Small molecule-controlled spontaneous growth of rose-like Se crystals at room temperature

Article abstract of DOI:10.1002/ejic.200701091

The spontaneous growth of rose-like Se crystals in aqueous solutions at room temperature is reported. The formation of rose-like Se crystals is based on the oxidation of Na₂Se in the presence of thioglycerol solution at pH = 11 in a dark ambient atmosphere. In alkaline solutions, the growth evolution of rose-like Se crystals with aging time was followed by scanning electron microscopy (SEM), and an interesting formation process from initial Se monomers to amorphous Se (a-Se) spheres, and to the final rose-like complex structures of Se crystals was observed. Seven kinds of small molecules with different structures, including 1-thioglycerol (TG), mercaptamine (MA), L-cysteine (L-cys), 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA), glycerol (GLY), and Lserine (L-ser), were used to manipulate the growth of Se crystals. The experimental results show that the structures of the small molecules play a key role in the growth of the Se crystals. The presence of thiols in the structure of the small molecules is favorable for the formation of the aggregates of Se crystals, and other termini, such as -NH₂, -OH, or -COO-, will determine whether the aggregates of Se crystals are made up of Se slices or Se prisms. These observations suggest that the ligand molecules have a crucial effect on the nucleation, monomers, and growth of nanocrystals. The selection of ligands can be extended to other important materials for further preparation of nanocrystals with desired shapes. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701091

FULL PAPER
DOI: 10.1002/ejic.200701091
Small Molecule-Controlled Spontaneous Growth of Rose-Like Se Crystals at
Room Temperature
Da-Wei Deng,[a,b] Jun-Sheng Yu,*[a,b] and Yi Pan[a]
Keywords: Selenium / Rose-like / Small molecules / Spontaneous growth / Crystal growth
The spontaneous growth of rose-like Se crystals in aqueous
solutions at room temperature is reported. The formation of
rose-like Se crystals is based on the oxidation of Na2Se in the
presence of thioglycerol solution at pH = 11 in a dark ambi-
ent atmosphere. In alkaline solutions, the growth evolution
of rose-like Se crystals with aging time was followed by scan-
tals. The experimental results show that the structures of the
small molecules play a key role in the growth of the Se crys-
tals. The presence of thiols in the structure of the small mole-
cules is favorable for the formation of the aggregates of Se
crystals, and other termini, such as –NH2, –OH, or –COO–,
will determine whether the aggregates of Se crystals are
ning electron microscopy (SEM), and an interesting forma- made up of Se slices or Se prisms. These observations sug-
tion process from initial Se monomers to amorphous Se (a-
Se) spheres, and to the final rose-like complex structures of
Se crystals was observed. Seven kinds of small molecules
with different structures, including 1-thioglycerol (TG), mer-
gest that the ligand molecules have a crucial effect on the
nucleation, monomers, and growth of nanocrystals. The se-
lection of ligands can be extended to other important materi-
als for further preparation of nanocrystals with desired
shapes.
captamine (MA),
acid (MPA), thioglycolic acid (TGA), glycerol (GLY), and
serine ( -ser), were used to manipulate the growth of Se crys-
L-cysteine (L-cys), 3-mercaptopropionic
L-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
L
structures (e.g. bipods and tripods). The chemical nature
and structure of the ligand molecules have been also consid-
ered as important factors for manipulating the growth of
nanocrystals.[7] To date, various shapes of semiconductor
nanocrystals including wires,[8] rods,[9] and multipods[10]
have been obtained by controlling the rate of growth of dif-
ferent faces of crystals in solution. However, under ambient
conditions of room temperature, can one obtain the desired
shape of nanocrystals only by depending on spontaneous
growth and control through small molecules?
