cellulose acetate porous films with enhanced photocatalytic activity for contaminants removal from wastewater

Journal of Hazardous Materials xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Highly flexible and stable carbon nitride/cellulose acetate porous films with enhanced photocatalytic activity for contaminants removal from wastewater Siyu Wanga, Fei Lia, Xiaohui Daia, Chuanjun Wanga, Xintao Lva, Geoffrey I.N. Waterhousea,b, Hai Fana,⁎, Shiyun Aia,⁎ a b

School of Chemistry and Material Science, Shandong Agricultural University, Taian, 271018, Shandong, PR China School of Chemical Sciences, The University of Auckland, Auckland 1142, New Zealand

G R A P H I C A L A B S T R A C T

Highly flexible and stable carbon nitride/cellulose acetate porous film with much enhanced photocatalytic activity for organic dye degradation and Cr (VI) reduction was prepared by a facile spreading method.

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

This study describes the successful fabrication of flexible photocatalytic films to remove contaminants from wastewater, the film is comprising sulfuric acid treated graphitic carbon nitride (SA-g-C3N4) embedded within a porous cellulose network (denoted here as CN/CA films). The SA-g-C3N4 content in the films was varied from 0 to 50 wt.%. The sulfuric acid treatment introduced carboxyl and sulfonyl groups on the surface of g-C3N4, which resulted in strong hydrogen bonding with the hydroxyl groups of cellulose acetate (so strong the partial delimination of the SA-g-C3N4 occurred on CN/CA film formation via solvent casting). The obtained films were around 10 μm in thickness, extremely flexible and durable, with the SA-g-C3N4 uniformly distributed throughout the cellulose acetate network. The CN/CA films showed excellent activities for aqueous dye degradation under direct sunlight, as well as outstanding performance for photocatalytic reduction of Cr (VI). The photocatalytic activity of the CN/CA films at the optimum SA-g-C3N4 content of 50 wt.% was far higher than that of pristine SAg-C3N4, highlighting a main advantage of the composite film fabrication strategy introduced here. Further, the CN/CA films showed excellent stability and reusability, with no loss in activity seen over 5 cycles of dye degradation.

Keywords: Graphitic carbon nitride Cellulose acetate Porous film Photocatalysis Water treatment

⁎

Corresponding authors. E-mail addresses: [email protected] (H. Fan), [email protected] (S. Ai).

https://doi.org/10.1016/j.jhazmat.2019.121417 Received 15 July 2019; Received in revised form 6 October 2019; Accepted 6 October 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Siyu Wang, et al., Journal of Hazardous Materials, https://doi.org/10.1016/j.jhazmat.2019.121417

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

1. Introduction

Further, poor adhesion between the metal oxide photocatalyst and polymers can lead to detachment of photocatalyst (Molinari et al., 2000; Teixeira et al., 2016), thereby creating a potential source of secondary pollution. Graphitic carbon nitride (g-C3N4) as one typical nonmetal semiconductor photocatalyst, has been widely used in energy conversion and environmental remediation (Tian et al., 2018; Liu et al., 2018b, c; Yang et al., 2019; Tian et al., 2019; Tang et al., 2019; Lu et al., 2018). As a polymeric photocatalyst, it is more suitable for g-C3N4 to form stable and flexible composite photocatalytic films with polymer materials (Chen et al., 2017). Herein, we aimed to develop flexible photocatalytic thin films by incorporating a graphitic carbon nitride (g-C3N4) photocatalyst within a porous cellulose acetate film. By treating a conventional g-C3N4 photocatalyst with sulfuric acid, we hypothesized that the surface −CO2H and –SO3H groups introduced, together with the –NH2 groups already present, would allow strong hydrogen bonding with the surface −OH groups of cellulose acetate, thus giving flexible, robust and photocatalytically active thin carbon nitride/cellulose acetate (CN/CA) films for aqueous dye degradation and reduction of Cr (VI) under direct solar or simulated solar irradiation. Further, the CN/CA films were expected to be easily recycled and reused, thereby offering a big advantage over conventional nanopowder-based photocatalyst systems. Graphitic carbon nitride is a low-cost, metal-free semiconductor photocatalyst with a strong visible-light response (Eg = 2.7–2.9 eV depending on the N content) that can readily be obtained by thermal polymerization of a number of nitrogen-rich organic molecules (e.g. melamine, urea, thiourea, dicyanamide or combinations thereof), thus justifying its use in the current study.

Currently enormous research effort is being devoted to the development of semiconductor photocatalysts for water treatment (e.g. oxidative degradation of aqueous dyes and heavy metal ions such as Cr (VI) reduction). A wide range of photocatalytic materials have been explored for water treatment, like organic dye degradation and Cr (VI) reduction (Zhou et al., 2018; Zhao et al., 2019a; Liu et al., 2017; Qiu et al., 2018). These kinds of materials usually have higher photocatalytic activities due to the small size (Martin and Sarkar, 2017; Sun et al., 2016; Han et al., 2017; Wang et al., 2015; Zhao et al., 2019b; Liu et al., 2019). However, nano-sized powders are not ideal for practical photocatalytic systems, especially water treatment, since they are difficult to remove from water after the photocatalytic treatment step, making recycling and reuse difficult (Zhao et al., 2015; Masih et al., 2017; Wan et al., 2018). Unless removed, the photocatalytic nanoparticles may themselves create a health hazard to humans or animals that drink the treated water. Accordingly, a number of different design strategies are being explored to overcome the shortcomings of conventional nano-sized photocatalyst powders. Assembling photocatalytic nanomaterials into three dimensional nanostructures is one important strategy (Jiang et al., 2016; Fan et al., 2015; Cho et al., 2018; Zhang et al., 2011). For example, 3D reduced graphene oxide aerogel-mediated Z-scheme photocatalytic system has been developed for highly efficient solar-driven water oxidation and removal of antibiotics (Liu et al., 2018a). Self-assembled g-C3N4 nanoarchitectures were prepared and showed boosted photocatalytic solar-to-hydrogen efficiency (Jiang et al., 2019). In our previous work, 3D ordered g-C3N4 nanostructures were successfully prepared (Fan et al., 2016a; Dai et al., 2018a). On account of their 3D structure and higher specific surface areas, 3D nanostructured photocatalytic materials usually showed superior photocatalytic activities and operational stabilities than their 2D counterparts. However, many of the 3D nanostructured photocatalytic systems developed to date are brittle and lose their initial structure and initial activity over time due to mechanical attrition. Nanostructured photocatalysts with high porosities and good flexibility are demanded. Porous and flexible photocatalytic thin films are especially desirable since they should have near optimal absorbing properties (2D like a leaf) and be easy recoverable for reuse (Shi et al., 2019). Little work has been done in this area to date, motivating a detailed investigation. Integrating photocatalytic nanomaterials within porous polymer films is arguably the simplest approach to achieve flexible photocatalytic thin films (Hu et al., 2019; Qian et al., 2018). A ZnO/PMMA nanocomposite was prepared and used in photocatalytic applications (Tang et al., 2006). TiO2-coated cellulose acetate monolithic structures were prepared for the photocatalytic reduction of Cr(VI) under direct sunlight (Marinho et al., 2017). However, the dispersion of metal oxide photocatalysts in polymer matrices tends not to be very uniform (Iketani et al., 2003; Chakraborty et al., 2013; Soumya et al., 2014).

