preparation of aqueous sols of tungsten oxide dihydrate

It is understood that the yellow gel precipitate deposited from the effluent of ion-exchange reaction after the ageing was a mixture of an amorphous product tungstic acids and a crystall

Preparation of aqueous sols of tungsten oxide dihydrate

from sodium tungstate by an ion-exchange method

Yong-Gyu Choia, Go Sakaib, Kengo Shimanoeb, Norio Miurac, Noboru Yamazoeb,* a

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,

Kasuga-shi, Fukuoka 816-8580, Japan

b

Department of Molecular and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan

c

Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan

Received 18 February 2002; received in revised form 16 May 2002; accepted 20 May 2002

Abstract

Aqueous sols of tungsten oxide dihydrate (WO3
2H2O) were prepared from Na2WO4by an ion-exchange method An aqueous solution of

Na2WO4was let to flow through a glass column packed with protonated cation-exchange resin The effluent, initially transparent, turned into

an opaque viscous fluid (pale yellow) in a few hours, before yellow precipitate deposited to completion in three days The precipitate was a mixture of a crystalline phase of WO3
2H2O and an amorphous phase, and the crystalline part could be separated from another by washing with deionized water and centrifuging The gel of WO3
2H2O thus obtained consisted of platelike crystallite 25 nm thick and 42 nm wide as evaluated from the X-ray diffractometer (XRD) peaks, and could be dispersed well into deionized water to form a stable suspension of colloidal particles with a mean diameter of about 30 nm The mean particle size as well as the crystallite size tended to increase gradually with the repetition of dispersion in water under ultrasonic wave agitation and gelling by centrifuging On heating, the gel (WO3
2H2O) changed to the monohydrate (WO3
H2O) at 100 8C, which in turn changed to the anhydride (WO3) at 240 8C Remarkably XRD patterns showed conspicuous preferred orientation of WO3
2H2O crystallites in (0 1 0) plane after the sol was centrifuged for a long time (10 h) and, upon dehydration, it was inherited by the dehydrated phases, resulting in the conspicuous orientation of WO3crystallites in (0 0 1)

# 2002 Elsevier Science B.V All rights reserved

Keywords: Tungsten oxide; Sol; Colloid suspension; Ion exchange; Sodium tungstate; Preferred orientation

1 Introduction

Tungsten trioxide (WO3) is known as a multi-functional

material applicable for electrochromic display device (ECD)

[1–3], semiconductor gas sensor [4–9], catalyst [9–11],

varistor [12,13] and so on It can combine with water

molecules to form crystalline phases of WO3
nH2O (n¼

2, 1 or 1/3), or amorphous phases of metatungstic acids and

isopoly tungstic acids These compounds take various

mole-cular and crystal structures, as reported in many literatures

[14–19] Such a variety in compound and structure appears

to be the origin of the multifunctionality of WO3 On the

other hand, this suggests that the functionality would depend

much on the method to prepare WO3 For applications to

semiconductor gas sensors, polycrystalline materials of

WO3have been prepared by various methods, i.e pyrolysis

of (NH4)10W12O41
5H2O, sputtering or evaporation from a

source of WO3, sol–gel method using W-alkoxide, etc It has been experienced well that the gas sensing performances differ significantly by the methods and conditions of WO3 preparation used, but why this is so has hardly been clarified well Tamaki et al have shown for sintered-block type gas sensors that the electrical resistance as well as the sensitivity

to NO2begins to increase sharply as the grain size (mean diameter) of WO3decreases to be less than a critical value and that the critical value would correspond to twice the surface space charge layer thickness of WO3 grains[20] There are still many other factors that should affect the gas sensing properties, such as porosity, pore size distribution, and gas sensing layer thickness These factors are deeply related with the material processing of WO3 We reported

[21]recently that preparation and control of SnO2sols in an aqueous medium were effective for controlling the physi-cochemical properties of SnO2-based sensors This finding prompted us to extend the same approach for the WO3 based sensors In the present case, however, the stable colloidal species in an aqueous medium is not WO3itself but its dihydrate, WO3
2H2O (monoclinic, a¼ 0:75 nm,

*

Corresponding author Tel.: þ81-92-583-7539;

fax: þ81-92-583-7538/7539.

