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TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification
Materials Chemistry and Physics xxx (2016) 1e8
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification Qiao Zhang, Zhufeng Lu, Shalang Jin, Yitian Zheng, Tongtong Ye, Duoduo Yang, Yiming Li, Longfeng Zhu, Lianwen Zhu* School of Biology and Chemical Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity were prepared. The TiO2-carbon monoliths have adsorption function and photocatalytic function. The TiO2-carbon macroscopic monoliths greatly facilitate organic wastewater purification.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2015 Received in revised form 1 February 2016 Accepted 10 February 2016 Available online xxx
In this paper, we have shown a facile strategy for the construction of porous TiO2 nanotube-carbon macroscopic monoliths (TNCMs). The resulting macroscopic materials not only possess multimodal porosity, large surface area, and good optical properties, but also combine the advantages of both adsorption materials and photocatalytic materials. Furthermore, the TNCMs can be easily recovered and the robustness of the TNCMs allows them to be reused. Therefore, the as-prepared TNCMs greatly facilitate the purification of organic wastewater, which should have potential for real-world water purification. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microporous materials Chemical synthesis Nanostructures Oxidation
1. Introduction Due to the rapid population growth and increasing environmental degradation, water crisis is becoming one of the top issues and is probably expected to be the most baffling problem in the future [1e3]. Therefore, increasing attention has been paid to the development of efficient water purification materials. In order to meet the demand of real world water purification, the essential features of * Corresponding author. E-mail address: [email protected] (L. Zhu).
advanced water purification materials should be high efficiency, easy recovery, high durability, low cost, and environment beneath [4,5]. To date, various kinds of functional materials have been applied to remove the pollutants from contaminated water, including photocatalytic materials, adsorption materials, and separation materials, etc [6e11]. Among all the functional materials proposed, adsorption materials is one of the most popular candidates, because adsorption is currently considered as an effective, efficient, and easy technique for water purification [9]. However, traditional powder adsorbents need to be dispersed into the polluted water and therefore it is impractical to use them to purify rivers or lakes
http://dx.doi.org/10.1016/j.matchemphys.2016.02.037 0254-0584/© 2016 Elsevier B.V. All rights reserved.
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because recovery is complicated and expensive. Furthermore, adsorption process only transfers pollutants from water phase to adsorbent rather than eliminating them from the environment. In addition, the regeneration of exhausted adsorbents by chemical and thermal procedure is required, which is expensive and highenergy consuming [9]. It is widely known that photocatalysts can effectively oxidize a wide range of organic contamination into non-toxic byproducts such as H2O, CO2, and mineral acids at ambient conditions [8]. Therefore, it is anticipated that the incorporation of adsorption materials with photocatalytic materials will give rise to a novel photocatalytic adsorbents, which would combine the advantages of both techniques. On one hand, photocatalysts can easily decompose the pollutants anchored on adsorbents under light illumination, therefore the problem about the high cost regeneration of exhausted adsorbents can be effectively addressed; on the other hand, large surface area adsorbents would concentrate the pollutants around the photocatalysts, leading to an enhancement of the photodecomposition rate [12]. Benefiting from their high surface area, porous structure, and special surface reactivity, carbon adsorbents (activated carbon, graphene, carbon nanotubes, carbon nanofibers, etc) have been the most popular candidate and are expected to be widely used in water treatment area [13e17]. Furthermore, comparing with other kind of adsorbents, carbon units can be easily assembled into macroscopic materials, including monoliths, membranes, sponge, aerogel and hydrogel, leading to the formation of easy recovery adsorption materials. TiO2 is one of the most popular photocatalysts with distinct photochemical activities, which for many years have been intensively exploited for the purification of contaminated water [18,19]. Therefore, combination of TiO2 with carbon macroscopic materials would give rise to a kind of advanced water purification materials with both adsorption and cleaning functions. To date, several macroscopic TiO2-carbon composites have been achieved, where TiO2 is composed of irregular nanoparticles [20,21]. Compared with nanoparticles, TiO2 nanotubes possess higher photocatalytic performance because of their superior charge transport properties [22], larger surface area, and multiple pathways for the diffusion of molecules and ions [23]. Furthermore, the pore structure of the reported TiO2-carbon composites is monotonous. Compared with irregular pore structures, multimodal porous structure permit faster and efficient diffusion of molecules and ions, leading to an obvious enhancement of the photocatalytic activity and adsorption performance [24e27]. Kazuya Nakata and coworkers reported that TiO2 monoliths with macropores and mesopores possess superior water purification performance [28]. Highly porous TiO2-based filters and membranes were used to fabricate effective photocatalytic reactor for environmental purification [29,30]. In addition, expensive CNTs or graphene oxide and organic titanium compounds were used as carbon source and titanium source in many previous reported macroscopic TiO2-carbon structure system, which increase the production cost [21,31]. Therefore, macroscopic TiO2 nanotube-carbon composites with multimodal pore structures are anticipated to be a kind of advanced water purification materials due to their several obvious advantages. (i) Easy recovery owing to the macroscopic size; (ii) Self-regeneration ability because of the photocatalytic feature of TiO2 nanotubes; (iii) Outstanding purification performance owing to the multimodal pore structures. However, as far as we know, the construction of multimodal porous TiO2 nanotube-carbon macroscopic composites is still a big challenge. In this work, a scalable approach is reported for the construction of multimodal porous TiO2 nanotube-carbon macroscopic monoliths, in which economic polyurethane (PU) sponge was employed as carbon source, P25 and SiO2 as pore-forming agents, phenolformaldehyde resin as adhesive materials to maintain the