2. Experimental section 2.1. Preparation of g-C3N4 and SA-g-C3N4 g-C3N4 powder was prepared by thermal polymerization of melamine at 520 °C in a tube furnace under air atmosphere (Li et al., 2013). The heating rate was 5 °C min−1 and the temperature held was at 520 °C for 4 h. To prepare the sulfuric acid treated g-C3N4 sample (denoted herein as SA-g-C3N4), the as-prepared g-C3N4 powder (1 g) was dispersed in 20 mL of H2SO4 (98 wt. %) and stirred for 8 h at room temperature. The product was subjected to ultrasonic treatment to break up any large particles and then collected by centrifugation (Xu et al., 2013). Following washing with water to remove any residual H2SO4, the SA-g-C3N4 product was finally dispersed in 40 mL of acetone for later use. 2.2. Preparation of the SA-g-C3N4/CA films The typical procedure for preparation of CN/CA film can be described as follows (Fig. 1). The SA-g-C3N4 dispersion above was centrifuged, the acetone supernatant discarded and the remaining solid

Fig. 1. Schematic diagram showing the fabrication process of the carbon-nitride/cellulose acetate (CN/CA) film, along with a diagram showing photocatalytic organic dye oxidation and hydrogen production over the CN/CA film under visible light irradiation. 2

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

3. Results and discussion

dried in air. Next, in a typical experiment, 0.5 g of the SA-g-C3N4 was dispersed in 15 mL acetone and then cellulose acetate (1 g) was added. Ultrasonication for 5 h was then applied to obtain a uniform SA-g-C3N4/ CA (CN/CA) dispersion. Finally, the dispersion was caste onto a clean glass plate and spread to a uniform thickness using a razor blade (i.e. a variant of the doctor blade method). After evaporation of the acetone, CN/CA films were obtained which could be easily peeled off the glass dish. By this approach, CN/CA films with different SA-g-C3N4 contents could easily be prepared simply by varying the amount of CA (0, 0.5, 1, 2, 4.5 g) added to the acetone dispersion of SA-g-C3N4. In the text, we use the notation CN/CA film-x to describe the different samples, where x is the weight fraction of SA-g-C3N4 in the films.

3.1. Preparation and characterization of the SA-g-C3N4/cellulose acetate (CN/CA) films The process used to fabricate the CN/CA composite films, along with their mechanism of action for dye degradation under solar and reduction of Cr (VI) under solar irradiation, are shown in Fig. 1. Due to the high porosity and low specific density of the CN/CA films, they floated on the surface of aqueous solutions. This meant that under light irradiation, the film can be fully photo-excited, with organic dyes in the solution being easily oxidized through contact with the SA-g-C3N4 photocatalyst embedded in the CA network, and with the oxidation of dyes involved valence band holes in SA-g-C3N4 (Molinari et al., 2000; Teixeira et al., 2016; Tian et al., 2018; Liu et al., 2018b). The porous structure of the CN/CA films was also beneficial for Cr (VI) reduction, where electrons photo-excited into the conduction band of SA-g-C3N4 reduced aqueous Cr (VI) to Cr (III). Thus, the prepared CN/CA composite films were expected to be useful for water treatment (i.e. dye removal via photocatalytic advanced oxidation process, and Cr (VI) reduction). The morphology and porosity of the CA film and the CN/CA composite film (CN/CA film-0.33) were examined using SEM. The CA film was highly porous (Fig. 2a), especially when viewed in cross section (Fig. 2b and c). The pore size decreased for films containing SA-g-C3N4 (Fig. 2d–f), though the composite films remained highly porous. In the magnified image of the CA film (Fig. 2f), SA-g-C3N4 nanoparticles can be seen on the surface of the porous film network, meaning that the SAg-C3N4 will be available to participate in photoreactions when the films are subjected to light irradiation. From Fig. 2e, the CN/CA composite film thicknesses were estimated to be around 10 μm. Given the high optical transparency of the CN/CA films at visible wavelengths (explored below), it is anticipated that all SA-g-C3N4 contained in the films would be photoexcited under direct sunlight or irradiation from a Xe lamp. To confirm the importance of the SA-g-C3N4 for the successful formation of the CN/CA film, pure g-C3N4 without treatment by sulphuric acid was also used to prepare film. Fig. S1 clearly demonstrated that large g-C3N4 particles with diameter of 500 nm - 5 μm can be observed on the surface of the film, which is consistent with the morphologies of the g-C3N4 (Fig. S2a, b, c). Furthermore, it is difficult to combine g-C3N4 and cellulose acetate together due to the low interaction ability, which results in the low loading amount of g-C3N4 (the maximum loading amount is only 10%) and low photocatalytic activity. Transmission electron microscope (TEM) was further used to characterize the structures and morphologies of the samples. When g-C3N4 was treated by sulfuric acid, the obtained SA-g-C3N4 (Fig. S2d, e) becomes thinner and porous compared to g-C3N4 (Fig. S2c), which is favorable for the insertion of acetic acid molecules. For the CA and CN/ CA films, it is difficult to conduct TEM characterizations because of the large size of the films, thus, we have to treat the films in water by mechanical force in order to obtain the low size of the samples. During the process, the morphologies of the films may alter; however, the interaction of CA and g-C3N4 can be still characterized. In Fig. S2f, the CA film shows the amorphous structure nature of the polymer with low transmittion contrast. For the TEM image of the CN/CA film (Fig. S2g, h), it can be clearly observed that the SA-g-C3N4 nanosheets less than 100 nm are uniformly distributed in the cellulose acetate due to the interaction of CA and SA-g-C3N4. The above results clearly indicate that CA has important influence on the size decrease of the SA-g-C3N4. Fourier transform infrared (FT-IR) spectra for the CA film, SA-gC3N4 and CN/CA composite films are shown in Fig. 3A. The spectrum of the CA film (curve c in Fig. 3A) is dominated by peaks at 1735, 1366, 1209 and 1024 cm−1, which can readily be assigned to C]O stretching vibration, the stretching vibration of acetylmethyl group, the stretching vibration of acetyl ester and CeO stretching vibrations, repsectively (de

2.3. Photocatalytic measurements To evaluate the photocatalytic activity of the CN/CA films for aqueous rhodamine B (RhB) degradation, a piece of CN/CA film containing 10 mg of SA-g-C3N4 was floated on the surface of an aqueous solution containing rhodamine B (10 mg L−1, 30 mL). As a contrast, the same amount of SA-g-C3N4 as that in CN/CA film was used here. To allow an adsorption-desorption equilibrium to be established between the film and aqueous RhB, the dye solution was stirred continuously for 60 min in the dark. The solution was then exposed to direct sunlight (about 0.37 to 0.44 sun, AM 1.5 G) without stirring and 1 mL solution was collected at regular intervals. The concentration of RhB in the aliquots were determined by a UV–vis spectrometry (i.e. measuring the absorption at 553 nm). Further experiments were conducted using aqueous solutions of other dyes, including methylene blue (MB), crystal violet (CV) and malachite green (MG). All test conditions were the same as those described for RhB (except that the absorption wavelength used for dye quantification was adjusted to match the dye). To evaluate the photocatalytic activity of the CN/CA films for aqueous Cr (VI) (K2Cr2O7) reduction, a piece of CN/CA film-0.5 (containing about 6 mg of SA-g-C3N4) was floated on the surface of an aqueous solution containing Cr (VI) (5 mg L−1, 30 mL). And the pH of the suspension was adjusted by using 0.1 M HCl solution and 0.1 M NaOH solution. To allow an adsorption-desorption equilibrium to be established between the film and aqueous Cr (VI) (K2Cr2O7), the Cr (VI) solution was stirred continuously for 60 min in the dark. The solution was then irradiated by a 300 W Xe lamp (380–750 nm) and 1 mL solution was collected at regular intervals. The concentration of Cr (VI) was determined by the diphenylcarbazide (DPC) method (Jin et al., 2014).

