Preparation of TiO2 Nanofibrous Membranes with Hierarchical

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Preparation of TiO Nanofibrous Membranes with Hierarchical Porosity for Efficient Photocatalytic Degradation Jin Zhang, Yi Bing Cai, Xuebin Hou, Huimin Zhou, Hui Qiao, and Qufu Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00555 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Preparation of TiO2 Nanofibrous Membranes with Hierarchical Porosity for Efficient Photocatalytic Degradation

Jin Zhang,a,b Yibing Cai,a,b Xuebin Hou,a,b Huimin Zhou,a,b Hui Qiao,a* Qufu Weia*

a

Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China b

College of Textile and Clothing, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China

E-mail: [email protected]

*Corresponding Author Qufu Wei, Ph.D. Tel.: (+86) 510-85913653, Fax: (+86) 510-85913100, E-mail: [email protected]

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: In order to solve the complex spinneret and unstable structure of multifludic electrospinning and side-by-side electrospinning, TiO2 nanofibers with hierarchical porosity were fabricated via microemulsion electrospinning, which can be realized just by single-spinneret electrospinning. The morphology, crystal and specific surface area of the TiO2 nanofibers were investigated by scanning electron microscopy (SEM), X-ray Diffraction (XRD) and physisorption analyzer. The photocatalytic activities of TiO2 nanofibers calcined at 500, 700 and 900 °C were examined. The experimental results indicated that TiO2 nanofibers were hollow, and the wall of nanofibers possessed abundant mesopores. The specific surface area of TiO2 nanofibers calcined at 500 °C was approximately 41.4 m2/g, which was favorable for increasing the photocatalytic reaction sites and the separation efficiency of electron-holes. The photocatalytic results demonstrated that compared with solid TiO2 nanofibers, the photocatalytic activities of the TiO2 nanofibers prepared via microemulsion electrospinning were significantly enhanced. Particularly, the TiO2 nanofibers calcined at 500 °C could decompose methyl blue (MB) solution completely within 70 min. The results verified that microemulsion electrospinning was indeed a simple, versatile and convenient method to prepare porous inorganic nanofibers for various applications.

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INTRODUCTION A serious water pollution problem has aroused immense attention in modern society due to excessive sewage discharge from dye and textile industries1-3. A variety of methods including biological degradation 4, filtration 5, precipitation 6, chemical oxidation and adsorption have been investigated widely to tackle organic pollutants from wastewater

7-8

. Photocatalysis, as a cost-effective “green” energy, demonstrated

great potential in solar energy conversion and environmental remediation9-12. TiO2 is considered to be one of the most promising photocatalysts due to its nontoxicity, low cost as well as the long-term stability against chemical- and photo-corrosion13-17. However, there still remains many challenges for decreasing the recombination rate of holes and electrons as well as increasing the solar energy conversion18-22. Fabrication of nanosized TiO2 materials with high porosity is one of the effective methods to overcome those shortcomings due to the larger adsorption capacity and higher specific surface area compared with bulk counterparts 23-24. In the past few decades, electrospinning has become a versatile and straightforward method to prepare multichannel and hollow nanofibers with high porosity and large specific surface area25-29. Xing et al.

30

fabricated TiO2 nanotubes

with electrospun carbon nanofibers as template. The method was time-consuming and intricate. Chang et al.

31

investigated the photocatalytic activities of hollow TiO2

nanofibers prepared by coaxial electrospinning. The spinneret for coaxial electrospinning consists of two concentric needles for outer tube and inner capillary to keep the separation of core and shell liquid, and coaxial electrospinning requires the 3



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coaxial feeding of core and shell liquid simultaneously to form core-shell nanofibers, subsequently, the core is removed via calcination to obtain hollow nanofibers. It is complicated and difficult to fabricate hollow nanofibers via coaxial electrospinning. Zhao et al.

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successfully prepared TiO2 nanofibers with interior hollow channels via

multifluidic electrospinning method for photocatalytic degradation of gaseous acetaldehyde. Multifluidic electrospinning is similar to coaxial electrospinning. This method requires complicated design of spinneret and precursor solutions, and the nanostructures are easily affected by humidity and temperature, thus inducing unstable nanostructures. Hence, it is of great significance to develop a convenient and versatile method to fabricate TiO2 nanofibers with hierarchical porosity. In this work, TiO2 nanofibers with hierarchical porosity were fabricated by microemulsion electrospinning for photocatalytic degradation of methyl blue and phenol. This method eliminates the intricate spinneret and unstable structure compared with the above methods. More interestingly, the inner structure of nanofibers can be tailored easily by changing the ratio of continuous phase and oil phase 32. And TiO2 nanofibers fabricated with microemulsion electrospinning can be obtained just by electrospinning and calcination, which is time-saving, simple, versatile and cost-effectiveness

33

.