E-mail address: yamazoe@mm.kyushu-u.ac.jp (N Yamazoe).

0925-4005/02/$ – see front matter # 2002 Elsevier Science B.V All rights reserved.

PII: S 0 9 2 5 - 4 0 0 5 ( 0 2 ) 0 0 2 1 8 - 6

solution of Na2WO4 by an ion-exchange method; the

Na2WO4solution was let to flow through a column packed

with a cation-exchange resin converted into the proton type

in advance This method is a modification of the

acidifica-tion method, but may be more advantageous because no

sodium salt is left in the effluent It has been reported[22]

that, in the acidification method, a trace amount of

remain-ing Naþ ions gives significant effects on the kinds and

properties of the acidification products This paper aims

at reporting the ion-exchange based preparation of

WO3
2H2O sols from Na2WO4 Formation and

character-ization of the sols as well as the effects of post treatments are

described together with the thermal behavior of the

corre-sponding gels

2 Experimental

A commercial cation-exchange resin (Diaion SK 1B,

Mitsubishi Chemical Co.) was immersed in an acid solution

(HNO3) for 1 h to convert it from Naþtype to Hþtype After

washing with distilled water five times, the resin was packed

uniformly in a glass column and washed again with distilled

water repeatedly until pH of the effluent came close to 7 The

ion-exchange capacity (content of protons) of the resin was

about 2 meq./cm3, as evaluated from the titration with an

NaOH solution

Sodium tungstate was purchased as its dihydrate

(Na2WO4
2H2O) and used without further purification Its

aqueous solution was let to flow down through the glass

column at a fixed rate, and the effluent was collected into a

beaker After standing for 3 days, the effluent precipitated a

yellow gel containing WO3
2H2O The particle size

distri-butions of sols were analyzed on a laser particle size

analyzer (Photal Otsuka Electronics, LPA 3100) The

crys-talline compounds were identified for dried or calcined

samples by using an X-ray diffractometer (Rigaku RINT

2100), while their crystallite sizes were evaluated from the

full width of half maximum intensity (FWHM) values of

X-ray diffractometer (XRD) peaks by using Scherrer’s

equa-tion In some cases, colloidal particles were subjected to

direct observation on a transmission electron microscope

(JEOL JEM-4000EX) The content of Naþions in the gels

was analyzed by fluorescence X-ray spectroscopy (Rigaku,

Fluorescence X-ray Spectrometer 3270)

at room temperature The resulting titration curves are shown in Fig 1 The curves exhibited two inflections in the pH ranges of 7–5 and 4–2, respectively The color of the solution changed from none to light yellow at the first inflection, while a deep yellow precipitate deposited at the second These titration curves were confirmed to be essentially the same as those obtained in the hydrolysis with sulfuric acid It is known that WO4 ions, stable in an alkaline solution, condense together with a lowering in pH; the condensation products are paratungstate ions like [W12O41]10 and [H2W12O40]6 in the pH range of 7–4, while metatungstic acids ((WO3)n
xH2O) and related ions prevail in the pH range of 4–1[19] Based on this informa-tion, the first inflection seems to reflect the condensation to paratungstate ions, for instance, as follows:

12WO4 þ 14Hþ! ½W12O4110þ 7H2O (1) 12WO4 þ 18Hþ! ½H2W12O406þ 8H2O (2) The resin to Na2WO4equivalent ratio at the first inflection, 1.2–1.4, coincides well with these condensation reactions The second inflection, on the other hand, seems to reflect the formation of metatungstic acids (and related ionic species), for example, as follows

nWO4 þ 2nHþ! ðWO3Þn
xH2Oþ ðn  xÞH2O (3) Here, metatungstic acids are expressed by (WO3)n
xH2O, but they are in fact a mixture of homologous compounds different in n and x The resin/Na2WO4 equivalent ratio observed (1.8) is slightly lower than expected (2.0), possi-bly due to slow equilibration of the foregoing paratungstate ions

