Thermal inkjet printing of polyaniline on paper

Thin Solid Films 520 (2012) 7200–7204

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Thermal inkjet printing of polyaniline on paper T.C. Gomes, C.J.L. Constantino, E.M. Lopes, A.E. Job, N. Alves ⁎ Departamento de Física, Química e Biologia, Univ Estadual Paulista — UNESP Presidente Prudente, São Paulo, Brazil

a r t i c l e

i n f o

Article history: Received 18 January 2012 Received in revised form 25 July 2012 Accepted 26 July 2012 Available online 2 August 2012 Keywords: Polyaniline Inkjet printing Paper substrate Humidity Sensor

a b s t r a c t The development of organic devices requires the fabrication of thin films, and inkjet printing has been shown to be a suitable method to reach this goal. This work describes the printing process and characterisation of polyaniline (PANI) printed on bond and photographic papers using a desktop inkjet thermal printer. To enable printing, a solution composed by PANI, n-methyl-2-pyrrolidone, ethylene glycol, alcohol and water must be prepared. PANI is printed on bond and photographic paper and then doping of PANI is performed by hydrochloric acid vapour exposure. Micro-Raman spectroscopy showed that PANI printed on paper keeps its basic characteristics. The results from electrical measurements showed that the surface resistivity of the printed PANI samples decreases by increasing the printing number, i.e. the number of layers that were deposited, and depends slightly on the paper type. A stretched semicircle followed by a linear upward tail, attributed to Warburg impedance combined with other intrinsic mechanisms of PANI on porous media, are always present on the Cole–Cole plots obtained for doped-PANI on bond paper. It was shown that these parameters significantly change with the relative humidity, opening the possibility to apply PANI/ paper-based devices as humidity sensors. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The development of organic electronics began with the discovery of conjugated molecules, oligomers and polymers, whose conductivities can be easily tuned through chemical manipulation, thereby allowing the production of materials with characteristics of conductors, semiconductors or insulators. These materials combine the electrical properties of inorganic semiconductors with the typical properties of plastics, such as low cost, versatility of chemical synthesis, ease of processing and flexibility. Organic electronic devices, such as solar cells, light emitting diodes, displays, field effect transistors and sensors, are practical applications of these materials [1]. Preparing organic devices requires the fabrication of thin- or ultra-thin-films, usually prepared by thermal evaporation [2,3], spin-coating [4,5], dip-coating [6], Langmuir–Blodgett assembly [7], electro-polymerisation [8], casting [9], printing [10–13], etc. Among these techniques, inkjet printing has been demonstrated to be an attractive method for film preparation, providing low cost, high production speed, selectivity, compatibility with different materials and substrates, low material loss, deposition over large areas and the possibility of large-scale production [14,15]. The desired geometric patterns for devices can be designed using software and printed directly onto the substrate without direct contact, making inkjet printing a simpler process than photolithography and suitable for industrial-scale production

⁎ Corresponding author. Tel.: +55 1832295760. E-mail address: [email protected] (N. Alves). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.07.119

and prototype development. Several works address the use of desktop printers for developing commercial prototypes [16–18] and describe devices prepared exclusively by inkjet printer or associated with other techniques [19–23]. In the inkjet printing process, the term ink refers to a liquid solution constituted by pigment, solvent and chemical compounds to fit the necessary viscosity, surface tension and boiling point. Basically, commercial desktop printers use a piezoelectric or a thermal system for ink ejection. With the piezoelectric systems, there is no contact between the ink and the printhead electrical circuit, while on thermal printers there are micro-resistors that heat the ink through direct contact at temperatures of approximately 300 °C for some microseconds [14]. The heating produces bubbles that expand until it collapse, causing ink ejection through tiny nozzles on the print cartridge. This printing process requires ink with the appropriate values of viscosity, surface tension and boiling point, typically 2.5 mPas, 33 mN/m and 90–95 °C, respectively, which demands water-based inks [16,24,25]. The application of inkjet printing to fabricate organic electronic devices requires the preparation of ink that must be constituted by a solvent and the desired polymer. Conducting polymers, such as polyaniline (PANI), polypyrrole, polythiophenes, which are materials of technological interest, are insoluble in water and common organic solvents [26,27]. Thus, the preparation of ink in a way that the polymer can be dissolved without the resulting solution damaging the cartridge is the main challenge [14,16]. PANI can be doped without changing the number of electrons associated with the polymer chain. In this process, the nitrogen atoms in the imine portion of the molecules are protonated or deprotonated,

