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Materials Chemistry and Physics 63 (2000) 104–108
Lanthanum doped SnO2and ZnO thin films sensitive
to ethanol and humidity
I. Stambolovaa,∗, K. Konstantinova, S. Vassilevb, P. Pesheva, Ts. Tsachevac
aBAS, Inst. of General and Inorganic Chemistry, G. Bonchev Blvd., 1113 Sofia, Bulgaria
bBAS, Central Lab. of Electrochemical Power Sources, 1113 Sofia, Bulgaria
cBAS, Institute of Physical Chemistry, G. Bonchev Blvd., 1113 Sofia, Bulgaria
Received 18 January 1999; received in revised form 6 August 1999; accepted 9 August 1999
Abstract
Thin SnO2and ZnO films doped with 0–7 at.% La were deposited by spray pyrolysis. The electrical resistance change of films doped
with different lanthanum concentrations was investigated in the presence of 1.5 vol% ethanol vapour in the air at temperatures varying
between 150 and 400oC. The ethanol sensitivity of 3% La-doped SnO2films was found to be about 30. Zinc oxide films doped with 3%
La showed a much higher sensitivity (S=350). All the films exhibited very short response and recovery times (less than 60 s.) at 300◦C.
Increasing of La content in the films up to 7% deteriorate the ethanol sensitivity. The enhancement of ethanol sensitivity for ZnO and SnO2
films doped with 3 at.% lanthanum can be explained by its double action on the films surface – increasing of surface basicity and decreasing
of the size of crystallites. The undoped and doped SnO2films show small change in resistance in the range of relative humidities 20–95%
RH. The electrical resistance of La doped ZnO films is more strongly influenced by the relative humidity than the resistance of the doped
SnO2films, but these dependencies are not linear. ©2000 Elsevier Science S.A. All rights reserved.
Keywords: Films; Tin oxide; Zinc oxide; Sensors
1. Introduction
Ethanol is one of the most used and widespread alco-
hols, which imposes the necessity of developing sensors
for its detection. Checkers for drives control and sensors
monitoring the ethanol concentration in air belong to the
most usual devices. Among oxide materials, perovskites
(Ln, M)BO3(Ln=rare earth metal; M=alkaline earth
metal; B =transition metal) [1,2] and bulk or thin film SnO2
[3–6] are most studied as ethanol sensors. It has been found
[3] that the introduction of some basic oxides and noble
metals into the SnO2matrix promotes its ethanol sensing
properties. For instance, La2O3additives significantly im-
prove the sensitivity and the response and recovery times.
In the present paper this effect has been investigated on
films prepared by spray pyrolysis of solutions.
Zinc oxide is also a promising material for fabrication
of gas sensitive thin films. Pure and doped ZnO films have
been investigated as sensors for oxygen [7], hydrogen [8]
and NOx[9], but data on their ethanol sensitivity are not
available. We have studied this sensitivity on La2O3-doped
∗Corresponding author.
ZnO films obtained by the same procedure as the SnO2
films.
2. Experimental
The spraying mixtures consisted of appropriate amounts
of 0.2M ethanol solution of SnCl4and ethanol-water (1: 1)
solutions of Zn(CH3COO)2(0.25M) and La(CH3COO)3
(0.05M), respectively. They had La : Sn or La :Zn atomic
ratios of 0:10, 0.2 : 9.8, 0.3 : 9.7, 0.5: 9.5 and 0.7 : 9.3 and
were deposited by spray pyrolysis at regular time intervals
of 10s onto silica substrates heated up to 350◦C in the case
of SnO2films and onto glass plates heated up to 300◦C for
ZnO films. The total amount of sprayed solution was 10ml.
After the deposition, the SnO2films were heated at 1000◦C
in oxygen for 30min, while ZnO films were annealed in air
at 400◦C for 60min. The thickness of the films obtained is
in the range of 100–250 nm. To prepare sensor elements two
vacuum evaporated Ag electrodes with a distance of 1mm
between them were deposited on the samples with dimen-
sions 5 ×5mm were obtained. The films were characterised
by SEM+EDX (Scanning Electron Microscopy+Energy
Dispersive Analysis) and XRD (X-ray diffraction). The
0254-0584/00/$ – see front matter ©2000 Elsevier Science S.A. All rights reserved.
