Conductive Polymer Synthesis with Single

materials Article

Conductive Polymer Synthesis with Single-Crystallinity via a Novel Plasma Polymerization Technique for Gas Sensor Applications Choon-Sang Park 1,† , Dong Ha Kim 1,† , Bhum Jae Shin 2 , Do Yeob Kim 3 , Hyung-Kun Lee 3 and Heung-Sik Tae 1, * 1 2 3

* †

School of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu 702-701, Korea; [email protected] (C.-S.P.); [email protected] (D.H.K.) Department of Electronics Engineering, Sejong University, Seoul 143-747, Korea; [email protected] Nano Convergence Devices Research Department, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Korea; [email protected] (D.Y.K.); [email protected] (H.-K.L.) Correspondence: [email protected]; Tel.: +82-53-950-6563 These authors contributed equally to this work.

Academic Editor: Der-Jang Liaw Received: 7 July 2016; Accepted: 26 September 2016; Published: 30 September 2016

Abstract: This study proposes a new nanostructured conductive polymer synthesis method that can grow the single-crystalline high-density plasma-polymerized nanoparticle structures by enhancing the sufficient nucleation and fragmentation of the pyrrole monomer using a novel atmospheric pressure plasma jet (APPJ) technique. Transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM) results show that the plasma-polymerized pyrrole (pPPy) nanoparticles have a fast deposition rate of 0.93 µm·min−1 under a room-temperature process and have single-crystalline characteristics with porous properties. In addition, the single-crystalline high-density pPPy nanoparticle structures were successfully synthesized on the glass, plastic, and interdigitated gas sensor electrode substrates using a novel plasma polymerization technique at room temperature. To check the suitability of the active layer for the fabrication of electrochemical toxic gas sensors, the resistance variations of the pPPy nanoparticles grown on the interdigitated gas sensor electrodes were examined by doping with iodine. As a result, the proposed APPJ device could obtain the high-density and ultra-fast single-crystalline pPPy thin films for various gas sensor applications. This work will contribute to the design of highly sensitive gas sensors adopting the novel plasma-polymerized conductive polymer as new active layer. Keywords: atmospheric pressure plasma; plasma-polymerized pyrrole; single-crystalline; gas sensor; iodine doping

1. Introduction Nanostructured conductive polymers, such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and poly(3,4-ethylenedioxythiophene) (PEDOT), have been studied as active layers of gas sensors [1–12]. Conductive polymers can be synthesized through the chemical method, electrochemical synthesis, electrospinning, seeding polymerization, interfacial polymerization, and low pressure plasma polymerization techniques [13–16]. However, almost all polymerization techniques have high synthesis temperatures (100–600 ◦ C) with solutions or the wet process [16]. Therefore, conventional conducting polymers have a generally amorphous state or poly-crystalline characteristics