In the earlier mentioned experiment, the hydrolysis of

Na2WO4was controlled by the rate of resin addition In the actual ion-exchange reaction, a solution of Na2WO4is let to flow through an ion-exchange column The hydrolysis is now controlled by the rate of ion exchange, allowing the pH

of the solution to decrease rapidly down to the final value

expected that the primary product of the ion-exchange reaction would be metatungstic acids (and related ions) It has been reported[23]that, in the acidification of Na2WO4

with HCl, WO3
2H2O is formed as a secondary product derived from metatungstic acids (primary product) when the final pH of the solution is set to 1–2 Thus WO
2H O would

be formed when the ion-exchanged effluent is kept for a

certain period

3.2 Products of ion exchange and ageing

A volume of 200 cm3 of sodium tungstate solution

(0.152 M, pH¼ 8:3) was allowed to flow through a glass

column packed with 125 cm3 of the protonated

cation-exchange resin at a rate of 2 cm3/min at room temperature

In this setting, the resin/Na2WO4equivalent ratio was 4.40

and the contact time of Na2WO4solution was 62.5 min The

effluent solution after the reactionðpH ¼ 0:3Þ, colored light

yellow, was stable for a short while before it was

trans-formed into an opaque soft gel like a pudding in a few hours

Soon, the soft gel began to separate into a yellow gel and a

transparent liquid, going to completion in 3 days

In order to check the degree of ion exchange, the whole

effluent solution was dried up As analyzed by fluorescence

X-ray spectroscopy, the Naþcontent of the resulting powder

was below the detection level, confirming that the

ion-exchange reaction between Na2WO4and the resin was

com-plete The same check was carried out for the effluents

obtained at larger rates of flow of the Na2WO4 solution

The Naþcontent was again below the detection level at a flow

rate of 20 cm3/min (contact time 6.25 min), while a

signifi-cant level of Naþwas detected at 100 cm3/min (contact time

1.25 min) The flow rate was fixed at 2 cm3/min hereafter

For identification, the yellow gel was collected by

decan-tation and dried in vacuum at room temperature As shown in

Fig 2, the gel as dried hardly exhibited XRD peaks The gel

obtained was then calcined at selected temperatures from

100–600 8C for 2 h After calcination at 100 8C, crystalline

phases of WO3
1/3H2O (orthorhombic, a¼ 0:7359 nm, b ¼

1:251 nm, c¼ 0:7704 nm) and WO3
H2O appeared The

latter phase disappeared almost completely at 200 8C, while

the former phase remained up to 400 8C and was converted into WO3 at 430 8C As just mentioned, the gel dried at room temperature hardly showed XRD peaks It is possible that crystalline products, even if formed, might be covered too thick by an amorphous material Thus the yellow gel was suspended in deionized water (320 cm3) under agita-tion briefly (3 min) and centrifuged for 1 h The resulting gel and liquid were vacuum-dried at room temperature separately.Fig 3(a)shows the XRD patterns of the gel as dried and as calcined Remarkably, a clear XRD pattern

of WO3
2H2O phase was observed after drying This phase was converted to WO3
H2O and WO3 after cal-cination at 100 and 300 8C, respectively The correspond-ing XRD patterns for the liquid part are shown inFig 3(b) The sample remained almost amorphous after drying at room temperature as well as after calcination at 100 8C Crystalline phases of WO3
1/3H2O and unidentified com-pound(s) appeared after calcination at 200–400 8C, and those were converted to WO3completely after calcination

at 500 8C

It is understood that the yellow gel precipitate deposited from the effluent of ion-exchange reaction after the ageing was a mixture of an amorphous product (tungstic acids) and

a crystalline product of WO3
2H2O, and that the mixture could be separated from each other by the washing and centrifugal treatment Yields of WO3
2H2O and tungstic acids were 72.1 and 20.3 mol%, on the basis of starting