PII: S0254-0584(99)00193-5
I. Stambolova et al./Materials Chemistry and Physics 63 (2000) 104–108 105
measurement set consists of chamber in which two heaters
and fan are fitted. The liquid alcohol is introduced into the
chamber with a micropipette and evaporated from the first
heater kept at 80◦C. The second heater is used to main-
tain the necessary temperature of the film surface, which
is controlled by fine thermocouple mounted directly on the
film. The fan is used to ensure fast equilibration of alcohol
vapours. The film resistance in absence (Rair) and presence
(Rgas) of 1.5 vol% of ethanol vapour in the air was measured
at 150–400◦C. The sensitivity (S) are defined as Rair/Rgas
ratio. The response and recovery times are important pa-
rameters of the sensors and are defined as the time needed
to reach 90% of the final resistance. The humidity sensitiv-
ity of the films was obtained by introducing different steam
amounts into the dry air in the test chamber at 25◦C. The
relative humidities were measured using a conventional
hygrometer with an accuracy of ±4%. The value of the
electrical resistance Rof the films was determined for each
RH value and R=f(RH) dependencies were plotted.
3. Results and discussion
The XRD data show that the basic phase in the SnO2films
is ␣-SnO2(cassiterite) without preferred orientation. In the
case of ZnO films the analysis reveals that the main phase is
hexagonal ZnO with strong 00L orientation and the highest
peak is the (002) one. In both the cases the introduction of
lanthanum above 5 at.% results in the appearance of traces of
impurity phase – La2O3. The crystallite size was calculated
according to the following equation [10]:
δ⊥(hkl)=sλ
βcosθ
where δis the crystallite size perpendicular to the hkl planes,
βis the true broadening due to the particle size and sis con-
stant, called the shape factor and usually sis taken equal to
unity. The error in determining the crystallite size was cal-
culated to be at a maximum ±0.3nm. The results obtained
show that the size of crystallites of ZnO films decreases
from 34nm to 14nm for doping level 0–7 at.% La, respec-
tively. The ZnO films doped with 3 at.% La which exhibit
the best sensing properties have 25nm crystallites. For un-
doped SnO2films the size of crystallites is 18 nm. Their size
decreases to 11nm for SnO2with 3 at.% La and slightly
increases to 15nm for films with 7 at.% La.
The SEM photographs of SnO2films are similar to those
of doped SnO2films deposited by spray pyrolysis as ob-
served in a previous paper [11]. The film morphology is
characterised by fine-grained basic layer with a few large
crystallites located on the surface. The SEM observations
of doped ZnO films show that introduction of lanthanum
above 5 at.% leads to the appearance of film inhomogeneity
(Fig. 1). The EDX analyses of the SnO2and ZnO films have
confirmed what was previously established [11] about the
SnO2film: the concentration of the dopant (i.e. the La) in the
Fig. 1. SEM photographs of ZnO films doped with (a) 5 at.% and (b) 7
at.% La.
basic layer is usually lower than in the spraying solution
(Fig. 2). In the large crystallites the dopant concentration is
several times higher as compared to the film. This fact can be
explained by the precipitation of the dopant as a secondary
phase (La2O3).
Fig. 2. Relationship between the concentrations of La in the film and in
the solution for SnO2(䊏) and ZnO (䊉) films.
106 I. Stambolova et al./Materials Chemistry and Physics 63 (2000) 104–108
Fig. 3. Ethanol sensitivity versus temperature of SnO2films doped with
0% (䉬), 3% (䊉), 4% ( ) and 7% (䊏) La.
The gas sensing characteristics of undoped and La-doped
SnO2films are shown in Fig. 3. It is evident that the intro-
duction of La leads to an increase in ethanol sensitivity, the
best properties being observed with 3% La-doped SnO2.At
higher or lower La concentrations, the sensitivity is close to
that of undoped films. The maximum sensitivity of doped
films appears at a lower temperature than is the case of un-
doped samples.
Fig. 4 presents the response times of undoped and 3%
La-doped SnO2films at temperatures where the sensitivity
has its maximum value. At 300◦C the response and recovery
times for doped samples are around 10s. The undoped films
have longer response and recovery times (20s).
The R/Tmeasurements on undoped ZnO films have re-
vealed a strong temperature dependence of their resistance.
The latter also exhibits some instability with time at a con-
stant temperature. The films doped with 3% La have the
best sensitivity (S=350) and a response time of about 60s.
Further increase of the dopant concentration decreases the
ethanol sensitivity and deteriorates the response time (Figs.
5 and 6).
Fig. 4. Response times of (a) undoped and (b) 3% La-doped SnO2films.
