Directed Synthesis of Hierarchical Nanostructured TiO2 Catalysts and

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Environ. Sci. Technol. 2008, 42, 2342–2348

Directed Synthesis of Hierarchical Nanostructured TiO2 Catalysts and their Morphology-Dependent Photocatalysis for Phenol Degradation L U L I U , * ,† H U A J I E L I U , ‡ Y A - P I N G Z H A O , * ,§ Y U Q I U W A N G , † YUEQIN DUAN,4 GUANDAO GAO,† M I N G G E , † A N D W E I C H E N * ,† Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300071, China, National Center for Nanoscience and Technology, Beijing 100080, China, Department of Environmental Science, East China Normal University, Shanghai 200062, China, and College of material Science and Engineering, University of Polytechnology, Tianjin, Tianjin 300191, China

Received April 25, 2007. Revised manuscript received January 7, 2008. Accepted January 16, 2008.

Nanostructured TiO2 with different hierarchical morphologies were synthesized via a warmly hydrothermal route. The properties of the products were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, N2 adsorption, UV–vis spectroscopy, etc. Two of the products, TiO2 1D nanorods (one-dimensional rutile TiO2 nanorods) and TiO2 3D0D microspheres (three-dimensional anatase TiO2 nanoparticle-assembled microspheres) exhibited superior photocatalytic effects on phenol degradation under UV illumination, compared with TiO2 3D1D microspheres (three-dimensional rutile TiO2 nanorods-assembled microspheres). Moreover, TiO2 3D0D was superior to TiO2 1D, as indicated by a 30% higher mineralization of dissolved phenol. Dihydroxybenze, 4,4′dihydroxybiphenyl, benzoquinone, maleic anhydride, etc. were identified as the degradation intermediates. The excellent catalytic effect was attributed to the structural features of TiO2 1D nanorods and TiO2 3D0D microspheres, that is, a larger amount of surface active sites and a higher band gap energy resulted in more efficient decomposition of organic contaminants.

Introduction Efficient, environmentally benign, and low-cost approaches for removal of recalcitrant organic compounds have been the focus of current research. Specifically, advanced oxidation processes (AOPs) have drawn great attention during the last two decades. One of the promising AOPs is complete mineralization of organic substances (to carbon dioxide and water) by irradiating TiO2 slurries under ultraviolet (UV) light. * To whom correspondence may be addressed e-mail: liul@ nankai.edu.cn (L.L), [email protected] (Y.-P.Z.), chenwei@ nankai.edu.cn (W.C.). † Nankai University. ‡ National Center for Nanoscience and Technology. § East China Normal University. 4 University of Polytechnology. 2342

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TiO2 was found to be an efficient heterogeneous photocatalyst because it is photostable, nontoxic, low-cost, and readily available, and can be adapted to specially designed reactor systems (1–3). In heterogeneous photocatalysis, oxidation reactions usually take place on the surface of TiO2. The degradation rate of organic compounds is related primarily to the amount of catalytical surface active sites, which depend on (1) the surface area of the catalyst, (2) the mass of the catalyst, or (3) the number of surface OH groups on a photocatalyst. Other factors affecting degradation rates include structural properties (such as phase composition, crystalline quality, particle size, size distribution, and band gap energy) and various parameters including temperature, pH, initial concentration of the pollutant, light intensity, the reactor geometry, agglomeration, stirring, etc. (4). Therefore, the morphology of the catalyst, which determines both surface area and the structural properties mentioned above, plays a key role in catalytic activity (5–7). However, much more research is still needed in this area. In particular, catalytic properties of nanostructured TiO2 with hierarchical morphologies have not been reported. Controlling morphology of inorganic materials via simple methods is of great interest in many fields. Even though TiO2 nanoparticles, nanorods, nanowires, and nanotubes have great potential applications in highly efficient sensors, photocatalysis, and photovoltaic cells (8–13), it has been difficult to prepare nanostructured TiO2 with high dimensionality (e.g., with hierarchical morphologies assembled from low-dimensional nanoparticles/nanorods) (14, 15). Recently, we have reported directed growth of TiO2 microspheres with curved 3D structures using a surfactant-assisted solvothermal route (16). In this work, we extended our previous work by evaluating the morphology-dependent photocatalytic effects of these hierarchical nanostructured TiO2 (TiO2 1D nanorods, TiO2 3D0D microspheres, and TiO2 3D1D microspheres), using the photocatalytic degradation of phenol as the model reaction. The degradability and degradation kinetics for the three TiO2 products were evaluated. The intermediate degradation products were identified to understand the reaction pathways.

