Formation of Inorganic Nanofibers from Preformed TiO2 Nanoparticles

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Formation of Inorganic Nanofibers from Preformed TiO2 Nanoparticles via Electrospinning Claas Wessel,†,‡ Rainer Ostermann,† Roland Dersch,‡ and Bernd M. Smarsly*,† UniVersity of Giessen, Institute of Physical Chemistry, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, and Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany ReceiVed: August 29, 2010; ReVised Manuscript ReceiVed: NoVember 10, 2010

The study demonstrates that mesoporous anatase nanofibers can be prepared via electrospinning of preformed TiO2 nanoparticles in an adequate solvent mixture without the use of any surfactant. A systematic study of several parameters influencing the fiber formation, that is, solvent composition, molecular weight of the polymer, TiO2 source, and humidity, was conducted to investigate the underlying physicochemical principles that determine the formation of the porous structure. The porosity of the fibers was characterized by nitrogen physisorption and SEM investigations with consistent results. It is proposed that the microphase separation of nanoparticles from the polymer produces a mesoscopic hybrid, resulting in pronounced mesoporosity after calcination. We believe that our study thus presents a general methodology to generate nonwovens of metal oxides with intrinsic mesoporosity based on electrospinning of preformed nanoparticles. 1. Introduction Mesoporous metal oxides have received significant attention in the past years. Various strategies have been developed to achieve the desired mesoporosity combined with high crystallinity of the metal oxide. One of the most important strategies is based on sol-gel approaches using block copolymer templates,1,2 which is particularly suitable for generating thin films and powders with a well-defined porosity. However, although they do offer high surface areas, mesoporous materials usually suffer from poor accessibility, and mass transport is only possible via diffusion. Transport problems can be solved by hierarchical pore systems (i.e., architectures possessing both small and large pores) that allow fast diffusion or even convection in macropores and exhibit high surface areas through mesoporosity. This concept has been realized by using a dense packing of latex beads as an additional macropore template for mesoporous powders and thin films.3-5 Another method of creating such hierarchical structures is a loose packing of one-dimensional nanostructures, as illustrated in Scheme 1. Recently, electrospinning has emerged as a versatile means of generating mats of fibers with typical diameters from 50 to 500 nm.6,7 Although the focus had originally been placed on polymeric systems, now also metals and metal oxides can be obtained by using suitable starting materials and postspinning treatments. Previously, nanofibers of metal oxides, such as TiO2, were prepared from polymer solutions containing suitable precursors, such as Ti(OiPr)4, and stabilizers, such as acetic acid or acetylacetonate.8 Solutions of this kind are sufficiently stable and their viscosity can be adjusted by molecular weight and concentration of the polymer, allowing them to be electrospun into well-defined composite nanofibers. Oxidic nanofibers are obtained by removing the carrier polymer by calcination. A modified approach uses an aged sol-gel solution, which is, however, more difficult to control and yields less uniform * To whom correspondence should be addressed. Tel: (+49)6419934590. Fax: (+49)641-9934509. E-mail: [email protected]. † University of Giessen, Institute of Physical Chemistry. ‡ Philipps-Universita¨t Marburg.

fibers, but templates, such as block copolymers, are compatible with this pure sol-gel process and allow the generation of mesoporosity comparable to sol-gel powders and thin films.9-11 One major disadvantage of these approaches is the instability of the spinning solution regarding hydrolysis and gelation. In the present study, we have developed a different approach using preformed TiO2 nanoparticles and polymers suitable for electrospinning. This concept was first demonstrated for the preparation of silica nanofibers by electrospinning of silica nanoparticles in an aqueous solution of poly(vinyl alcohol) (PVA), but to the best of our knowledge, this method has not been used for other inorganic fibers so far.12 Sol-gel chemistry is separated from fiber formation, which is a great advantage. Thus, the spinning solutions can be prepared with prolonged colloidal stability. Additionally, upon replacement of one metal oxide by another, the colloidal system is equally stable and there is no negative impact from the high reactivity of sol-gel precursors as in nonparticle approaches. Moreover, the use of preformed nanoparticles with a certain degree of crystallinity could permit lower sintering temperatures that are otherwise needed in order to obtain the metal oxide in a highly crystalline form. In addition to these advantages, our nanoparticle approach is fundamentally different from the point of view of colloid science, as the nanoparticles only disperse well in certain solvents. Similarly, the carrier polymers are only soluble in suitable solvents. The idea of the present study is to select appropriate carrier polymers and solvents that enable a microphase separation between the nanoparticles and the polymer upon solvent evaporation during the electrospinning, thus resulting in a nanostructured TiO2/polymer hybrid. The concept of preparing porous fibers via electrospinning by choosing a spinning solution that undergoes a phase separation process has been theoretically investigated in the case of pure binary polymer/solvents systems.13 A similar concept has recently been demonstrated for crystalline poly-L-lactide (PLLA) fibers by adding a poor solvent for the polymer,14 but this might not be generally applicable to amorphous polymer systems.

10.1021/jp108202b  2011 American Chemical Society Published on Web 12/21/2010

Formation of Inorganic Nanofibers from TiO2 NPs

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SCHEME 1: Illustration of Hierarchical Nanofiber System with Structuring over Several Orders of Magnitudea

a The software used only allows the generation of apparently ordered pores, which does not correspond to the disordered mesoporosity seen in the fibers.

