Programmable Structure Control in Cigarlike TiO2 Nanofibers and UV

Jul 20, 2016 - Programmable Structure Control in Cigarlike TiO2 Nanofibers and. UV-Light Photocatalysis Performance of Resultant Fabrics. Wenjia Sheng...
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Programmable Structure Control in Cigarlike TiO2 Nanofibers and UV-Light Photocatalysis Performance of Resultant Fabrics Wenjia Sheng,† Jingxin Zhao,† Zhouli Chen,† Quanlin Ye,‡ Xuxin Yang,‡ Keke Huang,§ Changmin Hou,§ Jichun You,*,† and Yongjin Li*,† †

College of Material, Chemistry and Chemical Engineering and ‡Department of Physics, Hangzhou Normal University, No. 16 Xuelin Road, Xiasha High-tech Zone, Hangzhou 310036, People’s Republic of China § State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: Novel cigarlike nanofibers with an outer-shell and innercontinuous-pore structure and resultant fabrics have been fabricated by coupling the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) containing titanium precursors with the electrospinning technique in our previous work [You et al. ACS Appl. Mater. Interfaces 2013, 5, 2278]. In the current work, the structure control in these nanofibers has been investigated in detail using scanning electron microscopy, focused ion beam, and small angle Xray scattering. Our results indicate that electrospinning conditions, the adopted solvent, the volume fraction of PS-b-PEO block copolymer, and the amount of titanium tetraisopropoxide in the mixture produce significant effects on both outer-shell and inner-continuous structures in the nanofibers. The parameters discussed above make it possible to achieve programmable structure control in the aspect of the diameter, thickness of the outer shell, and inner continuous pore. As a result, both micropores among fibers and nanopores in certain fibers are under their control. Furthermore, the photocatalytic activity of resultant TiO2 fabrics was investigated by taking the photodegradation of Rhodamine B as an example. The results suggest that the degradation efficiency and rate constant exhibit sensitivity on the structure of nanofibers. “evaporation-induced self-assembly (EISA)”.13 In these reported approaches, however, it is still a big challenge to “programmably” fabricate and control the connected hierarchically porous structures in different scales, although these structures correspond to higher efficiency in photocatalytic activity because of the higher surface area and effective contact with the reactant.14 In our previous work, the cigarlike nanofibers and resultant fabrics were fabricated by coupling the self-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) containing the precursors of TiO2 (titanium tetraisopropoxide, TTIP) with the electrospinning technique. During the fabrication of this structure, the interaction between PEO block and TTIP, validated by means of differential scanning calorimetry (DSC),15 acts as the base for the selective distribution of TTIP and resultant TiO2. In these fibers, there are outer-shell and inner-continuous-pore structures. This result can serve as the starting point for preparing three-dimensional connected hierarchically porous structures. Especially this strategy makes it possible to adjust the structure in two scales: On one hand, the

1. INTRODUCTION Titanium dioxide with high specific surface area has been widely employed in many fields involving photonic crystals, sensors, power sources, catalysts, and various other applications.1−5 The structure of porous TiO2 can produce an obvious effect on its photocatalytic activity. Therefore, it is significant to control its grain size, morphology, crystallization behaviors, resultant surface area, and the porosity.6−8 There are two major approaches to preparing porous materials: “top-down” is wellknown as the templating method, whereas “bottom-up” has the advantage of self-assembly.9 As a typical example of top-down preparation, colloidal crystal templating (CCT) is an excellent method to fabricate three-dimensionally porous materials by means of precursor infiltration followed by template removal.10 Cao and co-workers controlled the morphologies of the TiO2 hollow nanofibers by changing the heating rate, tetrabutyl titanate (the precursor of TiO2) amount, and the solvent composition in the combination of electrospinning technique and sol−gel process.11 Kumar et al. achieved tunable TiO2 nanostructures including one-dimensional regular fibers, hollow tubes, porous rods, and spindles from electrospun composite fibers by controlled annealing.12 By contrast, Brinker et al. obtained various porous TiO2 films with the help of selfassembly of a block copolymer containing a precursor of inorganic oxide at the liquid−air interface, which is called © 2016 American Chemical Society

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April 1, 2016 June 16, 2016 July 20, 2016 July 20, 2016 DOI: 10.1021/acs.iecr.6b01240 Ind. Eng. Chem. Res. 2016, 55, 8292−8298

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Industrial & Engineering Chemistry Research

tometer (UV-1201) and estimated from the absorbance at its characteristic wavelength (λ = 665 nm).

