Photocatalytic efficacy of supported tetrazine on MgZnO nanoparticles for the heterogeneous photodegradation of methylene blue and ciprofloxacin

MgZnO@SiO2-tetrazine nanoparticles were synthesized and their photocatalytic efficiency was demonstrated in the decomposition of ciprofloxacin and methylene blue (MB). This new heterogeneous nanocatalyst was characterized by FT-IR, XRD, UV-vis, DRS, FE-SEM, ICP, and CHN. Distinctive variables including photocatalyst dose, pH, and degradation time were investigated. Up to 95% photodegradation was gained under the optimum conditions (20 mg photocatalyst, 3.5 ppm MB, pH 9) by using MgZnO@SiO2-tetrazine nanoparticles after 20 min. An elementary kinetic study was carried out, and a pseudo-first-order kinetic with a reasonably high rate-constant (0.068 min−1) was derived for the MB decay. Photoluminescence (PL) studies confirmed that the photocatalytic activity of MgZnO@SiO2-tetrazine was almost consistent with the Taugh plots. Thus, it can be envisaged that the photocatalytic activity is closely related to the optical absorption. Furthermore, a photoreduction mechanism was suggested for the degradation process. Addition of scavengers and some mechanistic studies also revealed that O2˙− is the original radical accounting for the degradation of MB, considering this latter compound as a model type pollutant. Finally, efficacy of the present photocatalytic process was assessed in the degradation of ciprofloxacin as a model drug under the optimum reaction conditions.


Introduction
Photocatalytic destruction of organic contaminants with semiconductor-based nanomaterials has attracted substantial attention, because it offers a plausible solution to environmental pollution issues. 1,2 It is well-known that absorption of photons with an energy higher or equal to the band gap of semiconductors can promote electrons from the valence to conduction bands and induces a large number of holes in the valence shell. If this charge segregation is valid, the generated holes and electrons can be utilized for the photocatalytic degradation of various organic contaminants. The produced holes are eligible to react with the surface-bound water molecules to generate strong oxidizing species such as the hydroxyl radical. Furthermore, the conduction band electrons might be absorbed by the dissolved molecular oxygen and form superoxide anion-radicals, which can behave as a strong oxidizing agent for most organic compounds. 3,4 Nowadays, many strategies have been introduced to modify the photocatalytic efficiency of a wide range of semi-conductors by improving the host structure with various metallic and/or organic dopants. [5][6][7][8][9] Various parameters such as dopant concentration, electronic structure of the semi-conductor, electronic conguration of dopant, and intensity of the illuminating source can strongly affect the photocatalytic efficiency of a system. 10,11 Moreover, the electronic features of a semiconductor can be signicantly inuenced by the dopant characteristics. For example, the photoenergy threshold of a semiconductor can change and recombination of the photoinduced charge carriers can occur by doping. 12 It is worth noting that the absence of localized d-electrons in the alkaline earth metals mark them as good candidates to alter the photocatalytic properties of a semi-conductor.
Among various semi-conductors, ZnO has been suggested as a perfect photocatalyst for decomposition of water pollutants. Development of the photocatalytic activity of ZnO nanoparticles with metal doping and combination of this material with organic modiers has already attracted much consideration. [13][14][15][16][17] Among different organic modiers, tetrazine ring 18 which is a very small organic uorophore with distinct electrochemical and photochemical properties, is selected to be graed on the surface of ZnO nanoparticles doped with Mg in this report. The tetrazine ring is strongly electron decient and can be reversibly reduced into its radical anion at a high reduction potential. 19,20 Therefore, the photocatalytic efficiency of MgZnO nanoparticles can be modied by the surface modication of MgZnO with 4-((6-chloro-1,2,4,5-tetrazine-3yl)oxy)butyl propylcarbamate (abbreviated as tetrazine). The obtained nanocatalyst was used in the photodegradation of MB and ciprooxacin and effects of some operational variables are investigated.

