Synthesis and Transformation of Hollow Rutile Titania Wires by Single Spinneret Electrospinning with Sol-Gel Chemistry

fibers

Article Synthesis and Transformation of Hollow Rutile Titania Wires by Single Spinneret Electrospinning with Sol-Gel Chemistry

Chin-Shuo Kang and Edward Evans *

Department of Chemical, Biomolecular and Corrosion Engineering, The University of Akron, Akron, OH 44325, USA; [email protected] * Correspondence: [email protected]

Abstract: The work described below was carried out to understand how to control the morphology of nanostructured titania calcined from electrospun nanofibers. This is the first report of hollow rutile nanofibers synthesized from electrospun nanofibers with short calcination time. Titanium isopropoxide was incorporated into the nanofibers as the titania precursor. The electrospinning technique was used to fabricate ceramic/polymer hybrid nanofibers. The electrospun nanofibers were then calcined to produce rutile titania nanofibers with different morphologies (hollow or solid nanofibers), which were characterized by SEM and TEM. The initial concentration of ceramic precursor and the calcination time were shown to control the morphology of the nanofiber. The hollow morphology was only obtained with a concentration of the precursor within a certain level and with short calcination times. The heat treatment profile contributed to particle growth. At longer times, the particle growth led to the closure of the hollow core and all the nanofibers resembled strings of solid particles. A formation mechanism for the hollow nanofibers is also proposed.

Keywords: electrospinning; TiO2; hollow nanoﬁber; nanoﬁber formation

Citation: Kang, C.-S.; Evans, E. Synthesis and Transformation of Hollow Rutile Titania Wires by Single 1. Introduction Spinneret Electrospinning with One-dimensional titania nanostructures have been used in different applications based Sol-Gel Chemistry. Fibers 2021, 9, 18. on their morphology. Applications include dye-sensitized solar cells [1–3], water splitting https://doi.org/10.3390/fib9030018 cells [4], gas sensors [5–7], and lithium batteries [8]. One-dimensional TiO2 structures are typically categorized into one of several morphologies, including nanotubes, nanorods, Academic Editor: Sushanta Ghoshal nanowires, and nanofibers. Nanotubes and nanorods are straighter, whereas nanofibers and nanowires have significant curvature [9,10]. Nanotubes can be produced as either single- Received: 24 January 2021 layer [11] or asymmetric multi-layer tubes [12] depending on the synthesis technique. Accepted: 26 February 2021 Published: 6 March 2021 With the thickness of a single layer being a few nanometers [9,13,14], the surface area and pore volume per unit volume are significantly increased and this enhances surface

Publisher’s Note: MDPI stays neutral reactivity [14]. Titania nanorods [15] are believed to provide an effective and long-distance with regard to jurisdictional claims in electron-transport pathway for photogenerated electrons. [6,16,17] Nanofibers, defined as published maps and institutional affil- fibers with diameter ranges below 1000 nm, are usually obtained by calcining a polymer iations. nanofiber containing a ceramic precursor to remove the organic polymer. The applications vary significantly depending on the fiber and grain size [18–21]. At present, many ceramic materials have been made into hollow fiber by the electrospinning route [11,18,19,22–30]. Different formation mechanisms have been proposed. From the synthesis of hollow CuO [31] and ZnO [32] fibers made by electrospinning with Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. sol–gel chemistry, a mechanism involving the phase separation of a ceramic dense shell This article is an open access article and polymer dense core was proposed. It was suggested that a concentration gradient of distributed under the terms and ceramic precursor is established as the metal organic near the fiber surface is consumed and conditions of the Creative Commons crystallized into pure ceramic material. It was proposed that this concentration gradient Attribution (CC BY) license (https:// drives the ceramic precursor’s diffusion outward, with the polymer remaining in the core. creativecommons.org/licenses/by/ After full pyrolysis of the polymer core, a hollow structure formed. In their study of tita- 4.0/). nium and vanadium oxide complexes, Zhao et al. [27] proposed a multi-layer tube-in-tube

Fibers 2021, 9, 18. https://doi.org/10.3390/ﬁb9030018 https://www.mdpi.com/journal/ﬁbers Fibers 2021, 9, 18 2 of 14

