- Email: [email protected]
Highly textured ZnO thin films grown using sol-gel route for gas sensing application
Accepted Manuscript Highly textured ZnO thin films grown using sol-gel route for gas sensing application Sumati Pati PII:
S0925-8388(16)33897-X
DOI:
10.1016/j.jallcom.2016.11.414
Reference:
JALCOM 39911
To appear in:
Journal of Alloys and Compounds
Received Date: 26 June 2016 Revised Date:
6 November 2016
Accepted Date: 30 November 2016
Please cite this article as: S. Pati, Highly textured ZnO thin films grown using sol-gel route for gas sensing application, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.11.414. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Highly textured ZnO thin films grown using sol-gel route for
Sumati Pati
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gas sensing application
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M AN U
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N. C. (Auto.) College, Jajpur-755001, India
Key words: Thin films; Sol-gel processes; Crystal structure; Micro structure; Gas sensor * Corresponding author. Mobile: +91 9438855121; Email: [email protected]
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Abstract The crystalline orientation of metal oxide thin films often modulates their structural, optical as well as surface related properties. Viewing in the same line, in the present work we
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have systematically investigated the structural, micro structural, optical and gas sensing characteristics of highly textured zinc oxide (ZnO) thin films oriented along (002) crystalline planes. The ZnO thin films have been grown on quartz substrates by spin coating the precursor
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sol prepared through cost effective wet chemical synthesis route. The structural and micro structural characteristics of the grown films are studied using X-ray diffraction (XRD) and field
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emission scanning electron microscopy (FESEM) respectively. The optical properties of the synthesized films are investigated by analyzing the ultraviolet-visible (UV-VIS) and photoluminescence (PL) spectroscopy. From the recorded absorption spectra band gap energies of the films are estimated. The gas sensing characteristics of the grown films are investigated
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upon exposure to various toxic and combustible gases at different operating temperatures. Finally, the effect of crystalline orientation on the structural and optical properties and gas
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sensing characteristics of the grown ZnO thin films are explored.
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1. Introduction The structural, optical and surface related properties of metal oxide thin films depend largely
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on their crystallographic orientations [1-2]. For example, as reported by Delgado et al. [1] optical properties of tin oxide thin films change with the changes in crystallographic orientations. If all the grains are oriented along a particular direction, say along c-axis then the optical properties
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are considerably different than the case where they are oriented normal to c-axis. Also the surface properties such as the gas sensing performances of metal oxides significantly influenced
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by their effective surface area, grain and pore size, morphology as well as inter-grain contact, which in turn are also related with the crystallographic orientation. As explained by Korotcenkov et al. [2] if the grain growth is along vertical direction then the pores are straight pores, and hence require smaller time for the adsorption and desorption of gases. As reported in the
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literature several techniques such as metal organic chemical vapor deposition, molecular beam epitaxy, sputtering, and so on have been used to grow thin films of desired orientations [3-5]. Most of these techniques are though expensive and require careful handling, their competent
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features for controlling the deposition parameters and hence the frequencies of nucleation precisely, make them very useful for growing thin films along specific crystallographic
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orientations. However, it is difficult to grow such textured films from direct deposition of sols like in sol-gel process, as in this process due to lack of precise control on the frequencies of nucleation, grains get coalescence and hence instead of z-direction, growth is predominant along x-y direction. Among the metal oxides, ZnO is perhaps considered to be most studied due to its favorable properties such as high electron mobility, wide band gap, good transparency, etc. [6] and hence its versatile use in different emerging applications including optoelectronic
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applications, solar cells, protective coatings, ultrasonic oscillators and gas sensors [7-8]. For some of these applications (e.g. optoelectronic, gas sensor, etc.) the optical and surface properties of ZnO are considered to play an important role. Though ZnO is a well explored material, but as
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reflected in the literature there are few reports on the growth of textured ZnO thin films using sol-gel route [9-11] and limited research efforts have so far been made to study the gas sensing characteristics of such sol-gel grown textured ZnO thin films in presence of various toxic and
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combustible gases. In the present work, thus, we have attempted to grow preferentially c-axis oriented ZnO thin films, on an amorphous substrate (quartz), by spin coating the ZnO precursor
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sol synthesized using cost effective wet chemical synthesis route with an aim to study the effect of preferential orientation on the structural and optical characteristics and to evaluate its suitability as a gas sensor. 2. Experimental details
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In the present work, we have used the simple sol-gel spin coating technique to synthesize ZnO thin films on transparent fused quartz substrates. First the precursor sol (concentration ~ 0.4 M) was prepared by dissolving zinc acetate dihydrate [Zn (CH3CO2)2·2H2O] in 2-
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methoxyethanol. Mono-ethanolamine (MEA) was added to the mixture to stabilize the solution. The molar ratio of MEA to zinc acetate was maintained at 1: 1 ratio. The solution was stirred at
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60oC for 2 h and was used as precursor sol after cooling. The flow chart for the synthesis of the ZnO thin films is shown in Fig. 1. The precursor sol was spin coated over quartz substrate at 3000 rpm for 30 s using a commercial spin coating unit (SCU 2007, Apex Instruments Co, Kolkata). Prior to coating, the quartz substrates were cleaned ultrasonically in warm trichloro ethylene (TCE), acetone, methanol, and deionized water bath. Just after deposition, the film was
