CdS nanocomposites for the degradation of organic pollutants

Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112129

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Exploring the visible light driven photocatalysis by reduced graphene oxide supported Ppy/CdS nanocomposites for the degradation of organic pollutants Nafees Ahmad, Saima Sultana, Suhail Sabir, Mohammad Zain Khan

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Environmental Research Laboratory, Department of Chemistry, Faculty of Sciences, Aligarh Muslim University, Aligarh, 202 002, Uttar Pradesh, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanocomposite Polymerization Photodegradation Dye

Reduced Graphene oxide (rGO) supported polypyrrole/CdS nanocomposite (Ppy/CdS-NF) was prepared using facile chemical route. The synthesis of material was confirmed by characterization using advanced instrumentation techniques like XRD, FTIR, SEM-EDX and TEM. Thermal characteristic of nanocomposite was studied by TGA and DTA. Electrochemical Surface Area (ECSA) was calculated by Cyclic Voltammetry. The photocatalytic activities of the materials were demonstrated by degradation of Rhodamin B (Rh B), Reactive Blue-171(RB-171) dye and toluene under the influence of visible light irradiation. The photodegradation is attributed to efficient charge transfer between the composites thereby generating reactive oxygen species (ROS) which causes efficient photodegradation of the dye. Photoluminescence study confirmed the enhancement in photogenerated e− and h+ pair separation and reduction in recombination rate. Quenching experiments was performed to determine reactive groups in the photodegradation and stability of nanocomposite was confirmed through recycling experiments. The composite of Ppy with CdS/rGO exhibited higher photodegradation efficiency over pure CdS-NF and Ppy which is attributed to the formation of more interfacial reaction sites between Ppy and CdS-NF. Further the mineralization of the degraded sample was also confirmed by monitoring the reduction of chemical oxygen demand as a function of time. Atomic Absorption Spectroscopy was used to detect the amount of Cd2+ ion and no significant amount of Cd2+was detected in the degraded sample of dye.

1. Introduction Environmental problems related with toxic pollutants due to the rapid advancement and industrialization has become enormous and there is an immediate need to control water pollution by reducing the unwanted materials [1–3]. The major source of the pollutants includes textiles dyes, drugs, fertilizers etc which requires pretreatment before being disposed off. Among these toxic pollutants, dyes such as direct yellow 7, acid blue 92, rhodamine B, reactive blue 171 etc. are the major sources of water pollutants. Rhodamine B (Rh B) a cationic and reactive blue 171 (RB-171) an anionic dyes are quite toxic in nature and harmful for human being and aquatic species as well. It causes respiratory problems by inhalation and irritation to the skin and eyes and also disturbs the biological metabolic process [4]. Apart from dyes, many colorless pollutants such as toluene, nitrophenol, xylene etc also have harmful effect to the human beings and to the environment as well. Due to the stability of these pollutants against chemical and biological remediation it has now become a tough task to remove these

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pollutants from aqueous medium [5–8]. Conventional methods are associated with certain drawbacks such as high cost, slow degradation and possibility of secondary pollution etc. Nanomaterials as nanocatalyst is an advanced class of material finds application in photodegradation are considered to be most efficient technique to degrade industrial dyes. For the past decades, polymer based nanocomposites has emerged as effective photocatalyst for the environmental remediation [9,10]. The intensive exploration of the photocatalyst has fulfilled the demands for the environmental purification and energy conversion [11]. Polymers nanocomposites based on conducting polymers polyaniline (PANI), polypyrrole (Ppy), polythiophene (PTh) and polyfuran are associated with their characteristic and functional properties (such as electrical conductance, thermal stability and facile synthesis) are the origin of tremendous commercial applications [12,13]. Polypyrrole (Ppy) is a visible-light active conducting polymer catalyst due to electrochemical reversibility and high polarizibility [14,15]. In addition, polypyrrole with its properties can be applied for the photodegradation

Corresponding author. E-mail address: [email protected] (M.Z. Khan).

https://doi.org/10.1016/j.jphotochem.2019.112129 Received 24 May 2019; Received in revised form 19 September 2019; Accepted 30 September 2019 Available online 01 October 2019 1010-6030/ © 2019 Published by Elsevier B.V.

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2.2.2. Synthesis of polypyrrole Polymerisation of pyrrole was carried out using anhydrous ferric chloride (FeCl3) as anoxidant, sodium lauryl sulphate (NaC12H25SO4) as stabilizer and methanol used as a solvent. Anhydrous ferric chloride 1gand sodium lauryl sulphate 5.45 g was added in 80 mL of DW and kept stirring for 2 h. The reaction mixture became yellow color by the formation of Fe(LS)3Cl3.Thereafter1 mL pyrrole and 100 mL of methanol was added to obtained yellow mixture and again the whole mixture was kept for stirring for 12 h. After that, the precipitate was filtered and kept at 60 °C for 2 h to get the desired product [39].

