# Synthesis and Characterization of Uniform Spherical Nanoporous TiO2 Aerogel Templated by Cellulose Alcohol-Gel with Enhanced Photocatalytic Activity

Authors: Zhiming Liu; Peng Wu; Shaoli Yang; Haiying Wang; Chunde Jin
Journal: International Journal of Polymer Science
 (2016)

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## Abstract

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).


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## Body

## 1. Introduction

In the early 1990s, the ordered mesoporous silica was found for the first time [1, 2]. Since then, mesoporous materials have become one of the most important materials for catalysis [3–5]. With the fascinating properties of high specific surface areas, tunable large pore sizes, large pore volumes, controllable framework compositions, and alternative pore shapes, mesoporous materials can also be used in drug delivery, separation, sorption, fuel cells, gas separators, and magnetism areas [6–8]. Recently, researchers have paid much more attention to the mesoporous materials, such as TiO2, ZnO, and SnO2, which have the photocatalytic oxidation abilities to degrade the organic pollutants in water and air [9–11].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 [12, 13]. However, practical applications of titanium dioxide are still quite limited, mainly because of its low quantum efficiency and the broad bandgap which responds only to UV light [14]. In addition to the VB position, the pore configuration and particle size distribution of photocatalyst also influence the activity of a photocatalyst. In order to obtain nanoporous structure and high dispersibility of TiO2, there are three approaches: the template synthesis [15–19], the anodic oxidation [20–22], and the hydrothermal synthesis [23–25]. Among those, the template-based synthesis route takes the advantage of straightforward controlling over the morphology of the resulting TiO2 nanoporous structured materials and maintaining TiO2 good disperse stability on the surface of the template [19]. Recently, cellulose fiber has been used as a template to synthesize nanoporous structured materials, such as TiO2, CdS, and Fe2O3. Furthermore, cellulose alcohol-gel or hydrogel not only takes over cellulose fiber’s excellent characteristics, including high tensile strength, high water holding capacity, high crystallinity, and good biocompatibility but also generates the dimensional and porous structure and the uniform nanopore size [26].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.


## 2. Materials and Methods

### 2.1. Materials

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.


### 2.2. Preparation of Spherical Cellulose Alcohol-Gels

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.


### 2.3. Preparation of Spherical TiO2 Aerogels

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.


### 2.4. Characterization of Spherical TiO2 Aerogels

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-Kα radiation (45 kV, 40 mA, λ = 1.54178 Å) ranging from 4° to 90° (2θ angle). The pore size and the BET specific surface areas of the samples were calculated using N2 adsorption-desorption isotherms measured at −196°C with an ASAP 2020 instrument (Micromeritics, American). The elemental compositions and states were tested by X-ray photoelectron spectroscopy (XPS, Chanhassen, MN, USA) with Al-Kα radiation (h
ν = 1486.6 eV).


### 2.5. Photocatalytic Experiment

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.


## 2.1. Materials

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.


## 2.2. Preparation of Spherical Cellulose Alcohol-Gels

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.


## 2.3. Preparation of Spherical TiO2 Aerogels

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.


## 2.4. Characterization of Spherical TiO2 Aerogels

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-Kα radiation (45 kV, 40 mA, λ = 1.54178 Å) ranging from 4° to 90° (2θ angle). The pore size and the BET specific surface areas of the samples were calculated using N2 adsorption-desorption isotherms measured at −196°C with an ASAP 2020 instrument (Micromeritics, American). The elemental compositions and states were tested by X-ray photoelectron spectroscopy (XPS, Chanhassen, MN, USA) with Al-Kα radiation (h
ν = 1486.6 eV).


## 2.5. Photocatalytic Experiment

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.


