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A floating macro/mesoporous crystalline anatase TiO2 ceramic with enhanced photocatalytic performance for recalcitrant wastewater degradation† Zipeng Xing,a,b Wei Zhou,a Fan Du,b Yang Qu,a Guohui Tian,a Kai Pan,a Chungui Tiana and Honggang Fu*a A macro/mesoporous anatase TiO2 ceramic floating photocatalyst has been successfully synthesized using highly thermally stable mesoporous TiO2 powder as a precursor, followed by a camphene-based freeze-casting process and high-temperature calcinations. The ceramics are characterized in detail by X-ray diffraction, Raman spectra, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption isotherms. The results indicate that the TiO2 ceramics present hierarchical macro/ mesoporous structures, which maintain high porosity and high compressive strength at the optimal sintering temperature of 800 °C. The ordered mesoporous TiO2 network still possesses high thermal stability and inhibits the anatase-to-rutile phase transformation during calcinations. The obtained ceramics exhibit good adsorptive and photocatalytic activity for the degradation of octane and rhodamine B, and the total organic carbon removal ratio is up to 98.8% and 98.6% after photodegradation for 3 h, respectively. The roles of active species in the photocatalytic process are compared using different types of active species scavengers, and the degradation mechanism is also proposed. Furthermore, the ceramics are
Received 4th September 2013, Accepted 6th October 2013
recyclable, and no clear changes are observed after ten cycles. In addition, the ceramics are also active in
DOI: 10.1039/c3dt52433g
the photodegradation of phenol, thiobencarb, and atrazine. Therefore, these novel floating photocatalysts will have wide applications, including the removal of floating organic pollutants from the waste-
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water surfaces or the removal of soluble organic pollutants from wastewater.
Introduction Nowadays, organic contamination purification is one of the most interesting challenges in catalysis. A subject of global concern is the presence of harmful compounds in water supplies and in the discharge of wastewater from chemical industries, power plants, and agricultural sources.1–3 Some of these compounds are biologically recalcitrant and inhibitory organics, thus greatly reducing the ability of microorganisms
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P.R. China. E-mail: [email protected]; Fax: +86-451-8667-3647; Tel: +86-451-8660-4330 b Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin 150080, P.R. China † Electronic supplementary information (ESI) available: Fig. S1 Schematic of the fabrication of macro/mesoporous crystalline TiO2 ceramic by camphene-based freeze-casting. Fig. S2 Images of floating photocatalysts based on hierarchical porous TiO2 ceramic in 10 mg L−1 RhB solution before (A) and after reaction (B). Table S1 Composition of TiO2 ceramic/camphene slurries used in this study. See DOI: 10.1039/c3dt52433g
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to biodegrade these compounds during treatment or in nature.4 All the present chemical treatment processes use either high-energy ultraviolet light or strong oxidants of seriously hazardous and undesirable nature.5,6 Numerous intermediates are formed in these processes, and due to the very low efficiency, the overall treatment cost becomes higher if the destruction of intermediates and complete mineralization are to be achieved. Therefore, water contaminated with these toxic and biologically recalcitrant organics requires new treatment technologies.7 The use of heterogeneous photocatalysts to degrade hazardous organic and inorganic compounds in wastewater is the most promising water-treatment technology in terms of exploitation and utilization.8–11 One of the most advanced oxidation processes that couples low-energy ultraviolet light with semiconductors such as TiO2 acting as photocatalysts is heterogeneous photocatalysis. Growing attention has been focused on this technology on account of the possibility of using solar radiation as the energy source for the decontamination of these effluents.12 From the environmental perspective, the use of solar energy to drive the destruction of pollutants has an absolute advantage.13,14 It also reduces the costs of the process
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which makes it competitive with other technologies for wastewater treatment, such as ozone or H2O2/UV-C.15 However, three severe restrictions hinder the practical application of TiO2, namely the separation of photoinduced charges and holes,16–20 the improvement of photon utilization21–24 and photocatalyst recycling.25–28 A large amount of energy and effort has been devoted to this subject, and more practical ways to use this technology on an industrial scale are needed. In particular, before the TiO2 particles can be used in applications, a simple and low-cost filtration step as well as easy aggregation, particularly at high slurry concentrations, must be found.29 To overcome the separation problem, a great deal of effort has been made in the past several years.30–32 Considering the intimate interaction between light and photocatalysts, floating photocatalysts are advantageous owing to their optimization of the illumination/light utilization process, particularly in a system that uses solar irradiation, and maximization of photocatalyst oxygenation on the basis of its proximity to the air–water interface, particularly for nonstirred reactions.33,34 The optimization of illumination and oxygenation should result in higher radical formation rates and oxidation efficiencies.35 The floating photocatalysts can be applied in solar remediation in situ (i.e., directly in the contaminated wastewater reservoirs located in remote places without any special equipment or installation). Floating photocatalysts can be used for more efficient destruction of suspended insoluble organic contaminants (e.g., in oil-spill accidents). To date, various materials have been used as lightweight substrates for the immobilization of TiO2 catalysts.36–38 However, an apparent drawback of these TiO2-coated substrates is the gradual detachment of TiO2 particles from the substrate surface by collisions and hydrodynamic turbulence.39 In this paper, we report on the design and fabrication of novel macro/mesoporous crystalline anatase TiO2 ceramics as floating photocatalysts by a camphene-based freeze-casting process for the first time. Our previously prepared floating photocatalyst40 had demonstrated that camphene as a threedimensionally interconnected frozen vehicle network could sublime and in turn leave macroporous channels in the ceramic body. These open-pore structures were of particular interest and importance owing to their excellent permeability, large surface area, low density, and high specific strength, which were conducive to the transfer and diffusion of pollutants. In addition, our previously synthesized well-ordered mesoporous TiO241 with high thermal stability was used as the precursor, which would improve photon utilization and organic molecule adsorption and increase the photocatalytic activity. Ordered mesoporous oxides had been widely used in the potential application fields of photocatalysis, due to their large surface area and ordered pore networks.42–44 In particular, mesoporous nanocrystalline TiO2 had been demonstrated to be a more effective photocatalyst, because of its environmental friendliness, large surface area, ordered porous structure and large pore volume, which results in increased surface reactive sites and improved photon absorption and organic molecule adsorption.45 Within the hierarchical porous ceramic, some
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perlite granules were added to provide closed pores and maintain the floatability of the photocatalysts. The final prepared macro/mesoporous anatase TiO2 ceramic exhibited both good mechanical stability and high photocatalytic activity.
Materials and methods Materials All reagents [i.e., polystyrene (PS), octane, rhodamine B (RhB), phenol, thiobencarb, and atrazine] were of analytical grade and used without further purification. Camphene (C10H16) (95% purity, Shanghai Haiqu Co. Ltd, Shanghai, China) was used as the freezing vehicle, and Texaphor 3250 (Guangzhou Haichuan Co. Ltd, Guangzhou, China) was used as the dispersant (density at 25 °C of 0.89–0.91 g cm−3). Ethanediamine (EN) modified well-ordered mesoporous anatase TiO2 powder with a remarkably high thermal stability and improved crystallinity was synthesized in our previous work.41 Preparation of the floating photocatalysts A schematic diagram of the fabrication of hierarchical porous crystalline TiO2 ceramic is presented in Fig. S1.† The prepared TiO2 powder/camphene/PS binder/dispersant slurries were prepared by ball-milling at 60 °C for 24 h (The planetary ball mill was used. The ball mill jar and ball was agate, and the compound-to-ball weight ratio was 0.84. The raw material was added into the ball mill jar, and then the agate ball was added. The jar and jar cover was pressed compactly by the bolt. The rotation speed was set to 100 r min−1. During the milling, the jar and jar cover had to be kept sealed, as otherwise the liquid camphene would volatilize.), and the compositions of the TiO2/camphene slurries used in this study are summarized in Table S1.† Next, the granular expanded perlite (20–30 mesh) was added into the slurries and mixed uniformly before being poured into gum-packing paper molds (1 × 2 × 6 cm) for freezing. The samples were left to cool at room temperature [(20 ± 2) °C] for 30 min. After solidification, the green body was removed from the molds. Then, the green body was cut into uniform size pieces (5 × 5 × 5 mm), which were pressed into sheets by mechanical force with 0.5 MPa. The samples were then sublimed for 48 h at room temperature to completely remove the camphene. Following sublimation, the green bodies were sintered at 1 °C min−1 up to 600 °C followed by dwelling time for 1 h. They were then heated at 2 °C min−1 up to 700, 800, and 900 °C for 2 h, respectively. As shown in Fig. 1, the obtained ceramic had a hierarchical macro/mesoporous structure. Fig. S2(A)† indicates that the hierarchical porous crystalline TiO2 ceramic, which was floatable in water, was successfully synthesized. Its average dimension was 8 × 8 × 2 mm, and the average mass of one piece was 20 mg. Characterization of the floating photocatalysts X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-IIIB diffractometer using Cu-Kα radiation (λ = 1.5406 Å).
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Fig. 1 Schematic diagram of the hierarchical macro/mesoporous crystalline TiO2 ceramic structure.
