Reaction Spinning Titanium Dioxide Particle-Coated Carbon Fiber for Photoelectric Energy Conversion

fibers

Article Reaction Spinning Titanium Dioxide Particle-Coated Carbon Fiber for Photoelectric Energy Conversion

Leonardo Yuan 1, Xupeng Wei 1, Jenny P. Martinez 1, Christina Yu 2, Niousha Panahi 2, Jeremy B. Gan 3, Yongping Zhang 4 and Yong X. Gan 1,*

1 Department of Mechanical Engineering, California State Polytechnic University Pomona, Pomona, CA 91768, USA; [email protected] (L.Y.); [email protected] (X.W.); [email protected] (J.P.M.) 2 Department of Natural, Physical and Health Sciences, Citrus College, Glendora, CA 91741, USA; [email protected] (C.Y.); [email protected] (N.P.) 3 Diamond Bar High School, Diamond Bar, CA 91765, USA; [email protected] 4 Department of Civil Engineering, California State Polytechnic University Pomona, Pomona, CA 91768, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-909-869-2388

Received: 24 April 2019; Accepted: 20 May 2019; Published: 23 May 2019

Abstract: In this paper, a titanium dioxide particle coated carbon fiber was prepared by reaction spinning. Polyacrylonitrile (PAN) was used as the precursor to generate a continuous carbon nanofiber. A solution containing 10% wt PAN polymer dissolved in dimethylformamide (DMF) was made as the core fluid. The sheath fluid contains 10% titanium (IV) isopropoxide, 85% ethanol, and 5% acetic acid. The two solutions were co-spun onto an aluminium plate covered with a layer of soft tissue paper. A titanium hydroxide layer formed at the surface of the PAN fiber through the hydrolysis of titanium isopropoxide due to the moisture absorption in the co-spinning process. The reaction spun fiber was converted to a partially carbonized nanofiber by the heat treatment in air at 250 ◦C for two hours, then in hydrogen at 500 ◦C for two hours. During the early stage of the heat treatment, the titanium hydroxide decomposed and produced titanium dioxide nanoparticles at the surface of the carbon fiber. The structure and composition of the carbonized fiber were studied by scanning electron microscopy (SEM). The photosensitivity of the particle-containing fiber was characterized by measuring the open circuit voltage under visible light excitation. The photoelectric energy conversion behavior of the fiber was confirmed by open circuit potential measurement. The potential applications of the composite fiber for photovoltaics and photonic sensing were discussed.

Keywords: titanium dioxide particle coated carbon ﬁber; reaction spinning; heat treatment; microstructure; photovoltaics; photoelectric energy conversion; photosensitive behavior

1. Introduction Electrohydrodynamic casting or electrospinning is a simple, versatile, economic, and efficient strategy in preparing fibers in nanometer scales for various polymers and oxidized metals [1]. To incorporate oxide nanoparticles into electrospun fibers, metal- organic compounds are typically used. Titanium oxide nanoparticles can be prepared through the hydrolysis reaction of titanium isopropoxide in humid air with highly concentrated acetic acid as a stabilizer. Titanium oxide is manufactured in the industry as a main functional component due to its high photocatalytic activity [2]. Examples of its applications include in solar cell technology and environmental science [3]. The combination of the excellent properties of nanostructured titanium dioxide and its high surface area gives the titanium-oxide-containing nanofibers vast applicability in cosmetics, scaffolds for tissue engineering, catalytic devices, sensors, solar cells, and optoelectronic devices. During electrospinning,

Fibers 2019, 7, 49; doi:10.3390/fib7050049 www.mdpi.com/journal/fibers Fibers 2019, 7, 49 2 of 10 the electrostatic charging of the droplet results in the formulation of the well-known Taylor cone. This allows for nanofiber formation from a mixture of solutions with multiple components. To examine the photovoltaic responses of the resultant fiber, the fiber itself must be conductive. Therefore, partially carbonizing the precursor through a series of heat treatments has been performed as shown in earlier studies [4–8]. The high photoactive phase of the TiO2 nanofiber is anatase, and it makes the nanofiber viable for application in sensors, solar cells, etc. [9]. In general, a higher degree of molecular orientation in the original PAN precursor fiber results in carbon fibers with better mechanical properties—particularly the tensile modulus. Based on this fact, the polyacrylonitrile (PAN) and dimethylformamide (DMF) solution was considered the base polymer for forming the nanofiber [10]. However, the PAN and DMF solution itself is an excellent material to form porous nanocomposites through the electrospinning process. By incorporating the titanium dioxide into the fibers, enhancement on the photosensitive characteristic should be possible. It has been reported that graphene-based materials can be used as the counter electrode or photoanode for making dye-sensitized solar cells (DSSCs) [11]. Specifically, the photovoltaic performance of the DSSCs consisting of graphene composites with carbon nanotubes (CNTs), titanium dioxide (TiO2), and other substances, such as organic semiconductors and ionic liquids, were compared with standard reference solar cells. One of the interesting findings was that the graphene TiO2 nanocomposite anode had better photovoltaic properties than the commonly used pure TiO2 photoanode. Uddin et al. [12] made solid-state dye-sensitized photovoltaic micro-wires (DSPMs) with carbon nanotubes yarns (CNYs) as the counter electrode. Through optimizing the numbers of CNYs and CNYs–TiO2 interface, the open circuit voltage and current density of the micro-wire solar cells. The advantages of the cells include: high strength, high flexibility, and excellent electrical conductivity. But the energy conversion efficiency remains to be increased. This work presents the usage of a new composite fiber: TiO2 coated carbon fiber for making a flexible photoelectric energy converter. The TiO2 nanoparticles were sporadically coated at the fiber surface to form a composite fiber. This composite fiber showed fast response to visible light which allowed us to measure the open circuit voltage. The microstructure of the composite fiber was examined and its composition was analyzed by the scanning electron microscopy (SEM).

