Scalable synthesis of three-dimensional interconnected mesoporous TiO2 nanotubes with ultra-large surface area

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ScienceDirect Acta Materialia 93 (2015) 138–143 www.elsevier.com/locate/actamat

Scalable synthesis of three-dimensional interconnected mesoporous TiO2 nanotubes with ultra-large surface area Yizao Wan,a Ping Liu,a Zhiwei Yang,a Sudha R. Raman,b Guangyao Xiongc and Honglin Luoa,

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a

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China b Department of Community and Family Medicine, Duke University, NC, USA c School of Mechanical and Electrical Engineering, East China Jiaotong University, Nanchang 330013, China Received 12 December 2014; accepted 24 March 2015

Abstract—Mesoporous nanotubes represent a unique class of nanostructures with numerous applications including biomaterials, lithium ion battery anodes, and photocatalysts. Herein we describe for the first time a general, scalable, and reproducible approach for the production of 3D interconnected mesoporous TiO2 nanotubes with ultrahigh specific surface area and large pore volume by using a nanofibrous template of bacterial cellulose (BC). The obtained samples were characterized by Brunauer–Emmett–Teller (BET) surface area measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results demonstrated that TiO2 nanotubes sustained the 3D interconnected structure of pristine BC and exhibited an average diameter of 36 nm and a mesoporous wall with a mean mesopore size of 3.2 nm. The particular structure endows TiO2 nanotubes with ultra-large surface area (1629 m2 g1) and high photocatalytic activity. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanostructured materials; Oxide; Photocatalysis; Sol–gel

1. Introduction Since the discovery of carbon nanotubes in 1991 by Iijima [1], researchers have realized that the materials in nanotube form have novel properties that are not found in conventional forms. Soon after the synthesis of carbon nanotubes, boron nitride nanotubes were fabricated successfully by arc-discharge method in 1995 [2]. Afterward, numerous type of nanotubes were developed, including nanotubes made of metal oxides such as TiO2, SiO2, ZrO2, Al2O3, V2O5, and MoO3 [3–9]. In recent years, TiO2 nanotubes have attracted great interest due to their high surface-to-volume ratio and improved properties in many applications compared to nanofibers, colloids, films, and other forms of TiO2 [10,11]. TiO2 nanotubes can be used in many areas such as gas sensor [12], high-power lithium ion battery (LIB) anodes [10,13], photovoltaic cells [14], photocatalysts [15], and biomaterials [16,17]. Tremendous attention has been paid to the preparation of TiO2 nanotubes worldwide. To date, many methods have been developed including template synthesis [10], chemical treatment [18], electrochemical anodic oxidation [19], and hydrothermal treatment [20]. Among the various

⇑ Corresponding

author. Tel.: +86 22 2740 3045; fax: +86 22 2740 4724; e-mail: [email protected]

methods available for the production of TiO2 nanotubes, the template method has been proven to be a versatile and inexpensive technique [6]. So far, various templates have been used for the preparation of TiO2 nanotubes, including carbon nanofiber [13], ZnO nanorod [21], bacterial flagella [22], amino acid amphiphile fibers [23]. However, despite these significant achievements, until now, there have been no reports about the fabrication of 3D interconnected mesoporous TiO2 nanotubes with an extraordinary large surface area and a diameter of less than 100 nm. Materials possessing an extraordinarily high surface area will be invaluable for many applications. For instance, in biomedical fields, a large surface area facilitates the spreading and proliferations of cells as well as the formation of apatite [24], and improves photocatalytic activities for a photocatalyst [15]. It is expected that certain characteristics of the 3D interconnected mesoporous TiO2 nanotubes, such as the ability of the porous wall structure to significantly enhance the mass transportation between nanotubular matrix and the environment, will be highly desirable in such fields as biomaterials [25], 3D electrodes for all-solid-state micropower sources [26,27] and LIB anodes [28]. For example, the mesoporous TiO2 nanotubes prepared by Wang et al. displayed a high rate of charge– discharge which was attributed to the 3D network structure of mesoporous titania nanotubes with high surface area [10]. Therefore, it is of great importance to seek a scalable route to fabricate 3D interconnected mesoporous TiO2

http://dx.doi.org/10.1016/j.actamat.2015.03.059 1359-6462/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Y. Wan et al. / Acta Materialia 93 (2015) 138–143

nanotubes with a diameter down to nano-scale (less than 100 nm) and ultrahigh surface area. Herein we describe for the first time a general, scalable, and reproducible approach for the production of 3D interconnected mesoporous TiO2 nanotubes with ultrahigh specific surface area and large pore volume by using a nanofibrous template of bacterial cellulose (BC). The asprepared TiO2 nanotubes exhibit ultra-large surface area (1629 m2 g1) and pore volume (2.565 cm3 g1), and in particular they exhibit high photocatalytic activity. The present procedure can provide a general way for reproduction of 3D interconnected nanotubes by using 3D porous interconnected BC nanofibers as a template.

