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Hydrothermal Preparation and Photocatalytic Activity of Hierarchically Sponge-like Macro-/Mesoporous Titania Jiaguo Yu,* Lijuan Zhang, Bei Cheng, and Yaorong Su 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 ReceiVed: January 30, 2007; In Final Form: May 18, 2007
Trimodally sponge-like macro-/mesoporous titania was prepared by hydrothermal treatment of precipitates of tetrabutyl titanate (Ti(OC4H9)4) in pure water. Effects of hydrothermal time on the phase composition, porosity, and photocatalytic activity of hierarchically porous titania were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) and N2 adsorption-desorption measurements. All TiO2 powders prepared at 180 °C showed trimodal pore-size distributions in the macro-/mesoporous region: fine intraparticle mesopores with peak pore diameters of ca. 3.7-6.9 nm, larger interparticle mesopores with peak pore diameters of ca. 23-39 nm, and macropore with pore diameter of ca. 0.5-3 µm. With increasing hydrothermal time, crystallinity, and average anatase crystallite size, pore size and pore volume increased, while specific surface area decreased. The hierarchically porous titania prepared at 180 °C for 24 h displayed an especially high photocatalytic activity probably due to its special pore-wall structure, and its photocatalytic activity was about three times higher than that of Degussa P-25. This trimodally sponge-like macro-mesostructured titania could find its varieties of potential applications in photocatalysis, catalysis, solar cell, and separation and purification processes. A new concept “biomemitic photocatalysis” has been proposed, which may provide new insight into preparation of advanced photocatalytic materials by mimicking surface structures of plant leaves.
1. Introduction In recent years, materials with hierarchically multimodal poresize distributions have attracted more and more attention,1,2 because they combine the benefits of high surface area microand mesoporosity with the accessible diffusion pathways of macroporous networks and play key roles in industrial processes, from catalysis and catalytic supports to gas storage, purification, and separation. The controlled synthesis of hierarchically porous materials such as crystalline oxide semiconductor, amorphous silica, metallic gold, and elemental carbon should therefore lead to higher separation and catalytic efficiency and new applications in large-molecule catalysis, biomolecule separations, miniaturization of electronic devices, and chromatographic supports.3,4 To date, hierarchically porous materials with combinations of micro-/mesopores,5 meso-/macropores,6,7 micro-/macropores,8 and micro-/meso-/macropores9 have been prepared usually by employing dual templates with appropriate reaction solutions or nanoparticle building blocks. In general, the control of the bimodal porosity is achieved by combining suitable templates for the required length scale organization; for example, tetraalkylammonium ions are used for directing microporosity (50 nm).4 However, the template-free synthesis of high-surface-area materials with multiple porous structures still remains a great experimental challenge. * To whom correspondence should be addressed. E-mail: jiaguoyu@ yahoo.com.
Titania (TiO2) is a very important and multifunctional material with a wide variety of potential uses in diverse areas such as photovoltaic cell, gas senor, air purification, water disinfection, hazardous waste remediation, and pigments. Although titania powders and sols can be readily prepared by conventional hydrothermal or sol-gel methods, synthetic routes to hierarchically porous titania are less common. Recently, a lot of research groups have demonstrated that hierarchically ordered macro-/ mesoporous titania can be prepared by using surfactant templates.7,10-12 However, posttreatment removal of organic templates to produce the titania replica requires additional processing steps that can be costly, wasteful, and of environmental concern. Clearly, these problems would be easily solved if spatial patterning and hierarchically porous structures of the inorganic phase could be achieved in the absence of auxiliary organic templates or additives, for example by coupling the hydrothermal reactions with physical phenomena such as microphase separation, transient hydrodynamic gradients, timedependent diffusion gradients, and restructure and phase transformation of amorphous TiO2.13 To solve the increasingly serious problems of environmental pollution, various catalytic techniques are being applied in the fields of environmental protection. Oxide semiconductor photocatalysis is one technique that has great potential to control aqueous organic contaminates or air pollutants. It is believed to have several advantages over conventional oxidation processes, such as the following: (1) complete mineralization of the pollutants, (2) use of the near-UV or solar light, (3) no addition of other chemicals, (4) operation at near room temperature, (5) low cost, and so on.14,15 Although photocatalytic
