International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.91 (2015) © Research India Publications; http/www.ripublication.com/ijaer.htm
Ammonia Sensing Characteristics of p-CuO and n-ZnO Thin Films: A Comparative Study Anju Anna Jacoba, L. Balakrishnanb,*, S. R. Meherb, R. Sivacoumara, K. Shambavic and Z. C. Alexa a
MEMS and Sensor Division, School of Electronics Engineering, VIT University, 632 014, Vellore, India. b Materials Physics Division, School of Advanced Sciences, VIT University, 632 014, Vellore, India. c Photonics and Microwave Division, School of Electronics Engineering, VIT University, 632 014, Vellore, India. *
[email protected].
major advantages of using p-type oxides like CuO for gas sensors. This improves the life time and stability of sensors [5]. CuO has considerably good electrical properties, and is a good catalytic material also [6, 7]. NH3 gas sensors are widely studied due to its applications in different areas like environmental gas analysis, automotive industry, chemical industry, medical field, etc. [8]. Kumar et al. have reported the detection of low ppm level NH3 vapour in the working ambient at room temperature using spray deposited nanostructured ZnO thin films [9]. Room temperature NH3 sensing characteristics of CuO nanowires were investigated by Shao et al. and it shows high sensitivity compared to SnO2 nanowires [10]. Though there are some works reported on NH3 gas sensing using ZnO and CuO thin films, nanostructures etc., still studies are required for better sensitivity and easy fabrication of sensors at low cost and room temperature operation. This work focuses on a comparative study of sensing response of n-type ZnO and ptype CuO thin films deposited using RF magnetron sputtering technique, towards NH3 gas.
Abstract—Ammonia (NH3) is the second most extensively used chemical in the world. Its major sources are decomposition of manure, combustion from plants and automobiles, use in refrigeration system, production of fertilizers etc., High sensitive ammonia detectors with simple and low cost fabrication method are of wide research interest due to its various applications in areas like chemical, automotive, medical industries etc., Semiconducting metal oxides based chemiresistive sensors can be fabricated easily by low cost methods, and it senses gases by simple operating principle where the resistivity of the sensing film varies according to the concentration of gas to be detected. In this work, chemiresistive sensors based on p-type CuO and ntype ZnO thin films were fabricated for the detection of ammonia (NH3) gas. Thin films of CuO and ZnO were prepared on glass substrates by RF magnetron sputtering technique. The structural, morphological and optical properties of the films were investigated using X-ray diffraction, scanning electron microscope and UV-Vis analysis, respectively. The average crystallite size of deposited ZnO and CuO films were determined as 12 nm and 36 nm, respectively. Comparative study on the room temperature sensing characteristics of both the films was analyzed at different NH3 concentrations. An increase in resistance with concentration was observed for p-type CuO and vice-versa for n-type ZnO. From the analysis, it has been found that CuO thin films showed good sensitivity to NH3 compared to ZnO thin films. Keywords—gas sensors; thin film; sputtering; oxides.
II. EXPERIMENTAL ZnO and CuO films were deposited using RF reactive magnetron sputtering technique on glass substrate of size 2×2 cm2. Metallic Zn and Cu targets were used for the fabrication of ZnO and CuO thin films, respectively. The deposition conditions were same for both the films. Argon was used as the sputtering gas whereas oxygen the reactive gas. The Argon gas pressure was maintained as 0.01 mbar and oxygen as 0.02 mbar in the chamber. The target to substrate distance was kept as 6 cm. The films were deposited for 20 min at a substrate temperature of 4500 C at the RF power of 80 W. For gas sensing studies, electrical contacts of aluminum were made on the films using thermal evaporation technique. X-ray diffraction (XRD) and Scanning electron microscope (SEM) were used to study the structure and morphology of the films. The optical transmittance of ZnO and CuO thin films was measured by UV-Vis spectrophotometer in the wavelength range from 300 to 800 nm and calculated the optical band gap energy using Tauc’s plot. The electrical characteristics of the films was studied using Agilent B2901A Precision Source/Measure Unit. The change in electrical resistance is measured as a function of time for known amount of exposed NH3 gas. The chemical interaction of the target gases results in resistance
I. INTRODUCTION Chemiresistive gas sensors based on semiconducting metal oxides (SMO) have attracted wide research interest due to their small dimensions, low cost, measurement simplicity, ease of fabrication, etc. The adsorbed atmospheric oxygen on the surface of the semiconductor traps electrons from the semiconducting material, and on interaction with reducing/oxidizing gases, conductivity of the material changes due to the release of trapped electrons. This conductivity change upon exposure to different target gases serves as the sensing mechanism of SMO based sensors [1, 2]. Among the SMOs, ZnO an n-type semiconductor has received much attention due to its unique optoelectronic properties. As a gas sensing material, ZnO shows good sensitivity and also has good response towards a wide range of chemicals [3,4]. Cupric oxide (CuO) is a p-type semiconductor with a narrow energy band gap of 1.2 eV. The easy exchange of lattice oxygen with air and less dependency of conductivity on temperature at higher ranges are some
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International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.91 (2015) © Research India Publications; http/www.ripublication.com/ijaer.htm
variation. Aluminum contacts were made on the films to take leads for resistance measurement. The sensitivity (S) was calculated from the resistance obtained, using the following relation, R -R gas air (1) S[ ] 100 R air where, Rair is the resistance of the film in air ambience and Rgas is the resistance of the film upon gas exposure.
found to be about ~11 nm. Small grain size leads to large effective surface area for oxygen adsorption which makes it suitable for gas sensing. The SEM image of CuO thin film shows a porous appearance composed of agglomerated nano grains of size ~37 nm. This kind of mesoporous surface morphology composed of nano grains makes the films suitable for efficient gas sensing.
