Article pubs.acs.org/Langmuir
Porous TiO2 Materials through Pickering High-Internal Phase Emulsion Templating Xiaodong Li,† Guanqing Sun,† Yecheng Li,† Jimmy C. Yu,† Jie Wu,‡ Guang-Hui Ma,*,‡ and To Ngai*,† †
Department of Chemistry, The Chinese University of Hong Kong, Shatin N.T., Hong Kong National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
‡
S Supporting Information *
ABSTRACT: We report a facile method for preparing porous structured TiO2 materials by templating from Pickering high-internal phase emulsions (HIPEs). A Pickering HIPE with an internal phase of up to 80 vol %, stabilized by poly(N-isopropylacrylamide)-based microgels and TiO2 solid nanoparticles, was first formulated and employed as a template to prepare the porous TiO2 materials with an interconnected structure. The resultant materials were characterized by scanning electron microscopy, X-ray diffraction, and mercury intrusion. Our results showed that the parent emulsion droplets promoted the formation of macropores and interconnecting throats with sizes of ∼50 and ∼10 μm, respectively, while the interfacially adsorbed microgel stabilizers drove the formation of smaller pores (∼100 nm) throughout the macroporous walls after drying and sintering. The interconnected structured network with the bimodal pores could be well preserved after calcinations at 800 °C. In addition, the photocatalytic activity of the fabricated TiO2 was evaluated by measuring the photodegradation of Rhodamine B in water. Our results revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules.
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INTRODUCTION Hierarchical porous materials occur widely in nature, including, for example, the lungs of animals, the leaves of tree, and diatom skeletons, which accelerate air and solar energy exchange.1 Since the parallel development of mesostructuring and macrostructuring routes in the 1990s, the synthesis of hierarchical materials containing macropores along with smaller pores has become an exciting area of material chemistry, because of their potential uses in diverse applications such as catalysis,2,3 adsorption and separations,4 drug delivery,5,6 sensors,7 and tissue engineering.8,9 Template synthesis is commonly used for the preparation of porous materials. Hierarchical porous materials containing interconnected porous structures with different length scales can be prepared by a single surfactant route10 or macroscopic phase separations.11 However, dual templating proved to be the most effective way because this allows direct and independent control of the pore dimensions, and as a result, the final pore structures can be easily tailored for different applications.12 Currently, surfactant micelles are generally employed as the structure-directing agents of mesostructures, while large-scale systems such as colloid particles,13 emulsions,14,15 biomaterials,16,17 and ice crystals18 are added to govern the production of macroporous structures. In practical applications, the introduction of smaller pores into a macroporous structure is important. This is because the presence of macropores allows the mass transfer of the bulk of the substances like liquids and gases, while the smaller pores create a large internal surface area © 2014 American Chemical Society
that greatly enhances the host−guest interactions, selectivity, and catalytic or ion-exchange properties.19 Recently, a class of macroporous materials that have attracted a great deal of attention was obtained from high-internal phase emulsions (HIPEs). HIPEs are commonly defined as very concentrated emulsions, in which the volume fraction of the internal phase (Φ) exceeds 0.74.20−22 Polymerization of the continuous phase of HIPEs and the subsequent removal of the oil and water can lead to a highly porous, cross-linked polymer materials,23 which are known as PolyHIPEs. Under the right conditions, a small interconnecting throat or windows are also formed between adjacent emulsion droplets after drying. This produces highly porous and permeable materials that have great potential for use in diverse applications such as supported catalysis,24 electrochemical sensors,7 cell culture,25 hydrogen storage,26 and reaction containers.27 However, it is worth mentioning that conventional HIPEs are commonly stabilized by large amounts (5−50 vol %) of small molecule surfactants in a monomer solution.21,28 Over the past few years, there has been an increasing level of interest in the use of solid particles to stabilize the internal phase (droplets) of the HIPEs. Unlike their surfactant-stabilized counterparts, particle-stabilized emulsions (or so-called Pickering emulsions) are less susceptible to coalescence, Ostwald ripening, and creaming, thus resulting in Received: December 24, 2013 Revised: February 13, 2014 Published: February 24, 2014 2676
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then SEM images were taken with a FEI Quanta 400 FEG microscope operating at 10 kV. It can be seen that the synthesized microgel particles are spherical and monodispersed (Figure S2 of the Supporting Information). Preparation and Freeze-Drying of Pickering HIPEs. The oilin-water (o/w) emulsions with different internal phase volumes were initially produced by mechanically shearing the dispersed phase, hexane with an aqueous solution containing the solid TiO2 particles (33.37 wt %, 150 nm in diameter) and fluorescent PNIPAM-co-MAA microgel particles (2.0 wt %, 294 nm in diameter) (step 1 in Figure 1).
extremely stable HIPEs. Manufacturing porous structures from Pickering HIPEs has been explored in various recent studies.29,30 However, there is a need to develop this HIPE templating process for the fabrication of porous materials with controllable pore structures and surface functionality to open new applications. Recently, we have reported a simple and flexible method for obtaining a three-dimensionally interconnected, highly porous silica monolith with different length scales via Pickering HIPE templating.31 Here we report a significant advance that allows us to extend this technique to produce the structured TiO2 porous materials and evaluate their activity in photodegrading organic molecules. By templating through a Pickering HIPE stabilized by both soft microgels and TiO2 solid nanoparticles, we obtained three-dimensionally interconnected porous TiO2 monoliths with macropores (50 μm) and interconnecting throats (∼10 μm). More importantly, we found that microgel stabilizers drove the formation of smaller pores with a size of ∼100 nm throughout the macroporous walls after sintering. Unlike the mostly studied TiO2 suspension and coated film systems, TiO2 materials with such hierarchical structures are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores along the macroporous walls might have a specific large surface area and can greatly enhance the catalytic or ion-exchange properties. We thereby investigated the photocatalytic activity of the fabricated TiO2 by measuring the photodegradation of Rhodamine B in water.