macroscopic shape. The as-prepared TiO2 nanotube-carbon macroscopic monoliths (TNCMs) possess both photocatalytic function and adsorption function, which should have potential in water purification area. 2. Experimental 2.1. Materials and methods PU sponges were commercially available (purchased from Jiaxing Longteng Sponge Co., Ltd.). The other reagents were all purchased from Shanghai Chemical Co. and used as received. The morphology observations of the monoliths and the nanotubes were carried out on a HITACHI S-4800 scanning electron microscope (SEM). Transmission electron microscopy (TEM) measurement was performed using a FEI Tecnai G2 instrument operating at 200 kV. To prepare TEM samples, TiO2 nanotube was detached from carbon backbone by ultrasonic method, dispersed in ethanol, and deposited onto a carbon-coated copper grid. X-ray diffraction (XRD) patterns of the monoliths was recorded on an X'Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu-Ka radiation (l ¼ 1.5418 Å). Diffuse reflectance spectra (DRS) and UVevis absorption spectra were recorded on a Cary 5000 UVevisenearinfrared spectrophotometer fitted with an integrating sphere. The samples for SEM, XRD, and DRS characterizations were the monolith directly. 2.2. Preparation of TNCMs Before use, PU sponge was cleaned with 1 M NaOH solution and distilled water and was dried in an oven at 90 C. The clean PU was immersed into mixed solution containing P25, SiO2, and water soluble phenol-formaldehyde resin in a weight ratio of 4:2:1 for 5 min, and thus the interior of PU was filled by mixed solution. When PU was taken out and kept in hot oven, water would volatilize rapidly and P25, SiO2, and phenol-formaldehyde resin would be left on the sponge. This wetting-dry process was repeated twenty times in order to increase the loading capacity of P25, SiO2, and phenol-formaldehyde resin. After that, the sponge coated by P25, SiO2, and phenol-formaldehyde resin was calcined at 400 C under nitrogen atmosphere for 2 h and then placed into an autoclave filled with 10 M NaOH solution. After heated at 160 C for 48 h, the autoclave was cooled down to room temperature and the sponge was taken out, washed alternatively with 0.1 M HCl and distilled water for five times, and calcined at 600 C under nitrogen atmosphere for 2 h. Consequently, the multimodal porous TNCMs are successfully formed. 2.3. Photocatalytic activity test A piece of TNCMs (0.5 g) was immersed into 100 mL MB, RhB and MO solution (MB: 10 mg/L; RhB: 10 mg/L; MO: 5 mg/L) under irradiation by a 10 W UV lamp (wavelength: 254 nm, light intensity: 0.08 mW/cm1). The concentration of the dye solution was monitored by Cary 5000 UVevisenear-infrared spectrophotometer per 15 min. The absorbance measurement of the sample solution was repeated for three times and no obvious absorbance change was found during the repeated test. 2.4. Adsorption test A piece of TNCMs (0.5 g) was immersed into 100 mL MB solution with various concentration (8 mg/L; 12 mg/L; 24 mg/L; 60 mg/L) in the dark. The concentration of the dye solution was monitored by Cary 5000 UVevisenear-infrared spectrophotometer per 2 h. The
Please cite this article in press as: Q. Zhang, et al., TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.037
Q. Zhang et al. / Materials Chemistry and Physics xxx (2016) 1e8
absorbance measurement of the sample solution was repeated for three times and no obvious absorbance change was found during the repeated test. 2.5. Adsorptioneliftingeirradiation process for water purification A piece of TNCMs (0.5 g) was immersed into 100 mL MB solution (concentration: 24 mg/L) in the dark for 10 min. The TNCMs was lifted from the solution and irradiated by a 10 W UV lamp (wavelength: 254 nm, light intensity: 0.08 mW/cm2). During the irradiation, the TNCMs was placed 10 cm away from the lamp, and its each side was irradiated 40 min. The existence of the MB molecules on the surface of TNCMs was monitored by measuring the diffuse reflection spectrum. The above adsorption-lifting-irradiation process was repeated ten times to demonstrate the high durability of TNCMs. 3. Results and discussion Scheme 1 describes the fabrication process of the porous TNCMs. Firstly, a piece of PU sponge is soaked in the mixed solution containing P25, SiO2, and water soluble phenol-formaldehyde resin. The sizes of the sponge are unrestricted as long as it can be held by the reactor. After the adsorption process, the sponge is taken out and put in hot oven. Water volatilizes rapidly, P25, SiO2 and phenol-formaldehyde resin are introducted into the skeletons of PU sponge (Fig. 1def). The wetting-dry process is repeated several times to increase the loading capacity of P25 and SiO2 on the sponge (Fig. 1gei). SiO2 and P25 would serve as the sacrificial templates to form macropores and as the raw materials to prepare TiO2 nanotubes, respectively. Pure PU sponge (Fig. 1c) and SiO2 spheres (Fig. S1) possess very clean and smooth surfaces, while that in the skeletons have rough surfaces (Fig. 2f and i), indicating that phenol-formaldehyde resin led to the formation of a thin layer of coating on the surface of SiO2, TiO2, and PU sponge, which firmly anchored SiO2 and TiO2 on the skeletons of PU sponge. Secondly, the resulting sponge was carbonized in tubular furnace, in which gas intermediates escaped from the skeleton of the PU sponge, giving rise to the formation of numerous micropores [32,33]. Then the carbonized sponge was allowed to react with 10 M NaOH solution under hydrothermal condition for the dissolve of SiO2 spheres to create macropore structures and convert P25 particles into interconnected titanate nanotubes framework [34]. Sodium titanate nanotubes were converted into TiO2 nanotubes through the post-treatment of acid washing and calcination [35]. Consequently, the multimodal porous TNCMs are successfully formed. The optical image of the as-prepared TNCMs is shown in the inset of Fig. 2a and the shape and size can be optionally
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manipulated according to the specific demands. As is shown in Fig. 2 is the SEM characterizations of TNCMs. Low magnification SEM images (Fig. 2a and b) reveal that the structures of TNCMs are totally different from pure PU sponge (Fig. 1a and b). The backbones of TNCMs possess numerous sub-micron size holes (Fig. 2c and d) formed by sacrificial SiO2 spheres (Fig. S2), while PU sponges have solid and smooth backbone. Furthermore, high magnification SEM images (Fig. 2e and f) indicate that the TNCMs were filled with interpenetrated nanotube frameworks and there not only exist abundant nanoscale interstices among the neighboring nanotubes but also sub-micron grade of holes (Indicated by red circles in Fig. 2e and f) templated by sacrificial SiO2 (Fig. S3). There are micro scale gaps between the fillings and the backbones as indicated by yellow arrow in Fig. 2b. The inset in Fig. 2f is the TEM image of the TiO2 nanotubes, which reveals that the nanotubes have open terminations and the diameter of nanotube is ~9 nm. To determine the average width of the interstice, nitrogen adsorptionedesorption characterizations of the TNCMs were performed. The nitrogen isotherm of the TNCMs (Fig. 3a) exhibits combined characteristics of type I/II with a BrunauereEmmetteTeller (BET) surface area of 125 m2/g, a total pore volume of 0.227 cm3/g, a micropore volume