T.C. Gomes et al. / Thin Solid Films 520 (2012) 7200–7204

allowing doping reversibility and, consequently, conductivity tuning [28,29]. This property is the reason why PANI has been widely explored as the active layer in sensors for the detection of gases, such as methanol, benzene and ammonia [6,8,26,30,31]. Moreover, its optical properties, chemical stability, processability, ease of synthesis and low cost make PANI very attractive as a basis for devices. Doped PANI produced by conventional synthesis using hydrogen chloride (HCl) is not soluble in common solvents or water, which makes it difficult to use doped PANI in printable inks [32]. Different strategies to enable PANI printing by inkjet printers are described in the literature using the following: i) a silver nitrate aqueous solution onto a substrate previously immersed in aniline monomer and exposing it to UV light [21], ii) polyaniline/poly(4-styrenesulfonated) nanoparticles dispersed in water [12], iii) an aqueous nanodispersion of PANI doped with acid dodecylbenzenesulfonic [32], iv) an aqueous oxidant solution of ammonium persulphate (APS) and exposing it to aniline and HCl vapours [33] and v) a PANI-silica colloidal nanoparticle solution [13]. Commercial inks for inkjet printers contain n-methyl-2-pyrrolidone (NMP), which is an appropriated solvent for undoped PANI. Thus, this particularity favours the preparation of ink to print PANI, which can be doped after printing to obtain different conductivities. This work reports on a procedure to print PANI onto paper substrates using a desktop inkjet printer with a thermal head system. The printing is achieved using an aqueous solution with dedoped PANI (in this text dedoped PANI will be referred as the PANI that was submitted to a dedoping process) that, after printing, has its conductivity changed by exposure to HCl vapour. PANI was printed on white copy and photographic papers and was characterised by micro-Raman spectroscopy and electrical measurements. It is shown that PANI covers the paper uniformly, and exhibits the expected structures and good electrical properties. The viability to apply a sample with PANI-printed paper as humidity sensor is also shown. 2. Experimental details 2.1. Synthesising PANI To synthesise PANI (emeraldine salt), two solutions were initially prepared: 1) 23 g of APS was dissolved in 400 mL of HCl aqueous solution (1 mol/L) under constant agitation and 2) 40 mL of distilled aniline was mixed in 600 mL of HCl aqueous solution (1 mol/L), also under agitation. Solution 1 was added to solution 2 for polymerisation, and the mixture was filtered to obtain the PANI powder in the doped state. To dissolve PANI in NMP, it is necessary to promote dedoping, which was performed by immersing the PANI powder in a solution 1 M of ammonium hydroxide (NH4OH).