10.1021/jp0707889 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
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degradations of trace toxic organic compounds in water or air have been investigated intensively in the past decade, there still remain some problems in practical applications.14-16 Fundamental research regarding the preparation of photocatalyst with highly photocatalytic activity, the immobilization of powder photocatalyst, and the improvement of photocatalyst performance are priorities to be considered.16 To solve these problems, crystalline anatase TiO2 powders with hierarchically bimodal macro-/mesoporous structures and high BET specific surface areas are desirable to improve the photocatalytic activity, since such hierarchically ordered porous structures not only provide a readily accessible pore-wall system and high specific surface area for light and reactants but also optimize the transport of matter by minimizing the pressure drop over the monolithic material.10 In this work, we describe a simple and environmentally benign method for the synthesis of hierarchically spongelike trimodal macro-/mesoporous TiO2 materials with anatase and brookite biphase composition by hydrothermal method at 180 °C without using templates and additives. The prepared samples show especially high photocatalytic activity. A new “biomemitic photocatalysis” concept is proposed by mimicking surface structures and photosynthesis of green leaves. 2. Experimental Methods 2.1. Preparation. All chemicals used in this study were used as received from Shanghai Chemical Regent Factory of China without further purification. Distilled water was used in all experiments. Tetrabutyl titanate (Ti(OC4H9)4, TBOT) was used as titanium source for the synthesis of hierarchically trimodal macro/mesoporous TiO2 powders. In a typical synthesis, 20 mL of TBOT was rapidly added dropwise to 200 mL of distilled water without stirring. After aging for 24 h, the white precipitates were filtrated and dried at 80 °C in a vacuum oven for 10 h. Subsequently, 1.0 g of the dry samples was mixed with 150 mL of distilled water. Then the mixtures were put into a 200 mL stainless steel autoclave with a Teflon liner and kept at 180 °C for different times (1, 3, 10, 24, and 36 h). Finally, the obtained samples were centrifuged and washed with distilled water for five times and then dried in a vacuum oven at 80 °C for 8 h. 2.2. Characterization. The X-ray diffraction (XRD) patterns obtained on an X-ray diffractometer (type HZG41B-PC) using Cu KR radiation at a scan rate of 0.05° 2θ s-1 were used to characterize the crystalline phase, phase composition, and crystallite size of the TiO2 powders. The accelerating voltage and the applied current were 15 kV and 20 mA, respectively. If the sample contains anatase and brookite phases, the mass fraction of brookite can be calculated according to the following equation:17
WB )
2.721AB 0.886AA + 2.721AB
(1)
where AA and AB represent the integrated intensities of the anatase (101) and brookite (121) peaks, respectively. With eq 1, the phase contents of anatase and brookite in TiO2 samples can be calculated. The average crystallite sizes of anatase and brookite were determined according to the Scherrer equation using the fwhm (full width at half-maximum) data of each phase after correcting the instrumental broadening.16 Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images, which were obtained using a JEOLTEM-2010F at an acceleration voltage of 200 kV, were used to observe or determine the morphology, size, and
identity of nanocrystal particles in the as-prepared TiO2 powders. All the samples were degassed at 180 °C prior to BET measurements. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) was determined by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA) via a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05-0.3. The desorption isotherm was used to determine the pore-size distribution using the Barret-Joyner-Halender (BJH) method, assuming a cylindrical pore modal.18 The nitrogen adsorption volume at the relative pressure (P/P0) range of 0.997 was used to determine the pore volume and average pore size. 2.3. Measurement of Photocatalytic Activity. Acetone, formaldehyde, benzene, and other volatile organic compounds (VOCs) are common indoor air pollutants in modern houses, which have been the subject of numerous complaints regarding health disorders, such as leukemia, nausea, headache, and fatigue.19 These volatile harmful gases usually come from the paint, plywood, particleboard, and adhesives for wall clothes, which have been used in construction and furnishing. To improve indoor air quality (IAQ), these VOCs must be eliminated. Therefore, we chose acetone as a model contaminate chemical. The photocatalytic oxidation decomposition of acetone in air is based on the following reaction:20-22
CH3COCH3 + 4O2 f 3CO2 + 3H2O
(2)