of 0.027 cm3/g, a micropore to total pore volume ratio of 0.19, and an average pore diameter of 7.13 nm. In Fig. 3a, the initial rise at low pressure is ascribed to the adsorption in micropores, while the strong adsorption at relative pressure (P/P0) around 0.5e0.9 is originated from capillary condensation of nitrogen in the mesopores [36]. The H4 hysteresis reveals that the TNCMs possess several different types of pores with a broad size distribution owing to the packing of the carbon nanoparticles and nanotubes [37]. The steep adsorption at high pressure (P/P0) around 0.9e1.0 is assigned to the macropores generated by the SiO2 sub-microsphere template [36]. The pore-size distribution derived using nonlocal density functional theory (DFT) is given in Fig. 3b. Two regions can be identified: 1) micropores with size of 0.5 nm and 1.4 nm; and 2) mesopores with size of 2.9 nm, 6.8 nm and 9.3 nm. The pores centered at 0.5 nm, 1.4 nm and 2.9 nm were formed from the release of small molecules in the carbonization process of phenolformaldehyde resin, which is in accord with the previously reported data for phenolic resin carbons [38]. Mesopores centered at 6.8 nm were originated from the carbonization of PU skeletons [32], while mesopores centered at 9.3 nm were originated from TiO2 nanotubes, which is in good agreement with the TEM results. Thus, both morphological analysis and pore structure characterization indicate that the proposed strategy (Scheme 1) is able to develop a total of three levels of pores with pore width spanning over the scale of micrometer, sub-micron and nanometer. The crystallographic structure of the as-obtained TNCMs was
Scheme 1. Schematic diagram showing the process for the fabrication of TNCMs.
Please cite this article in press as: Q. Zhang, et al., TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.037
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Fig. 1. (aec) SEM images of PU sponge. The inset in Figure 1a is the digital picture of PU sponge. (dei) SEM images of PU sponge coated by P25, SiO2 and phenol-formaldehyde resin. (def) Coated by five times; (gei) Coated by twenty times; The inset in Figure 1d and g is the digital picture of the coated PU sponge.
determined by X-ray diffraction (XRD) measurements. From the XRD patterns shown in Fig. 4a, we found that peaks located at 25.5 , 38.3 , 48.5 , 56.7, 60.8 are belong to TiO2-A, while peaks centered at 27.8 , 29.7, 33.3 , 46.6 are traced to TiO2eB, indicating the coexist of TiO2-A and TiO2eB in the final products. To obtain TiO2 nanotubes-carbon monoliths, the monoliths obtained from the hydrothermal process was repeatedly washed with 0.1 M HCl and annealed at 600 C. It is reported that the protonated titanate is firstly transformed to a metastable monoclinic modification of TiO2 (TiO2eB) and then to anatase TiO2 (TiO2-A) under calcination [39]. Therefore, the coexistence of TiO2-A and TiO2eB in the products is attributed to the transformation process of protonated titanate to TiO2. Notably, no typical diffraction peaks belonging to carbon are observed in the TNCMs in the XRD spectrum, which is attributed to the fact that the main characteristic peak of carbon may be shielded by the peak of anatase TiO2 at 25.5 [39]. Thermogravimetric analysis (TGA) is an effective analytical technique to evaluate the mass ratio of carbon to TiO2. The TGA curve (Fig. 4b) of TNCMs shows 11% weight loss below 350 C, which is due to the loss of free water and bound water. The weight loss from 350 C to 550 C is 11% and belongs to the carbon oxidation process, while the 78% residue above 550 C is associated with TiO2 nanotubes. According to TGA curve, the weight ratio of carbon to TiO2 nanotubes is 0.14. Fig. 4c shows the surface diffuse reflection spectra of the pure TiO2 and TNCMs. Both TiO2 and TNCMs exhibit an intense absorption towards light below 400 nm, which is attributed to the wide band gap of TiO2. No obvious absorption edge shift was observed for TNCMs, indicating the same light absorption range of TiO2 and TNCMs. The reflection in the region of visible lights (400e800 nm) is decreased by 60% for TNCMs relative to pure TiO2, which is consistent with the black color of the product. Similar light enhancement phenomenon has also been observed in other TiO2/ carbon composite systems [40e42]. Benefiting from the high surface area, unique multimodal pore structure and good optical properties, the as-prepared TNCMs is
anticipated to exhibit excellent performance in adsorption and photodegradation test, which may serve as a kind of advanced water purification materials. As a chemically stable and poorly biodegradable dye, MB is a main contaminant in wastewater. In this work, we use the adsorption and photodegradation of MB as a model reaction to evaluate the great advantages of TNCMs in the purification of polluted water. Fig. 5 shows the photodegradation behavior of the TNCMs towards MB. MB was completely photodegraded within 120 min, demonstrating the high photocatalytic efficiency of TNCMs. The average degradation rate k is 0.5 mg/h (k ¼ m/t, where m and t indicate the weight of the degraded pollutant and the irradiation time, respectively), which is much higher than the reported TiO2 nanotubes/carbon fiber sheets [34]. The higher photocatalytic performance is owing to the unique multimodal pore structures. For TNCMs, there are three kinds of porous structure, including micropores, mesopores and macropores, while TiO2 nanotubes/carbon fiber sheets only have mesopores because of the solid structure of carbon fibers. Comparing with TiO2 nanowire membranes [43,44], the as-prepared TNCMs exhibited lower photocatalytic activity because the high content of carbon skeleton. For TNCMs, only the TiO2 nanotubes can utilize ultraviolet light for photodegradation of MB, because carbon backbones do not have photocatalytic activity [4]. For TiO2 nanowire membranes, all the building units of the membrane can serve as active sites to photodecompose MB. In addition, the TNCMs had good reuse capacity; even after five times reuse, the degradation ratio of MB is still as high as ca. 90% (Fig. 5c). The decrease in the efficiency can be explained by the free TiO2 nanotube loss during the recovery operation. SEM test reveals that carbon backbone was filled with interpenetrated TiO2 nanotube frameworks. Owing to the strong van der Waals interaction between nanotubes, the TiO2 in the interpenetrated nanotube frameworks were firmly anchored with each other. Unfortunately, a small amount of free TiO2 nanotubes existed in the carbon framework, which may fall off the backbone during the photocatalytic test, leading to the decrease in
Please cite this article in press as: Q. Zhang, et al., TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.037
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Fig. 2. SEM images of the TNCMs material. The inset in Fig. 2a is the digital picture of TNCMs. Red circles in Fig. 2e and f indicate the pores templated by SiO2 spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. Nitrogen adsorptionedesorption isotherm (a) and pore-size distribution (b) of the TNCMs material.