In the present work, a systematic investigation was performed studying the miscibility between different TiO2 particles and poly(ethylene oxide) (PEO) and the interaction during electrospinning by varying the synthesis conditions, molecular weight of the PEO, and the solvents used. The final porous TiO2 nanofibers were expected to be an imprint of the hybrid phaseseparated structure. Thus, the final internal fiber pore morphology represents the status of the as-spun hybrid consisting of polymer and nanoparticles. In particular, a systematic study was performed addressing the origin of the different internal structures observed in fibers prepared under different conditions, that is, the occurrence of mesoporous versus compact fibers. Because the dispersibility of TiO2 nanoparticles is known to depend on the degree of crystallinity (e.g., owing to differences in the surface concentration of hydroxyl groups, ligands, etc.), dispersions of TiO2 nanoparticles were synthesized both in a state of low crystallinity (close to amorphous oxide) and high crystallinity, by applying suitable syntheses. The TiO2 nanofibers obtained were characterized by diverse techniques, such as scanning electron microscopy, X-ray diffraction, and physisorption, providing insights into the interaction occurring during solvent evaporation. 2. Experimental Methods 2.1. Materials. All materials were of analytical grade and used as received: poly(vinyl pyrrolidone) (PVP, 360 kg/mol, Carl Roth), poly(ethylene oxide) (PEO, 300 kg/mol, Acros Organics; 600 kg/mol, Sigma Aldrich; 900 kg/mol, Acros Organics), hydrochloric acid (37 wt %), 1,3-propanediol, benzyl alcohol, Ti(OiPr)4, TiCl4, methanol (MeOH), dichloromethane (CH2Cl2), chloroform (CHCl3). 2.2. Synthesis of TiO2 Nanoparticles. 2.2.1. Synthesis of “Amorphous” Particles (Method 1). To prepare the TiO2 nanoparticles, Ti(OiPr)4 (2.60 g, 9.15 mmol) was dissolved in 8.1 g of ethanol. After adding 520 mg of hydrochloric acid (37 wt %), this solution was poured into a solution of 264 mg (3.47 mmol) of 1,3-propanediol in 20.0 g of benzyl alcohol. (Caution! Hydrochloric acid is strongly corrosive and should be handled with care.) The reaction mixture was stirred at 80 °C for 8 h. After cooling to room temperature, this solution was added to 100 mL of cold diethylether. The resulting white

precipitate was isolated by centrifugation, washed once with diethylether, and redispersed in methanol. A dispersion containing 3.41 wt % TiO2 was obtained, as determined after a small sample of the dispersion was dried and calcined at 550 °C. XRD measurements were conducted on a small sample of this dispersion after drying for at least 3 h at 80 °C. In another approach, TiCl4 (4.06 g, 21.4 mmol) was used as a sol-gel precursor and dissolved in 8.1 g of ethanol instead of Ti(OiPr)4 and hydrochloric acid. (Caution! TiCl4 is strongly corrosive and should be handled with care.) Otherwise, the same synthetic procedure was followed, as mentioned above. The use of different precursors in method 1 resulted in similar particles, as revealed by XRD; see Figure 1 and Table 1. In terms of thermodynamics, they are expected to have similar properties due to their similar crystallinity and size so that they are not distinguished in the following discussion. 2.2.2. Synthesis of “Crystalline” Particles (Method 2). To prepare TiO2 nanoparticles with substantial crystallinity, TiCl4 (4.06 g, 21.4 mmol) was dissolved in 8.1 g of ethanol. (Caution! TiCl4 is strongly corrosive and should be handled with care.) This solution was poured into a solution of 483 mg (6.35 mmol) of 1,3-propanediol in 40.5 g of benzyl alcohol. The reaction mixture was stirred at 110 °C for 2 h, that is, at higher temperature than in the case of the synthesis described in “method 1”. After cooling to room temperature, this solution was added to 100 mL of cold diethylether. The resulting white precipitate was isolated by centrifugation, washed once with diethylether, and redispersed in methanol with a droplet of water to obtain a dispersion containing 4 wt % TiO2, as determined after a small sample of the dispersion was dried and calcined at 550 °C. XRD measurements were conducted on a small sample of this dispersion after drying for at least 3 h at 80 °C. 2.3. Preparation of Spinning Solutions. 2.3.1. Nanoparticle-Containing Solutions. The poly(ethylene oxide) (PEO) was dissolved in a suitable solvent, for example, dichloromethane (CH2Cl2), chloroform (CHCl3), or water (H2O), to give a concentrated solution of ca. 5 wt % after stirring overnight. The TiO2 nanoparticle dispersion in MeOH was then added to this solution. This mixture was diluted with pure solvents to produce a spinning solution containing 2 wt % PEO and 1 wt % TiO2; see Table 2.

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Wessel et al. employing the adsorption branch and applying the kernel of metastable adsorption isotherm based on the spherical/cylindrical pore model for the system nitrogen (T ) 77.4 K)/silica. This approach yields more realistic results for larger mesopores than the Barrett-Joyner-Halenda approach. All samples were outgassed at 100 °C under vacuum for 12 h before measurement. SEM images were collected on a Leo Gemini 982 and on a Jeol JSM-7500F after sputtering the samples with platinum. The acceleration voltage was 10 kV for the pictures presented in Figures 2, 4, 5, and 7; 5 kV in Figure S1 (Supporting Information); and 4 kV in Figures 3, 6, and S2 (Supporting Information). 3. Results and Discussion

Figure 1. XRD patterns of the synthesized TiO2 nanoparticles: (a) “crystalline” particles obtained from TiCl4 as a precursor (synthesis: 2 h at 110 °C), (b) “amorphous” particles obtained from Ti(OiPr)4 as a precursor (synthesis: 8 h at 80 °C), and (c) “amorphous” particles obtained from TiCl4 as a precursor (synthesis: 8 h at 80 °C).

