The role of water in the conductivity of vanadium pentoxide xerogel films

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The role of water in the conductivity of vanadium pentoxide xerogel films

T. Szörényi, K. Bali, I. Hevesi

To cite this version:

T. Szörényi, K. Bali, I. Hevesi. The role of water in the conductivity of vanadium pentoxide xerogel

films. Journal de Physique, 1985, 46 (3), pp.473-477. �10.1051/jphys:01985004603047300�. �jpa-

00209987�

The role of water in the conductivity ^{of vanadium} pentoxide xerogel ^films

T. Szörényi, ^{K. Bali and I. Hevesi}

Research Group ^on Luminescence and Semiconductors of the Hungarian Academy ^of Sciences, H-6720 Szeged, ^{Dóm tér 9.,} Hungary

(Reçu ^{le 11} juillet ^1984, révisé le 30 octobre, accepte ^{le 12 novembre} 1984)

Résumé.

Nous avons mesuré entre 200 ^et 600 K la conductivité à l’air libre ou sous un vide de 5 ^x 10-7 _torr, de couches minces obtenues à partir ^de gels ^de pentoxydes de vanadium. Ces mesures ont montré _que les chan- gements réversibles de la conductivité sont liés à des phénomènes d’hydratation ^{et de} déshydratation. ^Le départ

de l’eau faiblement absorbée entraîne, ^à température ambiante, ^une diminution de la conductivité de ~ 2 S/m ^à 0,3 S/m. Un traitement thermique ^{entre 430 et} 550 K dans une atmosphère d’oxygène ^{conduit à une} phase ^de déshydratation ^maximum avec 03C3 ~ 9 ^x 10-3 S/m à 300 K. Le fait que les énergies d’activation sont similaires dans les différentes phases suggère que le processus d’hydratation entraîne seulement une modification de la concentration des porteurs ^de charge.

Abstract

The measurement of DC conductivity ^of thin films deposited from vanadium pentoxide gels ^between

200 and 600 K in air, oxygen and ^{a vacuum} of 5 x 10-7 torr has revealed that reversible changes ⁱⁿ conductivity

are determined by hydration/dehydration phenomena. The removal of weakly ^{bonded water} results in a conduc-

tivity decrease from ~ 2 S/m ^{to ~ 0.3} S/m ^{at room} temperature. ^{Heat treatment} between 430 and 550 K in oxygen leads ^to the maximally dehydrated phase ^{in which 03C3}

^{9 10-3} S/m ^at 300 K. The essentially unchanged

activation energies in all of the phases suggest ^that hydration affects the charge carrier concentration only.

Classification

Physics ^Abstracts

72.20F - 73.60F

1. Introduction.

The physical chemistry of vanadium pentoxide gels

and colloids has been extensively ^studied mainly by ^{French teams} during the last few _years [1-17].

Since it was demonstrated that vanadium pentoxide gels ^were promising candidates as host structures for

intercalating ^{a wide} variety ^of guest species [6-8], ^most

of the research activities have concentrated on the elaboration of this possibility [9-15] and less attention has been paid ^{to the} semiconducting properties ^of

films deposited from these gels.

Bullot et ale measured conductivity ^values ranging

from 80 to 100 S/m ^{at 292 K on} layers deposited ^from

a solution of c = V" /(V" + V I +) = 0.06-0.097 [3, 4]

which was obtained by quenching molten vanadium

pentoxide ^{in water.} Sanchez and his coworkers

reported ^room temperature conductivities of ⁶

15 and 60 S/m ^for layers deposited ^from gels (c = 0.0 1) prepared by polymerization ^of decavanadic acid

[16, 17]. ^These conductivity ^{values are at} least four orders of magnitude higher than those of amorphous

vanadium pentoxides [18-25] ^{and two orders of} magnitude higher than the value of Y 20S single crystals along the c-axis [18, 19, 26, 27]. ^{Since the} gels,

like paints, ^are easily deposited/sprayed ^{onto substrates}

of large ^surface they ^have already ^{found an} application

in the photographic industry ^as antistatic coating [28].

