Materials Transactions, Vol. 46, No. 8 (2005) pp. 1942 to 1949
#2005 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Enhancing the Sensitivity of Oxygen Sensors through the Photocatalytic
Effect of SnO2 /TiO2 Film
Hsiao-Ching Lee and Weng-Sing Hwang
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
This paper investigates the effect of photocatalysis on the sensitivity of oxygen sensors constructed with SnO2 /TiO2 thin films. An R.F.
magnetron sputtering system is employed to fabricate SnO2 /TiO2 double-layer films. The thin films are deposited with SnO2 /TiO2 thickness
ratios of 250/50, 200/100, 150/150, 100/200, and 50/250 nm, respectively. During deposition, the Ar:O2 flow rate is fixed at 4:1. To stabilize
the material properties, the films are annealed for four hours at a temperature of either 550 or 650 C. The increase in sensitivity of the SnO2 /
TiO2 thin films when irradiated by UV light with a wavelength of 365 nm is investigated. The results indicate that the annealed samples have
higher oxygen sensitivities than the as-deposited samples. The sensitivity of the non-annealed samples increases from 0.70 to 1.15 under UV
irradiation, while the sensitivity of the annealed samples increases from 7.17 to 10.60. Therefore, it is clear that UV irradiation causes the
sensitivity of the SnO2 /TiO2 thin films to increase significantly. Finally, it is found that the oxygen sensitivity of the SnO2 /TiO2 thin films
increases as the SnO2 /TiO2 ratio is reduced.
(Received April 14, 2005; Accepted July 11, 2005; Published August 15, 2005)
Keywords: photocatalytic effect, oxygen sensors, SnO2 , TiO2 , sensitivity
1.
Introduction
Photocatalysis is a catalytic reaction prompted by incident
photons. During the reaction, the photon energy is absorbed
and a high-energy state is formed. This energy is transmitted
to a reactant, where it induces a chemical reaction. In most
reaction processes, it is difficult to stop the chemical reaction
before it reaches its conclusion. However, in photocatalysis,
the photocatalytic reaction ceases when the irradiating light
source is removed.
It has been reported that metallic ions and metal complexes
are viable photocatalytic materials. Semiconductors such as
GaP, CdS, GaAs, TiO2 , ZnO, SrTiO3 , WO3 and SnO2 are
widely used in sensor applications. Semiconductors are
generally insulators and conduction occurs only when its
electrons are excited (thermally, optically, etc.) into higher
unfilled bands. A number of the electrons in the semiconductor (generally a very small, but not negligible number)
can be optically excited from the valence band to the
conduction band by an irradiating UV light with an
appropriate wavelength.
Sensor applications have attracted growing interest over
recent decades. Sensors are now widely used to automate and
control a variety of industrial processes. In addition to
industrial applications, sensors are now playing an increasingly important role in pollution control, biomedical applications, and safety routines, etc. Semiconductor oxides such
as TiO2 , ZnO and SnO2 are suitable materials for the
detection of various gases, including CO, SO2 , NOx , NH3 ,
CH4 , and C2 H5 OH.1–4) Oxygen sensors find extensive
application in a wide variety of fields where the analysis
and control of oxygen is required. For example, oxygen
sensors are used in mines and in hospital incubators to detect
the presence of oxygen, in large-scale furnaces for combustion control, in various biomedical applications, and as
analytical instruments in various research and development
fields.
TiO2 has three stable phases at different temperatures,
namely anatase (tetragonal structure), brookite (orthorhombic structure), and rutile (tetragonal structure). The anatase
phase of TiO2 has a higher sensitivity than the two other
phases because of its relatively greater surface activity. TiO2
films are commonly used in photocatalytic and gas sensor
fields due to their durability, sensitivity to gases, chemical
stability, and low cost.5) SnO2 is also an excellent material for
gas sensor applications. SnO2 has only one stable phase,
namely rutile (a tetragonal structure). This results in high
electrical conductivity, high transparency in the visible
region, and high thermal, mechanical and chemical stabilities. Accordingly, SnO2 has attracted intensive research
efforts since 1964.6,7) Although SnO2 is a photocatalytic
material, its applications in the photocatalytic field are rarely
discussed in the literature.