## 3. Results and Discussion

### 3.1. Morphology Analysis

Figure1 shows the photographs of TiO2 aerogel prepared from different concentrations of TBOT. With the increasing concentration of TBOT, TiO2 aerogel was shifted to spherical structure, and these TiO2 aerogels had the uniform shapes showed in Figure 1(c). For the morphology of TiO2 aerogel, the cellulose alcohol-gel played a role of template and morphology guiding in the preparation of nanoporous TiO2 aerogel.Figure 1
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 2(a)–2(c), respectively, represented morphology of TiO2 aerogel prepared from different concentrations of TBOT. The formation of the network structure of TiO2 aerogel could be attributed to the accumulation of the TiO2 nanoparticles. When the concentration of TBOT was increasing, the network structure of TiO2 aerogel was more obvious. Moreover, the external surface of spherical TiO2 aerogel was filled with pores shown in Figure 2(d). These pores mainly inherited the porosity of spherical cellulose aerogels matrix while the spherical TiO2-cellulose complex aerogel was calcined. Detailed information concerning the pore size and the specific surface area was analyzed in pore structure.Figure 2
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 3. The shape of TiO2 particle prepared from 0.1 mL of TBOT was irregular and the agglomeration could be clearly observed in Figure 3(a). But when the concentration of TBOT was added to 0.5 mL of TBOT, the TiO2 particle’s shape was almost in conformity and had minor agglomeration shown in Figure 3(b). The shape of TiO2 particle was uniform sphere and scarcely had agglomeration until the concentration of TBOT was up to 5.0 mL, which was shown in Figure 3(c). Furthermore, the diameter of TiO2 particle was about 15–20 nm, implying that the single TiO2 particle that consisted of TiO2 aerogel was nanoparticle.Figure 3
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.


### 3.2. Pore Structure

Figure4 shows nitrogen adsorption-desorption curves (inset) and corresponding pore-size distribution of TiO2 aerogel prepared from different concentrations of TBOT. The isotherms were measured to be Type IV, according to the IUPAC classification. The hysteresis loops were obviously observed at P
/
P
0 = 0.8–1.0, and the shapes of these hysteresis loops conformed to H3 hysteresis loops, indicating that obtained TiO2 aerogels were mesoporous materials [27–30]. The pore-size distribution results show that every sample had a bimodal distribution (2-3 nm and 10–100 nm), and the peak pore was around 2.3 nm and 34 nm. The different diameters of the pores may result from the different stacking styles of the TiO2 particles, which were confirmed by SEM.Figure 4
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.
(a)
(b)
(c)Detailed information in terms of the specific surface areas and the porosities of the TiO2 aerogels was summarized in Table 1. With the increasing concentration of TBOT, the shape of the TiO2 aerogel was shifted to sphere, leading to an increase in the specific surface areas and a drop in the pore size. When the consumption of TBOT was 5.0 mL, the specific surface area of obtained TiO2 aerogel can reach 149.95 m2/g, and the corresponding pore size was 12.96 nm. In addition, the increased specific area of spherical TiO2 aerogel compared with the aspheric TiO2 aerogels mainly came from these mesoporous areas, which agreed with the pore volume and pore-size distributions.Table 1
Specific surface area and pore structure parameters of TiO2 aerogel prepared from different concentrations of TBOT.
Samples
S
B
E
T (m2/g)
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


### 3.3. XRD and XPS Results

The X-ray diffraction (XRD) patterns of TiO2 aerogel are shown in Figure 5. With the increasing concentration of TBOT, the (101) peak position of TiO2 aerogel shifted to the higher-angle region shown in Figure 5(a). And the obvious characteristic diffraction peaks of TiO2 aerogel could be seen in Figure 5(b), which was attributed to anatase TiO2 crystals [31]. No peaks from other impurities were detected in this XRD pattern. Furthermore, the strong and sharp diffraction peaks shown in Figure 5 indicated that the obtained spherical TiO2 aerogel had high crystallinity [32].Figure 5
XRD patterns of Ti diffraction peak at2
θ = 25.26° (a) and TiO2 aerogels calcinated at 500°C for 3 h.
(a)
(b)The chemical compositions and elemental environments of the catalysts were tested by X-ray photoelectron spectroscopy (XPS); the specific spectra are shown in Figure6. Three elements, Ti, O, and C, were revealed in Figure 6(a). Among them, the existence of C elements may result from trace organic pollutants in the vacuum measurement of XPS instrument. High-resolution spectra of Ti 2p and O 1s region were separately shown in Figures 6(b) and 6(c). The obvious peaks, where binding energies were 458.59 eV and 464.34 eV, were the characteristic peaks of Ti 2p1/2 and Ti 2p3/2. Furthermore, the binding energy of Ti 2p1/2 was 5.75 eV lower than that of Ti 2p3/2, which was the important criterion of existence of Ti4+ [33, 34]. Figure 6(c) shows the XPS spectra of O 1s with a broad and asymmetrical peak, suggesting that the oxygen species on the surface of spherical TiO2 aerogel was not a single species. The O 1s region could be fitted with three peaks, whose binding energies were 529.78, 531.11, and 531.98 eV. These peaks were in accordance with the binding energy data of lattice oxygen, hydroxyl, and physically adsorbed oxygen [35]. O 1s mainly existed as lattice oxygen in Ti-O-Ti bonds observed from the areas of the peaks. This reveals that the obtained spherical TiO2 aerogel consisted of titania and some Ti-OH generated in the reaction process [35].Figure 6
XPS spectra of TiO2 aerogel (a) total spectrum; high-resolution spectrum of the (b) Ti 2p and (c) O 1s region.
(a)
(b)
(c)


