Introduction

Dyes play a major role in various branches of dying industries such as bleaching, textile, medicine and paper industries (Chauhan et al. 2014; Shanavas et al. 2017). The dye-based industries liberating ~ 10% of non-bio degradable dye wastages with highly colored water, which are deleterious to the environment (Ahmedchekkat et al. 2011; Bazin et al. 2012). The existing dye water leads to interfere with the penetration of light and thus reducing the photosynthesis of aquatic plants which severely annihilate the aquatic ecosystem (Ni et al. 2007). Many organic dyes are toxic and carcinogenic like azo dyes; Congo red, methylene blue and Rhodamine B these may lead to the jeopardizing aquatic organism and also human health (Wong et al. 2004). Generally, the degradation of organic pollutants are mainly classified into three types such as (1) physical technologies: adsorption, reverse osmosis, ultra-filtration. (2) Chemical process: chlorination, ozonation, precipitation, catalyst-chemical oxidation. (3) Biological process: enzymatic decomposition, biodegradation.

In this regards over the past decades, photocatalytic degradation of organic pollutants and water splitting using nano heterogeneous photocatalyst have immense important due to its feasibility and low cost (Ajmal et al. 2014; Chen and Mao 2007). TiO2 is one of the most accepted photocatalysts because of biologically and chemically inert (Boobas et al. 2017) and its high photostability, nontoxicity, and low cost, but it has some drawbacks of wide band gap, low quantum yield and only under ultraviolet region (Kudo and Miseki 2009). In case of metal chalcogenides, particularly metal sulfides, have much higher photocatalytic activity (e.g., CdS, ZnS, SnS2) although metal sulfides are subjected to oxidize by photogenerated holes known as photocorrosion. Among them, various narrow band gap semiconductors, CdS (~ 2.4) can directly absorb visible light at wavelengths ~ 550 nm, which is a good candidate to apply as a photocatalyst. Even though the pure CdS suffered by rapid recombination rate of photogenerated charge carriers and photocorrosion during the catalytic action and restricting its potential application. Many researchers focused to overcome the above problem by various technologies (Wang et al. 2016a; Yu et al. 2016). Incorporation of metal sulfides into interlayer and mesoporous materials have a long time efficient for stabilizing the metal sulfides (Shangguan and Yoshida 2002; Shen and Guo 2006). In the midst of various semiconductor photocatalyst, aurivillius family and its layered structure have attractive future and much attention because of hybridized valance band O 2p and Bi 6s (Tang et al. 2007; Garcia-Perez et al. 2012; Zhu et al. 2013; Dong et al. 2014a). Aurivillius oxide photocatalytic semiconductors (Bi2 An−1 B n O3n+3) A = Ca, Sr, Ba, Pb, Na, K and B = Ti, Nb, Ta, Mo, W, Fe (Guo et al. 2013; Zhang et al. 2009; Fu et al. 2005). Bismuth oxyhalide (Chatterjee and Dasgupta 2005; Wang et al. 2011) has the abundant head of the researchers, in the midst of these, Bi2WO6 (Saison et al. 2013; Zhuo et al. 2013; Dumrongrojthanath et al. 2013; Saison et al. 2011) has much higher photocatalytic activity and stability in visible and ultraviolet light irradiation and has a wide band gap. However, it also suffered by rapid recombination of photogenerated electrons and holes and unsuitable conduction band edge position (Ren et al. 2015). Meanwhile, among various conjugated polymers, nanocomposites on the polymer substrates or polymer-assisted nanocomposites are attractive to solve the problems concerning the recovery processes during the photocatalytic action. PVA is a hydrophilic polymer which has been explored in a wide range of application including biomaterials and drug delivery system because of biocompatibility and nontoxicity (Jung and Kim 2014; Liu et al. 2015a; Moon et al. 2013; Krishnakumar et al. 2017). Coupling of one or more active components to effectively make heterostructures like CaWO4/Bi2WO6 composite, Bi2WO6/CeO2 hybrid nanoparticles and Ag2CO3/Bi2WO6 heterojunction (Guo et al. 2012a, 2014; Bao et al. 2015), are a more powerful method to overcome the demerits of all traditional photocatalyst. In the last few years, making binary nanomaterials using aurivillius oxide semiconductors (Zhang et al. 2016; Liu et al. 2015b) and ternary (CNNS/CdS/rGO), (CeO2/SnO2/rGO), (La2CuO4/CeO2/rGO) (Zhou et al. 2016; Priyadharsan et al. 2017; Shanavas et al. 2017) plays a vital role in dye degradation. The heterostructure of nanomaterials have much higher efficiency compared to above all, because of interfacial charge transfer between the heterostructure, therefore, it can highly retain the recombination of photogenerated charge carriers.

In this research work, we analyzed the structure and morphology-based many metal chalcogenides, and aurivillius family heterostructure (Kundu et al. 2017; Luo et al. 2016; Kandi et al. 2017; Xiao et al. 2016; He et al. 2011), CdS and Bi2WO6 (Tang et al. 2015) have well-matched energy levels and efficient photocatalytic degradation of organic dyes but also had some drawbacks, especially metal sulfides suffered by photocorrosion. The mixing of photocatalyst nanocomposite with the polymeric substrate is a new way to solve the current problem; recovery of photocatalyst and inhibition of photocorrosion. In our work, as-synthesized Bi2WO6–CdS heterostructures; dispersed into polymer (PVA) matrix. The cost-effective transparent PVA assisted Bi2WO6–CdS nanocomposite film exhibit a recycling stability and ease of recovery in the photodegradation process.