Raman measurements were performed using a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. The laser beam was focused with a 50× objective lens to a ca. 1 μm spot on the surface of the sample. Scanning electron microscopy (SEM) micrographs were obtained using a Philips XL-30-ESEM-FEG instrument operating at 20 kV. The TiO2 ceramic sample was cut into pieces, and one piece was chosen to paste on the steel plate by using double coated copper tape. Then the steel plate was coated with gold by sputtering for 5 min. The prepared steel plate samples were analyzed by SEM. The apparent density and porosity of sintered bodies were measured by the Archimedes method. Test samples of TiO2 ceramics were loaded onto a testing machine (Instron 5569, Instron Corp., Canton, OH) to measure the compressive strengths with a crosshead speed of 0.2 mm min−1. The average of at least five samples was used for each measurement. Transmission electron microscopy (TEM) was performed using a JEM-2100 electron microscope (JEOL, Japan). The TiO2 ceramic samples were first ground by an agate mortar. The obtained ceramic samples were dispersed in absolute alcohol by ultrasound for 30 min. One or two dispersed drops were applied to a copper mesh microgrid. After drying, the prepared micro-grid samples were analyzed by TEM. Nitrogen adsorption–desorption isotherms at 77 K were collected with an AUTOSORB-1 (Quantachrome Instruments) nitrogen adsorption apparatus. The Brunauer–Emmett–Teller (BET) equation was used to calculate the specific surface area. Total organic carbon (TOC) analysis was carried out using a TOC-VCPN (Shimadzu, Japan) analyzer with the minimum detection limit of 0.05 mg L−1 in accordance with standard methods.46 Photocatalytic activity test As shown in Fig. S2(A),† the prepared TiO2 ceramics could float on the water well (i.e., they were slightly submerged, which was conducive to light utilization). Eighty pieces of the prepared TiO2 ceramic were covered on a glass culture dish reactor (9 cm diameter) with a reaction solution (30 mL). The reactor was open to the ambient air to achieve air-equilibration (Fig. 2(B)). A 20 W, 365 nm UV lamp was placed closely above the reactor with a 240 μW cm−2 average light intensity. Octane and rhodamine B (RhB) had been found to be potentially toxic and carcinogenic. Thus, octane and RhB were mainly chosen as the model recalcitrant pollutants to evaluate the photocatalytic activity of the as-prepared floating photocatalysts. After different photocatalytic reaction durations, the
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Fig. 2 XRD patterns of TiO2 ceramic prepared at 700, 800, and 900 °C, respectively.
octane concentration was analyzed by Shimadzu Model GC-14B gas chromatography and the RhB concentration was measured at 553 nm using a Shimadzu Model UV2550 spectrophotometer. 1 mL and 0.1 mol L−1 of ammonium oxalate, benzoquinone and tert-butyl alcohol were added into the photocatalytic system to examine the photocatalytic mechanism, respectively. In addition, the mineralization ratios of octane, RhB, phenol, thiobencarb, and atrazine were also examined by TOC-VCPN after 2 h of adsorption and 3 h of photocatalytic reaction. All photocatalytic experiments were performed at room temperature [(20 ± 2) °C].
Results and discussion Structure and characteristics of macro/mesoporous TiO2 ceramic The anatase-to-rutile phase transformation in TiO2 had attracted much attention because the phase structure of TiO2 nanostructures largely determined their suitability for practical applications.47 In general, the crystal phase and crystallization of TiO2 depended strongly on the sintering temperature. Fig. 2 shows the XRD patterns of the TiO2 ceramics prepared at different calcination temperatures. 700, 800 and 900 °C calcined samples are denoted as S700, S800 and S900, respectively. In Fig. 2 (S700 and S800), five characteristic XRD peaks of anatase TiO2 were observed at 25.2, 37.8, 48.1, 53.9 and 56.1°, which could be indexed to (101), (004), (200), (105), and (211), respectively. The rutile phase was present (S900) during calcinations at 900 °C, which was reasonable because rutile TiO2 was more thermodynamically stable. That is to say, rutile was the final phase of TiO2 after being calcined at high temperature. Anatase could be further crystallized during the calcination; the crystallinity of anatase TiO2 was improved obviously (as shown in Fig. 2). According to our previous report,41 we knew that ethanediamine (EN) species could
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interact with crystalline TiO2 particles strongly and bind to the surface of TiO2. EN was also considered to be an effective protector to inhibit the undesirable grain growth and phase transformation of anatase TiO2. Therefore, EN species, which were strong alkalis with two primary amine groups and two positive charges, would attack the surface of TiO2 particles strongly. Under the protection of EN, the TiO2 nanoparticles could not come into direct contact with each other, which would thus prolong the improvement of crystallinity for anatase and retard the phase transformation of anatase to rutile. The anatase-torutile phase transformation began when the sintering temperature exceeded 800 °C. This result further suggested that the EN species indeed protected the TiO2 particles, thus improving the crystallinity of anatase and slowing the anatase-to-rutile transformation. Raman spectroscopy, which was very sensitive to the crystallinity and microstructure of materials, was usually used to unambiguously discriminate the local order characteristics of TiO2.48 In order to further confirm the high crystallinity of the obtained macro/mesoporous TiO2 samples, Raman detection was carried out. Fig. 3 shows the Raman spectra of hierarchical porous TiO2 ceramic with different calcination temperatures. Under the calcination temperature of 800 °C, we could clearly see that five high intensity Raman peaks at 149, 199, 393, 513, and 639 cm−1 could be ascribed to Eg, Eg, B1g, A1g(B1g), and Eg modes, respectively, which were characteristic of anatase TiO2. Apart from that, no other Raman peaks could be observed, indicating the phase-pure anatase. Moreover, the intensity of Raman peaks increased with the increase of calcination temperature, suggesting the improvement of crystallinity of anatase, which was in good agreement with the XRD results. When the calcination temperature was over 900 °C, four different Raman peaks at 143, 244, 442, and 609 cm−1 could be observed and ascribed to B1g, the multi-photon process, Eg, and A1g modes, respectively, which were characteristic of rutile TiO2. This confirmed that the transformation was from anatase to rutile under this calcination
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Fig. 4 SEM (A) and magnified SEM images (B) of the hierarchical porous TiO2 ceramics at a sintering temperature of 800 °C.
temperature. These results were well consistent with the XRD characterization. The microstructure evolution of sintered TiO2 ceramic at 800 °C was further studied by SEM analysis. Many macropores could be observed in Fig. 4(A), and these macropores were interconnected. It just originated from the sublimation of camphene crystals and leaves pore channels in the ceramic body. In order to value the pore size, SEM micrographs in Fig. 4(B) were enlarged and the pore size was measured. The pore size of the calcined TiO2 ceramic was 0.1–2 μm, implying that the materials were in the macroporous region. The sintering temperature was important in determining the porosity and mechanical strength of the sintered samples. The effect of the sintering temperature on the porosity and compressive strength was also investigated, and the results are shown in Table 1. When the sintering temperature was increased from 700 to 900 °C, the porosity decreased from 98.5 to 86.7%, and the compressive strength increased from 0.27 to 1.19 MPa. It was widely accepted that higher porosity would improve pollutant transmission and the permeability of porous ceramic, and
Table 1 Porosity and compressive strength of porous TiO2 ceramic after being calcined at 700, 800, and 900 °C, respectively
Fig. 3 Raman spectra of TiO2 ceramic with different calcination temperatures.
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Parameters
S700
S800
S900
Porosity (%) Compressive strength (MPa)
98.5 0.27
94.2 0.66
86.7 1.19
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Fig. 5 TEM (A, B, C) and HRTEM (D) images of hierarchical porous TiO2 ceramics prepared at 800 °C (anatase: d101 = 0.35 nm).
a compressive strength of at least 0.6 MPa was desirable for floating photocatalysts to resist hydrodynamic turbulence. This illustrated that the obtained floating photocatalysts possessed high mechanical stability. Therefore, high ceramic porosity and compressive strength could be obtained after being calcined at 800 °C. The morphology and porosity of the sample were further investigated by TEM. Fig. 5 showed the TEM and HRTEM images of porous TiO2 samples after being calcined at 800 °C. TEM images of TiO2 ceramic after calcinations exhibited a 2D hexagonal mesostructure with P6mm symmetry, which possessed highly ordered mesopores of about 10 nm in diameter. It also showed a high degree of periodicity of 2D hexagonal arrangements over large domains. From the HRTEM images (Fig. 5(D)), we could see that the lattice fringes corresponding to the (101) (d101 = 0.35 nm) crystallographic planes of anatase were most frequently observed in S800, which indicated the high crystallinity of the pore walls of mesoporous TiO2. The nanocrystalline nature of anatase TiO2 was well defined. These results were consistent with the XRD and Raman findings. Such a high anatase crystallinity of the mesoporous TiO2 was highly desirable in photocatalysis. Fig. 6 showed the N2 adsorption–desorption isotherms (A) and the corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plots (B) of mesoporous TiO2 with different calcination temperatures. At a relatively high pressure, the curves exhibited small hysteresis loops, which were attributed to type IV isotherms and were representative of mesoporous materials. The calcined hierarchical porous TiO2 ceramic had a narrow BJH adsorption pore size distribution with a mean value of 10 nm, implying that the materials had very regular pore channels in the mesoporous region. With the increase of
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Fig. 6 (A) N2 adsorption–desorption isotherms and (B) the corresponding BJH pore size distribution plots of mesoporous anatase TiO2 with different calcination temperatures.
calcination temperature, the pore size distribution of the samples becomes smaller, as shown in Fig. 6(B). The specific BET surface area also decreased from 142 to 63 m2 g−1 from S700 to S900. The total pore volumes decreased from 0.29 to 0.0043 cm3 g−1. These differences were attributed to the collapse of some mesopores, contraction of others, and the agglomeration of particles during the thermal treatment process. Interestingly, S800 possessed an integrated and wellordered mesoporous framework, with a specific BET surface area of 97 m2 g−1 and total pore volumes of 0.15 cm3 g−1. Due to the large density of crystalline TiO2, this value (BET surface area) was very high for mesoporous TiO2 with high thermal stability and crystallinity. In fact, we could also find from Fig. 5 that the contraction of mesopores during the thermal treatment process was not serious. This was because the bonded N-containing species on a mesostructured network protected the framework of mesoporous TiO2, and subsequently maintained a relatively high surface area and integrated porous structure, which were beneficial for photocatalytic reactions.
