chapter 6 preparation and characterization of zno: al...

Chapter 6

Preparation and Characterization of ZnO: Al

thick films

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Poster Presentation: 1) Influence of Al Doping on ZnO Thick Film Gas Sensors, International

workshop and Symposium on the Synthesis and Characterization of Glass

/ Glass-ceramics (IWSSCGGC-2010), July 7-10, 2010, Orgnized by-

Centre for Materials for Electronics (C-MET), Pune, India in Association

with Materials Research Society of India (MRSI) Pune, Mumbai, Kolkata

& Gujarat Chapters.

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185

Chapter 6

Preparation and Characterization of ZnO: Al thick films ------------------------------------------------------------------------------- 6.1 Introduction

Nowadays, there is a great interest in implementing sensing devices in order

to improve environment and safety control of gases. There is also a great need of

these kind of sensors to carry out the optimization of combustion reactions in the

emerging transport industry and in domestic and industrial applications. Solid

state gas sensors show fast sensing response, simple implementation and low

prices [1, 2, 3]. Transition metal oxides such as TiO2, SnO2, WO3, ZnO, In2O3

etc. appear to be best candidates for semiconductor gas sensors [1, 2, 4]. The

sensitivity of these devices is based on the dependence of conductivity of metal

oxides on the surrounding atmosphere. Nevertheless, full development of such

devices requires an improvement of their characteristics by the introduction of

metal additives [5]. These additives enhance the material sensitivity, selectivity

and decrease the response time and the operating temperature of the sensing

layer [6,7]. Another important property is that the presence of metallic additives

can also modify the growth kinetic. The amount and distribution of the metal is

the most important parameter to be controlled in order to obtain the highest

sensitivity [8].

ZnO is an n-type semiconducting oxide of the group-II metal zinc and

belongs to the P63mc space group [9]. The undoped ZnO has high n-type

conductivity due to defects such as oxygen vacancies and Zn interstitials, which

form donor levels [10]. ZnO have high transparency in the visible and near-

ultraviolet spectral regions, wide conductivity range and conductivity changes

under photoreduction/oxidation condition [11]. Accordingly this binary

compound has wide applications in chemical sensors, heterojunction solar cells,

electrophotography, surface acoustic wave devices, conductive transparent

conductors and many others. However, pure ZnO films lack stability in terms of

thermal edging in air or corrosive environments [12]. Therefore polycrystalline

ZnO films have been doped with group-II and group-III metal ions such as

186

indium (In), aluminum (Al), gallium (Ga), copper (Cu), cadmium (Cd) etc. to

enhance their structural, optical and electrical properties [13]. Doping is

particularly done to get high transparency, stability and high conductivity.

Aluminium doping is particularly suitable for this purpose. Aluminium doped

ZnO (AZO) thin films have high transmittance in the visible region, and a low

resistivity, and the optical band gap can be controlled by using Al doping

amount [14]. Accordingly they have got potential applications in solar cells,

antistatic coatings, solid-state display devices, optical coatings, heaters,

defrosters, chemical sensors etc. [14-16]. Al has been used to improve the

electrical conductivity and thermal stability of ZnO films. For this work, Al3+

substitution on Zn2+ was chosen due to the small ion size of Al3+ compared to

that of Zn2+ (rAl3+ = 0.054 nm and rZn

2+=0.074 nm) [17].

The effect of Al doping on the electrical properties of ZnO varistor has

been investigated by several reports [18-21]. It has been observed that Al

increased the current density in the upturn region and shifted its onset to lower

fields and, consequently, the grain conductivity of ZnO is increased [22]. Since

the conductivity in the upturn region depends on the grain conductivity of ZnO,

a systematic study of the effect of Al doping on the conductivity of ZnO is,

therefore, necessary. Al doped films showed the potential to increase the gas

response of ZnO films, since their performance is directly related to the exposed

surface area, electrical and sensitivity characteristics [23-25].

This chapter consists, the explanation of electrical, structural and gas

sensing performance of ZnO: Al thick films.

