Synthesis of nanocrystalline cobalt selenide in nonaqueous solvent

Article abstract of DOI:10.1006/jssc.2000.8727

Cobalt selenide nanocrystals have been synthesized by a solvothermal reaction between metallic cobalt and selenium at 140°C. The products obtained have been characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). According to the XRD analysis, the as-produced cobalt selenide adopted a NiAs structure analogous to that of Ni_0.85As. Therefore, it was termed Co_0.85Se. Elemental analysis showed a cation-deficient composition of about Co_0.844Se. TEM observation indicated that the obtained Co_0.85Se nanocrystals were of foliate morphology. IR analysis suggested that the product was free from contaminants such as ethylenediamine. The influence of various solvents on this synthetic process has been discussed. Ni_0.85As has also been synthesized with a similar synthetic process. (C) 2000 Academic Press.

Full text of DOI:10.1006/jssc.2000.8727

Journal of Solid State Chemistry 152, 537}539 (2000)
doi:10.1006/jssc.2000.8727, available online at http://www.idealibrary.com on
Synthesis of Nanocrystalline Cobalt Selenide in Nonaqueous Solvent
J. H. Zhan,ꢀ X. G. Yang, S. D. Li, Y. Xie, W. C. Yu, and Y. T. Qianꢀ
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People+s Republic of China
Received December 27, 1999; in revised form March 14, 2000; accepted March 27, 2000
containing cobalt cations (6), or molecular precursors (7).
Cobalt selenide nanocrystals have been synthesized by a sol- Either the composition fails to be controlled or only
vothermal reaction between metallic cobalt and selenium at amorphous products are obtained. The electronic structure
1
403C. The products obtained have been characterized by X-ray
powder di4raction (XRD) and transmission electron microscopy
TEM). According to the XRD analysis, the as-produced cobalt
and magnetic and optical properties of cobalt selenide have
been reported (8). In this work, we apply this solvothermal
method, analogous to the method we reported previously
(
selenide adopted a NiAs structure analogous to that of Ni0.85As.
Therefore, it was termed Co0.85Se. Elemental analysis showed
a cation-de5cient composition of about Co0.844Se. TEM observa-
tion indicated that the obtained Co0.85Se nanocrystals were of
foliate morphology. IR analysis suggested that the product was
free from contaminants such as ethylenediamine. The in6uence
(9), to the synthesis of cobalt selenide. The elemental reac-
tion between metallic cobalt and selenium results in the
formation of monophase cobalt selenide nanocrystals in
nonaqueous solvents. The structure and composition of the
product will be discussed. The in#uence of the solvents has
of various solvents on this synthetic process has been discussed. been also studied.
Ni0.85As has also been synthesized with a similar synthetic pro-
cess. ( 2000 Academic Press
2
. EXPERIMENTAL PROCEDURES
Metallic cobalt powder (2 mmol) and equivalent metallic
selenium powder (99.99%) were ground together in an agate
roller. The mixed precursors were added to a Te#on-lined
autoclave of capacity 40 ml, which was then "lled with
anhydrous ethylenediamine up to 90% of the total volume.
The autoclave was maintained constantly at 1403C for 12 h
and then cooled to room temperature naturally. A black
product was collected, "ltered, washed with absolute
ethanol three times, and "nally dried in a vacuum box at
1
. INTRODUCTION
Inorganic solids are generally synthesized by repeated
grinding and "ring at high temperature. High reaction tem-
perature is indispensable to overcoming the long di!usion
path in solid reactions. It is di$cult to control particle size
or phase homogeneity at these temperatures. Recently, there
has been a marked trend to lower reaction temperatures to
prepare technologically important materials with tailored
properties (1) Much attention has been focused on lower-
temperature approaches such as the sol}gel method (2) and
soft chemistry including cationic exchange, dehydration,
dehydroxylation, hydrolysis, redox, intercalation, and dein-
tercalation (3). Intermediate-temperature synthetic tech-
niques, including #ux and hydrothermal methods, have also
been developed to prepare novel materials with unusual
electronic, optical, and catalytic properties. The solvother-
mal method, one of the most promising intermediate-tem-
perature synthetic techniques, has recently been extensively
applied to the synthesis and design of materials with novel
structures and properties (4).
6
03C for 4 h. The yield of the product was about 92%, based
on the molar weight of selenium.
The obtained samples were characterized by X-ray pow-
der di!raction (XRD). The XRD analysis was carried out
with a Japan Rigaku D/max-c rotation anode X-ray di!rac-
tometer, using Ni-"ltered CuKa radiation (j"1.5418 A
s
).
A scanning rate of 0.053/s was used to record the patterns in
the 2h range 203}803. Transmission electron microscopy
(TEM) was used to determine the morphology and particle
size of products. The images were taken with a Hitachi
H-800 transmission electron microscope, using an accelerat-
ing voltage of 200 kV. The infrared (IR) spectra were re-
corded on a Magana IR-750 FT spectrometer at room
temperature using KBr pressed pellets as self-supporting
wafers. Elemental analysis of the sample was carried out on
a Perkin}Elmer/6500 ICP-AES (inductive coupling plasma-
atomic emission spectroscopy) instrument. The obtained
Generally, cobalt selenide is synthesized by the common
solid-state reaction (5), selenization of aqueous solutions
ꢀ
To whom correspondence should be addressed: E-mail: jhzhan@mail.
ustc.edu.cn.
5
37
0
022-4596/00 $35.00
Copyright ( 2000 by Academic Press
All rights of reproduction in any form reserved.

