The important role of K2SO4 salt in the WO3 nanowires synthesis has been demonstrated.. Thus far, preparation of single-crystalline, 1D nanostructured tungsten oxide in mass quantity has
Trang 1Large-scale hydrothermal synthesis of WO 3 nanowires
a
Department of Chemistry, Fujian Normal University, Fuzhou 350007, P.R China
b
Coll Chem Engn & Mat Sci, Zhejiang University Technol, Hangzhou, Zhejiang 310014, P.R China
Received 7 July 2006; accepted 21 December 2006 Available online 30 December 2006
Abstract
WO3 nanowires were fabricated by a hydrothermal method in the presence of K2SO4 The nanowires exhibit a well crystallized one-dimensional structure with 10 nm in diameter and several microns in length Effects of other alkali salts (KNO3, NaNO3and Na2SO4) on the morphologies of WO3 nanocrystals were also investigated The important role of K2SO4 salt in the WO3 nanowires synthesis has been demonstrated
© 2006 Elsevier B.V All rights reserved
Keywords: Hydrothermal; WO 3 ; Nanowires; K 2 SO 4
1 Introduction
Over the past few years, much effort has been devoted to the
synthesis of semiconductor nanowires, nanorods, and nanobelts,
because of the importance of understanding the dimensionality
confined transport phenomena and fabricating nanodevices and
nanosensors[1–4] Many attempts have been made to synthesize
one-dimensional nanostructured materials[5–14] Of the
meth-ods used in 1D nanostructure synthesis, hydrothermal processes
have emerged as powerful tools for the fabrication of anisotropic
nanomaterials with some significant advantages, such as
controllable particle size and low-temperature, cost-effective,
and less-complicated techniques Under hydrothermal conditions,
many starting materials can undergo quite unexpected reactions,
which are often accompanied by the formation of nanoscopic
morphologies that are not accessible by classical routes
Among various metal oxides, WO3is a versatile wide
electrochromic devices [16], and photocatalyses[17] Thus far,
preparation of single-crystalline, 1D nanostructured tungsten
oxide in mass quantity has been accomplished by heating a
tungsten foil, covered by SiO2plate, in an argon atmosphere at
1600 °C[18]or recently by electrochemically etching a tungsten
employed harsh conditions, contamination by platelets, and uncontrolled size hamper systematic investigations on size-dependent properties of 1D nanostructured tungsten oxide itself
as well as of inorganic derivatives prepared from the oxide Recently, the hydrothermal synthesis of ultralong and single-crystalline Cd(OH)2nanowires using alkali salts as mineralizers was reported by Tang et al.[20] The 1D nanostructure synthesis using inorganic salt instead of surfactant and water-soluble high molecule has strong points in non-pollution, low-cost, easy-cleanout and recovery Herein, we describe a facile inorganic
solution This novel method is based on treating freshly prepared
conditions at 180 °C for 12 h
2 Experimental 2.1 Synthesis of WO3
form a transparent solution A (3 mol l− 1) HCl solution was added dropwise into the above solution under continuous
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doi: 10.1016/j.matlet.2006.12.055
Trang 2stirring until tungstenic acid was precipitated thoroughly Next,
the centrifuged precipitate was dissolved in 30 ml deionized
water, 40 g K2SO4was added to the system and agitated to form
starchiness, and then transferred into Teflon-lined autoclave
with a capacity of 50 ml Hydrothermal treatments were carried
out at 180 °C for 12 h After that, the autoclave was allowed to
cool down naturally The finally products were collected and
washed with deionized water and ethanol several times and
dried in air at 80 °C The WO3nanowires were finally obtained
2.2 Characterization
The morphologies were characterized using scanning
electron microscopy (SEM, Hitachi S-4700 II, 25 kV) and
transmission electron microscopy (TEM, JEM200CX, 120 kV) The composition of the product was analyzed by energy dispersive X-ray detector (EDX, Thermo Noran VANTAG-ESI,
120 kV) The X-ray diffraction (XRD, Thermo ARL SCINTAG
analyze the crystallinity
3 Results and discussion
The morphologies of the final products were demonstrated in Fig 1a–c On the basis of the SEM check, the proportion of the nanowire morphology was estimated to be about 100% (Fig 1a) As shown in the SEM images, the average diameter of these uniform nanowires was about 10 nm and the length was up to several microns (Fig 1b) Therefore, the nanowires reached a high aspect ratio of more than 500 A TEM image of a single nanowire with diameter of about
10 nm was shown in Fig 1c The selected area electron diffraction (SAED) taken from different parts of nanowires indicated that the sample was single crystalline with a preferential growth in the (001) direction The energy dispersive spectrometry (EDS) analysis was employed to determine the composition of the tungsten oxide nanowires As shown inFig 2, only oxygen and tungsten elements existed in the nanowires with molar ratio of about 3 (O/W) A representative XRD pattern for our synthesized tungsten oxide nanowire was displayed inFig 3 All the main peaks can be indexed undisputedly to hexagonal WO3 (JCPDS card 35-1001), which are consistent with general features of nanomaterials Diffraction peaks of
Fig 1 (a) SEM, (b) SEM, (c)TEM images of WO 3 nanowires synthesized at
180 °C for 12 h with 40 g K SO
Fig 2 EDS patterns of WO 3 nanowires.