2. Experimental 2.1. Preparation of BC aerogels The preparation procedure of BC pellicles was described in our previous work [29]. Briefly, the bacterial strain, Acetobacter xylinum X-2, was grown in the culture media containing 0.3 wt.% analytic grade green tea powder and 5 wt.% analytic sucrose for 7 days. The pH of the medium was adjusted to 4.5 by acetic acid. BC pellicles were purified by soaking in deionized water at 90 °C for 2 h followed by boiling in a 0.5 M NaOH solution for 15 min. The pellicles were then washed with deionized water several times and soaked in 1 wt.% NaOH for 2 days. After rinsing with deionized water until neutrality, the BC pellicles were finally freeze-dried to obtain BC aerogels. 2.2. Preparation of TiO2 nanotubes Fig. 1 shows the preparation of the TiO2 nanotubes. A BC aerogel (approximately 25 mg) (Fig. 1a) was immersed in a solution of isopropanol/titanium tetra-n-butyl (Ti(OBu)4) (10 mL:0.34 mL) for 24 h at room temperature under closed conditions to allow the adsorption of colloidal dispersion onto BC nanofibers (Fig. 1b). Afterward, the product was taken out and rapidly rinsed with isopropanol twice and then immersed in a closed vessel containing a solution of isopropanol and deionized water (volume ratio 9:1) for 48 h at room temperature to complete the hydrolysis reaction and yield TiO2-coated BC nanofibers (denoted as TiO2/BC hereinafter) (Fig. 1c). The TiO2/BC aerogel was washed with deionized water, freeze-dried in vacuum for 24 h, heated to 600 °C at a heating rate of 1 °C/min and held

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in air for 6 h to combust the template and to crystallize TiO2, resulting in TiO2 nanotubes (Fig. 1d). For comparison, some TiO2/BC samples were calcined at 800 °C. 2.3. Characterization Pristine BC, TiO2/BC, and TiO2 nanotubes were characterized by scanning electron microscopy (SEM, Nova Nanosem 430) from which the fiber diameter was determined by measuring at least 100 randomly selected fiber segments, transmission electron microscopy (TEM, Tecnai G2F-20) at an acceleration voltage of 200 kV, and X-ray diffraction (XRD, Rigaku D/Max 2500 v/pc) with Cu Ka radiation. Brunauer–Emmett–Teller (BET) surface area of TiO2 nanotubes was evaluated from nitrogen adsorption isotherms at 77 K using a surface area analyzer (NOVA 2200e). For photocatalytic activity determination, suspensions of the samples (5 mg, in powder form) in 10 mL methyl orange (MO) solution (10 ppm) were placed in the dark for 30 min to reach adsorption–desorption equilibrium prior to illumination. The stirred suspensions were irradiated from above by the UV light. The concentration of MO at different time intervals was determined by measuring the UV–vis absorption of the suspensions after removing the samples.

3. Results and discussion 3.1. Morphology of TiO2 nanotubes Fig. 2a shows a typical SEM image of pristine BC. Clearly, the pristine BC shows a 3D porous fibrous network structure consisting of randomly interconnected BC nanofibers with an average diameter of around 37 nm (Fig. 2b). Similar to BC, TiO2/BC also shows a 3D porous fibrous network structure consisting of interconnected BC/TiO2 nanofibers (Fig. 1c). TiO2 coating led to an increase of the diameter up to 58 nm (Fig. 2d). After calcination, TiO2 nanotubes with 3D porous interconnected network structure were obtained, which had a large macroscopic size in millimeter scale (Fig. 2e and inset). Fig. 2f reveals that the average diameter of TiO2 nanotubes decreased to around 36 nm, clearly indicating shrinkage due to calcination. TEM was further employed to acquire more detailed information about the structure of TiO2 nanotubes (Fig. 3). Fig. 3a shows a porous interconnected structure

Fig. 1. Preparation of TiO2 nanotubes (a) BC aerogel has 3D interconnected porous structure; (b) colloidal dispersion was adsorbed onto BC in the solution of Ti(OBu)4/isopropanol, (c) TiO2 was formed by hydrolysis in isopropanol/water, leading to amorphous TiO2-coated BC, (d) calcination resulted in crystalline TiO2 nanotubes.