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Photocatalytic performance for recalcitrant wastewater degradation When the sintering temperature was 700 °C, the prepared porous TiO2 ceramic began to fall to pieces in the water easily. It was due to the low compressive strength of S700. To better understand the relationship between the sintering temperature and photocatalytic performance, the effect of 800 and 900 °C sintering temperatures on floating octane degradation was examined. As shown in Fig. 7, after 2 h of dark adsorption equilibrium, the initial octane could be removed 43.5 and 25.7% for S800 and S900, respectively. The mesoporous structure of TiO2 ceramic could be beneficial for pollutant adsorption. The adsorption performance of S800 was better than S900; the main reason was that the specific BET surface of S800 (97 m2 g−1) was higher than that of S900 (63 m2 g−1). After 3 h of photocatalysis, the total octane removal ratio of S800 was significantly better than that of S900. S800 achieved a very high octane removal ratio of 99.9%. However, S900 removed only 75.4% of octane. This result could be attributed to the fact that S800 had a better adsorption performance than S900 on the one hand. In general, the large specific surface area facilitated the absorption and utilization of light, and also offered more active sites. Previous studies41 illustrated that a suitable conformation of pores allowed light waves to penetrate the catalyst deeply and leads to high mobility of photogenerated charges. It could be speculated that the vertical mesopores in the porous TiO2 ceramic are beneficial for the penetration of light waves and octane molecules deep into the photocatalyst, which could promote their photocatalytic performance greatly. On the other hand, S800 maintained high crystallinity and pure anatase phase, which had a higher photocatalytic activity. The high crystallinity meant fewer defects, which could be responsible for the lower recombination of photogenerated hole–electron pairs. Although the crystallinity of S900 was the best one, the rutile phase could decrease the photocatalytic activity. In addition, when the octane solution was exposed to the UV lamp alone, the octane concentration remained constant for 5 h, which suggested that
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UV photolysis could not decompose octane directly. Therefore, the main octane degradation mechanism included the adsorption and photocatalysis process. The photocatalytic kinetics had been proposed to follow a pseudo-first-order model. The integration of the equation under this assumption with boundary conditions of C = C0 at t = 0 yields (see eqn (1)) lnðC=C0 Þ ¼ kapp t
ð1Þ
in which C0 is the initial substrate concentration and kapp is the apparent first-order reaction rate. Many photocatalytic reactions exhibited this behavior.49,50 Fig. 8 showed the plots of ln(C/C0) as a function of reaction time with different catalysts or without catalysts for octane degradation. A linear trend was observed for both S800 and S900, which suggested that the photocatalytic degradation of octane under these reaction conditions followed pseudo-firstorder kinetics. The calculated apparent rate constant (the slopes from the plots) and the corresponding coefficient of determination (R2) are shown in Table 2. The apparent rate constant for S800 was 6.11 times higher than that of S900 implying the high photocatalytic performance. To further validate the floating photocatalyst performances, the 10 mg L−1 of soluble RhB degradation was also studied by S800 and S900. As shown in Fig. 9, after 2 h of dark adsorption equilibrium, the initial RhB could be removed 32.2 and 11.1% for S800 and S900, respectively. After 3 h photocatalysis, S800 achieved a very high RhB removal ratio of 99.4%. However, S900 removed only 63.6% of RhB. No matter their adsorption
Fig. 8 Value of ln(C/C0) versus reaction time with different catalysts or without a catalyst on octane degradation.
Table 2 Reaction kinetics equations and linear regression coefficients with photocatalysts prepared at different sintering temperatures for octane degradation
Fig. 7 Time profiles of the photocatalytic degradation of octane with different catalysts and without a catalyst.
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Samples
Reaction kinetics equations
kapp (h−1)
R2
S800 S900
ln(C/C0) = −2.20t + 0.0434 ln(C/C0) = −0.36t + 0.0368
2.20 0.36
0.9952 0.9912
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Fig. 9 Time profiles of the photocatalytic degradation of RhB with different catalysts and without a catalyst.
or photocatalysis ability, the S800 was better than S900. The obtained results were well consistent with octane degradation. Fig. 10 shows the plots of ln(C/C0) as a function of the reaction time with RhB degradation for photocatalysis, thereby revealing that RhB removal reactions also followed pseudofirst-order kinetics. Table 3 summarizes the apparent rate constant and the corresponding coefficient of determination (R2) for different photocatalysts after 3 h of UV illumination; the apparent rate constant for S800 was 5.14 times higher than that of S900. By comparison, we found that the apparent rate constant of octane removal was higher than RhB removal for S800 and S900, respectively. It indicated that the
Fig. 10 Value of ln(C/C0) versus reaction time with different catalysts or without a catalyst on RhB degradation.
Table 3 Reaction kinetics equations and linear regression coefficients with photocatalysts prepared at different sintering temperatures for RhB degradation
Samples
Reaction kinetics equations
kapp (h−1)
R2
S800 S900
ln(C/C0) = −1.49t + 0.0509 ln(C/C0) = −0.29t + 0.0123
1.49 0.29
0.9930 0.9897
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Fig. 11 Time profiles of the photocatalytic degradation of RhB with different active species scavengers. (BQ: benzoquinone, AO: ammonium oxalate, and TBA: tert-butyl alcohol.)
photodegradation rate of octane was faster than RhB. One possible explanation was that the octane was floating on the water surface and it could close contact with the floating photocatalyst more favorably than soluble RhB. It was helpful to understand the photocatalytic mechanism of floating TiO2 ceramic by the analysis of the photocatalytic degradation process. As shown in eqn (2)–(8), the hierarchical porous TiO2 ceramic could generate a large number of radical species with a strong oxidation capability in the water.51,52 To further evaluate the role of these active species such as hVB+, O2•− and •OH, different types of active species scavengers were added to the catalyst system. Fig. 11 shows the photocatalytic activity of porous TiO2 ceramic toward the degradation of RhB under different conditions. Benzoquinone (BQ) had the ability to trap O2•− by a simple electron transfer mechanism. After BQ was added into the reaction system, the rate of degradation of RhB over the TiO2 ceramic was remarkably decreased. Hierarchical porous TiO2 ceramic could generate the electrons and holes under UV light irradiation. The electrons could react with molecules of O2 on the surface of TiO2 to form a superoxide anion radical (O2•−). The prepared floating TiO2 ceramic was at the air–water interface; thus the generated electrons could react with the dissolved O2 easily to form the O2•− radical. It played an important role in the photocatalytic reaction. The addition of ammonium oxalate (AO) as a hole-scavenger provoked partial inhibition of the RhB degradation as shown in Fig. 11. After tert-butyl alcohol (TBA) as a scavenger for •OH was added in the system, it did not obviously affect the decomposition rate at all over the photocatalytic system. So, RhB oxidation was driven by the contribution of •OH radicals to a lesser extent. To sum up, through the comparison, we could conclude that RhB oxidation was driven mainly by the participation of O2•− and hole radicals, and to a lesser extent by the contribution of •OH radicals. TiO2 þ hν ! hVB þ þ eCB
ð2Þ
hVB þ þ Organic pollutants ! ! CO2 þ H2 O
ð3Þ
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eCB þ O2 ! O2 •
ð4Þ
O2 • þ Organic pollutants ! ! CO2 þ H2 O þ
•
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hVB þ H2 O ! OH þ H
•
þ
ð5Þ ð6Þ
O2 • þ H2 O ! •OH þ OH
ð7Þ
OH þ Organic pollutants ! ! CO2 þ H2 O
ð8Þ
The superiority of floating TiO2 ceramic was in its high recyclability. Therefore, its long-term stability during multiple cycling was also examined. Fig. 12 shows the photocatalytic degradation of RhB with floating TiO2 ceramic S800 over ten cycles. With 2 h of adsorption equilibrium and 3 h of UV irradiation per cycle, all RhB removal efficiencies were higher than 97%. In addition, for octane after ten cycles, all octane removal efficiencies were higher than 99%. Thus, the prepared floating porous TiO2 photocatalysts were easily recycled and retained high photocatalytic activity during reuse. In addition, besides octane and RhB the degradation of other types of organic pollutants was also investigated,
Fig. 12 Effect of floating photocatalyst cycle numbers on 10 mg L−1 RhB degradation by S800.
Fig. 13 Effect of floating photocatalysts on the TOC removal of 10 mg L−1 pollutants by S800.
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including phenol, thiobencarb, and atrazine. Fig. 13 indicates that a total organic carbon (TOC) removal efficiency of 98.8, 98.6, 97.9, 98.5, and 98.0% could be achieved for octane, RhB, phenol, thiobencarb, and atrazine, respectively. This result indicated that the prepared macro/mesoporous TiO2-ceramicbased floating photocatalysts had very wide applicability for the removal of organic pollutants from wastewater and could mineralize these pollutants completely. Therefore, these novel floating photocatalysts will likely trigger a strong interest in this field of research.