Experimental

Synthesis of CdS nanoparticle

All the chemical reagents and organic targets were of analytical grades and used without further purification. CdS NPs were synthesized by a previously reported hydrothermal route (Cheng et al. 2016), the required molar ratio of [Na2S·xH2O] sodium sulfide solution was added dropwise into the [CdCl2·2½ H2O] cadmium chloride solution and stirred vigorously for 8 h. The stirred solution was left aging for another 4 h to get the product. Furthermore, it was ultrasonicated with water and transferred into 100 ml Teflon-lined autoclave kept in 24 h at 180 °C. Finally, the yellow color precipitate was washed several times with ethanol and distilled water and the product was dried at 100 °C for 10 h for further characterization.

Synthesis of Bi2WO6–CdS nanocomposite

In the typical synthesis of Bi2WO6–CdS nanocomposite 0.12 g of [Bi(NO3)3·5H2O] bismuth nitrate pentahydrate was dissolved in 50 ml of ethylene glycol. On the other hand 0.06 g of [Na2WO4·2H2O] sodium tungstate was dissolved in 80 ml deionized water and stirred for 3 h, these two solutions were mixed and stirred until to obtain a homogeneous suspension. Afterwards, the required amount of CdS nanoparticles was dispersed by ultrasonication for 10 min and mixed with this solution. Then the suspension was transferred into the Teflon-lined autoclave kept at 180 °C for 20 h. The product was washed several times using ethanol and distilled water to remove moistures; the finally obtained precipitate is dried at 100 °C for 4 h. The same procedure is repeated for preparation of Bi2WO6 nanoparticles without the addition of CdS.

Synthesis of PVA assisted Bi2WO6–CdS nanocomposite film

PVA assisted Bi2WO6–CdS nanocomposite film was prepared via hydrothermal method. The cold soluble polymer was [–CH2CHOH–] n stirred for 2 h to get a homogenous solution, the prepared Bi2WO6–CdS nanocomposite was ultrasonicated for 30 min and then mixed with the polymer, continuously stirred for another hour to get a composition. It was transferred into 100 ml Teflon-lined autoclave and gave heat treatment at 180 °C for one hour. Then colloidal solution is poured into silica glass plate and dried to evaporate the solvent.

Characterization methods

Phase structure of the synthesized catalyst was examined by powder X-Ray diffraction using Rigaku X-ray diffractometer Cu-Kα radiation (K = 1.54 Å) in the 2θ range of 20°–80° at room temperature. The surface chemical elemental analysis was conducted on an X-ray photoelectron spectroscopy (XPS) (SPECS GmbH spectrometer with Phobios 100MCD Energy analyzer) depth profiling facility. Fourier transform infrared (FT-IR) spectra of all the samples were recorded using a Bruker Tensor 27 spectrophotometer with a resolution of 2 cm−1 in the range of 4000–400 cm−1 at room temperature. Whereas the morphology of the sample was analyzed by field emission scanning electron microscopy [FESEM] with the feature of energy dispersive X-ray diffraction spectrum. Optical properties namely UV–Vis DRS and photoluminescence of the all the prepared catalyst were evaluated by Ocean Optics PX-2 and Perkin Elmer LS-45 fluorescence spectrometer, respectively.

Photodegradation assessment and setup

Methylene blue and metronidazole acted as target substrates for examining the photocatalytic activity of as-synthesized photocatalyst under visible light irradiation. In a typical process 2 × 2 cm3 of nano-photocatalyst was dipped (100 ml) in 20 ppm solution of methylene blue. For metronidazole 3 × 3 cm3 for 100 ml in 0.1 mg were used, simultaneously the pH value would be monitored. Photodecomposition activity was carried out using 500 W halogen lamp as visible light source and the glassware with double walled for the purpose of circulating water system to maintain a sustained environment of temperature, before the irradiation of visible light. To ensure the formation of adsorption and desorption equilibrium of the organic pollutant with catalyst, the solution was ultrasonicated for 20 min and then stirred for 1 h in dark environment. During the process of decomposition at every 20 min, 6 ml of solution were sampled and centrifuged to remove the catalyst. The photocatalytic efficiency was investigated by the following equations (Mirhoseini and Salabat 2015):

$$ {\text{Decomposition}}\;{\text{efficiency}}\;(\% ) = \left( {\frac{{\mathop C\nolimits_{0} - C}}{{\mathop C\nolimits_{0} }}} \right) \times 100, $$
(1)

where C0 and C are initial and variable reaction concentration, respectively. In the degradation process, the absorption intensity for all the catalyst would be A0 and the significant time interval of the reduced absorption be A, the efficiency was calculated as follows:

$$ {\text{Efficiency}}\;\eta = \left( {\frac{{A_{0} - A}}{{A_{0} }}} \right) \times 100. $$
(2)

To get better understanding of kinetics reaction, the experimental data fitted by a pseudo-first-order model. The equation for calculation is as follows:

$$ \ln \left( {{\raise0.7ex\hbox{$c$} \!\mathord{\left/ {\vphantom {c {c_{0} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${c_{0} }$}}} \right) = - K_{\text{app}} t, $$
(3)

where C0 is the initial concentration of dye, C is the concentration at desired time interval Kapp is the apparent first-order rate constant.