2. Materials and Methods Polyacrylonitrile (PAN) with an approximate molecular weight of 150,000 was supplied by Scientific Polymer Products, Inc., Ontario, NY, USA. Dimethylformamide (DMF) was purchased from Alfa Aesar, Ward Hills, MA, USA. Titanium (IV) isopropoxide (95%) was also purchased from Alfa Aesar. A high voltage direct current power source was supplied by Spellman, Inc., Hauppauge, NY, USA. A Fusion 200 precision syringe pump was purchased from Chemyx, Inc., Stafford, TX, USA. An OTF-1200X compact split tube furnace made by MTI Corporation, Richmond, CA, USA was used for heat treating the prepared materials. Electrohydrodynamic casting or electrospinning was the process used to fabricate the fibers in the experiment. This method has the advantage of easy deposition and is versatile for the manufacturing of polymeric materials, composites, and ceramics [13]. This process provides sufficient electrostatic force by applying a very large DC voltage difference between the solution and collector to overcome and surface tension of the PAN and titanium isopropoxide solutions, which made the titanium oxide particle-forming solution and the PAN solution co-spun or co-cast onto the collector, and formed a composite microfiber in the reaction spinning process. Figure1a shows the layout of the equipment and Figure1b illustrates the spinneret for the fiber processing. The following forms of materials were used to make the solutions for this process. Polyacrylonitrile polymer (PAN) was supplied in powder form. Dimethylformamide (DMF) was used as the solvent for PAN. Titanium oxide (TiO2) (Cal Poly Pomona, Pomona, CA, USA) was made in nanoscale powder from the hydrolysis of titanium metal-organic compound. Acetic acid (CH3 COOH) (Alfa Aesar, Ward Hill, MA, USA) was used as the stable agent. The first step was to prepare the titanium oxide powder Fibers 2019, 7, 49 3 of 10 mixed into acetic acid with no critical percentage ratio because the solution would absorb water from ambient air. Roughly 10 mL of ethanol was used to dissolve about 1.0 g of titanium isopropoxide. Then, several drops of acetic acid were added into the solution as the stabilizer. The dissolving process Fibers 2019, 7, x FOR PEER REVIEW 3 of 10 took 4 h in a test tube mixer to ensure the solution quality. The next step was to prepare the PAN solution,solution, with with a 1 a to 1 9to ratio; 9 ratio; 1.0 1.0 g of g polyacrylonitrileof polyacrylonitrile polymer polymer per per 9 mL 9 ml of dimethylformamideof dimethylformamide solvent solvent to obtainto obtain the the concentration concentration of of 10% 10% PAN PAN solution. solution. This This mixing mixing process process was was done done in in the the test test tube tube mixer. mixer.