Selenium has promising photoelectrical and semiconduc-
tor properties and it has been successfully used in electrical
rectifiers, xerography, and solar cells.[11] Nano- and micro-
scale Se crystals with highly complex structures may be de-
sired functional materials with new or improved photocon-
ductivity, nonlinear optical response, thermoelectric or
piezoelectric responses, and catalytic activity compared to
bulk selenium. Recently, a great number of solution-based
chemical synthetic strategies have been reported for the
preparation of Se 1-D nanostructures, such as nanowires,[12]
nanorods,[13] nanotubes,[14] and nanobelts.[15] The successful
synthesis of these selenium nanomaterials was attributed to
the unique helical conformation of the selenium crystal
structure which provided a natural template to guide the
growth of the one-dimensional shape.[11] In the above cases,
most of the previous strategies were carried out with the
assistance of structure-directing reagents or externally
added forces such as ultrasonic, microwave, and heat-
Introduction
The preparation of shape-controlled inorganic nanocrys-
tals and their assembly into complex patterns have attracted
considerable attention because the geometrical uniqueness
of nanocrystals may translate into similarly unique proper-
ties.[1] Among several growth techniques, the solution-phase
chemical methods have distinct advantages in the cost,
throughput, and potential for large-scale production. How-
ever, the difficulty of chemical reactions in isotropic solu-
tion is how to effectively control the rates of nucleation and
growth of nanocrystals directed toward a controlled
shape.[2] The solution-phase chemical synthesis of shape-
controlled nanocrystals can normally be accomplished
through the design of special experimental conditions, such
as using organic solvents at high temperature,[3] using tem-
plate molecules,[4] and with ultrasonic, microwave, or heat-
ing[5] in the aqueous phase. Under these conditions, the in-
trinsic anisotropic properties of nanocrystals generally play
a key role in the growth of the desired shapes,[6] for example
nanocrystals with one-dimensional (1D) and branched
[a] School of Chemistry and Chemical Engineering, Nanjing Uni-
versity,
Nanjing, 210093, P. R. China
Fax: +86-25-83594957
E-Mail: jsyu@nju.edu.cn
[b] Key Laboratory of Analytical Chemistry for Life Science,
Nanjing University,
Nanjing, 210093, P. R. China
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ing.[12–15] However, room-temperature spontaneous growth
of Se crystals has been insufficiently studied and small
molecule-controlled spontaneous growth of Se crystals into
unusual shapes in aqueous solution at room temperature
has not yet been reported.
Herein, we present the first example of Se crystals with
rose- and orchid-like shapes synthesized in aqueous solu-
tions at room temperature. Since trigonal Se (t-Se) has a
strong tendency to grow into one-dimensional structures
due to its unique natural template property which defines
and guides growth along the c-axis of the hexagonal lat-
tice,[11] the formation of Se crystals with rose- and orchid-
like shapes in aqueous solution is quite unexpected. The
growth evolution of rose-like Se crystals with aging time
was followed by scanning electron microscopy (SEM), and
an interesting formation process for the rose-like Se crystals
was observed. To explore the formation mechanism of Se
roses, seven kinds of small molecules with different struc-
tures were used to manipulate the spontaneous growth of
these Se crystals, including 1-thioglycerol (TG), mercapt-
amine (MA), -cysteine (-cys), 3-mercaptopropionic acid
(MPA), thioglycolic acid (TGA), glycerol (GLY), and -ser-
ine (-ser). We discuss the role of these small molecules dur-
ing the formation of Se crystals with unusual shapes.
Results and Discussion
Figure 1. A–D: SEM images of rose-like microstructured Se crys-
tals obtained in the presence of TG (6 m) at room temperature.
E: EDX spectrum of rose-like Se crystals.