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inserted into a preheated furnace (kept at 300oC) for 5 min and then quenched to room temperature. The coating and drying cycles were repeated to increase the film thickness. Films of different thicknesses were prepared by varying the no. of coating (5, 10, 15 and 20
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times) and drying cycles. After final coating and drying cycle the films were annealed at 600oC for 1 h in air and cooled down to room temperature by normal furnace cooling. The structural characteristics of the films were studied by X-ray diffraction (Ultima III, Rigaku, Japan) analysis
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using Cu Kα radiation in the 2θ range 20-80° at a scanning rate of 3° min-1. For X-ray diffraction measurements, the accelerating voltage and current were maintained at 40 kV and 30 mA
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respectively. The surface and cross sectional morphology of the synthesized films was characterized using FESEM (SUPRA-40, Carl Zeiss, Germany). The optical properties of the synthesized films were investigated by analyzing the absorption spectra recorded in ultravioletvisible (UV-VIS) range using UV-VIS absorption spectrometer (Lambda 750, Perkin Elmer,
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USA). PL spectroscopy was used as a characterization tool to investigate the defect structure of the synthesized films and the PL spectra of these films were recorded using He–Cd laser as an excitation source, operating at 325 nm. The gas sensing characteristics (H2, CO, and CH4) were
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evaluated using a dynamic flow gas sensing measurement system described elsewhere in details. Using combustible gases the response%, response/ recovery times and stability of the
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synthesized films were evaluated by varying the film thickness, operating temperature (250 – 380oC), and test gas concentration (1-1660 ppm). 3. Results and discussion
3.1 . Structural characterization by X-ray diffraction Fig. 2 (a) shows the X-ray diffraction patterns of the sol-gel grown ZnO thin films of various thicknesses, grown on quartz substrates. As shown in figure in each pattern a sharp peak
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appears around 2θ ~ 34.92o, corresponding to the diffraction of (002) planes of hexagonal ZnO crystal structure. No other diffraction peaks are observed in the pattern indicating that all the synthesized films are phase pure and highly textured along (002) planes, i.e. perpendicular to the
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substrate. This preferential orientation along c-axis in ZnO thin films could be attributed to the lowest surface energy density of the (002) planes of ZnO wurzite crystal structure [12]. As explained by Fujihara et. al. [13] the probable mechanism underlying the c-axis orientation is the
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initial orientation due to nucleation and the final growth orientation at the film/ substrate interface and the successive growth in the direction perpendicular to the substrate. In addition,
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the improvement in crystallinity with increase in number of coatings is apparent in the diffraction pattern (in Fig. 2 (a)), i.e., the intensity of the (002) diffraction peak increases with the number of coatings and the full width at the half maximum (FWHM) of peaks decreases from 0.288o to 0.196o as the number of coatings increases from 5 to 20.
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The crystallite size D is estimated using the Debye-Scherer formula [14]: D = 0.9λ / β cosθ
(1)
Where λ (= 0.154 nm) is the wavelength of the X-ray radiation used, θ is the Bragg diffraction
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angle of the X-ray diffraction peaks and β is the measured full width at half maximum (FWHM) of diffraction peaks measured in radian. These values of crystallite size and lattice strain (%)
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(obtained from the XRD pattern) are plotted separately in Fig. 2 (b). From the figure, it is observed that with increasing number of coatings, the crystallite size increases and the lattice strain (%) decreases. As the films become thicker with increase in the number of coatings, it is relaxed thus reducing the lattice strain (%).
3.2. Micro-structural evaluation by FESEM
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The surface and cross sectional morphologies of all the synthesized ZnO thin films are investigated using field emission scanning electron microscopy. Fig. 3 (a-d) shows the surface morphology of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films.
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As envisaged in the micrograph all these films are porous, crack-free and have homogeneous grain size distribution. However, there is a slight variation in the morphology of the films with the variation in number of coatings. The grain size appears to increase with the increase in
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number of coatings. The same trend in the variation of micro structure was reported by Mridha et al. [15]. This is also clearly understood from the XRD pattern, where the intensity of (002) peak
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increases with increasing number of coatings. For five times coated (minimum number of coatings) ZnO thin film, the surface morphology is granular, whereas the morphology becomes more compact with increasing number of coatings. The cross-sectional morphologies, representing the thickness of the films, are shown in the right hand side of the respective films in
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Fig. 3 (e-h). As inferred from this figure there is only a small variation in the thickness of the films with the variation in number of coatings. Thicknesses of the films are found to be 320 nm, 353 nm, 393 nm and 420 nm for 5, 10, 15, and 20 times coated films respectively, indicating a
3.3.
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linear variation of thickness with the number of coatings. Optical Characterization
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Owing to its wide band gap ZnO is found to be a very good optical material. So to study the optical properties of the ZnO thin films two characterization techniques, namely UV-Vis and PL spectroscopy, are investigated. 3.3.1. UV-Vis Spectroscopy Ultra-violet to visible (UV-Vis) spectroscopy has been employed to record the optical transmittance and absorbance spectra of all the synthesized ZnO thin films. From the
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transmittance spectra (not shown here) the films are found to be transparent in the visible region (400-800) nm with average transmittance of (60-80) % and a steep fall off at 376 nm, whose corresponding energy is close to the intrinsic band gap of ZnO (3.37 eV). Also with increasing
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film thickness, the average transmittance of the films is decreased. This is probably due to the increase in absorption for higher thickness, thus reducing the transmittance.
