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J. Phys. Chem. B 2005, 109, 5554-5560
Synthesis of Highly Active Sulfate-Promoted Rutile Titania Nanoparticles with a Response to Visible Light Qiujing Yang,† Chao Xie,† Zili Xu,*,† Zhongmin Gao,‡ and Yaoguo Du† College of EnVironment and Resources and State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130023, China ReceiVed: September 23, 2004; In Final Form: January 26, 2005
Highly active sulfate-promoted rutile titania (SO42-/TiO2) with smaller band gap was prepared by an in situ sulfation method, that is, under moderate conditions, sulfate-promoted rutile titania was directly obtained via precipitating Ti(SO4)2 in NaOH solution followed by peptizing in HNO3 without the phase transformation from anatase to rutile. Thus, the negative impacts of phase transformation from anatase to rutile on the structure, surface, and photoactivity properties of the catalysts due to higher calcination temperature can be avoided. The catalysts were characterized by means of thermal analysis, Brunauer-Emmett-Teller analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, FT-IR pyridine adsorption, and temperature-programmed desorption (TPD). The results show sulfate species are sensitive to the variation of calcination temperature. In the process of peptizing, sulfate species are homogeneously dispersed throughout the bulk of catalysts, allowing sulfate species to penetrate into the network of TiO2 effectively. After being calcined at 300 °C, sulfate species occupy oxygen sites to form Ti-S bonds, as evidenced by XPS results. As calcination temperature is further increased to 600 °C or above, the active sulfate species on the catalyst surface are destroyed, and the sulfate species in the network of TiO2 are expelled out onto the surface to form inactive sulfate species. Thus, Ti3+ defects will be produced on the catalyst surface. Accompanying this process, surface area is decreased promptly, and crystalline size is greatly increased via two fast growth phases due to the decomposition of sulfate species with different binding forces. Most importantly, the band gap of SO42-/TiO2 is remarkably shifted to the visible light region due to the formation of Ti-S bonds, and with increasing calcination temperature the visible light absorption capability is reduced due to breakage of Ti-S bonds. The excellent photoactivity of 300 °C calcined SO42-/TiO2 can be explained by its small crystalline size, high surface area, loose and porous microstructure, and the generation of Brønsted acidity on its surface.
1. Introduction Titanium dioxide (TiO2) is a polymorphic compound that has two major crystal phases: anatase and rutile. In comparison with rutile, anatase TiO2 has drawn considerable attention due to its better activity for photodegradation of volatile organic compounds.1 On the other hand, rutile TiO2 has some advantages over anatase such as chemical stability, higher refractive index, and cheaper production.2 Furthermore, rutile phase has a narrower band gap (3.0 eV) than anatase (3.2 eV), and hence should be easily shifted to the visible light region by suitable modification, in contrast to anatase. Therefore, rutile TiO2 may be more suitable for practical use in the field of environmental cleanup. However, there are two major obstacles for practical application of rutile TiO2. The first obstacle is how to efficiently prepare highly active rutile TiO2. Generally, rutile TiO2 is produced by calcining anatase TiO2 at higher temperature, at which the photoactivity of rutile TiO2 is significantly impaired due to its large crystalline size and small surface area.1 Therefore, it is necessary to prepare rutile TiO2 under moderate conditions to improve the photoactivity. Moreover, the acid property of catalysis, considered as a very important factor to its photoactivity, is sensitive to * Corresponding author: e-mail
[email protected]. † College of Environment and Resources. ‡ State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry.
