ELSEVIER

Sensors and ActuatorsB 30 (1996) 95-99

Humidity sensor using potassium hexagonal tungsten bronze synthesizedfrom peroxo-polytungstic acid
Isao Tsuyumoto, Tetsuichi Kudo
Institute of lndustrial Science, Universityof Tokyo, 7-22-1 Roppongi, Tokyo 106, Japan
Received29 September1994; acceptedin revised from 15 May 1995

Abstract
We have found that potassium hexagonal tungsten bronze synthesized by a new synthetic route at low temperature (500 °C) shows aremarkable increase in resistivity in a humid atmosphere at room temperature. The conductometry performed in various humidities(Pa2o = 2.0 × 10- 3-I 4.6 mmHg) shows that the equilibrium values of resistivity are a clear function OfPH2o. It is alSOfound that the responsetime as a humidity sensor is improved from about 5-10 hours to 10-20 minutes by increasing the porosity of the sample pellet from 25 to
35%. The mechanism of resistivity change and the application to a humidity sensor are discussed in this connection.
Keywords:Humiditysensors; Potassiumhexagonaltungsten bronze

1. Introduction
In recent years there has been a great deal of interest in the
low-temperature synthesis of inorganic materials using precursors [ 1 ]. A typical example is the sol-gel technique basedon hydrolysis and condensation of metal alkoxide. The formation of a hexagonal form of tungsten trioxide from a hydrothermally prepared precursor ( 3 W O 3 • H 2 0 ) is pointed out as
an instance close to the present study [2]. Such syntheticroutes are advantageous over conventional ones in that they
enable us to synthesize various metastable phases because of
the lower formation temperature and better compositional
uniformity of products. Moreover, special properties can be
expected of substances formed at low temperature because
defect structures are often different from those of high-temperature formation even though the basic structures are thesame. Materials synthesis based on such concepts is called'soft chemistry' or 'chimie douce' [3].
We have reported that various tungstates with a defective
hexagonal or pyrochlore-type framework can be synthesized
through a soft chemical route using precursors precipitatedfrom a peroxo-polytungstic acid (isopoly acid, IPA) solution[4,5]. It has also been shown that tungsten bronze phaseswith the same framework are formed at relatively low temperature (500-700 °C) by heating such precursors in a reducing atmosphere ( 10% H2) [6]. Of these, pyrochlore typeCsxWO3 is a new bronze formed for the first time by this lowtemperature synthetic process [7 ].
0925-4005/96/$15.00 © 1996 ElsevierScience S.A. All rights reserved
SSDI0925-4005(95)O1755-K

Potassium hexagonal tungsten bronze (K-HTB, KxWO3;
x = 0.3) thus formed at 500 °C is not different in basic crystal
structure from conventional K-HTB synthesized by wellknown solid-state reactions around 1000 °C. Its electricalconductivity is metallic as usual when left in a dry atmosphere
after preparation. However, we have found that it shows aremarkable increase in resistivity when it is left in a humid
atmosphere at room temperature [6]. This is the first observation of a resistivity increase of inorganic materials by incorporation of water. Infrared spectroscopic observations anddensity measurements have indicated that this unusual property is probably associated with a special defect structure in
the present K-HTB.
The purpose of this paper is to investigate its electricalproperties quantitatively in relation to water vapour pressure
to demonstrate its applicability to a humidity sensor.

2. Experimental
2.1. Synthetic procedure
Peroxo-polytungstic acid was prepared according to thepreviously reported method [ 8]. Metallic tungsten was dissolved in 15% aqueous H202 to yield a pale yellow acidic
solution. It was then dried at room temperature after removing
excess H202 catalytically with a platinized platinum net,
resulting in a yellow glassy substance (IPA) with the approximate empirical formula 2 W O 3 • H 2 0 2 • 2H20.