The optical absorbance spectra of the films are shown in Fig. 4 (a). From the absorbance
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spectra, it is apparent that all the films have a low absorption in the visible range and a high absorption in the ultra-violet range. Fig. 4 (b) shows the Tauc plot representing the values of the
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optical band gap energy (Eg) of the ZnO thin films. Eg of the ZnO thin films is determined by extrapolation of the linear portion of (αhν) equation [16].
2
versus hν plots at ߙ2 = 0 using the following
(2)
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Where α is absorption coefficient, hν is the photon energy and A' is a constant. In case of allowed direct transitions (as in ZnO) the value of n is equal to ½. The intercepts of these plots on the energy axis represents the energy band gaps, since
when (
)
2
= 0. The measured
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energy band gap values are in the range of 3.23–3.25 eV, which is close to the band gap of
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intrinsic ZnO semiconductor. As observed, the value of band gap reduces with the increase in thickness. This is attributed to the bigger grain size for thicker films. 3.3.2. PL Spectroscopy
It is known that ZnO is a semiconductor at room temperature and shows n-type conductivity even in the absence of doping. This is due to the presence of oxygen vacancies and zinc interstitials, which are common donor type defects, exist in ZnO [6]. Hence to investigate this defect chemistry of the synthesized ZnO thin films, photoluminescence (PL) spectroscopy at
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room temperature is used as a characterization tool. Fig. 5 shows the room temperature PL spectra of the synthesized films recorded using He–Cd laser as an excitation source operating at
energy of ZnO (3.3 eV) in order to understand the defect emission.
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325 nm. The excitation energy (~3.81 eV) has been chosen to be higher than the band-gap
As revealed from the figure, each spectrum consists of two peaks at about ~389 nm, and ~545 nm. The very small peak at 389 nm (UV luminescence) is attributed to near band edge
defect level (DL) emission, primarily oxygen vacancies (
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(NBE) emission, whereas the dominant peak at ~545 nm (green luminescence) is due to the ), of the synthesized films [17]. In
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addition, there is a variation of intensity of peaks with the variation in number of coatings, which may be due to the difference in concentration of donor defect centers produced due to micro structural variations [18]. This enhanced defect concentration may be useful for gas sensing application of ZnO thin films. Gas Sensing Characteristics
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3.4.
The synthesized ZnO thin films are characterized in terms of their carbon monoxide (CO), hydrogen (H2), and methane (CH4) sensing characteristics. Initially, the gas sensing
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characteristics of all the synthesized ZnO films are studied at various operating temperatures (in the range between 250-380oC), on exposure to a fixed concentration (~ 1660 ppm) of H2 gas.
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Optimum temperature corresponding to the maximum response is observed. Fig. 6 (a) shows the variation of response% of ZnO thin film (of different thicknesses) gas sensors with the operating temperature in presence of H2 gas at ~ 1660 ppm. As observed from the figure the response% increases with increase in operating temperature up to 350oC (the optimum temperature) and reduces with further increase in the operating temperature. A good number of research reports in support of this behavior (existence of
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optimized operating temperatures corresponding to the maximum response towards a specific gas) for a number of SMO sensors are available in the literature [19-20]. The observed behavior can be explained using the concept of activation energy. At the lower temperature the reducing
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gas molecules do not have sufficient energy to react with the adsorbed oxygen species. However, with increasing operating temperature the energy is sufficient to provide the activation energy of the reaction, which increases the response. While the reduction of response with further increase
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in temperature (beyond optimum temperature) could be due to the higher desorption rates of the surface adsorbed oxygen species [20]. From this figure the optimum thickness (15 times coated
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film) is also estimated corresponding to the maximum response.
The response and recovery times (τres/τrec), estimated from the response transients of ZnO thin film gas sensors in presence of H2 gas for a fixed concentration (~ 1660 ppm) at different operating temperatures are plotted in Fig. 6 (b) and (c), respectively. Note that thinner film
response time.
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exhibits lower response time. As observed, for all these films recovery time is more than the
After optimizing the thickness (393 nm, for 15 times coated film) the thin film sensing
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element is further exposed to different test gas (H2, CO and CH4) environments at a particular concentration (~1660 ppm), to observe the effect of test gas on the response behavior. Fig 7 (a)
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shows the response% of 15 times coated ZnO thin film sensing element on exposure of 1660 ppm of H2, CO and CH4 gas at various operating temperatures. From this figure it is observed that at each temperature the response% is maximum in presence of H2, and then reduced for CO and minimum for CH4. The observed results may be explained on the basis of theory of gas diffusion. It is reported that in a gas sensor, the mechanism of gas diffusion depends on the size of the pores present in the sensing element. Consequently, surface
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diffusion, Knudsen diffusion and molecular diffusion take place in order with the increase in pore size. For a pore size in the range 2 to 50 nm (mesoporous region) Knudsen diffusion is known to take place. In this study, Knudsen diffusion is presumed to take place, owing to the
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grain size of the films (obtained from FESEM images) in the range 2 to 50 nm (as it is reported that pore size and grain size are comparable [21]). Knudsen diffusion constant (Dk) is determined
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from the following relation [22]:
(3)
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Where T is temperature, r is pore radius, R is gas constant and M is the molecular weight of the diffusing gas. For a given film (if r is assumed to be same for all the pores) (4)
So for gases with lower molecular weight, have higher diffusion coefficient. As a result these
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gases can enter upto the bottom of the film, thus increasing the utility factor and hence the response. However, for gases with higher molecular weight the utility factor is less, which reduces the response. In line to the above explanation, the response% in presence of H2, CH4 and
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CO gases are found to be 62, 53 and 50 respectively, at their respective optimized temperatures. The response and recovery time of this gas sensor, in presence of all the reducing gases are
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plotted in Fig. 7 (b) and (c) respectively. As observed, both the response and recovery time reduces with increase in temperature, being minimum during CH4 detection and maximum for CO at each temperature. 4. Conclusions In the present work we have successfully grown highly c- axis oriented ZnO thin films on quartz substrates using the low cost sol-gel spin coating technique. All the grown films exhibit
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only (002) diffraction peak, indicating the textured behavior of ZnO on the substrate. Thicknesses of films are varied by varying the number of coatings. The structural, micro structural and optical properties of the grown films are found to depend upon the number of
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coatings and hence on the thickness of the films. The gas sensing characteristics of the grown films are studied upon exposure of H2, CO, and CH4 gases at various operating temperatures. It is observed that irrespective of the operating temperature these ZnO thin film gas sensors are
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selective to H2 gas. Hence it is argued that this study can pave the way for device fabrication in detecting the highly flammable and explosive hydrogen gas in presence of other gases in the
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environment. Finally, an attempt is made to correlate the gas sensing characteristics of these films to their structural and optical properties. Acknowledgements
The author gratefully acknowledged IIT Kharagpur for this whole work.