Trapping of active species

The main active species produced in the photodecomposition process of MB and metronidazole confirmed by the trapping agents such as benzoquinone, ethylenediaminetetraacetate, isopropyl alcohol and silver nitrate. They were introduced into to the degradation process to trap superoxide (O ·−2 ), holes (h+), hydroxyl (OH·), and electron (e−), respectively. All these trapping agents were in the quantity of 5 mmol except BQ which was in 1 mmol to prevent the causes of dye during the reaction. Further, the hydroxyl radicals of all the samples were confirmed by the aid of photoluminescence of terephthalic acid.

Results and discussion

X-ray diffraction analysis

The crystalline structure, appropriate particle size and phase purity of as-synthesized catalysts examined by powder X-ray diffraction pattern. The prepared bare CdS, Bi2WO6, Bi2WO6–CdS and PVA assisted Bi2WO6–CdS nanocomposite film are separately shown in Fig. 1. The sharp peak in the powder X-ray diffraction pattern exposed the well crystalline nature of the prepared sample. Bare Bi2WO6 nanoparticle exhibit intense diffraction peaks at 28.16, 32.68, 35.70, 47.07, 51.08, 55.02, 58.54, 68.74, 75.89, 78.30, which corresponds to (113), (006), (115), (206), (224), (313), (226), (400), (139), (406) planes of the russellite (orthorhombic) structure and also exactly matches the JCPDS card No 73-1126. The pure CdS reveals peaks at 24.66, 26.39, 28.30, 36.42, 43.58, 47.64, 51.68, 58.02, 66.74, 69.39, 70.91, 72.35, 75.40,79.99. Which could be well indexed to the (100), (002), (101), (102), (110), (103), (200), (202), (203), (210), (211), (114), (105), (300) crystal planes of wurtzite (hexagonal) phase [JCPDS card No 06-0314]. In both cases, no additional peaks were found, which shows the absence of other phases or any other impurities. Whereas the XRD pattern of the as-synthesized Bi2WO6–CdS heterostructure, and PVA assisted Bi2WO6–CdS nanocomposites film, possess both Bi2WO6 and CdS nanomaterials with less intensity. It could be due to the synergistic effect of the two nanomaterials and additional peaks at 13.86, 16.72 were observed in PVA assisted Bi2WO6–CdS nanocomposite film due to the crystalline and semi-crystalline nature of PVA. The appropriate crystallite size of all the samples calculated using Debye–Scherrer’s equation

$$ D = \left[ {\frac{K\lambda }{\beta \cos \theta }} \right], $$
(4)

D mean diameter of the crystallite, K = 0.96 is constant, \( \lambda \) is wavelength (1.54 Å), β full width half maximum, θ is diffraction Braggs angle.

Fig. 1
figure 1

Powder X-ray diffraction pattern of prepared samples. a Bi2WO6, b CdS, c Bi2WO6–CdS and d PVA assisted Bi2WO6–CdS nanocomposite film

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The structural parameters of the samples like dislocation density (δ), stacking faults (SF), intercrystalline separation (R), are calculated by the following equation (Hemalatha and Rukmani 2016). The values are tabulated in the Table 1.

$$ {\text{Dislocation}}\;{\text{density}}\;\delta = \frac{1}{{D^{2} }}, $$
(5)
$$ {\text{Stacking}}\;{\text{fault}}\;{\text{SF}} = \left[ {\frac{{2\pi^{2} }}{{45(\tan \theta )^{{\frac{1}{2}}} }}} \right]\beta , $$
(6)
$$ {\text{Intercrystalline}}\;{\text{distance}}\;R = \frac{5\lambda }{8\sin \theta }. $$
(7)
Table 1 Structural parameters of the catalyst
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The lattice parameters calculated using Eqs. (5) and (6) and their values are tabulated in Table 2:

$$ {\text{Hexagonal}}\;\frac{1}{{d^{2} }} = \left[ {\frac{4}{3}(h^{2} + k^{2} + hk) + l^{2} ({a \mathord{\left/ {\vphantom {a c}} \right. \kern-0pt} c})^{2} } \right]{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {a^{2} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${a^{2} }$}}, $$
(8)
$$ {\text{Orthorhombic}}\;\frac{1}{{d^{2} }} = \frac{{h^{2} }}{{a^{2} }} + \frac{{k^{2} }}{{b^{2} }} + \frac{{l^{2} }}{{c^{2} }}. $$
(9)
Table 2 Lattice parameters
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Thus, the hexagonal phases of CdS possess a longer absorption in the visible light region and higher activity compared to cubic phases (Silva et al. 2008; Tian et al. 2012). The bare CdS nanoparticles have to aggregate due to their instabilities; therefore, reducing the aggregation by their heterostructures and doping would enhance the surface area and thus would help to improve the catalytic performance (Kumar and Ojha 2016). Composites show less crystalline nature compared to the pure CdS and Bi2WO6 thus it could enhance the crystal surface imperfection like dislocation, stacking faults which turn to increase reaction centers (Zhang et al. 2007). The rate of structural defects could increase the number of chemically reactive sites, and the defects and disorders could change the equilibrium coordination number, that’s why chemical reaction would increase further defects tend to lower the bandgap because of the introduction of new band position (Li et al. 2017).

