AlOOH

Sep 15, 2009 - PL spectral changes observed during illumination of sample E in a 5 × 10−4M basic solution of terephthalic acid (excitation at 315 n...
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J. Phys. Chem. C 2009, 113, 17527–17535

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Synthesis of Hierarchical Flower-like AlOOH and TiO2/AlOOH Superstructures and their Enhanced Photocatalytic Properties Xiaoxiao Yu,† Jiaguo Yu,*,† Bei Cheng,† and Mietek Jaroniec*,‡ State Key Laboratory of AdVanced Technology for Material Synthesis and Processing, Wuhan UniVersity of Technology, Luoshi Road 122#, Wuhan 430070, People’s Republic of China, and Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: July 23, 2009; ReVised Manuscript ReceiVed: August 23, 2009

Hierarchical flower-like boehmite superstructures (HFBS) composed of intermeshed nanoflakes were synthesized by hydrothermal treatment from as-formed aluminum hydroxide microspheres, which were prepared by adding aluminum spheres into a sodium metaaluminate solution. A two-step mechanism for the formation of HFBS is proposed, which involves initial formation of Al(OH)3 microspheres followed by their transformation to AlOOH. The as-prepared HFBS superstructures can be easily transformed into γ-Al2O3 superstructures without morphology change by calcination at 500 °C for 2 h. Furthermore, the TiO2 nanoparticles can homogeneously deposited on the surface of HFBS by the vapor-thermal method. The TiO2 nanoparticles coated on HFBS showed higher photocatalytic activity for photocatalytic decolorization of rhodamine B (RhB) aqueous solution than Degussa P25 (P25) and TiO2 powders prepared at the same experimental conditions. A significant enhancement of the photocatalytic activity can be related to several factors, including hierarchical porous structure, high dispersion of TiO2 particles, and the increased lightharvesting abilities. Moreover, TiO2-coated HFBS can settle naturally within 5 min, which is beneficial for separation and recycling, considering their future applications in wastewater treatment. 1. Introduction The optical and electrical properties and catalytic performance of materials strongly depend on their morphology and structure. Preparation of high-quality nanocrystals of desired morphology is of great fundamental and technological interest. In particular, direct fabrication of complex nanostructures with controlled morphologies, crystalline orientations, grain size, and surface architectures is one of the most challenging topics because of their promising functions.1 Scientists have paid more and more attention to the design and fabrication of functional materials showing long-range ordering of nanostructured domains. Hierarchical self-assembly of nanoscale building blocks (e.g., cluster, wires, belts, sheets, tubes, etc.) into novel architectures of higher dimensionality is highly desirable due to the fact that these structures could possess new properties that can be valuable for fabrication of advanced nanodevices and other nanotechnology applications.2 Therefore, many types of hierarchical superstructures (e.g., multipods,3 snowflakes,4 hyperbranches,5 saws,6 windwills,7 dendrites,8 and hierarchical structures9) have been reported. These include metals, metal oxides, and others. Numerous chemical and physicochemical methods, such as the Kirkendall effect, Ostwald ripening, self-assembly method, template-sacrificial method, and chemically induced self-transformation,10-15 have been developed to fabricate various hierarchical superstructures. Among them, the sacrificial templatedirected chemical transformation has been shown to be an effective strategy. Reactive sacrificial templates, which act as both reactive precursors and templates, do not require the template removal step and do not impose morphological * To whom correspondence should be addressed. E-mail: jiaguoyu@ yahoo.com and [email protected]. † Wuhan University of Technology. ‡ Kent State University.

limitations characteristic for hard templates. Furthermore, it is noteworthy that hydrothermal methods show special advantages for the synthesis of a variety of hierarchical architectures in the presence of different templates owing to the mild synthesis conditions, potential for scale-up, economic factors, simplicity, and ease of operation.16 Various transition aluminas with high surface area and pore volume, which refer to the group of partially dehydrated aluminum hydroxides, have been widely used in the industry as adsorbents, catalysts, and catalyst carriers. Among them, boehmite (γ-AlO(OH)) has been the most important precursor or intermediate for the synthesis of alumina and alumina-derived materials and has attracted a lot of attention.17 Boehmite exhibits a lamellar structure (orthorhombic symmetry), in which the Al3+ ions exist in distorted, edge-sharing octahedral arrays of oxide ions that form a double layer with layers being connected by zigzag chains of H-bonds. Because of the large number of -OH groups on the surface, boehmite is inclined to interact with foreign molecules, thus a variety of composite functional materials can be prepared.18 Boehmite can be prepared by three main procedures, leading to different shapes, morphologies, and surface properties:19 solid state decomposition of gibbsite Al(OH)3; preparation in Al-containing aqueous solution under acidic or basic conditions; and the sol-gel method usually involving aluminum alcoholates precursors. Heterogeneous photocatalysis of TiO2 semiconductor has been extensively studied and applied in environmental decontamination such as air purification, water disinfection, hazardous waste remediation, and water purification due to its biological and chemical inertness, strong oxidizing power, cost effectiveness, and long-term stability against photocorrosion and chemical corrosion.20 Though there are many effective ways to enhance the photocatalytic activity by minimizing the particle size in order to achieve higher surface area and create more active sites,