electrospinning serves as the template (top-down) for generating fibers with diameters ranging from the submicrometer to the micrometer scale.16 Micropores among these fibers can be controlled through electrospinning parameters (e.g., the viscosity of mixture solution, the adopted voltage). On the other hand, the self-assembly of PS-b-PEO (bottom-up) leads to the formation of nanopores (on the order of several tens of nanometers), which were under the control of microphase separation.17 In this work, therefore, the achievements of programmable structure control in two scales in the TiO2 fabrics were investigated by employing various parameters of electrospinning (e.g., the adopted solvent of the solution and the content of titanium tetraisopropoxide, TTIP%) and microphase separation (e.g., the volume fraction of PS-bPEO). Moreover, the photocatalysis activity of the TiO2 fabrics has been studied by taking the photodegradation of Rhodamine B (RhB) as an example.

3. RESULTS AND DISCUSSION 3.1. Structure Control in Nanofibers. The diameter of prepared fibers plays an important role in the hierarchically porous materials. For one thing, micropores resulting from electrospun fibers are sensitive to the diameter;15 for another thing, self-assembly in confined size is response for the formation of nanopores in certain fibers.18 According to the reported results, the diameter of electrospun nanofibers can be controlled by means of electrospinning parameters including the distance between the collector and the needle, the concentration of the solution, the diameter of the needle, and the adopted voltage. For convenience, in this work we prepared nanofibers with various diameters by adjusting the first parameter (i.e., the distance). It is easy to understand that longer distance can result in thinner fibers due to the enhanced polarization upon the static electric field, which has been validated in the results from Scarlet et al.19 The structures of fiber surface and cross section are shown in Figure 1 and Figure

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedure. The procedure used to fabricate TiO2 nanofibers can be described as follows. Polystyrene-blockpoly(ethylene oxide) (Mw ∼ 59000-b-31000, polydispersity index (PDI) = 1.05; Mw ∼ 190000-b-60000, PDI = 1.07; Mw ∼ 38000-b-102000, PDI = 1.1; purchased from Aldrich) and titanium tetraisopropoxide (TTIP) were mixed at 1/2, 1/1, and 1/0.5 (weight ratio) in chloroform or N,N-dimethylformamide (0.3 g/mL) and stirred for 10 h. The PS, PEO, and TTIP blend solution was prepared by mixing PS (Mw = 3.5 × 104 g/mol, PDI ∼ 1.10), PEO (Mw = 3.5 × 104 g/mol, PDI ∼ 1.09), and TTIP with the weight ratio of 1/1/1. The polymer solution (1 mL) was electrospun from a 5 mL syringe at a feeding rate of 0.4 mL/h driven by a syringe pump (KDS 200, KD Scientific, USA). A voltage of 12−15 kV was applied to the needle. The as-spun fibers were collected by a collector, i.e., a copper-plate electrode covered with a polished silicon wafer, which was placed 15 cm from the needle. As-spun fibers were annealed at 120 °C for 48 h in a vacuum, followed by calcination at 450 °C for 2 h to remove the polymer. 2.2. Characterization. A Hitachi S-4800 field emission scanning electron microscope (FESEM) was used for morphology measurements at an accelerating voltage of 5.0 kV. A HELIOS NanoLab 600i (FEI, USA) focused ion beam (FIB) that provides a 30 keV Ga+ ion beam with intensities ranging from 1 to 30 pA was adopted to “cut” and scan the profile and the surface of fibers. In situ small-angle X-ray scattering (SAXS) measurement was performed at the BW4 beamline of HASYLAB (DESY, Hamburg, Germany), using a monochromatic X-ray beam with an energy of 8.979 keV. The distance between the sample and the detector was 3267 mm. The samples were annealed and calcined in the vacuum (10−3 Pa) for 6 h (at 120 °C) and 3 h (at 400 °C), respectively. During this process, in situ X-ray scattering intensity patterns were acquired by a two-dimensional detector array (2048 × 2048 pixels). 2.3. Evaluation of Photocatalytic Activity. To assess the photocatalytic activity of TiO2 fabrics from nanofibers, the photocatalytic degradation of Rhodamine B (RhB) under UV irradiation was investigated. In our experiment, 5 mg of TiO2 fabrics was dispersed in 25 mL of a 1.25 × 10−5 mol·L−1 RhB aqueous solution. After stirring in the dark for 60 min, the mixture was irradiated with UV light (the wavelength peak at 365 nm) upon continuous stirring. The concentration of RhB during degradation was monitored on a UV−vis spectropho-

Figure 1. SEM images of TiO2 fibers with diameters of 180 (A), 320 (B), 550 (C), and 1300 nm (D). Scale bars are 200 nm in all images.