SnO2 /TiO2 double-layer films are easily fabricated using a
number of different techniques, including sol-gel methods
and sputtering processes. However, these double-layer films
have only seldom been employed for gas sensor applications.5,8–10)
This study employs the R.F. sputtering technique to
prepare various SnO2 /TiO2 double-layer thin films. The
aim of this study is to investigate the enhancement of the
oxygen sensitivity of these double-layer films by photocatalytic effects under different oxygen concentrations.
2.
Method
2.1 Fabrication of thin films
Using an R.F. sputtering system, SnO2 /TiO2 thin films
were fabricated on wafers which had been coated previously
with SiO2 in a thermal oxidation process. The R.F. sputtering
process was performed using a reaction gas comprising a
mixture of argon and oxygen gases. The total flow rate of the
reaction gas was fixed at 50 sccm. The Ar:O2 flow ratio was
maintained at a constant 4:1 ratio, and the total gas pressure
was approximately 0.3724 Pa. Sputtering was performed
using TiO2 and SnO2 targets, with R.F. powers of 300 Watts
Enhancing the Sensitivity of Oxygen Sensors through the Photocatalytic Effect of SnO2 /TiO2 Film
Table 1
Material
TiO2
SnO2
1943
SnO2 and TiO2 material properties.
Phase
Crystal Temp.
Band gap
Anatase
T < 800 C
Rutile
T > 800 C
3.20 eV
3.00 eV
Rutile
Only stable phase
3.60 eV
(a)
Ag electrode
SnO 2 thin film
TiO 2 thin film
SiO 2
Si
Fig. 2
Schematic illustration of sensitivity measurement system.
different samples were prepared, namely an as-deposited
sample, a sample annealed at 550 C for four hours, and a
sample annealed at 650 C for four hours.
(b)
Ag electrode
SnO 2 / TiO 2 film
3mm
SiO2
1mm
Fig. 1 Structure of SnO2 /TiO2 thin-film gas sensor. (a) Cross-section, and
(b) Top view.
Table 2 Thickness of SnO2 /TiO2 double-layer films.
Thickness of TiO2
Sample
Thickness of SnO2
A
250 nm
50 nm
B
200 nm
100 nm
C
150 nm
150 nm
D
100 nm
200 nm
E
50 nm
250 nm
and 150 Watts, respectively. The material properties of SnO2
and TiO2 are shown in Table 1. In fabricating the doublelayer, the TiO2 film was sputtered on the SiO2 isolation layer,
and the SnO2 film was then deposited over the TiO2 film.
Figure 1 illustrates the structure of a representative SnO2 /
TiO2 thin film gas sensor. It can be seen that the sensor
incorporates Ag electrodes to facilitate measurement of the
sensor’s electrical resistance. The active area of the sensor is
approximately 3 mm2 and the total thickness of the doublelayer film is 300 nm. Five different sensors (A–E) were
constructed in this study. As shown in Table 2, the SnO2 /
TiO2 thickness ratios of Samples A–E were 250 nm/50 nm,
200 nm/100 nm, 150 nm/150 nm, 100 nm/200 nm, and
50 nm/250 nm, respectively. For each sample type, three
2.2 Material analysis and resistance detection
The crystalline structure of each sample was examined by
the Grazing Incident X-Ray Diffraction technique (GID,
Rigaku D/MAX2500) using an X-ray incident angle of 1 , a
scanning speed of 1 /min, and a value of 2 in the range 20
to 70 . The surface morphology of the various samples was
observed through a Field Emission Scanning Electron
Microscope (FESEM, Philips XL40). The elemental composition of each sample was determined by an EDS microscope
(FESEM, Philips XL40).
The sensitivity of each sample was measured using the
apparatus shown in Fig. 2. The sensor was placed on a
hotplate, whose temperature was maintained at a constant
300 C. During the gas sensing tests, the variation in the
electrical resistance of the sensor was measured by probes
connected at one end to the electrodes shown in Fig. 1 and at
the other to an HP-34401A multimeter.
There are many ways in which sensitivity can be defined
and measured.11–15) Figure 3 illustrates the variation in the
sensor resistance over the course of a typical analysis cycle.
