CHAPTER 3
H2S AND Cl2 SENSING PROPERTIES OF Cr2O3 THIN
FILMS*
This chapter deals with preparation, characterization, H2S and Cl2 sensing
characteristics of Cr2O3 thin films. The films were found to exhibit p-type response
characteristics with a temperature dependent selectivity for H2S and Cl2 at
operating temperatures of 100 and 220°C respectively. The temperature dependent
selectivity provides flexibility to use the same film for sensing Cl2 as well as H2S. The
chapter ends by proposing a sensing mechanism on basis of different chemisorbed
oxygen species i.e. O2- and O- present at the surface of films due to temperature
and/or interaction with H2S/Cl2.
*
Part of the work described in this chapter has been published in “Sensors & Actuators B 157
(2011) 466-472”
65
3.1
INTRODUCTION
Cr2O3 is the most stable oxide among the various oxides of Chromium such as CrO2,
CrO3 etc. existing in a wide range of temperatures and pressure (Kubota, 1961). The
material properties of Cr2O3 are summarized in Table 3.1. Cr2O3 exhibits many attractive
tribological properties, such as hardness (29.5 GPa) and melting point (~ 2300°C)
chemical inertness, low friction coefficient, high wear resistance, and high temperature
oxidation resistance (Celik et al., 2003; Zhang et al., 1998). The most striking property of
Cr2O3 is the spontaneous nonreciprocal reflection of light which was first observed in
1993 on α-Cr2O3 (Krichevtsov et al., 1993). It also shows a high solar absorption
coefficient and low thermal emissivity (Maruyama and Akagi, 1996). It is one of the
principal oxides of chromium and is used as a pigment owing to its considerable stability.
Regarding magnetic behavior, Cr2O3 is antiferromagnetic with a Néel temperature of
307 K (Yu et al., 2003); however, the antiferromagnetic character can be changed to
weak ferromagnetic (Makhlouf, 2004) and even super-paramagnetic (Balachandran et al.,
1995) when chromia nanoparticles are considered. On the other hand, despite its intrinsic
insulating nature, Cr2O3 films can present either p or n-type semiconductor behavior,
depending on the growth conditions (Kofstad and Lillerud, 1980). It occurs in nature as a
rare mineral, Eskolaite which was discovered in late fifties (Kouvo and Vuorelainen,
1958) and named after a Finnish scientist P. Eskola. It crystallizes in trigonal-hexagonal
-
scalenohedral crystal system, space group R 3c and is iso-structural with α-Al2O3
(corundum), α-Fe2O3 (hematite) and V2O3 (karelianite). The lattice adopts corundum
structure, consisting of a hexagonal close packed (HCP) array of oxygen anions, in which
four out of every six available octahedral sites are occupied by Cr. The octahedral and
tetrahedral sites are located directly above one another in the HCP lattice and the
tetrahedral sites are empty. The octahedra share faces along a 3-fold axis and are
distorted to trigonal antiprisms because of the Cr−Cr repulsion across the shared face.
The crystal structure of Cr2O3 is depicted in fig. 3.1. This atomic arrangement gives Cr3+
ions the centro-symmetric D3d point group symmetry and leads to a highly dense
structure offering high polarizability, high refractive index, and intense color. With their
3d3 electronic configuration, chromium ions experience strong crystal field stabilization
energy. The atomic structure and electronic configuration confer to this sesquioxide a
66
Table 3.1
Material Properties of Cr2O3
Molecular formula
Cr2O3
Appearance
light to dark green
Density
5.22 g/cm3
Melting point
2435 °C
Refractive index
2.551
Crystal structure
Hexagonal
Cr
O
Fig. 3.1 Cr2O3 molecule and its crystal structure along c-axis
67
unique combination of electronic, optical, and magnetic properties. Its electronic
structure is of particular interest because of the simultaneous characteristics of a chargetransfer insulator (band gap 4.7-5 eV) and a Mott−Hubbard regime due to the location of
Cr ions in the middle of the first-row transition series (Catti et al., 1996). The
convergence of mechanical, physical and chemical properties in a single material makes
Cr2O3 a key material for the development of a broad range of industrial applications.
