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Durable Superhydrophobic/Superoleophilic Polyurethane Sponges Inspired by Mussel and Lotus Leaf for the Selective Removal of Organic Pollutants from Water [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) The low stability of most superhydrophobic materials is one of the hurdles faced in terms of their practical applications. Inspired by mussel and lotus leaf, we report the fabrication of durable superhydrophobic/superoleophilic polyurethane (PU) sponges for the selective removal of organic pollutants from water. The superhydrophobic/superoleophilic PU sponges are prepared through in-turn covalent modification of PU sponges with polydopamine, silver nanoparticles, and dodecyl mercaptan. The morphology and surface chemical composition of the sponges are characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, the wetting behaviors, stability, and oil/water separation performance of the superhydrophobic sponges are studied. The modified PU sponges show promising superhydrophobic and superoleophilic properties. The sponge has a water contact angle of about 155° and a shedding angle of approximately 3°, whereas the contact angle of oil is around 0°. In addition, the superhydrophobic coating on the PU sponge exhibits excellent mechanical, chemical, and environmental stability, for example, upon laundering, intensive scalpel scratching, long-duration immersion in organic liquids and oils, and subjection to harsh temperatures and UV irradiation. Moreover, the superhydrophobic sponge can be used repeatedly for the selective and quick absorption of various insoluble organic pollutants (for example, petrol, crude oil, and chloroform) from water with high separation efficiency. Keywords : dopamines · hydrophobic effect · oil/water separation · polyurethane sponge · superhydrophobic Introduction With the growth in oil production and transportation, industri- al oily wastewater and oil-spill accidents have become the most significant threat to the environment and people's health. The large amount of oily wastewater makes oil/water separation an important challenge. Traditional techniques for oil/water separation include absorption,[1] booms,[2] and disper- sion.[3] However, these traditional techniques have many prob- lems such as low separation efficiencies and high operation costs. Natural phenomena such as hydrophobicity and self-assem- bly help human beings solve many of today's engineering problems.[4, 5] Various functional materials have been developed by mimicking animals and plants in the natural world.[6, 7] Lotus-leaf-inspired materials with both superhydrophobic and superoleophilic properties have received considerable atten- tion in recent years as a solution to the problems of traditional materials for oil/water separation.[8] Such materials are super- repellent toward water, but can be wetted easily by oils. The use of superhydrophobic/superoleophilic materials by either filtration or selective absorption of oils from water is an effec- tive and facile way to achieve oil/water separation.[9-12] The typical superhydrophobic/superoleophilic materials for the removal of oils from water are in the forms of meshes,[13, 14] films,[10,15, 16] sponges[12, 17, 18] aerogels,[14, 19] and so on. Jiang and co-workers reported superhydrophobic/superoleophilic polyte- trafluoroethylene (PTFE)-coated meshes for effective oil/water separation.[20] In comparison with other materials such as metal meshes, polymer films, and graphene aerogel, polyurethane (PU) sponges are good substrates for the preparation of super- hydrophobic materials for oil/water separation owing to their large pore volume, low density, high flexibility, and mechanical stability under harsh practical conditions, as well as their low price.[21, 22] Li et al. prepared a superhydrophobic sponge using a conjugated polymer, which showed high oil absorbency.[12, 23] Tai et al. reported a strategy for the fabrication of superhydro- phobic graphene-based sponges through a facile dip-coating method.[18] However, acid etching of PU sponges is a necessary step for the preparation of most of the reported superhydro- phobic PU sponges.