Nevertheless the reason for this high conductivity ^is

not yet clearly understood. In their pioneering ^work

Bullot et al. interpreted ^the high conductivity ^of

the gel ^{as an} intrinsic feature of the material and, disregarding ^the presence of water, explained ^the conductivity ^as essentially determined by ^{the V4 +}

content [3]. However, ^{in the} light ^{of more recent} thermoanalytical and structural results [6, 10, 11, 29]

it has become apparent ^{that the} water content of the

gels ^must be considered to account for the unique semiconducting properties. ^{In a recent} communication Barboux et ale concluded that vanadium pentoxide gels ^{were mixed} conductors in which the idnic part ^of the conduction arises from diffusion of protons through ^the gel [30].

In this _paper the temperature dependence ^{of the} conductivity ^{of films} deposited from vanadium _pen- toxide gels ^is reported in the interval 200-600 K in different atmospheres. ^We demonstrate that the

conductivity ^is primarily determined by ^{the water}

content, and less decisively by ^{the V4+ content. This}

conclusion is substantiated by following ^well repro-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01985004603047300

474

ducible changes ^{in the} conductivity ^{due to} the removal and readdition of intercalated water and freezing/

melting phenomena. The critical temperatures ^observ- ed in the temperature dependence ^{of the} conductivity

agree with those observed by thermoanalytical ^me-

thods [6, 29].

2. Experimental ^details.

Amorphous ^vanadium pentoxide ^was prepared by

chemical _vapour deposition ^of VOCl3 ^with H20 ^at

room temperature [31]. By dissolving ^{the obtained}

material in water a dark red solution ^was formed which became ^{more and more viscous and was} slowly

converted to a colloidal solution _upon storage ^{for a} few days. The solution was 0.01-0.05 M in vanadium with _very low reduced vanadium content c=0.005 + 0.001, determined by titrimetric method [32]. ^After

dilution with water 1 ml aliquots ^were deposited ^into microscope slides which had been coated with vacuum

evaporated platinum electrodes in ^a coplanar geometry (electrode gap 1.5 mm). ^The samples ^{were allowed to}

evaporate ^to dryness ^{at room} temperature ^{in an} exsiccator over silica gel. ^The water content of the films was determined by thermogravimetric analysis.

Film thicknesses were measured by ^a Talystep

mechanical stylus instrument (Rank Taylor ^Hobson Ltd.). Temperature dependent conductivity ^measure-

ments were carried out in a liquid nitrogen ^cooled quartz cryostat in different atmospheres (air, oxygen and a vacuum of 5 x 10-’ torr). Heating ^and cooling cycles ^{were recorded at rates between 0.5 and}

4 Kmin-1. Sample resistance was evaluated from measurement of the current flowing through ^the sample ^{under constant DC} voltage (0.1-1.0 V). ^Below

1 V linear current-voltage characteristics were register-

ed at all temperatures.

3. Results and discussioa

3.1 TEMPERATURE DEPENDENCE OF THE CONDUCTIVITY IN AIR.

The DC conductivity ^{of films} deposited

from vanadium pentoxide gels displayed ^a hysteresis

behaviour between 220 and 360 K as shown in

figure ^1. Although details of the loop depended slightly ^on sample history, ^water vapour pressure

(14-18 ^{torr at 300} K) ^and heating/cooling rate, ^all the characteristic conductivity changes ^{could be} brought ^about reversibly by changing ^{the scan} speeds between 0.5 and 4 Kmin -1. In In a vs. 1 / T representation ^the hysteresis ^{curve is defined} by ^two parallel straight ^{lines of W}

0.22 ± 0.02 eV acti- vation energy corresponding ^{to two} different conduc-

tivity ^states (broken ^{lines in} Fig. 1), ^{and, transition}

curves connecting ^them.

Let us first consider the effect of heating ^{from 300}

to 360 K and a subsequent cooling ^{back to room} temperature. ^The conductivity (2 S/m ^{at 300} K) ^after

a brief increase starts decreasing ^{at - 315 K and with}

further elevation of the temperature ^the sample

Fig. ^1.

Temperature dependence ^{of DC} conductivity ^of

an initially V 20 5 ^{x 3} H20 xerogel ^film during subsequent heating/cooling ^scans between 210 and 360 K in air. Film thickness : 960 _nm; scan speed : ^{2 Kmin-1.}

reaches to lower conductivity ^{state at} approx. 360 K

(heating ^{scan in} Fig. 1). Thermoanalytical ^studies provided unambiguous ^evidence that in this tempe-

rature range dehydration ^takes place [6, 29]. ^Conse- quently, the removal of weakly ^{bonded water} [6, 29]

results in decreasing conductivity. ^{In the lower} conductivity ^{state of the} V205 ^x nH20 composition (1.3 ⁿ 1.7) (phase II) ^the conductivity ^varies exponentially ^with temperature between 360 and

~

335 K. Further decrease of the temperature ^leads

to an increase in conductivity ^and by completion

of the cooling ^{scan at} around 300 K the sample practically returns to the initial high conductivity

state (phase I). ^Its composition ^is V205 ^x nH20 (3 ⁿ 5), indicating that the removal of the

weakly ^bonded water content is fully reversible.

