In the last several decades, a wide variety of hollow inorganic nanostructures have received considerable attention [1, 2, 3, 4, 5, 6, 7, 8, 9] because of their promising applications, including drug delivery, photocatalytic redox reactions and water splitting, clean energy conversion and storage, gas sensing, and heavy-metal ion sequestration. TiO2 is a very important multifunctional material because of its peculiar and fascinating physicochemical properties and has a wide variety of potential uses in diverse fields, such as solar energy conversion, environmental purification and water treatment [10, 11, 12].
Recently, TiO2 hollow microspheres (TiO2-HMSs) have received increasing attention owing to their low density, high specific surface areas and good photocatalytic activity [5, 13, 14, 15, 16, 17, 18, 19]. Many strategies for the fabrication of TiO2-HMSs have been reported, including templating methods [20, 21, 22] and templating-free methods [17, 23, 24]. For example, the fabrication of TiO2-HMSs using SiO2 microspheres as templates and TiF4 as the precursor, where sacrifice of the template (SiO2 microspheres) was achieved by in situ hydrofluoric acid (HF) etching, has been reported [21]. Yu et al. [13, 25] also reported the fabrication of TiO2-HMSs by fluoride-induced self-transformation (FIST) in the presence of fluorides, such as (NH4)2TiF6 and NH4F, a method based on the template-free Ostwald ripening process. Our group also reported the fabrication of TiO2 hollow structures by the H2O2-assisted FIST method [18, 26, 27]. However, the presence of F can produce highly corrosive and toxic HF during the synthesis for the etching of solid cores, thus decreasing the practicality of this synthetic approach [23, 28]. Therefore, the challenge remains for the synthesis of TiO2-HMSs in a F-free process.
Herein, we report a facile F-free method for the fabrication of TiO2-HMSs using Ti(SO4)2 as precursor and K3PW12O40 (KPW) as template. It should be noted that the template, KPW, can be easily removed in diluted NaOH solution [29].
KCl solution (25 mL, 2.5 mol/L) was mixed with H3PW12O40 solution (5 0 mL, 0.4 mol/L) in a 250-mL flask under magnetic stirring to form a white milk-like KPW suspension. Ti(SO4)2 (2-25 mmol) was added into the flask and heated under reflux (approximately 125 °C) for 8 h. After cooling to room temperature, the solid products were collected by filtration, and the resultant cake was dispersed in NaOH solution (75 mL, 1.0 mol/L). After stirring for 1 h, the precipitate was collected by filtration and sequentially washed by distilled water until the pH of the filtrate was between 6 and 7. The prepared TiO2 samples were labeled as Tx, where x represents the amount of Ti(SO4)2 used in reaction (Table 1). For example, T4 represents the sample prepared in the presence of 4 mmol of Ti(SO4)2 in the presence of the KPW template. For comparison, TiO2 particles (Tp), by direct hydrolysis of Ti(SO4)2 in the absence of the KPW template, were also prepared.
The X-ray diffraction (XRD) patterns were obtained on a D8-advance X-ray diffractometer (Bruker, German) using Cu Kα radiation at a scanning rate of 0.02°/s and a step size of 0.02° in the 2θ range of 10°-70°. The accelerated voltage and applied current were 15 kV and 20 mA, respectively. The morphologies of the photocatalysts are characterized by transmission electron microscopy (TEM, Tecnai G20, USA) using an acceleration voltage of 200 kV and field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan) with an acceleration voltage of 10 kV. Fourier transform infrared spectroscopy (FTIR) was performed using a NEXUS-470 infrared spectrometer (Nicolet Co., USA). Diffuse reflectance spectroscopy (DRS) was performed on dry-processed disk samples using a ultraviolet-visible (UV-Vis) spectrophotometer (Lambda, Bio 35, PE Co., USA). X-ray photoelectron spectroscopy (XPS) measurements were obtained using a Multilab 2000 XPS system with a monochromatic Mg Kα source and a charge neutralizer. All the binding energies were referenced to the C 1s peak at 284.4 eV of the surface adventitious carbon.