Nevertheless, there were few research about the investigation of microemulsion electrospinning.

2. EXPERIMENTAL 2.1 Materials. Cetyl trimethyl ammonium bromide (CTAB), absolute ethanol, 4


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acetic acid, paraffin oil and tetrabutyl titanate (TBT) were purchased from Shanghai Chemical Reagents Co. (Shanghai, China). Moreover, polyvinylpyrrolidone (PVP) was obtained from Tianjin Bodi Chemical Reagent Co., Ltd. (Tianjin, China). All the chemical reagents in this research were used directly without further purification. 2.2 Preparation of Spin Dopes. 0.5 g PVP was first dissolved in a mixed solution of 0.3 g acetic acid and 6.5 g absolute ethanol, and then 0.3 g CTAB was added slowly with stirring until dissolving completely. Subsequently, 1.5 g TBT was added dropwise into the above solution followed by being stirred for another 2 h to mix uniformly. 1.0 g paraffin oil was added dropwise into the obtained solution, after stirring vigorously, the ultimate microemulsion solution was prepared. 2.3 Preparation of Hollow Tio2 Nanofibers. A typical electrospinning setup is composed of high-voltage power, syringe pump and roller, covered with aluminum-foil. In the electrospinning process, the microemulsion solution was filled in a 20 mL syringe equipped with a blunt-end stainless-steel needle of which the inner diameter was about 0.3 mm. Flow rate of syringe pump was set at 2.0 mL·h-1, an electrical potential of 20 kV was applied in the stainless-steel needle. The distance between the grounded collector and the stainless-steel needle was about 20 cm. The electrospinning process was carried out for 12 h to obtain the composite nanofibrous mats. Subsequently the composite nanofibrous mats were calcined at different temperature (500 °C, 700 °C and 900 °C) for 3 h with the heating rate of 1 °C·min-1, and then naturally cooling down to the room temperature, thus hollow TiO2 nanofibers will be obtained. The samples calcined at 500, 700 and 900 °C were denoted as 5



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TiO2-500, TiO2-700 and TiO2-900, respectively. 2.4

Characterization.

Differential

Thermogravimetric

Thermogravimetric

(DTG)

were

analysis applied

(TGA/1100SF) to

characterize

and the

decomposition process of PVP/TiO2 composite nanofibers from ambient temperature to 850 °C. X-ray Diffraction (XRD) patterns were performed on Bruker D8 Advance X-ray diffractometer with Cu target and Kα radiation (wavelength λ=1.54 Å) to identify the phase structure of TiO2-500, TiO2-700 and TiO2-900 samples. Field emission scanning electron microscopy (FE-SEM, SU4800) was employed to examine the morphology of PVP/TiO2 composite nanofibers and hollow TiO2 nanofibers. Prior to the examination, all the samples were coated with gold to avoid charge accumulation. EDX-Mapping was examined to characterize the element distribution of TiO2 nanofibers. Transmission electron microscopy (TEM) was performed on JEOL JEM 2100 instrument at an accelerating voltage of 120 kV to observe

the

morphology

and

structure

of

TiO2-500.

The

nitrogen

adsorption-desorption isotherms were measured with a physisorption analyzer (ASAP 2020, Micromeritics) to analyze the Brunauer-Emmett-Teller specific surface area and pore diameter distribution. 2.5 Photocatalytic Activities. The photocatalytic activities of the samples were examined by decomposing MB aqueous solution in a closed box. Prior to decomposition, the samples of 50 mg were impregnated into 50 mL MB solution with concentration of 10 mg·L-1, and then slightly stirred for 1 h to reach the adsorption-desorption equilibrium between the samples and organic dye molecules. 6


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Afterwards, the decomposition was conducted with a xenon lamp of 500 W as light source. During the entire decomposition process, the MB solutions were magnetically stirred. After the given illumination interval of 10 min, the upper solution of 6 ml was taken out and centrifuged for 3 min for photocatalytic analysis. The concentration change of MB solution was measured with a T6 spectrophotometer. Additionally, the photocatalytic decomposition of phenol was also conducted to examine the photocatalytic activity of hierarchical TiO2 nanofibers. The supernatant of phenol was tested on a high-performance liquid chromatograph (HPLC), which the wavelength was set at 270 nm. The other conditions and procedures were the same as the decomposition of MB.