The photocatalytic activity measurements on the as-prepared TiO2 powders, ground TiO2 powders, and Degussa P-25 (P25) powders for the oxidation decomposition of acetone in air were performed at ambient temperature using a 15 L photocatalytic reactor. The photocatalysts were prepared by coating an aqueous suspension of TiO2 powders (60 mL) onto three dishes with diameters of 7.0 cm. The weight of the photocatalyst used for each experiment was kept at about 0.3 g. The TiO2 photocatalysts were pretreated in an oven at 100 °C for about 2 h and then cooled to room temperature before use. After the dishes coated with TiO2 powder photocatalysts were placed in the reactor, a small amount of acetone was injected into the reactor. The reactor was connected to a dryer containing CaCl2 that was used for controlling the initial humidity in the reactor. The analysis of acetone, carbon dioxide, and water vapor concentration in the reactor was conducted on line with a Photoacoustic IR multigas monitor (INNOVA Air Tech Instruments Model 1312). The acetone vapor was allowed to reach adsorption equilibrium with the catalyst in the reactor prior to an experiment. The initial concentration of acetone after the adsorption equilibrium was about 400 ( 20 ppm, which remained constant until a 15 W, 365 nm UV lamp (Cole-Parmer Instrument) in the reactor was switched on and its intensity striking on the coating measured with a UV radiometer (Model UV-A, made in the Photoelectric Instrument Factory of Beijing Normal University) was 2.5 mW/cm2. The initial concentration of water vapor was 1.20 ( 0.01 vol %, and the initial temperature was 25 ( 1 °C. During the photocatalytic reaction, a near 3:1 ratio of carbon dioxide products to acetone destroyed was observed, and the acetone concentration decreased steadily with an increase in UV illumination time. Each set of experiments was followed for 60 min. The photocatalytic activity of the TiO2 samples can be quantitatively evaluated by comparing the apparent reaction rate constants. The photocatalytic oxidation of acetone is a pseudo-first-order reaction, and its kinetics may be expressed as follows: ln(C0/C) ) kt,23 where k is the apparent reaction rate constant and C0 and C are the initial concentration and the reaction concentration of acetone, respectively.
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Figure 1. XRD patterns of hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 0 (a), 1 (b), 3 (c), 10 (d), 24 (e), and 36 h (f).
3. Results and Discussion 3.1. Hydrothermal Preparation of Hierarchically Spongelike Macro-/Mesoporous Titania. Hierarchically sponge-like macro-/mesoporous titania was prepared by hydrothermal treatment of the precipitates of tetrabutyl titanate (Ti(OC4H9)4) in pure water at 180 °C (see Experimental Methods). XRD was used to investigate the changes of phase structures and crystallite sizes of the TiO2 samples with hydrothermal time. The asprepared samples before hydrothermal treatment are amorphous phase (see Figure 1a). This is due to the fact that the hydrolysis reaction of TBOT is not complete at room temperature and large amounts of unhydrolyzed alkyls still remain in the powders, which prevent crystallization to anatase by adsorbing on the surface of TiO2 particles.16d Effects of hydrothermal time on phase structures of TiO2 samples prepared at 180 °C are displayed in Figure 1. At 1 h, the phase transformation of amorphous to anatase occurs and all diffraction peaks of XRD pattern (Figure 1b) can be readily indexed to anatase TiO2 (JCPDS No. 21-1272; space group, I41/amd (141)). The broadening of the diffraction peaks is due to the small crystallite size (5.2 nm) and weak crystallization of the sample. With increasing hydrothermal time to 3 h, the peak intensities of anatase increase, indicating the enhancement of crystallization. Meanwhile, the width of the (101) plane diffraction peaks becomes narrow, showing the increase in anatase crystallite size (from 5.2 to 6.6 nm) (see Table 1). Further observation indicates that a minor peak at 2θ ) 30.7° corresponding to the (121) plane diffraction of the brookite phase (JPCDS No. 29-1360; space group, Pcab (61)) appears. Usually, the presence of weak acid or a low-pH environment is favorable for the formation of brookite.22 However, in this study, the whole synthesis was performed at a neutral-pH environment without any additives. The only explanation is that the hydrothermal environment can also promote the formation of brookite phase. The mass fraction of the brookite phase is about 15%. With increasing hydrothermal time to 10 h, the mass fraction of the brookite phase increases and reaches 30%. A further increase of the hydrothermal time has no obvious influence on the phase structures and composition of the samples except the crystallite size slightly increases (see Figure 1 and Table 1). At 36 h, brookite content in the sample slightly decreases (see Table 1). The XRD results reveal that the hydrothermal treatment promotes the phase transformation of amorphous to anatase, growth of crystallite, and the formation of brookite phase at a relatively low temperature, probably due to cooperative effects of pressure, temperature, and water as a catalyst.15c The nitrogen adsorption-desorption isotherms of TiO2 samples prepared at different hydrothermal time are presented