ZnO
(111)
(002)
III. RESULTS AND DISCUSSIONS A. Structural Properties Fig. 1 shows the XRD pattern of the deposited ZnO and CuO thin films. XRD pattern of ZnO thin film shows (002) preferential growth with corresponding diffraction peak at 2θ value 34.30. This indicates the formation of polycrystalline hexagonal wurtzite structure. XRD pattern of CuO thin film confirmed its polycrystalline nature with monoclinic structure. The diffraction peak at 2θ value of 37.70 corresponding to (111) orientation of CuO indicates the preferential growth of film along (111) plane. The average crystallite size (D) of the films were calculated from the X-ray diffraction patterns using Scherrer’s relation,
Intensity (a. u.)
CuO
Fig. 2. SEM image of (a) ZnO and (b) CuO thin film.
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20
30
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C. Optical Properties The UV-Vis transmission spectra of CuO and ZnO films are shown in Fig. 3 (a) and (b), respectively. ZnO thin films shows maximum transmittance of 95% whereas CuO thin films shows 57%. Optical band gap (Eg) of both the films were calculated from Tauc’s plot and are shown in the inset. For direct transition Eg can be calculated from the following Tauc’s relation, 1/2 αhυ B(hν Eg ) (3) where, hν is photon energy, B is a constant, α is absorption co-
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2(degree) Fig. 1. XRD pattern ZnO and CuO thin films
D
k
(2) cos where, k is the shape factor generally it has been taken as 0.89 for spherical shape, β is the FWHM of the peak measured in radians. The average crystallite sizes were calculated as 12 nm and 36 nm for ZnO and CuO films, respectively. This confirms the formation of nano crystalline thin films.
efficient and is calculated using the following relation, Abs (4) α(ν) 2.303( ) t where, Abs is the optical absorbance and t is the film thickness. The optical band gap of ZnO and CuO thin films obtained were 3.2 eV and 2.4 eV, respectively. The slight
B. Morphological Studies Surface morphology of ZnO and CuO thin films were studied using SEM and are shown in Fig. 2 (a) and (b), respectively. Morphology of ZnO shows a uniform distribution of nanostructured grains. The average grain size is
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International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.91 (2015) © Research India Publications; http/www.ripublication.com/ijaer.htm
variation in the optical band gap from the bulk values may be due to Burstein-Moss effect and quantum confinement [11].
compared to ZnO thin films. This can be due to the mesoporous morphology of CuO thin films and enhanced catalytic reactivity of CuO thin films towards ammonia than ZnO. This is due to the fact that in oxide thin film based sensors both adsorbed oxygen and lattice oxygen takes place in catalytic reaction [12]. From this fact, it has been understood that in CuO thin films presence of more copper vacancies also responsible for the high sensitivity.
(a)
7x1014 6x1014
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(h)2 (cm-2eV2)
Transmittance (%)
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5x1014 4x1014
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3x1014 2x10
14
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1x1014
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0
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Energy (eV)
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ZnO sensor CuO sensor
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2 (cm-2eV2)
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( h )
Transmittance (%)
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2.4x10
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1.6x10
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1.2x10
15
8.0x10
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4.0x10
14
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Concentration (ppm) Fig. 4. Sensitivity of ZnO and CuO thin films towards different concentration of NH3 gas.
0.0 1.0
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Energy (eV)
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IV. CONCLUSION CuO and ZnO thin films were grown by RF magnetron sputtering. The structural, morphological, optical and NH3 sensing characteristics of the films were studied. Both films exhibited increase in sensitivity to increasing NH3 gas concentration at room temperature whereas the sensitivity response of CuO films were found to be comparatively high due to the presence of mesoporous morphology and copper vacancies. Hence, it has been concluded that CuO thin films are suitable candidate to be used as NH3 sensor at room temperature.
1200
Wavelength (nm) Fig. 3. UV Transmittance spectra of (a) ZnO and (b) CuO thin films. Inset: Tauc’s plot of ZnO and CuO films.
D. Gas sensing response The sensitivity of the films were measured from change in resistivity on exposure to different concentrations of NH3. On exposure to reducing NH3 gas, decrease in resistance was observed for ZnO and increase in resistance was observed for CuO film. This is due to the difference in electrical behavior of n-type and p-type semiconducting oxides. For p-type CuO, adsorption of oxygen species traps electrons from CuO and hence the conductivity increases due to increase in hole concentration. Upon exposure reducing to NH3, the adsorbed oxygen species reacts with NH3 and release electrons, which reduces the hole concentration leads to the increase in resistance. In contrast, the adsorbed oxygen on the surface of n-type ZnO traps the electrons from the conduction band, potential barrier increases and thereby decreases the conductivity. On exposure to NH3 gas, the reaction between NH3 and adsorbed oxygen at the surface releases electrons, thus carrier concentration increases and hence resistivity decreases. Sensing response of CuO and ZnO thin films with increasing NH3 concentration at room temperature is shown in Fig. 4. The sensitivity of CuO films was found to be more
Acknowledgment The authors would like to acknowledge Department of Science and Technology (DST), New Delhi, India for providing the financial support through FIST (Fund for Improvement of S&T Infrastructure in Higher Educational Institutions) project [SR/FST/ETI-015/2011].
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