Effect of Doping and Grain Size on Butane gas sensing properties of In2O3 doped
SnO2
Mahdi Hassan Suhail* Manal Madhat Abdullah* and Sabah Ibrahim Abbas **
*Dept. of Physic, College of Science, University of Baghdad, Iraq
** Dept. of Physics, College of science, University of Wassit, Iraq
E:mail mhsuhail@yahoo.com
Abstract: Undoped and doped SnO2 with In2O3 (1 wt.%, 2 wt.%, 3 wt.%) thin films
were prepared by using the thermal spray pyrolysis method from SnCl2.2H2O and InCl3
dissolved in isopropyl mixing with water solution (1:1) on the glass substrate heated at
400 °C - 450 °C. The structure properties of In2O3-doped SnO2 were investigated by Xray diffraction patterns. The morphology and crystallite size was evaluated by using
Atomic Force Microscope. The band gap energy was 3.6 eV for pure SnO2. These films
were tested in 5 vol % butane in air and 5 bias volts. The optimal temperature for Butane
sensing is found to be 470 °C for pure SnO2 and 450 °C for doping with In2O3. Maximum
sensitivity is found (85.3%) for the (1%) doping concentration and response time and
recovery time is 4 s, 72 s respectively.
Keyword: SnO2 Thin films; spray pyrolysis; Butane gas sensors; sensitivity.
1. Introduction
Tin dioxide SnO2 is a wide band gap 3.6 eV n-type semiconductor with rutile bulk
structure [1]. SnO2 has been shown to have major potential for use as a transparent
conducting oxide; it has transparency in the visible range and relatively high electrical
conductivity [2, 3]. SnO2 exhibits a highly sensitive surface which enables its use as a
sensor material [4, 5]. Additionally, various electrical applications such as electrode
materialism display [6], light-emitting diodes [7], and transparent thin film transistors [8].
The space charge layer control of nanostructures, including nano-SnO2, makes them
particularly interesting since conduction can change drastically with expansion and
contraction of the layer in the presence of different gases [9, 10]. The energy required for
electrons or other charged species from adsorbed gas molecules to conduct through the
material can be greatly impacted by the magnitude of the band gap and whether or not
any energy level exists within the forbidden gap. When SnO2 based sensor is exposed
reducing gas it reduces the surface oxygen atoms and decreases the space charge layer
thickness around the nanocrystaline SnO2 grains [11]. Doping SnO2 with other elements
has been provided the ability to tailor its electrical and microstructural properties [12-14].
SnO2 is used in the form of nanosized grains since the surface to volume ratio is much
larger at the nanoscale [15].
The ability of a sensor to sense the presence of gas depends on the nature of the
interaction between the gas molecules and the surface atoms of the sensing film. The
reactivity of the surface is critically dependent on its doping and the defect structure. It
was reported that Malyshev et al [16] observed highest sensitivity of sputtered undoped
SnO2 thin films at 450 °C in presence of butane and propane.
Studies from Shukla et al
[10] had also shown an increase in hydrogen detection sensitivity of nanocrystalline SnO2
when doped with indium In3+. Indium doping creates even more of the desired oxygen
vacancies by substitution on a tin lattice site, which in equilibrium expects a 4 + charge to
occupy the lattice site. This happens because a 3+ on a 4+ site makes the tin site feel a
negative charge. To compensate, an oxygen vacancy will form which causes the previous
oxygen site to experience a positive charge. This charge compensation can drastically
enhance SnO2 gas sensing abilities. Charge modification can be shown using KrogerVink notations as [17].
1
𝑂𝑜𝑥 → 2 𝑂2(𝑔) + 𝑉𝑜𝑜𝑜 + 2𝑒
(1)
𝑥
2𝐼𝑛3+ + 2𝑆𝑛𝑆𝑛
+ 𝑂𝑜𝑥 → 2𝐼𝑛𝑆𝑛 + 𝑉000 + 𝑆𝑛𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (2)
Thus, successful doping of the indium atoms results when it replaces tin into the SnO2
matrix and this enhances the surface reactivity of the nanoparticles toward gas
sensing.Doping of the indium in the SnO2 matrix, corresponding change in free carrier
concentration, and variations in the space charge layer of the nanocrystalline SnO2.
In this work SnO2 and In2O3/SnO2 thin films has been prepare by spray pyrolysis
method, and demonstrates of the butane sensing properties.