2.4. Sample characterization The morphology and microstructure of the CN/CA composite films, as well as the starting materials used to fabricate the films, were analyzed by scanning electron microscopy (SEM) on a QUANTA250 scanning electron microscope (FEI, USA) equipped with an energy dispersive spectrometer (EDS) operating at 20 kV. X-ray powder diffraction (XRD) patterns were collected using a Rigaku DLMAX2550 V diffractometer (40 kV, Cu Kα, λ =1.54056 Å). Ultravioletvisible (UV–vis) absorbance spectra were collected using a UV–vis spectrophotometer (UV-2540 Shimadzu, Japan). Fourier transform infrared spectra (FT-IR) were collected over the range 400-4000 cm−1 on a Thermo Nicolet-380 IR spectrophotometer (USA). The KBr tablet technique was used to prepare the sample for analysis. Photoluminescence spectra were collected using a Cary Eclipse spectrophotometer (VARIAN, USA). Spectra were excited at 320 nm, with emission data collected over the range of 400–550 nm.

3

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

Fig. 2. SEM images of (a) CA film surface; (b, c) CA film cross section; (d) CN/CA film surface; and (e, f) CN/CA film cross section.

Moraes et al., 2015). The peak at ∼3500 cm−1 is an OeH stretching mode of hydroxyl groups in cellulose acetate. In the spectrum of SA-gC3N4 (curve b in Fig. 3A), the peaks around 1572 and 1632 cm−1 can be attributed to C]N stretching vibrations. The three peaks at 1253, 1320 and 1425 cm−1 correspond to aromatic C–N stretching modes. The peak at 807 cm−1 can be assigned to the a triazine ring breathing mode in condensed CN heterocycles (Xu et al., 2013). The broad peak between 3000–3500 cm−1 is due to uncondensed terminal amino groups (-NH2 or = NH group) (Xu et al., 2013). The peaks at 3344, 1760 and 920 cm−1 are due to the carboxylic acid groups, and assigned to OeH stretching, C]O stretching a non-planar rocking vibration of intermolecular hydrogen bonded OeH groups, respectively (Geng et al., 2009). Furthermore, the peaks at 1162, 1042, 667 and 572 cm−1 are characteristic of a sulfonic acid group introduced during the sulfuric acid treatment step used in the synthesis of SA-g-C3N4 (Yang et al., 2002), which can not be observed in the pure g-C3N4 (curve a in

Fig. 3A). The presence of amino groups, carboxyl groups and the sulfonic acid groups on the surface of SA-g-C3N4 offer plenty of sites for hydrogen bonding interactions with the hydroxyl groups on the cellulose acetate, thereby ensuring good interactions when SA-g-C3N4 and cellulose acetate were dispersed in acetone (Ramamoorthy and Ulbricht, 2003). The FTIR spectrum of the CN/CA composite film (curve d in Fig. 3A) contained peaks typical of the CA and SA-g-C3N4 components, indicating the successful formation of CN/CA composite films. Again, the hydrogen bonding interactions between the CA and SA-g-C3N4 components were expected to ensure a uniform dispersion of SA-g-C3N4 matrix. X-ray powder diffraction (XRD) was used to analyse the phase composition of the products. The XRD patterns of the as-prepared gC3N4 and SA-g-C3N4 samples contain two peaks at 13.0° and 27.4° due to (100) and (002) reflections of g-C3N4 (Fig. 3B). The former relates to the structural packing of tri-s-triazine units in the conjugated 2D planes,

Fig. 3. (A) FT-IR spectra for (a) g-C3N4; (b) SA-g-C3N4; (c) CA film; and (d) CN/CA-0.5 film; and XRD patterns for (B) g-C3N4 and SA-g-C3N4; and (C) SA-g-C3N4, the CA film and CN/CA film. (D) Elemental mapping images and (E) EDS spectrum of the CN/CA film. 4

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

semiconductor photocatalysts following photoexcitation. Under UV irradiation (320 nm), g-C3N4 showed an intense photoluminescence peak at around 440 nm (curve a in Fig. 4B) (Rong et al., 2016; Wang et al., 2016; Dai et al., 2018b). Compared with pure g-C3N4, SA-g-C3N4 (curve b in Fig. 4B) and CN/CA film (curve c in Fig. 4B) have the emission peak at about 430 nm. This blue shift of PL spectra is the result of band gap enlargement.

whilst the latter is related to the graphite-like interlayer stacking (Dai et al., 2018a). For SA-g-C3N4, the intensity of the (100) peak increased greatly and that of the (002) peak has an obvious decrease comparing to that of the g-C3N4. Mainly caused by the tendency of the bulk g-C3N4 to conjugate to a two-dimensional planar structure by sulfuric acid. The CA film was amorphous, evidenced by a broad diffraction peak between 15-30° (red curve in Fig. 3C) (Fan et al., 2013). The XRD pattern of the CN/CA composite film (blue curve in Fig. 3C) contained contributions from both CA and SA-g-C3N4, with the presence of the latter evidenced by the (002) reflection at 27.1° (Fan et al., 2016a). The data confirmed the formation of a CN/CA composite film. It is worth noting that the intensity of the (002) peak at 27.1° in the CN/CA composite films was weaker than expected based on the nominal SA-gC3N4 loading. This can be attributed to strong interfacial contact (Hbonding) between the SA-g-C3N4 and the CA, which acts to further strip nanosheets from the SA-g-C3N4. As a result, the SA-g-C3N4 in the composite film has a much smaller size than the SA-g-C3N4 starting material, resulting in a weaker (002) peak. It is expected that having a small size of SA-g-C3N4 photocatalyst should enhance both light absorption and the photocatalytic activity of the composites. In the elemental mapping and EDS images (Fig. 3D, E), it clearly shows that C, N, O, S elements can be observed. It is worth to note that the existence of the N and S elements confirms that SA-g-C3N4 has been successfully dispered in the CA film. The surface chemical compositions and states of as-prepared CN/CA film-0.5 were further examined by XPS. In the C 1s spectrum (Fig. S3a), the peaks at 284.8, 286.1, and 288.8 eV correspond to CeC, CeCeO, and C]O bonds arising from CA, respectively (Li et al., 2017a). The peak at 287.7 eV can be assigned to the carbon atoms bonded with three N neighbours (N2eC]Ne) whereas the peak at 285.4 eV is attributed to adventitious C arising from SA-g-C3N4 (Mane et al., 2017). The dominant N 1s signal peak (Fig. S3b) at 397.8 eV can be attributed to sp2 hybridized nitrogen involved in triazine rings (Kang et al., 2017). The peak at 398.8 eV is attributed to nitrogen bonded with two carbon atoms in a graphitic sp2 network (Fig. S3b) whereas the peak at 399.8 eV corresponds to the overlap of signals from bridging nitrogen atoms such as tertiary N (-N <), and amino groups (-NHx), revealing the presence of tris-triazine ring in the CN framework (Mane et al., 2017). The O 1s peaks (Fig. S3c) with binding energies of 531.8 and 532.9 eV are assigned to the C]O and CeOH/CeOeC (Wang et al., 2017). The O 1s peak of 532.4 eV corresponds to sulphate (Zhou et al., 2000). These facts prove that SA-g-C3N4 is well combined with cellulose acetate. The N 1s spectrum of the pure g-C3N4 is also conducted, as shown in Fig. S3d. It is worth to note that the peak at 404.6 eV disappears while the other three peaks are shifted to 399.8, 398.7 and 397.8 eV, compared to those of the SA-g-C3N4, which can be attributed to the hydrogen bond interaction with CA (Fan et al., 2016b).

