Photolytic preparation of tellurium nanorods

Article abstract of DOI:10.1039/b912434a

Well-defined tellurium nanorods have been prepared by the photolysis of ^tBu₂Te₂ in an aqueous micellar system incorporating dodecanethiol as an auxiliary morphology-directing agent.

Full text of DOI:10.1039/b912434a

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Photolytic preparation of tellurium nanorodsw
David H. Webber and Richard L. Brutchey*
Received (in Berkeley, CA, USA) 24th June 2009, Accepted 27th July 2009
First published as an Advance Article on the web 11th August 2009
DOI: 10.1039/b912434a
Well-defined tellurium nanorods have been prepared by the
photolysis of tBu2Te2 in an aqueous micellar system incorporating
dodecanethiol as an auxiliary morphology-directing agent.
was stable in the aqueous micellar solution for a period of
several weeks.
The high dispersibility of the resulting Te nanocrystals
in water likely results from a double layer of amphiphilic
surfactant molecules, or a double layer of an inner layer of
dodecanethiol and an outer layer of surfactant. Acetone was
used to successfully precipitate the nanocrystals from aqueous
solution. Nanocrystal precipitation likely occurs by reduction
of the hydrophobic effect (i.e., the driving force for aggregation
of amphiphilic molecules into micelles or double layers).
Non-hydrogen bonded polar solvents are known to dissolve
amphiphiles into a molecular rather than micellar or double
layer form.11 Thus, as acetone is added to the aqueous
suspension, the double layer surrounding the nanocrystals is
likely disrupted, thereby causing nanocrystal agglomeration
and precipitation. Once precipitated, the nanocrystals may be
completely redispersed in water with gentle agitation.
Elemental tellurium has long been of interest because it is a
p-type semiconductor (Eg E 0.35 eV) with many technologically
useful properties, such as photoconductivity, piezoelectricity,
and thermoelectricity.1 Moreover, tellurium can be used as
a starting material for the preparation of semiconductor
4
tellurides (e.g., PbTe,2 CdTe,2 Ag2Te3 and Bi2Te3 ), and as a
sacrificial reductive template for the formation of noble
metals.5 The crystal structure of trigonal tellurium (t-Te) is
composed of helical chains of Te atoms that typically leads to
anisotropic growth, and as a result, 1-D nanorods, nanowires,
and nanotubes have been successfully prepared via chemical
reductive routes,6 or by the disproportionation of tellurite.7
For example, Qian and co-workers prepared well-defined Te
nanorods (aspect ratio E 21) by reducing ammonium
sulfotellurate with sodium sulfite in the presence of a surfactant.8
There is still much to be achieved, however, in terms of
developing solution routes to 1-D tellurium nanostructures
that do not rely upon chemical reductants, yet still impart a
high level of morphological uniformity.
As a consequence of the low enthalpies of the Te–Te
and Te–C bonds (ca. 149 and 195 kJ molÀ1, respectively),12
visible light is able to cause homolytic bond cleavage in
diorganoditellurides with the extrusion of elemental Te and
organic by-products. The photolytic decomposition of
organotellurium compounds has received some application
in the production of Te and telluride thin films;13 however,
we believe this is the first example of a photolytic preparation
of well-defined Te nanocrystals. To probe the decomposition
kinetics of tBu2Te2 in the micellar solution, the intensity of the
As part of our general investigation into the utility of dialkyl
dichalcogenides as starting materials for the preparation of
semiconductor nanocrystals,9 we found di-tert-butyl ditelluride
(tBu2Te2) to be a convenient precursor to Te nanorods upon
photolytic decomposition in a micellar environment. Using a
modified literature procedure,10 tBu2Te2 was synthesized by
t
262 nm Bu2Te2 UV band was monitored over the course of
the reaction (Fig. 1). Aliquots of the aqueous micellar solution
were taken from the reaction at regular time intervals and the
Te nanorods were removed via precipitation and centrifugation
before the solution was analyzed by UV-vis spectrophotometry.
t
the reaction of BuLi with elemental Te, followed by aqueous
quenching and oxidation in air (see ESIw). Sublimation of the
crude product afforded a crystalline solid with excellent purity
that is stable in the solid state for one year (at 8 1C in the
dark). Di-tert-butyl ditelluride possesses UV absorption bands
at ca. 195, 262, and 367 nm (see ESIw), making it well suited
for photolytic decomposition at a wavelength of 254 nm. As
such, the photolytic decomposition of tBu2Te2 (50.0 mg,
0.135 mmol) was performed at 254 nm in a micellar
solution composed of 5.0 mL of a non-ionic surfactant
(polyoxyethylene(12) tridecyl ether), 0.10 mL of dodecanethiol,
and 50 mL of water. Tellurium formation occurred over a
period of 12 h, resulting in a dark brown suspension that
Department of Chemistry and the Center for Energy Nanoscience and
Technology, University of Southern California, Los Angeles,
CA 90089, USA. E-mail: brutchey@usc.edu
w Electronic supplementary information (ESI) available: Experimental
details; UV-vis spectra of tBu2Te2 and Te nanostructures; TEM
images of Te nanorods; high-resolution XPS of Te nanorods. See
DOI: 10.1039/b912434a
t
Fig. 1 Temporal change in the UV-vis spectrum of Bu2Te2 during
the photolytic decomposition with 254 nm irradiation over 12 h.
t
Apparent first-order reaction kinetics with respect to Bu2Te2 (inset).
