Physical Characteristics of Titania Nanofibers Synthesized by Sol-Gel ...

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ABSTRACT. Titania nanofibers were successfully synthesized by sol-gel coating of electrospun polymer nanofibers followed by calcining to form either the pure ...
Physical Characteristics of Titania Nanofibers Synthesized by Sol-Gel and Electrospinning Techniques Soo-Jin Park, Yong C. Kang, Ju Y. Park, Ed A. Evans, Rex D. Ramsier, and George G. Chase University of Akron, Akron, OH UNITED STATES Correspondence to: George G. Chase email: [email protected]

ABSTRACT Titania nanofibers were successfully synthesized by sol-gel coating of electrospun polymer nanofibers followed by calcining to form either the pure anatase or rutile phases. Characterization of these materials was carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and UV-vis spectroscopy techniques. The average diameter of these ceramic nanofibers was observed to be around 200 nm for both the rutile and anatase forms. The valence band structure and optical absorption thresholds differ, however, indicating that nanofibrous mats of titania can be selectively developed for different applications in catalysis and photochemistry. INTRODUCTION Nanostructured metal oxide materials have attracted considerable attention because of their potential applications in many areas such as electronics, photonics, sensors and catalysis.1-6 Nanosized ceramics with large surface area per unit mass can have enhanced electrical, physical, and chemical properties with respect to their bulk counterparts.7,8 In nanofiber form these materials can be incorporated into fabrics, textiles, and filter media. Titania, for example, is an important ceramic material used in practical applications such as microporous membranes,9,10 photocatalysts,11 catalysts12,13 and chemical sensors.14 The two most commonly used titania forms are anatase and rutile. Control of the phase content is important since the optical and electrical properties of the anatase and rutile phases differ. Rutile titania has applications in high quality paints, cosmetics and ultraviolet absorbents because of its high refractive index and UV absorption crossJournal of Engineered Fibers and Fabrics Volume 5, Issue 1 - 2010

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section.15 On the other hand, the anatase phase is chemically and optically active, thus it is generally used for catalysis and catalytic supports.16,17 Various approaches for the preparation of nano- and micro-structures of titania have been reported, such as sol-gel processes, pyrolysis, electrospinning, chemical vapor deposition and hydrothermal methods.18-22 In particular, our research focuses on electrospinning as one of the simplest and most versatile methods for the fabrication of polymeric or ceramic nanofiber mats.23 In this paper, titania nanofibers were fabricated by dip-coating electrospun polymeric fibers into sol-gel precursors. The fibers are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy techniques. Characterization of mechanical strength properties are a topic of future work. It was found that titania nanofibers formed different crystal structures (anatase or rutile) depending on the temperature during post-coating annealing, as verified by several different methods. Most importantly, we compare the electronic structure and optical properties of the two phases of titania coated nanofibers to demonstrate that our synthesis methods can be used to produce nanofibers with different physical properties which can therefore be tailored for specific applications. EXPERIMENTAL Titania nanofibers were prepared by sol-gel coating of a template made from electrospun nylon nanofibers. Nylon-6 (Aldrich, MW. 4,322) was dissolved in formic acid (Fisher Scientific) at a 20:80 wt % ratio. This solution was electrospun into http://www.jeffjournal.org

