Zheng Zhang,Kai Chen,Qiang Zhao,＊,Mei Huang,Xiaoping Ouyang,c,d

a Beijing Key Laboratory of Passive Safety Technology for Nuclear Energy,North China Electric Power University,Beijing,102206,China

b School of Nuclear Science and Engineering,North China Electric Power University,Beijing,102206,China

c Northwest Institute of Nuclear Technology,Xi'an,710024,China

d School of Materials Science and Engineering,Xiangtan University,Xiangtan,411105,China

Keywords:

ABSTRACT To maximize the catalytic performance of MoS2 in the hydrogen evolution reaction,we investigate the electrocatalytic and photocatalytic performance of monolayer MoS2 doped with noble metal(Ag,Au,Cu,Pd,and Pt)using first principles calculation combined with the climbing image nudged elastic band method.We find the band gap of the monolayer MoS2 is reduced significantly by the noble metal doping,which is unfavorable to improving its photocatalytic performance.The optical absorption coefficient shows that the doping does not increase the ability of the monolayer MoS2 to absorb visible light.The monolayer MoS2 doped with the noble metal is not a potential photocatalyst for the hydrogen evolution reaction because the band edge position of the conduction band minimum is lower than-4.44 eV,the reduction potential of H+/H2.Fortunately,the band gap reduction increases the electron transport performance of the monolayer MoS2,and the activation energy of water splitting is greatly reduced by the noble metal doping,especially the Pt doping.On the whole,noble metal doping can enhance the electrocatalytic performance of the monolayer MoS2.

1.Introduction

In recent years,due to the increasingly prominent problem of energy supply,the whole society is paying more and more attention to the development of renewable energy.Hydrogen energy may become important in the world energy arena in the future.Hydrogen has high combustion heat value,3 times that of gasoline,3.9 times that of alcohol and 4.5 times that of coke[1].The product of hydrogen combustion is water,which is the cleanest energy in the world.At present,there are three main hydrogen production methods:hydrogen production from fossil fuel[2],hydrogen production from biomass[3],and hydrogen production by water splitting[4].Hydrogen production from fossil fuels is widely used,but it is restricted by resource,environment,and other factors.Hydrogen production from biomass faces the problems of photosynthesis efficiency,soil and water area,and storage and transportation cost.The main methods of hydrogen production include the electrocatalytic,photocatalytic,and thermocatalytic.Splitting water into hydrogen and oxygen to produce hydrogen energy is a simple and long-standing idea.Among the three methods of hydrogen production by water splitting,electrocatalytic and photocatalytic hydrogen production have received more attention because the hydrogen production process has no pollutant emission.Since Fujishima and Hongda realized photocatalytic hydrogen production on the titanium dioxide(TiO2)single crystal electrode in 1972[5],more and more attention has been paid to photocatalytic hydrogen production.Compared with the traditional hydrogen production methods,the photocatalytic method of producing hydrogen from water splitting can make full use of abundant solar energy and water to produce hydrogen[6-8],and it can effectively avoid the disadvantages of traditional hydrogen production technology.As a result,electrocatalytic hydrogen production technology is becoming more practical[9-11],especially in energy storage and conversion.More importantly,with rapid advances in power generation technology and more new sources being used to generate electricity,access to electricity has become more convenient and cheaper than ever.

Many studies have shown that the research and development of catalytic materials have become important in photocatalytic and electrocatalytic hydrogen production.Research has shown that many materials can be used in photocatalytic hydrogen production,including almost all elements in the s,p,d regions of the periodic table and the lanthanide system.Presently,improving the hydrogen production efficiency and reducing the manufacturing costs of catalytic materials is the focus of photocatalytic hydrogen production research.The best electrocatalysts for the hydrogen evolution reaction(HER)are platinum and other noble metals or alloys.However,their large-scale deployment is greatly limited by their high cost and scarcity.For this reason,the successful preparation of graphene has set off a wave of research into the use of two dimensional materials[12].Because they can provide more active sites than bulk materials[13-15],they are widely considered appropriate catalytic materials for HER,ammonia synthesis,and carbon dioxide conversion.In recent years,monolayer MoS2has been shown by experimental[16,17]and theoretical research[18,19]to be a promising catalytic material for HER.The dissociation of adsorbed water(H2O)molecules occurs at the exposed edge sites of the pristine and doped monolayer MoS2[20]in addition to the defective monolayer MoS2with triple vacancy defects[21,22].Furthermore,monolayer MoS2has a high specific surface area,which might be an ideal surface for HER.Some research using experiments has shown that monolayer MoS2is a potential electrocatalytic material for HER[23,24].Large areas and high-quality MoS2atomic layers can be prepared using chemical vapor deposition(CVD)technology[25,26].It would be of special interest to develop economical monolayer MoS2-based electrocatalytic or photocatalytic materials for HER.

