Synthesis of a Polyaniline Nanoparticle Using a Solution Plasma Process with an Ar Gas Bubble Channel

Article Synthesis of a Polyaniline Nanoparticle Using a Solution Plasma Process with an Ar Gas Bubble Channel

Jun-Goo Shin 1,†, Choon-Sang Park 1,†, Eun Young Jung 1, Bhum Jae Shin 2 and Heung-Sik Tae 1,* 1 School of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu 41566, Korea; [email protected] (J.-G.S.); [email protected] (C.-S.P.); [email protected] (E.Y.J.) 2 Department of Electronics Engineering, Sejong University, Seoul 05006, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-53-950-6563 † These authors contributed equally to this work.

Received: 10 December 2018; Accepted: 7 January 2019; Published: 9 January 2019

Abstract: This work researched polymerization of liquid aniline monomer by solution plasma with a gas bubble channel and investigated characteristics of solution plasma and polyaniline (PANI). The injected gas bubble channel in the proposed solution plasma process (SPP) played a significant role in producing a stable discharge in liquid aniline monomer at a low voltage and furthermore enhancing the contact surface area between liquid aniline monomer and plasma, thereby achieving polymerization on the boundary of the liquid aniline monomer and plasma. Solution plasma properties were analyzed with voltage–current, optical emission spectroscopy, and high-speed camera. Conductivity, percentage yield, and firing voltage of PANI nanoparticle dispersed solution were measured. To investigate the characteristics of synthesized PANI nanoparticles, field emission scanning electron microscopy, dynamic light scattering, transmission electron microscopy, selective area electron diffraction (SAED) pattern, Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography, 1H-nuclear magnetic resonance (1H-NMR), and X-ray photo spectroscopy (XPS) were examined. The FTIR, 1H-NMR, and XPS analysis showed the PANI characteristic peaks with evidence that some quinoid and benzene rings were broken by the solution plasma process with a gas bubble channel. The results indicate that PANI nanoparticles have a spherical shape with a size between 25 and 35 nm. The SAED pattern shows the amorphous pattern.

Keywords: solution plasma; polyaniline nanoparticle; gas bubble channel; polymerization

1. Introduction Many scientists or engineers have investigated many considerable advances and changes in the synthesis and characterization of conducting polyaniline (PANI) for the last sixty years [1–7]. PANI nanoparticles have received great attention due to their potential applications in optoelectronics, chemical sensors, electromagnetic shielding, molecular electronics, and bionanotechnologies. Conventionally, PANI nanoparticles have been synthesized via chemical, electrochemical, electrospinning, seeding polymerization, ultrasonic assisted polymerization, hard and soft templates, and interfacial polymerization methods [8–20]. Among these methods, the electrochemical solution plasma process (SPP) is a new, simple, and useful synthesis method of polymer nanoparticles, carbon nanomaterials, metal nanoparticles, surface treatment of polymers, puriﬁcation of wastewater, and degradation of organic compounds, because this nonequilibrium plasma can provide rapid reaction and synthesis due to the reactive chemical species and radicals [21–27]. Electrical discharge in the solution phase is known as “SPP” and has recently been studied to be an effective synthesis method. In addition, SPP does not include any strong chemical reagents. Therefore, the removal of chemical residue is not

Polymers 2019, 11, 105; doi:10.3390/polym11010105 www.mdpi.com/journal/polymers Polymers 2019, 11, 105 2 of 14 required. However, most plasma materials grown using SPP without a gas bubble channel tend to show poor nanoparticle qualities with weak chemical stabilities, which would inherently result from the use of plasma with unstable discharge in solution conditions. Moreover, it is difficult to obtain polymer nanoparticles with uniform size due to the small contact surface area and unstable discharge. To obtain a uniform size of polymer nanoparticles via the conventional SPP method, it is very important to increase the contact surface area and to produce the stable discharge [28,29]. Therefore, in this work, we propose an advanced SPP with an injected gas bubble channel to increase the contact surface area and to produce a stable discharge during the electrochemical plasma synthesis of PANI nanoparticles from the aniline monomer solution in SPP devices. A gas bubble channel is often used to enhance the solution plasma performance in pulsed discharge systems. The injection of gas bubbles can significantly enhance the contact surface area of the gas and monomer solution and increase higher-energy electron production. In addition, gas bubbles can increase the mass transfer rate and can enhance the efficiency of the diffusion of reactive species into the solution [30]. Consequently, the advanced SPP with an Ar gas bubble channel can produce stable plasma in the fragmentation region, and as such increase the mass transfer rate and efficiency of the diffusion of reactive species between the gas and monomer solution. The gas channel formation and related plasma discharge are monitored by using a high speed camera, voltage–current probes, and optical emission spectroscopy (OES). The firing voltage, percentage yield, and conductivity of the monomer solution with PANI nanoparticles synthesized via the advanced SPP are also measured. To characterize the PANI nanoparticles synthesized by the advanced SPP with an Ar gas bubble channel, field emission-scanning electron microscopy, dynamic light scattering, transmission electron microscopy, Fourier transform infrared, 1H nuclear magnetic resonance, gel permeation chromatography, and X-ray photoelectron spectroscopy are also used.

2. Materials and Methods

2.1. Experimental Setup The plasma reactor consisted of a glass cylinder with an outer diameter (O.D.) and inner diameter (I.D.) of 40 and 34 mm, respectively, and a height of 150 mm. In the two tungsten electrodes, the diameter of each electrode was 0.5 mm, and the distance between the tungsten electrodes was 1 mm. The I.D. of the capillary glass tube for the Ar gas channel was 3 mm. The argon (Ar) gas was injected along with two tungsten electrodes in parallel, thus forming a gas bubble channel between two electrodes. The gap of the Ar gas channel between the tungsten electrode and the capillary glass tube was 2.5 mm. The amount of liquid aniline monomer was 26 mL, and the Ar ﬂow rate was 100 standard cubic centimeters per minute (sccm). A bipolar pulse with an amplitude of 16.4 kV and a frequency of 5 kHz was generated by a high voltage ampliﬁer (20/20C-HS, Trek, Inc., Lockport, NY, USA) and pulse generator (AFG-3102, Tektronix Inc., Beaverton, OR, USA). The bipolar pulse duty ratio was 60 µs and the process time for polymerization was 50 min.