10.1021/jp906992r CCC: $40.75  2009 American Chemical Society Published on Web 09/15/2009

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TABLE 1: Experimental Conditions for the Preparation of the Samples and the Corresponding Surface Areas no.

compositions

phase

hydrothermal

anneal

TiO2 coating

A B C1 C4 C24 D E F G

Al Al(OH)3 Al(OH)3/AlOOH AlOOH AlOOH Al2O3 TiO2/AlOOH TiO2 P25

aluminum bayerite bayerite/boehmite boehmite boehmite γ-Al2O3 anatase/boehmite anatase anatase/rutile

no no 180 °C, 1 h 180 °C, 4 h 180 °C, 24 h 180 °C, 24 h 180 °C, 24 h 180 °C, 24 h no

no no no no no yes yes yes no

no no no no no no yes yes no

this approach also has some disadvantage. Namely, reduction of the particle size to nanoscale creates some difficulties in the separation and recycling of photocatalyst from suspension after application in water and wastewater treatment, which hinders industrial applications of TiO2 nanoparticles that are too fine to be removed by gravitational settling due to the strong Brownian motion. To overcome the above inconvenience, many attempts have been made to effectively separate TiO2 photocatalysts, for example, by sol-gel immobilization of powder photocatalysts on various substrates21 and separation of magnetic photocatalysts from the system under an external magnetic field.22 However, the sol-gel method used for coating of the catalyst over glass, zeolite, silica, and ceramic often induces agglomeration of particles of the catalyst during heat treatment. On the other hand, the introduction of magnetic particles into photocatalysts may cause a rapid decrease in the photocatalytic activity. Therefore, the search for photocatalysts with high activity, which can be easily separated by natural settlement and reused, is still a big challenge. Such photocatalysts would be more promising than conventional nanosized powder photocatalysts for various applications such as remediation of water pollutants. In this work, HFBS were synthesized by hydrothermal treatment of as-formed aluminum hydroxide microspheres, which were prepared by adding aluminum spheres into sodium metaaluminate solution. TiO2-coated HFBS samples were further fabricated by the vapor-thermal method, using tetrabutylorthotitanate (Ti(OC4H9)4, TBOT) as the titanium resource. The resulting TiO2/AlOOH composite superstructures show higher photocatalytic activity than P25 and TiO2 powders prepared by the vapor-thermal method at the same experimental conditions. To the best of our knowledge, this is the first report on the fabrication and photocatalytic activity of TiO2/AlOOH composite superstructures. Our work provides new insights and understanding on the control of morphology and enhancement of photocatalytic activity of TiO2, and should be of significant interest for catalysis, sensing, separation technology, biomedical engineering, and nanotechnology. 2. Experimental Section 2.1. Chemicals. Metallic aluminum powders (4-6 µm) were purchased from Henan Yuan Yang Aluminum Industry Co, China. Al(NO3)3 · 9H2O, NaOH, tetrabutylorthotitanate (TBOT), and absolute ethanol were purchased from Shanghai Chemical Regent Co, China. All the reagents used were analytical grade without further purification. Deionized water was used for all synthesis and treatment processes. 2.2. Synthesis of Hierarchical Flower-like Boehmite Superstructures (HFBS). Metallic aluminum powders in spherical shape were used as the aluminum source and labeled as A (see Table 1). In a typical synthesis, 1.6 mmol of Al(NO3)3 · 9H2O and 40 mmol of NaOH were dissolved in 40 mL of deionized water to form a NaAlO2 solution. Then 1.26 g of Al powder

