J. Phys. Chem. C 2007, 111, 18965-18969
18965
Nanocrystalline Fe/TiO2 Visible Photocatalyst with a Mesoporous Structure Prepared via a Nonhydrolytic Sol-Gel Route Jian Zhu, Jie Ren, Yuning Huo, Zhenfeng Bian, and Hexing Li* Department of Chemistry, Shanghai Normal UniVersity, 100 Guilin Road, Shanghai, China 200234 ReceiVed: July 1, 2007; In Final Form: September 19, 2007
Fe-doped nanocrystalline TiO2 (Fe/TiO2) with a mesoporous structure was prepared via a facile nonhydrolytic sol-gel (NSG) route. During photodegradation of methylene blue under visible light irradiation, as-prepared Fe/TiO2 exhibited a higher activity than either the undoped TiO2 or the Fe/TiO2 obtained via the traditional hydrolytic sol-gel route. The correlation of the photocatalytic performance to the structural characteristics is discussed based on detailed characterizations. The promoting effect of the Fe-doping on the photocatalytic activity could be attributed to the formation of intermediate energy levels that allow Fe/TiO2 to be activated easily in the visible area. The optimum Fe-content in Fe/TiO2 is determined as 0.1% (Fe/Ti molar ratio). The nonhydrolytic sol-gel method is superior to the traditional hydrolytic sol-gel method owing to the controllable reaction rate and lack of surface tension, which ensures the formation of mesopores and well-crystallized anatase in the Fe/TiO2 sample, leading to a higher activity since the reactant molecules are easily adsorbed and the recombination between the photoelectrons and the holes is effectively inhibited. Meanwhile, the nonhydrolytic sol-gel process could strengthen the incorporation of Fe-dopants in the TiO2 network, which may further enhance the promoting effect of Fe-doping on the photocatalytic activity.
Introduction Photocatalysis has attracted considerable attention owing to its potential in environmental cleaning and energy regeneration. Most studies have involved TiO2-based photocatalysts owing to their low cost, nontoxicity, high stability, and easy preparation.1-4 However, pure TiO2 could be activated only by UV light due to its energy gap (3.0 to ∼3.2 eV) and thus, only less than 5% solar light can be utilized. Besides, pure TiO2 also exhibits a low quantum efficiency due to easy recombination between photoelectrons and holes. Doping TiO2 is one of the promising ways to extend light absorption to the visible region and reduce the recombination of photoinduced electrons and holes.5-8 Among various dopants, the Fe3+-dopant is most frequently employed owing to its unique half-filled electronic configuration,9-11 which might narrow the energy gap through the formation of new intermediate energy levels and also diminish recombination of photoinduced electrons and holes by capturing photoelectrons. Up to now, nearly all Fe-doped TiO2 photocatalysts have been prepared based on the traditional solgel method induced by traces of water12-14 (i.e., the hydrolysis and condensation of metal halides or metal alkoxides). However, the fast rate of hydrolysis is unable to form a homogeneous metal-oxygen-metal network in gels. Therefore, the doping based on this method makes it difficult to obtain a uniform structured oxide with a well-dispersed dopant. Recently, the nonhydrolytic (e.g., alcoholytic, etc.) sol-gel (NSG) method has been widely developed by using organic solvents instead of water as the oxygen donor, which shows a superiority over the hydrolytic sol-gel (HSG) in achieving a high activity of the TiO2 photocatalyst owing to a controllable reaction rate.15-18 When used in the synthesis of doped TiO2 photocatalysts, this method might also strengthen the interaction * Corresponding author. E-mail:
[email protected]; fax: +86 21 64322272.
