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Chapter 6 Synthesis of nanocrystalline SrTiO3 powder by hydrothermal method for gas sensing application

Publication:

D. D. Kajale, G. E. Patil, V. B. Gaikwad, S. D. Shinde, D. N. Chavan, N. K.

Pawar, S. R. Shirsath, G. H. Jain, “Synthesis of SrTiO3 nanopowder by sol-gel

hydrothermal method for gas sensing application” International Journal on Smart

Sensing and Intelligent System Vol. 5, No. 2, June 2012, pp.382-400.

189

Chapter 6 Synthesis of nanocrystalline SrTiO3 powder by hydrothermal method for gas sensing application

6.0 Introduction…………………………………………………………………. 192

6.1 Experimental.................................................................................................... 198

6.1.1 Preparation of the precursors................................................................ 198

6.1.1.1 Preparation of Ti precursor ......................................................... 198

6.1.1.2 Preparation of Sr precursor.......................................................... 198

6.1.2 Synthesis of nanocrystalline SrTiO3 (ST) powder……………………. 198

6.1.3 Sensor Fabrication ................................................................................. 199

6.2 Characterization results……………………………………………………… 199

6.2.1 Structural properties…………………………………………………. 200

6.2.1.1 Structural analysis by XRD......................................................... 200

6.2.1.2 Surface morphology.................................................................... 200

6.2.1.3 Microstructural analysis.............................................................. 201

i) Transmission electron microscopy ……………………….. 201

ii) Electron diffraction pattern ……………………………….. 202

6.2.1.4 Elemental analysis....................................................................... 203

6.2.1.5 Thickness measurements............................................................. 204

6.2.2 Thermal properties ……………………............................................... 204

6.2.3 Optical properties of nanocrystalline SrTiO3......................................... 205

6.2.3.1 UV-visible spectroscopy analysis……………………………… 205

6.2.3.2 FTIR spectra of nanocrystalline SrTiO3 particles……………. 206

6.2.4 Electrical properties of the sensor.......................................................... 207

6.2.4.1 I-V characteristics........................................................................ 207

6.2.4.2 Electrical conductivity................................................................ 208

6.3 Gas sensing performance of ST thick films ................................................... 209

6.3.1 Gas response with operating temperature................................................ 209

6.3.2 Selectivity................................................................................................ 210

6.3.3 Response and recovery time.................................................................... 211

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6.3.4 Variation of H2S gas response with gas concentration........................... 211

6.4 Gas sensing mechanism ........................................................................... 212

6.5 Conclusions........................................................................................................ 215

References………………………………………………………………………… 216

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Figure captions Fig. 6.1 : Hydrothermal technology in the 21st century

Fig. 6.2 : Pressure temperature map of materials processing techniques

Fig. 6.3 : XRD patterns of as-prepared SrTiO3 nanopowder

Fig. 6.4 : SEM images of the nanocrystalline SrTiO3 thick films

Fig. 6.5 : TEM images of the nanocrystalline SrTiO3 powder

Fig. 6.6 : SAED pattern of SrTiO3 nanoparticles

Fig. 6.7 : TGA–DTA curves of as-prepared SrTiO3 nanopowder

Fig. 6.8 : UV-visible absorption spectra recorded for SrTiO3 thick film

Fig. 6.9 : Plot of the (αhν)2 verses photon energy (hν) for SrTiO3 thick film

Fig. 6.10 : FTIR spectrum of as-prepared SrTiO3 nanopowder

Fig. 6.11 : I–V characteristics plot of the Nano ST thick film

Fig. 6.12 : Variation of conductivity with operating temperature

Fig. 6.13 : Variation of gas response of ST thick film with operating temperature

Fig. 6.14 : Selectivity of ST thick film

Fig. 6.15 : The typical response transients of SrTiO3 thick films with time at 150 oC for

the exposure of 80 ppm H2S

Fig. 6.16 : Variation of gas response of ST thick film with H2S gas concentration

Fig. 6.17 : Sensing mechanism of thick film consisting of nanograins: (a) in air

atmosphere and (b) in presence of target gas

Table captions Table 6.1 : Lattice spacing for different crystal planes as measured from XRD and

SAED pattern

Table 6.2 : Elemental analysis of nanocrystalline SrTiO3 thick films

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6.0 Introduction

The hydrothermal technique is becoming one of the most important tools for

advanced materials processing, particularly owing to its advantages in the processing of

nanostructural materials for a wide variety of technological applications such as

electronics, optoelectronics, catalysis, ceramics, magnetic data storage, biomedical,

biophotonics, etc. The hydrothermal technique not only helps in processing

monodispersed and highly homogeneous nanoparticles, but also acts as one of the most

attractive techniques for processing nano-hybrid and nanocomposite materials. The term

‘hydrothermal’ is purely of geological origin. It was first used by the British geologist,

Sir Roderick Murchison (1792-1871) to describe the action of water at elevated

temperature and pressure, in bringing about changes in the earth’s crust leading to the

formation of various rocks and minerals. It is well known that the largest single crystal

formed in nature (beryl crystal of >1000 g) and some of the large quantity of single

crystals created by man in one experimental run (quartz crystals of several 1000s of g)

are both of hydrothermal origin.