In recent years,the first principles calculation method has made great progress in many fields of scientific research,including condensed matter physics,materials science,computer science,and chemistry.This calculation method has become common in materials science research.Scientists use it not only to find new materials[27-29],but also to find mechanisms to explain many experimental phenomena[30-32],including theoretical research on monolayer MoS2[33-35].Because all of the atoms on monolayer MoS2can act as active sites,some previous theoretical research has suggested that monolayer MoS2is an excellent catalytic material for HER[16,36].To improve the electrocatalytic and photocatalytic performance of monolayer MoS2in HER,more attention should be paid to the catalytic mechanism of the water splitting on the monolayer MoS2.Many studies have shown that doping can enhance the chemical reactivity of monolayer MoS2,and single-atom catalysis has become a research hotspot in recent years[37,38].Single noble metal atom doping might be a good way to improve the electrocatalytic and photocatalytic performance of monolayer MoS2in hydrogen production by water splitting.

Our previous research has shown that element doping can enhance the adsorption capacity of monolayer MoS2to gas molecule and metal ions[39-42].Here,we investigate the electrocatalytic and photocatalytic performance of monolayer MoS2with noble metal doped in HER by means of the first principles calculation method.Formation energy is calculated to estimate the difficulty of the noble metal doping on the monolayer MoS2.Band structure,as well as optical properties,are calculated to describe the electrocatalytic and photocatalytic performance in HER.Adsorption energy is calculated to represent the interaction between the H2O molecule and the monolayer MoS2with/without noble metal doped.The activation energy(reaction barrier)is calculated to determine the difficulty of HER on various doped monolayer MoS2.Based on the study of these key parameters,we can analyze and discuss the effect of noble metal doping on the electrocatalytic and photocatalytic performance of monolayer MoS2in the hydrogen reduction reaction.

2.Computational method

Fig.1.The most favorable sites for the pristine(a)and noble metal doped monolayer MoS2(b,c,d).(Color figure online).

We perform all the calculations using the Vienna Ab-initio Simulation Package(VASP)code[43,44]based on the density functional theory(DFT).We use the projector-augmented wave(PAW)pseudopotentials method to describe the interactions between the atomic core and valence electrons.The exchange-correlation potential is represented by generalized gradient approximation(GGA)in the form of the Perdew-Burke-Ernzerhof(PBE)functional[45].The electron wave function is expanded in a basis set of plane waves.We set the kinetic energy cutoff to 450 eV,which is sufficient for the noble metal doped monolayer MoS2system.The position of all the atoms is fully relaxed in the configuration optimization process.A Monkhorst-Pack k-point grid of 3×3×1 is used for the geometry optimization.The electronic iterations convergence is set to 1.0×10-6eV.We obtain the optimized structure when the Hellman-Feynman force is less than 0.02 eV/?.For the self-consistent calculations,the Monkhorst-Pack k-point grid is set to 5×5×1.First,we model the substrate by 5×5×1 unit cells.Then,we give a vacuum layer of 16?thickness in the c direction to avoid layer-layer interaction.Previous research[46]has shown that adsorption is a good way to form the monolayer MoS2with the noble metal doped;the position of the noble metal atoms on the monolayer MoS2was investigated experimentally[47]and theoretically[48].Based on previous research,we first determined the most favorable adsorption sites on the monolayer MoS2,and then placed noble metal atoms at these locations,as shown in Fig.1.In this paper,five noble metals Ag,Au,Cu,Pd,and Pt doped monolayer MoS2are considered,and they are named MoS2(Ag),MoS2(Au),MoS2(Cu),MoS2(Pd),and MoS2(Pt),respectively.The formation energy of the doped monolayer MoS2is defined as:

