Selective synthesis and magnetic properties of ?±-MnSe and MnSe2 uniform microcrystals

Article abstract of DOI:10.1021/jp020635f

A facile and selective route based on a precipitate slow-release controlled process was developed to produce both high-quality MnSe₂ and ?±-MnSe uniform microcrystals in aqueous solution at low temperatures (100-180 ?°C). The magnetic properties of the two samples were investigated in detail. This synthetic method will have potential applications in material science, especially in fabricating diluted magnetic semiconductors. These uniform microcrystals might be useful in microdevices, seed crystals, and magnetic cells.

Full text of DOI:10.1021/jp020635f

J. Phys. Chem. B 2002, 106, 9261-9265
9261
Selective Synthesis and Magnetic Properties of r-MnSe and MnSe2 Uniform Microcrystals
Qing Peng,† Yajie Dong,† Zhaoxiang Deng,† Huizhong Kou,† Song Gao,‡ and Yadong Li*,†
Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China, and State Key Laboratory of
Rare Earth Materials Chemistry and Application, College of Chemistry & Molecular Engineering,
Peking UniVersity, Beijing 100871, P. R. China
ReceiVed: March 6, 2002
A facile and selective route based on a precipitate slow-release controlled process was developed to produce
both high-quality MnSe2 and R-MnSe uniform microcrystals in aqueous solution at low temperatures (100-
180 °C). The magnetic properties of the two samples were investigated in detail. This synthetic method will
have potential applications in material science, especially in fabricating diluted magnetic semiconductors.
These uniform microcrystals might be useful in microdevices, seed crystals, and magnetic cells.
Introduction
(N2H4‚H2O, 80%) were A. R. regents from the Beijing Chemical
Factory, China. They were used without any pretreatment.
To prepare R-MnSe and MnSe2 microcrystals, MnSO4 (1.51
g, 0.01 mol) was put into a Teflon-lined autoclave of 100 mL
capacity and dissolved in 60 mL of deionized water. A total of
0.86 g (0.005 mol) of Na2SeO3 and 10 mL of hydrazine hydrate
(N2H4‚H2O) were added into the autoclave. After stirring for 5
min, the autoclave was sealed and heated at 100 °C (for MnSe2)
or 180 °C (for R-MnSe) for about 24 h. The autoclave was
allowed to cool to room temperature naturally after heat
treatment. The system was transferred into 200 mL of citric
acid (1 M) solution and stirred until Mn(OH)2 precipitation
dissolved. The sample (black color for MnSe2 and dark brown
color for R-MnSe) was collected by filtration, washed with
deionized water and absolute ethanol, and then dried at 60 °C.
The yields of both of the two products are about 90% based on
the calculation of Na2SeO3.
The obtained samples were characterized on a Brucker D8-
advance X-ray powder diffractometer (XRD) with Cu KR
radiation (λ ) 1.5418 Å). Scanning electron microscopy (SEM)
and energy-dispersive X-ray spectroscopy (EDS) were taken on
a JEOL JSM-6301F scanning electron microscope operated at
20 kV. DC magnetic susceptibility measurements were made
on a Maglab 2000 superconducting quantum interference device
(SQUID) magnetometer.
Transition metal chalcogenides and dichalcogenides have
drawn considerable attention because of their important optical,
electrical, and transport properties.1,2 Their conductive behavior
ranges from semiconductor to metallic, and they evidence a wide
range of magnetic properties.3,4 MnSe and MnSe2 have attracted
great interest especially because of their critical magnetic
functions, for example, in the application of fabricating diluted
magnetic semiconductors (DMSs).5-7 With the introducing of
Mn2+, the host semiconductor will exhibit unique optical and
magnetic properties based on the exchange interaction of sp-d
band electrons.
Molecular beam epitaxy (MBE)8 or organometallic vapor-
phase epitaxy (OMVPE)9 method is often employed in the
traditional routine to synthesize R-MnSe and MnSe2, including
the introduction of Mn2+ into DMSs. Decker et al.10 have
synthesized R-MnSe by using high-purity Mn metal reacting
with elemental Se at 900 °C in a quartz tube for 20 h with the
existence of catalyst (I2). MnSe2 can also be obtained by using
stoichiometric Mn and Se at 550 °C through this method.11
(NH4)2Se or H2Se is often used to provide Se source in the
solution-phase synthesis of MnSe.11 These methods usually need
complex apparatus, severe reaction conditions, and toxic and
sensitive raw materials, which may restrict the wide development
of researches and applications.
In this paper, we designed a precipitate slow-release controlled
synthetic method in aqueous solution, which can conveniently
produce both high-quality MnSe2 and R-MnSe uniform micro-
crystals at low temperatures (100-180 °C). Na2SeO3, MnSO4,
and the reductant N2H4‚H2O are used as reactants in this
hydrothermal method. The high yields, simple reaction ap-
paratus, and low reaction temperature give this novel method a
good prospect in future applications.
Results and Discussion
MnSe crystallizes in three forms (R-, â-, and γ-MnSe). The
R-MnSe, which has the NaCl structure (Figure 1a), is the
thermodynamically stable phase.11 MnSe2 has a cubic pyrite
structure (Figure 1b), which has a close resemblance with NaCl
structure and contains discrete Se2 groups. The Mn atoms are
on octahedral sites with six nearest-neighbor Se atoms, whereas
the Se coordination is tetrahedral (consisting of one selenium
and three manganese atoms). If the Se2 groups are changed by
Se atoms in the middle of the edges, MnSe structure will be
obtained.
Figure 2 shows the XRD patterns obtained from the two as-
synthesized samples of MnSe2 and R-MnSe microcrystals, which
indicate that they are both pure-phase compounds. The strong
and narrow peaks show good crystallinity of them. The lattice
parameters of MnSe2 (Figure 2a) and R-MnSe (Figure 2b) were
Experimental Section
All chemicals in this work, such as manganese sulfate
(MnSO4), sodium selenite (Na2SeO3), and hydrazine hydrate
* To whom the correspondence should be addressed. E-mail: ydli@
tsinghua.edu.cn. Fax: 8610-62788765.
† Tsinghua University.
‡ Peking University.
10.1021/jp020635f CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/09/2002