Results and discussion
2.1. Characterization and properties of MgZnO@SiO 2tetrazine 2.1.1. FT-IR spectroscopy. The fabricated nanomaterials were characterized by FT-IR spectroscopy. FT-IR spectra of the prepared samples involving MgZnO, MgZnO@SiO 2 -Cl, and MgZnO@SiO 2 -tetrazine are shown in Fig. 1. This study designated the covalent binding and anchoring of tetrazine onto the surface of MgZnO@SiO 2 -Cl. The exceptional absorption bands in the domain of 400-600 cm À1 are specic for ZnO. The FT-IR of MgZnO comprised a powerful absorption band at 445 cm À1 and a shoulder at 501 cm À1 which are due to Zn-O stretching vibrations. Albeit, a small amount of the surface segregated MgO had approximately no apparent inuence on the infrared optical properties of ZnO. 21 The MgZnO was functionalized with CPTS in reuxing toluene to obtain MgZnO@CPTS. Graing of tetrazine onto the surface of MgZnO@CPTS was approved by the observation of the aliphatic C-H bands around 2844 and 2916 cm À1 in the FT-IR spectrum. 22 Appearance of tetrazine characteristic bands in the FT-IR spectrum of MgZnO@SiO 2tetrazine conrmed that the structure of tetrazine was unchanged aer immobilization. Observation of weak bands at 1652, 673 and 649 cm À1 conrmed graing of tetrazine moiety onto the surface of MgZnO. However, small shis in Fig. 1c compared to 1d, would be due to the interaction of tetrazine with the surface functional groups of the solid support.
2.1.2. CHN, ICP, FESEM, and EDX of MgZnO@SiO 2 -tetrazine. The CHN analysis of MgZnO@SiO 2 -tetrazine revealed the contents of C, 1.78%; H, 0.29%; N, 0.69%, which pointed to the amount of 2.84% for the loaded tetrazine. Moreover, Mg content of MgZnO@SiO 2 -tetrazine sample obtained from ICP analysis was 6.736%. The surface morphology of MgZnO and MgZnO@SiO 2 -tetrazine nanoparticles was further studied by FE-SEM (Fig. 2) and showed disordered irregular nanoscale semi-spherical aggregates of nearly 50 nm in size with a narrow size distribution. It seems that, functionalization with CPTS and anchoring of tetrazine had no signicant impacts on the microstructure of MgZnO. EDX analysis conrmed the coexistence of Mg, O, Si, and Zn in MgZnO@SiO 2 nanoparticles (Fig. 3).   23 The approximately unaltered XRD templates for MgZnO@SiO 2 -tetrazine and MgZnO nanoparticles established that no apparent loss of crystallinity was detected aer tetrazine graing. However, rare and small variations in the Bragg intensities were observed.
2.1.4. Photoluminescence (PL) and diffuse spread reectance (DRS) spectra. PL spectroscopy can be regarded as a powerful technique to characterize the recombination behaviors of the photogenerated electron-hole pairs. The photoluminescence (PL) spectra of the samples are shown in Fig. 5. A UV lamp with the wavelength of 385 nm was used to excite the samples in this experiment. The samples exhibited a broad emission in the wavelength domain of 400-480 nm. Compared to tetrazine and MgZnO, the PL response for MgZnO@SiO 2tetrazine showed the strongest emission, suggesting a greatly enhanced radiative recombination of the photogenerated  carriers. This result can be attributed to the reduced separation of charge carriers due to formation of the heterojunctions between MgZnO and tetrazine.
Optical properties of MgZnO@SiO 2 -tetrazine were also examined by UV-vis diffuse reectance (DRS) at room temperature (Fig. 6a). The high visible light absorbance demonstrated that MgZnO@SiO 2 -tetrazine nanoparticles would have high visible-light utilization efficiency. As a result of tetrazine anchoring to the surface of MgZnO, the visible-light absorption edge was slightly shied from $425 nm for MgZnO 24 to about 405 nm, may be due to the presence of tetrazine. Moreover, the UV-vis spectra of MgZnO@tetrazine, MgZnO, and tetrazine are also shown in Fig. 6b.
2.1.5. TGA analysis. The thermal stability of MgZnO@SiO 2tetrazine nanocatalyst was investigated by carrying out TGA. As shown in Fig. 7, the sample exhibited three stages of decomposition. The rst gradual and continuous weight-loss occurred below 200 C, which was most likely due to water desorption from the surface of particles and loss of moisture content. Then, tetrazine moiety was started destruction at about 210 . Further loss at higher temperatures was probably corresponded to the progressive burning of the alkyl remnants which was accompanied with the crystallization of zinc oxide up to 530 C. 25,26

