Preparation of hexagonal WO 3 from hexagonal ammonium tungsten bronzeImre Miklo´s Szila´gyia,* , Lisheng Wangb, Pelagia-Irene Goumab, Csaba Bala´zsic, Ja´nos Madara´szd, Gyo¨rgy Pokold a
Trang 1Preparation of hexagonal WO 3 from hexagonal ammonium tungsten bronze
Imre Miklo´s Szila´gyia,* , Lisheng Wangb, Pelagia-Irene Goumab, Csaba Bala´zsic, Ja´nos Madara´szd, Gyo¨rgy Pokold
a
Materials Structure and Modeling Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, H-1111 Budapest, Szt Gelle´rt te´r 4, Hungary
b Department of Materials Science and Engineering, 314 Old Engineering Building, SUNY, Stony Brook, NY 11794-2275, USA
c
Ceramics and Nanocomposites Laboratory, Research Institute for Technical Physics and Materials Science, H-1121 Budapest, Konkoly-Thege u´t 29-33, Hungary
d
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt Gelle´rt te´r 4, Hungary
1 Introduction
As sensors to various gases (NH3, NO2, H2S, etc.) [1–3],
chromogenic (electro-, photo- and thermochromic) materials
[4–6]and catalysts in several acid-catalysed[7]or photocatalytic
[8]reactions, tungsten oxides are attracting continuous attention
Among a series of polymorphs of tungsten oxides, the
open-tunnel structure and intercalation chemistry, which is quite
different from ReO3-like stable phases The h-WO3 is usually
prepared by acidification and hydrothermal treatment (with
various promoting reactants) of alkali tungstates[9–13] However,
since a hydrothermal step is involved, the small dimension of
autoclaves used for research purposes limits the yield of h-WO3
powders In addition, these methods are time consuming because
the hydrothermal reaction can take hours or days, and very often
calcination is needed to obtain crystalline products
Besides wet chemical methods, thermal annealing of
bronzes[16,17]is also a viable way to prepare h-WO3 This latter route has the advantage that it requires less time and larger
which is not completely the same as the one produced by hydrothermal synthesis This is shown by that the differently
Nevertheless, a comparison of the published atomic coordinates
[10,14]of these differently prepared h-WO3samples show that the structures are basically the same (i.e both are built up by corner sharing octahedra, which form hexagonal and trigonal channels along the c-axis), and the differences between them are minor Therefore the application characteristics of the h-WO3 samples prepared by these two ways should not differ from each other significantly
Up to now, among the several application possibilities of h-WO3 prepared by thermal annealing, only the ion intercalation was studied[14,16] However, as gas sensors they were not tested and
it was still unknown whether thermal annealing of ammonium polytungstates and hexagonal ammonium tungsten bronzes could yield nanosize h-WO3
A R T I C L E I N F O
Article history:
Received 3 June 2008
Received in revised form 2 July 2008
Accepted 5 August 2008
Available online 12 August 2008
Keywords:
A Oxides
Semiconductors
C X-ray diffraction
Electron microscopy
D Electrical properties
A B S T R A C T Hexagonal tungsten oxide (h-WO3) was prepared by annealing hexagonal ammonium tungsten bronze, (NH4)0.07(NH3)0.04(H2O)0.09WO2.95 The structure, composition and morphology of h-WO3were studied
by XRD, XPS, Raman,1H MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and
BET-N2specific surface area measurement, while its thermal stability was investigated by in situ XRD The
h-WO3sample was built up by 50–100 nm particles, had an average specific surface area of 8.3 m2/g and was thermally stable up to 450 8C Gas sensing tests showed that h-WO3was sensitive to various levels (10–50 ppm) of NH3, with the shortest response and recovery times (1.3 and 3.8 min, respectively) to
50 ppm NH3 To this NH3concentration, the sensor had significantly higher sensitivity than h-WO3 samples prepared by wet chemical methods
ß2008 Elsevier Ltd All rights reserved
* Corresponding author Tel.: +36 1 463 4047; fax: +36 1 463 3408.
E-mail address: imre.szilagyi@mail.bme.hu (I.M Szila´gyi).
