synthesis and characterization of wo3 nanostructures prepared by an aged-hydrothermal method

Synthesis and characterization of WO 3 nanostructures preparedby an aged-hydrothermal method R.. The growth direction of the tungsten oxide nanostructures was determined along [010] axis

Synthesis and characterization of WO 3 nanostructures prepared

by an aged-hydrothermal method

R Huirache-Acuñaa,b,⁎, F Paraguay-Delgadoc,1, M.A Albiterd, J Lara-Romerod,

aCFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico

bUniversidad La Salle Morelia, Av Universidad 500, Mpio Tarímbaro Mich., 58880, Mexico

cCentro de Investigación en Materiales Avanzados, S.C CIMAV, Laboratorio Nacional de Nanotecnología-Chihuahua, Miguel de Cervantes

120, Complejo Industrial Chihuahua, Chih., 31109, Mexico

d

Facultad de Ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, Morelia Mich., 58000, Mexico

A R T I C L E D A T A A B S T R A C T

Article history:

Received 7 November 2008

Received in revised form 5 March 2009

Accepted 9 March 2009

Nanostructures of tungsten trioxide (WO3) have been successfully synthesized by using an aged route at low temperature (60 °C) followed by a hydrothermal method at 200 °C for 48 h under well controlled conditions The material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) Specific Surface Area (SBET) were measured by using the BET method The lengths of the WO3

nanostructures obtained are between 30 and 200 nm and their diameters are from 20 to

70 nm The growth direction of the tungsten oxide nanostructures was determined along [010] axis with an inter-planar distance of 0.38 nm

© 2009 Elsevier Inc All rights reserved

Keywords:

Nanostructures

Tungsten trioxide (WO3)

Hydrothermal method

1 Introduction

In recent years, studies of transition metal oxides

nanos-tructures have become important due to the different

potential applications, such as nanoelectronics[1], gas sensors

[2,3], optical devices [4], electrochromic windows [5,6], and

catalysts [7,8] Nanostructures of tungsten oxide having

nanometer scale had been formed by using different

condi-tions and preparation methods: electrospinning[9], oxidation

of a substrate under appropriate conditions and the deposition

of tungsten oxide from a tungsten foil heated in the presence

of oxygen [10], by heating a tungsten filament in a partial

oxygen atmosphere[11], by reacting WO(OMe)4under

auto-genic pressure at elevated temperature followed by annealing [12], by hot filament chemical vapor deposition[13], physical vapor deposition process[14]and by ultrasonic spray and laser pyrolysis techniques[15–17] Recently, Therese et al.[18]and

Ha et al.[19]reported the synthesis of WO3nanostructures by following a hydrothermal route They used as raw materials ammonium salts and some additives to control the formation

of nanomaterials In comparison with the methods mentioned before, the hydrothermal route is an economical preparation method of nanostructures since it does not require an expensive experimental setup In this work, we report the synthesis and characterization of WO3 nanostructures pre-pared by following an easy two step aged-hydrothermal

⁎ Corresponding author CFATA-UNAM, Boulevard Juriquilla 3001, Juriquilla Querétaro, 76230, Mexico Tel.: +52 442 238 11 43; fax: +52 442 238 11 65

E-mail address:rafael_huirache@yahoo.it(R Huirache-Acuña)

1Present address: National Institute for Nanotechnology, 11421 Saskatchewan Drive Edmonton (AB) Canada T6G 2M9