where E(MoS2+M)is the total energy of the monolayer MoS2with noble metal atom doped,E(MoS2)is the energy of the pristine monolayer MoS2,and E(M)is the energy of an isolated noble metal atom,respectively.A negative value illustrates that the formation of noble metal doped monolayer MoS2is an exothermic process,while a positive value shows an endothermic process.The more negative the formation energy,the easier the adsorption doping.The formation energy can also show the stability of the doped monolayer MoS2,a more negative forming energy means higher stability.

Fig.2.Band structure of the monolayer MoS2(Ag)(a),MoS2(Au)(b),MoS2(Cu)(c),MoS2(d),MoS2(Pd)(e),and MoS2(Pt)(f).

For the adsorption of the H2O molecule,there are four adsorption sites on the monolayer MoS2,as shown in Fig.1,TMo(top of the Mo atom),TS(top of the S atom),Hex(center of a hexagon of S and Mo atoms)TX(top of the doped atom,and‘X’=Ag,Au,Cu,Pd,and Pt).The adsorption energy(Ead)of the H2O molecule on the noble metal doped monolayer MoS2 is defined as:

where E(MoS2+M+H2O)is the total energy of the noble metal atom doped monolayer MoS2with a H2O molecule,E(MoS2+M)is the total energy of monolayer MoS2with the noble metal atom doped,and E(H2O)is the energy of a H2O molecule,respectively.The more negative the adsorption energy,the stronger the adsorption.Band structure and optical absorption coefficient are calculated to explore the feasibility of hydrogen production by photocatalytic hydrolysis of the doped monolayer MoS2.The activation energy(reaction barrier)of the H2O molecule dissociation with different reaction paths on the noble metal doped monolayer MoS2is calculated by the climbing image nudged elastic band(CI-NEB)method[49].

3.Results and discussion

3.1.Basic parameters of the pristine monolayer MoS2

First,we calculate the structural and electronic properties of the pristine monolayer MoS2to validate our calculation models.The lattice constant of monolayer MoS2is 3.166?,the bond length of Mo-S is 2.408?,and the distance between S layers in the sandwich structure is 3.137?.These data are in good agreement with the previous theoretical calculations[50,51]and experiments[52,53].The band structure of the monolayer MoS2is shown in Fig.2(d).The monolayer MoS2is a direct band gap material with the band gap 2.064 eV.The band gap value agrees with previous experiments[52,53](1.98 eV and 1.90 eV)and theoretical calculations[50,51](1.80 eV and 1.70 eV).All the results have verified the accuracy of our calculation models.

3.2.Formation energy of monolayer MoS2 with noble metal doped

The formation energy of Ag,Au,Cu,Pd,and Pt doped monolayer MoS2is-0.996 eV,-1.172 eV,-1.931 eV,-2.656 eV,and-3.781 eV,respectively.These values are in agreement with previous theoretical results[46];the formation energy means that the doping on the monolayer MoS2is an exothermic process.According to the formation energy,the synthesizing difficulty of the doped monolayer MoS2is MoS2(Ag)>MoS2(Au)>MoS2(Cu)>MoS2(Pd)>MoS2(Pt),and the stability is reversed.

3.3.Band structure of monolayer MoS2 with the noble metal doped

In general,the electronic transfer capacity of a semiconductor can be characterized by conductivity,which is closely related to the band gap of the material.That is to say,the wider the band gap,the lower the conductivity of the semiconductor,and vice versa.Therefore,the conductivity of the semiconductor can be determined by the band structure.Fig.2 shows the band structure of monolayer MoS2with the noble metal doped.From Fig.2,the band gaps of monolayer MoS2(Ag),MoS2(Au),MoS2(Cu),MoS2(Pd),and MoS2(Pt)are 1.422 eV,1.123 eV,1.661 eV,1.634 eV,and 1.220 eV,respectively.Compared to the undoped monolayer MoS2,the valence band and the conduction band of the doped monolayer MoS2shift in the direction of energy reduction.The impurity levels lead to decrease in the band gap of the monolayer MoS2.The phenomenon proves that doping with noble metal can regulate and control the band structure of the monolayer MoS2.The band gap reduction caused by the noble metal doping makes the monolayer MoS2more conducive for electronic transfer,and the change is a benefit for improving the electrocatalytic performance of monolayer MoS2in HER.

