A First-Principles Study on Hydrogen Sensing Properties of Pristine and Mo-Doped Graphene

.e adsorption of H2 on the pristine and Mo-doped graphene was investigated by density functional theory (DFT) calculations. .e structural and electronic properties of H2-graphene systems were studied to understand the interaction between H2 molecule and graphene-based material. Our calculation results showed the pristine graphene was not an ideal sensing material to detect H2 molecule as it ran far away from the pristine graphene surface. Different with pristine graphene, the Mo-doped graphene presented much higher affinities to the H2 molecule. It was found that the placed H2 molecules could stably be chemisorbed on the Mo-doped graphene with high binding energy. .e electronic property of Mo-doped graphene was significantly affected by the strong interaction and orbital hybridization between H2 and Mo-doped graphene sheet. .e H2 molecule would capture more charges from the doped graphene than the pristine system, indicating the higher sensitivity for the graphene doped with Mo.


Introduction
Over the past decade, the two-dimensional layered graphene has inspired great interests due to its unique mechanical, optical, transport, and outstanding electronic properties since it was reported in by Professors Novoselov et al. [1].It has been widely used in lithium-ion battery, solar cell, light emitting diodes, and gas sensors [2][3][4].Previous studies showed that the intrinsic graphene could not have high efficiency to interact with the small gas molecules, such as CO, NO, NO, H 2 S, CO 2 , NH 3 , and H 2 , due to the inert nature of the π-electron conjugation formed by carbon atoms of graphene [5][6][7][8].Several methods have been constructed to overcome this limitation.Among various methods, doping with transition metal has been confirmed to be proved to be one of the most facial and effective methods to improve the gas sensing property of graphene.Very recently, Cortés-Arriagada et al. has found that Fedoped graphene (FeG) is a superior material for adsorption and sensing of CO, CO 2 , SO 2 , and H 2 S compared to pristine graphene [9].Clearly, the previous theoretical studies showed that metal dopant could be used to enhance the gas sensing behavior of graphene.
Hydrogen gas (H 2 ) has been considered to be an ideal energy resource, always used in the industrial processes and energy-generating devices [10,11].However, the light, colorless, odorless, and tasteless properties of H 2 make it easily leak out and difficultly be detected by humans.And, it has the high possibility of explosion due to the low ignition energy and wide flammable range [12,13].erefore, it is of great importance to assemble the devices to detect and control their exposure.Recently, many studies have focused on the hydrogen sensing performance of carbon-based material theoretically and experimentally [14].Lotfi and his coworkers have studied the enhanced adsorption of hydrogen gas on the transition metal-doped graphene [15].Chanukorn has also reported that the graphene nanosheets doped with the group 8B transition metal could be used as a more reliable and efficient hydrogen sensor than the bare graphene [16].Motivated by these research studies, we studied the interactions between hydrogen gas and Modoped graphene sheet (MoGs) in this work on the basis of density functional theory calculations.e calculation results showed that the adsorption of hydrogen molecules on the graphene sheet could also be enhanced through doping with Mo elements, leading to the higher sensitivity towards H 2 .

Computational Details
All the density functional theory (DFT) calculations were performed with CASTEP in Materials Studio (Version 8.0) of Accelrys Inc.
e graphene sheet is modeled using a certain number of graphene supercells, consisting of 72 atoms, with a vacuum spacing of 15 Å. e exchange correlation contribution to the total electronic energy was treated with the Perdue-Burke-Ernzerhof (PBE) functional.
e cutoff energy of 550 eV is used in all relaxation processes.
e k-point set in all the slabs is 4 × 4 × 1. e convergence criteria of optimization for the energy and force were 0.02 meV/atom and 0.05 eV/ Å, respectively.e MoGs are constructed by replacing one C atom with one Mo atom (concentration of Mo 1.4% atomic ration).e optimized structures of pure graphene and MoGs are shown in Figure 1.C 1 , C 2 , and C 3 were used to label the three C atoms around the center C (Mo) atom, respectively.e calculation showed that the length of bond elongated from 1.419 Å for the three C-C bonds to 1.921 Å, 1.930 Å, and 1.926 Å for Mo-C 1 , Mo-C 2 , and Mo-C 3 , respectively.
e transformation of chargers within the adsorbed H 2 molecule and sensing material was studied through the calculation of electron density difference.
e density of states of the constructed geometries was also investigated to research the interaction between the H 2 molecule and sensing material.

