3.1. Investigation of Synthesis of Ru (II) complex/ g-C3N4 NTs
So far, various methods have been introduced for the synthesis of g-C3N4, all of which use nitrogen-rich precursors including cyanamide, dicyandiamide, melamine, urea, thiourea, triazine derivatives, and heptazine [31, 32]. One of the simplest and most common methods for synthesizing this substance is the condensation method [33–36]. In this method, the mentioned precursors are converted to g-C3N4 at 550 ° C. But the main challenge of this method is the variable condensation efficiency, which leads to the production of a product with a small surface area and, of course, limits its application to photocatalytic processes [37]. However, the surface area of g-C3N4 depends on the type of precursor and synthesis conditions, and by changing the above factors, a high surface area can be achieved. Typically, the use of melamine precursors results in the production of g-C3N4 with a high N/C ratio [38]. The higher the N/C ratio, the lower the energy bandgap and the higher the photocatalytic property. If melamine is used in combination with an oxygen-containing precursor to making g-C3N4, the product has more porosity and a higher surface area [39]. The use of chlorine-containing precursors has a similar effect, increasing the porosity and surface area, possibly due to the evaporation of chlorine ions in the form of HCl [40]. The second approach to achieve a high surface area is to change the synthesis conditions to create nanostructures. Most reports on nanostructures derived from graphite carbon-nitride are about nanoporous and nanoplate [41, 42]. In addition to these two structures, nanotubes have recently been introduced as a new morphology of g-C3N4, which, due to their high specific surface area, regular crystalline walls available for mass transfer, have desirable semi-conductivity properties [43]. There are two general methods for synthesizing g-C3N4 NTs: First method, hard templating method [44], which is done by polymerizing the precursors in the created space of the template (such as silica) and then removing the template by reagents that dissolve the silica. Although the g-C3N4 NTs obtained from this method has a high surface area, its crystallinity has decreased. Crystallinity is an effective parameter against the separation of excited charges in the photocatalytic process [45]. The second method is the soft templating [46], which its general mechanism is based on the polymerization of nitrogen-rich precursors in the space that has been created by soft template such as trithion X10, P12, and F124 [47]. The disadvantage of this method is the high polymerization temperature, which leads to the loss of the template before the complete formation of the nanotube. In the present project, g-C3N4 NTs were synthesized by the joint polymerization of nitrogenous (melamine) and chlorinated (cyanuric chloride) precursors through the hydrothermal process without the use of any template. The cyanuric chloride is unstable under aqueous conditions therefore; it was converted to cyanuric acid as an oxygenated precursor created melamine-cyanurate crystals through a series of hydrogen bonds with melamine. Finally, the growth of these crystals within 6 hours and calcination at 450°C, it cause to produce g-C3N4 NTs. Because of incomplete polymerization of melamine, there are free amine groups at the outer edges of the prepared g-C3N4 NTs and these defects in the structure cause to balance in the energy bandgap and also it facilitates the functionalization of the surface. So here, the functionalization was done in the through formation imine band between carbonyls of phenantroline-5,6-dione ligand and free amines of g-C3N4 NTs. In the end, the coordination of immobilized bidentate ligands with dichloro(p-cymene)ruthenium(II) led to the stabilization of the relevant complex on the outer edges of g-C3N4NTs. Scheme1 depicted the steps of the synthesis of the photocatalyst.