[17, 21, 22, 24] This step will inevitably weaken the mechanical properties of the PU sponges and cause secon- dary pollution. Therefore, it is necessary to explore an alterna- tive strategy to solve the abovementioned problems. The adhesive proteins of mussel contain high levels of l-3,4- dihydroxyphenylalanine, which is believed to contribute to the crosslinking of the proteins and the formation of strong cova- lent/noncovalent interactions with surfaces.[25] Messersmith et al. demonstrated that dopamine could polymerize and de- posit on all kinds of substrates.[6] In addition, the as-formed polydopamine (PDA) shows great chemical versatility, and can be used as a platform for a diverse range of secondary reac- tions. For example, Xu et al. developed a versatile method for the fabrication of highly water-repellent microparticles under mild conditions.[25] Liu et al. fabricated multifunctional gra- phene composite paper through the fusion of nacre, mussel, and lotus leaf.[26] The effective oil/water separation of superhydrophobic ma- terials is caused by the superhydrophobic coatings on the sub- strates. However, most of the reported superhydrophobic ma- terials are mechanically and chemically instable. On one hand, the microscale and/or nanoscale roughness are important in the fabrication of superhydrophobic surfaces, but inherently weak toward mechanical abrasion. On the other hand, materi- als with low surface energies for the fabrication of superhydro- phobic surfaces are often chemically unstable toward acids, bases, organic solvents, and so on. The low stability of super- hydrophobic materials is the issue we are facing in terms of their practical applications, for example, for oil/water separa- tion. Herein, we report the fabrication of durable superhydropho- bic PU sponges for the selective removal of oils and other or- ganic pollutants from water, as inspired by the mussel adhe- sive protein and the lotus leaf (Figure 1). The fabricated PU sponges show good superhydrophobicity/superoleophilicity and high absorption capacities for various organic pollutants. Moreover, the superhydrophobic coatings feature high me- chanical, chemical, and environmental stabilities, which ensure that the superhydrophobic PU sponges can be used repeated- ly, even under harsh practical conditions. Results and Discussion Preparation of superhydrophobic PU sponges In wet conditions, mussels can achieve long-lasting adhesion on a variety of natural and synthetic substrates (Figure 1 a). The adhesive proteins have been discovered to contain high levels of dopamine (Figure 1 b).[6] It is well known that the lotus leaf has self-cleaning and water-repellent properties. The water-repellent behavior comes from the low surface energy of the botanical wax on the leaf and the unique micro-/nano- hierarchical structure (Figure 1 c). In tris(hydroxymethyl) aminomethanehydrochloride (Tris-HCl) aqueous solution at pH 8.5, dopamine polymerized spontane- ously into PDA and was deposited on the surface of a PU sponge. The yellow PU sponge became a brown PU@PDA sponge, and the contact angle (CA) of the PU sponge de- creased from 1268 to 998 (Figure 1 d,e). Subsequently, the PU@PDA sponge was modified with Ag nanoparticles (Ag-NPs) by reduction with n-butylamine in ethanol. In comparison with the PU@PDA sponge, the PU@PDA@Ag sponge was darker with a metallic luster. The water CA on the PU@PDA@Ag sponge increased to 1358 (Figure 1 d,e). Then, the superhydro- phobic sponge was produced simply by modifying the PU@P- DA@Ag sponge with an ethanol solution of dodecyl mercaptan (DM) to decrease the surface tension further. The PU@P- DA@Ag@DM sponge is superhydrophobic with a water CA of 1558 and a water shedding angle (WSA) of 38 (Figure 1 d,e). It should be noted that the errors for water CA measurements are very high ; it is impossible to get an accurate outline of the water drops (Figure S1, Supporting Information) and then mea- sure the CA and CA hysteresis exactly because the sponge sur- face is macroscopically rough, pliant, and nonreflective.