Materials 2016, 9, 812; doi:10.3390/ma9100812

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with non-porous properties due to high temperature, and present difficulties in their incorporation in highly sensitive gas sensors. However, nano-porous polymers can be synthesized by atmospheric pressure plasma jets (APPJs) due to the process of removing thermal solutions [17–22]. However, it is difficult to synthesize polymers using conventional APPJs because the plasma produced in ambient air conditions has low energy [23–26]. As a result, they would not feed enough energy for the nucleation of the monomers. Accordingly, most polymer films synthesized by the conventional APPJs tend to show poor film qualities, such as a low molecular weight and weak chemical stabilities with an amorphous state. More importantly, conductive polymer nanoparticles and nanofibers synthesized by the APP technique have non-crystalline or poly-crystalline characteristics [27–30]. To the authors’ knowledge, there have been no previous reports on the synthesis of porous conductive polymer nanoparticles with single-crystalline characteristics using APP polymerization techniques at room temperature due to insufficient plasma energy for the nucleation of monomers. Therefore, it is necessary to enhance the plasma energy to synthesize high-density conductive polymer nanoparticles with single-crystallinity as the active layers for gas sensors on various substrates at room temperature. We have recently reported a new polymer synthesis method using APPJs with an additional plastic tube and bottom cap [31]. The plasma-polymerized aniline (pPANI) nanofibers and nanoparticles were reported to be successfully obtained with high molecular weights and poly-crystalline characteristics [31]. However, previous APPJs with plastic tubes were not sufficiently completed for the synthesis of single-crystalline characteristics. In this study, we introduce an initial synthesis of high-density pyrrole (pPPy) nanoparticles with single-crystallinity on various substrates, such as glass, plastic, and interdigitated gas sensor electrodes, using a novel atmospheric pressure plasma polymerization technique at room temperature using an additional glass tube with a higher dielectric constant to increase the charged particles by the plasmas. In addition, the synthesized pPPy was doped with iodine to introduce charge carriers into the plasma-polymerized structures to render them conductive [32–34]. In particular, we examined the conductivity variation of the iodine-doped pPPy grown on the interdigitated gas sensor electrodes in order to check the suitability of the active layer for the electrochemical toxic gas sensors. Our experimental results show that the single-crystalline high-density pPPy nanoparticles can be obtained using the novel APPJ device with a single bundle of three glass tubes to enhance the plasma jets in the nucleation region. 2. Experimental Section 2.1. Plasma Polymer Synthesis and Measurement Argon gas (99.999%) was used as the plasma discharge gas at a flow rate of 1300 standard cubic centimeters per minute (sccm). Liquid pyrrole monomer (Mw = 67 g·mol−1 , Sigma-Aldrich Co., St. Louis, MO, USA) was vaporized using a glass bubbler, which was supplied by argon gas with a flow rate of 130 sccm. The three glass jets were tied and wrapped by copper tape electrode at 10 mm from the end of jet. The same sinusoidal power was applied to the powered electrode with a peak value of 12 kV with a frequency of 30 kHz on both outside and inside cases [31]. In both outside and inside cases, each glass jet had an inner diameter of 1.5 mm and an outer diameter of 3 mm. The center-to-center distance between each jet was 3 mm. The additional glass tube (or glass tube) had an inner diameter of 20 mm and a length of 60 mm. The polytetrafluoroethylene (PTFE) insulating substrate holder with an outer diameter of 15 mm was located outside or inside. The optical emission spectrometer (OES) was used to analyze the optical intensity and spectra of reactive nitrogen and oxygen peaks, respectively, for estimating the variations in the plasma energy states [35,36]. The surface temperature of the substrates was measured with an infrared thermometer (568 IR Thermometer, Fluke, Everett, WA, USA) using a special glass tube with a small hole. The power and monomer feeding system except for the additional glass tube employed in the novel APPJs in this research has been described in detail in [31].


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2.2. Iodine Doping on Plasma Polymer Films The plasma-polymerized pyrrole (pPPy) nanoparticle thin films on the substrates of interdigitated gas sensor electrodes were doped by iodine to introduce charge carriers into the plasma polymerized structures to render them conductive. The pPPy films were doped by placing samples in a sealed container containing solid iodine (1 g) under various iodine exposure (doping) times. 2.3. Scanning Electron Microscopy The top and cross-section views images of pPPy nanoparticle thin films were measured by scanning electron microscopy (SEM, Hitachi SU8220, Tokyo, Japan) with accelerating voltage and current of 5 kV and 10 mA, respectively. The samples for SEM were imaged with a conductive platinum coating, which played a role in preventing the substrate from charging during the imaging process. 2.4. Trasmission Electron Microscopy The high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were taken with a Titan G2 ChemiSTEM Cs Probe (FEI Company, Hillsboro, OR, USA) transmission electron microscope, operating at 200 kV. A TEM sample of pPPy nanoparticles was prepared by depositing a 6-µL solution (ultrasonically dispersed in DI water) on carbon-coated copper grids, and dried in air. 2.5. Fourier Transform Infrared Spectroscopy The Fourier transform infrared spectroscopy (FT-IR) was used to determine the chemical changes introduced by the plasma. The FTIR were taken with a Perkin–Elmer Frontier spectrometer (PerkinElmer, Waltham, MA, USA) between 650 and 4000 cm−1 . 2.6. X-ray Photoelectron Spectroscopy The X-ray photoelectron spectroscopy (XPS) was carried out on a K-ALPHA surface analysis system (Thermo Fisher Scientific, Waltham, MA, USA), using a monochromatic Al Kα X-ray source (hυ = 1486.71 eV) operated at 15 kV and 20 mA. The pressure in the analyzing chamber was maintained at 10−7 Pa or lower during analysis, and the size of the analyzed area was 5 mm × 5 mm. Spectra were acquired with the angle between the direction of the emitted photoelectrons and the surface equal to 60◦ . The estimated analyzing depth of the used XPS set up was 8 to 10 nm. The high-resolution spectra were taken in the constant analyzer energy mode with a 200 eV for survey scan and a 50 eV pass energy for element scan, respectively. The value of 285.8 eV of the C1s core level was used for calibration of the energy scale. 3. Results and Discussion As shown in Figure 1a, the newly proposed glass tube and cylindrical insulating substrate holder were introduced to minimize the quenching from ambient air and increase the plasma energy in the nucleation region. It is noted that the proposed glass tube and insulating substrate holder were installed at the jet end to confine the jet flow in the nucleation region and produce an intense and broad plasma. Moreover, it is well advised to leave a gap between the end of the glass tube and the substrate with the holder for a smoother jet flow. As shown in the plasma images (inside case), the profile of the produced plasma was dramatically changed, i.e., the strong, intense, and broad plasma was produced by adopting a jet with a glass tube, with a higher dielectric constant, instead of a plastic tube with a low dielectric constant [31]. Accordingly, thanks to an additional glass tube with a higher dielectric constant, the proposed APPJs can produce more efficient and intensive plasma in the nucleation region during plasma polymerization. As shown in Figure 1a, in the case of a jet whose insulating substrate holder is placed outside the glass tube (outside case), the streamer-like short plasmas were only produced in the nucleation region. On the contrary, the strong and intense plasma plumes were produced broadly and extended farther downstream in the case of a jet whose