It has been reported that obtaining stoichiometric Cr2O3 thin films is generally
difficult as there are several stable oxide phases of chromium i.e. CrO, Cr2O, CrO2,
Cr2O3, Cr3O4 etc. (Abu-Shagir et al., 2010). Therefore, in order to deposit thin films of
Cr2O3, presence of oxygen during deposition and annealing plays a dominant role. Cr2O3
thin films have been widely used as protective coatings against wear, corrosion and
oxidation. In recent years, p-type Cr2O3 is gaining importance for sensor applications as it
has an energy band gap of ~ 3.4 eV, resistant to chemical attack and is widely being used
in a variety of applications such as catalytic reactions, optical coating and infrared
sensors (Morrison, 1977; El-Molla, 2005; Cantalini, 2004; Cellard et al., 2009). For gas
sensing applications, there have been some reports based on the Cr2O3 thick films for
vapor sensing (e.g. ethanol) (Cantalini, 2004; Patil et al., 2007; Yoo and Wachsman,
2007; Suryawanshi et al., 2008; Patil and Patil, 2009). However, chemiresistive sensing
characteristics of Cr2O3 thin film for oxidizing/reducing toxic gases have not been
investigated. In this chapter, we investigate in detail the gas sensing properties of Cr2O3
thin films prepared by thermal oxidation of Cr films prepared by electron-beam
deposition (EBD).
3.2
EXPERIMENTAL
Cr2O3 thin films (thickness ~ 100 nm) were prepared in the following two steps.
In the first step, Cr films were prepared on pre-cleaned fused silica substrate using EBD
under a base vacuum of ~1.3x10-4 Pa and substrate temperature of 300°C. High purity
(99.99%) Cr chunks were used as the source material for deposition. In the second step,
deposited Cr films were transferred to a tubular furnace for oxidation of Cr films. It was
found that annealing of Cr films at 700°C for 2 h under oxygen flow (flow rate: 50 sccm)
leads to the formation of -Cr2O3 phase. The oxidation temperature and duration was
selected from the literature (Dong et al., 2002)
68
3.3
3.3.1
RESULTS AND DISCUSSION
Structural and Morphological Characterization
Fig. 3.2 shows a photograph of an oxidized film with a typical green color,
indicating the formation of Cr2O3 (Sousa et al., 2005). Fig. 3.3 (a) shows typical surface
morphology of grown Cr2O3 thin film imaged using AFM. The film morphology consists
of elongated grains with a width in the range of 0.5-2 µm and length varying between 2 and 8
µm. Analysis of the height profile (fig. 3.3 (b)) of AFM image shows that the average surface
roughness of the film is ~ 100 nm, indicating a rather rough surface of grown films. The
diffraction peaks of XRD pattern recorded for the film is shown in fig 3.4 could be indexed to
α-Cr2O3 phase. Absence of any unassigned peak in XRD pattern indicated that films do not
contain any other phase of chromium oxide. The least-square fitting of the XRD pattern
indicates monoclinic unit cell structure with lattice parameters a = b = 4.95 Å, and c =13.95
Å [PCPDF# 38-1479], which is in agreement with the reported values for α-Cr2O3
(Vayssieres and Manthiram, 2003).
The core level Cr-2p and O-1s XPS peaks of freshly prepared films are depicted in
fig. 3.5 (a) and (b). Cr-2p peak has well resolved 2p3/2 and 2p1/2 peaks at 576.4 and 585.9 eV
respectively, which are assigned to Cr3+. In addition, energy separation of 9.5 eV between
2p3/2 and 2p1/2 peaks indicates presence of Cr2O3 (Hassel et al., 1996; Moulder et al., 1995).
O-1s peak could be de-convoluted into two peaks at 530 and 532 eV, which correspond to
lattice and chemisorbed oxygen at Cr2O3 film surface (Moulder et al., 1995). The SEM
micrographs at three different magnifications are shown in fig. 3.6 (a,b,c) and the
corresponding composition measurements using EDX are shown in fig. 3.7. Grains of
different shapes and sizes were observed in SEM while EDX spectrum clearly shows that Cr
metal transformed to chromium oxide upon annealing.
3.3.2
Optical and Electrical Characterization
3.3.2.1 UV-Vis Spectroscopy
Typical UV-visible spectrum of Cr2O3 films is shown in fig. 3.8. It shows
characteristic absorption peaks of Cr2O3 at 380, 460 and 595 nm, which are known to
originate from the 4A2g →4T1g, 4A2g →4T2g and 2A2g →2T1g transitions, respectively (Ivanova
et al., 2001). The optical band gap of grown films was estimated from the absorption
spectrum using Tauc’s plot by plotting (E)2 as a function of Ein accordance with equation
2.10. he estimated value of Eg was found to be ~3.30 eV, which is nearly same as reported
in literature for Cr2O3 (Ivanova, et al., 2001). Therefore, the results of physical, structural
and optical studies unambiguously confirm that the grown films in this study are of -phase
Cr2O3.
