Influence of In doping on the structural, optical and acetone sensing properties of ZnO nanoparticulate thin films

Materials Science in Semiconductor Processing 16 (2013) 200–210

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Influence of In doping on the structural, optical and acetone sensing properties of ZnO nanoparticulate thin films C.S. Prajapati, P.P. Sahay n Department of Physics, Motilal Nehru National Institute of Technology, Allahabad 211 004, India

a r t i c l e in f o

abstract

Available online 19 May 2012

Indium-doped zinc oxide (ZnO) nanoparticle thin films were deposited on cleaned glass substrates by spray pyrolysis technique using zinc acetate dihydrate [Zn(CH3COO)2 2H2O] as a host precursor and indium chloride (InCl3) as a dopant precursor. X-ray diffraction results show that all films are polycrystalline zinc oxide having hexagonal wurtzite structure. Upon In doping, the films exhibit reduced crystallinity as compared with the undoped film. The optical studies reveal that the samples have an optical band gap in the range 3.23–3.27 eV. Unlike the undoped film, the In-doped films have been found to have the normal dispersion for the wavelength range 450–550 nm. Among all the films investigated, the 1 at% In-doped film shows the maximum response 96.8% to 100 ppm of acetone in air at the operating temperature of 300 1C. Even at a lower concentration of 25 ppm, the response to acetone in this film has been found to be more than 90% at 300 1C, which is attributed to the smaller crystallite size of the film, leading to sufficient adsorption of the atmospheric oxygen on the film surface at the operating temperature of 300 1C. Furthermore, In-doped films show the faster response and recovery at higher operating temperatures. A possible reaction mechanism of acetone sensing has been explained. & 2012 Elsevier Ltd. All rights reserved.

Keywords: ZnO nanoparticulate thin films Structural and optical properties Acetone sensing properties

1. Introduction Zinc oxide (ZnO), an n-type semiconductor with wide band gap of 3.37 eV at room temperature (300 K) and high exciton binding energy of 60 meV [1], has been shown to be one of the most attractive materials for gas sensor applications due to its thermal and chemical stability, abundance in nature, low cost and absence of toxicity [2–6]. ZnO has been exploited in various forms such as single crystal, sintered pellets, thick films, thin films and hetero-junctions [7–11]. More recently, this material has received a growing attention as a nanostructured material [12–15]. Morphology, size and size distribution of ZnO nanoparticles play an important role in deciding the

n

Corresponding author. Tel.: þ 91 532 2271260; fax: þ91 532 2545341. E-mail address: [email protected] (P.P. Sahay).

1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.04.015

properties of nanoparticulate thin films. One important method to modify the characteristics of the films is the introduction of dopants in the parent system, which, in turn, influences the performance of the gas sensors based on these films. Various dopants like Al, In, Cu, Sn, etc. have been used to improve sensitivity and selectivity performance of the gas sensors based on ZnO thin films [16–20]. Acetone has been in common use as a solvent and as an extracting regent in industry for several decades. Its speedy evaporation and toxic nature can make high concentrations in air dangerous to human health. At high concentration ( 410, 000 ppm) in air, acetone may cause such symptoms as cephalalgia, nausea and so on. Further, Acetone has been identified as a biomarker for type-I diabetes (T1D) [21]. In future, the possibility exists to use an acetone sensor as a means of monitoring the diabetic patient condition. Thus, there is the need of development of a reliable and selective acetone sensor. To the best of