XPS analysis

The oxidation states and surface chemical composition of the nanocomposite film were characterized by X-ray photoelectron spectroscopy (XPS). Typical survey XPS spectrum of Bi2WO6–CdS/PVA indicates that Bi, W, Cd, S, C and O elements could all be detected as shown in Fig. 2a. The high-resolution XPS spectra of Bi 4f, W 4f, Cd 3d, S 2p, C 1s and O 1s is shown in Fig. 2b–g. Figure 2b shows the two bands at 159.15 and 164.16 eV ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, which can be attributed to Bi3+ in the crystal structure. The peak at 36.2 and 37.6 eV as shown in Fig. 2c, corresponding to W 4f7/2 and W 4f5/2, respectively, can assign to be a W6+ oxidation state of Bi2WO6–CdS/PVA (Zhang et al. 2006). Figure 2d represents the binding energies of 405.28 and 412.60 eV corresponds to the Cd 3d5/2 and Cd 3d3/2 respectively (Li et al. 2013). The S 2p spectrum was divided into two different peaks is shown in Fig. 2e. The peak at 161.15 eV could be a credit to the S2− anion, and the other one at 167.12 eV could be assigned to the oxidized SO42− form (Bandyopadhyay et al. 1997). From Fig. 2f, it was observed that the peaks with binding energies of 284.2 and 287.65 could be assigned to the C 1s spectra of C–C and C–O, respectively. The C–C bond bond confirm the main chain of PVA (Wei et al. 2011). Furthermore, Fig. 2g show that two peaks fitted the O 1s spectrum. The peak at 530.35 eV can be due to the metal oxides, the broad peak at 533.04 eV indicates the binding energy of C–OH bond in PVA (Gu et al. 2012). Based on these observations and analyses the results exhibit that the formation of Bi2WO6–CdS/PVA composite occurs, which is in better agreement with the results of the EDX and XRD analysis.

Fig. 2
figure 2

XPS spectra of PVA assisted Bi2WO6–CdS nanocomposite film. a Survey spectrum; b Bi 4f; c W 4f; d Cd 3d; e S 2p; f C 1s; g O 1s

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FT-IR analysis

The chemical state of as-synthesized Bi2WO6, CdS, Bi2WO6–CdS and PVA assisted Bi2WO6–CdS nanocomposite film explored via FT-IR spectroscopy and their spectrograms are shown in Fig. 3. The spectrum reveals the band absorption peak at 3000–3600 cm−1 in bare nanoparticles is assigned to the OH stretching vibration of a physically observed water molecule. In Bi2WO6 several characteristic peaks were observed at wavenumbers 1395, 725, 583 cm−1 are due to the W–O–W bridging, W–O stretching, Bi–O stretching vibration, respectively (Liu et al. 2015c; Wei et al. 2014; Guo et al. 2012b). In the case of CdS, Cd–S stretching vibration occurred below 700 cm−1. Meanwhile, PVA assisted Bi2WO6–CdS nanocomposite film shows vibrations at 3000–3109 cm−1 are due to the O–H stretching vibration hydroxyl groups due to polymeric or water molecules, and then CH, C=O, C=C, and C–C stretching, rocking vibrations are assigned at 2970, 1746, 1605, 1525 and 900 cm−1, respectively (Krishnakumar and Shanmugam 2012; Chen 2002; Rao et al. 2015; Mallakpour and Vahabi 2015). In PVA assisted Bi2WO6–CdS nanocomposite film, the content of nanoparticle loading is very low, therefore, the interaction of nano heterostructures with nanostructured polymer was unpredictable from the FT-IR spectrum.

Fig. 3
figure 3

FT-IR spectrum of the prepared samples

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Morphology analysis

The surface morphology of all prepared samples was characterized by FESEM and their micrographs are shown in Fig. 4. It was observed that the bismuth tungstate shows spheroid structure. CdS show loosely agglomerated well-defined spherical shape with the high surface area, meanwhile, in the case of Bi2WO6–CdS nanocomposite CdS nanospheres were sprinkled on the core Bi2WO6 structure. The CdS nanosphere agglomeration get suppressed when it is adhered to the surface of Bi2WO6 and their particle size could enhance by deposition of CdS. In the case of PVA assisted Bi2WO6–CdS nanocomposite film, the dark spot shows a shapeless surface passivated structure which is attributed to the crystalline nature of polymer in nanoscale, the bright spot indicates the surface modified agglomerated Bi2WO6–CdS nano-heterostructures. This type of morphology might enhance the photocatalytic degradation performance.