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plumes were produced broadly and extended farther downstream in the case of a jet whose insulating substrate substrate holder holder was was placed placed about about 55 mm mm inside inside the the glass glass tube tube (inside (inside case) case) even even when when the the insulating applied voltages and total currents (i.e., input power) were the same, as shown in Figure 1b. applied voltages and total currents (i.e., input power) were the same, as shown in Figure 1b.

(a) Schematic Schematic diagram diagram of of experimental experimental setup setup in in this this study study and and images images of of plasmas plasmas produced produced Figure 1. (a) in the nucleation region; and (b) applied voltages and total currents of novel atmospheric pressure in nucleation region; and voltages holders (or(or polytetrafluoroethylene (PTFE) bottom cap) plasma jets jets (APPJs) (APPJs)whose whoseinsulating insulatingsubstrate substrate holders polytetrafluoroethylene (PTFE) bottom are placed outside or inside the glass tube.tube. cap) are placed outside or inside the glass

Figure nucleation Figure 22 shows shows the the optical optical emission emission spectra spectra from from 300 300 to to 880 880 nm, nm, measured measured in in the the nucleation region, further indicating that the excited N 2 , Ar, and carbonaceous species exist in the region, further indicating that the excited N2 , Ar, and carbonaceous species exist in the plasma plasma plumes. excited nitrogen nitrogen second second positive positive (N (N22;; 337, plumes. Interestingly, Interestingly, the the excited 337, 357, 357, and and 380 380 nm) nm) peaks peaks and and the the carbonaceous (CN; 388 increased in in the the inside inside case. case. The The various various N N22 peaks carbonaceous (CN; 388 nm) nm) peak peak were were significantly significantly increased peaks indicate higher concentration concentrationof ofreactive reactivenitrogen nitrogenspecies species(RNS), (RNS),which which have been shown to play indicate aa higher have been shown to play an an important role in obtaining a high-quality polymer layer. In addition, spectra from the important role in obtaining a high-quality polymer layer. In addition, spectra from the carbonaceous 2Σ → X2Σ), emitted during the nucleation processes of 2Σ → carbonaceous species, such as BCN (388 species, such as CN (388 nm X2nm Σ), Bemitted during the nucleation processes of the pyrrole the pyrrole monomer, were significantly increased in the inside OES data that the monomer, were significantly increased in the inside case. Thesecase. OESThese data show that show the proposed proposed APPJs can be suitable for sufficient nucleation and fragmentation of the pyrrole monomer

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APPJs can be suitable for sufficient nucleation and fragmentation of the pyrrole monomer and the and the efficient generation of a new polymer without any high-temperature process or thermal efficient generation of a new polymer without any high-temperature process or thermal damages. damages.

