During last few decades, environmental especially water pollution has become aserious concern, due to discharge of industrial effluents such as textile, leather cosmetic and they are detrimental to human life^[1]. Di Nitro Phenol (DNP) has vast applications in agriculture as pesticide and as polymerisation inhibitor for vinyl aromatics in petrochemical industry but other side it is carcinogenic and toxic when exceeds permissible limit, thus US Environmental Protection Agency (EPA) restricted and declared to cost effective and efficient method is necessary for removal of DNP from contaminated water^[1,2]. Researchers have been reported that photocatalysis is suitable method for removal of organic dye pollutants^[3]. However, semiconductor nanoparticles has shown great result in photocatalysis due to cost effective, its optical, electrical, biochemical properties with wider bandgap which captures the solar light and chance to ruggedize in visible light active.

Zinc oxide (ZnO) is a versatile semiconductor with large surface area, high catalytic activity and wide bandgap (3.37 eV), thus widely used in many applications such as catalytic, gas sensing, biological activity^[4]. However, the major drawback in photocatalysis due to faster the recombination of electron and hole pair because of presence of wide bandgap. To resolve this concern, doping or hybrid or composite with additional material is essential. Besides that Graphene oxide (GO) has great chemical, electronic and thermal properties and has numerous applications in biological, chemical, catalytic fields due to it has zero bandgap thus it slower the recombination of electron-hole pair^[5]. However, recently researchers are more interested in the preparation of RGO based metal oxide composites. As a result, GO based semiconductor composites as photocatalysts have been widely used in photocatalytic reactions. Moreover, the reports of ZnO/RGO prepared by different methods such as chemical reduction, precipitation, solve thermal and microwave show an excellent photocatalytic activity on the degradation of various organic dyes such as malachite green, rhodamine B and industrial dyes^[6]. However hydrothermal method is more effective due to it can be generating nanomaterials with controlled temperature and minimum loss of materials.

Herein, we report the photocatalytic degradation of 2, 4-Dinitrophenol under visible light irradiation by hydrothermally prepared ZnO/RGO nanocomposite. The photocatalytic degradation was studied with optimised conditions such as effect of pH, catalyst dose and initial concentration of pollutant.

Materials

From Sigma Aldrich India, collected the Graphite flakes, potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), sulphuric acid (H₂SO₄), hydrogen peroxide (H₂O₂), hydrochloric acid (HCl), sodium hydroxide (NaOH) and zinc acetate (Zn(CH₃COO)₂). 2, 4-Dinitrophenol dye stuff was received from Hitech media, India. Milli Q water was used in the synthesis and preparation.

Synthesis of GO: GO was prepared via universal modified Hummer’s method^[7]. In this, one gram of graphite flakes was sonicated for 30 minutes prior to oxidation. The modified graphite residue was dried at 60°C for 120 min. Then, sonicated graphite (1 g) and NaNO₃ (0.5 g) were suspended in 69 ml concentrated sulphuric acid in an ice bath (to maintain temperature below 5°C) in a 500 ml round bottom flask under gentle magnetic stirring for 15 min. Then 3 g of KMnO₄ was added continuously pinch by pinch by keeping the temperature of the solution did not exceed than 20°C. Then, the ice bath was removed and stirred the solution at 35°C under a reflux condenser for 3 h. After that, 120 ml of distilled water was added and stirring continued for an additional one hour. Excess unreacted KMnO₄ was removed by 10 ml of 30% H₂O₂. The complete removal of KMnO₄ was indicated by a colour change from dark to yellow. As prepared GO was carefully washed with Milli Q water and dried overnight in oven at 70°C.

Synthesis of Zno / RGO Composite: For preparation of ZnO/RGO composite^[8], 10 mL of 0.1 M zinc acetate solution and 10 mL NaOH aqueous solution were taken in beaker and 50 mg of GO added and then stirring for 30 min, and the slurry solutions were poured into Teflonlined autoclaves and hydrothermally heated at 180°C for 24 h. The obtained yield was washed, centrifuged and dried in oven at overnight. In absence of GO, the same procedure was used for the synthesis of pure ZnO.

