coatings

Article Sol-Gel and Electrospinning Synthesis of Niobate-Silica Nanofibers

Jesús Alberto Garibay-Alvarado 1, Rurik Farías 2 and Simón Yobanny Reyes-López 1,*

1 Instituto de Ciencias Biomédicas, Universidad Autónoma de Ciudad Juárez, Ciudad Juárez 32315, Mexico; [email protected] 2 Instituto de Ingeniería y Tecnología, Universidad Autónoma de Ciudad Juárez, Ciudad Juárez 32310, Mexico; [email protected] * Correspondence: [email protected]; Tel.: +52-656-688-2100 (ext. 5124)

 Received: 17 January 2019; Accepted: 12 February 2019; Published: 26 March 2019 

Abstract: Lithium niobate-silica fibers were produced by the combination of the sol-gel method and the electrospinning technique. Two sol-gel solutions starting from -lithium ethoxide and tetraethyl orthosilicate were prepared and then mixed with polyvinylpyrrolidone; the solutions were electrospun in a coaxial setup. The obtained lithium niobate-silica polymeric fibers were approximately 760 nm in diameter. Raman spectroscopy confirmed the composite composition by showing signals corresponding to lithium niobate and silica. Scanning electron microscopy showed coaxial fibers with a diameter of around 330 nm arranged as a fibrillar membrane at 800 ◦C. At 1000 ◦C the continuous shape of fibers was preserved; the structure is composed of silica and lithium niobate within the fibers. The formation of crystalline lithium niobate and amorphous SiO2 phase was also confirmed by XRD peaks.

Keywords: electrospinning; sol-gel; lithium niobite; nanofibers

1. Introduction

Lithium niobate (LiNbO3, LN) is an innovative material which has been researched extensively in the last few years due to its physicochemical properties. It has a fusion temperature close to 1200 ◦C, a Curie temperature of 1206 ◦C[1], and a perovskite-type crystalline structure, that is, it is composed of two cations A = Li and B = Nb, and an anion X = O [2]. The crystalline organization in perovskites allows the movement of B cations provoking a polarization of the while enabling high pyroelectric, photo-refractive, ferro-electric, electro-optic, piezo-optic, acusto-optic and foto-voltaic coefficients, and nonlinear electric susceptibilities [3–5]. Some of the methods adopted for the synthesis of lithium niobate include a water-soluble reaction which solves problems such as the use of toxic compounds, but has a long reaction time and limited control of the reaction conditions [6]; Liu et al. [7] used a combustion method to synthesize high purity LN powders, however, while this approach uses a low reaction temperature and zero solvents, the end product applications may be limited because of the morphological and macroscopic characteristics. Zeng and Tung [8] had a different approach by using the sol-gel method with niobium (V) ethoxide and lithium nitrate as precursors, which is cheaper to achieve and requires a low calcination temperature as well in comparison with an all-alkoxide reaction, however, this synthesis use acetylacetone as a chelant, which is a highly toxic reactant. Wang et al. [9] also used the sol-gel method with dihydrate lithium acetate and niobium chloride as precursors, but the latter is highly reactive and toxic for humans and the environment and requires special handling. Some of the advantages of the sol-gel method are that due to the relative proximity of the mixed components of the gel, inferior temperatures are required for the formation of the ceramic and .

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Furthermore, it yields a high purity and homogeneous product and electrospinning solutions for the obtainment of fibers. The most important factors influencing the sol-gel process and the preparation of ceramic fibers by electrospinning are the type and concentration of precursor [10–13]. Organometallic compounds, such as niobium ethoxide (Nb(OCH2CH3)5) and (LiOH), have been recently used in the processing route. For instance, acid catalysts are necessary, hydrolysis is difficult to control and requires additional steps and a long time for heating, and stirring is needed. In this works is reported that lithium niobate nanofibers were formed by the coalescence of colonies of particles at low temperature, giving rise to nanofibers with two different morphologies, cylindrical and necklace-like nanofibers [14]. The oriented growth in the matrix of the nanofibers is hindered by the fracture of the fibers, so slow heating or treatment at a higher temperature is necessary when using a polymer as a matrix. Recently, there has been an intense research effort on electrospinning of ceramics, we were not able to find many reports on the synthesis of lithium niobate nanofibers by electrospinning. Precursor concentration is a crucial parameter for the efficient preparation of fibers by electrospinning and sol gel because it affects viscosity and surface tension. In the specific case of ceramics, it has been reported that characteristics like diameter, grain growth rate, and sinterability are influenced by concentration values [13]. In this article, the synthesis of LiNbO3 and SiO2 were carried out using the sol-gel method by avoiding the use of harmful reactants while keeping the synthesis as brief as possible.