6.2. Experimental Procedure

ZnO:Al composite thick films were prepared by screen printing technique

and fired at 700oC as discussed in chapter-2. The attempt has made to study the

electrical and structural, and gas sensing characteristics of the ZnO: Al

composite thick films. These properties were found to be functions of the

operating temperatures, non-stoichiometery, gas concentration, type of gas, and

grain size of the material. The results are divided in to the following parts:

187

1. Structural characterization by XRD, SEM, EDAX.

2. Electrical Characterization

• Measurement of change in resistance of thick films at different operating

temperatures in an air atmosphere.

• Estimation of resistivity, TCR and activation energy.

3. Gas sensing studies

• Measurement of change in resistance with operating temperature for the

gases H2S, CO2, NH3, NO2, Ethanol vapours and LPG.

• Estimation of sensitivity, selectivity, optimum operating temperature,

active region (ppm), response and recovery time for the gas that shows

maximum response etc.

6.3. Result and Discussion

6.3.1. Drying and Firing of the films

Drying and firing stage of the films has been carried out by the same way

as described in previous chapters. Drying process removes the organic solvents,

make the printed film adhere to the substrate and be relatively immune to

smudging. The high temperature firing cycle is designed to remove the

remaining organic binders, to develop the structural and electrical properties of

the film and to bond the film to the substrate [26, 27].

6.3.2. Structural characterization

The study of structural characteristics of ZnO: Al thick films include (i)

Structural characterization using XRD (ii) Surface morphology using SEM (iii)

Elemental analysis using EDAX.

6.3.2.1. X-Ray diffraction analysis of ZnO: Al thick films

To understand the structure and phases of ZnO: Al films, the films were

characterized by XRD. X-ray diffraction analysis of ZnO: Al thick film samples

were carried out in the 20-80O range using CuKα radiation.

Figure 6.1(a,b,c,d,and e) shows XRD pattern of 1%,3%,5%,7% and 10%

Al doped ZnO thick films fired at 700oC and Table 6.1(a,b,c,d and e) shows

corresponding XRD data of such films.

Figure 6.1 (a): XRD of 1% Al doped ZnO thick film Sample fired at 700 0C

Table 6.1(a): XRD data for 1% Al doped ZnO thick film Sample fired at

700 0C

Plane (hkl) 2 θ FWHM d-values Intensity I/Io Z- 100 31.8 0.353 2.8116 883 65 Z- 002 34.6 0.235 2.6196 415 31 Z- 101 36.9 0.463 2.4402 1347 100 Z- 102 48.1 0.353 1.8900 254 19 Z- 110 56.9 0.235 1.6327 489 36 Z- 103 63.1 0.353 1.4679 310 23 Z- 200 66.4 0.235 1.4105 285 21 Z- 112 68.3 0.235 1.3668 476 35 Z- 201 69.1 0.353 1.3513 175 13 Z- 004 72.6 0.235 1.3011 41 3 Z- 202 77.3 0.235 1.2333 57 4 Al2O3 58.0 0.235 1.5888 1186 88

188

Figure 6.1 (b): XRD of 3% Al doped ZnO thick film Sample fired at 700 0C

Table 6.1(b): XRD data for 3% Al doped ZnO thick film Sample fired at

700 0C


189

Figure 6.1 (c): XRD of 5% Al doped ZnO thick film Sample fired at 700 0C

Table 6.1(c): XRD data for 5% Al doped ZnO thick film Sample fired at

700 0C


190

Figure 6.1 (d): XRD of 7% Al doped ZnO thick film Sample fired at 700 0C

Table 6.1(d): XRD data for 7% Al doped ZnO thick film Sample fired at

700 0C


191

Figure 6.1 (e): XRD of 10% Al doped ZnO thick film Sample fired at 700 0C

Table 6.1(e): XRD data for 10% Al doped ZnO thick film Sample fired at

700 0C


From the XRD pattern of Al doped ZnO films of different doping

concentrations, the observed diffraction peaks correspond to the hexagonal

wurtzite structure of ZnO (JCPDS 36- 1451). It has been observed that ZnO

phase has preferred orientation in (101) plane. XRD pattern of all the films seem

to be qualitatively similar. No metallic Al characteristic peak was observed.