Photocatalytic activity of MgZnO@SiO 2 -tetrazine
To evaluate the photocatalytic activity of MgZnO@SiO 2 -tetrazine, photodegradation of MB was picked up in the absence of catalyst and <10% discoloration was reached aer 40 min irradiation; whereas, aer addition of MgZnO@SiO 2 -tetrazine, the degradation efficiency was clearly increased. However, no signicant degradation was attained without irradiation, even, in the presence of MgZnO@SiO 2 -tetrazine (16% degradation aer 30 min). The bare tetrazine moiety was almost ineffective even in the presence of light. Some preliminary experiments were achieved to study of the photocatalytic efficiency of various catalysts for the decolorization of an aqueous solution of MB, and the results are detailed in Fig. 8. Before each run, the surface adsorption of MB was calculated in the dark.
According to the above results, a sufficient amount of the photocatalyst (5 mg) was added to a 40 mL solution of MB (4 ppm) and the suspension was stirred for 30 min before sampling. Absorbance of the solution before and aer adsorption was applied to determine the surface adsorbed dye. Finally,  This journal is © The Royal Society of Chemistry 2019 the best discoloration efficiency was achieved with MgZnO@SiO 2 -tetrazine.
Previous DFT calculations conrmed that the photocatalytic activity of Zn 1Àx Mg x O is highly dependent on the Mg content. Substitution of Mg ions at Zn sites, shis the conduction band to higher energies and enhances the photocatalytic activity; while, incorporation of Mg 2+ at the interstitial sites, diminishes the photocatalytic activity. 27 Therefore, the effects of Mg 2+ doping on the photocatalytic degradation of MB were also explored. 2.2.1. Effect of initial dye concentration. Effect of the primary dye concentration (2.5, 3, 3.5, 4, 4.5, 5 and 5.5 ppm) was studied on the photodecolorization process in the presence of MgZnO@SiO 2 -tetrazine ( Fig. 9). As shown, an optimum concentration of 3.5 ppm was determined for efficient discoloration of MB. Presumably, at low MB concentration, less dye molecules can approach the catalyst surface, where the hydroxyl radicals are present, and the produced radicals will be deactivated before reaction with the dye molecules, leading to decreasing of the discoloration efficiency. 28 Thus, with enhancing dye concentration to 3.5 ppm, more MB molecules reacted with the produced hydroxyl radicals; thus, the discoloration was grew up. Under high concentration of MB, dye molecules acted as a lter for the incident light and did not permit the desired light intensity to reach the semi-conductor surface within a limited time lapse; therefore, the photodiscoloration had been decreased. 29 At this condition, more dye molecules were adsorbed onto the surface of photocatalyst and the active sites of the catalyst would be reduced. Hence, the number of accommodated substrate ions in the interlayer spacing was increased and deactivation of the photocatalyst occurred.   Thereby, the amount of reactive O 2À and OH À free radicals attacking dye molecules was decreased and lower photodecolorization efficiency was observed. 28, 29 2.2.2. Effect of photocatalyst amount. The inuence of photocatalyst dosage on the degradation of MB was monitored in the range of 5-25 mg. The represented results in Fig. 10, demonstrated the best decomposition in the presence of 20 mg of MgZnO@SiO 2 -tetrazine nanoparticles. This observation can be explained by the fact that with enhancing the catalyst amount, more active MgZnO@SiO 2 -tetrazine centers would be existed to receive photons and produce hole-electron pairs. 30 Also, by increasing of the catalyst dosage, the catalyst surface area, light absorption, and the number of active species were increased, hence, degradation of dye was inclined. 31 Nevertheless, at higher doses outside the optimum value, the solid particles can block penetration of photons; therefore, the overall number of photons approaching the catalyst surface to generate radicals was declined. 32,33 Moreover, some parts of the catalyst may appear in the dark part and cause diminishing of the light penetration. Inactivation of the activated molecules by collision with ground state molecules, light scattering, screening effects, and aggregation of nanoparticles may also reduce the photocatalytic activity under a high catalyst concentration.
2.2.3. Effect of pH. pH Plays a signicant role in the photodegradation efficiency. 34 Therefore, pH of a series of 3.5 ppm aqueous solutions of MB containing 20 mg of nanocatalyst was changed from 2.0-11.0 and degradation extent of MB was monitored (Fig. 11). The pH pzc for pure ZnO was provided at $9; whereas, the value of 10 was attained for MgZnO@SiO 2 -tetrazine. 35 The surface of MgZnO@SiO 2 -tetrazine has negative and positive charges at pHs higher and lower than pH pzc , respectively. Therefore, in strongly acidic conditions, the protonated MB was repelled from the positively charged catalyst surface and degradation was reduced. Thus, with increasing of pH toward pH pzc , MB molecules became deprotonated and the catalyst surface felt less charge; hence, more degradation was developed. Moreover, in strongly acidic condition, contamination of the solution with chloride anions from HCl increased concentration of OClc by the reaction of Cl À with OHc. Thus, considering the lower reactivity of OClc compared to OHc, degradation of MB was alleviated in strong acidic conditions. 36 Furthermore, the supported ZnO may be dissolved in a strong acidic pH and diminish the photodegradation efficiency. 33 At strong basic pHs, free-electron pairs of nitrogen atoms in MB molecules can be repelled from the negatively charged catalyst surface and diminish degradation efficiency. Our ndings proved that at pH 9, this repulsion is minimum and a sufficient amount of hydroxyl anions can be generated to produce hydroxyl radicals; so, the best degradation had been observed under this pH.