2.3.2. Sol-Gel Solution. The PEO was dissolved in a suitable solvent, for example, CH2Cl2, to obtain a concentrated solution of 5.1 wt % after stirring overnight. After diluting 492 mg of this solution with 126 mg of CH2Cl2 and 591 mg of MeOH, 52.8 mg of Ti(OiPr)4 and 22.8 mg of hydrochloric acid (37 wt %) were added. (Caution! Hydrochloric acid is strongly corrosive and should be handled with care.) Before starting the spinning experiments, this mixture was aged for 80 min under air. This aged solution contained 2 wt % PEO, 4.1 wt % Ti(OiPr)4 (equals 1.2 wt % TiO2), and a CH2Cl2/MeOH solvent ratio of 1/1 (by weight). 2.4. Preparation of TiO2 Fibers. The TiO2 fibers were prepared by electrospinning of the TiO2-nanoparticle-containing polymeric spinning solution, followed by calcination. The spinning solution was loaded into a 1 mL plastic syringe and fed through a capillary tip with a diameter of 0.6 mm using a syringe pump. The metal capillary was connected to a highvoltage power supply and aluminum foil as a collector to another one with opposite polarity. The voltages were set to create an electric field of approximately 1 kV/cm, given a gap of ca. 10 cm between capillary tip and collector. This setup was placed in a chamber where the humidity could be controlled by purging with dry or humid compressed air. The as-spun polymer/TiO2 nanoparticle composite fibers were calcined at 550 °C in air to obtain pure TiO2 nanofibers. SEM samples were collected by mounting a piece of silicon wafer onto the collector. 2.5. Characterization. XRD measurements were performed on a PANalytical X’pert Pro. (Cu KR radiation, λ ) 154.18 pm) equipped with a X’Celerator counter. The nitrogen sorption measurements were carried out on a Quantachrome Autosorb 6 at 77 K. The surface area was determined via the BET method and the pore size distribution via the nonlocal density functional theory (NLDFT) approach

TiO2 fibers were prepared via electrospinning using different TiO2 sources. On the one hand, we spun a solution containing the sol-gel precursor, Ti(OiPr)4, and hydrochloric acid as a stabilizer to obtain a reference sample. On the other hand, polymeric solutions containing dispersed preformed nanoparticles of TiO2 were prepared using a modification of the approach introduced by Niederberger,15 with different degrees of crystallinity (i.e., amorphous versus highly crystalline nanoparticles; see the Experimental Methods). The obtained as-spun fibers were calcined at 550 °C to remove the polymer and all other organic residua. Depending on the chemical properties of the spinning solutions, the obtained fibers showed different morphologies, as discussed in the following. One of the major goals of the present study was to understand the underlying physicochemical principles that determine the formation of these different morphologies (e.g., dense versus porous fibers) by varying the aforementioned parameters in a systematic manner. Because the solvent composition and the spinning conditions are further important parameters influencing the structure formation process, these parameters were also investigated systematically. TiO2 nanoparticles were prepared via two preparation methods by varying the molecular precursor (TiCl4 or Ti(OiPr)4) and the reaction time and temperature (see the Experimental Methods). “Amorphous” particles were obtained after stirring for 8 h at 80 °C using either TiCl4 or Ti(OiPr)4 as a molecular precursor, as revealed by XRD; see Figure 1, patterns b and c. The XRD patterns of these particles show a broad reflection occurring in the range of 2θ ) 17-42° that indicates the predominantly amorphous character of the samples. A weak shoulder at 2θ ) 25.5° suggests the presence of a small portion of crystalline TiO2 in the anatase modification besides amorphous TiO2. As the size and crystallinity of these two kinds of “amorphous” particles are similar with respect to their XRD patterns, these particles are not further differentiated in the discussion of fiber morphology. Both kinds of particles will be denominated as “amorphous particles” in the following. When the synthesis was carried out at 110 °C for 2 h with TiCl4 as a precursor, the XRD pattern in Figure 1, pattern a, shows the typical reflections of the TiO2 anatase modification

TABLE 1: Overview of the TiO2 Particle Syntheses method

precursor

reaction time [h]

temperature [°C]

result

comment

1

Ti(OiPr)4

8

80

“amorphous” particles

particles are similar after calcination, as revealed by XRD (see Figure 1)

1 2

TiCl4 TiCl4

8 2

80 110

“amorphous” particles “crystalline” particles

crystallite size ) 3.4 nm

PEO

PEO

PEO

PEO

PEO

PEO PEO PEO PVP

2

3

4

5

6

7 8 9 10

1.0 1.0 1.0 1.0 1.0 1.0 without 1.0 1.0 3.1

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 4.9

300

300

300

300

300

300

300 600 900 360 method 2, amorphous particles method 2, amorphous particles sol-gel precursor: Ti(OiPr)4, according to Li et al.8

sol-gel precursor: Ti(OiPr)4

method 3, crystalline particles

method 3, crystalline particles

method 1, amorphous particles



Ti source/synthesis method for particles

CH2Cl2/MeOH (2/1) CH2Cl2/MeOH (1/1) CH2Cl2/MeOH (1/1) MeOH

CH2Cl2/MeOH (1/1)

CHCl3/MeOH (1/1)

CH2Cl2/MeOH (1/1)

CH2Cl2/MeOH (1/1)

CHCl3/MeOH (1/1)

H2O/MeOH (1/1)

solvent

Figures S1 and S2 are found in the Supporting Information. b n. m. ) not measured. c See Figures 8 and S3 in the Supporting Information.

PEO

1

a

polymer

sample

TiO2 concentration [wt %]

concentration [wt %]

Mw [kg/mol] rough fibers, but not porous (Figure 2) rough fibers, but not porous (Figure 5) mesoporous in a certain humidity range (Figure 4) highly mesoporous in a wide humidity range (Figure 6) highly mesoporous in a wide humidity range (Figure 7) mesoporous, but poor fiber formation (Figure S2) smooth fibers (Figure S1) not porous (not shown) not porous (not shown) not porous (not shown)

SEMa

TABLE 2: Composition of the Polymer/TiO2 Nanoparticle Solutions for Spinning Experiments and Characterization by SEM and Physisorption Analysis

n. m. n. m. n. m. 68

n. m.