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References
[1] F. Paraguay-Delgado, M. Miki-Yoshida, W. Antunez, J. Gonzalez-Hernandez, Y.V. Vorobiev, E. Prokhorov, Thin Solid Films 516 (2008) 1104–1111.
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[2] G. Korotcenkov, M. DiBattista, J. Schwank, V. Brinzari, Mater. Sci. Eng. B 77 (2000) 33–39. [3] S. Pati, S. B. Majumder, and P. Banerji, Sens. Actuators A 213 (2014) 52–58.
176.
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[4] C.-Y.J. Lu, L. Chang, K. H. Ploog, and M. M. C. Chou, J. Cryst. Growth, 378 (2013) 172-
[5] S. Elmas and S. Korkmaz, J. Non-Cryst. Solids, 359 (2013) 69–72. [6] A. Janotti, C. G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501 (29). [7] R. Groenen, J. Loffler, P.M. Sommeling, J.L. Linden, E.A.G. Hamers, R.E.I. Schropp, M.C.M. van de Sanden, Thin Solid Films 392 (2001) 226-230.
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[8] G.N. Dar, A. Umar, S. Zaidi, S. Baskoutas, S.W. Hwang, M. Abaker, A. Al-Hajry, S.A. AlSayari, Talanta 89 (2012) 155–161. [9] T. Yasuda, Y. Obata, M. Sato, J. Korean Phys. Soc. 53 (2008) 2921-2924.
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[10] C.H. Chia, W.C. Tsai, W.C. Chou, J. Lumin. 148 (2014) 111-115.
[11] W. Chebil, M.A. Boukadhaba, A. Fouzri, Superlattice. Microst. 95 (2016) 48-55. [12] D. Bao, H. Gu and A. Kuang, Thin Solid Films, 312 (1998) 37-39.
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[13] S. Fujihara, C. Sasaki, T. Kimura, Appl. Surf. Sci., 180 (2001) 341-350.
[14] B. D. Cullity, (2001), Elements of X-ray Diffraction, 3rd ED. Prentice Hall, New York.
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[15] S. Mridha, D. Basak, Mater. Res. Bull. 42 (2007) 875–882.
[16] A. K. Chawla, D. Kaur and R. Chandra, Opt. Mater. 29 (2007) 995–998. [17] B.-Z. Dong, G.-J. Fang, J.-F. Wang, W.-J. Guan and X.-Z. Zhao, J. Appl. Phys. 101 (2007) 033713 (7).
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[18] P. M. R. Kumar, C. S. Kartha, K. P. Vijayakumar, T. Abe, Y. Kashiwaba, F. Singh and D. K. Avasthi, Semicond. Sci. Technol., 20 (2005) 120-126. [19] C. Aifan, H. Xiaodong, T. Zhangfa, B. Shouli, L. Ruixian and L. C. Chiun, Sens. Actuators
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B, 115 (2006) 316-321.
[20] S. Pati, P. Banerji and S. B. Majumder, RSC Advance, 5 (2015) 61230-61238.
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[21] M. Ando, S. Suto, T. Suzuki, T. Tsuchida, C. Nakayama, N. Miura and N. Yamazoe, J. Mater. Chem. 4 (1994) 631-633. [22] G. Sakai, N. Matsunaga, K. Simanoe and N. Yamazoe, Sens. Actuators B, 80 (2001) 125131.
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Figure captions Fig.1. Flow chart showing the synthesis procedure of ZnO thin films using sol-gel spin coating technique
Fig.2. (a) X-ray diffraction patterns of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films grown
(obtained from x-ray diffraction pattern) with the number of coatings
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by sol-gel spin coating technique, and (b) Plot showing the variation of crystallite size and lattice strain
Fig.3. Scanning electron micrographs of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films and
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their cross sectional morphologies are shown, in the right hand side of the respective films, in Fig. 3 (e-h)
Fig.4.(a) Absorbance spectra of ZnO thin films of various number of coatings, synthesized by sol-gel spin
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coating technique on quartz substrates, and (b) Tauc plot [(a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films], showing the band gap of the corresponding films
Fig.5. Room temperature photoluminescence spectra of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films
Fig.6. Variation of (a) response%, (b) response time, and (c) recovery time of ZnO thin film [(a) 5, (b) 10,
operating temperature
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(c) 15, and (d) 20 times coated)] gas sensors towards the detection of 1660 ppm of H2 gas as a function of
Fig.7. Variation of (a) response%, (b) response time, and (c) recovery time of ZnO thin film (15 times
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temperature
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coated) gas sensor towards the detection of 1660 ppm of H2, CO, and CH4 as a function of operating
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Highlights
Highly textured ZnO thin films are grown using sol-gel spin coating technique.