Figure2.2. Optical emission spectra usingusing opticaloptical emission spectrometer (OES) measured the nucleation Figure Optical emission spectra emission spectrometer (OES)inmeasured in the region of novel whose insulating holders are placed outside or inside the glass tube. the nucleation regionAPPJs, of novel APPJs, whosesubstrate insulating substrate holders are placed outside or inside

glass tube.

Figure 3 shows the top-view and cross-section view SEM images of the pPPy nanofibers with a nanoparticle thin film, grown for 30 min, on plastic (for top-view) and glass (for cross-section view) Figure 3 shows the top-view and cross-section view SEM images of the pPPy nanofibers with substrates. The surface temperature of the substrates during the plasma deposition process in the a nanoparticle thin film, grown for 30 min, on plastic (for top-view) and glass (for cross-section inside case was about 30 °C. As shown in Figure 3, no nanoparticles or amorphous state was observed view) substrates. The surface temperature of the substrates during the plasma deposition process in the outside case because the weak plasma was produced in the nucleation region. However, in the ◦ ininside the inside about 30 As shown inwith Figure 3, no nanoparticles or amorphous state case, case manywas nanofibers andC.nanoparticles porous states were observed to be linked was observed in the outside case because the weak plasma the nucleation region. together in uniform and upright networks, meaning that was the produced proposed in APPJs can efficiently However, in the inside case, many nanofibers and nanoparticles with porous states were observed synthesize the uniform nanofibers and nanoparticles. Generally, plasma polymer structures have to bemany linked togethercross-linked in uniform networks and upright meaning However, that the proposed can efficiently irregular andnetworks, porous networks. as shownAPPJs in Figure 3, our synthesize the uniform nanofibers and nanoparticles. Generally, polymer structures have many SEM images confirm that these polymer structures had regularplasma networks without irregular crossirregular cross-linked networks and porous networks. However, as shown in Figure 3, our SEM images linked networks. This experimental result is quite noticeable in that the height and density of the pPPy nanofibers nanoparticles were significantly increased by the novel APPJ technique atnetworks. room confirm that theseand polymer structures had regular networks without irregular cross-linked temperature. Furthermore, proposed method also shows that the deposition rate was significantly This experimental result is the quite noticeable in that the height and density of the pPPy nanofibers −1 under the room-temperature process. In other words, the increased by approximately 0.93 µm·min and nanoparticles were significantly increased by the novel APPJ technique at room temperature. proposed method indicates that the nano-size polymer can be grown rapidly during plasma Furthermore, the proposed method also shows that the deposition rate was significantly increased − 1 thermal solution process. bypolymerization approximatelywithout 0.93 µmthe ·min under the room-temperature process. In other words, the proposed Figure 4a shows imagespolymer of the pPPy grown in the inside casepolymerization of Figure 1a. method indicates thatthe theTEM nano-size cannanoparticles be grown rapidly during plasma The pPPy nanoparticles, with a diameter range of 5–25 nm, were clearly observed. The selected area without the thermal solution process. electron diffraction pattern of pPPy nanoparticles (Figure 4a, inset) reveals the clear diffraction spot Figure 4a shows the TEM images of the pPPy nanoparticles grown in the inside case of Figure 1a. indicating the single-crystalline structure. The characteristics of single-crystalline pPPy nanoparticles The pPPy nanoparticles, with a diameter range of 5–25 nm, were clearly observed. The selected area are attributed to regular-uniform and upright networks, as shown in Figure 3. The prepared electron diffraction pattern of pPPy nanoparticles (Figure 4a, inset) reveals the clear diffraction spot nanostructures of pPPy visualized by energy dispersive X-ray spectroscopy (EDS) elemental indicating the single-crystalline structure. The characteristics of single-crystalline pPPy nanoparticles mapping and high-angle annular dark-field scanning transmission electron microscopy (HAADFare attributed to regular-uniform and upright networks, as shown in Figure 3. The prepared STEM) are shown in Figure 4b. EDS and elemental mapping results reveal that the pPPy nanostructures of pPPy visualized by energy dispersive spectroscopy (EDS) elemental mapping nanoparticles were exclusively composed of C, O, and N.X-ray Accordingly, the proposed device of APPJs and annular dark-fieldtoscanning transmission electron microscopy (HAADF-STEM) canhigh-angle provide versatile advantages the synthesis of polymer nanoparticle structures: (i) the plasma are shown in Figure 4b. EDS and elemental mapping results reveal that the pPPy nanoparticles were exclusively composed of C, O, and N. Accordingly, the proposed device of APPJs can provide versatile