From 5 wt. % to 10 wt. % Al doping, the slight increase in the intensity of strong

orientation of alumina (Al2O3) was observed. This may be due to Al replacing

192

zinc substitutionally in the lattice or Al segregation to the non-crystalline regions

in the grain boundary. From 5wt. % to 10 wt. %, the excess Al was suppose to

be converted into Al2O3. To confirm that Al3+ has been substituted for Zn2+ in

the unit cell, the lattice parameters (a and c) of the samples are calculated. They

are decreased by the addition of Al, probably due the smaller ionic size of Al3+

(0.54 Å) than that of Zn2+ (0.74 Å) [16, 22]. The‘d’ spacing decreases upto 3 wt.

% Al doping and then remain constant. FWHM (β) decreases upto 3 wt. % Al

doping and then increases upto 10 wt. % Al doping. Therefore crystallite size

increases upto 3 wt. % Al doping and then decreases for the further doping

concentrations.

The average crystallite size was calculated from XRD pattern using

following Debye Scherer’s formula [28],

θβλ

cos9.0

=D ..... (6.1)

where, β = Full angular width of diffraction peak at the at half maxima peak

intensity, λ = wavelength of X-radiation.

Table 6.1(f) shows the variation of crystallite size with doping concentration of

Al.

Table 6.1(f): Variation of grain size with doping concentration of Al

Doping Conc. of Al (%) Average grain size(nm) 0 18.637 1 18.90 3 19.41 5 18.35 7 17.90 10 17.47

From the XRD pattern, the lattice constants of ZnO phase for all doping

concentrations can be calculated using the equation [28, 29],

22101

2

11341

cad+⎟

⎠⎞

⎜⎝⎛= ..... (6.2)

where d is interplanar distance and a, c are lattice constants (being hexagonal

structure, )38=

ac .

193

194

The lattice constant of Al2O3 phase can not be determined because for 5 wt.

% to 10 wt. % doping, slight increase in the strong orientation of alumina has

been observed. The calculated values of lattice constants are given in Table 6.1

(g),

Table 6.1(g): Lattice constants of ZnO phase of ZnO: Al films

Al concentration (%) d (Ao) a=b (Ao) c (Ao) 0 2.4402 3.1894 5.208 1 2.4397 3.1888 5.2073 3 2.4391 3.1880 5.2060 5 2.4388 3.1876 5.2053 7 2.4388 3.1876 5.2053 10 2.4388 3.1876 5.2053

From the Table 6.1(g), it has been observed that there is variation of lattice

constants from JCPDS value (a = 3.25Ao, c = 5.207 Ao) of ZnO.

6.3.2.2 Scanning Electron Microscopy (SEM) Analysis of ZnO:Al thick films

Scanning electron microscopy was used to study the surface morphology

of ZnO: Al thick films. A scanning electron microscopy SEM [Model JOEL

6300(LA) Germany] was employed to characterize the samples. For SEM, all

ZnO: Al samples were coated with a very thin conducting gold layer (few 100Å)

using vacuum evaporation/sputtering technique to avoid charging of the samples.

Figure 6.2(a, b, c, d and e) shows the SEM micrographs of ZnO: Al

thick films fired at 700oC. All the SEM images are recorded at 10000X

magnification for comparison.

Formation of submicrometer crystallites was distributed more or less

uniformly over the surface. No cracks were detected. Some holes indicating

porosity is present. Agglomeration of small crystallites also seem to be present

in the certain region. The contrast difference is due to different orientations of

the crystallites. The microstructure observation indicated that the Al addition

decreased the grain growth at higher doping concentration.

