152

Chapter 5 Study of synthesis and gas sensing performance of nano Fe2O3

Publication:

1] N. K. Pawar, D. D. Kajale, G. E. Patil, V. G. Wagh, V. B. Gaikwad, M. K. Deore

and G. H. Jain, “Nanostructured Fe2O3 thick film as an ethanol sensor”,

International Journal on Smart Sensing and Intelligent System Vol. 5, No. 2, June

2012, pp. 441-457.

153

Chapter 5 Study of synthesis and gas sensing performance of nano Fe2O3

5.0 Introduction to nano technology ………………………………………….… 156

Section I

5.1 Gas sensing performance of nano Fe2O3 …………………………………… 161

5.1.1 Introduction ………………………………………………………….. 161

5.1.2 Experimental …………………………………………………………. 161

5.1.3 Results and discussion ……………………………………………….. 162

5.1.3.1 Structural characterization ……………………………….. 162

i) Thickness measurement ………………………………… 162

ii) X-ray diffraction analysis ……………………………… 162

iii) Optical absorption properties of nano Fe2O3 thick films

by UV spectroscopy ……………………………………. 163

iv) Scanning electron microscopy analysis ………………… 165

v) EDAX analysis …………………………………………. 166

vi) Transmission electron microscopy analysis…………….. 166

5.1.3.2 Electrical properties ……………………………………….. 168

i) I-V characteristic ………………………………………. 168

ii) Temperature dependent electrical conductivity ………. 168

5.1.3.3 Gas sensing response measurement ……………………… 169

i) Gas response of the sensor …………………………….. 170

ii) Selectivity of the sensor ……………………………….. 170

iii) Long term stability of the sensor ………………………. 171

iv) Response and recovery times …………………………. 172

5.1.4 Gas sensing mechanism ……………………………………………… 172

5.1.5 Conclusions …………………………………………………………… 174

154

Section-II

5.2 Synthesis, characterization and gas sensing properties of nano α-Fe2O3 …176

5.2.1 Introduction …………………………………………………………... 176

5.2.2 Experimental …………………………………………………………. 177

5.2.2.1 Synthesis of nano Fe2O3 by chemical route ……………….. 177

5.2.2.2 Preparation of thick film …………………………………….. 178

5.2.3 Result and discussion ………………………………………………… 179

5.2.3.1 Characterization …………………………………………… 179

i) X-ray diffraction analysis ……………………………… 179

ii) Scanning electron microscopy analysis ………………… 180

vii) Transmission electron microscopy analysis …………….. 181

5.2.3.2 Electrical characteristics …………………………………... 181

i) I-V characteristics ……………………………………… 181

ii) Temperature dependent electrical conductivity ………. 182

5.2.3.3 Gas sensing response measurement ……………………… 183

i) Gas response …………………………………………… 183

ii) Selectivity ……………………………………………… 183

iii) Variation in gas response with ethanol and H2S gases … 185

iv) Long term stability …………………………………….. 185

v) Response and recovery ………………………………… 186

5.2.4 Gas sensing mechanism …………………………………………….. 187

5.2.5 Conclusions ………………………………………………………….. 188

5.3 References …………………………………………………………………. 189

155

Figure captions Sr. No. Descriptions Page No. Fig. 5.1 XRD image of nano Fe2O3 163

Fig. 5.2 Optical absorbance of nano Fe2O3 by UV spectroscopy 164 Fig. 5.3 Plot of (αhν)2 versus hv 165 Fig. 5.4 SEM image of nano Fe2O3 thick film. 165 Fig. 5.5 TEM image showing the nano sized particles of nano Fe2O3 167 Fig. 5.6 Selected area electron diffraction pattern of nano Fe2O3 167 Fig. 5.7 I-V characteristics of nano Fe2O3 thick film 168 Fig. 5.8 Temperature dependent electrical conductivity 169 Fig. 5.9 Gas response of nano Fe2O3 film to all tested gases 170 Fig. 5.10 Selectivity to various gases at 350 oC 171 Fig. 5.11 Long term stability 171 Fig. 5.12 Response and recovery profile 172 Fig. 5.13 Schematic illustration of the mechanism of n-type materials 173 Fig. 5.14 Routes of oxidization of ethanol vapor 173 Fig. 5.15 Flow chart of method used for synthesis of nano Fe2O3 178 Fig. 5.16 X-ray diffractogram of synthesized material (Fe2O3) 179 Fig. 5.17 SEM image of synthesized nano Fe2O3 thick film 180 Fig. 5.18 TEM image of nano Fe2O3 181 Fig. 5.19 I-V characteristics of nano Fe2O3 thick film 182 Fig. 5.20 Electrical conductivity of Fe2O3 thick film 182 Fig. 5.21 Gas response of Fe2O3 thick film 183 Fig. 5.22 Selectivity of Fe2O3 thick film at 200oC 184 Fig. 5.23 Selectivity of Fe2O3 thick film at 300oC 184 Fig. 5.24(a) Variation in H2S gas response with gas concentration at

200oC 185

Fig. 5.24(b) Variation in H2S and ethanol vapor response with gas concentration at 300oC

185

Fig. 5.25 Stability of Fe2O3 thick film. 186 Fig. 5.26 Response and recovery time for H2S gas at 200 oC 186

Table captions Table 5.1 X-ray diffraction analysis data of sample 163 Table 5.2 Elemental analysis of functional material using EDAX 166 Table 5.3 X-ray diffraction analysis data of sample 180

156

5.0 Introduction

The nano technology: it may be viewed as the study of microstructures of

materials using electron microscopy and the growth and characterization of thin films as

nanotechnology.