Yu et al. in Figure 2. For the as-prepared amorphous TiO2 sample before hydrothermal treatment, the isotherm is of types I and IV (BDDT classification). At low relative pressure, the isotherm exhibits high adsorption, indicating the presence of micropores (type I).18 However, at high relative pressure range between 0.4 and 0.8, the curve exhibits a very small hysteresis loop indicating the presence of mesopores (type IV). This is also confirmed by its corresponding pore-size distribution (see Figure 3a). Hydrothermal treatment displays a significant influence on the isotherms of the samples. After hydrothermal treatment at 180 °C for 1 h, the isotherm is of type IV (BDDT classification) and has two hysteresis loops, indicating bimodal pore-size distributions in the mesoporous and macroporous region. The shapes of the two hysteresis loops are different from each other. At low relative pressure between 0.4 and 0.7, the hysteresis loop is of type H2, suggesting the existence of ink-bottle pores with narrow necks and wider bodies (ink-bottle pores).18 However, at high relative pressure between 0.8 and 1.0, the shape of the hysteresis loops is of type H3 associated with aggregates of platelike particles, giving rise to slitlike pores.18 According to the previous reports,16a,24,25 a bimodal pore-size distribution is due to two different aggregates in the powders. The hysteresis loop in lower relative pressure range (0.4 < P/P0 < 0.7) is related to finer intraaggregated pore formed between intraagglomerated primary crystallites, and that in higher relative pressure range (0.8 < P/P0 < 1) is associated with larger interaggregated pore produced by interaggregated secondary particles. This bimodal pore-size distribution is further confirmed by its corresponding pore-size distribution in Figure 3. The powders contain small mesopores (peak pore, ca. 3.7 nm) and larger mesopores with a peak pore diameter of ca. 39 nm. The hydrothermal time exerts obvious influences on the pore structure and BET surface areas of the obtained products (see Table 1). With increasing hydrothermal time, the shapes of nitrogen adsorption and desorption isotherms show two obvious changes, implying the great variation of pore structures. First, the isotherms corresponding to the samples obtained at 10 and 24 h (as compared with that at 1 h) show higher absorption at high relative pressure (P/P0) range (approaching 1), indicating the formation of macropores and/or the increase of their pore volume. This may be related to the enhancement of crystallization of TiO2 (as shown in Figure 1 and Table 1). Second, with increasing hydrothermal time, the two separate hysteresis loops gradually join together as one, implying that the pore-size distributions of intraaggregated pores and interaggregated ones tend to overlap, as confirmed in Figure 3. This is associated with the growth of the crystallites and the shrinkage of the aggregates, resulting in the right shift of intraaggregated pores from 3.7 to 6.9 nm and left shift of interaggregated ones from 39 to 23 nm, respectively. N2 adsorption-desorption analyses could not provide macroporous information of the TiO2 samples. Therefore, the macroporous structure of the TiO2 sample is observed directly by scanning electron microscopy (SEM). Figure 4 shows SEM images of hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 24 h. The prepared TiO2 samples exhibit a disordered worm-like macroporous frameworks with continuous walls that consisted of closely packed aggregates of nanocrystalline titania particles. The size of 3-dimensional continuous macropores and wall thickness are ca. 0.5-3 µm and ca. 0.5-1.5 µm, respectively. These three-dimensional continuous macroporous channels may serve as ideal lighttransport routes for introducing photoenergy and gas molecules into the interior space of titania.10 Therefore, SEM and N2
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TABLE 1: Effects of Hydrothermal Time on Physical Properties of TiO2 Powders Prepared at 180 °C time (h)
phasea
SBETb (m2/g)
pore volc (cm3/g)
av pore sizec (nm)
porosityd (%)
0 1 3
Am A A (85%) B (15%) A (70%) B (30%) A (70%) B (30%) A (75%) B (25%)
460.5 225.9 200.2
0.24 0.24 0.28
1.98 3.50 5.65
47.9 47.9 51.8
168.6
0.33
6.90
55.9
156.2
0.35
7.68
57.4
150.1
0.36
8.23
57.8
10 24 36
crystalline sizee (nm) A: A: B: A: B: A: B: A: B:
5.2 (1.0) 6.6 (1.3) 4.2 7.6 (1.5) 4.8 8.8 (1.8) 5.2 9.0 (1.9) 6.0
a Am, A, and B denote amorphous, anatase, and brookite, respectively. b The BET surface area was determined by a multipoint BET method using the adsorption data in relative pressure (P/P0) range of 0.05-0.3. c Pore volume and average pore size were determined by nitrogen adsorption volume at P/P0 ) 0.997. d The porosity is estimated from the pore volume determined using the desorption data at P/P0 ) 0.997. e Average crystalline size of TiO2 was determined by XRD using the Scherrer equation. Relative anatase crystallinity: the relative intensity of the diffraction peak from the anatase (101) plane (indicated in parentheses, reference ) the sample hydrothermal-treated at 180 °C for 1 h).