164

111 (ca. 15 nm pores)

143 (ca. 20 nm pores)

n. m.

83

BET surface area [m2/g]b,c

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Figure 2. SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “amorphous” TiO2 nanoparticles in H2O/MeOH (1/1) at different humidities: (A) 61, (B) 50, (C) 41, (D) 28, (E) 21, and (F) 10% and subsequent calcination at 550 °C. Scale bars are 10 µm (A1-F1) and 500 nm (A2-F2).

at 2θ ) 25.5, 37.9, 48.1, 54.1, and 62.7°.16 Analysis of the reflection at 2θ ) 25.5° by the Scherrer equation revealed an average crystallite size of ca. 3.4 nm.16 These particles will be denominated as “crystalline” particles in the following. 3.1. Nanoparticle-Based Preparation of Electrospun TiO2 Fibers with Different Morphologies. In our first experiments, poly(ethylene oxide) (PEO) was chosen as the polymer, as it can be well-spun from various solvents, such as dichloromethane (CH2Cl2), chloroform (CHCl3), or water (H2O). A dispersion of TiO2 nanoparticles in methanol was added to a solution of

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Figure 3. SEM images of nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “amorphous” TiO2 nanoparticles in CH2Cl2/MeOH (1/1) (A) before calcination, (B) after calcination at 550 °C (low magnification), and (C, D) after calcination at 550 °C (high magnification). Scale bars are 5 µm (A, B) and 200 nm (C, D).

PEO in H2O (sample 1), in CHCl3 (sample 2), and in CH2Cl2 (sample 3). In each case, PEO with a molecular weight (Mw) of 300 kg/mol was used in a concentration of 2 wt %, to obtain a solution with a viscosity that is sufficient for electrospinning. The TiO2 content of the spinning solution was set to 1 wt %, and the solvent composition was about 50 wt % MeOH and 50 wt % of the other respective solvent. 3.1.1. Variation of the SolWent for the Polymer. It was found that a solution containing 2 wt % PEO (Mw ) 300 kg/mol) and 1 wt % amorphous TiO2 nanoparticles in H2O/MeOH (1/1) was suitable for producing TiO2 nanofibers, which remained stable in shape after removal of the polymer by calcination at 550 °C. Smooth fibers with relatively uniform diameters in the range of 50-200 nm were obtained if the spinning experiments were conducted at a relative humidity lower than approximately 30%, as can be seen in Figure 2D-F. Bead formation was observed at a relative humidity of 35%, and relative humidity levels above approximately 50% resulted in suppression of fiber formation. This finding is consistent with earlier investigations on electrospinning of aqueous solutions of PEO that show that these solutions can only be spun at a relative humidity below approximately 50% because of axisymmetrical instabilities that cause bead formation and fiber disaggregation at increased humidity.17 The calcined fibers showed a high number of cracks due to mechanical stress caused by the shrinkage of the PEO/ TiO2 composite fibers during the pyrolysis of the polymer; see Figure 3B. Prior to the thermal treatment, the fibers were free of cracks, as shown by SEM in Figure 3A. Owing to sintering of TiO2 during thermal treatment, the fibers were interconnected so that the nonwovens were mechanically relatively stable despite the cracks. It was also observed that fibers in direct contact with the silicon substrate often lost their shape and disintegrated into

particles on the substrate upon calcination. There was no collapse in the upper layers of samples with multiple layers of fibers on the substrate. This finding might be explained by taking surface energies into account: Electrospun fibers with diameters in the range of hundreds of nanometers exhibit large surface energies, which might be reduced to a minimum during the calcination process. Fibers in contact with the substrate might collapse to particles when the interaction between the TiO2 particles of the polymer/inorganic composite fibers and the substrate becomes energetically more preferable compared with the particle-particle interaction. Fibers with no contact to the substrate cannot reduce surface energy by interaction with the substrate; therefore, particle-particle interaction is the only way to reduce the surface energy, thus preserving the fibrous morphology. Interestingly, by using CH2Cl2 as a solvent (sample 3) in a certain humidity range, it was possible to produce fibers with mesopores of approximately 15 nm in diameter coupled with a substantial mesopore volume (as revealed by SEM (see Figures 3 and 4) and nitrogen sorption experiments (see Figure 8)). The TiO2 particles used in this case were the amorphous ones prepared according to method 1 (as revealed by XRD; see Figure 1). Mesoporosity was achieved when the spinning experiments were conducted at humidities of approximately 20-55%. Higher humidities impeded fiber formation, whereas rough fibers without a pronounced mesoporosity were obtained when the spinning was conducted at a humidity below 20%. Because no surfactant template was used to generate mesoporosity, the observation of a significant degree of mesoporosity was surprising. The origin and formation of this mesoporosity were then investigated by systematically changing several parameters influencing fiber generation.



Figure 4. SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “amorphous” TiO2 nanoparticles in CH2Cl2/MeOH (1/1) at different humidities: (A) 63, (B) 54, (C) 40, (D) 29, (E) 21, and (F) 15% and subsequent calcination at 550 °C. Scale bars are 10 µm (A1-F1) and 500 nm (A2-F2).

Figure 5. SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “amorphous” TiO2 nanoparticles in CHCl3/MeOH (1/1) at different humidities: (A) 65, (B) 54, (C) 45, (D) 35, (E) 23, and (F) 14% and subsequent calcination at 550 °C. Scale bars are 10 µm (A1-F1) and 500 nm (A2-F2).