Hydrothermal processing can be defined as any heterogeneous reaction in the

presence of aqueous solvents or mineralizers under high pressure and temperature

conditions to dissolve and recrystallize (recover) materials that are relatively insoluble

under ordinary conditions. Definition for the word hydrothermal has undergone several

changes from the original Greek meaning of the words ‘hydros’ meaning water and

‘thermos’ meaning heat. Recently, Byrappa and Yoshimura define hydrothermal as any

heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-

aqueous) above the room temperature and at pressure greater than 1 atm in a closed

system [1]. However, there is still some confusion with regard to the very usage of the

term hydrothermal. For example, chemists prefer to use a term, viz. solvothermal,

meaning any chemical reaction in the presence of a non-aqueous solvent or solvent in

supercritical or near supercritical conditions. Similarly there are several other terms like

glycothermal, alcothermal, ammonothermal, and so on. Further, the chemists working in

the supercritical region dealing with the materials synthesis, extraction, degradation,

treatment, alteration, phase equilibria study, etc., prefer to use the term supercritical fluid

technology. However, if we look into the history of hydrothermal research, the

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supercritical fluids were used to synthesize a variety of crystals and mineral species in the

late 19th century and the early 20th century itself [1]. So, a majority of researchers now

firmly believe that supercritical fluid technology is nothing but an extension of the

hydrothermal technique. Hence, here the authors use only the term hydrothermal

throughout the text to describe all the heterogeneous chemical reactions taking place in a

closed system in the presence of a solvent, whether it is aqueous or non-aqueous.

The term advanced material is referred to a chemical substance whether organic or

inorganic or mixed in composition possessing desired physical and chemical properties.

In the current context the term materials processing is used in a very broad sense to cover

all sets of technologies and processes for a wide range of industrial sectors. Obviously, it

refers to the preparation of materials with a desired application potential. Among various

technologies available today in advanced materials processing, the hydrothermal

technique occupies a unique place owing to its advantages over conventional

technologies. It covers processes like hydrothermal synthesis, hydrothermal crystal

growth leading to the preparation of fine to ultra fine crystals, bulk single crystals,

hydrothermal transformation, hydrothermal sintering, hydrothermal decomposition,

hydrothermal stabilization of structures, hydrothermal dehydration, hydrothermal

extraction, hydrothermal treatment, hydrothermal phase equilibria, hydrothermal

electrochemical reactions, hydrothermal recycling, hydrothermal microwave supported

reactions, hydrothermal mechanochemical, hydrothermal sonochemical, hydrothermal

electrochemical processes, hydrothermal fabrication, hot pressing, hydrothermal metal

reduction, hydrothermal leaching, hydrothermal corrosion, and so on. The hydrothermal

processing of advanced materials has lots of advantages and can be used to give high

product purity and homogeneity, crystal symmetry, metastable compounds with unique

properties, narrow particle size distributions, a lower sintering temperature, a wide range

of chemical compositions, single-step processes, dense sintered powders, sub-micron to

nanoparticles with a narrow size distribution using simple equipment, lower energy

requirements, fast reaction times, lowest residence time, as well as for the growth of

crystals with polymorphic modifications, the growth of crystals with low to ultra low

solubility, and a host of other applications.

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In the 21st century, hydrothermal technology, on the whole, will not be just limited to

the crystal growth, or leaching of metals, but it is going to take a very broad shape

covering several interdisciplinary branches of science. Therefore, it has to be viewed

from a different perspective. Further, the growing interest in enhancing the hydrothermal

reaction kinetics using microwave, ultrasonic, mechanical, and electrochemical reactions

will be distinct [2]. Also, the duration of experiments is being reduced at least by 3-4

orders of magnitude, which will in turn, make the technique more economic. With an

ever-increasing demand for composite nanostructures, the hydrothermal technique offers

a unique method for coating of various compounds on metals, polymers and ceramics as

well as for the fabrication of powders or bulk ceramic bodies. It has now emerged as a

frontline technology for the processing of advanced materials for nanotechnology. On the

whole, hydrothermal technology in the 21st century has altogether offered a new

perspective which is illustrated in Fig. 6.1 It links all the important technologies like

geotechnology, biotechnology, nanotechnology and advanced materials technology. Thus

it is clear that the hydrothermal processing of advanced materials is a highly

interdisciplinary subject and the technique is popularly used by physicists, chemists,

ceramists, hydrometallurgists, materials scientists, engineers, biologists, geologists,

technologists, and so on.