Figure 1. Equipment layout of the reaction spinning process: (a) schematic of the reaction spinning; (b)Figure photograph 1. Equipment of the co-axial layout spinneret of the reaction for the spinning reaction spinning.process: (a) schematic of the reaction spinning; (b) photograph of the co-axial spinneret for the reaction spinning. With the completion of preparing two solutions, the PAN and titanium isopropoxide solutions were filledWith into the twocompletion separated of syringes.preparing Thesetwo solutions, two 15 mL the syringesPAN and were titanium connected isopropoxide with a coaxialsolutions spinneret.were filled The into PAN-DMF two separated core fluid syringes. was pumped These by two a Chemxy 15 ml precisionsyringes syringewere connected pump and with the syringea coaxial wasspinneret. connected The to PAN-DMF the 20-gauge core inner fluid needle. was pumped The syringe by a thatChemxy contained precision titanium syringe isopropoxide pump and the solution syringe waswas attached connected to a to flexible the 20-gauge PVC tube inner and needle. a 16-gauge The syring spinneret.e that Bothcontained needles titanium were bondedisopropoxide together solution to formwas a coaxialattached spinneret to a flexible setup, PVC which tube is and shown a 16-gauge in Figure sp1inneret.b. Since Both the titanium needles isopropoxidewere bonded solutiontogether to formedform thea coaxial outer-layer spinneret flow, setup, it reacted which with is shown water in in the Figure air to 1b. produce Since the titanium titanium hydroxide. isopropoxide This setupsolution providedformed a the structure outer-layer where flow, the titaniumit reacted isopropoxide with water in would the air generate to produce a titanium titanium hydroxide-containing hydroxide. This setup shellprovided layer on a structure the PAN-DMF where jet. the titanium isopropoxide would generate a titanium hydroxide-containing shellWhen layer the on fiber the PAN-DMF was finally jet. produced, it was necessary to go to the second step—i.e., the heat treatmentWhen step the of thefiber experiment. was finally Before produced, putting it thesewas nece twossary syringes to go into to thethe programmablesecond step—i.e., precision the heat syringetreatment pump, step the of air the was experiment. purged from Before both putting of the these syringes two to syringes ensure theinto consistency the programmable of flow rateprecision of solution.syringe Apump, distance the ofair 15 was cm purged was kept from between both of thethe tipsyringes of the to needles ensure and the theconsistency fiber collector of flow plate. rate of A DCsolution. power A supply distance was of linked15 cm was in between kept between the tip the of thetip of coaxial the needles spinneret and and the thefiber final collector collector plate. plate. A DC Thispower power supply supply was was linked connected in between to a high the tip voltage of the AC-DC coaxial converter, spinneret whichand the had final its collector positive plate. lead on This thepower spinneret, supply and was negative connected lead on to thea high collector voltage plate. AC-DC This setupconverter, facilitated which the had electrospinning its positive lead process on the underspinneret, room temperatureand negative and lead generic on the atmospheric collector plat conditions.e. This setup The facilitated injection the speeds electrospinning of both syringes process wereunder adjusted room between temperature 0.001 and to 0.01 generic mL/ minatmospheric depending cond onitions. the collecting The injection condition. speeds A DCof both voltage syringes of 15were kV was adjusted applied between to electrify 0.001 theto 0.01 solutions, ml/min and depending this potential on the dicollectingfference condition. led the charged A DC voltage jet to cast of 15 fiberskV onwas the applied collector to plate.electrify The the resultant solutions, microfiber and this was potential sprayed difference and collected led the on charged a tissue jet paper to cast which fibers wason placed the collector on the stainless-steelplate. The result collectorant microfiber plate, which was sprayed gave the and advantage collected of on separating a tissue paper the fabricated which was fiberplaced easier on than the itself.stainless-steel The entire collector electrospinning plate, which and ga collectingve the advantage process took of separating approximately the fabricated 2 h so that fiber theeasier fiber matthan could itself. haveThe entire the desired electrospinning thickness andand strengthcollecting for process the later took heat approximately treatment process. 2 hours It wasso that measuredthe fiber that mat thecould collected have the fiber desired mat samplesthickness after and stre electrospinningngth for the later reached heat atreatment thickness process. of 400 µItm. was Themeasured fiber has that an average the collected diameter fiber in mat the samples micron meterafter electrospinning range. The yield reached of the electrospinninga thickness of 400 process µm. The is aboutfiber has 90%. an average diameter in the micron meter range. The yield of the electrospinning process is aboutHeat 90%. treatment of the fabricated microfiber caused oxidation of PAN and complete dehydration of titaniumHeat hydroxide treatment by settingof the fabricated the specimen microfiber into a quartz caused tube oxidation furnace of at PAN 250 ◦andC for complete 2 h. The dehydration air in the quartzof titanium furnace hydroxide was replaced by setting with hydrogen the specimen after theinto heating a quartz temperature tube furnace was at raised250 °C to for 500 2 hours.◦C and The the air heatingin the process quartz continuedfurnace was for replaced an additional with 2hydrogen h before cool after down. the heating At this temperature high temperature was raised in hydrogen, to 500 °C and the heating process continued for an additional 2 hours before cool down. At this high temperature in hydrogen, the PAN fiber was converted into a partially carbonized state. Also, the titanium hydroxide was completely converted into dioxide. During the cooling process, the specimen remained in its position inside the furnace tube; this process could take different amounts of time due to different ambient temperature. Figure 2 represents the complete time-temperature profile associated with the

Fibers 2019, 7, 49 4 of 10 the PAN fiber was converted into a partially carbonized state. Also, the titanium hydroxide was completely converted into dioxide. During the cooling process, the specimen remained in its position Fibersinside 2019 the, 7, furnace x FOR PEER tube; REVIEW this process could take different amounts of time due to different ambient4 of 10 temperature. Figure2 represents the complete time-temperature profile associated with the heat heattreatment treatment process. process. After theAfter heat the treatment, heat treatment, the average the diameteraverage ofdiameter the fiber of was the reduced fiber was significantly. reduced significantly.

