TiO2 nanocomposite membrane with enhanced adsorption and photocatalytic degradation for dyes under ultraviolet-visible irradiation

Journal of Colloid and Interface Science 562 (2020) 21–28

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Three-dimensional bacterial cellulose/polydopamine/TiO2 nanocomposite membrane with enhanced adsorption and photocatalytic degradation for dyes under ultraviolet-visible irradiation Luyu Yang, Chuntao Chen, Ying Hu, Feng Wei, Jian Cui, Yuxiang Zhao, Xuran Xu ⇑, Xiao Chen ⇑, Dongping Sun ⇑ Institute of Chemical Biology and Functional Materials, School of Chemical Engineering, Nanjing University of Science and Technology, 200 Xiao Ling Wei, Nanjing 210094, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 26 September 2019 Revised 2 December 2019 Accepted 3 December 2019 Available online 4 December 2019 Keywords: Bacterial cellulose Titanium dioxide Polydopamine Photocatalyst Adsorption

a b s t r a c t Cleaner production of photocatalyst with efficient property and stable reusability is of great importance for the elimination of organic pollutants in wastewater. Herein, we present a bacterial cellulose (BC)based nanocomposite membrane with enhanced adsorption and photocatalytic degradation for dyes under UV radiation, by using BC nanofibers as a three-dimensional soft template, coated with the polydoamine (PDA) as a functional layer and a protective agent for immobilization of titanium dioxide (TiO2) nanoparticles. Compared with commercial P25, the as-prepared BC/PDA/TiO2 composite presented higher adsorption capacity for methyl orange, Rhodamine B and methylene blue, and the photocatalytic properties within 30 min after irradiation were further improved. BC/PDA/TiO2 also showed good stability, proved by the 5.5% reduction in photocatalytic capability after five cyclic tests. We expect our design could provide a facile and green approach with excellent photo-degradation performance for organic pollutants, providing further applications for photocatalysis and wastewater treatment. Ó 2019 Published by Elsevier Inc.

⇑ Corresponding authors. E-mail addresses: [email protected] (X. Xu), [email protected] (X. Chen), [email protected] (D. Sun). https://doi.org/10.1016/j.jcis.2019.12.013 0021-9797/Ó 2019 Published by Elsevier Inc.

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1. Introduction In the past few decades, the fast development of industrialization has brought critical environmental issues, especially the water contamination, which is very harmful to human health [1]. Among the various pollutants, organic dyes are difficult to treat by traditional physicochemical or biochemical methods owing to their chemical stability and poor biodegradability. Photocatalytic technology is one of the most common approaches for the elimination of organic dyes, with the advantages of eco-friendly operation, high photo-degradation efficiency, and reusability. At present, using titanium dioxide (TiO2) as the photocatalyst has become one of the most effective means to remove organic contaminants from water [2,3]. However, there are still some problems restricting its large-scale applications, such as the easy agglomeration, the difficulty of recycle, and high energy cost, which in turn may adversely increase the burden on the environment. It is an effective solution to load TiO2 onto various substrates, such as non-woven fabric [4], activated carbon [5], porous ceramics, molecular sieves and natural minerals [6] as well as polysaccharides in order to improve their photocatalytic properties, and more importantly, to facilitate the recycle and reuse of the catalyst. Cellulose is considered to be the most extensive and renewable polysaccharide on Earth, possessing the advantages such as hydrophilicity, biodegradability, and ease of extensive chemical modifications [7]. Bacterial cellulose (BC), a high-molecular polysaccharide secreted during microbial growth and metabolism, is composed of microfibers with a diameter of 40–60 nm [8]. The three-dimensional network structure of the bacterial cellulose membrane provides high specific surface area, high mechanical strength, and abundant hydroxyl groups on the surface of the fibers, making BC an ideal carrier for TiO2 nanoparticles. Previously our group prepared a BC/TiO2 photocatalytic material by hydrothermal treatment of tetrabutyl titanate and BC in a reaction kettle, and studied its ability to degrade organic dyes [9]. Moreover, Li et al. showed that immobilization of TiO2 on the oxidized bacterial cellulose (OBC) exhibited a good photocatalytic effect on organic dyes under UV-light [10]. However, there are still some drawbacks in these methods. For example, loading TiO2 nanoparticles at high temperature or irradiation of UV-light may cause the degradation of the cellulose substrate. Therefore, it is necessary to develop a BC/TiO2 composite with enhanced adsorption ability, photocatalytic performance and long-term recycling use. Recently, the bionic mussel-chemistry represented by the oxidation and self-polymerization of dopamine has injected great vitality into the development of material interface science and technology [11], especially in the designs of composite materials. It has been reported that PDA could serve as a secondary reaction platform through creating non-covalent and covalent bonds with nanoparticles [12]. Recent studies have confirmed that PDA presents extremely high adsorption properties for organic dye molecules due to the bisphenol structure on the dopamine benzene ring [13,14]. Liu’s group have modified natural cellulose by coating a PDA layer, which showed very high adsorption capacity for Pb2+ and methyl orange [15]. In addition, PDA can promote the separation of photo-generated carriers at the interface between PDA and TiO2 to improve the photocatalytic activity. A piece of evidence comes from Sun’s group, who found that after modifying with a layer of PDA, the TiO2@PDA exhibited stronger absorption of light [16,17]. Finally, the deposited nanoparticles have a strong affinity with the cellulose fibers due to the ‘‘bridge” effect of the PDA as a binder [18]. Given the advantages of BC carriers and PDA coating in improving the photocatalytic properties of nanoparticles, herein we present a facile approach to prepare BC/PDA/TiO2 composite by using three-dimensional BC network scaffold as the soft template