Fig. 6.1: Hydrothermal technology in the 21st century [1].

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The hydrothermal processing of materials is a part of solution processing and it can

be described as super heated aqueous solution processing. Fig. 6.2 shows the pressure

temperature (PT) map of various materials processing techniques [2]. According to this,

the hydrothermal processing of advanced materials can be considered as environmentally

benign. Besides, for processing nanomaterials, the hydrothermal technique offers special

advantages because of the highly controlled diffusivity in a strong solvent media in a

closed system. Nanomaterials require control over their physico-chemical characteristics,

if they are to be used as functional materials. As the size is reduced to the nanometer

range, the materials exhibit peculiar and interesting mechanical and physical properties:

increased mechanical strength, enhanced diffusivity, higher specific heat and electrical

resistivity compared to their conventional coarse grained counterparts due to a

quantization effect [3].

Fig. 6.2: Pressure temperature map of materials processing techniques [3].

Further, the technique facilitates issues like energy saving, the use of larger volume

equipment, better nucleation control, avoidance of pollution, higher dispersion, higher

rates of reaction, better shape control, and lower temperature operations in the presence

of the solvent. In nanotechnology, the hydrothermal technique has an edge over other

materials processing techniques, since it is an ideal one for the processing of designer

particulates. The term designer particulate refers to particles with high purity, high

crystallinity, high quality, monodispersed and with controlled physical and chemical

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characteristics. Today such particles are in great demand in the industry. In this respect

hydrothermal technology has witnessed a seminal progress in the last decade in

processing a great variety of nanomaterials ranging from microelectronics to micro-

ceramics and composites.

Synthesis of size and shape controlled metal oxide nanostructures is very important

in controlling their physical and chemical properties, and crucial for their potential uses.

Recently, considering the properties of the materials are greatly affected by their

morphologies, wide range of metal oxide with different morphologies providing great

opportunities for the discovery of new properties and potential uses have been

synthesized via different methods. Among these methods, hydrothermal approach [1, 5]

has great advantages in synthesizing metal oxide crystals through relative low

temperature and simple equipment, which makes the method more suitable and economic

for large-scale production

Nanostructured materials have been generating tremendous interests in the past years

due to their fundamental significance for addressing some basic issues in fundamental

physics, as well as their potential applications as advanced materials with collective

properties [6]. Perovskite-type oxides are some of the most fascinating materials in

condensed-matter research. Strontium titanate, SrTiO3 (ST), is arguably the prototypical

member of this structure family, not only because it can be made to exhibit a diverse

range of unusual properties itself. Moreover, ST is an important n-type semiconductor

with band gap of about 3.2 eV [7], and it has been widely studied not only because of its

variety of outstanding physical properties (stability, wavelength response, and current–

voltage) but also for its practical applications, such as their high static dielectric constant

and good insulation [8,9], their use in grain boundary barrier-layer capacitors [10],

resistive oxygen gas sensors [11,12], solar cells [13], solid oxide electronic device [14],

at large scale, as substrate at the time of growth of thin films perovskite compounds [15],

and promising candidate for efficient photocatalysts [16] and photoelectrodes [17,18].

It is well known that the properties of nanoparticles depend not only of the synthesis

method and chemical composition but also on their structure, shape, and size [19-20].

Therefore, the ability to tune the size and shape of ST particles is significant for

fundamental studies as well as for the preparation of ceramics and composite materials

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with tailored properties. There were many synthesis methods applied to prepare pure and

doped ST, including solid-state reaction [21], sol-gel method [22, 23], micro-emulsion

method [24], hydrothermal synthesis [25–28], and the polymeric precursor method [29–

31]. Recently, controlled homogeneity of the precursor gel in the synthesis of ST

nanoparticles by an epoxide- assisted sol-gel route was reported by Cui et al. [32].

Among these various synthetic methods, hydrothermal or chemical reaction methods

are of great interest because they are safe and environmentally friendly synthesis

performed at moderate temperatures (around 200oC) and they are effective methods for

creating novel architectures or hierarchical structures based on nanocrystals. [33]

The development of gas sensors has received considerable attention in recent years,

especially in the monitoring of environmental pollution. It is well known that

performance of gas sensors are regulated by their sensitivity, selectivity,

response/recovery speed, stability, and reproducibility [34–36].

