prepared by a hydrothermal route Helen Annal Theresea, Jixue Lib, Ute Kolbb, Wolfgang Tremela, ∗ aInstitut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universit
Trang 1prepared by a hydrothermal route Helen Annal Theresea, Jixue Lib, Ute Kolbb, Wolfgang Tremela, ∗
aInstitut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg 10-14, 55099 Mainz, Germany
bInstitut für Physikalische Chemie, Welderweg 11, 55099 Mainz, Germany
Received 17 June 2004; received in revised form 27 July 2004; accepted 8 October 2004
Available online 13 December 2004
Abstract
Hexagonal WO3nanorods of 5–50 nm in diameter and 150–250 nm in length have been synthesised in gram quantities by a low temperature hydrothermal route using citric acid as a structural modifier and hexadecylamine as a templating agent The ratio of [A]/[W] play an important role on WO3 nanorods formation These WO3nanorods were found highly suitable as a precursor for the synthesis of a good yield of multiwalled WS2nanotubes by reducing them with H2S at 840◦C for 30 min The length and the wall thickness of the WS2nanotubes could
be altered by controlled reduction of the oxide precursor The morphology, structure and the composition of the WO3nanorods and WS2 nanotubes were characterised by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX) and by selected area electron diffraction techniques (SAED)
2004 Elsevier SAS All rights reserved
Keywords: WO3; Nanorods; Hydrothermal route; WS2; Nanotubes
1 Introduction
Following the discovery of carbon nanotubes in 1991[1],
nanostructured inorganic materials and their syntheses have
attracted tremendous attention due to their superior
mechan-ical properties, their unique electronic behaviour and their
high potential in making technologically advanced
nanode-vices The interesting properties of these nanomaterials arise
from their enormous surface area, strength and the
quan-tum size Among nanomaterials, nanowires and nanotubes
are good candidates for studying the phenomena such as
electrical resistivity, strength, magnetic and optical
proper-ties in one dimension During the past few years, nanotubes
of various materials with graphite-like layered structures
has been synthesised successfully using techniques such as
arc discharge [2], laser ablation [3], electron beam
irradi-ation[4], sonochemical[5], hydrothermal reaction[6]and
* Corresponding author.
E-mail address:tremel@mail.uni-mainz.de (W Tremel).
iodine transport[7], etc Among the various classes of non-carbon nanotubes, transition metal chalcogenides of the gen-eral formula MQ2 (M = W, Mo, V, Nb, Ta, Zr; Q = S, Se) are significant materials and reveal interesting electronic and optical properties[8–10] Inorganic fullerene like MoS2 nanoparticles and MoS2 nanotubes exhibit excellent lubri-cating properties[11]and show high scope as tips for scan-ning probe microscopes[12] Recent electrochemical studies
on MoS2 nanotubes revealed that the nanotubes can store relatively large amounts of gaseous hydrogen by electro-chemical storage[13] MoS2nanotubes of up to 5 µm length and 10–20 nm in diameter were first synthesised by Tenne and co-workers by reducing MoO3in the atmosphere of a mixture of H2/N2and H2S gas[14]
Recently, WO3 nanorods were found to be a versatile precursor in the synthesis of WS2 nanotubes apart from its utility in electrochromic devices[15], gas sensors[16], rechargeable lithium batteries [17], memory devices [18], etc For example WS2 nanotubes were synthesised by an-nealing in a H2S stream a nanostructured tungsten oxide pre-cursor, which was produced by heating tungsten filaments
1293-2558/$ – see front matter 2004 Elsevier SAS All rights reserved.