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advantages to 9,the synthesis of polymer nanoparticle structures: (i) the plasma volume and intensity Materials 2016, 6 of Materials 2016, 9,812 812 6 of 1111 is increased due to the effect of the proposed glass tube and insulating substrate holder under ambient air volume and isisincreased to effect the proposed proposed glassfor tube and insulating substrate volume and intensity increaseddue due to the the plasma effect of ofenergies the glass tube and insulating substrate (‘inside case’ ofintensity Figure 1a), resulting in higher required the synthesis of high-density holder under ambient air (‘inside case’ of Figure 1a), resulting in higher plasma energies required for holder under ambientwith air (‘inside case’ of Figure resulting in(ii) higher plasma energies required for polymer nanoparticles single-crystalline characteristics; the polymer particles are changed ofofhigh-density polymer with single-crystalline characteristics; (ii) the thesynthesis synthesis high-density polymer nanoparticles nanoparticles characteristics; (ii) the to the nanometer sizes due to the increased nucleation ofwith the single-crystalline monomer caused by the intense plasma polymer particles are changed to nanometer sizes to the increased nucleation of the monomer polymer particles are changed to nanometer due to the increased nucleation of the monomer produced; and (iii) there is no thermal damage to the plastic substrate from the atmospheric pressure caused no thermal thermaldamage damagetotothe theplastic plasticsubstrate substrate causedby bythe theintense intenseplasma plasmaproduced; produced; and and (iii) there is no plasma polymerization due to its low temperature ionized discharge. fromthe theatmospheric atmosphericpressure pressureplasma plasma polymerization polymerization due from due to to its its low lowtemperature temperatureionized ionizeddischarge. discharge.

Figure 3. Changes in top and cross-section views of scanning electron microscopy (SEM) images of

Figure 3. 3.Changes in top and cross-sectionviews views ofscanning scanning electron microscopy (SEM) images Figure Changes in top and cross-section electron microscopy plasma-polymerized pyrrole (pPPy) nanoparticle of thin films prepared via proposed(SEM) APPJsimages after aof of plasma-polymerized pyrrole(pPPy) (pPPy)nanoparticle nanoparticle thin films prepared via proposed APPJs after plasma-polymerized thin films prepared proposed APPJs after a a deposition of 30 min pyrrole in case of an adopting jet whose insulating substratevia holders are placed outside deposition of 30 min in case of an adopting jet whose insulating substrate holders are placed outside or deposition of 30 mintube. in case ofbar an adopting or inside the glass Scale = 2 µm. jet whose insulating substrate holders are placed outside inside the glass tube. Scale bar = 2 µm. or inside the glass tube. Scale bar = 2 µm.

Figure 4. (a) Transmission electron microscopy (TEM) images of pPPy nanoparticles prepared via proposed APPJs, whose insulating substrate holder is placed inside the glass tube. High-resolution Figure Transmissionelectron electronmicroscopy microscopy (TEM) (TEM) images viavia Figure 4. 4. (a)(a) Transmission images of of pPPy pPPynanoparticles nanoparticlesprepared prepared TEM images of single-crystalline pPPy nanoparticles; insets in (a) represent the selected area electron proposed APPJs, whose insulating substrate holder is placed inside the glass tube. High-resolution proposed APPJs, whose insulating substrate holder is placed inside the glass tube. High-resolution diffraction (SAED) pattern of pPPy nanoparticles; (b) High-angle annular dark-field scanning TEM images single-crystallinepPPy pPPynanoparticles; nanoparticles; insets insets in represent the area electron TEM images of of single-crystalline in (a) (a) represent theselected selected area electron transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) diffraction(SAED) (SAED)pattern patternofofpPPy pPPy nanoparticles; nanoparticles; (b) High-angle annular dark-field scanning diffraction (b) High-angle annular dark-field scanning elemental mapping images of C, O, and N. Scale bar = 2 nm (left) and 20 nm (right). transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping images of C, O, and N. Scale bar = 2 nm (left) and 20 nm (right).

elemental mapping images of C, O, and N. Scale bar = 2 nm (left) and 20 nm (right).


