New fabrication of zinc oxide nanostructure thin film gas sensors

Superlattices and Microstructures 66 (2014) 23–32

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New fabrication of zinc oxide nanostructure thin film gas sensors A.A. Hendi ⇑, R.H. Alorainy Physics Department, Sciences Faculty for Girls, King Abdulaziz University, Jeddah, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 2 November 2013 Accepted 12 November 2013 Available online 26 November 2013 Key words: ZnO semiconductor Gas sensor Single oscillator model

a b s t r a c t The copper doped zinc oxide thin films have been prepared by sol–gel spin coating method. The structural and morphology properties of the Cu doped films were characterized by X-ray diffraction and atomic force microscope. XRD studies confirm the chemical structure of the ZnO films. The optical spectra method were used to determined optical constants and dispersion energy parameters of Cu doped Zno thin films. The optical band gap of undoped ZnO was found to be 3.16 eV. The Eg values of the films were changed with Cu doping. The refractive index dispersion of Cu doped ZnO films obeys the single oscillator model. The dispersion energy and oscillator energy values of the ZnO films were changed with Cu doping. The Cu doped ZnO nanofiber-based NH3 gas sensors were fabricated. The sensor response of the sensors was from 464.98 to 484.61 when the concentration of NH3 is changed 6600–13,300 ppm. The obtained results indicate that the response of the ZnO film based ammonia gas sensors can be controlled by copper content. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Metal oxide semiconductors have been received a great attention due to their interesting electrical and optical properties for various electronic device applications [1–4]. One of metal oxide semiconductors is ZnO and it has prepared by various techniques like RF magnetron sputtering [1,2], chemical vapor deposition [3], pulsed laser deposition [4] and sol–gel process [5,6]. The sol–gel technique is one of these techniques to prepare large-area coating for technological applications [5–10]. The optical properties of the metal oxides play an important from the technological viewpoint [11]. The refractive index is an optical constant and it is related to the electronic polarizability of the ⇑ Corresponding author. Tel.: +966 905337623815. E-mail address: [email protected] (A.A. Hendi). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.11.009

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materials. For optic devices applications, the refractive indices is important parameter. This parameter can be related to porosity in the metal oxide semiconductors. The porosity properties of any material are important for its gas sensing properties. Ammonia (NH3) is a colorless gas with a distinctive odor. For the detection of gaseous NH3, the potentiometric electrodes have been used [12]. The fabrication of new ammonia gas sensors is important to detect various concentrations, because this gas has been used in various applications such as fire power plants, food processing, chemical technology, medical diagnosis, fertilizers, and environmental protection. The metal oxide semiconductors such as ZnO, SnO2, Cr2O3, TiO2 and WO3 have been used in fabrication of the ammonia gas sensors [13–19]. In present study, nanostructure ZnO based gas sensors can be prepared and their gas response properties can be improved by controlling of nanosize and nanostructure. With this aim, we have prepared the nanostructure Cu doped ZnO films by sol gel method to fabricate ammonia gas sensors and improve the gas response characteristics. The optical and structural properties of Cu doped ZnO films were investigated using various methods. The gas sensing characteristics of the Cu doped ZnO films based gas sensors were investigated by current–time method. 2. Experimental details The copper doped ZnO films were prepared using the precursors: zinc acetate dihydrate [Zn(CH3CO2)22H2O;ZnAc], 2-methoxethanol and monoethanolamine (MEA). The ZnO films were doped with the various atomic ratios of 0.1 at.%, 0.5 at.%, 1 at.%, 2 at.% and 5 at.% copper contents. Firstly, the solutions were prepared for various Cu contents and were stirred at 60 °C for 2 h. After the cleaning procedure of the glass substrates in methanol, acetone and deionized water baths, the substrates were coated using the prepared solutions at 2000 rpm for 30 s using a spin coater. After the 5 times coating procedure, the films were preheated at 150 °C for 10 min in a furnace to evaporate the solvent and remove organic residuals and the solid films were obtained and the films were annealed at 400 °C for 1 h. The optical spectral curves such as diffuse reflectance, transmittance, and absorbance were measured using an integrating sphere for the SHIMADZU UV–VIS–NIR 3600 spectrophotometer. Barium sulfonate BaSO4 was used as reference to provide a nominal 100% reflectance measurement. X-ray diffraction patterns of the films were performed using a Bruker X-ray diffractometer. The gas sensors were prepared using interdigitated contact mask with 50 lm wide and 100 lm. The schematic diagram of the cu doped ZnO film/glass substrate sensors is given in Fig. 1. The sensors characteristics were performed computerized gas sensor system. The gas concentrations of NH3 gas were controlled using digital gas flow meters. 3. Results and discussion 3.1. Structural properties of the Cu doped ZnO films Fig. 2 shows the AFM images of Cu doped ZnO films. The undoped and Cu doped ZnO films are formed from fibers. The particle sizes of the films were determined using a PARK system XEI software

Fig. 1. Schematic diagram of the Cu doped ZnO film/glass substrate sensors.