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Figure 1. Schematic illustration of the three-step process used to obtain hierarchical porous TiO2-based materials. The emulsion type was determined by measuring the conductivity and by observing whether a drop of emulsion dispersed when it was added to a small volume of pure water or oil. In some cases, a dye molecule, perylene, was added to the oil phase to check the type of the formed emulsions via confocal microscopy. The confocal microscopy images were taken on a Nikon Eclipse Ti inverted microscope. Lasers with wavelengths of 543 and 408 nm were used to excite the fluorescent microgel particles (with Rhodamine B groups) and perylene molecules, respectively. An oil immersion objective (60×, NA = 1.49) was used to view the samples. The HIPEs were placed on the cover slides, and a series of x−y layers were scanned. The as-prepared HIPEs were first immersed in liquid nitrogen to solidify the entire HIPEs and then dried under vacuum overnight. After the HIPEs had been freeze-dried, the hexane and water were removed and the resultant TiO2/microgel composite porous materials were very stable. Thermogravimetric analysis (TGA) of the TiO2/microgel composite porous materials was performed with a Hi-Res TGA 2950 thermogravimetric analyzer (TA Instruments), where the temperature ramp was 10 °C/min under N2. The calcination of the TiO2/microgel composite via the removal of the microgel particles and fusion of the TiO2 particles was conducted in air using a furnace (Ney Vulcan 3-400 HTA). The temperature was ramped from room temperature to various temperatures (600, 700, 800, 900, and 1200 °C) at a rate of 100 °C/h, and the calcinations were conducted at that specific temperature for 2 h. After that, the temperature was decreased to room temperature at a rate of 100 °C/h. The X-ray diffraction (XRD) patterns were obtained on a HZG41B-PC X-ray diffractometer using Cu Kα radiation at a scan rate of 0.05° (2θ) s−1 and used to determine the identity of the crystalline phase and their crystallite size. The accelerating voltage and the applied current were 35 kV and 20 mA, respectively. Photocatalytic Activity Experiment. Photocatalysis of organic molecules was conducted over the various sintered TiO2-based products. In our experiments, Rhodamine B (C28H31ClN2O3) was chosen for photocatalysis experiments using the prepared hierarchical TiO2 materials. Typically, 20 mL of a Rhodamine B solution at a concentration of 2.0 × 10−5 M was placed in a cell (radius of 50 mm,
EXPERIMENTAL SECTION
Materials. N-Isopropylacrylamide [NIPAM (Fluka)] was recrystallized three times from a toluene/n-hexane mixture. N,N′-Methylene bisacrylamide [MBAA (Fluka)], methacrylic acid [MAA (Merck)], perylene (Aldrich), the fluorescent dye methacryloxyethyl thiocarbamoyl rhodamine B [MRB (Polysciences, Inc.)], and titanium dioxide nanoparticles (TiO2 NPs) with a diameter of ∼150 nm were used as received. Potassium persulfate [KPS (Merck)] and hexane oils were used without further purification. Deionized or MillQ (Millipore) water was used in all the experiments. Synthesis of the PNIPAM-Based Microgel Particles at a Stabilizer. The soft particles used to stabilize HIPEs are poly(Nisopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgel particles. They were synthesized using the well-known precipitation polymerization method. Copolymerization with MAA provides the carboxylic groups that allow variation of the surface charge of the particles, via a change in either the solution pH or the salt concentration. Typically, 3.0 g of NIPAM, 0.105 g of MBAA, 0.105 g of MAA, and 2 mg of MRB were dissolved into 145 mL of deionized water in a 250 mL two-neck flask fitted with nitrogen bubbling and a reflux condenser and stirred with a magnetic stir bar. The solution mixture was adjusted to pH 10.0 with a 0.1 M sodium hydroxide solution. After the solution had been stirred for 1 h at 70 °C under nitrogen bubbling, the precipitation polymerization was initiated by adding 0.035 g of KPS dissolved in 5 mL of deionized water. The reaction mixture was kept at 70 °C for 7 h. The pH of the microgel dispersion after reaction was ∼9. The resultant microgels were dialyzed for 7 days to remove the unreacted reagents. The final pH of the microgel was ∼6.0. Then the microgel suspension was concentrated at 50 °C by reducing half of its original volume for further use. Dynamic laser light scattering shows that the PNIPAM-coMAA microgel particles coated with carboxylic acid groups have an average hydrodynamic radius of ∼146 nm with a solution pH of 6.0 (Figure S1 of the Supporting Information). Scanning electron microscopy (SEM) was also used to characterize the synthesized PNIPAM-co-MAA microgel particles. The purified microgel particles were dried on silica substrates at room temperature overnight, and 2677
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height of 80 mm) that contained a water cooling bath and was surrounded by a UV lamp with an emission wavelength of 365 nm and a power of 15 W. Twenty milligrams of our prepared porous TiO2 materials was weighed and placed in the cell. The mixture was magnetically stirred at ∼100 rpm. Approximately 1.5 mL of the solution was taken at regular intervals and then centrifuged at 14000 rpm for 1 min to fully remove the suspended TiO2 materials. The concentration of the dye solution was immediately analyzed by UV− visible spectroscopy.
2b) shows that the remarkable stability of the emulsion can likely be attributed to the adsorption of fluorescent microgels and solid TiO2 particles at the interface acting as a barrier against oil droplet coalescence, which is in agreement with our recent work.31 In addition, the perlyene-loaded emulsion (Figure 2c) confirms the presence of polydisperse hexane oil droplets, whose size varies from around several micrometers to tens of micrometers. On the other hand, it is worth noting that no flow of emulsion is observed even though the vial is inverted (Figure 2a), indicating that the formed emulsion is a typical gel emulsion. This result can be ascribed to the fact that during the adsorption of particles to the oil−water interfacial area produced by the forceful mixing process, the microgels and TiO2 particles in the aqueous phase are probably contiguous with these adsorbed, thus serving to bind the oil droplets together into a three-dimensional network, in turn inhibiting the gravity-induced separation.32,33 The second step in the fabrication process (Figure 1) was the elimination of the water and oil components by freeze-drying. As shown in Figure 2d, freeze-drying resulted in a porous material consisting of cellular macropores (20−50 μm in diameter) with interconnecting throats approximately ∼10 μm in diameter (Figure 2e). The monolithic materials had typical polymerized HIPE-type interconnected macroporous textures. The average cavity sizes in the macroporous structures are comparable to the oil droplet diameters of the parent emulsions (Figure 2b,c), thus suggesting that these cavities result from a loss of the oil component. Moreover, the cell walls are expected to be densely decorated with a layer of mixed TiO2 and microgel particles. More importantly, high-magnification SEM image (Figure 2f) further revealed the coexistence of small pores (∼100 nm) throughout the macropore walls, which are likely imparted by the collapse of the interfacial adsorbed microgel particles. Note that microgel particles in the bulk solution are ∼300 nm in diameter. The microgel-induced pores are much smaller than the microgels used to stabilize the emulsion. This can be attributed to compression of the soft and deformable microgels at the oil−water interface by the adsorbed solid TiO2 particles. To improve the physical properties of the resulting hierarchically porous structured materials, the final step of the process (Figure 1) was to sinter the dried TiO2 monoliths at different temperatures. TGA (Figure S3 of the Supporting Information) for the TiO2/microgel composite porous materials showed that the mass (microgel) loss continued up to 600 °C, after which the mass remained constant, indicating the complete removal of microgel particles. The composite porous materials were therefore sintered at 600, 700, 800, and 1200 °C to yield robust TiO2 monoliths. It is expected that the sintering can remove the microgel particles to generate nanoscale pores and at the same time fuse the TiO2 building blocks together, which eventually enhances the mechanical properties of the porous materials. The resulting materials were fully characterized by SEM and mercury intrusion porosimetry. Figure 3a shows the representative SEM images of the obtained robust hierarchical porous TiO2 materials after they had been sintered at 600 °C for 2 h. It can be seen that the resulting materials consist of the cellular macropores ∼50 μm in diameter with interconnecting throats approximately ∼10 μm in diameter, indicating that the cavities do not shrink during heating. Moreover, a high-magnification SEM image (Figure 3a2) further revealed the retention of the nanoscale pores (∼100 nm) throughout the macroporous walls, which were
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RESULTS AND DISCUSSION Emulsion−Template Fabrication of Hierarchical Porous TiO2-Based Materials. Figure 1 shows a schematic illustration of the three-step process used to obtain the hierarchical porous TiO2 monolith. The first step was the generation of a Pickering HIPE using both soft PNIPAM-coMAA microgels and solid TiO2 nanoparticles as particulate stabilizers. Oil-in-water (o/w) Pickering HIPEs with different volume fractions were initially formed by mechanically shearing the dispersed phase, hexane, and aqueous solution containing the TiO2 particles (33.37 wt %, 150 nm in diameter) and fluorescent PNIPAM-based microgels (2 wt %, 294 nm in diameter). The appearance of the typically formed Pickering HIPE containing with a volume fraction of 75% is shown in Figure 2a. It is important to note that upon using TiO2 nanoparticles as solely a stabilizer, no stable HIPEs could be produced. However, the Pickering HIPE stabilized by binary particles was very stable, and the relative phase volume did not change for more than 6 months. The confocal image (Figure
Figure 2. (a) Typical HIPE with a volume fraction of 75% stabilized by binary nanoparticles in which the initial emulsions contain 0.7 mL of 2.0 wt % PNIPAM-based microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles. (b and c) Confocal images of the same emulsion excited by a laser with wavelengths of 543 and 408 nm, respectively. The fluorescence PNIPAM-based microgel shows a red color, while the oil phase shows a green color with dissolved perylene. (de and e) SEM images of the hierarchical macropores promoted by oil droplets and interconnected throats by the collapse of the thin film of the continuous water phase, respectively. (f) High-magnification SEM images revealing the small pores with a size of ∼100 nm throughout the cell walls due to the microgel particles occupying these positions. 2678
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Figure 4. Pore size distribution of the sintered porous TiO2 materials after sintering at 600, 700, and 800 °C for 2 h measured via mercury intrusion instruments. The parent Pickering HIPEs contained 0.7 mL of 2.0 wt % PNIPAM microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles, and the volume fraction was fixed at 80%.