First, PEO was electrospun without any TiO2 from CH2Cl2/ MeOH (weight ratio ) 2/1) at different humidities of 15, 30, 45, and 60% at ambient temperature. These experiments were carried out to study fiber formation from the pure polymer solution in the absence of TiO2 nanoparticles. None of the obtained PEO fibers showed an internal mesoporous structure, as documented by SEM in Figure S1 (Supporting Information), proving that all PEO fibers had a smooth surface. However, with increasing humidity, there was a tendency toward bead formation. This finding was similar to that of Reneker’s group

on the electrospinning of PEO from an aqueous solution (see above),17 indicating increased axisymmetric instabilities when PEO is electrospun from water at increased humidity. This finding may be generalized for electrospinning of PEO from several other solvent systems. Because the PEO fiber featured a smooth surface, it can be concluded that the mesoporous structure of the TiO2 fibers does not originate from a templating effect (in terms of a 1:1 copy of the polymeric structure) of a porous PEO fiber. Instead, microphase separation between the inorganic TiO2 particles and

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SCHEME 2: Compatibility of the Different Species Present in the Spinning Solutions Creating TiO2 Fibers with Different Morphologies

the solvent/polymer system must play a major role in the evolution of the porous structure of the inorganic fibers. Addressing this microphase separation, the solvent system of the spinning solution was changed in order to elucidate which properties of the solution were responsible for the onset of phase separation. It was not possible to produce fibers with a mesoporous morphology by replacing CH2Cl2 with the same amount of CHCl3 (sample 2; see Figure 5) or water (sample 1; see above), irrespective of the humidity during the spinning process. This finding suggested that a specific property of CH2Cl2 plays an important role in the formation of the porous fiber structure. Earlier investigations on the formation of porous structures of either electrospun polymer fibers18-21 or dip-coated thin films2,22 indicated that the low evaporation temperature of CH2Cl2 may be an essential precondition for the formation of mesopores. When CHCl3 was used as a solvent, the TiO2 fibers possessed a slightly uneven surface (see Figure 5, especially B-D) compared to the rather smooth TiO2 fibers obtained from an aqueous solution seen in Figure 2. It seems likely that the different morphologies of the TiO2 fibers arise from the different phase behavior of the abovementioned three solvent systems that are presented in Scheme 2: In aqueous solution, water is, on the one hand, the solvent for the polymer PEO, but on the other hand, it is also a good solvent to disperse the preformed TiO2 nanoparticles. By contrast, CHCl3 — and also CH2Cl2 — are only solvents for PEO, but are not able to disperse TiO2 nanoparticles. From these results, we suggest that a phase separation (e.g., spinodal decomposition) is responsible for the formation of the mesoporous structure, as illustrated in Scheme 3: At the beginning of the experiments, the spinning solutions are identical in composition (regarding weight fractions), differing only in the second solvent CH2Cl2, CHCl3, or H2O. When the jet solidifies on its way to the collector, the different spinning solutions exhibit a divergent solvent evaporation behavior. Of all solvents used, CH2Cl2 has the lowest evaporation temperature (40 °C). When CH2Cl2/MeOH is used as a solvent mixture (sample 3), evaporation of CH2Cl2 is favored; thus, depletion of this solvent in the spinning solution proceeds. When the solvent mixture CH2Cl2/MeOH reaches the point in the phase diagram where the spinning mixture becomes unstable, precipitation of PEO is the consequence. Microphase separation occurs the moment a phase boundary of the system is crossed. In view of the very rapid solvent evaporation in the jet, as also stated elsewhere,18 such demixing should more closely resemble spinodal decomposition rather than a binodal process. Such phase separation produces a microphase-separated PEO/TiO2 composite on the nanometer scale within the fiber that results in the observed mesoporous TiO2 fibers upon pyrolysis of the polymer.

SCHEME 3: Suggested Evolution of the Differently Shaped TiO2 Fibers Obtained after Calcination Starting from the Spinning Solutions with Different Solvent Systemsa

a The unstable regions for the spinning systems starting with either “crystalline” or “amorphous” TiO2 particles occur at different stages on the phase diagram.

Changing the solvent system to CHCl3/MeOH did not produce a mesoporous structure, as the evaporation temperatures of CHCl3 (61 °C) and MeOH (65 °C) differ only slightly. In this case, both solvents evaporate nearly simultaneously. Therefore, the composition of the solvent mixture remains, in first approximation, unchanged and no phase separation occurs because the unstable region of the phase diagram is not reached prior to complete solidification of the jet. The third solvent system used was H2O/MeOH. It becomes impoverished of MeOH first because H2O has a lower vapor pressure. The increasing H2O concentration in the solvent mixture dissolves the PEO and the TiO2 particles in the same manner, thus preventing phase separation in this system. In contrast to the other solvent systems, the depletion of MeOH may create a thermodynamically even more stable, homogeneous solution, resulting in rather smooth TiO2 fibers after calcination. This presumption is supported by the fact that both