Na2WO4, respectively, as evaluated from the masses of WO3 after calcination at 500 8C In order to check the material balance in more detail, the liquid part of the ion-exchange effluent remaining after the yellow gel (mixture) was sepa-rated off was also dried and calcined As a result, WO3was also found as the calcination product, and it accounted for 6.3 mol% of the starting Na2WO4 The sum of these found values amounted to 98.7 mol%, confirming that the reactant

Fig 1 Titration of Na 2 WO 4 solutions with protonated cation-exchanged resin.

Fig 2 XRD patterns of the as-precipitated gel after drying or calcining at the indicated temperatures.

Fig 3 XRD patterns of the powder samples derived from the solid part (a) and the liquid part (b) resulting when the as-precipitated gel was briefly washed and centrifuged The liquid was evaporated to dryness at room temperature.

or products remaining in the ion-exchange column should be

minimal

Based on these results, the reaction paths in the ion

exchange and ageing are summarized in the scheme shown

in Fig 4 It is assumed that the metatungstic acids as a

primary product of the ion-exchange reaction is soluble but

they combine together to form insoluble metatungstic acids

and WO3
2H2O gel during the ageing period (3 days) The

insoluble metatungstic acids are made soluble when washed

with water, probably because of an increase in pH A single

phase of WO3
2H2O gel is thus obtained at more than 70%

yield in the present method

3.3 Properties of gel and sol of WO3
2H2O freshly

prepared

The wet yellow gel of WO3
2H2O obtained earlier could

be easily dispersed in deionized water to form a suspension,

if the content of WO3
2H2O was about 5 wt.% or less on the

WO3 basis In addition, the suspension could be stored comfortably for more than 1 month at room temperature

Fig 5shows the mean particle size as well as the range of particle size distribution analyzed on LPA for a freshly prepared sol (content: 5 wt.% on the WO3 basis) as a function of storage time For the fresh suspension, the particle sizes were distributed in a fairly narrow range of 25–35 nm with a mean size of about 30 nm in diameter With

an increase in storage time up to 28 days, the mean size tended to increase somewhat, but the increment was minimal (only up to about 32 nm) and the range of particle size distribution was also fairly stable during the storage

In order to know whether the colloidal particles are provided with free crystallites of WO3
2H2O, the crystallite sizes were evaluated from the XRD peak widths of the corresponding WO3
2H2O gel dried at room temperature (Fig 3(a)) It has been reported that the WO3
2H2O crystal-lites tend to grow into platelets This tendency was also detected in the present study For the fresh gel, the crystallite

Fig 4 Reaction paths stating from Na 2 WO 4

Fig 5 Mean and range of particle sizes of the WO
2H O sol as a function of time of storage (measured on LPA).

3.4 Effects of washing and/or centrifugal treatments

It has been reported[23]that the WO3
2H2O platelets tend

to grow with a washing treatment This suggests a possibility

of controlling the crystallite size or colloidal particle size by

adequate post treatments The freshly prepared WO3
2H2O

gel was subjected to a washing and centrifugal treatment, i.e

dispersion into deionized water (320 cm3) under ultrasonic

wave agitation for 20 min followed by gelling back by

centrifuging for 1 h This treatment was repeated up to four

times After each treatment, a portion of the colleted gel was

vacuum-dried at room temperature for XRD analysis, while

another portion was subjected to the particle size distribution

analysis by LPA The resulting XRD data are shown inFig 6

Two kinds of changes can be discerned clearly First,

WO3
H2O phase began to appear in the gel after the third

treatment and became dominant after the fourth This

indi-cates that the dehydration from WO3
2H2O to WO3
H2O

proceeds gradually during the treatments Second, the XRD

intensities of (0 1 0) and (0 2 0) peaks of WO3
2H2O tended

to increase relative to the other peaks, e.g (2 0 0) and

(0 0 1), as the number of treatments increased

much more marked These results indicate that the partial conversion of WO3
2H2O to WO3
H2O during the earlier treatments was mainly caused by the washing treatment, and that the intensity distortion in XRD pattern was by the centrifugal treatment The intensity distribution will be discussed in detail later