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Trang 2Recently we managed to prepare hexagonal ammonium
tungsten bronze (HATB), (NH4)0.07(NH3)0.04(H2O)0.09WO2.95 by
(NH4)10[H2W12O42]4H2O in H2 for 6 h at 400 8C[18] This has
allowed us to synthesize h-WO3in good quality from this HATB
sample and study its gas sensitivity
In this paper, h-WO3particles were successfully prepared in
large batch and short time by heating the HATB precursor The
structure, composition and morphology of the product were
spinning) NMR and scanning electron microscopy (SEM), while its
thermal stability was investigated by in situ high temperature
powder XRD Gas sensing layer was prepared from as-produced
h-WO3, and its sensitivity was tested to NH3
2 Experimental
(NH4)0.07(NH3)0.04(H2O)0.09WO2.95in air (15 l h 1) at 10 8C min 1
to 470 8C and then keeping it there isothermally for 3 min in an
open aluminium crucible in a Du Pont 910 DSC instrument To
check the reproducibility of this preparation route, several batches
of h-WO3were prepared, and it was found that the characteristics
of different h-WO3batches did not differ from each other Since it
was enough for research purposes, usually 1 g of powder was
produced in a batch, but the batch size can be increased easily
Therefore it was demonstrated that – compared to the common
wet chemical methods – this preparation route of h-WO3required
less time (1 h or even less), was reproducible, and could yield large
batches of h-WO3
measured by a PANalytical X’pert Pro MPD X-ray diffractometer
equipped with an X’Celerator detector using Cu Karadiation In
situ high temperature XRD patterns of h-WO3in static air were
collected by the same X-ray diffractometer in an Anton Paar
scanning time was ca 3 min for each pattern and the heating rate
X-ray Photoelectron Spectroscopy (XPS) spectra were recorded
by a VG Microtech instrument consisting of a XR3E2 X-ray source, a
analyser using Mg Karadiation Detailed scans were recorded with
50 eV pass energy at (0.05 eV/1.5 s) The spectrometer was
calibrated with the binding energy of the C1s line (284.5 eV)
Raman spectra were collected by a Jobin Yvon Labram
instrument attached to an Olympus BX41 microscope Frequency
doubled Nd-YAG laser (532 nm) was applied as exiting source with
1 mW applied power The sample was located and examined with a
50 objective, thus individual crystals could be examined (laser
spot size was about 1.2mm) The backscattered light collected by
the objective was dispersed on an 1800 g/mm grating and detected
by a 1024 256 CCD detector
VARIAN/Chemagnetics probe.1H chemical shifts were referenced
to adamantane (d1H= 0 ppm) Spectra were recorded under the
same experimental conditions 16 transients were acquired at
12 kHz spinning rate and a recycle delay of 20 s was used
signals from the probe
Scanning electron microscopy (SEM) characterization was
performed by a LEO-1550 FEG SEM instrument
BET specific surface area measurement was carried on by
nitrogen adsorption at 77 K (Micromeritics Gemini 2375) after
degassing the sample, at least, for 1 h at 150 8C in nitrogen
For gas sensing test, as-synthesized h-WO3particles were well grinded into powders The gas sensing layer was produced by spin
Al2O3substrates with Au-metallization Sensing tests were carried out in the gas flow bench set-up at SUNY, Stony Brook The gases used in the sensing setup were UHP nitrogen (Praxair), UHP oxygen (Praxair), 1000 ppm ammonia in nitrogen (BOC gases) Concentra-tion of ammonia was varied by varying its flow rate in conjuncConcentra-tion with nitrogen/oxygen flow rates
3 Results and discussion Based on the XRD pattern (Fig 1), the h-WO3 sample was
crystalline structure Its cell parameters (a = 0.7324 nm and
c = 0.7638 nm) were almost the same as the ones published by
Oi et al.[14] The Raman bands[20–22]were also typical to h-WO3(Fig 2) The main bands at 783, 692, 648 cm 1were characteristic O–W–O stretching vibrations The bands at 325, 300 and 264 cm 1could be assigned to O–W–O deformation vibrations The band at 456 cm 1 was related to a small amount of reduced W atoms The peak at
184 cm 1was a lattice mode of h-WO3 In contrast with the h-WO3
sample, the precursor HATB contained a significant amount of reduced W atoms (see XPS results below) As a consequence the absorption bands were very broad in the Raman spectrum of HATB
Fig 1 XRD pattern of h-WO 3
Fig 2 Raman spectra of (a) HATB; (b) h-WO
Trang 3The oxidation state of tungsten atoms was investigated by XPS.