Conclusions Novel floating photocatalysts based on macro/mesoporous anatase TiO2 ceramics were successfully prepared using the high-thermal-stability and well-ordered mesoporous anatase TiO2 as a precursor. Sintering temperature is a key determinant of the pore structure, mechanical resistance, and photocatalytic activity of the floating photocatalysts. The photocatalytic activity of the samples was significantly affected by sintering temperature. The octane and RhB degradation process included adsorption and photocatalysis. The obtained photocatalysts exhibited excellent photocatalytic performance for both octane and RhB. The roles of active species in the photocatalytic process were compared by using different types of active species scavengers. At last, the floating photocatalysts could be reused several times with no significant decrease in RhB removal efficiency. Other organic pollutants, such as phenol, thiobencarb, and atrazine, could be mineralized effectively using the floating photocatalysts without any stirring or oxygenation. Therefore, the prepared floating photocatalyst has the potential to be used directly at the water surface using sunlight for the photocatalytic decontamination of recalcitrant organic pollutants in wastewater in the future.
Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (91122018, 21106035, 21101060, 21376065, and 21310402017), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708029), Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002 and 20112301120002), the Natural Science Foundation of Heilongjiang Province (QC2012C001 and QC2012C046), the Program for New Century Excellent Talents in University of Heilongjiang Province (1253-NECT-020), the China Postdoctoral Fund (2012M520780), the Harbin Science Foundation for Youth (2011RFQXS109 and RC2013QN001009), Heilongjiang Provincial Education Department Foundation (12511382), and the Heilongjiang University Youth Foundation (QL201018).
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Notes and references 1 S. Carbonaro, M. N. Sugihara and T. J. Strathmann, Appl. Catal., B Environ., 2013, 129, 1–12. 2 S. S. Lee, H. B. Z. Liu and D. D. Sun, Water Res., 2013, 47, 4059–4073. 3 Y. A. Chaban, M. A. E. Sayed, A. A. E. Maradny, R. K. A. Farawati and M. I. A. Zobidi, Chemosphere, 2013, 91, 307–313. 4 Z. Xing, D. Sun and X. Yu, Environ. Prog. Sustain., 2010, 29, 42–51. 5 M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas and A. M. Mayes, Nature, 2008, 452, 301–310. 6 T. Marino, R. Molinari and H. García, Catal. Today, 2013, 206, 40–45. 7 R. Daghrir, P. Drogui and D. Robert, Ind. Eng. Chem. Res., 2013, 52, 3581–3599. 8 H. Bai, Z. Liu, L. Liu and D. D. Sun, Chem.–Eur. J., 2013, 19, 3061–3070. 9 Q. Wu, J. Zhao, G. Qin, C. Wang, X. Tong and S. Xue, Appl. Catal., B Environ., 2013, 142–143, 142–148. 10 H. Sheng, Q. Li, W. Ma, H. Ji, C. Chen and J. Zhao, Appl. Catal., B Environ., 2013, 138–139, 212–218. 11 D. Chen, H. Zhang, Y. Liu and J. H. Li, Energy Environ. Sci., 2013, 6, 1362–1387. 12 A. Kubacka, M. F. García and G. Colón, Chem. Rev., 2012, 112, 1555–1614. 13 E. Baniasadi, I. Dincer and G. F. Naterer, Appl. Catal., A Gen., 2013, 455, 25–31. 14 H. S. Kibombo and R. T. Koodali, J. Phys. Chem. C, 2011, 115, 25568–25579. 15 M. Z. Chaleshtori, M. Hosseini, R. Edalatpour, S. M. S. Masud and R. R. Chianelli, Microchem. J., 2013, 110, 361–368. 16 Z. Xiong and X. S. Zhao, J. Mater. Chem. A, 2013, 1, 7738– 7744. 17 J. E. Yoo, K. Lee, M. Altomare, E. Selli and P. Schmuki, Angew. Chem., Int. Ed., 2013, 52, 7514–7517. 18 M. Sun, S. Xiong, X. Wu, C. He, T. Li and P. K. Chu, Adv. Mater., 2013, 25, 2035–2039. 19 D. Chen, H. Zhang, S. Hu and J. H. Li, J. Phys. Chem. C, 2008, 112, 117–122. 20 H. Wang, Y. Bai, H. Zhang, Z. Zhang, L. Guo and J. H. Li, J. Phys. Chem. C, 2010, 114, 16451–16455. 21 X. Luo, F. Deng, L. Min, S. Luo, B. Guo, G. Zeng and C. Au, Environ. Sci. Technol., 2013, 47, 7404–7412. 22 N. Zhou, L. Polavarapu, N. Gao, Y. Pan, P. Yuan, Q. Wang and Q. H. Xu, Nanoscale, 2013, 5, 4236. 23 C. Chen, P. Li, G. Wang, Y. Yu, F. Duan, C. Chen, W. Song, Y. Qin and M. Knez, Angew. Chem., Int. Ed., 2013, 52, 1–6. 24 H. Zhang, G. Wang, D. Chen, X. J. Lv and J. H. Li, Chem. Mater., 2008, 20, 6543–6549. 25 R. Chalasani and S. Vasudevan, ACS Nano, 2013, 5, 4093– 4104.
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26 Y. Tang, G. Zhang, C. Liu, S. Luo, X. Xu, L. Chen and B. Wang, J. Hazard. Mater., 2013, 252–253, 115–122. 27 H. Zhao, H. Li, H. Yu, H. Chang, X. Quan and S. Chen, Sep. Purif. Technol., 2013, 116, 360–365. 28 Y. Liu, J. Li, M. Wang, Z. Y. Li, H. Liu, P. He, X. R. Yang and J. H. Li, Cryst. Growth Des., 2005, 5, 1643–1649. 29 G. S. Pozan and A. Kambur, Appl. Catal., B Environ., 2013, 129, 409–415. 30 G. Zhang, W. Choi, S. H. Kim and S. B. Hong, J. Hazard. Mater., 2011, 188, 198–205. 31 A. Rey, D. H. Quiñones, P. M. Álvarez, F. J. Beltrán and P. K. Plucinski, Appl. Catal., B Environ., 2012, 111, 246–253. 32 A. A. Aziz, K. S. Yong, S. Ibrahim and S. Pichiah, J. Hazard. Mater., 2012, 199, 143–150. 33 H. Han and R. Bai, Ind. Eng. Chem. Res., 2009, 48, 2891– 2898. 34 L. C. R. Machado, C. B. Torchia and R. M. Lago, Catal. Commun., 2006, 7, 538–541. 35 F. Magalhães and R. M. Lago, Sol. Energy, 2009, 83, 1521– 1526. 36 L. C. R. Machado, C. B. Torchia and R. M. Lago, Catal. Commun., 2006, 7, 538–541. 37 S. C. Kim and D. K. Lee, Microchem. J., 2005, 80, 227–232. 38 R. J. Berry and M. R. Mueller, Microchem. J., 1994, 50, 28–32. 39 A. Modestov, V. Glezer, I. Marjasin and O. Lev, J. Phys. Chem. B, 1997, 101, 4623–4629. 40 Z. Xing, J. Li, Q. Wang, W. Zhou, G. Tian, K. Pan, C. Tian, J. Zou and H. Fu, Eur. J. Inorg. Chem., 2013, 2013, 2411– 2417. 41 W. Zhou, F. Sun, K. Pan, G. Tian, B. Jiang, Z. Ren, C. Tian and H. Fu, Adv. Funct. Mater., 2011, 21, 1922–1930. 42 Y. Shi, C. Hua, B. Li, X. Fang, C. Yao, Y. Zhang, Y. S. Hu, Z. Wang, L. Chen, D. Zhao and G. D. Stucky, Adv. Funct. Mater., 2013, 23, 1832–1838. 43 W. Zhou and H. Fu, ChemCatChem, 2013, 5, 885–894. 44 W. Li, Z. Wu, J. Wang, A. A. Elzatahry and D. Y. Zhao, Chem. Mater., 2013, DOI: 10.1021/cm4014859. 45 Z. Jiang, L. Kong, F. S. Alenazey, Y. Qian, L. France, T. Xiao and P. P. Edwards, Nanoscale, 2013, 5, 5396–5402. 46 APHA, Standard methods for the examination of water and wastewater, American Public Health Association, New York, 20th edn, 1998. 47 K. Cheng, W. Sun, H. Y. Jiang, J. Liu and J. Lin, J. Phys. Chem. C, 2013, 117, 14600–14607. 48 M. B. Sogo, M. Serra, A. Primo, M. Alvaro and H. Garcia, ChemCatChem, 2013, 5, 513–518. 49 A. J. Karabelas, V. C. Sarasidis and S. I. Patsios, Chem. Eng. J., 2013, 229, 484–491. 50 S. N. Hosseini, S. M. Borghei, M. Vossoughi and N. Taghavinia, Appl. Catal., B Environ., 2007, 74, 53–62. 51 Y. Lin, D. Li, J. Hu, G. Xiao, J. Wang, W. Li and X. Fu, J. Phys. Chem. C, 2012, 116, 5764–5772. 52 S. S. Umarea, A. Charanpaharia and R. Sasikalab, Mater. Chem. Phys., 2013, 140, 529–534.
