Novel CuO/chitosan nanocomposite thin film: facile hand-picking recoverable, efficient and reusable heterogeneous photocatalyst

Abstract

The present work demonstrates a new simple hand-picking technique for the 100% recovery of a photocatalyst. CuO nanospheres were synthesized by a simple wet chemical method and were subsequently embedded into the biopolymer matrix (chitosan) under mild conditions by the solution cast method and its photocatalytic application towards the degradation of organic pollutants was measured for the first time. The crystal structure, optical properties, surface and bulk morphology were discussed in detail. ICP-OES analysis showed 3.025% copper embedded in the chitosan (CS) matrix. Efficiency of the CuO/chitosan was evaluated against the degradation of rhodamine B dye as a probe. The combination of CuO nanospheres with chitosan leads to the higher efficiency of up to 99% degradation of the dye with 60 minutes of irradiation. This may be attributed to many features such as the slow electron hole pair recombination rate of nanosized CuO in the biopolymer matrix, the large surface area of the CuO and the high adsorption efficiency of the chitosan. The major advantage of this present protocol is that it is not only restricted to azo type dyes but can also be adopted for different kinds of organic pollutants. For all the types of organic contaminants tested, the CuO/chitosan nanocomposite thin film photocatalyst showed excellent activity. The facile hand-picking recovery and recyclability of this novel thin film likely opens up a new straightforward strategy in the effective photocatalytic degradation of organic contaminants.

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Introduction

Photocatalysis is a “green” technique which offers great potential in environmental remediation such as the photodegradation of the organic pollutants in the wastewater from industrial drains. In this regard, some metal oxide nanoparticles were reported with good photocatalytic performances.^1–3 However, in the case of support free heterogeneous metal oxide nanocatalysts, the separation⁴ and recycling of the utilized nanoparticles are very tedious tasks at present,⁵ especially in large scale treatment processes. Moreover, the turbidity of the highly concentrated catalyst suspensions is responsible for a light shadowing effect, which can drastically lower the photocatalytic activity. Therefore, development of high performance photocatalysts with easy recycling properties is highly desired for such applications.^6,7 In this regard, impregnation of the photocatalyst over a proper solid matrix might be an efficient strategy to recover the photocatalyst for effective recycling.

Embedding of an inorganic nanomaterial (photocatalyst) into a polymer matrix offers a number of potential advantages and presents significant value in environmental applications, including facile incorporation within continuous reactors and ease of recovery and reuse, especially on a large scale.⁸ Owing to their multifunctional behaviour such as easy processing, compatibility etc., polymers are currently being widely used as excellent solid supports for metal nanocomposites. Recently, a number of studies^8,9 have focused on the use of natural organic polymers as a supporting material for photocatalytic compounds, and more especially in the preparation of heterogeneous catalysts.^10–13

In recent years the chitosan (CS) biopolymer, in particular, has exhibited multifunctional performance with TiO₂, ZnO, and CdS in heterogeneous photocatalysis technology which gives a high percent photodegradation of environmental pollutants. In addition, immobilized nanosized photocatalysts on the chitosan bio-matrix can effectively prevent the agglomeration of nanoparticles during growth and also overcome the difficulty in recovery of the nanosized powder materials from the reaction medium.^14,15 Although in the solid matrix supported photocatalytic systems, recovery of the catalyst is carried forward either through filtration or by centrifugation which in turn causes loss of the photocatalyst to an extent. Hence, in the present study, an attempt has been made to develop a facile hand-picking technique for the biopolymer supported (CuO/CS) nanocomposite thin film (NCF) photocatalyst which was prepared by the solution cast method for the first time to enhance photocatalytic performance. Typical Rhodamine B (RhB) was selected as a model hazardous dye to evaluate the feasibility of the adsorption – photocatalytic degradation by the nano CuO/CS NCF under visible light irradiation. The high performance of the CuO/CS NCF was expected to result from the complementation of the advantages of each component hence providing an economically feasible and environmentally friendly method for the treatment of effective dye degradation. 100% recovery of the photocatalyst was demonstrated and reasonable reusability was observed.