3.3. The light transmission properties and flexibility of CN/CA films The light transmission properties and haze of the CA and CN/CA composite films were investigated. Ideally, the composite films should have good transparency at visible wavelengths to allow efficient photoexcitation of the contained SA-g-C3N4 photocatalyst. Fig. 5A (a–e) show digital images of the CA film (a) and CA/CN composite film with different SA-g-C3N4 contents (b–e). The CA film was transparent, with the films becoming progressively more opaque and of lower visible light transparency as the SA-g-C3N4 content increased. Transmission and haze data collected for the films is shown in Fig. 5B (b–e). The transmittance decreased, and the haze increased, as the SA-g-C3N4 content increased. With the content of SA-g-C3N4 at 50%, the transmittance decreased to 33% and the haze increased to near 100%. However, the increase in the haze suggests that light scattering (Fang et al., 2014) inside the film will increased, which can be expected to increase the absorption of sunlight by SA-g-C3N4, thus enhanceing the photocatalytic performance. A special feature of the CN/CA composite films was their flexibility. Fig. 5A (f) shows a digital photograph of the CN/ CA film-0.5 (containing 50% SA-g-C3N4 by weight) which can easily be curled without cracking or tearing. The film can also be kept in the curled state for extended periods (i.e. two weeks or more) without any damaged occuring, and readily rolled out again to give a flat sheet. When the content of SA-g-C3N4 in the composite film exceeds 50%, the prepared film will become fragile and hard to maintain the flexible and stable state of the film. Therefore, the maximum content of SA-g-C3N4 in the composite film is 50% in our experiment. Clearly, this flexibility is highly desirable for the design and fabrication of photocatalytic and photovoltaic devices. 3.4. Photocatalytic performances 3.4.1. Degradation of dyes activity of CN/CA film The photocatalytic activity of SA-g-C3N4 and the CN/CA composite films were first evaluated for photodegradation of the organic dyes. The CN/CA composite films with different SA-g-C3N4 contents were floated on the surface of an aqueous solution of rhodamine B (RhB) dye (10 mg L−1) (Fig. 6a). After magnetic stirring for 60 min in the dark, adsorption-desorption equilibrium between the films and the RhB dye was achieved (Fig. 6b). The SA-g-C3N4 reference material adsorbed only about 1% of RhB in the solution, whereas the CA film and CN/CA composite films adsorbed about 9% of the RhB in the solution. This enhanced adsorption is attributed to high surface area and enhanced dye adsorption abilities of the porous cellulose acetate network in the films (Yu et al. (2010)). Under direct sunlight, aqueous RhB showed some degradation due to UV absorption (blank test). The pure CA film displayed no obvious photocatalytic activity under sunlight. The SA-gC3N4 powder was active for RhB degradation, with a 60% removal of RhB achieved over 150 min (this 60% removal includes some adsorption, thus the amount of RhB removed photocatalytically is lower and probably around 51%). The RhB removal efficiency of the CN/CA composite films was superior to that of the SA-g-C3N4 powder, with the removal efficiency and photocatalytic activity increasing with the carbon nitride content. When the carbon nitride content reached 50 wt. %, ∼99% RhB removal was achieved in 150 min. The data suggests that the fabrication of CN/CA composite films is an excellent strategy for enhancing the performance of g-C3N4 for aqueous dye degradation. This can be attributed to the optimal light absorption/scattering properties

3.2. Optical properties The optical properties of the products were investigated as demonstrated in Fig. 4. Fig. 4A shows the UV–vis diffuse reflectance spectra of the products. For CA film, weak absorption is observed below 300 nm. For the g-C3N4 sample, a typical semiconductor absorption is observed in the blue light range, which is caused by band transitions. As to the SA-g-C3N4 and CN/CA film-0.5, the absorption edges shift remarkably to the shorter wavelength. In addition, an absorption tail in the long-wavelength region can be found for CN/CA film, which may be associated with the scattering of light among the texture and porous structure in CA film (Li et al., 2017b; Shen et al., 2017). The band gap for g-C3N4 is 2.77 eV; while those of SA-g-C3N4 and CN/CA film-0.5 are 2.98 eV, as shown in the inset image of Fig. 4A. The increase in the band gap is due to the protonation of the carbon nitride by sulphuric acid, which is similar to the results of the literature (Zhang et al., 2015). Photoluminescence spectroscopy is a useful technique for examining the rates of radiative electron-hole pair recombination in 5

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

Fig. 4. (A) UV–vis diffuse reflectance spectra of CA film, CN/CA film-0.5, SA-g-C3N4 and g-C3N4; the inset image is the bandgap determination plots. (B) Photoluminescence spectra for (a) g-C3N4; (b) SA-g-C3N4; (c) CA film; and (d) CN/CA-0.5 film. The photoluminescence spectra were excited at 320 nm.

Fig. 5. (A) Digital images and (B) transmittance/haze values for CN/CA composite films prepared with different contents of SA-g-C3N4: (a) 0%; (b) 10%; (c) 20%; (d) 33%; and (e) 50%; (f) (50%)-flex. test.

of the films, facile diffusion of dye to photocatalytically active g-C3N4 sites due to the high porosity of the films, and the thin g-C3N4 nanosheets in the composites (created via both sulfuric acid treatment of the as-prepared g-C3N4 and interactions with the CA). The performance of the CN/CA composites for the removal of other aqueous dyes in direct sunlight was also examined (Fig. 6c). The dyes tested included malachite green (MG), crystal violet (CV) and methylene blue (MB). The CN/CA film (50 wt.% SA-g-C3N4) showed excellent photocatalytic activity for the removal of all these dyes. This can be attributed to the low density of the CN/CA film, which means it can float on the surface of the dye solution (allowing maximum solar absorption), with the internal surfaces of the porous films being wetted with dye solution by capillary action, facilitated by the hydrophilicity of cellulose. Further, the SA-g-C3N4 is uniformly distributed throughout the films, thus offering lots of active sites for dye photo-oxidation. Importantly, the compositing with CA did not block the photocatalytic active sites of SAg-C3N4 (otherwise the photocatalytic activity would have been low). The immobilization of the SA-g-C3N4 photocatalyst in the form of a CN/CA composite film made recovery of the photocatalyst after the dye degradation tests very straightforward. The film could simply be picked up with a pair of tweezers, washed, and then reused. For powdered photocatalysts such as SA-g-C3N4, centrifugation or filtration will be needed to recover the photocatalyst, and the recovery procedures are usually time-consuming. To highlight the stability and reusability of the CN/CA film containing 50 wt.% SA-g-C3N4, five consecutive cycles of aqueous RhB degradation were performed under direct sunlight