2. Experimental
Pure SnO2 and doping with indium oxide (1 wt. %, 2 wt. %, 3wt. %) thin films
were deposited using an aqueous – isopropanol solution including SnCl2.2H2O (99.8%,
Aldrich) and InCl3 (98%) (0.1 M), H2O and CH3CH2OH (1:1) and a few ml of
hydrochloric acid by the spray pyrolysis technique. For film deposition, a glass with 1
mm thickness and 25 × 75 mm2 dimensions, were placed on the hot plate at temperatures
ranging from 440-460 °C, which is known to be the optimal range for the formation of
SnO2 films [18].
Then solution was sprayed at the following conditions: carrier-air
pressure: 1-2 atm., flow rate of solution: 6 ml/min, and substrate-to-nozzle distance: 35
cm. For preventing reduction at hot plate temperature, spraying was done in short time
intervals. The metallic salt solution, when sprayed onto a hot substrate, prolifically
decomposes and a chemical reaction takes place on the heated substrate and at least a thin
layer of SnO2 is deposited. The hot substrate provides the thermal energy for the thermal
decomposition and subsequent recombination of the constituent species. The
phenomenon for the preparation of a metal oxide thin film depends on surface hydrolysis
of metal chloride on a heated substrate surface [19].
The structural of the thin films were examined by X-ray diffractometer ( 6000Shimadzu) using CuKα radiation with a wavelength, λ=1.54060 Å .The morphological
of the films were analyzed using scanning Probe Microscope ( SPM , model AA3000
Angstrom Advanced .Inc). The optical absorbance of the films was measured using UVvisible spectrophotometer (SP-3000 Optima) in the wavelength range 200-1100 nm at
room temperature. The deposition of aluminum electrodes using masks where the finger
width is 1mm and distance between two fingers is 1mm. The gas sensing chamber had
been employed for testing of these films to gases. A fixed voltage of 5 V was applied
across the films. The current was measured using a Scope digital multimeters (UT81).
Films response to butane gas was studied by introducing the gas of known concentration
(5%) volume ratio to the air and recording current as a function of time.
3- Results and discussion
Figure.(1) shown the XRD patterns of the SnO2 thin films prepared with different
In2O3 doping concentrations at deposition temperature (Tpyr=400℃) for pure SnO2,
3wt.% and (Tpyr=450 ℃) for 1 wt.% and 2 wt.%. The analyses evidence that most of the
peaks belong to tin oxide except a few to indium oxide. With increasing doping
concentration, new peaks appear at 3 wt. % doping ratio for In2O3. The pattern shows that
all of the indium oxide doped SnO2 samples were nanocrystalline in nature. XRD data
shows that the SnO2 has formed by chemical spray was tetragonal, Dominant peaks at
(110), (101), and (211) parallel to the substrate. The (110) is the dominant crystal
structure of the low-index crystal faces for this material due to its stability. This is the
desired structure of SnO2 for sensing applications since its prevalent (110) growth plane
is extremely stable and can reject oxygen with little distortion [20]. Growth of this plane
helps in achieving high oxygen vacancy concentrations at low temperatures. The result is
in a good agreement with data mentioned in the literature (JCPDF card no 36-1451) [21].
Wideness of the peaks indicates that SnO2 films are composed of small nanoparticles as
shown in figure (1) [22].
Figure .1: XRD crystal structure of SnO2 doped with In2O3 (Pure, 1%, 2%, 3%) thin
films on glass substrate.
AFM is used to study the surface morphology with resolution of 0.1 nm. Figure (2)
illustrates the two and three-dimensional AFM images of samples. The grain size and
RMS roughness of these films is shown in table (1). The AFM images of all samples
displayed a granular structure. The granular films show higher surface area, which is
conducive for film-gas interaction and results in higher sensitivity in gas sensing
applications [23]. The grain size is decreased with increasing doping ratio. In the
undoped SnO2 a coarse and irregular surface with low grain density distribution is
domain also these grains are not tightly packed. The small spherical grains agglomerates
are shown in the figure for (1 wt. % , 2 wt.%).The smoother surface may obtain due to
optimization of the deposition conditions such as distance between the nozzle and the
substrate [22].
Figure .2: AFM image of undoped and doped SnO2 thin films deposited on glass
substrates at temperature 450 °C.
A lower surface roughness with uniform orientation is shown with (2 wt. %) doping,
this may corresponds to the columnar structure which is associated with the (110) SnO2
textured growth [24]. The grain density decreases and the roughness of these films are
increased for (3 wt.%) due to the existence of many hillocks, which are faceted and
distributed randomly on the relatively smooth surface [25] .While in crystallites with size
> 50- 70 nm , such planes as (101).(110) become dominating [26].