Reusability and recovery
Photocatalyst reuse is a privileged way to assess the ultimate cost of a photocatalytic path. The catalytic stability of the recycled MgZnO@SiO 2 -tetrazine nanoparticles was investigated via degrading MB under the optimum conditions over four consecutive runs. A simple proposed treatment ow chart for the reusability of MgZnO@SiO 2 -tetrazine is shown in Fig. 12. Degradation of MB was fullled at 25 C for 20 min in the attendance of MgZnO@SiO 2 -tetrazine. 95% Degradation was achieved at this stage. Aer that, the nanocatalyst was separated off and the reaction was continued with the ltrate for another 20 min under analogous reaction conditions. Only, 5% reduction in degradation was observed. This result demonstrated no signicant release of the catalyst components (principally tetrazine) during the reaction. Aer 4 th run, a smooth reduction in the photocatalytic activity was observed and degradation reached to 75%, may be due to the loss of reused catalyst during each time sampling, irreversible changes of the photocatalyst surface by pollutants, and probable leaking of the photoactive tetrazine moiety from the surface of MgZnO.

A brief kinetic study of MB degradation
A kinetic study was planned for the photocatalytic degradation of MB with MgZnO@SiO 2 -tetrazine as a function of time. As Fig. 14 shows, a pseudo-rst-order kinetic was developed for MB degradation according to the Langmuir-Hinshelwood model. 32 The photodegradation rate of MB can be shown by eqn (1): where C 0 is the initial concentration of MB (mg L À1 ), C is the MB concentration aer irradiation, k is the rate constant calculated from the slope of the straight line portion of the plot of log(C/C 0 ) versus t. The lower photocatalytic activity of tetrazine compared to MgZnO@SiO 2 -tetrazine would be explained by fast recombination of electrons and holes that are generated under the light illumination. Graing of tetrazine over MgZnO modied the charge separation and the photocatalytic activity of tetrazine was increased. Furthermore, dispersion of tetrazine onto the surface of MgZnO nanoparticles raised the number of active sites for the photoreduction of MB. According to Fig. 15, the rate constants, k, of 0.013, 0.022, and 0.068 min À1 were calculated for bare tetrazine, MgZnO, and MgZnO@SiO 2 -tetrazine, respectively.  2.6. Mechanistic aspects for the photodegradation of MB with MgZnO@SiO 2 -tetrazine 2.6.1. Effect of scavengers. When, holes and electrons get away from recombination, they can migrate to the surface and react with water, oxygen, and other molecules at the interface. Electrons can react with oxygen to form the oxidizing O 2 c À superoxide radical: However, O 2 c À is so reactive and can oxidize molecules and transforms itself into HOc via the following reactions: To identify the responsible species for the photodegradation activity of MgZnO@SiO 2 -tetrazine, some tests were performed with Cu 2+ and 2-propanol, as electron and hole scavengers, respectively. Data showed