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FT-IR and and XPS XPS were were used used to to determine determine the the chemical chemical changes changes introduced introduced by by the the plasma. plasma. Figure Figure 55 FT-IR shows the FT-IR spectrum of the pPPy nanofibers and nanoparticle thin films on the plastic substrates shows the FT-IR spectrum of the pPPy nanofibers and nanoparticle thin films on the plastic substrates after aa deposition depositiontime timeofof6060min. min.In In inside case, characteristic peaks were observed in after thethe inside case, thethe characteristic peaks were observed in the −1 (N–H stretching with hydrogen bonded secondary amino groups) the broad peak at 3285 cm broad peak at 3285 cm−1 (N–H stretching with hydrogen bonded secondary amino groups) and 2960 and cm−1C–H (aliphatic C–Habsorption). stretching absorption). A peak belonging to the =C–H aliphatic −1 2960 cm (aliphatic stretching A peak belonging to the =C–H aliphatic vibration was − 1 vibrationatwas located 2215 cm peaks some of therings pyrrole in the polymer located 2215 cm−1. atThese peaks. These indicate thatindicate some that of the pyrrole in rings the polymer were were fragmented In thespectra FT-IR spectra pPPy, the different absorptions corresponded to fragmented [37,38].[37,38]. In the FT-IR of pPPy,ofthe different absorptions corresponded to alkenes, − 1 − 1 alkenes, whichfrom resulted from broken and appeared as peaks 805730 cmcmand 730the cmC=C, and the −1, and which resulted broken rings andrings appeared as peaks 805 cm−1 and double −1 . The presence of these peak regions in C=C double bond was found in the sharp peak at 1690 cm −1 bond was found in the sharp peak at 1690 cm . The presence of these peak regions in pPPy films pPPy films that the performance electrical conductivity will be improved doping carrier implies thatimplies the performance in electricalinconductivity will be improved by dopingby carrier electrons electrons [38–40]. [38–40].

Figure 5. Changes in Fourier transform infrared spectroscopy (FTIR) spectra of pPPy nanofibers and Figure 5. Changes in Fourier transform infrared spectroscopy (FTIR) spectra of pPPy nanofibers and nanoparticles thin film prepared using the proposed APPJs after a deposition of 60 min on plastic nanoparticles thin film prepared using the proposed APPJs after a deposition of 60 min on plastic substrates substrates in in the the outside outside and and inside inside cases cases of of Figure Figure 1a, 1a, respectively. respectively.