2. Materials and Methods Lithium niobate sol-gel was prepared by dissolving Lithium-niobium ethoxide (Alfa Aesar 99%) in ethanol and using acetic acid as a catalyzer for hydrolysis, keeping a molar ratio of 1:10:6. Silica sol-gel was prepared by dissolving tetraethyl orthosilicate in ethanol using hydrochloric acid as a catalyzer for hydrolysis, keeping a molar ratio of 1:0.1. Simultaneously, 6 mL of a 10% Polyvinyl pyrrolidone, PVP (Alfa Aesar Mw 1,300,000) solution were prepared using ethanol (Jalmek 99.5%) as the solvent. This solution was divided into equal amounts and then added to both sol-gels and homogenized for 1 h. Precursor solutions of 0.003 (M003), 0.03 (M03) and 0.05 M (M05) of LiNbO3, and 10 w/v% SiO2 were obtained. The precursor solutions were transferred to 10 mL glass syringes connected through a hose to a coaxial nozzle and placed in a KD Scientific injection pump. The distance used between the nozzle and the collector was 10 cm. The feeding rate was 4 mL/h for the outer solution of SiO2 and 3 mL/h for the inner solution of LiNbO3, for samples M003 and M03; for the sample M05 1.5 mL/h for SiO2 and 3 mL/h for the LiNbO3 solution at 9 to 10 kV. Fibers were collected on a rotating drum covered with aluminum foil. Characterization of as-spun fibers was carried out by Fourier Transformed Infrared Spectroscopy (ALPHA Platinum, Bruker, Mexico City, Mexico) with 40 scans and a resolution of 4 cm−1 Morphology and diameters were determined by Scanning Electron Microscopy (JSM-6010 PLUS/LA, JEOL, Mexico City, Mexico). Before observation, the membrane samples were coated with gold using a sputter coating. Thermogravimetric Analysis, Differential Thermal Analysis and Differential Scanning Calorimetry were carried out simultaneously with an SDT Q600 instrument (SDT Q600 V20.9 build 20, TA Instruments, Mexico City, Mexico) using a 10 ◦C/min heating rate. The samples treated at 800 ◦C, 1000 ◦C, 1200 ◦C and 1400 ◦C were characterized by FTIR (ALPHA Platinum, Bruker, Mexico City, Mexico), SEM (JSM-6010 PLUS/LA, JEOL, Mexico City, Mexico), Raman spectroscopy (Alpha 300, Witec, Ulm, Germany) using a 532 nm scanning an area of 5 µm, and crystalline phases were identified by XRD with parameters Cu Kα1, 35 kV, 25 mA (X’Pert PRO, Malvern Panalytical, Mexico City, Mexico) and a scanning range from 20◦ to 40◦ at a 2◦/min scan speed.

3. Results and Discussions The SEM images obtained from as-spun samples M003, M03 and M05 are observed in Figure1. Figure1a,b show micrographs at 2000 and 50,000 × of the M003 sample obtained, the fibers were cylindric with a homogeneous surface and random orientation. The average diameter of fibers was Coatings 2019, 9, 212 3 of 8