The particle size distribution analyses were carried out for the sols dispersing the gels after these treatments The mean particle size and the range of particle size distribution are shown as a function of total centrifuging time in Fig 8

(upper) Note that the freshly prepared gel had already been centrifuged for 1 h and that each washing and centrifugal treatment included 1 h of centrifuging It is seen that the mean particle size as well as the upper and lower limit of particle sizes tend to increase gradually with an increase in total centrifuging time The sizes of WO3
2H2O crystallites evaluated from the widths of XRD peaks are also shown in

Fig 8 (lower) Since the platelike habit was evident as mentioned before, the dimension (thickness) normal to (0 1 0) was obtained by averaging the values based on (0 1 0) and (0 2 0), while that (width) parallel to (0 1 0) was done based on (2 0 0) and (0 0 1) These dimensions are seen to increase with an increase in total centrifuging time

Fig 6 Changes of XRD patterns of the gels with the repetition of the washing and centrifugal treatment.

Notably the upper and lower limits of particle sizes were

fairly close to the widths and thicknesses of WO3
2H2O

crystallites of the corresponding gels, respectively This

indicates that the sol particles consist of free (dissociated)

crystallites It is also obvious that the size of colloidal

particles (crystallites) can be controlled well by adjusting

these treatments As also indicated in the same figure

(Fig 8), the crystallites size of dehydrated phases, WO3
H2O

and WO3, increased gradually with increasing total

centri-fuging time This indicates that the crystallite size of WO3

included in the sensor device can also be controlled by these

treatments It is seen that for all of the investigated samples

of WO3, the dimensions normal to (2 0 0) are considerably

larger than those normal to (0 0 2), the crystallites are thus suggested to be considerably anisotropic

Fig 9shows a TEM image of WO3
2H2O particles for the sol obtained after the third washing and centrifugal treat-ment (total centrifuging time 4 h) Most of the crystallites are in the range of 25–50 nm in size, in fair agreement with the analyses based on LPA and XRD Although most of the crystallites overlap too heavily with each other, some not overlapping so heavily have their contrast (brightness) kept uniform inside their peripheries, suggesting the platelike nature of the crystallites

3.5 Preferred orientation

As mentioned in Section 3.4, the XRD patterns of

WO3
2H2O gels became distorted in intensity, that is, (0 1 0) and (0 2 0) peaks were unusually intense, after the repetition of washing and centrifugal treatments The distribu-tion was extremely marked after the prolonged (10 h) cen-trifuging (Fig 7), as also mentioned Such distortion in intensity is known to reflect preferred orientation of the crystallites involved It is considered that in the gelling process the WO3
2H2O crystallites (platelets) prefer to agglomerate together with their basal planes of (0 1 0) oriented in parallel to each other Notably the preferred orientation of WO3
2H2O was found to be inherited well by the dehydrated phases of

WO3
H2O and WO3 on calcination The XRD pattern of

WO3
H2O in Fig 7is strongly distorted by unusually high intensities of (0 2 0) and (0 4 0), indicating preferred orienta-tion in (0 1 0) plane Similarly that of WO3with unusually strong (0 0 2) indicates preferred orientation in (0 0 1) The preferred orientation of each phase is interrelated as follows ð0 1 0Þ of WO3
2H2O! ð0 1 0Þ of WO3
H2O

Fig 7 XRD patterns of the gel centrifuged for 10 h after drying or calcining at the indicated temperatures.