Experimental Section Preparation and Characterization of Catalysts. In a typical synthetic reaction of TiO2 3D0D microspheres, 5 mL of TiCl3 solution and 0.4 g of carbamide were added in a Teflon-lined stainless steel autoclave of 30 mL capacity. The autoclave was then filled with 5 mL of distilled water, 1 mL of surfactant span80, and 10 mL of butanol. The autoclave was maintained at 90 °C for 24 h without shaking or stirring during the heating period and allowed to cool to room temperature. The crude product was recovered by centrifugation. Then the white precipitate was collected and washed with distilled water and absolute ethanol to remove surfactant and organic impurities. Synthetic procedures of TiO2 3D1D microspheres were similar to those of TiO2 3D0D microspheres except that the synthesis was conducted at a higher temperature of 150 °C (16). The synthetic procedures of TiO2 1D nanorods were reported in a previous paper (16). The morphologies and structures of the products were characterized by a high-resolution transmission electron microscope (HRTEM, JEOL2010, Japan, acceleration voltage 200 kV), a scanning electron microscope (SEM, JSM-6301F, Japan), and an X-ray diffractometer (XRD, Rigaku D/Max2500, Japan, employing Cu KR radiation, λ ) 1.54056 Å). 10.1021/es070980o CCC: $40.75

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FIGURE 1. SEM images of TiO2 3D0D microspheres. Brunauer-Emmett-Teller (BET) surface area was measured by nitrogen adsorption at 77 K using a Micromeritics 2000 surface area analyzer (USA). Prior to analysis, 0.5-1.0 g of TiO2 samples were degassed at 473 K and 200 mmHg for 2 h. Total surface area was determined by curve-fitting the BET equation to 20-data-point adsorption isotherms of N2 at 77 K in the region of 0.05-0.99 relative pressure. The UV–vis spectra of the solid powder materials were measured on a JASCO V-560 UV–vis spectrophotometer (USA), equipped with an integrating sphere attachment (JASCO ISV-469). The powder was not diluted in any matrix to avoid a decrease of the absorbance. The spectra were recorded in diffuse reflectance mode and were transformed by the instrument software (JASCO) to equivalent absorption Kubelka–Munk units. The band gap energy of the TiO2 photocatalyst was calculated. Photodegradation Experiment. Photocatalytic activity was evaluated using phenol degradation experiments. The reactors used were open Pyrex reactors with a diameter of 12 cm and a height of 5 cm. A 400 mL portion of 50 mg/L phenol aqueous solution and 0.2 g of a TiO2 product were added to the reactor. The pH of the aqueous phenol solution was unadjusted. The suspension was stirred magnetically at 200 rpm at 25 °C for 3 h in the dark, and a small amount of the suspension was withdrawn to analyze equilibrium concentration of phenol in the solution. Then the suspension was exposed to UV light under a Philips 125 W high-pressure mercury lamp (Phillips GGY125Z ballast, luminous flux 4990 lm). The main emission of the lamp was 365 nm. The distance between the UV lamp and the surface of the solute was set at 10 cm. At selected time intervals, about 3 mL of the suspension was withdrawn for analysis. To analyze the concentration of phenol and degradation products, the suspension was first centrifuged and filtered through 0.45 µm Millipore membrane filters to remove the catalyst. (The membrane filters are made of mixed cellulose esters and had no effect on phenol concentration.). The concentrations of phenol were measured with a UV–vis spectrophotometer (Varian Cary 300) with UV absorbance in the range of 190–400 nm, and the absorbance at 269 nm corresponded to the maximal adsorption of phenol (17). The decrease of total organic carbon (TOC), which indicated the mineralization of phenol, was determined using a TOC analyzer (Shimadzu 5000A, Japan, T ) 650 °C) equipped with an ASI5000 autosampler. The calibration was performed using potassium hydrophthalate. Phenol degradation intermediates were determined with a liquid chromatography/triple quadrupole tandem mass spectrometry, equipped with turbo ion spray interface (API2000 LC/MS/ MS system, Applied Biosystems Asia Pte Ltd., USA). The HPLC series 1100 (Agilent) is equipped with a reverse-phase C18 analytical column (Zorbax SB-C18, USA) of 150 × 2.1 mm and 3.5 µm particle diameter. Column temperature was maintained at 22 °C. The mobile phase used for eluting phenol from the HPLC columns consisted of acetonitrile and water (30:70 v/v) at a flow-rate of 0.3 mL/min. The MS/MS turbo

FIGURE 2. TEM images of inside morphology of a blocked TiO2 3D0D microsphere. ion spray interface was operated in the negative ion mode at 4500 V and 450 °C.