760 ± 82 nm. Figure1c,d show micrographs at 5000 and 40,000 × of the M003 sample in which images ± Coatingsshows the2019 same, 9, x FOR morphology PEER REVIEW with an average diameter of 510 94 nm. The fibers present a diameter3 of of8 330 ± 83 nm distributed randomly. A further reduction in the inner flow to 1.5 mL/h to produce fibers with a cylindric morphology and homogeneous surface and an average diameterdiameter ofof 330330 ±± 118118 nm nm is shownshown in in Figure Figure 1e,f1e,f micrographs micrographs at at 1200 1200 and and 30,000× 30,000 of× theof the M05 M05 sample. sample. A few A fewdefects defects of round of round and elongatedand elongated shape shape are observed are observed with with small small accumulations. accumulations. It is It isseen seen that that at at a alow low concentration concentration of lithiumlithium niobate niobate the the fibers fibers appear appear to to be be polymer polymer only only because because the the precursor precursor solution solution of of sample sample M003 M003 isis mostly silica sol-gel. The The sample sample M03 M03 presents presents ruptures ruptures and discontinuit discontinuityy in the fibers.fibers. On On the other hand,hand, byby increasing increasing the the concentration concentration of lithiumof lithium niobate, niobate, continuity continuity is restored is restored due to due the greaterto the greaterstability stability of the injected of the injected flows as flows a result as a of re thesult increase of the increase in lithium in lithium concentration. concentration.

◦ Figure 1. TheThe SEM micrographs ofof LiNbOLiNbO33/SiO22 coaxialcoaxial fibers fibers samples samples at 100 °C:C: M003 (aa)) 20002000×× andand ((b)) 50,000×; 50,000× ;M03 M03 (c (c) )5000× 5000× andand (d ()d 40,000×;) 40,000× and; and M05 M05 (e) ( e1200×) 1200 and× and (f) (30,000×.f) 30,000 ×.

The sample sample obtained obtained from from solution solution M05 was was characterized characterized by DTA, DTA, IR, Raman Raman and and XRD XRD because because thisthis precursor precursor solution solution allowed allowed for for a a better better electros electrospunpun process and gives uniform fibers. fibers. Figure Figure 22 isis thethe thermogramthermogram byby differential differential scanning scanning calorimetry calorimetry and and thermogravimetric thermogravimetric analysis analysis for samplefor sample M05, ◦ M05,where where a loss ofa loss 8.07% of weight 8.07% atweight 91 C canat 91 be °C observed, can be correspondingobserved, corresponding to the loss of to water. the loss Subsequently, of water. ◦ Subsequently,the weight remains the weight stable remains up to 288 stableC whereup to 288 it descends °C where by it descends 11.59% corresponding by 11.59% corresponding to the organic to ◦ thecomponents organic components derived from derived the polymer. from the At 352polymer.C, the At weight 352 °C, loss the increases weight toloss 17.07%, increases temperature to 17.07%, at temperature at which a change in crystalline phase occurs and from this point and up to 454 °C the amount of weight is reduced by 52.4% due to a great loss of organic matter. At 773 and 917 °C, a loss of 15.97% and 0.98% occur, respectively. The analysis reveals that after heat treatment under a temperature of 920 °C, the composite loses up to 86.42% of its weight.

Coatings 2019, 9, 212 4 of 8 which a change in crystalline phase occurs and from this point and up to 454 ◦C the amount of weight is reduced by 52.4% due to a great loss of organic matter. At 773 and 917 ◦C, a loss of 15.97% and 0.98% occur, respectively. The analysis reveals that after heat treatment under a temperature of 920 ◦C, theCoatings composite 2019, 9, x loses FOR PEER up to REVIEW 86.42% of its weight. 4 of 8

FigureFigure 2. 2.The The thermogram thermogram of of the the LiNbO LiNbO3/SiO3/SiO22 coaxial fibers.fibers.