doi:10.1016/j.solidstatesciences.2004.10.006
Trang 2yielded WO3nanorods[25] Controlled removal of the
sur-factants from the mesolamellar precursors resulted in WO3
nanorods in large quantity[26] Recently, the synthesis of
WO3nanorods at a large scale[27]has been described by the
oxidation of W(CO)6with an amine oxide in a low-volatile
solvent (oleylamine) at 270◦C Most of the above mentioned
synthetic techniques are very tedious and require expensive
experimental setup or inert atmosphere Above all, obtaining
a single phase compound of similar morphology and
dimen-sions in considerable quantity still is a difficult task In this
article we report the synthesis of WO3nanorods of 5–50 nm
in diameter and 150–250 nm in length, in gram quantities
by a simple hydrothermal method WO3nanorods were
re-duced with H2S to obtain a very high yield of multiwalled
WS2nanotubes
2 Experimental
2.1 Synthesis of WO 3 nanorods
An aqueous solution containing a mixture of 1.32 g of
(NH4)10W12O41·7H2O and 2.10 g of citric acid was heated
around 120◦C under constant stirring for 4–5 h until a gel
was formed, which was allowed to stand overnight 2.45 g of
hexadecyl amine dissolved in ethanol was added as an
addi-tive to the gel and stirred for 10 h The resulting mixture was
transferred to a Teflon autoclave with a stainless steel
protec-tive outer body and heated at 180◦C for 7 days The product
obtained was washed with ethanol, cyclohexene, water and
finally with ethanol and dried at room temperature The
im-portance of citric acid as a structural modifier was studied
by replacing citric acid with hydrochloric acid, while
main-taining the pH of the reactant same as the corresponding
experiment carried with citric acid
2.2 Conversion of WO 3 nanorods to WS 2 nanotubes
An alumina crucible containing WO3 nanorods was
placed in a tubular furnace and heated up to 840◦C in Ar gas
flow, then switched to H2S gas for 30 min at 840◦C to allow
the complete conversion of oxide nanorods to tungsten
sul-phide nanotubes and finally cooled to room temperature in
nanotubes was characterised by high-resolution transmission electron microscopy (FEI Tecnai F30 ST operated at an ex-traction voltage of 300 kV, equipped with an EDXA energy dispersive X-ray spectrometer) and by selected area electron diffraction techniques (SAED)
For transmission electron microscopic (TEM) studies, the sample was prepared by crushing them mechanically with
a mortar and pestle followed by dispersing the powder ul-trasonically in absolute ethanol and placing a drop of this suspension on to a copper grid coated with a holey carbon films
3 Results and discussion
Representative TEM images of the tungsten oxide sam-ples obtained from two different trials of hydrothermal reac-tions are given inFig 1(with m and w given in the caption
of Fig 1) Fig 1a shows a part of a WO3 particle with many nonseparable rods protruding out TEM image of these rods are shown at a slightly higher magnification in the in-set ofFig 1a This sample consists of particles exclusively grown in a bunch-like fashion.Fig 1b shows very well sep-arated WO3 nanorods aggregated together due to the high surface energy owing to their nanosize The nanorod lengths
in Fig 1b range from 150 to 250 nm and their diameters vary from 5 to 50 nm A calculation of the particle size dis-tribution of these samples shows that more than 85% of the tungsten oxide rods are within the range of 15–50 nm in di-ameter
The powder X-ray diffraction pattern of the WO3 nano-rods (shown in Fig 2) could be well indexed based on a hexagonal cell of WO3with lattice constants a = 7.37 Å and
c = 3.77 Å The lattice parameters of the WO3rods reported here vary slightly from the reported (ICSD code= 32,001;
a = 7.298 Å, c = 3.899 Å) values for the hexagonal WO3 The reflection half-widths indicate the presence of nanoscale
WO3rods The particle size of the nanorods calculated from the XRD pattern using Scherrer’s formula varies between 50 and 185 nm
Fig 3 shows experimental and simulated HRTEM im-ages and SAED diffraction patterns of a WO3nanorod The lattice parameters of 0.38 and 0.63 nm correspond to the
d -spacings of (001) and (100) of the WO hexagonal cell
Trang 3Fig 1 TEM images of WO3nanorods synthesised by hydrothermal reactions at different mole percentages m = [W]/([A] + [W] + [HDA]) (%) and ratios
of w = [W]/[HAD] (%) ([W], [A] and [HAD] correspond to the number of moles of ammonium tungstate, citric acid and hexadecylamine) (a) Depicts the
bunch-like morphology of the WO3particle grown at m = 3.42 and w = 18.6, whereas (b) shows the morphology of WO3rods obtained at grown at m = 1.8
and w = 1.6 The WO3rods are shown at higher magnification in the inset w values < 5 and m values 1 resulted in the formation of nanorods.