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

our knowledge, the sensing behavior of the In-doped ZnO thin films towards acetone vapor has not yet been reported. Hence the study of acetone sensing properties of In-doped ZnO thin films has been carried out and reported in this paper. In addition, the structural and optical properties of these films have also been studied. The sensing responses of the undoped and doped ZnO materials towards acetone have been studied earlier by many researchers. Ge et al. [22] have investigated the CeO2-doped ZnO thin films for acetone sensing and found the maximum sensitivity ((Ra/Rg)¼23) at 325 1C for 100 ppm acetone concentration in air. Liu et al. [23] have reported the maximum response ((Ra/Rg)¼16) at 360 1C for 100 ppm acetone in case of the 0.5 wt% Co-doped ZnO nanofibres. Qia et al. [24] have found the maximum sensitivity ((Ra/Rg)¼16) at 300 1C for 50 ppm acetone in case of the dumbbell-like ZnO microcrystals. Maximum response ((Ra/Rg) ¼30.4) to acetone at 300 1C by the ZnO nanorods have been reported by Zeng et al. [25]. Fan and Jia [26] have reported selective detection of acetone and gasoline by temperature modulation in zinc oxide nanosheets sensors. Several deposition techniques have been used to grow the undoped and doped ZnO thin films which include thermal evaporation [27], sputtering [28,29], chemical vapor deposition [30], spray pyrolysis [31–33], the sol– gel method [34], dip coating [35], SILAR [36], etc. In the present investigation we have used the spray pyrolysis technique to deposit the ZnO thin films as this technique is simple and involves low cost equipment and raw materials. In this technique, a starting solution, containing the Zn and dopant precursors, is sprayed over a hot substrate. When the fine droplets arrive at the hot substrate, they decompose to form the ZnO thin films. The quality and the physical properties of the films depend on the deposition parameters such as substrate temperature, molar concentration of the starting solution, spray rate, type and rate of the carrier gas and geometric characteristics of the spray system. 2. Experimental details ZnO thin films were prepared on glass substrates (micro slides) by employing the spray pyrolysis technique. The precursor used was zinc acetate dihydrate [Zn(CH3COO)2 2H2O]. The indium chloride (InCl3) was the doping source. The mixture was dissolved in deionized water. The concentration of zinc acetate was 0.1 M and the In/Zn ratio in the starting solution was varied from 0 to 2 at%. The substrates were cleaned prior to deposition, firstly by freshly prepared chromic acid followed by deionized water, and then ultrasonically cleaned by trichloroethylene for about 30 min. After this, they were washed with deionized water again and finally dried in air. A schematic representation of the spray apparatus has been described elsewhere [15]. The precursor solution was sprayed through a locally designed glass nozzle over the hot substrates with pressurized air being the carrier gas. The various process parameters for the film deposition have been listed in Table 1. The thicknesses of the films were estimated by weight-difference method using an electronic precision balance (Citizen, Model: CX165).

201

Table 1 Process parameters for the spray deposition of the In-doped ZnO thin films. Spray process parameters

Optimum value/item

Substrate temperature Solvent Nozzle Substrate-nozzle distance Carrier gas pressure Solution flow rate Precursor solution concentration

400 710 1C Deionized water Glass 25 cm 3.2 kg/cm2  2 ml/min 0.1 M

Structural analyses of the films were carried out using ˚ PANalytical X’Pert PRO with CuKa radiation (l ¼1.5406 A) as the X-ray source at 30 mA, 30 kV, the scanning angle 2y varying from 301 to 651 at a scan speed of 0.021 per second. Atomic force microscopy (AFM) was used to analyze surface topography of the films, which gives the grain size, structure and surface roughness of the films. The AFM images of the films were acquired using a NTEGRA scanning probe microscope. The optical studies of the films were carried out with the help of Perkin Elmer Lambda 35 UV–vis spectrometer (UK) in the spectral range 350–700 nm. For electrical measurements, high conducting silver paste was applied on both ends of the films for making ohmic contacts. The acetone sensing properties of the films were carried out in an experimental setup shown in Fig. 1. The experimental setup was so designed that there was a complete dry air (free from humidity) in the surrounding areas of the experimental films to be examined. Therefore, the effect of humidity on the sensitivity characteristics of the films has not been taken into account in the present investigation. The film was mounted on a two-probe assembly placed into a silica tube, which was inserted coaxially inside a resistance-heated furnace. The electrical resistance of the films was measured before and after exposure to acetone vapor using a Keithley System Electrometer Model 6517B. The measurement of acetone concentration was carried out by taking the required amount of liquid acetone in a Hamilton micro syringe and then injecting it into the enclosure. The response of the film towards acetone vapor was studied at different operating temperatures in the range 200–300 1C for various concentrations ranging from 25 ppm to 100 ppm in air. 3. Results and discussion All the films were found to be almost clear and transparent in physical appearance, and well adherent to the glass substrates. The as-deposited films were annealed at 500 1C for 1 h to obtain highly uniform crystalline films. The film thicknesses were found to be in the range 250–300 nm. The films were then subjected to structural, optical and acetone sensing studies. 3.1. Structural and morphological characteristics Fig. 2 shows the XRD spectra of the undoped and Indoped ZnO thin films. The observed XRD patterns have

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Fig. 1. Experimental setup for acetone sensing studies.

