INTRODUCTION
Over the past decades, pollutants have been important challenges in environmental problems. Therefore, many researchers have been encouraged to provide solutions for air, water, and environmental pollution [1–4]. Industrial wastewater pollutants are one of the biggest problems in the world as it endangers life [5–7]. Synthetic dyes have been used in various industries such as cosmetics, dyeing, plastics, textile, and stationery [8,9]. For example, crystal violet as a cationic dye has been greatly applied in the textile industry. However, it has carcinogenic and mutagenic effects and may cause serious health problems [10]. Therefore, these dyes cause pollution of water and nature and their removal from industrial wastewater is crucial. Many techniques and methods have been suggested for dye removal from aqueous solutions such as physical adsorption, oxidation, biological methods, electrochemistry, membrane separation, filtration, and photocatalytic degradation [11–17]. Among them, photocatalytic reactions play a major role in pollution removal and in various compounds and composites have been developed for this purpose[15,16,18–21]. The nanostructured photocatalysts, especially, quantum dots (QDs) have attracted great attention because of their unique properties [1,22–24]. The QDs are semiconductor nanostructures with discrete quantized energy levels which can absorb electromagnetic irradiations and then emit them with high efficiency. The difference between energy levels inversely depends on the size of QDs and thus their emission can be seen in different colors depending on their size [25–29].
Graphene quantum dots (GQDs) consist of carbon (sp2 and sp3) and oxygen in the form of various functional groups. The GQDs have strong luminescence, conductivity, biocompatibility, and high solubility which make them useful in drug delivery, bio-imaging, catalytic systems, photovoltaic devices, solar cells, etc. [30–38]. The incorporation of hetero-atoms such as nitrogen in the structure of GQD gives it amended properties [39,40]. For instance, Palladium nanoparticles supported on N-GQD have been used as catalyst for oxygen reduction [41]. B-GQD and N-GQD as hybrid nanoplatelets have been used as efficient electrocatalysts for oxygen reduction [42]. Also, N-GQD itself has been used as a catalyst for oxygen reduction [32,43]. Moreover, composites of GQDs were synthesized and applied for various applications such as sensors, supercapacitors, photocatalysis, solar cell, adsorption, and antimicrobial activity [36 – 46]. Photocatalytic dye degradations have been done by various QDs as efficient photocatalysts [25]. Ding et al. reported degradation of rhodamine B (RhB), methyl orange (MO), and methylene blue (MB) by ZnO foam/carbon quantum dots nanocomposite as a photocatalyst under UV and visible light irradiation [55]. Photodegradation of methyl orange under visible light irradiation by CdS QDs/chitosan composite films was also reported [56]. Safardoust-Hojaghan et al. synthesized N-GQD/TiO2 and used it in the degradation of methylene blue [57]. Moreover, GQD/TiO2 composite has been used for the photodegradation of dye [14]. In our previous work, CdS quantum dot nanocomposite hydrogels (QD-NCH) were synthesized by in situ copolymerization cross-linking method using acrylic acid and κ-carrageenan and was applied for the adsorption of cationic dyes, crystal violet (CV) and malachite green (MG) [25].
In this work, N-GQD has been prepared by hydrothermal method and characterized by some analyses. Then, it was used for the photodegradation of CV under visible and UV irradiations. As the results were not satisfactory, MnO2/N-GQD nanocomposite was synthesized and applied for CV photodegradation under visible and UV irradiations. The results showed that the photodegradation process is fast and highly efficient. The equilibrium time is about 5-40 min depending on the amount of photocatalyst and initial dye concentration, and efficiency is 100%.
MATERIALS AND METHODS
Materials and instruments
Crystalline citric acid (C6H8O7), Urea (CH4N2O), manganese dioxide (MnO2), sodium hydroxide, and hydrochloric acid were prepared from Merck (Germany), crystal violet (C25N3H30Cl) was purchased from Sigma Aldrich (USA), and ethanol was prepared from Bidestan Company (Iran). All of the materials were used without further purification.