Results and discussion

The formation mechanism of the CuO/CS NCF prepared by the solution cast method is given in Scheme 1.

Scheme 1 Schematic representation of the synthesis of the novel CuO/CS NCF.

When copper acetate was added to the Millipore water, a blue colored transparent aqueous solution was obtained. Further, the CuO nanospheres were obtained by precipitation of aqueous copper acetate with sodium hydroxide followed by calcination. Finally the CuO was embedded in the CS matrix by the solution cast method.

Characterization of the CuO/CS NCF

Fig. 1 shows the WXRD of the pristine nano CuO, CS and the CuO/CS NCF. For the pure CuO nanocrystals, typical monoclinic diffraction patterns were found, which can be indexed as per the standard data (JCPDS no. 01-1117). The diffraction peaks at the 2θ regions of 35.5 and 38.7° were related to the monoclinic structure with (3̄13) and (1̄31) orientations, respectively. The same characteristic peaks are observed in the CuO/CS nanocomposite film with less intensity, which confirmed the formation of the CuO/CS nanocomposite. The peak at 20.2° is a characteristic diffraction peak for the CS, the peak is broad due to the amorphous nature of the polymer. Addition of the CuO to the CS matrix leads to an increase in the crystalline nature of the composite films and the peak is shifted towards a higher 2θ value. In addition, the crystallite size was calculated from Scherer’s equation¹⁶ using the reflections of plane (3̄13) and the value was 85.2 nm and 45.5 nm for the pristine CuO and the CuO/CS NCF respectively. The reduction in the crystallite size of the CuO by the formation of the CuO/CS NCF indicates better interaction between the CS and the CuO.

Fig. 1 XRD patterns of CS, CuO/CS NCF and CuO.

Fig. 2 shows the FESEM photomicrographs of the pristine CS film and the CuO/CS NCF. The surface of the CS film was pretty smooth as can be seen from Fig. 2(a). However, after the introduction of the CuO nanospheres into the CS matrix (Fig. 2(b)), some well dispersed and fine granules were also observed which confirmed the formation of the NCF of CuO/CS. The CS matrix could prevent the agglomeration of the CuO nanospheres and provide a larger surface area for the photocatalytic reaction via an adsorption process.

Fig. 2 FE-SEM micrographs of as synthesized (a) pure CS and (b) CuO/CS NCF.

The TEM images (Fig. 3) revealed that there existed many pleats on the surface of the CS.

Fig. 3 (a) TEM images of as synthesized CuO, (b) high magnification TEM images of as synthesized CuO/CS NCF (c) lattice fringes of as synthesized CuO/CS NCF.

The size of the CuO nanospheres embedded into the CS matrix was ∼50 nm as can be seen from the TEM micrograph (Fig. 3(b) and (c)) and the clear crystals were also observed. It also unveils the clear lattice fringes, thereby indicating that the nano CuO is structurally uniform and contains no defects such as dislocations or stacking faults after embedding into the CS matrix. The lattice spacing of 0.244 nm almost agrees with the (3̄13) plane of cubic-structured CuO (the lattice spacing is 0.247 nm for the (3̄13) according to the JCPDS file no. 01-1117).

Furthermore, the presence of the CuO in the CS matrix was confirmed by EDX analysis as shown in Fig. 4. In addition, ICP analysis was also carried out to find out the percentage of metal present in the CS matrix. It was confirmed that 3.025% copper was embedded in the CS matrix. This is well matched with the EDX spectrum and experimental procedure.

Fig. 4 EDX spectrum of as synthesized CuO/CS NCF.

The optical absorption properties and the migration of light induced electrons and holes of a semiconductor,¹⁷ which depends upon the material’s electronic structure, are known as key factors in determining the photocatalytic activity.¹⁸ As shown in Fig. 5(a), there is a significant absorption edge at around 560 nm revealing that the photocatalytic activity is in the visible range.