FigureFigure 2. Time-temperature 2. Time-temperature profile profile associated associated withthe the heat heat treatment treatment on on the the spun spun fibers. fibers. The photovoltaic tests were performed on the titanium dioxide coated carbon fiber composite. The photovoltaic tests were performed on the titanium dioxide coated carbon fiber composite. A fluorescent light tube was used to generate visible light. A CHI440C Electrochemical Workstation, A fluorescent light tube was used to generate visible light. A CHI440C Electrochemical Workstation, made by CH Instrument, Austin, TX, USA was used to record the voltage response data. The sample made by CH Instrument, Austin, TX, USA was used to record the voltage response data. The sample fiber was set on transparent glass slides and aluminum foil strips were used as the electrical fiber was set on transparent glass slides and aluminum foil strips were used as the electrical conducting conducting path. The light source was controlled as 25 s ON followed by 25 s OFF. The test results path. The light source was controlled as 25 s ON followed by 25 s OFF. The test results were obtained were obtained by plotting the open circuit potential versus time. by plotting the open circuit potential versus time. 3. Results and Discussion 3. Results and Discussion

3.1. Morphology and Composition The scanning electron microscopy (SEM) was us useded to observe the microstructure and determine the compositioncomposition of of the the specimen. specimen. Photoelectric Photoelectric energy energy conversion conversion properties properties of the partially of the carbonized partially carbonizednanofiber were nanofiber characterized were characterized using an electrochemical using an electrochemical workstation. Earlierworkstation. work on Earlier microstructure work on microstructureanalysis showed analysis that electrospun showed fibersthat electrospun were randomly fibers oriented were inrandomly different oriented layers [14 in]. Figuredifferent3a showslayers [14].the microstructure Figure 3a shows of the the microstructure particle coated of composite the particle carbon coated fiber composite specimen carbon under fiber SEM. specimen The diameter under SEM.of the The fibers diameter can be of adjusted the fibers by can varying be adjusted the rheological by varying properties the rheological of the properties solution and of the tuning solution the processingand tuning parametersthe processing as reportedparameters [15 ].as At reported a slightly [15]. higher At a slightly magnification, higher themagnification, SEM micrograph the SEM of micrographFigure3b revealed of Figure that 3b the revealed titanium that dioxide the titanium nanoparticles dioxide nanoparticles were laying on were the laying partially on the carbonized partially carbonizedPAN fiber. ThePAN sizes fiber. of The these sizes titanium of these dioxide titanium particles dioxide varied particles from severalvaried from tens toseveral several tens hundreds to several of hundreds of nanometers. During the second stage of the heat treatment at 500 °C, a size reduction of nanometers. During the second stage of the heat treatment at 500 ◦C, a size reduction of the nanofiber theoccurred nanofiber due tooccurred the evaporation due to the of evaporation smaller molecules of smaller in the molecules PAN polymer. in the The PAN PAN polymer. fiber coated The PAN with fibertitanium coated oxide with had titanium an average oxide diameter had an average of 4 µm. diam Aftereter the of heat4 µm. treatment, After the theheat fiber treatment, had an the average fiber haddiameter an average of 2 µ m.diameter The initial of 2 µm. size The and initial the heat-treated size and the fiber heat-treated size were fiber determined size were bydetermined microscopic by microscopic examination, and the results were obtained using the quantitative measurement on the dimension of the fiber filaments shown in typical SEM images. It is meaningful to reveal the composition profile of the microfiber. A typical area of the fiber mat as shown in the SEM image of Figure 4a was chosen for X-ray diffraction energy dispersive spectroscopic analysis. Only Ti, O, and C signals were observed on the X-ray diffraction energy dispersive spectrum as shown in Figure 4b. As mentioned, the results shown in Figure 4b were obtained by the area mapping analysis and the mapping region as defined by the whole area of Figure

Fibers 2019, 7, 49 5 of 10 examination, and the results were obtained using the quantitative measurement on the dimension of the fiber filaments shown in typical SEM images. It is meaningful to reveal the composition profile of the microfiber. A typical area of the fiber mat as shown in the SEM image of Figure4a was chosen for X-ray di ffraction energy dispersive spectroscopic analysis. Only Ti, O, and C signals were observed on the X-ray diffraction energy dispersive spectrum as shown in Figure4b. As mentioned, the results shown in Figure4b were obtained by the area mappingFibers 2019, analysis 7, x FOR PEER and REVIEW the mapping region as defined by the whole area of Figure4a. Since Figure5 of 410b only provides qualitative information, quantitative results from the elemental analysis are listed as a part4a. inSince Table Figure1. 4b only provides qualitative information, quantitative results from the elemental analysisFibers are2019 ,listed 7, x FOR as PEER a part REVIEW in Table 1. 5 of 10

4a. Since Figure 4b only provides qualitative information, quantitative results from the elemental analysis are listed as a part in Table 1.

