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Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films
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Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films Ganesh Kumar Mani, John Bosco Balaguru Rayappan ∗ Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) and School of Electrical & Electronics Engineering (SEEE), SASTRA University, Thanjavur 613 401, India
a r t i c l e
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Article history: Received 20 March 2014 Received in revised form 21 April 2014 Accepted 12 May 2014 Available online xxx Keywords: Thin films ZnO Nickel Gas sensor Ammonia
a b s t r a c t This research paper reports the deposition of nanostructured pure and Ni-doped ZnO thin films deposited at the substrate temperature of 523 K using simple and economical spray pyrolysis technique and subsequently post annealed at 673 K in air atmosphere for 3 h. Ni-doping greatly affected the crystallographic orientation, surface morphology, roughness and room temperature sensing response. Noticeable change in the crystallite size, transmittance and electrical properties was observed. The room temperature sensing characteristics like selectivity, response recovery studies, range of detection, stability and reproducibility of the undoped and Ni-doped ZnO thin films were investigated. Especially, the sensing elements exhibited an excellent selectivity towards ammonia. A lower detection limit of 5 and 25 ppm was observed for undoped and Ni-doped ZnO thin films respectively. The upper detection range was widened to 1000 ppm for the Ni-doped film. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ammonia is one of the most extensively produced chemicals in the world. Natural ammonia levels in the atmosphere are in the low ppb range. It is acutely toxic if inhaled above the exposure limit. Generally, short-term exposure over 15 min needs to be limited to 25–35 ppm and the Time-Weighted-Average (TWA) over an 8-h period should not exceed 25 ppm. Concentration for Immediate Danger to Life & Health (IDLH) is at 300 ppm [1–3]. The exploration of new, low cost and effective sensors for monitoring low to high concentrations of ammonia in the atmosphere is of great interest in many fields such as automotive, fertilizers, food processing industries, medical diagnostics and so on [4,5]. The development of chemiresistive gas sensors has attracted extensive interest in the past few decades due to their high sensitivity, selectivity, compactness, fabrication simplicity and low power consumption [6–8]. Majority of the commercially available gas sensors have been designed with the micro heater to enhance the response of the sensing element at higher operating temperatures. In the same frequent switching between heating and normal atmospheric conditions leads to structural transformation, phase separation, grain growth of the sensing element as well as the degradation of
∗ Corresponding author. Tel.: +91 4362 264101x2255; fax: +91 4362 264120. E-mail addresses: [email protected] (G.K. Mani), [email protected] (J.B.B. Rayappan).
contacts and heater performance resulting in-turn to poor stability and life time [9–11]. In this scenario, detection of target gases at room temperature would be one of the best solutions to solve the problem of degradation of the sensor and also such sensors can be employed for the in situ detection of gases like ammonia at low temperatures and explosive environments. ZnO is one of the widely used sensor materials for the detection of ammonia [12], trimethyamine [13], ethanol [14], acetaldehyde [15], formaldehyde [16], toluene [17], hydrogen peroxide [18], acetone [19], liquefied petroleum gas [20], hydrogen sulphide [21] and so on at room temperature as well as at elevated temperatures. Currently, several methods are being used to deposit ZnO thin films via sputtering, thermal evaporation, spray pyrolysis, sol gel, hydrothermal, solution growth, solvothermal, pulsed laser deposition, molecular beam epitaxy, etc. [22–24]. It is known that deposition of thin films by spray pyrolysis has many advantages like large area deposition, water as a solvent, low cost raw materials, relatively low processing temperature, effective stoichiometry control, cost effective and no need of vacuum [25,26]. Two feasible methods have been accepted to improve the sensing capability of the materials namely doping and tuning deposition parameters. Doping is an important and effective tool to modulate the properties of metal oxides. Transition metals (Fe, Co, Ni and Cu) doped has been one of the promising candidates to tune the electronic band structure and their applications [27–31]. In this context, the comparable ionic radii of Ni2+ (0.069 nm) and Zn2+ (0.074 nm) ions suggest that Ni2+ ions can substitute Zn2+ ions