structure with a polymer oxidation mechanism. Zhao believed that the presence of oxygen in the calcining environment is a crucial factor for the transformation of the precursor to titanium oxide. According to their mechanism, at the beginning, the oxygen diffuses from the gas phase into the surface of the polymer and causes oxidative degradation and subsequent removal of the polymer. As the removal of the polymer continues, the distance between inorganic grains is reduced, thus accelerating the crystallization of the inorganic grains. As a result, a rigidly crystallized shell is formed. Eid et al. [33] also reported an atmosphere inducible morphology change between solid and hollow fiber based on their study of iron oxide fibers made from electrospun precursors. Another approach to synthesizing hollow fibers through the electrospinning method was to precisely control the calcination profile. However, even with the same material, i.e., iron oxide, Cheng et al. [34] provided a different mechanism from the diffusion point of view. Cheng believed that the formation of hollow fibers was because of a gel layer on the surface of the polymer fiber. This gel layer was caused by solvent evaporating during calcination and controlling the diffusion of decomposed gas from inside out. If the decomposition rate of the polymer was higher than the diffusion rate, pressure built up inside the shell and sustained the hollow structure. Otherwise, the hollow structure collapsed because of a contraction force. Last but not least, the most complete research on TiO2 nanofibers was carried out by Lang [11]. ◦ Lang calcined the TiO2/PVP as spun fiber at 500 C for 4 h, resulting in a tube-in-tube structure with both anatase and rutile crystallinity. For the formation mechanism, Lang proposed an idea similar to the diffusion theory plus the concentration of the precursor. If the concentration of the precursor is high enough, the ceramic particles are able to fill the whole fiber after the decomposition of the polymer. Otherwise, hollow fibers are obtained. The work described below was carried out to understand how to control the morphology of electrospun nanostructured titania in rutile form. Synthesis of hollow anatase fibers has been previously reported by the single spinneret electrospinning method but this is the first report of hollow rutile fibers synthesized from electrospun fibers with a much shorter calcination profile.

2. Materials and Methods 2.1. Solution Preparation Ceramic/polymer fibers were electrospun with different concentrations (low, medium, and high) of ceramic precursor (Table1). Solutions prepared for electrospinning were made up of ceramic precursor solution and polymer solution. To obtain the titanium dioxide ceramic precursor solution, titanium isopropoxide (TIP) was mixed with an acetic acid–ethanol mixture (1:1, volume ratio) and stirred overnight to obtain a clear, light-yellow solution. The gelation of the ceramic precursor was fixed by stirring time and concentration of added acid solution, which served as a catalyst. By doing so, the degree of gelation and the size of the nuclei were controlled in similar conditions across the three recipes [35–37]. For the polymer solution, polyvinylpyrrolidone (PVP) was dissolved in either ethanol or a N,N-dimethylformamide (DMF)—chloroform mixture (1:1, volume ratio). Then, the precursor solution was mixed with the polymer solution with continuous stirring for 30 min. Concentration of ceramic precursor in this project is defined as the amount of ceramic precursor dispersed in the polymer with units of gram per gram of PVP and adjusted to 0.05, 0.62, and 1.5 g/g PVP.

Table 1. Recipe of as-spun green ﬁbers with ceramic precursor.

TIP/PVP As-Spun Fiber Diameter Formula Solvent Type (Mass Ratio) (nm) Low 0.05 Ethanol 1594 ± 16 Medium 0.62 Chloroform/DMF 1008 ± 59 High 1.50 Chloroform/DMF 877 ± 10 Fibers 2021, 9, 18 3 of 14

2.2. Materials Polyvinylpyrrolidone (PVP, MW 1300,000), titanium isopropoxide (TIP, 97%), chloroform (ACS reagent, ≥99.8%), and reagent alcohol (ethanol) (ACS grade) were purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF, 99.9%) was purchased from Fisher Chemical. Acetic acid (AA, 99 wt %) was purchased from EMD. All of the chemicals were directly used without further puriﬁcation.

2.3. Electrospinning The as-prepared electrospinning solution was loaded into a 5-mL syringe with a 21-gauge stainless steel needle and ﬁxed on the syringe pump. The applied voltage, tip- to-collector distance, and pumping rate were set depending on the solution (Table2). The humidity of the surrounding was kept under 15% relative humidity in a humidity- controlled chamber. For the grounded collector, a piece of aluminum foil was wrapped around a conductive roller that was then rotated at a constant rotation rate of 10 rpm. Due to the composition of the solution, the parameters of the electrospinning apparatus differed between recipes. The composition of the solution and parameters of electrospinning interacted with each other. The adjustment/tuning of the electrospinning parameters was done to produce nanoﬁbers with an average diameter of between 850 and 1600 nm and a smooth surface.

Table 2. Parameters for electrospinning apparatus.

Applied Tip-to-Collector Distance Pumping Rate Formula Voltage (kV) (cm) (mL/h) Low 5 5 1 Medium 7 7 1.5 High 8 10 1

2.4. Calcination Profiles The as-spun fibers were collected on the aluminum foil, moved to a ceramic dish, and then heated up to 950 ◦C and held (soaked) at that temperature to eliminate the polymer matrix and control the phase of the ceramic material. The different heating rates and soaking times shown in Table3 were used to determine the impact on particle size and fiber morphology.

Table 3. Calcination procedure for each proﬁle.

