Hierarchical TiO2 Nanoflakes and Nanoparticles Hybrid Structure for

Article pubs.acs.org/JPCC

Hierarchical TiO2 Nanoflakes and Nanoparticles Hybrid Structure for Improved Photocatalytic Activity Yuxin Tang,† Peixin Wee,† Yuekun Lai,† Xiaoping Wang,‡ Dangguo Gong,† Pushkar D. Kanhere,† Teik-Thye Lim,‡ Zhili Dong,*,† and Zhong Chen*,† †

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

‡

S Supporting Information *

ABSTRACT: Three-dimensional TiO2 microspheres with different hierarchical nanostructures were synthesized by the synergistic strategies of ultrafast electrochemical spark discharge spallation process followed by thermal treatment. The morphology, crystal structure, surface area, and photocatalytic activity of the hierarchical nanostructures were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, surface area analysis, and UV−vis spectroscopy respectively. The nanostructure of hierarchical microspheres undergoes three evolution steps, which includes the change from nanosheets into hybrid nanoflakes/ nanoparticles and finally to nanoparticles as calcination temperature increases, in line with the predicable trend of increase in crystallinity and decrease in specific surface area. Compared to other forms of calcined TiO2 samples (nanosheets and nanoparticles), the hybrid TiO2 nanoflake/ nanoparticle hierarchical porous structure exhibits a higher photocatalytic activity for the degradation of organic compounds (methyl orange and bisphenol A). This is attributed to their larger specific surface area (∼116 m2/g), more abundant porosity, and good crystallinity. On the basis of this hybrid structure, a visible light sensitive Ag/TiO2 microsphere photocatalyst is designed which shows faster degradation rate under the visible light illumination (>420 nm). The porous microspheric photocatalyst does not lose its activities after recycled use, showing great potential for practical application in environmental cleanup. thermal treatment37−45 of titanate nanomaterials is of paramount importance in designing ideal shape, size, exposed active facet, and phase of TiO2 materials with enhanced photocatalytic activity. However, it remains a great challenge to develop feasible top-down methods for synthesis of well-defined 3D TiO2 hierarchical nanostructures with large surface area. To further enhance the photocatalytic performance, 3D structures of single-crystalline anatase TiO2 building blocks are highly desired, because of their more efficient light harvesting due to excellent incident light scattering within the structures, high organic dye or pollutant adsorption, and unique hierarchical characteristics.46,47 In this work, a hybrid nanoflake/nanoparticle structure made up of hierarchical TiO2 anatase microsphere is thermal-derived from the high surface area (387.6 m2/g) of layered hydrogen titanate microspherulite (H-TMS), which is obtained through an ultrafast electrochemical spark discharge spallation. A visible light sensitive photocatalyst based on this hybrid structure is also demonstrated by Ag particle loading on the formed nanostructures. The thermally induced crystal structure, morphology transition,

1. INTRODUCTION In recent years, hierarchically structured materials with various morphologies have attracted great attention. Many investigations have demonstrated that hierarchical structures could improve the performance (optoelectronic, biomedical, energy storage, etc.) of materials because of their highly porous structure with a large specific surface area.1−6 In all these cases, the controllable synthesis of three-dimensional (3D) nanostructures plays an important role to achieve the advanced materials application targets.7−9 Titanium dioxide has been widely investigated during the past decades due to its important applications in photovoltaics, photocatalysis, photo/electrochromics, photonics, smart surface coatings, and sensors.10−17 So far, numerous synthetic strategies have been employed for the formation of TiO2-based nanomaterials in the form of particles, tubes, fibers/wires, belts, rods, etc., via sol−gel, hydrothermal, solvothermal, electrochemical anodization, electrodeposition, and templateassisted routes.11,18−28 Recently, research on titanate and its derived TiO2 nanostructures with large specific surface area has received great attention since the pioneering work by Kasuga et al.,26 who introduced an alkali hydrothermal method for preparing titanate nanotube without using any templates. In particular, the investigation on the thermal29−36 or hydro© 2012 American Chemical Society

Received: November 1, 2011 Revised: December 29, 2011 Published: January 4, 2012 2772

dx.doi.org/10.1021/jp210479a | J. Phys. Chem. C 2012, 116, 2772−2780

The Journal of Physical Chemistry C

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Figure 1. (a) TEM image and (b) XRD pattern of H-TMS. The inset in (b) is the EDS spectrum of the H-TMS.