The photocatalytic activity of the photocatalyst was evaluated by degradation of Brilliant Red X-3B (X3B), an anionic dye, under irradiation of a 3 W LED lamp (UVEC-4 II, Shenzhen Lamplic Tech. Co., Ltd., China) with a wavelength of 365±10 nm. Prior to irradiation, the suspensions were first sonicated, to ensure the photocatalyst was uniformly dispersed, and then shaken overnight in the dark, to establish the adsorption-desorption equilibrium. During the photocatalytic reaction, the reactor was mechanically stirred at a constant rate. The concentration of TiO2 was 1.0 g/L, and the initial concentration of dye used was 1.0×10−4 mol/L. At given intervals of irradiation, small volume aliquots were withdrawn and centrifuged. The supernatant fluid was then analyzed by an Agilent 8451 spectrometer at 510 nm for X3B.
Photoluminescence (PL) spectra were measured on a fluorescence spectrophotometer (F-7000, Hitachi, Japan). The excitation wavelength was 300 nm. Both of the widths for excitation and emission slits were 5.0 nm. The scanning speed was 1200 nm/min and the photomultiplier tube voltage was 700 V.
Figure 1(a) shows the TEM image of the prepared KPW templates. It can be seen that the templates are solid microspheres with diameters of approximately 0.5-1.0 μm. Observations at high magnification show that the surface of the templates is fairly smooth (Fig. 1(b)). Consistent with literature previous report [29], XRD characterization results (Fig. 1(c)) show that the prepared template is KPW. KPW microspheres were formed by the reaction of KCl and H3PW12O40 (Eq. (1)).
According to a previous report [29], KPW exhibits a weak solubility in water, which makes it a good candidate to be used as a template for the fabrication of TiO2-HMSs.
Phase structure and crystallization are important factors influencing the photocatalytic activity of TiO2 [30]. Fig. 2 shows the XRD patterns of the TiO2 photocatalysts. A broad peak at 2θ = 25.3°, corresponding to the (101) plane diffraction of anatase TiO2 (ICDD 21-1272), was observed for all the photocatalysts [31]. Upon increasing the amount of Ti(SO4)2 from 2 to 4 mmol, the relative crystallinity of anatase increased from 1.00 (T2) to 2.03 (T4), indicating an enhancement of crystallization. However, with a further increase in the amount of Ti(SO4)2 to 8 and 25 mmol, the relative crystallinity of anatase decreased to 1.47 (T8) and 1.35 (T25), respectively (Table 1). The relative crystallinity of anatase TiO2 for the Tp sample is 1.80, which is larger than that of T2 but smaller than that of the T4 sample. In the absence of KPW, too many crystal nucleation centers are formed by direct hydrolysis of Ti(SO4)2, which results in the formation of smaller TiO2 particle size (Tp). In the presence of the KPW template, however, nucleation of TiO2 is favored at the surface of KPW template owing to the strong interaction between TiO2 (positively charged) and KPW template (negatively charged). The limited nucleation centers lead to the formation of larger TiO2 particle sizes (T4). Therefore, it is understandable that T4 showed enhanced crystallization when compared with Tp, which was prepared in the same amount of Ti(SO4)2 precursor. However, with an increasing amount of Ti(SO4)2 precursor, the crystallization of TiO2-HMSs steadily decreases. This arises from the decreased relative concentration of KPW and increased acidity of solution. H2SO4 is formed during the hydrolysis of Ti(SO4)2 (Eq. (2)). The hydrolysis of Ti(SO4)2 is unfavorable in an acidic solution.