[27] Con- sequently, the classical CA measurements were unsuited to a reliable evaluation of the wetting properties of the surfaces. Thus, the WSA was used to evaluate water-repellent properties of the PU@PDA@Ag sponge in this study, instead of CA and CA hysteresis. The scanning electron microscope (SEM) images of the PU, PU@PDA, PU@PDA@Ag, and PU@PDA@Ag@DM sponges with different magnifications are shown in Figure 2. The macropo- rous PU sponge is composed of fibers with smooth surfaces (Figure 2 a). At a low magnification (left column), there is no clear difference in the surface morphology of the sponges. The pores of the PU sponges, which are closely related to the oil absorption capacity, are kept very well after in-turn modifica- tion with PDA, Ag NPs, and DM. At a higher magnification (middle and right columns), PDA can be seen on the surface of the fibers, and the surface becomes rough after modification with PDA (Figure 2 b). A large number of Ag NPs are observed on the surface of the PU@PDA@Ag sponge. Thus, the PU@P- DA@Ag sponge with a two-tier micro-/nanostructure is formed (Figure 2 c). The surface morphology of the PU@PDA@Ag@DM sponge is very similar to that of PU@PDA@Ag (Figure 2 d). The surface chemical composition of the sponges was iden- tified by X-ray photoelectron spectroscopy (XPS) (Figure 3). The C1s (285.08eV), N1s (399.08eV), and O1s (532.08 eV) peaks were detected on the surface of the PU sponge. As ex- pected, no clear change in the XPS spectrum was observed after the PU sponge was modified with PDA because the chemical elements of PU and PDA are the same. The new Ag peaks at 368.08, 374.08, 573.08, and 604.08 eV in the XPS spec- trum of the PU@PDA@Ag sponge confirmed the successful binding of Ag NPs on the surface of the PU@PDA sponge (Fig- ures 3 c and S2). Meanwhile, the intensities of the C 1s, N1s, and O 1s peaks decreased significantly compared with the spectrum of PU@PDA, which also indicates that the surface of the PU@PDA sponge is covered with a layer of Ag NPs. The new S 2p peak at 162.08 eV in the XPS spectrum of the PU@P- DA@Ag sponge confirmed the successful binding of DM on its surface. Wetting behavior of PU@PDA@Ag@DM sponges The wetting behavior of the PU@PDA@Ag@DM sponge is shown in Figure 4. The uncoated PU sponge can be wetted easily by water, whereas it changes into a superhydrophobic PU@PDA@Ag@DM sponge upon coating in turn with PDA, Ag NPs, and DM. Water drops are spherical in shape on the PU@P- DA@Ag@DM sponge, and roll off the slightly tilted ( ^ 38) sample easily (Figure 4 a). A jet of water from a pipet can bounce off the sponge surface without leaving a trace (Fig- ure 4 b). In addition, the PU@PDA@Ag@DM sponge can float on the surface of water for a few weeks and can be immersed in water upon application of an external force. The sponge sur- face was surrounded by an air cushion in water, exhibiting a silver mirror-like surface (Figure 4 c). After the external force was released, the PU@PDA@Ag@DM sponge floated immedi- ately on the water surface without any water absorption. Moreover, a 7 mL water droplet released from a height of 15 mm could bounce many times on the horizontal sponge (Movie S1), showing that the kinetic energy of the water drop- let is conserved well by the surface deformation and the dissi- pation of the kinetic energy by the work of adhesion is very low during the impact with the surface. No difference in water-repellent properties can be seen between the outside surface and the cross section of the PU@PDA@Ag@DM sponge (Figure 4 d), indicating that the PU sponge is coated uniformly. The coating of the PU sponge leads to changes in its wetta- bility. The uncoated PU sponge can be wetted easily by both water and oil. However, the PU@PDA@Ag@DM sponge is su- perhydrophobic and superoleophilic, and can only be wetted by oil (Figure 4 e-g). Oils, including petrol, diesel, and crude oil, can spread on the sponge in 6 ms. The unique wettability of the PU@PDA@Ag@DM sponge toward water and oil make it very promising as a material for oil/water separation. Stability of PU@PDA@Ag@DM sponges The PU@PDA@Ag@DM sponges show good mechanical, chem- ical, and environmental stability owing to the in-turn covalent binding of PDA, Ag NPs, and DM on the surface of the PU sponge. The laundering durability of the PU@PDA@Ag@DM sponge was evaluated. A laundering procedure is a combination of various mechanical interactions such as shearing forces with water and the wall of the container.