96

L Tsuyumoto, T. Kudo / Sensors and Actuators B 30 (1996) 95-99

A white amorphous substance was precipitated by adding
50 ml of an IPA solution (1.2 mol 1-1 as to tungsten) to 50
ml ofa KBr solution ( 1.2 mol 1- 1) with stirring. After storagefor 24 h at 20 °C, the precipitate was filtered, rinsed withwater and then dried at 70 °C. On heating the thus-formed
potassium peroxo-polytungstate in 10% H2 (N2 balance) at
500 °C for 1 h, dark blue potassium hexagonal tungsten
bronze (K-HTB) was obtained.
We prepared conventional K-HTB by the following solidstate reaction for comparison [9]:0.15K2WO4 -F 0.05W + 0.80WO 3) Ko.3WO 3(1)

Stoichiometric amounts of K 2 W O 4, W O 3 and W were intimately mixed and heated in N 2 at 850 °C for 4 h.

2.2. Structural aspects
The powder X-ray diffraction (XRD) profile was taken
for the present K-HTB using monochromatized Cu K a radiation. The 20 scanning was carried out stepwise with a widthof 0.02 ° and a measuring time of 5 s. In order to diminish apreferred orientation, the sample was well ground andattached softly on a sample holder.
The K / W ratio in this K-HTB (x in K~WO3) was determined by chemical analysis. The K-HTB was dissolved inhydrogen peroxide and the concentrations of both K ÷ andW 6+ in the solution were determined by inductively coupledplasma (ICP) emission spectrometry.

2.3. Conductometry
The electrical resistivity was measured by the conventional
a.c. two-probe, d.c. four-probe and d.c. two-probe methods.
The d.c. four-probe measurement was carried out as follows. A powder sample of K-HTB was pressed at 6 × l0 s Pa
for 8 min into a pellet 11.4 mm in length, 5.3 mm wide and
0.8 mm thick (Fig. 1). After gold electrodes were coated on
both sides and at two points on the plane with gold paste(Tokuriki Chemical Corporation No. 8570), this pellet was
reheated in 10% H 2 ( N 2 balance) at 500 °C for 1 h. The final
porosity of these sample pellets (calculated from apparent
and theoretical densities) was in the range 25-30%. Afterreheating, its resistivity was measured as a function of P.2o.
A constant current was made to flow from two outside electrodes with a TR6143 current source (Advantest Corporation) and the electric potential difference between the two
inside electrodes induced by the constant current was meas-

ured with an HP3457A electrometer (Hewlett-Packard). The
electric potential difference for the reverse current direction
was also measured in the same way. After confirming thatthese two values of potential difference were in good agreement, the average of these two values was used to calculate
the resistivity. The constant current density employed for thismeasurement was confirmed to show an ohmic V-I dependence. Thus the electrical properties we report in this paper
cannot be associated with interfacial phenomena between thepellet and the electrodes.
The d.c. two-probe measurement was carried out preliminarily using the HP3457A electrometer. We employed two
types of sample pellets for the d.c. two-probe measurement
in order to investigate how the porosity of the sample pellet
affects the resistivity change. One is the same as for the d.c.
four-probe measurement (porosity 25-30%) and the other isfabricated as follows. A powder sample of K-HTB waspressed at 3 × 107 Pa for 1 min into a pellet 2.5 mm in diameterand 2-3 mm thick. After gold electrodes were coated on bothsides using gold paste (Tokuriki Chemical Corporation No.
8570), this pellet was reheated in 10% H2 (N2 balance) at
500 °C for 1 h. The final porosity of these sample pellets wasin the range 30-40%.
The a.c. two-probe measurement was also carried out preliminarily on a part of this work using HP4192A apparatus(Hewlett-Packard). The frequency employed for thismeasurement was 100 Hz, at which the X-value (imaginary
part of the impedance) was negligible compared to the Rvalue (real part of the impedance). Thus the impedance
measured in this way is assumed to be identical with theresistance of the d.c. two-probe measurement.

2.4. Gravimetry
Gravimetric analysis was performed using a high-sensitivity balance (Cahn 2000 Recording Electrobalance).
Gravimetry of K-HTB at different P,2o values was carriedout at room temperature to investigate the relationshipbetween Pn2o and n (in KxWO3" nH20).
Thermogravimetry of K-HTB ( KxWO3 •nH20; n - 0.22)
in dry air (PH20=2.0× 10 -3 mmHg) at a heating rate 1 °C
min- 1 was also carried out to investigate the chemical potential of water molecules in K-HTB.