Fig. 5 (a) DRS UV-Visible absorption spectrum of CuO and CuO/CS NCF, (b) energy gap (Tauc plot) of CuO/CS NCF.

Moreover, the Tauc plot (Fig. 5(b)) is plotted to estimate the band gap of the prepared pristine CuO and CuO/CS NCF, and the band gap value is 2.10 and 2.60 eV, respectively. In general, CuO has a band gap value of around 1.4 eV but interestingly the band gap value is amplified when it is wrapped up in the CS matrix.

In order to understand more on the effect of porosity and the generated macro pores of the CuO/CS NCF on its optical and photoelectric properties, photoluminescence spectroscopy analysis was performed. Fig. 6 shows the room temperature PL emission spectra of pure CuO, bare CS and the CuO/CS NCF. CS shows a broad emission peak at 510 nm.

Fig. 6 Photoluminescence spectrum of pure CS, pure CuO and CuO/CS NCF.

It can be seen that there are three different emission bands in the PL spectrum of pure CuO. The peaks centered at 495, 555 and 600 nm are corresponding to the blue and green emission bands. The blue emission might be originate from the electron excitations and the green emission is due to the deep level defects. But in the case of the CuO/CS NCF, the emission intensities were almost negligible which might be due to the prevention of electron–hole pair recombination in CuO by CS. Because of this restricted recombination, the holes and the electrons were freely available for the generation of ˙OH and ˙OOH radicals which are responsible for the enhanced photocatalytic degradation.

The chemical composition of the synthesized CS, CuO and CuO/CS NCF was examined by FTIR-ATR spectroscopy as shown in Fig. 7. For the pristine CuO, the peak at 484 cm⁻¹ and 424 cm⁻¹ are the characteristic peaks of CuO. The stretching and bending vibrations of CS at 3437 cm⁻¹ and 2925 cm⁻¹ were attributed to the amino (–NH₂), hydroxyl (–OH), and –CH₂–, –CH₃ aliphatic groups, respectively.

Fig. 7 FT-IR ATR spectrum of as synthesized CuO, CS and CuO/CS NCF.

Strong absorption bands at 1643 cm⁻¹ and 1548 cm⁻¹ were attributed to the bending vibration of –NH₂ groups and the bending vibration of –OH groups, respectively. In the spectrum of the CuO/CS NCF, the 3437 cm⁻¹ peak of the N–H and O–H stretching vibrations of CS was shifted to 3370 cm⁻¹ and became wider, the peak at 1643 cm⁻¹ due to the N–H bending vibrations was shifted to 1637 cm⁻¹ and the 1548 cm⁻¹ peak of the O–H bending vibrations was shifted to 1550 cm⁻¹. Moreover, the shift from 484 cm⁻¹ to 469 cm⁻¹ and 424 cm⁻¹ to 419 cm⁻¹ for the characteristic peaks of CuO resulted from the strong interaction between the CS and the CuO. This result indicates that the CS and the CuO are well linked together.

Thermal stability of the pure CS, CuO and CuO/CS NCF were investigated by TG-DTA as shown in Fig. 8.

Fig. 8 TG-DTA spectrum of pure CS, CuO and CuO/CS NCF.

For pure chitosan the weight loss occurred at three stages. The first one started at 55 °C and ended at 157 °C with a weight loss of nearly about 10% which may be due to the loss of water in the chitosan films. The second stage started at 243 °C and ended at 352 °C with a weight loss of 52% which was assigned to the decomposition of the chitosan films. The third stage started at 392 °C and ended at 477 °C which may be attributed to the vaporization and elimination of volatile products.¹⁹ Inorganic metal oxide materials generally do not decompose in thermal conditions up to their melting point. The TGA curve of the CuO has no significant weight loss. That clearly indicates that the CuO exists in its pure form. The CuO/CS NCF has two distinct weight losses. The first stage with 3% weight loss started at 46 °C and ended at 109 °C which may be attributed to the removal of water from the CuO/CS NCF. The sudden weight loss of 51% (182 °C–365 °C) was the degradation of the polymer.²⁰ In addition, the residual weight percentage at 700 °C was proportional to the inorganic content and it is owing to the CuO. The residual weight of the CuO/CS NCF was 10% more than that of the pure chitosan film. The TG-DTA results shows the CuO nanoparticles may be immobilized onto the CS matrix.