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The photolytic decomposition of tBu2Te2 closely fits a first-order
rate law with a rate constant k = 0.68 hÀ1 and a half-life
t1/2 = 1.0 h under the conditions employed.
of (001) and (003) lattice planes normal to the growth direction
(see ESIw). Moreover, HRTEM analysis suggests that the
nanorods are single crystalline and without any appreciable
amorphous shell. An HRTEM image (Fig. 2b) of an individual
nanorod clearly shows distinct lattice spacings of 5.8
and 3.2 A, which correspond to the (001) and (011) planes
of t-Te, respectively. Powder X-ray diffraction (XRD)
corroborates that the nanorods are phase pure t-Te
(a = 4.46 A; c = 5.91 A), with no other crystalline impurities
observed (Fig. 3). The calculated lattice parameters agree well
with the literature values of a = 4.46 A and c = 5.93 A
(JCPDS 04-007-5290). X-ray photoelectron spectroscopy
(XPS) indicates that the nanorods are composed of elemental
tellurium, with a Te 3d5/2 binding energy of 573.1 eV (see
ESIw), which is in good agreement with literature values.15
Furthermore, XPS analysis did not reveal the presence of a
significant amount of oxidized Te4+ species, suggesting that
the Te nanorods are stable under the experimental conditions
described.
Transmission electron microscopy (TEM) revealed an
anisotropic nanorod morphology for the resulting Te
product (Fig. 2). The nanorods have an average diameter of
12.7 Æ 3.0 nm and an average length of 46.5 Æ 9.4 nm (aspect
ratio E 3.7). Although Te has a strong tendency toward
anisotropic growth, it was determined that dodecanethiol
is necessary for the growth of 1-D nanorods under these
t
conditions. The photolytic decomposition of Bu2Te2 in the
absence of dodecanethiol, but under otherwise identical
conditions (i.e., same concentration and photolysis time),
yielded larger ellipsoidal Te nanocrystals with very low aspect
ratios (B1.8, Fig. 2). The morphological change induced by
dodecanethiol may be a result of the (001) face of Te not
binding with the thiol as strongly as other faces. Alternatively,
the thiol may bind to all exposed faces, impeding the accretion
of Te atoms onto the growing nanorods, and giving thermo-
dynamic control rather than kinetic control to Te deposition.
The nanorods comprise lengthwise covalently bonded Te
chains which are capped with the (001) crystal faces, whereas
atoms deposited on the side faces are held together by weaker
van der Waals attractions;14 thus, a nanorod morphology
might be expected from thermodynamic shape-control.
UV-vis absorption spectra were obtained for the aqueous
micellar dispersion of the Te nanorods (see ESIw). The
spectrum revealed two distinct absorption bands with
lmax = 294 and 402 nm, and featureless control spectra of
the micelles after tellurium precipitation (see ESIw) prove that
the observed absorption bands are a result of the Te nano-
structures. Previous studies have assigned these two distinct
High-resolution TEM (HRTEM) analysis of the nanorods
indicates growth along the [001] direction (or crystallographic
c-direction) of the t-Te structure, as evidenced by the observation
absorptions to band transitions between p-bonding
-
p-antibonding bands (270 nm), and between p-lone-pair -
p-antibonding bands (617 nm).16 Similar absorption bands
have also been observed for Te thin films (i.e., 270 and
561 nm).17 It is possible that the blue shift observed for the
low energy band relative to literature values is a result of the
small diameter and/or elongated morphology; however, quantum
confinement effects in Te nanocrystals have not been studied
and further studies are warranted to explain the energy shift
of this band. The ellipsoidal nanocrystals formed in the
absence of dodecanethiol only show a single absorption band
(lmax = 352 nm) in the UV-vis spectrum, again suggesting that
the electronic transitions in Te nanostructures are sensitive to
their morphological properties.
Fig. 2 TEM images of Te nanorods at (a) low and (b) high resolution.
The high resolution image clearly shows the (001) lattice planes lying
normal to the nanorod growth direction. (c) TEM image of
the ellipsoidal Te nanocrystals formed without the addition of
dodecanethiol.
Fig. 3 XRD pattern of the nanorods (blue) and ellipsoidal nano-
crystals (red) indexed to the trigonal phase of tellurium.
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In summary, we have presented a novel photolytic preparation
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of well-defined t-Te nanorods. The synthesis is based on the
t
facile photolytic decomposition of Bu2Te2, which produces
elemental Te without the use of chemical reductants. Moreover,
control over the nanorod morphology was rendered possible
by addition of dodecanethiol to the aqueous micellar system.
The resulting nanorods are expected to be useful morphology-
directing synthons for a number of 1-D semiconductor
tellurides.
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This material is based upon work supported by the National
Science Foundation under DMR-0906745. This work was
also supported by generous financial assistance from the
Department of Chemistry and the College of Letters, Arts &
Sciences at the University of Southern California. We thank
the Anton Burg Foundation for contributing to the purchase
of the XRD. Acknowledgement is also made to the Molecular
Materials Research Center of the Beckman Institute at the
California Institute of Technology for use of XPS.
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