The gas sensing is a surface phenomenon of gas-solid interaction, where the

conductivity of semiconducting oxides can be altered by adsorption of gases from

ambient. It is well known that depending upon the morphology and operating

temperatures; the oxide surface hold various oxygen species, such as O-, O2-, and O2-.

Their number and distribution also plays an important role in the gas sensing

characteristics. The literature shows that the metal oxide nanoparticles enhance the

sensitivity of a gas sensing material, while the selectivity is achieved by doping on

surface or in the volume. However, recently Korotcenkov [37] suggested that the shape

control of the nanocrystallites can provide energetically different adsorption sites for the

test gases on different crystal facets. Thus existence of large surface to volume ratio in

the typical nanostructured material facilitates better response towards specific gases.

Moreover, morphology and particle size of nanomaterials depend upon their method of

preparation and sintering temperatures, and hence one can observe different responses

towards gases for the similar composition.

Hydrogen sulfide (H2S) is a corrosive, colorless, toxic, and flammable gas, occurring

naturally in crude petroleum, natural gas, volcanic gases, and hot springs with smell of

rotten eggs. It can also be produced from industrial activities that include food

processing, coking ovens, craft paper mills, tanneries, and petroleum refineries [38].

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To date, various semiconductor gas sensors have been employed to detect trace

concentrations of H2S, including those that use SnO2, CuO-doped SnO2, and In2O3 [39-

41]. It should be noted that the H2S sensors found in the literature often show slow or

irreversible recovery reactions. This hampers the application of H2S sensors to

commercial enterprises. From the viewpoint of applications, a small size and low power

consumption are other important issues, which can be best accomplished when the

micromachining technology is applied to the fabrication of a micro-heater and

microelectrodes.

In this chapter, we report a simple and novel approach to the fabrication of

nanocrystalline SrTiO3 powder by hydrothermal method. It is expected to explore a more

extensive potential application of perovskite oxides.

6.1 Experimental

6.1.1 Preparation of the precursors

6.1.1.1 Preparation of Ti precursor

TiCl4 (99.8 %, Sigma Aldrich, USA) was dissolved in deionized water to form a

TiOCl2 solution, in which the Ti concentration was 2.0 mol.dm-3. Under stirring, a NaOH

solution (12.0 mol.dm-3) was added dropwise to the TiOCl2 solution until the pH value of

the mixture was about 7. The white precipitate obtained was named as Ti precursor.

6.1.1.2 Preparation of Sr precursor

SrCl2.6H2O (99.8 %, Sigma Aldrich, USA) was dissolved in deionized water and

mixed with a stoichiometric amount of NaOH solution (12.0 mol.dm-3) to form

Sr(OH)2.nH2O by precipitation, which was then filtered and washed by deionized water

to obtain Sr precursor.

6.1.2 Synthesis of nanocrystalline SrTiO3 (ST) powder

Stoichiometric amounts of the Sr and Ti precursors were mixed and stirred to form

homogeneous slurry. Then, an amount of solid NaOH was added in while magnetically

stirring. Afterwards, 15.0 cm3 of the homogeneous slurry was poured into a 20.0 cm3

stainless steel autoclave with a Teflon liner. After heating under the autogeneous pressure

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for several hours at the designed temperatures, the product was obtained by filtration,

washed with deionized water and dried at 70°C for 6 h.

6.1.3 Sensor fabrication

The screen-printing technique was used to manufacture the sensors. In this process, a

thixotopic paste of as-synthesized white powder is pressed through a screen on to the

substrate using a rubber squeeze.The thixotropic paste of sensor material suitable for

screen-printing was formulated by adding 75 wt% of the fine powder of ST to 25 wt% of

the organic binder (solution of ethyl cellulose in a mixture of organic solvents such as

butyl cellulose, butyl carbitol acetate and terpineol etc.) The binder was used to provide

the necessary viscosity for the screen-printing process. After mixing the powder with the

organic binder, the paste was milled in a planetary ball mill in order to homogenize the

mixture. This thixotropic paste then used for screen-printing of thick films on glass

substrate in the desired pattern [42, 43].

The thickness of films was measured by using weight difference method. The

thicknesses of the films were observed in the range from 65-75 µm. The reproducibility

in the thickness of the films was possible by maintaining proper rheology and thixotropy

of the paste. The films were dried at 80-100 oC for 0.5 h. Sintering of the dried films was

carried out by heating at temperature 550 oC in the furnace for 30 min.