In general, nanotechnology can be understood as a technology of design,

fabrication, and applications of nanostructures and nanomaterials. It includes

fundamental understanding of physical properties and phenomena of nanomaterials and

nanostructures and relationships between physical properties and phenomena and

material dimensions in the nanometer scale, it is also referred to as nanoscience. Overall

nano technology can be defined as being “concerned with materials and systems whose

structures and components exhibit novel and significantly improved physical, chemical

and biological properties, phenomena and processes due to their nanoscale size”

Brief idea of nature of nano material

One nanometer is approximately the length equivalent to ten hydrogen or five

silicon atoms aligned in a line.

It has been observed and well-known that the size of the matter affects the

physical and chemical properties of the material. Materials in the micrometer scale

mostly exhibit physical properties the same as that of bulk form; however, materials in

157

the nanometer scale may exhibit physical properties distinctively different from that of

bulk. In order to observe the effect of small structure and size of the material on gas

sensing properties of the material, nano material is used for testing its gas sensing

performance. Nanotechnology is emerging technology that deals with small structures or

small sized materials in which typical dimension spans from sub nanometer to several

hundred nanometers [1].

Material in this size range exhibits some remarkable specific properties, for example,

i) Crystals in the nanometer scale have a low melting point (the difference can

be as large as 1000oC)

ii) Reduced lattice constants, since the number of surface atoms or ions becomes

a significant fraction of the total number of atoms or ions and the surface

energy plays a significant role in the thermal stability.

iii) Crystal structures stable at elevated temperatures are stable only at much

lower temperatures in nanometer sizes, so ferroelectrics and ferromagnetics

may lose their ferroelectricity and ferromagnetism when the materials are

shrunk to the nanometer scale.

iv) Bulk semiconductors become insulators when the characteristic dimension is

sufficiently small (in a couple of nanometers).

v) Although bulk gold does not exhibit catalysis properties, Au nanocrystal

demonstrates to be an excellent low temperature catalyst.

Nanostructured materials: Nanostructured materials are the materials those have at

least one dimension falling in nanometer scale, and include nanoparticles (including

quantum dots, when exhibiting quantum effects), nanorods and nanowires, thin films, and

bulk materials made of nanoscale building blocks or consisted of nanoscale structures.

Fabrication technologies for nanostructures and nanomaterials

Many technologies have been explored to fabricate nanostructures and

nanomaterials. These technical approaches can be grouped in several ways. One way is to

group them according to the growth media:

i) Vapor phase growth, including laser reaction pyrolysis for nanoparticle

synthesis and atomic layer deposition (ALD) for thin film deposition

158

ii) Liquid phase growth, including colloidal processing for the formation of

nanoparticles and self-assembly of mono layers

iii) Solid phase formation, including phase segregation to make metallic particles

in glass matrix and two-photon-induced polymerization for the fabrication of

three-dimensional photonic crystals

iv) Hybrid growth, including vapor–liquid–solid (VLS) growth of nanowires.

Another way is to group the techniques according to the form of products:

i) Nanoparticles by means of colloidal processing, flame combustion, and phase

segregation

ii) Nanorods or nanowires by template-based electroplating, solution–liquid–

solid growth (SLS), and spontaneous anisotropic growth

iii) Thin films by molecular beam epitaxy (MBE) and atomic layer deposition

(ALD)

iv) Nanostructured bulk materials, for example photonic band gap crystals by

self-assembly of nanosized particles.

The invention and development of scanning tunneling microscopy (STM) in the

early 1980’s and subsequently other scanning probe microscopy (SPM) such as atomic

force microscopy (AFM) have opened up new possibilities for the characterization,

measurement, and manipulation of nanostructures and nanomaterials. Combining with

other well-developed characterization and measurement techniques such as transmission

electron microscopy (TEM), it is possible to study and manipulate the nanostructures and

nanomaterials in great detail and often down to the atomic level.

Challenges in nanotechnology include

i) Integration of nanostructures and nanomaterials into or with macroscopic

systems that can interface with people.

ii) Building and demonstration of novel tools to study at the nanometer level

what is being manifested at the macro level.

iii) The small size and complexity of nanoscale structures make the development

of new measurement technologies more challenging than ever.

159

iv) New measurement techniques need to be developed at the nanometer-scale

and may require new innovations in metrological technology.

v) Measurements of physical properties of nanomaterials require extremely

sensitive instrumentation, while the noise level must be kept very low.

vi) As the dimensions of materials shrink from centimeter or millimeter scale to

nanometer scale, the system properties would change accordingly, and mostly

decrease with the reducing dimensions of the sample materials. Such a

decrease can be easily as much as six orders of magnitude as sample size

reduces from centimeter to nanometer scale.

Effect of doping

The above mentioned challenges arise in the nanometer scale, but are not found in

the macro level. For example, doping in semiconductors has been a very well established

process. However, random doping fluctuations become extremely important at nanometer

scale, since the fluctuation of doping concentration would be no longer tolerable in the

nanometer scale. Any distribution fluctuation of dopants will result in a totally different

functionality of device in such a size range. Making the situation further complicated is

the location of the dopant atoms. Surface atom would certainly behave differently from

the centered atom.

The challenge will be not only to achieve reproducible and uniform distribution of

dopant atoms in the nanometer scale, but also to precisely control the location of dopant

atoms. To meet such a challenge, the ability to monitor and manipulate the material

processing in the atomic level is crucial.

Furthermore, doping itself also imposes another challenge in nanotechnology,

since the self-purification of nanomaterials makes doping very difficult.

For the fabrication and processing of nanomaterials and nanostructures, the

following challenges must be met:

i) Overcome the huge surface energy, a result of enormous surface area or large

surface to volume ratio

160

ii) Ensure all nanomaterials with desired size, uniform size distribution,

morphology, crystallinity, chemical composition, and microstructure, that

altogether result in desired physical properties, and

iii) Prevent nanomaterials and nanostructures from coarsening through either

Ostwald ripening or agglomeration as time evolutes.

