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Zhang, Hong-ping, Hu, Wei, Du, Aijun, Li, Xiong, Zhang, Ya-Ping, Zhou,Jian, Lin, Xiaoyan, & Tang, Youhong(2018)Doped phosphorene for hydrogen capture: A DFT study.Applied Surface Science, 433, pp. 249-255.
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https://doi.org/10.1016/j.apsusc.2017.09.243
Accepted Manuscript
Title: Doped phosphorene for hydrogen capture: A DFT study
Authors: Hong-ping Zhang, Wei Hu, Aijun Du, Xiong Lu,Ya-ping Zhang, Jian Zhou, Xiaoyan Lin, Youhong Tang
PII: S0169-4332(17)32902-1DOI: https://doi.org/10.1016/j.apsusc.2017.09.243Reference: APSUSC 37322
To appear in: APSUSC
Received date: 26-6-2017Revised date: 21-9-2017Accepted date: 28-9-2017
Please cite this article as: Hong-ping Zhang, Wei Hu, Aijun Du, XiongLu, Ya-ping Zhang, Jian Zhou, Xiaoyan Lin, Youhong Tang, Dopedphosphorene for hydrogen capture: A DFT study, Applied SurfaceScience https://doi.org/10.1016/j.apsusc.2017.09.243
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1
Doped phosphorene for hydrogen capture: A DFT study
Hong-ping Zhang a, d, *, Wei Hu b, Aijun Du c, Xiong Lu d, Ya-ping Zhang a, Jian Zhou
e, Xiaoyan Lin a, Youhong Tang f, *
a Engineering Research Center of Biomass Materials, Ministry of Education, School
of Materials Science and Engineering, Southwest University of Science and
Technology, Mianyang, Sichuan 621010, China
b Computational Research Division, Lawrence Berkeley National Laboratory,
Berkeley, California 94720, United States
c School of Chemistry, Physics and Mechanical Engineering, Queensland University
of Technology, Queensland 4001, Australia
d Key Laboratory of Advanced Technologies of Materials, Ministry of Education,
School of Materials Science and Engineering, Southwest Jiaotong University,
Chengdu 610031, China
e School of Life Science and Engineering, Southwest University of Science and
Technology, Mianyang, Sichuan 621010, China
f Centre for NanoScale Science and Technology and College of Science and
Engineering, Flinders University, South Australia 5042, Australia
* Corresponding authors. Tel: +86-816-6089009, Fax: +86-816-6089009, E-mail:
[email protected] (H. P. Zhang) and Tel: +61-8-82012128, Fax: +61-8-82013618,
E-mail: [email protected] (Y Tang)
Highlights
2
The interactions between H2 and pristine or doped phosphorenes were studied
by DFT calculations.
Metallic dopants significantly improved the hydrogen capture ability of
phosphorene, whereas nonmetallic dopants produced no such effect.
Among the metallic dopants Ti, V, Fe, Ni, Co, Cr, Pt, Pd, Ag, and Au, Pt was
distinctive. Pt-doped phosphorene helped the H2 molecule to dissociate after
about 2 ps.
It is concluded that phosphorene could be a promising candidate for hydrogen
storage, especially Pt-doped phosphorene.
Abstract
Hydrogen capture and storage is the core of hydrogen energy application. With its
high specific surface area, direct bandgap, and variety of potential applications,
phosphorene has attracted much research interest. In this study, density functional
theory (DFT) is utilized to study the interactions between doped phosphorenes and
hydrogen molecules. The effects of different dopants and metallic or nonmetallic
atoms on phosphorene/hydrogen interactions is systematically studied by adsorption
energy, electron density difference, partial density of states analysis, and Hirshfeld
population. Our results indicate that the metallic dopants Pt, Co, and Ni can help to
improve the hydrogen capture ability of phosphorene, whereas the nonmetallic
dopants have no effect on it. Among the various metallic dopants, Pt performs very
differently, such that it can help to dissociate H2 on phosphorene. Specified doped
phosphorene could be a promising candidate for hydrogen storage, with behaviors
superior to those of intrinsic graphene sheet.