3. Results and discussion

3.1. Structural aspects

Au pasteI-~-5.3mm -~Fig. 1. Schematic view of the sample pellet for the d.c. four-probe measurement.

The present K-HTB shows a sharp powder XRD profileas shown in Fig. 2, indicating that it has high crystallinity
despite the low formation temperature (500 °C). This profile
was taken with a sample left in a humid atmosphere(Pn2o -- 5-15 mmHg) at room temperature for more than aweek. Therefore it is estimated to have contained a considerable amount of water and to have turned from metallic to

L Tsuyumoto, T. Kudo/Sensors and Actuators B 30 (1996) 95-99

10

2[)

30

40
50
2e (Cu K~)

60

70

Fig. 2. Powder XRD profile oftbe K-THB synthesized by heating potassium
peroxo-polytungstate in H2 at 500 °C.

97

It was found that samples with relatively low water content(and therefore low resistivity) only ceased to increase inresistivity when a dry gas (dew point - 7 0 °C, PH2O=
2.0X 10 -3 mmHg) was introduced instead of a humid one(Fig. 4). On the other hand, those with high water content(therefore, high resistivity) reduced their resistivity reversibly (Fig. 5), Such behaviour can be accounted for by assuming that the PH=O in equilibrium with K=WO3 •n H 2 0 is given
by a function ofn schematically illustrated in Fig. 6, in which
In P-2o is related to the chemical potential (/z) of the watermolecule in the KxWO3 phase by

semiconducting (see Section 3.2). It has been confirmed thata fresh and water-free sample shows almost the same profile
except for slight changes in lattice parameters. These finechanges in the crystal structure accompanied by water incorporation are now being investigated to understand the mechanism of the resistivity change.
We performed the Rietveld structural refinement to check
how the present K-HTB is different in structural details fromthe conventional one prepared at higher temperature. There
were no distinguishable differences between these K-HTBs,
except that distortion of the WO6 coordination octahedron in
the present K-HTB is somewhat greater [ 10]. Therefore, it
may be considered not to be special in the basic structure of
the framework. The distorted nature of WO6 might be related
to the fact that tungsten vacancies exist in the K-HTB, which
has been confirmed by the presence of an IR band at 960980 c m - 1 due to vibration of W = O [6].
The K / W ratio was determined as 0.3 from ICP analysis,
which is not significantly different compared with that of theconventional K-HTB formed by the solid-state reaction.

In P,2o = (tz-tz°)/(kT)3.2. Incorporation of water into K-HTB

Fig. 4. Resistivity change of K-HTB when a dry gas was introduced at theinitial stage of water incorporation (a.c. two-probe measurement).

A fresh water-free sintered sample pellet of the present K HTB showed a metallic resistivity as low as 0.1 12 cm at room
temperature in a dry atmosphere. However, its resistivity was
dramatically increased in a wet atmosphere (Fig. 3). The
saturated value of resistivity reached 200-8001~ cm depending on the water-vapour pressure (P,2o) of the atmosphere.
At the same time, a significant amount of water incorporationby the sample pellet was gravimetrically observed. The watercontent n in KxWO3.nH20 in equilibrium with P,2o = 12mmHg was about 0.2.

(2)where /z° is the p. in equilibrium with the standard P.2o
( = 760 mmHg). Let us assume, for simplicity that the/z isrepresented by a familiar form,~ = E + RTln[ O/ (1-O) ](3)

where E is the internal energy associated with water adsorption or intercalation at a set of sites in KxWO3, and 0 the
occupancy of the sites (i.e., O=n/n*, n* being the numberof sites per mole). If there is only one kind of site and E does
1.0

PH20
E

o.

PH20

0.(]
100

200 300t / min

400

500

800
E 700

dry

dry

6oo
5.8mmHg
50O
0

500
1000
t / rain

1500

Fig. 5. Resistivity change of K-HTB when a dry gas was introduced at thesaturated stage of water incorporation (a.c. two-probe measurernent).

4OO r

300

tPH2~

"'O..
~:
:'.~-

~

o+7

01/ . . . . . .
0

1000

2000
t / minsite 2.........................................