The electron-hole recombination that helps improve the photocatalytic ability of monolayer MoS2to HER can also be benefited by the energy bandgap reduction.As a potential photocatalyst for HER,a suitable energy band gap is necessary.Previous research has shown that the band gap of semiconductors should be wider than 1.23 eV[54].To cover a broad solar spectrum for the catalysis of chemical reactions that reduce H+to H2and oxidize water to O2,the ideal band gaps for photocatalysts should be from 1.8 eV to 2.3 eV.According to the band gap of the noble metal doped monolayer MoS2,monolayer MoS2(Ag),MoS2(Cu),and MoS2(Pd)are the candidate photocatalysts for HER,and pristine monolayer MoS2is an ideal photocatalyst.As a photocatalyst,the band edge position of the conduction band minimum(CBM)should be higher than the reduction potential of H+/H2(-4.44 eV),and the band edge position of the valence band maximum(VBM)should be much lower than the oxidation potential of O2/H2O(-5.67 eV)[55,56].We calculate both positions of the CBM and VBM by the following equations[57]:

Fig.3.The band edge position of CBM and VBM of the pristine and noble metal doped monolayer MoS2.

Fig.4.The optical absorption coefficient of the pristine and noble metal doped monolayer MoS2.

whereχis the Mulliken electronegativity of the doped monolayer MoS2,Egis its energy band gap.We show the band edge position of CBM and VBM of the noble metal doped monolayer MoS2in Fig.3.The band edge position of the pristine monolayer MoS2meets the requirement of photocatalytic HER to produce hydrogen,hence the pristine monolayer MoS2can be a potential photocatalyst for HER.However,except for the pristine MoS2,the band edge positions of VBM are all lower than the oxidation potential,so the noble metal doped monolayer MoS2is not a potential photocatalyst for the reduction reaction of H+to H2.The reason is that the band gap of the monolayer MoS2decreased significantly after the doping.Overall,the noble metal doped monolayer MoS2cannot be a potential photocatalytic HER to produce hydrogen[58,59].

3.4.Optical absorption coefficient of doped monolayer MoS2

Additional energy is required for HER catalyzed by pristine and noble metal doped monolayer MoS2.In order to study whether it is possible to use solar energy to provide additional energy for the water splitting reaction,we calculate the optical absorption coefficients of monolayer MoS2with/without noble metal doped.We use the dielectric function to represent the relationship between the interband transition and optical properties.The optical absorption coefficient is derived from the real part ε1(ω)and imaginary partε2(ω)of the dielectric function as the following expression:

The calculated optical absorption coefficient of monolayer MoS2with/without noble metal doped is presented in Fig.4.In the visible region,the coefficient of the monolayer MoS2is little changed by the noble metal doping.Hence the doping can not enhance the absorbing ability of the monolayer MoS2to the visible light.That is to say,the photocatalytic performance of the monolayer MoS2to HER could not be improved significantly by the noble metal doping.

3.5.Adsorption of H2O molecule on doped monolayer MoS2

Adsorption of the H2O molecule on the doped monolayer MoS2is the key step of the electrocatalytic and photocatalytic HER.Before investigating the water splitting reaction,we computed the adsorption energy of H2O,intermediate products(OH,H,and O),and H2on monolayer MoS2.Various adsorption configurations have been considered to find the most stable one for each adsorbate.The adsorption energy of H2O,intermediate products(OH,H,and O),and H2on monolayer MoS2is shown in Fig.5.The adsorption energy of H2O,OH,and H2is negative;it means that H2O,OH,and H2can be adsorbed on monolayer MoS2.The adsorption energy of both the H and the O atoms is positive,showing that adsorption of the H and O atoms is difficult.However,the adsorption of the H and O atoms on Ag,Au,Cu,and Pt doped monolayer MoS2is more stable due to the more negative adsorption energy.The absolute value of the adsorption energy can be used to judge the adsorption intensity.When the value is higher than 0.50 eV,it belongs to chemical adsorption