SBET (m2 g-1) 1.2 100 82 66 34 25 56 56 53

was added to the above solution under stirring at room temperature to produce Al(OH)3 microspheres. A large number of H2 bubbles rapidly escaped from the solution due to the reaction of Al and NaOH. At the end of reaction, a small amount of Al(OH)3 sample was separated from the solution and labeled as B. Afterward, the suspension was transferred into an autoclave with an inner Teflon lining and maintained at 180 °C for 1, 4, and 24 h, respectively. The white precipitate was recovered by centrifugation and washed three times with deionized water and ethanol to remove the possible remaining cations and anions before drying the sample in oven at 60 °C for 6 h. The resulting samples were labeled C1, C4, and C24, respectively. The conversion of the as-prepared AlOOH HFBS (C24) to γ-Al2O3 superstructure was performed by thermal dehydration in Muffle at 500 °C for 2 h under air. The calcined sample was labeled D. The detailed experimental conditions are shown in Table 1. 2.3. Synthesis of TiO2-Coated HFBS. The TiO2-coated HFBS were prepared by using TBOT as the titanium source. The vapor-thermal method has been reported in a previous report.23 In the typical synthesis, 0.3 g of as-prepared boehmite powder (C24) was dispersed in the solution of ethanol and TBOT (15 mL ethanol, 90 µL TBOT) and transferred into a 50 mL Teflon container, which was placed into a 200 mL stainless steel autoclave with a Teflon linear. The free space between two linear walls was filled with 6 mL of distilled water. At the beginning, distilled water did not contact with TBOT. After sealing, the autoclave was heated to 150 °C for 12 h. During the reaction, distilled water vaporized and contacted with TBOT, which results in its hydrolysis. At the end the autoclave was cooled to room temperature, the supernatant was decanted, and the resulting precipitate was repeatedly washed with distilled water and ethanol three times, and then dried at 80 °C in a vacuum oven for 10 h. Finally, the dried precipitate was calcined in an oven under air at 500 °C for 2 h. The calcined product was labeled E. In addition, pure TiO2 powder (sample F) was also prepared by using the same method but without HFBS. The detailed experimental conditions are listed in Table 1. 2.4. Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a D/Max-RB X-ray diffractometer (Rigaku, Japan), using Cu KR radiation at a scan rate of 0.05° 2θ s-1. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were performed by an S-4800 Field Emission SEM (FESEM, Hitachi, Japan) at an accelerating voltage of 5 kV and linked with an Oxford Instruments X-ray analysis system. High-resolution transmission electron microscopy (HRTEM) analysis and selected area electron diffraction (SAED) were conducted with use of a JEM 2100F microscope at an accelerating voltage of 200 kV. A small amount of product was dispersed in ethanol by ultrasonic treatment for 30 min, and then one drop of the resulting solution was placed onto a carbon-coated copper grid and dried at room temperature for HRTEM observation. Nitrogen adsorption

Flower-like AlOOH and TiO2/AlOOH Superstructures isotherms were measured on a Micromeritics ASAP 2020 gas adsorption apparatus (USA). All the samples were degassed at 180 °C prior to nitrogen adsorption measurements. The BET surface area was determined by a multipoint BET method, using the adsorption data in the relative pressure (P/P0) range of 0.05-0.3. The adsorption branch of nitrogen adsorption-desorption isotherms was used to determine the pore size distribution by the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore model.24 To improve the BJH pore size analysis we used the statistical film thickness derived on the basis of adsorption data for MCM-41 materials.25 The volume of nitrogen adsorbed at the relative pressure (P/P0) of 0.994 was used to determine the total pore volume. 2.5. Measurement of Photocatalytic Activity and Hydroxyl Radicals. The evaluation of photocatalytic activity of the prepared sample E for the photocatalytic decolorization of RhB aqueous solution was performed at ambient temperature, as reported in our previous studies.26 Experiments were as follows: 0.1 g of sample E was dispersed in a 20 mL RhB aqueous solution with a concentration of 1 × 10-5 M in a rectangle cell (52 W × 155 L × 30 H mm3). The solution was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, RhB, and water before UV light irradiation. A 300 W xenon lamp was used as a light source to carry out the photocatalytic reaction. The average light intensity striking the surface of the reaction solution was about 30 mW cm-2, and the amount of light center was standardized in order to keep the same light intensity for testing different samples. The concentration of RhB was determined by an UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). After UV irradiation for 10 min, the reaction solution was filtrated to measure the concentration change of RhB. The photocatalytic activities of P25 and sample F were also measured for reference. The formation of hydroxyl radicals (•OH) on the surface of the UV-illuminated samples E, F, and P25 was detected by a photoluminescence (PL) method, using terephthalic acid as a probe molecule.27 The experimental procedure was similar to the measurement of photocatalytic activity except that the RhB aqueous solution was replaced by the 5 × 10-4 M terephthalic acid aqueous solution with a concentration of 2 × 10-3 M NaOH. The PL spectra of generated 2-hydroxyterephthalic acid were measured on a Hitachi F-7000 fluorescence spectrophotometer. After UV irradiation for 10 min, the reaction solution was filtrated to measure the increase in the PL intensity at 425 nm excited by 315 nm light. 3. Results and Discussion 3.1. XRD Studies. Wide-angle XRD was used to identify crystallographic phases of the samples prepared at different experimental conditions. Figure 1 presents the evolution of the XRD patterns reflecting different stages of the HFBS formation from Al powder to the final product. As can be seen from Figure 1a the raw material (sample A) is a pure aluminum (space group Fm3m j ; JCPDS card no. 4-787). After adding Al powder into NaAlO2 solution, this powder has been immediately transformed into bayerite (see Figure 1b, space group P21/a; JCPDS card no. 20-11). After 1 h of hydrothermal treatment, the XRD pattern (Figure 1c) of the C1 sample shows bayerite and AlOOH phases, indicating the phase transformation of bayerite to AlOOH. After 4 h of hydrothermal treatment the XRD pattern shows only pure AlOOH, indicating the complete phase transformation from Al(OH)3 to AlOOH (see Figure 1d for the C4 sample). A further increase of the hydrothermal treatment time resulted in the improvement of the XRD pattern of AlOOH, which is reflected by enlargement of the intensity of the XRD peaks and

