preparation of aqueous sols of tungsten oxide dihydrate

Preparation of aqueous sols of tungsten oxide dihydrate from sodium tungstate by an ion-exchange method Yong-Gyu Choi a , Go Sakai b , Kengo Shimanoe b , Norio Miura c , Noboru Yamazoe b,* 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 (WO 3 Á2H 2 O) were prepared from Na 2 WO 4 by an ion-exchange method. An aqueous solution of Na 2 WO 4 was 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 WO 3 Á2H 2 O and an amorphous phase, and the crystalline part could be separated from another by washing with deionized water and centrifuging. The gel of WO 3 Á2H 2 O 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 (WO 3 Á2H 2 O) changed to the monohydrate (WO 3 ÁH 2 O) at 100 8C, which in turn changed to the anhydride (WO 3 ) at 240 8C. Remarkably XRD patterns showed conspicuous preferred orientation of WO 3 Á2H 2 O 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 WO 3 crystallites 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 (WO 3 ) 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 WO 3 ÁnH 2 O(n ¼ 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 WO 3 . On the other hand, this suggests that the functionality would depend much on the method to prepare WO 3 . For applications to semiconductor gas sensors, polycrystalline materials of WO 3 have been prepared by various methods, i.e. pyrolysis of (NH 4 ) 10 W 12 O 41 Á5H 2 O, sputtering or evaporation from a source of WO 3 , 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 WO 3 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 NO 2 begins to increase sharply as the grain size (mean diameter) of WO 3 decreases to be less than a critical value and that the critical value would correspond to twice the surface space charge layer thickness of WO 3 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 WO 3 . We reported [21] recently that preparation and control of SnO 2 sols in an aqueous medium were effective for controlling the physi- cochemical properties of SnO 2 -based sensors. This finding prompted us to extend the same approach for the WO 3 based sensors. In the present case, however, the stable colloidal species in an aqueous medium is not WO 3 itself but its dihydrate, WO 3 Á2H 2 O (monoclinic, a ¼ 0:75 nm, Sensors and Actuators B 87 (2002) 63–72 * 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 0925-4005(02)00218-6 b ¼ 0:693 nm, c ¼ 0:37 nm, b ¼ 90:58), which is dehy- drated on heating to WO 3 ÁH 2 O (orthorhombic, a ¼ 0:5238 nm, b ¼ 1:070 nm, c ¼ 0:5120 nm) and WO 3 (monoclinic, a ¼ 0:7297 nm, b ¼ 0:7539 nm, c ¼ 0:7688 nm, b ¼ 90:918). It has been reported that a colloidal suspen- sion of platelets of WO 3 Á2H 2 Oisformedbyacidification of an Na 2 WO 4 solution with a mineral acid (HCl), but little infor- mation has been accumulated on how to prepare and control the suspension from a standpoint of gas sensor applications. In the present study, we examined the possibilities of obtaining aqueous sols of WO 3 Á2H 2 O from an aqueous solution of Na 2 WO 4 by an ion-exchange method; the Na 2 WO 4 solution 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 Na þ ions gives significant effects on the kinds and properties of the acidification products. This paper aims at reporting the ion-exchange based preparation of WO 3 Á2H 2 O sols from Na 2 WO 4 . 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 (HNO 3 ) for 1 h to convert it from Na þ type to H þ type. 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./cm 3 , as evaluated from the titration with an NaOH solution. Sodium tungstate was purchased as its dihydrate (Na 2 WO 4 Á2H 2 O) 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 WO 3 Á2H 2 O. 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 Na þ ions in the gels was analyzed by fluorescence X-ray spectroscopy (Rigaku, Fluorescence X-ray Spectrometer 3270). 3. Results and discussion 3.1. Hydrolysis behavior of Na 2 WO 4 The hydrolysis of Na 2 WO 4 with a mineral acid (like HCl) is known to be rather complex, giving different products or intermediates depending on the pH of the reaction medium. As a preliminary test, the hydrolysis behavior of Na 2 WO 4 with the protonated cation-exchanged resin was investi- gated. The resin was added bit by bit to the solutions of Na 2 WO 4 (0.1, 0.15 and 0.2 M) under agitation with a stirrer 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 WO 4 2À ions, stable in an alkaline solution, condense together with a lowering in pH; the condensation products are paratungstate ions like [W 12 O 41 ] 10À and [H 2 W 12 O 40 ] 6À in the pH range of 7–4, while metatungstic acids ((WO 3 ) n ÁxH 2 O) 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: 12WO 4 2À þ 14H þ !½W 12 O 41  10À þ 7H 2 O (1) 12WO 4 2À þ 18H þ !½H 2 W 12 O 40  6À þ 8H 2 O (2) The resin to Na 2 WO 4 equivalent 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. nWO 4 2À þ 2nH þ !