that presence of Cu 2+ slowed down the degradation kinetics. Cu 2+ reacted with electrons to yield Cu + , and this reaction was in competition with reaction (2). The presence of Cu 2+ may lead to diminishing of the generated O 2 c À and, therefore, decreasing of the degradation kinetics. The photodegradation efficiency of MB was decreased from 77 to 22% in the presence of Cu 2+ (0.01 M) aer 40 min under UV-vis light irradiation. Also, in the absence of O 2 , the reactivity of MgZnO@SiO 2 -tetrazine for MB degradation was diminished and deg% decreased from 77 to 50% aer 40 min irradiation. 38 Moreover, other experiments were performed in the presence of 2-propanol to scavenge the holes and HOc radicals. The experiments in the presence of O 2 and 2propanol did not show any signicant decrease of photodegradation. Additional experiments were planned with other scavengers such as EDTA and H 2 O 2 as holes (h + ), and HOc scavengers, respectively. Fig. 16 shows that EDTA and H 2 O 2 did not inhibit the photocatalytic activity of MgZnO@SiO 2 -tetrazine. All these experiments proved that photodegradation of MB by MgZnO@SiO 2 -tetrazine was mainly caused by O 2 c À .  To assign the optical band gaps, the optical absorbance measurements were planed at room temperature and the absorption spectra of tetrazine, MgZnO, and MgZnO@SiO 2tetrazine were attained. Absorbance spectra showed an absorption peak around 282 nm for tetrazine, 335 nm for MgZnO, and 332 nm for MgZnO@SiO 2 -tetrazine. To compute the direct band gap, the Tauc relation was utilized: 39 where a is the absorption coefficient and "A" is a constant. An extrapolation of the linear region of the plot of (ahy) 2 vs. hn gave the values of optical band gaps as 4.2, 3.82, and 3.75 eV for tetrazine, MgZnO, and MgZnO@SiO 2 -tetrazine, respectively, as shown in Fig. 17. 40,41 The Taugh plots indicated that the optical band gap of MgZnO@SiO 2 -tetrazine is relatively smaller than that of MgZnO and tetrazine. It means that the electrons may transfer to the conductive band with lower energies and the nanocatalyst could absorb photons with longer wavelengths. Therefore, the absorption band was expanded to the visible region of spectrum and the absorption edge shied to longer wavelengths. It means the optical absorption has been increased. [42][43][44][45] The photocatalytic activity and, therefore, quantum efficiency of MgZnO@SiO 2 -tetrazine was almost consistent with the Taugh plots. 46,47 Thus, it can be envisaged that the photocatalytic activity is closely related to the optical absorption. As the optical absorption is increased, the photocatalytic activity was also enhanced.
A plausible photocatalytic mechanism with charge separation in MgZnO@SiO 2 -tetrazine involving both hole oxidizing water and electron reducing oxygen is proposed in Scheme 1. O 2 produced at the valence band (VB) can taking part in the O 2 reduction at the conduction band (CB). Moreover, the generated electrons and holes contributed to the degradation process.

