Conference-CIF ’ 15-Preparation of Electrically Conducting Self-Organized Micro-Hybrid Structures

We used a self-organized polymer microhoneycomb structure as a template to form polyaniline structures that contain hexagonal voids. The polyaniline was synthesized in situ and adsorbed spontaneously on a polymer honeycomb structure that was immersed in the solution. Successive adsorption cycles led to thicker polyaniline coating. The characteristic honeycomb structure could be preserved during the process. Electrical measurements showed that the film resistance is the sum of contact and sheet resistance. [DOI: 10.1380/ejssnt.2015.19]


INTRODUCTION
Honeycomb films were reported by Francois et al. in 1994 [1] and since then have been the topic of intense research in various fields and for various applications [2]. The outstanding features of the honeycomb films are the regularity of their porous structure, the ease of fabrication [3], the control of the pore parameters (diameter and rim width) [4], and the wide range of applicable materials [5]. Applications range from improved photocatalyst activity [6] to cell culturing [7].
Polyaniline (PANI) is a now widely known semiconducting polymer that has first been reported by Letheby in 1830 [8]. At that time, its potential for sensing devices was completely unknown. With the birth of the molecular sensor concept in 1960 [9], however, PANI returned into the spotlight when McDiarmid investigated the pH dependence of PANI conductivity [10] and subsequently various devices that use PANI have been reported [11].
Here we report on the fabrication of PANI-coated honeycomb structures and their electrical conductivity. Those structures may be useful as chemical sensors. The highly porous structure increases the active surface area of the sensing material, PANI, while the mesh structure of the honeycomb provides robustness of the device -even if parts of the honeycomb structure are damaged, electricity can flow 'around' the damages areas.

II. EXPERIMENTAL
Honeycomb films are prepared by casting a polymer solution under high humidity. The organic solvent should have a low boiling point (i.e. should evaporate easily) * This paper was presented at the 15th Chitose International Forum "Nanotechnology -From Synthesis to Devices," Chitose Institute of Science and Technology, Chitose, Japan, October 2-3, 2014. † Corresponding author: kart@photon.chitose.ac.jp ‡ Present address: Jokoh Co. Ltd., Jokoh Building 3-19-4, Hongo, Bunkyo-ku, Tokyo113-0033, Japan and the polymer solution needs to have a miscibility gap with water. A common combination is polystyrene and ethyl acetate. In order to stabilize the condensed water droplets, an amphiphilic polymer is added, in our case a poly ion complex made of poly(styrene sulfonate) and N, N-dioctadecyl-N, N-dimethyl ammonium [12]. A 10:1 mixture of those two polymers (5 mg of polystyrene-comaleic anhydride and 0.5 mg of the polyion complex in 1 ml of ethyl acetate) was cast on a glass substrate and allowed to evaporate at around 80% relative humidity. The strongly light scattering white films were characterized by optical and electron microscopy. Polyaniline was prepared by oxidative polymerization of aniline in acidic aqueous solution that contained an emulsifier, dodecylbenzene sulfonate (DBSA), that was added to stabilize the polyaniline suspension [13]. Aniline (0.045 mL), DBSA (0.45 g), and conc. HCl (1.5 mL) were added to 50 mL of deionized water in a 100 mL standard glass beaker and stirred for 20 minutes. Ammomium persulfate (0.1 g) was added and a honeycomb-coated glass slide was vertically immersed in the mixture and stirred for another 60 minutes during which the solution turned green.

III. RESULTS AND DISCUSSIONS
Polyaniline exists in different oxidation states that show different degree of conductivity. Leucoemeraldine is the completely reduced form that shows low conductivity. The semi-oxidized emeraldine base is conductive when doped with acid. The completely oxidized perningraniline, again, exhibits low conductivity [10]. The green color indicates the half-oxidized emeraldine. When the honeycomb template is removed and washed with deionized water, it is very slightly colored green. The photograph of this film and the corresponding electron microscopy image are shown in Fig. 1B. After drying, the same honeycomb film was again immersed in freshly prepared solutions, as described in the previous paragraph. Figure 1 shows the photographs and electron micrographs of honeycomb films before and after PANI adsorp- tion. The pristine honeycomb film (Fig. 1A) shows a characteristic uniform diameter of the air holes and a smooth polymer surface between the holes. After one PANI adsorption step, the optical photograph shows a very slight green color. A slight roughening of the honeycomb top layer can be seen in the electron micrograph. After two adsorption cycles, both the overall color and the microscopic surface roughening becomes more prominent. Both features become much more pronounced after three adsorption cycles, indicating a nonlinear PANI adsorption characteristics for subsequent steps. It seems that a thin priming layer is formed in the first PANI adsorption aides the adsorption of further PANI layers. Also, with increasing adsorption cycles, the air-hole diameter shrinks. That means that PANI nanoaggregates are formed in solution and attach in a diffusion-controlled manner freely from various angles onto the honeycomb film. Figure 2 shows the cross-sectional view of the hobeycomb film, illustrating that the polymer of the honeycomb film is surrounded  the polystyrene, which is surrounded by an approximately 200 nm thick PANI layer (B). The bottom layer of the honeycomb is also covered with a thin PANI layer (C).
Gold electrodes were deposited onto the samples through a mask with two voids that were 5mm wide and 10 mm long. The electrodes had distances of 5, 10, 15 and 20 mm, respectively. The rectangular measurement area was separated from the rest of the honeycomb film by a we could estimate the sheet resistance of the film. The total resistance Ω tot of the film is the sum of the sheet resistance Ω s and the contact resistance Ω c . By measuring the Ω tot at various electrode distances, we can deconvolute Ω s and Ω c . Table I shows the data for Ω tot of PANI honeycomb films after two, three and four cycles of PANI adsorption. As expected, Ω tot increases with electrode distance for a given number of PANI layers, because Ω s increases while Ω c should be constant. On the other hand, Ω tot does not change in a predictable way when comparing the data for the same electrode distance and different number of PANI layers. That means that the films show a certain range of electric conductivity that depends on the preparation conditions of the films. The most likely candidate for such a variation of conductivity is proton-doping, which is difficult to control during film deposition. Figure 3 is a graphical representation of the obtained conductivity data. The dotted lines are guides to the eye. The intersection with the y-axis shows Ω c and the slope of the lines indicate Ω s . Table II summarizes the calculated  values for Ω s and Ω c from Fig. 3. Even though the data shows some scatter, the linear regression has a mean root square error of >0.98, which is a sufficiently high value to conclude the vality of the data. Even though Ω s shows some variation, the sheet resistance value of 0.5-3 kΩ/cm is a typical value for PANI. Also the value of 2 kΩ for the contact resistance Ω c is reasonable.

IV. CONCLUSIONS
We prepared so-called honeycomb films by a bottomup self-organization method that relies on the formation of breath figures by condensation of water droplets on an evaporating polymer solution. These honeycomb films were used as templates for the deposition of in-situ prepared polyaniline. Optical and electron microscopy imaging showed that PANI was homogeneously adsorbed onto the honeycomb template. The thickness of the PANI coating can be controlled by the number of adsorption cycles. The sheet conductivity of the pristine films is in the kΩ/cm range and they thus might be used as pH sensors.