Liquid-phase adsorption of RhB by CuO/CS NCF

Liquid phase adsorption studies were carried out for the CS, CuO and CuO/CS NCF to find out the adsorption efficiency of the catalyst and the results are shown in Fig. 9. Among the three, both the CS and the CuO/CS NCF showed almost equal adsorption efficiency. Indeed, the photocatalytic efficiency of the CuO/CS NCF was superior which is owing to the high adsorption behaviour.

Fig. 9 Liquid phase adsorption by CuO, CS and CuO/CS with (a) RhB, (b) CV and (c) CR.

Photocatalytic degradation of RhB solution under visible light irradiation

The potential efficiency of the CuO/CS NCF was evaluated by the photocatalytic activity towards the degradation of RhB dye. RhB is relatively stable in aqueous solution upon visible light irradiation. Furthermore, the photocatalytic studies confirmed that catalyst and light source both are necessary for the photocatalysis reaction to occur. Therefore, the RhB dye degradation is caused by the photocatalytic reaction of the CuO/CS NCF. The characteristic absorption peak of RhB at around 553 nm was used to monitor the photocatalytic degradation process. Fig. 10(a) shows the UV-Vis absorption spectra of the aqueous solution of RhB with CuO/CS NCF as the catalyst for various durations.

Fig. 10 Absorption spectra of different dye solution under visible light illumination with CuO/CS NCF with (a) RhB, (b) CV and (c) CR.

The color of the dispersed solution disappeared after 60 min. Hence the CuO/CS NCF photocatalyst degraded ∼99% of RhB at 60 min under visible light illumination which clearly pointed out the efficacy of the catalyst.

In addition to the azo dye, (Crystal violet) CV was also chosen to evaluate the photocatalytic activity of the CuO/CS NCF. Like RhB, the CV solution also showed no decolorization in the presence of the catalyst alone under dark conditions, or without the catalyst under a light source. The UV-Vis spectra change significantly during the photocatalytic degradation of CV in the presence of the CuO/CS NCF as shown in Fig. 10(b). On irradiation with visible light the absorption peak at 582 nm of the CV solution decreases gradually and the solution (10 μM) becomes colorless after 50 minutes irradiation.

The photocatalytic degradation study was also carried out for (Congo red) CR. The UV-Vis spectra change notably during the photocatalytic degradation of CR in the presence of the CuO/CS NCF as shown in Fig. 10(c). During the visible light irradiation, the characteristic absorption bands of the dye (488 nm and 339 nm) decrease gradually and the solution (10 μM) was almost colorless after 50 minutes irradiation. All these results imply that the CuO/CS NCF photocatalyst can be used as an efficient degradation catalyst for different kinds of dyes including the azo type which is an added advantage of the present catalyst.

Moreover, comparisons of photocatalytic activity among the pure CS film, the pure CuO and the CuO/CS NCF are shown in Fig. 11. Among all, the CuO/CS NCF shows the highest photocatalytic degradation rate for all the dyes used. The degradation efficiency of the pristine CS and the pure CuO were much lower than those of the CuO/CS NCF and the corresponding C/C₀ values. Lowering of the C/C₀ values for the pristine CS films may be attributed to its high adsorption ability towards the dye molecules. Therefore, the photocatalysis and adsorption had a remarkable synergistic effect on the dye degradation.

Fig. 11 Photodegradation with CuO, CS and CuO/CS with (a) RhB, (b) CV and (c) CR.