http://dx.doi.org/10.1016/j.apsusc.2014.05.075 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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in the crystal structure of ZnO within the solubility limit [32,33]. By doping Ni in ZnO, one can engineer its energy levels and surface states [34]. Since gas sensing is a surface phenomenon where energy bands of the sensing element bends upwards or downwards based on its interaction with oxidising or reducing gas [35]. Therefore the influence of Ni-dopants on the surface states of ZnO and hence the gas sensing response need an interrogative insight. Though many reports are available on the magnetic behaviour of Ni-doped ZnO thin films, it is rare to find literature on the room temperature sensing behaviour. Ahn et al. [36] studied UV enhanced acetone sensing characteristics of Ni-doped ZnO nanorods at room temperature and higher sensitivity was observed at 6 at% Ni-doped films, also advised that higher concentration of Ni-doping deteriorates the sensitivity. Jain [37] studied the room temperature H2 S sensing (1 ppm) characteristics of Ni-doped ZnO nanorods and concluded that 6 at% Ni-doped nanorods showed the maximum response. Wang et al. [38] investigated the acetylene sensing properties of Ni-doped ZnO nanofibers upto 2000 ppm and higher sensitivity was found to be at 5 at% Ni-doping at 250 ◦ C. To our best knowledge only Rambu et al. [39] studied the ammonia sensing characteristic of spin coated Ni-doped ZnO films at an operating temperature of 190 ◦ C. In our previous work, undoped ZnO thin film was found to exhibit a very good response (S = 233) towards 25 ppm of ammonia with the response and recovery times of 20 and 25 s respectively [12]. Also the ammonia sensing characteristics of copper doped ZnO film was investigated and the response was found to be much better than that of undoped ZnO film [40]. With this background, the detailed investigation on the structural, morphological, optical and room temperature ammonia sensing properties of undoped and Ni-doped ZnO thin films have been studied and compared. 2. Materials and methods Spray pyrolysis technique was employed to deposit undoped and Ni-doped ZnO thin films on glass substrates. Glass substrates (Blue Star, Mumbai) were ultrasonically cleaned by using acetone, isopropanol and deionised water for 15 min and then dried in hot air oven at 373 K for 1 h. Then the substrates were carefully located on the substrate heater. The deposition was done through fully automated spray pyrolysis unit (HOLMARC, HO-TH-01, India). Compressed air was used as carrier gas. The detailed film deposition procedure was reported in our previous papers [12,40,41]. The 0.05 M of zinc acetate dihydrtae (Zn(CH3 COO)2 ·2H2 O) (Sigma–Aldrich, USA, Purity 99%) was dissolved in 50 mL of deionised water (Millipore, USA) and continuously stirred for 1 h. The prepared solution was loaded in the dispenser and fixed the heater temperature at 523 K. Ni-doped ZnO thin film was obtained through mixing 0.005 M (5 wt.%) of nickel acetate tetrahydrate (Ni(CH3 COO)2 ·4H2 O) (Sigma–Aldrich, USA, Purity 98%) into the host precursor solution. The doping concentration was measured with the help of molarity equation (Eq. (1)) [42], Weight =
Molarity × Molarity weight × Volume 1000
(1)
The crystal structure of the thin film was investigated using X-ray Diffractometer (Bruker, D8 Focus, Germany) with Cu K␣ radi˚ with the 2 range between 30◦ to 60◦ with a ation ( = 1.5418 A) ◦ scan rate of 1 /min. The morphological features were examined using Field Emission Scanning Electron Microscope (FE-SEM)(JEOL, 6701F, Japan) and Atomic Force Microscopy (Veeco Caliber, USA). Elemental composition was analysed using Energy Dispersive Xray Spectroscopy (EDS) along with FE-SEM system. Thickness of the undoped and Ni-doped ZnO thin films was measured by profilometer (Mitutoyo, SJ 301, USA) and found to be 500 nm and
540 nm respectively. The optical properties of the films were studied using UV–vis spectrophotometer (Perkin Elmer, Lambda 25, USA) in the wavelength range of 200–800 nm with the scan rate of 100 nm min−1 . The sensing properties of undoped and Ni-doped ZnO thin films were studied using our customised vapour sensing chamber [12]. The detailed description about gas sensing set up available with our previous reports [12,43]. Sensing measurements were performed at room temperature (303 K and 55% RH). 3. Results and discussion 3.1. Structural studies The XRD patterns of the undoped and Ni-doped ZnO thin films are shown in Fig. 1(a). The acquired XRD pattern confirmed that the deposited films were polycrystalline with wurtzite crystal structure and well matched with JCPDS card no. 36-1451. Moreover no peaks corresponding to Ni or NiO was observed in the doped film which confirmed that the doping was within the solubility limit. The decrease in peak width (Fig. 1(b)) and increase in intensity of the peaks (Fig. 1(c)) suggested that the crystal quality of the film was improved by Ni-doping. The lattice constant for (0 0 2) ˚ film was larger than that plane of the Ni-doped ZnO (c = 5.1146 A) ˚ It emphasises the fact that the Ni of undoped ZnO (c = 5.1118 A). ions exist in the interstitial sites of ZnO. The average crystallite size using Scherrer’s formula [44] was found to be 28 and 33 nm for undoped and Ni-doped ZnO thin films respectively. 3.2. Morphological and elemental studies Scanning electron micrographs of the undoped and Ni-doped ZnO thin films are shown in Fig. 2(a) and (b). It is clear that the surface morphology was modified by doping. Undoped ZnO thin films has uniformly distributed and tightly packed spherical grains with clear grain boundaries. Ni-doped ZnO thin film was found to be composed of non-uniform grains. It may be due to the formation of high nucleation density enhanced by Ni-dopants which in turn decreased the crystallite size and led to the aggregation of crystallites. Furthermore the deposition was uniform throughout the substrate and the film surface did not contain cracks and any other serious defects. The EDS results (Fig. 3(a)) proved the presence of zinc, oxygen in the undoped film and in addition nickel in the Nidoped ZnO thin film. The presence of Si and Au in the film could be ascribed to the effect of substrate and sputter coating to eliminate charging effects respectively. 