Calcination Ramping Rate Soaking Time Cooling Rate Proﬁle (◦C/h) (h) (◦C/h) R2400-0h 2400 0 150 R600-0h 600 0 150 R600-24h 600 24 150

2.5. Characterization A Vecstar Ltd. (model no. MF4) furnace was used to calcine the fibers. FTIR was used to determine the composition of as-spun fiber and calcined fibers. A scanning electron microscope, SEM (FEI Quanta 200 at 30 kV and HITACHI TM3000 at 15 kV), and a transmission electron microscope (TEM FEI Tecnai G2-F20) were used to obtain the morphology of as-spun and ceramic nanofibers. The FibraQuant 1.3 software (nanoScaffold Technologies, LLC, Chapel Hill, NC, USA) was adapted to measure the fiber size and particle size distribution. The particle size was manually measured from SEM images, as shown Figure1, and confirmed with transmission electron microscopy. The crystal structure was analyzed by X-ray diffraction (copper X-ray with wavelength 1.540 Angstrom, 40 kV, 35 mA). Fibers 2021, 9, x FOR PEER REVIEW 4 of 14

Fibers 2021, 9, x FOR PEER REVIEW 4 of 14

2.5. Characterization A Vecstar Ltd. (model no. MF4) furnace was used to calcine the fibers. FTIR was used 2.5.to determine Characterization the composition of as-spun fiber and calcined fibers. A scanning electron microscope,A Vecstar SEM Ltd. (FEI (model Quanta no. MF4)200 at furnace 30kV and was HITACHI used to calcine TM3000 the atfibers. 15kV), FTIR and was a trans- used tomission determine electron the microscopecomposition (TEM of as-spun FEI Tecnai fibe rG2-F20) and calcined were used fibers. to Aobtain scanning the morphol- electron microscope,ogy of as-spun SEM and (FEI ceramic Quanta nanofibers. 200 at 30kV The and FibraQuant HITACHI 1.3 TM3000 software at (nanoScaffold 15kV), and a trans-Tech- missionnologies, electron LLC, Chapel microscope Hill, USA) (TEM was FEI adapted Tecnai G2-F20) to measure were the used fiber to sizeobtain and the particle morphol- size ogydistribution. of as-spun The and particle ceramic size nanofibers. was manually The measuredFibraQuant from 1.3 SEMsoftware images, (nanoScaffold as shown Figure Tech- nologies,1, and confirmed LLC, Chapel with Hill,transmission USA) was electron adapted microscopy. to measure The the fibercrystal size structure and particle was ana-size Fibers 2021, 9, 18 distribution.lyzed by X-ray The diffraction particle size (copper was manuallyX-ray with measured wavelength from 1.540 SEM Angstrom, images, as 40 shown kV, 35 4Figure ofmA). 14 1, and confirmed with transmission electron microscopy. The crystal structure was analyzed by X-ray diffraction (copper X-ray with wavelength 1.540 Angstrom, 40 kV, 35 mA).

Figure 1. Schematic for particle size measurement.

FigureFigure3. Results 1. 1.Schematic Schematic for for particle particle size size measurement. measurement. 3.3.1. Results Ceramic Nanofiber Confirmation 3. Results 3.1. CeramicThe FTIR Nanofiber spectrum Confirmation confirmed that after calcination, the fiber was completely con- 3.1. Ceramic Nanofiber Confirmation vertedThe to FTIR titania. spectrum In Figure confirmed 2a, characteristic that after IR calcination, absorption the peaks fiber for was pure completely PVP and con-com- vertedmercialThe to TiO titania. FTIR2 powder spectrum In Figure are presented.confirmed2a, characteristic Forthat PVP, after IR the absorptioncalcination, absorption peaks the peaks fiber for between purewas PVPcompletely 1250 and and com- 1650con- vertedcm−1 corresponded to titania. In to Figure C-N and 2a, C=Ocharacteristic stretching IR motion, absorption respectively. peaks for The pure other PVP two and absorp- com- mercial TiO2 powder are presented. For PVP, the absorption peaks between 1250 and 1650mercialtion cm peaks− TiO1 corresponded around2 powder 1409 are to andpresented. C-N 2876 and cm For C=O−1 PVP,are stretching from the absorptionC–H motion, bonding. peaks respectively. [38] between For TheTiO 12502 other, the and broad two 1650 cm−1 corresponded to C-N and C=O stretching−1 −1 motion, respectively. The other two absorp- absorptionabsorption peaks peaks around centered 1409 at 664 and cm 2876 are cm attributedare from to stretching C–H bonding. of the [ 38Ti-O-Ti] For TiO bond.2, the The broadtionabsorption peaks absorption aroundat 3000–3500 peaks 1409 centered andcm− 1 2876was at cmdue 664−1 to cmare the− from1 arehydroxyl attributedC–H bonding. vibration. to stretching [38] The For as-spun TiO of the2, fiberthe Ti-O-Ti broad with bond.absorptionceramic The precursor absorption peaks centered shows at 3000–3500 peaks at 664 from cm cm−1− bothare1 was attributed TiO due2 and to the toPVP stretching hydroxyl (Figure vibration.of2b). the After Ti-O-Ti Thecalcination bond. as-spun The at −1 fiberabsorption650 with°C, peaks ceramic at 3000–3500of PVP precursor almost cm shows disappeared,was due peaks to the from indicating hydroxyl both TiOthe vibr2 decompositionandation. PVP The (Figure as-spun of 2PVP.b). fiber After At with 950 ◦ calcinationceramic°C, only precursorpeaks at 650 attributedC, shows peaks peaksto of TiO PVP from2 were almost both detected, disappeared, TiO2 and indicating PVP indicating (Figure that the 2b). the polymer After decomposition calcination matrix was of at ◦ PVP.650fully At°C, pyrolyzed. 950 peaksC, of only PVP peaks almost attributed disappeared, to TiO 2indicatingwere detected, the decomposition indicating that of the PVP. polymer At 950 matrix°C, only was peaks fully attributed pyrolyzed. to TiO2 were detected, indicating that the polymer matrix was fully pyrolyzed.