6.2 Characterization results

The obtained powders were characterized by X-ray powder diffraction (XRD)

(Bruker D8) with Cu-Kα radiation (λ = 1.5406 Å) in the 2θ range from 20o to 80o with

0.02o min−1. TG-DTA analyses were carried out with a Netzsch-409 STA apparatus with

a heating rate of 20oC min−1 under flowing air. Microstructural characterization was

performed by scanning electron microscopy (SEM, JEOL 2300) and transmission

electron microscopy (Philips EM 200 make) with selected area electron diffraction

(SAED). The samples for transmission electron microscope (TEM) were prepared by

ultrasonically dispersing the powder in methanol and allowing a drop of this to dry on a

carbon-coated copper grid. A UV–visible absorption measurement was performed to

analyze the optical properties of the prepared sample. The dispersions of the

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nanoparticles were prepared in deionized water for the UV–visible absorption

measurements.

6.2.1 Structural properties

6.2.1.1 Structural analysis by XRD

The typical room temperature XRD pattern of the as-synthesized white powder is

shown in Fig. 6.3 All the peaks are indexed for cubic SrTiO3 single phase material. The

lattice constant calculated from the XRD data is 3.903Å that agrees with the reported

XRD data in JCPDS file (JCPDS 35-0734) for SrTiO3. No secondary phase was observed

in the XRD patterns in the as-repared powder, thereby indicating that SrTiO3 phase

formation was complete during the process itself. In contrast, all the previous wet

chemical methods including combustion synthesis produced a single phase material only

after calcination at temperature ≥1000 oC.

Fig. 6.3: XRD patterns of as-prepared SrTiO3 nanopowder.

6.2.1.2 Surface morphology

Fig. 6.4 depicts the SEM images representing surface morphology of the

nanostructured SrTiO3 thick films fired at 550 oC. The particles get agglomerated due to

firring process which was done for removal of organic binder. The average particle sizes

obtained from the SEM images are 10-20 nm. It is found that the SrTiO3 thick films have

relatively porous morphology.

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Fig. 6.4: SEM images of the nanocrystalline SrTiO3 thick films

6.2.1.3 Micro-structural analysis

i) Transmission electron microscopy

Fig. 6.5: TEM images of the nanocrystalline SrTiO3 powder.

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Fig. 6.5 shows the TEM micrograph of the synthesized product. The particles were

submicron-sized agglomerates consisting of nanocrystallites. The TEM showed well

faceted particles with sharp boundaries, thereby indicating that no amorphous secondary

phases are segregated at the grain boundaries. The average particle size calculated from a

number of TEM images was 34 nm with a standard deviation of 14 nm. It is worth noting

here that by using electrochemical impedance spectroscopy Balaya et al [44] showed that

SrTiO3 particles up to a mean diameter of ∼80 nm behaves as electrically mesoscopic.

SrTiO3 particles synthesized in the present work also fall within this size regime

ii) Electron diffraction pattern

Fig. 6.6 shows the selected area electron diffraction (SAED) pattern of SrTiO3

powder. It is clear from the figure that the SrTiO3 particles are crystalline in nature. The

electron diffraction patterns show diffuse but continuous ring patterns without any

additional diffraction spots and rings of secondary phases revealing their crystalline

structure.

Fig. 6.6: SAED pattern of SrTiO3 nanoparticle.

Six fringe pattern corresponding to planes: (110), (111), (200), (211), (220) and (310)

are consistent with the peaks observed in XRD pattern. In agreement with the XRD

results, all the rings in the SAED pattern were indexed for cubic perovskite structure.

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Table 6.1 compares the lattice spacing (d values) calculated from XRD and SAED

patterns. A deviation of ∼3% was observed in the d-values, which most likely arise from

the difference in wavelengths of X-ray and electron beam used for recording the

diffraction pattern.

Table 6.1: Lattice spacing for different crystal planes as measured from XRD and SAED pattern.

h k l dXRD (Å) dED(Å)

1 1 0 2.767 2.837

1 1 1 2.257 2.316

2 0 0 1.955 2.008

2 1 1 1.595 1.635

2 2 0 1.381 1.422

3 1 0 1.232 1.242

6.2.1.4 Elemental analysis

The quantitative elemental compositions of nanocrystalline SrTiO3 thick films are

presented in Table 6.2. Stochiometrically (theoretically) expected wt % of cations Sr, Ti

and anions O are 47.76, 26.08 and 26.16 respectively. The wt % of constituent cations

and anions in as prepared nanocrystalline SrTiO3 is not as per the stoichiometric

proportion and it is observed to be oxygen deficient, leading to semiconducting nature of

SrTiO3.This oxygen deficiency would promote the adsorption of relatively larger amount

of oxygen species favorable for higher gas response [45].

Table 6.2: Elemental analysis of nanocrystalline SrTiO3 thick films.

