Z. Yao et al. / Materials Research Bulletin 46 (2011) 1938–1941
1939
Table 1
with 1:1 Mo:P ratio. After evaporating and drying in air at 110 8C
for 12 h, the precursor was treated by above-mentioned TPR in
pure H2 (150 ml/min) to obtain MoP sample. In order to investigate
the mutual transformations of Mo carbide, nitride and phosphide,
the P/Mo2C and P/Mo2N precursors (1:1 Mo:P ratio) were prepared
by the same impregnation technique described previously. Then,
the precursors were dried at RT for further use.
The structural stability of samples was characterized by TPT
(temperature-programmed treatment) experiments. The sample
was directly heated in a given gas (He, H2 or air) at a rate of 15 8C/
min to a given temperature. The transformations of these Mo-
based compounds were investigated by the nitridation of Mo2C
and MoP, carbonization of Mo2N and MoP, and reduction of P/Mo2C
and P/Mo2N according to the TPR procedure mentioned previously.
The structure characterization of samples was carried out by X-
ray diffraction (XRD, D/max-IIIA, Rigaku). The morphology of
samples was characterized by scanning electron microscopy (SEM,
LEO1530VP) equipped with energy dispersive X-ray (EDX)
spectroscopy.
Phase composition of the samples obtained by heat treatment under different gases.
Gas
Temperature (8C)
XRD phases
Mo2C
Mo2N
MoP
He
He
He
H2
H2
H2
Air
Air
Air
Air
750
850
950
750
850
950
400
500
600
700
Mo2C
Mo2N
MoP
Mo2C, Mo
Mo2C, Mo
Mo2C
Mo2N, Mo
Mo
MoP
MoP
Mo2N, Mo
Mo
MoP
Mo2C
MoP
Mo2C
Mo
MoP
Mo2C
Mo2N, MoO3
MoO3
MoP
Mo2C, MoO3
MoO3
MoP
MoO3
MoP
MoO3
MoO3
MoP, MoOPO4
while the Mo2C and MoP samples retained stable structure even at
950 8C. However, in air, all of them suffered from bulk oxidation
and threshold oxidation temperatures of 400, 500 and 700 8C were
observed for Mo2N, Mo2C and MoP, respectively. It was clear from
Table 1 that the order of structural stability of these Mo-based
materials was as follows: Mo2N < Mo2C < MoP, regardless of the
heating atmosphere.
3. Results and discussion
3.1. Structural stability
3.2. Mutual transformations
XRD patterns of Mo carbide, nitride and phosphide obtained
from MoO3 are shown in Fig. 1(A, C and E). The peak positions were
Fig. 1 also shows the XRD patterns corresponding to samples (B,
D, F, G, H and I) obtained by the nitridation/carbonization/
reduction of various Mo precursors. In the case of Mo2C heated
consistent with the crystalline phases of
39.5, 52.3 and 61.88, JCPDS72-1683), -Mo2N (2
75.7 and 79.78, JCPDS25-1366) and MoP (2 = 27.9, 32.2, 43.1, 74.3
b
-Mo2C (2u = 34.5, 38.1,
g
u
= 37.4, 43.5, 63.1,
under NH3 atmosphere, a new MoN (2u = 31.9, 36.2 and 49.08,
u
JCPDS25-1367) phase was found for the sample B, indicating the
nitridation of a portion of the Mo2C phase. The result was
consistent with what has been reported in our recent study [16]. As
for Mo2N heated under CH4 + H2 gas mixture, the carbonization
reaction readily occurred, yielding a single-phase sample D
and 85.78, JCPDS24-0771). There were no peaks that can be
assigned to Mo oxide(s) or other Mo carbide, nitride and phosphide
phases, indicating that these Mo-based compounds were phase-
pure.
Subsequently, the structural stability of these Mo-based
compounds was estimated under He, H2 and air, respectively.
Table 1 gives phase composition of the samples obtained by heat
treatment under different atmospheres. It appeared that the
structural changes of these Mo-based compounds depend on
temperature and atmosphere. Under the treatment in He, the Mo2C
(Mo2N) was transformed to Mo–Mo2C (Mo–Mo2N) mixture at
850 8C, and Mo2N was decomposed to a single-phase Mo at 950 8C.
In contrast, the MoP was thermally stable and kept its structure up
to 950 8C. When they were heated in H2 gas, the Mo2N sample
readily underwent phase transformation (Mo2N ! Mo) at 750 8C,
(MoC1ꢀx, 2u = 36.7, 42.7, 62.0, 74.1 and 77.78) as reported by
Xiang et al. [17]. It was worthy of note that, both P/Mo2C and P/
Mo2N can be transformed to MoP (samples F and G) after H2
reduction. In contrast, after carbonizing and nitriding of MoP, their
crystal phases (samples H and I) persisted well. In fact, CH4 was
also used as an effective reductant instead of H2 for MoP synthesis
[18]. In summary, both Mo2C and Mo2N can be transformed to
MoP, whereas the reverse transformations did not occur;
transformations occurred between Mo carbide and Mo nitride,
and the transformation of Mo nitride to Mo carbide was much
easier.
It is well known that the morphology of transition metal
interstitial compounds depends highly on the nature of precursors
[19]. In the present study, MoO3, Mo2C and Mo2N can be used as
Mo sources to obtain MoP, and the resultant products were
designated as o-MoP, c-MoP and n-MoP. The morphologies of these
MoP samples obtained from different Mo sources were compared.
SEM showed that the o-MoP consisted of aggregates of irregularly
shaped nanoparticles (Fig. 2a), which was similar to the
morphology of MoP prepared from molybdenum phosphate
[20]. While both c-MoP and n-MoP showed
a weak self-
aggregation tendency (Fig. 2b and c). Especially, it was clear that
very small nanoparticles can be seen for the c-MoP sample. The
EDX results (Fig. 2d) of o-MoP, c-MoP and n-MoP samples gave P/
Mo mole ratios of 0.97, 0.95, and 0.96, respectively, which were
close to the expected stoichiometric value of 1/1 for MoP.
In general, the synthesis of MoP from oxidic precursors by TPR
method involves two solid reactants (MoO3 and POx) and one
gaseous (H2) reactant [21]. At higher temperatures, phosphorus
oxides were reduced to produce volatile phosphorus species, such
as PH3 or P, which migrated on the solid surface to react with Mo
oxide(s), leading to the formation of MoP [21]. In the present study,
Fig. 1. XRD patterns of as-prepared Mo2C, Mo2N and MoP, as well as the products
obtained from different precursors during TPR processes: (A) Mo2C, (B) nitrided
Mo2C, (C) Mo2N, (D) carbonized Mo2N, (E) MoP, (F) reduced P/Mo2C, (G) reduced P/
Mo2N, (H) carbonized MoP and (I) nitrided MoP.