3000

Fig. 3. Resistivity change of K-HTB in a humid atmosphere, PH2O= 8.0
mmHg (a.c. two-probe measurement).

0

0.1
0.2
n (in KxWO3onH20)

0.3

Fig. 6. Schematic expression of P,2o vs, n in KxWO3. nH20.

98

L Tsuyumoto, T. Kudo / Sensors and Actuators B 30 (1996) 95-99
0.201

60C

o.2oo

50C

o.2

E

o.199

.........................................

01198

i

30C

=

[~H20/14.6rnmHg

40C~ 5.1mmHg

E

20C
0.19;

o

;o

,(

lOO

1;o

2o0

.

1000
t / min220

2000

temperature / °C
Fig. 7. Water-desorption curve for the present K - H T B with water content
n = 0.22 in dry air ( PH2o= 2 X 10- 3 mmHg) at a heating rate o f 1 °C min - ~.

Fig. 8. Resistivity change of K-HTB at room temperature when P~o wasincreased stepwise (d.c. four-probe measurement).
60C

not depend on n, the relationship of Eq. (2) is reduced to theLangmuir isotherm. On the other hand, a function in Fig. 6

can be derived by postulating that the K-HTB phase has two
kinds of water-adsorption sites (sites 1 and 2), the internalenergies of which are far separated from each other (i.e.,
E=- E~ >> RT). In order to confirm the function in Fig. 6,
water desorption from K, WO3. nH=O (n = 0.22) was investigated thermogravimeUically in dry air (Pa2o = 2.0 X 10 -3mmHg). As shown in Fig, 7, the desorption curve has a kinkat about 110 °C, indicating that phenomena at lower andhigher temperatures are different in nature. Water loss at thelower-temperature region is due to its desorption from site 2
at a higher energy level because the equilibrium value of n inthis atmosphere at 20 °C was determined experimentally as
about 0.15. Accordingly, the curve in the higher-temperature
region represents the desorption from site 1 at a lower energy
level, and can be fitted satisfactorily by the theoretical n-T
relationship under equilibrium. E~ is estimated to be - 50 to
40 kJ mol- ~ from this fitting. Ez is estimated to be about
10 kJ mol-~ because the water incorporation into site 2 isreversible in the PH20 range 5-15 mmHg at room temperature.
Details of the PH2o-n isotherm are under investigation, andwill be published together with discussion on the mechanismof water incorporation.
--

3.3. Resistivity change by incorporation of water
It is evident from the above discussion that, in sensing
humidity in a usual environment, the adsorption and desorption processes of water are involved with site 2. Therefore,
we investigated the applicability to a humidity sensor as follows. After reheating the sample pellet under the same condition as that for the synthesis, a wet gas (PH2o = 20 mmHg)
was introduced. The resistivity in equilibrium with PH2o= 20
mmHg at 25 °C (RH 84%) reached about 600 fl cm. At thisstage a dry gas (dew point - 7 0 °C, PH2o=2.0×10 -3mmHg) was introduced to the sample. Then, a new equilibrium for this PH~o was attained at a resistivity of about 250
f/cm, which corresponds to the water content n = 0.15. From
this equilibrium state, the resistivity increase was observedin response to stepwise increases in PH20. AS shown in Fig.
8, clear equilibrium values of the resistivity were observed

50C

PH20 9.0mrnHg
S.8mmHg

E
40C

30C

20C

1000
t / rnin

2000

Fig. 9. Resistivity change of K-HTB at room temperature when Pr~o wasdecreased stepwise (d.c. four-probe measurement).

for each value of Pn2o. For stepwise decreases in PH20, its
resistivity decreased as shown in Fig. 9, indicating goodreversibility of the process expressed by
H20(gas)-"H20(K-HTB)(4)

These results, which were performed by the d.c. four-probe
method using a sample pellet with a relative density of 75%,
demonstrate the applicability of the present K-HTB to ahumidity sensor. The response time using this pellet was
about 5-10 h, which is too slow for practical use. In order toimprove the response time, we formed sample pellets with ahigher porosity of 35% (relative density 65%) and measured
their resistivity by the d.c. two-probe method. As shown in
Fig. 10, the response time was much improved to about 1020 min. This means that the response time is controlled by
the bulk diffusion of water in the sample pellet. Therefore,much faster response can be expected by forming K-HTB
into a thin film.
7OO

600

S

9.3rnmHg

500
~.

ao
6.8mmHg

400 /

300

0

10

20
30
t / rain

40

50

Fig. 10. Resistivity change of K-HTB pellet with 35% porosity (d.c. twoprobe measurement).