◦ TheThe infraredinfrared spectrum of of as-spun as-spun fibers fibers M05 M05 is issh shownown in inFigure Figure 3. 3At. At25 °C 25 theC spectra the spectra show showabsorption absorption bands bands related related to functional to functional grou groupsps in inthe the PVP, PVP, silica silica and lithium-niobiumlithium-niobium ethoxide/ethanol/aceticethoxide/ethanol/acetic acid acid solution solution produced produced by by sol-gel. sol-gel. PVP PVP vibration vibration bands bands can can be be observed, observed, a −1 ahydroxyl hydroxyl stretching stretching band band of of H H22OO is is present present at 3420 cm−1, ,followed followed by by C–H C–H stretching stretching bands atat 2900,2900, −1 16501650 andand 1410 1410 cm cm−1. In In the the fingerprint fingerprint zone, thethe stretchingstretching ofof carbonylcarbonyl (C=O)(C=O) bondsbonds cancan bebe observedobserved −1 −1 atat 16551655 cmcm−1,, while scissoring bands ofof methylmethyl groupsgroups areare presentpresent at at 1441, 1441, 1415 1415 and and 1379 1379 cm cm−1 andand −1 otherother thethe characteristiccharacteristic vibrationvibration ofof thethe C=O C=O groupgroup appearsappears atat 1291 1291 and and 650 650 cm cm−1 toto thethe nitrogennitrogen ofof −1 −1 thethe tertiarytertiary amineamine inin PVP, PVP, C–O C–O bands bands at at 1000–1050 1000–1050 cm cm−1and andCH CH bands bands a a 1400–1500 1400–1500 cm cm−1 cancan bebe −1 observed.observed. AA bandband correspondingcorresponding toto C–NC–N bonds bonds is is observed observed at at 1291 1291 cm cm−1 andand IRIR spectraspectra showshow peakspeaks −1 forfor thethe bonds bonds Nb–O Nb–O at at 500–700 500–700 cm cm−1.. TheThe infrared infrared analysis analysis of of the the SiO SiO2 fibers2 fibers (black), (black), LiNbO LiNbO3/SiO3/SiO22 ◦ ◦ coaxialcoaxial fibersfibers atat 800800 °CC (blue),(blue), andand 11001100 °CC (green)(green) andand LiNbOLiNbO33 powders (red) are shown in Figure2 2.. ◦ TheThe intensityintensity ofof carbonylcarbonyl vibrationvibration bandsbands decreasesdecreases atat 200200 °C,C, whilewhile C–HC–Halmost almostdisappears. disappears. BothBoth ◦ phenomenaphenomena areare causedcaused byby polymerpolymer decompositiondecomposition atat 180180 °C.C. OrganicOrganic mattermatter isis lostlost completelycompletely andand ◦ characteristiccharacteristic bands bands of of silica silica appear appear at 500 at C.500 Vibration °C. Vibration bands ofbands Si–O bondsof Si–O are bonds present are as symmetricpresent as −1 −1 −1 (1220symmetric and 800 (1220 cm and) and 800 asymmetric cm−1) and asymmetric (1080 cm ) (1080 stretching, cm−1) and stretching, the scissoring and the (460 scissoring cm ) modes (460 cm [12−].1) ◦ Twomodes important [12]. Two changes important took changes place at 800took C,place first, at chemical 800 °C, first, stability chemical is reached stability by the is reached ceramic fibers,by the representedceramic fibers, by therepresented loss of the by Si–OH the loss and of M–ORthe Si–OH bonds. and Figure M–OR3 showsbonds. theFigure vibrations 3 shows corresponding the vibrations −1 ◦ tocorresponding Nb–O at 900, to 700 Nb–O and at 550 900, cm 700 andat 800 550 C.cm The−1 at peaks800 °C. become The peaks defined become as thedefined temperature as the temperature increases. Theincreases. spectrum The of spectrum LiNbO3/SiO of 2LiNbOcoaxial3/SiO fibers2 coaxial is compared fibers with is compared the crystalline with lithium the crystalline niobate powders. lithium ◦ niobateThe powders. coaxial fibers samples M03 and M05 at 800 C were characterized by Raman spectroscopy, the continuousThe coaxial structure fibers samples of the fibers M03 canand beM05 observed at 800 °C in thewere Raman characterized spectroscopy by Raman map inspectroscopy, Figure4a,c. Figurethe continuous4b,d show structure the Raman of the spectrum fibers can corresponding be observed in to the lithium Raman niobate, spectroscopy showing map vibrations in Figure of 4a,c. the −1 −1 bondFigures Nb–O 4b,d in show the range the Raman of 670–550 spectrum cm ,corresponding the bond O–Nb–O to lithium at 432 cmniobate,strongly showing coupled vibrations to the of Li–O the −1 andbond O–Li–O Nb–O bonds,in the range while weakof 670–550 signals cm at−1 238cm, the bondare O–Nb–O assigned at to 432 the cm deformation−1 strongly ofcoupled Nb–O. to Vibrations the Li–O correspondingand O–Li–O bonds, to SiO 2whileare detected weak signals in the outsideat 238cm of−1 the are fibers, assigned while to vibrations the deformation corresponding of Nb–O. to LiNbOVibrations3 are corresponding detected in the insideto SiO of2 are the fibers,detected according in the tooutside the coaxial of the configuration. fibers, while In vibrations addition, −1 −1 vibrationscorresponding were detectedto LiNbO at3 800,are619 detected cm and in fromthe inside 200 to 559of cmthe fibers,, corresponding according to to the the vibrations coaxial −1 ofconfiguration. the O–Si–O bonds In addition, at 810 cm vibrations. It is noticeable were detected that the at higher 800, 619 the cm concentration−1 and from of 200 lithium to 559 niobate, cm−1, thecorresponding greater the definition to the vibrations of the silica of the edges O–Si–O of the bonds fibers. at 810 cm−1. It is noticeable that the higher the concentration of lithium niobate, the greater the definition of the silica edges of the fibers.