Fig. 4
figure 4

The FESEM images of a Bi2WO6, b CdS, c Bi2WO6–CdS and d PVA assisted Bi2WO6–CdS film

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EDX analysis

The representative EDX spectra of synthesized samples examined by the integrated feature of FESEM, and their graphs are shown in Fig. 5. In Bi2WO6 the Bi, W, O elements were confirmed and their wt% are 48.15, 35.09, 16.76, respectively. The established CdS spectra show Cd, S element with 76.10, 23.10 wt% correspondingly. Whereas the Bi2WO6–CdS shows the presence of above-mentioned all elements with slight changes in wt% (Cd, Bi, W, O, S with 29.78, 26.98, 25.55, 9.08, 8.60 wt% singly). But PVA assisted Bi2WO6–CdS nanocomposite film confirms the presence of H, C, O, Cd, Bi, W, S elements with 51.73, 23.57, 15.97, 1.65, 0.26, 6.15, 0.69 wt%, respectively. No other impurities are found in all the samples.

Fig. 5
figure 5

EDX spectrum of a Bi2WO6, b CdS, c Bi2WO6–CdS and d PVA assisted Bi2WO6–CdS nanocomposite film

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UV–Vis diffuse reflectance spectra

All the prepared photocatalysts subjected to optical characterization of UV–Vis DRS techniques using Ocean Optics PX-2 spectrophotometer, and the results are shown in Fig. 6. The Bi2WO6 nanoparticle exhibited the strong absorption edge ultraviolet and visible region approximately 420 nm is owing to the band gap photoexcitation of Bi2WO6. On the other hand, CdS exhibit the intensive absorption edge around 600 nm due to its response to visible light. When adding CdS to Bi2WO6 the absorption edge shows blue shifts 575 nm and it could gather few number of photons compared to bare Bi2WO6 and CdS (Zou et al. 2016). Then the PVA assisted Bi2WO6–CdS composite film also exhibited absorption edge in visible region 560 nm. This blue shift of absorption indicates that the composites exhibit quantum confinement effect (Jing and Guo 2007). From above obtained results PVA assisted Bi2WO6–CdS nanocomposite film has higher absorption maximum in the visible region. The band gaps of all the samples were calculated with the help of tauc plot and shown in inset Fig. 6 using the following equation (Zheng and Liu 2007)

$$ \left( {\alpha h\nu } \right)^{2} = A(h\nu - E_{\text{g}} ), $$
(10)

α is the absorption coefficient, h is the plank constant, ν is the frequency of light, Eg is the band gap, A is the proportionality constant, respectively, n = (1, 4) depend on the type of optical transition whether direct or indirect. Therefore, plotting between (αhν)2 and (hν) by extrapolating the linear portion of the curve to zero absorption coefficient. The band gap of pure Bi2WO6, CdS, Bi2WO6–CdS and PVA assisted Bi2WO6–CdS nanocomposite film calculated, and their values are given by 3.32, 2.16, 2.25 and 2.41 eV, respectively. The PVA assisted Bi2WO6–CdS nanocomposites film show low band gap compared to that of Bi2WO6 materials, because of changes in optical properties suffered by the interaction between the pure CdS and Bi2WO6 nanomaterials (Guo et al. 2016). Although the band gap of PVA assisted Bi2WO6–CdS nanocomposites film, due to the intermediate transaction of electrons in between the composite materials the recombination rate will be declined and the observed result is further confirmed through PL study. These results illustrate that the PVA assisted Bi2WO6–CdS nanocomposite film can be excited in visible light region due to the deposition of CdS nanoparticles on its surface.

Fig. 6
figure 6

UV–Vis DRS spectra of a Bi2WO6, b CdS, c Bi2WO6–CdS and d PVA assisted Bi2WO6–CdS nanocomposite film

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Photoluminescence spectroscopy

Photoluminescence spectroscopy is used to assess the recombination efficiency of photogenerated electrons and holes in photocatalytic reaction. Photoluminescence properties of all the products are shown in Fig. 7. The more intense peak assigned to CdS and thus more emission occurs due to a large amount of recombination of photogenerated charge carriers. The Bi2WO6 and PVA could reduce the photoemission, and the composite often indicates a drastic decrease in its intensity and confirmed the sensible suppression and recombination of electron and hole which improved photocatalytic activity. PVA/Bi2WO6–CdS has narrow optical band gap than Bi2WO6, this could give highly harvesting photons from UV–Vis light, and it could retain the recombination of generated charge carriers. The PL intensity of PVA assisted nanocomposite film shows less intensity compared to all other prepared photocatalyst, thus result from most of the surface states are passivated by making the availability of the electron for catalytic action (Wang et al. 2016b). The weaker PL intensity is, the higher separation efficiency of photoinduced charge (electron–hole separation), so that higher photocatalytic degradation behavior was preferred. Thus, the result indicates that compared with pure CdS and Bi2WO6 samples, the photocatalytic efficiency of PVA/Bi2WO6–CdS photocatalyst may be greatly improved.