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25

Fig. 2. AFM images of Cu doped ZnO films (a) undoped, (b) 0.1 at.% Cu, (c) 0.5 at.% Cu, (d) 1 at.% Cu, (e) 2 at.% Cu, (f) 5 at.% Cu.

programming and they were found to be 100–64 nm. Another important parameter of the films for gas sensor applications, the surface roughness of the films and it was determined by the following relation,

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 i¼1 ðZ i  Z av e Þ Rq ¼ N

ð1Þ

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where Rq is the root mean square roughness, Zi is the Z value of each point, Zave is the average of the Z values and N is the number of points. The Rq values of the Cu doped ZnO films were determined by XE 100E Park system software programming and are given in Table 1. As seen in Table 1, the Rq value is changed with Cu content and the obtained Rq values are high due to the presence of the fibers on substrate. These values are suitable for gas sensor applications, because the high surface roughness increases the adsorption of gas. The crystal structures of the Cu doped ZnO were performed using X-ray diffraction method. The Typical X-ray diffraction spectra of the films are shown in Fig. 3a and f. All the diffraction peaks in the spectrum at 2h position of 31.80°, 34.46°, 36.19°, 47.54°, 56.59°, corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) respectively. The obtained peaks belong to a hexagonal crystal structure and they are in agreement with the peaks JCPDS Card No. 89-7102 [20]. No characteristics reflection peaks related to Cu and other related impurities copper phases were detected by X-ray pattern. This confirms that Cu doped ZnO films have a high purity. The main differences were observed for (0 0 2) and (1 0 1) planes with Cu contents. This indicates that Cu ions were substituted by Zn sites entire the lattices of ZnO crystal [21]. The sharp peaks and high intensity indicate that Cu-doped ZnO nanoparticles are formed from well crystallines. For hexagonal crystal structure, the lattice constants, (a) and (c), is given:

1 2

dhkl

2

¼

2

2

4ðh þ hk þ k Þ l þ 2 3a2 c

ð2Þ 2

h where hkl are the miller indices, d is the lattice spacing parameter ðd12 ¼ 4 sin Þ and k is the wavelength k2 hkl

of the X-ray source. The obtained lattice constants, (a) and (c), are given in Table 2. The lattice constants of the Cu doped znO films were changed with Cu dopant concentration. The average grain size of the films was calculated using the Scherrer’s equation [22,23]:

Ds ¼

kk b cos h

ð3Þ

where k is the wavelength of incident X-ray, h is the Bragg angle of diffraction peak, k is the shape factor and b is the full width at half maximum height. The crystallite sizes, Ds, of the Cu doped ZnO films was determined using Eq. (3) and are given in Table 2. The Ds values of the films are decreased with Cu doping except for 0.5 at.% Cu sample. The obtained Ds values indicate that the Cu doped ZnO films are the nanostructure materials. 3.2. Determination of the optical band gap of n-type semiconductor ZnO film We used the optical spectra to determine optical band gap and optical constants of the Cu doped ZnO films. For this, the transmittance and reflectance spectra of the films are shown in Fig. 4a and b. The transmittance of the ZnO film is higher than that of Cu doped ZnO films and The maximum transmittance was found to be 1 at.% Cu content. All transmittance spectra exhibited an absorption edge and its position is changed with Cu doping. The type of direct optical transitions occurred in Cu doped ZnO films can be determined by the following relation [24,25],

Table 1 Surface morphology parameters obtained from the AFM images of Cu doped ZnO films. Cu doped ZnO films

D (nm)

Rq (nm)

Undoped ZnO 0.1 at.% Cu 0.5 at.% Cu 1 at.% Cu 2 at.% Cu 5 at.% Cu

98.96 77.682 66.628 64.314 82.270 71.844

114.807 139.073 110.780 70.042 152.456 114.807

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A.A. Hendi, R.H. Alorainy / Superlattices and Microstructures 66 (2014) 23–32

(101)