Initially, the chamber was vacuumed down to a pressure of
0.665 Pa. Subsequently, pure air (N2 :O2 = 4:1) with a
volume, V, was introduced. The gas entrance valve was then
closed. Once the measured resistance attained a steady state,
the pure air was pumped out and replaced with pure oxygen
gas with the same volume, V. When the resistance reached a
steady state, the oxygen gas was pumped out and the UV light
was turned on. Pure oxygen gas with a volume, V, was then
introduced into the chamber. Since the volumes of the pure
air and the oxygen gas introduced into the chamber were
identical, the oxygen partial pressure in the pure oxygen gas
was higher than the oxygen partial pressure in the pure air. As
shown in Fig. 3, the resistance under pure oxygen gas (Rg ) is
higher than the resistance under pure air (Ra ).
The data acquisition rate for the resistance measurement
was one datum per second. The sensitivity of the film to
oxygen was calculated from the measured change in
resistance.
1944
H.-C. Lee and W.-S. Hwang
a b
c
d
e
f
(a)
g
Resistance, R /Ω
600000
400000
pure
air
200000 in
oxygen
pure in
air
out
oxygen
in under
UV radiation
oxygen
out
oxygen
out under
UV radiation
0
0
2000
4000
6000
8000
10000
12000
14000
16000
Time, t /s
Fig. 3 Typical dynamic response of resistance during testing cycle. (a) In
vacuum. (b) SnO2 /TiO2 thin film exposed to pure air (N2 :O2 = 4:1).
Oxygen molecules are adsorbed and receive electrons creating depletion
layer (O ) on surface of SnO2 /TiO2 thin film. Depletion layer increases
resistance, Ra , of SnO2 /TiO2 thin film. (c) In vacuum. (d) SnO2 /TiO2 thin
film exposed to pure O2 gas (O2 ). More oxygen molecules are adsorbed
and electrons are consumed causing a higher resistance (Rg ) of SnO2 /TiO2
thin film. (e) In vacuum. (f) SnO2 /TiO2 thin film in oxygen environment
under irradiation by UV light. Electrons are excited by adsorbing energy of
photons. Hence, electron density on surface of film is enhanced, leading to
higher resistance (Rg ). (g) In vacuum.
(b)
*
*
Intensity (a.u.)
*
*
C
*
(c)
B
A
20°
30°
40°
50°
60°
70°
2θ
Fig. 4 GID patterns of as-deposited and annealed SnO2 /TiO2 thin films.
A: As-deposited film, B: Film annealed at 550 C, and C: Film annealed at
650 C. indicates rutile SnO2 , remaining peaks correspond to anatase
TiO2
The sensitivity of the sensor in an oxygen environment was
calculated as:
S¼
Rg
Ra
ð1Þ
where Ra is the sensor resistance under pure air and Rg is the
sensor resistance in an oxygen environment.
In order to assess the influence of UV irradiation on the
SnO2 /TiO2 film resistance, the sensitivity under UV irradiation is defined in this study as:
S¼
Rg
Ra
ð2Þ
where Ra is the electrical resistance under pure air and Rg is
that under an oxygen environment with UV irradiation.
Fig. 5 FE-SEM images of SnO2 /TiO2 thin films. (a) as-deposited SnO2 /
TiO2 film, (b) SnO2 /TiO2 film annealed at 550 C for 4 h, (c) SnO2 /TiO2
film annealed at 650 C for 4 h.
3.
Experimental Results and Discussion
Figure 4 shows the GID (Grazing Incident X-Ray Diffraction) patterns of SnO2 /TiO2 thin films with and without
heat treatment. Besides the rutile SnO2 diffraction peaks,
anatase TiO2 diffraction peaks are also evident. The results
indicate that anatase TiO2 and rutile SnO2 are present in both
the as-deposited films and the annealed SnO2 /TiO2 thin
Enhancing the Sensitivity of Oxygen Sensors through the Photocatalytic Effect of SnO2 /TiO2 Film
68
(a)
as-deposited film
Film with heat treatment at 550°C
Film with heat treatment at 650°C
66
64
62
e-
Φs
Ec
χg
60
Grain size /nm
1945
Ef
58
56
54
Ev
52
50
Energy
Barrier
n-type semiconductor
48
46
(b)
44
42
-
O
O
40
38
A
B
C
D
E
Φs
χg
Ec
Sample
Ef
Fig. 6 Relationship between thickness ratio and grain size for as-deposited
and annealed SnO2 /TiO2 thin films.
Energy
O2
Barrier
Ev
Oxidizing gas
n-type semiconductor
Si
O2 + e −
1 / 2O2 + e
O2−
−
CPS
1 / 2O2 + 2e
Energy, E /keV
Fig. 7 EDS spectrum of SnO2 /TiO2 thin films.
films. It is observed that the SnO2 and TiO2 peaks become
sharper following heat treatment, particularly in the TiO2
layer.