TiO2 monoliths sintered at 600, 700, and 800 °C were calculated to be 93.94, 92.59, and 93.2%, respectively, indicating that the original HIPE structure (∼80% volume fraction) was completely retained. For sintering at 600 °C, the resulting material exhibited three distinct peaks around 72, 10, and 12 nm, which can likely be linked to the macropores, throats that connect two adjacent macropores, and smaller pores along the cellular walls, respectively. However, the contributions of the macropores and throat sizes inside the produced TiO2 materials were significantly reduced from 72 to ∼30 μm and from 10 to ∼2 μm, respectively, when the composite microgel/TiO2 materials were sintered at high temperatures (e.g., 700 and 800 °C). These results correlate well with the SEM images (Figure 3) because strong sintering causes the TiO2 nanoparticles to melt and fuse together, resulting in the collapse of pores. The small 100 nm pores even disappeared in the pore size distribution curves when the TiO2 materials were sintered at 800 °C. Other data, including the total intrusion volume, surface area, and bulk density of the TiO2 porous monoliths, were also obtained from mercury intrusion and are summarized in Table 1. XRD was also used to investigate the effect of sintering temperature on the phase structure of the porous TiO2 materials. Figure 5 shows that the as-synthesized TiO2 porous materials were similar to commercially used TiO2 powder (P25), in which a mixture of rutile and anatase crystalline phase
Figure 3. SEM images of hierarchical macropores and interconnected throats templating from Pickering HIPEs stabilized by microgels and TiO2 particles after being sintered at (a) 600, (b) 700, (c) 800, and (d) 1200 °C for 2 h. High-magnification SEM images indicating the nanoscale pores throughout the cell walls due to the removal of microgel particles after sintering at 600 (a2), 700 (b2), 800 (c2), and 1200 °C (d2) for 2 h. The initial HIPEs contained 0.7 mL of 2.0 wt % PNIPAM microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles, and the volume fraction was fixed at 80%.
likely related to the removal of the interfacial adsorbed microgel particles during sintering. Such porous materials with interconnected pores of different length scales are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores have a large specific surface area and can greatly enhance the catalytic or ionexchange properties. The TiO2/microgel composite porous materials were also sintered at 700 °C and characterized via SEM (Figure 3b1,b2). Macropores, interconnected throats, and nanoscale pores along the cellular walls were also observed. Increasing the sintering temperature from 700 to 800 °C enhances the mechanical stability of the resultant hierarchical porous TiO2 materials. However, further increasing the sintering temperature causes shrinkage of the whole structure without retaining the porous shape of the resultant materials, especially for the nanoscale pores throughout the macroporous walls (Figure 3c1,c2). Exquisite structures such as 100 nm pores and interconnected throats disappeared because TiO2 particles began to melt and fuse together when the composite porous materials were heated at 1200 °C (Figure 4d1,d2). The prepared hierarchical porous TiO2 monoliths after being sintered at various temperatures were also characterized via mercury intrusion porosimetry. Figure 4 shows the pore size distribution of the porous TiO2 materials after they had been sintered at various temperatures. The degrees of porosity of the
Table 1. Effect of Sintering Temperature on the Physicochemical Properties of Hierarchical Porous TiO2 Materials
total intrusion volume (mL/g) surface area (m2/g) bulk density (g/mL) porosity (%) nanoscale pore size (nm) window size (μm) macropore size (μm) 2679
TSintering = 600 °C
TSintering = 700 °C
TSintering = 800 °C
6.26
5.12
5.6
28.78 0.15 93.94 12 9 72
9.34 0.18 92.59 26 1.9 31
4.88 0.17 93.2 − 2.7 30
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Figure 5. XRD patterns of TiO2. (a) Commercially used P25, (b) dried HIPE at room temperature, and (c) HIPE sintered at 600 °C for 2 h, (d) 700 °C for 2 h, and (e) 800 °C for 2 h.
was observed. After the heat treatment, the peaks at 25° and 38° that are associated with an anatase gradually disappeared within the XRD profile, indicating that the phase transition occurred from anatase to rutile by sintering. The transformation of crystalline rutile can certainly enhance the photocatalytic activity of the resulting materials. More importantly, it is worth noting that compared with those of conventional TiO2 porous materials that are generally unstable upon sintering above 600 °C, the morphology and porous structure of our synthesized porous materials are well preserved after sintering at 600 °C (Figure 3a). For heating at a higher temperature (e.g., 800 °C), the intensity of the peaks related to the crystalline phase was increased significantly, indicating that obtained materials have a higher degree of crystallinity. However, the SEM pictures (Figure 3b,c) showed that sintering at such a high temperature could lead to the shrinkage of the whole structure without retention of the porous shape of the resultant materials. Photocatalytic Activity of the TiO2-Based Hierarchically Structured Porous Materials. TiO2 is one of the most prominent materials for photocatalysis and solar energy application because of its strong redox power, high photocorrosion resistance, and long-term stability.34 Over the past few decades, a great effort has been made to prepare crystalline TiO2 nanoparticles for photocatalysis. This is because the small TiO2 nanocrystals have excellent photocatalytic properties because of the higher level of photon to electron conversion and efficient charge carrier separation and migration.35 However, the smaller surface area and stronger tendency for clustering of nanoparticles hinder their application, particularly in catalysis, separation, and molecular sensing. Apart from these discrete nanoparticles, three-dimensional interconnected networks of TiO2 containing meso-macropores with a crystalline anatase phase and large surface area promise a different kind of porous materials with advantageous characteristics.2,36,37 The large surface areas derived from the introduction of mesoporosity into TiO2 materials can provide more active adsorption sites and reaction centers, which render these materials attractive for applications such as in size-selective adsorption and photocatalysis. Inspired by that, we have tested the feasibility of using our synthesized three-dimensionally interconnected, highly porous TiO2 materials in photocatalysis. Figure 6 shows the comparison of photocatalytic activity of resulting TiO2 materials after sintering at different temperatures. It can be seen that the sintering temperature had a significant effect on the photocatalytic activity of resultant TiO2 materials. The porous materials sintered at 600 °C exhibited the best photocatalytic performance and could fully photodegrade the organic molecules within 25 min, which was even faster than the fully crystallized commercial P25 sample.
Figure 6. First-order degradation rate curves using the hierarchical porous TiO2 materials sintered at 600, 700, and 800 °C. The photodegradation curve of organic dyes using commercial P25 power was also plotted for comparison.
However, for the products sintered at higher temperature such as 700 and 800 °C, they could photodegrade only ∼60% of the organic molecules in 120 min. The sintering of TiO2 materials at higher temperatures is expected to have a higher degree of crystallinity, and the result thus suggests that the photodegrading performance is not purely associated with the TiO2 crystalline phase. We speculate that this is affected by the morphology and structure of the resulting materials. Although sintering at higher temperature results in a higher degree of crystallinity of the prepared TiO2 samples, the exquisite structures including the interconnected throats and small pores along the macroporous walls would be destroyed, resulting in a dramatic decrease in photocatalytic activity. Therefore, we ascribe the highest photocatalytic activities of the 600 °C sintered samples to the fact that our products contain a three-dimensional interconnected macropore incorporated with the smaller pores, in which large pores allow high gas flow rates while smaller pores ensure a high level of substrate−gas contact in a connected network. This result agrees well with recent reports that meso-macroporous TiO2 exhibited an outstanding photocatalytic activity for degradation reactions of various organic molecules because of the existence of macropores.2,37,38 The reason is that the macropores presented in the porous TiO2 can act as a light transfer path for the introduction of incident photon flux onto the inner surface of porous materials, which allowed light to penetrate deeper inside the photocatalyst. Moreover, the existence of macropores allows easy diffusion of reactants and products, making the photocatalytic reaction more efficient. To confirm that the presence of macropores could enhance the photocatalytic performance of TiO2, we investigated the photodegradation of Rhodamine B by using TiO2 with different sized macropores. Other properties such as the surface area and porosity are kept constant. These samples were prepared by varying the initial internal phase volume while maintainng the concentrations of TiO2 and microgel particles. Figure 7 shows that the size of the macropores greatly decreased from 72 to 14 μm with a decrease in the internal phase volume from 80 to 66.7%, indicating that the final pore structures were highly dependent on the internal phase volume. Other data, including the total intrusion volume, surface area, and bulk density of the porous monoliths, were also obtained from mercury intrusion 2680
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organic compounds in 25 min. This high efficiency was likely related to the improved accessibility of the samples because of the presence of the macroporous structure. For porous material that consisted of smaller macrochannels (14 μm) at a high internal phase volume of 66.7%, it took ∼80 min to fully photodegrade the organic molecules, which had photocatalytic activity ∼3 times lower than that of the materials obtained at 80%. This work thus further confirms that the existence of macropores plays key roles in increasing photoabsorption efficiency and allowing efficient diffusion of organic molecules, resulting in the enhancement of the photocatalytic performance of titania.