3.3. Topographical studies AFM measurements were performed to examine the differences in surface morphology between undoped and Ni-doped ZnO thin films. AFM images were taken by scanning over 10 × 10 m2 area and is shown in Fig. 4(a) and (b). A strong difference between undoped and Ni-doped ZnO thin film surface morphologies were observed. Undoped ZnO thin film has uniformly distributed homogeneous smaller spherical grains. On the other hand, Ni-doped films were formed as clusters comprising of tiny grains with non-uniform morphology. The Root Mean Square (RMS) roughness were found to be 22.2 and 31.5 nm for undoped and Ni-doped ZnO films respectively. The increase in surface roughness in Ni-doped ZnO thin film might be due to the enhancement in the number of nucleation sites by Ni dopants [45]. 3.4. Optical studies The optical transmittance spectra of undoped and Ni-doped ZnO thin films was shown in Fig. 5(a). The sharp absorption edge and no impurity peaks is another proof for the formation of high
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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crystalline quality ZnO thin films even in the case of Ni-doped ZnO thin films. The transmittance was found to decrease with Ni-doping and this may be associated with the impurity levels created below the conduction band as a result of Ni-doping. The calculated optical band gap (Fig. 5(b)) using Tauc’s plot [46] was found to be 3.30 and 3.25 eV for undoped and Ni-doped ZnO thin films respectively. The decrease in the band gap value might be due to the influence of film thickness, exchange interaction between d electrons of the Ni ions and the host s and p electrons of Zn ions and lowering of a conduction band induced by Ni-doping [47].
Fig. 3. EDX spectra of (a) undoped and (b) Ni-doped ZnO thin films.
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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Fig. 4. The 2D and 3D topography of (a) undoped and (b) Ni-doped ZnO thin film surfaces.
3.5. Ammonia sensing performance The sensing performance of the undoped and Ni-doped ZnO thin films were carried out systematically using the home-made setup and a high resistance electrometer (Keithley 6517A, USA). The electrical contacts were made on the surface of thin films using conductive silver paste following our previous work [12,43] and the precise geometry of electrical contacts is shown in Fig. 6. The baseline resistance was recorded at room temperature for both the undoped and Ni-doped sensing elements. The response to the sensing elements to test vapour at room temperature was calculated using the relation, S = Ra /Rg , where Ra and Rg are the resistance of the sensor measured in the dry air atmosphere and test vapour respectively. 3.5.1. Selectivity The room temperature sensing properties of the undoped and Ni-doped ZnO thin films were carried out after determining their selectivity trend. In this study, the response of the films towards 100 ppm concentration of various commonly interfering vapours namely ethanol, methanol, 2-propanol, benzyl alcohol, acetone, acetic acid and ammonia were observed and are shown in Fig. 7(a) and (b). From the observed results, it was found that the response of the films was significant towards ammonia than for other vapours. Based on this observation, a complete sensing performance of the Ni-doped and undoped ZnO films towards ammonia was carried out. Since we have already reported the ammonia sensing performance of the undoped ZnO at the room temperature [12,40], the sensing characteristics of Ni-doped ZnO sensing element has been compared with that of undoped one.
3.5.2. Response and recovery characteristics The initial resistance (base resistance) of the sensor was measured in dry air atmosphere and found to be stable for longer time. The response of the undoped and Ni-doped films towards 5–1000 ppm of ammonia was estimated by recording the change in resistance with reference to baseline resistance. The change in resistance trend is shown in Fig. 8(a) and (b) for undoped and Ni-doped ZnO films respectively. Since ammonia is a reducing gas and ZnO is an n-type semiconductor, the interaction of with the ZnO film led to a decrease in the resistance of the film with increase in the concentration of ammonia. The transient resistance response of Ni-doped ZnO thin film towards 25–1000 ppm of ammonia is shown in Fig. 9. The correlation between sensor responses and ammonia concentration for undoped and Ni-doped ZnO thin films is shown in Fig. 10(a) and (b). The response of the undoped ZnO was found to be saturated for 25 ppm of ammonia but for Ni-doped ZnO film, the response was not saturated even at 1000 ppm.
3.5.3. Speed of response and recovery The interaction rate of the target gas with the sensor element determines the response and recovery time of the sensor. The response (t90% (air to target gas) ) and recovery times (t90% (target gas to air) ) are defined as the time taken to attain 90% of its steady state value. The response and recovery time trend of the undoped and Ni-doped ZnO thin films is shown in Fig. 11(a) and (b). Fig. 11(c) shows the transient resistance response of the Ni-doped film towards 750 ppm of ammonia. The response and recovery times were found to be 46 and 14 s respectively. But they were found to be 12 and 39 s respectively for the undoped film.