2, respectively. It is obvious that the structure of the prepared TiO2 nanofibers depends significantly on their diameters. Thinner fibers (Figure 1A,B) exhibit a more pronounced surface roughness than those with a higher diameter (Figure 1C,D). It is reasonable to attribute the formation of wrinkled surface and small holes on it to the surface density fluctuations and the superposition of them at certain positions in various directions during the solvent evaporation in the electrospinning and thermal annealing.20,21 Figure 2 provides the images of the cross section of a broken fiber (caused by either an incident or a focused ion beam, FIB). It is worth noticing that there are three distinct diameter regions. In region one (150−1000 nm), there are bicontinuous structures in the fibers, as shown in Figure 2A; in region three (>4000 nm), compact nanoparticles with an average diameter of ∼85 nm in spindle-shaped beads (produced with a shorter distance between the needle and the collector during electrospinning) are observed. The result is consistent with not only the encapsulated silicon oxycarbide 8293

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structure in the cigar. It is reasonable to explain the results as follows: Under the influence of electrostatic field, a pendant droplet of the precursor solution at the capillary tip was deformed into a conical shape (Taylor cone) during the electrospinning process. At 15 kV, the electrostatic forces overcame the surface tension of the polymer blend, producing some TTIP/PS-b-PEO composite nanofibers with the help of evaporation of the solvent.23,24 In this process, the evaporation rate of the solvent plays a key role in the formation of the fibers. Therefore, the good (in the case of chloroform) and poor (in the case of DMF) evaporation rates are responsible for the nice cigarlike nanofibers (Figure 3C) and microclusters (Figure 3A), respectively. Of course, there must be a transitional state, i.e., fibers containing nanospheres in Figure 3B in the case of mixed solvent. During the combination of the self-assembly of polystyreneblock-poly(ethylene oxide) (PS-b-PEO) containing titanium precursors with the electrospinning technique, the microphase separation of PS-b-PEO serves as the soft template for the TiO2 precursor and the formation of consequent production during the electrospinning and calcination process. According to the reported results,25 the structure of microphase separation is mainly controlled by the component ratio of two blocks. Therefore, we tried to investigate the volume fraction (of PS-bPEO) dependence of resultant TiO2 fibers. Figure 4 shows the SEM images of the fiber profile along the radial or axial direction fabricated with different PS-b-PEOs. Block copolymers with the molecular weights of 38000-b-102000 and 59000-b-31000 lead to the TiO2 fibers with an inner bicontinuous structure (including continuous TiO2 and continuous pores) and a compact shell around the fibers (Figure 4A1,A2,B1,B2), which has been discussed elsewhere.15 In the case of PS-b-PEO with a molecular weight of 190000-b60000, we can obtain the inner-fiber (along the fiber) and outer-shell structure (Figure 4A3,B3), named as “double fiber”. During electrospinning, the polymer was stretched intensely by the high voltage of 15 kV, which was accompanied by the microphase separation of the PS block and the PEO block together with TTIP since the PEO block shows better interaction with it in the mixed system. Probably, the smaller fiber (with the diameter of ∼40 nm) comes from the combination of the perforated layer (HPL) structure of microphase separation and the stretch.26 At the same time, PEO block and TTIP move to the fiber surface due to the lower surface energy,15 resulting in their enrichment on the fiber surface. After calcination, both PS and PEO blocks are removed while TTIP is transferred into TiO2, leading to the formation of double-fiber and outer-shell structures. To detect the effect of the amount of TTIP on the structure of TiO2 fiber, various weight ratios of PS-b-PEO to TTIP were

Figure 2. SEM images (in the direction of cross section) of TiO2 fibers with diameters of 400 (A), 1800 (B), and 5600 nm (C). Scale bars are 200 (A), 1000 (B), and 500 nm (C).