and PDA as the functional layer. The adsorption efficiency and degradation rate of BC/PDA/TiO2 toward traditional organic dyes such as methyl orange, methylene blue and Rhodamine B were investigated. Compared with commercial P25 and BC/TiO2, BC/ PDA/TiO2 presents higher adsorption, faster photocatalytic degradation rate as well as efficient reusability and stability, which provides a simple design of a TiO2-based photocatalyst with low energy consumption, economic and environmental sustainability. 2. Experimental 2.1. Materials Dopamine-HCl (98%), tris(hydroxymethyl)aminomethane (99%) and (NH4)2TiF6 (ammonium hexafluorotitanate) were obtained from Aladdin Reagent Co. Urea, ethanol, methyl orange (MO), methylene blue (MB), Rhodamine B (RhB) and deionized water were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). P25 (Commodity TiO2, 80% anatase and 20% rutile, average particle size 25 nm, BET specific surface area 50 m2/g) was purchased Degussa, Germany. All chemicals were used as received without further purification. 2.2. Preparation of BC/PDA complex film BC films were produced by using acetobacter xylinum NUST4.2 stocked in our laboratory through a static fermentation process at 30 °C for 24 h. BC/PDA was prepared by chemical oxidative polymerization of dopamine with BC in aqueous solution. Typically, BC film (30 mm
30 mm) was immersed into the dopamine hydrochloride solution (0.02 mol/L), fully dissolved under ultrasonic conditions. Tris was added to adjust the pH of the solution to 8.5 and placed in a constant temperature shaker at 30 °C for 12 h to obtain BC/PDA composite membrane. 2.3. Preparation of BC/PDA/TiO2 photocatalytic complex film The BC/PDA/TiO2 samples were prepared as the following processes. Individual BC/PDA films were pre-soaked in a 100 mL mixture containing 0.3 g of (NH4)2TiF6 and 3 g of urea, and the mixture was kept in water-bath at 85 °C for different durations of time to obtain the BC/PDA/TiO2 complex film. The obtained product was washed by deionized water, and freeze dried. BC/PDA/TiO2 produced with 45, 50, 55 and 60 min reaction time was noted in the following studies as S1, S2, S3 and S4, respectively. In addition, BC/TiO2 complex prepared without PDA under the same conditions was used as control for comparative studies. 2.4. Instruments and methods The surface morphology of the as-prepared materials was observed by SEM (Hitachi S-4800) at 25 kV after coating with a thin layer of evaporated gold. FT-IR spectra were collected on a Thermo Scientific Nicolet iS5 spectrometer with scanning wavelengths from 500 to 4000 cm1. The XRD patterns were obtained by using a Bruker AXS D8 advanced diffractometer with Cu Ka radiation (k = 1.5418 Å) at 40 kV and 35 mA. XPS spectra were collected using an RBD-upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg K radiation (h = 1253.6 eV). The UV–vis diffuse reflectance spectra of the materials were obtained by a spectrophotometer (Cary 5000). The contact angle of the materials was measured using a video optical contact angle meter (Dataphysics OCA25L, Germany). Mechanical properties of the materials were tested by an electronic universal testing machine (TY8000-A) at a speed of 2 mm/min. The point of zero charge (PZC) of the materials