2 4 Commercial TiO2 M950 M650 PVP M350 M0

2 4 1.5 Commercial TiO2 3 M950 M650 PVP M350 M0

1.5 3 1 2 N o rm a liz e d in te n s ity N o rm1 a liz e d in te n s ity 2 0.5 1 N o rm a liz e d in te n s ity N0.5 o rm a liz e d in te n s ity 1 0 0 1000 2000 3000 4000 1000 2000 3000 4000 W ave length (1/cm ) W ave length (1/cm) 0 (a) 0 (b) 1000 2000 3000 4000 1000 2000 3000 4000 W ave length (1/cm ) W ave length (1/cm) Figure 2. FTIR peak information(a) for the (a) raw material, namely TiO2 and PVP, (b) as-spun ﬁber(b) after calcination. (The number after the M stands for the soaking temperature with unit ◦C.)

3.2. Nanofiber Morphology In order to have a systematic observation with regard to the concentration of precursor and heat treatment, fiber samples with different precursor concentrations were calcined under the different calcination profiles. The morphology of the final products was observed by SEM and TEM. The maximum soaking temperature was chosen as 950 ◦C in order to fully pyrolyze the PVP and form pure rutile titania. The calcination profiles followed were shown in Table3. Green as-spun polymer/ceramic hybrid fiber and samples from recipe R2400-0h (2400 ◦C/h heating rate without soaking), R600-0h (600 ◦C/h heating Fibers 2021, 9, x FOR PEER REVIEW 5 of 14

Figure 2. FTIR peak information for the (a) raw material, namely TiO2 and PVP, (b) as-spun fiber after calcination. (The number after the M stands for the soaking temperature with unit °C.)

3.2. Nanofiber Morphology In order to have a systematic observation with regard to the concentration of precursor and heat treatment, fiber samples with different precursor concentrations were calcined under the different calcination profiles. The morphology of the final products was Fibers 2021, 9, 18 observed by SEM and TEM. The maximum soaking temperature was chosen as 9505 of °C 14 in order to fully pyrolyze the PVP and form pure rutile titania. The calcination profiles followed were shown in Table 3. Green as-spun polymer/ceramic hybrid fiber and samples ratefrom without recipe soaking), R2400-0h and (2400 R600-24h °C/h heating (600 ◦C/h rate heatingwithoutrate soaking), with 24 R600-0h h soaking) (600 were°C/h heating used forrate comparison. without soaking), All of the and samples R600-24h were (600 cooled °C/h heating at 150 ◦ rateC/h. with The 24 corresponding h soaking) were total used heatfor treatment comparison. time All was of increasedthe samples from were 6.6 cooled to 31.7 at h, 150 which °C/h. corresponds The corresponding to R2400-0h total toheat R600-24htreatment in calcinationtime was increased profile. from 6.6 to 31.7 h, which corresponds to R2400-0h to R600- 24hAs in shown calcination in the profile. leftmost images in Figure3, the as-spun fibers for all samples were smooth.As When shown exposed in the leftmost to the rapidimages ramping in Figure calcination 3, the as-spun profile fibers (R2400-0h), for all samples the low- were concentrationsmooth. When sample exposed presented to the rapid a mixture ramping of incomplete calcination wall profile and (R2400-0h), hollow fibers the withlow-concen- very thintration walls, sample while medium-concentrationpresented a mixture of samplesincomplete produced wall and more hollow uniform fibers hollow with very fibers thin withwalls, thicker while walls. medium-concentration The sample with a highsamples concentration produced more of ceramic uniform precursor hollow formedfibers with a solidthicker fiber walls. shape, The with sample shell wallwith thickness a high concen equaltration to the fiberof ceramic radius. precursor The as-spun formed fiber a forsolid allfiber samples shape, showed with shell a significant wall thickness reduction equal into radiusthe fiber at radius. this stage, The as-spun as shown fiber in Figurefor all sam-4. Thisples was showed considered a significant to be due reduction to the loss in radius of the at polymer this stage, matrix as shown and the in condensationFigure 4. Thisof was ceramicconsidered particles. to be due to the loss of the polymer matrix and the condensation of ceramic particles.