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was possible to add water into the solution until it reached as much as 70–75% without PANI precipitation. 2.3. Printing PANI A HP Deskjet 2460 commercial inkjet printer was used to print PANI. A cartridge HP number 21 was opened to remove the original ink and the sponge. After a careful cleaning using isopropyl alcohol and deionised water, the final PANI solution was added in the small compartment close to the printhead. With the ink in the cartridge, black squares with the desired area were designed and printed using Microsoft Word settings (Microsoft Windows XP version 2002 Professional). The tonality setup in Microsoft Word combined with the printing speed options in the HP driver allows the setting of the printing quality. In Microsoft Word, the draws were set to RGB model on (0, 0, 0) configuration using the colour menu. In the printing menu, the maximum printing quality was chosen, which means a larger number of pixels per inch that can be achieved by the printer. To increase the amount of printed PANI at the same paper position, successive printings ranging from 1 up to 10 were performed. 2.4. Characterising the printed PANI The procedure to characterise the printed PANI consists of the following: i) perform the desired measurements on the dedoped sample, ii) promote sample doping and iii) perform a new set of measurements on the doped sample. The doping was achieved by leaving the samples for 10 min inside a closed glass compartment (~2500 cm3) in the presence of HCl vapour from 1 mL HCl aqueous solution (1 mol/L). Before performing the second set of measurements, the samples were left in a desiccator with silica for 24 h to achieve complete drying. The structural PANI characterisation was performed by Raman measurements using a Renishaw spectrograph (model in-Via), with laser line excitation at 785 nm, coupled to a Leica optical microscope. Current–voltage (I–V) measurements were performed using a Keithley 617 electrometer, after the evaporation of 3-mm-separated gold electrodes over the samples with 10× 10 mm printed area. These measurements were performed by scanning the voltage from 0 to 5 V in steps of 1 V and recording the current value 5 min after the start of the voltage application. Impedance measurements were performed using a Solartron 1260 impedance analyser coupled to a dielectric interface A1296, after the evaporation of lateral gold electrodes (~3 mm wide) over the samples with a 20× 20 mm printed area. These measurements were performed by applying an ac voltage of 100 mV with frequencies ranging from 10−1 to 10 6 Hz. 3. Results and discussion

2.2. Preparing the PANI ink

3.1. Visual evaluation

The PANI ink was prepared with the properties and compositions of commercial inks for Hewlett Packard (HP) inkjet printers (thermal head system) as a reference. The ink density should be between 1 and 2 g/mL, the pH should range from 7.8 to 8.4, the boiling point should be between 90 and 95 °C and the viscosity should be higher than 2 mPas. In black commercial ink, these values are achieved with the following approximated composition (in mass): 5% of carbon black, 5% of isopropyl alcohol, 5% of ethylene glycol, 15% of NMP and 65– 70% of water [24]. Thus, the appropriate PANI ink was obtained by dissolving dedoped PANI powder in NMP at 13 mg/mL by stirring for 24 h with a magnetic stirrer, followed by sonication for 1 h. Continuing to perform sonication on the PANI/NMP solution, ethylene glycol, alcohol and ultra-pure water (Millipore Milli Q system) were added very slowly. Through this slow addition, 2% by volume of ethylene glycol and 2% of alcohol were added, and with this process, it

Fig. 1 shows photos (taken with a digital camera) of representative samples, printed on bond paper with one, five and ten printings before and after the doping process, that were used for the impedance measurements. The images show that the printed PANI has good uniformity with blue or green hues, which is characteristic for PANI in its dedoped and doped states, respectively. Also, by increasing the printing number, i.e. the number of layers that were deposited, an enhancement of the sample tonality is observed. Similar quality is achieved for photographic paper, aside from two points: i) the ink spreads in the fibres on bond paper, while the ink remains more concentrated on photographic paper, thereby increasing the resolution and ii) 10 printings are possible on bond paper without significant changes on the absorption paper capability, while a “coffee-stain-like” effect starts to segregate PANI around the previously printed regions on photographic paper.

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1 printing

5 printings

10 printings

Printed films Dedoped PANI

Printed films Doped PANI

Fig. 1. Photos of PANI printed onto bond paper, before and after doping, for one, five and ten printings. The photos correspond to the samples used for impedance characterisation.