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Figure 1. Evolution of the XRD patterns for transformation of Al powder to final products: (a) raw material A, (b) sample B, (c) sample C1, (d) sample C4, (e) sample C24, and (f) sample D.

Figure 2. (a) SEM images of C24 with inset showing the cross section of a single microsphere. (b) High-magnification SEM images of individual spherical superstructure (C24). (c) SEM images of D (γAl2O3). (d) HRTEM image and SAED pattern (inset in panel d) of the individual nanoflake of C24.

simultaneous reduction of their widths; in addition, the peak positions slightly shift to the right, suggesting the formation of larger AlOOH crystallites. The XRD pattern of C24 (Figure 1e) indicates the presence of a pure boehmite phase with orthorhombic crystal structure, which well matches the JCPDS powder diffraction pattern 21-1307 [space group Amam (63), unit-cell parameters a ) 3.700 Å, b ) 12.227 Å, and c ) 2.868 Å at 25 °C]. This boehmite phase can be subsequently changed into γ-Al2O3 by calcination in air at 500 °C for 2 h, as shown in Figure 1f. 3.2. SEM and TEM Images. The SEM image (Figure 2a) of C24 shows that the highly yielded flower-like microspheres have a diameter of about 3-6 µm with a similar particle size distribution as that of aluminum spheres (not shown here). Some peanut- or dumbbell-like particles are formed via coalescence of two particles. Other shapes are also produced by agglomeration of three or more particles. The high-magnification SEM image of an individual microsphere is shown in Figure 2b, clearly indicating the presence of hierarchical flower-like superstructures consisting of two-dimensional thin nanoflakes having a thickness of several tens of nanometers and widths and lengths in the range of 1-2 µm. These nanoflakes are aligned perpendicularly to the spherical surface, pointing toward center. More importantly, these nanoflakes form a 3D structure having pores of different sizes, which may serve as transport paths for small molecules. The inset in Figure 2a shows the

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Figure 3. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve (inset) for the sample C24.

cross section of a microsphere, confirming that the aforementioned nanoflakes originate from the same core. Individual nanoflakes can be obtained from AlOOH microspheres by sonication for 30 min in an ultrasonic water bath. Highresolution TEM image and the corresponding ED pattern (Figure 2d) were obtained for an individual AlOOH nanoflake. The ED pattern of a single nanoflake is characteristic for orthorhombic AlOOH, which is consistent with the XRD investigation. The individual AlOOH nanoflake is actually single crystal as revealed by clear lattice planes of the corresponding HRTEM image. This HRTEM image indicates that the lattice distance is 0.60 nm, which agrees well with the {010} lattice distance (0.611 nm) of the orthorhombic AlOOH crystal. These results also suggest that the basal plane is a (010) plane. It should be pointed out that the same morphology was observed for the entire sample of HFBS having hierarchical structure with high surface-to-volume ratio, which makes it an attractive material for catalysis, gas sensing, and so on. When the as-prepared AlOOH HFBS (C24) was dehydrated in air at 500 °C for 2 h, the product (D) retained hierarchical flower-like structure as shown in Figure 2c. The γ-Al2O3 superstructures inherited the size and flower-like morphology of the AlOOH precursor. The observed transformation from boehmite to γ-Al2O3 is attributed to the loss of water molecules by internal condensation of protons and hydroxyl ions. 3.3. Nitrogen Adsorption Studies. To characterize the specific surface area and porosity of the as-prepared HFBS (C24), N2 adsorption analysis was carried out. Figure 3 displays the N2 adsorption-desorption isotherm and the corresponding pore size distribution curve for the C24 sample. The desorption branch of the isotherm shows a stepwise behavior and ends at the limiting pressure of the hysteresis closure, implying nonuniformity of pore openings associated with pore constrictions and/or ink-bottle pores with narrow necks.24 The hysteresis loop in the relative pressure range between 0.4 and 0.9 is probably related to the pores present in HFBS, which are formed between primary particles. The high-pressure part of the hysteresis loop (0.9 < P/P0 < 1) is probably associated with textural larger pores that can be formed between secondary particles due to aggregation of nanoflakes into the flower-like superstructures. Since the hysteresis loop ends at the limiting pressure, only adsorption data can be used for the calculation of pore size distribution (PSD). Therefore, adsorption branch has to be used to calculate the PSD curve, which is shown in the inset of Figure 3. As can be seen from this inset the PSD curve is quite broad and bimodal with small mesopores (∼3.5 nm) and larger ones (∼13.5 nm). The smaller mesopores reflect porosity within nanoflakes, while