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was determined with Zetasizer Nanoseries ZS instrument (Malvern Instruments). Room temperature photoluminescence spectra (PL) were detected with an RF6000 Shimadzu fluorescence spectrophotometer with the excitation wavelength of 325 nm.

2.4.1. Adsorption of MO, MB and RhB The adsorption properties were studied under dark conditions at 25 °C. A certain mass of the materials with the same quantity of TiO2 (30 mg) was added to 50 mL solution containing the organic dye at a concentration of 20 mg/L. The concentration (Qt, mg/g) of dye in each sampling solution at the timepoint t postreaction was measured by recording its absorbance on an UV–vis spectrophotometer (UV2500, Xu Jiang electromechanical plant, Nanjing Chian) at 465 nm (MO), 664 nm (MB) and 554 nm (RhB). Qt was calculated using Eq. (1):

Qt ¼

ðC 0  C t ÞV m

ð1Þ

where C0 (mg/L) represents the initial concentration of the dye, Ct (mg/L) is the concentration of the dye at the timepoint t. Qt (mg/ g) is the adsorption of dye at the timepoint t. V is the volume of the adsorption solution, and m is the mass of the material.

2.4.2. Photocatalysis of MO, MB and RhB The photocatalytic capacity and rate of the material was carried out under ultraviolet light. A certain mass of the material with the same quantity of TiO2 (30 mg) was place into MO, MB or RhB aqueous solution in a quartz tube and stirred for 60 min in dark to reach the adsorption–desorption equilibrium. After that, the quartz tube was placed under ultraviolet lamp (Hg lamp, ASSATUV, 500 W, >320 nm). At different timepoints after irradiation, the concentration of MO, MB and RhB in the solution was monitored by an ultraviolet spectrophotometer (UV2500, Xu Jiang electromechanical plant, Nanjing China) at 465 nm, 664 nm and 554 nm, respectively.

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3. Results and discussion The experimental design of this study is illustrated in Scheme 1. In the first step, the PDA layer was coated onto the surface of BC via oxidation and self-polymerization of DA in mild conditions (Fig. S1). One can see that the PDA gradually deposits and agglomerates on the BC surface as the harvesting time increases, eventually leading to complete coating of PDA on the BC film (Fig. S2). Subsequently, the TiO2 nanoparticles are obtained by slow hydrolysis of (NH4)2TiF6. According to the literature, PDA coating could serve as a good platform for TiO2 nanoparticles loading [19], for the hydroxyl groups of PDA can react with the metal ion to form metal oxide nanoparticles [1], improving the binding affinity between TiO2 nanoparticles and the nanofibers [20]. Therefore, we assume that the uniformly dispersed TiO2 nanoparticles in the three-dimensional network structure of the BC membrane, together with the good hydrophilicity of BC membrane, would serve as an effective adsorbent during the photocatalytic degradation of organic pollutants. 3.1. Preparation of BC/PDA/TiO2 The surface morphology of BC, BC/PDA and BC/PDA/TiO2 was obtained by SEM. Fig. 1a shows the heavily interwoven threedimensional nanofibrous structures with the fiber diameters of ca. 40 nm in the pristine BC membrane. After the coating of PDA layer, the diameter of nanofibers increased to ca. 50 nm (Fig. 1b). One can also see that the PDA coating layer is rather uniform along the BC nanofibers, which is expected to facilitate the further loading of active layer [21]. In the figure of sample BC/PDA/TiO2, the distribution of TiO2 nanoparticles with size of ca. 200 nm along the nanofibers could be observed (Fig. 1c). Compared with the sample BC/TiO2 without PDA coating (Fig. 1d), it can be seen that spherical TiO2 nanoparticles was more evenly dispersed in BC/ PDA/TiO2 sample. In addition, it can be seen from Fig. S3a that the TiO2 nanoparticles were uniformly dispersed on the nanofibers of the BC/PDA without any agglomeration appearing, which facilitates the diffusion of the organic dye molecules into the BC film

Scheme 1. Preparation of BC/PDA/TiO2 by immobilizing TiO2 NPs on PDA-modified BC membranes and use of BC/PDA/TiO2 for the photocatalytic degradation and adsorption of organic dyes.

