Novel room temperature method to nanocrystalline Ag2Se

Article abstract of DOI:10.1016/S0025-5408(99)00083-5

Nanocrystalline Ag₂Se was synthesized at room temperature through the reaction of AgNO₃, Se, and KBH₄ in pyridine. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-r

Full text of DOI:10.1016/S0025-5408(99)00083-5

Materials Research Bulletin, Vol. 34, No. 6, pp. 877–882, 1999
Copyright © 1999 Elsevier Science Ltd
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0025-5408/99/$–see front matter
PII S0025-5408(99)00083-5
A NOVEL ROOM TEMPERATURE METHOD TO NANOCRYSTALLINE Ag2Se
Wenzhong Wang1,2, Yan Geng2, Yitai Qian1,2*, Mingrong Ji1, and Yi Xie2
1Structure Research Laboratory
2Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, P.R. China
(Refereed)
(Received June 2, 1998; Accepted August 13, 1998)
ABSTRACT
Nanocrystalline Ag2Se was synthesized at room temperature through the
reaction of AgNO3, Se, and KBH4 in pyridine. The products were character-
ized by X-ray diffraction (XRD), transmission electron microscopy (TEM),
and X-ray photoelectron spectroscopy (XPS). TEM revealed that the as-
prepared Ag2Se grains were homogeneous and spherical, and their average
size was about 25 nm. XPS analysis showed that the purity was good.
Different solvent media were tested, and pyridine was found to be the most
suitable one. The possible mechanism of the formation of Ag2Se grains is
proposed. This novel mild method will be extended to prepare other nanoc-
rystalline chalcogenides at room temperature. © 1999 Elsevier Science Ltd
KEYWORDS: A. chalcogenides, A. nanostructures, B. chemical synthesis, C.
electron microscopy, C. photoelectron spectroscopy
INTRODUCTION
The preparation and characterization of selenides has attracted much attention because of the
special properties of these materials. Ag2Se is a well-known superionic conductor [1–3]. It
has been used as thermochromic material for nonlinear optical devices [4], photochargeable
secondary batteries [5], and multipurpose ion-selective electrodes [6]. Many methods have
been used to obtain Ag2Se, including mechanical alloying method using a high-energy ball
*To whom correspondence should be addressed.
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FIG. 1
XRD diffraction patterns of as-prepared samples: Ag2Se, Ag2Se ϩ Se (indexed by *), and
Ag2Se ϩ Ag (indexed by ϩ).
mill [7], microemulsions [8], electrolysis [9], and a hydrothermal route at elevated temper-
ature (Ͼ260°C) and pressure (100 atm) [10]. However, there are few reports that nanocrys-
talline Ag2Se has been synthesized.
Since properties are known to vary with size, it has become important to decrease the size
of materials to nanoscale. Most researchers are also seeking milder conditions for preparing
nanocrystalline materials. This paper reports a novel method to obtain nanocrystalline Ag2Se
at room temperature, with the reduction of KBH4. Compared with other methods, this method
is simpler and milder, and it can be extended to obtain other chalcogenides.
In past years, most studies of the chemistry of BHϪ4 reduction focused on preparing
amorphous metals [11], nanocrystalline magnetic materials [12,13], catalysts [14], borides
[15], and transition-metal oxides such as MoO2, VO2, NaxWO3, and KxMO3 [16–18]. In
these studies, boron was always incorporated into the solid products [15,19]. Most products
were amorphous or crystallized after post-treatment at high temperature (Ͼ350°C) in inert
atmosphere [15–18]. To our knowledge, this is the first time that reduction of KBH4 has been
used to prepare nanocrystalline selenide at room temperature in organic solvent.
EXPERIMENTAL
Appropriate amounts of Se, AgNO3, and KBH4 powders were put into a flask, and the flask
was filled with pyridine to 80% of its capacity. Next, the flask was sealed and the system was