(BPA) was selected to investigate the photocatalytic degradation of the calcined H-TMS. The batch experiment was first conducted in the dark. A beaker, wrapped with an aluminum foil, was filled with 25 mg of the heat-treated sample dissolved in 25 mL of 10 mg/L MO or BPA solution. The solution was stirred at 500 rpm at room temperature throughout the experiment to maintain homogeneity. After 30 min in the dark to attain adsorption/desorption equilibrium, the aluminum foil was unwrapped and the solution was exposed to UV−visible light with intensity 95 mW/cm2 from a 300 W xenon lamp (HAL-320, Asahi Spectra Co. Ltd.) for degradation study. For the visible light degradation of MO, the loading of photocatalysts is 1.0 mg/1.0 mL. A filter was applied to cut away light below 420 nm, and the light intensity is 83 mW/cm2. The quantitative measurement of the MO concentration at different time intervals can be determined by the absorption peak generated under a Perkin-Elmer UV−vis-NIR Lambda 900 spectrophotometer. For the quantitative measurement of BPA, aliquots of the solution filtered using cellulose acetate syringe membrane filters (Iwaki, 0.45 μm) were analyzed using a highperformance liquid chromatograph (HPLC, Perkin-Elmer Series200) at designated time intervals. The HPLC analysis was carried out using an Inertsil ODS-3 column and a Series 200 UV/vis detector at 225 nm, with acetonitrile and water (70/30, v/v) as effluent at a flow rate of 1 mL/min.

and photocatalytic activity of the hybrid nanostructures have been evaluated in this work.

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 Microspheres and Ag/TiO2 Microspheres. Preparation of TiO2 Microspheres. The layered titanate hierarchical microspherulite were prepared using a starting titanium foil via a rapid electrochemical spark discharge spallation (ESDS) method48 in an electrolyte of 10 M NaOH in aqueous solution with a platinum counter electrode. The anodization was conducted at current density of 0.5 A/cm2 at room temperature, with a distance of 3.0 cm between the two electrodes. After completion of the experiment, a graywhite precipitate was collected from the solution, yielding the sodium titanate microspherulite (Na-TMS). The hydrogen titanate microspherulite (H-TMS) was obtained by soaking the precipitates in HCl solution (0.1 M) for several times, followed by washing in deionized water and absolute ethanol before drying in air. The as-synthesized H-TMS samples were annealed at certain temperature (300, 400, 500, 600, and 700 °C) for 1 h with a heating rate of 5.0 °C/min. Preparation of Ag/TiO2 Microspheres. The as-prepared HTMS was added into a mixed solution composed of 0.1 M of silver nitrate (1 mL), 20 mL of methanol (99.98% Tedia), and 50 mL of deionized water. Photoreduction is carried out for 2 h during which silver ions are reduced to form silver nanoparticles on the surface of H-TMS. The solution is then washed, centrifuged with deionized water, and dried in an oven to form Ag/H-TMS materials. Finally, Ag/H-TMS powders were annealed at 500 °C for 1 h with a heating ramp rate of 5.0 °C/min, generating the Ag/TiO2 microspheres. Ag loading on commercially obtained samples for comparison followed the same photoreduction procedure. 2.2. Characterization. The phase composition of the sample was identified by X-ray diffraction using a Shimadzu 6000 X-ray diffractometer with a Cu Kα radiation (λ = 1.54178 Å). The morphologies were examined using a field emission scanning electron microscope (FESEM, JEOL JSM-7600F) and transmission electron microscope (TEM, JEOL JEM-2010). Nitrogen adsorption/desorption isotherms were measured at 77 K using ASAP2000 adsorption apparatus from Micromeritics. The UV−visible absorption spectra were recorded with a Perkin-Elmer UV−vis-NIR Lambda 900 spectrophotometer. 2.3. Photocatalytic Degradation of Methyl Orange and Bisphenol A. Methyl orange (MO) and bisphenol A