sulfate addition.3-5 Although it has been widely reported that sulfate modification of anatase TiO2 can markedly enhance its photoactivity, no special research result regarding sulfatepromoted rutile TiO2 is reported. So, we believe investigating the effects of sulfate modification on structure, surface, and photoactivity properties of rutile TiO2 under different calcination temperatures may obtain some interesting results. The second obstacle is how to significantly extend its optical response to the visible light region. For achieving this aim, various studies upon transition-metal ion dopants have been performed. But, the disadvantage of metal ion dopants is that they can serve as recombination centers for photogenerated electron-hole pairs and therefore reduce the activity.6 Thus, the nonmetal dopants, such as sulfate,4,7,8 may be more appropriate for improving the activity as well as extending the optical response to the visible light region. In general, the traditional method of preparing sulfate-promoted TiO2 is to impregnate the metal hydroxide obtained via a sol-gel method in sulfuric acid.4 However, this traditional impregnation method has some drawbacks. For efficiently anchoring sulfate species onto the surface, TiO2 with large surface area and porous structure is required, which needs expensive and complicated techniques, such as supercritical drying.5 It has been widely accepted that preparation method has a great influence on the surface properties and photoactivity of the resulting materials.4 So, in this paper we adopted a novel
10.1021/jp045676l CCC: $30.25 © 2005 American Chemical Society Published on Web 03/03/2005
Sulfate-Promoted Rutile Titania Nanoparticles
J. Phys. Chem. B, Vol. 109, No. 12, 2005 5555
and simple method that is different from the traditional impregnation method to synthesize sulfate-promoted rutile TiO2. Furthermore, SO42--removed/TiO2 was prepared and its photoactivity was tested for comparison as well. The aim of this study is to make clear the impacts of calcination temperature on the nature of sulfate species, physicochemical properties of the catalysts (crystalline size, surface area, content of surface hydroxyl, and sulfur content in the samples) and the photoactivity for the photodegradation of heptane. Additionally, the discrepancies between the previous literature and this paper are also discussed. 2. Experimental Section 2.1. Preparation of Catalysts. All chemicals in this study were used as received without any further purification. First, white precipitate A was obtained by slowly adding 1.3 M NaOH solution [the molar ratio of NaOH to Ti(SO4)2 is 8.402:1] into Ti(SO4)2 solution (analytical grade, Beijing chemical reagent factory) under vigorous stirring at room temperature. After being centrifuged and washed by double-distilled water until the supernatant liquid was neutral, the precipitate A was dissolved by 70 wt % HNO3 to form a colloid and then heated in a water bath to form precipitate B. Precipitate B was aged for 24 h at room temperature, and also centrifuged and washed by doubledistilled water until the supernatant liquid was neutral. Finally, precipitate B was dried under an infrared lamp and ground, followed by calcining at a given temperature for 2 h. For obtaining SO42--removed/TiO2, 0.74 M BaCl2 solution was added into Ti(SO4)2 solution to remove SO42- ions until no BaSO4 precipitate was detected. After BaSO4 precipitate was separated from the supernatant, NaOH solution was added to form the precipitate that was centrifuged and washed by doubledistilled water until the supernatant liquid was neutral and free of Cl ions (negative AgNO3 test). The subsequent procedure was the same as for SO42-/TiO2. The samples were labeled as SO42-/TiO2-T and SO42--removed/TiO2-T, respectively, where T stands for the calcination temperature. 2.2. Catalyst Characterization. Differential thermal analysis (DTA) was carried out on a Netzsch STA 449C instrument. BET surface area was measured on a MIKE Micromeritics sorptometer ASAP 2010 with N2 as the adsorbent. The crystalline phase was characterized by a Bruker D8 GADDS X-ray diffractometer by use of Cu KR radiation (λ ) 1.540 56 Å). In addition, crystalline size D was estimated from the width of lines in the X-ray pattern with the aid of the Scheerer formula: D ) Kλ/(β cos θ), where λ is the wavelength of the X-ray used, β is the width of the line at half-maximum intensity, and K is a constant. The morphology of aggregated particles was observed by scanning electron microscopy (JSM-6700F). FTIR spectra for the samples were obtained by utilizing a Nicolet Impact 410 Fourier transform infrared spectrophotometer in the range 4000-400 cm-1. UV-vis spectra were recorded on a Perkin-Elmer Lambda 20 spectrometer and transformed to a magnitude proportional to the extinction coefficient (K) through the Kubelka-Munk function. X-ray photoelectron spectroscopy (XPS) measurements were performed in a VG Escalab MKΠ X-ray photoelectron spectrometer. The X-ray source emitted Mg KR radiation (1253.6 eV). For all the binding energy obtained, the pressure was maintained at 6.3 × 10-7 Pa. Binding energies were calibrated with respect to the signal for adventitious carbon (binding energy ) 284.6 eV). Quantitative analysis was carried out by use of the sensitivity factors supplied with the instrument. The surface hydroxyl content was determined by calculating the ratio of hydroxyl oxygen to total oxygen by analyzing the
Figure 1. DTA curve of as-prepared SO42-/TiO2.