of organic pollutants. Synergistic effect of conducting polymer with semiconductor nanoparticle like ZnO, Fe2O3, ZnS, CdS, WO3, Ni has been the most promising host for the photodegradation of organic pollutants [16–18]. CdS is a IIeVI semiconductor material owing to its direct bandgap (Eg = 2.4 eV) and high absorptivity has been used as a visible light photocatalyst [19,20]. Apart from photodegradation, CdS is a suitable semiconductor material for the CO2 and water reduction [21] and has been used in optical and gas sensors, solar cells and display devices etc [22,23]. Incorporation of these semiconductor nanomaterials using graphene oxide (GO), carbon nanotubes (CNT) became an effective strategy to enhance the photocatalytic activity due to high electron mobility, effective adsorption properties and greater surface area [24]. Reduced GO due to its appreciable chemical, thermal, optical and mechanical properties finds applications in various fields [25]. It is considered as an efficient photocatalyst due to the existence of number of active adsorption sites, reaction centre, electron reservoir and pathway of the electron transfer which minimizes the recombination rate of the charge carriers [26–28]. The higher surface area of the rGO will help the dye molecule to bind through π-π stacking on the surface [29]. The properties of rGO can be tailored by incorporating conducting polymer chain to produce a synergistic effect. Reduced GO and graphene based nanocomposites, with excellent electro conductivity and efficient charge separation are rational in terms of photocatalysis. Other form of carbon constituents such as Carbon nanotubes, fullerenes, and 3D graphene aerogels can also be used for the photocatalysis with highly efficient activity due to multidimensional structure and porous structure assisting in the transportation of electron [30–32]. For the last few years, number of research has been carried out for the photodegradation of organic pollutants by CdS doped with metal oxide nanoparticles in form of binary and ternary nanocomposites [33–37]. However rGO doped CdS nanoparticle with conducting polymer are still very limited for the photocatalytic applications. The present study aims to prepare ternary nanocomposites of Ppy, CdS-nanoflower, and rGO in different proportion to understand the complete mechanism of photocatalytic activity against Rhodamin B (Rh B), Reactive Blue-171(RB-171) dye and toluene. Special focus has been given to Rh B as it is more toxic and widely distributed in aqueous streams. Efforts have been made to correlate the electrochemical surface area with photocatalytic activity. Various scavengers have been used to identify the primary reactive species during photodegradation of Rh B.

2.2.3. Synthesis of Ppy/CdS and Ppy/CdS/rGO nanocomposites Ppy/CdS (PC) was synthesized through in situ polymerization of pyrrole in the presence of CdS nanoparticles. Briefly, the above synthesized CdS nanoparticle was added along with anhydrous ferric chloride and sodium lauryl sulphate in distilled water under the continuous stirring. Thereafter pyrrole and methanol was introduced to the above mixture and whole mixture was again kept for stirring for 12 h and the final product was obtained. The same method was followed for the preparation of polymer nanocomposites (Ppy/CdS/rGO- PCG) in which graphene oxide (0.1 g) prepared by hummer’s method [39] was dissolved in 50 mL of DW followed by ultrasonication for 1 h and different weight percent of CdS named as (PCG-1 and PCG-2 (10weight %CdS/Ppy/rGO and 20 wt %CdS/Ppy/rGO) was added during the polymerization of pyrrole where GO was converted to the rGO by the thiourea reductant [40]. The preparation pathway of nanocomposites is presented in Scheme 1. 2.3. Materials characterization The synthesized materials were characterized by X-ray diffraction technique (XRD- BRUKER D8 ADVANCE 30 kV and 15 mA) to study the crystal size and geometry and the analysis of functional groups was performed by FTIR (fourier transform infrared spectroscopy- Perkin Elmer spectrum-2, USA) in range of 400–4000 cm−1. Surface morphology and elemental composition was studied by SEM-EDX (JSM 6510 L V JEOL Japan) and TEM (JEM 2100 JEOL Japan) was operated to study the structure and size of the prepared materials. Bandgap of the materials was calculated by UV–diffuse reflectance spectroscopy (Perkin Elmer Lambda 35 USA) and electrochemical surface areas of the synthesized materials were calculated cyclic voltammetry (Auto lab 204 Netherland).UV Visible spectroscopy (Thermo Scientific Evolution 201USA) was used to check the absorbance of the degraded aliquots from the reactor and the recombination behavior of photo induced charge carriers was examined by photoluminescence intensity through fluorescence spectroscopy (Hitachi- F-2500- Japan) at an excitation wavelength of 590 nm. Traces of Cd2+ ion in the degraded sample of dye was detected using atomic absorption spectrometer (Model No. GBC 932 Plus, GBC Scientific, Australia).

2. Materials and methods 2.1. Reagents and chemicals Thiourea (CH4N2S) and cadmium nitrate Cd(NO3)2.4H2O was purchased from Thermo Fisher Scientific Pvt. Ltd (USA). Rhodamine B was purchased from CDH Chemical Ltd (India). Pyrrole was obtained from Sigma Aldrich (USA) while other chemicals like disodium ethylenediamminetetraacetic acid, isopropyl alcohol, p-benzoquinone etc were procured from Fisher Scientific (USA).