2.2. Preparation of Polyaniline The polyaniline (PANI) nanoparticles were synthesized from liquid aniline monomer using a solution plasma process (SPP). After the synthesis process was carried out for 50 min, the PANI nanoparticles were dispersed within processed liquid aniline monomer. For ﬁltration and cleaning of the PANI grown from aniline monomer, processed liquid aniline monomer (5 mL) and ethanol (15 mL) were mixed together. The mixed solution was rotated using a centrifugal separator for 20 min at 10,000 rpm and the by-product, except precipitated PANI, was removed. The distilled water was added to rinse the precipitated PANI and rotated again at the same rotation conditions. Each step mentioned above was repeated twice. The ﬁnal step was to dry the precipitated PANI nanoparticles at 60 ◦C for 12 h in an oven. Hereinafter, the synthesized PANI nanoparticles prepared for measurement are called samples or powder samples. Polymers 2019, 11, 105 3 of 14

2.3. Voltage–Current (V–I) Measurement To examine discharge characteristics, a high voltage probe (P6015A, Tektronix Inc., Beaverton, OR, USA) was connected to the powered electrode and a current probe (Pearson Elec. Inc., Palo Alto, CA, USA) was connected to the ground electrode.

2.4. Optical Emission Spectroscopy The optical emissions ranging from 300 to 900 nm emitted from the Ar plasma and the decomposed aniline during solution plasma polymerization process were observed using an optical emission spectrometer (OES, Ocean Optics Inc., Dunedin, FL, USA) with a wide area lens.

2.5. High Speed Camera The formation of a gas bubble channel in liquid aniline monomer and related discharge phenomena were investigated with a high-speed camera (Phantom Miro C110, AMETEK, Wayne, NJ, USA). In this case, two lenses (Nikon AF Nikkor 105 mm, 1:2.8 D, Nikon, Tokyo, Japan and Nikon-AF 36 mm DG) were used at 5000 frames per second (fps) with a shutter time of 200 µs. Resolution was 272 × 256.

2.6. Scanning Electron Microscopy The morphology and size of the synthesized PANI nanoparticles were monitored by ﬁeld emission-scanning electron microscopy (FE-SEM: SU8220, Hitachi Korea Co. Ltd., Seoul, Korea). For measurement, samples were ﬁxed with paste. Energy dispersive X-ray spectroscopy (EDS) (SU8220, Hitachi Korea Co. Ltd., Seoul, Korea) was employed to identify the element composition of samples.

2.7. Dynamic Light Scattering The size distribution and average particle size of PANI nanoparticles ranging from 0.6 nm to 10 µm were determined by dynamic light scattering (DLS, Otsuka Electronics Co. Ltd., Osaka, Japan). Before the measurement of size distribution, the dispersions of PANI nanoparticles were prepared by 20 min sonication in ethanol.

2.8. Transmission Electron Microscopy PANI nanoparticles were also measured using transmission electron microscopy (TEM) for the precise shape of the image and selected area electron diffraction (SAED) for diffraction patterns. This information was taken using a Titan G2 ChemiSTEM CS Probe (FEI Company, Hillsboro, OR, USA) operating at 200 kV. Samples were dispersed in ethanol and obtained with a carbon-coated copper grid. EDS (FEI Company, Hillsboro, OR, USA) was used to identify the components of the sample.

2.9. Fourier Transform Infrared Spectroscopy Fourier transform infrared spectroscopy (FTIR) spectra of the powder sample was conducted with a PerkinElmer spectrometer (Frontier, Waltham, MA, USA). The measurement method was attenuated total reﬂection-FTIR (ATR-FTIR) and the spectra range was 400–4000 cm−1.

2.10. Nuclear Magnetic Resonance Spectroscopy The detailed electronic structures of PANI nanoparticles were obtained by nuclear magnetic resonance spectroscopy (NMR) (Bricker Company, Billerica, MA, USA) that gave a signal to excite the local magnetic ﬁeld to nuclei of atoms and detected the resultant resonant signal. The detected signal was changed depending on the environment around the atoms. For measurement, the sample was dissolved in deuterated dimethylformamide (DMF-d7) solvent. Polymers 2019, 11, x FOR PEER REVIEW 4 of 15