Figure 2. Nitrogen adsorption-desorption isotherms of hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 0 (a), 1 (b), 10 (c), and 24 h (d).
Figure 3. Pore-size distribution curves of TiO2 powders after hydrothermal treatment at 180 °C for 0 (a), 1 (b), 10 (c), and 24 h (d).
adsorption analyses reveal that the current materials exhibit the trimodal pore structure of a meso-/macroporous system. Our SEM observation further indicates that the percentage of the prepared particles containing macroporous structures is above 80% (not shown here). TEM and HRTEM were used to further study the microstructures and crystallization of the hierarchically sponge-like macro-/mesoporous titania powders. Figure 5a shows a typical TEM image of the TiO2 powders prepared at 180 °C for 24 h. It can be seen that the primary particle size is about 9 nm, which is in agreement with the value of the crystallite size determined by XRD (8.8 nm) (as shown in Table 1). Further observation indicates that a large number of mesopores come from the aggregation of primary particles (or crystallites). Figure 5b shows the corresponding HRTEM image of the same sample. It shows clear lattice fringes, which allows for the identification
of crystallographic spacing and indicates that the prepared anatase TiO2 powders are well crystalline. The fringes of 0.35 nm match that of the (101) crystallographic plane of anatase TiO2. Figure 6 presents SEM images of as-prepared amorphous titania powders by hydrolysis of TBOT before hydrothermal treatment. The amorphous TiO2 powders exhibit relatively homogeneous and long-range periodical macropores with pore diameter of about 2-4 µm and pore wall thickness of 1-3 µm (Figure 6a,b). Figure 6a also reveals that the walls of the macroporous TiO2 are composed of many small interconnected microporous TiO2 spherical particles with sizes of 300-700 nm. The micro-/mesoporosity of the amorphous TiO2 powders is confirmed by type I N2 adsorption-desorption isotherms and corresponding pore size distribution curve (see Figures 2 and 3). Further observation shows that the ultralong macroporous channels are arranged parallel to each other and perpendicular to the outer surface of the particles (Figure 6b). This result is consistent with those reported in the literatures.13,26,27 On the basis of the above XRD, nitrogen adsorption, SEM, and TEM investigation results of TiO2 powders before and after hydrothermal treatment and report results of others,13,26 a possible multistep formation mechanism is proposed to account for the production of trimodal sponge-like nanocrystalline macro-/ mesoporous TiO2 powders. When TBOT drops are added into pure water without stirring, TBOT liquid droplets can exist in water due to immiscibility. The contact between the TBOT droplets and water immediately produces a thin, dense, semipermeable titania membrane at the droplets interface due to hydrolysis reactions occurring very quickly. This membrane will separate TBOT within droplets and water and compartmentalize the subsequent hydrolysis and condensation reactions. Then, the above reactions proceed inwardly, and approximately perpendicular to the external surface of the particles, as the distilled water diffuses through the spherical outer membrane.13 With the progress of the reaction, more and more alcohol molecules will form, which gather together leading to the formation of larger water/ethanol macrochannels inside the structure. Meanwhile, the polymerization generates large amounts of titania spherical particles. The self-aggregation of these spherical particles gives the interparticular mesoporosity. Inside these spherical particles another micro- or mesoporosity will form.26 The amount of water/ethanol trapped inside the particle creates high pressures, which in turn causes the splitting of the particle resulting in formation of hierarchical meso-(micro-)/macroporous titania particles (see Figure 6).13,26,27 Our previous investigation results have showed that the macroporous structures can be well preserved, and the pore size and wall thickness have no obvious
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Figure 4. SEM images of hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 24 h.
Figure 5. TEM (a) and HRTEM (b) of hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 24 h.