2.4. Sample preparation for cyclic voltammetry Electrochemical nature of the synthesized nanocomposites was evaluated through CV. From cyclic voltammograms data, ECSA was calculated using electrical double layer capacitance (EDLC). Cyclic Voltammetry was performed using three electrode systems in the applied potential range of −1.5− +1.5 V. Platinum (Pt) wire was taken as counter electrode, Ag/AgCl was used as the reference electrode and glassy carbon electrode was used as working electrode. A solution of KOH (0.1 M) was used as an electrolyte in the experiment. Prior to analysis, the sample was purged with nitrogen in order to maintain an inert atmosphere. The sample fabrication on the GCE surface was done in accordance to Saquib et al [41]. Typically, 0.2 mg of each photocatalyst Ppy, Ppy/CdS and Ppy/CdS/rGO was taken along with 5 μl isopropyl alcohol and water and a solution of chitosan in glacial acetic acid was taken as binder. The samples were then fabricated on the GCE

2.2. Synthesis of CdS nanoparticle,Ppy and Ppy/CdS/rGO nanocomposites 2.2.1. Synthesis of cadmium sulphide nanoflower Cadmium sulphide was synthesized via hydrothermal method [38]. Briefly, a fixed concentration of thiourea (0.2 M) was dissolved to 50 mL of DW and kept for stirring for 10 min. Further 0.2 M cadmium nitrate Cd(NO3)2.4H2O was added to this mixture followed by continuous stirring for 30 min. Further this solution was shifted to a teflon lined stainless steel autoclave and kept at 180°C for 12 h. After 12 h, the precipitate was washed and dried at 70 °C for 12 h and the final product was obtained. 2

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Scheme 1. Reaction scheme of the synthesis of nanocomposites.

sample was taken and diluted for 5 times with water to make the total volume upto 2 mL and then 2.8 mL of H2SO4 and 1.2 mL of 0.25 N K2Cr2O7 was added to this solution. The colour of the mixture turns yellow. This mixture was then digested for 2 h in a COD digester. After 2 h, the sample is allowed to cool and the absorbance of the sample is recorded. The concentration of the sample from the observed absorbance at wavelength of 600 nm can be calculated by plotting a calibration curve.

surface by the dip casting method and kept for drying in open air for 30 min. 2.5. Assessment of the photocatalytic activity and quenching experiments Photocatalytic evaluation of the nanocomposites was carried out by monitoring the degradation of Rh B, RB-171dye and toluene in a photocatalytic reactor made of glass consisting of triple jacket, visible lamp (power 500 W) and an oxygen pump for the supply of oxygen to the sample. A fixed amount of 300 ml of the dyes and toluene solution with 0.3 g of photocatalyst were taken into the reactor. The whole mixture was stirred for 20 min in absence of visible irradiation to approach the adsorption-desorption equilibrium. Thereafter the aliquots were taken out of the reactor at regular interval of time under irradiation. The concentration of degraded aliquots was calculated by measuring the absorbance using UV–vis spectrophotometer. The following formula was used to calculate the degradation efficiency of the photocatalyst,

Degradation efficiency =

C0 − Ct x 100 (%) C0

3. Results and discussion 3.1. X-ray diffraction analysis The Powder XRD was used for the determination of phase and size. Fig. 1. Shows the XRD spectra of synthesized materials. CdS nanoparticle with hexagonal phase wurtzite-type structure is obtained with planes; (100), (002), (101), (102), (110), (103), (112),(203),(211) and (105) with corresponding 2θ angles of 24.80°, 26,30°, 28.12°,36.30°, 43.7°, 47.82°, 51.82°, 65.6°, 71.1° and 75.8°, respectively [44]. The spectra of the polypyrrole with a broad hump shows its amorphous nature at 2θ value of 20.7° with (104) plane [45,46]. Graphene oxide with plane (101), (002),(102),(201),(210) and (220) covered corresponding value of 26.2°, 31.8°, 36.2°,45.6°, 56.6° and 75.3° [29]. The Ppy/CdS spectra show three characteristic peaks with plane (002), (110) and (112) at corresponding 2θ angle of 26.3°, 43.6° and 51.9°. The disappearances of some CdS peaks in nanocomposites are might be due to lower amount of the CdS and change in angle strain. As the doping amount of CdS increased, characteristics peaks appeared in PCG-1 and PCG-2. The planes appeared are (100), (002), (101), (102), (110), (103), (112),(203),(211) and (105) with corresponding 2θ angle of 24.80°, 26.3°, 28.12°, 36.3°, 43.°, 47,82°, 51.82°, 65.6°, 71.1° and 75.8° in both the PCG-1 and PCG-2 respectively [47]. The high intensity in PCG-2 is due to higher amount of the CdS nanoparticle. The difference in XRD pattern of PC, PCG-1 and CdS ensures the substantiation of the dopant. The average crystal size of the nanocomposites was calculated by Scherrer’s formula [48,49] as given below:

(1)

Where C0 and Ct is the initial and the concentration at time‘t’ respectively. Moreover, quenching experiment was conceded to assure the determination of major reactive species during photodegradation. Various scavengers like p-benzoquinone, disodium ethylenediamminetetraacetic acid (EDTA) and isopropyl alcohol (IPA) were taken as trapping agents to quench %O2−, h+ and %OH respectively. 2 mM of each scavenger were added to the solution of the Rh B dye before the addition of photocatalyst so as to identify reactive species. In order to verify the stability of the nanocomposites, the photodegradation experiments of Rh B were repeated consecutively without replacing the photocatalyst. 2.6. Chemical oxygen demand measurements Chemical oxygen demand (COD) was calculated with the help of the standard APHA method [42,43]. Briefly, a fixed volume (0.4 mL) of the 3

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Fig. 1. XRD spectra of the prepared nanoparticle and nanocomposites.

d = 0.9λ/βCosθ

oxidation of graphite into GO [29]. The incorporation of GO into PCG-1 and PCG-2samples results into the shifting of absorption bands of these functional group.