Polymers2.11. Gel2019 Permeation, 11, 105 Chromatography 4 of 14 Gel permeation chromatography (GPC) was conducted using an Agilent 1200 series instrument 2.11.(Santa Gel Clara, Permeation CA, USA). Chromatography The sample was dissolved in dimethylformamide (DMF, HPLC, Sigma- AldrichGel permeationCo., St. Louis, chromatography MO, USA). Polydispersi (GPC) was conductedty index (PDI) using was anAgilent performed 1200 with series polystyrene instrument (Santastandard. Clara, CA, USA). The sample was dissolved in dimethylformamide (DMF, HPLC, Sigma-Aldrich Co., St. Louis, MO, USA). Polydispersity index (PDI) was performed with polystyrene standard. 2.12. X-ray Photoelectron Spectroscopy 2.12. X-ray Photoelectron Spectroscopy XPS was performed to measure the surface components of PANI nanoparticles using a ESCALABXPS was 250Xi performed instrument to measure (Thermo the Scientific surface components Instruments of Co., PANI Ltd., nanoparticles Waltham, usingMA, USA) a ESCALAB with a 250Xihigh resolution instrument of (Thermo 3 μm. The Scientific source Instrumentsof XPS was a Co., micro-focu Ltd., Waltham,sing monochromator MA, USA) with of a aluminum high resolution with ofa hemispherical 3 µm. The source energy of XPS analyzer was a with micro-focusing a mean radius monochromator of 150 mm and of aluminumenergy range with from a hemispherical 0 to 5000 eV energy analyzer with a mean radius of 150 mm and energy range from 0 to 5000 eV 3. Results and Discussion 3. Results and Discussion 3.1. Discharge Properties with Ar Gas Bubble Channel in Liquid Aniline Monomer 3.1. Discharge Properties with Ar Gas Bubble Channel in Liquid Aniline Monomer Figure 1 shows a schematic diagram of the solution plasma cylinder and measurement equipmentFigure 1used shows in athis schematic study. Discharge diagram of without the solution a gas plasmabubble cylinderchannel andwas measurementvery difficult equipmentto produce usedin liquid in this aniline study. monomer. Discharge In addition, withouta most gas bubbleparticles channel and materials was very synthesized difficult to using produce streamer in liquid and anilineunstable monomer. discharge In without addition, a mostgas bubble particles channel and materials tended synthesizedto show poor using nanoparticle streamer and quality unstable and dischargeproperties withoutdue to the a gasproduction bubble channelof wear tendedout or erosion to show of poor electrodes. nanoparticle To overcome quality these and propertiesproblems, dueAr gas to the was production injected to of wearthe discharge out or erosion region of electrodes.in liquid aniline To overcome monomer these between problems, two Ar tungsten gas was injectedelectrodes to thethrough discharge two capillary region in glass liquid tubes. aniline The monomer distance between between two two tungsten tungsten electrodes electrodes through was 1 twomm. capillary In the case glass that tubes. the distance The distance between between the two two tungsten tungsten electrodes electrodes is more was 1 than mm. 1 Inmm, the the case initial that thedischarge distance voltage between is very the two high tungsten and the electrodes discharge is is more difficult than to 1 mm,form the in initialthe discharge discharge region. voltage The is verysolution high plasma and the cylinder discharge was is connected difficult to with form two in thecapillary discharge glass region. tubes for The supplying solution plasmaAr gas between cylinder wastwo connectedtungsten electrodes. with two capillaryAr gas bubbles glass tubes flowing for supplyingthrough a capillary Ar gas between glass tu twobe formed tungsten a gas electrodes. bubble Archannel gas bubbles between flowing the two through electrodes, a capillary thus enabling glass tube the formeddischarge a gas to be bubble produced channel more between easily at the a twolow electrodes,voltage, even thus in enablingliquid aniline. the discharge The distance to be betw producedeen the more two capillary easily at glass a low tubes voltage, for injection even in liquid of Ar aniline.gas was The3 mm. distance In the betweencase that the twodistance capillary between glass two tubes capillary for injection glass tubes of Ar is gas more was than 3 mm. 3 mm, In the caseAr gas that bubble the distance channel between from two two capillary glassglass tubestubes is is more difficult than to 3 mm,form the in Arthe gas discharge bubble channelregion. fromApplied two peak capillary voltage, glass bipolar tubes is rectangular difficult to form pulse in thewidth, discharge and frequency region. Applied were 16.4 peak kV, voltage, 60 μs, bipolar and 5 rectangularkHz, respectively. pulse width,A bipolar and pulse frequency width wereshorter 16.4 than kV, 60 60 μµss, can and limit 5 kHz, generation respectively. of discharge. A bipolar Applied pulse widthand corresponding shorter than 60 dischargeµs can limit peak generation voltage and of discharge. currents were Applied monitored. and corresponding Optical emissions discharge during peak voltagedischarge and were currents detected were with monitored. OES, whereas Optical a high emissions speed during camera discharge was used were to monitor detected the with behavior OES, whereasof gas bubbles a high and speed related camera discharge. was used to monitor the behavior of gas bubbles and related discharge.

Figure 1.1. Schematic diagram of the solution plasma process in liquid aniline monomer with the proposed argonargon (Ar)(Ar) gasgas bubblebubble channelchannel andand measurementmeasurement setupsetup usedused inin thisthis study.study.

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FigureFigure2 2 shows shows the the images images of of gas gas bubble bubble evolution evolution in in liquid liquid aniline aniline monomer monomer with with and and without without discharge,discharge, usingusing aa highhigh speedspeed camera.camera. ForFor thethe non-dischargenon-discharge case,case, thesethese imagesimages clearlyclearly showshow thatthat thethe ArAr gasgas flowingflowing throughthrough eacheach capillarycapillary glassglass tubetube formedformed aa bubblebubble withinwithin thethe liquidliquid aniline,aniline, andand waswas combinedcombined intointo aa largerlarger bubble,bubble, finallyfinally formingforming aa gasgas bubblebubble channelchannel betweenbetween twotwo tungstentungsten electrodeselectrodes (Video(Video S1). S1). This This gas gas bubble bubble channel channel enveloped enveloped two tw tungsteno tungsten electrodes electrodes and and provided provided a discharge a discharge path betweenpath between the two the tungsten two tungsten electrodes. electrodes. The blue The color blue in Figurecolor in2b Figure shows the2b shows plasma the region plasma generated region duegenerated to the Ardue gas to bubblethe Ar channelgas bubble (Video channel S2). Since(Video the S2). rectangular Since the bipolarrectangular pulse bipolar had a pulsepulse widthhad a ofpulse 60 µ widths at a frequencyof 60 μs at ofa frequency 5 kHz, the of high 5 kHz, voltage the high bipolar voltage pulse bipolar was continually pulse was applied continually irrespective applied ofirrespective the formation of the of formation a gas bubble of channel.a gas bubble At t =channe 6.8 ms,l. At no t discharge = 6.8 ms, wasno discharge observed was in liquid observed aniline in monomerliquid aniline because monomer a gas bubble because channel a gas wasbubble not yetchannel formed was despite not yet the formed fact that despite the rectangular the fact that bipolar the pulserectangular was applied. bipolar Atpulse t = was 10.2 applied. ms, however, At t = 10.2 plasma ms, washowever, observed plasma to be was generated observed in to liquid be generated aniline monomerin liquid aniline thanks monomer to the formation thanks to of the a gas formation bubblechannel, of a gas asbubble shown channel, in Figure as 2shownb. The in gas Figure bubble 2b. channelThe gas facilitatedbubble channel a production facilitated of plasmaa production due to of the plasma lowering due of to a the firing lowering voltage of in a the firing liquid voltage aniline in environment.the liquid aniline The environment. resultant plasma The resultant discharge plasma was propagated discharge was in a propagated direction of in the a direction Ar gas bubble of the channel,Ar gas bubble implying channel, that the implying Ar gas bubblethat the channel Ar ga playeds bubble a rolechannel in providing played a a role discharge in providing path at aa relativelydischarge lowpath voltage at a relatively condition. low voltage condition.