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Fig. 2. SEM images and fiber diameter distribution of BC (a and b), TiO2/BC (c and d), and TiO2 nanotubes (e and f), and EDS results of TiO2/BC (g) and TiO2 nanotubes (h).

of TiO2 nanotubes, which agrees well with the observations using SEM. The TEM image of a randomly selected individual TiO2 nanotube (Fig. 3b) reveals that the tube walls consist of tiny nanoparticles and the wall surface is not very smooth. The wall thickness is around 14 nm and the outer diameter of this TiO2 nanotube is around 48 nm, falling into the diameter range of 22–58 nm as measured by SEM (Fig. 2f). Furthermore, TEM observation of this typical nanotube (Fig. 3b) reveals its porous nature. Clearly, the formation of the porous structure of this nanotube is due to the voids between aggregated TiO2 nanoparticles,

which is more clearly shown in the high-resolution TEM (HRTEM) image (Fig. 3c). Note that these nanoparticles are loosely connected with each other and that the mesopores are abundant in the tube walls. The average pore size is around 3.2 nmp(Fig. 3c). As calculated by the interstice ffiffiffi model [30] (d ¼ ð 2  1Þ  D, where D represents the average diameter of the TiO2 spheres, around 7 nm), the average pore size is 2.9 nm, which is comparable to the value from HRTEM measurement. Fig. 3c also reveals clear lattice fringes with an interplanar spacing of 0.352 nm, corresponding to the (1 0 1) plane of the anatase phase. The

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BC Anatase ƽ Rutile 

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a

Intensity (a.u.)



b Ƶ

c

ƽ

10

20

Ƶ Ƶ Ƶ

Ƶ ƵƵ

d 30

Ƶ

ƽ

40 2θ (degree)

50

60

70

Fig. 5. XRD patterns of pristine BC (a), TiO2/BC (b), and TiO2 nanotubes calcined at 600 °C (c) and 800 °C (d).

Fig. 3. TEM images of TiO2 nanotubes (a), an individual TiO2 nanotube (b) and its HRTEM image (c), and SAED pattern (d) (HRTEM showing the walls of TiO2 nanotubes consist of TiO2 nanoparticles).

corresponding selected area electron diffraction (SAED) pattern (Fig. 3d) reveals that the TiO2 nanoparticles are highly crystalline. 3.2. Structure of TiO2 nanotubes FTIR analysis was conducted to determine the surface structure of BC, TiO2/BC, and TiO2 nanotubes. The FTIR spectra shown in Fig. 4 reveal that the spectrum of TiO2/BC is the mixture of BC and TiO2. However, a careful comparison between the spectra of BC and TiO2/BC reveals that the peak at 1640 cm1 (corresponding to C@O) in the spectrum of pristine BC has been intensified and slightly shifted to 1632 cm1 in the spectrum of TiO2/BC, which is ascribed to the characteristic band of Ti–O–C [31]. A similar phenomenon was observed by Sun and co-workers [30]. The existence of Ti–O–C indicates that a chemical bond between BC and TiO2 precursors has been formed.

3.3. Pore structure of TiO2 nanotubes

BC

Transmittance (a.u.)

C=O

TiO2/BC C=O/Ti-O-C

TiO2

4000

XRD analysis was further performed to determine the structure of pristine BC, TiO2/BC, and TiO2 nanotubes (Fig. 5). Pristine BC displays the typical XRD pattern of cellulose I with the main diffraction peaks at around 2h = 14.6°, 16.9°, and 22.9°, corresponding to the crystalline planes of (1  1 0), (1 1 0), and (0 2 0) planes of cellulose type I [32,33]. After coating with TiO2, the typical peaks of BC are weakened due to the presence of TiO2. However, the diffraction peaks of TiO2 are not observed although Ti element is confirmed by EDS results (Fig. 2g and h). This indicates that the coated TiO2 is amorphous. After calcination at 600 °C for 6 h, the transformation from amorphous to crystalline was noted as revealed by the XRD pattern of TiO2 nanotubes. Note that there are six strong characteristic peaks located at 25.3°, 37.9°, 47.9°, 53.8°, 55.0°, and 62.6° in the XRD pattern of TiO2 nanotubes, which are assigned to diffraction planes of (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), and (2 0 4), respectively, of anatase TiO2 (JCPDS 21-1272). This implies that the crystal structure of TiO2 nanotubes is of the anatase type. Since anatase TiO2 shows better photocatalytic activities than the TiO2 with other types of crystal structure [34,35] and anatase TiO2 is considered to be the most appropriate phase for lithium insertion hosts [31,36], this material is attractive as a photocatalyst and an anode for LIBs. Fig. 5 reveals the existence of rutile TiO2 when the samples were calcined at 800 °C. A similar transition of anatase to rutile TiO2 has been reported by many previous researchers [37].

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig. 4. FTIR spectra of BC, TiO2/BC, and TiO2 nanotubes.