3. RESULTS AND DISCUSSION 3.1 Morphological and Structural Properties. Figure 1 demonstrates TG and DTG curves of PVP/TiO2 mats. It can be observed from Figures 1 a and 1 b that the entire decomposition process could be divided into four steps. From ambient temperature to 150 °C, the weight loss was approximately 7.1%, which should be ascribed to the volatilization of physically absorbed moisture and residual solvent. There was a weight loss of 41.7% in the range of 150 to 290 °C, which corresponded to the partial decomposition of paraffin oil and TBT molecules. The weight loss of 16.8% from 300 to 390 °C should be ascribed to the decomposition of main chain of PVP. The final decomposition process occurred from 390 to 500 °C, which should be attributed to the complete decomposition of PVP, paraffin oil and TBT. It can be 7



observed that the weight was approximately constant after 500 °C, thus indicating the formation of TiO2 nanofibers. Hence, it was reasonable to choose 500 °C as the calcination temperature.

Figure 1 TG and DTG curves of as-spun PVP/TiO2 composite nanofibers SEM images and TEM images of hollow TiO2 nanofibers calcined at 500 °C as well as XRD patterns calcined at 500, 700 and 900 °C are illustrated in Figure 2. It was evident from Figure 2a that the TiO2 nanofibers distributed uniformly and the fiber diameter was relatively homogeneous. As demonstrated in Figure 2 b, when the calcination temperature was 500 °C, the diffraction peaks located at 2θ of 25.2° and 48.0° were attributed to the (101) and (200) crystallographic planes of anatase TiO2 nanofibers. For TiO2 nanofibers annealed at 700 and 900 °C, the peaks centered at 27.4°, 36.1° and 54.3° can be indexed into the (110)，(101) and (211) crystallographic planes of rutile TiO2 nanofibers. It was noteworthy that the diffraction peak acquired from TiO2 nanofibers annealed at 900 °C was sharper than that annealed at 700 °C, suggesting that the TiO2 nanofibers annealed at 900 °C possessed higher crystallinity. As illustrated in Figures 2 c and 2 d, it was prominent that the resultant nanofibers possessed hollow structure and almost all the nanofibers were hollow, which can be 8


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observed in Figure 2d. The hollow structure mainly stemmed from the remove of paraffin oil in the process of calcination. Moreover, there were numerous pores existing on the surface of nanofibers, and the fiber wall was thoroughly mesoporous. TEM was applied to investigate further the inner structure of hollow nanofibers. The hollow channel can be observed clearly by the striking contrast between the hollow structure and mesoporous wall, and the hollow channel was axial-aligned. From Figure 2f it was evident that the hollow TiO2 nanofibers were composed of pure anatase phase.

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Figure 2 Surface SEM image (a), cross-sectional SEM images (c-d), TEM image (e) and HRTEM

image (f) of hollow TiO2 nanofibers annealed at 500 °C; XRD pattern

of hollow TiO2 nanofibers annealed at 500, 700 and 900 °C. Figure 3 demonstrates the schematic illustration of the supposed mechanism about the formation of the hollow TiO2 nanofibers. When the precursor solution was stirred thoroughly, all phases including TBT, paraffin oil and PVP were distributed uniformly in the solution, as demonstrated in Figure 3 a. Once the solution was spun out from the spinneret, the paraffin oil on the surface would volatilize rapidly with ethanol and acetic acid, thus forming a region enriched PVP and TBT near the surface of nanofibers, as shown in Figure 3 b. After calcination, the paraffin oil in the core was removed. Hence, the hollow TiO2 nanofibers would then be successfully fabricated.

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Figure 3 The schematic illustration about the formation of hollow TiO2 nanofibers. Figure 4 demonstrates element mapping and EDX spectrum of the hollow TiO2 nanofibers annealed at 500 °C. The mapping images of O and Ti demonstrated a homogeneous distribution along the fiber body. EDX spectrum examined from different nanofibers and different positions along the single nanofiber indicated that the chemical composition of nanofibers was identical and mainly composed of O and Ti. The minute amount of element C should be attributed to the contamination of TEM grid during the process of sample preparation. Moreover, as shown in Figure 4 d, the atom ratio of Ti and O was approximately 1:2, thus verifying the formation of TiO2 nanofibers.

