Visible-light-driven photocatalytic degradation of rhodamine B using Bi2WO6/GO deposited on polyethylene terephthalate fabric

The environmental repercussions of wastewater from the dye process mean that it is very important to obtain an eco-friendly photocatalyst that would degrade wastewater. Herein, bismuth tungstate/graphene oxide (Bi2WO6/GO) composites are fabricated through in-situ hydrothermal reaction and then the Bi2WO6/GO photocatalysts are deposited onto polyethylene terephthalate (PET) fabric. The obtained Bi2WO6/GO deposited PET fabrics are then characterized through XPS, Raman, SEM, TEM, XRD, UV-vis, BET method and photoluminescence spectroscopy (PL) to investigate their chemical and crystal structures, morphology, optical property, surface area and photochemical properties. Photocatalytic performance is studied through examining the rate of degrading rhodamine B (RhB) under visible light. Surface of PET fibers is densely covered with Bi2WO6/GO. Bi2WO6/GO deposited PET fabrics show a broad absorption band in the visible spectra. Removal rate of RhB on the Bi2WO6/GO deposited PET fabric is the highest with the GO content of 2 g/L (labeled as Bi2WO6/2 g/LGO). The result of active species experiment shows that superoxide radicals (·O2−) plays a major role in the degradation of RhB. Moreover, Bi2WO6/2 g/LGO deposited PET fabric shows excellent cycle stability of photocatalytic degradation for RhB. The findings in this work can be extended to preparation other types of composite on the textile for photocatalysis, which can be applied to remove dyes in the wastewater produced by the textile or leather industry.


Introduction
It is well known that industries can contribute to the contamination of water bodies, such as textile and leather industries because a large volume of water is used in the process, generating large amounts of effluents. The dye wastewater from these industries is resistant to biodegradation [1]. Therefore, industrial dye wastewater treatment has received much attention. Among numerous pollutant treatment approaches, photocatalysis is one of the most promising methods for wastewater treatment [2][3][4]. In recent years, Bi-based photocatalysts have attracted much attention [5][6][7]. Bi 2 WO 6 is reported as an effective photocatalyst for detoxification in wastewater as well as polluted air among Bi-based semiconductors. Additionally, Bi 2 WO 6 is stable with high activity because the octahedron of ceratoid WO 6 is situated in the sandwich of (Bi 2 O 2 ) 2+ and can improve the separation of photo-generated charges [8,9]. Nevertheless, the photocatalytic performance of pristine Bi 2 WO 6 is restricted due to its low light absorption, difficult migration and rapid recombination of photo-generated electron-hole pairs [10,11]. In this sense, various efficient methods and technologies have been reported such as doping, substitution, heterostructure building with a narrow-bandgap semiconductor and coupling with a carrier to increase the photoinduced electron-hole pair's separation and transfer of Bi 2 WO 6 [12][13][14].
Graphene oxide (GO) contains functional groups like hydroxyl and epoxide groups on the basal plane and carboxyl groups at the edge [15]. The presence of πconjugation systems and oxygen groups cause GO to absorb visible light and impart high hydrophilicity to GO. In addition, GO can be readily dispersed in water at the molecular level, which exhibits biocompatibility and possesses tunable band gap. These can inspire this study to explore its potential as a photocatalytic material [16,17]. To date, there have been some reports on the fabrication of bismuth tungstate/graphene oxide (Bi 2 WO 6 /GO) for photocatalytic degradation of organic dyes [18,19].
Up to date, most researchers mainly focus on the study of the photocatalytic property of Bi 2 WO 6 /GO powders. However, it is not easy for the Bi 2 WO 6 /GO powder to be separated and recycled from the dye solution during the photocatalysis. In order to overcome these difficulties, some photocatalysts have deposited on textile products to investigate the photocatalytic activity of deposited fabrics in our previous reports such as Bi 2 WO 6 and Bi 2 WO 6 /TiO 2 [20,21]. Up to now, there are no reports about Bi 2 WO 6 /GO composite photocatalysts depositing on textiles for the dye degradation.
In this work, Bi 2 WO 6 and GO was complexed and treated by a facile hydrothermal method to obtain Bi 2 WO 6 /GO and they were then deposited on the PET fabric. The structure, photo-adsorption characteristics and specific surface area of the as-Bi 2 WO 6 /GO deposited PET fabrics were studied by SEM, TEM, XPS, Raman, XRD, UV-vis, PL and BET. The photocatalytic performance and recyclability of the Bi 2 WO 6 /GO deposited PET fabrics were investigated by the decomposition of RhB under visible light. Besides, effect of GO concentration on the photocatalytic properties of Bi 2 WO 6 /GO deposited PET fabric was studied.