Results and Discussion Characterization of Catalysts. Nanostructured materials with hierarchical morphologies assembled from low-dimensional building blocks are expected to exhibit special properties. In our previous report (16), a one-step synthetic approach has proven effective in preparing hierarchical TiO2 3D1D microspheres with TiO2 1D nanorods as building blocks. Two factors have been found to be important in our studies: first, surfactant is crucial in the assembly of TiO2 3D microspheres; second, synthetic temperature affects the morphology of the building blocks and, therefore, the morphology of the final product. Thus, it is possible that the morphology of TiO2 can be controlled by adjusting synthetic conditions carefully. Figure 1 shows the SEM images of as-prepared TiO2 3D0D microspheres. The microspheres appear to be uniform and to have sphere-like morphologies with diameters from 100 to 250 µm (Figure 1a). Higher magnification SEM clearly revealed that the microspheres consist of nanoparticles with many pores (Figure 1, panels b and c). The TEM images of TiO2 3D0D microspheres (Figure 2) shows that the inner structure of TiO2 microspheres is the aggregates of nanoparticles with diameters of 6–10 nm. The high-resolution TEM image of a nanoparticle reveals the single crystalline structure (Figure 2b). The powder X-ray diffraction (XRD) patterns (Figure 3) show that the as-prepared TiO2 3D0D microspheres are in conformity with anatase TiO2 form [Joint Committee on Powder Diffraction Standards (JCPDS) No. 21–1272]. No characteristic peaks of other impurities such as rutile and brookite were observed. The broadening of the (101) diffraction peak reveals that the building blocks of these TiO2 3D0D microspheres are in nanoscale. According to Scherrer’s equation, the building blocks of these TiO2 3D0D microspheres are 6–8 nm in size, which is consistent with the TEM results. Figure 4 shows the controlled morphologies of the asprepared TiO2 products under different synthetic conditions. First, no TiO2 microspheres were formed in the absence of surfactant span80. Only TiO2 anatase nanoparticles were formed at 90 °C (Figure 4a), and only rutile TiO2 1D nanorods VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. XRD pattern of TiO2 3D0D microspheres.

FIGURE 4. Images of TiO2 under different synthetic conditions: (a) in the absence of surfactant span80 at 90 °C (TiO2 0D nanoparticles); (b) in the absence of surfactant span80 at 150 °C (TiO2 1D); (c) XRD of TiO2 1D nanorods; (d) in the presence of surfactant span80 at 150 °C (TiO2 3D1D). were obtained at 150 °C (Figure 4b). The XRD patterns of rutile TiO2 1D nanorods are shown in Figure 4c (JCPDS No. 21–1276). Second, TiO2 rutile microspheres could be formed by self-assembly of TiO2 1D nanorods if the temperature was kept at 150 °C (Figure 4d). The BET surface areas of the catalysts are 53.63 m2/g for TiO2 1D-nanorods, 22.36 m2/g for TiO2 3D0D microspheres, and 20.16 m2/g for TiO2 3D1D microspheres, respectively. Optical absorption spectra of the TiO2 3D0D microspheres and TiO2 1D nanorods are shown in Figure 5. For both samples, the maximum absorbing peak appeared at 325 nm, and the corresponding band gap energy was estimated to be about 3.81 eV. However, the band gap energy of TiO2 3D1D microspheres was only 3.39 eV (16), a 0.42 eV blue-shift compared to the other two products. Photocatalytic Degradation of Phenol. No detectable degradation of phenol was observed without TiO2 as the catalyst. For all three TiO2 samples, adsorption was below 4% over the initial 3 h pre-equilibration period carried out in the dark, indicating that adsorption did not contribute considerably to the observed phenol removal. Thus, the initial concentration was taken to be 50 mg/L in all cases. The initial point, t ) 0, was taken after addition of the catalyst. Figure 6 shows the change of phenol absorbance with time when using TiO2 1D nanorods, TiO2 3D0D microspheres, 2344