Figure 6. SEM images of as-prepared amorphous titania powders by hydrolysis of TBOT before hydrothermal treatment.
change during calcinations at 300-500 °C.27 However, hydrothermal treatment greatly changed the morphology and pore structures of the above samples. About formation of trimodal sponge-like macro-/mesoporous TiO2 powders, one possible explanation is that trimodal sponge-like macro-/mesoporous
structures are most probably from the cooperative effect of hydrodynamic flow of water, and restructure and phase transformation of amorphous TiO2 during hydrothermal treatment.24,26 Of course, the detailed formation mechanism on hierarchically sponge-like macro-/mesoporous structures still
Sponge-like Macro-/Mesoporous Titania
Figure 7. (a) Comparison of the apparent reaction rate constants of P25 and hierarchically sponge-like macro-/mesoporous titania prepared at 180 °C for 0, 1, 3, 10, 24, and 36 h. (b) ln(C0/C) as a function of UV irradiation time in the presence of TiO2 photocatalysts prepared at 180 °C for different times and P25.
needs to be further investigated in our future work. If possible, the in-situ observation may be the best method for investigating structural evolution of the above sample during hydrothermal treatment by optical microscopy.26 The above results highlight a facile hydrothermal process for the synthesis of hierarchically sponge-like trimodal macro-/ mesoporous structures in the absence of templates and additives. The prepared TiO2 powders are composed of at least three levels of structural organization: (i) TiO2 crystallization at the nanoscale (nanocrystalline), (ii) mesoscale aggregation and packing of the TiO2 nanocrystallites (several nanometer-size) in the sponge-like wall to produce bimodal peak mesopores 2-50 nm in size, and (iii) sponge-like macroporous frameworks with continuous mesoporous walls at micrometer scale. 3.2. Photocatalytic Activity of Hierarchically Macro-/ Mesoporous Titania. The photoctalytic activity of sponge-like macro-/mesopoeous titania was evaluated by gaseous photocatalytic oxidation decomposition of acetone. Figure 7a displays the comparison of photocatalytic activity (or apparent reaction rate constant) of the titania samples before and after hydrothermal treatment at 180 °C and P25. It can be seen that the hydrothermal time has a great effect on the photocatalytic activity of TiO2. Although the as-prepared samples before hydrothermal treatment have the largest specific surface areas (at 0 h) (see Table 1), there is no obvious photocatalytic activity observed presumably due to the as-prepared TiO2 powders consisting of amorphous phase. For the sample prepared at 180 °C for 1 h, it shows obvious photocatalytic activity due to the formation of anatase phase. With increasing hydrothermal time, the photocatalytic activity of the samples greatly increase due to the enhancement of the crystallization. At 24 h, the highest photocatalytic activity is observed and the k value reaches 9.12 × 10-3 min-1. With further increasing hydrothermal time to 36 h, the k value decreases due to the decrease in specific surface
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10587 area. The k value of P25 is determined to be 3.01 × 10-3 min-1 at the same experimental conditions, which is well-known for its superior photocatalytic activity. This is mainly ascribed to the fact that P25 contains anatase (78% in mass fraction) and rutile (22%) two phases (determined by XRD). Usually, the composite of two kinds of semiconductors or two phases of the same semiconductor is beneficial in reducing the combination of photogenerated electrons and holes and enhancing photocatalytic activity.22,28 Furthermore, the specific surface area (53.2 m2/g) and crystallite size (30 nm) of P25 are also characterized, and its pore structure is monomodal with a maximum pore diameter at 34 nm.16a The highest photocataytic activities of the 24 h samples might be explained by the composite structures of anatase and brookite phases, high specific surface areas, and hierarchically trimodal macro-/ mesoporous structures. Usually, for anatase TiO2, a large surface area can offer more active adsorption sites and photocatalytic reaction centers. The hierarchically macro-/mesoporous structures are beneficial for enhancing the adsorption efficiency of light and the flow rate of the gas molecules. Furthermore, we inferred from this investigation and our previously experimental results that the biphase composite structures of anatase and brookite are more beneficial for enhancing photocatalytic activity than the biphase composite structures of anatase and rutile because of the former usually having a larger specific surface area.16a,22 Of course, this deduction needs to be further confirmed by more experiments. Therefore, it is not surprising that the 3 h sample shows higher photocatalytic activity than the 1 h sample due to the former containing