FigureFigure 2.2. Changes in the gas bubble in (a) a non-discharge non-discharge case without applying aa rectangular bipolarbipolar pulse,pulse, andand inin ((bb)) aa dischargedischarge casecase withwith applyingapplying aa rectangularrectangular bipolarbipolar pulsepulse relativerelative toto solutionsolution plasmaplasma processprocess time.time. TheseThese imagesimages werewere obtainedobtained byby highhigh speedspeed cameracamera withwith 50005000 fpsfps andand exposureexposure timetime ofof 197197µ μs.s.

FigureFigure3 3 shows shows the the applied applied peak peak voltage, voltage, discharge discharge peak peak voltage, voltage, and and current current measured measured during during thethe solutionsolution plasmaplasma processprocess ofof liquidliquid anilineaniline monomer.monomer. InIn FigureFigure3 3a,a, the the applied applied peak-to-peak peak-to-peak voltage voltage betweenbetween twotwo tungstentungsten electrodeselectrodes waswas 16.416.4 kVkV underunder nono dischargedischarge condition,condition, wherewhere thethe pulsepulse widthwidth µ waswas 6060 μss atat aa frequencyfrequency ofof 55 kHz.kHz. FigureFigure3 3bb shows shows the the discharge discharge peak peak voltages voltages measured measured relative relative to to processprocess timetime fromfrom 1010 toto 5050 minmin atat anan intervalinterval ofof 1010 minmin whenwhen plasmaplasma waswas producedproduced throughthrough thethe gasgas bubblebubble channelchannel between between two tungstentwo tungsten electrodes. electrodes. As the processingAs the processing time increased, time the increased, corresponding the dischargecorresponding peak discharge voltage also peak increased voltage fromalso increased 2 to 5 kV, from indicating 2 to 5 kV, that indicating it was difficult that it towas produce difficult a dischargeto produce with a discharge an increase with in an the incr processease in time. the process In other time. words, In withother an words, increase with in an the increase process in time, the formingprocess time, a gas forming bubble channel a gas bubble between channel two tungstenbetween two electrodes tungsten was electrodes suppressed, was and suppressed, as such, higherand as firingsuch, higher voltage firing was requiredvoltage was togenerate required discharge,to generate thereby discharge, resulting thereby in resulting an increase in an in increase the discharge in the peakdischarge voltage. peak In Figurevoltage.3c, In the Figure corresponding 3c, the corresponding discharge currents discharge show thecurrents tendency show opposite the tendency to the dischargeopposite to voltage, the discharge meaning voltage, that the decreasemeaning in that the dischargethe decrease current in the would discharge be caused current by an would increase be ofcaused resistance by an inincrease plasma of region resistance depending in plasma on region process depending time. Figure on 4process shows time. the variations Figure 4 shows of firing the voltage,variations percent of firing yield, voltage, and conductivity percent yield, of theand synthesized conductivity PANI of the nanoparticle synthesized dispersed PANI nanoparticle solution in liquiddispersed aniline solution monomer in liquid during aniline process monomer times upduring to 50 process min. In times Figure up4a, to the 50 firingmin. In voltage Figure tended 4a, the tofiring increase voltage with tended an increase to increase in the wi processth an increase time during in the process,process time implying during that process, a gas bubbleimplying channel that a inducedgas bubble by thechannel flow ofinduced the Ar gasby the bubbles flow wasof the difficult Ar gas to bubbles form, as was mentioned difficult in to the form, previous as mentioned discussion in onthe the previous tendency discussion of discharge on the peak tendency voltage of withdischa processrge peak time. voltage In Figure with4 processb, the percentage time. In Figure yield of4b, the percentage yield of PANI relative to the process time tended to increase, thus inducing the increase in the viscosity, which can affect formation of a gas bubble channel [31]. In Figure 4c, the

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Polymers 2019, 11, x FOR PEER REVIEW 6 of 15 PANI relative to the process time tended to increase, thus inducing the increase in the viscosity, which canconductivity affect formation of the of synthesized a gas bubble PANI channel nanoparticle [31]. In Figure dispersed4c, the solution conductivity in liquid of the aniline synthesized monomer PANIduring nanoparticle process times dispersed up to solution50 min intended liquid to aniline increase. monomer From the during results process of Figure times 4a–c, up to when 50 min the tendedmedium to increase. spaced Frombetween the two results tungsten of Figure electrodes4a–c, when was the converted medium spacedfrom liquid between aniline two tungsteninto PANI electrodesnanoparticle was dispersed converted solution from liquid due anilineto SPP, intoboth PANI conductivity nanoparticle and viscosity dispersed were solution increased, due to thereby SPP, bothresulting conductivity in increases andviscosity in the firing were voltage. increased, This thereby phenomenon resulting confirms in increases that the in plasma the ﬁring discharge voltage. in Thisliquid phenomenon aniline monomer conﬁrms strongly that the plasmadepends discharge on the formation in liquid aniline of a gas monomer bubble stronglychannel dependsbetween ontwo thetungsten formation electrodes. of a gas bubble channel between two tungsten electrodes.

Figure 3. (a) Applied voltage, (b) discharge voltage, (c) discharge current measured during solution plasmaFigure process 3. (a) Applied with gas voltage, bubble channel(b) discharge in liquid voltage, aniline (c monomer) discharge relative current to measured process time. during solution plasma process with gas bubble channel in liquid aniline monomer relative to process time.