(a) (b)

Figure 3. Cont. Fibers 2021, 9, 18 6 of 14 Fibers 2021, 9, x FOR PEER REVIEW 6 of 14

(e) (f)

(g) (h)

(i) (j)

Figure 3. Cont. Fibers 2021, 9, 18 7 of 14 Fibers 2021, 9, x FOR PEER REVIEW 7 of 14 Fibers 2021, 9, x FOR PEER REVIEW 7 of 14

(k) (l) (k) (l) Figure 3. SEM images of fiber samples with low, medium, and high concentrations under different calcination profiles. (a) FigureFigure 3. 3.SEM SEM imagesimages of fiber ﬁber samples samples with with low, low, medium, medium, and and high high concentrations concentrations under under different different calcination calcination profiles. proﬁles. (a) Low-as-spun, (b) Low-R2400-0h, (c) Low-R600-0h, (d) Low-R600-24h, (e) Medium-as-spun, (f) Medium-R2400-0h, (g) Me- (aLow-as-spun,) Low-as-spun, (b) ( bLow-R2400-0h,) Low-R2400-0h, (c) (Low-R600-0h,c) Low-R600-0h, (d) (Low-R600-24h,d) Low-R600-24h, (e) (Medium-as-spun,e) Medium-as-spun, (f) Medium-R2400-0h, (f) Medium-R2400-0h, (g) Me- (g) dium-R600-0h, (h) Medium-R600-24h, (i) High-as-spun, (j) High-R2400-0h, (k) High-R600-0h, and (l) High-R600-24h. Medium-R600-0h,dium-R600-0h, (h) ( hMedium-R600-24h,) Medium-R600-24h, (i) ( iHigh-as-spun,) High-as-spun, (j) ( jHigh-R2400-0h,) High-R2400-0h, (k) (k) High-R600-0h,High-R600-0h, and and (l) ( lHigh-R600-24h.) High-R600-24h.

Figure 4. Measurement of fiber diameter for as-spun fiber and after calcination. FigureFigure 4.4.Measurement Measurement ofof fiberfiber diameter diameter for for as-spun as-spun fiber fiber and and after after calcination. calcination. The same relation between the concentration of precursor and morphology of cal- TheThe same same relation relation between between the the concentration concentration of precursorof precursor and and morphology morphology of calcined of calcined fiber with the rapid ramping calcination profile observed by SEM was also found fibercined with fiber the with rapid the rapid ramping ramping calcination calcination profile profile observed observed by SEM by SEM was was also also found found by by TEM (Figure 5). The low-concentration sample produced an incomplete fiber or flake- TEMby TEM (Figure (Figure5). The 5). low-concentrationThe low-concentration sample sample produced produced an incomplete an incomplete fiber fiber or flake-like or flake- like material with a more transparent wall. For the medium-concentration sample, the materiallike material with awith more a transparentmore transparent wall. For wall. the For medium-concentration the medium-concentration sample, sample, the hollow the hollow fiber morphology was more defined. The contrast between wall and core indicated fiberhollow morphology fiber morphology was more was defined. more defined. The contrast The contrast between between wall andwall core and indicatedcore indicated the the hollow structure. With increased concentration of ceramic precursor, the calcined fiber hollowthe hollow structure. structure. With With increased increased concentration concentrat ofion ceramic of ceramic precursor, precursor, the calcined the calcined fiber fiber had had a fiber morphology without contrast in the TEM image, suggesting that a solid fiber ahad fiber a morphologyfiber morphology without without contrast contrast in the in TEM the TEM image, image, suggesting suggesting that athat solid a solid fiber wasfiber was formed. When total heating was prolonged from 6.6 to 7.7 h by slowing the ramping formed.was formed. When When total heatingtotal heating was prolonged was prolonged from 6.6from to 6.6 7.7 hto by7.7 slowing h by slowing the ramping the ramping from from◦ 2400 °C/h down◦ to 600 °C/h, the morphology for each sample did not change signif- 2400from C/h2400 down°C/h down to 600 toC/h, 600 °C/h, the morphology the morphology for each for sample each sample did not did change not change significantly. significantly. Lastly, when the samples were soaked for 24 h, all samples ended up in a solid Lastly,icantly. when Lastly, the when samples thewere samples soaked were for soaked 24 h, all for samples 24 h, all ended samples upin ended a solid up fiber in a form. solid fiber form. The calcined fibers for all concentrations of precursor at this stage appeared to Thefiber calcined form. The fibers calcined for all concentrationsfibers for all concentrat of precursorionsat of this precursor stage appeared at this stage to be appeared composed to be composed of a string of particles. ofbe a composed string of particles. of a string of particles. Fibers 2021, 9, 18 8 of 14 Fibers 2021, 9, x FOR PEER REVIEW 8 of 14

(a) (b)