Figure 3. Scanning electron microscopic (SEM) analysis of the reaction spun composite fibers: (a) a Figure 3. Scanning electron microscopic (SEM) analysis of the reaction spun composite fibers: (a) a lowerlower magnification magnification image image showingshowing thethe microfibermicrofiber on the substrate; (b) (b) a a higher higher magnification magnification image image Figure 3. Scanning electron microscopic (SEM) analysis of the reaction spun composite fibers: (a) a showingshowing the the titanium titanium dioxidedioxide nanoparticlesnanoparticles locatedlocated atat the surface of the carbon fibers. fibers. lower magnification image showing the microfiber on the substrate; (b) a higher magnification image It mustshowing be indicated the titanium that dioxide at temperatures nanoparticles lo highercated at than the surface 450 C,of the the carbon crystallization fibers. of TiO particles It must be indicated that at temperatures higher than 450 °C,◦ the crystallization of TiO22 particles converts the rutile form to anatase [16]. Therefore, the obtained TiO2 nanoparticles in this work should converts Itthe must rutile be indicated form to that anatas at temperaturese [16]. Therefore, higher thanthe 450obtained °C, the TiOcrystallization2 nanoparticles of TiO 2in particles this work be in the anatase form with high photosensitivity. In the SEM analysis, since the titanium element has shouldconverts be in thethe rutile anatase form form to anatas with ehigh [16]. photosenTherefore,sitivity. the obtained In the TiOSEM2 nanoparticles analysis, since in thisthe worktitanium aelement highershould atomichas be ina mass thehigher anatase than atomic any form other masswith elements highthan photosen any in PAN,otsitivity.her itelements generates In the SEM in stronger analysis,PAN, backscatteringit since generates the titanium stronger electron signals.backscatteringelement Therefore, has electron a thehigher regionssignals. atomic containing Therefore,mass than titanium thanye regions other dioxide elementscontaining showed in PAN,titanium the highestit generatesdioxide brightness showedstronger in the the micrograph.highestbackscattering brightness The partially electronin the micrograph. carbonizedsignals. Therefore, The fibers partially formedthe regions carbonized the containing grey fibers tone duetitanium formed to the thedioxide relatively grey showedtone lowdue the atomicto the massrelativelyhighest of carbon. low brightness atomic It must in mass bethe notedmicrograph. of carbon. that the TheIt heatmust partially treatment be noted carbonized couldthat fibersthe result heat formed in treatment the the morphology grey could tone dueresult changing to the in the of relatively low atomic mass of carbon. It must be noted that the heat treatment could result in the themorphology composite changing fiber as also of the indicated composite in [ 17fiber]. as also indicated in [17]. morphology changing of the composite fiber as also indicated in [17].

FigureFigure 4. Energy 4. Energy dispersive dispersive spectroscopic spectroscopic analysis analysis of the microfiber microﬁber specimen: specimen: (a) (aa Scanning) a Scanning Electron Electron Figure 4. Energy dispersive spectroscopic analysis of the microfiber specimen: (a) a Scanning Electron MicroscopeMicroscope (SEM) (SEM) image image showing showing the selectedthe selected area forarea mapping; for mapping; (b) spectrum (b) spectrum from from the area the mapping.area Microscopemapping. (SEM) image showing the selected area for mapping; (b) spectrum from the area mapping. Energy Dispersive Spectrum (EDS) of X-ray diffraction spot analysis was used to identify the elementalEnergy Dispersive composition Spectrum of the selected (EDS) locations.of X-ray diffraIn thection SEM spotimage analysis as shown was by used Figure to 5a,identify three the elementaldifferent composition locations were of pickedthe selected to perform locations. the spot In analysis the SEM for comparison. image as shown The results by Figurefrom the 5a, spot three differentanalysis locations are shown were in picked Figure to5b–d. perform The top the detect spot analysised elements for arecomparison. Ti, O, and TheC, which results matches from the the spot analysischemical are showncompositions in Figure of 5b–d.the fabricated The top particle-containingdetected elements compositeare Ti, O, andnanofiber. C, which No matchesnitrogen the chemicalelement compositions was detected of through the fabricated this process particle-containing indicated the experiment composite was nanofiber.successful inNo view nitrogen of converting the PAN polymer into a carbonized fiber. element was detected through this process indicated the experiment was successful in view of converting the PAN polymer into a carbonized fiber.

