the surface of SiCNCs is applied at three different hydrogenation sites H S1 , H S2 and H S3 . The hydrogen atom can be adsorbed on silicon atom (named Type 1) or can be adsorbed on carbon atom (named Type 2), as shown in Figure 2. From Table 4 and

Valence Orbitals
The energy gaps of mono-hydrogenated SiNCs at three different hydrogenation sites H S1 , H S2 and H S3 for each disclination angel are studied, see Table 4 and Table 5. From Table 4, it is found that the energy gaps for mono-hydrogenated SiCNCs-Type 2 are always smaller than the energy gaps for mono-hydrogenated SiCNCs-Type 1. The smallest and the largest energy gaps are found to be 0.01 eV for Si 47 C 47 H 14 -H S3 -Type 2 with disclination angle 240˚, and 0.10 eV for Si 28 C 28 H 10 -H S3 -Type 1 with disclination angle 120˚, respectively. From Table 5, the energy gaps for mono-hydrogenated SiCNCs-Type 2 are always smaller than SiCNCs-Type 1. The smallest and the largest energy gaps are found to be 0.01eV and 0.12 eV for Si 41 C 49 H 10 -H S2 -M1-Type 2 with disclination angle 300˚ and Si 34 C 36 H 20 -H S2 -M1-Type 1 with disclination angle 60˚, respectively. In addition, it can be reported that 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 and the energy gaps before adsorption hydrogen are larger than the energy gaps after adsorption hydrogen, in a good agreement with previous observations [27] [28] [29]. From Figure 3, the electron density is found Graphene to be located at the tip of the cone for LUMO before adsorption hydrogen and after adsorption hydrogen on Type 1. However it is found to be located at the tip of the cone for HOMO after adsorption hydrogen on Type 2. Also, it is noticed that the electron density is located at terminal of the cone for the HOMO before adsorption hydrogen and after adsorption hydrogen on LUMO Type 2. The electron density is found to be distributed over most of cone for HOMO after adsorption hydrogen on Type 1. Furthermore, the intensities of the LUMOs are found to be larger than the intensities of HOMOs before and after the adsorption hydrogen on Type 1. The intensity of the LUMO is smaller than the intensity of HOMO after adsorption hydrogen on Type 2.

Conclusion
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 Si 41 N 49 H 10 -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 Si 41 C 49 H 10 -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 Si 49 N 41 H 10 -H S2 (M2) when the hydro-Graphene genation 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.

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