9262 J. Phys. Chem. B, Vol. 106, No. 36, 2002
Peng et al.
surface of some microcrystals, these cubes have good crystal-
linity, which corresponds to the XRD results. The average edge
size of these cubes is about 30 µm (Figure 3c), for which was
calculated the average volume of about 27000 µm3. From the
staircase surface of some cubes, it seems that MnSe2 crystals
may grow on the basis of layer structures. The sample of
R-MnSe is an aggregation of uniform ball-like microcrystals
(Figure 3d-f), with average radius of about 3 µm. The
uniformity and similar dimension of them show a good growth
environment of R-MnSe crystals, which indicate the good
crystallinity of these spheres from the other aspect. Figure 4
shows the energy dispersion spectroscopy (EDS) measurements
on these two samples. The x-axis shows the energy of the
X-quantum photons, which can be used to identify the elements.
Different elements will have different energy of the X-quantum
photons. The y-axis shows the quantities of the X-quantum
photons, which can reflect the relative content of elements. The
characteristic peaks for both Se and Mn are observed (Mn, about
6 keV; Se, about 11 and 1.5 keV). The atomic ratios are
calculated to be 2:1 (MnSe2) and 1:1 (R-MnSe) by the
comparisons of relative areas under the peaks of Se and Mn.
Using soluble Na2SeO3 as the Se source to synthesize
selenides is a novel and effective method. Na2SeO3 will
2-
Figure 1. Crystal structures of (a) R-MnSe and (b) MnSe2.
dissociate itself and form SeO3 in aqueous solution, which
can be reduced to Se very quickly upon heating.12 Compared
to commercial Se powder, the newly produced Se (even Sen
molecules dissolved in solution before their aggregation) is much
more reactive, and it is easier to produce Se2- through
disproportionation under alkaline conditions.13 In our experi-
ments, the whole process (from SeO32- to Se2-) is found to be
completed in 2 h in the presence of N2H4 as reductant (pH ≈
11) at 100-180 °C.14 Nothing but unreacted Se and Mn(OH)2
had been obtained in our experiments if commercial Se powder
was used as raw material instead of Na2SeO3.
Before the heat treatment, Na2SeO3 will first react with Mn2+
and form MnSeO3 (Ksp ) 5.4 × 10-8) precipitate, which can
dissociate in water and form SeO32- slowly. The proposed steps
to form MnSe2 (at 100 °C) can be described as follows:
Na2SeO3 + Mn2+ f 2Na+ + MnSeO3(V)
(1)
MnSeO3 7slow release8 Mn2+ + SeO3
(2)
(3)
(4)
(5)
2-
SeO32- + N2H4 f Se + N2 + H2O + 2OH-
3Se + 6OH- f 2Se2- + SeO32- + 3H2O
Se2- + Se + Mn2+ f MnSe2 (V)
2-
The slow-released SeO3 will soon be reduced in step 3. The
in-situ produced ultrafine Se particles with high reactivity form
Se2- much more easily through the disproportionating reaction
(step 4). The Se2- formed in step 4 will combine with one Se
atom produced in step 3 to form Se2 (Se + Se2- f Se22-).
2-
This is just like the formation of polysulfide ions in aqueous
solution. The MnSe2 product is finally obtained through the
Figure 2. Powder X-ray diffraction patterns of (a) MnSe2 and (b)
R-MnSe.
2-
precipitation reaction of Se2 + Mn2+ f MnSe2 (step 5).
calculated as a ) 6.417 Å (JCPDS 73-1525) and a ) 5.462 Å
(JCPDS 11-0683), respectively.
Because the Ksp of MnSe2 is smaller than that of MnSeO3, step
5 might be the driving force of the whole set of reactions. The
SeO32- generated in step 3 will be reduced by hydrazine again
and is finally consumed completely to form the product because
of the excess of Mn2+. In the experiments, long reaction time
(24 h is selected) is needed to convert all MnSeO3 to MnSe2
and favor the growth of high-quality and large MnSe2 micro-
SEM images of the two samples, which can show information
on the quality of individual microcrystals, are given in Figure
3. The sample of MnSe2 (Figure 3a-c) is comprised of many
uniform cubic microcrystals with almost the same edge size.
Although desquamation phenomenon can be noticed on the