Figure 6.2(a): SEM of 1% Al doped ZnO thick film fired at 700oC

Figure 6.2(b): SEM of 3% Al doped ZnO thick film fired at 700oC

Figure 6.2(c): SEM of 5% Al doped ZnO thick film fired at 700oC

195

Figure 6.2(d): SEM of 7% Al doped ZnO thick film fired at 700oC

Figure 6.2(e): SEM of 10% Al doped ZnO thick film fired at 700oC

The particle size of pure ZnO films is more than Al doped ZnO films. As

Al concentration increased upto 3 wt. %, the particle size increases and the

surface roughness also increases. For the further doping concentration of Al upto

10 wt. %, the particle size decreases and the surface roughness decreases. It may

be due to segregation of Al to the non crystalline region in the boundary [30].

The specific surface area of ZnO: Al thick films were calculated using BET

method for spherical particles using the following equation [31, 32].

d

SW ρ6

= ..... (6.3)

where d is the diameter and ρ is the density of the particles.

Table 6.2 shows the change in particle size and Sp.surface area of ZnO: Al

thick films with doping concentration.

196

Table 6.2: Variation of particle size and Sp.surface area with doping concen.

wt. % of Al Particle size in nm Specific Surface Area in m2/g

1 383 2.77 3 385 2.79 5 301 3.61 7 263 4.17 10 233 4.79

The specific surface area of 10 wt. % doped films is observed to be

maximum. A high specific surface area facilitates the chemisorptions process by

increasing the adsorption and desorption rates [33].

6.3.2.3 EDAX –Energy dispersive X-ray analysis

The elemental analysis of the Al doped ZnO thick films fired at 700oC

was carried out using EDAX (JOEL, JED-2300, Germany). The EDAX analysis

shows presence of only Zn, Al and O as expected, no other impurity elements

were present in the ZnO: Al thick films. The EDAX result shows variation in

Zn/O ratio and Al/Zn ratio with variation in doping concentration. Figure 6.3(a,

b, c, d and e) shows the EDAX spectra of ZnO: Al thick films. Table 6.3 gives

quantitative elemental analysis of ZnO: Al thick films.

Figure 6.3 (a): EDAX spectra of 1 wt. % Al doped ZnO thick film

197

Figure 6.3(b): EDAX spectra of 3 wt. % Al doped ZnO thick film

Figure 6.3(c): EDAX spectra of 5 wt. % Al doped ZnO thick film

Figure 6.3(d): EDAX spectra of 7 wt. % Al doped ZnO thick film

198

Figure 6.3(e): EDAX spectra of 10 wt. % Al doped ZnO thick film

Table 6.3 Composition of the ZnO: Al films at different doping

concentrations

Al Element (mass %) 1wt. % 3wt. % 5wt. % 7wt. % 10wt. % O 21.38 22.14 21.12 20.66 20.43 Zn 77.97 75.15 74.17 73.42 72.44 Al 0.65 2.71 4.71 5.92 7.13

EDAX confirms that only Zn, O and Al are present in the samples. The

mass% of Zn and O in each sample was not as per stoichiometric proportion.

The entire samples were observed to be oxygen deficient. The film doped with

10 wt. % Al was observed to be most oxygen deficient. The deficiency or excess

of the constituent material results in distorted band structure with corresponding

increase in conductivity. Zinc oxide loses oxygen on heating so that the zinc is in

excess. The oxygen of course evolves as electrically neutral substance so that it

is associated with each excess zinc ion in the crystal; there will be two electrons

that remain trapped in the solid material, thus leading to non-stoichiometricity in

the solid. This leads to the formation of the semiconducting nature of the

material [34].

6.3.3 Electrical characterization (Resistivity, TCR and activation energy)

6.3.3.1 Resistivity of ZnO: Al thick films

The DC resistance of the ZnO: Al thick films fired at 7000C was measured

by using half bridge method as a function temperature. Figure 6.4(a, b, c, d and

199

e) shows the resistance variation of ZnO: Al thick films with operating

temperatures in an air atmosphere.

From Figure 6.4, it is observed that there is a decrease in resistance with

increase in temperature. It could be attributed to negative temperature coefficient

of resistance and semiconducting behavior of Al doped ZnO thick films, obeying

R=RO e-∆E/KT in the temperature range of 40 to 3500C.