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Figure 3. The infrared spectrum of the LiNbO3/SiO2 coaxial fibers. FigureFigure 3. 3.The The infraredinfrared spectrum spectrum of of the the LiNbO LiNbO33/SiO/SiO2 coaxialcoaxial fibers. fibers.

Figure 4. The Raman spectroscopy mapping and Raman spectrum of the LiNbO3/SiO2 coaxial fibers ◦ forFigure samples 4. The M03 Raman (subsections spectroscopy (a) and mapping (b)) and and M05 Raman (sunsections spectrum (c) andof the (d ))LiNbO sintered3/SiO at2800 coaxialC. fibers Figure 4. The Raman spectroscopy mapping and Raman spectrum of the LiNbO3/SiO2 coaxial fibers for samples M03 (subsections (a) and (b)) and M05 (sunsections (c) and (d)) sintered at 800 °C. Figurefor samples5 shows M03 samples(subsections after (a) thermal and (b)) treatmentand M05 (sunsections at 800 ◦C. ( Asc) and observed (d)) sintered in the at images, 800 °C. the mean diametersFigure and 5 shows standard samples deviation after decreasedthermal treatment considerably. at 800 For °C. instance, As observed the diameter in the images, in sample the mean M003 Figure 5 shows samples after thermal treatment at 800 °C. As observed in the images, the mean (Figurediameters5a,b) and decreased standard 47% deviation (diameter decreased 360 ± considerably.120 nm). Likewise, For instance the sample, the diameter obtained infrom sample solution M003 diameters and standard deviation decreased considerably. For instance, the diameter in sample M003 M03(Figure (Figure 5a,b)5 decreasedd,e) decreased 47% 54%(diameter to a mean 360 ± diameter120 nm). ofLikewise, 280 ± 76the nm. sample As observedobtained infrom Figure solution5f,g, (Figure 5a,b) decreased 47% (diameter 360 ± 120 nm). Likewise, the sample obtained from solution aM03 continuous (Figure 5d,e) and decreased smooth surface 54% to is a retained mean diameter in sample of 280 M05, ± 76 and nm. the As diameter observed decreases in Figure 57% 5f,g, toa M03 (Figure 5d,e) decreased 54% to a mean diameter of 280 ± 76 nm. As observed in Figure 5f,g, a 190continuous± 140 nm and. Assmooth observed surface in Figureis retained5, the in fibroussample continuousM05, and the morphology diameter decreases was lost 57% in samples to 190 ± continuous and smooth surface is retained in sample M05, and the diameter decreases 57% to 190 ± prepared,140 nm. As and observed lithium in niobate Figure particles5, the fibrous formed cont dueinuous to grain morphology growth and was glass lost in formation. samples prepared, However, 140 nm. As observed in Figure 5, the fibrous continuous morphology was lost in samples prepared, theand originallithium niobate fiber structure particles is formed observed due in to all grain areas. growth It was and observed glass formation. that lithium However, niobate the particles original and lithium niobate particles formed due to grain growth and glass formation. However, the original increasedfiber structure with is the observed increment in all in precursorareas. It was concentration. observed that At lithium low concentrations, niobate particles only increased a few with fiber structure is observed in all areas. It was observed that lithium niobate particles increased with arethe formed.increment In in comparison, precursor concentration. the amount of precursorAt low co availablencentrations, in more only concentrated a few crystals solutions are formed. caused In the increment in precursor concentration. At low concentrations, only a few crystals are formed. In comparison, the amount of precursor available in more concentrated solutions caused the higher comparison, the amount of precursor available in more concentrated solutions caused the higher