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CONCLUSIONS In summary, a direct synthetic route to three-dimensionally interconnected, highly porous TiO2 materials with ordering on two different scales via a templating from an emulsion stabilized by binary colloidal particles is described. We have demonstrated the versatility of this method by producing hierarchical porous materials with a controlled pore size from the initial highinternal phase emulsion droplets and the interfacial microgel particles. Preliminary catalytic experiments have revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules. Despite the fact that a great deal of research on the preparation of TiO2-based photocatalytic disinfectants has been conducted, it was mostly focused on two categories, including the TiO2 suspension system and the coated TiO2 film system. The disadvantages of the suspension approach are that the slurry reduces UV light penetration and the catalyst powders need to be removed after treatment. For the film system, the photoinduced conversion from hydrophilic to hydrophobic of the photocatalytic surfaces remains an as yet unsolved challenge. Our fabricated hierarchical titania contains macropores along with small pores along the cellular walls. Titania materials with such bimodal porous structures are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores have a specific large surface area and can be used to control and transform substances that encounter it. This kind of pore material thereby could drive the development of high-throughput flow-through reactors for applications ranging from large water treatment plants to a point-of-use system. Finally, the developed methodology can be readily extended to produce other porous materials by replacing silica particles with other solid particles.
Figure 7. Pore size distribution of the TiO2 porous materials templated from Pickering HIPEs stabilized by microgels and TiO2 particles after they had been sintered at 600 °C for 2 h. The internal phase volume of initial HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
and are summarized in Table 2. More importantly, it can be seen in Figure 8 that at an internal phase volume of 80%, the Table 2. Properties of the Hierarchical Porous TiO2 Materials after They Had Been Sintered at 600 °C for 2 ha total intrusion volume (mL/g) surface area (m2/g) bulk density (g/mL) porosity (%) nanoscale pore peak (nm) window peak (μm) macropore peak (μm)
Φdisp = 80%
Φdisp = 75%
Φdisp = 66.7%
6.26 28.78 0.15 93.94 12 9 72
3.65 20.31 0.24 86.1 12 10.6 30
4.37 22.98 0.21 90.88 12 4.5 14
a
The internal phase volume of initial HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
product had the largest macropores. It showed a highest photocatalytic activity and could completely photodegrade the
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of PNIPAM-co-MAA microgels, typical hydrodynamic radius (Rh) distribution of the synthesized PNIPAMco-MAA microgel particles measured by laser light scattering at an angle of 20° and 23 °C (Figure S1), SEM images of the synthesized PNIPAM-co-MAA microgel particles at an accelerating voltage of 10 kV (Figure S2), and TGA curves of a freeze-dried HIPE sample (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 8. First-order degradation rate curves using hierarchical porous TiO2 materials templated from Pickering HIPEs stabilized by microgels and TiO2 particles after they had been sintered at 600 °C for 2 h. The internal phase volume of initial Pickering HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. 2681
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Notes
conformation by a biotemplate-directed sol-gel technique. Nanotechnology 2006, 17 (15), 3968−3972. (17) Xia, Y.; Zhang, W. K.; Huang, H.; Gan, Y. P.; Xiao, Z.; Qian, L. C.; Tao, X. Y. Biotemplating of phosphate hierarchical rechargeable LiFePO4/C spirulina microstructures. J. Mater. Chem. 2011, 21 (18), 6498−6501. (18) Zhao, D. Y.; Yang, P. D.; Chmelka, B. F.; Stucky, G. D. Multiphase assembly of mesoporous-macroporous membranes. Chem. Mater. 1999, 11 (5), 1174−1178. (19) Iskandar, F.; Nandiyanto, A. B. D.; Yun, K. M.; Hogan, C. J.; Okuyama, K.; Biswas, P. Enhanced photocatalytic performance of brookite TiO2 macroporous particles prepared by spray drying with colloidal templating. Adv. Mater. 2007, 19 (10), 1408−1412. (20) Barbetta, A.; Cameron, N. R. Morphology and surface area of emulsion-derived (PolyHIPE) solid foams prepared with oil-phase soluble porogenic solvents: Three-component surfactant system. Macromolecules 2004, 37 (9), 3202−3213. (21) Williams, J. M. High Internal Phase Water-in-Oil Emulsions: Influence of Surfactants and Cosurfactants on Emulsion Stability and Foam Quality. Langmuir 1991, 7 (7), 1370−1377. (22) Menner, A.; Verdejo, R.; Shaffer, M.; Bismarck, A. Particlestabilized surfactant-free medium internal phase emulsions as templates for porous nanocomposite materials: Poly-Pickering-foams. Langmuir 2007, 23 (5), 2398−2403. (23) Cameron, N. R.; Sherrington, D. C. High internal phase emulsions (HIPEs): Structure, properties and use in polymer preparation. Adv. Polym. Sci. 1996, 126, 163−214. (24) Manoharan, V. N.; Imhof, A.; Thorne, J. D.; Pine, D. J. Photonic crystals from emulsion templates. Adv. Mater. 2001, 13 (6), 447−450. (25) Bokhari, M.; Carnachan, R. J.; Przyborski, S. A.; Cameron, N. R. Emulsion-templated porous polymers as scaffolds for three dimensional cell culture: Effect of synthesis parameters on scaffold formation and homogeneity. J. Mater. Chem. 2007, 17 (38), 4088−4094. (26) Su, F.; Bray, C. L.; Tan, B.; Cooper, A. I. Rapid and reversible hydrogen storage in clathrate hydrates using emulsion-templated polymers. Adv. Mater. 2008, 20 (14), 2663. (27) Solans, C.; Esquena, J.; Azemar, N. Highly concentrated (gel) emulsions, versatile reaction media. Curr. Opin. Colloid Interface Sci. 2003, 8 (2), 156−163. (28) Haibach, K.; Menner, A.; Powell, R.; Bismarck, A. Tailoring mechanical properties of highly porous polymer foams: Silica particle reinforced polymer foams via emulsion templating. Polymer 2006, 47 (13), 4513−4519. (29) Ungureanu, S.; Birot, M.; Laurent, G.; Deleuze, H.; Babot, O.; Julian-Lopez, B.; Achard, M. F.; Popa, M. I.; Sanchez, C.; Backov, R. One-pot syntheses of the first series of emulsion based hierarchical hybrid organic-inorganic open-cell monoliths possessing tunable functionality (Organo-Si(HIPE) series). Chem. Mater. 2007, 19 (23), 5786−5796. (30) Gurevitch, I.; Silverstein, M. S. Polymerized Pickering HIPEs: Effects of Synthesis Parameters on Porous Structure. J. Polym. Sci., Part A: Polym. Chem. 2010, 48 (7), 1516−1525. (31) Li, Z. F.; Wei, X. L.; Ming, T. A.; Wang, J. F.; Ngai, T. Dual templating synthesis of hierarchical porous silica materials with three orders of length scale. Chem. Commun. 2010, 46 (46), 8767−8769. (32) Li, Z. F.; Ming, T.; Wang, J. F.; Ngai, T. High Internal Phase Emulsions Stabilized Solely by Microgel Particles. Angew. Chem., Int. Ed. 2009, 48 (45), 8490−8493. (33) Sun, G. Q.; Li, Z. F.; Ngai, T. Inversion of Particle-Stabilized Emulsions to Form High-Internal-Phase Emulsions. Angew. Chem., Int. Ed. 2010, 49 (12), 2163−2166. (34) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95 (3), 735−758. (35) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. Rev. 2007, 107 (7), 2891−2959.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of this work by the Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK403210 and 2130237) and the NSFC/RGC Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the National Natural Science Foundation of China (N-CUHK454/11 and 2900350) is gratefully acknowledged.