5
38
ZHAN ET AL.
samples were also characterized by X-ray photoelectron
spectra (XPS). XPS measurements were performed on an
ESCLAB MKII with MgKa (hl"1253.6 eV) as the exciting
source. The binding energy obtained in the XPS analysis
was corrected for specimen charging by referencing the C1s
to 284.5 eV.
TABLE 1
X-Ray Di4raction Data of the As-Obtained Co0.85Se
(
s
k ؍ 1.54178 A)
D/A
s
I/I
hkl
ꢁ
2
2
.6912
.0241
100
74
59.7
23.4
23.4
22.1
10.2
101
102
110
103
112
202
004
3
. RESULTS AND DISCUSSION
1.8037
1.5316
1.4985
1.3443
1.3213
A typical XRD pattern of the as-obtained sample is
shown in Fig. 1. The XRD Bragg re#ection data are listed in
Table 1. All the peaks could be indexed as for a hexagonal
structure. No other impurities, such as metallic cobalt or
selenium, were detected in the XRD pattern. After re"ne-
ment, the cell constants are given as a"3.615(0) A
.283(2) A, c/a"1.460 (space group, P6 /mmc (194)). This ment in this NiAs structure. The cobalt and selenium are
structure is analogous to that of hexagonal Ni
s
, c" number, they do not have the same coordination environ-
5
s
ꢅ
Se with both six-coordinate. The selenium atoms have a hexagonal
NiAs structure (JCPDS 18-888). Therefore, the as-formed close-packed arrangement, and the cobalt atoms occupy the
ꢁ
ꢂꢃꢄ
product is termed Co Se. The broadening of peaks in the octahedral interstitial sites (O sites) with the tetrahedral sites
ꢁ
ꢂꢃꢄ
XRD pattern indicates the nanocrystalline nature of the (¹ and ¹ sites) empty. The selenium atoms have six
>
>
sample. The crystallite size is about 19 nm, as estimated cobalt neighbors arranged as a trigonal prism. It is a charac-
according to (101) re#ection by the Scherrer equation with teristic feature for this structure to show a variable composi-
a shape factor of 0.9 applied (10).
The morphology and particle size of the as-obtained is about Co
samples were characterized with TEM. The image shows troscopy (AES) measurements. This result was also con-
a larger particle size than that estimated with the Scherrer "rmed by the XPS analysis. These experimental results
equation. If discriminated carefully, these crystallites have showed that the obtained cobalt selenide is a little cation-
a foliate shape. This morphology is very common in crystal- de"cient.
tion. The product obtained with this solvothermal method
Se, as determined by atomic emission spec-
ꢁ
ꢂꢃꢆꢆ
lites with NiAs structure (11). On the other hand, it could
IR analysis of the as-formed products was carried out to
explaine the di!erence between the Scherrer estimation and investigate attachment of ethylenediamine to cobalt sele-
TEM observation for crystal size because of the lack of nide, as shown in Fig. 2. The absorption peaks at about
spherical symmetry. These foliate crystallites are also very 3431 and 1633 cm\ꢀ, which correspond to the O}H stretch-
inclined to stack on each other.
ing vibration [l(O}H)] and H}O}H bending vibration
The obtained cobalt selenide has a NiAs structure. Al- [d(OH )], respectively, could be due to absorption by H O
ꢇ
ꢇ
though the cobalt and selenium have the same coordination absorbed on these samples. In fact, absorption by water is
very common for powder samples with high surface area
which have been exposed to atmosphere. These results
FIG. 1. XRD pattern of the obtained Co
Se.
ꢂꢃꢄ
FIG. 2. IR spectrum of the obtained sample.
ꢁ

SYNTHESIS OF NANOCRYSTALLINE COBALT SELENIDE
539
suggest that the as-obtained products are free from selenide nanocrystals have a foliated morphology. This
ethylenediamine. morphology is generally observed in compounds with NiAs
The synthetic procedure has also been carried out at structure. In this synthetic process, various solvents have
temperatures ranging from 120 to 1803C. The reaction a considerable in#uence on the formation of cobalt selenide.
could proceed rapidly with the elevation of the reaction The coordinating ackylamine is favorable for the reaction.
temperature, but the experiments showed that the reaction Chelating ethylenediamine is especially favorable. By a sim-
temperature had no drastic in#uence on the composition of ilar synthetic approach, nickel selenide has also been pro-
the obtained products.
duced with a structure as reported in the literature
Se, JCPDS 18-888).
To investigate their in#uence on the reaction, we replaced (Ni
ethylenediamine with other solvents such as benzene,
triethylamine, and pyridine, keeping other reaction factors
constant. No obvious reaction was observed in the case of
benzene, for a large amount of cobalt remained in the
ꢁ
ꢂꢃꢄ
ACKNOWLEDGMENTS
Financial support from the National Natural Science Foundation of
product. It was necessary to prolong the reaction time with China through the National Outstanding Youth Fund and the Chinese
triethylamine or pyridine substituting for ethylenediamine. National Funds of Natural Science Research is gratefully acknowledged.
It has been reported that the low-temperature elemental
reaction of metals with chalcogens could be promoted in
coordinating alkylamine solvents, as these solvents could
activate the metal or chalcogen surface, e!ectively allowing
good contact between the elements (12}14). Accordingly, it
was reasonable that no obvious reaction occurred in ben-
zene. Ethylenediamine could promote the reaction faster
than triethylamine and pyridine since it is a bidentate
chelating agent and has stronger coordinating ability. Con-
sequently, ethylenediamine was selected as the synthetic
medium in this solvothermal process.
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(
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