3.2. Catalyst characterization
3.2.1. Fourier transform-infrared spectroscopy (FT-IR)
The functional groups of melamine-cyanuric acid, g-C3N4 NTs, Ru(II)complex/g-C3N4 NTs samples were confirmed by FTIR spectroscopy. The melamine-cyanuric acid spectrum (Fig. 1a) is extremely crowded, which can be attributed to the formation of a complicated network of hydrogen bonds. The appeared peaks at 3088, 3230, and 3391cm− 1 are due to the symmetric stretching vibrations of the amine groups of melamine cations that have shifted to shorter wavelengths (blue shift) than pure melamine [48–49]. The asymmetric stretching vibrations of the amine groups attributed to bored peaks around 2500–2700 cm− 1. In additional, the intense peak at 1731 cm− 1 is assigned to carbonyl stretching vibrations of cyanurate anions which produced from hydrolysis of cyanuric chloride. The strong peaks in region 1400–1700 cm− 1 corresponded to bending vibrations of N-H and stretching vibrations of C-N and C = N. These peaks in the assembly indicate the successful synthesis of melamine-cyanuric acid crystals. After pyrolysis treatment, the arrangement of the absorption bands was changed (Fig. 1b). For example, the intensity of absorption bands attributed to N-H groups dropped sharply in region > 3000 cm− 1, and also the assigned peak of carbonyl at 1731 cm− 1 disappeared which confirmed the release of NH2 and H2O during pyrolysis treatment to the formation of g-C3N4. Figure 1c again shows the decreasing of the vibrations of the amino groups, which, together with the new appeared band at 1650 cm− 1, proves the chemical modification of the nanotubes through the formation of the imine band with phenantroline-5,6-dione ligand.
3.2.2. Ultraviolet and visible absorption spectroscopy (UV-Vis)
To get the UV-Vis absorption spectrum of g-C3N4 Nts, this material was dispersed in ethanol by ultrasonic bath for 20 minutes and then its adsorption was measured. As shown in Fig. 2, g-C3N4 NTs have maximum adsorption at 368 nm, which has a blue shift (the transition to a higher energy level or shorter wavelength) relative to the bulk sample [50]. In nanomaterials, the distance between the electron-hole is controlled by the particle size, so that as the particle size shrinks to the nanometer scale, the movement of excitons are limited and a transition in the optical spectrum is observed [51]. In general, the transfer of optical spectrum to higher energies due to the decreasing of particle size means an increase in the forbidden band energy. The energy difference (in units of electron volts) between the highest valence band and the lowest conduction band is called the energy bandgap [52]. To calculate the bandgap energy of a material, the absorption coefficient parameter of the material must first be obtained. The absorber coefficient is an important parameter in optical applications and is calculated for a transparent thin film from the following relation:
\(\alpha =\frac{1}{d}\text{ln}\frac{1}{T}\) ( 4)
Here, d is the thickness of the thin film in nanometers and T is the percentage of light transmission. Since g-C3N4 has a direct energy gap, so by direct electron transfer between the capacitance band and the conduction band, the band gap energy (Eg) is obtained from the Tuac Eq. (5).
(𝛼ℎ𝜈)(1/ 𝑛) = 𝐴(ℎ𝜈 − 𝐸𝑔) (5)
Here, A is constant, α is the absorption coefficient, Eg is the width of the energy gap, and hν is the energy of the emitting photon with respect to the absorption spectrum of the synthesized g-C3N4 NTs. Also, n for g-C3N4 NTs with a straight and permissible bandgap is equal to 1/2. According to Fig. 2, the energy gap of g-C3N4 NTs can be calculated by drawing (αhν)2(eV.cm− 1)2 in versus of hν (eV) and extrapolation its linear part. This energy gap was obtained 2.96 eV.
3.2.3. X-ray diffraction (XRD)
The prepared melamine-cyanuric acid sample and its Ru(II)complex/g-C3N4 NTs have been characterized by X-ray powder diffraction technique (Fig. 3). The presence of sharp diffraction peaks in the XRD of the synthesized melamine-cyanuric acid sample during the hydrothermal process confirms that the obtained product is highly crystalline [53]. The X-ray diffraction pattern changed completely after the pyrolysis process and the formation of g-C3N4 NTs nanotubes, followed by chemical modification.
New peaks have appeared around 2θ = 13.1 and 27.2 and confirm the formation of graphite nitride carbon phases in the sample [54]. An important point that can be seen in this image is the decrease in the intensity of the peaks after heat treatment, which indicates the percentage of crystallinity of the sample is reduced, or in other words, the long-range order of the structure is reduced.
During the heat treatment, the carbon nitride surfaces appear to be oxidized and the layers separated. Also, due to the oxidation process, the structure is defective and its effects in reducing the peak intensity of this material have appeared. Another reason for the low intensity of peaks is the modification process when an element enters the structure of the compound through chemical bonding, causing the corresponding peaks to flatten or shift and due to the stabilization conditions of the complex, there is no possibility of ruthenium phases in the final structure.
