Syntheses and characterizations of bismuth nanofilms and nanorhombuses by the structure-controlling solventless method

Article abstract of DOI:10.1021/ic0615067

Substrate-free bismuth nanofilms with an average thickness of 0.6 nm (σ = ±14.1%) and monodisperse layered Bi nanorhombuses with an average edge length of 21.5 nm (σ = ±14.7%) and thickness of 0.9 nm (σ = ±25.8%) have been successively synthesized by structure-controlling solventless thermolysis from a new layered bismuth thiolate precursor with a 31.49 A spacing. The morphologies result from self-control at an atomic level by the layered Bi(SC₁₂H ₂₅)₃ crystal structure. The formation of the Bi nanofilm intermediate provides significant substantiation for this synthesis method, and detailed evidence on the conversion progress has been obtained. Both the films and the rhombuses have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), high-resolution TEM (HRTEM), and atomic force microscopy (AFM) measurements. Special UV-vis electronic absorption spectra of the nanoproducts have been studied.

Full text of DOI:10.1021/ic0615067

Inorg. Chem. 2007, 46, 586−591
Syntheses and Characterizations of Bismuth Nanofilms and
Nanorhombuses by the Structure-Controlling Solventless Method
Jing Chen,†,‡ Li-Ming Wu,† and Ling Chen*,†
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China, and
Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Received August 9, 2006
Substrate-free bismuth nanofilms with an average thickness of 0.6 nm (σ ) ±14.1%) and monodisperse layered
Bi nanorhombuses with an average edge length of 21.5 nm (σ ) ±14.7%) and thickness of 0.9 nm (σ ) ±25.8%)
have been successively synthesized by structure-controlling solventless thermolysis from a new layered bismuth
thiolate precursor with a 31.49 Å spacing. The morphologies result from self-control at an atomic level by the
layered Bi(SC12H25)3 crystal structure. The formation of the Bi nanofilm intermediate provides significant substantiation
for this synthesis method, and detailed evidence on the conversion progress has been obtained. Both the films and
the rhombuses have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM),
energy-dispersive X-ray spectrometry (EDX), high-resolution TEM (HRTEM), and atomic force microscopy (AFM)
measurements. Special UV−vis electronic absorption spectra of the nanoproducts have been studied.
Introduction
newly established chemical approach to nanomaterial syn-
thesis that has successfully generated diverse materials such
as sulfides and metals over a wide range of morphologies
(wires, belts, rods, fabric, disks, prisms, and spheres).2-6 We
have delineated three important steps in the solventless
method: (i) the preparation of a uniform complex/colloidal
precursor in solution, (ii) the separation of the precursor from
solution, and (iii) the decomposition of the solvent-free
precursor under well-defined conditions. The nanoproducts
are produced during the third step in which coagulative
growth caused by the molecule/particle diffusion is largely
reduced under a solventless condition.
Our previous studies on Cu2S revealed that the uniform
nanowires, rods, or spheres could be made from the corre-
sponding precursors obtained from the solutions with dif-
ferent viscosities.5 The viscosity is considered to be an
important indicator of the colloidal precursor polymerization
or association. This dependence of nanoproduct morphology
The booming development of nanoscience and technology
has been accelerated by the discoveries of the various special
properties associated with the size and shape of the nano-
materials. Well-controlled synthesis is one of the most
important factors in nanoscience, especially when applica-
tions are concerned, and the greatest challenge is the
development of the controlled synthetic methodologies.
Numerous chemical approaches have been explored, and
some have been proven to be successful in the control of
size and morphology of the nanoproduct. Generally, the
controlling factors are reactant concentration, solvent, tem-
perature, pressure, and so forth; however, other possible
factors are seldom discussed.1 The solventless method is a
* To whom correspondence should be addressed. E-mail: chenl@
fjirsm.ac.cn.
† Fujian Institute of Research on the Structure of Matter.
‡ Graduate School of the Chinese Academy of Sciences.