The optical transmittance of undoped and doped SnO2 films as a function of
wavelength ranging from 200-1100 nm is studied by UV-Vis spectrophotometer. Figure
(3) shows the optical transmittance spectra of thin films. All samples demonstrate more
than 83% transmittance at wavelengths longer than 380 nm. A sharp decrease in the
transmittance of the films at wavelength below 350 nm , is probably due to the absorption
edge in this region.
100
Transmittance %
80
60
40
pure
1%
2%
3%
20
0
280
480
680
880
Wavelength (nm)
1080
Figure .3: optical transmittance of undoped and doped SnO2 thin film.
This can also manifest the crystalline nature of the SnO2 films. An increase in the
transmittance of the pure films could be attributed to the decrease in free carriers [22].
The decrease in lattice disorder due to the reduction of population density of oxygen
vacancies enhances the crystalline nature of the films which in turn improves the
transmittance percentage [27]. The increase in transmittance and conductivity can be
attributed to the improved crystalline structure.
Figures (4) show the variation of (𝛼ℎʋ)2 against hʋ for all samples the nature of
the plot indicates the existence of direct optical transitions.
1E+10
pure
1%
2%
( αhʋ)2 (cm-2 eV 2 )
8E+09
6E+09
4E+09
2E+09
0
1
2
3
hʋ (eV)
4
Figure .4: (αhν)2 vs. hν of undoped and doped SnO2 thin films.
Band gap Values increase slightly for high doping (2%) concentration due to
improved structure and or enhanced quantum confinement. An oxygen-deficient film
usually has a wide bandgap, resulting in a blue shift of the optical transmission spectrum.
It should be noted here that at very high carrier densities for 3% doping, the electronelectron and electron-impurity scattering could cause a band-gap narrowing [28,29].
I – V characteristics of undoped and doped SnO2 films are deposited on the glass
substrate shows the forward bias exposure of air, and butane gas with concentration
levels 5 vol % butane to the air, at different operating temperature as shown in the figure
(5).
150
100
500
(1%)
400
300
200
50
100
0
0
0
900
2
4
6
Bias Voltage(V)
300 ◦C in air
400 ◦C in air
450 ◦C in air
300 ◦C in Butane
400 ◦C in Butane
450 ◦C in Butane
800
700
Current (μA)
300 ◦C in air
400 ◦C in air
450 ◦C in air
300 ◦C in Butane
400 ◦C in Butane
450 ◦C in Butane
600
600
8
0
10
350
300
250
500
400
300
2
4
6
Bias Voltage (V)
300 ◦C in air
400 ◦C in air
450 ◦C in air
300 ◦C in Butane
400 ◦C in Butane
450 ◦C in Butane
(2%)
Current(μA)
Current ( μA)
200
700
(Pure)
Current (μA)
300 ◦C in air
400 ◦C in air
450 ◦C in air
300 ◦C in Butane
400 ◦C in Butane
450 ◦C in Butane
250
8
10
8
10
(3%)
200
150
100
200
50
100
0
0
0
2
4
6
Bias Voltage(v)
8
10
0
2
4
6
Bias Voltage(V)
Figure.5: I-V characteristics of undoped and doped SnO2 (Pure, 1%, 2%, 3%) thin films
deposited on glass substrates at temperature 450 ◦C.
Figures of I-V characteristics in butane gas have shown increase in slopes of current
than the current in the air. The maximum current in the butane gas at temperature range
above 400 ℃, Butane It requires high temperature to dissociate in to Carbon-Carbon and
Carbon-hydrogen bonds are quite strong due to strong Vander Waals forces. It can be
observed that there is a decrease in the measured current as the temperature is further
raised above 400 ℃ indicating an increase in the film’s resistance.
All these samples exhibited a high resistivity; these curves are clearly non-linear,
which can be explained by the presence of an electron depleted layer at the grain
boundary, and the formation of a potential barrier. The non-linearity of the currentvoltage characteristics in Figures clearly show that the semiconductor-metal interface
forms Schottky barrier First oxygen from the ambient is adsorbed on the exposed surface
of the sensor, extracts electron from the surface states there by increasing the film
resistance. These results in the formation of ionic species such as O2-, O2- and O-. When
the sensor is exposed to a reducing gas like butane, the adsorbed butane reacts with the
adsorbed oxygen ions and releases the trapped electron back to the conduction band, and
thereby lowering the barriers height and resistance [30]. This effect is observed in the
chemisorption region at elevated temperatures [31], the doping affects on the I-V
characteristics are shown that 1% and 2% has a large current than others samples. That
result when the material Doping with trivalent atoms creates even more of the desired
oxygen vacancies or impurities by substitution on a tin lattice site [32]. When the doping
level increased which is comparatively high and leads to overlap of a large number of
energy states. In addition the size of the dopants ions In2O3 is large and overlap of energy
states can occur [33].