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(1) Soler-illia, G. J. D.; Sanchez, C.; Lebeau, B.; Patarin, J. Chemical strategies to design textured materials: From microporous and mesoporous oxides to nanonetworks and hierarchical structures. Chem. Rev. 2002, 102 (11), 4093−4138. (2) Chen, X. F.; Wang, X. C.; Fu, X. Z. Hierarchical macro/ mesoporous TiO2/SiO2 and TiO2/ZrO2 nanocomposites for environmental photocatalysis. Energy Environ. Sci. 2009, 2 (8), 872−877. (3) Ikeda, S.; Kobayashi, H.; Ikoma, Y.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Size-selective photocatalytic reactions by titanium(IV) oxide coated with a hollow silica shell in aqueous solutions. Phys. Chem. Chem. Phys. 2007, 9 (48), 6319−6326. (4) Ishizuka, N.; Minakuchi, H.; Nakanishi, K.; Soga, N.; Tanaka, N. Designing monolithic double-pore silica for high-speed liquid chromatography. J. Chromatogr., A 1998, 797 (1−2), 133−137. (5) Vallet-Regi, M. Ordered mesoporous materials in the context of drug delivery systems and bone tissue engineering. Chem.Eur. J. 2006, 12 (23), 5934−5943. (6) Andersson, J.; Rosenholm, J.; Areva, S.; Linden, M. Influences of material characteristics on ibuprofen drug loading and release profiles from ordered micro- and mesoporous silica matrices. Chem. Mater. 2004, 16 (21), 4160−4167. (7) Zhao, C.; Danish, E.; Cameron, N. R.; Kataky, R. Emulsiontemplated porous materials (PolyHIPEs) for selective ion and molecular recognition and transport: Applications in electrochemical sensing. J. Mater. Chem. 2007, 17 (23), 2446−2453. (8) Ma, H. Y.; Hu, J. A.; Ma, P. X. Polymer Scaffolds for SmallDiameter Vascular Tissue Engineering. Adv. Funct. Mater. 2010, 20 (17), 2833−2841. (9) Tampieri, A.; Sprio, S.; Ruffini, A.; Celotti, G.; Lesci, I. G.; Roveri, N. From wood to bone: Multi-step process to convert wood hierarchical structures into biomimetic hydroxyapatite scaffolds for bone tissue engineering. J. Mater. Chem. 2009, 19 (28), 4973−4980. (10) Yuan, Z. Y.; Vantomme, A.; Leonard, A.; Su, B. L. Surfactantassisted synthesis of unprecedented hierarchical meso-macrostructured zirconia. Chem. Commun. 2003, 13, 1558−1559. (11) Amatani, T.; Nakanishi, K.; Hirao, K.; Kodaira, T. Monolithic periodic mesoporous silica with well-defined macropores. Chem. Mater. 2005, 17 (8), 2114−2119. (12) Wang, Z. Y.; Stein, A. Morphology control of carbon, silica, and carbon/silica nanocomposites: From 3D ordered Macro-/Mesoporous monoliths to shaped mesoporous particles. Chem. Mater. 2008, 20 (3), 1029−1040. (13) Velev, O. D.; Kaler, E. W. Structured porous materials via colloidal crystal templating: From inorganic oxides to metals. Adv. Mater. 2000, 12 (7), 531−534. (14) Zhang, H. F.; Cooper, A. I. Synthesis and applications of emulsion-templated porous materials. Soft Matter 2005, 1 (2), 107− 113. (15) Zhang, H.; Hardy, G. C.; Khimyak, Y. Z.; Rosseinsky, M. J.; Cooper, A. I. Synthesis of hierarchically porous silica and metal oxide beads using emulsion-templated polymer scaffolds. Chem. Mater. 2004, 16 (22), 4245−4256. (16) Dong, Q.; Su, H. L.; Zhang, D.; Zhang, F. Y. Fabrication and gas sensitivity of SnO2 hierarchical films with interwoven tubular 2682
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Porous TiO2 Materials through Pickering High-Internal Phase Emulsion Templating Xiaodong Li,† Guanqing Sun,† Yecheng Li,† Jimmy C. Yu,† Jie Wu,‡ Guang-Hui Ma,*,‡ and To Ngai*,† †
Department of Chemistry, The Chinese University of Hong Kong, Shatin N.T., Hong Kong National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
‡
S Supporting Information *
ABSTRACT: We report a facile method for preparing porous structured TiO2 materials by templating from Pickering high-internal phase emulsions (HIPEs). A Pickering HIPE with an internal phase of up to 80 vol %, stabilized by poly(N-isopropylacrylamide)-based microgels and TiO2 solid nanoparticles, was first formulated and employed as a template to prepare the porous TiO2 materials with an interconnected structure. The resultant materials were characterized by scanning electron microscopy, X-ray diffraction, and mercury intrusion. Our results showed that the parent emulsion droplets promoted the formation of macropores and interconnecting throats with sizes of ∼50 and ∼10 μm, respectively, while the interfacially adsorbed microgel stabilizers drove the formation of smaller pores (∼100 nm) throughout the macroporous walls after drying and sintering. The interconnected structured network with the bimodal pores could be well preserved after calcinations at 800 °C. In addition, the photocatalytic activity of the fabricated TiO2 was evaluated by measuring the photodegradation of Rhodamine B in water. Our results revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules.
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INTRODUCTION Hierarchical porous materials occur widely in nature, including, for example, the lungs of animals, the leaves of tree, and diatom skeletons, which accelerate air and solar energy exchange.1 Since the parallel development of mesostructuring and macrostructuring routes in the 1990s, the synthesis of hierarchical materials containing macropores along with smaller pores has become an exciting area of material chemistry, because of their potential uses in diverse applications such as catalysis,2,3 adsorption and separations,4 drug delivery,5,6 sensors,7 and tissue engineering.8,9 Template synthesis is commonly used for the preparation of porous materials. Hierarchical porous materials containing interconnected porous structures with different length scales can be prepared by a single surfactant route10 or macroscopic phase separations.11 However, dual templating proved to be the most effective way because this allows direct and independent control of the pore dimensions, and as a result, the final pore structures can be easily tailored for different applications.12 Currently, surfactant micelles are generally employed as the structure-directing agents of mesostructures, while large-scale systems such as colloid particles,13 emulsions,14,15 biomaterials,16,17 and ice crystals18 are added to govern the production of macroporous structures. In practical applications, the introduction of smaller pores into a macroporous structure is important. This is because the presence of macropores allows the mass transfer of the bulk of the substances like liquids and gases, while the smaller pores create a large internal surface area © 2014 American Chemical Society
that greatly enhances the host−guest interactions, selectivity, and catalytic or ion-exchange properties.19 Recently, a class of macroporous materials that have attracted a great deal of attention was obtained from high-internal phase emulsions (HIPEs). HIPEs are commonly defined as very concentrated emulsions, in which the volume fraction of the internal phase (Φ) exceeds 0.74.20−22 Polymerization of the continuous phase of HIPEs and the subsequent removal of the oil and water can lead to a highly porous, cross-linked polymer materials,23 which are known as PolyHIPEs. Under the right conditions, a small interconnecting throat or windows are also formed between adjacent emulsion droplets after drying. This produces highly porous and permeable materials that have great potential for use in diverse applications such as supported catalysis,24 electrochemical sensors,7 cell culture,25 hydrogen storage,26 and reaction containers.27 However, it is worth mentioning that conventional HIPEs are commonly stabilized by large amounts (5−50 vol %) of small molecule surfactants in a monomer solution.21,28 Over the past few years, there has been an increasing level of interest in the use of solid particles to stabilize the internal phase (droplets) of the HIPEs. Unlike their surfactant-stabilized counterparts, particle-stabilized emulsions (or so-called Pickering emulsions) are less susceptible to coalescence, Ostwald ripening, and creaming, thus resulting in Received: December 24, 2013 Revised: February 13, 2014 Published: February 24, 2014 2676
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then SEM images were taken with a FEI Quanta 400 FEG microscope operating at 10 kV. It can be seen that the synthesized microgel particles are spherical and monodispersed (Figure S2 of the Supporting Information). Preparation and Freeze-Drying of Pickering HIPEs. The oilin-water (o/w) emulsions with different internal phase volumes were initially produced by mechanically shearing the dispersed phase, hexane with an aqueous solution containing the solid TiO2 particles (33.37 wt %, 150 nm in diameter) and fluorescent PNIPAM-co-MAA microgel particles (2.0 wt %, 294 nm in diameter) (step 1 in Figure 1).
