Preparation by Hydrothermal Techniques in a Tungstosilicate Acid Solution System and Optical Properties of Tellurium Nanotubes

Article abstract of DOI:10.1002/ejic.200900143

Single-crystalline tellurium nanotubes can be easily synthesised by a simple hydrothermal, economical, and green chemical route. The tungstosilicate acid (TSA) serves as not only an effective reducing agent but also a new morphology-directing agent for te

Full text of DOI:10.1002/ejic.200900143

FULL PAPER
DOI: 10.1002/ejic.200900143
Preparation by Hydrothermal Techniques in a Tungstosilicate Acid Solution
System and Optical Properties of Tellurium Nanotubes
[a]
[a]
[a]
Li Zhang,* Cong Wang, and Deyun Wen
Keywords: Nanotubes / Tellurium / Polyoxometalates
Single-crystalline tellurium nanotubes can be easily synthe-
sised by a simple hydrothermal, economical and green chem-
ical route. The tungstosilicate acid (TSA) serves as not only
an effective reducing agent but also a new morphology-di-
recting agent for tellurium nanotubes from sodium tellurate
tions. Te nanotubes grow along the [001] direction and have
excellent crystallinity. The synthetic conditions also investi-
gated for the tellurium nanotubes were molar ratio of sodium
tellurate to TSA, pH of reaction solution and reaction tem-
perature. The optical properties of the t-Te nanotubes have
been investigated. A possible mechanism for the growth of
tellurium nanotubes has been discussed.
2 3
(Na TeO ) precursor powders. Scanning electron microscopy
images show that most of the tellurium nanotubes have slop-
ing cross-sections and open ends with outer diameters of
1
00–500 nm, wall thicknesses of 30–100 nm and lengths of
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
30–50 µm. A few Te nanotubes have hexagonal cross-sec-
Introduction
Te nanotubes by in situ disproportionation of sodium tellu-
[6]
rite (Na TeO ) in an aqueous ammonia system at 180 °C.
2
3
In recent years nanomaterials, especially 1D nanostruc-
tures such as nanowires, nanorods, nanobelts and nano-
tubes have attracted much interest due to their distinct
Yu and coworkers have synthesised the single-crystalline tri-
gonal tellurium (t-Te) nanotubes by reducing TeO using
2
ethylene glycol (EG) as a reducing agent. The Te nanotubes
obtained in this approach display strong luminescent emis-
sion in the blue-violet region and strong corrosion resis-
physical and chemical properties and potential applications
in nanoscale devices.[
1,2]
Elemental tellurium is a narrow
bandgap semiconductor with a direct bandgap energy of
.35 eV. Trigonal tellurium has a highly anisotropic crystal
[10]
tance in ethanol. Recently, it has been shown that the Te
0
nanotubes synthesised by chemical methods could contain
structure consisting of helical chains of covalently bound
atoms which are in turn bound together through van der
tiny amounts of impurities which could result in dramatic
[10c]
changes in the conducting properties.
Komarneni and
[3]
Waals interactions into a hexagonal lattice. This inherent
anisotropy makes these materials ideal candidates for gener-
ating a 1D nanostructure.[ Trigonal tellurium exhibits
many useful and interesting properties for example, pho-
toconductivity and catalytic activity toward some reactions,
thermoelectricity, high piezoelectricity and nonlinear op-
coworkers reported a green chemical approach to the syn-
thesis of tellurium nanowires by using starch as a reducing
4]
[11]
agent
whereas Wang and coworkers reported the PEG-
mediated hydrothermal growth of single-crystal tellurium
[12]
nanotubes.
All these studies indicated that the growth
processes of tellurium nanotubes were still diverse under
different synthetic conditions. At present developing novel,
simple and convenient methods for synthesising nanomater-
ials, especially 1D nanostructures, is still a challenge for
chemists and materials scientists.