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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Fig. 5. (a) Optical transmittance spectra and (b) Tauc’s plot of undoped and Ni-doped ZnO thin films.
The response time of the Ni-doped ZnO thin films was longer than that of undoped ZnO film. But remarkably shorter recovery was observed for Ni-doped ZnO. 3.5.4. Stability and reproducibility Further, to confirm the repeatability and the stability of sensing element, the response, response and recovery times for film samples were recorded periodically for 60 days. The observed cyclic response is shown in Fig. 12(a) which revealed the reproducible capability of the sensing element. Also the long term stability Fig. 12(b) of the sensing element was tested not only in air, but
Fig. 8. Change in resistance trend with respect to concentration of vapours (a) undoped and (b) Ni-doped ZnO thin films. Inset shows the highlighted view of low concentration response of undoped ZnO thin film. Fig. 6. Geometry of the electrical contacts.
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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also towards 500 ppm of ammonia and the results showed a similar response trend and hence confirmed the stability. 3.5.5. Sensing mechanism In the ambient atmosphere, because of the interaction between oxygen species and the n-type ZnO sensing element, the surface resistance was found to be increased due to the oxidization process following the mechanism given below (Eq. (2)) [36]: O2 (atmosphere) + e− → O− 2 (Zno surface) (Zno surface)
(2)
Fig. 11. Typical response recovery characteristics of (a) undoped, (b) Ni-doped ZnO thin film towards various concentrations with respect to time and (c) transient resistance response of Ni-doped ZnO towards 750 ppm of ammonia.
This interaction leads to the formation of baseline resistance for the sensing measurement. When the ZnO sensing element was exposed to ammonia, the gas–solid interaction led to the following reaction mechanism (Eq. (3)):
Fig. 10. Relative sensing response for (a) undoped and (b) Ni-doped ZnO thin film.
4NH3 + 3O− → 2N2 + 6H2 O + 6e− 2 (ZnO surface)
(3)
Please cite this article in press as: G.K. Mani, J.B.B. Rayappan, Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.05.075
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Acknowledgements
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The authors wish to express their sincere thanks to the Department of Science & Technology, New Delhi, India for their financial support (Project ID: INT/SWD/VINN/P-04/2011). They also wish to acknowledge SASTRA University, Thanjavur for extending infrastructural support to carry out this work. We would like to thank Dr. R. Varadarajan, Professor, SEEE, SASTRA University for his linguistic corrections.
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Time (days) Fig. 12. (a) The cyclic transient response of sensing behaviour towards 750 ppm of ammonia and (b) long term stability of Ni-doped ZnO thin film tested over a period of 60 days towards 500 ppm.
During this interaction, the liberated electrons led to the increased conductivity of the film. The initial resistance of the undoped and Ni-doped ZnO was found to be 3.5 × 1011 and 4.8 × 108 respectively. The overall sensing response of the Ni-doped ZnO films was found to be lower than that of undoped one. The reduced sensor response of the Nidoped ZnO thin film might be due to the reduced number of carriers available for the interaction when compared with the undoped one and the same was confirmed by the decrease in resistance of the doped film with reference to the undoped one. The quick recovery time and wide range detection performance might be due to enhanced catalytic activity of Ni-doped ZnO thin films. 4. Conclusion In summary, pure and Ni-doped ZnO thin films were deposited on glass substrates and their structural, morphological and optical studies were studied. Individual and comparative sensing properties of the undoped and Ni-doped ZnO thin films were investigated. A significant improvement in the recovery time and extended detection range of 25–1000 ppm towards ammonia for the Nidoped ZnO sensor was observed. Hence Ni-doped ZnO sensing element can be used to detect ammonia in the presence of other interfering vapours that too at room temperature in fertilizer, textile, explosive and chemical industries where ammonia is used as a raw or additional chemical.