1044-5803/$– see front matter © 2009 Elsevier Inc All rights reserved

doi:10.1016/j.matchar.2009.03.006

method at low temperature by using ammonium

metatung-state as tungsten source and without the presence of

additives

2 Experimental

2.1 Synthesis of WO3Nanostructures

WO3nanostructures were synthesized by using an

aged-hydrothermal route A saturated aqueous solution of

ammo-nium metatungstate [(NH4)10W12O41xH2O] (0.15 mol of W) was

prepared and acidified with HNO32.2 N (Normal) to produce a

pH around 5 and then kept in a flask hermetically sealed with

stirring by one week at 60 °C Then, 5 ml of the aged solution

was deposited into a Teflon-lined stainless steel autoclave and

heated at 200 °C for 48 h The material obtained was filtered

and washed with deionized water and dried in the presence of

air at room temperature

2.2 Characterization

A scanning electron microscope (JEOL JSM 5800 LV) was used

to perform morphological analysis Several fields were analyzed

at different magnifications in order to get information of the

prevalent features The elemental composition was determined

using energy dispersive spectroscopy (EDS) (Oxford Inca

X-Sight) Specific surface area (SBET) determination was made with

a Quantachrome AUTOSORB-1 model by nitrogen adsorption at

−196 °C using the BET isotherm Samples were degassed under

flowing argon at 200 °C for 2 h before nitrogen adsorption TEM

and HRTEM micrographs were obtained in a Philips TECNAI F20 FEG transmission electron microscope operated at 200 kV X-ray analysis was made with a Philips X Pert MPD diffractometer, equipped with a graphite monochromator, copper Kα radiation with wavelengthλ=1.54056 Å, operated at 43 kV and 30 mA Raman spectroscopy was performed using a Labram system model Dilor micro-Raman equipped with a 20 mW He–Ne laser emitting at 632.8 nm and a holographic notch filter made by Kaiser Optical Systems, Inc (model supertNotch-Plus) with a

256 × 1024-pixel charge-coupled device (CCD) used as the detector; and a computer-controlled XY stage with a spatial resolution of 0.1 µm with two interchangeable gratings (600 and

1800 g mm− 1) and a confocal microscope with 10, 50, and 100× objectives All measurements were collected at room tempera-ture with no special sample preparation Oxidation state and surface composition were analyzed by an X-ray photoelectron spectroscope, Energy Spectrometer EA 11 MCD, using Mg monoenergetic soft X-ray (Kα=1253.6 eV)

3 Results and Discussion

The synthesis method presented in this work for procuring

WO3nanostructures is a modification of the method reported

by Albiter et al for MoO3nanorods[20]and Therese et al for

WO3 nanostructures [18] The main difference between Therese et al method and ours is that we did not use additives

to form the nanostructures On the other hand, nitric acid (HNO3) was not used before the hydrothermal treatment as reported by Albiter et al

To convert W12O41 10− anions to neutral W12O36, excess divalent oxygen anions must be removed Stoichiometrically, five divalent oxygen anions per W12O4110−must be combined with protons from the acidic medium:

W12O 10−

According to reaction (1), high concentrations of both

W12O41 −and H+would shift the reaction to the right ensuring the formation of WO , although many intermediate steps and

Fig 1– XRD pattern for WO3nanostructures Where all

reflections are indexed based on a hexagonal WO3cell

Table 1– Crystallite size (ϕ) determined by Scherer

equation (ϕ=Kλ/βCosθ), where K=0.9, λ=1.54 Å, β is

FWHM in radians andθ is the glancing angle

Fig 2– N2adsorption–desorption isotherm at −196 °C of tungsten oxide nanostructures

thus compounds and phases may exist It is thus anticipated

that the formation of WO3should have a strong dependence

on the acid medium Furthermore, the aging time in solution

and the hydrothermal treatment time inside the autoclave

have a strong influence in the formation of nanostructures

Paraguay-Delgado et al.[21], concluded that two conditions

are important for procuring nanostructures, the first being

that for an extended-time-aged solution a short hydrothermal

treatment is required (about 24 h) and the second being that

for a short-time-aged solution (1 week) at least 36 h of

hydrothermal treatment is required

The XRD pattern from WO3nanostructures is reported in

Fig 1where a well crystallized phase was observed The

half-widths reflection indicates the presence of nanoscale

tung-sten oxide which was corroborated measuring the crystallite

size (ϕ) for more intense and representative reflections (100),

(001) and (200) (Table 1) by using the Scherer equation:

The observed peaks could be indexed based in a

hex-agonal cell with inter-planar spacings for tungsten trioxide

(ICSD 32,001, JCPDS 33-1387; a = 7.298 Å, c = 3.899 Å, space group P6/mm)[18,19] It was also observed that there are not other impurity phase peaks

As observed,Fig 2shows the N2adsorption–desorption curve corresponding to a type IV isotherm (IUPAC Classifi-cation) with desorption step characteristic of mesoporous materials above the relative pressure (P/Po) of 0.4 and specific surface area (SBET) values between 34 and 35 m2

(Table 2) The formation of a mesoporous material is due to the water vapor pressure inside the autoclave at 200 °C, at this time the exactly mechanism of formation is not right known[22]