Fig 2 Powder X-ray diffraction pattern of WO3 nanorods All reflections
are indexed based on a hexagonal WO3cell with a = 7.37 Å, c = 3.77 Å.
These values are also in agreement with the lattice
parame-ters obtained from the powder XRD pattern All rods tend
to grow along the ‘c’ direction The high resolution image
filtered via FFT with DM3.6 (see inset (a)) is in good
agree-ment with the image of zone[010] (see inset (b)) simulated
by multislice method[28,29] (thickness of 37 Å, defocus
−855 Å, Cs= 1.2 mm) using Cerius[30] The same holds
for the dynamically calculated diffraction pattern compared
with the experimental SAED pattern Selected area energy
dispersive X-ray analyses (EDX) of individual nanorods
ex-hibit the existence of tungsten and oxygen in an atomic ratio
of 1:3
Orthorhombic WO3·1/3H2O needles has been
synthe-sised by Figlarz and co-workers[31]by hydrothermal
syn-thesis of tungstic gel at 120◦C These needles on heating at
250◦C yielded hexagonal WO
3, but there was nothing men-tioned about the yield of such needles In the present report,
it is important to mention that samples prepared for m 1
and w < 5 contain exclusively hexagonal WO3 nanorods
Recently, nanotubes of VOxhave been synthesised[32–34]
by low temperature sol–gel techniques In both cases of VOx
nanotube synthesis, hexadecyl amine used as a template gets
Fig 3 Experimental HRTEM image of a WoO3nanorod along b axis (top);
the filtered image is shown as inset (a) the corresponding simulated image as inset (b) together with experimental SAED pattern (bottom, left-hand side) and dynamically calculated ED pattern for zone [010] (bottom, right-hand
side).
intercalated into the vanadium oxide structure, resulting in
larger d -spacings ( ∼ 3 nm) From the lattice spacings of the
WO3 rods reported here one could see that the hexadecyl amine is not intercalated into the WO3nanorods This also helps us in availing this material as a precursor for WS2–NT synthesis Synthesis of WO3nanorods was also carried out using hydrochloric acid instead of citric acid, while main-taining the pH similar to the reaction which yielded WO3 nanorods This reaction resulted in a product containing a
Trang 4Fig 4 A low resolution TEM image of the WS2nanotubes (a) indicates the yield of nanotubes during the conversion process TEM image of hollow WS2 nanotubes (b) The sheet-like appearance in (b) are due to Moire patterns The open end of a nanotube is indicated by an arrow HRTEM micrograph of a typical MWNT (c) along with its SAED pattern (d) The chiral angle of the nanotubes could be calculated as ∼ 10◦based on the SAED.