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A floating macro/mesoporous crystalline anatase TiO2 ceramic with enhanced photocatalytic performance for recalcitrant wastewater degradation† Zipeng Xing,a,b Wei Zhou,a Fan Du,b Yang Qu,a Guohui Tian,a Kai Pan,a Chungui Tiana and Honggang Fu*a A macro/mesoporous anatase TiO2 ceramic floating photocatalyst has been successfully synthesized using highly thermally stable mesoporous TiO2 powder as a precursor, followed by a camphene-based freeze-casting process and high-temperature calcinations. The ceramics are characterized in detail by X-ray diffraction, Raman spectra, scanning electron microscopy, transmission electron microscopy and N2 adsorption–desorption isotherms. The results indicate that the TiO2 ceramics present hierarchical macro/ mesoporous structures, which maintain high porosity and high compressive strength at the optimal sintering temperature of 800 °C. The ordered mesoporous TiO2 network still possesses high thermal stability and inhibits the anatase-to-rutile phase transformation during calcinations. The obtained ceramics exhibit good adsorptive and photocatalytic activity for the degradation of octane and rhodamine B, and the total organic carbon removal ratio is up to 98.8% and 98.6% after photodegradation for 3 h, respectively. The roles of active species in the photocatalytic process are compared using different types of active species scavengers, and the degradation mechanism is also proposed. Furthermore, the ceramics are
Received 4th September 2013, Accepted 6th October 2013
recyclable, and no clear changes are observed after ten cycles. In addition, the ceramics are also active in
DOI: 10.1039/c3dt52433g
the photodegradation of phenol, thiobencarb, and atrazine. Therefore, these novel floating photocatalysts will have wide applications, including the removal of floating organic pollutants from the waste-
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water surfaces or the removal of soluble organic pollutants from wastewater.
Introduction Nowadays, organic contamination purification is one of the most interesting challenges in catalysis. A subject of global concern is the presence of harmful compounds in water supplies and in the discharge of wastewater from chemical industries, power plants, and agricultural sources.1–3 Some of these compounds are biologically recalcitrant and inhibitory organics, thus greatly reducing the ability of microorganisms
a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P.R. China. E-mail: [email protected]; Fax: +86-451-8667-3647; Tel: +86-451-8660-4330 b Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion, College of Heilongjiang Province, Heilongjiang University, Harbin 150080, P.R. China † Electronic supplementary information (ESI) available: Fig. S1 Schematic of the fabrication of macro/mesoporous crystalline TiO2 ceramic by camphene-based freeze-casting. Fig. S2 Images of floating photocatalysts based on hierarchical porous TiO2 ceramic in 10 mg L−1 RhB solution before (A) and after reaction (B). Table S1 Composition of TiO2 ceramic/camphene slurries used in this study. See DOI: 10.1039/c3dt52433g
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to biodegrade these compounds during treatment or in nature.4 All the present chemical treatment processes use either high-energy ultraviolet light or strong oxidants of seriously hazardous and undesirable nature.5,6 Numerous intermediates are formed in these processes, and due to the very low efficiency, the overall treatment cost becomes higher if the destruction of intermediates and complete mineralization are to be achieved. Therefore, water contaminated with these toxic and biologically recalcitrant organics requires new treatment technologies.7 The use of heterogeneous photocatalysts to degrade hazardous organic and inorganic compounds in wastewater is the most promising water-treatment technology in terms of exploitation and utilization.8–11 One of the most advanced oxidation processes that couples low-energy ultraviolet light with semiconductors such as TiO2 acting as photocatalysts is heterogeneous photocatalysis. Growing attention has been focused on this technology on account of the possibility of using solar radiation as the energy source for the decontamination of these effluents.12 From the environmental perspective, the use of solar energy to drive the destruction of pollutants has an absolute advantage.13,14 It also reduces the costs of the process
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which makes it competitive with other technologies for wastewater treatment, such as ozone or H2O2/UV-C.15 However, three severe restrictions hinder the practical application of TiO2, namely the separation of photoinduced charges and holes,16–20 the improvement of photon utilization21–24 and photocatalyst recycling.25–28 A large amount of energy and effort has been devoted to this subject, and more practical ways to use this technology on an industrial scale are needed. In particular, before the TiO2 particles can be used in applications, a simple and low-cost filtration step as well as easy aggregation, particularly at high slurry concentrations, must be found.29 To overcome the separation problem, a great deal of effort has been made in the past several years.30–32 Considering the intimate interaction between light and photocatalysts, floating photocatalysts are advantageous owing to their optimization of the illumination/light utilization process, particularly in a system that uses solar irradiation, and maximization of photocatalyst oxygenation on the basis of its proximity to the air–water interface, particularly for nonstirred reactions.33,34 The optimization of illumination and oxygenation should result in higher radical formation rates and oxidation efficiencies.35 The floating photocatalysts can be applied in solar remediation in situ (i.e., directly in the contaminated wastewater reservoirs located in remote places without any special equipment or installation). Floating photocatalysts can be used for more efficient destruction of suspended insoluble organic contaminants (e.g., in oil-spill accidents). To date, various materials have been used as lightweight substrates for the immobilization of TiO2 catalysts.36–38 However, an apparent drawback of these TiO2-coated substrates is the gradual detachment of TiO2 particles from the substrate surface by collisions and hydrodynamic turbulence.39 In this paper, we report on the design and fabrication of novel macro/mesoporous crystalline anatase TiO2 ceramics as floating photocatalysts by a camphene-based freeze-casting process for the first time. Our previously prepared floating photocatalyst40 had demonstrated that camphene as a threedimensionally interconnected frozen vehicle network could sublime and in turn leave macroporous channels in the ceramic body. These open-pore structures were of particular interest and importance owing to their excellent permeability, large surface area, low density, and high specific strength, which were conducive to the transfer and diffusion of pollutants. In addition, our previously synthesized well-ordered mesoporous TiO241 with high thermal stability was used as the precursor, which would improve photon utilization and organic molecule adsorption and increase the photocatalytic activity. Ordered mesoporous oxides had been widely used in the potential application fields of photocatalysis, due to their large surface area and ordered pore networks.42–44 In particular, mesoporous nanocrystalline TiO2 had been demonstrated to be a more effective photocatalyst, because of its environmental friendliness, large surface area, ordered porous structure and large pore volume, which results in increased surface reactive sites and improved photon absorption and organic molecule adsorption.45 Within the hierarchical porous ceramic, some
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perlite granules were added to provide closed pores and maintain the floatability of the photocatalysts. The final prepared macro/mesoporous anatase TiO2 ceramic exhibited both good mechanical stability and high photocatalytic activity.
Materials and methods Materials All reagents [i.e., polystyrene (PS), octane, rhodamine B (RhB), phenol, thiobencarb, and atrazine] were of analytical grade and used without further purification. Camphene (C10H16) (95% purity, Shanghai Haiqu Co. Ltd, Shanghai, China) was used as the freezing vehicle, and Texaphor 3250 (Guangzhou Haichuan Co. Ltd, Guangzhou, China) was used as the dispersant (density at 25 °C of 0.89–0.91 g cm−3). Ethanediamine (EN) modified well-ordered mesoporous anatase TiO2 powder with a remarkably high thermal stability and improved crystallinity was synthesized in our previous work.41 Preparation of the floating photocatalysts A schematic diagram of the fabrication of hierarchical porous crystalline TiO2 ceramic is presented in Fig. S1.† The prepared TiO2 powder/camphene/PS binder/dispersant slurries were prepared by ball-milling at 60 °C for 24 h (The planetary ball mill was used. The ball mill jar and ball was agate, and the compound-to-ball weight ratio was 0.84. The raw material was added into the ball mill jar, and then the agate ball was added. The jar and jar cover was pressed compactly by the bolt. The rotation speed was set to 100 r min−1. During the milling, the jar and jar cover had to be kept sealed, as otherwise the liquid camphene would volatilize.), and the compositions of the TiO2/camphene slurries used in this study are summarized in Table S1.† Next, the granular expanded perlite (20–30 mesh) was added into the slurries and mixed uniformly before being poured into gum-packing paper molds (1 × 2 × 6 cm) for freezing. The samples were left to cool at room temperature [(20 ± 2) °C] for 30 min. After solidification, the green body was removed from the molds. Then, the green body was cut into uniform size pieces (5 × 5 × 5 mm), which were pressed into sheets by mechanical force with 0.5 MPa. The samples were then sublimed for 48 h at room temperature to completely remove the camphene. Following sublimation, the green bodies were sintered at 1 °C min−1 up to 600 °C followed by dwelling time for 1 h. They were then heated at 2 °C min−1 up to 700, 800, and 900 °C for 2 h, respectively. As shown in Fig. 1, the obtained ceramic had a hierarchical macro/mesoporous structure. Fig. S2(A)† indicates that the hierarchical porous crystalline TiO2 ceramic, which was floatable in water, was successfully synthesized. Its average dimension was 8 × 8 × 2 mm, and the average mass of one piece was 20 mg. Characterization of the floating photocatalysts X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-IIIB diffractometer using Cu-Kα radiation (λ = 1.5406 Å).
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Fig. 1 Schematic diagram of the hierarchical macro/mesoporous crystalline TiO2 ceramic structure.
