The spherical nanoporous TiO2 aerogels were prepared by a simple ethanol-thermal method, using spherical cellulose alcohol-gel as the template. The morphology, crystalline structure, pore size, specific surface area, and the photocatalytic activity of obtained TiO2 aerogel were separately characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption isotherms, and double beam UV-VIS spectrophotometer. The characteristics of TiO2 aerogels presented uniform sphere shape, good internal structural morphology, high specific surface area (ranging from 111.88 to 149.95 m2/g), and good crystalline anatase phase. Moreover, methyl orange dye was used as the target pollutant to characterize the photocatalytic activities and the adsorption performance. The photocatalytic experiment shows that the obtained spherical TiO2 aerogels had a higher degradation ratio of 92.9% on methyl orange dye compared with aspherical TiO2 aerogels prepared from other concentrations of tetrabutyl orthotitanate (TBOT).
In the early 1990s, the ordered mesoporous silica was found for the first time [
Among various oxide semiconductor photocatalysts, TiO2 has successfully attracted a great deal of interest and also has been the most promising photocatalyst due to its strong oxidizing potential, the low cost, high chemical stability against photocorrosion, and excellent degradation for organic pollutants [
The unique structure of cellulose aerogel bestows it on unusual properties. Herein we reported our research work in the preparation of spherical nanoporous TiO2 aerogel using spherical cellulose aerogel as the host matrix, employing tetrabutyl orthotitanate (TBOT), urea, and dehydrated alcohol as the starting materials. The resulting TiO2 spheres were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and the nitrogen gas adsorption (Micromeritics, ASAP 2020 analyzer) techniques. The influence of the concentration of TBOT on the morphologies and size of TiO2 nanoparticles was investigated. Meanwhile, the excellent photocatalytic activity for the degradation of methyl orange dye (MO) in aqueous solution was also demonstrated under UV irradiation at room temperature. We hope to provide a novel method for easily creating nanoporous TiO2 aerogel. Our findings may provide a new and “green” pathway for the design and fabrication of photocatalytic materials to solve the problem of organic pollution.
The commercial natural bamboo fiber, which was manufactured by slicing, steaming, cooking, and enzymatic process, was used as raw materials for preparation of spherical cellulose alcohol-gels. The fiber with 1.5 D (denier) and 38 mm length was purchased from Mingtong Bamboo Charcoal Products Co., Ltd., China. All other chemicals were of analytical grade and used without further purification.
The spherical cellulose alcohol-gels were synthesized using the hand-dropping procedure as follows. A solution with NaOH/urea/H2O of 7 : 12 : 81 (mass ratio) was cooled to −12°C as solvent system. 2.0 g natural bamboo cellulose fiber was dispersed into 100 g solvent system under vigorous stirring to obtain the transparent cellulose solution. Then, the cellulose solutions were added drop by drop to the well-mixed regenerate solution with a certain proportion of trichloropropane, ethyl acetate, and acetic and solidified at room temperature for 10 min before rinsing under running deionized water for 12 h. Finally, the spherical cellulose alcohol-gels were obtained after adequate exchanging for several times with ethanol.
The spherical TiO2 aerogels were prepared by an ethanol-thermal method. Firstly, 0.1 g of urea was added to a 100 mL beaker with 40 mL of anhydrous ethanol under magnetic stirring. Meanwhile, 0.1 mL of TBOT was put into the mixed solution. When TBOT mixed the urea and anhydrous ethanol, 1.0 g of spherical cellulose alcohol-gels was added into the solution. After placing it for 2 h to form sol-gel at room temperature, the final reactant was transferred into a 50 mL Teflon-lined stainless steel autoclave through heat treatment at 120°C for 10 h. Then the products were separately washed with deionized water, ethanol, and t-butyl alcohol three times every day for two days and freeze-dried overnight at 30–40 Pa of vacuum. Finally, the TiO2 with spherical cellulose aerogel template was calcined in oxygen atmosphere at 500°C for 3 h (heating rate: 1°C/min). Similarly, various nanoporous TiO2 aerogels prepared from 0.5 mL and 5 mL of TBOT were also synthesized for comparison while the other experimental conditions were not changed.