Formation of Inorganic Nanofibers from TiO2 NPs PEO and TiO2 particles possess a higher compatibility to water than to the other solvents. 3.1.2. Influence of the Molecular Weight of PEO. The molecular weight of the PEO was investigated as a further parameter influencing the formation of mesoporous inorganic fibers. When the molecular weight of the PEO was increased to 600 or 900 kg/mol, while leaving all other parameters unchanged, we obtained smooth fibers without any pores from spinning of “amorphous” TiO2 particles from CH2Cl2/MeOH (1/1) (pictures not shown). This observation indicates that the increased viscosity of the spinning solution also prevented the phase separation process needed for the formation of mesoporous TiO2 fibers. It was not possible to reduce the molecular weight of the PEO, because the reduced viscosity of the solution aggravates the spinning process itself, as no continuous fibers can be produced from such inviscid solutions. 3.1.3. Influence of the TiO2 Particles. The morphology of the fibers also changed when the synthesis conditions and, hence, the crystallinity of the particles was varied. Figures 6 (sample 4) and 7 (sample 5) show SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “crystalline” TiO2 nanoparticles in CH2Cl2/ MeOH and CHCl3/MeOH (each 1/1), respectively, at different humidities and subsequent calcination at 550 °C. These experiments were analogous to the experiments mentioned above except that “crystalline” instead of “amorphous” particles were used; compare with Figures 4 and 5. It can be seen that, in the case of the CH2Cl2/MeOH solvent system, the fibers in Figure 6 had improved porosity compared with the fibers in Figure 4, as revealed by SEM. The reason for this change in the morphology must be attributable to the particle properties. The difference between the particles can be recognized in the XRD patterns of the particles in Figure 1, patterns a and b. The particles of sample 3 (see Figure 4) were almost amorphous, showing only one weak reflection that can be attributed to a small portion of crystalline TiO2 in the anatase modification. The particles used in the case of sample 4 (see Figure 6) were highly crystalline, and the XRD pattern did not indicate the presence of amorphous TiO2 (degree of crystallinity, ca. > 80%). In the case of the CHCl3/MeOH solvent system, the fibers in Figure 7, obtained from the “crystalline” particles, were mesoporous, in contrast to the fibers obtained with the same solvent system by using the “amorphous” particles, as shown in Figure 5. A possible explanation for the different porosity of the fibers could be related to the assumption that the more crystalline particles show a more pronounced microphase separation during the spinning experiment. This supposition is corroborated by the fact that these particles have a lower dispersibility/solubility than the more amorphous particles, as these crystalline particles were only dispersible in MeOH upon addition of a small quantity of water. For instance, the “amorphous” particles certainly possess a large concentration of hydroxyl groups on the surface. In the case of CHCl3/MeOH mixtures, the inferior dispersibility/ solubility of the “crystalline” particles enables the spinning solution to undergo a microphase separation, whereas the better soluble/dispersible “amorphous” particles do not undergo a microphase separation from the polymer. In terms of a phase diagram, it can be proposed that the unstable region of the spinning system containing the “crystalline” particles is shifted nearer to the starting composition of the spinning solution


Figure 6. SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “crystalline” TiO2 nanoparticles in CH2Cl2/MeOH (1/1) at different humidities: (A) 62, (B) 53, (C) 41, (D) 32, (E) 24, and (F) 10% and subsequent calcination at 550 °C. Scale bars are 10 µm (A1-F1) and 500 nm (A2-F2).

compared with the unstable region of the spinning system with the “amorphous” particle, as presented in Scheme 3. As a consequence, mesoporous fibers were obtained from the “crystalline” particles and dense fibers from the “amorphous” ones in the CHCl3/MeOH spinning system. Another advantage of the more crystalline particles was the production of well-defined porous TiO2 fibers over a wider range of humidity compared with the experiments with the more amorphous particles for both solvent systems investigated (CH2Cl2/MeOH and CHCl3/MeOH). This observation implies

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Figure 7. SEM images of TiO2 nanofibers prepared by electrospinning a solution of 2 wt % PEO with 1 wt % “crystalline” TiO2 nanoparticles in CHCl3/MeOH (1/1) at different humidities: (A) 49, (B) 44, (C) 36, (D) 31, (E) 22, and (F) 12% and subsequent calcination at 550 °C. Scale bars are 10 µm (A1-F1) and 500 nm (A2-F2).

that the phase separation process is less affected by the humidity in the spinning experiments, the more crystalline the particles are. 3.2. Preparation of Electrospun TiO2 Fibers from Sol-Gel Precursors. In light of the aforementioned finding that different nanoparticles resulted in different microstructures (mesoporosity) of the fibers, we further investigated the substitution of TiO2 nanoparticles by an analogous amount of a molecular sol-gel titania precursor compound. A solution containing 2 wt % PEO and 4.1 wt % Ti(OiPr)4 (equals 1.2 wt % TiO2) was electrospun

Wessel et al. in a solvent mixture of CH2Cl2/MeOH (1/1) after addition of some hydrochloric acid and aging of the solution for approximately 80 min in air. Although titania sol-gel solutions were previously electrospun,6,8,10,23 to the best of our knowledge, no experiments have been conducted using the above-mentioned solvent mixture. This solution was electrospun at different humidities in the range of 11-49%; see Figure S2 (Supporting Information). The obtained fibers were observed to be relatively smooth at the highest humidity of 49%. At higher humidities, the solution could not be spun at all, as no continuous fibers were accessible, probably due to the incompatibility of PEO for electrospinning at such high levels of humidity. When the relative humidity was below approximately 45%, the resulting fabrics exhibited a mesoporous structure similar to the porous fibers of sample 4; see Figure 6. One of the main disadvantages of this sol-gel approach was the inhomogeneity of fiber morphology. Whereas the sample spun at a humidity of 43% predominantly resulted in the formation of well-defined fibers, the samples spun at lower humidities merely produced porous agglomerates on the silicon wafer. Within this series of samples, a clear tendency can be observed: The lower the humidity, the more the fiber formation was impeded when a sol-gel precursor solution was used. This finding implies that the fiber formation from sol-gel precursors requires a precisely controlled humidity to ensure the proceeding of the sol-gel reaction. If the humidity was too low, the molecular precursors and some reacted agglomerates were not interconnected by covalent bonding within the PEO/ inorganic composite fiber so that the shape of the fiber degenerated during the pyrolysis of the PEO. 3.3. Characterization of the Mesoporous TiO2 Fibers. 3.3.1. Physisorption. Nitrogen physisorption was performed on different fiber samples at T ) 77 K. These experiments provided further proof of the morphology observed by SEM and, in addition, quantified the specific surface area and the mesoporosity of the samples. In Figure 8, the N2 adsorption-desorption isotherms and the nonlocal density functional theory (NLDFT) pore size distributions of some samples are summarized. Table 3 contains quantified evaluations. Further N2 adsorption-desorption isotherms and NLDFT pore size distributions as well as a more detailed discussion on all the physisorption results are available in the Supporting Information. N2 physisorption is much more feasible than, for example, transmission electron microscopy (TEM) measurements to prove the mesoporosity of the fibers because the fibers are too thick to be investigated by TEM directly without elaborate preparation steps. The disordered nature of the fiber mesoporosity is another reason why physisorption is the method of choice to characterize the porous structure of the fibers. As a reference, physisorption experiments were also conducted on the pure TiO2 nanoparticles after calcination at 550 °C in order to rule out the possibility that the mesoporosity measured by physisorption arises from the interparticular space of the sintered TiO2 nanoparticles. The isotherm of the calcined TiO2 particles in Figure 8a2 shows a type IV-like behavior with a H2-type hysteresis loop, being indicative of a broad distribution of pore sizes.24 This is corroborated by the NLDFT pore size distribution in Figure 8b2. The specific surface area of the calcined TiO2 particles was estimated to be 73 m2/g. This result revealed that some mesoporosity was present in this sample, but in contrast to the mesoporous fibers (see Figure 8a1,b1 and Table 3), the total mesopore volume was quite low (0.13 cm3/g), indicating dense