Fig. 7
figure 7

Photoluminescence spectra of the samples

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Photocatalytic degradation of metronidazole

Drugs occupy an important place in the water pollution due to the availability of many healthcare products such as antibiotics, analgesics, anti-inflammatories and hypnotics, and high consumption rate of a human being and veterinary medication (Kansal et al. 2014). The drugs occupy the aquatic environment from various ways such as pharmaceuticals, industrial effluents and domestic effluents, the disposal of rude discarding of extra drugs in households, medicinal care, and healing treatment of livestock (Farzadkia et al. 2014). High concentration of antibiotics can involve serious risk for the human being and natural system because high solubility causes them, long-term presence, more toxic, cancerogenic, less adsorption, non-biodegradability nature. The effluent of wastewater from pharmaceuticals industries must be treated adequately before entry into natural water streams, to remove the contaminant caused by antibiotics. In over the past decades antibiotic effluents from industries act as a target pollutants to degrade using photocatalyst such as quinoline content and levofloxacin (Kansal et al. 2014), amoxicillin, glycopeptide, macrolide, sulphonamide (Kummerer 2009), tetracycline (Safari et al. 2015; Zhao et al. 2013), sulfamethoxazole (Abellan et al. 2009; Abellan et al. 2007), chloramphenicol (Chatzitakis et al. 2008) and metronidazole (Farzadkia et al. 2014). Among them metronidazole has been studied by many researchers, metronidazole antibiotics (2-methyl-5-nitro-imidazole-1-ethanol) was a derivative of nitroimidazole, which was widely used in the treatment of infectious diseases like anaerobic bacteria, protozoans, Bacteroides. Continuos and increasing the content of metronidazole is more carcinogenic and mutagenic, researchers have been paid mandatory to remove this type of contaminants completely before they are discharged from industries (Farzadkia et al. 2014; Liu et al. 2013; Dong et al. 2014b) here by all the problems aforementioned we choose metronidazole as a target organic contaminants.

The photocatalytic activities of pure CdS, Bi2WO6, binary Bi2WO6–CdS and PVA–Bi2WO6–CdS nanocomposite were evaluated by the metronidazole shown in Fig. 8a. From the result, PVA–Bi2WO6–CdS nanocomposite shows enhanced photocatalytic efficiency when compared to other binary composites such as Bi2WO6–CdS and parent photocatalysts like Bi2WO6 and CdS. The parent photocatalyst shows the limited amount of catalytic efficiency, but after coupling PVA with of CdS–Bi2WO6, the photocatalytic degradation of metronidazole significantly enhanced, achieving almost 70% after the irradiation of visible light for 100 min. Due to the composition of CdS–Bi2WO6 with PVA can serve as charge traps thus could retarding electron–hole combination rate and thereafter enhancing the interfacial charge transfer for metronidazole degradation. In addition to this connection, PVA coupled with binary Bi2WO6–CdS composite the photocorrosion phenomenon would be minimized, so that the durability and photostability increased. To understand the photocatalytic activity of pure CdS, Bi2WO6, binary Bi2WO6–CdS, and PVA–Bi2WO6–CdS nanocomposite can be described by pseudo-first-order kinetics, in Fig. 8b shows a linear relationship between ln(C0/C) and reaction time, indicating that the photodegradation of metronidazole follows first-order kinetics. The calculated values of Kapp or K were given by 0.0120, 0.0110, 0.0090 and 0.0070 for PVA assisted Bi2WO6–CdS, Bi2WO6–CdS, Bi2WO6, and CdS, respectively. Indeed thus from the values of K it is clear that the PVA assisted Bi2WO6–CdS nanocomposite film shows highest photocatalytic activity. A nanocomposite film possesses more activity 1.7142, 1.333 and 1.0909 times higher than CdS, Bi2WO6 and Bi2WO6–CdS, respectively; this is due to highly suppressed photogenerated carriers. Furthermore, UV–visible spectra confirmed the enhanced photocatalytic degradation efficiency of PVA–Bi2WO6–CdS nanocomposite film an antibiotic solution of metronidazole. The absorption spectrum of efficient PVA–Bi2WO6–CdS nanocomposite film are shown in Fig. 9. The absorption spectra of metronidazole show the characteristic peak (absorption maximum) at approximately λmax 320 nm. The photocatalytic degradation of metronidazole observed for every 20 min time interval at constant irradiation of UV–visible light. From the observation of absorption spectrum, drastically decrease the absorption maximum, it strongly indicates decomposition nature of metronidazole solution. So that, PVA–Bi2WO6–CdS nanocomposite film has an excellent photocatalytic activity compared to the CdS, Bi2WO6, Bi2WO6–CdS catalyst.

Fig. 8
figure 8

Photocatalytic degradation of metronidazole a efficiency under the presence of different catalyst, b their first-order pseudo kinetics

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Fig. 9
figure 9

Photocatalytic degradation of metronidazole with PVA assisted Bi2WO6–CdS nanocomposite film

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Photocatalytic degradation MB

