Materials Research Bulletin, Vol. 31, No. 1, pp. 17-28, 1996
Copyright 0 1995 Elsevier Science Ltd
Printed in the USA. All rights reserved
0025-5408/96 $15.00 + .OO

Pcrgnmon0025-5408(95)00173-5

HUMIDITY SENSOR USING POTASSIUM HEXAGONAL TUNGSTEN
BRONZE SYNTHESIZED FROM PEROXO-POLYTUNGSTIC
ACID ANDITS RESISTIVITY CHANGE MECHANISM

Isao Tsuyumoto

and Tetsuichi

Kudo

Institute of Industrial Science, University of Tokyo
7-22-l Roppongi, Tokyo 106, Japan(Received May 25, 1995; Refereed)

ABSTRACT

We have found that potassium hexagonal tungsten bronze (K-HTB)
synthesized from potassium peroxo-polytungstate
at low temperature(5000C) shows a remarkable increase in resistivity in a humid atmosphereat room temperature. The mechanism of the resistivity increase byincorporation of water was investigated by gravimetry, complex impedance
spectroscopy, and powder XRD measurement. The powder XRD profiles
indicated that superlattice structure of the K-HTB emerges by incorporationof water. A layer structure model of the K-HTB was suggested from theseresults.
MATERIALS INDEX: tungsten, potassium, oxide, polyacid

INTRODUCTION

In recent years there has been a great deal of interest in the use of precursors for theformation of inorganic materials (1). The sol-gel techniques based on hydrolysis andcondensation of metal alkoxide is a typical example of such formation. The synthesis ofa hexagonal form of tungsten trioxide from a hydrothermally prepared precursor (3 WO,.H,O) is pointed out as an instance close to the present study (2). Such synthetic routes
generally have the advantages over conventional ones in that they enable us to synthesize

17

1. TSUYUMOTO

18

Vol. 31, No. 1

et al.

400
mm

p7

300
E
$j

.

200

a 100
0

I

1000

2000

3000

t/min

FIG. 1
Resistivity increase of the K-HTB in ahumid atmosphere ~,*o = 8.0 mmHg
(AC 2-probe measurement).

3f36(
0

0
200

400

600
t/min

800

10001200

FIG. 2Mass increase of the K-HTB by incorporation of water in a humid atmosphere pHzo
= 13 mmHg.

various metastable phases because of lower formation temperaturesand better compositional
uniformityof products. Moreover,
special properties
can be expected ofsubstances formed at low temperatures because defect structures are often different fromthose of high temperatureformation even though the basical structures are the same.
Materials synthesis on such concepts is called “soft chemistry” or “chimie deuce” (3).
We have reported that various tungstates with a defective hexagonal or pyrochlore typeframework are synthesized through a soft chemical route using precursors precipitatedfrom a peroxo-polytungstic
acid (IPA) solution with alkaline or alkaline earth halides (4,5).
It has also been shown that tungsten bronze phases with the same framework are formed

0

1000

2000

tlmin
FIG. 3
Resistivity increase of the K-HTB at room
temperaturewhen pHzo was increased
stepwise (DC 4-probe measurement).

FIG. 4.
Resistivity decrease of the K-HTB atroom temperaturewhen pHzo was
stepwise(DC
4-probedecreasedmeasurement).