69
Fig. 3.2 Photograph of a Cr2O3 film
250
Height (nm)
200
150
100
50
0
0
(a)
2
4
6
Length (m)
8
10
(b)
Fig. 3.3 (a) 10×10 µm2 AFM image showing the surface morphology of Cr2O3 films
and (b) height profile measured along the line
70
Fig. 3.4 XRD pattern recorded for as grown Cr2O3 films
71
Fig. 3.5 XPS data showing (a) Cr-2p and (b) O-1s peaks of the Cr2O3 films
72
(a)
(b)
(c)
Fig. 3.6 Morphology using SEM at three different magnifications of (a) 5 kX (b) 30
kX and (c) 100 kX
73
Element
Weight%
Atomic%
OK
9.03
24.39
Cr K
90.97
75.61
Total
100.00
100.00
Fig. 3.7 Compositional analysis of Cr2O3 films using EDX
Fig. 3.8 Optical absorption spectra of Cr2O3 films. Inset shows a plot of (αE)2 as a
function of E
74
3.3.3
Charge Transport Measurements
The conductance of grown -Cr2O3 films was measured as a function of
temperature. Typical room temperature I-V characteristics of Cr2O3 film, in the bias range
of ±50 V is shown in fig. 3.9 (a). Interestingly, this I-V curve is ohmic and nonhysteretic. In general for oxides materials due to presence of large numbers of vacancies
and structural disorders cause trapping of charge carriers, which result in non-ohmic and
hysteretic I-V curves (Shang et al., 2006). Linear I-V characteristics in the present case
indicates ohmic contact between Au/Cr2O3, which is due to nearly equal work functions
of Au (5.1 eV) and Cr2O3 (4.8 eV). The non-hysteretic character of I-V reveals that the
defect density is significantly low in Cr2O3 films. The absence of hysteresis in I-V also
implies that the base conductance of films will remain constant as a function of time,
which is very much desirable property for gas sensing application. It may be noted that if
I-V’s are hysteretic in nature, then base conductance keeps varying as a function of time
due to trapping and de-trapping of charge carriers. From the temperature dependent I-V
characteristics, conductivity has been calculated and plotted it as a function of operating
temperature, as shown in fig. 3.9 (b) which indicates semiconducting nature of grown
films. It is evident that conductivity increases with increase in temperature. In addition,
activation energy of the films has also been determined from the plot of logarithmic
conductivity versus inverse temperature as depicted in the inset of fig. 3.9 (b).
3.3.4
Hot-Probe Measurements
P-type semiconducting nature of grown Cr2O3 films has been determined using
thermo-power measurement. For this purpose, we measured thermo-emf developed
across the films. The results shows a negative sign (positive terminal of the nanovoltmeter connected to hot end and negative terminal connected to the cold end) of
voltage which indicates its p-type nature and is in agreement with literature (Nagai et al.,
1985).
3.4
H2S AND Cl2 SENSING CHARACTERISTICS
3.4.1
Optimization of Operating Temperature
The chemiresistive response of grown α-Cr2O3 films has been systematically
studied as a function of operating temperature for various oxidizing and reducing gases
i.e. Cl2, H2S, NH3, CH4, CO and NO. For this purpose, first of all a trial test is conducted
75
Fig. 3.9 (a) Room temperature current–voltage characteristics of Cr2O3 films and (b)
temperature dependence of electrical conductivity for these films with an inset
showing inverse temperature dependence of logarithmic conductivity
76
by selecting a particular temperature and exposing different gases one by one. By
performing the trial test it was found that the films were responding only to H2S and Cl2.
After this test, response for these two gases was independently checked as a function of
temperature to determine the operating temperatures for H2S and Cl2 detection. A plot of
response (for 5 ppm gas concentration) as a function of temperature is shown in fig. 3.10.
For H2S, the response maxima were found at an operating temperature of 100°C while for
Cl2, it was observed at 220°C. Hence 100 and 220°C were selected as operating
temperature for H2S and Cl2 detection respectively.