Keywords: Phosphorene, Pt-doped, Density functional theory, Hydrogen dissociation
1. Introduction
Under the challenge of the energy crisis, hydrogen has been extensively studied as a
3
candidate to replace fossil fuels. It is a clean, renewable, and efficient energy source
[1-3]. However, two dominant problems need to be solved to achieve scaled-up
application of hydrogen energy. On the one hand, a key issue is the exploration of
suitable hydrogen storage materials with high gravimetric density. On the other hand,
the adsorption energy per hydrogen molecule on the storage material should lie within
an appropriate range to guarantee free recycling of the hydrogen.
Carbon-based nanomaterials have attracted much research interest for hydrogen
energy storage because of their several excellent properties such as high specific area,
unique nanostructures, and chemical stability [4]. Notably, after the discovery of
graphene materials in 2004 by Novoselov and Geim [5], graphene has attracted
particular research interest by virtue of its special characteristics such as desirable
mechanical properties, excellent conductivity and thermal conductivity, and high
transparency. Thus, researchers have investigated graphene across a wide range of
study areas because of its attractive promising applications. These areas include
energy storage, ultrafiltration, electronic devices, sensors, and biological engineering
[6-8]. However, the weak interactions between hydrogen molecules and pure carbon
atoms hinder to some extent the application of carbon nanomaterials in hydrogen
storage. Metal-decorated carbon nanomaterials have been proposed to improve the
adsorption interactions of H2 on substrates [9], but exploring a low-cost technology to
achieve the mass production of graphene can present a significant obstacle for
industrial applications. Inspired by the success of graphene studies, various kinds of
2D materials have been discovered and fabricated in laboratories, with examples
including MXenes [10], hexagonal boron nitride (h-BN) [11], and elemental analogs
of carbon such as silicone, germanene, and stanene [12-14]. Among these materials,
phosphorene is particularly interesting because of its many natural properties.
Phosphorus has been known as an essential component of living systems and is
abundant in nature. Black phosphorus has a layered structure like that of graphite
[15-17]. The title ‘phosphorene’ refers to single-layered black phosphorus. It is
exciting to realize that the band gap of phosphorene can be tunable by adjusting the
layer numbers. Thus, phosphorene has been regarded as one of the best candidates for
4
graphene in electronic applications [18-19]. Graphene has attracted considerable
attention when applied as a gas sensor, due to its high specific area and its electronic
structure or chemical properties that are highly sensitive to gas adsorption. However,
the band gap of graphene is zero, a feature that has occasioned some problems for the
sensor responding process. These problems could be conquered by phosphorene
[20-22].
Very few studies have been published relating to gas molecule adsorption on
phosphorene, and the majority of those have focused on pristine phosphorene [23-27].
Small molecules such as CO, H2, H2O, NH3, NO, and NO2 are physisorbed on
phosphorene. There are also some theoretical studies of interactions between doped
phosphorene and small molecules. Lalitha et al. [25] studied the interactions between
small molecules and Ca-doped phosphorene. They demonstrated that Ca could help
improve the affinity of gas molecules on phosphorene. Duan et al. [21] studied the
adsorption of H2 on Li-doped phosphorene and reached a similar conclusion. The
same trend was deduced in earlier graphene-related studies. We have also published
similar work on the effect of dopants on the interactions between gas molecules and
graphene, demonstrating that metallic dopants could effectively enhance the affinity
of small molecules on graphene. Among the dopants, Ti was found to be of particular
interest.
Black phosphorus (BP) was regarded as the most stable allotrope among the group
including red, white and violet phosphorus. The successful fabrication of the
exfoliated BP and evaluation of its outstanding performance in field-effect transistors
has been reported in 2014 [28-29]. Generally, the methods widely used for the
fabrication of 2D materials such as graphene, MoS2, BN and so on could be classified
into two aspects, top-down methods and bottom-up methods. The most popular
top-down methods are cleavage with tape, liquid-phase exfoliation, and
plasma-assisted fabrication. The principle of these methods is to destroy the weak Van
der Waals interactions between the different materials layers. The bottom-up methods
include chemical vapor deposition (CVD) and hydrothermal synthesis. Compared
with bottom-up methods, top-down methods have been reported for successful
5
fabrication of phosphorene [30-34]. The band gap, electronic structures, reaction
activities of phosphorene could vary, which corresponds to the layer numbers, surface
defects, such as vacuum, doping, and so on, and configurations of phosphorene. All
these parameters could be directly related to the source of the phosphorus and the
fabrication methods. DFT calculations show that the band gap and electronic
properties could be tuned by adatoms doping including transition metallic and
nonmetallic atoms, such as S, Si, O, Ge. Compared with theoretical studies, few
experimental researches could be found due to the strict requirements of the
experimental conditions. Only Al and Cu were doped onto phosphorene for the
physical properties studies of phosphorene. The processing procedures were quite
similar, i.e., a silicon wafer was needed and physical vapor deposition or sputtering
methods were utilized. Comparing experimental studies, theoretical methods may be a
more suitable way for the phosphorene and adatoms doped phosphorene studies at this
stage.