the efficiency. After most of free TiO2 nanotubes detach from the backbone during the long-term stability test, the photodegradation efficiency of TNCMs become stable. The TNCMs were also capable of photodecomposing other organic contaminant, including Methyl orange, and Rhodamine B (Fig. S4). Fig. 6 shows the adsorption behavior of the TNCMs towards MB. When TNCMs was immersed in MB solution, the concentration of MB decreased with increased
contact time (Fig. 5 and Fig. S5). Maximum quantitative removal of MB from an aqueous solution was achieved in less than 72 h. Beyond 72 h, there was no increase in MB removal. This long contact time required for attaining equilibrium may be a result of diffusion processes of the MB into the porous structure of the adsorbent. Therefore, the as-prepared TNCMs is a kind of photocatalytic adsorbents since it possess both adsorption function and
Please cite this article in press as: Q. Zhang, et al., TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.037
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Fig. 4. (a) XRD patterns of TNCMs; (b) TGA curve of TNCMs; (c) Surface diffuse reflection spectra of pure TiO2 and TNCMs.
Fig. 5. Photodegradation behavior of the TNCMs towards MB. (a) Temporal spectral changes of MB in the photocatalytic process. (b) Relationships of the concentration of MB with illumination time. (c) Curve of the degradation ratio of MB versus reuse times of TNCMs.
Fig. 6. Adsorption behavior of the TNCMs towards MB (60 mg/L). (a) Temporal spectral changes of MB in the adsorption process. (b) Relationships of the concentration of MB with desorption time.
photodegradation function. To evaluate the great advantages of the as-prepared TiO2 nanotube-carbon photocatalytic adsorbents in the purification of polluted water, a facile adsorptioneliftingeirradiation experiment was designed. As is shown in Fig. 7a, multimodal porous TNCMs photocatalytic adsorbent was immersed into wastewater to capture MB. Then the TNCMs photocatalytic adsorbents anchored with MB was lifted from the water directly owing to its macroscopic shape and robustness feature. Finally the TNCMs photocatalytic adsorbents were irradiated to remove MB because of its photocatalytic function. Fig. 7b shows the diffuse reflection spectra of the TNCMs photocatalytic adsorbents at the three stages. Initially, only a sharp band around 400 nm was found, which is attributed to the TiO2 nanotubes on the monolith. At the second stage, two additional bands around 600 nm and 650 nm appeared, which are due to the chromophores in the MB molecules, indicating that the MB molecules in the solution was captured by porous TNCMs. When TNCMs is irradiated with ultraviolet light, conduction band electrons (e) and valence band holes (hþ) are generated on TiO2 nanotubes [32].
The photogenerated electrons and holes can react with H2O and O2 molecules, leading to the formation of reactive oxygen species, such as O2 and OH. The resulting reactive oxygen species can oxidize MB to CO2, H2O and mineral end-products (Fig. S6).8 After irradiation under UV light for 80 min, the diffuse reflection spectrum of the modified textile changed back to the original form (red line), indicating that the adsorbed MB molecules were removed. The adsorption-lifting-irradiation process can be repeated ten times while retaining the adsorption function and photodegradation function of TNCMs (Fig. 7c), confirming that TNCMs is a reusable and durable photocatalytic adsorbent, which has great potential in water purification. 4. Conclusions In this paper, we have shown a facile strategy for the construction of TiO2 nanotube-carbon macroscopic composites with multimodal porosity, large surface area, and good optical properties. Owing to the photocatalytic feature of TiO2 nanotube and
Please cite this article in press as: Q. Zhang, et al., TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.037
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Fig. 7. (a) Schematic diagram of the purification of the contaminated water with TNCMs. (b) Diffusion reflection spectra of TNCMs at the three stages (black curve: before adsorption; blue curve: after capturing MB; red curve: after irradiation). (c) Diffuse reflection spectra of TNCMs after ten times of adsorption and irradiation with UV light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
multimodal pore structures, the as-prepared TNCMs combine the advantages of both adsorption materials and photocatalytic materials. Furthermore, the macroscopic size and robustness feature allow TNCMs to be easily recovered and reused. Therefore, the asprepared TNCMs are a kind of efficient recyclable photocatalytic adsorbents for the purification of organic wastewater, which should have potential for real-world water purification. Acknowledgments This work was financially supported by Xinmiao Talent Project (851914004Z) of Zhejiang Province and the Natural Science Foundation of Zhejiang Province, China (LQ14B010002). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.037.