Preparation of Bi 2 WO 6 /GO deposited PET fabric
Firstly, PET fabric was cut into the size of 5 cm × 5 cm and then was put into the mixed solution containing ethanol and acetone with volume ratio of 1:1, cleaned under ultrasonic stirring at 50°C for 30 min and dried. 0.2 g as-prepared Bi 2 WO 6 /GO was dispersed in 50 mL deionized water (DI). Subsequently, PET fabric was dipped into Bi 2 WO 6 /GO suspension and shaked in the shaker (SHA-C, Jintan Kexi Instrument Co., Ltd.) for 2 h. The obtained Bi 2 WO 6 /GO deposited PET fabric samples were rinsed in DI water several times and dried. Additionally, 0.2 g Bi 2 WO 6 powder was also deposited on the PET fabric according to the similar procedure for comparison.
In detail, Bi 2 WO 6 /GO composites were prepared by a hydrothermal process. 15 mL GO solution (0.5 g/L, 1 g/ L, 2 g/L, 3 g/L and 4 g/L) were added into deionized water, being sonicated for 1 h to produce a homogeneous solution. 0.3 g Na 2 WO 4 ·2H 2 O and 0.1 g CTAB were then added into the GO solution. The mixture was then stirred and sonicated for 30 min. After that, 0.8 g Bi (NO 3 ) 3 ·5H 2 O was placed into the above mixture solution. Subsequently, the suspension was placed into a 50 mL Teflon-lined autoclave and heated at 120°C for 8 h. Finally, the product was rinsed in DI and ethanol, and dried at 65°C. Meanwhile, Bi 2 WO 6 photocatalyst was also synthesized without GO.

Characterizations
Chemical composition of Bi 2 WO 6 and Bi 2 WO 6 /2 g/L GO deposited PET fabrics were conducted by X-ray photoelectron spectroscopy (XPS) spectra with a spectrometer (Kratos XSAM800) using an Al Kα X-ray source (1486.6 eV photons). Raman spectra of Bi 2 WO 6 , GO and Bi 2 WO 6 /2 g/L GO powders were performed by a Raman spectrometer (LabRAM HR, France). Surface morphologies of Bi 2 WO 6 /GO deposited PET fabric and Bi 2 WO 6 / GO composite were obtained through SEM (JEOL JSM-6700F) and TEM (Tecnai G2 F20 S-TWIN), respectively. The XRD patterns of the products were analyzed using X' Pert PRO diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the 2θ ranging from 10°to 80°. UV-vis spectra were used to characterize the optical property of all the samples and the spectra of the Bi 2 WO 6 /GO deposited PET fabrics were examined by UV-vis spectrophotometer (UV-2700) from 200 to 800 nm. PL spectra of Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fabrics were recorded using a F-7000 fluorescence spectrophotometer. The surface areas of Bi 2 WO 6 /GO composite were investigated by BET equipment (Gemini VII 2390).

Photocatalytic activity evaluation
The photocatalytic properties were studied by degradation of RhB. A 500 W xenon lamp (GXZ500, Shanghai, China) with a 420 nm UV-cut filter was utilized as the visible light source. The typical experiment processes were referred to our previous studies [20]. All the samples were immersed in 10 mg/L of RhB (50 mL) aqueous solution. Absorbance of RhB was recorded by a UV-vis spectrophotometer (554 nm for RhB). Absorbance ratio (A t /A 0 ) of RhB was used to evaluate photocatalytic degradation properties of Bi 2 WO 6 /GO deposited PET fabrics. According to the A t and A 0 , removal rate of RhB (R RhB ) and k value were calculated according to the Eqs.
(1) and (2) [22,23]: where A 0 and A t are the initial absorbance of RhB; the absorbance of RhB at certain reaction time t (min), respectively; k and t are the rate constant and the total irradiation time, respectively. For the purpose of comparison, photoactivity decomposition of RhB with Bi 2 WO 6 deposited PET fabric was also evaluated under the similar conditions. Additionally, influences of deposit weight of Bi 2 WO 6 /2 g/L GO and initial concentration of dye on the removal rate of RhB were studied. In order to understand the main reactive species, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), benzoquinone (BQ) and isopropanol (IPA) were used as the scavengers of photoexcited holes (h + ), superoxide radical (·O 2 − ) and hydroxyl radical (·OH), respectively.

