Evolution of hydrogen gas sensing properties of sol–gel derived nickel oxide thin film

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Abstract

Hydrogen sensing properties of nanocrystalline NiO thin films are investigated. NiO thin films were fabricated by a sol–gel method with special efforts to change the porosity of coated films by a multi-step coating and annealing processes. The experimental results indicated that the multi-step annealed NiO thin films had a higher porosity than films fabricated by a typical sol–gel process. When the thicknesses of NiO films were increased, the grain size increased and the sheet resistance of the film decreased. The highest gas response was detected at 175 °C for two layered samples. NiO films fabricated by multi-step annealing showed a better response to hydrogen than NiO fabricated by a typical sol–gel process. The H2 gas response was decreased with increasing the thickness and decreasing the porosity of NiO films. A high response value of 68% was obtained for 3000 ppm of H2 at 175 °C for optimized sample by a multiple annealing process. The effect of humidity on the gas sensing performance of the sensor was studied and the devices were also tested for several cycles to study the repeatability of the sensors. The cross sensitivity measurements showed that the sensor could be used to monitor hydrogen in a likelihood mixture of reducing gases.

Introduction

Applications of nanocrystalline metal oxide in gas sensors have attracted much attention due to their high sensitivity, fast response and low operation temperature [1], [2], [3], [4], [5]. The working principal of metal oxide gas sensor (MOGS) is associated with the change of electrical conductivity due to adsorption/desorption of target gas in a given ambient. Many materials have been investigated for MOGS applications [2], [3], [4], [5], [6], [7]. NiO is an interesting p-type semiconductor, which has a band-gap around 3.6 eV [8], [9]. NiO has a wide range of applications, such as super capacitors, transparent electrochromic films, fuel cell electrodes and chemical gas sensors due to its chemical stability and nontoxicity [6], [7], [10], [11], [12]. Thin films of NiO can be fabricated by different physical or chemical techniques, such as RF sputtering [10], chemical vapor deposition [13], reactive pulsed laser deposition [14] and sol–gel technique [15]. Among these methods, sol–gel technique has few advantages over other methods, such as uniformity of grain size and simple manufacturing process [15].

It is known that the gas sensing behavior of MOGS is related to the microstructure of thin films. Several approaches have been studied to improve the gas sensing behavior of different metal oxides such as ZnO and SnO2 [16], [17], [18], [19]. Imawan et al. reported low response value of 10% for 500 ppm of H2 gas at different operating temperature of 200–500 °C for TiO2 modified NiO film [20]. Fasaki et al. reported the gas response of 50% under 10,000 ppm of H2 at 400 °C for PLD grown NiO samples [21]. Recently, Steinebach et al. reported the response values of 96.6% at 5000 ppm of H2 at high operating temperature of 500 °C for NiO thin films deposited by a sputtering method [22]. In practical viewpoint, low operation temperature of H2 sensor is required to use the sensor in ambient conditions because of explosive and flammable nature of H2 at elevated temperature. Thus, the need for a gas sensor that can be operated at relatively low operating temperature (<200 °C) with high sensitivity as well as fast response and recovery process is needed.

It is known that MOGSs are mostly non selective for different gases. Various techniques have been used to improve the sensitivity and selectivity of sensor devices such as using filters, and catalysts [23]. Moreover, in real application, humidity has an adverse effect on the gas sensing property of the device. Though numerous investigations have been carried out, there is only a limited information is given about the selectivity and reliability issues of gas sensor.

In this study, NiO thin films were fabricated using a sol–gel process. The fabricated films were uniform and the grain size was around 20 nm on the glass substrate. In order to enhance the sensor properties, porosity was increased by a multi-step annealing during the calcination. The samples were characterized for structural, electrical and optical properties. Moreover, the effects of operating temperature, humidity, thickness, grain size and porosity on the H2 sensing property of NiO thin films were investigated. Also, the cross sensitivity measurements showed that the sensor could be used to monitor a target gas in a mixed gas environment.

Section snippets

Experimental procedure

NiO thin films were deposited with a sol–gel process. The coating solution was prepared by dissolving 0.1 mol/L−1 nickel nitride hexahydrate [Ni(NO3)2·6H2O, Alfa] into 20 ml of equal amount of isopropanol alcohol and polyethylene glycol 200 [H(OCH2CH2)nOH, Alfa]. The solution was stirred at 25 °C for 2 h to yield a transparent solution. Nickel hydroxide colloid was produced in the solution by adding dilute ammonium hydroxide [NH4OH, Fisher] drop wise to the solution under magnetic stirring. In

Structure and surface morphology

Fig. 3 shows the XRD patterns of NiO samples. The peaks in the XRD patterns can be assigned to two crystallographic planes. Two peaks at 2θ = 37.4° and 43.4° were observed for all samples and assigned to (1 1 1) and (2 0 0) of simple cubic NiO, respectively. Both N-2L and P-2L samples showed approximately the same peak intensities. When the thickness was increased, the intensity of NiO peaks increased while the full width at half maximum (FWHM) values decreased. This can be attributed to the growth

Conclusions

Nanocrystalline NiO films were successfully deposited by a sol–gel process. Special efforts were made to enhance the porosity of NiO films. The structural, electrical and sensing properties of prepared films were investigated as a function of film fabrication conditions, testing temperature and H2 concentration. The gas sensor response was temperature dependent and the highest response was observed at relatively low temperature (175 °C) of H2 gas. It was found that the NiO films have several

Acknowledgment

This research was supported by a grant (Grant Number: 0933069) from National Science Foundation (NSF), USA.

Amir M. Soleimanpour is a Ph.D. candidate in Mechanical, Industrial and Manufacturing Engineering (MIME) at the University of Toledo, Ohio, USA. His research interest is in the fields of thin film processing, semiconductor devices, finite element analysis and material characterization. He received his BS and MS degrees in material science and engineering from the University of Tehran in 2005 and Sharif University of Technology in 2008, respectively.

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    Amir M. Soleimanpour is a Ph.D. candidate in Mechanical, Industrial and Manufacturing Engineering (MIME) at the University of Toledo, Ohio, USA. His research interest is in the fields of thin film processing, semiconductor devices, finite element analysis and material characterization. He received his BS and MS degrees in material science and engineering from the University of Tehran in 2005 and Sharif University of Technology in 2008, respectively.

    Yue Hou is a Ph.D. candidate in Mechanical, Industrial and Manufacturing Engineering (MIME) Department at the University of Toledo. Her research interest is in the fields of processing of materials, design of experiments and semiconductor devices. She has obtained her BS degree in Mechanical Engineering from Yantai University in 2009.

    Ahalapitiya H. Jayatissa (Jay) is a professor in MIME Department at the University of Toledo. His research interest is in the areas of processing of electronic materials, electronic devices, nanotechnology, MEMS and material characterization. He is a recipient of faculty early career award from the National Science Foundation (NSF) of USA in 2003. Also he is a recipient of distinguished research award in engineering at the University of Toledo in 2011. He has been serving as the principle investigator of numerous research projects funded by federal agencies and industries.

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