A new chemical route to prepare nanocrystalline cobalt monoarsenide

Article abstract of DOI:10.1016/S0025-5408(99)00098-7

A low-temperature solvothermal reduction process to nanocrystalline CoAs is reported here. All the precursors are easily accessible. The formation and crystalline temperature of CoAs could be as low as 140°C. Powder X-ray diffraction (XRD) shows a single phase of CoAs. The average particle size observed from transmission electron micrography (TEM) is about 18 nm. Elemental analysis and X-ray photoelectron spectroscopy (XPS) were utilized to determine the chemical composition of the product.

Full text of DOI:10.1016/S0025-5408(99)00098-7

Materials Research Bulletin, Vol. 34, No. 7, pp. 1129–1133, 1999
Copyright © 1999 Elsevier Science Ltd
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025-5408/99/$–see front matter
PII S0025-5408(99)00098-7
A NEW CHEMICAL ROUTE TO PREPARE NANOCRYSTALLINE
COBALT MONOARSENIDE
C. Wang, X.F. Qian, X.M. Zhang, Y.D. Li, Y. Xie, and Y.T. Qian*
Department of Chemistry, University of Science and Technology of China,
Hefei, Anhui, 230026, P.R. China
(Refereed)
(Received June 22, 1998; Accepted September 3, 1998)
ABSTRACT
A low-temperature solvothermal reduction process to nanocrystalline CoAs is
reported here. All the precursors are easily accessible. The formation and
crystalline temperature of CoAs could be as low as 140°C. Powder X-ray
diffraction (XRD) shows a single phase of CoAs. The average particle size
observed from transmission electron micrography (TEM) is about 18 nm.
Elemental analysis and X-ray photoelectron spectroscopy (XPS) were utilized
to determine the chemical composition of the product. © 1999 Elsevier Science Ltd
KEYWORDS: A. nanostructures, B. chemical synthesis, C. X-ray diffraction
INTRODUCTION
It is known that pnictide-rich phases of cobalt phosphide and arsenide exhibit semiconducting
behavior [1–3], whereas the cobalt-rich phases are expected to be metallic [4]. Cobalt
monoarsenide has MnP-type crystal structure with orthorhombic space group Pmcn at room
temperature [5]. A continuous phase transition between the MnP and NiAs structure occurs
at 1248 Ϯ 20 K [6].
CoAs can be prepared by heating a stoichiometric mixture of its constituent powdered
elements above its melting point (Ͼ1000°C) [7]. Mixed phases of cobalt arsenides can be
obtained by mixing cobalt and arsenic and heating the mixture at 470°C for several weeks
[8]. More recently, Parkin et al. [9] used a solid-state metathesis (SSM) reaction to prepare
certain pnictides; CoAs was obtained in their study. Physical deposition methods [10,11]
*
To whom correspondence should be addressed.
1
129

1
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C. WANG et al.
Vol. 34, No. 7
FIG. 1
Powder X-ray diffraction pattern of nanocrystalline CoAs.
have been utilized to prepare CoAs and related phases, and thin films of CoAs can be
prepared from the metalorganic chemical vapor deposition (MOCVD) of single-source
precursor [12,13]. All of these methods, however, require either a high processing temper-
ature or complex toxic organometallic reagents.
Recently, solution phase reactions for the preparation of metal pnictides have been
developed, especially for the synthesis of III–V semiconductors [14,15]. Our group has
developed [16] a co-reduction method to prepare nanocrystalline InAs in a nonaqueous
solution. Here we report an extension of this co-reduction method, for preparing nanocrys-
talline CoAs in absolute ethanol solution at a temperature as low as 140°C.
EXPERIMENTAL
Ϫ3
In a typical reaction, a homogeneous solution was formed after dissolving about 5 ϫ 10
Ϫ3
mol of AsCl (Aldrich Chemical Co. Ltd.) and 8 ϫ 10 mol of CoCl ⅐6H O (Shanghai
3
2
2
Chemical Co. Ltd.) in absolute ethanol. The solution was kept in a Teflon-lined autoclave.
After addition of about 0.04 mol zinc powder (99.5% purity, Shanghai Co. Ltd.), the
autoclave was heated in an oven at 140°C and maintained at that temperature for 16 h, then
allowed to cool to room temperature. After washing the resulting product with distilled water,
dilute HCl (1 mol/L) overnight, and distilled water again, a black precipitate was collected.
Finally, the product was desiccated at 80°C under vacuum for several hours.
The X-ray powder diffraction (XRD) pattern was recorded on a Rigaku ␥A X-ray
diffractometer using Cu K␣ radiation (␭ ϭ 1.54178 Å). Transmission electron microscopy
(TEM) images were taken with a Hitachi H-800 transmission electron microscope. Elemental
analysis was carried by atomic absorption by using a Perkin-Elmer 1100B atomic absorption
spectrometer. X-ray photoelectron spectra were obtained on a VGESCALAB MKII X-ray
photoelectron spectrometer, using nonmonochromatized Mg K␣ X-ray as the excitation
source.