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Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films Ganesh Kumar Mani, John Bosco Balaguru Rayappan ∗ Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) and School of Electrical & Electronics Engineering (SEEE), SASTRA University, Thanjavur 613 401, India
a r t i c l e
i n f o
Article history: Received 20 March 2014 Received in revised form 21 April 2014 Accepted 12 May 2014 Available online xxx Keywords: Thin films ZnO Nickel Gas sensor Ammonia
a b s t r a c t This research paper reports the deposition of nanostructured pure and Ni-doped ZnO thin films deposited at the substrate temperature of 523 K using simple and economical spray pyrolysis technique and subsequently post annealed at 673 K in air atmosphere for 3 h. Ni-doping greatly affected the crystallographic orientation, surface morphology, roughness and room temperature sensing response. Noticeable change in the crystallite size, transmittance and electrical properties was observed. The room temperature sensing characteristics like selectivity, response recovery studies, range of detection, stability and reproducibility of the undoped and Ni-doped ZnO thin films were investigated. Especially, the sensing elements exhibited an excellent selectivity towards ammonia. A lower detection limit of 5 and 25 ppm was observed for undoped and Ni-doped ZnO thin films respectively. The upper detection range was widened to 1000 ppm for the Ni-doped film. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ammonia is one of the most extensively produced chemicals in the world. Natural ammonia levels in the atmosphere are in the low ppb range. It is acutely toxic if inhaled above the exposure limit. Generally, short-term exposure over 15 min needs to be limited to 25–35 ppm and the Time-Weighted-Average (TWA) over an 8-h period should not exceed 25 ppm. Concentration for Immediate Danger to Life & Health (IDLH) is at 300 ppm [1–3]. The exploration of new, low cost and effective sensors for monitoring low to high concentrations of ammonia in the atmosphere is of great interest in many fields such as automotive, fertilizers, food processing industries, medical diagnostics and so on [4,5]. The development of chemiresistive gas sensors has attracted extensive interest in the past few decades due to their high sensitivity, selectivity, compactness, fabrication simplicity and low power consumption [6–8]. Majority of the commercially available gas sensors have been designed with the micro heater to enhance the response of the sensing element at higher operating temperatures. In the same frequent switching between heating and normal atmospheric conditions leads to structural transformation, phase separation, grain growth of the sensing element as well as the degradation of
∗ Corresponding author. Tel.: +91 4362 264101x2255; fax: +91 4362 264120. E-mail addresses: [email protected] (G.K. Mani), [email protected] (J.B.B. Rayappan).
contacts and heater performance resulting in-turn to poor stability and life time [9–11]. In this scenario, detection of target gases at room temperature would be one of the best solutions to solve the problem of degradation of the sensor and also such sensors can be employed for the in situ detection of gases like ammonia at low temperatures and explosive environments. ZnO is one of the widely used sensor materials for the detection of ammonia [12], trimethyamine [13], ethanol [14], acetaldehyde [15], formaldehyde [16], toluene [17], hydrogen peroxide [18], acetone [19], liquefied petroleum gas [20], hydrogen sulphide [21] and so on at room temperature as well as at elevated temperatures. Currently, several methods are being used to deposit ZnO thin films via sputtering, thermal evaporation, spray pyrolysis, sol gel, hydrothermal, solution growth, solvothermal, pulsed laser deposition, molecular beam epitaxy, etc. [22–24]. It is known that deposition of thin films by spray pyrolysis has many advantages like large area deposition, water as a solvent, low cost raw materials, relatively low processing temperature, effective stoichiometry control, cost effective and no need of vacuum [25,26]. Two feasible methods have been accepted to improve the sensing capability of the materials namely doping and tuning deposition parameters. Doping is an important and effective tool to modulate the properties of metal oxides. Transition metals (Fe, Co, Ni and Cu) doped has been one of the promising candidates to tune the electronic band structure and their applications [27–31]. In this context, the comparable ionic radii of Ni2+ (0.069 nm) and Zn2+ (0.074 nm) ions suggest that Ni2+ ions can substitute Zn2+ ions
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in the crystal structure of ZnO within the solubility limit [32,33]. By doping Ni in ZnO, one can engineer its energy levels and surface states [34]. Since gas sensing is a surface phenomenon where energy bands of the sensing element bends upwards or downwards based on its interaction with oxidising or reducing gas [35]. Therefore the influence of Ni-dopants on the surface states of ZnO and hence the gas sensing response need an interrogative insight. Though many reports are available on the magnetic behaviour of Ni-doped ZnO thin films, it is rare to find literature on the room temperature sensing behaviour. Ahn et al. [36] studied UV enhanced acetone sensing characteristics of Ni-doped ZnO nanorods at room temperature and higher sensitivity was observed at 6 at% Ni-doped films, also advised that higher concentration of Ni-doping deteriorates the sensitivity. Jain [37] studied the room temperature H2 S sensing (1 ppm) characteristics of Ni-doped ZnO nanorods and concluded that 6 at% Ni-doped nanorods showed the maximum response. Wang et al. [38] investigated the acetylene sensing properties of Ni-doped ZnO nanofibers upto 2000 ppm and higher sensitivity was found to be at 5 at% Ni-doping at 250 ◦ C. To our best knowledge only Rambu et al. [39] studied the ammonia sensing characteristic of spin coated Ni-doped ZnO films at an operating temperature of 190 ◦ C. In our previous work, undoped ZnO thin film was found to exhibit a very good response (S = 233) towards 25 ppm of ammonia with the response and recovery times of 20 and 25 s respectively [12]. Also the ammonia sensing characteristics of copper doped ZnO film was investigated and the response was found to be much better than that of undoped ZnO film [40]. With this background, the detailed investigation on the structural, morphological, optical and room temperature ammonia sensing properties of undoped and Ni-doped ZnO thin films have been studied and compared. 