Raman measurements were performed using a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. The laser beam was focused with a 50× objective lens to a ca. 1 μm spot on the surface of the sample. Scanning electron microscopy (SEM) micrographs were obtained using a Philips XL-30-ESEM-FEG instrument operating at 20 kV. The TiO2 ceramic sample was cut into pieces, and one piece was chosen to paste on the steel plate by using double coated copper tape. Then the steel plate was coated with gold by sputtering for 5 min. The prepared steel plate samples were analyzed by SEM. The apparent density and porosity of sintered bodies were measured by the Archimedes method. Test samples of TiO2 ceramics were loaded onto a testing machine (Instron 5569, Instron Corp., Canton, OH) to measure the compressive strengths with a crosshead speed of 0.2 mm min−1. The average of at least five samples was used for each measurement. Transmission electron microscopy (TEM) was performed using a JEM-2100 electron microscope (JEOL, Japan). The TiO2 ceramic samples were first ground by an agate mortar. The obtained ceramic samples were dispersed in absolute alcohol by ultrasound for 30 min. One or two dispersed drops were applied to a copper mesh microgrid. After drying, the prepared micro-grid samples were analyzed by TEM. Nitrogen adsorption–desorption isotherms at 77 K were collected with an AUTOSORB-1 (Quantachrome Instruments) nitrogen adsorption apparatus. The Brunauer–Emmett–Teller (BET) equation was used to calculate the specific surface area. Total organic carbon (TOC) analysis was carried out using a TOC-VCPN (Shimadzu, Japan) analyzer with the minimum detection limit of 0.05 mg L−1 in accordance with standard methods.46 Photocatalytic activity test As shown in Fig. S2(A),† the prepared TiO2 ceramics could float on the water well (i.e., they were slightly submerged, which was conducive to light utilization). Eighty pieces of the prepared TiO2 ceramic were covered on a glass culture dish reactor (9 cm diameter) with a reaction solution (30 mL). The reactor was open to the ambient air to achieve air-equilibration (Fig. 2(B)). A 20 W, 365 nm UV lamp was placed closely above the reactor with a 240 μW cm−2 average light intensity. Octane and rhodamine B (RhB) had been found to be potentially toxic and carcinogenic. Thus, octane and RhB were mainly chosen as the model recalcitrant pollutants to evaluate the photocatalytic activity of the as-prepared floating photocatalysts. After different photocatalytic reaction durations, the
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Fig. 2 XRD patterns of TiO2 ceramic prepared at 700, 800, and 900 °C, respectively.
octane concentration was analyzed by Shimadzu Model GC-14B gas chromatography and the RhB concentration was measured at 553 nm using a Shimadzu Model UV2550 spectrophotometer. 1 mL and 0.1 mol L−1 of ammonium oxalate, benzoquinone and tert-butyl alcohol were added into the photocatalytic system to examine the photocatalytic mechanism, respectively. In addition, the mineralization ratios of octane, RhB, phenol, thiobencarb, and atrazine were also examined by TOC-VCPN after 2 h of adsorption and 3 h of photocatalytic reaction. All photocatalytic experiments were performed at room temperature [(20 ± 2) °C].
Results and discussion Structure and characteristics of macro/mesoporous TiO2 ceramic The anatase-to-rutile phase transformation in TiO2 had attracted much attention because the phase structure of TiO2 nanostructures largely determined their suitability for practical applications.47 In general, the crystal phase and crystallization of TiO2 depended strongly on the sintering temperature. Fig. 2 shows the XRD patterns of the TiO2 ceramics prepared at different calcination temperatures. 700, 800 and 900 °C calcined samples are denoted as S700, S800 and S900, respectively. In Fig. 2 (S700 and S800), five characteristic XRD peaks of anatase TiO2 were observed at 25.2, 37.8, 48.1, 53.9 and 56.1°, which could be indexed to (101), (004), (200), (105), and (211), respectively. The rutile phase was present (S900) during calcinations at 900 °C, which was reasonable because rutile TiO2 was more thermodynamically stable. That is to say, rutile was the final phase of TiO2 after being calcined at high temperature. Anatase could be further crystallized during the calcination; the crystallinity of anatase TiO2 was improved obviously (as shown in Fig. 2). According to our previous report,41 we knew that ethanediamine (EN) species could
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interact with crystalline TiO2 particles strongly and bind to the surface of TiO2. EN was also considered to be an effective protector to inhibit the undesirable grain growth and phase transformation of anatase TiO2. Therefore, EN species, which were strong alkalis with two primary amine groups and two positive charges, would attack the surface of TiO2 particles strongly. Under the protection of EN, the TiO2 nanoparticles could not come into direct contact with each other, which would thus prolong the improvement of crystallinity for anatase and retard the phase transformation of anatase to rutile. The anatase-torutile phase transformation began when the sintering temperature exceeded 800 °C. This result further suggested that the EN species indeed protected the TiO2 particles, thus improving the crystallinity of anatase and slowing the anatase-to-rutile transformation. Raman spectroscopy, which was very sensitive to the crystallinity and microstructure of materials, was usually used to unambiguously discriminate the local order characteristics of TiO2.48 In order to further confirm the high crystallinity of the obtained macro/mesoporous TiO2 samples, Raman detection was carried out. Fig. 3 shows the Raman spectra of hierarchical porous TiO2 ceramic with different calcination temperatures. Under the calcination temperature of 800 °C, we could clearly see that five high intensity Raman peaks at 149, 199, 393, 513, and 639 cm−1 could be ascribed to Eg, Eg, B1g, A1g(B1g), and Eg modes, respectively, which were characteristic of anatase TiO2. Apart from that, no other Raman peaks could be observed, indicating the phase-pure anatase. Moreover, the intensity of Raman peaks increased with the increase of calcination temperature, suggesting the improvement of crystallinity of anatase, which was in good agreement with the XRD results. When the calcination temperature was over 900 °C, four different Raman peaks at 143, 244, 442, and 609 cm−1 could be observed and ascribed to B1g, the multi-photon process, Eg, and A1g modes, respectively, which were characteristic of rutile TiO2. This confirmed that the transformation was from anatase to rutile under this calcination
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Fig. 4 SEM (A) and magnified SEM images (B) of the hierarchical porous TiO2 ceramics at a sintering temperature of 800 °C.
temperature. These results were well consistent with the XRD characterization. The microstructure evolution of sintered TiO2 ceramic at 800 °C was further studied by SEM analysis. Many macropores could be observed in Fig. 4(A), and these macropores were interconnected. It just originated from the sublimation of camphene crystals and leaves pore channels in the ceramic body. In order to value the pore size, SEM micrographs in Fig. 4(B) were enlarged and the pore size was measured. The pore size of the calcined TiO2 ceramic was 0.1–2 μm, implying that the materials were in the macroporous region. The sintering temperature was important in determining the porosity and mechanical strength of the sintered samples. The effect of the sintering temperature on the porosity and compressive strength was also investigated, and the results are shown in Table 1. When the sintering temperature was increased from 700 to 900 °C, the porosity decreased from 98.5 to 86.7%, and the compressive strength increased from 0.27 to 1.19 MPa. It was widely accepted that higher porosity would improve pollutant transmission and the permeability of porous ceramic, and
Table 1 Porosity and compressive strength of porous TiO2 ceramic after being calcined at 700, 800, and 900 °C, respectively
Fig. 3 Raman spectra of TiO2 ceramic with different calcination temperatures.
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Parameters
S700
S800
S900
Porosity (%) Compressive strength (MPa)
98.5 0.27
94.2 0.66
86.7 1.19
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Fig. 5 TEM (A, B, C) and HRTEM (D) images of hierarchical porous TiO2 ceramics prepared at 800 °C (anatase: d101 = 0.35 nm).
a compressive strength of at least 0.6 MPa was desirable for floating photocatalysts to resist hydrodynamic turbulence. This illustrated that the obtained floating photocatalysts possessed high mechanical stability. Therefore, high ceramic porosity and compressive strength could be obtained after being calcined at 800 °C. The morphology and porosity of the sample were further investigated by TEM. Fig. 5 showed the TEM and HRTEM images of porous TiO2 samples after being calcined at 800 °C. TEM images of TiO2 ceramic after calcinations exhibited a 2D hexagonal mesostructure with P6mm symmetry, which possessed highly ordered mesopores of about 10 nm in diameter. It also showed a high degree of periodicity of 2D hexagonal arrangements over large domains. From the HRTEM images (Fig. 5(D)), we could see that the lattice fringes corresponding to the (101) (d101 = 0.35 nm) crystallographic planes of anatase were most frequently observed in S800, which indicated the high crystallinity of the pore walls of mesoporous TiO2. The nanocrystalline nature of anatase TiO2 was well defined. These results were consistent with the XRD and Raman findings. Such a high anatase crystallinity of the mesoporous TiO2 was highly desirable in photocatalysis. Fig. 6 showed the N2 adsorption–desorption isotherms (A) and the corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plots (B) of mesoporous TiO2 with different calcination temperatures. At a relatively high pressure, the curves exhibited small hysteresis loops, which were attributed to type IV isotherms and were representative of mesoporous materials. The calcined hierarchical porous TiO2 ceramic had a narrow BJH adsorption pore size distribution with a mean value of 10 nm, implying that the materials had very regular pore channels in the mesoporous region. With the increase of
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Fig. 6 (A) N2 adsorption–desorption isotherms and (B) the corresponding BJH pore size distribution plots of mesoporous anatase TiO2 with different calcination temperatures.
calcination temperature, the pore size distribution of the samples becomes smaller, as shown in Fig. 6(B). The specific BET surface area also decreased from 142 to 63 m2 g−1 from S700 to S900. The total pore volumes decreased from 0.29 to 0.0043 cm3 g−1. These differences were attributed to the collapse of some mesopores, contraction of others, and the agglomeration of particles during the thermal treatment process. Interestingly, S800 possessed an integrated and wellordered mesoporous framework, with a specific BET surface area of 97 m2 g−1 and total pore volumes of 0.15 cm3 g−1. Due to the large density of crystalline TiO2, this value (BET surface area) was very high for mesoporous TiO2 with high thermal stability and crystallinity. In fact, we could also find from Fig. 5 that the contraction of mesopores during the thermal treatment process was not serious. This was because the bonded N-containing species on a mesostructured network protected the framework of mesoporous TiO2, and subsequently maintained a relatively high surface area and integrated porous structure, which were beneficial for photocatalytic reactions.