Vol. 34, No. 7
NANOCRYSTALLINE COBALT ARSENIDE
1131
FIG. 2
XPS spectra of CoAs.
RESULTS AND DISCUSSION
The final product was characterized by X-ray diffraction. The XRD pattern (Fig. 1) showed
a single phase of CoAs, since all peaks could be indexed as the orthorhombic phase (JCPDS
9
-94). The broadening of the peaks indicated that the as-prepared product was comprised of
very small particles. No other peaks due to impurities such as Zn and Co were observed. The
average particle size determined by the Scherrer equation is about 15 nm.
Figure 2 shows the XPS spectra of the sample. The two strong peaks at 777.90 and 44.35
eV correspond to the binding energy of Co (3d) and As (3d), respectively. The quantification
of peaks gives a ratio As:Co of 47.3:51.9, which is consistent with 47.8:51.7 determined by
elemental analysis and near to the chemical composition of CoAs.

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C. WANG et al.
Vol. 34, No. 7
FIG. 3
TEM micrograph of nanocrystalline CoAs.
Figure 3 is the TEM image of an as-prepared sample and shows CoAs particles consisting
of uniform spherical crystallites. The average size is about 18 nm, which is in agreement with
that calculated from the Scherrer equation.
In a solid-state metathesis reaction (SSM), with the use of anhydrous CoCl and Na As as
2
3
precursors, Parkin [9] described the formation of CoAs as either a reductive recombination
pathway or an ionic metathesis route. In our study, the formation of CoAs is believed to be
mainly a reductive recombination pathway. The possible process might be described as
shown in the following equations:
2
CH CH OH ϩ Zn 3 2H ϩ Zn(CH CH O)
2
(1)
(2)
(3)
(4)
3
2
3
2
CoCl ϩ AsCl ϩ 5H 3 Co ϩ As ϩ 5HCl
2
3
Co ϩ As 3 CoAs
Zn(OCH CH ) ϩ 2HCl 3 ZnCl ϩ 2CH CH OH
2
3
2
2
3
2
Absolute ethanol acts both as the solvent for CoCl ⅐H O and AsCl and as a reactant. It may
2
2
3
react with zinc powder to produce reducing agent atomic hydrogen at elevated temperature,
2
ϩ
3ϩ
as shown in eq. 1. The atomic hydrogen reduces the Co and As to cobalt and arsenic.
The newly produced elements were both dispersed in ethanol solution and had high
reactivities, which benefits their combination to form cobalt monoarsenide. Although abso-
lute ethanol itself is a reducing agent in some chemical reactions, with the absence of zinc
powder, no phase of cobalt monoarsenide could be obtained in this system. The crystalline
water of the cobalt chloride does not have a significant effect on the final product. It can also
react with zinc powder to produce a reductive agent. But an excess amount of water should
be avoided, since it would cause the hydrolysis of AsCl , which would affect the final
3
products.
Other influences were also investigated in the experiments. If the temperature was lower
than 140°C or the reaction time less than 6 h, the formation of CoAs was incomplete or could

Vol. 34, No. 7
NANOCRYSTALLINE COBALT ARSENIDE
1133
not even be initiated. With an increase in temperature from 140 to 190°C, the size of the
product tended to be larger. Varying the reaction times in the range of 12–24 h did not affect
the size and crystallinity of the CoAs. When ethanol was replaced by other organic solvents,
such as benzene, toluene, or pyridine, no CoAs formed.
CONCLUSIONS
A novel solvothermal reduction method to crystalline cobalt monoarsenide at 140°C was
developed. High-temperature treatment was unnecessary to obtain the product. Average
particle size of the product was about 18 nm. The reaction conditions of this method can
easily be controlled.
ACKNOWLEDGMENT
This work was supported by the Climbing Program Research–National Foundation project.
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