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NANOCRYSTALLINE SILVER SELENIDE
879
FIG. 2
TEM image of nanocrystalline Ag2Se.
kept at room temperature (about 10°C) for 4 h. The black precipitate that formed was washed
with distilled water, and the final product was collected for characterization.
Powder X-ray diffraction (XRD) was performed on a Rigaku D/max ␥A rotation anode
X-ray diffractometer using Ni-filtered Cu K␣ radiation, to characterize the products. The
morphology and grain size were determined by transmission electron microscopy (TEM).
TEM images were taken with a Hitachi H-800 transmission electron microscope. The purity
and composition of the as-prepared nanocrystalline X-ray photoelectron spectra (XPS) were
recorded on an ESCALab MK2 using Al K␣ as the exciting source. The binding energies
obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s
to 284.50 kV.
RESULTS AND DISCUSSION
The XRD pattern of a typical sample (Fig. 1a) revealed that the as-prepared nanocrystalline
Ag2Se was orthorhombic and matched the literature data perfectly [20]. After refinement, the
cell parameters were a ϭ 0.43, b ϭ 0.71, and c ϭ 0.78 nm, which were also in accordance
with reported values [20]. The average crystallite size was 25 nm, which was determined by
the Scherrer equation based on the half-widths of XRD peaks. No impurities, such as Ag and
Se, were detected.
The TEM image of the grains of the as-prepared nanocrystalline Ag2Se is shown in Figure
2. From this microphoto, it is obvious that the grain morphology was homogeneous and
spherical; the grain sizes varied from 20 to 30 nm and the average size was about 25 nm.
XPS analysis results are shown in Figure 3. The two strong peaks, at 368.1 and 373.9 eV,
correspond to Ag 3d binding energy for Ag2Se. The strong peak at 53.6 eV corresponds to
Se 3d binding energy for Ag2Se. These results are close to that of bulk Ag2Se. No impurities,
such as Ag, Ag2O, Se, SeO2, or B, were detected; the product purity was good. From the XPS
analysis, Ag and Se contents were quantified by Ag 3d and Se 3d peak areas and the
composition of the product was determined to be Ag2.1Se.

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FIG. 3
XPS analysis of as-prepared nanocrystalline Ag2Se.
Different solvent media were tested to investigate their influence on the quality of the
nanocrystalline Ag2Se product. Pyridine is known to produce complex S(Py)2 with sulfur,
due to its base and N-chelation [21–23]. Possibly a similar complex Se(Py)2 is produced
when Se is put into pyridine, because many properties of Se are similar to those of S. This
was supported by the fact that when only selenium powder was added into pyridine, the
solution color slowly changed to brown and the selenium powder disappeared. This increased
the activity of the reactants and ensured the complete conversion of selenium and Ag to
Ag2Se, without high temperature; thus, this method differs greatly from solid-state reaction.
When other solvents were used, e.g., benzene, which have no chelation and cannot produce
a complex with selenium, a large amount of unreacted selenium remained in the product even
when the system temperature was elevated to 180°C for 3 days. (To ensure safety in these
experiments, a stainless steel autoclave with Teflon-liner was used instead of the flask,

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881
because the system pressure and temperature greatly increased.) The results are shown in
Figure 1b. Peaks indexed by an asterisk belong to Se. These experiments showed that
pyridine was the most suitable solvent.
That KBH4 was selected as the reductant was a key factor in Ag2Se being obtained at room
temperature in a short time. The existence of active HϪ in KBH4 activates the reaction
between Se and AgNO3 and makes the reaction happen easily at room temperature. Excessive
KBH4 was used to ensure that selenium and AgNO3 were completely converted. When KBH4
was substituted by other reductants, such as sodium, no obvious reactions happened under
identical conditions.
Experiments demonstrated that when stoichiometric AgNO3 was added to the system, only
Ag2Se was produced, as evidenced by the XRD pattern (Fig. 1a). When AgNO3 was
excessive, Ag peaks appeared in the XRD pattern and Ag and Ag2Se coexisted in the product.
In Figure 1c, the peaks indexed by the plus-sign belong to Ag. These results reveal that KBH4
might reduce AgNO3 to Ag first. The newly formed Ag grains dispersed in the solution; they
had a high surface area and were very active. They reacted with Se to produce nanocrystalline
Ag2Se at room temperature. To test this hypothesis, stoichiometric Ag powder was used
directly instead of AgNO3. The result was a large amount of Ag remained in the product and
little Ag2Se was produced. This may be due to the low activity of silver powder.
CONCLUSIONS
The reduction route discussed here has the advantage of obtaining metal selenides directly at
room temperature. XPS indicated that no boron was incorporated into the products when
KBH4 was used for the reduction. Furthermore, preparation of metal selenides through this
method does not require extreme precursors, such as the easily hydrolyzable Na2Se or toxic
H2Se.
ACKNOWLEDGMENTS
This work was supported by the Natural Science Foundation of China and the National
Nanometer Materials Climbing Project.
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