Fig 3a–c shows SEM micrographs at different magnifica-tions from separable WO3nanostructures with different size protruding out The oxide nanostructures had smooth sur-faces and a not well-defined rectangular cross section These particles were about 0.1 to 3 µm long, and 50–200 nm wide as determined from SEM images As illustrated inFig 3a–c this method led to the formation of nanostructures in a wide range

of thicknesses, most of which had shown an irregular shape The oxygen (O) and tungsten (W) atomic contents were determined by Energy Dispersive Spectroscopy (EDS) analysis (1% error) and the results are reported in Table 2 The EDS spectrum presented in Fig 3d reveals a 3:1 atomic ratio for oxygen and tungsten elements, which solely constitute the composition of WO3

Transmission electron microscopy (TEM) micrographs of

WO3nanostructures are reported (Fig 4a–b) A representative TEM image of the tungsten oxide nanostructures is given in

Table 2– Elemental analysis determined by EDS (% atomic)

and specific surface area (SBET)

Sample % at O % at W SBET(m2/g)

Fig 3– Scanning electron microscopy images of WO3nanostructures at different magnifications: (a) 5000×, b) 14,000× and c) 15,000× (d) EDS spectrum

Fig 4a This sample consists of very well separated particles

with irregular shape and lengths between 30 and 200 nm and

wide from 20 to 70 nm, aggregated together due to the high

surface energy owing to their nanosize.Fig 4b shows a TEM

micrograph at a higher magnification By using HRTEM we

observed that the growth direction of the tungsten oxide

nanostructures is along [010] axis with an inter-planar

distance of 0.38 nm (Fig 4b) It seems that the WO3growth

proceeds layer by layer increasing the thickness and the width

of the nanostructures

Raman spectroscopy was used to characterize this

material since this technique is suitable to obtain details

of the WO3chemical structure (Fig 5) Three broad bands

were clearly detected: high in the 900–1000 cm− 1 region,

medium in the 600–800 cm− 1region and low in the 200–

400 cm− 1 region The most intense peak is centered at

780 cm− 1with a shoulder at 730 cm− 1and they are attributed

to the symmetric and asymmetric vibrations of W6+–O

bonds (O–W–O stretching modes) Two peaks centered at

320 and 270 cm− 1can be found in the 200–400 range and correspond to W–O–W bending modes of the bridging oxygen [23–25] A peak at 910 cm− 1 with a shoulder positioned at 960 cm− 1 in the 900–1000 cm− 1 can be observed These peaks correspond to the WfO stretching mode of terminal oxygen atoms that are present on the surface of the cluster (dangling bonds) or at the boundaries

of nanometre grains[15,26] The small feature observed at

435 cm− 1is attributed to the characteristic band of crystal-line WO3[27] Note that these results confirm the formation

of hexagonal WO3since the main features corresponding to monoclinic WO3typically reported at 807 and 715 cm− 1are absent in the Raman spectrum[28–30]

The XPS collected spectra of the material for the peaks O1 s

and W4fare shown inFig 6a and b respectively Peaks position for O1 sis 530.3 eV and binding energy peak located at 35.4 and 40.6 eV is attributed to W4 f According to the literature[31]it is

WO3, this means that the surface of the material contains W6+

and not other oxidation state for this metal was detected This was also verified by calculating the O/W ratio using their relative peak areas (I) and atomic sensitivity factors (S), which

is shown below (Eq (3)):

Atomic Ratio O

W =

I 0

S 0

I W

S W

=

47483:97 0:66 63692:91 2:75

4 Conclusions

WO3nanostructures with hexagonal phase and mesopor-osity were obtained by using a two step aged-hydrothermal method By using HRTEM we observed that the growth direction of the tungsten oxide nanostructures is along [010]

Fig 4– Transmission electron microscopy (TEM) micrographs

of WO nanostructures

Fig 5– Typical Raman spectrum of hexagonal WO3

nanostructures obtained by the aged-hydrothermal method

axis with an inter-planar distance of 0.38 nm Raman, EDS and

XPS analysis confirmed that the chemical structure and

oxidation states belong to tungsten oxide (WO3)

Acknowledgements

The authors appreciate the valuable technical assistance

of M.C W Antúnez, M.I.Q Alicia del Real, M.C E Torres, Ing

C Ornelas, Dr A Medina, Dr Ismeli Alfonso and M.C F

Rodríguez Melgarejo This work was financially supported by

CIMAV, S.C., Universidad La Salle Morelia and Postdoctoral

fellowship-UNAM

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