Figure 8. Physisorption experiments on different fiber samples: (a) N2 adsorption-desorption isotherms (T ) 77 K) and (b) NLDFT pore size distribution (from adsorption branch). (1) Mesoporous fibers obtained from “amorphous” TiO2 particles electrospun with PEO in CH2Cl2/MeOH (1/1) after calcination at 500 °C (sample 3). (2) As-synthesized “crystalline” TiO2 particles calcined at 550 °C (sample NP). (3) Fibers obtained from “amorphous” TiO2 particles electrospun with PEO in H2O/MeOH (1/1) after calcination at 500 °C (sample 1).

TABLE 3: Overview of the Results from the Physisorption Experiments on Different TiO2 Samples

sample

calcination temperature [°C]

BET surface area [m2/g]

total pore volume [cm3/g] (up to a pore size of) [nm]

maximum of NLDFT pore size distribution [nm]

figurea

SEM figureb

NP NP 1 3 3 4 5 10

100 550 500 500 750 550 550 500

321 73 83 143 50 111 164 68

0.19 (52) 0.13 (51) 0.12 (48) 0.45 (38) 0.07 (54) 0.31 (46) 0.36 (50) 0.09 (56)

3 10 5 20 3 15 13 5

S3a1,b1 8a2,b2 + S3a2,b2 8a3,b3 + S3a6,b6 8a1,b1 + S3a3,b3 S3a8,b8 S3a4,b4 S3a5,b5 S3a7,b7

n. m. n. m. 2 4 n. m. 6 7 n. m.

a

Figure S3 is found in the Supporting Information. b n. m. ) not measured.

structures. The NLDFT pore size distribution of the calcined particles proved the presence of some small mesopores with diameters in the range of 7-15 nm, but the surface generated by these mesopores was not overly large, as can be concluded from the relatively small value for the BET surface area. The mesoporous fibers obtained from the CH2Cl2/MeOH spinning solution (sample 3; see Figure 8a1,b1) exhibited a sustainable large value for the total pore volume (0.45 cm3/g), which suggests the presence of additional internal surfaces, generated by the above-mentioned microphase demixing process. The phase separation leads to mesoporous fibers with relatively broad pore size distributions in the range of 10-30 nm. The NLDFT pore size distributions of the mesoporous fibers corroborated the aforementioned SEM analyses. The presence of a larger amount of big mesopores results in an improved BET surface area of 143 m2/g for the mesoporous fibers. Evidently, such large mesoporosity and also the size of the mesopores cannot be attributed to the replication of single polymer chains, but must be ascribed to a microphase separation (i.e., partial immiscibility) on the nanometer scale between the nanoparticles and the polymer. The fact that the mesoporosity of the electrospun fibers is not attributable to the replication of single polymer chains was proved by the N2 physisorption results on another fiber sample obtained from the H2O/MeOH spinning solution system (sample 1; see Figure 8a3,b3) that possesses no mesoporosity: These fibers had a specific surface area (83 m2/g) comparable to that of the pure calcined particles (see Table 3 and Figure 8a2,b2). The small total pore volume of this sample (0.12 cm3/g) and the NLDFT pore size

distribution indicate dense structures but do not show any hint of mesoporosity. As the polymer concentration of PEO in samples 1 and 3 was constant, but the resulting morphologies of the fibers differ from quite dense to mesoporous, it can be stated that a high polymer concentration in the spinning solution containing some TiO2 particles alone will not necessarily produce porous TiO2 fibers after calcination. 3.3.2. XRD Analysis on the Thermal EWolution of the Mesoporous Fibers. XRD measurements were performed on the porous TiO2 fibers obtained from the CH2Cl2/MeOH solvent system (sample 3) following calcination at 550 °C for 1 h, 700 °C for 1 h, and 750 °C for 2 h. The corresponding XRD patterns are shown in Figure 9. The reflections at 2θ ) 25.5, 37.9, 48.1, 54.1, and 62.7° demonstrate that anatase was formed in all cases.16 Using the Scherrer equation, the average crystallite size was estimated to be approximately 7 nm (10, 14 nm) after calcination at 550 °C (700, 750 °C). Such Ostwald ripening leads to a decrease in the specific surface area of the fibers, as revealed by nitrogen physisorption performed on the fibers calcined at 750 °C; see Figure S3 (Supporting Information). This sample showed a specific surface area of just 50 m2/g, which was significantly lower than the value for the same fibers calcined at 550 °C (111 m2/g). On the other hand, this surface area was in the same range as the surface area of the fibers obtained from molecular precursors after calcination at 500 °C (sample 10, 68 m2/g; see Figure S3 (Supporting Information)). From these results, one can conclude that the mesoporous fibers possess an improved thermal stability compared with sol-gel fibers, which showed a collapse of the surface area after thermal treatment at temperatures above 500 °C.