Vol. 31, No. 1

POTASSIUM TUNGSTEN BRONZE

19

at relatively low temperature (500-700°C) by heating such precursors in a reducing
atmosphere (H, 10%) (6). Of these, pyrochlore-type Cs,WO, is a new bronze synthesized
for the first time by this low temperature synthetic process (7).
Potassium hexagonal tungsten bronze (K-HTB, K xWO,; x = 0.3) thus synthesized at
500°C is not different in basical crystal structure from conventional K-HTB synthesized
by well-known solid state reaction around 1000°C (8). However, we have found that it
shows a remarkable increase in resistivity by water incorporation (6). A fresh water-free‘sample pellet of the present K-HTB shows resistivity as low as 0.1 ncm at room
temperature in a dry atmosphere like the conventional ones. However, its resistivity was
dramatically increased in a humid atmosphere at room temperature (Fig. 1). At the sametime, a significant amount of water incorporation by the sample pellet was gravimetricallyobserved (Fig. 2). It was also found that the present K-HTB shows a property as a humidity
sensor in the P,~, range of 2.0~10-~-14.6 mmHg at room temperature (9,lO). The
resistivity of the sample pellet was increased in response to stepwise increases in pHzo as
shown in Fig. 3, showing a clear equilibrium value of resistivity for each value of PHzo. For
stepwise decreases in P,,,, its resistivity was decreased as shown in Fig. 4, indicating goodreversibility. Infrared spectroscopic observations and density measurements have indicatedthat this unusual property is associated with the defect structure due to low temperatureformation. The purpose of this paper is to investigate the mechanism of resistivity increaseby water incorporation.
EXPERIMENT
Svnthetic Procedure. Peroxo-polytungstic acid was prepared according to the previously
reported method (11). Metallic W was dissolved in 15% H,O,aq to yield a pale yellow
acidic solution. It was then dried at room temperature after removing excess H,O* catalytically with a platinized Pt net, resulting in a yellow glassy substance (IPA) with anapproximate empirical formula of 2W03*H20,.2H20.
White amorphous substance was precipitated by adding 50 ml of an IPA solution (1.2mol/l as to tungsten) into 50 ml of a KBr solution (1.2 mol/l) with stirring. After storagefor 24 h at 20°C the precipitate was filtered, rinsed with water, and then dried at 70°C.
Heating thus formed potassium peroxo-polytungstate in H Z 10% (N z balance) at 500°C
for 1 h, dark blue potassium hexagonal tungsten bronze (K-HTB) was obtained (10).
Gravimetrv. Gravimetry of K-HTB at different P,*, was carried out at room temperatureusing high sensitive balance (Cahn 2000 Recording Electrobalance) to investigate therelationship between PHzo and n (n in K,WO,nH,O). The desired PH2, was made bymixing a dry gas with a wet one using mass flow controllers.
lmnedance Snectroscoov. The changes of complex impedance of the present K-HTB by
water incorporation were measured using impedance analyzer (Hewlett-Packard 4 192A)
with the oscillation level of 0.300 V over a frequency range of 5 Hz- 13 MHz. The sample
pellet of K-HTB was fabricated as follows. The powder sample of K-HTB was pressed at
3~10~ Pa for 1 min into a pellet with 2.5 mm in diameter and 2 mm in thickness. After thegold electrodes were coated on both sides with gold paste (Tokuriki Chemical Corporation
No. 8570) this pellet was reheated at the same condition as the synthesis for 1 h. The final

20

I. TSUYUMOTO

et al.

Vol. 3 1, No. 1

porosity of this sample pellet was ca. 35%. The changes of complex impedance in a wet
atmosphere ( pHZo = 11 mmHg, 28°C) were measured using this sample pellet.
Structural Analysis.
Structural change by incorporation of water was investigated bypowder XRD measurement using monochromatized Cu &radiation. The 2 13scanning was
carried out stepwise with the width of 0.02” and the measuring time of 0.6 sec. In order todiminish preferred orientation, the sample was well ground and attached softly on a sample
holder.
XRD spectra were taken using three types of powder samples: (a) taken immediately
after reheating (water-free), (b) left in a wet atmosphere (R.H. 100%) for 67 hours, and (c)left in a wet atmosphere (R.H. 100%) for 113 hours.
RESULTS AND DISCUSSION
Relationshin between Water Vanor Pressure and Water Content. Relationship between
water content in the K-HTB and water vapor pressure was investigated by gravimetricanalysis. Water content n (in K,WO,nH,O) was increased stepwise in response to increasein water vapor pressure P,*, , showing a clear equilibrium value of n as shown in Fig. 5.
Relationship between P HZo and the equilibrium value of n is shown in Fig. 6. The valueof n equilibrium with the dry gas (~,~o = 2.0~10~~ mmHg) was ca. 0.16. When used as ahumidity sensor at room temperature, water molecules are assumed to adsorb and desorb
the sites n>ca. 0.16. From TG and TPD spectrum it was found that water desorption occurs
in two steps: one is r.t. - ca. 120°C in the range of n>ca. 0.12 and the other over ca. 120°C
in the range of n = 0 - ca. 0.12 (6,10). An approximate theoretical fitting to the TG
spectrum indicated that adsorption sites in K-HTB consists of two types, one of which has
the adsorption energy ca. -5O--40 kJ/mol and the other ca. -10 kJ/mol (10). We canconclude that water adsorption sites of the K-HTB consist of two types with different
adsorption energy. In this paper we discuss thermodynamics using the n vs. ~,~orelationship at constant temperature.
200.4

co.2

c

PH20
‘0.002mmHg

199.4
0

0‘
P
-0.1
c

-0.15
0.0

100

200 300 400

500

t/min

FIG. 5
Increase of water content when PHz, wasincreased stepwise.