Yu et al.

Figure 4. Schematic illustration of the formation of C24. SEM images of Al powder and the samples before and after hydrothermal treatment at 180 °C for 1, 4, and 24 h, respectively. The scale bar is 2 µm.

larger mesopores can be related to the pores formed between stacked nanoflakes. The lack of plateau on the adsorption isotherm at the relative pressures approaching unity (which resembles the type II isotherm curve) indicates the presence of macropores between agglomerated nanofalkes, which is manifested by tailing of the PSD curve in the direction of large pore sizes. This macroporous structure can be directly observed on the SEM images of C24 shown in Figure 2, which cannot be accessed by N2 adsorption-desorption analysis. As mentioned above, the C24 sample exhibits a disordered slit-like macroporous framework created by stacking nanoflakes. Their unique threedimensional macroporous framework is well suited for harvesting photoenergy and introducing reactive molecules into the interior space of C24.28 The BET specific surface area of C24 is 33.8 m2/g, indicating that the sample has a relatively high surface-to-volume ratio. 3.4. Mechanism of HFBS Formation. For a complete view of the formation process of HFBS and their subsequent growth, the time-dependent evolution of morphology was elucidated by XRD and SEM, as shown in Figures 1 and 4, respectively. Experimental observation indicates that the gray aluminum powder turned white quickly after adding it into the NaAlO2 solution. This indicates that Al(OH)3 was initially formed before hydrothermal treatment. The as-formed Al(OH)3 sample (B) shows spherical morphology similar in shape and size to aluminum spheres (see Figure 4). After starting hydrothermal treatment, the phase transformation of Al(OH)3 to AlOOH spontaneously occurred. After 1 h, the surface of the microsphere became coarse and many small nanoflakes were formed. After increasing hydrothermal time to 4 h, the newly formed nanoflakes tended to grow larger epitaxially, resulting in the formation of an intermediate product composed of smaller nanoflakes. A further increase in the reaction time to 24 h caused the formation of hierarchical flower-like particles. In a broader sense, one can consider that the resulting boehmite flower-like conformation is “sculptured” chemically from a single particle of raw material (i.e., a single Al sphere). The formation of AlOOH under the experimental conditions studied involves two steps: (i) formation of Al(OH)3 microspheres and (ii) their conversion to AlOOH. Probably, this process involves the following reactions:

2H2O + 2Al + 2OH- f 2AlO2- + 3H2

(1)

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AlO2- + 2H2O S Al(OH)3 + OH-

(2)

Al(OH)3 f AlOOH + H2O

(3)