(2)

Where‘d’ is the size of the crystallite, λ is the x-ray wavelength, β is the full width at half maxima and θ is the diffraction angle between 10° to 80° degree. The calculated size of the CdS nanoparticle, Ppy/CdS, PCG1 and PCG-2 nanocomposites were found to be around 54, 84, 123 and 143 nm respectively.

3.3. Microscopic studies by SEM, TEM and EDX Surface morphologies, elemental composition and shapes of the nanomaterials have been studied by SEM, TEM and EDX. SEM images in Fig. 3(a and b) reveals nanoflower structure of the as synthesized CdS which resembles tiny petal of the flowers [38]. Further CdS nanoflowers are uniformly distributed and covered on the Ppy sheet which confirms the existence of CdS on the Ppy sheet. The plane and smooth surface of polypyrrole sheets are clearly shown in Fig. 3(c and d) that closely resembles the nano sheets synthesized by different research groups earlier [55]. SEM images clearly show the grain like CdS are well resolved and scattered over the Ppy sheets.SEM images in Fig. 3(e and f) shows the surface morphology of Ppy/CdS/rGO which shows the scattered CdS nanoparticle and Ppy matrix on the entire sheet of rGO. Incorporation of rGO to the CdS/Ppy matrix provide increased surface area and more reactive sites. It is clear from the Fig. 3(e and f) that scattered particle on the surface of rGO provide enough surface area for the transfer of electron and more reaction site which subsequently enhances the dye adsorption on the catalyst surface [56].Moreover the elemental compositions and purity of the sample nanocomposite were analyzed by EDX spectroscopy as shown in Fig. 4.The spectra obtained confirm the existence of cadmium nanoparticle in the nanocomposites.

3.2. Functional group analysis The specific functional groups of synthesized materials PPy, CdS, GO, PC, PCG-1, PCG-2 was confirmed by the FTIR spectra (Fig. 2). The presence of strong absorption band at 618 cm−1 is due to Cd-S stretching vibrations [50] and the vibration of C]C and CeC appeared at 1551 cm−1 and 1465 cm−1 of the pyrrole ring [51]. The stretching vibration of CeN and CeH of pyrrole are appeared at 1293 cm−1and 968 cm−1respectively [52] and the peak appeared at 3401 cm−1 is attributed to NeH bond of pyrrole [53]. Characteristic peaks of CeH inplane deformation appeared at 1172 cm−1 and the stretching vibration of CeH was appeared at 2852 cm−1 [47]. The stretching vibration at 3638 cm−1 is due to OeH bond in the water molecule [54]. Shifting of the characteristic bands position shows the chemical interaction of the composites samples - PC and PCG-1 and PCG-2. The characteristics peaks of the GO shows the C]O, alkoxy and epoxy groups at 1730 cm−1,1035 cm−1 and 1285 cm−1which confirms the complete

Fig. 2. FT-IR spectra of the prepared nanoparticle and nanocomposites. 4

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Fig. 3. SEM images of the CdS Nanoflower (a and b), Ppy/CdS (c and d) and Ppy/CdS/rGO (e and f).

Fig. 4. EDX images of the PCG nanocomposites. 5

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Fig. 5. TEM images (a–e) of the CdS nanoparticles, PC and PCG-2 nanocomposites and (f) SAED pattern of the PCG-2 nanocomposites.

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Fig. 6. Tauc plot for the bandgap calculation of the (a) Ppy and (b) CdS nanoparticle.

3.5. Thermal analysis

The TEM images of the materials are shown in Fig. 5. The CdS nanoparticles are embedded on the rGO sheet are shown in Fig. 5(a and b) and the scattered CdS nanoparticle on the rGO sheets with different size can easily seen in Fig. 5(c and d). The wurtazite hexagonal structures of CdS nanoparticle are shown in Fig. 5(d and e). Meanwhile the selected area electron diffraction pattern as shown in Fig. 5f reveals the semicrystalline nature of the nanocomposites.

Thermal analysis of CdS, Ppy, PC, PCG-1, PCG-2 nanocomposites have been carried out in the presence of nitrogen atmosphere in the temperature range 25–800 °C as shown in Fig. 7(a and b). Two exothermic and one endothermic peak appeared in the Ppy which shows corresponding weight loss in two regions as shown in TGA curve and endothermic peak is due to thermal decomposition of the nanocomposites (Fig. 7a). The first region of degradation is in the range of 210 °C–300 °C is due to the vaporization of adsorbed water molecules while the second region 300 °C -500 °Cis due to the thermal decomposition of polymeric matrix [47]. The weight loss between 500 °C–800 °C is due to pyrolyzation of carbon skeleton of polypyrrole [58]. In CdS nanoparticle one exothermic peak appeared shows the corresponding weight loss at around 750 °C while in the PC nanocomposites, two exothermic peaks appeared which is due to corresponding weight loss in two regions, first region is around to 200 °C–350 °C and second 550 °C–650 °C respectively. However in case of PCG-1 two exothermic peaks results in to the weight loss of two regions. The thermal stability of the polymer nanocomposites (PCG-1 and PCG-2) has been improved in the temperature range 25–300 °C as compared to bare Ppy which shows one exothermic peak appeared and very little weight loss [59]. Thus the thermal stability of the polymer nanocomposites samples have been substantially improved by incorporation of rGO and CdS nanoparticle (for both 10 and 20 wt %).