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Fig. 1. Representative SEM images of (a) pristine BC, (b) BC/PDA, (c) BC/PDA/TiO2, and (d) BC/TiO2. Inserts are optical images of corresponding samples.

interior and the adsorption behavior of the PDA layer. However, P25 nanoparticles agglomerated easily on the surface of BC (Fig. S3b). This plate-like structure appears on the surface of BC, which greatly hinders the adsorption behavior of composite materials on organic dyes. Fig. S4 showed the formation of TiO2 particles as the harvesting time increased. It can be seen that with the extension of reaction time, TiO2 nanoparticles mutually assembled into multi-stage structures (Fig. S4c), and finally assembled into spherical particles with a diameter of 200 nm (Fig. S4d). The chemical structure of BC, BC/PDA and BC/PDA/TiO2 was examined by FTIR with spectra shown in Fig. 2a. Compared to the BC fabrics, the FTIR spectrum of the BC/PDA contains a high intensity peak at 1612 cm1, which is attributed to the C@C stretch vibration of aromatic rings of dopamine [19,22], and the peak at 1517 cm1 corresponds to the NAH shearing vibration [23]. The absorption peak of the BC/PDA at 3345 cm1 is broadened due to the coating of PDA and the loading of TiO2 nanoparticles. The surface of PDA and TiO2 nanoparticles in sample BC/PDA/TiO2 contains a large amount of AOH, with the characteristic peak locating at the wavelength of 3200–3600 cm1 [24]. The peaks at around 1000– 1300 cm1 in BC/PDA/TiO2, due to CAOH stretching (1060 cm1) and CAOAC bending vibrations (1163 cm1), are weakened in comparison to the peaks in BC due to the coating of PDA and TiO2 nanoparticles. It is well known that anatase TiO2 shows better photocatalytic activity than other types of TiO2 [25]. To explore the effect of crystal structures of TiO2 in our samples, all samples were tested by XRD as presented in Fig. 2b. The characteristic peaks of the crystal structure of BC can be found at 14.5°, 16.6° and 22.5° corresponding to the 

crystallographic plane of (1 1 0), (1 1 0), (2 0 0), respectively, which can also be found in BC/PDA sample. After coating TiO2 on BC/PDA fibers, the intensity of the above peaks became weaker. In addition, the new strong peaks appeared at 25.3°, 37.9°, 48.0°, 53.8°, 55.1° and 62.7°corresponding to the crystallographic plane of (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) of anatase TiO2 (JCPDS

No. 21-1272) [9]. The average grain size of the TiO2 nanoparticles KY , where is about 40 nm (calculated by the Scherrer formula D ¼ bcosh D is the average thickness of the crystal grains perpendicular to the crystal plane direction, K is the Scherrer constant, b is the halfheight width of the diffraction peak of the measured sample, h is the Bragg diffraction angle, and c is the X-ray wavelength), which is similar to the grain size shown in Fig. S1a in SI. The chemical composition of BC, BC/PDA and BC/PDA/TiO2 was determined by XPS. As in Fig. 2c, all samples containing PDA showed a new N 1s peak at 396–400 eV. For BC/PDA/TiO2 sample, one could see a strong Ti 2p peak at 455–465 eV, corresponding to the orbital electron binding of the anatase TiO2 reported in the literature [26]. From the deconvolutions of these peaks in Fig. S5, the peak of N1s at about 400 eV is attributed to the presence of oxidized nitrogen in the form of TiAOAN bonds, which is an evidence of N-doping into the TiO2 lattice [27]. Thermogravimetric analysis (TGA) is an acceptable approach to study the effect of metal oxide on the thermal decomposition of cellulose. Fig. 2d shows TGA results of BC, BC/PDA, BC/TiO2, and BC/PDA/TiO2. The pure BC presents the initial decomposition temperature (Tonset) at 300.2 °C. This value of BC/TiO2 is decreased to 290 °C after the loading of TiO2 nanoparticles, which may be due to the catalytic effect of TiO2 nanoparticles [15,28]. We found that the initial decomposition temperature of BC/PDA and BC/PDA/TiO2 was higher than BC/TiO2, indicating that PDA played a role in protecting BC. Mechanical properties are also important in evaluating the membrane performance. As shown in Fig. 2e, both BC and BC/ PDA exhibit similar tensile strength of 0.7 MPa, and the PDA coating improves the elongation at break. However, tensile strength of BC/PDA/TiO2 decrease significantly (0.51 MPa), with typical brittle fracture behavior of materials. We hypothesize that the use of urea and thermo-reaction during the formation of TiO2 result in a slight degradation of BC, thereby reducing the mechanical properties of the treated nanofibers.