Characterization: The synthesized catalyst was characterized by Powder X-ray diffraction (XRD) (D8 Focus, Bruker instrument, Germany) with Cu Kα radiation (λ=1.5406 Ao), 2θ ranges from 10^o to 90^o with scanning speed of 0.02^oS^-1 to identify the crystalline phase of samples. Fourier Transform Infrared (FTIR) spectra were recorded on IR prestige 21 Shimadzu with scale of 400-4000 cm^-1. The morphology of samples was studied using Fourier Emission Scanning Electron Microscope (FESEM) with an acceleration voltage of 15 KeV and elemental analysis was done by Energy Dispersive X-ray (EDS) spectroscopy. The bandgap energy of synthesized samples was obtained through UV-visible absorption spectra taken from UV-DRS spectrophotometer (UV-2600R, Shimadzu, Jap.

Investigation of Photocatalytic efficacy: The photocatalytic activities of the prepared catalysts were examined by the degradation of the 2, 4-DNP (10 mg/L) solution under visible light irradiation. All the reactions were performed at room temperature and normal atmospheric pressure. 0.03 g of the catalyst was suspended in 100 mL dye solution. Prior to irradiation, the system was placed in a total dark environment and magnetically stirred for 30 min until adsorption-desorption equilibrium was reached. Following this, the photocatalytic reaction was started by the exposure of visible light to the system. 4 mL of mixture solution was taken out at regular intervals and centrifuged to remove the catalyst and concentration of the dye was determined by using a UV-vis spectrophotometer.

The photocatalytic activity of catalysts was measuring the absorbance spectra of dye solution. The degradation efficiency of photocatalytic reaction was calculated using equation(1)

Photo degradation efficiency (%) = C₀-C₀/C₀x 100 Equation(1)

Where ‘C₀’ are initial concentration or absorbance and ‘C_t’ after the photocatalytic reaction concentration or absorbance with various time’s.

Results and Discussion

Structural analysis: Figure.1 shows theXRD patterns of GO, ZnO, ZnO/RGO. GO showed a characteristic reflection major peak at 2θ = 10.6° ascribed to the (002) reflection plane^[8]. For ZnO, the major peaks located at 2θ =32.1° (100), 34.8° (002), 36.6° (101), 47.8° (102), 56.9°(110), 63.9°(200) and 68.2° (201), compared with JCPDS (36-1451). The XRD peaks of synthesized ZnO/RGO composite are same as in ZnO, specifying that the RGO doesn’t give any adverse result mainly alters in preferential orientations of ZnO with impure peaks which may be due to relatively low diffraction intensity of RGO, which is evident from previous^[9]. The average crystallite size of prepared samples was measured using Debye Scherer equation, D= kλ/βcosθ, where k, λ, β and θ are the respective shape factor (0.9), wave length (0.154 nm), full width at half maximum (FWHM) and incident angle of the X-rays. The obtained results declared that ZnO/RGO composite were found to be 17.8 nm which is smaller in size than ZnO with 26.3 nm.

Figure 1: XRD patterns of a) ZnO, b) GO and c) ZnO/RGO

Figure 2 shows the FTIR spectrum of GO in which the observed characteristic peaks at 1717.5 cm^-1 and 1586 cm^-1 could be assigned to C=O group and aromatic C=C group, the hydroxyl (-OH) group exhibit broad band centred around 3235.5 cm^-1[10]. ZnO/RGO shows a metal oxide peak at 514 cm^-1 but after hydrothermal process, vanished C=O and decrease the peak intensity of O−H and C−O. This indicates the detoriation of functional groups of oxygen meanwhile partial reduction of GO which means the formation of RGO in prepared composite^[8,11], proves ZnO was successfully lies on RGO.

Figure 2: FTIR spectra of prepared samples

Figure 3 shows the UV-vis DRS spectra of ZnO and ZnO/RGO nanocomposite. The band gap energy (E.g.) of the catalyst can be calculated from equation, Eg= 1240/λ Where, ‘Eg’ is band gap energy, ‘λ’ is wavelength (nm) corresponding to the absorption edge. The ZnO/RGO shows band gap energy of 3.24eV (381.9nm) whereas, in case of ZnO/RGO composite shows band gap energy 3.01 eV (402.7 nm)^[12]. The absorption edge wavelength of prepared ZnO/RGO composite slightly shifted towards visible region and thus possesses great absorption intensity than pure ZnO, due to an interaction of Zn–O–C in ZnO/RGO composite. Hence photocatalytic activity enhanced by ZnO composited with RGO due to it is more visible light respond material.

