ARTICLE pubs.acs.org/IECR
Facile Method To Prepare TiO2 Hollow Fiber Materials via Replication of Cotton Fiber Tao Zheng, Ze Tian, Bitao Su,* and Ziqiang Lei Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, No. 967 Anning East Road, Lanzhou 730070, People’s Republic of China ABSTRACT: A facile template method was employed to prepare TiO2 hollow fiber materials by using defatted cotton fibers as the template. The prepared samples were characterized by thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), BET analysis, and UV vis absorbance spectroscopy. The photocatalytic degradation of methylene blue (MB) was used as the model reaction to evaluate the photocatalytic activity of the samples under the irradiation of simulated sunlight. Results indicated that the samples replicated the morphology of original cotton fibers very well and had hollow fiber structure. The calcination temperatures had a direct impact on the phase composition, size, specific surface area, and morphology, which are closely related to the photocatalytic activity of the samples. The TiO2 sample containing about 2.93% rutile phase shows optimal photocatalytic activity.
1. INTRODUCTION Since Fujishima and Honda reported photochemical splitting of water by a TiO2 electrode,1 TiO2 has been extensively studied as a photocatalyst in many fields because of its high photocatalytic activity, robust chemical stability, low production costs, and nontoxicity.2 Many methods have been used to prepare TiO2 photocatalytic materials, such as sol gel,3 microemulsion,4 chemical vapor deposition,5 hydrothermal,6 and template method.7 Although different methods have been designed to prepare TiO2 controllable materials, template-directed preparation is a promising method due to its versatility and multiformity.8 It has been reported that various natural biological materials with special structures such as oak,9 silk,10 sisal fibers,8 and filter paper11 have been used as biological templates to prepare photocatalytic materials. Some other natural biological materials, such as cellulose fiber felts, naturally grown wood (such as rattan and pine), and corrugated cardboard structures,12 and different ash-forming biological materials, such as gills of mushrooms, cotton wool, spider silk, dog’s hair, and human hair,13 were also used in recent years as biological templates to prepare various materials with different structures. Natural biological templates have received so much attention due to their advantages such as that they are cheap, abundant, pollution-free, and renewable. In this paper, defatted cotton fiber is chosen as the template to prepare TiO2 hollow fiber materials. Defatted cotton fiber owns excellent wetting and adsorption properties and is easily removed by its combustion at a proper temperature. The effect of calcination temperatures on the phase composition, size, specific surface area, morphology, and photocatalytic activity of the obtained materials was investigated. The photocatalytic activity of the samples was evaluated by the photocatalytic degradation efficiency of methylene blue (MB) solution under the irradiation of simulated sunlight. r 2011 American Chemical Society
2. EXPERIMENTAL PROCEDURES 2.1. Preparation of the Samples. The samples were prepared via the template method and the preparation process included the following two steps. Step 1. The first step is preparing the precursor. Tetrabutyl titanate (TBT, AR) was used as the Ti source. A 1.5 mL volume of TBT was dropwise added into 75 mL of anhydrous alcohol (EtOH, AR) under vigorous stirring at room temperature. A 1.5 g sample of dried and loose cotton fibers used as template was dipped in the solution for 1 h and then naturally dried at room temperature. Finally, the precursor was obtained and the TBT was successfully adsorbed on it. Step 2. The second step includes removing the cotton fiber template and fulfilling the transformation of Ti4+ f TiO2 to prepare the goal samples by calcining the precursors at different temperatures. These precursors were respectively calcined at 450, 500, 550, 600, 650, 700, and 750 °C under air atmosphere for 2 h and then naturally cooled to room temperature. TiO2 hollow fiber materials were obtained and named as S-1, S-2, S-3, S-4, S-5, S-6, and S-7, respectively. 2.2. Characterization. Thermogravimetric (TG) analysis was implemented on a Diamond TG/DTA/SPAECTRUN ONE thermogravimetric analyzer. X-ray diffraction (XRD) patterns of the samples were measured by step scanning on a Rigaku D/Max-2000 X-ray diffractometer with Cu Kα radiation (λ = 0.154 056 nm) operated at 40 kV and 100 mA. The morphologies of the samples were observed by a JSM-5600LV scanning electron microscope (SEM), which was equipped with an energy dispersive spectrometer (EDS) for undertaking element Received: March 14, 2011 Accepted: December 19, 2011 Revised: October 5, 2011 Published: December 19, 2011 1391
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Figure 1. Scheme of the reactor setup used during the photocatalytic experiments. Figure 3. XRD patterns of S-1, S-4, and S-7.
absorbance value of the supernatant, from which the powders were removed, was measured at 664 nm (λmax of MB solution). The photocatalytic degradation efficiency Dt (%) was calculated by the equation Dt (%) = [(A0 At)/A0](100%), where A0 and At are the absorption values of MB solution at the initial time t = 0 and reaction time t, respectively.