extremely stable HIPEs. Manufacturing porous structures from Pickering HIPEs has been explored in various recent studies.29,30 However, there is a need to develop this HIPE templating process for the fabrication of porous materials with controllable pore structures and surface functionality to open new applications. Recently, we have reported a simple and flexible method for obtaining a three-dimensionally interconnected, highly porous silica monolith with different length scales via Pickering HIPE templating.31 Here we report a significant advance that allows us to extend this technique to produce the structured TiO2 porous materials and evaluate their activity in photodegrading organic molecules. By templating through a Pickering HIPE stabilized by both soft microgels and TiO2 solid nanoparticles, we obtained three-dimensionally interconnected porous TiO2 monoliths with macropores (50 μm) and interconnecting throats (∼10 μm). More importantly, we found that microgel stabilizers drove the formation of smaller pores with a size of ∼100 nm throughout the macroporous walls after sintering. Unlike the mostly studied TiO2 suspension and coated film systems, TiO2 materials with such hierarchical structures are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores along the macroporous walls might have a specific large surface area and can greatly enhance the catalytic or ion-exchange properties. We thereby investigated the photocatalytic activity of the fabricated TiO2 by measuring the photodegradation of Rhodamine B in water.
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Figure 1. Schematic illustration of the three-step process used to obtain hierarchical porous TiO2-based materials. The emulsion type was determined by measuring the conductivity and by observing whether a drop of emulsion dispersed when it was added to a small volume of pure water or oil. In some cases, a dye molecule, perylene, was added to the oil phase to check the type of the formed emulsions via confocal microscopy. The confocal microscopy images were taken on a Nikon Eclipse Ti inverted microscope. Lasers with wavelengths of 543 and 408 nm were used to excite the fluorescent microgel particles (with Rhodamine B groups) and perylene molecules, respectively. An oil immersion objective (60×, NA = 1.49) was used to view the samples. The HIPEs were placed on the cover slides, and a series of x−y layers were scanned. The as-prepared HIPEs were first immersed in liquid nitrogen to solidify the entire HIPEs and then dried under vacuum overnight. After the HIPEs had been freeze-dried, the hexane and water were removed and the resultant TiO2/microgel composite porous materials were very stable. Thermogravimetric analysis (TGA) of the TiO2/microgel composite porous materials was performed with a Hi-Res TGA 2950 thermogravimetric analyzer (TA Instruments), where the temperature ramp was 10 °C/min under N2. The calcination of the TiO2/microgel composite via the removal of the microgel particles and fusion of the TiO2 particles was conducted in air using a furnace (Ney Vulcan 3-400 HTA). The temperature was ramped from room temperature to various temperatures (600, 700, 800, 900, and 1200 °C) at a rate of 100 °C/h, and the calcinations were conducted at that specific temperature for 2 h. After that, the temperature was decreased to room temperature at a rate of 100 °C/h. The X-ray diffraction (XRD) patterns were obtained on a HZG41B-PC X-ray diffractometer using Cu Kα radiation at a scan rate of 0.05° (2θ) s−1 and used to determine the identity of the crystalline phase and their crystallite size. The accelerating voltage and the applied current were 35 kV and 20 mA, respectively. Photocatalytic Activity Experiment. Photocatalysis of organic molecules was conducted over the various sintered TiO2-based products. In our experiments, Rhodamine B (C28H31ClN2O3) was chosen for photocatalysis experiments using the prepared hierarchical TiO2 materials. Typically, 20 mL of a Rhodamine B solution at a concentration of 2.0 × 10−5 M was placed in a cell (radius of 50 mm,
EXPERIMENTAL SECTION
Materials. N-Isopropylacrylamide [NIPAM (Fluka)] was recrystallized three times from a toluene/n-hexane mixture. N,N′-Methylene bisacrylamide [MBAA (Fluka)], methacrylic acid [MAA (Merck)], perylene (Aldrich), the fluorescent dye methacryloxyethyl thiocarbamoyl rhodamine B [MRB (Polysciences, Inc.)], and titanium dioxide nanoparticles (TiO2 NPs) with a diameter of ∼150 nm were used as received. Potassium persulfate [KPS (Merck)] and hexane oils were used without further purification. Deionized or MillQ (Millipore) water was used in all the experiments. Synthesis of the PNIPAM-Based Microgel Particles at a Stabilizer. The soft particles used to stabilize HIPEs are poly(Nisopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgel particles. They were synthesized using the well-known precipitation polymerization method. Copolymerization with MAA provides the carboxylic groups that allow variation of the surface charge of the particles, via a change in either the solution pH or the salt concentration. Typically, 3.0 g of NIPAM, 0.105 g of MBAA, 0.105 g of MAA, and 2 mg of MRB were dissolved into 145 mL of deionized water in a 250 mL two-neck flask fitted with nitrogen bubbling and a reflux condenser and stirred with a magnetic stir bar. The solution mixture was adjusted to pH 10.0 with a 0.1 M sodium hydroxide solution. After the solution had been stirred for 1 h at 70 °C under nitrogen bubbling, the precipitation polymerization was initiated by adding 0.035 g of KPS dissolved in 5 mL of deionized water. The reaction mixture was kept at 70 °C for 7 h. The pH of the microgel dispersion after reaction was ∼9. The resultant microgels were dialyzed for 7 days to remove the unreacted reagents. The final pH of the microgel was ∼6.0. Then the microgel suspension was concentrated at 50 °C by reducing half of its original volume for further use. Dynamic laser light scattering shows that the PNIPAM-coMAA microgel particles coated with carboxylic acid groups have an average hydrodynamic radius of ∼146 nm with a solution pH of 6.0 (Figure S1 of the Supporting Information). Scanning electron microscopy (SEM) was also used to characterize the synthesized PNIPAM-co-MAA microgel particles. The purified microgel particles were dried on silica substrates at room temperature overnight, and 2677
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height of 80 mm) that contained a water cooling bath and was surrounded by a UV lamp with an emission wavelength of 365 nm and a power of 15 W. Twenty milligrams of our prepared porous TiO2 materials was weighed and placed in the cell. The mixture was magnetically stirred at ∼100 rpm. Approximately 1.5 mL of the solution was taken at regular intervals and then centrifuged at 14000 rpm for 1 min to fully remove the suspended TiO2 materials. The concentration of the dye solution was immediately analyzed by UV− visible spectroscopy.