[
5]
tical responses. In the past decade, 1D tellurium nano-
crystals have been synthesised by different routes such as
[6]
solvothermal or hydrothermal methods, microwave-as-
sisted methods, chemical (physical) vapour deposition[8]
etc. Xia and coworkers first reported that Te nanotubes
with blocking seeds (Te atom clusters) within could be ob-
tained by adding orthotelluric acid to pure ethylene glycol
heated to reflux at 197 °C.[ Mo and coworkers synthesised
[7]
Polyoxometalates of the Keggin structure have the gene-
(8–n)–
ral formula (XM O )
where ‘M’ stands for W or Mo
12
40
and ‘X’ stands for heteroatoms such as P, Si, Ge with ‘n’
9]
[13,14]
being the valency of X.
It is well known that Keggin
ions undergo stepwise multielectron redox processes with-
[
15,16]
[
a] Anhui Key Laboratory of Spin Electron and Nanomaterials out a structural change.
They may be reduced electro-
(
Cultivating Base), Department of Chemistry & Biology,
lytically, photochemically and chemically (with suitable re-
ducing agents). These metal oxygen cluster anions with well
defined structures and properties represent a large category
and they have extensive applications in the fields of analyti-
cal chemistry, biochemistry and solid state devices, and have
Suzhou University,
Suzhou 234000, P. R. China
Fax: +86-557-2871003
E-mail: zhlisuzh@163.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900143.
Eur. J. Inorg. Chem. 2009, 3291–3297
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3291

L. Zhang, C. Wang, D. Wen
FULL PAPER
been used as antiviral and antitumor reagents. Their redox NHE).[27] Furthermore, TSA ions can be utilised cyclically
chemistry is characterised by their ability to accept and re- as oxidising or reducing agents according to equations 1
4
–
lease a certain number of electrons, in distinct steps, with- and 2 whilst the structure of (SiW O ) remains un-
12
40
[
16]
out decomposition. Tungstosilicate acid (TSA) is one of changed.
the simplest polyoxometalates with the Keggin structure. The influence of the reaction temperature on the forma-
Recently, Papaconstantinou and coworkers discovered tion of tellurium nanotubes was examined. The reaction
that photochemically reduced polyoxometalates with the cannot take place when the temperature is below 180 °C.
3–
Keggin structure of phosphotungstic acid [(PW O ) ] In the XRD pattern, the characteristic Te diffraction peaks
12
40
lead to the formation of the corresponding metal nanopar- cannot be observed (see XRD pattern in Figure S1 in the
[
17]
3–
ticles.
Sastry and coworkers used PW O
ions to Supporting Information). If the temperature is up to
and they 200 °C, the t-Te tubes can be obtained (as shown in Fig-
as a template for the in situ growth ure 1).
1
2
40
[
18,19]
make phase-pure core-shell nanoparticles
also used PW O
40
of metal nanoparticles,
crystals
3
–
1
2
[20,21]
star-shaped calcium carbonate
[
22]
and CdS nanoparticles.[23] Yang and coworkers
prepared the 3D packed trigonal selenium microspheres by
a hydrothermal process[ and, recently, we have synthe-
sised a tungstosilicate acid (TSA)-Ag nanocomplex based
on the reduction of silver nitrate by a UV-irradiated tungs-
tosilicate acid solution.[ The size of the nanocomplex was
in the range of 20–60 nm, with a mean diameter of 45 nm.
Multi-charged TSA-Ag composite nanoparticles were alter-
nately deposited with oppositely charged chitosan in a
layer-by-layer assembly.
24]
25]
Herein we report a facile, economical and green chemical
route for the synthesis of tellurium nanotubes. Single-crys-
talline tellurium (t-Te) nanotubes with a sloping cross-sec-
tion or a hexagonal cross-section can be synthesised on a
Figure 1. XRD pattern of the prepared tellurium nanotubes.