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TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity as efficient recyclable photocatalytic adsorbents for water purification Qiao Zhang, Zhufeng Lu, Shalang Jin, Yitian Zheng, Tongtong Ye, Duoduo Yang, Yiming Li, Longfeng Zhu, Lianwen Zhu* School of Biology and Chemical Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TiO2 nanotube-carbon macroscopic monoliths with multimodal porosity were prepared. The TiO2-carbon monoliths have adsorption function and photocatalytic function. The TiO2-carbon macroscopic monoliths greatly facilitate organic wastewater purification.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2015 Received in revised form 1 February 2016 Accepted 10 February 2016 Available online xxx
In this paper, we have shown a facile strategy for the construction of porous TiO2 nanotube-carbon macroscopic monoliths (TNCMs). The resulting macroscopic materials not only possess multimodal porosity, large surface area, and good optical properties, but also combine the advantages of both adsorption materials and photocatalytic materials. Furthermore, the TNCMs can be easily recovered and the robustness of the TNCMs allows them to be reused. Therefore, the as-prepared TNCMs greatly facilitate the purification of organic wastewater, which should have potential for real-world water purification. © 2016 Elsevier B.V. All rights reserved.
Keywords: Microporous materials Chemical synthesis Nanostructures Oxidation
1. Introduction Due to the rapid population growth and increasing environmental degradation, water crisis is becoming one of the top issues and is probably expected to be the most baffling problem in the future [1e3]. Therefore, increasing attention has been paid to the development of efficient water purification materials. In order to meet the demand of real world water purification, the essential features of * Corresponding author. E-mail address: [email protected] (L. Zhu).
advanced water purification materials should be high efficiency, easy recovery, high durability, low cost, and environment beneath [4,5]. To date, various kinds of functional materials have been applied to remove the pollutants from contaminated water, including photocatalytic materials, adsorption materials, and separation materials, etc [6e11]. Among all the functional materials proposed, adsorption materials is one of the most popular candidates, because adsorption is currently considered as an effective, efficient, and easy technique for water purification [9]. However, traditional powder adsorbents need to be dispersed into the polluted water and therefore it is impractical to use them to purify rivers or lakes
http://dx.doi.org/10.1016/j.matchemphys.2016.02.037 0254-0584/© 2016 Elsevier B.V. All rights reserved.
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because recovery is complicated and expensive. Furthermore, adsorption process only transfers pollutants from water phase to adsorbent rather than eliminating them from the environment. In addition, the regeneration of exhausted adsorbents by chemical and thermal procedure is required, which is expensive and highenergy consuming [9]. It is widely known that photocatalysts can effectively oxidize a wide range of organic contamination into non-toxic byproducts such as H2O, CO2, and mineral acids at ambient conditions [8]. Therefore, it is anticipated that the incorporation of adsorption materials with photocatalytic materials will give rise to a novel photocatalytic adsorbents, which would combine the advantages of both techniques. On one hand, photocatalysts can easily decompose the pollutants anchored on adsorbents under light illumination, therefore the problem about the high cost regeneration of exhausted adsorbents can be effectively addressed; on the other hand, large surface area adsorbents would concentrate the pollutants around the photocatalysts, leading to an enhancement of the photodecomposition rate [12]. Benefiting from their high surface area, porous structure, and special surface reactivity, carbon adsorbents (activated carbon, graphene, carbon nanotubes, carbon nanofibers, etc) have been the most popular candidate and are expected to be widely used in water treatment area [13e17]. Furthermore, comparing with other kind of adsorbents, carbon units can be easily assembled into macroscopic materials, including monoliths, membranes, sponge, aerogel and hydrogel, leading to the formation of easy recovery adsorption materials. TiO2 is one of the most popular photocatalysts with distinct photochemical activities, which for many years have been intensively exploited for the purification of contaminated water [18,19]. Therefore, combination of TiO2 with carbon macroscopic materials would give rise to a kind of advanced water purification materials with both adsorption and cleaning functions. To date, several macroscopic TiO2-carbon composites have been achieved, where TiO2 is composed of irregular nanoparticles [20,21]. Compared with nanoparticles, TiO2 nanotubes possess higher photocatalytic performance because of their superior charge transport properties [22], larger surface area, and multiple pathways for the diffusion of molecules and ions [23]. Furthermore, the pore structure of the reported TiO2-carbon composites is monotonous. Compared with irregular pore structures, multimodal porous structure permit faster and efficient diffusion of molecules and ions, leading to an obvious enhancement of the photocatalytic activity and adsorption performance [24e27]. Kazuya Nakata and coworkers reported that TiO2 monoliths with macropores and mesopores possess superior water purification performance [28]. Highly porous TiO2-based filters and membranes were used to fabricate effective photocatalytic reactor for environmental purification [29,30]. In addition, expensive CNTs or graphene oxide and organic titanium compounds were used as carbon source and titanium source in many previous reported macroscopic TiO2-carbon structure system, which increase the production cost [21,31]. Therefore, macroscopic TiO2 nanotube-carbon composites with multimodal pore structures are anticipated to be a kind of advanced water purification materials due to their several obvious advantages. (i) Easy recovery owing to the macroscopic size; (ii) Self-regeneration ability because of the photocatalytic feature of TiO2 nanotubes; (iii) Outstanding purification performance owing to the multimodal pore structures. However, as far as we know, the construction of multimodal porous TiO2 nanotube-carbon macroscopic composites is still a big challenge. In this work, a scalable approach is reported for the construction of multimodal porous TiO2 nanotube-carbon macroscopic monoliths, in which economic polyurethane (PU) sponge was employed as carbon source, P25 and SiO2 as pore-forming agents, phenolformaldehyde resin as adhesive materials to maintain the