Figure 5 presents Field Emission Scanning Electron
Microscope (FESEM, Philips XL40) images of the asdeposited and annealed films. The surface morphology of
each film is reasonably flat. A slight increase in the grain size
is evident in the annealed samples. The grain size growth
caused by annealing is also shown in Fig. 6, which indicates
the average grain size of the as-deposited and annealed films
for each of the five samples. The grain sizes of the asdeposited SnO2 /TiO2 samples are estimated to be approximately 40 to 50 nm. A slight increase in the grain size is
observed as the SnO2 /TiO2 thickness ratio is reduced.
Following heat treatment at 550 C for four hours, the grain
size increases to approximately 40 to 60 nm. Similarly,
annealing at a temperature of 650 C causes the grain size to
increase to approximately 45 to 65 nm. The EDS spectrum
presented in Fig. 7 shows that titanium, tin and oxygen are all
present in the SnO2 /TiO2 films.
Two gas adsorption mechanisms exist, depletion adsorption and accumulation adsorption.16) In this study, depletion
adsorption occurs when the n-type semiconductor films (i.e.
the SnO2 /TiO2 coupled thin films) are exposed to oxidizing
gases (O2 ). When the electron affinity of oxygen is higher
O−
−
O 2−
Fig. 8 Illustration of depletion adsorption in n-type semiconductors. (a)
Energy band of n-type semiconductors before contact with oxidizing
gases. (b) Energy band of n-type semiconductors after contact with
oxidizing gases. When SnO2 /TiO2 thin films is exposed to oxygen,
oxygen molecules are adsorbed and receive electrons. Hence, depletion
layer (O ) is created on surface of SnO2 /TiO2 thin films. Depletion layer
increases resistance of film. Note: g is electron affinity of oxidizing gases,
s is work function of semiconductor, Ec is conduction band, Ev is valence
band, and Ef is Fermi energy band gap.
than the work function of the SnO2 /TiO2 coupled thin film,
oxygen molecules are captured by the surface electrons and
become adsorbed oxygen (O ). Different types of adsorbed
oxygen (O2 , O and O2 ) exist at different temperatures. In
general, the operational temperature of semiconductor gas
sensors is approximately 300 to 500 C. In this temperature
range, the most probable type of oxygen to be adsorbed on
the sensor surface is O . The adsorbed oxygen creates a
depletion layer on the surface of the film which continues to
grow until the Fermi energy bands of the oxygen reach the
same level as those of the SnO2 /TiO2 coupled thin film. The
depletion layer increases the energy barrier and hence
increases the electrical resistance of the films, as shown in
Fig. 8.16) It has been reported that the number of oxygen
adsorptions, and hence the resistance of the material,
increases as the number of electrons on the surface
increases.17) From Eq. (1), it can be seen that a higher
resistance increases the sensitivity of the sensor.
The incident irradiating light must be of an appropriate
wavelength if it is to excite electrons from the valence band
(VB) to the conduction band (CB). The required wavelength
can be calculated from:18,19)
hc
ð3Þ
hc 4:136 1015 3 1017
1240
¼
¼
: ð4Þ
¼
E
E(eV)
E
E ¼ h ¼
(nm)
1946
H.-C. Lee and W.-S. Hwang
(a)
OO-
(c)
Φ TiO2
Φ SnO2
EC
O-OOO
Φ TiO2
Φ
SnO2
EC
Ef
Ef
3.2eV
3.6eV
3.6eV
SnO 2
EV
UV light
EV
TiO2
SnO2
Surface
(b)
OOO-
Surface
Φ SnO2
Φ TiO2
EC
Ef
3.6eV
SnO 2
3.2eV
EV
TiO2
Surface
where E is the energy of the band gap (eV), h is the Planck
constant, is the frequency of the photon, c is the speed of
light in a vacuum (nm/s), and is the wavelength of the
incident light (nm).
The energy band gap of TiO2 (anatase phase) is 3.2 eV. As
a consequence, the electrons are excited from the valence
band to the conduction band when the wavelength of the
irradiating UV light is less than 380 nm. In this study, the
electrons of the SnO2 /TiO2 thin film are excited by UV light
with a wavelength of 365 nm.