nanofiber mats using a laboratory-scale electrospinning apparatus.24 The polymer mixture was loaded into a 5 ml plastic syringe with a flexible silicone tube at one end and a 21 gauge stainless steel needle at the other end. The needle was connected to a high voltage supply (Gamma High Voltage Research Inc. Ormond Beach, FL) operated at 20 kV DC. The feeding rate for the Nylon-6 solution was controlled using a syringe pump (World Precision Instruments, Sarasota, FL; SP1011) at 2.0 μl/min with the needle positioned about 15 cm above the grounded collector. The collector was aluminum foil wrapped around a 12.5 cm diameter cylindrical wire drum. The drum was rotated on its axis at a rate of about 1 revolution per minute. In order to deposit titania on the surface of the Nylon-6 nanofibers, the fiber mats were submerged into a sol-gel solution which consisted of 144 ml distilled water, 20 ml of 5 M nitric acid (Fisher Scientific), 10 ml of isopropyl alcohol (Fisher Scientific), and 2 ml of titanium isopropoxide (Aldrich). In order to make the anatase form of titania, the submerged nanofiber mat was heated from 60 ºC to 95 ºC at a rate of 5 ºC/min and held at 95 ºC for about 90 minutes until the solution turned to a milky white color. To make the rutile form, the coated nylon nanofibers were heated from room temperature to 60 ºC at a rate of 0.5 ºC/min and held at 60 ºC for 3 to 4 hrs until the solution became milky. This heating induced the growth of titania nanoparticles on the surfaces of the fibers. The titania-coated nanofibers were removed from the sol solution and washed several times with methanol to remove any residual alkoxides. The nanofibers were then heated to 350 ºC to form anatase titania and 700 ºC to form rutile titania. The morphology of the aforementioned titania fibers was observed by scanning electron microscopy (SEM; JEOL Ltd., JSM 5310, Tokyo, Japan) and by transmission electron microscopy (TEM, JEOL Ltd., JEM 1200XII, Tokyo, Japan). The vibrational spectra were acquired using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS; Bruker Optics, IFS 66v/S, Tucson, AZ). The crystalline phase of the titania fibers was identified by wide angle X-ray diffraction (WAXD; Rigaku Co., Ltd., Tokyo, Japan) in the reflection mode with Cu K radiation. The chemical nature of the titania nanofibers was investigated with X-ray photoelectron spectroscopy (XPS; VG ESCALAB MK II, West Sussex, UK). The base pressure in the analysis chamber was less Journal of Engineered Fibers and Fabrics Volume 5, Issue 1 - 2010

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than 1 × 10−9 mbar. The XPS system was equipped with a twin anode X-ray source, Mg Kα (1253.6 eV) and Al Kα (1486.6 eV), and a concentric hemispherical analyzer (CHA). During all experiments discussed here, spectra were obtained using the Al Kα X-ray source. The parameters used for XPS experiments were an anode voltage of 9 kV, an electron multiplier voltage of 2850 eV, anode current of 20 mA, filament current of 4.2 A, pass energy of 50 eV, dwell time of 100 ms, and energy step size of 0.5 eV in constant analyzer energy (CAE) mode for survey scans. High resolution scans were performed at an energy step size of 0.02 eV and a pass energy of 20 eV with all other parameters the same as used in survey scans. XPS data were collected from several different samples and all exhibited similar trends and reproducibility. In each binding energy region, spectra were signal averaged up to 15 times in order to improve the signal to noise ratios. The optical absorption edge of the nanofibers was measured using a UV-Vis spectrophotometer (Varian Inc., Cary 300, Palo Alto, CA). The optical absorbance spectra of the titania samples was used to calculate the band gap energy in the wavelength range of 200–800 nm. RESULTS AND DISCUSSIONS The morphology and diameter of sol-gel coated titania nanofibers were investigated by SEM and TEM, as shown in Figures 1 and 2. During the solgel process titania precipitates and coats the polymer nanofibers. The diameter of the titania coated Nylon6 fibers was in the range of about 250 nm after heating to 120 ºC (Figure 1A) and in the range of 200 nm after heating to 350 ºC (Figure 1B). Rutile titania nanofibers exhibited similar diameters after heating to 700 ºC. The decreasing diameter of fibers at higher temperature is due to the removal of solvent and hydrocarbons and the decomposition of the nylon fibers. The TEM images in Figure 2 show the fiber surface morphology is irregular and rough due to the crystal grains of the titania. In order to identify the chemical nature of the synthesized nanofibers, DRIFT measurements were carried out yielding spectra as shown in Figure 3. The characteristic vibrational bands in sol-gel coated Nylon-6 nanofibers appeared in the range of 35003300 cm-1 and 1650-1580 cm-1 for N-H stretching and bending modes, 2960-2850 cm-1 and 1470-1350 cm-1 for C-H stretching, scissoring and bending, 17601670 cm-1 for the C-O stretching bands, and 13401020 cm-1 for the C-N stretching band. In Figure 3 http://www.jeffjournal.org