(101)

(100)

(002)

35

40

45

(110)

(110)

(102)

30

(002)

Intensity (a.u)

Intensity (a.u)

(100)

50

55

(102)

60

30

o

45

50

(a)

(b)

55

60

Intensity (a.u)

Intensity (a.u)

(002)

40

(101) (100)

(110)

(102)

35

40

2θ

(101) (100) (002)

30

35

2θ ( )

45

50

55

(110)

(102)

60

30

2θ

35

40

45

50

55

60

2θ

(c)

(d) (002) (100)

Intensity (a.u)

(102)

30

35

(101)

Intensity (a.u)

(002)(101) (100)

40

45

50

(102)

(110)

55

60

30

35

40

45

2θ

2θ

(e)

(f)

(110)

50

55

60

Fig. 3. XRD spectra of Cu doped ZnO films (a) undoped, (b) 0.1 at.% Cu, (c) 0.5 at.% Cu, (d) 1 at.% Cu, (e) 2 at.% Cu, (f) 5 at.% Cu.

ahm ¼ Aðhm  Eg Þ2

ð4Þ

where A is an energy-independent constant and Eg is the optical band gap. For the determination of optical band gap, we plotted the curves of (ahm)2 vs. hm and are shown in Fig. 5 and the obtained Eg values are given in Table 3. The Eg values is changed with Cu content and are in agreement with that of ZnO thin films prepared on various substrates [25–27].

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Table 2 Crystallite size and unit cell parameters of Cu doped ZnO films. Film

Ds (nm)

a (nm)

c (nm)

Undoped ZnO 0.1 at.% Cu 0.5 at.% Cu 1 at.% Cu 2 at.% Cu 5 at.% Cu

47.101 37.934 135.814 37.743 36.583 35.954

36.64 36.62 36.64 36.62 36.64 36.67

68.79 68.76 68.79 68.74 68.74 68.78

undoped ZnO 0.1% at. Cu 0.5% at. Cu 1% at. Cu 2% at. Cu 5% at. Cu

(a) undoped ZnO 0.1% at. Cu 0.5% at. Cu 1% at. Cu 2% at. Cu 5% at. Cu

)

(b) Fig. 4. Transmittance and reflectance spectra of Cu doped ZnO films (a) transmittance, (b) reflectance.

3.3. Refractive index dispersion and optical constants of the Cu doped ZnO films The reflectance spectra of the Cu doped ZnO films are shown in Fig. 4b. The reflectance spectra exhibit a peak and the position of the peak is changed with Cu doping. The shifting in the peak position confirms the change in the optical band gap with Cu content. The refractive index of the Cu doped ZnO films was determined from reflectance spectra. The refractive index was determined by the following relation [28]

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undoped ZnO 0.1% at. Cu 0.5% at. Cu 1% at. Cu 2% at. Cu 5% at. Cu

Fig. 5. Plots of (ahm)2 vs. hm of the Cu doped ZnO films.

Table 3 Optical band gap and single oscillator model parameters of the Cu doped ZnO films. Cu doped ZnO films

Eg (eV)

Eo (eV)

Ed (eV)

ko (nm)

Undoped ZnO 0.1 at.% Cu 0.5 at.% Cu 1 at.% Cu 2 at.% Cu 5 at.% Cu

3.17 3.20 3.23 3.16 3.21 3.27

3.721 3.577 3.889 3.588 4.358 5.138

6.717 6.987 6.428 5.572 11.470 9.731

333.78 347.21 319.36 346.15 284.99 241.72

nðkÞ ¼

ð1 þ RðkÞÞ þ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4RðkÞ  ð1  RðkÞÞ2 kðkÞ2 1  RðkÞ

ð5Þ

where k is the extinction coefficient (k = ak/4p). The plots of the refractive index of the Cu doped ZnO films are shown in Fig. 6. A normal dispersion was observed for the Cu doped ZnO films and the refractive index is changed with Cu content and is changed from 1.58 to 3.1. The refractive index dispersion of the Cu doped ZnO films was analyzed by various methods. The first method is single oscillator model and in this model, the refractive index dependence of photon energy is expressed by the following relation [28,29],

n2 ¼ 1 þ

Ed Eo 2

E2o  ðhmÞ

ð6Þ

where n is the refractive index, h is Planck’s constant, m is the frequency, hm is the photon energy, Eo is the average excitation energy for electronic transitions and Ed is the dispersion energy. The plots of 1/n2  1 vs (hv)2 were shown in Fig. 7. The oscillator parameters were obtained via Fig. 7 and are given in Table 3. The oscillator parameters did not indicate a regular trend.The Ed and Eo values were calculated from the slope (EdEo)1 and intercept (Eo/Ed) of Fig. 7 and are given in Table 2. The average oscillator strength So and average oscillator wavelength ko parameters were calculated by the following relation [29],

