Polleux et al.[32]successfully synthesized tungsten oxide hydrate WO3H2O nanoplatelets by Contents lists available atScienceDirect journal homepage:www.elsevier.com/locate/jcrysgro Journ
Trang 1Synthesis, growth mechanism and room-temperature blue luminescence
Jinmin Wang, Pooi See Lee , Jan Ma
School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e i n f o
Article history:
Received 21 July 2008
Received in revised form
24 October 2008
Accepted 5 November 2008
Communicated by K Nakajima
Available online 13 November 2008
PACS:
68.70.+w
81.05.Dz
81.10.Dn
81.20.Ka
Keywords:
A1 Crystal morphology
A2 Growth from solutions
B1 Nanomaterials
B1 Oxides
a b s t r a c t
Uniform single-crystalline tungsten oxide (WO3) nanosheets have been synthesized in a large-scale with W powders as starting material The results from the thermal stability measurements show that the as-synthesized WO3nanosheets are anhydrous Their thickness and length are 80 and 500 nm, respectively They exhibit blue luminescence emissions at 431, 486 and 497 nm, UV emissions at 362 and 398 nm The blue emissions are resulted from the band–band indirect transition and the UV emissions should be attributed to the defect states of WO3 The growth mechanism of the two-dimensional WO3nanosheets is discussed
&2008 Elsevier Ltd All rights reserved
1 Introduction
Blue luminescence emission has attracted much attention due
to the applications in short-wavelength laser [1], light-emitting
diode (LED) [2] and white light source [3] A number of
blue-emission semiconductor materials have been studied, such as
GaAs, GaN, ZnSe [4–7] These materials are direct band-gap
semiconductors which readily emit luminescence On the other
hand, tungsten oxide (WO3) is an indirect band-gap
semiconduc-tor [8], which has been extensively studied due to their
applications in electrochromic [9], photocatalytic [10] and gas
sensing materials [11] Less attention has been paid to the
luminescence properties of WO3 because of the low emission
efficiency in conventional indirect band-gap semiconductors
However, recently, much progress has been realized for the
luminescence of WO3 Manfredi et al [12] reported the light
emission in WO3thin films, at which the excitation temperature is
at liquid nitrogen temperature Niederberger et al [13–15]
realized the room-temperature blue emission of WO3
nanoparti-cles in ethanol solution Feng et al [8]also reported the
room-temperature strong photoluminescence (PL) of WO3nanoparticles and W18O49 nanowires It is believed that the particle size, morphology and quantum-confinement effect played an impor-tant role for the room-temperature luminescence emission[16] Recently, a great deal of efforts has been focused on the morphology control of all kinds of nanostructures due to their morphology dependent properties [17–23] Nanosheets, one of two-dimensional (2D) nanostructures with one distinct thin thickness, have many special potential applications in electrical, optical, photochemistry, sensors and ion-exchange properties
[24] Relative to the comprehensive investigations in zero- and one-dimensional (1D) nanostructures, 2D nanostructures are almost neglected for the last decade One important reason is the synthesis of single-crystalline 2D nanostructures is more difficult to control However, much progress has been achieved recently for the synthesis of ZnO, TiO2, Fe3O4, Mn3O4, CoO, Ga2O3 and complex hydroxide nanosheets[24–31] It is well known that tungsten oxides contain many non-stoichiometric sub-oxides (WO3 x) Moreover, some hydrates often exist in the products from a wet chemical route for the synthesis of WO3, making it difficult to synthesize stoichiometric WO3nanosheets Only a few research groups reported the synthesis of WO3and tungsten oxide hydrates (WO3xH2O) nanosheets Polleux et al.[32]successfully synthesized tungsten oxide hydrate (WO3H2O) nanoplatelets by
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/jcrysgro
Journal of Crystal Growth
0022-0248/$ - see front matter & 2008 Elsevier Ltd All rights reserved.
Corresponding author Tel.: +65 67906661; fax: +65 67909081.
E-mail address: pslee@ntu.edu.sg (P.S Lee).
Trang 2heating the system of tungsten chloride (WCl6)-benzyl alcohol.