3.2. Micro-Raman spectroscopy Fig. 2a shows the PANI molecular structure in its dedoped (emeraldine base) and doped (emeraldine salt) states, and Fig. 2b shows its corresponding micro-Raman spectra, when printed on bond paper. The investigation of PANI using Raman spectroscopy is widely documented in the literature [34–38]. In Fig. 2, the main Raman bands related to PANI dedoped and doped states (dashed lines) are marked. For the dedoped PANI, the observed bands at 748 and 843 cm−1 are related to quinoid ring deformation, the band at 1167 cm−1 is associated with the C\H bending in the quinoid ring, the band at 1462 cm−1 is attributed to C_N stretching in the quinoid ring, and the band at 1589 cm−1 is assigned to C_C stretching in the quinoid ring. When the PANI is doped, the band at 1462 cm−1 is practically non-existent due to the transformation of quinoid to benzenoid rings, the band at 1167 cm−1 is shifted to 1176 cm−1, which is attributed to C\H bending in benzenoid ring, and the band at 843 cm−1 is overtaken by the band at 868 cm−1 that is assigned to benzenoid ring deformation, with the

a

same happening for the band at 1622 cm−1 (against 1589 cm−1) that is related to C\C stretching in benzenoid ring. In addition, there is the presence of the bands at 1339 cm−1, assigned to the radical cation (C\N + stretching), and at 1511 cm−1, attributed to N\H bending, that are characteristic of doped PANI. The band at 1339 cm−1, particularly, is directly related to the conducting state of PANI achieved by protonation. The latter confirms the doping efficiency, once the main bands associated with vibrational groups characteristic of doped state are observed. In a general way, the Raman results show that neither the ink preparation nor its printing on bond paper using a thermal printer changes the PANI molecular structure. A detailed investigation into the spectral analysis reveals the remnant of the band at 1339 cm−1 in the spectrum of the PANI emeraldine base, which indicates that the dedoping process through NH4OH was not fully achieved. This finding might explain the reduction in two orders of magnitude of the resistivity measured on bare bond paper after a single printing of dedoped PANI, as discussed in the next section. Besides, Temperini et al. have investigated the PANI molecular structure under annealing and observed the appearance of the bands at ca. 574 and 1381 cm−1 and a shoulder at 1643 cm−1 for PANI at a temperature of 100 °C, which became the most intense bands at 150 °C [37]. The bands were assigned to possible PANI inter-chain interaction (e.g., crosslink forming tertiary nitrogen) and/or oxidation (e.g., reaction between imine and oxygen). Here, we have found a similar effect considering the bands at 584 (weak), 1386 (strong) and 1640 (shoulder) cm−1 (solid lines), which is consistent with the temperature involved in the printing process. It is worth mentioning that this effect on PANI has been observed not only by annealing but also by chemical interactions in PANI blended films [39]. 3.3. Surface resistivity The surface resistivity was the parameter used to study the changes occurring on paper conductivity due to the printing of PANI. This parameter can be calculated from the electrical resistance (R), which is extracted from the I–V measurements. The surface resistivity is defined as:

400

600

800

1589 1643

1000

Wavenumber

1200

1400

1511

1622

Doped PANI

1339

1176

868

1462

748

843

Dedoped PANI

584

Intensity (arb. units)

Raman - 785 nm laser line

1386

b

1167

ρs ¼

1600

(cm-1)

Fig. 2. a) Emeraldine (y = 0.5) base (dedoped) and salt (doped) PANI molecular structures and b) micro-Raman spectra with baseline correction for dedoped and doped PANI films printed on bond paper. The excitation laser line is at 785 nm.

