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Studies on spray pyrolysis Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films for N[O.sub.2] gas sensing application.


Metal oxide semiconductor films are widely applied in gas sensors. The properties of such sensors including sensitivity, selectivity and response time depend critically on the film microstructure. A number of studies have shown that one promising approach to improve conductometric metal oxide sensors is to utilize semiconducting nanostructured composite materials consisting of metal oxides with different electronic structure and chemical properties . Studies of sensory phenomena in metal oxide composites have shown that there are certain optimum compositions for which these effects reach maximum values [1,2]. One of the important factors that determine the dependence of sensory phenomena in metal oxide composites on composition is the effect of composition on the morphology of the sensor film. Therefore, it is important to clarify the morphological features of the composite sensors that exhibit high sensitivity. These features depend on the nature of the components of the mixed metal oxide composite and the processing conditions The transparency of thin films so formed depends on parameters like substrate temperature, concentration of the precursor solution, spray duration, flow rate, pressure etc. and also affect the features of the thin film [3]. Transparent conductive oxide (TCO) thin films such as [In.sub.2][O.sub.3], ZnO, Sn[O.sub.2] and [In.sub.2][O.sub.3]: Sn[O.sub.2] are technologically important due to their high optical transparency in the visible region, wide band gap (in the range of 3.60-3.72eV for several pyrolysis temperature), photovoltaic devices [4] and good electrical conductivity. Among them, indium oxide thin film is one of the most significant TCO materials. [In.sub.2][O.sub.3] thin films are n-type semiconductors and has a tetragonal rutile structure [5].

Tin Oxide (Sn[O.sub.2]) is a n-type semiconductor with wide energy band gap (3.7 eV). Tin oxide thin films have some very beneficial properties, such as transparency for visible light, reflectivity for infrared light and a low electrical sheet resistance, making them suitable for a wide variety of applications as gas sensors, electrodes in solar cells, infrared reflectors for glass windows, transparent electrodes in electroluminescent lamps and displays etc because of its unique optical, catalytic, and electrical properties [6].

Metal oxide materials can be used as gas sensors, when heated to temperatures between 250[degrees]C and 550[degrees]C. At these temperatures, oxygen is adsorbed at the metal oxide surface by trapping electrons from the bulk material. The result is an overall decrease or increase in the metal oxide resistance, depending on whether the material is n-type or p-type, respectively [7]. By increasing the electron concentration in the material, thereby decreasing the electrical resistance. This change in resistance serves as sensing signal. The main drawback of tin oxide however is its low selectivity towards reducing gases and thus cross sensitivity between these gases is one of the major problems [8].

Tin dioxide or indium doped tin dioxide is the most used n-type semiconductor in gas sensing devices because of its capabilities to detect inflammable gases like C[H.sub.4], [H.sub.2], [C.sub.2][H.sub.5]OH, CO and so on [9]. Besides, tin-indium oxide (Sn[O.sub.2]-[In.sub.2][O.sub.3]) nano composites that exhibited superior thermal stability against grain growth have been reported [10]. It is difficult to control the size and morphology of the oxide composites, which have important influence on their physical and chemical properties.

In this study in order to study the structural, morphological, optical, electrical and gas sensing characteristics of spray pyrolysis Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films with (5, 10, 15 and 20)% ml of indium oxide which were prepared on glass substrate at temperature 300[degrees]C, various approaches have been employed to enhance the selectivity of sensors, which include the manipulation of sensing temperature.

II-Experimental Part:

Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films were deposited by the chemical spray pyrolysis technique from aqueous solutions containing tin chloride pentahydrate and indium chloride with molarities (0.1M) as a precursor using compressed air at pressure 1 bar as a carrier gas. The indium chloride and tin chloride solutions were prepared using the following procedure:
V (ml)Sn[O.sub.2]   V(ml)[In.sub.2][O.sub.3]

50                             0
45                             5
40                             10
35                             15
30                             20

Glass slides cut in small pieces (1x1) [cm.sup.2] were used as a substrate on which films are grown and cleaned using ethanol and distilled water, then these glass slides were ultrasonically cleaned. The substrates were then placed on the substrate heater of the spray equipment to provide proper heating with uniformity to films. The temperature for decomposition of Sn[O.sub.2]: [In.sub.2][O.sub.3] is 300[degrees]C. The films formed are of nanoparticle regime.