It is well known that the heterogeneous photocatalysis mainly occurs in the molecule’s interfacial layers, so the adsorption properties between the reactants and the photocatalyst surface play an important role in determining the overall reaction rate. It should be considered that the modification of the CS surface by embedding the CuO would provide an effective environment for increasing the surface active sites for the dye–semiconductor interaction, which may also increase the photo degradation process. In addition, the CS itself adsorbs dye molecules, which were continuously supplied to the CuO for degradation, and thus contributes to the increase in the efficiency of the CuO significantly. According to the observation from electron microscopy, the surface of the CuO/CS NCF was rough and porous, which could increase the surface area of the composite to improve its adsorption ability.²¹

In order to minimize the waste of chemicals it is necessary to make sure that the optimized amount of the photocatalyst required is used for the effective and greener degradation of these environmentally hazardous dyes. And also it is of paramount importance to evaluate the maximum efficiency of the CuO/CS NCF photocatalyst by increasing the concentration of the dyes.

The amount of the CuO/CS NCF photocatalyst needed for the effective degradation of dyes was investigated by using different dosages of the CuO/CS NCF varying from 25 mg to 75 mg as shown in Fig. 12. It is so obvious that the rate of the photodegradation of the dyes increased with increasing photocatalyst amount. This observation may be caused by the increase in the number of photons adsorbed on the photocatalyst or the number of activated molecules adsorbed on the photocatalyst surface increased with an increased amount of the photocatalyst.

Fig. 12 Photodegradation by CuO/CS at different amounts of catalyst loading with (a) RhB, (b) CV and (c) CR.

Initially, the increase of photocatalyst quantity increased the degradation percentage and the utmost efficiency was observed with 50 mg of the photocatalyst. But further increase of the catalyst loading showed a similar activity as like in other catalytic systems, i.e. after a certain amount of the catalyst the photocatalytic activity remained at same rate. This is due to the fact that excess loading of the catalyst prevents light penetration into the surfaces so that the reaction proceeds at a similar rate after reaching the optimum amount. Hence 50 mg of the photocatalyst was noted as the optimum catalyst dosage and was used for further photoreactions.

Similarly, it is necessary to know the effectiveness of the photocatalyst against the highly concentrated dye solution. Therefore the photodegradation of the dyes was studied by varying the concentration from 1 μM–25 μM. The degradation of the dyes over the concentration range from 1 μM–25 μM is shown in Fig. 13. It was worthy to note that as the dye concentration increases from 1 μM–15 μM, the C/C₀ value after continuous illumination reduces by around 99% for all three dyes.

Fig. 13 Photodegradation by CuO/CS at different concentrations of (a) RhB, (b) CV and (c) CR.

At a fixed light intensity, the decrease in the degradation of the dyes with an increase in the dye concentration can be attributed to the greater amount of dye competing for the degradation and the reduction in the light intensity that reaches the CuO/CS NCF surface. At very high concentrations, most of the light is screened by the solution and fewer photons are able to reach the CuO/CS NCF surface. Thus, the generation of electron–hole pairs is greatly reduced and in turn the dye degradation is reduced due to the absence of oxidizing species.

To examine the stability of the photocatalysts, recycling experiments were carried out and the results are presented in Fig. 14. For each cycle, the photocatalysts were reused for the degradation of a fresh RhB solution under visible light irradiation. The decolorized percentage of the RhB solutions for 5 cycles were 98.8, 78.3, 72.8, 70.6 and 69.2% after 60 minutes of irradiation, respectively. The results show that the catalytic activity of the CuO/CS NCF was decreased in the second run and subsequently maintained relative stability. The decrease of the degradation percentage may be due to the adsorption of intermediate products on the photocatalyst active sites, which rendered them unavailable for the degradation of fresh dye solution.¹³ However, 69.2% of RhB was decolorized successfully for the 5^th use of the photocatalyst, which indicates that the CuO/CS NCF catalyst could be reused.

Fig. 14 Reusability efficiency of CuO/CS NCF.