The Spontaneous Growth of Rose-Like Se Crystals
In a typical synthesis of rose-like Se crystals, NaOH solu-
tion containing thioglycerol was used to absorb H2Se gas
(pH = 11). H2Se gas was generated by the reaction of the
ethanol solution of NaHSe with dilute H2SO4 added (see
the Experimental Section). The concentrations of Se atoms
and thioglycerol in solution are 2 m and 6 m, respec-
tively. The obtained solution of Na2Se was then stored in
the dark at room temperature for ca. 10 h. During storage,
the solution would change gradually from nearly colorless
to brownish-red within ca. 1 h and at the same time the
solution became gradually slightly turbid. Subsequently, the
color of the solution became pale, and ca. 3 h later, the
precipitates started to appear at the bottom of the bottle.
After ca. 10 h storage, a large quantity of dark precipitates
of Se was obtained. At this time, the upper solution turned
colorless and clear. Then, the dark Se precipitates were di-
rectly transferred onto a Si substrate to assess their mor-
phology and composition by means of SEM. The SEM
images of the dark Se precipitate are shown in Figure 1.
The SEM images (Figure 1, A–D) show mainly an interest-
ing rose-like Se crystal morphology. The typical size of the
final rose-like complex structures of Se crystals is ca. 10–
Figure 2. A typical X-ray diffraction pattern of an as-prepared
rose-like complex structure.
The Evolution of Rose-Like Se Crystals with Aging Time
As noted above, the rose-like shape of the Se crystals
15 µm in diameter, and the typical thickness of leaves is obtained is very unique and unexpected. We used SEM to
ca. 400 nm. The energy-dispersed X-ray spectrum (EDX) follow further the growth processes of the Se complex struc-
confirms that the products are highly pure Se (Figure 1, E). ture. Five samples were collected at different intermediate
The XRD pattern in Figure 2 shows that the obtained reaction stages. Their SEM images are shown in Figure 3,
products were well-crystallized hexagonal Se (JCPDS: 06- which presents the time-dependent shape evolution of the
0362). All the diffraction peaks can be readily indexed to rose-like complex structures. In the early stages of aging
the hexagonal Se.
(0–40 min), Se2– anions are oxidized to Se monomers by
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Controlled Spontaneous Growth of Se Crystals
dissolved oxygen (the standard redox potentials for Se2–/Se and will twist continuously around the surfaces of the as-
and O2/OH– pairs are –0.924 V and 0.401 V, respec- formed twisted Se and form a bud-like shape. At this stage
tively[16]). As presented in the SEM images in parts A and of growth, a small quantity of precipitates would appear
B of Figure 3, a portion of the Se monomers aggregate to- gradually at the bottom of the bottle. Here, it should be
gether into amorphous Se (a-Se) spheres, whereas another mentioned that in the present synthetic system (pH Ն 8.5),
portion might crystallize spontaneously into bunches of tri- amorphous selenium colloids could transform spontane-
gonal Se slices due to its 1-D anisotropic growth nature. At ously into trigonal Se crystals because a-Se has a higher
this time, the dispersion is a complicated mixture of free energy (5–10 kJ/mol) relative to trigonal selenium.[11]
amorphous spheres and bunches of Se slices. It was also As observed in the SEM images in Figure 3 (A–F), the
observed that the initially nearly colorless solution turns quantity of amorphous Se spheres decreases obviously with
brownish-red and becomes slightly turbid. When aging pro- the growth of rose-like t-Se crystals. With the aging time
ceeded to 1 h, the formed bunches of Se slices start to twist increased to 5 h, the rose-like morphology of Se can be
spontaneously and form relatively incompact slice-twisting seen, and the surfaces of the rose-like Se have a large
Se (Figure 3, C and D) with an average diameter of ca. amount of Se silks (Figure 3, G and H) with the average
4 µm. These twist structures gradually become more com- diameter of the rose-like Se increasing to ca. 12 µm. When
pact as the twisting proceeds. Subsequently (after about aging time is close to 7 h, Se rose-like complex structures
3 h), as shown in Figure 3 (E and F), the newly formed have formed by and large (Figure 3, I and J), and only a
slices are rice-leaf like (their typical size is ca. 2–3 µm in small amount of Se silks exists on the surfaces. The average
length, ca. 200 nm in width, and ca. 50 nm in thickness), diameter of the rose-like Se is now ca. 15 µm. At this time,
the quantity of precipitates that have formed at the bottom
of the bottle increases greatly. As a result, the color of the
upper solution turns pale obviously. Finally, after ca. 10 h
storage, the upper solution becomes colorless and clear, and
a large quantity of dark precipitates is obtained at the bot-
tom of the bottle. As shown in the SEM images in Figure 1
(A–D), the dark precipitates obtained finally are uniform
rose-like Se crystals, and their surfaces are relatively
smooth. The size of the selenium microstructures has
reached a plateau in growth due to the depletion of the Se
component in solution.