2. Materials and methods Spray pyrolysis technique was employed to deposit undoped and Ni-doped ZnO thin films on glass substrates. Glass substrates (Blue Star, Mumbai) were ultrasonically cleaned by using acetone, isopropanol and deionised water for 15 min and then dried in hot air oven at 373 K for 1 h. Then the substrates were carefully located on the substrate heater. The deposition was done through fully automated spray pyrolysis unit (HOLMARC, HO-TH-01, India). Compressed air was used as carrier gas. The detailed film deposition procedure was reported in our previous papers [12,40,41]. The 0.05 M of zinc acetate dihydrtae (Zn(CH3 COO)2 ·2H2 O) (Sigma–Aldrich, USA, Purity 99%) was dissolved in 50 mL of deionised water (Millipore, USA) and continuously stirred for 1 h. The prepared solution was loaded in the dispenser and fixed the heater temperature at 523 K. Ni-doped ZnO thin film was obtained through mixing 0.005 M (5 wt.%) of nickel acetate tetrahydrate (Ni(CH3 COO)2 ·4H2 O) (Sigma–Aldrich, USA, Purity 98%) into the host precursor solution. The doping concentration was measured with the help of molarity equation (Eq. (1)) [42], Weight =
Molarity × Molarity weight × Volume 1000
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The crystal structure of the thin film was investigated using X-ray Diffractometer (Bruker, D8 Focus, Germany) with Cu K␣ radi˚ with the 2 range between 30◦ to 60◦ with a ation ( = 1.5418 A) ◦ scan rate of 1 /min. The morphological features were examined using Field Emission Scanning Electron Microscope (FE-SEM)(JEOL, 6701F, Japan) and Atomic Force Microscopy (Veeco Caliber, USA). Elemental composition was analysed using Energy Dispersive Xray Spectroscopy (EDS) along with FE-SEM system. Thickness of the undoped and Ni-doped ZnO thin films was measured by profilometer (Mitutoyo, SJ 301, USA) and found to be 500 nm and
540 nm respectively. The optical properties of the films were studied using UV–vis spectrophotometer (Perkin Elmer, Lambda 25, USA) in the wavelength range of 200–800 nm with the scan rate of 100 nm min−1 . The sensing properties of undoped and Ni-doped ZnO thin films were studied using our customised vapour sensing chamber [12]. The detailed description about gas sensing set up available with our previous reports [12,43]. Sensing measurements were performed at room temperature (303 K and 55% RH). 3. Results and discussion 3.1. Structural studies The XRD patterns of the undoped and Ni-doped ZnO thin films are shown in Fig. 1(a). The acquired XRD pattern confirmed that the deposited films were polycrystalline with wurtzite crystal structure and well matched with JCPDS card no. 36-1451. Moreover no peaks corresponding to Ni or NiO was observed in the doped film which confirmed that the doping was within the solubility limit. The decrease in peak width (Fig. 1(b)) and increase in intensity of the peaks (Fig. 1(c)) suggested that the crystal quality of the film was improved by Ni-doping. The lattice constant for (0 0 2) ˚ film was larger than that plane of the Ni-doped ZnO (c = 5.1146 A) ˚ It emphasises the fact that the Ni of undoped ZnO (c = 5.1118 A). ions exist in the interstitial sites of ZnO. The average crystallite size using Scherrer’s formula [44] was found to be 28 and 33 nm for undoped and Ni-doped ZnO thin films respectively. 3.2. Morphological and elemental studies Scanning electron micrographs of the undoped and Ni-doped ZnO thin films are shown in Fig. 2(a) and (b). It is clear that the surface morphology was modified by doping. Undoped ZnO thin films has uniformly distributed and tightly packed spherical grains with clear grain boundaries. Ni-doped ZnO thin film was found to be composed of non-uniform grains. It may be due to the formation of high nucleation density enhanced by Ni-dopants which in turn decreased the crystallite size and led to the aggregation of crystallites. Furthermore the deposition was uniform throughout the substrate and the film surface did not contain cracks and any other serious defects. The EDS results (Fig. 3(a)) proved the presence of zinc, oxygen in the undoped film and in addition nickel in the Nidoped ZnO thin film. The presence of Si and Au in the film could be ascribed to the effect of substrate and sputter coating to eliminate charging effects respectively. 3.3. Topographical studies AFM measurements were performed to examine the differences in surface morphology between undoped and Ni-doped ZnO thin films. AFM images were taken by scanning over 10 × 10 m2 area and is shown in Fig. 4(a) and (b). A strong difference between undoped and Ni-doped ZnO thin film surface morphologies were observed. Undoped ZnO thin film has uniformly distributed homogeneous smaller spherical grains. On the other hand, Ni-doped films were formed as clusters comprising of tiny grains with non-uniform morphology. The Root Mean Square (RMS) roughness were found to be 22.2 and 31.5 nm for undoped and Ni-doped ZnO films respectively. The increase in surface roughness in Ni-doped ZnO thin film might be due to the enhancement in the number of nucleation sites by Ni dopants [45]. 3.4. Optical studies The optical transmittance spectra of undoped and Ni-doped ZnO thin films was shown in Fig. 5(a). The sharp absorption edge and no impurity peaks is another proof for the formation of high
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crystalline quality ZnO thin films even in the case of Ni-doped ZnO thin films. The transmittance was found to decrease with Ni-doping and this may be associated with the impurity levels created below the conduction band as a result of Ni-doping. The calculated optical band gap (Fig. 5(b)) using Tauc’s plot [46] was found to be 3.30 and 3.25 eV for undoped and Ni-doped ZnO thin films respectively. The decrease in the band gap value might be due to the influence of film thickness, exchange interaction between d electrons of the Ni ions and the host s and p electrons of Zn ions and lowering of a conduction band induced by Ni-doping [47].