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Photocatalytic performance for recalcitrant wastewater degradation When the sintering temperature was 700 °C, the prepared porous TiO2 ceramic began to fall to pieces in the water easily. It was due to the low compressive strength of S700. To better understand the relationship between the sintering temperature and photocatalytic performance, the effect of 800 and 900 °C sintering temperatures on floating octane degradation was examined. As shown in Fig. 7, after 2 h of dark adsorption equilibrium, the initial octane could be removed 43.5 and 25.7% for S800 and S900, respectively. The mesoporous structure of TiO2 ceramic could be beneficial for pollutant adsorption. The adsorption performance of S800 was better than S900; the main reason was that the specific BET surface of S800 (97 m2 g−1) was higher than that of S900 (63 m2 g−1). After 3 h of photocatalysis, the total octane removal ratio of S800 was significantly better than that of S900. S800 achieved a very high octane removal ratio of 99.9%. However, S900 removed only 75.4% of octane. This result could be attributed to the fact that S800 had a better adsorption performance than S900 on the one hand. In general, the large specific surface area facilitated the absorption and utilization of light, and also offered more active sites. Previous studies41 illustrated that a suitable conformation of pores allowed light waves to penetrate the catalyst deeply and leads to high mobility of photogenerated charges. It could be speculated that the vertical mesopores in the porous TiO2 ceramic are beneficial for the penetration of light waves and octane molecules deep into the photocatalyst, which could promote their photocatalytic performance greatly. On the other hand, S800 maintained high crystallinity and pure anatase phase, which had a higher photocatalytic activity. The high crystallinity meant fewer defects, which could be responsible for the lower recombination of photogenerated hole–electron pairs. Although the crystallinity of S900 was the best one, the rutile phase could decrease the photocatalytic activity. In addition, when the octane solution was exposed to the UV lamp alone, the octane concentration remained constant for 5 h, which suggested that
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UV photolysis could not decompose octane directly. Therefore, the main octane degradation mechanism included the adsorption and photocatalysis process. The photocatalytic kinetics had been proposed to follow a pseudo-first-order model. The integration of the equation under this assumption with boundary conditions of C = C0 at t = 0 yields (see eqn (1)) lnðC=C0 Þ ¼ kapp t
ð1Þ
in which C0 is the initial substrate concentration and kapp is the apparent first-order reaction rate. Many photocatalytic reactions exhibited this behavior.49,50 Fig. 8 showed the plots of ln(C/C0) as a function of reaction time with different catalysts or without catalysts for octane degradation. A linear trend was observed for both S800 and S900, which suggested that the photocatalytic degradation of octane under these reaction conditions followed pseudo-firstorder kinetics. The calculated apparent rate constant (the slopes from the plots) and the corresponding coefficient of determination (R2) are shown in Table 2. The apparent rate constant for S800 was 6.11 times higher than that of S900 implying the high photocatalytic performance. To further validate the floating photocatalyst performances, the 10 mg L−1 of soluble RhB degradation was also studied by S800 and S900. As shown in Fig. 9, after 2 h of dark adsorption equilibrium, the initial RhB could be removed 32.2 and 11.1% for S800 and S900, respectively. After 3 h photocatalysis, S800 achieved a very high RhB removal ratio of 99.4%. However, S900 removed only 63.6% of RhB. No matter their adsorption
Fig. 8 Value of ln(C/C0) versus reaction time with different catalysts or without a catalyst on octane degradation.
Table 2 Reaction kinetics equations and linear regression coefficients with photocatalysts prepared at different sintering temperatures for octane degradation
Fig. 7 Time profiles of the photocatalytic degradation of octane with different catalysts and without a catalyst.
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Samples
Reaction kinetics equations
kapp (h−1)
R2
S800 S900
ln(C/C0) = −2.20t + 0.0434 ln(C/C0) = −0.36t + 0.0368
2.20 0.36
0.9952 0.9912
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Fig. 9 Time profiles of the photocatalytic degradation of RhB with different catalysts and without a catalyst.
or photocatalysis ability, the S800 was better than S900. The obtained results were well consistent with octane degradation. Fig. 10 shows the plots of ln(C/C0) as a function of the reaction time with RhB degradation for photocatalysis, thereby revealing that RhB removal reactions also followed pseudofirst-order kinetics. Table 3 summarizes the apparent rate constant and the corresponding coefficient of determination (R2) for different photocatalysts after 3 h of UV illumination; the apparent rate constant for S800 was 5.14 times higher than that of S900. By comparison, we found that the apparent rate constant of octane removal was higher than RhB removal for S800 and S900, respectively. It indicated that the
Fig. 10 Value of ln(C/C0) versus reaction time with different catalysts or without a catalyst on RhB degradation.
Table 3 Reaction kinetics equations and linear regression coefficients with photocatalysts prepared at different sintering temperatures for RhB degradation
Samples
Reaction kinetics equations
kapp (h−1)
R2
S800 S900
ln(C/C0) = −1.49t + 0.0509 ln(C/C0) = −0.29t + 0.0123
1.49 0.29
0.9930 0.9897
796 | Dalton Trans., 2014, 43, 790–798
Fig. 11 Time profiles of the photocatalytic degradation of RhB with different active species scavengers. (BQ: benzoquinone, AO: ammonium oxalate, and TBA: tert-butyl alcohol.)
photodegradation rate of octane was faster than RhB. One possible explanation was that the octane was floating on the water surface and it could close contact with the floating photocatalyst more favorably than soluble RhB. It was helpful to understand the photocatalytic mechanism of floating TiO2 ceramic by the analysis of the photocatalytic degradation process. As shown in eqn (2)–(8), the hierarchical porous TiO2 ceramic could generate a large number of radical species with a strong oxidation capability in the water.51,52 To further evaluate the role of these active species such as hVB+, O2•− and •OH, different types of active species scavengers were added to the catalyst system. Fig. 11 shows the photocatalytic activity of porous TiO2 ceramic toward the degradation of RhB under different conditions. Benzoquinone (BQ) had the ability to trap O2•− by a simple electron transfer mechanism. After BQ was added into the reaction system, the rate of degradation of RhB over the TiO2 ceramic was remarkably decreased. Hierarchical porous TiO2 ceramic could generate the electrons and holes under UV light irradiation. The electrons could react with molecules of O2 on the surface of TiO2 to form a superoxide anion radical (O2•−). The prepared floating TiO2 ceramic was at the air–water interface; thus the generated electrons could react with the dissolved O2 easily to form the O2•− radical. It played an important role in the photocatalytic reaction. The addition of ammonium oxalate (AO) as a hole-scavenger provoked partial inhibition of the RhB degradation as shown in Fig. 11. After tert-butyl alcohol (TBA) as a scavenger for •OH was added in the system, it did not obviously affect the decomposition rate at all over the photocatalytic system. So, RhB oxidation was driven by the contribution of •OH radicals to a lesser extent. To sum up, through the comparison, we could conclude that RhB oxidation was driven mainly by the participation of O2•− and hole radicals, and to a lesser extent by the contribution of •OH radicals. TiO2 þ hν ! hVB þ þ eCB
ð2Þ
hVB þ þ Organic pollutants ! ! CO2 þ H2 O
ð3Þ
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eCB þ O2 ! O2 •
ð4Þ
O2 • þ Organic pollutants ! ! CO2 þ H2 O þ
•
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hVB þ H2 O ! OH þ H
•
þ
ð5Þ ð6Þ
O2 • þ H2 O ! •OH þ OH
ð7Þ
OH þ Organic pollutants ! ! CO2 þ H2 O
ð8Þ
The superiority of floating TiO2 ceramic was in its high recyclability. Therefore, its long-term stability during multiple cycling was also examined. Fig. 12 shows the photocatalytic degradation of RhB with floating TiO2 ceramic S800 over ten cycles. With 2 h of adsorption equilibrium and 3 h of UV irradiation per cycle, all RhB removal efficiencies were higher than 97%. In addition, for octane after ten cycles, all octane removal efficiencies were higher than 99%. Thus, the prepared floating porous TiO2 photocatalysts were easily recycled and retained high photocatalytic activity during reuse. In addition, besides octane and RhB the degradation of other types of organic pollutants was also investigated,
Fig. 12 Effect of floating photocatalyst cycle numbers on 10 mg L−1 RhB degradation by S800.
Fig. 13 Effect of floating photocatalysts on the TOC removal of 10 mg L−1 pollutants by S800.
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including phenol, thiobencarb, and atrazine. Fig. 13 indicates that a total organic carbon (TOC) removal efficiency of 98.8, 98.6, 97.9, 98.5, and 98.0% could be achieved for octane, RhB, phenol, thiobencarb, and atrazine, respectively. This result indicated that the prepared macro/mesoporous TiO2-ceramicbased floating photocatalysts had very wide applicability for the removal of organic pollutants from wastewater and could mineralize these pollutants completely. Therefore, these novel floating photocatalysts will likely trigger a strong interest in this field of research.