macroscopic shape. The as-prepared TiO2 nanotube-carbon macroscopic monoliths (TNCMs) possess both photocatalytic function and adsorption function, which should have potential in water purification area. 2. Experimental 2.1. Materials and methods PU sponges were commercially available (purchased from Jiaxing Longteng Sponge Co., Ltd.). The other reagents were all purchased from Shanghai Chemical Co. and used as received. The morphology observations of the monoliths and the nanotubes were carried out on a HITACHI S-4800 scanning electron microscope (SEM). Transmission electron microscopy (TEM) measurement was performed using a FEI Tecnai G2 instrument operating at 200 kV. To prepare TEM samples, TiO2 nanotube was detached from carbon backbone by ultrasonic method, dispersed in ethanol, and deposited onto a carbon-coated copper grid. X-ray diffraction (XRD) patterns of the monoliths was recorded on an X'Pert PRO SUPER rA rotation anode X-ray diffractometer with Ni-filtered Cu-Ka radiation (l ¼ 1.5418 Å). Diffuse reflectance spectra (DRS) and UVevis absorption spectra were recorded on a Cary 5000 UVevisenearinfrared spectrophotometer fitted with an integrating sphere. The samples for SEM, XRD, and DRS characterizations were the monolith directly. 2.2. Preparation of TNCMs Before use, PU sponge was cleaned with 1 M NaOH solution and distilled water and was dried in an oven at 90 C. The clean PU was immersed into mixed solution containing P25, SiO2, and water soluble phenol-formaldehyde resin in a weight ratio of 4:2:1 for 5 min, and thus the interior of PU was filled by mixed solution. When PU was taken out and kept in hot oven, water would volatilize rapidly and P25, SiO2, and phenol-formaldehyde resin would be left on the sponge. This wetting-dry process was repeated twenty times in order to increase the loading capacity of P25, SiO2, and phenol-formaldehyde resin. After that, the sponge coated by P25, SiO2, and phenol-formaldehyde resin was calcined at 400 C under nitrogen atmosphere for 2 h and then placed into an autoclave filled with 10 M NaOH solution. After heated at 160 C for 48 h, the autoclave was cooled down to room temperature and the sponge was taken out, washed alternatively with 0.1 M HCl and distilled water for five times, and calcined at 600 C under nitrogen atmosphere for 2 h. Consequently, the multimodal porous TNCMs are successfully formed. 2.3. Photocatalytic activity test A piece of TNCMs (0.5 g) was immersed into 100 mL MB, RhB and MO solution (MB: 10 mg/L; RhB: 10 mg/L; MO: 5 mg/L) under irradiation by a 10 W UV lamp (wavelength: 254 nm, light intensity: 0.08 mW/cm1). The concentration of the dye solution was monitored by Cary 5000 UVevisenear-infrared spectrophotometer per 15 min. The absorbance measurement of the sample solution was repeated for three times and no obvious absorbance change was found during the repeated test. 2.4. Adsorption test A piece of TNCMs (0.5 g) was immersed into 100 mL MB solution with various concentration (8 mg/L; 12 mg/L; 24 mg/L; 60 mg/L) in the dark. The concentration of the dye solution was monitored by Cary 5000 UVevisenear-infrared spectrophotometer per 2 h. The
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absorbance measurement of the sample solution was repeated for three times and no obvious absorbance change was found during the repeated test. 2.5. Adsorptioneliftingeirradiation process for water purification A piece of TNCMs (0.5 g) was immersed into 100 mL MB solution (concentration: 24 mg/L) in the dark for 10 min. The TNCMs was lifted from the solution and irradiated by a 10 W UV lamp (wavelength: 254 nm, light intensity: 0.08 mW/cm2). During the irradiation, the TNCMs was placed 10 cm away from the lamp, and its each side was irradiated 40 min. The existence of the MB molecules on the surface of TNCMs was monitored by measuring the diffuse reflection spectrum. The above adsorption-lifting-irradiation process was repeated ten times to demonstrate the high durability of TNCMs. 3. Results and discussion Scheme 1 describes the fabrication process of the porous TNCMs. Firstly, a piece of PU sponge is soaked in the mixed solution containing P25, SiO2, and water soluble phenol-formaldehyde resin. The sizes of the sponge are unrestricted as long as it can be held by the reactor. After the adsorption process, the sponge is taken out and put in hot oven. Water volatilizes rapidly, P25, SiO2 and phenol-formaldehyde resin are introducted into the skeletons of PU sponge (Fig. 1def). The wetting-dry process is repeated several times to increase the loading capacity of P25 and SiO2 on the sponge (Fig. 1gei). SiO2 and P25 would serve as the sacrificial templates to form macropores and as the raw materials to prepare TiO2 nanotubes, respectively. Pure PU sponge (Fig. 1c) and SiO2 spheres (Fig. S1) possess very clean and smooth surfaces, while that in the skeletons have rough surfaces (Fig. 2f and i), indicating that phenol-formaldehyde resin led to the formation of a thin layer of coating on the surface of SiO2, TiO2, and PU sponge, which firmly anchored SiO2 and TiO2 on the skeletons of PU sponge. Secondly, the resulting sponge was carbonized in tubular furnace, in which gas intermediates escaped from the skeleton of the PU sponge, giving rise to the formation of numerous micropores [32,33]. Then the carbonized sponge was allowed to react with 10 M NaOH solution under hydrothermal condition for the dissolve of SiO2 spheres to create macropore structures and convert P25 particles into interconnected titanate nanotubes framework [34]. Sodium titanate nanotubes were converted into TiO2 nanotubes through the post-treatment of acid washing and calcination [35]. Consequently, the multimodal porous TNCMs are successfully formed. The optical image of the as-prepared TNCMs is shown in the inset of Fig. 2a and the shape and size can be optionally
3
manipulated according to the specific demands. As is shown in Fig. 2 is the SEM characterizations of TNCMs. Low magnification SEM images (Fig. 2a and b) reveal that the structures of TNCMs are totally different from pure PU sponge (Fig. 1a and b). The backbones of TNCMs possess numerous sub-micron size holes (Fig. 2c and d) formed by sacrificial SiO2 spheres (Fig. S2), while PU sponges have solid and smooth backbone. Furthermore, high magnification SEM images (Fig. 2e and f) indicate that the TNCMs were filled with interpenetrated nanotube frameworks and there not only exist abundant nanoscale interstices among the neighboring nanotubes but also sub-micron grade of holes (Indicated by red circles in Fig. 2e and f) templated by sacrificial SiO2 (Fig. S3). There are micro scale gaps between the fillings and the backbones as indicated by yellow arrow in Fig. 2b. The inset in Fig. 2f is the TEM image of the TiO2 nanotubes, which reveals that the nanotubes have open terminations and the diameter of nanotube is ~9 nm. To determine the average width of the interstice, nitrogen adsorptionedesorption characterizations of the TNCMs were performed. The nitrogen isotherm of the TNCMs (Fig. 3a) exhibits combined characteristics of type I/II with a