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure of H-TMS. During the electrochemical spark discharge spallation process, a fast anodic reaction on the titanium surface creates a layer of titanium dioxide that instantly breaks down by the applied electrical field into the solution in the form of titanium oxide particles. The spalled particles readily react with the heated NaOH electrolyte to form Na-TMS. The Na-TMS transforms to H-TMS with the same morphology after acid washing. The typical TEM image and XRD pattern of the as-prepared H-TMS are shown in Figure 1. The TEM image in Figure 1a shows that individual microspheres are highly porous, and the 3D microsphere consists of individual 2D sheets at the nanoscale, emanating from the core like a star forming a hierarchical structure. The low diffraction angle at ∼9° of the H-TMS indicates the product possesses a large interplane spacing, which is common for layered titanate structures. The broad peaks in the XRD pattern of H-TMS corresponds well with the orthorhombic titanate H2Ti2O5·xH2O (JCPDS No. 47−0124). The energydispersive X-ray spectroscopy (EDS) data in the inset of Figure 2773



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1b confirms that the sodium ions are exchanged completely after the acid washing process. 3.2. Morphology and Phase Evolutions of H-TMS. The XRD patterns of the pyrosynthesized H-TMS samples at different temperatures are shown in Figure 2. After annealing,

Table 1. Effect of Calcination Temperature on the Crystalline Size, BET-Specific Surface Area (SBET), Pore Volume, and Pore Size of Calcined H-TMS

a

calcination temperature (°C)

crystalline sizea (nm)

surface area (m2/g)

pore volume (cm3/g)

average pore size (nm)

H-TMS 300 400 500 600 700

NA 9.6 11.4 13.4 22.1 33.4

387.6 238.6 174.5 116.1 75.2 33.3

0.66 0.60 0.70 0.57 0.50 0.39

7.1 10.1 16.1 19.6 25.3 47.1

Average crystallite size was calculated from the anatase (1 0 1) peak.

was also found in the monoclinic phase of bulk titanate materials.52,53 It is noted that the titanate, TiO2(B), and anatase contain the same zigzag ribbons of TiO6 octahedra that share four edges with the others, and the common structural features make the phase transition from titanate to anatase or TiO2(B) relatively easy. We propose that the different phase transitions undergo different evolution schemes in Figure 3. The hydrogen

Figure 2. XRD patterns of H-TMS calcined at various temperatures, indicating the formation of anatase phase from the orthorhombic titanate H-TMS.

the orthorhombic titanate transforms to the anatase phase with a lattice constant a = 0.37 nm (JCPDS No. 21−1272). The diffraction peaks at 2θ = 25.3, 37.8, 48.0, 53.1, 55.4, 62.7, 68.8, 70.3, and 75.5° corresponds to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes of anatase TiO2, respectively. It is interesting that the onset temperature of phase transformation (300 °C) was lower than what were usually reported, and the anatase phase was stable up to 700 °C when the Na-free H-TMS titanate was used. With increasing calcination temperature, the peak intensity of anatase increases significantly, indicating the enhanced crystallization of anatase phase. Simultaneously, the width of the (101) peak becomes narrower, suggesting the growth of the anatase crystallites. The average crystallite size of the calcined sample is calculated using the Scherrer’s equation for the main diffraction peak via D = (0.9λ)/(β cos θ), where D is the crystallite size, λ the X-ray wavelength used, β the full-width at half-maximum intensity of the anatase peak, and θ the corresponding Bragg angle. By increasing calcination temperature from 300 to 700 °C, the crystallite size of anatase monotonically increased from 9.6 to 33.4 nm as shown in Table 1. In the previous studies, different phase transition sequences have been reported for hydrogen titanate transformation upon heat treatment. For examples, many researchers29,31,43 found that anatase phase was formed after the calcinations of hydrogen titanate nanotubes, which is in good agreement with our present results. Also, the similar trend was observed when we annealed the 2D hydrogen titanate nanosheet samples via hydrothermal approach (Figure S1 of Supporting Information). However, when the hydrogen titanate nanowires or fibers were heat treated, the TiO2(B) phase was always formed before the appearance of anatase phase.49−51 The same

Figure 3. Different schemes of phase transformation via calcined layered orthorhombic and monoclinic titanate materials. (1) Formation of tetragonal anatase TiO2 by delamination layered orthorhombic titanate splitting along the ⟨b⟩ axis. (2) The layered monoclinic titanate undergoes topotactic structural condensation (step ①), forming a TiO2(B)-like intermediate, and then monoclinic TiO2(B) is obtained via breaking the intermediate product along the connected corner (green line, step ②) of three TiO6 units of one chain.