(Fig. 6d). A removal efficiency of 99% was achieved in each cycle, no loss in photocatalytic activity occurred across the five cycles. During the cycles, the weight of the CN/CA film remained unchanged due to the protection of SA-g-C3N4 offered by the CA film. The digital pictures for the test of adsorption of RhB, photocatalytic degradation of RhB and easy recovery of the of the CN/CA composite film are shown in Fig. 7a. After the adsorption of RhB, the CN/CA composite film turned to be a pink colour. The pink dye-loaded film was then placed in water and exposed to direct sunlight. After 1 h of irradiation, the film becomes colourless. The results prove that the composite film is fully reusable. The corresponding SEM iamges of the three stage films were demonstrated in Fig. 7 (b, c, d). As we can see that the SEM images of the initial film, the film after dye adsorption and the film after dye photodegradation have almost the same morphologies, indicating the high stability of the film during the recycle experiment. The CN/CA composite films prepared with different contents of SA-gC3N4 were sonicated in water for 60 min (100 W), and the aqueous solution after ultrasonication was analyzed by fluorescence spectrum. Under the excitation light of 320 nm, no fluorescence emission peak of carbon nitride appeared (Fig. S4), which indicates CN/CA composite film can maintain its own stability in harsh environments, further demonstrating the stable combination of carbon nitride and cellulose acetate. Fig. 8 showed the first-order rate constant k (min−1) of SA-g-C3N4 and CN/CA film-0.5 for RhB, which was calculated by the following first-order kinetics equation (Yu et al., 2018): 6

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

Fig. 6. (a) Digital image showing the CN/CA film floating on the surface of the Rhodamine B solution; (b) Removal efficiency of RhB under direct sunlight in the absence or presence of CA film, SA-g-C3N4 and CN/CA composite films with different SA-g-C3N4 contents (each film contains about 10 mg of SA-g-C3N4); (c) Removal efficiency of different dyes on the CN/ CA film-0.5 under direct sunlight, (d) Recycling tests for RhB removal using the CN/CA film-0.5.

0.5 possessed better photocatalytic activities under visible-light irradiation. The enhancement mechanism of the photocatalytic activities for the CN/CA film-0.5 was also discussed. According to the characterization results, with the addition of the CA, the size of the SA-g-C3N4 nanosheets becomes much smaller and the distribution of the SA-g-C3N4 nanosheets becomes more uniform. Thus, the abundant porous

-ln(C/C0) = kt; where C0 is the initial concentration of RhB, C is the concentration of RhB at photocatalytic reaction time t, and k is the kinetic constant. Fig. 8 demonstrated that the kinetic rate constant (k) of RhB degradation with CN/CA film-0.5 (0.03761 min−1) was 4 times as large as that of SA-g-C3N4 (0.00771 min−1), which confirmed that CN/CA film-

Fig. 7. Digital pictures (a) and SEM images of the CN/CA-0.5 composite film at different stages of RhB removal: (b) Initial film; (c) after adsorption of RhB; (d) after complete photodegradation of RhB. 7

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

example. As shown in Fig. 9a, it can be seen that with the pH decreases, the reduction ability of the film for Cr (VI) gradually increases. When pH value equals to 2, Cr (VI) can be reduced by more than 95% in 100 min. Compared with CA film and SA-g-C3N4, CA/CN film-0.5 also showed much higher reduction properties (Fig. 9b, pH = 2). Furthermore, the composite film also showed strong acid resistance property without obvious decease of reduction ability for Cr (VI) after 5 circles. 4. Conclusions To summarize, we successfully prepared graphitic carbon nitridecellulose acetate (CN/CA) composite films with graphitic carbon nitride contents up to 50 wt.%. The films were highly flexible, porous and contained a uniform dispersion sulfuric acid treated g-C3N4 nanosheets (SA-g-C3N4). The cellulose acetate gave the films high optical transparency at low SA-g-C3N4 contents, though the films became opaque and hazy at high SA-g-C3N4 contents (e.g. 50 wt.%) due to enhanced light scattering at interfaces in the films. This light scattering and a high SA-g-C3N4 content imparted the composite films with outstanding photocatalytic performance for aqueous dye degradation and reduction of Cr (VI) relative to the pristine SA-g-C3N4 powder. Furthermore, CA/ CN film-0.5 exhibited a superior ability to reduce Cr (VI), achieving a reduction efficiency of 95% for Cr (VI) (5 mg/L) in 100 min. More importantly, the CN/CA composites showed outstanding stability and recyclability during the photocatalysts tests, with no loss in activity seen after 5 test cycles. Results encourage the wider development of porous composite films for photocatalytic applications.

Fig. 8. Kinetics of RhB photodegradation over SA-g-C3N4 and CN/CA film-0.5.

structure, the small size of the SA-g-C3N4 nanosheets and much exposed SA-g-C3N4 provide more active sites for photocatalysis. What’s more, the porous structure of CA films exhibit excellent visible light scattering property, the scattering light overlaps with the absorption light of SA-gC3N4, which can enhance the utility of incident light and produce more photoexcited electrons and holes, which greatly increase the photocatalytic activity of the CN/CA film (Li et al., 2017b). It is confirmed that the abundant porous structure, the small size of the SA-g-C3N4 nanosheets and much exposed SA-g-C3N4 provide more active sites for photocatalysis. According to the photocatalytic experimental results, the possible photocatalytic process is as follows. Firstly, cellulose acetate in the CN/CA film will adsorb organic dye molecules from the dye solution. Then, SA-g-C3N4 nanosheets dispersed throughout the film will contact with the absorbed dye molecules and photocatalytic degrade them under the sunlight irradiation. Because of the high stability of the CN/CA film, it can be kept on the surface of the polluted solution during the photocatalytic process.

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. Acknowledgments This work was supported by National Key R&D Program of China (2017YFD0801504), National Natural Science Foundation of China (No. 41771342, 51402175), Shandong Provincial Natural Science Foundation of China (No. ZR2019MEM018), Funds of Shandong “Double Tops” Program (SYL2017XTTD15) and A Project of Shandong Province Higher Educational Science and Technology Program (No. J18KA008). GINW acknowledges funding support from the Dodd Walls Centre for Photonic and Qunatum Technologies and the MacDiarmid Institute for Advanced Materials and Nanotechnology.