Absorbance measurements were employed using UV–Vis Photonix Ar 2015 spectrophotometer (Iran). FT-IR spectra of the samples in the form of KBr pellets were recorded using a Tensor 27, Bruker FT-IR spectrophotometer (Germany). The surface morphology of the samples was investigated using a field emission scanning electron microscope FESEM Mira III with SAMX detector (Czech Republic). X-ray diffraction analysis of samples was obtained using PW1730 (Netherland).
Preparation of Nitrogen doped graphene quantum dot (N-GQD)
A solution of crystalline citric acid (0.21 g) and urea (0.18 g) in 5 ml of deionized water was transferred into a 25 ml Teflon lined stainless steel autoclave and placed in the oven at 170 °C for 4 hours. After cooling, 40 ml of ethanol was added to the resulting mixture and centrifuged for 20-30 min with 8000 rpm. The precipitate was separated, washed with ethanol several times, centrifuged, and finally dried at room temperature.
Preparation of MnO2/N-GQD nanocomposite
To prepare MnO2/N-GQD nanocomposite, 15 mg of N-GQD was dispersed in 15 ml of deionized water and stirred for 30 min. Afterward, 50 mg of MnO2 powder was added and the mixture was stirred for 4 hours. Then, the mixture was transferred into the Teflon lined stainless steel autoclave and was heated to 110 °C for 2 hours. Finally, 40 ml of ethanol was added to the sample and the precipitate was separated with centrifugation.
Evaluation of the photocatalytic activity of the MnO2/N-GQD
To investigate the photocatalytic activity of MnO2/N-GQD nanocomposite, dye degradation was carried out under UV and visible irradiation in the presence of MnO2/N-GQD nanocomposite as photocatalyst. First, a solution containing 50 ml of crystal violet (10 ppm) was prepared and then 0.05 g of MnO2/N-GQD was added to the solution. The solution was placed in dark or in the presence of visible light (200-watt tungsten bulb) as well as ultraviolet light (Mercury lamp 125 W) at a distance of 20 cm from the solution upon stirring. After certain times, 3 ml of the solution was taken and centrifuged to separate the nanocomposite. The residual dye concentration was determined using UV-Vis absorption spectroscopy at λmax = 590 nm. It should be noted that the experiment was also performed by N-GQD and MnO2 and the results were compared with MnO2/N-GQD nanocomposite.
Investigation of the effect of various parameters on the photocatalytic dye degradation
To study the effect of pH on the photocatalytic dye degradation, dye solutions with various pH values from 2 to 10 were prepared by adding hydrochloric acid or sodium hydroxide solutions. To evaluate the effect of contact time on photocatalytic dye degradation, the absorption of dye solution was measured at certain times by UV-Vis spectroscopy. To investigate the effect of photocatalyst amount on the photocatalytic dye degradation, dye solutions with initial concentration of 10 ppm were prepared. Then, determined amount of the MnO2/N-GQD nanocomposite (m= 0.005 g to 0.1 g) was added to the solutions, separately. To examine the effect of initial dye concentration on the photocatalytic dye degradation, dye solutions with concentration of 5-50 ppm were prepared, and then 0.05 g of the MnO2/N-GQD was added to each solution. After the equilibrium is established, the residue of dye concentration was determined. To study the effect of temperature, 50 ml of CV solution (C0=10 ppm) was prepared and, 0.05 g of the MnO2/N-GQD was added to the solution (at pH= 5.5) and heated to 35, 40, 45, 50, 55 and, 60 °C and cooled by an ice bath for 20 °C.
The percent of degradation of dye was calculated by the following equation:
where, C0 (mg. L−1) and Ce (mg. L−1) are initial and equilibrium dye concentrations, respectively.
RESULTS AND DISCUSSION
Characterization of N-GQD and MnO2/ N-GQD nanocomposite
The FT-IR spectra of N-GQD, MnO2, and MnO2/N-GQD nanocomposite are shown in Fig. 1. As can be seen, the characteristic bands of N-GQD are observed at about 3450 cm-1 (stretching vibration of OH), 2930 cm-1 (stretching vibration of C-H), 1705 cm-1 (stretching vibration of C=O), 1636 cm-1 (bending of NH), and 1080 cm-1 (stretching vibration of C-O-C). The bands of MnO2 were appeared at 521 cm-1 attributed to Mn-O stretching vibration and 3450 cm-1 due to O-H stretching of associated H2O. FT-IR spectrum of MnO2/N-GQD nanocomposite exhibits all of the bands of N-GQD and MnO2, confirming the formation of nanocomposite.