Figure 6.4(a): 1 wt. % Al: ZnO film Figure 6.4(b): 3 wt. % Al: ZnO film

200

Figure 6.4(c): 5 wt. % Al: ZnO film Figure 6.4(d): 7 wt. % Al: ZnO film

Figure 6.4(e): 10 wt. % Al: ZnO film

The resistance of Al doped ZnO films falls suddenly in linear fashion

up to certain transition temperature and after that the resistance decreases

exponentially with increase in temperature and finally saturates to a steady level.

The transition temperature depends on doping concentration.

The resistivity of ZnO: Al thick films at constant temperature is calculated

using the relation,

mohml

tbR /⎟⎠⎞

⎜⎝⎛ ××

=ρ ..... (6.4)

201

where R = Resistance of ZnO: Al thick film at const. temperature

202

t = thickness of the film sample

l = length of the thick film

b = breadth of the thick film

The resistivity of ZnO:Al thick films decreases with increase in doping

concentration of Al up to 3 wt. % then increases with increase in doping

concentration of Al from 5 wt. % to 10 wt.%. Figure 6.5 shows the variation of

resistivity of ZnO: Al thick films with doping concentration of Al.

It was found that the resistivity of ZnO: Al films are strongly dependent on

the Al doping concentration. The resistivity of ZnO: Al films is found to

decrease with aluminum concentration up to 3 wt. %.

Figure 6.5: Variation of resistivity of ZnO:Al thick films with doping

concentretion of Al

This can be explained by the fact that ionic radius of aluminum (Al3+ =

0.053 nm) is smaller than that of Zn (Zn2+=0.074 nm). So the resistivity

decreased with the dopant concentration due to the replacement of Zn2+, by Al3+

ions and generation of a free charge carrier. But at concentrations of more than 3

wt. % Al, it will produce disorder in the lattice and increase scattering

mechanism (proton and ionized impurity) which increase the resistivity [35, 36].

However, after a certain level of doping, the extra aluminum atoms might not

occupy the correct places inside the zinc oxide crystallites because of the limited

solubility of aluminum inside ZnO. Therefore excess aluminum may occupy

interstitial positions [37] leading to distortion of the crystal structure. Hence, the

dopant atoms become ineffective as donors, as the higher levels of Al

incorporation lead to interstitial incorporation of Al in the form of Al2O3 [37]

giving rise to the greater electron scattering. The aluminum atoms may also

segregate to the grain boundaries in the form of Al2O3, which will increase the

grain boundary barrier and hence the resistivity increases after a certain level of

doping concentration [38, 39].

6.3.3.2 TCR of ZnO: Al thick films

Temperature coefficient of resistance (TCR) of ZnO: Al thick films fired at

700OC is calculated by using the following relation,

KTR

RTCR O

o

/1⎟⎠⎞

⎜⎝⎛∆∆

= ..... (6.5)

TCR of ZnO: Al thick film deposits are negative for all samples, indicating

semiconducting behaviour.

Figure 6.6 shows the variation of TCR with doping concentration of Al in

ZnO thick films. It is found that TCR is negative and its value increases with

increase in doping concentration up to 3 wt % of Al and then decreases with

increase in further doping concentration.Generaly for thick films, high resistivity

corresponds a low TCR value [40, 41].

Figure 6.6: Variation of TCR with doping concentration of Al in ZnO films 6.3.3.3 Activation energy of ZnO: Al thick films

Activation energies of ZnO: Al thick films are calculated from Arrhenius

plot for low and high temperature regions. Figure 6.7 (a, b, c, d and e) shows

Log(R) versus reciprocal of temperature (1/T) variation of 1, 3, 5, 7 and 10 wt.