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and TiO2 3D1D microspheres, respectively, as the catalyst. Figure 6a shows a significant decrease in phenol absorbance (85% in 150 min) in the presence of TiO2 1D nanorods catalyst; the intensity of the signal substantially decreased and finally disappeared after 2 h of reaction. Also, no apparent new absorbance peaks were observed, indicating little degradation intermediates remained at the end of the experiments. Figure 6b shows a similar pattern, that is, phenol absorbance decreased rapidly (91% in 150 min) in the presence of TiO2 3D0D microspheres; no new absorbance peaks appeared, indicating near complete mineralization of phenol. The higher photocatalytic activity of TiO2 3D0D microspheres than that of TiO2 1D nanorods indicates that TiO2 3D0D microspheres might have more catalytically active sites than TiO2 1D nanorods. Figure 6c shows the absorbance spectra of phenol in the presence of TiO2 3D1D microspheres. In contrast to Figure 6, panels a and b, however, only a small decrease in phenol absorbance was observed (40% in 150 min). In addition, the absorbance spectra shows another absorbance at λ) 249 nm as well as a tailing absorbance with a shoulder in the region of λ ) 300-400 nm. The new absorbance observed at λ ) 249 nm is probably related to an intermediate degradation product (presumably 4,4′-dihydroxybiphenyl), and the broad tailing absorption spectra are indicative of other aromatic degradation intermediates. Thus, Figure 6c

FIGURE 5. UV–vis absorption spectra of TiO2 3D0D microspheres (solid line) and TiO2 1D nanorods (dash line). indicates that when TiO2 3D1D microspheres were used as the catalyst, phenol was oxidized to a complex mixture of UV absorbing intermediates. The lower photocatalytic efficiency of TiO2 3D1D microspheres, compared with TiO2 1D nanorods and TiO2 3D0D microspheres, was probably due to its lower band gap energy. In comparison with TiO2 3D1D, TiO2 1D nanorods and TiO2 3D0D microspheres have higher band gap energy (due to the blue-shift of their absorption peaks) and could result in stronger oxidation and, thus, facilitated phenol decomposition. The difference in photocatalytic efficiency among TiO2 1D nanorods, TiO2 3D0D microspheres, and TiO2 3D1D microspheres can be further illustrated with Figure 7, which shows the phenol degradation kinetics data. The results clearly show the higher photocatalytic efficiencies of TiO2 1D nanorods and TiO2 3D0D microspheres over TiO2 3D1D microspheres. For example, Figure 7a shows that, after 150 min of UV irradiation, the concentration of phenol was only 7.5 and 4.5 mg/L with TiO2 1D nanorods and 3D0D microspheres as the catalysts, respectively, whereas the concentration was as high as 30 mg/L with TiO2 3D1D microspheres as the catalyst. After 180 min of irradiation, near complete mineralization of phenol was observed when using TiO2 1D nanorods or TiO2 3D0D microspheres as the catalyst, whereas the phenol concentration was still 20 mg/L when using TiO2 3D1D microspheres as the catalyst. Figure 7b shows the observed decrease of total organic content (TOC). Even though the phenol degradation curves under TiO2 1D nanorods and TiO2 3D0D microspheres nearly overlap (Figure 7a), TOC decreased more quickly in the reaction using TiO2 3D0D microspheres as the catalyst (90.6% in 180 min) than in the reactor using TiO2 1D nanorods as the catalyst (70.9% in 180 min). The discrepancy between Figure 7a and Figure 7b seems to suggest that phenol degradation was even more complete (i.e., with less intermediate degradation products formed) when using TiO2 3D0D microspheres as the catalyst, even though both products are highly effective. Figure 7b shows that the TOC decrease was only 49.7% when using TiO2 3D1D microspheres as the catalyst; this result is consistent with the phenol absorbent results shown in Figure 6. During this experiment, we have observed that the color of the solution and catalyst surface changed to brownish yellow. LC/MS/MS results show that possibly some polymeric compounds were formed, including 4,4′-dihydroxybiphenyl. The brownish yellow color on the surface of TiO2 3D1D microspheres suggests the formation of insoluble polymeric compounds sticking on the surface of the catalyst. At the same time, gradual congregation of TiO2 3D1D microspheres colloidal particles was noticed in the bottom of the reactor, which probably have caused a decrease in the number of surface active hydroxyl sites. Those might have been the