The Influence of Small Molecules on the Morphology of Se
Crystals
The interesting rose-like shape of the Se crystals inspired
us to consider factors that would effect the growth of the
Se crystals. Therefore, we examined further the oxidation of
Na2Se in the absence of any small molecules in the alkaline
solution at pH = 11. As shown in the SEM image in Fig-
ure 4 (A), when the NaOH solution containing no organic
small molecules was used as the absorption solution for the
H2Se gas (see the Exp. Sect.), the obtained products were
Se rods whose typical size was ca. 1–3 µm in length and ca.
1 µm in diameter. These Se products are very different in
morphology from the rose-like Se crystals obtained in the
Figure 3. SEM images of the evolution of a rose-like Se crystal with
aging time in the presence of TG (6 m) at room temperature; A,B:
0.5 h, C,D: 1 h, E,F: 3 h, G,H: 5 h, I, J: 7 h. Inset of panel B: mag-
nified SEM image of amorphous Se particles.
Figure 4. The typical SEM images of Se products obtained by the
oxidation of Na2Se in the absence of any organic small molecules
(A) and in the presence of glycerol (6 m) (B).
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presence of thioglycerol. This observation suggests that TG
plays an important role in controlling the spontaneous
growth of the Se complex structures. In order to further
understand the role of TG, another small molecule, glycerol
was used as a substitute for TG because the molecular
structure of glycerol is similar to that of TG except for the –
SH terminus. We found that the morphologies (Figure 4, B)
of the products obtained in the presence of GLY are very
similar to those obtained by the oxidation of Na2Se in the
absence of any organic small molecules (Figure 4, A), and
no rose-like Se crystals were observed. These experimental
results indicate strongly that the –SH terminus in the TG
molecule has an important role in the formation of rose-
like Se complex structures.
The results mentioned above encouraged us to further
investigate the role of small molecules in the growth of Se
crystals. Four thiols (MA, HSCH2CH2NH2; -cys,
HSCH2CH(NH2)COOH; MPA, HSCH2CH2COOH; and
TGA, HSCH2COOH) with various functional termini were
used. Parts A and B of Figure 5 show that when the NaOH
solution used for the absorption of the H2Se gas contains
MA, the major products are orchid-like Se crystals, besides
a portion of rose-like Se crystals. The typical diameter of
orchid-like Se crystals is ca. 7–10 µm and the typical thick-
ness of their leaves is ca. 500 nm. However, when -cysteine
was used, the product was aggregates of Se prisms (Fig-
ure 5, C and D). The typical diameter of the aggregates of
Se prisms is ca. 5–10 µm; the typical size of the Se prisms
is ca. 2–4 µm in length and ca. 2 µm in diameter. And if
MPA and TGA were used, respectively, the products (Fig-
ure 5, E and F) are similar to those obtained in the presence
of -cysteine; only the diameter of the aggregates of Se
prisms is relatively small (their typical diameter is ca. 3 µm).