R n□

ð1Þ

where n□ is the number of squares that can be counted on the film surface whose side is equal to the distance between the electrodes. To obtain reliable values for the surface resistivity, five identical samples were measured, and the average value was considered. In Fig. 3a, the dependence of the surface resistivity on the printing number is shown for doped and dedoped PANI printed on bond and photographic papers, and in Fig. 3b, the result for dedoped PANI printed on bond paper is shown in detail, with a linear abscissa axis and including the standard deviation. Fig. 3a also shows the results for bare paper (without PANI). The bare bond paper exhibits a surface resistivity about 10 12 Ω/□, and when the bare bond paper is “doped” due to the exposure to HCl, this value drops by about one order of magnitude, as it can be seen in Fig. 3a. After a single printing of dedoped PANI, the resistivity is reduced by approximately two orders of magnitude, while for doped PANI this reduction is approximately six orders of magnitude. This large reduction on the resistivity of doped PANI compared to the bare papers allows us to state that the measured surface resistivity is from the printed PANI. As shown in detail in Fig. 3b, a significant reduction of the surface resistivity is observed until the fourth printing of dedoped PANI in bond paper. Actually, as can be seen in Fig. 3a, this reduction is observed for both kinds of papers and for doped and dedoped PANI. This is due to the fact that until the fourth printing PANI stills penetrates into the paper, turning it less resistive; after saturation this point, the PANI provided by the following printings does not efficiently penetrate into the


Surface resistivity (Ω/ )

1013 bare papers

photograph paper - dedoped bond paper- dedoped

1011 109 107

photograph paper - doped bond paper - doped

105 103

0

2

4

6

8

10

Printing number

Surface resistivity (109 Ω/ )

b

12

9

6

3

0 0

2

4

6

8

10

Printing number Fig. 3. Surface resistivity as function of printing number for a) doped and dedoped PANI printed on bond and photographic papers (the results for bare paper are identified) and b) for dedoped PANI printed on bond paper in the linear scale, with the data showing the standard deviation.

paper and so the resistivity is little affected. The resistivity obtained for PANI printed on bond paper is three times lower than that printed on photographic paper. This difference is attributed to the fact that the morphology of photographic paper is cracked and after printing PANI the surface will result in separated conduction islands, as observed by optical microscopy (data not show here). The fact that this difference remains approximately constant shows that the characteristics of the paper surface have minor importance on the dependence of surface resistivity on the printing number. Others authors found similar values for the surface resistivity of printed PANI using different strategies: Morrim et al. found 4.7 × 103 Ω/□ by printing PANI nanoparticles on polyethylene terephthalate (PET) [27]; Cho et al. found values from 3.8×103 to 1.9×104 Ω/□, depending on the track thickness of PANI printed on PET or photographic paper substrates [33]; Kristian et al. reported a resistivity of 4.0×104 Ω/□ using multiple printings of multi-walled carbon nanotubes [40], and Small et al. found 1.0×105 Ω/□ using single-walled carbon nanotubes and poly(2-methoxyaniline-5-sulfonic acid) [41]. 3.4. Impedance characterisation Impedance measurements were performed on samples of PANI printed on bond paper after doping of the printed PANI. The bond paper was chosen because it absorbed PANI better and because the PANI distribution is more uniform when printed on it, as discussed earlier. Because the majority of sensors reported in the literature act by detecting substances that promote the dedoping of PANI [42–45], the discussion will be centred on the doped PANI samples. It is worth