O 1s peak of XPS spectra with curve-fitting software supplied with the instrument. Temperature-programmed desorption (TPD) was carried out with NH3 as probe molecules. In a standard procedure, 100 mg of fresh sample was first calcined at a given temperature under an Ar stream for 1 h and then cooled to 323 K and exposed to a given amount of ammonia for 0.5 h. After the system was purged with flowing Ar for 2 h at 323 K, the sample was heated at the rate of 10 K‚min-1 in He (30 mL/ min), and the concentration change of the desorbed NH3 was monitored by using an on-line thermal conductivity detector (TCD). In addition, the sulfur content of catalyst was determined by barium sulfate gravimetry. 2.3. Photocatalytic Reaction System. The photoactivity tests for the oxidation of heptane were carried out at room temperature in a 300 mL cylindrical quartz tube (4.4 cm i.d. and 20 cm length). In the experiment, 0.1 g of catalyst was spread uniformly over the internal surface of the reactor. After this, the reactor was vacuum-packed and then given amounts of heptane (0.1% v/v) and oxygen (20% v/v) were injected into the reactor. Finally, ultrapure nitrogen was mixed with the reactant in the reactor to 1 atm pressure. The reaction was started by turning on the light source (in the UV photoactivity experiment a 400 W high-pressure mercury lamp was used, and in the visible light photoactivity experiment a 350 W Xe lamp was used). Subsequently, the content of reactant in the reactor was measured by a HP4890GC (FID). The UV and visible light photoactivity of Degussa P25 without further calcination treatment was also measured as a reference to compare with the samples. In this paper, the percentage of adsorption toward heptane was calculated by (C0 - C1)/C0, where C0 is the concentration of heptane without catalyst and illumination, and C1 is the concentration of heptane after adsorption equilibrium but before the irradiation in the presence of catalyst. 3. Results and Discussion 3.1. Thermal Analysis and BET. Differential thermal analysis (DTA) was performed on the as-prepared SO42-/TiO2. We can see from Figure 1 that there are two endothermic peaks, one from room temperature to 500 °C and the other from 500 to 800 °C. In comparison with pure TiO2,9 the first endothermic peak is remarkably broad, demonstrating that this peak includes not only the desorption of molecular water adsorbed on the surface but also the decomposition of some sulfate species. The second endothermic peak belongs to the decomposition of the residual sulfate species with different structures. What is more, it should be noted there is an exothermic peak around 900 °C.
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Figure 2. XRD patterns of SO42--removed/TiO2 and SO42-/TiO2 calcined at different temperatures. From bottom to top: SO42--removed/ TiO2-300; uncalcined SO42-/TiO2; SO42-/TiO2-300 ∼ 900 at intervals of 100.
Figure 3. SEM micrograph of SO42-/TiO2-300.