•
(002) oriented films are grown with an aim to study the effect of preferential orientation on
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•
the structural and optical characteristics and to evaluate its suitability as a gas sensor.
Gas sensing parameters in presence of H2, CO and CH4 gases are studied by varying the
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•
operating temperature.
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The correlation between crystallographic orientation and other properties are discussed.
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•
S0925-8388(16)33897-X
DOI:
10.1016/j.jallcom.2016.11.414
Reference:
JALCOM 39911
To appear in:
Journal of Alloys and Compounds
Received Date: 26 June 2016 Revised Date:
6 November 2016
Accepted Date: 30 November 2016
Please cite this article as: S. Pati, Highly textured ZnO thin films grown using sol-gel route for gas sensing application, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2016.11.414. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Highly textured ZnO thin films grown using sol-gel route for
Sumati Pati
RI PT
gas sensing application
AC C
EP
TE D
M AN U
SC
N. C. (Auto.) College, Jajpur-755001, India
Key words: Thin films; Sol-gel processes; Crystal structure; Micro structure; Gas sensor * Corresponding author. Mobile: +91 9438855121; Email: [email protected]
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Abstract The crystalline orientation of metal oxide thin films often modulates their structural, optical as well as surface related properties. Viewing in the same line, in the present work we
RI PT
have systematically investigated the structural, micro structural, optical and gas sensing characteristics of highly textured zinc oxide (ZnO) thin films oriented along (002) crystalline planes. The ZnO thin films have been grown on quartz substrates by spin coating the precursor
SC
sol prepared through cost effective wet chemical synthesis route. The structural and micro structural characteristics of the grown films are studied using X-ray diffraction (XRD) and field
M AN U
emission scanning electron microscopy (FESEM) respectively. The optical properties of the synthesized films are investigated by analyzing the ultraviolet-visible (UV-VIS) and photoluminescence (PL) spectroscopy. From the recorded absorption spectra band gap energies of the films are estimated. The gas sensing characteristics of the grown films are investigated
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upon exposure to various toxic and combustible gases at different operating temperatures. Finally, the effect of crystalline orientation on the structural and optical properties and gas
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sensing characteristics of the grown ZnO thin films are explored.
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1. Introduction The structural, optical and surface related properties of metal oxide thin films depend largely
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on their crystallographic orientations [1-2]. For example, as reported by Delgado et al. [1] optical properties of tin oxide thin films change with the changes in crystallographic orientations. If all the grains are oriented along a particular direction, say along c-axis then the optical properties
SC
are considerably different than the case where they are oriented normal to c-axis. Also the surface properties such as the gas sensing performances of metal oxides significantly influenced
M AN U
by their effective surface area, grain and pore size, morphology as well as inter-grain contact, which in turn are also related with the crystallographic orientation. As explained by Korotcenkov et al. [2] if the grain growth is along vertical direction then the pores are straight pores, and hence require smaller time for the adsorption and desorption of gases. As reported in the
TE D
literature several techniques such as metal organic chemical vapor deposition, molecular beam epitaxy, sputtering, and so on have been used to grow thin films of desired orientations [3-5]. Most of these techniques are though expensive and require careful handling, their competent
EP
features for controlling the deposition parameters and hence the frequencies of nucleation precisely, make them very useful for growing thin films along specific crystallographic
AC C
orientations. However, it is difficult to grow such textured films from direct deposition of sols like in sol-gel process, as in this process due to lack of precise control on the frequencies of nucleation, grains get coalescence and hence instead of z-direction, growth is predominant along x-y direction. Among the metal oxides, ZnO is perhaps considered to be most studied due to its favorable properties such as high electron mobility, wide band gap, good transparency, etc. [6] and hence its versatile use in different emerging applications including optoelectronic
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applications, solar cells, protective coatings, ultrasonic oscillators and gas sensors [7-8]. For some of these applications (e.g. optoelectronic, gas sensor, etc.) the optical and surface properties of ZnO are considered to play an important role. Though ZnO is a well explored material, but as
RI PT
reflected in the literature there are few reports on the growth of textured ZnO thin films using sol-gel route [9-11] and limited research efforts have so far been made to study the gas sensing characteristics of such sol-gel grown textured ZnO thin films in presence of various toxic and
SC
combustible gases. In the present work, thus, we have attempted to grow preferentially c-axis oriented ZnO thin films, on an amorphous substrate (quartz), by spin coating the ZnO precursor
M AN U
sol synthesized using cost effective wet chemical synthesis route with an aim to study the effect of preferential orientation on the structural and optical characteristics and to evaluate its suitability as a gas sensor. 2. Experimental details
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In the present work, we have used the simple sol-gel spin coating technique to synthesize ZnO thin films on transparent fused quartz substrates. First the precursor sol (concentration ~ 0.4 M) was prepared by dissolving zinc acetate dihydrate [Zn (CH3CO2)2·2H2O] in 2-
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methoxyethanol. Mono-ethanolamine (MEA) was added to the mixture to stabilize the solution. The molar ratio of MEA to zinc acetate was maintained at 1: 1 ratio. The solution was stirred at