Fibers 2019, 7, 49 6 of 10

Energy Dispersive Spectrum (EDS) of X-ray diffraction spot analysis was used to identify the elemental composition of the selected locations. In the SEM image as shown by Figure5a, three different locations were picked to perform the spot analysis for comparison. The results from the spot analysis are shown in Figure5b–d. The top detected elements are Ti, O, and C, which matches the chemical compositions of the fabricated particle-containing composite nanofiber. No nitrogen element was detected through this process indicated the experiment was successful in view of converting the

PANFibers polymer 2019, 7, x FOR into PEER a carbonized REVIEW ﬁber. 6 of 10

FigureFigure 5. 5.The Thespeciﬁc specificlocation location selectionsselections forfor thethe spot analysis and the element analysis results: results: (a) (a) SEM SEM imageimage showing showing thethe locationslocations forfor EnergyEnergy DispersiveDispersive Spectrum (EDS) spot analysis; analysis; (b) (b) the the energy energy dispersivedispersive spectrum spectrum of of location location 001;001; ((c)c) thethe energyenergy dispersivedispersive spectrum of of location location 002; 002; (d) (d) the the energy energy dispersivedispersive spectrum spectrum of of location location003. 003.

FigureFigure5 b–d5b–d shows shows the the spot spot EDS EDS analysis analysis results results obtainedobtained fromfrom thethe analysisanalysis of specificspecific locations ofof Spots Spots 001, 001, 002, 002, and and 003, 003, asas indicatedindicated inin thethe SEMSEM imageimage of Figure 55a.a. TheThe resultsresults from Figure 55b–db–d cancan be be summarized summarized inin thethe lowerlower partpart ofof TableTable1 .1. Although Although thethe elementselements foundfound fromfrom thesethese locations areare identical, identical, there there exist exist didifferencesfferences inin thethe quantities.quantities. Such differences differences reveal whether whether a a location location is is a a particleparticle or or a a carbon carbon fiber. fiber. For For example, example, Spot Spot 002 002 is is a a particle particle cluster cluster area, area, the the titanium titanium concentration concentration is higheris higher than than in otherin other locations. locations.

TableTable 1. 1.Atomic Atomic massmass contentcontent fromfrom areaarea mappingmapping and spot analysis. Type of EDS Locations for analysis Carbon Oxygen Titanium Type of EDS Locations for Analysis Carbon Oxygen Titanium area mapping the entire region in Figure 4a 54.59% 28.40% 17.01% area mapping the entire region in Figure4a 54.59% 28.40% 17.01% spot analysis spot 001 shown in Figure 5a 54.43% 30.91% 14.66% spot analysis spot 001 shown in Figure5a 54.43% 30.91% 14.66% spotspot analysis analysis spot spot 001 001 shown shown in Figurein Figure5a 5a 34.57% 34.57% 33.97% 33.97% 31.47% 31.47% spotspot analysis analysis spot spot 001 001 shown shown in Figurein Figure5a 5a 37.82% 37.82% 42.42% 42.42% 19.77% 19.77%

In order to show the statistical results of the elemental analysis, more spot composition analyses of the fiber filaments and the oxide clusters were performed. More than 15 locations containing each microstructural feature (fiber or nanoparticle cluster) were examined. The X-ray diffraction energy spectrum data revealed the following statistical results. The mean and the standard deviation of the spot analysis results are listed in Table 2 and Table 3, respectively. From the data in Table 2, we can see that the fiber coated with titanium dioxide nanoparticle coating has much higher carbon content than the oxide cluster. The EDS elemental analysis data showed relatively high standard deviations for all the elements as listed in Table 3. One of the reasons for such behavior may be due to the limited point resolution of the X-ray diffraction instrument in submicron scale when a significant change in the depth of the locations for analysis exists. The other reason is the inhomogeneity of the material in

Fibers 2019, 7, 49 7 of 10

In order to show the statistical results of the elemental analysis, more spot composition analyses of the fiber filaments and the oxide clusters were performed. More than 15 locations containing each microstructural feature (fiber or nanoparticle cluster) were examined. The X-ray diffraction energy spectrum data revealed the following statistical results. The mean and the standard deviation of the spot analysis results are listed in Tables2 and3, respectively. From the data in Table2, we can see that the fiber coated with titanium dioxide nanoparticle coating has much higher carbon content than the oxide cluster. The EDS elemental analysis data showed relatively high standard deviations for all the elements as listed in Table3. One of the reasons for such behavior may be due to the limited point resolution of the X-ray diffraction instrument in submicron scale when a significant change in the depth of the locations for analysis exists. The other reason is the inhomogeneity of the material in view of both structure and composition. The complexity and the inhomogeneity of the composite fiber come from the reaction spinning process. During the coaxial electrospinning, the titanium organic compound was actively involved in moisture absorption to form titanium hydroxide. Since the hydrolysis is a multiphase reaction, the water moisture content is not constant in the space due to the whipping motion of the fiber. Therefore, the titanium hydroxide nanophase nucleation and growth were not so easy to control. This caused the uneven distribution of the titanium dioxide nanoparticles on or underneath the carbonized fiber surface. The carbonized fiber surface as shown in Figure3a,b, Figure4a, and Figure5a contains knots and clusters of oxide, which is due to this rapid but not so easy to control reaction during the spinning.