(e) (f) Figure 5. TEM images of fiber samples with low, medium, and high recipes with 2400 °C/h heating rate without soaking. Figure 5. TEM images of fiber samples with low, medium, and high recipes with 2400 ◦C/h heating rate without soaking. (R2400-0h). (a,b) Low, (c,d) Medium, and (e,f) High. (R2400-0h). (a,b) Low, (c,d) Medium, and (e,f) High. 3.3. Particle Size and Aspect Ratio 3.3. Particle Size and Aspect Ratio The particles grew as the concentration of ceramic precursor and soaking time in- The particles grew as the concentration of ceramic precursor and soaking time in- creasedcreased (Figure (Figure6). 6). The The higher higher the concentrationthe concentration of precursor of precursor and theand longer the longer heat treatment, heat treat- thement, bigger the the bigger particle. the particle. At the highest At the concentrationhighest concentration of precursor, of precursor, the first observed the first observed particle wasparticle large was enough large to enough fill the to tube fill volume,the tube whilevolume, fibers while appeared fibers appeared in hollow in structures hollow struc- at mediumtures at andmedium lowconcentrations and low concentrations of precursor. of precursor. FibersFibers 20212021,, 99,, 18x FOR PEER REVIEW 9 ofof 1414 Fibers 2021, 9, x FOR PEER REVIEW 9 of 14

Particle size Particle size 500 500 Low Medium High Low Medium High 400 400

300 300 200 200 particle (nm) size

100particle (nm) size 100 0 0 0 6.73 7.92 31.92 0 6.73 7.92 31.92 Time (hr) Time (hr) Figure 6. Measurement of particle size of as-spun ﬁberfiber and after calcination. Figure 6. Measurement of particle size of as-spun fiber and after calcination. However, thethe aspectaspect ratiosratios of of the the particles, particles, which which is is the the particle particle size size in in the the longitudinal longitudi- naldirection directionHowever, divided divided the by theaspectby sizethe ratios size in the in of radialthe the radial particles, direction, direction, which for all foris calcinedthe all particlecalcined samples size samples in were the werelongitudi- almost al- mostalwaysnal alwaysdirection equal equal to divided one to (exceptone by (except the in size high-concentration in in high-concentration the radial direction, sample sample for soaked all soaked calcined for for 24 samples h),24 h), indicating indicat- were al- ingthatmost that the alwaysthe growth growth equal rate rate ofto theofone the particles (except particles in in inhigh-concentration the the radial radial and and longitudinal longitudinal sample soaked direction direction for was24was h), similarsimilar indicat- (Figureing that7 7).). the growth rate of the particles in the radial and longitudinal direction was similar (Figure 7).

. . Figure 7. Measurement of aspect ratio for as-spun fiber and after calcination. FigureFigure 7. Measurement7. Measurement of of aspect aspect ratio ratio for for as-spun as-spun fiber fiber and and after after calcination. calcination. The measurement of aspect ratio also suggests that the ultimate maximum particle size isThe limitedThe measurement measurement by the fiber of ofdiameter aspect aspect ratio ratioin most also also suggestscases. suggests In no that that instance the the ultimate ultimate did the maximum finalmaximum particle particle particle size exceedsizesize is limitedis the limited initial by by thefiber the fiber diameter fiber diameter diameter and in the in most finalmost cases. particle cases. In In no size no instance wasinstance most did did sensitive the the final final particleto particlethe initial size size concentrationexceedexceed the the initial initial of precursor. fiber fiber diameter diameter Combined and and the the with final final the particle particle steady size sizeaspect was was most ratio, most sensitive growth sensitive toof totheparticles, the initial initial alongconcentrationconcentration with heat of oftreatment precursor. precursor. and Combined Combined the reduction with with the of the fiber steady steady size, aspect aspect it is ratio,evident ratio, growth growththat the of of particles,particles particles, grewalongalong toward with with heat theheat treatmentmiddle treatment of andthe and fiber the the reduction for reduction low and of of fibermedium fiber size, size, concentrations. it isit evidentis evident that that the the particles particles grewgrew toward toward the the middle middle of of the the fiber fiber for for low low and and medium medium concentrations. concentrations. 3.4. Surface Composition 3.4. Surface Composition To determine whether the ceramic precursor moved to the perimeter before calcination, twoTo sets determine of samples whether were theprepared ceramic from precurso the medium-concentrationr moved to the perimeter recipe, before including calcination, two sets of samples were prepared from the medium-concentration recipe, including Fibers 2021, 9, 18 10 of 14