In this process diffusing Al atoms at the liquid-solid interface turn into soluble metaaluminate ions (AlO2-), which subsequently react with water yielding solid phase, Al(OH)3. Because H2 is released, reaction 1 is irreversible, which provides an additional driving force for the complete transformation of Al spheres into aluminum oxide spheres. The initially formed Al(OH)3 microspheres have a similar size as aluminum spheres (see Figure 4), indicating that the in situ transformation occurred slowly from outside to inside. In this process, an introduction of AlO2- to the starting solution is a prerequisite to protect the Al(OH)3 solid microspheres, that is, to prevent equilibrium 2 from shifting to the left. The control experiment showed that when the Al spheres reacted directly with NaOH solution (i.e., in the absence of initial AlO2-), only unordered aggregation of nanoflakes occurred (not shown here) because the newly formed Al(OH)3 was dissolved again, resulting in the spontaneous nucleation of nanoflakes in solution instead of the in situ transformation of Al(OH)3 microspheres into HFBS. The first formation of Al(OH)3 microspheres plays an important role in the final growth of AlOOH microspheres, where Al(OH)3 microspheres might act as both the aluminum source during the phase transformation and the sacrificial template in the formation of HFBS. The high surface area (99.7 m2/g, see Table 1) and the coarse surface (see Figure 4) of asprepared Al(OH)3 microspheres indicate that the microspheres are composed of nanoparticles. This porous structure can provide numerous transport paths for small molecules to move through the material during phase transformation. Under the hydrothermal conditions studied, the phase transformation first occurs on the surface of Al(OH)3 microspheres, resulting in the formation of small nanoflakes. Then the phase transformation seems to proceed gradually from the outside to the inside due to exchange of reacting species through intercrystallite interstitials of microspheres, which lead to the complete phase transformation of Al(OH)3 to AlOOH and to the formation of intermediate products composed of smaller nanoflakes. Nonetheless, these intermediate products are metastable and readily converted to HFBS composed of larger nanoflakes upon longer reaction time due to Ostwald ripening. The observed decrease in the surface area (see Table 1) also proves the gradual phase transformation and crystalline growth. Generally, the morphology of crystals could be a result of synergism between the inner structure and experimental conditions.29 The following discussion will present the evidence why the growth of boehmite is inhibited in the direction perpendicular to the (010) crystal surface, resulting in the nanoflakes that could readily intermesh with each other to form a 3D dendritic porous structure. As shown in Figure 5, the octahedral boehmite double layers are oriented parallel to the unit cell axis. This structure consists of aluminum-centered octahedrons joined by sharing of edges in such a way that the oxygen atoms near the middle of the layer are common to four octahedrons and correspond to O2-, while the outer oxygen atoms are common to two octahedrons and correspond to (OH)-.18c The successive layers are held together by hydrogen bonds. This reveals that the interactions along the a-c plane, occurring between the oxygen atoms shared by four octahedrons and aluminum atoms, should be stronger than the interaction between two octahedral double layers. So the crystal grows along planes instead of perpen-

Figure 5. Projection of boehmite crystalline structure along the [001] direction.

dicular to them, that is, the crystal cleavage, should occur between double layers and not inside them, resulting in the crystal surfaces totally covered by hydroxyls bonded to aluminum atoms. The oxygen atoms of these surface hydroxyls have a free orbital that can react easily with atoms of an aqueous environment.30 Importantly, the large number of -OH groups on the surface is beneficial to interact with foreign molecules, thus the composite functional materials may be easily prepared.31 According to the above proposed formation mechanism, the effect of the reaction temperature on the formation of HFBS can be well explained. When hydrothermal temperature was below 120 °C, only irregular polyhedrons were obtained because there was not enough energy to trigger the crystal cleavage occurring between double layers of the boehmite unit cell. On the other hand, when the temperature rose to 200 °C, the morphology of HFBS became irregular and was partially destroyed; this could be attributed to the higher rate of spontaneous nucleation in solution and faster phase transformation. 3.5. TiO2 Coating on HFBS. Anatase TiO2/AlOOH composite superstructure can be fabricated by the vapor-thermal method and then calcined at 500 °C for 2 h. The SEM image shows that sample E still retains the hierarchical flower-like superstructure of C24 (not shown here). Local magnification of the SEM image (see Figure 6a) of E reveals that TiO2 nanoparticles are homogeneously deposited on the surface of nanoflakes. Figure 6b presents the XRD pattern of E, indicating that E contains anatase and boehmite phases. It is also noteworthy that in comparison to D, the support AlOOH does not convert into γ-Al2O3 after calcination. This is due to the fact that the existence of the anatase phase suppresses the phase transformation of AlOOH to γ-Al2O3.20 The chemical composition of E was determined by EDX. The EDX spectrum (the inset in Figure 6B) confirms the presence of Ti and Al elements and the Al/Ti molar ratio and weight ratio are 12.4 and 6.9, respectively. Figure 7 displays the N2 adsorption-desorption isotherm and the corresponding pore size distribution for sample E. The shape of the hysteresis loop is very similar to that of sample C24, indicating the pore structure was preserved during deposition of TiO2 nanoparticles and subsequent calcination. A small difference between isotherms of samples E and C24 shows in the former case higher adsorption at the entire relative pressure range, indicating the larger surface area of E. The BET specific

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Figure 8. Absorption changes of RhB aqueous solution at room temperature in the presence of sample E under UV irradiation.

Figure 6. (a) High-magnification SEM images of sample E. (b) XRD patterns and EDX (inset) of sample E.