203

% Al doped ZnO thick films fired at 700OC. This variation is reversible in both

heating and cooling cycles obeying the Arrhenius equation,

R=Ro e -∆E/KT ..... (6.6)

Where, Ro = Resistance at room temperature = constant,

∆E = the activation energy of the electron transport in the conduction band,

K = Boltzmann constant and

T = Absolute temperature

(a) 1wt. % Al doped ZnO:Al thick film (b) 3wt. % Al doped ZnO:Al thick

film

(c) 5wt. % Al doped ZnO:Al thick film (d) 7wt. % Al doped ZnO:Al thick

film

204

(e) 10 wt. % Al doped ZnO:Al thick film

Figure 6.7: Variation of Log(R) with reciprocal of temperature (1/T)

Activation energy can be thought of as the height of the potential barrier

or energy barrier separating two minima of potential energy. Activation energy

of ZnO: Al thick films calculated for low and high temperature regions decreases

with increase in doping concentration of Al up to 3 wt. %. From 5 wt. % to 10

wt. % doping concentration of Al, activation energies increase with increase in

doping concentration of Al. It may be because the barrier height and therefore

resistivity decreases up to 3 wt. % doping and then increases for further increase

in Al doping [38,42].

From above Arrhenius plot, it is observed that the curve for ZnO: Al

films have low temperature region and high temperature region. The change in

slope of the graph, from one region to other region occurs between two

temperatures, depending upon the wt. % of Al in ZnO: Al thick films. The

transition temperatures for the ZnO: Al thick films with 1wt. %, 3wt. %, 5wt. %,

7wt. % and 10wt. % Al doping are 250oC, 260oC, 220oC, 180oC and 170oC

respectively.

The variation of resistivity, TCR and activation energy of the ZnO: Al thick films fired at temperature of 700oC is summarized in Table 6.4, (Thickness

of samples = 21 µm)

Table 6.4: Variation of resistivity, TCR and Activation energy with doping

concentration

205

wt. % of Al

Resistivity (Ω )

TCR , (/oK)

Activation energy, eV

Al (Ωm) (/oK)

Low temperature region

High temperature region

1 2.067 X 102 0.00894 0.116 0.176

3 1.674 X 102 0.00899 0.114 0.149

5 1.685 X 104 0.00641 0.195 0.377 7 2.395 X 104 0.00512 0.201 0.41

10 3.816 X 104 0.00368 0.233 0.417

6.3.3.4 Sample to sample variation

Table 6.5: Electrical parameters of ZnO: Al film samples fired at

7000C

206

6.3.4 Gas sensing studies

Activation energy (eV) Wt.%

of Al

Sample Resistivity at room

temperature

(ρ ), Ω m

TCR, at

1000C

(/oK) Low temperature

region

High temperature

region

S1 2.098 X 102 0.00892 0.119 0.181

S2 2.056 X 102 0.00895 0.113 0.171

1%

S3 2.102 X 102 0.00887 0.122 0.186

S1 1.644 X 102 0.00901 0.112 0.148

S2 1.695 X 102 0.00898 0.117 0.152

3%

S3 1.672 X 102 0.00899 0.114 0.149

S1 1.648 X 104 0.00649 0.191 0.371

S2 1.682 X 104 0.00641 0.195 0.377

5%

S3 1.698 X 104 0.00638 0.199 0.387

S1 2.397 X 104 0.00512 0.201 0.41

S2 2.366 X 104 0.00519 0.194 0.38

7%

S3 2.385 X 104 0.00512 0.201 0.41

S1 3.839 X 104 0.00365 0.239 0.422

S2 3.811 X 104 0.00368 0.233 0.417

10%

S3 3.859 X 104 0.00342 0.245 0.467

The gas sensing study was initiated with a view to study the gas sensing

properties of ZnO thick film resistors with 1, 3, 5, 7 and 10 wt. % Al doping and

fired at 7000C. The main characterization is the optimization of operating

temperature of film sample for test gases. On the basis of measured data, the

sensitivity, selectivity, response and recovery time of thick film sensor for a

fixed gas concentration of 1000 ppm in air ambient condition are calculated.

Also the response of gas with variation of gas concentration at optimum

operating temperature is studied.