The stability of the CuO/CS before and after the photocatalytic reaction was evaluated using XRD which is shown in Fig. 15.

Fig. 15 Comparison of XRD patterns of CuO/CS NCF before and after the photocatalytic reaction.

The XRD pattern of the CuO/CS NCF after the photocatalytic reaction evidently suggests that the intensity of the characteristic peaks of CuO was decreased. The decrease in the intensity might be due to the photocorrosion and photo dissolution of the catalysts. These results suggest that the catalyst was relatively stable.

The rate of the degradation of the dyes in the presence of benzoic acid was investigated and is shown in Fig. 16. The results indicate that the addition of benzoic acid into the reaction medium suppresses the photo degradation since it scavenges the active ˙OH radicals and slows down the photo degradation process. When the concentration of the benzoic acid was increased, the photo degradation percentage was decreased further. This is because of the reduced amount of ˙OH radicals produced from the electron–hole pairs of the photocatalyst which leads to the decrease in the photo degradation of the dyes.

Fig. 16 Photodegradation by CuO/CS in the presence of different concentrations of benzoic acid.

To support the involvement of the ˙OH radicals in the photo degradation, the reaction was carried out by some more substances like the azide ion, acrylamide, triphenyl phosphine (TPP) and Rose Bengal (RB) (Fig. 17). It was found that acrylamide enhances the photodegradation in the presence of the catalyst indicating the absence of involvement of the superoxide mechanism. The azide ion and the rose bengal are singlet oxygen generators, which generally support the superoxide radical mechanism²² by creating singlet oxygen which may absorb an electron from the conduction band and produce the superoxide radical ion.

Fig. 17 Photodegradation by CuO/CS in the presence of different substrates.

Furthermore they may precede the photocatalytic mechanism in the conduction band by up taking the superoxide radical ion. The azide ion, a singlet oxygen quencher, fails to suppress the photodegradation indicating the absence of involvement of singlet oxygen in the surface photocatalysis. RB, a singlet oxygen generator decreases the photodegradation indicating that the superoxide anion is not involved in the degradation reaction. Sacrificial electron donor TPP generally enhances photocatalytic reactions which involve the superoxide radical ion.²³ In the present system it fails to enhance the photodegradation rate which clearly indicates that the reaction is led only by the formation of the ˙OH radicals and not by the superoxide radical anion.

CuO/CS + hv → CS/CuO (e_CB⁻ + h_VB⁺) (1)

CuO/CS (h_VB⁺) + OH⁻ → CS/CuO + OH˙ (2)

Dye + OH˙ → CO₂ + H₂O + salts (3)

CuO/CS (h_VB⁺) + dye → degradation products (4)

When the catalyst is irradiated with visible light, electrons (e⁻) in the valance band (VB) are excited to the conduction band (CB) with simultaneous generation of the same number of holes in the VB (eqn (1)). Similarly, the photo induced holes can be easily trapped by OH⁻ to produce a hydroxyl radical species (OH˙) (eqn (2)), which is an extremely strong oxidant for the partial or complete mineralization of organic pollutants (eqn (3) and (4)).¹⁴

The active electrons (e⁻) and holes (h⁺) are generated when the incident photon energy is greater than the band gap. The narrow band gap of CuO tends to mean that the electron (e⁻) and hole (h⁺) pairs recombine quickly [Fig. 18].

Fig. 18 Mechanism of the photodegradation of RhB.

However, when the nano CuO particle was embedded into the CS, more photo-generated electrons could be captured by the adsorbed O₂ on the surface of the CuO/CS NCF, which may restrain the recombination between the photo-generated electrons and holes to improve the photocatalytic activity of the CuO/CS NCF.²⁴

Experimental section

Materials

All chemicals and solvents used were of analytical grade from Merck. Commercial grade RhB, CV and CR were used. Throughout the experimental work, Millipore water was used.