Fig. 3. EDX spectra of (a) undoped and (b) Ni-doped ZnO thin films.
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Fig. 4. The 2D and 3D topography of (a) undoped and (b) Ni-doped ZnO thin film surfaces.
3.5. Ammonia sensing performance The sensing performance of the undoped and Ni-doped ZnO thin films were carried out systematically using the home-made setup and a high resistance electrometer (Keithley 6517A, USA). The electrical contacts were made on the surface of thin films using conductive silver paste following our previous work [12,43] and the precise geometry of electrical contacts is shown in Fig. 6. The baseline resistance was recorded at room temperature for both the undoped and Ni-doped sensing elements. The response to the sensing elements to test vapour at room temperature was calculated using the relation, S = Ra /Rg , where Ra and Rg are the resistance of the sensor measured in the dry air atmosphere and test vapour respectively. 3.5.1. Selectivity The room temperature sensing properties of the undoped and Ni-doped ZnO thin films were carried out after determining their selectivity trend. In this study, the response of the films towards 100 ppm concentration of various commonly interfering vapours namely ethanol, methanol, 2-propanol, benzyl alcohol, acetone, acetic acid and ammonia were observed and are shown in Fig. 7(a) and (b). From the observed results, it was found that the response of the films was significant towards ammonia than for other vapours. Based on this observation, a complete sensing performance of the Ni-doped and undoped ZnO films towards ammonia was carried out. Since we have already reported the ammonia sensing performance of the undoped ZnO at the room temperature [12,40], the sensing characteristics of Ni-doped ZnO sensing element has been compared with that of undoped one.
3.5.2. Response and recovery characteristics The initial resistance (base resistance) of the sensor was measured in dry air atmosphere and found to be stable for longer time. The response of the undoped and Ni-doped films towards 5–1000 ppm of ammonia was estimated by recording the change in resistance with reference to baseline resistance. The change in resistance trend is shown in Fig. 8(a) and (b) for undoped and Ni-doped ZnO films respectively. Since ammonia is a reducing gas and ZnO is an n-type semiconductor, the interaction of with the ZnO film led to a decrease in the resistance of the film with increase in the concentration of ammonia. The transient resistance response of Ni-doped ZnO thin film towards 25–1000 ppm of ammonia is shown in Fig. 9. The correlation between sensor responses and ammonia concentration for undoped and Ni-doped ZnO thin films is shown in Fig. 10(a) and (b). The response of the undoped ZnO was found to be saturated for 25 ppm of ammonia but for Ni-doped ZnO film, the response was not saturated even at 1000 ppm.
3.5.3. Speed of response and recovery The interaction rate of the target gas with the sensor element determines the response and recovery time of the sensor. The response (t90% (air to target gas) ) and recovery times (t90% (target gas to air) ) are defined as the time taken to attain 90% of its steady state value. The response and recovery time trend of the undoped and Ni-doped ZnO thin films is shown in Fig. 11(a) and (b). Fig. 11(c) shows the transient resistance response of the Ni-doped film towards 750 ppm of ammonia. The response and recovery times were found to be 46 and 14 s respectively. But they were found to be 12 and 39 s respectively for the undoped film.
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1.02
Ni-doped ZnO
2.5
0.8
0.35 0.06
0.0
3.25 eV
2.6
2.8
3.0
3.30 eV
3.2
3.4
3.6
M et ha no l 2Pr op an ol
0.2
0.0 2.4
1.07
3.0
1.0
0.4
1.07
1.01
0.27 0.02
Et ha Be no nz l yl Al co ho l Ac et on e Ac et ic Ac id Am m on ia
0 350
1.02
M et ha no l 2Pr op an ol
Transmittance (%)
60
Et ha Be no nz l yl Al co ho l Ac et on e Ac et ic Ac id Am m on ia
Response log (S)
100
70
3.8
hν (eV)
Fig. 7. Selectivity histogram of the (a) undoped and (b) Ni-doped ZnO thin films towards 100 ppm of various vapours.