Conclusions Novel floating photocatalysts based on macro/mesoporous anatase TiO2 ceramics were successfully prepared using the high-thermal-stability and well-ordered mesoporous anatase TiO2 as a precursor. Sintering temperature is a key determinant of the pore structure, mechanical resistance, and photocatalytic activity of the floating photocatalysts. The photocatalytic activity of the samples was significantly affected by sintering temperature. The octane and RhB degradation process included adsorption and photocatalysis. The obtained photocatalysts exhibited excellent photocatalytic performance for both octane and RhB. The roles of active species in the photocatalytic process were compared by using different types of active species scavengers. At last, the floating photocatalysts could be reused several times with no significant decrease in RhB removal efficiency. Other organic pollutants, such as phenol, thiobencarb, and atrazine, could be mineralized effectively using the floating photocatalysts without any stirring or oxygenation. Therefore, the prepared floating photocatalyst has the potential to be used directly at the water surface using sunlight for the photocatalytic decontamination of recalcitrant organic pollutants in wastewater in the future.
Acknowledgements We gratefully acknowledge the support of this research by the Key Program Projects of the National Natural Science Foundation of China (21031001), the National Natural Science Foundation of China (91122018, 21106035, 21101060, 21376065, and 21310402017), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (708029), Special Research Fund for the Doctoral Program of Higher Education of China (20112301110002 and 20112301120002), the Natural Science Foundation of Heilongjiang Province (QC2012C001 and QC2012C046), the Program for New Century Excellent Talents in University of Heilongjiang Province (1253-NECT-020), the China Postdoctoral Fund (2012M520780), the Harbin Science Foundation for Youth (2011RFQXS109 and RC2013QN001009), Heilongjiang Provincial Education Department Foundation (12511382), and the Heilongjiang University Youth Foundation (QL201018).
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Notes and references 1 S. Carbonaro, M. N. Sugihara and T. J. Strathmann, Appl. Catal., B Environ., 2013, 129, 1–12. 2 S. S. Lee, H. B. Z. Liu and D. D. Sun, Water Res., 2013, 47, 4059–4073. 3 Y. A. Chaban, M. A. E. Sayed, A. A. E. Maradny, R. K. A. Farawati and M. I. A. Zobidi, Chemosphere, 2013, 91, 307–313. 4 Z. Xing, D. Sun and X. Yu, Environ. Prog. Sustain., 2010, 29, 42–51. 5 M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Mariñas and A. M. Mayes, Nature, 2008, 452, 301–310. 6 T. Marino, R. Molinari and H. García, Catal. Today, 2013, 206, 40–45. 7 R. Daghrir, P. Drogui and D. Robert, Ind. Eng. Chem. Res., 2013, 52, 3581–3599. 8 H. Bai, Z. Liu, L. Liu and D. D. Sun, Chem.–Eur. J., 2013, 19, 3061–3070. 9 Q. Wu, J. Zhao, G. Qin, C. Wang, X. Tong and S. Xue, Appl. Catal., B Environ., 2013, 142–143, 142–148. 10 H. Sheng, Q. Li, W. Ma, H. Ji, C. Chen and J. Zhao, Appl. Catal., B Environ., 2013, 138–139, 212–218. 11 D. Chen, H. Zhang, Y. Liu and J. H. Li, Energy Environ. Sci., 2013, 6, 1362–1387. 12 A. Kubacka, M. F. García and G. Colón, Chem. Rev., 2012, 112, 1555–1614. 13 E. Baniasadi, I. Dincer and G. F. Naterer, Appl. Catal., A Gen., 2013, 455, 25–31. 14 H. S. Kibombo and R. T. Koodali, J. Phys. Chem. C, 2011, 115, 25568–25579. 15 M. Z. Chaleshtori, M. Hosseini, R. Edalatpour, S. M. S. Masud and R. R. Chianelli, Microchem. J., 2013, 110, 361–368. 16 Z. Xiong and X. S. Zhao, J. Mater. Chem. A, 2013, 1, 7738– 7744. 17 J. E. Yoo, K. Lee, M. Altomare, E. Selli and P. Schmuki, Angew. Chem., Int. Ed., 2013, 52, 7514–7517. 18 M. Sun, S. Xiong, X. Wu, C. He, T. Li and P. K. Chu, Adv. Mater., 2013, 25, 2035–2039. 19 D. Chen, H. Zhang, S. Hu and J. H. Li, J. Phys. Chem. C, 2008, 112, 117–122. 20 H. Wang, Y. Bai, H. Zhang, Z. Zhang, L. Guo and J. H. Li, J. Phys. Chem. C, 2010, 114, 16451–16455. 21 X. Luo, F. Deng, L. Min, S. Luo, B. Guo, G. Zeng and C. Au, Environ. Sci. Technol., 2013, 47, 7404–7412. 22 N. Zhou, L. Polavarapu, N. Gao, Y. Pan, P. Yuan, Q. Wang and Q. H. Xu, Nanoscale, 2013, 5, 4236. 23 C. Chen, P. Li, G. Wang, Y. Yu, F. Duan, C. Chen, W. Song, Y. Qin and M. Knez, Angew. Chem., Int. Ed., 2013, 52, 1–6. 24 H. Zhang, G. Wang, D. Chen, X. J. Lv and J. H. Li, Chem. Mater., 2008, 20, 6543–6549. 25 R. Chalasani and S. Vasudevan, ACS Nano, 2013, 5, 4093– 4104.
798 | Dalton Trans., 2014, 43, 790–798
Dalton Transactions
26 Y. Tang, G. Zhang, C. Liu, S. Luo, X. Xu, L. Chen and B. Wang, J. Hazard. Mater., 2013, 252–253, 115–122. 27 H. Zhao, H. Li, H. Yu, H. Chang, X. Quan and S. Chen, Sep. Purif. Technol., 2013, 116, 360–365. 28 Y. Liu, J. Li, M. Wang, Z. Y. Li, H. Liu, P. He, X. R. Yang and J. H. Li, Cryst. Growth Des., 2005, 5, 1643–1649. 29 G. S. Pozan and A. Kambur, Appl. Catal., B Environ., 2013, 129, 409–415. 30 G. Zhang, W. Choi, S. H. Kim and S. B. Hong, J. Hazard. Mater., 2011, 188, 198–205. 31 A. Rey, D. H. Quiñones, P. M. Álvarez, F. J. Beltrán and P. K. Plucinski, Appl. Catal., B Environ., 2012, 111, 246–253. 32 A. A. Aziz, K. S. Yong, S. Ibrahim and S. Pichiah, J. Hazard. Mater., 2012, 199, 143–150. 33 H. Han and R. Bai, Ind. Eng. Chem. Res., 2009, 48, 2891– 2898. 34 L. C. R. Machado, C. B. Torchia and R. M. Lago, Catal. Commun., 2006, 7, 538–541. 35 F. Magalhães and R. M. Lago, Sol. Energy, 2009, 83, 1521– 1526. 36 L. C. R. Machado, C. B. Torchia and R. M. Lago, Catal. Commun., 2006, 7, 538–541. 37 S. C. Kim and D. K. Lee, Microchem. J., 2005, 80, 227–232. 38 R. J. Berry and M. R. Mueller, Microchem. J., 1994, 50, 28–32. 39 A. Modestov, V. Glezer, I. Marjasin and O. Lev, J. Phys. Chem. B, 1997, 101, 4623–4629. 40 Z. Xing, J. Li, Q. Wang, W. Zhou, G. Tian, K. Pan, C. Tian, J. Zou and H. Fu, Eur. J. Inorg. Chem., 2013, 2013, 2411– 2417. 41 W. Zhou, F. Sun, K. Pan, G. Tian, B. Jiang, Z. Ren, C. Tian and H. Fu, Adv. Funct. Mater., 2011, 21, 1922–1930. 42 Y. Shi, C. Hua, B. Li, X. Fang, C. Yao, Y. Zhang, Y. S. Hu, Z. Wang, L. Chen, D. Zhao and G. D. Stucky, Adv. Funct. Mater., 2013, 23, 1832–1838. 43 W. Zhou and H. Fu, ChemCatChem, 2013, 5, 885–894. 44 W. Li, Z. Wu, J. Wang, A. A. Elzatahry and D. Y. Zhao, Chem. Mater., 2013, DOI: 10.1021/cm4014859. 45 Z. Jiang, L. Kong, F. S. Alenazey, Y. Qian, L. France, T. Xiao and P. P. Edwards, Nanoscale, 2013, 5, 5396–5402. 46 APHA, Standard methods for the examination of water and wastewater, American Public Health Association, New York, 20th edn, 1998. 47 K. Cheng, W. Sun, H. Y. Jiang, J. Liu and J. Lin, J. Phys. Chem. C, 2013, 117, 14600–14607. 48 M. B. Sogo, M. Serra, A. Primo, M. Alvaro and H. Garcia, ChemCatChem, 2013, 5, 513–518. 49 A. J. Karabelas, V. C. Sarasidis and S. I. Patsios, Chem. Eng. J., 2013, 229, 484–491. 50 S. N. Hosseini, S. M. Borghei, M. Vossoughi and N. Taghavinia, Appl. Catal., B Environ., 2007, 74, 53–62. 51 Y. Lin, D. Li, J. Hu, G. Xiao, J. Wang, W. Li and X. Fu, J. Phys. Chem. C, 2012, 116, 5764–5772. 52 S. S. Umarea, A. Charanpaharia and R. Sasikalab, Mater. Chem. Phys., 2013, 140, 529–534.
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