XPS and Raman spectra
Chemical compositions of Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fabrics were studied by XPS spectroscopy (Fig. 1a-e). It can be seen that the two samples mainly contain Bi, W, O and C elements as shown in Fig. 1a. High resolution W 4f XPS spectrum of the sample is presented in Fig. 1b. The peaks with binding energy at 34.8 eV and 36.9 eV correspond to W 4f 7/2 and W 4f 5/2 , respectively, which is attributed to a W 6+ oxidation state [24]. The binding energies at 158.6 eV and 164.0 eV are attributed to Bi 4f 7/2 and Bi 4f 5/2 (Fig. 1c), respectively. The result shows that Bi 3+ exists in Bi 2 WO 6 deposited PET fabric [25]. However, the peaks of W 4f and Bi 4f in the Bi 2 WO 6 /GO deposited PET fabric shift toward high binding energy, which suggests interaction between Bi 2 WO 6 and GO [26,27]. The C 1 s XPS spectrum of the Bi 2 WO 6 deposited PET fabric in Fig. 1d is deconvolved into three peaks due to C-C (284.6 eV), C-O (286.0 eV) and C=O (288.4 eV) in epoxy or hydroxyl forms, and the similar peaks are observed in the Bi 2 WO 6 /GO deposited PET fabric, indicating that GO exists in the composite. Compared with the Bi 2 WO 6 deposited PET fabric, the O 1 s spectrum of the Bi 2 WO 6 / GO deposited PET fabric also exhibits the higher binding energy (Fig. 1e). All these results indicate the interface interaction between GO and Bi 2 WO 6 , affecting the electronic structure of Bi 2 WO 6 . In order to further study the structure of Bi 2 WO 6 /GO composite, Raman was used and the result is shown in Fig. 1f. It can be found that the peaks at 800 and 826 cm − 1 are attributed to symmetric and antisymmetric modes of terminal O-W-O due to the presence of Bi 2 WO 6 [28]. The peak at about 400 cm − 1 is ascribed to the simultaneous motions of Bi 3+ and WO 6 6− . In the case of GO, the positions of 1334 cm − 1 and 1594 cm − 1 displays the characteristic peaks of D-band and G-band, which corresponds to the vibrational mode of E 2g phonons of two dimensional sp 2 hybridized carbon network in a six edged lattice with breathing mode of j-joint photons of A 1g symmetry [29]. Similarly, for the spectrum of Bi 2 WO 6 /GO composite, the characteristic peaks of GO and Bi 2 WO 6 can be observed, indicating that the Bi 2 WO 6 /GO composite is successfully synthesized.

Morphology
SEM and TEM were used to study surface morphology of Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fibers and detailed structure of the Bi 2 WO 6 /GO composite. The surfaces of the fibers become rough after deposition of Bi 2 WO 6 (Fig. 2a). Bi 2 WO 6 are aggregated together to form larger particles. Figure 2b shows that Bi 2 WO 6 /GO are deposited on the PET fibers densely and the morphology of the Bi 2 WO 6 /GO deposited PET fibers is similar to that of Bi 2 WO 6 deposited PET fibers. This indicates that the introduction of GO has no obvious effect on the Bi 2 WO 6 structure [30]. The structure of Bi 2 WO 6 /GO composite photocatalyst was observed under transmission electron microscope as shown in Fig. 2c and d. Therefore, Bi 2 WO 6 are randomly located on the surfaces or edges of GO, indicating the formation of Bi 2 WO 6 /GO.

Crystal structure
XRD patterns of Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fabrics were presented in Fig. 3. Figure 3a shows the crystal structures of the pristine Bi 2 WO 6 and   Fig. S1, GO exhibits a strong peak at 2θ = 10.36°which is ascribed to the (002) plane [31]. In addition, compared with pristine Bi 2 WO 6 particles, the diffraction peaks of Bi 2 WO 6 /GO don't change significantly. This result indicates that the crystal structure of Bi 2 WO 6 is not damaged after the introduction of GO because of the small amount of GO in the Bi 2 WO 6 /GO composite. Figure 3b illustrates the XRD patterns of the Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fabrics. The diffraction peaks of Bi 2 WO 6 can be observed on the PET fabric, and the peaks at 17.7°, 22.8°and 25.7°are attributed to the characteristic peaks of the pristine PET fabric in all the samples as show in Fig. S1 [32]. This result shows that the Bi 2 WO 6 /GO composite photocatalysts are deposited on PET fabric.

