Room-temperature conversion route to nanocrystalline mercury chalcogenides HgE (E = S,Se,Te)

Article abstract of DOI:10.1016/S0022-3697(98)00349-7

A convenient, simple route has been employed to synthesize the nanocrystalline HgE (E = S,Se,Te) in ethylenediamine at room temperature. Mercury oxide was used as mercury source and sulfur, selenium, tellerium, respectively, as chalcogenide sources. The obtained products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques. XRD patterns show that the products were hexagonal (wurtzite structure) HgS, cubic (zincblende structure) HgSe and HgTe. We also find that the ethylenediamine plays a crucial role in the conversion process.

Full text of DOI:10.1016/S0022-3697(98)00349-7

Journal of Physics and Chemistry of Solids 60 (1999) 965–968
Room-temperature conversion route to nanocrystalline mercury
chalcogenides HgE (E ˆ S,Se,Te)
*
Yadong Li , Yi Ding, Hongwei Liao, Yitai Qian
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei 230026,
People’s Republic of China
Received 7 September 1998; accepted 16 December 1998
Abstract
A convenient, simple route has been employed to synthesize the nanocrystalline HgE (E ˆ S,Se,Te) in ethylenediamine at
room temperature. Mercury oxide was used as mercury source and sulfur, selenium, tellerium, respectively, as chalcogenide
sources. The obtained products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy
(TEM), and X-ray photoelectron spectroscopy (XPS) techniques. XRD patterns show that the products were hexagonal
(wurtzite structure) HgS, cubic (zincblende structure) HgSe and HgTe. We also find that the ethylenediamine plays a crucial
role in the conversion process. ᭧ 1999 Elsevier Science Ltd. All rights reserved.
Keywords: A. Chalcogenides; A. Semiconductors; B. Chemical synthesis
1. Introduction
needed relatively lower temperature than solid-state reac-
tion and was expected to be a very mild route to form
metal chalcogenides, obtaining the vapor phase precusor
of organometallic compounds is very difficult. Kulifay has
described a preparative method of HgSe from HgO and Se
[8], which needed a two-step reaction with HgSeO3 as the
intermediate. Many scientists are still exploring new, safe,
simple route to II–VI semiconductors, with the goals of
lowering processing temperature, avoiding complex organo-
metallic reactions and toxic precursors, and searching for
milder preparation conditions.
It is known that mercury will react with sulfur at room
temperature [9]. However, elemental reaction at room
temperature to produce HgSe or HgTe is not reported.
Recently, a liquid ammonia route has been used to synthe-
size metal chalcogenides at room temperature [10]. But in
this method, very toxic metal mercury was employed and
the obtained HgS was amorphous, which needed post heat
treatment at 250ЊC–300ЊC.
There has been much interest in synthesis and physical
characterization of II–VI family nanoscale semiconductors.
Most studies in this field were focused on cadmium chalco-
genides and zinc chalcogenides [1–4]. However, owing to
the toxicity problem of mercuride, reports about mercury
chalcogenides are scarce.
Conventional methods for preparation of mercury chalco-
genides include solid-state reaction [5], reaction of metal
cation with hydrogen chalcogenides in aqueous solution
[6], molecular precursor methods [7]. Solid-state reaction
usually required elevated reaction temperature and the reac-
tion was not easily controllable, what is more, the products
from high temperature reaction were not nanosized. For
reaction of metal cation with H2E (E ˆ S,Se,Te) in aqueous
solution, although the processing temperature is low, the
hydrogen chalcogenides are gaseous (at standard tempera-
ture and pressure) and highly toxic. As for molecular precur-
sor methods, they usually involve the formation and thermal
decomposition process of a complicated organometallic
compound, using organometallic reagents and compounds
containing an R–E bond as precursors. Though this method
In this article, we report a convenient, room temperature
reaction to fabricate nanocrystalline mercury chalcogenides
of the type HgE (E ˆ S,Se,Te), with an emphasis on HgSe,
by conversion of HgO with chalcogene in ethylenediamine
(en). The ethylenediamine was found not only to act as
solvent but also to play an important role in producing the
* Corresponding author.
0022-3697/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0022-3697(98)00349-7