(1) (a) Cobden, D. H. Nature 2001, 409, 32-33. (b) Tang, Z.-Y.; Kotov,
N. A.; Giersig, M. Science 2002, 297, 237-240. (c) Volokitin, Y.;
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586 Inorganic Chemistry, Vol. 46, No. 2, 2007
10.1021/ic0615067 CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/21/2006

Bismuth Nanofilms and Nanorhombuses
large 0k0 spacings in the powder X-ray diffraction (XRD, see details
below). In addition, the suggested formula Bi(SC12H25)3 was further
confirmed by the element analysis on the dried sample. Anal.
Calcd: C, 53.18; H, 9.30; N, 0; S, 11.83. Found: C, 53.06; H,
9.09; N, 0.26; S, 11.79.
on the precursor state (viscosity) encouraged us to speculate
whether a precursor with a selected structure might generate
a nanoproduct with a controlled shape. Such speculation was
supported by our recent discoveries of the conversion of solid
layered precursors to layered silver nanodisks,6 for which
we proposed that the crucial item that determined the layered
morphology of the Ag nanodisks was the precursor crystal
structure. That is, the initial Ag nuclei concentration and
distribution and the consequent atom diffusion paths and
speeds are restricted by the precursor crystalline structure.
In other words, such a conversion could be regarded as a
growth mechanism self-constrained at the atomic level by
the precursor structure. Unfortunately, we could not establish
additional facts to support such an idea, so a great deal of
guessing appeared necessary to describe the conversion of
the layered crystalline structure to the layered morphology
of an individual nanoparticle. On the other hand, character-
ization of some intermediate state between the two end
materials (the crystalline precursor and the isolated layered
nanoproduct) would afford convincing support for our
structure-controlling conversion mechanism. In this paper,
we report a new example of a layered-precursor to layered-
nanoproduct conversion via this solventless method that is
significant because a Bi nanofilm intermediate has been
obtained.
Syntheses of Bi Nanoproducts. A 0.505 g sample of the yellow
precursor was sealed in a glass tubing of 1.3 cm diameter and about
12 cm length which was sealed at a residual pressure < 0.58 Pa.
The assembly was subsequently heated in a conventional tube
furnace at 90 °C for different times to produce black nanoproducts
with different morphologies. For example, a 2 h heating time
resulted in Bi nanofilms, and a 5 h heating time resulted in Bi
nanorhombuses. The products were usually dispersed in CHCl3 and
reprecipitated by the addition of excess ethanol. The yields are about
20% on the basis of Bi(NO3)3‚5H2O.
Characterizations. XRD, transmission electron microscopy
(TEM), high-resolution TEM (HRTEM), and atomic force micros-
copy (AFM) were used to characterize the structure, composition,
size, and shape of the synthesized nanoproducts, respectively. The
XRD patterns were collected at room temperature with the aid of
a D-MAX-2500 diffractometer and Cu KR radiation. The TEM
images were obtained using a JEM 2010 transmission electron
microscope equipped with a field emission gun operating at 200
kV. Images were acquired digitally using a Gatan multipole
scanning CCD camera with an imaging software system. Energy-
dispersive X-ray spectrometry (EDX) analyses of nanoproducts
were performed on a carbon-film-coated Cu grid with the aid of a
JEM 2010 transmission electron microscope equipped with an
Oxford INCA spectrometer. The elemental chemical analyses
were performed by Vario EL III (Elementar Co.). The UV-vis
spectra were measured on a Perkin-Elmer Lambda-900 spectro-
photometer.
Bismuth is a semimetal with a small indirect band gap
and an anisotropic effective electron mass and could convert
to a semiconductor when the size is decreased enough.7 Many
previous efforts have investigated the special properties of
Bi nanoparticles, it being thermoelectric for example.8
Nanoparticles with different morphologies such as wires10
and tubes9-11 have been prepared by a variety of methods.
In comparison, our Bi nanorhombuses exhibit similar
UV-vis absorption behaviors, whereas the nanofilms exhibit
unique electronic transitions in the same energy range.
Results and Discussion
Layered Crystal Structure of Bi(SC12H25)3 Precursor.