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undoped ZnO 0.1% at. Cu 0.5% at. Cu 1% at. Cu 2% at. Cu 5% at. Cu

Fig. 6. Plots of n vs. k of the Cu doped ZnO films.

undoped ZnO 0.1% at Cu 0.5% at Cu 1% at Cu 2% at Cu 5% at Cu

Fig. 7. Plots of 1/n2  1 vs (hv)2 of the Cu doped ZnO films.

n2  1 ¼

So k2o 1  ðko =kÞ2

ð7Þ

where k is the wavelength of incident light. Eq. (9) also can be transformed as,

 2 n21  1 ko ¼ 1  n2  1 k

ð8Þ

The parameters, ko and So values were determined from the slope and intercept of (n2  1)1 vs k2 curves plotted and are given in Table 3. 3.4. Gas sensing properties of Cu doped ZnO films based on gas sensors The gas sensing properties of Cu doped ZnO films based on gas sensors were investigated by current time characteristics. The sensing characteristics of the sensors are shown in Fig. 8a and f. The gas sensors were exposed to NH3 gas adsorption and desorption. The various amounts of the NH3 gas were injected to gas chamber. During gas injection, the current was measured as a function of time. As seen in sensing curves of the sensors, the response of the gas sensors is changed with Cu contents. The

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A.A. Hendi, R.H. Alorainy / Superlattices and Microstructures 66 (2014) 23–32

3.0x10

-6

0.1% at.Cu doped ZnO

6.0x10-5

ZnO 13300 ppm NH

3

2.5x10

5.0x10-5

-6

13300 ppm NH

3

10000 ppm NH

2.0x10 1.5x10 1.0x10 5.0x10

-6

Current (A)

Current (A)

3

6600 ppm NH

3

-6

-6

-7

4.0x10-5 3.0x10-5

10000 ppm NH

3

-5

2.0x10

1.0x10-5

air

6600 ppm NH

3

Air

0.0

0.0 0

25

50

75

100

125

150

175

0

25

50

75

100

(a) 6.0x10

4.0x10

-6

3.0x10

-6

2.0x10

-6

1.0x10

-6

175

200

(b) 2.5x10

1% at.Cu doped ZnO

0.5% at.Cu doped ZnO

5.0x10

150

-5

-6

-6

125

Time (s)

Time (s)

13300 ppm NH

3

13300 ppm NH

3

-5

2.0x10

10000 ppm NH

3

10000 ppm NH

Current (A)

Current (A)

3

6600 ppm NH

3

1.5x10-5

6600 ppm NH

3

1.0x10-5 5.0x10-6

air

air

0.0

0.0 0

25

50

75

100

125

150

175

200

0

50

75

100

125

Time (s)

(c)

(d) 5.0x10

1.0x10-5 2% at Cu doped ZnO

150

175

200

-6

5% at.Cu doped ZnO

13300 ppm NH3

13300 ppm NH

3

8.0x10-6

4.0x10

-6

3.0x10

-6

2.0x10

-6

1.0x10

-6

10000 ppm NH

3

6.0x10-6

6600 ppm NH

Current (A)

Current (A)

25

Times (s)

3

4.0x10-6 2.0x10-6

10000 ppm NH

3

6600 ppm NH

air

3

air

0.0

0.0 0

25

50

75

100

125

150

175

200

0

25

50

75

Time (s)

(e)

100

125

150

175

200

Time (s)

(f)

Fig. 8. The sensing characteristics of Cu doped ZnO gas sensors.

response values of the gas sensors are given in Table 4. The highest response was found to be 2 at.% Cu content based on the gas sensor. The obtained results indicate that the response of the gas sensors is controlled by Cu content.