FIG. 6
Relationship between P,*, andthe equilibrium value of watercontent n (in K,WO ,nH,O).

Vol. 31, No. 1

POTASSIUM TUNGSTEN BRONZE

21

Let us first discuss the n vs. P,~~ relationship theoretically when the adsorption sites for
H,O consist of two types. If the site energy of adsorption sites (site 1 and site 2) are
E, and E, (E,<E,<O), respectively,
the Helmholtz free energy F(n) of the K-HTB withwater content n isF(n) = E,n, + kT In N, Cn, + E,n,+ kT In N,Cn,
where n, and n2 are the molar number of H,O distributed
molar number of adsorbed H,O; therefore,(1)

to site 1 and site 2, and n the total

n=n,+n2(2)

N, and N2 are the maximum accommodation
number of site 1 and site 2, the mole of theadsorption host.
The adsorbed Hz0 molecules are distributed over the two sites so as to minimize theHelmholtz free energy F(n). We can find n, and n2 by using the Lagrange undetermined
multiplier under the condition of [2].
aF/an, = aF/an, = L
where 1 is an undetermined,uH *O as follows:

multiplier(3)

and its value is related to the chemical

I = aF/an, = aF/an, = ,uH,O = ,uH,O” + kT lnPHz,
Substituting

potential(4)[l] in [4], we obtain,uH,O = pH,O” + kT In pHzo = E, + kT In x,/(1-x,)(5)= E, f kT In xJ(I-xj(6)

where x, and x2 are the occupanciesof site 1 and site 2x, = n/N,,

By transforming[5] and [6], the equation

x2 = nJNI(7)

for the Langmuir

xi = K P,,,/(

K = exp(pH,O”
From [5] and [6], we can find the relationshipadsorption

isotherm

1+K P&- Ei )lkT(ibetweenis derived:(8)= 1,2)
P,?,(9)and Xi (i = 1,2).

In 'Hz0 = (E, -,uH,O”)lkT+ In x ,/( 1-x,)(10)= (E2 -,uH,O”)lkT+ In x,l( 1-x2)(11)

If E, - E, = 1 eV and T = r.t.-600 K, water molecules begin to occupy site 2 after site 1 isalmost occupied. These equations are none other than the Langmuir adsorption isotherm.
It can be said that the above is the thermodynamic
derivation of the Langmuir adsorption
isotherm.

22

I. TSUYUMOTO

Let us discuss the relationshipVol. 31, No. 1

et al.

by using the observed

data from Fig. 6:

n = 0.187 (= a) at P , = 4.45 mmHg

(12)n=0.206(=6)atP,=(13)

11.87mmHg

If we assume x, = 1 at these PHzo’s and In pHzo (dry gas)>>(E,-,uH,O”)lkT,uH ,O”<<-0.3 1 eV), the following can be derived:
N, = 0.154 ( = n equilibrium
Transformingwith the dry gas)(14)

= (E, - ,uH,O”)lkT + In n J(N2 - nz)(15)[ 111, we obtain
In P,,

Using P,, P,, a, b, and N,

in [ 151, we obtain

In P, = (E, - pH,O”)IkT+ ln(a - N,)I(N, + N 2 - a)

In P, = (E, - ,uH,O”)IkT + In (b - N,)l(N,
Subtracting(16)+ N, - b)(17)+ N, - a). (N, + NJ - b)l(b - N ,)(18)

[ 171 from [ 161, we obtain
P&P, = (a - N,)I(N,

Substituting(i.e., E , -[12], [13], and [14] into [18], we obtain

4.45/l 1.87 = (0.187-0.154)/(0.154+N,-0.187)~(0.154+N,-0.206)/(0.206-0.154)(19)