Fig. 5. Ethanol sensitivity versus temperature of ZnO films doped with
3% ( ), 5% (䊐) and 7% (4) La.
The data obtained show that doping with La promotes the
ethanol sensitivity of both SnO2and ZnO films. It is interest-
ing that the highest sensitivity is achieved at 3% La, but the
sensitivity of ZnO films exceeds that of SnO2films by one
order of magnitude. The doped SnO2films, however, have
shorter response times. The recovery times for both types
of films are similar (about 15–20s). The positive effect of
La doping on the alcohol sensitivity of SnO2and ZnO films
can be explained taking into account the chemical routes of
thermal decomposition of C2H5OH [3]. The selectivity of
two reactions is known to be influenced by acid-base prop-
erties on oxide surface-dehydrogenation to CH3CHO pro-
ceeds preferentially on basic surface:
C2H5OH +1
2O2→CH3CHO +H2(1)
and dehydration to C2H4which is favored on the acidic
surface:
C2H5→C2H4+H2O (2)
I. Stambolova et al./Materials Chemistry and Physics 63 (2000) 104–108 107
Fig. 6. Response times of ZnO films doped with (a) 3%, (b) 5% and (c) 7% La.
CH3CHO is known to have much higher molecular sensi-
tivity to a semiconductor gas sensor than C2H4. Therefore,
a basic oxide such as La2O3introduced in SnO2or ZnO
matrix will promotes the basicity of the surface such that
the dehydrogenation route is more favored than the hydra-
tion one and consequently the ethanol sensitivity increases.
Another reason for the improvement of ethanol sensitivity
of ZnO and SnO2films is the increasing surface area due
to the decreasing crystallite size when the La dopant con-
centration rises. The same effect has been reported by Xu
et al. [12]. Above 3 at.% La, however, the solubility limit is
reached and a secondary phase of La2O3precipitates on the
surface leading to its inhomogeneity and poor ethanol sensi-
tivity. On one hand with increasing of dopant concentration
the surface becomes more basic and with finer crystallites
which promotes the sensitivity. On the other hand, however,
appearance of precipitates leads to deterioration of the sur-
face quality and consequently decreases the ethanol sensi-
tivity and makes longer the response time above 3 at.% La
introduced in the films.
The R=f(RH) dependencies of undoped and doped SnO2
and ZnO films have also been studied. The SnO2films show
small change in resistance in the range of relative humidities
20–95% RH. This behaviour is typical for both undoped and
doped tin oxide films. The R=f(RH) relationship of 3% La
doped SnO2films is shown on Fig. 7a. The undoped ZnO
films exhibit instability in their resistance and have been ex-
cluded from this investigation. The resistance of lanthanum
doped ZnO films is more strongly influenced by the humid-
ity than the resistance of doped SnO2films, but these de-
pendencies are not linear and have step-like regions (Fig.
7b). The increasing of La content in the ZnO films does not
affect significantly the R=f(RH) dependencies.
4. Conclusions
The results of the measurements on the electrical resis-
tance at 150–400◦C of pure and La-doped thin films of
SnO2and ZnO in the presence of 1.5 vol% ethanol vapour
in air show that the lanthanum promotes their sensitivity.
The undoped ZnO films are not suitable for ethanol sensors
due to their strong R/Tdependence and the change of their
Fig. 7. Electrical resistance vs. relative humidity dependencies at 25oC
for (a) 3% La doped SnO2and (b) 3% La doped ZnO films.
resistance with time at a constant temperature. Such effects
are not observed with La-doped ZnO and SnO2films. At
the optimum dopant concentration, which is in both cases
3 at.% La, the sensitivity of zinc oxide films (S=300–350)
is by one order of magnitude higher than that of the SnO2
films. At 300◦C both types of films exhibit very short (less
than 60s.) response and recovery times. The enhancement
of ethanol sensitivity for ZnO and SnO2films doped with 3
at.% lanthanum can be explained by its double action on the
films surface increasing of surface basicity and decreasing
of the size of crystallites.
108 I. Stambolova et al./Materials Chemistry and Physics 63 (2000) 104–108
The study of humidity sensitivity of SnO2films show
that both undoped and doped films exhibit small change in
the resistance in the range of 20–95% RH. The lanthanum
doped ZnO films resistance is affected more strongly by
the humidity than the corresponding of SnO2films, but the
dependencies are not linear. The increasing of La content up
to 7 at.% in the ZnO films does not affect significantly the
R=f(RH) relationships.
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