Figures 3 and 4 present the TEM and SEM images of the photocatalysts, respectively. In the absence of the KPW template, direct hydrolysis of Ti(SO4)2 resulted in only irregular TiO2 particles (Fig. 3(a) and (b)). In the presence of the KPW template, TiO2-HMSs were formed (Fig. 3(c)-(e) and Fig. 4(b)-(e)) owing to the template-directed deposition and template-sacrificial dissolution (Scheme 1). When the concentration of Ti(SO4)2 was low, the obtained TiO2-HMSs were fragile (example of T2 in Fig. 3(c) and Fig. 4(b)), possibly owing to the fact that the wall of TiO2 was thin. However, only solid aggregations of irregular TiO2 microspheres were obtained if the concentration of Ti(SO4)2 was too high (example of T25 in Figs. 3(f) and 4(f)). This is because of the failure for etching of the KPW template by NaOH owing to the thick wall of TiO2 on the surface of the KPW template. Perfect TiO2-HMSs were obtained (Fig. 3(d) and Fig. 4(c)-(d)) in the presence of 4 mmol Ti(SO4)2, and the inner diameters of the hollow microspheres were approximately 0.5-1 μm, consistent with the diameters of the KPW templates.
Figure 5 shows the FT-IR spectra of TiO2 particles (Tp) and TiO2-HMSs (T4). It can be observed that the two samples showed similar spectra. The bands appearing at approximately 3414 cm−1 correspond to the stretching vibration of OH groups linked to Ti atoms (Ti-OH), and the bands at 1636 and 1619 cm−1 relate to the flexion vibration of OH groups in adsorbed water. These results confirm the presence of hydroxyl groups in the samples. The adsorbed water and hydroxyl groups are crucial to the photocatalytic reactions since they react with photo-excited holes on the catalyst surface to produce hydroxyl radicals, which are powerful oxidants. The band centered at 1383 cm−1 is assigned to the bending vibrations of the C-H bond in the species linking the -Ti-O-Ti- structural network. The intensity of the absorption at 1189 cm−1 is proposed to arise from Ti-O-Ti stretching. The peak at around 544 cm−1 was ascribed to the absorption bands of Ti-O and O-Ti-O flexion vibration [32].
Figure 6 compares the UV-Vis absorption spectra of TiO2 particles (Tp) and TiO2-HMSs (T4). It can be clearly observed that the spectrum of T4 is almost the same with that of the Tp sample. It was reported that TiO2 hollow structures allowed multiple reflections of UV-visible light within the interior cavity that facilitates more efficient use of the light source, which is important for an enhanced photocatalytic activity of TiO2 [33]. However, in our present study, these TiO2 samples showed similar light-harvesting ability with a bandgap of 3.18-3.20 eV (Table 1). The similar absorption spectra of these TiO2 photocatalysts also reflect that the possibility of doping TiO2-HMSs by hetero elements, such as W and P, can be ruled out.
The XPS spectra of TiO2 particles (Tp) and TiO2-HMSs (T4) are shown in Fig. 7(a). It can be determined that both the photocatalysts contain Ti, O and C with binding energies for Ti 2p at 458 eV, O 1s at 531 eV and C 1s at 284 eV. The C element is ascribed to the residual carbon from the precursor solution and the adventitious hydrocarbon from the XPS instrument [34]. A strong XPS signal of Na (binding energy of about 499 eV) and weak signal of element W can also be observed from the XPS survey spectrum of the T4 sample (Fig. 7). The element Na should arise from the residual NaOH on the surface of the photocatalyst, which was used to decompose the KPW template. High resolution XPS spectrum of the W element, shown in Fig. 7(b), shows a binding energy of approximately 249 eV (W 4d5/2) and 261 eV (W 4d3/2). The concentrations of W and P elements in the T4 sample were calculated to be 0.74 and 0.50 at%, respectively. This infers that KPW template was not completely removed after addition of NaOH. The UV-Vis absorption spectra and XPS characterization results suggest that W and P elements have not been doped into the lattice of TiO2-HMSs, and may still exist in the form of KPW. It has been reported that the photocatalytic activity of TiO2 can be enhanced after modification by a polyoxometalate (POM), such as H3PW12O40, owing to the efficient electron transfer of POM from the conduction band of TiO2 [12, 35, 36, 37, 38]. The presence of residual KPW template, the salt of H3PW12O40, should also facilitate the enhancement of the photocatalytic activity of TiO2-HMSs.