brookite phase. However, it is difficult to explain the change of photocatalytic activity of the hierarchically macro-/mesoporous titania with hydrothermal time based solely on the above three factors. For example, the 3 h sample possesses a large specific surface area of 200.2 m2/g. After the samples were treated at 180 °C for 10 and 24 h, the specific surface areas decrease to 168.6 and 156.2 m2/g, respectively. Such a large decrease in specific surface area should lead to a decrease in the photocatalytic activity. However, these two samples still exhibit relatively higher photocatalytic activity. The increase of photocatalytic activity at 10 and 24 h might be due to the enhancement of anatase crystallization and increase of brookite content in photocatalysts. The former is beneficial to reduce the recombination rate of the photogenerated electrons and holes due to the decrease in the number of the defects.28 The latter can enhance the transfer and separation of photogenerated electrons and holes,16a,29 implying that the mass ratio of brookite to anatase also obviously influence photocatalytic activity, and an optimal brookite to anatase mass ratio is probably 3:7. According to this deduction, it is easy to explain that the 36 h sample shows a lower photocatalytic activity than the 24 h sample due to the decrease of brookite content in photocatalysts. Figure 7b further shows the kinetics of photocatalytic degradation of acetone in the presence of TiO2 photocatalysts prepared at 180 °C for different time and P25. The same results can be observed for comparison of photocatalytic activity. To further investigate the beneficial influence of macrochannels on photocatalytic activity, trimodally sponge-like macro-/ mesoporous titania samples prepared at 180 °C for 24 h were ground into fine powders by an agate mortar to destroy the macroporous networks (or hierarchically macro-/mesoporous structures) of the samples, and then their photocatalytic activity were tested (see Experimental Methods). The ground samples still kept their mesoporous structures (see Figure 8a), but their macroporous structures or networks disappeared (see Figure 8b).
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Figure 9. SEM images of lotus leaf surface. Inset shows highmagnification SEM images of the valleys indicating a lotus leaf surface with sponge-like porous structures.
Figure 8. (a) Pore size distribution curve of ground titania powders prepared at 180 °C for 24 h, indicating that the sample still keeps its mesoporous structure. (b) SEM image of ground titania powders prepared at 180 °C for 24 h showing the collapse of the macrochannels.
The results indicate that, in the absence of macrochannels, there was about a 10% drop in photocatalytic activity (i.e., the reaction rate constant) observed after grinding. It can be clearly seen from the above results and previous research report that hierarchically macro-/mesoporous structures obviously enhance photocatalytic activity of titania.10,27 Biology has developed such highly efficient hierarchical structures at ambient temperature to achieve rapid gas exchange over large surface area and an optimal solar-light harvesting. For example, green leaves have such hierarchical and fractal network structures, which allow gas to be exchanged rapidly and maximize the amount of energy captured from the sun’s ray. Green leaves capture the solarlight energy available to them by means of the most important process for life on earth. This process, called photosynthesis, involves the trapping and ultimate storing of energy in sugar molecules that are constructed from ordinary water and from carbon dioxide present in the atmosphere.30 All the energy needs of living organisms ultimately depend on photosynthesis occurring on the surface of leaves. To enhance harvesting of solar light, the surfaces of leaves possess hierarchical structures and are dotted with tiny pores (stomata), which not only allow entry for the carbon dioxide gas needed for photosynthesis but also play a important in the diffusion out of the leaves of oxygen produced during photosynthesis.30 Figure 9 shows SEM images of the lotus leaf surface. It can be seen that the surface of the lotus leaf is dotted with 3-10 µm size protrusions and valleys uniformly. High-magnification SEM images (inset in Figure 9) of the valleys indicate that lotus leaf surface has sponge-like porous structures and a lot of nanosticks with an average diameter of about 100-150 nm and pores of 100-200 nm in size are randomly distributed on the subsurface layer. Therefore, it is reasonable to infer that lotus leaf surfaces possess hierarchical micro- and nanometer-size binary structures of at