Fibers 2021, 9, x FOR PEER REVIEW 3.4. Surface Composition 10 of 14

To determine whether the ceramic precursor moved to the perimeter before calcination, two sets of samples were prepared from the medium-concentration recipe, including as- ◦ ◦ as-spunspun ﬁbers fibers and and ﬁbers fibers heated heated at at 230 230C °C for for an an hour; hour; 230 230C °C heat heat treatment treatment was was chosen chosen as asit it is is above above the the glass glass transitiontransition temperaturetemperature but not high high enough enough to to degrade degrade the the polymer. polymer. ThisThis treatment treatment provides provides energy energy for for diffusion diffusion an andd allows allows for for phase phase separation separation if if it it occurs. occurs. AsAs part part of of the the XPS XPS analysis, analysis, both both of of the the samp samplesles were were etched etched in in argon argon for for 1 1 min, min, removing removing roughlyroughly 5 5nm nm from from the the surface, surface, to to eliminate eliminate surface surface contamination. contamination. Both Both of of the the samples samples appearedappeared to to have have similar similar surface surface compositions compositions of of elements, elements, including including 70% 70% carbon, carbon, 15% 15% oxygen,oxygen, 7% 7% nitrogen, nitrogen, and and 5% 5% titanium. titanium. The The su surfacerface composition composition of of both both samples samples was was also also quitequite similar similar to to the the recipe, recipe, which which was was 69% 69% carbon, carbon, 17% 17% oxygen, oxygen, 11% 11% nitrogen, nitrogen, and and 3% 3% of of titaniumtitanium (Figure (Figure 8).8). The The results results suggested suggested th thatat phase phase separation separation between between the the polymer polymer and and ceramicceramic precursor precursor did did not not happen happen during during electrospinning electrospinning or or in in the the initial initial heating. heating.

pp 16000 Atomic % C1s 70.5

−C1s O1s 17.6 14000 N1s 6.5 −O1s Ti2p 5.4 Si2p <.1

12000 −Ti2p −Ti2p1

10000 −Ti2p3 −Ti2s −O KLL −O 8000 c/s

6000 −N1s

4000

2000 −Ti3p −Si2p −Si2s

0 1100 1000 900 800 700 600 500 400 300 200 100 0 Binding Energy (eV)

(a) (b)

FigureFigure 8. 8. XPSXPS spectrum spectrum for for recipe recipe (a (a) )medium medium as-spun as-spun fiber ﬁber and and (b (b) )after after calcined calcined at at 230 230 °C◦C for for an an hour. hour.

3.5.3.5. Crystallography Crystallography TheThe crystal crystal structures structures of of all all samples samples were were identified identiﬁed by by powder powder X-ray X-ray diffraction diffraction with with scanningscanning intervals intervals from from 2 2θθ equalequal 10 10 to to 70 70 degrees. degrees. All All the the samples samples were were calcined calcined at at 950 950 °C◦C forfor different different soaking soaking times, times, from from zero zero hours hours to to twenty-four twenty-four hours. hours. In In order order to to compare compare the the crystallinitycrystallinity of of each each sample, sample, the the intensity intensity of of X-ray X-ray diffraction diffraction was was normalized normalized (Figure (Figure 9).9). OnlyOnly the the diffraction diffraction peaks peaks of of rutile rutile are are detected detected across across the the three three recipes, recipes, with with 2 2θθ equalequal to to 27.6827.68 (110), (110), 36.32 36.32 (101), (101), 41.51 41.51 (111), (111), 54.53 54.53 (211), (211), 56.84 56.84 (220), (220), and and 69.23 69.23 (301) (301) (JCPDS (JCPDS card card no. no. 21-1276).21-1276). Moreover, Moreover, the the lattice lattice volumes volumes for for al alll samples samples were were around around 60 60 cubic cubic Angstroms, Angstroms, ∼ ∼ withwith lattice lattice parameter parameter a a= =b b≅ 4.567= 4.567 and and c ≅ c2.947= 2.947 Angstroms. Angstroms. Transmission Transmission electron electron mi- croscopymicroscopy images images indicate indicate that that the theparticles particles are aresingle-crystal. single-crystal. Fibers 2021, 9, x FOR PEER REVIEW 11 of 14

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Low M edium R600-24h R600-24h R600-0h R600-0h R2400-0h R2400-0h In te n s ity In te n s ity

0 1020304050607080 0 1020304050607080 tw o th e ta tw o th e ta (a) (b)

High R600-24h R600-0h R2400-0h In te n s ity

0 1020304050607080 tw o th e ta (c)

Figure 9. X-ray diffraction with normalized peak intensity of TiO2 fiber samples with formula (a) low, (b) medium, and Figure 9. X-ray diffraction with normalized peak intensity of TiO2 ﬁber samples with formula (a) low, (b) medium, and (c)(c high.) high.