TABLE 1: Chemicophysical Properties of SO42-/TiO2 and SO42--removed/TiO2 SO42-/TiO2
SO42--removed/TiO2
temp (°C)
crystalline size (nm)
band gap (eV)
adsorption (%)
crystalline size (nm)
adsorption (%)
300 400 500 600 700 800
12.3 13.4 14.9 32.4 33.4 54.1
2.50 2.52 2.66 2.75 2.81 2.83
59.7 51.0 44.9 19.2 14.8 6.2
21.9 27.4 32.9 40.9 53.8 66.8
24.1 19.5 9.3 5.7 3.9 3.3
The surface area of SO42-/TiO2-300 is 219.0 m2/g. Such a high value can be explained by the fact that the network of TiO2 is strengthened by sulfate species. The surface area, however, promptly drops down to 18.9 m2/g upon calcination at 600 °C. In addition, the average pore diameter of SO42-/ TiO2-300 is 2.73 nm. 3.2. XRD and SEM Studies. The XRD patterns of SO42-removed/TiO2-300 and SO42-/TiO2 calcined at different temperatures are shown in Figure 2. It can be clearly seen that all samples display rutile phase even for uncalcined SO42-/TiO2. The formation of rutile TiO2 without calcination should be associated with the peptization of the precipitate (obtained by addition of NaOH solution into Ti(SO4)2) with HNO3. It is likely that H+ ions in the synthetic solution rearrange the amorphous precipitate and result in crystalline rutile TiO2. Recently, a growth unit model of anion coordination polyhedrons was proposed to describe the formation mechanism of crystals.10 Its main idea is that in one liquid system may exist several growth units, and from the aspect of energy and geometric configuration the growth unit with more stable structure appears more. In the process of peptization, there should exist a competition between the two growth units of anatase and rutile. The formation of rutile should surpass that of anatase in that rutile has low formation energy, intense structure, and short Ti-O bond length.1 The crystalline sizes of SO42-/TiO2 and SO42--removed/TiO2 were calculated and given in Table 1. We can see that, at the identical calcination temperature, the crystalline size of SO42-/ TiO2 is obviously smaller than that of SO42--removed/TiO2, confirming that the presence of sulfate species is capable of effectively retarding crystalline growth. Interestingly, the crystalline size of SO42--removed/TiO2 increases progressively as
Figure 4. FT-IR spectra of SO42-/TiO2 calcined at different temperatures in the range of (a) 4000-3000 cm-1 and (b) 2000-400 cm-1.
a function of calcination temperature, whereas for SO42-/TiO2 two sharp increments of crystalline size occur at the temperature range from 500 to 600 °C and from 700 to 800 °C. Figure 3 displayed the scanning electron micrograph of SO42-/TiO2-300, which clearly shows that this sample has uniform, compact primary particles with loose and porous microstructure. 3.3. FT-IR and X-ray Photoelectron Spectrum. For determining the impact of calcination temperature on the nature of sulfate species, FT-IR spectra were employed to characterize the samples. FT-IR spectra of SO42-/TiO2 in the range of 40003000 cm-1 and 2000-400 cm-1 were shown in Figure 4, respectively. The great absorption band of SO42-/TiO2-300 at 4000-3000 cm-1 shown in Figure 4a, which can be assigned to surface hydroxyl groups strongly bound to the catalyst surface, is gradually reduced as the calcination temperature is increased. In particular, when the calcination temperature is higher than 700 °C this absorption band cannot be observed. The above facts prove that higher calcination temperature is detrimental to the presence of surface hydroxyl groups, which is considered to play a key role in the photocatalytic reaction because the photoinduced holes can attack those surface hydroxyl groups and yield surface hydroxyl radicals with high oxidation capability As shown in Figure 4b, for SO42-/TiO2-300 the absorption band around 500-800 cm-1 attributed to the Ti-O bond is very weak, while this band becomes stronger with increasing calcination temperature. This fact verifies that the sulfate species dispersed in the network of TiO2 via the process of peptization