Considering the poor thermal stability of WO3H2O as it can
decompose and release water molecules at higher temperatures,
anhydrous WO3will be more stable for practical applications Kim
et al [33] reported a similar system of WCl6 in ethanol to
synthesize WO3 nanosheets Recently, Zhang et al [34]
synthe-sized ultrathin WO3nanodisks using poly(ethylene glycol) (PEG)
as a surface modulator However, the luminescence properties of
WO3 nanosheets were not studied and only the last paper
proposed a possible growth mechanism for the WO3 nanodisks
Moreover, when WCl6 was used as the starting material, the
reaction medium must be organic solvent because WCl6will quite
strongly hydrolyze to release hydrogen chloride in aqueous
solution Besides, it is expensive to use WCl6 as the starting
material So an aqueous approach using common tungsten source
must be developed for the large-scale synthesis of WO3
nanosheets Obviously, metal tungsten (W) is the ideal tungsten
source for the preparation of WO3 However, up to now, no one
has reported the synthesis of WO3 nanosheets using W as the
starting material
Herein, we report a much more economical approach to the
large-scale synthesis of uniform stoichiometric and anhydrous
WO3nanosheets from W powder and a successful demonstration
for the room-temperature blue emissions from the resultant WO3
nanosheets The blue emissions were attributed to arise from the
band–band indirect transition of WO3 And a new growth
mechanism for the as-synthesized WO3nanosheets was proposed
2 Experimental procedure
The precursor was prepared by oxidation of W using hydrogen
peroxide (H2O2)[35,36] In a typical synthesis, 6.5 g of W powder
was dissolved into a mixed solution of 40.0 mL of H2O2and 4.0 mL
of H2O with an ice-water bath and stirring After filtration, a light
yellow solution was obtained The solution was refluxed at 55 1C
for 6 h, then yellow concentrated sol was formed After aging at
room temperature, yellow precipitate was separated out and used
as the precursor A total of 0.4 g of precursor was added into
19.0 mL of de-ionized water to form a suspension and 3 M HCl was
dropped into the suspension until its pH value is up to 1.7 Then,
the suspension was transferred into a Teflon-lined autoclave with
a capacity of 45 mL The autoclave was placed into an oven and
heated at 180 1C for 24 h After cooling down to room temperature,
yellow product was obtained
The phase of the product was identified by X-ray powder
diffraction (XRD), using Cu Ka(l=0.15406 nm) radiation in a 2y
range from 101 to 801 at room temperature The morphologies of
the as-synthesized WO3nanosheets were characterized by
field-emission scanning electron microscopy (FESEM) The FESEM
sample was prepared by coating Pt using a sputtering machine
at a beam current of 20 mA for 45 s Transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM) images,
selected area electron diffraction (SAED) pattern of the WO3
nanosheets were obtained using an accelerating voltage of 200 kV
The thermal stabilities of the precursor and the product are
examined by thermal gravimetric analysis (TGA) in air The PL
properties of the as-synthesized WO3nanosheets were measured
on a fluorescence spectrophotometer with a Xe lamp as the
excitation source at room temperature
3 Results and discussion
The XRD patterns of the precursor and the product are shown
inFig 1a All the XRD peaks of the precursor can be identified as
tungsten oxide hydrate (WO3H2O) (JCPDS 18-1419) Compared to the XRD pattern of the precursor, significant differences can be found from the XRD pattern of the product, showing a chemical reaction has occurred in the process of hydrothermal treatment All of the XRD peaks of the product can be indexed to the orthorhombic structure of WO3 (JCPDS 71-0131) No non-stoichiometric tungsten oxides (WO3 x) and tungsten oxide hydrates (WO3xH2O) were detected, indicating pure orthor-hombic WO3 has been obtained Strong and sharp diffraction peaks also indicate good crystallinity of the hydrothermal product Fig 1b shows TGA curves of the precursor and the product It can be seen that the precursor has an obvious weight loss within 150–200 1C, corresponding to the decomposition of
WO3H2O and formation of WO3 In contrary, the TGA curve of the product is almost a straight line, which implies the product do not contain any hydrate This result from thermal analysis is well consistent with the XRD result
Fig 2a and b shows the FESEM image and TEM image of the as-synthesized WO3nanosheets Uniform WO3nanosheets with a thickness of 80 nm and length of 500 nm can be observed Further structural characterizations were carried out by HRTEM The clear crystal lattices show that the as-synthesized WO3 nanosheets are single crystals The calculated lattice spaces are 0.385 and 0.376 nm in the 2D plane of a single nanosheet, which correspond to the plane distances of (0 0 2) and (0 2 0) planes, respectively And the angle between the (0 0 2) and (0 2 0) plane is
901 These crystal parameters are well consistent with that from standard XRD data It can be regarded that the WO3crystals grew
(001)
(020) (200) (220) (021) (111)
10
100
98
96
product precursor
94
92
2 / (°°)
(121)
(221)
WO3 H2O
Fig 1 (a) XRD patterns and (b) TGA curves of the precursor and the product