A number of recent studies have coupled TiO2 with
various metal oxides in an attempt to enhance the TiO2
photocatalytic reaction. The coupling of TiO2 with SnO2 has
attracted particular attention. The results have shown that
SnO2 /TiO2 coupled films have high photocatalytic efficiency. It has also been reported that the photo-generated carriers
in the TiO2 film play a key role in photocatalytic reactions.9,20) Figures 9(a) and (b) show the electron transfer
paths before and after the TiO2 and SnO2 films are combined.
The quantity of electrons on the surface is increased by the
transfer of electrons from TiO2 to SnO2 . Two mechanisms
are involved in this transfer process. First, the conduction
band (CB) edge and the valence band (VB) of SnO2 are much
lower than their counterparts in TiO2 . This facilitates
Fig. 9 Illustration of electron transfer, where TiO2 is work function of
TiO2 and SnO2 is work function of SnO2 . (a) Energy band before SnO2
and TiO2 combined. (b) Energy band after SnO2 and TiO2 combined.
Conductivity is enhanced by electron transfer from TiO2 to SnO2 . Hence
more oxygen molecules are adsorbed (O ) on surface and resistance
increases. (c) Energy band under UV light irradiation. Electrons are
excited from valence band to conduction band. When SnO2 /TiO2 doublelayer film is excited by photons, more electrons transfer from TiO2 to
SnO2 . Hence more oxygen adsorptions occur and resistance increases.
interfacial electron transfer from TiO2 to SnO2 , while
simultaneously suppressing the transfer of electrons to
SnO2 .8) A heterojunction is formed at the interface between
the TiO2 and SnO2 films because electrons are transferred
from TiO2 to SnO2 as a result of the higher work function of
SnO2 and the higher electron affinity of TiO2 (4.33 eV).21–24)
The transfer of electrons continues until the Fermi energy
bands in the two films reach equivalent levels. The second
transfer mechanism arises because semiconductors such as
TiO2 and SnO2 can be excited by photons of suitable energy
to produce photo-generated electron/hole pairs:25)
TiO2 þ h ! TiO2 þ hþ þ e
ð5Þ
SnO2 þ h ! SnO2 þ hþ þ e :
ð6Þ
and
The fact that the SnO2 film shows a much lower photocatalytic activity than the TiO2 film is probably due to the
rapid recombination of the photo-generated electron/hole
pairs in the SnO2 film.26,27) The charge separation in the
coupled system can be improved by provoking a rapid
electron transfer process in the coupled layers by using two
semiconductors with different energy levels and conduction
band edges.28,29) In this study, TiO2 and SnO2 are both n-type
Enhancing the Sensitivity of Oxygen Sensors through the Photocatalytic Effect of SnO2 /TiO2 Film
(d)
20
as-deposited film
annealed film (550°C)
annealed film (650°C)
as-deposited film under UV light irradiation
annealed film under UV light irradiation (550°C)
annealed film under UV light irradiation (650°C)
18
16
Sensitivity (Rg/Ra)
14
12
20
as-deposited film
annealed film (550°C)
annealed film (650°C)
as-deposited film under UV light irradiation
annealed film under UV light irradiation (550°C)
annealed film under UV light irradiation (650°C)
18
16
Sensitivity (Rg/Ra)
(a)
10
8
6
14
12
10
8
6
4
4
2
2
0
500
1000
1500
2000
0
500
(e)
20
as-deposited film
annealed film (550°C)
annealed film (650°C)
as-deposited film under UV light irradiation
annealed film under UV light irradiation (550°C)
annealed film under UV light irradiation (650°C)
Sensitivity (Rg/Ra)
16
14
10
8
10
8
6
4
2
2
1000
1500
2000
0
500
1000
1500
2000
Oxygen Concentration (ppm)
20
as-deposited film
annealed film (550°C)
annealed film (650°C)
as-deposited film under UV light irradiation
annealed film under UV light irradiation (550°C)
annealed film under UV light irradiation (650°C)
18
16
Sensitivity (Rg/Ra)
12
4
Oxygen Concentration (ppm)
(c)
14
6
500
2000
as-deposited film
annealed film (550°C)
annealed film (650°C)
as-deposited film under UV light irradiation
annealed film under UV light irradiation (550°C)
annealed film under UV light irradiation (650°C)
16
12
0
1500
20
18
Sensitivity (Rg/Ra)
18
1000
Oxygen Concentration (ppm)
Oxygen Concentration (ppm)
(b)
1947
14
12
10
8
6
4
2
0
500
1000
1500
2000
Oxygen Concentration (ppm)
semiconductors with band gap energies greater than 3.0 eV,
and both strongly absorb UV light. Electrons transfer to the
SnO2 layer, while holes diffuse into the TiO2 layer. In other
words, the rather high photocatalytic activity of the SnO2 /
TiO2 double-layer film can be attributed to an enhanced
charge separation caused by the rapid transfer of electrons
from TiO2 to SnO2 .30) When the SnO2 /TiO2 double-layer
film is excited by photons, a greater number of electrons
transfer from the TiO2 film to the SnO2 film. This creates
more oxygen adsorptions and increases the resistance of the
material, as shown in Fig. 9(c).17,31) As stated previously, the
increase in resistance increases the value of the sensor’s
sensitivity. The increase in sensitivity under UV irradiation
Fig. 10 Gas sensitivity in different oxygen concentrations with and
without UV light irradiation of films deposited with different TiO2 /SnO2
thickness ratios: (a) Sample A, (b) Sample B, (c) Sample C, (d) Sample D
and (e) Sample E.