2b) shows that the remarkable stability of the emulsion can likely be attributed to the adsorption of fluorescent microgels and solid TiO2 particles at the interface acting as a barrier against oil droplet coalescence, which is in agreement with our recent work.31 In addition, the perlyene-loaded emulsion (Figure 2c) confirms the presence of polydisperse hexane oil droplets, whose size varies from around several micrometers to tens of micrometers. On the other hand, it is worth noting that no flow of emulsion is observed even though the vial is inverted (Figure 2a), indicating that the formed emulsion is a typical gel emulsion. This result can be ascribed to the fact that during the adsorption of particles to the oil−water interfacial area produced by the forceful mixing process, the microgels and TiO2 particles in the aqueous phase are probably contiguous with these adsorbed, thus serving to bind the oil droplets together into a three-dimensional network, in turn inhibiting the gravity-induced separation.32,33 The second step in the fabrication process (Figure 1) was the elimination of the water and oil components by freeze-drying. As shown in Figure 2d, freeze-drying resulted in a porous material consisting of cellular macropores (20−50 μm in diameter) with interconnecting throats approximately ∼10 μm in diameter (Figure 2e). The monolithic materials had typical polymerized HIPE-type interconnected macroporous textures. The average cavity sizes in the macroporous structures are comparable to the oil droplet diameters of the parent emulsions (Figure 2b,c), thus suggesting that these cavities result from a loss of the oil component. Moreover, the cell walls are expected to be densely decorated with a layer of mixed TiO2 and microgel particles. More importantly, high-magnification SEM image (Figure 2f) further revealed the coexistence of small pores (∼100 nm) throughout the macropore walls, which are likely imparted by the collapse of the interfacial adsorbed microgel particles. Note that microgel particles in the bulk solution are ∼300 nm in diameter. The microgel-induced pores are much smaller than the microgels used to stabilize the emulsion. This can be attributed to compression of the soft and deformable microgels at the oil−water interface by the adsorbed solid TiO2 particles. To improve the physical properties of the resulting hierarchically porous structured materials, the final step of the process (Figure 1) was to sinter the dried TiO2 monoliths at different temperatures. TGA (Figure S3 of the Supporting Information) for the TiO2/microgel composite porous materials showed that the mass (microgel) loss continued up to 600 °C, after which the mass remained constant, indicating the complete removal of microgel particles. The composite porous materials were therefore sintered at 600, 700, 800, and 1200 °C to yield robust TiO2 monoliths. It is expected that the sintering can remove the microgel particles to generate nanoscale pores and at the same time fuse the TiO2 building blocks together, which eventually enhances the mechanical properties of the porous materials. The resulting materials were fully characterized by SEM and mercury intrusion porosimetry. Figure 3a shows the representative SEM images of the obtained robust hierarchical porous TiO2 materials after they had been sintered at 600 °C for 2 h. It can be seen that the resulting materials consist of the cellular macropores ∼50 μm in diameter with interconnecting throats approximately ∼10 μm in diameter, indicating that the cavities do not shrink during heating. Moreover, a high-magnification SEM image (Figure 3a2) further revealed the retention of the nanoscale pores (∼100 nm) throughout the macroporous walls, which were
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RESULTS AND DISCUSSION Emulsion−Template Fabrication of Hierarchical Porous TiO2-Based Materials. Figure 1 shows a schematic illustration of the three-step process used to obtain the hierarchical porous TiO2 monolith. The first step was the generation of a Pickering HIPE using both soft PNIPAM-coMAA microgels and solid TiO2 nanoparticles as particulate stabilizers. Oil-in-water (o/w) Pickering HIPEs with different volume fractions were initially formed by mechanically shearing the dispersed phase, hexane, and aqueous solution containing the TiO2 particles (33.37 wt %, 150 nm in diameter) and fluorescent PNIPAM-based microgels (2 wt %, 294 nm in diameter). The appearance of the typically formed Pickering HIPE containing with a volume fraction of 75% is shown in Figure 2a. It is important to note that upon using TiO2 nanoparticles as solely a stabilizer, no stable HIPEs could be produced. However, the Pickering HIPE stabilized by binary particles was very stable, and the relative phase volume did not change for more than 6 months. The confocal image (Figure
Figure 2. (a) Typical HIPE with a volume fraction of 75% stabilized by binary nanoparticles in which the initial emulsions contain 0.7 mL of 2.0 wt % PNIPAM-based microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles. (b and c) Confocal images of the same emulsion excited by a laser with wavelengths of 543 and 408 nm, respectively. The fluorescence PNIPAM-based microgel shows a red color, while the oil phase shows a green color with dissolved perylene. (de and e) SEM images of the hierarchical macropores promoted by oil droplets and interconnected throats by the collapse of the thin film of the continuous water phase, respectively. (f) High-magnification SEM images revealing the small pores with a size of ∼100 nm throughout the cell walls due to the microgel particles occupying these positions. 2678
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Figure 4. Pore size distribution of the sintered porous TiO2 materials after sintering at 600, 700, and 800 °C for 2 h measured via mercury intrusion instruments. The parent Pickering HIPEs contained 0.7 mL of 2.0 wt % PNIPAM microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles, and the volume fraction was fixed at 80%.
TiO2 monoliths sintered at 600, 700, and 800 °C were calculated to be 93.94, 92.59, and 93.2%, respectively, indicating that the original HIPE structure (∼80% volume fraction) was completely retained. For sintering at 600 °C, the resulting material exhibited three distinct peaks around 72, 10, and 12 nm, which can likely be linked to the macropores, throats that connect two adjacent macropores, and smaller pores along the cellular walls, respectively. However, the contributions of the macropores and throat sizes inside the produced TiO2 materials were significantly reduced from 72 to ∼30 μm and from 10 to ∼2 μm, respectively, when the composite microgel/TiO2 materials were sintered at high temperatures (e.g., 700 and 800 °C). These results correlate well with the SEM images (Figure 3) because strong sintering causes the TiO2 nanoparticles to melt and fuse together, resulting in the collapse of pores. The small 100 nm pores even disappeared in the pore size distribution curves when the TiO2 materials were sintered at 800 °C. Other data, including the total intrusion volume, surface area, and bulk density of the TiO2 porous monoliths, were also obtained from mercury intrusion and are summarized in Table 1. XRD was also used to investigate the effect of sintering temperature on the phase structure of the porous TiO2 materials. Figure 5 shows that the as-synthesized TiO2 porous materials were similar to commercially used TiO2 powder (P25), in which a mixture of rutile and anatase crystalline phase
Figure 3. SEM images of hierarchical macropores and interconnected throats templating from Pickering HIPEs stabilized by microgels and TiO2 particles after being sintered at (a) 600, (b) 700, (c) 800, and (d) 1200 °C for 2 h. High-magnification SEM images indicating the nanoscale pores throughout the cell walls due to the removal of microgel particles after sintering at 600 (a2), 700 (b2), 800 (c2), and 1200 °C (d2) for 2 h. The initial HIPEs contained 0.7 mL of 2.0 wt % PNIPAM microgels and 0.3 mL of 33.37 wt % TiO2 nanoparticles, and the volume fraction was fixed at 80%.
likely related to the removal of the interfacial adsorbed microgel particles during sintering. Such porous materials with interconnected pores of different length scales are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores have a large specific surface area and can greatly enhance the catalytic or ionexchange properties. The TiO2/microgel composite porous materials were also sintered at 700 °C and characterized via SEM (Figure 3b1,b2). Macropores, interconnected throats, and nanoscale pores along the cellular walls were also observed. Increasing the sintering temperature from 700 to 800 °C enhances the mechanical stability of the resultant hierarchical porous TiO2 materials. However, further increasing the sintering temperature causes shrinkage of the whole structure without retaining the porous shape of the resultant materials, especially for the nanoscale pores throughout the macroporous walls (Figure 3c1,c2). Exquisite structures such as 100 nm pores and interconnected throats disappeared because TiO2 particles began to melt and fuse together when the composite porous materials were heated at 1200 °C (Figure 4d1,d2). The prepared hierarchical porous TiO2 monoliths after being sintered at various temperatures were also characterized via mercury intrusion porosimetry. Figure 4 shows the pore size distribution of the porous TiO2 materials after they had been sintered at various temperatures. The degrees of porosity of the
Table 1. Effect of Sintering Temperature on the Physicochemical Properties of Hierarchical Porous TiO2 Materials
total intrusion volume (mL/g) surface area (m2/g) bulk density (g/mL) porosity (%) nanoscale pore size (nm) window size (μm) macropore size (μm) 2679
TSintering = 600 °C
TSintering = 700 °C
TSintering = 800 °C
6.26
5.12
5.6
28.78 0.15 93.94 12 9 72
9.34 0.18 92.59 26 1.9 31
4.88 0.17 93.2 − 2.7 30
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Figure 5. XRD patterns of TiO2. (a) Commercially used P25, (b) dried HIPE at room temperature, and (c) HIPE sintered at 600 °C for 2 h, (d) 700 °C for 2 h, and (e) 800 °C for 2 h.