Element Mass % Atom %

Sr 55.88 30.33

Ti 31.06 30.84

O 13.06 38.83

SrTiO3 100.00 100.00

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6.2.1.5 Thickness measurement

The thickness of the film was measured by using a micro gravimetrical method

(considering the density of the bulk SrTiO3 [46]. The films were screen-printed on the

glass substrates whose mass was previously determined. After the deposition the

substrate was again weighted, determining the quantity of deposited SrTiO3. Measuring

the surface area of the deposited film, taking account of SrTiO3 specific weight of the

film, thickness (t) was determined using the relation:

t = [M SrTiO3 /A.ρ ]×104 µm (1)

Where A is the surface area of the film [cm2], M SrTiO3 is the quantity of the deposited

tin oxide and ρ is the specific weight of SrTiO3.

The thickness of the thick films was observed to be in the range from 11 to 14µm

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

achieved by maintaining the proper rheology and thixotropy of the paste

6.2.2 Thermal properties

Fig. 6.7: TGA–DTA curves of as-prepared SrTiO3 nanopowder.

The thermal characterization of the nanoparticles of SrTiO3 synthesized through the

hydrothermal process was carried out using differential thermal analysis and thermo

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gravimetric analysis up to 1200oC at heating rate of 20oC/min in nitrogen atmosphere.

Fig. 6.7 shows the DTA and TGA curves of the as-prepared powders of SrTiO3. No

notable changes were observed in the TGA as well as in DTA curves of the obtained

product. The total weight loss for a temperature range of 30-1200oC in the TGA curve is

< 3% and this weight loss can be due to the adsorbed moisture present in the sample.

6.2.3 Optical properties of nanocrystalline SrTiO3

6.2.3.1 UV-visible spectroscopy analysis

Fig. 6.8: UV-visible absorption spectra recorded for SrTiO3 thick film.

The optical energy band gap of SrTiO3 thick film film was estimated from optical

absorption measurement. The optical absorption spectrum for the SrTiO3 thick film film

is recorded in the wavelength range of 200–800 nm at room temperature shown in Fig.

6.8. The distinct sharp absorption at the band edge confirms that as prepared materials

have a crystalline nature. The steep shape of the UV edge and the strong absorption in the

UV region reveal that the absorption band of SrTiO3 is ascribed to the intrinsic transition

between the valence band (VB) and the conduction band (CB). As can be seen, the

nanocrystalline SrTiO3 thick film displays a wide absorption peak centered at around 310

nm (3.28) eV), which is slightly blue-shifted compared with the band gap of bulk SrTiO3

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thick film (as discussed in Chapter 3) [47]. The optical absorption data were analyzed

using the following classical relation of optical absorption in semiconductor near band

edge [48]:

αhν = A(hν-Eg)n (2)

where α is absorption coefficient, A is constant, Eg is the separation between bottom of

the conduction band and top of the valence band, hν the photon energy and n is a

constant. The value of n depends on the probability of transition; it takes values as 1/2,

3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden

transition respectively. Thus, if plot of (αhν)2 versus (hν) is linear the transition is direct

allowed. Extrapolation, of the straight-line portion to zero absorption coefficient (α = 0),

leads to estimation of band gap energy (Eg) values. Fig. 6.9 shows variation of (αhν)2 as a

function of photon energy (hν). The band gap energy, calculated from the spectrum for

film is 3.28 eV.

Fig. 6.9: Plot of the (αhν)2 verses photon energy (hν) for SrTiO3 thick film.

6.2.3.2 FTIR spectra of nanocrystalline SrTiO3

Fig. 6.10 shows the FT-IR spectrum of the as prepared product. The bands at ∼3400

and 1600cm−1 are due to the stretching and bending vibrations of water molecule. The

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band at 745 cm−1 is assigned to stretching vibrations of SrO [49] and band at 560cm−1

corresponds to stretching vibrations of TiO2 [50]. The sharp peak at ∼1400 cm−1 is an

instrumental artifact. Presence of water might result from adsorption of water vapor from

atmosphere due to the high surface area of the combustion product.

Fig. 6.10: FTIR spectrum of as-prepared SrTiO3 nanopowder.

6.2.4 Electrical properties of the thick film sensor

6.2.4.1 I–V characteristics

Fig. 6.11 depicts the conductivity of nanocrystalline SrTiO3 thick films at room

temperature. The symmetrical nature of the I–V characteristics for particular sample

shows that the silver contacts are Ohmic in nature. The Ohmic behavior is very important

from the point of sensing applications, especially with different concentration of gases or

other species.

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Fig. 6.11: I–V characteristics plot of the Nano ST thick film.

6.2.4.2 Electrical conductivity

Fig. 6.12: Variation of conductivity with operating temperature.