nanoparticles in fibers of a similar diameter reported by Lu et al.,22 but also the structures in blend film composed by block copolymer and TTIP (shown as Figure S1 in the Supporting Information). In region two (1000−4000 nm), a large number of nanoparticles attached on a continuous TiO2 frame in the fiber are obtained (Figure 2B), indicating the coexistence of the gyroid phase and the particles. Therefore, transitions discussed above make it clear that the diameter produces an important effect on the nanopore (in certain electrospun fibers) as well as micropores (among fibers). To investigate the solvent dependence on the morphology of TiO2 nanofibers, we synthesized TiO2 nanofibers using various solvents including chloroform, N,N-dimethylformamide (DMF), and their mixture (DMF and chloroform are miscible). In all experiments, the total volume of the solvent is kept constant (1 mL). Figure 3 shows the SEM images of the TiO2 nanofibers. As described in our previous work, novel cigarlike fibers with smooth outer-shell and inner-continuous-pore structure can be achieved if neat chloroform was adopted (Figure 3C). In the case of neat DMF, we cannot get any fibers but we get some “TiO2 microclusters” upon the same electrospinning condition (Figure 3A). However, fibers electrospun from the chloroform/DMF mixture with an equal volume ratio exhibit distinct structures from the “cigar”. For one thing, there is no continuous nanopore or continuous TiO2 frame, but some “nanospheres” attached to each other. For another thing, wrinkled surfaces take the place of the smooth outer-shell

Figure 3. SEM images of TiO2 fibers with different weight ratios of CHCl3/DMF: (A) 0/1, (B) 0.5/0.5, and (C) 1/0. Scale bars are 2 μm (A), 300 nm (B), and 400 nm (C). 8294

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Figure 4. SEM images of TiO2 fibers with different volume fractions of PS-b-PEO. (A) 38 000−102 000, (B) 59 000−31 000, and (C) 190 000− 60 000. Scale bars are 500 nm (A1, B1, A3), 300 nm (A2, B2), and 400 nm (B3).

Figure 5. SEM images of TiO2 fibers with different weight ratios of PS-b-PEO/TTIP: (1) 1/0.5, (2) 1/2, and (3) 1/4. Scale bars are 300 (A1), 200 (B1), and 400 nm (A2, B2, A3, and B3).

compact in the case of higher TTIP fraction while there are small pores on the wrinkled surface with lower TTIP content. Third, the thickness of the outer shell is very sensitive to the weight fraction of TTIP in the mixture. Moreover, in situ SAXS experiments were performed to track the structure evolution during thermal annealing and calcination.22,23 The results are displayed in Figure 6. There is only one peak at q = 0.156 nm−1 (black line, as-spun) in Figure 6A, which does not move after annealing at 140 °C for 6 h (red line). Upon calcination at 400 °C for 3 h, the peak moves to q = 0.230 nm−1 (green line). Figure 6B shows the in situ SAXS result with the weight ratio of 1/2 (PS-b-PEO to

employed, that is, 1/0.5, 1/2, and 1/4. In Figure 5A1,B1, SEM images with the ratio of 1/0.5, we cannot find the outer shells, but only bicontinuous structures. In Figure 5A2,B2 (with the weight ratio of 1/2), typical cigarlike nanofibers containing outer-shell and inner-bicontinuous structures were obtained. This result has good agreement with Figure 2A and Figure 3C. Upon further increase of TTIP in the mixtures, the outer shell becomes more smooth and thick while the inner structures coalesce. According to the discussion above, three points are obvious. First, we cannot find the obvious differences in the inner-bicontinuous structure in SEM images upon various TTIP contents. Second, the fiber surface is more smooth and 8295

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Figure 6. SAXS profiles for different weight ratios of PS-b-PEO/TTIP: (A) 1:0.5, (B) 1:2, and (C) 1:4. SAXS profiles for annealing at 120 °C for 6 h and calcining at 400 °C for 3 h.

TTIP). The only peak at q = 0.127 nm−1 remains stable and moves to q = 0.177 nm−1 when the fibers are annealed at 140 °C for 6 h (red line) and calcined at 400 °C for 3 h, respectively. With further increase of the weight fraction of TTIP, no obvious peak could be seen in Figure 6C. According to the discussion on SEM and SAXS results, we can get the following points. First, annealing at 140 °C produces no effect on the structure evolution since the characteristic peak remains stable in Figure 6A,B. Second, calcination at 400 °C for 3 h corresponds to the remarkable decrease of structure size. This is the reason for the movement of q to the higher direction in SAXS results. Third, the original value of q = 0.156 nm−1 (black and red lines) in Figure 6A is higher than that in Figure 6B (q = 0.127 nm−1), representing the smaller structure size with lower TTIP fraction. After preparation, the mixed solution of PS-b-PEO and TTIP was fed by the syringe pump. The droplet was transferred into a conical shape upon a static electric field, and some PS-b-PEO and TTIP composite fibers could be obtained. During this process, TTIP shows better interaction with the PEO block,26 producing two kinds of effect on our system: the formation of the complex structure (between TTIP and the PEO block) and the higher PEO composition fraction (relative to the neat PS-bPEO).27,28 The former is response for the stable structure upon annealing at 140 °C for 6 h. As a result, the peaks at q = 0.156 nm−1 in Figure 6A and q = 0.127 nm−1 in Figure 6B do not move. On the contrary, the latter leads to the increase of the microphase separation size, which is the reason for the lower peak in Figure 6B relative to Figure 6A.29 In the case of the highest weight fraction of TTIP (Figure 5A3,B3 and Figure 6C), the excess TTIP moves to the surface to form a thicker outer shell, accompanied by further increase of the innerstructure size. Upon calcination at 400 °C, the block copolymer was degraded completely and TTIP was transferred to TiO2. In this process, the weight loss is very high, up to 85% (data is shown in Figure S3 in the Supporting Information), corresponding to the remarkable shrinkage of the production.15