Coatings 2019, 9, 212 6 of 8 Coatings 2019, 9, x FOR PEER REVIEW 6 of 8 thecenter higher orientation center orientation of particles. of particles.These processes These processes lead to the lead formation to the formation of coaxial-fibers of coaxial-fibers of wider of widerdiameters diameters due to duethe toincorporation the incorporation of silica. of silica.Nanometric Nanometric particles particles of lithium of lithium niobate niobate inside insideof fibers of fiberswere obtained were obtained from precursor from precursor solutions. solutions.

Figure 5.5. The SEMSEM micrographsmicrographs ofof LiNbOLiNbO33/SiO/SiO22 coaxialcoaxial fibers fibers for for samples samples M003 M003 (incises (incises ( (aa)) and and ( b)), M03 (incises ((cc)) andand ((dd)))) andand M05M05 (incises(incises ((ee)) andand ((ff)))) sinteredsintered atat 800800 ◦°C.C.

◦ Figure6 6 showsshows the SEM micrograph micrograph zone zone obtained obtained from from the the coaxial coaxial fibers fibers M05 M05 at 1000 at 1000 °C. TheC. Thefibers fibers were were observed observed withwith the same the same morphology morphology of a continuous of a continuous structure structure composed composed of silica of silicawith withlithium lithium niobate niobate nanoparticles nanoparticles within within the fibers. the fibers. It is Itnoticeable is noticeable that thatat a athigher a higher temperature temperature the thediameter diameter of the of thefibers fibers increases increases to to637 637 ± ±127127 nm nm due due to to the the fusion fusion of of the the formed formed glass and thethe coalescence ofof thethe fibers.fibers. InIn thethe zoomzoom ofof thethe FigureFigure5 5cc to to 80,000 80,000×× lithium niobate particles are clearly seen inin whitewhite contrast.contrast. EDS analys analysisis in Figure 55cc showsshows thatthat thethe fibersfibers arearecomposed composed mainlymainly ofofO O (61.65(61.65 wtwt %),%), SiSi (20.36(20.36 wtwt %),%), CC (16.05(16.05 wtwt %),%), andand NbNb (1.93(1.93 wtwt %).%). TheThe resultingresulting concentrationconcentration isis 0.020.02 moles for LiNbOLiNbO33 approximately near to the initial precursor solutions.solutions. To confirmconfirm thethe existenceexistence ofof thethe lithiumlithium niobateniobate nanoparticlesnanoparticles withinwithin thethe fibers,fibers, anan XRDXRD analysisanalysis waswas performed,performed, showedshowed inin FigureFigure7 .7. XRDXRD ofof LiNbOLiNbO3 fibersfibers showed the formation of a curve ofof ◦ ◦ approximately 2020° to 3030° belonging to silica and crystalline lithium niobite peaks according to thethe Raman andand IRIR results.results. In In this this diffractogram, diffractogram, the the habitual habitual fluorescence fluorescence background background was was seen seen due due to the to the amorphous SiO2 phase, but also, a Quartz phase was found and characterized by their main diffracted peaks (100), (011) and (110) using JCP: 01-087-2096. The formation of crystalline lithium