3.4. Bandgap analysis by UV-diffuse reflectance spectroscopy The bandgap values of the synthesized materials were calculated by generating UV-diffuse reflectance spectra. The tauc plot for the band gap of CdS nanoparticle and Ppy as shown in Fig. 6 was calculated by Kubelka Munk Function using the following formula: (hν.α) = (Ahν-Eg)n/2

(3)

Where α is proportional to F(R), which is kubelka Munk function, Now the equation becomes, {(hν.F(R)} = (Ahν-Eg)n/2

(4)

Where ν is the frequency, A is the proportionality constant and Eg is the bandgap energy. The value of ‘n’ is determined by n = 1 or 4 for direct and indirect allowed transition. The indirect transition (n = 4) have been taken into account to calculate the bandgap of the Ppy and CdS from best fitting of tangent to the Eg axis.The calculated bandgap of the Ppy and CdS was found to 2.1 eV and 2.3 eV and found in accordance to those reported earlier [57,51] and the bandgap of rGO was calculated in our previous paper which was found to 0.9 eV [39].

3.6. Determination of electrochemical surface area of nanocomposites The ECSA of the materials was calculated by EDLC using the cyclic voltammetry technique [60,61]. The cyclic voltammograms were

Fig. 7. (a) TGA and (b) DTA curve of the as prepared materials. 7

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Fig. 8. (a) Electrochemical double layer capacitance of the PCG-2 nanocomposites at 10–80 mV s−1 and (b) calculated surface area of the as prepared materials.

Reduced GO sheets increase the surface area of the photocatalyst by providing less hindered path which improves the adsorption of the dye through the π-π stacking between dye and rGO (as confirmed from Fig. 8b). The shifting of the maximum absorbance wavelength toward shorter wavelength (blue region or hypsochromic shift) clearly shows that N-de-ethylation of dye and the destruction of conjugated structure which forms intermediate products by decreasing the photooxidation ability of the system [62–64]. However to check our hypotheses whether the nanocomposites used as photocatalyst are applicable to another dye, one more dye (Reactive Blue-171) was used for the photodegradation over PCG-2 and the appreciable degradation was observed. The UV visible spectra photo degraded sample of RB-171 are presented in Fig. 2S (supplementary Information). Moreover a transparent substrate, toluene was also used for the photodegradation over PCG-2 catalyst. The UV–vis spectra of the photo degraded samples of the toluene are presented in Fig. 10a and the kinetics of photodegradation of toluene is presented in Fig. 10b. COD removal percentage of the degraded aliquots is shown in Fig. 10c which confirms the photodegradation of the toluene. A considerable reduction in the COD value confirms the degradation of the toluene and around 57.24% of toluene was degraded in 120 min of irradiation time. To study the reaction kinetics of photodegradation, LangmuirHinshelwood method pseudo first order kinetics was used;

plotted in the applied potential of -1.5 to 1.5 V with different scan rates of 10mVs−1, 20mVs−1 and 30mVs−1 respectively and the cyclic voltammograms at scan rates 30mVs−1 are presented in Fig. 1S (supplementary Information). EDLC was investigated by again drawing the CV curves in the scan rates of 10-80mVs−1 (Fig. 8a) and from the EDLC data the obtained value of slope is the calculated ECSA which is found to be 0.00046477 mFcm−1, 0.000722208 mFcm−1, 0.000762845 mFcm−1 for Ppy, PC and PCG respectively (Fig. 8b). The calculated surface area controls the activity of the photocaytalyst. Higher surface area caused more adsorption of dyes on the surface of photocatalyst due to existence of more reaction sites. So, it can be conclude that higher electrochemical surface areas of the nanocomposites will results into the higher activity of photodegradation.

3.7. Photocatalytic evaluation and kinetics of photodegradation The photodegradation activities of CdS, Ppy and nanocomposites (PC, PCG-1, PCG-2) were determined by monitoring the degradation of solution of RhB dye (λmax554 nm) in visible light irradiation and rGO loaded Ppy/CdS (PCG-2 and PCG-1 (blank test)) were also carried out. The photodegradation processes were checked by measuring the absorbance of the degraded aliquots using UV–vis spectroscopy as shown in Fig. 9a.The continuous decline in the absorbance of the aliquots indicates the degradation of the Rh B dye molecules. The kinetics of the photodegradation (Ct/C0) is presented in Fig. 9b.When the sample was irradiated without catalyst, negligible degradation was observed confirming the stability of RhB dye against visible light. However upon addition of photocatalyst Ppy and CdS to the aqueous solution of the dye, an effective degradation was observed with irradiation time and the degradation efficiency was found to be increasing in presence of PC nanocomposites. The maximum degradation was found to be 99.1% in 180 min of irradiation when PC nanocomposites decorated rGO was used as photocatalyst. Reduced GO sheets provide the pathway for the transfer of electron which was helpful in the photodegradation.

