Effects of noble metal doping on hydrogen sensing performances of monolayer MoS2

To develop a new kind of hydrogen sensor based on monolayer MoS2, we investigated effects of noble metal doping on hydrogen sensing performances of the monolayer MoS2 by using the first principles calculation method. The Cu, Pd, and Pt doping decrease the adsorption energy of a hydrogen molecule on the monolayer MoS2, while Ag and Au doping have little effect on the adsorption energy. The adsorption energy change indicates that the Cu, Pd, and Pt doping strengthen the interaction between the hydrogen molecule and the monolayer MoS2. The density of states shows that the hybridization of H s, noble metals d, S p, and Mo d orbitals contributes to the adsorption of the hydrogen molecule on the noble metal doped monolayer MoS2. The changes in bader charge and charge density difference indicate that noble metal doping increases the charge transfer between the hydrogen molecule and the monolayer MoS2. All of the results demonstrate that noble metal doping can improve the hydrogen sensing performances of the monolayer MoS2, especially the Pd and Pt doping.


Introduction
Hydrogen is widely considered as one of the most potential clean energy sources which can replace the traditional fossil energy due to its advantages of no pollution and high calorific value [1]. However, hydrogen gas requires special caution for its combustible and high explosive properties [2]. For the safety reason, hydrogen monitoring runs through all stages of hydrogen energy utilization, including hydrogen production, hydrogen storage, and hydrogen transportation [3][4][5]. In nuclear energy production, the accumulation of hydrogen in the containment building can also cause a significant safety risk. During the normal operation of nuclear power plants, only a small amount of hydrogen is generated by the chemical reaction of zirconium alloy fuel cladding and water [6]. In the event of large break loss of coolant accident, the hydrogen production rate increases dramatically due to the chemical reaction between the zirconium fuel cladding and hot steam. If the hydrogen concentration in the containment building exceeds the explosion limit, the containment building might be destructed by hydrogen explosion, such as the severe nuclear accident happened in the Fukushima nuclear power plant in Japan in 2011 [7]. Therefore, the accurate monitoring of hydrogen in the containment building can help improve the safety level of nuclear power plants. As mentioned above, hydrogen monitoring is very important not only for the development and utilization of hydrogen energy [8], but also for the safety of nuclear power plants [9,10].
To monitor hydrogen in different environments accurately, scientists have developed many kinds of hydrogen sensing materials [2], and engineers have designed different types of hydrogen sensors based on the sensing materials, including optical sensor [11], electrochemical sensor [12], semiconductor sensor [13][14][15], and so on. In recent years, the research of two dimensional materials has received an increasing attention from scientists because of their unique band structure, semiconductor or superconducting properties, and excellent mechanical performance. The two dimensional materials are widely considered for use in electronic devices, catalysis, energy storage, tools used under extreme conditions, and other fields. Previous researches have shown that graphene is a candidate material which can be used for hydrogen sensing [16][17][18]. Meanwhile, scientists also found that monolayer MoS 2 has some adsorption capacity for toxic gas molecules [19][20][21]. When toxic gases adsorbed on monolayer MoS 2 , the electronic structure of the monolayer MoS 2 is changed, so the monolayer MoS 2 might be a candidate material for toxic gases sensing [22][23][24]. Based on the previous researches, the monolayer MoS 2 might also be a candidate material which can be used for hydrogen detection. Due to the introduction of impurity is a good strategy to improve the chemical activity of the monolayer MoS 2 [25][26][27], the adsorption of toxic gases on the monolayer MoS 2 is improved by doping [21]. However, whether doping can improve the hydrogen sensing performances of the monolayer MoS 2 has rarely been examined directly. More importantly, large areas and high quality MoS 2 atomic layers could be prepared through the chemical vapor deposition (CVD) technology [28,29]. Therefore, it is meaningful to develop cost-effective MoS 2 -based hydrogen sensing materials for monitoring the hydrogen concentration in different environments.
With the rapid progress of computer technology and computational materials science, first principles calculation method plays an increasingly important role in the research of two dimensional materials [30]. Based on our previous researches [31][32][33], we investigated the effects of noble metal doping on hydrogen sensing performances of the monolayer MoS 2 by using the first principles calculation method in this paper. Adsorption energy determines the interaction between the hydrogen molecule and the noble metal doped monolayer MoS 2 , density of states is calculated to understand the adsorption mechanism, and charge transfer between the hydrogen molecule and the noble metal doped monolayer MoS 2 is analyzed by bader charge and charge density difference. Through the calculations of these key parameters, we can get a further understanding of the hydrogen sensing performances of the noble metal doped monolayer MoS 2 .