Sulfate-Promoted Rutile Titania Nanoparticles have a significant influence on the structure of Ti-O bonds. As the calcination temperature increases, the sulfate species in the network of TiO2 will be expelled, and hence the influence of those sulfate species on the structure of Ti-O bond will be removed. SO42-/TiO2-300 exhibits several distinct absorption bands at 1000-1400 cm-1, hinting at the presence of different kinds of sulfate species in the samples. According to the previous literature,5 1384, 1211, and 1043 cm-1 absorption bands should be assigned to active sulfate species (more strongly covalent in nature and containing SdO bonds with a bond order close to 2). Meanwhile, the absorption band at 1142 cm-1 should be associated with inactive sulfate species (partially ionic in nature with SdO bond order less than 2). Note that at 600 °C only the absorption band assigned to inactive sulfate species can be observed, whose intensity is increased below 800 °C with the increased calcination temperature. This is to say, the active sulfate species obtained at lower temperature (300 °C) are destroyed as the calcination temperature is further increased, and inactive sulfate species are produced by expulsion of the sulfate species in the TiO2 network out onto the surface.5,8 This finding is contrary to the results reported by Ward and Ko5 that inactive sulfate species are converted to active forms through calcination at higher temperature (at least 500 °C).5 In their study, the catalyst before calcination is amorphous and therefore for activating sulfate species must be calcined at higher temperature to form a regular crystal structure that is beneficial for charge transfer and delocalization. Unfortunately, high temperature can lead to the decomposition of a large amount of active sulfate species. In our study the calcination temperature necessary for activating sulfate species is significantly lowered because crystalline TiO2 is produced in the peptization process. Thus, upon calcination at relatively low temperature (300 °C), we can activate sulfate species and retain as much as possible active sulfate species on the catalyst surface. The fact that the 1384 cm-1 absorption band disappears above 400 °C, the absorption bands at 1211 and 1043 cm-1 disappear above 500 °C, and the absorption band at 1142 cm-1 completely vanishes at 900 °C, which is in accordance with the exothermic peak around 900 °C (Figure 1) very well, suggests there should exist at least three kinds of sulfate species with different binding forces that can be decomposed at different temperatures. Now we have enough evidence to demonstrate the two fast growth phases of crystalline size mentioned early. We deduce that these two fast growth phases should be assigned to the decomposition of sulfate species with different binding forces as a function of calcination temperature. The first fast growth phase at 500600 °C can be attributed to the decomposition of active sulfate species. The second, between 700 and 800 °C, should be assigned to the decomposition of inactive sulfate species. To identify the effects of calcination temperature on the surface properties of SO42-/TiO2, XPS studies with regard to S 2p, Ti 2p, and O 1s were carried out. As can be seen from Figure 5, for uncalcined SO42-/TiO2 only a broad peak around 170 eV (labeled as A) can be observed. According to Roman et al.,11 this peak is associated with two states of sulfate species, that is, S6+ (SO42- ions) and S4+ (SO32- species). Upon calcination at 300 °C, a new peak with binding energy around 160 eV (labeled as B) assigned to S2- (Ti-S bonds) occurs, proving the generation of Ti-S bonds in the case of SO42-/TiO2-300. The evidence from XPS results of S 2p also supports the previous conclusion that in the catalyst there should exist at least three kinds of sulfate species with different binding forces. Table 2 displayed the values of Ti 2p3/2 of SO42--removed/ TiO2-300 and SO42-/TiO2 calcined at different temperatures.
J. Phys. Chem. B, Vol. 109, No. 12, 2005 5557
Figure 5. XPS spectra for the S 2p of (a) uncalcined SO42-/TiO2 and (b) SO42-/TiO2-300.
Figure 6. XPS spectra of the O 1s region for SO42-/TiO2-300 and SO42-/TiO2-600.
TABLE 2: Values of Ti 2p3/2, Ratio of Hydroxyl Oxygen to Total Oxygen, and Sulfur Content of the Catalysts catalysts SO42--removed/TiO2-300 SO42-/TiO2-300 SO42-/TiO2-600 SO42-/TiO2-800
Ti 2p3/2 hydroxyl oxygen: sulfur in the (eV) total oxygen (%) catalyst (wt %) 458.5 458.4 458.3 458.0
23.4 27.1 25.0 22.3