The composition and phase of the resultant samples were
large scale by the reaction of tungstosilicate acid and so- examined by XRD. Figure 1 shows the XRD pattern of the
dium tellurate solutions under hydrothermal conditions resultant sample. All diffraction peaks can be readily inde-
wherein the TSA plays the role of a reducing agent, a soft xed as the hexagonal phase of tellurium with lattice con-
template and a solvent. To the best of our knowledge, this stants a = 0.4459 nm and c = 0.5921 nm which are in agree-
is the first study of the synthesis of single-crystalline Te ment with the reported values of a = 0.4458 nm and c =
nanotubes by using templates of Keggin ion colloidal par- 0.5927 nm (JCPDS, 36–1452). No other impurity was indi-
ticles. The optical properties of t-Te nanotubes have also cated by the X-ray diffraction pattern. Compared with the
been investigated.
standard pattern of hexagonal phase tellurium, unusually
strong (h00) reflection peaks and weak (hkl) reflection
peaks (l ϶ 0) were observed in the XRD pattern.
Results and Discussion
The morphology and dimension of the samples were ex-
amined by FESEM. Figure 2 (a) shows that the product
obtained is composed of 1D nanomaterials with lengths in
the range of 30–50 µm. Parts b, c and d of Figure 2 show
the FESEM images with higher magnification indicating
that the resultant tellurium sample is mainly composed of
irregular hexagonal prism nanotubes with sloping cross sec-
tions. Some tellurium nanotubes show hexagonal cross sec-
tions (as indicated by arrow 1 in Figure 2, b). Most of the
nanotubes have outer diameters in the range of 100–500 nm
and wall thicknesses in the range of 30–100 nm. Figure 2
(b) shows that some tiny spherical nanoparticles can be
(1) found on the surface of the sample (as indicated by arrow
4
–
A representative Keggin structure (SiW O ) ion was
1
2
40
5
–
used to study the reaction between (SiW O ) ions and
1
2
40
2
–
5–
TeO3 ions. The (SiW O ) ion was obtained by hydro-
12
40
4
–
thermal treatment of a deaerated 2-propanol/(SiW O ) /
1
2
40
2
–
TeO3 aqueous solution, in the presence of, for instance, 2-
propanol as a sacrificial reagent [Equation (1)].[
17,26]
With
hydrothermal treatment, the above-described solution grad-
ually turned from colourless to silver-gray as TeO32– ions
were reduced to Te metal [Equation (2)].
40]4– + (CH
3 2
)
2[SiW12
O
CHOH Ǟ
[SiW12
40]5– + (CH
2
O
3
)
2
C=O + 2H+
40]4– + Te + 3H
2
O (2)
2
). It should be amorphous tellurium dioxide because of
[10]
40]5– + TeO
2–
+ 6H+ Ǟ 4[SiW12
the oxidation of Te tubes during the storage period.
4[SiW12
O
3
O
The microstructure and growth direction of the tellurium
There are two reasons for the result: one is the ability of nanotubes were studied by HRTEM and SAED. Figure 3
5
–
2–
the (SiW O ) to transfer electrons efficiently to TeO3
shows a typical transmission electron microscope (TEM)
1
2
40
ions. The other is the lower potential of the one-equivalent- image and high-resolution TEM (HRTEM) image of the
4
–
5–
reduced tungstate couple (SiW O ) /(SiW O )
(
obtained sample, along with the electron diffraction
1
2
40
–
12 40
2
0.057 V vs. NHE) relative to TeO3 /Te (0.827 V vs. (SAED) pattern of the selected area. The TEM images in
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Eur. J. Inorg. Chem. 2009, 3291–3297

Preparation and Optical Properties of Tellurium Nanotubes
Figure 2. FESEM images with different magnifications of the tel-
lurium nanotubes (a): low-magnification FESEM image of the pre-
pared tellurium nanotubes; (b) (c) (d): FESEM image at high mag-
nification.
Figure 3 (a, b) show that the Te nanostructures are tubelike
and the tubes are genuinely hollow and contain no blocking
seeds within. Figure 3 (b) shows an individual Te nanotube
with a well-defined tubelike morphology and an outer dia-
meter of ca. 300 nm. No obstruction was found within the
tube. The SAED pattern in Figure 3 (c) was taken on a
typical individual nanotube, showing that the nanotube is
single crystalline with growth along the [001] direction. The
Figure 3. (a) TEM images of the Te nanotubes and (b) an individ-
ual Te nanotube; (c) SAED pattern taken on the Te nanotube
shown in (b); (d) HRTEM image of the tellurium nanotube.