X-ray diffraction (XRD) pattern of N-GQD (Fig. 2) indicates a broad peak with a maximum at about θ = 27° indicating its weak crystalline structure. On the other hand, in the XRD pattern of nanocomposite, the sharp peaks which are consistent with the standard data (PDF#00-024-0735) indicate the presence of crystalline MnO2 in the nanocomposite.
Chemical composition of the N-GQD and MnO2/N-GQD nanocomposite were investigated by energy-dispersive X-ray (EDX) analysis. As can be seen in Fig. 3, the EDX spectra of the N-GQD and MnO2/N-GQD show characteristic peaks of carbon, oxygen, and nitrogen in the N-GQD structure and carbon, oxygen, nitrogen, and manganese in the MnO2/N-GQD nanocomposite. The surface morphology of the nanocomposite is shown in the SEM image and elemental distribution over the MnO2/N-GQD can be seen in the mapping image of the nanocomposite (Fig. 2).
In the UV-Vis absorption spectrum of the N-GQD (Fig. 4), three major bands can be seen at 230, 345, and 395 nm attributed to the π → π* transition of the C sp2 domains, n → π* transition of C=O bonds, n → π* transition of C=N, respectively. In the UV-Vis absorption spectrum of the MnO2/N-GQD nanocomposite, the absorption bands of N-GQD are observed again and a new peak at about 430 nm is appeared due to the presence of MnO2 component. The band gap of the N-GQD was calculated by Tauc equation:
where, α, hν, Eg, A, and n are absorption coefficient, the photon energy, band gap, a proportional constant, and an exponent which equals 1/2 for direct transitions and 2 for indirect transitions [58]. The Eg of the N-GQD is obtained as 2.6 eV which is according to the band gap of the N-GQD reported between 2.2 to 3.4 in the previous researches [33].
Comparison between various conditions for degradation (or removal) of dye solution
As can be seen in Fig. 5a, the dye degradation in the absence of catalyst showed a poor efficiency (about 2% at dark, 8% at UV light, and 10% at visible light). The results confirm that the catalyst has an important role in this reaction. Figs 5b-d also show a comparison between three catalysts i.e. MnO2, N-GQD, and MnO2/N-GQD nanocomposite in the dye degradation. In the given conditions, degradation (or adsorption) of dye in the presence of N-GQD has the lowest efficiency while in the presence of nanocomposite the maximum efficiency has been reached. The comparison of three catalysts at dark (Fig. 5d) confirms that the dye degradation is progressed through the photocatalytic process. Also, it can be concluded that the small part of dye removal is related to the adsorption on the catalyst as adsorbent. As can be seen in all graphs, the equilibrium time for the processes can be determined.
The effect of the various parameters on the photocatalytic degradation of dye
The effect of various parameters such as initial dye concentration, amount of photocatalyst, pH, and temperature on the photocatalytic degradation of dye were investigated and the results were shown in Fig. 6.
The effect of initial concentration
The percentage of photocatalytic degradation of CV decreased with an increase of initial CV concentration (Fig. 6a), which is due to the decreased surface of photocatalyst compared to dye molecules. It can be noted, the percentage of dye degradation was increased over time at higher concentration, indeed, equilibrium time increased with increasing of dye concentration.
The effect of dose of photocatalyst
With increasing the dose of photocatalyst, the number of available active sites increases which leads to an increased dye degradation percent and also increases the rate of degradation (Fig. 6b). This means that the equilibrium times are shorter at a higher amount of photocatalyst (Table. 1).
The effect of pH
The CV is sensitive to pH, this means that the structure of CV changes in acidic and basic pH, so the best pH for investigation of photocatalytic degradation is the neutral pH (5.5-7.5). In acidic pH, the maximum wavelength of UV absorption was changed and in basic pH, CV was degraded by hydroxyl groups without any catalyst (Fig. 6c).