In this study, using the DFT method, we systematically investigated the interactions
between hydrogen and phosphorene at the atomistic scale. The effect of selected
metallic and nonmetallic dopants on the interactions between hydrogen molecules and
phosphorene were carefully studied. Further, the adsorption and interaction
mechanisms of hydrogen on pure and doped phosphorene were studied and proposed.
We also investigated the interaction dynamics for H2/Pt-doped phosphorene,
H2/Pd-doped phosphorene, and H2/Ni-doped phosphorene systems by ab initio
dynamic simulation.
2. Computational methods
2.1 Simulation parameters
All the simulations in the study were performed by the DFT program Dmol3 in
Materials Studio (Accelrys, San Diego, CA), wherein the physical wave functions
were expanded in terms of numerical basis sets. The double numerical basis set with
polarization function (DNP) [35], that is comparable to the 6-31G** basis set, was
6
utilized during the simulations [35-37]. The core electrons were treated with DFT
semicore pseudopotentials. The exchange-correlation energy was calculated using the
Perdew-Burke-Ernzerhof generalized gradient approximation [38]. Special point
sampling integration over the Brillouin zone was employed using the Monkhorst-Pack
schemes with a 5 × 5 × 1 k-point mesh [39]. A Fermi smearing of 0.005 Ha (1 Ha =
27.21 eV) and a global orbital cutoff of 7 Å were employed. The convergence criteria
for the geometric optimization and energy calculation were set as follows: (1)
self-consistent field tolerance of 1.0 × 10−6 Ha/atom, (2) energy tolerance of 1.0 ×
10−5 Ha/atom, (3) maximum force tolerance of 0.002 Ha /Å, and (4) maximum
displacement tolerance of 0.005 Å. Dmol3 produces highly accurate results while
keeping the computational cost low. For comparative study, all the calculations were
also carried out with the Becke-Lee-Yang-Parr (BYLP) functional theory.
The built model of phosphorene is shown in Figure 1. It contains 42 phosphorus
atoms and exhibits an undulating appearance. Its area is about 9.9 Å×13.14 Å. A
vacuum layer of 30 Å thickness was added along the direction perpendicular to the
phosphorene surface to avoid interactions between the phosphorene substrate and its
periodic images. The Tkatchenko-Scheffler method was utilized to carry out the
dispersion corrections for non-covalent interactions, especially hydrogen bondings
and van der Waals interactions.
Doped phosphorene models were built based on the DFT optimized phosphorene
model. Four doping sites were considered: H site above P6 hexagon, B site above the
mid-point of the bottom P-P bond, B1 site above the mid-point of the top P-P bond,
and T site on the top of the P atom. To find the equilibrium doping configurations, all
the dopant-phosphorene systems were optimized by the DFT method. After that, the
adsorption energy between these dopants and phosphorene was used to judge the
equilibrating doping site of the different dopants. The optimized kinds of doped
phosphorene models are shown in Figure S1. The bond lengths of P-P or dopant-P are
listed in Table S1. Comparison of our calculation results with the reports of others
demonstrated that our optimized models agreed well with their findings. The
reproducibility of previously reported data validated the applicability of Dmol3 to the
7
present systems. To investigate the dynamic interactions between H2 and Pt-, Pd-, or
Ni-doped phosphorene, ab initio molecular dynamics simulations were performed at
the temperature of 298 K under the NVT ensembles. The Massive Nosé-Hoover
method was used to control temperature. A 1 fs time step was used to carry the 10 ps
dynamics calculations. The dispersion correction for the van der Waals interactions
was applied during all the dynamics simulations.