3.4.2
Sensor Response Curve
The response curve for sensor was recorded by exposing various known
concentrations of H2S and Cl2 at their respective operating temperatures of 100 and
220oC. The obtained response curves are shown in fig. 3.11 (a) and (b). The response
direction in the two cases has also been found to be opposite, as the conductance
increased on exposure to oxidizing gas (Cl2), while it decreased on exposure to reducing
gas (H2S). This kind of behavior further confirms p-type nature of Cr2O3 films which is
initially tested using thermo-power measurements. From the response curve, sensor
sensitivity has also been determined and is plotted in fig. 3.12 (a). It can be seen that
sensitivity (for both H2S and Cl2 gases) increases with increase in gas concentration but
Cl2 sensors exhibit higher sensitivity in comparison. In case of H2S, in the concentration
range 0.5-20 ppm, response curves were highly reproducible however, for higher
concentrations (>20 ppm) base conductance of sensor was found to drift a bit. In contrast
for Cl2, response curves were highly reproducible in the concentration range 0.5-30 ppm
without any drift in base conductance. It is seen that for H2S, the response varies almost
linearly with concentration in the range 0.5–5 ppm and at higher concentrations, the
response saturates/decreases whereas for Cl2 the response remains linear in the
concentration range 0.5-30 ppm as depicted in fig. 3.12 (a).The response and recovery
times of these sensors has also been calculated and was found to be 18 and 31 s for Cl2,
while for H2S it was 6 and 11 minutes respectively. The fast response and recovery in Cl 2
is attributed to fast kinetics of the involved processes. A plot of response and recovery
time as a function of gas concentrations is shown in fig. 3.12 (b).
77
Fig. 3.10 Response% plotted as a function of temperature for 5 ppm of H2S and Cl2
Fig. 3.11 The response curves recorded for different concentration of (a) H2S at
100°C and (b) Cl2 at 220°C. Dotted lines show the base conductance of the
films. Inset shows the enlarged view of response curve (to clearly show the
response / recovery time) on 5 ppm Cl2 exposure
78
Fig. 3.12 (a) Plot of sensitivity as a function of gas concentration and (b) response
and recovery times plotted as a function of concentration for H2S (at
100°C) and Cl2 (at 220°C)
79
3.4.3
Response in Gas Mixture
In order to investigate the response of films to a mixture of two gases (i.e. Cl2
and H2S) having opposite behavior. The response curves for a mixture of gases (2 ppm of
H2S and 2 ppm of Cl2) were recorded at their respective operating temperature of 100
and 220°C respectively. The obtained results are shown in fig. 3.13. It has been found
that at 100°C (fig. 3.13 (a)), the conductance of films first increases (indicating the
response for Cl2) and
after that conductance starts decreasing below the base
conductance value (indicating the effect of H2S) and finally recovers back to base
conductance value when exposed in air. This behavior is attributed to faster response and
recovery time for Cl2 as compared to that of H2S. At 220°C (fig. 3.13 (b)), as the
response for Cl2 is much higher than that of H2S, therefore as a combined effect, response
only for Cl2 is observed.
3.4.4
Sensor Selectivity
The films were found to show temperature dependent selectivity. The histograms
showing the response for 5 ppm of different gases (i.e. Cl2, NH3, CH4, H2S, CO and NO)
at 100 and 220°C are shown in Fig. 3.14 (a) and (b), respectively. The results clearly
suggest that -Cr2O3 films are selective to H2S at 100°C and to Cl2 at 220°C. It is
pertinent to mention here that pure oxide materials are generally non-selective and hence
respond to various gases even at a fixed operating temperature.
3.5
SENSING MECHANISM
In general, the sensor resistance has contributions from surface accumulation
layer, grain boundaries and electrode/semiconductor contacts. As explained earlier,
owing to ohmic and non-hysteretic nature of Au/Cr2O3 contacts, their contribution can be
neglected. Similarly, as in the present case since the average size of grains is 2-10 m
and electrode separation is 12 m, therefore only few grains will be present between the
two electrodes. As a result, the effect of grain boundaries can also be neglected.
Therefore, the main contribution to sensing in the present case comes from the surface
accumulation layer, and hence, as shown in Fig. 3.15. A sensing mechanism was devised
in which the change in conductance of Cr2O3 films is linked to the modification in the
surface charge arising due to temperature and/or adsorbed gases. Formally, six different
situations can be distinguish as follows:
80
Fig. 3.13 The response curves recorded for a mixture of 2 ppm H2S and 2 ppm Cl2 at
(a) 100°C and (b) 220°C
81
Fig. 3.14 Selectivity histogram of films for 5 ppm of different gases recorded at (a)
100°C and (b) 220°C
82
a)
No adsorbate at film surface: If there are no oxygen ions adsorbed at the surface
of Cr2O3 films then there are no surface states, and therefore, the energy bands
can be represented as flat bands, which is schematically shown in Fig. 3.15 (a).
b)
Chemisorbed oxygen at film surface: As Cr2O3 films are exposed in air; oxygen
is chemisorbed in form of O 2 at the surface of the films by capturing electrons
from the valence band of Cr2O3, in accordance with equation and is given by
equation 1.5 (i.e. O2(gas) +e-  O2ads ). The surface traps associated with
adsorption of O 2 results in an increase in hole concentration in the vicinity of
the surface which results in build-up of an accumulation layer. As a
consequence, film conductance increases in comparison to that of flat bands
situation. This process, as shown in fig. 3.15 (b), is described in terms of an
energy band representation as an upward band bending. The thickness of the
accumulation layer, also known as the Debye length (λD), is given by the
following expression:
λD =
εkT
n 0e2
(3.1)
where, ε is permittivity of material
k is Boltzmann constant
T is the absolute temperature and
n0 is free charge carrier concentration.