Figure 9. Comparison of photocatalytic activities of samples E, P25, F, and C24 for the photocatalytic decolorization of RhB aqueous solution at ambient temperature; Ct and C0 denote the reaction and initial concentration of RhB in the system, respectively. The inset shows a schematic illustration of multireflections within the intermeshed nanoflakes.

Figure 7. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve (inset) for sample E.

surface area of E is 56.3 m2/g and the observed increase may be attributed to the surface roughness caused by the deposition of TiO2 nanoparticles. 3.6. Photocatalytic Activity. To demonstrate the potential applicability of the composite sample E in photocatalysis and the effect of the composition and morphology on the photocatalytic activity, we examined its photocatalytic activity in relation to that of the pure TiO2 sample obtained by the vaporthermal method (sample F) and the commercial P25 by using photocatalytic degradation of RhB as a test reaction. Figure 8 shows changes in the absorption spectra of an RhB aqueous

solution exposed to UV light for various times in the presence of sample E. Under UV-light illumination, a rapid decrease of RhB absorption at a wavelength of 553 nm was observed, accompanied by an absorption band shift to shorter wavelengths. The degradation rate of RhB was very fast at the beginning of irradiation and then became slow. It is well-known that the RhB photodegradation occurs via two competitive processes: Ndemethylation and the destruction of the conjugated structure.32 In this study, these two processes occurred simultaneously and the later one dominated in the degradation of RhB. A sharp decrease and the observed shift of the major absorption band within 1 h indicate that sample E exhibited excellent photocatalytic activity in the degradation of RhB. Figure 9 shows the comparison of photocatalytic activities of samples E, P25, and F. The pure TiO2 sample is in the form of aggregates consisting of nanoparticles of similar size to those deposited on HFBS (not shown here). The blank reactions performed in the dark or in the presence of C24 under the same experimental conditions of irradiation showed negligible degradation of RhB. These results suggest that the decolorization of RhB aqueous solution was caused by photocatalytic reactions on sample E under UV illumination. As can be seen from Figure

Flower-like AlOOH and TiO2/AlOOH Superstructures

Figure 10. PL spectral changes observed during illumination of sample E in a 5 × 10-4 M basic solution of terephthalic acid (excitation at 315 nm). Each fluorescence spectrum was recorded every 10 min of UV illumination.

9, the photocatalytic activity of sample E was highest. The enhanced photocatalytic activity of sample E can be attributed to the combined effects of several factors: First, TiO2 nanoparticles are homogeneously deposited on the surface of nanoflakes, which eliminated the aggregation of pure TiO2 obtained under the same conditions. A good dispersion of TiO2 nanoparticles resulted in larger surface area accessible to the light and RhB as well as in higher activity of catalytic sites. Second, the unique flower-like superstructure is beneficial due to the porosity present between nanoflakes. These pores can be considered as transport ways for reactant and product molecules moving in or out of the material, so the chemical reactions can occur more easily.28,33 Third, as schematically illustrated in the inset of Figure 9, we believe that the intermeshed nanoflakes allow multiple reflections of UV light, which enhances light-harvesting and thus increases the quantity of photogenerated electrons and holes available to participate in the photocatalytic decomposition of the contaminants.34 To understand the involvement of active species in the photocatalytic process occurring on sample E, the formation of hydroxyl radicals (•OH) on the surface of UV-illuminated sample was detected by the PL technique by using terephthalic acid as a probe molecule. For comparison, samples P25 and F were also examined under the same conditions. Figure 10 shows the changes in the PL spectra for 5 × 10-4 M terephthalic acid solution in 2 × 10-3 M NaOH with irradiation time in the presence of sample E. As can be seen from this figure a gradual increase in the PL intensity at about 425 nm is observed with increasing irradiation time. However, no PL intensity increase is observed in the absence of UV light or sample E. This suggests that the fluorescence is caused by chemical reactions of terephthalic acid with •OH formed at the TiO2/water interface via photocatalytic reactions.35-37 Figure 11 shows a comparison of the induced PL intensity at 426 nm for samples E, P25, and F with irradiation time. It can be seen that the PL intensity increases linearly with time for all samples. Consequently, it can be inferred that •OH radicals produced at the TiO2 surface are proportional to the light irradiation time obeying zero-order reaction rate kinetics.35-37 The formation rate of •OH radicals can be expressed by the slope of these lines shown in Figure 11. These data indicate that the formation rate of •OH radicals

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Figure 11. Comparison of the induced PL intensity at 426 nm for samples E, P25, F, and C24.

