DFT Study of Se-Doped Nanocones as Highly Efficient Hydrogen Storage Carrier

We have investigated the high capacity of Selenium atom (Se) doped nanocones surfaces as hydrogen storage systems. Hydrogen is a clean source of energy and it is derived from diverse domestic and sustainable resources. Hence, it can use as a viable alternative to fossil fuels. Therefore, the hydrogen storage on pure and doped Se-CNCs, BNNCs and SiCNCs was studied by density functional theory (DFT) method. The obtained results show that the lowest adsorption energy and the highest surface reactivity are −31.03 eV and 39.73 Debye for Se-Si 34 C 41 H 9 -M1 with disclination angle 300˚, respectively. Therefore, one can conclude that the doped Se-SiCNCs are good candidate for hydrogen storage. This finding was also confirmed by using the molecular orbital analysis. It is found that doping NCs with Se atom results in increasing the electron density around the Se atom and leading to increase the hydrogen storage capacity. The new understanding of highly efficient hydrogen storage for doped Se-SiCNCs, will be useful for the future synthesis of nan-cones with high performance for H 2 energy storage.


Introduction
With the consumption of fossil fuels and the increasing environmental pollution, people urgently need to find a new type of clean, sustainable and environmentally friendly energy to replace the traditional energy carriers [1]- [10]. The energy derived from renewable energy, and alternative energy sources receive considerable research attention [11]- [21]. Hydrogen is an ideal gas for energy source [22]- [31]. As reported by the United States Department of Energy (DOE), the storage of hydrogen is still the most difficult of these technological challenges. The department of Energy was determined the required hydrogen storage density of 9 wt% to replace the petroleum-fueled vehicles by fuel-cells vehicles and in practical application a high wt% is required to make hydrogen available alternative to fossil fuel. The wt% is defined as the ratio of the mass of the stored hydrogen to the mass of the storage system [32] [33] [34].
H 2 can be obtained by chemical reaction and stored as liquid-hydrogen storage, solid-state conformable storage, and compressed fuel storage which they require very high-pressures, heavy machinery, and offers low storage efficiency.
However, nanomaterials are believed to be suitable for hydrogen storage applications due to their inexpensive, lightweight, chemically stable, in addition to their desirable hydrogen-adsorption and desorption energies.
Therefore, in this study, we have chosen three types of nanocones (NCs) as carbon nanocones (CNCs), boron nitride nanocones (BNNCs) and silicon carbide nanocones (SiCNCs) with two disclination angles 60˚ and 300˚. Each configuration of nanocones is doped by Selenium (Se) atom, then it is monohydrogenated. Selenium (Se), is a chemical element with atomic number 34. It is a chalcogenide element with unique photoelectric property and photoconductivity. In addition, it can use as an excellent semiconductor component for many applications such as solar cells, electrochemical sensors and electrocatalysis [40]- [47].
Due to the unique physical, chemical and biochemical properties of selenium, it participates in numerous important life processes and it is attracted numerous attentions in biosensing, catalysis, diagnosis as well as treatment of diseases.
Therefore, recent studies on Selenium-based nanomaterials are mainly interested on quantum dots or selenium nanoparticles while other applications as hydrogen storage and other novel selenium-based nanomaterials as Se-nanocones are less studied.
Hence, in this study, the adsorption energy, the energy gaps (Eg), the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) and the surface reactivity of pure and Se-doped CNCs, BNNCs and SiCNCs with disclination angles 60˚ and 300˚ are investigated.

Computational Details
The monohydrogenated of pure and Se-doped nanocones was performed with full geometry optimization using the Density Functional Theory (DFT). The Becke's three parameter hybrid functional with LYP correlation functional (B3LYP) [48] [49] and standard basis set as implemented in the Gaussian 03 W program [50] are applied. All calculations carried out using Gauss View 4 mole-

Se-Doped Nanocones
We investigate the optimized geometries of Se atom decorating the various types of nanocones, Se-CNCs, Se-BNNCs and Se-SiCNCs to study their stabilities and their abilities for hydrogen storage, see Figure 3. As shown in Figure  Once can report that the order of stability of nanocones as a function of binding energy is as the following: Se-Si 38

Adsorption Energy
To identify a suitable nanocone for hydrogen storage, the hydrogen atoms ad-    Both of the surface reactivity and the energy gaps are considered to be the most important properties which can provide with the fundamental information required for designing the next generation of nanocones devices. Hence, the surface reactivity and energy gaps for disclination angles 60˚ and 300˚ investigated for pure and Se-doped NCs and listed in Table 2.

Surface Reactivity and Energy Gaps
From Table 2, the surface reactivity of pure and doped Se-CNCs, Se-SiCNCs and Se-BNNCs structures is increased by increasing the disclination angles from 60˚ to 300˚. The smallest and largest surface reactivity for CNCs found to be 1.29 Debye and 19.63 Debye for Se-C 80 H 20 and C 75 H 9 CNCs with disclination angles 60˚ and 300˚, respectively. As well as for BNNCs, their surfaces reactivity are increased by increasing the disclination angles, the smallest and largest surfaces reactivity are found to be 8.51 Debye and 15.22 Debye for Se-B 42 N 38 H 20 -M2 and B 34 N 41 H9-M1 with disclination angles 60˚ and 300˚,respectively. For the SiCNCs, the surfaces reactivity are also increased by increasing the disclination angles where the smallest and largest surfaces reactivity are found to be 6.81 Debye and 39.73 Debye for Si 38 C 42 H 20 -M1 and Se-Si 34 C 41 H 9 -M1 with disclination angles 60˚ and 300˚, respectively. It's noticed that the surface reactivity increased by increasing the disclination angle because by increasing the disclination angle, the strain on the cone is increased, resulting in an increase in the surface reactivity. Also, it is clear the effect of Se atom in increasing the electron density around its position resulting in an increase of hydrogen adsorption and hydrogen storage, see Figure 5.

Conclusion
We used the DFT to study the hydrogen adsorption on pure and doped Se-CNCs, Se-BNNCs and Se-SiCNCs. The obtained results show that by increasing the disclination angle, the adsorption energy of hydrogen is enhanced. In addition, doping nanocones with Se-atom cause an increase in surface reactivity and hydrogen storage. The doped Se-SiCNC was found to be the best nanocone for hydrogen storage. The adsorption energy and the surface reactivity were obtained to be −31.03 eV and 39.73 Debye for Se-Si 34 C 41 H 9 -M1 with disclination angle 300˚, respectively. Finally, one can conclude that the doped Se-SiCNCs are expected to be the best candidate nanocones for highly efficient hydrogen storage capacity.