2.2. Analysis methods
The adsorption energy (a d s
E ), indicating the intensity of interaction between hydrogen
molecules and phosphorene surfaces, was derived according to Equation (1):
2 2
( )a d s p h o sp h o ren e H p h o sp h o ren e H
E E E E
(1)
where 2
p h o sp h o ren e HE
, p h o sp h o re n eE , and
2H
E represent the total energy of the
adsorption system, the energy of phosphorene, and the energy of H2, respectively. A
negative a d s
E corresponds to a stable adsorption and a more negative a d s
E results in
a more stable adsorption system. The electron density difference reveals the change in
electron density during adsorption and is calculated by subtracting the electron
density of the isolated H2 (2
H ) and the phosphorene surface (
p h o sp h o re n e ) from the
total electron density of the system (2
p h o sp h o re n e H
), as shown in Equation (2):
2 2p h o sp h o ren e H H p h o sp h o ren e
(2)
Density of states analysis can be an effective method of studying the interactions
between hydrogen molecules and phosphorenes at the electronic level. Partial density
of states (PDOS) analysis can provide information about electron interactions between
different orbits. We performed PDOS analysis to investigate the interactions between
H2 and different kinds of phosphorene. The Hirshfeld population analysis was used to
analyse the details of charge transfer during the H2 adsorption process.
3. Results and discussion
3.1 Pure phosphorene
8
Figures 1(a) and 1(b) show the optimized structure of a monolayer black phosphorus
(phosphorene) obtained by the DFT method. By combining the top and side views of
phosphorene, it can be found that the phosphorene is composed of repeating puckered
hexagonal structures, unlike flat structured graphene. To perform a meaningful
calculation, the structure optimization process of phosphorene is very important. In
our simulation model of pure phosphorene, the bond length of P-P is 2.22 Å in the
zigzag direction and 2.28 Å in the armchair direction. The distance between the
two-layer phosphorus atoms is about 2.17 Å. For comparison, 2.23 Å and 2.26 Å for
the corresponding P-P bond lengths in the zigzag and armchair directions respectively
and 2.21 Å for the layer distance were reported by Duan et al. [21], and 2.21 Å and
2.26 Å for the corresponding P-P bond lengths in the zigzag and armchair directions
respectively by were reported by Lalitha [25]. Our results are in good agreement with
those studies. The adsorption energy between H2 and pristine phosphorene is about
-0.13 eV, indicating the rarity of low interactions between phosphorene and H2, as
shown in Figure 2. The vertical distance between H2 and phosphorene is about 3.04
Å.
3.2 Doped phosphorene
To simulate the effect of various kinds of dopants on the interactions between H2 and
phosphorene, doped phosphorene models based on the pristine phosphorene were set
up and optimized. Figures 1(c) and 1(d) showed the optimized Pt-doped phosphorene
model. Details of the doping site, bond length, and binding energy between dopants
and phosphorene for different kinds of doped phosphorene are listed in Table S1. It
was found that the optimized doped phosphorene models were very similar to
previous reports [31-33]. The bond lengths of Ca-P were 2.78 Å and 2.92 Å in the
zigzag and armchair directions, respectively. The bond length of P-P in the zigzag
direction was 2.8 Å, indicating good agreement of our results with those of Lalitha et
al. [25]. For the Pt-doped phosphorene, the vertical distance between Pt and
phosphorene surface was 2.3 Å and the binding energy between Pt dopant and
phosphorene was -4.5 eV. The adsorption behavior of metal adatoms on single-layer
9
phosphorene was systematically studied by Wu et al [34], who found the vertical
distance of 2.3 Å between Pt and phosphorene and the binding energy of -4.7 eV
between Pt and phosphorene. It was concluded that our doped phosphorene models
were reasonable and could be used for the further study of H2 adsorption.
3.3 Interactions between hydrogen molecule and doped phosphorene
To explore the interactions between a hydrogen molecule and phosphorene, single H2
molecules were placed separately onto phosphorene and doped phosphorene surfaces.