Materials and methods
Starting solvents and materials were purchased from Sigma-Aldrich, Fluka, and Merck, and were utilized as received without further purication. A high-pressure mercury vapor lamp (NAVIFLUX, 400 W, Berlin, NARVA) with lamp operating current of 3.25 A and a nominal voltage of 230 V was employed for the photocatalytic experiments. Morphologies of the synthesized nanomaterials were studied by a Mira 3-XMU eld emission scanning electron microscope (FESEM). Fourier transform infrared (FT-IR) spectra were performed on a Shimadzu 8700 Fourier transform spectrophotometer in the range of 400-4000 cm À1 with KBr pellets. UV-visible spectra were recorded by using a Photonix UV-visible array spectrophotometer. Elemental analyses were done by a Thermo Finnigan Flash-1112EA microanalyzer. X-ray diffraction patterns (XRD) were acquired on an Xpert MPD diffractometer with Cu K a radiation at 30 mA and 40 keV under the scanning rate of 3 min À1 in the 2q domain from 5-80 . Thermogravimetric analyses were performed on a TGA 92 Setaram at a rate of 10 C min À1 . The chemical composition of the prepared materials was determined by using an inductively coupled plasma spectrometer (ICP-MS; model VARIAN VISTA-PRO). Diffuse reection-transmittance (DRS-DTS) spectra were recorded on an Avaspec-2048-TEC.

Preparation of Mg 0.007 Zn 0.093 O nanoparticles (denoted as MgZnO)
Doped MgZnO was prepared by the sol-gel process. A methanol solution of zinc acetate dihydrate (50 mL, 0.093 M) was blended with another methanol solution of magnesium acetate tetrahydrate (50 mL, 0.007 M). Then, 10 mL of ethylene glycol (C 6 H 6 O 2 ) was added to the nal solution and stirred for 30 min. Aer that, the solution was warmed up gradually to 70 C with continuous stirring until a clear white gel was developed. The obtained gel was kept-up at room temperature for 24 h and, then, was heated to 250 C for 6 h. Eventually, the solid product was well grounded and heated to 650 C for 10 h to obtain a white color Mg 0.007 Zn 0.093 O (MgZnO) nano-powder. A similar procedure was followed for the preparation of other Mg 1Àx Zn x O metal oxides by changing the respective proportion of the reactants.

Photochemical procedure
The photocatalytic activity of the heterogeneous MgZnO@SiO 2tetrazine nanoparticles was assessed by suspending the catalyst into a solution of MB inside a simple hand-made Pyrex reactor equipped with a water circulating system at ambient temperature. First, the heterogeneous solution was stirred for 30 min in the absence of light to obtain the adsorption-desorption equilibrium of the dye molecules onto the nanocatalyst surface. Aerward, the photoreactor was adjusted at a distance of 8 cm from the light source and illuminated at 24 AE 1 C for the required time. Diminishing of the dye concentration was monitored by a UV-vis spectrophotometer. The degradation efficiency of the analyte was calculated using eqn (7): Fig. 19 Synthetic scheme for the preparation of "tetrazine". Paper where A 0 and A are absorbances of the analyte before and aer irradiation, which can be easily correlated to the nal (C) and initial (C 0 ) concentrations of the dye, respectively.

Conclusions
MgZnO@SiO 2 -tetrazine nanoparticles were synthesized, characterized, and utilized as an effective heterogeneous photocatalyst for the photodegradation of MB and ciprooxacin under the irradiation of a high-pressure mercury lamp in the UV-vis region. The effects of several parameters on the photodegradation of MB had been investigated. Enhancing the photocatalyst amount up to 0.02 g at 40 mL of MB solution increased the photodegradation efficacy. However, the efficiency was diminished when a higher concentration was used; perhaps due to the aggregation of the solid nanoparticles. The optimum pH was about 9; whereas a higher pH reduced the photodegradation efficiency. High MB concentration up to 3.5 mg L À1 enhanced photodegradation; however, beyond a dye concentration of 3.5 ppm, the photocatalytic degradation was alleviated. Moreover, the optical absorbance measurements were done at room temperature and the optical band gaps of 4.2, 3.82, and 3.75 eV were attained for tetrazine, MgZnO, and MgZnO@SiO 2 -tetrazine. The catalytic stability of the recycled MgZnO@SiO 2 -tetrazine was evaluated, and a high stability and reusability was attained. Pseudo-rst-order kinetics with a high rate constant (0.068 min À1 ) were obtained for MB degradation. Possible photocatalytic mechanism, which contributes to the oxidation of MB may include both reduction of oxygen with electrons and holes oxidation of water molecules.

Conflicts of interest
There are no conicts to declare.