The optical transmission was recorded using a UV-Visible double-beam spectrophotometer (UV-265 Shimadza) between 300-1100 nm wavelength range. The x ray diffraction data were recorded on a Rigaku Miniflex X ray diffractometer using Cu[K.sub.[alpha]] radiation source ([lambda] = 1.5414 [Angstrom]) at 2[theta] values between 20-80[degrees] at room temperature. The accelerating voltage of 30 KV, emission current of 20 mA and scanning rate of 0.05% were used. The morphology of the films was detected by using AFM model (AA3000 Scanning Probe Microscope SPM, tip NSC35/AIBS from Angstrom Ad-Vance Inc).

The values of carrier concentration ([n.sub.H]) and Hall mobility ([[mu].sub.H]) were calculated using equations [11]:

[n.sub.H] = [1/[R.sub.H]e] (1)

[[mu].sub.H] = [sigma][absolute value of [R.sub.H]] (2)

where [R.sub.H] = [[V.sub.H]/I] [t/[B.sub.z]] (3)

[R.sub.H] is Hall coefficient, [V.sub.H] is Hall voltage, t is the sample of thickness, I is constant current, [sigma] is conductivity, and [B.sub.z] is magnetic field.

Gas response (S) is given by [12]:

S = ([G.sub.g] - [G.sub.a])/[G.sub.a] x 100 (4)

where [G.sub.a] and [G.sub.g] is the conductance of the sensor in air and gas respectively.

In this work, the gas responsitivity tests performed at room temperature. The time taken for the sensor to attain 90% of the maximum decrease in resistance on exposure to the target gas is the response time. The time taken for the sensor to get back 90% of original resistance is the recovery time. The test was performed at various sensing temperatures with 6 V bias voltage.


III-1--Structural properties:

Figure 1 show XRD patterns of the Sn[O.sub.2]: [In.sub.2][O.sub.3] (0, 5, 10, 15, 20) ml prepared by spray pyrolysis method on glass substrates. From the above XRD patterns it is clear that with increasing indium concentration in the mixing ratios, new values appear in the structure of the film formed [13].

At higher mixed ratio of indium oxide peaks become evident, indicating that it has formed a separate phase. Indium is much closer in size to a tin atom and will more easily substitute and dope into the Sn[O.sub.2] crystal lattice. The intensity of other peaks is small; it indicates that our films are textured, it is indicate that the change of predominant orientation of crystallites, confirms the cassiterite structure of crystalline Sn[O.sub.2] [14].

Table 1 show the grain size for (211) peak for Sn[O.sub.2]:[In.sub.2][O.sub.3] thin films.

Typical AFM images of Sn[O.sub.2]: [In.sub.2][O.sub.3] films deposited at different concentration of indium are shown in Figure 2. As shown in figure, the concentration of indium affects the surface morphology of the deposited films.

It is well understood that AFM depends on the structure and the stoichiometry of the prepared films [1]. AFM parameters contain average diameter, average roughness, average RMS roughness value for these samples have been shown in table 2.

The effect of the indium concentration on the average grain size is clearly depicted, the increasing of the mixed concentration cause decrease the average grain size.

III-2--Optical properties:

Figure 3 shows the optical transmittance curves as a function of the wavelength for Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films deposited at substrate temperature 300[degrees]C. There was a shift in the absorption edge to shorter wavelength for the optimum film, which was in accordance with Burstein- Moss shift [13].

The value of the optical energy gap calculated using the standard fundament absorption. Optical energygap of these thin films deposited on glass substrate, was calculated from the intercept on energy axis obtained by extrapolating the linear portion of the Tauc plot of [([alpha]hv).sup.2] vs photon energy (hv) as shown in Figure 4.

It can be observe that increasing of [In.sub.2][O.sub.3] concentration from 5 to 20 leads to small decrease in the optical band gap. As the size of semiconductor particles decreases to the nanoscale, the band gap of the semiconductor decreases, causing a blue shift in the UV-Vis absorption spectra due to quantum confinement [14,15]. Table 3 show the optical energy gap ([E.sub.g.sup.opt]) at different concentrations of [In.sub.2][O.sub.3].

III-3--Electrical properties (Hall effect):

The type of charge carriers, concentration (nH) and Hall mobility (pH), have been estimated from Hall measurements for un-doped Sn[O.sub.2] and mixed with [In.sub.2][O.sub.3] films at different concentration which were deposited on glass substrates at 300[degrees]C using spray pyrolysis technique and shown in table 4.