curve A and Figure 3 curve B, we see that the coated fibers, after heating to 120 ºC and 275 ºC, still have significant IR absorption features corresponding to vibrations of Nylon-6. Figure 3 curve C, shows that Ti-O vibrations dominate around 900 cm-1, indicating that pyrolysis of the nylon material has occurred after calcining these fibers to 350 ºC and the anatase titania phase has formed. Also, the formation of a sequestered form of carbon dioxide is evidenced by the feature around 2340 cm-1 .25 The absorption

bands at 1636 and 3000 cm-1 in this spectrum indicate the presence of hydroxyl groups. Rutile titania nanofibers (Figure 3 curve D) exhibited DRIFT spectra very similar to those from the anatase form.

1m

1m (A)

(A)

(B)

FIGURE 1. Scanning electron microscopy (SEM) images of, (A) titania sol-coated nylon fibers calcined at 120 ºC, and (B) calcined at 350 ºC. The coated nylon fibers shrink in diameter from about 250 nm to 200 nm in diameter after heating to 350 ºC.

(A)

(B)

FIGURE 2. Transmission electron microscopy (TEM) images of titania fibers calcined to 700 ºC. In photograph (A) distinct individual crystals grains are clearly observed on the surface of the fibers. From the TEM images it is uncertain whether the fibers are hollow. The photo in (B) is at a lower magnification than (A) but still shows the crystals. The surface morphology of the fibers appears to be rough and uneven due to the granular structure.

Wide angle X-ray diffraction was used to identify the crystal structures of the nanofibers, as illustrated in Figure 4. When the heating rate was fast (5 ºC/min), anatase titania was formed after calcination at 350 ºC (Figure 4A). It was found that all the sharp features observed at 2θ = 25.39, 38.11, 48.47, 54.5, 55.01, 62.5 Journal of Engineered Fibers and Fabrics Volume 5, Issue 1 - 2010

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and 68.5o in the XRD pattern are consistent with anatase (101), (004), (105), (200), (211), (204) and (116) Miller indices. The lattice constants determined for the anatase nanofibers were a = 0.3772 nm and c = 0.9505 nm. On the other hand, when the solution was slowly heated (0.5 ºC/min) and calcinated at 700 ºC, http://www.jeffjournal.org

High resolution XPS data from the Ti 2p and valence band regions comparing rutile and anatase titania are shown in Figures 5A and 5B, respectively. As seen in Figure 5A, the doublet peaks of Ti 2p were observed at 459 (Ti 2p3/2) and 465 (Ti 2p1/2) eV in both rutile and anatase titania. These are the characteristic Ti 2p XPS features of titania. The similar peak positions for both forms of nanofibers imply that the core level binding energy of Ti on the surface does not depend significantly on the crystal structure. However, the electronic structure of the valence band region was affected by structure as shown in Figure 5B. This structure dependence on the valence band observed in this work has also been identified by Rossi’s group in their investigation of a nickel-based alloy using XPS.28 They identified that the binding energies of Ni core levels, such as Ni 2p3/2 and Ni 2p1/2, were not affected by different crystal structures, but the valence level (the Ni 3d region in their case) was altered by changes in phase. In Figure 5B, the top two curves represent XPS data from rutile and anatase titania and the bottom trace means that anatase titania exhibits a higher density of states than rutile and that anatase has less covalent

shows the spectral intensity obtained by subtraction of the XPS intensity of rutile titania from that of anatase. It is clear that the band structure ca. 3 eV below the Fermi (EF) level shows positive peak intensity. This bond character. This is consistent with the rutile phase having the higher thermodynamic stability.27 The difference in electronic structure near the Fermi level distinguishes the two different titania forms and is supported by the UV-vis absorption spectra of Figure 6. The band gap wavelength was determined by extrapolation of the base line and the absorption edge to calculate the optical band gap.29 N-H C-O C-N C-H

o

o

(B) 275 C anatase OH CO2 Ti-O

OH

o

(C) 350 C anatase

CO2

OH

o

(D) 700 C rutile

800

1300

1800

2300

2800

3300

-1

Wavenumber (cm )