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CoatingsamorphousCoatings 2019 2019, ,9 SiO9, ,x x FOR FOR2 phase, PEER PEER butREVIEWREVIEW also, a Quartz phase was found and characterized by their main diffracted77 ofof 88 peaks (100), (011) and (110) using JCP: 01-087-2096. The formation of crystalline lithium niobate phase wasniobateniobate also phasephase confirmed waswas also byalso XRD confirmedconfirmed peaks. Crystallographicbyby XRDXRD peaks.peaks. CrystallographicCrystallographic indexation was obtained indexationindexation using waswas JCP: obtainedobtained 00-074-2238 usingusing JCP:forJCP: trigonal 00-074-223800-074-2238 (R3c) for indicatingfor trigonaltrigonal (012), (R3c)(R3c) (104), indicatingindicating and (110) (012),(012), as (104),(104), the principal andand (110)(110) directions. asas thethe principalprincipal directions.directions.

Figure 6. The SEM micrographs of the LiNbO3/SiO2 coaxial fibers at 1000 °C:◦ (a) 3000×; (b) 9000×; (c) Figure 6. The SEM micrographs of the LiNbO 33/SiO/SiO2 coaxial2 coaxial fibers fibers atat 1000 1000 °C:C: (a () a3000×;) 3000 ×(b;() b9000×;) 9000 (×c); (80,000×80,000×c) 80,000 andand× and ((dd)) EDS. (EDS.d) EDS.

◦ FigureFigure 7. 7.The The XRD XRD of of the the LiNbO LiNbO3/SiO3/SiO22 coaxial fibers fibers at 1000 °C.C. Figure 7. The XRD of the LiNbO3/SiO2 coaxial fibers at 1000 °C. 4. Conclusions 5.5. ConclusionsConclusions An alternative methodology was obtained for the synthesis of lithium niobite, which combines An alternative methodology was obtained for the synthesis of lithium niobite, which combines the sol-gelAn alternative and electrospinning methodology technique was obtained to produce for the a fibrillarsynthesis ceramic of lithium composite niobite, composed which combines of silica the sol-gel and electrospinning technique to produce a fibrillar ceramic composite composed of silica theand sol-gel lithium and niobate. electrospinning Characterization technique by infrared to produc spectroscopy,e a fibrillar ceramic Ramanspectroscopy, composite composed X-ray diffraction of silica andand lithium lithium niobate. niobate. CharacterizationCharacterization by by infraredinfrared sp spectroscopy,ectroscopy, Raman Raman spectroscopy, spectroscopy, X-ray X-ray diffraction diffraction andand scanningscanning electronelectron microscopymicroscopy showedshowed thethe formatformationion ofof ceramicceramic fibersfibers ofof lithium-silicalithium-silica niobate.niobate. TheThe continuouscontinuous shapeshape ofof fibersfibers waswas preservedpreserved withwith thethe presencepresence ofof discontinuitydiscontinuity ofof thethe lithiumlithium niobateniobate nucleusnucleus foundfound surroundedsurrounded byby silica.silica. ItIt waswas observedobserved thatthat thethe concenconcentrationtration ofof thethe precursorprecursor

Coatings 2019, 9, 212 8 of 8 and scanning electron microscopy showed the formation of ceramic fibers of lithium-silica niobate. The continuous shape of fibers was preserved with the presence of discontinuity of the lithium niobate nucleus found surrounded by silica. It was observed that the concentration of the precursor influences the morphology and stability of the fibers; the use of the silica improves the characteristics of the fibers as well, resulting in continuous fibers. The LiNbO3/SiO2 coaxial fibers were analyzed by the X-ray diffraction technique and the XRD of LiNbO3/SiO2 fibers, showing the formation a curve of approximately 20◦ to 30◦ belonging to the silica and crystalline lithium niobite peaks according to the SEM, Raman and IR results.