Figure 3: UV-DRS spectra of a) ZnO and b) ZnO/RGO nanocomposite

Morphological analysis: The structure and morphology of samples were studied using Fourier Emission Scanning Electron Microscopy (FESEM) and the corresponding images are shown in Figure 4. The surface of RGO sheets decorated with ZnO means hydrothermal route fascinates the formation of composite material from ZnO intact with RGO shown in figure 4 (c & d). The prepared ZnO in composite showed smaller size than pure ZnO, this is due to interaction of positively charged sites Zn²⁺ which are attracted by RGO sheets renders many negatively charged sites^[8]. There is no free RGO on the prepared ZnO/RGO nanocomposite was prepared by hydrothermal, best method for the formation of pure ZnO/RGO composite. The elemental composition of prepared materials was carried out using EDX spectroscopy. The corresponding EDX spectra (Figure 4e) unambiguously confirmed the ZnO/RGO were formed without impurities. However, the elements such as zinc (Zn), oxygen (O) and carbon (C) present in spectra. Due to incorporation of ZnO onto RGO sheets, the Zn atomic weight % in composite is decreased than in pure ZnO. These results were supported with XRD results confirmed the purity of composite structure.

Figure 4: FESEM and EDS images of preparedsamples

Photocatalysis studies: The photocatalytic activity of synthesized ZnO/RGO composite was studied on degradation of 2, 4-DNP organic pollutant under visible light illumination. Initially, the ZnO was tested under vigorous conditions such as 0.03 g of catalyst and 10 ppm of dye solution which is alkaline based on literature and showed less degradation efficiency (42%). Later degradation of 2, 4-DNP was tested using hydrothermally prepared ZnO/RGO composite under visible light irradiation and exhibited better result in degradation. In order to optimise the conditions such as effect of pH, catalyst amount and dye concentration solution were also studied.

Effect of pH of dye solution: The role of pH plays key factor in photocatalysis. In this study, three different pH such as 4, 7 and 10 were used to evaluation of photocatalytic activity of composite. Interestingly the maximum degradation efficiency was noticed at pH-10 under kept constant such as catalyst dose (0.03 g) and initial dye concentration (10 ppm) of 2, 4-DNP carried out. The degradation efficiencies of composite are 98, 75 and 54% at their respective pH- 4, 7 and 10, shown in Figure 5. Furthermore, when the pH is greater to pH-10, the degradation of 2, 4-DNP decreased with increase in pH due to pH played great role in photocatalysis. Hence, for better adsorption of the dye on the catalyst surface, the surface must be maintained the pH-10 for degradation of 2, 4-DNP solution by ZnO/RGO composite.

Figure 5: Effect of pH on degradation of 2, 4-DNP dye

Effect of Catalyst Dose: To study the effect of catalyst dosage on the degradation of 2,4-DNP, a series of experiments were conducted to get optimum catalyst loading by varying the selected catalyst dosage ranging from 0.01 g to 0.03 g at a fixed pH-10 and dye concentration (10 ppm) of 2, 4-DNP aqueous solution. With the increase of catalyst dosage up to 0.03 g rate of degradation increases (up to 98% but 0.01 and 0.02 g possess 64 and 81% respectively, (Figure 6), since the active surface becomes constant, the number of photons absorbed and the number of dye molecules adsorbed are increased with respect to an increase in the number of catalyst molecules. For complete degradation of 2, 4-DNP catalyst dose was increased to 0.05 g and showed better result. Beyond a certain number of catalyst dosage the dye molecules are not sufficient for adsorption by increased number of catalyst molecules. Therefore, overdose of catalyst powder is not effectively involved in the photocatalytic activity rather increase in the turbidity of the solution, which intercept the penetration of light transmission due to formation of agglomeration of the catalyst particles. Furthermore, it was explained with the previous literature^[13,14].