mixture of longer rods of various thickness and unevenly
shaped crystalline particles Similarly synthesis carried out
for m < 1 and w < 5 resulted in a mixed product with more
of highly crystalline WO3 particles and a few bunch-like
WO3particles
In our study, the role of citric acid as a structural
mod-ifier and the mechanism involved in the growth of WO3 is
not clear But from the above study we could infer that the
amount of citric acid plays an important role in the
forma-tion of WO3nanorods One could speculate that at higher m
values, the 3 carboxylate group of each citric acid could bind
to more than one WO6octahedron and helping in the olation
of WO6in a directed manner while hindering the oxolation
in all directions due to steric effects This could lead to the
formation of shorter and thinner rods On the other hand, at
lower m values, two or more oxygens of the WO6
octahe-dra could be contributed from a single citric acid molecule,
hence hindering both the olation and oxolation to a large extent During hydrothermal reaction at 180◦C, when citric acid decomposes the hydrophobic hexadecyl amine template could be helping in preserving the rod like structure of the
tungsten oxides For w values > 5 the surface coverage of
the growing WO3nanorods is not sufficient to limit the par-ticle growth and WO3bunches and rods are obtained When citric acid is fully replaced by hydrochloric acid condensa-tion of WO6 takes place rather very quickly resulting in a mixture of longer and thicker rods and crystals
The nanorods were converted to WS2nanotubes by heat-ing the WO3 nanorods in Ar gas up to 840◦C and then treating them in hydrogen disulphide atmosphere for 30 min TEM images of WS2 nanotubes obtained after H2S reduc-tion (Fig 4a) show the high yield of WS2nanotubes How-ever a manifold increase in the diameter and the length of the WS tubes compared to the WO starting material was
Trang 5Fig 5 The various defects formed in a nanotube during the reduction process (a) HRTEM micrograph of a typical open ended WS2nanotube encapsulated
by a WS2mantle (b) The shorter nanotubes obtained when the nanorods were reduced for a duration of 10 min under H2S are shown in (c) and (d).
observed The nanotube thicknesses range broadly from 20
to 200 nm and their length varies approximately from 1
to 8 µm A large fraction of the nanotubes has open ends
(Fig 4b) Studies on these nanotubes by HRTEM combined
with EDX analyses reveal the complete conversion of oxide
rods to sulphide tubes during the reduction process which
allows the synthesis of large amounts of multiwalled
nan-otubes (MWNTs) A HRTEM image of one such
represen-tative MWNT is shown inFig 4c The interlayer spacing
of 0.65 nm between the tubular walls is consistent with the
(002) d -spacing of 2H–WS2lattice The helicity of the
nan-otube (Fig 4c) could be calculated as∼ 10◦ based on the
selected area diffraction (SAED) pattern (Fig 4c) of the
mul-tiwalled WS2nanotube[35]
The synthesis of WS2 nanotubes by reduction of WOx
nanorods has been reported earlier, where tungsten
disul-phide tubes were produced by heating WOx particles first
in flowing H /N mixture and then in flowing H S gas
In consistency with WS2 tubes reported by Tenne and co-workers[20]the WS2 nanotubes reported in this contribu-tion also contain both open (see the tubes marked by an arrow in Fig 4b) and closed ends (Fig 5b), and exhibit plenty of defects One such nanotube with defects is shown
inFig 5a A mechanism for the growth of nanotubes from oxide whiskers and rods has been proposed previously[19, 20] According to this mechanism the growth of WS2layer starts by encapsulating the WOx particle anisotropically in the initial phase of the reaction with the H2/N2 and H2S flow During the course of the reaction this embryonic WS2
layer starts growing inward as well as slowly converting the oxide, which is continuously growing on the other end of the particles by the condensation of WOx from the vapour state A similar mechanism is plausible in the present nano-tube synthesis, where the role of the reducing H2/N2gas has been replaced by the pretreatment of the oxide with Ar gas
A TEM analysis of the oxide rods after the pretreatment with
Trang 6with 3–5 tube walls Some of these particles also can be
de-scribed as a slightly elongated fullerene structured WS2
4 Conclusions
In conclusion, we have synthesised WO3 nanorods in
large quantities by a low temperature sol–gel route These
nanorods were then converted by reduction with H2S to get
WS2nanotubes in large quantities This simple and
inexpen-sive approach might be extended to the synthesis of other
MS2 nanotubes It was also possible to control the wall
thickness and the length of the nanotubes by selecting
ap-propriate reaction times for the reduction
Acknowledgement
We are grateful to the Federal Ministry for Research and
Technology (BMBF) for the support of this research within
the program “Multifunctional Materials and Miniaturized
Devices” at the University of Mainz and the Deutsche
Forschungsgmeinschaft (DFG, SFB 625)
References
[1] S Iijima, Nature 354 (1991) 56.