After DFT calculations, the a d s
E , isosurface of electron density difference, and PDOS
were obtained. Table S2 lists the adsorption energy results of the
hydrogen-phosphorene systems, that can also be intuitively understood in Figure 2. As
mentioned, the a d s
E is about -0.13 eV when the H2 molecule interacts with the
pristine phosphorene. The adsorption energy between H2 molecule and phosphorene
can be improved by metal adatom dopants, whereas the nonmetallic dopants have a
negligible effect. However, the capacities of different metallic elements to improve
the interaction between H2 molecule and phosphorene differ. The transition metallic
dopants Ti, V, Fe, Ni, Co, Cr, Pt, Pd, Ag, and Au significantly improve the H2 capture
ability of phosphorene. It is worth noting the different effects of Pt and other metallic
dopants. The adsorption energy between phosphorene and H2 can be improved to 6 eV,
about 50 to 60 times greater than that between the pristine phosphorene and H2. The
high adsorption energy value indicates the strong interactions between Pt-doped
phosphorene and H2 molecule. Among the H2-metallic doped phosphorene systems,
the interaction between H2 and Ca-doped phosphorene is relatively low. When a H2
molecule interacts with a nonmetallic doped phosphorene sheet, the a d s
E is very
similar to that of the pristine phosphorene. Thus, the nonmetallic dopants, C, B, O, N,
Si, Se, and S did not strengthen the interaction between H2 and phosphorene. Hence,
considering the effect of metallic dopants and nonmetallic dopants, it was concluded
that metallic dopants could enhance the hydrogen capture ability of phosphorene. To
gain deeper understanding, electron density difference analysis was carried out. As
10
shown in Figure S1, the clear distinction of electron density difference between the
different H2-doped phosphorene systems can be perceived. Generally, there is a
significant electron transfer between the doped metallic atoms and H2 molecules. That
phenomenon is not found in the H2-nonmetallic doped phosphorene systems.
Figure 3 shows the results of isosurface of electron density difference analysis for the
Pt-doped phosphorene/H2, Pd-doped phosphorene/H2, Ni-doped phosphorene/H2, and
pristine phosphorene/H2 systems. The blue area in the figure represents accumulation
of electrons and the yellow area represents depletion of electrons. It is found that an
apparent accumulation of electrons occurs between the H2 molecule and the Pt-, Pd-,
or Ni- doped phosphorene, whereas there is no electronic interaction between the H2
molecule and the intrinsic phosphorene. Although there is apparent electron
accumulation between the H2 molecule and Pt-, Ni-, or Pd- doped phosphorene, the
amounts of transferred charge are quite different. A 0.562|e| charge is transferred in
the Pt-doped phosphorene system and 0.099 |e| and 0.171 |e| charges are transferred in
the Pd-doped and Ni-doped phosphorene systems respectively, as shown in Figure 3.
It is found that the H2 molecule has dissociated on the Pt-doped phosphorene surface.
PDOS analysis was also performed to illustrate the electronic interaction between the
H2 molecule and the Pt-, Pd-, and Ni-doped phosphorene sheet, respectively. The
interactions between the s orbital of a H atom of H2 (close to the doped phosphorene)
and the s, p, and d orbitals of a dopant (Pt, Pd, and Ni atoms, respectively) were
compared. Clear differences can be found in Figure 4 between the H2/Pt-doped
phosphorene and the H2/Pd-doped or H2/Ni-doped phosphorene systems. Overlapping
between the s orbital of the H and the d orbital of Pt occurs around the Femi energy
level, i. e., from 0 eV to -6.25 eV. For the Pd- and Ni-doped phosphorenes, however,
overlapping between the s orbital of the H and the d orbital of a dopant (Pd or Ni)
only occurs at -7.5 eV. These findings indicate that there are far more orbital hybrids
in the H2/Pt-doped phosphorene system than in the H2/Pd-doped or H2/Ni-doped
phosphorene systems. The PDOS results could be beneficial for understanding the
difference of H2 adsorption behaviors in different doped phosphorenes, such as Pt-,
Pd-, and Ni- doped phosphorenes.