300 In (2at%)_ZnO

JCPDS Card No.: 05-0664 (002) (101)

200

0 300

(110)

(102)

(101) (101)

ZnO

100

(103)

(102)

200

(110)

(100)

0 300

(002)

100

(002)

In (1at%)_ZnO

200

(100)

Intensity (a.u.)

100

0 30

40

50 2θ (Degree)

60

Fig. 2. XRD spectra of the undoped and In-doped ZnO thin films.

been found to match with JCPDS card (zinc oxide, 05–0664). XRD analyses confirm that all the films are polycrystalline zinc oxide possessing hexagonal wurtzite structure. No phase corresponding to indium or other indium compound is observed in the XRD spectra. Undoped zinc oxide film exhibits three prominent peaks corresponding to (100), (002) and (101) planes, in addition to weak intensities peaks at (102), (110) and (103). It is observed that the intensities of all the peaks are diminished strongly in the indium doped films, indicating their lower crystallinity as compared with the undoped zinc oxide film. Similar tendency has been observed earlier by many researchers [37–39]. Poor crystallinity in the indium doped films is due to the fact that the In incorporation in film network enables more nucleation sites which, in turn, inhibit the growth of crystal grains, resulting in an

increase in the lattice strain. The texture coefficient has been calculated to describe the preferential orientation (hkl) using the following relation [40]: TCðhklÞ ¼

IðhklÞ=Io ðhklÞ P : ð1=NÞ N IðhklÞ=Io ðhklÞ

ð1Þ

where N is the number of diffraction peaks, I(hkl) and Io(hkl) are, respectively, the measured and corresponding recorded intensities according to JCPDS (05–0664) card. The variations of TC along various lattice planes at different In concentrations are listed in Table 2. The highest TC value for (002) and (101) planes has been found for the 1 at% In-doped film while for other lattice planes it is more for the undoped ZnO film. As the In doping concentration is increased further, the value of TC goes down showing randomly oriented crystallites in the

3.37 5.2271 3.2775

24

7.0 5.2450 3.2834

27

7.65

Average crystallite size: 19.68 nm

Average crystallite size: 21.04 nm

0. 034 0.025 0.030 0.060 0.067 0.071 0.058 0.037 0.050 0.090 0.134 0.064 0.054 Average crystallite size: 33.54 nm

1.4098 1.6597 1.0278 0.3849 1.0165 0.5013 0.6101 2.3292 1.6865 0.2370 0.6192 0.435 1.000 100 002 101 102 110 103 100 002 101 102 110 002 101 2.8160 2.6020 2.4760 1.9110 1.6260 1.4770 2.8160 2.6020 2.4760 1.9110 1.6260 2.6020 2.4760 2.8013 2.5906 2.4667 1.9042 1.6226 1.4747 2.8411 2.6203 2.4930 1.9248 1.6328 2.6135 2.4944 2 at% In

1 at% In

31.921 34.596 36.390 47.722 56.682 62.981 31.463 34.192 35.995 47.179 56.300 34.313 35.978 Undoped (0 at% In)

203

film. In case of the undoped film, the most intense peak has been found at (1 0 1) plane whereas on doping with 1 at% indium, the most intense peak (but much lower in intensity compared to the undoped film) is observed at (002) plane. Similar results have been reported by Joseph et al. [41]. Another effect of In doping in ZnO film is the shift of the (002) and (101) peak positions towards lower values of 2y compared to the peak position observed in the undoped film. Such a decrease in 2y arises due the fact that the ionic radii of Zn2 þ and In3 þ are 0.074 and 0.094 nm, respectively and therefore the lattice spacing (d) between the planes corresponding to (002) and (101) is expected to increase when the In3 þ ions are substituted into the Zn2 þ sites in the films. This has been found in agreement with the XRD results where d-spacing of the lattice planes in the indium doped film increases. The lattice constants, a ( ¼b) and c, have been calculated from the prominent peaks using the formula [42]: 1