noting that these substances modify the resistivity of the active material as well the capacitance of the sample, and so the study of the impedance as a function of frequency provides important information that can be useful to design a sensor based on doped PANI samples. Fig. 4 shows the Cole–Cole (or Nyquist) plots for doped PANI printed on bond paper with one, five and ten printings. The measurements were performed in air, with the samples placed over a metal plate inside an electrically grounded closed metal box. The “High” (HI) dielectric interface terminal was connected to one gold electrode at the sample while the “Low” (LO) terminal was connected simultaneously to the other electrode and to the metal plate, as shown in the inset of Fig. 4. With this experimental setup, the measured impedance is related to a combination of the PANI resistance between the terminals and the resistance and capacitance across the sample. The Cole–Cole plots shown in Fig. 4 consist of a stretched semicircle at high frequency and an almost linear section in the low-frequency region. The semicircle in the high-frequency range probably results from a distribution of relaxations related with the charge transport in the bulk paper, while the low-frequency part is often attributed to Warburg impedance and/or other mechanisms intrinsic to the PANI when it is immersed on porous media [46–49]. The real impedance at the transition point between the two regimes (~100 Hz) corresponds approximately to the total resistance of the sample. With increasing printing number, the total resistance decreases due to deeper penetration of the PANI into the paper, i.e., the cross-sectional area of the paper wetted by the PANI ink increases, resulting in a lower resistance along the sample as well between the PANI and the metal plate (across the sample), leading to the reduction of the semicircle scale. To confirm this effect, Fig. 5 shows the capacitance at 1 kHz under a relative humidity of 40% as a function of the printing number. It can be observed in this figure that there is also an increase in capacitance with the printing number because the thickness of paper not wetted by the PANI decreases. This change in capacitance can also lead to reducing the scale of the semi-circle shown in Fig. 4. A sample with ten prints of PANI (doped) was submitted to impedance measurements while changing the relative humidity (RH%) of the environment, aiming to investigate the applicability of the sample as a humidity sensor. Conditions of 13%, 30% and 35% for RH% were achieved using environments with silica, sodium chloride and ambient air, respectively. The Cole–Cole plots for these measurements are shown in Fig. 6. The point that corresponds to the total resistance of the sample (transition from the semi-circle to the linear region) significantly changes as a function of RH%. In fact, shown in the inset of Fig. 6, there is a linear dependence of the total resistance with the sample humidity. Moreover, the semi-circle rescales with changing RH%, indicating that

180

Imaginary impedance (kΩ)

a

7203

150

Doped PANI Paper

120

Metal Plate

90 60 1 Print 5 Print 10 Print

30 0

0

100

200

300

400

500

600

700

Real impedance (kΩ) Fig. 4. Cole–Cole plots for doped PANI printed on bond paper with one, five and ten printings. The dependence of the capacitance on the printing number, measured at 1 kHz and at a relative humidity of ~40%, is shown.

7204


700

for the discussions regarding printing PANI using an inkjet printer; and Prof. Dr. José Alberto Giacometti and MSc. Fernando Pereira Sabino for the discussions regarding the electrical measurements.

Real capacitance (pF)

600 500

References

400 300 200 100

0

2

4

6

8

10

Printing number Fig. 5. Plots of the real capacitance at 1 kHz and at a relative humidity of 40% versus the printing number for doped PANI printed on bond paper.

Resistance (kΩ )

Imaginary impedance (kΩ)

60 50 40

20 10 0 0

30

10 Printing RH~90% RH~40% RH~13%

30

25

50

75

100

RH%

20 10 0

0

10

20

30

40

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60

Real impedance (kΩ) Fig. 6. Cole–Cole plots for doped PANI printed on bond paper with ten printings for three different values of RH%.

the capacitance is also humidity dependent. As the resistance and capacitance can be tuned changing the doping, the printing number and the printed area, a humidity sensor can be specifically designed and produced by printing PANI on paper using a thermal inkjet printer. 4. Conclusion A procedure for preparing dedoped PANI ink to be printed by commercial thermal inkjet printers was presented. Micro-Raman spectroscopy showed that the ink preparation and the thermal printing do not cause PANI degradation. After four printings of PANI on bond and photographic papers, the surface resistivity is 10 10 Ω/□ and 2.2 × 10 4 Ω/□ for dedoped and doped samples, respectively. The Cole–Cole plots obtained for doped PANI on bond paper showed a characteristic shape with a stretched semicircle (at high frequencies) followed by a linear region at low frequencies, attributed to the Warburg impedance and other intrinsic mechanisms of PANI on porous media. With increasing printing number, the resistance decreases, while the capacitance increases. These parameters changed significantly when a sample with ten printings was exposed to different RH%, indicating that a humidity sensor made by PANI printed on paper can be produced. Acknowledgements The authors thank the Fundação de Apoio à Pesquisa do Estado de São Paulo (FAPESP) for financial support; Prof. Dr. Rodrigo F. Bianchi

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