ln (C0/Ct)= Kapp t

(5)

Where‘k’ is the first order rate constant, C0 is the initial concentration and Ct is the concentration of the RhB dye at time‘t’. The Kapp values of the photodegradation were calculated by the first order kinetics and result shows the highest rate of degradation in case of PCG-2.The rate constant of the photocatalyst was found to be 0.0055 min−1, 0.0072 min−1, 0.026 min−1, 0.02961 min−1, 0.03616 min−1 for Ppy, CdS, PC, PCG-1 and PCG-2 respectively while the degradation rate was 0.65 × 10−3 min−1in absence of photocatalyst. The rate constants of rGO loaded Ppy/CdS (blank test) was found to be 0.0016 min−1 and

Fig. 9. (a) UV–vis spectra of the degraded sample and (b) kinetics of photodegradation of the various photocatalyst. 8

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Fig. 10. (a) UV–vis spectra of the degraded sample of the toluene (b) kinetics of the photodegradation of toluene and (c) COD removal percentage of the degraded aliquots.

Table 1 Comparative study of the % degradation of RhB dye by various composites of CdS, Ppy and rGO reported previously. S.No

Catalyst

%Degradation

Intensity of light

Irradiation Time

Reference

1 2 3 4 5 6 7 8 9 10

CdS/CQDs/BiOCl CdS QDs/npg-C3N4 WO3/RGO CdS-Graphene Fe2O3-TiO2-Graphene Aerogel CdS-Reduced Graphene Oxide CdS/TiO2 7% Ppy-BiOI 1:100 PPy/TiO2 Ppy/CdS/rGO

99.5% 88.2% 94.1% 91% 97.7% 95.2% 96.8% 80% 97% 99.1%

500 W Xe lamp and 500 W Hg Lamp 500 W Xe lamp 350 W Xe lamp 400 W lamp 500 W Xe lamp 500 W Xe lamp 300 W and 500 W Xe lamp λ > 400 nm 500 W tungsten-halogen lamp 500 W tungsten lamp

105 min 90 min 150 min 300 min 60 min 50 min 150 min 300 min 480 min 180 min

[65] [66] [67] [68] [69] [70] [71] [51] [72] Present Study

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Fig. 11. Proposed mechanism of the photodegradation of the Rhodamine B dye in presence of PCG-2.

0.00105 min−1 for PCG-2 and PCG-1 respectively. The results suggested that the photodegradation of Rh B was highest in the presence of polymer nanocomposite containing 20 wt% of CdS (i.e. PCG-2). Table 1 shows the percentage degradation of the Rh B dye in presence of the different photocatalyst as per the reports available in literature.

synergistic effect generates more reactive species (OH-, h+, e−) resulting into higher degradation of the Rh B. To overcome the problem of the recombination behavior of the photogenerated e− and h+ PCG-2 photocatalyst was used. Upon the irradiation of the visible light on the mixture of photocatalyst as well as Rh B dye, there is photosensitization in the Rh B molecules and the electrons ejected from the VB and drift to the CB but due to the higher CB potential (−1.42 eV) of the Rh B dye [73], the e- will move to the CB of the Ppy and subsequently move to the CB of CdS because of higher CB potential of Ppy than CdS. The CB potential of the Ppy and CdS are quite enough for the oxidation and the electron will react with the atmospheric oxygen and get converted into % O2-(superoxide radicals) and some of the electrons will drift over the rGO sheets. Reduced GO sheets have strong adsorption property and excellent capability of capturing and stores e− through the interfacial contact through CdS [74] which ultimately enhances the rate of transfer of e- by minimizing the recombination rate. Further H+ formed by the interaction of h+ to H2O molecules react with %O2− to convert into % OOH which again react with %O2− and H+ to form H2O2. These hydrogen peroxide molecules on reacting with %O2- will form %OH radicals. Thus the reactive oxygen species (ROS) formed in the whole process react to Rh B dye leads to de-ethylation followed to degradation into the simpler product [62,75]. The following proposed reaction occurs in the degradation of dye:

3.8. Proposed mechanism of photodegradation To study the mechanism of the photodegradation, valence and conduction edge band potential of the photocatalyst must be investigated. The valence band and conduction band edge potential of the CdS and Ppy was calculated by using following Eqs. (6) and (7): EVB= X-Ec+ 0.5 Eg

(6)

Where, X is the electronegativity of the constituent atoms, Eg is the band gap energy and Ec is the energy of free electrons i.e 4.5 eV. The ECB of the catalyst was estimated by using the following formula: ECB= EVB-Eg

(7)

The EVB and ECB of the CdS was found to be 1.89 and −0.41 eVand that of Ppy, EVB and ECB was calculated and found to be 1.15and −1.05 eV,respectively, and the values are found in accordance with the reported values in previous literature [51,64]. The possible mechanism of the photocatalytic degradation of the Rh B in presence of PCG-2 is shown in Fig. 11. Upon the irradiation of visible light to the CdS and Ppy separately there is fast recombination of the photogenerated e− in conduction band (CB) and h+ in the valence band (VB) which lowers the photocatalytic activity because of formation of less reactive species. Ppy show somewhat higher degradation than CdS due to characteristic properties of extended π-conjugated electron system which shows great promises to high mobility of the charge carrier and high adsorption coefficient. On enhancing the rate of transfer of e- by doping CdS nanoparticle to the Ppy, with their