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A.A. Hendi, R.H. Alorainy / Superlattices and Microstructures 66 (2014) 23–32 Table 4 Response values of Cu doped ZnO gas sensors. Films

S (response)

Undoped ZnO 0.1 at.% Cu 0.5 at.% Cu 1 at.% Cu 2 at.% Cu 5 at.% Cu

464.988 452.758 205.279 136.458 484.610 358.962

4. Conclusions The Cu doped ZnO films were prepared by sol method. The optical band gap and optical constants of the films were determined. It was found that the Cu doping controls the optical band gap and optical constants of the Cu doped ZnO films. We fabricated Cu doped ZnO nanofiber-based amonia gas sensors. The responses of the gas sensors were found to be 135.458 and 484.610, when the gas chamber was injected from 6600 ppm to 13,300 ppm NH3. The highest response was found to be for 2 at.% Cu doped ZnO film based gas sensor. Acknowledgments This paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under Grant No. (10-843-D1432). The authors, therefore, acknowledge with thanks DSR technical and financial support. References [1] P.P. Sahay, R.K. Nath, Sens. Actuat. B 134 (2008) 654. [2] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, Y.D. Park, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 1 (2003) 93. [3] T. Shuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125. [4] Y. Natsume, H. Sakata, Thin Solid Films 372 (2000) 30. [5] T.L. Yang, D.H. Zhang, J. Ma, H.L. Ma, Y. Chen, Thin Solid Films 326 (1998) 60. [6] Y. Zhou, P.J. Kelly, A. Postill, O. Abu-Zeid, A.A. Alnajjar, Thin Solid Films 447 (2004) 33. [7] T.M. Barnes, J. Leaf, C. Fry, C.A. Wolden, J. Cryst. Growth 274 (2005) 412. [8] J. Mass, P. Bhattacharya, R.S. Katiyar, Mater. Sci. Eng. B 103 (2003) 9. [9] V. Musat, B. Teixeira, E. Fortunato, Monteiro, P. Vilarinho, Surf. Coat. Technol. 180 (2004) 659. [10] G. Westin, M. Wijk, A. Pohl, J. Sol–Gel Sci. Technol. 31 (2004) 283. [11] Q.H. Li, D. Zhu, W.J. Liu, Y. Liu, X.C. Ma, Appl. Surf. Sci. 254 (2008) 2922. [12] M.E. Meyerhoff, Anal. Chem. 52 (1980) 1532. [13] A. Masami, T. Takashi, S. Suto, S. Takahiro, N. Chiaki, M. Norio, N. Yamazoe, J. Ceram. Soc. Jpn. 104 (1996) 1112–1116. [14] H. Nanto, T. Minami, S. Takata, J. Appl. Phys. 60 (1986) 482–484. [15] R. Sanjines, V. Demarne, F. Levy, Thin Solid Film 193 (194) (1990) 935–942. [16] P.T. Moseley, D.E. Williams, Sens. Actuat. B 1 (1990) 113–115. [17] M. Aslam, V.A. Chaudhary, I.S. Mulla, S.R. Sainkar, A.B. Mandale, A.A. Belhekar, K. Vijayamohanan, Sens. Actuat. 75 (1999) 162–167. [18] N. Yamazoe, N. Miura, J. Tamaki, Trans. Mater. Res. Soc. Jpn. 15A (1994) 111–116. [19] D. Manno, G. Micocci, R. Rella, A. Serra, A. Taurino, A. Tepore, J. Appl. Phys. 82 (1) (1997) 54–59. [20] M.H. Aslan, A.Y. Oral, E. Mensur, A. Gul, E. Basaran, Sol. Energy Mater. Sol. Cell 82 (2004) 543–552. [21] W.Y. Zhang, D.K. He, Z.Z. Liu, L.J. Sun, Z.X. Fu, Optoelectron. Adv. Mater. 4 (2010) 1651–1654. [22] W. Water, S.-Y. Chu, Y.-D. Juang, S.-J. Wu, Mater. Lett. 57 (2002) 998. [23] Zarycka, J. Ilczuk, D. Czekaj, Mater. Sci. 21 (2003) 439. [24] V.R. Shinde, T.P. Gujar, C.D. Lokhande, R.S. Mane, S.H. Han, Mater. Chem. Phys. 96 (2006) 326. [25] R.K. Gupta, M. Cavas, F. Yakuphanoglu, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 95 (2012) 107–113. [26] Sh.A. Mansour, F. Yakuphanoglu, Solid State Sci. 14 (2012) 121–126. [27] T. Ates, C. Tatar, F. Yakuphanoglu, Sensor. Actuat. A 190 (2013) 153–160. [28] Z. Serbetci, H.M. El-Nasser, Fahrettin Yakuphanoglu, Spectrochim. Acta Part A 86 (2012) 405–409. [29] M. DiDomenico, S.H. Wemple, J. Appl. Phys. 40 (1969) 720.