30.72 45.47 40.20 28.02 31.67 25.16 17.48 30.17 23.58 18.35 15.64 17.60 21.75

Crystallite size [nm] TC hkl d-spacing ˚ (JCPDS) [A] ˚ observed [A]

d-spacing

Positions [12h]

XRD analyses ZnO films

Table 2 X-ray diffraction and AFM analyses of the In-doped and undoped ZnO thin films.

36 5.1821 3.2348

Average grain size (nm) ˚ c [A] ˚ a¼ b [A] Lattice strain [%]

AFM analyses

Average surface roughness (nm)

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

2

dhkl

2

¼

4 l 2 2 ðh þ hk þk Þ þ 2 3a2 c

ð2Þ

The crystallite size (D) and the lattice strain (e) of the deposited films have been determined using the Debye– Scherrer formula (3) [42] and the tangent formula (4) [42]: D¼

e¼

0:9l ðb cos yÞ

b 4 tan y

ð3Þ

ð4Þ

˚ y is the where l is the X-ray wavelength equal to 1.5406 A, Bragg diffraction angle and b (radians) is the full-width at half maximum. The factor 0.9 is a typical value of the dimensionless shape factor which varies from 0.87 to 1 depending upon the shape of the crystallites. The lattice constants, the crystallite size and the lattice strain of the films thus obtained are listed in Table 2, the average crystallite size being included in vertical. It is observed that the crystallite size of the film decreases on In doping. This is due to enhancement in the densities of nucleation centers in the doped film which results in the formation of smaller crystallites [43]. The lattice constants, a ( ¼b) and c, are found to increase with the indium doping. Similar variation in lattice constants on In doping has been reported by Singh et al. [38]. Further, on doping with indium, the film undergoes the lattice disorder due to substitution of the In3 þ ions into the Zn2 þ sites, leading to an increase in the lattice strain (listed in Table 2). Fig. 3(a–c) shows the two-dimensional (2D) and threedimensional (3D) AFM images of the undoped, 1 at% and 2 at% In-doped films, respectively. The 2D images show that the films are uniform and the substrate surface is well covered with grains that are almost uniformly distributed over the surface. The 3D images exhibit large nicely separated conical columnar microstructure in the undoped and 1 at% In-doped films whereas coalescence of some columnar grains are seen in the 2 at% In-doped film. The average surface roughness and the average grain size, listed in Table 2, have been found to be maximum for the

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Fig. 3. Two-dimensional (2D) and three-dimensional (3D) AFM images of the undoped (a) 1 at% (b) and 2 at% (c) In-doped ZnO thin films.

undoped film and decreases with In doping. Surface roughness is directly proportional to the gas sensitivity of the film since larger roughness results in larger contact area with gaseous species [44]. Another feature associated with the columnar microstructure, which is also of crucial importance in the gas sensing applications, is the presence of voids. A few black patches observed in the 2D images of the undoped and 1 at% In-doped films corresponds to the presence of some voids on the film surface. 3.2. Optical properties The optical transmission and reflectance spectra of the films in the wavelength range 350–700 nm are shown in Fig. 4(a, b). Among all the films, the 1 at% In-doped film shows the highest transmittance ( 480%) and reflectance throughout the wavelength range. High optical transmission in the indium doped films has been reported earlier by many researchers [45,46]. Higher reflectance exhibited by the 1 at% In-doped film may be due to its larger refractive index. Usually, the optical absorption at absorption edge corresponds to the transmission from valence band to conduction band, while the absorption in the visible region corresponds to some localized energy states in the band gap. For allowed electronic transition in materials, the absorption coefficient a is given by [47]

ahn ¼ kðhn2Eg Þp

forbidden cases it will be 3 or more. Eg is the optical band gap and k is a constant given by [47] 2

