A safe low temperature route to nanocrystalline transition metal arsenides

Article abstract of DOI:10.1246/cl.2000.114

Nanocrystalline FeAs, CoAs and NiAs were prepared by keeping the mixture of FeCl₃, CoCl₂ or NiCl₂ with arsenic and KBH₄ in ethylenediamine (en) at 100 °C for 4 h, respectively. X-Ray powder diffraction (XRD) patterns and transmission electron microscope (TEM) images show that the products are orthorhombic FeAs, CoAs and hexagonal NiAs, respectively, and all are well crystalline in nanometers.

Full text of DOI:10.1246/cl.2000.114

114
Chemistry Letters 2000
A Safe Low Temperature Route to Nanocrystalline Transition Metal Arsenides
Yi Xie*, Jun Lu†, Ping Yan, Xuchuan Jiang, and Yitai Qian
Structure Research Laboratory and Department of Chemistry, University of Science and Technology
of China, Hefei, Anhui 230026, P. R. China
Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
(Received October 25, 1999; CL-990912)
Nanocrystalline FeAs, CoAs and NiAs were prepared by
the metal chlorides were purified by refluxing in thionyl chlo-
ride (SOCl2) for 45 min before used. FeAs was prepared as
follows : 40 ml of ethylenediamine, 0.8125 g (5 mmol) of
FeCl3 and 0.375 g (5 mmol) of arsenic were added to a 50 ml
flask, and then the mixture was ultrasonically mixed for 4 h
before putting 0.807 g (15 mmol) of KBH4 into the flask .The
mixture was transferred to a Teflon-lined autoclave at once. It
was maintained at 100 °C for 24 h, and then cooled to room
temperature naturally. The CoAs and NiAs were prepared
analogously to FeAs by using CoCl2 and NiCl2 with the molar
ratio MCl2 : As : KBH4 = 1 :1 : 2, (M = Co, Ni, respectively).
The products were filtered out, washed with distilled water
and absolute alcohol in sequence, and then dried in vacuum at
room temperature for 4h.
All the products have been characterized by X-ray diffrac-
tion (XRD) and transmission electron microscopy (TEM)
examinations.13 XRD patterns for FeAs, CoAs and NiAs are
shown in Figure 1, which are in agreement with literature.14
Average crystalline sizes as determined by Scherrer equation
based on the XRD linewidths are recorded in Table 1. TEM
images show nearly monodisperse plate morphology of the
keeping the mixture of FeCl3, CoCl2 or NiCl2 with arsenic and
KBH4 in ethylenediamine (en) at 100 °C for 4 h, respectively.
X-Ray powder diffraction (XRD) patterns and transmission
electron microscope (TEM) images show that the products are
orthorhombic FeAs, CoAs and hexagonal NiAs, respectively,
and all are well crystalline in nanometers.
Compound semiconductors, mainly metal pnictides, have a
wide variety of applications such as compact displayers,1 satel-
lite TV receivers2 and optical fiber.3 Compared with III-V
semiconductors, transition metal pnictides have many special
electrical, mechanical and anticorrodating properties.4 Among
them, iron arsenides have attracted scientists' attention for a
long time because of its technological importance as semicon-
ductors,5 secondary high-temperature batteries6 and catalysts.7
The most straightforward way to synthesize the transition
metal arsenides, as we know, is the direct combination of ele-
mental metal and arsenic at high temperature.4 Recently,
Parkin and co-workers synthesized a series of metal arsenides
by solid state metathesis (SSM) reactions with alkaline metal
pnictides and metal halides as precursors at a typical tempera-
ture of 500 °C. However, only a mixture of FeAs and Fe2As
was reported to be obtained by this method.8
In order to decrease the temperature and alleviate the time
and energy cost, some new approaches have been introduced to
prepare inorganic materials. For example, using a solution-liq-
uid-solid (SLS) mechanism, Trentler et al. obtained InP and
InAs at 111-201 °C and 203 °C respectively.9 However, the
very toxic gases, PH3 and AsH3 had to be used, in the process.
Solvothermal co-reductions (STCR) method was used to prepare
nanocrystalline FeAs10 at 150-180 °C, while another toxic
reagent, AsCl3 had to be used. The use of AsH3 and AsCl3 is of
particular concern due to their carcinogenic and mutagenic
properties. Considering the order of toxicity given by
Penrose,11 R3As(R = H, Me, Cl, etc.) > As2O3 (As(III)) >
(RAsO)n > As2O5 (As(V)) > RnAsO(OH)3-n (n =1, 2) > R4As+ >
As (0), we give a safe route in eq 1. to prepare the transition
metal arsenides such as FeAs, CoAs and NiAs at 100 °C. This
route has a further advantage that the reactants are readily
available.
Reduction of metal salts with hydroorganborates is very
useful to prepare finely divided powders of metals and alloys.12
Here we report potassium borohydride reducing route to pre-
pare MAs (M = Fe, Co and Ni) nanocrystalline.
In this approach, all reagents were of analytical grade, and
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
115
of Natural Science Research and Huo Yingdong Foundation
for Young Teachers is gratefully acknowledged.
References and Notes
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products. As examples, TEM images for FeAs and NiAs are
shown in Figure 2. Particle sizes determined by TEM are also
recorded in Table 1, which are in accordance with average size
estimated from XRD patterns.
In order to investigate the influence of different solvents,
we substituted ethylenediamine for benzene, heptane and CCl4,
respectively, under similar condition. We found that in ben-
zene, heptane and CCl4, no arsenides can be obtained.
Considering the difference in the coordinating ability of ethyl-
enediamine and benzene, heptane or CCl4, we proposed a pos-
sible formation mechanism as follows (taking the FeAs as an
example) :
7
8
9
The reaction mechanism can be rationalized and the inter-
mediate product is confirmed as elemental iron by the follow-
ing experiments: If the FeCl3, As, KBH4 and ethylenediamine
were added together into a flask, mixed by ultrasonic treatment
at room temperature, and then maintained at 100 °C for 24 h,
the product was not the metal arsenides, but the mixture of
metallic iron and arsenic powder, as confirmed by XRD
results. Since, at the room temperature, newly formed iron
atoms reduced with KBH4 cannot react with arsenic. However,
when the temperature is not lower than 100°C, the newly
formed iron atoms reduced with KBH4 are active enough to
combine with arsenic immediately to form corresponding
metal arsenides.
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13 XRD patterns were obtained on a Japan Rigaku D/Max γA
rotation anode X-ray diffractometer with Ni-filtered CuKα
radiation (λ=1.54178 Å). TEM measurements were made
on a Hitachi H-800 transmission electron microscope,
using an accelerating voltage of 200 KV.
14 Joint Committee on Powder Diffraction Standards
(JCPDS), File No.12-799 FeAs; 9-94 CoAs; 31-900 NiAs.
Financial support from the Chinese National Foundation