XRD patterns of Se products obtained in the presence of
MA, -cys, or MPA are shown in Figure 6, which indicates
clearly that these as-prepared products are all pure single-
crystalline trigonal Se. All the diffraction peaks in Figure 6
can be readily indexed to the hexagonal Se. Furthermore,
when -cysteine was substituted by -serineϪthe molecular
structures of which are similar except for a –SH ter-
minusϪthe morphology of the products obtained was also
Se rods (not shown), similar to that (Figure 4, A) obtained
by the oxidation of Na2Se in the absence of any organic
small molecules, and no aggregates of Se prisms were ob-
served, which confirms further the role of the –SH ter-
minus.
Figure 5. The typical SEM images of Se products obtained in the
presence of different small molecules (6 m); A,B: MA, C, D: -
cys, E: MPA, F: TGA.
Figure 6. Normalized XRD patterns of Se products obtained by
the oxidation of Na2Se in the presence of various small molecules
(6 m); A: MA, B: -cys, C: MPA, according to the intensity of
the (110) peak.
say, the aggregates of Se crystals are made up of Se slices;
whereas thiols containing the –COO– terminus control the
formation of aggregates of Se crystals made up of Se
prisms.
These experimental results show that in the present syn-
thetic system, the structural differences of small molecules
have a vital effect on the growth of Se crystals. In the ab-
sence of any organic small molecules, the products obtained
from the oxidation of Na2Se are Se rods, which indicates
that the growth process of Se crystals is spontaneously pro-
ceeding in aqueous solution at pH = 11. In the presence
Conclusions
In summary, Se crystals with complex structures such as
of small molecules, the spontaneous growth process of Se rose- and orchid-like shapes, and prism aggregates have
crystals was tuned selectively. Molecular structures contain- been successfully synthesized in aqueous solutions at pH 11
ing the –SH terminus promote the aggregation of Se crys- at room temperature. The interesting growth evolution of
tals; those including –SH and –NH2 or –OH termini favor a rose-like t-Se crystal with aging times was observed. In
the formation of flower-type complex structures, that is to addition, seven kinds of small molecules, including five
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Controlled Spontaneous Growth of Se Crystals
kinds of thiols, were used to explore the formation mecha-
nism of Se rose-like complex structures. According to these
experimental observations, several points should be ad-
dressed. (1) The growth process of Se crystals spontane-
ously proceeds under natural conditions of room tempera-
ture. (2) The small molecules might selectively manipulate
the spontaneous nucleation and growth process of Se crys-
tals at room temperature. It can be further concluded from
the study that the role of small molecules of thiols in the
growth of Se crystals is crucial in that: (i) the –SH terminus
dominates the formation of the aggregates of Se crystals;
(ii) other termini, such as –NH2, –OH, or –COO–, will de-
termine whether the aggregates of Se crystals are made up
of Se slices or Se prisms. The formation mechanism of these
Se crystals may help us understand ligand effects on nucle-
ation, monomers, and growth of nanocrystals. The selection
of ligands can be extended to other important materials for
further preparation of nanocrystals with desired shape.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (20275016), the National Science Fund for Creative
Research Groups (20521503), and the Scientific Research Founda-
tion for the Returned Overseas Chinese Scholars.
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Experimental Section
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Preparation of Se Crystals: The synthesis of Se crystals was carried
out following similar procedures to those developed in our recent
report.[17] Firstly, under N2, Se powder (8 mg) was reduced by ex-
cessive NaBH4 (8 mg) in ethanol at 40 °C, to form a colorless solu-
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precipitates were collected by centrifugation, washed several times
with distilled water to completely remove the adhered small organic
molecules, and then allowed to dry in air at room temperature. In
this study, the seven kinds of small molecules used include TG,
MA, -cys, MPA, TGA, GLY, and -ser. The concentrations of Se
atoms and organic small molecules in solution are 2 m and 6 m,
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Characterization: The scanning electron microscope images and en-
ergy dispersive X-ray spectra were obtained with a Philips FEI
Quanta 200 electron microscope. The specimens for SEM measure-
ments were prepared by depositing a drop of the dilute aqueous
suspension of the sample on a Si substrate, and drying in air at
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Received: October 10, 2007
Published Online: January 7, 2008
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