k ¼ ½e2 =ðpncmne h Þð2mr Þ3=2

ð6Þ

n

where me and mr are the effective and reduced masses of charge carriers, respectively. To decide whether the material is a direct or indirect band gap, (Ahu)2 versus hu or (Ahu)1/2 versus hu of a given material are traced, where A is the absorbance. The optical band gap is obtained by extrapolating the linear portion of the plot to the energy axis. The plots of (Ahu)2 against hu for the In-doped and ZnO thin films are shown in Fig. 5. It has been found that the optical band gap is reduced with In doping. This band gap narrowing is due to the increase in the band tail (Urbach tail) width which is related to the lattice disorder in the film network. Since the ionic radius of In is larger than that of Zn, the In introduction into the film is followed by the lattice distortion and consequently disorder creation which gives rise to the localized states near the conduction band in the energy band gap. The absorption coefficient a near the fundamental absorption edge is found to be exponentially dependent on the incident photon energy and obeys the well-known Urbach relation expressed as [48]   hu a ¼ ao exp ð7Þ Eo

ð5Þ

where p has discrete values like 1/2, 3/2, 2, or more depending on whether the transition is direct or indirect, and allowed or forbidden. In the direct and allowed cases p ¼1/2 whereas for direct but forbidden cases it is 3/2. But for the indirect and allowed cases p ¼2 and for the

where ao is a constant and Eo is a parameter describing the width of the tail of localized state in the band gap. In terms of absorption, the Eq. (7) can be written as   hu A ¼ Ao exp ð8Þ Eo

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

-1.0

ZnO ; Eo = 0.094 eV In (1at%)_ZnO ; Eo = 0.112 eV In (2at%)_ZnO ; Eo = 0.355 eV

100 -1.1

80

-1.2

70

-1.3

60

lnA

Transmittance (%)

90

50

-1.4 -1.5

40 30

-1.6

ZnO In (1at%)_ZnO In (2at%)_ZnO

20

-1.7

10 0 350

205

-1.8

400

450

500 550 600 Wavelength, λ (nm)

650

3.22

700

3.23

3.24

3.25 hν (eV)

3.26

3.27

3.28

Fig. 6. Plots of ln A versus hu.

16 ZnO In (1at%)_ZnO In (2at%)_ZnO

2.1

12 Refractive index, n

Reflectance (%)

14

10 8 6

ZnO In (1at%)_ZnO In (2at%)_ZnO ____ Cauchy fit

2.0

1.9

1.8

4 350

400

450

500 550 600 Wavelength, λ (nm)

650

700 450

Fig. 4. (a) Optical transmission and (b) reflectance spectra of the undoped and In-doped ZnO thin films.

60

(Ahν)2 X 109 (eV/cm)2

500 525 wavelength, λ (nm)

550

Fig. 7. Variation of refractive index with wavelength.

The refractive index, n, has been determined from reflectance data using the relation [47]:   n1 2 R¼ ð9Þ nþ1

ZnO ;Eg = 3.27 eV In (1at%)_ZnO ;Eg = 3.26 eV In (2at%)_ZnO ;Eg = 3.23 eV

50

475

40

Fig. 6 depicts the variation of refractive index of the films with wavelength in the range 450–550 nm. The dispersion of the refractive index is fitted to the Cauchy relation:

30 20 10

n ¼ aþ 0 -10 1.5

2.0

2.5

3.0 hν (eV)

3.5

4.0

Fig. 5. Plots of (Ahu)2 versus hu.

where Ao is another constant. Eo is estimated from the slope of the linear relationship ln A against hu, shown in Fig. 5. It is observed that the width of the localized states increases with In doping.

b l2

ð10Þ

where a and b are Cauchy’s constants and l is the wavelength of the light used. It is observed that the refractive index of the 1 at% In-doped film satisfies the relation 1.73þ6.1  104/l2 while that of the 2 at% doped film follows the relation 1.66þ 4.2  104/l2. This indicates that the Indoped films have the normal dispersion for the wavelength range 450–550 nm. However, no such normal dispersion is observed in case of the undoped film. The solid curve in Fig. 7 represents the Cauchy fit. The refractive index (n¼1.80–2.04) of the indium doped films in the visible