This is the reason for the movement of the characteristic wavenumber (q) in the higher direction (green lines in Figure 6). 3.2. Photocatalytic Performance of Resultant TiO2 Fabrics. With the help of XRD measurement, it has been confirmed that all TiO2 is in the anatase phase since there are strong diffraction peaks at 25, 38, and 48° (data shown in Figure S4 in the Supporting Information). The photocatalytic behaviors of the resultant crystals were investigated by monitoring the photocatalytic degradation of Rhodamine B (RhB), taking the samples prepared using different block fractions in the copolymer.30−32 Figure 7A shows the photocatalytic degradation efficiency of TiO2 fabrics prepared using different block copolymers and polymer blends, as well as the results without the photocatalyst. The TiO2 fabrics prepared using the block copolymer with the molecular weights of 190k−60k (labeled as sample-190−60, sic passim) exhibit the highest degradation activity, and almost all the RhB dye can be degraded within 50 min under UV irradiation. Furthermore, all TiO2 fabrics prepared by means of block copolymer as the structure agent show much higher activities than that from PS/ PEO blend (the morphology of these nanofibers is shown in Figure S2). The photodegradation conversion was enhanced from 77.43% (for sample-blend) to 99.6% (for sample-190−60) within 40 min irritation time. It has been confirmed that the photocatalytic degradation of RhB follows the Langmuir− Hinshelwood first-order kinetic model. The rate constant (k) can be calculated for the photocatalytic degradation of RhB under UV irradiation based on the equation of ln(C/C0) = −kappt, where kapp and t represent the rate constant and irradiation time (min), respectively. The kinetic plots of our samples are expressed in Figure 7B. The rate constant kapp of sample-190−60 reaches 83 × 10−3 min−1, which is 3 times that of sample-blend (28 × 10−3 min−1). Sample-59−31 and sample-38−102 have similar kapp values.The difference of photocatalytic activity comes from various structures. For instance, the double-fiber structures and resultant direct 8296

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Y.). *E-mail: [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51173036 and 21234007).

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Figure 7. (A) Photocatalytic degradation of RhB on as-prepared samples with different block ratios between PS and PEO. (B) Apparent first order linear transform ln(C/C0) = −kt of RhB degradation kinetic plots; C0 and C are the initial RhB concentration and concentration at irradiation time t (min), respectively.

electron transform are responsible for the highest degradation activity in sample-190−60. By contrast, the lower porosity and bigger pores are the reason for the poor performance in sampleblend.

4. CONCLUSION In this work, the electrospinning condition, the adopted solvent, the block fraction in copolymers, and TTIP% dependences of cigarlike structures including outer-shell and inner-bicontinuous structures have been investigated systematically. For one thing, the nanopores in certain fibers are sensitive to the inner structures of the cigarlike fibers, which is under the control of the volume fraction of the copolymer and the fiber diameter. For another thing, the micropores among fibers are sensitive to the electrospinning condition (e.g., the distance between the needle and the collector as well as the adopted voltage) and the solvent. These parameters make it possible for us to control the structure in two scales. Furthermore, the photodegradation activity of resultant TiO2 fabrics has been studied by taking RhB as an example. The results indicate that the nonwoven fabrics with “double fibers” exhibit the highest photocatalytic activity due to the direct electron transform resulting from the “double-fiber” structures.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01240. Figures S1−S4 and brief experimental section (PDF) 8297

DOI: 10.1021/acs.iecr.6b01240 Ind. Eng. Chem. Res. 2016, 55, 8292−8298

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DOI: 10.1021/acs.iecr.6b01240 Ind. Eng. Chem. Res. 2016, 55, 8292−8298