1D titanate nanotube, 2D nanosheets, and 3D H-TMS possess the same orthorhombic titanate phase H2Ti2O5·xH2O (JCPDS No. 47−0124).54,55 Compared with the monoclinic titanate, the trapped water molecules more easily escape through the parallel-layered TiO6 nanosheets in the orthorhombic titanate. Upon heat treatment, the orthorhombic titanate dehydrates and the large structural units such as zigzag ribbons remain almost unchanged, and would rearrange to form tetragonal anatase lattice when the water molecules escape from the interlayers (scheme 1 of Figure 3). For the hydrogen 1D titanate nanowire or fibers and the bulk titanate materials possessing the monoclinic phase (H2Ti3O7·xH2O),56 their dehydration proceeds through three distinct steps:52 an initial topotactic structural condensation, followed by an exothermic nucleation and growth step that results in the formation of a TiO2(B)-like intermediate, and a final, low-energy transformation, which yields monoclinic phase TiO2(B) (scheme 2 of Figure 3). The FESEM images (Figure 4) and TEM images (Figure 5) are provided to understand the morphology change of the 2774



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Figure 4. FESEM images of the (a) H-TMS calcined at different temperatures: (b) 300 °C, (c) 400 °C, (d) 500 °C, (e) 600 °C, and (f) 700 °C.

Figure 5. TEM images of the calcined H-TMS at different temperatures of (a) 300 °C, (b) 400 °C, (c, f) 500 °C, (d) 600 °C, and (e) 700 °C. (f) HRTEM image of c. The insets in parts a−e are taken at higher magnification, while the inset in f is the diffraction pattern of c.

pyrosynthesized H-TMS at different temperatures. A morphology evolution process under different temperatures is illustrated in Figure 6. During the heat treatment, the H-TMS starts to dehydrate, and the H-TMS undergoes the topotactic structural

condensation accompanying the shrinkage of the morphology. The H-TMS shows a 3D superstructure that is constructed by large quantities of fluffy nanosheets emanating from the centers in three dimensions. The as-prepared H-TMS sample (Figure 2775



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Figure 6. Schematic illustration of calcined H-TMS morphology evolution with increasing temperatures.

4a) after calcination at 300 °C still retains the hierarchical nanosheet structure (Figure 4b), while TEM image (Figure 5a) shows the nanosheets shrink and become curly. The higher annealing temperature leads to the wrap up of the fringes and the sheets tend to be fragile with small TiO2 crystalline conjunctions. At 400 °C, an oblong shape nanoflakes after bending and scrolling of nanosheets was observed with the formation of small portion of nanoparticles on its surface (Figures 4c and 5b), which indicates that H-TMS is unstable at 400 °C. This phenomenon was also found in the thermal treatment of titanate nanotube.29 The tendency of nanoparticles growth from nanosheets becomes significant for the 500 °C annealed sample as the high calcination temperature provides enough energy for the collapse of the nanosheets. It was observed that the microspheric surface has a branchlike structure composed of nanoflakes and nanoparticles (Figure S2 of Supporting Information and Figure 5c). When the calcination temperature increases to 600 °C (Figures 4e and 5d), the nanosheets completely transform into nanoparticles. The sample shows a heterogeneous assembly of small crystallites of 20 ± 4 nm (Figure 5d). Increasing the temperature to 700 °C leads to the formation of larger and more irregularly shaped crystals ranging from 20 to 65 nm, which are probably derived from the coalescence of smaller grains. These results are consistent with the X-ray diffraction data analysis (Figure 2, Table 1). HRTEM images in Figure 5f are taken from the inset in Figure 5c. The diffraction pattern (inset of Figure 5f) suggests that the calcined TiO2 microspheres are polycrystalline, and the interplanar spacing of the TiO2 crystals is ca. 0.36 nm, corresponding to the (101) crystal plane of anatase TiO2. The typical isotherms for nitrogen adsorption and desorption of the calcined H-TMS samples are shown in Figure 7a. The Barrett−Joyner−Halenda (BJH) method was employed to analyze the pore-size distribution, and the results are shown in Figure 7b and Table 1. Before calcination, the as-prepared HTMS material has a high Brunauer−Emmett−Teller (BET) specific surface area of 387.6 m2/g with a large pore volume of 0.66 cm3/g. The peak of pore diameter distribution below 10 nm is attributed to the nanogaps between the thin nanosheets inside the H-TMS, while the large pore diameter (>10 nm) is due to the aggregation of H-TMS particles. The BET surface area and pore size distribution of the calcined H-TMS samples strongly depends on the calcination temperatures. It can be seen that all calcined samples show type IV sorption isotherms according to IUPAC classification, indicating the presence of