The organothiolate anion (RS-) is a fundamental ligand in
metal complexes that can be compared with HS- and S2-.
Among the numerous thiolates, the metal ions are usually
transition metals, such as Zn2+, Cd2+, Hg2+, Cu+, Ag+, Au3+,
Fe2+, Co3+, Ni2+, and so forth, or main-group metal ions,
for example, Sn4+ and Pb2+. Although some bismuth com-
plexes have been postulated, for example, Bi(SPh)x(SePh)3-x
and Bi[(SC6F5)4]-, very few bismuth thiolates have been
synthesized and characterized.12 Herein, we have successfully
synthesized a novel bismuth dodecanethiolate, as described
above, and have studied its dominant structural motif.
Considering the charge balance, we assume the stoichiometry
of the precursor to be Bi3+[(SC12H25)-]3, as supported by
the elemental analysis results. The powder XRD experiment
revealed that the as-synthesized precursor is a pure phase
with a layered structure with a large interlayer d spacing
(Figure 1). The intense narrow reflections of the polycrys-
talline precursor contain all successive 0k0 orders for k )
2-16. The measured kd values of all 15 reflections fall in
the range of 31.46-31.59 Å with an average of 31.49 Å
(Supporting Information Table 1), about twice the expected
length of the alkyl chain (L). Such a layered motif is similar
to that of the silver analogue which has a 34.6 Å spacing.6
Experimental Section
The reactants were used without any further purification:
C12H25SH (Lancaster, 98%), Bi(NO3)3‚5H2O (A.R., Shanghai
Chemical Co.), ethanol (A.R., Shanghai Chemical Co.), chloroform
(A.R., Shanghai Chemical Co.), and dimethylformamide (DMF,
A.R., Shanghai Chemical Co.).
Synthesis of the Precursor. In a typical procedure, Bi(NO3)3‚
5H2O (2.328 g) was dissolved in 10 mL of DMF, and 2 mL of
1-dodecanethiol (DT, C12H25SH) was added with stirring. The
yellow precipitate was filtered 30 min later, washed with ethanol,
and dried at room temperature. The precursor was determined to
be a single-phase layered bismuth dodecanethiolate according to
(7) (a) Isaacson, R. T.; Williams, G. A. Phys. ReV. 1969, 185, 682-688.
(b) Black, M. R.; Lin, Y.-M.; Cronin, S. B.; Rabin, O.; Dresselhaus,
M. S. Phys. ReV. B 2002, 65, 195417.
(8) (a) Costa-Kra¨mer, J. L.; Garc´ıa, N.; Olin, H. Phys. ReV. Lett. 1997,
78, 4990-4993. (b) Black, M. R.; Hagelstein, P. L.; Cronin, S. B.;
Lin, Y.-M.; Dresselhaus, M. S. Phys. ReV. B 2003, 68, 235417. (c)
Liu, K.; Chein, L. C. Appl. Phys. Lett. 1998, 73, 1436-1438.
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7067-7072.
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Mater. 1999, 11, 1659-1665. (b) Wang, J.-W.; Wang, X.; Peng, Q.;
Li, Y.-D. Inorg. Chem. 2004, 43, 7552-7556.
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D.-P.; Yang, P.-D. J. Am. Chem. Soc. 2001, 123, 9904-9905.
(12) Dance, I. G. Polyhedron 1986, 5, 1037-1104.
Inorganic Chemistry, Vol. 46, No. 2, 2007 587

Chen et al.
indicates that the films are amorphous. Elemental analyses
by EDX on several different spots on the film (Figure 2a)
consistently reveal only Bi. A typical EDX spectrum recorded
on an individual film is shown in Supporting Information
Figure 3a. The layered characteristic is obvious in the TEM
observation in Figure 2a, which is further confirmed by the
AFM analysis shown in Figure 3, in which the thickness of
the film is measured to be 0.6 nm (σ ) (14.1%) (Supporting
Information Figure 6). The morphology of the Bi nanofilm
is found to be flat and rigid, which is different from the “rag”
sheet-like structure of MoS2.14 Such differences may come
from their different crystallographic structures. The bulk
MoS2 features a hexagonal layered structure in which the
motions between the layers are flexible and similar to that
in carbon; such flexibility may result in the soft rag-like
morphology. Contrarily, the bulk Bi is a three-dimensional
rhombohedral network, which may lead to the rigid nano-
film as shown in Figure 2a and Supporting Information
Figure 8.