FIGURE 6. Phodocatalytic degradation of a 50 mg/L phenol solution (400 mL) using (A) TiO2 1D nanorods, (B) TiO2 3D0D microspheres, and (C) TiO2 3D1D microspheres as catalysts (200 mg) illuminated with UV light, separately. Samples for UV analysis were taken at 0, 30, 60, 90, 120, 150, and 180 min intervals of reaction. causes of the observed slow photodegradation and TOC reduction. For the experiments using TiO2 3D0D microspheres and TiO2 1D nanorods as the catalysts, nonetheless, the color changed very little during the reaction, and no aggregation of nanomaterials were observed. Kinetic Studies. The Langmuir–Hinshelwood model (L-H) has been used to describe the rates of photocatalytic destruction of many organic compounds in many studies (18, 19). In the absence of external mass transfer limitations, VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Results obtained for degradation of phenol (50 mg/L) over TiO2 1D nanorods(-2-), TiO2 3D0D microspheres (-9-), and TiO2 3D1D microspheres (-f-) sample catalysts (200 mg) in phenol solution (400 mL); (a) phenol concentration, (b) total organic carbon concentration.

TABLE 1. Adsorption Constant Kads and Reaction Rate Constant kr adsorption constant

reaction rate constant

samples

Kads (L mol-1)

kr (mol L-1 s-1)

TiO2 1D nanorods TiO2 3D0D microspheres TiO2 3D1D microspheres

2.04 × 103 2.22 × 103 3.49 × 103

0.14 0.13 0.005

kinetic data of phenol photodecomposition under UV irradiation agree well with the L-H kinetic rate mode: r ) -dC/dt ) (kr × Kads × C)/(1 + Kads × C), where kr and Kads are the reaction rate constant in the aqueous solution and apparent adsorption coefficient, respectively. The rate form can be linearized for initial concentrations: 1/r0 ) (1/(kr × Kads)) × 1/C0 + 1/kr, where C0 is the initial phenol concentration and r0 is the initial degradation rate. The kr and Kads values can be obtained from the intercept and slope of the 1/r0 vs 1/C0 plot. The kr and Kads values obtained in this study are listed in Table 1. It can be seen from Table 1 that the kr value of phenol photodegradation using TiO2 1D nanorods or TiO2 3D0D microspheres as the catalyst was much higher than that using TiO2 3D1D microspheres as the catalyst. The higher kr value indicates a higher degradation rate of phenol. It has been reported that, even though both anatase and rutile are active in the photocatalytic reaction, the kinetics could be different: The rutile crystal resulted in a faster initial degradation of phenol, whereas anatase led to a much faster total degradation after an initial slower start (18). However, the experimental results in the present study showed that both anatase TiO2 3D0D microspheres and rutile TiO2 1D nanorods are active in the photocatalytic reaction, and very interestingly, the kinetics were also similar — both the anatase and the rutile crystal resulted in a faster initial degradation of phenol (Figure 7a). The Kads values indicate that the difference in apparent adsorption to TiO2 1D nanorods, TiO2 3D0D microspheres, and TiO2 3D1D microspheres was not very large. The higher Kads value further indicates that catalytic degradation occurred on the surface of TiO2. Mechanism of Photodegradation. The photocatalytic oxidation of phenol involves complicated multistage processes. It has been demonstrated that the photocatalytic process with TiO2 is energetically favorable for the decom2346