The effect of temperature
The results show that the higher temperature leads to the higher photocatalytic degradation of CV (Fig. 6d). On the other hand, the rate of photodegradation increases with the increased temperature.
Contact time of degradation of CV by MnO2/N-GQD nanocomposite
Fig. 7d confirms the effect of contact time on the activity of catalyst. In other words, this parameter determined the time of the end of reactions. As can be seen, the photocatalytic decolorization of CV solution increases remarkably up to 40 min under visible light and 55 min under UV irradiation, and then no significant change can be observed with irradiation time.
The kinetic study of photocatalytic degradation of CV
Kinetic studies aiming to determine the mechanism of processes are one of the valuable parts of researches. In this work, to investigate the kinetics of photocatalytic degradation of CV, three kinetic models were used and the experimental data were fitted by them. These models are the zero-order model, the first-order model, and the second-order model.
The zero-order kinetic model describe the rate of reactions independent of the concentration of reactants [59]. This means that the rate of the reaction is equal to the rate constant, k0, of that reaction. The differential form of the zeroth-order rate law is shown as the following equation:
and, the integrated form of this model:
where, Ct and C0 are reactant concentration at time t and initial reactant concentration, respectively, t shows time, and k0 is a rate constant. So, if the diagram of Ct versus time is plotted and this diagram is linear (the experimental data follows linear equation (5)), it can be said that the rate of reaction obeys the zero-order model [60]. Fig. 7a shows the fitting plot of the zero-order model.
In the first-order rate law, the rate of reaction is linearly proportional to the reactant concentration. The differential equation describing first-order kinetics is given below:
and, the integral representation:
where, k1 is a rate constant. The plot of ln Ct versus time is linear in this model. In other words, to test if the reaction is a first-order reaction, plot the natural logarithm of a reactant concentration versus time. If the graph is linear and has a negative slope, the reaction must be a first-order reaction. Fig. 7b shows the fitting diagram of the first-order model.
In the reactions that follow the second-order rate low, the relation between the inverse of reactant concentration and time is linear[61]. The differential rate law for the simplest second-order reaction in which 2A → products is as follows:
the integrated form of this model:
where, k2 is a rate constant. In this model, the graph of 1/Ct versus time is linear. Fig. 7c shows the fitting diagram of the second-order kinetic model. As can be seen, the first-order model has the best correlation coefficient comparison among two other models, it can be concluded that the rate of photocatalytic degradation of CV by synthesized photocatalyst (MnO2/N-GQD nanocomposite) follows the first-order kinetic model. The rate constants and R2 of the three models are shown in Table 2.it should be noted that the data for fittings was from the start of degradation to equilibrium time.
Photocatalytic mechanism
The mechanism of CV photocatalytic degradation process over MnO2/N-GQD nanocomposites is proposed in Fig. 8. Under UV or visible irradiated light, the electron of MnO2 is transferred from the valence band to the conduction band to form a hole. Holes reacted with hydroxyl ions to produce hydroxyl radicals[62]. On the other hand, transferred electrons reacted with oxygen molecules to produce superoxide radicals. These radicals can directly result in the mineralization of CV into CO2 and H2O or indirectly degrade the CV by forming other active species[63]. The N-GQD enhance light-driven photodegradation by allowing the excited electrons to be prevented from recombination[64]. As shown in Fig. 5, photocatalytic performance is highly enhanced by 1.6 times.
CONCLUSION
The synthesis of N-GQDs and MnO2/N-GQD nanocomposite was successfully done and the characterization techniques such as FT-IR spectroscopy, XRD, EDX, Map analysis, SEM, UV-Vis spectroscopy, and PL analysis confirmed their synthesis and their structures. Then, MnO2/N-GQD as a photocatalyst was used to degradation of CV from aqueous solutions. The results show that the photocatalyst has excellent performance under visible light as well as UV irradiation in a very short time (≈ 10 min) has an efficiency of 100%. The experiments confirm that the nanocomposite has excellent photocatalytic activity in comparison with individual N-GQD and MnO2. The kinetic study of the reaction depicts that this photocatalytic degradation obeys the first-order model.
ACKNOWLEDGMENT
The authors are thankful to Imam Khomeini International University for the financial support of this study.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.