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Figure 4 The element mapping of (a) Ti, (b) O, (c) SE, Ti, O; (d) a typical EDX spectrum of hollow TiO2 nanofibers calcined at 500 °C. The nitrogen adsorption-desorption isotherm and pore diameter distribution of TiO2 nanofibers annealed at 500 °C are displayed in Figure 5. According to the classification of IUPAC, the isotherm of hollow TiO2 nanofibers belonged to a representative type IV isotherm with H3 hysteresis. It was apparent that there were mainly two types of pores for hollow TiO2 nanofibers: mesopores in the wall of which the size was about tens of nanometers and hollow macropores of which the diameter was about 100~200 nm. The size of these two pores was not an order of magnitude. Pore size distribution was not uniform, thus resulting in the occurrence of H3 hysteresis. The specific surface area of hollow TiO2 nanofibers was about 41.4 m2/g, which should be assigned to the hollow structure and numerous pores existing on the walls of nanofiber. Based on the analysis of Barrett-Joyner-Halenda (BJH) pore diameter distribution, the pore diameter distributed in the range of 18 to 156 Å, and the average pore diameter was approximately 57.0 Å, the BJH adsorption cumulative volume of pores was about 0.1 m3/g. The higher specific surface area and pore volume was beneficial for improving the photocatalytic performances. 12


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Figure 5 (a) Nitrogen adsorption-desorption isotherms of hollow TiO2 nanofibers annealed at 500 °C; (b) the corresponding pore diameter distribution 3.2 Photocatalytic Activity. The photocatalytic activities of all the samples were evaluated by decomposing MB solution under the irradiation of xenon lamp at room temperature, and the results are demonstrated in Figure 6. As shown in Figure 6 a, the decomposition ratio of TiO2-500 can be reached 96% within 70 min. Moreover, the photocatalytic activities decreased from TiO2-500, TiO2-700 to TiO2-900. Similar results were also obtained by the photocatalytic degradation of gaseous acetaldehyde and ammonia with TiO2 nanofibers. It demonstrated that with the increase of calcination temperature, the grain size increased, and the nanofibers would be denser, thus causing the decrease of specific surface area and porosity 34. It was noteworthy that the samples prepared via microemulsion electrospinning displayed better photocatalytic activities than solid TiO2 nanofibers, which should be attributed to the larger specific surface area, increased separation efficiency of electron-holes and more photocatalytic reaction sites. For blank sample, the MB was degraded approximately 30% within 70 min, thus demonstrating MB possessed a certain self-cleaning function. In order to depict the decomposition rate more clearly, the Langmuir-Hinshelwood 13



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pseudo-first-order kinetics model was applied to investigate the kinetic analysis (eqn. (1)) 35-36. lnC0/Ct = kapp t

(1)

Where kapp is the first-order constant (min-1), C0 is the initial concentration of MB solution, Ct is the concentration at the time of t, all the kapp values are illustrated in Figure 6 b. The first-order constants were 0.045, 0.026, 0.011, 0.008, 0.005 for TiO2-500, 700, 900, solid nanofibers and blank sample, respectively. Obviously, the first-order constant of TiO2-500 was almost 6 times than that of the solid nanofibers, thus further verifying the significant improvement of photocatalytic activity for TiO2 nanofibers made via microemulsion electrospinning. Besides, UV-vis spectral of MB solution (the inset corresponding color change images) for TiO2-500 (c) and solid TiO2 nanofibers (d) are displayed in Figures 6 c and 6 d. It is noteworthy that the absorbance of MB solution at wavelength around 300-350 nm represents the same trend as wavelength at 664 nm. With the increase of time, the absorbance of MB solution decreased. The UV-vis spectral and color change images of MB solution for other samples are demonstrated in Figure S2.

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Figure 6 (a) Photocatalytic degradation profiles and (b) kinetic linear simulation curves of all the samples for MB; UV-vis spectral of MB solution versus photoreaction time of (c) TiO2-500 and (d) solid TiO2 nanofibers, the inset images corresponding the color change of MB solution at different time. In order to verify further the photocatalytic activity of hierarchical TiO2 nanofibers for colorless organic pollutants, the photocatalytic performances of all the samples for phenol were investigated, the relevant results were demonstrated in Figure S3. The results indicated that phenol can be decomposed completely within 180 min for TiO2-500. Photocatalytic performances decreased from TiO2-500, TiO2-700 to TiO2-900, which verified further the above theory. The comparison of photocatalytic performances based on different TiO2 nanofibrous mats form other literatures was listed in Table 1. All the photocatalytic activities were examined by decomposing MB. The first-order constant can reflect the decomposition rate of MB directly, hence, the first-order constants were used to compare the photocatalytic activities of all samples.