FIGURE 3. DRIFT spectra from titania-coated Nylon-6 nanofiber mats after heating to (A)120 ºC, (B) 275 ºC, (C) 350 ºC, and (D) 700 oC. R(110)

Intensity (a.u.)

R(211)

20

30

40

50

R(301)

70

R(310) R(002)

R(200)

R(220)

R(111)

R(101)

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A(116)

40 50 2 Theta (degrees)

A(204)

A(211) A(105)

30

A(200)

A(004)

Intensity (a.u.)

A(101)

20

N-H

C-H

(A) 120 C anatase Intensity (a.u.)

rutile titania resulted (Figure 4B). The (110), (101), (220), (111), (211), (220), (002), (310) and (301) features can be indexed to the rutile titania crystal structure with lattice parameters a = 0.4566 nm and c = 0.295 nm.26 Depending on the heating rate and temperature the crystal structures of the nanofibers can therefore be manipulated to yield distinct properties from the same starting materials.27

60

70

2 Theta (degrees)

(A) (B) FIGURE 4. XRD spectra for (A) anatase titania nanofibers calcined at 350 ºC, and (B) rutile titania fibers calcined at 700 ºC (A = anatase; R = rutile).

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(B) valence band

Ti 2p3/2

EF

relative intensity (cps)

relative intensity (cps)

(A) Ti 2p

Ti 2p1/2

R

R A

A A-R x3

468

466

464

462

460

458

10

456

8

6

4

2

0

-2

-4

Binding Energy (eV)

Binding Energy (eV)

(A) (B) FIGURE 5. High resolution XPS spectra of the Ti 2p region (A) and valence band region (B) comparing rutile and anatase titania. (A = anatase; R = rutile; A-R = subtraction of rutile XPS intensity from anatase XPS intensity).

Absorbance (a.u.)

E=hc/λ, where h is Planck’s constant (6.626×10-34 Js), c is the speed of light (2.998×108 m/s) and λ is the wavelength of light. These calculated band gap energies are consistent with data reported in the literature for other forms of titania materials.30

anatase (A)

350

rutile (R)

400

450

500

Wavelength (nm)

FIGURE 6. UV absorption spectra of anatase (A), and rutile (R) titania nanofibers.

Based on our data, anatase titania has a band gap of 3.2 eV, which corresponds to approximately 385 nm and rutile titania has band gap energy of 3.0 eV, which corresponds to approximately 420 nm. These band gap energies were calculated using the relationship of photon energy and frequency (c/λ):

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Nanofibers offer surface areas (per unit volume and mass) that are larger than bulk materials and approaching those of nanoparticles. One benefit over the latter, however, is the macroscopic handling capability of nanofiber mats which is not possible with nanoparticles. Thus, even though our materials may not exhibit surface areas and defect densities as large as nanoparticles having diameters less than 100 nm, we can employ these materials for catalysis and surface chemical reaction and sensor applications where nanoparticles are not as cost effective. For example, nanoparticles may be used for the chemical regeneration of used motor oils by surface anchoring of sulfate species thus removing them from the oil however the particles may themselves not be recovered and are therefore not recyclable. Nanofiber mats, or filters constructed thereof, may be able to perform the same surface chemistry as the nanoparticles, but offer the additional possibility of being cleaned and reused since they can be handled macroscopically. Controlling the titania nanofibers' crystalline phases to exhibit different optical and electronic properties could provide for the fabrication of advanced materials with significant industrial uses.