between the dopants and the TiO2 network, leading to an enhanced promoting effect. However, such work has not been reported so far. In this paper, we report a new Fe-doped TiO2 visible photocatalyst prepared by a -NSG route (Scheme 1), which shows an enhanced activity owing to both the mesoporous structure and the high crystallinity. Experimental Procedures Catalyst Preparation. The Fe-doped TiO2 samples (Fe/TiO2) were prepared by a previously reported -NSG route by Vioux et al.19 In a typical synthesis, 1.0 mL of TiCl4 was added dropwise into 20 mL of a t-butyl alcohol solution containing the required amount of Fe(NO3)3 at 313 K and allowed to stir for 3 h at 333 K until the formation of a translucent alcogel occurred. After being dried in air at 353 K for 12 h, the alcogel was calcined for 5 h at 573, 673, 773, and 873 K, respectively, followed by being crushed into nanoparticles and maintained in vacuum until the time of use. The as-prepared samples are denoted as xFe/TiO2(NSG), where x and NSG represent the Fe/ Ti molar ratio and the NSG- route, respectively. Undoped TiO2 (x ) 0) was obtained by the same procedure without adding Fe(NO3)3. For comparison, both the undoped TiO2 and the Fe/ TiO2 samples were also prepared by a conventional -HSG and are denoted as xFe/TiO2(HSG)-. Catalyst Characterization. X-ray diffraction (XRD) patterns were collected on a Rigacu Dmax-3C with Cu KR radiation. Selected area electron diffraction (SAED) images and transmission electronic microscopy (TEM) morphologies were recorded on a JEM-2010 instrument. Raman spectra, photoluminescence spectroscopy (PLS), and UV-vis diffuse reflectance spectra (UV-vis DRS) were conducted on Dilor Super LabRam II, Varian Cary-Eclipse 500, and MC-2530 instruments, respectively. The N2 adsorption-desorption isotherms were determined by the BET method on a NOVA 4000 instrument at 77
10.1021/jp0751108 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2007
18966 J. Phys. Chem. C, Vol. 111, No. 51, 2007
Figure 1. XRD patterns of 0.1% Fe/TiO2(NSG) sample calcined at different temperatures: (a) as-received, (b) 473 K, (c) 573 K, (d) 673 K, and (e) 773 K.
SCHEME 1 : Nonhydrolytic Route to Nanocrystalline Fe/TiO2 Visible Photocatalyst
K, from which the surface area (SBET), pore volume (Vp), and average pore diameter (dp) were calculated by using the BJH method. Activity Test. The liquid-phase photocatalytic degradation of methylene blue (MB) was carried out at 303 K in an 80 mL self-designed quartz reactor containing 0.050 g of catalyst and 50 mL of 10 mg/L MB aqueous solution. After stirring for 30 min until reaching adsorption equilibrium, the photocatalytic reaction was initiated by irradiating the system with three 150 W xenon lamps located 30 cm away from the reaction solution. All UV lights with wavelengths less than 420 nm were removed by a glass filter. Each run of reactions lasted for 3 h, and the MB was analyzed by a UV spectrophotometer (UV 7504/PC) at its characteristic wavelength (λ ) 665 nm) to determine the degradation yield. Preliminary tests demonstrated that only less than 7% MB decomposed after reaction for 3 h in the absence of either the photocatalyst or the light irradiation and thus could be neglected in comparison with MB degradation yields resulting from real photocatalysis. The reproducibility of the results was checked by repeating the results at least 3 times and was found to be within acceptable limits ((5%). Results and Discussion Figure 1 shows the XRD patterns of 0.1% Fe/TiO2(NSG) calcined at different temperatures. Unlike Fe/TiO2(HSG), which is present mainly in the amorphous state,20 the as-received Fe/ TiO2(NSG) was present in a well-crystallized anatase phase even without calcinations since the smooth reaction between TiCl4 and t-butyl alcohol during the -NSG process allows nucleation and crystallization of TiO2.16 Thus, calcination of the Fe/TiO2(NSG) sample from 473 to 673 K only slightly enhances the crystallization degree of the anatase. A further increase in the calcination temperature to 773 K causes a phase transformation from anatase to rutile. The phase transformation temperature of Fe/TiO2(NSG) was nearly 100 K higher than that of either the undoped TiO2(NSG) or the Fe/TiO2(HSG). The promoting
Zhu et al.