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Figure 4. Variations of (a) ﬁring voltage, (b) percent yield, and (c) conductivity of synthesized polyanilineFigure 4. (PANI)Variations nanoparticle of (a) firing dispersed voltage, solution (b) percent in liquid yield, aniline and monomer (c) conductivity during process of synthesized time up topolyaniline 50 min. (PANI) nanoparticle dispersed solution in liquid aniline monomer during process time up to 50 min. Figure5 shows the optical emissions measured from plasma discharge in liquid aniline when PANIFigure nanoparticles 5 shows were the optical synthesized emissions by the measured solution from plasma plasma process discharge with a in gas liquid bubble aniline channel. when MolecularPANI nanoparticles emission peaks were of synthesized CN, CH, and by C the2 were solu producedtion plasma by theprocess electron with impact a gas dissociationbubble channel. of liquidMolecular aniline emission monomer. peaks Swan of bandsCN, CH, of C and2 correspond C2 were produced to 471.52, 516.5,by the and electron 561 nm. impact Swan dissociation bands of C 2of areliquid commonly aniline observed monomer. in Swan organic bands material of C and2 correspond originate to from 471.52, conjugated 516.5, and carbon 561 ofnm. benzene Swan ringsbands of of liquidC2 are aniline commonly monomer. observed The in CN organic peaks ofmaterial 388 nm and are orig intenseinate peaks, from conjugated which are related carbon to of benzene benzene rings rings ofof liquid liquid aniline aniline monomer. monomer. The The CN CN violet peaks system of 388 has nm two are smaller intense peaks peaks, of which 360 and are 419 related nm, indicatingto benzene anrings amine of groupliquid attachedaniline monomer. to a benzene The ring. CN Hydrogenviolet system Balmer has linetwo ofsmaller Hα corresponding peaks of 360 toand 656.3 419nm nm, indicating an amine group attached to a benzene ring. Hydrogen Balmer line of Hα corresponding to

Polymers 2019, 11, x FOR PEER REVIEW 8 of 15

Polymers656.3 nm2019 also, 11, 105indicates a dissociation of liquid aniline monomer. Multiple excited Ar lines8 ofare 14 observed from 698 to 854 nm due to the Ar gas bubble injected to ignite plasma [32–34]. Polymers 2019, 11, x FOR PEER REVIEW 8 of 15 also indicates a dissociation of liquid aniline monomer. Multiple excited Ar lines are observed from 656.3 nm also indicates a dissociation of liquid aniline monomer. Multiple excited Ar lines are 698 to 854observed nm due from to 698 the to Ar 854 gas nm bubbledue to the injected Ar gas tobubble ignite injected plasma to ignite [32– 34plasma]. [32–34].

Figure 5.Figure5.Optical Optical 5. emissionsOptical emissions emissions measured measured measured from plasmafromfrom plasma discharge dischargedischarge in liquid in anilineinliquid liquid aniline when aniline PANIwhen nanoparticleswhenPANI PANI werenanoparticles synthesizednanoparticles were by weresynthesized solution synthesized plasma by bysolution processsolution plasma plasma with gas processprocess bubble with with channel. gas gas bubble bubble channel. channel.

3.2. Properties of Synthesized Polyaniline 3.2. Properties of Synthesized Polyaniline Figure 6 shows the synthesized and dispersed PANI nanoparticles in solution and dried PANI Figure6 shows the synthesized and dispersed PANI nanoparticles in solution and dried PANI Figurenanoparticles 6 shows from the thesynthesized solution. Af terand synthesizing dispersed PANI PANI nanopartic nanoparticlesles from inliquid solution aniline and monomer dried PANI nanoparticlesby solution from plasma the solution. with a gas Af Afterbubbleter synthesizing channel for 50 PANImin, the nanoparticlesnanopartic color of solutionles from was changed liquid aniline into black, monomer by solutionbecause plasma polyaniline with ananoparticles gas bubble werechannel dispersed for 50 in min, the solution the color color processed of of solution solution by solution was was changed changed plasma with into into black, black, becausethe polyaniline gas bubble nanoparticleschannel, and nanoparticles were dispersed were not in precipitated the solution on processed the bottom byof the solution bottle. When plasma with PANI nanoparticles were filtered and dried, the PANI nanoparticles agglomerated together to form the gas bubblebubble channel,channel, andand nanoparticlesnanoparticles were notnot precipitated on the bottombottom ofof thethe bottle.bottle. When bigger particles. PANI nanoparticlesnanoparticles were ﬁlteredfiltered andand dried,dried, the PANIPANI nanoparticlesnanoparticles agglomeratedagglomerated together to form bigger particles.

Figure 6. Dispersed PANI synthesized for 50 min and dried PANI nanoparticles.

SEM was used to measure morphology and size of PANI nanoparticles synthesized by solution plasma with gas bubble channel [35,36]. The SEM image in Figure 7a shows that the PANI nanoparticles synthesized have spherical shapes. The size distribution results in Figure 7b show that Figure 6. DispersedDispersed PANI PANI synthesized for 50 min and dried PANI nanoparticles.nanoparticles.

SEM was used to measure morphology and size of PANI nanoparticles synthesized by solution plasma withwith gas gas bubble bubble channel channel [35,36 [35,36].]. The SEMThe imageSEM inimage Figure in7a Figure shows that7a theshows PANI that nanoparticles the PANI synthesizednanoparticles have synthesized spherical have shapes. spheri Thecal size shapes. distribution The size results distribution in Figure results7b show in Figure that the 7b nano show sizes that of PANI nanoparticles are ranged from 25 to 35 nm with a narrow size distribution. The insets in

Polymers 2019, 11, x FOR PEER REVIEW 9 of 15 Polymers 2019, 11, 105 9 of 14 the nano sizes of PANI nanoparticles are ranged from 25 to 35 nm with a narrow size distribution. The insets in Figure 7a are the results of EDS, which confirm that the PANI nanoparticles involve Figurecomponents7a are theof carbon results and ofEDS, nitrogen. which In confirmparticular, that th thee presence PANI nanoparticles of nitrogen in involve the PANI components nanoparticles of carbonis confirmed. and nitrogen. From EDS In particular, results, thewhen presence adopting of nitrogenthe gas inbubble the PANI channel, nanoparticles the chemical is confirmed. element Fromcompositions EDS results, (wt%) when of tungsten adopting on the PANI gas bubblenanopartic channel,les were the remarkably chemical element reduced compositions from 6.36 to (wt%)1.37% ofcompared tungsten to on no PANI gas nanoparticlesbubble channel. were These remarkably results confirm reduced that from the 6.36 solution to 1.37% plasma compared process to nowith gas a bubblegas bubble channel. channel These can results produce confirm uniform that thePANI solution nanopa plasmarticles process with very with small a gas size bubble and channel can reduce can produceerosion of uniform the electrodes. PANI nanoparticles with very small size and can reduce erosion of the electrodes.