BrunauereEmmetteTeller (BET) surface area of 125 m2/g, a total pore volume of 0.227 cm3/g, a micropore volume of 0.027 cm3/g, a micropore to total pore volume ratio of 0.19, and an average pore diameter of 7.13 nm. In Fig. 3a, the initial rise at low pressure is ascribed to the adsorption in micropores, while the strong adsorption at relative pressure (P/P0) around 0.5e0.9 is originated from capillary condensation of nitrogen in the mesopores [36]. The H4 hysteresis reveals that the TNCMs possess several different types of pores with a broad size distribution owing to the packing of the carbon nanoparticles and nanotubes [37]. The steep adsorption at high pressure (P/P0) around 0.9e1.0 is assigned to the macropores generated by the SiO2 sub-microsphere template [36]. The pore-size distribution derived using nonlocal density functional theory (DFT) is given in Fig. 3b. Two regions can be identified: 1) micropores with size of 0.5 nm and 1.4 nm; and 2) mesopores with size of 2.9 nm, 6.8 nm and 9.3 nm. The pores centered at 0.5 nm, 1.4 nm and 2.9 nm were formed from the release of small molecules in the carbonization process of phenolformaldehyde resin, which is in accord with the previously reported data for phenolic resin carbons [38]. Mesopores centered at 6.8 nm were originated from the carbonization of PU skeletons [32], while mesopores centered at 9.3 nm were originated from TiO2 nanotubes, which is in good agreement with the TEM results. Thus, both morphological analysis and pore structure characterization indicate that the proposed strategy (Scheme 1) is able to develop a total of three levels of pores with pore width spanning over the scale of micrometer, sub-micron and nanometer. The crystallographic structure of the as-obtained TNCMs was
Scheme 1. Schematic diagram showing the process for the fabrication of TNCMs.
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Fig. 1. (aec) SEM images of PU sponge. The inset in Figure 1a is the digital picture of PU sponge. (dei) SEM images of PU sponge coated by P25, SiO2 and phenol-formaldehyde resin. (def) Coated by five times; (gei) Coated by twenty times; The inset in Figure 1d and g is the digital picture of the coated PU sponge.
determined by X-ray diffraction (XRD) measurements. From the XRD patterns shown in Fig. 4a, we found that peaks located at 25.5 , 38.3 , 48.5 , 56.7, 60.8 are belong to TiO2-A, while peaks centered at 27.8 , 29.7, 33.3 , 46.6 are traced to TiO2eB, indicating the coexist of TiO2-A and TiO2eB in the final products. To obtain TiO2 nanotubes-carbon monoliths, the monoliths obtained from the hydrothermal process was repeatedly washed with 0.1 M HCl and annealed at 600 C. It is reported that the protonated titanate is firstly transformed to a metastable monoclinic modification of TiO2 (TiO2eB) and then to anatase TiO2 (TiO2-A) under calcination [39]. Therefore, the coexistence of TiO2-A and TiO2eB in the products is attributed to the transformation process of protonated titanate to TiO2. Notably, no typical diffraction peaks belonging to carbon are observed in the TNCMs in the XRD spectrum, which is attributed to the fact that the main characteristic peak of carbon may be shielded by the peak of anatase TiO2 at 25.5 [39]. Thermogravimetric analysis (TGA) is an effective analytical technique to evaluate the mass ratio of carbon to TiO2. The TGA curve (Fig. 4b) of TNCMs shows 11% weight loss below 350 C, which is due to the loss of free water and bound water. The weight loss from 350 C to 550 C is 11% and belongs to the carbon oxidation process, while the 78% residue above 550 C is associated with TiO2 nanotubes. According to TGA curve, the weight ratio of carbon to TiO2 nanotubes is 0.14. Fig. 4c shows the surface diffuse reflection spectra of the pure TiO2 and TNCMs. Both TiO2 and TNCMs exhibit an intense absorption towards light below 400 nm, which is attributed to the wide band gap of TiO2. No obvious absorption edge shift was observed for TNCMs, indicating the same light absorption range of TiO2 and TNCMs. The reflection in the region of visible lights (400e800 nm) is decreased by 60% for TNCMs relative to pure TiO2, which is consistent with the black color of the product. Similar light enhancement phenomenon has also been observed in other TiO2/ carbon composite systems [40e42]. Benefiting from the high surface area, unique multimodal pore structure and good optical properties, the as-prepared TNCMs is
anticipated to exhibit excellent performance in adsorption and photodegradation test, which may serve as a kind of advanced water purification materials. As a chemically stable and poorly biodegradable dye, MB is a main contaminant in wastewater. In this work, we use the adsorption and photodegradation of MB as a model reaction to evaluate the great advantages of TNCMs in the purification of polluted water. Fig. 5 shows the photodegradation behavior of the TNCMs towards MB. MB was completely photodegraded within 120 min, demonstrating the high photocatalytic efficiency of TNCMs. The average degradation rate k is 0.5 mg/h (k ¼ m/t, where m and t indicate the weight of the degraded pollutant and the irradiation time, respectively), which is much higher than the reported TiO2 nanotubes/carbon fiber sheets [34]. The higher photocatalytic performance is owing to the unique multimodal pore structures. For TNCMs, there are three kinds of porous structure, including micropores, mesopores and macropores, while TiO2 nanotubes/carbon fiber sheets only have mesopores because of the solid structure of carbon fibers. Comparing with TiO2 nanowire membranes [43,44], the as-prepared TNCMs exhibited lower photocatalytic activity because the high content of carbon skeleton. For TNCMs, only the TiO2 nanotubes can utilize ultraviolet light for photodegradation of MB, because carbon backbones do not have photocatalytic activity [4]. For TiO2 nanowire membranes, all the building units of the membrane can serve as active sites to photodecompose MB. In addition, the TNCMs had good reuse capacity; even after five times reuse, the degradation ratio of MB is still as high as ca. 90% (Fig. 5c). The decrease in the efficiency can be explained by the free TiO2 nanotube loss during the recovery operation. SEM test reveals that carbon backbone was filled with interpenetrated TiO2 nanotube frameworks. Owing to the strong van der Waals interaction between nanotubes, the TiO2 in the interpenetrated nanotube frameworks were firmly anchored with each other. Unfortunately, a small amount of free TiO2 nanotubes existed in the carbon framework, which may fall off the backbone during the photocatalytic test, leading to the decrease in
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Fig. 2. SEM images of the TNCMs material. The inset in Fig. 2a is the digital picture of TNCMs. Red circles in Fig. 2e and f indicate the pores templated by SiO2 spheres. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. Nitrogen adsorptionedesorption isotherm (a) and pore-size distribution (b) of the TNCMs material.