3.6.3.6. Proposed Proposed Fiber Fiber Formation Formation Mechanism Mechanism TheThe results results indicate indicate that that the the formation formation of of hollow hollow fibers fibers is is controlled controlled by by both both the the initial initial concentrationconcentration of of the the ceramic ceramic precursorprecursor andand th thee calcination calcination profile. profile. The The morphology morphology of ofthe theceramic ceramic fiber fiber is believed is believed to todevelop develop in three in three main main events, events, including including surface surface pyrolysis, pyrolysis, par- particleticle growth, growth, and and gasification gasification of of polymer polymer (Figure 1010).). OurOur XPSXPS results results indicate indicate that that the the ceramicceramic precursor precursor is is evenly evenly distributed distributed throughout throughout the the fiber fiber prior prior to to any any heating heating and and even even afterafter mild mild heating heating (Stage (Stage 1).1). As the temperatur temperaturee is is increased, increased, pyrolysis pyrolysis begins begins and and the the asas-spunspun fiber fiber is is burned burned from from the outside in, whichwhich leadsleads totoa a reduction reduction in in fiber fiber diameter diameter (Stage(Stage 2). 2). During During thisthis period,period, the morphology morphology transformation transformation is is dominated dominated by by surface surface py- pyrolysisrolysis until until a dense a dense solid, solid, ceramic ceramic shell shell forms forms (Stage (Stage 3). The 3). shell The is shell formed is formed with particles with particlesthat appear that appearto have to a haveminimum a minimum diameter diameter of 20 nm. of 20 After nm. the After shell the forms, shell forms, during during Stage 3 Stageand 3Stage and 4, Stage the 4,polymer the polymer degradation degradation continues continues toward toward the middle the middle of the offiber the but fiber the butmechanism the mechanism switches switches from frompyrolysis pyrolysis to thermal to thermal gasification. gasification. The gasification The gasification of the of poly- the polymermer builds builds up upa positive a positive pressure pressure inside inside the the shell. shell. Because Because of of the the pressure pressure difference difference in in between the inside and the outside of the shell, the gasified gas tends to bring any remaining ceramic particles outward until they reach the shell. The gas escapes through Fibers 2021, 9, x FOR PEER REVIEW 12 of 14

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between the inside and the outside of the shell, the gasified gas tends to bring any remaining ceramic particles outward until they reach the shell. The gas escapes through the shell the shell while particles are filtered. The thickness of the shell then grows outside in until while particles are filtered. The thickness of the shell then grows outside in until there is there is no further gasification of polymer (Stage 4). When the calcination temperature no further gasification of polymer (Stage 4). When the calcination temperature reaches 950 reaches 950 ◦C, grain growth dominates the process. As a result, the hollow fiber contracts °C, grain growth dominates the process. As a result, the hollow fiber contracts and closes and closes up the core of the hollow fiber and leaves a solid fiber (Stage 6). up the core of the hollow fiber and leaves a solid fiber (Stage 6).

FigureFigure 10. 10.Overall Overall fiber fiber transformation transformation mechanism. mechanism. 4. Conclusions 4. Conclusions Experimental results have been presented that provide a method for producing rutile Experimental results have been presented that provide a method for producing rutile titania nanofibers with controlled morphology. Results indicate that the concentration of titania nanofibers with controlled morphology. Results indicate that the concentration of precursor plays an important role in determining the final morphology. By adjusting the precursor plays an important role in determining the final morphology. By adjusting the concentration of ceramic precursor, incomplete hollow nanofiber, hollow nanofiber, and concentration of ceramic precursor, incomplete hollow nanofiber, hollow nanofiber, and solid nanofiber were obtained after calcination of as-spun nanofibers. SEM characterization solid nanofiber were obtained after calcination of as-spun nanofibers. SEM characteriza- indicates that the ceramic particle size was restricted by the radius of the fiber. A possible tion indicates that the ceramic particle size was restricted by the radius of the fiber. A mechanism for the formation of rutile titania nanofibers that includes six stages was possible mechanism for the formation of rutile titania nanofibers that includes six stages proposed. The transformation started from the surface pyrolysis of the polymer that leaves a was proposed. The transformation started from the surface pyrolysis of the polymer that solid ceramic wall, followed by gasification of the polymer inside the wall, which may result inleaves a hollow a solid or solid ceramic morphology. wall, followed This mechanism by gasification can help of the researchers polymer inside to develop the wall, methods which formay controlling result in the a hollow synthesis or solid of other morphology. ceramic materials This mechanism from electrospun can help nanofibers. researchers to develop methods for controlling the synthesis of other ceramic materials from electrospun Authornanofibers. Contributions: All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript. Author Contributions: All authors contributed equally to this work. All authors have read and Funding:agreed toThis the research published received version no of external the manuscript. funding. DataFunding: Availability This research Statement: receivedNot applicable. no external funding. Acknowledgments:Acknowledgments:We We would would like like to to acknowledge acknowledge thethe technicaltechnical support from from Min Min Gao Gao at at the the Ad- Advancedvanced Materials Materials and and Liquid Liquid Crysta Crystall Institute, Institute, Kent Kent State State University University and and the the filtration filtration group group led led by byProfessor Professor George George Chase, Chase, Chemical, Chemical, Biomolecular Biomolecular,, and and Corrosion Corrosion Engineering Engineering Department, Department, The The Uni- Universityversity of of Akron. Akron. ConflictsConflicts of Interest:of Interest:The The authors authors declare declare no no conflict conflict of interest.of interest.

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