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60oC for 2 h and was used as precursor sol after cooling. The flow chart for the synthesis of the ZnO thin films is shown in Fig. 1. The precursor sol was spin coated over quartz substrate at 3000 rpm for 30 s using a commercial spin coating unit (SCU 2007, Apex Instruments Co, Kolkata). Prior to coating, the quartz substrates were cleaned ultrasonically in warm trichloro ethylene (TCE), acetone, methanol, and deionized water bath. Just after deposition, the film was
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inserted into a preheated furnace (kept at 300oC) for 5 min and then quenched to room temperature. The coating and drying cycles were repeated to increase the film thickness. Films of different thicknesses were prepared by varying the no. of coating (5, 10, 15 and 20
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times) and drying cycles. After final coating and drying cycle the films were annealed at 600oC for 1 h in air and cooled down to room temperature by normal furnace cooling. The structural characteristics of the films were studied by X-ray diffraction (Ultima III, Rigaku, Japan) analysis
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using Cu Kα radiation in the 2θ range 20-80° at a scanning rate of 3° min-1. For X-ray diffraction measurements, the accelerating voltage and current were maintained at 40 kV and 30 mA
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respectively. The surface and cross sectional morphology of the synthesized films was characterized using FESEM (SUPRA-40, Carl Zeiss, Germany). The optical properties of the synthesized films were investigated by analyzing the absorption spectra recorded in ultravioletvisible (UV-VIS) range using UV-VIS absorption spectrometer (Lambda 750, Perkin Elmer,
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USA). PL spectroscopy was used as a characterization tool to investigate the defect structure of the synthesized films and the PL spectra of these films were recorded using He–Cd laser as an excitation source, operating at 325 nm. The gas sensing characteristics (H2, CO, and CH4) were
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evaluated using a dynamic flow gas sensing measurement system described elsewhere in details. Using combustible gases the response%, response/ recovery times and stability of the
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synthesized films were evaluated by varying the film thickness, operating temperature (250 – 380oC), and test gas concentration (1-1660 ppm). 3. Results and discussion
3.1 . Structural characterization by X-ray diffraction Fig. 2 (a) shows the X-ray diffraction patterns of the sol-gel grown ZnO thin films of various thicknesses, grown on quartz substrates. As shown in figure in each pattern a sharp peak
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appears around 2θ ~ 34.92o, corresponding to the diffraction of (002) planes of hexagonal ZnO crystal structure. No other diffraction peaks are observed in the pattern indicating that all the synthesized films are phase pure and highly textured along (002) planes, i.e. perpendicular to the
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substrate. This preferential orientation along c-axis in ZnO thin films could be attributed to the lowest surface energy density of the (002) planes of ZnO wurzite crystal structure [12]. As explained by Fujihara et. al. [13] the probable mechanism underlying the c-axis orientation is the
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initial orientation due to nucleation and the final growth orientation at the film/ substrate interface and the successive growth in the direction perpendicular to the substrate. In addition,
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the improvement in crystallinity with increase in number of coatings is apparent in the diffraction pattern (in Fig. 2 (a)), i.e., the intensity of the (002) diffraction peak increases with the number of coatings and the full width at the half maximum (FWHM) of peaks decreases from 0.288o to 0.196o as the number of coatings increases from 5 to 20.
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The crystallite size D is estimated using the Debye-Scherer formula [14]: D = 0.9λ / β cosθ
(1)
Where λ (= 0.154 nm) is the wavelength of the X-ray radiation used, θ is the Bragg diffraction
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angle of the X-ray diffraction peaks and β is the measured full width at half maximum (FWHM) of diffraction peaks measured in radian. These values of crystallite size and lattice strain (%)
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(obtained from the XRD pattern) are plotted separately in Fig. 2 (b). From the figure, it is observed that with increasing number of coatings, the crystallite size increases and the lattice strain (%) decreases. As the films become thicker with increase in the number of coatings, it is relaxed thus reducing the lattice strain (%).
3.2. Micro-structural evaluation by FESEM
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The surface and cross sectional morphologies of all the synthesized ZnO thin films are investigated using field emission scanning electron microscopy. Fig. 3 (a-d) shows the surface morphology of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films.
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As envisaged in the micrograph all these films are porous, crack-free and have homogeneous grain size distribution. However, there is a slight variation in the morphology of the films with the variation in number of coatings. The grain size appears to increase with the increase in
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number of coatings. The same trend in the variation of micro structure was reported by Mridha et al. [15]. This is also clearly understood from the XRD pattern, where the intensity of (002) peak
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increases with increasing number of coatings. For five times coated (minimum number of coatings) ZnO thin film, the surface morphology is granular, whereas the morphology becomes more compact with increasing number of coatings. The cross-sectional morphologies, representing the thickness of the films, are shown in the right hand side of the respective films in
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Fig. 3 (e-h). As inferred from this figure there is only a small variation in the thickness of the films with the variation in number of coatings. Thicknesses of the films are found to be 320 nm, 353 nm, 393 nm and 420 nm for 5, 10, 15, and 20 times coated films respectively, indicating a
3.3.