The semiconducting nature of nanocrystalline SrTiO3 is observed from the

measurements of conductivity with operating temperature. The semiconductivity in

SrTiO3 must be due to large oxygen deficiency in it. The material would then adsorb the

oxygen species at higher temperatures (O2−→ 2O−→ O2−).

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It is clear from Fig. 6.12 that the conductivity of nanocrystalline SrTiO3 thick film

increase with an increase in operating temperature in ambient, indicating a negative

temperature coefficient of resistance. This behavior confirmed the semiconducting nature

of ST.

6.3 Gas sensing performance of ST thick films

6.3.1 Gas response with operating temperature

Gas response of a sensor was defined as the ratio of the change in conductance of a

sample on exposure to the test gas to the conductance in air.

Gas Response Gg Ga G

Ga Ga− ∆

= = (3)

Where Gg & Ga are conductance of a sample in the presence and absence of a test gas

respectively & ∆G is the change in conductance.

Fig. 6.13: Variation of gas response of ST thick film with operating temperature.

It has been firstly investigated that the optimal operating temperatures of the sensor,

nano SrTiO3 thick film, to different testing gases. Fig. 6.13 shows the gas response plots

of the SrTiO3 sensor towards 80 ppm H2S and 200 ppm of other different testing gases

like C2H5OH, ammonia (NH3), CO2, CO, Cl2, LPG, H2, NO2, O2 at different operating

temperature. The response of SrTiO3 sensor towards H2S increases rapidly and reaches

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its maximum at the operating temperature of 150oC, and then decreases with further

increasing the temperature. The same behavior is observed in the case of other testing

gases, and their maximum responses appear at different temperature. Among the testing

gases, the SrTiO3 sensor shows the highest response towards H2S. The magnitude of the

response descends in the order of H2S, C2H5OH, and NH3, which seems to be correlated

with the interaction strength between the testing gas and the sensing layer [51]. The

response of the SrTiO3 sensor to H2S reaches the maximum value of 543 at 150oC, which

is about 5.43 and 20.74 times higher than the responses of C2H5OH, and NH3 achieved

at the 350oC, respectively. This operating temperature of 150oC could be useful for an

improved selectivity of gas sensor to H2S.

6.3.2 Selectivity of ST thick film

The ability of a sensor to respond to a certain gas in presence of other gases is

known as selectivity. A good sensor will discern a particular signal by allowing

adsorption of the desired gas while remaining insensitive to others.

Fig. 6.14: Selectivity of ST thick film.

Fig. 6.14 depicts the selectivity of nano SrTiO3 thick films to 80 ppm of H2S gas

against various gases (200 ppm) at 150oC. It is clear from figure that, nano SrTiO3 thick

films shows not only enhanced response towards H2S but also very high selectivity.

211

6.3.3 Response and recovery time

Fig. 6.15 shows the response and recovery of the nanocrystalline SrTiO3 based thick

films sensors to 80 ppm H2S gas at an operating temperature 150oC. The

response/recovery time is an important parameter, used for characterizing sensors. It is

defined as the time required to reach 90% of the final change in resistance, when the gas

is turned ON or OFF, respectively.

The response was quick (~10 s) to while the recovery was fast (~25 s). The quick

response may be due to faster oxidation of gas. Its high volatility explains its quick

response and fast recovery to its initial chemical status.

Fig. 6.15: The typical response transients of SrTiO3 thick films with time at 150 oC for

the exposure of 80 ppm H2S.

6.3.4 Variation of gas response with H2S gas concentration

The dependence of gas response of ST thick films with the H2S concentration at an

operating temperature of 150oC is shown in Fig. 6.16. It is observed that the Gas response

increases linearly as the H2S concentration increases from 10–80 ppm and then decreases

with further increase in the H2S concentration. The linear relationship between the Gas

response and the H2S concentration at low concentrations may be attributed to the

availability of sufficient number of sensing sites on the film to act upon the H2S. The low

gas concentration implies a lower surface coverage of gas molecules, resulting into lower

surface reaction between the surface adsorbed oxygen species and the gas molecules. The

212

increase in the gas concentration increases the surface reaction due to a large surface

coverage. Further increase in the surface reaction will be gradual when saturation of the

surface coverage of gas molecules is reached. Thus, the maximum sensitivity was

obtained at an operating temperature of 150oC for the exposure of 80 ppm of H2S. The

SrTiO3 is able to detect up to 10 ppm for H2S with reasonable sensitivity at an operating

temperature of 150 oC. The linearity of the sensitivity in the low H2S concentration range

(10–80 ppm) suggests that the SrTiO3 can be reliably used to monitor the concentration

of H2S over this range.