Since HATB was partly reduced, W4+(5.4%) and W5+(13.8%) atoms
were also observed besides W6+(80.9%) atoms[18] When HATB
was heated in air at 470 8C, the product h-WO3showed an almost
fully oxidized structure (96.8% W6+, 1.8% W5+, 1.4% W4+) This was
also supported by the yellow color of h-WO3, while the partly
reduced HATB was dark blue
which remained in the solid structure, by solid-state1H MAS NMR
spectroscopy[18] After curve fitting the1H MAS NMR spectra, the
molecules respectively The1H MAS NMR results showed that the
amount of NH4 ions and NH3molecules decreased significantly in
h-WO3compared to HATB, but they were still present Recently we
determined the amounts of NH4 and NH3in h-WO3, which were
ca 0.11 and 0.04 wt.%, respectively[17]
precursor of h-WO3, i.e HATB was built up by aggregated 50–
100 nm particles (Fig 3a) The annealing of HATB did not affect the
built up by 50-100 nm particles, which were aggregated intomm scale blocks (Fig 3b and c)
BET measurement showed that the average specific surface area (SSA) of the particles was about 8.3 m2/g and the BET equivalent average diameter (dBET) was 100 nm, which is consistent with above SEM results Here, dBETis calculated as dBET= 6/(SSA rp), whererpis the weighted density of h-WO3(7.16 g/cm3) For gas sensing tests it was advisable to study the thermal stability of h-WO3 Based on in situ high temperature XRD patterns (Fig 4), the starting h-WO3structure remained nearly the same up
to 450 8C Then between 500–550 8C the hexagonal structure
43-1035), which later around 750 8C transformed reversibly into
showed that h-WO3was stable up to 450 8C[17], which made it safe to test h-WO3at 300 8C as a gas sensor
concentrations of NH3gas is shown inFig 5 The measurements were carried out at 300 8C The film showed a decrease in resistance on exposure to NH3, which is characteristic to an n-type semiconductor If we define the sensitivity as the ratio of baseline resistance to gas-responding resistance, its value was 2 when the
concentration to 20 ppm and 50 ppm step by step, the sensitivity increased to 4 and 6, respectively This means the h-WO3sensor is quite sensitive to different concentrations of NH3gas at 300 8C; in
hydrothermal route, which had a sensitivity of 3 for 50 ppm NH3at
300 8C[23] Besides, the response time was quite fast (3.3, 1.5, 1.3 min for 10, 20 and 50 ppm NH3, respectively), especially when
lowered down afterwards, the sensor resistance increased gradually accordingly It is clear to see that the sensor had almost the same resistance value at the same NH level as the first half part
Fig 3 SEM images of (a) HATB; (b and c) h-WO 3
Fig 4 In situ high temperature XRD patterns of h-WO 3 in static air recorded from r.t to 900 8C.
Trang 4of the test, respectively This means the sensor performance is
reversible and recoverable, which is important for its application
In addition, the recovery time was not long (9.0, 6.0, 3.8 min for 10,
20 and 50 ppm NH3, respectively) At the end of the test, the sensor
went to its baseline value when we stopped the NH3gas flow
4 Summary and conclusions
tungsten bronze, (NH4)0.07(NH3)0.04(H2O)0.09WO2.95 Compared to
the common wet chemical methods, this preparation route required
less time, it was reproducible, and it could yield large batches of
crystalline structure It contained small amounts of reduced
tungsten atoms, as well as residual NH4 ions and NH3molecules
The h-WO3sample was built up by aggregated 50–100 nm particles
and was thermally stable up to 450 8C Gas sensing layers were
prepared from as-produced h-WO3, and the gas sensor test showed
that h-WO3was sensitive to various levels (10–50 ppm) of NH3 Its
sensitivity to 50 ppm NH3 (6) was significantly higher than the
synthesis The sensor had the fastest response and recovery times
(1.3 and 3.8 min, respectively) also for 50 ppm NH3 Therefore, it
was demonstrated that this preparation route resulted in a h-WO3
sample whose gas sensing parameters were twice better than those
h-WO3samples, which were prepared by wet chemical methods In
addition, this route required less time, it was reproducible, and large
batches of h-WO3could be obtained
Acknowledgments A.L To´th (Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, Budapest, Hungary), A Szabo´ (Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, Budapest, Hungary), K Varga-Josepovits (Department of Atomic Physics, Budapest University of Technology and Economics, Budapest, Hungary) as well as P Kira´ly and G Ta´rka´nyi (Institute
of Structural Chemistry, Chemical Research Center of the Hungarian Academy of Sciences, Budapest, Hungary) are acknowledged for their help in performing the SEM, Raman,
thankful to Particle Technology Laboratory in ETH Zurich, Switzerland for providing BET surface analysis equipment A diffractometer purchase grant from the Agency for Research Fund Management (KPI-EU-GVOP-3.2.1.-2004-04-0224/3.0 KMA) is gratefully acknowledged
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