Author Contributions: S.Y.R.-L. and R.F. generated conceptualization; S.Y.R.-L. designed methodology, performed the experimental work, characterized the materials and devices analyzed results, supervised the entire research work; and J.A.G.-A., S.Y.R.-L. and R.F. wrote, reviewed and edited the manuscript. Funding: This research no received external funding. Acknowledgments: Thanks to PRODEP, Universidad Autónoma de Ciudad Juárez and CONACYT for supporting this investigation. Reyes-Lopez thanks Carlos Rodriguez for SEM images and Alejandra Peña and Hector Martinez for investigation. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Volk, T.; Wöhlecke, M. Lithium Niobate: Defects, Photorefraction and Ferroelectric Switching; Springer Science & Business Media: Berlin, Germany, 2008. 2. Wong, K.K. Properties of Lithium Niobate; INSPEC: London, UK, 2002. 3. Smith, R.T.; Welsh, F.S. Temperature dependence of the elastic, piezoelectric, and dielectric constants of and lithium niobate. J. Appl. Phys. 1971, 42, 2219–2230. [CrossRef] 4. Arizmendi, L. Photonic applications of lithium niobate crystals. Phys. Status Solidi A 2004, 201, 253–283. [CrossRef] 5. Cudney, R.S.; Ríos, L.A.; Orozco Arellanes, M.J.; Alonso, F.; Fonseca, J. Fabricación de niobato de litio periódicamente polarizado para óptica no lineal. Revista Mexicana de Física 2002, 48, 548–555. 6. Camargo, E.R.; Kakihana, M. Low temperature synthesis of lithium niobate powders based on water-soluble niobium malato complexes. Solid State Ion. 2002, 151, 413–418. [CrossRef] 7. Liu, M.; Xue, D.; Zhang, S.; Zhu, H.; Wang, J.; Kitamura, K. Chemical synthesis of stoichiometric lithium niobate powders. Mater. Lett. 2005, 59, 1095–1097. [CrossRef] 8. Zeng, H.C.; Tung, S.K. Synthesis of lithium niobate gels using a metal alkoxide-metal nitrate precursor. Chem. Mater. 1996, 8, 2667–2672. [CrossRef] 9. Wang, L.H.; Yuan, D.R.; Duan, X.L.; Wang, X.Q.; Yu, F.P. Synthesis and characterization of fine lithium niobate powders by sol-gel method. Cryst. Res. Technol. 2007, 42, 321–324. [CrossRef] 10. Eichorst, D.J.; Howard, K.E.; Payne, D.A.; Wilson, S.R. of lithium niobium ethoxide (LiNb

(OCH2CH3)6: A precursor for lithium niobate ceramics. Inorg. Chem. 1990, 29, 1458–1459. [CrossRef] 11. Garibay-Alvarado, J.A.; Espinosa-Cristóbal, L.F.; Yobanny, S. Fibrous silica-hydroxyapatite composite by electrospinning. Int. J. Res. Granthaalayah 2017, 5, 39–47. [CrossRef] 12. Roque-Ruiz, J.H.; Martínez-Máynez, H.; Zalapa-Garibay, M.A.; Arizmendi-Morquecho, A.; Farias, R.;

Reyes-López, S.Y. Surface enhanced Raman spectroscopy in nanofibers mats of SiO2-TiO2-Ag. Results Phys. 2017, 7, 2520–2527. [CrossRef] 13. Roque-Ruiz, J.H.; Medellín-Castillo, N.A.; Reyes-López, S.Y. Fabrication of α-alumina fibers by sol-gel and electrospinning of aluminum nitrate precursor solutions. Results Phys. 2018, 12, 193–204. [CrossRef] 14. Maldonado-Orozco, M.C.; Ochoa-Lara, M.T.; Sosa-Márquez, J.E.; Olive-Méndez, S.F.; Espinosa-Magaña, F.

Synthesis and characterization of electrospun LiNbO3 nanofibers. Ceram. Int. 2015, 41, 14886–14889. [CrossRef]

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