6.3.4.1 Optimization of operating temperature and Sensitivity of gas sensor

207

The gas sensing behavior of ZnO: Al thick films fired at 700oC was

studied by using static measurement system. The DC resistance of ZnO:Al thick

film resistors was measured by using half bridge method as a function of

temperature in air as well as in H2S, LPG, CO2, NH3, NO2, and Ethanol vapour

gas atmosphere (1000ppm). The operating temperature was varied at the interval

of 50oC. From the measured resistance in air as well as in gas atmosphere, the

sensitivity of gas was determined at particular operating temperature using the

equation,

100)( ×−

=a

ga

RRR

SySensitivit ..... (6.7)

where Ra – resistance of thick film in air atmosphere,

Rg – resistance of thick film in gaseous atmosphere

Figure 6.8 (a, b, c, d, e and f) shows variation of sensitivity of ZnO: Al

thick films with different doping concentrations of Al and fired at 700oC as a

function of operating temperature for different gases. From Figure 6.8, it has

been observed that the Al doped ZnO thick films are more sensitive to CO2 than

other tested gases - NH3, LPG, H2S, Ethanol and NO2.

The ZnO thick films doped with 10 wt. % Al has shown higher sensitivity

to CO2 than other doping concentrations. These films have shown 87.30 %

sensitivity to CO2 at 250oC operating temperature. The films doped with 10 wt.

% Al have also shown some considerable sensitivity to H2S gas at 300oC. Films

showed 87.30 % sensitivity to CO2 at 250oC and 52.50 % sensitivity to H2S gas

at 300oC.

208

Figure 6.8(a) Figure 6.8(b)

Figure 6.8 (c) Figure 6.8 (d)

Figure 6.8 (e) Figure 6.8 (f)

Figure 6.8: Variation of sensitivity of ZnO: Al thick films with operating

temperature at different doping concentrations of Al

209

210

CO2 sensor

Due to environmental concerns, carbon dioxide in the atmosphere needs to

be monitored and controlled regularly. The atmosphere contains 0.038% CO2

while the concentration of CO2 in exhaled air was approximately 6%. The

regulations from OSHA (Occupational Safety and Health Administration)

limited CO2 in the workplace to 0.5% for a prolonged period, and the U.S.

National Institute for Occupational Safety and Health limited brief exposures (up

to ten minutes) of CO2 to 3 % [43]. Recently, carbon dioxide (CO2) gas sensor

have attracted considerably attention for applications in greenhouse gases

(GHGs) monitoring. This type of sensors is also required for quality assurance of

carbon dioxide used in soft drinks, mineral waters and beers according to The

International Society of Beverage Technologists (ISBT) [44]. To assess the

potential for such applications we investigated screen printed ZnO: Al films as

sensing material for a novel carbon dioxide sensor.

The adsorption–desorption sensing mechanism is proposed on the base of

reversible chemisorptions of the carbon dioxide on the ZnO surface. It produces

a reversible variation in the resistivity with the exchange of charges between

carbon dioxide and the ZnO surface leading to changes in the depletion length

[23, 45]. Thus, one way to improve sensitivity is to increase the change in the

surface/volume ratio by adding Al like dopants [23, 46]. It is well known that

oxygen is adsorbed on a ZnO surface as O- or O2- by capturing electrons [23,

47].

When Al-doped zinc oxide gas sensor is exposed to gas, the reversible

chemisorptions can take place,

CO2 + V··o + 2e’ ↔ CO + Oo

x ..... (6.8)

where V··o is a double positive charged oxygen vacancies, e’ is negatively

charged electron, Oox is neutral oxygen in oxygen site according to the Kroger–

Vink notation [48].

It is obvious from the figure that operating temperature plays a vital role in

determining the response of the film. In fact, there exists an optimum operating

temperature of a sensor to achieve the maximum response to a gas of interest,

211

the temperature being dependent upon the kind of gases, i.e. the mechanism of

dissociation and further chemisorptions of a gas on the particular sensor surface

[49]. Also, the adsorption of atmospheric oxygen on the film surface depends

upon the operating temperature.

The temperature (thermal energy) at which the gas response is maximum is

the actual thermal energy required to activate the material for progress in

reaction. However the response decreases at higher operating temperature, as the

oxygen adsorbates are desorbed from the surface of the sensor [50]. Also at

higher temperature, the carrier concentration increases due to intrinsic thermal

excitation and Debye length decreases. This may be one of the reasons for

decreased gas response at higher temperature [51].