11
Figures 5 to 7 show the dynamic configurations of H2/doped phosphorene systems at
different times from 0 ps to 10 ps. It can be seen that the adsorption process of a H2
molecule on a doped phosphorene is almost stable after 2 ps, by which time the H2
molecule has already dissociated from the Pt-doped phosphorene. Tables 1 to 3 list the
bond lengths between the H-H of a H2 molecule and the dopant-H of Pt-, Pd-, and
Ni-doped phosphorene systems. It can be found that bond length of H-H for a H2
molecule varies from 0.7 Å to about 2.0 Å after 2 ps on the Pt-doped phosphorene and
varies around 2 Å in the subsequent 8 ps. However, the bond length of H-H for a H2
molecule varies from 0.7 Å to about 0.8 Å after 2 ps, and varies around 0.8 Å in the
subsequent 8 ps. Thus, H2 can be dissociated on the Pt-doped phosphorene. From
Tables 1 to 3, it is also found that the bond lengths between different dopants (Pt, Ni,
and Pd) and the H atom of a H2 molecule are quite different. The distance between the
H2 molecule and a doped phosphorene can be sequenced as: Pd-doped > Ni-doped >
Pt-doped. These results are in good agreement with the results for adsorption energy,
electron density difference, PDOS, and charge transfer.
4. Conclusion
In this study, we undertook a search based on first-principle calculations for
high-capacity hydrogen storage media among phosphorene-based materials. We
investigated the interactions between a H2 molecule and phosphorene bya d s
E , electron
density difference, PDOS analysis, Hirshfeld population, and first-principle dynamics
simulations. It was found that the interactions between H2 and metallic doped
phosphorenes were notably greater than those between H2 and intrinsic phosphorene.
Metallic dopants significantly improved the hydrogen capture ability of phosphorene,
whereas nonmetallic dopants produced no such effect. Among the metallic dopants Ti,
V, Fe, Ni, Co, Cr, Pt, Pd, Ag, and Au, Pt was distinctive. Pt-doped phosphorene helped
the H2 molecule to dissociate after about 2 ps. It is concluded that phosphorene could
12
be a promising candidate for hydrogen storage, especially Pt-doped phosphorene.
Acknowledgements
H. P. Zhang is grateful for the support of the China Scholarship Council (CSC) with a
visiting scholar program to Flinders University. This research was supported by the
National Natural Science Foundation of China (NSFC; Grant No. 31300793,
31400809).
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16
Figure 1. Optimized (a) and (b) pristine phosphorene models; (c) and (d) Pt-doped
phosphorene models; (a) and (c) top views; (b) and (d) side views.
17
Figure 2. Comparison of the interactions between a hydrogen molecule and a doped
phosphorene with two different methods.
Figure 3. Isosurface of the electron density difference of H2-phosphorene interaction
systems with the isovalue of 0.004. Colors: yellow indicates electron depletion and
blue indicates electron accumulation.
18
Figure 4. PDOS of H-dopant for different H2/doped phosphorene systems.
Figure 5. Dissociation evolution of H2 on the Pt-doped phosphorene surface.
19
Figure 6. Dissociation evolution of H2 on the Pd-doped phosphorene surface.
Figure 7. Dissociation evolution of H2 on the Ni-doped phosphorene surface.
Table 1. Bond length evolution of the H2/Pt-doped phosphorene system
Time 0 ps 2 ps 4 ps 6 ps 8 ps 10 ps
H-H bond (Å) 0.76 2.13 1.93 2.18 1.93 2.03
Pt-H bond (Å) 2.43 1.65 1.54 1.74 1.63 1.56
Pt-H bond (Å) 2.45 1.58 1.59 1.64 1.72 1.60
20
Table 2. Bond length evolution of the H2/Pd-doped phosphorene system
Time 0 ps 2 ps 4 ps 6 ps 8 ps 10 ps
H-H bond (Å) 0.75 0.75 0.79 0.78 0.80 0.8
Pd-H bond (Å) 2.47 2.2 2.35 2.37 2.21 2.35
Pd-H bond (Å) 2.29 2.47 2.34 2.43 2.46 2.34
Table 3. Bond length evolution of the H2/Ni-doped phosphorene system.
Time 0 ps 2 ps 4 ps 6 ps 8 ps 10 ps
H-H bond (Å) 0.75 0.86 0.93 0.88 0.80 0.83
Ni-H bond (Å) 2.54 1.69 1.59 1.78 1.79 1.60
Ni-H bond (Å) 2.55 1.62 1.54 1.67 1.71 1.71