HRTEM image in image d in Figure 3 show lattice spacings mation of Te nanotubes. This behaviour is similar to that
of ca. 2.23 and 5.95 Å corresponding to the lattice spacings found in many chemical reduction approaches to nanosys-
of the (110) and (001) planes, respectively, for trigonal tel- tems because the nucleation and growth sequences are af-
lurium. The combination of the results by HRTEM, XRD fected both by the relative concentrations of the reducing
[
24]
and the SAED pattern confirm that the axis of the nano- agent and the precursor.
tube is along the [001] direction. This result confirms that The dependence of the products on the pH of the solu-
single-crystalline tellurium nanotubes with a sloping cross tion was also studied. The FESEM images show the pre-
sections can be synthesised on a large scale from a commer- pared white products corresponding to pH values of 3.62
2
–
cial TeO3 precursor by using TSA as a reducing agent.
(a) and 4.45 (b) (as shown in Figure S2 in the Supporting
Figure 4 shows the FESEM images of tellurium nano- Information). The products are composed of irregular and
tubes synthesised with different molar ratios of TeO32– ions jujube-like particles. In the XRD pattern (see XRD pattern
to TSA at 200 °C for 20 h. Parts a–h of Figure 4 show four in Figure S3 in the Supporting Information) all peaks can
samples of Te nanotubes corresponding to molar rations of be indexed to the orthorhombic phase of paratellurite
1
:2, 1:4, 1:10 and 4:1. There are some morphology changes (TeO ) with cell parameters a = 12.0 Å, b = 5.46 Å and c
2
between this samples and the sample of Figure 2. The = 5.61 Å which are in good agreement with the standard
lengths of these samples are in the range of 3–30 µm. As literature data (JCPDF card number: 74–1131). The en-
demonstrated in images a and b, many small nanoparticles hanced relative intensities of the (h00) diffraction peaks
have been found on the surfaces of the sample which may compared with those for the standard indicated the possible
be amorphous tellurium dioxide formed during the storage preferential orientation of the crystals. The stability of TSA
2
–
period. When the molar ratio of TeO3 ions to TSA is 1:4, changes with the pH of the solution Ϫ controlling the pH
4
–
the Te nanotubes with a sloping cross-sections were ob- of (SiW O ) solution is important in the synthesis nano-
12
40
[28]
tained (images c and d in Figure 4). When the molar ratio structure materials. TSA is stable up to a pH of about 5,
of TeO3 ions to TSA is to 1:10, a remarkable change in the pH values of the 5 m TSA and 5 m sodium tellurate
2
–
morphology occurred. A tri-tipped tubular structure was solutions were maintained at 1.82 in this experiment. We
2
–
observed. When the molar ratio of TeO3 ions to TSA is suppose that TSA decomposed when the pH value of the
:1, the Te nanotubes with an irregular cross-sections were solution was adjusted to 3.62 or 4.45 using NaOH. An in-
4
obtained. However, the lengths of the tubes is diverse and crease in the pH value does alter the redox power of TSA
in a wide range and some flakes are also obtained (images and this leads to the synthesis of TeO not Te.
2
f and h in Figure 4). The above results indicate that the
molar ratio of TeO3 ions to TSA is influential in the for- tions of the products, X-ray photoelectron spectra (XPS)
To obtain further evidence for the purities and composi-
2
–
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L. Zhang, C. Wang, D. Wen
FULL PAPER
2
–
Figure 4. FESEM images of tellurium nanotubes synthesised with different molar ratios of TeO
3
ions to TSA at 200 °C for 20 h: (a, b)
2–
3
1:2, (c, d) 1:4, (e, f) 1:10 and (g, h) 4:1. ([TeO ] = 5 m).