### 3.4. Photocatalytic Activity

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 7). As a control, the activities of aspherical TiO2 aerogels were also examined under the same conditions. Results of the photocatalytic investigation are shown in Figure 7. Obviously, the photocatalytic activity of spherical TiO2 aerogel prepared from 5.0 mL of TBOT was higher than the aspherical TiO2 aerogels prepared from 0.1 mL of TBOT and 0.5 mL of TBOT. In the process of adsorption equilibrium, the first reaction time of 30 min, the removal rate of MO with the spherical TiO2 aerogels was higher than that of aspherical TiO2 aerogels. This phenomenon may result from the fact that spherical TiO2 aerogels had the higher specific surface area than aspherical TiO2 aerogels. Moreover, with the increase in degradation time, the concentration of MO decreased rapidly, and, after 90 min of irradiation, the decomposition ratio of MO over the spherical TiO2 aerogels prepared from 5.0 mL of TBOT was about 92.9%, while those of aspherical TiO2 aerogels prepared from 0.1 mL and 0.5 mL of TBOT were 87.8% and 88.8%. Therefore, both the spherical TiO2 aerogels and the aspherical TiO2 aerogels had excellent photocatalytic activity of MO, which can be attributed to special anatase phase, specific surface areas, and unique structure [7].Figure 7
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.


## 3.1. Morphology Analysis

Figure1 shows the photographs of TiO2 aerogel prepared from different concentrations of TBOT. With the increasing concentration of TBOT, TiO2 aerogel was shifted to spherical structure, and these TiO2 aerogels had the uniform shapes showed in Figure 1(c). For the morphology of TiO2 aerogel, the cellulose alcohol-gel played a role of template and morphology guiding in the preparation of nanoporous TiO2 aerogel.Figure 1
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 2(a)–2(c), respectively, represented morphology of TiO2 aerogel prepared from different concentrations of TBOT. The formation of the network structure of TiO2 aerogel could be attributed to the accumulation of the TiO2 nanoparticles. When the concentration of TBOT was increasing, the network structure of TiO2 aerogel was more obvious. Moreover, the external surface of spherical TiO2 aerogel was filled with pores shown in Figure 2(d). These pores mainly inherited the porosity of spherical cellulose aerogels matrix while the spherical TiO2-cellulose complex aerogel was calcined. Detailed information concerning the pore size and the specific surface area was analyzed in pore structure.Figure 2
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 3. The shape of TiO2 particle prepared from 0.1 mL of TBOT was irregular and the agglomeration could be clearly observed in Figure 3(a). But when the concentration of TBOT was added to 0.5 mL of TBOT, the TiO2 particle’s shape was almost in conformity and had minor agglomeration shown in Figure 3(b). The shape of TiO2 particle was uniform sphere and scarcely had agglomeration until the concentration of TBOT was up to 5.0 mL, which was shown in Figure 3(c). Furthermore, the diameter of TiO2 particle was about 15–20 nm, implying that the single TiO2 particle that consisted of TiO2 aerogel was nanoparticle.Figure 3
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.


## 3.2. Pore Structure

Figure4 shows nitrogen adsorption-desorption curves (inset) and corresponding pore-size distribution of TiO2 aerogel prepared from different concentrations of TBOT. The isotherms were measured to be Type IV, according to the IUPAC classification. The hysteresis loops were obviously observed at P
/
P
0 = 0.8–1.0, and the shapes of these hysteresis loops conformed to H3 hysteresis loops, indicating that obtained TiO2 aerogels were mesoporous materials [27–30]. The pore-size distribution results show that every sample had a bimodal distribution (2-3 nm and 10–100 nm), and the peak pore was around 2.3 nm and 34 nm. The different diameters of the pores may result from the different stacking styles of the TiO2 particles, which were confirmed by SEM.Figure 4
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.
(a)
(b)
(c)Detailed information in terms of the specific surface areas and the porosities of the TiO2 aerogels was summarized in Table 1. With the increasing concentration of TBOT, the shape of the TiO2 aerogel was shifted to sphere, leading to an increase in the specific surface areas and a drop in the pore size. When the consumption of TBOT was 5.0 mL, the specific surface area of obtained TiO2 aerogel can reach 149.95 m2/g, and the corresponding pore size was 12.96 nm. In addition, the increased specific area of spherical TiO2 aerogel compared with the aspheric TiO2 aerogels mainly came from these mesoporous areas, which agreed with the pore volume and pore-size distributions.Table 1
Specific surface area and pore structure parameters of TiO2 aerogel prepared from different concentrations of TBOT.
Samples
S
B
E
T (m2/g)
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