Therefore,
N, = 0.079,

E, - pH,O” = -0.125 eV(20)This result is consistent with the assumption expressed between Eq. 13 and Eq. 14. The
x2 (occupancy of site 2) at P, and P, can be calculated from this result.
at P,, x2 = 0.42

(21)

at P,, x1 = 0.66

(22)

Let us examine whether these results agree with the observed data at intermediate
Fig. 6. From Fig. 6,
n = 0.199

at Pgo= 7.75 mmHg

pHzo

in

(23)

Thus,
In P,*o = -4.59(E, - ,uH,O”)lkT+

In n2/(N2 - n ) = -4.55

(24)
(25)

Vol. 3 1, No. 1

POTASSIUM TUNGSTEN BRONZE

23

The observed data are almost coincident
with equation [15]. Similarly,
the otherobserved data are also coincident
with equation [ 151. Since equation [ 151 is anotherexpressionof the Langmuir adsorption
isotherm, this result indicates that the water
adsorption into site 2 follows the Langmuir adsorption isotherm and the adsorption energyof site 2 is constant. In other words, it was demonstrated by thermodynamic
discussion thatwater adsorption into site 2 follows the Langmuir adsorption isotherm. This result suggeststhat site 2 is located in a homogeneousplane-likeinterlayer because it has constantadsorption energy. More studies concerning the n vs. PH2, relationship(i.e., n vs. pH,O)
are underway.
ImDedance Snectroscopv.
The changes of the K-HTB in complex impedance plot by water
incorporation
are shown in Figs. 7a-7f. The complex impedance plot of K-HTB withrelative low water content is characterized by its positive values of Z” (imaginary part ofimpedance),
which can be explained by assuming a component of coil in the equivalentcircuit of K-HTB.
It is not quite certain so far why the equivalent circuit apparently includes a componentof coil. However, since samples with high electronic conductivity
such as wire made ofcopper, plate of aluminum,etc., exhibit a complex impedance plot represented
by anequivalent circuit including coil (as shown in Fig. S), we can empirically conclude that thisis the nature of samples with high electronic conductivity.
The frequency dependence of
the impedance (2) of the equivalent circuit in Fig. 8 is represented by
Z = joL

+ R,.

(26)

The complex impedance plot of the equivalent circuit in Fig. 8 is shown in Fig. 9. Thisis consistent with that of the water-free K-HTB immediately after reheating in Fig. 7a. This
indicates that the water-free K-HTB has high electronic conductivity
like metal and likethe conventional
K-HTB by a solid state reaction. On the other hand, the complex
impedance plot after water incorporation is consistent with that of equivalent circuit in Fig.
10. Thus, we interpreted the profiles of the impedance diagrams by using the equivalentcircuit in Fig. 10.
The frequency
dependenceof the impedance of the equivalentcircuit in Fig. 10 isrepresented by
Z = joL+ R,. -I-Rr(l - jocR,)/(I+02c2R,-2)(27)

R,, and R,. are estimated from Figs. 7a-7f and are plotted as a function of time in Fig. 11.
R,,is dramatically increased from 6 Q to 20000 n, whereas R,, remains almost constant (7R-30 n). (The resistance of wire used for this measurement
was 0.4 n. Thus it isnegligible
compared to R,~.) This indicates that a new component of high resistivity andhigh capacitanceis generated in the circuit by incorporationof water and the newcomponent
hinders the conductionof electrons, whereas the part with high electronic
conductivity
remains unchanged. This suggests that the component of condenser (R,, C)
corresponds
to interlayer or grain boundary of the sample pellet and that of coil (R ,_, L)corresponds to grain. In other words, water molecules are incorporated in the interlayer orgrain boundaryand separate the path of electrons, which results in the increase ofresistivity.

24

I. TSUYUMOTO et al.

Vol. 31, No. 1

20.0

OJ)

I

I

0

40

60
120
Z/Ohm

-150'
0

50

100
150
Z’/Ohm

,

I

I

160

200

0.00
-20.0
f
5 -40.0-20.0

i; -60.0

E
P
&

-40.0

' -80.0* -60.0

-60.0

-100
-120 0

-120
40

60
120
T/Ohm

160

200

50.0

20.0
0.00

0.00

-20.0
2

0

E

-40.0

Q

i; -60.0-50.0$' -60.0-100-120'.
0

I

'
40

I

I

I

00
120
Z’/Ohm

160

200

I

t

I

200

250

( r)

3000
16000

2000

cv

E 1000p 0.000:;.2;;

t

,

,

0

2000

4000
i”/Ohmt=

27fnlin

6000

{

6000

FIG. 7The complex impedance plots of the K-HTES. The measurement was carried out using a sample
pellet left in a humid atmosphere (pxJo = 11 mm Hg) for (a) 0 min, (b) 8 min, (c) 24 min, (d)54 mitt, (e) 274 min, and (I) 1739 mm.