On basis of all aforementioned photocatalytic performance of catalysts like individuals and their composites were evaluated by the degradation of MB exposed to visible light irradiation, before that the adsorption test was carried. There was no degradation observed in the absence of catalysts, which was obviously the stability of MB under self-photolysis. For comparison purpose, the photocatalytic activity of bare CdS, Bi2WO6, binary Bi2WO6–CdS and PVA–Bi2WO6–CdS nanocomposite against MB is shown in Fig. 10. Where net visible, light irradiating time was to be 100 min, and the absorption intensity was measured peak position at 664 nm, during the reaction absorption intensity was monitored for every 20 min. The results show that the bare CdS and Bi2WO6 shows low activity against degradation MB for 100 min that was 72 and 86%, respectively. Meanwhile, PVA assisted Bi2WO6–CdS nanocomposite film gave much higher efficiency (92%) compared to binary Bi2WO6–CdS and neat catalysts. Further, the higher degradation efficiency of PVA assisted Bi2WO6–CdS nanocomposite film, confirmed in Fig. 11a. Clearly, the photocatalytic activity of binary Bi2WO6/CdS composites boosts with the addition of CdS when compared to pure Bi2WO6. The difference in photocatalytic activity may relate to visible light absorption properties, the separation efficiency of photogenerated electrons and holes and efficient charge transfer. CdS are normally charge carriers, both electrons and holes quickly recombine and only a fraction are contributed in the photocatalytic reaction process causing low reactivity and also Bi2WO6 nanoparticles exhibit the lowest photocatalytic activity, which may result from its large band gap. According to the band gap calculation above, the band gap of Bi2WO6 is 3.32 eV, indicating poor absorbance of light in the visible region. The degradation rate of MB was analyzed by the pseudo-first-order kinetic process over CdS, Bi2WO6, Bi2WO6–CdS and PVA assisted Bi2WO6–CdS nanocomposite film and the results shown in Fig. 11b. Photoreaction kinetics rate of the CdS, Bi2WO6, Bi2WO6–CdS and PVA assisted Bi2WO6–CdS nanocomposite film were 0.012, 0.015, 0.022 and 0.024, respectively. The PVA assisted Bi2WO6–CdS nanocomposite film were 1.0909 times higher than Bi2WO6–CdS heterostructures and 1.6, 2 times higher than bare Bi2WO6, CdS.

Fig. 10
figure 10

Photocatalytic degradation of MB. a Bi2WO6, b CdS, c Bi2WO6–CdS, d PVA assisted Bi2WO6–CdS nanocomposite film

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Fig. 11
figure 11

Photocatalytic degradation of MB. a Efficiency of all the prepared photocatalyst. b First-order pseudo kinetic study

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Proposed possible mechanism

Based on the above discussion, the possible photocatalytic mechanism for PVA assisted Bi2WO6–CdS nanocomposite film was proposed and shown in Fig. 12. Under the visible light, irradiation of CdS was excited and produced the photogenerated charge carriers. When the excited electron in the CB of CdS where transferred into the CB of Bi2WO6. While excited holes in the VB of Bi2WO6 shifted to the CB of CdS. There was a suppression of photogenerated charge carriers and free transfer of separated electron and holes towards the surface for the availability of carriers for oxidation and reduction. The conduction band (CB) and valence band (VB) edges were determined using the following equations:

$$ E_{\text{CB}} = \chi - E_{\text{e}} - 0.5E_{\text{g}} , $$
(11)
$$ E_{\text{CB}} = E_{\text{VB}} - E_{\text{g}} , $$
(12)

χ is the absolute electronegativity of semiconductor which is geometric mean of constitute of atoms Ee is the energy free electrons on the hydrogen scale (~ 4.5 eV), the absolute electronegativity was 5.19 for CdS (Zou et al. 2016) and 6.36 for Bi2WO6 (Wang et al. 2017) and the calculated band edge positions are tabulated in Table 3. The degradation of model pollutants such as metronidazole and MB described in the following reaction

Fig. 12
figure 12

Schematic illustration of band edge position and interfacial charge transfer of the PVA assisted Bi2WO6–CdS nanocomposite film

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Table 3 Calculation of band edge position
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$$ {\text{CdS}} + h\vartheta \to {\text{CdS}}\left[ {{\text{e}}_{\text{CB}}^{ - } + {\text{h}}_{\text{VB}}^{ + } } \right], $$
(13)
$$ {\text{CdS}}[{\text{e}}_{\text{CB}}^{ - } ] + {\text{Bi}}_{2} {\text{WO}}_{6} \to {\text{CdS}} + {\text{Bi}}_{2} {\text{WO}}_{6} \left[ {{\text{e}}_{\text{CB}}^{ - } } \right], $$
(14)
$$ {\text{Bi}}_{2} {\text{WO}}_{6} [{\text{e}}_{\text{CB}}^{ - } ] + {\text{O}}_{2} \to {\text{Bi}}_{2} {\text{WO}}_{6} + O_{2}^{ \cdot - } , $$
(15)
$$ {\text{Bi}}_{2} {\text{WO}}_{6} \left[ {{\text{h}}_{\text{VB}}^{ + } } \right] + {\text{CdS}} \to {\text{Bi}}_{2} {\text{WO}}_{6} + {\text{CdS}}\left[ {{\text{h}}_{\text{VB}}^{ + } } \right], $$
(16)
$$ {\text{CdS}}\left[ {{\text{h}}_{\text{VB}}^{ + } } \right] + {\text{H}}_{2} {\text{O}} \to {\text{OH}}^{ \cdot } , $$
(17)
$$ {\text{O}}_{2}^{ - } + {\text{Bi}}_{2} {\text{WO}}_{6} \left[ {{\text{e}}_{\text{CB}}^{ - } } \right] + 2{\text{H}}^{ + } \to {\text{H}}_{2} {\text{O}}_{2 } , $$
(18)
$$ {\text{H}}_{2} {\text{O}}_{2} + {\text{e}}^{ - } \to {\text{OH}}^{ \cdot } + {\text{OH}}^{ - } , $$
(19)
$$ {\text{h}}^{ + } + {\text{Bi}}_{2} {\text{WO}}_{6} \to {\text{OH}}^{ \cdot } , $$
(20)
$$ {\text{OH}}^{.} + {\text{Dye}} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}}, $$
(21)
$$ {\text{O}}_{2}^{ - } + {\text{Dye}} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} . $$
(22)