was observed. After the heat treatment, the peaks at 25° and 38° that are associated with an anatase gradually disappeared within the XRD profile, indicating that the phase transition occurred from anatase to rutile by sintering. The transformation of crystalline rutile can certainly enhance the photocatalytic activity of the resulting materials. More importantly, it is worth noting that compared with those of conventional TiO2 porous materials that are generally unstable upon sintering above 600 °C, the morphology and porous structure of our synthesized porous materials are well preserved after sintering at 600 °C (Figure 3a). For heating at a higher temperature (e.g., 800 °C), the intensity of the peaks related to the crystalline phase was increased significantly, indicating that obtained materials have a higher degree of crystallinity. However, the SEM pictures (Figure 3b,c) showed that sintering at such a high temperature could lead to the shrinkage of the whole structure without retention of the porous shape of the resultant materials. Photocatalytic Activity of the TiO2-Based Hierarchically Structured Porous Materials. TiO2 is one of the most prominent materials for photocatalysis and solar energy application because of its strong redox power, high photocorrosion resistance, and long-term stability.34 Over the past few decades, a great effort has been made to prepare crystalline TiO2 nanoparticles for photocatalysis. This is because the small TiO2 nanocrystals have excellent photocatalytic properties because of the higher level of photon to electron conversion and efficient charge carrier separation and migration.35 However, the smaller surface area and stronger tendency for clustering of nanoparticles hinder their application, particularly in catalysis, separation, and molecular sensing. Apart from these discrete nanoparticles, three-dimensional interconnected networks of TiO2 containing meso-macropores with a crystalline anatase phase and large surface area promise a different kind of porous materials with advantageous characteristics.2,36,37 The large surface areas derived from the introduction of mesoporosity into TiO2 materials can provide more active adsorption sites and reaction centers, which render these materials attractive for applications such as in size-selective adsorption and photocatalysis. Inspired by that, we have tested the feasibility of using our synthesized three-dimensionally interconnected, highly porous TiO2 materials in photocatalysis. Figure 6 shows the comparison of photocatalytic activity of resulting TiO2 materials after sintering at different temperatures. It can be seen that the sintering temperature had a significant effect on the photocatalytic activity of resultant TiO2 materials. The porous materials sintered at 600 °C exhibited the best photocatalytic performance and could fully photodegrade the organic molecules within 25 min, which was even faster than the fully crystallized commercial P25 sample.
Figure 6. First-order degradation rate curves using the hierarchical porous TiO2 materials sintered at 600, 700, and 800 °C. The photodegradation curve of organic dyes using commercial P25 power was also plotted for comparison.
However, for the products sintered at higher temperature such as 700 and 800 °C, they could photodegrade only ∼60% of the organic molecules in 120 min. The sintering of TiO2 materials at higher temperatures is expected to have a higher degree of crystallinity, and the result thus suggests that the photodegrading performance is not purely associated with the TiO2 crystalline phase. We speculate that this is affected by the morphology and structure of the resulting materials. Although sintering at higher temperature results in a higher degree of crystallinity of the prepared TiO2 samples, the exquisite structures including the interconnected throats and small pores along the macroporous walls would be destroyed, resulting in a dramatic decrease in photocatalytic activity. Therefore, we ascribe the highest photocatalytic activities of the 600 °C sintered samples to the fact that our products contain a three-dimensional interconnected macropore incorporated with the smaller pores, in which large pores allow high gas flow rates while smaller pores ensure a high level of substrate−gas contact in a connected network. This result agrees well with recent reports that meso-macroporous TiO2 exhibited an outstanding photocatalytic activity for degradation reactions of various organic molecules because of the existence of macropores.2,37,38 The reason is that the macropores presented in the porous TiO2 can act as a light transfer path for the introduction of incident photon flux onto the inner surface of porous materials, which allowed light to penetrate deeper inside the photocatalyst. Moreover, the existence of macropores allows easy diffusion of reactants and products, making the photocatalytic reaction more efficient. To confirm that the presence of macropores could enhance the photocatalytic performance of TiO2, we investigated the photodegradation of Rhodamine B by using TiO2 with different sized macropores. Other properties such as the surface area and porosity are kept constant. These samples were prepared by varying the initial internal phase volume while maintainng the concentrations of TiO2 and microgel particles. Figure 7 shows that the size of the macropores greatly decreased from 72 to 14 μm with a decrease in the internal phase volume from 80 to 66.7%, indicating that the final pore structures were highly dependent on the internal phase volume. Other data, including the total intrusion volume, surface area, and bulk density of the porous monoliths, were also obtained from mercury intrusion 2680
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organic compounds in 25 min. This high efficiency was likely related to the improved accessibility of the samples because of the presence of the macroporous structure. For porous material that consisted of smaller macrochannels (14 μm) at a high internal phase volume of 66.7%, it took ∼80 min to fully photodegrade the organic molecules, which had photocatalytic activity ∼3 times lower than that of the materials obtained at 80%. This work thus further confirms that the existence of macropores plays key roles in increasing photoabsorption efficiency and allowing efficient diffusion of organic molecules, resulting in the enhancement of the photocatalytic performance of titania.
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CONCLUSIONS In summary, a direct synthetic route to three-dimensionally interconnected, highly porous TiO2 materials with ordering on two different scales via a templating from an emulsion stabilized by binary colloidal particles is described. We have demonstrated the versatility of this method by producing hierarchical porous materials with a controlled pore size from the initial highinternal phase emulsion droplets and the interfacial microgel particles. Preliminary catalytic experiments have revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules. Despite the fact that a great deal of research on the preparation of TiO2-based photocatalytic disinfectants has been conducted, it was mostly focused on two categories, including the TiO2 suspension system and the coated TiO2 film system. The disadvantages of the suspension approach are that the slurry reduces UV light penetration and the catalyst powders need to be removed after treatment. For the film system, the photoinduced conversion from hydrophilic to hydrophobic of the photocatalytic surfaces remains an as yet unsolved challenge. Our fabricated hierarchical titania contains macropores along with small pores along the cellular walls. Titania materials with such bimodal porous structures are particularly intriguing because the presence of macropores allows ready mass transfer, while the smaller pores have a specific large surface area and can be used to control and transform substances that encounter it. This kind of pore material thereby could drive the development of high-throughput flow-through reactors for applications ranging from large water treatment plants to a point-of-use system. Finally, the developed methodology can be readily extended to produce other porous materials by replacing silica particles with other solid particles.
Figure 7. Pore size distribution of the TiO2 porous materials templated from Pickering HIPEs stabilized by microgels and TiO2 particles after they had been sintered at 600 °C for 2 h. The internal phase volume of initial HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
and are summarized in Table 2. More importantly, it can be seen in Figure 8 that at an internal phase volume of 80%, the Table 2. Properties of the Hierarchical Porous TiO2 Materials after They Had Been Sintered at 600 °C for 2 ha total intrusion volume (mL/g) surface area (m2/g) bulk density (g/mL) porosity (%) nanoscale pore peak (nm) window peak (μm) macropore peak (μm)
Φdisp = 80%
Φdisp = 75%
Φdisp = 66.7%
6.26 28.78 0.15 93.94 12 9 72
3.65 20.31 0.24 86.1 12 10.6 30
4.37 22.98 0.21 90.88 12 4.5 14
a
The internal phase volume of initial HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
product had the largest macropores. It showed a highest photocatalytic activity and could completely photodegrade the
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of PNIPAM-co-MAA microgels, typical hydrodynamic radius (Rh) distribution of the synthesized PNIPAMco-MAA microgel particles measured by laser light scattering at an angle of 20° and 23 °C (Figure S1), SEM images of the synthesized PNIPAM-co-MAA microgel particles at an accelerating voltage of 10 kV (Figure S2), and TGA curves of a freeze-dried HIPE sample (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 8. First-order degradation rate curves using hierarchical porous TiO2 materials templated from Pickering HIPEs stabilized by microgels and TiO2 particles after they had been sintered at 600 °C for 2 h. The internal phase volume of initial Pickering HIPEs ranged from 66.7 to 80%, while the concentration of the TiO2 nanoparticles was 23.4 wt % in the aqueous solution.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. 2681
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of this work by the Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK403210 and 2130237) and the NSFC/RGC Joint Research Scheme sponsored by the Research Grants Council of Hong Kong and the National Natural Science Foundation of China (N-CUHK454/11 and 2900350) is gratefully acknowledged.
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