3.4.2. Photocatalytic reduction of Cr (VI) According to previous reports (Jin et al., 2014), Cr (VI) ion (Cr2O72−) was generally the predominant species at medium to low pH values, the reduction of Cr (VI) by photogenerated electrons could be described as follows: Cr2O72− + 14H+ + 6e- → 2Cr (III) + 7H2O

(1)

As shown in Eq. (1), the pH could strongly influence the reaction direction, and the decrease of pH value could expectedly increase the reduction ability of Cr (VI). By adjusting the pH values, the degradation experiment of Cr (VI) was carried out by taking CA/CN film-0.5 as an

Appendix A. Supplementary data Supplementary material related to this article can be found, in the

Fig. 9. (a) Effect of pH on the reduction of Cr (VI) (5 mg/L) with CN/CA film-0.5 under Xe lamp irradiation (380–750 nm); (b) Photocatalytic reduction of Cr (VI) (5 mg/L) over CA film, SA-g-C3N4 and CN/CA film-0.5 under Xe lamp irradiation (380–750 nm). 8

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121417.

Liu, Q., Shen, J., Yu, X., Yang, X., Liu, W., Yang, J., Tang, H., Xu, H., Li, H., Li, Y., Xu, J., 2019. Unveiling the origin of boosted photocatalytic hydrogen evolution in simultaneously (S, P, O)-Codoped and exfoliated ultrathin g-C3N4 nanosheets. Appl. Catal. B: Environ. 248, 84–94. https://doi.org/10.1016/j.apcatb.2019.02.020. Liu, Q., Shen, J., Yang, X., Zhang, T., Tang, H., 2018a. 3D reduced graphene oxide aerogel-mediated Z-scheme photocatalytic system for highly efficient solar-driven water oxidation and removal of antibiotics. Appl. Catal. B: Environ. 232, 562–573. https://doi.org/10.1016/j.apcatb.2018.03.100. Liu, W., Shen, J., Liu, Q., Yang, X., Tang, H., 2018b. Porous MoP network structure as cocatalyst for H2 evolution over g-C3N4 nanosheets. Appl. Surf. Sci. 462, 822–830. https://doi.org/10.1016/j.apsusc.2018.08.189. Liu, W., Shen, J., Yang, X., Liu, Q., Tang, H., 2018c. Dual Z-scheme g-C3N4/Ag3PO4/ Ag2MoO4 ternary composite photocatalyst for solar oxygen evolution from water splitting. Appl. Surf. Sci. 456, 369–378. https://doi.org/10.1016/j.apsusc.2018.06. 156. Lu, X., Wang, Y., Zhang, X., Xu, G., Wang, D., Lv, J., Zheng, Z., Wu, Y., 2018. NiS and MoS2 nanosheet co-modified graphitic C3N4 ternary heterostructure for high efficient visible light photodegradation of antibiotic. J. Hazard. Mater. 341, 10–19. https:// doi.org/10.1016/j.jhazmat.2017.07.004. Mane, G.P., Talapaneni, S.N., Lakhi, K.S., Ilbeygi, H., Ravon, U., Al-Bahily, K., Mori, T., Park, D.-H., Vinu, A., 2017. Highly ordered nitrogen-rich mesoporous carbon nitrides and their superior performance for sensing and photocatalytic hydrogen generation. Angew. Chemie Int. Ed. English 56, 8481–8485. https://doi.org/10.1002/anie. 201702386. Marinho, B.A., Cristóvão, R.O., Djellabi, R., Loureiro, J.M., Boaventura, R.A.R., Vilar, V.J.P., 2017. Photocatalytic reduction of Cr(VI) over TiO2-coated cellulose acetate monolithic structures using solar light. Appl. Catal. B: Environ. 203, 18–30. https:// doi.org/10.1016/j.apcatb.2016.09.061. Martin, A., Sarkar, A., 2017. Overview on biological implications of metal oxide nanoparticle exposure to human alveolar A549 cell line. Nanotoxicology 11, 713–724. https://doi.org/10.1080/17435390.2017.1366574. Masih, D., Ma, Y., Rohani, S., 2017. Graphitic C3N4 based noble-metal-free photocatalyst systems: a review. Appl. Catal. B: Environ. 206, 556–588. https://doi.org/10.1016/j. apcatb.2017.01.061. Molinari, R., Mungari, M., Drioli, E., Di Paola, A., Loddo, V., Palmisano, L., Schiavello, M., 2000. Study on a photocatalytic membrane reactor for water purification. Catal. Today 55, 71–78. https://doi.org/10.1016/S0920-5861(99)00227-8. Qian, D., Chen, D., Li, N., Xu, Q., Li, H., He, J., Lu, J., 2018. TiO2/sulfonated graphene oxide/Ag nanoparticle membrane: in situ separation and photodegradation of oil/ water emulsions. J. Membrane Sci. 554, 16–25. https://doi.org/10.1016/j.memsci. 2017.12.084. Qiu, J., Zhang, X.-F., Zhang, X., Feng, Y., Li, Y., Yang, L., Lu, H., Yao, J., 2018. Constructing Cd0.5Zn0.5S@ZIF-8 nanocomposites through self-assembly strategy to enhance Cr(VI) photocatalytic reduction. J. Hazard. Mater. 349, 234–241. https:// doi.org/10.1016/j.jhazmat.2018.02.009. Ramamoorthy, M., Ulbricht, M., 2003. Molecular imprinting of cellulose acetate-sulfonated polysulfone blend membranes for Rhodamine B by phase inversion technique. J. Membrane Sci. 217, 207–214. https://doi.org/10.1016/S0376-7388(03)00133-9. Rong, M., Cai, Z., Xie, L., Lin, C., Song, X., Luo, F., Wang, Y., Chen, X., 2016. Study on the Ultrahigh Quantum Yield of Fluorescent P,O-g-C3N4 Nanodots and its Application in Cell Imaging. Chem.-Eur. J. 22, 9387–9395. https://doi.org/10.1002/chem. 201601065. Shen, X., Zhang, T., Xu, P., Zhang, L., Liu, J., Chen, Z., 2017. Growth of C3N4 nanosheets on carbon-fiber cloth as flexible and macroscale filter-membrane-shaped photocatalyst for degrading the flowing wastewater. Appl. Catal. B: Environ. 219, 425–431. https://doi.org/10.1016/j.apcatb.2017.07.059. Shi, Y., Huang, J., Zeng, G., Cheng, W., Hu, J., 2019. Photocatalytic membrane in water purification: is it stepping closer to be driven by visible light? J. Membrane Sci. 584, 364–392. https://doi.org/10.1016/j.memsci.2019.04.078. Soumya, S., Mohamed, A.P., Paul, L., Mohan, K., Ananthakumar, S., 2014. Near IR reflectance characteristics of PMMA/ZnO nanocomposites for solar thermal control interface films. Sol. Energy Mater. Sol. Cells 125, 102–112. https://doi.org/10.1016/ j.solmat.2014.02.033. Sun, J., Fan, H., Wang, N., Ai, S., 2016. Controlled synthesis of Sn doped ZnO microspheres stringed on carbon fibers with enhanced visible-light photocatalytic activities. Sep. Purif. Technol. 160, 67–72. https://doi.org/10.1016/j.seppur.2016.01. 013. Tang, E., Cheng, G., Ma, X., 2006. Preparation of nano-ZnO/PMMA composite particles via grafting of the copolymer onto the surface of zinc oxide nanoparticles. Powder Technol. 161, 209–214. https://doi.org/10.1016/j.powtec.2005.10.007. Tang, H., Wang, R., Zhao, C., Chen, Z., Yang, X., Bukhvalov, D., Lin, Z., Liu, Q., 2019. Oxamide-modified g-C3N4 nanostructures: tailoring surface topography for highperformance visible light photocatalysis. Chem. Eng. J. 374, 1064–1075. https://doi. org/10.1016/j.cej.2019.06.029. Teixeira, S., Martins, P.M., Lanceros-Méndez, S., Kühn, K., Cuniberti, G., 2016. Reusability of photocatalytic TiO2 and ZnO nanoparticles immobilized in poly(vinylidene difluoride)-co-trifluoroethylene. Appl. Surf. Sci. 384, 497–504. https://doi. org/10.1016/j.apsusc.2016.05.073. Tian, L., Yang, X., Liu, Q., Qu, F., Tang, H., 2018. Anchoring metal-organic framework nanoparticles on graphitic carbon nitrides for solar-driven photocatalytic hydrogen evolution. Appl. Surf. Sci. 455, 403–409. https://doi.org/10.1016/j.apsusc.2018.06. 014. Tian, L., Yang, X., Cui, X., Liu, Q., Tang, H., 2019. Fabrication of dual direct Z-scheme gC3N4/MoS2/Ag3PO4 photocatalyst and its oxygen evolution performance. Appl. Surf. Sci. 463, 9–17. https://doi.org/10.1016/j.apsusc.2018.08.209. Wan, W., Zhang, R., Ma, M., Zhou, Y., 2018. Monolithic aerogel photocatalysts: a review.