Computational details
All calculations were performed by using the Vienna Ab-initio Simulation Package (VASP) [34,35] based on the project augmented wave (PAW) formalism of density functional theory. Exchange-correlation terms were computed by using the Perdew-Burke-Ernzerhof [36] functional with generalized gradient approximation (GGA). The electron wave functions were expanded by a plane-wave basis set with a cutoff energy of 400eV. A Gaussian smearing of 0.1 eV and the Monkhorst-Pack k-point mesh of 3×3×1 (7×7×1) were employed for supercell geometry optimizations (total energy calculation). To study the interaction between hydrogen (H 2 ) molecule and the noble metal doped monolayer MoS 2 more accurately, the DFT-D2 method [37,38] was adopted to account for the van der Waals interaction. In geometry optimization, the maximum Hellman-Feynman force acting on each atom was less than 0.02eV/Å, and energy convergence criterion was chosen as 1.0×10 −5 eV. In order to avoid interlayer interactions, a vacuum layer of 16 Å was used in the c direction.
In this paper, five kinds of noble metal doped monolayer MoS 2 are considered, and they are named as MoS 2 (Ag), MoS 2 (Au), MoS 2 (Cu), MoS 2 (Pd), and MoS 2 (Pt), respectively. According to the theoretical research [39], sorption is a good way for noble metal doping on the monolayer MoS 2 , and the most stable configuration of the five noble metals doped monolayer MoS 2 is shown in figure 1. For a H 2 molecule on the noble metal doped monolayer MoS 2 , there are four adsorption sites, named as H ex (center of the hexagon), T Mo (top of the Mo atom), T S (top of the S atom), and T NM (top of the noble metal atom). Due to the molecular structure of the H 2 molecule, two models are considered as the initial adsorption configurations, H-model (the H 2 molecule parallels to the noble metal doped monolayer MoS 2 ), V-model (the H 2 molecule perpendiculars to the noble metal doped monolayer MoS 2 ), and they are labeled as 'P' and '⊥' in this paper, respectively. To estimate the difficulty of the noble metal doping, we calculated the formation energy of the noble metal doped monolayer MoS 2 , and the formation energy is defined as: ) and E MoS 2 ( )is the energy of the noble metal doped and undoped monolayer MoS 2 , E (NM) is the energy of an isolated noble metal atom, respectively. The more negative the formation energy is, the easier the noble metal doping is. At the same time, the formation energy also indicates the stability of the noble metal doped monolayer MoS 2 , a more negative forming energy means the noble metal doped monolayer MoS 2 has a higher stability. Adsorption energy determines the interaction strength between the H 2 molecule and the noble metal doped monolayer MoS 2 , the adsorption energy of a H 2 molecule on the noble metal doped monolayer MoS 2 is calculated by the following equation: ) denote the total energy of the noble metal doped monolayer MoS 2 with and without an adsorbed H 2 molecule, and E H 2 ( )is the energy of a free H 2 molecule, respectively.
The more negative the adsorption energy is, the stronger the adsorption of the H 2 molecule on the noble metal doped monolayer MoS 2 is. To understand the microcosmic mechanism of the noble metal doping and the adsorption mechanism of hydrogen molecule on the noble metal doped monolayer MoS 2 , density of states of different adsorption systems was calculated. Bader charge and charge density difference were calculated to analyze the charge transfer between the H 2 molecule and the noble metal doped monolayer MoS 2 in quantity. The charge density difference (∇ρ) was acquired to analyze the electron transfer direction by a 3D visualization program VESTA [40]. The charge density difference is defined as: ) , and H 2 r ( )represent the electron density of the adsorption system, the electron density of the noble metal doped monolayer MoS 2 , and the electron density of the H 2 molecule, respectively. Furthermore, the change of band structure caused by hydrogen adsorption leads to the change of the conductivity of the monolayer MoS 2 , the hydrogen concentration can be detected through the change of the conductivity.

Results and discussion
To ensure the accuracy of our results in this paper, we calculated the structural parameters and electronic properties of the monolayer MoS 2 . The lattice constant of the monolayer MoS 2 is 3.166 Å, and the bond length of Mo-S is 2.408 Å, and the distance between S layers in the sandwich structure is 3.137 Å, respectively. They are in good agreement with the previous theoretical results [41,42] and experimental data [43,44].  [39], which also shows that our results are reliable. The negative formation energy indicates that the five noble metal doped monolayer MoS 2 have high stability. According to the formation energy, the synthesizing  Previous research has shown that a chemisorption exists between adsorbate and absorbent when the absolute value of adsorption energy is higher than 0.50 eV [47]. The adsorption of the H 2 molecule on the Pd and Pt doped monolayer MoS 2 belongs to chemisorption because the absolute value of the adsorption energy is greater than 0.5eV. The distance between two H atoms after adsorption was calculated, it shows that the distance is not  To explore the adsorption mechanism of the H 2 molecule on the noble metal doped monolayer MoS 2 , the density of states of the noble metal doped monolayer MoS 2 with a H 2 molecule at the most favorable site were calculated. Figure 5 shows that the density of states of the noble metal doped monolayer MoS 2 with a H 2 molecule at the favorable adsorption site. The s orbital of the H 2 molecule adsorbed on the pristine and Ag, Au doped monolayer MoS 2 is mainly distributed in the valence band. When the H 2 molecule adsorbed on the Cu, Pd, and Pt doped monolayer MoS 2 , the s orbital of the H 2 molecule splits in two parts, one is in the valence band and the other is in the conduction band. For the H 2 molecule adsorbed on the Ag, Cu, Pd, and Pt doped

Electronic structure change after hydrogen adsorption
Previous researches [48][49][50] have shown that the more significant the change of electron structure of material after hydrogen adsorption is, the better the hydrogen sensing performance of the material is. To investigate the effect of noble metal doping on the H 2 sensing performances of the monolayer MoS 2 , we calculated the electronic structure of the noble metal doped monolayer MoS 2 with a H 2 molecule at the most favorable site. Figure 2 shows the band structure of the noble metal doped monolayer MoS 2 without and with the adsorbed H 2 molecule, and the adsorbed H 2 molecule is at the most favorable adsorption site as shown in the figure 4. Table 2