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Fig. 2. (a) FTIR spectra, (b) XRD patterns, (c) XPS spectra, (d) TGA curves, (e) Stress-strain curves, (f) photographs of water droplets on the surface at 1s after contact of BC, BC/ PDA and BC/PDA/TiO2 samples. (g) N2 adsorption-desorption isotherms of BC, BC/PDA and BC/PDA/TiO2. (h) UV–vis diffuse reflectance spectra of P25, BC/TiO2 and BC/PDA/ TiO2.

The surface hydrophilicity of BC, BC/PDA and BC/PDA/TiO2 was characterized by measuring the water contact angle. As can be seen from Fig. 2f, BC exhibits a contact angle of 8.2° while the other two samples exhibit non-observable water contact angle (after one second). It is shown that the modification on BC with PDA and TiO2 remarkably enhances the surface hydrophilicity of BC, and this change is good for adsorption performance in water environment.

The porous properties of BC, BC/PDA and BC/PDA/TiO2 were investigated by N2 adsorption-desorption experiments (Fig. 2g). Three samples exhibit type IV isotherms with an H1 loop (IUPAC classification), indicating a uniform mesoporous structure [29]. The BET specific surface area of BC/PDA/TiO2 is calculated to be 170 m2/g, which is 5.2 times higher than that of pure BC (32.4 m2/g) and 1.7 times higher than BC/PDA (98.5 m2/g), suggest-

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ing that the coating of TiO2 and PDA layers could effectively increase the exposed active surface area for adsorption. Fig. 2h shows the UV–vis diffuse reflectance spectra of commercial TiO2 (P25) powder, BC/TiO2 and BC/PDA/TiO2. It can be observed that the absorbance in the visible light region is significantly enhanced and shifted for BC/PDA/TiO2, mainly because the synergistic effect of PDA and TiO2 promotes the migration efficiency of photogenerated carriers at the interface between PDA and TiO2 [30]. The band gap of these samples can be estimated by the following equation: Eg = 1240 , where kg is the wavelength corresponding to kg the intersection point of the linear part and horizontal axis of the spectrum [9]. The Eg of BC/PDA/TiO2 (2.97 eV) is lower than that of P25 (3.10 eV) and BC/TiO2 (3.10 eV), suggesting a red shift occurred in the light absorption sideband of BC/PDA/TiO2, thus expanding the utilization range of UV light to UV–visible light.

3.2. Adsorption behavior and ultraviolet photocatalysis of BC/PDA/TiO2 The adsorption and photocatalytic properties of BC/PDA/TiO2 samples towards organic dyes (MO, RhB, MB) were examined. Meanwhile, BC/TiO2, BC/PDA and P25 were also tested as control groups. Sample S4 was chosen for the following experiments owing to its highest photo-degradation efficiency of MO, RhB and MB as shown in Fig. S6.