Table 2. Composition analysis statistical results showing the mean of atomic mass content from the energy dispersive X-ray diﬀraction spot analysis of 15 samples.

Locations for Analysis Carbon Oxygen Titanium coated ﬁbers 49.42% 34.23% 16.21% oxide clusters 38.11% 37.86% 23.98%

Table 3. Composition analysis statistical results showing the standard deviation of atomic mass content from the energy dispersive X-ray diﬀraction spot analysis of 15 samples.

Locations for Analysis Carbon Oxygen Titanium coated ﬁbers 9.07% 7.75% 6.24% oxide clusters 8.55% 7.08% 8.35%

It must be noted that the nanoparticle cluster shows higher titanium and oxygen concentrations as revealed by the data in Table2. This should not be considered as an advantage because the uniform doping of carbon in the titanium dioxide will be hard to realize. Such a disadvantage may be alleviated by controlling the relative humidity of the fiber spinning environment. By reducing the humidity, the hydrolysis reaction will slow down, which allows the PAN polymer to mix with the titanium hydroxide more sufficiently in the nanofiber. The electrospinning in a non-ambient humidity environment may be helpful to better understand the titanium oxide nanoparticle growth kinetics within the carbon fiber, which is one of our future study topics.

3.2. Photovoltaic Property The photovoltaic response of the processed oxide coated carbon fiber composite is demonstrated in Figure6. A 120 W fluorescent light was used to generate the time-dependent voltage in this work. The voltage responses were recorded using an electrochemical workstation, CHI440C, made by CH Instrument, Austin, TX, USA. When the sample fiber was exposed under the fluorescent lamp which was placed 1.5 m away from it, positive charging was found. There were 4 total cycles recorded and the peak value voltage of 0.04 V was observed. From the figure, we can see that every cycle includes 25 s ON followed by 25 s OFF. The test revealed an identical behavior of generating electricity under Fibers 2019, 7, 49 8 of 10 photon energy excitation. In addition, positively charged holes allow for the measured voltage to spike into a more positive region. This concludes that the titanium oxide containing nanofiber has a typical p-type semiconductor behavior as also reported in [18]. The fast response as seen from Figure6 also indicated the high photocatalytic activity of the fiber composite. It must be noted that the carbon nanofiber made from PAN without the TiO2 nanoparticle coating has a different photovoltaic response as shown in Figure7. As can be seen from Figure7, at the beginning of the photovoltaic test, the light source was OFF, the open circuit potential was almost zero, i.e., the system in an equilibrium state. When the light was ON, the open circuit potential changed rapidly to the negative values. This is due to the electron generation at the working electrode induced by the photonic energy excitation. It was also found that the recombination of the electron-hole charge Fibers 2019, 7, x FOR PEER REVIEW 8 of 10 pairsFibers was 2019 quite, 7, x FOR quick PEER because REVIEW the open circuit potential decayed quickly as time elapsed. 8 of 10

Figure 6. Photovoltaic response of the partially carbonized fiber coated with the titanium oxide FigureFigure 6. 6.Photovoltaic Photovoltaic response response of of the the partially partially carbonized carbonized ﬁber fiber coated coated with with the the titanium titanium oxide oxide nanoparticle coating to visible light showing the p-type semiconducting behavior. nanoparticlenanoparticle coating coating to visibleto visible light light showing showing the the p-type p-type semiconducting semiconducting behavior. behavior.

FigureFigure 7.7. PhotovoltaicPhotovoltaic responseresponse of of the the partially partially carbonized carbonized ﬁber fiber made made from from Polyacrylonitrile Polyacrylonitrile (PAN) (PAN) to Figure 7. Photovoltaic response of the partially carbonized fiber made from Polyacrylonitrile (PAN) theto the visible visible light light showing showing the the n-type n-type semiconducting semiconducting behavior. behavior. to the visible light showing the n-type semiconducting behavior.