R-MnSe and MnSe2 Uniform Microcrystals
J. Phys. Chem. B, Vol. 106, No. 36, 2002 9263
Figure 3. SEM images of as-synthesized (a, b, c) MnSe2 and (d, e, f) R-MnSe microcrystals with different magnifications.
Figure 5. The evolution of the XRD patterns of the products with
different reaction times at 180 °C: (/) MnSe2; (#) R-MnSe.
was increased to 180 °C and maintained for another 24 h.
R-MnSe was found to be the final product. When MnSe2 was
isolated after the first reaction step at 100 °C, put into an
autoclave only containing deionized water, and heated at 180
°C for 24 h, no reaction happened. This indicates that the
reductant N2H4 and the excess Mn2+ in the system play an
important role in the formation of R-MnSe at higher tempera-
tures. The R-MnSe product may result from the transition of
MnSe2.
To verify the formation mechanism of R-MnSe, the product
was taken out after reaction for 3, 6, and 12 h at 180 °C and
characterized by XRD (after dissolving the impurities of
Mn(OH)2 and MnSeO3 by using excess ammonium citrate
solution (5 M)). Figure 5 shows the XRD patterns of the
products obtained at different stages. Only MnSe2 peaks are
found in the XRD pattern of the product after 3 h reaction. With
the elongation of reaction time, the peaks of R-MnSe become
stronger, along with the weakening of MnSe2 peaks, which
indicates the transition from MnSe2 to R-MnSe. R-MnSe has
become the main phase of the product after 12 h reaction. After
24 h, all MnSe2 has been consumed and a pure phase of R-MnSe
Figure 4. EDS analysis of MnSe2 and R-MnSe samples. Characteristic
peaks for both Se and Mn are observed. The molar ratios of Se and
Mn are shown in the inset. The peak of Au is observed because the
samples are coated by Au film for good electrical conductivity.
crystals. On the basis of our experimental results, step 2 may
play an important role in the formation of uniform MnSe2
microcrystals. The slow dissociation of MnSeO3 into Mn2+ and
2-
SeO3 makes the formation of the initial MnSe2 nuclei very
slow, which forms a good basis for the crystallization of MnSe2.
Well-crystallized uniform microcrystals will be produced after
a long period reaction.
In our experiments, MnSe2 is only found for syntheses below
about 120 °C. When the reaction temperature is raised to 140
°C, the product only contains R-MnSe. To get phase-pure
R-MnSe and improve the crystallinity of the product, 180 °C is
chosen for the synthesis of R-MnSe. To understand the reaction
process more clearly, additional experiments have been per-
formed. The autoclave was first put into a baking oven at 100
°C, just the same condition as MnSe2. After 24 h, temperature