Interestingly, similar treatment of the layered Bi(SC12H25)3
precursor at the same temperature but for an elongated time
(5 h) generates uniform rhombuses with layered structural
motifs (Figure 2b-f). Remarkably, the individual rhombuses
are organized in a quasi closed-packed two-dimensional
array. A selected area electron diffraction (SAED) investiga-
tion of them shows sharp diffraction spots (Figure 2d) and
thence their crystalline characteristics. The XRD patterns
(Figure 5) of the rhombuses formed in 5 h show sharp intense
reflection peaks that can be well-indexed as rhombohedral
(ICSD 64705; space group R3hm, 166; a ) b ) 4.533(3) Å,
c ) 11.797(6) Å). There is no evidence for a preferred
orientation in agreement with the TEM and SAED observa-
tions. Compared with the film formed in 2 h, the rhombuses
are naturally better crystallized, which is reasonable because
of the extended reaction time. The XRD patterns of the
rhombuses indicate single-phase bismuth and no other
phases. Such singularity is confirmed by the EDX spectrum
(Supporting Information Figure 3b). The rhombuses show
nice uniformity of shape and size under these conditions,
with edge lengths of around 21.5 nm (σ ) (14.7%) (Figure
2, Supporting Information Figure 5) and the thicknesses of
about 0.9 nm (σ ) (25.8%) according to AFM measure-
ments (Figure 4, Supporting Information Figure 7). The layer
motifs are clearly seen in Figure 4 (bottom) as well. The
HRTEM image of the single rhombus in Figure 2f shows
interplanar distances of 3.25 Å, which correspond to the d102
and d012 of rhombohedral Bi phase.
Figure 1. XRD pattern of the Bi(SC12H25)3 precursor.
The structure of the as-synthesized Bi(SC12H25)3 is also
proposed to consist of a central layer of Bi/S with the alkyl
chain tails all extending on both sides of the central layer,
making the interlayer distance about 2L. A difference
between the silver and the bismuth salts will originate from
different details of the intralayer structure, particularly
because the two metals possess a different coordination
number (CN) of thiol ligands. That for silver is probably 4,
whereas that for bismuth is probably 4, 5, or even 6. A
trigonal bipyramid (CN ) 5) is probably more likely. The
(h0l) and so forth reflections are all too weak in the powder
data to allow for the investigation of the intralayer structural
details; on the other hand, the absence of any other non-
axial reflections is consistent with a preferred crystallographic
registry of layers. Although powder diffraction data give
good evidence for the layered motif, the structure needs to
be confirmed by single-crystal diffraction studies. However,
the common problem for most of the associated metal thiolate
polymers, the insolubility of Bi(SR)3, makes it extremely
difficult to obtain crystals with a quality sufficient for single-
crystal diffraction analysis.
As far as our structure-controlling solventless thermolysis
process is concerned, knowing the anisotropic layered nature
of the Bi(SC12H25)3 precursor already provides enough
information for us to understand the conversion process.
Structures and Morphologies of the Bi Nanoproducts.