The morphology of spherical TiO2 aerogels was observed on the scanning electron microscopy (SEM; Quanta 200, FEI, Hillsboro, OR, USA) and the transmission electron microscopy (TEM; JEOL 2011, FEI Holland). X-ray diffraction (XRD) patterns of spherical TiO2 aerogels were measured by a Rigaku D/Max-rB diffractometer (Tokyo, Japan) with Cu-K
The photocatalytic activities of the spherical nanoporous TiO2 aerogel were investigated in terms of the photocatalytic degradation of 10 mg/L methyl orange dye (MO) solution under illumination of UV light at 254 nm. Before the illumination, 50 mg of the spherical TiO2 aerogels was first added to photocatalytic device filled with 150 mL of 10 mg/L MO, and the mixture was stirred for 30 min to reach a saturated state. Simultaneously, the adsorption/desorption of MO and O2 molecules on the spherical TiO2 aerogels surface reached an equilibrium in the darkness. Then the stirring solution was illuminated by the vertically incident UV light. During the photocatalytic reaction, samples were carried from the supernatant solution at every 15 min and were immediately centrifuged at 2000 r/min for 5 min. The concentration of MO after catalyzing was measured by a TU-1901 UV-visible spectrometer at 460 nm.
Figure
Photographs of TiO2 aerogel: (a) aspherical TiO2 aerogel-0.1 mL of TBOT, (b) aspherical TiO2 aerogel-0.5 mL of TBOT, and (c) spherical TiO2 aerogel-5.0 mL of TBOT.
To observe the internal structure of the TiO2 aerogel, these TiO2 aerogels were broken into two parts, and the surface of the fracture surface could be imaged by scanning electron microscopy (SEM). Figures
SEM images of TiO2 aerogel: (a) internal structure of aspherical TiO2 aerogel-0.1 mL of TBOT, (b) internal structure of aspherical TiO2 aerogel-0.5 mL of TBOT, (c) internal structure of spherical TiO2 aerogel-5.0 mL of TBOT, and (d) external surface structure of spherical TiO2 aerogel.
The TEM images of these TiO2 aerogels were shown in Figure
TEM images of TiO2 aerogel: (a) aspherical TiO2 aerogel-0.1 mL of TBOT, (b) aspherical TiO2 aerogel-0.5 mL of TBOT, and (c) spherical TiO2 aerogel-5.0 mL of TBOT.
Figure
N2 adsorption-desorption isotherm curves and corresponding pore-size distribution of (a) aspherical TiO2 aerogel-0.1 mL of TBOT, (b) aspherical TiO2 aerogel-0.5 mL of TBOT, and (c) spherical TiO2 aerogel-5.0 mL of TBOT.
Detailed information in terms of the specific surface areas and the porosities of the TiO2 aerogels was summarized in Table
Specific surface area and pore structure parameters of TiO2 aerogel prepared from different concentrations of TBOT.
Samples |
|
Mesoporous volume (cm3/g) | Average pore diameter (nm) |
---|---|---|---|
TiO2-0.1 mL of TBOT | 129.32 | 0.52 | 16.20 |
TiO2-0.5 mL of TBOT | 111.88 | 0.42 | 15.04 |
TiO2-5.0 mL of TBOT | 149.95 | 0.49 | 12.96 |
The X-ray diffraction (XRD) patterns of TiO2 aerogel are shown in Figure
XRD patterns of Ti diffraction peak at
The chemical compositions and elemental environments of the catalysts were tested by X-ray photoelectron spectroscopy (XPS); the specific spectra are shown in Figure
XPS spectra of TiO2 aerogel (a) total spectrum; high-resolution spectrum of the (b) Ti 2p and (c) O 1s region.
The photocatalytic activities of the spherical nanoporous TiO2 aerogel were investigated in terms of the photocatalytic degradation of 10 mg/L MO under illumination of UV light within 90 min (Figure
The degradation curves of MO in the presence of the TiO2 aerogels prepared from 0.1 mL of TBOT, 0.5 mL of TBOT, and 0.5 mL of TBOT.
Spherical TiO2 aerogels were prepared by a simple ethanol-thermal method, using spherical cellulose aerogel as the template and TBOT as raw material. The obtained TiO2 aerogels consisted of TiO2 nanoparticles with the diameter 15–20 nm. The high specific surface area, ranging from 111.88 m2/g to 149.95 m2/g, and good porosity of the network structures provided a large number of active sites for photocatalysis. The highest UV light activity, giving methyl orange degradation of 92.9%, was achieved by spherical TiO2 aerogel prepared from 5.0 mL of TBOT under the calcination condition of 500°C for 3 h.
The authors declare that there are no competing interests regarding the publication of this paper.
This research was financially supported by the Industry Research Special Funds for Public Welfare Project under Grant no. 201504602, the Key Laboratory of Wood Science and Technology, Zhejiang Province, under Grant no. 2014lygcz002, and the Fundamental Research Funds for the Central Universities under Grant no. 2572014EB01-02. Special thanks are due to Professor Shujun Li for her equipment and analysis.