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region determined in the present investigation is consistent with the results reported by other authors [45]. 3.3. Acetone sensing properties On injecting the required amount of acetone in the gas sensing enclosure to achieve the desired concentration, the film resistance was found to decrease. When the enclosure was opened i.e., the film was exposed to air, the film resistance recovered its original value. In the present study, the sensor response (S) of the film has been defined as the ratio of change in film resistance upon exposure to acetone to the film resistance in air (at the same operating temperatures) and is given by the equation: S¼

Ra Rg  100% Ra

ð11Þ

where Ra is the film resistance in air and Rg is the resistance upon exposure to acetone vapor. The response characteristics of the undoped, 1 at% and 2 at% In-doped ZnO thin films as a function of the

operating temperature at four different concentrations viz. 25, 50, 75 and 100 ppm of acetone vapor in air are shown in Fig. 8(a–c). At a low operating temperature of 200 1C, the response of the film to acetone is restricted by the speed of the chemical reaction because the acetone molecules do not have enough thermal energy to react with the surface adsorbed oxygen species. In fact, due to adsorption of atmospheric oxygen on the film surface, an electron-depleted space-charge layer is produced in the surface region of the film, causing a potential barrier to charge transport. At operating temperatures beyond 200 1C, the responses of all the films have been found to increase with the increasing operating temperature except in case of the undoped film where the response to 75 ppm acetone concentration first attains a maximum at 275 1C and then decreases with a further rise in the operating temperature. Increase in response with the operating temperature is attributed to the enhanced surface adsorbed oxygen species at the higher operating temperatures, which react effectively with the acetone vapor molecules. Due to this chemical reaction, electrons

100

100

Undoped ZnO

90

25ppm 50ppm 75ppm 100ppm

80

In (1at%)_ZnO 25ppm 50ppm 75ppm 100ppm

80 Response (%)

Response (%)

70 60 50 40 30 20

60

40

20

10 0

0 200

275 225 250 Operating temperature (°C)

300

200

240 260 220 280 Operating temperature (°C)

300

100 In (2at%)_ZnO

90 80

25ppm 50ppm 75ppm 100ppm

Response (%)

70 60 50 40 30 20 10 0 200

225 250 275 Operating temperature (°C)

300

Fig. 8. (a–c): Response characteristics of the (a) undoped,and (b) 1 at% and (c) 2 at% In-doped ZnO thin films as a function of the operating temperature.

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

are liberated into the conduction band which, in turn, reduces the film resistance and consequently the response increases. In case of the undoped film, the response to acetone is found to be more at a concentration of 75 ppm in the operating temperature range 200–275 1C in comparison to those observed at 100 ppm concentration in the same operating temperature range. This is attributed to increase in the surface coverage of acetone molecules on the film at higher concentration which prevents subsequent adsorption of atmospheric oxygen on the film surface, causing chemical reaction to proceed slowly and thus a decrease in response. However, at the operating temperature of 300 1C, the film exhibits more response at 100 ppm concentration of acetone in air. Among all the doped films, the 1 at% In-doped film shows the maximum response (96.8%) to 100 ppm of acetone at 300 1C, shown in Fig. 9. Even at a lower concentration of 25 ppm, the response to acetone in this film has been found to be more than 90% at 300 1C. This is attributed to the smaller crystallite size as well as the distribution of crystallites present in the film. As we know, smaller crystallite size leads to the larger surface area of the film, resulting in a greater oxygen adsorption and thus creation of a large number of sensing sites. Surface roughness as well as the presence of voids in the film also contributes to the enhancement in the sensor response since they result in larger contact area with the gaseous species. In the 1 at% In-doped film, at all concentrations of acetone, the response to acetone increases rapidly with increasing the operating temperature up to 250 1C and beyond this temperature, the response increases gradually, indicating that the film becomes more or less saturated. The rapid increase in response is attributed to the availability of sufficient adsorbed oxygen species on the film surface as well as the enhanced chemical activation of acetone molecules. It has been observed that saturation of the response is more favorable to the higher level of the acetone concentration because the surface coverage of acetone molecules on the film