Fig 3 XRD patterns of WO nanowires.
Trang 3(001) are stronger compared with the rest, indicating that the [001] is
the major growth direction This agrees well with the SAED results
The morphologies of synthesized WO3with different amounts of
K2SO4were shown in Fig 4a–c Fig 4a is the TEM image of the
sample obtained without K2SO4, which has a lamellar structure with
diameter of about 100 nm With the addition of 15 g K2SO4, the
morphologies of synthesized WO3shown inFig 4b are the mixture of
nanorods and nanoparticles Nanorods and nanowires were obtained
for the products synthesized with 30 g K2SO4(Fig 4c) As the content
of K2SO4reaches 40 g,Fig 4d shows the TEM image of the solid
sample, where nanowires with lengths around several micrometers and
diameter of about 10 nm are the major product From the results, it can
be speculated that the content of the assisted K2SO4plays an important
role in the specific morphologies of WO3 No nanowires of WO3could
be obtained without K2SO4, and the purity of the nanowires only
depends strongly on the content of K2SO4 We have also carried out
synthesis with 40 g K2SO4at 160 and 200 °C individually It could be
found that short WO3nanorods but generally mixed with nanoparticles
were produced at 160 °C However, the nanowires were obtained at
200 °C, and the morphology was similar with that obtained at 180 °C
In addition, we have carried out analogous experiments with
different inorganic salts for comparison Fig 5displayed the TEM
image of the obtained WO3with the addition of NaNO3, KNO3and
Na2SO4 It can be seen that no WO3nanowires could be obtained with any content of KNO3 With an increase in the content of KNO3, the dimension of nanoparticles only became smaller Similarly, the nanorods and nanoparticles were obtained in the products with the addition of NaNO3 or Na2SO4 The nanowires of WO3 were not obtained at last with any content of KNO3and Na2SO4 It could be concluded from the results mentioned above that K2SO4 plays an important role in the synthesis of WO3nanowires
The XRD patterns for WO3 synthesized with different kinds of inorganic salts were compared inFig 6 It is obvious that the crystalline phases for WO3nanocrystals are discriminatory at different conditions The hexagonal phase of WO3 (JCPDS card 33-1387) was obtained without salts or in the presence of KNO3, NaNO3and NaSO4(seeFig
6a–d) Among this, the intensity for the diffraction peaks grew weaker
as follows: no saltNKNO3NNaNO3NNaSO4 On the other hand,Fig
6e exhibited the hexagonal reflections (JCPDS card 35-1001) in the presence of K2SO4 Combined with the TEM results, it could be concluded that the inorganic salts had a significant effect on the crystalline phase and the corresponding morphology of WO3 The morphologies and dimensions of synthesized nanocrystals were controlled not only by the inner structure, but also affected by the
Fig 4 TEM images of WO 3 synthesized at 180 °C for 12 h with different amounts of K 2 SO 4 : (a) 0 g, (b) 15 g, (c) 30 g, and (d) 40 g.
Trang 4surrounding conditions such as temperature, pressure, and composition
of the solution [21] The formations of nanowires first need the
anisotropy during the growth process for the nanoparticles In our
experiments, the presence of the salts is also an important factor
influencing the crystallization process and the growth of the WO3
nanowires With the addition of different inorganic salts, the
compositions and the corresponding properties of the solution are
different in the hydrothermal conditions The changes in the
surrounding conditions would affect the crystalline phase, and further
affect the morphologies and dimensions of the WO3nanocrystals As
the reaction mechanism and hydrothermal conditions are complicated,
the exact reason for the 1D nanostructure synthesized in the presence of inorganic salts needs further investigations
4 Conclusion
In summary, tungsten oxide nanowires with relatively uniform diameters ranging from 10 to 20 nm and lengths up
to several micrometers were synthesized on a large scale With the distinctive and promising properties of tungsten oxide, the as-synthesized nanowires may serve as functional materials in the fabrication of nanosized sensors and flat panel display systems The important role of K2SO4salt in the synthesis has been demonstrated This aqueous route should be feasible for large-scale production of low-dimensional nanostructured tungsten oxide
Acknowledgment
We wish to acknowledge the financial support from the Natural Science Foundation of Fujian Province (no: 2006J0153) References
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