The Hall effect measurements were determined using room temperature in the van der Pauw configuration [16]. The positive sign of the Hall coefficient confirms the p-type conductivity. Electrical properties as shown semi-stable with the change of mixing ratio and the stability of the temperature. It could also relate to an improvement of crystallinity leading to a decrease of donor sites trapped at the dislocations and grain boundaries [17].

III-4--Gas sensors:

Response of sensors depends on two factors, namely: the speed of chemical reaction on the surface of the grains, and the speed of the diffusion of gas molecules to that surface. At low temperatures the sensor response is restricted by the speed of chemical reactions. At higher temperature the sensor response is restricted by the speed of the diffusion of gas molecules to that surface. At some intermediate temperature the speed values of two processes become equal, and at that point the sensor response reaches its maximum. According to this mechanism for every gas there is a specific temperature at which the sensor response attains its peak value [18]. Figure 5 shows the Sensing characteristics of Sn[O.sub.2] and [In.sub.2][O.sub.3] thin film towards N[O.sub.2] gas.

The optimum operating temperature for the Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films N[O.sub.2] gas sensor was found to be around 300[degrees]C.

3-4-1-Response and recovery of the sensor:

Figure 6 and 7 shows the relation between the response time and the Recovery time as a function of operation temperature at different etching time for the pure Sn[O.sub.2] and their compositions with (45:5,40:10,35:15,30:20) for Sn[O.sub.2]: [In.sub.2][O.sub.3] deposited on glass with 3 % N[O.sub.2]: air and bias voltage of 6V.

The high oxidizing ability of adsorbed oxygen species on the surface particles and high volatility of desorbed by-products explain the quick response to [No.sub.2] and fast recovery.

The reveals that the decrease of response/recovery time with increasing of operation temperature. The figure show that the (10 min) etching time sample exhibits a fast response speed of (29.82s) and recovery time (10.72s) at 300[degrees]C operation temperature. This revealed that a (10min) etching time is the best one to achieve fast response sensor. The gas responsitivity tests performed at room temperature showed lowest variation on the film conductivity, The gradual increase in the operating temperature led to an improvement of the films responsitivity [19]. There is an increase and decrease in the sensitivity indicates the adsorption and desorption phenomenon of the gas. The higher sensitivity may return to the optimum number of inequality on the porosity, largest surface area, larger rate of oxidation and the optimum surface roughness [20]. The sensitivity as well as response time depend on operating temperature since the chemical kinetics in solid-gas reaction is governed by the dependence of temperature [21].

The values of Response time, recovery time and sensitivity of un-doped Sn[O.sub.2] and mixed with different ratio of [In.sub.2][O.sub.3] are shown in table 5.


The Sn[O.sub.2]: [In.sub.2][O.sub.3] composition thin films were successfully deposition on glass substrates by chemical spray pyrolysis technique. The structural and microstructural properties confirm that the as-prepared films are polycrystalline in structure and nanocrystalline in nature. The energy gap increased with the increase of [In.sub.2][O.sub.3] percentage of the composition . Gas sensor measurement of pure Sn[O.sub.2] and mixed have high resistivity for N[O.sub.2]. The nanocrystalline Sn[O.sub.2]: [In.sub.2][O.sub.3] composition thin films exhibits rapid response-recovery which is one of the main features of this sensor. The results obtained by chemical spray pyrolysis technique are promising for the preparation of sensitive and low cost gas sensor operating at low temperatures. Advantages of this method for deposition of gas sensing metal oxide films will be used in future work.


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(1) Othman Abad Fahad, (1) Hamid S. Al-Jumaili and (2) Mahdi Hasan Suhail

(1) Dept. of physics, College of Education for pure science, University of anbar-Iraq

(2) Dept. of physics, college of science, University of Baghdad-Iraq

Address For Correspondence:

Mahdi Hasan Suhail, Dept. of physics, college of science, University of Baghdad-Iraq


Received 12 August 2016; Accepted 17 December 2016; Available online 22 December 2016

Caption: Fig. 1: show the XRD of Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films for different concentration of indium.

Caption: Fig. 2: Typical AFM images of Sn[O.sub.2]: [In.sub.2][O.sub.3] films deposited at different concentration of indium

Caption: Fig. 3: transmittance curves for Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films deposited at substrate temperature 300[degrees]C.

Caption: Fig. 4: Optical energy gap for Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films deposited with different percentage.

Caption: Fig. 5: shows the Sensing characteristics of Sn[O.sub.2]: [In.sub.2][O.sub.3] thin film towards N[O.sub.2] gas.

Caption: Fig. 6: The variation of Response time with operating temperature of Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films for N[O.sub.2] gas.