During repeated heating/cooling ^{scans between 300}

and 360 K, ^the sample ^shuttles between the two

conductivity ^states (phase ^I

phase ^II transition) according ^{to its} water content (see ^{inset in} Fig. 1).

If the heating ^{scan is} interrupted between 320 and 360 K and immediately ^{switched to} cooling ^{a shrunken} hysteresis loop ^can be obtained (see inset) ^since only ^a partial removal of the water content results.

Isothermal heat treatment at any temperature ^between 320 and 360 K produces transition between the above

conductivity ^{states. An} (interrupted) heating brings

about ^a phase ^I

phase ^II transition whereas an

(interrupted) cooling induces the reverse transition

(vertical ^{arrows a and b in} Fig.1). ^{This zero} speed ^case

was performed ⁱⁿ temperature dependent conductivity

measurements on xerogel samples prepared by poly-

merization of decavanadic acid by Barboux et al. [30].

The fair agreement in the characteristic temperature

[30] ^is noteworthy.

Upon cooling ^{from room} temperature the conduc-

tivity rapidly ^decreases again and, ^{at about 260} K, the temperature dependence of the lower conductivity

state is regained indicating that from the point ^{of view}

of the conduction process this ^state is, ⁱⁿ fact, analogous

to phase ^{II. There is, of course, a} significant ^difference

as to the removal of the weakly ^{bonded water. The}

same amount of water which was removed by heating

above 315 K is removed below room temperature by excluding ^{from the} gel ^structure [29]. The excluded water forms a separate phase which then makes no

contribution to the conduction. With further lowering

of the temperature ^this phase ^freezes [29].

During heating ^{scans the} low-to-high conductivity

transition commences exactly ^{at the} temperature (225 K) ^{at which the onset of a} broad endothermic

peak is observed in the DTA curves [29]. ^{This is} again

a strong evidence that water molecules as they ^rein-

tercalate into the gel ^{structure resume} their role in the conduction _process. At a scan speed ^{of 2 Kmin-1}

this rearrangement ^is completed ^{at about 240 K and}

then the conductivity displays ^the exponential ^beha-

viour of phase ^I.

It should be emphasized that, apart ^{from minor} variations during the first few _scans, the conductivity changed, fully reversibly, during subsequent heating

and cooling cycles. ^Our previous ^{remarks on inter-} rupted ^{scans also} apply ^{to the low} temperature part of the hysteresis loop, e.g. ^an interrupted cooling ^above

220 K and a subsequent heating ^{to room} temperature results in a shrunken hysteresis loop.

When the sample ^was heated above 360 K, ^the conductivity ^{exceeded the} exponential ^increase and, in the subsequent cooling ^and heating ^{scans, increased}

conductivities were measured. The hysteresis loop irreversibly ^{shifted to} higher ^values (Fig. 2a). ^The higher ^the completion temperature ^{and the} longer

the heat treatment, ^{the more} pronounced this increase in the conductivity (Fig. 2b). ^{Note that} during ^sub-

sequent ^{scans the} slope of the In Q vs. 1/T ^curves slightly ^decreased.

Fig. ^2.

Temperature dependence ^{of DC} conductivity ^of

an initially V205

³ H20 ^{xerogel film} during subsequent heating/cooling ^{scans between} (a) : 300 and 400 K; (b) :

300 and 430 K in air. Film thickness : 920 nm; ^scan speed :