ThereThere areare twotwo possiblepossible reasonsreasons forfor thethe highhighphotovoltaic photovoltaic sensitivitysensitivity ofofthe theTiO TiO22coated coated carboncarbon There are two possible reasons for the high photovoltaic sensitivity of the TiO2 coated carbon ﬁbersfibers toto thethe visiblevisible light.light. OneOne reasonreason isis duedue toto thethe carboncarbon dopingdoping inin thethe TiOTiO22.. ThisThis changeschanges thethe energyenergy fibers to the visible light. One reason is due to the carbon doping in the TiO2. This changes the energy bandwidth of the oxide. Pure titanium oxide is only sensitive to UV light, but the carbon doping can bandwidthbandwidth of ofthe the oxide. oxide. Pure Pure titanium titanium oxide oxide is isonly only sensitive sensitive to toUV UV light, light, but but the the carbon carbon doping doping can can narrownarrow the the band band gap gap of ofthe the oxide. oxide. This This is isthe the major major reason reason for for the the visible visible light light sensitivity. sensitivity. Another Another reasonreason is isfrom from the the conjugated conjugated structure structure of ofthe the carbonized carbonized fiber, fiber, which which causes causes the the localization localization of ofthe the charges.charges. Such Such a localization a localization allows allows the the carbon carbon fiber fiber to toshow show unique unique semiconducting semiconducting behavior. behavior. Tuning Tuning thethe band band gap gap of ofthe the carbon carbon fiber fiber is ispossible possible by by co controllingntrolling the the intercalation intercalation state state in inthe the carbon carbon fiber fiber andand tailoring tailoring the the conductivity conductivity of ofthe the material. material. A Ahigher higher Ti Ticomposition composition could could improve improve the the intensity intensity of ofthe the photovoltaic photovoltaic response response beca becauseuse of ofstronger stronger light light absorption. absorption. However, However, too too much much titanium titanium oxide oxide in inthe the composite composite fiber fiber could could damage damage the the band band gap gap for for the the visible visible light light absorption. absorption. Because Because titanium titanium oxideoxide is isintrinsically intrinsically an an ultraviolet ultraviolet (UV) (UV) light light abso absorber.rber. The The sporadic sporadic and and uniform uniform distribution distribution of ofthe the titaniumtitanium oxide oxide nanoparticles nanoparticles at atthe the surface surface and and in inthe the composite composite can can signi significantlyficantly enhance enhance visible visible lightlight absorption, absorption, as asrevealed revealed by by the the results results in inFigure Figure 6. 6.This This fast fast photoelectric photoelectric response response property property is is importantimportant for for potential potential applications applications on on flexible flexible photovoltaic photovoltaic solar solar panels panels and/or and/or photon photon sensing sensing detectorsdetectors for for monitoring monitoring traffic traffic flow flow in inthe the night night time. time.

Fibers 2019, 7, 49 9 of 10 narrow the band gap of the oxide. This is the major reason for the visible light sensitivity. Another reason is from the conjugated structure of the carbonized fiber, which causes the localization of the charges. Such a localization allows the carbon fiber to show unique semiconducting behavior. Tuning the band gap of the carbon fiber is possible by controlling the intercalation state in the carbon fiber and tailoring the conductivity of the material. A higher Ti composition could improve the intensity of the photovoltaic response because of stronger light absorption. However, too much titanium oxide in the composite fiber could damage the band gap for the visible light absorption. Because titanium oxide is intrinsically an ultraviolet (UV) light absorber. The sporadic and uniform distribution of the titanium oxide nanoparticles at the surface and in the composite can significantly enhance visible light absorption, as revealed by the results in Figure6. This fast photoelectric response property is important for potential applications on flexible photovoltaic solar panels and/or photon sensing detectors for monitoring traffic flow in the night time.

4. Conclusions

Electrospinning, the processing method of the TiO2-PAN microfiber, has delivered the desired functional composite fiber material successfully. Subsequent heat treatment carbonized the PAN fiber and generated anatase titanium dioxide nanocrystal particles on the fiber. The special structure and composition ensured the photovoltaic responses of this fiber. The elemental composition analysis confirmed all the elements in the resultant fiber; no contamination had occurred during the entire casting process. SEM observation shows the distribution of the titanium dioxide particles on the partially carbonized microfibers. The morphology of the composite material reveals the random orientation of the fibers. Photovoltaic responses show the p-type semiconducting behavior of the heat-treated composite material. It is highly sensitive to visible light. The fabricated microfiber shows fast photoelectric responses. Therefore, partially carbonized TiO2 coated carbon fiber has the potential to be used for photoelectric energy conversion and photonic sensing.

Author Contributions: Conceptualization, Y.X.G.; Data curation, J.B.G.; Formal analysis, Y.X.G.; Funding acquisition, Y.X.G.; Investigation, L.Y., X.W., J.P.M., C.Y., N.P., J.B.G., Y.Z. and Y.X.G.; Methodology, Y.X.G.; Project administration, Y.X.G.; Resources, Y.X.G.; Supervision, Y.X.G.; Writing – original draft, Leonardo Yuan. Funding: This work was supported in part by the National Science Foundation (NSF) under Grant Number CMMI-1333044. The SEM images were made possible through the NSF MRI grant DMR-1429674. The two summer undergraduate researchers, N.P. and C.Y., were supported by the Citrus College Race to STEM Program co-managed by Marianne Smith and Winny Dong. We also acknowledge the support by the California State University Pomona 2016–2017, 2017–2018, and 2018–2019 Provost’s Teacher-Scholar program, the 2016–2017 SPICE grant program, and the 2018–2019 RSCA grant program. Acknowledgments: Anan Hamdan is appreciated for his assistance in the SEM experiment. Conﬂicts of Interest: There is no conﬂict of interest regarding the publication of this paper.

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