Gas sensing mechanism of nanocrystalline MOS gas sensor

The gas response of any metal oxide semiconductor to a particular gas increases

with decrease in the size of crystallites/ grains due to increase in surface to volume ratio

and therefore the reactivity [2]. Crystallite/grain sizes and microstructures of the sensor

affect the gas sensing performance of the sensor. It was found that, if the grain size of the

sensor material is sufficiently small, the area of active surface sites is larger, and the

response and selectivity for a particular gas enhances largely. Several recent research

reports have confirmed the benefits of “nano-scale materials” on sensor performance [3,

4]. The sensor showed good response by controlling its’ particle size below 10-60 nm. It

was also observed that the response was decreased with increasing the particle size by

sintering at high temperature [5, 6]. Proper control of grain size remains a key challenge

for high sensing performance.

Nano-ZnO material would be expected to show much better gas sensing

performance as compared with the sensor fabricated from conventional methods such as

thick film, thin film, pellets, etc [7, 8]. It was reported that the competitive sensor using

ZnO of with nano-sized grains in LPG and ethanol sensors improves their response [8].

These examples highlight the improvements which can be made through nanotechnology

research and development. At this stage the use of nanoparticles is limited and still under

development, but the possibilities are limitless. It is therefore, a key objective of

researchers in this field is to create and use structures, devices and systems that have

novel properties and functions because of their small and/or intermediate size. It may be,

due to the smaller grain sizes (< 100 nm) of oxides, arranged in the manner so that, the

effective surface area becomes undoubtedly, explosively largest.

161

Section I 5.1 Gas sensing performance of nano Fe2O3

5.1.1 Introduction

In recent years considerable attention has been focused on use of metal oxide

semiconductors for the purpose of gas sensing application. Iron oxide, metal oxide

semiconducting material, can exist in various forms such as α-Fe2O3, γ-Fe2O3 and Fe3O4.

The gas sensing properties of α & γ forms are still not established and contrasts are

available in iron oxide literature [9, 10]. Some papers attribute to gas–sensing properties

of γ-Fe2O3 and Fe3O4 rather than to α-Fe2O3. The α-Fe2O3 form has been recognized as

having minimal gas-sensing response [9]. It has been reported that the thermal stability of

the γ-Fe2O3 limits its use as gas sensor [10]. Iron titanium oxide solid solutions have

shown response to ethanol [11]. Some report says α-Fe2O3, the most stable iron oxide

with n–type semiconducting properties under ambient conditions, is extensively used as

gas sensor, catalysts [12, 13-17]. In metal oxide semiconductor thick film gas sensor,

surface structure of the film and surface to volume ratio play very important role in

sensing performance.

In present work nano Fe2O3, being smaller in size, was especially studied to

observe the effect of change in surface to volume ratio on the gas sensing performance of

the material. As it is known, a specific area is sharply increased with decrease of grain

size. A high specific surface area and comparability of grain size (D) with the thickness

surface charge layer can take great advantage for development of high–sensitive gas

sensors [18]. It is known that the surface of nano structure with high surface to volume

ratio is very unstable and it easily adsorbs foreign molecules for stabilization [17, 18].

Structural factor for nanoscaled material is complicated conception and apart from size,

crystallite shape, nanoscopic structure, crystallographic orientation of nanocrystallites

planes forming gas sensing surface affect sensing performance of the sensing material

[19].

5.1.2 Experimental

The AR grade nano Fe2O3 powder was taken and its thixotropic paste was

formulated for printing the films. Thixotropic paste was formulated by mixing nano

162

Fe2O3 powder with the organic binder. The inorganic to organic part was kept 75:25 in

formulating the paste. Thick films of the material were prepared by screen printing

technique. The prepared thick films were fired at 550 oC for 30 min for removal of

organic binder. The thickness of the film was measured by weight difference method.

Gas-sensing measurements were carried out by the static gas-sensing system [20].

5.1.3 Results and discussion

5.1.3.1 Structural characterization

i. Thickness measurement

The thickness of the film prepared by screen printing technique was measured by

the weight difference method [21]. The substrate was weighed before deposition (screen

printing) of the film. After depositing the material on the substrate, the film was dried and

sintered and again its weight was taken. The weight difference, density of the material

and the area of the film were used to identify the thickness of the film

Thickness of the film, t = M/A. ρ …………………………(1)

Where M is difference between weight of the substrate after and before deposition

of the film, A is the area of the film deposited in cm3 and ρ is the density of the material

deposited in gmcm-3. The thickness of the film observed was 27 µm.

ii. X-ray diffraction analysis

The structural properties of the film was studied using X-ray diffractometer

(Bruker D 8 Advance, France) with Cu Kα radiation of wavelength 1.5404 Å. Fig. 5.1

shows the XRD of the nano Fe2O3. The peaks with the plane (2 2 0), (3 1 1), (4 0 0),

(5 1 1), (4 4 0) and all other found to be exactly matched with the standard peaks with

corresponding planes values. XRD of the material was taken to assure the state of the

material.

163

Fig. 5.1: XRD image of nano Fe2O3.

The material found was Maghemite-C with cubic-system, primitive lattice. Well

defined peaks for Fe2O3 were seen in the diffractogram. The XRD found well matched

with the file no.39-1346 of JCPDS data and all peaks matched well with the JCPDS data.

The d spacing from the XRD was calculated it also found well matched with the standard

value. The standard value for d spacing for (3 1 1) plane is 2.52 and the calculated value

from the XRD was found 2.5177. This indicates the phase of the material matches well

with standard material.

Table 5.1: X-ray diffraction analysis data of sample.

(hkl) planes Angle, 2θθθθ (degree)

d spacing (Å)

FWHM Crystallite size (nm)

(220) 30.30 2.9474 7.704 4 (311) 35.60 2.5198 4.353 2 (400) 43.30 2.0879 1.617 6 (511) 57.30 1.606 2.796 4 (440) 63.00 1.4743 1.929 5

iii. Optical absorption properties of nano Fe2O3 thick films by UV spectroscopy

The optical density (αt) of the film was recorded in the wavelength range 250 to

700 nm. Fig. 5.2 shows the variation of relative absorbance (αt) with wavelength (λ) for

Fe2O3 thick film. Absorption coefficient is of the order of 104 cm−1. In order to confirm

164

the nature of the film's optical transition as direct allowed or direct forbidden, the optical

data was analyzed using the classical relation [22]:

αhν = A (hν-Eg) n……………………………….. (2)

where A is a constant depending upon the type of the transition that prevails.

