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

Abstract

The adsorption of H₂ on the pristine and Mo-doped graphene was investigated by density functional theory (DFT) calculations. The 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. The electronic property of Mo-doped graphene was significantly affected by the strong interaction and orbital hybridization between H₂ and Mo-doped graphene sheet. The H₂ molecule would capture more charges from the doped graphene than the pristine system, indicating the higher sensitivity for the graphene doped with Mo.

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1. 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 [2–4]. Previous studies showed that the intrinsic graphene could not have high efficiency to interact with the small gas molecules, such as CO, NO, NO, H₂S, CO₂, NH₃, and H₂, due to the inert nature of the π-electron conjugation formed by carbon atoms of graphene [5–8]. 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 Fe-doped graphene (FeG) is a superior material for adsorption and sensing of CO, CO₂, SO₂, and H₂S 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 (H₂) 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 H₂ 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]. Therefore, 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 Mo-doped graphene sheet (MoGs) in this work on the basis of density functional theory calculations. The 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 H₂.

2. Computational Details

All the density functional theory (DFT) calculations were performed with CASTEP in Materials Studio (Version 8.0) of Accelrys Inc. The graphene sheet is modeled using a certain number of graphene supercells, consisting of 72 atoms, with a vacuum spacing of 15 Å. The exchange correlation contribution to the total electronic energy was treated with the Perdue–Burke–Ernzerhof (PBE) functional. The cutoff energy of 550 eV is used in all relaxation processes. The k-point set in all the slabs is 4 × 4 × 1. The convergence criteria of optimization for the energy and force were 0.02 meV/atom and 0.05 eV/Å, respectively. The MoGs are constructed by replacing one C atom with one Mo atom (concentration of Mo 1.4% atomic ration). The optimized structures of pure graphene and MoGs are shown in Figure 1. C₁, C₂, and C₃ were used to label the three C atoms around the center C (Mo) atom, respectively. The 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–C₁, Mo–C₂, and Mo–C₃, respectively. The transformation of chargers within the adsorbed H₂ molecule and sensing material was studied through the calculation of electron density difference. The density of states of the constructed geometries was also investigated to research the interaction between the H₂ molecule and sensing material.

[figures omitted; refer to PDF]

3. Results and Discussion

Firstly, we have calculated the binding energy between the Mo atom with graphene sheet (E_bing) as follows: E_bing = E_G + E_Mo − E_Mo-G, 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]. The 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 H₂ molecule was introduced into the system. There are four possible geometries constructed for the intrinsic and Mo-doped graphene systems: H₂ 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). The optimized results are shown in Figure 2. The adsorption energy (E_ads) of the established adsorption modes are calculated with the following equation:$$\begin{array}{}\text{(1)}& E_{\text{ads}}=E_{\text{adsorbate}‐\text{substrate}}\mathrm{-}{E}_{\text{substrate}}\mathrm{-}{E}_{{\text{H}}_2},\end{array}$$where E_{adsorbate-substrate} is the total energy of the adsorbate-substrate system in the equilibrium state and E_substrate and $E_{{\text{H}}_2}$ are the the total energies of the sensing material and adsorbed H₂ present, respectively [17]. The 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 (−0.57 eV). The negative value of adsorption energy corresponds to a stable adsorption structure. Similar calculation results were also found in the studies of H₂CO in the Al-doped graphene and the H₂S in the Al-doped graphene sheet [18].

[figures omitted; refer to PDF]

As can be seen in Figures 2(a) and 2(b), the H₂ molecules move farther away in modes A1 and A2 than those in modes M1 and M2 from the sensing material surface. The optimized results showed that the adsorption of H₂ does not have a significant effect in the structural distortion of intrinsic graphene. The 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 H₂ molecule remains 0.753 Å as the free-standing H₂ molecules, indicating that H₂ 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 H₂ 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. The initial structures of a H₂ 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 H₂ adsorbed on Mo-doped graphene being not affected by the orientation of the placed H₂ molecules. For the H₂ 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 H₂ 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 H₂ molecules have a stronger interaction with the MoGs. The larger elongation of Mo–C bond and higher adsorption energy of the Mo-doped system imply that the H₂ molecule has a chemical bond with the MoG sheet. Different with the doped system, the H₂ molecule only shows the physical adsorption on intrinsic graphene with weak van der Waals interaction between them [20].

Then, we study the accumulation of charges between the adsorbed H₂ 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. The results mean that the bond in the intrinsic graphene and H₂ are of covalent nature. There is no obvious charge accumulation between H₂ and graphene observed. However, in the modes M1 and M2, the H₂ captures electrons from the doped graphene system, further confirming the binding between adsorbed H₂ and Mo-doped graphene. As can be seen in Figure 3, the H₂ 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. The other charges mainly comes from the C neighbors. The calculated results reveal that the chemisorbed H₂ molecule in the modes M1 and M2 will capture electrons from the doped graphene, agreeing well with the results in previously studies [2, 21].

[figures omitted; refer to PDF]

The electronic densities of states (DOS) for the pristine and Mo-doped graphene systems were also studied to better understand the interaction between the H₂ molecule and sensing material (modes V1 and M1), as shown in Figure 4. The calculation results of modes V2 and M2 are shown in Figures S3 and S4, respectively. The molecular orbitals of the adsorbents are recognizable in the supercell DOS, meaning that there is no obvious hybridization between the H₂ molecule and intrinsic graphene sheet. The calculation results further prove the placed H₂ has little effect on the graphene state, indicating that the intrinsic graphene presents poor sensing performance to H₂ [20, 22]. In the doped systems of M1 and M2, the doped graphene state is obviously affected by the adsorption of the H₂ molecule. As can be seen, the electronic structure of the material changes a lot after the H₂ adsorbed on the material’s surface. The calculation of PDOS (partial density of states) for adsorbed H₂, Mo, and C atoms in the doped system is also shown in Figures 4(b) and S4. The adsorbed H₂ molecule obviously hybridizes with the doped Mo atom and carbon atoms of the graphene sheet [22–24]. And, the PDOS of H₂ also reveals the H₂ in M1 and M2 is no longer a free one compared with that in Figure S3, demonstrating the direct interaction between the H₂ molecule and Mo-doped graphene. The 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 effective material to sensing the H₂ molecule.

[figures omitted; refer to PDF]

4. Conclusion

The first-principles calculations of the interaction between H₂ and graphene-based material show that the H₂ sensing behavior of graphene can be highly enhanced by doping with Mo. The H₂ 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 H₂ molecules can be stably chemisorbed on the Mo-doped graphene with shorter distance from the sensing surface. Both the changes in the electronic structure and the orbital hybridization between H₂ and MoGs confirm the promising H₂ sensing property of the graphene doped with Mo. Therefore, our work gives a new insight into the design of hydrogen sensor based on the Mo-doped graphene.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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