As shown in Fig. 3a, c and e, the adsorption capacity of BC/PDA/ TiO2 towards MO, RhB and MB was calculated to be 11.8 mg/g, 13.4 mg/g, and 28.9 mg/g, respectively, which is larger than the values of the BC/TiO2 and P25. The adsorption capacity of MO, RhB and MB on BC/TiO2 was extremely low, while the adsorption capacity of BC/PDA/TiO2 was significantly improved (6.4, 5.7, and 4.8 times, respectively). The adsorption capacity of BC/PDA/TiO2 toward all organic dyes at varied solution pH was shown in Fig. S7a. The adsorption capacity of MB and RhB were low at low pH, but dramatically increased when the solution pH increased to 7, while that of MO showed the opposite tendency. Surface charge is an important factor affecting the interactions between sorbents and organic dyes, so the zeta potentials of BC/PDA/TiO2 at varying pH were determined, and the results were shown in Fig. S7b. The PZC of BC/PDA/TiO2 was found to be around 5.5, which provides a reasonable interpretation for the variation of adsorption capacity of organic dyes at different pH. In addition, although MB and RhB are both cationic dyes, BC/PDA/TiO2 has different adsorption capacity for either of them. From Fig. S7c we can see that RhB has a larger molecular weight (MB:373.9, RhB:479.01, MO:327.33) and a more complex molecular structure, suggesting the influence of space resistance as a reasonable explanation for the higher adsorption of MB than RhB [31]. Interestingly, BC/PDA has the largest adsorption capacity for MO, RhB and MB (14.8 mg/g, 15.9 mg/g and 38.7 mg/g), however, the adsorption

Fig. 3. Adsorption capacity of BC/TiO2, BC/PDA, BC/PDA/TiO2 and P25 towards MO (a), RhB (c), and MB (e); photocatalytic activity of BC/TiO2, BC/PDA/TiO2 and P25 towards MO (b), RhB (d), and MB (f), respectively.

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capacity of BC/PDA/TiO2 decreased, mainly because TiO2 support reduced the relative content of PDA that played the main adsorption role of organic dyes. The photocatalytic degradation performances of MO, RhB and MB were evaluated under UV radiation after 60 min of adsorption in the dark. As shown in Fig. 3b, d and f, BC/PDA/TiO2 presents the fastest photo-degradation rate for all dyes (MO, RhB, MB) compared with BC/TiO2 and P25. In addition, the adsorption and photocatalytic properties were studied for BC/PDA/P25 in the same conditions. Compared with BC/PDA/TiO2, the ability of BC/PDA/ P25 to adsorb and degrade organic dyes was greatly reduced as shown in Fig. S3c and d. The UV–vis spectra of MO over BC/PDA/ TiO2 BC/TiO2 and P25 after different irradiation times are shown in Fig. S8. In Table 1, we summarized the results from recent representative studies of other TiO2-based catalysts and compared the time needed for the complete degradation of various molecules under similar conditions. It is clearly to conclude our sample showed excellent photocatalytic reactivity for organic pollutant degradation. The photocatalytic process is a complex interaction between substrate, carrier and catalyst [15]. Based on the above results and discussion, there are three main factors contributing to the high photo-degradation rate of organic dyes in BC/PDA/TiO2. Firstly, more uniformly dispersed nano-sized TiO2 on BC/PDA/ TiO2 can reduce the nanoparticles agglomeration and makes them in full contact with dye molecules. Secondly, according to the PDA adsorption mechanism reported previously, which mainly depends on electrostatic attraction and the hydrogen bonding between the aromatic rings and organic dye, the PDA layer presents efficient adsorption capacity for organic dye molecules [32,33]. Finally, PDA can promote the separation of photoexcited electron hole pairs at the interface between PDA and TiO2, thus improving the photocatalytic activity. The effect of the PDA layer in BC/PDA/ TiO2 on separation of the photoexcited electron hole pairs was evidenced by the PL spectra (Fig. S9). The intensity of PL emission of BC/PDA/TiO2 was decreased significantly compared with those of BC/TiO2, suggesting efficient separation of the electron hole pairs in the presence of the PDA layer. Taken together, we assumed that PDA layer could help to improve both adsorption and photocatalytic performance of TiO2 nanoparticles, due to the synergy between PDA and TiO2 [20]. 3.3. Reusability of BC/PDA/TiO2 The reusability of photocatalyst is another important aspect in photodegradation. Fig. 4 depicts the long-term stability of the BC/PDA/TiO2, BC/TiO2 and P25 evaluated by the repeated removal of MO under ultraviolet light for 30 min. At the end of each test, the samples were cleaned and recovered with deionized water. The removal ratio of MO in BC/PDA/TiO2 decreased by only 5.5%

Fig. 4. (a) Removal efficiency of P25, BC/TiO2 and BC/PDA/TiO2 towards MO over five consecutive cycles.