Figure 2. XRD patterns of (a) TiO2(HSG), (b) 0.1% Fe/TiO2(HSG), (c) TiO2(NSG), and (d) 0.1% Fe/TiO2(NSG) samples calcined at 573 K.
effect of the Fe-doping on the thermal stability of the anatase could be attributed to the coverage of the TiO2 surface by Fe2O3 species that might diminish the heating effect.21 Meanwhile, partial Fe-dopants incorporated into the TiO2 network might also stabilize the anatase phase owing to enhanced lattice strain.22 Fe/TiO2(NSG) exhibits a higher thermal stability of anatase than Fe/TiO2(HSG) owing to the uniform porous structure and larger particles (as discussed next), which could inhibit the rearrangement and agglomeration of the TiO2 particles.16 No significant XRD peaks indicative of Fe-species were observed even after being calcined at 773 K, indicating a high dispersion of the Fe-dopants in TiO2. The XRD patterns in Figure 2 further confirm that the -NSG route is favorable for the crystallization of anatase, while Fe-doping provides very little or even no improvement on the crystallization degree of the anatase phase. As shown in Figure 3, the attached SAED images also confirm that both the TiO2(NSG) and the Fe/TiO2(NSG) samples are in the highly crystallized anatase. The TEM morphologies reveal that both TiO2(NSG) and 0.1% Fe/TiO2(NSG) display larger particles with a uniform worm-like mesoporous structure, while TiO2(HSG) displays only an irregular porous structure, corresponding to type IV and type II N2 adsorption-desorption isotherms, as shown in Figure 4. t-Butyl alcohol may act as a structural directing agent to construct the mesoporous structure in the precursors of TiO2(NSG) and Fe/TiO2(NSG), which could be preserved after being calcined at high temperatures owing to the lack of surface tension in the absence of water. Table 1 summarizes the surface area (SBET), pore volume (Vp), and pore diameter (dp) of different samples calculated by the BJH method based on the N2 adsorption-desorption isotherms. First, 0.1% Fe/TiO2(NSG) calcined at 573 K exhibits lower SBET values than undoped TiO2(NSG) due to an increase in the particle size (see Figure 3). Second, TiO2(NSG) and 0.1% Fe/ TiO2(NSG) calcined at 573 K exhibit smaller dp values than the corresponding TiO2(HSG) and 0.1% Fe/ TiO2(HSG) since the former samples contain only mesopores with a relatively uniform pore size around 3.0 nm, while the latter samples contain irregular pores with diameters ranging from 0.5 to 20 nm. Although having smaller dp values, TiO2(NSG) and 0.1% Fe/TiO2(NSG) exhibit much higher Vp values than the corresponding TiO2(HSG) and 0.1% Fe/TiO2(HSG) since more pores are generated during the -NSG process (see the TEM morphologies in Figure 3), which could also account for their higher
Nanocrystalline Fe/TiO2 Visible Photocatalyst
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Figure 3. TEM morphologies of (a) TiO2(HSG), (b) TiO2(NSG), and (c) 0.1% Fe/TiO2(NSG) samples calcined at 573 K. Insets are the SAED images.
Figure 4. N2-sorption isotherms of different photocatalysts calcined at 573 K.