3. RESULTS AND DISCUSSION
Figure 2. TG curve of the precursor.
analysis. The specific surface areas of the samples were determined via the multipoint BET method on a NOVA 2000e surface area and pore size analyzer. The UV vis spectrum of the sample was recorded on a Shimadzu UV-2550 UV visible spectrophotometer. 2.3. Evaluation of Photocatalytic Activity. Photocatalytic experiments were performed on an XPA-7(G8) photocatalytic reactor (Xujiang, Nanjing). See Figure 1. An 800 W xenon lamp (70.8 103 lx) built in the reactor was used to simulate sunlight, and the lamp was surrounded by a quartz circulating water jacket to cool it. Catalytic reaction was carried out in a group of quartz tubes. These quartz tubes were placed parallel to the lamp, and the light distance was about 10 cm. Photocatalytic activity of the samples was evaluated by the photocatalytic degradation efficiency of 10 mg/L MB solution. Sample powders of 50 mg were added to 50 mL of MB solution. The system was stirred under dark conditions for 30 min to establish adsorption desorption equilibrium. Then oxygen was bubbled into the reactor to offer an O2 atmosphere. After that the system was stirred and illuminated at the same time. At a certain interval, 5 mL of the suspension was separated by centrifugation (9000 rpm) for 20 min to remove the photocatalyst; the supernatant was pumped by an injection syringe into a cuvette. The
3.1. TG Analysis. The TG curve of the precursor is illustrated in Figure 2. As shown in Figure 2, there are two main weight loss processes. The first loss (15.06%) below 300 °C is attributed to the release of absorbed EtOH and H2O on the precursor, whereas the second sharp loss (73.45%) from 300 to 500 °C may be due to the transformation of Ti4+ f TiO2 and the combustion release of the cotton template. Therefore, it is better to calcine the precursor above 500 °C for fully removing the template and preparing TiO2. 3.2. XRD Analysis. The X-ray diffraction technique was used to study the phase composition, weight fractions of each phase, and grain size of the samples. Figure 3 shows the XRD patterns of samples S-1, S-4, and S-7. All the peaks could be indexed to TiO2 of the anatase phase (JCPDS Card No. 21-1272) and rutile phase (JCPDS Card No. 21-1276). The sharp intense peak at the Bragg angle (2θ) of 25.3° is representative for anatase (101) phase reflections in all samples. XRD peaks at 2θ = 27.4° (110), 36.1° (101), and 54.3° (211) confirm the presence of the rutile phase in S-7. The patterns demonstrate clearly that all of the samples are well crystallized, and the widened peaks indicate the nanosize and some defects of the samples. It can be also seen that the phase composition and grain size vary with the calcination temperatures. The phase composition of sample S-1 which calcined at 450 °C is only anatase phase; however, there are two phases, anatase and rutile, when the temperature increases to 600 °C, and the weight fraction of rutile (listed in Table 1) increases from 2.93 to 44.21% with further increase of the calcination temperatures from 600 to 750 °C. Weight fractions of each phase and grain size of the TiO2 samples are listed in Table 1. The grain size of TiO2 particles is calculated by using the Scherrer equation on the anatase (101) and rutile (110) diffraction peaks. This result 1392
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indicates that the size of the sample particles is nanosized and increases with increasing calcination temperature. From Figure 3, it is observed that the transformation of anatase to rutile occurs when the calcination temperature reaches 600 °C. The low transformation temperature tells the nanosize of anatase TiO2 particles and some defects of the TiO2 lattice. XRD results show that TiO2 with different nanosizes, phase compositions, and some lattice defects can be prepared by calcining the precursors under air atmosphere and different temperatures. 3.3. SEM Observation. Figure 4 shows SEM micrographs of S-1, S-4, and S-7. From the low-magnification SEM (Figure 4a c), the prepared samples replicate the morphology of original cotton fibers very well. The prepared fibers, especially at 600 °C, are fractured due to the gas momentum from the combustion of the cotton template. In the magnified SEM (Figure 4d f), it is observed that the fibers are of hollow structure with inner diameter ranging from 3 to 10 μm. On the surface of the fibers, there are a large number of particles and the size of those increases gradually with the increase of calcination temperatures. It is noteworthy that the surface of S-1 is wrinkled and its wall is relatively compact. The EDS result of sample S-1, obtained at 450 °C, demonstrated that carbon still remains in S-1 and the weight fraction of carbon is about 20%. From what has been discussed, the calcining temperature must be higher than 450 °C in order to fully remove the cotton template. The wall of S-4 is basically composed of particles with narrow size distribution, while S-7 is formed by compact and rolled sheets. It has been reported that fine TiO2 particles have high catalytic activity due
to the fine structure and large surface area.14 Therefore, as a catalytic material S-4 will show good catalytic activity because of its special structure which can offer a large and effective surface area for the adsorption and catalytic reaction. 3.4. Specific Surface Area. Table 2 shows the comparison of specific surface areas of S-1, S-4, and S-7. It can be seen that the surface areas of the samples decreased with the increase of the calcined temperature due to the growth of their grains (seen in Table 1). 3.5. UV Vis Spectrum. Figure 5 shows the UV vis spectrum of S-4. This spectrum illustrates that TiO2 is photoactive in the Table 2. Specific Surface Areas and Pore Parameters of S-1, S-4, and S-7 sample sp surf. areaa(m2/g)
S-1 11.61
S-4 8.003
S-7 6.553
a Specific surface area calculated from the linear part of the BET plot (P/P0 = 0.1 0.25).
Table 1. Weight Fractions of Each Phase and Grain Size of S-1, S-4, and S-7 weight fraction of each phase (%)
grain size (nm)
sample
anatase
rutile
anatase
S-1 S-4
100 97.07
2.93
13.69 24.47
S-7
55.79
44.21
31.09
rutile
average 13.69 24.47
34.41
32.75
Figure 5. UV vis spectrum of S-4.
Figure 4. SEM micrographs of S-1 (a, d), S-4 (b, e), and S-7 (c, f). 1393
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Figure 6. Photocatalytic degradation efficiencies of MB solution on S-1 S-7.
UV region (