From the mechanism pathway, PVA assisted Bi2WO6–CdS nanocomposite film produced a large amount of highly reactive species like OH˙ than that of less reactive species such as ·O2− radical. So that photocatalytic activity drastically increases PVA assisted Bi2WO6–CdS nanocomposite film, because of the large amount and highly reactive OH· radical production. Meanwhile, the contribution of PVA on Bi2WO6–CdS composite immense fashion, due to the special character of minimizing photocorrosion and photostability.

Trapping of active species

The active species trapping measurements is significant to examine the active species in the photocatalytic process for a better understanding of photocatalytic mechanism. In degradation of dyes, several active radical species such as hole (h+), hydroxyl radicals (OH·), superoxide anion radical (·O2−) and electron (e−) plays major contribution. The active species trapping experiments toward PVA assisted Bi2WO6–CdS nanocomposite film are carried out, and the results are shown in Fig. 13. Primarily, all the four established species with a photogenerated hole (h+), hydroxyl (OH·) radical superoxide anion radical (·O2−) and electron (e−) are active during the photocatalytic process. Almost adverse change exist in the presence of IPA, implying that OH· radicals had a large effect on the degradation process under visible light illumination. In contrast, an apparent reduction was observed upon the addition of BQ compared to the case IPA, and a radical inhibition was superficial after the addition of EDTA, suggesting that ·O2− radicals and/or h+ were the main reactive species of the PVA assisted Bi2WO6–CdS nanocomposite film and played significant roles in the photocatalytic process under visible light illumination. Moreover, the photocatalytic degradation rate of model pollutants solution using EDTA and AgNO3 was lower than that using IPA, revealing that OH· played the most significant role in the photocatalytic process under visible light illumination.

Fig. 13
figure 13

Traping of active species

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Detection experiment for hydroxyl radicals

In general, the hydroxyl radicals (OH·), a highly reactive species with strong oxidation ability, were considered mainly responsible for the degeneration of pollutant molecules in the photocatalytic reaction. The concentration of hydroxyl radicals was detected during the reaction with the help of terephthalic acid by photoluminescence spectra. The taken terephthalic acid is reacting with OH· radicals when produced at the reaction and convert into a highly luminescent 2-hydroxy terephthalic acid. The intensity of PL spectra is directly proportional to the formation of 2-hydroxy terephthalic acid in the reaction of terephthalic acid with OH· in photocatalysis system (Sarkar et al. 2016; Zeng et al. 2017). The UV–Vis absorbance and detection of OH· radicals for PVA assisted Bi2WO6–CdS nanocomposite film using PL spectra is shown in Fig. 14. The UV–Vis absorbance gives a strong absorbance at 233 nm, then PL spectra of the produced 2-hydroxy terephthalic acid were monitored two peaks at approximately 345, 420 nm, respectively. PVA Bi2WO6–CdS show more PL intensity, due to producing a significant amount of OH· radicals. The same experiment was carried out with the bare photocatalyst like CdS and Bi2WO6. From the OH· radical detection processes, the PL intensity of Bi2WO6 and CdS has low when compared with PVA assisted Bi2WO6–CdS nanocomposite film. Figure 14b–d shows the PL intensity of formation of 2-hydroxy terephthalic acid in presence of Bi2WO6 and CdS.

Fig. 14
figure 14

Detection of hydroxyl radical using terephthalic acid as probe molecule

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Recycling exploration

In the practical applications of photocatalysis, the reusability, as well as stability, had a more important which when act as a catalyst. The recycling experiment was carried out for the prepared PVA assisted Bi2WO6–CdS nanocomposite film under an aforementioned same reaction condition; here the nanocomposite film was separated after the degradation of organic pollutant. Then it was dipped into another 100 ml of MB solution. The efficiency of Photodegradation of MB for every cycle is shown in Fig. 15. There was no obvious loss until seven cycles after that obviously minimum decline in photocatalytic activity.

Fig. 15
figure 15

Recycling exploration of PVA assisted Bi2WO6–CdS nanocomposite film

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Conclusion

In summary, PVA assisted Bi2WO6–CdS nanocomposite film photocatalyst was successfully synthesized through hydrothermal method. The PVA assisted Bi2WO6–CdS nanocomposite film showed better photocatalytic activity and photostability than that of pure CdS and pure Bi2WO6 toward the degradation of both MB and metronidazole under visible-light irradiation. The attractive nature of PVA assisted Bi2WO6–CdS nanocomposite film was easy to recover from the degradation system after the photocatalytic degradation performance. The PVA play a vital role in the retired recombination rate of photon-generated carriers, accordingly promote the activity and stability of the catalyst. This work will deliver a new way for efficient photocatalyst and reduced photocorrosion.