Adsorption of H 2 on Ga 24 N 24 cluster; A density functional theory investigation M. Aarabi, Z. Mahdavifar, S. Noorizadeh * Chemistry Department, Faculty of Sciences, Shahid Chamran University of Ahvaz, Ahvaz, Iran article info Article history: Received 5 May 2017 Received in revised form 8 June 2017 Accepted 9 June 2017 Available online 15 June 2017 Keywords: Hydrogen storage Adsorption Gallium nitride clusters Fukui function DFT Reduced density gradient abstract Molecular adsorption of hydrogen on Ga 24 N 24 cluster is explored using Density Functional Theory. It is shown that the adsorption of H 2 molecules on gallium atoms surface of the cluster is a quasi-molecular adsorption with E ad ¼19.7 kJ mol 1 , which is favorable for hydrogen-storage materials. This adsorption is nearly position-independent but is not dissociative. The natural bond orbital analysis, reduced density gradient scatter graphs as well as Fukui functions and density of states spectra confirm the weak interaction between s-bonding electrons of H 2 molecule with the gallium atom of the cluster. The results may open a window to new hydrogen-storage devices at moderate pressures. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction During the past few decades, many attempts have been made to replace fossil fuels with greener and more sustainable alternatives. Since the hydrogen fuels do not create any carbon pollutants, they are among the most investigated energy sources [1]. But, hydrogen has some physical characteristics that make it difficult to store in large quantities without taking up a significant amount of space. Therefore, hydrogen adsorption on different substrates, including transition metal clusters [2], carbon nanotubes [3,4], boron nitride sheets and nanotubes [5], fullerene-like clusters [6e8] and metal- organic frameworks [9e13] have been explored as a possible route towards removing this difficulty [14]. Computational in- vestigations show that all of the mentioned adsorbents do not behave in the same manner. Thermodynamic calculations predict that the adsorption energies that would lead to an efficient cyclic adsorption/desorption process at room temperature and moderate pressures are in the narrow range of 0.1e0.6 eV per hydrogen molecule (9.6e57.9 kJ mol 1 ) [15,16]. In fact, this range of energy is sufficient to compensate the unfavorable decrease of entropy dur- ing the adsorption process [17]. The III-V group semiconductor compounds show a wide range of both technological [18e22] and biological [23] applications. Although both ring- and cage-type configurations could be assumed for B n N n clusters, a crossover from the ring to the cage is observed for n > 12 [24]. Also, it is theoretically shown that the fullerene-like B 12 N 12 ,B 16 N 16 and B 28 N 28 clusters are magic stable Boron Nitride (BN) cages [25], and B 12 N 12 spheroid, which is built by eight hexagons and six squares, is energetically the most stable one [26]. Bekenev and Pokropivny reported the first synthesis of stable covalent crystals built by B 12 N 12 fullerenes [27]. Therefore, Matxian et al. [28] attempted to investigate the stability of different B 12 N 12 fullerene dimers. It is shown that the dimer formed by co- valent interactions facing squares (S-S) are more stable than those, which are due to hexagon facing a hexagon (H-H). It should be mentioned that during the formation of a dimer, the structure of each monomer remains nearly unaltered. In fact, instability of H-H dimer could be due to form less stable B-B and N-N bonds. A comparison between Ga n N n and B n N n shows that gallium nitride fullerene-like structures could be also possible for the same cluster sizes as have been predicted or observed for boron nitrides [29]. An investigation is also made between thermodynamic sta- bility of medium-sized Ga n N n clusters (n ¼ 4e12), and it is found that the Ga 12 N 12 nanocluster is the most stable one among them [26]. Therefore, many attempts have been made to synthesize different GaN nanostructures [30e35]. Zhao ad coworkers [26] * Corresponding author. E-mail address: noorizadeh_s@scu.ac.ir (S. Noorizadeh). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2017.06.021 0042-207X/© 2017 Elsevier Ltd. All rights reserved. Vacuum 143 (2017) 209e216