ðWO 3 Þ n Á xH 2 O þðn À xÞH 2 O (3) Here, metatungstic acids are expressed by (WO 3 ) n ÁxH 2 O, but they are in fact a mixture of homologous compounds different in n and x.Theresin/Na 2 WO 4 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 Na 2 WO 4 was controlled by the rate of resin addition. In the actual ion-exchange reaction, a solution of Na 2 WO 4 is 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 ( 1) observed in the titration experiment (Fig. 1). It is thus 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 Na 2 WO 4 with HCl, WO 3 Á2H 2 O 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 3 Á2H 2 O would 64 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 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 cm 3 of sodium tungstate solution (0.152 M, pH ¼ 8:3) was allowed to flow through a glass column packed with 125 cm 3 of the protonated cation- exchange resin at a rate of 2 cm 3 /min at room temperature. In this setting, the resin/Na 2 WO 4 equivalent ratio was 4.40 and the contact time of Na 2 WO 4 solution was 62.5 min. The effluent solution after the reaction ðpH 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 Na þ content of the resulting powder was below the detection level, confirming that the ion- exchange reaction between Na 2 WO 4 and the resin was com- plete. The same check was carried out for the effluents obtained at larger rates of flow of the Na 2 WO 4 solution. The Na þ content was again below the detection level at a flow rate of 20 cm 3 /min (contact time 6.25 min), while a signifi- cant level of Na þ was detected at 100 cm 3 /min (contact time 1.25 min). The flow rate was fixed at 2 cm 3 /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 WO 3 Á1/3H 2 O (orthorhombic, a ¼ 0:7359 nm, b ¼ 1:251 nm, c ¼ 0:7704 nm) and WO 3 ÁH 2 O appeared. The latter phase disappeared almost completely at 200 8C, while the former phase remained up to 400 8C and was converted into WO 3 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 cm 3 ) 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 WO 3 Á2H 2 O phase was observed after drying. This phase was converted to WO 3 ÁH 2 OandWO 3 after cal- cinationat100and3008C, respectively. The correspond- ing XRD patterns for the liquid part are shown in Fig. 3(b). The sample remained almost amorphous after drying at room temperature as well as after calcination at 100 8C. Crystalline phases of WO 3 Á1/3H 2 O and unidentified com- pound(s) appeared after calcination at 200–400 8C, and those were converted to WO 3 completely 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 WO 3 Á2H 2 O, and that the mixture could be separated from each other by the washing and centrifugal treatment. Yields of WO 3 Á2H 2 O and tungstic acids were 72.1 and 20.3 mol%, on the basis of starting Na 2 WO 4 , respectively, as evaluated from the masses of WO 3 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, WO 3 was also found as the calcination product, and it accounted for 6.3 mol% of the starting Na 2 WO 4 . 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. Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 65 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. 66 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 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 WO 3 Á2H 2 O 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 WO 3 Á2H 2 O gel is thus obtained at more than 70% yield in the present method. 3.3. Properties of gel and sol of WO 3 Á2H 2 O freshly prepared The wet yellow gel of WO 3 Á2H 2 O obtained earlier could be easily dispersed in deionized water to form a suspension, if the content of WO 3 Á2H 2 O was about 5 wt.% or less on the WO 3 basis. In addition, the suspension could be stored comfortably for more than 1 month at room temperature. Fig. 5 shows 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 WO 3 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 WO 3 Á2H 2 O, the crystallite sizes were evaluated from the XRD peak widths of the corresponding WO 3 Á2H 2 O gel dried at room temperature (Fig. 3(a)). It has been reported that the WO 3 Á2H 2 O 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 3 Á2H 2 O sol as a function of time of storage (measured on LPA). Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 67 size evaluated from (0 1 0) or (0 2 0) was about 25 nm, while that from (2 0 0) or (0 0 1) was about 42 nm. This shows that each crystallite is a platelet with its basal plane parallel to (0 1 0) developed about two times as large as its thickness normal to (0 1 0). Obviously the mean particle size (30 nm) of the sol as well as the range of particle size distribution (25–35 nm) is placed between the width (42 nm) and the thickness (25 nm) of the crystallites. It is thus rational to conclude that each colloidal particle is in fact provided with a single crystallite of WO 3 Á2H 2 O. This conclusion was supported by TEM observation as described later. 3.4. Effects of washing and/or centrifugal treatments It has been reported [23] that the WO 3 Á2H 2 O 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 WO 3 Á2H 2 O gel was subjected to a washing and centrifugal treatment, i.e. dispersion into deionized water (320 cm 3 ) 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 in Fig. 6. Two kinds of changes can be discerned clearly. First, WO 3 ÁH 2 O phase began to appear in the gel after the third treatment and became dominant after the fourth. This indi- cates that the dehydration from WO 3 Á2H 2 OtoWO 3 ÁH 2 O proceeds gradually during the treatments. Second, the XRD intensities of (0 1 0) and (0 2 0) peaks of WO 3 Á2H 2 O tended to increase relative to the other peaks, e.g. (2 0 0) and (0 0 1), as the number of treatments increased. In order to know the origins of these changes in more detail, the time of washing and that of centrifuging were prolonged independently in separate experiments by using freshly prepared gels of WO 3 Á2H 2 O. After the washing for 10 h, the gel contained WO 3 ÁH 2 O partially or totally as judged from the XRD patterns, while the XRD peak inten- sities were not distorted. After the centrifuging for 10 h, on the other hand, all the XRD peaks could be ascribable to WO 3 Á2H 2 O but the peak intensities were extremely dis- torted, as shown at the bottom in Fig. 7. The distortion was in the same tendency as observed previously but was much more marked. These results indicate that the partial conversion of WO 3 Á2H 2 OtoWO 3 ÁH 2 O 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 WO 3 Á2H 2 O 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. 