Fig. 5. (a) Optical transmittance spectra and (b) Tauc’s plot of undoped and Ni-doped ZnO thin films.
The response time of the Ni-doped ZnO thin films was longer than that of undoped ZnO film. But remarkably shorter recovery was observed for Ni-doped ZnO. 3.5.4. Stability and reproducibility Further, to confirm the repeatability and the stability of sensing element, the response, response and recovery times for film samples were recorded periodically for 60 days. The observed cyclic response is shown in Fig. 12(a) which revealed the reproducible capability of the sensing element. Also the long term stability Fig. 12(b) of the sensing element was tested not only in air, but
Fig. 8. Change in resistance trend with respect to concentration of vapours (a) undoped and (b) Ni-doped ZnO thin films. Inset shows the highlighted view of low concentration response of undoped ZnO thin film. Fig. 6. Geometry of the electrical contacts.
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6 8
6x10
8
5x10
7
1x10
0
1000
2000
3000
4000
5000
6000
1000 ppm
indicates Ammonia OFF
750 ppm
indicates Ammoia ON
500 ppm
8
1x10
300 ppm
8
2x10
200 ppm
100 ppm
75 ppm
8
3x10
50 ppm
25 ppm
Resistance (Ω)
8
4x10
7000
8000
Time (s) Fig. 9. Transient response curves of Ni-doped ZnO thin film as a function of concentration from 25 ppm to 1000 ppm.
also towards 500 ppm of ammonia and the results showed a similar response trend and hence confirmed the stability. 3.5.5. Sensing mechanism In the ambient atmosphere, because of the interaction between oxygen species and the n-type ZnO sensing element, the surface resistance was found to be increased due to the oxidization process following the mechanism given below (Eq. (2)) [36]: O2 (atmosphere) + e− → O− 2 (Zno surface) (Zno surface)
(2)
Fig. 11. Typical response recovery characteristics of (a) undoped, (b) Ni-doped ZnO thin film towards various concentrations with respect to time and (c) transient resistance response of Ni-doped ZnO towards 750 ppm of ammonia.
This interaction leads to the formation of baseline resistance for the sensing measurement. When the ZnO sensing element was exposed to ammonia, the gas–solid interaction led to the following reaction mechanism (Eq. (3)):
Fig. 10. Relative sensing response for (a) undoped and (b) Ni-doped ZnO thin film.
4NH3 + 3O− → 2N2 + 6H2 O + 6e− 2 (ZnO surface)
(3)
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Acknowledgements
8
5.8x10
Ammonia ON
8
4.8x10
The authors wish to express their sincere thanks to the Department of Science & Technology, New Delhi, India for their financial support (Project ID: INT/SWD/VINN/P-04/2011). They also wish to acknowledge SASTRA University, Thanjavur for extending infrastructural support to carry out this work. We would like to thank Dr. R. Varadarajan, Professor, SEEE, SASTRA University for his linguistic corrections.
8
8
2.8x10
Ammonia OFF
Resistance ( )
3.8x10
8
1.8x10
References
(a)
7
8x10
0
1000
2000
3000
4000
5000
Time (s) 5.0
8.0
4.9
7.5
4.6
7.0
6.5
4.5
6.0
4.4
Resistance x x10
Resistance x 10
500 ppm
4.7
7
4.8 8
7
5.5
4.3
In air
(b)
4.2
5.0 0
10
20
30
40
50
60
Time (days) Fig. 12. (a) The cyclic transient response of sensing behaviour towards 750 ppm of ammonia and (b) long term stability of Ni-doped ZnO thin film tested over a period of 60 days towards 500 ppm.
During this interaction, the liberated electrons led to the increased conductivity of the film. The initial resistance of the undoped and Ni-doped ZnO was found to be 3.5 × 1011 and 4.8 × 108 respectively. The overall sensing response of the Ni-doped ZnO films was found to be lower than that of undoped one. The reduced sensor response of the Nidoped ZnO thin film might be due to the reduced number of carriers available for the interaction when compared with the undoped one and the same was confirmed by the decrease in resistance of the doped film with reference to the undoped one. The quick recovery time and wide range detection performance might be due to enhanced catalytic activity of Ni-doped ZnO thin films. 4. Conclusion In summary, pure and Ni-doped ZnO thin films were deposited on glass substrates and their structural, morphological and optical studies were studied. Individual and comparative sensing properties of the undoped and Ni-doped ZnO thin films were investigated. A significant improvement in the recovery time and extended detection range of 25–1000 ppm towards ammonia for the Nidoped ZnO sensor was observed. Hence Ni-doped ZnO sensing element can be used to detect ammonia in the presence of other interfering vapours that too at room temperature in fertilizer, textile, explosive and chemical industries where ammonia is used as a raw or additional chemical.
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