[2] N.G Chopra, R.J Luyken, K Cherrey, V.H Crespi, M.L Cohen, S.G.
Louie, A Zettl, Science 269 (1995) 966.
[3] T Guo, P Nikolaev, A Thess, D.T Colbert, R.E Smalley, Chem.
Phys Lett 243 (1995) 49.
[4] D Golberg, Y Bando, K Kurashima, T Sasaki, Appl Phys Lett 72
(1998) 2108.
4025.
[13] J Chen, S.L Li, Z.L Tao, J Alloys Compd 356 (2003) 413 [14] Y Feldman, E Wasserman, D.J Srolovitz, R Tenne, Science 267 (1995) 222.
[15] C Santato, M Odziemkowski, M Ulmann, J Augustynski, J Amer Chem Soc 123 (2001) 10,639.
[16] W.M Qu, W Wlodarski, Sens Actuators B 64 (2000) 42.
[17] P Poizot, S Grugeon, L Dupont, J.-M Tarascon, Nature 407 (2000) 496.
[18] I Turyan, U.O Krasovec, B Orel, T Saraidorov, R Reisfeld, D Man-dler, Adv Mater 12 (2000) 330.
[19] A Rothschild, G.L Frey, M Homyonfer, R Tenne, M Rappaport, Mater Res Innovat 3 (1999) 145.
[20] A Rothschild, J Sloan, R Tenne, J Amer Chem Soc 122 (2000) 5169.
[21] Y.Q Zhu, W.K Hsu, N Grobert, B.H Chang, M Terrones, H Ter-rones, H.W Kroto, D.R.M Walton, Chem Mater 12 (2000) 1190 [22] Y.Q Zhu, W Hu, W.K Hsu, M Terrones, N Grobert, J.P Hare, H.W Kroto, D.R.M Walton, H Terrones, Chem Phys Lett 309 (1999) 327 [23] Z Liu, Y Bando, C Tang, Chem Phys Lett 372 (2003) 179 [24] Y.B Li, Y Bando, D Golberg, K Kurashima, Chem Phys Lett 367 (2003) 214.
[25] G Gu, B Zheng, W.Q Han, S Roth, J Liu, Nano Lett 2 (2002) 849 [26] X.-L Li, J.-F Liu, Y.-D Li, Inorg Chem 42 (2003) 921.
[27] K Lee, W.S Seo, J.T Park, J Amer Chem Soc 125 (2003) 3408 [28] J.M Cowley, A.F Moodie, Acta Crystallogr 10 (1957) 609 [29] W.O Saxton, M.A O’Keefe, D.J.H Cockayne, M Wilkens, Ultrami-croscopy 12 (1983) 75.
[30] Cerius2, version 4.6MS, Molecular modeling environment, Accelrys Inc., 9685 Scranton Road, San Diego, CA 92121-3752, USA [31] M Figlarz, Rev Chim Miner 22 (1985) 177.
[32] F Krumeich, H.-J Muhr, M Niederberger, F Bieri, B Schnyder, R Nesper, J Amer Chem Soc 121 (1999) 8324.
[33] M Niederberger, H.-J Muhr, F Krumeich, F Bieri, D Günther, R Nesper, Chem Mater 12 (2000) 1995.
[34] G.T Chandrappa, N Steunon, S Cassaignon, C Bauvais, P.K Biswas,
J Livage, J Sol-Gel Sci Technol 26 (2003) 593.
[35] R Rosentsveig, A Margolin, Y Feldman, R Popovitz-Biro, R Tenne, Appl Phys A 74 (2002) 367.