can also be observed in Fig. 10.
Figure 10 illustrates the sensitivity of the as-deposited and
annealed SnO2 /TiO2 films in different oxygen concentrations under irradiated and non-irradiated conditions. Figure 10(a) shows the sensitivity of the coupled films with a
SnO2 /TiO2 ratio of 250:50. It is clear that there is no
significant change in the sensitivity of the as-deposited or
annealed films under UV irradiation. The sensitivity of the
as-deposited SnO2 /TiO2 coupled films ranges from 0.94–
0.99, and improves only marginally to 0.99–1.00 when the
surface is irradiated by UV light. The sensitivity of the
annealed coupled films lies in the range 0.96–1.58, and
improves slightly to 1.17–2.22 when irradiated by UV light.
H.-C. Lee and W.-S. Hwang
20
18
Sample A
Sample B
Sample C
Sample D
Sample E
Sample A annealed at 550°C
Sample B annealed at 550°C
Sample C annealed at 550°C
Sample D annealed at 550°C
Sample E annealed at 550°C
Sample A annealed at 650°C
Sample B annealed at 650°C
Sample C annealed at 650°C
Sample D annealed at 650°C
Sample E annealed at 650°C
Sensitivity (Rg/Ra)
16
14
12
10
8
6
4
2
0
0
500
1000
1500
2000
Oxygen Concentration (ppm)
Fig. 11 Gas sensitivity in different oxygen concentrations with UV light
irradiation of films deposited with different TiO2 /SnO2 thickness ratios.
Sample A
Sample B
Sample C
Sample D
Sample E
8000000
7000000
6000000
Resistance, R/Ω
Figure 10(b) shows the results obtained for the coupled films
with a SnO2 /TiO2 ratio of 200:100. In this case, there is a
slight increase in the sensitivity of the as-deposited and
annealed SnO2 /TiO2 coupled films under UV irradiation.
The sensitivity of the as-deposited SnO2 /TiO2 films ranges
from 1.03–1.38, and increases slightly to 1.21–1.44 when the
surface is irradiated by UV light. The sensitivity of the
annealed coupled films varies from 1.05–2.63, and improves
to 2.45–4.42 when irradiated by UV light. Figure 10(c)
illustrates the sensitivity of the coupled films with a SnO2 /
TiO2 ratio of 150:150. Again, it can be seen that there is a
gradual increase in the sensitivity of the as-deposited and
annealed SnO2 /TiO2 coupled films when the surface is
irradiated by UV light. The sensitivity of the as-deposited
SnO2 /TiO2 films varies from 0.70–1.35, and improves to
1.15–1.48 under UV irradiation. The sensitivity of the
annealed SnO2 /TiO2 coupled films lies in the range 1.64–
3.62, and increases to 1.79–5.96 when irradiated by UV light.
Figure 10(d) shows the sensitivity of the SnO2 /TiO2 coupled
films with a SnO2 /TiO2 ratio of 100:200. The sensitivity of
the as-deposited SnO2 /TiO2 coupled films ranges from 0.69–
5.7, and improves considerably to 0.72–8.23 when the
surface is irradiated by UV light. The sensitivity of the
annealed SnO2 /TiO2 coupled films varies from 1.59–10.21,
and improves to 1.82–12.68 under UV light irradiation.