Synthesis of nano CuO

CuO was synthesized by a simple wet chemical method. 20 mL of 0.1 M copper acetate solution in deionized water was taken and the pH of the solution was adjusted to 6 by adding dilute acetic acid. Then the reaction mixture was mixed with 20 mL of 0.5 M NaOH solution to obtain a brownish precipitate. Then it was washed with water and ethanol several times. After washing, the precipitate was dried at 120 °C for 12 h and then it was subjected to calcination at 500 °C for 4 h.

CuO/CS NCF preparation

In a typical experiment, 0.5 g of CS was dissolved in 20 mL of 1% acetic acid and it was stirred vigorously to attain the homogeneous gelatinous form. Then the desired amount of nano CuO was initially dispersed into the CS matrix by sonication. The reaction mixture was stirred vigorously until it became a homogeneous mixture. To the above reaction mixture, 0.2 M NaOH was added in order to neutralize the solution. Then the homogeneous mixture was poured into a Petri dish and was dried at room temperature. Further the films were dried in an oven at 60 °C and subsequently dried films were peeled off from the Petri dish.

Characterization

Surface morphology of the nanocomposites films was studied by FESEM (Model SUPRA 40 Field Emission Scanning Electron Microscope) with an acceleration voltage of 30 kV. The bulk morphology, dispersion and distribution of nano CuO in the CS matrix were observed through transmission electron microscopy (TEM) using a JEM 2100. Wide angle X-ray diffraction (WXRD) was performed to examine the crystal structure of the nano CuO and its nanocomposites using a Philips PW-1710 X-ray diffractometer (Eindhoven, The Netherlands). ICP-OES was performed to find out the percentage of copper in the composite using Optima 5300 DV ICP-OES. A Shimadzu UV-Vis-1800 spectrometer was used to analyze the samples.

Liquid-phase adsorption of RhB by CuO/CS NCF

Since the photo-oxidation reaction usually takes place on the photocatalyst surface, the adsorption of the pollutant on the photocatalyst is important for the photocatalytic process. 50 mg of the CuO/CS NCF was added to 100 mL of RhB solution (10 μM of initial concentration). The adsorption system was continuously stirred in the dark. The supernatant was withdrawn periodically and analyzed using a maximum absorption wavelength (λ_max) of 553 nm with the help of the UV-Visible spectrometer.

Photocatalytic degradation of RhB solution under visible light irradiation

The photocatalytic activity of the CuO/CS NCF was evaluated by employing an immersion type photo reactor (Heber, India) in the region of 450 nm. For the effective degradation of the RhB dye, 50 mg of the catalyst was added to 100 mL (10 μM) of the RhB dye solution in a 150 mL reaction vessel. At given time intervals, 5 mL aliquots were collected. The degraded solutions were analyzed using the absorption wavelength (λ_max) of 553 nm. After the degradation, the catalyst was separated from the reaction mixture by filtering the reaction mixture and the catalyst was washed with ethanol and water several times and dried to carry out the reusability tests.

Conclusions

Novel CuO/CS NCF were synthesized using the solution cast method. The structure, morphology and optical properties were investigated in detail. Entrapping of CuO into the CS matrix amplified the band gap of the CuO. The photocatalytic activity and adsorption capacity of the CuO/CS NCF were evaluated. The combinatorial effect of adsorption and photo degradation was reflected in the enhanced photo reactivity which justified the importance of adsorption in photocatalysis. This excellent behaviour may be attributed to the large surface area of the CuO/CS NCF and the small crystalline size of CuO, which significantly facilitates diffusion and mass transportation of the RhB molecules and delays the recombination time of the photo-generated electron–hole pairs in the photocatalysis. The photocatalytic degradation system was not only restricted with RhB but was also extended to CV and CR dyes. On the basis of the results, the present system provides an economically feasible and environmentally friendly method for the treatment of environmental pollutants.

Acknowledgements

The authors express their sincere thanks to the College managing board, Principal and Head of the Department, VHNSN College for providing necessary research facilities. The authors are also thankful to DST and saif IIT madras for providing ICP-OES facility.

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