The mesoporous nature of the TiO2 nanotubes was further examined by Brunauer–Emmett–Teller (BET) measurements conducted at 77 K. The pore structure, volume, and pore size of TiO2 nanotubes are shown in Fig. 6. From the nitrogen adsorption and desorption isotherm (Fig. 6a), we can see that the as-prepared TiO2 nanotubes show a typical type IV isotherm with type H3 hysteresis loop according to BDDT classification [38], indicating the presence of mesopores (2–50 nm). A similar isotherm has been recently reported by Chattopadhyay and co-workers [39]. Moreover, the observed hysteresis loop approaches P/P0 = 1, suggesting the presence of macropores (>50 nm) [40]. As shown in Table 1, the surface area of TiO2 nanotubes after burning off the BC nanofiber core is 1629 m2 g1 and the pore volume is 2.565 cm3 g1, which are much larger as compared to many previously reported

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Y. Wan et al. / Acta Materialia 93 (2015) 138–143 1800

(a)

1600

Methyl Orange C/C0

1400

Volume@STP (cc/g)

a

1.0

1200 1000 800 600 400 200

0.8 0.6 0.4 0.2 b 0.0

0 0.0

0.2

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0.6

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1.0

c 0

10

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30

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Fig. 7. Photocatalytic activity of TiO2/BC (a) and TiO2 nanotubes calcined at 800 °C (b) and 600 °C (c).

(b) 0.08

dV/dr (cc/g/nm)

3.4. Photocatalytic activity of TiO2 nanotubes 0.06

0.04

0.02

0.00 0

20

40

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Fig. 6. Nitrogen adsorption–desorption isotherm (a) and Barret– Joyner–Halenda (BJH) pore size distribution (b) of 3D mesoporous TiO2 nanotubes.

Table 1. Comparison of various relevant TiO2 nanomaterials. Morphology of TiO2

TiO2 nanotubes TiO2 nanotubes Mesoporous TiO2 nanotubes TiO2 nanotubes Wormhole-like mesoporous TiO2 Mesoporous TiO2 TiO2 powder 3D interconnected mesoporous TiO2 nanotubes

Surface area

Pore volume

(m2 g1)

(cm3 g1)

137 356 400 400 1256 853 750 1629

0.26 1.55 0.94 n/a 0.653 n/a 0.84 2.565

Refs.

[13] [45] [10] [18] [44] [41] [43] This work

values (Table 1) [10,18,41–45]. The extraordinarily large surface area of TiO2 nanotubes synthesized in this work is due to the 3D porous interconnected structure, ultra-fine tube diameter, and the mesoporous walls. The Barrett– Joyner–Halenda (BJH) pore size distribution (Fig. 6b) obtained from the isotherm indicates a relatively narrow pore distribution centered at 3.3 nm, which is very close to the HRTEM measurement (Fig. 3c). The pore structure measurement demonstrates that the present route is suitable for the synthesis of 3D interconnected mesoporous nanotubes with extremely high surface area.

Fig. 7 presents the photocatalytic activity of various materials. The TiO2/BC does not show photocatalytic activity toward photodegradation of MO due to the amorphous nature of TiO2. The anatase TiO2 nanotubes calcined at 600 °C took only 40 min to totally decompose the MO, indicating excellent photocatalytic activity. However, 6.5% MO molecules were still left after 60 min UV irradiation with the anatase and rutile mixed TiO2 nanotubes calcined at 800 °C. This suggests that the crystalline structure of TiO2 nanotubes has an effect on their photocatalytic activity, which agrees with previous reports [34,42]. This work presents the first evidence that 3D interconnected TiO2 nanotubes with an ultra-large surface area can be obtained via a scalable and cost-effective method. This lays a solid foundation for the large scale application of these nanotubes in various fields such as sensors, photovoltaic cells, LIB anodes, and tissue engineering scaffolds, which will be the focus of our future work. Furthermore, we believe that this approach can be extended to the synthesis of other metal oxide nanotubes such as ZnO, SnO2, and Fe3O4. 4. Conclusions In summary, a template based route has been developed for the synthesis of 3D interconnected mesoporous TiO2 nanotubes with ultra-large surface area and pore volume by using natural BC as the template without any surface modification. The as-synthesized TiO2 nanotubes exhibit distinct tubular structure with ultrafine outer diameter (ranging from 22 to 58 nm) and sustain the 3D porous interconnected structure of BC with limited shrinkage. The obtained interconnected mesoporous TiO2 nanotubes have a very high surface area of 1629 m2 g1 and a pore volume of 2.565 cm3 g1 with mesopores centered at 3.3 nm. A preliminary application study indicates that the as-prepared TiO2 nanotubes show very high photocatalytic activity in the decomposition of the model dye (MO). It is expected that the ultra-large surface area and the unique structure offered by such materials make them a promising candidate as catalysts, as well as pseudocapacitors, batteries, and sensors. It is also expected that the synthesis approach is generic and can be conducted at a low cost,

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which make the process translatable to large-scale production and extendable to the synthesis of other nanotubular metal oxides with 3D interconnected structure. Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant No. 51172158).

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