To demonstrate the complex structure-function correlation, we examined the photocatalytic activity of the prepared samples using X3B degradation as a probe reaction. Fig. 8(a) shows the degradation profiles of X3B in the presence of TiO2 under UV irradiation. It can be observed that the X3B dye is very stable, which shows little photo-bleaching in the absence of the TiO2 photocatalyst [39].
From Fig. 8(a), it is clearly observed that, in the presence of TiO2, the concentration of X3B in solution decreases linearly with increasing the irradiation time, which obeys the kinetics of a pseudo-zero-order reaction rate equation. The slopes for the degradation curve (rate constants), which represent the photocatalytic activity of the photocatalyst, increase first and then decrease with increasing the amount of Ti(SO4)2 (Fig. 8(b)). The highest photocatalytic activity was achieved for the T4 sample (rate constant of 0.015 μmol/(L·min)), which is approximately 2.1 times higher than that of Tp (0.0073 μmol/(L·min)).
The reasons for TiO2-HMSs of T4 showed the highest photoreactivity can be explained as the follow. First, good crystallization (Table 1) of the T4 sample is beneficial to reduce the number of defects, thus the recombination rates of photo-generated electrons and holes for the TiO2-HMSs are smaller. Therefore, it is not surprising that the photocatalytic activity of TiO2-HMSs increased with increasing the amount of Ti(SO4)2 from 2 to 4 mmol owing to enhanced crystallinity. Second, residual KPW template facilitates the removal of photo-generated electrons from the conduction band of TiO2, retarding the recombination (Fig. 7). Third, the photocatalytic activity can be enhanced owing to entrapment of organic pollutants into the hollow interiors [27].
However, the hydrolysis reaction of Ti(SO4)2 was restrained if the concentration of Ti(SO4)2 was too high owing to the formation of highly concentrated H2SO4 (Eq. (2)). Therefore, it is not surprising to observe that the crystallization of the TiO2 sample decreased from 2.03 to 1.36 with the increase in the amount of Ti(SO4)2 from 4 to 25 mmol, which is consistent with the photocatalytic activity (Fig. 8). It is noteworthy that the relative crystallinity of Tp is 1.80, which is higher than that of the T8 and T25 samples. Hence, it is understandable that the photocatalytic activities of TiO2-HMSs of T8 and T25 are lower than Tp.
Although SiO2 microspheres have been widely used as a hard template [21], the synthetic procedures are time- and energy-consuming. Additionally, surfactants, such as cetyltrimethylammonium bromide (CTAB) and Brij-56, are usually needed to fabricate monodispersed SiO2 [40, 41], which are costly and not environmentally-friendly. In the present study, however, the KPW template can be formed by simply mixing solutions of KCl and H3PW12O40, which is very convenient.
Further experiments also indicate that hollow microspheres are readily separated from the slurry system by filtration or sedimentation after the photocatalytic reaction and can then be reused, which make them good photocatalysts for environmental purification.
A simple method for the fabrication of TiO2-HMSs was developed by using KPW microspheres as templates and Ti(SO4)2 as the precursor at 125 °C. The photocatalytic activity of anatase TiO2-HMSs, prepared in the presence of 4 mmol Ti(SO4)2, showed the highest photocatalytic activity, which is 2.1 times higher than that of TiO2 nanoparticles. This enhancement is ascribed to the synergistic effect of good crystallinity, coupling with residual KPW template and the special hollow structure. The proposed route for the preparation of TiO2-HMSs has the merits of being simple, reproducible, and easily scaled-up, and is expected to find wide application in photocatalysis, catalysis, electrochemistry, separation, purification, drug delivery and so on.
2015, Vol. 36 Issue (12): 2237-2243
PDF (1336 KB)
2015, Vol. 36 Issue (12): 2237-2243
PDF (1336 KB)