372


Wessel et al. metal oxides requiring a large surface area in the form of selfstanding materials combined with low back pressure for any throughput of gases or liquids. Acknowledgment. Prof. Greiner and Prof. Wendorff are thanked for the usage of the equipment at University of Marburg. We thank Gwyneth Schulz for carefully revising the manuscript regarding the English language. R. Ostermann gratefully acknowledges the German National Merit Foundation for a PhD scholarship. Supporting Information Available: Additional SEM figures, N2 physisorption data, and NLDFT pore size distributions on different fiber samples and more detailed discussions of the physisorption results. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. XRD patterns of the porous TiO2 fibers (sample 3) after calcination at (a) 550, (b) 700, and (c) 750 °C.

4. Conclusion Highly mesoporous anatase nanofibers were prepared via electrospinning of preformed TiO2 particles in a solution of PEO in CH2Cl2/MeOH (1/1) and CHCl3/MeOH (1/1) without the use of any surfactant. A systematic study of several parameters influencing the fiber formation, for example, solvent composition, molecular weight of the polymer, TiO2 source, and humidity, was conducted to investigate the underlying physicochemical principles that determine the formation of this porous structure. The results suggest that the formation of mesoporosity is a consequence of a microphase separation (e.g., a spinodal microphase demixing) between a polymer-rich phase and a phase containing the inorganic TiO2 particles during the solidification of the jet in the electrospinning process. Furthermore, it was shown that the nature of the TiO2 particles has a substantial impact on the evolution of the fiber morphology. Our study systematically addressed the influence of various parameters on this microphase separation, in particular, the solvent and the nanoparticle properties. Comparative SEM investigations and N2 physisorption experiments on different electrospun fibers revealed that the evolution of porous TiO2 fibers is neither a consequence of the general loss of large amounts of polymer during the calcination of the composite polymer/inorganic fibers nor a consequence of interparticular space between the sintered TiO2 nanoparticles. The obtained fibers possessed a large specific surface area of up to 164 m2/g after calcination at 550 °C and pores with a diameter in the range of 10-30 nm. Our study thus demonstrated that the usage of preformed oxidic nanoparticles represents a promising conceptual methodology for generating fibers featuring intrinsic mesoporosity. Because electrospinning allows production of nonwoven mats, the obtained material might be of interest for applications of

References and Notes (1) Brinker, C. J.; Lu, Y.; Sellinger, A. AdV. Mater. 1999, 11, 579– 585. (2) Grosso, D.; Cagnol, F.; Soler-Illia, G. J.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. AdV. Funct. Mater. 2004, 14, 309–322. (3) Zhou, Y.; Antonietti, M. Chem. Commun. 2003, 2564–2565. (4) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244–2246. (5) Lebeau, B.; Fowler, C. E.; Mann, S.; Farcet, C.; Carleux, B.; Sanchez, C. J. Mater. Chem. 2000, 10, 2105–2108. (6) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151–1170. (7) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670– 5703. (8) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555–560. (9) Macı´as, M.; Chacko, A.; Ferraris, J. P.; Balkus, K. J. Microporous Mesoporous Mater. 2005, 86, 1–13. (10) Madhugiri, S.; Sun, B.; Smirniotis, P. G.; Ferraris, J. P.; Balkus, K. J. Microporous Mesoporous Mater. 2004, 69, 77–83. (11) Zhan, S.; Chen, D.; Jiao, X.; Tao, C. J. Phys. Chem. B 2006, 110, 11199–11204. (12) Kanehata, M.; Ding, B.; Shiratori, S. Nanotechnology 2007, 18, 315602. (13) Dayal, P.; Kyu, T. J. Appl. Phys. 2006, 100, 043512. (14) Qi, Z.; Yu, H.; Chen, Y.; Zhu, M. Mater. Lett. 2009, 63, 415–418. (15) Polleux, J.; Pinna, N.; Antonietti, M.; Hess, C.; Wild, U.; Schlo¨gl, R.; Niederberger, M. Chem.sEur. J. 2005, 11, 3541–3551. (16) Trung, T.; Ha, C.-S. Mater. Sci. Eng., C 2004, 24, 19–22. (17) Tripatanasuwan, S.; Zhong, Z.; Reneker, D. H. Polymer 2007, 48, 5742–5746. (18) Bognitzki, M.; Czado, W.; Frese, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2001, 13, 70–72. (19) Casper, C. L.; Stephens, J. S.; Tassi, N. G.; Chase, D. B.; Rabolt, J. F. Macromolecules 2004, 37, 573–578. (20) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Macromolecules 2002, 35, 8456–8466. (21) Bognitzki, M.; Frese, T.; Steinhart, M.; Greiner, A.; Wendorff, J. H. Polym. Eng. Sci. 2001, 41, 982–989. (22) Gao, C.-Y.; Li, A.; Feng, L.-X.; Yi, X.-S.; Shen, J.-C. Polym. Int. 2000, 49, 323–328. (23) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446–2461. (24) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619.

JP108202B

Formation of Inorganic Nanofibers from Preformed TiO2 Nanoparticles

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