Results and Discussion
Firstly, we have calculated the binding energy between the Mo atom with graphene sheet (E bing ) as follows: , where E G , E Mo , and E Mo-G are defined as total energies of the graphene with one C vacancy, the single Mo atom, and the Mo-doped graphene, respectively [8]. e calculation results showed the binding energy of Mo in the Mo-doped graphene was ∼4.99 eV, indicating the possibility of Mo-doping.To investigate the interaction between the target gas molecule and our built-up material, one H 2 molecule was introduced into the system.
ere are four possible geometries constructed for the intrinsic and Mo-doped graphene systems: H 2 perpendicular to and parallel along X axis to the sensing surface (modes V1 and V2 for graphene and modes M1 and M2 for Mo-doped graphene).e optimized results are shown in Figure 2. e adsorption energy (E ads ) of the established adsorption modes are calculated with the following equation: where E adsorbate-substrate is the total energy of the adsorbatesubstrate system in the equilibrium state and E substrate and E H 2 are the the total energies of the sensing material and adsorbed H 2 present, respectively [17].e final adsorption energy for V1 and V2 are −0.056 and −0.048 eV, respectively, much lower than those for M1 (−0.55 eV) and M2 (−0.57eV).e negative value of adsorption energy corresponds to a stable adsorption structure.Similar calculation results were also found in the studies of H 2 CO in the Al-doped graphene and the H 2 S in the Al-doped graphene sheet [18].
As can be seen in Figures 2(a) and 2(b), the H 2 molecules move farther away in modes A1 and A2 than those in modes M1 and M2 from the sensing material surface.e optimized results showed that the adsorption of H 2 does not have a significant effect in the structural distortion of intrinsic graphene.e bond lengths of C-C in both V1 and V2 slightly elongated to be 1.420 Å.Moreover, the length of the bond in the H 2 molecule remains 0.753 Å as the freestanding H 2 molecules, indicating that H 2 would not interact directly with intrinsic graphene surface.Our results agree well with the precious study [19].Compared with intrinsic graphene system, the Mo-C bonds obtained from the initial structure with H 2 perpendicular to the doped surface were lengthened to be 1.927 Å, 1.977 Å, and 1.931 Å for Mo-C1, Mo-C2, and Mo-C3 in the mode M1 (shown in Figure 2(c)), respectively.e initial structures of a H 2 molecule adsorbed on the Mo-doped graphene are shown in Figure S1.It should be noted that we found the finally optimized geometries for the constructed modes of H 2 adsorbed on Mo-doped graphene being not affected by the orientation of the placed H 2 molecules.For the H 2 molecules parallel along X axis to the sensing surface, the corresponding values for Mo-C1, Mo-C2, and Mo-C3 bonds are 1.930 Å, 1.975 Å, and 1.931 Å, respectively.In the modes M1 and M2, the bond lengths of H 2 expand from original 0.735 Å to 0.857 Å and 0.858 Å, respectively.Based on the above research, it can be seen that the H 2 molecules have a stronger interaction with the MoGs.e larger elongation of Mo-C bond and higher adsorption energy of the Mo-doped system imply that the H 2 molecule has a chemical bond with the MoG sheet.Different with the doped system, the H 2 molecule only shows the physical adsorption on intrinsic graphene with weak van der Waals interaction between them [20].
en, we study the accumulation of charges between the adsorbed H 2 and graphene systems with the method of calculating the deformation electron density.In the intrinsic graphene modes of V1 and V2, the majority of electron accumulations sites are distributed in the bond, as shown in Figure S2.e results mean that the bond in the intrinsic graphene and H 2 are of covalent nature.ere is no obvious charge accumulation between H 2 and graphene observed.However, in the modes M1 and M2, the H 2 captures electrons from the doped graphene system, further confirming the binding between adsorbed H 2 and Modoped graphene.As can be seen in Figure 3, the H 2 molecules in modes M1 and M2 gain 0.28 e and 0.29 e, respectively.Accordingly, the Mo-dopant loses electronic charge of 0.27 e and 0.26 e in modes M1 and M2. e other charges mainly comes from the C neighbors.e calculated results reveal that the chemisorbed H 2 molecule in the modes M1 and M2 will capture electrons from the doped graphene, agreeing well with the results in previously studies [2,21].

2
Journal of Nanotechnology e electronic densities of states (DOS) for the pristine and Mo-doped graphene systems were also studied to better understand the interaction between the H 2 molecule and sensing material (modes V1 and M1), as shown in Figure 4.
e calculation results of modes V2 and M2 are shown in Figures S3 and S4, respectively.e molecular orbitals of the adsorbents are recognizable in the supercell DOS, meaning that there is no obvious hybridization between the H 2 molecule and intrinsic graphene sheet.e calculation results further prove the placed H 2 has little e ect on the graphene state, indicating that the intrinsic graphene presents poor sensing performance to H 2 [20,22].In the doped systems of M1 and M2, the doped graphene state is obviously a ected by the adsorption of the H 2 molecule.As can be seen, the electronic structure of the material changes a lot after the H 2 adsorbed on the material's surface.e calculation of PDOS (partial density of states) for adsorbed H 2 , Mo, and C atoms in the doped system is also shown in Figures 4(b) and S4. e adsorbed H 2 molecule obviously hybridizes with the doped Mo atom and carbon atoms of the graphene sheet [22][23][24].And, the PDOS of H 2 also reveals the H 2 in M1 and M2 is no longer a free one compared with that in Figure S3, demonstrating the direct interaction between the H 2 molecule and Mo-doped graphene.e research implies that the hydrogen sensing performance could be enhanced through doping with Mo, and the Mo-doped   graphene has the prospect of being an e ective material to sensing the H 2 molecule.

Conclusion
e rst-principles calculations of the interaction between H 2 and graphene-based material show that the H 2 sensing behavior of graphene can be highly enhanced by doping with Mo.
e H 2 molecule runs away from the pristine graphene and is more likely to be physically adsorbed on intrinsic graphene due to weak van der Waals interaction between them.In contrast, there is clear evidence showing that H 2 molecules can be stably chemisorbed on the Modoped graphene with shorter distance from the sensing surface.Both the changes in the electronic structure and the orbital hybridization between H 2 and MoGs con rm the promising H 2 sensing property of the graphene doped with Mo. erefore, our work gives a new insight into the design of hydrogen sensor based on the Mo-doped graphene.

Figure 2 :
Figure 2: Calculations of structure of H 2 molecules (white color) on the graphene-based material surfaces (a) with H 2 perpendicular to graphene surface (V1); (b) with H 2 parallel along X axis to graphene surface (V2); (c) with H 2 perpendicular to Mo-doped graphene surface (M1); (d) with H 2 parallel along X axis to Mo-doped graphene surface (M2).

Figure 3 :
Figure 3: e plots of the deformation electron density for M1 and M2 modes (blue color: electron trap; yellow color: electron release).

Figure 1 :
Figure 1: Optimized geometries of intrinsic and Mo-doped graphene: gray and light blue spheres are denoted as C and Mo, respectively.