Figure 7. (a) Nitrogen adsorption−desorption isotherms. (b) Pore size distribution (BJH desorption) of H-TMS and the calcined samples at different heating temperatures.

mesopores (2−50 nm). As the calcination temperature increases, the hysteresis loops shift toward higher relative pressure and the area of the hysteresis loops gradually decrease, indicating the decrease of BET surface area and pore volume. Also, the peaks of pore size distribution shifts toward larger pore diameter. After calcination at 300 and 400 °C, the isotherm presents a typical hysteresis loop of type H2 (Figure 7a) which is attributed to the difference in the adsorption and desorption mechanisms occurring in the “ink-bottle” pores. This hysteresis loop is similar with H-TMS, indicating that the pore structure (Figure 7b) did not change much under low-temperature calcinations. The specific surface area of calcined sample at 300 °C reaches 238.6 m2/g with an average pore size of 10.1 nm (calculated from BJH model) and a pore volume of 0.60 cm3/g. Also, the 400 °C heated sample possesses a high surface area around 174.5 m2/g with a pore volume of 0.70 cm3/g. The decrease of surface area is due to the destruction of the sheetlike structure and the growth of TiO2 crystallites size, which corresponds to the morphology changes in schematic illustration of Figure 6. When the temperature increased to 500 °C, the shape of hysteresis loop became narrow, indicating that the increase of mesopores size among the nanostructures. The TEM (Figure 5c) and FESEM (Figure S2 of Supporting Information) observation shows that the more than half of the nanosheets structure has been transformed to nanoparticles, and a branchlike structure (schematic model in Figure 6) is 2776



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Figure 8. (a) Photocatalytic degradation of MO solution (25 mL, 10 mg/L) under UV−vis light irradiation (intensity 95 mW/cm2) by commercial Degussa P25, commercial anatase powder, H-TMS calcined at various temperatures (300, 400, 500, 600, and 700 °C) at a constant heating rate (5 °C/min). (b) Photocatalytic degradation of BPA solution (25 mL, 10 mg/L) by commercial P25, commercial anatase powder, H-TMS calcined at various temperatures (300, 400, 500, 600, and 700 °C) at a constant heating rate (5 °C/min). (c) Cyclic runs of the photocatalytic degradation of MO in the presence of the hierarchical TiO2 microspheres calcined at 500 °C under the UV−vis light irradiation. The running time is 60 min for every cycle.

formed with a surface area around 116.1 m2/g. Upon further increase of the calcination temperature to 600 and 700 °C, the hysteresis loop shift to high relative pressure and the loop shape changes from the type H2 to type H3, reflecting the appearance of the macrospore structures. Compared with other temperatures, their crystalline size increase rapidly as well as the average pore size (Table 1), and the pore size ranges from 5 to 20 nm decreases abruptly (Figure 7b). This is due to the complete conversion from the original nanosheet structure to nanoparticles structure (parts e and f of Figure 4 and parts d and e of Figure 5). Their specific surface area was drastically decreased to 75.2 and 33.3 m2/g, respectively. At 700 °C, the sorption curve of the sample exhibited a smaller hysteresis loop at P/P0 ≈ 0.9, and the pore diameter distribution curve is spread more evenly throughout a wider range of pore diameters. This indicates that the coarsening of the nanoparticles occurs at higher temperatures. FESEM and TEM observations also confirmed the coarsening. 3.3. Photocatalytic Activity of Calcined H-TMS. It is known that the photoactivity depends on the crystallinity, phase, and the surface area of the semiconductor materials. For TiO2, anatase phase has a higher photocatalytic activity compared with the other polymorphs of TiO2.11 In this work, the photocatalytic activities of the as-synthesized anatase hierarchical TiO2 microspheres under different heat treatment conditions were evaluated by photocatalytic degradation of MO and BPA. Experiments in the dark and under UV−visible light were performed to distinguish the contribution of adsorption