[28] There have only been a few papers reporting the laundering durabilities of superhy- drophobic materials. No change in water CA on the PU@P- DA@Ag@DM sponge was detected after five machine-wash cycles. The changes in WSA of the PU@PDA@Ag@DM sponge with laundering cycles are shown in Figure 5 a. The WSA in- creases gradually with increasing numbers of laundering cycles, but remains below 118, which indicates excellent laun- dering durability of the superhydrophobic coating. A 7 mL water drop could bounce off the 128 tilted sample easily even after five laundering cycles, which indicates the very weak in- teraction between the washed PU@PDA@Ag@DM sponge and the water drop. In addition, other important aspects of mechanical durability, such as scratching with a scalpel and twisting by hand, were also studied qualitatively (Figure 5 b,c); no clear change in WSA on the superhydrophobic PU@PDA@Ag@DM sponge was ob- served after intensive scratching with a sharp scalpel and twist- ing by hand. It is of primary importance that a coating does not affect the mechanical properties of the substrate, for example, its tensile strength and flexibility. The frequently used strong acid corrosion of the PU sponge for the preparation of superhydro- phobic sponges will inevitably weaken the mechanical proper- ties of the PU sponges and cause secondary pollution. Unlike strong acid activation, the mild coating approach used in this study surprisingly improves the mechanical properties of the PU sponge (Table 1). In comparison with the pristine PU sponge, the tensile strength and elongation at break were en- hanced by 23.2 % and 24.5 %, respectively, owing to the uni- form and covalent modification of the PU sponge. In respect of the environmental and chemical durability of the superhydrophobic PU@PDA@Ag@DM sponge, a series of experiments was performed under various conditions (Fig- ure 5 d and Table 2). The superhydrophobic sponges show ex- cellent environmental and chemical stability, and are stable against, acid, salt and alkali solutions, UV irradiation (200- 400 nm), and very high and low temperatures. Only slight in- creases in WSA were detected after these treatments. The PU@PDA@Ag@DM sponges are also resistant to various organ- ic solvents and oils owing to their superhydrophobicity. Pro- longed immersion has a very small effect on the superhydro- phobicity of the samples. The WSA is below 78 after immersion in these organic solvents and oils for seven days. The water drops are spherical in shape and roll off the samples easily after the stability tests in organic solvents and oils. Selective removal of organic pollutants The outstanding mechanical, chemical, and environmental sta- bility of the superhydrophobic PU@PDA@Ag@DM sponges pave the way for selective oil absorption and oil/water separa- tion. The PU@PDA@Ag@DM sponges absorbed oils selectively from water, as shown in Figure 6. Notably, no water uptake was observed during the absorption of oils, indicating excel- lent absorption selectivity of the as-prepared sample. Further- more, no dripping of organic pollutants was observed in the handling process, indicating firm absorption by the PU@P- DA@Ag@DM sponge. If the PU@PDA@Ag@DM sponge was placed on the surface of an oil/water mixture, the oil floating on the surface of the water was absorbed within a few seconds (Figure 6 a and Movie S2). In addition, the PU@PDA@Ag@DM sponge could be used to remove organic liquids with higher density than water, for example, chloroform. Upon placing the PU@PDA@Ag@DM sponge close to the chloroform in water, the chloroform was absorbed rapidly upon contact (Figure 6 b and Movie S3). The absorption capacities of the PU@PDA@Ag@DM sponge for var- ious organic pollutants are shown in Figure 6 c. The absorption capacity for chloroform is as high as 43 g g^1, and other ab- sorption capacities are in the range 18 to 43 g g^1 depending on the density of the oils and organic solvents. The influence of repeated