The effect of the operation temperature on the thin films sensitivity was studied with
the aim of optimizing the operation temperature to the lowest possible value. The
operating temperature is defined as temperature where the resistance of the sensor
reaches the constant value; the changing of resistance is just only influenced by the
presence of amount of some gases of interest [34].
Figure (6) shows the sensitivity as a function of operating temperature in the range
(300-500 °C) for SnO2 pure and doping with In2O3 which are deposited on glass
substrate. A 5 % Butane: air mixing ratio and bias voltage of 5 Voltage were applied on
the
all
samples.
90
PURE
1%
2%
3%
80
Sensitivity %
70
60
50
40
30
20
10
0
300
340
380
420
460
Operating Temperature
500
◦C
Figure .6: Variation of sensitivity with operating temperature of undoped and doped SnO2
thin films of 5 vol% butane: air mixing ratio and bias voltage of 5 v.
The gas sensitivity began to butane gas for all samples that doping with indium
oxide at 300 °C. The increase in the operating temperature leads to an improvement of the
films sensitivity. It can be seen in Figures that the sensitivity of all these films increases
with the increasing in the operating temperature results from an increase in the rate of
surface reaction with the target gas. The sensitivity factor (S %) at various temperatures
was calculated by equation [35].
𝑆%=(
( 𝑅𝑎𝑖𝑟 –𝑅𝑔𝑎𝑠 )
𝑅𝑎𝑖𝑟
) ∗ 100
(3)
At the optimal temperature, the activation energy may be enough to complete the
chemical reaction [36].The optimal temperature is 470°C for pure SnO2, and it decreased
to 450 °C for Indium Oxide doping SnO2. This blue shift in the optimal temperature
explains the occurrence of chemical reactivity at lower temperature. This decreased in
temperature is agreement with Ansari et al [35]. After 450 °C temperature, the surface
would be unable to oxidize the gas so intensively and the butane gas may burn before
reaching the surface of the film at higher temperature. Thus, the gas sensitivity decreases
with increasing temperature [36]. The higher sensitivity may be attributed to the optimum
number of misfits on the surface, porosity, largest surface area and the larger rate of
oxidation of butane at 450 °C for film.
Figure (7) shows the variation of Current with time of undoped and doped SnO2 as
exposed to 5 % Butane in the air ambient injected into stainless steel chamber and bias
voltage are keeping at 5 Volt, at optimal operating temperature of each sample.
7.5
pure
1%
2%
3%
gas off
6.5
Sensitivity
5.5
4.5
3.5
2.5
1.5
gas on
0.5
0
25
50
75
100
125
150
Time (s)
Figure.7: The variation of Sensitivity with time of undoped and doped SnO2
of 5 vol% butane:air mixing ratio and bias voltage of 5 v.
thin films
The high sensitivity measurement in 1% doping with indium, and decreased when
increased doping ratio due to decrease number of active adsorption sites and reduce
sensor’s current. Undoped and doped SnO2 samples working above 400 ◦C and atomic
oxygen ( 𝑂− ) became a dominant specie on the surface of SnO2 [37]. Where the oxygen
is adsorbed at the surface of the metal oxide that enable an electron trapping, hence the
charge carrier density is reduced which leads to an increase in the resistance of the
undoped and doped SnO2. This reaction can be expressed as follows:
−
𝑂2 (𝑎𝑡𝑚) + 2𝑒 − (𝑐𝑜𝑛𝑑 𝑏𝑎𝑛𝑑) → 2𝑂𝑎𝑑𝑠
(4)
Where O2 is the adsorbed oxygen molecules, O- is the chemisorbed oxygen and e- is the
trapped electrons from the undoped and doped SnO2 surface. 𝑂− Species on the surface,
acts as electron acceptors, and leads to the formation of depletion layer extending to the
particles as well as surface barrier. As the butane is it requires high temperature (~ 350450 ◦C) to dissociate into lower alkanes. Butane gets oxidized to CO2 and H2O.
Ultimately converting the alkanes to carbon dioxide and water as [36]:
−
−
𝐶4 𝐻10 (𝑔𝑎𝑠) + 13𝑂(𝑓𝑖𝑙𝑚
𝑠𝑢𝑟𝑓𝑎𝑐𝑒) → 4𝐶𝑂2 (𝑔𝑎𝑠) + 5𝐻2 𝑂(𝑔𝑎𝑠) + 13𝑒 (𝑐𝑜𝑛𝑑 𝑏𝑎𝑛𝑑) (5)
Each of butane molecular reacts with 13 atomic of atomic oxygen and returned 13
electrons to conduction band.
