D. Liu et al. / Journal of Molecular Structure 980 (2010) 66–71
67
theory (DFT), the calculations of the crystal structure of MoS
2
and
into a 2 ꢀ 1 super cell, to form a surface layer (1 0 0) [abbreviated
molecular dynamics simulation of its thermal decomposition were
carried out. The geometric crystal structure, energy band, the
density of states and other various properties can be predicted
accurately by this program. The generalized gradient approxima-
tion–Perdew Burker Ernzerhof (GGA–PBE) [12,13] functional,
exchange–correlation functional, were adopted in this calculation,
which is better than local density approximation (LDA) [14,15]
functional and the preferred method used in the studies of molec-
ular and materials properties currently. Compared with LDA, all the
parameters in Perdew Burker Ernzerhof (PBE) are fundamental
constants. The exchange part of this function is similar with Becke
formula while the correlation part is close to Perdew–Wang func-
tion. In addition, this function has a strong physics background and
reliable numerical results which are frequently used in DFT calcu-
lations. Ultra-soft pseudo-potential was used and the spin-polari-
zation of the system was ignored in the calculation with its plane
wave 270 eV cutoff and the thickness of the two adjacent vacuum
layers 1 nm. Whether the energy and charge density distribution of
the system is based on convergence in the self-consistent process
are the criteria, the accuracy of the energy convergence is better
as (1 0 0) slab]; five layers were chosen along the crystal face
(1 1 0) of MoS
a surface layer (1 1 0) [abbreviated as (1 1 0) slab]; and two layers
were chosen along the crystal face (0 0 1) of MoS and then ex-
2
and then expanded into a 1 ꢀ 3 super cell, to form
2
panded into a 2 ꢀ 1 super cell, to form a surface layer (0 0 1)
[abbreviated as (0 0 1) slab].
The dynamics simulation calculations of the optimized 4MoS
slab and 2Mo slab were carried out under experimental condi-
2
2 3
S
tions (1823 K, 100 kPa). It was simulated by NPT system, with the
simulation time 1.5 ps and time step size 1.0 fs, total 1500 steps,
the 500 steps simulation for equilibration phase in initial stage,
and later 1000 steps for the dynamic simulation results. The Nosé
thermostat [17] and Andersen barostat [18] was used in the
dynamic simulation. The molecular dynamics (MD) simulation in
CASTEP is based on the velocity Verlet algorithm for integration
of the equation of motion. Molecular dynamics in total-energy
DFT schemes is implemented in essentially the same way as in con-
ventional forcefield based methods. The main difference is that the
energy and atomic forces are derived by solving DFT equations
rather than from empirical potentials of interatomic interactions.
Electrons are kept on the Born–Oppenheimer surface by means of
explicit electronic structure optimization after each MD step. A side
effect of this is that evaluation of force and energy from first princi-
ples is always the most computationally expensive part of ab initio
MD. As a result, the efficiency of the MD step itself has no impact on
the speed of the calculation [19,20].
ꢁ5
ꢁ1
than 2 ꢀ 10 a.u. and the quality k-point separation in 0.05 Å
MoS belongs to hexagonal crystal system and layered structure
see Fig. 1) [16], D crystal system and P63 space group. The lattice
.
2
4
6
(
h
constants are a = 0.315 nm, c = 1.230 nm and Z = 2. In the crystal
structure, the surface network composed by Mo4 was sandwiched
+
2ꢁ
between the upper and lower surface network formed by the S
which formed a trigonal column coordinated structure layer to-
gether. [MoS ] formed a trigonal column coordinated polyhedron
,
The calculation model of MoS
simulation was obtained as follows: it was cut to a depth of
0.666 along the crystal face (0 0 1) of MoS and then expanded into
a 2 ꢀ 2 super cell, to form a surface layer (0 0 1) including four Mo
atoms and eight S atoms [abbreviated as four MoS slab]. Accord-
ing to the lattice constants of MoS , the MoS unit cell was mod-
eled in CASTEP and was further optimized to form a surface layer
0 0 1) including four Mo atoms and six S atoms [abbreviated as
two MoS slab] along the crystal layer (0 0 1). And all the atoms
2
molecular dynamics Mulliken
6
and this structure layer was composed of and linked with empty
octahedral layers.
2
Therefore, the cutting scheme of the calculation model of the
2
nature of MoS
chosen along the crystal face (1 0 0) of MoS
2
crystal structure were as follows: three layers were
and then expanded
2
2
2
(
2
in the studied structures were fully allowed to relax during the
optimization procedure.
2.2. Experiment
2
In order to further validate the thermal decomposition of MoS ,
the experiment was carried out with MoS (>98.5%) produced by
2
Tianjin Chemical Reagent Factory as raw materials which was
listed in Table 1. Differential thermal analysis–thermogravimetry
(
DTA–TG) curves were obtained by German NETZSCH STA 409
PC/PG simultaneous thermal analyzer, with oxygen as the shield-
ing gas to eliminate the influence of oxygen and flow 30 mL/min,
and heating rate 5 K/min.
3
. Results and discussion
3.1. Crystal geometries, energy band structure, electron density,
electron density difference diagram, projection drawing of density of
states and Mulliken overlap population of MoS
slab and (0 0 1) slab
2
(1 0 0) slab, (1 1 0)
The periodic density functional theory was adopted to optimize
the crystal geometries of MoS (1 0 0) slab, (1 1 0) slab and (0 0 1)
2
Fig. 1. The crystal structure of MoS .
2
Table 1
The ingredients of raw materials (MoS
2
).
Ingredients
MoS
2
Fe
<0.1
SiO
2
MnO
<0.2
2
Acid insoluble materials
<0.5
Oils
Contents (%)
P98.5
<0.001
<0.2