absorption/desorption of oils on the oil absorption capacity of the PU@PDA@Ag@DM was also investigated to evaluate its reusability (Figure 6 d), which is an important criterion for practical applications. After five absorp- tion/desorption cycles, the oil absorption capacities of the ma- terials for the three oils were still comparable with those of the freshly coated sample, indicating excellent recyclability. Almost all of the absorbed oil in the sponge could be removed by squeezing. In addition, no measurable weight loss of the sam- ples was detected after soaking in the three oils and the re- peated absorption tests. This is attributed to the covalent modification of the PU sponge. The oil/water separation efficiency of the PU@PDA@Ag@DM sponge is shown in Figure 7 and Movie S4. A mixture of n- octane and 5 ppm methylene blue (MB) aqueous solution was poured slowly onto the sponge ; the oil penetrated through the material quickly and dropped into the flask beneath it. Meanwhile, more and more water was collected on the surface of the PU@PDA@Ag@DM sponge. The water concentration in the collected n-octane determined using a Compact Karl Fisch- er Coulometer (C20, Mettler Toledo, Switzerland) was below 60 ppm, which is in the range of the normal water concentra- tion in n-octane. Moreover, no oil was detected in the collect- ed water. These results indicate the very high oil/water separa- tion efficiency of the superhydrophobic sponge. Conclusion We have demonstrated a versatile method for the preparation of durable superhydrophobic/superoleophilic PU sponges for the selective removal of organic pollutants from water. The su- perhydrophobic PU sponge is prepared through in-turn cova- lent modification of a PU sponge with PDA, Ag NPs, and DM. The modification not only introduces nanoscale roughness on the skeleton of the PU sponge, but also decreases the surface tension. The frequently used acid etching approach for the preparation of superhydrophobic PU sponges is avoided by using PDA as a platform for further reactions. Consequently, the mechanical properties of the PU sponge are improved at the same time as making it superhydrophobic. The superhy- drophobic PU sponge exhibits good superhydrophobicity/su- peroleophilicity, and outstanding mechanical, chemical, and environmental stability. In addition, the superhydrophobic PU sponge can absorb various oils and organic liquids quickly, and can separate oil/water mixtures with high separation efficiency. The superhydrophobic PU sponge is very stable in oils and can be used repeatedly for oil/water separation. Thus, we expect that the durable superhydrophobic PU sponges will be adopt- ed widely for the selective removal of organic pollutants from water. Experimental Section Materials PU sponge was supplied by Shaoxing Chengfeng Foam Co. Dopa- mine hydrochloride (98 %) was purchased from Shanghai DEMO Medical Tech Co. , China. Tris-HCl, AgNO3, NaOH, n-butylamine, an- hydrous ethanol, n-octane, DM, and MB were purchased from China National Medicines Co. All chemicals were used as received without further purification. Fabrication of PU@PDA sponge Dopamine (1.5 mg mL^1) was dissolved in 10 mm Tris-HCl and the pH was adjusted to 8.5 using 1 m NaOH. A piece of PU sponge (1^ 1^ 1 cm) was added to 15 mL of the above dopamine solution. The solution was stirred mechanically overnight at 25 8C. Then, the PU@PDA sponge was washed several times with deionized water and dried in an oven at 60 8C. Fabrication of PU@PDA@Ag sponge AgNO3 (0.68 mg mL^1) was dissolved in anhydrous ethanol under vigorous stirring in a conical flask. A piece of PU@PDA sponge (1 × 1 × 1 cm) was immersed in the AgNO3 solution. Then, 10 mL of n - butylamine solution (10 mm in ethanol) was added to the AgNO3 solution at 50 8C. The conical flask was shaken in a thermostatic shaker (THZ-98A, Chincan, Zhejiang, China) at 50 8C and 120 rpm for 50 min to form the PU@PDA@Ag sponge. The PU@PDA@Ag sponge was washed several times with ethanol and dried in an oven at 60 8C. Fabrication of PU@PDA@Ag@DM sponge A piece of the PU@PDA@Ag sponge was immersed in the DM eth- anol solution (2 % v/v) at room temperature for 24 h. The as-pre- pared