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linear variation of thickness with the number of coatings. Optical Characterization
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Owing to its wide band gap ZnO is found to be a very good optical material. So to study the optical properties of the ZnO thin films two characterization techniques, namely UV-Vis and PL spectroscopy, are investigated. 3.3.1. UV-Vis Spectroscopy Ultra-violet to visible (UV-Vis) spectroscopy has been employed to record the optical transmittance and absorbance spectra of all the synthesized ZnO thin films. From the
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transmittance spectra (not shown here) the films are found to be transparent in the visible region (400-800) nm with average transmittance of (60-80) % and a steep fall off at 376 nm, whose corresponding energy is close to the intrinsic band gap of ZnO (3.37 eV). Also with increasing
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film thickness, the average transmittance of the films is decreased. This is probably due to the increase in absorption for higher thickness, thus reducing the transmittance.
The optical absorbance spectra of the films are shown in Fig. 4 (a). From the absorbance
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spectra, it is apparent that all the films have a low absorption in the visible range and a high absorption in the ultra-violet range. Fig. 4 (b) shows the Tauc plot representing the values of the
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optical band gap energy (Eg) of the ZnO thin films. Eg of the ZnO thin films is determined by extrapolation of the linear portion of (αhν) equation [16].
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versus hν plots at ߙ2 = 0 using the following
(2)
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Where α is absorption coefficient, hν is the photon energy and A' is a constant. In case of allowed direct transitions (as in ZnO) the value of n is equal to ½. The intercepts of these plots on the energy axis represents the energy band gaps, since
when (
)
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= 0. The measured
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energy band gap values are in the range of 3.23–3.25 eV, which is close to the band gap of
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intrinsic ZnO semiconductor. As observed, the value of band gap reduces with the increase in thickness. This is attributed to the bigger grain size for thicker films. 3.3.2. PL Spectroscopy
It is known that ZnO is a semiconductor at room temperature and shows n-type conductivity even in the absence of doping. This is due to the presence of oxygen vacancies and zinc interstitials, which are common donor type defects, exist in ZnO [6]. Hence to investigate this defect chemistry of the synthesized ZnO thin films, photoluminescence (PL) spectroscopy at
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room temperature is used as a characterization tool. Fig. 5 shows the room temperature PL spectra of the synthesized films recorded using He–Cd laser as an excitation source operating at
energy of ZnO (3.3 eV) in order to understand the defect emission.
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325 nm. The excitation energy (~3.81 eV) has been chosen to be higher than the band-gap
As revealed from the figure, each spectrum consists of two peaks at about ~389 nm, and ~545 nm. The very small peak at 389 nm (UV luminescence) is attributed to near band edge
defect level (DL) emission, primarily oxygen vacancies (
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(NBE) emission, whereas the dominant peak at ~545 nm (green luminescence) is due to the ), of the synthesized films [17]. In
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addition, there is a variation of intensity of peaks with the variation in number of coatings, which may be due to the difference in concentration of donor defect centers produced due to micro structural variations [18]. This enhanced defect concentration may be useful for gas sensing application of ZnO thin films. Gas Sensing Characteristics
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3.4.
The synthesized ZnO thin films are characterized in terms of their carbon monoxide (CO), hydrogen (H2), and methane (CH4) sensing characteristics. Initially, the gas sensing
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characteristics of all the synthesized ZnO films are studied at various operating temperatures (in the range between 250-380oC), on exposure to a fixed concentration (~ 1660 ppm) of H2 gas.
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Optimum temperature corresponding to the maximum response is observed. Fig. 6 (a) shows the variation of response% of ZnO thin film (of different thicknesses) gas sensors with the operating temperature in presence of H2 gas at ~ 1660 ppm. As observed from the figure the response% increases with increase in operating temperature up to 350oC (the optimum temperature) and reduces with further increase in the operating temperature. A good number of research reports in support of this behavior (existence of
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optimized operating temperatures corresponding to the maximum response towards a specific gas) for a number of SMO sensors are available in the literature [19-20]. The observed behavior can be explained using the concept of activation energy. At the lower temperature the reducing
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gas molecules do not have sufficient energy to react with the adsorbed oxygen species. However, with increasing operating temperature the energy is sufficient to provide the activation energy of the reaction, which increases the response. While the reduction of response with further increase
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in temperature (beyond optimum temperature) could be due to the higher desorption rates of the surface adsorbed oxygen species [20]. From this figure the optimum thickness (15 times coated
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film) is also estimated corresponding to the maximum response.
The response and recovery times (τres/τrec), estimated from the response transients of ZnO thin film gas sensors in presence of H2 gas for a fixed concentration (~ 1660 ppm) at different operating temperatures are plotted in Fig. 6 (b) and (c), respectively. Note that thinner film
response time.