Specifically, for an allowed, direct allowed transition n is 1/2. Fig. 4 shows the variation

of (αhν)2 versus hν, which is almost a straight line, indicating that direct transition is the

dominant transition involved. The energy gap is obtained by extrapolating the linear

portion of the (αhν)2 versus hν plot to α = 0. The band gap energy is found to be 2.1 eV.

Fig. 5.2: Optical absorbance of nano Fe2O3 by UV spectroscopy.

Optical absorption spectrum of the sample was recorded from UV- VIS

spectrometer (Shimadzu Japan Model 2450). The variation in the absorbance with respect

to wave length is shown in the graph in Fig. 5.2.

Fig. 5.3 shows the graph of (αhν)2 versus energy hν. From the slope of the graph

the energy gap of Fe2O3 observed is 2.1 eV.

iv. Scanning electron microscopy analysis

Fig. 5.4:

For examining the surface morphology of the film and percentage of constituent

particles in the film the scanning electron micrographs along with energy dispersive

analysis were taken using JOEL 2300 model (Japan).

Fig. 5.4 shows typical SEM micrograph

screen printing technique

Fig. 5.3: Plot of (αhν)2 versus hv.

canning electron microscopy analysis

. 5.4: SEM image of nano Fe2O3 thick film.

For examining the surface morphology of the film and percentage of constituent

particles in the film the scanning electron micrographs along with energy dispersive

analysis were taken using JOEL 2300 model (Japan).

shows typical SEM micrograph of nano Fe2O3 thick film

screen printing technique. The SEM image depicts the surface morphology of the film.

165

For examining the surface morphology of the film and percentage of constituent

particles in the film the scanning electron micrographs along with energy dispersive

film prepared by

he SEM image depicts the surface morphology of the film.

166

The film shows small circular shaped grains of the Fe2O3 along with the porous nature of

the film. This porosity, which causes increase in surface to volume ratio eventually

causes increase in interaction with gas molecule, is beneficial for gas sensing properties

of the film.

v. EDAX analysis

Table 5.2 shows the elemental analysis (EDAX) which clearly depicts the

percentage of O and Fe present in the film. No noticeable amount of impurities is

found in the film material.

Table 5.2: Elemental analysis of functional material using EDAX.

Elements Wt %

O 7.85

Fe 92.15

Total 100.00

vi. Transmission electron microscopy analysis

In order to verify the nano size and crystal structure of the material TEM images

of the material were taken. TEM images were recorded from transmission electron

microscope (Philips CM 200 Make with point resolution 2.8 Å). TEM image shows the

nano structure of the material and surface morphology of the sensing layer. In TEM sharp

regular angular faces were seen indicating that the material was well crystallized.

Fig 5.5 shows the nano particles observed in the TEM image. Particles of size

ranging from 51.29 nm to 43.62 nm were observed in TEM image. The average size of

the particle observed was 25.69 nm.

167

Fig. 5.5: TEM image showing the nano sized particles of nano Fe2O3.

Fig. 5.6: Selected area electron diffraction pattern of nano Fe2O3.

Fig. 5.6 shows a selected area electron diffraction pattern of Fe2O3 nanoparticle,

the observed ring pattern reveals the crystalline structure of the material. The d spacing

observed in the diffraction pattern is in consistence with the standard values and the

values obtained from XRD. The electron diffraction patterns shows continuous ring

patterns without any additional diffraction spots and rings of secondary phases revealing

their crystalline structure.

168

5.1.3.2 Electrical properties

i. I-V Characteristics

It is well known that there is strong correlation between electronic transport and

structural characteristics of the films. During the heat treatment the structure of the film

may change eventually the I-V relation may vary. Fig. 5.7 represents I-V characteristics

of the Fe2O3 thin film under testing at room temperature. It is clear from the I-V

characteristics graph that the contacts fabricated on the film were ohmic in nature [23].

The current was measured for varying the bias voltage i.e. increased from 0 to 25

V and again decreased to zero volts. The measurement was repeated with negative bias

voltage. Every value of the current measured during voltage increase nearly coincided

with that measured during voltage decrease. From the graph, as the nature is almost

linear, it is clear that the contact on the film was ohmic in nature.

Fig. 5.7: I-V characteristics of nano Fe2O3 thick film.

ii. Temperature dependent electrical conductivity of the film

The electrical conductivity of Fe2O3 film with varying film temperature was

carried out on gas sensing system in the temperature range 323-723 K. Several heating

and cooling cycles were repeated for the appropriate observations. Successive heating

and cooling cycles resulted in the stabilization of surface resistance in the temperature

range were studied. Temperature dependent electrical conductivity of the film is shown

169

in the Fig. 5.8. It is observed that the conductivity of the film almost increases linearly

with the increase in temperature.

The conductivity of the sample is observed to be increasing with an increase in

temperature. The increase in conductivity with increase in temperature could be attributed

to negative temperature coefficient of resistance and semiconducting nature of Fe2O3

film. This increase in electrical conductivity is attributed to improvement of charge

density and semiconducting nature of the film. Temperature dependence of electrical

conductivity (σ) of the sample is expressed in terms of the Arrhenius model and is given

by the relation:

σ = σ0 exp (Ea/ kT) ……………………………… (3)

σ0 is pre-exponential factor, Ea is the activation energy, k is Boltzmann constant and T is

the absolute temperature.

Fig. 5.8: Temperature dependent electrical conductivity.