2 Kmin-’. The symbols ^{are as} defined in figure ^1.

During ^such high temperature ^{scans the} initially bright red colour of the films changed irreversibly ^to

somewhat yellowish indicating reduction. Quanti-

tative EPR analyses proved ^{that the V4+ content of}

the films had a parallel increase with temperature/

time above 360 K in air. Thus, ^we conclude that the exposure ^to temperatures above 360 K leads to irreversible reduction of the initially ^{near stoichio-}

metric vanadium pentoxide films. The increase in reduced vanadium content results in an increase of the 6 values but does not affect the character of the

temperature dependence ^{of the} conductivity ^below

360 K.

3.2 TEMPERATURE DEPENDENCE OF THE CONDUCTIVITY IN VACUUM. - At _any temperature below 360 K the _exposure to vacuum (5 ^{x 10-’} torr) brought

about a rapid decrease in the conductivity; ^within

few minutes the ⁶ value of the samples approached

that of phase ^II (in air) ^{with a} difference of less than 10 % (vertical ^{arrows in} Fig. 3). During subsequent heating ^and cooling ^{scans between 360 and 200 K}

the conductivity displayed ^a fully reversible _expo- nential behaviour with an activation energy of W

0.022 ± 0.02 ^eV. This value was precisely ^the

same as that measured in phase ^{II in air.} Figure ³ clearly shows the difference between the temperature dependence ^{of the} conductivity ^{in air} (full line) ^and

in vacuum (curve 1). Unlike in air the loss of water in

vacuum is irreversible and thus there is no way for a

transition from the stable phase ^II.

During heating ^{scans above 360 K the} conductivity

exceeded the exponential increase, just ^{as observed}

in air, ^and during subsequent cooling ^{the In 6 vs.}

Fig. ^3.

Temperature dependence ^{of DC} conductivity ^of

an initially V205

³ H20 xerogel ^film during subsequent heating/cooling ^scans between 250 and 500 K in vacuum.

Film thickness : 950 _nm; scan speed : ^{2 Kmin-1. For} comparison ^the temperature dependence of DC conduc-

tivity in air is also given (full line).

476

1 / T curves were displaced ^to higher ^{a values and had} slightly ^decreased slopes. ^The higher ^the completion temperature ^{of the} heating ^{scans, the} larger ^the conductivity increase and the lower the activation energy (curves ^{2 and 3 in} Fig. 3). ^EPR analysis ^demon-

strated that these changes ^were also connected with irreversible increase of V4 + content. After each

heating ^{scan a more reduced state was} achieved which led to concomitant increase in the conductivity.

We note that this conductivity ^increase displayed

saturation at higher ^{V4+ contents.}

The observations that there is an instantaneous

change ^{in the} conductivity ^when the film is exposed

to vacuum and that the conductivity ^{in vacuum is,}

within experimental ^{error, the same as that of} phase ^II

in air further substantiate that the unique features of the gel layers in air below 360 K are due to reversible removal and recapture ^of weakly ^bonded (adsorbed)

water [6, 29, 30].

3. 3 TEMPERATURE DEPENDENCE OF THE CONDUCTIVITY IN OXYGEN.

To prevent ^chemical reduction, ^{i.e. to}

maintain a constant oxidation state of the films even

during high temperature heating ^{scans, all} subsequent

measurements were made in an oxygen atmosphere.

The room temperature conductivity value of the films in oxygen ^was somewhat lower than in air : J = 1.5 S/m

was measured at 300 K. Below 360 K a hysteresis loop ^was observed similar to the one measured in air (cf. full line in Fig. 4). After removal of the weakly

bonded water the conductivity ⁱⁿ phase ^II (curve ^I

in Fig. 4) ^increased nearly exponentially up ^{to -} 430 K, then the increase was slowed down and above 510 K

Fig. ^4.

Temperature dependence ^{of DC} conductivity ^of xerogel ^films during subsequent heating/cooling ^scans

between 260 and 600 K in oxygen atmosphere. ^This plot

was constructed by collecting the data of three samples ^of nearly ^{the same thickness} (d

^800-900 nm). ^Scan speed :

2 Kmin-1. Full line : the hysteresis loop under 360 K. The

symbols ^{are as} defined in figure ^1.

there was a steep decrease. This transient decrease

was completed ^{at - 550 K and at} higher temperatures increasing ^{In Q vs.} 1 / T ^{curves were} regained. ^{In the} light ^of previous thermoanalytical ^results [6, 29] ^this

behaviour is quite ^{natural. The slow} loss of the more

strongly ^{bonded water} commencing ^{at - 430 K first}

decelerates the conductivity increase, then, ^{with the}

advance of this _process at about 510 K, the conducti-

vity drastically drops.