References Chakraborty, H., Sinha, A., Mukherjee, N., Ray, D., Protim Chattopadhyay, P., 2013. A study on nanoindentation and tribological behaviour of multifunctional ZnO/PMMA nanocomposite. Mater. Lett. 93, 137–140. https://doi.org/10.1016/j.matlet.2012.11. 075. Chen, J., Dong, C.-L., Zhao, D., Huang, Y.-C., Wang, X., Samad, L., Dang, L., Shearer, M., Shen, S., Guo, L., 2017. Molecular design of polymer heterojunctions for efficient solar–hydrogen conversion. Adv. Mater. 29, 1606198. https://doi.org/10.1002/ adma.201606198. Cho, S., Ahn, C., Park, J., Jeon, S., 2018. 3D nanostructured N-doped TiO2 photocatalysts with enhanced visible absorption. Nanoscale 10, 9747–9751. https://doi.org/10. 1039/C8NR02330A. Dai, X., Han, Z., Waterhouse, G.I.N., Fan, H., Ai, S., 2018a. Ordered graphitic carbon nitride tubular bundles with efficient electron-hole separation and enhanced photocatalytic performance for hydrogen generation. Appl. Catal. A-Gen. 566, 200–206. https://doi.org/10.1016/j.apcata.2018.09.001. Dai, X., Han, Z., Fan, H., Ai, S., 2018b. Sulfur doped carbon nitride quantum dots with efficient fluorescent property and their application for bioimaging. J. Nanopart. Res. 20, 315. https://doi.org/10.1007/s11051-018-4424-y. de Moraes, A.C.M., Andrade, P.F., de Faria, A.F., Simões, M.B., Salomão, F.C.C.S., Barros, E.B., Gonçalves, Md.C., Alves, O.L., 2015. Fabrication of transparent and ultraviolet shielding composite films based on graphene oxide and cellulose acetate. Carbohyd. Polym. 123, 217–227. https://doi.org/10.1016/j.carbpol.2015.01.034. Fan, Y., Ma, W., Han, D., Gan, S., Dong, X., Niu, L., 2015. Convenient recycling of 3D AgX/Graphene aerogels (X = Br, Cl) for efficient photocatalytic degradation of water pollutants. Adv. Mater. 27, 3767–3773. https://doi.org/10.1002/adma.201500391. Fan, G., Wang, M., Liao, C., Fang, T., Li, J., Zhou, R., 2013. Isolation of cellulose from rice straw and its conversion into cellulose acetate catalyzed by phosphotungstic acid. Carbohyd. Polym. 94, 71–76. https://doi.org/10.1016/j.carbpol.2013.01.073. Fan, H., Wang, N., Tian, Y., Ai, S., Zhan, J., 2016a. Acetic acid induced synthesis of laminated activated carbon nitride nanostructures. Carbon 107, 747–753. https:// doi.org/10.1016/j.carbon.2016.06.082. Fan, X., Su, Y., Zhao, X., Li, Y., Zhang, R., Ma, T., Liu, Y., Jiang, Z., 2016b. Manipulating the segregation behavior of polyethylene glycol by hydrogen bonding interaction to endow ultrafiltration membranes with enhanced antifouling performance. J. Membrane Sci. 499, 56–64. https://doi.org/10.1016/j.memsci.2015.10.026. Fang, Z., Zhu, H., Yuan, Y., Ha, D., Zhu, S., Preston, C., Chen, Q., Li, Y., Han, X., Lee, S., Chen, G., Li, T., Munday, J., Huang, J., Hu, L., 2014. Novel nanostructured paper with ultrahigh transparency and ultrahigh haze for solar cells. Nano Lett. 14, 765–773. https://doi.org/10.1021/nl404101p. Geng, W., Nakajima, T., Takanashi, H., Ohki, A., 2009. Analysis of carboxyl group in coal and coal aromaticity by Fourier transform infrared (FT-IR) spectrometry. Fuel 88, 139–144. https://doi.org/10.1016/j.fuel.2008.07.027. Han, Z., Wang, N., Fan, H., Ai, S., 2017. Ag nanoparticles loaded on porous graphitic carbon nitride with enhanced photocatalytic activity for degradation of phenol. Solid State Sci. 65, 110–115. https://doi.org/10.1016/j.solidstatesciences.2017.01.010. Hu, C., Wang, M.-S., Chen, C.-H., Chen, Y.-R., Huang, P.-H., Tung, K.-L., 2019. Phosphorus-doped g-C3N4 integrated photocatalytic membrane reactor for wastewater treatment. J. Membrane Sci. 580, 1–11. https://doi.org/10.1016/j.memsci. 2019.03.012. Iketani, K., Sun, R.-D., Toki, M., Hirota, K., Yamaguchi, O., 2003. Sol–gel-derived TiO2/ poly(dimethylsiloxane) hybrid films and their photocatalytic activities. J. Phys. Chem. Solids 64, 507–513. https://doi.org/10.1016/S0022-3697(02)00357-8. Jiang, W., Liu, Y., Wang, J., Zhang, M., Luo, W., Zhu, Y., 2016. Separation-free Polyaniline/TiO2 3D hydrogel with high photocatalytic activity. Adv. Mater. Interfaces 3, 1500502. https://doi.org/10.1002/admi.201500502. Jiang, Y., Qu, F., Tian, L., Yang, X., Zou, Z., Lin, Z., 2019. Self-assembled g-C3N4 nanoarchitectures with boosted photocatalytic solar-to-hydrogen efficiency. Appl. Surf. Sci. 487, 59–67. https://doi.org/10.1016/j.apsusc.2019.05.056. Jin, Z., Zhang, Y.-X., Meng, F.-L., Jia, Y., Luo, T., Yu, X.-Y., Wang, J., Liu, J.-H., Huang, X.J., 2014. Facile synthesis of porous single crystalline ZnO nanoplates and their application in photocatalytic reduction of Cr(VI) in the presence of phenol. J. Hazard. Mater. 276, 400–407. https://doi.org/10.1016/j.jhazmat.2014.05.059. Kang, S., Zhang, L., Yin, C., Li, Y., Cui, L., Wang, Y., 2017. Fast flash frozen synthesis of holey few-layer g-C3N4 with high enhancement of photocatalytic reactive oxygen species evolution under visible light irradiation. Appl. Catal. B: Environ. 211, 266–274. https://doi.org/10.1016/j.apcatb.2017.04.050. Li, T., Zhao, L., He, Y., Cai, J., Luo, M., Lin, J., 2013. Synthesis of g-C3N4/SmVO4 composite photocatalyst with improved visible light photocatalytic activities in RhB degradation. Appl. Catal. B: Environ. 129, 255–263. https://doi.org/10.1016/j.apcatb. 2012.09.031. Li, W., Li, T., Li, G., An, L., Li, F., Zhang, Z., 2017a. Electrospun H4SiW12O40/cellulose acetate composite nanofibrous membrane for photocatalytic degradation of tetracycline and methyl orange with different mechanism. Carbohyd. Polym. 168, 153–162. https://doi.org/10.1016/j.carbpol.2017.03.079. Li, H., Zhang, T., Pan, C., Pu, C., Hu, Y., Hu, X., Liu, E., Fan, J., 2017b. Self-assembled Bi2MoO6/TiO2 nanofiber heterojunction film with enhanced photocatalytic activities. Appl. Surf. Sci. 391, 303–310. https://doi.org/10.1016/j.apsusc.2016.06.167. Liu, F., Yu, J., Tu, G., Qu, L., Xiao, J., Liu, Y., Wang, L., Lei, J., Zhang, J., 2017. Carbon nitride coupled Ti-SBA15 catalyst for visible-light-driven photocatalytic reduction of Cr (VI) and the synergistic oxidation of phenol. Appl. Catal. B: Environ. 201, 1–11. https://doi.org/10.1016/j.apcatb.2016.08.001.