Novel Bi nanofilms free of any substrate have been obtained
upon heating the as-synthesized layered Bi(SC12H25)3 precur-
sor in a sealed glass tube at 90 °C for 2 h. Similar to the
formation of the metallic Ag disks from the Ag(SR)
precursor,6 the Bi3+ here is thought to be reduced by the
thiolate anion (SR-) through the electron transfer from the
thiolate (SR-), the two (SR•) radicals yielding the disulfide.13
It is interesting that when heating is carried out in a N2
atmosphere up to 150 °C for 2 h, pure Bi is also produced
(Supporting Information Figure 1), but at a higher temper-
ature, for example, 250 °C, Bi2S3 is formed (Supporting
Information Figure 2), which is consistent with the results
in the literature.4 The uniformity of the Bi nanofilm is
demonstrated in Figure 2a. This is evidently the first time
that thin, uniform, and free-standing bismuth nanofilms have
been reported. Such films give no powder XRDs, which
UV-Visible Absorption Spectra of the Nanofilms and
Nanorhombuses. The UV-vis absorption spectra of the Bi
films and rhombuses suspended in CHCl3 (1 mg of Bi in 5
mL of CHCl3) are shown in Figure 6. The rhombuses show
a sharp absorption band at approximately 274 nm, in good
agreement with the 280 nm absorption for Bi nanoparticles
with 13 nm diameters found by Wang et al.9 The films show
two absorption maxima at approximately 257 nm and 362
nm. The former is similar to the 253 nm peak found in an
(13) Oae, S. Organic Sulfur Chemistry: Structure and Mechanism; CRC
Press: Boca Raton, FL, 1991; Chapter 6, pp 204-206.
(14) Chianelli, R. R.; Prestridge, E. B.; Pecoraro, T. A.; Deneufville, J. P.
Science 1979, 203, 1105-1107.
588 Inorganic Chemistry, Vol. 46, No. 2, 2007

Bismuth Nanofilms and Nanorhombuses
Figure 2. TEM images of Bi nanoparticles: (a) image of nanofilm, (b) low-magnification TEM image of the nanorhombuses monolayer array, (c) high-
magnification TEM image of the nanorhombuses, (d) SAED pattern of the nanorhombus from c, (e) corresponding HRTEM image of a single nanorhombus
in c, and (f) an enlarged image of the selected part of panel e.
Figure 4. (top) AFM image of the Bi nanorhombuses produced at 90 °C
for 5 h, and (bottom) the height profile along the line in the top figure. The
height is about 0.9 nm.
Figure 3. (top) AFM image of a Bi nanofilm produced at 90 °C for 2 h,
and (bottom) the height profile along the line in the top figure. The thickness
is about 0.6 nm.
with the theoretical predictions in the usual UV-vis range.15
The Bi-Bi distance in bulk bismuth is around 3 Å; therefore,
a film of 0.6 nm thickness is only two to three atoms thick.
The quantum confinement effect in such a film would be
significant, and the as-synthesized film might be an ideal
candidate for the study of the enhancement of the thermo-
electric figure of merit for bismuth.1f
aqueous solution of Bi particles with mean diameters of 20
nm by Gutie´rrez and Henglein.15a The 362 nm absorption
has not been reported and might be related to the unique
film morphology. The overall trend for both films and
rhombuses is an increasing absorption with decreasing
wavelength in the 200-800 nm range which is consistent
(15) (a) Gutie´rrez, M.; Henglein, A. J. Phys. Chem. 1996, 100, 7656-
7661. (b) Foos, E. E.; Stroud, R. M.; Berry, A. D.; Snow, A. W.;
Armistead, J. P. J. Am. Chem. Soc. 2000, 122, 7114-7115.
Layered Precursor to Layered Nanorhombus: Con-
version via an Intermediate Nanofilm. The layered Bi
Inorganic Chemistry, Vol. 46, No. 2, 2007 589

Chen et al.
Figure 7. TEM images of the sample prepared at 90 °C over (a) 3 h and
(b) 4 h.
anisotropic hindrance role. Second is the subsequent particle
crystallization and growth by atom diffusion guided by the
concentration gradient of Bi atoms constrained in the film.
Naturally, the anisotropic atom diffusion and interparticle
aggregation lead to the layered crystalline rhombuses.