TABLE 1: Structural Parameters of Undoped TiO2 and Fe-Doped Samples sample TiO2(NSG) 0.1% Fe/TiO2(NSG) 0.1% Fe/TiO2(NSG) 0.1% Fe/TiO2(NSG) 0.1% Fe/TiO2(NSG) 0.05% Fe/TiO2(NSG) 0.2% Fe/TiO2(NSG) TiO2(HSG) 0.1% Fe/TiO2(HSG)
temp (K) SBET (m2/g) dp (nm) Vp (cm3/g) 573 473 573 673 773 573 573 573 573
185 197 156 130 79 177 129 27 35
3.0 2.5 3.1 4.2 8.1 3.2 2.0 5.4 5.4
0.36 0.29 0.30 0.28 0.21 0.35 0.20 0.040 0.050
SBET values. Calcination of 0.1% Fe/TiO2(NSG) at elevated temperatures makes the SBET and the Vp values decrease while the dp value increases. The increase in dp could be attributed to the collapse of the small pores, corresponding to a slight decrease in Vp. The decrease in SBET could be accounted for by considering both the agglomeration of TiO2 nanoparticles and the collapse of the porous structure at high calcination temperatures. The Raman spectra (Figure 5) demonstrate that, in good accordance with XRD patterns, all the TiO2(HSG), TiO2(NSG), and 0.1% Fe/TiO2(NSG) samples calcined at 573 K are present in the unique anatase phase, corresponding to characteristic peaks around 143 cm-1.23 The peak position shifts toward a higher wavenumber in the order of TiO2(HSG), TiO2(NSG), and 0.1% Fe/TiO2(NSG), indicating an increase in the number of surface oxygen vacancies.23 The more oxygen vacancies induced by Fe-doping could be attributed to the incorporation of Fe-dopants into the TiO2 lattice, which reduces the O/Ti ratio, taking into account that the Fe-dopant is present in the +3 oxidation state while Ti is present in the +4 oxidation state.
Figure 5. Raman spectrum of (a) TiO2(HSG), (b) TiO2(NSG), and (c) 0.1% Fe/TiO2(NSG) samples calcined at 573 K.
The TiO2 precursor obtained via -the NSG method contains more organic species that may induce a stronger stress and strain effect in nanocrystalline TiO2 during calcination, leading to more oxygen vacancies in TiO2(NSG) than in TiO2(HSG).24-27 Figure 6 reveals that all the TiO2(HSG), TiO2(NSG), and 0.1% Fe/ TiO2(NSG) samples calcined at 573 K displayed two kinds of PLS peaks. The peak around 382 nm could be attributed to an emission peak from band edge free excitation, and the increase of the peak intensity in the order TiO2(HSG), TiO2(NSG), and 0.1%Fe/TiO2(NSG) further confirms the increase of the surface oxygen vacancies and/or surface defects.28 The PLS peak around 560 nm is assigned to the light absorption coefficient, known as a dual-frequency peak. The decrease in the peak intensity from TiO2(HSG), TiO2(NSG), to 0.1%Fe/TiO2(NSG) represents the enhanced ability for light absorbance.29 The UV-vis DRS spectra (Figure 7) further confirm that both the TiO2(NSG) and the 0.1% Fe/TiO2(NSG) samples display stronger absorbance for UV light (200 to ∼350 nm) than the corresponding samples obtained via the traditional -HSG method, while the Fe-doping could enhance the light absorbance. In addition, both TiO2(NSG) and TiO2(HSG) display no significant absorbance for visible light, obviously due to the large energy band gap (3.2 eV) of anatase. Fe-doping induces the absorbance for visible light owing to the formation of intermediate energy levels, leading to a decrease in the energy band gap.30,31 As shown in Figure 8, the activity of 0.1% Fe/TiO2(NSG) in MB mineralization increases with the calcination temperature increasing to 573 K. As the SBET value decreases, the increase in the activity could be mainly attributed to the enhanced crystallization degree of anatase that may facilitate electron transfer from bulk to surface and thus inhibit recombination with photoinduced holes, leading to a higher quantum efficiency.
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Figure 6. PLS spectra of (a) TiO2(HSG), (b) TiO2(NSG), and (c) 0.1% Fe/TiO2(NSG) samples calcined at 573 K.
Figure 7. UV-vis DRS spectra of (a) TiO2(HSG), (b) TiO2(NSG), (c) 0.05% Fe/TiO2(NSG), (d) 0.1% Fe/TiO2(NSG), and (e) 0.2%Fe/ TiO2(NSG) samples calcined at 573 K.