9

Journal of Hazardous Materials xxx (xxxx) xxxx

S. Wang, et al.

synthesize pyramid-like g-C3N4 arrays for efficient degradation of rhodamine B under visible-light irradiation. Chem. Eng. J. 334, 1869–1877. https://doi.org/10.1016/j. cej.2017.11.133. Zhang, X., Lu, X., Shen, Y., Han, J., Yuan, L., Gong, L., Xu, Z., Bai, X., Wei, M., Tong, Y., Gao, Y., Chen, J., Zhou, J., Wang, Z.L., 2011. Three-dimensional WO3 nanostructures on carbon paper: photoelectrochemical property and visible light driven photocatalysis. Chem. Commun. 47, 5804–5806. https://doi.org/10.1039/C1CC10389J. Zhang, J., Zhang, M., Lin, L., Wang, X., 2015. Sol processing of conjugated carbon nitride powders for thin-film fabrication. Angew. Chemie Int. Ed. English 54, 6297–6301. https://doi.org/10.1002/anie.201501001. Zhao, Z., Sun, Y., Dong, F., 2015. Graphitic carbon nitride based nanocomposites: a review. Nanoscale 7, 15–37. https://doi.org/10.1039/C4NR03008G. Zhao, H., Li, G., Tian, F., Jia, Q., Liu, Y., Chen, R., 2019a. g-C3N4 surface-decorated Bi2O2CO3 for improved photocatalytic performance: theoretical calculation and photodegradation of antibiotics in actual water matrix. Chem. Eng. J. 366, 468–479. https://doi.org/10.1016/j.cej.2019.02.088. Zhao, C., Chen, Z., Xu, J., Liu, Q., Xu, H., Tang, H., Li, G., Jiang, Y., Qu, F., Lin, Z., Yang, X., 2019b. Probing supramolecular assembly and charge carrier dynamics toward enhanced photocatalytic hydrogen evolution in 2D graphitic carbon nitride nanosheets. Appl. Catal. B: Environ. 256, 117867. https://doi.org/10.1016/j.apcatb. 2019.117867. Zhou, C., Lai, C., Huang, D., Zeng, G., Zhang, C., Cheng, M., Hu, L., Wan, J., Xiong, W., Wen, M., Wen, X., Qin, L., 2018. Highly porous carbon nitride by supramolecular preassembly of monomers for photocatalytic removal of sulfamethazine under visible light driven. Appl. Catal. B: Environ. 220, 202–210. https://doi.org/10.1016/j. apcatb.2017.08.055. Zhou, J., Wu, Z., Zhang, Z., Liu, W., Xue, Q., 2000. Tribological behavior and lubricating mechanism of Cu nanoparticles in oil. Tribol. Lett. 8, 213–218. https://doi.org/10. 1023/A:1019151721801.

J. Mater. Chem. A Mater. Energy Sustain. 6, 754–775. https://doi.org/10.1039/ C7TA09227J. Wang, N., Fan, H., Ai, S., 2015. Lignin templated synthesis of porous carbon–CeO2 composites and their application for the photocatalytic desulphuration. Chem. Eng. J. 260, 785–790. https://doi.org/10.1016/j.cej.2014.09.051. Wang, F., Chen, P., Feng, Y., Xie, Z., Liu, Y., Su, Y., Zhang, Q., Wang, Y., Yao, K., Lv, W., Liu, G., 2017. Facile synthesis of N-doped carbon dots/g-C3N4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of indomethacin. Appl. Catal. B: Environ. 207, 103–113. https://doi.org/10.1016/j.apcatb.2017.02. 024. Wang, N., Fan, H., Sun, J., Han, Z., Dong, J., Ai, S., 2016. Fluorine-doped carbon nitride quantum dots: ethylene glycol-assisted synthesis, fluorescent properties, and their application for bacterial imaging. Carbon 109, 141–148. https://doi.org/10.1016/j. carbon.2016.08.004. Xu, J., Zhang, L., Shi, R., Zhu, Y., 2013. Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis. J. Mater. Chem. A Mater. Energy Sustain. 1, 14766–14772. https://doi.org/10.1039/C3TA13188B. Yang, X., Tian, L., Zhao, X., Tang, H., Liu, Q., Li, G., 2019. Interfacial optimization of gC3N4-based Z-scheme heterojunction toward synergistic enhancement of solar-driven photocatalytic oxygen evolution. Appl. Catal. B: Environ. 244, 240–249. https://doi. org/10.1016/j.apcatb.2018.11.056. Yang, J.C., Jablonsky, M.J., Mays, J.W., 2002. NMR and FT-IR studies of sulfonated styrene-based homopolymers and copolymers. Polymer 43, 5125–5132. https://doi. org/10.1016/S0032-3861(02)00390-7. Yu, S., Liu, M., Ma, M., Qi, M., Lü, Z., Gao, C., 2010. Impacts of membrane properties on reactive dye removal from dye/salt mixtures by asymmetric cellulose acetate and composite polyamide nanofiltration membranes. J. Membrane Sci. 350, 83–91. https://doi.org/10.1016/j.memsci.2009.12.014. Yu, Y., Wang, C., Luo, L., Wang, J., Meng, J., 2018. An environment-friendly route to

10