(from 95.1% to 89.8%) after five cycles, suggesting good stability. By contrast, BC/TiO2 showed a poor reusability, for the removal ratio of MO was reduced by 23.8% (from 68.3% to 44.5%) after 5 cycles. This is because BC/TiO2 samples were degraded due to the lack of PDA protection in the recycling process, and the BC fiber is catalytically degraded by TiO2, which causes the TiO2 to fall off [15]. We also saw some of the BC nanofibers broken and TiO2 fallen off in BC/TiO2 (indicated by the red circle) in Fig. S10a and b, which is a major cause of poor cycle performance of BC/TiO2. Contrarily, the micromorphology of BC/PDA/TiO2 composites did not change before and after cycling (Fig. S10c and d), and the elemental content of nitrogen in BC/PDA/TiO2 did not change much after five cycles (Fig. S11 and Table S1). 4. Conclusion In summary, the soft photocatalytic adsorbent BC/PDA/TiO2 nanocomposite film was synthesized by using BC as a threedimensional network subtract. The morphological results showed that the TiO2 nanoparticles could be uniformly distributed along the PDA-modified cellulose nanofiber, which may greatly improve the surface area as well as the active sites for the removal of the dyes molecules. Furthermore, our sample not only exhibited better photo-degradation performance than commercial P25 toward MO, RhB and MB (6.4, 5.7 and 4.8 times faster, respectively), but are also much more efficient than some TiO2-based photocatalysts in previous studies [17,34,37,38]. Meanwhile, the BC/PDA/TiO2 composite presented excellent reusability and stability, with only 5.5% decrease in the photo-degradation efficiency after five cyclic

Table 1 Comparison of adsorption performances of MB, MO and RhB by various different TiO2-based photocatalyst. Catalyst

Weight

Target & concentration

Time

photo-degradation efficiency

References

BC/PDA/TiO2

30 mg

35 mg 40 mg

30 min 60 min 20 min 240 min 150 min

CNNS/TiO2 N-doped TiO2 NTAs oxided TiO2@PDA

30 mg 25 mg 10 mg

CdS/PDA/TiO2

5 mg

TiO2/cellulose

10 mg

95.1% 100% 99.5% 100% 63% 76% 100% 40% 95.2% 93% 97% 96% 99.5%

This work

TiO2/polypropylene fabric TiO2@PDA

MO 20 mg/L RhB 20 mg/L MB 20 mg/L MO 15 mg/L MO 10 mg/L MB 10 mg/L RhB 10 mg/L RhB 5 mg/mL MO 10 mg/L RhB 10 mg/mL MB 10 mg/L RhB 10 mg/mL MO 30 mg/L

60 min 180 min 90 min 60 min 80 min 80 min 100 min

[34] [17] [35] [36] [37] [20] [38]

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tests. Therefore, our work provides a facile, green, and low-cost approach for produce photocatalysis and wastewater treatment. In addition, our future work will focus on the studies of improving the photocatalytic reactivity of TiO2-based materials toward phenolic compound by tuning down their size or optimization of modification method. CRediT authorship contribution statement Luyu Yang: Data curation, Writing - original draft. Chuntao Chen: Investigation. Ying Hu: Data curation. Feng Wei: Software. Jian Cui: Resources. Yuxiang Zhao: Resources. Xuran Xu: Investigation. Xiao Chen: Data curation. Dongping Sun: Conceptualization, Validation.

[13]

[14]

[15]

[16]

[17]

[18]

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment We thank the financial support from the National Natural Science Foundation of China (51572124, 51873087 and 51803092), National Natural Science Foundation of China Jiangsu Province (BK20180490), the Fundamental Research Funds for the Central Universities (30920130121001), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, China).

[19]

[20]

[21]

[22]

[23]

[24]

Appendix A. Supplementary material

[25]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.12.013.

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