In addition, the enhanced interaction between the Fe-dopants and the TiO2 after calcinations could also increase the absorbance for visible light, responsible for the higher photocatalytic activity in the visible area. A further increase of the calcination temperature from 573 to 773 K had little improvement on the crystallization degree (see Figure 1) but caused an abrupt decrease in SBET (see Table 1), which could account for the abrupt decrease in photocatalytic activity. Figure 9 shows the degradation yields of MB over different photocatalysts calcined at 573 K. On one hand, both TiO2(NSG) and Fe/TiO2(NSG) exhibited a much higher activity than the corresponding TiO2(HSG) and Fe/TiO2(HSG), showing the promoting effect of the -NSG method on photocatalytic activity. One possible reason is the higher surface area and/or mesoporous structure favoring the adsorption of reactant molecules and the light absorbance to generate more holes. Besides, the enhanced crystallization degree of anatase facilitates the transfer of photoelectrons from the bulk to the surface32 and thus could reduce the recombination probability between photoinduced electrons and holes. Furthermore, the increase of surface oxygen vacancies and/or defects could capture more photoelectrons and thus reduce their recombination with photoinduced holes. These factors could enhance the quantum
Figure 8. Dependence of activity of 0.1% Fe/TiO2(NSG) photocatalyst on calcination temperature. Reaction conditions: 0.050 g of photocatalyst, 50 mL of 10 mg/L MB solution, three 150 W xenon lamps (cutoff at 420 nm by a glass filter) 30 cm away from the reaction solution, reaction temperature 303 K, stirring rate 1000 rpm, and reaction period 3 h.
efficiency, leading to higher photocatalytic activity. Fe-doped TiO2 shows a much higher activity than undoped TiO2, obviously owing to light absorbance in the visible area (see Figure 7). Meanwhile, Fe-doping could also enhance the quantum efficiency of photocatalysis because the recombination between photoinduced electrons and holes is effectively inhibited owing to the increased number of oxygen vacancies and/or defects (see Figures 5 and 6) and the presence of Fe2O3 species for capturing photoelectrons. The stronger interaction between Fe-dopants and TiO2 in Fe/TiO2(NSG) could strengthen the promoting effect of Fe-doping, which might be another important factor accounting for the higher activity of Fe/TiO2(NSG) than that of Fe/TiO2(HSG). A very high Fe-content (>0.2%) is harmful for photocatalytic activity since too many Fe2O3 species in the TiO2 network may serve as the centers for recombination between photoinduced electrons and holes, leading to an abrupt decrease in quantum efficiency of photocatalysis.9 0.1% Fe/TiO2(NSG) calcined at 573 K exhibited maximum activity, which is nearly 6 times higher than commercially available P-25, showing a good potential in practical applications.
Nanocrystalline Fe/TiO2 Visible Photocatalyst
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18969 References and Notes
Figure 9. Photocatalytic degradation yield of MB over different photocatalysts calcined at 573 K. Reaction conditions are given in Figure 8 caption.
Conclusion The present work supplies a facile -NSG approach to prepare an Fe-doped TiO2(Fe/TiO2) with a uniform mesoporous structure surrounded by a wall of crystalline anatase. Such Fe/TiO2 exhibits photocatalytic activity under irradiation with visible light owing to the formation of intermediate energy levels resulting from Fe-doping, which narrows the energy gap and thus allows the absorbance of visible light. As-prepared Fe/ TiO2 is more active than either the undoped TiO2 or the corresponding Fe/TiO2 obtained via the traditional -HSG method in the presence of water, which could be explained by considering the high surface area, the well-crystallized anatase, the strong absorbance for visible light, and more surface oxygen vacancies and/or defects that could reduce recombination between photoelectrons and holes, leading to the enhanced quantum efficiency of photocatalysis. On the basis of the present method, more effective TiO2-based and even non-TiO2-based photocatalysts could be designed, which offer more opportunities for the practical application of photocatalysis in environmental cleaning. Acknowledgment. This work is supported by the Science and Technology Ministry of China (2005CCA01100 and 065412070) and the Shanghai Leading Academic Discipline Project (T0402).
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