5.1.3.3 Gas sensing response measurement

The Fe2O3 sample was tested for its gas sensing performance for various gases at

different temperatures ranging from 150 oC to 400 oC. The sensing was done using static

gas system. The variation in gas concentration was studied by observing the variation in

current Ia (air) and current Ig (testing gas). The gas response to the particular gas at

particular temperature was calculated by the relation [24-25].

170

���������� =�� ����

��………………….. (4)

i. Gas response of the sensor

Fig. 5.9 shows the variation in the gas response at different temperatures for

various gases. It also reveals that the selectivity of ethanol vapors as compared to other

gases. The nano Fe2O3 film showed maximum gas response (180) to ethanol vapors

(100ppm) at 350oC temperature, whereas response to other gases was very low as

compared to response to ethanol.

Fig. 5.9: Gas response of nano Fe2O3 film to all tested gases.

ii. Selectivity of the sensor

Selectivity or specificity is defined as the ability of a sensor respond to a certain

(target) gas in the presence of other gases. Percent selectivity [26-27] of one gas over

others is defined as the ratio of the maximum response of target gas (e.g. H2S) to the

maximum response to other gas at optimum temperature of target gas [28].

% Selectivity = (Sother gas / STarget gas)× 100 ……………… (5)

Fig. 5.10 shows the selectivity of the sensor. In present work the test material

showed maximum response to ethanol vapors at 350 oC while at the same temperature the

gas response to all other gases tested was very low as compared to ethanol vapors. The

sensor found highly selective to ethanol vapors at 350 oC as compared to other gases.

0

50

100

150

200

150 200 250 300 350 400

Gas r

esp

on

se

Temperature (oC)

co CO2

NH3 LPG

H2 Cl2

Ethanol H2s

Gas concentration: 100 ppm

171

Fig. 5.10: Selectivity to various gases at 350 oC.

iii. Long term stability of the sensor

Working life of the sensor is one of the most important parameter for its practical

application. The long term stability test of the sensor was conducted to observe the

variation in its gas response corresponding to its aging period. After every five days time

span the response of the film was tested. The process of testing was carried for 70 days.

The gas response was dropped from 180 to 168. Fig. 5.11 shows the variation in the gas

response with respect to the period in days.

Fig. 5.11: Long term stability.

0

20

40

60

80

100

120

140

160

180

200

CO CO2 NH3 LPG H2 Cl2 Ethanol H2S

Gas r

esp

on

se

Gas

Operating temperature: 350 oCGas concentration: 100 ppm

172

iv. Response and recovery time

These are the important parameters for designing sensor for the desired operation.

The response time is defined as the time taken for the sensor to attain 90 % of the

maximum change in resistance on exposure to the test gas and the recovery time is

defined as the time taken by the sensor to get back to 10 % of the value of its resistance at

the time of maximum resistance.

Fig. 5.12: Response and recovery profile.

In present work the response and recovery times are defined as the times required

for a sensor to reach 90 % of its full response. From Fig. 5.12, the response and recovery

time for the sensing response can be clearly observed. The response and recovery times

for ethanol vapors at 350 oC at 100 ppm gas-concentration were found to be 7 s and 32 s.

5.1.4 Gas sensing mechanism

Electrical conduction mechanism of n-type material and ethanol

Fig. 5.13 shows mechanism of electrical conduction due to the gas sensing in n-

type material [29-31]. The majority carriers in n-type Fe2O3 are electrons in the

conduction band. When exposed to the air, the atmospheric oxygen molecules are

adsorbed on the surface of the functional material causing the depletion layer at the

surface of the material. As a result the material shows high resistan

C2H5OH, the interaction of C

Fig. 5.13: Schematic illustration of the mechanism of n

C2H5OH + O2− → CH

CH3CHO + 5O2−→ 2CO

Releasing electrons to the depletion layer and increasing the electrical conductance of the

semiconductor decreases the resistance. The

promoted by basic oxides [33

performance should be related to the oxidation of ethanol vapor.

Fig. 5.14:

As mentioned in c

reaction roots i.e. dehydrogenation to CH

the acidic surface. These intermediates are consecutively oxidized to CO

surface of the material. As a result the material shows high resistance. When exposed to

OH, the interaction of C2H5OH with the surface chemisorbed O2− takes place [32

Schematic illustration of the mechanism of n-type materials.

CH3CHO + H2O + 2e………………………… (6)

2CO2 +2H2O + 10e− ………………………… (7)

Releasing electrons to the depletion layer and increasing the electrical conductance of the

semiconductor decreases the resistance. The response to ethanol vapor is grea

promoted by basic oxides [33]. Being specific to the ethanol vapor, the sensing

performance should be related to the oxidation of ethanol vapor.

. 5.14: Routes of oxidization of ethanol vapor.

catalytic chemistry (Fig. 5.14) ethanol vapor is oxidized via two

dehydrogenation to CH3CHO on the basic surface and dehydration on

the acidic surface. These intermediates are consecutively oxidized to CO2

173

ce. When exposed to

takes place [32].

type materials.

Releasing electrons to the depletion layer and increasing the electrical conductance of the

to ethanol vapor is greatly

the ethanol vapor, the sensing

) ethanol vapor is oxidized via two

CHO on the basic surface and dehydration on

2 and H2O.