begins to attain saturation, leading to a gradual increase in response. In case of the 2 at% In-doped film, the response has been found to increase with rise in the operating temperature and attains a maximum (88.7%) at 300 1C for a concentration of 100 ppm of acetone. However, at this concentration, the response is found to be low at the other operating temperatures of 225, 250 and 275 1C (shown in Fig. 8(c)), which may be due to enhanced surface coverage of acetone molecules on the film at higher concentration, leading to a barrier in the subsequent adsorption of atmospheric oxygen on the film surface. Further, it has been observed that the rate of increase of response varies with the operating temperature as well as the acetone concentration. This is due to the variation in the number of adsorbed oxygen species on the film surface and the strength of chemical activation of acetone molecules at different operating temperatures and concentrations of acetone. Fig. 10 shows the response characteristics of the 1 at% In-doped ZnO film as a function of acetone concentration at different operating temperatures. At 225 1C there is a quite significant increase in response with the increasing acetone concentration as the surface reaction increases due to enhanced surface coverage of acetone molecules on the film. Whereas at the other operating temperatures, the response appears to get saturated with increasing the concentration because the surface coverage of acetone molecules on the film begins to attain saturation as well as the enhanced surface coverage acts as a barrier to subsequent adsorption of oxygen on the film, resulting in a gradual increase in response. The sensing mechanism of the ZnO thin films to acetone may be described as follows: When ZnO thin films are heated in air at a temperature higher than 200 1C, at first atmospheric oxygen is absorbed on the film surface. In fact, at lower temperatures the surface reactions proceed too slowly to be useful, while at higher temperatures the increased promotion of electrons into the conduction band tends to obscure the effects of the 100

100

In (1at%)_ZnO

300°C 275°C 250°C

90

90

80 Response (%)

80 Response (%)

207

70 60 50 40

70

225°C

60 50 40 30

30 Operating temperature: 300°C

20

Undoped_ZnO 1.0at% In_ZnO 2.0at% In_ZnO

10 0

20 200°C 10 25

25

50 75 Acetone concentration (ppm)

100

Fig. 9. Comparative responses of the three films as a function of the acetone concentration at 300 1C.

50 75 Acetone concentration (ppm)

100

Fig. 10. Response characteristics of the 1 at% In-doped ZnO thin film as a function of the acetone concentration at different operating temperatures.

208

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

gases to be detected. The adsorption of oxygen forms ionic  species such as O2  , O2 and O  which acquire electrons from the conduction band and which desorb from the surface at 80, 150 and 500 1C, respectively. So in the temperature range used in the present investigation, only O  species react with the acetone molecules. The reaction kinetics is as follows [6,15]:

100 In (1at%)_ZnO Operating temperature: 225°C 25ppm 50ppm 75ppm 100ppm

90

Response (%)

80 70 60 50 40 30 20

O2 (air)2O2 (ads)

(12)



(13)

O2 (ads) þe  2O2 (ads)

10 0 0

2

4

6

8 10 12 14 16 18 20 Time (min)

100 In (1at%)_ZnO Operating temperature: 300°C 25ppm 50ppm 75ppm 100ppm

90

Response (%)

80 70 60 50



O2 (ads)þe  22O  (ads)

(14)

The reaction mechanism between acetone and ionic oxygen species may take place by two different ways [49,50] CH3COCH3 (gas)þ O  -CH3CO þ H2 þOH  þe 

(15a)

CH3COCH3 (gas)þ OH  -CH3CHO þCH3O 

(15b)

CH3CHO þO (bulk)-CH3CHOHþO (vacancies)

(15c)

CH3COCH3 (gas)þ O  -CH3C þ OþCH3O 

(16a)

CH3C þ O-C þ H3 þ CO

(16b)

COþ O  -CO2 þ e 

(16c)

40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Time (min)

Fig. 11. (a, b): Dynamic response characteristics of the 1 at% In-doped ZnO film.