were recorded. The spectroscopic survey of the products
stored in air for one month showed the presence of C1s (BE the tellurium nanostructures was reported previously.[
284.82 eV), Te4d (BE = 41.0 eV), Te3d (BE = 573.19 eV) The concentration dependence of the photoluminescence
Room-temperature photoluminescence spectroscopy of
6c]
=
and O1s (BE = 531.07 eV) core levels with no evidence of spectra of the obtained Te nanotubes is depicted in Fig-
impurities (see Figure 5, a). Part b of Figure 5 shows the ure 6. The photoluminescence intensity increases with the
Te3d core-level spectrum with four peaks centred at binding concentration of the obtained Te nanotubes in absolute eth-
energies of 573.19, 576.65, 583.6 and 587.15 eV. Two strong anol solution. The excitation spectrum of the t-Te nanotu-
0
peaks at 573.19 and 583.6 eV correspond to the Te 3d bes show one strong peak and one weak peak at 275 and
binding energy, two weak peaks at 576.65 and 587.15 eV 301 nm, respectively, at a wavelength of 332 nm (Figure 6,
IV
can be assigned to the Te 3d binding energy. The spec- a) which is consistent with that for ultrathin nanowires re-
[
6c]
trum in Figure 5 (b) shows the presence of both the elemen- ported by S. H. Yu and coworkers. Two strong photolu-
tal tellurium 3d5/2 peak (573.19 eV) and a small amount of minescence emission peaks at 335 and 405 nm with an exci-
oxidised tellurium (576.65 eV) which is consistent with that tation wavelength of 273 nm were observed for the t-Te
[10]
for TeO reported by S. H. Yu and coworkers.
nanotubes (Figure 6, b). Apparently, different from the re-
2
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Preparation and Optical Properties of Tellurium Nanotubes
Figure 5. XPS core level spectra recorded from the Tellurium nano-
tubes after being stored in air for one month. (a) Survey of the
sample; (b) survey of the Te3d region.
Figure 6. (a) Concentration dependence of photoluminescence exci-
tation spectrum of the obtained Te nanotubes with emission at
3
32 nm. (b) Concentration dependence of the photoluminescence
emission spectrum of Te nanotubes with an excitation wavelength
of 273 nm. The curves from bottom to top correspond different
concentration of Te nanotubes in absolute ethanol solution
sults obtained by H. T. Chang and coworkers,[29] deconvol-
–
5
Ϫ1
–4
Ϫ1
ution of the photoluminescence band for 1D t-Te nanowires (1,
6.5308ϫ10 molL
;
2,
1.3062ϫ10 molL
;
3,
–
4
Ϫ1
–4
Ϫ1
obtained at 120 min yields four Gaussian peaks centred at 1.9592ϫ10 molL ; 4, 2.6123ϫ10 molL ).
34, 397, 460,and 507 nm.[ The differences in the emis-
29]
3
sion peaks might be associated with the thickness of the
nanostructures and crystallisation behaviour of 1D nano-
structures.
The Raman scattering spectrum taken for the synthesised
t-Te is depicted in Figure 7. The characteristic vibration
–
1
peaks at 119.7 and 135.4 cm were observed at room tem-
perature. A similar observation was made by S. H. Yu and
–1 [10a]
coworkers who reported peaks at 114.8 and 134.4 cm .
It is reasonable to attribute these bands to A1 bond bending
and A1 bond stretching modes or the E mode of the t-Te
–
1
tubes, respectively. The peaks at 260.05 cm can be as-
signed as the second-order spectra of t-Te.
Although the exact mechanism of the formation of tel-
lurium nanotubes is difficult to know, we think that the Figure 7. Raman scattering spectrum of tellurium nanotubes.
