Advanced Materials Research Vol. 506 (2012) pp 39-42 © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.506.39

Novel Flexible NH 3 Gas Sensor Prepared By Ink-Jet Printing Technique

C. Wongchoosuk 1,a , P. Jangtawee 2,b , S. Lokavee 3,c ,S. Udomrat 3,e,

P. Sudkeaw 2,f and T. Kerdcharoen 4,5,g 1Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand 2Department of Chemistry, Faculty of Science, Prince of Songkla University, Songkhla 90112, Thailand 3Materials Science and Engineering Programme, Faculty of science, Mahidol University, Bangkok 10400, Thailand 4Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand 5NANOTEC Center of Excellence at Mahidol University, National Nanotechnology Center, Thailand [email protected],[email protected], [email protected], [email protected], [email protected], [email protected]

Keywords: Inkjet printing, PEDOT/PSS, Ammonia Gas sensor, Polymer/metal interface

Abstract. We have fabricated a low-cost and flexible NH 3 gas sensor using thermal ink-jet printing. The poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonated acid (PEDOT/PSS) with thickness of ~ 2 µm was used as a sensing film. The interdigitated electrode using patterned aluminum plate was attached over the sensing film. Atomic force microscopy results show the high homogeneous film and only small roughness is presented on the sensing film. This sensor exhibits high selectivity and sensitivity to NH 3 at room temperature. The sensor response works linearly with gas concentrations between 100-1000 ppm. The modulation of conducting polymer/metal electrode interface plays a role in the sensing mechanism of NH 3. Changes in the position of interdigitated electrodes can change the dominant sensing mechanism of typical polymer gas sensor.

Introduction

Ammonia (NH 3) is highly toxic. Inhalation of NH 3 at high level may cause respiratory distress or failure. Therefore, detection of NH 3 is important in the industrial, medical, and environmental applications. Up to now, there are several techniques to prepare NH 3 gas sensors such as spin coating [1], reactive sputtering [2], Langmuir-Blodgett technique [3], thermal evaporation [4], and chemical vapor deposition [5]. Although these techniques have been successfully used in the fabrication of NH 3 gas sensors, many of them do not support the preparation of film on flexible substrates such as paper or plastic. Moreover, some of these techniques have disadvantages such as high cost, complicated and long-time operations, low reliability, and low productivity. To overcome the problems, inkjet printing technique has recently attracted considerable attention as a powerful technology for fabricating the gas sensor devices [6,7]. However, few studies on the NH 3 gas sensors based on the inkjet printing technique have been published [8,9]. Thus, most studies have relied on similar gas sensor structure (i.e. deposition of sensing film on top of electrode). The new design of sensor structure is still requested since atomic force microscopy (AFM) showed that the vapor can redissolve the sensing film [7] that may cause a change in sensing properties and sensor drift effects. In this paper, we have reported the fabrication of low-cost and flexible NH 3 gas sensor using inkjet printing technique on a basis of a novel sensor structure and electronic ink formula. The sensing mechanism of the flexible gas sensor will be discussed in more details.

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Experimental Details Preparation of Electronic Ink. The electronic ink was prepared based on the conjugated polymer poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonated acid (PEDOT/PSS) (Baytron P). The PEDOT-PSS was dissolved in a mixture of solvents in the weight ratio of 89.82% PEDOT/PSS to 10.18% solvent. The 10.18 wt.% solvent includes 5.98 wt.% dimethyl sulfoxide (DMSO), 3.99 wt.% ethylene glycol (EG), and 0.199 wt.% triton x-100. It should be noted that the selective DMSO can give the low baseline resistance at low temperature and good sensitivity to

NH 3 [10] while the EG and the triton x-100 were added for improvement of the viscosity and surface tension as well as prevention of dryness and clogginess in the printer head [11].The final mixture solution was thoroughly stirred for 45 min at room temperature. Fabrication of Gas Sensor. A flexible gas sensor was fabricated by using modified Canon PIXMA iP4600 thermal printer with a resolution (BW) of 600 × 600 dpi. The original ink in printer cartridge was removed. The printer cartridge was thoroughly rinsed with deionized water and dried with nitrogen gas prior to be refilled with the prepared electronic ink. Schematic diagram of sensor structure is displayed in Fig. 1a.