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Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 965–968
Fig. 1. X-ray diffraction (XRD) patterns of obtained samples; (a) HgS (b) HgSe (c) HgTe.
active mercury and chalcogene species in solution. In this
method, toxic mercury and complex organomercurial
reagents are successfully avoided.
12ЊC) for 6–16 h. Red (HgO ϩ S) and black (HgO ϩ
Se,Te) precipitates were collected and washed with distilled
water and absolute ethanol, respectively. After that, the
obtained samples were dried in vacuum at 80ЊC for 2 h.
The yields ranged from 95%–98%. X-ray powder diffrac-
tion (XRD) patterns were carried out on a Japan Rigaku
Dmax rA X-ray diffractionmeter with graphite monochro-
2. Experimental section
˚
matized CuKa radiation (l ˆ 1.5418 A). TEM was
All reagents were of high purity. Element S (SP,
99.999%), Se (AP, 99.95%), and Te (SP, 99.999%) with
0.005 mole HgO powder (AP,99.95%) in appropriate
proportions were mixed in ethylenediamine. We chose the
amount of elemental chalcogene in excess of 2%–8% to that
of the mercury oxide so as to enable a complete reaction of
mercury oxide. Each of the solutions was added into a coni-
cal flask, then kept stirring at room temperature (about
performed on a Hitachi Model H-800 transmission electron
microscope, using an accelerating voltage of 200 kV. X-ray
photoelectron spectroscopy (XPS) was carried out on a
VGESCALAB MKII X-ray photoelectron spectrometer,
using nonmonochromatized MgKa X-ray as the excitation
source and choosing C1s (284.60 eV) as the reference line.
3. Results and discussion
The XRD patterns of obtained HgE are shown in Fig. 1.
All the reflections in Fig. 1(a) can be indexed to be a pure
hexagonal (wurtzite structure) phase HgS with a lattice
parameter a ˆ 4.142 c ˆ 9.509, which is equal to the
reported data (JCPDS card, No. 6-256). However, the
diffraction peaks in Fig. 1(b) and (c) are recognized, respec-
tively, to be cubic (zincblende structure) phase HgSe and
HgTe. That the samples of HgS crystallites do not exhibit
the zincblende structure, we think, is because b-HgS is
more stable than a-HgS.
Elemental analyses by atomic absorption (Perkin-Elmer
1100B Atomic Absorption Spectrophotometer) show the
presence of Hg and chalcogene in a 1.02–1.08 : 1.00 ratio
which is generally consistent with the result of chemical
titration.
Fig. 2 shows the TEM image of the obtained HgSe
sample. Although the HgSe crystallites are not well
Fig. 2. TEM micrograph of nanocrystalline HgSe.

Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 965–968
967
reaction is most likely caused by the disproportionation of
chalcogen in alkali solutions, as hydroxy (OHϪ) ion may be
generated by a trace of water in the system.
HgO ϩ 2NH2CH2CH2NH2 ϩ H2O ! Hgꢀen†2ϩ ϩ 2OHϪ
2
3EꢀS; Se; Te† ϩ 6OHϪ ! 2E2Ϫ ϩ EO32Ϫ ϩ 3H2O
It seemed that only elemental sulfur could react in this
way while for selenium and tellerium it was very difficult.
We also did not detect the existence of SO32Ϫ using chemical
analysis. Lelieur [12] and Parkin [10] have investigated the
solubilization process and related reaction with metals of
sulfur in liquid ammonia. They pointed out in sulfur–ammo-
nia solutions there were various sulfur imido anions (S2NϪ,
S3NϪ, S4NϪ) as well as sulfur polyanions (S62Ϫ, S42Ϫ) and
zero oxidation sulfur species (SxNH3, x ˆ 1–8). One or
more of these species could be responsible for the redox
reactions. We also recalled that sulfur could dissolve in
organo-amine to produce colored N,N0-diamine-polysulfide
solution and hydrosulphuric acid [13]. So we considered
that the ethylenediamine might not only act as a coordinat-
ing alkali solution but also plays an important role in gener-
ating the active sulfur species, such as S2Ϫ
.
The conversion process could be described in the follow-
ing steps. We would see the important effect ethylenedia-
mine and a trace of water had on the whole reaction process.
The first step of the reaction is the formation of the mercuric
complex in ethylenediamine-solution in the presence of a
small amount of water. Ethylenediamine as a strong biden-
tate ligand could coordinate mercury in HgO into a relative
stable Hg2Ϫ complex (the stability constant log b2 ˆ 23.3).
Fig. 3. XPS spectra of nanocrystalline HgSe; (a) Close-up surveys
for Hg 4f core, (b) close-up surveys for Se 3d core.
dispersed, we can clearly see that the particles are of sphe-
rical morphology with the average size ranging from 15 to
20 nm, which is consistent with the XRD analysis using the
Scherrer equation [11].
HgO ϩ 2en ϩ H2O ! Hgꢀen†2ϩ ϩ 2OHϪ
2
At the same time, the disproportionation of chalcogen in
ethylenediamine might proceed as follows (taking sulfur
as an example):
Further evidence for the formation of HgSe can be
obtained from the X-ray photoelectron spectra (XPS) of
the product. Fig. 3(a) and (b) are high-resolution XPS spec-
tra taken for the Hg and Se regions of samples as prepared.
The two strong peaks in Fig. 3(a) at 99.95 and 103.90 eV
correspond to Hg (4f) binding energy. The peak at about
54.10 eV (Fig. 3(b)) is attributed to Se (3d). No obvious
peak for mercury oxide or elemental selenium is observed.
Peak area of the Hg and Se cores were measured and the
quantification of the peaks gave the ratio of Hg to Se as
1.08 : 1.
Though this reaction seems to be oversimplified, the sulfur
anion was thought to be produced at least in a similar
process.
The aforesaid two reactions would be promoted by the
second step – the nucleation of the mercury chalcogenides.
‰Hgꢀen† Š2ϩ ϩ 2OHϪ ϩ H2E Y HgE # ϩ2en ϩ 2H2O
2
In order to investigate the effect ethylenediamine has on
the reaction, we designed following related experiments.
Appropriate amounts of elemental chalcogen, mercury
The reaction of chalcogen-ethylenediamine solutions
with HgO is obvious of the redox type with the elemental
chalcogen being reduced to E2Ϫ. But what species is
oxidized in this reaction? We have proposed that the
oxide
(HgO)
powder,
and
mercury
nitrate
[Hg(NO3)2·0.5H2O] were dissolved in ethylenediamine,
and kept stirring at similar reaction conditions mentioned
earlier. We found that the S–en solution quickly changes to