Fig 8 Mean and range of particle sizes of WO 3
2H 2 O sols (LPA) and

crystallite sizes of the corresponding gel after dried or calcined at the

designated temperatures (XRD) as a function of total centrifuging time.

This phenomenon can be understood well if topotaxy is

assumed for the respective dehydration steps

To discuss preferred orientation more quantitatively,

pre-ferred orientation index (POI) was defined for each phase on

the basis of the intensities (heights) of selected diffraction

peaks as follows

where I (h k l) means the intensity of (h k l) peak, and the suffixes (observed (obsd.) and studied (std.)) indicate the intensity ratios for actual and standard (undistorted) XRD patterns, respectively The intensity ratios of the standard XRD patterns were obtained by referring to JCPDS; 3.33 for WO3
2H2O (JCPDS, 18-1420), 0.80 for WO3
H2O

Fig 9 A TEM image of WO 3
2H 2 O particles of the sol after the third washing and centrifugal treatment (total centrifuging time 4 h).

POI¼

½Ið0 1 0Þ=Ið0 0 1Þobsd:=½Ið0 1 0Þ=Ið0 0 1Þstd:for WO3
2H2O

½Ið0 2 0Þ=Ið1 1 1Þobsd:=½Ið0 2 0Þ=Ið1 1 1Þstd:for WO3
H2O

½Ið0 0 2Þ=Ið2 0 0Þobsd:=½Ið0 0 2Þ=Ið2 0 0Þstd:for WO3

8

>

>

9

>

Fig 10 Preferred orientation index as a function of total centrifuging time for the powder samples of WO 3
2H 2 O, WO 3
H 2 O or WO 3 after dried or calcined

at the indicated temperatures.

(43-0679), 1.01 for WO3(43-1035) The POI values thus

evaluated are summarized as a function of total centrifuging

time in Fig 10 For WO3
2H2O, POI is seen to increase

progressively with prolonging centrifuging time As shown

previously (Fig 8), the crystallites (platelets) of WO3
2H2O

grew gradually in both thickness and width during these

treatments It is likely that the growth of the basal plane

coupled with the centrifugal force causes the crystallites

to agglomerate together with their basal planes oriented

in parallel It is notable that the behavior of preferred

orientation with a change in total centrifuging time is very

similar for the three phases This POI behavior as well as

that shown in Fig 7 clearly indicates that the topotaxy

mentioned earlier is maintained firmly at each dehydration

step of WO3
2H2O! WO3
H2O! WO3

4 Conclusions

Nano sized crystallites of WO3
2H2O were prepared from

Na2WO4at more than 70% yield by an ion-exchange method

coupled with the ageing of the effluent (3 days) followed by

brief washing and centrifuging The crystallites, platelike in

shape, could be easily dispersed in deionized water to form a

stable sol The size of the crystallites increased gradually by

washing and/or centrifugal treatments of the sol, providing a

method to control the crystallite size Because of the

plate-like nature of the crystallites, preferred orientation became

marked for the WO3
2H2O crystallites after prolonged

centrifuging Notably the preferred orientation was inherited

well by the dehydrated phases of WO3
H2O and WO3,

indicating the existence of topotaxy at the dehydration steps

Acknowledgements

The authors would like to thank Dr M Uehara of faculty

of Engineering, Kyushu University, for technical support in

TEM observation

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Biographies

Yong-Gyu Choi received his BE degree in materials science and engineering in 1996 and ME degree in 1998 from the Kyungsung University in Korea Now, he is a doctoral course student of majoring of molecular and materials sciences in the Kyushu University His current research interest is development of an NO x sensor by spin coating method with WO 3 sol provided by ion-exchange method.

Go Sakai has been a research associate at the Kyushu University since

1996 He received his BE degree in applied chemistry in 1991, ME degree

in 1993 and PhD in engineering 1996 from the Kyushu University His