Figure (8) shows that the sensitivity of the undoped and indium doping SnO2 sensor
is increases when exposed to the butane gas with higher concentration levels from 5% to
15 % ,and this relationship can be empirically represented as [38].
𝛽
𝑆𝑔 = 𝐴𝑃𝑔
(6)
where 𝑃𝑔 is the target gas partial pressure, which is directly proportional to its
concentration, and the response value is characterized by the A and 𝛽
2.5
(Pure)
15%
2.3
5%
5%
2.1
Sensitivity
1.9
Sensitivity
15%
10%
2.1
1.7
1.5
1.3
.
(1%)
10%
1.9
1.7
1.5
1.3
1.1
1.1
0.9
0.9
0.7
0
100
200
0
300
100 200 300 400 500 600 700
Time (s)
TIme (s)
2.3
(2%)
2.1
5%
10%
1.7
1.5
1.1
1.1
0.9
200
Time (s)
300
400
300
400
5%
1.5
1.3
100
(3%)
1.7
1.3
0
15%
10%
1.9
Sensitivity
Sensitivity
1.9
2.1
15%
0.9
0
100
200
Time (s)
Figure .8: variation of Sensitivity with time for different Concentration of Butane: Air
mixing ratio for undoped and doped SnO2 with In2O3 at bias voltage of 5 V
The value of the constant 𝛽 depends on the sensor material, the type of gas the sensor
is exposed to and the operating temperature, which is normally around either (0.5 or 1),
depending on the charge of the surface species and the stoichiometry of the elementary
reactions on the surface [38]. Increases in sensitivity with increases of gas concentration
due to the target gas partial pressure.
Figure (9) and table.1. Show the reduction in the particle size with doping
concentration and increases in sensitivity. The smaller particle size, higher is the surface
to volume ratio, The reduction in the grain size allows the space charge to cover large
volume of the grain and the large number of grain boundaries providing large area for
adsorption 𝑂− ,𝑂−2 . be hence large variation in the barrier and resistance which can
enhance the reactivity at lower temperature. Also, the density of surface states increases
with reduction in the particle size hence, the density of surface states can help in lowering
8
160
7
140
6
120
5
100
4
80
3
60
2
40
sensitivity
Grain size (nm)
1
Grain Size (nm)
Sensitivity
the operating temperature.
20
0
0
0
1
2
3
Indium oxide doping ratio %
4
Figure .9: variation of Grain Size and Sensitivity with Indium Oxide doping ratio for 5 %
Butane: air mixing ratio & bias voltage of 5 V.
The other possibility may be the Fermi level pinning, which may reduce with doping.
The Fermi level pinning, can help in lowering the temperature and also can lead to large
variation in surface barrier and, hence the resistance. This may also be the reason for the
higher sensitivity with small size particles. When increase in the doping level (2%, 3%)
of SnO2 with indium should decrease number of active adsorption sites and reduce
sensor’s sensitivity [39].Table.1 show the exhibits a fast response speed (4 s ) and
recovery time (72 s) for doping with 1% doping concentration In2O3 that revealed that
when small quantity of impurities is the best doping ratio to achieve fast response sensor.
The quick response sensor for butane gas may be due to faster oxidation of gas [36].
Table.1: Physical parameters of spray pyrolytically grown undoped and doped SnO2 thin
films on glass substrate. 5 % Butane: air mixing ratio and bias voltage of 5 V.
Doping
ratio
Thickness
(nm)
Roughness
Average
(nm)
Eg
(eV)
281
Average
Crystallite
sizes from
AFM (nm)
150.84
SnO2 Pure
SnO2:In2O3
(1 wt.%)
SnO2:In2O3
(2 wt.%)
SnO2:In2O3
(3 wt.%)
Sensitiv Response
ity %
Time(s)
Recovery
Time(s)
2.59
3.6
82.5
14
58
388
112.37
1.85
3.1
85.3
4
72
228
121.26
1.21
3.95
79.16
8
59
350
125.48
1.63
3.56
55.4
15
115
4. Conclusions
We have successfully prepared In2O3 – doped SnO2 with different concentration by
spray Pyrolysis method, the average Crystallite sizes and Roughness was decrease with
doping by indium oxide. This reduction in particle size might also be responsible for
decreases of the optimal operating temperature to 450 ℃. Enhancement sensitivity occurs
at low level doping concentration (1%) due to the smaller particle size; higher is the
surface to volume ratio. Fast response time (4 s) occurs also, and quick recovery time is
(72 s).
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