L Tsuyumoto, T. Kudo ~Sensors and Actuators B 30 (I 996) 95-99

The mechanism o f resistivity increase by water incorporation is now being investigated in relation to the defect structure. Since tungsten complex oxides with hydrated water orwith a layer structure are reported in the literature, we areproposing a model based on a layer structure. Briefly, thepresent K - H T B has a layer structure and the electrical pathis separated by intercalation o f water between the layers.
More studies are needed for details.

Acknowledgement
This work was supported by a Grant-in Aid for Scientific
Research from the Japanese Minister of Education, Science
and Culture (04205024).

References[ 1] C.D. Chandler, C. Roger and M.J. Hampden-Smith,Chemical aspectsof solution routes to perovskite-phasemixed-metaloxides from metalorganic precursors, Chem. Rev., 93 (1993) 1205-1241.
[2] B. Gerand, G. Nowogrocki, J. Guenot and M. Figlarz, Structural studyof a new hexagonal form of tungsten trioxide, J. SolidState Chem., 29(1979) 429-434.
[3] M. Figlarz, Chimie douce, a new route for the preparation of newmaterials: some examples, Chemica Scripta, 28 (1988) 3-7.
[4] T. Kudo, A. Kishimoto, J. Oi and H. Inoue, Synthesis of new WO3based complex oxides with tunnels by a chemical mixing process using
peroxo-polytungstates,Solid State lonics, 40/41 (1990) 567-571.
[5] T. Kudo, J. Oi, A. Kishimoto and M. Hiratani, Three kinds offramework structures of comer-sharing WO~ octahedm derived fromperoxo-polytungstates as precursors, Mater. Res. Bull., 26 (1991)
779-787.

99[6] i. Tsuyumoto, A. Kishimoto and T. Kudo, Hexagonal tungsten bronze
synthesized from potassium pemxo-polytungstate and its electricalproperties, Solid State lonics, 59 ( 1993) 211-216.
[7] J. Oi, A. Kishimoto and T. Kudo, Hexagonal and pyrochlore-type
cesium tungstate synthesized from cesium peroxo-polytungstate andtheir redox chemistry, J. SolidState Chem., 103 (1993) 176-185.
[8] T. Kudo, H. Okamoto, K. Matsumoto and Y. Sasaki, Peroxopolytungstic acids synthesized by direct reaction of tungsten or tungsten
carbide with hydrogen peroxide,lnorg. Chim. Acta, 111 (1986) L27L28.
[9] M.F. Pye and P.G. Dickens, A structural study of the hexagonal
potassium tungsten bronze, Ko.26WO3,Mater. Res. Bull., 14 (1979)
1397-1402.
[10] I. Tsuyumoto, A. Kishimoto and T. Kudo, in B.V.R. Chowdari, M.
Yahaya, I.B. Talib and M.M. Salleh (eds.), Solid State Ionic Materials.
World Scientific,Singapore, 1994, pp. 387-392.

Biographies
lsao Tsuyumoto is a Ph.D. candidate at the Department ofApplied Chemistry, Division of Engineering, University ofTokyo. He received a B.Sc. degree in 1991 and an M.Sc.
degree in 1993, both from the University of Tokyo. Sincejoining the Institute of Industrial Science, University ofTokyo, in 1991, he has engaged in research on the synthesis
and characterization of new inorganic materials.
Tetsuichi K u d o is a professor at the Institute of Industrial
Science, University of Tokyo. He received a B.Sc. degree in
1964, an M.Sc. degree in 1966, and a Ph.D. degree in 1978,
all from the University of Tokyo. F r o m 1966 to 1988, he wasa researcher at the Central Research Laboratory, Hitachi Ltd.