and degradation. Commercial available anatase nanoparticles (A-TiO2, surface area ∼9.6 m2/g) with a particle size of 50−200 nm from Sigma-Aldrich and the Degussa P25 titania nanoparticles (P25, surface area ∼50 m2/g with a particle size of ∼25 nm) were selected for comparison. As shown in Figure 8a, all the samples show little adsorption of MO in dark, and all the calcined samples can efficiently photodegrade MO under UV− visible light irradiation. In the absence of TiO2 photocatalyst, the self-degradation of MO is almost negligible under UV− visible illumination. H-TMS calcined at 300 °C has shown relatively lower photocatalytic activity, which is due to its poor crystallinity despite that the sample possessed a high surface area (Table 1). With the increase in the calcination temperature to 400 and 500 °C, the photocatalytic activity of the sample is significantly increased, which can be ascribed to their improved crystallinity (Figure 3) coupled with high surface areas and pore volumes (Table 1). Larger specific surface area allows more aqueous reactants to be absorbed onto the surface of the photocatalyst, while higher pore volume results in a more rapid diffusion of various aqueous products during the photocatalytic reaction. It is observed that the 500 °C treated photocatalyst completely degrade MO at a shorter time than the commercial A-TiO2 nanoparticles, as shown in Figure 8a. The performance is comparable with P25, which is known for its excellent photocatalytic activity due to its unique mixed-phase crystal structure. On the basis of the different activities shown, we believe that the dominant factor to influence the photoactivity is the crystallinity when the heating temperature is ≤500 °C. 2777



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Figure 9. (a) FESEM image and (b) low- and (c) high-magnification TEM images of the as-prepared Ag/TiO2 microspheres. The inset image in part c is the high-magnification TEM images of a individual Ag nanoparticle. (d) Diffuse reflectance UV−vis absorption spectra of P25, A-TiO2, H-TMS, TiO2 microsphere, Ag/P25, Ag/A-TiO2, and Ag/TiO2 microsphere. (e) Photocatalytic degradation of MO solution (25 mL, 10 mg/L) of the commercial P25, A-TiO2 nanoparticles, TiO2 microspheres, Ag/P25, Ag/A-TiO2, and Ag/TiO2 microspheres. The loading of photocatalysts is around 1.0 mg/1.0 mL. The light intensity is 83 mW/cm2 under the visible light region (>420 nm).

for loading noble metals or visible light sensitive semiconductor (Fe2O3, CuO, CdS, etc.) nanoparticles to construct heterostructures for visible light degradation. For demonstration, the heterostructures composed of Ag nanoparticles and TiO2 microsphere was fabricated via photoreduction and postannealing treatment. The Ag nanoparticles were in situ grown on the H-TMS to form Ag/H-TMS first, and then heterostructures Ag/TiO2 microspheres was obtained by calcination of the Ag/ H-TMS materials. Following the same procedure, Ag loaded on P25 (Ag/P25) and Ag loaded on TiO2 anatase nanoparticles (Ag/A-TiO2) were selected for comparison. The FESEM image in Figure 9a and TEM images in parts b and c of Figure 9 confirm that the silver nanoparticles are uniformly distributed on the surfaces of calcined H-TMS samples. The diameter of the silver nanoparticles is around 9 ± 4 nm (Figure 9c). The lattice distance of 0.23 nm in the inset of Figure 9c corresponds to the (111) plane of silver nanocrystals. The absorption spectra of A-TiO2, H-TMS, Ag/A-TiO2, and hierarchical Ag/ TiO2 microspheres are shown in Figure 9d. It was found that the silver-decorated P25, A-TiO2, and TiO2 microspheres have a distinct red-shift to visible region compared with other TiO2 samples. The absorption spectra of Ag/P25 (black color) and Ag/A-TiO2 (dark gray color) clearly show significant increase of absorption in the visible and near-infrared regions, and in particular with wavelengths larger than 450 nm. The Ag/TiO2 microspheres sample (gray color) shows a shallow peak with higher intensities observed at around 500 nm. The enhanced absorption of Ag/P25, Ag/A-TiO2, and Ag/TiO2 microspheres