UV-vis DRS
UV-vis DRS of as-prepared Bi 2 WO 6 /GO deposited PET fabrics were investigated as presented in Fig. 4a. Bi 2 WO 6 /GO deposited PET fabrics have a wider absorption than pristine Bi 2 WO 6 deposited PET fabric. In addition, when the content of GO is lower than 2 g/L, the absorption intensity of Bi 2 WO 6 /GO deposited PET fabrics increases as the content of GO rises. However, the absorption of the deposited fabric does not move to long wavelengths when GO continues to increase, which is in consistent with previous reports [33,34]. The band gaps are estimated from the (αhυ) 2 versus photon energy (hυ) plots and the results are 2.69 eV (Bi 2 WO 6 /2 g/L GO deposited PET fabric) and 2.81 eV (Bi 2 WO 6 deposited PET fabric), respectively, as shown in Fig. 4b. The phenomenon can be explained by the fact that the introduction of GO can effectively narrow down the band gap, thus electron-hole pairs of the sample can separate more easily, further the transfers of photogenerated carriers can be promoted. In order to determine the positions of the valence band (VB) and conduction band (CB) edges, the ultravioletvisible diffuse reflectance spectra (DRS), Mott-Schottky plot and valence band X-ray photoelectron spectroscopy (VB XPS) of Bi 2 WO 6 /2 g/L GO composite were performed and the results are shown in Fig. 4c-e. The results show that the Fermi energy level (E f ), the energy gap between the valence band and the Fermi level (E vf ) and the band gap for Bi 2 WO 6 /2 g/L GO composite are 0.01 eV (vs NHE), 1.90 eV and 2.69 eV, respectively. Therefore, combining with the results of DRS, Mott-Schottky plot and VB XPS, VB and CB edges can be calculated to be 1.91 eV and − 0.78 eV for the Bi 2 WO 6 /2 g/L GO composite, respectively.

Surface area
The surface areas of Bi 2 WO 6 and Bi 2 WO 6 /GO composite photocatalyst particles are presented in Fig. 5. It is found that the specific surface areas of pure Bi 2 WO 6 and Bi 2 WO 6 /GO particles are 51.52, 46.15, 47.15, 54.06, 48.82 and 42.44 m 2 /g, respectively. This result shows that the surface area of Bi 2 WO 6 rises and then reduces with an increase of content of GO, which contributes to adsorb more active substances and further could enhance the photocatalytic properties [35].

PL analysis
The charge transfer and separation behaviors of the Bi 2 WO 6 and Bi 2 WO 6 /2 g/L GO samples were investigated and the PL spectra of Bi 2 WO 6 and Bi 2 WO 6 /2 g/L GO deposited PET fabric are shown in Fig. 6. It can be found that the two samples have strong emission peaks at 455 nm. However, the intensity of Bi 2 WO 6 /2 g/L GO deposited PET fabric is lower than that of Bi 2 WO 6 deposited PET fabric, indicating that Bi 2 WO 6 /2 g/L GO deposited PET fabric exhibits a high separation rate of photoinduced electron-hole pairs. Thus, introduction of GO can inhibit recombination of photo-generated carriers of Bi 2 WO 6 and improve the photocatalytic properties, which is in good agreement with other reports [36].