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Table 1. Comparisons of Photocatalytic Activities Based on Different Tio2 Nanofibers Samples

Preparation method

Light source

The first-order constant (min-1)

TiO2 nanofibers

Electrospinning

UV light

0.038

9

Porous TiO2 nanofibers Hollow TiO2 nanofibers Hollow TiO2 nanofibers Hierarchical TiO2 nanofibers

Carbon nanospheres as template Coaxial electrospinning

UV light

0.035

37

UV light

0.081

31

Electrospinning

UV light

0.035

38

Electrospinning

Visible light

0.045

This work

References

3.3 The Mechanism of Enhanced Photocatalytic Activity for Porous Tio2 Nanofibers. The mechanism of enhanced photocatalytic activity for porous TiO2 nanofiber is displayed in Figure 7. As we all know, when TiO2 is irradiated by light source, some of the photons which the energy is more than or equal to the band gap of TiO2 will be absorbed by the electrons on the valence band, and the electrons (e-CB) subsequently will jump from valence band to conduction band, thus leaving behind a hole on the valence band. The generated electrons and holes either react with the electrons acceptors/donors or recombine with each other. The electrons react with O2 adsorbed on the surface of TiO2 nanofibers to generate superoxide radical anion (O2·-) while the holes will react with H2O or –OH to generate OH· . These free radicals will react with organic dye molecules to make them completely decompose. The TiO2 nanofibers fabricated by microemulsion electrospinning possessed larger specific surface area and higher porosity. When the photocatalyst was irradiated with light, a part of photons would be absorbed by the porous TiO2 nanofibers. 16


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However, some of the photons could be reflected. It was worth noting that the majority of the photons were remained inside the porous TiO2 nanofibers until completely absorbed. Hence, it was beneficial for absorbing more photons with larger specific surface area to generate more effective electron-hole pairs. Additionally, high porosity was favorable for adsorbing more oxygen and H2O on the surface of the TiO2 nanofibers to produce more free radicals. This is the other important reason for porous TiO2 nanofibers showing significantly improved photocatalytic activity than solid nanofibers. Moreover, TiO2 nanofibers with larger specific surface area and higher porosity are favor of promoting the electron-hole separation efficiency by shortening the diffusion path, and increasing the available active sites between organic dye molecules and free radicals.

Figure 7 The mechanism of enhanced photocatalytic activity for porous TiO2 nanofiber

4. CONCLUSIONS In this work, TiO2 nanofibers with hierarchical porosity were fabricated via 17



single-spinneret electrospinning combined with calcination for efficient photocatalytic degradation. Thereafter, the photocatalytic performances of TiO2 nanofibers calcined at 500, 700 and 900 °C were examined in detail. The results indicated that the TiO2 nanofibers were hollow with abundant mesopores in the wall of nanofibers, which the specific surface area were approximately 41.4 m2/g. Moreover, all the porous TiO2 nanofibers prepared via microemulsion electrospinning demonstrated better photocatalytic activities than solid TiO2 nanofibers, and the photocatalytic performances decreased with the increase of calcination temperature. Especially, the first-order constant of the photocatalytic activities of TiO2-500 was almost 6 times than that of solid nanofibers, which implied the significantly enhanced photocatalytic activities. The results verified that microemulsion electrospinning is indeed a simple, convenient and versatile method to fabricate hollow TiO2 nanofibers with hierarchical porosity.

Supplementary Information: Figure S1 SEM image (a) and the fiber diameter distribution (b) of as-spun PVP/TiO2 composite nanofiber Figure S2 UV-vis spectral of MB solution versus photoreaction time of (a) TiO2-700, (b) TiO2-900 and (c) blank sample, the inset images corresponding the color change of MB solution at different time Figure S3

(a) Photocatalytic degradation profiles and (b) kinetic linear simulation

curves of all the samples for phenol 18


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Acknowledgements This research was financially supported by National Key R&D Program of China (2017YFB0309100), the Fundamental Research Funds for the Central Universities (JUSRP51621A), Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX17_1438), the China Postdoctoral Science Foundation (2017M610296) and Hubei Key Laboratory of Low Dimensional Optoelectronic Material and Devices (HLOM161002 and HLOM161005).

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Preparation of TiO2 Nanofibrous Membranes with Hierarchical

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