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FIGURE 8. Photocatalytic degradation of 400 mL of a 50 mg/L phenol solution using TiO2 3D0D microspheres as catalysts (200 mg) illuminated with UV light. Samples for LC/MS/MS analysis were taken at 180, 120, and 60 min (peak of retention time (RT) 3.40 min of phenol showed from low to high): RT 0.99 min, maleic anhydride; RT 1.76 min, benzoquinone; RT 2.05 min, 4,4′-dihydroxybiphenyl; RT 2.40 min, dihydroxybenzene; and RT 3.40 min, phenol. position of phenolic compounds and that two types of oxidizing species — hydroxyl radicals and positive holes — are involved in the transformation of aromatic compounds in oxygenated aqueous TiO2 suspensions (21, 22). Various aromatic intermediates products were formed in the initial stage. The intermediate products then undergo further photocatalytic oxidation to induce ring cleavage to form aliphatic acids, and finally, they completely degrade to CO2 and H2O. Thus, it can be seen that the lifespans of the intermediates formed at different stages of the reaction are short, because the intermediates can undergo further fast oxidation. It has been proposed that rutile and anatase result in different photodegradation intermediates, indicating different decomposition mechanisms (20). However, LC/MS/MS results (Figure 8 and Supporting Infomation Figure I and Table 1) obtained in the present study indicate that the reactions catalyzed by the three TiO2 products followed the same degradation pathway. Dihydroxybenzene (including mixtures of the m,o,p-dihydroxybenzene isomers), benzoquinone, 4,4′-dihydroxybiphenyl, and maleic anhydride were the main intermediates identified. Because no other intermediates (e.g., aliphatic acids) were observed, it appears that the intermediates mentioned above further degraded to more

treatment. (See Supporting Information Figure II and Annotation II for detailed comparisons).

Supporting Information Available Additional HPLC/MS/MS data and annotations provide further information on compounds and processes mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments

FIGURE 9. Scheme of phenol degradation with reaction pathways. polar compounds such as carboxylic acid and eventually degraded to CO2 (23, 24). The further degradation of the intermediates was likely via several reaction pathways (including oxidative hydroxylation, ring cleavage, oxidative decarboxylation, etc.) that were simultaneously operative. These proposed photodegradation pathways are shown in Figure 9. The photodegradation pathways proposed in Figure 9 are consistent with the results of other research groups with anatase TiO2 nanoparticles as catalyst (25–27). For example, Azevedo reported that three compounds — catechol, hydroquione, and 4,4′-dihydroxybiphenyl — were formed during photodegradation in function of salinity (25). Catechol, hydroquinone, and 1,2,3-trihydroxybenzene were identified as primary and secondary hydroxylation products (21). 1,2,4-trihydroxybenzene and benzoquinone have also been reported as the intermediate species during phenol mineralization (22). Under different reaction conditions, the primary aromatic intermediates could be different. For example, during oxidation of phenol in alkaline medium, catalyzed by PCS, p-benzoquinone is one of the primary intermediate products. The p-benzoquinone was easily oxidized in alkaline solution (26, 27). The photodegradable activity and mineralization of phenol over TiO2 3D1D microspheres was far below than that of TiO2 3D0D microspheres and TiO2 1D nanorods. It is known that the photocatalytic processes are driven by the band gap energy — the higher the band gap energy, the higher ultraviolet energy that can be absorbed to activate the photocatalyst; accordingly, oxidation is enhanced, and degradation of organic compounds is accelerated. The band gap energy values of TiO2 3D0D microspheres, TiO2 1D nanorods, and TiO2 3D1D microspheres are 3.81, 3.81, and 3.39 eV, respectively; the lowest band gap energy of TiO2 3D1D microspheres related well to the lowest photodegradability of phenol. It is necessary to note that, although the surface area of TiO2 1D nanorods is about twice as high as that of TiO2 3D0D microspheres, the photocatalytic activity of TiO2-3D0D microspheres was indeed better than that of TiO2 1D nanorods. This seems to suggest that the surface area cannot be the only controlling factor for the observed photocatalytic activity, and the crystal structure was likely the most important parameter controlling catalytic effect of the catalysts. It is necessary to further explore the mechanism(s) controlling the higher photocatalytic activity of TiO2 3D0D microspheres than that of TiO2 3D1D microspheres and TiO2 1D nanorods. Finally, an important implication of the present study is that the synthetic TiO2 3D0D microspheres have a significant advantage over commercially available products such as Degussa P25, because this new product is much larger in size and, thus, is much easier to separate/recover after

This work is supported by the National Natural Science Foundation of China (50272034, 20577024), Tianjin Municipal Science and Technology Commission (Grant No. 06TXTJJC14000), Ministry of Education of China (Grant No. NCET-05-0228), Fok Ying Tung Education Foundation (Grant No. 101081), Startup Funds of Nankai University, and China-US Center for Environmental Remediation and Sustainable Development.

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