Photocatalytic properties of Bi 2 WO 6 /GO deposited PET fabrics
Before carrying out the photocatalytic experiment, the adsorption properties of different samples in the dark were explored first and the result is shown in Fig. 7a. It can be seen that all the samples exhibit good adsorption property for RhB. The adsorption of RhB by the samples has reached saturation for 120 min. Especially, the adsorption efficiency of RhB over Bi 2 WO 6 /GO deposited PET fabric reaches the highest when the concentration of GO is 2 g/L due to strong hydrophilic properties of GO. The photocatalytic degradation for RhB over Bi 2 WO 6 /GO deposited PET fabrics was then tested under visible light and the results are shown in Fig. 7b and c. The RhB degradation rates for Bi 2 WO 6 and Bi 2 WO 6 /GO deposited PET fabrics with different concentrations of GO are 82.1%, 88.5%, 89.5%, 94.8%, 92.3% and 90.6%, respectively, when subjected to visible light irradiation for 120 min. Compared with the Bi 2 WO 6 deposited PET fabric, the Bi 2 WO 6 /GO deposited PET fabrics exhibit improved photodegradation performance. In addition, it can be observed that the photocatalytic efficiency of the Bi 2 WO 6 /GO deposited PET fabric increases gradually and then decreases when the concentration of GO increases. And the Bi 2 WO 6 /GO deposited PET fabric exhibits the best photocatalytic property when the concentration of GO reaches 2 g/L because the increase of GO content can widen the absorption of visible light, thus resulting in the enhancement of photocatalytic properties. However, degradation rate of RhB on Bi 2 WO 6 /GO deposited PET fabric decreases when the concentration of GO further rises due to limitation of visible light absorption of Bi 2 WO 6 . Meanwhile, the photodegradation process fits with the pseudo-first-order kinetics and the maximum rate constant k (Bi 2 WO 6 /2 g/LGO deposited PET fabric: 0.024 min − 1 ) is 1.7 times than Bi 2 WO 6 deposited PET fabric (0.014 min − 1 ). The UV-vis spectra of RhB over Bi 2 WO 6 /2 g/LGO deposited PET fabric during the photodegradation process were measured and the result is presented in Fig. 7d. It can be seen that the characteristic peak of RhB is 554 nm and a rapid decline of RhB adsorption is detected, while the spectral maximum shifts from 554 to 496 nm. This is because the release of N-ethyl groups and elimination of the conjugated structure from RhB during photodegradation make the spectrum shift to shorter wavelength [37]. The result demonstrates that RhB is degraded to CO 2 , H 2 O and other small molecules. The effect of Bi 2 WO 6 /2 g/L GO dosage and concentration of the dye solution on removal rate of RhB in the presence of the deposited PET fabric is shown in Fig. 8a and b. The removal rate of RhB rises from 86.8% to 97.3% when the amount of Bi 2 WO 6 /2 g/LGO photocatalyst on the fabric increases from 0.10 g to 0.25 g as shown in Fig. 8a. However, the removal rate of RhB decreases when the dosage of Bi 2 WO 6 /2 g/L GO photocatalyst is further increased. This phenomenon may be explained by that the rise of amount of Bi 2 WO 6 /2 g/L GO will increase the reaction sites that can generate more reactive oxidants. However, the overweight of photocatalysts would increase light scattering and reduce the transmittance, resulting in a decrease in the degradation efficiency of contaminants [20]. From Fig. 8b, the obtained catalysis rates of the samples are 0.028, 0.020, 0.014, 0.013 and 0.011 min − 1 , respectively, according to the Formula (2). It can be included that the catalysis rate reduces when the concentration of RhB rises because low concentration of dye increases the path length of the photons which enters the solution of dyes, and finally increase amount of photon absorption of photocatalyst [38].
3.9 Recycle stability and trapping experiment of Bi 2 WO 6 / 2 g/L GO deposited PET fabric The photocatalytic recycle stability of Bi 2 WO 6 /2 g/L GO deposited PET fabric under visible light is presented in Fig. 9a. Removal rate of RhB in the presence of Bi 2 WO 6 /2 g/L GO deposited PET fabric is 95% after the first cycle. The removal rate of RhB is 89% after the third cycle. The results show that the photodegradation efficiency of Bi 2 WO 6 /2 g/L GO deposited PET fabric slightly reduces after three times recycle under visible light irradiation. This is because the photocatalyst deposited on the fabric will be lost to a certain extent during its repeated use. Meanwhile, the surface of the photocatalyst on the fabric will absorb the by-product after the reaction, making the active site reduce. However, the stability of Bi 2 WO 6 /2 g/L GO deposited PET fabric is good. In order to study the main active species during the photocatalytic process, the trapping experiment was carried out and EDTA-2Na, BQ and IPA were used as h + , ·O 2 − and ·OH scavenger, respectively. Figure 9b shows the effect of different scavengers on the degradation of RhB over Bi 2 WO 6 /2 g/L GO deposited PET fabric. The result shows that the degradation rate of RhB declines significantly in the presence of BQ. However, the addition of IPA and EDTA-2Na has only a weak inhibitory effect on the degradation of RhB. Therefore, the result demonstrates that ·O 2 − plays dominant role in the photodegradation of RhB.

Conclusions
In conclusion, Bi 2 WO 6 /GO composite photocatalysts were successfully fabricated via a simple hydrothermal process and they were then deposited on the PET fabric. Compared with the Bi 2 WO 6 deposited PET fabric, the Bi 2 WO 6 /GO deposited PET fabrics show enhanced photocatalytic activity for RhB. The removal rate of RhB with Bi 2 WO 6 /GO deposited PET fabric is highest when the content of GO reaches to 2 g/L. Additionally, Bi 2 WO 6 /2 g/L GO deposited PET fabric shows excellent cycle stability for RhB degradation. This study provides insights into the preparation of modified photocatalysts deposited textiles for organic wastewater treatment.
Additional file 1: Fig. S1. XRD patterns of (a) GO and (b) pure PET fabric.