Figure 10(e) shows the results obtained for the SnO2 /TiO2
coupled films with a SnO2 /TiO2 ratio of 50:250. A
significant increase in the sensitivity of the as-deposited
and annealed SnO2 /TiO2 coupled films is observed when the
surface is irradiated by UV light. The sensitivity of the asdeposited SnO2 /TiO2 coupled films ranges from 1.1–1.27,
and improves to 1.31–1.51 under UV irradiation. The
sensitivity of the annealed SnO2 /TiO2 coupled films varies
from 2.13–8.52 and improves to 2.78–10.59 when irradiated
by UV light. The results of Fig. 10 indicate that the
sensitivity of the as-deposited SnO2 /TiO2 coupled films
increases only marginally when the surface is irradiated by
UV light. However, the sensitivity of the annealed SnO2 /
TiO2 coupled films increases significantly under UV irradiation, particularly for Samples C to E. In general, Fig. 10
indicates that the sensitivity increases as the SnO2 /TiO2
thickness ratio decreases. Furthermore, it is observed that the
annealed films have a higher oxygen sensitivity than their asdeposited counterparts.
Figure 11 shows the relationship between the thickness
ratio of the SnO2 /TiO2 films and their sensitivity in different
oxygen concentrations when irradiated by UV light. It can be
seen that the annealed samples have a higher sensitivity than
the as-deposited samples. Surface oxygen vacancies are
known to act as n-type donors.32) When the surface oxygen
vacancy concentration increases, the surface oxygen vacancies introduce donor levels in the gap and free electrons are
produced by the thermal treatment. Hence, the conductivity is
enhanced by the increased surface oxygen vacancy concentration.33,34) As shown in Fig. 12, the heat treatment process
lowers the resistance of the annealed samples significantly.
Since the sensitivity of the SnO2 /TiO2 films in an oxygen
environment is dependent upon the change in resistance of
the film (i.e. rather than upon the absolute value of the
resistance), it is apparent that the annealing process is
5000000
4000000
3000000
annealed films
as-deposited films
2000000
1000000
0
0
50
100
150
200
250
300
350
Time, t/s
Fig. 12 Resistance of as-deposited and annealed SnO2 /TiO2 thin films in
pure air. A high annealing temperature creates more surface oxygen
vacancies and hence reduces the resistance of the annealed SnO2 /TiO2
thin films, thereby enhancing their sensitivity.
8
∆S (S Under Light-S Without Light)
1948
as-deposited SnO2 / TiO2 films
annealed at 550°C
annealed at 650°C
7
6
5
4
3
2
1
0
A
B
C
D
E
Sample
Fig. 13 Graph of sensitivity difference (SUnder Light –SWithout Light ) for different thickness ratios of as-deposited and annealed SnO2 /TiO2 thin films.
instrumental in enhancing the sensitivity of the double-layer
films.
Figure 13 presents the difference in sensitivity (S) before
and after irradiation by UV light in 2000 ppm oxygen gas for
each of the as-deposited and annealed samples. The results
show that S increases with decreasing SnO2 /TiO2 thick-
Enhancing the Sensitivity of Oxygen Sensors through the Photocatalytic Effect of SnO2 /TiO2 Film
ness ratio, irrespective of whether or not heat treatment is
performed. However, it can be seen that S increases more
noticeably when an annealing treatment is performed.
Finally, the elevated annealing temperature has a particularly
significant effect on the sensitivity of the sample with the
lowest SnO2 /TiO2 thickness ratio.
4.
Conclusion
The present study has confirmed that the photocatalytic
effect significantly improves the surface electron density of
SnO2 /TiO2 films. Consequently, the photocatalytic effect
provides an effective enhancement of the sensitivity of the
current gas sensors in oxygen environments. It has been
shown that the sensitivity of the gas sensors increases with
decreasing SnO2 /TiO2 thickness ratio. Surface oxygen
vacancies are known to be n-type donors and the quantity
of surface oxygen vacancies is enhanced by heat treatment.
Hence, a greater number of surface electrons causes a higher
variation of the sensor resistance and therefore improves the
sensitivity of the films.
Acknowledgements
This work has been supported by the National Science
Council in Taiwan (NSC 92-2216-E-006-041), for which the
authors are grateful.
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