Preparation and characterization of molybdenum particles by reducing MoO3 nano-fibers

Article abstract of DOI:10.1080/15533170802023551

One-dimensional nanostructures of orthorhombic molybdenum trioxide have been synthesized in the forms of fibers under hydrothermal conditions at 170-180°C. It has been found that the dimensions of the fibers are about 50 nm in thickness, 100-200 nm in width and a few tens of micrometers in length under our experimental settings. Using H2 stream, the MoO3 fibers prepared can be converted completely to Mo metal powders at 600°C. The original fiber-like morphology is not well-retained, but in some special cases, fiber-like morphology can be partial retained, although the aspect ratio of the oxide template is reduced upon the reduction treatment. Copyright

Full text of DOI:10.1080/15533170802023551

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 38:318–321, 2008
Copyright # 2008 Taylor & Francis Group, LLC
ISSN: 1553-3174 print/1553-3182 online
DOI: 10.1080/15533170802023551
Preparation and Characterization of Molybdenum Particles
by Reducing MoO3 Nano-fibers
Zhao Peng,1,2,3 Li Jing,2 Wang Lin,2 Zhao Baohua,2 Yang Liuxiao,2
and Fan Huiqing3
1New Materials Research Institute of Chang’an University, Xi’an, China
2JinDuiCheng Molybdenum Group Co., Ltd., Xi’An, China
3State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, China
An efficient method to produce nanoscopic molybdenum
One-dimensional nanostructures of orthorhombic molyb- oxide fibers was reported. The procedure is based on the inter-
denum trioxide have been synthesized in the forms of fibers
under hydrothermal conditions at 170–1808C. It has been found
that the dimensions of the fibers are about 50 nm in thickness,
100–200 nm in width and a few tens of micrometers in length
under our experimental settings. Using H2 stream, the MoO3
calation of primary amines into the layered structure of
molybdic acid and subsequent transformation of the lamellar
molybdenum oxide–amine intermediate into the fibrous
product.[7]
fibers prepared can be converted completely to Mo metal
powders at 6008C. The original fiber-like morphology is not
well-retained, but in some special cases, fiber-like morphology
can be partial retained, although the aspect ratio of the oxide
template is reduced upon the reduction treatment.
A simple hydrothermal method for preparation of nanos-
tructured a-MoO3 with morphology control on either ribbons
or rods at only 140–2008C. 2H-MoS2 nanorods indeed can
be prepared from the resultant MoO3 nanorods through sulfida-
tion conversion.[8] Molybdenum trioxide nanobelts and micro-
balls were prepared by oxidization of molybdenum metal in an
ambient atmosphere,[9] but nano Mo metal powders prepared
from MoO3 nanorods or nanofibers have not been reported in
the literatures. Here, we report the process of MoO3 nanofiber
prepared by hydrothermal method, and the nano Mo metal
powder indeed can be reduced from the resultant MoO3 nano-
fibers through reduction conversion by using H2 stream.
Keywords metal Mo powder, MoO3, nano-fibers, reduction
New methods of making new or existing inorganic materials
are always interesting and challenging for materials scientists.
Development of one-dimensional (1D) materials has become a
focal area in nanostructured materials research, owing to their
special characteristics which differ from those of respective bulk
crystals. These highly anisotropic 1D materials include elemental
carbon, metals, semiconductor, alloys, sulfides, oxides, hydrox-
ides, and so forth. Among the important layered transition metal
oxidesandchalcogenideshavebeenextensivelyinvestigated.[1–5]
EXPERIMENTAL
Materials Preparation
The a-MoO3 nanofibers were synthesized by a hydrother-
MoO3 and its derivatives are widely used in industry as cat- mal route. Ammonium heptamolybdate tetrahydrate (AHM;
.
alysts, display devices, sensors, smart windows, lubricants, (NH4)6Mo7O24 4H2O) and nitric acid were the two starting
battery electrodes. In particular, MoO3 has been prepared reagents. A saturated solution of precursor compound AHM
into the forms of carbon-metal-oxide nanocomposites, nano- was prepared at room temperature. For each run of experiment,
tubes, and nanorods using carbon nanotubes (CNTs) as a host 30.0 ml of the saturated solution was diluted with deionized
material or template. The MoO3 nanofibers obtained with this water (30.0 ml) and then further acidified using the 2.2M
novel method are up to 15 mm long with their diameters nitric acid to a total of 70–85.0 ml in volume. The resultant
ranging from 50 to 150 nm.[6]
solution was transparent and transferred to a Teflon-lined stain-
less steel autoclave and heated at 170–1808C for 40 h. The
product precipitate was filtered and rinsed with deionized
water, followed by drying at 628C for 5 h. The dried sample
was fibrous and pale yellowish.
Received 15 June 2007; accepted 18 October 2007.
The authors gratefully acknowledge research funding (50472028)
supported by NSFC.
The above as-growna-MoO3 crystals were further used as
metal oxide precursors for reduction investigation. The
Address correspondence to Zhao Peng, JinDuiCheng Molybdenum
Group Co., Ltd., Xi’An 710068, China. E-mail: zyzhao@chd.edu.cn
318