174

5.1.5 Conclusions

To understand the effect of size of the particles of the test material that causes

change in surface to volume ratio of the thick film on gas sensing performance thick film

gas sensor nano Fe2O3 material films were prepared and studied.

i) From the XRD of the material it was confirmed that the material of the film

was Maghemite-C with cubic-system, primitive lattice, observed peaks, d spacing

and plane values were well matched with file no.39-1346 of JCPDS

ii) From the UV spectroscopy, with the reference of graph of (αhν)2 versus energy

hν, The energy gap of the functional material observed was 2.1 eV. Which

matches with the standard value of energy gap of Fe2O3

iii) The thickness of the film observed was 27 µm.

iv) The SEM images and analysis shows small circular shaped grains of the Fe2O3

along with the porous nature of the film. This porosity, which causes increase in

surface to volume ratio, may enhance the interaction rate of the gas with sensing

material.

v) TEM images confirms the nano nature of the material and the average size of the

particle observed was 24 nm which further helps to increase surface to volume

ratio which is one of the most important parameter for enhancing the gas

response.

vi) During testing the gas sensing ability of the film the contacts fabricated on the

film were found ohmic in nature and the conductivity of the sample is observed to

be increasing with an increase in temperature. The increase in conductivity with

increase in temperature could be attributed to negative temperature coefficient of

resistance and semiconducting nature of Fe2O3 film. This increase in electrical

conductivity is attributed to improvement of charge density and semiconducting

nature of the film.

vii) The nano Fe2O3 film showed maximum response (180) to ethanol vapors at 350oC

temperature, whereas response to other gases was very low as compared to

response to ethanol.

viii) The sensor found highly selective to ethanol vapors at 350 oC as compared to

other gases.

175

ix) The process of testing was carried for 70 days. The response was dropped from

180 to 168. The observed change in response was about 6.66% decrease in

response.

***

176

Section-II 5.2 Synthesis, characterization and gas sensing properties of nano α-Fe2O3

5.2.1 Introduction The Fe being a transition metal, use of iron oxide as a metal semiconductor gas

sensor is an important issue to be studied thoroughly. There are several views about the

use of Fe2O3 as a gas sensing material. Several experiments have been undertaken

previously regarding to use of α- Fe2O3 as a gas sensor. α-Fe2O3 is an n-type metal oxide

semiconductor, and has been used as gas sensing material since the 1980s of the last

century [34, 35]. There have been many reports about good response and selectivity of α-

Fe2O3 sensors to combustible gases and organic vapors in recent years, such as ethanol,

acetone, gasoline and LPG, etc. [36, 37]. Some reports say Hematite (α-Fe2O3) is the

most stable iron oxide with n-type semiconducting properties (band gap Eg = 2.1eV) at

ambient conditions [38]. Its applications have been explored in the field of gas sensors

[39].

Nano iron oxide has been synthesized by various techniques and its gas sensing

performance has been tested. The effect of technique of synthesis of Fe2O3 on its gas

sensing Performance is studied as well. Gong’s group [40] has synthesized α- Fe2O3

crystals with different morphologies by changing the pH value in an aqueous reaction

system.

Developing a simple and more accurate method to synthesize α-Fe2O3 crystals

with various morphologies is still in need. There are several methods reported for

synthesis of nano Fe2O3 material.

Besides the most popular gas-sensitive oxides, e.g. SnO2 and ZnO, iron oxides,

either pure or doped, have been investigated for sensor applications by several authors,

and promising results have been obtained [41–58]. Iron oxide thin film gas sensors have

been prepared by, e.g. radio frequency sputtering [44], evaporation [45], chemical vapor

deposition [46, 47], sol–gel processing [50, 51, 54], spray pyrolysis [52] and liquid-phase

deposition [56]. In present study nano Fe2O3 is synthesized using chemical route method.

The thick films of the material are prepared and tested for their gas sensing performance.

177

5.2.2 Experimental

5.2.2.1 Synthesis of nano Fe2O3 by chemical route technique

Previously several attempts are made to synthesize nano Fe2O3 through different

techniques such as Fe2O3 was prepared by the method [59] in which Ferric hydroxide

was precipitated from an aqueous solution of FeCl3 with ammonia, washing with water,

and calcinations in air at 600 oC for 3 h. The CaO, Al2O3 and ZrO2 were also prepared by

the same method. A wet chemical method used to synthesize the WO3, was as, 0.1M

HNO3 was added to 0.1M (NH4)5H5[H2(WO4)6] H2O solution rapidly with vigorously

stirring to adjust the pH to 3. Yellow precipitation generated after 10 min and was filtered

and washed with de-ionized water for five times. The precursor was dried at 100 oC and

calcined at 600 oC in muffle furnace for 3 h. Au loaded WO3 was prepared according to

literature [60].

In present study, nano Fe2O3 was synthesized in the laboratory using chemical

root technique. All the chemicals FeCl3, FeSO4.7H2O, ethylene glycol, NaOH and

methanol used for the synthesis of nano Fe2O3 used were of AR grade. Initially 1.39 gm

of FeSO4 7H2O and moderate quantity of ethylene glycol was mixed in one beaker and

1.62 gm of FeCl3 and moderate quantity of ethylene glycol were mixed in another beaker.

Both the solutions were slowly mixed together in the beaker A. The solution of 5 gm of

NaOH in 20gm of H2O was prepared in another beaker B. The beaker A was slowly

heated and the solution was thoroughly stirred. During the process of stirring and heating

the solution of NaOH (beaker B) was added drop by drop in beaker A. the pH of the

solution was continuously monitored and controlled by addition of NaOH solution drop

by drop. The addition of NaOH was controlled to maintain the pH of solution basic. The

temperature was maintained between 70 to 80 oC during maintaining the pH. There after

the temperature was slowly raised up to 200 oC. The temperature was maintained till all

the liquid got evaporated. The drying period was quite long about 48 hrs during which

the temperature was maintained between 200 to 220 oC. Finally the charred dense

material was formed. This material was then washed 4 to 5 times with methanol and

finally the material was isolated using centrifugal. The powder was then dried. This

powder was nano sized Fe2O3. Fig. 5.15 shows the flowchart of method used for

synthesis of nano Fe2O3

178

Fig. 5.15: Flow chart of method used for synthesis of nano Fe2O3.