PU@PDA@Ag@DM sponge was washed several times with ethanol and dried in a vacuum oven at 40 8C. Measurements of CA and WSA The CA and WSA measurements were performed with a Contact Angle System OCA20 (Dataphysics, Germany) equipped with a tilt- ing table. The syringe was positioned such that the liquid drop (7 mL) contacted the surface of the samples before leaving the needle. For the WSA measurements (Figure S3), the samples were fixed onto a sample holder and placed on the tilting table. The sy- ringe was positioned with the needle tip 10 mm above the tilted sample such that a drop falling from the needle would contact the substrate 8 mm from the bottom end of the sample. The needle with an inner diameter of 110 mm was used to produce liquid droplets with a volume of 7 mL. For the determination of the WSA, measurements were started at an inclination angle of 508. Droplets of water were released onto the sample at a minimum of three dif- ferent positions. If all drops bounced or rolled completely down the sample, the inclination angle was reduced by 28 and the proce- dure repeated until one or more of the droplets did not roll down the surface completely. The lowest inclination angle at which all the drops rolled down or bounced off the surface completely was taken as the WSA. Stability tests For the laundering tests, the samples were washed in a washing machine with ten pieces of cotton textiles (20^ 20 cm) for five cycles (30 min each) at room temperature. After each washing cycle, the sponges were washed in turn with deionized water and absolute ethanol three times, and then dried in an oven at 60 8C before WSA measurement. For the determination of the stability in oils and organic solvents, a piece of the sample was immersed in oil or organic solvent for seven days. Subsequently, the sample was washed with n-hexane and dried in an oven at 60 8C before the WSA measurement. Selective oil absorption A piece of sample was immersed in oil or organic solvent at room temperature. The sample was taken out after 1 min and wiped with filter paper to remove excess oil. The absorption capacity k of the sample was determined by weighing the sample before and after absorption and calculated according to Equation (1), in which Winitial and Wsaturated are the weight of the dry sample and weight of the sample at absorption equilibrium, respectively, and k is calcu- lated as grams of oil per gram of sample. k = (Wsaturated-Winitial)/Winitial initial (1) Oil/water separation For oil/water separation, a mixture of n-octane (20 mL) and MB aqueous solution (30 mL, 5 ppm) was poured slowly onto the custom-built setup. The PU@PDA@Ag@DM sponge was fixed be- tween a glass tube and a flask. Characterization Micrographs of the samples were obtained by SEM (JSM-6701F, JEOL). Before SEM observation, all samples were fixed on alumi- num stubs and coated with gold (^ 7 nm). The XPS spectra were obtained with a VG ESCALAB 250 Xi spectrometer equipped with a monochromated AlK a X-ray radiation source and a hemispherical electron analyzer. The spectra were recorded in the constant-pass energy mode with a value of 100 eV, and all binding energies were calibrated using the C 1s peak at 284.6 eV as the reference. The me- chanical properties of the PU and PU@PDA@Ag@DM sponges were measured with a universal testing machine (CMT4304, Shenzhen SANS Test Machine Co. , Shenzhen, China) equipped with a 200 N load cell at room temperature with a gauge length of 3 mm and crosshead speed of 2 mm min^1. All the tests were performed in triplicate. 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Received : November 27, 2013 Published online on April 10, 2014 Bucheng Li,[a] Lingxiao Li,[a] Lei Wu,[a, b] Junping Zhang,*[a] and Aiqin Wang[a] [a] B. Li, L. Li, L. Wu, J. Zhang, A. Wang Center of Eco-material and Green Chemistry Lanzhou Institute of Chemical Physics Chinese Academy of Sciences Tianshui Middle Road 18 Lanzhou 730000 (P. R. China) Fax: (+ 86) 931 8277088 E-mail : [email protected] [b] L. Wu Graduate University of the Chinese Academy of Sciences Beijing 100049 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201300403. (c) 2014 Blackwell Publishing Ltd. |