MOLYBDENUM PARTICLES
319
FIG. 1. The SEM and HRTEM of MoO3 nanofibers: (a) SEM 3000X; (b) HERTEM 50000X; (c) 200000X; and (d) 2000000X.
˚
reactions were carried out in a tubular quartz reactor at 400– XRD-6000, Cu Ka, l ¼1.5406 A). Crystal sizes and the mor-
6008C using H2 stream under normal atmospheric pressure.
phology of crystal samples was examined before and after the
reduction reactions with scanning electron microscopy (SEM,
JSM-5600LV, 15 kV).
Materials Characterization
High-resolution analytical transmission electron microscopy
(TEM, JEM-2010, 200 kV) was also used to examine crystal-
line a-MoO3 and its reduced products. The specimens for
Crystallographic information of samples including reduced
ones was investigated with X-ray diffraction (XRD; Shimadzu
FIG. 2. SEM of MoO3 nanofibers reduced at different temperatures: (a) Before reduced; (b) 5308C for 1 h; and (c) 6008C for 1 h.

320
Z. PENG ET AL.
observed in Figure 3. All the strong diffraction peaks corre-
spond to (h00) reflections.
The crystal growth process of MoO3 under hydrothermal
conditions could be explained according to literatures.[8–10]
The MoO3 nanofibers were reduced at different tempera-
tures under H2 stream atmosphere. From Figure 3, the XRD
patterns indicated that MoO3 nano fibers could be reduced
as MoO2 at 5308C for 1 hour, the materials had a pure
phase of MoO2. That means the MoO3 nanofibers have been
reduced as MoO2 completely at 5308C. Corresponding to
Figure 2a and Figure 2b, the morphology of the MoO3 nano-
fibers have been changed, the fibers-like morphology was
partial retained.
Figure 3c showed that the MoO3 nanofibers had been reduced
as Mo powders completely when the reduced temperature was
increased to 6008C, corresponding Figure 2c, the morphology
of Mo powders is not fibers-like, most of metal Mo powders
are irregular shape, neither the ball-like, nor fibers-like.
But in some cases, the nanofiber-like shape of MoO3 was
retained, and the result could be seen in Figure 4. From
Figure 4b, it could be found that needle-like metal Mo
powder was formed. The reason of forming needle-like metal
Mo powder could be explained that the MoO3 nanofiber was
restricted in a free space, with no other MoO3 nanofiber
contact with each other, although orthorhombic crystal struc-
ture of MoO3 changed as FCC crystal structure of metal Mo,
no other metal Mo particles to combine with, so the nanofi-
bers-like shape was retained.
FIG. 3. XRD patterns of MoO3 nanofibers reduced at different temperatures.
TEM imaging study were prepared by suspending solid samples
in acetone.
RESULTS AND DISCUSSION
The morphology investigations were characterized with SEM
and HRTEM. A SEM image of a typical product (Figure 1a)
showed the sample as an entirely fiber-like structure with a
width of about 50 to 200 nm, and a length of about a few tens
microns. Nearly rectangle-like cross section of the nanofibers
could be seen from the images. HRTEM image of an individual
nanofiber (Figures 1b–d) provided further insight into the struc-
ture of these products. HRTEM images recorded perpendicular
to the growth axis of the single nanofibers could be attributed
to the [010] of orthorhombic MoO3, and suggested that the nano-
fibers grew along the [001] direction.
According to the reason mentioned above, we using small
bulk (about 2–3 mm) MoO3 nanofiber as raw materials
instead of the MoO3 nanofibers powders. The small bulk
MoO3 was reduced at 6008C for 1 h, XRD indicated that
MoO3 had been completely changed as metal Mo powder.
SEM results are shown in Figure 5. Much more needle-like
metal Mo powders were formed. It means nanofiber-like
MoO3 could be reduced as needle-like metal Mo powder in
some special cases.
X-ray diffraction showed that samples consisted only of
strongly oriented orthorhombic a-MoO3 (a ¼ 1.385 nm,
b ¼ 0.3696 nm, c ¼ 0.3966 nm, JCPDS 89–7112). It can be
FIG. 4. SEM of Mo powder reduced from MoO3 nanofibers: (a) 5000X; and (b) amplifying (a).

MOLYBDENUM PARTICLES
321
original fiber-like morphology is not well-retained, but in
some special cases, fiber-like morphology can be partial
retained, although the aspect ratio of the oxide template is
reduced upon the reduction treatment.
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