68 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 Notably the upper and lower limits of particle sizes were fairly close to the widths and thicknesses of WO 3 Á2H 2 O 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, WO 3 ÁH 2 O and WO 3 , increased gradually with increasing total centri- fuging time. This indicates that the crystallite size of WO 3 included in the sensor device can also be controlled by these treatments. It is seen that for all of the investigated samples of WO 3 , 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. 9 shows a TEM image of WO 3 Á2H 2 O 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 WO 3 Á2H 2 O 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 WO 3 Á2H 2 O 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 WO 3 Á2H 2 O was found to be inherited well by the dehydrated phases of WO 3 ÁH 2 OandWO 3 on calcination. The XRD pattern of WO 3 ÁH 2 OinFig. 7 is 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 WO 3 with unusually strong (0 0 2) indicates preferred orientation in (0 0 1). The preferred orientation of each phase is interrelated as follows. ð010Þof WO 3 Á 2H 2 O !ð010Þ of WO 3 Á H 2 O !ð001Þ of WO 3 (4) 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 Á2H 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. Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 69 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 (hkl) means the intensity of (hkl) 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 WO 3 Á2H 2 O (JCPDS, 18-1420), 0.80 for WO 3 ÁH 2 O Fig. 9. A TEM image of WO 3 Á2H 2 O particles of the sol after the third washing and centrifugal treatment (total centrifuging time 4 h). POI ¼ ½Ið010Þ=Ið001Þ obsd: =½Ið010Þ=Ið001Þ std: for WO 3 Á 2H 2 O ½Ið020Þ=Ið111Þ obsd: =½Ið020Þ=Ið111Þ std: for WO 3 Á H 2 O ½Ið002Þ=Ið200Þ obsd: =½Ið002Þ=Ið200Þ std: for WO 3 8 > < > : 9 > = > ; (5) Fig. 10. Preferred orientation index as a function of total centrifuging time for the powder samples of WO 3 Á2H 2 O, WO 3 ÁH 2 OorWO 3 after dried or calcined at the indicated temperatures. 70 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 (43-0679), 1.01 for WO 3 (43-1035). The POI values thus evaluated are summarized as a function of total centrifuging time in Fig. 10. For WO 3 Á2H 2 O, POI is seen to increase progressively with prolonging centrifuging time. As shown previously (Fig. 8), the crystallites (platelets) of WO 3 Á2H 2 O 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 WO 3 Á 2H 2 O ! WO 3 Á H 2 O ! WO 3 . 4. Conclusions Nano sized crystallites of WO 3 Á2H 2 O were prepared from Na 2 WO 4 at 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. 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Soc. 141 (1994) 2207–2210. [21] N.S. Baik, G. Sakai, N. Miura, N. Yamazoe, Hydrothermally treated sol solution of tin oxide for thin-film gas sensor, Sens. Actuators B 63 (2000) 74–79. [22] Y.M. Li, M. Hibino, M. Miyayania, T. Kudo, Proton conductivity of tungsten trioxide hydrates at intermediate temperature, Solid State Ionics 134 (2000) 271–279. [23] C. Bala ´ zsi, J. Pfeifer, Structure and morphology changes caused by wash treatment of tungstic acid precipitates, Solid State Ionics 124 (1999) 73–81. 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 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 71 current research work is focused on development of chemical sensors as well as functional inorganic materials. Kengo Shimanoe has been an associate professor at the Kyushu University since 1999. He received his BE degree in applied chemistry in 1983 and ME degree in 1985 from the Kagoshima University and the Kyushu University, respectively. He joined the advanced materials and technology laboratory in Nippon Steel Corp. and studied the electronic characteriza- tion on semiconductor surface and the electrochemical reaction on materials. He received PhD in engineering in 1993 from the Kyushu University. His current research interests include the development of chemical sensors and the analysis of solid surface. Norio Miura joined the Kyushu University as an associate professor in 1982 and was promoted to professor in 1999. He received his BE degree in applied chemistry in 1973, ME degree in 1975 from the Hiroshima University and PhD in engineering in 1980 from the Kyushu University. His current research concentrates on development of new chemical sensors as well as other electrochemical functional devices. Noboru Yamazoe has been a professor at the Kyushu University since 1981. He received his BE degree in applied chemistry in 1963 and PhD in engineering in 1969 from the Kyushu University. His current research interests include the development and application of the functional inorganic materials. 72 Y G. Choi et al. / Sensors and Actuators B 87 (2002) 63–72 . Preparation of aqueous sols of tungsten oxide dihydrate from sodium tungstate by an ion-exchange method Yong-Gyu. 2002 Abstract Aqueous sols of tungsten oxide dihydrate (WO 3 Á2H 2 O) were prepared from Na 2 WO 4 by an ion-exchange method. An aqueous solution of Na 2 WO 4 was