Figure 12. (a) Cyclic degradation curve for sample E under UV irradiation, using 1 × 10-5 M RhB as a probe. (b) The experiment of natural settlement: sample E suspended in water (left); settled naturally after 5 min (right).

on sample E is higher than that on samples F and P25, which further proves the former sample shows higher photocatalytic activity. To test the repeatability of RhB degradation on sample E, we carried out the degradation experiment repeatedly five times. As shown in Figure 12a, the RhB is quickly degraded after every injection of the RhB solution, and the photocatalyst is stable under repeated application with nearly constant photodecomposition rate, showing that sample E was not deactivated during the photocatalytic oxidation of the pollutant molecules. Moreover, because the microscopic particle size of E and its rapid

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rate of settling, this material can be easily separated from the reaction system. Thus, sample E not only showed enhanced photocatalytic activity, but also could settle naturally in 5 min, as shown in Figure 12b. Therefore, sample E can be regarded as an attractive photocatalyst for large-scale environmental purification because it can be separated from the slurry system by filtration or sedimentation after photocatalytic reaction and reused more easily than the conventional nanosized powder photocatalytic materials. 4. Conclusions Hierarchical flower-like boehmite superstructures (HFBS) can be synthesized by hydrothermal treatment of as-formed aluminum hydroxide microspheres, which are prepared by adding aluminum spheres into a sodium metaaluminate solution. The as-prepared HFBS can be easily converted into γ-Al2O3 without morphology change by calcination in air at 500 °C for 2 h. Furthermore, TiO2 nanoparticles can be homogeneously deposited on the surface of the as-prepared HFBS by using the vaporthermal method. These TiO2/AlOOH composite superstructures show higher photocatalytic activity than conventional TiO2 and P25 particles for the photocatalytic decolorization of Rhodamine B aqueous solution at ambient temperature, partly due to their specific hierarchical structure easily accessible to light and reactants. After five recycles of the RhB photodegradation, sample E retained its activity, proving high stability of the TiO2/ AlOOH composite superstructures. This study provides some new insights into the design and fabrication of advanced photocatalytic materials with complex hierarchical architectures with enhanced photocatalytic activity. It should be noted that the synthesis strategy may be extended to the preparation of complex functional structures with controlled physicochemical properties for a variety of applications such as wastewater treatment. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (50625208, 20773097, and 20877061), CHCL09006, and the National Basic Research Program of China (2007CB613302). References and Notes (1) (a) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (b) Wang, Z. L.; Pan, Z. W. AdV. Mater. 2002, 14, 1029. (c) Shen, G. Z.; Bando, Y.; Ye, C. H.; Yuan, X. L.; Sekiguchi, T.; Golberg, D. Angew. Chem., Int. Ed. 2006, 45, 7568. (2) (a) Zhang, J.; Sun, L. D.; Yin, H. L.; Su, H. L.; Liao, C. S.; Yan, C. H. Chem. Mater. 2002, 14, 4172. (b) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (c) Gao, F.; Lu, Q. Y.; Xie, S. H.; Zhao, D. Y. AdV. Mater. 2002, 14, 1537. (d) Fang, X. S.; Zhang, L. D. J. Mater. Sci. Technol. 2006, 22, 1. (e) Yan, C. L.; Xue, D. F. J. Phys. Chem. B 2006, 110, 7102. (f) Shen, G. Z.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2006, 88, 123107. (3) Mokari, T.; Rothenberg, E.; Popov, L.; Costi, R.; Banin, U. Science 2004, 304, 1787. (4) (a) Lu, Q. Y.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54. (b) Zhu, L. Y.; Xie, Y.; Zheng, X. W.; Liu, X.; Zhou, G. E. J. Cryst. Growth 2004, 260, 494. (c) Wu, Z. C.; Pan, C.; Yao, Z. Y.; Zhao, Q. R.; Xie, Y. Cryst. Growth Des. 2006, 6, 1717. (5) Wang, D.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (6) Shen, G. Z.; Bando, Y.; Liu, B. D.; Tang, C. C.; Huang, Q.; Golberg, D. Chem.sEur. J. 2006, 12, 2987. (7) Ma, C.; Moore, D.; Li, J.; Wang, Z. L. AdV. Mater. 2003, 15, 228. (8) Balakrishnan, S.; Gun’ko, Y. K.; Perova, T. S.; Moore, R. A.; Venkatesan, M.; Douvalis, A. P.; Bourke, P. Small 2006, 2, 864. (9) (a) Yada, M.; Taniguchi, C.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. AdV. Mater. 2004, 16, 1448. (b) Yu, J. G.; Zhao, X. F.; Liu, S. W.; Li, M.; Mann, S.; Ng, D. H. L. Appl. Phys. A: Mater. Sci. Process. 2007, 87, 113.

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