TSA plays an important role in the formation of tellurium
nanotubes. A reasonable route is presented. First, TeO32–
Conclusions
ions are absorbed onto the surfaces of the TSA colloidal
particles’ templates then converted quickly to Te atoms in
In summary, single-crystalline trigonal tellurium (t-Te)
a short time under hydrothermal conditions. The tellurium nanotubes with a sloping cross-section or a hexagonal
atoms then recrystallise into spherical seeds. It is known cross-section can be synthesised on a large scale by reaction
that tellurium has infinite helical chain conformation in its of a mixture of tungstosilicate acid and Na TeO solutions
2
3
crystal structure. Hence it has a strong tendency to grow under hydrothermal conditions, wherein the TSA plays the
along the c axis direction into a 1D structure. Under hydro- role of a reducing agent and a soft template. Different mor-
thermal conditions, the spherical tellurium seeds will keep phologies of the single-crystalline tellurium can be success-
on growing with continuous feeding of tellurium atoms and fully synthesissed by varying the reaction conditions. With
finally grow into tellurium nanotubes.
unique fluorescence properties, the resultant t-Te nanowires
Eur. J. Inorg. Chem. 2009, 3291–3297
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L. Zhang, C. Wang, D. Wen
FULL PAPER
hold great potential for fabrication of nanodevices. This X-ray Diffraction (XRD) Measurements: The XRD of pattern was
examined on a Philips X’Pert PRO SUPER X-ray diffractometer
method enlarges the application of polyoxometalates to the
green synthesis of nanostructures with low dimensionality
and by using the above method, other nanomaterials could
α
equipped with graphite monochromated Cu-K radiation (λ =
1.54178 Å).
be possibly synthesised under very mild and economical Raman Spectroscopic Studies: The Raman spectroscopic studies
were performed with 488 nm laser excitation with a micro-Raman
conditions.
system which was modified by coupling an Olympus microscope to
a Jobin–Yvon-HR800 spectrometer with a CCD detector.
Experimental Section
Photoluminescence Spectroscopic Studies: Photoluminescence spec-
troscopy of the products was monitored with a F-4500 model pho-
toluminescence spectrophotometer (Japan Hitachi Co.).
Materials: Tungstosilicate acid (H
4
SiW12
O
40·xH
CH(OH)CH
2
O, TSA), sodium
] were all
tellurate (Na
2
TeO
3
), and 2-propanol [CH
3
3
Supporting Information (see also the footnote on the first page of
this article): Three figures showing XRD patterns FESEM images.
A.R. grade and obtained from Shanghai Reagent Co. All the rea-
gents were used without further purification.
Synthesis of t-Te Nanotubes: In a typical experiment, aqueous de-
aerated solutions of TSA (50 mL, 5 m) and aqueous Na
50 mL, 5 m) were taken in a test beaker. An aliquot of 2-propa-
nol (5 mL) was added to the 100 mL mixed aqueous deaerated
solution of Na TeO and TSA under continuous stirring for 10 min
2
TeO
3
Acknowledgments
(
This work is supported by the National Natural Science Founda-
tion of China (NSFC) (20871089), the Key Project of Anhui Prov-
incial Education Department (KJ2007A076) and the Scientific Re-
search Foundation for the Professors (Doctors) of Suzhou Univer-
sity (2006jb03).
2
3
and the solution was then allowed to age for 30 min. The mixed
®
solutions (20 mL) were added into a 25 mL Teflon -lined stainless-
steel autoclave and the reaction mixture formed a homogeneous
white suspension under vigorous stirring. The autoclave was sealed
and maintained in an oven at 200 °C for 20 h and then naturally
cooled to room temperature. Silver-gray solids were obtained and
collected by centrifugating the reaction mixture. The particles were
then washed with distilled water and absolute ethanol several times
and dried in a vacuum at 60 °C for 6 h before further characterisa-
tion.
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To discount the possibility of hydrothermal 2-propanol being the
2–
reducing agent for TeO
3
ions and thus for tellurium nanotubes
formation, a control experiment was performed in which 2-propa-
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2
–
sponsible for the reduction of TeO
3
.
[
[
Transmission Electron Microscopy (TEM) Measurements: Trans-
mission electron microscope (TEM) images were taken with a Hita-
chi H-800 transmission electron microscope at an acceleration volt-
age of 200 kV. High resolution transmission electron microscope
(HRTEM) analysis and selected area electron diffraction (SAED)
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Received: February 11, 2009
Published Online: July 1, 2009
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3297