Fig. 6.16: Variation of gas response of ST thick film with H2S gas concentration.

6.4 Gas sensing mechanism

Concerning the gas sensing mechanism of resistance-type semiconductor oxide

materials, the sensing mechanism and change in electrical transport properties are usually

involved with the adsorption and desorption process of oxygen molecules on the surface

of materials [52–59]. When SrTiO3 sensors are exposed to air, the oxygen molecules

(O2) of circumstance atmosphere can be adsorbed on the surface of the SrTiO3 film to

form adsorbed oxide ions (O2−, O− or O2−) via capturing electrons from the conduction

band, which decreases the concentration of electrons in the conduction band and results

in a higher resistance. When H2S is introduced at the moderate temperature, the surface

213

of SrTiO3 film is exposed to the traces of the reductive gas. The interaction would occur

between these adsorbed oxygen species and the reductive H2S [58], which reduces the

concentration of oxygen ions, releases free electrons to the film surface and thus

increases the electron concentration (Eq. (4), eventually increases the conductivity of the

SrTiO3 sensor. The reaction kinetics may be explained by the following reactions:

(4)

(5)

The presence of chemically adsorbed oxygen could cause electron depletion in the

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

conductance of film decreased to a minimum. The SrTiO3 thick 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) −.

(6) Actually, the theory for sensing mechanism of such sensors involves the adsorption

and desorption processes, which occur at the surface of the sensing materials [54].

Therefore, the surface accessibility and high surface area are crucial to maintaining the

high sensing properties of nanomaterial [57]. Thus the SrTiO3 thick film sensor shows

large surface accessibility, which may lead to higher sensing performance.

More gas would be adsorbed by the 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 sensitivity increases with

operating temperature. However, the sensitivity decreases at higher operating

temperature, as the oxygen adsorbates are desorbed from the surface of the sensor [60].

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 sensitivity at higher temperature [61].

Nanocrystallinity effect on gas response and speed of response

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

the decrease in the size of nanocrystallites due to an increase in surface to volume ratio

214

and therefore the reactivity [62]. 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 sensitivity

and selectivity for a particular gas enhances largely. Nanocrystalline material would be

expected to show much better gas sensing performance as compared with the sensor

fabricated from bulk SrTiO3 [63, 64].

The resistance of the nanocrystalline materials decreases as gas flows into the test

chamber and adsorbed on the surface of the nanocrystalline material. However, as shown

in Fig. 6.17(a), when a nanocrystalline thick film consisting of fine grains is exposed to

air, the depletion layer would extend throughout the entire layer of nanocrystalline

materials on the film, and its resistance would become strikingly large.

Fig. 6.17: Sensing mechanism of thick film consisting of nanograins:

(a) in air atmosphere and (b) in presence of target gas.

In a target gas environment (see Fig. 6.17(b)), the depleted layer would shrink quickly

as it obtains conduction electrons due to reaction between gas-adsorbed oxygen, and the

resistance of the nanocrystalline materials would experience a large change.

215

6.5 Conclusion

Advanced materials processing using hydrothermal technology has lots of advantages

owing to the adaptability of the technique, which is also environmentally benign SrTiO3

nanomaterial was successively synthesized using a sol-gel-hydrothermal method and its

gas sensing performance was tested. The following conclusions were drawn from the

present investigation:

i. The X-ray diffraction studies of the nano powder of SrTiO3 synthesized through

this hydrothermal route have shown that the as-prepared powder was single phase,

crystalline, and has a cubic perovskite structure (ABO3) with a lattice constant a =

3.903 Å.

ii. The particle size calculated from FWHM is ∼22 nm. The phase purity of SrTiO3

nanopowders has been confirmed using differential thermal analysis, thermo

gravimetric analysis, and UV-visible abortion spectroscopy.

iii. The elemental analysis confirmed that the as prepared Thick films were non-

stoichiometric in nature.

iv. The transmission electron microscopic investigation has shown that the particle

size of the as-prepared powder has a mean particle size of 34 nm with standard

deviation 14 nm.

v. The band gap values obtained from the absorption spectra was found to be 3·28

eV.

vi. The maximum sensitivity was obtained at an operating temperature of 150oC for

the exposure of 80 ppm of H2S.

vii. The thick films of nanocrystalline SrTiO3 showed remarkable improvement of

H2S gas response over the reported sensors.

viii. The nanocrystalline SrTiO3 thick films exhibit quick response (~10 s) and fast

recovery (~25 s) which is one of the main features of SrTiO3 based sensor.

ix. The results of the SrTiO3 films sensing studies reveal that the as prepared

material and films are a suitable for the fabrication of the H2S gas sensor.

216

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