least two levels, which are beneficial to enhancing the photosynthesis and superhydrophobicity of their surfaces. Nature has created a wide variety of hierarchically porous structures with specific morphologies, organizations, and textures in order to defend against the surrounding environment and to accomplish tasks efficiently.26 For example, lungs of animals have such hierarchical and fractal network structures, which allow air to be exchanged rapidly,31 and trees have such hierarchical structures from the stem to the leaves to accelerate the movement of water absorbed by roots and transported throughout the plant.30 Therefore, to further develop highly efficient photocatalytic materials and to improve the performance of photocatalyst, an efficient way will be to create the hierarchically porous structures in photocatalytic materials by mimicking nature. Based on the above-described hierarchically sponge-like multimodal porous structures and enhanced photocatalytic activity, and inspired by these marvellous natural structures, a new biomemitic photocatalysis concept has been proposed in the field of photocatalysis. This new concept may provide new insight into preparation of advanced photocatalytic materials and design of highly efficient photocatalytic reaction by mimicking the hierarchical surface structures and photosynthesis of green plant leaves. Possible advantages of the biomimetic photocatalysis concept include the following: enhanced photocatalytic efficiency, reduced energy consumption, and improved ecological harmlessness. 4. Conclusions (1) Trimodally sponge-like macro-/mesoporous titania with biphase (anatase and brookite) structures was successfully synthesized by hydrothermal treatment of the precipitates of tetrabutyl titanate (Ti(OC4H9)4) in pure water without using any templates and additives. (2) All hierarchically porous titania powders prepared at 180 °C for different times showed three levels of pore organization in the macro-/mesoporous region: fine intraparticle mesopores with peak pore diameters of ca. 3.7-6.9 nm, larger interparticle mesopores with peak pore diameters of ca. 23-39 nm, and macropore with pore diameter of ca. 0.5-3 µm. (3) The hydrothermal time exhibited a strong effect on the microstructure and photocatalytic activity of the powders. With increasing hydrothermal time, the crystallinity, average anatase size, pore size, and pore volume increased. However, the BET specific surface area decreased.
Sponge-like Macro-/Mesoporous Titania (4) The hierarchically porous titania powders prepared at 180 °C for 24 h displayed an excellent photocatalytic activity probably due to their special pore-wall structure, and their photocatalytic activity was 3 times higher than that of P25. This novel trimodal macro-/mesostructured titania should also find its widely potential applications in multiple fields including photocatalysis, catalysis, solar cells, separation and purification processes, and so on because the textural mesopores and intrinsic interconnected pore systems of macrostructures should efficiently transport guest species and light to framework binding sites. (5) The biomemitic photocatalysis concept may provide new insight into environmentally benign preparation of novel photocatalytic materials and design of highly efficient photocatalytic reactions. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (Grants 20473059 and 50625208). This work was also financially supported by the Key Research Project of Chinese Ministry of Education (Grant No. 106114) and PCSIRT (Grant No. IRT0547). References and Notes (1) (a) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (b) Walsh, D.; Arcelli, L.; Ikoma, T.; Tanaka, J.; Mann, S. Nat. Mater. 2003, 2, 386. (2) Yuan, Z. Y.; Su, B. L. J. Mater. Chem. 2006, 16, 663. (3) Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 1174. (4) Zhang, B.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369. (5) (a) Tosheva, L.; Valtchev, V.; Sterte, J. Microporous Mesoporous Mater. 2000, 35-36, 621. (b) Kloetstra, K. R.; van Bekkum, H.; Jansen, J. C. Chem. Commun. (Cambridge) 1997, 2281. (c) Landau, M. V.; Tavor, D.; Regev, O.; Kaliya, M. L.; Herskowitz, M. Chem. Mater. 1999, 11, 2030. (6) (a) Antonietti, M.; Berton, B.; Goeltner, C.; Hentze, H.-P. AdV. Mater. 1998, 10, 154. (b) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (c) Lebeau, B.; Fowler, C. E.; Mann, S.; Farcet, C.; Charleux, B.; Sanchez, C. J. Mater. Chem. 2000, 10, 2105. (d) Yu, J.; Guo, H.; Davis, S. A.; Mann, S. AdV. Funct. Mater. 2006, 16, 2035. (7) (a) Leonard, A.; Blin, J. L.; Su, B. L. Chem. Commun. (Cambridge) 2003, 2568. (b) Yuan, Z. Y.; Vantomme, A.; Leonard, A.; Su, B. L. Chem. Commun. (Cambridge) 2003, 1558. (c) Yuan, Z. Y.; Ren, T. Z.; Su, B. L. AdV. Mater. 2003, 15, 1462. (d) Blin, J. L.; Leonard, A.; Yuan, Z. Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 2872. (8) (a) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308. (b) Zhang, B.; Davis, S. A.; Mendelson, N. H.; Mann, S. Chem. Commun. (Cambridge) 2000, 781. (c) Anderson, M. W.; Holmes, S. M.;
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