The interfacial phenomena between the electrode and the sample pellet is negligible
because the change of resistivity
was observed by DC 4-probe measurement.
Themechanism will be discussed in more detail later.

Vol. 31, No. 1

POTASSIUM TUNGSTEN BRONZE

L

25

RL
Z’/OhmFIG. 8The equivalent circuit representing
samples with
conductivity.

highelectronic

FIG. 9The complex impedance plot of theequivalent circuit in Fig. 8.

Structural Analvsis. Slight changes in lattice parameters are observed by incorporationof water. The water-free K-HTB immediately after reheating has lattice parameters a =7.3726 A, c = 7.5570 A (Fig. 12a); and the saturated K-HTB after 113 hours of water
adsorption, a = 7.3868 A, c = 7.5085 A (Fig. 12~). Lattice parameter a is increased by ca.
0.01 A, and lattice parameter c is decreased by ca. 0.05 A. Moreover, superlattice structure
every six layer along c axis emerged by incorporation of water as shown in Figs. 12b and12~. After reheating the saturated sample in Fig. 12c, the XRD spectrum returned to thatof the water-free one in Fig. 12a. In other words, these structural changes are reversible inresponse to adsorption and desorption of water molecules.
The mechanism of the resistivity-change in this connection is discussed as follows.
Oxygen vacancies exist every six layers along c axis, as shown in Fig. 13. Water molecules
are incorporated into these vacancies as coordinated water or structural water, and

20000

0

0

E
;

0

g 10000Im.a 5000.0d60.000015000

0i-5000.0 0

FIG. 10The equivalent circuit used for theinterpretation of the impedance diagramsin Figs. 7a-7f.

o

0
n

Rc
RL

o

500

1000
timin

1500

2000

FIG. 11
The changes of R (, and R 1,in Fig. 10 byincorporation of water. R. and R,, areestimated from Figs. 7a-7f.

a=7.373A

c=7.557A

10K I
z
3

OKj-

ii

,A~

,
20.0

28/Ka

40.0

(4

IOK

r
a=73854c=7.513A

A&_,?

OK

2e/Ka

40.0

60.0

10KI

The powder
incorpora-tion
incorporation

FIG. 12XRD profiles (a) immediately after reheating, (b) after 67 hours of waterin a humid atmosphere R.H. 100%; and (c) after 113 hours of waterin a humid atmosphere R.H. 100%.

Vol. 3 1, No. 1

POTASSIUM

water-free

TUNGSTEN

BRONZE27

after water-incorporation

FIG. 13
The layer structure model suggested in this study. A rhombus corresponds to a WO,
octahedron.

structural changes (as shown in Fig. 13) take place. Because of these structural changes,
the path of electrons along c axis is separated. This model is coincident with the result of
the impedance spectroscopy discussed earlier in this paper. . In this structural model, watermolecules can be accommodated into vacancies up to H,O/W = 0.167. Judging from this
H,O/W ratio, these oxygen vacancies corresponds to site 1 (see discussion on water vapor
pressure/water content relationship). Site 2 is assumed to be located between watermolecules in site 1 and terminal oxygen (W = 0). It is unlikely that site 2 is located in thegrain boundary because the n vs. pqo relationship indicates that site 2 has constantadsorption energy. More studies are needed to provide more details.
ACKNOWLEDGEMENT
This work was supported by a Grant-in Aid Scientific Research from the Japanese Ministerof Education, Science and Culture (04205024).

28

I. TSUYUMOTO

et al.

Vol. 31, No. 1

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1.
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3.
4.
5.
6.
7.
8.
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C.D. Chandler, C. Roger and M.J. Hampden-Smith,
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I. Tsuyumoto, A. Kishimoto and T. Kudo, Solid State tonics 59, 21 I (1993).
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