3.2.4. Scanning Electron Microscopy (SEM)
Scanning electron microscopy provides useful information from topography and morphology of the prepared photocatalyst. Scanning electron microscope images of the sample are shown in Fig. 5. These images were taken with different magnifications to determine the overall morphology of the sample and how the nanotubes are arranged. As you can see, the study sample contains clusters of orderly nanotubes and the amount of unwanted particles in that is very low.
To determine the composition of the elements in the prepared photocatalyst, the distribution of active sites, and to determine whether the ruthenium complex is embedded in the surface of the nanotube matrix or in the inner part of the matrix, two complementary analyzes of energy-dispersive X‐ray pattern and element distribution map (EDS & Map) were used. The EDS spectrum confirmed the presence of the elements carbon, nitrogen, ruthenium, and chlorine with the appearance of the corresponding peaks and the mapping analysis of polymer shows uniform distribution and non-accumulation of elements that enhance the properties of the photocatalyst.
3.2.5. Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) images showed that the structure of g-C3N4 is in the form of hollow curved tubes with an outer diameter of less than 10 nm and a length of several hundred nanometers (Fig. 5). It appears that after chemical functionalization of nanotubes and loading of ruthenium (II) complex, failure has occurred in areas where defects such as pentagons and heptagons. In addition, the modification process has resulted further distance between the nanotubes, which has a constructive effect on increasing the surface area of the photocatalyst. The Ru(II) complex/g-C3N4 NTs show accumulations in some areas, which possibly due to the agglomeration of graphitic nanotubes. This re-stacking of graphitic nanotubes is dependent on strong van der Waals forces and π-π interactions of sp2 structures and hydrogen bonding.
3.3. Investigation of effective factors in optical degradation of tetracycline
3.3.1. Influence of amount of photocatalyst
As shown in Fig. 6, the degradation efficiency of tetracycline increased with increasing photocatalyst concentration to 30 mg, after which the degradation efficiency remained almost constant. This can be attributed to the fact that when all the antibiotic molecules landed on the nanotubes, the excess amount of photocatalyst due to the lack of antibiotic molecules had no effect on the percentage of degradation and even slightly reduced the reaction rate. The decreasing in reaction rate at a dose of 40 mg was caused by two main factors. First; g-C3N4 nanotubes tend to agglomeration due to their nanometric size and high surface energy, and when the concentration of these nanotubes exceeds a certain limit, the activated nanotubes will be deactivated by contact with the base catalyst, and to follow it, the catalytic efficiency decreases. Second, with the increasing amount of photocatalyst, the turbidity increased and the light scattering occurs due to the collision of optical rays with the catalyst particles scattered in the solution and a number of light photons lose their energy and thus the efficiency of photocatalytic processes decreases.
3.3.2. Influence of irradiation time
To investigate the effect of irradiation time on degradation efficiency, a certain concentration of tetracycline was exposed to an optimized amount of photocatalyst (30 mg) and a frequency of 60 watts at different times. The results are shown in Fig. 7. As you can see, the percentage of antibiotic degradation increased with increasing reaction time, so that in 90 minutes, more than 99% of the tetracycline molecule was broken down into smaller components. This step was followed by monitoring the UV spectrum. But the degradation of the small organic components until producing carbon dioxide and water takes longer so that after 6 hours, 30% of the smaller molecules are still present in the solution. Chemical Oxygen Demand (COD) technique was used to determine the concentration of organic components in the second step. In this method, the sample is strongly refluxed by acidic solutions and a certain amount of potassium dichromate (K2Cr2O7). After digestion of the sample to determine COD, the residual and unoxidized amount of potassium dichromate is titrated with ammonium sulfate to determine the amount of potassium dichromate consumed and the oxidized material is calculated in the oxygen equations.