Rh B + hν → Rh B*

(8)

Rh B* + 2-PCG → Ppy (e-) + %Rh B+ −

(9) −

PCG-2 + hν → Ppy/CdS (e /h+), rGO (e ) −

%

Ppy/CdS(e ) + O2 → O2 H %

+

+

%

O2−

OOH + H +

H2O2 + 10

O2−

%

O2−

(10) (11)

%

→ OOH

+

%

−

(12)

→ O2 + H2O2 ∙

→ O2 + OH- + OH

(13) (14)

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Fig. 14. Recycling experiments for the photodegradation of Rh B dye in presence of PCG-2. Fig. 12. Fluorescence spectra of the different photocatalyst.

%

O2−/H+/e−/%OH + Rh B → Degradation Product

confirms that %O2- are although accountable but not the prime reactive species in photodegradation mechanism. Further, the stability of synthesized photocatalyst is quite significant for the practical purposes. To confirm the stability and reusability of photocatalyst, the PCG-2 photocatalyst was recovered after the first experiment, centrifuged and the washed with DW and acetone and again used for the four consecutive cycles of photodegradation. Fig. 14 shows the photodegradation of Rh B up to 4 cycling runs over the PCG-2 photocatalyst. On comparing the activity of the 1stand 4th cycle, the degradation efficiency was only decreased by 12.8%. So it can be concluded that the photocatalyst was quite stable even after 4th cycle.

(15)

Moreover, to check the recombination behavior of the photogenerated e−-h+ pair, fluorescence spectroscopy was carried out to measure photoluminescence (PL) intensity of the photocatalyst. The rate of recombination of e- in CB and h+ in VB was found directly proportional to the PL intensity and higher recombination rate (high PL intensity) leads to lower photocatalytic activity. The PL spectra of the different photocatalyst Ppy, PC, PCG-1 and PCG-2 are presented in Fig. 12 at the excitation wavelength of 590 nm. Highest PL intensity in case of Ppy suggested higher recombination rate which in turn lowered the rate of photodegradation while the lower PL intensity in case of PCG-2 photocatalyst causes higher rate of photodegradation of Rh B.

3.10. Chemical oxygen demand (COD) test and detection of Cd2+ ion The COD test was performed to ensure the mineralization of the Rh B aliquots in the presence of the PCG-2 photocatalyst. It was observed that the COD of the aliquots calculated from photodegradation at 20 min interval showed decreasing trend during the course of reaction (Fig. 3S supplementary Information). The percentage removal efficiency of the chemical oxygen demand was found to be 71.7%. Further Cd2+ ion in the degraded solution was detected and it was found that only 0.3% Cd2+ ion to its initial concentration was left. A small amount of Cd2+ ion was associated with the degraded solution after taking part in the photocatalytic mechanism. So it can be concluded that using the photocatalyst based on the CdS nanocomposites, there is no adverse effect on the water quality due to Cd2+ ions toxicity.

3.9. Identification of primary reactive species and stability of the photocatalyst The primary reactive species were identified by adding various scavengers and simultaneously monitoring the rate of photodegradation. 2 mM of each EDTA (for trapping of h+), IPA (for trapping of %OH) and p-benzoquinone (for trapping of %O2−) were used [76,77]. As shown from the Fig. 13, rate constant is higher when no scavengers is introduced, but upon the incorporation of EDTA and IPA the rate constant for the photodegradation is decreased significantly which shows that generated h+ and %OH radicals are primary radicals causing significant effect in the photocatalytic activity. However in case of pbenzoquinone, there is a little effect on the rate constant which

4. Conclusion A series of rGO supported CdS/Ppy nanocomposite with different weight proportion of CdS have been effectively used for the photodegradation of RhB, RB-171 and toluene. The synergistic effect of CdS nanoparticle and rGO sheet in the PCG nanocomposites showed significant effect by enhancing the separation of photogenerated charge carriers with improved redox properties of the photocatalyst. It has been found that PCG-1 and PCG-2 showed higher percentage in photodegradation of pollutants which is due to higher charge separation and low recombination rate of the reactive species. Large surface area of the PCG-2 also favors the photodegradation of dye by providing more reactive sites. Holes (h+) and the hydroxyl radicals (%OH) were the main reactive species in the photodegradation of Rh B by the as prepared nano photocatalyst. Incorporation of CdS to rGO sheet retards the recombination behavior of the catalyst and thus improved the photocatalytic activity. The photocatalytic degradation was further confirmed by decreasing COD of the aliquots. The stability of the photocatalyst shows that the same catalyst can be used an effective material

Fig. 13. Effect of scavengers on the photodegradation of dye in presence of PCG-2. 11

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for the treatment of organic pollutants repeatedly.