FigureFigure 7.7. ((aa)) ScanningScanning electronelectron microscopemicroscope (SEM)(SEM) imageimage andand energyenergy dispersivedispersive X-rayX-ray spectroscopyspectroscopy (EDS),(EDS), and and (b )(b size) size distribution distribution by dynamic by dynamic light spectroscopy light spectroscopy (DLS) of PANI(DLS) nanoparticles of PANI nanoparticles synthesized bysynthesized solution plasma by solution with plasma a gas bubble with a channel; gas bubble insets channel; of (a) insets indicate of ( carbona) indicate and carbon nitrogen and measured nitrogen bymeasured EDS. by EDS.

The TEM image of Figure8a shows a single PANI nanoparticle synthesized using solution plasma The TEM image of Figure 8a shows a single PANI nanoparticle synthesized using solution process with gas bubble channel. Figure8a is a magnified image of a single PANI nanoparticle for plasma process with gas bubble channel. Figure 8a is a magnified image of a single PANI nanoparticle investigating the PANI nanoparticle in detail. A certain crystal structure in the PANI nanoparticle for investigating the PANI nanoparticle in detail. A certain crystal structure in the PANI nanoparticle is not observed in the TEM image. The SAED measurement in the inset of Figure8a confirms that is not observed in the TEM image. The SAED measurement in the inset of Figure 8a confirms that the the PANI nanoparticle has an amorphous pattern [37,38]. Figure8b shows an image of agglomerated PANI nanoparticle has an amorphous pattern [37,38]. Figure 8b shows an image of agglomerated PANI nanoparticles. The EDS analysis in the insets of Figure8b shows that the PANI nanoparticles PANI nanoparticles. The EDS analysis in the insets of Figure 8b shows that the PANI nanoparticles synthesized have carbon and nitrogen elements, as shown in insets of Figure7, because PANI synthesized have carbon and nitrogen elements, as shown in insets of Figure 7, because PANI nanoparticles are composed of a benzene ring, quinoid ring, and amine group. nanoparticles are composed of a benzene ring, quinoid ring, and amine group. Figure9 shows the FTIR spectra of PANI nanoparticles synthesized by the solution plasma process with a gas bubble channel. FTIR spectra shown in Figure9 reveal peaks of C=C bending at 684 cm−1, C–H bending at 744 cm−1, C–H bending at 818 cm−1, C–H bending at 872 cm−1, C–N stretching aromatic amine at 1261 cm−1, C–C stretching aromatic ring at 1493 cm−1, C=C stretching conjugated at 1588 cm−1, and N–H stretching aliphatic primary amine at 3332 cm−1. In particular, N–H stretching aliphatic primary amine at 3332 cm−1 is broad due to the amorphous nature of the PANI nanoparticle. [39,40]. Figure 10 shows the 1H-NMR spectrum of PANI nanoparticles dissolved in 1 DMF-d7 solvent. H-NMR is a useful method to confirm molecules in detail. Two high intensity peaks between 2–4 ppm are 1H-NMR peaks in DMF solvent. 1H-NMR peaks in PANI are mainly centered at 7 ppm and 7, 8 ppm due to protons on pheylene and a disubstituted pheylene unit, as shown in the inset of Figure 10. Peaks of primary amine (–NH2) connected to a benzene ring neighboring imine (–C=N) nitrogen connected to a quinoid ring is observed at 6.4–6.5 ppm. Three sharp peaks at 7.08, 7.18, and 7.28 reveal NH+ that is 1H coupled to 14N confirming the protonated state. The α indicates broad benzene ring photon peaks of PANI nanoparticles at 7.0 ppm and β indicates quinoid ring photon peaks in the range of 7.2–7.9 ppm [41,42]. Table1 shows the result of GPC. Although PANI nanoparticles synthesizedFigure 8. by Transmission solution plasma electron have microscope low molecular (TEM) imag weighte and (Mw) energy and dispersive low molecular X-ray spectroscopy number (Mn), the PANI(EDS) nanoparticles and selected area have electron excellent diffraction polydispersity (SAED) pattern (PDI) of characteristics PANI synthesized of about by solution 1.19. Asplasma a result, PANIwith nanoparticles gas bubble channel. using solution (a,b) TEM plasma images with with adifferent gas bubble magnifications; channel were inset successfully of (a) is SAED synthesized. pattern, This iswhereas one of insets methods of (b suitable) are carbon for and obtaining nitrogen polymer elements nanoparticles using EDS. under liquid monomer [43,44].

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the nano sizes of PANI nanoparticles are ranged from 25 to 35 nm with a narrow size distribution. The insets in Figure 7a are the results of EDS, which confirm that the PANI nanoparticles involve components of carbon and nitrogen. In particular, the presence of nitrogen in the PANI nanoparticles is confirmed. From EDS results, when adopting the gas bubble channel, the chemical element compositions (wt%) of tungsten on PANI nanoparticles were remarkably reduced from 6.36 to 1.37% compared to no gas bubble channel. These results confirm that the solution plasma process with a gas bubble channel can produce uniform PANI nanoparticles with very small size and can reduce erosion of the electrodes.