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CONCLUSIONS Titania nanofibers with diameters of 200 nm were synthesized by sol-gel coating of electrospun nylon nanofibers followed by calcination. The XRD results show that the crystalline phases were either anatase or rutile forms of titania depending on the heating method. Photoelectron emission data demonstrate that while the crystal structures of titania affected the electronic structure of the valence band region, it did not affect the core level structure, such as Ti 2p3/2 and Ti 2p1/2. Optical absorption spectra exhibited band gap differences as expected for these materials.

[8] Matsui, T., Harada, M., Ichihashi, Y. et al., Effect of noble metal particle size on the sulfur tolerance of monometallic Pd and Pt catalysts supported on highsilica USY zeolite, Appl. Catal. A 286 (2005) 249257. [9] Sekulic, J., Ten, E.J.E., Blank, D.H.A., Synthesis and characterization of microporous titania membranes, J. Sol-Gel Sci. Tech. 31 (2004) 201-204. [10] Kumar, K.N.P., Keizer, K., Burggraaf, A.J. et al., Synthesis and textural properties of unsupported and supported rutile (TiO2) membranes, J. Mater. Chem. 3 (1993) 923-929.

ACKNOWLEDGMENTS We thank the Coalescence Filtration Nanomaterials Consortium (Ahlstrom, Cummins Filtration, Donaldson Filtration Solutions, Hollingsworth & Vose Company, and Parker filtration) for financial support. We acknowledge E.T. Bender and P. Katta for measuring the DRIFT spectra and assistance in sol-gel coating of the nanofibers, respectively.

[11] Onozuka, K., Ding, B., Tsuge, Y. et al., Electrospinning processed nanofibrous TiO2 membranes for photovoltaic applications, Nanotechnology 17 (2006) 1026-1031.

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modified electrospun titania nanofibers, Sol. Energy Mater. Sol. Cells 85 (2005) 477-488.

AUTHORS’ ADDRESS

[21] Pradhan, S.K., Reucroft, P.J., Yang, F. et al., Growth of TiO2 nanorods by metalorganic chemical vapor deposition, J. Cryst. Growth 256 (2003) 83-88.

Soo-Jin Park Yong C. Kang Ju Y. Park Ed A. Evans Rex D. Ramsier George G. Chase

[22] Yang, J., Mei, S., Ferreira, J.M.F., Hydrothermal synthesis of nanosized titania powders: Influence of tetraalkyl ammonium hydroxides on particle characteristics, J. Am. Ceram. Soc. 84 (2001) 16961702. [23] Li, D., Xia, Y., Electrospinning of nanofibers: Reinventing the wheel?, Adv. Mater. 16 (2004) 11511170.

University of Akron Chemical Engineering 185 E. Mill Street Whitby Hall 411A Akron, OH 44325-3906 UNITED STATES

[24] Reneker, D.H., Yarin, A.L., Fong, H. et al., Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys. 87 (2000) 4531-4547. [25] Bender, E.T., Katta, P., Lotus, A. et al., Identification of CO2 sequestered in electrospun metal oxide nanofibers, Chem. Phys. Lett. 423 (2006) 302-305. [26] Dhage, S.R., Gaikwad, S.P., Ravi, V., Synthesis of nanocrystalline TiO2 by tartarate gel method, Bull. Mater. Sci. 27 (2004) 487-489. [27] Gopel, M., Chan, W.J.M., Jonghe, L.C.D., Room temperature synthesis of crystalline metal oxides, J. Mater. Sci. 32 (1997) 6001-6008. [28] Elsener, B., Atzei, D., Krolikowski, A. et al., From chemical to structural order of electrodeposited Ni22P alloy: An XPS and EDXD study, Chem. Mater. 16 (2004) 4216-4225. [29] Martínez-Castañón, G.A., Sánchez-Loredo, M.G., Dorantes, H.J. et al., Characterization of silver sulfide nanoparticles synthesized by a simple precipitation method, Mater. Lett. 59 (2005) 529-534. [30] Navrotsky, A., Kleppa, O.J., Enthalpy of the anatase-rutile transformation, J. Am. Ceramic, Soc. 50 (1967) 626-626.

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