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exhibits lower response time. As observed, for all these films recovery time is more than the
After optimizing the thickness (393 nm, for 15 times coated film) the thin film sensing
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element is further exposed to different test gas (H2, CO and CH4) environments at a particular concentration (~1660 ppm), to observe the effect of test gas on the response behavior. Fig 7 (a)
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shows the response% of 15 times coated ZnO thin film sensing element on exposure of 1660 ppm of H2, CO and CH4 gas at various operating temperatures. From this figure it is observed that at each temperature the response% is maximum in presence of H2, and then reduced for CO and minimum for CH4. The observed results may be explained on the basis of theory of gas diffusion. It is reported that in a gas sensor, the mechanism of gas diffusion depends on the size of the pores present in the sensing element. Consequently, surface
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diffusion, Knudsen diffusion and molecular diffusion take place in order with the increase in pore size. For a pore size in the range 2 to 50 nm (mesoporous region) Knudsen diffusion is known to take place. In this study, Knudsen diffusion is presumed to take place, owing to the
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grain size of the films (obtained from FESEM images) in the range 2 to 50 nm (as it is reported that pore size and grain size are comparable [21]). Knudsen diffusion constant (Dk) is determined
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from the following relation [22]:
(3)
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Where T is temperature, r is pore radius, R is gas constant and M is the molecular weight of the diffusing gas. For a given film (if r is assumed to be same for all the pores) (4)
So for gases with lower molecular weight, have higher diffusion coefficient. As a result these
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gases can enter upto the bottom of the film, thus increasing the utility factor and hence the response. However, for gases with higher molecular weight the utility factor is less, which reduces the response. In line to the above explanation, the response% in presence of H2, CH4 and
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CO gases are found to be 62, 53 and 50 respectively, at their respective optimized temperatures. The response and recovery time of this gas sensor, in presence of all the reducing gases are
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plotted in Fig. 7 (b) and (c) respectively. As observed, both the response and recovery time reduces with increase in temperature, being minimum during CH4 detection and maximum for CO at each temperature. 4. Conclusions In the present work we have successfully grown highly c- axis oriented ZnO thin films on quartz substrates using the low cost sol-gel spin coating technique. All the grown films exhibit
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only (002) diffraction peak, indicating the textured behavior of ZnO on the substrate. Thicknesses of films are varied by varying the number of coatings. The structural, micro structural and optical properties of the grown films are found to depend upon the number of
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coatings and hence on the thickness of the films. The gas sensing characteristics of the grown films are studied upon exposure of H2, CO, and CH4 gases at various operating temperatures. It is observed that irrespective of the operating temperature these ZnO thin film gas sensors are
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selective to H2 gas. Hence it is argued that this study can pave the way for device fabrication in detecting the highly flammable and explosive hydrogen gas in presence of other gases in the
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environment. Finally, an attempt is made to correlate the gas sensing characteristics of these films to their structural and optical properties. Acknowledgements
The author gratefully acknowledged IIT Kharagpur for this whole work.
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References
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[2] G. Korotcenkov, M. DiBattista, J. Schwank, V. Brinzari, Mater. Sci. Eng. B 77 (2000) 33–39. [3] S. Pati, S. B. Majumder, and P. Banerji, Sens. Actuators A 213 (2014) 52–58.
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[8] G.N. Dar, A. Umar, S. Zaidi, S. Baskoutas, S.W. Hwang, M. Abaker, A. Al-Hajry, S.A. AlSayari, Talanta 89 (2012) 155–161. [9] T. Yasuda, Y. Obata, M. Sato, J. Korean Phys. Soc. 53 (2008) 2921-2924.
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[10] C.H. Chia, W.C. Tsai, W.C. Chou, J. Lumin. 148 (2014) 111-115.
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[13] S. Fujihara, C. Sasaki, T. Kimura, Appl. Surf. Sci., 180 (2001) 341-350.
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[16] A. K. Chawla, D. Kaur and R. Chandra, Opt. Mater. 29 (2007) 995–998. [17] B.-Z. Dong, G.-J. Fang, J.-F. Wang, W.-J. Guan and X.-Z. Zhao, J. Appl. Phys. 101 (2007) 033713 (7).
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[18] P. M. R. Kumar, C. S. Kartha, K. P. Vijayakumar, T. Abe, Y. Kashiwaba, F. Singh and D. K. Avasthi, Semicond. Sci. Technol., 20 (2005) 120-126. [19] C. Aifan, H. Xiaodong, T. Zhangfa, B. Shouli, L. Ruixian and L. C. Chiun, Sens. Actuators
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[21] M. Ando, S. Suto, T. Suzuki, T. Tsuchida, C. Nakayama, N. Miura and N. Yamazoe, J. Mater. Chem. 4 (1994) 631-633. [22] G. Sakai, N. Matsunaga, K. Simanoe and N. Yamazoe, Sens. Actuators B, 80 (2001) 125131.
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Figure captions Fig.1. Flow chart showing the synthesis procedure of ZnO thin films using sol-gel spin coating technique
Fig.2. (a) X-ray diffraction patterns of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films grown
(obtained from x-ray diffraction pattern) with the number of coatings
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by sol-gel spin coating technique, and (b) Plot showing the variation of crystallite size and lattice strain
Fig.3. Scanning electron micrographs of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films and
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their cross sectional morphologies are shown, in the right hand side of the respective films, in Fig. 3 (e-h)
Fig.4.(a) Absorbance spectra of ZnO thin films of various number of coatings, synthesized by sol-gel spin
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coating technique on quartz substrates, and (b) Tauc plot [(a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films], showing the band gap of the corresponding films
Fig.5. Room temperature photoluminescence spectra of (a) 5, (b) 10, (c) 15, and (d) 20 times coated ZnO thin films
Fig.6. Variation of (a) response%, (b) response time, and (c) recovery time of ZnO thin film [(a) 5, (b) 10,
operating temperature
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(c) 15, and (d) 20 times coated)] gas sensors towards the detection of 1660 ppm of H2 gas as a function of
Fig.7. Variation of (a) response%, (b) response time, and (c) recovery time of ZnO thin film (15 times
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temperature
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Highlights
Highly textured ZnO thin films are grown using sol-gel spin coating technique.
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(002) oriented films are grown with an aim to study the effect of preferential orientation on
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the structural and optical characteristics and to evaluate its suitability as a gas sensor.
Gas sensing parameters in presence of H2, CO and CH4 gases are studied by varying the
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operating temperature.
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The correlation between crystallographic orientation and other properties are discussed.
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