5.2.2.2 Preparation of thick film

The as prepared nano Fe2O3 powder was taken for the preparation of the thick

film for testing the gas sensing performance of the material. The thixotropic paste was

formulated by mixing the fine powder of Fe2O3 with a solution of ethyl cellulose (a

temporary binder) in a mixture of organic solvents such as butyl cellulose, butyl carbitol

acetate and terpinol etc. The ratio of the inorganic to organic part was kept at 75:25 in

179

formulating the paste. This paste was screen printed on a glass substrate in a desired

pattern [61]. The films were fired at 550oC for 30min. Silver contacts were made for

electrical measurements.

5.2.3 Results and discussions

5.2.3.1 Characterization

i) X-ray diffraction analysis

The powder was characterized by XRD for confirmation of the material. In order

to get the information of phase formation, the powder X-ray diffraction was carried out

by Bruker D8 Advance diffractometer using CuKα radiation source. The average

crystallite size, D, was estimated from line broadening analysis of the diffraction peaks

by using the Scherrer equation as follows:

D= 0.9λ/ βcosθ where λ, β and θ are the wavelength of X-ray (λ = 1.5406 Å), the full

width at half maximum (FWHM) of the diffraction peak and the Bragg’s diffraction

angle, respectively.

Fig. 5.16 shows the X-ray diffractogram of the synthesized nano Fe2O3 by

chemical route method. The product obtained by the method was annealed for 550 oC for

proper formation of crystallite. All the peaks observed in the XRD are well matched with

the JCPDS data card no: 33-0664. The peaks indicate the hexagonal form of the α-Fe2O3.

The average grain size calculated is 24 nm.

Fig. 5.16: X-ray diffractogram of synthesized material (Fe2O3).

180

Table 5.3: X-ray diffraction analysis data of sample.

(hkl) planes Angle, 2θθθθ (degree)

d spacing (Å)

FWHM Crystallite size (nm)

(0 1 2) 24.133 2.3842 1.1717 21 (1 0 4) 33.152 2.0879 0.254 28 (1 1 0) 35.611 1.7426 0.262 22 (1 1 3) 40.852 1.6042 0.225 28 (0 2 4) 49.483 1.5118 0.286 25 (1 1 6) 54.093 1.4030 0.255 26 (2 1 4 ) 62.452 3.4770 0.423 19 (3 0 0) 63.987 1.456 0.403 23

ii) Scanning electron microscopy analysis

Fig. 5.17: SEM image of synthesized nano Fe2O3 thick film.

The microstructural compositions of the films were analyzed using a scanning

electron microscope. Scanning electron microscopic (SEM) studies were carried out by

using JEOL 6300 (LA) Japan. Fig. 5.17 shows the scanning electron micrograph of the

thick film of the synthesized Fe2O3. SEM analyses were performed to examine the

morphology of the sample. SEM image of as-prepared α-Fe2O3 thick film reveals the

nature of surface structure of the distribution of particles. The micro observation reveals

that the porosity available on the surface which may contribute to enhancing the gas

interaction due to porosity of the film. Further the material was studied using TEM for

determining the practical size.

181

iii) Transmission electron microscopy analysis

Gas sensing is the phenomena that depend on the surface to volume ratio. Larger

the surface to volume ratio more the adsorption is. The Smaller size of the particles

increases surface to volume ratio. The material was characterized by TEM to confirm the

reduction in particle size. The average particle size observed to be 24 nm.

Fig. 5.18: TEM image of nano Fe2O3.

5.2.3.2 Electrical characteristics

i) I-V characteristics

Fig. 5.19 shows I-V characteristics of the film. As voltage increases the

corresponding current also increases. This tells that the resistance of the film decreases

with the rise in temperature.

182

Fig. 5.19: I-V characteristics of nano Fe2O3 thick film.

ii) Temperature dependent electrical conductivity of Fe2O3 thick film

Fig. 5.20 represents the variation of conductivity with temperature for the Fe2O3

thick film sample. The conductivity varied nonlinearly with temperature, showing

negative temperature coefficient.

Fig. 5.20: Electrical conductivity of Fe2O3 thick film.

-200

-150

-100

-50

0

50

100

150

200

-30 -20 -10 0 10 20 30

Cu

rren

t( p

A)

Voltage (volt)

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

1.5 1.75 2 2.25 2.5 2.75

log

(co

nd

ucti

vit

y)M

ho

/cm

1000/T(1/K)

183

5.2.3.3 Gas sensing response measurement

i) Gas response

The gas sensing performance was tested using the static gas sensing system.

Various gases were tested for gas response at different temperatures. Fig. 5.21 shows the

gas sensing performance of the nano Fe2O3 thick film. The various gases tested were O2,

H2, NH3, CO, CO2, Cl2, H2S, and LPG. The film showed good sensing response to

ethanol vapors as well as H2S gas. The gas response for H2S(150 ppm) observed was

82.32 at 200 oC temperature while at the temperature 300 oC the film showed higher gas

response (142) to ethanol vapors(150ppm). Fig. 5.21 shows the gas response of the film

for various gases at different temperatures.

.

Fig. 5.21: Gas response of Fe2O3 thick film.

ii) Selectivity

As the film showed notable gas response to ethanol at 300 oC and to H2S at

200oC, the film was tested for observing the selectivity to other gas at both the

temperatures.

Fig. 5.22 shows the gas response performance of the film for various gases at

200oC temperature.

0

20

40

60

80

100

120

140

160

150 200 250 300 350 400 450

Gas r

esp

on

se

Temperature (oC)

H2 COCO2 NH3LPG O2CL2 EthanolH2S

184

Fig. 5.22: Selectivity of Fe2O3 thick film at 200oC.

Fig. 5.23 shows the gas response to the various tested gases at 300 oC

temperature. The film showed maximum response to ethanol. As well as the film showed

gas response to H2S gas also.

Fig. 5.23: Selectivity of Fe2O3 thick film at 300oC.

From the graphs presented in both the figures it is observed that at 200 oC the film

was sensing H2S gas only. And at 300 oC the gas response was quite high to ethanol but

at the same temperature the film was responding to H2S gas also.