9264 J. Phys. Chem. B, Vol. 106, No. 36, 2002
Peng et al.
respectively. Plots of øm and ømT vs T are shown in Figure 6.
The room-temperature ømT values (2.5 emu K mol-1 for
R-MnSe, 1.9 emu K mol-1 for MnSe2) are significantly lower
than what was expected for a noncoupling high-spin Mn2+ ion
(4.375 emu K mol-1 with g ) 2.0). In both cases, ømT decreases
with the decrease of the temperature and vanishes when T
approaches zero. The magnetic susceptibility obeys the Curie-
Weiss law in the temperature range 120-300 K for R-MnSe
and 50-300 K for MnSe2 giving the Weiss constants of -216.8
K for the former and -549.7 K for the latter. These features
are characteristic of a considerably strong antiferromagnetic
interaction between adjacent Mn2+ ions through Se2- bridges.
The øm-T curves (H ) 10 000 G) show a shallow maximum
at 120 K for R-MnSe and an apparent maximum at 45 K for
MnSe2, and much more marked maxima are observed in the
øm-T curves measured in 5000 G (Figure 5b). The presence of
the maxima indicates the occurrence of long-range antiferro-
magnetic order.15
Below TN, both compounds exhibit complicated magnetism.
The susceptibilities in 5000 G undergo an abrupt upturn and
display another maximum at liquid helium temperatures,
whereas the 10 000 G data do not show such maxima. This
magnetic behavior may suggest the occurrence of spin flop in
the antiferromagnetic state, which would account for the abrupt
increase in øm. This supposition could be confirmed by the
measurements of isothermal field dependence of the magnetiza-
tion as shown in Figure 6c. The magnetization at 1.8 K increases
linearly with the field reaching 0.23 Nâ for R-MnSe and 0.12
Nâ for MnSe2 at the highest accessible field of 7 T. These values
are much lower than the saturation value of 4 Nâ for high-spin
octahedral Mn2+ compounds, indicating an antiferromagnetic
behavior for both compounds. The magnetization value at 7 T
for MnSe2 is lower than that for R-MnSe, which may be due to
the stronger antiferromagnetic interaction in MnSe2 as implied
by the Weiss constants.
Conclusion
In summary, we have demonstrated a precipitate slow-release
controlled synthesis method, which could produce high-quality
R-MnSe and MnSe2 microcrystals conveniently on a large scale.
These microcrystals might have potential applications in mi-
crodevices, seed crystals, and magnetic cells among others. This
synthetic method surely could be applied in fabricating diluted
magnetic semiconductors (DMSs) and be carried out to syn-
thesize other high-quality selenides, especially those that are
difficult to obtain through conventional methods.
Figure 6. Magnetic susceptibility, øm, and ømT vs temperature T plots
for R-MnSe and MnSe2: (a) H ) 10 000 G; (b) H ) 5000 G; (c)
isothermal field dependence of the magnetization.
is produced (not shown in Figure 5). This transition can be
explained from the crystal structures of them. From both energy
and space-filling points of view, R-MnSe with the NaCl structure
(Se/Mn ) 1:1) is more stable than MnSe2 with the pyrite
structure (Se/Mn ) 2:1). The Se2 groups in the MnSe2 structure
are more reactive and the Se-Se bonds may be broken under
appropriate conditions (N2H4 and excess of Mn2+). The released
Se atoms will be converted to selenide in our reaction system.
On the basis of the experiments above, long reaction time is
also needed to convert MnSe2 to R-MnSe completely, and the
slow transition rate results in the good crystallinity of R-MnSe.
On the other hand, the rate of the disproportionating reaction
of Se (step 4) will be faster at 180 °C, which favors the
formation of Se2-. It is also possible that some of the R-MnSe
is formed directly through the reaction of Mn2+ + Se2- f MnSe
after step 4.
Acknowledgment. This work was supported by NSFC
(Grants 20025102, 50028201, 20151001), the Foundation for
the Author of National Excellent Doctoral Dissertation of P. R.
China, and the state key project of fundamental research for
nanomaterials and nanostructures.
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