The conductivity during subsequent cooling ^scans

will be determined by how far the preceding heating

scan went. If the _upper bound of the heating ^{scan was}

less than 430 K then the In J vs. 1/T curves, during cooling, repeated ^the appropriate heating ^curve (curve ¹

in Fig. 4) and, below 360 K, followed the cooling

branch of the hysteresis loop (lower ^full line). ^{If the} heating ^{scan was} completed between 430 and 550 K then the cooling ^{curves were} straight ^lines running essentially parallel ^{with a} shift towards lower conduc- tivities depending ^{on the final} temperature ^{of the} heating ^scan (curves ^{2 and} 3). ^{As the} cooling ^scans

reached - 330 K the exponential decrease slowed down and with further decrease of the temperature the conductivity gradually approached ^the cooling

branch of the hysteresis loop indicating that the loss of the more strongly ^{bonded water above 430 K is also}

reversible similarly ^{to the} phase Ipphase ^II transitions.

Isothermal heat treatment (i.e. interrupting ^the heating scan) ^at any temperature ^{between 430 and} 550 K resulted in a conductivity decrease. Curve 4 in

figure ⁴ shows the temperature dependence ^{of the} conductivity during ^a cooling ^{scan after a heat}

treatment of 24 hours at 540 K (vertical ^{arrow a in} Fig. 4). During ^{this treatment} all removable water is lost and so curve 4 displays ^the temperature depen-

dence of the conductivity ^{in the} maximally dehydrated phase ^III. If in this case the cooling ^{scan is} interrupted

below 330 K the conductivity slowly increases until the cooling branch of the hysteresis loop ^is again

reached (phase ^III

phase ^I transition : vertical

arrow b); indicating ^{that the} phase I => phase ^III (via phase II) transition is also reversible. Of course, similar isothermal transitions can be recorded starting

from intermediate (partly dehydrated) ^{states and} going ^to phase ^{I below 330 K} (vertical ^arrow c).

With the present conductivity measurements in an

oxygen atmosphere ^it may be demonstrated that the loss of the more strongly ^{bonded water between 430}

and 550 K, similarly ^{to the} phase ^I

phase ^{II tran-} sition, brought ^{about a} considerable decrease in the

conductivity. ^The higher ^the temperature ^{and the} longer ^{the heat treatment the more} pronounced ^this

reversible ^decrease. In the maximally dehydrated phase ^{III a} conductivity ^{of ~ 9 x 10- 3} S/m ^was

measured at 300 K. This value

which is

170 times less than that of phase ^I

agrees surprisingly ^well

with the values of Barboux et al. (Q = 10-2-10-3 S/m

for V205 ^{x 0.5} H20 ^{at 300} K) ⁱⁿ spite ^of differences in

preparation ^and measurement techniques [30].

4. Conclusions.

The characteristic changes ^{in the DC} conductivity

of films deposited ^{from vanadium} pentoxide gels ⁱⁿ

well defined temperature ranges ^are associated with

phase transitions due to the removal/separation ^and

readdition of intercalated water. The removal of

weakly ^bonded water, ^either by ^{heat treatment between}

320 and 360 K, ^or by exposure ^{to vacuum} and the

separation ^{of the} weakly ^bonded water content from the gel ^structure just ^{below room} temperature ^results

in a conductivity ^decrease by ^{almost an order of} mag- nitude. The consequence of the removal of the more

strongly ^{bonded water at 430-550 K} (in ^an oxygen

atmosphere) ^{is a} yet ^more pronounced decrease in

conductivity. All these alterations in the degree ^of hydration ^{and -} consequently

ⁱⁿ conductivity ^are

fully reversible until the material remains non-

crystalline.

These results reveal that the conductivity ^{of films} deposited from vanadium pentoxide gels ^is primarily

determined by the intercalated water content of the

sample. ^The surprisingly high conductivity ^{of the} xerogel films in air at ambient temperature [3, 4, 16, 17, 30] ^{is due to the} maximally hydrated ^state (phase I).

The essentially unchanged thermal activation energies

in _any of the phases corresponding ^{to different} hydra-

tion degrees suggest ^that hydration ^affects only ^the charge carrier concentration but not the mechanism of the conduction _process. Although ^{from the} present data no direct conclusion can be drawn as to the nature (electronic, ^{ionic or} mixed) ^{of the} conduction,

our results further corroborate the picture ^{of the} charge ^carrier transport given by Barboux et al. [30].

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