(a) (b) Fig. 1 (a) Schematic diagram and (b) Photograph of a flexible gas sensor

The electronic ink as a sensing film with rectangle geometry (1.5 cm x 2.5 cm) was directly printed on flexible substrate (photo paper). The film thickness can be controlled by variation in the number of printed layers. The thickness of the sensing film in our experiment was ~ 2 µm. The simple interdigitated electrodes made from patterned aluminum plate with thickness of 35 µm were attached over the sensing film. The photograph of fabricated gas sensor is shown in Fig. 1b.

Results and Discussion Surface Morphology. AFM was used to investigate the morphology of PEDOT/PSS sensing film printed on paper substrate (See Fig. 2). The results indicate that the film surface is well uniform and only tiny defects are presented with a scan size area of 7.5 µm x 7.5 µm. The surface of inkjet printed film is rather smooth and continuous with the average surface roughness of ~ 7 nm. Moreover, it was revealed that nano-sized grains exist on the sensing film to enhance gas sensitivity due to the increase of the surface area by inclusion of nanostructures.

Fig. 2 AFM images of an inkjet printed film of PEDOT/PSS on paper. Advanced Materials Research Vol. 506 41

Sensor Response. Fig. 3a shows the sensing behavior of the PEDOT/PSS gas sensor to NH 3 vapor (25% w/w) under an air flow of 0.8 l/min at room temperature. It can be observed that the resistance of gas sensors decreases in the presence of vapor whereas other PEDOT/PSS gas sensors exhibited an increase in resistance after exposure to NH 3 gas [10]. This effect arises from the different sensor structure leading to difference of the dominant sensing mechanism. The details of the sensing mechanisms will be discussed in the next section. It can be seen that the sensor offers a good reproducibility with relatively minor deviations over four replications. To demonstrate the sensitivity and selectivity of the PEDOT/PSS gas sensor, its sensing response to various gas vapors was measured and plotted in Fig. 3b. The sensor response (S) of the gas sensor is defined as the percentage of resistance change: ( S(%) = (R 0-R)/ R 0 x 100 ) where R 0 and R are the average resistance of the gas sensor in pure air and test gas, respectively. From Fig. 3b, it can be seen that

PEDOT/PSS gas sensor exhibits a strong response to NH 3, and much weaker responses to ethanol, acetone, and toluene at room temperature. The sensor response works linearly with gas concentrations between 100-1000 ppm. Accordingly, it can be concluded that the flexible gas sensor prepared by our technique exhibits high sensitivity and selectivity to NH 3.

(a) (b)

Fig. 3 (a) Sensing behavior and (b) Sensing response of the gas sensor at room temperature

Sensing Mechanism. A PEDOT/PSS gas sensor is a polymer composite sensor such that swelling of the polymer sensing film from physisorption/chemisorption of vapor molecules becomes the main basis of sensing mechanism. In such a case, the resistance of PEDOT/PSS gas sensor increases in the presence of gas vapors due to separation of the conduction path caused by swelling.

However, our gas sensor has exhibited a decrease in resistance after exposure to NH 3. Therefore, we have proposed another sensing mechanism based on the conduction at the metal/polymer interface [12-14].

Fig. 4 Schematic band diagram of the band bending for a metal-polymer interface without (L-side) and with gas molecule adsorption (R-side). 42 Biomaterials and Applications

Fig. 4 shows the band diagrams of the metal/polymer interface without and with gas molecule adsorption. When metal and polymer with different local work functions (Φ) are placed in electrical contact, the Fermi levels of metal and polymer are forced to coincide resulting in a gradient formation of the electron potential in the depletion layer (band bending). Therefore, the gas sensor gives a high resistance value. In the presence of gas vapors, the adsorbed gas molecules at the metal -polymer interface introduce an excess of charge states, leading to a redistribution of charges in the depletion region and reduction of the degree of band bending. Consequently, the barrier height decreases with the increasing gas concentration.

Conclusion The low-cost and flexible PEDOT/PSS gas sensor has been successfully fabricated using thermal ink-jet printing technique. Direct print of sensing film on substrate offers high homogeneous film over typical gas sensor. With our electronic ink formula and new design of sensor structure, the sensor exhibits high selectivity and sensitivity to NH 3 at room temperature. The sensing mechanism based on to the band bending at the metal/polymer interface was proposed. It is hoped that this work will be useful for advanced future technology such as a wearable toxic gas sensor.

Acknowledgements Kasetsart University, Mahidol University and NECTEC are acknowledged for supports.

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