[60].Therefore,the H2O molecule is chemisorbed on the Cu,Pd,and Pt doped monolayer MoS2,and the H2molecule is chemisorbed on the Pd and Pt doped monolayer MoS2.Compared to the undoped monolayer MoS2,the adsorption energy of H2O,OH,H,O,and H2on the doped monolayer MoS2is decreased.This indicates that the adsorption of H2O,OH,H,and H2on the monolayer MoS2is strengthened,which is a benefit to improving the electrocatalytic and photocatalytic HER on the monolayer MoS2.Under the change of adsorption energy at different adsorption sites,we can find the favorable adsorption site for each adsorbate,and it can provide some useful information about the photocatalytic reaction path of the H2O molecule on the monolayer MoS2.

3.6.Dissociation of H2O molecule on monolayer MoS2 with noble metal doped

In order to reveal the dissociation mechanism of adsorbed H2O molecules,we use the CI-NEB method to evaluate the minimum energy reaction path(MEP).We use EIS,ETS,and EFSto denote the total energy of the initial state(IS),transition state(TS),and final state(FS),respectively.So,the activation energy Eacan be defined as:

And the reaction energy Ercan be defined as:

Fig.5.The adsorption energy of H2O(a),OH(b),H(c),O(d),and H2(e)on pristine and noble metal doped monolayer MoS2.

Fig.6.The activation energy and reaction energy of the H2O molecule dissociation on the monolayer MoS2.

A positive Eameans that the dissociation of H2O molecules is an endothermic process,and vice versa.Fig.6 shows the MEP of the adsorbed H2O molecule on the pristine monolayer MoS2.Fig.6 also shows the activation energy of the H2O molecule dissociation on the monolayer MoS2is 3.308 eV,and the reaction energy is 3.639 eV.For comparison with the undoped monolayer MoS2,we also show the activation energy and reaction energy of the H2O molecule dissociation on the doped monolayer MoS2in Table 1.The activation energy and reaction energy of the H2O molecule dissociation on the monolayer MoS2are decreased due to the noble metal doping.It means that the H2O molecule dissociation becomes easier after noble metal doping.The Pt doping has the greatest effect on the decrease of the activation energy,followed by Au,Pd,and Cu doping.The activation energy decrease caused by the Ag doping is the least.Compared to the undoped monolayer MoS2,the H2O molecules are easier to dissociate into the H adatom and OH group on the doped monolayer MoS2.After the dissociation,the H atoms migrate to form H2molecules.Thus,hydrogen gas is formed on noble metal doped monolayer MoS2by the water splitting reaction,which is driven by electrocatalysis.The result demonstrates that the noble metal doping can enhance the electrocatalytic performance of the monolayer MoS2in water splitting for hydrogen production.

4.Conclusion

To improve the electrocatalytic and photocatalytic performance of monolayer MoS2in HER,we investigated the effects of noble metal doping.According to the formation energy,we find that the stability of the doped monolayer Mo2is MoS2(Pt)>MoS2(Pd)>MoS2(Cu)>MoS2(Au)>MoS2(Ag).The band gap of the monolayer MoS2is significantly reduced by the noble metal doping,which is unfavorable to improving its photocatalytic performance in HER.The monolayer MoS2with the noble metal doped is not a potential photocatalyst for HER.The optical absorption coefficient shows that the doping does not increase the ability of the monolayer MoS2to absorb the visible light.Fortunately,the band gap reduction enhances the electron transport performance of the monolayer MoS2.The activation energy of the water splitting is greatly reduced by the noble metal doping,especially the Pt doping.In general,the results demonstrate that the electrocatalytic performance of the monolayer MoS2in HER can be enhanced by noble metal doping.

Table 1The activation energy and reaction energy of the H2O molecule dissociation on monolayer MoS2.

Declaration of competing interest

None.

Acknowledgements

This work was supported by the Joint Funds of the National Natural Science Foundation of China(Grant No.U1967212),the National Science and Technology Major Project of China(Grant No.2019XS06004009),the Fundamental Research Funds for the Central Universities(Grant No.2018ZD10).