Structure-Controlling Solventless Method. We present
here some new experimental facts to illustrate our structure-
controlling solventless method for converting layered precur-
sors to the layered products. Compared with our previous
report on Ag nanodisks,6 the thermolysis temperature for Bi
is remarkably lower than for Ag nanodisks, 90 °C vs 180
°C, a logical difference in itself. This makes it possible for
us to isolate a kinetic intermediatesa bismuth nanofilmsto
further support the proposed layered-precursor to layered-
product conversion mechanism. The observation of the
nanofilm might originate with the lower thermolysis tem-
perature. The significance of the as-synthesized film lies in
its demonstration that the distribution of the Bi atoms first
formed really inherits the layered pattern of the crystal
structure of the precursor as we speculated. Furthermore, the
in situ film not only serves as the source of bismuth but
also restricts its aggregation and diffusion during the
subsequent crystallization and growth stages of the to-be-
formed rhombuses. These experiment facts further establish
that our structure-controlling solventless method is a useful
bridge between the crystal structure and the nanoproducts.16
Figure 5. XRD pattern of the nanorhombuses produced at 90 °C in 5 h.
Figure 6. Room-temperature UV-vis absorption spectra of a chloroform
suspension of a collection of Bi nanofilms produced at 90 °C for 2 h and
nanorhombuses formed at 90 °C over 5 h.
rhombuses have been repeatedly obtained by the thermal
decomposition of Bi(SC12H25)3 at 90 °C for 5 h (see Figure
2b-f). Such an observation is a good analogue of our
previous discovery of the conversion of a layered silver
precursor to Ag nanodisks. In order to understand the
formation of the present rhombuses, we have explored
different annealing times. Annealing at 90 °C for less than
2 h results in a product that retains mainly the precursor.
An extension of the duration to 2 h produces a black powder
with no XRD pattern, which is pure bismuth and possesses
a film morphology (Figure 2a). The XRD patterns of the
samples produced in elongated periods of 3, 4, and 5 h are
all those of pure rhombohedral bismuth. (Figure 5, Support-
ing Information Figure 4). The products heated for 3 h are
still mostly films, but the X-ray detectable rhombuses
appeared also (Figure 7a, Supporting Information Figure 4a).
With the time increasing to 4 h, the products are mainly
nanorhombuses accreted on the film (Figure 7b, Supporting
Information Figure 4b). Pure nanorhombuses are made after
being annealed for 5 h. Thus, the formation of the rhombuses
takes place in two stages during such a process: the
formation of the amorphous film and the crystallization and
growth of the rhombuses. At first, Bi(SC12H25)3 decomposes
to bismuth atoms in a distribution that inherits the layered
pattern of the mother structure, and amorphous films are
formed during which the long-chain alkyl ligands play an
Conclusions
In summary, we have successfully synthesized the novel
bismuth nanofilms and layered nanorhombuses from the
layered bismuth thiolate precursor. The Bi nanofilms are
found to be an intermediate between the precursor crystal
structure and the layered nanorhombuses. Both the Bi films
and the rhombuses are produced on a gram scale with
average thicknesses of 0.6 and 0.9 nm, respectively. Interest-
ingly, in the UV-vis range, the rhombuses show an usual
274 nm absorption; the film has two maxima at 257 and
362 nm, and the former peak is a known absorption for
Bi nanoparticles while the latter one is new. Some exten-
sions to O- and N-containing ligands and to other binary/
ternary alloys and compounds are ongoing, and other
possible structural motifs for the conversion are under
exploration.
(16) Meyer, G.; Naumann, D.; Wesemann, L. Inorganic Chemistry in Focus
III; WILEY-VCH: Weinheim, Germany, 2006; Chapter 20, pp 295-
303.
590 Inorganic Chemistry, Vol. 46, No. 2, 2007

Bismuth Nanofilms and Nanorhombuses
Supporting Information Available: Interlayer spacings, XRD
patterns, EDX analyses, particle size distribution, thickness distribu-
tion, particle height distribution, and TEM images. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
National Natural Science Foundation of China under Project
Nos. 20401014, 20401013, and 20521101, by the State Key
Laboratory Science Foundation (050086 and 050097), NSF
of Fujian Province (2004J042, 2005HZ01-1, and 2006J0271),
and a ‘‘One Hundred Talent Project” from CAS.
IC0615067
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