Table 3 Response of acetone sensors based on zinc oxide material. Formula used for response calculation

Response for acetone vapor

Reference

Sensing element

Deposition technique; Material/Precursor used

Operating temperature (1C)

In-doped ZnO thin film

Spray pyrolysis; Zn (CH3COOH)2 2H2O (host precursor) þInCl3 (dopant precursor)

300

Ra Rg  1000=0 96.8% for Ra 100 ppm

This work

0.5 wt% Co-doped ZnO nanofibers

Electrospinning; Zn(NO3)2  6H2O (host precursor) þCo(NO3)2  6H2O (dopant precursor)

360

Ra Rg

16 for 100 ppm

[23]

Dumbbell –like ZnO

Facile solution method; Zn (CH3COOH)2 2H2O

300

Ra Rg

16 for 50 ppm

[24]

300

Ra Rg

30.4 for 100 ppm

[25]

ZnO nanorods thin Hydrothermal process; ZnCl2 2H2O film ZnO thin film

Spray pyrolysis; Zn (CH3COOH)2 2H2O

300

Ra Rg  1000=0 55% for Ra 3000 ppm

[50]

ZnO thin films

RF sputtering; Zn metal

400

Ra Rg  1000=0 99% for Ra 500 ppm

[51]

ZnO (Thickness  80 nm) thin film

Sol–gel method; Zn (CH3COOH)2 2H2O

200

Ra Rg

10.08 for 1000 ppm

[52]

Fe(1 at%) ZnO nanosheet

Hydrothermal process; Zn(NO3)2  6H2O (host precursor) þFe(NO3)3  9H2O (dopant precursor)

400

Ra Rg

44 for 500 ppm

[53]

C.S. Prajapati, P.P. Sahay / Materials Science in Semiconductor Processing 16 (2013) 200–210

Fig. 11(a, b) presents the dynamic response characteristics of the 1 at% In-doped film at two different operating temperatures of 225 1C and 300 1C, respectively. It has been observed that the film exhibits fast response and recovery to acetone at the higher operating temperatures. At the low operating temperature of 225 1C, the response time is found to be more than the recovery time for all concentrations while at the higher operating temperatures the case is reversed i.e., the response time is less than the recovery time. This may be due to insufficient thermal energy supplied to the acetone molecules at the low operating temperature, resulting in a slow response process. At the higher operating temperatures, the acetone molecules get enough thermal energy to react effectively with the adsorbed oxygen species and therefore the response is fast. A comparison of this work with other acetone sensors based on zinc oxide material is presented in Table 3; clearly, In-doped ZnO thin film grown by spray pyrolysis is a promising candidate for acetone sensors. 4. Conclusion In-doped ZnO thin films prepared by the spray pyrolysis using Zn(CH3COO)2 2H2O are found to have hexagonal wurtzite structure. The intensities of all peaks are significantly diminished as indium doping in films increases, indicating their reduced crystallinity as compared with undoped zinc oxide film. Narrowing of the optical band gap (3.26 eV in 1 at% and 3.23 in 2 at% Indoped films) is attributed to the increase in the band tail (Urbach tail) width which is related to the lattice disorder in the film network. Unlike the undoped film, the Indoped films are found to have the normal dispersion for the wavelength range 450–550 nm. Among all the films examined, at 300 1C the 1 at% In-doped film shows the maximum response (96.8%) to 100 ppm of acetone in air while the undoped film exhibits 43.8% response at the same operating temperature and concentration. Even at a lower concentration of 25 ppm, the response observed in case of the 1 at% In-doped is more than 90%. High response to acetone is attributed to the smaller crystallite size of the said film. The 1 at% Indoped film exhibits fast response and recovery to acetone at the higher operating temperatures. It is observed that at the higher operating temperatures the response time is less than the recovery time. The acetone sensing mechanism of the film has been explained.

Acknowledgments The authors are thankful to the Director, National Center of Experimental Mineralogy and Petrology, University of Allahabad, India for providing the facilities for XRD characterization. They are also thankful to the Head, Institute Instrumentation Center, Indian Institute of Technology, Roorkee, India for providing AFM facilities. The financial support provided by the Department of Science and Technology, Government of India, in the form

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