It is clear that the film was more selective at 200 oC temperature while at 300 oC the film

was poorly selective.

0

10

20

30

40

50

60

70

80

90

100

H2 CO CO2 NH3 LPG O2 CL2 ETHANOL H2S

Gas r

esp

on

se

Gases

Gas Concentration 150 ppmTemp 200 oC

0

20

40

60

80

100

120

140

160

H2 CO CO2 NH3 LPG O2 CL2 ETHANOL H2S

Gas r

esp

on

se

Gases

Gas concentration 150 ppmTemp. 300 oC

185

iii) Optimization of gas concentration

Fig 5. 24 (a) and (b) show the variation in gas response with variation in gas

concentration at optimized temperatures.

Fig. 5.24 (a): Variation in H2S gas response with gas concentration at 200 oC

Fig. 5.24(b): Variation in H2S and ethanol vapors response with gas concentration at

300oC.

iv) Long term stability

The gas sensing performance was tested fro its long term stability. Fig. 5.25

shows the response. The response for ethanol vapor was tested at 300 oC temperature and

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200

Gas r

esp

on

se

Gas comcentration (ppm)

Gas H2STemp 200 oC

0

20

40

60

80

100

120

140

160

0 50 100 150 200

Gas r

esp

on

se

Gas concentration(ppm)

Ethanol

H2S

Temp 300 oC

186

the response for H2S gas was tested at 200 oC temperature of the period of 70 days. The

sensing performance showed slight fall of about 10 % in ethanol sensing and about 5 %

in H2S sensing during the testing period of 70 days

Fig. 5.25: Stability of Fe2O3 thick film.

v) Response and recovery time

The response/ recovery time is an important parameter used for characterizing a

sensor. It is defined as the time required to reach 90% of the final change in current,

when the gas is turned on and off respectively.

Fig. 5.26: Response and recovery time for H2S gas at 200 oC.

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60 70 80

Gas r

esp

on

se

Time( day)

Ethanol

H2S

187

5.2.4 Gas sensing mechanism

The gas sensing mechanism belongs to the surface controlled type which is based

on the change of the electrical conductance of the semiconducting material upon

exposure to H2S gas [62].

The gas response is a function of grain size, surface state and oxygen adsorption

[63]. The surface area generally provides more adsorption-desorption sites and thus the

higher gas response. The H2S sensing mechanism is based on the change in conductance

of Fe2O3 film, which is controlled by H2S species and the amount of chemisorbed oxygen

on the surface. It is known that atmospheric oxygen molecules are adsorbed on the

surface of semiconductor oxides in the form of O2–, O– or O2–. The reaction kinematics

may be explained by the following reactions:

O2(gas) + e– → O2– (ads) ……………………… (8)

O2–

(ads) + e- → 2O–

(ads) ……………………… (9)

The presence of chemical adsorbed oxygen could cause electron depletion in the thin

film surface and building up of Schottky surface barrier; consequently, the electrical

conductance of the thin film decreased to a minimum. The SnO2 thin film interacts with

oxygen by transferring the electron from the conduction band to adsorbed oxygen atoms.

The response to H2S can be explained as a reaction of gas with the O2 (ads) –.

H2S + 3O–(ads)

→ H2O (g) + SO2 (g) + 3e– ……. (10)

With this reaction, many electrons could released to thin film surface. This could

make the Schottky surface barrier decrease, with the depletion layer thinner;

consequently, the electrical conductance of the thin film increases. More gas would be

adsorbed by the thin film surface; consequently, the gas response was enhanced. Increase

in operating temperature causes oxidation of large number of H2S molecules, thus

producing very large number of electrons. Therefore, conductivity increases to a large

extent. This is the reason why the gas response increases with operating temperature.

However, the gas response decreases at higher operating temperature, as the oxygen

adsorbates are desorbed from the surface of the sensor [64]. Also, at higher temperature,

the carrier concentration increases due to intrinsic thermal excitation and the Debye

length decreases. This may be one of the reasons for decreased gas response at higher

temperature [65].

188

Principle of ethanol sensing mechanism

The mechanism of the ethanol detection by CTO thin film can be described as

follows: At first oxygen is adsorbed on the oxide layer when the sensitive film is heated

at ambient at a temperature of 50 oC –550 oC. The adsorption of the oxygen forms ionic

species such as O2–, O2 – and O–. These oxygen species when desorbed (desorption of O2,

O2 – and O– take place at 50 oC, 100 oC and 550 oC, respectively), result in the increase or

decrease of the conductance of thin film layer depending on the nature of gas. Its

conductivity increases when the incoming gas is reducing type and decreases when it is

oxidizing type. At the higher temperature range only O– species will react with the

contaminant gas. The reaction kinematics will proceed like this:

O2 (gas) ↔ O2 (ads) ……………………………. (11)

O2 (ads) + e– ↔ O2–

(ads) ……………………. (12)

O2 (ads) + e– ↔ 2O– (ads) ……………………… (13)

The reaction between ethanol and ionic oxygen species takes place by two different

ways:

C2H5OH (gas) + O ↔ CH3CHO + H2O + e– …....... (14)

C2H5OH (gas) ↔ H + C2H5O (surface) ……………. (15)

C2H5OH ↔ H + CH3CHO ………………… (16)

CH3CHO + O (bulk) ↔ CH3COOH + O (vacancies) ………….. (17)

5.2.5 Conclusions

Nano Fe2O3 material was synthesized by chemical root method. The thick film

prepared was tested for its gas sensing performance. The gas sensing performance was

tested for various operating temperature.

i) At 200 oC temperature the film showed maximum response to H2S gas.

ii) The film was highly selective to H2S gas at the operating temperature 200 oC

iii) At 300 oC temperature the same film showed maximum response to ethanol

iv) At 300 oC temp the film was observed poorly selective. At 300 oC

temperature along with ethanol it also showed response, though weak, to H2S

***

189

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