Figure 6 contains XPS spectra and elemental composition (Figure 6a, insets) of the atomic Figure 6incontains XPS spectra and composition insets) of the after atomic distribution the pPPy nanofibers andelemental nanoparticle thin film (Figure on the 6a, glass substrates a distributionofin60the pPPy nanofibers nanoparticle thin film the glass after a deposition deposition min in outside andand inside cases. As shown inon Figure 6a, insubstrates the XPS survey spectrum, of 60 min in outside and cases. AsNshown in Figure 6a,Oin1sthe XPSeV) survey spectrum, signals signals corresponding to Cinside 1s (285.8 eV), 1s (399.8 eV), and (532.4 electronic orbitals can corresponding to C 1s (285.8 eV), N 1s (399.8 eV), and O 1s (532.4 eV) electronic orbitals can be be observed. These results suggest that there are C, N, and O atoms in the pPPy thin film; the C and observed. These results suggest that there are C, N, and O atoms in the pPPy thin film; the C and N atoms belong to the pyrrole structure, but the O atoms could have originated in the oxidation of N atoms to the pyrrole structure, the O atoms havelines originated in the oxidation of the pPPy belong from ambient air. However, the but additional weakcould emission at the binding energies of the (1072 pPPy eV), fromNa ambient air. and However, the additional weak emission lines at the binding energies of Zn (500 eV), Si (100–200 eV) can be only observed in the survey spectra of the Zn (1072 eV), Na (500 eV), and Si (100–200 eV) can be only observed in the survey spectra of the outside case, which would be presumably due to the easy contamination from the ambient exposure outside case, which would be presumably duethe to the easy contamination from the ambient exposure and chamber. To obtain further insight into chemical polar functional groups present on the and chamber. To obtain further insight thehigh-resolution chemical polarCfunctional groups present on the surface of the pPPy thin films, curve fittinginto of the 1s peaks can be performed. Note surface of the pPPy thin films, curve fitting of the high-resolution C 1s peaks can be performed. Note that chemical assignments for deconvoluted peaks are based on the binding energies reported in the that chemical assignments for deconvoluted peaks areand based on eV the(O=C–N) binding energies reported in the literature; 285.5 eV (C–C, C–H), 286.2 eV (C–N, C≡N), 288.1 [23,41,42]. As shown in literature; 285.5 eV (C–C, C–H), 286.2 eV (C–N, C ≡ N), and 288.1 eV (O=C–N) [23,41,42]. As shown Figure 6b, the 288.1 eV peak corresponding to carbons with oxidation from ambient air, such as O=C– in Figure 6b, the 288.1 eV peakincorresponding carbons with fromwhich ambient air, such as N, was remarkably decreased the inside casetocompared to theoxidation outside case, showed more O=C–N, wascharacteristics remarkably decreased in the inside compared case, hydrophobic [43]. In addition, in thecase inside case, thetoCthe 1s, outside N 1s, and O 1swhich peaksshowed had an more hydrophobic characteristics [43]. In addition, in the inside case, the C 1s, N 1s, and O 1s, 1s peaks atom percent of 73.0%, 13.4%, and 13.6%, respectively. Whereas, in the outside case, the C N 1s, had an atom percent of 73.0%, 13.4%, and 13.6%, respectively. Whereas, in the outside case, the 1s, and O 1s had an atom percent of 69.3%, 12.1%, and 18.6%, respectively. The C 1s and N 1s inCthe N 1s, and 1s had an atomtopercent of 69.3%, due 12.1%, and presence 18.6%, respectively. The Cglass 1s and N 1s in inside caseOwere observed have increased to the of the proposed tube and the inside case were observed to have increased due to the presence of the proposed glass tube and insulating substrate holder. This indicates that the fragmented pyrrole rings in the inside case insulatingcompared substrate holder. Thisof indicates that case. the fragmented pyrrole the inside case observed increased increased with those the outside However, the O 1srings in theininside case was to have decreased significantly, implying that the proposed glass tube and cylindrical insulating


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compared with those of the outside case. However, the O 1s in the inside case was observed to have 8 of 11 decreased significantly, implying that the proposed glass tube and cylindrical insulating substrate holder contributed to minimizing the oxidation pPPy from air and therefore increasing the substrate holder contributed to minimizing theofoxidation ofambient pPPy from ambient air and therefore plasma energy in the nucleation region. increasing the plasma energy in the nucleation region. Materials 2016, 9, 812

Figure 6. Differences in the inside and outside cases of pPPy nanofibers and nanoparticle thin film Figure 6. Differences in the inside and outside cases of pPPy nanofibers and nanoparticle thin film prepared using proposed APPJs after a deposition of 60 min on glass substrates. (a) X-ray prepared using proposed APPJs after a deposition of 60 min on glass substrates. (a) X-ray photoelectron photoelectron spectroscopy (XPS) survey spectra and detailed (b) C 1s (high resolution); (c) N 1s, and spectroscopy (XPS) survey spectra and detailed (b) C 1s (high resolution); (c) N 1s, and (d) O 1s spectra. (d) O 1s spectra. Insets in (a) represent the atom percent in pPPy film. The XPS data is based in 1s Insets in (a) represent the atom percent in pPPy film. The XPS data is based in 1s orbitals. orbitals.