Beyond that, the surface area becomes more important since the samples have all reached high crystallinity state. This is confirmed by the significantly decreased photoactivity of the samples cacined at 600 and 700 °C. With the same experiment condition, the as-prepared samples were also tested for degradation of colorless BPA aqueous solution without having strong photoabsorption to UV−visible light under illumination. Similar trend was observed for the degradation of BPA solution, commercial A-TiO2, P25, and the 500 °C calcined sample displayed similar time for complete degradation of BPA. This experiment indicates that the as-prepared TiO2 hierarchical microsphere photocatalyst is also efficient to degrade the colorless organic molecule in aqueous solution. Comparing the degradation rate between MO and BPA under the same condition, it is clear that BPA is more easily decomposed. The hierarchical TiO2 porous microspheres can be easily recycled by a simple sedimentation or filtration. After four recycles for the photocatalytic degradation of MO, the photocatalysts calcined at 500 °C still showed good photocatalytic activity (Figure 8c), and the hierarchical porous nanostructure remained (Figure S3 of Supporting Information). The slight decrease in the degradation rate could be explained by the loss of TiO2 particles during sampling and filtration processes. Our study indicates that the photocatalyst possesses excellent stability and reusability, which is important for practical applications such as environmental depollution. In addition, this hybrid H-TMS material with hierarchical nanostructure and open porous networks offer a favorable host 2778



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in the visible light region are attributed to the surface plasmon resonance (SPR) effect of spatially confined electrons in the Ag nanoparticles.57,58 Although the Ag/P25 and Ag/A-TiO2 show the higher visible light absorption, the photocatalytic performance is far below the Ag/TiO2 microspheres (Figure 9e). A possible reason for the higher photocatalytic activity of the Ag/ TiO2 microspheres is believed to be due to the smaller TiO2 crystallite size and large number of pores present, which enables the formation of larger number of Ag-TiO2 heterojunctions. Because of the comparable particle size of Ag and TiO2, a single Ag nanoparticle could be in contact with multiple TiO2 nanoparticles (parts b and c of Figure 9). Such uniformly formed or trapped (in the pore) Ag particles on the hierarchical TiO2 hybrid-structure could enhance the electron−hole separation between Ag and the TiO2.59,60 Whereas in the case of commercial P25 and anatase samples, because of the relatively large particle size, several Ag nanoparticles are grown on an individual TiO2 particle (Figure S4 of Supporting Information).

In summary, we have synthesized hierarchical TiO2 microsphere structures with large specific surface area derived from heat treatment of titanate microspherulite, which was prepared via an ultrafast and straightforward electrochemical spark discharge spallation method. Upon the heat treatment, the morphology transforms from nanosheets to hybrid nanoflakes/ nanoparticles. Because of the high surface area, open porous networks, and good crystallinity, the synthesized hybrid TiO2 nanoflake/nanoparticle microspheres demonstrated a significantly improved photocatalytic performance for both MO and BPA. In addition, the as-prepared microspheres can be easily recycled and reused. It can also be made visible light active by loading Ag nanoparticles. The general method described in this work provides a facile strategy to synthesize 3D hierarchical materials with improved photocatalytic activity.

ASSOCIATED CONTENT

S Supporting Information *

Additional characterizations of the titanate nansheets, highmagnification FESEM image of the calcined H-TMS, TEM images of the H-TMS sample calcined at 500 °C after photocatalytic degradation cycling, and TEM images of the Ag/ A-TiO2 and Ag/P25 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +65 6790 4256, +65 6790 6727. Fax: +65 6790 9081. E-mail: [email protected] (Z.C.); [email protected] (Z.L.D.).

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REFERENCES

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ACKNOWLEDGMENTS

The authors thank the Environment and Water Industry Programme Office under the National Research Foundation of Singapore (Grant No. MEWR651/06/160) for the financial support of the work. The authors thank Dr. Pierre Pichat, Dr. Simo Pehkonen, and Dr. Alexander Orlov for advice and numerous constructive discussions. 2779



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