3.3.3. Influence of temperature
The influence of temperature on tetracycline degradation was shown in Fig. 8. Changes in process velocities in the range of 0 to 75 ° C indicate that degradation rate is directly related to reaction temperature (Fig. 8). In fact, the reaction temperature, on the one hand, contributes to the production of dissolved O2 and the degradation of H2O2, which ultimately leads to the production of active hydroxyl radicals, and on the other hand, it provides the activation energy for the reaction. Based on this, it can be said that the catalytic process is exothermic and by drawing ln k (rate constant) versus the inverse of the temperature (kelvin) and in accordance with Arrhenius equation [55] (Eq. 6), the activation energy of the reaction was obtained from the slope of the straight line. The value of the slope is equal to -Ea/R where R is a constant equal to 8.314 J/mol K. (Fig. 9).
$$\text{ln}\text{k}=\text{ln}\text{A}-\frac{{\text{E}}_{\text{a}}}{\text{R}\text{T}} \left(6\right)$$
The activation energy (Ea) of tetracycline degradation is estimated to be 0.94 kJ/mol.
3.3.4. Influence of pH
In optical degradation processes, pH can affect the desired decomposition rate of the contaminant. Previous studies have shown that pH plays an important role in the breakdown and elimination of antibiotics. As shown in Fig. 5, there is a significant difference between the percentage of tetracycline degradation at different pHs and the rate of decomposition at alkaline pH is significantly higher and optimized at pH = 9. The pH variable affects the adsorption and dissociation capacity of the target compounds, the electric charge distribution on the surface of the catalysts, and the oxidation potential of the conduction band.
Since the isoelectric point of g-C3N4 based materials is around pH = 9 [56] and the tetracycline antibiotic acidic, the effect of pH on the photocatalytic process can be justified by the presence of electrostatic forces between the surface of g-C3N4 NTs and tetracycline. Thus, at pHs > 9, the g-C3N4 NTs surface has a negative charge and the tetracycline molecules also have a negative charge, therefore, the force between them is repulsive and thay don’t have any reluctant to react and this cause to decrease resulting in the yield of photodegradation decreased at the pH = 11. At pHs < 9, the g-C3N4 NTs surface has a positive charge and the tetracycline molecules also have a positive charge due to protonation. Therefore, the electrostatic force between the photocatalyst and the tetracycline is the repulsive force, which leads to a decrease in degradation efficiency. Although, the highest percentage of pesticide degradation was obtained at an alkaline pH but, due to the economic aspect and ease of operation treatment at the neutral pH, investigation of other parameters in photocatalytic degradation of the mentioned pesticides was performed at pH = 7
3.3.5. Effect of chemical scavengers
To probe role of reactive species in the photodegradation of tetracycline in the present Ru(II) complex/g-C3N4 NTs, trapping tests were investigated under optimal condonation. For this purpose, isopropyl alcohol as a hydroxyl radical scavenger, ethylenediamine tetra acetic acid as a hole scavenger and sodium sulphate as an electron scavenger were added separately to the reaction mixture and result compared to no scavenger addition. In all three cases, the addition of scavengers reduced the rate and degradation percentage. According to these results, it can be deduced that the hydroxyl radical (•OH) and species generated via hole carriers as key drivers are involved in the photodegradation of tetracycline.
3.3.6. Investigation of synergistic effect
In order to study the synergistic effect, tetracycline degradation reaction in the presence of g-C3N4 NTs and Ru(II) complex/g-C3N4 NTs was investigated separately and the results are reported in Table1. According to the results, g-C3N4 nanotubes alone are able to destroy 62% of tetracycline in 90 minutes. After loading the ruthenium complex, the degradation percentage was higher than 99% for the same period. It seems combining two photon-active substances; g-C3N4 NTs and Ru(II) complex has been able to optimize the prepared photocatalyst bandgap. Therefore, the synergistic effect of Ru(II) complex and g-C3N4 NTs enhances the photocatalytic properties.
Table 1
The kinetic parameters of photodegradation of tetracycline in the present of different catalyst components
Entry
|
|
Tetracycline
|
Photocatalyst components
|
Degradation(%)
|
k (1/min)
|
1
|
g-C3N4 NTs
|
62
|
0.0109
|
2
|
Ru (II) complex/g-C3N4 NTs
|
99
|
0.0417
|
Reaction condithion: tetracycline concentration: 10 mg/L; photocatalyst dosage: 30 mg; irradiation time: 90 min; light intensity: 60 Watt; temperature: 25°C; pH:7