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The XPS analysis of synthesized PANI nanoparticles using solution plasma with a gas bubble Figure 7. (a) Scanning electron microscope (SEM) image and energy dispersive X-ray spectroscopy channel was conducted. As shown in Figure 11a, the elements C 1s (at 284.5 eV), N 1s (at 398.4 eV), (EDS), and (b) size distribution by dynamic light spectroscopy (DLS) of PANI nanoparticles and O 1ssynthesized (at 531.8 eV)by solution were plasma measured with a on gas the bubble PANI channel; nanoparticles. insets of (a) indicate TheXPS carbon spectra and nitrogen of C 1s, N 1s, and O 1smeasured indicates by thatEDS. PANI nanoparticles consist of carbon, nitrogen and oxygen. The atomic percentage of PANI nanoparticle is 83.4% for C 1s, 6.0% for N 1s, and 10.6% for O 1s. Carbon and nitrogen areThe components TEM image ofof PANI,Figure whereas8a shows oxygen a single is PANI added nanoparticle from atmospheric synthesized air after using polymerization. solution In Figureplasma 11 processb, six carbon-containingwith gas bubble channel. component Figure 8a is peaks a magnified of C 1simage at 284.1, of a single 284.6, PANI 285.7, nanoparticle 286.4, 286.9, for investigating the PANI nanoparticle in detail. A certain crystal structure in the PANI nanoparticle and 287.5 eV are conﬁrmed to be corresponding to C–C/C–H, C=C, C–N, C–O, C=O, and O=C–O, is not observed in the TEM image. The SAED measurement in the inset of Figure 8a confirms that the respectively. Figure 11c shows the N 1s spectra of the PANI nanoparticles where the three peaks PANI nanoparticle has an amorphous pattern [37,38]. Figure 8b shows an image of agglomerated centeredPANI at nanoparticles. 398.7, 399.7, The and EDS 401.1 analysis eV correspond in the insets to of the Figure quinoid 8b shows imine that (–N–), the PANI benzenoid nanoparticles amine-like + (–NH–)Polymerssynthesized structure, 2019, 11 have, x and FOR carbon positivelyPEER REVIEW and nitrogen charged elements, nitrogen as (NH shown2 )[ 45in, 46insets], respectively. of Figure 7, Peaks because of10 CPANI of 1s, 15 N 1s, and Onanoparticles 1s are summarized are composed in Tables of a benzen2 and3e. ring, quinoid ring, and amine group. Figure 9 shows the FTIR spectra of PANI nanoparticles synthesized by the solution plasma process with a gas bubble channel. FTIR spectra shown in Figure 9 reveal peaks of C=C bending at 684 cm−1, C–H bending at 744 cm−1, C–H bending at 818 cm−1, C–H bending at 872 cm−1, C–N stretching aromatic amine at 1261 cm−1, C–C stretching aromatic ring at 1493 cm−1, C=C stretching conjugated at 1588 cm−1, and N–H stretching aliphatic primary amine at 3332 cm−1. In particular, N–H stretching aliphatic primary amine at 3332 cm−1 is broad due to the amorphous nature of the PANI nanoparticle. [39,40]. Figure 10 shows the 1H-NMR spectrum of PANI nanoparticles dissolved in DMF-d7 solvent. 1H-NMR is a useful method to confirm molecules in detail. Two high intensity peaks between 2–4 ppm are 1H-NMR peaks in DMF solvent. 1H-NMR peaks in PANI are mainly centered at 7 ppm and 7, 8 ppm due to protons on pheylene and a disubstituted pheylene unit, as shown in the inset of Figure 10. Peaks of primary amine (–NH2) connected to a benzene ring neighboring imine (–C=N) nitrogen connected to a quinoid ring is observed at 6.4–6.5 ppm. Three sharp peaks at 7.08, 7.18, and 7.28 reveal NH+ that is 1H coupled to 14N confirming the protonated state. The α indicates broad benzene ring photon peaks of PANI nanoparticles at 7.0 ppm and β indicates quinoid ring photon peaks in the range of 7.2–7.9 ppm [41,42]. Table 1 shows the result of GPC. Although PANI FigurenanoparticlesFigure 8. Transmission 8. Transmissionsynthesized electron electronby solu microscope microscopetion plasma (TEM)(TEM) have imagimage lowe molecular and and energy energy weight dispersive dispersive (Mw) X-ray X-rayand spectroscopy low spectroscopy molecular (EDS)number(EDS) and (Mn), selectedand selectedthe PANI area area electron nanoparticles electron diffraction diffraction have (SAED) (SAED) excellen pattern patternt polydispersity of of PANI PANI synthesized synthesized (PDI) characteristics by solution by solution plasma of plasma about 1.19. As a result, PANI nanoparticles using solution plasma with a gas bubble channel were with gaswith bubble gas bubble channel. channel. (a, (ba),b TEM) TEM images images withwith different magnifications; magniﬁcations; inset inset of (a of) is (a SAED) is SAED pattern, pattern, successfully synthesized. This is one of methods suitable for obtaining polymer nanoparticles under whereaswhereas insets insets of ( bof) are(b) are carbon carbon and and nitrogen nitrogen elements elements using EDS. EDS. liquid monomer [43,44].

FigureFigure 9. Fourier 9. Fourier transformation transformation infrared infrared (FTIR)(FTIR) spec spectrumtrum of of synthesized synthesized PANI PANI nanoparticles nanoparticles by by solutionsolution plasma plasma process process with with gas gas bubble bubble channel. channel.

Polymers 2019, 11, 105 11 of 14 Polymers 2019, 11, x FOR PEER REVIEW 11 of 15

1 FigureFigure 10. 10.H-nuclear 1H-nuclear magnetic magnetic resonance (NMR) (NMR) spectrum spectrum of PANI of PANI nanoparticles nanoparticles synthesized synthesized by by solutionsolution plasmaplasma withwith gas bubble channel, channel, PANI PANI nanoparticles nanoparticles were were dissolved dissolved by deuterated by deuterated

dimethylformamidedimethylformamide (DMF-d (DMF-d7)7 solvent.) solvent.

TableTable 1. Measured 1. Measured PANI PANI nanoparticles nanoparticles by gel gel permeation permeation chromatography. chromatography.