## 3.3. XRD and XPS Results

The X-ray diffraction (XRD) patterns of TiO2 aerogel are shown in Figure 5. With the increasing concentration of TBOT, the (101) peak position of TiO2 aerogel shifted to the higher-angle region shown in Figure 5(a). And the obvious characteristic diffraction peaks of TiO2 aerogel could be seen in Figure 5(b), which was attributed to anatase TiO2 crystals [31]. No peaks from other impurities were detected in this XRD pattern. Furthermore, the strong and sharp diffraction peaks shown in Figure 5 indicated that the obtained spherical TiO2 aerogel had high crystallinity [32].Figure 5
XRD patterns of Ti diffraction peak at2
θ = 25.26° (a) and TiO2 aerogels calcinated at 500°C for 3 h.
(a)
(b)The chemical compositions and elemental environments of the catalysts were tested by X-ray photoelectron spectroscopy (XPS); the specific spectra are shown in Figure6. Three elements, Ti, O, and C, were revealed in Figure 6(a). Among them, the existence of C elements may result from trace organic pollutants in the vacuum measurement of XPS instrument. High-resolution spectra of Ti 2p and O 1s region were separately shown in Figures 6(b) and 6(c). The obvious peaks, where binding energies were 458.59 eV and 464.34 eV, were the characteristic peaks of Ti 2p1/2 and Ti 2p3/2. Furthermore, the binding energy of Ti 2p1/2 was 5.75 eV lower than that of Ti 2p3/2, which was the important criterion of existence of Ti4+ [33, 34]. Figure 6(c) shows the XPS spectra of O 1s with a broad and asymmetrical peak, suggesting that the oxygen species on the surface of spherical TiO2 aerogel was not a single species. The O 1s region could be fitted with three peaks, whose binding energies were 529.78, 531.11, and 531.98 eV. These peaks were in accordance with the binding energy data of lattice oxygen, hydroxyl, and physically adsorbed oxygen [35]. O 1s mainly existed as lattice oxygen in Ti-O-Ti bonds observed from the areas of the peaks. This reveals that the obtained spherical TiO2 aerogel consisted of titania and some Ti-OH generated in the reaction process [35].Figure 6
XPS spectra of TiO2 aerogel (a) total spectrum; high-resolution spectrum of the (b) Ti 2p and (c) O 1s region.
(a)
(b)
(c)


## 3.4. Photocatalytic Activity

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 7). As a control, the activities of aspherical TiO2 aerogels were also examined under the same conditions. Results of the photocatalytic investigation are shown in Figure 7. Obviously, the photocatalytic activity of spherical TiO2 aerogel prepared from 5.0 mL of TBOT was higher than the aspherical TiO2 aerogels prepared from 0.1 mL of TBOT and 0.5 mL of TBOT. In the process of adsorption equilibrium, the first reaction time of 30 min, the removal rate of MO with the spherical TiO2 aerogels was higher than that of aspherical TiO2 aerogels. This phenomenon may result from the fact that spherical TiO2 aerogels had the higher specific surface area than aspherical TiO2 aerogels. Moreover, with the increase in degradation time, the concentration of MO decreased rapidly, and, after 90 min of irradiation, the decomposition ratio of MO over the spherical TiO2 aerogels prepared from 5.0 mL of TBOT was about 92.9%, while those of aspherical TiO2 aerogels prepared from 0.1 mL and 0.5 mL of TBOT were 87.8% and 88.8%. Therefore, both the spherical TiO2 aerogels and the aspherical TiO2 aerogels had excellent photocatalytic activity of MO, which can be attributed to special anatase phase, specific surface areas, and unique structure [7].Figure 7
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.


## 4. Conclusions

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.



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