Trang 3along two perpendicular directions, [0 0 2] and [0 2 0] to form 2D
nanosheets
The formation process of WO3 nanosheets from W can be
divided into three parts: the formation of precursor, the formation
of WO3by decomposition of WO3H2O and growth of WO3crystal
nucleus Peroxy-tungstates were formed by the action of H2O2on
W [35,36] After ageing, the precursor WO3H2O precipitated
from the solution, which has been proved by XRD data (Fig 1a)
The results from thermal analysis show the precursor WO3H2O
can decompose into anhydrous WO3at lower temperature than
the applied hydrothermal temperature So WO3 can be easily
formed in our experimental conditions However, the subsequent
crystal growth is dominant for the formation of WO3nanosheets
According to the existing steps of the WO3nanosheets (Fig 2a, b
and d), it is believed that the obtained WO3nanosheets grow by
step-growth mechanism which resulted from the periodic bond
chain (PBC) theory developed by Kossel, Stranki and Volmer[37]
According to the step-growth theory[37], an atom adsorbed onto
a facet will diffuse randomly on the surface until it reaches an energetically favorable site and will subsequently be incorporated into the crystal structure by generating chemical bonds This growth process will reduce the total energy of the atom and the crystal If more chemical bonds can be generated at some site, that site will be the preferred growth site Compared with a terrace (smooth surface), a corner of a given crystal can provide more unsatisfied bonds to generate new chemical bonds with an incorporated atom Thus, the crystal will be in a thermodynami-cally stable state Why did the WO3crystal nucleus grow into 2D nanosheets instead of 3D crystals? This is due to the distinct growth rate of different crystal facets with different surface energy It is well known that the facets have different atomic densities and unsatisfied bonds, resulting in variations of surface energy The facet with a larger surface area has a smaller surface atom density, which results in a lower surface energy For orthorhombic WO3with a=7.341, b=7.570 and c=7.754, the (2 0 0) facet has the largest surface area and lower surface energy, resulting in a lowest growth rate in the [2 0 0] direction In contrary, the other directions of [0 0 2] and [0 2 0] have higher growth rate, resulting in the preferred growth along [0 0 2] and [0 2 0] directions Hence the resultant shape of the crystals is a 2D nanosheet This mechanism differs from WO3 nanorods or nanodisks growth [23,34]which emphasizes the use of surface modulators or structure-directing agents For the growth of WO3 nanorods, some capping agents (Cl ions) cap some facets of WO3 crystal nuclei, resulting in slow growth rates of these facets and one fast growth rate of a special direction (c-axis)[23] For the growth of WO3nanodisks[34], it is believed that the formation
of WO3 nanodisks is driven by the preferential adsorption of poly(ethylene glycol) (PEG—1 0 0 0 0) onto the (0 1 0) crystal facets
of WO3, thereby inhibiting crystallographic growth However, the growth process of the as-synthesized WO3 nanosheets does not dependent on any surface modulators or structure-directing agents, which belongs to facet growth
The PL properties of the as-synthesized WO3nanosheets were measured using a Xe lamp as the excitation source at room temperature.Fig 3shows the excitation and emission spectrum When the excitation wavelength is 315 nm (Fig 3a), the emission peaks of the as-synthesized WO3nanosheets consist of two UV emissions at 362 and 398 nm and three blue luminescence emissions at 431 (2.88 eV), 486 (2.55 eV) and 497 nm (2.49 eV) (Fig 3b) The blue emissions (2.88, 2.55, 2.49 eV) are in the range
of reported band-gap energies of WO3 [12,38,39], so the blue emissions should be attributed to the indirect band–band
Fig 2 (a) FESEM, (b) TEM and (c), (D) HRTEM images of the as-synthesized WO 3
nanosheets.
275 Wavelength (nm)
Wavelength (nm)
Trang 4transition of WO3and the UV emissions should arised from the
defect states of WO3 Niederberger et al [13] also suggested
that the blue emission resulted from band–band transition of
WO3 Recently, Zhao et al.[40] also found similar PL properties
in WO3 x nanowire networks and attributed it to the
above-mentioned explanation by measuring the changes of emission
peaks with the changing excitation wavelengths
4 Conclusions
In summary, uniform single-crystalline WO3nanosheets have
been synthesized in a large-scale with W powders as starting
material by a facile hydrothermal process The thermal stabilities
of the precursor and the as-synthesized WO3 nanosheets were
studied The results show that the WO3nanosheets are
stoichio-metric and anhydrous whereas the precursor contains hydrates
The developed novel process for the synthesis of WO3nanosheets
is much more economical than the previous methods The growth
mechanism of the 2D WO3 nanosheets is proposed The
as-synthesized WO3 nanosheets successfully exhibit blue
lumines-cence emissions at 431, 486 and 497 nm, UV emissions at 362
and 398 nm The blue emissions are resulted from the band–band
indirect transition and the UV emissions should be attributed to
the defect states of WO3
Acknowledgement
The authors thank Mr Liap Tat Su for the assistance in PL
measurements
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