SamplesSamples Mw Mw (kDa) (kDa) Mn Mn (kDa) (kDa) Polydispersive Polydispersive Index Index (PDI) (PDI) AnilineAniline monomer monomer 0.093 0.093 - - - - PANIPANI 9.7059 9.7059 8.1299 8.1299 1.19 1.19

Polymers 2019, 11, x FOR PEER REVIEW 12 of 15 The XPS analysis of synthesized PANI nanoparticles using solution plasma with a gas bubble channel was conducted. As shown in Figure 11a, the elements C 1s (at 284.5 eV), N 1s (at 398.4 eV), and O 1s (at 531.8 eV) were measured on the PANI nanoparticles. The XPS spectra of C 1s, N 1s, and O 1s indicates that PANI nanoparticles consist of carbon, nitrogen and oxygen. The atomic percentage of PANI nanoparticle is 83.4% for C 1s, 6.0% for N 1s, and 10.6% for O 1s. Carbon and nitrogen are components of PANI, whereas oxygen is added from atmospheric air after polymerization. In Figure 11b, six carbon-containing component peaks of C 1s at 284.1, 284.6, 285.7, 286.4, 286.9, and 287.5 eV are confirmed to be corresponding to C–C/C–H, C=C, C–N, C–O, C=O, and O=C–O, respectively. Figure 11c shows the N 1s spectra of the PANI nanoparticles where the three peaks centered at 398.7, 399.7, and 401.1 eV correspond to the quinoid imine (–N–), benzenoid amine-like (–NH–) structure, and positively charged nitrogen (NH2+) [45,46], respectively. Peaks of C 1s, N 1s, and O 1s are summarized in Tables 2 and 3.

FigureFigure 11. X-ray 11. X-ray photoelectron photoelectron spectra spectra (XPS) (XPS) ofof synthesizedsynthesized PANI PANI nanoparticles nanoparticles by solution by solution plasma plasma with with gas bubblegas bubble channel channel (a) (a XPS) XPS wide wide scan scan spectra,spectra, ( (bb)) C C 1s 1s spectra, spectra, (c) ( cN) N1s 1sspectra, spectra, and and(d) O (d 1s) O 1s spectraspectra of synthesized of synthesized PANI PANI nanoparticles. nanoparticles.

Table 2. Peak assignment and envelop composition of various C 1s core level spectra of PANI observed in X-ray photoelectron spectra (XPS) in Figure 11b.

C 1s Peaks Assignment (eV) and Envelopment Composition (%) Sample 284.6 285.1 285.7 286.4 286.9 287.5 C=C C–C/C–H C–N C–O C=O O–C=O PANI 25.4 38.7 17.2 10.7 4.3 3.7

Table 3. Peak assignment and envelop composition of various N 1s core level spectra of PANI observed in X-ray photoelectron spectra (XPS) in Figure 11c.

N 1s Peaks Assignment (eV) and Envelopment Composition (%) Sample 398.7 399.7 401.1 –N– –NH– –NH2– PANI 31.0 52.6 16.4

4. Conclusions PANI nanoparticles were successfully synthesized from liquid aniline monomer by solution plasma with a gas bubble channel. Polymerization was achieved at the boundary of the gas bubble channel and liquid monomer aniline. Plasma is very difficult to generate in liquid aniline. To solve this problem, a gas bubble was provided externally during the solution plasma process, such that the plasma discharge was effectively produced to synthesize the PANI nanoparticle from liquid aniline monomer successfully. The FTIR, NMR, and XPS reveal that the synthesized nanoparticles are PANI. The synthesized PANI nanoparticles are observed by SAED to be an amorphous pattern. The SEM, DLS, and TEM confirm that the size of the PANI nanoparticle is estimated to be about tens of nanometers and has a spherical shape.

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Table 2. Peak assignment and envelop composition of various C 1s core level spectra of PANI observed in X-ray photoelectron spectra (XPS) in Figure 11b.

C 1s Peaks Assignment (eV) and Envelopment Composition (%) Sample 284.6 285.1 285.7 286.4 286.9 287.5 C=C C–C/C–H C–N C–O C=O O–C=O PANI 25.4 38.7 17.2 10.7 4.3 3.7

Table 3. Peak assignment and envelop composition of various N 1s core level spectra of PANI observed in X-ray photoelectron spectra (XPS) in Figure 11c.

N 1s Peaks Assignment (eV) and Envelopment Composition (%) Sample 398.7 399.7 401.1 –N– –NH– –NH2– PANI 31.0 52.6 16.4

4. Conclusions PANI nanoparticles were successfully synthesized from liquid aniline monomer by solution plasma with a gas bubble channel. Polymerization was achieved at the boundary of the gas bubble channel and liquid monomer aniline. Plasma is very difﬁcult to generate in liquid aniline. To solve this problem, a gas bubble was provided externally during the solution plasma process, such that the plasma discharge was effectively produced to synthesize the PANI nanoparticle from liquid aniline monomer successfully. The FTIR, NMR, and XPS reveal that the synthesized nanoparticles are PANI. The synthesized PANI nanoparticles are observed by SAED to be an amorphous pattern. The SEM, DLS, and TEM conﬁrm that the size of the PANI nanoparticle is estimated to be about tens of nanometers and has a spherical shape.

Supplementary Materials: Video S1: gas bubble channel formation without a bipolar rectangular pulse, Video S2: plasma generation through a gas bubble channel when applying a bipolar rectangular pulse. Author Contributions: J.-G.S., C.-S.P., and H.-S.T. conceived and designed the study; J.-G.S. and C.-S.P. performed the experiments; J.-G.S., C.-S.P., and E.Y.J. helped to conduct the plasma experimental setup; J.-G.S., E.Y.J., B.J.S., C.-S.P., and H.-S.T. analyzed the data; J.-G.S., C.-S.P., and H.-S.T. wrote the majority of the paper and all authors reviewed and approved the final version. Funding: This research was funded by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. 2017R1A4A1015565) and the Korean government (MOE) (No. 2016R1D1A1B03933162). Acknowledgments: The authors would like to thank Sang-Geul Lee and Weon-Sik Chae at the Korea Basic Science Institute (Daegu) for useful discussion and taking the FTIR data. Conflicts of Interest: The authors declare no conflicts of interest.

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