Figure 77 shows shows the the resistance resistance (R) (R) of of the the pPPy pPPy nanofibers nanofibers with with nanoparticle nanoparticle thin thin films films on on the the Figure substrates of of interdigitated interdigitated gas gas sensor sensor electrodes electrodes under under various various iodine iodine exposure exposure (doping) (doping) times timesunder under substrates the same sheet thickness and area. The doping with iodine had the objective to introduce charge the same sheet thickness and area. The doping with iodine had the objective to introduce charge carriers into into the the pPPy pPPy structures structures for for enhancing enhancing its its electrical electrical conductive conductive characteristics. characteristics. The The resistance resistance carriers 7 of the the pPPy pPPy thin thin film film was was over over 99 × 107 ΩΩwithout withoutiodine iodinedoping. doping.As Asthe thedoping dopingtime timewas wasincreased, increased, of × 10 the corresponding resistance decreased sharply over a very short period, namely, 15 min, and was was also the corresponding resistance decreased sharply over a very short period, namely, 15 min, and 5 Ω after 60 min. The as-synthesized pPPy is not electrically conductive, but saturated to be about 3 × 10 also saturated to be about 3 × 105 Ω after 60 min. The as-synthesized pPPy is not electrically its conductivity be induced through iodine doping Moreover, resistance of the pPPy conductive, but can its conductivity can bean induced throughprocess. an iodine dopingthe process. Moreover, the nanofibers and nanoparticle thin films prepared using the proposed APPJs can be easily controlled resistance of the pPPy nanofibers and nanoparticle thin films prepared using the proposed APPJs can byeasily simply doping with iodine underwith various doping which allows theirallows application in be controlled by simply doping iodine undertimes, various doping times,for which for their application in electrochemical toxic gas sensors due to their single-crystalline and porous nature. With this method, a detailed parametric study depending on humidity and temperature using various toxic gases will be carried out in the near future to measure the gas-sensing characteristics with electric conductivity of the nano-sized single-crystalline plasma polymers.


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electrochemical toxic gas sensors due to their single-crystalline and porous nature. With this method, a detailed parametric study depending on humidity and temperature using various toxic gases will be carried out in the near future to measure the gas-sensing characteristics with electric conductivity of the nano-sized single-crystalline plasma polymers. Materials 2016, 9, 812 9 of 11

Figure Figure 7. Changes Changes in in the the resistance resistance of of pPPy pPPy nanofibers nanofibers and and nanoparticle nanoparticle thin thin films films on on substrates substrates of of interdigitated interdigitated gas gas sensor sensor electrodes electrodes under under various various iodine iodine exposure exposure (doping) (doping) times times prepared prepared using using proposed APPJs with the insulating substrate holder placed inside the glass tube.

4. 4. Conclusions Conclusions We We demonstrated that the single-crystalline high-density high-density pPPy pPPy nanoparticle nanoparticle structures structures were successfully synthesized on the glass, plastic and interdigitated gas sensor electrode substrates successfully glass, plastic and interdigitated gas sensor electrode substrates using using novel novel APPJs APPJs at at room room temperature. temperature. As As aa result, result, the the proposed proposed APPJ APPJ device device can can obtain obtain the the high-density high-density and ultra-fastpPPy pPPy with single-crystalline nanoparticles forgasvarious gas sensor and ultra-fast thinthin filmsfilms with single-crystalline nanoparticles for various sensor applications. applications. Furthermore, we also expect that the new pPPy nanoparticles with the single-crystalline Furthermore, we also expect that the new pPPy nanoparticles with the single-crystalline property property grown under a low-temperature (30 °C) can process can aprovide a versatile advantage for gas grown under a low-temperature (30 ◦ C) process provide versatile advantage for gas sensors, sensors, molecular electronics, future technologies, display technologies, optoelectronics, and bio-nanotechnology. molecular electronics, future display optoelectronics, and bio-nanotechnology. by by thethe National Research Foundation of Korea (NRF) grant Acknowledgments: This This work workwas wassupported supported National Research Foundation of Korea (NRF)funded grant by the Korea (MSIP) (No. 2016R1C1B1011918). funded by thegovernment Korea government (MSIP) (No. 2016R1C1B1011918). Author Contributions: Contributions: Choon-Sang Choon-Sang Park, Park, Dong Dong Ha Ha Kim, Kim,and andHeung-Sik Heung-SikTae Tae conceived conceived and anddesigned designedthe thestudy; study; Author Choon-Sang Park and Dong Ha Kim performed the experiments; Choon-Sang Park, Dong Ha Kim, Bhum Jae Shin, Choon-Sang Park and DongLee, Ha Kim performed Tae the experiments; Park, Dong Ha Kim, Jae Do Yeob Kim, Hyung-Kun and Heung-Sik analyzed the Choon-Sang data; Choon-Sang Park, Dong Ha Bhum Kim, and Shin, Do Yeob and Heung-Sik analyzed the data; Choon-Sang Dong Ha Kim, Heung-Sik Tae Kim, wroteHyung-Kun the majorityLee, of the paper, and allTae authors reviewed and approved thePark, final version. and Heung-Sik Tae wrote the majority of the paper, and all authors reviewed and approved the final version. Conflicts of Interest: The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

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