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Y. Li et al. / Journal of Physics and Chemistry of Solids 60 (1999) 965–968
Fig. 4. UV-Vis absorption spectra of HgO–en and Hg(NO3)2–en
solutions; (a) HgO–en, (b) Hg(NO3)2–en.
Fig. 5. UV-Vis absorption spectra of S–en solutions.
dark-green and became colorless transparent after 6 h, while
the Se–en solution showed dark-red color at first, and then
brown transparent after 6 h, and golden transparent after 8 h.
Though the Te–en solution became greyish-white and
remained turbid even after 10 h, we were sure that ethyle-
nediamine did have an important effect in the nucleation of
the very stable HgTe. To enhance the understanding of the
reaction mechanism proposed before, we recorded absorp-
tion spectra of some of these solutions.
studies are needed to investigate the solubilization process
of the chalcogene in ethylenediamine. This conversion
method is very convenient to be carried out as it is less
toxic, low-temperature, fast and easily controlled. This
route is expected to be applicable to fabricate other semi-
conductor nanoparticles and nano-semiconductor films.
Ultraviolet absorption spectroscopy were collected with a
Shimadzu UV-visible Recording spectrophotometer (UV-
240) at room temperature. In Fig. 4, we see that the absorp-
tion spectra of HgO–en and Hg(NO3)2–en solutions almost
have the same shape – both having a sharp peak at 258 nm.
This should be the contribution of the mercuric complex,
[Hg(en)2]2ϩ. Fig. 5 is the absorption spectra of S–en solu-
tion after 10 h. The absorption band located at ca. 260 nm
might result from the depicted polysulfide imido species (*)
[12]. The absorption shoulder at ca. 300 nm refers to the
active sulfur species [12] which is related to the final forma-
tion of HgS.
Acknowledgements
We would like to acknowledge Prof. Zhou Guien and Ji
Mingrong for valuable assistance with the XRD and XPS
analysis. This work is supported by the National Natural
Science Foundation of China and the National Nanometer
Materials Climbing Program.
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195.
From this discussion on the formation of nanocrystalline
HgE, we could see that ethylenediamine had a crucial effect
almost in whole reaction process. We know that the solubi-
lity product constants (pKsp) of the HgO, HgS and HgSe are
about 25.4, 52.4 and 59.8, respectively, (the pKsp of HgTe
should also be very large) while the stability constant
(log b2) of [Hg(en)2]2ϩ is about 23.3. In ethylenediamine
solution, the conversion from HgO to [Hg(en)2]2ϩ would
take place, and instantly, the conversion from [Hg(en)2]2ϩ
to HgE took place in the presence of newly-produced E2Ϫ
,
(also yielded by ethylenediamine) as HgE was more stable.
4. Conclusion
In conclusion, an oxide direct-conversion route for the
formation of nanocrystalline HgS, HgSe and HgTe at
room temperature in ethylenediamine is presented. Solvent
ethylenediamine is found to have a very important role in
the electron-transfer process of the whole reaction. Further
[13] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 3,
John Wiley and Sons, Inc, 1972.