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Declaration of Competing Interest On the behalf of all authors, corresponding author declares no conflicts of interest. Acknowledgments We are highly thankful to Chairperson, Department of Chemistry, Aligarh Muslim University, for providing instrumentation facilities. Authors are also thankful to University Sophisticated Instrumentation Facility (USIF) for providing instrumentation facilities of microscopic studies and Department of Mechanical Engineering, AMU, Aligarh for extending XRD facility. SS thanked the Uttar Pradesh Council of Science and Technology, for providing financial assistance. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2019. 112129. References [1] Y. Tang, P. Wee, Y. Lai, X. Wang, D. Gong, P.D. Kanhere, T. Lim, Z. Dong, Z. Chen, Hierarchical TiO2 nanoflakes and nanoparticles hybrid structure for improved photocatalytic activity, J. Phys. Chem. C 116 (4) (2012) 2772–2780. [2] F. Shen, W. Que, Y. He, Y. Yuan, X. Yin, G. Wang, Enhanced photocatalytic activity of ZnO microspheres via hybridization with CuInSe2 and CuInS2 nanocrystals, ACS Appl. Mater. Interfaces 4 (8) (2012) 4087–4092. [3] M. Kong, Y.Z. Li, X. Chen, T.T. Tian, P.F. Fang, F. Zheng, X.J. Zhao, Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency, J.Am. Chem. Soc. 133 (2011) 16414–16417. [4] V.K. Gupta, Application of low-cost adsorbents for dye removal–A review, J. Environ. Manage. 90 (8) (2009) 2313–2342. [5] A. Houas, H. Lachheb, M. Ksibi, E. Elaloui, C. Guillard, J. Herrmann, Photocatalytic degradation pathway of methylene blue in water, Appl. Catal. B: Environ. 31 (2001) 145–157. [6] H. Fan, W. Chung, C. Chen, Degradation pathways of Crystal Violet by Fenton and Fenton-like systems: condition optimization and intermediate separation and identification, J. Hazard. Mater. 171 (2009) 1032–1044. [7] C. Wen, Y. Zhu, T. Kanbara, H. Zhu, C. Xiao, Effects of I and F codoped TiO2 on the photocatalytic degradation of methylene blue, Desalination 249 (2009) 621–625. [8] C.C. Chen, H.J. Liao, C.Y. Cheng, C.Y. Yen, Y.C. Chung, Biodegradation of crystal violet by Pseudomonas putida, Biotechnol. Lett. 29 (2007) 391–396. [9] S.K. Moosvi, M. Kowssr, T. Ara, Study of thermal, electrical and photocatalytic activity of Iron complex doped polypyrrole and polythiophene nanocomposites, Ind. Eng. Chem. Res. 56 (15) (2017) 4245–4257. [10] P.D. Bui, H.T. Huy, K. Fei, W. Ya-Fen, M.C. Thi, Y. Sheng-Jie, H.V. Nam, P.V. Viet, Insight into the photocatalytic mechanism of tin Dioxide/Polyaniline nanocomposites for NO degradation under solar light, ACS Appl. Nano Mater. 1 (10) (2018) 5786–5794. [11] L. Wang, J. Shang, W. Hao, S. Jiang, S. Huang, T. Wang, Z. Sun, A dye-sensitized visible light photocatalyst-Bi24O31Cl10, Sci. Rep. 4 (2014) 7384. [12] X. Li, D. Wang, G. Cheng, Q. Luo, Preparation of polyaniline-modified TiO2 nanoparticles and their photocatalytic activity under visible light illumination, Appl. Catal. B: Environ. 81 (2008) 267–273. [13] Y. Yang, J. Wen, J. Wei, R. Xiong, J. Shi, C. Pan, Polypyrrole-decorated Ag-TiO2 nanofibers exhibiting enhanced photocatalytic activity under visible light illumination, ACS Appl. Mater. Interfaces 5 (13) (2013) 6201–6207. [14] T.A. Kandiel, R. Dillert, D.W. Bahnemann, Enhanced photocatalytic production of molecular hydrogen on TiO2 modified with Pt – polypyrrole nanocomposites, Photochem. Photobiol. Sci. 8 (5) (2009) 683–690. [15] K. Huang, M. Wan, Y. Long, Z. Chen, Y. Wei, Multi-functional polypyrrole nanofibers via a functional dopant-introduced process, Synth. Met. 155 (2005) 495–500. [16] W. Zhao, Z. Bai, A. Ren, B. Guo, C. Wu, Sunlight photocatalytic activity of CdS modified TiO2 loaded on activated carbon fibers, Appl. Surf. Sci. 256 (2010) 3493–3498. [17] S. Bharathi, D. Nataraj, D. Mangalaraj, Y. Masuda, K. Senthil, K. Yong, Highly mesoporous α -Fe2O3 nanostructures : preparation, characterization and improved photocatalytic performance towards Rhodamine B (RhB), J. Phys. D Appl. Phys. 43 (1) (2009) 015501. [18] O. Mehraj, N.A. Mir, B.M. Pirzada, S. Sabir, Fabrication of novel Ag3PO4/BiOBr heterojunction with high stability and enhanced visible-light-driven photocatalytic activity, Appl. Surf. Sci. 332 (2015) 419–429. [19] J. Jin, J. Yu, G. Liu, P.K. Wong, Single crystal CdS nanowires with high visible-light photocatalytic H2-production performance, J. Mater. Chem. 36 (2013)

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