the efficiency. After most of free TiO2 nanotubes detach from the backbone during the long-term stability test, the photodegradation efficiency of TNCMs become stable. The TNCMs were also capable of photodecomposing other organic contaminant, including Methyl orange, and Rhodamine B (Fig. S4). Fig. 6 shows the adsorption behavior of the TNCMs towards MB. When TNCMs was immersed in MB solution, the concentration of MB decreased with increased
contact time (Fig. 5 and Fig. S5). Maximum quantitative removal of MB from an aqueous solution was achieved in less than 72 h. Beyond 72 h, there was no increase in MB removal. This long contact time required for attaining equilibrium may be a result of diffusion processes of the MB into the porous structure of the adsorbent. Therefore, the as-prepared TNCMs is a kind of photocatalytic adsorbents since it possess both adsorption function and
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Fig. 4. (a) XRD patterns of TNCMs; (b) TGA curve of TNCMs; (c) Surface diffuse reflection spectra of pure TiO2 and TNCMs.
Fig. 5. Photodegradation behavior of the TNCMs towards MB. (a) Temporal spectral changes of MB in the photocatalytic process. (b) Relationships of the concentration of MB with illumination time. (c) Curve of the degradation ratio of MB versus reuse times of TNCMs.
Fig. 6. Adsorption behavior of the TNCMs towards MB (60 mg/L). (a) Temporal spectral changes of MB in the adsorption process. (b) Relationships of the concentration of MB with desorption time.
photodegradation function. To evaluate the great advantages of the as-prepared TiO2 nanotube-carbon photocatalytic adsorbents in the purification of polluted water, a facile adsorptioneliftingeirradiation experiment was designed. As is shown in Fig. 7a, multimodal porous TNCMs photocatalytic adsorbent was immersed into wastewater to capture MB. Then the TNCMs photocatalytic adsorbents anchored with MB was lifted from the water directly owing to its macroscopic shape and robustness feature. Finally the TNCMs photocatalytic adsorbents were irradiated to remove MB because of its photocatalytic function. Fig. 7b shows the diffuse reflection spectra of the TNCMs photocatalytic adsorbents at the three stages. Initially, only a sharp band around 400 nm was found, which is attributed to the TiO2 nanotubes on the monolith. At the second stage, two additional bands around 600 nm and 650 nm appeared, which are due to the chromophores in the MB molecules, indicating that the MB molecules in the solution was captured by porous TNCMs. When TNCMs is irradiated with ultraviolet light, conduction band electrons (e) and valence band holes (hþ) are generated on TiO2 nanotubes [32].
The photogenerated electrons and holes can react with H2O and O2 molecules, leading to the formation of reactive oxygen species, such as O2 and OH. The resulting reactive oxygen species can oxidize MB to CO2, H2O and mineral end-products (Fig. S6).8 After irradiation under UV light for 80 min, the diffuse reflection spectrum of the modified textile changed back to the original form (red line), indicating that the adsorbed MB molecules were removed. The adsorption-lifting-irradiation process can be repeated ten times while retaining the adsorption function and photodegradation function of TNCMs (Fig. 7c), confirming that TNCMs is a reusable and durable photocatalytic adsorbent, which has great potential in water purification. 4. Conclusions In this paper, we have shown a facile strategy for the construction of TiO2 nanotube-carbon macroscopic composites with multimodal porosity, large surface area, and good optical properties. Owing to the photocatalytic feature of TiO2 nanotube and
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Fig. 7. (a) Schematic diagram of the purification of the contaminated water with TNCMs. (b) Diffusion reflection spectra of TNCMs at the three stages (black curve: before adsorption; blue curve: after capturing MB; red curve: after irradiation). (c) Diffuse reflection spectra of TNCMs after ten times of adsorption and irradiation with UV light. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
multimodal pore structures, the as-prepared TNCMs combine the advantages of both adsorption materials and photocatalytic materials. Furthermore, the macroscopic size and robustness feature allow TNCMs to be easily recovered and reused. Therefore, the asprepared TNCMs are a kind of efficient recyclable photocatalytic adsorbents for the purification of organic wastewater, which should have potential for real-world water purification. Acknowledgments This work was financially supported by Xinmiao Talent Project (851914004Z) of Zhejiang Province and the Natural Science Foundation of Zhejiang Province, China (LQ14B010002). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.037.
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