Caption: Fig. 7: The variation of Recovery time with operating temperature of Sn[O.sub.2]: [In.sub.2][O.sub.3] thin films for N[O.sub.2] gas.
Table 1: XRD parameters for (211) peak Sn[O.sub.2]:
[In.sub.2][O.sub.3] thin films.

sample of Sn[O.sub.02]:   2[theta]   FWHM     [d.sub.hkl] Exp.
[In.sub.2][O.sub.3]       (Deg.)     (Deg.)     ([Angstrom])

pure                      51.8593    0.4020        1.7616
45:5                      51.5443    0.3650        1.7716
40:10                     51.5863    0.8223        1.7703
35:15                     51.1847    0.8664        1.7832
30:20                     52.3842    0.9576        1.7452

sample of Sn[O.sub.02]:   G.S (nm)    hkl      [d.sub.hkl]
[In.sub.2][O.sub.3]                          Std.([Angstrom])

pure                        22.0     (211)        1.7642
45:5                        24.2     (211)        1.7642
40:10                       10.7     (211)        1.7642
35:15                       10.2     (211)        1.7642
30:20                       9.2      (211)        1.7642

Table 2: average diameter, average roughness and average R.M.S.
roughness value for Sn[O.sub.2]: [In.sub.2][O.sub/3] Thin Film.

sample of Sn[O.sub.2]:   Average     Average      R.M.S
[In.sub.2][O.sub.3]      diameter   roughness   Roughness
                           (nm)       (nm)        (nm)

45 : 5                    80.96       3.58        4.31
40 : 10                   102.06      4.39        5.33
35 : 15                   100.38      5.83        6.83
30 : 20                   96.68       4.04        4.98

Table 3: Optical energy gap variation of Sn[O.sub.2] mixing with
[In.sub.2][O.sub.3] mixed ratio.

sample of Sn[O.sub.2]:   [E.sub.g.sup.opt]
[In.sub.2][O.sub.3]            (eV)

45 : 5                         3.50
40 : 10                        3.30
35 : 15                        3.39
30 : 20                        3.40

Table 4: show the Hall coeficient ([R.sub.H]), concentration
([n.sub.H]), conductivity ([[sigma].sub.D.c]) and Hall mobility

Sample of      [R.sub.H]        [n.sub.H]
Sn[O.sub.2]:   ([cm.sup.3]/c)   ([cm.sup.-3])
[In.sub.2]     X[10.sup.6]

45: 5          1.877            3.326 x [10.sup.+12]
40: 10         0.0068           9.242 x [10.sup.+15]
35: 15         1.831            3.409 x [10.sup.+24]
30: 20         0.0575           1.086 x [10.sup.+13]

Sample of      [[sigma].sub.D.c]   [[micro].sub.H]
Sn[O.sub.2]:   at R.T              ([cm.sup.-2]/
[In.sub.2]                         v.sec)

45: 5          3.376 x [10.sup.-3]   6336
40: 10         2.149 x [10.sup.-1]   145.2
35: 15         2.539 x [10.sup.+7]   46.49
30: 20         2.761 x [10.sup.-5]   15.87

Table 5: sensitivity, response time and recovery time of un-doped
Sn[O.sub.2] and mixed with different ratio of [In.sub.2][O.sub.3].

Sensitivity %

Sample of             0       5 mL     10 mL    15mL     20 mL

RT                    15.79   2.641    2.436    0.372    0.519
100 ([degrees]C)      20.68   2.236    1.289    0.809    1.935
200 ([degrees]C)      32.42   9.270    2.179    2.404    7.227
300 ([degrees]C)      20.65   33.632   48.199   24.160   12.300

Response time

RT                    28.6    25.7     39.0     38.6     33.4
100 ([degrees]C)      17.5    30.0     22.0     36.1     29.2
200 ([degrees]C)      15.1    20.5     21.7     22.3     34.2
300 ([degrees]C)      17.9    21.1     20.1     24.5     27.8

Recovery time

RT                    67.2    55.      59.0     44.1     57.7
100 ([degrees]C)      53.7    41.      60.2     45.5     47.8
200 ([degrees]C)      41.7    57.      31.8     29.9     36.6
300 ([degrees]C)      44.6    42.      52.3     47.7     41.8
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Author:Fahad, Othman Abad; Jumaili, Hamid S. Al-; Suhail, Mahdi Hasan
Publication:Advances in Environmental Biology
Article Type:Report
Date:Dec 1, 2016
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