At a low operating temperature, the response of the films to CO2 is restricted

by the speed of the chemical reaction because the gas molecules do not have

enough thermal energy to react with the surface adsorbed oxygen species. In

fact, during adsorption of atmospheric oxygen on the film surface, a potential

barrier to charge transport is developed.

The higher response may be attributed to the optimum porosity and largest

effective surface area available to react with the gas. The response could be

attributed to the adsorption–desorption type of sensing mechanism. The amount

of oxygen adsorbed on the surface would depend on the number of Al or Al2O3

misfits to adsorb the oxygen which in turn would oxidize the exposed gas. When the optimum amount of Al (10 wt. %) is incorporated on the surface

of the ZnO film, Al or Al2O3 species would be distributed uniformly throughout

the surface of the film. As a result the initial resistance of the film is high and

this amount would also be sufficient to promote the catalytic reaction effectively

and the overall change in the resistance on the exposure of CO2 gas leading to an

increase in the sensitivity. When the amount of Al or Al2O3 on the surface of the

film is less than the optimum, the surface dispersion may be poor and the

sensitivity of the film is observed to be decreased since the amount may not be

sufficient to promote the reaction more effectively.

6.3.4.2 Selectivity of other gases against CO2

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

certain gas in the presence of other gases. Percent selectivity [52, 53] of one gas

over others is defined as the ratio of the maximum response of other gas to the

maximum response of the target gas at optimum temperature,

%100%arg

×⎟⎟⎠

⎞⎜⎜⎝

⎛=

gasett

gasother

SS

ySelectivit ..... (6.9)

Table 6.6 shows the sensitivity and corresponding selectivity of other gases

against CO2 at 250oC.

Table 6.6: Selectivity of ZnO: Al (10wt %) thick film for different gases

against CO2at 2500C

Gas Sensitivity in % Selectivity in % NH3 10.21 11.69 H2S 42.97 49.22 CO2 87.30 100

Ethanol 16.25 18.61 LPG 7.12 8.15 NO2 12.67 14.51

Figure 6.9 shows the selectivity of 10 wt. % Al doped ZnO thick films for

different gases against CO2 at 250oC. The selectivity of CO2 is considered as

100 %.

Figure 6.9: Selectivity of 10 wt.% Al doped ZnO thick films

212

6.3.4.3 Variation of sensitivity with gas concentration (ppm)

Figure 6.10 shows variation of sensitivity of 10 wt. % Al doped ZnO

thick films with CO2 concentration in ppm level at the optimum temperature

250oC.

Figure 6.10: Variation of sensitivity with gas concentration at 250oC

The 10 wt. % Al doped ZnO films were exposed to different concentrations

of CO2. The sensitivity values were observed to have increased by increasing the

gas concentration up to 1000 ppm. The response was highest for 1000 ppm of

CO2. From the above graph, it has been observed that the sensitivity of thick

films for CO2 at 250oC increases linearly with increase in gas concentration upto

500 ppm. Above 500 ppm the increase in sensitivity is slow and almost saturates

at 1000 ppm. Thus the active region of the sensor would be up to 500 ppm. At

lower gas concentrations, the unimolecular layer of gas molecules would be

formed on the surface of the sensor which could interact more actively giving

larger response. At the higher gas concentrations, the multilayer of gas

molecules may formed that would result into saturation in response beyond 500

ppm. The excess gas molecules would remain idle and would not reach the

surface active sites of the film. So, the response at higher concentrations of the

gas was not expected to increase in large extent.

6.3.4.4 Response and recovery time

The response and recovery times of 10 wt. % Al doped ZnO thick films

are represented in Figure 6.11. The response was quick (~ 25 sec) to 1000 ppm

of CO2 while the recovery was comparatively slow (~ 110 sec). The quick

213

response may be due to faster oxidation of gas. The aluminum species catalyses

the reaction promoting the rapid electron transfers between the adsorbate and

substrate.

Fig.6.11 Response and recovery of the 10 wt. % Al doped ZnO thick films at

250oC

214

215

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