Here no was estimated from the relation:
σ=n 0eμ
where
(3.2)
ζ is conductivity
e is electronic charge (1.6x 10-19C) and
µ is mobility
Using experimentally measured value of  (3.1×10-2 mho m-1) and µ (1.8×10-4
m2/Vs: measured using transient voltage pulse technique (Tiwari and Greenham,
2009). The estimated value of n0 was found to be 1.1×1021 m-3. Using literature
value of ε (i.e. 1.2×10-10 F/m), the calculated value for λD comes out to be ~130
83
(a)
(b)
Vacuum
Φa
Φb
Surface states
CB
CB
Surface states
EF
VB
++ + + + +
(c)
Vacuum
(d)
Vacuum
Vacuum
Φd
Φc
Surface states
Surface states
CB
EF
++ + + + + VB
+
(e)
++ + + + + +VB
Vacuum
Φf
CB
Surface states
Surface states
CB
EF
(f)
Vacuum
Φe
∆Φ
EF
+
VB
+
++++ + + + +
EF
+
+++++++ VB
CB
EF
+++
++++++++ VB
Fig. 3.15 Energy band representation of the surface process accompanied to
reaction with ambient oxygen, with increase in temperature, and on
exposure to the oxidizing (Cl2)/ reducing gas (H2S). Here χ represents
electron affinity, Φ is the work function, CB is conduction band, VB is
valance band and EF is the Fermi level. (a) Flat band situation for a clean
Cr2O3 surface; (b) Trapping of electron due to chemisorbed oxygen ( O 2 );
(c) The situation after heating the films at 100°C; (d) Situation after
exposing the films to H2S at 100°C; (e) The situation after heating the
films at 220°C; (f) Situation after exposing films to Cl2 at 220°C
84
nm, which is in agreement with the reported literature value for metal oxide
semiconductors (Dube et al., 2007; Kiss et al., 2001). A smaller λD as compared
to the grain size indicates that the accumulation region is formed only at the
surface region of the grains.
c)
Film at 100C: If Cr2O3 film is heated to higher temperatures (e.g. 100C),
partial desorption of O 2 takes place from its surface, which decreases
concentration of holes in the vicinity of the surface i.e. lowering of band
bending, see Fig. 3.15 (c).
d)
Interaction of film with H2S at 100C: It is known that H2S, a reducing gas react
with O2- via the reaction:
2H2S+3O2-  2H2O+2SO2 +6e-
(3.3)
which takes place vigorously with increasing temperature (Ramgir et al., 2010).
This reaction leads to a decrease in the concentration of O 2 at the film surface,
which is described in energy band representation as a lowered band bending. As
explained earlier, with increasing film temperature there is lesser O 2 available at
the surface to react with H2S, and this explains the reduction of H2S response with
increasing temperature, see fig. 3.11 (a).
e)
Film at 220C: At high temperature e.g. 220C, most of the O 2 is desorbed, and
hence the energy band diagram is very close to the flat bands, see fig. 3.15 (e).
During desorption of O 2 , captured electrons return to the film surface and hence
hole concentration decreases. As a result, conductance decreases to a very low
value at 220C.
f)
Interaction of film with Cl2 at 220C: At 220C, since most of the O 2 is
desorbed, film does not exhibit response for H2S. However, in this temperature
range, highly oxidizing Cl2 can be chemisorbed at the film surface (Tamaki et
al., 2006). The resulting negative charge at the film surface results in an upward
band bending, and hence, an increase in hole concentration in the vicinity of the
surface, which results in conductance increase, see fig. 3.11 (b).
85
3.6
CONCLUSION
A systematic study on gas sensing characteristics of polycrystalline α-Cr2O3 thin
films prepared by thermal oxidation of electron-beam evaporated Cr films. The main
inferences are summarized as:
 Cr2O3 films were found to exhibit a temperature dependent selectivity to H2S and
Cl2 at an operating temperature of 100 and 220°C respectively. This temperature
dependent selectivity provides a flexibility to use the same film for sensing Cl 2 as
well as H2S.
 The mechanism has been explained on basis of different chemisorbed oxygen
species i.e. O 2 and O- present at the surface of films due to temperature and/or
interaction with Cl2/H2S.
86