Figure 6: Effect of catalyst dosage on % degradation of 2, 4-DNP

Effect of Dye Initial Concentration: The effect of initial 2, 4-DNP concentration on degradation using ZnO/RGO nanocomposite is illustrated and Figure 7. The influence of initial concentration of 2, 4-DNP on the rate degradation was studied from 10, 15 and 20 ppm at a fixed dosage of selected photocatalyst (0.03 g) using a solution of pH-10. Increasing of photocatalytic degradation occurred with an increase in the concentration of dye up to 20 ppm, and degradation rate decrease after increasing in excessive dye concentration. This may be due to concentration of dye increase, more dye concentration will be available for excitation and energy transfer^[4] which enhances the degradation percentage, but above the limit due to the fact that at the higher concentration dye start covering the active surface of photocatalyst as blanket and intercepting from light intensity^[15] that decreases degradation.

Figure 7: Effect of initial dye concentration on % degradation of 2, 4-DNP

The simulated visible light photoreaction of the 2, 4-DNP dye solution with the selected nanocomposite shows relatively satisfactory results. But in the absence of nanocomposite, no dye degradation observed. These rate values indicate the rate of degradation of 2, 4-DNP in alkaline medium. The optimised conditions for degradation of 2, 4-DNP using ZnO/RGO composite are 0.03 g catalyst dose, pH-10 and initial concentration 10 ppm of dye solution.

Proposed mechanism: A proposed mechanism for improved photocatalytic degradation efficiency of ZnO/RGO nanocomposite under visible light irradiation was schematically explained in Figure 8. The ZnO/RGO nanocomposite kept in visible light illumination; initially ZnO undergoes charge separation process that leads to promotion of electrons from valence band (VB) to conduction band (CB) and leaving a hole in the VB. Due to the formation of heterojunction, the photo generated elections in CB of ZnO transferred to CB of ZnO. Meanwhile, the excited holes at the valence band (VB) of ZnO were transferred to VB of ZnO. Finally, the photo generated electrons are captured by the RGO sheets through semiconductor carbon heterojunction which was dramatically enhanced photogenerated charge separation efficiency in ZnO/RGO nanocomposite. Simultaneously an equal amount of holes have been formed in semiconductor nanocomposite. These separated electrons and holes directly reacts with oxygen and water to generate highly reactive superoxide radicals (^•O₂-) and hydroxyl radicals (•OH). These energetic reactive radicals consequently react with surface adsorbed MB molecules and degraded them into small intermediate molecules, such as CO₂ and H₂O. Based on the above mechanism explanation for photocatalytic degradation of dye in the presence of ZnO/RGO nanocomposite under visible light irradiation was shown by the following equations 3-10.

ZnO + hvZnO(e⁻) + ZnO(h⁺)                                                3

ZnO(h⁺)+H₂O ZnO+ ^•OH + H⁺                                          4

ZnO(e⁻) + RGO RGO (e^-) + ZnO                                         5

RGO (e^- ) + O₂ RGO + ^•O-₂                                                    6

H2O + ^•O-₂HO₂^•+ OH-                                                          7

2HO₂^• H₂O₂ + O₂                                                                       8

H₂O₂ + ^•O-₂ OH− + ^•OH + O₂                                                    9

^•OH/^•O-₂/ HO₂^•+dye Degradation products   equation 10

Figure 8: Photocatalytic degradation of dyes using ZnO/RGO composite

The synthesized reduced graphene oxide based zinc oxide nanocomposite shown better photocatalytic activity due to increases the number of impurity energy levels between the VB and CB which helps for generation of more electron-hole pairs, thus decrease the recombination of photo generated holes and electrons by reduced graphene oxide ^[1-3] to eliminate the harmful effect of defect bands.

Conclusion

In this paper, we have discussed about the hydrothermal synthesis of ZnO/RGO nanocomposite. The structure, morphology and thermal studies of synthesized ZnO/RGO nanocomposite using various instrumental techniques. The photocatalytic activity of as prepared ZnO/RGO composite was tested using the degradation of 2,4-DNP. With the comparison of ZnO experimental results, ZnO/RGO nanocomposite shows excellent photocatalytic activity under visible light irradiation.

Conflicts of interest

There are no conflicts of interest.

Acknowledgement

The author B. Sathish Mohan acknowledges the UGC (University Grants Commission), Delhi for Rajiv Gandhi National Fellowship (RGNF-SC-2016-17-AND-9309).

References

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